Invited: Li-Air Flow Batteries

Although Li-air batteries have an extremely large theoretical energy density they suffer from several drawbacks: (1) The Li₂O₂/Li₂O discharge product deposits on the air side of the electrode reducing the pore size and limiting  access of the O₂ in the cathode. The discharge products deposit mostly near the air side of the electrode because the O₂ concentration is higher on this side. This inhomogenous deposition of reaction products severely limits the usage of cathode volume, which limits the maximum capacity and energy density of the battery; (2) the cyclability and energy efficiency of Li-air batteries are poor due to the lack of effective catalysts to convert solid Li₂O₂/Li₂O discharge products into Li ions; and (3) 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 solution.

We have demonstrated a novel rechargeable Li-air flow battery (Fig. 1&2), which consists of a lithium-ion conducting glass-ceramic membrane sandwiched by a Li-metal anode in organic electrolyte and a carbon nanofoam cathode through which oxygen-saturated aqueous electrolyte (0.85 M CH₃COOH in deionized water) 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 physical structure of the air cathode is crucial for electrochemical performance. Most conventional air-cathode structures are prepared via a traditional “brick-and-mortar” fabrication approach based on mixing and pressing powders of carbon, catalyst particles, and a polymeric binder into a composite electrode that exhibits an ad-hoc porous structure. Carbon paper supported carbon nanofoam electrode exhibits the following desirable properties: (i) electrical conductivity (10-200 S cm⁻¹); (ii) mechanical integrity; (iii) high specific surface areas (300-500 m² g⁻¹); and (iv) through-connected porosity in three dimensions with macroporous free volume, which is critical in maintaining facile water/air transport throughout the volume of the air cathode.

Fig. 3(a) displays the charge-discharge curves at various current densities. The discharge and charge voltages keep at 3.2 V and 3.9 V, respectively, at a current density of 1 mA cm^-2. Even at the current density of 5 mA cm^-2, the discharge and charge voltages still keep at 1.5 V and 5.2 V, respectively. With the growth of applied current density, the discharge voltage linearly decreases, while its power density sharply increases as shown in Fig. 3(b). At the current density of 4 mA cm^-2, a Li-air flow battery reaches its maximal power density of 7.64 mW cm^-2.

In Li-air flow batteries, a Li-metal foil was used as the anode electrode. The safety of the Li metal is always an important consideration. Table 1 shows theoretical energy densities of Li-air flow batteries if different anode materials such as Li metal, silicon, and graphite carbon are used. The theoretical specific energy was calculated based on EC reaction as^1,2:

4Li+O₂+4CH₃COOH « 4CH₃COOLi+2H₂O                             (1)

The biggest difference between the new Li-air flow batteries and conventional Li-air batteries^1,3 is that oxygen is supplied from the aqueous electrolyte instead of diffusing from the window in the cathode side. Hence, the thickness of cathode utilized is not limited by the oxygen diffusion length 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 improved⁴.

References:

¹J.P. Zheng, P. Andrei, M. Hendrickson, and E.J. Plichta, J. Electrochem. Soc. 158, A43 (2011).

²T. Zhang, N. Imanishi, Y. Shimonishi, A. Hirano, Y. Takeda, O. Yamamoto, and N. Sammes, Chem. Commun., 46, 1661 (2010).

³P. He, Y. Wang, H. Zhou, Electrochemistry Communications, 12, 1686 (2010).

⁴Andrei P, Zheng J P, Hendrickson M, Plichta E J., J. Electrochem. Soc., 2012, 159(6): A770-A780.