The growing demand for lithium-ion batteries, particularly LiFePO4 batteries, has led to significant environmental challenges due to improper disposal. This study investigates a green hydrometallurgical approach using a ternary deep eutectic solvent (DES) composed of choline chloride (ChCl), ascorbic acid (AA), and ethylene glycol (EG) to recover lithium (Li) and iron (Fe) from spent LiFePO4 battery cathodes. The DES system leverages synergistic interactions between hydrogen bond donors and acceptors to enhance metal dissolution while minimizing environmental impact.
1. Experimental Methodology
Spent LiFePO4 batteries were manually disassembled, and cathode materials were separated for leaching experiments. The DES was prepared by mixing ChCl, AA, and EG at molar ratios of 1:1:2 to 1:1:6. Leaching efficiency was evaluated under varying conditions:
| Parameter | Range |
|---|---|
| DES molar ratio (ChCl:AA:EG) | 1:1:2 – 1:1:6 |
| Liquid/solid ratio (mL/mg) | 0.05 – 0.4 |
| Temperature (°C) | 50 – 90 |
| Time (min) | 5 – 60 |
The leaching efficiency was calculated using:
$$ x = \frac{\rho_B}{\rho_{B0}} \times 100\% $$
where \( \rho_B \) and \( \rho_{B0} \) represent metal concentrations in leachate and digested sample, respectively.
2. Results and Discussion
2.1 Optimization of Leaching Conditions
The recovery of Li and Fe from spent LiFePO4 batteries demonstrated strong dependence on DES composition and process parameters:
| Parameter | Optimal Value | Li Recovery | Fe Recovery |
|---|---|---|---|
| DES Ratio | 1:1:3 | 96% | 98% |
| Liquid/Solid | 0.1 mL/mg | Maximized dissolution | |
| Temperature | 80°C | Optimal kinetic energy | |
| Time | 60 min | Reaction equilibrium | |
2.2 Kinetic Analysis
Leaching kinetics of LiFePO4 battery cathode materials were modeled using four control mechanisms:
- External diffusion: \( x = k_1 t \)
- Internal diffusion: \( 1 – \frac{2}{3}x – (1 – x)^{2/3} = k_2 t \)
- Chemical reaction: \( 1 – (1 – x)^{1/3} = k_3 t \)
- Mixed control: \( (1 – x)^{-1/3} – 1 + \ln(1 – x)^{1/3} = k_4 t \)
The chemical reaction model showed the best fit (\( R^2 > 0.97 \)), confirming surface-controlled kinetics. Activation energies were derived using the Arrhenius equation:
$$ k = A \exp\left(-\frac{E_a}{RT}\right) $$
where \( E_a \) values of 42.3 kJ/mol (Li) and 38.7 kJ/mol (Fe) further supported chemical reaction dominance.
2.3 Morphological Changes
SEM analysis revealed significant structural modifications in LiFePO4 battery cathode materials post-leaching. Pristine particles exhibited smooth surfaces (Fig. 1a), while leached residues showed extensive cracking and surface erosion (Fig. 1b), indicating successful metal extraction through DES interaction.
3. Mechanistic Insights
The ternary DES system facilitates LiFePO4 battery cathode dissolution through three synergistic effects:
- Proton donation from ascorbic acid: \( \text{LiFePO}_4 + \text{H}^+ \rightarrow \text{Li}^+ + \text{Fe}^{2+} + \text{HPO}_4^{2-} \)
- Chloride complexation: \( \text{Fe}^{2+} + 4\text{Cl}^- \rightarrow [\text{FeCl}_4]^{2-} \)
- EG-mediated solvation: Enhanced metal ion stabilization
This mechanism enables high recovery rates while avoiding toxic reagents typically used in LiFePO4 battery recycling.
4. Conclusion
The ChCl/AA/EG ternary DES system demonstrates exceptional efficiency in recovering valuable metals from spent LiFePO4 batteries, achieving 96% Li and 98% Fe recovery under optimized conditions. The chemical reaction-controlled process, combined with the green solvent characteristics, positions this method as a sustainable alternative to conventional hydrometallurgical approaches for LiFePO4 battery recycling. Future research should focus on DES regeneration and scale-up feasibility to enable industrial adoption.
| Method | Acid Consumption | Temperature | Li Recovery | Fe Recovery |
|---|---|---|---|---|
| Ternary DES (This work) | 0 | 80°C | 96% | 98% |
| Traditional H2SO4 | 2-4 M | 90°C | 92% | 95% |
| Pyrometallurgy | N/A | >800°C | 85% | 90% |