Innovative Recovery of Battery-Grade Ferric Phosphate from Lithium Iron Phosphate Battery Residue

The rapid growth of lithium iron phosphate battery production and recycling has generated substantial residue containing ferric phosphate (FePO₄) and aluminum impurities. Traditional lithium recovery methods often discard this residue due to challenges in separating FePO₄ from aluminum. This study presents a novel nitric acid leaching process combined with methanol electrolysis and hydrofluoric acid precipitation to recover high-purity FePO₄, meeting the stringent requirements of battery-grade materials.

Experimental Methodology

The residue, sourced from spent lithium iron phosphate battery cathodes, underwent sequential treatment steps:

Pretreatment: Grinding and sieving to achieve uniform particle size (D₅₀ = 45 μm).
Nitric Acid Leaching: Optimized conditions derived from parametric studies:
$$\text{FePO}_4 + 3 \text{HNO}_3 \rightarrow \text{Fe(NO}_3\text{)}_3 + \text{H}_3\text{PO}_4 + \text{H}_2\text{O}$$
Electrolytic Reduction: Methanol-assisted Fe³⁺ → Fe²⁺ conversion:
$$\text{Fe}^{3+} + \text{e}^- \rightarrow \text{Fe}^{2+} \quad (E^0 = 0.77 \text{ V vs. SHE})$$
Aluminum Removal: Hydrofluoric acid precipitation:
$$\text{Al}^{3+} + 3 \text{F}^- \rightarrow \text{AlF}_3 \downarrow$$

Table 1. Optimization of Leaching Parameters
Parameter	Range	Optimal Value	Fe Leaching Efficiency
Temperature (°C)	20-90	70	93.4%
Time (min)	20-80	60	94.1%
HNO₃ Concentration	1-5M	3M	91.8%

Mechanistic Insights

Cyclic voltammetry revealed methanol’s critical role in suppressing Fe²⁺ oxidation during electrolysis. The current density (5 A/cm²) and reaction time (30 min) were optimized using the Nernst equation:

$$ E = E^0 – \frac{RT}{nF}\ln Q $$

where Q represents the reaction quotient for Fe³⁺/Fe²⁺ redox couple.

Table 2. Impurity Analysis of Recovered FePO₄
Element	Content (ppm)	HG/T 4701-2021 Limit
Al	72	100
Cu	12	50
Ca	22	200

Process Economics

The developed method demonstrates superior cost-effectiveness for lithium iron phosphate battery recycling:

$$\text{Process Efficiency} = \frac{m_{\text{FePO}_4}}{m_{\text{Residue}}} \times 100\% = 89.7\% $$

Compared to conventional methods, this approach reduces chemical consumption by 40% while achieving 98.5% aluminum rejection.

Conclusion

This study successfully addresses the technical barriers in lithium iron phosphate battery residue valorization. The integrated process achieves:

93.4% Fe recovery efficiency
Al content reduction to 72 ppm
Energy consumption < 2 kWh/kg FePO₄

The recovered FePO₄ meets all specifications for reuse in lithium iron phosphate battery production, establishing a closed-loop solution for sustainable energy storage systems.