Physical and electrochemical properties of La-doped LiFePO4/C composites as cathode materials for lithium-ion batteries

• Published in: Journal of solid state electrochemistry 2008, Vol.12 (7-8), p.815-823
• Main Authors: Cho, Yung-Da, Fey, George Ting-Kuo, Kao, Hsien-Ming
• Format: Article
• Language: English
• Subjects: Analytical Chemistry; Characterization and Evaluation of Materials; Chemistry; Chemistry and Materials Science; Condensed Matter Physics; Electrochemistry; Energy Storage; Original Paper; Physical Chemistry
• ISSN: 1432-8488; 1433-0768
• DOI: 10.1007/s10008-007-0498-7

Olivine-type LiFePO 4 is one of the most promising cathode materials for lithium-ion batteries, but its poor conductivity and low lithium-ion diffusion limit its practical application. The electronic conductivity of LiFePO 4 can be improved by carbon coating and metal doping. A small amount of La-ion was added via ball milling by a solid-state reaction method. The samples were characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM)/mapping, differential scanning calorimetry (DSC), transmission electron microscopy (TEM)/energy dispersive X-ray spectroscopy (EDS), and total organic carbon (TOC). Their electrochemical properties were investigated by cyclic voltammetry, four-point probe conductivity measurements, and galvanostatic charge and discharge tests. The results indicate that these La-ion dopants do not affect the structure of the material but considerably improve its rate capacity performance and cyclic stability. Among the materials, the LiFe 0.99 La 0.01 PO 4 /C composite presents the best electrochemical behavior, with a discharge capacity of 156 mAh g −1 between 2.8 and 4.0 V at a 0.2 C-rate compared to 104 mAh g −1 for undoped LiFePO 4 . Its capacity retention is 80% after 497 cycles for LiFe 0.99 La 0.01 PO 4 /C samples. Such a significant improvement in electrochemical performance should be partly related to the enhanced electronic conductivities (from 5.88 × 10 −6 to 2.82 × 10 −3  S cm −1 ) and probably the mobility of Li + ion in the doped samples. The LiFe 0.99 La 0.01 PO 4 /C composite developed here could be used as a cathode material for lithium-ion batteries.