| As the increasing applications of lithium ion batteries, the cathode materials for lithiumion battery are intensively researched. Lithium transition metal phosphates have attractivedwide attention as promising positive electrodes because of their high structural stability,reliability and abundance resources. LiFePO4 and Li3V2(PO4)3 are the most promising ascathode materials, and will probably take place of the currently commercialized LiCoO2. ButLiFePO4 and Li3V2(PO4)3 have a common disadvantage, low electronic conductivity, whichleads to poor performance at high-current. In this paper, we prepared carbon coatedLi3V2(PO4)3 by an one-step solid-state method using different carbon sources and calcinationtemperatures. Ag-coating modification method has been also researched. ThexLiFePO4·yLi3V2(PO4)3/C composites were also investigated, especially how the effective ofdiphase structure on the physical and electrochemical performances.Comparing different carbon sources between petroleum coke and polypropylene,petroleum coke has a better effect on the appropriate range of temperature (650℃- 800℃),which is related to the degree of graphitization. The best calcination temperature forpetroleum coke is 750℃; while is 700℃for polypropylene. However, the capacities of thecomposites decrease remarkably when calcination temperature increase to 750℃. The reasonis believed to be that PO43- can be reduced to pyrophosophoric acid by hydrogen frompolypropylene at the high temperature, which also affect the electrochemical performance ofpure Li3V2(PO4)3.Li3V2(PO4)3/(Ag+ C) composite cathodes are synthesized by carbon-thermal reduction(CTR) method using petroleum coke as both reduction agent and carbon source, and chemicalplating on Li3V2(PO4)3/C as well. The silver and carbon co-modification improves its kineticsin terms of discharge capacity and rate capability. Discharge capacity is improved from 162mAh g-1 for Li3V2(PO4)3/C to 172 mAh g-1 for Li3V2(PO4)3/(Ag + C), and it exhibites goodcycle performance (140.5 mAh g-1 at 50th cycle under 1 C rate, 97.3% of initial dischargecapacity) and rate behavior (120.5 mAh g-1 under 5 C rate for initial discharge). Electrochemical impedance spectroscopy (EIS) measurements show that the carbon and silverco-modification decreases the charge transfer resistance of Li3V2(PO4)3/(Ag+ C) cathodes.Diphase 9LiFePO4·Li3V2(PO4)3/C composite are synthesized by a simple one-stepmethod using micro-sized petroleum cokes as both the reduction agent and carbon source.The composite shows excellent electrochemical performance at high rate and wide workingtemperature. At rate of 10 C, 9LiFePO4·Li3V2(PO4)3/C can exhibit high reversible dischargecapacity of 125 mAh g-1 even after 150 cycles. And at the temperature of -20℃, thereversible capacity of the composite can maintain 75% of the capacity at room temperatureafter 150 cycles. The improved electrochemical performance of 9LiFePO4·Li3V2(PO4)3/C isattributed to:a) After complex part of LiFePO4 is substituted by Li3V2(PO4)3 with high ionconductivity, which highly increases ion conductivity of the composite.b) The V/Fe mutually doping in the diphase structure and the formatting of LiFePO4,Li3V2(PO4)3, LimFem-2V3PO4 and Li3V2-m(PO4)3 phases enhance the electricalconductivity and improve the insertion/extraction behavior of lithium.c) The grain size is small and uniform after complex. |
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