Efficient separation of the NCM cathode material and Al foil from spent lithium-ion batteries with oxalic acid under mild conditions

Abstract

In the recycling process of spent lithium-ion batteries (SLIBs), rapid and effective separation of the cathode material and current collector aluminium (Al) foil is a significant and difficult step. However, traditional separation methods have some drawbacks, including high energy consumption and cost and toxicity. In this study, we selected oxalic acid (OA), a green and low-cost simple organic acid, as the separating agent. Within 6 minutes of oxalic acid treatment, more than 99% of the nickel-manganese-cobalt (NCM) cathode materials are separated from the current collector Al foil. The mechanism analysis shows that the reaction of oxalic acid with the surface of the Al foil destroys the connection between the Al foil and the adhesive, while the oxalate protective layer formed on the surface of the NCM cathode material prevents further corrosion of the NCM cathode material, maintaining a good structural integrity. This green and efficient separation method provides an economical and viable solution for SLIB regeneration or upcycling.

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New concepts

Here, the green and low-cost oxalic acid (OA) solution is proven to be a highly efficient agent for separating the NCM cathode layer and the Al foil of spent lithium-ion batteries (SLIBs). The reaction of the OA solution with the surface of the Al foil destroys the connection between the Al foil and the adhesive, while the oxalate protective layer formed on the surface of the NCM cathode material prevents further corrosion of the NCM cathode material, maintaining a good structural integrity, which differs from the reaction-passivation separation method driven by other organic acid systems. After calcination with a lithium source, the separated NCM with the oxalate protective layer can be regenerated into new cathode active materials. Our results provide a practical and efficient recycling method of the NCM cathode materials that is economic and environmentally friendly and has potential application in the recycling industry of spent LIBs.

1. Introduction

Since they were first produced and used by Sony in 1991, lithium-ion batteries (LIBs) have been widely used in smartphones and electric vehicles (EVs) due to their advantages of high energy density, light weight, small size, long cycle life, high operating voltage and absence of memory effects.^1–3 The global EV market has been growing rapidly in recent years, and considering the 5–10 years lifespan of LIBs, a large number of spent LIBs will be retired in the next few years. Significantly, SLIBs contain valuable metals including lithium (Li), nickel (Ni), cobalt (Co), and manganese (Mn), and the element content in SLIBs is much higher than that in natural minerals.⁴ Therefore, the recycling of LIBs is vital for resource utilization, environmental protection and supply-chain stability.^5–7

Large-scale recycling methods for SLIB cathode materials require a combination of low-cost, good processability, and environmental friendliness. For industrial and lab scales, the main recycling methods include pyrometallurgy,^8–10 hydrometallurgy^11–13 and direct recycling methods.^14–16 Pyrometallurgy, with the advantages of simple equipment and short processing time, has been applied in industries earlier, but this approach causes high energy consumption and increased emission of pollutants. In comparison, hydrometallurgy offers low energy consumption, high product purity and wide applicability, making it become the main method for SLIB recycling by replacing pyrometallurgy. However, hydrometallurgy requires large amounts of reagents, and the separation process is complex, increasing the cost and metal loss in the whole process. The newly emerging direct recovery method can simply and directly repair and regenerate spent cathode materials, but it is still limited by impurity content and crystal structure defects in the cathode active materials before recycling, making it hard to be applied in an industrial scale. Considering that metal impurities play a critical role in industrial recycling of SLIBs, efficiently separating the cathode material layer from the current collector during the pretreatment process can be an efficacious and key method to reduce the content of metal impurities and simplify the recycling process.^17,18

Common separation methods include mechanical separation,^19–23 thermal treatment,^24–26 organic solvent dissolution,^27–29 alkali dissolution^30–32 and other methods.^33–35 In the mechanical separation method, the cathode material layer and Al foil are separated by a mechanical force, such as ultrasonication, grinding and washing, but the separation is usually incomplete and easily leads to Al impurities in the cathode materials. Thermal treatment methods decompose the organic binder, such as polyvinylidene difluoride (PVDF), at high temperatures, causing the adhesion between the cathode material layer and the Al foil to fail. This is a straightforward operation but requires high energy consumption and produces harmful gases such as hydrogen fluoride (HF). As an organic polymer, the PVDF binder can be dissolved by specific organic solvents, including N-methyl pyrrolidinone (NMP), dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), but these solvents are typically toxic or expensive. Although some non-toxic and inexpensive solvents, such as molten salt and deep eutectic solvents, have been reported recently, their applications are still limited by difficult treatment conditions and operations.^36–38

In addition, organic acids can be used in hydrometallurgy as selective leaching agents in the presence of a reductant and heating, while some of them with specific acidities can selectively react with the Al foil under mild conditions,^39–41 making it possible to separate the cathode material layer from the Al foil. Chen X et al. found that H⁺ ions in the solution can generate ˙OH free radicals through ultrasound assistance.⁴² After changing the ultrasonic medium from deionized water to an acidic solution, the ultrasonic-assisted separation efficiency of the cathode material increased from 46% to over 99%. Chen Z et al. used aqueous phytic acid to separate the cathode materials from the Al foil in 5 min.⁴³ The processed Al foil was intact due to its surficial passivation. The authors proposed a reaction-passivation driven mechanism, using an aqueous solution of organic phytic acid (PA) to separate the cathode active material layer from the Al foil within 5 minutes, with a separation efficiency of over 99.9%. The cathode material layer showed low metallic dissolution loss during the process, while the reaction between PA and Al formed a dense Al–PA layer, which prevented continuous corrosion between PA and Al foil. Similarly, Gong et al. reported more organic phosphonic acids with similar delamination ability to PA.⁴⁴ It is worth mentioning that Han et al. demonstrated that a methanol and citric acid (MeOH–CA) solution was a highly efficient solvent for separating cathode materials from current collectors.⁴⁵ Mechanism analysis indicated that the separation was caused not only by the reaction between MeOH–CA and Al foil but also by the inactivation of PVDF by the abundant hydrogen bonds of the ester produced in the MeOH–CA solvent. The AlF₃ passivation layer and the ester film generated on the surface of the Al foil suppressed further corrosion of the Al foils, leading to a separation efficiency of over 99.5% in 15 minutes. Nevertheless, the organic acids mentioned above generally have a higher cost, which is not conducive for large-scale industrial applications.

As the simplest organic dicarboxylic acid, oxalic acid (OA) shows advantages of moderate acidity, chelating ability, low cost, low toxicity, and high solubility.⁴⁶ Benefiting from its reducibility at higher temperatures, OA can be used for hydrometallurgy leaching of SLIBs cathodes with and without reductant.^47–49 Under appropriate conditions, OA can selectively leach Li from the cathode⁵⁰ and form oxalate precipitates on the surface of the NCM cathode, which can impede further dissolution of NCM materials.⁵¹ It can also completely dissolve Al foil at higher temperatures and longer leaching times.⁴⁰ Inspired by these previous reports, in this study, we used green and low-cost OA as a selective acid agent to separate the NCM cathode layer and the Al foil of SLIBs under mild conditions. As schematically shown in Fig. 1a and b, the strongly acidic OA produces enough H⁺ to react with surface Al₂O₃ and metallic Al of the Al foil, which weakens the contact between the NCM material layer and the Al foil. Meanwhile, oxalate precipitates of transition metals are produced and generate a dense protective layer, hindering element loss and structural damage that could be caused by further corrosion of the NCM material. The OA solution shows a clear separation of the cathode material layer and the Al foil within 6 minutes at only 40 °C, and retains its separation ability for 5 cycles. This separated NCM can be easily regenerated into new cathode active materials with good performance after calcination with lithium carbonate (Li₂CO₃). The regenerated NCM displays an initial capacity of 118 mAh g⁻¹ at 0.5C with a capacity retention of 80.1% after 100 cycles.

Fig. 1 (a) Schematic of the separation mechanism using OA solution. (b) Chemical reactions of separation. (c)–(e) Digital images of the disassembled NCM cathode (dNCM), separated Al foil (sAl), and separated NCM material (sNCM). (f) Digital images of the separation process.

2. Experimental section

2.1. Materials and reagents

Spent TAFEL LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523) LIBs were completely discharged and then disassembled to cathode strips, anode strips, separators and metal shells. The main chemical reagents in this study, including oxalic acid and Li₂CO₃, were analytical grade and produced by Shanghai Aladdin Biochemical Technology Co., Ltd.

2.2. Separation method

Pretreatment of the cathode strip: the disassembled NCM523 cathode strip was dried in a vacuum oven for 12 h at 60 °C. Then a paper knife was used to cut the strip into square pieces of 5.0 × 5.0 cm², as shown in Fig. 1c.

Separation of the cathode material layer and the Al foil using OA solution: The NCM523 cathode piece was immersed in OA solution (s/l = 20 g L⁻¹) at the chosen temperature with stirring until the NCM layer and Al foil were separated, as shown in Fig. 1f.

Separation and purification of the cathode material layer and Al foil: the separated NCM layer and the Al foil were respectively collected and washed with deionized (DI) water, then separately dried for 6 h at 60 °C, as shown in Fig. 1d and e.

2.3. Calculation of the separation efficiency

In this study, the separation efficiency could be expressed and calculated by the following formulas:where α₁ and α₂ are the recovery yields of the NCM layer and the Al foil, respectively, m₀ is the weight of the cathode piece, and m₁ and m₂ are the actual weights of sNCM and sAl, respectively,where β₁ and β₂ are the recovery rates of an element in the NCM layer and the Al foil, respectively, M₀ is the element weight in the cathode piece, and M₁ and M₂ are element weights in sNCM and sAl, respectively.

2.4. Regeneration of cathode materials

The separated NCM cathode materials were ground into powder with a ball mill and mixed with Li₂CO₃ at a molar ratio of Li:(Ni + Co + Mn) = 1.10 : 1. Then, the mixture was heated in a tube furnace at 850 °C for 10 hours with a heating rate of 5 °C min⁻¹ in an O₂ atmosphere, producing regenerated NCM cathode materials.

2.5. Material characterizations

The chemical composition and microstructure characterizations of the materials were measured with a Jena PQ9000 inductively coupled plasma optical emission spectrometer (ICP-OES), a GA1 thermogravimetric analysis instrument (TG), a Bruker D8-advance X-ray powder diffractometer (XRD), a JSM7100F field emission scanning electron microscope (FE-SEM), and a Thermo Fisher Scientific Escalab 250Xi X-ray photoelectron spectroscope (XPS).

2.6. Electrochemical measurements

The regenerated NCM materials, 4 wt% of polyvinylidene fluoride (PVDF) and Super-P were mixed with a mass ratio of 8 : 1 : 1 and evenly applied to the Al foil, then dried in a vacuum oven at 120 °C for 12 h. After drying, it was pressed into a round piece with a diameter of 12 mm; the loading mass of the active substance was about 2.6–3.2 mg cm⁻². When assembling the experimental cells, the above-mentioned regenerated NCM materials, Celgard 2325 diaphragm, and lithium metals were respectively used as the cathodes, separators and anodes, while ethylene carbonate (EC) and dimethyl carbonate (DMC) mixed in 1 M lithium hexafluorophosphate (LiPF₆) with a volume ratio of 1 : 1 was used as the electrolyte. All electrochemical measurements were conducted using CR2025 coin-type cells fabricated in an argon-filled glove box. The cycle and rate performance were tested in the voltage range of 2.8–4.3 V with a LAND battery tester (1C = 170 mAh g⁻¹).

3. Results and discussion

3.1. Process used to separate the Al foil and cathode material

To ensure sufficient infiltration of the cathode pieces by the OA solution, a solid–liquid ratio (s/l) of 20 g L⁻¹ was selected for the separation process. L’ea M. J. Rouquette et al. reported that the concentration of OA solution, temperature, and reaction time play important roles in the leaching of metal elements in OA solution.⁴⁰ Therefore, it is necessary to appropriately adjust the concentration of the OA solution and the reaction temperature in order to promote the separation process between the cathode material and the Al foil while suppressing the leaching of metals by the OA solution. We investigated the effects of solution concentration and reaction temperature on the separation time in order to optimize the separation conditions of the OA-mediated separation process, where the separation time refers to the time required for the cathode material to be completely separated from the Al foil. As depicted in Fig. 2a and b, the promotion of separation efficiency can be ignored when the concentration of OA is over 0.5 M. If the reaction temperature exceeds 60 °C, a significant leaching effect on the NCM cathode material is observed, as shown in Fig. 2d and e. According to these results, the optimal reaction conditions were set as an OA concentration of 0.5 M and a reaction temperature of 40 °C. Under the optimal conditions, the OA solution cleanly separated the cathode material layer from the Al foil within 6 minutes, with the two accounting for 88% and 8% of the original mass of the cathode piece, respectively. These results indicate that it is possible to separate the cathode material layer from the Al foil in a short time while minimizing the leaching of cathode metals by controlling the reaction conditions of the OA solution.

Fig. 2 Effect of OA concentration on recovery yield (a) and separation time and element contents (b). (c) XRD patterns of sNCM separated with different concentrations of OA. Effect of temperature on the recovery yield (d) and separation time and element contents (e). (f) XRD patterns of sNCM separated at different temperatures.

XRD measurements were further employed to investigate the sNCM products separated under different conditions. As displayed in Fig. 2c and f, the characteristic peaks including clear (018)/(110) and (006)/(012) split peaks of different sNCM (detailed shown in Fig. S3) indicate their well-crystallized layered structure.⁴⁴ Significantly, the SEM images of sNCM in high concentration (1.0 M) or high temperature (70 °C) display damaged surface morphology, as depicted in Fig. S1 and S2, indicating element loss and structural damage (as shown in Fig. S3) due to over-reactions.

To explore the cycling ability, the OA solution was repeatedly used for the separation process under the optimal conditions with an initial s/l of 20 g L⁻¹. The separation time increased from 6 minutes to 15 minutes in the first five cycles, resulting in a slight change in the color of the solution. After the 6th cycle, the separation did not finish completely within 30 minutes, while white suspended impurities caused by the dissolution of Al foil appeared in the solution, as displayed in Fig. S4. This can be explained by considering that the OA solution loses its ability to mediate rapid separation after multiple cycles because of the depletion of H⁺ and oxalate ions. However, the used oxalic acid solution can be treated by evaporation or by direct addition of oxalic acid powder to increase the concentration of the solution, which can partly restore its acidity and separation ability.

sNCM separated during the different cycles keeps stable element recovery rates and microstructure, as depicted in Fig. 3 and Fig. S5. These results indicate that the OA solution can retain its separation ability for 5 cycles while 90% of the initial volume was retained, indicating the good cycling performance and low cost of the OA-mediated separation method.

Fig. 3 (a) Separation time and element ratio of sNCM separated by repeatedly using the same OA solution (set molar ratio of Ni + Co + Mn = 1). (b) XRD patterns of sNCM separated by repeatedly using the same OA solution.

3.2. Characterization of separated NCM material

The SEM images of dNCM and sNCM are shown in Fig. 4a and b. It is obvious that sNCM maintains its morphology after separation, demonstrating that the OA solution did not visibly dissolve NCM, which is consistent with the XRD results and the element contents mentioned above. As shown in Fig. 4c and d, the element contents determined by ICP indicate that the separated NCM contains <0.15 wt% Al element (much lower than 2–3 wt% Al content in the separated NCM powders for mechanical separation methods), while there is nearly no residual cathode material on the surface of sAl. The element recovery rates shown in Fig. S6 were calculated from the recovery yield of sNCM and sAl, and compared with the outcome of comparable separation experiments using the PA solution, proving that the OA separation method achieves similar metallic recovery rates to the PA method.

Fig. 4 SEM images of dNCM (a) and sNCM (b). Element contents of dNCM and sNCM (c), dNCM and sAl (d) tested by ICP.

It is reported that the OA solution can form oxalate precipitates on the surface of the NCM cathode under certain conditions, preventing further reaction of NCM materials.^40,51 To further explore the effect of the OA solution on sNCM, the chemical states of dNCM and sNCM were investigated by XPS measurements, as depicted in Fig. 5. In comparison with dNCM, the peak of Al–O disappears, proving the clean and thorough separation of the Al foil from the NCM layer. A distinct peak of O–C=O appears after the separation, while other C 1s peaks remain, indicating the new formation of the oxalate protective layer on the surface of sNCM. The peak of Li–F is probably generated by the residues of the electrolyte on the NCM strips; the intensity of this peak is greatly reduced in sNCM, which suggests that these electrolytes are mostly removed during pretreatment and separation. Besides, the peaks of –(CH₂–CF₂)_n– still exist after the separation, implying a considerable portion of the PVDF remains in sNCM.^45,52

Fig. 5 XPS spectra of Al 2p (a), C 1s (b) and F 1s (c) for dNCM and Al 2p (d), C 1s (e) and F 1s (f) for sNCM.

Taking account of the thermal stability of oxalate, the oxalate protective layer on the surface of sNCM will be decomposed at higher temperature.^53,54 It has been reported that these oxalates, together with unreacted NCM material, can be directly calcined to regenerate the NCM cathodes after mixing with a moderate amount of Li₂CO₃.⁵¹ Hence, TG analysis was conducted under an O₂ atmosphere to explore the transformation in the thermal decomposition process of sNCM. As displayed in Fig. 6a, the TG result of sNCM manifests three evident weight loss stages. The first stage occurs at 140–200 °C, representing the evaporation of crystalline water. The second stage, appearing at 240–340 °C, is assigned to the decomposition of oxalates to metal oxides and carbon oxides, while the third stage, which ended at 550 °C, can be considered as the thermal degradation of PVDF binder. Furthermore, the sNCM samples were separately heated in a muffle furnace for an hour at temperatures of 200 °C, 400 °C, 550 °C and 850 °C. As exhibited in Fig. 6b and Fig. S7, the EDS element contents and SEM images of unheated and heated sNCM reveal that oxalate on the surface can be basically decomposed above 400 °C. Notably, as shown in Fig. 6c, the XRD peaks of sNCM heated to excessive temperatures display obvious shifts to lower angles, while the (018)/(110) and (006)/(012) split peaks evidently reduce their peak spacing width, indicating that the structure of sNCM cannot be fully restored without adding lithium resources.

Fig. 6 (a) Thermal analysis of dNCM and sNCM. (b) Element contents of unheated and heated sNCM measured by EDS. (c) XRD patterns of unheated and heated sNCM.

3.3. Regeneration of active material

In order to assess the validity of the separation, sNCM was directly regenerated to the active cathode material using a high-temperature solid-state method. As depicted in Fig. 7a, the regenerated NCM (rNCM) exhibits a crystalline structure with evenly distributed elements, while the ICP results shown in Fig. 7b indicate an appropriate element ratio without the impurities of Al and P. In Fig. 7c, comparison with the XRD patterns of sNCM, rNCM presents two pairs of split peaks at (018)/(110) and (006)/(012) with reduced peak spacing width, demonstrating that the layer structure has been effectively restored and the lithium deficiency in sNCM has been recovered after regeneration.

Fig. 7 (a) SEM and EDS images of rNCM. (b) Element contents of dNCM, sNCM and rNCM determined by ICP. (c) XRD patterns of sNCM and rNCM. (d) Voltage-capacity profiles, (e) rate performance, and (f) cycle performance of sNCM and rNCM.

The electrochemical performances of sNCM and rNCM were recorded in a voltage range of 2.8 to 4.3 V at a 1C current rate (1C = 170 mAh g⁻¹). The regenerated NCM exhibits a higher discharging capacity than the separated NCM. As displayed in Fig. 7d and e, the rNCM reveals discharge capacities of 142, 124, 110.7, 84.4, 66 and 54 mAh g⁻¹ at 0.1C, 0.2C, 0.5C, 1.0C, 2.0C and 3.0C, respectively. After three activation cycles at 0.1C, the initial capacity of rNCM at 0.5C is 118 mAh g⁻¹, while the capacity retention after 100 cycles is 80.1%, as shown in Fig. 7f. The improved electrochemical performance of rNCM compared with sNCM indicates a successful regeneration.

Although the feasibility of the OA separation-regeneration method has been demonstrated, the rNCM still performs at a slightly lower capacity than the common commercial NCM523 products. Thus, the fabrication conditions for the high-temperature solid-state method should be further optimized.

3.4. Environmental and economic analysis

Here, the use of OA solutions for the efficient separation of the NCM cathode material and Al foil is demonstrated. OA is a simple organic acid with advantages of low cost, low toxicity and ready availability, which are beneficial for the economic and environmental aspects of recycling. Thus, the environmental impacts and economic benefits of the Pyro, Hydro, and General Direct methods and direct recycling with the OA separation method (OA-direct) were evaluated using the EverBatt model based on a feedstock of 10 000 tons of spent NCM LIBs per year, as shown in Fig. 8 and Fig. S8, and listed in Table S1. The OA-direct process reveals low energy consumption and low greenhouse gas emission, as expected, but it still needs a certain amount of water consumption for the preparation of the solution. Attributed to the cheap price of the OA raw material, the OA-direct process shows the highest profit of 19.70 $ kg⁻¹ cell. In summary, the OA separation method provides an economic and environmentally friendly route for the efficient recycling of LIB cathode materials, which has a clear industry application potential.

Fig. 8 (a) Energy consumption, (b) GHG emission, (c) water consumption, (d) cost, (e) revenue and (f) profit for the pyro, hydro, direct and OA-direct methods.

4. Conclusions

In this study, the green and low-cost OA solution has been proven to be a highly efficient agent for separating the NCM cathode layer and Al foil of SLIBs. Through adjustment of the reaction conditions, the OA solution shows a clear separation of the cathode material layer and Al foil within 6 minutes and retains its separation ability over 5 cycles. The separated Al foil exhibits high purity, while the separated NCM maintains its morphology and elemental composition. Mechanism analysis demonstrates that the reaction of the OA solution occurs on the surface of the Al foil and weakens the contact between the NCM material layer and the Al foil. Furthermore, an oxalate protective layer can be produced on the surface of the NCM material, which inhibits structural damage and element loss due to overreaction. After calcination with a lithium source, the separated NCM with the oxalate protective layer can be regenerated into new cathode active materials that display an initial capacity of 118 mAh g⁻¹ at 0.5C with a capacity retention of 80.1% after 100 cycles. This work provides a practical and efficient recycling method for NCM cathode materials that is economic and environmentally friendly, and has potential applications in the spent LIB recycling industry.

Author contributions

W. L. and G. Y. supervised the project and designed the experiments. Y. X. and Z. G. performed all the experiments and characterization. Y. X., W. P., Y. T. and S. P. contributed to the writing and revision of the manuscript. B. Y., R. H. and W. M. H. have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

The partial XRD patterns of sNCM separated by different concentrations of OA and different temperatures; the SEM images of sNCM separated by different concentrations of OA, different temperatures and different cycles; the element recovery rate of PA and OA solution; the LCA and TEA results of different recycling approaches are provided in the supplementary information  (SI).

Supplementary information is available. See DOI: https://doi.org/10.1039/d5mh01615k.

Acknowledgements

This work was financially supported by the National Key Research and Development Program of China (No. 2024YFB4709700) and Guangdong Basic and Applied Basic Research Foundation (2023B1515040011).

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