Battery Characteristics

For this discussion, batteries have been divided into four thematic groups: lead acid, alkaline, high temperature, and solid electrolyte. Various battery designs have been examined that would fall under the latter three types, and obtaining comprehensive data on their current development status and characteristics is challenging; a listing of the various types under development and their developers is given in table 3-11. The discussion focuses on batteries that are potential winners according to the current consensus, but it should be noted that the list of “winners” has changed considerably during the last five years. For example, in 1991, the nickel-iron and sodium-sulphur batteries were considered the most promising, but are no longer the leading contenders.

Lead acid

Lead acid batteries have been in existence for decades, and more advanced traction batteries with improved specific power and energy, as well as durability, are under development. Delco Remy’s VRLA battery is perhaps is the most advanced battery commercially available (though in limited quantity), and it has claimed the following characteristics per battery module: a specific energy of 35 Wh/kg, specific power of 210 W/kg (filly charged) and 150 W/kg at 20 percent charge, and over 800 cycles of life at 50 percent DoD. Delco also offers a “battery package” including fill thermal and electrical management. An entire 312V system with 26 modules and battery management has a net specific energy of 30.5 Wh/kg.

Other recent developments include the woven grid pseudo-bipolar lead acid battery from Horizon, which has a demonstrated specific energy of 42 Wh/kg and peak power of 500 W/kg at fill charge and 300 W/kg at 80 percent DoD at the cell level. Horizon claims life in excess of 900 cycles at C/2 and has begun delivery of complete batteries from a pilot production plant.70 Horizon anticipates additional improvements to specific energy levels over 48 Wh/kg at the module level, and expects other benefits, such as fast charging, owing to the batteries’ low internal resistance.

Bipolar lead acid batteries under development offer even higher power densities and energy densities than the Horizon battery, with specific power of 900 W/kg and specific energy of 47 Wh/kg demonstrated by ARIAS Research at the module level. The traditional problem with bipolar batteries has been with corrosion at the electrode interfaces, and it is not yet clear whether this problem has been solved over the life of the batteries. Nevertheless, the new designs show promise in providing significant improvements in power and energy density, but providing reasonable life may still be a serious problem.

Alkaline Systems

The three most successful candidates in this category are nickel-cadmium, nickel-iron and nickel-metal hydride. Nickel-cadmium (Ni-Cd) batteries are available commercially, but the major problem has been their relatively modest improvement in specific energy over advanced lead acid batteries in comparison to their high cost. Modern Ni-Cd batteries have specific energy ratings up to 55 Wh/kg, which is about 25 percent better than the Horizon lead acid battery. They cost at least four times as much,72 but these higher costs will be offset to an extent by Ni-Cd batteries’ longer cycle lives. High-energy versions of these batteries require maintenance and their capacity changes with charge/discharge cycles. Sealed Ni-Cd batteries that are maintenance free have significantly lower specific energy (35 to 40 Wh/kg), although there is ongoing research to avoid this penalty. In addition, concerns about the toxicity of battery materials and the recyclability of the battery has resulted in reduced expectations for this battery.

Nickel-iron batteries received considerable attention a few years ago, but interest has faded recently. Their specific energy is about 50 Wh/kg, and their costs are similar to, or slightly lower than, those for Ni-Cd batteries. Although they have demonstrated good durability, they require a sophisticated maintenance system that adds water to the batteries and prevents overheating during charge. In addition, they cannot be sealed, as they produce hydrogen and oxygen during charging, which must be vented and pose some safety problems. The formation of hydrogen and oxygen also results in reduced battery charging efficiency, and these features account for the lack of current interest in this battery.

Nickel-metal hydride batteries have received much recent attention lately, and Ovonic and SAFT are the leading developers of such batteries. The maintenance-free Ovonic batteries have demonstrated specific energy values in excess of 80 Wh/kg at the module level and specific power densities of over 200 W/kg.74 However, auto manufacturers have stated that these batteries have high internal self-discharge rates, especially at high ambient temperatures, with losses of 32 percent over 5 days at 40oC.75 Auto manufacturers have also noted that Ovonic batteries have capacity limitations at low temperatures when discharged quickly, and they are worried about hydrogen build-up during charging. Nevertheless, the Ovonic batteries’ demonstrated capabilities and the potential to overcome these problems has led to optimism about their prospects for commercialization. GM and Ovonics have entered into a joint venture to produce the battery, and pilot production may occur in late-1996. It should be noted that a complete battery to power an EV has only recently become available, and prototype testing will demonstrate the battery’s durability in an EV environment.

 Auto manufacturers do not believe that the Ovonic battery can be manufactured at low cost, especially as other battery manufacturers developing nickel metal hydride batteries do not support Ovonic’s cost claims. Ovonic has suggested that the batteries can be manufactured at $235/kWh and perhaps below, whereas others expect costs to be twice as high (~$500/kWh) in volume production.76 It should also be noted that the batteries are not yet easily recyclable, as the complex metal hydride used by Ovonic can only be regenerated today by an expensive process

High-temperature batteries

This category includes sodium sulfur, sodium-nickel chloride and lithium-metal disulfide batteries. All high-temperature batteries suffer from the fact that temperature must be maintained at about 300°C, which requires a sophisticated thermal management system and battery insulation and imposes a lack of packaging flexibility. Moreover, thermal losses must be compensated by electrical heating when the vehicle is not in use, so that these electrical losses are similar to self discharge. Hence, these losses may significantly increase total electrical consumption for lightly used vehicles. Meanwhile, these batteries offer much higher levels of energy storage performance than lead acid or alkaline systems and are insensitive to ambient temperature effects.

Sodium sulfur batteries have been in operation for more than a decade in Europe and offer high specific energy (100 Wh/kg) with relatively low-cost battery materials. They have the favorable characteristic of their specific power’s not declining significantly with the state-of-charge, although the specific power value is a relatively low 130 W/kg. 77 More recently, Silent Power has unveiled a new design, the MK6, with a specific energy of 120 Wh/kg and specific power of about 230 W/kg.78 However, the corrosivity of the battery materials at high temperature has led to limited calendar life (to date), and reliability is affected if the battery “freezes.” Even now, a leading manufacturer, ABB, claims a battery life of less than three years for its sodium sulfurbattery. Silent Power has estimated a selling price of $250/kWh in volume production of 1050 units/month for its MK6 battery.

Sodium-nickel chloride batteries have many of the sodium sulfur batteries’ favorable characteristics along with reduced material corrosivity, so that they may have longer calendar life. These batteries are being extensively tested in Europe, and the latest versions (dubbed ZEBRA in Europe) have shown energy densities over 80 Wh/kg and specific power of over 110 W/kg at full charge. 79 Other advancements are expected to increase both specific energy and specific power. However, specific power drops to nearly half the fully charged value at 80 percent DoD, and possibly is also reduced with age or cycles used. Despite this problem, this battery type has emerged as a leading contender in Europe owing to its potential to meet a life goal of five years.

Lithium-metal sulfide bipolar batteries hold the promise of improvements in specific energy and power relative to the other “hot” batteries, but they are in a very early stage of their development. Work by Argonne National Laboratories has shown very good prospects for this type of battery. It is lithium’s low equivalent weight that gives lithium batteries their high-energy content of three to five times that of a lead acid battery. Research efforts on lithium-metal sulfide batteries of the bipolar type are being funded by the USABC, and battery developers hope to achieve specific energy levels of over 125 Wh/kg and power levels of 190 W/kg.80 Initial tests on cells have indicated approximately constant power output with battery DoD, and the system also holds the potential for long life and maintenance free operation, but substantial research is still required to meet these goals. Problem areas include corrosion and thermal management, as well as durability. At this point, an EV-type battery or module has not yet been fabricated.

Lithium-Ion

 This battery type has many supporters who consider it a leading long term candidate for EV power. The battery has been studied at the cell level and has demonstrated the following advantages

· high specific energy of about 100 to 110 Wh/kg,

· good cycle performance with a life of over 1,000 cycles at 100 percent DoD,

· maintenance free system,

· potential for low cost.

The battery developer, SAFT, has used a lithium-nickel oxide alloy (LiNiO2) as the anode and a carbon cathode, with an electrolyte of confidential components to demonstrate a prototype cell with the above properties. SAFT has publicly stated that it can attain a specific power of about 200 W/kg, and costs near the $150/KWh goal, similar to the statements of other battery developers. Nevertheless, there is much development work to be done, as the current system is seriously degraded by overcharge or overdischarge, and a mass production process for the anode material is not well developed.82 The battery holds promise for commercialization in the post2005 time frame.

Solid electrolyte batteries

These batteries are potentially extremely “EV friendly” batteries in that they are spillage proof and maintenance free. A schematic of the lithium polymer battery is shown in figure 3-4, and the battery can be manufactured as “sheets” using manufacturing technology developed for magnetic tape production. Many problems still remain to be resolved for lithium-polymer rechargeable batteries including the need for reversible positive electrode materials and stable high conductivity polymers as well as scale-up problems associated with high voltages and current. Researchers at Oak Ridge National Laboratory (ORNL) have projected specific energy and power of 350 Wh/kg and 190 W/kg, respectively, but these figures are based on laboratory cell performance data.83 Actual data from Westinghouse and 3M suggest that the specific energy and power from an entire battery may be at half the levels projected by ORNL for a single cell. 84 Other researchers have suggested that sodium-polymer batteries may be superior to lithium-polymer versions, and could have lower costs. However, even a prototype EV size battery is possibly several years away.

As noted, the previous discussion covers only those battery types that are highly regarded today, but there are numerous other electrochemical couples in various stages of development with the potential to meet USABC goals. These include nickel-zinc, zinc-bromine, and sodiumpolydisulfide systems; these are being actively researched but need considerable development before they can become serious contenders. Nickel-zinc and zinc-bromine batteries have energy densities comparable to Ni-MH batteries but significantly lower power densities of about 100 W/kg, so that they can compete only if costs are low and they have long life. 86 Sodiumpolydisulfide batteries are in a very early stage of development and little is publicly known about their performance parameters.

Table 3-12 provides a summary of the state-of-the-art for batteries of different types. It is important to note that the actual usable specific energy and power can differ significantly from the values listed for some batteries. Lead acid batteries should not be discharged to below 80 percent DoD, for example, so that usable specific energy is only (40x 0.8) or 32 Wh/kg for the advanced lead acid battery.