Synthesis and electrochemical properties of Co-doped Li9V3(P2O7)3(PO4)2/C as cathode materials for lithium-ion batteries

• Co-doped Li9V3(P2O7)3(PO4)2/C cathode materials have been synthesized by using sol-gel method for the first time.
• The X-ray photoelectron spectroscopy analysis reveal that V4+ ion exists in the V3+ parent when Co2+ ion-doping is presented in Li9V3-xCox(P2O7)3(PO4)2/C compounds.
• Proper amounts of Co ions can be incorporated into the crystal structure of Li9V3(P2O7)3(PO4)2/C and the resulting compounds exhibit smaller particle size, lower charge-transfer resistance and faster lithium-ion diffusion.
• Co-doped Li9V2.96Co0.04(P2O7)3(PO4)2/C compound exhibits the best cyclic performance and rate ability among all the samples.

A series of Co-doped Li9V3 − xCox(P2O7)3(PO4)2/C (x = 0.00–0.10) compounds have been prepared by sol–gel method and the Rietveld refinement results indicate that pure-phase Li9V3 − xCox(P2O7)3(PO4)2/C (x = 0.00–0.10) compounds with trigonal structure can be obtained. Their electrochemical performance has been investigated and the results show that, although the initial specific capacity decreased with Co doping at a lower current rate, both cycle performance and rate capability have excited improvement with proper Co-doping content. Li9V2.96Co0.04(P2O7)3(PO4)2/C compound presents the best cyclic ability and rate performance. The enhancement of cyclic ability and rate performance may be attributed to enhanced specific surface area and improved lithium-ion diffusion during the proper amount of Co-doping (x = 0.04) in V sites.