High-rate Na⁺ diffusion in solid polymer electrolytes enabled by high-concentration Na salt aggregates and size-selective anion immobilization

Solid polymer electrolytes present transformative potential for all-solid-state sodium metal batteries (ASSMBs) owing to their molecular tailorability, cost-effectiveness and ease of processability, yet face critical challenges including sluggish Na⁺ diffusion kinetics at room temperature, high-voltage oxidation tendency, and uncontrolled Na dendrite growth. To address these limitations, we propose an anion-regulation strategy through a hierarchically engineered composite polymer electrolyte (CPE) that strategically integrates (1) a high-concentration NaTFSI⁻ optimized polyethylene oxide (PEO) matrix (EO:Na⁺ = 6:1) and (2) a mechanically reinforced polyacrylonitrile (PAN) scaffold embedded with uniformly dispersed UIO-66 metal–organic framework nanoparticles (UIO-66@PAN). High-concentration TFSI⁻ dissociation activates mobile anion-Na⁺ cluster species that form dynamic percolation networks, establishing low-energy-barrier pathways for accelerated Na⁺ migration (1.01 mS/cm at 30 °C). Meanwhile, the UIO-66@PAN scaffold delivers exceptional tensile resilience (3.46 MPa) for the CPE membrane, meanwhile sub-nanometer precision of porous architecture (7.5–8.1 Å in UIO-66 vs. 7.9 Å for TFSI⁻) enables steric-electronic dual-anchoring effects for TFSI⁻ immobilization, achieving a high Na⁺ transference number (t_Na⁺ = 0.76). In a solvent-free, layer-stacked ASSMB assembly with sodium vanadium phosphate (NVP) cathode (1 mAh/cm²), 25 μm CPE and the Na foil, the prototype achieves 91.3 % capacity retention over 200 cycles at room temperature, rate behavior up to 1 C as well as robust cyclability across a wide temperature range (25–80 °C). This molecular-scale engineering of anion behavior in the CPE design thus establishes a new paradigm for the practical ASSMB prototyping.