An Original Empirical Method for Simulating V L_2,3 Edges: The Example of KVPO₄F and KVOPO₄ Cathode Materials

Reversible chemical reactions are the most common mechanism storing electrochemical energy in M-ion batteries (M = Li, Na, K....). At the positive electrode, these transformations consist in the solid-state oxidation or reduction of transition metal ions going along with the reversible (de)intercalation of an alkali cation from the crystal structure. X-ray spectroscopies are among the most suitable tools to unveil and monitor these reactions. The interpretation of experimental spectra, however, is not trivial. This is particularly true in V-based positive electrodes since the variety of oxidation states of vanadium as well as its coordination and bonding geometries may lead to complex spectroscopic features that call for ab initio modeling to understand the spectra. Here we show not only that V L2,3-edge X-ray Raman spectra can effectively be modeled by a full ab initio approach but also that the empirically obtained Hamiltonian parameters can reproduce shape and intensity of the experimental spectra from a given coordination geometry. In a broader context, our study shows that inexpensive empirical calculations provide highly reliable information and help solve the electronic structure of transition metal oxides compounds, which governs the electrochemical behavior in M-ion batteries. The promising results shown here underline the efficiency of this strategy for X-ray spectroscopy data analysis, which can be generalized and extended to the wider family of vanadium phosphate-based polyanionic compounds. Such an approach can in principle be extended to any transition metal-based material.