A rosin-based polyurethane binder, its preparation method, and its application in lithium-sulfur batteries.

Abstract

This invention discloses a rosin-based polyurethane binder, its preparation method, and its application in lithium-sulfur batteries. First, rosin is reacted to obtain hydroxymethyl rosin as a chain extender. This chain extender is then added to a polyurethane prepolymer prepared from diisocyanate and polyether / polyester diol for polymerization. Finally, a branching agent and LiOH are added to graft onto the carboxyl groups or double bonds of the rosin, yielding the rosin-based polyurethane binder. This invention utilizes the reaction of hydroxyl groups in hydroxymethyl rosin with isocyanate groups in the polyurethane prepolymer to form a linear main chain structure. Then, the carboxyl groups and carbon-carbon double bonds in the rosin are modified to obtain a branched network. Lithium carboxylate, thioether, or silane coupling groups are introduced, effectively improving the binder's adhesion to battery active materials, conductive agents, and current collectors, as well as its ability to capture polysulfides. This improves battery stability, effectively suppresses volume expansion during charge and discharge, and enhances battery cycle performance.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical energy storage materials technology, specifically relating to binders for lithium-sulfur batteries, and particularly to a rosin-based polyurethane binder, its preparation method, and its application in lithium-sulfur batteries. Background Technology

[0002] With the rapid development of the new energy industry, lithium-sulfur batteries, with their advantages of ultra-high theoretical specific capacity, abundant sulfur resources, and environmental friendliness, have become a key research direction for next-generation high-energy-density energy storage devices. However, sulfur cathodes suffer from key problems during charge and discharge processes, such as polysulfide shuttle effect, severe volume expansion, and easy pulverization of the electrode structure, which seriously restrict their cycle life and practical applications. As a core component of the sulfur cathode, the binder not only needs to firmly adhere the active material and conductive agent to the current collector, but also needs to achieve polysulfide anchoring, buffer volume deformation, and ensure the overall conductivity of the electrode through molecular structure design. This is a crucial breakthrough in overcoming the bottlenecks in lithium-sulfur battery applications. Currently, the widely used polyvinylidene fluoride (PVDF) binder has limited mechanical properties and lacks functional polar sites, making it unable to effectively anchor polysulfides. Therefore, developing novel binders with excellent mechanical properties, polysulfide adsorption capacity, and green, low-carbon characteristics has become an important direction for overcoming the technological bottlenecks of lithium-sulfur batteries.

[0003] In recent years, research on polymer binders prepared from renewable biomass has gradually emerged, especially bio-based polyurethane binders. These materials combine flexible molecular chains with abundant polar functional groups, which can simultaneously buffer electrode volume deformation and suppress shuttle effects. Rosin, as a natural forest resource with abundant reserves and wide sources in my country, is characterized by its low price and molecular structure rich in active functional groups such as carboxyl groups and double bonds. It can efficiently adsorb lithium polysulfides through chemical modification and hydrogen bonding, effectively suppressing the shuttle effect. The combination of the rigid structure of tricyclic diterpenes and the flexible block of polyurethane can also adapt to the volume expansion and contraction during the cycling process of sulfur cathodes, maintaining the integrity of the electrode structure. Summary of the Invention

[0004] Technical problem to be solved: To address the problem that existing lithium-sulfur battery binders have insufficient anchoring ability for polysulfides and are unable to effectively buffer volume expansion during charge and discharge, resulting in poor battery cycle stability, this invention provides a rosin-based polyurethane binder, its preparation method, and its application in lithium-sulfur batteries. This binder can significantly improve the adhesion to active materials and current collectors. At the same time, by introducing lithium carboxylate, thioether, or silane coupling groups, it effectively captures polysulfides, thereby suppressing the shuttle effect and mitigating electrode volume changes, thus improving the battery cycle performance.

[0005] Technical solution: A rosin-based polyurethane adhesive, comprising a polyurethane backbone and rosin units connected to the polyurethane backbone, wherein the rosin units have at least one branch selected from lithium carboxylate groups, thioether groups, silane coupling groups, and thioctic acid groups, as shown in the following structural formula:

[0006] .

[0007] The silane coupling group mentioned above is derived from KH560, the thioether group is derived from KH590, and the thioctic acid group is derived from thioctic acid.

[0008] The rosin-based polyurethane adhesive described above is made from reactants including diisocyanate, polyether / polyester diol, hydroxymethyl rosin, and at least one branching agent selected from KH560, KH590, thioctic acid, and LiOH.

[0009] A method for preparing the rosin-based polyurethane adhesive includes the following steps: mixing diisocyanate, polyether / polyester diol, a catalyst amount of dibutyltin dilaurate and anhydrous inert organic solvent, heating to 80°C and stirring to react, then adding hydroxymethyl rosin, maintaining the temperature to react, and finally adding a branching agent and LiOH, heating to 90-120°C to react.

[0010] The diisocyanate mentioned above is at least one of isophorone diisocyanate, diphenylmethane diisocyanate, dicyclohexylmethane diisocyanate, hexamethylene diisocyanate, dimer acid diisocyanate, or lysine diisocyanate.

[0011] The aforementioned polyether / polyester diol is at least one of polytetrahydrofuran ether diol, polycaprolactone diol, hydroxyl-terminated polybutadiene, or polytetrafluoroethylene ether diol; the anhydrous inert organic solvent is N,N-dimethylacetamide.

[0012] The aforementioned branching agent is at least one of KH560, KH590, or thioctic acid, and the amount of LiOH used is equimolar with the carboxyl content in the system.

[0013] The molar ratio of the isocyanate group of the above diisocyanate to the hydroxyl group of the polyether / polyester diol and hydroxymethyl rosin is 1.05 to 1.10, the molar ratio of the polyether / polyester diol to hydroxymethyl rosin is (0.3 to 0.7):(0.7 to 0.3), and the addition of dibutyltin dilaurate accounts for 0.05% to 0.3% of the total mass of the reactants.

[0014] When the branching agent is KH560, the molar ratio of KH560 to hydroxymethyl rosin is 1:1; when the branching agent is KH590, the molar ratio of KH590 to hydroxymethyl rosin is (0.5~2):1; when the branching agent is lipoic acid, the molar ratio of lipoic acid to hydroxymethyl rosin is (0.5~2):1.

[0015] The above-mentioned rosin-based polyurethane binder is used in the preparation of lithium-sulfur batteries, wherein the rosin-based polyurethane binder is used as a binder for the positive electrode of lithium-sulfur batteries.

[0016] Beneficial Effects: In this invention, the rigid tricyclic diterpenoid skeleton of rosin and the flexible molecular segments of polyurethane work together. On the one hand, the rigid structure of rosin enhances the mechanical strength of the binder; on the other hand, the flexible segments of polyurethane buffer the volume changes of the sulfur cathode during charging and discharging. More importantly, after hydroxymethyl rosin participates in the construction of the polyurethane backbone as a chain extender, the carboxyl groups and carbon-carbon double bonds retained on its molecule can still undergo secondary functionalization. By introducing branching agents such as KH560, KH590, or thioctic acid, and combining with LiOH to convert the carboxyl groups into lithium carboxylate, the same binder molecule simultaneously contains siloxane groups that can form strong interactions with current collectors and active materials, thioether groups or thioctic acid groups that can capture polysulfides, and lithium carboxylate groups that can conduct lithium ions. The peel strength of the cathode sheet of the rosin-based polyurethane binder modified by branching is improved. At the same time, the introduction of thioether or siloxane branches can more effectively suppress polysulfide shuttle, and this improvement effect exceeds the expected level of single-component modification. Furthermore, the synergistic effect of multiple functional groups can more uniformly disperse the internal stress of the electrode. Therefore, this invention not only utilizes rosin, a unique forestry resource, but also, through molecular design, enables different structural units to mutually promote each other, ultimately obtaining a rosin-based polyurethane material with significantly superior overall performance compared to existing PVDF binders. Detailed Implementation

[0017] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, and not all embodiments.

[0018] Example 1

[0019] 11.05 g of dicyclohexylmethane diisocyanate, 30.62 g of polycaprolactone diol with a molecular weight of 2000 g / mol, and 0.1% (5 drops) of dibutyltin dilaurate by mass of the total reactants were added to a reaction flask. The mixture was heated to 80 °C and stirred for 3 h. 8.3 g of hydroxymethyl rosin was added in batches, and the reaction was maintained at this temperature for 4 h. 500 mL of N,N-dimethylacetamide (water content ≤50 ppm) was added throughout the reaction. The chemical structure of the polymer was characterized using infrared spectroscopy, showing that at 3200 cm⁻¹... -1 Characteristic peaks belonging to urethane bonds (-NH-CO-) appear at 2200 cm⁻¹, while dicyclohexylmethane diisocyanate shows peaks at 2200 cm⁻¹. -1 The disappearance of the -NCO absorption peak at 1692 cm⁻¹ indicates that the hydroxyl groups of the polyol and the NCO of the isocyanate have successfully undergone a polymerization reaction; furthermore, the absorption peak at 1692 cm⁻¹... -1 A carbonyl group (C=O) belonging to a carboxylic acid appears at 893 cm. -1 The presence of a characteristic peak at the point of origin, belonging to the olefinic proton (C=CH), indicates that rosin retains its complete structure and is incorporated into the polyurethane molecular chain. Its structural formula is shown in Formula 1.

[0020]

[0021] Formula 1

[0022] Example 2

[0023] 11.05 g of dicyclohexylmethane diisocyanate, 30.62 g of polycaprolactone diol with a molecular weight of 2000 g / mol, and 0.1% (5 drops) of dibutyltin dilaurate by mass of the total reactants were added to a reaction flask. The mixture was heated to 80 °C and stirred for 3 h. 8.3 g of hydroxymethyl rosin was added, and the reaction was maintained at this temperature for 4 h. Then, 0.55 g of LiOH was added, and stirring was continued for 0.5 h before heating was stopped. Throughout the reaction, 500 mL of N,N-dimethylacetamide (water content ≤50 ppm) was added. Infrared spectroscopy characterization of the polymer showed that at 1600 cm⁻¹... -1 and 1411 cm -1 Absorption peaks at 1692 cm⁻¹ appeared, belonging to the antisymmetric and symmetric stretching vibrations of the carbonyl group in the carboxylic acid anion, respectively. -1 The absorption peak at the carbonyl group (C=O) of the carboxylic acid has disappeared, indicating that the carboxylic acid has been completely converted into lithium carboxylate, the structural formula of which is shown in Formula 2:

[0024]

[0025] Formula 2

[0026] Example 3

[0027] 12.63 g of dicyclohexylmethane diisocyanate, 26.26 g of polycaprolactone diol with a molecular weight of 2000 g / mol, and 0.1% (5 drops) of dibutyltin dilaurate (by mass of the total reactants) were added to a reaction flask. The mixture was heated to 80 °C and stirred for 3 h. 11.11 g of hydroxymethyl rosin was added, and the reaction was maintained at this temperature for 4 h. Then, 6.31 g of KH560 was added, and the mixture was heated to 120 °C and reacted for 6 h. Throughout the reaction, 500 mL of N,N-dimethylacetamide (water content ≤50 ppm) was added. Infrared spectroscopy characterization of the polymer's chemical structure showed that at 1692 cm⁻¹... -1 The absorption peak of the carbonyl group (C=O) belonging to the carboxylic acid shifts to 1724 cm⁻¹. -1 and at 756cm -1 The presence of an absorption peak at the point of origin, belonging to Si-C, indicates that the carboxyl group has successfully undergone esterification with KH560, and its structural formula is shown in Formula 3:

[0028]

[0029] Formula 3

[0030] Example 4

[0031] 8.03 g of dicyclohexylmethane diisocyanate, 38.95 g of polycaprolactone diol with a molecular weight of 2000 g / mol, and 0.1% (5 drops) of dibutyltin dilaurate by mass of the total reactants were added to a reaction flask. The mixture was heated to 80 °C and stirred for 3 h. 3.02 g of hydroxymethyl rosin was added, and the reaction was maintained at this temperature for 4 h. Then, 4.5 g of KH590 and 0.09 g of azobisisobutyronitrile were added, and the mixture was heated to 100 °C and reacted for 8 h. Finally, 0.55 g of LiOH was added, and the mixture was stirred for another 0.5 h before heating was stopped. Throughout the entire reaction process, 500 mL of N,N-dimethylacetamide (water content ≤50 ppm) was added. Infrared spectroscopy characterization of the polymer showed that at 893 cm⁻¹... -1 The characteristic peak at 756 cm⁻¹, which was attributed to the alkene proton (C=CH), disappeared. -1 The site is classified as Si-C, 1600 cm. -1 and 1411 cm -1 The presence of characteristic peaks belonging to carboxylates indicates that the double bonds and carboxyl groups on rosin have successfully reacted, as shown in Formula 4:

[0032]

[0033] Formula 4

[0034] Example 5

[0035] 9.82 g of dicyclohexylmethane diisocyanate, 34 g of polycaprolactone diol with a molecular weight of 2000 g / mol, and 0.1% (5 drops) of dibutyltin dilaurate by mass of the total reactants were added to a reaction flask. The mixture was heated to 80 °C and stirred for 3 h. 6.16 g of hydroxymethyl rosin was added, and the reaction was maintained at this temperature for 4 h. Then, 7.01 g of thioctic acid was added, and the mixture was heated to 110 °C and reacted for 8 h. Finally, 1.22 g of LiOH was added, and the mixture was stirred for another 0.5 h before heating was stopped. 500 mL of N,N-dimethylacetamide (water content ≤50 ppm) was added throughout the entire reaction. Infrared spectroscopy characterization of the polymer showed that at 893 cm⁻¹... -1 The characteristic peak at 1600 cm⁻¹, which was attributed to the alkene proton (C=CH), disappeared. -1 and 1411 cm -1 The presence of characteristic peaks belonging to carboxylates indicates that the double bonds on rosin have successfully reacted with lipoic acid, and that the carboxyl groups in the molecular chain have been completely converted into lithium carboxylate, as shown in Formula 5:

[0036]

[0037] Formula 5

[0038] The polyurethane solutions obtained in Examples 1-5 can be directly used as binders for lithium-sulfur batteries, which can be achieved through the following steps:

[0039] A sulfur-carbon composite cathode material (75% sulfur content), Ketjen Black ECP-600JD, and rosin-based polyurethane binder were mixed in a mass ratio of 8:1:1 to form a uniform slurry. The solid content of the slurry was controlled at ~60%. The slurry was then uniformly coated onto carbon-coated aluminum foil using a wet film coater. The coated electrode wet film was placed in a 40 ℃ oven for overnight drying. It was then cut into electrode sheets of appropriate size and dried in a 60 ℃ vacuum drying oven for 12 h to obtain the cathode electrode sheet. The cathode electrode sheet, Celgard2500 battery separator, and negative lithium sheet were assembled into a battery in sequence in a glove box, and an appropriate amount of electrolyte was added. The battery was then sealed using a hydraulic sealing device to obtain a lithium-sulfur battery.

[0040] Comparative example: The polyurethane adhesive of the present invention was replaced with a commercially available PVDF adhesive, and the remaining steps were the same as in the example.

[0041] 1. Electrode peel strength test

[0042] The peel strength of the positive electrode sheets in Examples 1-5 and the comparative examples was tested according to the national standard GB / T 2792-2014: Test method for peel strength of adhesive tapes. The results are shown in Table 1.

[0043] 2. Battery performance test

[0044] The cycle stability of the lithium-sulfur batteries in Examples 1-5 and the comparative example was tested using the Blue Electric Test System at a test current density of 0.5C. The results are shown in Table 1.

[0045] 3. Electrode Expansion Rate Test

[0046] Electrode expansion rate test (%) = (T-T0) / T0×100%;

[0047] In the above formula, T0 is the thickness (μm) of the positive electrode sheet before assembly into a battery, and T is the thickness (μm) of the positive electrode sheet after 50 charge-discharge cycles at room temperature and 0.5C. The results are shown in Table 1.

[0048] Table 1 Performance of Examples 1-5 and Comparative Examples

[0049]

[0050] According to the data in Table 1, the peel strength of the positive electrode sheets in Examples 1-5 is significantly higher than that in the comparative examples, indicating that the adhesion between the rosin-based polyurethane binder and the active material, conductive agent and current collector is better than that of commercial polyvinylidene fluoride (PVDF). Furthermore, the first-cycle discharge capacity of the batteries in Examples 1-5 reaches more than 816 mAh / g, and the capacity retention rate after 500 cycles at room temperature and 0.5C is 84-91%. At the same time, the electrode sheet expansion rate is ≤3.41%, indicating that the rosin-based urethane binder of the present invention can effectively suppress the volume expansion of sulfur-carbon positive electrode materials during charge and discharge, so that the battery has excellent cycle stability. Furthermore, in Examples 1-5: compared to Example 1, Example 2 converted the carboxyl groups in the rosin fragment into lithium carboxylate, resulting in improved first-cycle discharge capacity and cycle capacity retention of the battery; Example 3 grafted silane coupling agent onto the carboxyl groups, significantly improving the peel strength and expansion rate of the positive electrode sheet; Example 4, while converting the carboxyl groups into lithium carboxylate, further significantly improved the peel strength and expansion rate of the positive electrode sheet, as well as the first-cycle discharge capacity and cycle capacity retention of the battery, compared to Examples 1-3, by grafting polythiooctanoic acid onto the rosin double bonds, introducing a large amount of lithium carboxylate and flexible branches, further improving the overall performance of the battery; these results indicate that secondary functionalization of the carboxyl groups and carbon-carbon double bonds retained in the rosin fragment, utilizing the synergistic effect of multiple functional groups, can further improve battery performance.

[0051] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A rosin-based polyurethane adhesive, characterized in that, The rosin-based polyurethane binder comprises a polyurethane backbone and rosin units attached to the polyurethane backbone. Each rosin unit has at least one branch selected from lithium carboxylate groups, thioether groups, silane coupling groups, and thioctic acid groups. The specific structural formula is shown below: 。 2. The rosin-based polyurethane adhesive according to claim 1, characterized in that, The silane coupling group is derived from KH560, the thioether group is derived from KH590, and the thioctic acid group is derived from thioctic acid.

3. The rosin-based polyurethane adhesive according to claim 1, characterized in that, The rosin-based polyurethane adhesive is made from reactants including diisocyanate, polyether / polyester diol, hydroxymethyl rosin, a branching agent selected from KH560, KH590, thioctic acid, and LiOH.

4. A method for preparing the rosin-based polyurethane adhesive according to any one of claims 1-3, characterized in that, Includes the following steps: Diisocyanate, polyether / polyester diol, a catalyst amount of dibutyltin dilaurate and anhydrous inert organic solvent are mixed and heated to 80°C with stirring. Then hydroxymethyl rosin is added and the reaction is maintained at this temperature. Finally, a branching agent and LiOH are added and the reaction is heated to 90-120°C.

5. The method according to claim 4, characterized in that, The diisocyanate is one of isophorone diisocyanate, diphenylmethane diisocyanate, dicyclohexylmethane diisocyanate, hexamethylene diisocyanate, dimer acid diisocyanate, or lysine diisocyanate.

6. The method according to claim 4, characterized in that, The polyether / polyester diol is one of polytetrahydrofuran ether diol, polycaprolactone diol, hydroxyl-terminated polybutadiene, or polytetrafluoroethylene ether diol; the anhydrous inert organic solvent is N,N-dimethylacetamide.

7. The method according to claim 4, characterized in that, The branching agent is at least one of KH560, KH590 or thioctic acid, and the amount of LiOH used is equimolar with the carboxyl content in the system.

8. The method according to claim 4, characterized in that, The molar ratio of the isocyanate group of the diisocyanate to the hydroxyl group of the polyether / polyester diol and hydroxymethyl rosin is 1.05~1.10, and the molar ratio of the polyether / polyester diol to hydroxymethyl rosin is (0.3~0.7):(0.7~0.3).

9. The method according to claim 7, characterized in that, When the branching agent is KH560, the molar ratio of KH560 to hydroxymethyl rosin is 1:1; when the branching agent is KH590, the molar ratio of KH590 to hydroxymethyl rosin is (0.5~2):1; when the branching agent is lipoic acid, the molar ratio of lipoic acid to hydroxymethyl rosin is (0.5~2):

1.

10. The use of the rosin-based polyurethane binder according to any one of claims 1-3 in the preparation of lithium-sulfur batteries, characterized in that, The rosin-based polyurethane binder is used as a binder for the positive electrode of a lithium-sulfur battery.

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