Structural evolution of Lithium Manganate cathodes during charge/discharge cycles

Surface Nanoparticles modifications have been observed on alumina coated LiMn2O4(LMO) cathodes with respect to the uncoated nanoparticles cathodes during the first charge/discharge cycles. During discharging (lithiation), at low voltages the disproportionation reaction is favoured, scanning electron microscopy has revealed the presence of holes formation in LMO nanoparticles while similar effects were not visible in other phases of the voltammogram. We put in relation this observation with the nonuniform Al2O3coating on the nanoparticles used for the electrode where the channel of ions transport is activated, contrarily to uncoated nanoparticles, showing a deep reaction of the carbon-based electrolyte with the oxide, on a larger extent than usually considered for the cathode electrolyte interphase. The non- thermodynamic proton transfer process at the basis of the present effect was recently predicted by Choi et at [1] by first-principle density functional calculations. The x-ray absorption analysis (Mn L2,3, F and O K-edge) was also able to describe the chemistry at the interphase formed at the high voltages when strong fluorinated by-products are differently observed along with the vertical profile at variance with the coatings. Al2O3 coating results fully effective for stopping the strong reduction processes occurring during charging at a high voltage of the battery, when O-Mn couple react with organic species, making Mn capture and electrolyte reaction with the oxide surface possible, and electrolyte interphase and Mn dissolution might occur. We followed the transition by Raman scattering to detect the signatures of the transformation, observing a reversible modification of the surface character of the nanoparticle from spinel to a layered. In conjunction with soft X-ray spectroscopy study is particularly suitable to detect modification of the active material at the frontier of the nanoparticle, where the interphase between the electrolyte and active nanoparticle is formed, during phases of oxidation/reduction that are crucial for the cyclability and safety of the whole battery.