Enabling Stable Cycling of 4.2 V High‐Voltage All‐Solid‐State Batteries with PEO‐Based Solid Electrolyte

Poly(ethylene oxide) (PEO)‐based solid electrolytes are expected to be exploited in solid‐state batteries with high safety. Its narrow electrochemical window, however, limits the potential for high voltage and high energy density applications. Herein the electrochemical oxidation behavior of PEO and the failure mechanisms of LiCoO2‐PEO solid‐state batteries are studied. It is found that although for pure PEO it starts to oxidize at a voltage of above 3.9 V versus Li/Li+, the decomposition products have appropriate Li+ conductivity that unexpectedly form a relatively stable cathode electrolyte interphase (CEI) layer at the PEO and electrode interface. The performance degradation of the LiCoO2‐PEO battery originates from the strong oxidizing ability of LiCoO2 after delithiation at high voltages, which accelerates the decomposition of PEO and drives the self‐oxygen‐release of LiCoO2, leading to the unceasing growth of CEI and the destruction of the LiCoO2 surface. When LiCoO2 is well coated or a stable cathode LiMn0.7Fe0.3PO4 is used, a substantially improved electrochemical performance can be achieved, with 88.6% capacity retention after 50 cycles for Li1.4Al0.4Ti1.6(PO4)3 coated LiCoO2 and 90.3% capacity retention after 100 cycles for LiMn0.7Fe0.3PO4. The results suggest that, when paired with stable cathodes, the PEO‐based solid polymer electrolytes could be compatible with high voltage operation. The root cause for the failure of poly(ethylene oxide) (PEO)‐based solid‐state lithium batteries at high voltages has been identified as the strong oxidizing ability of oxide cathodes that catalyze the decomposition of PEO. PEO‐based solid‐state lithium batteries using Li1.4Al0.4Ti1.6(PO4)3 coated LiCoO2 and phosphate LiMn0.7Fe0.3PO4 cathodes show significantly improved high‐voltage cycle performances at 4.2 V.