Li10GeP2S12-Type\nStructured Solid Solution Phases in the Li9+δP3+δ′S12–kOk System: Controlling Crystallinity by Synthesis\nto Improve the Air Stability

Understanding the\nfast Li ionic conductors of oxygen-substituted\nthiophosphates is useful for developing all-solid-state batteries\nbecause these compounds possess a high electrochemical stability and\nthus may be applied as solid electrolytes. In this study, we synthesized\nthe Li9+δP3+δ′S12–kOk series of solid solution\nphases with the same structure as the Li10GeP2S12 superionic conductor and characterized their crystallinity,\nsolid solution range, and chemical stabilities. Two methods (mechanochemical\nand melt quenching) were used for sample synthesis. Mechanochemical\nsynthesis was used to obtain samples within a wide range of sulfur/oxygen\nsubstitution degrees, and the solid solution range was determined\nto be 0 < k ≤ 3.6 based on their lattice\nparameter variation. Meanwhile, the melt-quenched Li9P3S9O3 phase exhibited a high degree of\ncrystallinity up to its particle surface and was thus selected for\nneutron crystal structure analysis, which revealed the oxygen distribution\nrelated to the solubility limit. The highly crystalline melt-quenched\nLi9P3S9O3 showed better\nstability in the air atmosphere compared to the mechanochemically\nsynthesized counterpart with a low crystallinity, implying that sample\ncrystallinity is an important parameter in evaluating the air stability\nof thiophosphates. The promising electrochemical properties of the\nsolid solution series were demonstrated by the stable charge–discharge\ncycling of an all-solid-state lithium metal cell using the Li9+δP3+δ′S12–kOk electrolyte with k = 0.9 and a conductivity of >1 × 10–3 S cm–1 at 300 K.