Review of candidate batteries for electric vehicles

Review of candidate batteries for electric vehicles

Energy Conversion. Vol. 15, pp. 9 5 - 1 1 2 . P e r g a m o n Press, 1976. P r i n t e d in G r e a t B r i t a i n Review of Candid e Bafleries f...

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Energy Conversion.

Vol. 15, pp. 9 5 - 1 1 2 .

P e r g a m o n Press, 1976.

P r i n t e d in G r e a t B r i t a i n

Review of Candid e Bafleries for Electric Vehicles SIDNEYGROSS Boeing Aerospace Co., Seattle, Washington 98124, U.S.A. (Received 4 March 1975)

Abstract---Short summaries are presented of most of the battery systems that can be considered for electric vehicles. Many little known systems axe included, some with little or no experimental background, and thus axe worth considering for future research. Electric vehiclebattery requirements are postulated, and based on these requirements the battery candidates axe evaluated for their near-term and long-term prospects. Accumulators Batteries Battery research Electricpower chemical power sources Power sources Primary batteries Secondary batteries Storagecells

Introduction

This paper presents brief summaries of most of the battery types that can be considered as candidates for electric vehicle propulsion. Included in addition to the well known systems are many that are little known; some have little or no experimental background, and thus are worth considering for future research. Requirements for electric vehicle batteries are postulated, and based on these requirements the battery candidates are evaluated for their near-term and long-term prospects. A q u e o u s Batteries

Most of the candidate aqueous electrolyte batteries are well developed, and some have been used in the operation of commercial and research vehicles. While new high performance nonaqueous battery systems are being developed, the conventional aqueous systems are also being improved and will be very competitive for traction applications. Vehicle designers are finding that significant weight savings are possible in the vehicle design, and very high energy transfer eiticiencies are attainable. For example, trucks are being fabricated for the Electric Vehicle Council capable of 68 miles range, using leadacid batteries. An experimental electric vehicle operated with lead-acid batteries has obtained 145 miles range at steady speed, and 65 miles range in stop-go traffic [1]; acceleration was unacceptably low, however. This performance shows that conventional systems will continue to be competitive for powering electric vehicles.

Electric vehicles Electricvehicle power ElectroPropulsive power sources Rechargeablebatteries

cost for materials and manufacturing. Discussions on this system and its use in electric vehicles are abundant [2-7]. The attainable energy density of lead-acid batteries is dependent on the discharge rate, as illustrated in Fig. 1. This rate dependency is caused primarily by mass transport and ionic diffusion limitations. Crystals of PbSO4 deposit on the surface and in the pores of the electrodes, reducing surface area available for reaction, and causing a decrease in pore size that limits access of electrolyte. Simultaneously, the HzSO4 within the pores becomes depleted and diluted. Cycle life of lead-acid batteries is relatively low for high energy density designs at deep depths of discharge. For example, some electric vehicles made in England are equipped with LSI batteries guaranteed to provide 200 cycles at 90 per cent depth of discharge, which will be good for approx. 12,000 miles. It is possible to

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The lead-acid system has an open circuit potential of approximately 2.05 V, a theoretical energy density of 76 W hr/lb, and an achieved energy density of 10-24 W hr/lb. This system clearly dominates as today's main battery source of medium to high power. Characteristics that permit this domination are high current capabifity, operation over a wide temperature range, good charge retention, high efficiency, long life, and in particular, low

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SIDNEY GROSS

increase life by compromising on energy density. How- have been made in the development of grid alloys to ever, at least one vehicle manufacturer claims to have replace the conventional lead-antimony cast grids [17]. experimental proof that it is more economical for a Antimony causes self-discharge by migration from the vehicle to use light, high energy density batteries with positive grid to the negative plate, and is also becoming short life, than to use heavier, longer-lived designs. scarce and costly. The improved lead alloys are proA substantial effort is being made to develop better prietary, but are known to contain calcium and tin. lead-acid batteries for the increasing market in electric Wrought processes have been developed instead of castgolf carts, garden tractors, lawn mowers and electric ing, resulting in a micro-structure that is much stronger vehicles. Light-weight cases have been developed, using and much more corrosion resistant than conventional polypropylene and intercell connector path lengths have cast grids. Consequently, much thinner, lighter grids can been shortened using through-the-partition connectors be used, permitting substantial weight savings. In an [8]. Improved water-filling devices have also been experiment to verify this design approach, an energy developed. One worthwhile development [9] is a new grid density of approximately 24 W hr/lb was attained. The design, called laminar grid, that is designed for high problem remaining is adequate adhesion of active charge and discharge current needs such as for electric material to the grid. vehicles. Active material is held in a sandwich between Other approaches to development of improved grids are latticed grid segments, thus improving utilization and the use of plastics and titanium. Early work on plastic minimizing shedding. Results from bench tests are grids [18] showed charge retention was excellent and encouraging, for up to 1500 cycles at 80 per cent depth of proved that l0 per cent weight savings could be made. This discharge have been obtained [10]. No significant improve- approach is being pursued with some success [14]. ments in energy density have resulted from this design, Titanium is also a promising grid material for the positive however. electrode, resulting not only in weight savings but also in Another noteworthy lead-acid battery development is improved utilization of the active material [19]. an experimental design which produces 16 W hr/lb at the Another approach [20] is the use of expanded titanium 1.25 hr rate, and approximately 20 W hr/lb at the 5 hr grids nitrided to provide passivity to sulfuric acid. This rate [11]. However, cycle life with this battery is poor. In development was completed, yielding approximately Sweden, a long life lead-acid battery using tubular positive 20 W hr/lb; also, due to intimate contacts to reactants, plates has been developed giving 17 W hr/lb at the 6 hr the battery was capable of high rate discharge. However, rate, and a life expectancy of 1500-2000 cycles. Another the relatively high cost of titanium is a deterent to widedeveloped battery has achieved 20 W hr/lb at the 17 hr spread commercial use. Since then, a low cost metal alloy rate, and yet another has achieved 18.2 W hr/lb at the system has been developed which is essentially proved 19 hr rate. Japan has an extensive program to develop out, but information on operating characteristics and life better lead-acid batteries, with research being conducted is not yet available. on multilayer electrodes, on porous sheet electrodes, and Much progress is being made in the development of on circulating electrolyte systems. Many companies have maintenance-free lead-acid batteries [21]. These batteries programs on improved lead-acid batteries, but have not can be sealed and thus require no service throughout their yet revealed their results. lifetime. With the newer lead-calcium-tin grids, the Acceleration and regenerative braking needs may oxygen overvoltage is low enough that gassing is negligible impose high charge rate and discharge rate requirements at normal charging voltages, permitting a sealed design. on batteries for some applications. A bipolar high rate Catalytic recombination devices have been developed [22pulse battery has been developed [12] capable of 20,000 24] that can be used for conventional lead-antimony grid cycles operation at shallow depths of discharge. The initial batteries. A sealed semi-starved design has also been development has been done toward a semi-bipolar lead- developed that combines oxygen on the negative electrode, acid battery [13, 14] that should provide 100W/lb permitting overcharge safely at the C/3 rate [25]. Sealed (typical LSI batteries provide as little as 7 W/lb), and lead-acid batteries are now starting to be used in internal energy density approaching 20 W hr/lb at the C/5 rate. A combustion engine automobiles, which will help advance pile-type lead-acid battery is also being developed that this technology. delivers high power levels with conventional energy Another approach being taken to improve the energy density, but is capable of many cycles at deep depths of density of lead-acid batteries is to use potassium perdischarge. Analyses have been conducted of optimum chlorate as an electrolyte instead of sulfuric acid. This plate designs for high power, short pulse operation [15] gives greater energy density, and also improves perforand power density of 140-160 W/lb was obtained on an mance at low temperature. The major problem has been experimental cell. In spite of these efforts, it is uncertain difficulty in recharging these batteries, and therefore they whether there will continue to be a requirement for high have been used mostly as primary batteries so far. rate capability, based on recent assessments that a fly- Research in Germany is being conducted to improve wheel is more effective than a battery for acceleration and recharge, and if successful should result in some weight deceleration energy [16]. An experimental program is reduction. now underway to evaluate the flywheel approach. Energy density attained on some advanced lead-acid Much research is underway to improve energy density batteries is shown in Fig. 2, being noticeably greater for and life of lead-acid batteries. Important improvements long discharge durations. Most of these battery systems

Review of Candidate Batteries for Electric Vehicles

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growth. However, KOH electrolyte of low concentration with lithium hydroxide added has been found to minimize the swelling and give satisfactory life. Sintered nickel positives have also been used successfully [30, 33]. In order to seal the nickel-zinc cell, the oxygen evolved from the nickel positive during charge must be recombined on the zinc negative to form zinc oxide. Teflon bonded auxiliary electrodes with either silver, silver amalgam or active carbon can be used for this purpose. Continuous overcharge at the C/IO rate was found to be acceptable with such recombination electrodes [34]. What little hydrogen is formed generally diffuses out of the cell. Cycle life of the nickel-zinc system is limited by the zinc electrode. Due to solubility of zinc oxide, zinc dendrites develop during charging, causing shorts. This solubility also causes redistribution of active material, called shape change, resulting in capacity loss (Fig. 4, from [27]). If cycle life can be increased by a factor of 3-6, then the nickel zinc system could prove to be very useful for electric vehicles.

Fig. 2. Energy density of lead-acid batteries.

are still under development, so the data must be used with caution. It is seen that energy density of more than 20 W hr/lb has been attained with lead-acid batteries.

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Nickel-zinc battery The nickel-zinc system has an open circuit potential of 1.706 V, a theoretical energy density of 146 W hr/lb, and an achieved energy density of over 30 W hr/lb. Over 300 cycles at 65 per cent depth of discharge have been attained [26, 27], and up to 1500 cycles at 50 per cent depth of discharge have been attained in the laboratory [28]. This system is of great interest for electric vehicles of the future, for the theoretical and practical energy density is nearly twice that of lead-acid batteries, it has good high-rate capability [29], it can be sealed, and the cost of materials is low. Typical performance is shown in Fig. 3 [30]. Non-sintered nickel positive electrodes have been developed for this system to improve energy density and lower costs. These electrodes are generally of the pressed type, using nickel grids and carbon to improve electrical conduction [31, 32]. Swelling reduces life and can be severe on such pressed plates, with up to 40 per cent Basis: Cycle 25 2491.9 > ~ 1.8_

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Much research is being conducted to improve the cycle life of the zinc electrode, not only for the nickel-zinc system but also for other battery systems that use zinc negatives [35-37]. Electrodes are being optimized for cycling by addition of additives, binders and by contouring. One unique approach [38] is the use of KaBOa in the electrolyte to reduce solubility of the zinc species. A different approach is to permit high zincate concentration, but to suppress dendrites by adding potassium molybdate and lead monoxide to the electrolyte [39]. The charging method can also have a major influence on cycle life, with important improvements possible from interrupted d.c. charging. Improved separators are essential to long life. Recently discovered is the importance of separator geometry and electrolyte quantity in promoting long life [40]. Inorganic separators offer the most promise, and present developments with at least three types are very encouraging; in one laboratory test, over 1500 cycles at 50 per cent depth of discharge was obtained.

Zinc-bromine battery The zinc-bromine system has an open circuit potential of 1.8 V and a theoretical energy density of 196 W hr/lb.

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An energy density of 22 W hr/lb has been achieved on small batteries at the C/2 discharge rate [41], but 30 W hr/ lb should be attainable. Over 200 cycles have been reached, but the possible lifetime could be much greater. High energy density, high rate capability, and low cost of materials have prompted development of this unique system. However, insufficient research has been expended to fully evaluate the system's potential. For example, a flow version of this system might be very promising. The negative electrode for this battery is zinc, the positive electrode is bromine absorbed on activated carbon, and the electrolyte is slightly acidic (pH 3) aqueous zinc bromide with special additives to enhance uniform deposition of zinc during charge [42-47]. The bromine electrode presently limits cycle life due to capacity degradation. Bromine is a relatively safe material, for though it is toxic, its strong odor gives ample warning long before it becomes injurious. A recent design [48] employs a bipolar battery configuration.

Nickel-iron battery

The nickel-iron system has an open circuit potential of 1.370 V and a theoretical energy density of 121 W hr/lb. This is a very old system that is low in cost and very long lived. After Thomas Edison discontinued efforts to perfect the nickel-iron system for automobile engine starting, development on the system stopped, and the batteries marketed today are of essentially the same design established by Edison many years ago. Nevertheless, nickeliron cells are the most rugged and last longer than any type in use today. It is not uncommon to find nickel-iron batteries that have been in continuous use for 20 yr or more. They are virtually foolproof electrically, with little damage by being deeply discharged, short-circuited, reversed or overcharged. The batteries can be stored for very long periods without corrosion or deterioration. One manufacturer gives a 5 yr guarantee on his nickel-iron batteries, based on one discharge to 100 per cent depth each day. The major shortcomings of the classical nickel-iron battery are poor high rate capability, and only mediocre Charging energy density. Even so, the energy density of commercial 2.5 nickel-iron batteries is as high as 16.2 W hr/lb at low Open circuit w 2-0 rates [49], giving some indication of its potential. Since the __ T . - - ~l,, ~ pol'en1'ia I efficiency of commercial iron electrodes is only about oneOpen c.irc-it ~ 1.5 third of theoretical [50], there is substantial room for potential Load v o l t a g e ~ -~ IO-~ improvement. 2.5A _I_ 2.0 A _ U charge I discharge One unique development [51, 52] is an electrode system 0'5 f formed of felted and sintered microribbons of nickel and I I I I I I I I I iron. This design approach is based on an electroforming 0 0.5 1.0 1.5 2.0 2,5 5.0 5.5 4.0 4-5 process to produce microribbon fibers easily and economiTime, hr cally [53]. The key point of this concept is that a thin Fig. 5. Typical operating characteristics of a zinc-bromine cell. metal ribbon substructure can provide greater surface area and porosity than a powder-based structure. This Typical operating characteristics are shown in Fig. 5 work is not continuing at present, however. [44]. The cell displays a voltage rise at the end of charge Another advanced electrode is the nickel fiber electrode that can be used to terminate charge. Water is consumed [54]. This electrode is made up of interconnecting fibers, during charge at approximately the same rate as with lead- each fiber of which is composed of a single elongated acid batteries. As with other battery systems, charge must chain of small, interconnected, nearly perfect spheres. be terminated when full charge has been reached to avoid Such a structure has a high surface area, desired for good thermal problems. performance with low weight. Whereas most secondary batteries with zinc anodes The most advanced nickel-iron battery development have life problems due to zinc dendrites, this difficulty uses metal fiber substrates for the electrodes, resulting in does not occur with the zinc-bromine battery. As lightweight designs with high surface area, and hence dendrites tend to form in the separator, they are dis- good high rate capability. An important feature of the sipated due to reaction with the bromine. As a con- iron electrode is activation of the iron oxides with sequence, long cycle life should be attainable with this elemental sulfur; this helps corrode the iron to cause system. Long storage in the discharge state is also breakdown of the protective passivating film on the iron achieved with little degradation. surface, thus keeping the iron active material in an active The major drawback of the zinc-bromine system is its state [55]. Rather than simply employing an additive such high self discharge rate, losing nearly 50 per cent charge in as FeS which tends to separate as well as slowly decomtwo days. Continuous trickle charge at C/35 will counter pose electrolytically, a technique has been developed that this self-discharge, and the trickle rate can be reduced permits elemental sulfur, selenium or tellurium to form a significantly if only a 70 per cent state of charge is main- fusion coating on the surface of the iron oxide particles, tained. It should be possible to solve the self-discharge resulting in long lifetimes for the electrodes. An alternate problem with the use of a selective permeability separator. approach shown to be effective is the use of extremely If this deficiency is corrected, then the zinc--bromine pure iron [56]; this would be impractical for most battery could be an important contender for electric applications, however. vehicle power, for cost is estimated to be equal to the leadThe research on this battery has been completed, and it acid battery. is now in the pilot plant phase. 22 W hr/lb and 900 cycles

Review of Candidate Batteries for Electric Vehides >

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The nickel-hydrogen system has an open circuit potential of 1.358 V and a theoretical energy density of 177 W hr/lb. This is a recently developed system [63-66] but the technology is advancing rapidly because the nickel electrode is well developed from nickel--cadmium battery 4 hr rate 2.0technology, and the hydrogen electrode is well developed 2 hr rate > I hr r o t e from hydrogen-oxygen fuel cell technology. The Teflonated hydrogen electrode negative functions best with 1.0 platinum catalyst, but nickel is satisfactory for electric k3" vehicle applications, and is used in Russian designs [65]. = 0.5-Both electrodes are known to be long-lived, so very long I [ I I 0.5O 0.75 1.0 0 0125 operating lifetimes are expected with this system. An Fraction of rated copacity energy density of 25 W hr/lb has been achieved on prototype cells, and design studies show up to 40 W hr/lb Fig. 6. Typical performance characteristicsof nickel-iron cells. should be attainable. Power density of 40 W/lb has been realized, and could be increased to 200 W/lb on an at 100 per cent depth of discharge have been demonoptimized design. strated at the 2-hr rate [57]. Power density is approxiA nickel-hydrogen battery consists of a series stack of mately 40 W/lb. Cost is anticipated to be approximately nickel and hydrogen electrodes installed inside a pressure $2.20/lb, equivalent to $0.10/Whr. Typical charge-vessel filled with hydrogen gas. The dectxodes are discharge characteristics are shown in Fig. 6. separated by gas diffusion screens, but the hydrogen gas One unique characteristic of the nickel-iron battery is need not be isolated from the nickel electrodes since they hydrogen gassing during charge. One approach to this will not react chemically. The battery operates positiveproblem is to plumb the cells together within an external limited, and is completely sealed. Figure 7 shows a typipump system that circulates the electrolyte during charge. cal battery module design [64]. Except for the pressure Makeup water is added to each cell through the common vessel, no costly materials are used. Typical operating manifold; after charge is completed, each cell is closed off, characteristics of a single cell are shown in Fig. 8 and are being essentially sealed for the discharge operation. u

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Nickel-cadmium battery The nickel-cadmium system has a theoretical energy density of 100 W hr/lb and an open circuit potential of approximately 1.35V, depending on state-of-charge. Considerable literature is available on this system [49]. Nickel--cadmium cells permit very high power drain, have outstanding cycle life, and the energy density can be a little better than the lead-acid system. Improved separators have greatly extended the cycle life of vented batteries [58], providing further inducement to this system's use. High capacity nickel electrodes have been developed that do not use nickel plaque [27], thus permitting high energy density for lower cost. Improved nickel electrodes have also been developed using nickel or nickel-plated steel fibers [54, 59, 60]. For the negative electrodes, sponge plates provide an important weight saving [61]. Copper can also be used as a replacement for nickel plaque in the cadmium negative electrodes. As a result of this progress, nickel--cadmium cells are now available with energy density of 25 W hr/lb at the 5-hr rate [62]. The main drawback to widespread use of nickel--cadmium batteries is the limited world supply of cadmium; cost is also high enough to restrict its use. In spite of these shortcomings, nickel-cadmium batteries will continue to be used for some traction applications because of their high energy and power capability, though they will not likely serve the mass market. Recycling of batteries would

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seen to be comparable to nickel cadmium cell performance. An important attribute of the nickel-hydrogen system is that it is inherently protected against damage by reversal, permitting the full capacity of the battery to be used. Hydrogen pressure drops to approx. 100 psi during continuous reversal. Also significant is the ability of the system to tolerate overcharge without ill effect, though suitable means of heat removal must be provided. Hydrogen pressure increases to a maximum of 400-500 psi during continuous overcharge. The major disadvantage of the nickel-hydrogen system is the need for a pressure vessel to contain the hydrogen, which comprises a significant share of the battery weight. Structural safety factors will have to be relatively high for commercial applications, limiting the overall energy density. For example, it is calculated [66] that a 1000 W battery for traction applications would deliver 35 W hr/lb with a titanium pressure vessel, or 30 W hr/lb with an Inconel pressure vessel. Even with the lower value, this is quite attractive for electric vehicles in view of the good electrical performance and expected long life. A possible improvement to the nickel-hydrogen battery would be development of a satisfactory means of storing hydrogen in solids. Metal hydrides are able to store hydrogen even more compactly than liquid hydrogen. Using this principle, an electrochemically reversible hydrogen electrode has been developed in which the hydrogen is stored on interstitial sites of a metal lattice, using Ti~Ni and TiNi intermetallic phases [67]. The electrode is capable of very high energy density, and conceivably could eliminate or minimize the need for a pressure vessel with the nickel-hydrogen battery.

Zinc-chlorine hydrate battery The zinc--chlorine hydrate system has an open circuit potential of 2.12 V and a theoretical energy density of 209 W hr/Ib (zinc and chlorine yield 375 W hr/lb). The reaction is: Zn -t- C12.6H~O ~ ZnCI2 q- 6H~O. Refrigeration below 9 °C is necessary to keep the chlorine hydrate from decomposing into chlorine and water. This system is a flow type, using circulating electrolyte, a chlorine hydrate forming device, refrigeration and storage units [68]. A prototype system was built and operated in a Vega test vehicle, achieving a distance of 152 miles on a speedway. Typical discharge behavior of a zinc-chlorine hydrate cell is shown in Fig. 9. An energy density of 30 W hr/lb has been achieved [69], with projections to 50 W hr/lb at the 4-hr rate. At least 2 yr are expected before the system will be commercially available.

Antimony redox battery The antimony redox battery is quite unusual, having two inert carbon electrodes and a participating electrolyte with antimony and chloride ions, possibly separated into positive and negative compartments [70]. When the battery is charged, the antimony is plated out, and during discharge it goes into solution; this rejuvenation feature gives

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promise for long cycle life. Another appealing characteristic is no gassing during charge, resulting in good charge efficiency and the ability to be sealed. Also, either terminal can be selected to serve as the positive terminal when the battery is in its uncharged state. This battery development is still in the initial laboratory phase, so very little information is available. However, energy density estimates range from 25 to 40 W hr/lb. Metal-Air Batteries

Theoretical considerations Metal-air systems use a metal for the negative electrode and a gas electrode using oxygen from the air for the positive side. These systems can function as primary batteries [71], as conventional secondary batteries [72], as mechanically rechargeable batteries by replacement of electrodes or active materials [73-79], or as electromechanically recharged batteries using fluidized bed electrodes [80, 81], tape anodes [82], or a rotating particle bed [83]. Electric vehicles using zinc have been built and operated with most of these types of approaches, and the resulting technology presumably could be applied to other metals also. Another possible direction is high temperature operation with a solid electrolyte. Required for all metal-air systems are improved high-rate air electrodes with non-noble metal catalysts. High energy-density metal-air batteries have severe thermal problems. The metal-air battery can overheat, or for high temperature designs, give problems in initial heating.Also, water can be lost by evaporation from the electrolyte [74]. An improved oxygen electrode is an important task in the development of metal-air batteries. To attain a high voltage efficiency, an effective catalyst must be incorporated into a suitable porous structure. Both the catalyst and the supporting structure are subjected to a high positive potential, so conventional carbon-bearing or silver-bearing electrodes do not last long. Mechanical stress caused by oxygen evolution must also be considered in the electrode design [84]. Considerable effort has been directed to development of a suitable rechargeable oxygen electrode. For example, multilayer oxygen electrodes have been developed [85, 86] in which the carbon layer performs the oxygen reduction

Review of Candidate Batteries for Electric Vehicles Table 1. Candidate metal-oxygen systems

101

Failure of the zinc-air system to live up to early expectations for vehicle applications caused a number of the Energy Energy sponsors to terminate development, even after considerdensity cost Metal W hr/lb $/kW hr Comment able investment. In the view of one investigator [93], the problems are basic and not likely to yield with additional Beryllium 8142 6"63 Poisonous,difficult Lithium 6045 1.352 Studied development. These problems [94] include zinc electrode Aluminum 3718 0.124 Studied passivation under short term peak power loads; difficulty Magnesium 3090 0.1167 Primarydeveloped of producing during charge a zinc deposit compact Titanium 2162 0-217 Difficult Calcium 2075 0.164 Studied enough to avoid interelectrode shorting; difficulty in Sodium 1644 0.267 Studied achieving a good air electrode capable of high current Chromium 1192 0.396 Difficult densities at low gas pressure; loss of water in the air Manganese 868 0.403 Difficult Zinc 614 0' 554 Primary,developed exhausted from the air electrodes; and thermal problems. Iron 556 0.449 Secondary,developed In addition, energy density would be about 35 W hr/lb, Cadmium 262 5.72 Secondary,developed which is less than anticipated, and cost would also be Lead 110 I. 272 Studied relatively high. Thus, the outlook for the zinc-air system is not encouraging for electric vehicle applications. (discharge) and the nickel layer catalyses oxygen liberation (charge). Two-layer electrodes have been found to be Aluminum-air battery suitable for this purpose, whereas three-layer electrodes Aluminum is very attractive for metal-air batteries are sometimes destroyed by oxygen evolution within the from both weight and cost standpoints. It has been used electrode [85]. successfully in primary batteries [95-97], but shows little Though zinc has received most of the attention in promise as a secondary material in aqueous electrolyte. metal-air batteries, many other metals can also be con- Aluminum apparently can be cycled in non-aqueous sidered. Table 1 lists in order of decreasing theoretical electrolyte, but with reduced energy density. A major energy density the more promising candidate metals, with problem with aluminum is its rapid corrosion. Research the exotic metals eliminated from consideration. Using in Japan is presently being done on protective surface typical costs in 1972 for metals in a relatively pure state, layers that break down on discharge and reform on stand. the metal raw material cost per unit of battery theoretical Another approach being developed is to drain the electroenergy has been calculated, and ranges from $0.117/kW lyte away from the aluminum during stand. hr for magnesium to $6.63/kW hr for beryllium. The An aluminum-air battery currently under development theoretical energy density [87] includes no penalty for is a 2 kW system that is mechanically recharged by oxygen taken from the air, and ranges from 8142 W hr/lb replacing the aluminum and the alkaline electrolyte. for beryllium to 110 W hr/lb for lead. Comparing the Corrosion during stand is prevented by separating the candidate metals to zinc, it is seen that, of the 13 metals electrolyte from the aluminum anode. The air cathode listed, nine are superior in theoretical energy density and uses platinum catalyst, though non-noble metals could be eight are lower in metal cost per unit of theoretical energy. substituted for commercial applications. Energy density Thus, many metals other than zinc would be promising if estimates [98] for a 5 kW system are 400 W hr/lb at the the technical problems could be solved. The methodology 20 hr rate, and 100 W hr/lb at the 4 hr rate. offered by Conway [88] is suggested as a promising Iron-air battery approach to these problems. Cost of metal-air systems is strongly influenced by cost The iron-air battery is one of the more promising types of the air cathode. Estimates range from $10/ft 2 to for electric vehicles, with an expected energy density t, $1.50/ft 2, with power densities of 50 W/ft ~ to 100 W/ftL 50 W hr/lb. Considerable progress has been made in the Thus, assuming 75 W/ft 2, the cost of the cathodes for a development of improved iron electrodes (see section on 5 kW system would be in the range of $100-$667. nickel-iron batteries), which eases that task in development of iron-air batteries. Zinc-air battery One of the most important iron-air developments is the Zinc has almost completely dominated the field of work in Sweden [99]. Development work has progressed metal-air batteries. The zinc-air system has a theoretical to the point where an experimental 30 kW hr battery has energy density of 614 W hr/lb and an open circuit potential been built and used to power a small truck, achieving of 1.65 V, with demonstrated performance for primaries 35 W hr/lb, with an energy density of at least 45 W hr/lb of up to 150 W hr/lb. Much effort has been spent on expected in production models. The air cathode is made applying this system to experimental electric vehicles with from pressed nickel powder with silver catalyst. A relatively little evidence that the complex recharge pumped KOH electrolyte is used, providing temperature approaches developed are appropriate. Nevertheless, the control and the makeup water that is needed periodically. zinc-air technology has become highly developed as a Forced air provides the needed oxygen, with scrubbers to consequence and has been competently described and remove COs, and pressure control over the air cathodes. analysed many times [72-92]. In situ recharge has been an The auxiliary systems comprise ca. 10 per cent of the elusive goal, the solution of which can have a major impact total system weight. Batteries have lifetimes of ca. 500 cycles, limited by the cathode. on electric vehicle power systems.

102

SIDNEY GROSS

An iron-air battery developed in the United States is expected to obtain 50Whr/lb [100]. A high current density oxygen electrode has been obtained without the use of noble metal catalyst. Very active development of an iron-air battery is also occurring in Germany with generous funding for this program. Both two-layer and three-layer oxygen electrodes have been developed for this purpose [85, 86], but apparently an even better oxygen electrode has also been developed. Typical performance of one of these iron-air cells [85] is shown in Fig. 10.

1.0

0"9

0"8 > • 0"7 - -

"6

°t 0

I

I0

I

2O

I

30

I

40

Discharge current density,

I

5O

I

6O

A c m -2

Fig. lO. Typical discharge behavior of an iron-air cell.

Lead--air battery Lead offers the lowest theoretical energy density of those metals listed in Table 3, and would also be relatively costly. The system has been studied, however [101], using a sulfuric acid electrolyte. Projected weights for this system showed a possible energy density improvement of about 35 per cent over lead-acid batteries. The main problems experienced were poor life and performance of the rechargeable air cathode, though up to 100 cycles were obtained.

Other metal--air systems Near-term prospects are not good for the remaining metal-air systems shown in Table 3, though solution of a few technical problems could result in significant breakthroughs. The magnesium-air system, for example, is very attractive from both weight and cost standpoints. This system has been developed as a primary [102-105], but suffers from the inexpedience of a sludge discharge product, the problem of corrosion, and the concomitant heating problem. Titanium forms a tenacious protective coating but would be very attractive if this problem could be solved; perhaps the key is operation at elevated temperature [106]. Calcium has good theoretical possibilities, but in the brief studies of this system [105] the efficiency was low. Sodium has been studied [107, 108], but amalgamation with mercury was required, resulting in system complexities. Chromium poses difficult electrochemical problems, as does manganese. Cadmium has

been developed as a secondary air battery [109] but cycle life is limited and cost is high. High Temperature Batteries High temperature batteries are attractive for traction applications because they offer both high power density and high energy density. High energy density is achieved by selecting reactants of low equivalent weight and high electronegativity difference. High power capability is attained by use of low resistance electrolyte materials such as fused salts, and by operating at elevated temperature, which increases the exchange current density. Use of high temperature imposes difficult materials and special seal problems. The large increase in volume (up to 25 per cent) when electrolyte salts melt poses a design problem, and heating the battery prior to use poses an operational problem. However, a major advantage of high temperature in many of the candidate systems is use of electrode materials in the liquid state, thus avoiding morphological changes that occur with conventional solid electrodes and thereby offering promise of exceptionally long life. High temperature operation is also an asset for heat rejection, though maintenance of operating temperature can be a problem.

Sodium-sulfur battery The sodium-sulfur cell has a theoretical energy density of 312 W hr/lb and an open circuit potential ranging from 1.8 to 2.1 V, depending on state of charge. Operation is at approximately 300°C, with all reactants and products in the liquid state. Discharge must be stopped part way, after formation of Na2Sa, to prevent the formation of solid Na2Ss. Standby temperature cannot be lower than 230°C. A solid electrolyte is used with this battery. One type is the ceramic called beta-aluminat with a composition range from Na20.5AIzO3 to Na20. l lAleOs. The high conductivity is attributed to highly mobile sodium ions in cleavage planes. Battery development with this electrolyte has been reported in the United States [110-112] in England [113, 114], in France [115], and in Japan [116]; battery application studies have also been made [I 17, 118]. A major problem is accumulation of metallic sodium in the grain boundaries of the beta-alumina, causing shorts and also weakening the separator material [119]. Also, there is a tendency of the beta-alumina to lose sodium after long durations at high temperature. Considerable effort continues to he expended in obtaining better understanding of this ceramic separator and in improving its physical properties [120-124]. Longest life is obtained by use of very pure and dense material, preferably with density at least 95 per cent of theoretical. Addition of impurities such as CuO increase density and thus decrease the resistance [124], the trick being to avoid shortening of life by such additions. Over 1200 hr operation in single cells has been obtained [I 15]. Energy density realized on a t Na20.lIAlaOa has been called beta-alumina, sodium--aluminate, and aluminum--sodium oxide.

Review of Candidate Batteries for Electric Vehicles

small laboratory battery unit was 43 W hr/lb, with a power density of 95 W/lb [111]. A second type of solid electrolyte used is a highly basic glass heavily loaded with sodium oxide, with conduction by transfer of sodium ions [125-127]. The glass is in the form of capillaries 100/~m in diameter with walls approx. 15/~m thick; sodium, the negative material, is contained inside the capillaries. Many thousands of these fibers are paralleled to give the needed volume and surface area, for a large surface area is needed to compensate for the relatively high resistance of the glass. Construction and life problems have not yet been solved, however. Conceptual designs of this approach show that 100 W hr/lb and 100 W/lb can be obtained. Typical operating characteristics of this cell are shown in Fig. 11.

f

> 2.C _._o 1.9

1.7 --

L)

I-E -_l_

0

Potassium-sulfur battery The potassium-sulfur system has a theoretical energy density of 307 W hr/lb and an open circuit potential of 2.34 V. A solid electrolyte is being developed in the form of glass capillaries that will conduct potassium ions, similar to the Dow sodium-sulfur technology. An attractive attribute of this system is that it can be kept under a standby condition as low as 110°C. It is expected that this system will prove to be a strong competitor to the sodium-sulfur system.

The sodium-selenium system has not been studied experimentally to the knowledge of this writer. However, it has an attraction because of a high energy density, a high power density due to use of selenium, and a welldeveloped separator technology using beta-alumina (sodium aluminate). Uncertainty in the cost and availability of selenium would tend to limit interest in this system.

Z'I

~

such as P4Sz0 that reduce viscosity, permitting better diffusion of cell products away from the positive electrode during discharge and improving utilization of active material.

Sodium-selenium battery

2,4-2"3

103

I

_1

2 Operating

dural'ion,

3

4

hr

Fig. 11. Typi¢=i operating clum~eristics of Dow sodium-sulfur cell.

One characteristic of the sodium-sulfur system is that the current efficiency is essentially 100 per cent. As a consequence, there is no overcharge mechanism. Cells will either have to be exceptionally well matched or, more likely, sophisticated charge control methods will have to be used, incorporating bypass of individual cells. Because of this problem, sodium-sulfur cells with third electrodes have been studied with the intent of improving battery charging [128]. Of all the high temperature battery systems, the sodium-sulfur battery shows the most promise for electric vehicles, for energy density and power density are high, and solution of the technical problems is in sight. The ultimate cost of the sodium-sulfur battery is much in question, however, for though sodium and sulfur are very inexpensive, beta-alumina could be relatively costly. The Dow design uses no high cost materials, and thus has good possibilities of being made cheaply. In any case, achieving low cost on the sodium-sulfur battery is as important a challenge as solution of the technical problems.

Sodium/phosphorus-sulfur battery This system has not been investigated to the knowledge of this writer. It has possibilities as an approach to improve on the utilization of sulfur in the sodium-sulfur system. Adding phosphorus to sulfur forms compounds

Lithium-sulfur battery The lithium-sulfur system has a theoretical energy density of 700 W hr/lb when discharging to Li2S~. Open circuit potential is 2.25 V, and operating temperature is 375--425°C. Approximately 150Whr/lb are expected from this system. The electrolyte is molten LiI-KI-LiC1 eutectic salt, and both the lithium and the sulfur reactants are in the molten state. The system has a high rate capability [129-132] and should be particularly useful in large power installations [133]. However, the very high energy and power densities offered will come at the expense of severe engineering problems. As a result, a relatively long time may be expected for completion of development. Major problems of this system are materials to withstand high temperature corrosion, retention of electrode materials during repeated cycling, and low efficiency (40 per cent) of the sulfur electrode. Adequate wetting on the lithium negative electrode is a problem, caused by reaction of the supporting structure with sulfur. Some of the problems with the negative might be circumvented by the use of a properly designed lithium-aluminum alloy electrode [134-137] for a small sacrifice in efficiency, with the advantage of having a solid negative electrode at the high operating temperature. Mixing selenium with the sulfur has been found to be helpful to the positive electrode [138]. Sulfur electrodes of the carbon type tend to disintegrate with cycling, possibly due to the expansion of active material during discharge. Copper sulfide and iron sulfide electrodes are presently commanding the most attention. A cell design was investigated that featured the lithium negative electrode held under the molten electrolyte and the sulfur floating on top [139], but was not successful. Work is also being done to obtain a solid

104

SIDNEY GROSS

electrolyte of high conductivity with lithium ions [140], analogous to beta-alumina which conducts sodium ions very well. The lithium-sulfur system has a good potential for low cost, provided the corrosion problems can be solved without the need for exotic or costly materials. Materials for this system are estimated to cost $5-$8 kW hr [133]. If cell costs are approximately eight times material costs, then the battery will cost approximately $0.06/W hr, essentially the same cost as lead acid batteries.

Lithium-tellurium tetrachloride battery The lithium-tellurium tetrachloride system has an open circuit potential of 3.1 V and a theoretical energy density of 510 W hr/lb. The electrolyte is a molten eutectic of lithium chloride and potassium chloride operating at about 400°C [140-142]. The active lithium negative material, rather than being a liquid, is alloyed with aluminum to form a solid [134-136], and when encapsulated in a screen [137], gives in excess of 2000 cycles without lithium dendrites or other problems [143]. Current densities up to 2 A/cm 2 can be attained on this electrode without serious polarization. The positive electrode is a porous, high surface area microcrystalline carbon polymer with tellurium tetrachloride additive [144, 145]. In this system, the lithium and the tellurium tetrachloride react to form lithium chloride and tellurium, with some lithium telluride produced also. Long cycle life has been proven with this battery, with an attained energy density of 38 W hr/lb and a projected level of 60 W hr/lb. Tellurium, a by-product of the lead and copper industries, has a production of only 100-200 tons/yr, which is not sufficient to make all the projected electric vehicle batteries, but is enough nevertheless to produce batteries in substantial numbers.

Lithium-chlorine battery The lithium-chlorine system has a very high theoretical energy density of 1050Whr/lb and an open circuit potential of 3.46 V. The system operates at a temperature of 650°C when the electrolyte used is lithium chloride, which is also the reaction product. Most of the research effort has been toward development of a primary battery [146], though development of a secondary system is the principal interest for electric vehicles. The lithium negative reactant is maintained in the liquid state. The chlorine positive is porous carbon, which absorbs the chlorine gas, The reaction rate is limited by the chlorine positive electrode reaction occurring much slower than the lithium negative reaction. Feasibility of quickly heating to operating temperature either electrically or pyrotechnically has been demonstrated with this system. The major problems with the lithium-chlorine system are attack of materials by the hot molten lithium, inefficient storage of the chlorine gas, and safety due to high temperature and use of chlorine gas. Another approach uses a molten eutectic electrolyte of lithium chloride and potassium chloride and operates at 450°C [147]. However, the lithium is alloyed with zinc to avoid problems with the use of molten lithium. Good

cycling performance was attained, with a very low selfdischarge rate. Current densities used during cycling were 0.2 and 0.5 A/cm 2 for charge and discharge, respectively. The energy density of large batteries is expected to be 120 W hr/lb at the 2-hr rate [148].

Lithium-selenium battery The lithium-selenium system has a theoretical energy density of 550 W hr/lb and an open circuit potential of 2.1 V [149, 150]. Very high power densities are possible, 2 W/cm 2 having been attained on laboratory models. The electrolyte is a molten salt such as LiF-LiC1-LiI eutectic which has been immobilized in the form of a stiff paste by mixing with lithium aluminate powder. The lithium negative material is contained within a stainless steel felt electrode. The selenium positive material is contained within niobium mesh and can be alloyed with thallium (a poisonous material) and sulfur to discourage transport of selenium through the paste electrolyte. Due to expected high cost and the limited availability of selenium, batteries of this type will be limited to special applications, such as military uses or in systems using small amounts of selenium mixed with sulfur.

Aluminum-chlorine battery The aluminum--chlorine system has a theoretical energy density of 650 W hr/lb and an open circuit potential of 2.1 V [151-154]. The electrolyte is molten A1Cla--KC1NaC1, the latter two constituents being a binary eutectic and the amount of A1Cla varying with cell state of charge. This electrolyte can melt as low as 70°C, but a realistic operating temperature is 150-250°C. The experimental chlorine positive electrode was vitreous carbon partially immersed in electrolyte. Problems observed with cells operating at 150°C were formation of aluminum dendrites and excessive blockage of the aluminum by the AICla discharge product; operating above 200°C should help solve both of these problems, though use of a ceramic separator may also be necessary. An added incentive to the further development of this system is its possible use for utility power load leveling, a potentially large commercial battery market. Overall efficiencies of 87 per cent were obtained for this purpose [155]. Should the technical problems become solved, then the combination of low operating temperature and high energy density will make this system very attractive for electric vehicles.

Lithium-phosphorous/sulfur battery The lithium/phosphorous-sulfur system has an open circuit potential of 2.5 V and operates at approx. 351425°C. This system has been studied [156] as an approach to improve on the utilization of sulfur in the lithiumsulfur system. Sulfur forms long-chain polymers at high temperature, causing viscosity to be high; this inhibits diffusion of cell products away from the positive electrode during discharge, limiting utilization of the active material. Adding phosphorous to sulfur forms compounds such as P4S10 that reduce viscosity. Tests showed that at current densities higher than about 0.6 A/cm z the

Review of Candidate Batteries for Electric Vehicles

phosphorous-sulfur cathode is capable of greater capacity densities than sulfur alone.

Magnesium-chlorine battery The magnesium-chlorine system has a theoretical energy density of 689Whr/lb and an open circuit potential of 2.71 V. The system has been operated as a primary at 480°C with a molten salt electrolyte of NaC1KCI-MgCI2 [157]. Power densities of 2W/cm 2 were produced, which demonstrates the high power capiablity of this system. Since the technology of chlorine cathodes has been successfully developed for use with lithium anodes, it should be possible to apply that technology to the magnesium--chlorine system, sacrificing theoretical energy density in order to use low-cost materials.

Calcium~barium-chlorine battery The calcium-chlorine system has a theoretical energy density of 848 W hr/lb and an open circuit potential of 3.9 V (room temperature values; less at elevated temperature). In order to lower the solubility of calcium in calcium chloride, a molten salt electrolyte of CaClz--CaF~ BaCI~ has been examined, which results in electrochemical participation of barium [157]. Initial experiments failed due to presence of water in the system. Good techniques for water removal are available, however, which suggests that this system might be feasible.

Other high temperature systems The lithium-tellurium system has a theoretical energy density of 612 W hr/lb and an open circuit potential of 1.73 V. Development work on this system [158] showed very high current density and good prospects. However, the short supply of tellurium and the greater attractiveness of sulfur and selenium positives have led to the discouragement of this development. The sodium-air system has a theoretical energy density of 1940 W hr/lb and an open circuit potential of 2.6 V. This system was found to be impractical [107, 108], so development has been discontinued in favor of other more promising approaches. Of great practical interest are batteries with molten salt electrolytes that operate at intermediate temperatures, preferably below 200°C. There are many salt mixtures that melt below 100°C, especially the alkalide halidealuminum chloride mixtures; for example, the A1CIaNaC1-KCI eutectic melts at 70°C. Such melts have very high ionic conductivities and avoid many of the problems of most high temperature systems. Battery systems developed from this premise would be very attractive. Also of great value would be the development of good solid state ionic conductors for use as battery separators, for example systems employing tantalum oxide and niobium oxide. Solid-state Batteries

An all-solid battery would be very attractive for its long operating life, its long-term storage, and its easy maintenance. Problems avoided or minimized by solid-state batteries are corrosion of electrode materials, the need for

105

special seals, and operation over a limited temperature range. The technology for such batteries is in its infancy, using expensive materials, producing small discharge currents, and achieving low energy density. Nevertheless, the future possibilities are good. A major difficulty in the development of an all-solid battery is attaining a suitable electrode geometry. Since there is no fluid to assure good contact at the reaction sites, the solid structure must afford good conduction throughout and provide a good geometrical match between electrolyte and active material [159, 160]. Thin layers are commonly used to avoid high internal resistance, but development of high performance batteries will require, in addition, a well-defined electrode geometry. Most efforts to date on all-solid batteries have concentrated on battery systems that for traction applications would be too expensive, too low in energy density, and produce too low discharge currents. For example, the RbAg415 solid electrolyte has high electrical conductivity and is commonly used with couples such as Ag/I or Ag/RbIz [161]. The theoretical energy density for the latter couple is only 27.9 W hr/lb. The lithium/silver iodide couple with Li solid electrolyte [162, 163] has higher theoretical energy density of 130 W hr/lb, but this is still too low. Barium-stabilized magnesium selenide solid electrolyte shows much promise as the basis for future solid-state batteries [164], but is only in the very early stages of development.

Calcium-nickelfluoride battery Appearing to be the most promising all-solid battery for traction applications is the Ca/NiF2 couple with doped CaF2 for electrolyte. This system has an open circuit potential of 2.82 V and a theoretical energy density of 496 W hr/lb. If 25 per cent of this can be attained, then this would be an attractive approach. Conductivity of the CaF2 electrolyte is increased by doping with monovalent or trivalent fluorides such as NaF and YF8 and results in a cubic structure [160]. The resulting conductivity is still lower than for many other solid electrolytes and would require operation at 400500°C and use of very thin electrolyte and electrode layers, approx. 50 t~m thick. Improvements in conductivity may be possible by doping with LiF and YbFa or by use of a ternary system such as CaFz-NaFYFa [164]. Research is continuing on this system, but is considered proprietary [165].

Other solid-state systems Using a doped CaF solid electrolyte, three other possibilities for an all-solid traction battery are the Mg/NiF2 couple, the Mg/FeF2 couple, and the Mg/CrF2 couple [160]. Theoretical energy density for these is a little less than with the Ca/NiF2 couple, being 450 W hr/lb for Mg/NiF2, with an open circuit potential of 2.21 V. More important, though, is that use of magnesium would require a nonporous electrolyte to prevent transport of magnesium vapor at high temperatures. Still another possible solid-state battery is an approach that uses stabilized zirconia both as an electrolyte and as a

106

SIDNEY GROSS

source of electrode reactants [166, 167]. This system has an open circuit potential of 1.70 V and operates at 1000°C. The cell has been shown to be reversible with reproducible results. The electrochemical reaction is not known, but apparently the zirconium is cycled between two of its many oxidation states. A major problem observed in the experimental work was electrode cracking at high rate charge due to internal strains created. Use of thinner electrodes or a different cell geometry should alleviate this problem. Organic Electrolyte Batteries

such as SO2 which can passivate lithium, preventing its corrosion, thus opening up the possibilities for soluble positive electrodes.

Lithium-sulfur dioxide battery The lithium-sulfur dioxide battery has a theoretical energy density of 495 W hr/lb and an open circuit potential of 2.95 V. The practical energy density is very high, however, for small primary cells deliver 120Whr/lb. Since the sulfur dioxide is soluble in the electrolyte, typically lithium perchlorate or lithium bromide, the system is capable of operating at high rates. The unique characteristic of this system [171] is that the lithium and sulfur react on the lithium electrode forming a passivating film that is porous to anodic ions, and functions as a semipermeable membrane. The passivating film protects the lithium from corrosion, permitting 5-10 yr storage life with low self discharge. This is presently produced as a primary battery [172], but is capable of being produced as a secondary battery. Typical discharge characteristics of the lithium-sulfur dioxide battery are shown in Fig. 12.

Much effort has been expended to develop secondary high energy density organic electrolyte batteries, based on the use of lithium as the high energy negative electrode. Even assuming a number of these efforts will be successful, there is some question as to the applicability of such batteries to electric vehicles. One problem is that most of the systems being investigated are inherently limited to low discharge rates and low charge rates, which is a drawback for electric vehicle use; other high rate batteries in parallel may be necessary for acceleration. Basis, C-Cell, 41 g A wide operating temperature range is an important 7s*r asset of organic electrolyte batteries, especially when the 2.8 thermal problems of the competitive high temperature batteries are considered. High rate organic electrolyte batte- ~> 2"6 ries are not without their thermal problems either, for the 0 2"4 energy loss is concentrated as well as the energy source. 2.2 In spite of considerable research, few promising o 2 C secondary systems have emerged. Up to 100 or 200 cycles =~ (',.J 18 have been obtained on systems such as lithium-copper I [ I I I I J I I I I I I chloride and other systems. One problem has been I I I 4 5 6 7 8 9 I0 rl 12 13 14 J5 16 0 I 2 3 formation of lithium dendrites which ultimately short the Discharge "time, hr cell. Other problems are explosions on charge due to cell Fig. 12. Discharge performance of lithium--SO2 cell. imbalance and a host of problems associated with the positive electrode, including transport of positive Lithium-lamellar dichalcogenide structure batteries electrode materials to the negative electrode and passivaInteresting possibilities are also presented by a new tion of positive electrodes. class of rechargeable lithium inorganic electrolyte systems The approach that has commanded the most attention utilizing lamellar transition metal dichalcogenides such as is the use of highly insoluble positives. Few demands are niobium diselenide as host structures for cathodic nonplaced on the separator, but the positive electrode metals such as iodine and sulfur [173]. Cells using lithium development is plagued with problems of insufficient anodes and iodine-niobium diselenide cathodes, with electrochemical activity, too much solubility, chemical open circuit potential of 3.0 V, have operated more than instability, too low conductivity, and surface passivation. 1100 cycles at low current densities and at ambient A goal has been to obtain good solubility of the lithium temperature. Propylene carbonate was the electrolyte. salts, the discharge product, yet obtain very low solubility Significant improvements reportedly have been made of the metal salts; this may not be possible with metal using a variation of the lamellar structure [174]. halides. A second approach is to use positive plate materials that are highly soluble in the electrolyte. This permits the Lithium-bromine battery high charge and discharge rates required for traction The lithium-bromine system has an open circuit applications. Metal dendrites can form at high charge potential of 4.05 V and a theoretical energy density of rates, but this appears to have been minimized by binders 504Whr/Ib. This system has been investigated [175] in the lithium electrode and by use of mixed electrolytes. using propylene carbonate electrolyte and microporous Chemical discharge during stand is a more serious polyethylene separator to retard diffusion of the bromine problem. Good progress has been made in the develop- to the lithium electrode. An experimental cell attained ment of separators to avoid that problem [168-170], and 1785 cycles of 90 rain each, though current efficiency was such separators may ultimately prove important for only 30 per cent at the end of the cycling. The long life of traction batteries. One approach is the use of positives this system is due in part to a bromine shuttle mechanism

Review of Candidate Batteries for Electric Vehicles

which limits self discharge. It would be expected that since the value of this high quality separator has now been demonstrated that it would find application in other high energy density systems.

Other organic electrolyte systems With a suitable electrolyte, many metal halides are reversible and thus long have been considered candidates for the positive electrode of a secondary battery, though with only slight success. Sometimes Lewis acids are added to form ionic complexes [176]. Dissolved gases such as COs, NHa or H2S can also function as secondary batteries with lithium negatives [177]. Other positive electrodes claimed to be suitable for secondary batteries are BisOn, CrOa, and MnO2 [178], and carbon composites of tungsten [179]. Organic materials have good possibilities for positive electrodes, but relatively little effort has been aimed in this direction. One possibility is m-dinitrobenzene [180]. Electron exchange organic polymers offer a promising approach [181]. Particularly promising are quinones and hydroquinones, especially ehloranil which has been shown to be completely reversible and capable of high rate discharge [182, 183]. The initial research has been started on a lithiumgraphite secondary system [184]. Open circuit potential is very high, ranging from 4.3 to 4.8 V. The positive electrode is a reinforced pyrolytic graphite consisting of a porous carbon matrix on which small crystallites of pyrolytic graphite are deposited. The electrolyte used was dimethyl suifite saturated with lithium perchlorate. Excessive swelling of the positive electrode caused its eventual disintegration, resulting in poor cycle life. Wet stand capability was also poor. Molybdenum trioxide (MOO3) shows promise as a positive electrode with lithium negatives [185, 186]. This system has an open circuit potential of 2.7 V and an undetermined but high theoretical energy density. The positive electrode tested was MoOs mixed with graphite for electrical conduction, and the electrolyte was lithium aluminum chloride in butyrolactone (LiA1CI4-BL). A high performance separator is necessary with this system, for MoO8 has a high solubility. Primary Batteries Rechargeable (secondary) batteries are the most important type for electrical vehicles. However, even with successfui development of suitable secondary batteries, there will be some need for high energy density primary batteries. Applications will include emergency power and special extended range uses. Candidate primary batteries include the metal-air systems previously described, iron and aluminum being especially attractive. The task of developing primary batteries is considerably easier than developing secondary batteries and there are many more possible systems. There are today a number of high energy density primary batteries essentially fully developed, many of which are proprietary and not publicized. However, in order for a primary battery to

107

have any significant use for traction applications, it will have to be of moderate cost. This will probably exclude lithium batteries from consideration except for special applications. Unfortunately, many of the primary systems developed are lithium-based and may not meet the low-cost requirement.

Aluminum-trichlorotriazinetrione primary battery This primary battery is a reserve type that is activated by the addition of water. The theoretical energy density is 740 W hr/lb, and the open circuit potential is 2.2 V. The anode is aluminum with 5 per cent zinc and 0.05 per cent indium or mercury. The cathode is trichlorotriazinetrione having the formula ClaNsCaO3. The original work on this cathode was done by Monsanto, but the development currently is being undertaken by Admiralty Materials Laboratory, England [187]. An energy density has been demonstrated of 85 W hr/lb dry, or 55 W hr/lb wet, and discharges in less than 1 hr have been demonstrated in pile-type cells. Other features that make this very attractive as an emergency or extended range battery for electric vehicles are the very low cost of materials and a prolonged storage life.

Lithium-polycarbon monofluoride primary battery The lithium-polycarbon monofluoride battery has a theoretical energy density of 911 W hr/lb and an open circuit potential of 2.82 V [188-192]. Graphite intercalation compounds such as polycarbon monofluoride are very attractive because they have high energy content and are practically insoluble in most electrolytes. These batteries are expected to be commercially available soon [193]. The energy density available from practical batteries is calculated to be 117 W hr/lb [190]. However, this system is intrinsically limited to low discharge rates, thus severely restricting its application to electric vehicles.

Other lithium primary batteries A number of proprietary lithium batteries have been developed, but are unpublicized. Lithium batteries under development by the U.S. Army are expected to provide 90 W hr/lb at the discharge rates and temperatures encountered by electric vehicles. Possibly the best today is the lithium-sulfur dioxide system previously described. One of the better developed lithium primary systems capable of the high rates needed by electric vehicles is the lithium--copper chloride system with a theoretical energy density of 503 W hr/lb and an open circuit potential of 3.07 V [194, 195]. Typical discharge performance is shown in Fig. 13, yielding 60-63 W hr/lb on small batteries. This battery system would have to be used as a reserve, with a simple activation of electrolyte prior to use. The lithium-water system has a high theoretical energy density, and is capable of very high rates, with sustained outputs in the laboratory of 3.2 W in -2 [5]. Utilization of the lithium electrode is low and thus is one of the areas requiring further improvement. Some primary lithium systems described in the literature are the lithium-nickel sulfide system (using NiaS~) with a theoretical energy density of 388 W hr/lb and an

108

SIDNEY GROSS

>

coo,,o0

o

hr/Lb

(J

I

I

5

I0

I

15

Discharge

L

20

1

25

durotion,

I

30

1

35

I

40

45

rnin

Fig. 13. Discharge of 40 A-hr Li-CuCh cell at 20 A. open circuit potential of 1.8 V [196]; the lithium--cupric sulfide system with a theoretical energy density of 500 W hr/lb (unadjusted for its two-plateau behavior) and an open-circuit potential of 2.55 V [197-200]; the lithium-copper vanadate system with a theoretical energy density of 580 W hr/lb and an open circuit potential of 3.5 V [201]; the lithium-vanadium pentoxide system with a theoretical energy density of 826 W hr/lb and an open circuit potential of 3.5 V [202]; a room-temperature lithium-sulfur system with a theoretical energy density of 1336 W hr/lb and an open circuit potential of 2.5 V [203]; the lithium--chlorine trifluoride system with a theoretical energy density of 1380 W hr/Ib and an open circuit potential of 5.76 V [204]; and the lithium-silver chromate cell with an open circuit potential of 3.35 V [205]. E v a l u a t i o n of C a n d i d a t e Batteries

Battery requirements To evaluate the electric vehicle prospects of the many development batteries, the requirements listed in Table 2 were postulated. Though there will be many kinds of electric vehicles with differing requirements, personal highway vehicles that supplement or replace internal combustion engine automobiles are the principal reference point in this evaluation. Table 2. Requirements for electrical vehicle batteries

• • • •

High energy density (at least 20 W hr/lb at 2-hr rate) Capableof low manufacturing cost Long life with low maintenance Long activated stand capability with low self discharge or

• •

Good high rate capability for acceleration and hill climbing Efficientlyand quickly recharged with little or no special equipment Smallsize Safeduring accidents or charge control failure Easilyreplaced Little or no special handling equipment

degradation

• • • •

High energy density is certainly an important requirement for electric vehicle batteries. Unfortunately, considerable publicity has been given to a calculated requirement of 100 W hr/lb. Though such batteries would be most welcome in the electric vehicle field, it is unnecessarily demanding in view of the good technical progress

demonstrated with vehicles using conventional lead acid batteries having energy density less than 18 W hr/lb [l]. Any battery system with achievable energy density of at least 20 W hr/lb at the 2-hr rate is considered an important prospect for the electric vehicle battery market. Of major importance is the ability of a potential battery system to be manufactured at low cost. The raw materials must be inexpensive, and the manufacturing technique must be able to be automated, requiring a minimum of manpower. A technically superb battery that is costly will find very little public acceptance. Long battery life is important, but there is a limit to the increase in first cost that will be acceptable for especially long lifetimes. If battery costs are low enough, replacement during the vehicle lifetime will very likely find public acceptance. Once a battery has been fully charged, it is important that the battery retain most of the charge during inactive periods. Also, activated stand should not contribute excessively to wearout. A self discharge rate of 1 per cent per day is believed to be an acceptable upper limit. Good high rate capability is an important feature of electric vehicle batteries, being needed for acceleration and hill climbing. Battery systems that lack this capability may need to be supplemented by auxiliary high-rate batteries. Development of flywheel systems [16] may reduce the importance of high rate capability in batteries. Efficient recharge is most important, for this affects total energy consumption. Fast recharge capability will permit quick boost charging during long trips, and also would allow charge to be delayed to utility off-peak hours. Small battery size is important, especially for small vehicles. Battery systems with auxiliary components such as pumps, fans and holding tanks generally require much space. Safety is of paramount importance, and batteries using molten or dangerous materials must be capable of being built to be safe during accidents or charge control failure. The battery should be easily maintained or replaced, and little or no special handling equipment should be needed for servicing batteries.

Evaluation summary Based on the preceding requirements of electric vehicle batteries, an evaluation was made of the candidate battery systems. The more promising candidates are listed in Table 3 with an evaluation of their near-term prospects, 0--5 yr, and their long-term prospects, 5-15 yr. Battery systems not included on this list are considered to have very dim hopes. In making forecasts such as these, one must be aware that unforeseen technological breakthroughs can occur that cause obscure systems to be suddenly very attractive; conversely, very promising systems may encounter unsurmountable technical obstacles or go wanting for lack of research funds. In reviewing the near-term prospects, the three best are the lead-acid system, the nickel-iron system, and the nickel-hydrogen system. The lead-acid system is today's leader in electric vehicle batteries, and is expected to continue its dominance at least for the near future. The

109

Review of Candidate Batteries for Electric Vehicles Table 3. Evaluation of selected candidate secondary batteries

Near-term prospects 0-5 yr

Long-term prospects 5-15 yr

Low cost; better energy density and life needed Need improved cycle life, better oxygen transport Need separator for long activated stand Uncertain cost High cost, limited cadmium supply Cost and hydrogen tank weight Weight, cost and reliability Weight, cost and life Rechargeable aluminum electrode System complexity, cost, life Weight and life Solid separator, life, cost Solid separator, life, cost Limited selenium supply, cost, improved energy density Improved sulfur electrode, life Improved energy density Improved energy density

Excellent Fair Poor Good Poor Good Poor Poor Poor Fair Very poor Fair Poor Very poor Poor Poor Poor

Good Good Fair Good Very poor Good Fair Poor Fair Good Very poor Good Fair Poor Good Good Good

Limited selenium supply, cost, improved energy density Improved aluminum electrode Improved sulfur electrode, life Requires more basic research Requires more basic research Improved solid electrolyte doping, better electrode geometry Cost, high rate capability Cost, high rate capability Requires more research and development Solid separator, life, cost Requires more research and development

Poor Poor Poor Very poor Very poor Very poor Poor Poor Poor Fair Poor

Fair Good Good Poor Poor Fair Poor Poor Fair Good Good

Battery

Major factors affecting possible use

Lead-acid Nickel-zinc Zinc-bromine Nickel-iron Nickel--cadmium Nickel-hydrogen Zinc--chlorine hydrate Zinc--air Aluminum-air Iron-air Lead-air Sodium-sulfur Sodium-phosphorus/sulfur Sodium-selenium Lithium-sulfur Lithium-chlorine Lithium-tellurium tetrachloride Lithium-selenium Aluminum--dalorine Lithium-phosphorous/sulfur Magnesium-chlorine Calcium/barium-chlorine Calcium-nickel fluoride Lithium-sulfur dioxide Lithium-lamellar structure Antimony rcdox Potassium-sulfur Lithium-bromine

nickel-iron system is technologically satisfactory, but its future rests on the uncertainties of cost. The nickelhydrogen system is paced m o s t by cost uncertainties but also weight uncertainties, particularly the hydrogen tank. The other promising near-term prospects are the nickelzinc system, the iron-air system and the sodium-sulfur system. Limited cycle life is the principal drawback o f the nickel-zinc system, but significant improvements might be obtained. The iron-air system is in the final stages o f development, and its future will depend on the resulting complexity, cost and life. O f all the workers on the sodium-sulfur system, only the Japanese have claimed to be near a final solution, so this system might be available earlier than generally expected. Also worthy o f mention is the nickel-cadmium system which performs very well [206], but suffers f r o m high cost (ten times the cost o f lead-acid) and a limited world supply o f c a d m i u m ; recycling may make these disadvantages tolerable, and warrants further study. F o r long-term prospects, it is evident that the field is wide open, there being 20 candidates rated fair or better. It would be unwise to speculate on which of these systems will finally emerge as the front-runners. Certainly, those that become well-funded have the best opportunity to succeed. Acknowledgement--This paper is based on work sponsored by Seattle City Light as part of Project 72-8. Permission to publish this paper is gratefully acknowledged. References

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