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

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We report on the surface and bulk chemistry of Li x Mn 2 O 4 (0 ≤ x ≤ 1) spinel oxide electrode for the selective extraction of LiCl from natural salt lake brines using an electrochemical method based on LiMn 2 O 4 (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 Mn 2 O 4 films and composite LMO/carbon black electrodes. Cyclic voltammetry (CV), XPS/UPS, XRD, chrono-amperometry, and galvanostatic intermittent titration technique (GITT) experiments in model LiNO 3 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 cm 2 ·s –1 . Photoelectron spectroscopy showed Mn/O surface stoichiometry close to 1:2 and initial 1:1 Mn IV /Mn III ratio with Mn III depletion during oxidation at 1.1 V vs Ag/AgCl and recovery of surface Mn III 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.