Resonant Inelastic X-ray Scattering of Energy Materials: from Fundamental Understandings to Practical Developments*

The pressing demand of improving the energy device performance calls for new concepts that require characterization techniques beyond conventional probes. With much improved throughput, ultra-high efficiency mapping of Resonant Inelastic X-ray Scattering (mRIXS) has opened up RIXS as such a powerful technique. This has become a timely and critical solution to characterize the novel electronic states of both transition-metals and oxygen involved in electrochemical devices, i.e., batteries, for energy applications.

In this presentation, we first show that the electrochemical performance of batteries is fundamentally defined by many intrinsic physics parameters of the materials, e.g., spin states, ionization energy, crystal field etc., which could be approached through conventional spectroscopy [1]. However, modern concepts based on novel states in batteries are often difficult to be clarified and require tools that are more incisive. We demonstrate that these unconventional states could be clearly addressed through mRIXS experiments [2]. In particular, mRIXS could distinguish the critical lattice oxygen activities in batteries, and precisely quantify its contributions, reversibility, and cyclability upon electrochemical cycling. mRIXS findings clarify many misunderstandings of the mysterious oxygen activities in battery electrodes and indicate independent oxygen behaviors that are intrinsically determined by the material physics, something that has been indiscreetly taken for granted [3]. At the end, we show that mRIXS results have gone beyond fundamental understandings and led to guidelines for future material developments and optimizations.
[1] Wu et al., JACS 139, 18358 (2017)
[2] Yang & Devereaux, J Power Sources 389, 188 (2018)
[3] (N&V) Yang, Nat Energy 3, 619 (2018)