The pressing demand for high-performance energy storage in today's sustainable energy applications, especially electric vehicles and power grids, requires both conceptual breakthroughs and practical developments of alkali-ion (e.g. Li+ or Na+) batteries . In the past few years, oxygen (O) redox in battery electrodes is a novel conceptual breakthrough beyond conventional transition metal (TM) redox, and has been considered as a promising strategy to enhance the energy and power density. Several critical challenges regarding both the fundamental understanding and practical utilization of O redox still remain formidable . While electrochemical and structural characterizations have been extensively conducted, the O redox reaction is actually an electronic process, and thus a reliable and direct characterization of the O electronic states is essential. Though soft X-ray absorption spectroscopy (sXAS) is an elemental and chemical sensitive probe to 3d TMs and has been widely utilized in the investigations of 3d TM redox , it suffers intrinsic limitations on characterizing the O states in battery electrodes .
In this project, we study the O redox via advanced mapping technique of resonant inelastic X-ray scattering (mRIXS). Technically, we clarify the superiority of mRIXS over sXAS on fingerprinting the non-divalent O states, and establish a spectroscopic method based on mRIXS to quantitatively analyze the O redox. Fundamentally, we take a scrutiny on the O redox process during electrochemical cycling, and reveal several important scientific discoveries regarding the reversibility, performance decay, voltage hysteresis and kinetics. Practically, we systematically explore the possible approaches for tuning and improving the reversibility of the oxygen redox, which would potentiate the utilization of this novel strategy in the battery electrodes.
 Goodenough, J.B. Energy storage materials: A perspective. Energy Storage Materials 1, 158 (2015)
 Assat, G., and Tarascon, J.-M. Fundamental understanding and practical challenges of anionic redox activity in Li-ion batteries. Nature Energy 3(5), 373 (2018)
 Yang, W., and Devereaux, T.P. Anionic and cationic redox and interfaces in batteries: Advances from soft X-ray absorption spectroscopy to resonant inelastic scattering. Journal of Power Sources 389, 188 (2018)
 J. Wu et al. Modification of transition-metal redox by interstitial water in hexacyanometalate electrodes for sodium-ion batteries. Journal of the American Chemical Society 139(50), 18358 (2017)
 J. Wu et al. Dissociate lattice oxygen redox reactions from capacity and voltage drops of battery electrodes. Science Advances 6(6) (2020)
 K. Dai et al. High reversibility of lattice oxygen redox quantified by direct bulk probes of both anionic and cationic redox reactions. Joule 3(2), 518 (2019)