Preparation method of silicon-based negative electrode plate, silicon-based negative electrode plate and application of silicon-based negative electrode plate

Abstract

The invention provides a preparation method of a silicon-based negative electrode plate, the silicon-based negative electrode plate and application of the silicon-based negative electrode plate, and the preparation method comprises the following steps: S1, preparing negative electrode slurry; the negative electrode slurry comprises negative electrode main material particles, and the negative electrode main material particles comprise first silicon-based main material particles, second silicon-based main material particles and third graphite main material particles; d50 of the first silicon-based main material particles is 3-8 [mu] m; the D50 of the second silicon-based main material particles is 7-12 [mu] m; the D50 of the third carbon-based main material particles is 13-16 [mu] m; s2, coating a negative electrode current collector with the negative electrode slurry to obtain a wet pole piece blank, and performing forced air drying on the wet pole piece blank to obtain a silicon-based negative pole piece; after the wet pole piece blank is subjected to forced air drying, the silicon-based negative pole piece is obtained without a rolling process; in the forced air drying process, the drying temperature is 40-90 DEG C, and the air blowing speed is 10-30 Hz. The silicon-based negative electrode plate disclosed by the invention does not need to be subjected to a rolling process, and the battery prepared by utilizing the silicon-based negative electrode plate still has relatively high cycling stability, energy density and fast charging performance.

Description

Technical Field

[0001] The present application belongs to the technical field of lithium-ion batteries, and specifically relates to a method for preparing a silicon-based negative electrode sheet, a silicon-based negative electrode sheet and applications thereof. Background Art

[0002] The main solution to the development of high energy density batteries lies in the selection of active materials in the battery system, that is, positive and negative electrode materials. At present, the most widely used secondary battery negative electrode material in commercial use is graphite negative electrode material, whose theoretical gram capacity is only 372mAh / g. The existing technology is extremely close to its theoretical gram capacity, so it is urgent to develop negative electrode materials with higher gram capacity. The positive electrode material in high energy density batteries is represented by high nickel ternary, while the main development trend of negative electrode materials is silicon-based negative electrode materials. Silicon-based negative electrode materials have extremely high gram capacity, with a theoretical specific capacity of 4200mAh / g, which is more than 10 times that of graphite negative electrode materials (372mAh / g), and have a lower lithium deintercalation potential. It is considered to be one of the most promising negative electrode materials in the next generation of lithium-ion batteries. However, silicon has severe lattice expansion in the process of lithium deintercalation, and the volume expansion rate is greater than 300%, which can easily cause cracking of active materials, and even lead to loss of electrical contact between silicon and current collector, repeated damage and repair of solid electrolyte phase (SEI), etc., which causes irreversible consumption of limited lithium ions and serious attenuation of cycle life. In addition, silicon-based negative electrode materials also have the problem of low intrinsic conductivity, which restricts the fast charging performance of silicon-based negative electrode system batteries.

[0003] At present, many methods have been published to solve the expansion problem of silicon-based negative electrodes, mainly through structural regulation to improve the expansion problem of silicon-based negative electrodes, including porous structure, yolk structure, core-shell structure, and coating layer control. In terms of material structure, these modification methods, through the design of silicon-based material structure, reserve space inside to alleviate the expansion of silicon materials, and effectively improve the inherent problems of silicon-based negative electrodes. However, in terms of the structure of the negative electrode of the battery cell, there are few methods to alleviate the expansion of silicon negative electrodes.

[0004] For example, vapor-deposited silicon carbon is one of the important means of mass-producing low-expansion silicon-based materials in the current market. The specific method is to deposit nano-silicon on a porous hard carbon matrix to synthesize a silicon-carbon composite material. Part of the pore structure is reserved inside the material to alleviate the expansion, which can reduce the expansion of silicon materials by at least 20%. However, due to the design of the hollow structure inside the material particles, the pressure resistance of silicon-carbon materials is significantly reduced. The silicon-carbon materials currently prepared, especially those prepared by vapor deposition, all need to go through a rolling process to optimize the negative electrode surface quality, energy density, bonding strength between materials, battery performance, etc. This makes it easy for silicon-carbon particles to break during the rolling process, reducing their particle integrity. After the subsequent assembly of the battery cell, the electrolyte may penetrate into the silicon-carbon particles, increasing the active sites of the electrolyte, causing more side reactions and capacity decay. Summary of the invention

[0005] In order to solve the problems and deficiencies in the prior art, the present application provides a method for preparing a silicon-based negative electrode sheet, a silicon-based negative electrode sheet and its application. The silicon-based negative electrode sheet prepared by the preparation method provided in the present application has a high porosity and a low expansion rate of the negative electrode sheet. Therefore, the volume expansion effect of silicon particles in the silicon-based negative electrode sheet during the charge and discharge cycle can be well alleviated, thereby effectively improving the cycle stability of the battery. At the same time, the silicon-based negative electrode sheet in the present application does not need to go through a rolling process, and the battery prepared using the same still has high cycle stability, energy density and fast charging performance.

[0006] According to the first aspect of the present application, a method for preparing a silicon-based negative electrode sheet is provided, comprising the following steps: S1. preparing a negative electrode slurry; the negative electrode slurry comprises negative electrode main material particles, and the negative electrode main material particles comprise first silicon-based main material particles, second silicon-based main material particles, and third graphite main material particles; the D50 of the first silicon-based main material particles is 3-8 μm; the D50 of the second silicon-based main material particles is 7-12 μm; the D50 of the third carbon-based main material particles is 13-16 μm; S2. coating the negative electrode slurry on the negative electrode collector to obtain a wet blank of the electrode sheet, and subjecting the wet blank of the electrode sheet to air drying to obtain a silicon-based negative electrode sheet; after the wet blank of the electrode sheet is air dried, a silicon-based negative electrode sheet is obtained without undergoing a rolling process; during the air drying process, the drying temperature is 40-90°C, and the air speed is 10-30 Hz.

[0007] In the current preparation of silicon-based negative electrode sheets, the rolling process is required to be consistent with the conventional negative electrode sheet preparation process. It is generally believed that the rolling process is of vital necessity in the production process of lithium-ion batteries. The main functions of the rolling process are as follows: (1) It can reduce the pores between the negative electrode material particles and arrange the materials more closely to improve the volume energy density of the negative electrode sheet, reduce the contact resistance and improve the structural stability of the negative electrode sheet; (2) Provide uniform thickness of the negative electrode sheet, so that the negative electrode sheet has good flatness and flexibility, and is easier to carry out subsequent processing steps such as cutting and welding; (3) Enhance the adhesion of the negative electrode sheet and reduce active shedding and damage to the electrode structure.

[0008] However, the rolling process can easily destroy the crystal structure of the negative electrode material, especially the silicon-carbon material in the silicon-based negative electrode sheet. The material particles are hollow in structure. The rolling process can easily destroy the integrity of the particles, causing them to break, etc., which affects the cycle stability and increases the active sites of the electrolyte, causing more side reactions and capacity attenuation. Moreover, after rolling, the porosity of the negative electrode sheet will become smaller, especially for the silicon-based negative electrode sheet, which has a significant volume expansion effect during the cycle. Too small a porosity is not conducive to alleviating the volume expansion of the silicon-based negative electrode sheet. In addition, the rolling process can easily cause surface defects such as wrinkles and scratches on the negative electrode sheet, or poor contact between the active material and the current collector, which will increase the internal resistance of the electrode. At the same time, the density of the negative electrode sheet after rolling is too high, which may narrow or block the diffusion channel of lithium ions inside the electrode, resulting in a decrease in the diffusion rate of lithium ions and affecting the rate performance of the battery.

[0009] Therefore, this application is different from the traditional practice in the past and proposes a new design idea for the preparation of silicon-based negative electrode sheets. Specifically, this application selects at least three or more negative electrode main material particles with different D50, two of which are silicon-based main material particles and one is carbon-based main material particles, and then uses specific blast drying conditions to enable the electrode wet blank to obtain a directly usable silicon-based negative electrode sheet after only a drying process, that is, a directly usable silicon-based negative electrode sheet can be obtained without a rolling process.

[0010] This is because, by selecting three specific D50 ranges of silicon-based main material particles and carbon-based main material particles, the present application can effectively improve the packing density and reduce the porosity. For example, particles with smaller D50 can be filled into the gaps between larger D50 particles, so that the mixed particle system is more densely packed, thereby improving the overall packing density, and thus improving the energy density, cycle performance and rate performance of the battery. And the mixing of three specific D50 ranges of negative electrode main material particles can be combined with the subsequent specific drying process, that is, a slower solvent mass transfer process (lower drying temperature and lower blast drying wind speed), which is conducive to a tighter combination of electrode structures and full contact of electrode components; and the mixed particles are nested and supported with each other to form a more stable structure, thereby improving the bonding force and overall mechanical properties of the silicon-based negative electrode sheet; at the same time, such a slow solvent mass transfer process is conducive to regulating the pore structure of the coating surface, making the coating surface smooth and uniform in thickness. Therefore, the above-mentioned specific design idea enables the wet blank of the electrode sheet to be used directly after only the drying process, and has good energy density, bonding performance and cycle stability without the need for a rolling process.

[0011] Moreover, for silicon-based negative electrode sheets, because they contain silicon particles, the volume expansion effect of silicon-based negative electrode sheets is large, and the structural stability of silicon-based materials is also poor. In this application, the process without rolling is relatively equivalent to leaving more pores, that is, having a higher porosity, which can effectively alleviate the volume expansion effect of silicon-based materials during the cycle, effectively enhance the structural stability of silicon-based materials, and further optimize its cycle stability and rate performance. In addition, without the rolling process, the particles of silicon-based materials are more complete, less likely to break during the cycle, and can also reduce the active sites of the electrolyte, reduce adverse side reactions and capacity decay.

[0012] Preferably, in S2, the running speed of the electrode wet blank is 0.5-3 m / min. Controlling the running speed of the electrode wet blank within the above range is more conducive to a more appropriate solvent mass transfer process during the drying process, which is more conducive to improving the uniform distribution of the electrode negative electrode main material particles and other components, and is also more conducive to forming a tighter and higher strength structure, so as to further optimize the electrode bonding force, energy density, and cycle performance.

[0013] Preferably, in S2, during the air drying process, the drying temperature is 40-60°C.

[0014] Preferably, the first silicon-based main material particles and the second silicon-based main material particles independently include at least one of silicon-carbon / graphite mixed main material particles, silicon-carbon composite main material particles, silicon-oxygen composite main material particles, silicon-oxygen / silicon-carbon mixed main material particles, and silicon-oxygen / graphite composite main material particles; the third carbon-based main material particles include at least one of graphite, hard carbon, soft carbon, intermediate phase carbon microspheres, and graphene.

[0015] Preferably, the first silicon-based main material particles and the second silicon-based main material particles independently include at least one of silicon-oxygen / silicon-carbon composite main material particles and silicon-carbon / graphite composite main material particles.

[0016] Preferably, the third carbon-based main material particles include at least one of graphite and mesophase carbon microspheres.

[0017] Preferably, the first silicon-based main material particles include silicon-oxygen / silicon-carbon mixed main material particles, the second silicon-based main material particles include silicon-carbon / graphite mixed main material particles; and the third carbon-based main material particles include mesophase carbon microspheres.

[0018] Preferably, the mass ratio of the first silicon-based main material particles, the second silicon-based main material particles, and the third carbon-based main material particles is 1-5:5-9:0.5-30. Furthermore, controlling the mass ratio of the three negative electrode main material particles within the above range is more conducive to obtaining a pole piece that has a rich internal pore structure and high adhesion and mechanical strength. Therefore, it can further alleviate the volume expansion of the silicon-based negative electrode material, optimize the lithium ion transmission performance, and improve the structural stability of the pole piece, thereby further optimizing the relevant performance of the battery. Preferably, in the negative electrode slurry, the mass ratio of the negative electrode main material particles, the conductive agent, and the binder is 80-98:2-10:3-15.

[0019] Preferably, the conductive agent includes at least one of conductive carbon black, multi-walled carbon nanotubes, single-walled carbon nanotubes, and vapor-grown carbon fibers; the binder includes at least one of sodium hydroxymethyl cellulose, styrene-butadiene rubber, acrylic polymers, acrylonitrile copolymers, and sodium alginate.

[0020] According to a second aspect of the present application, a silicon-based negative electrode sheet is provided, which is prepared by the above-mentioned method for preparing the silicon-based negative electrode sheet.

[0021] Preferably, the porosity of the silicon-based negative electrode sheet is 30-70%. Controlling the porosity of the silicon-based negative electrode sheet within the above range can not only ensure that there is enough space inside the silicon-based negative electrode sheet to alleviate the volume expansion of the silicon-based negative electrode material, but also avoid excessive porosity affecting the overall mechanical strength and bonding performance of the silicon-based negative electrode sheet, so that the silicon-based negative electrode sheet has good comprehensive performance, which is conducive to better performance of its electrochemical performance. At the same time, relatively speaking, because the silicon-based negative electrode sheet provided in the present application does not undergo a rolling process, the porosity will be higher than that of the silicon-based negative electrode sheet that usually undergoes a rolling process. Therefore, the internal pore structure of the silicon-based negative electrode sheet provided in the present application is richer, enriching the contact area between the electrolyte and the active material, and providing contact sites for the transmission of lithium ions at the liquid-solid interface, so that the battery has good ionic conductivity, which is conducive to improving the fast charging performance of the battery.

[0022] According to the third aspect of the present application, a battery is provided, comprising the silicon-based negative electrode sheet prepared by the above-mentioned method for preparing the silicon-based negative electrode sheet or the above-mentioned silicon-based negative electrode sheet. The battery prepared using the above-mentioned silicon-based negative electrode sheet has high energy density, cycle stability and rate performance.

[0023] Preferably, the above-mentioned battery includes a positive electrode sheet, the positive electrode sheet includes positive electrode main material particles, and the positive electrode main material particles include at least one of ternary nickel cobalt lithium manganese oxide main material particles, ternary nickel cobalt aluminum oxide main material particles, lithium cobalt oxide main material particles, lithium manganese oxide main material particles, lithium iron phosphate main material particles, and lithium-rich manganese-based main material particles.

[0024] Preferably, the positive electrode main material particles include ternary nickel cobalt lithium manganese oxide main material particles.

[0025] In summary, first of all, the method for preparing the silicon-based negative electrode sheet provided in the present application adopts three kinds of negative electrode main material particles of specific D50, and controls the slow solvent mass transfer volatilization process during the drying process, so that it is different from the traditional electrode sheet manufacturing process, and there is no need to roll the coated electrode sheet before the subsequent electrode sheet manufacturing process can be carried out, which can effectively improve the efficiency of electrode sheet manufacturing and save labor and time costs. Secondly, the silicon-based negative electrode sheet prepared by the method of the present application has a rich pore structure inside the electrode sheet, which is conducive to alleviating the volume expansion of the silicon-based negative electrode material and improving the safety of the battery cell. In addition, the silicon-based negative electrode sheet synthesized by the method of the present application has a porous structure of the electrode sheet coating with a rich pore structure, which enriches the contact area between the electrolyte and the active substance, provides contact sites for the transmission of lithium ions at the liquid-solid interface, and makes the battery have good ionic conductivity, which is conducive to improving the fast charging performance of the battery cell. DETAILED DESCRIPTION

[0026] In order to enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below. Obviously, the described embodiments are only embodiments of a part of the present application, rather than all of the embodiments.

[0027] Example 1

[0028] 1. Preparation of silicon-based negative electrode sheets

[0029] The silicon-based negative electrode sheet of this embodiment is prepared according to the following steps:

[0030] S1. Prepare negative electrode slurry; use water as solvent, mix negative electrode main material particles, conductive agent SP, and binder PAA in a mass ratio of 95:2.5:2.5 to obtain negative electrode slurry, and control the solid content of the negative electrode slurry between 38% and 42%; the negative electrode active slurry includes negative electrode main material particles, and the negative electrode main material particles include first silicon-based main material particles, second silicon-based main material particles, and third graphite main material particles; the D50 of the first silicon-based main material particles is 5μm; the D50 of the second silicon-based main material particles is 9μm; the D50 of the third carbon-based main material particles is 14μm; the first silicon-based main material particles are silicon-oxygen / silicon-carbon mixed main material particles, the second silicon-based main material particles are silicon-carbon / graphite mixed main material particles, and the third carbon-based main material particles are graphite; the mass ratio of the first silicon-based main material particles, the second silicon-based main material particles, and the third carbon-based main material particles is 3:7:15;

[0031] S2. Coat the negative electrode slurry on the negative electrode current collector copper foil to obtain a wet electrode blank, and place the wet electrode blank in a forced air drying oven for forced air drying. The drying temperature is 50°C and the forced air speed is 20 Hz. After drying, a silicon-based negative electrode sheet is obtained without the need for a rolling process.

[0032] 2. Preparation of batteries

[0033] Preparation of positive electrode sheet: NMP is used as solvent, and the ternary material NCM811, conductive agent SP, and binder PVDF are mixed evenly in a mass ratio of 97:1.2:1.8, coated on aluminum foil, dried, and rolled to obtain a positive electrode sheet.

[0034] The above positive electrode sheet, separator and negative electrode sheet are assembled into a soft-pack 396389 battery, and the electrolyte is 1 mol / L LiPF 6 / EC+DMC+EMC, the volume ratio is 1:1:1, and the diaphragm is Celgard2400 diaphragm.

[0035] Example 2

[0036] 1. Preparation of silicon-based negative electrode sheets

[0037] The preparation of the silicon-based negative electrode sheet in this embodiment is different from that in Embodiment 1 in that, in S2, the running speed of the wet blank of the electrode sheet is 4 m / min. The rest of the operations are the same as those in Embodiment 1.

[0038] 2. Preparation of batteries

[0039] The preparation of the battery in this example is consistent with that in Example 1.

[0040] Example 3

[0041] 1. Preparation of silicon-based negative electrode sheets

[0042] The preparation of the silicon-based negative electrode sheet in this embodiment is different from that in Embodiment 1 in that, in S2, the running speed of the wet blank of the electrode sheet is 0.3 m / min. The rest of the operations are the same as those in Embodiment 1.

[0043] 2. Preparation of batteries

[0044] The preparation of the battery in this example is consistent with that in Example 1.

[0045] Example 4

[0046] 1. Preparation of silicon-based negative electrode sheets

[0047] The preparation of the silicon-based negative electrode sheet in this embodiment is different from that in Embodiment 1 in that, in S1, the third carbon-based main material particles are adjusted to mesophase carbon microspheres. The remaining operations are the same as those in Embodiment 1.

[0048] 2. Preparation of batteries

[0049] The preparation of the battery in this example is consistent with that in Example 1.

[0050] Example 5

[0051] 1. Preparation of silicon-based negative electrode sheets

[0052] The preparation of the silicon-based negative electrode sheet in this embodiment is different from that in Embodiment 1 in that, in S1, the first silicon-based main material particles are adjusted to silicon-oxygen mixed main material particles, and the third carbon-based main material particles are adjusted to hard carbon. The rest of the operations are the same as in Embodiment 1.

[0053] 2. Preparation of batteries

[0054] The preparation of the battery in this example is consistent with that in Example 1.

[0055] Example 6

[0056] 1. Preparation of silicon-based negative electrode sheets

[0057] The preparation of the silicon-based negative electrode sheet in this embodiment is different from that in Embodiment 1 in that, in S1, the first silicon-based main material particles are adjusted to silicon-oxygen mixed main material particles, and the third carbon-based main material particles are adjusted to soft carbon. The rest of the operations are the same as in Embodiment 1.

[0058] 2. Preparation of batteries

[0059] The preparation of the battery in this example is consistent with that in Example 1.

[0060] Example 7

[0061] 1. Preparation of silicon-based negative electrode sheets

[0062] The preparation of the silicon-based negative electrode sheet in this embodiment is different from that in Embodiment 1 in that, in S1, the first silicon-based main material particles are adjusted to silicon-oxygen mixed main material particles, the second silicon-based main material particles are adjusted to silicon-carbon mixed main material particles, and the third carbon-based main material particles are adjusted to graphene. The remaining operations are the same as in Embodiment 1.

[0063] 2. Preparation of batteries

[0064] The preparation of the battery in this example is consistent with that in Example 1.

[0065] Example 8

[0066] 1. Preparation of silicon-based negative electrode sheets

[0067] The preparation of the silicon-based negative electrode sheet in this embodiment is different from that in Embodiment 1 in that, in S1, the mass ratio of the first silicon-based main material particles to the second silicon-based main material particles is adjusted to 0.5:9.5. The remaining operations are the same as those in Embodiment 1.

[0068] 2. Preparation of batteries

[0069] The preparation of the battery in this example is consistent with that in Example 1.

[0070] Example 9

[0071] 1. Preparation of silicon-based negative electrode sheets

[0072] The preparation of the silicon-based negative electrode sheet in this embodiment is different from that in Embodiment 1 in that in S1, the mass ratio of the first silicon-based main material particles, the second silicon-based main material particles, and the third carbon-based main material particles is adjusted to 3:7:40. The remaining operations are the same as those in Embodiment 1.

[0073] 2. Preparation of batteries

[0074] The preparation of the battery in this example is consistent with that in Example 1.

[0075] Comparative Example 1

[0076] 1. Preparation of silicon-based negative electrode sheets

[0077] The preparation of the silicon-based negative electrode sheet in this comparative example is different from that in Example 1 in that in S1, the D50 of the second silicon-based main material particle is 4.5 μm. The rest of the operations are the same as in Example 1.

[0078] 2. Preparation of batteries

[0079] The preparation of the battery in this comparative example is consistent with that in Example 1.

[0080] Comparative Example 2

[0081] 1. Preparation of silicon-based negative electrode sheets

[0082] The preparation of the silicon-based negative electrode sheet in this comparative example is different from that in Example 1 in that, in S1, the D50 of the second silicon-based main material particles is adjusted to 15 μm. The remaining operations are the same as those in Example 1.

[0083] 2. Preparation of batteries

[0084] The preparation of the battery in this comparative example is consistent with that in Example 1.

[0085] Comparative Example 3

[0086] 1. Preparation of silicon-based negative electrode sheets

[0087] The preparation of the silicon-based negative electrode sheet in this comparative example is different from that in Example 1 in that in S1, the D50 of the first silicon-based main material particles is adjusted to 9.5 μm, and the D50 of the third carbon-based main material particles is adjusted to 10 μm. The rest of the operations are the same as in Example 1.

[0088] 2. Preparation of batteries

[0089] The preparation of the battery in this comparative example is consistent with that in Example 1.

[0090] Comparative Example 4

[0091] 1. Preparation of silicon-based negative electrode sheets

[0092] The preparation of the silicon-based negative electrode sheet in this comparative example is different from that in Example 1 in that, in S2, the temperature of the forced air drying is adjusted to 95° C. The remaining operations are the same as those in Example 1.

[0093] 2. Preparation of batteries

[0094] The preparation of the battery in this comparative example is consistent with that in Example 1.

[0095] Comparative Example 5

[0096] 1. Preparation of silicon-based negative electrode sheets

[0097] The preparation of the silicon-based negative electrode sheet in this comparative example is different from that in Example 1 in that the wind speed of the blast drying is adjusted to 5 Hz in S2. The remaining operations are the same as those in Example 1.

[0098] 2. Preparation of batteries

[0099] The preparation of the battery in this comparative example is consistent with that in Example 1.

[0100] Comparative Example 6

[0101] 1. Preparation of silicon-based negative electrode sheets

[0102] The silicon-based negative electrode sheet of this embodiment is prepared according to the following steps:

[0103] S1. Prepare negative electrode slurry; using water as solvent, mix the negative electrode main material particles, conductive agent SP, and binder PAA in a mass ratio of 95:2.5:2.5 to obtain negative electrode slurry, and control the solid content of the negative electrode slurry between 38 and 42%; the negative electrode active slurry includes negative electrode main material particles, the negative electrode main material particles include silicon oxygen main material particles and graphite main material particles, the mass ratio of silicon oxygen main material particles and graphite main material particles is 2:8, and the D50 of silicon oxygen main material particles and graphite main material particles are 7μm and 14μm respectively;

[0104] S2. The negative electrode slurry was coated on the negative electrode current collector copper foil to obtain a wet electrode blank. The wet electrode blank was placed in a blast drying oven for blast drying at a drying temperature of 80°C and a blast wind speed of 40Hz. After drying, the blast was rolled and the compaction density was 1.1g / cm 3 , and obtain a silicon-based negative electrode sheet.

[0105] 2. Preparation of batteries

[0106] The preparation of the battery in this example is consistent with that in Example 1.

[0107] Test Case

[0108] 1. Experimental Construction Method

[0109] (1) Porosity test

[0110] The porosity of the silicon-based negative electrode sheets prepared in all the above embodiments and comparative examples was tested. The specific testing method is as follows: the porosity of the electrode sheet is tested by mercury intrusion method, and the total volume of mercury before mercury intrusion is recorded. m Then, after applying a certain pressure, the total volume of mercury after immersion, V, is recorded. s The porosity can be calculated by the following formula: Porosity = (V m -V s ) / V m (The volume here deducts the volume of the current collector).

[0111] (2) Expansion rate test

[0112] The expansion rate of the silicon-based negative electrode sheets prepared in all the above embodiments and comparative examples was tested, and the expansion rate of the silicon-based negative electrode sheets is the expansion rate of the thickness of the fully charged silicon-based negative electrode sheets after rolling. The specific testing method is as follows: the thickness of the electrode sheet before the battery cell is injected with liquid is recorded as L1, and after the formation and capacity division processes, the thickness of the electrode sheet when fully charged and disassembled is L2, and the electrode sheet expansion rate = (L2-L1) / L1.

[0113] (3) Peel force test

[0114] The peeling force of the silicon-based negative electrode sheets prepared in all the above embodiments and comparative examples was tested. The specific test method is as follows: a tensile tester is used to peel the electrode sheet 180° at a peeling speed of 100 to 300 mm / min, and the electrode sheet peeling force is tested and recorded.

[0115] (4) Energy density test

[0116] The energy density test was carried out on the batteries prepared in all the above embodiments and comparative examples, and the specific test method was as follows: a) the battery was discharged at a constant current of 0.5C to 2.5V; b) it was allowed to stand for 5 minutes; c) the battery was charged at a constant current and constant voltage of 0.5C, with a cut-off voltage of 4.2V and a cut-off current of 0.05C; d) it was allowed to stand for 5 minutes; e) the battery was discharged at a constant current of 0.5C to 2.5V; f) steps b) to e) were repeated 3 times; the energy density was calculated based on the last discharge capacity. The energy density calculation formula is: energy density = (capacity * voltage) / mass (here is mass energy density).

[0117] (5) Capacity retention test at room temperature (25±2℃) 1C / 1C cycle for 1000 cycles

[0118] The batteries prepared in all the above embodiments and comparative examples were tested for capacity retention rate of 1C / 1C cycle for 1000 weeks at room temperature. The specific testing method is as follows: a charge and discharge test cabinet was used for battery testing, 1C constant current and constant voltage charging, a cut-off current of 0.33C, standing for 10 minutes, and then discharged at 1C to 2.5V, which was considered a charge and discharge cycle. The capacity ratio of the battery at 1C discharge (4.2-2.5V) to the 1000th week to the capacity ratio of the first week was the capacity retention rate of the battery cell for 1000 cycles.

[0119] (6) 8C charging constant current charging ratio test

[0120] The batteries prepared in all the above embodiments and comparative examples were subjected to an 8C charging constant current charging ratio test. The specific testing method is as follows: a charge and discharge test cabinet is used to test the battery, the battery is discharged to 2.5V at 1C, and charged to 4.2V at 8C constant current. The ratio of the constant current charging capacity to the total constant current and constant voltage charging capacity is the 8C charging constant current charging ratio.

[0121] 2. Experimental results

[0122] The test results of the silicon-based negative electrode sheets and battery-related performances prepared in all the above embodiments and comparative examples are shown in Table 1.

[0123] Table 1 Test results of silicon-based negative electrode sheets and battery performance prepared in the examples and comparative examples

[0124]

[0125]

[0126] As shown in Table 1, the silicon-based negative electrode sheet prepared by the preparation method provided by the present application has a high porosity, a low expansion rate, and a high peeling force. At the same time, the battery prepared by the method also has a high energy density, room temperature cycle performance, and high rate performance (8C). Moreover, in the process of preparing the silicon-based negative electrode sheet of the present application, there is no need to go through a rolling process, which not only simplifies the process and reduces the production cost, but also further optimizes the performance of the silicon-based negative electrode sheet and the battery. For details, refer to Examples 1 to 9.

[0127] In Comparative Example 1, the D50 of the second silicon-based main material particle is too small, in Comparative Example 2, the D50 of the first silicon-based main material particle is too large, and in Comparative Example 3, the D50 of the first silicon-based main material particle is too large and the D50 of the third carbon-based main material particle is too small; all of the above factors cause the decrease in the normal temperature cycle capacity retention rate and the 8C charging constant current charging ratio in Comparative Examples 1 to 3, and the negative electrode sheet expansion rate of Comparative Example 1 is relatively high, and the negative electrode sheet peeling force (adhesion force) of Comparative Example 2 is relatively low. This shows that it is necessary to control the particle size of the first silicon-based main material particle, the second silicon-based main material particle, and the third carbon-based main material particle separately, so that the mixed particle system can be stacked more tightly, thereby improving the overall stacking density, and at the same time making the synergistic effect of the three particles better, thereby optimizing the energy density, cycle performance and rate performance of the battery. Moreover, keeping the three particles within a specific D50 range is more conducive to making the particle distribution in the negative electrode sheet more uniform, and is more conducive to alleviating the volume expansion of silicon in the negative electrode sheet, which is conducive to further improving the stability of the silicon-based negative electrode sheet and optimizing the battery performance.

[0128] In Comparative Example 4, the drying temperature in S2 is too high; in Comparative Example 5, the blast wind speed in S2 is too low; Comparative Example 6 is a conventional negative electrode sheet preparation process, and it needs to undergo a roller pressing process. The change in the above process conditions has resulted in a decrease in the room temperature cycle capacity retention rate and / or the 8C charging constant current charging ratio in Comparative Examples 4 to 6. In addition, the negative electrode sheet expansion rate in Comparative Example 4 is high, the negative electrode sheet peeling force is low, and the battery energy density in Comparative Example 6 is low. These all illustrate that, first, the process conditions in the drying process need to be controlled within a specific range in this application, so that the negative electrode sheet can form a more stable structure during the gradual drying process, and it has a higher peeling force (adhesive force), so that it can obtain a battery with better performance in terms of energy density, room temperature cycle performance, and high rate performance without roller pressing; second, controlling the process conditions in the drying process within a specific range is conducive to forming more uniform pores, better relieving the volume expansion of silicon-based materials, and improving the result stability of silicon-based negative electrode sheets, and further optimizing the performance of the battery.

[0129] Further comparing Example 1 with Examples 2-3, the speed of the wet electrode blanks in Examples 2 and 3 is too slow and too fast respectively, and the performance of the silicon-based negative electrode sheets and batteries in Examples 2 and 3 are worse than that in Example 1. This shows that controlling the speed of the wet electrode blanks (or other process condition parameters) within a specific range is more conducive to forming more uniform pores during the drying process and making the stacking of each particle more uniform and compact, which is more conducive to obtaining silicon-based negative electrode sheets and batteries with better performance.

[0130] Comparing Examples 1 to 4 to 7, it can be seen that when the types of the first silicon-based main material particles, and / or the second silicon-based main material particles, and / or the third carbon-based main material particles are changed, the performance of the silicon-based negative electrode sheet and / or the battery will decrease to a certain extent, which shows that the selection of specific materials for the three particles is more conducive to obtaining silicon-based negative electrode sheets and batteries with better performance. At the same time, it can also be seen that when the first silicon-based main material particles are silicon-oxygen / silicon-carbon mixed main material particles, the second silicon-based main material particles are silicon-carbon / graphite mixed main material particles, and the third carbon-based main material particles are graphite, such a combination is more conducive to obtaining silicon-based negative electrode sheets and batteries with better performance.

[0131] Comparing Example 1 with Examples 8 to 9, the mass ratio of the first silicon-based main material particles, the second silicon-based main material particles, and the third carbon-based main material particles in Examples 8 and 9 is not in the range of 1-5:5-9:0.5-30, resulting in a decrease in the performance of the silicon-based negative electrode sheets and batteries in Examples 8 and 9. This also shows that it is necessary to control the mass ratio of the above three particles within a specific range to obtain a negative electrode sheet with better performance, thereby further optimizing the performance of the battery.

[0132] The above embodiments are only used to illustrate the technical solution of the present application rather than to limit the protection scope of the present application. Although the present application is described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the technical solution of the present application can be modified or equivalently replaced, but these modifications or replacements are all within the protection scope of the present application.

Claims

1. A method for preparing a silicon-based negative electrode sheet, characterized in that: The steps include: S1. Prepare negative electrode slurry; the negative electrode slurry includes negative electrode main material particles, the negative electrode main material particles include first silicon-based main material particles, second silicon-based main material particles, and third graphite main material particles; the D50 of the first silicon-based main material particles is 3 to 8 μm; the D50 of the second silicon-based main material particles is 7 to 12 μm; the D50 of the third carbon-based main material particles is 13 to 16 μm; S2. The negative electrode slurry is coated on the negative electrode current collector to obtain a wet electrode blank, and the wet electrode blank is subjected to air drying to obtain a silicon-based negative electrode sheet; after the wet electrode blank is subjected to air drying, the silicon-based negative electrode sheet is obtained without undergoing a rolling process; during the air drying process, the drying temperature is 40 to 90°C, and the air speed is 10 to 30 Hz.

2. The method for preparing a silicon-based negative electrode sheet according to claim 1, characterized in that: In S2, the running speed of the wet electrode blank is 0.5-3 m / min.

3. The method for preparing a silicon-based negative electrode sheet according to claim 1, characterized in that: The first silicon-based main material particles and the second silicon-based main material particles independently include at least one of silicon-carbon / graphite mixed main material particles, silicon-carbon composite main material particles, silicon-oxygen composite main material particles, silicon-oxygen / silicon-carbon mixed main material particles, and silicon-oxygen / graphite mixed main material particles; The third carbon-based main material particles include at least one of graphite, hard carbon, soft carbon, mesophase carbon microspheres, and graphene.

4. The method for preparing a silicon-based negative electrode sheet according to claim 1, characterized in that: The mass ratio of the first silicon-based main material particles, the second silicon-based main material particles, and the third carbon-based main material particles is 1-5:5-9:0.5-30.

5. The method for preparing a silicon-based negative electrode sheet according to claim 1, characterized in that: In the negative electrode slurry, the mass ratio of the negative electrode main material particles, the conductive agent, and the binder is 80-98:2-10:3-15.

6. The method for preparing a silicon-based negative electrode sheet according to claim 5, characterized in that: The conductive agent includes at least one of conductive carbon black, multi-walled carbon nanotubes, single-walled carbon nanotubes, and vapor-grown carbon fibers; The binder includes at least one of sodium hydroxymethyl cellulose, styrene-butadiene rubber, acrylic polymer, acrylonitrile copolymer and sodium alginate.

7. A silicon-based negative electrode sheet, characterized in that: The silicon-based negative electrode sheet is prepared by the method for preparing the silicon-based negative electrode sheet according to any one of claims 1 to 6.

8. The silicon-based negative electrode sheet according to claim 7, characterized in that: The porosity of the silicon-based negative electrode sheet is 30-70%.

9. A battery, characterized in that: It includes the silicon-based negative electrode sheet prepared by the method for preparing the silicon-based negative electrode sheet as described in any one of claims 1 to 6 or the silicon-based negative electrode sheet as described in any one of claims 7 to 8.

10. The battery according to claim 9, characterized in that: It includes a positive electrode sheet, which includes positive electrode main material particles. The positive electrode main material particles include at least one of ternary nickel cobalt lithium manganese oxide main material particles, ternary nickel cobalt aluminum oxide main material particles, lithium cobalt oxide main material particles, lithium manganese oxide main material particles, lithium iron phosphate main material particles, and lithium-rich manganese-based main material particles.

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