Negative electrode sheet, electrochemical device, and electronic device

Abstract

The application relates to a negative pole piece, an electrochemical device and an electronic equipment, and belongs to the technical field of electrochemical energy storage. The negative pole piece comprises a negative pole current collector and a first active layer, a second active layer and a third active layer arranged on at least one surface of the negative pole current collector in sequence; the first active layer comprises a first active material, and the first active material comprises first silicon carbon and graphite; the second active layer comprises a second active material, and the second active material comprises second silicon carbon; and the third active layer comprises a third active material, and the third active material comprises hard carbon and lithium titanate. The negative pole piece provided by the application solves the problem of unstable interface of high-silicon materials, enables the battery to maintain high energy density, improves the cycle life, and inhibits lithium precipitation.

Description

Technical Field

[0001] This application relates to the field of electrochemical energy storage technology, and in particular to negative electrode plates, electrochemical devices and electronic equipment. Background Technology

[0002] Silicon-based materials are considered ideal anode materials for next-generation high-energy-density lithium-ion batteries due to their high specific capacity. However, silicon undergoes volume expansion (>300%) during charge and discharge, leading to the pulverization of active materials, continuous growth and rupture of the solid electrolyte interphase (SEI) film, ultimately resulting in rapid capacity decay and shortened cycle life of lithium-ion batteries.

[0003] Existing technologies propose setting a buffer layer with low or no silicon content on the negative electrode current collector, and then covering it with a main active layer with high silicon content. However, the outermost high silicon active layer of this structure is still in direct contact with the electrolyte, and its unstable interface problem has not been fundamentally solved. Some studies have used carbon materials as a protective layer, but the mechanical strength and ionic conductivity of a single carbon layer are limited, which is not effective in protecting the high silicon content core and may sacrifice the fast charging performance of lithium-ion batteries. Summary of the Invention

[0004] The purpose of this application is to overcome the shortcomings of the prior art and provide a negative electrode, an electrochemical device, and an electronic device. The negative electrode provided by this application can maintain the high energy density of the battery, improve cycle life, and improve lithium plating.

[0005] To achieve the above objectives, the technical solution adopted in this application is as follows: The first aspect of this application provides a negative electrode sheet, including a negative electrode current collector and a first active layer, a second active layer and a third active layer sequentially disposed on at least one surface of the negative electrode current collector; The first active layer includes a first active material, which includes a first silicon carbide and graphite; the second active layer includes a second active material, which includes a second silicon carbide; the third active layer includes a third active material, which includes hard carbon and lithium titanate.

[0006] As an embodiment of this application, the lithium titanate has a mass percentage content of 20-80% in the third active material.

[0007] As an embodiment of this application, the hard carbon in the third active material has a mass percentage content of 20-80%.

[0008] As an embodiment of this application, the mass percentage of silicon in the first active material is w1, the mass percentage of silicon in the second active material is w2, and w2 > w1.

[0009] As an embodiment of this application, the mass percentage of silicon in the first active material is w1, and 0 ≤ w1 ≤ 15%.

[0010] As an embodiment of this application, the mass percentage of silicon in the second active material is w2, and 20%≤w2≤50%.

[0011] As an embodiment of this application, the mass percentage of lithium titanate in the third active material is w3, and 0 ≤ (w3-20%) / w2 ≤ 2.

[0012] As an embodiment of this application, the mass percentage of lithium titanate in the third active material is w3, and 0.6 ≤ (w3-20%) / w2 ≤ 0.9.

[0013] As an embodiment of this application, the thickness of the third active layer is D μm, and 0.12≤ D / (100 w2)≤0.7.

[0014] As an embodiment of this application, the thickness of the third active layer is D μm, and 0.2 ≤ D / (100 w2) ≤0.3.

[0015] As an embodiment of this application, the mass percentage of the first active material in the first active layer is 95-98%.

[0016] As an embodiment of this application, the second active layer contains 86-92% silicon carbon by mass.

[0017] As an embodiment of this application, the third active layer contains 95-98% by mass of the third active material.

[0018] As an embodiment of this application, the first active layer, the second active layer, and the third active layer each independently include a conductive agent and a binder.

[0019] As an embodiment of this application, the method for preparing the electrode sheet described in this application includes the following steps: S1. A first active material, a conductive agent, and a binder are mixed in a solvent to obtain a first slurry; a second active material, a conductive agent, and a binder are mixed in a solvent to obtain a second slurry; a third active material, a conductive agent, and a binder are mixed in a solvent to obtain a third slurry; S2. Using a multi-layer continuous coating process, the first slurry, the second slurry, and the third slurry are sequentially coated onto the negative electrode current collector, and then cold-pressed and slit to obtain the negative electrode sheet.

[0020] A second aspect of this application provides an electrochemical device comprising the electrode as described above.

[0021] A third aspect of this application provides an electronic device, including the electrochemical device described above.

[0022] Compared with the prior art, the beneficial effects of this application are as follows: This application provides a negative electrode sheet with a first, second, and third active layer, forming a three-layer gradient structure design consisting of a bottom layer, a high-silicon layer, and a composite protective layer. By optimizing the composition of the active materials in each layer and establishing the relationship between the silicon content of the second active layer and the composition and thickness of the third active layer, a synergistic effect is achieved between the hard carbon and lithium titanate in the outer protective layer. This not only solves the fundamental problem of interface instability of high-silicon materials but also enables the battery to maintain high energy density, achieves a cycle life far exceeding that of traditional structures, and significantly improves lithium plating. Detailed Implementation

[0023] To better illustrate the purpose, technical solution, and advantages of this application, the following detailed description will be provided in conjunction with specific embodiments. Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods; and unless otherwise specified, the materials and reagents used are commercially available.

[0024] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0025] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0026] Unless otherwise specified, all components, raw materials, or instruments used in the embodiments and comparative examples of this application are commercially available, and the components and raw materials used in each parallel experiment are the same.

[0027] In the following description, all figures disclosed herein are approximate values, regardless of whether the terms "about" or "approximately" are used in conjunction. They may vary by 1%, 2%, 5%, or sometimes 10% to 20%. Whenever a range of values ​​with a lower limit RL and an upper limit RU is disclosed, any values ​​falling within that range are specifically disclosed. Specifically, the following values ​​within this range are specifically disclosed: R = RL + k * (RU - RL), where k is a variable with a 1% increment from 1% to 100%, i.e., k is 1%, 2%, 3%, 4%, 5%, ..., 50%, 51%, 52%, ..., 95%, 96%, 97%, 98%, 99%, or 100%. Furthermore, any range of values ​​defined by the two R values ​​as defined above are also specifically disclosed.

[0028] General definition The term "active material" refers to a core functional material that can achieve charge storage and release through reversible electrochemical reactions (such as lithium insertion / deintercalation, alloying, redox reactions, etc.), and serves as a carrier for negative electrode electrochemical reactions.

[0029] The term "silicon-carbon" refers to a composite active material for lithium-ion batteries, which uses silicon as a high-capacity active component and carbon materials as a dispersion / buffer / conductive carrier, and is prepared through composite processes (ball milling, vapor deposition, in-situ polymerization, etc.).

[0030] The term "graphite" refers to graphite composed of carbon atoms arranged in sp... 2 The layered crystalline structure of carbon materials (hexagonal crystal system) formed by hybridization is bonded between layers by van der Waals forces.

[0031] The term "hard carbon" refers to amorphous or low-crystallinity carbon materials that lack a distinct layered structure, have a cross-linked and disordered carbon skeleton, and are difficult to graphitize at high temperatures (>2000 ℃).

[0032] The term "lithium titanate" refers to lithium titanate with the chemical formula Li₄Ti₅O₂. 12 Spinel-structured oxide.

[0033] I. Negative electrode plate To address the contradiction between high energy density, fast charging performance, and cycle life in existing silicon-based anodes, this application provides an anode sheet.

[0034] The negative electrode sheet provided in this application includes a negative current collector and a first active layer, a second active layer and a third active layer sequentially disposed on at least one surface of the negative current collector; The first active layer includes a first active material, which includes a first silicon carbide and graphite; the second active layer includes a second active material, which includes a second silicon carbide; the third active layer includes a third active material, which includes hard carbon and lithium titanate.

[0035] In some embodiments, the lithium titanate in the third active material has a mass percentage content of 20-80%, for example, it can be 20%, 25%, 27%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, 60%, 70%, 75%, 80%, or a range of any two of these values.

[0036] In some embodiments, the hard carbon in the third active material has a mass percentage content of 20-80%, for example, it can be 20%, 25%, 27%, 30%, 35%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, 73%, 75%, 80%, or a range of any two of these values.

[0037] In some embodiments, the mass percentage of silicon in the first active material is w1, the mass percentage of silicon in the second active material is w2, and w2 > w1.

[0038] This application designs a negative electrode with a three-layer active layer structure. The outermost layer introduces a hard carbon-lithium titanate composite protective layer. By adjusting the ratio of active materials in the outermost third active layer and the relationship with the silicon content of the first and second active layers, the energy density, fast charging performance and cycle life are synergistically improved.

[0039] The negative electrode active layer of this application consists of three layers from the inside out: a first active layer (bottom layer), a second active layer (middle layer), and a third active layer (protective layer). The silicon content of the second active layer is higher than that of the first active layer, forming a silicon content gradient. The active material hard carbon in the third active layer provides toughness and ion channels, while lithium titanate provides structural strength and safety potential. When the two are combined as the third active material, they can produce a 1+1>2 effect, solving the problem that a single material such as pure graphite or pure LTO cannot achieve the best balance in terms of interface stability, ionic conductivity, and mechanical strength when used as a protective layer.

[0040] The negative electrode sheet provided in this application solves the problem that the traditional negative electrode double-layer structure cannot simultaneously achieve high energy density, fast charging and long cycle life. It maintains the advantages of high energy density and fast charging of the traditional structure, and solves the problem of interface instability through the composite protective layer, thus achieving a significant improvement in cycle performance.

[0041] In some embodiments, the mass percentage of silicon in the first active material is w1, and 0 ≤ w1 ≤ 15%, for example, it can be 0%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15% or any two of these values.

[0042] In some embodiments, the mass percentage of silicon in the second active material is w2, and 20% ≤ w2 ≤ 50%, for example, it can be 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, or a range of any two of these values.

[0043] In some embodiments, the mass percentage of lithium titanate in the third active material is w3, and 0 ≤ (w3 - 20%) / w2 ≤ 2, for example (w3 - 20%) / w2 ≤ 2. The value of %) / w2 can be 0.00, 0.05, 0.10, 0.15, 0.20, 0.23, 0.25, 0.30, 0.35, 0.40, 0.43, 0.45, 0.50, 0.55, 0.60, 0.65, 0.69, 0.70, 0.73, 0.75, 0.80, 0.85, 0.90, 0.95, 1.00, 1.05, 1.07, 1.10, 1.13, 1.15, 1.17, 1.20, 1.25, 1.30, 1.40, 1.50, 1.60, 1.70, 1.80, 1.90, 2.0, or a range of any two of these values.

[0044] In some embodiments, the mass percentage of lithium titanate in the third active material is w3, and 0.6 ≤ (w3-20%) / w2 ≤ 0.9.

[0045] In some embodiments, the thickness of the third active layer is D μm, and 0.12 ≤ D / (100 w2) ≤ 0.7. For example, the value of D / (100 w2) can be 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.30, 0.35, 0.40, 0.43, 0.45, 0.50, 0.55, 0.60, 0.65, 0.67, 0.69, 0.70, or a range of any two values ​​thereof.

[0046] In some embodiments, the thickness of the third active layer is D μm, and 0.2 ≤ D / (100 w2) ≤0.3.

[0047] This application further combines the instability of the second active layer (silicon content) to precisely adjust the lithium titanate and hard carbon content of the protective layer, thereby improving the chemical protection strength and achieving dynamic optimization; at the same time, it also precisely adjusts the thickness of the protective layer according to the aggressiveness of the second active layer (silicon content), thereby improving the physical protection strength, and comprehensively further improving the energy density and cycle life, while suppressing lithium plating.

[0048] In some embodiments, the mass percentage of the first active material in the first active layer is 95-98%, for example, it can be 95.0%, 95.1%, 95.2%, 95.3%, 95.4%, 95.5%, 95.6%, 95.7%, 95.8%, 95.9%, 96.0%, 96.1%, 96.2%, 96.3%, 96.4%, 96.5%, 96.6%, 96.7%, 96.8%, 96.9%, 97.0%, 97.1%, 97.2%, 97.3%, 97.4%, 97.5%, 97.6%, 97.7%, 97.8%, 97.9%, 98.0%, or a range of any two of these values.

[0049] In some embodiments, the second active layer contains 86-92% silicon carbon by mass, for example, 86%, 86.5%, 87%, 87.5%, 88%, 88.5%, 89%, 89.5%, 90.0%, 90.1%, 90.2%, 90.3%, 90.4%, 90.5%, 90.6%, 90.7%, 90.8%, 90.9%, 91.0%, 91.1%, 91.2%, 91.3%, 91.4%, 91.5%, 91.6%, 91.7%, 91.8%, 91.9%, 92.0%, or a range of any two of these values.

[0050] In some embodiments, the third active material in the third active layer has a mass percentage content of 95-98%, for example, it can be 95.0%, 95.1%, 95.2%, 95.3%, 95.4%, 95.5%, 95.6%, 95.7%, 95.8%, 95.9%, 96.0%, 96.1%, 96.2%, 96.3%, 96.4%, 96.5%, 96.6%, 96.7%, 96.8%, 96.9%, 97.0%, 97.1%, 97.2%, 97.3%, 97.4%, 97.5%, 97.6%, 97.7%, 97.8%, 97.9%, 98.0%, or a range of any two of these values.

[0051] In some embodiments, the first active layer, the second active layer, and the third active layer each independently include a conductive agent and a binder.

[0052] In some embodiments, the first active layer contains 0.5-1.5% by mass of the conductive agent and 2-3% by mass of the binder.

[0053] In some embodiments, the second active layer contains 0.5-1.5% by mass of the conductive agent and 7-9% by mass of the binder.

[0054] In some embodiments, the third active layer contains 0.5-1.5% by mass of conductive agent and 2-3% by mass of binder.

[0055] In some embodiments, the thickness of the third active layer is 3.5-20 μm, for example, it can be 3.5 μm, 4.0 μm, 5.0 μm, 6.0 μm, 7.0 μm, 8.0 μm, 9.0 μm, 10.0 μm, 11.0 μm, 12.0 μm, 13.0 μm, 14.0 μm, 15.0 μm, 16.0 μm, 17.0 μm, 18.0 μm, 19.0 μm, 20.0 μm or any two of these values.

[0056] In some embodiments, the thickness of the first active layer is 20-120 μm, for example, it can be 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm or any two of these values.

[0057] In some embodiments, the thickness of the second active layer is 20-120 μm, for example, it can be 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm or any two of these values.

[0058] In this application, there are no particular restrictions on the negative electrode current collector, as long as it can achieve the purpose of this application. For example, it can be copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, foamed copper, or composite current collector, etc.

[0059] II. Preparation method of negative electrode sheet This application provides a method for preparing the negative electrode sheet, comprising the following steps: S1. A first active material, a conductive agent, and a binder are mixed in a solvent to obtain a first slurry; a second active material, a conductive agent, and a binder are mixed in a solvent to obtain a second slurry; a third active material, a conductive agent, and a binder are mixed in a solvent to obtain a third slurry; S2. Using a multi-layer continuous coating process, the first slurry, the second slurry, and the third slurry are sequentially coated onto the negative electrode current collector, and then cold-pressed and slit to obtain the negative electrode sheet.

[0060] III. Electrochemical Device This application provides an electrochemical device including a negative electrode as described above. In some embodiments, the electrochemical device further includes a positive electrode, a separator, and an electrolyte.

[0061] The electrochemical device of this application includes any device in which an electrochemical reaction occurs, and specific examples include all kinds of primary batteries, secondary batteries, fuel cells, solar cells, or capacitors. In particular, the electrochemical device is a lithium secondary battery, including lithium metal secondary batteries, lithium-ion secondary batteries, lithium polymer secondary batteries, or lithium-ion polymer secondary batteries.

[0062] positive electrode The electrochemical device of this application includes a positive electrode, wherein the positive electrode includes a positive electrode current collector and a positive electrode active layer disposed on at least one surface of the positive electrode current collector.

[0063] In some of these embodiments, the type of positive current collector is not particularly limited, and it may be any material known to be suitable for use as a positive current collector.

[0064] In some embodiments, the positive current collector includes metallic materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum, as well as carbon materials such as carbon cloth and carbon paper.

[0065] There are no particular restrictions on the form of the positive electrode current collector. When the positive electrode current collector is a metallic material, it can be in the form of metal foil, metal cylinder, metal strip, metal plate, metal foil, metal mesh, stamped metal, foamed metal, etc. When the positive electrode current collector is a carbon material, it can be in the form of carbon plate, carbon film, carbon cylinder, etc.

[0066] In some embodiments, the positive electrode active layer includes a positive electrode active material, a positive electrode binder, and a positive electrode conductive agent.

[0067] diaphragm The separator separates the negative and positive electrodes and provides a pathway for lithium-ion migration. The use of the separator is not particularly limited, as long as it is a separator commonly used in lithium-ion secondary batteries. In particular, separators with low resistance to electrolyte ion movement and excellent electrolyte permeability are preferred. Specifically, porous polymer membranes can be used, such as porous polymer membranes formed from polyolefin-based polymers (e.g., ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, ethylene / methacrylate copolymers, etc.) or laminated structures with two or more layers. Alternatively, nonwoven fabrics formed from conventional porous nonwoven fabrics (e.g., glass fibers with high melting points, polyethylene terephthalate fibers, etc.) can be used. Furthermore, coated separators containing ceramic components or polymer materials to ensure heat resistance or mechanical strength can be used, and can optionally be used as single-layer or multi-layer structures.

[0068] Generally, a diaphragm includes a substrate and a coating applied to the surface of the substrate.

[0069] electrolyte The electrolyte includes electrolyte salts and organic solvents. The specific types and compositions of the electrolyte salts and organic solvents are not limited. It may include positive electrode film-forming additives, negative electrode film-forming additives, and additives to improve cycle performance and low temperature.

[0070] IV. Electronic Equipment This application provides an electronic device, including the electrochemical device described above.

[0071] The electronic device described in this application is not particularly limited and may be any electronic device known in the prior art.

[0072] The application of the electrochemical device in this application is not particularly limited, and it can be used in any electronic device known in the prior art. According to some embodiments of this application, the electronic device includes, but is not limited to, mobile phones, smartphones, laptops, tablets, wearable devices, smartwatches, smart bracelets, smart glasses, power banks, televisions, game consoles, game controllers, digital cameras, smart speakers, headphones, keyboards, mice, monitors, drones, audio equipment, home appliances, toys, power tools, automobiles, motorcycles, electric bicycles, bicycles, robots, robot dogs, industrial robots, android robots, etc.

[0073] The following uses a lithium-ion battery as an example and combines specific embodiments to illustrate the preparation of lithium-ion batteries. Those skilled in the art will understand that the preparation methods described in this application are merely examples, and any other suitable preparation methods are within the scope of this application.

[0074] In the following examples and comparative examples, the battery disassembly and pretreatment methods are as follows: the battery is frozen at -20 ℃ for 12 hours, then dissected at room temperature, and the electrode sheets are dried in a vacuum drying oven for 12 hours to remove residual electrolyte; In the following examples and comparative examples, the test method for the mass percentage content of lithium titanate and hard carbon in the third active material is as follows: the titanium content in the third active layer is determined by inductively coupled plasma optical emission spectrometry (ICP-OES), and then converted into the mass content of lithium titanate according to the stoichiometry of lithium titanate. In the following examples and comparative examples, the test method for the mass percentage of hard carbon in the third active material is as follows: a thermogravimetric analyzer (TGA) is used to heat the material from room temperature to 800°C in an air atmosphere. Based on the difference in thermal stability between lithium titanate and hard carbon, the mass percentage of hard carbon is calculated through the weight loss curve. In the following examples and comparative examples, the method for testing the mass percentage of silicon in the first and second active materials is as follows: Using glow discharge emission spectroscopy (GD-OES) to obtain an accurate composition-depth quantitative curve, the precise measurement of silicon content is achieved through the following steps: The negative electrode sample is used as the cathode and placed in the vacuum chamber of the GD-OES; plasma is generated using argon gas, and the plasma is sputtered onto the sample surface at a constant rate, forming a micrometer-sized crater; during the sputtering process, the characteristic spectral intensity emitted by the sputtered atoms (e.g., Si 288.16 nm, C 156.14 nm) is detected in real time; through standard sample calibration, the spectral intensity is converted into elemental concentration, and the sputtering time is converted into depth; the instrument software uses the pre-input calibration curve and sputtering rate data to convert the "signal intensity-sputtering time" curve into an "elemental concentration-depth" curve; In the following examples and comparative examples, the testing method for the thickness of the first, second, and third active layers is as follows: Scanning electron microscopy (SEM) cross-sectional analysis method. After the electrode sample is subjected to liquid nitrogen embrittlement, the cross-section is observed using a scanning electron microscope, and the thickness of the current collector, the first active layer, the second active layer, and the third active layer are measured respectively. At least 10 different positions are measured for each sample and the average value is taken.

[0075] Examples 1-20 The preparation methods of the lithium-ion batteries in Examples 1-20 of this application all include the following steps: (1) Preparation of negative electrode S1, Slurry for the first active layer: The first active material (graphite and first silicon carbide), conductive agent Super P, binder styrene-butadiene rubber (SBR) and thickener sodium carboxymethyl cellulose (CMC) are mixed in deionized water at a mass ratio of 96.5:1.0:1.5:1.0 and stirred at high speed until uniform to obtain the first slurry with a solid content of 48%. The slurry for the second active layer: The second active material (second silicon carbide), conductive agent Super P, binder styrene-butadiene rubber (SBR) and thickener sodium carboxymethyl cellulose (CMC) are mixed in deionized water at a mass ratio of 91.0:1.0:7.0:1.0 and stirred at high speed until uniform, to obtain a second slurry with a solid content of 35%. The slurry for the third active layer: The third active material (hard carbon and lithium titanate), conductive agent Super P, binder styrene-butadiene rubber (SBR) and thickener sodium carboxymethyl cellulose (CMC) are mixed in deionized water at a mass ratio of 96.5:1.0:1.5:1.0 and stirred at high speed until uniform, to obtain a third slurry with a solid content of 32%. S2. Using multi-layer continuous coating technology, three types of first, second and third slurries are sequentially coated onto a copper current collector with a thickness of 8μm to form a three-layer structure with a total thickness of 120μm. Then, the negative electrode sheet is obtained by cold pressing and slitting.

[0076] (2) Preparation of positive electrode: The active material LiCoO2, conductive agent acetylene black, conductive carbon nanotubes and binder polyvinylidene fluoride (PVDF) are fully dispersed and uniformly dispersed in N-methylpyrrolidone solvent system at a weight ratio of 98.2:0.3:0.5:1.0, coated onto aluminum current collector, and then cold-pressed into strips to obtain positive electrode sheet.

[0077] (3) Separator: The surface is coated with ceramic mixture and the PE with a thickness of 7μm is used as the separator; (4) Preparation of electrolyte: Ethyl carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC) and propyl propionate (PP) are mixed in a volume ratio of 1:1:4:4. Then, fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent at a ratio of 1 mol / L to prepare the electrolyte.

[0078] (5) Battery assembly: The above positive electrode, separator and negative electrode are prepared into a wound cell by winding, top and side sealing, liquid injection, formation and second sealing.

[0079] The specific parameters of the active layer in Examples 1-20 are shown in Table 1 below, wherein the mass percentage of silicon in the first active material is w1, the mass percentage of silicon in the second active material is w2, the mass percentage of lithium titanate in the third active material is w3, the mass percentage of hard carbon in the third active material is w4, and the thickness of the third active layer is D. Compared with Example 1, the parameter changes in other examples are as follows: Example 2 involves simply changing the ratio of graphite to silicon carbon in the first active material, thereby changing the mass percentage w1 of silicon in the first active material. Example 3 involves changing the ratio of graphite to silicon carbon in the first active material and using second silicon carbon with different silicon contents to make the mass percentage of silicon in the first and second active materials the same, w1 and w2. Example 4 involves simply changing the ratio of graphite to silicon carbon in the first active material, so that the mass percentage of silicon in the first active material, w1, is reduced to 0. Examples 5-8 involve using second silicon-carbon materials with different silicon contents to change the mass percentage w2 of silicon in the second active material, adjusting the mass percentage w3 and w4 of lithium titanate and hard carbon in the third active material, and adjusting the coating amount of the third active layer to obtain different thicknesses D of the third active layer. Examples 9-15 involve adjusting the mass percentages w3 and w4 of lithium titanate and hard carbon in the third active material. Examples 16-20 show how different thicknesses D of the third active layer are obtained by simply adjusting the coating amount of the third active layer.

[0080] Table 1 Comparative Examples 1-3 The difference between Comparative Examples 1-3 and Example 1 lies in the use of different active layers in the negative electrode sheet, as detailed below: The only difference between Comparative Example 1 and Example 1 is that the slurry without coating the third active layer forms only the first and second active layers; The only difference between Comparative Example 2 and Example 1 is that a certain proportion of hard carbon and lithium titanate in the third active material is replaced with lithium titanate alone. The only difference between Comparative Example 3 and Example 1 is that a certain proportion of hard carbon and lithium titanate in the third active material is replaced with a single hard carbon.

[0081] Example of effect To investigate the performance of the negative electrode and lithium-ion battery provided in this application, the following tests were conducted: (1) Cyclic performance test: At 25 °C, charge and discharge according to the following method: charge to 4.10 V at a rate of 2.5 C, then charge to 4.20 V at a rate of 2.5 C, charge to 4.30 V at a rate of 1.5 C, charge to 4.53 V at a rate of 1.0 C, and then discharge to 3.0 V at a rate of 0.7 C. Cycle 500 times, calculate the cycle capacity retention rate = (discharge capacity of the 500th cycle / discharge capacity of the first cycle) × 100%; (2) Lithium plating level test of lithium-ion batteries: After taking the parallel samples of the cells in each example and comparative example and performing step (1) cycle, disassemble the cells, remove the encapsulation film, unfold them along the winding direction, separate the separator and the electrode, and observe the interface of the negative electrode. If there is no lithium plating, it is marked as level 0; if there is point-like lithium plating, it is marked as level 1; if there is linear lithium plating but it is not continuous, it is marked as level 2; if there is linear lithium plating and it is continuous, it is marked as level 3. There are 5 parallel samples in each group of tests, and the highest level among the samples is counted.

[0082] The test results are shown in Table 2 below.

[0083] Table 2 As shown in Table 2: The negative electrode sheet provided in this application solves the fundamental problem of interface instability of high-silicon materials in the prior art, enabling the battery to maintain an energy density of over 800 Wh / kg and achieve a cycle life higher than that of traditional structures, with a capacity retention rate of >80% after 500 cycles, and can suppress lithium plating. In contrast, Comparative Example 1 uses a traditional double-layer active layer, which, although having a higher energy density, has poor cycle stability and poor lithium plating suppression effect; Comparative Examples 2 and 3 use a single lithium titanate and a single hard carbon as the active material of the third active layer, respectively, and the lithium plating level is still relatively high.

[0084] In summary, the negative electrode sheet provided in this application establishes a three-layer gradient structure of bottom layer-high silicon layer-composite protective layer, and optimizes the relationship between silicon content and the composition and thickness of the protective layer, thereby achieving precise control of protective characteristics. The excellent comprehensive performance of the negative electrode sheet provided in this application not only retains the high energy density and fast charging advantages of traditional structures, but also solves the problem of interface instability through composite layer, significantly improves cycle life, and improves lithium plating suppression effect, thus having high application value.

[0085] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A negative electrode sheet, characterized in that, It includes a negative electrode current collector and a first active layer, a second active layer and a third active layer sequentially disposed on at least one surface of the negative electrode current collector; The first active layer includes a first active material, which includes a first silicon carbide and graphite; the second active layer includes a second active material, which includes a second silicon carbide; the third active layer includes a third active material, which includes hard carbon and lithium titanate.

2. The negative electrode sheet as described in claim 1, characterized in that, The lithium titanate in the third active material has a mass percentage content of 20-80%; And / or, the hard carbon in the third active material has a mass percentage content of 20-80%.

3. The negative electrode sheet as described in claim 1, characterized in that, The mass percentage of silicon in the first active material is w1, and the mass percentage of silicon in the second active material is w2, where w2 > w1.

4. The negative electrode sheet as described in claim 1, characterized in that, The mass percentage of silicon in the first active material is w1, and 0 ≤ w1 ≤ 15%.

5. The negative electrode sheet as described in claim 1, characterized in that, The mass percentage of silicon in the second active material is w2, and 20% ≤ w2 ≤ 50%.

6. The negative electrode sheet as described in claim 5, characterized in that, The mass percentage of lithium titanate in the third active material is w3, and 0 ≤ (w3-20%) / w2 ≤ 2.

7. The negative electrode sheet as described in claim 5, characterized in that, The thickness of the third active layer is D μm, and 0.12 ≤ D / (100 w2) ≤0.

7.

8. The negative electrode sheet as described in claim 1, characterized in that, In the first active layer, the mass percentage of the first active material is 95-98%; And / or, in the second active layer, the mass percentage of the second silicon-carbon is 86-92%; And / or, in the third active layer, the mass percentage of the third active material is 95-98%.

9. The negative electrode sheet as described in claim 1, characterized in that, The first active layer, the second active layer, and the third active layer each independently include a conductive agent and a binder.

10. An electrochemical device, characterized in that, Includes the negative electrode sheet as described in any one of claims 1-9.

11. An electronic device, characterized in that, Includes the electrochemical device as described in claim 10.

Images