The rapid expansion of the new energy vehicle and energy storage sectors, driven by global carbon neutrality goals, has led to an unprecedented surge in the production and consumption of lithium-ion batteries. Graphite remains the dominant anode material in commercial lithium-ion batteries due to its favorable electrochemical properties. However, this growth faces a dual threat: the increasing cost and finite reserve of critical raw materials like graphite, and the impending wave of end-of-life battery management which poses significant environmental risks if not handled properly. The strategic importance of graphite has been recognized globally, with many countries imposing controls on its mining and export. Therefore, developing efficient recycling technologies for graphite from spent lithium-ion batteries is not merely an economic imperative but a crucial strategy for ensuring resource security, mitigating environmental impact, and supporting a sustainable circular economy for the battery industry.
The performance of regenerated graphite from spent lithium-ion batteries is influenced by a complex interplay of factors. These can be categorized into impurity-related issues and structural degradation. Impurities include inorganic residues such as metal ions (Ni, Fe, Cu, Al) from cathode cross-contamination and current collector fragments, and organic residues like binders (e.g., PVDF) and conductive agents (e.g., carbon black). Structural degradation encompasses the destruction of the graphite crystal lattice due to repeated lithium intercalation/deintercalation cycles, particle cracking during battery use or the recycling process, and the formation of a defective or excessively thick solid electrolyte interphase (SEI). This study systematically investigates these influencing factors and develops an optimized process flow involving purification and repair to restore the electrochemical performance of waste graphite. The core of the regeneration strategy lies in effective impurity removal via acid leaching and particle reconstruction through carbon coating and high-temperature treatment.
1. Materials and Experimental Methods
1.1 Raw Materials and Reagents
Two primary categories of waste graphite feedstock were used: “cycled graphite” derived from the black mass of dismantled and crushed batteries containing electrolyte, and “uncycled graphite” obtained from precision-disassembled anode scraps or production waste from cell manufacturing. Commercial graphite (AML-400) was used as a baseline and for simulated contamination experiments. Contaminants like commercial NCM (LiNixCoyMnzO2) and LFP (LiFePO4) cathode powders, copper powder, conductive carbon black (Super P), and binder (Polyvinylidene fluoride, PVDF) were used to study impurity effects. Chemical reagents included hydrochloric acid (HCl, AR), sulfuric acid (H2SO4, AR), and nitric acid (HNO3, AR). Pitch with a softening point of 250°C was used as the carbon precursor for coating.
1.2 Equipment and Characterization
The experimental setup included a tube furnace, carbonization furnace, acid leaching stir tank, centrifuge, vibrating sieve, and a high-speed mixer. Characterization was performed using a scanning electron microscope (SEM) for morphology, a specific surface area analyzer (BET) for surface area measurement, an inductively coupled plasma optical emission spectrometer (ICP-OES) for elemental analysis, and an X-ray diffractometer (XRD) for crystal structure analysis. Electrochemical performance was evaluated using CR2032 coin-type half-cells (vs. Li/Li+). The electrode slurry was composed of 91.6 wt% active material, 6.6 wt% PVDF binder, and 1.8 wt% Super P. Cells were tested within a voltage window of 0.005-2.0 V at 0.1C rate and 45°C.
1.3 Experimental Procedures
1.3.1 Influence and Removal of Metallic Inorganic Impurities
To evaluate the efficiency of different acids, commercial graphite was artificially contaminated with 1 wt% of individual impurities (Cu, NCM, or LFP). Acid leaching was conducted using 200 g/L H2SO4, 200 g/L HCl, 200 g/L HNO3, or a 50 g/L-50 g/L HCl-H2SO4 mixed acid. The solid-to-liquid ratio was maintained at 1:10 (w/v). The residual metal content was analyzed by ICP-OES. This procedure was repeated with a mixed impurity system (1 wt% total of Cu+NCM+LFP) using the optimal mixed acid identified. Finally, actual waste graphite powders from industrial sources were treated with the optimal acid leaching conditions to validate the process.
1.3.2 Influence and Removal of Organic Impurities
The impact of organic residues was studied by subjecting pure PVDF binder, Super P conductive agent, and commercial graphite to a two-step treatment: pre-carbonization at 650°C under N2 atmosphere followed by graphitization at 2400°C. Their respective mass retention, BET surface area, and simulated electrochemical contribution (as a contaminant in graphite) were assessed. Furthermore, commercial graphite was mixed with 2 wt% PVDF or Super P, and after pre-carbonization at 650°C, the change in BET was measured to understand the effect of in-situ residue formation.
1.3.3 Repair of Graphite Particles
To repair structural defects and reduce surface area, a carbon coating process was employed. Waste graphite powder was thoroughly mixed with different weight percentages (0%, 2%, 3%, 4%, 5%) of pitch. The mixture was then carbonized at 1200°C for 2 hours under a nitrogen atmosphere. The specific surface area of the resulting material was measured to determine the optimal pitch ratio. Two full regeneration processes were then executed:
1. For uncycled graphite: Sieving/classification → optimal acid leaching → pitch coating/carbonization (4% pitch).
2. For cycled graphite: Sieving/classification → optimal acid leaching → graphitization (high T) → pitch coating/carbonization (4% pitch).
The final regenerated products were comprehensively characterized and their electrochemical performance was compared with a commercial graphite reference.
2. Results and Discussion
2.1 Optimization of Inorganic Impurity Removal by Acid Leaching
The efficacy of acid leaching is highly dependent on the nature of the metallic impurity and the oxidizing power of the leaching medium. The results for single-impurity systems are summarized below.
| Impurity Type | Target Element | 200 g/L H2SO4 | 200 g/L HCl | 200 g/L HNO3 | 50g/L-50g/L HCl-H2SO4 |
|---|---|---|---|---|---|
| 1% NCM | Ni (mg/kg) | 4 | 23 | 4 | 12 |
| 1% LFP | Fe (mg/kg) | 18 | 21 | 18 | 22 |
| 1% Cu | Cu (mg/kg) | High | High | <10 | Medium |
All acids were effective in dissolving NCM and LFP, reducing residual Ni and Fe to below 50 mg/kg. For copper, a strong oxidant is required to dissolve metallic Cu. Nitric acid was the most effective due to its inherent oxidizing nature, but it generates hazardous NOx gases. The mixed acid showed moderate efficacy. However, in a complex, multi-impurity system resembling real black mass (containing Cu, NCM, LFP), the presence of oxidizing NCM significantly enhanced Cu removal when using the mixed acid, bringing residual Cu below 100 mg/kg. Therefore, considering overall efficiency, safety, and environmental impact, the 50 g/L-50 g/L HCl-H2SO4 mixed acid was selected as the optimal leaching agent.
Treating industrial waste graphite powders with the optimal mixed acid confirmed its robustness. Key impurity elements (Ni, Fe, Cu, Al) were reduced from initial levels (often <1 wt%) to consistently below 50 mg/kg post-leaching. This purification step also led to a decrease in BET surface area and an improvement in initial coulombic efficiency (ICE), indicating the removal of high-surface-area impurities and electrochemically active metal ions that contribute to irreversible capacity loss.
2.2 Impact and Mitigation of Organic Impurities
Organic components undergo complex transformations during thermal treatment. Pure PVDF and Super P were subjected to pre-carbonization (650°C) and graphitization (2400°C). The results are critical for understanding their role as contaminants.
| Material | Mass Retention after 650°C (%) | BET after 650°C (m2/g) | Mass Retention after 2400°C (%) | BET after 2400°C (m2/g) | ICE (Simulated, %) | Capacity (Simulated, mAh/g) |
|---|---|---|---|---|---|---|
| Graphite | ~100 | 1.59 | ~100 | ~1.5 | High | ~350 |
| PVDF Residue | ~20 | 2.77 | ~5 | N/A | Very Low | Very Low |
| Super P Residue | ~98 | 3.23 | ~95 | 61.53 | Very Low | Very Low |
PVDF decomposes substantially, leaving a small carbonaceous residue that slightly increases BET. Super P, being a stable conductive carbon, retains most of its mass and its extremely high surface area persists even after high-temperature treatment. When mixed with graphite, these residues act as detrimental impurities. Super P is particularly harmful due to its high, persistent surface area, which increases electrolyte decomposition and irreversible Li+ consumption, severely lowering ICE. The liberation of gases from PVDF decomposition can also increase porosity and BET. Fortunately, the significant difference in particle size after treatment—PVDF forming larger agglomerates and Super P remaining as fine particles—allows for their effective partial removal through mechanical processes like sieving and air classification prior to the main regeneration steps.
2.3 Structural Repair via Carbon Coating and Graphitization
The repair process addresses two main issues: crystallographic disorder and excessive surface area/defects from fractured particles. High-temperature graphitization (>2500°C) is effective in restoring long-range order in the graphite crystal, which is especially necessary for cycled graphite that has suffered lattice distortion. For particle-level repair, pitch coating and carbonization were employed. The relationship between pitch content and the resulting BET surface area of the regenerated graphite is given by:
$$ \text{BET}_{\text{final}} = \text{BET}_{\text{initial}} – k \cdot C_{\text{pitch}} $$
where $k$ is a positive constant representing the coating efficiency, and $C_{\text{pitch}}$ is the weight percentage of pitch. Experimentally, as pitch content increased from 0% to 5%, the BET decreased monotonically. A 4% pitch addition was chosen as the optimal compromise, achieving a sufficiently low BET (~1-2 m2/g) for good electrochemical performance without excessive cost.
| Pitch Content (%) | 0 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|
| BET (m2/g) | ~4.5 | ~3.2 | ~2.3 | ~1.8 | ~1.5 |
The SEM analysis of the final regenerated products (from both uncycled and cycled feedstocks) showed smooth surfaces with significantly reduced fines and debris. The carbon coating effectively “glued” some small particles together, forming more robust secondary particles. This morphology enhances isotropy and improves the tap density and cycling stability of the material. XRD patterns confirmed that the regenerated graphite exhibited a sharp and strong (002) diffraction peak, comparable to that of commercial graphite, indicating a high degree of graphitization and well-restored crystal structure. The graphitization degree ($G$) can be estimated from the XRD pattern using the formula:
$$ G = \frac{I_{002}}{I_{001}} \times 100\% $$
where $I_{002}$ and $I_{001}$ are the integrated intensities of the (002) and (001) peaks, respectively. The regenerated samples showed $G$ values exceeding 90%, confirming effective structural repair.
2.4 Electrochemical Performance of Regenerated Graphite
The ultimate test of the regeneration process is the electrochemical performance in a lithium-ion battery half-cell. The results for the optimally processed materials are summarized below.
| Sample | Process | First Discharge Capacity (0.1C, mAh/g) | Initial Coulombic Efficiency – ICE (%) |
|---|---|---|---|
| Regenerated (Uncycled Feed) | Sieving → Mixed Acid Leach → 4% Pitch Coating | 349.57 | 94.13 |
| Regenerated (Cycled Feed) | Sieving → Mixed Acid Leach → Graphitization → 4% Pitch Coating | 350.88 | 94.06 |
| Commercial Graphite (Ref.) | – | 341.36 | 92.52 |
The data demonstrates the outstanding success of the regeneration strategy. Both regenerated graphite products surpassed the commercial reference in terms of specific capacity and, more importantly, initial coulombic efficiency. An ICE exceeding 94% is indicative of a very clean surface with minimal active sites for parasitic electrolyte reduction and a well-repaired structure, which is critical for the energy density and longevity of a full lithium-ion battery. The necessity of the high-temperature graphitization step for the cycled feedstock is highlighted by its performance matching that of the less-degraded uncycled feedstock.
3. Conclusion
This study presents a comprehensive and effective pathway for regenerating high-performance graphite anode material from spent lithium-ion batteries. The key to successful regeneration lies in addressing the two major categories of issues: impurity contamination and structural degradation.
- Purification: Metallic impurities (Ni, Fe, Cu, Al) can be efficiently removed using a 50 g/L-50 g/L HCl-H2SO4 mixed acid leach, bringing residual levels below 50 mg/kg. Organic impurities (PVDF, carbon black) require a combination of mechanical separation (sieving/classification) and thermal treatment to mitigate their negative impact on surface area and electrochemical reactivity.
- Repair: The structural damage inherent in spent graphite, particularly from cycled batteries, is repaired through a two-pronged approach. High-temperature graphitization restores crystallographic order. Subsequent pitch-derived carbon coating (at an optimal 4 wt%) effectively reduces specific surface area, repairs particle cracks, and forms a protective layer, leading to a morphology of smooth-surfaced secondary particles.
The regenerated graphite anodes delivered exceptional electrochemical performance, with a first-cycle discharge capacity around 350 mAh/g and an initial coulombic efficiency exceeding 94%, outperforming a commercial benchmark. This work validates a practical and scalable process that contributes significantly to closing the loop for critical graphite resources in the lithium-ion battery industry, promoting environmental sustainability and resource security.