Protracted Relaxation Dynamics of Lithium Heterogeneity in Solid-State Battery Electrodes

The lithium (Li) heterogeneity formed in the composite electrodes has a significant impact on the performance of solid-state batteries (SSBs). Whereas the influence of various factors on the Li heterogeneity, such as (dis)charge currents, ionic and/or electronic conductivity of the constituent materials, and interfacial charge transfer kinetics, is extensively studied, the influence of the relaxation on the Li heterogeneity in SSB electrodes is largely unexplored, despite its unignorable impact on the battery performance. Here, we performed a three-dimensional operando evaluation of the relaxation dynamics of the electrode-scale Li heterogeneity in a composite SSB electrode under open-circuit conditions after charging using the computed tomography combined with X-ray absorption near-edge structure spectroscopy (CT-XANES). In contrast to the electrode for the liquid-based Li-ion batteries, the Li heterogeneity formed in the composite SSB electrode during charging was not fully relaxed, even after a long open-circuit hold, leaving both higher and lower Li content regions. Such protracted relaxation dynamics in the composite SSB electrode may be due to the high interfacial resistance between active material particles as well as between active material and solid electrolyte particles and is potentially an essential issue for SSBs. This work demonstrated that our CT-XANES technique can three-dimensionally resolve the relaxation dynamics of Li heterogeneity within SSB electrodes, which has only been analyzed indirectly by conventional electrochemical methods such as electrochemical impedance spectroscopy. Our technique can be a valuable tool for identifying detrimental factors affecting the battery performance, ultimately contributing to the development of high-performance SSBs.