Sulfur-Induced Local Structure Reconstruction and Weak Na-Anion Interactions Enable Fast Na+ Transport in xNa2S-TaCl5 Electrolytes.

Abstract

Amorphous solid electrolytes offer opportunities for fast ion transport, yet the structural origins governing their performance remain difficult to resolve due to the absence of long-range order. While mixed-anion strategies have recently emerged to improve ion transport in sodium halide electrolytes, how sulfur incorporation modifies the local structure and Na+ transport remains poorly understood. Here, we develop a series of xNa2S-TaCl5 mixed anion solid electrolytes to elucidate the impact of sulfur on the structure and Na+ transport. Structural analyses reveal that sulfur incorporation reconstructs the connectivity of Ta-centered polyhedra while progressively disrupting the long-range crystalline ordering to form an amorphous framework. More importantly, neutron pair distribution function analysis reveals the sulfur-induced reconstruction of the local Ta-centered structure, with the emergence of coexisting edge-sharing- and corner-sharing-like motifs from the isolated polyhedra in crystalline NaTaCl6. Simulations further show that sulfur shifts the local polyhedral connectivity toward a larger proportion of edge-sharing configurations relative to the O-containing analogue. Matched-composition machine-learning molecular dynamics (MLMD) comparing O- and S-containing amorphous models suggest that sulfur weakens Na-anion binding, shortens Na-anion residence times, and lowers the barrier for Na+ migration relative to the O analogue. As a result, the optimized composition achieves a high room-temperature ionic conductivity of 3.25 mS cm-1 with a low activation energy of 0.28 eV. When evaluated in all-solid-state sodium batteries with an uncoated NaNi1/3Mn1/3Fe1/3O2 cathode, the cell delivers a discharge capacity of 112 mAh g-1 after 100 cycles (82.6% retention) with a high average Coulombic efficiency of 99.8%, demonstrating stable interfacial compatibility with uncoated layered oxide cathodes.

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