Lithium extraction from salt lake via rocking-chair flow electrode capacitive deionization with monovalent selective membrane

This work developed a novel rocking-chair flow electrode capacitive deionization (R-FCDI) system that employs the commercial monovalent selective membrane (CMSM) to realize extremely efficacious separation of Li+/Mg2+ ions with extracting lithium from salt lake brine. The R-FCDI system exhibits exceptional lithium extraction performance, obtaining an electrosorption efficiency (ESR) of 98.4 %, an average electrosorption rate (AESR) of up to 7.77 mu mol cm- 2 min- 1, a high charge efficiency (CE) of 93.44 %, and energy consumption (Em) as low as 0.07 kWh mol- 1 Li. Especially, the Em is considerably lower than that of electrodialysis (ED, 0.13-3.795 kWh mol- 1 Li), making it a suitable option for industrial lithium extraction by reducing the operational costs. Moreover, the selectivity coefficient of the system reached a maximum value of 15.93 with low Em (0.037 kWh mol- 1 Li) when the magnesium/lithium mass ratio was 10 under a low applied voltage of 0.8 V. The electrochemical measurements reveal that the selective mechanism of Li+/Mg2+ ion in the proposed R-FCDI system is ascribed to the significantly higher capacitance of lithium ions (1.13-fold as greater as that of magnesium ions) and the substantially lower transmembrane transfer resistance of lithium ions (2.48-fold lower compared to that of magnesium ions). Interestingly, this study found that the initial ion concentration of the recovered solution has a marked consequence on the selectivity coefficient of the R-FCDI system, in which the highest selectivity coefficient of 34.99 with low Em (0.042 kWh mol-1 Li) was achieved when the ion concentration is 500 mg L-1. In an experimental study of the natural brine from Golmud salt lake, China, the maximum of selectivity coefficient is 24.74 and the Em is 3.03 kWh mol-1 Li at optimal process parameters.