Phosphonate‐Pillared High‐Entropy MXene Separator Enabling Ion‐Sieving, Flame‐Retardant, and Energy/Power‐Dense Lithium Metal Pouch Cells

ABSTRACT Coupling Ni‐rich cathodes with lithium metal anodes offers a compelling route to high‐energy‐density batteries, yet cation crosstalk from cathode dissolution destabilizes the anode interface, accelerates dendritic protrusion, and can trigger thermal runaway. Herein, we report an ion‐sieving, flame‐retardant separator based on a phosphonate‐pillared high‐entropy (HE) MXene (TiVNbMoC 3 /Tppm) functional layer that addresses these coupled failure modes. Through topological exfoliation, tetraphosphonate (TppmH 8 ) ligands act as molecular pillars to expand the TiVNbMoC 3 interlamellar spacing to 18.5 Å, enabling a 95% yield of few‐layer (< 5 layers) nanosheets. The HE architecture constructs rapid and homogeneous Li + conduction pathways with a diffusion barrier of 0.179 eV while sequestering 82% of dissolved transition metals. The composite separator delivers an Li + transference number of 0.77, tensile strength of 95.17 MPa, and thermal stability at 180°C. The regulated nanochannels also facilitate stable interfacial chemistry at the Li‐metal anode. In 1.0 Ah NCM811||Li pouch cells under lean‐electrolyte conditions, it achieves 87.1% capacity retention after 200 cycles, gravimetric/volumetric energy densities of 411.8 Wh kg −1 /838.2 Wh L −1 , and a power density of 1127.0 W kg −1 . Phosphonate‐derived PO· radicals and MXene‐derived ceramic char synergistically suppress thermal propagation, enabling stable operation during thermal chamber testing.