Research On Preparation,Electrochemical Performance And Mechanism Of LiVPO₄F/C Cathode Material For Lithium-Ion Batteries

In order to reduce the environmental problems caused in the application process of energy by human,China has put forward the goal of"carbon peak"and"carbon neutrality"in 2020,which urgently requires the development of green and low-carbon energy industry.Lithium-ion batteries have taken a dominant position in consumer batteries,energy storage power stations and new energy vehicles due to their advantages of environmental friendliness,high energy density and long service life.Although a variety of cathode materials have been commercialized,a balance ha d to be made between the energy density and cyclic stability.As a fluorinated polyanionic cathode material,LiVPO₄F/C has attracted much attention due to the advantages of higher operating voltage,energy density and better cycling stability,but its development is seriously hampered by its drawbacks such as low electronic conductivity and difficulties in the preparation of pure phases.Addressing these problems,in order to improve the comprehensive electrochemical p erformance of LiVPO₄F/C,the modifications such as coating carbon,B-doping pyrolytic carbon,combining fluorine complement and conductive agent and modulatin g inorganic salt have been taken according to different preparation methods in this paper.The main research results are as follows:（1）The LiVPO₄F/C cathode materials were prepared by one-step rapid reduction sol-gel method using epoxy resin,soluble starch and polyvinylidene fluoride（PVDF）as carbon sources,respectively,and V₂O₅,LiF and NH₄H₂PO₄ as raw materials.The results show that there are fewer defects in graphite microcrystals formed by epoxy resin pyrolysis than in PVDF pyrolytic carbon,and the conductivity of epoxy pyrolytic carbon is higher than that of PVDF pyrolytic carbon.However,the samples prepared with carbon sources PVDF has lower content of impurity phase Li₃V₂（PO₄）₃/C,longer4.2 V voltage plateau and excellent rate performance delivering the specific capacities of 141.9,136.4 and 115.0 mAh g^-1 at 0.2,1 and 5 C,respectively.This is attributed to the fact that PVDF produces HF gas during the pyrolysis,which inhibits the volatilization of fluoride and replenishes the loss of fluorine in the precursor.However,the hydrogen radicals released from the epoxy resin and soluble starch can combine with fluorine,thus accelerating the volatilization of fluorine and leading to the increase of impurity phase Li₃V₂（PO₄）₃/C.（2）The B atoms were doped into the PVDF pyrolysis carbon layer on the surface of LiVPO₄F by a two-step carbon thermal reduction method using boric acid as the B source.The results show that the B-doped LVPF/C-B₃ sample has the best electronic conductivity,Li⁺diffusion rate and exchange current density(0.536 S cm^-1,1.127×10^-10 cm² s^-1 and 0.150 m A cm^-2),resulting in the excellent rate and cycling performance.The specific capacities at 0.2,5,10 and 15 C are 148.1,132.9,128.4 and125.6 mAh g^-1,respectively.After 800 cycles at 6 C,the capacity retention rate is up to 85.0%and the crystal structure is still intact.This is because the formation of pyridine-like B（BC₂O and BCO₂）after the doping of B-atom generates defective active sites,while graphite-like B（BC₃）promotes the growth of graphite-like carbon hexamer rings,which facilitate the rapid transfer of Li⁺and electrons.The theoretical calculations based on the first-principles show that B-atom doping increases the density of states near the Fermi energy level of pyrolytic carbon,and pyridine-like B transforms LiVPO₄F/C from a p-type semiconductor to a metal-like structure,thus improving the conductivity and electrochemical p erformance of LiVPO₄F/C cathode materials.（3）The LiVPO₄F/C cathode materials were prepared by a combination of one-step rapid reduction sol-gel method and spray drying pyrolysis method using polytetrafluoroethylene（PTFE）emulsion as the fluorine complement and multi-walled carbon nanotubes（MWCNT）as the conducting agent.Spherical LiVPO₄F/C cathode materials with good performance are obtained by adding 5 g PTFE emulsion and 2%MWCNT.The reason is that the fluoride or fluoride ions released from PTFE during the sintering process can inhibit the volatilization o f fluorine and supplement the fluorine source,thus inhibiting the formation of Li ₃V₂（PO₄）₃/C.The sample has higher conductivity,lower charge transfer impedance and electrochemical polarization after the addition of MWCNT.Its discharge specific capacities at 2,5 and 10 C rates are132.9,121.6 and 100.9 mAh g^-1,respectively,with the capacity retention of 77.3%after 1000 cycles.This is attributed to the three-dimensional conductive network with good conductivity is formed on the surface of LiVPO₄F under the intertwining effect of MWCNT and pyrolytic carbon.（4）LiVPO₄F/C cathode material was prepared by one-step rapid reduction sol-gel method using NH₄F as regulator and PVDF as carbon source.LiVPO₄F/C prepared with 0.025 mol NH₄F has the best Li⁺diffusion coefficient,conductivity,exchange current density,and excellent rate and cycling performance.The discharge specific capacities are 144.8,127.5,120.0,and 112.7 mAh g^-1 at 1,5,10,and 15 C,respectively.This is because NH₄F can suppress the formation of impurity phase Li₃V₂（PO₄）₃/C,refine the particles and dope N element.The"Combination-Protection-Release"mechanism proposed in this paper can well explain the role of NH₄F.The generation of F^-can combine with Li⁺and F^-ionized by LiF to form Li HF₂,which can remain stable and does not decompose at temperatures below220℃,and release LiF and HF until 220℃,thus achieving the effect of inhibiting fluorine volatilization and supplementing fluorine source.The in-situ XRD analysis reveals that the phase transition process of LiVPO₄F/C is LiVPO₄F（?）Li_0.67VPO₄F（?）VPO₄F and VPO₄F（?）LiVPO₄F during the charging and discharging process,respectively.The LiVPO₄F/C full cell with high energy density and excellent rate performance can be obtained by matching it with suitable graphite anode and electrolyte.