The global transition toward electric mobility has made the traction battery one of the most critical components in modern vehicles. In China, the government has actively promoted new energy vehicles as a national strategy to secure energy supply, reduce carbon emissions, and mitigate air pollution. With the rapid expansion of the electric vehicle market, the number of retired traction batteries has increased dramatically. A typical traction battery has a service life of roughly five to six years, and once its capacity decays to about 70–80% of the initial value, it is usually considered unsuitable for further automotive use. Nevertheless, the remaining capacity is still adequate for less demanding applications, such as grid storage, backup power, and low-speed electric vehicles. This has made the disassembly and recycling of used traction battery packs an urgent environmental and economic issue.
Within a traction battery pack, many individual cells must be mechanically secured and electrically insulated. Polyurethane structural adhesives are widely used to bond the battery cells to metal cooling plates, side plates, and module frames. These adhesives provide high bonding strength, excellent toughness, thermal stability, vibration resistance, and sealing performance. They also help maintain good thermal contact between cells and cooling surfaces, which is necessary for safe operation under high current loads. However, the same properties that make polyurethane structural adhesives valuable during cell assembly also make them extremely difficult to remove during repair, dismantling, and recycling. If the adhesive cannot be efficiently degraded or separated, the whole traction battery module cannot be reused or remanufactured economically. Thus, the development of a reliable, low-temperature, and environmentally friendly method to remove polyurethane structural adhesives is highly relevant to the sustainable recycling of traction batteries.
Several removal strategies have been tested in industry, including mechanical prying, ultrasonic cleaning, and thermal decomposition. Mechanical methods are slow and can easily damage the battery case or the separator foil, leading to electrolyte leakage and safety hazards. Thermal incineration at elevated temperatures releases toxic gases, including carbon monoxide, hydrogen cyanide, formaldehyde, ammonia, and various volatile organic compounds, while destroying valuable cathode and anode materials. Ultrasonic cleaning has limited applicability because many traction battery components are not water-resistant and immersion can cause short circuits. Therefore, chemical degradation routes offer a promising alternative because they can selectively decompose the polyurethane network without exposing the battery to excessive temperatures or harsh oxidizing conditions.
The aim of this research was to identify practical chemical degradation systems suitable for removing polyurethane structural adhesives from traction battery components. We first tested a sodium hydroxide-catalyzed alcoholysis route using diethylene glycol as the degradation solvent. We then designed a series of Lewis acidic ionic liquids and evaluated their catalytic activity in the alcoholysis of polyurethane. The influence of temperature, time, catalyst amount, ionic liquid composition, and degradation solvent was systematically investigated. Several characterization tools, including Fourier-transform infrared spectroscopy, viscometry, and electrospray ionization mass spectrometry, were used to analyze degradation products and confirm catalyst structure. This work provides a new perspective on closed-loop recycling and remanufacturing of traction batteries.
Chemical Degradation Methods for Polyurethane
Polyurethane is synthesized by the polyaddition reaction between polyols and polyisocyanates. The resulting polymer contains carbamate, urea, ester, and ether linkages, all of which are susceptible to cleavage under certain chemical conditions. Common chemical degradation methods include alcoholysis, aminolysis, hydrolysis, ammonolysis, alkali-catalyzed hydrolysis, phosphorolysis, and thermal cleavage. Among these, alcoholysis has been widely applied because the degradation products can be reused as polyols for new polyurethane formulations. In the presence of a catalyst, diethylene glycol can attack the carbamate linkage, resulting in chain scission and formation of lower molecular weight fragments.
The main alcoholysis reaction can be written schematically as follows:
$$
\mathrm{R_1-NH-COO-R_2 + HO-R_3-OH \xrightarrow{\Delta,\,catalyst} R_1-NH-COO-R_3 + HO-R_2}
$$
Similarly, aminolysis with diethanolamine or other amines may occur:
$$
\mathrm{R_1-NH-COO-R_2 + H_2N-R_3 \xrightarrow{\Delta} R_1-NH-CO-NH-R_3 + HO-R_2}
$$
For comparative analysis, Table 1 summarizes the major chemical degradation methods with the common reaction media and typical operating temperatures.
| Method | Degradation agent | Typical temperature range (°C) | Representative cleavage reaction |
|---|---|---|---|
| Alcoholysis | Glycols, diols | 150–250 | Carbamate + diol → urethane fragment + alcohol |
| Aminolysis | Amines | 20–150 | Carbamate + amine → urea derivative |
| Hydrolysis | Water/steam | 200–300 | Carbamate + water → amine + CO2 + alcohol |
| Ammonolysis | Ammonia | ~220 | Carbamate + NH3 → urea + alcohol |
| Alkali-catalysis | NaOH/KOH with glycol | 125–180 | Base-promoted ester exchange |
| Ionic-liquid catalysis | Lewis acidic ionic liquid + glycol | 120–175 | Synergistic cation/anion transition-state catalysis |
Although traditional alkali catalysts can efficiently degrade polyurethane at high temperatures, such temperatures are undesirable for traction battery disassembly because they can degrade electronic components, damage the polymer separators, and trigger hazardous reactions with the non-aqueous electrolyte. Therefore, the present research was divided into two stages: first, establishing the baseline performance of sodium hydroxide as a catalyst; second, replacing sodium hydroxide with task-specific ionic liquids to lower the reaction temperature and improve catalytic activity.
Experimental Approach
Materials and Degradation Solvent
All experiments were conducted with a commercial two-component polyurethane structural adhesive consisting of black adhesive and white adhesive, supplied for battery module assembly. For generality, three additional commercial polyurethane AB adhesives intended for potting, resin casting, and structural bonding were also examined. Diethylene glycol and diethanolamine were used as degradation agents. Sodium hydroxide was used as a conventional alkaline catalyst. Ionic liquids were synthesized from imidazolium chlorides and anhydrous metal chlorides: FeCl3, ZnCl2, and CoCl2. The synthesized ionic liquids are listed in Table 2 with their abbreviations.
| Ionic liquid | Cation | Anion | Abbreviation |
|---|---|---|---|
| 1-allyl-3-methylimidazolium tetrachloroferrate | [Amim]+ | [FeCl4]− | [Amim][FeCl4] |
| 1-butyl-3-methylimidazolium tetrachloroferrate | [Bmim]+ | [FeCl4]− | [Bmim][FeCl4] |
| 1-hexyl-3-methylimidazolium tetrachloroferrate | [Hmim]+ | [FeCl4]− | [Hmim][FeCl4] |
| 1-allyl-3-methylimidazolium trichlorozincate | [Amim]+ | [ZnCl3]− | [Amim][ZnCl3] |
| 1-allyl-3-methylimidazolium trichlorocobaltate | [Amim]+ | [CoCl3]− | [Amim][CoCl3] |
Synthesis of Ionic Liquids
The ionic liquids were prepared by mixing the appropriate imidazolium chloride salt with the corresponding anhydrous metal chloride at selected molar ratios. All reactions were carried out under reflux at 50 °C for 5 hours. After completion, the crude product was vacuum-dried at 60 °C for 24 hours to remove residual moisture. For example, the formation of [Bmim][FeCl4] can be represented as:
$$
\mathrm{[Bmim]Cl + FeCl_3 \rightarrow [Bmim][FeCl_4]}
$$
Similarly, [Amim][ZnCl3] and [Amim][CoCl3] were formed by reacting [Amim]Cl with one equivalent of ZnCl2 or CoCl2, respectively. The molar fraction of the metal-containing anion in the ionic liquid, \(x\), was calculated using the general expression
$$
x_{\mathrm{anion}} = \frac{n_{\mathrm{metal\ chloride}}}{n_{\mathrm{metal\ chloride}} + n_{\mathrm{imidazolium\ chloride}}}
$$
This fraction was varied over the range 0.40, 0.50, 0.67, and 0.75 by adjusting the initial amounts of metal chloride and imidazolium chloride. The resulting ionic liquids were used directly as catalysts in the degradation experiments.
Polyurethane Degradation Procedure
In a typical experiment, 40–60 mL of degradation solvent was placed in a three-necked flask equipped with a magnetic stirrer and a thermometer. A measured amount of catalyst was added, and the mixture was heated to the desired temperature. A piece of polyurethane adhesive with an initial mass \(m_1\) was then immersed in the hot mixture. After a prescribed time, the remaining polyurethane was removed, washed with ethanol, dried, and weighed to obtain \(m_2\). The degradation performance was quantified using the mass loss and the degradation rate:
$$
\Delta m = m_1 – m_2
$$
$$
\text{Degradation rate} (\%) = \frac{m_1 – m_2}{m_1} \times 100
$$
Sodium Hydroxide Catalyzed Degradation of Polyurethane
Effect of Temperature on Complete Degradation
In the first series of experiments, diethanolamine was used as the degradation solvent and sodium hydroxide as the catalyst. The black and white polyurethane adhesives were completely dissolved at different temperatures, and the time required for complete disappearance of the solid piece was recorded. The results are presented in Table 3.
| Temperature (°C) | Complete degradation time of black adhesive (min) | Complete degradation time of white adhesive (min) |
|---|---|---|
| 125 | 446 | 486 |
| 145 | 195 | 235 |
| 165 | 90 | 110 |
| 185 | 42 | 38 |
| 205 | 22 | 18 |
| 225 | 13 | 9 |
| 245 | 10 | 5 |
The data clearly show that raising the reaction temperature strongly accelerates the degradation. At 125 °C nearly 8 hours were required for complete degradation, while at 245 °C the same task was accomplished in only 5–10 minutes. This behavior can be explained by the increase in the fraction of activated molecules and hence the higher frequency of effective collisions as the temperature rises. However, temperatures above 200 °C are not compatible with the safe handling of traction battery modules, because the aluminum-plastic film and polymer separators inside the cells are vulnerable to thermal damage. Therefore, a catalyst that could operate at significantly lower temperatures was needed.
Swelling Behavior
To understand the structural differences between the black and white adhesives, swelling tests were performed using water, diethylene glycol, and diethanolamine at 30 °C until equilibrium was reached. The swelling ratio was calculated from the mass change:
$$
\text{Swelling ratio} (\%) = \frac{m_{\mathrm{equil}} – m_{\mathrm{initial}}}{m_{\mathrm{initial}}} \times 100
$$
| Sample | Solvent | Initial mass (g) | Equilibrium mass (g) | Swelling ratio (%) |
|---|---|---|---|---|
| Black adhesive | Diethylene glycol | 0.298 | 0.305 | 2.35 |
| Black adhesive | Diethanolamine | 0.301 | 0.306 | 1.66 |
| Black adhesive | Water | 0.308 | 0.312 | 1.30 |
| White adhesive | Diethylene glycol | 0.469 | 0.475 | 1.28 |
| White adhesive | Diethanolamine | 0.536 | 0.542 | 1.12 |
| White adhesive | Water | 0.525 | 0.530 | 0.95 |
Diethylene glycol produced a higher swelling ratio than water or diethanolamine for both adhesives. The black adhesive showed greater swelling than the white adhesive, indicating that its crosslink density was lower and its network was more open. This structural feature explains why the black adhesive degraded slightly faster than the white adhesive at the same temperature. Consequently, the white adhesive, with its more tightly crosslinked network, was chosen as the model material for subsequent optimization studies.
Effect of Degrading Solvent Composition
The effect of diethylene glycol content in a diethylene glycol/diethanolamine blend was studied at 165 °C for 1 h with sodium hydroxide as catalyst. The concentration of diethylene glycol in the mixed solvent was varied from 0% to 100%. The mass loss of white adhesive is shown in Table 4.
| Diethylene glycol volume fraction (%) | Mass loss after 1 h (g) |
|---|---|
| 0 (pure diethanolamine) | 1.856 |
| 17 | 1.520 |
| 33 | 1.210 |
| 50 | 1.372 |
| 67 | 1.850 |
| 83 | 2.250 |
| 100 (pure diethylene glycol) | 2.578 |
The mass-loss curve first decreased, reached a minimum, and then increased with increasing diethylene glycol fraction. This non-monotonic trend reflects the competing nucleophilic and solvent effects of diethanolamine and diethylene glycol. When diethylene glycol was used alone, the mass loss was the greatest, demonstrating that diethylene glycol was more effective than diethanolamine for the base-catalyzed alcoholysis of polyurethane. Therefore, pure diethylene glycol was selected as the degradation solvent for all further experiments.
Optimization of Sodium Hydroxide Amount, Temperature, and Time
A series of single-factor experiments was performed using pure diethylene glycol and white adhesive as the model substrate. The effects of reaction temperature, catalyst amount, and reaction time on mass loss were investigated.
First, the temperature was varied from 135 to 175 °C. The mass loss after 1 hour is reported in Table 5.
| Temperature (°C) | Mass loss (g) |
|---|---|
| 135 | 0.445 |
| 145 | 0.962 |
| 155 | 1.763 |
| 165 | 2.578 |
| 175 | 2.599 |
Because the difference between 165 °C and 175 °C was small, 165 °C was selected as the best temperature to achieve fast degradation while minimizing energy consumption.
Second, the amount of sodium hydroxide was varied from 0.1 to 0.5 g while keeping the temperature at 165 °C and the time at 1 h. The results are shown in Table 6.
| NaOH amount (g) | Mass loss (g) |
|---|---|
| 0.1 | 1.852 |
| 0.2 | 2.578 |
| 0.3 | 2.203 |
| 0.4 | 1.718 |
| 0.5 | 1.510 |
The maximum mass loss occurred when 0.2 g NaOH was used. Beyond this value, excess alkali likely shifted the reaction equilibrium, or promoted side reactions that reduced the efficiency of the alcoholysis. Therefore, the optimal catalyst dosage was fixed at 0.2 g.
Third, the influence of degradation time was investigated at 165 °C with 0.2 g NaOH. The data are presented in Table 7.
| Time (min) | Mass loss (g) |
|---|---|
| 10 | 0.318 |
| 20 | 0.891 |
| 30 | 1.593 |
| 40 | 2.102 |
| 50 | 2.330 |
| 60 | 2.578 |
The mass loss increased monotonically with time. The instantaneous slope was highest between 30 and 40 min. Thus, 40 min was taken as the optimal time for comparing different catalyst systems.
Universal Applicability of the NaOH/Diethylene Glycol System
To test whether the proposed formulation could degrade different types of polyurethane structural adhesives used in the market, three additional two-component polyurethane AB adhesives were prepared and cured. They were subjected to complete degradation using diethylene glycol and 0.2 g NaOH at selected temperatures. The complete degradation times are listed in Table 8.
| Commercial adhesive | 145 °C (min) | 155 °C (min) | 165 °C (min) | 175 °C (min) |
|---|---|---|---|---|
| Polyurethane potting AB adhesive | 135 | 78 | 35 | 18 |
| Polyurethane PU adhesive | 160 | 85 | 42 | 22 |
| Polyurethane resin AB adhesive | 192 | 112 | 55 | 28 |
The results demonstrate that the sodium hydroxide/diethylene glycol system can degrade all tested polyurethane-based adhesives despite differences in chemical formulation, filler content, and crosslink density. The primary limitation was the relatively high temperature required to obtain short degradation times. This limitation motivated the use of ionic liquids as more active catalysts.
Characterization of NaOH-Catalyzed Degradation Products
Fourier-transform infrared spectra were measured for the degradation products obtained at various temperatures. In the region of 3200–3600 cm−1, a broad absorption band indicated the stretching vibration of hydroxyl groups. A band near 2800–3000 cm−1 was assigned to saturated C–H stretching vibrations, while the strong band around 1000–1100 cm−1 corresponded to C–O stretching. The major absorption peaks did not change with degradation temperature, suggesting that the functional groups of the degradation products remained the same. However, lower-molecular-weight fragments were generated at higher temperatures, which was supported by viscosity measurements.
The shear viscosity of degradation products was measured over a range of shear rates. For each product, the viscosity was almost constant, meaning the products behaved as Newtonian fluids. Nevertheless, the average viscosity decreased as the degradation temperature increased. This indicates that higher reaction temperatures promoted deeper chain scission and produced smaller oligomeric fragments. Such products could potentially be reused as renewable polyols or as chemical feedstocks.
Ionic Liquid Catalyzed Degradation of Polyurethane
Structure Confirmation of Ionic Liquids by ESI-MS
The synthesized ionic liquids were characterized by electrospray ionization mass spectrometry in both positive and negative ion modes. For [Amim][FeCl4] and [Bmim][FeCl4], the positive-ion spectra revealed the intact cations [Amim]+ and [Bmim]+ at m/z 123.9 and 139.1, respectively. In the negative-ion mode, a distinct peak for the tetrachloroferrate anion [FeCl4]− appeared near m/z 197.8. A weaker fragment peak at m/z 162.3 was attributed to [FeCl3]−, which may arise from reduction by the glycol solvent or from collision-induced dissociation in the mass spectrometer. Table 9 summarizes the observed signals.
| Ionic liquid | Positive ion species | m/z | Negative ion species | m/z |
|---|---|---|---|---|
| [Amim][FeCl4] | [Amim]+ | 123.9 | [FeCl4]− | 197.8 |
| [Bmim][FeCl4] | [Bmim]+ | 139.1 | [FeCl4]− | 197.8 |
These results confirmed that the desired ionic liquid structures were successfully synthesized and that no major impurities were present.
Comparison of NaOH and Ionic Liquid Catalysts
The catalytic performance of sodium hydroxide and two iron-based ionic liquids was compared under identical conditions: 40 mL diethylene glycol, 2 g catalyst, 170 °C, and 40 min. The degradation rates are shown in Table 10.
| Catalyst | Degradation rate (%) |
|---|---|
| NaOH | 26.7 |
| [Bmim][FeCl4] | 67.4 |
| [Amim][FeCl4] | 69.5 |
The ionic liquids more than doubled the degradation rate compared with sodium hydroxide. This significant enhancement demonstrates the strong synergy between the imidazolium cations and the tetrachloroferrate anions in activating the carbamate bonds of polyurethane.
A plausible catalytic cycle involves cooperative activation of both reactants. The imidazolium cation coordinates with the carbonyl oxygen of the urethane group, thus withdrawing electron density from the carbonyl carbon and making it more electrophilic. Simultaneously, the metal-chloride anion forms a hydrogen bond with the hydroxyl group of diethylene glycol, increasing the nucleophilicity of the alcohol oxygen. These two effects allow a six-membered transition state to form, followed by cleavage of the C–O bond in the urethane group and the release of lower-molecular-weight products. This mechanism is illustrated by the following scheme:
$$
\mathrm{[Cat]^+ \cdots O=C(NH)-O-R}
$$
$$
\mathrm{[Anion]^- \cdots HO-CH_2-CH_2-O-CH_2-CH_2-OH}
$$
After the transition state collapses, the polyurethane chain is broken and the ionic liquid is regenerated in its original chemical form, allowing it to participate in further catalytic cycles.
Influence of Anion Fraction in Ionic Liquids
The fraction of [FeCl4]− anions in [Bmim][FeCl4] was varied by changing the initial FeCl3: [Bmim]Cl molar ratio. The degradation rate of polyurethane after 170 °C and 40 min is presented in Table 11.
| [FeCl4]− molar fraction | Degradation rate (%) |
|---|---|
| 0.40 | 51.2 |
| 0.50 | 67.4 |
| 0.67 | 58.7 |
| 0.75 | 46.3 |
The maximum degradation rate was obtained when the cation-to-anion ratio was 1:1, corresponding to a molar fraction of 0.50. At this stoichiometry, the cation and anion are balanced and can jointly coordinate with the polyurethane carbonyl group and the glycol hydroxyl group in an optimal manner. When the anion fraction was too low, insufficient anions were available to activate the diethylene glycol molecules; when it was too high, the excess anions likely disrupted the coordination balance and hindered the attack on the carbonyl group.
Influence of Ionic Liquid Mass
The amount of [Amim][ZnCl3] catalyst was varied from 0.5 to 4 g while keeping the degradation solvent volume fixed at 40 mL. The resulting degradation rates at 170 °C for 40 min are reported in Table 12.
| Ionic liquid mass (g) | Degradation rate (%) |
|---|---|
| 0.5 | 48.5 |
| 1.0 | 62.1 |
| 1.5 | 73.8 |
| 2.0 | 81.2 |
| 2.5 | 81.7 |
| 3.0 | 82.0 |
The degradation rate increased sharply when the ionic liquid dosage was increased from 0.5 g to 2.0 g, and then leveled off above 2.0 g. Once the catalyst concentration reached a saturation level, all accessible polyurethane bonds were already participating in the alcoholysis reaction, so further addition of the ionic liquid had little effect. Therefore, 2 g of ionic liquid was selected as the standard catalyst loading.
Influence of Degradation Solvent
Diethylene glycol and diethanolamine were compared as degradation solvents using [Bmim][FeCl4] with a 1:1 cation-anion ratio at 170 °C for 40 min. The results are shown in Table 13.
| Degradation solvent | Degradation rate (%) |
|---|---|
| Diethylene glycol | 67.4 |
| Diethanolamine | 39.8 |
Diethylene glycol was clearly the better degradation solvent. Its two primary hydroxyl groups have similar nucleophilicity, and the central ether oxygen enhances the electron density of the alcohol oxygen through its inductive effect. In contrast, the secondary amine in diethanolamine can reduce the nucleophilicity of the hydroxyl groups through resonance and may form urea linkages that increase crosslinking. Thus, all subsequent degradation experiments were conducted in pure diethylene glycol.
Lowest Possible Degradation Temperature
One of the main goals was to lower the degradation temperature so that the method could be safely applied to traction battery pack disassembly. Table 14 compares the degradation rates of three catalysts at different temperatures after 40 min.
| Temperature (°C) | NaOH rate (%) | [Amim][FeCl4] rate (%) | [Amim][ZnCl3] rate (%) |
|---|---|---|---|
| 125 | 8.2 | 32.1 | 37.8 |
| 140 | 15.6 | 49.6 | 57.2 |
| 155 | 24.0 | 62.4 | 72.5 |
| 170 | 26.7 | 69.5 | 81.2 |
Under sodium hydroxide catalysis, a measurable but low degradation rate was obtained even at 125 °C, but long reaction times would be required for complete removal. In contrast, the iron- and zinc-based ionic liquids gave substantial degradation at 125 °C, indicating that these ionic liquids can activate the urethane bond at considerably lower temperatures. This makes them much more promising candidates for the selective removal of structural adhesives from traction battery components.
Effect of Cation Structure
To understand the role of the cation, three ionic liquids containing the same tetrachloroferrate anion but different cations — [Amim]+, [Bmim]+, and [Hmim]+ — were compared. Degradation was carried out in diethylene glycol at 155 °C for 40 min. The results are shown in Table 15.
| Ionic liquid | Degradation rate (%) |
|---|---|
| [Amim][FeCl4] | 62.4 |
| [Bmim][FeCl4] | 58.1 |
| [Hmim][FeCl4] | 50.7 |
The catalytic activity followed the order [Amim]+ > [Bmim]+ > [Hmim]+. The allyl substituent in [Amim]+ possesses a carbon-carbon double bond with relatively high electron density, causing the positive charge to be localized and therefore promoting stronger electrostatic interaction with the carbonyl oxygen of the urethane group. In the case of [Hmim]+, the longer hexyl chain creates a larger steric hindrance that impedes the approach of the polar reactant molecules. Consequently, shorter side chains and unsaturated groups are preferred for high catalytic activity in polyurethane alcoholysis.
Effect of Anion Structure
Anions with different Lewis acidities were compared while keeping the cation as [Amim]+. Degradation experiments were performed at different temperatures, and the degradation rates at 155 °C and 170 °C are summarized in Table 16.
| Ionic liquid | Anion Lewis acidity | Rate at 155 °C (%) | Rate at 170 °C (%) |
|---|---|---|---|
| [Amim][ZnCl3] | Weak | 72.5 | 81.2 |
| [Amim][CoCl3] | Medium | 69.3 | 77.4 |
| [Amim][FeCl4] | Strong | 62.4 | 69.5 |
Surprisingly, the catalytic activity was inversely correlated with the Lewis acidity of the anion: the weaker Lewis acidic [ZnCl3]− anion gave the highest degradation rate, while the strongly acidic [FeCl4]− anion gave the lowest rate among the three. This can be explained by the requirement for a balanced ion-pair interaction. If the anion is too Lewis acidic, it may bind too strongly to the hydroxyl proton or form stable adducts that prevent the nucleophilic oxygen atom of diethylene glycol from attacking the carbonyl carbon. A moderately weak Lewis acidic anion like [ZnCl3]− can promote the reaction without over-stabilizing the transition state.
Removal of Polyurethane Adhesive from a Metal Substrate at Low Temperature
In practical traction battery recycling, the polyurethane structural adhesive is often applied on aluminum cooling plates or steel module trays. To simulate this situation, we immersed a metal plate covered with cured polyurethane adhesive in a mixture of 60 mL diethylene glycol and 2 g [Amim][ZnCl3] at 50 °C for 40 min. After the mild treatment, the adhesive was sufficiently softened that it could be easily scraped from the metal surface with a plastic spatula. This experiment demonstrates that the ionic liquid formulation can effectively promote the disintegration and detachment of the structural adhesive layer at a temperature far below the decomposition threshold of traction battery cells.
Recycling and Stability of the Ionic Liquid
To evaluate catalyst stability and service life, [Amim][ZnCl3] was repeatedly used in successive degradation runs. A fresh piece of polyurethane was added in each run under identical conditions (170 °C, 40 min). After each run, the remaining piece was removed and the mass loss was recorded. The results are summarized in Table 17.
| Cycle number | Mass loss after 40 min (g) | Remarks |
|---|---|---|
| 1 | 3.24 | Fresh ionic liquid |
| 5 | 3.20 | Stable performance |
| 10 | 3.19 | Stable performance |
| 15 | 3.16 | Stable performance |
| 18 | 3.14 | Stable performance |
| 20 | 3.01 | Slight decrease |
| 22 | 2.42 | Significant decrease |
| 24 | 1.68 | Progressively deactivated |
| 25 | 1.10 | Very poor performance |
After about 20 cycles, a gradual loss of activity was observed. This was likely due to the accumulation of degradation products in the solvent, which could dilute the ionic liquid and interfere with the active sites. When fresh diethylene glycol was added to the system, the activity recovered partially, implying that the ionic liquid itself had not decomposed but was blocked by high-molecular-weight oligomeric species. After further cycles, the accumulated residues eventually reduced the catalytic efficiency to an unacceptable level. Nevertheless, the long service life of more than 20 cycles is a significant advantage over sodium hydroxide, which could not be easily reused in the same way.
Conclusion and Outlook
In this thesis, we systematically investigated chemical methods for the degradation and removal of polyurethane structural adhesives used in traction battery assembly. Two catalytic systems were studied: conventional sodium hydroxide and newly synthesized metal-containing ionic liquids. The main conclusions are summarized in Table 18.
| Comparison | NaOH catalyst | Ionic liquid catalyst |
|---|---|---|
| Degradation solvent | Diethylene glycol | Diethylene glycol or diethanolamine |
| Catalyst amount | 0.2 g | 2 g |
| Optimal temperature | 165 °C | 170 °C for high rate; 125 °C still effective |
| Degradation rate in 40 min at 170 °C | 26.7% | 67.4%–81.2% |
| Reusability | Poor | Up to 20 cycles |
| Main advantage | Low cost | High activity, mild conditions, tunable structure |
| Main drawback | High temperature needed | Higher initial material cost |
For sodium hydroxide-catalyzed alcoholysis, the optimal degradation conditions were found to be pure diethylene glycol as the solvent, 0.2 g sodium hydroxide per 60 mL solvent, and 165 °C. Under these conditions, the polyurethane adhesive was degraded rapidly, and the formulation worked on several commercial polyurethane-based adhesives, confirming broad applicability. However, the sodium hydroxide system required temperatures above 160 °C for practical degradation times, which is incompatible with sensitive traction battery structures.
To overcome this limitation, we synthesized a family of imidazolium-based ionic liquids containing Fe, Co, and Zn chloride anions. These ionic liquids exhibited excellent catalytic performance for the alcoholysis of polyurethane. The best catalyst among those studied was [Amim][ZnCl3], which achieved more than 80% degradation in 40 min at 170 °C and still produced significant degradation at 125 °C. The ionic liquid acted as a recyclable catalyst and could be used for 20 consecutive runs before its activity declined. Electrochemical and spectroscopic measurements indicated that the degradation products were lower-molecular-weight species containing hydroxyl and carbonyl functional groups. The ionic liquid-mediated degradation mechanism involves the cooperative activation of the urethane carbonyl and the hydroxyl group of the degradation solvent through cation-anion synergy.
The results of this work provide a scientific foundation for designing green and efficient removal processes of polyurethane structural adhesives in traction battery recycling. By fine-tuning the cation and anion of ionic liquids, it is possible to achieve high degradation rates at temperatures low enough to preserve the internal structure of the battery cells. This progress ultimately supports the more efficient reuse and remanufacturing of traction battery packs and contributes to the circular economy in the electric vehicle sector.
Future work will focus on scaling up the optimized ionic liquid system and validating the process on complete traction battery modules. The recovery of intact cells, the regeneration of the degradation solvent, and the purification of the degraded polyurethane fragments into reusable polyols will be important next steps. Additionally, the compatibility of ionic liquids with aluminum, copper, steel, and plastic components commonly found in traction battery packs must be evaluated to ensure that the chemical removal step is both safe and non-destructive. Continued development of low-temperature alcoholysis methods will advance the sustainable management of retired traction batteries and reduce the environmental footprint of the emerging electric vehicle industry.