Synchrotron-Based X-Ray Spectroscopy Study Of Anionic Redox Reaction In Layered Oxide Cathodes For Sodium-Ion Batteries

With the advantages of abundant sodium salt resources and low cost,sodium-ion batteries（SIBs）are considered as one of the most promising candidates for future large-scale energy storage applications.As a key component in SIBs,cathode material affects the energy density,cycle life and safety of SIBs,which plays a decisive role in the overall performance of the batteries.Therefore,it is important to develop high-performance cathode materials for advanced SIBs.Mn-based layered oxide（P2-Na_xMn O₂）is one of the most suitable cathode materials for commercialization of SIBs.More importantly,the anionic redox reaction can be activated in the layered oxide,which can effectively bridge the energy density gap with lithium-ion batteries（LIBs）,thus providing a promising pathway to achieve high-energy cathode materials for SIBs.However,anionic redox is often accompanied by a number of problems,such as irreversible oxygen release,irreversible transition metal（TM）migration,voltage decay,voltage hysteresis and sluggish kinetics.Therefore,the in-depth investigation of the intrinsic reaction mechanism of anionic redox in layered oxides,the effective regulation of the reversibility of anionic redox,and the deeper understanding of the inherent correlation between anionic redox and various side reactions are important for the practical application of cathode materials for SIBs with synergistic anionic and cationic reactions.Based on the above research background,the representative P2-type Mn-based layered oxide is selected as the prototype material,and the corresponding anionic redox reaction is explored by using in-situ/ex-situ multi-dimensional synchrotron spectroscopy characterization techniques,mainly including soft X-ray absorption spectroscopy（s XAS）,hard X-ray absorption spectroscopy（h XAS）and mapping of resonant inelastic X-ray scattering（m RIXS）.We aim to solve the problems of irreversible oxygen release,irreversible TM migration,voltage decay and other side reactions from the root cause,and finally realize the application of cathode materials for SIBs with high energy density and long cycle life.The anionic redox reaction of P2-Na_2/3Ni_1/3Mn_2/3O₂（NNMO）cathode material is firstly studied systematically.NNMO is synthesized as a prototype material by doping electr^ochemically active Ni²⁺into P2-Na_0.67Mn O₂（NMO）,which has attracted much attention owing to the high specific capacity and high air stability.However,NNMO suffers from voltage fade and capacity decay that have not been fully understood so far.By using O K-edge m RIXS,we show that NNMO is charge compensated by irreversible oxygen release rather than lattice oxygen redox reaction during cycling,resulting in capacity decay.In addition,by developing inverse partial fluorescence yield（i PFY）technique,the charge compensation mechanism of the TM cationic redox reaction is qualitatively and quantitatively analyzed by a combination of s XAS and m RIXS spectra.The quantitative results show that the Ni redox is mainly from the Ni²⁺/Ni³⁺couple rather than the traditionally wisdom of the Ni²⁺/Ni⁴⁺couple.Furthermore,the valence state of the TM gradually decreases upon cycling,which leads to the voltage decay of NNMO.Subsequently,P2-Na_0.6Mg_0.3Mn_0.7O₂（NMMO）cathode material is synthesized with electrochemically inactive Mg²⁺and Mn⁴⁺,both of which are considered unstable under higher oxidation states.The cationic and anionic redox reactions of NMMO are systematically studied by a combination of s XAS spectra and electrochemical analyses.We reveal that only oxygen oxidation is involved in the charge compensation during the initial charge process,while both manganese and oxygen undergo reduction reactions to contribute capacity during the following discharge process.In addition,a gradient distribution of valence state of Mn from the surface to the bulk is unveiled by comparing the results of surface-sensitive TEY spectra and bulk-sensitive TFY spectra,which further clarifies that the initial charge capacity is mainly achieved with irreversible oxygen activity.It is crucial to understand the mechanism of anionic redox reaction and to improve its reversibility.Here we propose a strategy to improve the reversibility of anionic redox by modulating transition metal-oxygen covalency.The layered oxide P2-Na_0.6Mg_0.15Mn_0.7Cu_0.15O₂（NMMCO）cathode material is obtained by using highly electronegative Cu²⁺instead of low electronegative Mg²⁺,and the reversibility of the anionic redox reaction is substantially increased from 73%to 95%by directly quantified through m RIXS spectra.Density functional theory（DFT）calculations reveal that the Cu3d and O 2p states are highly overlapping,which strengthens the rigidity of the TM-O framework and thus effectively alleviates the irreversible anionic redox reaction and TM migration.Furthermore,the electrode exhibits a complete solid solution reaction with a volume change of only 0.45%and reversible metal migration during cycling,which together ultimately ensure the improved electrochemical performance.Although single-metal element doping can effectively modify the reversibility of anionic redox as mentioned above,the improvement is limited on other aspects,such as operating voltage,Na⁺transport kinetics,etc.Here NNMO is used as a prototype material,and an optimization strategy of Zn/Cu co-doping is proposed to synergistically modulate the crystal structure and electronic structure of layered oxides for SIBs.By using inert Zn to partially replace Ni with similar ionic radii but different Fermi energy levels,the Na⁺/vacancy ordering arrangement is disrupted and the unfavorable“P2-O2”phase transition is suppressed.In addition,the introduction of electrochemically active Cu not only improves the working potential but also alleviates the irreversible anionic redox through enhanced TM-O orbital hybridization.Furthermore,Zn and Cu cooperatively suppress the Ni/Mn honeycomb ordering arrangement,which effectively facilitate the transportation of Na⁺.Thanks to the co-doping synergistic optimization,the resulting P2-Na_0.67Ni_0.21Mn_0.67Cu_0.05Zn_0.07O₂（NNMCZO）cathode displays an absolute solid solution reaction during the whole charge/discharge process,a high working voltage beyond 3.6V,an ultra-low voltage decay rate and an excellent rate performance of 84.1 m Ah/g at a high current of 20 C.In addition to the elemental doping that can affect the anionic redox reaction,the introduction of metal vacancies directly into the TMO₂ layer can also stimulate the anionic redox activity by constructing the"Na-O-vacancy"configuration.However,it is still elusive whether the reversible anionic redox or irreversible oxygen release excited by the metal vacancies.Additionally,the mechanism of interaction between the metal vacancies and the doping elements are still unknown.In order to investigate the correlations among doping element species,metal vacancy concentrations and anionic redox reversibility,we prepare layered oxides with different synthesis conditions by regulating metal vacancy concentrations at different cooling rates and introducing Al and Zn elements with different bonding energy,respectively.The synchrotron-based in-situ XRD results show that vacancies can improve the reversibility and flexibility of the layered structure,thus reducing the lattice stress caused by Na⁺extraction/insertion.While the reversibility of anionic redox excited by vacancies varies considerably due to the introduction of different elements.The strong Al-O bonding energy stabilizes the reversibility of anionic redox through the Al-O-Mn lattice network,however,the weaker Zn-O bonding energy causes the poor reversibility of anionic redox excited by vacancies.