Electrochemical Mechanism Study And Doping Modification Of Li₃VO₄ Anode For Lithium Ion Batteries

With a large capacity and low,safe voltage,Li₃VO₄ is considered to be a promising new insertion type anode for lithium-ion batteries.In recent years,many works have been made to improve the electrochemical performance such as reducing the particle size via different synthesis methods,enhacing the conductivity by carbon coating or compositing.However,there still remains some challenges for its practical application.For example,its reaction mechanism is not clear,the Li insetion sites/amounts and the structure change need further discussion.Besides,Li₃VO₄ poesseses low temperature?phase and high temperature?phase,the latter owns higher ionic conductivity.However,the present works mainly focus on the modification of?-Li₃VO₄,?-Li₃VO₄ is rarely seen since it is only stable in high temperature.Therefore,we investigate the surface/interface and structure evolution of Li₃VO₄ during the lithiation/de-lithiation process systematically,and combine the first principle calculation to compute the reaction process and expansion ratio of cell volume,figure out the capacity loss mechanism of Li₃VO₄ upon cycles.On this basis,we further stabilize high temperature phase?-Li₃VO₄ by doping Si,and study the relative position and doping mechanism of doped Si.Electrochemical measurements prove that Si doped?-Li₃VO₄ possesses higher conductivity and more stable Li insertion/de-insertion structure.Fisrt,we use solid state reaction method to synthsis Li₃VO₄ and inverstige its electrochemical reaction process and mechanism.By controlling the depth of discharge,studing its surface/interface and structure evolution,it is found that the first irreversible capacity loss is related to the formation of SEI film as well as the structure distortion initiated by first lithiation.The first principle calculation gives the Li insetion sites and corresponding cell volume expansion,it demonstrates that the volume expansion and crystal distortion become more irreversible with the increasing depth of discharge.The experimental results also prove the inferior cycling performance of Li₃VO₄ at deep discharge depth.Along with cycle number,the accumulated structure deterioration results in the decline of material crystallization and structure orders leading to capacity loss upon cycles.At last,we study the kinetics of Li₃VO₄ during the first cycle by GITT and EIS measurements,the low Li⁺diffusion at voltage plateau may relate to the high barrier caused by phase change and structure distortion.Furthermore,we use Si-doping to stabilize high temperature phase?-Li₃VO₄ and study its strucutre,properties as well as electrochemical performances.By adjusting the ratio of reaction precursor,it is found that the pure?-Li₃VO₄ can be obtained when Li is excess and doped Si?x?is in the range of 0.05⁰.15.Both the experimental and the first principle calculation results indicate the doping mechanism of interstitial Li,which corresponds to the formula of Li_3+xV_1-xSi_xO₄.The furthure measurments demonstrate that the defects such as inersitial Li⁺will increase with the Si doping,which results in the declined sturcture ordes and imoprved Li⁺diffusion kinetics,it is may account for the higher ion conductivity of?-Li₃VO₄.The electrochemical measurments show the conductivity of?-Li₃VO₄ is2.37×10^-6 S/cm,which is three orders of magnitude higher than?-Li₃VO₄.What's more,?-Li₃VO₄ exhibits only a pair of redox peaks,impling the enhanced stability of Li⁺insertion/extraction process.And with the increasing concentration of Si,the capacity of Li₃VO₄ decreases while the cycling stability is improved,it may relate to the more stable structure of?-Li₃VO₄ after Si-doping.Therefore,Si doped?-Li₃VO₄ can be a good choice to enhance the conductivity and structure stability of Li₃VO₄ anode.