Optimizing the electrochemical activity and understanding the reaction mechanism of Li3.27FeII0.19FeIII0.81V(PO4)3 cathode material for lithium-ion batteries

This work reports the synthesis of a NaSICON-type structure material, a cost-efficient electrode for lithium-ion batteries, by the substitution of vanadium with cost-effective and eco-friendly iron. Li3.27FeII0.19FeIII0.81V(PO4)3 was synthesized using sol-gel followed by a calcination at 450 °C under argon. XRD has confirmed the successful preparation of a compound isostructural to Li3V2(PO4)3 which crystallizes in a monoclinic structure with the P21/c space group. The electrochemical properties of the novel phosphate were explored at different cut-off voltage windows (2.0–4.2 V, 2.8–4.5 V, 1.4–4.5 V, and 1.0–4.6 V), and using three electrolytes (LP30, LP40, and LP50). The best electrochemical performance was obtained at C/2 over 1.4–4.5 V voltage range and using LP40 delivering a reversible capacity of 150 mAh g−1 after 55 cycles with coulombic efficiency and capacity retention of 99% and 79%, respectively. 57Fe Mӧssbauer spectroscopy evidenced the contribution of Fe2+/Fe3+ redox couple upon the electrochemical reaction. Magnetic measurements revealed the oxidation of the transition metals during the charge process. XAFS study around V and Fe at K-edge revealed the successful substitution of Fe in the V site with a reorganization of the local structure at short and medium range order.