Silicon-based thick electrode binder and preparation method and application thereof

Abstract

The invention relates to the technical field of silicon-based negative electrode binders, in particular to a silicon-based thick electrode binder and a preparation method and application thereof. The invention provides a silicon-based thick electrode binder and a preparation method and application thereof. The aqueous binder is simple in synthesis process, has the advantages of high adhesive force, high ion conductivity and high electron conductivity, is applied to a high-load silicon-based negative electrode, can greatly relieve the volume effect of a silicon-based negative electrode material in a thick electrode in a lithium intercalation and de-intercalation process, promotes effective transmission of electrons / ions in the thick electrode, improves the reversible specific capacity of the silicon-based thick electrode, and has a good application prospect. The reversibility and the stability under the conditions of high multiplying power and long circulation are improved, and a theoretical basis is provided for preparation of a high-energy-density silicon negative electrode.

Description

Technical Field

[0001] The present invention relates to the technical field of silicon-based negative electrode binders, and in particular to a silicon-based thick electrode binder, a preparation method thereof, and applications thereof. Background Art

[0002] As new energy vehicles continue to demand higher range, lithium battery anode materials are also developing towards higher specific capacity. Currently, graphite is the primary anode material for lithium batteries, but the specific capacity of commercially available graphite is approaching its theoretical limit (372 mAh / g). Silicon anodes, however, possess a higher theoretical capacity (3579 mAh / g), approximately 10 times that of current graphite-based anode materials. The use of silicon materials can significantly improve the energy density of lithium batteries, making them the most promising next-generation anode materials. However, intrinsic defects in silicon anodes limit their commercial application. Key challenges include: 1. Large volume expansion during lithium insertion and extraction (volume change >300%); 2. Poor electronic conductivity; and 3. Instability of the SEI film. Currently, three main approaches are being considered to address these challenges: 1. Modifying silicon materials to mitigate volume expansion and improve conductivity; 2. Selecting suitable binders to enhance adhesion between active materials and between active materials and the current collector, mitigating volume expansion; and 3. Developing suitable electrolytes to stabilize the SEI film.

[0003] Researchers have conducted a lot of modification research on silicon-based negative electrode materials. Through methods such as silicon nano-sizing, porosification, carbon coating and alloying, the volume expansion, poor conductivity and unstable SEI film of silicon have been alleviated, making silicon-based negative electrode materials more suitable for thin electrodes (active material loading <1 mg / cm 2 ) showed good cycle stability and rate performance. However, in order to better improve the energy density of silicon-based negative electrodes, it is required to reduce the content of inactive substances in the electrode and increase the surface loading of the electrode active substances, that is, to build thick electrodes (active material loading> 1mg / cm 2 However, the significant increase in thickness of silicon-based thick electrodes leads to slow ion and electron transport kinetics within the electrode. Furthermore, the accumulated stress from lithium insertion and deintercalation causes the active material in the thick electrode to fracture and delaminate, resulting in poor mechanical stability. Consequently, these thick silicon-based electrodes suffer from low effective specific capacity, short cycle life, and poor rate performance. Therefore, constructing high-performance thick electrodes for silicon-based anode materials with large volume deformation is extremely challenging.

[0004] In the construction of thick electrodes, binders are one of the essential materials for preparing electrodes. Their function is to bond the active material and the conductive agent together and fix them on the current collector, which has a great impact on the electrochemical performance of silicon negative electrode materials. Polyvinylidene fluoride (PVDF) binders are widely used in commercial electrodes due to their (electro)chemical stability and processability. However, PVDF binders rely on weak supramolecular interactions (van der Waals forces) and cannot adapt to the huge hoop stress generated during the electrochemical reaction of high-capacity negative electrode materials, which causes severe electrode cycle decay. In addition, the dissolution of PVDF binders requires the use of N-methylpyrrolidone (NMP) as a solvent, which is not environmentally friendly and difficult to recycle due to its high cost. Aqueous binders such as sodium alginate (SA), carboxymethyl cellulose, guar gum, polyacrylic acid and thiourea make the electrochemical performance of high-capacity negative electrodes better than traditional oil-based binders. These substances form hydrogen bonds with polar functional groups on the silicon surface (such as -OH, -COOH and -NH2), showing good adhesion ability. Some can even provide self-healing ability to restore interrupted (or damaged) interactions and dissipate mechanical stress. At the same time, different aqueous binders can form covalent cross-links, dynamic cross-links, 3D networks and highly elastic networks, effectively buffering the stress caused by volume deformation during the deintercalation and insertion of lithium in the silicon-based negative electrode, thereby constructing a stable high-capacity negative electrode and extending the battery cycle life. Li et al. (Li Z, Wan Z, Lin Z, et al. “A highly elastic and Li-ion conductive binder enables stable operation of silicon microparticle anodes in high-capacity and high-energy-density pouch cells” Energy & Environmental Science, 2025, 18: 2035) placed guar gum (GG) in a four-necked flask and dissolved it in deionized water by magnetic stirring. Then, (NH4)2S2O8 and NaHSO3 initiators were added and heated in a water bath with stirring. When the water bath temperature reached 60°C, acrylic acid (AA) was added for in-situ polymerization to obtain a GG-g-PAA composite binder. The SiMP electrode constructed with this binder has an active material ratio of 80wt%, a binder ratio of 10wt%, and a loading capacity of 1.6mg / cm 2 The initial surface capacity at 0.1C (1C = 4200 mA / g) is 2.4 mAh / cm 2. However, the synthesis of this binder requires in-situ polymerization, the process is complicated, and the surface capacity of the obtained silicon-based material is low, which limits the large-scale application of silicon materials. In order to better meet the practical application of silicon materials, based on low-cost SA binders, researchers Li et al. (Li J, Hu X, Zhao H, et al. "Cross-Linked Sodium Alginate-Sodium Borate Hybrid Binders for High-Capacity Silicon Anodes in Lithium-Ion Batteries" Langmuir, 2022, 38: 402) used sodium alginate (SA) and inorganic crosslinker sodium borate (SB) for esterification reaction condensation to obtain the binder SA-SB. When the active material accounts for 80wt% and the binder accounts for 10wt%, the nano-silicon electrode prepared using this binder has a loading of 0.8mg / cm 2 , at a current density of 0.2 A / g, the capacity retention rate after 100 cycles is 64.1%, while the capacity retention rate of the original Si-SA electrode is only 50.6%. In addition, the D Li + Compared with the original SA binder, it has been improved by an order of magnitude. + The transport kinetics have been improved to a certain extent, but the electrode loading is only 0.8 mg / cm 2 , it is still difficult to meet high loading requirements. At present, compared with the widely reported PAA-based binders, SA-based binders do not require complex in-situ polymerization and are low-cost. However, the electrodes prepared with the reported SA-based binders still have problems with insufficient ion conductivity, electronic conductivity and interface compatibility. The charge transfer dynamics of the electrodes are limited, and it is difficult to effectively withstand the large volume effect of silicon-based negative electrode materials under high mass loading, which makes it difficult to effectively improve the long cycle and rate characteristics of silicon-based thick electrodes.

[0005] Currently, there is a lack of binders for silicon-based thick electrodes that offer excellent combined adhesion, electron-ion, and interfacial compatibility, along with simple preparation methods and low cost. Therefore, developing a binder for silicon-based thick electrodes with strong adhesion, excellent mechanical properties, and good electron / ion transport capabilities is of great research value for improving the energy density of silicon-based anodes in lithium-ion batteries. Summary of the Invention

[0006] The present invention provides a silicon-based thick electrode binder, its preparation method, and its application. This aqueous binder has a simple synthesis process and exhibits the advantages of high adhesion, high ion conductivity, and high electron conductivity. When applied to high-load silicon-based anodes, it can significantly alleviate the volume effect of silicon-based anode materials during lithium insertion and extraction in thick electrodes, promote efficient electron / ion transport in the thick electrode, enhance the reversible specific capacity of silicon-based thick electrodes, and improve their reversibility and stability under high-rate and long-cycle conditions. This provides a theoretical basis for the preparation of high-energy-density silicon anodes.

[0007] In order to achieve the above object, the present invention provides the following technical solutions:

[0008] The present invention provides a silicon-based thick electrode binder, comprising sodium alginate, polyvinyl alcohol with a relative molecular mass of 31,000 to 50,000, xanthan gum and water; the mass ratio of the sodium alginate, polyvinyl alcohol with a relative molecular mass of 31,000 to 50,000 and xanthan gum is (5-10): (1-5): (1-5).

[0009] The present invention uses sodium alginate (SA) as the primary binder, adding polyvinyl alcohol (PVA) and xanthan gum (XG) to create a multi-component composite binder. The combination of long-chain SA, short-chain PVA, and the double-helix structure of XG significantly increases the binder's viscosity, creating a three-dimensional bonding network.

[0010] Specifically, there are a large number of hydrophilic groups -COOH and -OH in the hydrophilic molecular chain of SA, which will form intramolecular or intermolecular hydrogen bonds; PVA is a flexible molecule with hydroxyl hydrogen bonds between chains; SA and PVA are compounded, and the addition of SA will weaken the hydrogen bonding between PVA molecules, enhance the hydrogen bonding between SA molecules and PVA molecules, and further improve the mechanical strength of the binder. SA is compounded with XG, and the SA aqueous solution is viscous, while the XG aqueous solution behaves as a pseudoplastic fluid. After mixing the two, the viscoelasticity is enhanced. Therefore, due to the interaction between the complex functional groups in the different binder components, the ternary silicon-based thick electrode binder of the present invention with excellent electrochemical and physicochemical properties is finally obtained, which can be applied to silicon thick electrodes with high loading.

[0011] Furthermore, extensive experiments have revealed that a high-performance ternary silicon-based thick electrode binder is only achieved when polyvinyl alcohol (PVA) of a specific molecular weight is blended with sodium alginate and xanthan gum. Furthermore, the ratio of these three components significantly influences the final performance of the ternary silicon-based thick electrode binder. This ternary silicon-based thick electrode binder is only achieved using the raw materials and raw material ratios provided by the present invention.

[0012] The experimental results show that the ternary silicon-based thick electrode binder has high adhesion and good mechanical stability, and is applied to silicon negative electrode sheets (loading capacity 2.0-3.0 mg / cm 2) can effectively promote the electron / ion transmission of the electrode sheet and is expected to be widely used as a high-load, high-current-density lithium-ion battery.

[0013] Preferably, the solid content of the silicon-based thick electrode binder is 1 to 5 wt %.

[0014] Preferably, based on the total weight of the silicon-based thick electrode binder, 0.1-1 wt% of component A is further included, and the component A is at least one of polyacrylic acid, sodium carboxymethyl cellulose, guar gum, gelatin, konjac gum, carrageenan, and polyethyleneimine.

[0015] After obtaining the ternary silicon-based thick electrode binder provided by the present invention, other common binder components, such as polyacrylic acid, sodium carboxymethyl cellulose, guar gum, gelatin, konjac gum, carrageenan, or polyethyleneimine, can be added. In the amounts specified by the present invention, the original three-dimensional bonding network is not destroyed, and a multi-component silicon-based thick electrode binder with other excellent properties can be obtained.

[0016] The invention provides a method for preparing a silicon-based thick electrode binder. The silicon-based thick electrode binder is obtained by hydrothermal crosslinking of sodium alginate, polyvinyl alcohol with a relative molecular mass of 31,000 to 50,000, and xanthan gum.

[0017] Preferably, the thermal cross-linking method is: sodium alginate, polyvinyl alcohol with a relative molecular mass of 31,000 to 50,000, xanthan gum and water are mixed and heated to 60 to 110°C.

[0018] Preferably, the thermal cross-linking method is: polyvinyl alcohol with a relative molecular mass of 31,000 to 50,000 and water are dissolved at 60 to 110° C., and then sodium alginate and xanthan gum are added.

[0019] The appropriate thermal cross-linking temperature can promote the cross-linking reaction between sodium alginate, polyvinyl alcohol and xanthan gum, forming a stronger bonding network. However, too high or too low a temperature will destroy the bonding network and reduce the key properties of the silicon-based thick electrode binder, such as reversible specific capacity.

[0020] The invention provides a silicon negative electrode sheet, comprising a silicon-based negative electrode material, a conductive agent and a silicon-based thick electrode binder.

[0021] Preferably, the conductive agent is one or more of conductive graphite, carbon nanotubes and carbon nanofibers.

[0022] Preferably, the mass ratio of the silicon-based negative electrode material, the conductive agent and the silicon-based thick electrode binder is (60-80):(10-20):(10-20).

[0023] The present invention provides a method for preparing a silicon negative electrode sheet. Silicon-based negative electrode material, a conductive agent and a silicon-based thick electrode binder are mixed, coated on an electrode sheet substrate, and dried to obtain a silicon negative electrode sheet.

[0024] Preferably, the electrode sheet substrate is copper foil.

[0025] Preferably, the loading amount of the silicon negative electrode sheet is 2.0-3.0 mg / cm 2 .

[0026] The present invention provides a lithium ion battery using a silicon negative electrode sheet.

[0027] Therefore, the present invention has the following beneficial effects:

[0028] (1) The silicon-based thick electrode binder provided by the present invention utilizes long-chain sodium alginate, short-chain polyvinyl alcohol and double-helix structured xanthan gum for thermal cross-linking, thereby increasing the viscosity of the binder and constructing a three-dimensional bonding network. The obtained silicon-based thick electrode binder has high adhesion and good mechanical stability.

[0029] (2) The present invention selects polyvinyl alcohol with a specific molecular weight and cross-links it with sodium alginate and xanthan gum in a specific ratio to obtain a silicon-based thick electrode binder with excellent performance.

[0030] (3) The silicon negative electrode sheet made of the silicon-based thick electrode binder provided by the present invention is tightly anchored on the surface of the silicon negative electrode material through the action of hydrogen bonds, giving the active material and the current collector a strong binding force. When applied to high-load silicon negative electrode materials, it effectively reduces the shedding and peeling of the active material in the silicon negative electrode sheet during the charge and discharge process, thereby improving the cycle stability of the battery.

[0031] (4) In the silicon negative electrode sheet made of the silicon-based thick electrode binder provided by the present invention, there is a strong hydrogen bond between the rigid sodium alginate and the flexible polyvinyl alcohol, and the addition of xanthan gum with an elastic double helix structure forms a three-dimensional interpenetrating network. The synergistic effect between the "rigid-flexible-elastic" three elements can provide effective energy dissipation and buffer the stress generated by the silicon material during the lithium insertion and deinsertion process, thereby maintaining the structural stability of the silicon negative electrode sheet during the cycle.

[0032] (5) The method for preparing the silicon-based thick electrode binder proposed in the present invention is low-cost and simple to synthesize. It does not require a complex purification and polymerization process and can be achieved by simple heating and mixing. The synthesized binder is a water-based binder and does not involve toxic organic solvents. It is safe and environmentally friendly and suitable for large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1Cycling performance curve of the battery assembled with binary binder and SA at a current density of 150 mA / g;

[0034] Figure 2 Cycling performance curves of batteries assembled with different ternary binders at a current density of 150 mA / g;

[0035] Figure 3 Areal capacity diagram of the battery assembled for SVX;

[0036] Figure 4 Cycling performance curves of batteries assembled with SVX binders obtained at different temperatures at a current density of 150 mA / g;

[0037] Figure 5 Comparison of rate performance of lithium-ion batteries assembled with SA, SV, SX, and SVX binders;

[0038] Figure 6 The peel strength test diagram of the electrode sheets prepared with SA, SV, SX, and SVX adhesives;

[0039] Figure 7 The viscosity comparison chart of SA, SV, SX and SVX adhesives is shown below;

[0040] Figure 8 Impedance diagram of lithium-ion batteries assembled with SA, SV, SX, and SVX binders after 100 cycles at a current density of 150 mA / g;

[0041] Figure 9 Cycling stability curves of lithium-ion batteries assembled with SA, SV, SX, and SVX binders at a current density of 300 mA / g. DETAILED DESCRIPTION

[0042] The present invention will be further described below with reference to specific embodiments. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below generally represent only a portion of the present invention, rather than all of the embodiments. Therefore, all other embodiments derived by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0043] In this section, the sources of raw materials are as follows: polyvinyl alcohol (relative molecular weight 31000-50000) was purchased from McLean, CAS number 9002-89-5; sodium alginate, polyacrylic acid, sodium carboxymethyl cellulose, guar gum, gelatin, konjac gum, carrageenan, xanthan gum, and polyethyleneimine were all purchased from Aladdin; silicon tin powder was purchased from Zhongnuo New Materials; conductive graphite, SFG-6, was purchased from Cyber ​​Electrochemistry; carbon nanofiber, CNF, was purchased from Showa, Japan; copper current collector, was purchased from Cyber ​​Electrochemistry; metal lithium sheet, was purchased from Tianjin Lithium Industry.

[0044] [Example]

[0045] Example 1

[0046] Weigh 0.0625g of polyvinyl alcohol (PVA) with a relative molecular mass of 31,000-50,000 and add it to 24.25g of deionized water. Heat and stir at 80°C until it is completely dissolved. Then, add sodium alginate and xanthan gum in the order of 10:1:1. Magnetic stirring is continued at 1000 rpm for 24 hours. The resulting binder has a solid content of 3wt%, and is designated SA-PVA-XG(SVX).

[0047] Comparative Example 1

[0048] This comparative example is basically the same as Example 1, except that xanthan gum and polyvinyl alcohol with a relative molecular mass of 31,000 to 50,000 are replaced by sodium alginate, and a total of 0.75 g of sodium alginate is added to finally obtain a pure sodium alginate binder with a solid content of 3 wt%, which is recorded as SA.

[0049] Comparative Example 2

[0050] This comparative example is basically the same as Example 1, except that xanthan gum is replaced by an equal mass of polyacrylic acid, carrageenan, gelatin or polyacetimide, and the resulting binder solid content is 3 wt %, which are respectively recorded as SA-PVA-PAA (polyacrylic acid), SA-PVA-KCG (carrageenan), SA-PVA-GN (gelatin), and SA-PVA-PEI (polyacetimide).

[0051] Comparative Example 3

[0052] This comparative example is basically the same as Example 1, except that the heating temperature is adjusted from 80°C to 50°C or 120°C.

[0053] Comparative Example 4

[0054] 0.125 g of polyvinyl alcohol with a relative molecular mass of 31,000 to 50,000 was weighed and added to 24.25 g of deionized water. The mixture was heated and stirred at 80°C. After it was completely dissolved, sodium alginate powder was added, wherein the mass ratio of sodium alginate to polyvinyl alcohol was 5:1. The mixture was magnetically stirred at 1,000 rpm for 24 hours. The solid content of the obtained binder was 3 wt%, and was recorded as SA-PVA (SV).

[0055] Comparative Example 5

[0056] This comparative example is basically the same as comparative example 4, except that the polyvinyl alcohol with a relative molecular mass of 31,000 to 50,000 is replaced by an equal mass of polyacrylic acid, which is recorded as SA-PAA.

[0057] Comparative Example 6

[0058] This comparative example is basically the same as comparative example 4, except that the polyvinyl alcohol with a relative molecular mass of 31,000 to 50,000 is replaced by an equal mass of sodium carboxymethyl cellulose, which is recorded as SA-CMC.

[0059] Comparative Example 7

[0060] This comparative example is basically the same as comparative example 4, except that the polyvinyl alcohol with a relative molecular mass of 31,000 to 50,000 is replaced by guar gum of equal mass, which is recorded as SA-GG.

[0061] Comparative Example 8

[0062] This comparative example is basically the same as comparative example 4, except that the polyvinyl alcohol with a relative molecular mass of 31,000 to 50,000 is replaced by gelatin of equal mass, which is recorded as SA-GN.

[0063] Comparative Example 9

[0064] This comparative example is basically the same as comparative example 4, except that the polyvinyl alcohol with a relative molecular mass of 31,000 to 50,000 is replaced by konjac gum of equal mass, which is recorded as SA-KG.

[0065] Comparative Example 10

[0066] This comparative example is basically the same as comparative example 4, except that the polyvinyl alcohol with a relative molecular mass of 31,000 to 50,000 is replaced by an equal mass of carrageenan, which is recorded as SA-KCG.

[0067] Comparative Example 11

[0068] This comparative example is basically the same as comparative example 4, except that the polyvinyl alcohol with a relative molecular mass of 31,000 to 50,000 is replaced by xanthan gum of equal mass, which is recorded as SA-XG (SX).

[0069] Comparative Example 12

[0070] This comparative example is basically the same as comparative example 4, except that the polyvinyl alcohol with a relative molecular mass of 31,000 to 50,000 is replaced by polyethyleneimine of the same mass, which is recorded as SA-PEI.

[0071]

Performance test

[0072] The silicon negative electrode sheets were prepared using the binders obtained in Example 1 and Comparative Examples 1 to 11 and the batteries were assembled. The specific process was as follows: silicon tin powder, SFG-6, CNF and binder were weighed in a mass ratio of 72:4.5:13.5:10, and deaerated and stirred. The deaeration and stirring parameters were 600 rpm, 120 s, 2000 rpm, 2400 s, and 2200 rpm, 180 s. The operation was repeated 3 times to form a uniformly mixed slurry. The slurry was then coated on the copper current collector with a 0.25 mm scraper. After natural drying, it was placed in a vacuum oven at 70 ° C for 9 hours. After cooling to room temperature, it was cut into 10 mm discs. The loading capacity of a single electrode sheet was 2.5 mg / cm 2 , 2.7mg / cm 2 or 3.0 mg / cm 2 The cells were then vacuum-dried in a 105°C oven for 11 hours. After cooling to room temperature, they were transferred to an argon-filled glove box. A button-type lithium-ion battery was assembled using the prepared electrode as the positive electrode, a metal lithium sheet as the negative electrode, and 1M LiPF6 (EC / EMC / DEC volume ratio of 1:1:1) as the electrolyte. The assembled battery was aged in a 45°C oven for 23 hours. Electrochemical performance was measured using a constant current method with a voltage window of 0.01 to 1.5 V.

[0073] 1. Binary adhesive

[0074] Before preparing the ternary silicon-based thick electrode binder, the inventor explored the various components. In Comparative Examples 4 to 12, the inventor explored the performance of the binder obtained by compounding sodium alginate with other components. According to the above method, the silicon negative electrode sheet was prepared and the battery was assembled. The loading of the silicon negative electrode sheet was 2.5 mg / cm 2 The performance results obtained from the test are shown in Table 1 and Figure 1 shown.

[0075] Table 1 Performance comparison of binary binders

[0076]

[0077] From Table 1, we can see that the first coulombic efficiency of the electrodes prepared by SA-PVA and SA-XG are 71.55% and 81.54%, respectively, which are much higher than SA (64.90%). It can be seen that the silicon electrodes prepared by SA and PVA, and SA and XG have good first efficiency and capacity retention, and good interfacial compatibility with the silicon negative electrode. Here, in addition to screening the raw material components, the inventors screened out PVA and XG that are more suitable for compounding with SA, and tested PVA and XG of different molecular weights. It was found that when the molecular weight of PVA is controlled between 31,000 and 50,000, the performance results are better.

[0078] Furthermore, the cycle performance test of the silicon negative electrode prepared by the binary binder obtained in Comparative Examples 4 to 12 and a single SA was carried out. The test results are as follows: Figure 1 The results showed that when the loading amount was 2.5 mg / cm 2 When the current density is 150mA / g, the first reversible specific capacity of the electrode material obtained by SA-PVA binder is 2305.33mAh / g, the reversible specific capacity after 100 cycles is 1552.56mAh / g, and the capacity retention rate is 67.35%; the electrode sheet prepared by SA-XG has an initial reversible specific capacity of 2690.32mAh / g, and the reversible specific capacity after 100 cycles is 1742.87mAh / g, and the corresponding capacity retention rate is 64.78%; the specific capacity and capacity retention rate of the electrodes prepared by SA-PVA and SA-XG binders after 100 cycles are higher than those of SA binder (1350.88mAh / g, 61.95%). The cyclic stability of SA-PAA, SA-CMC, SA-GG, SA-GN, SA-KG, SA-KCG and SA-PEI was not significantly improved compared with SA binder. Among them, SA-PVA and SA-XG had better cyclic stability, which was mainly due to the introduction of flexible polyvinyl alcohol or elastic xanthan gum into rigid sodium alginate. The formed three-dimensional network can better buffer the stress generated by the volume expansion and contraction of the silicon-tin material during the cycle, thereby giving the silicon electrode excellent cyclic stability.

[0079] 2. Selection of ternary components

[0080] In addition, the inventors found that in many attempts to explore the binary binder, no matter what type of raw materials or how to adjust the ratio, it was impossible to achieve a higher level of performance improvement. Based on this, the inventors considered constructing a ternary component to prepare a ternary binder. The binder obtained in Example 1 and Comparative Example 2 was used to prepare a silicon negative electrode sheet and assemble a battery in the above manner. The loading of the silicon negative electrode sheet was 2.5 mg / cm 2 The performance results obtained from the test are shown in Table 2 and Figures 2-3 shown.

[0081] Table 2 Performance comparison of ternary binders

[0082]

[0083] From Table 2, we can see that the first charge specific capacity and first coulombic efficiency of SVX are the best. Figure 2 As shown, when the loading amount is 2.5 mg / cm 2 When the current density is 150mA / g, the first reversible specific capacity is 2395.82mAh / g. After 100 cycles, there is still a reversible specific capacity of 1820.15mAh / g, corresponding to a capacity retention rate of 75.97%. The capacity retention rates of the electrodes prepared by SA-PVA-PAA, SA-PVA-KCG, SA-PVA-GN, and SA-PVA-PEI binders after 100 cycles are 29.00%, 13.51%, 17.94%, and 31.30%, respectively. By comparison, it can be seen that the electrode prepared by the ternary binder SVX has the best cycle stability. This may be because there is a strong hydrogen bond between the rigid sodium alginate and the flexible polyvinyl alcohol, and the addition of the elastic double helical structure of xanthan gum forms a three-dimensional interpenetrating network. The synergistic effect between the "rigid-flexible-elastic" three can provide effective energy dissipation, buffer the stress generated by the silicon material during the lithium insertion and deintercalation process, thereby maintaining the structural stability of the silicon negative electrode sheet during the cycle.

[0084] In addition, if Figure 3 As shown, this ternary binder SVX is used for silicon negative electrode, and the initial surface capacity reaches 6 mAh / cm 2 , further confirming that the ternary composite binder is suitable for the preparation of high energy density silicon negative electrodes.

[0085] 3. Preparation process

[0086] In order to explore the effect of different temperatures on the electrode prepared by the ternary binder SVX, the binder heat treatment temperatures of 50℃ and 120℃ were used as comparisons. The cycling stability results of the electrode are shown in Figure 2. Figure 4 As shown in the figure, it can be seen that the cycling stability of the electrode prepared at a heat treatment temperature of 80°C is better than that of the other two temperatures. It can also be seen that the appropriate heat treatment temperature can make the cross-linked network formed by the binder stronger, thereby achieving better cycling stability.

[0087] 4. Performance of ternary adhesive

[0088] The binders obtained in Example 1, Comparative Example 1, Comparative Example 4 and Comparative Example 11 were used to prepare silicon negative electrode sheets and assemble batteries in the same manner as described above. The loading capacity of the four binders (SA, SV, SX and SVX) was 2.7 mg / cm 2 The electrodes are assembled into half cells for rate performance testing. The results are as follows Figure 5 As shown in the figure, it can be found that the rate performance of the silicon negative electrode prepared by the ternary binder (SVX) is significantly better than that of the silicon electrode prepared by the binary binder (SV, SX) and the original sodium alginate (SA), and the silicon electrode prepared by the ternary binder (SVX) still has a specific capacity of 678.02mAh / g at a current density of 1500mA / g. This is mainly because the bonding network constructed by the ternary binder (SVX) can effectively connect the silicon tin particles with the conductive agent, so that they have better electrical contact, thereby obtaining excellent rate performance.

[0089] The peeling test of silicon anode electrode sheets made of four different binders (SA, SV, SX and SVX) was carried out to evaluate their mechanical stability. Figure 6 As shown in the figure, the average peeling force of the SVX electrode sheet is 0.539N, while the average peeling forces of the SX, SV and SA electrode sheets are 0.293N, 0.067N and 0.0233N respectively. By comparison, it is found that the peeling strength of the SVX binder electrode is 23 times higher than that of the SA binder, indicating that the SVX binder has good mechanical properties. When applied to silicon negative electrode, it has high mechanical strength and can effectively relieve the stress generated by the volume effect of silicon tin particles during the lithium insertion and extraction process.

[0090] Viscosity tests were performed on four adhesives to illustrate their adhesion. Figure 7 This is a viscosity test comparison chart, where the viscosity values ​​of SA, SV, SX and SVX binders are 11470mPa·s, 9039mPa·s, 12710mPa·s and 14210mPa·s respectively. SVX has the highest viscosity value, indicating that the composite binder composed of rigid sodium alginate, flexible polyvinyl alcohol and elastic xanthan gum has a synergistic and complementary effect under the action of covalent bonds and hydrogen bonds, which improves the viscosity of the binder and can more effectively combine with silicon tin particles and conductive agents, and has stronger adhesion with the copper current collector, thereby maintaining the integrity of the electrode structure during the cycle.

[0091] The binders obtained in Example 1, Comparative Example 1, Comparative Example 4 and Comparative Example 11 were used to prepare silicon negative electrode sheets and assemble batteries in the same manner as above. 2 The impedance test of the electrode was carried out after 100 cycles at a current density of 150 mA / g. Figure 8It shows that the SEI impedances corresponding to SA, SV, SX and SVX binder electrodes are 44.9Ω, 22.3Ω, 14.1Ω and 5.4Ω, respectively, and the charge transfer impedances are 174.9Ω, 133.6Ω, 127.7Ω and 107.1Ω, respectively. The impedances of the SVX binder electrode are all smaller than those of the electrodes prepared with the other three binders, indicating that the ternary binder electrode has faster electron and ion transmission during the cycle and has better kinetic performance.

[0092] In order to better explore the universality of the ternary binder (SVX) in high-load silicon anodes, four different binders, SA, SV, SX and SVX, were prepared with a loading of 3.0 mg / cm 2 The electrode sheets were assembled into button cells for electrochemical performance testing. The initial charge and discharge specific capacity and initial coulombic efficiency are shown in Table 3 and Figure 9 As shown in Table 3, at a current density of 300 mA / g, the initial coulombic efficiency of the silicon anode materials prepared with binary (SV, SX) and ternary binders (SVX) is superior to that of the electrode prepared with the original SA binder. The capacity retention after 100 cycles is also much higher than that of the SA electrode, with the ternary binder (SVX) electrode showing the best cycling stability, with a capacity retention of 61.78%.

[0093] Figure 9 The cycling performance test results of electrodes prepared with four different binders. As can be seen from the figure, at a current density of 300mA / g, the first reversible specific capacity of the silicon negative electrode material prepared with the ternary binder is 2041.57mAh / g. After 100 cycles, it still has a specific capacity of 1261.36mAh / g, while the silicon negative electrode materials prepared with SA, SV and SX binders retain reversible specific capacities of 381.73mAh / g, 743.52mAh / g and 1012.62mAh / g, respectively. This shows that the SA binder cannot withstand the volume effect of silicon tin under high load, resulting in faster capacity decay. When the ternary binder (SVX) is applied to a high-load silicon negative electrode, it has better adhesion, and the constructed three-dimensional network structure can maintain the structural stability of the silicon tin particles and adapt to the volume stress generated by the silicon tin particles during the cycle.

[0094] Table 3 Performance comparison of adhesives

[0095]

[0096]

Claims

1. A silicon-based thick electrode binder, characterized in that: include: Sodium alginate, polyvinyl alcohol with a relative molecular mass of 31,000 to 50,000, xanthan gum and water; The mass ratio of the sodium alginate, the polyvinyl alcohol with a relative molecular mass of 31000-50000 and the xanthan gum is (5-10): (1-5): (1-5).

2. The silicon-based thick electrode binder according to claim 1, wherein The solid content of the silicon-based thick electrode binder is 1 to 5 wt %.

3. The silicon-based thick electrode binder according to claim 1 or 2, characterized in that: Based on the total weight of the silicon-based thick electrode binder, the binder further comprises 0.1-1 wt% of component A, wherein the component A is at least one of polyacrylic acid, sodium carboxymethyl cellulose, guar gum, gelatin, konjac gum, carrageenan, and polyethyleneimine.

4. A method for preparing a silicon-based thick electrode binder according to any one of claims 1 to 3, characterized in that: Sodium alginate, polyvinyl alcohol with a relative molecular weight of 31000 to 50000, xanthan gum and hydrothermal cross-linking are used to obtain a silicon-based thick electrode binder.

5. The preparation method according to claim 4, wherein The thermal cross-linking method is as follows: sodium alginate, polyvinyl alcohol with a relative molecular mass of 31,000 to 50,000, xanthan gum and water are mixed and then heated to 60 to 110° C.

6. The preparation method according to claim 4, wherein The thermal cross-linking method is as follows: polyvinyl alcohol with a relative molecular mass of 31,000 to 50,000 and water are dissolved at 60 to 110° C., and then sodium alginate and xanthan gum are added.

7. A silicon negative electrode sheet, characterized in that: It comprises a silicon-based negative electrode material, a conductive agent and the silicon-based thick electrode binder according to any one of claims 1 to 3 or the silicon-based thick electrode binder prepared by the preparation method according to any one of claims 4 to 6; Preferably, the conductive agent is one or more of conductive graphite, carbon nanotubes and carbon nanofibers.

8. The silicon negative electrode sheet according to claim 7, wherein: The mass ratio of the silicon-based negative electrode material, the conductive agent and the silicon-based thick electrode binder is (60-80): (10-20): (10-20).

9. A method for preparing a silicon negative electrode sheet according to claim 7 or 8, characterized in that: After the silicon-based negative electrode material, conductive agent and silicon-based thick electrode binder are mixed, they are coated on the electrode sheet substrate and dried to obtain a silicon negative electrode sheet; Preferably, the electrode sheet substrate is copper foil; Preferably, the loading amount of the silicon negative electrode sheet is 2.0-3.0 mg / cm 2 .

10. A lithium ion battery, characterized in that: Use the silicon negative electrode sheet as described in claim 7 or 8 or the silicon negative electrode sheet prepared by the preparation method as described in claim 9.

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