Coordination-Disorder Engineering of Amorphous Halide Superionic Conductors for Long-Cycle All-Solid-State Sodium Batteries

Sodium superionic conductors are critical enablers for advancing energy density and operational safety in next-generation sodium-ion batteries. While conventional crystalline sodium-halide-based electrolytes have demonstrated promising electrochemical stability, their ionic conductivity has been fundamentally constrained by reliance on vacancy-mediated transport mechanisms inherent to ordered crystalline frameworks. Here, we present a cation engineering strategy that induces structural coordination disorder to develop amorphous chloride conductors (A_2-xM_1-xTa_xCl₆ and NaNb_1-xTa_xCl₆; A = Li/Na, M = Zr/Hf; 0 < x < 1), achieving ionic conductivities surpassing 10^-3 S cm^-1 at ambient conditions. The optimized Na_1.4Zr_0.4Ta_0.6Cl₆ composition exhibits room-temperature conductivity of 1.95 × 10^-3 S cm^-1 at 25 °C, coupled with enhanced oxidative stability (>4.0 V) and mechanical robustness enabled by its disordered configuration and broadened ion migration channels. Implementation in all-solid-state sodium cells with Na₃V₂(PO₄)₃ cathodes demonstrates good rate performance and long-cycling stability (86% after 1000 cycles under 0.5 C). This work establishes amorphous-phase engineering through cation substitution as a transformative paradigm for designing sodium superionic conductors beyond the limitations of crystalline frameworks.