Operando Time-of-Flight Secondary Ion Mass Spectrometry Visualization of Reversible and Irreversible Reactions in Oxide-Based Solid-State Batteries

Operando time-of-flight secondary ion mass spectrometry (TOF-SIMS) was applied to solid-state batteries (SSBs) to visualize the dynamic electrode reactions. The SSBs consist of a LiNi0.8Co0.15Al0.05O2 (NCA) composite positive electrode, a Li4Ti5O12 (LTO) composite negative electrode, and a Li2OHCl0.9F0.1/Li0.31La0.56TiO3 composite solid electrolyte, which were fabricated by a pressure-assisted melt infiltration method. Reversible changes in 6Li+, 58Ni+, and 48Ti+ intensities were observed during charge–discharge processes. The intensity change of transition metal secondary ions is attributed to changes in ionization efficiency (matrix effects) and correlates with the state of charge. Reaction inhomogeneity was detected in the NCA composite electrode, indicating that the solid electrolyte infiltration has a positive effect on the electrochemical performance, whereas the LTO electrode exhibited uniform reaction distribution. These findings demonstrate that operando TOF-SIMS can provide critical insights into reaction distribution and irreversible capacity in SSBs, thereby guiding the optimization of electrode architectures.