Minute-Scale Ultrafast Synthesis of Superionic Sodium Halide-Borate Electrolytes for Solid-State Batteries

Halides have emerged as promising solid electrolytes for solid-state batteries, owing to their good oxidative stability and favorable mechanical deformability. Nevertheless, the practical application of sodium chloride-based electrolytes has been severely hindered by sluggish Na⁺-ion transport and a strong reliance on prolonged mechanochemical processing, which substantially increases manufacturing cost. Here, we report a mixed-anion sodium halide-borate solid electrolyte, Na_1+xTa(B₄O₇)_xCl_6-x, that effectively addresses these limitations. Partial substitution of Cl^- by [B₄O₇]^2- units creates mixed Ta-Cl-O coordination polyhedra that induce significant local structural distortion and promote rapid amorphization. As a result, a room-temperature ionic conductivity as high as 3.1 mS cm^-1 is achieved within only 10 min of mechanochemical milling. Ab initio molecular dynamics simulations reveal that the incorporation of borate weakens Na⁺-ion coordination, activates coupled cation-anion dynamics, and enlarges Na⁺-ion transport bottleneck, which synergistically flattens the cation migration energy landscape for rapid Na⁺-ion diffusion. Benefiting from fast Na⁺-ion transport and favorable compatibility with cathode materials, Na_1+xTa(B₄O₇)_xCl_6-x enables solid-state sodium batteries employing a NaCu_0.12Ni_0.22Fe_0.33Mn_0.33O₂ cathode to sustain stable cycling for over 800 cycles with a capacity retention of 81.6%. These results establish Na_1+xTa(B₄O₇)_xCl_6-x as a new class of halide-based superionic conductors and highlight mixed-anion engineering as an effective strategy for developing high-performance solid electrolytes.