Lithium transport processes in electrodes on the basis of Li3V2(PO4)3 by constant current chronopotentiometry, cyclic voltammetry and pulse chronoamperometry

Lithium transport in electrode material on the basis of lithium vanadium phosphate (Li3V2(PO4)3) with the NASICON structure was studied by the constant current chronopotentiometry (also called as galvanostatic charge-discharge technique), cyclic voltammetry (CV) and pulse chronoamperometry (PITT). The shape of the cyclic voltammograms of the electrode under study at various potential scan rates and galvanostatic charge-discharge curves at various currents as well as the character of their change during cycling were established. PITT current transients of the Li3V2(PO4)3 electrode had a characteristic and non-trivial shape, splitting into short, middle, and long-time fragments. The limiting current obtained by means of the zero-moment extrapolation of the short-time j,t fragment does not depend on the lithium concentration in the intercalate, which pointed to its relation to the solid electrolyte interphase (SEI), whereas the shape of the current transients over the whole temporal scale was strongly influenced by the lithium content in the electrode. By integration of current for each PITT step and subsequent summation of charges the intercalation isotherm of the Li3V2(PO4)3 electrode was determined. The differential form of the intercalation isotherm is in agreement with the cyclic voltammogram shape at low potential scan rates (≤ 0.05 mV s−1). Within the whole j,t-curves of the Li3V2(PO4)3 electrode the short-time j,t fragments can be located where semi-infinite diffusion conditions are preserved so that the mathematical treatment in the framework of such diffusion type approximation with surface control is justified. The values of the lithium diffusion coefficients (D) were found and the dependence of D on the electrode potential was demonstrated. The lithium diffusion coefficient ranges found by CV and PITT overlap and are within the limits of 10−15–10−12 cm2 s−1.