Vacancy Regulation And Surface Modification Of High-capacity Cathode Materials With Oxygen Redox

Anionic redox has been proved an effective strategy to improve the energy density of cathode materials.The activity and reversibility of oxygen redox are affected with the configurations around the O atoms.Introduction of vacancies in the transition metal layers of the material can form an A-O-□configuration and activate the reversible anionic redox in the materials.This thesis comprehensively investigated the native vacancies enhanced structural stability,the vacancies activated anionic redox and the universality of these effects in the cathode materials for the Li-ion and Na-ion batteries.In addition,we proposed a surface-modification strategy for the Li-rich layered oxide cathode materials,effectively improving the structural stability.Na_2/3[Mn_7/9Mg_1/9□_1/9]O₂,characteristic of equal numbers of Mg²⁺and native vacancies in the transition metal（TM）layer was designed as a cathode material for the Na-ion batteries.This specifically designed structure permits us to compare the superiority of these two configurations in triggering the anionic redox.X-ray diffraction,aberration-corrected scan transmission electron microscopy（STEM）and simulations to the STEM imaging demonstrate the presence of the vacancies in the TM layer.The soft X-ray absorption spectroscopy（s XAS）characterization and density function theory（DFT）calculations identified that the native vacancy facilitates the anionic redox at low voltage.The electrochemical evaluations show that this material delivers a reversible capacity of 212 m Ah g^-1 between 1.5 and 4.5 V and with a capacity retention of 100%in 100 cycles between 2.1 and 4.4 V.The in situ XRD results indicate that the vacancy-induced asymmetric MO₆octahedron ensures the structural reversibility and flexibility.Na_2/3[Mn_7/9Zn_1/9□_1/9]O₂ with equal native vacancies and Zn²⁺ions was prepared and compared with the vacancy-free Na_2/3[Mn_7/9Zn_2/9]O₂ in structural stability during cycling.It was proved that TM vacancies tailor the Mn³⁺ion spinning states,stabilize the structure and improve the kinetic performance of the material.Measurements to the temperature-dependent magnetic susceptibility demonstrate the increase of the low-spin Mn³⁺ions in Na_2/3[Mn_7/9Zn_1/9□_1/9]O₂ at low potential suppresses the Jahn-Teller effect and structural deformation.The DFT calculations indicate the Mn-O bond length is shorter in Na_2/3[Mn_7/9Zn_1/9□_1/9]O₂ than in Na_2/3[Mn_7/9Zn_2/9]O₂,consistent with the experimental results.X-ray absorption spectroscopy identifies that both the Na-O-□and Na-O-Zn configurations can activate the anionic redox,but the vacancies facilitate the oxygen redox at a lower potential than the Zn²⁺ions do and Na_2/3[Mn_7/9Zn_1/9□_1/9]O₂shows better cycling performance in the whole potential window.TM vacancies were successfully introduced into the spinel-structured cathode material Li₂Mn₃O_7-x by the ion-exchange method.The atomic-scale structure was investigated by STEM,XRD and Raman spectroscopy.The DFT calculations indicate that the vacancies on the tetrahedral sites are more stable than those on octahedral sites of the spinel.The in situ XRD patterns demonstrated its“zero-strain”feature during cycling.XAS and EXAFS further proved the oxygen redox and the local structural stability.These studies that the TM vacancies can facilitate the oxygen redox and stabilize the structure of both the layered and the spinel oxides for the Li-ion batteries and the Na-ion batteries.Surface doping of the Al³⁺ion was proposed to regulate the oxygen redox and stabilize the bulk structure of the Li-rich layered cathode materials.The STEM imaging identified that the doping of the Al³⁺ions is⁴ nm in depth and alleviates the surface aggregation of Ni.The surface doping enhances the initial coulombic efficiency of the material from 85%to 91%,increases its capacity retention to 87.4%in 100 cycles,lowers its setting-up potential of the oxygen oxidation and alleviates the voltage hysteresis.XAS demonstrates the boosted reversibility of the oxygen redox and structural variation after Al³⁺-ion surface doping.The effects of surface doping on the thermodynamic and kinetic properties of the material were investigate by the DFT calculations.