Anomalous Thermal Decomposition Behavior of Polycrystalline LiNi_0.8Mn_0.1Co_0.1O₂ in PEO‐Based Solid Polymer Electrolyte

Abstract Replacing liquid electrolytes (LEs) with polymer electrolytes has been considered a promising approach to developing next‐generation lithium‐ion batteries (LIBs) with high energy density and superior safety. Nevertheless, compared with the extensive research on the electrochemical stability of the cathode/polymer electrolyte interfaces, reports on their thermal behaviors are rare to date. Herein, this work systematically investigates the thermal stability of two typical layered oxide cathodes, LiCoO₂ (LCO) and LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811), with poly(ethylene oxide) (PEO) electrolyte and with carbonate LEs, respectively. It is found that the oxygen release from the cathodes plays a central role in thermal runaway. Replacing the LE with PEO electrolyte can considerably improve the thermal stability of LCO, but surprisingly, deteriorate that of NMC811. The reason is that the surface of single‐crystalline LCO particles can be effectively passivated by the PEO electrolyte during heating, but PEO cannot sufficiently passivate all the primary particles of NMC811 owing to insufficient interface wettability of PEO electrolyte within the polycrystalline secondary NMC811 particles. The findings in this work collectively formulate valuable guidance for improving the safety of polymer‐electrolyte‐based as well as other types of all‐solid‐state lithium‐ion batteries.