The pressing demand of high-capacity and high-power batteries leads to extensive efforts on various material and technical developments, such as high-voltage electrodes and anionic redox. In these systems, batteries often operate beyond the thermodynamic stability of the battery materials, which leads to unusual surface/interface behavior and bulk redox reactions. The mechanism of such complex systems is often under debate and requires incisive tools for measuring the chemical states of specific elements. Recent developments of soft X-ray spectroscopy (SXS), especially new types of soft X-ray absorption (sXAS) and resonant inelastic X-ray scattering (RIXS), have provided us an inherently elemental and chemical sensitive technique for probing both the surface/interface and the bulk (100 nm probe depth) materials of batteries.
The focus of this presentation is to provide extensive examples on probing the key electron and oxidation states of both the transition metals and oxygen for studying the battery electrodes and interfaces. In particular, recent technical developments of various sXAS detection channels extracted from high-effeciency RIXS, including high-resolution inverse partial fluorescence yield (iPFY) and super partial florescence yield (sPFY), will be introduced. We will discuss the methodology for quantitatively define the oxidation states of transition metals through sXAS and calculations. For some novel transition metal redox reactions that cannot be resolved by conventional sXAS, RIXS provides extra sensitivity to clarify the mechanism by further resolving the energies of emitted photons. For light elements involved in battery interfaces, we show that sXAS is also a sensitive probe for specific chemical bonds. We then showcase some recent examples on clarifying the intrinsic oxygen state evolution (oxygen redox) in battery materials through soft X-ray RIXS. In summary, due to the energy range of the dipole-allowed transitions in both the low-Z elements, e.g., C, N, O, and 3d Transition metals, SXS is the most direct probe of the valence states of these elements, which provides unique information on the chemical states of battery electrodes and interfaces.