Electric-field-assisted Co²⁺ reconstruction on lamellar MXene/δ-MnO₂ membrane surfaces for efficient Li⁺/Co²⁺ separation

Efficient recovery of Li⁺ from lithium-ion batteries (LiBs), which also include Co²⁺, Ni²⁺, or Mn²⁺ in the cathode material, remains a formidable challenge because the sizes and physicochemical properties of these ions. Composite lamellar membranes are emerging as highly ion-selective nanochannels with controllable interlayers. Herein, Li⁺/Co²⁺ was precisely separated through two-dimensional MXene/δ-MnO₂ composite nanochannels under an electric field. The nanochannels were constructed on tubular ceramic membranes via hydroxyl crosslinking. The membrane layer spacing can be extended to 5.7 Å by electrostatic repulsion between MXene and δ-MnO₂ nanosheets, which is suitable for Li⁺/Co²⁺ separation. Under an electric field of 2 mA cm⁻², Li⁺/Co²⁺ was nearly completely separated, with a high Li⁺ flux of 0.0094 mol m^-2h⁻¹, and the energy consumption was ∼0.315 kW h (mol Li⁺)^-1. Similarly, near-complete separation of Li⁺/Ni²⁺ and Li⁺/Mn²⁺ can be achieved at a current density of 2 mA cm⁻². The transport of Li⁺ in solution was accelerated under a positive electric field and by partial dehydration in the nanopores. Meanwhile, the zeta potential was distinctly more negative in the composite membranes than in individual MXene or δ-MnO₂. Consequently, Co²⁺ was rapidly adsorbed on the negatively charged membrane surface through electrostatic interactions and complexation, forming a positively charged Co²⁺ layer that electrostatically repelled further Co²⁺ adsorption, ultimately achieving near-complete separation of Co²⁺. Owing to their stable and robust structure, the MXene/δ-MnO₂ membranes maintained high separation efficiency in acidic solutions containing Li⁺ and Co²⁺ and remained stable over eight cycles (up to 14 h) of ion separation under an electric field. This approach can potentially realize 2D membrane composites for efficient ion separation in practical applications.