Sulfide-based solid electrolytes have attracted much attention due to their high conductivities, which are far beyond those of oxide-based solid electrolytes.^[1,2] However, They (Li₂S-P₂S₅ system, i.e., LPS) have been normally synthesized by solid state synthesis such as mechanical ball milling. These methods require rigorous control of reaction environment as well as high temperature heat treatment and repeated pelletizing steps. In contrast, solution-based synthesis methods can induce chemical reaction among precursor particles (Li₂S and P₂S₅) at low temperatures resulting in the formation of conductive phases of β-Li₃PS₄ and Li₃P₇S₁₁ with only moderate thermal treatment.^[3,4] The method deserves great attention since it simplifies synthesis process, yields products of great purity, and may facilitate the fabrication of composite electrodes with improved interfaces.

In this work, we have developed an efficient method to form a thin solid electrolyte layer directly on Li metal using the liquid coating techniques. The formation of LPS (Li₂S-P₂S₅) based electrolyte is achieved by rational design of the solvent and the Li, P, and S precursor ratios. The solution electrolyte can be directly coated and formed on Li metal through the in-situ formation of the solid electrolyte layer, which does not require the complex synthesis process and high temperature sintering step. Layers of thickness of < 50 mm can be fabricated and electrochemical cycling of lithium is achieved. This liquid-phase coating is a simple and straightforward technique for making a thin solid electrolyte and can be applicable to anode surface with complex contours. The new liquid coating technique holds the promise to overcome the limitations of current state solid electrolytes.

[1] Kamaya, N. et al. A lithium superionic conductor. Nature Mater. 10, 682-686 (2011).

[2] Yamane, H. et al. Crystal structure of a superionic conductor, Li₇P₃S₁₁. Solid State Ion. 178, 1163-1167 (2007).

[3] Ito, S. et al. A synthesis of crystalline Li₇P₃S₁₁ solid electrolyte from 1,2-dimethoxyethane solvent. J. Power Sources 271, 342-345 (2014).

[4] Liu, Z. et al. Anomalous High Ionic Conductivity of Nanoporous β-Li₃PS₄. J. Am. Chem. Soc. 135, 975-978 (2013).