Precisely quantifying bulk transition metal valence evolution in conventional battery electrode by inverse partial fluorescence yield

Precisely quantifying transition metal (TM) redox in bulk is a key to understand the fundamental of optimizing cathode materials in secondary batteries. At present, the commonly used methods to probe TM redox are hard X-ray absorption spectroscopy (hXAS) and soft X-ray absorption spectroscopy (sXAS). However, they are both facing challenges to precisely quantify the valence states of some transition metals such as Mn. In this paper, Mn-L iPFY (inverse partial fluorescence yield) spectra extracted from Mn-L mRIXS (mapping of resonant inelastic X-ray scattering) is adopted to quantify Mn valence states. Mn-L iPFY spectra has been considered as a bulk-sensitive, non-distorted probe of TM valence states. However, the exact precision of this method is still unclear in quantifying practical battery electrodes. Herein, a series of LiMn2O4 electrodes with different charge and discharge states are prepared. Based on their electrochemical capacity (generally considered to be very precise), the precision of Mn iPFY in quantifying bulk Mn valence state is confirmed, and the error range is unraveled. Mn-L mRIXS iPFY thus is identified as one of the best methods to quantify the bulk Mn valence state comparing with hXAS and sXAS.