Progress of hydrogen storage alloys for Ni-MH rechargeable power batteries in electric vehicles: A review

Progress of hydrogen storage alloys for Ni-MH rechargeable power batteries in electric vehicles: A review

Accepted Manuscript Progress of hydrogen storage alloys for Ni-MH rechargeable power batteries in electric vehicles: A review Liuzhang Ouyang, Jianli...

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Accepted Manuscript Progress of hydrogen storage alloys for Ni-MH rechargeable power batteries in electric vehicles: A review

Liuzhang Ouyang, Jianling Huang, Hui Wang, Jiangwen Liu, Min Zhu PII:

S0254-0584(17)30511-4

DOI:

10.1016/j.matchemphys.2017.07.002

Reference:

MAC 19809

To appear in:

Materials Chemistry and Physics

Received Date:

16 November 2016

Revised Date:

19 June 2017

Accepted Date:

01 July 2017

Please cite this article as: Liuzhang Ouyang, Jianling Huang, Hui Wang, Jiangwen Liu, Min Zhu, Progress of hydrogen storage alloys for Ni-MH rechargeable power batteries in electric vehicles: A review, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.07.002

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ACCEPTED MANUSCRIPT Progress of hydrogen storage alloys for Ni-MH rechargeable power batteries in electric vehicles: A review Liuzhang Ouyang1,2, Jianling Huang1,2, Hui Wang1,2, Jiangwen Liu1,2, Min Zhu1,2* 1School

of Materials Science and Engineering, Key Laboratory of Advanced Energy Storage Materials of

Guangdong Province, South China University of Technology, Guangzhou, 510641, PR China 2China-Australia

Joint Laboratory for Energy & Environmental Materials, South China University of Technology, Guangzhou, 510641, PR China

Abstract As clean energy materials, hydrogen storage alloys have been widely investigated and applied as negative electrodes for nickel-metal hydride (Ni-MH) rechargeable batteries due to their high energy densities and environment-friendliness. This review details the progress made in the last few decades on hydrogen storage alloys, such as AB5-type alloys, AB2-type alloys, Mg-based alloys, Ti-V-based alloys and RE-Mg-Ni (rare earth abbreviated as RE) alloys, for Ni-MH rechargeable batteries. The principles of Ni-MH batteries and the relationship between electrochemical performance and hydrogen storage properties have been narrated in detail. The achieved research results, existing problems and development direction are discussed systematically. The relationship between alloying compositions, crystal structures and electrochemical properties for each alloy type are also noted and analyzed with the emphasis on power batteries. Finally, the challenges of Ni-MH batteries are discussed in the context of developing electric vehicles. Keywords: Electric vehicles; Ni-MH batteries; Hydrogen storage alloys; Electrochemical properties

*Corresponding author: Min Zhu, E-mail: [email protected] Tel.:86-20-87113924.

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1. Introduction With increasing energy demand, the consumption of fossil fuels, such as coal, oil and natural gas, has become a serious burden to the environment and energy resources [1]. Therefore, it is essential to develop new green energies and energy saving technologies. Developing clean electric vehicles (EVs), fuel-efficient hybrid electric vehicles (HEVs) and distributed energy storage stations is an appropriate approach to ease these problems, and reliable batteries, as power conversion and storage devices, play a key role [2]. Several types of rechargeable batteries, such as lead (Pb)-acid batteries, nickel-cadmium (Ni-Cd) batteries, nickel-metal hydride (Ni-MH) batteries and lithium (Li)-ion batteries, have been developed and practically applied [3,4]. Among them, as shown in Fig. 1 [3] and Table 1 [5], Li-ion batteries have relatively higher energy density, but as an energy carrier for electric vehicles, they are still a safety hazard. Ni-MH batteries have high power capability, tolerance to overcharge/discharge, environmental compatibility and safety, which make them appropriate for portable power tools and HEVs, although their energy density is relatively low compared to Li-ion batteries[6]. A Ni-MH battery consists of a metal hydride (MH) negative electrode, a Ni(OH)2 positive electrode and an alkaline electrolyte (KOH). Figure 2(a) shows the principles of a Ni-MH battery. The electrochemical reactions that occur for the charge-discharge process in a Ni-MH battery are as follows [5–7]: On positive electrode:

(1-1)

On negative electrode:

(1-2)

Overall reaction:

(1-3)

When the battery is overcharged or overdischarged, the reactions occurring at the positive and negative electrodes can be expressed as follows: Overcharge on positive electrode: 4OH  2H 2O+O 2 +4e -

2

-

(1-4)

ACCEPTED MANUSCRIPT On negative electrode: 2H 2O+O 2 +4e  4OH -

-

(1-5)

4MH+O 2  4M+2H 2O

(1-6)

Overall reaction: 4MH+O 2  4M+2H 2O

(1-7)

Overdischarge on positive electrode: 2H 2O+2e  H 2 +2OH -

On negative electrode: H 2 +2OH  2H 2O+2e -

-

-

(1-8) (1-9)

χH 2 +2M  2MH χ

(1-10)

Overall reaction: χH 2 +2M  2MH χ

(1-11)

Therefore, in general, the capacity of the negative electrode is set higher than that of the positive electrode, and the oxygen produced from the positive electrode can be reduced at the surface of the MH electrode during the overcharge process (oxygen consumption reaction). As for the overdischarge process, correspondingly, the reduction product, hydrogen, is absorbed by the MH electrode (hydrogen elimination reaction). Hence, Ni-MH batteries exhibit excellent tolerability during the overcharge and overdischarge processes. The hydriding and dehydriding of electrodes involves a series of reactions and mass transport [8,9], as illustrated in Fig. 2(b). It is obvious that the hydrogen is supplied from the electrolyte. The H2O dissociates into H(ad) and OH- at the interface of the solid, and then, H(ad) combines with the hydrogen storage alloys to form M-H(ad) and the adsorbed hydrogen diffuses into the bulk of the alloys to form a solid-solution phase (α phase). Finally, the α phase transforms into a hydride phase with further increases in H concentration. The reverse reactions take place during the electrochemical dehydriding reaction. According to the principles of Ni-MH batteries and their operating conditions, a hydrogen storage electrode alloy must satisfy the following basic requirements [6,10–14]: (1) high reversible hydrogen storage capacity and suitable absorption/desorption plateau pressure, (2) good electrochemical catalysis for hydrogen absorption and desorption, and excellent electrochemical stability in alkaline electrolyte, (3) good charge/discharge kinetics for

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ACCEPTED MANUSCRIPT efficient operation and long cycle life, (4) a wide working temperature range and (5) resource-rich, cheap and easy to industrially produce. The electrochemical performances of Ni-MH batteries strongly depend on the intrinsic properties of the electrode materials. So far, only a small number of hydrogen storage alloys, including AB5-type alloys, AB2-type alloys, Mg-based alloys, Ti-V-based alloys and A2B7-type or AB3-type rare earth (RE)-Mg-Nibased superlattice alloys, have been developed as anode materials for Ni-MH batteries, and significant efforts have been devoted to promote their electrochemical properties, such as energy density, high rate discharge ability(HRD) and cycle life. The Ni-MH battery industry has undergone rapid development since Japan began to produce Ni-MH batteries in large batches in 1990, and well-known battery manufacturers in several countries have also accelerated the industrial development of Ni-MH batteries. The gravimetric and volumetric energy densities of AA-size Ni-MH cells have been raised from 54 to 110 Wh/kg and 190 to 490 Wh/L, respectively [15]. Their power has increased from under 200 to 1200 W/kg commercially, and up to 2000 W/kg at a development level [16]. In 2000, the total output of small Ni-MH batteries was up to 1 billion in Japan. China was one of the few countries to participate in the early development of Ni-MH batteries. Since 1995, China has built a number of production bases for the largescale production of Ni-MH batteries, such as the Tianjin Peace Bay Company, the Shenyang Sanpu Company, the Shenzhen BYD Company, the Shenzhen HPJ Company and the Shenzhen Great Power Company. Now, the production and export of Ni-MH batteries in China are more than in Japan, meaning that China is ranked first in the production of Ni-MH batteries in the world. The export volume from China was 0.69 billion in 2012, 0.627 billion in 2013 and 0.573 billion in 2014 (Fig. 3). Due to the impact of the global economic crisis and the rise of Li-ion batteries, the market for Ni-MH batteries has been declining in recent years. At present, 85% of the listed HEVs are based on Ni-MH batteries as power sources. However, the requirements for energy density show an increasing trend over time. In particular, with the rapid development of HEVs, high power and a long cycle life are required

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ACCEPTED MANUSCRIPT for Ni-MH batteries. Hence, efforts to improve the properties of Ni-MH batteries are still ongoing. Here, we detail the relationships between electrochemical performances and properties of hydrogen storage alloys used as the negative electrode in Ni-MH batteries, with the emphasis on how the alloy compositions and crystal structures contribute to the electrochemical properties of the main types of hydrogen storage alloys. This review will provide a deeper understanding and elucidate the challenges of hydrogen storage alloy anodes for NiMH batteries and provide some references and enlightenment for future research.

2. Relationship between electrochemical performance and hydrogen storage properties The most important electrochemical properties of hydrogen storage alloys, with respect to practical applications, are activation performance, maximum discharge capacity (Cmax), capacity retention rate and high rate

dischargeability (HRD). In general, Cmax is achieved after several charge-discharge cycles [17–19], and the cycle number (Na) to reach Cmax is related to the so-called activation performance. Because of the dissolution of some elements in alkaline solution, such as La and Mg, and pulverization caused by expanding/contracting of the unit cell volume during the charge–discharge processes of alloys, the capacity decreases with cycling. The capacity retention rate is calculated by the formula   C n / C max  100% , where Cn is the discharge capacity for the nth cycle[20,21]. The electrochemical performance of MHs is determined by their hydrogen storage capacity, kinetics and thermodynamic stability. The thermodynamic properties are normally characterized by the formation enthalpy (ΔH) and entropy (ΔS) of the hydride. Kleperis at al. [5] stated that ΔH should range between −25 and −50 kJ mol−1 for an alloy as a candidate for battery applications. The values for ΔH and ΔS can be derived from the pressurecomposition-isothermal (PCI) at different temperatures using the van’t Hoff equation [22]:

ln[PH 2 / P  ]  H / RT  S / R

(2-1)

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ACCEPTED MANUSCRIPT where PH2 is the equilibrium plateau pressure of the PCI, which is usually determined by the mid-plateau values, and Pθ is the standard pressure. As shown in Fig.4[10], the plateau region corresponds to the saturated α phase transforming into the β phase in the domain of αmax to βmin in PCI curve. The electrode potential in Ni-MH batteries is closely related to the plateau pressure of the PCI and can be calculated by the Nernst equation [23]:

E eq  E 0 

RT ln(PH 2 )  0.9324  0.0291 ln(PH 2 ) 2F

(2-2)

where E0 = −0.9324 V (vs. Hg/HgO) and F is the Faraday constant. The theoretical electrochemical capacity of a hydrogen storage alloy anode is determined by the H capacity in the plateau of the PCI and can be calculated using Eq.(2-3) derived from Faraday’s law, where F, CH and MW represent the Faraday constant, the atoms per formula unit (H/f.u.) and the molecular weight of the alloy in g/f.u., respectively.

C (mAh/g) =

F  CH 3.6  M W

(2-3)

The electrochemical reaction kinetics are another important factor that controls the electrochemical properties, especially the HRD of Ni-MH batteries [5]. In general, the kinetics are mainly controlled by both the chargetransfer process on the electrode/electrolyte interface and the hydrogen diffusivity within the bulk of the alloy [10]. It is known that the electrochemical reactions inside the hydrogen storage alloy electrodes are related to the mass transfer, charge transfer and hydrogen diffusion processes. The charge transfer and hydrogen diffusion occur simultaneously during the charging/discharging processes of the alloy electrode, and both of them limit the rate of electrode reactions. The charge transfer process can be characterized by the charge-transfer resistance (Rct) and the exchange current density (I0), and the hydrogen diffusivity can be evaluated through the limiting current density (IL) and hydrogen diffusion coefficient (D) [6,27]. Table 2 shows the relationship of the HRD to the charge transfer rate and hydrogen diffusivity for some representative compounds of each type of hydrogen storage alloy. Obviously, the HRD is determined by both the charge transfer rate and hydrogen diffusivity. Therefore, the charge

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ACCEPTED MANUSCRIPT transfer rate and hydrogen diffusivity must be high enough to ensure a high charge/discharge rate, otherwise the charging or discharging processes are retarded, and the electrochemical properties decrease for the anodes of the Ni-MH battery. The Rct of alloy electrodes can be evaluated by electrochemical impedance spectroscopy (EIS). Figure 5 [30] is a typical EIS graph that has a smaller semicircle, a larger semicircle and a straight line, reflecting the contact impedance between the alloy particles and the current collector, the charge-transfer resistance of the alloy electrode and the Warburg impedance, respectively [31,32]. To quantitatively analyze the Rct, an equivalent circuit, which can be seen in Fig. 5, is used to fit EIS data proposed by Kuriyama et al. [31,32]. The exchange current density (I0) is used to characterize the electrocatalytic activity of charge-transfer reactions on the interface of the alloy electrodes[9]. The I0 can be obtained by measuring the currents at different overpotentials (linear polarization). When the overpotential changed within a small range, the I0 can be calculated through the following equation[9]:

I0 

I d RT F

(2-4)

where R is the gas constant, T is the absolute temperature (K), Id is the applied current density (mA/g) and η is the total overpotential (mV). It can be found that RT/F is constant at a given temperature, and there is a linear relationship between the current density and the overpotential (Fig. 6[30]). Therefore, the I0 can be obtained according to Eq.(2-4) by fitting a linear slope. The hydrogen diffusion rate is a measure of how fast hydrogen spontaneously diffuses into a material, which can be evaluated by D (cm2/s), which is usually measured by potential-step methods. According to the model of Zheng et al. [33], the D of hydrogen inside the alloy particles can be estimated by fitting the slope of the linear portion of the response curves using the following equation:

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logi  log[

6FD 2 D ( C — C ) ] — ( )( 2 )t 0 s 2 da 2.303 a

(2-5)

where i (A/g) is the diffusion current density, C0 (mol/cm3) is the initial hydrogen concentration in the bulk of the alloy, Cs (mol/cm3) is the hydrogen concentration on the surface of the alloy particles, a (cm) is the alloy particle radius, d (g/cm3) is the density of the hydrogen storage alloy and t (s) is the discharge time. In summary, as negative electrode materials for Ni-MH power batteries, hydrogen storage alloys must have high reversible hydrogen storage capacities, sufficiently high electrochemical reaction rates and high corrosion resistance in alkaline solution to ensure excellent electrochemical performance.

3. Main hydrogen storage alloys for Ni-MH batteries Hydrogen storage electrode alloys consist of two types of metal elements in different stoichiometries, i.e., A and B elements with positive and negative affinity to H, respectively. Depending on the ratio of A to B, the hydrogen storage alloys can be principally classified as AB5-type alloys, AB2-type alloys, A2B7-type or AB3-type RE-Mg-Ni-based superlattice alloys. The performances of each type of alloy differ greatly in terms of hydrogen storage capacity, discharge capacity, activation and stability due to their different compositions and structures. Some properties of these alloys are shown in Table 3. With a single CaCu5-type hexagonal structure, AB5-type hydrogen storage alloys have a low electrochemical capacity. AB2-type hydrogen storage alloys have higher electrochemical capacity and the typical representative phases of AB2-type alloys are the hexagonal C14 and cubic C15 Laves phases. However, binary AB2-type compounds show poor electrochemical properties in alkaline electrolytes due to the high stability of their hydrides. Optimizing the C14/C15 phase abundance and forming third phases by multielements and optimizing the composition can improve their performances. RE-Mg-Ni-based hydrogen storage alloys, as novel negative electrode materials for Ni-MH batteries, mainly contain the LaNi5 and (La,Mg)Ni3 phases. In RE-Mg-Ni-based

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ACCEPTED MANUSCRIPT alloys, the LaNi5 phase is not only a hydrogen-absorbing phase, but also acts as a catalyst to activate the (La,Mg)Ni3 phase to absorb/desorb hydrogen, and the (La,Mg)Ni3 phase could enhance the electrode reaction kinetics, so that RE-Mg-Ni-based hydrogen storage alloys show better electrochemical properties. For Ti-V-based alloys, both the Ti-based C14-type Laves and V-based solid-solution phases have high hydrogen storage capacity. The V-based solid-solution phase shows poor electrochemical performance due to its stable hydride, but the C14 Laves phase can work as a catalyst to catalyze the V-based solid-solution phase to achieve reversible electrochemical hydrogen absorption/desorption. Mg-based hydrogen storage alloys have very high hydrogen storage capacities of 7.6 and 3.6 wt.% for MgH2 and Mg2NiH4, respectively. Nevertheless, crystalline Mg-based alloys show poor electrochemical properties at room temperature, meaning that high temperatures are required for dehydriding, in addition to their serious corrosion in alkaline electrolytes. In this section, these alloys will be evaluated in detail, and other novel and high energy density alloys, such as Co-based alloys, are also discussed.

3.1. AB5-type hydrogen storage alloys The typical representative of the AB5-type hydrogen storage alloy is LaNi5. With a CaCu5-type hexagonal structure (space group P6/mmmwith La (1a) in (0,0,0), Ni (2c) in (1/3,2/3,0) and Ni (3g) in (1/2,0,1/2), as shown in Fig. 7 [6,7], there are three octahedral sites and three tetragonal sites, and H preferentially occupies the tetrahedral sites containing A2B2, AB3 and B4. One LaNi5 unit cell can absorb six hydrogen atoms to form the LaNi5H6 hydride, which is equivalent to a theoretical electrochemical capacity of 372 mAh/g. Van Vucht et al. [34] first reported the LaNi5 alloy with reversible hydrogen absorption/desorption properties in the late 1960s. In 1973, Ewe et al. [35] reported the electrochemical performance of the LaNi5 alloy; however, it showed poor cycling performance due to pulverization and dissolution during the charge/discharge process. Soon after, the COMSAT laboratory developed a Ni-MH battery that used a LaNi5 alloy as a negative electrode, but the cycling life still did

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ACCEPTED MANUSCRIPT not meet the practical application requirements. From then on, great efforts have been made to improve the cycle life of LaNi5 alloys using alloy element substitution both for La and Ni. In 1984, a breakthrough was accomplished for the life cycling of LaNi5 through partial substitution of Ni by Co [36]. This discovery acted as the key for NiMH batteries to enter the market. Since then, significant research has been completed to improve the overall performances and reduce the cost of this type of alloy. In brief, the A side (La) was substituted by mischmetal (cerium-rich mischmetal (Mm) or lanthanum-rich mischmetal (Ml)) and the B side (Ni) was partially substituted by Co, Mn, Al, Sn, Fe, Cr or Cu. The capacity of AB5-type alloys are in the range of 250 to 350 mAh/g (Table 4) [37– 43]. Studies have shown that plateau pressures within the range of 0.01–1 bar are suitable for the reversible electrochemical reaction. Partial substitutions can obtain larger reversible electrochemical capacities and better kinetics than the LaNi5 parent compound, which is attributed to the suitable plateau pressure. As shown in Fig. 8 [7], the AB5 alloy with partial substitution of Mn, Al, Co achieve suitable plateau pressure (0.01–1 bar), and La0.62Ce0.27Pr0.03Nd0.08Ni3.55Co0.75Mn0.4Al0.3 is widely applied even in today’s Ni-MH batteries and has adequately met the requirements for a practical battery [44]. Unfortunately, the cost of the raw materials in this alloy is high as they contain Co, Pr and Nd, even though their contents are at low levels. Cobalt, the key element to maintain long life in the commercial hydrogen storage alloy mentioned above, accounts for about 40–50% of the total cost of this typical alloy. Although Pr and Nd can improve the activation properties of hydrogen storage alloys and increase their high-rate dischargeability and cyclic stability, the price of Pr and Nd are about 5–10 times of that of La-Ce. Therefore, it is important to reduce the content of high cost Co, Pr and Nd to increase the market competitiveness of Ni-MH batteries. As a result, significant work has been concentrated on developing high performance, Co/Pr/Nd-free or low-Co/Pr/Nd hydrogen storage alloys [45–49]. For instance, Wei et al. [45] reported that the low-Co, Pr/Nd-free La0.9Li0.1Ni3.2Co0.3Al0.3 alloy has a maximum discharge capacity of 328 mAh/g and retained 71.5% after 230 charge/discharge cycles.

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ACCEPTED MANUSCRIPT Balogun et al. [47] investigated the electrochemical properties of a series of La-Ni-Co-Mn-Al alloys and noted that the LaNi4.2Co0.3Mn0.3Al0.2 alloy has a maximum discharge capacity of 330.4 mAh/g and showed a good cycle life. The Pr/Nd-free La9.5Ce6.4Ni69Co4.7Mn4.3Al5.7Zr0.1Si0.3 alloy prepared by induction melting has a maximum discharge capacity of about 340 mAh/g, the same as the commercial AB5 alloy containing Pr and Nd [49]. Another challenge is the poor high-rate dischargeability of hydrogen storage electrode materials for the applications of Ni-MH batteries in high-power fields. Numerous efforts have been implemented to improve the electrochemical performance of AB5-type alloys, such as annealing treatment, surface treatment, additives and nonstoichiometry[50–54].

Shen

et

al.[55]

studied

the

surface

modification

of

an

AB5-type

alloy

(La0.64Ce0.25Pr0.03Nd0.08Ni4.19Mn0.31Co0.42Al0.23) electrode with polyaniline by electroless deposition. The result revealed that through the polyaniline-coating, the high-rate discharge ability increased from 8.5 to 45.0% at a discharge current density of 1440mA/g. Li et al.[56] reported the synthesis of a composite by coating hydrogen storage alloys (HSAs) with reduced graphite oxide via a top-down route. The high-rate dischargeability was distinctly enhanced with the capacity retention rate reaching 51.3% at a discharge current density of 3000 mA/g, which is almost four times that of the bare HSA electrode (13.5%). Recently, Zhou et al. [57] indicated that after hot-alkali treatment and duplex hot-alkali treatments with reducing agents, the MmNi3.7Co0.7Mn0.3Al0.3 alloy shows excellent high-rate output performance, even at low temperature. So far, commercial AB5-type hydrogen storage alloys have achieved a reversible capacity of ~320–350 mAh/g. Obviously, the electrochemical capacity of AB5-type hydrogen storage alloys has small space for improvement, which is an unfortunate situation that is attributed to their structure. Therefore, novel electrode alloys with higher hydrogen storage densities need to be developed to match the requirements of increasing energy density in Ni-MH batteries.

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ACCEPTED MANUSCRIPT 3.2. AB2-type alloys AB2-type alloys are considered as second-generation electrode alloys for Ni-MH batteries due to their higher energy densities and the earliest investigations were based on binary compounds with A = Zr or Ti and B = V, Cr or Mn. The Laves phases, mainly the hexagonal C14 phase (MgZn2), the cubic C15 phase (MgCu2) and the hexagonal C36 phase (MgNi2), are typical representatives of AB2-type intermetallic compounds, corresponding to a compact stacking for an atomic ratio of RA/RB = 1.225 [58,59]. Both the C14 phases, such as ZrMn2 and TiMn2, and C15 phases, such as ZrV2, are good hydrogen absorbers, while the C36 phase is a poor hydrogen absorber and is therefore not discussed here. There are the same number of interstitial tetrahedral sites for hydrogen occupation in the C14 or C15 structures with environments A2B2, AB3 and B4 (Fig. 9) [6]. The A2B2 sites are the most occupied sites owing to their high coordination with A-type elements and they show the strongest affinity to hydrogen. Owing to the high stability of their hydrides, binary AB2-type compounds show poor electrochemical properties in alkaline electrolytes. However, interest soon turned to multi-element pseudo binary intermetallics and modifying the A/B stoichiometry. The basic multi-element AB2-type alloys mostly contain Ti, Zr, V, Ni. Cr, Co, Al and Fe. Ti, Zr and V are the hydride forming elements, Co and Mn provide the surface activity, and Cr, Al and Fe increase the corrosion resistance. It should be emphasized that Ni is the key element in all Ni-MH battery materials, including AB5, AB3-3.5, AB2 and AB. In the case of AB2 compounds, adding Ni can improve the catalytic activity of the redox reaction. The discharge capacity of the multi-element AB2-type alloys ranges from 370 to 450 mAh/g, which is much higher than that of AB5-type alloys, but with slow activation and poor cycle life. These drawbacks are caused by surface passivation, in which a very dense layer of metal oxides forms on the surface during the charge/discharge cycling process, which blocks the electrochemical reaction and hydrogen diffusion, and increases electrical resistance.

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ACCEPTED MANUSCRIPT To improve the overall electrochemical properties of AB2-type alloys, more work has been concentrated on the optimization of the composition. The alloy Zr0.8Ti0.2Mn0.8V0.2Ni0.8C0.15Al0.05, with the C14 structure, exhibited a reversible electrochemical capacity of 380 mAh/g after activation [60]. The ZrMn0.5Cr0.2V0.1Ni1.2, with the C15 structure, also deliver a capacity of 380 mAh/g, and has been established as a reference compound [61]. As an anode material developed by the Ovonic battery company, the V5Ti9Zr26.2Ni38Cr3.5Co1.5Mn15.6Al0.4Sn0.8 alloy provides an energy density of 1057 W/kg with an excellent stability within 1000 cycles [62–64]. Further studies have found that the formation of ZrNi and TiNi phases played important roles in improving the overall electrochemical performances of AB2-type compounds [65,66]. Optimizing the C14/C15 phase abundance can also improve their activation, capacity retention rates and high-rate dischargeability. However, a clear relationship between crystal structure and electrochemical properties has not yet been established. Moreover, composites with other types of alloys to introduce some catalytic structures is an effective way to improve the performance of AB2type compounds. For instance, for a Ti0.9Zr0.2Mn1.5Cr0.3V0.3 (AB2) composite with LaNi3.8Mn0.3Al0.4Co0.5 (AB5) formed by ball milling, the maximum discharge capacity increases from 48.6 to 310.4 mAh/g, and that of a Ti0.9Zr0.2Mn1.5Cr0.3V0.3-La0.7Mg0.25Zr0.05Ni2.975Co0.525 (AB3.5) composite reaches 314.0 mAh/g [67]. Compared to the AB5-type hydrogen storage alloys, the AB2-type compounds still show slower activation and lower rate capabilities. AB2-type compounds with higher energy densities, higher rate capabilities, faster activation and lower cost are still desired for Ni-MH batteries.

3.3. A2B7-type and AB3-type RE-Mg-Ni-based superlattice alloys Some binary La-Ni alloys, such as LaNi (AB-type), LaNi2 (AB2-type), LaNi3 (AB3-type) and La2Ni7 (A2B7type), have higher theoretical electrochemical capacities than the LaNi5 alloy, but the high stability of their hydrides restricts their development. In 1997, Kadir et al. [68] reported a series of new ternary RMg2Ni9 alloys

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ACCEPTED MANUSCRIPT (where R = La, Ce, Pr, Nd, Sm or Gd) prepared by sintering. Later studies [69–80] found that RE-Mg-Ni-based alloys contained the (La,Mg)2Ni7 phase with a rhombohedral PuNi3-type structure or the (La,Mg)2Ni7 phase with a hexagonal Ce2Ni7-type structure. Figure 10 shows that their structure can be considered as the stacking of AB5 units (CaCu5-type structure) and AB2 units (MgCu2 structure) along the c-axis direction with ratios of n:1[74]. In addition, many studies have shown that all the AB2C9-type alloys (A = RE or Ca, B = Mg or Ca, C = Ni) have the same structure as LaMg2Ni9 alloys, so they can be also labeled as AB2C9-type or AB3-type structures [69–80]. In 2000, Chen et al. [72] evaluated LaCaMgNi9, CaTiMgNi9, LaCaMgNiAl3 and LaCaMgNiMn3 alloys and all of them showed good activation performance. The maximum discharge capacity of LaCaMgNi9 reached 356 mAh/g, which is higher than that of AB5-type alloys, but they exhibit poor cyclability and low high-rate dischargeability. Around the same time, Kohno et al. [73] reported that the La0.7Mg0.3Ni2.8Co0.5 alloy has a maximum discharge capacity of 410 mAh/g, which is much higher than that of commercial Ni-MH batteries. Zhang et al. [81] investigated the structure and electrochemical properties of La0.7Mh0.3Ni2.975-xCo0.525Mnx (x = 0, 0.1, 0.2, 0.3, 0.4) alloys and found that all these alloys are mainly composed of the La(La,Mg)2Ni9 phase with the rhombohedral PuNi3-type structure and the LaNi5 phase with the CaCu5-type structure. The alloys have a maximum discharge capacity of 330–360 mAh/g and excellent high-rate dischargeability but poor cycle life. A La1.5Mg0.5Ni7 alloy has a double-phase structure with the Gd2Co7-type and Ce2Ni7-type phases, and its electrochemical capacity could be up to 390 mAh/g, as reported also by Zhang et al. [82]. Thus, both A2B7-type and AB3-type RE-Mg-Nibased hydrogen storage alloys are attracting more and more attention as negative electrode materials for Ni-MH batteries. Significant efforts have been made to improve the electrochemical properties of RE-Mg-Ni-based hydrogen storage alloys. Liao et al. [75,76] studied the ternary LaxMg3-xNi9 (x = 1.0–2.2) alloys and found that the discharge capacities and cycle lives of LaxMg3-xNi9 alloys show an increasing trend with increasing Mg content. In particular,

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ACCEPTED MANUSCRIPT La2MgNi9 has a maximum discharge capacity of 397.5 mAh/g and HRD1200 = 52.7%, exhibiting good overall electrochemical performance. Guo et al. [77] obtained the same conclusion. Pan et al. [78] investigated the electrochemical properties of a series of La0.7Mg0.3(Ni0.85Co0.15)x (x = 2.5–5.0) alloys and found that this type of alloy showed high discharge capacities, outstanding high-rate dischargeability and good kinetics, but poor capacity retention rates (Fig. 11(a)). Based on the aforementioned studies, Liu et al. [79] formulated the La0.7Mg0.3Ni3.4xCoxMn0.1

(x = 0–1.6) alloys and suggested that the capacity retention rate of alloy electrodes increases with

increasing Co content (Fig. 11(b)). The alloy achieved the best electrochemical properties at x = 0.75. Later [80], they partially substituted Ni with Al to obtain a series of La0.7Mg0.3Ni2.65-xCo0.75Alx (x = 0–0.5) alloys and found that the discharge capacity and high-rate dischargeability only marginally decrease, but the capacity retention rate improved distinctly from 32.0% (x = 0) to 73.8% (x = 0.3) after 100 charge/discharge cycles (Fig. 11(c)). Further studies suggested that there is a dense Al2O3 layer formed on the alloy surface during the cycling process, which improves the capacity retention rate by restricting pulverization and dissolution of the active materials [80]. In order to reduce the alloy’s cost, Tang et al. [83] substituted La with a Ml and studied the electrochemical properties of a Ml0.7Mg0.2Ni2.8Co0.6 alloy. As a result, this alloy is mainly composed of the CaCu5 and PuNi3 phases and has a capacity of 380 mAh/g and excellent cycle stability, exhibiting good overall electrochemical performance. Research has been ongoing to further improve the overall electrochemical performance of RE-Mg-Ni-based alloys and Table 5 lists part of the alloys developed so far. Yang et al. [97] investigated the surface coating of La0.88Mg0.12Ni2.95Mn0.10Co0.55Al0.10 alloy powder with Ni-Cu-P by an electroless composite plating treatment. The discharge capacity, cycle life and high-rate dischargeability were all noticeably improved by this treatment. Similarly, polyaniline electroless-deposition was applied to a La0.80Mg0.20Ni2.70Mn0.10Co0.55Al0.10 hydrogen storage alloy powder to improve electrochemical and kinetic properties [98]. Owing to the polyaniline coating, both the capacity retention rate and high-rate dischargeability were increased. The present author [99] added graphene to the

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ACCEPTED MANUSCRIPT AB3-type R-Mg-Ni-based alloy by a plasma milling method, finding that both the discharge capacity and high-rate dischargeability were significantly improved, and it is worth mentioning that the HRD1750 was increased from 53.7% to 89.6%. This is important for using this type of alloy electrode in power batteries. In order to further decrease the cost of the alloy, Cao et al. [100] prepared a high-Sm, Pr/Nd-free and low-Co La0.95Sm0.66Mg0.40Ni6.25Al0.42Co0.32 alloy by an induction melting. The alloy shows high discharge capacity and excellent cyclic properties with the maximum discharge capacity reaching 338.0 mAh/g and 80% of the capacity being retained after 239 cycles at 1C. More recently, Yan et al.[95] studied new La-Y-Ni system alloys prepared by the induction-melting rapidquenching method. The A2B7-type LaY2Ni9.7Mn0.5Al0.3 alloy has a maximum discharge capacity of 385.7 mAh/g and a 76.6% capacity retention rate after 300 cycles, which has a higher discharge capacity and better cycle stability than the AB5-type alloy. Liu et al.[96] reported that the La0.78Mg0.22Ni3.73 alloy prepared by step-wise annealing the as-cast alloy sample has a maximum discharge capacity of 372 mAh/g and achieves a cycle life of above 400 cycles (the discharge capacity decreases to 60% of the maximum discharge capacity), and the alloy exhibits an excellent kinetic performance with a high-rate dischargeability of 62.4% at 1500 mA/g. Nowadays, RE-Mg-Ni-based alloys have already been used as commercial negative electrode materials in NiMH batteries, but new types of RE-Mg-Ni-based alloys with higher electrochemical capacities and longer cycle lives still need to be developed to meet the increasing energy demands.

3.4. Ti-V-based alloys As a result of their higher hydrogen storage capacities, both Ti-based C14-type Laves phase and V-based solid-solution type alloys have attracted extensive attention. Generally, V-based solid-solution phase alloys have high hydrogen absorption capacities, but their reversible hydrogen absorption/desorption capacity is low due to the

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ACCEPTED MANUSCRIPT formation of stable VH, and they do not have high reversible electrochemical discharge capacity due to poor electrocatalytic activity in alkaline electrolytes [101,102]. The Ti-based C14 type Laves phase has a high electrocatalytic activity [101,102] and it can catalyze the V-based solid-solution phase to achieve reversible electrochemical hydrogen absorption/desorption. Of course, the Ti-based C14 type Laves phase itself has a high electrochemical hydrogen storage capacity. Thus, Ti-V-based alloys have the synergistic effect of Ti-based alloys with V-based alloys and exhibit high electrochemical properties. Tsukahara et al. [103] reported that the V3TiNi0.56 alloy has a capacity of 420 mAh/g, but with a very poor cycle life, with the discharge capacity decreasing to 0 mAh/g after 77 cycles. They noted that forming a second phase, such as the C14 Laves or TiNi phases, could catalyze the V-based solid-solution phase to absorb and desorb a large amount of hydrogen, and the loss of reversibility maybe due to the disappearance of the secondary phase during the charge/discharge cycling process [103,104]. Thus, it is believed that creating multiphase alloys is a feasible approach to improve the electrochemical properties of Ti-V-based hydrogen storage alloys. Following this idea, Pan et al. [105] and Zhu et al. [106] studied the electrochemical properties of Ti-V-based alloys with partial substitution by a series of metal elements, such as Zr, Fe, Ni, Cr, Mn and Pd. The Ti1-xZrxV1.6Mn0.32Cr0.48Ni0.6 (x = 0.2–0.5) alloys mainly consist of the hexagonal structured C14 type Laves phase and the body centered cubic structured V-based solid-solution phase [106]. After partial substitution of Ti by Zr, the capacity retention rate significantly improved but the activation, maximum discharge capacity and high-rate dischargeability decreased. By partial substitution of V with Fe, a Ti0.8Zr0.2V2.7-xMn0.5Cr0.8Ni1.0Fex (x = 0–0.5) alloy was created, and both the cyclic stability and high-rate dischargeability were improved [107]. In addition, the cost of the alloy was deceased as

pure

vanadium

is

very

expensive.

They

further

studied

a

series

of

superstoichiometric

Ti0.8Zr0.2(V0.533Mn0.107Cr0.16Ni0.2)x (x = 2–6) alloys containing a C14 Laves phase and a V-based solid solution phase, and demonstrated that the Ti0.8Zr0.2(V0.533Mn0.107Cr0.16Ni0.2)5 alloy has a maximum discharge capacity of 380

17

ACCEPTED MANUSCRIPT mAh/g [105]. Here again, the V-based solution phase is the major hydrogen absorbing phase and the C14 Laves phase acts not only as a hydrogen absorbing phase, but also as a catalyst for the electrochemical hydrogenation and dehydrogenation of the V-based phase [108]. Recently, Wang et al. [109] reported that the cycling stability of a Ti0.10Zr0.15V0.35Cr0.10Ni0.30 alloy was distinctly improved after compositing with LaNi5. Some properties of Ti-V-based alloys are listed in Table 6. By summarizing the obtained results, we can obtain the following principle for Ti-V-based alloys. As essential elements of Ti-V-based alloys, Ti, Zr and V are the primary hydrogen absorption elements, Ni provides the catalytic activity for the redox reaction, Co and Mn improve surface activity and Cr, Fe and Al increase the anti-corrosion ability. Optimizing the processing of an alloy, e.g., rapid solidification, can refine the microstructure and improve the uniformity, and thus prevent pulverization of the alloy particles and improve the anti-corrosion ability [115,116]. However, the cyclic stability and the high-rate dischargeability are still not satisfactory for practical application, although the Ti-V-based alloys have been extensively investigated and their overall electrochemical performances have been markedly improved. Further studies are still required and composition optimization, annealing treatment and composites with other types of alloys are effective ways to improve the overall electrochemical properties of Ti-V-based alloys.

3.5. Co-based alloys Although the electrochemical properties of hydrogen storage alloy anode materials for Ni-MH batteries have improved after vast research, their capacity is relatively low in comparison with, in particular, Li-ion batteries. Recently, Co-based alloys, especially CoB alloys, were found to have relatively high discharge capacities, excellent cycle lives and extremely high catalytic activities when used as negative electrodes in an aqueous KOH electrolyte. Wang and coworkers [117] demonstrated that ultra-fine particles of the Co-B amorphous alloy could be successfully prepared through the reduction of Co(SO4)2 by NaBH4. As an anode material in an alkaline

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ACCEPTED MANUSCRIPT rechargeable battery, the discharge capacity of Co-B is more than 600 mAh/g for the initial cycle. In addition, the cycling ability and high rate capability are fairly good. Liu et al. [118] found that the capacity retention of CoxB (x = 1, 2 and 3) alloys prepared by a magnetic levitation melting method was more than 93% after 100 charge/discharge cycles. Co-B prepared by Tong et al. [119] in a water/cetyl-trimethyl-ammonium bromide/nhexanolmicroemulsion has a maximum discharge capacity of 357 mAh/g and this is still maintained after 50 discharge cycles. Using a vacuum freeze-drying method, the as-prepared Co-B alloy has a maximum discharge capacity of 438 mAh/g [120]. In a recent study, an amorphous Co-B alloy prepared by the reduction of CoSO4 by KBH4 has an extremely high discharge capacity of 968.6 mAh/g and the introduction of NaS2O3 into the alkaline electrolyte is helpful for storing the battery for long durations (Fig. 12) [121]. Additionally, in a 6 M KOH+0.09 M ethylenediamine electrolyte, the CoB alloy prepared from the precursor of CoCl2·6H2O exhibits a high discharge capacity and a long cycle life at an elevated temperature of 55 ºC, i.e., the discharge capacity is still up to 601.7 mAh/g after 100 discharge cycles [122]. Other Co-based alloys, including Co-P, Co-Si, Co-BN and Co-Si3N4, also show high discharge capacities. For instance, the Co-BN alloy prepared by ball milling has a maximum discharge capacity of 450 mAh/g, which remains at 280 mAh/g after 50 charge/discharge cycles [123]. By the same preparation method, the Co-Si3N4 alloy shows equivalent electrochemical performances [124]. However, there are two different viewpoints on the principles of the electrochemical reaction of Co-based alloys used as electrode materials in batteries. Mitov et al. [125] studied the Co-B nano-alloy prepared by a chemical reduction method via cyclic voltammetry (CV) testing and found that the CV curve of this alloy is similar to that of hydrogen storage electrode alloys. Thus, they judged that the principles of the electrochemical reaction of Co-B alloys were based on hydriding/dehydriding. Wang et al. [117] agreed with this view after carrying out a similar investigation. Later on, Liu et al. [126] studied two types of Co-B alloys prepared by high temperature solid state reaction and an arc melting method, respectively, they considered that boron plays a role for hydrogen

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ACCEPTED MANUSCRIPT absorbing and the electrode reaction is given by: (3-1) He et al. [127,128] and Cao et al. [129] investigated the electrochemical properties of Co-Si and Co-P alloys, respectively, and they all approved the hydriding/dehydriding principle. They reported that the reactions proceed as follows: (3-2) (3-3) In those studies, the researchers who supported the electrochemical hydrogen storage mechanism generally believed that the non-metallic elements P, Si and B in the Co-based alloys play an important role in hydrogen absorbing. But there are also a few studies that deem that Co can absorb and release hydrogen at room temperature, because the measured capacity is actually equivalent to the electrochemical hydrogen storage of Co. Chung et al. [130,131] compared the X-ray diffraction (XRD) patterns of a Co electrode before and after charging, finding that the structure of Co transformed from a stable HCP phase to an unstable FCC phase. Thus, they concluded that the charge/discharge process occurred in the Co electrode via the following reaction: (3-4) Another viewpoint is the same as for the Fe, Cd and Zn electrodes, belonging to the dissolution-precipitation mechanism. Liu et al. [118] used scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and XRD to investigate a series of Co-B alloys before and after charge–discharge, finding that both Co and B were oxidized in the process of discharging, but only Co can be reduced, the oxidation of B is irreversible, and the oxidation products of B is gradually eluted into the electrolyte during the charge–discharge cycling process. This point has also been confirmed by other studies [132–134]. They favor the reaction occurring in the bulk of the CoB alloy electrode at the beginning of the discharge process as follows:

20

ACCEPTED MANUSCRIPT 3

CoB  OH   e   Co(OH) 2  BO3  H 2 O

(3-5)

After B is oxidized completely, the later charge–discharge process is the same as that of a Cd electrode: (3-6) Wherein B plays an important role for the electrochemical properties of the Co-B alloy: (1) at the beginning of the discharge process, B is oxidized to BO33-, which will increase the discharge capacity of the alloy electrode. (2) BO33-, the oxidation product of B, is continuously dissolved in the electrolyte, so that the new surface of Co is exposed, reducing the passivation of Co, which will increase the discharge capacity and high-rate dischargeability. This means that B plays the role of an activator in the Co-B alloy. Lu et al. [123] and Yao et al. [124] also support the dissolution–precipitation mechanism after investigating the electrochemical properties of the Co-BN and CoSi3N4 alloys, respectively. Obviously, further research is necessary to clarify this argument.

3.6. Mg-based alloys High-energy density, long endurance and excellent rate performance are the standards of ideal electric vehicles, as determined by the performance of the onboard battery. As stated previously, the progress of industries advances rapidly, traditional energy sources are dwindling and environment concerns are increasing, therefore, the development of an ideal battery with a high capacity, long life, outstanding high charge/discharge rate, environmentally friendliness, safety and low cost for electric vehicles to ease these problems is imperative. Mgbased hydrogen storage alloys have a very high hydrogen storage capacity, with 7.6 and 3.6wt.% for MgH2 and Mg2NiH4, respectively, corresponding to theoretical capacities of ~2200 and 999 mAh/g, respectively [135–137]. This is much higher than that of AB5, AB3, AB2 and Ti-V-based hydrogen storage alloys. In addition, Mg is an abundant and cheap element. Therefore, Mg-Ni based alloys were regarded as very promising candidates as hydrogen storage alloys for Ni-MH batteries. Originally, Mg-based alloys showed poor electrochemical properties

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ACCEPTED MANUSCRIPT due to their poor hydriding/dehydriding kinetics at room temperatures. By 1986, Sapru et al. [138] prepared the Mg52Ni48 amorphous alloy using a sputtering method and the maximum discharge capacity of the alloy was 500 mAh/g at the discharge current density of 50 mA/g. Lei et al. [139] reported that the Mg50Ni50 amorphous alloy prepared by ball milling showed good catalytic activity at room temperature and the first discharge capacity can reach 500 mAh/g. Iwakura et al. and Nohara et al. [140,141] further increased the discharge capacity of Mg50Ni50 to higher than 1000 mAh/g. Lee et al. [142] noted that the surface energy of the amorphous or nanocrystalline electrode is higher than the crystalline one, and the hydrogen diffusion and charge transfer reaction is enhanced. Since then, amorphization and nanocrystallization have been regarded as very effective ways for activating Mgbased alloy electrodes. To further improve the electrochemical properties of Mg-based alloys, alloying with other elements was attempted. Notten et al. [143,144] investigated the electrochemical properties of Mg-X (X=Sc, Ti, V and Cr) binary alloys. They exhibited superior high reversible capacity ranges from 1300 to 1800 mAh/g, five times higher than commercial AB5-type materials. Tian et al. [145,146] reported a series of Mg0.9-xTi0.1PdxNi (x = 0.04, 0.06, 0.08, 0.10) alloys for electrode materials. Among them, Mg0.9Ti0.1Ni1-xPdx (x = 0, 0.5, 0.10, 0.15) amorphous alloys showed good cycling stability, in which a Mg0.8Ti0.1Pd0.1Ni alloy maintained a discharge capacity of 200 mAh/g after 80 charge/discharge cycles. By adopting special preparation methods, Mg-based alloys have been obtained with unique structures. For example, a quaternary nanocrystalline Mg1.95Y0.05Ni0.92Al0.08, which was prepared by sintering followed by ball milling with Ni powder, showed an excellent cycling life that maintained nearly 96% of the initial capacity retention rate of 385 mAh/g after 140 cycles (Fig. 13) [147]. Mixing Mg-based alloys with some other hydrogen storage alloys or compounds with high catalytic activity or good anti-corrosion capability is another strategy to improve the performance of Mg-based anodes. A significant number of studies have focused on the effect of the addition of AB5-type alloys and the results show that the hydrogen storage performances of Mg-based

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ACCEPTED MANUSCRIPT alloys improve distinctively after alloying with AB5-type alloys [148–150]. For example, when MgH2 is ball milled with commercial MmNi5 powders, the composites show faster hydriding/dehydriding kinetics, especially at low temperature hydrogenation, than the original MgH2 [148]. Mg-Ni-based alloys introduced with carbon by a mechanical alloying method showed a discharge capacity of 415 mAh/g at the first cycle and it only dropped by 5.3% in the second cycle[151]. Recently, Huang et al. [152] reported that the electrode of MgNi-10 wt.% TiNi0.56Co nanocomposition has a maximum discharge capacity of 397 mAh/g with a capacity retention rate of 62% (S50). Although progress has been achieved, the cyclability of Mg-Ni-based anodes is still unfortunately poor due to rapid dissolution of magnesium in alkaline solution [153,154]. Therefore, the practical application of Mgbased alloys is not yet possible. Recently, a series of new Mg-M (M = Ni, La, Nd, Ag, Y and mischmetal)-based hydrogen storage alloys have been developed via different methods and they show fast kinetics and reasonably high capacity [155–161]. In addition, Mg-based amorphous alloys have also been developed with a high hydrogen storage capacity. This provides new possibilities for developing negative electrode alloys. Nevertheless, it is still a great challenge to develop Mg-based alloys as negative electrodes for Ni-MH batteries.

4. Summary Developing clean and fuel-efficient HEVs, electric vehicles and distributed energy storage stations are appropriate approaches to realize a green energy society. Ni-MH rechargeable batteries are preferred for portable power tools and HEVs, owing to their comparatively high-energy density and good environmental compatibility. As anode materials for Ni-MH batteries, hydrogen storage alloys have been extensively studied in the past several decades. In this review, we have primarily presented the development of hydrogen storage alloys with the focus on Ni-

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ACCEPTED MANUSCRIPT MH power batteries. The principles of Ni-MH batteries, electrochemical hydrogen storage thermodynamics and electrochemical hydrogen storage kinetics have been discussed at first, and then, the relationships between alloy compositions, crystal structures, microstructures and electrochemical properties have also been evaluated. In particular, the recent advance in hydrogen storage electrode alloys have been summarized with the emphasis on AB5- and AB2-type alloys. The AB5-type alloys with a typical composition of MmNi3.55Co0.75Mn0.4Al0.3 (Mm = La0.62Ce0.27Pr0.03Nd0.08) are most popular and marketable electrode alloys used for Ni-MH batteries. However, these alloys have small room to grow due to their low theoretical electrochemical capacity of 372 mAh/g. Although AB2type alloys have a higher capacity, they have relatively poor cyclic stability and are costly to produce. An effective way to improve their performances is to combine AB5 and AB2 units to get alloys with AB3- or A2B7-typed structures. In addition, more Mg is added into alloys. This has led to the development and commercialization ofAB3- or A2B7-type RE-Mg-Ni-based alloys as negative electrode materials in Ni-MH batteries in recent years. Owing to their higher capacity, these materials are regarded as the new generation of materials replacing AB5 based alloys. To satisfy the requirements of power batteries, significant efforts, including composition optimization, additives, heat treatment, surface treatment and ball milling treatment, have been extensively conducted to improve the overall electrochemical properties of RE-Mg-Ni-based hydrogen storage alloys. In addition to the above anode alloys, Mg-, Ti-V- and CoB-based alloys have been extensively studied due to their higher energy density in recent decades. Although there have been many successful studies, whereby their properties were improved, it has proven difficult to improve their hydrogenation/dehydrogenation kinetics and rate capacities. The poor cyclability, low high-rate dischargeability and high cost of Ti-V- and CoB-based alloys are preventing their practical application. The stability of Mg-based alloys in alkaline solutions still cannot meet the requirements of industrialization. Nevertheless, once the breakthrough for the stability in electrolytes is accomplished, Mg-based hydrogen storage alloys should make excellent negative electrode materials for Ni/MH

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ACCEPTED MANUSCRIPT batteries. In any case, however, new ideas and advanced research are still needed to develop novel electrochemical active hydrogen storage alloys with higher energy densities, faster activation, better rate capacities and lower costs.

Acknowledgements This work was supported by the Fund for Innovative Research Groups of the National Natural Science Foundation of China (No. NSFC51621001), the National Natural Science Foundation of China Projects (Nos. 51431001) and by the Project Supported by Natural Science Foundation of Guangdong Province of China (Nos. 2016A030312011 and 2014A030311004). The Project Supported by Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2014) is also acknowledged. References 1. J. Chow, R.J. Kopp, P.R. Portney. Energy resources and global development. Science, 2003, 302: 1528. 2. F. Beck, P.R. etschi. Rechargeable batteries with aqueous electrolytes. Electrochim Acta, 2000, 45: 2467-2482. 3. J.M. Tarascon, M. Armand. Issues and challenges facing rechargeable lithium batteries. Nature, 2001, 414: 359. 4. J.Q. Kang, F.W. Yan, P. Zhang, C.Q. Du. Comparison of comprehensive properties of Ni-MH (nickel-metal hydride) and Li-ion (lithium-ion) batteries in terms of energy efficiency. Energy, 2014, 70, 618-625. 5. J. Kleperris, G. Wojcik, A. Czerwinski, J. Skowronski, M. Kopczyk, M. Beltowska-Brzezinska. Electrochemical behavior of metal hydrides. J Solid State Electrochem, 2001, 5: 229-249. 6. Y.F. Liu, H.G. Pan, M.X. Gao, Q.D. Wang. Advanced hydrogen storage alloys for Ni/MH rechargeable batteries. J Mater Chem, 2011, 21: 4743-4755. 7. F. Cuevas, J.M. Joubert, M. Latroche, A. Percheron-Guégan. Intermetallic compounds as negative electrodes of Ni/MH batteries. Appl Phys A, 2001, 72: 225-238. 8. P.H.L. Notten, P. Hokkeling. Double-Phase Hydride Highly Electrocatalytic Materials. J Electrochem Soc, 1991, 138: 1877-1885. 9. P.H.L. Notten, R.E.E. Einerhand. Electrocatalytic Hydride-Forming Compounds for Rechargeable Batteries. Advanced Metarials, 1991, 3: 343-350. 10. M. Tliha, C. Khaldi, S. Boussami, N. Fenineche, O. El-Kedim, H. Mathlouthi, J. Lamloumi. Kinetic and thermodynamic studies of hydrogen storage alloys as negative electrode materials for Ni/MH batteries: a review. J Solid State Electrochem, 2014, 18: 577-593. 11. J. Kleperis, G. Wójcik, A. Czerwinski, J. Skowronski, M. Kopczyk, M. Beltowska-Brzezinska. Electrochemical behavior of metal hydrides. J Solid State Electrochem, 2001, 5: 229-249. 12. F. Feng, M. Geng, D.O. Northwood. Electrochemical behavior of intermetallic-based metal hydrides used in Ni/metal hydride (MH) batteries: a review. Int J Hydrogen Energy, 2001, 26: 725-734. 13. Z.S. Wronski. Materials for rechargeable batteries and clean hydrogen energy sources. Int Mater Rev, 2001, 46: 1-49.

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ACCEPTED MANUSCRIPT

Fig. 1 Comparison of the different battery technologies in terms of volumetric and gravimetric energy density and some hydrogen storage materials [3].

Fig. 2 Schematic diagram of the electrochemical reaction process of a Ni-MH battery (a) and the hydride formation/decomposition process occurring via electrochemical charge transfer reaction (b) [6].

ACCEPTED MANUSCRIPT

Fig. 3 The sales amount and export volume of Ni-MH batteries from 2012 to 2014 in China.

Fig. 4 Typical PCT curves for hydrogen absorption–desorption in intermetallic compounds [10].

ACCEPTED MANUSCRIPT

Fig. 5 The electrochemical impedance spectra (EIS) of the La0.7Mg0.3Ni3.5alloy and La0.7Mg0.3Ni3.5Ti0.17Zr0.08V0.35Cr0.1Ni0.3 composite electrodes at 50% DOD and 303 K [30].

Fig. 6 Linear polarization curves (LP) of the La0.7Mg0.3Ni3.5alloy andLa0.7Mg0.3Ni3.5Ti0.17Zr0.08V0.35Cr0.1Ni0.3composite electrodes at 50% DOD and 303 K [30].

ACCEPTED MANUSCRIPT

Fig. 7 Crystal structure of LaNi5 alloy [6,7].

Fig. 8 Linear dependence of the plateau pressure as a function of the intermetallic cell volume for various La1−yRyNi5-xMx compounds at room temperature [7].

ACCEPTED MANUSCRIPT

Fig. 9 Crystal structures of C14 (a) and C15 (b) Laves phases and theirpossible interstitial sites for hydrogen occupancy [6].

Fig. 10 Crystal structure of AB3-type alloys. (a) Interrelation of stacking layers, (b) some of the interstitial sites [74].

ACCEPTED MANUSCRIPT

Fig. 11 Cyclic stability of different alloys. (a) forLa0.7Mg0.3(Ni0.85Co0.15)x (x = 2.5-5.0) alloys, (b) for La0.7Mg0.3Ni3.4-xCoxMn0.1 (x = 0-1.6) alloys and (c) for La0.7Mg0.3Ni2.65-xCo0.75Alx (x = 0-0.5) alloys [78-80].

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Fig.12 (a) CV curves of the Co2.012BHy sample, (b) Discharge curves of the Co2.012BHy sample at different cycles at 60 mA/g, (c) XRD patterns of the CoOOH precipitate (insert: SEM image of the precipitate), and (d) Cycling stability of the Co2.012BHy sample in the alkaline electrolyte with different additions of Na2S2O3 at a charge and discharge current density of 150 mA/g [121].

Fig.13 Capacity decay of the composite magnesium alloy electrodes with different crystalline sizes [147].

ACCEPTED MANUSCRIPT 1. The prerequisites for Ni-MH battery are summarized. 2. The Relationship between electrochemical performance and hydrogen storage properties are noted. 3. The relationship between alloying compositions, crystal structures and properties for each type alloy are narrated. 4. The achieved research results, existing problems and development direction are discussed.

ACCEPTED MANUSCRIPT Table. 1 Rechargeable batteries selected for application in electric vehicles [5]. Technology

Environmental

Nominal voltage (V)

Costs ($/Wh)

Specific enengy (Wh/kg)

Cycle life

Lead-acid

Toxic

2.0

0.1-0.3

20-35

100-500

Ni-Cd

Toxic

1.2

0.5-1.5

30-50

1000

Ni-MH

Low toxicity

1.2

1.0

60-120

500

Lithium-ion

Hazardous

1.5-3.9

0.2-0.3

115-265

400-1200

Table. 2 The relationship of HRD to charge transfer rate and hydrogen diffusivity. Samples

Properties

Rct

I0

IL

D or D/a2

HRD

Ref.

La0.78Ce0.22Ni3.73Mn0.30Al0.17Co0.80 La0.78Ce0.22Ni3.73Mn0.30Al0.17Fe0.80

AB5-type

~0.6 Ω ~2.1 Ω

~140 mA/g ~62 mA/g

-

D=0.207×10-10cm2/s D=0.186×10-10cm2/s

HRD600=43.47 % HRD600=54.73 %

[25]

Ti12.5Zr21V10Cr8.5Mn2.6Co1.5Ni43.9 Ti12.5Zr21V10Cr8.5Mn23.6Co1.5Ni22.9

AB2-type

-

25.5 mA/g 17.98 mA/g

-

D=2.05×10-10cm2/s D=1.54×10-10cm2/s

HRD50=92.8 % HRD50=83.9 %

[26]

La0.7Mg0.3Ni2.65Mn0.1Co1.05 La0.7Mg0.3Ni2.65Mn0.1Co1.15

RE-Mg-Ni-based

35.4 mΩ 44.9 mΩ

327.8 mA/g 306.3 mA/g

1912.0 mA/g 1653.8 mA/g

D=0.87×10-10cm2/s D=0.79×10-10cm2/s

HRD1750=61.4 % HRD1750=50.2 %

[27]

Ti0.8Zr0.2V2.7Mn0.5Cr0.8Ni0.75 Ti0.8Zr0.2V2.7Mn0.5Cr0.8Ni1.25

V-based

-

72.8 mA/g 85.6 mA/g

501.9 mA/g 778.6 mA/g

D=0.257×10-10cm2/s D=0.498×10-10cm2/s

HRD800=~25 % HRD800=~70 %

[28]

Mg-based

5.54 Ω 0.86 Ω

50.05 mA/g 127.07 mA/g

-

D/a2=2.048×10-5cm2/s D/a2=3.623×10-5cm2/s

HRD400=51 % HRD400=65 %

[29]

Mg50Co50 Mg50Co45Pd5

Table. 3 Typical characteristics of hydrogen storage alloys for Ni-MH battery application [4-15]. Compound (structure type)

Types

LaNi5 (Hexagonal) TiMn2 (Hexagonal or cubic) LaNi3 (Rhombohedral) VTi

AB5 AB2 RE-Mg-Ni-based V-based

(C14-type Laves phase and V-based solid-solution)

Mg-based

Mg2Ni (Cubic)

Hydrides

Hydrogen storage capacity (wt.%)

Discharge capacity (mAh/g)

Advantages

Disadvantages

LaNi5H6

1.4

320

Resource-rich

Low capacity

TiMn2H2

1.7

440

High capacity

Poor activation performance

LaNi3H5

1.6

410

Good activation performance

Poor cycle life

VTiH2

2.0

420

High capacity

Poor activation performance, high cost

Mg2NiH4

3.6

500

Resource-rich, high capacity

Poor stability

Table. 4 Hydrogen capacity for various pseudo-binary AB5-type alloys [10]. Composition

Csg (H/f.u), (PH2(bar))

Cel (mAh/g)

LaNi5

6.24, (25)

387

LaNi4.7Al0.3

5.85, (10)

370

LaNi4.25Co0.75

6.23, (10)

386

LaNi3Co2

5.39, (10)

334

LaNi4Cu

5.45, (10)

334

LaNi4Fe

5.24, (10)

327

LaNi4.6Mn0.4

6.21, (10)

386

LaNi4Mn

5.82, (10)

364

LaNi4.5Sn0.5

5.25, (10)

304

LaNi3.55Mn0.4Al0.3Co0.75

5.60, (10)

334

MmNi3.55Co0.75Mn0.4Al0.3

5.30, (10)

332

ACCEPTED MANUSCRIPT Table. 5 Electrochemical properties of various R–Mg–Ni-based alloy electrodes. Maximum discharge capacity (mAh/g)

High-rate dischargeability

Capacity retention

Ref.

410

-

-

[73]

La0.7Mg0.3(Ni0.85Co0.15)3.5

395.6

HRD1000 = 85.8%

S60 = 45.9%

[78]

La2MgNi9

397.5

HRD1200 = 52.7%

S100 = 60.6%

[76]

La0.7Mg0.3Ni3.5

352.8

HRD800 = 79.4%

S100 = 58.4%

[84]

La0.7Mg0.3Ni2.9(Al0.5Mo0.5)0.6

397.6

HRD1200 = 70.5%

S70 = 70.8%

[85]

La0.4Nd0.4Mg0.2Ni3.2Co0.2Al0.2

372

HRD1200 = 39.2%

S100 = 82.3%

[86]

La0.6Pr0.1Mg0.3Ni2.45Co0.75Mn0.1

372.7

HRD1000 = 70.1%

S100 = 78.4%

[87]

La1.8Ti0.2MgNi8.7Al0.3

339.6

HRD1400 = 61.1%

S205 = 60%

[88]

349

HRD1500 = 41.5%

S100 = 82.8%

[89]

377.5

-

S100 = 95.72%

[90]

Pr2MgNi9

342

HRD1500 = 56.7%

S100 = 86.3%

[91]

Nd2MgNi9

341

HRD1500 = 58.7%

S100 = 84.5%

[91]

Alloy La0.7Mg0.3Ni2.8Co0.5

La0.75Mg0.25Ni3.5 La0.4Pr0.4Mg0.2Ni3.15Co0.2Al0.1Si0.05

La4MgNi19 La0.6Gd0.2Mg0.2Ni3.05Co0.25Al0.1Mn0.1 La0.75Mg0.25Ni3.05Co0.20Al0.10Mo0.15 LaY2Ni9.7Mn0.5Al0.3 La0.78Mg0.22Ni3.73

367

HRD1500 = 66.4%

S100 = 85.7%

[92]

391.2

HRD900 = 75.3%

S100 = 89.8%

[93]

372

HRD1500 = 60.1%

S100 = 80.9%

[94]

385.7

-

S300 = 76.6%

[95]

372

HRD1500 = 62.4%

S100 = 85.3%

[96]

Table. 6 Electrochemical properties of some Ti-V-based alloy electrodes. Maximum discharge capacity (mAh/g)

High-rate dischargeability

Capacity retention

Ref.

Ti0.8Zr0.2V2.7Mn0.5Cr0.8Ni1.25

328.6

-

S200 = 73.13%

[28]

Ti0.17Zr0.08V0.34Pd0.01Cr0.1Ni0.3

Alloy

317

-

S100 = 86%

[110]

Ti0.17Zr0.08V0.35Cr0.1Ni0.25Mn0.05

374.1

-

S600 = 62.9%

[111]

Ti0.8Zr0.2V2.7Mn0.5Cr0.6Ni1.15Co0.1Fe0.2

333.4

HRD600 = 62.5%

S200 = 79.8%

[112]

Ti0.7Y0.1Zr0.2V2.7Mn0.5Cr0.6Ni1.25Fe0.2

360.2

HRD600 = 67.9%

S200 = 69.8%

[113]

Ti0.17Zr0.08V0.35Cr0.1Ni0.3B0.1

329.0

HRD800 = 72.5%

S200 = 89.4%

[17]

421

-

S30 = 41.07%

[114]

Ti0.26Zr0.07V0.235Mn0·1Ni0.33Gd0.005