| With the development of social economy,lithium-ion batteries with increasing costs can no longer meet the demand for energy storage batteries in industries such as mobile terminals and electric vehicles.Sodium-ion batteries are one of the effective alternatives to lithium-ion batteries,but the theoretical capacity provided by the redox of only transition metals(TM)in the sodium-ion layered oxide cathodes cannot exceed the current commercially available lithium-ion layered oxide cathode materials.Since the anionic redox reaction(ARR)can provide additional capacity,the layered sodium transition metal oxide cathode materials with anionic redox activity have become the focus of current research.This thesis uses NMR and EPR technology to characterize two typical layered transition metal oxide cathode materials with oxygen redox activity.The electronic state changes of oxygen and TM,the evolution of O2 and the evolution of local structures in the oxygen redox process were studied in depth to fully understand the oxygen redox reaction from the perspective of atoms and electron spins,so as to lay a solid foundation for the design of sodium-ion cathode materials with high reversible oxygen redox reaction and non-hysteretic behavior.The specific works are as follow:(1)At first,P2-Na0.66Li0.22Mn0.78O2 archetypical material was optimized by trace F substitution modification method.The trace F-substituted P2-Na0.65Li0.22Mn0.78O1.99F0.01 showed excellent cycle stability and rate performance,much higher than the unsubstituted P2-Na0.66Li0.22Mn0.78O2.Through systematic bulk phase/surface spectroscopic characterizations,we demonstrate that the substitution of trace F effectively inhibits irreversible oxygen release and undesired Jahn-Teller distortion,thereby maintaining a highly reversible oxygen redox reaction.(2)On the basis of the above work,P2-Na0.66Li0.22Mn0.775Sn0.005O2 and P2-Na0.66Li0.22Mn0.775Zr0.005O2 materials were synthesized by Sn/Zr substitution modification method,showing a significantly improved reversibility of oxygen redox reactions and significantly enhanced electrochemical performance.With various experimental characterizations and theoretical calculations,the role of Sn/Zr substituents was studied in detail,and it is proved that they could effectively attenuate the anisotropic(O2)n-—Mn4+coupling and curb the irreversible lattice oxygen loss.In addition,systematic 23Na and ~7Li MAS NMR characterization revealed the local structural transformation during the oxygen redox reaction.(3)Furtuer,We introduced a novel highly fluorinated electrolyte,which successfully constructed a uniform and robust fluorine-rich CEI film between P2-Na0.66Li0.22Mn0.78O2 cathode material and the electrolyte.The in situ formed CEI effectively refrains the irreversible Li/Mn dissolution and O2 release caused by the oxygen redox reaction,thus facilitating a highly reversible oxygen ion redox reaction and showing an impressive electrochemical performance.In addition,the formation of"trapped"molecule O2 in the bulk phase of desodiated P2-Na0.66Li0.22Mn0.78O2 was revealed for the first time by using EPR technique.In addition,the results of EPR also proved that peroxo-like(O2)n-are generated before the molecule O2 during the charging process and coexist with the molecule O2 in the fully charged state at 4.5 V.(4)Finally,We employed operando EPR to deeply study the oxygen redox process of Na2Mn3O7 cathode with an abnormal small voltage hysteresis,and found that the material presented a highly reversible local electronic structure evolution during the oxygen redox reaction.In addition,the low temperature EPR tests showed that the oxygen redox process of this system does not occur with the dimerization of O—O,but with the presence of oxidized O species in the form of localized electron holes(O-·).We hypothesized that the stable electronic structure of Na2-xMn3O7,which is associated with Mn4+?O-·,might be the cause of the abnormally small voltage hysteresis in its electrochemical process. |
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