Modification And Lithium Storage Properties Of NASICON-type Lithium Vanadium Phosphate Cathode Materials

Recently,lithium-ion batteries（LIBs）have been widely used in portable electronic devices,transportation,and storage of renewable energy for smart grid due to its high energy density.Cathode materials play a key role in high-performance LIBs.Monoclinic lithium vanadium phosphate has been considered as a promising cathode material due to its high operating voltage,large theoretical specific capacity,structural stability and abundant reserves.However,the poor electronic conductivity(10^-7 S cm^-1)and poor lithium-ion diffusion limit its practical applications.A series of studies are carried out in this paper,which mainly include coating with conductive carbon or doping with foreign atoms and nano-structure design.The detail contents are as follows:1.Nitrogen-doped carbon coated Li₃V₂（PO₄）₃（LVP/NC）and its electrochemical performance.The uniform and continuous nitrogen-doped carbon coating was obtained by the oxidative self-polymerization of dopamine on Li₃V₂（PO₄）₃ surface and subsequent carbonization.Nitrogen doping improves the electronic conductivity and increase the electrochemical activity.Moreover,the defects in the nitrogen-doped carbon coating also provide Li+ diffusion paths.Consequently,the resultant LVP/NC exhibits high rate capability(10 C,74 mA h g^-1)and excellent cycle stability（capacity retention after 100 cycles was 95.4% at 1 C）.2.Li₃V₂（PO₄）₃/nitrogen-doped reduced graphene oxide（LVP/N-RGO）nanocomposite with enhanced lithium storage properties.The three-dimensional（3D）porous LVP/N-RGO composite was prepared by a facile one-pot hydrothermal method followed by heat treatment using urea as nitrogen precursor.Firstly,the introduction of N atoms further improves the conductivity of RGO and promotes electron transfer during the electrochemical reaction.Secondly,LVP nanoparticles robustly attached on graphene,especially nitrogen-doped graphene,which enhanced intimate contact and provide continuous electron transport pathways.Thirdly,the unique 3D porous architecture accommodates the strain induced by the volume change during lithium de/insertion and favor electrolyte penetration which facilitating fast Li+ migration between electrode and electrolyte.Additionally,nano sized LVP particles further shorten the ion diffusion length,resulting in accelerating Li+ diffusion in LVP.The resultant LVP/N-RGO composite exhibit superior rate properties(92 mA h g^-1 at 30 C)and outstanding cycle performance（97.3% capacity retention after 300 cycles at 5 C）.3.Ru-doped Li₃V₂（PO₄）₃/carbon（Li3V2-x Rux（PO4）3/C）nanocomposite and its electrochemical performance.Micelle formation of lauric acid surfactant in the preparation process inhibits colloid agglomeration.The size of the synthesised Li₃V₂（PO₄）₃/C was about 80 nm.Ru3+ was doped into the lattice structure of Li₃V₂（PO₄）₃ and improved the intrinsic conductivity.Li₃V_1.98Ru_0.02（PO₄）₃/C nanocomposite exhibits superior specific capacity(0.2 C,127.3 mA h g^-1),high rate capability(5 C,107.4 mA h g^-1;10 C,97.3 mA h g^-1;20 C,85.4 mA h g^-1)and good cyclic stability（96.6% capacity retention after 500 cycles at 5 C）.4.Organic-phase synthesis of Li₃V₂（PO₄）₃@carbon（LVP@C）nanocrystals and their lithium storage properties.The formation mechanism of nanocrystals could be attributed that high-boiling-point OA and OAm as the reducing and capping agents adsorb on precursor of Li₃V₂（PO₄）₃,and limit crystal growth.Through a subsequent carbon coating process,the LVP@C nanocrystals were synthesised.The resultant LVP@C nanocrystals exhibit perfect rate performance(84 mA h g^-1 at a rate of 20 C)and outstanding cyclic stability（96.2% capacity retention after 200 cycles at 5 C）.5.Li₃V₂（PO₄）₃ embedded in hierarchically porous 3D carbon matrix（（LVP@C）@pC）for high-rate and ultralong-life lithium-ion batteries.High boiling point tetraethylene glycol as solvent,vanadium acetylacetonate as vanadium precursor will decompose and translate into carbon coated nanocore.Tetraethylene glycol as the capping agent inhibits particle growth and turns into porous carbon matrix during the heat treatment.（LVP@C）@pC was obtained as cathode which delivered with a excellent lithium storage properties(129 mA h g^-1 at 0.2 C,96.0% capacity retention after 500 cycles at 10 C).6.Enhanced electrochemical properties of Mg F2 and C co-coated Li₃V₂（PO₄）₃（MgF2-LVP/C）composite for Li-ion batteries.MgF2 layer can prevent the electrode surface from HF attack,which enhances the structural stability of Li₃V₂（PO₄）₃.Meanwhile,the carbon layer enormously improves the conductivity of Li₃V₂（PO₄）₃.Thus the cycle performance of Mg F2-LVP/C has been significantly enhanced via the synergetic of MgF2 layer and carbon coating（capacity retention after 100 cycles was 95.8% at 1 C from 3.0 V to 4.8 V）.