Engineering Li-Al LDH with anions and metals modulations for durable lithium recovery

The rise of lithium-ion battery use necessitates the development of efficient, eco-friendly, and cost-effective lithium (Li⁺) recovery systems. In this study, lithium–aluminum layered double hydroxides (LDH) were modified by introducing different interlayer anions (CO₃²⁻, Cl⁻, SO₄²⁻, NO₃⁻, CH₃COO⁻, and PO₄³⁻) and first row divalent metal cations (Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺) and their performance was systematically evaluated. The results revealed that the LDH material doped with Fe²⁺ and intercalated with PO₄³⁻ (Fe-LDH-P) showed the optimal configuration for Li⁺ recovery. These modifications synergistically enlarge the interlayer spacing through PO₄³⁻ intercalation and enhance structural stability by Fe²⁺ doping, thereby preventing structural collapse during repeated lithium adsorption–desorption cycles. Adsorption experiments in Li⁺-spiked seawater showed that Fe-LDH-P achieved a maximum adsorption capacity (q_max) of 11.23 mg/g, significantly exceeding that of LDH (6.95 mg/g). Nonlinear isotherm analysis indicated the Langmuir model best describes the adsorption behavior (R² = 0.993), confirming monolayer adsorption and strong selectivity over competing ions. Kinetic analysis revealed pseudo-second-order (PSO) behavior (R² = 0.94, K₂ = 0.143 g/ mg min, RMSE = 0.267, χ² = 6.76), indicating a chemisorption-controlled Li⁺ adsorption mechanism. Density functional theory (DFT) analysis revealed that phosphate intercalation significantly expands the LDH interlayer spacing from 7.59 to 8.86 Å, enhancing Li⁺ accessibility and structural stability, whereas iron doping induces a slight contraction while strengthening Li⁺ binding and suppressing deintercalation and gibbsite formation. This synergistic effect suppresses Li⁺ deintercalation and gibbsite formation, thereby enabling durable and efficient Li⁺ recovery.