Knock Off Diffusion Mechanism,Structural Design,and Electrochemical Behavior Of Lithium Vanadate Electrode Material

Li⁺diffusion is the most important process in lithium ion batteries（LIBs）,which has directly determined the rate performance.Understanding Li⁺diffusion in Li-intercalation compounds at atomic level can help to develop a targeted design for optimization of Li⁺diffusion rate.Vanadate LiV₃O₈ with high energy density was studied in this thesis.The crystal structure of LiV₃O₈ would be adjusted to attain accelerated Li⁺diffusion after thoroughly clarifying the Li⁺diffusion mechanism.This thesis was devoted to developing LiV₃O₈ electrode both with high energy density and high power density.The main results in this thesis are as follows:1.Li⁺diffusion mechanism was carefully studied and knock off mechanism was proposed for LiV₃O₈.The development of LiV₃O₈ electrode had been largely restricted for the unclear Li⁺diffusion mechanism.The migration energies of direct hopping mechanism and knock off mechanism had been calculated in all the possible diffusion paths of LiV₃O₈.Results showed that it was impossible for Li⁺migrating along[001]and[100]direction due to the serious steric hindrance in those paths.Compared with the classical site-to-site direct hopping mechanism,Li⁺preferentially tended to diffuse along[010]direction via Li（a）-Li（1）-Li（b）-Li（1）path by the distinctive knock off mechanism.The knock off mechanism was more energy favorably than the direct hopping mechanism.2.The excellent kinetics of knock off mechanism had been explored.We had noticed that the excellent kinetic in knock off mechanism was a common phenomenon in solid-state ionology.Therefore,to explain this phenomenon,a general and universal diffusion model had been constructed and the knock off mechanism was revealed at atomic level.Knock off mechanism performed more improved electrode kinetics relying on an"excitation effect"that Li⁺at a low-energy site could be activated and driven to next high-energy site by another Li⁺through coulomb interactions.Furthermore,the relation between"excitation effect"and atomic distance was quantified by introducing formula in our work.In summary,based on the new insight into knock off mechanism,our work would provide meaningful theoretical guidance for the subsequent development of electrode materials with ultrafast Li⁺diffusion behavior.3.The influence of structural factors upon knock off mechanism in LiV₃O₈ had been studied and the relation of"diffusion mechanism-structural adjustment-optimized performance"had been established.Relation 1:The influence of variable cell parameters on Li⁺diffusion kinetic was studied.Theoretical calculation shown that the migration energies decreased with increase of the cell parameters.In particular,the migration energies were greatly affected by the cell parameters a and c,while was less sensitive to cell parameters b.Thus,solid-state Li⁺diffusion kinetic would be enhanced by expanding（010）plane.Relation 2:Different performance for Li⁺migration in bulk and at the solid/liquid interface was studied.The whole Li⁺intercalation process from outside to inside of the crystal structure was investigated by establishing slab model.Results shown that the migration energies for Li⁺diffusion at the interface and in the bulk were 0.61 e V and 0.36e V respectively.It meant that the slow Li⁺diffusion at the interface was the rate-determining step for LiV₃O₈ electrode.Relation 3:The relationship between lithium content in bulk and the Li⁺diffusion kinetic was studied.Results showed that the migration energies decreased with the increase of lithium content,and more excellent Li⁺diffusion kinetic was shown inβphase.Actually,with the increase of lithium content,the atomic distance between two Li⁺in knock off mechanism was further reduced,consequently raising the"excitation effect"in diffusion process.Li⁺diffusion mechanism in LiV₃O₈ had been fully understood and the the relation of"diffusion mechanism-structural adjustment-optimized performance"had been developed through our theoretical studies.Based on above results,the Li⁺diffusion kinetic in LiV₃O₈would be effectively improved through more targeted structural design.4.Based on relation 1,accelerated solid-state diffusion would be realized by expanding（010）plane through Ca doping.The effect of Ca-doping content on the crystal structure of LiV₃O₈ was systematically studied.The optimum Ca-doping content was determined as 7%,and the（010）plane was expanded from 75.92（?）²to 76.44（?）² wich had greatly diminished the steric hindrance for Li⁺diffusion.The specific capacity of LiV₃O₈ and Ca-doped LiV₃O₈ was 40 m Ah g^-1 and 150 m Ah g^-1 at the high current density of 1500 m A g^-1.Moreover,theα-βphase transition during charging and discharging had been suppress by Ca doping,which led to low lattice strain and excellent cycling performance.Further insight into Ca doping effect on Li⁺storage and phase transition mechanism were realized by experiment and theoretical calculation.To sum up,the Li⁺diffusion kinetic had been improved and the cyclability had been optimized for Ca-doped LiV₃O₈.5.Based on relation 2,the isomorphic NaV₃O₈/LiV₃O₈ core-shell structure was designed.Excellent Li⁺solid diffusion kinetics was the intrinsic advantage of the knock off mechanism.However,the slow Li⁺intercalation process at the solid/liquid interface was the rate-determining step for LiV₃O₈ electrode.On the contrary,the direct hopping mechanism was more conducive to Li⁺intercalation at the interface.Therefore,the NaV₃O₈/LiV₃O₈ core-shell structure was successfully prepared by ion-exchange method.This structure took the advantages both from knock off mechanism and direct hopping mechanism.In detail,Li⁺intercalation at interface was greatly improved by direct hopping mechanism through the NaV₃O₈"shell structure",and excellent Li⁺solid diffusion would be obtained by knock off mechanism through the LiV₃O₈"core structure".The activation energy of interfacial reaction decreased from 35.47 k J mol^-1 to 26.25 k J mol^-1 and the migration energies by theoretical calculation decreased from 0.61 e V to0.40 e V.Therefore,the excellent rate performance was shown for NaV₃O₈/LiV₃O₈ core-shell structure.Moreover,the isomorphic NaV₃O₈ and LiV₃O₈,with similar chemical components and lattice parameters,had high lattice matching and core-shell binding force,and thus shown good cycling performance.6.Based on relation 3,a new fast-charging negative electrode material Li₅V₃O₈had been developed.As positive electrode material,the high voltage range of LiV₃O₈～Li₅V₃O₈ had been studied and investigated by our group and many other researchers.Relation 3 indicated that the kinetic of knock off mechanism would be further promoted at high lithium content.Therefore,the electrochemical performance of Li₅V₃O₈at low voltage range of 0.1 V～2.0 V would be studied in this thesis.Results showed that Li₅V₃O₈ had a capacity of 256 m Ah g^-1and an average potential of 0.7 V.The charging and discharging mechanism had been carefully investigated,such as structural evolution,the change of chemical valence and the formation of SEI.In-situ XRD pattern showed that the crystal structure of Li₅V₃O₈ exhibited excellent reversibility during charging and discharging,and had extremely low lattice strain.As a result,this vanadium-based negative electrode material exhibited outstanding cyclability（capacity retention of 80%after 11,000 cycles at 20C）.Moreover,Li₅V₃O₈,which adopted a distinctive knock-off mechanism,was capable of intercalating considerable lithium ions at extremely high rates（delivering 60%of capacity at 40C）.It can be concluded that Li₅V₃O₈ is a new fast-charging negative electrode material for lithium-ion batteries with great development potential.In conclusion,Li⁺diffusion mechanism of LiV₃O₈ had been studied systematically and the electrochemical performance of LiV₃O₈ was effectively optimized via targeted design.Our studies provide valuable insight for the future optimization of vanadate for facilitating their practical use in LIBs.