As the global demand for clean energy solutions escalates, energy storage lithium batteries have become indispensable in powering everything from portable electronics to electric vehicles and grid storage systems. The rapid expansion of these applications has led to a surge in end-of-life batteries, posing significant environmental and resource challenges. In this study, we explore an advanced recycling methodology using deep eutectic solvents (DES) to recover valuable metals from spent energy storage lithium batteries, focusing on lithium cobalt oxide (LiCoO₂) cathode materials. Our approach emphasizes green chemistry principles, high efficiency, and economic viability, addressing critical gaps in current recycling technologies for energy storage lithium battery systems.
The proliferation of energy storage lithium battery waste underscores the urgency for innovative recycling techniques. Traditional methods, such as pyrometallurgy and hydrometallurgy, often involve high energy consumption, toxic reagents, and low metal recovery rates, particularly for lithium. For instance, pyrometallurgical processes typically achieve lithium recovery efficiencies below 60%, while hydrometallurgical methods generate acidic waste streams that require neutralization. In contrast, DES-based systems offer a promising alternative due to their low volatility, biodegradability, and tunable properties. We developed a ChCl-OA-H₂O DES system, where choline chloride (ChCl) and oxalic acid (OA) form a eutectic mixture with water, enabling selective leaching and precipitation of metals without additional chemicals. This method aligns with the circular economy goals for energy storage lithium battery management, reducing environmental footprint and conserving scarce resources.
Our experimental investigation began with synthesizing the DES by mixing ChCl, OA, and water in varying molar ratios. The viscosity of the DES was critical for practical application, as high viscosity can impede mass transfer during leaching. We measured the viscosity across different water contents and temperatures, noting a significant reduction with increased water addition. For example, at 25°C, the viscosity dropped from 700 mPa·s for anhydrous DES to 11 mPa·s for ChCl-OA-8H₂O, facilitating higher solid-liquid ratios. The leaching process involved reacting spent LiCoO₂ powder with the DES at optimized conditions, leading to the dissolution of lithium and cobalt, followed by selective precipitation. The leaching efficiency was calculated using the formula: $$\text{Leaching Efficiency} = \frac{C \times V}{m \times w} \times 100\%$$ where \(C\) is the metal concentration in the leachate (g/L), \(V\) is the volume of the leachate (L), \(m\) is the mass of the spent battery material (g), and \(w\) is the mass percentage of the metal in the material. Similarly, recovery efficiency was determined by: $$\text{Recovery Efficiency} = \frac{M}{M_0} \times 100\%$$ where \(M_0\) is the initial mass of the metal in the sample (g) and \(M\) is the mass of the metal in the precipitate (g).
We systematically evaluated the effects of DES molar ratio, solid-liquid ratio, and temperature on metal recovery. The optimal conditions were identified as a ChCl-OA-8H₂O molar ratio, a solid-liquid ratio of 100 g/L, and a temperature of 90°C, under which lithium leaching efficiency exceeded 99.4%, and cobalt was preferentially precipitated as CoC₂O₄·2H₂O with a recovery efficiency of 97.8%. The mechanism involves the breakdown of Li-O and Co-O bonds by H⁺ ions from the DES, reduction of Co³⁺ to Co²⁺ by C₂O₄²⁻, and subsequent coordination and precipitation steps. The reactions can be summarized as: $$2\text{LiCoO}_2(s) + \text{C}_2\text{O}_4^{2-} + 8\text{H}^+ \rightarrow 2\text{Co}^{2+} + 2\text{Li}^+ + 2\text{CO}_2 + 4\text{H}_2\text{O}$$ $$\text{Co}^{2+} + 4\text{ChCl} \rightarrow [\text{CoCl}_4]^{2-} + 4\text{Ch}^+$$ $$[\text{CoCl}_4]^{2-} + 4\text{Ch}^+ + 6\text{H}_2\text{O} \rightarrow [\text{Co(H}_2\text{O)}_6]^{2+} + 4\text{ChCl}$$ $$[\text{Co(H}_2\text{O)}_6]^{2+} + \text{C}_2\text{O}_4^{2-} \rightarrow \text{CoC}_2\text{O}_4 \cdot 2\text{H}_2\text{O} \downarrow + 4\text{H}_2\text{O}$$ $$\text{Li}^+ + \frac{1}{2}\text{C}_2\text{O}_4^{2-} \rightarrow \frac{1}{2}\text{Li}_2\text{C}_2\text{O}_4 \downarrow$$ This sequential process ensures high selectivity, with lithium recovered as Li₂C₂O₄ after water evaporation. The purity of the recovered products was exceptional, reaching 99.72% for CoC₂O₄·2H₂O and 99.93% for Li₂C₂O₄, as confirmed by ICP analysis.
To illustrate the performance under varying conditions, we present the following tables summarizing key results. Table 1 shows the impact of DES molar ratio on leaching and recovery efficiencies, highlighting the optimal point at ChCl-OA-8H₂O. The data demonstrate that water content plays a dual role in reducing viscosity and modulating metal coordination, crucial for efficient energy storage lithium battery recycling.
| Molar Ratio (ChCl:OA:H₂O) | Li Leaching Efficiency (%) | Co Leaching Efficiency (%) | Li Recovery Efficiency (%) | Co Recovery Efficiency (%) |
|---|---|---|---|---|
| 1:1:5 | 97.3 | 7.2 | 78.5 | 92.8 |
| 1:1:6 | 97.6 | 6.6 | 80.2 | 93.4 |
| 1:1:8 | 99.0 | 4.4 | 87.9 | 97.6 |
| 1:1:10 | 96.9 | 5.9 | 76.9 | 94.2 |
| 1:1:15 | 95.7 | 7.3 | 74.3 | 92.7 |
Table 2 details the influence of solid-liquid ratio on recovery performance, emphasizing the trade-off between efficiency and practicality. At 100 g/L, we achieved a balance, enabling high-throughput processing for energy storage lithium battery recycling without compromising recovery rates.
| Solid-Liquid Ratio (g/L) | Li Leaching Efficiency (%) | Co Leaching Efficiency (%) | Li Recovery Efficiency (%) | Co Recovery Efficiency (%) |
|---|---|---|---|---|
| 80 | 96.3 | 3.8 | 79.7 | 96.1 |
| 100 | 99.4 | 2.7 | 84.2 | 97.8 |
| 120 | 97.0 | 3.1 | 77.3 | 95.5 |
| 140 | 95.3 | 4.8 | 70.7 | 94.2 |
| 160 | 94.1 | 6.9 | 68.4 | 93.1 |
Temperature optimization was another critical factor, as shown in Table 3. The data indicate that 90°C maximizes lithium recovery while minimizing cobalt in the leachate, essential for selective separation in energy storage lithium battery recycling processes.
| Temperature (°C) | Li Leaching Efficiency (%) | Co Leaching Efficiency (%) | Li Recovery Efficiency (%) | Co Recovery Efficiency (%) |
|---|---|---|---|---|
| 60 | 44.1 | 56.9 | 34.4 | 41.1 |
| 70 | 60.8 | 48.3 | 49.5 | 51.9 |
| 80 | 90.7 | 14.5 | 77.5 | 84.4 |
| 90 | 94.9 | 3.2 | 88.3 | 96.3 |
| 100 | 55.2 | 3.4 | 53.1 | 96.0 |
The DES regeneration study revealed excellent sustainability, with the solvent maintaining performance over multiple cycles. After six regeneration cycles, lithium and cobalt recovery efficiencies remained at 78.1% and 92.8%, respectively, demonstrating the robustness of the system for long-term energy storage lithium battery recycling. The Fourier-transform infrared (FT-IR) spectroscopy confirmed that the DES structure remained intact after regeneration, with key functional groups preserved. This reusability reduces operational costs and environmental impact, making it a viable solution for large-scale applications. The overall process efficiency can be modeled using kinetic equations, such as the shrinking core model, to predict leaching rates: $$\frac{dX}{dt} = k(1-X)^{2/3}$$ where \(X\) is the fraction of metal leached, \(t\) is time, and \(k\) is the rate constant dependent on temperature and DES composition.
In conclusion, our DES-based strategy offers a green and efficient pathway for recycling critical metals from spent energy storage lithium batteries. By optimizing water content and process parameters, we achieved high recovery efficiencies and product purity, with the DES being reusable multiple times. This approach not only addresses the environmental challenges associated with battery waste but also contributes to the sustainable supply chain for energy storage lithium battery production. Future work will focus on scaling up the process and adapting it to other battery chemistries, further enhancing the circular economy for energy storage systems. The integration of such innovative technologies is pivotal for meeting the growing demands of the energy storage lithium battery market while minimizing ecological footprint.
Furthermore, the economic and environmental benefits of this method are substantial compared to conventional techniques. For instance, the avoidance of strong acids and bases reduces chemical consumption and waste treatment costs. The high solid-liquid ratio of 100 g/L improves throughput, making it suitable for industrial-scale energy storage lithium battery recycling. Life cycle assessment (LCA) studies could quantify the reduced carbon footprint, but preliminary estimates suggest significant savings in energy and emissions. As the adoption of energy storage lithium batteries continues to rise, driven by renewable energy integration and electric mobility, sustainable recycling methods like this will play a crucial role in ensuring resource security and environmental protection. We envision that further research will explore hybrid DES systems and automation to optimize the process for diverse battery types, ultimately supporting a greener future for energy storage technologies.