Depth profile analysis of a cycled lithium ion manganese oxide battery electrode via the valence state of manganese, with soft X-ray emission spectroscopy
A 50-μm thick lithium manganese oxide (parent material LiMn2O4) battery electrode (positive electrode; cathode) was charged, slightly discharged and then sliced with a scotch tape test-type method. A selected number of slices was then subject to synchrotron soft X-ray emission spectroscopy near the Mn Lα,β emission lines in order to determine changes in the oxidation state of the manganese as a function of sampling depth. The emission spectra showed a minute yet noticeable and systematic chemical shift of up to 0.25 eV between the layer near the current collector and the layer near the electrolyte separator. The average manganese oxidation state near the separator was smaller than the average oxidation state in the interior of the electrode, or near the current collector. Since the data provide an oxidation state depth profile of the cathode, a Li+ depth profile can be inferred. This method provides information on the spatial chemical inhomogeneity of electrodes prior to and after electrochemical cycling, and thus can aid in degradation studies.