Synthesis and electrochemical properties of layered lithium monodiphosphate Li9V3−xAlx(P2O7)3(PO4)2 solid solutions

We recently proposed Li-rich vanadium monodiphosphate Li9V3(P2O7)3(PO4)2 as a novel polyanion-type cathode for rechargeable Li batteries. Herein, the Li9V3−xAlx(P2O7)3(PO4)2 solid solutions are obtained in different ratios of V and Al via solid-state reaction at 750 °C in high-purity N2. The cell parameters (including a, c and V) of Li9V3−xAlx(P2O7)3(PO4)2 decrease in a linear way with Al-doping content. The intrinsic electronic conductivity of Al-doped samples reaches ∼10−7 S cm−1, which is more than one order of magnitude higher than that of the undoped one. The electrochemical insertion/extraction properties of the trigonal Li9V3−xAlx(P2O7)3(PO4)2 (x = 0.00, 0.10, 0.25, and 0.50) phases are also presented. The Al-doped phases are demonstrated the better capacity retention and higher output voltage than that of the pristine one. After 30 cycles, Li9V2.75Al0.25(P2O7)3(PO4)2 can keep 92.4%, 87.7%, and 85.5% of the initial capacity under the current rates of 0.2 C, 0.5 C and 1.0 C, respectively. The favorable voltage polarization, enhanced electronic conductivity, and dramatically contractible particle size after Al doping play significant contributions of comparatively good electrochemical performance of Li9V2.75Al0.25(P2O7)3(PO4)2 sample.