Engineering Interfacial Immunity in High‐Nickel Batteries with a Mediating Biodegradable Separator

While high‐nickel cathodes offer compelling energy density, their cycle life is severely compromised by transition metal (TM) dissolution and subsequent interfacial degradation at the anode. Herein, we engineer a fully biodegradable composite separator comprising poly(lactic acid) and poly(ethylene glycol) (PLA/PEG) to address these issues through multifunctional integration. The ether‐oxygen‐rich PEG chains not only effectively scavenge dissolved TM ions through strong Lewis acid‐base interactions and enhance interfacial ion transport, but also facilitate hydrolytic breakdown of the PLA matrix, collectively contributing to a self‐mediating interphase, while the mutually‐reinforced degradability of both polymers ensures environmental benignity. The Graphite || NCM811 full cell employing the PLA/PEG separator demonstrates exceptional cyclability, retaining 70.4% capacity after 2000 cycles at 5 C with a cathode loading of 10.56 mg cm −2 . This work presents a paradigm of eco‐adaptive separator design that concurrently enhances electrochemical durability and environmental sustainability.