Deterioration Mechanisms Due to Exposing to Humidity-Controlled Air in All Solid-State Battery Revealed By TEM
Authors: Mori Shigeo , Tsukasaki Hirofumi 
Published: December 2024

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 durability.
As a promising SE for sulfide-based all-solid-state batteries, we focused on the argyrodite. Argyrodite-type crystals Li6PS5X (X = Cl, Br or I) have high ionic conductivities of 10−2–10−3 S cm−1 at room temperature [2, 3]. In particular, Li6PS5Cl is reported to have a wide electrochemical window of up to 7.0 V vs. Li/Li+ [3]. Furthermore, the all-solid-state cells using Li6PS5Cl and positive electrode materials such as Li2S and LiNi1/3Co1/3Mn1/3O2 exhibit high electrochemical performance [4, 5]. Thus, Li6PS5Cl is a promising candidate SE for all-solid-state sulfide-based batteries. However, the moisture stability and structural changes it undergoes when exposed to humidified air are still unknown.
Recently we developed a transmission electron microscope (TEM) system to evaluate the air stability of battery materials. Using a vacuum transfer double-tilt TEM holder with a gas-flow system, the in situ observation of the degradation process was conducted for a sulfide-based Li4SnS4 glass ceramic under an air-flow environment. In this study, we conducted transmission electron microscope (TEM). observations in investigating the microstructure and morphology of argyrodite Li7-aPS6-aCla (a»1, D50 = 3.5 um, Mitsui Mining & Smelting, Japan) to understand its deterioration mechanism during air exposure. Figure 1(a) shows the electron diffraction pattern and its intensity profile after exposure to moisture at −20°C d.p. for 24 h. Diffraction peaks of Li2CO3 and LiCl are detected. These reaction products indicate that Li7-aPS6-aCla reacts with CO2 and H2O in the air [6]. In addition. We took the corresponding hollow-cone dark-field (HCDF) images and investigated the spatial distribution of the reaction products. Figures 1(b) and (c) show HCDF images obtained using the Debye-Scherrer rings of Li2CO3 and LiCl, respectively. Debye-Scherrer rings, which were used to capture the HCDF images, are indicated by the yellow highlighted areas in the ED patterns. Crystallite size is approximately 20–50 nm, as denoted by the bright contrast. Each reaction product is randomly distributed and deterioration seems to progress from the particle surface. In the presentation, the main factor of the decreased ionic conductivity and the deterioration process were discussed based on the reaction products caused by air exposure.
References:
[1] Y. Kato et al., Nat. Energy 1 (2016), 16030. doi: 10.1038/nenergy.2016.30
[2] M. A. Kraft et al., J. Am. Chem. Soc. 139(31) (2017), 10909–10918. doi: 10.1021/jacs.7b06327
[3] S. Boulineau et al., Solid State Ionics 221 (2012), 1–5. doi: 10.1016/j.ssi.2012.06.008
[4] F. Han et al., Nano Lett. 16 (2016), 4521−4527. doi: 10.1021/acs.nanolett.6b01754
[5] J. Zhang et al., J. Power Sources 391 (2018), 73−79. doi: 10.1016/j.jpowsour.2018.04.069
[6] H. Tsukasaki et al., J. Power Sources 524 (2022), 231085. doi: 10.1016/j.jpowsour.2022.231085
Acknowledgement:
The authors thank all of the collaborators; Profs. A. Hayashi, A. Sakuta (Osaka Metropolitan University), Drs. H. Sano, Y. Morino (LIBTEC), Drs. K. Igarashi, A. Wakui, T. Yaguchi (Hitachi High-Tech Corporation). This study is based on results obtained from the project of “Development of Fundamental Technologies for All Solid State Battery applied to Electric Vehicles” (SOLiD-EV, JPNP18003) commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Figure 1