Systematic investigation on determining chemical diffusion coefficients of lithium ion in Li_1 + x VPO₄F (0 ≤ x ≤ 2)

Abstract

The chemical diffusion coefficients of lithium ion (${D}_{{\mathrm{Li}}^{+}}$) in Li_1 + x VPO₄F (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.