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

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 (TiVNbMoC₃/Tppm) functional layer that addresses these coupled failure modes. Through topological exfoliation, tetraphosphonate (TppmH₈) ligands act as molecular pillars to expand the TiVNbMoC₃ 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.