Rechargeable batteries with aqueous electrolytes

In recent years, rechargeable batteries (RB's) have found important new applications in rapidly expanding markets, such as portable computers (laptops), telecommunication equipment (handies), camcorders and tools. The interest in electric vehicles has continued to stimulate research on RB's having improved specific energy. Attention has been focussed on nonaqueous battery systems, in particular on lithium batteries. Small rechargeable lithium batteries, available on the market today, show specific energies of 100-120 Wh kg(-1). The upper limit surpasses those of aqueous electrolyte counterparts (nickel-cadmium or nickel-metal hydride) by a factor of 1.5-2. Is there still a future for aqueous electrolyte battery systems? This paper reviews the present status of the various RB's with aqueous electrolytes and analyses their outlook for the future. Traditionally, lead, cadmium and iron were used as active materials in the negative electrode. In recent years, research on rechargeable zinc electrodes has been intensified. The development of metal hydride negative electrodes has resulted in RB's with improved specific energy (60-70 Wh kg(-1), under some conditions even 80 Wh kg(-1)). Similar values can be reached with nickel-zinc batteries, the latter having, however, a considerably lower cycle life. Rechargeable zinc-air batteries, with bifunctional air electrodes, are capable of specific energies matching those of lithium batteries. Their cycle life is, however, up to now still totally unsatisfactory. Systems with 'mechanical recharge' of the highly porous zinc electrodes have been tested in electric vehicles. Although the specific energy during discharge exceeded 200 Wh kg(-1), this system has not gained any large scale pl actical acceptance, All metal-air batteries suffer, in principle, from a poor energy efficiency (eta < 50%). At present, it appears unlikely that alternative negative electrode materials, such as anthraquinone, polypyrrole, or specially passivated lithium, can be adapted to permit long-term cycling in aqueous electrolytes. Regarding positive electrodes, no alternatives to lead dioxide and nickel-oxy hydroxide are in sight. Manganese dioxide electrodes tend to rapidly loose capacity during cycling. So far, active materials such as bromine, chlorine and insertion electrodes based on carbon, have not, and will probably not in the near future, see commercial use. The main advantages of aqueous electrolyte systems are low cost and ability to withstand overcharge (due to the establishment of the socalled oxygen cycle). This simplifies charging equipment. Due to high electrolyte conductivity, relatively thick electrodes can be used. Lead-acid batteries, in particular maintenance-free or sealed types, will remain 'work horses' in SLI and stationary (standby) applications. Large stationary redox cells may possibly find use for the storage of solar energy. Of all the aqueous systems, the metal hydride batteries might have the brightest future. Chances are good that their specific energy can be further improved. Negative and positive electrodes an both of the 'insertion type', thus avoiding a dissolution redeposition step. This results in excellent cycle life. As to electric cars, aqueous-electrolyte fuel cell systems might some day become power sources being capable to beat lithium batteries with respect to specific energy and driving range. (C) 2000 Elsevier Science Ltd. All rights reserved.