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

ABSTRACT 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 0.8 Co 0.1 Mn 0.1 O 2 (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.