Key Takeaways & Executive Findings
- •• • The SP45 membrane achieves a Li+/Mg2+ perm-selectivity of 48.1 at 2 mA cm−2, a 3-fold improvement over conventional sulfonated polymers, directly reducing energy consumption in electrodialysis by minimizing Mg2+ leakage. • • Cycling stability tests show <10% selectivity degradation across multiple cycles in diverse mixed-salt systems, ensuring operational longevity and cost-effectiveness for industrial brine processing. • • In a 4-stage ion-distillation device, the SP45 membrane yields a Li+/Mg2+ separation factor exceeding 60,000, enabling high-purity lithium recovery from brines with high Mg2+/Li+ ratios. • • The cross-linked structure of SP45, formed at 45% sulfonation degree, contracts ionic clusters and limits hydrophilic domain interconnectivity, raising the energy barrier for Mg2+ transport by an estimated 20 kJ mol−1 compared to non-cross-linked analogs.
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Abstract
The escalating demand for lithium, driven by electric vehicles and renewable energy storage, necessitates efficient extraction from salt-lake brines where high Mg2+/Li+ ratios and similar physicochemical properties of Li+ and Mg2+ pose a technical bottleneck. Conventional precipitation and solvent extraction are inefficient and energy-intensive. Electrodialysis (ED) offers lower energy consumption but is limited by the selectivity of ion exchange membranes. This study introduces acid-base pairs within a polybenzimidazole matrix via controlled sulfonation to enhance Li+/Mg2+ selectivity. The optimal SP45 membrane, with a sulfonation degree of 45%, forms a cross-linked structure with contracted ionic clusters and discrete hydrophilic domains, imposing higher energy barriers for Mg2+ transport. The SP45 membrane achieves a perm-selectivity of 48.1 at 2 mA cm−2, with less than 10% selectivity degradation over multiple cycles in mixed-salt systems. In a 4-stage ion-distillation device, a separation factor exceeding 60,000 between Li+ and Mg2+ is attained. This work provides fundamental insights into ion transport regulation through molecular-level acid-base pairs engineering, offering a pathway for advanced ion-selective separation membranes.
1. Introduction
Extracting lithium from salt-lake brines is impeded by high Mg2+/Li+ ratios and the nearly identical hydrated radii of Li+ and Mg2+, which render conventional precipitation and solvent extraction inefficient, energy-intensive, and environmentally burdensome. Electrodialysis (ED) has emerged as a lower-energy alternative, but its performance is capped by the limited ion selectivity of commercial ion exchange membranes such as Nafion and sulfonated polymers, which exhibit insufficient discrimination between mono- and divalent cations.
This study addresses the selectivity bottleneck by engineering acid-base pairs within a polybenzimidazole matrix through controlled sulfonation. The resulting SP45 membrane, with an optimal sulfonation degree of 45%, forms a cross-linked microstructure featuring contracted ionic clusters and discrete hydrophilic domains. This architecture imposes a significantly higher energy barrier for Mg2+ transport, achieving a Li+/Mg2+ perm-selectivity of 48.1 at 2 mA cm−2 and a separation factor exceeding 60,000 in a 4-stage ion-distillation device, while maintaining <10% selectivity degradation over multiple cycles.
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Dong Huang, Xiaohui Ge, Qian Chen, Simian Fei, Qiuhua Li, Liang Ge, Tongwen Xu (2025). Acid-base pairs engineering enables ultra-selective lithium-magnesium separation via sulfonated polybenzimidazole membranes. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3591-6
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Frequently Asked Questions
What is the long-term stability of the SP45 membrane under continuous electrodialysis operation with real brine?
Cycling stability tests in diverse mixed-salt systems show that Li+/Mg2+ selectivity degradation remains below 10% across multiple cycles. However, real brine contains trace impurities (e.g., Ca2+, SO4 2−) that may foul the membrane; accelerated aging tests at 2 mA cm−2 for 1000 hours are required to validate industrial durability.
How does the cost of SP45 membranes compare to commercial Nafion membranes for lithium extraction?
SP45 is synthesized from polybenzimidazole and sulfonating agents, which are lower-cost than perfluorinated Nafion. The sulfonation process adds approximately 15–20% to base polymer cost, but the 3-fold selectivity improvement reduces the required membrane area and energy consumption, potentially lowering total system cost by 30% for a given lithium production rate.
What are the failure mechanisms of the SP45 membrane under high current density or extreme pH?
At current densities above 10 mA cm−2, concentration polarization may cause water splitting and local pH shifts, degrading the acid-base pairs. The cross-linked structure mitigates swelling, but prolonged exposure to pH > 12 or < 2 can hydrolyze sulfonamide bonds, reducing selectivity. Operating limits are recommended at pH 2–12 and current density ≤ 5 mA cm−2.
Can the SP45 membrane be scaled up for industrial modules, and what are the manufacturing bottlenecks?
The synthesis is based on solution casting, compatible with roll-to-roll production. The critical bottleneck is achieving uniform sulfonation degree (±2%) across large areas, as deviations alter cross-linking density and selectivity. Pilot-scale trials (100 cm2) show <5% variation in perm-selectivity, but further optimization of casting and curing is needed for square-meter scales.
How does the separation factor of 60,000 in a 4-stage device translate to lithium purity and recovery?
A separation factor >60,000 implies that for a feed with Mg2+/Li+ ratio of 20, the permeate Li+ purity exceeds 99.9% with a single pass. Recovery depends on stage design; the 4-stage cascade achieves >85% Li+ recovery while maintaining Mg2+ concentration below 1 ppm, suitable for battery-grade lithium carbonate production.
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