Analytical Protocols for Recovered Electrolyte Characterization in Battery Recycling

The recovery and reuse of lithium-ion battery electrolytes present significant economic and environmental advantages, but require rigorous analytical verification to ensure composition and purity meet performance standards. Three principal techniques—Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), and nuclear magnetic resonance (NMR)—form the core of electrolyte characterization protocols. Each method provides complementary data for identifying functional groups, quantifying contaminants, and assessing degradation products.

Raman Spectroscopy for Structural Fingerprinting
Raman spectroscopy is employed for non-destructive analysis of electrolyte solvents and lithium salts. The technique detects symmetric vibrational modes, making it ideal for identifying carbonate solvents like ethylene carbonate (EC) and dimethyl carbonate (DMC). Characteristic peaks for EC appear at 715 cm⁻¹ (ring breathing) and 890 cm⁻¹ (CO symmetric stretch), while DMC shows a strong band at 830 cm⁻¹ (O–C–O stretch). Detection limits for common contaminants such as hydrofluoric acid (HF) are approximately 100 ppm, with sensitivity limited by weak Raman scattering in non-polar species. Water contamination can be identified near 3400 cm⁻¹ (O–H stretch), but quantification below 500 ppm requires supplemental techniques.

FTIR for Functional Group Analysis
FTIR complements Raman by detecting asymmetric vibrations and polar functional groups. The carbonyl stretch (C=O) of carbonates appears between 1740–1800 cm⁻¹, while LiPF6 decomposition products like PF5 and POF3 exhibit peaks at 850–950 cm⁻¹. FTIR achieves lower detection limits for HF (50 ppm) due to strong H–F absorption at 4000 cm⁻¹. Water contamination is quantifiable down to 200 ppm using the O–H bending mode at 1640 cm⁻¹. Calibration curves constructed from known mixtures of virgin and degraded electrolytes enable semi-quantitative analysis of decomposition products such as organic acids (e.g., formic acid, acetic acid).

NMR for Quantitative Compositional Analysis
¹H and ¹⁹F NMR provide atomic-level resolution for quantifying solvent ratios and salt degradation. In ¹H NMR, EC protons resonate at 4.5 ppm, while DMC methyl groups appear at 3.7 ppm. Molar ratios of solvent components can be determined with <5% error using internal standards like tetramethylsilane (TMS). ¹⁹F NMR is critical for assessing LiPF6 integrity; the PF6⁻ anion produces a singlet at -70 ppm, while decomposition byproducts like HF (δ = -120 ppm) and PF5 (δ = -80 ppm) are detectable at 10 ppm concentrations. Purity thresholds for reuse typically demand <100 ppm HF and <200 ppm water.

Standardization and Contaminant Thresholds
Industry standards for recovered electrolytes specify maximum permissible levels of key contaminants:
- HF: <50 ppm (cell corrosion risk)
- H2O: <200 ppm (SEI formation interference)
- Organic acids: <300 ppm (accelerated aging)
- Metal ions (Fe, Cu, Al): <1 ppm (dendrite nucleation)

ASTM E222-17 and IEC 62321-8 provide guidelines for sample preparation and measurement reproducibility. Cross-validation between Raman, FTIR, and NMR is recommended to mitigate false positives/negatives.

Decision Trees for Quality Grading
A tiered grading system determines reuse potential:
Grade A (Virgin-equivalent): Contaminants below threshold, solvent/salt ratios within ±5% of specification. Suitable for direct reuse in high-performance cells.
Grade B (Blend-ready): HF <100 ppm, H2O <500 ppm, minor decomposition. Requires blending with virgin electrolyte at ≤30% recovered fraction.
Grade C (Non-recoverable): Excessive degradation or contamination. Diverted to neutralization or material recovery.

Blending strategies for Grade B electrolytes involve:
1. Adjusting solvent ratios via distillation or additive correction.
2. Neutralizing residual HF with Li2CO3 or molecular sieves.
3. Verifying electrochemical stability via linear sweep voltammetry (LSV) to ensure >4.5 V anodic stability.

Equipment Cost-Benefit Analysis
Capital and operational costs for analytical setups vary by technique:
Technique Capital Cost ($) Throughput (samples/day) Operating Cost ($/sample)
Raman spectrometer 80,000–150,000 20–30 50–100
FTIR spectrometer 40,000–100,000 30–50 20–50
NMR spectrometer 300,000–600,000 10–15 150–300

High-volume recycling facilities (>10,000 tons/year) benefit from integrated FTIR-Raman systems, achieving 80 samples/day with 30% cost reduction versus standalone units. Mid-scale operations (1,000–10,000 tons/year) prioritize FTIR for cost efficiency, while NMR serves as a confirmatory tool for borderline cases.

Implementation in Recycling Workflows
Optimal analytical sequences follow:
1. Rapid FTIR screening of all incoming electrolyte batches.
2. Raman analysis for batches failing FTIR thresholds.
3. NMR validation for compositional ambiguity or high-value streams.

Automated spectral libraries and machine learning classifiers reduce interpretation time by 70%, with false rejection rates maintained below 5%.

Future directions include miniaturized sensors for inline monitoring during recovery processes, though current limitations in sensitivity restrict deployment to final quality control stages. Advances in cavity-enhanced Raman spectroscopy may lower HF detection limits to 10 ppm, further closing the gap between recycled and virgin electrolyte performance.

The integration of these protocols ensures recovered electrolytes meet stringent performance criteria while maximizing resource efficiency in closed-loop battery ecosystems.