Phase Fraction Modulation Enhances Li ⁺ /Na ⁺ Diffusion Disparity in Spent LiFePO ₄ Cathodes for Efficient Lithium Extraction from Brine

ABSTRACT Traditional spent LiFePO ₄ (SLFP) recycling faces challenges such as high energy consumption, environmental pollution, and low profitability. A novel direct cascaded utilization strategy was proposed for SLFP, where phase fraction modulation in the SLFP lattice enabled efficient lithium extraction from secondary‐treated lithium‐containing brine. The approach precisely controlled LiFePO ₄ phase (x in Li _x FePO ₄ ) by regulating the oxidant dosage within a pH‐maintained reversible phase transition framework. Moreover, the contraction of the crystal structure and the amplified disparity in Li ⁺ /Na ⁺ diffusion energy barriers synergistically improve structure stability and ion selectivity. The Li ⁺ /Na ⁺ diffusion energy barrier difference was up to 0.56 eV. During the (de)intercalation process, Li _0.19 FePO ₄ exhibited excellent structure stability and Li/Na separation capability. The dissolution loss rates of Fe and P atoms were only 0.27% and 0.58%. And the maximum Li ⁺ recovery rate can reach 99% within 40 min. Furthermore, the economic and environmental analysis indicated that the new strategy yields 528% higher profit ($7.23 per kg of SLFP), 40.5% reduction in energy demand and 67.8% reduction in greenhouse gas (GHG) emissions compared to conventional pyrometallurgy. It bypasses the need for pre‐sorting SLFP, enhancing SLFP recycling efficiency and promoting high‐value use of lithium‐containing brine through “urban mining”.