Structure And Performance Of Multiple Ions Doping LiFePO₄/C

LiFePO4has been investigated as a potential cathode material for power lithium ion battery due to its rich raw material sources, low cost, high theoretical specific capacity(170mAh·g-1) and operating voltage, excellent high-temperature behavior and safety performance. However, LiFePO4has poor rate performance because of its low electronic conductivity and lithium-ionic diffusion velocity, which limits its practical application. At present, many methods have been used to solve this problem, including narrowing partical size, coating the conductive agent on the partical surface and doping ions.In this work, the effects of reducing temperature and time on LiFePO4/C prepared by Carbothermal Reduction Technology (CRT), was analyzed by X-ray diffraction (XRD), charge-discharge test, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The LiFePO4/C sample prepared at650℃for8.5h has the olivine structure with the space group Pnmb. Electrochemical tests indicated that this LiFePO4/C sample delivered a discharge special capacity of121.8mAh·g-1with a capacity retention rate100%after100cycles at10C, exhibiting good rate and cycle performance.The LiFePO4/C doped by Ti4+or V3+were characterized by X-ray diffraction(XRD), X-ray photoelectron spectroscopy (XPS) and magnetic property. The results indicated that Ti4+or V3+were successfully introduced into the LiFePO4lattice, and formed interstitial solid solution or substitutional solid solution at low-level, respectively. The impurity phases formed at the high doping level, which include Li4P2O7, TiP2O7, LiTi2(PO4)3or Li3V2(PO4)3. Among the Ti4+or V3+doped samples, LiFe0.95Ti0.05PO4/Cand LiFe0.63V0.07PO4/C exhibited excellent electrochemical performance and delivered a discharge capacity of131.3mAh·g-1and137.4mAh·g-1with a capacity retention rate of100.7%and100.3%after100cycles at10C, respectively.On the basis of the single ion doping, the structure and electrochemical performance of the LiFePO4/C co-doped by Ti4+and V3+were researched via response surface method (RSM). The results demonstrated that the relationship between the initial discharge special capacity of LiFe(1-x-y)TixVyPO4/C at10C and V3+amounts (x1) and Ti4+amounts (x2) relationship can be expressed by quadratic model-Y=144.54+1.86x1-0.89x2-0.38x1x2-4.84x12-3.24x22. The interactions between x1and x2was statistically significant. The initial discharge special capacity of LiFe0.899Ti0.027V0.074PO4/C was143.5mAh·g-1at10C, and the capacity retention rate was98.5%after100cycles, exhibiting good rate and cycle performance. The reasons for this mainly were that the V and Ti ions had been successfully doped into the LiFePO4phase with solid solution formed, increased cationic vacancy defect concentrations, and Li3V2(PO4)3with good ionic conduction existed because of additional V. Therefore Li+-diffusion rate, conductivity and the reversibility of electrode reaction were enhanced, and the rate and cycle performance were improved.