Molecular Crystalline Electrolyte Based on Li{N(SO2CF3)2} and Succinonitrile with Closely Contacted Grain Boundary Interfaces Exhibiting Selective Li-Ion Conductivity and 5 V-Class Electrochemical Stability

Abstract

Achieving all-solid-state batteries requires the development of solid electrolytes with high ionic conductivities, high Li-ion transference numbers, and wide-range electrochemical stabilities. Molecular crystals, which combine a moderate flexibility similar to that of polymer electrolytes with ion conduction paths resembling those of ceramic electrolytes, have attracted attention as promising candidates for innovative solid electrolytes. Improving the properties of molecular crystalline electrolytes requires clarifying the correlations between their hierarchical structures and electrolyte properties, as well as establishing material design guidelines. Herein, we report the organic molecular crystal Li₂{N(SO₂CF₃)}₂(NCCH₂CH₂CN)₃, hereafter referred to as Li₂(TFSA)₂(SN)₃, as a promising solid electrolyte. This molecular crystal exhibits an ionic conductivity of 3.6 × 10^–5 S cm^–1 at 30 °C with a considerably high Li-ion transference number of 0.98. In addition, we confirmed the wide electrochemical stability of the 5 V-class cathodes and their compatibility with Li metal anodes. Scanning electron microscopy observations revealed the formation of tightly contacted grain boundaries in the powder-molded pellets of Li₂(TFSA)₂(SN)₃. Notably, the previously reported molecular crystalline electrolyte Li(PF₆)(NC(CH₂)₄CN)₂ (Li(PF₆)(ADN)₂) formed an interface containing liquid components of substantial thickness with a Li-ion transference number of only 0.54. These results highlight that both, the selection of constituent molecules and anions and the design of the grain boundary interface, play crucial roles in achieving superior electrochemical stability and selective Li-ion conductivity in the development of molecular crystalline electrolytes.