Ionic transport kinetics of selective electrochemical lithium extraction from brines

Abstract

Lithium extraction from brines by electrochemical De-intercalation/Intercalation method with LiFePO4 as host materials has attracted extensive attention, but the ionic transport properties and rate-determining step of this process are not fully understood. Therefore, to address this knowledge gap, we study the electrode process kinetics of LiFePO4/Li1-xFePO4 (0 <x < 1) in brines by electrochemical impedance spectroscopy. The results reveal that the diffusion of Li+ in electrode is the rate-determining step, the apparent diffusion coefficient DLi shows a trend of first increasing and then decreasing with the increase of electrode coating density, and the maximum value is obtained as 2.23 × 10−11 cm−2·s−1 at 55 mg·cm−2. The charge transfer resistance Rct and Warburg impedance Zω decrease with increasing temperature and do not change significantly with Li+ concentration, but Mg2+ has an inhibitory effect on Li+ charge exchange. Furthermore, the Li+ charge transfer at electrode/solution interface is differentiated for different de-lithiated Li1-xFePO4 electrodes, the Rct reaches a minimum around Li0.3FePO4. The diffusion of other competing cations is more difficult than Li+ in electrodes, which gives theoretical support for selective lithium extraction from brines from a kinetic point of view.