Preparation and characterization of Ti4+-doped LiFePO4 cathode materials for lithium-ion batteries

Olivine structured LiFePO4 (lithium iron phosphate) and Ti4+-doped LiFe1−xTixPO4 (0.01 ≤ x ≤ 0.09) powders were synthesized via a solution route followed by heat-treatment at 700 °C for 8 h under N2 flowing atmosphere. The compositions, crystalline structure, morphology, carbon content, and specific surface area of the prepared powders were investigated with ICP-OES, XRD, TEM, SEM, EA, and BET. Capacity retention study was used to investigate the effects of Ti4+ partial substitution on the intercalation/de-intercalation of Li+ ions in the olivine structured cathode materials. Among the prepared powders, LiFe0.97Ti0.03PO4 manifests the most promising cycling performance as it was cycled with C/10, C/5, C/2, 1C, 2C, and 3C rate. It showed initial discharge capacity of 135 mAh g−1 at 30 °C with C/10 rate. From the results of GSAS refinement for the prepared samples, the doped-Ti4+ ions did not occupy the Fe2+ sites as expected. However, the occupancy of the doped Ti4+ ions are still not clear, and theoretical calculations are needed for further studies. From the variation of lattice parameters calculated by the least square method without refinement, it suggested that Ti4+-doped LiFePO4 samples formed solid solutions at low doping levels while TiO2 was also observed with TEM in samples prepared with doping level higher than 5 mol%.