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

Abstract

Electrical and ionic conductivity has proven to be a major limiting factor in the adoption of high-voltage polyanion cathode materials, specifically LiMPO 4 (M = Mn, Co, Ni). While the aliovalent substitution of V 3+ for Fe 2+ in LiFePO 4 has been shown recently, no conclusive evidence is available for aliovalent substitutions in the analogous LiCoPO 4 . This study presents a low-temperature microwave-assisted solvothermal synthesis and characterization of aliovalently substituted LiCo 1–3 x /2 V x □ x /2 PO 4 for x ≤ 0.07. The as synthesized LiCo 1–3 x /2 V x □ x /2 PO 4 samples belonging to the Pn2 1 a space group are transformed to the Pnma polymorph via calcination in an inert atmosphere at 525 °C. The LiCo 1–3 x /2 V x □ x /2 PO 4 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 LiCo 1–3 x /2 V x □ x /2 PO 4 samples presented here experienced a systematic decrease in the unit cell volume with increasing vanadium substitution. The electrochemical performance and cyclability of LiCo 1–3 x /2 V x □ x /2 PO 4 is greatly improved compared to the unsubstituted LiCoPO 4 .