Study Of Charge-discharge Mechanism And Carbon Coating/Cationic Doping Modification Of Li₃V₂(PO₄)₃

Monoclinic lithium vanadium phosphate(?-Li₃V₂(PO₄)₃)is a promising high performance cathode material with stable structure,high electrochemical reaction platform voltage,high theoretical capacity,and good safety.However,some disadvantages,including low electronic conductivity and the irreversible electrochemical process during cycle within 3.0-4.8V voltage window,greatly hinder further application of lithium vanadium phosphate.At present,most of the research efforts on lithium vanadium phosphate are focusing on improving electrochemical performance but lack of fundamental understandings about mechanisms and structure-function correlation.In this thesis,monoclinic lithium vanadium phosphate has been taken as the main research object,investigations has been carried out in the following three aspects:electrochemical charge and discharge mechanism,structrue of carbon coating,and the function of cationic doping.In order to illustrate the origin of the irreversible charge-discharge of lithium vanadium phosphate,the structure evolution and dynamics during the charge-discharge process of lithium vanadium phosphate has been systematically studied by combining multiple characterization techniques including X-ray diffraction(XRD)and solid state nuclear magnetic resonance(SSNMR).Our results indicate that multiple phase transitions take place within the charging(delithium)voltage window of 4.3-4.8 V from Li₂V₂(PO₄)₃ to V₂(PO₄)₃ via Li₁V₂(PO₄)₃,which is a crystalline phase with highly distorted structure.Under the non-equilibrium rapid charging,the material tends to avoid the distorted Li₁V₂(PO₄)₃ structure.In contrast,the distorted Li₁V₂(PO₄)₃ structure is formed under a quasi-equilibrium slow charging process.The subsequent discharging(lithiation)process(4.8-4.3V)follows a solid reaction routes to avoid massive framework distortion,in which lithium ions are gradually inserted to form local domains of Li₂V₂(PO₄)₃ directly.It has also been demonstrated that the charge ordering distribution between adjacent V(1)/V(2)ion pair generally exists in Li₂V₂(PO₄)₃,Li₁V₂(PO₄)₃ and V₂(PO₄)₃ phases,refreshing the previous understanding that V(1)⁴⁺/V(2)³⁺charge ordering only exists in Li₂V₂(PO₄)₃ phase.Efforts have also been made to optimize the electrochemical performance of carbon-coated lithium vanadium phosphate electrode.The roles of non-active components in an electrode,including carbon coating,conductive additives(Super P)and binder(PVDF)was firstly investigated by taking lithium vanadium phosphate/reduced graphene oxide composite as an example.The results indicte that coating carbon and reduced graphene oxide can not only improve the electronic conductivity of material,but also help to maintain the integrity of electrode.Although the conductive agent(Super P)can improve electronic conductivity,it will reduce Li⁺diffusion rate and increase polarization.PVDF binder has low intrinstic conductivity and hinders electrolyte penetration,thus compromising the high rate performance of electrode.Then,a systematic study has been conducted to investigate how the electrostatic interaction between the functional groups(carboxyl,hydroxyl,etc.)of a carbon source and the building units of Li₃V₂(PO₄)₃(Li⁺,VO²⁺,PO₄^3-,etc.)in the original precursor affects the structure of Li₃V₂(PO₄)₃-carbon interface in the final composite.It has been demonstrated that the types and concentrations of electro-negative functional groups in a carbon source play an important role in controlling not only the morphology of the product,but also the composition,the crystallinity and the micro-structure of Li₃V₂(PO₄)₃-carbon interface;and in turn the electrochemical behavior of Li₃V₂(PO₄)₃/C composite.The third part of the thesis focuses on investigating the function of cation doping in lithium vanadium phosphate.Al³⁺and Sc³⁺doped lithium vanadium phosphate materials with different doping ratios were prepared.By combining structural characterization techniques of SSNMR and XRD refinement,DFT calculation and electrochemical performance tests,structural changes caused by doping and their correlation with performance are systematically studied.Results of structural characterization indicate that with the increase of Al³⁺and Sc³⁺doping amount,the occupied sites of Al³⁺and Sc³⁺gradually change from interstitial position to vanadium position.Experimental and theoretical results also suggest that Al³⁺doping changes the electron spin ordering/orbital ordering structure,resulting in the changes of magnetic property and intrinsic electronic conductivity.Electrochemical performance tests indicate that appropriate amount of cationic(Al³⁺or Sc³⁺)doping can improve the specific discharge capacity,cycle and rate performance of lithium vanadium phosphate.