Preparations Of Vanadium Modified Lithium Iron Silicates As Cathode Materials And Vanadium Analyses

Lithium iron silicate(Li2FeSiO4)has been considered as an ideal cathode material for lithium ion batteries owing to a nominal capacity of 330 mAh/g.The first principles calculations show that the deintercalation voltage during exchanging the second lithium ion is 4.8 V.The substitution of divalent iron with the equal amount of divalent vanadium to form Li2Fe0.85V0.15SiO4/C can reduce the voltage of delithiating the second lithium ion,thereby improving discharge capacity.However,experimental preparations the high amount of vanadium modified lithium iron silicates are extremely difficult owing to the diversified valence states of vanadium and the difficulty to obtain V2+.The existing form and impact of vanadium on the modified material remain unclear.In this work,a series of different amount of vanadium modified Li2FeSiO4/C were synthesized by sol-gel precursor and solid state reaction using ammonium metavanadate,vanadium trioxide and vanadium oxalate as vanadium sources,with the impact and the existing form of vanadium on Li2FeSiO4/C analyzed.The experimental results show that vanadium modified Li2FeSiO4/C using NH4VO3 and V2O3 as sources exhibited the main crystal structure indexed to Pmn21 space group,with impurities,such as Li2SiO3,Fe and Fe3O4.A mixed Pmn21 and P21 phase,with Li2SiO3 and Fe3O4,were identified in the vanadium modified Li2FeSiO4/C using C4O8V as vanadium source.The initial and the maximum discharge capacity were 215.2 and 242.2 mAh/g at 0.1C rate with 15%vanadium modified Li2FeSiO4/C using NH4VO3 as vanadium source,corresponding to 1.30 and 1.46 moles per formula unit,respectively,remaining 161.4 mAh/g after 31 cycles.The initial discharge capacities were 179,226.5,222.3 and 238.5 mAh/g at 0.1C rate with 35%,40%,45%and 50%vanadium modified Li2FeSiO4/C using V2O3 as vanadium source.But the capacity decreased very fast.The initial discharge capacity was 179.8 mAh/g with 25%vanadium modified Li2FeSiO4/C using C4O8V as vanadium source,and the capacity reached 153.5 mAh/g after 30 cycles.In contrast,50%vanadium modified Li2FeSiO4/C can be prepared using V2O3 as vanadium source,with the initial discharge capacity corresponding to 1.43 moles per formula unit.Further analysis shows that the modification of vanadium is beneficial for the carbon residue using ammonium metavanadate,vanadium trioxide and vanadium oxalate as vanadium sources.Graphitized carbon and amorphous carbon were found on the surface and between particles of Li2FeSiO4.The charge-transfer resistances between the electrolyte and the active particles were obviously reduced.Quadrivalent vanadium predominated at 150 nm below the surface.The graphitization degree of carbon decreased with the increase of vanadium using NH4VO3 and V2O3 as vanadium sources,containing V2+.However,the graphitization degree increased first and then decreased using C4O8V as vanadium source,containing V3+,V4+ and V5+.Silicon ions may be replaced by high-valence vanadium,while iron ions may be replaced by low-valence vanadium.