Toward elucidation of delithiation mechanism of zinc-substituted LiFePO4

Abstract

Results of research on the mechanism of delithiation of LiFe0.75Zn0.25PO4 cathode material having olivine structure are presented. Introduction of zinc into Fe-sublattice of LiFePO4 caused a decrease of unit cell volume, however, shrinkage of unit cell parameters a and b was accompanied by an increase of parameter c. Analysis of phase composition during delithiation process indicated presence of single phase in the range of lithium content from 1 to 0.5 in LixFe0.75Zn0.25PO4. This effect is accompanied by observed sloping shape of charge/discharge curves and results of Mössbauer studies. Introduction of zinc increased electrical conductivity of the material, as well as caused increase of chemical diffusion coefficient of lithium D, comparing to pure LiFePO4. Surface activated material exhibited discharge capacity of 88mAhg−1 at C/10 rate (70% of the theoretical value for LiFe0.75Zn0.25PO4). Cyclic voltammetry studies showed single oxidation and reduction peaks with mean potential value of 3.47V, and peak current vs. scan potential dependence characteristic for diffusional limitation of the electrode reaction. Determination of D, based on GITT method is presented. Additionally, results of PITT-type measurements are given, which allowed to simultaneously determine D and surface exchange reaction coefficient K. The measured values of chemical diffusion coefficient of lithium are of the order of 10−13 to 10−10cm2s−1, depending on an assumption about effective diffusion length.