Anion Activity and Metastable Phase Formation in Li FePO Investigated Using Soft-to-Hard X-ray Absorption and Emission Spectroscopy.

We systematically investigate the intricate roles of cations as well as anions during phase transformation, specifically, the formation of a metastable phase in phospho-olivine, LiFePO (LFP). The cation- and anion-specific electronic structures are studied using a combination of high-resolution soft-to-hard X-ray absorption and emission spectroscopy. Our findings reveal that the formation of the metastable phase at higher states-of-charge (SoC) is associated with a decreased oxidation state of iron, assisted by oxygen release. Additionally, we find that phosphorus is active in the charge process, exhibiting reduction, resulting from an electron density redistribution between oxygen and its neighboring iron and phosphorus atoms. Furthermore, the phase transformation process in LFP impacts its magnetic properties, with iron retaining its high-spin configuration along with an increased average spin during its transformation into FePO (FP).