| Due to the advantages of high energy density and high conversion efficiency,lithium ion battery has been widely used in the field of portable mobile power supply,power battery and renewable energy storage.The large-scale application of lithium ion battery requires higher energy density for lithium ion battery,and the cathode material is the key factor for higher energy density.Therefore,developing high energy density cathode materials is the most important thing for improving the electrochemical performance of lithium ion battery.Among cathode materials,the lithium-rich manganese-based cathode material has received extensive attention due to their very high specific capacity of more than 300 mAh/g.Besides,manganese,the major element in this material,is cheap and environmentally friendly.However,the electrochemical problems of lithium-rich manganese-based cathode material,such as low Coulomb efficiency,capacity decay,voltage decay and poor rate capacity,directly hinder its commercial application.In recent years,it has been found that both the high capacity and electrochemical problem of lithium-rich materials are closely related to the lattice oxygen redox reaction(including the reversible oxidation/reduce and irreversible oxygen release).Therefore,in-depth exploration of the oxygen redox reaction mechanism,and effectively regulation of the lattice oxygen redox reaction is important for lithium-rich manganese-based cathode materials.Based on the background,our work starts from the electrochemical problem of lithiumrich cathode,and aim to regulate/explore the redox reaction mechanism of lattice oxygen through material modification.Our work choose representative lithium-rich manganese-based cathode material Li2MnO3 and 0.5Li2MnO3·0.5LiNi0.4Co0.2Mn0.4O2 as the research material,introduce proton insertion and layered-spinel coexist phase construction as modification method,and combine synchrotron-based resonant inelastic X-ray scattering(RIXS),solid-state nuclear magnetic resonance(ssNMR)spectroscopy,operando differential electrochemical mass spectrometry(DEMS)to analysis the effect of phase modified and rate control on electrochemical process and lattice oxygen reaction.The regulation method and reaction mechanism of lattice oxygen redox in lithium-rich manganese-based cathode materials are discussed.In order to investigate the low initial Coulombic efficiency for lithium-rich material,acid treatment method is applied in Li2MnO3 system.The experimental results show that acid treatment is useful for improving the electrochemical performance.Compared with pristine Li2MnO3(initial Coulombic efficiency of 55.2%,discharge capacity of 194 mAh/g),the treatment material presents an unprecedented initial Coulombic efficiency of 99.2%with a greatly improved discharge capacity of 302 mAh/g in current density of 10 mA/g.Combining thermal gravity analysis mass spectrometry(TGA-MS)and ssNMR method,the results show that proton inserts into the bulk and the amount of proton insertion is inversely related to the pH value.Furthermore,the,neutron diffraction(ND)experimental results,density functional theory(DFT)and bond valence sum(BVS)calculation results confirm that inserted protons locate in the lithium octahedral vacancies in the lithium layer.To further explore the proton-insertion mechanism,we apply bulk-sensitive RIXS technique and DFT calculation to explore the lattice oxygen redox evolution in the initial charge-discharge process.RIXS has been confirmed as one of the most powerful technique for probing the oxygen redox reaction.Our RIXS results show as a parent compound of Li-rich electrodes,pristine Li2MnO3 has no reversible oxygen redox and suffers notoriously low Coulombic efficiency due to oxygen release and related surface reaction in initial charge process.After acid treatment,a moderate coupling between the introduced protons and lattice oxygen at the oxidized state is revealed via RIXS,which stabilizes the oxygen activities during electrochemical process.Besides,the theoretical calculation results show that the influence of proton-oxygen coupling is mainly worked in the latter stage of high-potential charge process.The moderate coupling between proton and oxygen can effectively inhibit the undesired lattice oxygen release.In addition,combined with electrochemical impedance spectroscopy(EIS)experiment and lithium ion diffusion barrier theoretical calculation,the lithium ion migration battier is reduced.To solve the voltage decay issue in lithium-rich material,we applied a combined electrochemical conditioning and thermal treatment method to construct layer-spinel coexist phase.Compared with pristine material,modified material shows improved electrochemical performance,the capacity retention after 35 cycles increases from the 40%for the pristine material to 80%for the treated material.In terms of voltage,pristine LMO delivers an average voltage of 3.1 V for the initial cycle and decreases to 2.8 V after 35 cycles.In contrast,the treated sample presents an averaged discharge voltage of 3.2 V in the initial cycle,and the average voltage value is sustained at the thirty-fifth cycle.By combining atomic-sensitive ssNMR,DEMS and RIXS methods,we disclose that treatment triggers cationic redistribution to form three-coexisting phases,i.e.,lithium-rich layered,spinel and defect spinel phase,which enables improved reversibility of the oxygen redox activity and enhanced manganese redox reactions in the initial cycle.Our findings suggest the key role of local structure on the voltage decay problem and provide insights for material optimizations towards lithium-rich and manganese-based cathodes without voltage decay.To investigate the poor rate capacity for lithium-rich material,the bulk-sensitive RIXS was applied on lithium-rich system to reveal the influence of rate capacity on lattice oxygen redox reaction.Lithium-rich 0.5Li2MnO3·0.5LiNi0.4Co0.2Mn0.4O2 material is investigated under different current density.The synchrotron based RIXS data reveals that 0.5Li2MnO30.5LiNi0.4Co0.2Mn0.4O2 material presents reversible oxygen redox with reversibility of 77.3%in the initial cycle and 92.6%sustained at the twentieth cycle with current density of 25 mAlg.Compared with that at low current density(25 mA/g),the reversible oxygen redox reaction is sustained at high current density(2500 mA/g).Meanwhile,the quantitative tracking on the unique 4.5 V charge plateau shows the plateau ration is reduced when the current density switches from 25 mA/g to 2500 mA/g.However,the RIXS area of reversible oxidized oxygen for low and high current density is 0.0176 and 0.0180,respectively.Such results indicate that reversible oxygen redox reaction is not the main reason for poor rate capacity for lithium-rich cathode.In addition,the DEMS data reveals that compared with low rate capacity,high rate capacity delivers reduced oxygen release. |
PDF Full Text Request