Anionic Redox In Lithium Ion Batteries Studied By Soft X-ray Spectroscopy

It is critical to develop high energy-density,and low-cost lithium ion batteries to effectively address energy and environmental challenges.Stable and reversible anionic redox is meaningful in practice for enhancing the energy density of a battery.Unfortunately,the fundamental understanding on anionic redox is still ambiguous and under active debate.It is crucial to decipher the fundamental mechanism of anionic redox to achieve high energy density cathode materials of lithium ion batteries.The key to understand the fundamental mechanism of anionic redox is to provide direct experimental evidence of oxidized oxygen state upon the electrochemical cycling.Synchrotron based soft X-ray spectroscopy can directly probe the electronic states in the vicinity of the Fermi level.In this work,facilitated by soft X-ray spectroscopy,we performed a comprehensive analysis on oxygen redox activities in the transition metal oxides systems.Firstly,we surveyed 47 different transition metal oxides with O-K sXAS,and concluded that the TM-O hybridization feature dominates in the O-K sXAS,suggesting it was deceptive to probe anionic redox activities through O-K sXAS.The O-K sXAS also provided the chance to plot different transition metal states on the same common energy scale,mapping out redox sequence of transition metals.Secondly,we provided a benchmark of mRIXS,coupling with simulations,for fingerprinting the different oxygen states.A specific intra-band excitonic feature was found in the mRIXS,which referred to partially unoccupied O-2p states in oxidized oxygen systems.The aforementioned specific mRIXS feature provided much prospect of probing and understanding anionic redox in transition metal oxides electrode materials.Thirdly,mRIXS was employed to resolve the intrinsic nature of oxygen redox activities of Li2RuO3.The aforementioned specific excitation feature was observed in the RIXS spectra,which referred to oxidized oxygen state.Moreover,the excitation feature was reversible following the electrochemical cycling.It is believed that negative charge transfer,which reconstructs ground state electronic configuration and leads to oxidized oxygen state,is the fundamental mechanism of oxygen redox.Fourthly,facilitated by mRIXS technique,a comprehensive analysis was performed on cationic and anionic redox activities in the model system Li₂MnO₃.Irreversible oxygen reaction activities,other than reversible lattice oxygen redox,took place in the initial charging process.Mn redox was activated in the extended cycling,and became more and more activated.Our work suggested there was an intrinsic nature difference between the reversible lattice oxygen redox and irreversible oxygen reaction.