Solid electrolytes are key materials to enable solid-state rechargeable batteries, a promising technology that could address the safety and energy density issues. Here, we report a sulfide sodium-ion conductor, Na2.88Sb0.88W0.12S4, with conductivity superior to that of the benchmark electrolyte, Li10GeP2S12. Partial substitution of antimony in Na3SbS4 with tungsten introduces sodium vacancies and tetragonal to cubic phase transition, giving rise to the highest room-temperature conductivity of 32 mS cm−1 for a sintered body, Na2.88Sb0.88W0.12S4. Moreover, this sulfide possesses additional advantages including stability against humid atmosphere and densification at much lower sintering temperatures than those (>1000 °C) of typical oxide sodium-ion conductors. The discovery of the fast sodium-ion conductors boosts the ongoing research for solid-state rechargeable battery technology with high safety, cost-effectiveness, large energy and power densities.

h i g h l i g h t sLi 6 PS 5 Cl solid electrolyte was prepared from ethanol solution. LiCoO 2 was coated with Li 6 PS 5 Cl solid electrolyte by using the solution. All-solid-state batteries using the electrolyte-coated LiCoO 2 operated reversibly. battery Lithium secondary battery Sulfide solid electrolyte Liquid phase method a b s t r a c t A Li 6 PS 5 Cl solid electrolyte was successfully prepared by dissolution-reprecipitation process via ethanol solution. An ionic conductivity of the Li 6 PS 5 Cl solid electrolyte from the homogeneous ethanol solution was 1.4 Â 10 À5 S cm À1 at room temperature. LiCoO 2 particles were coated with the Li 6 PS 5 Cl electrolyte via ethanol solution to form favorable electrode-electrolyte interface with a large contact areas. An allsolid-state cell using the electrolyte-coated LiCoO 2 operated as a rechargeable battery and showed the initial discharge capacity of 45 mAh g À1 at 25 C.

An argyrodite Li6PS5Br electrolyte is synthesized using tetrahydrofuran and ethanol solvents and shows the highest conductivity of 3.1 mS cm−1.

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Argyrodite-type crystals, Li 6 PS 5 X (X = Cl, Br, I), are promising solid electrolytes (SEs) for bulk-type allsolid-state lithium-ion batteries with excellent safety and high energy densities because of their high ionic conductivities and electrochemical stabilities. However, these advantageous features alone are not sufficient to achieve good cell performance. It is also critically important to have a simple and effective synthetic route to SEs and techniques for forming favorable solid−solid interfaces with large contact areas between the electrode and electrolyte particles. Here, we report an effective route for the preparation of argyrodite-type crystals using a liquid-phase technique via a homogeneous ethanol solution to improve cell performance using an SE-coating on the active material. The preparation conditions, such as appropriate halogen species and alcohol solvents, dissolution time, and drying temperature, are examined, finally resulting in Li 6 PS 5 Br with a lithium-ion conductivity of 1.9 × 10 −4 S cm −1 . Importantly, the obtained solution forms a favorable solid−solid electrode−electrolyte interface with a large contact area in the all-solid-state cells, resulting in a higher capacity than conventional techniques such as hand mixing using a mortar.