Study On Construction And Doping And Lithium Storage Behaviour Of Highly Stable Lithium Vanadium Phosphate

The polyanionic Li₃V₂(PO₄)₃ has a stable structure,in which all three Li ions in the structure can participate in(de)intercalation,and the redox potential is higher.However,it is still facing challenges as follows:low intrinsic electron conductivity and Li ion mobility coefficient,as well as poor cycle stability in a wide voltage range(3.0?4.8 V).Taking monoclinic Li₃V₂(PO₄)₃ as the research object,we systematically clarify the evolution of the electronic structure and ion migration behavior during Li(de)intercalation processes,and design a hierarchical plate-like Li₃V₂(PO₄)₃/C composite.In addition,the effect mechanism of interstitial Li introduction and cation doping on the structure and electrochemical behavior of Li₃V₂(PO₄)₃ is further studied.First-principles calculations and molecular dynamics simulations are employed to investigate the mesophase configuration,charge compensation mechanism,Li ion migration channel and energy barrier of Li₃V₂(PO₄)₃ during Li ion(de)intercalation.The computational results disclose that the ion(de)intercalation behavior of Li₃V₂(PO₄)₃ is basically consistent with the experimental results.The last Li ion extraction can cause a part of O to participate in charge transfer due to the significantly enhanced V-O covalency.As an indirect-gap semiconductor,its electronic conductivity depends greatly on the carbon coating.Li ion migration presents obvious anisotropic properties and is dependent on the Li content in the structure,wherein the[100]direction is fast channel for Li ion diffusion.Considering the anisotropic ion transport behavior and the characteristics of the carbon coating,we prepare a hierarchical Li₃V₂(PO₄)₃/C composite through the modified carbothermal reduction method by introducing hydrogen peroxide(H₂O₂)and polyacrylamide(PAM)as reducing agent and chelating agent,respectively.Comparative experiments show that H₂O₂ can convert V₂O₅ to VO²⁺,and the amide group can effectively bind with VO²⁺ions,which helps the precursor to be fully mixed.During the subsequent high-temperature sintering process,the sufficient pyrolysis of sucrose and PAM can reduce the precursor to plate-like Li₃V₂(PO₄)₃material and form a carbon coating layer with N defects.Benefitting from the structural advantages,the composite material exhibits excellent electrochemical performance,and its capacity retention after 1000(200)cycles in the range of 3.0?4.3V(3.0?4.8 V)reaches 93.2%(84.8%).In order to seek a new way for improving the performance,non-stoichiometric Li₃V₂(PO₄)₃ materials are prepared by regulating the amount of Li:V in the starting materials.The Li_3+3xV_2-x(PO₄)₃(0?x?0.05)compounds can maintain the monoclinic phase with a certain lattice shrinkage,and the continuing increase in Li content and calcination above 700 ^oC may produce the Li₃PO₄ phase.Structural analysis demonstrates that the excess Li ions occupy both the octahedral and interstitial sites.Subsequent electrochemical tests and theoretical calculations manifest that although the Li excess strategy cannot provide additional capacity,the excess Li ions can induce an alternative Li⁺diffusion mechanism and enhance the Li⁺diffusion characteristics.In addition,the Li excess strategy can stabilize the bulk and interface structure,thereby synergistically alleviating structural degradation during electrochemical cycling.Li_3.15V_1.95(PO₄)₃ can achieve a capacity retention of 82.5%after 1000 charge-discharge cycles at 5C rate in the range of 3.0?4.8 V.To further improve the electrochemical performance of Li₃V₂(PO₄)₃,Mg²⁺ion doped Li₃V₂(PO₄)₃ materials were prepared,and the relationship between the structural evolution induced by doping and the electrochemical behavior is studied.Structural characterization shows that Mg²⁺ions can replace Li and V sites,and the charge is compensated by vacancy/internal Li introduction instead of polaron formation,and its doping sites can be determined by the relative abundance of the starting materials.Electrochemical performance tests show that the V-site doping of Mg²⁺ions can enhance the ion transport characteristics of Li₃V₂(PO₄)₃ and improve its cycling stability.The optimized V-site Mg²⁺ion doped Li₃V₂(PO₄)₃ can achieve a capacity retention of 88.2%after 1000 cycles at 5C rate in the range of 3.0?4.8 V.Theoretical calculations indicates that the electronic conduction mechanism of Li₃V₂(PO₄)₃ is mainly polaron migration,and the common cation doping has little effect on the concentration of polaron,so it can hardly improve its electronic conductivity.Secondly,introducing the interstitial Li⁺ions into Li₃V₂(PO₄)₃ can be used as a transition state to further enhance the Li ion transmission rate.In addition,the p-band center of O can be used to describe the stability of the Li₃V₂(PO₄)₃ doping system.The negative shift of the p-electronic-state center away from the Fermi level is believed to inhibit oxygen from participating in charge transfer at high potentials,thereby increasing the structrual stability of Li₃V₂(PO₄)₃.