Mechanically Adaptive Polyimide Interfaces for Stable High-Voltage NCM-Sulfide All-Solid-State Batteries

The practical implementation of Ni-rich layered oxide-sulfide all-solid-state batteries (ASSBs) is limited by rapid capacity fading from interfacial contact loss and parasitic reactions. Conventional inorganic coatings provide limited mechanical compliance, failing to address these intertwined electrochemical-mechanical degradations. Here, we report a smart responsive cathode-electrolyte interface realized through the stepwise construction of a conformal polyimide (PI) coating on single-crystal LiNi_0.8Co_0.1Mn_0.1O₂ (sNCM). The grafted PI layer establishes robust carboxylate-transition metal coordination bonds, effectively eliminating surface lithium residues and suppressing oxygen release. Owing to its viscoelastic nature, the PI coating reduces the surface modulus of sNCM by 26.6%, accommodating anisotropic volume changes and mitigating intragranular microcrack propagation. Most significantly, the PI interface functions as an electrochemically driven self-optimizing system during cycling, progressively decreasing total interfacial impedance by 38%. Consequently, the sNCM@PI0.05 cathode delivers 83.6% capacity retention after 400 cycles at 1C under a 4.3 V cutoff voltage, significantly outperforming unmodified sNCM (33.2% retention). It also maintains 90.2% of its initial capacity after 200 cycles at 4.5 V. This reactive polymer interphase design harmonizes chemical passivation and mechanical adaptability, providing a transformative strategy for durable, high-energy ASSBs.