Revealing Phase Transitions in Poly(Ethylene Oxide)-Based Electrolyte for Room-Temperature Solid-State Batteries

Abstract

Solid-state electrolytes (SSEs) offer enhanced safety, extended cycle life, and increased energy density when replacing flammable liquid electrolytes in lithium-ion batteries. Poly(ethylene oxide) (PEO)-based SSE is the only candidate that has been commercially implemented in electric vehicles. However, the equipped battery needs to operate at temperatures above 50 °C, and its phase transitions at room temperature are still unclear. Herein, the solidification of the PEO-lithium bis(trifluoromethanesulfonyl)imide (PEOn-LiTFSI) system is revisited. Contrary to the prevailing view of forming PEO(6)-LiTFSI spherulites, the presence of crystalline PEO(8)-LiTFSI complexes is quantitatively confirmed. The nucleation and growth processes of crystalline PEO and PEO(8)-LiTFSI spherulites are also visually elucidated, and phase transitions with the impedance change are correlated. In addition, it is demonstrated that the crystalline PEO shell surrounding the PEO(8)-LiTFSI spherulites hinders the kinetics of crystal growth, thereby enabling the highest ionic conductivity at n = 10. Importantly, it is pointed out that instead of the poor ionic conductivity of the electrolyte layer, the heterogeneous nucleation of PEO(8)-LiTFSI within the electrodes is the limiting factor in constructing room-temperature all-solid-state batteries.