Operando X-ray Transmission Microscopy of Dynamic Concentration Change of Electrolyte Salt in Composite Electrodes of Lithium-ion Battery

• Operando X-ray microscopy visualizes electrolyte salt redistribution in LIBs. • Cross-sectional imaging reveals salt dynamics during high-rate discharge. • High-rate discharge induces transient salt accumulation in porous anodes. • Concentration gradients relax by diffusion after the current is stopped. Spatiotemporal evolution of electrolyte salt concentration in lithium-ion battery composite electrodes during charge-discharge is investigated using synchrotron-based operando X-ray transmission microscopy. A custom-designed operando electrochemical cell enables cross-sectional imaging of the anode and separator regions under fast charge-discharge battery operating conditions. By exploiting the absorption contrast of the PF 6 − anion, relative changes in electrolyte salt concentration are visualized in real time. During high-rate discharge, lithium ions are released from the anode, leading to transient salt accumulation inside the porous electrode and the formation of pronounced concentration gradients. In contrast, concentration variations across the separator are smaller and relax more rapidly. The measurements further reveal that such concentration inhomogeneities are strongly rate-dependent and persist on time scales comparable to fast cycling, but gradually relax once the current is stopped. These results demonstrate that electrolyte concentration polarization dynamically develops during charge-discharge and can significantly influence ionic transport and cell performance, highlighting the critical role of electrolyte transport properties in high-rate lithium-ion batteries.