Pressure-Dependent Vacuum Fractional Distillation for DMC-Rich Fraction Recovery from a Model Lithium-Ion Battery Electrolyte Solvent Mixture​

Technical Summary: Pressure-Dependent Vacuum Fractional Distillation for DMC-Rich Fraction Recovery

Problem Statement
The rapid expansion of the lithium-ion battery (LIB) industry has created an urgent need for efficient recycling technologies to handle the projected million-ton scale of spent batteries by 2030. While significant research focuses on recovering cathode metals, the organic carbonate electrolyte solvents—typically a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC)—have received less attention. These solvents constitute 10–15 wt% of a cell and pose safety and environmental risks if released uncontrolled. Although vacuum distillation is a scalable method for solvent recovery, existing applications often rely on simple distillation, which yields a DMC/DEC mixture rather than separated components, sacrificing purity for recovery efficiency. Furthermore, the application of fractional distillation under reduced-pressure conditions for LIB electrolytes remains systematically uninvestigated. Key gaps include the lack of data comparing Antoine-predicted boiling points with experimental onset temperatures under vacuum, the absence of analysis regarding the bubble-point behavior of ternary carbonate systems, and the unclear relationship between operating pressure and the practical separation window (ΔT) for these solvents.

Methodology
This study employed a pressure-controlled vacuum fractional distillation apparatus equipped with a quartz-packed fractionation column (20 mm ID) and a reflux divider to separate a model ternary mixture (EC/DMC/DEC at 1:1:1 v/v/v). The research was conducted in two stages:

1. Pure-Component Distillation: Individual solvents (DMC, DEC, and EC) were distilled at 100, 300, and 500 Torr to characterize their boiling behaviors and compare experimental recovery windows against theoretical predictions derived from the Antoine equation.
2. Ternary Mixture Distillation: The 1:1:1 mixture was distilled at 300 Torr (selected based on pure-component results) under varying temperature and reflux ratio conditions (2:1, 5:1, and 10:1).

The distillates and residues were analyzed using Gas Chromatography with a Flame Ionization Detector (GC-FID) for relative compositional analysis (normalized peak areas) and Fourier-Transform Infrared (FTIR) spectroscopy for functional group identification. Theoretical vapor-liquid equilibrium (VLE) was estimated using Raoult's law under an ideal-solution assumption to predict the initial bubble-point temperature and equilibrium vapor composition.

Key Results

• Pressure-Dependent Boiling and Recovery Windows: Experimental data confirmed that boiling points for DMC, DEC, and EC increase with pressure (e.g., DMC boiling points shifted from 38 °C at 100 Torr to 78 °C at 500 Torr). While the theoretical boiling point gap (ΔT_theo) between DMC and DEC remained nearly constant (32–35 °C), the apparatus-dependent operational recovery window (ΔT_exp)—defined as the temperature gap between complete DMC recovery and the onset of DEC recovery—increased with pressure (10 °C at 100 Torr, 20 °C at 300 Torr, and 30 °C at 500 Torr). This suggests that column transport effects (vapor flow, liquid holdup, and interfacial contact) significantly influence separation efficiency alongside thermodynamic volatility.
• Selection of Operating Conditions: 300 Torr was selected as the optimal operating pressure for the ternary mixture. It provided a sufficient practical separation interval (20 °C) while avoiding the incomplete DEC recovery and higher temperatures observed at 500 Torr.
• Distillation Performance: At 300 Torr, distillation near the calculated bubble point (89.3 °C) effectively retained high-boiling EC in the liquid residue. The recovered distillates were consistently enriched in DMC, accounting for 92.4–96.1% of the combined DMC and DEC FID peak area, with DEC comprising the remainder. EC was not detected in the distillate chromatograms.
• Process Variables: Within a fixed 1-hour operation, increasing the temperature (from 80 to 90 °C) increased the collected distillate volume, while increasing the reflux ratio decreased the volume. However, variations in temperature and reflux ratio had minimal impact on the relative DMC/DEC distribution within the distillate, which remained consistently DMC-rich.
• Analytical Confirmation: FTIR analysis confirmed the presence of characteristic linear carbonate bands (C=O and C–O–C) in the distillate, while the residue spectrum exhibited a distinct split carbonyl peak near 1800 cm⁻¹, characteristic of EC, confirming its retention in the residue.

Significance and Claims
The authors claim that this study establishes a thermodynamic and engineering basis for designing selective solvent recovery processes for LIB recycling. The work demonstrates that pressure-dependent vacuum fractional distillation is a viable primary separation step for producing a DMC-rich distillate and an EC-rich residue from carbonate solvent mixtures. By integrating experimental fractionation results with VLE analysis, the study clarifies that while simple vacuum distillation often yields mixed solvents, fractional distillation under controlled pressure can achieve high DMC enrichment. The findings highlight that the operational recovery window is not solely a function of intrinsic thermodynamic volatility but is also influenced by column hydrodynamics, necessitating careful selection of operating pressure to balance separation efficiency and recovery yield. The study concludes that this approach supports the development of closed-loop material cycles for battery recycling by enabling the recovery of high-value organic solvents without interference from lithium salt decomposition or aged electrolyte impurities.