Resonant inelastic X-ray scattering and X-ray absorption spectroscopy on the negative electrode material Li_0.5Ni_0.25TiOPO ₄ in a Li-ion battery

We have studied the first lithiation/delithiation cycle of the Li-ion battery electrode material Li_xNi_0.25TiOPO₄ applying X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS). A set of ten identical Li_xNi _0.25TiOPO₄ battery electrodes have been cycled and left in different states of charge in the range of x = 0.5 ... 2.5, before disassembly in an Ar filled glove box. We find that Ni-, Ti-, and O-ions are affected simultaneously, rather than sequentially, upon lithiation of the material. In particular, Ni is reduced from Ni²⁺ to Ni⁰ but only partially re-oxidized to Ni¹⁺, again, by delithiation. Overall, there is considerable "crosstalk" between the different atomic species and non-linearity in the response of the electronic structure during the lithiation/delithiation process. Fortuitously, the background variation in Ni L-XAS shows to contain valuable information about solid-electrolyte interface (SEI) creation, showing that the SEI is a function of the degree of lithiation.