Direct tracking of reaction distribution in an all-solid-state battery using operando scanning electron microscopy with energy dispersive X-ray spectroscopy

抄録

The reaction distribution in the composite cathode of an all-solid-state battery (ASSB) was directly tracked by in situ scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDX). Contact between an electrode active material and a solid electrolyte is important for improving the properties of ASSBs as a promising next-generation battery. An in situ analysis is significant for establishing strategies to obtain sufficient contact areas between the active material and solid electrolyte particles. SEM-EDX has the advantages of in-situ measurement in spatial/time resolution, non-destruction, and versatility. We investigated the sensitivity of EDX to the Na signal and distinguishable distance to ensure sufficient spatial/time resolution. The acceleration voltage of 5 kV for the electron beam provided the highest sensitivity to the Na signal among all acceleration voltages. The distinguishable distance decreased with increasing magnification owing to the decrease in pixel size. Cross-sectional SEM-EDX images of the TiS₂–Na₃PS₄/Na₃PS₄/Na–Sn cell were collected during charge/discharge. The time variation of Na signal intensity confirms the deintercalation of Na⁺ in the TiS₂–Na₃PS₄ cathode layer. Moreover, intercalation on the solid electrolyte side proceeded faster than that on the current collector side. This was because the rate-determining step was ionic conductivity rather than electronic conductivity based on the difference between ionic and electronic conductivities. Ex situ observations detected only a uniform distribution in the composite after Na⁺ diffusion had relaxed. Operando SEM-EDX is a new tool to directly explore the intermediate conditions of electrode materials under ASSB operation.