Adhesive for lithium ion batteries and method for preparing the same

Abstract

The application discloses a kind of binder for lithium ion battery and preparation method thereof.The binder is prepared by polycondensation reaction of bisphenol A type diether dianhydride and functional group-containing diamine in polar aprotic solvent;Wherein, functional group-containing diamine is composed of 50-80% amido diamine monomer and the balance 2-(4-aminophenyl)-5-amino benzimidazole by mole percentage, and amido diamine monomer is 4,4'-diaminobenzanilide and / or N,N'-bis(4-aminophenyl) terephthalamide.The battery prepared by applying the binder to lithium battery positive / negative slurry has good initial coulomb efficiency and cycle stability.

Description

Technical Field

[0001] This invention relates to lithium-ion batteries, and more specifically to an adhesive for lithium-ion batteries and its preparation method. Background Technology

[0002] Lithium-ion batteries mainly consist of four components: positive electrode, negative electrode, electrolyte, and separator. The positive and negative electrodes typically consist of active materials, conductive agents, binders, and current collectors. The binder constitutes a relatively small proportion of the lithium-ion battery electrode (usually 1.5–10 wt%), and its main function is to bond the active materials together and to the current collector, preventing the active materials from peeling off from the current collector; therefore, it is essential.

[0003] Traditional binders used in industrial applications for ternary cathode materials (NCM) and / or NCA) and graphite and / or silicon-carbon composite anodes mainly include solutions of polyvinylidene fluoride (PVDF) and N-methylpyrrolidone (NMP), and aqueous dispersions of styrene-butadiene rubber (SBR) and sodium carboxymethyl cellulose (CMC). However, the use of these traditional binders often leads to damage to the active material layer (cracks and micronization) or delamination of the current collector from the active material layer due to volume changes during lithium-ion battery charge-discharge applications, ultimately resulting in abnormally reduced battery cycle characteristics.

[0004] To address the aforementioned issues, existing technologies propose using polyimide resin as a binder in ternary cathode materials containing lithium iron phosphate, NCM, and / or NCA, as well as in anode active materials containing silicon. Typically, an electrode slurry is prepared by mixing the polyimide precursor (polyamic acid solution) or a polyimide solution (containing a polar aprotic solvent) with the electrode active material. This slurry is then coated onto the current collector, followed by high-temperature heating to dehydrate and cyclize (imidize) or drying to remove the solvent, thereby forming the electrode layer. For example, invention patent CN113429927A discloses a polyimide binder prepared by copolymerization of benzimidazole polyamic acid salt and polyamide salt, wherein the weight ratio of benzimidazole polyamic acid salt to polyamide salt is 8-9:1-2; the raw materials for synthesizing benzimidazole polyamic acid salt include: diamine monomers containing benzimidazole groups (2-(3-aminophenyl)-5-aminobenzimidazole, 2-(4-aminophenyl)-5-aminobenzimidazole) and tetracarboxylic acid dianhydride monomers; the raw materials for synthesizing polyamide salt include: dianhydride monomers and water-soluble diamine monomers. This invention improves the bonding performance of polyimide by introducing benzimidazole groups into polyimide, and improves the flexibility of the binder by introducing an appropriate amount of polyamide structure into polyimide, further improving the problem of large volume change of silicon-carbon anode during charging and discharging. Moreover, the acyl groups in polyamide can form hydrogen bonds with silicon-carbon active materials, further improving the bonding strength and improving the expansion problem of silicon-carbon anode. Although the adhesive of this invention can effectively improve the problem of electrode expansion, its battery cycle stability is not ideal (the capacity retention rate after 100 cycles is not ideal, about 65%). Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide an adhesive for lithium-ion batteries and a method for preparing the same. Applying the adhesive of the present invention to lithium-ion batteries can enable the batteries to obtain good initial coulombic efficiency and cycle stability.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] An adhesive for lithium-ion batteries is prepared by condensation polymerization of bisphenol A type diether dianhydride (BPADA) and a functional diamine in a polar aprotic solvent; wherein the functional diamine comprises 50-80 mol% of an amide-based diamine monomer and the balance 2-(4-aminophenyl)-5-aminobenzimidazole (APBIA) by molar percentage, and the amide-based diamine monomer is 4,4'-diaminobenzoylaniline (DABA) and / or N,N'-bis(4-aminophenyl)terephthalamide (BPDPA).

[0008] The adhesive described in this application uses bisphenol A-type diether dianhydride (BPADA) with a bisphenol A structure, and functional groups such as diamine 2-(4-aminophenyl)-5-aminobenzimidazole (APBIA), 4,4'-diaminobenzoylaniline (DABA), and / or N,N'-bis(4-aminophenyl)terephthalamide (BPDPA) as polymerization monomers. Flexible chain segments such as -O- and -C(CH3)2-, and functional groups such as amide and imidazole groups are introduced into the main chain and side chains of polyimide macromolecules. Its low internal rotation barrier (increasing the flexibility of the main chain and the mobility of the molecular chain) and non-planar structure effectively reduce the intermolecular interaction. The synergistic effect of multiple chain conformations reduces the molecular chain interaction and the degree of close packing, thereby increasing the free volume of the polyimide molecular chain (increasing the voids inside the polymer to form ordered porosity), which can effectively enhance lithium-ion transport (high ion diffusion rate) and improve the electrochemical performance of lithium-ion batteries.

[0009] In this application, the polyimide molecular chains with a specific bisphenol A structure (the electron-donating group isopropyl: causing the electron cloud density on the nitrogen atom to partially shift to the electron-withdrawing group, thereby reducing the electron cloud density on the nitrogen atom) can promote a decrease in polarizability due to the increased interchain spacing and free volume, weaken the electrostatic interaction and electronic conjugation between molecular chains, destroy the conjugated planar segments and π-π stacking of the polyimide, decouple the electronic conjugation, reduce the carrier migration between molecular chains, and effectively reduce electrical conductivity loss. At the same time, the unique structure of bisphenol A type diether dianhydride (BPADA) also increases the chain length of the polyimide molecular unit structure, reduces the density of the imide ring in the polyimide molecular chain, improves the thermoelasticity (high toughness) and adhesion properties of the polyimide (making it have stronger metal adhesion), and also optimizes the dielectric properties (dielectric constant and loss factor) of the adhesive itself and reduces the water absorption rate due to the influence of the polyimide molecular structure (flexible bond and steric hindrance effect) and the local change of dipole moment, further improving the lithium-ion battery characteristics (high capacity and charge-discharge cycle stability).

[0010] The functional groups such as amide and imidazole groups in the diamine used in this application have strong polarity (high polarizability), which can generate greater cohesion and easily promote the formation of new chemical bonds (multiple hydrogen bonds, covalent bonds, van der Waals forces, etc.) between the polyimide molecular chain structure and the electrode active material. These chemically bonded active groups on the surface of the electrode active material can form a strong adhesive force (intermolecular association force), suppressing the volume expansion and contraction cycle changes of the active material powder during charging and discharging, reducing the internal stress of the active material, and eliminating the damage to the adhesive itself, the active material layer, the interface between the negative electrode active material and the adhesive, and the peeling from the current collector caused by the expansion and contraction. This further optimizes the cycle stability characteristics of lithium-ion batteries. At the same time, it can intrinsically dissociate under the action of an electric field to generate conductive ions, optimize the conductivity of the adhesive (ionic conductivity and electrical conductivity), and further enhance electrochemical performance (such as the redox potential of polyimide).

[0011] On the other hand, the 2-(4-aminophenyl)-5-aminobenzimidazole (APBIA) diamine used in this application contains an imidazole functional group, which has a self-catalytic imidization effect. It can effectively promote the positive reaction of polyamic acid in the adhesive structure system at low temperature, that is, cyclization into polyimide. This can effectively control the degree of imidization of the adhesive molecular chain (i.e., the ratio of polyamic acid to polyimide structure), thereby promoting better adhesive performance of the adhesive. At the same time, it can also effectively reduce the high-temperature treatment in the subsequent electrode drying process, effectively reducing costs.

[0012] Furthermore, the adhesive molecular structure described in this application can simultaneously possess abundant aggregated structures such as flexible amorphous regions (provided by bisphenol A type diether dianhydride), rigid amorphous regions, and ordered oriented regions (provided by functional diamine groups), effectively reducing the bulk resistance and interfacial resistance of the adhesive (improving lithium-ion diffusion coefficient, electronic conductivity, etc.), and reducing the internal resistance of the solid electrolyte interphase (SEI) film and charge transport during electrode cycling. This is beneficial for optimizing the electrochemical performance of the battery during charge and discharge, and improving the battery's initial coulombic efficiency and charge-discharge cycle stability. At the same time, the covalent bonds and van der Waals forces existing between polymer molecular chains can further optimize the charge-discharge cycle stability.

[0013] Therefore, the lithium-ion battery adhesive prepared by the present invention using bisphenol A-type diether dianhydride (BPADA) with a bisphenol A structure and 2-(4-aminophenyl)-5-aminobenzimidazole (APBIA) containing functional diamine groups, 4,4'-diaminobenzoylaniline (DABA) and / or N,N'-bis(4-aminophenyl)terephthalamide (BPDPA) as polymerization monomers exhibits high adhesion, elasticity and chemical stability, that is, it has excellent surface chemical properties, high adaptability to volume change and excellent ion / electron transport properties.

[0014] In the above-mentioned technical solution for the adhesive for lithium-ion batteries, the diamine containing functional groups is further preferably composed of 60-70 mol% amide-diamine monomer and the balance 2-(4-aminophenyl)-5-aminobenzimidazole by molar percentage.

[0015] In the above-mentioned technical solution for the adhesive for lithium-ion batteries, the molar ratio of the bisphenol A type diether dianhydride and the diamine containing the functional group is usually 0.98 to 1.05:1, preferably 0.99 to 1.02:1.

[0016] The above-mentioned lithium-ion battery adhesive is prepared by: placing bisphenol A type diether dianhydride and a functional group-containing diamine in a polar aprotic solvent and carrying out a condensation polymerization reaction under an inert atmosphere; wherein, the functional group-containing diamine consists of 50-80 mol% of an amide-based diamine monomer and the balance 2-(4-aminophenyl)-5-aminobenzimidazole by molar percentage, and the amide-based diamine monomer is 4,4'-diaminobenzoylaniline and / or N,N'-bis(4-aminophenyl)terephthalamide.

[0017] In the above preparation method, the molar ratio of the bisphenol A type diether dianhydride and the diamine containing the functional group, and the preferred composition of the diamine containing the functional group are the same as those described above.

[0018] In the above preparation method, the amount and selection of the polar aprotic solvent, as well as the parameters involved in the condensation polymerization reaction, such as temperature and time, are the same as in the prior art. Preferably, the polar aprotic solvent can be one or a combination of two or more selected from N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), N,N-diethylacetamide, N-methyl-2-pyrrolidone (NMP), and N-ethyl-2-pyrrolidone. The amount of the polar aprotic solvent is preferably such that the solid content in the polycondensation-obtained material is 10–30 wt%. The condensation polymerization reaction is usually carried out under an inert atmosphere, and the reaction temperature is usually 10–60 °C, more preferably 20–35 °C. When the condensation polymerization reaction is carried out at 10–60 °C, the reaction time is usually controlled within 6–24 hours.

[0019] The amount of adhesive added during application is the same as in the prior art, preferably controlling the amount of solid components in the adhesive to be 0.5 to 5 wt% of the total solid components in the negative electrode slurry.

[0020] The adhesive described in this invention is suitable for lithium iron phosphate, NCM or NCA ternary cathodes, and also for graphite and silicon-containing anodes.

[0021] Compared with the prior art, the present invention is characterized by:

[0022] 1. This invention uses bisphenol A type diether dianhydride as the dianhydride monomer to copolymerize with a diamine composed of 2-(4-aminophenyl)-5-aminobenzimidazole and an amide group. The applicant discovered in experiments that using bisphenol A type diether dianhydride as the dianhydride monomer not only improves the ionic conductivity of the binder but also enhances the adhesive performance, thereby improving the initial coulombic efficiency and charge-discharge cycle stability of the lithium-ion battery. A high-density amide-based diamine monomer is selected for the diamine, utilizing the property of the amide group to improve adhesive performance, further enhancing the adhesive performance. An appropriate amount of 2-(4-aminophenyl)-5-aminobenzimidazole is introduced to act as a self-catalyzing imidization agent to regulate the degree of imidization (i.e., the structural ratio of polyamic acid to polyimide), thus giving the adhesive better adhesive performance.

[0023] 2. The introduction of the 2-(4-aminophenyl)-5-aminobenzimidazole structure in the system plays a role in autocatalytic imidization, which allows the polyamic acid obtained from the condensation polymerization reaction to be further converted into polyimide at room temperature. This can reduce the imidization temperature (i.e., reduce the temperature at which the electrode mixture containing the binder is coated onto the current collector and then heated at high temperature to dehydrate and cyclize (imidize) or dry and remove the solvent), thus optimizing the process and reducing costs.

[0024] 3. The battery prepared by applying the binder described in this invention to the positive / negative electrode slurry of a lithium battery has an initial coulombic efficiency of ≥90% and a capacity retention rate of ≥92% after 100 cycles. Detailed Implementation

[0025] To better explain the technical solution of the present invention, the present invention will be further described in detail below with reference to the embodiments, but the implementation of the present invention is not limited thereto.

[0026] Example 1

[0027] 1. Preparation of adhesives

[0028] 9.13 g (0.040 mol) of 4,4'-diaminobenzoyl aniline (DABA) and 6.00 g (0.027 mol) of 2-(4-aminophenyl)-5-aminobenzimidazole (APBIA) were dissolved in 450 g of N-methylpyrrolidone (NMP) by stirring. Then, a total of 34.86 g (0.067 mol, added in 3 portions) of bisphenol A type diether dianhydride (BPADA) was added (the molar percentage of DABA to APBIA was 60%:40%, and the molar ratio of dianhydride to diamine was 1:1). The mixture was stirred and reacted at room temperature under a nitrogen atmosphere for 24 h to obtain the binder. An appropriate amount of the obtained adhesive was placed at 50℃ and dried for 24 hours. The infrared spectrum of the obtained material was tested at room temperature using a Nicolet 560 infrared spectrometer. The proportion of polyimide structure was calculated with reference to the literature (Musto P, Ragosta G, Scarinzi G, et al. Polyimide-silica Nanocomposites: Spectroscopic, Morphological and Mechanical Investigations[J]. Polymer, 2004, 45(5):1697-1706.). The results showed that the proportion of polyimide structure was approximately 45.8%.

[0029] 2. Preparation of positive and negative electrode sheets for lithium batteries

[0030] 2.1 Negative electrode plate:

[0031] Take 16.31g of the binder prepared in this embodiment (3wt% by solids content), 50g of the negative electrode active material (30g graphite, 20g nanoporous silica powder, graphite:nanoporous silica powder = 6:4), and 2.72g of acetylene black (binder:active material:conductive agent = 3:92:5, weight ratio), stir and mix evenly. Add N-methylpyrrolidone (NMP) solvent to adjust the system to an appropriate viscosity (5000±500cp), and grind the resulting mixture three times using a three-roll mill to obtain the negative electrode slurry. Use a doctor blade to coat the negative electrode slurry onto copper foil to a thickness of 28μm±2.0μm. Copper foil coated with negative electrode paste was placed in an inert gas atmosphere oven and heated at 80°C for 1 hour under the condition of flowing argon gas and oxygen concentration below 20 ppm. Then, the temperature was increased to 180°C at a rate of 3.5°C / min and held at 180°C for 0.5 hours to obtain the negative electrode sheet.

[0032] 2.2 Positive electrode sheet:

[0033] The active materials ternary cathode (NCM721), polyvinylidene fluoride (PVDF), and acetylene black were mixed evenly in a weight ratio of 94:3:3. NMP solvent was added to adjust the viscosity to an appropriate level (6000±500 cp). The mixture was then ground in a three-roll mill for 4 hours and dispersed at high speed for 2 hours to obtain the cathode slurry. The cathode slurry was coated onto aluminum foil using a doctor blade. The gap between the coating rollers (doctor blade) was adjusted to control the thickness of the cured cathode slurry to be 85±3.0 μm. The coated aluminum foil was placed in an oven and kept at 120℃ for 2 hours under air circulation to obtain the cathode sheet.

[0034] 3. Battery manufacturing

[0035] To reduce the gaps between active materials, the aforementioned lithium battery negative and positive electrode sheets were appropriately rolled using a rolling mill. The rolled negative and positive electrode sheets were then cut into 14mm diameter round pieces using a punching machine. CR2032 coin cells were assembled in an argon glove box (H2O < 0.01ppm, O2 < 0.01ppm). The negative electrode shell, negative electrode sheet, separator, positive electrode sheet, nickel foam, spring sheet, and positive electrode shell were assembled sequentially. 1ml of electrolyte was added to each end of the separator. The electrolyte was a 1.0mol / L LiPF6 solution dissolved in a mixture of EC and DMC (EC:DMC = 1:1, volume ratio). The assembled battery was then sealed in a sealing machine at a pressure of 75MPa. After standing for 24 hours, the corresponding electrochemical performance was tested.

[0036] 4. Charge and discharge characteristics test

[0037] The batteries prepared by the above method were subjected to cyclic charge-discharge characteristic tests. The batteries were subjected to charging and discharging tests and cyclic tests at 25°C. The experiment used a 0.1C current charge-discharge test with a voltage window of 0.005 to 1.5V. The amount of electricity flowing from the start of charging or discharging to the end was defined as the charging capacity or discharging capacity.

[0038] Test its charge-discharge efficiency after the first and 100 cycles [where charge-discharge efficiency = (discharge capacity / charge capacity) * 100%].

[0039] The test results were as follows: the initial coulomb efficiency was 94%, and the capacity retention rate was 96% after 100 cycles.

[0040] Comparative Example 1

[0041] Same as Example 1, except that: the diamine 2-(4-aminophenyl)-5-aminobenzimidazole (APBIA) was omitted, and 4,4'-diaminobenzoylaniline (DABA) was used entirely. The resulting adhesive was tested for its infrared spectrum and the proportion of polyimide structure was calculated according to the same procedure as in Example 1. The results showed that the proportion of polyimide structure in the obtained material was approximately 6.74%.

[0042] The test results were as follows: the initial coulomb efficiency was 89%, and the capacity retention rate was 86% after 100 cycles.

[0043] Comparative Example 2

[0044] Same as Example 1, except that the total amount of diamine remains the same, but the molar percentage of 4,4'-diaminobenzoylaniline (DABA) and 2-(4-aminophenyl)-5-aminobenzimidazole (APBIA) is 45%:55%. The resulting adhesive was tested for its infrared spectrum and the proportion of polyimide structure was calculated using the same procedure as in Example 1. The results showed that the polyimide structure accounted for approximately 51.7% of the obtained material.

[0045] The test results were as follows: the initial coulomb efficiency was 88.4%, and the capacity retention rate after 100 cycles was 89.1%.

[0046] Comparative Example 3

[0047] Same as Example 1, except that the total amount of diamine remains the same, but the molar percentage of 4,4'-diaminobenzoylaniline (DABA) and 2-(4-aminophenyl)-5-aminobenzimidazole (APBIA) is 85%:15%. The resulting adhesive was tested for its infrared spectrum and the proportion of polyimide structure was calculated using the same procedure as in Example 1. The results showed that the polyimide structure accounted for approximately 32.9% of the obtained material.

[0048] The test results were as follows: the initial coulomb efficiency was 85.7%, and the capacity retention rate after 100 cycles was 83.6%.

[0049] Comparative Example 4

[0050] Same as Example 1, except that 4,4'-terephthalodioxyphthalic anhydride (HQDA) was used instead of bisphenol A type diether dianhydride (BPADA). The resulting adhesive was tested for its infrared spectrum and the proportion of polyimide structure was calculated according to the same procedure as in Example 1. The results showed that the proportion of polyimide structure in the obtained material was approximately 42.6%.

[0051] The test results were as follows: the initial coulomb efficiency was 78%, and the capacity retention rate was 82% after 100 cycles.

[0052] Comparative Example 5

[0053] Same as Example 1, except that bisphenol AF dianhydride was used instead of bisphenol A type diether dianhydride (BPADA). The resulting adhesive was tested for its infrared spectrum and the proportion of polyimide structure was calculated using the same procedure as in Example 1. The results showed that the proportion of polyimide structure in the obtained material was approximately 41.8%.

[0054] The test results were as follows: the initial coulomb efficiency was 83%, and the capacity retention rate was 79% after 100 cycles.

[0055] Comparative Example 6

[0056] 1. Preparation of adhesives

[0057] 11.769 g (0.059 mol) of 4,4'-diaminodiphenyl ether (4,4'-ODA) was dissolved in 970 g of NMP by stirring. Then, 18.232 g (0.059 mol, added in 15 portions) of 4,4'-diphenyl ether tetracarboxylic dianhydride (s-ODPA) (dianhydride to diamine molar ratio of 1:1) was added. The mixture was stirred and reacted at room temperature under a nitrogen atmosphere for 24 h to obtain the adhesive. The infrared spectrum of the obtained adhesive was tested according to the same procedure as in Example 1, and the proportion of polyimide structure was calculated. The results showed that the proportion of polyimide structure in the obtained material was approximately 5.29%.

[0058] The remaining parts: the preparation of the positive and negative electrode sheets of the lithium-ion battery, the preparation of the battery, and the charging and discharging characteristic testing are all the same as in Example 1.

[0059] The test results were as follows: the initial coulomb efficiency was 73%, and the capacity retention rate was 82% after 100 cycles.

[0060] Example 2

[0061] 1. Preparation of adhesives

[0062] 13.87 g (0.061 mol) of 4,4'-diaminobenzoyl aniline (DABA) and 3.42 g (0.015 mol) of 2-(4-aminophenyl)-5-aminobenzimidazole (APBIA) were dissolved in 323 g of N-methylpyrrolidone (NMP) by stirring. Then, 40.51 g (0.078 mol, added in 5 portions) of bisphenol A type diether dianhydride (BPADA) (the molar percentage of DABA to APBIA was 80%:20%, and the molar ratio of dianhydride to diamine was 1.02:1) were added. The mixture was stirred and reacted at room temperature under a nitrogen atmosphere for 12 h to obtain the adhesive. The infrared spectrum of the obtained adhesive was tested according to the same procedure as in Example 1, and the proportion of polyimide structure was calculated. The results showed that the proportion of polyimide structure in the obtained material was approximately 40.2%.

[0063] 2. Preparation of positive and negative electrode sheets for lithium batteries

[0064] 2.1 Negative electrode plate:

[0065] Same as Example 1, except that 10.87g of the adhesive prepared in this example (3wt% of solid content) was taken.

[0066] The remaining parts: the preparation of the positive electrode sheet of the lithium-ion battery, the preparation of the battery, and the charging and discharging characteristic test are the same as in Example 1.

[0067] The test results were as follows: the initial coulomb efficiency was 90%, and the capacity retention rate was 92% after 100 cycles.

[0068] Example 3

[0069] 1. Preparation of adhesives

[0070] 17.36 g (0.076 mol) of 4,4'-diaminobenzoyl aniline (DABA) and 17.13 g (0.076 mol) of 2-(4-aminophenyl)-5-aminobenzimidazole (APBIA) were dissolved in 323 g of N-methylpyrrolidone (NMP) by stirring. Then, 77.92 g (0.150 mol, added in three portions) of bisphenol A type diether dianhydride (BPADA) (the molar percentage of DABA to APBIA was 50%:50%, and the molar ratio of dianhydride to diamine was 0.98:1) were added. The mixture was stirred and reacted at room temperature under a nitrogen atmosphere for 12 h to obtain the adhesive. The infrared spectrum of the obtained adhesive was tested according to the same procedure as in Example 1, and the proportion of polyimide structure was calculated. The results showed that the proportion of polyimide structure in the obtained material was approximately 49.2%.

[0071] 2. Preparation of positive and negative electrode sheets for lithium batteries

[0072] 2.1 Negative electrode plate:

[0073] Same as Example 1, except that 5.44g of the adhesive prepared in this example (3wt% of solid content) is taken.

[0074] The remaining parts: the preparation of the positive electrode sheet of the lithium-ion battery, the preparation of the battery, and the charging and discharging characteristic test are the same as in Example 1.

[0075] The test results were as follows: the initial coulomb efficiency was 92%, and the capacity retention rate after 100 cycles was 93.5%.

[0076] Example 4

[0077] 1. Preparation of adhesives

[0078] 34.08 g (0.150 mol) of 4,4'-diaminobenzoyl aniline (DABA) and 14.42 g (0.063 mol) of 2-(4-aminophenyl)-5-aminobenzimidazole (APBIA) were dissolved in 640 g of N-methylpyrrolidone (NMP) by stirring. Then, 110.39 g (0.212 mol, added in 8 portions) of bisphenol A type diether dianhydride (BPADA) (the molar percentage of DABA to APBIA was 70%:30%, and the molar ratio of dianhydride to diamine was 0.99:1) was added. The mixture was stirred and reacted at room temperature under a nitrogen atmosphere for 24 h to obtain the adhesive. The infrared spectrum of the obtained adhesive was tested according to the same procedure as in Example 1, and the proportion of polyimide structure was calculated. The results showed that the proportion of polyimide structure in the obtained material was approximately 43.6%.

[0079] 2. Preparation of positive and negative electrode sheets for lithium batteries

[0080] 2.1 Negative electrode plate:

[0081] Same as Example 1, except that 8.15g of the adhesive prepared in this example (3wt% by solids content) is taken.

[0082] The remaining parts: the preparation of the positive electrode sheet of the lithium-ion battery, the preparation of the battery, and the charging and discharging characteristic test are the same as in Example 1.

[0083] The test results were as follows: the initial coulomb efficiency was 92%, and the capacity retention rate after 100 cycles was 94.1%.

[0084] Example 5

[0085] 1. Preparation of adhesives

[0086] 19.06 g (0.055 mol) of N,N'-bis(4-aminophenyl)terephthalamide (BPDPA) and 8.23 ​​g (0.037 mol) of 2-(4-aminophenyl)-5-aminobenzimidazole (APBIA) were dissolved in 425 g of N-methylpyrrolidone (NMP) by stirring. Then, 47.72 g (0.092 mol, added in 5 portions) of bisphenol A type diether dianhydride (BPADA) (the molar percentage of DABA to APBIA was 60%:40%, and the molar ratio of dianhydride to diamine was 1:1) were added. The mixture was stirred and reacted at room temperature under a nitrogen atmosphere for 24 h to obtain the adhesive. The infrared spectrum of the obtained adhesive was tested according to the same procedure as in Example 1, and the proportion of polyimide structure was calculated. The results showed that the proportion of polyimide structure in the obtained material was approximately 47.1%.

[0087] 2. Preparation of positive and negative electrode sheets for lithium batteries

[0088] 2.1 Negative electrode plate:

[0089] Same as Example 1, except that 10.87g of the adhesive prepared in this example (3wt% of solid content) was taken.

[0090] The remaining parts: the preparation of the positive electrode sheet of the lithium-ion battery, the preparation of the battery, and the charging and discharging characteristic test are the same as in Example 1.

[0091] The test results were as follows: the initial coulomb efficiency was 94.7%, and the capacity retention rate after 100 cycles was 97.2%.

[0092] Example 6

[0093] 1. Preparation of adhesives

[0094] Same as Example 1.

[0095] 2. Preparation of positive and negative electrode sheets for lithium batteries

[0096] 2.1 Negative electrode plate:

[0097] Same as Example 1.

[0098] 2.2 Positive electrode sheet:

[0099] Take 16.31g of the adhesive prepared in this embodiment (3wt% by solids content), and mix the active material ternary cathode (NCM622), adhesive, and acetylene black in a weight ratio of 94:3:3 until homogeneous. Add NMP solvent to adjust the system to an appropriate viscosity (6000±500cp), and grind in a three-roll mill for 4 hours and disperse at high speed for 2 hours to obtain a cathode slurry. Use a doctor blade to coat the cathode slurry onto an aluminum foil, adjusting the gap of the coating roller (doctor blade) to control the thickness of the cathode slurry after curing to 85±3.0μm. Place the coated aluminum foil in an oven and keep it at 160℃ for 1 hour under air circulation to obtain the cathode sheet.

[0100] The rest of the process, including the preparation of the lithium-ion battery and the testing of its charge-discharge characteristics, is the same as in Example 1.

[0101] The test results were as follows: the initial coulomb efficiency was 95%, and the capacity retention rate was 97.8% after 100 cycles.

[0102] Example 7

[0103] 1. Preparation of adhesives

[0104] Same as Example 5.

[0105] 2. Preparation of positive and negative electrode sheets for lithium batteries

[0106] 2.1 Negative electrode plate:

[0107] A silicon-carbon composite negative electrode active material (80g graphite, 40g nanoporous silicon powder, graphite:nanoporous silicon powder = 2:1):polyacrylic acid (PAA):acetylene black was mixed evenly in a weight ratio of 94:3:3. Deionized water was added to adjust the system to an appropriate viscosity (5000±500cp). The resulting mixture was then milled three times using a three-roll mill to obtain a negative electrode slurry. Using a doctor blade, the negative electrode slurry was coated onto copper foil to a thickness of 28±2.0μm. The copper foil coated with the negative electrode slurry was placed in an inert gas atmosphere oven and heated at 80℃ for 1 hour under flowing argon gas and an oxygen concentration below 20ppm. The temperature was then increased to 120℃ and held at 120℃ for 1.5 hours to obtain the negative electrode sheet.

[0108] 2.2 Positive electrode sheet:

[0109] Take 10.87g of the adhesive prepared in this embodiment (3wt% by solids content), and mix the ternary positive electrode active material (NCA811), adhesive, and acetylene black in a weight ratio of 94:3:3 until homogeneous. Add NMP solvent to adjust the system to an appropriate viscosity (6000±500cp), and grind in a three-roll mill for 3 hours and disperse at high speed for 1 hour to obtain a positive electrode slurry. Use a doctor blade to coat the positive electrode slurry onto an aluminum foil, adjusting the gap of the coating roller (doctor blade) to control the thickness of the positive electrode slurry after curing to 85±3.0μm. Place the coated aluminum foil in an oven and keep it at 180℃ for 0.5 hours under air circulation to obtain the positive electrode sheet.

[0110] The rest of the process, including the preparation of the lithium-ion battery and the testing of its charge-discharge characteristics, is the same as in Example 1.

[0111] The test results were as follows: the initial coulomb efficiency was 95.8%, and the capacity retention rate after 100 cycles was 95.4%.

[0112] Example 8

[0113] 1. Preparation of adhesives

[0114] 15.69 g (0.069 mol) of 4,4'-diaminobenzoyl aniline (DABA), 23.91 g (0.069 mol) of N,N'-bis(4-aminophenyl)terephthalamide (BPDPA) and 20.64 g (0.092 mol) of 2-(4-aminophenyl)-5-aminobenzimidazole (APBIA) were dissolved in 820 g of N-methylpyrrolidone (NMP). Then, a total of 119.76 g (0.230 mol, added in 5 portions) of bisphenol A type diether dianhydride (BPADA) was added (the molar percentages of DABA, BPDPA and APBIA were 30%:30%:40%, and the molar ratio of dianhydride to diamine was 1:1). The mixture was stirred and reacted at room temperature under a nitrogen atmosphere for 24 h to obtain the binder. The resulting adhesive was tested for its infrared spectrum and the proportion of polyimide structure was calculated using the same procedure as in Example 1. The results showed that the proportion of polyimide structure in the adhesive prepared in this example was approximately 46.3%.

[0115] 2. Preparation of positive and negative electrode sheets for lithium batteries

[0116] 2.1 Negative electrode plate:

[0117] Take 21.61g of the binder prepared in this embodiment (5wt% by solids content), 70g of the negative electrode active material (35g of graphite, 35g of nanoporous silica powder, graphite:nanoporous silica powder = 5:5), and 3.89g of acetylene black (binder:active material:conductive agent = 5:90:5, weight ratio), stir and mix evenly. Add N-methylpyrrolidone (NMP) solvent to adjust the system to an appropriate viscosity (5000±500cp), and grind the resulting mixture three times using a three-roll mill to obtain the negative electrode slurry. Use a doctor blade to coat the negative electrode slurry onto copper foil to a thickness of 30±2.0μm. Copper foil coated with negative electrode paste was placed in an inert gas atmosphere oven and heated at 80°C for 1 hour under the condition of flowing argon gas and oxygen concentration below 20 ppm. Then, the temperature was increased to 180°C at a rate of 3.5°C / min and held at 180°C for 0.5 hours to obtain the negative electrode sheet.

[0118] The remaining parts: the preparation of the positive electrode sheet of the lithium-ion battery, the preparation of the battery, and the charging and discharging characteristic test are the same as in Example 1.

[0119] The test results were as follows: the initial coulomb efficiency was 94.1%, and the capacity retention rate was 98% after 100 cycles.

[0120] As can be seen from the comparison, the lithium-ion battery adhesives prepared in Examples 1-8 using bisphenol A-type diether dianhydride (BPADA) with a bisphenol A structure and functional diamine 2-(4-aminophenyl)-5-aminobenzimidazole (APBIA) with 4,4'-diaminobenzoylaniline (DABA) and / or N,N'-bis(4-aminophenyl)terephthalamide (BPDPA) as polymerization monomers can effectively improve the initial coulombic efficiency and charge-discharge cycle stability of the battery due to their high adhesion, elasticity, and chemical stability. In contrast, the adhesives prepared in Comparative Examples 1-6, which did not use the polymerization monomers of this application or only partially used them, did not have the desired adhesive properties, resulting in lower initial coulombic efficiency and poorer charge-discharge cycle stability of the batteries. It is evident that the characteristics of lithium-ion batteries (especially initial coulombic efficiency and charge-discharge cycle stability) can be effectively improved by introducing specific structures, functional groups, and reasonable proportioning. However, the application effect of adhesives prepared using conventional (Comparative Example 6) or even structurally similar (Comparative Example 5) polymer monomers is reduced due to the significant differences in the distribution of flexible and rigid amorphous regions and ordered orientation regions within the adhesive itself. Furthermore, the adhesive prepared in this application can be converted into polyimide at a relatively low temperature (drying at 50°C for 24 hours) and achieves a high polyimide conversion rate (the proportion of polyimide structure in the obtained material reaches over 40%). The adhesive with a high proportion of polyimide structure can effectively reduce the temperature of subsequent positive and negative electrode drying processes (e.g., holding at 160–180°C for 1–3 hours), thereby reducing production costs. Normally, the conversion of polyamic acid to polyimide and the achievement of a high conversion rate require a high temperature. As reported in the literature (Enterprise Technologist's Polyimide, High Performance and Functional Design, Part 1: Basic Polyimide, Chapter 3: Synthesis of Polyimide, p. 83, Figure 3-29, Kohei Goto, 2020), polyamic acid can achieve an imidization conversion rate of about 90% in just a few minutes at 300°C, but it takes 20 minutes to achieve an imidization conversion rate of 60% at 200°C, and the imidization rate is only slightly over 20% when held at 150°C for 1 hour.

Claims

1. An adhesive for lithium-ion batteries, characterized in that, It is prepared by condensation polymerization of bisphenol A type diether dianhydride and a functional diamine in a polar aprotic solvent; wherein the functional diamine consists of 50-80 mol% of an amide-based diamine monomer and the balance 2-(4-aminophenyl)-5-aminobenzimidazole by molar percentage, and the amide-based diamine monomer is 4,4'-diaminobenzoylaniline and / or N,N'-bis(4-aminophenyl)terephthalamide.

2. The adhesive for lithium-ion batteries according to claim 1, characterized in that, The functional diamine comprises 60-70 mol% amide-diamine monomer and the balance 2-(4-aminophenyl)-5-aminobenzimidazole by molar percentage.

3. The adhesive for lithium-ion batteries according to claim 1, characterized in that, The molar ratio of the bisphenol A type diether dianhydride and the diamine containing the functional group is 0.98 to 1.05:

1.

4. The method for preparing the adhesive for lithium-ion batteries according to claim 1, characterized in that, Bisphenol A type diether dianhydride and a diamine containing a functional group are placed in a polar aprotic solvent and subjected to a condensation polymerization reaction under a protective atmosphere to obtain the product; wherein, The diamine containing the functional group is composed of 50-80 mol% of an amide-diamine monomer and the balance of 2-(4-aminophenyl)-5-aminobenzimidazole, wherein the amide-diamine monomer is 4,4'-diaminobenzoylaniline and / or N,N'-bis(4-aminophenyl)terephthalamide.

5. The preparation method according to claim 4, characterized in that, The functional diamine comprises 60-70 mol% amide-diamine monomer and the balance 2-(4-aminophenyl)-5-aminobenzimidazole by molar percentage.

6. The preparation method according to claim 4, characterized in that, The molar ratio of the bisphenol A type diether dianhydride and the diamine containing the functional group is 0.98 to 1.05:

1.

7. The preparation method according to claim 4, characterized in that, The condensation polymerization reaction was carried out at 10–60 °C.

8. The preparation method according to claim 7, characterized in that, The time for the condensation polymerization reaction is 6 to 24 hours.