Aliovalent Substitution of V3+ for Co2+ in LiCoPO4 by a Low-Temperature Microwave-Assisted Solvothermal Process

Electrical and ionic conductivity has proven to be a major limiting factor in the adoption of high-voltage polyanion cathode materials, specifically LiMPO4 (M = Mn, Co, Ni). While the aliovalent substitution of V3+ for Fe2+ in LiFePO4 has been shown recently, no conclusive evidence is available for aliovalent substitutions in the analogous LiCoPO4. This study presents a low-temperature microwave-assisted solvothermal synthesis and characterization of aliovalently substituted LiCo1-3sx/2Vx square x/2PO4 for x <= 0.07. The as synthesized LiCo1-3x/2Vx square x/2PO4 samples belonging to the Pn2(1)a space group are transformed to the Pnma polymorph via calcination in an inert atmosphere at 525 degrees C. The LiCo1-3x/2Vx square x/2PO4 samples are characterized with scanning electron microscopy, X-ray diffraction, Rietveld refinement, inductively coupled plasma optical emissions spectroscopy, Raman and infrared spectroscopy, galvanostatic charge/discharge, electrochemical impedance spectroscopy, and cyclic voltammetry. Unlike the prior reports of aliovalent substitution via high-temperature methods, the LiCo1-3x/2Vx square x/2PO4 samples presented here experienced a systematic decrease in the unit cell volume with increasing vanadium substitution. The electrochemical performance and cyclability of LiCo1-3x/2Vx square x/2PO4 is greatly improved compared to the unsubstituted LiCoPO4.