Magnetic Resonance Study Of Li,mn-based Layered Oxide Cathode Materials Based On Oxygen Anion Redox

Driven by the rapid development of electric vehicle industry and energy storage field,it’s urgent to find more advanced Lithium-ion battery materials（LIBs）with higher energy density to meet the increasing demand of the market.In the field of high-energy-density LIBs,layered oxide cathode materials have attracted a lot of attention from researchers because of the additional capacity provided by redox reactions originating from oxygen anions that are activated at high voltage.Li-rich Mn-based layered oxide cathodes（LRMC）are featured by the special structure and anionic redox reaction that can provide a high specific capacity of more than 250 m Ah/g,and are considered to be a promising material to meet the commercial demand for high-energy-density batteries.In this paper,with the help of electron paramagnetic resonance（EPR）and nuclear magnetic resonance（NMR）techniques,a variety of different types of LRMC materials were systematically analyzed,and a large amount of information was obtained on the change of elemental valence,oxygen anionic redox reversibility,voltage hysteresis,evolution of local structure,and the chemical state of the oxidized oxygen species during the charging and discharging process.The electrochemical reaction mechanism of LRMC in terms of atomic spin and electron spin has been analyzed in detail,which deepened the understanding of oxygen anionic redox reaction involved in materials.A solid foundation for the future design and development of LRMC materials with more reversible oxygen anionic redox reaction has been laid,and the related works of the study are as follows:（1）Based on the advantages of O2-type Li-rich materials with a face-sharing structure,we first synthesized the benchmark material O2-Li_5/6(Li_0.2Ni_0.2Mn_0.6O2(Li_1.033Ni_0.2Mn_0.6O₂,LLNMO)using an ion-exchange strategy.To facilitate a deeper understanding of the material,we have used high-resolution ⁷Li pj-MATPASS（projection of magic-angle turning phase-adjusted spinning sidebands）NMR,quantitative ⁶Li MAS（magic-angle spinning）NMR and EPR techniques to systematically reveal the correlation between the local structural transition of O2-LLNMO and the electrochemical reaction mechanism during charging and discharging process（2.0-4.8 V）.The existence of“six-fold hyperfine splitting”of high-spin Mn⁴⁺associated with O^（2-α）-（0α2）during the initial and subsequent cycles was first identified via EPR,and the reversible redox process of the oxygen anions confirmed the advantage of the O2-type structure in facilitating the redox of the oxygen anions.The local structural evolution that occurs during charging and discharging was analyzed using NMR techniques.In the O2-type structure,it has proved that the irreversible out-of-plane transition metal（TM）migration is suppressed compared to the O3-type structure.The appearance of the tetrahedral Li（Litet）in the lithium layer after the first cycle and the irreversible（de）intercalation of Li（Li TM）in the TM layer are also demonstrated.Furthermore,the（de）intercalation of Litet and the evolution of the local structure in subsequent cycles were found to be highly reversible,thus explaining the small voltage hysteresis changes in the O2-type structure during the long-term cycling.（2）In the previous work,we demonstrated the inhibition effect of the irreversible out-of-plane cation migration in O2-Li_1.033Ni_0.2Mn_0.6O₂.On this basis,we conducted an in-depth study on whether the generation of molecule O2 under high voltage can be eliminated in the charging and discharging process of O2-type LRMC.The comparison with other classic system of Li-rich materials(layered O3-Li1.2Ni0.2Mn0.6O2,O3-Li1.2Ni0.13Co0.13Mn0.54O2 and cationic disordered rock-salt Li_1.2Ti_0.4Mn_0.4O₂)were carried out.We have used the EPR technique to discover for the first time the reversible generation and disappearance of the EPR signal representing the"trapped"molecular O2 during the initial charge/discharge and subsequent cycles of these four systems,confirming the reversibility of the oxygen anion redox and the importance of suppressing in-plane cation disorder.In addition,the simultaneous characterization with⁷Li pj-MATPASS NMR illustrates the local structural evolution of the materials during the formation of the"trapped"molecular O2.The results confirm that the key to improving the local structural reversibility of oxygen anion redox is the control of cationic disorder,rather than the formation of specific oxidized oxygen species.（3）Based on the study of O2-Li_1.033Ni_0.2Mn_0.6O₂,we confirmed the importance of suppressing in-plane TM migration for electrochemical performance enhancement.Therefore,we optimized O2-Li_1.033Ni_0.2Mn_0.6O₂ by introducing vacancy（"□"）,the modified O2-Li1.033Ni0.2[□_0.1Mn0.5]O2 exhibits significantly better cycling stability.The effect of vacancy introducing was analyzed by NMR in detail.Firstly,the reversibility of the local structural transformation has been significantly improved and the Litet has not been detected in the electrochemical process.It is speculated that the reservation of"□"provides additional space for the reintercalation of Li⁺.In addition,the in-plane migration of irreversible Mn,which causes the generation of"trapped"molecular O2 has been effectively suppressed.（4）In the previous works,we have carried out detailed studies for Li-rich materials with a lithium content>1.In this work,we attempted to analyze the Li-rich materials with a lithium content of<1.O2/O3-Li0.6Li0.2Mn0.8O2（LLMO）with a lithium content<1 were synthesized using a similar ion-exchange method.Interestingly,the electrochemical stability of O2-LLMO in the range of 2.0-4.8 V is much less than that of O3-LLMO,which is obviously contradictory to the previous knowledge.NMR and Raman spectroscopy were used to analyze the evolution of the local environment around Li in the charging and discharging process.It was confirmed that the formation of Litet in the O2 phase and the disappearance of the chemical environment of part of lithium in the Li layer led to the intensification of structural instability,which were the fundamental reasons for the difference in cycle stability compared with O3 phase induced by the material phase transition.In addition,we have also explored the electrochemical performance of the O2/O3-type Li-rich materials with several chemical compositions（all with Li content<1）and found that the O3 phase consistently exhibits superior electrochemical performance among the same chemical compositions.It is therefore concluded that the electrochemical properties of LRMC are determined synergistically by the phase structure and the Li content in the initial material.