Hidden Subsurface Molecular Bubbles in Graphite Anodes for LIBs

The interplay between solvent co-intercalation, solid–electrolyte interface (SEI) formation, and gas evolution at the graphite anode–electrolyte interface plays a critical role in battery performance; yet, it remains poorly understood at the nanoscale. In this study, we introduce ultrasound-based operando atomic force microscopy (AFM), which breaks the spatial-resolution limitation of ultrasound-based techniques, to visualize the dynamics of solvent co-intercalation, SEI formation, and subsurface gas evolution in graphite anodes for lithium-ion batteries. Remarkably, we observe that gas evolution leads to the formation of ‘‘subsurface molecular bubbles’’—gaseous pockets trapped between graphite layers—that compromise interfacial stability during battery formation cycles. AFM and density functional theory calculation results revealed that these subsurface molecular bubbles are primarily induced by the co-intercalation and decomposition of Li+(EC)4 solvation complexes. We also found that the solvent co-intercalation and interlayer decomposition effects can be fully suppressed by incorporating a low-permittivity, non-solvating diluent solvent (fluoride benzene) through optimizing the de-solvation energy and the interfacial molecular architectures. By applying this optimized electrolyte in both graphite/Li half-cells and lithium cobalt oxide (LCO)/graphite full-cells, we achieve stable cycling with negligible molecular bubble formation, compact SEI growth, and high coulombic efficiency (493%) during high-rate (0.5C) battery formation