Oxygen loss and surface degradation during electrochemical cycling of lithium-ion battery cathode material LiMn₂O₄

Electrode surfaces play a critical role in determining the electrochemical performance of lithium-ion batteries, and uncovering how surface chemistry and structure evolve during cycling, particularly at the atomic level, is necessary for improved battery materials design. We report a scanning transmission electron microscopy (STEM) investigation into the atomistic mechanisms behind surface reconstruction induced by electrochemical cycling of cathode material LiMn₂O₄. Direct STEM observations reveal that surface layers of as-synthesised LiMn₂O₄ thin films are subject to considerable compressive lattice strain as a result of oxygen deficiency. During the first charge, the lattice strain increases significantly, resulting in a reconstruction reaction to form Mn₃O₄ with further loss of oxygen from the topmost layers. Continued cycling leads to deterioration of surface crystallinity. The observed irreversible structure changes affect charge transfer reaction kinetics at LiMn₂O₄ surfaces because Li pathways become blocked by Mn atoms, contributing to a reduction in long-term cycle life and energy capacity. The ability to observe atomic-level changes at electrode surfaces at different stages of cycling provides a more robust understanding of electrode processes that can accelerate development of safer and longer-lasting battery materials.