Composite binder and application thereof

Abstract

The invention discloses a composite binder and application thereof, and provides a composite binder adopting a semi-interpenetrating polymer network (c-IPN) binder strategy, the composite binder comprises cationic polyionic liquid, a cross-linking agent and a modified binder, and the electrostatic phenomenon in an electrode is adjusted. Compared with a traditional neutral linear binder (such as PVDF and the like), the c-IPN suppresses electrode cracking caused by solvent drying and improves the dispersion state of electrode components due to electrostatic repulsion driven by surface charges and mechanical toughness of the c-IPN, and the cationic polyion liquid and the modified binder form a semi-interpenetrating polymer network structure after crosslinking, so that the performance of the electrode is improved, and the service life of the electrode is prolonged. And the stability of the positive thick pole piece is effectively improved. The cationic polyion liquid fixes anions of the liquid electrolyte in the electrode through electrostatic attraction, so that conduction of Li < + > is promoted, a stable positive electrode-electrolyte interface is formed, the impedance of the battery is reduced, and the cycle performance of the battery is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of battery technology, and specifically to a composite binder and its application. Background Art

[0002] Since the commercialization of lithium-ion batteries in the 1990s of the last century, after more than 30 years of rapid development, they have penetrated into all aspects of our lives. From mobile phones, laptops to electric vehicles and aerospace, lithium batteries play an irreplaceable role in various fields. High-energy-density lithium-ion batteries can improve the battery life of related electronic products, reduce the inconvenience caused by frequent charging, and improve the energy utilization efficiency, which has always been the focus of lithium-ion battery research.

[0003] Currently, the energy density of lithium-ion batteries is mainly improved from aspects such as the cathode and anode materials, new battery structures, and lightweight designs. In addition to continuously searching for advanced electrode active materials, the design of high areal density electrodes has also attracted people's attention. High areal density electrodes reduce the use of current collectors and increase the energy density of the battery without the need to synthesize new electrode active materials, which is the simplest and most effective method to improve the energy density of lithium-ion batteries. However, the larger the areal density of the electrode, the greater the corresponding thickness of the electrode sheet. Due to the use of thick electrodes (physical problems) and uneven charge transfer throughout the electrode (electrochemical problems), traditional electrodes cannot meet this requirement. In particular, the processing solvents (such as N-methylpyrrolidone (NMP) and water) dry during the manufacturing process of thick electrodes, usually resulting in cracks and delamination, thus limiting the increase in the areal density of the electrode sheet. In addition, as the electrode thickness increases, the charge transfer in the electrode active material often shows uneven and slow reaction kinetics in the thickness direction of the electrode, resulting in a loss of the specific capacity of the material. Especially for the cathode material with a relatively low specific capacity, it is more challenging and urgent to increase the areal density and develop thick electrodes. Summary of the Invention

[0004] The purpose of the present invention is to overcome the deficiencies of the prior art and provide a composite binder. The technical solutions adopted are as follows:

[0005] A composite binder includes a cationic polyionic liquid, a cross-linking agent, and a modified binder, and has repeating units of formula I and formula II, where p and q respectively represent the molar ratio of the corresponding repeating units in the whole polymer:

[0006]

[0007] In formula I and formula II, R2, R3, R4, R'2, R'3, R'4 are selected from one of H, C 1-6 alkyl;

[0008] R6 is selected from One of them, * represents a connection point, and R1 is selected from H, C 1-6 One of alkyl groups, X n- is selected from anions, X is selected from at least one of TFSI, FSI, Tf or PF6, and n is the valence number of the anion;

[0009] R'6 is selected from One of them, * represents a connection point, and R'5 is C 1-6 alkylene; Y m- is selected from anions, Y is selected from at least one of TFSI, FSI, Tf or PF6, and m is the valence number of the anion.

[0010] As a preferred embodiment of the composite binder of the present invention, the structural formula of the cationic polyionic liquid is:

[0011]

[0012] As a preferred embodiment of the composite binder of the present invention, the crosslinking agent in the composite binder includes at least one of epoxy-terminated PEG, epoxy-terminated POSS, and glycerol triglycidyl ether.

[0013] As a preferred embodiment of the composite binder of the present invention, the modified binder includes at least one of polyvinylidene fluoride, polyvinylidene fluoride homopolymer, and polyvinylidene fluoride-hexafluoropropylene copolymer. The modified binder accounts for 40% to 90% of the total mass of the composite binder, and the cationic polyionic liquid and the corresponding epoxy crosslinking agent account for 10% to 60% of the total mass of the composite binder. The weight average molecular weight (Mw) of the modified binder is 5000 to 5 million.

[0014] As a preferred embodiment of the composite binder of the present invention, the repeating unit shown in Formula I accounts for 75 to 99 mol% of the total repeating units of the cationic polyionic liquid. Correspondingly, the molar ratio of the repeating unit shown in Formula II to the total repeating units of the cationic polyionic liquid is 1 to 25 mol%.

[0015] The weight average molecular weight (Mw) of the cationic polyionic liquid is 10 million to 12 million.

[0016] As a preferred embodiment of the composite binder of the present invention, the crosslinking agent is selected from one of the compounds of Formula I(c), Formula I(d), Formula I(e), Formula I(h), Formula I(f), Formula I(g), Formula I(h),

[0017]

[0018] wherein, R is m is the number of the repeating units.

[0019] As a preferred embodiment of the composite binder of the present invention, the mass ratio of the cationic polyion liquid, the crosslinking agent, and the modified binder is: 0.1335 - 0.801: 0.0106 - 0.099: 0.6 - 1.35.

[0020] As a preferred embodiment of the composite binder of the present invention, the molar ratio of the amino group in the cationic polyion liquid to the epoxy group in the crosslinking agent is 0.5 - 2:1. When the ratio is below 0.5:1, insufficient crosslinking will occur due to too little amino group. When the ratio is above 2:1, insufficient crosslinking will occur due to too much amino group.

[0021] As a preferred embodiment of the composite binder of the present invention, the epoxy group on the crosslinking agent reacts with the amino group on the cationic polyion liquid at a temperature above 25 °C to achieve crosslinking. The reaction of the epoxy group and the amino group generates a structure as shown in formula II(a), where * represents the connection point

[0022]

[0023] The present invention also provides a positive electrode sheet, which includes a positive electrode current collector and a positive electrode active material layer located on at least one surface of the positive electrode current collector. The positive electrode active material layer includes the above-mentioned composite binder.

[0024] As a preferred embodiment of the positive electrode sheet of the present invention, the composite binder accounts for 0.2 - 25 wt% of the total mass of the positive electrode active material layer. Preferably, 0.5 - 15 wt%, and more preferably, 1 - 5 wt%.

[0025] As a preferred embodiment of the positive electrode sheet of the present invention, the positive electrode active material layer further includes a positive electrode active material and a conductive agent.

[0026] As a preferred embodiment of the positive electrode sheet of the present invention, the positive electrode current collector is a single-sided smooth aluminum foil, a double-sided smooth aluminum foil, or a porous aluminum foil.

[0027] As a preferred embodiment of the positive electrode sheet of the present invention, the positive electrode active material is one or more of lithium iron phosphate, ternary positive electrode materials (NCM622, NCM811, NCA), lithium cobaltate, lithium manganese iron phosphate, and lithium manganate.

[0028] As a preferred embodiment of the positive electrode sheet of the present invention, the conductive agent is at least one of conductive graphite, carbon black, acetylene black, graphene, and carbon nanotubes.

[0029] The present invention also provides a battery, which includes the above-mentioned composite binder, or the battery includes the above-mentioned positive electrode sheet.

[0030] The present invention also provides a method for preparing the above positive electrode sheet, which includes the following steps:

[0031] Mix the positive electrode active material, the conductive agent, and the above-mentioned composite binder evenly to obtain a positive electrode paste; coat the positive electrode paste on the surface of the current collector, and after baking, the positive electrode sheet is obtained.

[0032] The present invention also provides an application of the above composite binder in a battery.

[0033] The present invention provides a battery, which includes the above positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte.

[0034] The present invention provides a method for preparing the above battery, which includes the following steps: winding or laminating the positive electrode sheet, the negative electrode sheet, and the separator by a commonly used method in the industry to form an electrode core, then encapsulating it with an aluminum-plastic film, and then successively performing baking, injecting the electrolyte, forming, and secondary sealing processes to obtain a lithium-ion battery.

[0035] As a preferred embodiment of the battery of the present invention, the negative electrode active material in the negative electrode sheet includes one or more of elemental silicon, silicon monoxide, natural graphite, artificial graphite, mesophase carbon fiber, mesophase carbon microsphere, soft carbon, hard carbon, and metallic lithium.

[0036] Advantages of the present invention:

[0037] The present invention provides a composite binder adopting a semi-interpenetrating polymer network (c-IPN) binder strategy to regulate the electrostatic phenomenon in the electrode. Compared with traditional neutral linear binders (such as PVDF, etc.), due to its surface charge-driven electrostatic repulsion and mechanical toughness, c-IPN inhibits the electrode cracking caused by solvent drying, improves the dispersion state of electrode components, and after cross-linking, the cationic polyionic liquid and the modified binder form a semi-interpenetrating polymer network structure, effectively improving the stability of the positive electrode thick electrode sheet. The cationic polyionic liquid fixes the anions of the liquid electrolyte inside the electrode through electrostatic attraction, thereby promoting the conduction of Li + , forming a stable positive electrode-electrolyte interface, reducing the battery impedance, and improving the battery cycle performance. Specific embodiments

[0038] In order to more clearly understand the above objects, features, and advantages of the present invention, the present invention will be further described in detail below in conjunction with specific embodiments. It should be noted that, without conflict, the embodiments of the present application and the features in the embodiments can be combined with each other.

[0039] In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, the present invention may be practiced in other ways than those described herein, and thus, the scope of the present invention is not limited by the specific embodiments disclosed below.

[0040] Unless otherwise specified, the experimental methods used in the following examples are all conventional methods; the reagents, materials, etc. used in the following examples can all be obtained from commercial sources unless otherwise specified.

[0041] The present invention provides a method for preparing a battery, comprising the following steps:

[0042] S1: Preparation of the positive electrode sheet

[0043] Mix the positive electrode active material lithium cobalt oxide (LCO), composite binder, conductive agent acetylene black, and single-walled carbon nanotubes in a weight ratio of 96.5:1.5:1.5:0.5, add them to NMP solvent, and stir under the action of a vacuum mixer until the mixed system becomes a homogeneous and flowable positive electrode slurry; uniformly coat the positive electrode slurry on the current collector aluminum foil (aluminum foil thickness 10 μm); after baking the coated aluminum foil in an oven, then dry it in an oven at 120 °C for 8 h, and then obtain the required positive electrode sheet through rolling and slitting, with the areal density of the positive electrode sheet being 28 mg / cm 2 , and the compaction being 4.15 g / cm 3 . Observe whether cracks occur on the electrode sheet during the coating process and whether powder drops off during the rolling process.

[0044] S2: Preparation of the negative electrode sheet

[0045] Mix the graphite negative electrode active material, thickening agent sodium carboxymethyl cellulose (CMC-Na), binder (Zeon 451B), and conductive agent acetylene black in a weight ratio of 96.5%:1.2%:1.5%:0.8%, add deionized water, and obtain a negative electrode slurry under the action of a vacuum mixer; uniformly coat the negative electrode slurry on a high-strength carbon-coated copper foil (copper foil thickness 6 μm) to obtain an electrode sheet; after air-drying the obtained electrode sheet at room temperature, transfer it to an oven at 80 °C and dry it for 10 h, and then obtain the negative electrode sheet through rolling and slitting, with the areal density being 14.5 mg / cm 2 , and the compaction being 1.65 g / cm 3 .

[0046] S3: Preparation of the electrolyte

[0047] In a glove box filled with inert gas (argon) (H2O < 0.1 ppm, O2 < 0.1 ppm), a solution was prepared by mixing EC (ethylene carbonate), EMC (ethyl methyl carbonate), DEC (diethyl carbonate), and FEC (fluoroethylene carbonate) in a mass ratio of 20:50:20:10. Then, thoroughly dried lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide salt (LiFSI) were quickly added thereto, with mass fractions in the system being 11.4% and 3.1% respectively. They were dissolved in a non-aqueous organic solvent, stirred evenly, and after passing the moisture and free acid tests, an electrolyte solution was obtained.

[0048] S4: Preparation of the separator

[0049] An 8-μm thick (5 + 3) Zhuogao mixed-coated separator was selected.

[0050] S5: Preparation of the lithium-ion battery

[0051] The prepared positive electrode sheet, separator, and negative electrode sheet were stacked in sequence to ensure that the separator was between the positive and negative electrode sheets to play a separating role, and then a non-injected bare battery core was obtained through winding; the bare battery core was placed in an outer packaging foil, and the prepared corresponding electrolyte solution was injected into the dried bare battery core. After vacuum packaging, standing, forming, shaping, and sorting processes, the corresponding lithium-ion battery was obtained.

[0052] Example 1: According to the above method for preparing a battery, the mass ratio of polyionic liquid (as shown in formula III(a), a = 40, molecular weight: 100,000), crosslinking agent (as shown in formula I(c), molecular weight: 1000), and polyvinylidene fluoride (molecular weight: 1,000,000) in the composite binder was: 0.445:0.055:1.

[0053]

[0054] Example 2: Referring to Example 1, the difference was that the mass ratio of the composite binder was changed. The specific mass ratio of polyionic liquid (as shown in formula III(a)), crosslinking agent (as shown in formula I(c), molecular weight: 1000), and polyvinylidene fluoride was: 0.801:0.099:0.6.

[0055] Example 3: Referring to Example 1, the difference was that the mass ratio of the composite binder was changed. The specific mass ratio of polyionic liquid (as shown in formula III(a)), crosslinking agent (as shown in formula I(c), molecular weight: 1000), and polyvinylidene fluoride was: 0.1335:0.0165:1.35.

[0056] Example 4: It was carried out with reference to Example 1, except that the mass ratio of the composite binder and the type of crosslinking agent were changed. The mass ratio of the polyionic liquid (as shown in Formula III(a)), the crosslinking agent (as shown in Formula I(d), molecular weight: 260.3), and polyvinylidene fluoride was specifically: 0.4894:0.0106:1.0.

[0057] Example 5: It was carried out with reference to Example 1, except that the mass ratio of the composite binder and the type of crosslinking agent were changed. The mass ratio of the polyionic liquid (as shown in Formula III(a)), the crosslinking agent (as shown in Formula I(e), molecular weight: 434.5), and polyvinylidene fluoride was specifically: 0.4815:0.0185:1.0.

[0058] Example 6: It was carried out with reference to Example 1, except that the mass ratio of the composite binder and the type of crosslinking agent were changed. The mass ratio of the polyionic liquid (as shown in Formula III(a)), the crosslinking agent (as shown in Formula I(h), molecular weight: 1200), and polyvinylidene fluoride was specifically: 0.4821:0.0179:1.0.

[0059] Example 7: It was carried out with reference to Example 1, except that the type of polyionic liquid in the composite binder was changed. The mass ratio of the polyionic liquid (as shown in Formula III(b), b is 40), the crosslinking agent (as shown in Formula I(c), molecular weight: 1000), and polyvinylidene fluoride was specifically: 0.448:0.052:1.

[0060]

[0061] Example 8: It was carried out with reference to Example 1, except that the type of polyionic liquid in the composite binder was changed. The mass ratio of the polyionic liquid (as shown in Formula III(c), b is 40), the crosslinking agent (as shown in Formula I(c), molecular weight: 1000), and polyvinylidene fluoride was specifically: 0.448:0.052:1.

[0062]

[0063] Comparative Example 1: It was carried out with reference to Example 1, except that the composite binder was only replaced with a commercial oil-based PVDF (model: Arkema HSV900) as the binder. The cathode active material lithium cobaltate (LCO), polyvinylidene fluoride, conductive agent acetylene black, and single-walled carbon nanotubes were formulated according to a weight ratio of 96.5:1.5:1.5:0.5 to prepare the cathode sheet, and other processes remained unchanged.

[0064] Comparative Example 2: It was carried out with reference to Example 1, except that the composite binder was only replaced with a polyionic liquid (as shown in Formula III(a), a = 40) and a crosslinking agent (as shown in Formula I(c), molecular weight Mw: 1000), and the mass ratio was 0.445:0.055. The cathode active material lithium cobaltate (LCO), polyvinylidene fluoride, conductive agent acetylene black, and single-walled carbon nanotubes were formulated according to a weight ratio of 96.5:1.5:1.5:0.5 to prepare the cathode sheet, and other processes remained unchanged.

[0065] For the cathode sheets prepared in the above Examples 1-8, Comparative Example 1, and Comparative Example 2, the peel strength test was carried out:

[0066] Preparation of samples: First, use a flat paper cutter to cut the rolled cathode sheet into strips with a length of 170 mm and a width of 30 mm; then wipe the uncalibrated steel ruler with degreased gauze to make it clean without stains and dust; then, horizontally stick a 60-mm-wide transparent tape at the bottom of the dried uncalibrated steel ruler with the end faces flush; then, stick a 25-mm-wide double-sided tape on the transparent tape with the same length as the width of the transparent tape and centered; finally, stick the test sample on the double-sided tape with the end faces flush, and roll back and forth on the surface of the electrode sheet with a pressing wheel (2 kg) with a diameter of 84 mm and a height of 45 mm.

[0067] Peel strength test: After folding the free end of the cathode sheet in the experimental sample by 180°, clamp it on the upper clamp of the AG-X plus electronic universal material testing machine, and clamp the uncalibrated steel ruler on the lower clamp. Under the conditions of 22-28 °C and a humidity of less than 25%, prepare several 30-mm-wide negative electrode sheets, and the stretching speed of the electrode sheet is 50 mm / min. The average value of stretching 25-80 mm is taken for testing, and the cathode sheet is peeled. When the current collector and the coating are completely separated, read the test result of the coating peel strength. Peel strength calculation method: Peel strength = Peel force / Electrode sheet width. The test results are shown in Table 1.

[0068] Table 1

[0069]

[0070] For the batteries prepared in the above Examples 1-8, Comparative Example 1, and Comparative Example 2, the performance test was carried out:

[0071] (1) 25 °C cycle test: Place the battery in a constant temperature environment of 25 °C and charge it at a constant current of 1C to 4.5V, with a cut-off current of 0.05C, and then discharge it at 0.5C to 3V. The charge and discharge cycle is 500 times. Record the cycle discharge capacity and divide it by the discharge capacity of the first cycle to obtain the room temperature cycle capacity retention rate. Record the cycle capacity retention rates of the 100th / 500th / 1000th cycles respectively. The test results are shown in Table 1.

[0072] (2) Multi-rate charging test: Test the voltage, internal resistance, and thickness of the sample at 25 ± 5°C (it is necessary to ensure that the voltage of the sample is between 3.8 and 3.85V. If it is not within this range, it is necessary to charge and discharge to the voltage within the range before testing the sample data). 1) Let it stand for 10 minutes; 2) Discharge at 0.2C to the lower limit voltage; 3) Let it stand for 10 minutes; 4) Charge at a certain rate (the rates are as follows), with a cut-off current of 0.05C; 5) Let it stand for 10 minutes; 6) Discharge at 0.2C to the lower limit voltage; 7) Let it stand for 10 minutes. Repeat steps 4) - 7) until all multi-rate charging tests are completed; Charging rates: 0.2C / 1C / 2C. The test results are shown in Table 2.

[0073] (3) DCIR test in fully charged state (100% SOC): The test process is as follows. 1) Let it stand at 0°C ± 2°C for 1 hour; 2) Discharge at 0.1C to the lower limit voltage; 3) Let it stand for 10 minutes; 4) Charge at 0.1C to the upper limit voltage, with a cut-off current of 0.05C, and let it stand for 10 minutes; 5) Discharge at 0.1C to the lower limit voltage (for the initial capacity); 6) Let it stand for 10 minutes, charge at 0.1C to the upper limit voltage, with a cut-off current of 0.05C; 7) Let it stand for 10 minutes, discharge at 0.1C for 10s (sampling every 200ms), discharge at 1C for 360s (sampling every 200ms); 8) Let it stand for 15 minutes; Repeat steps 7) - 8) until the voltage drops to the lower limit voltage;

[0074] The discharge currents in steps 7) - 8) are calculated using the initial capacity (Note: The 0.1C current and the 1C current are both calculated based on the same capacity. The 0.1C current and the 1C current are both calculated based on the same capacity. The terminal voltage of discharging at 0.1C for 10s is V1, and the voltage corresponding to discharging at 1C for 5S is V2); Collect the DICR at 100% SOC. The DCIR calculation method: (V1 - V2) / (1C - 0.1C). The test results are shown in Table 2.

[0075] Table 2 Multi-rate charging test and DCIR internal resistance test

[0076]

[0077]

[0078] It can be seen from the comparative examples and examples in Table 1 and Table 2 that the positive electrode sheets of Examples 1 - 8 did not show cracks at a surface density as high as 28mg / cm 2 and did not shed powder under a compaction of 4.15g / cm 3 The cycle capacity retention rate of the battery made from this positive electrode sheet was significantly improved, the multi-rate charging performance of the battery was significantly improved, and the DC resistance of the battery was significantly reduced, proving the effectiveness of the solution.

[0079] Obviously, the above-mentioned embodiments of the present invention are merely examples for clearly illustrating the present invention, rather than limitations on the implementation manners of the present invention. For those of ordinary skill in the art, various obvious changes, re-adjustments and substitutions can be made without departing from the protection scope of the present invention. It is not necessary and impossible to enumerate all the implementation manners here. Any modifications, equivalent substitutions and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the claims of the present invention.

Claims

1. A composite binder, comprising a cationic polyionic liquid, a crosslinking agent, and a modified binder, characterized in that, The cationic poly(ionic liquid) includes repeating units represented by Formula I and repeating units represented by Formula II: In Formula I and Formula II, R2, R3, R4, R'2, R'3, R'4 are selected from one of H, C 1-6 one of alkyl groups; R6 is selected from one of them, * represents a connection point, R1 is selected from H, C 1-6 one of alkyl groups, X n- is selected from anions, X is selected from at least one of TFSI, FSI, Tf or PF6, and n is the valence number of the anion; R'6 is selected from one of them, * represents the connection point, and R'5 is C 1-6 alkylene; Y m- is selected from anions, Y is selected from at least one of TFSI, FSI, Tf or PF6, and m is the valence number of the anion.

2. The composite binder according to claim 1, wherein The crosslinking agent in the composite binder includes at least one of epoxy-terminated PEG, epoxy-terminated POSS, and glycerol triglycidyl ether.

3. The composite binder according to claim 1, wherein, The modified binder includes at least one of polyvinylidene fluoride, polyvinylidene fluoride homopolymer, and polyvinylidene fluoride-hexafluoropropylene copolymer. The modified binder accounts for 40% to 90% of the total mass of the composite binder, and the weight-average molecular weight of the modified binder is 5,000 to 5 million.

4. The composite binder according to claim 1, wherein The repeating units represented by Formula I account for 75 to 99 mol% of the total repeating units of the cationic poly(ionic liquid); The repeating units represented by Formula II account for 1 to 25 mol% of the total repeating units of the cationic poly(ionic liquid). The weight-average molecular weight of the cationic poly(ionic liquid) is 10 million to 12 million.

5. The composite binder according to claim 1, characterized in that, The crosslinking agent is selected from one of the compounds of formula I(c), formula I(d), formula I(e), formula I(h), formula I(f), formula I(g), and formula I(h). wherein, R is m is the number of the repeating units.

6. The composite binder according to claim 3, wherein, The mass ratio of the cationic poly(ionic liquid), crosslinking agent, and modified binder is: 0.1335 to 0.801: 0.0106 to 0.099: 0.6 to 1.

35.

7. The composite binder according to claim 1, characterized in that, The molar ratio of the amine group in the cationic poly(ionic liquid) to the epoxy group in the crosslinking agent is 0.5 to 2:

1.

8. Use of the composite binder according to any one of claims 1-7, characterized in that, The composite binder is used to prepare a carbon-coated slurry for an aluminum foil current collector.

9. Use of the composite binder according to claim 8, characterized in that, The carbon-coated slurry for the aluminum foil current collector is used to prepare a positive electrode sheet.

10. The application of the composite binder according to claim 9, characterized in that, The positive electrode sheet is used to prepare a battery.

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