Negative pole piece and lithium ion battery

Abstract

The invention relates to the technical field of batteries, in particular to a negative pole piece and a lithium ion battery. According to the negative pole piece, through the special double-active-layer structure, the energy density and the dynamic performance of the battery can be considered, and the k value of the battery can be reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a negative electrode sheet and a lithium ion battery. BACKGROUND

[0002] The existing negative electrode material is mainly graphite-based carbon-based material, and its theoretical specific capacity is low (about 372 mAh / g), which is difficult to meet the demand of high energy density. In recent years, silicon-based materials have attracted widespread attention due to their ultra-high theoretical specific capacity (about 4200 mAh / g). However, the introduction of silicon elements is beneficial to improve the energy density, but the semiconductor characteristics of silicon will cause the blockage of electron transmission and the lengthening of active ion diffusion path, thereby reducing the rate performance and limiting the kinetic performance.

[0003] Therefore, it is of great significance to balance the energy density and kinetic performance through multi-dimensional synergistic effect, and to provide a feasible technical path for the industrialization of high energy density batteries. SUMMARY

[0004] The negative electrode sheet and the lithium ion battery provided by the present application can not only balance the energy density and kinetic performance of the battery, but also reduce the k value of the battery and improve the micro-short circuit test pass rate of the battery.

[0005] The present application provides a kind of negative electrode sheet, including current collector and the negative electrode active layer of at least one surface of the current collector, the negative electrode active layer along first direction, sequentially including first negative electrode active layer and second negative electrode active layer, the first direction is the direction of current collector to negative electrode active layer;

[0006] The first negative electrode active layer includes a first negative electrode active material, and the first negative electrode active material includes a first carbon-based material;The second negative electrode active layer includes a second negative electrode active material, and the second negative electrode active material includes a second carbon-based material and a silicon-based material;The Dv50 of the first carbon-based material is greater than the Dv50 of the second carbon-based material;

[0007] The surface of the negative electrode active layer is provided with a plurality of recesses, and the adjacent recesses form protrusions, and the surface of the negative electrode active layer includes a plurality of protrusions;The ratio A of the projection area of the protrusion on the surface of the second negative electrode active layer to the total surface area of the second negative electrode active layer satisfies: 90%≤A≤99.5%;The sphericity of the silicon-based material is B, which satisfies: 0.95≤A / B≤1.8, 0.5≤B≤1;Wherein, the total surface area of the second negative electrode active layer refers to the projection area of the second negative electrode active layer along the second direction, and the second direction is opposite to the first direction.

[0008] The Dv50 of the first carbon-based material is 10-15 μm, and the Dv50 of the second carbon-based material is 8-13 μm.

[0009] The Raman spectrum ID / IG value G of the first carbon-based material satisfies: 0.03≤G≤0.2, and the Raman spectrum ID / IG value F of the second carbon-based material satisfies: 0.3≤F≤0.6.

[0010] The mass percentage C of silicon element satisfies: 0.4%≤C≤30% based on the total mass of the negative electrode active layer.

[0011] The silicon-based material includes at least one of elemental silicon, silicon-oxygen material and silicon-carbon material.

[0012] The silicon-based material is silicon-carbon material, and the amount H of floating silicon on the surface of the silicon-carbon material satisfies: H≤1.8.

[0013] The particle size distribution value P of the silicon-based material satisfies: 0.8≤P≤1.2.

[0014] The negative electrode active layer further includes a first binder, the first binder includes styrene-butadiene rubber, the weight average molecular weight of the styrene-butadiene rubber is K, and the mass percentage of the styrene-butadiene rubber based on the total mass of the negative electrode active layer is J, satisfying: 900≤J K≤1500.

[0015] The negative electrode active layer further includes a first binder, the first binder includes styrene-butadiene rubber, the weight average molecular weight of the styrene-butadiene rubber is K, and the mass percentage of the styrene-butadiene rubber based on the total mass of the negative electrode active layer is J, satisfying: 900≤J

[0016] The porosity L of the negative electrode sheet satisfies: 25%≤L≤35%.

[0017] The weight loss rate α of the negative electrode active layer satisfies: 0.5%≤α≤3%, and / or the depth of the recess is less than or equal to the thickness of the second negative electrode active layer, and / or the thickness ratio of the first negative electrode active layer to the second negative electrode active layer is (1:9)-(9:1), and / or the width of the recess is 0.05-0.2 mm, and / or the pitch of the recess is 0.5-5 mm.

[0018] The negative electrode sheet described above includes at least one of the following: artificial graphite, natural graphite, mesophase carbon microspheres, petroleum coke, pitch coke, soft carbon, and hard carbon; the negative electrode active layer also includes a conductive agent, which includes one or more of the following: conductive carbon black, acetylene black, Ketjen black, carbon-based negative electrode olefin, carbon fiber, single-walled carbon nanotubes, and multi-walled carbon nanotubes; the negative electrode active layer also includes a second binder, which includes at least one of the following: polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, polyimide, polyamide-imide, and polyvinylidene fluoride.

[0019] The negative electrode sheet described above, wherein the first negative electrode active layer comprises a first carbon-based material, a silicon-based material, a conductive agent, and a binder, and the mass ratio of the four is (50-98):(1-50):(0.1-3):(0.1-3), and the second negative electrode active layer further comprises a silicon-based material, wherein the second negative electrode active layer comprises a second carbon-based material, a silicon-based material, a conductive agent, and a binder, and the mass ratio of the four is (50-98):(1-50):(0.1-3):(0.1-3); wherein the binder comprises a first binder.

[0020] The present invention also provides a lithium-ion battery, comprising a negative electrode sheet and an electrolyte, wherein the negative electrode sheet is the aforementioned negative electrode sheet; the electrolyte comprises fluoroethylene carbonate, and based on the total mass of the electrolyte, the mass percentage I of the fluoroethylene carbonate satisfies: 0.1% ≤ I ≤ 20%.

[0021] The negative electrode sheet provided by this invention improves the energy density of the battery cell by introducing silicon-based materials into the negative electrode active layer. It employs a double-layer active layer structure, with the second negative electrode active layer using a smaller-particle-size second carbon-based material and a surface design of protrusions and concave sections to synergistically improve kinetic performance. The larger-particle-size first carbon-based material in the first negative electrode active layer compensates for the energy density loss caused by the second negative electrode active layer. By synergistically controlling the proportion of the projected area of ​​the protrusions on the surface of the second negative electrode active layer with the sphericity of the silicon-based material, the K-value and short-circuit risk are reduced. Through the above synergistic effect, the negative electrode sheet provided by this invention reduces the battery's K-value and improves the battery's micro-short-circuit test pass rate, thereby improving the battery's safety performance, while maintaining both energy density and kinetic performance. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the longitudinal section of the negative electrode sheet described in Embodiment 1 of the invention.

[0023] In the figure, 1 is the current collector, 2 is the first negative electrode active layer, and 3 is the second negative electrode active layer;

[0024] 31 convex, 32 concave. Detailed Implementation

[0025] To enable those skilled in the art to better understand the solutions of this invention, the following provides a further detailed description of this application. The specific embodiments listed below are merely descriptions of the principles and features of this invention; the examples are only for explaining the invention and are not intended to limit its scope. Based on the embodiments of this invention, other embodiments obtained by those skilled in the art without creative effort are all within the scope of protection of this invention.

[0026] Technical terms:

[0027] The K-value refers to the decrease in battery voltage per unit time (usually measured in mV / h or mV / d). It is used to quantify the self-discharge rate of a battery in a resting state. Its magnitude directly reflects the internal chemical stability of the battery. A smaller K-value indicates a lower self-discharge rate and better storage performance. An abnormally high K-value may indicate problems such as micro-short circuits, electrolyte decomposition, or impurity contamination, which can affect battery life and safety. Therefore, the K-value is a key indicator for evaluating battery consistency, screening problematic cells, and predicting battery health status.

[0028] To address the numerous problems associated with silicon-based materials, current optimization techniques often result in over-optimization of certain properties at the expense of others. Therefore, this application aims to overcome the current limitations by implementing multi-dimensional synergistic optimization to break through the existing challenges in balancing battery energy density, kinetic performance, and safety performance, thus providing a feasible technical path for the industrialization of high-energy-density lithium-ion batteries.

[0029] Specifically, the present invention provides a negative electrode sheet, such as... Figure 1 As shown, it includes a current collector 1 and a negative electrode active layer disposed on at least one surface of the current collector. The negative electrode active layer includes a first negative electrode active layer 2 and a second negative electrode active layer 3 in sequence along a first direction. The first direction is the direction in which the current collector points to the negative electrode active layer.

[0030] The first negative electrode active layer includes a first negative electrode active material, which includes a first carbon-based material. The second negative electrode active layer includes a second negative electrode active material, which includes a second carbon-based material and a silicon-based material. The Dv50 of the first carbon-based material is greater than that of the second carbon-based material.

[0031] Multiple recesses 32 are formed on the surface of the second negative electrode active layer, and protrusions 31 are formed between adjacent recesses. The ratio A of the positive projection area of ​​the protrusions on the surface of the second negative electrode active layer to the total surface area of ​​the second negative electrode active layer satisfies 90%≤A≤99.5%. The sphericity of the silicon-based material is B, and satisfies: 0.95≤A / B≤1.8, 0.5≤B≤1. Wherein, the total surface area of ​​the second negative electrode active layer refers to the positive projection area of ​​the second negative electrode active layer along the second direction, which is opposite to the first direction (i.e., the second direction is the direction in which the negative electrode active layer points to the current collector).

[0032] The electrode provided by this invention can balance the energy density and dynamic performance of the battery, reduce the k-value of the battery, and improve the pass rate of the battery's micro-short circuit test.

[0033] Specifically, introducing silicon-based materials into the negative electrode active layer is beneficial to improving the energy density of the battery cell, but it will reduce the kinetic performance. However, by adopting a double-layer active layer structure, the smaller particle size of the second carbon-based material in the second negative electrode active layer can shorten the diffusion path of active ions and enhance the rapid insertion / extraction capability of active ions, thereby improving the kinetic performance. The larger particle size of the first carbon-based material in the first negative electrode active layer has a higher tap density and fewer internal pores, and can fill more active material in the same volume, thereby compensating for the energy density loss caused by the second negative electrode active layer.

[0034] The protrusions and depressions on the surface of the second negative electrode active layer can improve the wettability of the electrode to the electrolyte, thereby increasing ion transport efficiency and further improving kinetic performance. However, when the protrusion area is relatively small, the pressure at the protrusions is high during hot pressing or expansion. Silicon-based materials with poor sphericity have more sharp edges, making them more likely to puncture the separator, resulting in poor K-value and short circuits. Therefore, by synergistically controlling the proportion of the positive projection area of ​​the protrusions on the surface of the second negative electrode active layer with the control of the sphericity of the silicon-based material, the K-value and short circuit risk can be reduced, the pass rate of micro-short circuit tests can be improved, and the safety performance of the battery can be enhanced.

[0035] In summary, the negative electrode sheet provided by this invention, through the aforementioned synergistic effect, reduces the battery's k-value while balancing the battery's energy density and kinetic performance, improves the battery's micro-short circuit test pass rate, and thus enhances the battery's safety performance.

[0036] In the above, the total surface area of ​​the second negative electrode active layer is the sum of the orthogonal projections of the protrusions and concave parts on the surface of the second negative electrode active layer in the second direction (excluding the longitudinal surface area of ​​the protrusions).

[0037] In this invention, the Dv50 of the first carbon-based material and the second carbon-based material can be obtained by laser particle size analyzer. Specifically, the first carbon-based material and the second carbon-based material can be obtained directly and subjected to the above-mentioned detection. Alternatively, the battery can be discharged to 0% SOC, the negative electrode sheet can be disassembled and removed, or the negative electrode sheet can be obtained directly. After polishing the cross-section of the negative electrode sheet with an argon ion mill, the cross-section image can be obtained in a scanning electron microscope (SEM). Then, particle size measurement and distribution statistics can be performed using electron microscope image analysis software such as ImageJ to obtain the size of its Dv50.

[0038] The sphericity B of silicon-based materials can be achieved through image analysis (such as taking pictures of particles with a scanning electron microscope, extracting the contours with software, and calculating the area equivalent roundness or volume equivalent sphericity) or laser diffraction (using a particle size analyzer to measure the equivalent spherical diameter and shape factor of the particles). For example, the battery is discharged to 0% SOC, the negative electrode is removed, or the negative electrode is directly removed. The negative electrode is polished with an argon ion mill to form a flat surface. The resulting flat surface is imaged using a scanning electron microscope (SEM) in backscatter imaging mode. Then, the silicon-carbon image in the SEM image (backscatter mode) at a certain magnification (e.g., 2500x) is analyzed using image processing software such as Image Pro Plus to obtain the perimeter and area of ​​each particle. The perimeter equivalent radius r1 and area equivalent radius r2 of each silicon-based material particle are calculated. The sphericity S of each particle is then calculated as r2 / r1. Finally, the sphericity of each particle is weighted and averaged to obtain the sphericity.

[0039] The projected area of ​​the protrusions on the second active layer can be obtained by taking an SEM image and then calibrating the number of protrusions and their length and width on a single electrode. The total area of ​​the second negative electrode active layer can be calculated by measuring the length and width of the negative electrode.

[0040] In some embodiments, the Dv50 of the first carbon-based material is 10 μm-15 μm, and the Dv50 of the second carbon-based material is 8 μm-13 μm.

[0041] Specifically, the Dv50 of the first carbon-based material can be 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, or any two of the above values, and the Dv50 of the second carbon-based material can be 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, or any two of the above values.

[0042] When the Dv50 of the first carbon-based material and the second carbon-based material are controlled within the above range, it is beneficial to better balance energy density and kinetic performance.

[0043] In some embodiments, the Raman spectral ID / IG value G of the first carbon-based material satisfies: 0.03≤G≤0.2; and the Raman spectral ID / IG value F of the second carbon-based material satisfies: 0.3≤F≤0.6.

[0044] The ID / IG ratio in the Raman spectrum of carbon-based materials is the intensity ratio of the D peak to the G peak in the Raman spectrum of carbon-based materials, used to quantify the defect density and graphitization degree of the material. A lower ID / IG ratio indicates fewer defects or disordered structures in the material, a higher degree of graphitization, and a more regular and ordered structure, which means that the material has better electrical, thermal, and mechanical properties. A higher ID / IG ratio reflects more defects or disordered structures in the material, a lower degree of graphitization, and this structure can improve the ion insertion / extraction activity of energy storage materials.

[0045] When the Raman spectrum ID / IG values ​​of the first carbon-based material and the second carbon-based material meet the above range, their high-temperature performance can be improved while further enhancing their kinetic properties.

[0046] In this application, the ID / IG ratio of the first carbon-based material and the second carbon-based material can be detected by a laser microscope confocal Raman spectroscopy. Specifically, the first carbon-based anode material and the second carbon-based material can be directly obtained and subjected to the above-mentioned detection. Alternatively, the battery can be discharged to 0% SOC, the anode sheet can be disassembled and removed, or the anode sheet can be directly obtained. After polishing the cross-section of the anode sheet with an argon ion mill to form a plane, the cross-section can be imaged using a scanning electron microscope-Raman combined system. The anode particles can be identified by SEM-EDS, and then Raman spectroscopy tests can be performed on the upper and lower carbon-based anodes respectively using a Raman system.

[0047] In some embodiments, based on the total mass of the negative electrode active layer, the mass percentage C of silicon element satisfies: 0.4% ≤ C ≤ 30%.

[0048] Specifically, the mass percentage C of silicon can be 0.4%, 1%, 5%, 10%, 15%, 20%, 25%, 30%, or any value between any two of the above ranges.

[0049] In the above, the total mass of the negative electrode active layer includes the mass of the first negative electrode active layer and the mass of the second negative electrode active layer. The mass percentage C of silicon element refers to the proportion of the total mass of silicon element in the first and second negative electrode active layers in the total mass of the negative electrode active layer. The mass percentage of silicon element means that silicon element can be distributed only in the second negative electrode active layer or simultaneously in both the first and second negative electrode active layers.

[0050] The mass percentage of silicon can be calculated by first quantitatively detecting the mass of silicon, then detecting the total mass of the negative electrode sheet, and finally calculating the mass percentage. Quantitative detection of silicon can be achieved using acid digestion-inductively coupled plasma mass spectrometry (ICP-MS) or X-ray fluorescence spectrometry (XRF). Specifically, the battery can be discharged to 0% SOC, the negative electrode sheet can be removed, immersed in a DMC (dimethyl carbonate) solution for 10 minutes, dried, and then the negative electrode powder can be scraped off. The silicon content can be determined by using XRF (X-ray fluorescence spectrometry) to utilize the linear relationship between the characteristic X-ray fluorescence intensity and the content of silicon. The total mass of the negative electrode active layer can be obtained by subtracting the mass of the current collector from the mass before scraping off the negative electrode powder.

[0051] In this invention, the silicon-based material includes at least one of elemental silicon, silicon-oxygen materials, and silicon-carbon materials.

[0052] In some embodiments, the silicon-based material is a silicon-carbon material, that is, a composite material obtained by depositing silicon particles in a porous carbon matrix.

[0053] In some embodiments, the amount of floating silicon H on the surface of the silicon-carbon material satisfies: H ≤ 1.8. Floating silicon refers to silicon that has not been deposited into the porous carbon framework and is exposed to the outside of the carbon material. Due to the lack of spatial protection and confinement by the porous carbon, its expansion will be very large. By controlling the amount of floating silicon, the volume expansion can be reduced, and the cycle performance can be further improved.

[0054] The amount of floating silicon, H, can be obtained by using silicon-carbon materials to make lithium coin cells, conducting charge-discharge tests on the coin cells, obtaining the charge-discharge curve of the silicon-carbon coin cells, plotting the DQ / DV curve (differential differential capacity curve) using the specific capacity of the silicon-carbon materials, and then dividing the DQ / DV value at 0.45V by the reversible capacity (usually the discharge capacity of the second cycle). For the amount of floating silicon, H, on the surface of the silicon-carbon material in the electrode, it can be measured as follows: after dissolving the electrode by immersing it in an NMP solution, evaporating the NMP in an oven, scraping off the negative electrode active layer to obtain negative electrode active material powder, and then obtaining the silicon-carbon material by flotation. Taking a conventional graphite + silicon-carbon system as an example, the true density of graphite is greater than that of silicon-carbon. The negative electrode active material is placed in a solution of 1 part diiodomethane (density 3.33 g / cm³). 3 In a mixed solution of 9 parts of graphite and 9 parts of carbon tetrachloride (density 1.59 g / cm3), the density of the mixed solution is much lower than that of graphite and silicon carbon. All active materials are deposited at the bottom of the solution. On this basis, diiodomethane solution is added until the powder floats significantly while some powder settles at the bottom of the solution. At this time, the suspended powder is collected and dried to obtain silicon carbon powder. The above test is performed on a coin cell using silicon carbon powder to obtain the amount of floating silicon H on the surface of silicon carbon material in the electrode.

[0055] In some embodiments, the particle size distribution value P of the silicon-based material satisfies: 0.8 ≤ P ≤ 1.2.

[0056] The P-value reflects the uniformity of the particle size distribution in silicon-based materials, P = (Dv90 - Dv10) / Dv50. A lower P-value indicates a more concentrated particle size distribution, while a higher P-value indicates a more dispersed particle size distribution. A more concentrated particle size distribution can better alleviate stress concentration during the charging expansion process of silicon materials, preventing pulverization and breakage. However, an overly concentrated particle size distribution can lead to insufficient filling of the gaps between larger particles by smaller particles, resulting in a decrease in the compaction density of the material and a corresponding decrease in energy density. Controlling the P-value between 0.8 and 1.2 can improve cycle stability while maintaining energy density.

[0057] The P-value can be obtained by detecting Dv50, Dv90, and Dv10 using a laser particle size analyzer, and then calculated according to the formula P=(Dv90-Dv10) / Dv50.

[0058] Understandably, in order to achieve adhesion of the negative electrode active layer, the negative electrode active layer also includes a first binder, which includes styrene-butadiene rubber (SBR). The weight-average molecular weight of SBR is K, and based on the total mass of the negative electrode active layer, the mass percentage of SBR is J, satisfying: 900 ≤ J. K≤1500.

[0059] The polymer network structure of the binder can encapsulate the active material to form a three-dimensional constraint layer, restricting the displacement of particles when their volume changes (i.e., inhibiting particle expansion), while uniformly transferring local expansion stress to the surrounding structure to avoid particle breakage or pulverization caused by stress concentration, thereby maintaining the integrity of the electrode structure. However, a large amount of binder will reduce the kinetic performance and energy density of the battery, while a small amount of binder is difficult to achieve a good bonding effect. Controlling the mass ratio of styrene-butadiene rubber to meet the above conditions can simultaneously take into account kinetic performance, energy density and bonding effect.

[0060] In some embodiments, 80,000 ≤ K ≤ 180,000, and 0.5% ≤ J ≤ 1.8%. In this case, it is possible to better balance dynamic performance, energy density, and bonding effect.

[0061] Styrene-butadiene rubber (SBR) can be obtained by the following method: Discharge the battery to 0% SOC, disassemble and remove the negative electrode sheet or obtain the negative electrode sheet directly, soak the negative electrode sheet in dimethyl carbonate (DMC), and then use a TG thermogravimetric analyzer. There is a weight loss peak at 350-650℃. The weight loss rate at 350-650℃ is the SBR content.

[0062] The molecular weight of styrene-butadiene rubber (SBR) can be determined by the following method: discharge the battery to 0% SOC, disassemble and remove the negative electrode sheet or directly obtain the negative electrode sheet, remove the negative electrode powder, dissolve the negative electrode powder in cyclohexane solvent, remove the solid matter to obtain a cyclohexane / SBR solution, and then place the cyclohexane / SBR solution into a gel permeation chromatography instrument to obtain the weight average molecular weight of SBR.

[0063] In some embodiments, the porosity L of the negative electrode sheet satisfies: 25% ≤ L ≤ 35%. When the porosity is maintained within the above range, the particles inside the negative electrode are more tightly bonded, which can better maintain the electronic conductivity network, while also ensuring sufficient pore space to wet the electrolyte, taking into account both electronic and ionic conductivity, and further improving the kinetic performance.

[0064] The porosity of the negative electrode sheet can be tested by the following method: discharge the battery to 0% SOC, disassemble and remove the negative electrode sheet or directly obtain the negative electrode sheet, test the actual volume, measure the true volume of the electrode sheet using a true density meter, and then obtain the porosity of the electrode sheet by (actual volume - true volume) / actual volume.

[0065] In some embodiments, the weight loss rate α of the negative electrode active layer satisfies: 0.5% ≤ α ≤ 3%.

[0066] Specifically, the recesses on the surface of the negative electrode active layer can be achieved through laser wire bonding, forming protrusions between adjacent recesses. The weight loss rate is the percentage of mass removed by the laser wire bonding. Controlling the weight loss rate within the range of 0.5%-3% can improve wettability while reducing the internal resistance of the cell.

[0067] Specifically, the weight loss rate can be 0.5%, 1%, 2%, 3%, or any value between any two of the above ranges.

[0068] For a negative electrode sheet that has already been wire-bonded, its weight loss rate can be measured by the following method: Measure the volume of the protruding part and the total mass of the negative electrode sheet. Then, use a cutting device such as a laser wire bonding machine or an argon ion laser to cut off the protruding part. After that, measure the total mass of the remaining negative electrode sheet and the mass of the cut-off protruding part. The density of the protruding part can be obtained from the volume and weight of the protruding part. The density of the concave and convex parts is the same. Then, based on the volume and density of the concave part, the weight of the concave part (i.e., the mass of the missing second negative electrode active layer) can be obtained. Weight loss rate = mass of the missing second negative electrode active layer / (total mass of the remaining negative electrode sheet + mass of the concave part + mass of the protruding part).

[0069] The volume of the protrusions and recesses can be obtained by measuring the length, width, and height and then calculating. In detail, the area of ​​the protrusions and the total area of ​​the second negative electrode active layer can be obtained by taking SEM images and then calibrating the number of protrusion lines and their length and width for each individual electrode.

[0070] In some embodiments, the depth of the recess is less than or equal to the thickness of the second negative electrode active layer, meaning that wire bonding is only performed on the second negative electrode active layer. This is because the principle behind the improvement in kinetics due to the recessed and raised portions lies in concentration polarization, which mainly occurs in the region of the negative electrode sheet near the separator. By forming recessed and raised portions through wire bonding, the contact area between the electrode sheet and the electrode liquid is increased, alleviating the concentration polarization in the region near the separator. At the same time, the upper material is a fast-charging material, and its performance is more significantly improved when combined with the recessed and raised portions formed by wire bonding. The lower layer is an energy-type material, and concentration polarization does not mainly occur here. Therefore, if the wire bonding penetrates the second negative electrode active layer, it will not significantly improve the kinetics, but will instead cause a loss of energy density.

[0071] The depth of the recessed portion on the surface of the second negative electrode active layer can be tested using the following method: discharge the battery to 0% SOC, disassemble and remove the negative electrode sheet, polish its cross-section with an argon ion polisher, obtain a cross-sectional image in a scanning electron microscope (SEM), and then measure the wire bonding depth using electron microscope image analysis software such as ImageJ.

[0072] In some embodiments, the thickness ratio of the first negative electrode active layer and the second negative electrode active layer is 1:9 to 9:1. Specifically, the mass ratio of the two can be (1-9):1 or 1:(1-9).

[0073] In some embodiments, the width of the recess is 0.05mm-0.2mm, and the spacing between the recesses is 0.5mm-5mm.

[0074] Specifically, the width of the recess can be 0.05mm, 0.08mm, 0.10mm, 0.15mm, 0.18mm, 0.20mm, or any two of the above values, and the spacing between the recesses can be 0.05mm, 0.08mm, 0.10mm, 0.15mm, 0.18mm, 0.20mm, or any two of the above values.

[0075] There are no specific limitations on the carbon-based materials; any carbon-based material available in the art may be used. For example, the first carbon-based material and / or the second carbon-based material includes at least one of artificial graphite, natural graphite, mesophase carbon microspheres, petroleum coke, pitch coke, soft carbon, and hard carbon.

[0076] It is also understood that the negative electrode active layer also includes a conductive agent. There are no specific limitations on the conductive agent; any conductive agent available in the art can be used. For example, the conductive agent includes one or more of conductive carbon black, acetylene black, Ketjen black, carbon-based negative electrode olefin, carbon fiber, single-walled carbon nanotubes, and multi-walled carbon nanotubes.

[0077] In some embodiments, the negative electrode active layer further includes a second binder, which includes at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, polyimide, polyamide-imide, and polyvinylidene fluoride.

[0078] The current collector is not specifically limited and any current collector available in the art can be used. For example, the current collector includes one or more of copper foil, chromium foil, nickel foil, titanium foil, carbon-coated copper foil, and perforated copper foil.

[0079] In some embodiments, the first negative electrode active layer comprises a first carbon-based material, a silicon-based material, a conductive agent, and a binder, wherein the mass ratio of the four components is (50-98):(1-50):(0.1-3):(0.1-3), and the second negative electrode active layer comprises a second carbon-based material, a silicon-based material, a conductive agent, and a binder, wherein the mass ratio of the four components is (50-98):(1-50):(0.1-3):(0.1-3). The binder includes a first binder, SBR.

[0080] The present invention also provides a lithium-ion battery, comprising the above-mentioned negative electrode and electrolyte;

[0081] The electrolyte contains fluoroethylene carbonate (FEC), and based on the total mass of the electrolyte, the mass percentage I of fluoroethylene carbonate (FEC) satisfies: 0.1% ≤ I ≤ 20%.

[0082] Specifically, the mass percentage I of fluoroethylene carbonate (FEC) can be 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 10%, 15%, 20%, or any range between any two of the above values.

[0083] As an anode film-forming additive, FEC can more easily form an inorganic SEI film rich in Li and F on the silicon surface. The higher inorganic content significantly improves the mechanical strength of the SEI film, thereby enhancing the cycle stability of the silicon anode. Furthermore, FEC can penetrate into the silicon particles, generating numerous nano-sized lithium fluorides during the electrochemical process, effectively hindering the formation of crystalline Li. 15 The formation of Si4 will result in the generation of many amorphous Li. 15 Si4, amorphous Li 15Si4 has low internal resistance and minimal volume effect, which greatly reduces the "lithium absorption" phenomenon inside the silicon particles and minimizes the loss of active lithium. However, FEC has the disadvantage of easily generating gas at high temperatures, so as to ensure expansion performance, the amount of FEC should be minimized. Considering the stability of the SEI film and the increased risk of gas generation due to excessively high FEC content, the FEC mass percentage is limited to 0.1% ≤ I ≤ 20% to balance the cycle performance and storage performance of the battery.

[0084] The mass percentage of FEC can be determined by the following method: Electrolyte extraction is performed at 25°C and humidity below 1%. The finished battery is fixed in a hydraulic press, and the pressure is gradually increased to 2 MPa. Fresh electrolyte is extracted from the periphery of the battery, and the FEC content is determined by gas chromatography (GC). In the later stage, the pressure is continued to be increased to extract electrolyte (e.g., 0.5 mL), and the FEC content is determined by GC.

[0085] In some embodiments, the electrolyte is a non-aqueous electrolyte, which also includes other carbonate solvents and lithium salts.

[0086] In some embodiments, other carbonate solvents include one or more of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). Lithium salts include one or more of LiPF6, LiBF4, LiSbF6, LiClO4, LiCF3SO3, LiAlO4, LiAlCl4, Li(CF3SO2)2N, LiBOB, and LiDFOB.

[0087] In some embodiments, the electrolyte includes ethylene carbonate, dimethyl carbonate, fluoroethylene carbonate, and lithium salt, wherein, based on the total mass of the electrolyte, the mass percentage of ethylene carbonate is 10-40%, the mass percentage of dimethyl carbonate is 30-50%, the mass percentage of lithium salt is 10-20%, and the mass percentage of fluoroethylene carbonate is 0.1-20%.

[0088] It is understood that the lithium-ion battery also includes a positive electrode sheet. This invention does not strictly limit the positive electrode active material in the positive electrode sheet; it can be a commonly used positive electrode active material in lithium-ion batteries, such as at least one composite oxide of lithium with cobalt, manganese, nickel, or combinations thereof. More specifically, it can be at least one of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium manganese oxide, nickel-cobalt-manganese ternary materials, nickel-cobalt-aluminum ternary materials, lithium iron phosphate (LFP), lithium nickel manganese oxide, lithium-rich manganese-based materials, etc.

[0089] It is also understood that the lithium-ion battery also includes a separator. The present invention does not strictly limit the choice of separator material. It can be a separator material commonly used in lithium-ion batteries, such as polypropylene separator (PP), polyethylene separator (PE), polypropylene / polyethylene double-layer composite membrane (PP / PE), polyimide electrospun separator (PI), polypropylene / polyethylene / polypropylene triple-layer composite membrane (PP / PE / PP), cellulose nonwoven fabric separator, and separator with ceramic coating.

[0090] In the preparation of lithium-ion batteries, the positive electrode, separator, and negative electrode are wound or stacked to obtain a bare cell, which is then packaged into a pre-stamped aluminum-plastic film bag. After the packaged battery is dried at 85°C, the electrolyte is injected into the dried battery. The battery undergoes resting, formation, and secondary sealing to complete the preparation of the lithium-ion battery.

[0091] The technical solution of this application will be further explained below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions or as recommended by the manufacturer. Unless otherwise specified, all reagents used are commercially available or obtained through public channels.

[0092] Example 1

[0093] This embodiment provides a negative electrode sheet, including a current collector (specifically a copper foil) and negative electrode active layers disposed on two surfaces of the current collector along the thickness direction. The negative electrode active layers include a first negative electrode active layer and a second negative electrode active layer in sequence along a first direction, where the first direction is the direction from the current collector to the negative electrode active layer.

[0094] The first negative electrode active layer comprises a first carbon-based material, a silicon-based material, a conductive agent (conductive carbon black), a binder (styrene-butadiene rubber (SBR), and a thickener (sodium carboxymethyl cellulose (CMC), with a mass ratio of 85.2:10:2:1.2:1.6.

[0095] The second negative electrode active layer comprises a second carbon-based material, a silicon-based material, a conductive agent (conductive carbon black), a binder (styrene-butadiene rubber (SBR), and a thickener (sodium carboxymethyl cellulose (CMC)) in a mass ratio of 85.2:10:2:1.2:1.6.

[0096] The first carbon-based material is graphite with a Dv50 of 12 μm and a Raman spectral ID / IG value G of 0.08. The second carbon-based material is also graphite with a Dv50 of 10 μm and a Raman spectral ID / IG value F of 0.5. The silicon-based material is silicon-carbon with a Dv50 of 12 μm, a P value of 1.04, a sphericity B of 0.8, a floating silicon content H of 1.7, and a silicon element mass ratio of 43% (the silicon element mass ratio in the active layer is 4.3%). The weight-average molecular weight K of the styrene-butadiene rubber is 100,000, and the relationship between the SBR content J and the weight-average molecular weight K is J×K=1200.

[0097] The width of the recesses on the second negative electrode active layer is 0.08 mm, the spacing is 1.5 mm, the wire bonding weight loss rate is 1.5%, and the ratio A of the projected area of ​​the protrusions on the surface of the second negative electrode active layer to the total surface area of ​​the second negative electrode active layer is 97% (obtained by calibrating the number and length and width of the protruding lines of a single electrode after SEM imaging). A / B is 1.21, the porosity L of the negative electrode active layer is 30%, the ratio of the thickness O of the first negative electrode active layer to the thickness N of the second negative electrode active layer is 5:5, and the depth M of the recesses is less than N.

[0098] The above-mentioned negative electrode sheet is prepared by the following method:

[0099] Preparation of the first negative electrode active layer slurry: The first carbon-based material, silicon-based material, conductive agent conductive carbon black, binder styrene-butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC) are dissolved in water in a mass ratio of 85.2%:10%:2%:1.2%:1.6% to prepare the first negative electrode active layer slurry.

[0100] Preparation of the second negative electrode active layer slurry: The fast-charging carbon-based negative electrode material, silicon-carbon composite material, conductive carbon black, binder styrene-butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC) are dissolved in water at a mass ratio of 85.2%:10%:2%:1.2%:1.6% to prepare the second negative electrode active layer slurry.

[0101] The first and second negative electrode active layer slurries were sequentially and uniformly coated onto a 6 μm thick copper foil. The coating thickness ratio of the first to the second negative electrode active layer slurries was 5:5, and the single-sided density of the negative electrode was 8 mg / cm³. 2 After drying the negative electrode sheet, it was pressed twice with a 10 MPa pressure roller to achieve a compaction density of 1.6 g / cm³. 3 The rolled electrode sheet is then passed through a laser wire forming machine to etch grooves, resulting in a weight loss of 1.5%. Finally, the wire-formed electrode sheet is die-cut to obtain the final negative electrode sheet (see...). Figure 1 ).

[0102] This embodiment also provides a lithium-ion battery, including a positive electrode, a separator, the aforementioned negative electrode, and an electrolyte. The positive electrode includes a current collector (specifically an aluminum foil) and a positive active layer disposed on two surfaces of the current collector along its thickness direction. The positive active layer includes lithium iron phosphate (LFP), a conductive agent (SP), and a binder (polyvinylidene fluoride (PVDF)) in a mass ratio of 96.5:2.0:1.5.

[0103] The diaphragm is a polyethylene diaphragm. The electrolyte consists of an organic solvent composed of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a 1:1 mass ratio, 17 wt% LiPF6, and 4 wt% fluoroethylene carbonate (FEC).

[0104] The above-mentioned lithium-ion battery is prepared by the following method:

[0105] Lithium iron phosphate (LFP), conductive agent (SP), and binder polyvinylidene fluoride (PVDF) are dissolved in N-methylpyrrolidone (NMP) at a mass ratio of 96.5:2.0:1.5 to obtain a positive electrode active slurry. The positive electrode active slurry is coated onto the aluminum foil of the positive electrode current collector, rolled, and die-cut to obtain the positive electrode of the full cell.

[0106] In a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm, Ar atmosphere), based on the total mass of the electrolyte, ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a mass ratio of 1:1, and then LiPF6 and fluoroethylene carbonate (FEC) were added and mixed to obtain the electrolyte. The molar concentration of LiPF6 was 1 mol / L, and fluoroethylene carbonate (FEC) accounted for 4% of the total mass of the electrolyte.

[0107] The separator is a polyethylene separator. The positive electrode is designed to have a capacity of 140 mAh / g, and the negative electrode is designed to have a capacity based on half-cell capacity test results, with a CB value of 1.15. The above-mentioned positive electrode, separator, negative electrode, and electrolyte are assembled into a pouch battery.

[0108] Examples 2-24, Comparative Examples 1-2

[0109] Examples 2-24 and Comparative Examples 1-2 were prepared using the same method as Example 1, with the differences shown in Tables 1 and 2 below. Specifically, in Example 19, the mass ratio of the first carbon-based material, silicon-based material, conductive agent (conductive carbon black), binder (styrene-butadiene rubber (SBR), and thickener (sodium carboxymethyl cellulose (CMC)) was 94.2:1:2:1.2:1.6. In Example 20, the mass ratio of the first carbon-based material, silicon-based material, conductive agent (conductive carbon black), binder (styrene-butadiene rubber (SBR), and thickener (sodium carboxymethyl cellulose (CMC)) was 55.2:40:2:1.2:1.6. In Example 21, the mass ratio of the first carbon-based material, silicon-based material (silicon content 64.5%), conductive agent (conductive carbon black), binder (styrene-butadiene rubber (SBR), and thickener (sodium carboxymethyl cellulose (CMC)) was 55.2:40:2:1.2:1.6.

[0110] Table 1

[0111]

[0112] Table 2

[0113]

[0114] Performance testing:

[0115] The batteries in the above embodiments and comparative examples were subjected to the following performance tests, and the results are shown in Table 3 below:

[0116] Short circuit rate test method: Discharge the battery to 0% SOC, let it stand for 2 hours, and then apply a voltage of 100±5V at room temperature (25±2℃) to test the battery resistance (test time is 4±1s). If the resistance is ≤2MΩ, the battery is judged to be short circuit. Test 100 cells in parallel and calculate the pass rate.

[0117] Energy density: The battery under test is charged at 0.33C to the cutoff voltage and then charged at constant voltage for 180 minutes. The battery capacity at this time is multiplied by the average voltage to obtain the cell energy, and the cell energy is divided by the cell volume to obtain the volumetric energy density.

[0118] 4C fast charging cycle test method at room temperature: Charge the battery under test at 4C constant current and constant voltage to the cutoff voltage of 3.8V, then discharge at 1C constant current to 2V, repeating this cycle three times. The highest discharge capacity in the three cycles is taken as the initial capacity, denoted as C0. Charge the battery at a rate of 4×C0 at constant current and constant voltage to the cutoff voltage of 3.8V, then discharge at 1×C0 constant current to 2V for 500 cycles. Divide the discharge capacity of the 500th cycle by C0 to obtain the capacity retention rate after 500 cycles of 4C fast charging. Measure the cell thickness at this point and divide it by the initial battery thickness to obtain the cell expansion rate after 500 cycles of 4C fast charging.

[0119] 45℃ 100% SOC 30-day Capacity Recovery Retention Rate: The battery under test was charged to 3.8V at a constant current and constant voltage rate of 0.33C at 25℃, then discharged to 2.0V at a constant current rate of 0.33C, repeating this cycle three times. The highest discharge capacity of the three cycles was recorded as the initial capacity, denoted as C0. Subsequently, it was charged again to 3.8V at a constant current and constant voltage rate of 0.33C and placed in a 45℃ constant temperature and humidity chamber for 30 days. After 30 days, the battery was removed, cooled to 25℃, and discharged to 2.0V at a constant current rate of 0.33C, then charged to 3.8V at a constant current and constant voltage rate of 0.33C, and finally discharged to 2.0V at a constant current rate of 0.33C. The capacity recovery retention rate was recorded as C1. Capacity recovery retention rate = C1 / C0.

[0120] Table 3

[0121]

[0122] As can be seen from the above results, the short-circuit pass rate of the lithium-ion batteries in Examples 1-24 is higher than that of Comparative Examples 1 and 2, and the energy density, capacity retention rate after 500 cycles at 4C, expansion rate, and capacity recovery retention rate are comparable to those of the comparative examples. This indicates that the negative electrode sheet provided by the present invention reduces the k value of the battery while taking into account the energy density and dynamic performance of the battery, and improves the short-circuit test pass rate of the battery, which is beneficial to improving the safety performance of the battery.

[0123] A comparison of Examples 1 and 6-13 shows that when the binder includes styrene-butadiene rubber, and the weight-average molecular weight K and mass percentage J of the styrene-butadiene rubber satisfy 900≤J*K≤1500, the power performance and cycle expansion rate of the battery can be better balanced.

[0124] Finally, it should be noted that other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein, and is not limited to what has been described above. Various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A negative electrode sheet, characterized in that, It includes a current collector and a negative electrode active layer disposed on at least one surface of the current collector. The negative electrode active layer includes a first negative electrode active layer and a second negative electrode active layer in sequence along a first direction. The first direction is the direction in which the current collector points to the negative electrode active layer. The first negative electrode active layer includes a first negative electrode active material, which includes a first carbon-based material; the second negative electrode active layer includes a second negative electrode active material, which includes a second carbon-based material and a silicon-based material; the Dv50 of the first carbon-based material is greater than the Dv50 of the second carbon-based material. The surface of the negative electrode active layer is provided with a plurality of recesses, and a protrusion is formed between adjacent recesses; the ratio A of the orthogonal projection area of ​​the protrusion on the surface of the second negative electrode active layer to the total surface area of ​​the second negative electrode active layer satisfies: 90%≤A≤99.5%; the sphericity B of the silicon-based material satisfies: 0.95≤A / B≤1.8, 0.5≤B≤1; wherein, the total surface area of ​​the second negative electrode active layer refers to the orthogonal projection area of ​​the second negative electrode active layer along a second direction, which is opposite to the first direction.

2. The negative electrode sheet according to claim 1, characterized in that, The Dv50 of the first carbon-based material is 10μm-15μm, and the Dv50 of the second carbon-based material is 8μm-13μm; And / or, the Raman spectrum ID / IG value G of the first carbon-based material satisfies: 0.03≤G≤0.2; the Raman spectrum ID / IG value F of the second carbon-based material satisfies: 0.3≤F≤0.

6.

3. The negative electrode sheet according to claim 1 or 2, characterized in that, Based on the total mass of the negative electrode active layer, the mass percentage C of silicon element satisfies: 0.4% ≤ C ≤ 30%; and / or The silicon-based material includes at least one of elemental silicon, silicon-oxygen materials, and silicon-carbon materials; and / or The silicon-based material is a silicon-carbon material, and the amount of floating silicon H on the surface of the silicon-carbon material satisfies: H ≤ 1.8; and / or The particle size distribution value P of the silicon-based material satisfies: 0.8≤P≤1.

2.

4. The negative electrode sheet according to any one of claims 1-3, characterized in that, The negative electrode active layer further includes a first binder, which comprises styrene-butadiene rubber (SBR) with a weight-average molecular weight of K. Based on the total mass of the negative electrode active layer, the mass percentage of SBR is J, satisfying: 900 ≤ J. K≤1500.

5. The negative electrode sheet according to claim 4, characterized in that, The following conditions must be met: 80,000 ≤ K ≤ 180,000, 0.5% ≤ J ≤ 1.8%.

6. The negative electrode sheet according to any one of claims 1-5, characterized in that, The porosity L of the negative electrode sheet satisfies: 25% ≤ L ≤ 35%.

7. The negative electrode sheet according to any one of claims 1-6, characterized in that, The weight loss rate α of the negative electrode active layer satisfies: 0.5% ≤ α ≤ 3%; And / or, the depth of the recess is less than or equal to the thickness of the second negative electrode active layer; And / or, the thickness ratio of the first negative electrode active layer to the second negative electrode active layer is (1:9)-(9:1). And / or, the width of the recess is 0.05mm-0.2mm; And / or, the spacing between the recesses is 0.5mm-5mm.

8. The negative electrode sheet according to any one of claims 1-7, characterized in that, The first carbon-based material and / or the second carbon-based material includes at least one of artificial graphite, natural graphite, mesophase carbon microspheres, petroleum coke, pitch coke, soft carbon, and hard carbon; The negative electrode active layer also includes a conductive agent, which includes one or more of the following: conductive carbon black, acetylene black, Ketjen black, carbon-based negative electrode olefin, carbon fiber, single-walled carbon nanotube, and multi-walled carbon nanotube. The negative electrode active layer further includes a second binder, which includes at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, polyimide, polyamide-imide, and polyvinylidene fluoride.

9. The negative electrode sheet according to claim 8, characterized in that, The first negative electrode active layer comprises a first carbon-based material, a silicon-based material, a conductive agent, and a binder, and the mass ratio of the four is (50-98):(1-50):(0.1-3):(0.1-3). The second negative electrode active layer further comprises a silicon-based material. The second negative electrode active layer comprises a second carbon-based material, a silicon-based material, a conductive agent, and a binder, and the mass ratio of the four is (50-98):(1-50):(0.1-3):(0.1-3). The binder includes a first binder.

10. A lithium-ion battery, characterized in that, It includes a negative electrode sheet and an electrolyte, wherein the negative electrode sheet is the negative electrode sheet according to any one of claims 1-8; The electrolyte includes fluoroethylene carbonate, and based on the total mass of the electrolyte, the mass percentage I of the fluoroethylene carbonate satisfies: 0.1% ≤ I ≤ 20%.

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