Lattice‐Level Negative Thermal Expansion Enables Thermomechanical Self‐Compensation in High‐Rate Lithium Batteries

Localized Joule heating in high‐rate lithium metal batteries often induces severe thermomechanical failure of polymer separators and accelerates thermal runaway in cells with Ni‐rich NCM cathode. Here we report a multiscale separator design strategy that translates lattice‐level negative thermal expansion into macroscopic thermomechanical self‐compensation. In this design, Cu 2 P 2 O 7 is coupled with the PVDF‐HFP matrix through a PDA mediated interface. Temperature‐dependent structural analysis and thermal‐response tests demonstrate the anisotropic lattice contraction of Cu 2 P 2 O 7 , which relieves the thermally induced deformation of PVDF‐HFP through PDA mediated interfacial interactions. The amine‐rich PDA layer further suppresses corrosive hydrofluoric acid during the cycling, improving interfacial durability. The resulting composite membrane maintains complete dimensional stability up to 200°C while preserving high ionic conductivity and a uniform Li + flux. As a result, Li||Li symmetric cells with the self‐compensating separator operate for more than 3000 h at 1 mA cm −2 . When paired with Ni‐rich NCM811 cathodes, full cells maintain 1000 stable cycles at 5 C and deliver 135 mAh g −1 at 60°C after 100 cycles. This work demonstrates a solid‐state negative thermal expansion strategy for active separator protection, providing a materials route for mitigating thermomechanical failure in high‐power lithium metal batteries.