Researches On The Synthetic Methods And Electrochemical Performances Of Cathode Material Li₃V₂?PO₄?₃/C For Lithium Ion Batteries

Lithium ion battery is a kind of electrochemical storage devices which is used in communication equipment,electric vehicles,household appliances and military affairs.In the cost structure of Li-ion batteries,the manufacturing cost of the cathode material is about 30-40% which determines the quality of batteries mostly.The polyanion compound Li₃V₂?PO₄?₃ owns the stable structure,rapid Li ion migration rate and high electro-chemical potential.It exhibits the highest theoretical capacity among the polyanion cathode materials.But the octahedron VO 6 is separated by tetrahedron PO4 in its structure thus the electrical conductivity is poor.Furthermore,its capacity decays rapidly when it cycles under high potential?⁴.8V?.Therefore,synthesizing Li₃V₂?PO₄?₃ with high capacity and long cycle life is the research hotspot of this material.In this paper,firstly the solid synthesizing method of Li₃V₂?PO₄?₃ was optimized.Then under the optimized solid reaction process,the carbon-coating resources of Li₃V₂?PO₄?₃ were researched.Metal ion doping and carbon coating of Li₃V₂?PO₄?₃ using sol-gel method were systematically researched to synthesize the Li₃V₂?PO₄?₃/C composite material with high electric conductivit y and better rate capacity.The details are summarized as follows.?1?The orthogonal experiment was adopted to research the influence of pre-decomposition to the electrochemical performance of Li₃V₂?PO₄?₃.The final predecomposition is as follows.Ball mills the raw material for 3 hours and then decomposite it at 350? for 5 hours under gas flow of 100 m L/min.After that,the sintering temperature and time was discussed.The experiments proved that the cathode material will obtain the monoclinic crystal system,small and uniform particles and the best electrochemical pe rformances when sintering at 800 ? for 16 hours.Charge/discharge capacity is 129.2m Ah/g and 124.4m Ah/g at 0.2C rate respectively and still remains 121.3 m Ah/g after 30 cycle times.?2?Acetylene black,glucose,epoxy resin and phenolic resin were adopted f or carbon source to synthesize the carbon coated Li₃V₂?PO₄?₃/C.It was proved that macromolecule carbon source can be in-situ pyrolysized.Furthermore,the carbon film with high conductivity and large superficial area improves the rate and cycle performance of cathode material.Then,the influence of carbon content to the electrochemical performance was discussed by adopt phenolic resin as carbon source.It was proved that the most suitable content is 5 wt.%.The particle size is small and evenly.The discharge capacity is respecitively 129.1m Ah/g and 103.9m Ah/g at 0.2C and 5C rate.The charge transfer resisitance is 59.04 ?.?3?The improved sol-gel method was used to synthesize Li₃V₂?PO₄?₃/C compound.Firstly the sol-gel method was used to synthesize the precursor.Then the precursor and carbon source was ball-milled together and finally the mixture was sintered at 750? for 10 h to synthesize the cathode material.Carbon sourses include glucose,phenolic resin and PVDF.The samples using phenolic resin and PVDF as carbon source have outstanding cycle performance with capacity about 120 m Ah/g at 2C,110 m Ah/g at 5C and 95 m Ah/g at 15 C.In addition,the capacity does not attenuate when cycle 50 times at 5C rate.The EIS test shows the transfer resistance of them are 41.2? and 36.8? respectively.?4?Cathode materials Li₃V_2-?4/3?x Tix?PO₄?₃ using PVDF as carbon source are synthesized via an sol-gel method.X-ray diffraction?XRD?shows that samples exhibit pure monoclinic structure but the crystal parameters and cell volume changed slightly.Elemental analysis shows that Ti⁴⁺ distributes among the particles of cathode evenly.Benefitting from the enhancement of electronic conductivity and Li ion diffusion coefficient,t he discharge capacities and cyclic performance are improved at higher current density by a proper amount of Ti⁴⁺ doping?x=0.03⁰.06?.Charge/discharge capacity is about 110 m Ah/g at 15 C and without decay after cycle 50 times at 5C.?5?Adopting PVDF as carbon source,a novel cathode material of Li-ion batteries in which Na ion was doped in the location of Li ion,L i2.5Na0.5V2?PO₄?₃/C,was synthesized using sol-gel method.From XRD result we know orthorhombic Li₂ Na V₂?PO₄?₃ was formed.Its charge/discharge curve and CV curve is different from Li₃V₂?PO₄?₃ and the work potential is 3.7-3.8V.Comparing with Li₃V₂?PO₄?₃ which has three charge-discharge plateaus,Li_2.5Na_0.5V₂?PO₄?₃ has more stable charge/discharge potential.It has capacity about 80-90 m Ah/g at 12 C.Then,the cathode material Li_3-x Na_x V₂?PO₄?₃?x=0,0.01,0.03,0.05,0.07?with microscope Na-doping was researched.The results show that this material keeps the monoclinic crystal structure.At 12 C rate,the sample with x=0.03 has capacity less than 100 m Ah/g and its Li-ion diffusion coefficient is 10-8-10-7cm²/s.The electrochemical performance is worse than Li₃V_1.92Ti_0.06?PO₄?₃/C.