| As polyanion cathode materials for lithium ion batteries,monoclinic lithium vanadium phosphate[Li3V2(PO4)3]and olivine-type lithium iron phosphate(LiFePO4) are attractive for stable structure,excellent cycle performance and safety.However, their low electronic conductivity impedes their use as electrode materials.In this paper,pure Li3V2(PO4)3 was prepared by high temperature solid-state hydrogen reducing method,Li3V2(PO4)3/C was respectively synthesized by carbothermal reaction and sol-gel method,and Fe2P/LiFePO4/C was prepared by sol-gel method. The physical properties of Li3V2(PO4)3,Li3V2(PO4)3/C and Fe2P/LiFePO4/C were investigated by XRD,FTIR,SEM,TEM,EDAX,XPS and Raman spectroscopy,and their electrochemical performances were investigated by galvanostatic current charge-discharge,cyclic voltammetry,electrochemical impedance spectroscopy.The effects of preparation conditions on the physical properties and electrochemical performances of active materials were investigated.Pure Li3V2(PO4)3 was synthesized at 600-900℃by solid-state hydrogen reducing method,using V2O5 and mixture of Ar and H2 as vanadium source and reductant,respectively.Li3V2(PO4)3,synthesized by calcination at 850℃for 16 h, has smooth surface and the average particle size of 2μm.In the voltage range of 3.0-4.3 V,the reversible capacity of pure Li3V2(PO4)3 is 107.8 mAh·g-1at 0.1 C and 62 mAh·g-1at 2C,respectively.While in the voltage range of 3.0-4.8 V,the capacity is 140.1 mAh·g-1at 0.1C and 84 mAh·g-1at 2C,respectively.The low intrinsic electronic conductivity of pure Li3V2(PO4)3 is responsible for its low capacity at high rates.Li3V2(PO4)3/C composite materials were synthesized by carbothermal method, using acetylene black,activated carbon and carbon black as carbon source and reductant,respectively.It was found that acetylene black and activated carbon have more strong reducing ability than carbon black.It is difficult to prepare the composite material Li3V2(PO4)3/C with carbon black as reductant.The Li3V2(PO4)3/C sample synthesized with acetylene black has coarse surface,an average particle size of about 700 nm and better electrochemical performance.In the voltage range of 3.0-4.3 V,the reversible capacity of Li3V2(PO4)3/C is 116 mAh·g-1at 0.1C,and 86 mAh·g-1at 2C, respectively.While in the voltage range of 3.0-4.8 V,the reversible capacity is 151 mAh·g-1at 0.1C,and 105 mAh·g-1at 2C,respectively.Comparing with pure Li3V2(PO4)3,Li3V2(PO4)3/C cathode material displays better electrochemical performance because the coated carbon can enhance the electronic conductivity of composite material and suppress the growth of Li3V2(PO4)3 crystal.A sol-gel method with citric acid,V2O5 powder,H2O2,NH4H2PO4 and LiOH as starting materials was used to prepare the carbon-coated Li3V2(PO4)3 with excellent electrochemical performance.As a complexing agent,citric acid can make all kinds of raw materials mixed in molecular level,and it can also decompose into carbon in higher calcination temperatures.The in situ produced carbon uniformly distributed in the mixture is beneficial for reaction and can suppress the growth of Li3V2(PO4)3 crystal.In this work,V2O5 powder was dissolved by H2O2 and reacted with citric acid and other raw materials to get precursor gel,hence it is not necessary to take long time to prepare the V2O5 gel.In the formation procedure of the precursor gel,citric acid and H2O2 may reduce V5+to V4+.This is advantageous in reducing calcination temperature and shortening reaction time for the preparation of Li3 V2(PO4)3.The carbon-coated Li3V2(PO4)3 synthesized by calcining the precursor at 800℃displays coarse surface,an average particle size of about 400 nm and better electrochemical performance.The electronic conductivity of composite materials increases with the increase of calcination temperature,and the sample calcined at 800℃for 8 h presents higher electronic conductivity and better electrochemical performance.In the voltage range of 3.0-4.3 V,at 0.1C,the reversible capacity of the carbon-coated Li3V2(PO4)3 is 128.1 mAh·g-1at 25℃and 130 mAh·g-1at 55℃, respectively;at 2C,the reversible capacity is 108.9 mAh·g-1at 25℃and 113.3 mAh·g-1at 55℃,respectively.In the voltage range of 3.0-4.8 V,the reversible capacity at 0.1C is 177 mAh·g-1.The electrochemical performance of the above carbon-coated Li3V2(PO4)3 is better than that of samples prepared by hydrogen reducing reaction and carbothermal method.This may result from its higher electronic conductivity and smaller particle size.The effects of various preparation methods on the electrochemical kinetics of Li3V2(PO4)3 and Li3V2(PO4)3/C samples were investigated.The exchange current density(j0),charge transfer resistance(Rt)and lithium ion diffusion coefficient (DLi+)were determined by linear polarization,EIS and PITT(potentiostatic intermittent titration technique),respectively.The values of j0 and DLi+ increase and the Rt value decreases in sequence of hydrogen reducing,carbothermal and sol-gel methods.This is in general agreement with the results of the electrochemical test.The Fe2P/LiFePO4/C composite material,synthesized by a sol-gel method using citric acid,iron nitrate,lithium acetate and ammonium dihydrogen phosphate as starting materials,presents better electrochemical performance.The Fe2P/LiFePO4/C sample synthesized at 650℃displays the best electrochemical performance.At 25℃,its initial reversible capacity is 158 and 125 mAh·g-1at 0.1 and 1C,respectively; and the corresponding retention rate of capacity is 95.3%and 92.2%after 30 cycles, respectively.At 55℃,its initial reversible capacity is 162 and 140 mAh·g-1at 0.1 and 1C,respectively;and the corresponding retention rate of capacity is 96.4%and 93.2%after 30 cycles,respectively.The lithium diffusion coefficient increases with the increase of temperature,from 1.71×10-14cm2·s-1(25℃)to 7.83×10-14cm2·s-1 (55℃).The increase of lithium diffusion coefficient may be partially responsible for the improvement in the electrochemical performance of the sample. |
PDF Full Text Request