Electrolyte recovery in lithium-ion battery recycling has gained significant attention due to the high value of functional additives like vinylene carbonate (VC), fluoroethylene carbonate (FEC), and propane sultone (PS). These compounds improve battery performance by enhancing solid electrolyte interphase (SEI) stability, reducing gas generation, and improving low-temperature operation. Efficient recovery of these additives from spent electrolytes requires specialized separation techniques to meet the stringent purity requirements for reuse in new battery electrolytes.

Chromatographic separation methods are widely employed for additive recovery due to their high selectivity. High-performance liquid chromatography (HPLC) with polar stationary phases, such as silica or amino-modified columns, effectively separates VC, FEC, and PS from degraded electrolyte mixtures. Gradient elution using acetonitrile and water as mobile phases achieves optimal resolution, with retention times varying based on molecular polarity. Size-exclusion chromatography (SEC) is another viable approach, particularly for separating PS from smaller organic carbonate molecules. The purity of recovered additives via chromatographic methods typically exceeds 99.5%, meeting industry benchmarks for reuse. However, challenges arise from co-elution of structurally similar degradation products, such as ethylene carbonate (EC) dimers or linear carbonates formed during battery aging.

Azeotropic distillation offers a scalable alternative for additive recovery, leveraging differences in boiling points and azeotrope formation. VC, with a boiling point of 162°C, can be isolated from dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) using toluene as an entrainer. The process involves fractional distillation under reduced pressure to minimize thermal degradation. FEC, which forms a binary azeotrope with EMC at 92°C, requires precise temperature control to prevent decomposition. Distillation columns with high reflux ratios (10:1 or greater) achieve FEC purity levels above 99%. PS recovery is more complex due to its thermal instability; vacuum distillation at temperatures below 80°C is necessary to prevent ring-opening reactions. Despite its effectiveness, azeotropic distillation struggles with removing trace degradation products like alkyl phosphates or HF-contaminated species, necessitating additional purification steps.

Crystallization strategies provide high-purity recovery for additives with favorable solubility properties. VC can be selectively crystallized from spent electrolyte solutions by cooling to -20°C in the presence of anti-solvents like n-hexane. The process yields VC crystals with purity exceeding 99.7%, though residual LiPF6 salts must be removed via washing with anhydrous solvents. FEC recovery via crystallization relies on its low solubility in diethyl ether at room temperature, achieving recoveries above 90% with less than 0.3% impurity content. PS presents unique challenges due to its hygroscopic nature; anhydrous conditions and controlled cooling rates are critical to prevent hydrate formation. The addition of seed crystals improves yield and purity, with industrial-scale processes reporting PS recovery rates of 85-90% at purities suitable for battery reuse.

Degradation product removal remains a critical hurdle in additive recovery. Electrochemical aging generates species like polycarbonates, organofluorophosphates, and acidic contaminants that co-extract with target additives. Solvent extraction with alkaline washes (e.g., Na2CO3 solutions) effectively removes acidic degradation products but risks hydrolyzing PS. Adsorption on activated alumina or molecular sieves reduces polycarbonate content below 100 ppm, though some capacity loss occurs for VC due to competitive adsorption. Membrane filtration using nanofiltration membranes with 200-300 Da molecular weight cutoffs shows promise for removing larger degradation products while allowing additives to permeate.

Purity benchmarks for recovered additives align with commercial electrolyte specifications. VC must maintain purity above 99.5% with water content below 50 ppm to prevent SEI instability. FEC requires less than 0.1% total impurities and fluoride content below 10 ppm to avoid gas generation in cells. PS specifications are more stringent, demanding 99.8% purity and sulfonate contamination below 5 ppm to ensure effective passivation. Meeting these benchmarks often requires hybrid approaches, such as combining distillation with final polishing via chromatography or crystallization.

Economic considerations favor integrated recovery processes where multiple additives are extracted sequentially. A typical flowsheet might involve initial distillation to recover volatile carbonates, followed by chromatographic separation of VC and FEC, and concluding with PS crystallization. Such systems achieve overall recovery efficiencies of 75-85% for each additive while minimizing energy consumption. The scalability of these methods has been demonstrated in pilot plants, with production capacities exceeding 1 ton/day of recovered additives.

Future advancements may focus on solvent-resistant chromatographic materials and low-energy distillation configurations to further improve recovery rates. The development of selective adsorbents for degradation products could simplify purification steps, reducing operational costs. As battery chemistries evolve, recovery processes must adapt to handle new additive formulations while maintaining stringent purity standards for closed-loop electrolyte recycling.

The successful recovery of high-value electrolyte additives hinges on optimizing separation techniques to balance purity, yield, and cost. Chromatography, distillation, and crystallization each offer distinct advantages, with hybrid approaches providing the most robust solutions for industrial-scale implementation. Overcoming degradation product contamination remains pivotal to ensuring the recycled additives meet the performance requirements of next-generation lithium-ion batteries.