The rechargeable lithium-air battery is promising for the power source of electrical vehicles, because of its high specific energy density. Now two types of the lithium-air battery have been developed, namely aqueous and non-aqueous. The aqueous system has larger gravimetric energy density of 3505 Wh kg^-1 than that of the non-aqueous system of 1917 Wh kg^-1. However, the none-aqueous system has severe problems to be solved such as electrolyte decomposition. The gravimetric specific energy density of the aqueous system is comparable to that of internal conversion engines. The severe problem of the electrolyte decomposition would be removed in the aqueous system.

  The aqueous lithium-air battery consists of a lithium metal anode, a water stable lithium conducting solid electrolyte, an aqueous electrolyte and an air electrode. The concept was proposed by Visco et al. in 2004¹. The key material for the aqueous system is the water stable solid electrolyte. A few types of the lithium conducting solid electrolytes such as NASICON-type Li_1+xAl_xTi_2-x(PO₄)₃ (LATP), perovskite-type Li_2/3-xLi_3xTiO₃ (LLTO) and garnet-type Li₇La₃Zr₂O₁₂ (LLZ) are known to be stable in aqueous solutions with LiCl. LLZ is stable in contact with lithium metal, but it has serious problem of a lithium dendrite formation on lithium at a high current density. LATP and LLTO are unstable in contact with lithium metal and an interlayer should be used between lithium and LATP (or LLTO). As the interlayer, polyethylene oxide (PEO) based polymer electrolytes have been successfully used, but the electrical conductivity of the PEO based electrolyte is low at room temperature. The Li/PEO₁₈Li(CF₃SO₂)₂N/LATP/ aqueous 1M LiCl/Pt, air cell was successfully discharge and charge at 60 ⁰C and 0. 5 mA cm^-2 for 24 h.² Imanishi and co-workers³ have proposed a lithium dendrite formation free liquid electrolyte of a solvate ionic liquid of tetraethylene glycol dimethyl ether (TEGDME) and Li(FSO₂)₂N (LiFSI). A metallic lithium anode could deliver a high area capacity of 12 mAh cm^-2 for 40 cycles in TEGDME-2LiFSI with a current density of as high as 6 mA cm^-2 at 60 ^oC. The novel electrolyte as the interlayer between lithium metal and LATP allowed favorable cyclability for the aqueous lithium-oxygen at 0.64 mA cm^-2 and 60 ⁰C. To operate the lithium dendrite free electrolyte at room temperature, 1,3-dioxolane (DOL) was added into TEGDME-2LiFSI. TEGDME-2LiFSI-50 vol.% DOL showed a low interface resistance between lithium metal and the electrolyte. No short circuit was observed in the Li/TEGDME-2LiFSI-50 vol% DOL/Li cell at 1.0 mA cm^-2 and 25 ^oC for 40 h polarization. The TEGDME-2FSI-50 vol% DOL electrolyte is quite attractive interlayer between the lithium metal electrode and LATP for the lithium-air battery.

References

1. S.J. Visco, E. Nimon, B. Katz, L.C. De Jonghe and M.Y. Chu, 12^th International Meeting on Lithium Batteries, Abst. #53, Nara, Japan (2004)

2. T. Zhang, N. Imanishi, S. Hasegawa, A. Hirano, J. Xie, Y. Takeda, O. Yamamoto, and N. Sammes,  Electrochem. and Solid State Lett., 12, A132 (2009)

3. H. Wang, S. Sunahiro, M. Matsui, P. Zhang, Y. Takeda, O. Yamamoto, and N. Imanishi, ChemElectroChem, 2, 1144 (2015)