The recycling of spent lithium iron phosphate (LiFePO4) batteries has gained significant attention due to growing environmental concerns and the need for sustainable resource utilization. This study investigates the leaching kinetics of lithium (Li) and iron (Fe) from spent LiFePO4 electrode powder using sulfuric acid, focusing on optimizing process parameters and establishing a robust kinetic model.
Experimental Methodology
The electrode powder, derived from dismantled LiFePO4 batteries, primarily contained LiFePO4, Li3Fe2(PO4)3, Fe2O3, and carbon. Leaching experiments were conducted under controlled conditions with sulfuric acid (H2SO4) as the lixiviant. The effects of acid concentration (1.5–3.5 mol/L), temperature (55–95°C), particle size (0.095–0.375 mm), and agitation speed (200–600 rpm) were systematically analyzed.
Optimization of Leaching Parameters
Optimal leaching efficiency (>99.8% for both Li and Fe) was achieved under the following conditions:
- H2SO4 concentration: 2.5 mol/L
- Temperature: 75°C
- Particle size: 0.14 mm
- Agitation speed: 400 rpm
| Factor | Level | Fe | Li | ||
|---|---|---|---|---|---|
| n | ln k | n | ln k | ||
| Acid Conc. (mol/L) | 1.5 | 0.150 | -0.482 | 0.240 | 0.719 |
| 2.0 | 0.181 | -0.363 | 0.246 | 0.797 | |
| 2.5 | 0.231 | -0.254 | 0.285 | 0.855 | |
| 3.0 | 0.251 | -0.155 | 0.326 | 0.940 | |
| 3.5 | 0.273 | -0.029 | 0.354 | 1.008 | |
Kinetic Analysis
The leaching process followed the Avrami model, expressed as:
$$ -\ln(1 – x) = kt^n $$
where \( x \) = leaching fraction, \( k \) = rate constant, and \( n \) = reaction characteristic parameter. The average \( n \) values of 0.2349 (Fe) and 0.2867 (Li) indicated diffusion-controlled kinetics.
Activation Energy Calculation
The Arrhenius equation revealed low apparent activation energies:
$$ \ln k = \ln A – \frac{E_a}{RT} $$
where \( E_a \) = 11.03 kJ/mol (Fe) and 8.45 kJ/mol (Li), confirming external diffusion dominance.
Comprehensive Kinetic Equations
The derived rate equations for LiFePO4 battery electrode powder leaching:
For Fe:
$$ -\ln(1 – x) = 1.25C^{0.5228}D^{-0.3191}W^{0.3718}\exp\left(-\frac{1326.6}{T}\right)t^{0.2349} $$
For Li:
$$ -\ln(1 – x) = 0.68C^{0.3381}D^{-0.2208}W^{0.5640}\exp\left(-\frac{1016.3}{T}\right)t^{0.2867} $$
Mechanistic Insights
The hierarchical leaching behavior originates from structural differences in LiFePO4 battery components:
- Li+ ions intercalated in carbon layers dissolve rapidly (complete within 10 min)
- Fe dissolution from crystalline LiFePO4 requires acid penetration through carbon coatings
- Oxidized Fe2O3 phases dissolve through surface reaction-limited pathways
Industrial Implications
This kinetic model enables precise control of LiFePO4 battery recycling processes:
- Reduces acid consumption by 22% compared to conventional over-leaching practices
- Shortens leaching duration to 90 min while maintaining >99.8% metal recovery
- Facilitates closed-loop regeneration of battery-grade FePO4 precursors
Conclusion
The established kinetic framework provides fundamental guidance for sustainable recovery of strategic metals from spent LiFePO4 batteries. The low activation energies and diffusion-controlled mechanism suggest potential for energy-efficient industrial implementation. Future research should focus on interface engineering to enhance leaching rates while minimizing carbon matrix decomposition.