Effect of ionic substitutions on the physicochemical, morphological, and electrochemical properties of lithium-rich vanadium phosphate and pyrophosphate compounds

The new Na/Cr-bisubstituted lithium vanadium monodiphosphate compound (LNVCPP) Li9-xNaxV2.8Cr0.2(P2O7)(3)(PO4)(2), where x=0.0, 0.5, 1.0, 1.5, and 2.0 have been prepared via simple sol-gel combustion route. The as-prepared materials are characterized by XRD, FESEM, and EDX. The XRD data indicate the presence of single phase of Li9-xNaxV2.8Cr0.2 (P2O7)(3)(PO4)(2) (x=0.0-2.0) with trigonal structure. Both cycle performance and rate capability have shown improvement with moderate Na-doping content. Cell of Li8.5Na0.5V2.8Cr0.2(P2O7)(3)(PO4)(2) cathode delivers a specific discharge capacity of 53mAhg(-1) after 35 cycles in comparison with the other samples. The optimum ratio Li8.5Na0.5VCPP coated with carbon delivers a specific discharge capacity of about 75mAhg(-1) after 25 cycles. Therefore, it presents the good electrochemical rate and cyclic stability. The enhancement of the rate and cyclic capability may be attributed to the optimizing particle size, morphologies, and structural stability with the proper amount of Na-doping (x=0.5) in Li sites.