A High-Rate Rechargeable Li-Air Flow Battery
The experimental Li-air flow battery was shown in Fig. 1 which consists of two units: the electrochemical (EC) reaction unit; and a combined electrolyte storage/oxygen exchange unit. The EC reaction unit is similar to conventional Li-air batteries using hybrid electrolytes, which converts chemical energy into electrical energy and vice versa. However, a major difference is that the cathode electrode does not open directly to the atmosphere to receive oxygen; instead it circulates the electrolyte continuously between the EC reaction unit and electrolyte storage unit. Hence, the thickness of cathode utilized is not limited by the slow oxygen diffusion in the electrolyte any more. Instead, by using a thick cathode the reaction rate which is proportional to the volume of cathode can be increased, so the cell’s power performance can be improved2. Furthermore, the energy and power capabilities can be totally separated according to the load requirements. The maximum output power of the system is given by the maximum current density and the electrode size of the EC reaction unit; the electrolyte storage unit determines the maximum energy storage and delivery capacity; and the oxygen exchange unit regenerates (i.e. refreshes) the electrolyte to become EC reactive.
Fig. 2 displays the charge-discharge curves at various current densities. With the growth of applied current density, the discharge and charge voltage difference linearly increases. According to the electrochemical impedance spectra (EIS) analysis, this voltage difference is mainly attributed to the resistance of LIC-GC membrane and causes hydrogen evolution at high current density during discharge. At the current density of 4 mA/cm2, a Li-air flow battery reaches its maximal power density of 7.64 mW/cm2. The theoretical cell specific energy is calculated as 477 Wh/kg based on the weight of Li-metal, acid and H2O3.
In conclusion, a rechargeable Li-air flow battery is demonstrated to be cycled at a rate up to 5 mA/cm2, and the power performance can be further improved by reducing the resistance of LIC-GC membrane.
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- J.P. Zheng, P. Andrei, M. Hendrickson, and E.J. Plichta, J. Electrochem. Soc. 158, A43 (2011).