Oxide Coating Role on the Bulk Structural Stability of Active LiMn₂O₄ Cathodes

The protective coating of the electrode materials is a known source of improvement of the cycling performances in battery devices. In the case of the LiMn₂O₄ cathodes, the coating with a thin alumina layer has been proven to show performance efficiency. However, the precise mechanism of its effect on the performance improvement of the electrodes is still not clear. In this work we investigate alumina-coating-induced effects on the structural dynamics of the active materials in correlation to the modified solid electrolyte interface dynamics. The local structures of coated and uncoated samples at different galvanostatic points are studied by both soft X-ray absorption measurements at the Mn L-edges and O K-edge (in total electron yield mode) and hard X-ray absorption at the Mn K-edge (in transmission mode). The different probing depths of the employed techniques allowed us to study the structural dynamics both at the surface and within the bulk of the active material. We demonstrate that the coating successfully hinders the Mn³⁺ disproportionation and, hence, the degradation of the active material. Side products (layered Li₂MnO₃ and MnO) and changes in the local crystal symmetry with formation of Li₂Mn₂O₄ are observed in uncoated electrodes. The role of alumina coating on the stability of the passivation layer and its consequent effect on the structural stability of the bulk active materials is discussed.