As lithiation occurs alongside oxidation, we track Mn oxidation state via Mn K-and L-edge XAS. In situ K-edge difference spectra (SI Figure ) agree with TGA that the most rapid lithiation (and oxidation) occur between 300 °C and 500 °C. Given experimental (temperature > 800 °C was not accessible) and analytical (self absorption) limitations with in situ XAS, we turned to ex situ characterization of quenched samples for more quantitative analysis. Details of oxidation state quantification from Mn K-edge and L-edge XAS data are available in SI Figure .