Elastomeric Core‐Conductive Shell Slurry Additive Stabilizes High‐Voltage NCM811 Cathodes via Mechanical‐Electrical Dual Regulation

High‐voltage (>4.3 V) Ni‐rich layered cathodes are central to high‐energy‐density lithium‐ion batteries, yet their practical operation is limited by coupled mechanical degradation, sluggish interfacial charge transport, and parasitic cathode‐electrolyte reactions. Herein, we report a slurry‐compatible strategy to construct a phase‐separated mosaic‐like interphase on LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) cathodes using poly(acrylonitrile‐styrene‐acrylate) (ASA) as a multifunctional additive. During N‐methyl‐2‐pyrrolidone (NMP)‐based slurry casting and electrode drying process, the core‐shell ASA precursor undergoes structural reorganization, forming an ASA‐derived interphase composed of poly (butyl acrylate) (PBA)‐rich elastomeric regions and poly(styrene‐co‐acrylonitrile) (SAN)‐rich polar/aromatic regions. The PBA‐rich component buffers volume‐change‐induced stress and suppresses particle cracking, while the SAN‐rich component contributes to Li + ‐related interfacial affinity and enhances electron conductive. This phase‐separated, mosaic‐like interphase promotes stress dissipation from intercalation (deintercalation), enabling simultaneous regulation of mechanical integrity, interfacial chemistry, and electrode kinetics. Consequently, the optimized cathode exhibits reduced polarization, enhanced Li + diffusion, suppressed transition‐metal dissolution, and stabilized electrode interphases. In practical NCM811||graphite pouch cells under 4.5 V operation, capacity retention increases from 55.2% to 78.7% after 500 cycles, increased by 43%. This strategy highlights the importance of regulating the phase organization of polymeric slurry additives, offering a practically compatible route for stabilizing high‐voltage layered oxide cathodes.