Solid electrolytes (SE) have been considered as alternative components to organic liquid electrolytes in lithium batteries (LIB). They promise safety enhancement by preventing the safety risks (e.g., leakage and ignition) [1]. In addition, the possibility of replacing graphite anode with lithium can lead to higher energy densities. Among various SE types, Li₂S-P₂S₅ based sulfide SEs consisting of PS_x polyhedral anions have been extensively explored due to its superionic conductive performances [2]. Previously, the SE was fabricated as pellets from Li₂S-P₂S₅ powders via sintering or cold-pressing to reduce the grain boundary resistance. However, the processing costs of the high-energy ball milling and/or sintering at high temperature are considerable. In the meantime, the SE pellets are very thick (> 200 μm) due to the need to maintain mechanical integrity during processing and handling, resulting in cell energy density reduction, and the SE pellet brittleness prevents efficient device integration. Therefore, a desirable SE synthesis process should be designed to meet the following requirements: 1) low processing cost, 2) easy device integration, and 3) high energy density. In this work, we describe a Li₂S_y-P₂S₅ solution coating process for the SE synthesis on the basis of aforementioned three requirements. Morphologies, compositions, crystal structures, and microstructures of the synthesized SE layers (Figure 1) were thoroughly examined using SEM, Raman spectroscopy, XRD, and XPS, and electrochemical performances were assessed using electrochemical impedance spectroscopy (EIS) and lithium ion battery cycling performance. We will describe how the fine tuning of solution composition and reaction condition can offer solid electrolyte layers inherently amenable to device integration.

[1] Teragawa, S. et al., J. Mater. Chem. A 2, 5095 (2014)

[2] Ohara, K. et al., Sci. Rep. 6, 21302 (2016)

Figure 1. Solution process based solid electrolyte (SE) layers formed on a metal electrode: a) photograph, and b) cross-sectional SEM image, and EDX images of c) phosphorus and d) sulfur in cross-section shown in b).