A negative electrode sheet and a battery

Abstract

This invention belongs to the field of lithium-ion battery technology, specifically relating to a negative electrode sheet and a battery. The negative electrode sheet of this invention includes a negative electrode material, which includes silicon; the compressive strength of the negative electrode material is a MPa; the mass percentage of silicon in the negative electrode material is b%; the surface roughness of the negative electrode sheet is c μm; and a, b, and c satisfy the relationship shown in Equation I: 0.003 ≤ b × c / a ≤ 1.9 Equation I. Compared with the prior art, the negative electrode sheet provided by this invention has less expansion and higher structural stability, and the battery prepared from it can achieve both excellent cycle life and fast charging performance.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to a negative electrode and a battery. Background Technology

[0002] With the rapid development of new energy vehicles, large-scale energy storage, and portable electronic devices, higher demands are being placed on the energy density, cycle life, and cost of lithium-ion batteries. Developing electrode materials with high specific capacity is one of the key ways to improve battery energy density. In the field of anode materials, silicon (Si)-based materials stand out due to their extremely high theoretical specific capacity (up to 3579 mAh / g at room temperature, approximately 10 times that of traditional graphite anodes) and moderate delithiation potential (approximately 0.4V vs. Li / Li). + With its abundant natural reserves, it is considered one of the most promising next-generation anode materials.

[0003] However, despite the significant theoretical advantages of silicon-based materials, silicon undergoes volume expansion during charging and discharging, leading to the breakage of active particles, collapse of electrode structures, repeated rupture and regeneration of the SEI film, and exacerbation of side reactions with the electrolyte, resulting in a decrease in battery cycle life. This problem severely restricts the large-scale application of silicon-based batteries.

[0004] Therefore, providing a silicon-based battery with good cycle performance has become a problem that needs to be solved. Summary of the Invention

[0005] In view of this, the technical problem to be solved by the present invention is to provide a negative electrode and a battery. The silicon-based battery prepared by the negative electrode provided by the present invention has good cycle performance.

[0006] This invention provides a negative electrode sheet, the negative electrode sheet comprising a negative electrode material, the negative electrode material comprising silicon.

[0007] The crushing force of the negative electrode material is a MPa; the mass percentage of silicon in the negative electrode material is b%; and the surface roughness of the negative electrode sheet is c μm.

[0008] a, b, and c satisfy the relationship shown in equation I:

[0009] 0.003≤b×c / a≤1.9 Formula I.

[0010] The present invention also provides a battery comprising the aforementioned negative electrode sheet.

[0011] The present invention also provides an electronic device including the battery described above.

[0012] Compared with existing technologies, the negative electrode sheet provided by this invention has less expansion and higher structural stability, and the battery prepared by it can achieve both excellent cycle life and fast charging performance. Attached Figure Description

[0013] Figure 1 This is a schematic diagram showing the sampling points for testing the surface roughness of the negative electrode. Detailed Implementation

[0014] This invention provides a negative electrode sheet, the negative electrode sheet comprising a negative electrode material, the negative electrode material comprising silicon.

[0015] The crushing force of the negative electrode material is a MPa; the mass percentage of silicon in the negative electrode material is b%; and the surface roughness of the negative electrode sheet is c μm.

[0016] a, b, and c satisfy the relationship shown in equation I:

[0017] 0.003≤b×c / a≤1.9 Formula I.

[0018] This invention effectively improves the cycle performance and fast-charging performance of a battery by controlling the relationship between the mass percentage of silicon in the negative electrode material, the crushing force of the negative electrode material, and the surface roughness of the negative electrode sheet, which satisfies the relationship shown in Equation I. Specifically, as the mass percentage of silicon in the negative electrode material increases, the volume expansion effect of the battery during charging and discharging intensifies, leading to the breakage of active particles, increased side reactions with the electrolyte, and weakened cycle performance. By increasing the strength of the negative electrode material, i.e., its crushing force, the breakage of negative electrode material particles during charging and discharging or rolling can be avoided, thereby reducing side reactions between the negative electrode and the electrolyte and improving the battery's cycle performance. However, excessive crushing force means a large particle size or sphericity of the negative electrode material, which increases the likelihood of lithium ion (Li) breakage. + The diffusion path inside the particles is lengthened, which leads to a decrease in ion migration rate and a decrease in the fast charging performance of the battery. Further, by reducing the surface roughness of the negative electrode sheet, the number of active sites on the surface of the negative electrode material in contact with the electrolyte is reduced, thereby reducing the side reactions between the negative electrode and the electrolyte and avoiding the problem of decreased fast charging performance caused by excessive crushing pressure.

[0019] Therefore, by controlling 0.003≤b×c / a≤1.9, the negative electrode sheet provided by this invention can have small expansion and high structural stability, and the battery prepared can achieve both excellent cycle life and fast charging performance.

[0020] In this invention, the calculated value of b×c / a can be 0.003, 0.008, 0.01, 0.015, 0.016, 0.02, 0.04, 0.05, 0.1, 0.2, 0.3, 0.32, 0.38, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.99, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, or any value between 0.003 and 1.9.

[0021] In some preferred embodiments of the present invention, 0.016 ≤ b × c / a ≤ 0.38. The calculated value of b × c / a can be 0.016, 0.02, 0.04, 0.05, 0.1, 0.2, 0.3, 0.32, 0.38, or any value between 0.016 and 0.38.

[0022] In this invention, the crushing force of the negative electrode material is a MPa.

[0023] In this invention, the value of a MPa is controlled to be in the range of 130~1050 MPa, and can be any value between 130 MPa, 200 MPa, 300 MPa, 400 MPa, 500 MPa, 600 MPa, 700 MPa, 800 MPa, 900 MPa, 1000 MPa, 1050 MPa, or 130~1050 MPa.

[0024] In some preferred embodiments of the present invention, the value of a MPa is controlled to be in the range of 380~820 MPa, and can be 380 MPa, 400 MPa, 450 MPa, 500 MPa, 550 MPa, 600 MPa, 650 MPa, 700 MPa, 750 MPa, 800 MPa, 820 MPa, or any value between 380 and 820 MPa.

[0025] Therefore, by controlling the a MPa value within the range of 130~1050 MPa, this invention can, on the one hand, avoid the problem of excessively low porosity of the negative electrode material layer (excessive crushing force means that the particles are compressed more tightly during the rolling process, significantly reducing the porosity of the negative electrode material layer), which would obstruct the wetting channels of the electrolyte in the negative electrode, ensuring smooth migration of lithium ions in the negative electrode and improving the fast-charging performance of the battery. On the other hand, it can improve the strength of the negative electrode material, preventing cracking during the rolling process of electrode preparation or during the charging and discharging process of the battery, reducing side reactions with the electrolyte, and improving the cycle life of the battery.

[0026] For example, the testing method for a includes the following steps:

[0027] The specific testing process is as follows:

[0028] 1) The lithium-ion battery was discharged to 2.5V at 0.33C. The negative electrode sheet in its empty state was obtained by disassembly. It was then immersed in dimethyl carbonate (DMC) solution at room temperature for 4 hours. After immersion, the negative electrode sheet was removed and dried in a vacuum environment. The negative electrode material on the surface of the electrode sheet was scraped off with a ceramic knife to obtain negative electrode material powder. 2) The particle size distribution of the powder was determined according to GB / T 43091-2023 to obtain the median particle size (Dv50, where Dv50 refers to the equivalent particle size value corresponding to 50% of the particle volume in the particle size distribution). The negative electrode material powder was evenly placed on the surface of the test platform. The test platform was transferred to the objective lens of a micro compression tester and fixed. Appropriate loading pressure and loading rate were set according to the characteristics of the negative electrode material powder. The loading pressure range was 0~500mN, the loading rate range was 0.1~0.5μm / s, and the displacement stability was controlled within ±0.01μm. Adjust the position of the test platform and observe the particles within the viewing area of ​​the plane indenter. Select particles with a representative target particle size, which is close to the median particle size (Dv50) of the negative electrode material powder. Adjust the magnification of the eyepiece or objective lens so that a single particle is located in the center of the plane indenter's viewing area. Fine-tune the height of the test platform to make the edge of the target particle clear. Measure the particle size. Fine-tune the height of the test platform again to move the target particle to the apex and focus. Move the test platform under the plane indenter and test and obtain the test force versus compressive displacement curve. Analyze the abrupt change point of the curve to obtain the crushing force F (mN) of the negative electrode material. Test the crushing force of 50 particles and finally calculate the average value to obtain 'a'.

[0029] In this invention, the method of adjusting 'a' is not limited. The crushing force of the negative electrode material can be adjusted by changing the sphericity (0.6~1.0) of the negative electrode material particles and the Dv50 (8~20 μm) of the negative electrode material. For example, the sphericity of the negative electrode material can be controlled by changing the temperature (480~540°C) and heating rate (5~10°C / min) during the calcination of the porous carbon matrix. The Dv50 of the negative electrode material is controlled by controlling the rotational speed (1800~3500 rpm) of the classifying wheel in the air jet mill sieving process.

[0030] In this invention, the mass percentage of silicon in the negative electrode material can also affect the cycle performance of the battery.

[0031] The present invention controls the value of b% to be in the range of 0.5% to 30%, and can be 0.5%, 1%, 1.5%, 2%, 4%, 5%, 6%, 8%, 10%, 12%, 14%, 15%, 16%, 18%, 20%, 22%, 24%, 25%, 26%, 28%, 30%, or any value between 0.5% and 30%.

[0032] In some preferred embodiments of the present invention, the value of b% is controlled to be in the range of 1.8% to 20%, and can be 1.8%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, or any value between 1.8% and 20%.

[0033] In this invention, by controlling the value of b% to be within the range of 0.5% to 30%, the integrity of the electrode conductive network can be improved, the charge transfer resistance reduced, the lithium-ion diffusion coefficient increased, and the fast-charging performance of the battery improved. Furthermore, the excessive volume expansion effect of silicon during charging and discharging can be controlled, reducing the breakage of active particles, decreasing side reactions between the negative electrode material and the electrolyte, and improving the cycle life of the battery.

[0034] For example, the testing method for b includes the following steps:

[0035] 1) The lithium-ion battery was discharged to 2.5V at 0.33C. The negative electrode sheet in the empty state was obtained by disassembling it. It was soaked in dimethyl carbonate (DMC) solution at room temperature for 4 hours. After soaking, the negative electrode sheet was taken out and dried in a vacuum environment. The negative electrode material on the surface of the electrode sheet was scraped off with a ceramic knife to obtain negative electrode material powder.

[0036] 2) Under oxygen-containing atmosphere, a certain mass of negative electrode material powder (mass m0) is placed in a crucible (initial crucible mass m1), and then calcined to constant weight. The total mass m2 of the calcined product and the crucible is weighed. The molar mass of Si is 28.09 g / mol, and the molar mass of SiO2 is 60.09 g / mol.

[0037] Based on the mass of the reaction product SiO2, the mass of silicon in the original sample can be calculated, thus obtaining the silicon content in the silicon-carbon material.

[0038] The silicon content in the sample is calculated using the following formula (2):

[0039] (2)

[0040] Specifically, the oxygen-containing atmosphere is selected from oxygen or air environment;

[0041] Calcination is carried out using a box-type resistance furnace or an atmosphere furnace, under the following conditions:

[0042] The temperature is raised to 500℃ at a certain heating rate and held for 2 hours to remove volatile substances and organic components; the temperature is then raised to 600℃ and held for 1 hour to decompose and stabilize the carbon structure, preventing residual carbon from interfering with subsequent silicon content testing; the temperature is then raised to 1000℃ and held for 1 hour to ensure that silicon is fully oxidized to SiO2. The heating rate is 5~10℃ / min, and can be any value between 5, 6, 7, 8, 9, 10, or 5~10℃ / min.

[0043] In this invention, the mass percentage of silicon in the negative electrode material is adjusted in the following ways:

[0044] Method 1: By adjusting the amount of silane gas feedstock introduced during the preparation of silicon-carbon composite materials (the volume ratio of N2 to silane is 1:(1~4)).

[0045] Method 2: Increase the pore volume of the carbon framework in silicon-carbon composite materials (0.8-1.1 cm³). 3 / g), which allows it to hold more silicon.

[0046] In this invention, the surface roughness of the negative electrode sheet can also affect the battery's cycle performance and fast charging performance. The surface roughness of the negative electrode sheet is c μm.

[0047] The present invention controls the value of c μm to be in the range of 5~10μm, which can be 5μm, 5.5μm, 6μm, 6.5μm, 7μm, 7.5μm, 8μm, 8.5μm, 9μm, 9.5μm, 10μm, or any value between 5 and 10μm.

[0048] In some preferred embodiments of the present invention, the value of c is controlled to be in the range of 6.5~8.5μm, and can be 6.5μm, 7μm, 7.5μm, 8μm, 8.5μm, or any value between 6.5 and 8.5μm.

[0049] This invention controls the surface roughness c μm of the negative electrode sheet to be 5~10 μm. On the one hand, it can avoid excessive accumulation of electrolyte on the surface of the negative electrode material, reduce side reactions between the electrolyte and the negative electrode material, and improve the cycle life of the battery. On the other hand, it can ensure the adhesion between the negative electrode sheet and the separator, improve the lithium ion transport rate, and improve the fast charging performance of the battery.

[0050] For example, the testing method for c includes the following steps:

[0051] The lithium-ion battery was discharged to 2.5V at 0.33C, and the negative electrode in its empty state was obtained after disassembly.

[0052] The negative electrode was placed on a smooth glass plate, and its four corners were fixed. The surface roughness of the electrode was measured using an SJ210 roughness instrument at six sampling points. The average of these six points was calculated to obtain the surface roughness of the electrode. (See also...) Figure 1 , Figure 1 This is a schematic diagram showing the sampling points for testing the surface roughness of the negative electrode sheet. In this invention, the surface roughness of the negative electrode sheet is adjusted in the following ways:

[0053] Method 1: Increase the compaction density of the negative electrode sheet, reduce the gaps between particles, make the particles more tightly packed, make the surface smoother, and reduce surface roughness.

[0054] Method 2: Increase the amount of adhesive used. Its rheological properties can be utilized to penetrate into the tiny gaps between the active material particles, reducing the unevenness of the surface caused by the gaps and lowering the surface roughness.

[0055] In this invention, the Dv90 (Dv90 refers to the equivalent particle size value corresponding to 90% of the particle volume in the particle size distribution) of the negative electrode material is controlled to be in the range of 12~18μm, and can be 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, or any value between 12~18μm. By controlling the Dv90 range of the negative electrode material to 12~18μm, this invention can improve particle strength and reduce the risk of particle breakage.

[0056] In this invention, the particle size distribution span value (Dv90-Dv10) / Dv50 of the negative electrode material ranges from 0.5 to 0.9, and can be any value between 0.5, 0.6, 0.7, 0.8, 0.9, or 0.5 to 0.9. By controlling the particle size distribution span value (Dv90-Dv10) / Dv50 of the negative electrode material to a range of 0.3-1.5, this invention achieves highly uniform particle size distribution in the negative electrode material, thereby improving the absorption of external rolling pressure by the negative electrode material particles and preventing larger particles from being subjected to greater pressure and thus breaking.

[0057] In this invention, the uniformity coefficient Dv60 / Dv10 of the negative electrode material ranges from 1.05 to 1.20, and can be any value between 1.05, 1.06, 1.07, 1.08, 1.09, 1.1, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16, 1.17, 1.18, 1.19, 1.2, or 1.05 to 1.20. Dv60 refers to the equivalent particle size value corresponding to 60% of the particle volume in the particle size distribution, and Dv10 refers to the equivalent particle size value corresponding to 10% of the particle volume in the particle size distribution. Among these factors, an excessively high uniformity coefficient (Dv60 / Dv10), accompanied by a lack of intermediate particle size, results in a "polarized" packing structure. This leads to decreased filling efficiency, increased electrode porosity, and increased susceptibility of material particles to breakage under external pressure. Furthermore, it exacerbates side reactions with the electrolyte during charge and discharge, resulting in significant consumption of active lithium and deteriorating battery cycle performance. Additionally, the uniformity of the negative electrode material is related to the surface roughness of the electrode; the worse the uniformity of the negative electrode material, the greater the surface roughness of the electrode.

[0058] In this invention, the maximum displacement / particle size Dv50 during crushing of the negative electrode material ranges from 0.8 to 1.5, and can be any value between 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, and 1.5. The maximum displacement during crushing refers to the maximum compressive deformation experienced by the material under compressive load from the initial contact point until significant damage, a sudden drop in force, or reaching a preset failure criterion. This parameter is one of the important indicators for evaluating the compressive strength and structural stability of the material. By controlling the ratio of the maximum displacement / particle size Dv50 during crushing of the negative electrode material, this invention ensures the flexibility of the negative electrode material particles, that is, guarantees the ability of the negative electrode material particles to withstand deformation after expansion.

[0059] In this invention, the sphericity of the negative electrode material ranges from 0.6 to 1.0, and can be any value between 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1, or 0.6-1.0. The sphericity of the negative electrode material is the ratio of the shortest axis to the longest axis of the particles. By controlling the sphericity of the negative electrode material within the range of 0.6-1.0, this invention can, on the one hand, improve the particle strength of the negative electrode material, reduce the volume expansion effect during charging and discharging, and improve the cycle life of the battery; on the other hand, it can shorten the lithium-ion transport path and improve the fast-charging performance of the battery.

[0060] In this invention, the negative electrode material comprises a silicon-carbon composite material, which includes porous carbon and silicon embedded in the pores of the porous carbon. This invention utilizes porous carbon as a buffer, with silicon deposited within the porous carbon, which reduces the risk of particle breakage during charging and discharging.

[0061] The graphitization degree of the negative electrode material is 80%~96%, and can be any value between 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, or 80%~96%. In this invention, it is necessary to control the graphitization degree of the negative electrode material to be 80%~96%. Within this range, the layered structure of the negative electrode material is more regular, ensuring smooth lithium-ion migration channels and improving the fast-charging performance of the battery. It also reduces surface defects and active sites in the negative electrode material, thereby reducing side reactions between the negative electrode material and the electrolyte, reducing the consumption of active lithium, and improving the cycle life of the battery.

[0062] In some specific embodiments of the present invention, in order to alleviate the stress caused by the expansion of silicon in the electrochemical reaction, improve the conductivity of the negative electrode material, and reduce the side reactions between the negative electrode material and the electrolyte, a carbon coating layer is provided on the surface of the negative electrode material.

[0063] This invention does not impose any particular limitation on the preparation method of the silicon-carbon composite material; any preparation method of silicon-carbon composite material known to those skilled in the art is acceptable. As an example, the preparation method of the silicon-carbon composite material includes the following steps:

[0064] ① Porous carbon material is used as the carbon matrix. It is added to a rotary kiln, nitrogen is introduced to remove oxygen, and the material is heated and calcined. The nitrogen flow rate, calcination temperature and heating rate, and rotary kiln rotation speed are common parameters in this field. For example, the nitrogen flow rate can be set to 5~15 L / min, the calcination heating rate can be 5~10℃ / min, the temperature can be 480-540℃, and the rotary kiln rotation speed can be 1~5 rpm.

[0065] ② Keep the roasting temperature constant and introduce a mixed gas. The mixed gas includes nitrogen and silane in a volume ratio of 1:(1-4). The flow rate of the mixed gas can be set to 0.2-0.6 L / min, and the introduction time can be 4-8 h.

[0066] ③ After the reaction is complete, turn off the silane gas and only pass nitrogen gas. Calcinate in a nitrogen atmosphere at a temperature of 500-600℃. Then, introduce acetylene gas to carry out the reaction. The gas flow rate can be set to 1-3 L / min. The deposition time is 14-18 h. After the reaction is complete, turn off the gas and allow the material to cool naturally in a nitrogen atmosphere. After the material has cooled completely, remove it and crush, sieve, and demagnetize it to obtain the carbon-coated silicon-carbon composite material.

[0067] In this invention, the negative electrode material further includes carbon materials. This invention does not specifically limit the type of carbon material and can select it according to actual needs. As examples, the carbon material can be natural graphite, artificial graphite, mesophase microcarbon spheres (MCMB), hard carbon, or soft carbon.

[0068] In some specific embodiments of the present invention, the mass ratio of the carbon material and the silicon-carbon composite material is (56~93):(3~40), which can be any value between 93:3, 90:6, 80:16, 70:30, 60:36, 56:40, or (56~93):(3~40).

[0069] In this invention, the negative electrode sheet includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The negative electrode material layer includes a negative electrode material, and the negative electrode material may further include a conductive agent, a binder, and / or a thickener.

[0070] This invention does not impose specific limitations on the types of conductive agents, binders, and thickeners in the negative electrode material, and they can be selected according to actual needs. As an example, the conductive agent is one or more of graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers; the binder is one or more of styrene-butadiene rubber, polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl butyral, waterborne acrylic resin, and carboxymethyl cellulose; and the thickener is selected from carboxymethyl cellulose.

[0071] The present invention does not impose any particular limitation on the negative electrode current collector, as long as it has high conductivity and does not cause adverse chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium or silver, or aluminum-cadmium alloy can be used.

[0072] This invention does not impose any particular limitation on the preparation method of the negative electrode sheet; any preparation method known to those skilled in the art is acceptable. The preparation can be carried out according to the following method:

[0073] The negative electrode active material, binder, conductive agent and thickener are mixed evenly according to the mass ratio, solvent is added, and the mixture is stirred under vacuum until the system is homogeneous to obtain a negative electrode slurry with a certain solid content; the negative electrode slurry is coated on the surface of the negative electrode current collector and then rolled and cut to obtain a negative electrode sheet.

[0074] The mass ratio of the negative electrode active material, binder, conductive agent and thickener is 92%-97%: 3%-5%: 0.5%-2%: 0%-1.5%.

[0075] In this invention, the porosity of the negative electrode sheet is 35%~45%, and can be any value between 35%, 37%, 40%, 42%, 45%, or 35%~45%; the compaction density of the negative electrode sheet is 1.1-1.6 g / cm³. 3 It can be 1.1 g / cm³. 3 1.2 g / cm 3 1.3 g / cm 3 1.4 g / cm 3 1.5 g / cm 3 1.6 g / cm 3 or 1.1-1.6 g / cm³ 3 Any value between; the areal density of the negative electrode is 40-100 g / m³. 2 It can be 40 g / m 2 45 g / m 2 50 g / m 2 55 g / m 2 60 g / m 2 65 g / m 2 70g / m 2 75 g / m 2 80 g / m 2 85 g / m 2 90 g / m 2 95 g / m 2 100 g / m 2 or 40-100g / m 2 Any value between.

[0076] The present invention also provides a battery comprising the aforementioned negative electrode sheet.

[0077] The battery provided by the present invention further includes a positive electrode sheet, wherein the positive electrode sheet includes a positive current collector and a positive electrode material layer composited on the surface of the positive current collector.

[0078] The positive electrode material layer includes a positive electrode material, which includes a positive electrode active material. In this invention, the positive electrode active material includes at least one of lithium nickel cobalt manganese oxide and lithium iron phosphate.

[0079] Lithium nickel cobalt manganese oxide satisfies the general formula Li a Ni b Co c Mn d M e O f R gWherein, 0.75≤a≤1.2, 0<b<1; 0<c<1, 0<d<1, b+c+d=1; 0≤e≤0.2, 1≤f≤2.5, 0≤g≤1, f+g≤3; M is selected from at least one of B, Zr, Ti, Si, Mo, La, Ce, Te, Nb, Ta, Al, W, Sr, V, Y, Mg, Co, and Li; R includes, but is not limited to, at least one of N, F, S, and Cl.

[0080] Lithium iron phosphate (LFP) is a cathode active material with an olivine-type crystal structure, offering advantages such as low cost and high safety. The general chemical formula for lithium iron phosphate is LiFe. 1-x M x PO y Where x≤0.1, 3.85≤y≤4, 0≤z≤0.05, and the doping element M includes, but is not limited to, one or more of Mn, Ni, Co, Cr, Cu, Bi, and Sb.

[0081] When the positive electrode active material is selected from lithium nickel cobalt manganese oxide, the value of a ranges from 150 to 1000 MPa.

[0082] In some specific embodiments of the present invention, when the molar percentage of nickel in the total content of transition metal elements in the lithium nickel cobalt manganese oxide is greater than or equal to 0.9, the value of a ranges from 180 to 1000 MPa.

[0083] When the positive electrode active material is selected from lithium iron phosphate, the value of a ranges from 130 to 850 MPa.

[0084] In this invention, the positive electrode active material layer further includes a binder, a conductive agent, and a dispersant.

[0085] The binder is used to improve the adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Therefore, the binder suitable for use in the embodiments is a fluorinated polyolefin binder, which may include, but is not limited to, polyvinylidene fluoride (PVDF), PVDF copolymers, or their modified (e.g., modified with carboxylic acid, acrylic acid, acrylonitrile, etc.) derivatives.

[0086] In this invention, a conductive agent is used to provide conductivity in the electrode. The conductive agent used in this invention can be any conductive agent without particular limitation, as long as it has suitable electronic conductivity and does not cause adverse chemical changes in the battery. Preferably, the conductive agent includes one or more of carbon black, graphite, carbon nanotubes, and graphene; the carbon black includes acetylene black (AB), Super P (conductive carbon black), and Ketjenblack; the graphite includes natural graphite and artificial graphite; the artificial graphite includes KS-6; the carbon nanotubes include single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs); and the graphene includes graphene oxide (GO), reduced graphene oxide (rGO), and graphene nanosheets (GNPs).

[0087] The dispersant is used to improve the uniformity and stability of the slurry, ensuring that the positive electrode active material, conductive agent, and binder are uniformly dispersed in the solvent, avoiding agglomeration or sedimentation, thereby improving the electrochemical performance of the electrode. In this invention, the dispersant is selected from one or more of acrylic acids, acrylates, polyether esters, phosphate esters, small molecule alkanolamines, polyurethanes, modified styrene / maleic anhydride, hydrogenated nitrile butadiene rubber (HNBR), polyvinylpyrrolidone (PVP), and polyethylene glycol (PEG).

[0088] In this invention, the positive electrode active material constitutes the main material, and the mass ratio of the main material, conductive agent, binder and dispersant is 94.5~98.5:0.5~1.5:1~2:0~2.

[0089] This invention does not impose any particular limitation on the preparation method of the positive electrode sheet; any preparation method known to those skilled in the art is acceptable. The preparation can be carried out according to the following method:

[0090] The positive electrode active material, binder, conductive agent, and dispersant are mixed evenly according to the mass ratio, and a solvent is added. The mixture is stirred under vacuum until the system is homogeneous to obtain a positive electrode slurry with a certain solid content. The positive electrode slurry is coated on the surface of the positive electrode current collector and then rolled and cut to obtain a positive electrode sheet.

[0091] The present invention does not have any particular limitation on the type of solvent, which can be N-methylpyrrolidone (NMP).

[0092] The solid content of the positive electrode slurry is 50%-70%, and can be any value between 50%, 55%, 60%, 65%, 70%, or 50%~70%.

[0093] The final positive electrode sheet has an areal density ranging from 250 to 450 g / m². 2 The compaction density ranges from 3 to 4 g / cm³. 3 .

[0094] In this invention, there is no particular limitation on the type of positive current collector. For example, the current collector can be a metal foil or a composite current collector.

[0095] Specifically, the metal foil can be aluminum or an aluminum alloy;

[0096] Specifically, the composite current collector includes an intermediate high-molecular layer and metal layers disposed on both sides of the polymer layer. The polymer layer includes polymer materials, including at least one of polyamide (PA), polyterephthalate, polyimide (PI), polyethylene (PE), polypropylene (PP), polystyrene (PPE), polyvinyl chloride (PVC), aramid, acrylonitrile-butadiene-styrene copolymer (ABS), polybutylene terephthalate (PET), poly(p-phenylene terephthalamide) (PPTA), polypropylene (PPE), polyoxymethylene (POM), epoxy resin, phenolic resin, polytetrafluoroethylene (PTEE), polyvinylidene fluoride (PVDF), silicone rubber, polycarbonate (PC), polyvinyl alcohol (PVA), polyethylene glycol (PEG), cellulose, starch, protein, their derivatives, their crosslinks, and their copolymers. The metal layer may include at least one of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy.

[0097] The battery provided by this invention also includes an electrolyte. The electrolyte of this invention can be any electrolyte suitable for electrochemical energy storage devices in the art.

[0098] In this invention, the viscosity of the electrolyte is in the range of 2~5 mPa·s, and can be 2 mPa·s, 2.5 mPa·s, 3 mPa·s, 3.5 mPa·s, 4 mPa·s, 4.5 mPa·s, 5 mPa·s, or any value between 2 and 5 mPa·s. When the viscosity of the electrolyte exceeds the above range, it affects the wetting effect on the negative electrode of the battery, thereby affecting the film formation effect, resulting in a rougher SEI film and an increase in the roughness of the electrode sheet.

[0099] In this invention, the electrolyte comprises an electrolyte, a solvent, and additives. The electrolyte typically includes a lithium salt.

[0100] Specifically, the lithium salt includes at least one selected from lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP). The concentration of the electrolyte in the electrolyte solution can be 0.5~5 mol / L.

[0101] The solvent is selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone, acetonitrile, dimethyl glycol ether (DME), dimethyl propylene glycol ether (PGME), and tetrahydrofuran. The solvent accounts for 70%-90% of the mass of the electrolyte.

[0102] The additives can be negative electrode film-forming additives, positive electrode film-forming additives, or additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature performance, additives that improve battery low-temperature performance, etc.

[0103] Specifically, positive electrode film-forming additives include: anisole derivatives, polymeric monomers (such as xylene and phenylcyclohexane), and boron / nitrogen heterocyclic compounds (such as LiBOB).

[0104] Negative electrode film-forming additives: vinylene carbonate (VC), fluoroethylene carbonate (FEC), vinyl ethylene carbonate (VEC), 1,3-propanesulfonate lactone (1,3-PS), vinyl sulfite (ES), and vinyl sulfate (DTD). The content of these additives in the electrolyte ranges from 0.1% to 5%, and can be any value between 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, or 0.1% to 5%.

[0105] The battery may further include a separator located between the positive and negative electrodes to separate them and prevent short circuits. The separator may be at least one of glass fiber, nonwoven fabric, polyethylene (PE), polypropylene (PP), and polyvinylidene fluoride. A coating may also be provided on the separator surface, which may be an inorganic coating and / or an organic coating. The inorganic coating material includes at least one of alumina, silicon oxide, titanium oxide, magnesium oxide, zirconium oxide, and boehmite; the organic coating includes at least one of aramid coating and polyvinylidene fluoride (PVDF) coating.

[0106] In this invention, when the battery is a cylindrical battery, the positive and negative electrodes and the separator are formed into a cell by winding. The high winding stress of the electrodes will further aggravate the volume expansion effect of the negative electrode material, increasing the risk of breakage. Therefore, it is necessary to control the value of 'a' to be within the range of 180~1000MPa.

[0107] The present invention also provides an electronic device comprising the battery described above. The electrochemical device serves as the power source for the electronic device.

[0108] The electronic device refers to any device that can utilize electrical energy and convert it into mechanical energy, thermal energy, light energy, or one or more other energy forms, such as electric motors, electric heaters, and electric light sources. Specifically, it can include, but is not limited to, mobile devices, electric vehicles, electric trains, ships and satellites, and energy storage systems. Mobile devices can include mobile phones, laptops, drones, robot vacuum cleaners, e-cigarettes, etc.; electric vehicles can include pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.

[0109] To further understand the present invention, the negative electrode sheet and a battery provided by the present invention will be described below with reference to the embodiments. The scope of protection of the present invention is not limited by the following embodiments.

[0110] Example 1

[0111] 1) Preparation of positive electrode sheet: The prepared positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 O2, conductive agent acetylene black, and binder PVDF are mixed at a mass ratio of 98:1:1, and solvent NMP is added. The mixture is stirred under vacuum until the system is homogeneous to obtain a positive electrode slurry. The positive electrode slurry is then coated onto one surface of the positive electrode current collector aluminum foil, and then onto the other side. After drying at room temperature, it is transferred to an oven for further drying. Finally, it is cold-pressed and slit to obtain the positive electrode sheet. The areal density of the positive electrode sheet is 330 g / m³. 2 The compacted density is 3.55 g / cm³. 3.

[0112] 2) Negative electrode preparation:

[0113] Preparation of negative electrode materials:

[0114] ① Porous carbon material is used as a carbon matrix. It is added to a rotary kiln, nitrogen is introduced to remove oxygen, and the temperature is raised for calcination. The nitrogen flow rate is set to 10 L / min, the calcination heating rate is 8℃ / min, the temperature is 510℃, and the rotary kiln rotation speed is 1 rpm.

[0115] ② Keep the roasting temperature constant and introduce a mixed gas. The mixed gas includes nitrogen and silane in a volume ratio of 1:3. The flow rate of the mixed gas can be set to 0.4 L / min and the flow time is 6 h.

[0116] ③ After the reaction is complete, turn off the silane gas and only pass nitrogen gas. Calcinate in a nitrogen atmosphere at a temperature of 600℃. Then, introduce acetylene gas to carry out the reaction. The gas flow rate can be set to 2L / min. The deposition time is 15h. After the reaction is complete, turn off the gas and allow the material to cool naturally in a nitrogen atmosphere. After cooling is complete, remove the material and crush, sieve (the classifier wheel speed of the air jet mill is 2200rpm), and demagnetize to obtain the carbon-coated silicon-carbon composite material.

[0117] Preparation of the negative electrode sheet: The negative electrode active material (graphite and silicon-carbon composite material, mass ratio of graphite to silicon-carbon composite material: 70:30), conductive agents (acetylene black and SWCNT, mass ratio of acetylene black to SWCNT: 95:5), thickener (CMC), and binder (SBR) are mixed at a mass ratio of 95:1:0.5:3.5. Deionized water is added as a solvent, and the mixture is stirred under vacuum until homogeneous to obtain a negative electrode slurry. The negative electrode slurry is uniformly coated onto both surfaces of the negative electrode current collector copper foil (controlling surface density), air-dried at room temperature, and then transferred to an oven for further drying. Finally, it is rolled and slit to obtain the negative electrode sheet. The areal density of the negative electrode sheet is 91.23 g / m³. 2 The compacted density is 1.35 g / cm³. 3 .

[0118] 3) Preparation of electrolyte

[0119] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, fully dried lithium salt LiPF6 was dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L. The mass fractions of additives VC and FEC in the electrolyte were 2% and 1%, respectively.

[0120] 4) Preparation of the separating membrane

[0121] The diaphragm is commercially available. Its base membrane is a PE membrane with a thickness of 9μm. One side surface of the base membrane is sequentially provided with an alumina coating with a thickness of 3μm and a PVDF coating with a thickness of 2μm. The other side surface of the base membrane is provided with a PVDF coating with a thickness of 2μm.

[0122] 5) Assembly and formation

[0123] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The cells are then wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a lithium-ion battery is obtained.

[0124] Examples 2-17 and Comparative Examples 1-4

[0125] Examples 2-17 and Comparative Examples 1-4 each provide a lithium-ion battery. The preparation method is similar to that of Example 1. The difference is that the process parameters involved in adjusting a, b, and c are different. See Table 1 and Table 2 for details.

[0126] Table 1

[0127]

[0128] Table 2

[0129]

[0130] The lithium-ion batteries prepared in the above embodiments and comparative examples were subjected to performance tests, and the specific items and methods are as follows:

[0131] Performance Test Method 1: Cyclic Performance Test

[0132] Capacity setting: Place the test subject in a 25°C chamber and perform the following operations: charge at 0.33C to the upper limit voltage of 4.25V, then charge at a constant voltage to the cutoff current of 0.05C; let stand for 30 minutes, then discharge at 0.33C to the lower limit voltage of 2.5V; repeat the above operation 3 times, and use the discharge capacity of the third cycle as the battery's capacity setting; then proceed with the following steps:

[0133] 1) Charge at a constant current rate of 0.5C to the upper limit voltage of 4.25V, and then charge at a constant voltage until the current drops to 0.05C;

[0134] 2) Let it stand for 20 minutes;

[0135] 3) Discharge to 2.5V at a 1C rate;

[0136] 4) Let it stand for 20 minutes.

[0137] Perform cycle tests according to steps 1)-4) until the capacity of the lithium-ion battery is less than 80% of the initial capacity, and record the number of cycles.

[0138] Performance 2 Test Method: DCR Test

[0139] 1) Place the secondary battery in the fixture, apply a force of 3000N, charge it at a constant current of 0.33C at room temperature (25℃) to the upper limit voltage of 4.25V, and let it rest for 30 minutes until thermal equilibrium is reached;

[0140] 2) Discharge at a constant current of 0.33C to the lower limit voltage of 2.5V, let stand for 30 minutes, repeat the cycle 3 times, and record the discharge capacity Cs of the third discharge as the basis for load adjustment. Adjust the capacity to 50% SOC and let stand in the temperature chamber for 2 hours until thermal equilibrium is reached.

[0141] 3) Perform a 1C pulse discharge for 18 seconds; let it rest for 40 seconds, then perform a 1C pulse discharge for 10 seconds. Record the battery voltage U2, current I before the discharge stops, and battery voltage U1 after the battery voltage stabilizes. Calculate the DC internal resistance R1 according to the formula R1=(U2-U1) / I. This R1 is the discharge DCR.

[0142] The performance test results are shown in Table 3.

[0143] Table 3

[0144]

[0145] For the lithium-ion batteries prepared in the various embodiments of the present invention, when the battery satisfies 0.003≤b×c / a≤1.9, its cycle count is ≥400 cycles. At the same time, the DCR test is 2~10 mΩ. It can be seen that the lithium-ion battery of the present invention has good cycle performance and also has good fast charging performance.

[0146] Based on the test results of Examples 1-6 and Examples 9-10 combined with Examples 7-8 and Examples 11-17, it can be seen that a) meets the preferred range, which can ensure the smooth transport channel of lithium ions and avoid the cracking of the negative electrode material during charging and discharging, and reduce the side reactions with the electrolyte. b) meets the preferred range, which can ensure the integrity of the conductive network and reduce the volume expansion effect of silicon during charging and discharging, and reduce the risk of electrode shedding. c) meets the preferred range, which can improve the adhesion between the negative electrode and the separator, shorten the diffusion path of lithium ions, and avoid the increase of side reactions caused by excessive enrichment of electrolyte in the negative electrode.

[0147] When the formula is further controlled to satisfy 0.016≤b×c / a≤0.38, the fast charging performance and cycle performance of lithium-ion batteries are relatively better.

[0148] As can be seen from Comparative Examples 1 to 4, when the battery does not meet the range of 0.003≤b×c / a≤1.9, even if a, b, and c meet their respective maximum ranges, the lithium-ion battery cannot simultaneously achieve both cycle performance and fast charging performance.

[0149] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A negative electrode sheet, characterized in that, The negative electrode sheet includes a negative electrode material, and the negative electrode material includes silicon. The crushing force of the negative electrode material is a MPa; the mass percentage of silicon in the negative electrode material is b%; and the surface roughness of the negative electrode sheet is c μm. a, b, and c satisfy the relationship shown in equation I: 0.003≤b×c / a≤1.9 Formula I; In Formula I, the value of a MPa ranges from 130 to 1050 MPa; The value of b% ranges from 0.5% to 30%. The value of c μm ranges from 5 to 10 μm.

2. The negative electrode sheet according to claim 1, characterized in that, 0.016≤b×c / a≤0.

38.

3. The negative electrode sheet according to claim 1, characterized in that, The value of a MPa ranges from 380 to 820 MPa; And / or, the value of b% ranges from 1.8% to 20%; And / or, the value of c μm ranges from 6.5 to 8.5 μm.

4. The negative electrode sheet according to claim 1, characterized in that, The Dv90 of the negative electrode material is 12-18 μm.

5. The negative electrode sheet according to claim 1, characterized in that, The particle size distribution span value (Dv90-Dv10) / Dv50 of the negative electrode material ranges from 0.5 to 0.

9.

6. The negative electrode sheet according to claim 1, characterized in that, The uniformity coefficient Dv60 / Dv10 of the negative electrode material ranges from 1.05 to 1.

20.

7. The negative electrode sheet according to claim 1, characterized in that, The maximum displacement / particle size Dv50 of the negative electrode material during crushing is 0.8~1.

5.

8. The negative electrode sheet according to claim 1, characterized in that, The sphericity of the negative electrode material is 0.6-1.

0.

9. The negative electrode sheet according to claim 1, characterized in that, The negative electrode material includes a silicon-carbon composite material, which includes porous carbon and silicon incorporated in the pores of the porous carbon.

10. The negative electrode sheet according to claim 9, characterized in that, The graphitization degree of the negative electrode material is 80%~96%.

11. The negative electrode sheet according to claim 9, characterized in that, The surface of the silicon-carbon composite material also includes a carbon coating layer.

12. The negative electrode sheet according to claim 1, characterized in that, The porosity of the negative electrode is 35-45%.

13. The negative electrode sheet according to claim 1, characterized in that, The compaction density of the negative electrode sheet is 1.1-1.6 g / cm³. 3 .

14. The negative electrode sheet according to claim 1, characterized in that, The areal density of the negative electrode sheet is 40-100 g / m³. 2 .

15. A battery, characterized in that, Includes the negative electrode sheet as described in any one of claims 1 to 14.

16. The battery according to claim 15, characterized in that, The battery further includes a positive electrode sheet, which includes a positive electrode active material, and the positive electrode active material includes at least one of lithium nickel cobalt manganese oxide and lithium iron phosphate.

17. The battery according to claim 16, characterized in that, When the positive electrode active material is selected from lithium nickel cobalt manganese oxide, the aMPa is 150~1000MPa.

18. The battery according to claim 17, characterized in that, In the lithium nickel cobalt manganese oxide, when the molar percentage of nickel in the total content of transition metal elements is greater than or equal to 0.9, the value of a MPa ranges from 180 to 1000 MPa.

19. The battery according to claim 16, characterized in that, When the positive electrode active material is selected from lithium iron phosphate, the value of aMPa ranges from 130 to 850 MPa.

20. The battery according to claim 15, characterized in that, When the battery is a cylindrical battery, the value of a MPa ranges from 180 to 1000 MPa.

21. The battery according to claim 15, characterized in that, The battery also includes an electrolyte with a viscosity range of 2-5 mPa·s.

22. An electronic device, characterized in that, Includes the battery as described in any one of claims 15 to 21.

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