Researches On The Doping Modification The Performances Of LiVPO₄F/C Cathode Material For Lithium Ion Batteries

New cathode LiVPO4F for lithium-ion batteries possesses the advantages of high voltage,high specific capacity,good thermal stability,excellent cycle performance and high safety performance.Therefore,it becomes a research focus all over the world.However,a series of problem,the pure LiVPO4F is difficult to synthesize,low electrical conductivity and low diffusion coefficient of lithium ion,restricts its industrial application.Currently,the modification research of LiVPO4F is mainly focus on seeking novel synthesis method,coating or doping of metalic ions to substitute vanadium.The influences of doping on the structure,morphology and electrochemical performance of LivPO4F are investigated.Ti4+,Mn4+ and Al3+ ions are selected to dope the V position of LiVPo4F.A13+ doped LiV0.97Al0.03PO4F/C cathode material is first synthesized by one-step hydrothermal method.Then,the formation mechanism of LiVPO4F and sphere are further analyzed.The results are given as follows:(1)The Ti4+ doped LiVPO4F is synthesized by sol-gel method.The specific discharge capacity under 0.2 C of sample LVPF-T3 is 134.1 mAh/g when the doping amount is 3%.The capacity retention rate at 5 C is 96.40%after 50 cycles.The cyclic performance at 0.5 C of the sample with 2%doping is better than that of LVPF-T2.(2)The Mn4+ doped LiVPO4F shows that the electrochemical performance is the best when the doping ratio is 2%,The discharge specific capacity at 0.2 C is 131.2 mAh/g.The cycle performance under 0.5 C and 5 C are obviously improved.With the increase of the doping rate,the charge transfer resistance of cathode is significantly reduced.(3)The LiAIPO4F formed after doping AIF3 posesses the similar structure to LiVPO4F.They are easily form solid solution and the conductivity of intrinsic crystal is improved in return.The doping does not change the triclinic structure of LiVPO4F.Meanwhile,fluoride ion can inhibit the generation of impurity Li3V2(PO4)3.When the doping amount is 3%,LiVO.97Al0.03PO4F/C delivers a maximum discharge specific capacity of 140.9 mAh/g at 0.2 C.The diffusion coefficient of lithium ion is significantly improved and the cyclic performance reaches an excellent level.The material delivers excellent high and low temperature performance at-10 ? and 55 ?.The XRD patterns show that LoVPO4F still possesses whole crystal structure after charge/discharge experiments at above condition.(4)The novel one-step hydrothermal method was first used to synthesize LIVO,97Al0.03PO4F/C cathode material.When the ammonium hydroxide is 4 mL and the pH of solution reaches 6.41,the sample synthesized delives spherical morphology and uniform size.The specific discharge capacity at 0.1 C is 91.0 mAh/g.When the mole ratio of Li is 1.4,a large number of spherical particles are formed.The electrochemical performance of cathode reaches an excellent level.The specific discharge capacity at 0.1 C and 5 C are 135.1 mAh/g and 104.7 mAh/g,respectively.