Oxidation state of cross-over manganese species on the graphite electrode of lithium-ion cells

It is well known that Li-ion cells containing manganese oxide-based positive electrodes and graphite-based negative electrodes suffer accelerated capacity fade, which has been attributed to the deposition of dissolved manganese on the graphite electrodes during electrochemical cell cycling. However, the reasons for the accelerated capacity fade are still unclear. This stems, in part, from conflicting reports of the oxidation state of the manganese species in the negative electrode. In this communication, the oxidation state of manganese deposited on graphite electrodes has been probed by X-ray absorption near edge spectroscopy (XANES). The XANES features confirm, unequivocally, the presence of fully reduced manganese (Mn⁰) on the surface of graphite particles. The deposition of Mn⁰ on the graphite negative electrode acts as a starting point to understand the consequent electrochemical behavior of these electrodes; possible reasons for the degradation of cell performance are proposed and discussed.