Unusual Spinel-to-Layered Transformation in LiMn₂O₄ Cathode Explained by Electrochemical and Thermal Stability Investigation

Distorted surface regions (5-6 nm) with an unusual layered-like structure on LiMn₂O₄ cathode material were directly observed after it was cycled (3-4.9 V), indicating a possible spinel-to-layered structural transformation. Formation of these distorted regions severely degrades LiMn₂O₄ cathode capacity. As we attempt to get a better understanding of the exact crystal structure of the distorted regions, the structural transformation pathways and the origins of the distortion are made difficult by the regions' nanoscopic size. Inspired by the reduction of Mn⁴⁺ to Mn³⁺ in surface electronic structures that might be associated with oxygen loss during cycling, we further investigated the atomic-level surface structure of LiMn₂O₄ by heat-treatments between 600 and 900 °C in various atmospheres, finding similar surface spinel-to-layered structural transformation only for LiMn₂O₄ heat-treated in argon atmosphere for a few minutes (or more). Controllable and measurable oxygen loss during heat-treatments result in Mn³⁺ for charge compensation. The ions then undergo a disproportionation reaction, driving the spinel-to-layered transformation by way of an intermediate LiMn₃O₄-like structure. The distortion of the surface regions can be extended to the whole bulk by heat-treatment for 300-600 min, ultimately enabling us to identify the bulk-level structure as layered Li₂MnO₃ (C2/m). This work demonstrates the critical role of Mn³⁺ in controlling the kinetics of the structural transformation in spinel LiMn₂O₄ and suggests heat-treatment in argon as a convenient method to control the surface oxygen loss and consequently reconstruct the atomic-level surface structure.