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

Abstract

Olivine-type LiFePO4 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 LiFePO4 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 LiFe0.99La0.01PO4/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 LiFePO4. Its capacity retention is 80% after 497 cycles for LiFe0.99La0.01PO4/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 LiFe0.99La0.01PO4/C composite developed here could be used as a cathode material for lithium-ion batteries.