Degradation Mechanisms of All-Solid-State Lithium-Ion Batteries with Sulfide-Type Electrolytes Based on High-Temperature Studies

Abstract

Introduction Sulfide-based all-solid-state Li-ion batteries (LIBs) are potential next-generation energy storage devices due to their increased safeties and energy densities compared to those of conventional LIBs with liquid electrolytes. However, our understanding of the durabilities and degradation mechanisms of such batteries remains limited. Via industry-academia collaboration, a project that aims to develop a sulfide-based all-solid-state LIB for use in electric vehicles (NEDO “Development of Fundamental Technologies for All-Solid-State Battery Applied to Electric Vehicles (SOLiD-EV),” 2018-2023) has been undertaken in Japan. Under this project, we conducted high-temperature cycling and storage studies using a SOLiD-EV prototype cell to gain insight into the durability and degradation mechanism of the sulfide-based all-solid-state LIB. [1] Experimental The SOLiD-EV prototype cell, which was a developed laminate-type single-layer all-solid-state LIB with a capacity of approximately 8 mAh, was assembled by combining LiNbO3-coated LiNi0.5Co0.2Mn0.3O2 (NCM, Sumitomo Metal Mining, Tokyo, Japan) as the positive active material, graphite (Mitsubishi Chemical Holdings, Tokyo, Japan) as the negative active material, and argyrodite-structured Li(7–x)PS(6–x)Cl x (x ≈ 1.2 × 10−3 S cm−1; Mitsui Mining & Smelting, Tokyo, Japan) as the solid electrolyte (SE). All assembly processes were conducted in a glove box under an Ar atmosphere with a dew point below −76 °C and an O2 concentration of <1 ppm.In the high-temperature cycling study, the prototype cell was charged at a rate of C/10 to 4.2 V in constant current mode and further charged in constant potential voltage (CC-CV) mode. It was then discharged at a rate of C/3 to reach 3.0 V in constant current (CC) mode at 100 °C. In the high-temperature storage study, the prototype cell was stored in a thermostatic chamber at 60, 80, or 100 °C after CC-CV charging. In both studies, the following performance measurements were conducted at intervals of 7 days: charge/discharge evaluations at the same current condition as that used in the high-temperature cycling study at 25 °C, charge/discharge evaluations at a rate of C/20 at 45 °C, and electrochemical impedance spectroscopy at a depth of discharge (DOD) of 50% at 25 °C. Both studies were conducted for a total of 28 days.X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and inductively coupled plasma atomic emission spectrometry (ICP-AES) component analysis of the SE at each electrode were performed using the prototype cells that were degraded in the high-temperature studies as disassembly analyses. Results and discussion Each evaluation reduces the capacity of the SOLiD-EV prototype cell, and the loss of capacity at high temperatures is faster than that at low temperatures, as shown in Figure 1. This reduction in the capacity during the cycling study at 100 °C is lesser than that during the storage study at 100 °C in the first 7 days; however, the reduction during the cycling study increases after 14 days. The degradation of the electrodes was caused by the repeated charging and discharging of the prototype cell. Increases in internal resistance are also confirmed, and the trend of the temperature dependence is almost identical to the capacity trend. The partial substitution of PS4 3− in Li(7–x)PS(6–x)Cl x by P–S–P, –S–S–, and S components, as revealed by XPS, and the decrease in the Li/P ratio, as revealed by ICP-AES, indicate that the SE (Li(7–x)PS(6–x)Cl x ) is oxidatively decomposed at the positive electrode due to its (noble) potential. S, which is a product of oxidative decomposition, is a poor conductor of Li+ ions and may increase the charge transfer and path resistances in the positive electrode. The substitution of PS4 3− in Li(7–x)PS(6–x)Cl x with Li3P and Li2S and the increase in the Li/P ratio indicate that the SE (Li(7–x)PS(6–x)Cl x ) is reductively dissolved at the negative electrode due to the negative electrode (low) potential. The capacity degradation of the cell is due to the loss of active Li+ ions caused by the reduction of the sulfide electrolyte at the negative electrode. Furthermore, the change in the composition of the SE, as determined via ICP-AES (Figure 2) before and after the high-temperature storage study, suggests that the SE degradation reactions at the positive and negative electrodes are temperature-dependent and roughly follow the Arrhenius law, with different activation energies. Acknowledgement This study was supported by the “Development of Fundamental Technologies for All Solid State Battery Applied to Electric Vehicles” project (SOLiD-EV, JPNP18003) commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Reference [1] K. Ando, T. Matsuda, T. Miwa, M. Kawai, D. Imamura, Battery Energy, 20220052 (2023). Figure 1