Local Structural And Electrochemical Study On Li-Rich Layered Oxide Cathode Materials

Li-ion batteries are widely used because of their high energy density and low cost.However,as the development of large-scale energy storage devices and electric vehicles,the energy density of Li-ion batteries is urgently needed to be improved.At present,compared with commercial anode materials,the lower capacity of cathode materials is the primary factor for restricting the improvement of energy density of Li-ion batteries.Therefore,the innovation of cathode materials with higher specific capacity has become a research focus.Li-rich layered oxides（LLOs:with a nominal chemical formula of x Li₂Mn O₃-（1-x）Li TMO₂（TM=Ni,Co,Mn））are regarded as one of the promising cathode materials for the next-generation high-energy Li-ion batteries,due to their ultra-high specific capacity(>250 m Ah g^-1).The high specific capacity of LLOs comes from the synergistic anion/cation charge compensation.However,this special electrochemical process is partly irreversible and involves slow atoms migration and rearrangement,which results in large voltage hysteresis and lead to low Coulombic efficiency,low energy efficiency and poor rate capability.On the other hand,the irreversible migration of atoms may lead to structure transition,thereby reducing the cycle stability.It is worth noting that recent studies have shown that the charge compensation mechanism of lattice oxygen in LLOs is closely related to the structural characteristics of the materials.Although the researches on structure of LLOs have been tremendously developed in the past 20 years,the intrinsic corelation between oxygen activity and structure characteristics of LLOs has still not been revealed,due to the limitations of research methods and characterization techniques.Illuminating local structure of LLOs is a more challenging research topic because it is often impossible to obtain reliable results by using sole characterization technique or analytical method.In addition,the consistency of the material system are also worth paying attention to.Inconsistent material samples may lead to inconsistent research results,which further increases the difficulty of structural study of LLOs.Therefore,in this dissertation,we prepared a series of LLOs in large scale and carried out a systematic investigation on corelation between oxygen activaty and local structure of LLOs.The main research contents and results are as follows:1.Investigation of local structural changes in LLOs during synthesisThree carbonate precursors with different Mn content,Mn CO₃,Ni_1/6Co_1/6Mn_1/3CO₃and Ni_0.25Co_0.25Mn_0.5CO₃,were synthesized by co-precipitation method.Then the LLOs,Li₂Mn O₃,Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ and Li_1.08Ni_0.22Co_0.22Mn_0.45O₂,were prepared by high-temperature solid sintering after mixing with Li₂CO₃.In-situ X-ray diffraction（in-situ XRD）results show that the three LLOs have undergone a structure transition from spinel-like to layered structure during the synthesis process,with the increasing ordering of Li and transition metal ions.The transition mainly occurs in the high-temperature ramping and holding stage,which means that high-temperature sintering play a vital role in eliminating the Li/TM mixing.However,high-temperature sintering also leads to the volatilization of Li on the surface of the material,thereby inducing the surface structure reconstruction from layered to spinel/rocksalt structure.This phenomenon implies the importance of precise control of the sintering temperature.The analysis of superlattice peaks shows that the probability of stacking faults in Li₂Mn O₃domain has no solid relationship with the sintering temperature and composition,but the domain size becomes large when temperature increases.Besides,the addition of Ni and Co can effectively suppress the increase of domain size.In-situ hard X-ray absorption spectroscopy（in-situ h XAS）and total scattering for atom pair distribution function（PDF）indicate that the Mn coordination environment changes with the composition,which proves that the configuration of Li Mn₆ increases with more Li content.Electrochemical performance tests show that the composition and Li₂Mn O₃domain size play a crucial role in the reversible oxygen redox of LLOs.This work highlights the importance of conditions control during the synthesis of LLOs.2.Investigation of structural basis for oxygen redox activity in LLOs.A series of LLOs,Li_1+Y[Ni_2/（3+3X）Co_2/（3+3X）Mn_{2X/（3+3X）}]O₂（NCM11X:X=1,2,3,4,6;Y≥0）,were prepared.By using a combination of synchrotron radiation X-ray diffraction（SXRD）and time-of-flight neutron powder diffraction（TOF-NPD）techniques,the structure parameters such as the unit cell parameters,phase composition and atomic position of the series of samples were analyzed.Extended X-ray absorption fine structure（EXAFS）was used to analyze the coordination environment of TM ions.The results show that the atomic coordination environments of the three transition metal elements are quite different.The coordination environment of Ni in all samples is very similar.It can be considered that the coordination of Ni ions is not easily affected by the composition,and Ni ions are likely to segregate in the material and form a conventional layered phase（specifically,Ni ions coordinate with 6 Li ions and 6transition metal ions in the nearest neighbor cationic coordination）.Co ions show a unique coordination environment.Their coordination environment changes in dependence on composition.Specifically,Co ions can partly participate in the formation of Li₂Mn O₃ phase and affect the charge transfer path of lattice oxygen.Combined with the results of electrochemical tests,these findings reveal that the oxygen redox activity of LLOs is highly correlated with the Co coordination environment.The EXAFS results for cycled samples further confirmed the findings.This work clarifies the role of local structure in oxygen redox activity,and illuminates the correlation between electrochemical performance of LLOs and element composition.3.Investigation of local structural changes in LLOs during cyclingA new LMLO,Li_1.12Ni_0.22Co_0.13Mn_0.52O₂,was synthesized by tuning the composition of the typical LMLO Li_1.14Ni_0.13Co_0.13Mn_0.54O₂.Electrochemical performance tests show that the modified material has higher energy efficiency and higher capacity retention,and exhibits slower voltage decay rate and excellent rate performance.Electrochemical impedance spectroscopy（EIS）and Galvanostatic intermittent titration technique（GITT）show that the better electrochemical performance of Li_1.12Ni_0.22Co_0.13Mn_0.52O₂ is due to the fast kinetics of redox reaction,especially the oxygen redox kinetics.The electrochemical in-situ characterization techniques（insitu XRD and insitu h XAS）were perfomed to reveal that the oxygen redox kinetics is tuned by the reversibility and kinetics of transition metal migration.Appropriately increasing Ni content can effectively limit the extension of Li₂Mn O₃ domain.Moreover,the Li/Ni mixing at high voltage charge state may make Ni ions block the migration path of Mn ions by the electrostatic shielding effect,thereby alleviating the hysteresis of atom arrangement.These findings provide new insights into understanding the evolution of local structure in LLOs,and provide design principle for LLOs with high energy efficiency and high cycling stability.