Study On Modification Of Lithium Vanadium Phosphate Cathod Material For Li-ion Batteries

Monoclinic structure of Li₃V₂?PO₄?₃ has attracted great attention due to its high theoretical capacity?197 mAh/g?,excellent thermal stability and cyclability.However,the poor electrical conductivity and low lithium ion diffusivity limite Li₃V₂?PO₄?₃ to be put into use.In this paper,the precursor was prepared by grinding-sol-gel method,choosing different temperature and time to roast the materials.And then,the Li_3-2xMg_xV₂?PO₄?₃/C and Li₃V_2-xAl_x?PO₄?₃/C composites with single Mg or Al doping,and Li_2.9Mg_0.05V_1.9Al_0.1?PO₄?₃/C composites with single Mg and Al doping were obtained through grinding-sol-gel method.we also prepared the precursor under different pH values.Then using XRD and SEM to analyze their crystal structure and morphology,and the electrochemical performances are investigated by galvanostatic current charge-discharge,cyclic voltammetry and electrochemical impedances spectroscopy.The mam contents are summarized as follows:The Li₃V₂?PO₄?₃/C materials were prepared by grinding-assisted sol-gel method with different calcination temperature and sintering time.The optimum experimental conditions were as follows: CH₃COOLi·2H₂O as the lithium source,NH₄VO₃ as the vanadium source,citric acid as the carbon source,the precursor was kept at 80? for 12 h,and then grinding3 h,the initial heating is up to 350? for 5 h and then heating performed at 750? for 10 h.The pure Li₃V₂?PO₄?₃/C show the best performance: in the voltage range of 3.0-4.3 V,the discharge capacity of 118 mAh/g can be reached at 0.2 C,charge discharge efficiency is97.8 %.Then,the effects of Mg²⁺ and Al³⁺ doping were also studied in the paper.The Li_3-2xMg_xV₂?PO₄?₃/C and Li₃V_2-xAl_x?PO₄?₃/C were synthesized by grinding-assisted sol-gel method using magnesium nitrate as the magnesium source,aluminum nitrate and aluminium oxide as the aluminum source.The structure,morphology and electrochemical performance of the Li_3-2xMg_xV₂?PO₄?₃/C and Li₃V_2-xAl_x?PO₄?₃/C were characterized.Theresults showed that the discharge specific capacity and cyclic stability of the composites were enhanced after doping.Li2.9Mg0.05V₂?PO₄?₃/C has the best electrochemical performance.The initial discharge capacity of Li2.9Mg0.05V₂?PO₄?₃/C is 125.4 m Ah/g at 0.2C rate and the retention rate of capacity is 98.1 % after 50 cycles.The best aluminum source is aluminium oxide and When x=0.1,the Li3V1.9Al0.1?PO₄?₃/C has the best electrochemical performance.The initial discharge capacity of Li3V1.9Al0.1?PO₄?₃/C is121.6 m Ah/g at 0.2 C rate and the retention rate of capacity is 98.6 % after 50 cycles.The Li_2.9Mg_0.05V_1.9Al_0.1?PO₄?₃/C was synthesized based on the optimal doping ratio by grinding-assisted sol-gel method.The results proved that the electrochemical performances of the Li_2.9Mg_0.05V_1.9Al_0.1?PO₄?₃/C were enhanced.In the voltage of 3.0-4.3 V,the initial discharge specific capacity of Li_2.9Mg_0.05V_1.9Al_0.1?PO₄?₃/C at the rate of 0.2 C were 130 m Ah/g,and the retention rate of capacity is 98.9 % after 50 cycles.The capacity of Li_2.9Mg_0.05V_1.9Al_0.1?PO₄?₃/C was 121.2 mAh/g even at the high rate of 10 C.The Li₃V₂?PO₄?₃/C with different pH was synthesized by grinding-assisted sol-gel method.The precursor solution ph is adjusted to pH 3,5,7,9 with aqueous ammonia and acetic acid.The results show that the electrochemical properties of Li₃V₂?PO₄?₃/C composites are the best under neutral conditions.In the voltage of 3.0-4.3V,the initial discharge specific capacity at the rate of 0.2 C was 130.4 mAh/g,and the retention rate of capacity is 98.9 % after 50 cycles,and the capacity was 119.6mAh/g even at the high rate of 10 C.