Surface structure evolution of LiMn₂O₄ cathode material upon charge/discharge

Surface dissolution of manganese is a long-standing issue hindering the practical application of spinel LiMn₂O₄ cathode material, while few studies concerning the crystal structure evolution at the surface area have been reported. Combining X-ray photoelectron spectroscopy, electron energy loss spectroscopy, scanning transmission electron microscopy, and density functional theory calculations, we investigate the chemical and structural evolutions on the surface of a LiMn₂O₄ electrode upon cycling. We found that an unexpected Mn₃O₄ phase was present on the surface of LiMn₂O₄ via the application of an advanced electron microscopy. Since the Mn₃O₄ phase contains ¹/₃ soluble Mn²⁺ ions, formation of this phase contributes significantly to the Mn²⁺ dissolution in a LiMn₂O₄ electrode upon cycling. It is further found that the Mn₃O₄ appears upon charge and disappears upon discharge, coincident with the valence change of Mn. Our results shed light on the importance of stabilizing the surface structure of cathode material, especially at the charged state. The understanding of the manganese dissolution reaction that occurs in the LiMn₂O₄ can certainly be extended to other oxide cathodes.