Chlorine-containing polycarbonate-based polyurethane for lithium battery binder and method of making same

Abstract

The application discloses a kind of chloro-containing polycarbonate type polyurethane for lithium battery binder and preparation method thereof.The polyurethane is with the carbon dioxide-based polycarbonate diol containing chlorine side group as raw material, and is formed by diisocyanate and small molecule diol chain extension.The polyurethane itself has good mechanical properties, excellent heat resistance and low temperature flexibility, and the polyurethane as the binder of lithium battery contains more polar carbonate groups and chlorine groups, which is convenient for lithium ion migration, has high ionic conductivity, and the chlorine atom of side group also gives the binder good flame retardance, improves the safety performance of battery.Because the soft segment is carbon dioxide-based copolymer, compared with the commonly used PVDF binder, the polyurethane binder also has the advantages of biodegradability, low cost and easy recovery of electrode material.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, specifically to a binder for lithium batteries and its preparation method. Background Technology

[0002] A lithium-ion battery is a battery that uses lithium metal or lithium alloy as the negative electrode material and a non-aqueous electrolyte solution. Its basic structure consists of five parts: a positive electrode, a negative electrode, a separator, an electrolyte, and a casing. The battery electrodes are the core components of a lithium-ion battery, playing a crucial role in battery performance. They are divided into positive and negative electrodes, which work together to achieve battery charging and discharging. Here is a detailed description: Positive Electrode: Made by uniformly mixing positive active materials (such as lithium cobalt oxide, lithium iron phosphate, ternary materials, etc.), conductive agents (such as carbon black, graphene, etc.), and binders (such as PVDF), and coating them onto an aluminum foil current collector. The active materials release lithium ions during charging, which is key to storing and releasing electrical energy; the conductive agents enhance conductivity; and the binders ensure that all components are tightly bound and adhered to the current collector. Negative Electrode: Mainly made by mixing negative active materials (such as graphite, silicon-based materials, etc.), binders (such as SBR, CMC, etc.), and additives, and coating them onto a copper foil current collector. During charging, lithium ions are embedded in the active materials for storage and released during discharging.

[0003] Lithium-ion binders are key materials in the manufacture of lithium-ion battery electrodes. Their role is to enhance the bonding force between the active material and the conductive agent and current collector in the electrode, and to maintain the integrity of the electrode structure.

[0004] Currently, most commercially available lithium battery cathode plates use polyvinylidene fluoride (PVDF) as a binder. However, PVDF binder is neither conductive nor ion-conducting, which easily causes polarization and is not conducive to the performance of battery electrochemical properties. Moreover, it is expensive, generally exceeding 100,000 yuan / ton. Its production raw material is dichlorofluoroethane, whose production capacity is limited due to environmental issues, making it difficult to expand the production capacity of PVDF. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a chlorine-containing polycarbonate-type polyurethane with high ionic conductivity, good flame retardancy, good mechanical properties, good adhesion properties and biodegradability as a binder for lithium batteries, so as to solve the problems of traditional binders mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: This invention introduces a third monomer, epichlorohydrin, into the copolymerization of carbon dioxide and propylene oxide to obtain a carbon dioxide-based polycarbonate diol (PPC) with chlorine-containing side groups. Cl-DA) is used as the soft segment of this polyurethane binder. The soft segment contains a large number of polar carbonate groups, which is beneficial for lithium-ion migration. The presence of chlorine atoms with larger atomic diameters also makes this soft segment more flexible than conventional polycarbonate diols, thus giving this polyurethane relatively high ionic conductivity. Chlorine atoms also impart good flame retardancy to this binder, improving the safety performance of lithium-ion batteries. Furthermore, in PPC... Cl Based on DA, isocyanate and small-molecule diol chain extenders are introduced for further polymerization into thermoplastic polyurethane, providing the binder with good mechanical properties and good adhesion. Since the urethane groups can form intermolecular hydrogen bonds, the binder also provides a certain degree of self-healing ability to buffer the volume changes of the positive electrode active material during lithium-ion intercalation / deintercalation. Furthermore, this chlorine-containing polycarbonate polyurethane degrades rapidly and completely within a narrow temperature range of approximately 243°C, which is highly beneficial for the recycling and reuse of electrode active materials and conductive agents. Compared to commonly used PVDF binders, this binder, based on carbon dioxide-based copolymers, has better biodegradability, which can reduce the cost of lithium battery manufacturing and decrease carbon dioxide emissions and environmental pollution.

[0007] A chlorinated polycarbonate-type polyurethane for lithium battery binders is polymerized from soft segments and hard segments, and its structure is shown in formula (1); the soft segment is a carbon dioxide-based polycarbonate diol with chlorinated methyl side groups, and its structure is shown in formula (2); the hard segment is composed of diisocyanate and small molecule diol chain extender, and its structure is shown in formula (3).

[0008] Preferably, the diisocyanate (R1) is one or more of toluene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), naphthalene 1,5-diisocyanate (NDI), hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), and dicyclohexylmethane-4,4'-diisocyanate (HMDI); The small molecule diol chain extender (R2) is one or more of 1,4-butanediol (BDO), 1,3-propanediol (PDO), ethylene glycol (EG), 1,5-pentanediol (PDO), and 1,6-hexanediol (HDO).

[0009] Preferably, the carbon dioxide-based polycarbonate diol (PPC) containing chloromethyl side groups Cl The structure of -DA) is shown in equation (4), where (a+b) / (c+d)>1, (a+c) / (b+d) = 1~10, and a, b, c and d are all integers.

[0010] Preferably, the method for preparing the carbon dioxide-based polycarbonate diol containing chloromethyl side groups includes the following steps: Epichlorohydrin monomer, propylene oxide, chain transfer agent, solvent, and catalyst were added to a high-pressure reactor, carbon dioxide was introduced, and the mixture was heated to induce copolymerization. After the reaction, the product was poured into deionized water and purified by stirring at least three times. Finally, the mixture was separated and dried to obtain carbon dioxide-based polycarbonate diol (PPC) containing chloromethyl side groups. Cl -DA); the solvent is one or more of tetrahydrofuran, isopropanol, chloroform, butyl ether, cyclohexane, dimethylformamide, and dimethyl sulfoxide; the catalyst is a combination of a borane catalyst and an organic amine catalyst, wherein the borane catalyst is one or more of triethylboron (TEB), triphenylboron (TPB), and tributylboron (TBB), and the organic amine catalyst is one or more of triethylamine (TEA), N,N-diisopropylethylamine (DIPEA), and N,N-dimethylcyclohexylamine (DMCHA); the chain transfer agent is one or more of 1,4-butanediol (BDO), 1,6-hexanediol (HDO), and 1,3-propanediol (PDO).

[0011] A battery electrode includes an active material, a carbon-containing conductive additive, a binder, and a current collector; the active material and the conductive additive are fixed to the current collector by the binder, wherein the binder is the aforementioned chlorine-containing polycarbonate type polyurethane.

[0012] Preferably, the solvent used in the adhesive is one or more of N-methylpyrrolidone, dimethylformamide, dimethyl sulfoxide, tetrahydrofuran, chlorobenzene, acetonitrile, succinic acid, toluene, anhydrous ethanol, and deionized water, or no solvent is used. The active materials include one or more of the following: lithium nickel oxide, lithium cobalt oxide, lithium titanium oxide, lithium manganese oxide, lithium iron phosphorus oxide, nickel cobalt multi-element oxide, artificial graphite, natural graphite, silicon-based materials, silicon-carbon composite materials, and lithium titanate. The carbon-containing conductive additive is one or more of acetylene black, graphene, carbon nanotubes, carbon fiber, and Ketjen black. The current collector is one of copper, aluminum, titanium, nickel, stainless steel, carbon paper, or carbon-coated aluminum foil.

[0013] Compared with the prior art, the present invention has the following beneficial effects: 1. The PPC of the present invention Cl -DA, as the soft segment of this polyurethane adhesive, contains more polar carbonate groups, which is conducive to lithium ion migration. The presence of chlorine atoms with larger atomic diameters also makes this soft segment more flexible than conventional polycarbonate diols. Therefore, this polyurethane has relatively high ionic conductivity.

[0014] 2. The PPC of the present invention Cl -DA introduces chloromethyl side groups, and the presence of chlorine atoms can give the binder good flame retardancy, thus improving the safety performance of lithium-ion batteries.

[0015] 3. This invention is based on PPC Cl Based on DA, isocyanate and small molecule diol chain extender are introduced to further polymerize into thermoplastic polyurethane, which provides the binder with good mechanical properties and good adhesion properties. Since the urethane groups can form intermolecular hydrogen bonds, they can also provide the binder with a certain self-healing ability to buffer the volume change of the positive electrode active material during the lithium ion insertion and extraction process.

[0016] 4. The polyurethane described in this invention degrades rapidly and completely within a narrow temperature range of approximately 243°C, which is highly beneficial for the recycling and reuse of electrode active materials and conductive agents. Compared to commonly used PVDF binders, this binder, made from carbon dioxide-based copolymers, exhibits better biodegradability, reducing the cost of lithium-ion battery manufacturing and minimizing carbon dioxide emissions and environmental pollution. Attached Figure Description

[0017] The accompanying drawings are provided to further illustrate and explain the technical solutions and embodiments of the present invention, and do not constitute a limitation thereof. In the drawings: Figure 1 The 1H NMR spectrum of Example 2 Figure 2 This is a flame retardant test diagram from Example 2; Figure 3 The tensile test diagrams for Example 2 and PVDF are shown. Figure 4 This is a thermogravimetric curve of Example 2; Figure 5 Impedance test diagrams for Example 3 and Comparative Example 1; Figure 6 The graphs show the battery cycle performance test results for Example 3 and Comparative Example 1. Figure 7 The graphs show the battery rate performance test results for Example 3 and Comparative Example 1. Figure 8 The images show the 180° peel test results of the LiFePO4 cathode sheets of Example 3 and Comparative Example 1. Detailed Implementation

[0018] The present invention can be further explained and illustrated in conjunction with the following specific embodiments. However, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0019] Unless otherwise specified, all experimental materials selected were commercially available. Example 1

[0020] Preparation of carbon dioxide-based polycarbonate diols with chlorinated side groups: A 50 mL high-pressure stainless steel reactor with a PTFE liner and magnetic stirrer was dried overnight in a 110 °C oven and then immediately transferred to a glove box. After cooling to room temperature, the reactor was moved into the glove box under a nitrogen atmosphere. BDO, propylene oxide, epichlorohydrin, and TEB catalyst were added sequentially to the reactor, which was then sealed and removed from the glove box. The reactor was then heated in an oil bath and copolymerized under a CO2 pressure of 2.5 MPa. After the reaction, the reactor was rapidly immersed in ice water to cool to room temperature, and residual CO2 was slowly released. A small amount of 5 wt% hydrochloric acid-ethanol solution was added dropwise to the remaining product and stirred to quench the reaction. The crude product was then dissolved and diluted with dichloromethane, deionized water was added, and the mixture was stirred at high speed for 30 min. After removing the supernatant, fresh deionized water was added and stirring continued. The washing process was repeated 5 times to remove residual initiator and cyclic carbonate byproducts. After washing the product, the dichloromethane solvent was removed by rotary evaporation and then transferred to vacuum drying at 80 °C to constant weight, finally obtaining the diol product. Example 2

[0021] Synthesis of Chlorine-Containing Polycarbonate-Type Polyurethane: The carbon dioxide-based polycarbonate-type diol with chlorine-containing side groups synthesized in Example 1 was transferred to a three-necked flask equipped with a vacuum port, a mechanical stirrer, and a nitrogen protection device. The diol was mechanically stirred at 110°C and subjected to vacuum treatment for 3 h to thoroughly remove residual moisture. After the system cooled to 90°C, isocyanate (HMDI) containing stannous isooctanoate catalyst (catalyst mass accounting for 2 wt% of the diol mass) was added according to the set formula, and the reaction was carried out under mechanical stirring for 3 h to complete the prepolymerization process, obtaining an NCO-terminated prepolymer. Subsequently, ultra-dry DMF was added at a ratio of 60 wt% of the system solids content to reduce the viscosity of the prepolymer. After stirring evenly, a measured amount of BDO was slowly added as a chain extender, and the reaction system was then heated to 95°C and stirred for another 3 hours to complete the chain extension process. After the reaction was completed and cooled to room temperature, an appropriate amount of dichloromethane was added to dissolve the polyurethane, and the mixture was precipitated with ethanol under high-speed mechanical stirring to complete the purification. Finally, the polyurethane was dried under reduced pressure in a 60 °C oven for 24 h to obtain a chlorinated polycarbonate type polyurethane adhesive. Example 3

[0022] Preparation of wet-process LiFePO4 positive electrode sheet: 1.92g LiFePO4 and 0.04g conductive carbon black were placed in a mortar and ground evenly. Then, 0.04g of the chlorine-containing polycarbonate type polyurethane binder prepared in Example 2 (polyurethane dissolved in N-methylpyrrolidone, mass fraction of 15%) was added and grinding was continued for 0.5-1 h to obtain a uniform slurry. The slurry was coated onto the surface of carbon-coated aluminum foil by scraping and then dried in an oven at 60 °C for 6 h. The dried electrode sheet was cut, weighed, and then dried in a vacuum oven at 120 °C for 12 h. It was then transferred to a glove box filled with nitrogen to obtain the wet-process LiFePO4 positive electrode sheet.

[0023] Battery assembly: The obtained positive electrode and lithium sheet were assembled to obtain a 2025 type coin lithium battery. The electrolyte used was a LiFP6 concentration of 1 mol / L, and the solvent was an electrolyte of ethylene carbonate (EC): dimethyl carbonate (DMC) = 1:1.

[0024] Comparative Example 1: The difference between Comparative Example 1 and Example 3 lies in the adjustment of the binder. Specifically, the binder is replaced with a PVDF binder. All other raw material types, electrode preparation processes, and battery assembly processes remain the same as in Example 3. Example 4

[0025] The difference between this embodiment and Example 2 lies in the selection of raw materials. Specifically, the diisocyanate used is hexamethylene diisocyanate (HDI), and the small molecule diol chain extender used is 1,3-propanediol (PDO). The remaining raw material ratios, synthesis steps, and post-processing are consistent with those of Example 2. Example 5

[0026] The difference between this embodiment and Example 2 lies in the selection of raw materials and the preparation process. Specifically, the diisocyanate used is toluene diisocyanate (TDI), and the small molecule diol chain extender used is 1,6-hexanediol (HDO). The chlorinated carbon dioxide-based polycarbonate diol with side groups synthesized in Example 1, TDI, HDO, and stannous isooctanoate catalyst are mixed in a twin-screw extruder and heated at 120 °C for 3 h to obtain a chlorinated polycarbonate polyurethane adhesive in a single step. The post-processing is consistent with that of Example 2. Example 6

[0027] Preparation of dry-process LiFePO4 positive electrode sheet: LiFePO4, carbon-containing conductive additives (conductive carbon black and carbon nanotubes) and the chlorine-containing polycarbonate type polyurethane binder prepared in Example 2 were mixed and ground in a mass ratio of 88:2:8:2. Then, the mixture was added to a ball mill jar at room temperature and ball milled at 150 r / min for 12 h to obtain a uniformly mixed positive electrode powder. The positive electrode powder was then calendered into sheets and hot-pressed in a hot press at 8 MPa and 120 °C for 10 min to obtain a solvent-free LiFePO4 positive electrode sheet.

[0028] Battery assembly: The obtained positive electrode and lithium sheet were assembled to obtain a 2025 type coin lithium battery. The electrolyte used was a LiFP6 concentration of 1 mol / L, and the solvent was an electrolyte of ethylene carbonate (EC): dimethyl carbonate (DMC) = 1:1.

[0029] Test Example 1: 180° Peel Test The 180° peel test is a standard quantitative method for evaluating the adhesion strength between the coating of a lithium-ion battery electrode and the current collector. Its core principle is to fold the free end of the electrode 180° in the opposite direction, apply tensile force using a universal testing machine, and measure the change in force during the peeling process to calculate the average peel strength per unit width. The LiFePO4 cathodes prepared in Example 3 and Comparative Example 1 were tested.

[0030] The testing steps are as follows: 1. Install the test specimen: Securely fix the prepared test plate to the base (lower clamp) of the testing machine, fold the free end of the electrode in the opposite direction by 180° and clamp it in the upper clamp.

[0031] 2. Perform the test: Start the test program, and the equipment will automatically record the force curve within a peel length ≥100 mm. Peel speed: set to 100 mm / min. Test environment: standard temperature and humidity conditions (23±2℃, 50±5% RH).

[0032] 3. Data processing: Remove data from the beginning and end of the peeling curve that are not stable (eliminate edge effects), and take the force value of the stable segment in the middle.

[0033] Strength calculation: Peel strength (N / cm) = Average peel force (N) / Sample width (cm) according to Figure 1 The NMR results shown demonstrate that the polyurethane obtained in Example 2 has the structure of formula (1).

[0034] according to Figure 2 The flame retardant test results shown indicate that the chlorine-containing polycarbonate type polyurethane adhesive of Example 2 prepared by the present invention did not burn after 10 seconds of flame contact and did not spontaneously combust after the flame source was removed, indicating that the adhesive has good flame retardant properties.

[0035] Tensile tests were performed on the polyurethane adhesive prepared in Example 2 and commonly used PVDF adhesives, such as... Figure 3 As shown, the tensile strength of the adhesive in Example 2 is 26.4 MPa, which is higher than that of PVDF (16.3 MPa), proving that the adhesive has good mechanical properties.

[0036] Table 1. Tensile strength of Example 2 and PVDF

[0037] Figure 4 The thermogravimetric curves show that the polyurethane adhesive degrades rapidly and completely within a narrow temperature range of around 243 °C, which is beneficial for the recycling and reuse of electrode active materials and conductive agents.

[0038] Figure 5 , Figure 6 and Figure 7 The results show that the battery assembled using polyurethane as a binder in Example 3 has lower electrochemical impedance, better cycle performance, and better rate performance compared to the battery assembled using conventional PVDF as a binder in Comparative Example 1. This indicates that the high ionic conductivity of the binder provides superior performance for the lithium battery.

[0039] Table 2. Battery performance of Example 3 and Comparative Example 1

[0040] Figure 8The 180° peel test showed that the average peel strength of the LiFePO4 positive electrode sheet of Example 3 was 0.308 N / cm, and the average peel strength of the LiFePO4 positive electrode sheet of Comparative Example 1 was 0.330 N / cm. The two remained at the same level, indicating that the electrode sheet prepared using polyurethane as a binder has good bonding performance.

[0041] Table 3. Electrode peel strength of Example 3 and Comparative Example 1

Claims

1. The application of chlorinated polycarbonate-type polyurethane as a binder in lithium batteries, characterized in that... The chlorinated polycarbonate polyurethane is polymerized from soft and hard segments. The soft segment is a carbon dioxide-based polycarbonate diol with chloromethyl side groups, and the hard segment is composed of diisocyanate and a small-molecule diol chain extender. The preparation method of the carbon dioxide-based polycarbonate diol with chloromethyl side groups includes the following steps: adding epichlorohydrin monomer, propylene oxide, chain transfer agent, solvent, and catalyst to a high-pressure reactor, introducing carbon dioxide, heating to copolymerize, and after the reaction, pouring the product into deionized water and stirring to purify it at least 3 times, and finally separating and drying to obtain the chlorinated polycarbonate diol. The side group is a carbon dioxide-based polycarbonate diol; the solvent is one or more of tetrahydrofuran, isopropanol, chloroform, butyl ether, cyclohexane, dimethylformamide, and dimethyl sulfoxide; the catalyst is a combination of a borane catalyst and an organic amine catalyst, wherein the borane catalyst is one or more of triethylboron, triphenylboron, and tributylboron, and the organic amine catalyst is one or more of triethylamine, N,N-diisopropylethylamine, and N,N-dimethylcyclohexylamine; the chain transfer agent is one or more of 1,4-butanediol, 1,6-hexanediol, and 1,3-propanediol.

2. The application of the chlorinated polycarbonate-type polyurethane as described in claim 1 as a lithium battery binder, characterized in that, The diisocyanate is one or more selected from toluene diisocyanate, diphenylmethane diisocyanate, naphthalene 1,5-diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, and dicyclohexylmethane-4,4'-diisocyanate; the small molecule diol chain extender is one or more selected from 1,4-butanediol, 1,3-propanediol, ethylene glycol, 1,5-pentanediol, and 1,6-hexanediol.

3. A battery electrode, comprising an active material, a carbon-containing conductive additive, a binder, and a current collector; wherein the active material and the conductive additive are fixed to the current collector by the binder, characterized in that, The adhesive is the chlorine-containing polycarbonate type polyurethane as described in claim 1.

4. The battery electrode as described in claim 3, characterized in that, The solvent used in the adhesive is one or more of N-methylpyrrolidone, dimethylformamide, dimethyl sulfoxide, and tetrahydrofuran, or no solvent is used; The active materials include one or more of the following: lithium nickel oxide, lithium cobalt oxide, lithium titanium oxide, lithium manganese oxide, lithium iron phosphorus oxide, nickel cobalt multi-element oxide, artificial graphite, natural graphite, silicon-based materials, silicon-carbon composite materials, and lithium titanate. The carbon-containing conductive additive is one or more of acetylene black, graphene, carbon nanotubes, carbon fiber, and Ketjen black. The current collector is one of copper, aluminum, titanium, nickel, stainless steel, carbon paper, or carbon-coated aluminum foil.

5. A battery, characterized in that, The battery includes the battery electrode as described in claim 3.

Images