Preparation And Research Of Polyanionic Cathode Materials Li₉M₃P₈O₂₉?M=V/Fe?

A series of Li₉M₃?P₂O₇?₃?PO₄?₂ ?M=V/Fe? polyanionic cathode materials were prepared successfully by sol-gel method in the paper. First, We studied the influence of sintering temperature of the target product Li₉V₃?P₂O₇?₃?PO₄?₂, analyzed the history of the material in the process of temperature change. Finally, The best temperature was 750?, pure phase Li₉V₃?P₂O₇?₃?PO₄?₂ belonged to the layered structure and space point group P3c1, the influence of temperature on the structure and electrochemical properties and analysis found that discharge specific capacity of material Li₉V₃?P₂O₇?₃?PO₄?₂was 110 mAh/g.at 0.2C ratio.Second, the research of magnesium doped lead to oxygen defects effects on morphology and the electrochemical properties of Li₉V₃?P₂O₇?₃?PO₄?₂.Mg doping amount x=0.1 or less, still belonged to the single layered structure, space group P3c1, constant a, c,V increased after reducing and the c/a value corresponding the change along with the increase of theMg doped content lattice , The better of three-dimensional layered crystal structure, the better lithium ion embedded process.The increase of Mg content is a symbol of [VO₆] content decreased and [MgO₆] structure increased, the V³⁺/V⁴⁺ redox electricity to reduce oxygen defect structures, however, the material structure stability enhanced, lead to improvement of electrochemical performance.The average particle size of composite material Li₉V_2.9Mg_0.1?P₂O₇?₃?PO₄?₂/C was 1?m by the SEM scan,the first discharge specific capacity was as high as 115 mAh/g at 0.2C ratio in 2.0-4.6V,capacity 113 mAh/g after 40 cycling times, capacity retention 95%, when x=0.2, the material appeared mixed phase,structure stability decline dramatically, the electrochemical properties decreased significantly. So the best doping Mg content was x=0. 1.In addition,the amount of phosphorus defects ??=0.0,0.1,0.2,0.3,0.4,0.5? for polyanionic material Li₉V₃?P₂O₇?₃?PO₄?₂ was studied, when the defect quantity ?<0.4,it contained only special trace impurity phase, which could be ignored basically, characteristic peak sharp and clear, but the material with defect quantity ?=0.5 contained obvious impurity peak and its electrochemical properties fell sharply. The material of defect quantity ?=0.4 had stable specific capacity 105mAh/g in the second cycle at 2C, higher specific capacity and stability in 103mAh/g after 40 cycling times and capacity remained 96% of first capacity. The sample with ?=0.4 showed the minimum Li/P disorder,the highest capacity,excellent cycle performance and good rate capability.Finally,for the first time double phosphate materials Li₉Fe₃?P₂O₇?₃?PO₄?₂ was prepared successfully by the gel-sol method in the experiment, the precursor at different sintering temperature ?700 ?, 750?, 800?, 850?? calcined under 8 h, We studies the influence of synthesis temperature on the final product,and We reseachedthe physical properties of the polyanion anode material Li₉Fe₃?P₂O₇?₃?PO₄?₂ in the comprehensive study for the first time.Results showed that the best sintering temperature was 750 ? and could attain pure phase at it,SEM scan demonstrated nanostructures. Precursor's TG/DSC differential thermal testing proved it had obviously two endothermic peak at 797 ? and 842 ?, corresponding to the high temperature decomposition. pure phase XPS shows that material surface properties of Fe oxidation state is +3.