Layered hydrated-titanium-oxide-laden reduced graphene oxide composite as a high-performance negative electrode for selective extraction of Li via membrane capacitive deionization

In this work, we initially prepared layered lithium titanate (Li₂TiO₃) using a solid-state reaction. Then Li⁺ of Li₂TiO₃ were acid-eluded with Hydrochloric acid to obtain hydrated titanium oxide (H₂TiO₃). Different weight percentages (50%, 60%, 70%, 80%, and 90%) of the as-prepared H₂TiO₃ were deposited on a conductive reduced graphene oxide (rGO) matrix to obtain a series of rGO/ H₂TiO₃ composites. Of the prepared composites, rGO/H₂TiO₃-60% showed excellent current density, high specific capacitance, and rapid ion diffusion. An asymmetric MCDI (membrane capacitive deionization) cell fabricated with activated carbon as the anode and rGO/H₂TiO₃-60% as the cathode displayed outstanding Li⁺ electrosorption capacity (13.67 mg g⁻¹) with a mean removal rate of 0.40 mg g⁻¹ min⁻¹ in a 10 mM LiCl aqueous solution at 1.8 V. More importantly, the rGO/H₂TiO₃-60% composite electrode exhibited exceptional Li⁺ selectivity, superior cyclic stability up to 100,000 s, and a Li⁺ sorption capacity retention of 96.32% after 50 adsorption/desorption cycles. The excellent Li⁺ extraction obtained by MCDI using the rGO/H₂TiO₃-60% negative electrode was putatively attributed to: (i) ion exchange between Li⁺ and H⁺ of H₂TiO₃; (ii) the presence of narrow lattice spaces in H₂TiO₃ suitable for selective Li⁺ capture; (iii) capture of Li⁺ by isolated and hydrogen-bonded hydroxyl groups of H₂TiO₃; and (iv) enhanced interfacial contact and transfer of large numbers of Li⁺ ions from the electrolyte to H₂TiO₃ achieved by compositing H₂TiO₃ with a highly conductive rGO matrix.