Batteries for Low-Power Applications

Pier Paolo Prossini, Rita Mancini et al., Li4Ti50i2 as anode in all-solid-state, plastic, lithium-ion batteries for low-power applications, Solid State Ionics (September 2001), pp. 185-192. 

Performance characteristics of batteries for low-power applications will be somewhat different from the performance characteristics of batteries required for low-power electronics applications. This chapter identifies the major difference between the performance characteristics of batteries deployed for these two different applications. Progress in both battery development and technology is strictly tied to progress in electronics. Low-power electronics applications are referred as microelectronics, whereas low-power electronics are called low-power electronic devices, such as disc players, subnotebook computers, and minicomputers. 

Performance Capabilities of Lithium-Based Batteries for Low-Power Applications... 

The anode, cathode, and electrolyte play a critical role in maintaining the performance of lithium-ion (Li-ion) batteries for low-power applications [1]. The anode uses the spinel (lithium-titanium-oxide [Li TijOjj]) material for its fabrication because it offers the optimum electrochemical performance for the battery. The spinel is considered to be a complex chemical compound. This anode can deliver a very high retention capability in excess of 150 mAh/g or 150 mAh/kg. This kind of energy density and retention capacity is remarkable and can significantly reduce the weight and size of the battery. As a matter of fact, spinel-based... 

According to some projections, the energy densities of DMFCs could be considerably higher than those of even lithium-ion batteries, so that DMFCs could find applications for low power applications (laptop computers, backpack power sources for soldiers). IFC in the United States, Siemens and Daimler in Germany have designed, constructed, and tested kW-size DMFC power plants. Research studies in several government laboratories, universities, and industries have shown the prospects of attaining a current density of 400 mA cm-2 at a cell potential of 0.5 V. 

There are also disadvantages. First, litliium batteries are very expensive. Compared on tiie basis of equal energy content, they may cost three to five times more than Leclanch cells. More serious is the matter of safety. For low power applications, these batteries are quite safe, but high power lithium batteries have been known to explode. For example, accidental heating may melt the lithium (m.p. 180.5°C). This can rupture the protective SET layer, leading to a violent reaction between the metal and the solvent, and eventually to explosion. 

Another problem is the cost of the fuel - sodium borohydride costs 55/kg. The fuel contribution to the cost of electricity (COE) for the DBEC will be 9.7/kWh, even if the operating voltage of the DBFC can reach 1.0 V. It is 100 times that of the hydrogen gas. At present it is very difficult to apply the DBEC to high power applications such as a car or a home power generator. However, it is very possible that DBFCs could be used in place of batteries in low power applications such as primary batteries or secondary batteries for notebook computers, cordless electric tools and so on because the COE for batteries is rather high. For example, the COE for an alkaline primary battery is up to 450/kWh. 

Therefore preferably low-temperature PEM fuel cells (PEFC NT) used as these are able to deploy their rated power very quickly (<1 min), because the operating temperature of about 60-80 °C is not far above the usual ambient temperature. So for low power applications, even today supercaps are sufficient to bridge the startup phase, and thus batteries are omitted entirely. Supercaps are more expensive to buy, but maintenance-free and life-cycle components. 

Lithium-based, Ni-Cd, and Ni-MH batteries were developed and evaluated for low-power applications. The anode of this battery is a metal hydride. The cathode is nickel hydroxide. The electrolyte is KOH. This battery offers impressive power and energy performance with minimum weight and size. 

The authors of Solid State Ionics [1] eloquently describe the performance of the all-solid-state, plastic, Li-ion batteries and the cycling behavior for the Li4Ti50,2 cell and for the LiMn204 cell as a function cycler number. The battery composite structure best suited for low power applications consists of the Li4TijOj2 anode... 

Fabrication Aspects of Batteries for Low-Power Electronic Device Applications... 

Power MEMS [6] is the term used to describe miniaturized sources of electrical power. Batteries remain as the major source of portable electrical power, but MEMS-type turbines [6,154] and fuel cells are investigated as alternatives. The main limitations of batteries, for autonomous MEMS applications such as microrobots, are their low energy and power densities. Turbines and fuel cells have higher densities, but these systems are not yet available on the microscale. Fuel cells have, however, been miniaturized to some extent, with the size of a pack of cards already available commercially [154] for applications like powering cell phones. 

As already pointed out, ionic conductivity is only one of the many parameters required to achieve high performance LPBs. Although the performance of the room temperature system is adequate for several medium to low power applications (e.g. laptop computers, etc.), there are limitations in power density and cycle life which, at present, make it unsuitable for EV batteries. Since these batteries must operate at temperatures as low as — 40°C, thermal insulation will be required in the final EV battery module. 

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