In recent years, high lithium ion conducting solid electrolytes have appealed to many researchers due to their outstanding performance in safety and electrochemical stability in comparison to the flammable liquid electrolyte, which is widely used in current lithium-ion batteries¹. A large number of solid electrolytes have been reported over the past several decades such as Li₃N, Li_3.25P_0.95S₄, Li₁₀GeP₂S_12, Li_1.4Al_0.4Ge_0.2Ti_1.6(PO₄)₃ and Li₇La₃Zr₂O₁₂ whose electrical conductivities are all more than 10^-4 S cm^-1 at 25 ^oC. Among them, Li_1.4Al_0.4Ge_0.2Ti_1.6(PO₄)₃ (LAGTP) exhibits the most prominent potential as an electrolyte in various kinds of batteries because of its high conductivity (10^-3 S cm^-1 at room temperature), excellent stability in atmosphere, and the ease with which it can be prepared into thin films²; however, the main obstacle to its development and application in batteries is the instability of the interface between LAGTP and lithium electrode, which is attributed to the reduction reaction of Ti⁴⁺ by metal lithium. Li₃N and polymer protective layers are typically introduced into the architecture design of electrolytes to address this problem by spatially separating LAGTP and lithium electrode. Even though polymer electrolytes are well known for their relatively low conductivity at room temperature, their brief and adjustable preparation process is still considered as a major advantage.

In this study, a novel composite electrolyte of lithium stable polymer and water impermeable LAGTP was proposed. This composite solid electrolyte consisted of a thin lithium stable polyethylene oxide (PEO) based polymer film and a high lithium ion conducting LAGTP film. The LAGTP film was prepared via the tape casting method in which the preceding fine powder was synthesized via the sol-gel method. Afterwards, epoxy resin gradually grew in the pores of the LAGTP film through polymerization with polymer monomers at 170 ^oC for 24 hours. A PEO solution dispersed in acetonitrile with lithium bis(fluorosulfonyl)imide (LiFSI), tetraethylene glycol dimethyl ether (G4) and the fine powder of BaTiO₃ was gradually dropped on the surface of the LAGTP films, where the mole ratio of PEO/LiFSI/G4 was 8/1/2 and the content of BaTiO₃ was 10 wt%. The composite solid electrolyte with 4 wt% PEO based electrolyte showed a high electrical conductivity of about 1×10^-4 S cm^-1 at 25 ^oC and 6×10^-4 S cm^-1 at 60 ^oC. The composite electrolytes also displayed a good stability with metal lithium and suppressed the formation of the lithium dendrite in the cell of Lithium/composite electrolyte of PEO coated LAGTP/Lithium at 1.0 mA cm^-2for 40 h at room temperature. This lithium-stable and water-insensitive composite solid electrolyte with high ion conductivity is most likely to become an attractive candidate for all solid lithium batteries and lithium-air batteries in the near future.

References:

(1) B. Scrosati, J. Garche, J. Power Sources, 195, 2419 (2010).

(2) P. Zhang, H. Wang, Y-G. Lee, M. Matsui, Y. Takeda, O. Yamamoto, N. Imanishi, J. Electorchem. Soc., 162, A1265 (2015).