An anode sheet and application thereof

Abstract

This invention discloses a negative electrode sheet. The structure of the negative electrode sheet includes a silicon-based material-lithophile metal composite layer disposed on the outermost layer of at least one side of the current collector. The silicon-based material-lithophile metal composite layer includes a silicon-based material and a lithiumophile metal. The silicon-based material is Si / SiO2@C1@C2, where Si is silicon, SiO2 is silicon oxide, and C1 is a porous fumed carbon layer coating Si / SiO2 particles. Si, SiO2, and C1 constitute a hollow "core-shell" structure of Si / SiO2@C1. C2 is a pitch carbon shell, which coats the outer ring of Si / SiO2@C1 to form a second buffer layer, thus constituting a hollow "egg-shell" structure of Si / SiO2@C1@C2. The silicon-based material-lithophile metal composite layer of this invention, when applied to the structure of a battery negative electrode sheet, can solve the problems of capacity decay, thermal runaway, and safety caused by the formation of lithium dendrites in the negative electrode sheet.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery anode technology, specifically to an anode sheet and its application. Background Technology

[0002] In recent years, with the rapid development of society, electronic products have had a revolutionary impact on our lives. The widespread use of consumer electronics (mobile phones, tablets, laptops) has brought convenience to our lives. The emergence of new energy vehicles has not only solved people's travel problems but also reduced their reliance on gasoline, saving on living costs and contributing to environmental improvement. As people's demands for the performance and functionality of electronic devices increase, they also place higher demands on the performance of lithium-ion batteries. People not only want smaller, thinner, and larger-capacity batteries but also faster charging speeds.

[0003] Graphite anodes have essentially reached their limit with a theoretical capacity of only 372 mAh / g, making further breakthroughs difficult. Improving battery energy density requires a redesign from a materials perspective. Silicon-based anodes offer higher capacity, with a theoretical capacity of up to 4200 mAh / g. However, silicon-based anodes suffer from low initial charging efficiency. During the first charge of a lithium-ion battery, the organic electrolyte undergoes reduction and decomposition on the anode surface, forming a solid electrolyte interface film. This irreversibly consumes a large amount of lithium ions from the cathode, resulting in a low coulombic efficiency in the first cycle. Furthermore, the volume change of silicon during charge-discharge cycles further affects the contact between the anode current collector and the active material, leading to rapid capacity decay. Summary of the Invention

[0004] This invention addresses the problems in the prior art by disclosing a negative electrode sheet and its application. The invention yields a silicon-based material that overcomes the technical problem of volume expansion of silicon during battery charge-discharge cycles. To further improve the application of this silicon-based material in battery negative electrodes and enhance its lithium affinity during fast charging, a lithium affinity metal is added to the silicon-based material, resulting in the negative electrode sheet structure of this invention. When applied to batteries, this negative electrode sheet structure can improve the fast-charging lithium intercalation reaction speed of lithium-ion batteries and reduce capacity decay, thermal runaway, and safety issues caused by lithium dendrites generated at the negative electrode.

[0005] This invention is achieved through the following technical solution:

[0006] This invention provides a negative electrode sheet, the structure of which includes a silicon-based material-lithophile metal composite layer disposed on the outermost layer of at least one side of the current collector. The silicon-based material-lithophile metal composite layer includes a silicon-based material and a lithiumophile metal. The silicon-based material is Si / SiO2@C1@C2, wherein Si is silicon, SiO2 is silicon oxide, and C1 is a porous gaseous carbon layer coating Si / SiO2 particles. Si, SiO2, and C1 constitute a hollow "core-shell" structure of Si / SiO2@C1. C2 is a pitch carbon shell, which coats the outer ring of Si / SiO2@C1 to form a second buffer layer, thus constituting a hollow "egg-shell" structure of Si / SiO2@C1@C2.

[0007] The present invention provides a hollow "egg-shell" structured silicon-based material Si / SiO2@C1@C2. The "egg-shell" structure includes an egg structure and a shell structure, with the shell structure encapsulating the egg structure. The shell structure is C2, and the egg structure is a hollow "core-shell" structure formed by C1 encapsulating Si and SiO2 particles. When the silicon-based material of the present invention is used as a negative electrode, the hollow structure in the egg structure provides space for the expansion of Si, effectively solving the pulverization phenomenon caused by expansion and improving cycle performance. C2 can fill the pores generated on the surface of C1 by carbothermic reduction gas, reducing the specific surface area and thus reducing side reactions with the electrolyte, which is beneficial to improving the cycle performance of the battery.

[0008] While the "egg-shell" structure of silicon-based materials in this invention helps to overcome the volume expansion of silicon-based materials during battery charge-discharge cycles, due to the presence of the "shell structure" of the silicon-based materials, we expect to further improve the lithium insertion / extraction capability of the silicon-based materials. This would allow the "egg-shell" structure of the silicon-based materials to rapidly accept lithium during battery fast charging, thereby reducing the formation of lithium dendrites and thus reducing the battery capacity decay, thermal runaway, and safety issues caused by lithium dendrites.

[0009] The addition of a lithium-affinity metal to the silicon-based material in this invention enhances the affinity between silicon and lithium, thereby accelerating the fast-charging lithium intercalation reaction. Furthermore, when trace amounts of lithium plating occur due to defects or uneven charge distribution on the negative electrode, the metal adsorbs the generated elemental lithium, preventing lithium dendrite formation. This elemental lithium can also be released during delithiation at the negative electrode, preventing dead lithium formation and thus avoiding capacity loss. This invention not only solves the capacity decay, thermal runaway, and safety issues caused by lithium dendrite formation at the negative electrode during fast charging of lithium-ion batteries, but also improves the battery's energy density and reduces capacity decay and safety issues caused by negative electrode expansion during charge-discharge cycles.

[0010] The silicon-based material Si / SiO2@C1@C2 in this invention. Here, " / " is generally used to indicate a heterogeneous structure within the material structure; "@" is generally used to indicate a structure in which the material following "@" encapsulates the material preceding "@".

[0011] As a further embodiment, the particle size of the SiO2 is in the range of 200nm-1000nm; the thickness of the C2 is 8nm-12nm; and the thickness of the C1 is 1nm-4nm.

[0012] As a further embodiment, the silicon-based material Si / SiO2@C1@C2 exhibits characteristic diffraction peaks of 22°, 26°, and 28° in its X-ray powder diffraction pattern expressed at a diffraction angle of 2θ.

[0013] As a further embodiment, the structure also includes one or more layers of graphite, carbon, or graphite-carbon composite material disposed on at least one side of the current collector outward to the silicon-based material-lithophile metal composite layer.

[0014] As a further embodiment, the graphite material in the graphite layer and the graphite-carbon composite layer includes one or more of artificial graphite, natural graphite, and composite graphite.

[0015] As a further embodiment, the carbon material in the carbon material layer and the graphite-carbon material composite layer includes one or more of hard carbon and soft carbon.

[0016] As a further embodiment, the lithium-loving metal in the lithium-loving metal layer may include one or more of Ag, Mg, Sn, Ti, and metal-organic frameworks.

[0017] As a further improvement, the lithium-loving metal is Ag.

[0018] As a further embodiment, the amount of the lithium-loving metal added is no more than 5% of the total mass of the total active material, wherein the total mass of the total active material is the total mass of one or more of the graphite material in the graphite layer coated on the surface of the current collector, the carbon material in the carbon material layer, the silicon-based material, the graphite material in the graphite-carbon composite layer, and the carbon material.

[0019] As a further embodiment, the graphite layer has a thickness of 10-150 μm.

[0020] As a further embodiment, the thickness of the carbon material layer is 1-20 μm.

[0021] As a further embodiment, the thickness of the graphite-carbon composite layer is 1-20 μm.

[0022] As a further embodiment, the thickness of the silicon-based material-lithophile metal composite layer is 1-100 μm.

[0023] As a further embodiment, in the structure, from the current collector outwards to the silicon-based material-lithophile metal composite layer, the D50 of the active material in the layered structure decreases layer by layer away from the current collector. The D50 design of each layer in this invention is beneficial for improving the Li-type performance during fast charging of the negative electrode structure. + The insertion and extraction rate promotes high-rate fast charging.

[0024] As a further preferred embodiment, the structure includes a graphite layer, a carbon material layer, and a silicon-based material-lithophile metal composite layer sequentially coated outward from at least one side of the current collector;

[0025] The D50 of the graphite layer is 10-20 μm; the D50 of the carbon material layer is 8-12 μm; the D50 of the silicon-based material in the silicon-lithophile metal composite layer is <9 μm; and the D50 of the graphite layer is greater than that of the carbon material layer, which in turn is greater than that of the silicon-based material in the silicon-lithophile metal composite layer. This negative electrode structure, during fast charging, creates a gradient distribution of lithium-ion concentration from high to low in the direction from the electrolyte to the current collector, effectively improving the overall rate performance of the negative electrode. Furthermore, the reduced orderliness, combined with a smaller particle distribution design, further facilitates high-rate fast charging.

[0026] As a further option, the current collector includes copper foil current collectors, PET-copper composite current collectors, Al foil current collectors, titanium foil current collectors, and Ag foil current collectors. It is not easily reduced under low voltage.

[0027] The present invention also provides a method for preparing the silicon-based material Si / SiO2@C1@C2, comprising:

[0028] S1: grinding SiO2;

[0029] S2: Gas phase coating, SiO2 is coated using a gas phase carbon source to obtain SiO2@C1;

[0030] S3: Heat-treat the generated SiO2@C1 to convert some of the SiO2 into Si, and obtain Si / SiO2@C1;

[0031] S4: The obtained Si / SiO2@C1 is mixed with asphalt and heated to the carbonization temperature to obtain the final product Si / SiO2@C1@C2. When the gaseous carbon source coats the surface of SiO2, a reaction occurs at a certain temperature. At the same time, some SiO2 is converted into Si, and the volume of SiO2 decreases and the particles become finer. The surface coated with the gaseous carbon source also forms a porous gaseous carbon layer due to the overflow of CO generated during the reaction, thus forming Si / SiO2@C1 with a hollow "core-shell" structure. Asphalt is conducive to the formation of C2 with a certain strength, which can not only fill the pores of C1 in S3, but also reserve space between C1 and C2, and can also buffer the volume expansion when silicon-based materials are used in the negative electrode of the battery. In this invention, coating SiO2 with a gas-phase carbon source makes it easier to achieve uniform coating, which is more conducive to reducing the volume and refining the particles of SiO2, thus forming a "core-shell" structure. Secondly, using a gas-phase carbon source for coating results in a relatively thinner coating layer compared to other methods, which facilitates the overflow of CO generated during the reaction process from the gas-phase carbon layer, thereby reserving the volume for silicon expansion and forming a hollow "core-shell" structure. Due to the porous gas-phase carbon layer formed, in order to further stabilize the structural stability of the silicon-based material in the battery negative electrode, we added asphalt with a certain strength as a second buffer layer. This reduces the specific surface area and has a strong binding effect on the volume expansion of silicon during battery cycling, while also having a certain degree of elasticity, thereby reducing the battery expansion rate and improving the battery safety performance.

[0032] As a further embodiment, the gaseous carbon source includes one or more of CH4, C2H2, C2H4, C2H6, C3H3, C3H6, and C3H8.

[0033] In the preparation of silicon-based materials Si / SiO2@C1@C2, each step has a certain correlation with the final product. By comprehensively adjusting the conditions during the preparation process, the silicon-based materials prepared in this invention can be successfully obtained. Among these factors, the quality of the asphalt, the gas flow rate of the gaseous carbon source, the heating rate of the heat treatment in step S3, the temperature range of the heat treatment, and the heat treatment time play a more significant role in the electrochemical performance of the resulting silicon-based materials, enabling adjustments to the Si content and the thickness of C1 and C2. Therefore, under the basic condition of successfully preparing silicon-based materials, the performance of the obtained silicon-based materials can be improved by further optimizing the quality of the asphalt, the gas flow rate of the gaseous carbon source, the heating rate of the heat treatment in step S3, the temperature range of the heat treatment, and the heat treatment time.

[0034] As a further embodiment, the mass of the asphalt is 400g-800g; the gas phase coating conditions in S2 are that the gas flow rate of the gas phase carbon source is 800mL / min-1800mL / min; the heating rate of the heat treatment in S3 is 1℃ / min-8℃ / min, the temperature range of the heat treatment is 1600℃-2500℃, and the heat treatment time is 0.5h-3h. The gas flow rate of the gaseous carbon source is directly related to the thickness of the generated C1. Controlling the gas flow rate of the gaseous carbon source can obtain a suitable C1 thickness. In S3, the purpose of rapid heating is to prevent the generation of excessive Si, which would lead to excessive expansion, causing C1 to crack and affecting cycling. If the holding time is too short, the reaction is not sufficient, resulting in less Si and a smaller capacity of the final product. If the reaction time is too long, excessive Si is generated. When silicon-based materials are used in the negative electrode of the battery, the battery expands significantly during cycling, eventually causing C1 to crack and C2 to deform, thus affecting the performance of the silicon-based materials. Therefore, it is necessary to have a C2 of a certain thickness to reduce the expansion of the silicon-based materials. The thickness of C2 can be controlled by controlling the quality of the asphalt, which helps to reduce the expansion rate of the silicon-based materials.

[0035] As a further improvement, the mass of the asphalt is 600g-800g. This allows for the formation of a suitable "egg-shell" structure with C2 thickness, which helps reduce the battery's expansion rate.

[0036] As a further refinement, the gas-phase coating conditions in S2 are such that the gas flow rate of the gaseous carbon source is between 1000 mL / min and 1800 mL / min. This results in a more suitable C1 thickness and better electrochemical performance of the battery.

[0037] As a further refinement, the conditions for gas-phase coating in S2 also include a temperature range of 1000℃-1400℃ and a heating rate of 1℃ / min-5℃ / min. Temperature variations will affect the coating effect of the gas-phase carbon source on SiO2 to some extent. When the gas-phase coating temperature is too low, more Cl impurities may be formed, leading to more side reactions; when the gas-phase coating temperature is too high, energy consumption is excessive. Conversely, when the heating rate is too low, energy consumption is excessive; when the heating rate is too high, the uniformity of Cl coating is affected.

[0038] As a further improvement, the heating rate in S3 is 1℃ / min-5℃ / min, the temperature range is 1800℃-2500℃, and the heat treatment time is 1h-3h. This generates an appropriate amount of Si, which not only helps to improve the battery capacity but also ensures that the thickness of C1 is within a suitable range, thereby reducing the battery expansion rate.

[0039] As a further embodiment, the carbonization temperature in step S4 is 1000℃-1400℃, and the heating rate is 1℃ / min-5℃ / min. The carbonization temperature has a certain impact on the uniformity of the generated C2 coating. When the temperature is too low, more C2 impurities may be formed, leading to more side reactions; when the carbonization temperature is too high, energy consumption is too high. Conversely, a heating rate that is too low will result in excessive energy consumption, while a heating rate that is too high will affect the uniformity of the C2 coating.

[0040] As a further embodiment, the mass ratio of Si / SiO2@C1 to asphalt is (95-99):(1-5).

[0041] The present invention also provides an electrochemical device having the negative electrode, which can be used in end consumer products. The end consumer products applied for include, but are not limited to, mobile phones, laptops, pen input computers, mobile computers, e-book players, portable phones, portable fax machines, portable copiers, and portable printers.

[0042] The present invention also provides an electrochemical device having the aforementioned negative electrode, which can be used in electrical equipment, including large portable electrical equipment and small portable electrical equipment. Small portable electrical equipment includes consumer products, wearable electronic devices, or portable electronic devices; large portable electrical equipment includes transportation equipment. Transportation equipment includes, but is not limited to, vehicles such as automobiles, motorcycles, electric bicycles, bicycles, buses, subways, high-speed trains, airplanes, and ships; wearable electronic devices or portable electronic devices include, but are not limited to, headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, drones, motors, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors. The negative electrode of the battery of the present invention is used in an electrochemical device, which can be housed in an electrical device in the form of an electrochemical device. Typically, the electrochemical device includes a battery pack or / multiple battery modules or / single battery module or / single cell and a management system for controlling them.

[0043] The features and beneficial effects of this invention are as follows:

[0044] (1) In the core-shell structure, some SiO2 is reduced to Si and CO to form a C1-coated SiO2 / Si hollow structure, which reserves space for the expansion of Si, effectively solves the pulverization phenomenon caused by expansion, alleviates the overall cell expansion, and improves cycle performance.

[0045] (2) By preparing a silicon-based material-lithophile metal composite layer, the silicon-based material is applied to the negative electrode sheet of the battery, and a lithiophile metal material is added. The prepared silicon-based material not only controls the volume change of silicon, but the added lithiophile metal material can also improve the lithium extraction and insertion capabilities of the silicon-based material. The silicon-based material-lithophile metal composite layer of the present invention, when applied to the negative electrode sheet structure of the battery, can not only solve the battery capacity decay, thermal runaway and safety problems caused by the formation of lithium dendrites on the negative electrode sheet, but also solve the capacity decay and safety problems caused by the volume change of silicon during the battery charge and discharge cycle. Attached Figure Description

[0046] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0047] Figure 1 This is a schematic diagram showing the expansion of silicon-based materials in the negative electrode sheet before and after charging, according to an embodiment of the present invention.

[0048] Figure 2 The image shows the XRD pattern of the silicon-based material Si / SiO2@C1@C2 provided in the embodiments of the present invention.

[0049] Figure 3 The capacity retention rate for 0.33C cycle performance is provided for the embodiments and comparative examples of the present invention.

[0050] Figure 4 This is a schematic diagram of the negative electrode structure provided in an embodiment of the present invention.

[0051] Figure 5 This is a schematic diagram of the negative electrode structure provided in an embodiment of the present invention.

[0052] Figure 6 This is a schematic diagram of the negative electrode structure provided in an embodiment of the present invention.

[0053] Figure 7 This is a schematic diagram showing lithium plating occurring after fast charging of negative electrode sheets with different structures provided in the embodiments and comparative examples of the present invention.

[0054] The above figures include the following reference numerals:

[0055] 1-Current collector; 2-Graphite layer; 3-Carbon material layer; 4-Silicon-based material-lithophile metal composite layer; 5-Graphite-carbon material composite layer. Detailed Implementation

[0056] To facilitate understanding of the negative electrode sheet of the present invention, a more comprehensive description of the negative electrode sheet of the present invention will be given below, and embodiments of the present invention will be provided, but this does not limit the scope of the present invention.

[0057] (1) Methods for preparing silicon-based materials, including:

[0058] S1: SiO2 is selected as the raw material, and SiO2 is ground into powder with a particle size of 200nm-1000nm;

[0059] S2: Using a gas phase coating method, one or more of the following gas phase carbon sources are selected: CH4, C2H2, C2H4, C2H6, C3H3, C3H6, and C3H8. The gas flow rate is controlled at 800 mL / min-1800 mL / min, the temperature is set at 1000℃-1400℃, and the heating rate is 1℃ / min-5℃ / min to obtain SiO2@C1.

[0060] S3: Rapidly heat the SiO2@C1 generated in S2 to 1600℃-2500℃, with the heating rate controlled at 1℃ / min-8℃ / min. After reaching the highest temperature, the holding time is controlled at 0.5h-3h to obtain a hollow "core-shell" structure of Si and SiO2 coated with C1 layer—Si / SiO2@C1.

[0061] S4: After the reaction in S3 is complete, mix with asphalt and heat. Set the carbonization temperature to 1000℃-1400℃ and the heating rate to 1℃ / min-5℃ / min, so that the outermost layer is coated with C2. The final product is a silicon-based material Si / SiO2@C1@C2 with a hollow "egg-shell" structure, as shown in the schematic diagram below. Figure 1 As shown. From Figure 1 It can be observed that in silicon-based materials, there can be reserved space between C1 and C2. When C1 and C2 are partially connected, the unconnected parts of C1 and C2 form the reserved space.

[0062] The mass of asphalt is 400g-800g, and the mass ratio of Si / SiO2@C1 to asphalt is (95-99):(1-5).

[0063] Based on this, we optimized the conditions of the silicon-based material to obtain a silicon-based material with better electrical and safety performance for the battery. We prepared a slurry with the silicon-based material of this invention, binder, and conductive agent, and coated it on both sides of the negative electrode current collector of the battery, and then studied the electrical performance of the battery, as shown in Examples 1-13 in Table 2. We further added a lithiophilic metal to the slurry and then coated it on both sides of the negative electrode current collector of the battery, obtaining a silicon-based material-lithophilic metal composite layer on the surface of the current collector. We applied the silicon-based material-lithophilic metal composite layer in the negative electrode structure of the battery, as shown in Examples 14-16 in Table 2, to further study the improvement of the battery's electrical and safety performance by the combined effect of the lithiophilic metal material and the silicon-based material. Among them, Ag was selected as the lithiophilic metal, and the amount of lithiophilic metal added was 1% of the total mass of the active material. In Examples 14-16, the amount of lithiophilic metal added was 1% of the silicon-based material.

[0064] We also investigated the lithium deposition on the negative electrode and the battery capacity retention when silicon-based materials and lithium-based metal composite layers, obtained by combining silicon-based materials and lithium-based metals with graphite layers, carbon material layers, and graphite-carbon material composite layers during fast charging. We obtained a silicon-based material-lithophile metal composite layer using the method in Example 14, and then combined the obtained silicon-based material-lithophile metal composite layer with graphite layers, carbon material layers, and graphite-carbon material composite layers to prepare negative electrode sheets as described in Examples 17-20. We also set up comparative examples for comparison.

[0065] Example 17: The negative electrode adopts the above three-layer coating, from the inside out as follows: First layer: graphite layer, using artificial graphite, D50 is 13μm, and areal density is 136g / m³. 2 The second layer is a carbon material layer made of hard carbon with a D50 of 9 μm and an areal density of 8 g / m³. 2 The third layer is a silicon-based material-lithophile metal composite layer, wherein the silicon-based material has a D50 of 6 μm and an areal density of 16 g / m³. 2 The lithium-loving metal is Ag, added at 1% of the total active material, with a particle size of 100 nm.

[0066] Example 18: The negative electrode uses the above four-layer coating, from the inside out as follows: First layer: graphite layer, made of artificial graphite, with a D50 of 13μm and an areal density of 136g / m³. 2 The second layer is a carbon material layer, made of soft carbon with a D50 of 8μm and an areal density of 8g / m³. 2 The graphite-carbon composite layer has a graphite to carbon material mass ratio of 1:1, using artificial graphite and hard carbon, with a D50 of 7μm and an areal density of 2g / m³. 2 Fourth layer: Silicon-based material-lithophile metal composite layer, wherein the silicon-based material has a D50 of 6μm and an areal density of 16g / m³.2 The lithium-loving metal is Ag, added at 1% of the total active material, with a particle size of 100 nm.

[0067] Example 19: The negative electrode uses the above two-layer coating, from the inside out: First layer: graphite layer, made of artificial graphite, with a D50 of 13μm and an areal density of 136g / m³. 2 The second layer is a silicon-based material-lithophile metal composite layer, wherein the silicon-based material has a D50 of 7 μm and an areal density of 18.286 g / m³. 2 The lithium-loving metal is Ag, added at 1% of the total active material, with a particle size of 100 nm.

[0068] Example 20: The negative electrode uses the above two-layer coating, from the inside out: First layer: carbon material layer, using hard carbon, D50 is 9μm, and areal density is 50g / m³. 2 The second layer is a silicon-based material-lithophile metal composite layer, wherein the silicon-based material has a D50 of 6 μm and an areal density of 90 g / m³. 2 The lithium-loving metal is Ag, added at 1% of the total active material, with a particle size of 100 nm.

[0069] Comparative Example 1: The negative electrode uses a three-layer coating, from the inside out: First layer: graphite layer, made of artificial graphite, with a D50 of 13μm and an areal density of 136g / m³. 2 The second layer is a carbon material layer, made of hard carbon acrylic with a D50 of 9μm and an areal density of 8g / m³. 2 The third layer is a silicon-based material layer, wherein the silicon-based material has a D50 of 6μm and an areal density of 16g / m³. 2 .

[0070] Comparative Example 2: The negative electrode uses a three-layer coating, from the inside out: First layer: silicon-based material-lithophile metal composite layer, wherein the silicon-based material has a D50 of 6μm and an areal density of 16g / m³. 2 The second layer is a graphite layer made of artificial graphite with a D50 of 13μm and a surface density of 136g / m³. 2 The third layer is a carbon material layer made of hard carbon with a D50 of 9μm and an areal density of 8g / m³. 2 The amount of Ag added is 1% of the total active substances.

[0071] We first applied the obtained silicon-based material to the negative electrode of a battery to study the improvement of the battery's electrochemical and safety performance by the silicon-based material in this invention, as shown in Examples 1-13 in Table 2. We further added a lithiophilic metal to the slurry to further study the improvement of the lithiophilic properties of the silicon-based material and its effect on the battery's electrical performance, as shown in Examples 14-16 in Table 2.

[0072] (2) Battery manufacturing method:

[0073] Preparation of negative and positive electrode sheets: A negative electrode sheet is made from silicon-based materials Si / SiO2@C1@C2:SWCNT (single-walled carbon nanotubes):PAA (polyacrylic acid):SP (conductive carbon black) in a mass ratio of 94:0.2:3:2.8, and a positive electrode sheet is made from NCM811 (lithium nickel cobalt manganese oxide):PVDF (polyvinylidene fluoride):SP (conductive carbon black) in a mass ratio of 95:2:3. The electrodes are then assembled into a pouch cell.

[0074] Based on the above-mentioned method for preparing the negative electrode sheet, the preparation of the silicon-based material-lithophile metal composite layer involves adding a lithiophile metal, wherein the lithiophile metal is selected from Ag, and the amount of Ag added is 1% of the total mass of the total active material.

[0075] (3) Battery capacity test: Cyclic performance test was conducted at room temperature (0.33°C).

[0076] (4) A positive electrode sheet was made of silicon-based material Si / SiO2@C1@C2:SP:CMC (sodium carboxymethyl cellulose):SBR (styrene-butadiene rubber) in a ratio of 91:3:3:3. A Li sheet was used as the negative electrode to assemble a coin cell. The cell was discharged at a constant current of 0.05C to 5mV and then charged at a constant current of 0.05C to 2V. The first-cycle capacity of the Si / SiO2@C1@C2 was obtained by testing the ratio of the first-cycle charging capacity to the mass of Si / SiO2@C1@C2.

[0077] (5) The test method for electronic conductivity shall comply with GB / T30835-2014.

[0078] (6) Test method for battery cell expansion rate: First, measure the overall thickness in the empty state before cycling with a micrometer and record it as h1. After the cell has cycled to a certain number of times, charge it at 0.5C to obtain the cell thickness h2. Cell expansion rate = (h2-h1) / h1.

[0079] The silicon-based materials were tested using the above testing methods, and the test results are shown in Table 2.

[0080] (7) The prepared battery was subjected to low-rate charge-discharge test and 50-cycle fast charging test at room temperature:

[0081] The low-rate charge / discharge test is conducted under the following conditions: 1) Let stand for 10 minutes; 2) Discharge at 0.1C to 3V; 3) Let stand for 10 minutes; 4) Charge at 0.1C to 4.48V and maintain constant voltage at 0.025C; 5) Repeat the above steps twice.

[0082] The fast charging cycle test was conducted 50 times under the following conditions: 1) Rest for 10 minutes; 2) Discharge at 1.5C to 3V; 3) Rest for 10 minutes; 4) Charge at 7C to 4.2V, and maintain constant voltage at 5C; 5) Charge at 5C to 4.3V, and maintain constant voltage at 4C; 6) Charge at 4C to 4.4V, and maintain constant voltage at 3C; 7) Charge at 3C to 4.48V, and maintain constant voltage at 0.025C; 8) Repeat the above steps 50 times.

[0083] We categorized lithium plating into 10 levels based on severity, with higher values ​​indicating more severe plating. The overall characteristics of the 10 levels are shown in Table 1.

[0084] Table 110 Lithium Plating Status by Grade

[0085] grade Lithium plating 0 No lithium plating; 1 Point-like lithium deposition, with lithium coverage on the negative electrode surface between 0% and 3%; 2 Extremely slight lithium plating, with lithium coverage on the negative electrode surface between 3% and 5%; 3 Slight lithium plating, with lithium coverage on the negative electrode surface between 5% and 15%; 4 Lithium plating occurs in a very small portion, with lithium coverage on the negative electrode surface ranging from 15% to 30%. 5 Lithium plating occurred in a small portion, with lithium coverage on the negative electrode surface ranging from 30% to 45%. 6 Lithium plating occurred in some areas, with lithium coverage on the negative electrode surface ranging from 45% to 60%. 7 The vast majority of them exhibit lithium plating, with lithium coverage on the negative electrode surface ranging from 60% to 75%. 8 Lithium is deposited on almost the entire surface, with a lithium coverage of 75-90% on the negative electrode surface; 9 Lithium deposition is achieved across the entire surface, with a lithium coverage rate of ≥90% on the negative electrode surface;

[0086] Table 2 Application results of different silicon-based materials

[0087]

[0088] Table 3. Structures of negative electrode sheets with different structures

[0089] —— First layer Second floor Third layer Fourth floor Example 14 Silicon-based material-Ag composite layer —— —— —— Example 17 Artificial graphite Hard carbon Silicon-based material-Ag composite layer —— Example 18 Artificial graphite soft carbon Artificial graphite-hard carbon composite layer Silicon-based material-Ag composite layer Example 19 Artificial graphite Silicon-based material-Ag composite layer —— —— Example 20 Hard carbon Silicon-based material-Ag composite layer —— —— Comparative Example 1 Artificial graphite Hard carbon Silicon-based materials —— Comparative Example 2 Silicon-based material-Ag composite layer Artificial graphite Hard carbon —— Example 1 Silicon-based materials —— —— ——

[0090] Table 4 Cell Test Data

[0091]

[0092] Using the preparation method of this invention, we prepared Examples 1-13 with the same SiO2 mass (20 kg), selected different gaseous carbon sources, and explored the effects of the silicon-based materials Si / SiO2@C1@C2 obtained under different conditions on the electrochemical and safety performance of the battery anode. The results are shown in Table 2. We successfully prepared the silicon-based materials Si / SiO2@C1@C2. In the X-ray powder diffraction pattern expressed as a diffraction angle of 2θ, characteristic diffraction peaks at 22°, 26°, and 28° were observed, which are the characteristic peaks of SiO2, C, and Si, respectively. Figure 2 As shown. We compared the cycle performance of batteries using silicon-based materials obtained by the method of this invention with that obtained by conventional methods at a rate of 0.33C at room temperature, as follows. Figure 3 As shown, we found that the capacity retention rate of the battery containing silicon-based material obtained by the method of the present invention is about 90% after about 200 cycles, while the capacity retention rate of the battery obtained by the conventional method is about 82%. It can be seen that the method of the present invention to obtain silicon-based material is beneficial to improving the cycle performance of the battery.

[0093] We further optimized the application of silicon-based materials prepared under different conditions in the battery anode. As shown in Table 2, different conditions have different degrees of improvement on the electrochemical performance of the battery. The gas flow rate of the gaseous carbon source, the heating rate, temperature, and holding time in preparation process S3, and the amount of asphalt used are all closely related to the thickness of C1 and C2, as well as the electrochemical performance and safety performance of the battery. The gas flow rate of the gaseous carbon source is directly related to the final thickness of C1. It can be seen that when the gas flow rate of the gaseous carbon source is small, the final thickness of C1 is small, and the electronic conductivity of the battery will decrease. However, in S3, the gaseous carbon source reacts with SiO2 to generate Si, which also indirectly affects the thickness of the generated C1. Furthermore, the reaction between the gaseous carbon source and SiO2 also affects the amount of Si generated, thus affecting the battery's first-cycle capacity, as verified in Examples 11 and 12. It can also be observed that the holding time in S3 has the greatest impact on the amount of Si generated. The battery's expansion rate is also directly related to the thickness of C2 and the amount of Si generated. When the amount of Si generated increases, it leads to a decrease in the number of empty structures during battery cycling, resulting in structural changes, such as... Figure 1 As shown; however, the present invention can control the thickness of the final C2 by controlling the amount of asphalt used. It is evident that the individual factors in Table 2 are closely related in the preparation of silicon-based materials. To obtain better silicon-based materials, we further optimized the condition parameters of the silicon-based materials, using a battery expansion rate of no more than 10% and a first-cycle capacity of no less than 1500 mAh / g as standards.

[0094] As shown in Table 2, the electrochemical and safety performance of the batteries obtained from the silicon-based materials of Examples 1-10 prepared in this invention are superior to those of Examples 11-13. We believe that the hollow "egg-shell" structure of the silicon-based material prepared in this invention not only reserves space for the expansion of Si, but also effectively solves the pulverization phenomenon caused by the expansion of Si in conjunction with the internal core-shell structure. It also reduces the expansion of the battery cell, thereby improving the cycle performance, capacity, and safety performance of the battery. Figure 1 As shown.

[0095] We further explored the optimization of preparation parameters under different conditions. First, we further optimized the gas flow rate of the gas phase carbon source, which affects the thickness of C1 in the generated "core-shell" structure. When the gas flow rate is higher, the thickness of the generated C1 is greater, and when the gas flow rate is lower, the thickness of the generated C1 is smaller. Comparing Examples 1, 2, and 11, we found that Example 2 obtained the largest C1 thickness, while Example 11 obtained the smallest C1 thickness. We further discovered that the thickness of C1 affects the electrochemical performance of the battery. A thicker C1 layer is less conducive to the capacity advantage of Si in C1, but it helps to overcome the volume change of silicon. However, if the gas flow rate is lower, the thickness of the formed C1 layer is lower, which may increase the battery expansion rate. We believe that this is primarily because the gaseous carbon source reacts with SiO2. While SiO2 generates Si, its volume decreases and its particles become finer, and CO gas is also generated, thus reducing the thickness of C1. Secondly, because the gas flow rate of the gaseous carbon source is lower, it cannot uniformly cover the SiO2 surface. The generated CO overflows, resulting in the formation of C1 with more pores. As shown in the comparison between Example 1 and Example 2, although Example 2 achieved better cycle performance, electronic conductivity, and lower battery cell expansion rate, the first-cycle capacity of the battery in Example 2 was significantly lower than that in Example 1. Considering the overall electrochemical performance of the battery, we further selected a gas flow rate of 1000 mL / min to 1400 mL / min for the gaseous carbon source.

[0096] Based on this, we further investigated the optimal conditions for generating the core-shell structure—Si / SiO2@C1—in S3 by heating rate, temperature, and holding time. Increasing the heating rate, lowering the temperature, and reducing the holding time can effectively prevent an increase in Si formation. Although an increase in Si content is beneficial for utilizing Si's capacity and thus improving the battery's first-cycle capacity, as seen in the comparison between Examples 3-5 and Examples 1-2 and Examples 6-10, the silicon-based materials obtained in Examples 3-5 yielded the highest first-cycle capacity, but their electronic conductivity was lower than that of the other examples. We believe this may be because excessive Si formation leads to increased volume changes during battery cycling, which in turn damages the negative electrode structure, resulting in reduced electronic conductivity and cycle performance. We verified our hypothesis by observing the expansion rate of the battery cells in Examples 3-5, and we can further verify our hypothesis through Example 1. However, when the amount of Si formation decreases, the thickness of the generated C1 also increases. Although the volume change of the silicon-based material can be controlled by reducing the amount of Si formation, it will also directly lead to a significant decrease in battery capacity, as seen in Example 12. We further discovered that although Examples 1 and 3-5 had the same gas flow rate of gaseous carbon source (1000 mL / min), the thickness of the generated Cl was different. We believe this may be because, due to the need to generate more Si, the amount of Cl and SiO2 participating in the reaction increases, resulting in a decrease in the final Cl coating thickness. We further selected the following settings for S3: a heating rate of 1℃ / min-5℃ / min, a temperature of 1800℃-2500℃, and a holding time of 1h-3h.

[0097] Based on this, we verified the range of asphalt dosage, as shown in the comparison between Examples 1 and 6. We found that the more asphalt used, the thicker the final C2 layer, resulting in an "egg-shell" structure. A thicker C2 layer helps overcome the volume change of silicon-based materials, thus reducing the expansion rate of the battery cell. However, excessive C2 thickness can also lead to a decrease in battery capacity. We also verified this in Example 13, finding that a smaller amount of asphalt resulted in a thinner C2 layer, but with a higher expansion rate and capacity. We found that when the asphalt dosage was between 600g and 800g, the battery expansion rate was consistently low.

[0098] Finally, we selected various gaseous carbon sources in different proportions to obtain different silicon-based materials. We found that the type and proportion of gaseous carbon sources had little impact on the electrochemical and safety performance of the battery, as shown in Examples 1 and 7-10.

[0099] In summary, the hollow structure of silicon-based materials Si / SiO2@C1@C2 not only provides space for the expansion of Si when silicon-based materials are used in battery anodes, effectively solving the problem of electrode pulverization caused by silicon expansion, but also balances capacity and cycle life.

[0100] Based on this, we used the silicon-based material of this invention in a battery. During the fast charging process, we observed lithium plating, as shown in Example 1 of Table 4. To address this, we further added a lithium-loving metal to the slurry containing the silicon-based material. After uniformly mixing the slurry, we coated it onto both sides of the current collector to obtain a silicon-based material-lithophile metal composite layer, which was then assembled into a battery. We further found that the silicon-based material-lithophile metal composite layer obtained by this invention helps reduce lithium plating during fast charging, as shown in Example 14 of Table 4. Furthermore, it significantly improves the battery's first-cycle capacity and cell expansion rate, and further improves the battery's cycle performance and electronic conductivity, as shown by comparing Examples 1 and 14 in Table 2. We can also verify this through comparisons of Examples 3 and 15, and Examples 5 and 16, that adding a lithium-loving metal to the silicon-based material can further improve the battery's electrical and safety performance. We believe this is likely due to the presence of the "shell structure" of the silicon-based material, while the lithium-loving metal material can further enhance the lithium-loving properties of the silicon-based material. Therefore, during fast charging of batteries, lithium-loving metals can, on the one hand, enhance the affinity between silicon-based materials and lithium, thereby increasing the rate of lithium insertion / extraction reaction at the negative electrode. On the other hand, when a small amount of lithium is deposited due to defects and uneven charge distribution in a localized area of ​​the negative electrode, lithium can be adsorbed to prevent the formation of lithium dendrites. At the same time, this portion of lithium can continue to be extracted during the delithiation process at the negative electrode without causing dead lithium, thus avoiding capacity loss and improving battery capacity.

[0101] We also attempted to further investigate the interaction between the silicon-based material layer and other layer structures by preparing other layer structures. As shown in Comparative Example 1 in Table 3, and comparing Comparative Example 1 with Example 1 in Table 4, it was found that the interaction between the silicon-based material layer and other layer structures can also improve lithium plating during battery fast charging to some extent. We believe this may be because the D50 of the active material in each layer of the negative electrode structure in Comparative Example 1 gradually decreases in the direction away from the current collector, and the conductivity gradually increases from the inside to the outside. This is beneficial for electrolyte wetting and electron transport to the outer coating, improving the electronic and ionic conductivity of the electrode, thereby increasing the overall reaction rate of the negative electrode with lithium and reducing lithium plating during battery fast charging.

[0102] Building upon this, to further explore the potential of combining silicon-based material-lithophile metal composite layers with other layer structures to improve lithium plating during fast charging, as shown in Examples 17-20 of Table 3, we examined the results in Table 4. No lithium plating occurred in Examples 17-20. We believe this is because the lithiophile coating facilitates the insertion / extraction of lithium from the silicon-based material and can adsorb trace amounts of lithium plating caused by defects and uneven charge distribution in the negative electrode, thus preventing lithium dendrite formation and reducing the risk of lithium plating. Furthermore, the combination of the lithiophile coating with the D50 (which gradually decreases in the direction away from the current collector) of the active material in each layer of the negative electrode structure further enhances electron and ion conduction, further reducing lithium plating during fast charging.

[0103] We further investigated whether the coating order of the layer structure on the current collector had an impact. The coating order and method of the negative electrode sheets on the current collector differed between Comparative Example 2 and Examples 17-20. We conducted low-rate charge-discharge tests and 50-cycle fast charging tests on Examples 17-20 and Comparative Example 2. The test results are shown in Table 4. From Table 4, we can see that no lithium plating occurred on the negative electrode of Examples 17-20 after 50 cycles, while lithium plating occurred in Comparative Example 2 (e.g., ...). Figure 7 As shown, Example 17 is a representative example. We further verified that when the outermost layers on both sides of the current collector are provided with silicon-based material-lithophile metal composite layers (such as... Figures 4-6 As shown, the design of the active material D50 in the middle layer structure of the negative electrode sheet is necessary to reduce lithium plating during fast charging, thereby improving the battery's electrical and safety performance.

[0104] Further comparison of Examples 17-20 revealed that Example 17 exhibited better electrical performance than the other examples. We believe this is likely due to the ordered arrangement of the graphite layer internally, while the carbon material layer exhibits a highly disordered internal structure, and the negative electrode structure shows a decreasing trend in orderliness. The disordered structure within the carbon material generates numerous defects, thus providing a potential source of electrical energy for Li... + Li provides numerous embedding sites, and due to the large interlayer spacing and randomly arranged crystal structure of the carbon material layers, Li... +The diffusion coefficient in the carbon material layer is significantly higher than that in the graphite layer, resulting in better rate performance for the carbon material layer. This decreasing order within the negative electrode structure creates a gradient distribution of lithium-ion concentration from high to low in the direction from the electrolyte to the current collector during fast charging, effectively improving the overall rate performance of the negative electrode. Furthermore, the decrease in order, combined with a smaller particle distribution design, further promotes high-rate fast charging. We further optimize the current collector by sequentially coating a graphite layer, a carbon material layer, and a silicon-based material-lithophile metal composite layer from at least one side outwards. The D50 of the graphite layer is 10-20 μm; the D50 of the carbon material layer is 8-12 μm; the D50 of the silicon-based material in the silicon-based material-lithophile metal composite layer is <9 μm; and the D50 of the graphite layer is greater than that of the carbon material layer, which in turn is greater than that of the silicon-based material-lithophile metal composite layer.

[0105] In summary, the negative electrode sheet with silicon-based material-lithophile metal composite layer of the present invention, when applied in a battery, can not only reduce capacity decay and safety issues caused by negative electrode expansion during battery charge and discharge cycles, but also reduce capacity decay, thermal runaway and safety issues caused by lithium dendrites during fast charging.

[0106] It should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A negative electrode sheet, characterized in that, The structure of the negative electrode includes a silicon-based material-lithophile metal composite layer disposed on the outermost layer of at least one side of the current collector. The silicon-based material-lithophile metal composite layer includes a silicon-based material and a lithiumophile material. The silicon-based material is Si / SiO2@C1@C2, where Si is silicon, SiO2 is silicon oxide, and C1 is a porous gaseous carbon layer covering Si / SiO2 particles. Si, SiO2, and C1 constitute Si / SiO2@C1 with a hollow "core-shell" structure. C2 is a pitch carbon shell. C2 covers the outer ring of Si / SiO2@C1 to form a second buffer layer, thus constituting Si / SiO2@C1@C2 with a hollow "egg-shell" structure. SiO2 is used as a raw material, and its particle size ranges from 200nm to 1000nm. SiO2 was coated with a gaseous carbon source to obtain SiO2@C1; the generated SiO2@C1 was then heat-treated at a temperature range of 1600℃-2500℃. The lithiophilic material includes one or more of the lithiophilic metals Ag, Mg, Sn, Ti, and metal-organic frameworks.

2. The negative electrode sheet according to claim 1, characterized in that, The silicon-based material Si / SiO2@C1@C2 exhibits characteristic diffraction peaks at 22°, 26°, and 28° in its X-ray powder diffraction pattern, expressed as a diffraction angle of 2θ.

3. The negative electrode sheet according to claim 1, characterized in that, The structure of the negative electrode sheet also includes one or more layers of graphite layer, carbon material layer, and graphite-carbon material composite layer disposed between at least one side of the current collector and the silicon-based material-lithophile metal composite layer.

4. A negative electrode sheet according to claim 3, characterized in that, The graphite material in the graphite layer and the graphite-carbon composite layer includes one or more of artificial graphite, natural graphite, and composite graphite.

5. A negative electrode sheet according to claim 3, characterized in that, The carbon material in the carbon material layer and the graphite-carbon composite layer includes one or more of hard carbon and soft carbon.

6. A negative electrode sheet according to claim 3, characterized in that, The amount of the lithium-loving metal added is no more than 5% of the total mass of the total active material, and the total mass of the total active material is the total mass of one or more of the graphite material in the graphite layer, the carbon material in the carbon material layer, the silicon-based material, the graphite material in the graphite-carbon composite layer, and the carbon material coated on the surface of the current collector.

7. A negative electrode sheet according to claim 3, characterized in that, The graphite layer has a thickness of 10-150 μm; the carbon material layer has a thickness of 1-20 μm; the graphite-carbon material composite layer has a thickness of 1-20 μm; and the silicon-based material-lithophile metal composite layer has a thickness of 1-100 μm.

8. A negative electrode sheet according to claim 3, characterized in that, In the structure of the negative electrode sheet, from the current collector outward to the silicon-based material-lithophile metal composite layer, the D50 of the active material in the layer structure of the negative electrode sheet decreases layer by layer in the direction away from the current collector.

9. A negative electrode sheet according to claim 1, characterized in that, The structure of the negative electrode sheet includes a graphite layer, a carbon material layer, and a silicon-based material-lithophile metal composite layer sequentially coated outward from at least one side of the current collector; The D50 of the graphite layer is 10-20 μm; the D50 of the carbon material layer is 8-12 μm; the D50 of the silicon-based material in the silicon-lithophile metal composite layer is <9 μm; and the D50 of the graphite layer is greater than the D50 of the carbon material layer, and the D50 of the carbon material layer is greater than the D50 of the silicon-based material in the silicon-lithophile metal composite layer.

10. A negative electrode sheet according to claim 1, characterized in that, The current collectors include copper foil current collectors, PET-copper composite current collectors, Al foil current collectors, titanium foil current collectors, and Ag foil current collectors.

11. An electrochemical device, characterized in that, The electrochemical device includes the negative electrode structure as described in any one of claims 1-10.

12. An electrical appliance, characterized in that, The electrical equipment includes the electrochemical device as described in claim 11.

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