Disentangling the ligand and electronic structure in KVPO4F1-xOx positive electrode materials by valence-to-core X-ray emission spectroscopy

Understanding the intricate crystal structure of polyanionic positive electrode materials is essential for elucidating the mechanisms involved during cycling and predicting the working potential of the electrode. To achieve this goal, a clear comprehension of how the coordination environment of the transition metal ion influences the ionicity/covalency of the metal-ligand bonds is necessary. Yet, discriminating these ligands poses challenges due to the limited sensitivity to light elements of common characterization techniques such as X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) analysis. To address this issue, we employed valence-to-core Kβ X-ray emission spectroscopy combined with ab initio modelling and conducted a systematic investigation using potassium vanadium oxyfluoride phosphate compounds with the general formula KVPO4F1-xOx (x = 0, 0.25, 0.5, 0.75, 1). Our approach allows distinguishing the contributions of V-F, V-O, and V=O bonds at the Kβ" region, with intensities highly correlated to the F− and O2− anion composition. Additionally, the evolution of the features at the Kβ2,5 region is highly correlated to the presence of short V=O bonds, strongly influencing the electrode potential of the material. Density of state (DOS) analysis based on ab initio modelling of the end member compounds KVPO4F and KVOPO4 further supports the existence of V-F and V=O bonding through the mixing of F/O p-DOS with V d-DOS. Overall, we present a detailed and reliable approach for understanding the occupied electronic states of the bulk material, proving valuable for a thorough comprehension of the structure of positive electrode materials in batteries.