LiNi0.5Mn1.5O4-based hybrid capacitive deionization for highly selective adsorption of lithium from brine

In recent years, the demands for lithium are increasing sharply with the fast development of electrical vehicles, new green energy and portable electronic devices. To meet the demands, selective recovery of lithium from brine/seawater has attracted extensive attention. LiMn2O4 (LMO) is a good redox material for lithium recovery. However, the dissolution of Mn is inevitable during the redox process which can lead to severe capacity decay and poor cycle performance. In this paper, LiNi0.5Mn1.5O4 (LNMO) nanoparticles were prepared through one-step solid-state sintering and applied as a positive electrode in the hybrid capacitive deionization cell for lithium recovery. Cyclic voltammetry (CV) scan of the LNMO electrode in LiCl aqueous solution presented clear redox peaks, illustrating the reversible intercalation/de-intercalation of Li+ into/from LNMO; while the Na+, K+, Mg2+ and Ca2+ cations had negligible intercalation. Combined with an activated carbon (AC) negative electrode, the hybrid capacitive deionization cell (LNMO||AC) was assembled to selectively adsorb lithium, arriving at the adsorption amount of ~260 µmol/g, almost three times higher than that of the symmetric AC cell (AC||AC). Even in the Mg2+ and Li+ mixed binary salt solution at a high CMg2+/CLi+ ratio of 30, LNMO remained high selectivity to Li+ with the separation factor of ~104 achieved. In the synthetic brine water containing various salts, the LNMO||AC cell kept excellent selectivity to Li+ with the adsorption amount still remaining ~230 µmol/g. And the cell can preserve a retention of ~86% after 100 cycles, much higher than the LMO||AC cell (~50%). Meanwhile, neither Mn2+ nor Ni2+ were found in the cycling solution of LNMO||AC cell. Nevertheless, the LMO||AC cell deteriorated gradually with the dissolution loss ratio of Mn in LMO close to 40%. Such excellent selectivity and high stability of LNMO enable it applicable for practical lithium recovery application, superior to LMO.