Evolution And Modification Of Local Structure In Layered Oxide Cathode Materials For Lithium Ion Batteries

With the rapid development of the society and economy,energy crisis and environmental pollution have become the common challenges in the world.Developing electric vehicles can effectively reduce the dependence on the fossil fuels and alleviate the aggravating environmental pollution.The high-energy density lithium ion battery is the ideal power source for the electric vehicles,the cathode material of which plays a critical role in determining the energy density of the battery.In comparison to the spinel-type and olivine-type cathode materials,the layered oxides with higher energy densities are much more applicable as the cathode materials for power batteries with long duration.However,the severe voltage hysteresis and initial capacity loss of the Li-rich materials and the ubiquitous Li⁺/Ni²⁺mixing in Ni-rich ternary materials limit their electrochemical performances and hinder their commercial applications in the power battery systems.On the basis of the background hereinbefore,this thesis studied the origins of the voltage hysteresis and the initial capacity loss in two prototype materials in the viewpoint of local structure.As for the Li⁺/Ni²⁺mixing in the Ni-rich ternary materials,we proposed the strategy that introducing the sintering aid with strong oxidation to the sintering process,which could observably reduce the cation mixing and improve the electrochemical performance of the synthesized material.It is still under controversy whether the voltage hysteresis is caused by the transition metal?TM?migration or the anionic redox.The obvious voltage hysteresis in the cation disordered materials indicates that the TM migration is not required for the voltage hysteresis.Therefore,it is necessary to clarify whether the anionic redox is indispensable for the voltage hysteresis.Due to the entanglement of the TM migration and the anionic redox upon lithium extraction at high potentials in Mn-based Li-rich materials,it is difficult to recognize the origin of these issues in conventional Li-rich layered oxides.We chose Li₂Mo O₃ as a prototype material to uncover the reason for the voltage hysteresis since both the TM migration and anionic redox can be eliminated below 3.6 V versus Li⁺/Li in this material.On the basis of the comprehensive investigations by the neutron powder diffraction,X-ray diffraction,Raman spectroscopy,scanning transmission electron microscopy,synchrotron X-ray absorption spectroscopy and density functional theory calculations,it is clarified that the hysteresis can take place even without the anionic redox or the interlayer Mo migration.The ordering-disordering transformation of the Mo₃O₁₃ clusters induced by the intralayer Mo migration is responsible for the remarkable voltage hysteresis in the first cycle.Moreover,the initial capacity loss in the Li-rich material is usually attributed to the irreversible anionic redox and its resultant structural degradation.However,little attention has been paid to the evolution of the local structures,especially the structural decay when the material is cycled below the anionic redox potential.Herein,we disclosed the correlation between the anomalous electrochemical reversibility and the evolution of local structure below the anionic redox plateau in Li₂Ru O₃,an archetypical lithium-rich layered oxide.On the basis of the comprehensive performance evaluation,physical characterization and theoretical calculations,it is found that the anomalous electrochemical reversibility is closely related to the evolution of stacking fault density in different voltage ranges.The large amount of stacking faults in the low delithiated state and the further increased stacking fault density during the lithiation would impede the reinsertion of Li _TM,thus resulting in the severe initial capacity loss in 2.00-3.66 V.However,the disappeared stacking faults in high delithiated state would benefit the reinsertion of Li _TM,therefore the good electrochemical reversibility is presented in2.00-4.00 V.Due to the unsolved drawbacks,the application of Li-rich materials is still a long uphill journey.In contrast,the Ni-rich ternary materials with high energy density are regarded as rational choices.However,the electrochemical performance of Ni-rich ternary materials is subject to the ubiquitous Li⁺/Ni²⁺mixing.To alleviate the cation mixing,we introduced?Li₂O₂?as a sintering aid of Ni-rich ternary materials.This additive promotes the oxidation of Ni²⁺and reduces the oxygen vacancies in the synthesized material.Moreover,the synthesized material has much less cation mixing and the primary particles present radial arrangement.In comparison with the materials synthesized without the sintering aid,the optimized Ni-rich ternary material displays high discharge specific capacity,excellent rate performance and outstanding cycling stability.The first two parts of this thesis focused on the local structure evolution and its influence on the macroscopic electrochemical properties of the material.As for Li₂Mo O₃,the ordering-disordering transformation of the Mo₃O₁₃ clusters in the local structure and the continuously decreased quantity of Mo₃O₁₃ clusters lead to the obvious voltage hysteresis between 2.0 and 3.5 V and the voltage decay in the cycling process,respectively.As for Li₂Ru O₃,we found that the local stacking faults and their evolution can significantly influence the macroscopic electrochemical reversibility.The evolution of local defect would have important inspiration in understanding the kinetic and thermodynamic performance decay of the conventional stoichiometric oxide cathode materials as well as the lithium-rich cathode materials.To solve the ubiquitous Li⁺/Ni²⁺mixing in Ni-rich ternary materials,we introduced the sintering aid?Li₂O₂?to effectively reduce the cation mixing in synthesized material.In addition,the optimized Ni-rich ternary material has superior electrochemical performance,this simple strategy would widen the road for the Ni-rich/Co-poor ternary materials.