Electrochemical Response and Structural Stability of the Li+ Ion Battery Cathode with Coated LiMn2O4 Nanoparticles

Mn dissolution is the main drawback of LiMn2O4 cathodes, leading to capacity fading and anode poisoning. It is well known that improved capacity/cycling performances have been obtained by the Al2O3 coating. It is less clear what is the effect of the coating from the point of view of the fundamental processes occurring within the active material and on the interface with the active material, especially during the first cycle, when a dynamical interaction at a high voltage with an electrolyte and a binder leads to the formation of a passivation layer. We present here the close comparison of coated and uncoated electrodes' X-ray absorption analysis at the interface during the measurements of several charged/discharged states of the electrode. The Al2O3 coating is significantly effective for stopping the high voltage instability of the battery, especially, when the Mn–O couple reacts with organic species, limiting Mn capture and the electrolyte reaction with the oxide surface. In the low-voltage discharge, on the other hand, more complex structure/electronic modifications occur. The presence of the coating limits disproportionation, preventing a general corrosion with dissolution of the Mn2+ species, and hence improves the electrode performance. From the structural point of view, the signatures of the transformations and a reversible modification of the surface character of the nanoparticles from a spinel to a defective phase are observed, while no charge transfer between the coating and manganese oxide is found. The role of nonthermodynamic interphase formation by means of proton transfer is enhanced for the coated oxide particles.