All-solid-state sodium-ion batteries are promising candidates for large-scale energy storage applications. The key enabler for an all-solid-state architecture is a sodium solid electrolyte that exhibits high Na(+) conductivity at ambient temperatures, as well as excellent phase and electrochemical stability. In this work, we present a first-principles-guided discovery and synthesis of a novel Cl-doped tetragonal Na3PS4 (t-Na3-xPS4-xClx) solid electrolyte with a room-temperature Na(+) conductivity exceeding 1 mS cm(-1). We demonstrate that an all-solid-state TiS2/t-Na3-xPS4-xClx/Na cell utilizing this solid electrolyte can be cycled at room-temperature at a rate of C/10 with a capacity of about 80 mAh g(-1) over 10 cycles. We provide evidence from density functional theory calculations that this excellent electrochemical performance is not only due to the high Na(+) conductivity of the solid electrolyte, but also due to the effect that "salting" Na3PS4 has on the formation of an electronically insulating, ionically conducting solid electrolyte interphase.

In this work, we investigated the interface between the sodium anode and the sulfide-based solid electrolytes Na₃SbS₄ (NAS), Na₃PS₄ (NPS), and Cl-doped NPS (NPSC) in all-solid-state-batteries (ASSBs). Even though these electrolytes have demonstrated high ionic conductivities in the range of 1 mS cm^-1 at ambient temperatures, sulfide sold-state electrolytes (SSEs) are known to be unstable with Na metal, though the exact reaction mechanism and kinetics of the reaction remain unclear. We demonstrate that the primary cause of capacity fade and cell failure is a chemical reaction spurred on by electrochemical cycling that takes place at the interface between the Na anode and the SSEs. To investigate the properties of the Na-solid electrolyte interphase (SSEI) and its effect on cell performance, the SSEI was predicted computationally to be composed of Na₂S and Na₃Sb for NAS and identified experimentally via X-ray photoelectron spectroscopy (XPS). These two compounds give the SSEI mixed ionic- and electronic-conducting properties, which promotes continued SSEI growth, which increases the cell impedance at the expense of cell performance and cycle life. The SSEI for NPS was similarly found to be comprised of Na₂S and Na₃P, but XPS analysis of Cl-doped NPS (NPSC) showed the presence of an additional compound at the SSEI, NaCl, which was found to mitigate the decomposition of NPS. The methodologies presented in this work can be used to predict and optimize the electrochemical behavior of an all-solid-state cell. Such joint computational and experimental efforts can inform strategies for engineering a stable electrolyte and SSEI to avoid such reactions. Through this work, we call for more emphasis on SSE compatibility with both anodes and cathodes, essential for improving the electrochemical properties, longevity, and practicality of Na-based ASSBs.