Separation of Lithium Isotopes: Electromigration Coupling with Crystallization Li⁶

This work reports the design, development, and validation of an integrated system that couples electromigration-based lithium isotope separation with controlled crystallization of lithium carbonate enriched in lithium-6 (⁶Li). The research aims to provide a sustainable and scalable technological alternative for isotope enrichment processes, with direct relevance to nuclear materials and next-generation battery applications.

The experimental setup combines a dual-module electromigration cascade with a thermostated crystallization unit, interconnected through a fully automated robotic and fluidic transfer system. This configuration enables continuous operation, real-time process control, and zero-loss transfer of the Li⁶-enriched cathodic solution. Four types of ion-conducting membranes were investigated: Nafion 212, Nafion 117, Celgard 2325, and a novel PEG-crosslinked PS-9 prototype developed at ICSI ENERGY. All membranes were tested under constant potential (5 V) for 120 h, ensuring comparable electrochemical conditions.

Following separation, the Li⁶-enriched cathodic solution underwent a crystallization protocol optimized for pH (7.2–7.5), concentration, and temperature (4 °C). Ammonium carbonate ((NH₄)₂CO₃) was used as a controlled nucleating agent, promoting selective precipitation of Li₂CO₃ (⁶Li). FTIR analysis confirmed the presence of Li–O and CO₃²⁻ vibrational bands, indicating pure lithium carbonate formation. SEM characterization revealed uniform prismatic microcrystals (1–3 µm), with morphology dependent on membrane architecture, well-faceted for Nafion, compact for PS-9, and large aggregates for Celgard.

Crystallization kinetics were evaluated using the Avrami model, yielding n ≈ 2.8 and k = 0.3 h⁻ⁿ, consistent with instantaneous nucleation followed by three-dimensional diffusion-controlled growth. The entire process achieved a Li⁶ recovery of 95.7 ± 0.3% and >98% crystalline purity.

The results confirm the system’s efficiency, reproducibility, and full automation, demonstrating a novel technological pathway for lithium isotope enrichment through synchronized electromigration–crystallization coupling.