A High-Rate Rechargeable Li-Air Flow Battery

The Li-air batteries have been attracted much attention because of the possibility of extremely high theoretical energy density. However, the practical energy density of Li-air batteries is much less than the theoretical predication. In addition to the energy density, Li-air batteries also suffered from other major deficiencies including extremely low power (or current) density and poor cyclability, and sensitivity to the moisture. In particular, the current and power densities of Li-air batteries are much lower compared to conventional batteries due to an extremely low oxygen diffusion coefficient in liquid solution¹. A solid Li-ion conducting glass ceramic (LIC-GC) membrane has not only a good conductivity for Li-ions but also good chemical stability in both non-aqueous and aqueous solutions, as well as the ability to isolate the two electrolytes. This membrane is used in the Li-air battery which consists of dual electrolytes - a non-aqueous electrolyte and an aqueous electrolyte in the anode and cathode electrodes, respectively. This dual electrolyte configuration can avoid the solid discharge product in the organic electrolyte configuration resulting in improved rechargeability. In this study, a new structure of Li-air flow battery system for possible large scale applications is proposed. It consists of a LIC-GC membrane sandwiched by a Li-metal anode in organic electrolyte and a carbon nanofoam cathode through which oxygen-saturated aqueous electrolyte flows. It features a flow cell design in which aqueous electrolyte is bubbled with compressed air, and is continuously circulated between the cell and a storage reservoir to supply sufficient oxygen for high power output.

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.

References

1. P. Andrei, J. P. Zheng, M. Hendrickson, and E. J. Plichta, J. Electrochem. Soc., 157, A1287 (2010).
2. P. Andrei, J. P. Zheng, M. Hendrickson, and E. J. Plichta, J. Electrochem. Soc., 159(6), A770 (2012).
3. J.P. Zheng, P. Andrei, M. Hendrickson, and E.J. Plichta, J. Electrochem. Soc. 158, A43 (2011).