Lithium iron phosphate cathodes for brackish water desalination: A proof-of-concept and techno-economic analysis toward direct reuse of end-of-life lithium-ion batteries

• Circular SBDI desalination addresses battery waste and water scarcity simultaneously • Spent LFP cathodes demonstrate non-monotonic SAC peaking at 14.19 mg g⁻¹ SAC over 1000 cycles • Direct electrode reuse bypasses 75 % energy-intensive recycling steps • IEM costs dominate SBDI CapEx (∼60 %), but SAC improvement offers the greatest marginal LCoW reduction The renewable energy transition demands efficient energy storage, particularly lithium-ion batteries (LIBs), creating an urgent need for spent LIB recycling toward a circular economy. This work proposes direct cathode/anode reuse from dominant LIB types, nickel-manganese-cobalt (NMC) and lithium iron phosphate (LFP), via a two-channel spent battery electrode deionization (SBDI) cell. Among the four electrode types tested, the LFP cathode demonstrates the most promising desalination performance, with salt adsorption capacity (SAC) peaking at 14.19 mg g⁻¹ at ∼500 cycles before declining to 6.96 mg g⁻¹ by 1000 cycles (3-min half-cycles, ±0.8 V), exhibiting a non-monotonic activation-and-decay pattern. Techno-economic analysis of the SBDI process reveals that while the ion exchange membrane (IEM) component dominates capital cost (∼60 % of baseline CapEx), SAC improvement offers the greatest marginal reduction in levelized cost of water (LCoW), making electrode performance optimization the principal research target. As a potential circular economy solution for LIB electrode waste that simultaneously addresses persistent global freshwater shortages, the SBDI concept shows a pathway to economic viability contingent on electrode performance improvements and IEM cost reductions.