The interphasial degradation of 4.2 V-class poly(ethylene oxide)-based solid batteries beyond electrochemical voltage limit

Solid-state polymer electrolytes (SPEs) have attracted increasing attention due to good interfacial contact, light weight, and easy manufacturing. However, the practical application of SPEs such as the most widely studied poly(ethylene oxide) (PEO) in high-energy solid polymer batteries is still challenging, and the reasons are yet elusive. Here, it is found that the mismatch between PEO and 4.2 V-class cathodes is beyond the limited electrochemical window of PEO in the solid LiNi1/3Mn1/3Co1/3O2 (NMC)-PEO batteries. The initial oxidation of PEO initiates remarkable surface reconstruction of NMC grains in solid batteries that are different from the situation in liquid electrolytes. Well-aligned nanovoids are observed in NMC grains during the diffusion of surface reconstruction layers towards the bulk in solid batteries. The substantial interphasial degradation, therefore, blocks smooth Li+ transport across the NMC-PEO interface and causes performance degradation. A thin yet effective LiF-containing protection layer on NMC can effectively stabilize the NMC-PEO interface with a greatly improved lifespan of NMC|PEO|Li batteries. This work deepens the understanding of degradations in high-voltage solid-state polymer batteries.