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.