An adhesive, a preparation method, a negative electrode sheet and a lithium-ion battery

Abstract

The present invention belongs to the technical field of lithium-ion batteries, and particularly relates to a binder, a preparation method, a negative electrode sheet and a lithium-ion battery. When the binder provided by the present invention is applied to the negative electrode sheet, based on the strong physical cross-linking effect, the binder can exhibit high rigidity, which is beneficial to suppressing the stress deformation of the negative electrode active material, thereby reducing the volume expansion of the electrode sheet; different from the existing synthetic cross-linked polymer technology, in the present invention, through the breakage of physical cross-links of hydrogen bonds and ionic bonds, the stress energy received by the material can be dissipated, thereby greatly enhancing the toughness of the negative electrode material. After the stress is removed, the physical cross-links based on hydrogen bonds and ionic bonds can be reversibly restored; the binder of the present invention has both excellent expansion inhibition effect and flexibility improvement, making the negative electrode sheet and lithium-ion battery prepared based on it have excellent performance in various aspects.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and particularly relates to a binder, a preparation method, a negative electrode sheet and a lithium-ion battery. Background Art

[0002] Due to its advantages such as high energy density, long cycle life and environmental friendliness, lithium-ion batteries have been applied to various portable electronic devices and used as the power source for hybrid electric vehicles and electric vehicles. They are currently very widely used secondary batteries. In order to further improve the energy density of lithium batteries, silicon negative electrode materials have been favored by researchers due to their advantages such as high theoretical capacity, low working voltage and rich resources. However, during the charge and discharge process of the silicon negative electrode, it will suffer a huge volume expansion (>300%), resulting in the pulverization of silicon particles, poor electrical contact with the current collector, and the fragmentation of the solid electrolyte interface (SEI) film. The fragmentation of the SEI film will exacerbate the occurrence of electrolyte side reactions, which will lead to a rapid decay of the battery capacity.

[0003] In the prior art, structural designs such as nanosizing, eggshell, porous and hollow can effectively buffer the volume expansion of silicon materials. However, the high cost and complex process limit their commercial applications. In contrast, modifying the binder is an economical and practical method to improve the volume expansion of silicon materials.

[0004] An ideal silicon negative electrode binder should have the following two elements: (1) have a strong interaction with the material to effectively inhibit the expansion of silicon particles; (2) have good toughness itself to adapt to the stress generated by the expansion of silicon particles. However, traditional lithium battery binders, such as polyvinylidene fluoride (PVDF), are only connected to the active material / current collector by van der Waals forces and cannot provide sufficient bonding strength for the electrode sheet; although sodium carboxymethyl cellulose (CMC) and polyacrylic acid (PAA) binders can inhibit the expansion of silicon particles to a certain extent, the prepared electrode sheets have poor flexibility and are prone to cracking during cycling, seriously affecting the battery performance.

[0005] In view of this, it is urgent to develop a binder that can effectively inhibit the volume expansion of silicon particles and has good flexibility at the same time. Summary of the Invention

[0006] Therefore, the technical problem to be solved by the present invention is to overcome the above defects in the prior art, so as to provide a binder, a preparation method, a negative electrode sheet and a lithium-ion battery.

[0007] To this end, the present invention provides the following technical solutions:

[0008] The present invention provides a binder, the raw materials of which include organic compound A and polymer B with a mass ratio of 1:(10-100);

[0009] Among them, organic compound A contains more than 2 guanidine groups, and polymer B is a natural polysaccharide and its derivatives containing functional groups capable of forming ionic bonds with guanidine groups.

[0010] Optionally, the functional groups capable of forming ionic bonds with guanidine include one or more of carboxyl groups, carboxylate groups, sulfonate groups or phosphate groups.

[0011] Optionally, the natural polysaccharides and their derivatives containing carboxylate groups include at least one of sodium carboxymethyl cellulose, sodium alginate, xanthan gum;

[0012] And / or, the natural polysaccharide and its derivative containing a carboxyl group is carboxymethyl chitosan;

[0013] And / or, the natural polysaccharides and their derivatives containing sulfonate groups include at least one of sodium lignosulfonate, sodium cellulose sulfonate, sulfonated guar gum or carrageenan;

[0014] And / or, the natural polysaccharide and its derivative containing a phosphate group is sodium cellulose phosphate.

[0015] Optionally, the organic compound A includes at least one of chlorhexidine gluconate, chlorhexidine acetate, guanamine, metformin, phenformin.

[0016] The present invention also provides a method for preparing the above-mentioned binder, comprising the following steps:

[0017] S1, dissolving organic compound A in water to obtain an aqueous solution of organic compound A;

[0018] S2, adding polymer B to the aqueous solution of organic compound A, stirring to obtain a mixed material, and spray-drying to obtain the binder.

[0019] Optionally, the solid content of the mixed material is 5wt% - 15wt%.

[0020] Optionally, in step S2, polymer B is added to the aqueous solution of organic compound A in portions.

[0021] Optionally, in step S2, polymer B is added in 2 - 5 portions;

[0022] And / or, the stirring time is 3 - 12h.

[0023] The present invention also provides a negative electrode plate, comprising the above-mentioned binder or the binder prepared by the above-mentioned preparation method;

[0024] Optionally, the negative electrode plate is a silicon-based negative electrode plate.

[0025] The present invention also provides a lithium-ion battery, which includes the above-mentioned negative electrode sheet.

[0026] The other components and preparation methods of the negative electrode sheet and the lithium-ion battery provided by the present invention are all well-known in the art.

[0027] Typically and non-limitingly, taking a button battery as an example, its specific manufacturing method includes:

[0028] S1: Prepare the negative electrode sheet. Graphite, a silicon-based material, a conductive agent (such as SP), and a binder material are mixed at a weight ratio of 72:24:1:3 (the mass ratio of graphite to the silicon-based material is 3:1), and dispersed in deionized water to form a uniform slurry. Subsequently, the slurry is cast on a copper foil by a doctor blade method and dried at 70-90 °C, and the thickness of the active material is 40-60 μm. Cold pressing, edge trimming, and slicing are carried out, and then dried in a vacuum at 80-100 °C for 12-24 h to obtain the negative electrode sheet.

[0029] S2: Assemble a 2032-type button half-cell in a glove box filled with an inert gas, where the contents of H2O and O2 are both less than 0.1 ppm. A lithium metal sheet is used as the counter electrode, and PP / PE (polypropylene / polyethylene) is used as the separator. 1M LiPF6 (lithium hexafluorophosphate) (EC (ethylene carbonate): EMC (ethyl methyl carbonate): DMC (dimethyl carbonate) = 1:1:1 vol%) + 1-3% FEC (fluoroethylene carbonate) is used as the electrolyte.

[0030] The technical solution of the present invention has the following advantages:

[0031] The raw materials provided by the present invention include an organic compound A and a polymer B with a mass ratio of 1:(10-100); wherein, the organic compound A contains more than 2 guanidyl groups, and the polymer B is a natural polysaccharide and its derivatives containing functional groups capable of forming strong physical cross-linking with the guanidyl groups. In the binder provided by the present invention, a synergistic force of hydrogen bonds and ionic bonds can be formed between the organic compound A and the polymer B, generating a strong physical cross-linking effect; compared with the method of cross-linking a polymer containing amide and imide groups with a carboxylic acid polymer in the existing patent, the organic compound A used in the present invention (a small molecule guanidine salt containing 2 or more guanidyl groups, different from the easy entanglement and aggregation of polymer molecular chains) is more likely to form hydrogen bonds and ionic bonds with the polymer B containing sulfonate groups, phosphate groups, and carboxyl / carboxylate groups, and the intermolecular force is much greater than the intermolecular force between amide, imide groups and carboxylic acid groups, and can more effectively limit the expansion of the silicon negative electrode. At the same time, different from the existing synthetic cross-linked polymer technology, through the breakage of physical cross-linking hydrogen bonds and ionic bonds in the present invention, the stress energy received by the material can be dissipated, thereby greatly enhancing the toughness of the negative electrode material.

[0032] When the binder provided by the present invention is applied to the negative electrode sheet, based on the strong physical cross-linking effect, the binder can exhibit high rigidity, which is beneficial to suppressing the stress deformation of the negative electrode active material, thereby reducing the volume expansion of the electrode sheet. Different from the existing synthetic cross-linked polymer technology, in the present invention, through the breakage of physical cross-links of hydrogen bonds and ionic bonds, the stress energy received by the material can be dissipated, thereby greatly enhancing the toughness of the negative electrode sheet. After the stress is removed, the physical cross-links based on hydrogen bonds and ionic bonds can be reversibly restored. The binder of the present invention has both excellent expansion suppression effect and flexibility improvement, making the negative electrode sheet and lithium-ion battery prepared based on it have excellent performance in various aspects. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required to be used in the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0034] Figure 1 It is a schematic structural diagram of the winding needle test of the present invention.

[0035] Reference numerals:

[0036] 1, active material layer; 2, current collector; 3, winding needle. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0037] The following embodiments are provided to better further understand the present invention. It is not limited to the best embodiment, and does not limit the content and protection scope of the present invention. Any product obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with other prior art features and being the same as or similar to the present invention falls within the protection scope of the present invention.

[0038] For those embodiments in which specific experimental steps or conditions are not specified, the operations or conditions of the conventional experimental steps described in the literature in this field can be followed. For the reagents or instruments whose manufacturers are not specified, they are all conventional reagent products that can be obtained through commercial purchase.

[0039] Example 1

[0040] This example provides a binder, and its specific raw material composition and preparation method are as follows:

[0041] Weigh 10 g of dodine and add it to 1 kg of deionized water, stir to fully dissolve it to obtain an aqueous solution of dodine; add 100 g of sodium lignosulfonate to the above aqueous solution of dodine in 3 batches. After the sodium lignosulfonate is completely dissolved, continue to stir for 3 h, and obtain a binder with a cross-linked structure through spray drying.

[0042] Example 2

[0043] This example provides an adhesive, which is similar to the preparation process of the adhesive in Example 1, except that 10 g of dodine is replaced with 10 g of metformin.

[0044] Example 3

[0045] This example provides an adhesive, which is similar to the preparation process of the adhesive in Example 1, except that 10 g of dodine is replaced with 5 g of dodine and 5 g of chlorhexidine acetate.

[0046] Example 4

[0047] This example provides an adhesive, which is similar to the preparation process of the adhesive in Example 1, except that the mass of dodine in organic compound A is changed from 10 g to 5 g.

[0048] Example 5

[0049] This example provides an adhesive, which is similar to the preparation process of the adhesive in Example 1, except that the mass of dodine in organic compound A is changed from 10 g to 1 g.

[0050] Example 6

[0051] This example provides an adhesive, which is similar to the preparation process of the adhesive in Example 1, except that 100 g of sodium lignosulfonate is changed to 50 g of sodium lignosulfonate and 50 g of cellulose phosphate.

[0052] Example 7

[0053] This example provides an adhesive, which is similar to the preparation process of the adhesive in Example 1, except that 100 g of sodium lignosulfonate is changed to 50 g of sodium lignosulfonate and 50 g of sodium alginate.

[0054] Example 8

[0055] This example provides an adhesive, which is similar to the preparation process of the adhesive in Example 1, except that 100 g of sodium lignosulfonate is changed to 50 g of sodium lignosulfonate and 50 g of carrageenan.

[0056] Example 9

[0057] This example provides an adhesive, which is similar to the preparation process of the adhesive in Example 1, except that 100 g of sodium lignosulfonate is changed to 100 g of carrageenan.

[0058] Comparative Example 1

[0059] This comparative example provides a binder, which is similar to the preparation process of the binder in Example 1, except that the binder does not contain organic compound A.

[0060] Comparative Example 2

[0061] This comparative example provides a binder, which is similar to the preparation process of the binder in Example 1, except that the binder does not contain polymer B, and organic small molecule A cannot be used as a binder alone.

[0062] Comparative Example 3

[0063] This comparative example provides a binder, which is similar to the preparation process of the binder in Example 1, except that the mass of organic compound A, guazatine, is changed from 10 g to 20 g. During the experiment, it was found that too much organic compound A led to too strong cross-linking effect and a uniform binder mixed solution could not be obtained.

[0064] Comparative Example 4

[0065] This comparative example provides a binder, which is similar to the preparation process of the binder in Example 1, except that the mass of organic compound A, guazatine, is changed from 10 g to 0.5 g.

[0066] Comparative Example 5

[0067] This comparative example provides a binder, which is similar to the preparation process of the binder in Example 1, except that 10 g of organic compound A, guazatine, is changed to hexamethylenediamine.

[0068] Test Example

[0069] 1. Manufacture of coin cell

[0070] The coin cells assembled with the binder materials prepared in each example and comparative example were respectively subjected to full charge expansion and electrochemical performance test analysis, as follows:

[0071] S1: Preparation of negative electrode sheet. Graphite, silicon-based material, carbon black and binder material were mixed at a weight ratio of 72:24:1:3, and dispersed in deionized water to form a uniform slurry. Subsequently, the slurry was cast on copper foil by doctor blade method and dried at 70 °C. Then, cold pressing, edge trimming and slicing were carried out, and then dried under vacuum at 100 °C for 12 h to obtain the negative electrode sheet.

[0072] S2: Assemble a 2032-type coin half-cell in a glove box filled with inert gas, where the contents of H2O and O2 are both less than 0.1 ppm. A lithium metal sheet was used as the counter electrode, PP / PE was used as the separator, and 1M LiPF6 (EC:EMC:DMC = 1:1:1 vol%) + 3% FEC (fluoroethylene carbonate) was used as the electrolyte.

[0073] 2. Coin Cell Testing

[0074] (1) Flexibility Testing

[0075] Wind the negative electrode sheet around mandrels of different diameters (diameters: 1 mm, 1.5 mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm). Align the wound part of the electrode sheet forcefully. The winding method is as shown in Figure 1 . Keep the current collector 2 close to the mandrel 3 and the active material layer 1 away from the mandrel 3. Observe whether there are cracks on the wound electrode sheet and record the minimum mandrel diameter at which the negative electrode sheet can be wound without cracking. The smaller the corresponding value, the better the flexibility of the electrode sheet.

[0076] (2) Cycling Performance Testing

[0077] Take 3 batteries from each group of the batteries prepared above and repeat charging and discharging of the batteries through the following steps, and calculate the discharge capacity retention rate of the batteries.

[0078] First, in an environment of 25°C, discharge at 0.5C to 5 mV, 0.05C to 5 mV, 0.02C to 5 mV, 0.01C to 5 mV, and then charge at 0.1C to 1.5V, and record the charging specific capacity of the first cycle. Then perform 200 discharge and charge cycles and record the charging specific capacity of the 200th cycle. According to the following formula:

[0079] Cycling capacity retention rate = (Specific capacity of the first cycle / Specific capacity of the 200th cycle) × 100%.

[0080] Calculate the average capacity retention rate of each group of batteries after cycling, as shown in Table 1.

[0081] (3) Thickness Expansion Rate Testing of the Negative Electrode Sheet in the Fully Inserted State

[0082] Measure the thickness of the negative electrode sheet before assembling the battery, denoted as D0. Keep the assembled battery in an environment of 25°C, discharge at 0.1C to 5 mV, 0.02C to 5 mV to make the negative electrode sheet in the fully inserted state. Disassemble the battery and test the thickness of the negative electrode sheet in the fully inserted state, denoted as D1. The thickness of the foil used is 9 μm. According to the following formula:

[0083] Thickness expansion rate = (D1 - D0) / (D0 - 9) × 100%.

[0084] Calculate the thickness expansion rate of the negative electrode sheet in the fully inserted state, as shown in Table 1.

[0085] Table 1

[0086]

[0087] As can be seen from the above results, compared with the comparative example, the cross-linked binder with stress dissipation effect prepared in the embodiment of the present invention can balance the contradiction between the rigidity and toughness of the binder material, and the prepared negative electrode sheet has both excellent expansion inhibition effect and good flexibility, thus greatly improving the performance of the battery.

[0088] Obviously, the above embodiments are only examples for clear illustration and not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to enumerate all the implementation manners here. And the obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A negative electrode sheet, characterized in that, The negative electrode plate is a silicon-based negative electrode plate, and the negative electrode plate includes a binder; The binder is prepared from raw material organic compound A, polymer B and water, and the mass ratio of organic compound A to polymer B in the raw materials is 1:(10 - 100); Among them, organic compound A contains more than 2 guanidine groups, and polymer B is a natural polysaccharide and its derivatives containing functional groups capable of forming ionic bonds with guanidine groups.

2. The negative electrode sheet according to claim 1, characterized in that, The functional groups capable of forming ionic bonds with guanidine include one or more of carboxyl groups, carboxylate groups, sulfonate groups or phosphate groups.

3. The negative electrode sheet according to claim 2, characterized in that, The natural polysaccharides and their derivatives containing carboxylate groups include at least one of sodium carboxymethyl cellulose, sodium alginate, xanthan gum; And / or, the natural polysaccharide and its derivative containing carboxyl groups is carboxymethyl chitosan; And / or, the natural polysaccharides and their derivatives containing sulfonate groups include at least one of sodium lignosulfonate, sodium cellulose sulfonate, sulfonated guar gum or carrageenan; And / or, the natural polysaccharide and its derivative containing phosphate groups is sodium cellulose phosphate.

4. The negative electrode sheet according to any one of claims 1-3, characterized in that, The organic compound A includes at least one of chlorhexidine gluconate, chlorhexidine acetate, dodine, metformin, phenformin.

5. The negative electrode sheet according to claim 1, characterized in that, The preparation method of the binder includes the following steps: S1, dissolving organic compound A in water to obtain an aqueous solution of organic compound A; S2, adding polymer B to the aqueous solution of organic compound A, stirring to obtain a mixed material, and spray-drying to obtain the binder.

6. The negative electrode sheet according to claim 5, characterized in that, The solid content of the mixed material is 5wt% - 15wt%.

7. The negative electrode sheet according to claim 5, wherein In step S2, the polymer B is added to the aqueous solution of organic compound A in portions.

8. The negative electrode sheet according to claim 7, wherein, In step S2, the polymer B is added in 2 - 5 portions; And / or, the stirring time is 3 - 12h.

9. A lithium-ion battery, characterized in that, Including the negative electrode plate according to any one of claims 1 - 8.

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