Flow through negatively charged, nanoporous membranes separates Li + and K + due to induced electromigration.

Flow through negatively charged nanopores separates Li ⁺ and K ⁺ with selectivities of up to 70 and Li ⁺ passages from 20% to above 100%. Remarkably, both the Li ⁺ /K ⁺ selectivity and Li ⁺ passage initially increase with flow rate, breaking the permeability/selectivity trade-off. Modelling demonstrates that flow through the membranes creates electric fields that retard transport of cations. Selectivity increases with flow rate because the K ⁺ electromigration velocity exceeds its convective velocity, but for Li ⁺ electromigration is weaker than convection. Modelling also shows the importance of controlling concentration polarization. With further work, related separations might provide highly pure Li salts for battery manufacturing.