Diffusion Channel Engineering of Spinel Cathodes for Selective Lithium Extraction from Low-Grade Brine

The surging demand for lithium to power electric vehicles and grid-scale energy storage highlights the urgent need for sustainable extraction from unconventional resources, such as low-grade salt lake brines. Electrochemical lithium extraction offers an energy-efficient pathway, yet its progress is constrained by the limited selectivity and poor durability of the cathode material, such as LiMn2O4 (LMO). Here, we demonstrate that Nb doping effectively tunes the three-dimensional Li+ diffusion channels of LMO, simultaneously lowering the Mn valence, enlarging the lattice constant, and strengthening the Mn-O framework. The structural optimization delivers a discharge capacity of 107 mAh g-1 and markedly reduces the Li+ diffusion resistance, thereby lowering the energy consumption of lithium extraction (4.83 Wh mol-1). In highly complex Qarhan brine with an extremely high ratio of competing ions, Nb-doped LMO exhibits considerable Li+ extraction capacity (5.21 mmol g-1 in Qarhan raw brine and 3.79 mmol g-1 in Qarhan old brine), superior Li+ selectivity (Li+/Mg2+ = 48.47 and Li+/Na+ = 44.42), and enhanced cyclic stability (Li+ intercalation capacity retention 72.09% after 50 cycles). Depth-resolved TOF-SIMS and DFT analyses reveal that the broadened diffusion channels suppress bulk diffusion of Na+ and Mg2+ while reducing the Li+ migration barrier, underpinning the high selectivity. These results suggest that Nb-doped LMO is a robust and scalable cathode for electrochemical lithium extraction, offering a viable strategy to unlock lithium from low-grade brines.