High rate capability by sulfur-doping into LiFePO4 matrix

Abstract

Enhanced electrochemical performance of LiFePO₄ for Li-ion batteries has been anticipated by anion doping at the O-site rather than cation doping at the Fe-site. We report on the electrochemical performance of S-doped LiFePO₄ nanoparticles synthesized by a solvothermal method using thioacetamide as a sulfur source. S-doping into the LiFePO₄ matrix expands the lattice due to the larger ionic radius of S^2- than that of O^2-. The lattice parameters a and b increase by around 0.2% with sulfur content, while that of c remains almost unchanged with only 0.03% increase. The S-doping also contributes to the suppression of antisite defects (Fe occupying Li sites), which facilitates the easy migration of Li in the diffusion channels without blockage. Owing to these effects of S-doping, the S-doped LiFePO₄ nanoparticles show enhanced electrochemical properties with a high discharge capacity of ∼113 mA h g^-1 even at a high rate of 10C.

Fig. 1. (a) XRD patterns of LFP and LFP-S-x (where x = 0.02, 0.07, 0.11, 0.22, 0.44 or 0.67) (silicon was used as a standard). (b) Changes in a and c lattice parameters of LFP-S-x (x: from 0 to 0.67).

Fig. 2. SEM images of LFP (a), LFP-S-0.22 (b) and LFP-S-0.67 (c and d).

Fig. 3. Secondary electron image (SEI), EDS mapping images and line scan profile of LFP-S-0.22 for P, O, Fe and S. The line scan profile was collected in the red dot line of the SEI image.

Fig. 4. XPS spectra for the S 2p (a) and P 2p (b) electrons of LFP, LFP-S-0.22 and LFP-S-0.67.

Fig. 5. (a) Charge–discharge voltage curves of LFP and LFP-S-0.22 at 5C. (b) Rate performance of LFP and LFP-S-0.22.

Fig. 6. FTIR spectra of LFP and LFP-S-0.22.