With the rapid growth of the global energy storage sector, lithium-ion batteries, particularly energy storage lithium battery systems, have become indispensable due to their high efficiency and reliability. Among these, lithium iron phosphate (LiFePO₄) batteries dominate the market for energy storage lithium battery applications, owing to their safety, long cycle life, and cost-effectiveness. However, the escalating volume of spent energy storage lithium battery units poses significant environmental challenges, necessitating the development of sustainable recycling methods. In this study, we focus on the separation of cathode materials from aluminum current collectors in spent LiFePO₄ batteries, a critical step in the recycling process. Traditional methods, such as strong alkali treatments, often damage the cathode structure, reducing the material’s recyclability. Here, we explore the use of organic quaternary ammonium bases, specifically tetrapropylammonium hydroxide (TPAH), as a green alternative for efficient separation while preserving the integrity of the cathode materials for reuse in energy storage lithium battery systems.
We conducted a series of experiments to compare the separation efficiency of cathode materials using NaOH, ammonia water, tetramethylammonium hydroxide (TMAH), and TPAH. The separation rate was calculated using the formula: $$ \eta = \frac{\rho_1}{\rho_2 \times 0.935} \times 100\% $$ where $\rho_1$ is the dry mass of the recovered cathode material and $\rho_2$ is the dry mass of the cathode sheet. This formula allowed us to quantitatively assess the performance of each alkaline solution under varying conditions, ensuring accurate comparisons for energy storage lithium battery recycling applications.
Our experimental setup involved discharging spent energy storage lithium battery units in a 1.5 mol/L NaCl solution for 5 hours to reach a safe voltage, followed by manual disassembly to isolate cathode sheets. These sheets were cut into squares of varying sizes (1.5 cm to 3.5 cm per side) and treated with different alkaline solutions under controlled conditions. We optimized parameters such as pH, solution volume, and cathode size to maximize separation efficiency. The reactions involved, particularly for TPAH, can be represented by the equation: $$ \text{Al} + 4\text{TPAH} \rightarrow [\text{Al(TPAH)}_4]^+ + 3\text{H}_2\text{O} $$ This highlights the role of TPAH in selectively dissolving the aluminum current collector while minimizing damage to the cathode material, a key advantage for energy storage lithium battery recycling.
The results demonstrated that TPAH outperformed other alkaline substances, achieving a separation rate of up to 98.3% under optimal conditions: temperature of 25°C, TPAH solution volume of 100 mL, pH of 12.0, and cathode sheet side length of 3.0 cm. In contrast, NaOH, ammonia water, and TMAH showed lower efficiencies, with ammonia water being particularly ineffective due to its weak alkalinity. The superior performance of TPAH is attributed to its long carbon chain structure and steric hindrance effects, which enhance selectivity and reduce corrosion, making it ideal for energy storage lithium battery applications where material integrity is paramount.
| Alkaline Solution | Separation Rate (%) | Optimal pH | Reaction Time (min) |
|---|---|---|---|
| NaOH | 88.2 | 12.0 | 40 |
| Ammonia Water | ~0 | 12.0 | 60 |
| TMAH | 74.0 | 12.0 | 40 |
| TPAH | 98.3 | 12.0 | 40 |
Further analysis using scanning electron microscopy (SEM) revealed that TPAH-treated cathode materials retained a relatively smooth surface with uniform particle distribution, whereas NaOH caused significant surface collapse and agglomeration. This structural preservation is crucial for maintaining the electrochemical performance of recycled materials in energy storage lithium battery systems. Additionally, X-ray diffraction (XRD) patterns confirmed that TPAH-treated samples exhibited clear LiFePO₄ peaks (PDF#40-1499) with minimal structural degradation, unlike NaOH-treated samples where peak intensity decreased due to lithium ion leaching and lattice disruption.
Inductively coupled plasma optical emission spectroscopy (ICP-OES) was employed to analyze the elemental composition of the residual solutions after treatment. The results, summarized in Table 2, show that TPAH leaching resulted in the lowest concentrations of Li, Fe, and P, indicating minimal dissolution of cathode materials. This further underscores the suitability of TPAH for energy storage lithium battery recycling, as it reduces the loss of valuable components and minimizes environmental impact.
| Leaching Solution | Li | Fe | Al | P |
|---|---|---|---|---|
| NaOH | 0.800 | 0.900 | 15.86 | 5.200 |
| Ammonia Water | 0.267 | 0.567 | 1.400 | 2.300 |
| TMAH | 0.267 | 0.467 | 8.267 | 1.933 |
| TPAH | 0.167 | Not Detected | 10.134 | 1.867 |
To evaluate the practical applicability of the recovered cathode materials, we fabricated CR2016 coin cells and conducted electrochemical tests. The cells exhibited a stable specific capacity of approximately 125 mAh/g at a 0.5 C rate over 20 charge-discharge cycles, with a Coulombic efficiency consistently around 98.3%. The rate capability tests, performed between 2.8 V and 4.2 V, showed high reversibility and stability, with Coulombic efficiencies ranging from 96% to 98%. Cyclic voltammetry (CV) curves further confirmed the electrochemical activity, with distinct oxidation and reduction peaks at 3.569 V and 3.297 V, respectively, indicating excellent reversibility and minimal polarization. These results highlight the potential of TPAH-based recycling for producing high-quality materials for energy storage lithium battery applications.
The enhanced performance of TPAH can be explained by its molecular properties. The long alkyl chains in TPAH create a steric hindrance that selectively targets the aluminum current collector without attacking the cathode material. This is described by the free energy change equation: $$ \Delta G = \Delta H – T\Delta S $$ where the favorable entropy change ($\Delta S$) due to the formation of soluble complexes promotes the reaction kinetics. Moreover, the mild alkalinity of TPAH reduces the risk of side reactions, preserving the LiFePO₄ structure. This makes TPAH an ideal candidate for large-scale recycling of energy storage lithium battery systems, aligning with sustainability goals.
In conclusion, our study demonstrates that organic quaternary ammonium bases, particularly TPAH, offer a green and efficient approach for recycling spent LiFePO₄ cathode materials. The high separation rate, minimal material damage, and excellent electrochemical performance of the recovered materials underscore the viability of this method for energy storage lithium battery recycling. Future work could focus on scaling up the process and integrating it with other recycling steps to create a closed-loop system for energy storage lithium battery management. By adopting such innovative techniques, we can address the environmental challenges associated with battery waste while supporting the growth of renewable energy storage solutions.