| Commercialized cathode materials in lithium ion batteries are mainly lithium transition-metal oxides;however,high cost and security problem limit their large-scale use.Among newly-exploited cathode materials,vanadium-based phosphates,with stable structure and high theoretical specific capacity,have been attracting much research interest.In this work,two vanadium-based phosphates, Li3V2(PO4)3 and LiVOPO4,were prepared and extensively investigated.Since the electrochemical performances of both materials are poor due to their low inherent electronic conductivity and Li ion diffusion,we designed and prepared core-shell structure nanocomposites for Li3V2(PO4)3/C andβ-LiVOPO4/RuO2.We also preparedα-LiVOPO4 material through a hydrothermal route,and studied the electrochemical performance of LiVOPO4 as an anode material for Li ion batteries.We synthesized Li3V2(PO4)3 with a sol-gel method,and then prepared core-shell Li3V2(PO4)3/C through a hydrothermal process.TEM images exhibit that Li3V2(PO4)3 particles are encapsulated with a carbon shell with 10 nm in thickness.The electronic conductivity was improved for the presence of the carbon.Compared with pure Li3V2(PO4)3,core-shell Li3V2(PO4)3/C material presented enhanced electrochemical Li+ intercalation performances.The discharge capacity of Li3V2(PO4)3/C was 126 mAh/g after 50 cycles at a current density of 28 mA/g,much higher than that of pure Li3V2(PO4)3(68 mAh/g after 30 cycles),and the retention rate in discharge capacities was 98.5%for Li3V2(PO4)3/C after 50 cycles,but only 62.8%for pure Li3V2(PO4)3 after 30 cycles.β-LiVOPO4 was synthesized with one-pot sol-gel method,and then we preparedβ-LiVOPO4/C andβ-LiVOPO4/RuO2 composite materials.The EDS results reveal that Ru and C are homogeneously distributed amongβ-LiVOPO4 particles.Compared with pristineβ-LiVOPO4,β-LiVOPO4/RuO2 composite presented better electrochemical Li+ intercalation performances due to the enhanced electrical conductivity and Li ion diffusion.The discharge capacity ofβ-LiVOPO4/RuO2 composite was 119 mAh/g after 30 cycles,and the retention rate in discharge capacities was～100%forβ-LiVOPO4/RuO2 composite after 30 cycles,but only 82.9%for pristineβ-LiVOPO4.Even at a higher current density of 80 mA/g,β-LiVOPO4/RuO2 composite showed higher discharge capacity than pristineβ-LiVOPO4 at the current density of 10 mA/g;after 30 cycles the discharge capacity retained 86 mAh/g,and the retention rate still attained 96.2%.Also,we studied the electrochemical performances of LiVOPO4 as an anode material for Li ion batteries.Through the XRD patterns tested at different charge-discharge states we speculated the mechanisms for Li recycle:VPO4+3Li+ +3e-(?)V+Li3PO4.Theβ-LiVOPO4 anode material showed an initial discharge capacity of 726 mAh/g and a reversible capacity higher than 380 mAh/g during 30 cycles;the coulombic efficiency exceeded 94%.The higher discharge capacity and cyclic ability may be due to the large PO43- ions that can alleviate the change of the cell volume during the charge/discharge processes,and the formation of Li2O during the first discharge process that can buffer the volume change in the electrodes.Finally,α-LiVOPO4 hollow microspheres were prepared through the one-pot hydrothermal reaction of LiOH·H2O,V2O5,H3PO4,and N2H4·H2O at 250℃for 48 h. During this procedure,the reactant,V2O5 particles,served as the template and was consumed when the hydrothermal reaction was completed.Therefore,such a self-sacrifice template method makes large-scale production ofα-LiVOPO4 hollow spheres feasible,α-LiVOPO4 hollow spheres were exploited as both cathode and anode materials for lithium ion batteries.Vanadium-based phosphates display remarkable electrochemical and thermal stability as well as higher discharge plateaus vs.Li+/Li;therefore,they have been attracting much interest as promising cathode materials for Li ion batteries.However, they show poor electrochemical performances due to the low electrical conductivity. Core-shell composites proposed in this paper improved the performances of vanadium-based phosphate materials;however,economic and effective methods are still necessarily exploited to promote their practical applications to 4 V Li ion batteries. |
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