Research on High Rate Capabilities B-Substituted LiFePO4

LiFePO4 is currently recognized as one of the most promising electrode materials for large-scale application of lithium ion batteries. However, the limitation of rate capability is believed to be intrinsic to this family of compounds due to the existence of larger tetrahedral (PO4)(3-) unit and quasi-hexagonal close-packed oxygen array. This paper report here a systematic investigation of the enhancement of rate performance by partly substitution of light small triangle oxyanion, (BO3)(3-), for the larger tetrahedral (PO4)(3-) units in LiFePO4. Cathode electrode materials LiFeBXO(1-x)O4-delta, in which X = 0, 3, 6 and 9, mol%, were synthesized by solid-state method. The as-synthesized products were characterized by X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM) and Electrochemical Measurements. The results showed that 6 mol% of boron substitution had no effect on the structure of LiFePO4 material, but significantly improved its rate performance. The initial discharge capacity of the LiFeB0.06P0.94O4-delta sample was 145.62 mAh/g at 0.1 C, and the capacity retention ratios of 81% at 2 C and 76% at 5 C were obtained, demonstrating that a proper amount of boron substitution (lower than 6 mol%) could significantly improve the rate performance of LiFePO4 cathode material.