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 (PEO n ‐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.