SOC Imbalance of the Electrodes in All-Solid-State LTO/LCO Batteries Examined by Three-Electrode Cell
Authors: Furukawa Kazuki , Yamada Masayuki , Ariyoshi Kingo 
Published: December 2024

All-solid-state lithium-ion batteries (LIBs), which have overwhelmingly higher thermal stability and longer calendar life compared to liquid-based LiBs, are suitable for environments where conventional liquid-based LIBs cannot be used, leading to dramatically expand their applications. In all-solid-state LIBs, sulfide-type solid electrolytes have been most extensively examined owing to their excellent conductivity and flexibility. Although applying the solid electrolytes enables to stable operation of the batteries even at 100oC, capacity moderately decreases during cycling under such severe conditions.
This study examines high-temperature long-term cycle tests of all-solid-state Li[Li1/3Ti5/3]O4/LiCoO2 (LTO/LCO) batteries in order to clarify the origin of capacity fading at high temperature. For cycle tests, a three-electrode cell (MX-3E-5T-CWS, Tomcell, Japan) were used to monitor state-of-charges (SOCs) of LCO positive and LTO negative electrodes during cycling, because the imbalance of SOCs of the electrodes causes capacity fading even without material deterioration. All-solid LIBs used in this study were consisted of LTO negative and LCO positive electrodes, and argyrodite-type solid electrolyte (Li7-xPS6-xClx). The LTO and LCO electrodes were consisted of LTO:SE = 65:35 wt% and LCO:SE:AB = 73:26:1 wt%, respectively. The size of the electrodes was the same (9 mm diameter and 0.85 mm 9 thickness) and electrode weight was 0.10 g for LTO and 0.15 g for LCO electrodes. Li metal placed on the solid electrolyte was used as the reference electrode.
Fig. (a) shows charge-discharge curves of the LTO/LCO cell before and after the cycle test. After cycling, the charge-discharge curves of both the LCO and LTO electrodes shifted to the right (higher capacity side), and the capacity decreased by the difference in the amount of shift between the two electrodes. This means that the capacity fading of the all-solid-state battery is caused by the imbalance of SOCs between LCO positive and LTO negative electrode, which is essentially the same capacity fading mechanism as that of liquid-based LIBs.
To quantitatively analyze the SOC imbalance, side reaction current at both electrodes were estimated from a cumulative capacity plot (Fig. b). In the figure, the derivative of cumulative capacity at the end of charge and discharge corresponds to the side reaction current at LTO negative and LCO positive electrodes, respectively. As can be seen in Fig. (b), larger side reaction current at LCO than LTO electrodes indicates SE decomposition at LCO is faster than that at LTO. The capacity fading mechanism due to side reactions in all-solid-state LTO/LCO batteries is schematically illustrated in Fig. (c). Because of the formation of a local cell at each electrode consisting of Li insertion and side reactions, the current due to the Li insertion reaction, namely current efficiency differs between the electrodes. The difference in current efficiency between LTO and LCO electrodes induces the deviation of the SOC balance between the electrodes, resulting in a capacity fading of the battery without deterioration of active materials.
From the above results, we will discuss the quantitative prediction of the lifetime for all-solid-state batteries based on the mass balance in the cell.

Figure 1