Systematic investigation on determining chemical diffusion coefficients of lithium ion in Li1 + x VPO4F (0 ≤ x ≤ 2)

Abstract

The chemical diffusion coefficients of lithium ion (\( {D}_{{\mathrm{Li}}^{+}} \)) in Li1 + xVPO4F (0 ≤ x ≤ 2) between 3.0 and 0.01 V are systematically analyzed by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic intermittent titration technique (GITT). The results indicate that the \( {D}_{{\mathrm{Li}}^{+}} \) values depend heavily on the voltage state. Based on the results from EIS and GITT, the diffusion coefficients (\( {D}_{{\mathrm{Li}}^{+}} \)) measured in a single-phase region below 1.7 V have relatively steady values of about 10−9 (EIS) and 10−10 (GITT) cm2 s−1, respectively, while the \( {D}_{{\mathrm{Li}}^{+}} \) values in the single-phase region above 1.9 V decrease rapidly from 10−9 to 10−11 cm2 s−1 due to concentration of lithium ions in the bulk LiVPO4F. The Li+ chemical diffusion coefficients measured in the two-phase region by GITT range a lot from 10−9 to 10−14 cm2 s−1, while the \( {D}_{{\mathrm{Li}}^{+}} \) values in the two-phase region determined by CV are around 10−10 cm2 s−1. By the GITT, the \( {D}_{{\mathrm{Li}}^{+}} \) values in the two-phase region vary in non-linear shape with the charge–discharge voltage, which is ascribed to strong interactions of Li+ with other ions.