Synthesis and electrochemical properties of Co-doped Li 3V 2(PO 4) 3 cathode materials for lithium-ion batteries

Abstract

Co-doped Li 3V 2− x Co x (PO 4) 3/C ( x = 0.00, 0.03, 0.05, 0.10, 0.13 or 0.15) compounds were prepared via a solid-state reaction. The Rietveld refinement results indicated that single-phase Li 3V 2− x Co x (PO 4) 3/C (0 ≤ x ≤ 0.15) with a monoclinic structure was obtained. The X-ray photoelectron spectroscopy (XPS) analysis revealed that the cobalt is present in the +2 oxidation state in Li 3V 2− x Co x (PO 4) 3. XPS studies also revealed that V 4+ and V 3+ ions were present in the Co 2+-doped system. The initial specific capacity decreased as the Co-doping content increased, increasing monotonically with Co content for x > 0.10. Differential capacity curves of Li 3V 2− x Co x (PO 4) 3/C compounds showed that the voltage peaks associated with the extraction of three Li + ions shifted to higher voltages with an increase in Co content, and when the Co 2+-doping content reached 0.15, the peak positions returned to those of the unsubstituted Li 3V 2(PO 4) 3 phase. For the Li 3V 1.85Co 0.15(PO 4) 3/C compound, the initial capacity was 163.3 mAh/g (109.4% of the initial capacity of the undoped Li 3V 2(PO 4) 3) and 73.4% capacity retention was observed after 50 cycles at a 0.1 C charge/discharge rate. The doping of Co 2+into V sites should be favorable for the structural stability of Li 3V 2− x Co x (PO 4) 3/C compounds and so moderate the volume changes (expansion/contraction) seen during the reversible Li + extraction/insertion, thus resulting in the improvement of cell cycling ability.