Oxygen Substitution of Li3PS4–LiF Electrolytes for Stable Li Metal / Solid Electrolyte Interface in All-Solid-State Batteries

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Recently, demands for rechargeable batteries with higher energy density and safety have soared with the growing interest in electric vehicles for sustainable society. All-solid-state batteries can achieve higher safety by using a nonflammable inorganic solid electrolyte instead of the organic electrolyte used in conventional lithium-ion batteries. Sulfide electrolytes have been studied extensively as solid electrolytes for all-solid-state batteries because of their high conductivity and formability, making them suitable for practical use. At the same time, the application of Li metal negative electrode is also desired since Li metal has the lowest electrochemical potential (−3.04 V versus the standard hydrogen electrode) and high theoretical capacity (3860 mA h g−1). However, the penetration of Li dendrites (filaments) through the solid electrolyte layer causes short circuits in all-solid-state Li metal batteries. Our group has investigated the short-circuit mechanism in all-solid-state Li metal batteries using sulfide electrolytes [1,2]. First, at the interface between the Li3PS4 electrolyte and Li metal, Li3PS4 is reductively decomposed into Li3P and Li2S [1]. The decomposition causes the volume expansion and the formation of microcracks. Then, Li metal electrochemically precipitates along the cracks, causing short circuits through repeated reductive decomposition and crack formation [2]. Therefore, it is necessary to improve the reduction tolerance of the solid electrolyte to prevent short circuits. In addition, it is known that high conductivity of the solid electrolyte is also required for high short-circuit resistance [3]. One strategy to improve reduction tolerance is to add lithium halides, which are thermodynamically stable to Li metal. Adding LiF, which has a wider electrochemical window among lithium halides, may improve reduction resistance. Also, as another strategy, it improves the stability of the PS4 units to Li metal by replacing a part of the S in the PS4 units with O. However, it is typical for these approaches to decrease the conductivity of the solid electrolyte. Therefore, a process recently developed by our group to precipitate α-Li3PS4 by rapidly heating and quenching Li3PS4 glass [4] has the potential to produce solid electrolytes that combine both high conductivity and high reduction tolerance.In this study, Li3PS4−x O x ·LiF electrolytes were prepared by replacing a part of S in Li3PS4-LiF electrolytes with O. The α-Li3PS4 analog phase was precipitated by heat treatment, and the conductivity and reduction resistance of the fabricated glass-ceramics were investigated. We also studied the heat treatment conditions to precipitate the α-Li3PS4 analog phase.The Li3PS4−x O x ·LiF glass-ceramics were prepared by a mechanochemical process and subsequent heat treatment. The XRD pattern of the milled samples showed that they were almost amorphous, although the peaks attributed to LiF remained. Rapid heating and rapid cooling of Li3PS4·LiF glass precipitated the α-Li3PS4 analog phase, while slow heating and slow cooling precipitate crystals attributed to β-Li3PS4. However, the oxygen-substituted Li3PS3.8O0.2·LiF glass precipitated an α-Li3PS4 analog phase even after crystallization by slow heating and cooling. As a result, Li3PS3.8O0.2·LiF glass-ceramics exhibited a high conductivity of 1.3 × 10−3 S cm−1 at room temperature. Li symmetric cells using Li3PS3.8O0.2·LiF glass-ceramics showed excellent Li stripping/plating performance. In addition, the formation of the reaction layer at the Li/solid electrolyte interface after operation was suppressed compared to that before oxygen substitution.In summary, Li3PS3.8O0.2·LiF glass-ceramics exhibited XRD peaks attributed to α-Li3PS4 analog, even if they were prepared by heat treatment with slow heating and slow cooling. Li3PS3.8O0.2·LiF glass-ceramics exhibited high conductivity and high reduction tolerance, suggesting that it is a suitable solid electrolyte material for all-solid-state Li metal batteries.