Highly Thermal Conductive Separator with In‐Built Phosphorus Stabilizer for Superior Ni‐Rich Cathode Based Lithium Metal Batteries

The movement toward cobalt‐free cathode materials has served as a motivation for increased research in layered nickel‐rich cathodes for next generation metal batteries. Unfortunately, Ni‐rich cathode materials suffer from low capacity retention and poor thermal stability due to phase transition that results in issues such as the oxygen evolution reaction, hindering its extensive implementation. Herein, highly pliable separators with a 3D porous structure are prepared via a facile phase‐inversion method from an inorganic phosphorus‐based flame retardant and a thermally conductive graphene oxide additive. Benefiting from its 3D porous structure, in‐built radical scavenger, and uniform thermal distribution, the obtained separator enables a near‐single Li + migration ( t Li + = 0.8) by blocking large‐size anions, driving the LiNi 0.8 Mn 0.1 Co 0.1 O 2 /Li metal batteries to 188.8 mAh g −1 at 0.2 C, and demonstrating a capacity retention of 82.2% versus 41.4% for commercial polyolefin separators after 200 cycles, as well as excellent dendrite‐suppressing capabilities by reducing localized temperature hotspots and enabling sufficient mass transfer. This work also suggests a new alternative pathway for stabilizing reactive electrode materials for other high‐energy battery systems.