Post-lithium-ion battery chemistries for hybrid electric vehicles and battery electric vehicles

Post-lithium-ion battery chemistries for hybrid electric vehicles and battery electric vehicles

Post-lithium-ion battery chemistries for hybrid electric vehicles and battery electric vehicles 7 P. Kurzweil University of Applied Sciences, Amberg...

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Post-lithium-ion battery chemistries for hybrid electric vehicles and battery electric vehicles

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P. Kurzweil University of Applied Sciences, Amberg, Germany

7.1  The dawn of batteries succeeding lithium-ion This chapter deals with next-generation batteries designed to be the powerful successors of today’s lithium-ion technology (Scrosati et al., 2013; Ritchie and Howard, 2006). As a mid-term solution, lithium-ion batteries entered the market of small power sources in the 1990s. In recent years, lithium-ion has ejected nickel-metal hydride batteries and high-temperature batteries from the market and is about to find its way into electric vehicles—until fuel cells fulfill the long-held visions of future road traffic and hydrogen economy in the long term.

7.1.1  Requirements for electric propulsion According to their press releases from 2013, Toyota Motors Corporation intends to replace the current “liquid” lithium-ion system with commercial solid-state batteries by 2020, followed by the lithium-air battery technology several years later (Greimel, 2013). The solid-state battery is predicted to be three to four times, and lithium-air more than five times, more powerful than the current lithium-ion battery of same weight. Solid electrolytes allow for connecting the single cells without the need for individual casings, which results in a most compact packaging. Future metal-air batteries will be designed to work on ambient “air” as the cathode, so that battery weight is essentially determined by the anode. Toyota’s research strategy aims at batteries having energy densities approaching that of gasoline. In other words, the energy density of lithium batteries must be improved by a factor of 50—to match a tank of gasoline. In urban transport, the range d of a compact electric car, having a curb weight of 1500 kg, and carrying a 300 kg battery, depends linearly on the specific energy of the battery W:

(

)

d /km = 2W / Whkg -1 . Today’s performance values between 80 and 120 Wh kg−1 correspond to a range of about 160–240 km. Long-distance driving at a constant speed of 80 km h−1 permits a theoretical range of d = 4.5 W. Advances in Battery Technologies for Electric Vehicles. http://dx.doi.org/10.1016/B978-1-78242-377-5.00007-8 © 2015 Elsevier Ltd. All rights reserved.

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The goals of the U.S. Advanced Battery Consortium (USABC) (Neubauer et al., 2014) for advanced battery cells for battery electric vehicles (BEVs) read: 350 Wh kg−1 (C/3), 750 Wh L−1 (C/3), 300 W kg−1 (10 s pulse), 700 W kg−1 (30 s pulse), life: 1000 cycles, operating environment −30 °C to +52 °C, recharge time < 7 h, high-rate charge to 80% state of charge (SoC) in 15 min, self-discharge < 1% per month. Argonne National Laboratory forecasts estimated energy densities of future lithium batteries in the range of 200 Wh kg−1 (375 Wh L−1) after 2020, and 300 Wh kg−1 (550 Wh L−1) toward 2030. Today’s energy densities must thereby be increased by a factor of 2.5 in the next two decades. The HORIZON 2020 program of the European Commission is currently pursuing nanotechnologies, advanced materials, and the production of post-lithium-ion batteries for electric automotive applications. The electrification of road transport has been identified as the key to environmentally friendly mobility in urban areas. Improved cost-competitive and sustainable storage technologies for electrified vehicles will more closely match the performance and driving range of internal combustion vehicles. This is in line with the road map of the European Green Vehicle Initiative (EGVI). Further, the commission considers it important for the European competitiveness that these next generations of batteries be produced in Europe. At the same time, energy density, power density, thermal stability, charging speed, and inherent safety of the battery cells have to be improved. New cell chemistries have to be found, and the aging mechanisms have to be thoroughly understood. Moreover, the future battery has to have a competitive cost, and it has to be produced in an environmentally friendly way, including sound recycling concepts and life-cycle assessment.

7.1.2  Shortcomings for advanced 5-V lithium-ion materials Unfortunately, no lithium-ion cell chemistry has yet been found that is able to meet the challenges of BEV propulsion over long distances such as the fossil fuels are already doing in today’s internal combustion engines. Some advanced lithium-ion technology providing energy densities of 250–300 Wh kg−1 by 20,125 or later face the 12,000 Wh kg−1 of gasoline (Danielson, 2011). Therefore, the following sections cannot be more than a momentary overview of the state-of-the-art of leading concepts of battery technology, which might possibly be able to replace today’s lithium-ion batteries in the next decades.

7.1.2.1  Positive electrode (cathode) Conventional 4-V metal oxides such as LiCoO2 (LCO, ~150 Ah kg−1, 3.9 V vs. Li|Li+, low power), LiNiO2 (LNO, ~170 Ah kg−1, 3.8 V), LiNi0.8Co0.15Al0.05O2 (NCA, ~190 Ah kg−1, 3.6 V, powerful), Li(NiMnCo)0.33O2 (NMC, ~160 Ah kg−1, 3.8 V) pose safety problems and are expensive.

5-V cathode materials Promising materials are compiled in Figure 7.1. ●

Spinels (Santhanam and Rambabu, 2010; Kraytsberg and Ein-Eli, 2012): Commercial LMO (LiMn2O4) yields about 4 V and 120 Ah kg−1, but shows low stability, although power, cost,

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Electrode potential vs. Li|Li+ (V)

5.5

800 Wh kg–1 1200 Wh kg–1 600 Wh kg–1 1000 Wh kg–1 Olivine fluorides 5.0 (Li2MPO4F) Olivines (LiMPO4) Spinels (LiM2O4) 4.0

Layered oxides (LiMO2) Li2MO3-LiMO2 Solid solutions

Silicates (Li2MSiO4)

3.0 2.5 100

Vanadium oxides (V2O5, LiV3O8) 200

300

Sulfur 1600

1700

Specific capacity (Ah kg–1) Figure 7.1  Performance data of advanced cathode materials currently under research.

and safety are good. The potential can be increased by replacing the manganese with transition metals. Computations revealed that the transition metal cations do not compromise the spinel structure but modify the electronic properties (Kawai et al., 1999). The redox reactions of transition metal ions improve the spinel structure LiMxMn2− xO4 (M = Ni, Co, Cr, Cu, Fe) by an additional discharge plateau. The spinel oxide Li[Ni0.5Mn1.5]O4 delivers 148 Ah kg−1 and an average working voltage for the NiII/NiIV couple of 4.75 V vs. a lithiated graphite negative electrode, and a first potential step at 4.1 V for the MnIII/MnIV couple.

Li éë Ni 0.5 II Mn1.5 ùû O 4 ® éë Ni 0.5 IV Mn1.5 ùû O 4 + Li + + e





The substitute works as a redox couple (Ni2+/Ni4+), and the electrochemically inert manganese stabilizes the structure (Ohzuku et al., 1999). More examples: LiFe0.5Mn1.5O4 (4.9 V), LiCo0.5Mn1.5O4 (5.0 V), and LiCoMnO4 (5.0 V). Unfortunately, these spinels are not stable in conventional electrolytes owing to cathode–electrolyte interaction. The formation of a badly conducting solid–electrolyte interface (SEI) can be improved by surface modification by AlPO4, ZnO, or Al2O3. Lithium excess materials: Li2MO3–LiMO2 solid solutions, are called layered-layered composites xLi2MnO3⋅(1 − x)LiMO2, or layered-spinel composites xLi2MnO3⋅(1 − x)LiM2O4, wherein M denotes a metal ion such as Ni, Mn, Co, or Cr. These materials exhibit potentials of about 4.8 V and a specific capacity of 220–280 Ah kg−1, but power capability is low, and voltage drops dramatically during its lifetime. In the first cycle, capacity drops irreversibly because the extracted Li-ions cannot move back into the layered Li2MnO3 lattice having lost its oxide vacancies (Xu et al., 2011). This noncommercial material was mainly developed by ANL and is currently under research. Vanadates: Vanadium oxides (V2O3, LiV3O8) provide voltages around 3 V. Lithium nickel vanadate (LiNiVO), an inverse spinel yields 4.8 V and 60 Ah kg−1.

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Phosphates: Lithium manganese phosphate (LiMnPO4), lithium cobalt phosphate (LiCoPO4, olivine, 4.8 V), lithium nickel phosphate (LiNiPO4, olivine, ~5.2 V) provide 160–171 Ah kg−1. The moderate ionic and electronic conductivity is improved by coatings and using small particles with guarantee short diffusion pathways.

Two-electron cathode materials ●





Olivine fluorides: Li2Mn(PO4)F provides a theoretical energy of 1218  Wh  kg−1 and −1 3708 Wh L . The demonstrated capacity of LiFeSO4F at a C/10 rate is only 130 mAh g−1 (Cheng and Scott, 2011). Two-lithium compounds isostructural to the natural mineral tavorite LiFe(PO4)(OH)34 encompass a broad range of chemical compositions LiM(YO4)X, combining a redox active metal M, a p-block element Y, and X = O, OH, or F. Promising candidates of tavorite-structured oxyphosphates, fluorophosphates, oxysulfates, and ­fluorosulfates might be V(PO4)F, MoO(PO4), WO(PO4), and NbO(PO4) (Mizuno et al., 2010). Two-lithium cycling has to be demonstrated in tavorite materials yet. Pyrophosphates: Li2MP2O7 (M = Fe, Mn, Co) (Barpanda and Nishimura, 2012; Quartarone and Mustarelli, 2011) suggest potential directions for future studies, although two-lithium cycling has not been demonstrated yet. Calculations reveal that the second lithium can be extracted from Fe and Mn pyrophosphates only at voltages above 5 V, for which practical electrolytes do not yet exist. Silicates of the general formula Li2MSiO4 (M = Fe, Mn, Co) can reversibly intercalate two-­ lithium ions per transition metal atom. They adopt a large number of polymorphs and promise theoretical capacities above 200 Ah kg−1. However, the manganese and cobalt silicates tend to rapid capacity fade. Iron silicate Li2FeSiO4 achieves reversible capacities of 120–140 Ah kg−1 (Huang et al., 2014; Li et al., 2013; Wang et al., 2013a). As kinetics is slow, elevated temperatures are required for high discharge rates. Unfortunately, the capacity is accessible only in a narrow cell voltage range, because the two distinct redox processes lie not far from each other.

Mixed cathode materials ●

Blended materials (Chikkannanavar et al., 2014) comprise physical mixtures of different active materials combining specific advantages. An expensive and heat-sensitive, but powerful and durable material (such as LiNixCo1− x − yAlyO2) can be combined with a low-­capacity material having good thermal stability, high-voltage, and high-rate capability (such as LiMn2O4). Typical blends are: – Spinel and layered oxides (LCO, NCA, NMC). – Olivines (LiFePO , LiMnPO ) and layered oxides (LCO, NMC). 4 4 – LiCoO with NMC, NCA, or spinels. For example, LiCoO  + LiNi Mn Co O yields 2 2 0.33 0.33 0.33 2 180 Ag kg−1 and 3.9 V vs. Li|Li+, compared to 151 and 153 Ah kg−1 of the components. – Blends lithium excess materials. Core–shell materials (Myung et al., 2013): Two materials of similar composition are incorporated into one particle, for example, the compounds Li(NixCoyMnz)O2 with x + y + z = 1. Nickel creates high capacity at the expense of thermal and cycling stability; cobalt leads to a good structural stability and conductivity; manganese achieves thermal and cycling stability at the expense of low capacity. A more or less smooth concentration gradient from Li(Ni0.8Co0.1Mn0.1)O2 (high capacity) in the particle bulk to Li(Ni0.46Co0.23Mn0.31)O2 (high thermal stability) in the outer layer avoids an abrupt change of composition and thus r­ educes mechanical stress at the interphase between shell and core during charge and discharge. Materials having 200 Ah kg−1 which retain 88% of the capacity after 1500 cycles were demonstrated (Noh et al., 2013).









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Disordered cathode materials: Generally, the reversible intercalation of lithium ions is thought to require ordered materials. However, disordered electrode materials such as Li1.211Mo0.467Cr0.3O2 allow excess lithium beyond the stoichiometric limit in a percolation network providing specific lithium transport pathways (Lee et al., 2014).

7.1.2.2  Negative electrode (anode) With reference to pure lithium (3850 Ah kg−1, 0 V), the state-of-the-art anode materials have reached performance data which hardly fulfill the requirements of electric vehicles. ●







Graphite (~350 Ag kg−1, 0.05–0.2 V vs. Li|Li+) combines moderate power capability, low cost, excellent stability, and safety. Amorphous carbon (~200 Ag kg−1, 0.1–0.7 V vs. Li|Li+) offers power capability and safety, however, at the expense of higher cost and moderate stability. Lithium titanate (Li4Ti5O12, ~150 Ah kg−1, 1.4–1.6 V vs. Li|Li+) promises excellent power capability, lifetime, stability, and safety, against the background of increased cost. Metal alloys are expected to be compact and light, cheap, and powerful. Owing to high crystallographic densities, their energy densities exceed that of graphite by a factor of 10. However, these materials are prone to low stability and safety problems, especially as unwanted volume changes occur during charge and discharge. By mechanical stress, the alloy electrode is gradually pulverized, and a growing SEI is continuously formed at the unpassivated alloy surfaces. Tin alloys have reached a commercial state (Inoue, 2011). Silicon–­ carbon composites range in the area of roughly ~1500 Ah kg−1 at potentials below 0.6 V vs. Li|Li+. Examples and theoretical values (Zhang, 2011): Li4.4Si Li4.4Sn Li3Sb LiAl LiMg







(4200 Ah kg−1, 9786 Ah L−1, 0.4 V vs. Li, volume change 300%) (994 Ah kg−1, 7246 Ah L−1, 0.05–0.6 V vs. Li, 260%) (660 Ah kg−1, 4422 Ah L−1, 0.9 V vs. Li, 200%) (993 Ah kg−1, 2681 Ah L−1, 0.3 V vs. Li, 96%) (3350 Ah kg−1, 4355 Ah L−1, 0.1 V vs. Li, 100%)

Nanosized materials, primarily based on silicon nanoparticles, nanowires, nanotubes, and hollow particles, guarantee small absolute volume changes even when strain is high. Furthermore, nanostructures improve the kinetics of lithium-ion transport during intercalation and deintercalation. Nanosized alloys provide high capacity but tend to limited lifetime below 750 cycles, so that consumer applications are preferred for the time being. With forming the alloy, a metastable amorphous Li-Si phase formed first, which can be stabilized in a restricted potential window, and then a moderately stable, crystalline intermetallic equilibrium phase (Limthongkul et al., 2003). Unfortunately, the energy density of nanosized composite anodes is limited because of large voids in the hollow structures, the low tap density of nanomaterials, and the addition of 50% and more carbon. A novel type of smart designed particles (aggregated nanoparticles) will overcome these problems. Composite electrodes (Santhanam and Rambabu, 2010), mainly silicon with graphite, combine the electrochemical properties of both material: silicon for high capacity and graphite for electronic conductivity and elasticity during volume changes. The composites are prepared by milling or coating the nanostructures. The performance data and lifetime of the composites depend on structure, morphology, and composition (Terranova et al., 2014). Metal oxides (~750 Ag kg−1, 0.8–1.6 V vs. Li|Li+), as well, do not meet the requirements of stability, safety, and cost.

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7.1.3  State-of-the-art vehicles For consumer application, more than 3 billion of lithium-ion cells are produced every year, mostly based on the 18,650 cylindrical cell using LiCoO2/graphite. According to Panasonic (PANASONIC, 2010), 3-Ah devices are based on LiNiO2/graphite, until future LiNiO2/SiC system will work in 4-Ah capacities. Panasonic’s NCR18650B battery (LiNiCoAlO2/graphite) already provides 266 Wh kg−1 and 691 Wh L−1. For transportation, LiFePO4 (LFP) seems to be one of the most suitable materials for small electric vehicles with respect to high power, low cost, high stability, and safety. Such lithium-ion cells are currently manufactured, for example, by LEJ (21 Ah, prismatic, 3.3 V, 108 Wh kg−1) and A123 (20 Ah, pouch, 3.3 V, 135 Wh kg−1) for plug-in hybrid electric vehicles (HEVs). Nickel oxide mixtures are used by Sanyo, Samsung, and LG in prismatic (~112 Wh kg−1) and pouch cells (~149 Wh kg−1, 3.7 V). AESC presented a 23-Ah pouch configuration (NCO-type/graphite, 3.75 V, 151 Wh kg−1). BEVs have been recently realized by several car manufacturers using ­semi­commercial batteries, weighing 50–75% of the total car and providing package energies of about 90 Wh kg−1. ●

















Daimler Smart: 17 kWh, 3.1 kW kWh−1, 101 Wh kg−1 (Li-TEC, pouch, graphite/NMC, 52 Ah, 3.65 V, 152 Wh kg−1, 316 Wh L−1). BMW i3: 22 kWh, 6.7 kW kWh−1, 94 Wh kg−1 (Samsung). Renault Zoe: 26 kWh, 3.1 kW kWh−1, 93 Wh kg−1 (LG Chem, pouch, graphite/NMC-LMO, 36 Ah, 3.75 V, 157 Wh kg−1, 275 Wh L−1). Fiat 500: 24 kWh, 5.2 kW kWh−1, 88 Wh kg−1 (Samsung-Bosch, prismatic, graphite/NMCLMO, 64 Ah, 3.7 V, 132 Wh kg−1, 243 Wh L−1). Nissan Leaf: 24 kWh, 3.8 kW kWh−1, 82 Wh kg−1 (AESC, pouch, graphite/LMO-NCA, 33 Ah, 3.75 V, 155 Wh kg−1, 309 Wh L−1). Mitsubishi i-MEV: 16 kWh, 3.8 kW kWh−1, 80 Wh kg−1 (Li Energy Japan, prismatic, graphite/ LMO-NMC, 50 Ah, 3.7 V, 109 Wh kg−1, 218 Wh L−1). Honda Fit: (Toshiba, prismatic, LTO-NMC, 20 Ah, 3.3 V, 89 Wh kg−1, 200 Wh L−1). Coda EV: (Lishen Tianjin, prismatic, graphite/LFP, 16 Ah, 3.25 V, 116 Wh kg−1, 226 Wh L−1). Tesla Model S: (Panasonic, prismatic, graphite/NCA, 3.1 Ah, 3.6 V, 248 Wh kg−1, 630 Wh L−1).

7.1.4  Future visions beyond lithium-ion Future visions focus on completely new approaches to meet the challenge of energy densities above 200 Wh kg−1. Critics complain the target of 400 Wh kg−1 by end of the decade is unrealistic. Such batteries would be five times more energy dense than the standard of the day—especially as the energy density of rechargeable batteries has risen only sixfold since the early prototypes at the beginning of the twentieth century: 1. Metal-sulfur batteries: Lithium-sulfur promises a theoretical specific energy of 2500 Wh kg−1. 2. Metal-air batteries: Metal-air batteries use the oxidation of ignoble metals in air. Practical cell voltages range between Li (2.4 V) > Ca (2.0 V) > Al (1.6 V) > Mg (1.4 V) > Zn (1.2 V) > Fe (1.0 V). Operation in pure oxygen raises cathode potentials by about 50%. The theoretical limits equal: lithium-air 11,600 Wh kg−1, aluminum-air 5000–8000 Wh kg−1, zinc-air 960 Wh kg−1, iron-air 764 Wh kg−1. 3. Chemistries based of cations – Sodium-ion batteries – Lithium dual-ion cells



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4. Chemistries based on anions – Fluorine batteries

7.2  Lithium-sulfur battery Lithium-sulfur batteries (Kim et al., 2013; Chen and Shaw, 2014; Ding et al., 2014) have been studied since the late 1960s. At best, the possible reduction of the “dead weight” by the inactive host materials in lithium-ion batteries, the lithium-­sulfur system promises an energy density of 600 Wh kg−1, and a theoretical capacity of 1675 Ah kg−1 (elemental sulfur), at a most favorable price (about US$25 per ton of sulfur). However, there is still a long road ahead before current research results will bring marketable products. Any forecasts, whether a mature lithium-sulfur battery will propel electric vehicles by 2020 or 2025, seem still vague and uncertain—­ especially as the first Li-S batteries were posited 40 years ago and did not survive past about 100 cycles until recently. The production of cheap commercial cell that works over a wide range of temperatures seems all but simple. Nevertheless, ­lithium-sulfur is considered by most experts to have great potential as the next-generation high-­ capacity battery. Basic cell reaction: A combination of solid lithium and chemically active sulfur should deliver about 2.5 V according to the following simplified basic cell reactions and standard potentials in aqueous solution:

(−) Anode: (+) Cathode: Cell reaction:

2 Li  2 Li + + 2e S + 2e –  S2 2 Li + S  Li 2S

Standard potential

Specific capacity

E0 = −3.040 V E0 = −0.476 V ΔE0 = 2.564 V

3861 Ah kg−1 1673 Ah kg−1

Actually, natural sulfur appears as S8, and elemental lithium and sulfur do not prefer reversible reactions. In practice, the cathode reaction proceeds via several intermediates with different potentials. Soluble Li-S compounds can seep into the electrolyte and the cell gums up. Lithium metal anodes pose safety risks caused by dendrite growth. On the other hand, sulfur is cheap and nontoxic and works in a safe potential range (1.5–2.5 V vs. Li|Li+). Challenges: (1) Sulfur, an electrical insulator, must be supported by a conductive matrix that allows ions and electrons to diffuse on its surface. (2) Intermediate polysulfides dissolve into the organic electrolyte. Li2S and Li2S2 from the cathode deposit on the lithium anode and undergo undesired parasitic reactions. (3) During discharging, the sulfur volume expands by up to 80, which causes pulverization of the cathode.

7.2.1  Lithium polysulfide battery Lithium sulfide (Li2S8) provides much higher energy density than lithium-ion metal oxide chemistries. Specific energy is estimated at 2600 Wh kg−1 (theoretically) and 150–378 Wh kg−1 (in practice). Unfortunately, the specific power of present ­lithium-sulfur battery is rather low, because sulfur is an electric insulator (conductivity: 5 × 10−30 S cm−1 at 25 °C) and tends to form a variety of polyanions.

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7.2.1.1  Cell chemistry The lithium-sulfur battery consists of a lithium anode (−), a sulfur cathode (+), and a nonaqueous electrolyte. During discharge lithium sulfides are formed, and Li2S is deposited on the carbon matrix. During charging, Li2S does not bring back sulfur, but forms polysulfide anions [Sx]2− which diffuse through the electrolyte as a shuttle (see Figure 7.2a and b). S + 2 Li + ® Li 2S + 2e - (sulfur is reduced during discharge) S8 ® Li 2S8 ® Li 2S6 ® Li 2S4 ® Li 2S3 ® Li 2S2 ® Li 2S 2 Li ® 2 Li + + 2e S8 ® Li 2S8 ® Li 2S6 ® Li 2S4 ® Li 2S3

(+) Cathode: (−) Anode: Self-discharge

At the sulfur cathode, between S8 (fully charged) and the formation of Li2S, different reduced species occur depending on the depth-of-discharge: Li2S8 at 12.5% DoD (2.4 V), Li2S4 at 25% DoD (2.2 V), insoluble Li2S2 at 50% DoD, and finally insoluble Li2S at 100% discharge (2.05 V). The chemical reaction proceeds more and more into the sulfur grain with rising DoD. The cell voltage equals only 2.1 V, but lithium-sulfur cells tolerate overvoltage. At room temperature, two voltage plateaus at 2.3–2.4 V and 2.1 V correspond to the electrochemical reduction (acceptance of electrons) from [S8]2− + 2e− → 2[S4]2− and [S4]2− + 4e− → [S2]2− + 2S2−, respectively. On the lithium anode a SEI is formed. Passivation and soluble products: Unfortunately, some lithium-sulfur intermediates are soluble in the electrolyte and can react directly with the lithium electrodes.

Charge Discharge

Li2S Discharge

S2–

[Sn]2–

Li2Sn Li+ Charge

(a)

S8

E vs. Li/Li / V

II

III

2.1

+

Li+

Sulfur/graphite

Lithium

+

I

2.4

S8

Li2S8 Li2S4

0% 12.5% 25%

Li2S2

Li2S

50%

100%

Depth of discharge (%)

(b)

S8

S8

Li2Sn (n ≥ 4)

(–) anode

Porous carbon Sulfur

(c)

Lithium

Li2Sn (n < 4)

Li2S

cathode (+)

Discharge Charge

Pore (1 – 2 nm)

(d) Lithium Interlayer

Electrolyte

Sulfur

Figure 7.2  Lithium-sulfur battery: (a) cell design and electrode reactions, (b) stages during discharge, (c) nanostructured carbon-sulfur cathode, and (d) lithium hybrid anode with graphite layer adjacent to the electrolyte space. See text.

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The sulfides Li2S2 and Li2S are insoluble in the electrolyte and cause a passivation layer on the electrode surface. Therefore, the depth of discharge must be limited in practice. The lithium polysulfides Li2S8 to Li2S3 are soluble in the electrolyte and can be oxidized and reduced by chemical reactions that do not provide electrical current; they cause considerable self-discharge (6–15% per month) and low efficiency. For instance, Li2S4 +  6 Li → 4 Li2S.

Technical measures, such as protecting layers on the lithium electrode, membranes instead of porous separators, gel electrolytes, and solvents, which reduce the solubility and transport rate for sulfides, represent attempts to reduce self-discharge. Lewis acids such as BF3 have been found to suppress polysulfide formation.

7.2.1.2  Sulfur cathodes The perfect cathode should have sufficient sulfur content, good conductivity, and a flexible structure to buffer the volume changes, and retain polysulfide intermediates within the electrode. As sulfur is an insulator (5 × 10−30 S cm−1 at 25 °C), it must be incorporated into an electronically conducting structure such as carbon (powder or multiwall nanotubes) with the help of a polymer binder. Carbon and sulfur are preferably mixed in a weight ratio of 1:2 in the electrode. Replacing carbon black with fine graphite powders mitigates the fragileness of electrode at high sulfur loading. Typically, a mixture of poly(vinylidene fluoride) (PVDF) and N,Ndimethylformamide (or PVA/acetonitrile, PVP/isopropanol) is used as a binder. In recent studies, sublimed sulfur is heated with polyacrylonitrile (PAN) to form heterocyclic decomposition products with intercalated sulfur. Nanostructured sulfur cathodes, such as porous carbon-sulfur composites or sulfur-containing nanotubes, provide increased surface-to-volume ratio and short pathways for ions and electrons. Macroporous carbon (pore size > 50 nm) cannot effectively retain sulfur and polysulfides. Sufficient pore volume requires mesopores ranging from 2 to 50 nm. Initial capacitances of up to 1400 Ah kg−1 were reported. Microporous carbon (<2 nm) with narrow pore size distribution immobilizes sulfur effectively and prevents intermediate polysulfides from outflowing into the electrolyte. The pores should not completely be filled with sulfur to save channels for Li+ migration. In a bimodal porous carbons, the large pores accommodate the liquid electrolyte while the small pores host sulfur and confine polysulfides. Different from natural sulfur S8, the metastable sulfur allotropes S2, S3, and S4 fit in carbon pores of the size 0.5 nm. Linear chains of sulfur even display metallic conductivity. Hydrophilic carbon, by adding mesoporous silica, helps to trap more polysulfides. Carbon nanotubes and carbon nanofibers are flexible, quasi-two-dimensional matrices for sulfur. Capacities of 900 Ah kg−1 and more are obtained, if sulfur is not coated on the surface, but brought into the hollow structures by template synthesis or chemical vapor deposition. Polyvinylpyrrolidone (PVP) seems to hinder the detachment of polysulfides from the carbon nanotube surface. Polyaniline nanotubes (PANI-NT) with encapsulated sulfur were described.

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Graphene, its sheetlike shape and open structure is rather inappropriate for loading sulfur and retarding polysulfide dissolution. Nevertheless, sandwich structures and coatings with sulfur, polyethylene glycol (PEG), a sulfur and carbon black have been tested. Three-dimensional (3D) nanostructured sulfur composites provide porosity and accommodate volume changes, for example, by using 3D carbon nanotubes, polymer nanotubes, or metal organic frameworks. Core–shell structures are coatings around spherical sulfur particles, that is, the hollow within the shell is completely filled with sulfur. In yolk-shell structures the void space is not completely filled with the sulfur core (Seh et al., 2013) (Figure 7.2c). Usually, sulfur is encapsulated in a conductive material (e.g., carbon, polymer, TiO2); otherwise capacity retention is poor. Insufficient coating results in polysulfide dissolution into the electrolyte, and fracture of the shell structure during volume expansion and constriction. Ultrafine sulfur, double shells, soft shells, and yolk-shells were realized. Conducting polymer-sulfur nanocomposites improve electrical conductivity and adsorption of polysulfides, for example, sulfur in polypyrrole or poly(pyrrole-co-­ aniline). Unfortunately, capacity retention and cycle stability are low. In aqueous solution, the self-assembling PVP molecules form a double-layer structure with an interior backbones (which include sulfur nanospheres), and an exterior wall of hydrophilic amide groups. Self-healing polymers, useful for volume changes, comprise (1) soft polymers with dynamic bonds to replace binders, or (2) micron-sized capsules with embedded polymer agent that will leak out to heal any fractures. Lithium sulfide (Li2S), offers a theoretic capacity of 1166 Ah kg−1, and can be paired with anodes of silicon or tin (instead of lithium), but it is a poor electronic and ionic conductor. At an activation potential of about 1 V, Li2S is converted to lithium, sulfur, and polysulfides. Ball-milled Li2S with carbon black, carbon-coated Li2S, porous materials filled Li2S, heat-treated Li2S–carbon–polyacrylonitrile mixtures, the superionic-conducting Li2S–Li3PS4 core–shell structure (from Li2S and P2S5 in tetrahydrofuran), and Li2S/ microporous carbon cathodes (by vaporizing sulfur molecular into the matrix, and subsequently spraying lithium metal powder) were reported. Additives in cathodes shall (1) increase the ionic and electronic conductivity, and (2) adsorb intermediate polysulfides to mitigate its dissolution and the shuttle phenomenon. ●





Porous silica and titanium in carbon-sulfur composites improve cycling stability, because they adsorb and desorb hydrophilic lithium polysulfides reversibly. Nanosized Mg0.6Ni0.4O, in sulfur–PAN composites absorbs polysulfides and promotes redox reactions and displays about 100% coulombic efficiency over 100 cycles at 0.1 °C. Polypyrrole (PPy)  +  poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAAMPSA) is an ionic-electronic conductor for sulfur cathodes with improved discharge capacity (500 Ah kg−1 at 1 °C after 50 cycles) and cycling stability.

Binders maintain the stable structure of the sulfur cathodes. Binders shall (1) disperse sulfur and carbon uniformly, (2) have low electric resistance, (3) buffer the volume expansion when discharging and contraction when charging, and (4) retain polysulfides and limit its dissolution.

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PVDF is an electrochemically stable adhesive, which is usually dissolved in N-methyl-2pyrrolidone (NMP) at elevated temperature above 80 °C, where sulfur sublimes. Polyvinylpyrrolidone + polyethyleneimine, gelatin, and styrene butadiene rubber (SBR) +  sodium carboxyl methyl cellulose (CMC) suppress the agglomeration of sulfur and carbon.

7.2.1.3 Anodes Lithium metal exhibits the high specific energy and cell voltage, but it reacts with common electrolyte media, organics and polysulfides, forms a passivating layer (SEI) and tends to the growth of dendrites through the separator, shorting the cell, and causing thermal runaway and fire. Protection layers, metal-free anodes, and lithiated silicon were considered at the cost of cell voltage: 1.5–2 V.

7.2.1.4  Current collectors The current collector is a highly conducting substrate on which the active electrode materials are coated. Nano-cellular carbon foam and carbonized eggshell membranes are able to store sulfur and localize dissolved polysulfides in abundant pores.

7.2.1.5 Electrolytes The electrolyte (Scheers et al., 2014) is of great importance for cell cyclability, rate capability, safety, and life span of lithium-sulfur batteries. Liquid organic electrolytes contain lithium salts in organic solvents. They (1) dissolve polysulfides well, (2) are chemically stable against polysulfide anions and radicals and lithium, and (3) have low viscosity for fast ion and charge transports. Carbonate solvents are usually not suitable: ●



The state-of-the-art Li-S battery electrolyte of today is a 1 M solution of LiTFSI in 1,3dioxolane (DIOX)  +  1,2-dimethoxyethane (DME) (1:1, v/v)  + possibly LiNO3. The choice of the salt LiTFSI in place of LiCF3SO3 (LiTf) generally doubles the electrolyte conductivity. LiNO3 supports the formation of a stable interface film at the Li-metal surface. Ternary solvent mixtures have seldom been used, although the addition of diglyme (G2 = CH3[OCH2CH2]2OCH3) seems to be advantageous. Cyclic and linear ethers, such as tetrahydrofuran (THF), mixtures of DIOX, and DME, and tetra(ethylene glycol) dimethyl ether (TEGDME, tetraglyme, G4), are suitable due to their polysulfide dissolution. LiCF3SO3 is a useful conducting salt. Fluoroethers, such as CHF2CF2OCH2CF2CF2H, form a surface film on lithium, thus impeding polysulfide reduction and alleviating the unwanted redox shuttle. A mixture of TEGDME and DIOX has been found to be appropriate to form a stable SEI. The dramatic capacity fade at low temperatures (~1300 Ah kg−1 at 20 °C to 360 Ah kg−1 at −10 °C) can be alleviated by adding 5% methyl acetate.

Solid-state electrolytes fundamentally eliminate the problem of polysulfide ions. They must (1) exhibit high lithium-ion conductivity, (2) be stable with lithium metal anodes, and (3) provide a large contact area with the electrodes. Polyethylene oxide (PEO) with lithium salts and fillers (ZrO2 nanoparticles, LiAlO2) show high ­coulombic

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­efficiency. Li2S–SiS2 powers, thio-LiSICON, Li2S–P2S5 glass-ceramics, and so on, are outlined in Section 7.4. Gel polymer electrolytes comprise a conductive liquid electrolyte in a solid matrix, which provides mechanical strength, is impermeable to polysulfides, and suppresses dendrite formation. The polymer matrix is cast or hot-pressed to form a membrane, which is soaked with electrolyte; for example, EC/DMC and LiPF6 in a matrix of PEO/LiCF3SO3. Electrospun nanofibrous polymer membranes combine interfacial compatibility, oxidation stability, and ionic conductivity. Tris(methoxypolyethyleneglycol)-borate ester (PEG-B) can increase ionic conductivity and Li-ion transference number. Blends of poly(methyl methacrylate), containing trimethoxysilane domains, and PVDF-HFP deliver higher ionic conductivity and less capacity decay compared to the nonfunctionalized polymer. Ionic liquids inhibit the formation of lithium dendrites and the dissolution of polysulfides, and they change the morphology of the SEI. Methyl-n-butyl-piperidinium bis(trifluoromethanesulfonyl) imide (PP14TFSI) suppresses the dissolution of polysulfides in the electrolyte. 1-Ethyl-3-methylimidazolium bis(tri-fluoro-methyl-sulfonyl) imide (EMITFSI) and n-methyl-n-­allylpyrrolidinium bis(tri-fluoro-methane-sulfonyl) imide (P1A3TFSI) exhibit superior capacity performance. A mixture of n-­methyl-nbutyl-piperidinium bis(tri-fluoro-methane-sulfonyl) imide (PYR14TFSI) + TFSI + LiT FSI + PEGDME is a very promising electrolyte for Li-S cells. Electrolyte additives: LiNO3 is believed to enable the formation of a protective film on lithium anode, thereby suppressing polysulfides and improving capacity retention. However, insoluble reduction products of LiNO3 affect the reversibility of the sulfur cathode adversely (below a discharge voltage of 1.6 V). Interestingly, excess lithium polysulfide anions in the electrolyte diminish the dissolution of these ions at the electrodes.

7.2.1.6  Membrane separators The membrane functions as ionic conductor and electronic insulator to avoid internal short circuit and provides mechanical strength and flexibility. In lithium-sulfur batteries, the separator must hinder migration of polysulfides to the anode side and thus prevent the shuttling. Lithiated Nafion ionomer and Nafion-coated polypropylene were tested with respect to inhibiting the transport of polysulfide anions. Interlayers between the sulfur cathode and the separator shall effectively retain sulfur-based materials within the cathode. Useful are carbon paper with hydroxyl functional groups, multiwalled carbon nanotubes, meso- and microporous carbon, carbonized eggshell membranes, and atomic layer deposited Al2O3. A graphite layer can be used as a shield at the anode side, as shown in Figure 7.2d (Huang et al., 2014).

7.2.1.7  Experimental cell data Current research focuses novel sulfurous materials with good conductivity, and all solid-state batteries. As metallic lithium anodes pose a problem of dendrite growth, tin-carbon alloys are discussed. An incomplete overview on current achievements is given in Table 7.1.

of some lithium-sulfur systems

Anode

Cathode

Electrolyte

Performance data

Lithium metal

sulfur + carbon

Polyethers in dioxolane

Lithium-tin-carbon alloys Lithium metal

Li2S–C composite

Composite gel polymer membrane

<1300 Ah kg−1, usually 140–170 Ah kg−1 ~600 Ah kg−1, ~2 V

Sulfur from petroleum processing

~1200 Ah kg−1 (60 °C), slow charge

Lithium metal

Sulfur in a PAN network

Solid lithium-conducting polysulfidophosphates (3 × 10−5 S cm−1 at 25 °C): Li3 éë( Sx )3 P = Sùû + 2 Li  Li3 [S3 P = S] + Li 2Sx Ion liquids: [Li(glyme-4)][TFSA]

Lithium metal

Carbon + PVDF + NMP

Polysulfide catholyte: TEGDME + 1 M LiCF3SO3 + 5% Li2S8 + 0.4 M LiNO3

2 V, 430–700 Ah kg−1 LiNO3 stabilizes under extended voltage

References

Hassoun and Scrosati (2010) Lin et al. (2013)

Terada et al. (2014) Agostini et al. (2014)

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Table 7.1  Overview

M, molar (mol L−1); glyme, CH3O(CH2CH2O)nCH3; TFSA, bis(tri-fluoro-methyl-sulfonyl)amide; TEGDME, tetraethyleneglycol dimethyl ether; PAN, polyacrylonitrile; LiTFSI, lithium bis(tri-fluoro-methane-sulfonyl)imide; DME, 1,2-dimethoxyethane; DOL, 1,3-dioxolane.

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Lithium-sulfur prototypes by Sion Power (2.2 Ah) have provided specific energies above 350 Wh kg−1 (350 Wh L−1). Cycle life, self-discharge, and novel binder systems (apart from PVDF) are subject to further research.

7.2.2  Lithium-organosulfur battery A low-cost approach, which differs in many ways from the more common lithium-ion batteries, takes advantage of the reversible redox cleavage of the sulfur–sulfur bond to give lithium thiolate: R – S – S – R + 2 Li + + 2e - ® 2 LiSR where R denotes an arbitrary molecule fragment of the electrode material. The system presently suffers from solubility of the thiolates in the electrolyte, leading to self-­ discharge and bad cycle life. 2,5-Dimercapto-1,3,5-thiadiazole yields a theoretical capacity of 362 Ah kg−1 and a discharge voltage of 2.5–2.75 V.

7.3  Lithium-air battery A future option of energy storage is given by the lithium-oxygen systems (Quartarone and Mustarelli, 2011; Li et al., 2013; Wang et al., 2013a) in organic or aqueous electrolytes, that pull in oxygen from the air. Practical specific energy is estimated at about 500 Wh kg−1, roughly a tenth of the theoretical capability. With regard to metallic lithium, the Li–O battery can, in theory, store energy as densely as a petrol engine. However, such low current densities as 0.1 mA cm−2 are still a long way from fruition of a lithium-air system in electric vehicles. The lithium-air technology can be divided into (1) nonaqueous lithium/air, (2) aqueous lithium/air, (3) all solid-state electrolyte, and (4) submersible lithium-air systems.

7.3.1  Dry and nonaqueous lithium-air systems History: Presumably the first lithium-air battery in alkaline aqueous solution was described in 1976 by researchers at Lockheed (Littauer and Tsai, 1976). In 1987, a high-temperature system was proposed to work at 600–850 °C, similar to a solid oxide fuel cell (Tsai et al., 1990). In 1996, a nonaqueous Li-air system was outlined by Abraham and Jiang (1996). The rechargeability was explored in 2006 (Ogasawara et al., 2006). Historically, the Canadian company MOLI created the first commercial Li-metal cell (Li-MoS2) in 1989, which was the market’s first rechargeable lithium cell. Safety problems caused by dendrite growth caused the cells to be removed from the market. Polymer electrolytes solved the problem, however, at the price that operating temperatures of 60–80 °C were required to provide sufficient ionic conductivity. Today’s ­lithium-metal polymer systems for electric vehicles are developed by the France company Balloré, from which subsidiary Batscap has worked for about 15 years in this field. The state of the art is a 30-kWh lithium-metal polymer battery (Li-polyoxyethylene/ LiTFSI-V2O5/C) for their “Blue Car” electric vehicle. Specific performance values

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are 100 Wh kg−1, 100 Wh L−1, and 875 Ah (at C/4) at a battery voltage of 300–435 V, a nominal voltage of 410 V, and a battery mass of 300 kg. However these lithium-metal batteries should not be confused with lithium-air batteries. Cell chemistry: The basic cell design, comprising lithium electrode, solid or liquid electrolyte, and oxygen gas-diffusion electrodes, is shown in Figure 7.3a. In nonaqueous electrolyte, the oxidation of dry lithium runs as the main reaction and generates mainly lithium peroxide (Li2O2), which is hardly soluble.

– Interlayer

Charging

4.5

Gas diffusion electrode

C Au

4.0 E vs. Li / Li+ / V

Lithium

Catholyte

Gas diffusion electrode

+

Pt

3.5

PtAu

3.0

Au PtAu

2.5

O2 Seal

(a)

LISICON

2.0 Solid electrolyte

(b) LISICON

Organic electrolyte

Pt 0

500

C 1500

1000

Q / Ah kg–1 (carbon) Seal: deforms with cycling

Aqueous electrolyte

(–) Lithium metal

Lithium

(c)

Li2O2 CO2

O2

O2

LiOH•(H2O)x

CO2

H2O CO2

Li2O3

Interlayer Li+ Conducting solid electrolyte

H2O

(d)

Organic interlayer (hybrid technology)

(e)

Figure 7.3  Lithium-air battery: (a) cell design. (b) charge–discharge characteristics of lithium-air cells with carbon-supported gold, platinum, and PtAu catalysts at 100 A kg−1. Dashed line: nominal equilibrium potential of O2/Li2O2 (modified from Lu et al. (2010)). (c) Nonaqueous cell configuration. (d) Aqueous and hybrid cell configuration (with solid, liquid, or gel interlayer). LISICON = lithium-ion conducting electrolyte, e.g., pinhole-free Li1.3Al0.3Ti1.7(PO4)3. (e) Protected lithium electrode (PLE) after PolyPlus Battery Company.

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The basic cell reactions for discharge with reference to metallic lithium (0 V = −3.040 V SHE) read as follows: (−) Anode: (+) Cathode:

(1) (a) (b) (c) (2) (3)

Cell reaction:

2 Li  2 Li + + 2e O2 + 2 Li + + 2e -  Li 2 O2 O2 + e - ® O2 - + Li  LiO2 2 LiO2 ® Li 2 O2 + O2 LiO2 + Li + + e - ® Li 2 O2 O2 + 4 Li + + 4e - ® 2 Li 2 O Li 2 O2 + 2 Li + + 2e - ® 2 Li 2 O 2 Li + O2  Li 2 O2

(

)

(0 V) (3.10 V, reverse on charging)

(2.91 V, irreversible) (2.72 V, irreversible) ΔE0 ≈ 3 V

Unfortunately, carbonate electrolytes do not withstand the nucleophilic attack by superoxide anion radical (O2−), which are formed as intermediates in the oxygen reduction (Aurbach et al., 1991). Solvent decomposition and the oxidation of degradation products is a major problem of nonaqueous lithium-air systems (Mizuno et al., 2010; McCloskey et al., 2011). Specific energy: The theoretical specific energy of lithium-air batteries often calculated with respect to metallic lithium. Specific capacity accounts for 3842 Ah kg−1 (lithium), since the unlimited feed of oxygen from air at the positive gas electrode is considered to contribute no weight to the battery. Wm = U ×

2.96 V × 1 × 96, 485 C mol -1 zF = » 11, 400 Wh kg -1 M 0.00694 kg mol -1 × 3600 C Ah -1

A more realistic value is obtained, if the reaction product Li2O2 is considered, that is, a mixture of 0.5 Li2O2 and 0.5 Li (average molar mass: 26.6 g mol−1) at 50% stateof-charge, excluding the mass of cell components (electrolyte, current collector, container). Wm =

2.96 V × 2 × 96485 C mol -1

( 0.0264 kg mol

-1

× 3600 C Ah -1

)

» 6000 Wh kg -1 ( at SOC 50% ) .

Practical batteries achieve about 11–16% of the theoretical specific energy, so that the lithium-air system might yield roughly 650–1500 Wh kg−1 one day (without air-­ cleaning system). Specific power: Unfortunately, the internal resistance leads to a low specific power of roughly 0.003 W cm−2 (compared to Li-ion: 0.03 W cm−2). Therefore, power applications will require the Li-air hybrid systems with different batteries or supercapacitors (Wang et al., 2011). Lifetime: Current Li-air cells withstand about 100 charge–discharge cycles. Service life is determined by the low reversibility of Li/Li2O2 reaction, dendrite growth, and electrolyte decomposition. Unwanted side reactions of carbon in the electrolyte and electrode material with the lithium and oxygen form lithium carbonate, so that in every cycle, some percent of the battery capacity is lost.

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Price: Li-air cells promise costs of approximately 80% of the Li-ion cells. The air cathodes especially are competitive, as the cathode determines 30% of the costs of a Li-ion cell. Shortcomings: Nonaqueous electrolytes pose the problem that the insoluble discharge products may precipitate in pore spaces of the gas-diffusion cathode. Li2O2 and Li2O exhibit a limited solubility in common electrolytes up to about 1 mol L−1. At the positive electrode, the actual composition of LiO2 (instable), Li2O, and Li2O2 during discharge depends on the carbon-based electroactive material, the electrolyte and the current density. To summarize, the problems of nonaqueous lithium-air systems are as follows: (1) Evaporation of solvent from the air electrode to the environment. (2) Uptake of water by hydrous oxygen feed. (3) Irreversible formation of Li2CO3 (from Li2O2 and CO2) and loss of capacity at the O2 electrode. (4) Slow oxygen transport due to flooding of the air electrode with electrolyte. (5) Clogging of the pores of the air electrode by insoluble Li2O2.

A closed system and anhydrous oxygen (no water or CO2 contamination) may solve some problems at THE cost of system energy density. Aprotic electrolytes: All known electrolytes tend to decompose in the lithium-air battery within some few cycles: ●









Low-viscous organic alkylene carbonates exhibit a high oxidation potential (HOMO) at about 4.7 V; for example, propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate, and dimethyl carbonate (DMC). The phase transfer catalytic properties of quaternary ammonium salt—such as the triflate [NBu4]SO3CF3—improve the reduction of Li2O2 into Li2O. As well, trifluoroethylphosphates (TFP) support the oxygen solubility at high discharge currents. Boron esters of the type [(CH3 or NO2)–C(CH2O and COO)3]B favor the dissolution of Li2O2 and Li2O in solvents such as EC/DMC and DMF. Tris(pentafluorophenyl)borane in LiTFSI/ PC-EC solutions, as well as the 12-crown-4 ether improve the solubility of lithium salts. Low-volatile ethers—such as DME and crown ethers—withstand lithium metal and small concentrations of Li2O2 at up to 4.5 V. Li2O2 only forms on the first discharge; then Li2CO3 is found. Ethers decompose, giving a mixture of Li2CO3, HCO2Li, CH3CO2Li, polyethers, polyesters, CO2, and H2O. Good performances were obtained with LiN(SO2CF3)2 (LiTFSI) in 1,3-dioxolane and 1,2-dimethoxyethane (1:1). Inflammable ionic liquids—such as 1-ethyl-3-methylimidazolium bis(tri-fluoromethane-sulfonyl)-imide (EMITFSI)—tolerate oxidation potentials of 5.3 V vs. Li|Li+. The ionic liquids can be added to poly(vinylidene fluoride)-based gel electrolytes. A ­propylimidazolinium-TFSI-silica-PVDF/HFP gel (Zhang et al., 2010) delivers higher discharge capacity than pure ionic liquids; probably because the lithium anode is protected by a stabilized interface. Polysiloxanes might provide a better chemical stability against the oxygen reduction than carbon compounds.

Protected electrodes: Moisture from air is critical to electrode life. Protected lithium anodes can be applied in a mixture of ethylene glycol, dimethylformamide, and a conducting salt (such as LiTFSI). Unfortunately, the oxygen cathode polarizes after

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a short time (≥0.05 Ah cm−2). Ex situ passivation of lithium by carbonate electrolytes (Peng et al., 2012) or oxidizing additives (Walker et al., 2013) such as LiNO3 were proposed to form a protective SEI in aggressive solvents such as amides and sulfoxides. Solid electrolytes suffer from the poor lithium-ion conductivity, for example, lithium aluminum-phosphate glass-ceramic (LISICON), and Li3− xPO4− yNy (LiPON). Water-stable electrodes, see Section 7.3.4. Current collector: Nickel was found to promote the decomposition of alkylene carbonate electrolytes (above 3.5 V vs. Li|Li+). Aluminum and graphite are discussed as alternatives.

7.3.2  Aqueous lithium-air systems Cell chemistry: The anode consists of lithium metal, the cathode is a gas-diffusion electrode made of porous carbon and coated with an oxygen reduction catalyst. Theoretically, the oxygen reduction runs at a potential of +1.229 V SHE, and the lithium oxidation 0 0 at −3.040 V SHE, so that open cell voltage equals D E 0 = Ecathode - Eanode » 4.27 V in acid solution. The discharge reactions read (Kowalczk et al., 2007): (−) Anode: (+) Cathode:

Alkaline cell: Acid cell: Unwanted:

2 Li  2 Li + + 2e O2 + 2e - + H 2 O  OH - + HO2 2 Li + + 2OH -  2 LiOH O 2 + 4e - + 4H +  2H 2 O 4 Li + O2 + 2H 2 O  4 LiOH 4 Li + O2 + 4H +  4 Li + + 2H 2 O 2 Li + 2H 2 O  2 LiOH + H 2

(alkaline) (acid) (3.45 V, pH 14) (4.27 V, pH 0) (2.22 V, pH 14)

The hydrolysis of water is usually unwanted, but, in bold visions, dissolved oxygen might be fed from seawater, providing 2.56–3.79 V at pH 7.2. Alkaline systems: Corrosion of lithium in aqueous electrolytes of some A cm−2 drops to mA cm−2 in highly basic media. In contrast to organic systems, alkaline batteries have the advantage that the generated lithium hydroxide solves well in the solution. Unfortunately, alkaline electrolytes absorb carbon dioxide from air, so that K2CO3 may clog the porous air electrode and block oxygen transport. Acidic systems: Protected lithium electrodes allow the use of acid solutions. The current collector of nickel, useful in alkaline solutions, must then be replaced. Owing to protected electrodes, open-circuit voltages (OCVs) of 3.5 V, and 2.6–2.8 V at 0.5 mA cm−2, coulometric efficiencies of 99%, negligible self-discharge, and 500– 800 Wh kg−1 (excluding cell housing) have been achieved. Volume changes: In practical cells, compliant seals around the current collectors at the bottom and at the top of the protected lithium anode must accommodate the shrinkage of the lithium volume (by 8–20%) due to dissolution during discharge. The oxygen cathode expands due to the accumulation of LiOH; weight increases by 8–13%. Performance data: Theoretically, the aqueous lithium hydroxide system should provide 5000 Wh kg−1, and still 3400 Wh kg−1 at the end of discharge by lithium depletion. However, Li-air batteries require bulky electrolyte and electrode materials. Depending on the porosity of the air electrode (e.g., 70%), the maximum theoretical cell

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c­ apacities and energies are 435 Ah kg−1 (1300 Wh kg−1) and 509 Ah L−1 (1520 Wh L−1) in basic electrolyte, and 378 Ah kg−1 (1400 Wh kg−1) and 452 Ah L−1 (1680 Wh L−1) in acidic electrolyte (Zheng et al., 2008). In contrast, nonaqueous Li-air batteries do not consume electrolyte during the discharge process. Air electrode structure: Under nonaqueous conditions, the air electrode is usually flooded, and anhydrous oxygen is required. For aqueous systems, a gas-diffusion electrode is required with hydrophobic domains creating channels for O2 gas transport. The three-phase boundaries throughout the aqueous air electrode guarantees faster oxygen transport and O2 reduction, and therefore higher current densities.

7.3.3  Water-stable lithium anodes Aqueous electrolytes avoid clogging of the oxygen electrodes, but require a dense, pinhole-free Li+ conducting layer on the lithium metal electrode. The invention of the water-stable protected lithium electrode (PLE) by PolyPlus Battery Company opens up a world of aggressive electrolytes that would normally react with lithium metal (Visco et al., 2009). The three-layer lithium anode for aqueous media consists of (1) a water-stable Nasicon-type lithium-conducting solid glass electrolyte, and (2) a protective interlayer, such as LiPON, between lithium and a conductive layer. Solid ­electrolyte-protected lithium electrodes can be used in protic or aprotic solvents: ●







Lithium super ionic conductor (LISICON) is the lithium analog of NASICON (Goodenough et al., 1976), Li1 + x + y(M, Al, Ga)x (Ge1 − zTiz)2− xSiyP3− yO12, with the typical composition Li1.3Al0.3Ti1.7(PO4)3 (LATP) produced by Ohara Corporation (Japan) and Corning Inc. (United States) and having a conductivity of 0.1–1 mS cm−1. As LISICON is stable to water, but not stable to lithium, the incorporation of a solid-state (i.e., Li3N), polymeric, ionic liquid, or nonaqueous liquid electrolyte interlayer is required, which is called a dual electrolyte system. Preferably, the direct reaction of water at the metallic lithium electrode is prohibited by a dense lithium-ion conducting solid-state film. In the hybrid Li-ion configuration, the electrical contact resistance at the Li|LISICON interface is reduced by an organic lithium salt solution placed in between. The solid electrolyte separates the lithium anode in the aprotic solvent from the air cathode in the aqueous solution. Oxide glasses are poor ion conductors. Sulfide glasses are unstable in water, but can be protected by a lithium phosphorous oxynitride (LiPON) glassy electrolyte. Lithium nitride (Li3N) exhibits a conductivity of 0.001 S cm−1 at room temperature, but has a narrow voltage stability window (~0.45 V), reacts very slowly with water within some years, and is unstable to reduction (≤2.4 V vs. Li/Li+), so that it cannot be contacted directly to lithium. Lithium metal phosphate, Na1 + xM2 − xIIIMIVP3O12 (LMP), is the lithium analog of the sodium superionic conductor (Nasicon, Na1+ xZr2SixP3− xO12, 0 ≤ x ≤ 3). The water-stable and dense glass-ceramic membrane Li1+ x + y(L,Al,Ga)x(Ge1− zTiz)2− xSiyP3− yO12, produced by Ohara Corporation in Japan, where L is a lanthanide element, provides a Li+ conductivity of 10−4 S cm−1. A 100-μm-thick membrane exhibits a resistance of 100 Ω cm2.

7.3.4  Lithium-water battery Lithium/seawater batteries work on the reduction of dissolved oxygen or water, so that a mixed potential is measured. Seawater serves as both oxidant and electrolyte. Lithium corrodes at a rate of 19,000 mA cm−2, so that protected anodes are r­equired, unless

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high-rate applications are not intended. An average voltage of 3.0 V at 0.5 mA cm−2 has been achieved, until the lithium is depleted. Aqueous lithium/air chemistry has been designed so far for primary batteries. The lithium-air system requires detailed research to increase reversibility and to reduce dendrite growth.

7.3.5  Advanced electrodes 7.3.5.1  Dendrite growth (anode) As well as in lithium-metal cells, the incomplete recharge of lithium and dendrite growth pose enormous problems (Brandt, 1994), which shall be overcome by advanced concepts: ●





Alloys (such as Li–Al) and nanostructured Sn–C composites. Self-healing electrostatic shield mechanism: Additive cations (Cs+, Rb+) in low concentration adsorb on lithium dendrite tips like a positively charged shield, and force further deposition of lithium to adjacent regions of the anode (Ding et al., 2013). Inert additives (e.g., SiO2) in a gel electrolyte block the dendrite growth mechanically.

7.3.5.2  Oxygen reduction catalysts (cathode) The 1-V gap between charge and discharge must be reduced by the help of a proper electrocatalyst. The specific power of the lithium-air battery is determined by (1) the slow oxygen reduction reaction and (2) the slow oxygen and lithium-ion transport across the electrolyte. Catalysts fix the electrode potential below 4.2 V, which, however, provides higher lifetime by preventing the electrolyte from oxidation. Some catalysts improve the capacity, as they influence the structure of the precipitated lithium oxides or the catalyst support material (carbon): ●





Carbon materials: On discharge (oxygen reduction), carbon black (Ketjen black) yields 850 Ah kg−1, carbon nanotubes yield 590 Ah kg−1, nitrogen-doped carbon nanocapsules in aqueous solutions (Chen et al., 2012) yield 866 Ah kg−1, and doped graphene yields 9000– 12,000 Ah kg−1. However, there is no obvious charging capacity, so that applications are restricted to primary batteries. If a catalyst is really necessary for the charge process (oxygen evolution), its nanostructure is relevant. Metal oxides: Among the transition metal oxides, chalcogenides, carbides, nitrides, oxynitrides, and carbonitrides (Chen et al., 2011), nanosized MnO2 type catalysts (Mizuno et al., 2010; Quartarone and Mustarelli, 2011) show better specific capacity than platinum and RuO2. On discharge, Fe2O3 exhibits high initial capacity; Fe3O4, CuO, and CoFe2O4 provide a good capacity retention; Co3O4 offers a good compromise between capacity and retention. On charge, MnO2 works at a low potential of 3.8 V; CoMn2O4 works bifunctionally both for oxygen reduction and for oxygen evolution. Noble metals: The kinetically inhibited reactions at the oxygen electrode require high charging and discharging voltages, which limit efficiency. Platinum facilitates the oxygen evolution during charge, gold enhances the oxygen reduction reaction at discharge (Figure 7.3b). At carbon, oxygen is reduced rather inefficiently, even at low current, densities coulombic efficiency is low, that is, η = Uout/Uin = 2.5 V/4.5 V ≈ 55% (at 0.04 mA cm−2). A carbon-­ supported PtAu catalyst improves the round-trip efficiency to Uout/Uin = 2.7 V/3.7 V ≈ 73%.

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For ­comparison, the efficiency of Li-ion system amounts to > 90%. The high cost is the major concern for applications in electric vehicles. Other classes: N4-macrocycle complex and transition metal nitrides exhibit a high oxygen reduction activity, but are poor materials for the charging reaction. Bifunctional electrodes: Oxygen evolution is even slower than oxygen reduction, especially as some catalysts suffer a reduced activity in the oxidized state (at high potential). Bifunctional catalysts are useful for both oxygen reduction and evolution (Jörissen, 2009).

Discharge potentials: The potentials (vs. Li|Li+) of the oxygen reduction in alkylene carbonate solution (mostly 1 M LiClO4 in propylene carbonate) increase in the row: CuFe (2.5)  > MnOx, Pd, Fe3O4, CoFe2O4 (2.6)  <  Au, Pt, Li5FeO4 (2.7)  < Ru (2.75) < Li2MnO3⋅LiFeO2 (2.8) < β-MnO2/Pd (2.9) < MoN (3.1) Charge potentials: The potentials (vs. Li|Li+) of the oxygen-evolution reaction in alkylene carbonate solution (mostly 1 M LiClO4 in propylene carbonate) increase in the row: β-MnO2/Pd (3.6) < Pd (3.9) < Ru (3.95) < Pt, MoN, Li2MnO3⋅LiFeO2 (4.0) < Fe3O4, Li5FeO4 (4.1) < Au, MnOx (4.2) < CoFe2O4 (4.3). A key point of future development is a mesoporous carbon cathode, and a membrane to prevent the CO2 and moisture migration into the air electrode while allowing O2 diffusion and mass transport. Table 7.2 compiles recent advances of cell performance data.

7.4  All-solid-state batteries Most current all-solid-state cells are thin-film microbatteries based on the phosphorous oxynitride LiPON. Current vacuum productions methods (PVD, CVD), low degree of automation, and low cell productions force rather high costs such as US$25,000 per Ah or US$1250 per Wh. Moreover, large cells with high-capacity electrodes need a thick separator having a conductivity of at least 10−4 S cm−1 at 25 °C. Advanced solid electrolytes require structural compatibility with the active masses, without forming alloys and creating electrolyte/electrode junctions that are detrimental to lithium-ion diffusion. Although some companies have reached prototype manufacturing, the production of large cells for electric vehicles at reasonable costs is not expected in this and the next decade.

7.4.1  Advantages of solid electrolytes for vehicles Liquid electrolytes tend to decompose at high voltages, so that high-voltage cathode materials cannot be used. Solid-state electrolyte promise electrochemical and thermal stability, high power, and extended cycle life and shelf life (Table 7.3). Solid electrolytes for Li-ion batteries (Quartarone and Mustarelli, 2011; Fergus, 2010) consist of (1) organic polymer electrolytes or (2) inorganic solid electrolytes. Conductivities fall in the range of 10−7–0.1 S cm−1 at room temperature (Figure 7.4). Solid polymer and gel polymer electrolytes have already been commercialized, whereas ceramic and glass electrolytes are still in the research stage.

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Table 7.2  Technological Anode (−)

overview of recent lithium-air system

Electrolyte, cathode, and cell reaction

Performance data

References

Solid state: Air as oxidant, no lithium dendrites; low Li-ion conductivity, low energy density Lithium metal

2 Li + O2  Li 2 O2 ceramic glasses (LiPON)

Nonaqueous aprotic: High energy density, rechargeability; insoluble products, instable electrolytes, ineffective catalysis Lithium metal

Lithium Lithium

U0 ≈ 3 V

Abraham and Jiang (1996)

Theory: 2.96 V, 3450 Wh kg−1, 8000 Wh L−1

Visco et al. (2014)

(a) Dried DMF + LiTFSI + TPFB as anion receptor and Li2O2 solubility improver (Xie et al., 2008; Li et al., 2009; Shanmukaraj et al., 2010) (b) Diglyme and tetraglyme + LiTFSI (c) Sulfolane + ethyl acetate + LiBF4 salt Tetra(ethylene)glycol dimethyl ether-lithiumtriflate (TEGDME-LiCF3SO3) LiClO4 in DMSO, nanoporous gold

Practical discharge: 0.1–0.25 mA cm−2, 1 mAh cm−2, poor cycling stability

~100 cycles, 5000 Ah kg−1 (carbon), fast Li+ diffusion comparable to Li-ion batteries 300–500 Ah kg−1 (gold). Capacity retention: 95% of initial capacity after 100 cycles. Faster Li2O2 oxidation at gold than on carbon

Jung et al. (2012) Peng et al. (2012)

Advances in Battery Technologies for Electric Vehicles

Lithium: (a) Metallic bare (b) Protected (PLE) Li/LATP protected

Li+ conductive polymer membrane, organic carbonate electrolyte; dry oxygen, cobalt phthalocyanine catalyzed carbon electrode: 2 Li + O2  Li 2 O2 2 Li + O2  Li 2 O2 (hardly soluble and reactive: Li 2 O2 + CO2 ® Li 2 CO3 + ½ O2 )

Lithium: (a) Metallic bare (b) Protected (PLE) Li/LATP protected: Li ® Li + + e -

Aqueous: 4 Li + O2 + 2H 2 O  4 LiOH

Theory: 3.45 V, 3850 Wh kg−1, 7000 Wh L−1

(a) NH4Cl/LiCl and NH4NO3/LiNO3: Not useful for rechargeable cells. 2 Li + 2 NH 4 Cl + ½ O2 ® 2 NH 3 + H 2 O + 2 LiCl 2 Li + ½ O2 + H 2 O ® 2 LiOH (b) Citric acid/imidazole, malonic acid: 4 ROOH + O2 + 4e - ® 4 ROO - + 2H 2 O

(a) 0.1–0.5 mA cm−2, 0.2 Ah cm−2, 600–800 Wh kg−1. Instability of LATP in strong acids and bases. (b) 1 mA cm−2, 5 Ah cm−2, 2.4–4.2 V, ~75 cycles. Air electrode: pH 1 (fully charged) → pH 12 (fully discharged)

Visco et al. (2014)

U0, practical open-circuit voltage; DMF, dimethyl formamide (monoglyme); DMSO, dimethyl sulfoxide; LiTFSI, lithium bis(trifluoromethylsulfonyl)imide; TPFB, tris(pentafluorophenyl) borane; LATP, Li1.3Al0.3Ti1.7(PO4)3 (LISICON-type).

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Aqueous: Soluble products; bad ion-conducting membrane and charging behavior, SEI formation

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Table 7.3  Comparison of liquid and solid electrolytes in lithium-ion batteries Parameter

Liquid cell

Solid-state cell

Separator Production Ionic conductivity

Flexible Low price, large format High, near room temperature Flammable, volatile Yes, affects cycle life No, decomposition

Rigid and brittle ceramic High price, small format Moderate, broad temperature range

Poor Bad, limited shelf life Sensitive Yes Low

Excellent Good, longer shelf life Abuse tolerant Limited, extended cycle life High, less inactive materials

Electrolyte safety SEI layer High-voltage cathode materials Thermal stability Self-discharge Overcharge Dendrite growth Specific energy

Nonflammable, safe, nonvolatile None, longer cycle life Yes

7.4.2  Solid polymer electrolytes Polyethylene oxide (PEO) is widely used. A dry mixture of bis(trifluoromethanesulfonyl)imide lithium salt (LiN(SO2CF3)2, LiTFSI) and the aprotic PEO matrix yields an ionic conductivity of 10−4 S cm−1 above 60 °C. By adding inorganic fillers, such as nanoscale SiO2, improve the conductivity up to 1.4 × 10−4 S cm−1 at room temperature. Unfortunately, the transference number of Li+ transport amounts to t ≈ 0.25 only, although the lithium ions are intended to carry the total electric current across the T (°C) 800 500

1

LISICON Li14Zn(GeO4)4

200

100

27

–30

–100

New Li10 GeP2S12 solid electrolyte Glass electrolyte Li2S-SiS2-Li3PO4

Ionic liquid electrolyte 1 M LiBF /EMIBF 4 4

0.1

Protective coating

Glass-ceramic electrolyte Li7P3S11 Doped Li3N Li

Li-b-alumina

10–3

Li Si P O 36 0.6 0.4 4

Ge0.25P0.75S4

325

Gel electrolyte 1 M LiPF6 EC-PC(50:50 vol %) +PVDF-HFP (10 wt%)

Li3N

Polymer electrolyte PEO-LiCIO 4 (10 wt% added TIO2)

10–5

Polymer electrolyte LiN(CF3SO2)2/(CH2CH2O)n(n = 8)

10–7

1

2

Cathode current collector

La0.5Li0.5TiO3 Organic electrolyte 1 M LiPF6/EC-PC (50:50 vol%)

10–6

(a)

Lithium anode

Glass electrolyte Li2S-P2S5

10–4

3 103 T –1(K –1)

Electrolyte Cathode

LIPON

4

5

15 µm

s (S cm–1)

0.01

6

Substrate

Anode current collector

(b)

Figure 7.4  (a) Conductivity of lithium-ion conducting electrolytes. (b) Solid-state thin-film microbattery.

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­electrolyte (i.e., t = 1). Current research activities aim at the immobilization of anions in the polymer chains (Golodnitsky, 2009; Meziane et al., 2011). Gels contain 60–95% liquid electrolyte and are two to five times less conductive than liquid solutions. With polyethers (electron pair donors) as gelling agents, most of the solvation takes place through the polymer chains rather than carbonate solvents. Thus, the lightly plasticized systems can be used in a lithium-metal battery, whereas the soft gels work only in a lithium-ion configuration.

7.4.3  Inorganic solid electrolytes Inorganic solid electrolyte materials may be classified into four groups: (1) Perovskites, for example, (Li,La)TiO3. (2) Garnet structures, for example, Li5La3M2O12 (M = transition metal). (3) Glassy electrolytes based on lithium nitrides, sulfides, borates, and phosphates. (4) Superionic conductors, for example, LISICON, LiMIV2(PO4)3 (MIV = Ti, Zr, Ge).

7.4.3.1 Garnets The Garnet class, Li5La3M2O12 (M = Nb, Ta, Zr) (Buschmann et al., 2012), exhibits the currently best chemical and electrochemical stability against metallic lithium, in combination with a moderate Li+ conductivity (~10−4 S cm−1 at room temperature): ●



Niobium-doped lithium-lanthanum zirconate, Li7La3Zr2O12 (LLZO), achieves 8 × 10−4 S cm−1 (Ohta et al., 2011), a low interfacial resistivity and a decomposition voltage of ~ 6 V vs. Li|Li+. The conductivity is due to the disorder of delocalized Li+ in the cubic garnet crystal lattice, which must be stabilized by a small amount of aluminum. Li6.75La3 Zr1.75Nb0.25O12 (LLZONb) was screen-printed by Toyota in Li/LLZONb/LiCoO2 cells. To improve the interfacial contact, the LiCoO2 cathode layer was formed with the help of the Li+ conductor Li3BO3. Approximately 74% of the theoretical capacity, high coulombic efficiency, and a resistance comparable to that of physical vapor deposited layers were achieved (Ohta et al., 2013). Mixtures of garnets, polymer glue layers, and LiTFSI provide 5.5 × 10−4 S cm−1 at room temperature and long-term stability even in water. This allows applications as ion-­sensitive separator in lithium-bromine flow-batteries (4 V, 335 Ah kg−1) (Wang and Goodenough, 2012).

7.4.3.2  Glassy nitrides The glassy thin layers of current microbatteries consist of amorphous lithium ­phosphorous oxynitride (LiPON, Li2.9PO3.3N0.46), an electronic insulator made by sputtering of LiPO4 in N2 atmosphere, which exhibits a lithium-ion conductivity of 3.3  10−6 S cm−1 at 25 °C and a good electrochemical stability at cell potentials up to 5.5 V vs. lithium. LiPON is used in commercial microbatteries, mostly based on LiCoO2, by Cymbet, Front Edge, and Infinit Power (Jones, 2011). Automotive applications are pursued by Sakti3 and Toyota. Further, Bathium, Solicor, and Solid Energy are also developing all-solid-state cells.

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7.4.3.3  Sulfidic glasses In contrast to oxide, the larger sulfide ions exhibit higher polarizability and ion mobility. Because of the low interfacial resistivity, solid electrolyte layers can be formed by cold pressing. Unfortunately, 5-V cathode materials react with sulfidic electrolytes, reducing the power capability: ●



The three-dimensional framework of Li10GeP2S12 allows one-dimensional Li+ conduction along the c axis. The ionic conductivity amounts to 0.012 S cm−1 at 27 °C and 0.001 S cm−1 in the cold at −30 °C. Toyota’s In/Li10GeP2S12/LiCoO2 cells achieved a discharge capacity of 120 Ah kg−1, and a nearby 100% discharge efficiency after the second cycle. Li2S–SiS2–LiI, Li2S–SiS2–Li3PO4, and Li2S–P2S5 exhibit bad Li+ conductivity, are unstable in water, and form badly conducting layers at the positive electrode.

7.4.3.4  Super ionic conductors The sulfide Li3.25Ge0.25P0.75S4, the most stable member of the thio-LISICON (lithium super ionic conductor) family, has an ionic conductivity of 0.0022 S cm−11 at 25 °C. Li10GeP2S12 provides at least 0.02 S cm−1 at 27 °C. Thio-LiSICON is employed by Planar Energy in automotive cells using CuS cathodes and SnO2 anodes. Hybrid electrolytes exhibit reduced resistivity. Both the lithium anode and the cathode are coated with a solid-state electrolyte (SSE) and then assembled to a cathode|SSE|liquid electrolyte|SSE|Li cell. The SSE coatings block SEI layer formation and increase cycle life.

7.5  Conversion reaction materials This section outlines the use of phases that react through conversion reactions and displacement reactions as both positive and negative electrode materials in advanced batteries (Cabana et al., 2010; Bruce et al., 2008; Amatucci and Pereira, 2007). Beyond classical intercalation reactions, a variety of low-cost compounds promises specific capacities that are two to five times larger than those attained with currently used graphite and LiCoO2. The conversion reaction concept was developed at the beginning of this century. However, much more technical progress needs to occur, especially the reorganization of particles during charge and discharge. Energy storage beyond intercalation: In lithium-ion cells, the Li+ guest is reversibly placed into and withdrawn from a host lattice, which remains more or less unchanged during charging and discharging (Figure 7.5). Many transition metal compounds do not have any vacant sites for intercalation processes and have therefore been disregarded for batteries so far. Modern R&D approaches search for transition metal oxides, fluorides, nitrides, sulfides, phosphides, or hydrides that employ more than one electron per metal atom for energy storage. M a X b + bxLi  aM + bLi x X

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Insertion: Li+ + e– + MO2 → Li MO2 → Li+ + e– + MO2

5 4

Discharge

+

Charge

+





Conversion: 2 Li + 2e + MX2 → 2 LiX + M → 2 Li + 2e + MX2 30-50 Å M Discharge

Charge

Electrode potential vs. Li|Li+ (V)

M

2 1 0

(b)

0

0.75

4

1.5 2.25 x in LixFeF2

3 0.6 V

2 1 0

(a)

1.5 V

3

0

1

2

3

x in LixCoS

Figure 7.5  (a) Lithium insertion in contrast to structure reorganization in conversion materials. (b) Discharge/charge curves of conversion materials: iron fluoride and cobalt sulfide (after Amatucci).

M transitions metal, x formal oxidation number of the anion X. Negative electrodes are based primarily on phosphides, nitrides, or oxides; positive electrodes are based on fluorides. The fluorides, oxides, and sulfides of copper, manganese, iron, cobalt, and nickel achieve theoretical potentials up to 3.5 V vs. Li|Li+ (Poizot et al., 2000): MnS + 2e – + 2 Li +  Mn + Li 2S FeF2 + 2e – + 2 Li +  Fe + 2 LiF CoF2 + 2e – + 2 Li +  Co + 2 LiF NiF2 + 2e – + 2 Li +  Ni + 2 LiF CuF2 + 2e – + 2 Li +  Cu + 2 LiF

(<1.1 V) (<2.6 V) (<2.7 V) (<3.0 V) (<3.5 V)

Performance data: The potential is mostly determined by the electronegativity of anions. The highest potentials were measured with CuF2 (~3 V), CoF2 (2.1 V), and FeF2 (1.9 V). The best experimental sulfides are Cu2S (1.6 V), NiS (1.4 V), Co0.9S (1.3 V), and FeS (1.2 V). The potentials of oxides range usually below 1 V vs. Li|Li+. The theoretical capacity of conversion materials exceeds that of intercalation materials and compensates the slightly lower potentials. Theoretical capacities above 1500 Ah kg−1 promise CoP3, MnP4, CrN, and NiP3. More than 1000 Ah kg−1 were calculated for CoN, NiP2, FeP2, Cr2O3, MoO3, MnO2, Mn2O3, Fe2O3, RuO2, Co3O4, and MnS. The three-valent compounds FeF3 and BiF3 achieve 800 Wh kg−1 (material) at 2.5 V. Such fluoride phases are free of lithium, so that mixtures of LiF and iron have been used as the positive electrode in experimental lithium-ion cells. Reaction kinetics: Unfortunately, the structure reorganizations during charging and discharging of these materials is slow. The high internal resistance causes a strong hysteresis of the charge/discharge characteristics and leads to low energy efficiencies (Figure 7.5b). Electronegative anions provide high cell voltage, but on the other hand

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the strong M–X bounds increase the internal resistance (polarization resistance, overpotential) in the order: fluorides > oxides > sulfides > phosphides. In addition to the lattice changes, the diffusion of lithium ions and the charge transfer of electrons are kinetically inhibited. Reduced diffusion pathways in nanostructures might improve the kinetics of conversion reactions. Moreover, the particle reorganization during charge and discharge leads to a gradual pulverization of the materials.

7.6  Sodium-ion and sodium-air batteries Some developments of high-temperature batteries in the 1970s and 1980s are currently undergoing a revival. Lithium resources are unevenly distributed in South America, and the price of the raw materials has roughly doubled from 1991 to the present due to global demand for lithium-ion batteries. In 2010, the worldwide production of sodium (1011 kg a−1) and magnesium (6 × 109 kg a−1) exceeded that of lithium (2.5 × 107 kg a−1) by orders of magnitude (Vesborg and Jaramillo, 2012). Viable alternatives for a cheap, high-performance rechargeable battery of the future might run on sodium: (1) sodium-ion batteries in aqueous and nonaqueous electrolytes and (2) sodium-oxygen batteries.

7.6.1  Sodium-ion systems Among their advantages, sodium-ion batteries (Ellis and Nazar, 2012) promise to be more durable and cheaper to manufacture. The basic cell design is shown in Figure 7.6a. The major challenge is the negative electrode (anode) and its passivation. (−) Anode: (+) Cathode:

Na ( C )  C + Na + + e Host + Na + + e -  Na ( Host )

Being in an early developmental phase, the performance data of sodium-ion systems are currently not up to par with lithium batteries: capacity, 1.16 Ah kg−1; voltage, 2.7 V SHE; ionic radius, 980 pm; melting point, 97.7 °C. Sodiums’s ionic volume is almost double and its atomic mass is triple than that of lithium. It is difficult to find spacious host materials for multiple insertion and extraction of the large sodium-ion.

7.6.1.1  Negative electrode materials (anode) Metallic sodium poses security problems and cannot be applied in aqueous solutions. Alternative materials are required (Figure 7.6b). Carbon materials: Graphite is not able to accommodate the spacious Na+ ions. Sodium intercalation was observed in petroleum cokes, soft carbons (small regions of ordered graphene), and hard (disordered) carbons. Hard carbon, equivalent to graphite in lithium-ion batteries, appears to be the material with the most potential to realize commercial sodium-ion batteries. Unfortunately, the specific capacity is low (~280 Ah kg−1) (Komaba et al., 2011). Hard carbon by pyrolysis of glucose or sucrose (1500 °C) demonstrated 300 Ah kg−1 at a slow intercalation rate (C/80).

Layered oxides NaMO2

Na+ e–

e–

Electrode potential vs. Na|Na+ (V)

Graphite

Anode (–)

Electrolyte

Cathode (+)

Na0.85Li0.17Ni0.21Mn0.54O2

Na2FePO4F

Na3V2(PO4)3 Na(Mn0.5Fe0.5)PO4

3.0

Na2FePO4F

Olivines

NaFePO4 NaTi2(PO4)3

2.0

NASICONs

1.0

Na2Ti3O7

Anodes Na2Ti3O7 Hard carbon

0

(b)

Na3V2(PO4)2F3

Na3V2(PO4)2F3

4.0

0.0

(a)

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Cathodes

5.0 e–

50

100

150

200 250

300

Specific capacity (Ah kg–1)

Figure 7.6  (a) Cell design of a sodium-ion battery. (b) Performance data of advanced materials for Na-ion systems.

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Porous, nongraphitic carbon based on a porous silica template delivers 180 Ah kg−1 (1st cycle, C/5 rate) to 80 Ah kg−1 after 125 cycles (Wenzel et al., 2011). Tin-doped hard carbon is capable to deliver high capacity, whereas the cycling stability of tin is poor (Yamamoto et al., 2012). The anodes are prepared from 15% SnCl2 and sucrose in a tube furnace under argon atmosphere. A capacity of ~350 Ah kg−1 over 70 cycles was reported in organic solution (1 mol L−1 NaClO4 in propylene carbonate/ethylene carbonate, 1:1). Metal phosphates: NASICON-type NaTi2(PO4)3 achieves ~120 Ah kg−1 (theory 133 Ah kg−1) at 2.1 V vs. Na|Na+ in nonaqueous and aqueous electrolytes. Metal oxides: Sodium is able to intercalate into wide, amorphous TiO2 nanotubes (inner diameter > 45 nm). Layered Na2Ti3O7 intercalates Na+ already around 0.3 V vs. Na|Na+. As well, the O3 phase of NaVO2 allows deintercalation at a low voltages (Na0.66VO2, → Na0.5VO2, 126 Ah kg−1, 1.2–2.4 V); unfortunately, the material is extremely sensitive to oxygen. Alloys: Little research has been done thus far on sodium alloy materials as negative electrodes. Lead is able to suffer an electrochemical loading of up to 3.75 Na per atom. Na15Sn4, NaSi, and NaGe are a known compounds.

7.6.1.2  Positive electrode materials (cathode) Most electrode materials (Palomares et al., 2013) result from the idea, that lithium is known battery materials is replaced by sodium. Two-dimensional layered and three-dimensional materials with corner sharing matrices are preferred (Figure 7.6b). Manganese oxides: The wide tunnels in NaxMnO2 (0.25 < x < 0.65) allow reversible Na+ insertion at capacities as high as 140 Ah kg−1 over several voltage steps and six phase transitions within a potential range of 2–3.8 V. Irreversibility occurs at charging beyond Na0.25MnO2. Single-crystal Na0.44MnO2 nanowires provide 128 Ah kg−1 at a C/10 rate and good capacity retention over 1000 cycles. Na0.44MnO2 works as well in a sodium-ion polymer battery with PEO8NaAsF6 electrolyte, and even in aqueous media. λ-MnO2 (by chemical delithiation of LiMn2O4) gets amorphous when cycled vs. sodium, but can store up to 0.6 Na per formula between 2 and 4 V, and is suitable for aqueous solutions. The spinel phase NaMn2O4 is thermodynamically unstable and tends to form Mn2+ above 55 °C. Electrochemical desertion of lithium from LiMn2O4 followed by insertion of sodium brings about poor cycle performance and structure rearrangement. Alternatively, a mixture of excess Na2O2 and Mn2O3, heated at 950 °C under pressure, shows charge/discharge plateaus at about 3 V vs. Na|Na+, which can be attributed to the redox couple Mn4+/Mn3+. Sodium-layered oxide phases: Monoclinic α-NaMnO2, known since 1985, is more stable than LixMnO2, can reversibly intercalate about 0.8 Na, and yields a capacity of 200 Ah kg−1 with good capacity retention (electrolyte: NaPF6 in ethylene carbonate/ dimethyl carbonate). NaxCoO2 for x ≥ 0.50 (Berthelot et al., 2011) exhibit single-phase domains with different Na+/vacancy patterns depending on the sodium concentration, for example, Na0.5CoO2 and Na0.66CoO2. The synthesis of these materials with a very narrow

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e­ xistence range can be carried out electrochemically. The P2 phase Na0.66CoO2 shows sodium extraction and intercalation via several steps at 40 °C (Figure 7.7a). Small particle sizes improve electrochemical performance. II Na0.85[Li0.17Ni0.21 MnIV0.64]O2 is able to extract 0.42 Na (112 Ah kg−1) within a single phase and was reported to deliver 80 Ah kg−1 at a 10 °C discharge rate, and 65 Ah kg−1 at a 25 °C rate (in about 3 min: 670 W kg−1). Nax[Fe0.5Mn0.5]O2 was proposed as an electrode material with low overpotential at room temperature; cycle life has yet to be improved. Sodium-ion conductors: NASICON-like insertion materials have been known since 1992, for example, NaNb-Fe(PO4)3, Na2TiFe(PO4)3, and Na2TiCr(PO4)3. The low Fe3+/ Fe2+ redox potential around 2.4 V vs. Na/Na+ makes them less attractive as positive electrodes. Na3V2(PO4)3 might be a candidate for symmetric cells, owing to the range of oxidation states of vanadium (90 Ah kg−1, poor cycling stability). Olivines: Amorphous FePO4 is able to reversibly intercalate Na-ions providing 100 Ah kg−1. NaFePO4 (maricite) is electrochemically inactive as it interrupts sodium-ion migration pathways along the Fe and Na octahedra during insertion and de-insertion. The olivine framework of NaFePO4 suffers a 15% volume contraction on Na+ extraction. The solid-solution-like potential of Nax(Fe0.5Mn0.5) PO4, synthesized from molten salts, depends on the sodium content x = 0–0.6 (4.3–2 V). Sodium vanadium fluorophosphate: The V3+/V4+ redox transition provides high cell voltages. NaVPO4F releases Na+ over two voltage plateaus (3.0 V and 3.7 V vs. Na), indicative of a structural transformation. Na3(VO)2(PO4)2 yields 87 Ah kg−1 after 400 cycles (initially 120 Ah kg−1) over the course of two voltage plateaus at 3.6 V and 4.0 V vs. Na. This sodium-containing positive electrode may be paired with graphite in an Li+ electrolyte. 7

2

3.2 3

Na0.7C002

456

2.8

7 8

2.6

9

2.4

Cha

rge

Dis cha rge

2.2

∆x

Capacity (mAh/g)

3.4

3.0

120

1

3.6

Electrode potential vs. Na|Na+ (V)

Electrode potential vs. Na|Na+ (V)

3.8

6

5

100 80 60 40 20 0

4

0

2 4 6 8 10 Number of cycle

3

2

2.0

(a)

0.4

0.5

0.6 0.7 Sodium content (x)

0.8

0.9

(b)

1

1.2

1.4 1.6 1.8 X in Nax FePO4F

2

Figure 7.7  (a) Phase transitions of Na0.7CoO2 on electrochemical cycling. (b) Electrochemical profile of Na2FePO4F cycled vs. sodium at a rate of C/5.

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Layered sodium iron fluorophosphates: Na2FePO4F has been known since 2007. Na+ ions reside between or close to the iron phosphate sheet layers; the volume changes only by 3.7% on extraction of 1 mol Na. Charge and discharge run over two two-phase plateaus at 2.90 V and 3.05 V vs. Na/Na+, and 80% of the theoretical capacity (120 Ah kg−1) is sustained on cycling (Figure 7.7b). Tavorite sodium iron fluorosulfate: The structure of nano-crystalline NaFeSO4F (from FeSO4⋅H2O and NaF in glycol) is similar to the mineral tavorite (LiFePO4OH). Na+ reside in tunnels along the [1 1 0] direction and hop between them, that is, the material is a 1D ion conductor, and suffers volume changes on redox reactions.

7.6.2  Sodium-oxygen batteries Air-breathing batteries guarantee a huge weight advantage because they do not have to carry around the oxidant. The Na-air system (Peled et al., 2011), in Figure 7.8a, shows high overpotential and low energy efficiencies in carbonate-based sodium electrolytes, as sodium peroxide Na2O2 is formed. Nevertheless, an Na–O battery recharges without complicating side-reactions known from Li–O. Cell chemistry: The cell reactions read: Na + + O2 + e - ® NaO2 2 Na + + O2 + 2e - ® Na 2 O2 4 Na + + O2 + 4e - ® Na 2 O

(2.263 V) (2.330 V) (1.946 V)

Temperatures above the melting point of sodium (98 °C) mitigate the formation of metallic dendrites on the negative electrode during charge, and exclude the adsorption of water vapor by the cell components. The cell is charged at about 2.9 V and discharges at 1.8 V. 3.0

Anode –

Cathode +

O2 Na metal

Electrode potential vs. Na|Na+ (V)

Load

2.8 2.6 2.4

Charge

2.2 Discharge 2.0 0.5

0.2 mA cm–2

0.3

0.12

1.8

(a)

NaOx + catalyst

(b)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Q (mAh)

Figure 7.8  (a) Cell design of a Na-O2 battery on discharge. (b) Charge/discharge characteristics of Na-O2 with das diffusion (GDL) electrode at different currents. Dashed line: E0(NaO2) = 2.27 V.

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Sodium superoxide: NaO2 (Hartmann et al., 2013) in an ether-based electrolyte, although less thermodynamically stable than Na2O2, can be charged and discharged reversibly at current densities of up to 0.2 mA cm−2 against a pure carbon cathode. The one-electron step, O2 + e¬ → [O2]−, is kinetically preferred; so that very low overpotentials (<200 mV) were observed.

7.7  Multivalent metals: magnesium battery Sodium, magnesium (Saha et al., 2014), calcium (Datta et al., 2014), and aluminum (Wang et al., 2013b) were proposed as future material for metal-air batteries. The idea behind reads: Replacing the monovalent lithium with heavier ions that carry two charges, doubles the energy carried per volume. The theoretical specific capacity of magnesium (2205 Ah kg−1) is lower, and the potential (−2.36 V SHE) and density (1.74 g cm−3) are less good compared with lithium. Magnesium batteries get heavier, but smaller than lithium systems. Major electronics firms such as Toyota, LG, Samsung, and Hitachi are working on such higher-charge-carrying cells. Toyota’s magnesium-sulfur battery using non-nucleophilic electrolyte (hexamethyldisilazide magnesium chloride) has been envisioned to be implemented for hybrid vehicles (Kim et al., 2011).

7.7.1  Cell chemistry The pioneering work of Aurbach et al. (2000) proposed a prototype rechargeable battery using MgxMo6S8 electrodes in an electrolyte of magnesium organochloro aluminate in tetrahydrofuran or polyethers (glymes), which delivered 60 Wh kg−1 for over 2000 cycles with little fade (Figure 7.9). The electrochemical processes during discharge of a rechargeable Mg–Mo6S8 comprise the dissolution of magnesium (0 V), and the intercalation of magnesium-ions into the host lattice (~1.1 V vs. Mg|Mg2+). (−) Anode: (+) Cathode: (a) (b)

Mg  Mg 2 + + 2e Mg 2 + + 2e - + Mo6S8  Mg x Mo6S8 Mo6S8 + Mg 2 + + 2e - ® MgMo6S8 MgMo6S8 + Mg 2 + + 2e - ® Mg 2 Mo6S8

Due to the strong electrostatic interaction of the divalent Mg-ions with the host material, the diffusion of Mg2+ proceeds very slowly. The slightly different crystal structure of MgxMo6Se8 allows higher Mg2+ mobility at reduced potential (~0.9 V vs. Mg|Mg2+).

7.7.2 Electrolyte Magnesium forms passive layers in polar aprotic electrolytes (such as esters, alkyl carbonates, amides, and acetonitrile) that do not have any Mg2+ ion conductivity—as it is observed for Li+ in the SEI of lithium-ion batteries. Grignard compounds (RMgX), as conducting salt in ethers and tetrahydrofuran, avoid passivation (Gofer et al., 2009), but the electrochemical window is small. Aromatic complexes, such as Ph2Mg⋅3AlCl3

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2.0

Mg0

Mg+2 + 2e– + Mo6S8

MgxMo6S8

Deintercalation

Dissolution

S Mo

1.0

Mg

1.5 0.5

0.0

0.0

–0.5 –1.0

–0.5

–1.5 –2.0 –0.5

(a)

Deposition 0.0

Intercalation

0.5

1.0

1.5

2.0

E (V) vs. Mg/Mg2+

2.5

–1.0 3.0

(b)

Cell voltage (V)

0.5

/ (mA cm–2)

/ (mA cm–2)

1.0

(c)

1.9 1.7 1.5 1.3 1.1 0.9 0.7 0.5

0

1

2

2500

Cycle number

Figure 7.9  Magnesium battery: (a) Blue line: stability of the electrolyte solution 0.25 M Mg(AlCl2BuEt)2 in THF at a platinum electrode at 20 mV s−1. Red line: Reversible Mg deposition and dissolution into Mo6S8 at 0.005 mV s−1. (b) Crystal structure of the Chevrel phase MgxMo6S8 (positive electrode) with 12 site for magnesium insertion. (c) Cycle stability of a rechargeable Mg/Mo6S8 battery at constant current rate (C/8).

in THF, provide a voltage window of 3.0 V and almost 100% cycling efficiency. Binary ionic liquids with low passivation and a wide voltage window were reported (Kakibe et al., 2012). Nonaqueous electrolytes: Magnesium cannot be reversibly deposited from simple magnesium salt solutions such as Mg(ClO4)2 in acetonitrile, propylene carbonate, or N,N-dimethylformamide, because a dense passivating layer of decomposition products is formed on the electrode surface. Traces of water generate MgO and Mg(OH)2. NaClO4 in formamide + acetonitrile (1:1) is good for magnesium dissolution at low overpotentials, but does not support the electrodeposition of magnesium. Magnesium trifluoromethanesulfonate, Mg(CF3SO3)2 or Mg(TFSI)2, dissolved in dimethylacetamide (DMA) or acetonitrile (0.2–2.8 V vs. steel) proved to be inappropriate for magnesium deposition. Grignard reagents (RMgX, R = alkyl; X = Br, Cl: Figure 7.10) can electrochemically be reduced, but they are instable above 1.5 V vs. Mg, and incompatible for insertion cathodes and solvents besides ethers. Magnesium tetrabutyl borate, Mg(BBu4)2, in tetrahydrofuran or N-methylaniline supports magnesium dissolution and deposition at reasonable overpotentials, however, at the cost of coulombic efficiency and instability in the presence of transition metal oxides or sulfides. Aurbach’s ethereal solutions of magnesium organochloro aluminates, Mg(AlCl2 or R ) are the reaction products of the Lewis bases RxMgCl2− x (x = 0–2) and Lewis 3 1 or 2 2 acids R ¢y AlCl3- y (y = 0–3). A stoichiometric mixture of Bu2Mg and EtAlCl2 (1:2 in THF) achieved the best anodic stability (2.10 V vs. Mg) and a 95% reversible magnesium deposition. Bu2Mg/2⋅AlCl3 yields a higher decomposition potential (2.40 V) at the expense of reversibility (75%). Aromatic rests improve the voltage window up to 3.3 V vs. Mg.

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2+ THF

Cl

Mg THF Mg

Mg THF

Cl R

2 2Mg2+

THF

Pt

THF Al

R

Cl

R

Cl

2

2Mg2+



Al R

Cl

6 THF 4 e–

V

Figure 7.10  Reaction mechanism and intermediates of magnesium deposition on platinum in 0.25 M solution of Mg(AlCl2EtBu)2 in THF.

An anionic species is oxidized above the highest occupied molecular orbital (HOMO) energy level, that is, the more negative the HOMO energy, the higher the oxidative stability of the electrolyte. Examples include AlCl4− (HOMO: −7.53 eV, LUMO: +1.01 eV), (HMDS)AlCl3− (−5.670 eV and +0.061 eV), (C6F5)3BPh− (−5.559 eV and −0.422 eV), (Ph4)3B− (−4.819 eV and −0.536 eV). In practice, the Hauser base HDMS (hexamethyldisilazide magnesium chloride) (Muldoon et al., 2012) electrolyte (3 HMDSPhMgCl⋅AlCl3) in THF is stable up to 3.3 V vs. Mg. The hydrogen storage material Mg(BH4)2, in diglyme (better than in dimethoxyethane and THF), allows magnesium deposition and stripping at a coulombic efficiency of 77%. Pellion Technologies’ (Doe et al., 2014) electrolyte Mg2AlCl7 (synthesized from 2 MgCl2 + AlCl3) in ethereal solvents is anodically stable up to 3.5 V vs. Mg. Among the mixtures of magnesium salts MgCl2:Mg(TFSI)2 (2.5:1 in 1,2-dimethoxyethan) provides the best conductivity of 5.80 mS cm−1 (at 28 °C). Polymer electrolytes: Contrary to liquids, polymers do not fail by internal short circuit, electrolyte leakage, and combustion. Unfortunately, Mg2+ conductivity, electrochemical stability, and compatibility with electrodes are not sufficient. Gel electrolytes may use the butyl-ethyl complex salt [Mg(AlCl2EtBu)2], dissolved in PVDF or PEO as the polymer matrix, and tetraglyme as the plasticizer. The polymer complexes MgCl2⋅(PEO)8, 12, 16, or 24, and Mg(ClO4)2⋅(PEO)16 possess ionic conductivity comparable to LiCF3SO3⋅(PEO)9 complex salt. Mixtures of polymer (PVDF better than PAN) and magnesium trifluoromethane sulfonate Mg(Tf)2 in alkylene carbonates (PC, EC) achieve conductivities up to 2.7 mS cm−1 at 20 °C, although the cycling behavior is poor. A polymer gel of magnesium bis(trifluoromethylsulfonyl)imide Mg(TFSI)2 and ionic liquids (EMITFSI) in PEO modified polymethacrylate achieves 3.5 mS cm−1 (at 60  °C). Ionic liquids—such as N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium-bis(trifluoromethanesulfonyl)imide (DEMETFSI)—in the gel enhance ­ the conductivity of Grignard reagents EtMgBr l (1:3 in THF) to 7.4 mS cm−1 at 25 °C.

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A Mn2+ conducting gel consisting of a magnesium triflate [Mg(Tf)2] and 1-­ ethyl-3-methyl imidazolium trifluoromethanesulfonate (EMITf) in poly (­vinylidenefluoride-co-hexafluoropropylene) [PVDF-HFP] offers a conductivity of ~3 mS cm−1 at room temperature, a voltage window of ~4 V, and thermal stability between −30 °C and 110 °C, but a poor transport number of t(Mg2+) ≈ 0.26, that is, the triflate anions are more mobile. Dispersing 10% MgO microparticles and 3% fumed silica in gel improves the conductivity to 6 mS cm−1 and 11 mS cm−1, respectively.

7.7.3 Electrodes Anode: Metallic magnesium offers a theoretical capacity of 2205 Ah kg−1 and does not form dendrites because of a solution-precipitation mechanism. However, passivation involves slow kinetics. Nano magnesium and alloys (Al–Zn–Mg, Si–Mg, Sn–Mg, Bi– Mg) were proposed. Intercalation anodes do not passivate, but provide poor capacity. In conventional electrolytes, metallic bismuth yields 384 Ah kg−1 at intercalation potentials of +0.23/0.32 V vs. Mg and tin 903 Ah kg−1 at +0.15/0.20 V vs. Mg (Singh et al., 2013; Arthur et al., 2012). Cathode: The cathodes currently used exploit Mg2+ intercalation phenomena in oxides, phosphates, and sulfides such as Co3O4, V2O5, Mg0.5Ti2(PO4)3, and TiS2: ●









The Chevrel phase Mo6S8 provides a potential of 1.1 V vs. Mg|Mg2+, and a theoretical specific capacity of 128 Ah kg−1, whereby from the second cycle onward only 60–70% of the total capacity is available. Doping with TiS2, SrS2, MoS2, RuO2, Co3O4, V2O5 MoO3, and MgxMnO2 (Levi et al., 2010) provide viable specific energies. Mixed Chevrel phases, Mo6S8− ySey; (y = 1, 2) and CuxMo6S8 (100 Ah kg−1 at C/6), show better reversibility, but lower specific discharge capacity than Mo6S7 (75 Ah kg−1 at C/5). Vanadium oxide: Mg2+ insertion into bulk V2O5 is extremely slow, requires water molecules in the electrolyte, and is accompanied by a rapid capacity fade (up to 300 Ah kg−1, Mg(ClO3)2 in aqueous acetonitrile). Metal oxides, sulfides, and borides tend to irreversibly form MgO and MgS. High capacity is involved with low voltage in the row: TiB2 (324 Ah kg−1, 1.25 V vs. Mg|Mg2+), ZrB2 (313 and 1.2), MoB2, TiS2, VS2, WO3, MoO3 Co3O4, RuO2, ZrS2, V2O5 (194 and 2.66), Mn2O3 (224 and 2.40), Mn3O4, Pb3O4, PbO2 (56 and 3.10). nano-MgMn2O4, havening a diffusion energy barrier below 0.8 eV, delivers 290 Ah kg−1 at 2.9 V. Mg0.5MoO3 is estimated at 142 Ah kg−1 and 2.28 V. α-MnO2 (Zhang et al., 2012) offers initial 280 Ah kg−1 in HMDSMgCl electrolyte. Magnesium-sulfur conversion cathodes: According to the reaction Mg (anode) + S (cathode) → MgS promise 1.77 V, 1671 A kg−1, and 3459 Ah L−1. Unfortunately, suitable electrolytes are not known, although hexamethyldisilazide magnesium chloride (HMDSMgCl)t in tetrahydrofuran is able to prevent sulfur dissolution.

Current collectors should be chemically inert, very thin, light, rigid, excellent electrical conductors. Copper, nickel, stainless steel, titanium, and aluminum hinder the reversible magnesium deposition by overpotentials and do not withstand Grignard reagents. Platinum, glassy carbon, molybdenum, and tungsten proved anodic stability in ethers.

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Performance data: In 2000, a cell based on Mg anode, Mo6S8 cathode, and a Grignard-THF electrolyte survived more than 3500 cycles, at low self-discharge, and a wide range of operating temperatures (Aurbach et al., 2003), although the low voltage limited specific energy to 60 Wh kg−1 (cell). Toyota’s concept of a magnesium-sulfur battery proved a specific capacity of 1200 Ah kg−1 in the first cycle. Model calculations of Pellion Technologies, USA, project specific energies of future magnesium batteries of more than 400 Wh kg−1 and energy densities above 1200 Wh L−1.

7.8  Halide batteries The following battery type is far from any near-future technical applications and market launches.

7.8.1  Fluoride battery The extraordinary electronegativity and standard potential of fluorine (E0 = +2.87 V SHE) promises cell voltages of up to 6 V in combination with metals. In contrast to the aggressive and highly reactive gaseous fluorine (F2), fluoride ions (F−) are employed as the mobile species for current transport between the electrodes (Figure 7.11). Cell chemistry: During discharge, metal fluorides are formed at the negative electrode, whereas fluoride is released at the positive electrode.

Load e–

Electrolyte

+

– F–

Discharge Metal fluoride MFx

F–

Charge

Figure 7.11  General cell design of a fluoride battery.

Current collector Metal M

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M + xF -  MFx + xe M′ Fx + xe −  M′ + xF − M¢Fx + M  M¢ + MFx

(−) Anode: (+) Cathode: Cell:

The metals M and M′ may be divalent or trivalent, so that it is possible to store more than one electron per metal atom. Theoretical performance data: Base metals such as lithium, calcium, and lanthanum, in combination with noble metal fluorides, allow cell voltages of more than 3 V and specific energies in the range of 400 Wh kg−1 (Table 7.4). Challenges for future research are the low power, limited cycle life, and high operating temperatures of halide batteries. Electrode materials: Both the anode and the cathode material have to be excellent electronic and fluoride-ion conductors, which can hardly be produced. In practice, conducting materials and electrolytes are used as additives: ●





Anode materials: Li, Ca, La, Ce Cathode materials: MnF2, CoF3, CuF2, BiF3, KBiF4, SnF2 A concept of Reddy and Fichtner (2011) uses fluoride ions between the electrodes instead of Li+. La0.9Ba0.1F2.9 forms the solid electrolyte. The electrodes consist of BiF3, SnF2, CuF2, and cerium. During cycling CeF3 and metals (Bi, Sn, Cu) are formed. Unfortunately, reversibility is possible only at 150 °C and small current densities.

Fluoride-conducting electrolyte: The major challenge of a future battery is a liquid or solid electrolyte, which provides sufficient ionic conductivity at room temperature. So far, only solid electrolytes have been known in the literature: ●



Fluorites MF2 (M = Ca, Sr, Ba) and tysonites MF3 (M = La, Ce) achieve acceptable fluoride conductivity by doping. The conductivity of LaF3 of 10−8 S cm−1 at room temperature can be increased to more than 10−6 S cm−1 by doping with Ca2+, Sr2+, Ba2+, or Eu2+. La0.9Ba0.1F2.9 yields 2.8  10−4 S cm−1 at 160 °C, and 10−6 S cm−1 at 30 °C. Different compositions, such as La0.85Ba0.1F2.9 and La0.95Ba0.05F2.95, behave less potent.

Experimental cells: Fluoride batteries realized so far exhibit poor power capabilities: ●

The cell (−) Ce|La0.9 Ba0.1 F2.9|CuF2 (+) brings about 10 μA cm−2 at about 2.5 V and 150 °C, and exhausts above 200–300 Ah kg−1 (Reddy and Fichtner, 2011). Composites of BiF3 or SnF2 or KBiF4 proved to be even grimmer.

Table 7.4  Theoretical

performance data of fluoride systems

Cell system

Open-circuit voltage (V)

Specific capacity (Ah kg−1)

Specific energy (Wh kg−1)

Energy density (Wh L−1)

Ca + MnF2  CaF2 + Mn 3Ca + 2CoF3  3CaF2 + 2Co 2 Li + CuF2  2 LiF + Cu La + CoF3  LaF3 + Co

1.89 3.59 3.46 3.31

403 456 464 315

761 1639 1607 1044

2104 4068 2838 5241

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Ce|La 0.9 Ba0.1 F2.9|CuF2 cells bring about a capacity of 322 Ah kg−1 (61% of theoretically 527 Ah kg−1), an average discharge voltage of ~2.5 V, and practical specific energies of about 800 Wh kg−1. Ce|La0.9Ba0.1 F2.9|BiF3 (solid solution) proved 190 Ah kg−1 in the first, and 50 Ah kg−1 after the 30th cycle. The fast capacity loss is caused by contact problems and volume changes at the phase boundary between active mass and solid electrolyte. Liquid electrolytes might help to solve this problem.

7.8.2  Chloride battery Chloride-ion systems using chloride-conducting electrolytes are under development (Zhao et al., 2014).

7.9  Ferrite battery The technology is mostly used for primary batteries in aqueous electrolyte systems and is far from any market launch. Super-iron batteries (Yu and Licht, 2007a,b) use alkali and alkali earth ferrates(VI) as cathode materials, which allow three-electron charge storage reactions, such as K2FeO4 (theoretically 601 Ah kg−1), Na2FeO4 (485 Ah kg−1), K2FeO4 (406 Ah kg−1), SrFeO4 (388 Ah kg−1), BaFeO4 (313 Ah kg−1). Cell chemistry: Iron(VI) is reduced in a three-electron step. The anode is usually zinc. The potential amounts to 0.5–0.65 V SHE. (+) Cathode: (−) Anode:

FeO 24 - + 3H 2 O + 3e - ® FeO ( OH ) + 5OH FeO 24   2.5H 2 O  3e   ½ Fe 2 O3  5OH  Zn ® Zn 2 + + 2e -

The silver ferrate(VI) Ag2FeO4 cathode exhibits a five-electron cathodic charge storage. Ag 2 FeO 4  2.5H 2 O  5e   2 Ag  ½ Fe 2 O3  5OH  Electrode materials: Small particle graphite and compressed carbon black are employed as conductive matrix for the super-iron cathodes. Fluorinated polymer graphites act both as a conductive and also add intrinsic capacity to the cathode: ●



Inorganic additives (such as SrTiO3) improve the faradaic efficiency of Fe(VI) reduction. Solid-phase MnO2 reduces the Fe(VI) potential by ~200 mV, whereas Co2O3 increases it by ~150 mV. K2FeO4 passivates in alkaline media, so that the charge transfer is inhibited. Composites of Fe(VI)/Mn(IV or VII), Fe(VI)/Ag(II), or stabilizing zirconia coatings solve this problem and provide much higher specific power than the untreated cathodes.

Rechargeable Fe(VI) batteries: Nanometer-thin Fe(VI/III) films exhibit a certain ­degree of reversibility, whereas thicker films tend to passive toward the cathodic

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Fe(VI) charge transfer. A conductive matrix of high-surface area platinum, titanium, or gold facilitates submicrometer thick films. Fe(VI) salts as the cathode material can be coupled with a nickel-metal hydride or a lithium anode in nonaqueous media.

7.10  Redox-flow batteries In flow batteries, the fuel consists of two liquids that pass ions to each other through a membrane; for example, vanadium ions of different oxidation states. The liquids reside in tanks outside the battery and are pumped in when needed. From the present vantage point, these systems will not conquer electric vehicles in the near future, because they require a large volume. Organic cells based on quinones, partnered to a standard liquid electrode such as bromine, were proposed recently (Huskinson et al., 2014). Future visions pursue flow-batteries using liquid Li-S and solid lithium.

7.11  Proton battery Future visions project proton-conducting electrode materials, which allow extremely rapid charge storage. Today’s supercapacitors based on platinum metal oxides pursue this idea, although cell voltage is limited to less than 1.5 V in aqueous solution, and specific energy is a factor of 100 lower than that of a current lithium-ion battery. The inverse mass ratio battery (Augé, 2012) is an energy source that is driven by the separation of charged species in saline solutions through an energy conversion process. It is known from biologic electron transport chain mechanisms that form proton gradients. The principle is used for experimental medical scalpels. A localized alternating current splits water, liberates fuel cell-like redox reactions, and sets up a proton pump that maintains regional proton gradients within the saline solution ambient to biological tissues. 2H 2 O + Cl - + energy  H 2 + ½ O2 + HCl + OH -

References Abraham, K.M., Jiang, Z., 1996. A polymer electrolyte‐based rechargeable lithium/oxygen battery. J. Electrochem. Soc. 143, 1–5. Agostini, M., Lee, D.-J., Scrosati, B., Sun, Y.K., Hassoun, J., 2014. Characteristics of Li2S8tetraglyme catholyte in a semi-liquid lithium-sulfur battery. J. Power Sources 265, 14–19. Amatucci, G.G., Pereira, N., 2007. Fluoride based electrode materials for advanced energy storage devices. J. Fluorine Chem. 128, 243–262. Arthur, T.S., Singh, N., Matsui, M., 2012. Electrodeposited Bi, Sb and Bi1− xSbx alloys as anodes for Mg-ion batteries. Electrochem. Commun. 16, 103–106.

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Appendix: abbreviations and symbols Ah DoD E F

Ampere-hour: 1 Ah = 3600 C depth of discharge: discharged electrical energy divided by the total stored energy reversible cell voltage (in V): electric potential difference of a galvanic cell; open-circuit voltage (OCV), formerly: electromotive force (emf) Faraday constant: 96,485 C mol−1

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I P PEO Q R SoC SPE LiTFSI T U W W kg−1 Wh kg−1 W L−1 Wh L−1 x Z(ω) %

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electric current (A) electric power (W) polyethylene oxide capacity, specific electric charge (in Ah kg−1 = mAh g−1) molar gas constant: 8.3144 J mol−1 K−1 state of charge: available energy for discharging divided by the total stored energy. solid polymer electrolyte bis(tri-fluoro-methane-sulfonyl)imide lithium salt, LiN(SO2CF3)2 absolute temperature (K) electric voltage (V) vs. versus a reference electrode electric energy (J = Ws) Watts per kilogram (specific power) Watt-hours per kilogram (specific energy) Watts per liter (power density) Watt-hours per liter (energy density): 1 Wh L−1 = 3.6 kJ dm−3 mole fraction impedance, depending on circular frequency (in Ω) percent by weight, mass fraction