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.

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Charging a spinel LiMn2O4cathode material to high voltage (>4.3 V) is a convenient way to obtain more lithium ions for formation of an anodic solid-electrolyte-interface in a full cell.

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Li-rich layered Li1+xMnyM1-x-yO2 (or denoted xLi2MnO3·(1 -x)LiMO2, M = Ni, Co, Mn, etc.) are promising cathode materials for high energy-density Li-ion batteries. However, their commercial applications suffer from problems such as a drop in the capacity and discharge voltage during cycling. In this work, the cycling performance of a layered oxide Li1.2Ni0.13Co0.13Mn0.54O2 is improved by integration with spinel LiNi0.5Mn1.5O4 to obtain a layered-spinel composite. Characterization by powder X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) as well as cyclic voltammetry (CV) indicates that delayed degradation of layered Li2MnO3 and the suppressed growth of LiMn2O4-like spinel are responsible for the performance improvement.