Composite binder as well as preparation method and application thereof

Abstract

The invention relates to a composite binder as well as a preparation method and application thereof. The composite binder is prepared from the following raw materials: a beta-cyclodextrin derivative and ferrocene-grafted polyethyleneimine, the total mass of the beta-cyclodextrin derivative and the polyethyleneimine grafted with ferrocene is 100%, the mass content percentage of the beta-cyclodextrin derivative is 30%-80%, and the mass content percentage of the polyethyleneimine grafted with ferrocene is 20%-70%. The composite binder prepared by the invention has a large number of cyclodextrin inner cavities and polar groups of-OH,-Fc and-NH2, can adsorb polysulfide generated by a sulfur positive electrode, and inhibits the shuttle effect of a battery; the hydrogen-bond interaction and supramolecular interaction in the composite binder can resist the stress caused by electrode expansion, and the pulverization and shedding phenomena of the electrode are reduced. The lithium-sulfur battery prepared by using the composite binder as a sulfur-based positive electrode raw material has stable cycle performance, and the specific capacity of the battery is improved.

Description

Technical Field

[0001] The present invention belongs to the field of lithium-sulfur batteries, and particularly relates to a composite binder, a preparation method thereof, and an application thereof. Background Art

[0002] Lithium-ion batteries have been widely used in portable electronic products such as mobile phones, tablets, and laptop computers. In recent years, with the booming development of new energy vehicles, higher requirements for the energy density of lithium-ion batteries have been put forward. However, restricted by the limitation of the electrode specific capacity, the energy density of lithium-ion batteries at the present stage has approached its theoretical value and cannot meet the demand for a higher cruising range. Therefore, it is urgent to develop the next generation of high-energy batteries. Lithium-sulfur batteries use elemental sulfur with a high theoretical specific capacity (1672 mAh / g) as the positive electrode and metallic lithium (3860 mAh / g) as the negative electrode, and their theoretical energy density can reach 2600 Wh / kg. At the same time, sulfur has the advantages of rich reserves, low price, and environmental friendliness, and is considered to be one of the next generation of high-energy batteries with great development prospects.

[0003] However, some problems existing in lithium-sulfur batteries during the cycling process have hindered their commercial application. On the one hand, about 70% volume expansion of the sulfur positive electrode during the cycling process causes the destruction of the electrode structure, ultimately leading to the shedding of active substances and the decline of the cycling performance. On the other hand, the discharge intermediate product polysulfide lithium (LiPS) during the cycling process is soluble in the electrolyte and migrates to the negative electrode and reacts with it under the action of the concentration gradient. This "shuttle effect" leads to the loss of the positive electrode capacity and the continuous progress of the irreversible reaction of the negative electrode.

[0004] As an important component of the electrode, the polymer binder not only needs to maintain good electrical contact between the active substance and the conductive agent, but also needs to have excellent mechanical properties and polysulfide fixation ability. Therefore, binder modification is an effective way to maintain the integrity of the electrode structure and avoid the shuttle effect. At present, most lithium-sulfur batteries still use commercial polyvinylidene fluoride (PVDF) and sodium carboxymethyl cellulose-styrene butadiene rubber (CMC-SBR) binders. Although these binders have relatively high mechanical strength, their linear structure results in poor deformation adaptability and weak adsorption ability for LiPS, making it difficult to achieve a high loading and ensure the cycling stability of the battery.

[0005] CN102074704A discloses carbonylated β-cyclodextrin as a cathode binder for lithium-sulfur secondary batteries. By modifying β-cyclodextrin, water-soluble carbonylated β-cyclodextrin is obtained, which greatly improves its solubility in water. Even after the water is evaporated, no crystals precipitate, thus effectively enhancing the contact between the binder and the sulfur-based material and being very suitable as a cathode binder for secondary batteries. The cathode made of carbonylated β-cyclodextrin as the binder and the metal lithium anode form a secondary lithium-sulfur battery with a high capacity retention rate and stable battery cycling performance.

[0006] CN106159272A discloses a nitrogen-containing binder for lithium-sulfur batteries and its preparation method. Introducing nitrogen-containing functional groups effectively inhibits the dissolution and shuttle effect of polysulfide ions in the electrolyte. There are a large number of hydroxyl groups in cyclodextrin, which have strong hydrogen bond interactions with the cathode material particles, can better maintain the stability of the electrode sheet, and improve the specific power, specific energy and cyclicity of the battery.

[0007] Although the above binders have all made improvements in enhancing the adhesion of the electrode sheet and increasing the stability of the electrode sheet, there are still deficiencies in inhibiting the dissolution and migration of polysulfide ions in the electrolyte, etc., and the problem of poor cycling performance. Summary of the Invention

[0008] To solve the above technical problems, the present invention provides a composite binder, its preparation method and application. The composite binder has excellent adhesion performance and water solubility, and can adsorb polysulfides generated by the sulfur cathode, inhibiting the shuttle effect of the battery; the hydrogen bond interaction and supramolecular interaction in the composite binder can resist the stress caused by electrode expansion, reducing the pulverization and shedding of the electrode; the lithium-sulfur battery prepared by using the composite binder described in the present invention as the sulfur-based cathode raw material has stable cycling performance and improves the specific capacity of the battery.

[0009] To achieve this purpose, the present invention adopts the following technical solutions:

[0010] In the first aspect, the present invention provides a composite binder, and the preparation raw materials of the composite binder include β-cyclodextrin derivative and polyethyleneimine grafted with ferrocene;

[0011] Based on the total mass of the β-cyclodextrin derivative and the polyethyleneimine grafted with ferrocene being 100%, the mass content percentage of the β-cyclodextrin derivative is 30%-80%, and the mass content percentage of the polyethyleneimine grafted with ferrocene is 20%-70%.

[0012] Among them, the mass content percentage of the β-cyclodextrin derivative can be 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75% or 80%, and the mass percentage of the polyethyleneimine grafted with ferrocene can be 20%, 30%, 40%, 50%, 60% or 70%.

[0013] On the one hand, the composite binder provided by the present invention has a three-dimensional network structure, which can buffer the volume expansion of the sulfur positive electrode during the charge and discharge process. The composite binder prepared by the present invention contains rich -OH, -Fc (ferrocene group), -NH2 and other groups. The -Fc group is present in the inner cavity of the cyclodextrin derivative and is dynamically cross-linked with the cyclodextrin derivative. When the sulfur-based particles expand, the relative position of the -Fc group and the cyclodextrin derivative is readjusted, and the two are always in a complex state. This dynamic cross-linking structure can effectively resist the stress caused by electrode expansion, and the hydrogen bonding between the -NH2 group and the -OH in the inner cavity of the cyclodextrin derivative is conducive to strengthening the interaction between the groups, thereby avoiding the powdering and shedding of the electrode. On the other hand, these groups can adsorb LiPS generated during the cycle, which is conducive to reducing the capacity decay caused by the dissolution and diffusion of LiPS.

[0014] The polyethyleneimine molecules used in the present invention contain abundant -NH2, which can react with the aldehyde group in formaldehyde ferrocene to form a Schiff base reaction, thereby grafting ferrocene onto the polyethyleneimine skeleton. The grafted product improves the flexibility of polyethyleneimine, and the cyclopentadiene ring group in ferrocene and Li + π-Li + bond, which improves ionic conductivity. In addition, the Fe 2+ There is an electrostatic interaction between the lithium salt anion and the electrolyte, which can effectively reduce the concentration polarization and strengthen the Li + Transmission rate, improve battery rate performance.

[0015] Preferably, the β-cyclodextrin derivative includes any one of β-cyclodextrin polymer, hydroxypropyl-β-cyclodextrin or carbonylated-β-cyclodextrin, or a combination of at least two thereof.

[0016] The present invention adopts beta-cyclodextrin derivatives as the raw materials for preparing the composite binder. The inner cavity size and structure of cyclodextrin will affect its complexing ability with ferrocene. The inner cavity size of beta-cyclodextrin is better matched with ferrocene, and the inclusion constant is higher. Therefore, the structure of beta-cyclodextrin can form a stable complex structure with ferrocene, while the inner cavity of alpha-cyclodextrin is smaller, and the inclusion ability of ferrocene is slightly weaker. The inner cavity of gamma-cyclodextrin is larger, and there is a certain space redundancy after complexing with ferrocene, resulting in poor stability.

[0017] The β-cyclodextrin derivative used in the present invention has more excellent water solubility compared with β-cyclodextrin. The solubility of β-cyclodextrin in water at room temperature is only 1.85 g / 100 mL. Although heating can increase the solubility of β-cyclodextrin in water, as the water evaporates, β-cyclodextrin will recrystallize and cannot achieve the effect of bonding the thiol-based material.

[0018] Preferably, the number-average molecular weight of the polyethyleneimine > 5000, for example, it can be 5500, 6000, 6500, 7000, 7500, 8000, 8500, 9000, 9500 or 10000, etc.

[0019] Preferably, the raw materials for preparing the composite binder further include a solvent, preferably deionized water.

[0020] Preferably, the preparation method of the β-cyclodextrin polymer includes the following steps:

[0021] (1) Alkalize β-cyclodextrin;

[0022] (2) Carry out a cross-linking reaction between the alkalized β-cyclodextrin and epichlorohydrin;

[0023] (3) After the reaction, neutralize the system to obtain the β-cyclodextrin polymer.

[0024] Preferably, the mass ratio of β-cyclodextrin to epichlorohydrin is 1:(0.5 - 0.8), for example, it can be 1:0.5, 1:0.6, 1:0.7 or 1:0.8.

[0025] Preferably, in step (1), the alkaline treatment is to mix β-cyclodextrin with an alkaline solution.

[0026] Preferably, the alkaline solution includes a sodium hydroxide solution.

[0027] Preferably, the mass ratio of β-cyclodextrin to the volume of the alkaline solution is 1 g:(6 - 10) mL, for example, it can be 1 g:6 mL, 1 g:7 mL, 1 g:8 mL, 1 g:9 mL or 1 g:10 mL.

[0028] Preferably, the mass fraction of sodium hydroxide in the sodium hydroxide solution is 25 - 35 wt%, for example, it can be 25 wt%, 27 wt%, 29 wt%, 30 wt%, 32 wt%, 34 wt% or 35 wt%.

[0029] Preferably, the reaction temperature of the crosslinking reaction in step (2) is 25 - 35°C, for example, it can be 25°C, 27°C, 29°C, 30°C, 32°C, 34°C or 35°C, and the reaction time is 8 - 10 h, for example, it can be 8 h, 8.5 h, 9 h, 9.5 h or 10 h.

[0030] Preferably, the neutralization treatment in step (3) is to mix the system with an acidic solution.

[0031] Preferably, the acidic solution includes a hydrochloric acid solution.

[0032] Preferably, the concentration of the hydrochloric acid solution is 5 - 7 mol / L, for example, it can be 5 mol / L, 5.5 mol / L, 6 mol / L, 6.5 mol / L or 7 mol / L.

[0033] Preferably, the stirring time in step (3) is 22 - 26 h, for example, it can be 22 h, 23 h, 24 h, 25 h or 26 h.

[0034] Preferably, the pH value of the system after the neutralization treatment in step (3) is 6.5 - 7.5, for example, it can be pH = 6.5, pH = 6.8, pH = 7, pH = 7.3 or pH = 7.5.

[0035] Preferably, the preparation steps of the carbonylated-β-cyclodextrin include: mixing β-cyclodextrin with an oxidant and reacting to obtain the carbonylated-β-cyclodextrin.

[0036] Preferably, the oxidant includes an aqueous hydrogen peroxide solution.

[0037] Preferably, the mass fraction of hydrogen peroxide in the aqueous hydrogen peroxide solution is 25 - 35 wt%, for example, it can be 25 wt%, 27 wt%, 29 wt%, 30 wt%, 32 wt%, 34 wt% or 35 wt%.

[0038] Preferably, the reaction temperature is 75 - 85°C, for example, it can be 75°C, 77°C, 79°C, 80°C, 82°C, 84°C or 85°C, and the reaction time is 20 - 28 h, for example, it can be 20 h, 22 h, 24 h, 26 h or 28 h.

[0039] Preferably, the preparation method of the ferrocene-grafted polyethyleneimine includes: mixing polyethyleneimine with ferrocene formaldehyde and reacting to obtain the ferrocene-grafted polyethyleneimine.

[0040] Preferably, the mass ratio of polyethyleneimine to ferrocene formaldehyde is 1:(0.25 - 0.5), for example, it can be 1:0.25, 1:0.3, 1:0.35, 1:0.4, 1:0.45 or 1:0.5.

[0041] Preferably, the reaction is carried out in a solvent, and the solvent is preferably methanol.

[0042] Preferably, the temperature of the reaction is 10 - 30 °C, for example, it can be 10 °C, 15 °C, 20 °C, 25 °C or 30 °C, and the reaction time is 1 - 4 h, for example, it can be 1 h, 2 h, 3 h or 4 h.

[0043] In a second aspect, the present invention provides a method for preparing the composite binder as described in the first aspect. The method for preparing the composite binder includes: mixing a β - cyclodextrin derivative with polyethyleneimine grafted with ferrocene, and carrying out a reaction to obtain the composite binder.

[0044] Preferably, the reaction is carried out in a solvent, and the solvent is preferably deionized water.

[0045] Preferably, the temperature of the reaction is 15 - 30 °C, for example, it can be 15 °C, 20 °C, 25 °C or 30 °C, and the reaction time is 2 - 4 h, for example, it can be 2 h, 2.5 h, 3 h, 3.5 h or 4 h.

[0046] In a third aspect, the present invention provides a sulfur - based positive electrode, which includes the composite binder as described in the first aspect, a sulfur - based positive electrode active material, a conductive agent and a current collector.

[0047] Preferably, the mass of the composite binder is calculated based on the total mass of the β - cyclodextrin derivative and polyethyleneimine grafted with ferrocene in the composite binder. The mass ratio of the composite binder, the sulfur - based positive electrode active material and the conductive agent is (0.8 - 1.2):(7 - 9):(0.8 - 1.2).

[0048] Wherein "0.8 - 1.2" can be 0.8, 0.9, 1, 1.1 or 1.2, and "7 - 9" can be 7, 7.5, 8, 8.5 or 9.

[0049] Preferably, the sulfur - based positive electrode active material includes any one or at least two combinations of elemental sulfur, lithium sulfide, sulfur - carbon composite material or organic sulfide, and is preferably a sulfur - carbon composite material.

[0050] Preferably, the conductive agent includes any one or at least two combinations of conductive carbon black, carbon nanotubes, graphene, acetylene black or Ketjen black.

[0051] Preferably, the current collector includes any one or at least two combinations of aluminum foil, carbon - coated aluminum foil or aluminum foam.

[0052] In a fourth aspect, the present invention provides a method for preparing the sulfur - based positive electrode as described in the third aspect. The method for preparing the sulfur - based positive electrode includes the following steps:

[0053] (1) Mix the composite binder, sulfur-based cathode active material and conductive agent to obtain an electrode slurry;

[0054] (2) Coat the electrode slurry on the current collector, and then dry and slice it to obtain the sulfur-based cathode.

[0055] Preferably, the drying temperature is 60 - 90 °C, for example, it can be 60 °C, 65 °C, 70 °C, 75 °C, 77 °C, 79 °C, 80 °C, 82 °C, 84 °C, 85 °C or 90 °C, and the drying time is 2 - 12 h, for example, it can be 2 h, 4 h, 6 h, 8 h, 10 h or 12 h.

[0056] The preparation method of the sulfur-carbon composite material includes: mixing Ketjen black and elemental sulfur in a mass ratio of 1:(2 - 8), for example, it can be 1:2, 1:3, 1:4, 1:5, 1:6, 1:7 or 1:8, and calcining at 150 - 160 °C (for example, it can be 150 °C, 152 °C, 154 °C, 156 °C, 158 °C or 160 °C) for 10 - 14 h (for example, it can be 10 h, 11 h, 12 h, 13 h or 14 h) to obtain the sulfur-carbon composite material.

[0057] In the fifth aspect, the present invention provides a lithium-sulfur battery, and the lithium-sulfur battery includes the sulfur-based cathode as described in the third aspect.

[0058] Compared with the prior art, the present invention has at least the following beneficial effects:

[0059] 1. The composite binder provided by the present invention has a three-dimensional network structure, which can buffer the volume expansion phenomenon that occurs during the charge and discharge process of the sulfur cathode. When the sulfur-based particles expand, the dynamic cross-linking structure of the -Fc group and the cyclodextrin derivative can effectively resist the stress caused by the electrode expansion, and the hydrogen bond interaction between the -NH2 group and the -OH in the cyclodextrin inner cavity is also beneficial to strengthening the interaction between the groups, thereby reducing the pulverization and shedding phenomena of the electrode.

[0060] 2. The rich cyclodextrin inner cavity and groups such as -OH, -Fc, -NH2 in the composite binder provided by the present invention can adsorb the LiPS generated during the cycle, which is beneficial to reducing the capacity attenuation caused by the dissolution and diffusion of LiPS.

[0061] 3. The present invention grafts ferrocene on the polyethyleneimine skeleton. After grafting, the flexibility of polyethyleneimine is improved. The cyclopentadiene ring group in ferrocene forms a π-Li + bond with Li + , which improves the ionic conductivity. In addition, Fe 2+The electrostatic interaction with the lithium salt anion also increases the ionic conductivity, which is beneficial to reducing the concentration polarization in the electrode and improving the rate performance of the battery.

[0062] 4. The composite binder provided by the present invention has excellent binding performance, which can enhance the force between the active material and the current collector, and is beneficial to reducing the interfacial resistance of the battery.

[0063] 5. The composite binder provided by the present invention has excellent water solubility, using water as a solvent to avoid the use of toxic and expensive organic solvents. Description of the Drawings

[0064] Figure 1 is the test effect diagram of the stripping strength of the sulfur-based positive electrode prepared in Example 1 and Comparative Example 1 of the present invention;

[0065] Figure 2 is the specific capacity-cycle curve diagram of the sulfur-based positive electrode prepared in Example 1 and Comparative Example 1 of the present invention when applied to a lithium-sulfur battery. Detailed Embodiments

[0066] The technical solutions of the present invention will be further described below in conjunction with the drawings and through specific embodiments. However, the following examples are only simple examples of the present invention and do not represent or limit the scope of the protection of the rights of the present invention. The scope of protection of the present invention shall be subject to the claims.

[0067] The sources of the raw materials used in the following examples are as follows:

[0068] 1. β-cyclodextrin polymer:

[0069] Dissolve β-cyclodextrin (1.0 g, purchased from Aladdin) in 8 mL of sodium hydroxide solution with a mass fraction of 33 wt%, and stir at 30 °C for 12 h. Then add epichlorohydrin (0.57 g), stir at 25 °C for 4 h, and then add 5 mL of acetone and continue to stir for 4 h. Remove the acetone, and then dropwise add 6 mol / L hydrochloric acid until the pH of the system is close to 7, and continue to stir for 24 h. The product is dialyzed with a dialysis bag with a molecular cut-off of 3500 for 72 h and dried to obtain the β-cyclodextrin polymer;

[0070] 2. Hydroxypropyl-β-cyclodextrin, purchased from Aladdin;

[0071] 3. Carbonylated-β-cyclodextrin:

[0072] Dissolve β-cyclodextrin (2 g) in 6 g of hydrogen peroxide aqueous solution with a mass fraction of 30 wt%, stir at 80 °C for 24 h, and place the product in a blast oven at 80 °C to evaporate the solvent to obtain the carbonylated-β-cyclodextrin.

[0073] 4. Polyethyleneimine grafted with ferrocene:

[0074] Polyethyleneimine (1.24 g, purchased from Aladdin, with a number-average molecular weight of 70,000) was dissolved in 30 mL of methanol, and ferrocene formaldehyde (0.46 g) was dissolved in 18 mL of methanol. The methanol solution of ferrocene formaldehyde was added dropwise to the methanol solution of polyethyleneimine, and the mixture was stirred at 25 °C for 2 h. The product was dialyzed for 72 h using a dialysis bag with a molecular weight cut-off of 3500 Da to remove the methanol solvent in the solution, and the polyethyleneimine grafted with ferrocene was obtained.

[0075] 5. Sulfur-carbon composite material:

[0076] Ketjen black (3 g) was mixed with elemental sulfur (7 g) and calcined at 150 °C for 12 h to obtain the sulfur-carbon composite material.

[0077] Example 1

[0078] A composite binder, the preparation method of the composite binder includes: dissolving β-cyclodextrin polymer (0.08 g) in deionized water, and stirring to prepare a 5% β-cyclodextrin polymer solution; then adding polyethyleneimine grafted with ferrocene (0.02 g) to the above β-cyclodextrin polymer solution, and reacting at 25 °C for 3 h to obtain the composite binder.

[0079] A sulfur-based positive electrode, the preparation method of the sulfur-based positive electrode includes the following steps:

[0080] All of the above-prepared composite binder was mixed with sulfur-carbon composite material (0.8 g), conductive carbon black (0.1 g) and deionized water, stirred evenly to obtain an electrode slurry (solid content of 40%), and the electrode slurry was coated on carbon-coated aluminum foil and dried in a forced-air oven at 80 °C for 12 h, and then roll-pressed and sliced to obtain the sulfur-based positive electrode.

[0081] Example 2

[0082] A composite binder, the preparation method of the composite binder includes: dissolving carbonylated-β-cyclodextrin (0.03 g) in deionized water, and stirring to prepare a 5% carbonylated-β-cyclodextrin solution; then adding polyethyleneimine grafted with ferrocene (0.07 g) to the above carbonylated-β-cyclodextrin solution, and reacting at 25 °C for 3 h to obtain the composite binder.

[0083] The preparation method of the sulfur-based positive electrode is the same as that in Example 1.

[0084] Example 3

[0085] A composite binder, the preparation method of the composite binder comprising: dissolving hydroxypropyl-β-cyclodextrin (0.05 g) in deionized water, and stirring to prepare a 5% hydroxypropyl-β-cyclodextrin solution; then adding polyethyleneimine grafted with ferrocene (0.05 g) to the above-mentioned hydroxypropyl-β-cyclodextrin solution, and reacting at 25 °C for 3 h to obtain the composite binder.

[0086] The preparation method of the sulfur-based positive electrode is the same as that in Example 1.

[0087] Example 4

[0088] A composite binder, the preparation method of the composite binder comprising: dissolving 0.05 g of hydroxypropyl-β-cyclodextrin (0.03 g) in deionized water, and stirring to prepare a 5% hydroxypropyl-β-cyclodextrin solution; then adding polyethyleneimine grafted with ferrocene (0.07 g) to the above-mentioned hydroxypropyl-β-cyclodextrin solution, and reacting at 25 °C for 3 h to obtain the composite binder.

[0089] The preparation method of the sulfur-based positive electrode is the same as that in Example 1.

[0090] Comparative Example 1

[0091] A binder, the preparation method of the binder comprising: dissolving β-cyclodextrin polymer (0.1 g) in deionized water, and stirring to prepare a 5% β-cyclodextrin polymer solution, which is the binder.

[0092] The preparation method of the sulfur-based positive electrode is the same as that in Example 1.

[0093] Comparative Example 2

[0094] A composite binder, the preparation method of the binder comprising: dissolving β-cyclodextrin polymer (0.08 g) in deionized water, and stirring to prepare a 5% β-cyclodextrin polymer solution; then adding polyethyleneimine (0.05 g) to the above-mentioned β-cyclodextrin polymer solution, and reacting at 25 °C for 3 h to obtain the composite binder.

[0095] The preparation method of the sulfur-based positive electrode is the same as that in Example 1.

[0096] Comparative Example 3

[0097] A binder, the preparation method of the binder comprising: dissolving polyvinylidene fluoride (0.1 g) in N-methylpyrrolidone, and stirring to prepare a 5% polyvinylidene fluoride solution, which is the binder.

[0098] The preparation method of the sulfur-based positive electrode is the same as that in Example 1.

[0099] Comparative Example 4

[0100] An adhesive, the preparation method of the adhesive comprising: fully dissolving β-cyclodextrin (0.1 g) in deionized water to obtain the adhesive.

[0101] The preparation method of the sulfur-based cathode is the same as that in Example 1.

[0102] Testing method

[0103] Preparation of the electrolyte:

[0104] Prepare a mixed solution by mixing 1,3-dioxolane and ethylene glycol dimethyl ether in a volume ratio of 1:1, then add 1.0 M lithium bis(trifluoromethanesulfonyl)imide and 0.2 M LiNO3, stir evenly, and place it in a glove box for standby after the lithium salt is fully dissolved.

[0105] Assembly and testing of CR2025 batteries:

[0106] Cut the sulfur-based cathodes prepared in Examples 1-4 or Comparative Examples 1-4 into small round pieces with a diameter of 14 mm as the positive electrode sheets, put them into the positive electrode case, then place the Celgard 2400 separator and the lithium sheet in the battery case in sequence, and inject 75 μL of the above electrolyte. The assembled battery is left to stand for 12 h, and the assembled battery is subjected to constant current charge and discharge testing at different current densities (0.1 C when testing the first-cycle discharge specific capacity) using a CT2001A Blue Electric Battery Tester. The testing temperature is 25 °C, and the voltage range is 1.7 - 2.8 V.

[0107] 100-cycle capacity retention rate (%): After the test battery is formed at a rate of 0.1 C for 2 cycles, it is then cycled 100 times at 0.5 C, and the ratio of the discharge specific capacities before and after is calculated.

[0108] Peeling strength (N): Prepare two kinds of electrodes into strips with a specification of 100 mm in length and 20 mm in width. Stick a tape on the surface of the electrode membrane, and place one end of the tape and one end of the current collector on the fixture of a universal tensile machine, and conduct a 180° peeling test at a constant speed of 10 mm / min. The magnitude of the peeling force during the test is used to characterize the bonding strength of the adhesive.

[0109] Test results

[0110] The test results of Examples 1-4 and Comparative Examples 1-4 are shown in Table 1.

[0111] Table 1

[0112]

[0113] It can be seen from the test results that:

[0114] (1) It can be seen from Examples 1 to 4 that the composite binder provided by the present invention enables the battery to have a high initial capacity. The specific discharge capacity in the first cycle can reach more than 1500 mAh / g, and the capacity retention rate after 100 cycles can reach more than 95%. It has excellent stability and cycling performance of the positive electrode.

[0115] (2) It can be seen from Example 1 and Comparative Examples 1-2 that through the hydrogen bond and supramolecular interactions between the composite binders of the present invention, the binding performance as a binder for the sulfur-based positive electrode is effectively improved, thereby enhancing the stability and cycling performance of the positive electrode.

[0116] (3) It can be seen from Example 1 and Comparative Example 3 that compared with the commonly used polyvinylidene fluoride binder in the prior art, the composite binder prepared by the present invention has more excellent stability and cycling performance of the positive electrode.

[0117] (4) In the binder provided by Comparative Example 4, due to the poor solubility of β-cyclodextrin in water, it cannot achieve the effect of binding the sulfur-based material.

[0118] Figure 1 is the test effect diagram of the stripping strength of the sulfur-based positive electrodes prepared in Example 1 and Comparative Example 1 of the present invention. Through Figure 1 it can be seen that compared with Comparative Example 1, the stripping force of the sulfur-based positive electrode provided by Example 1 is greater than 0.9 N, the electrode is firmly combined with the substrate, and it is not easy to fall off during charge and discharge, and the stability of the battery is excellent.

[0119] Figure 2 is the specific capacity - cycle curve graph of the sulfur-based positive electrodes prepared in Example 1 and Comparative Example 1 of the present invention applied to lithium-sulfur batteries. Through Figure 2 it can be seen that compared with Comparative Example 1, the sulfur-based positive electrode provided by Example 1 assembled into a CR2025 battery has excellent stability and cycling performance of the positive electrode.

[0120] The applicant declares that the above description is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and public scope of the present invention.

Claims

1. A composite adhesive, characterized in that: The raw materials for preparing the composite binder include β-cyclodextrin derivatives and polyethyleneimine grafted with ferrocene; Taking the total mass of the beta-cyclodextrin derivative and the polyethyleneimine grafted with ferrocene as 100%, the mass content of the beta-cyclodextrin derivative is 30%-80%, and the mass content of the polyethyleneimine grafted with ferrocene is 20%-70%.

2. The composite adhesive according to claim 1, characterized in that: The β-cyclodextrin derivative includes any one of β-cyclodextrin polymer, hydroxypropyl-β-cyclodextrin or carbonyl-β-cyclodextrin or a combination of at least two thereof; Preferably, the number average molecular weight of the polyethyleneimine is greater than 5000; Preferably, the raw materials for preparing the composite binder also include a solvent, preferably deionized water.

3. The composite adhesive according to claim 1 or 2, characterized in that: The preparation method of the β-cyclodextrin polymer comprises the following steps: (1) alkalizing β-cyclodextrin; (2) the alkaline-treated β-cyclodextrin undergoes a cross-linking reaction with epichlorohydrin; (3) after the reaction is completed, the system is neutralized to obtain the β-cyclodextrin polymer; Preferably, the mass ratio of β-cyclodextrin to epichlorohydrin is 1:(0.5-0.8); Preferably, the alkaline treatment in step (1) is mixing β-cyclodextrin with an alkaline solution; Preferably, the alkaline solution comprises sodium hydroxide solution; Preferably, the mass ratio of the β-cyclodextrin to the volume ratio of the alkaline solution is 1 g: (6-10) mL; Preferably, the mass fraction of sodium hydroxide in the sodium hydroxide solution is 25-35wt%; Preferably, the reaction temperature of the cross-linking reaction in step (2) is 25-35° C., and the reaction time is 8-10 h; Preferably, the neutralization treatment in step (3) is mixing the system with an acidic solution; Preferably, the acidic solution comprises a hydrochloric acid solution; Preferably, the concentration of the hydrochloric acid solution is 5-7 mol / L; Preferably, the neutralization treatment time in step (3) is 22-26 hours; Preferably, the pH value of the system after the neutralization treatment in step (3) is 6.5-7.

5.

4. The composite adhesive according to any one of claims 1 to 3, characterized in that: The preparation steps of the carbonylated β-cyclodextrin include: mixing β-cyclodextrin with an oxidant, and reacting to obtain the carbonylated β-cyclodextrin; Preferably, the oxidant comprises aqueous hydrogen peroxide; Preferably, the mass fraction of hydrogen peroxide in the aqueous hydrogen peroxide solution is 25-35wt%; Preferably, the reaction temperature is 75-85°C, and the reaction time is 20-28h.

5. The composite adhesive according to any one of claims 1 to 4, characterized in that: The preparation method of the polyethyleneimine grafted with ferrocene comprises: mixing polyethyleneimine with formaldehyde ferrocene, and reacting to obtain polyethyleneimine grafted with ferrocene; Preferably, the mass ratio of polyethyleneimine to formaldehyde ferrocene is 1:(0.25-0.5); Preferably, the reaction is carried out in a solvent, and the solvent is preferably methanol; Preferably, the reaction temperature is 10-30°C, and the reaction time is 1-4h.

6. A method for preparing the composite adhesive according to any one of claims 1 to 5, characterized in that: The preparation method of the composite binder comprises: mixing a β-cyclodextrin derivative with polyethyleneimine grafted with ferrocene, and reacting the mixture to obtain the composite binder; Preferably, the reaction is carried out in a solvent, and the solvent is preferably deionized water; Preferably, the reaction temperature is 15-30°C and the reaction time is 2-4h.

7. A sulfur-based positive electrode, characterized in that The invention comprises a composite binder as described in any one of claims 1 to 5, a sulfur-based positive electrode active material, a conductive agent and a current collector.

8. The sulfur-based positive electrode according to claim 7, characterized in that The mass of the composite binder is calculated based on the total mass of the β-cyclodextrin derivative and the polyethyleneimine grafted with ferrocene in the composite binder, and the mass ratio of the composite binder, the sulfur-based positive electrode active material and the conductive agent is (0.8-1.2):(7-9):(0.8-1.2); Preferably, the sulfur-based positive electrode active material comprises any one or a combination of at least two of elemental sulfur, lithium sulfide, sulfur-carbon composite material or organic sulfide, preferably a sulfur-carbon composite material; Preferably, the conductive agent comprises any one of conductive carbon black, carbon nanotubes, graphene, acetylene black or Ketjen black, or a combination of at least two thereof; Preferably, the current collector comprises any one of aluminum foil, carbon-coated aluminum foil or foamed aluminum, or a combination of at least two of them.

9. A method for preparing a sulfur-based positive electrode as claimed in claim 7 or 8, characterized in that: The preparation method of the sulfur-based positive electrode comprises the following steps: (1) mixing a composite binder, a sulfur-based positive electrode active material and a conductive agent to obtain an electrode slurry; (2) The electrode slurry is coated on a current collector, and then dried and sliced ​​to obtain the sulfur-based positive electrode.

10. A lithium-sulfur battery, characterized in that: The lithium-sulfur battery comprises the sulfur-based positive electrode according to claim 7 or 8.

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