Preparation and Characterization of Fe‐substituted Li 3 V 2 (PO 4 ) 3 Cathodes for Li‐ion Batteries

In this study, Li 3 V 2(1‐x) Fe 3x (PO 4 ) 3 (0 ≤ x ≤ 0.09) powders are prepared via a solution method and heattreated at 700 °C for 8 hours. The chemical composition, morphology, crystalline structure, and valence of iron in the prepared samples were investigated with ICP, SEM, XRD, and EXAFS. The effects of Fe‐substitution for vanadium sites on the electrochemical properties as cathode materials for lithium ion batteries are studied with capacity retention study and incremental capacity analysis. In addition to the monoclinic Li 3 V 2 (PO 4 ) 3 phase, olivine LiFePO 4 is also observed when x ≥ 0.03. The valence of iron in the prepared powders is predominately 3+ at low substitution level (x ≤ 0.05), while the ratio of the Fe 2+ to Fe 3+ increases abruptly when the substitution level is higher than x = 0.07. Among the prepared samples, Li 3 V 1.86 Fe 0.21 (PO 4 ) 3 shows the best cycling performance at rates higher than 1C. It demonstrates discharge capacities of 141.2 mAh/g for second cycle and 126 mAh/g after 30 cycles at 5C rate.