Chemistry Department Colloquium

The pressing demand of improved energy storage systems, especially for electric vehicles and green-grid, calls for speedy strategies for developing materials based on advanced analytic tools. Synchrotron based soft x-ray core-level spectroscopy is one of such incisive tools that probes the key electronic states pertaining to the performance of batteries. This colloquium starts with an in-depth introduction of conventional soft X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) with its applications in detecting the critical electron states in battery materials from binder to electrodes. The experimental results provide both general understandings and quantitative analysis of the transition-metal (TM) reactions at different electrochemical states, through direct probes of the K-edges (2p states) of low-Z elements such as C, O, N, and the L-edges (3d states) of 3d TMs. More importantly, however, we clarify that conventional spectroscopic experiments based on XAS do not really provide the claimed "elemental sensitivity" in either the O-K or the TM-L in the bulk-sensitive photon-in-photon-out mode, thus failing to detect the true signature of the bulk redox reactions of lower TMs, and especially, Oxygen [1]. This naturally requires advanced spectroscopic probes beyond conventional XAS for disentangle the mixed signals in oxides. We show that high-efficiency mapping of resonant inelastic X-ray scattering (mRIXS) beautifully solves the problems in both TM-L and O-K edge characterizations, providing clear experimental signatures of both the TM [2] and Oxygen [3] redox that cannot be distinguished in conventional XAS. This colloquium does not focus on technical discussions of a specific scientific study, instead, the focus will be on clarifying the principle and on how to correctly interpreting soft X-ray spectroscopic data instead of following popular misinterpretations. We finally note that recent advances in bo