Complex Hydride Solid Electrolytes of the Li(CB₉H₁₀)-Li(CB₁₁H₁₂) Quasi-Binary System: Relationship between the Solid Solution and Phase Transition, and the Electrochemical Properties

Closo-type complex hydrides have recently received much attention as promising solid electrolyte systems for all-solid-state batteries, because of the high lithium ion conductivity of their high-temperature (high-T) phases, excellent stability against a lithium metal anode, and a highly deformable nature. However, the superionic conductivity of closo-type complex hydrides is achieved in only a few materials; therefore, an understanding of the material factors involved in the formation of the high-T phase at room temperature and experimental demonstration of their battery applications are required. Here, we report the relationship between the solid solution and formation of the high-T phase of the Li(CB9H10)-Li(CB11H12) quasi-binary system, and the electrochemical properties as a solid electrolyte for all-solid-state Li-TiS2 batteries. The single-phase solid solutions, Li(CB9H10)-based phase in which [CB9H10]- is partially substituted with [CB11H12]- and Li(CB11H12)-based phase in which [CB11H12]- is partially substituted with [CB9H10]-, are obtained at compositions with low- and high-x in the (1 - x)Li(CB9H10)-xLi(CB11H12) (0.1 ≤ x ≤ 0.9) system. The effect of the solid solution on structural changes is more noticeable at low x, whereby a superionic conducting phase is formed with an identical structural framework as that of the high-T phase of Li(CB9H10) at room temperature. In addition, the 0.7Li(CB9H10)-0.3Li(CB11H12) (x = 0.3) solid electrolyte exhibits high chemical/electrochemical stability against a TiS2 cathode, which leads to superior performance in the rate capability and cycle life of all-solid-state Li-TiS2 batteries. The results presented here offer insights into strategies for the design of complex hydride lithium superionic conductors and for the development of all-solid-state batteries with these solid electrolytes.