Improved Performance of a Ni, Co-Doped LiMn₂O₄ Electrode for Lithium Extraction from Brine

Electrochemical extraction of Li⁺ from salt lakes has the characteristics of high efficiency and low energy consumption, now becoming a research hotspot. The asymmetric lithium capacitor composed of de-lithiated Li_1-xNi_0.025Co_0.025Mn_1.95O₄ (LNCMO) and activated carbon is used as a system for electrochemical recovery of Li⁺. The consequence of cyclic voltammetry shows that lithium-ion diffusion is a process control step and Ni, Co doping is beneficial to improving the diffusion of lithium ions in the low-potential oxidation process and high-potential reduction process. The discharge capacity of LNCMO during a 50-cycle process is 122 mAh g^-1 with a retention rate of 97.93% at 1 C, which is higher than that of the LiMn₂O₄ (LMO)/AC system (107 mAh g^-1 with a retention rate of 83.92%), proving that Ni, Co doping restrains the disproportionation reaction of LMO and increases the Li⁺ diffusion coefficient of LMO, leading to a higher capacity and better cycle performance in brine. In 30 mM LiCl solution, the Li⁺ extraction capacity of 1.86 mmol g^-1 is obtained with an energy of 3.97 Wh mol^-1 Li⁺ under the optimal extraction conditions (i = 0.75 mA, T = 20 min). After 15 times of Li⁺ insertion/extraction in simulated brine/recovery solution, the purity of the recovery solution reached 94.59%, indicating that the LNCMO/activated carbon system has good selectivity for Li ions and stability.