Sulfur-Induced Local Structure Reconstruction and Weak Na–Anion Interactions Enable Fast Na+ Transport in x Na2S-TaCl5 Electrolytes
Jiwei Wang, Anika Tabassum Promi, Xiaohu Zhang, Yogeshwaran Agilan, Man Chen, Mengting Ye, Feng Lin, Enyuan Hu, Hui Wang, Jianming Bai, Jue Liu, Joshua W. Gallaway, KyuJung Jun, Hongli ZhuAbstract
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.