TEM Observation of the Deterioration and Thermal Recovery Process of Argyrodite-type Solid Electrolytes under Dry-Room-Simulated Condition

All-solid-state lithium-ion batteries exhibit even higher energy densities owing to their flame retardancy and high thermal stability. Sulfide-based solid electrolytes (SEs) have recently attracted attention due to their high ionic conductivity comparable to that of an organic electrolytic solution [1]. However, their low moisture stability is a common disadvantage. Air exposure causes a significant decrease in their ionic conductivity, together with a toxic H2S gas generation. This is a major barrier to all-solid-state cells fabrication process cost and its application to electric vehicles. Clarifying their deterioration mechanism during air exposure is necessary to develop sulfide-based SEs with high moisture resistance. As a sulfide-based SE, in this study, we focused on the argyrodite. Argyrodite Li6PS5Cl exhibits high ionic conductivities of 10−2–10−3 S cm−1 at room temperature as well as a wide electrochemical window of up to 7.0 V vs. Li/Li+ [2]. The all-solid-state cells using Li6PS5Cl and positive electrode materials such as Li2S and LiNi1/3Co1/3Mn1/3O2 exhibit high electrochemical performance [3, 4]. In addition, Morino et al. have recently reported on good low-temperature performances of the all-solid-state half cells using argyrodite-structured SEs (Li7-xPS6-xClx) and LiNi0.5Co0.2Mn0.3O2 [5]. In order to further improve the cell performance, it is necessary to understand the degeneration mechanism in a humidity environment such as dry rooms for battery manufacturing. To clarify the deterioration mechanism under humid conditions, we conducted transmission electron microscope (TEM) observations in investigating the microstructure and morphology of argyrodite Li7-xPS6-xClx (x≈1, D50 = 3.5 um, MITSUI MINING & SMELTING, Japan) [6, 7]. Fig. 1 shows the TEM observation results for the pristine samples. The electron (ED) pattern in the inset was obtained from the particles in the bright-field image, as the yellow circles indicated. Particle size is approximately 2 µm. The ED pattern for the pristine Li7-xPS6-xClx exhibits [110]-zone-axis single-domain pattern, indicating high crystallinity. Fig. 2 shows the ex-situ TEM observation results after exposure to moisture at −20°C d.p. for 1 h. The ED pattern in Fig. 2(b) shows Debye-Scherrer rings comprising diffraction spots, different from Fig. 1. Diffraction peaks in the corresponding intensity profile are consistent with the simulated X-ray diffraction (XRD) pattern of argyrodite. The HCDF image in Fig. 2(a) was taken using Debye-Scherrer rings of argyrodite in Fig. 2(b). Bright-contrast areas in the image denote argyrodite crystallites. The crystallite size is approximately 20–80 nm, as shown by the arrows in Fig. 2(a), which is considerably smaller than the pristine sample. That is, the crystallinity deteriorates when exposed to moisture at −20°C d.p.. In the presentation, the correlation between ionic conductivity, and the deterioration and thermal recovery process of Li7-xPS6-xClx were discussed based on the TEM observation results. Since a degenerated sulfide-based SE was reported to recover its conductivity upon heating [8], we also examined the thermal recovery process of Li7-xPS6-xClx [9]. BF images and the corresponding ED patterns of the pristine Li7-xPS6-xClx sample. The ED patterns in the inset were obtained from the areas denoted by the circles in the BF images. (a) HCDF image and (b) the corresponding ED intensity profile of the Li7-xPS6-xClx sample after exposure to moisture of −20°C d.p. for 1 h.