Effects of Na+ and Cl- co-doping on electrochemical performance in LiFePO4/C

Na+ and Cl- co-doped LiFePO4/C composites were prepared via a simple solid state reaction. The structure, valence state and electrochemical performance were carefully investigated. Rietveld refinement on X-ray diffractions reveals that Na+ and Cl- have successfully been introduced into the lattice of LiFePO4. X-ray photoelectron spectroscopy proves that the co-doping of Na+ and Cl- does not change the chemical state of Fe(II). Experimental results further show that the co-doping contributes to induce the lattice distortion, modify the particle morphology, and increase the electronic conductivity. Considerably enhanced capacity, coulombic efficiency and rate capability were obtained in the co-doped LiFePO4. The specific capacities are 157 mAh g(-1) at 0.2 C, 115 mAh g(-1) at 10 C and 98 mAh g(-1) at 20 C for the (Na+, Cl-) co-doped LiFePO4/C cathode material. The improvement can be ascribed to the enhanced electronic conductivity and electrode kinetics due to the micro-structural modification promoted by co-doping. (C) 2011 Elsevier Ltd. All rights reserved.