Surface Chemistry and Lithium-Ion Exchange in LiMn2O4 for the Electrochemical Selective Extraction of LiCl from Natural Salt Lake Brines

We report on the surface and bulk chemistry of Li x Mn2O4 (0 ≤ x ≤ 1) spinel oxide electrode for the selective extraction of LiCl from natural salt lake brines using an electrochemical method based on LiMn2O4 (LMO) lithium intercalation electrode and polypyrrole (PPy) reversible chloride electrode. Both the surface composition and insertion/release of Li ions into/from the crystal structure have been studied with pulsed laser deposited (PLD) thin Li x Mn2O4 films and composite LMO/carbon black electrodes. Cyclic voltammetry (CV), XPS/UPS, XRD, chrono-amperometry, and galvanostatic intermittent titration technique (GITT) experiments in model LiNO3 solutions and natural brines from Salar de Olaroz (Jujuy, Argentina) have been used. Repetitive CV and GITT experiments showed reversible extraction/intercalation of Li ions in LMO with high selectivity and electrode stability in natural brine, while PPy is reversible to chloride ions. Chronoamperometry for time-bound diffusion in small nanocrystals with interference of concentration profiles yielded D Li + ∼ 10–10 cm2·s–1. Photoelectron spectroscopy showed Mn/O surface stoichiometry close to 1:2 and initial 1:1 MnIV/MnIII ratio with MnIII depletion during oxidation at 1.1 V vs Ag/AgCl and recovery of surface MnIII after reduction at 0.4 V. Coadsorption of Na+ was detected which resulted in slower ion exchange of Li ions, but there was no evidence of Na+ intercalation in the Mn oxide electrode.