Lithium Manganate Wrapped with Ion-Selective Graphene Coatings for Thermally Assisted Lithium Extraction from Simulated Seawater

Lithium (Li) is a critical element for various energy storage devices. Extracting Li from the ocean by electrochemical ion pumping using lithium manganate (LMO) could solve the potential Li shortages. In particular, a thermally assisted electrochemical Li+ extraction process using low-grade heat can speed up extraction and reduce energy consumption. However, LMO suffers from fast extraction capacity fading and insufficient selectivity for Li+ over Na+. Here, we show that the performance of LMO can be significantly improved by wrapping LMO with ion-selective reduced graphene oxide (rGO) coatings. The rGO coatings were assembled uniformly and controllably on LMO particles via a poly(diallyldimethylammonium) chloride interfacial binder. At the optimum mass ratio of 0.125 wt %, the rGO-coated LMO demonstrates a higher capacity retention of 75.1% than uncoated LMO over 50 charge/discharge cycles. When paired with a nickel hexacyanoferrate electrode for Li+ extraction from simulated seawater with a high Na+/Li+ molar ratio of 20 000 at 60 °C, rGO-coated LMO shows much less capacity loss of 23.13% and yields a higher Li purity of 65.49% than uncoated LMO. The improved performance can be attributed to the Donnan effect of rGO nanosheets in attracting cations from low Li concentration simulated seawater on electrode surfaces and the size exclusion effect by restricting the mass transfer of large ions. Less cointercalation of competing Na+ helps to ease the Jahn–Teller effect of LMO, resulting in enhanced crystalline structure stability. The thermally assisted Li extraction process has the potential to be further improved with innovative electrode material designs for practical applications.