Negative plate, lithium ion battery and electric equipment

Abstract

The invention relates to the technical field of batteries, and discloses a negative plate, a lithium ion battery and electric equipment. The negative electrode plate comprises a negative electrode current collector and a negative electrode active layer arranged on the surface of at least one side of the negative electrode current collector, and the negative electrode active layer comprises a first negative electrode active layer arranged on the negative electrode current collector and a second negative electrode active layer arranged on the side, away from the negative electrode current collector, of the first negative electrode active layer; the first negative electrode active layer comprises first graphite, the first graphite comprises secondary particle graphite and single particle graphite, and the mass ratio of the secondary particle graphite to the single particle graphite is (1: 9)-(9: 1); the second negative electrode active layer comprises second graphite, the second graphite is secondary particle graphite coated with carbon nanotubes, and the specific surface area of the second graphite is larger than that of the first graphite. According to the invention, through the design of the double-layer coated negative electrode, both the fast charging performance and the high-temperature performance can be better considered.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a negative electrode sheet, a lithium-ion battery and electrical equipment. Background Art

[0002] Currently, the demand for power battery energy density is increasing. Regarding cathode materials, compared to the structurally stable but lower-capacity lithium iron phosphate, higher-capacity ternary materials play a crucial role in high-energy-density systems, with high-nickel materials leading the pack in terms of gram capacity. Regarding anode materials, the gram capacity of commercially available graphite is already close to the theoretical value of 372 mAh / g, making further breakthroughs difficult. Therefore, we are increasingly turning our attention to higher-capacity silicon anodes.

[0003] Mixing silicon-based materials into graphite as the negative electrode can increase the gram capacity of the negative electrode material, thereby increasing the energy density of the lithium battery. However, the kinetics of the negative electrode after mixing silicon materials into graphite are poor. When charging at a high rate, lithium deposition is likely to occur on the surface of the negative electrode, resulting in a decay of the cycle capacity. In addition, due to the material properties of graphite, it is difficult to achieve both fast charging and high-temperature performance. Summary of the Invention

[0004] In view of this, the present invention provides a negative electrode sheet, a lithium-ion battery and an electrical device to improve the problem that when the silicon-doped graphite negative electrode increases the specific capacity of the negative electrode material, its fast charging and high temperature performance cannot be well balanced, so as to achieve the purpose of balancing the fast charging and high temperature performance of the silicon-doped graphite negative electrode.

[0005] In a first aspect, the present invention provides a negative electrode sheet comprising a negative electrode current collector and a negative electrode active layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active layer comprises a first negative electrode active layer disposed on the negative electrode current collector and a second negative electrode active layer disposed on a side of the first negative electrode active layer away from the negative electrode current collector;

[0006] The first negative electrode active layer includes a first negative electrode active material, the first negative electrode active material includes a silicon material and a first graphite, the first graphite includes secondary graphite particles and single graphite particles, and the mass ratio of the secondary graphite particles to the single graphite particles is 1:9 to 9:1;

[0007] The second negative electrode active layer includes a second negative electrode active material, the second negative electrode active material includes a silicon material and a second graphite, and the second graphite is a secondary particle graphite with an outer layer coated with carbon nanotubes.

[0008] The specific surface area of ​​the second graphite is greater than the specific surface area of ​​the first graphite.

[0009] In an optional embodiment, the specific surface area of ​​the second graphite is 1.0 to 1.4 m2 / g, the specific surface area of ​​the first graphite is 0.8 to 1.1m 2 / g;

[0010] And / or, the OI value of the first graphite is greater than the OI value of the second graphite; preferably, the OI value of the first graphite is 6.2-6.7, and the OI value of the second graphite is 5.5-6.0;

[0011] And / or, the Dv50 of the first graphite is greater than the Dv50 of the second graphite; preferably, the Dv50 of the first graphite is 11-20 μm, and the Dv50 of the second graphite is 8-15 μm.

[0012] In an optional embodiment, the coating amount of the carbon nanotubes is 0.05-0.5% based on the total amount of graphite in the second negative electrode active layer;

[0013] and / or, the carbon nanotubes have an average length of 5 to 15 μm and an average diameter of 1 to 5 nm;

[0014] And / or, in the first negative electrode active layer, a soft carbon coating layer is further present in the secondary graphite particles, and the coating amount of the soft carbon coating is 0.5% to 2%;

[0015] And / or, in the second negative electrode active layer, a soft carbon coating layer is further present in the secondary graphite particles, and the coating amount of the soft carbon coating is 0.5% to 2%.

[0016] In an optional embodiment, the compaction density of the negative electrode active layer is 1.4-1.65 g / cm 3 ;

[0017] And / or, the surface density of the first negative electrode active layer and the second negative electrode active layer is 1.8 to 6.5 mg / cm 2 , preferably, the ratio of the surface density is 3:7 to 7:3;

[0018] And / or, the Dv50 of the silicon material is 6 to 12 μm.

[0019] In an optional embodiment, the first negative electrode active layer and the second negative electrode active layer further include a conductive agent, a binder and a thickener;

[0020] Preferably, the conductive agent comprises conductive carbon black in an amount of 0.5% to 0.7% and carbon nanotubes in an amount of 0.05% to 0.25%;

[0021] And / or, the binder comprises 1.5% to 2.2% of polyacrylic acid (PAA) and 1.8% to 2.5% of styrene-butadiene rubber (SBR);

[0022] and / or, the thickener comprises 0.45% to 0.8% carboxymethyl cellulose (CMC);

[0023] And / or, the mass proportion of silicon material in the first negative electrode active material is 10% to 30%, and the mass proportion of silicon material in the second negative electrode active material is 10% to 30%.

[0024] In an optional embodiment, the negative electrode current collector includes a negative electrode foil and a carbon coating layer provided on the surface of the negative electrode foil; the surface density of the carbon coating layer is 0.3 to 1.3 g / m 2 , thickness is less than or equal to 3.0μm.

[0025] Preferably, the thickness of the negative electrode foil is 4 to 10 μm;

[0026] And / or, the tensile strength of the negative electrode foil is TD ≥ 400 MPa, MD ≥ 400 MPa; the elongation of the negative electrode foil is TD ≥ 4.5%, MD ≥ 4.5%.

[0027] In a second aspect, the present invention further provides a lithium-ion battery comprising the above-mentioned double-layer coated negative electrode.

[0028] The lithium-ion battery provided by the present invention also includes a positive electrode sheet; the positive electrode sheet includes a positive electrode current collector and a positive electrode active layer, the positive electrode active layer includes a positive electrode active material, and the positive electrode active material is a ternary material LiNi x Co y Mn 1-x- y O2, wherein x≥0.8, y≥0.02, 1-xy>0; preferably, x≥0.92.

[0029] In an optional embodiment, the high nickel ternary material LiNi x Co y Mn 1-x-y O2 includes single crystal particles and polycrystalline particles, and the mass ratio of the single crystal particles to the polycrystalline particles is (0.33-3):1;

[0030] And / or, the positive electrode active layer further includes a conductive agent and a binder; the conductive agent includes 0.8% to 1.5% by weight of conductive carbon black and 0.7% to 1.1% by weight of carbon nanotubes, and the binder is 0.9% to 1.5% by weight of polyvinylidene fluoride;

[0031] And / or, the compaction density of the positive electrode active layer is 3.2 to 3.4 g / cm 3 .

[0032] In a third aspect, the present invention further provides an electrical device comprising the above-mentioned lithium-ion battery.

[0033] Beneficial effect: The negative electrode sheet of the present invention is designed with a double-layer coating, wherein the surface layer is selected from secondary particle graphite coated with carbon nanotubes and the specific surface area of ​​the graphite is optimized to improve the fast charging performance, and at the same time, the bottom layer is selected from high-temperature graphite composed of secondary particle graphite and single particle graphite to improve the high-temperature performance; by improving the fast charging performance of the secondary particle graphite coated with carbon nanotubes, and cooperating with the high-temperature graphite that improves the high-temperature performance, the silicon-doped graphite negative electrode is realized to comprehensively improve the fast charging and high-temperature performance of the battery, thereby achieving the purpose of taking into account both fast charging and high-temperature performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the specific implementation methods of the present application or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the specific implementation methods or the description of the prior art. Obviously, the drawings described below are some implementation methods of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0035] Figure 1 This is an SEM image of the surface of the negative electrode sheet in Example 1 of the present invention. DETAILED DESCRIPTION

[0036] To make the purpose, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative efforts shall fall within the scope of protection of the present invention.

[0037] The following describes an embodiment of the present invention in combination with the above solutions.

[0038] According to an embodiment of the present invention, on the one hand, the present invention provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active layer disposed on at least one side of the negative electrode current collector, wherein the negative electrode active layer comprises a first negative electrode active layer disposed on the negative electrode current collector and a second negative electrode active layer disposed on a side of the first negative electrode active layer away from the negative electrode current collector;

[0039] The first negative electrode active layer includes a first negative electrode active material, the first negative electrode active material includes a silicon material and a first graphite, the first graphite includes secondary graphite particles and single graphite particles, and the mass ratio of the secondary graphite particles to the single graphite particles is 1:9 to 9:1;

[0040] The second negative electrode active layer includes a second negative electrode active material, the second negative electrode active material includes a silicon material and a second graphite, and the second graphite is a secondary particle graphite with an outer layer coated with carbon nanotubes.

[0041] The specific surface area of ​​the second graphite is greater than the specific surface area of ​​the first graphite.

[0042] The secondary graphite particles are graphite particles with a larger particle size range obtained by bonding single graphite particles with a binder. The second negative electrode active layer is the surface layer, and the first negative electrode active layer is the bottom layer.

[0043] In the present invention, the theoretical gram capacity of the graphite negative electrode is relatively low, only 372mAh / g, which is difficult to meet the current market demand for high-energy-density lithium batteries, while the theoretical gram capacity of the silicon-based negative electrode is as high as 4200mAh / g. Mixing silicon material in graphite as the negative electrode can increase the gram capacity of the negative electrode material, thereby increasing the energy density of the lithium battery; after the graphite is mixed with silicon material, the kinetics of the negative electrode are poor, and lithium precipitation is prone to occur on the surface of the negative electrode when charging at a higher rate, resulting in a decay of the cycle capacity, and based on the material properties of graphite, it is difficult to take into account both fast charging and high-temperature performance. In the present invention, by designing a double-layer coating on the negative electrode, while selecting secondary particle graphite coated with carbon nanotubes for the surface layer and optimizing the specific surface area of ​​the graphite to improve the fast charging performance, the bottom layer selects high-temperature graphite composed of secondary particle graphite and single particle graphite to improve the high-temperature performance, which can comprehensively improve the fast charging and high-temperature performance of the battery, thereby achieving the purpose of taking into account both fast charging and high-temperature performance.

[0044] Specifically, coating the graphite surface with carbon nanotubes can enhance its conductivity. The carbon nanotube coating forms a three-dimensional conductive network, promoting the rapid transmission of lithium ions and electrons, reducing the polarization problem caused by the low conductivity of silicon, and thus more efficiently utilizing the capacity of silicon. At the same time, the surface graphite has a larger specific surface area, and the surface is coated with carbon nanotubes, which is conducive to the increase of active sites on the material surface. Moreover, they are all secondary particles with a small OI value and a short lithium ion deintercalation path. These can all increase the lithium deintercalation speed of the negative electrode and improve the material kinetics. The bottom layer of graphite has a smaller specific surface area, fewer surface active sites, and is a mixed product of secondary particles and single particles. The OI value is larger and the kinetics are poor, but it has fewer side reactions with the electrolyte and better high-temperature stability. During the charging process of lithium batteries, the active material on the surface of the negative electrode preferentially intercalates lithium. When the charge rate increases, a large overpotential is generated, resulting in a decrease in the surface potential, which makes it easy for lithium to be deposited. Therefore, the combination of secondary particle graphite coated with carbon nanotubes with fast charging performance can ensure fast charging performance. The bottom layer of the negative electrode uses relatively high-temperature resistant graphite, that is, high-temperature graphite formed by mixing secondary particle graphite and single particle graphite in proportion. On the one hand, the bottom layer embeds lithium slower than the surface layer, and the fast charging requirement for the negative electrode is relatively lower; on the other hand, the high-temperature graphite has fewer side reactions with the electrolyte, and being located at the bottom layer can reduce contact with the electrolyte and improve the overall high-temperature resistance. In summary, by controlling the mass ratio of the bottom secondary particle graphite and the single particle graphite to 1:9 to 9:1 to form high-temperature graphite, and cooperating with the secondary particle graphite coated with carbon nanotubes on the outer layer of the surface, combined with the design of the specific surface area, it is possible to improve the fast charging performance while not significantly affecting the high-temperature performance, thereby achieving the purpose of balancing fast charging and high-temperature performance.

[0045] As an example, the mass ratio of needle coke secondary graphite particles to petroleum coke single graphite particles in the first graphite can be 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, 9:1 or within the range of any two of the above values.

[0046] In an optional embodiment, the specific surface area of ​​the second graphite is 1.0 to 1.4 m 2 / g, the specific surface area of ​​the first graphite is 0.8 to 1.1m 2 / g.

[0047] The increase in the specific surface area of ​​the graphite in the present invention helps to improve the fast charging performance, but too high a specific surface area will have a negative impact on the high temperature performance, mainly due to the decrease in the initial charge and discharge efficiency caused by the thickening of the SEI film. In the present invention, the specific surface area of ​​the second graphite is preferably 1.0 to 1.4 m 2 / g, the specific surface area of ​​the first graphite is 0.8 to 1.1m 2 / g.

[0048] As an example, the specific surface area of ​​the first graphite may be 0.9 m 2 / g, 1.0m 2 / g, 1.1m 2 / g or within the range of any two of the above values; the specific surface area of ​​the second graphite can be 1.0m 2 / g, 1.1m 2 / g, 1.2m 2 / g, 1.3m 2 / g, 1.4m 2 / g or within the range formed by any two of the above values.

[0049] The specific surface areas of the first graphite and the second graphite can be measured using a conventional gas adsorption method or a liquid adsorption method, such as the BET method, the Langmuir method, and the t-plot method.

[0050] In an optional embodiment, the OI value of the first graphite is greater than the OI value of the second graphite; preferably, the OI value of the first graphite is 6.2-6.7, and the OI value of the second graphite is 5.5-6.0;

[0051] The Oriented Index (OI) of a graphite anode is a key performance metric for lithium-ion batteries. The OI directly impacts the battery's discharge capacity and endurance. The OI of a graphite anode can be controlled by adjusting its particle size and surface area. Lowering the OI of a graphite anode can improve fast-charging performance. Therefore, setting the OI of the second graphite lower than that of the first graphite can improve fast-charging performance.

[0052] As an example, the OI value of the first graphite may be 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, or a range between any two of the above values; the OI value of the second graphite may be 5.5, 5.6, 5.7, 5.8, 5.9, 6.0, or a range between any two of the above values.

[0053] OI=I004 / I110, wherein I004 is the peak intensity of the 004 crystal plane of graphite during X-ray diffraction, and I110 is the peak intensity of the 110 crystal plane of graphite during X-ray diffraction.

[0054] In an optional embodiment, the Dv50 of the first graphite is greater than the Dv50 of the second graphite; preferably, the Dv50 of the first graphite is 11 to 20 μm, and the Dv50 of the second graphite is 8 to 15 μm.

[0055] A smaller Dv50 value means a smaller average particle size, which is generally beneficial to improving the fast charging performance of lithium-ion batteries; therefore, the present invention can effectively further improve the fast charging performance by setting the Dv50 of the first graphite to be greater than the Dv50 of the second graphite.

[0056] As an example, the Dv50 of the first graphite may be 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, or within a range formed by any two of the above values; the Dv50 of the second graphite may be 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, or within a range formed by any two of the above values.

[0057] Dv50 is measured using conventional laser diffraction or dynamic light scattering methods.

[0058] In an optional embodiment, the coating amount of the carbon nanotubes is 0.05-0.5% based on the total amount of graphite in the second negative electrode active layer;

[0059] If the carbon nanotube coating is too low, the purpose of increasing surface active sites will not be achieved, and the improvement in fast charging performance will not be obvious. If the carbon nanotube coating is too high, it will have a significant impact on the high-temperature structural stability and reduce the overall thermal stability of the material. Therefore, by controlling the carbon nanotube coating amount in the second negative electrode active layer to 0.05-0.5%, a better balance between high-temperature performance and fast charging performance can be achieved.

[0060] As an example, the coating amount of the carbon nanotubes may be 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, or a range between any two of the above values.

[0061] The morphology of carbon nanotubes can be directly observed using a transmission electron microscope (TEM).

[0062] In an optional embodiment, the carbon nanotubes have an average length of 5 to 15 μm and an average diameter of 1 to 5 nm;

[0063] Carbon nanotubes with larger diameters and longer lengths help improve their stability in high-temperature environments, but their poor conductivity is detrimental to fast charging performance. The present invention better achieves the goal of balancing fast charging and high-temperature performance by controlling the average length of carbon nanotubes to 5-15 μm and the average diameter to 1-5 nm.

[0064] As an example, the average length of the carbon nanotubes can be 5μm, 8μm, 10μm, 12μm, 15μm or within the range of any two of the above values, and the average diameter of the carbon nanotubes can be 1nm, 2nm, 3nm, 4nm, 5nm or within the range of any two of the above values.

[0065] In an optional embodiment, in the first negative electrode active layer, a soft carbon coating layer is further present in the secondary graphite particles, and the coating amount of the soft carbon coating is 0.5% to 2%; and / or, in the second negative electrode active layer, a soft carbon coating layer is further present in the secondary graphite particles, and the coating amount of the soft carbon coating is 0.5% to 2%.

[0066] Soft carbon is a type of amorphous carbon. Compared to the interlayer structure of graphite, it has more and wider channels, enabling better insertion and extraction of lithium ions, thereby improving their dynamics. Carbon nanotubes also enhance electrical conductivity. This invention, by coating the graphite with an appropriate amount of soft carbon, improves the thermal stability and electrical conductivity of the graphite material, thereby enhancing the battery's cycling performance and fast-charging performance in high-temperature environments.

[0067] As an example, the coating amount of the soft carbon coating may be 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2% or a range formed by any two of the above values.

[0068] The soft carbon-coated coating can be observed with the aid of EDS+Raman, and the coated carbon layer can be actually observed by TEM to observe the lattice fringes (short-range ordered lattice fringes can be seen in soft carbon, with an interlayer spacing of approximately 0.34-0.35 nm).

[0069] In an optional embodiment, the compaction density of the negative electrode active layer is 1.4-1.65 g / cm 3 When the compaction is greater than 1.65g / cm 3 When the compaction is less than 1.4g / cm, the silicon particles may be broken, especially the surface deformation will be more serious, resulting in increased side reactions between the negative electrode and the electrolyte and increased internal resistance. 3 When the density difference between the surface and bottom layers is too large, the particles are not tightly bound together, the electron mobility of the negative electrode sheet is low, and the internal resistance increases. When the surface and bottom layers differ too much, the coating becomes more difficult and the yield decreases. The slurry between the surface and bottom layers easily mixes, making double-layer coating meaningless. Therefore, the setting of the compaction density in the present invention is beneficial for reducing the internal resistance and polarization of the battery, thereby improving the cycle stability. However, the performance difference within this range is not significant, and the verification of different compaction densities will not be carried out in the subsequent examples.

[0070] As an example, the compaction density of the negative electrode active layer can be 1.4 g / cm 3, 1.45g / cm 3 , 1.5g / cm 3 , 1.55g / cm 3 , 1.6g / cm 3 , 1.65g / cm 3 Or within the range formed by any two of the above values.

[0071] The compaction density of the negative electrode active layer can be tested using the method disclosed in GB / T 24533-2019.

[0072] In an optional embodiment, the surface density of the first negative electrode active layer and the second negative electrode active layer is 1.8 to 6.5 mg / cm 2 Preferably, the ratio of the surface density of the first negative electrode active layer to the surface density of the second negative electrode active layer is 3:7 to 7:3;

[0073] A lower surface density will affect the energy density, and a higher surface density will increase the internal resistance of the battery, thereby reducing the fast charging efficiency; the present invention effectively ensures the energy density and fast charging performance of the battery by controlling the surface density of the first negative electrode active layer and the second negative electrode active layer and the surface density ratio range of the first negative electrode active layer and the second negative electrode active layer.

[0074] As an example, the surface density of the first negative electrode active layer and the second negative electrode active layer can both be 1.8 g / cm 2 , 2g / cm 2 , 3g / cm 2 , 4g / cm 2 , 5g / cm 2 , 6g / cm 2 、6.5g / cm 2 Or within the range formed by any two of the above values; the surface density ratio can be 3:7, 4:6, 5:5, 6:4, 7:3 or within the range formed by any two of the above values.

[0075] The test method for the surface density of the first negative electrode active layer and the second negative electrode active layer is as follows:

[0076] First, the negative electrode compaction = active material layer mass / active material layer thickness is measured, then the thickness of each of the two negative electrode active layers is measured, and then the surface density of each of the two negative electrode active layers is obtained through surface density = compaction × thickness.

[0077] In an optional embodiment, the Dv50 of the silicon material is 6 to 12 μm.

[0078] A smaller Dv50 value for silicon material means finer particles, which may help alleviate structural damage caused by thermal expansion, thereby improving high-temperature performance to a certain extent. At the same time, a smaller Dv50 value for silicon material provides more active sites and shorter lithium-ion transmission paths, which is generally beneficial to increasing the diffusion rate of lithium ions. However, a silicon material with an excessively small Dv50 will increase the specific surface area and the contact area between the silicon material and the electrolyte, thereby exacerbating the occurrence of side reactions and causing the battery's performance to deteriorate at high temperatures. At the same time, a too-small Dv50 will also increase the volume expansion of the silicon material during the lithium insertion process, leading to particle breakage and poor electrical contact, forming an "island effect," which in turn affects fast-charging performance. Therefore, by having a silicon material with a Dv50 of 6 to 12 μm, a better balance between fast-charging and high-temperature performance can be effectively achieved.

[0079] As an example, the Dv50 of the silicon material may be 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, or within a range formed by any two of the above values.

[0080] In an optional embodiment, the first negative electrode active layer and the second negative electrode active layer also include a conductive agent, a binder and a thickener.

[0081] Preferably, the conductive agent comprises conductive carbon black in an amount of 0.5% to 0.7% and carbon nanotubes in an amount of 0.05% to 0.25%;

[0082] In the present invention, the carbon nanotubes and the conductive carbon black are used together, which can reduce the cost and increase the content of the negative electrode active material, and can also avoid the difficulty of dispersion during the mixing process and the influence of the coating quality due to the excessive viscosity.

[0083] As an example, the content of the conductive carbon black can be 0.5%, 0.6%, 0.7% or within the range of any two of the above values, and the content of the carbon nanotubes can be 0.05%, 0.1%, 0.15%, 0.2%, 0.25% or within the range of any two of the above values.

[0084] Preferably, the binder comprises 1.5% to 2.2% of polyacrylic acid (PAA) and 1.8% to 2.5% of styrene-butadiene rubber (SBR). Preferably, the content of styrene-butadiene rubber is greater than that of polyacrylic acid.

[0085] Among the binders in the present invention, SBR has a high viscosity and has a better expansion inhibition effect on highly silicon-doped negative electrode sheets. However, SBR has poor mechanical stability. If the binder is purely SBR, the stability of the slurry will be reduced. Therefore, a certain proportion of PAA needs to be added to balance it. The present invention sets the SBR content to be greater than the PAA content and utilizes the lower glass transition temperature of SBR to construct an elastic bonding network, thereby better inhibiting the expansion of the negative electrode sheet during the charge and discharge process, preventing the active material from losing adhesion to the current collector due to the large expansion of the silicon-doped negative electrode, and improving the cycle stability.

[0086] As an example, the content of the polyacrylic acid can be 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2% or within the range formed by any two of the above values, and the content of the styrene-butadiene rubber can be 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5% or within the range formed by any two of the above values.

[0087] Preferably, the thickener comprises carboxymethyl cellulose (CMC) in an amount of 0.45% to 0.8%.

[0088] As an example, the content of the carboxymethyl cellulose may be 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, 0.8% or within a range formed by any two of the above values.

[0089] Preferably, the mass proportion of silicon material in the first negative electrode active material is 10% to 30%, and the mass proportion of silicon material in the second negative electrode active material is 10% to 30%.

[0090] In the present invention, if the proportion of silicon material is too low, the energy density will be affected; if the proportion of silicon material is too high, the volume will expand severely during the charge and discharge process. This expansion will cause damage to the internal structure of the battery, affecting high-temperature performance. At the same time, the volume change during the insertion and extraction of lithium ions will cause the silicon particles to break and pulverize, thereby affecting the fast-charging performance of the battery. The present invention can balance energy density, high-temperature performance, and fast-charging performance by controlling the mass proportion of the silicon material in the first negative electrode active material and the silicon material in the second negative electrode active material within an appropriate range.

[0091] As an example, the mass proportion of the silicon material in the first negative electrode active material can be 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30% or within the range formed by any two of the above values, and the mass proportion of the silicon material in the second negative electrode active material can be 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30% or within the range formed by any two of the above values.

[0092] The contents of the above-mentioned conductive carbon black, carbon nanotubes, polyacrylic acid, styrene-butadiene rubber and carboxymethyl cellulose are all the contents in the corresponding negative electrode active layer.

[0093] In an optional embodiment, the negative electrode current collector includes a negative electrode foil and a carbon coating layer provided on the surface of the negative electrode foil; the surface density of the carbon coating layer is 0.3 to 1.3 g / m 2 , thickness is less than or equal to 3.0μm.

[0094] In the present invention, since silicon undergoes a huge volume change when lithium is embedded, the internal stress of the negative electrode is large during the battery cycle, which can easily cause the negative electrode material to crack or even pulverize, and lose contact with the current collector, causing rapid decay of the battery capacity; after the negative electrode surface is coated with carbon, the adhesion between the active material and the current collector can be improved, thereby improving the cycle stability.

[0095] As an example, the surface density of the carbon coating layer can be 0.3 g / m 2 , 0.4g / m 2 , 0.5g / m 2 , 0.6g / m 2 , 0.7g / m 2 , 0.8g / m 2 , 0.9g / m 2 , 1.0g / m 2 , 1.1g / m 2 , 1.2g / m 2 , 1.3g / m 2 Or within the range formed by any two of the above values, the thickness of the carbon coating layer may be 0.1 μm, 0.5 μm, 1.0 μm, 1.5 μm, 2.0 μm, 2.5 μm, 3.0 μm or within the range formed by any two of the above values.

[0096] Preferably, the thickness of the negative electrode foil in the present invention is 4 to 10 μm. Thinner foil can reduce heat accumulation within lithium-ion batteries, thereby improving battery safety and stability. It can also reduce battery internal resistance and improve lithium ion transmission efficiency, thereby enhancing the battery's fast charging capability. Within the above thickness range, the effects achieved in the present invention are basically the same, and thickness verification will not be performed in subsequent examples.

[0097] As an example, the thickness of the negative electrode foil may be 4 μm, 4.5 μm, 5.0 μm, 5.5 μm, 6.0 μm, 6.5 μm, 7.0 μm, 7.5 μm, 8.0 μm, 9.0 μm, 10.0 μm, or within a range formed by any two of the above values.

[0098] In an optional embodiment, the tensile strength of the negative electrode foil is TD≥400 MPa, and MD≥400 MPa; the elongation of the negative electrode foil is TD≥4.5%, and MD≥4.5%.

[0099] MD (Machine Direction) refers to the longitudinal direction; TD (Transverse Direction) refers to the transverse direction. Because the silicon-doped negative electrode in this invention expands significantly, a negative electrode current collector with high tensile strength and elongation provides stronger mechanical support. Therefore, the tensile strength and elongation of the negative electrode foil in this invention are set to be higher than conventional ones. This reduces damage to the overall electrode structure caused by silicon expansion, thereby reducing the risk of current collector fracture and avoiding conductive network breakage caused by repeated expansion and contraction.

[0100] The present invention provides a lithium ion battery comprising the above-mentioned double-layer coated negative electrode.

[0101] The lithium-ion battery provided by the present invention also includes a positive electrode sheet; the positive electrode sheet includes a positive electrode current collector and a positive electrode active layer, the positive electrode active layer includes a positive electrode active material, and the positive electrode active material is a ternary material LiNi x Co y Mn 1-x- y O2, wherein x≥0.8, y≥0.02, 1-xy>0; preferably, x≥0.92.

[0102] The ternary material LiNi of the present invention x Co y Mn 1-x-y In O2, x≥0.8 has a high nickel content and a higher gram capacity, which can improve the battery energy density.

[0103] As an example, the x may be 0.80, 0.82, 0.84, 0.86, 0.88, 0.9, 0.92, 0.94, 0.96, 0.97, or a range between any two of the above values, and the y may be 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18, 0.19, or a range between any two of the above values.

[0104] In an optional embodiment, the high nickel ternary material LiNi x Co y Mn 1-x-y O2 includes single crystal particles and polycrystalline particles, and the mass ratio of the single crystal particles to the polycrystalline particles is (0.33-3):1;

[0105] Single crystal ternary materials are crystals grown from a single nucleus, and their internal structure is basically a complete lattice. They have a small particle size, a Dv50 of 3 to 6 μm, and a low gram capacity. At the same time, their structure is relatively stable and not easily broken during high-temperature cycling, resulting in good cycling stability. However, they have relatively few reactive sites on their surface, so the lithium ion deintercalation rate is slow and their rate performance is poor. Polycrystalline materials are secondary spherical particles composed of many primary particles with different orientations. They have a larger particle size, a Dv50 of 8 to 12 μm, and a relatively high gram capacity. Polycrystalline ternary materials have more orientations and more reactive sites on their surface, resulting in better rate performance. However, during high-temperature cycling, fine cracks will form inside the particles, causing more side reactions with the electrolyte, consuming active lithium, resulting in a decrease in capacity retention, and generating gas. In the present invention, by mixing single crystal and polycrystalline ternary materials in a certain proportion as positive electrode active materials, the characteristics of single crystal and polycrystalline can be fully utilized to achieve the goal of taking into account both high-temperature stability and kinetic performance.

[0106] As an example, the mass ratio of single crystal particles to the polycrystalline particles can be 0.33:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, 1.5:1, 2:1, 2.5:1, 3:1 or within the range of any two of the above values.

[0107] In an optional embodiment, the positive electrode active layer also includes a conductive agent and a binder; the conductive agent includes 0.8% to 1.5% by mass of conductive carbon black and 0.7% to 1.1% by mass of carbon nanotubes, and the binder is 0.9% to 1.5% by mass of polyvinylidene fluoride (PVDF).

[0108] In this invention, conductive carbon black and carbon nanotubes are primarily used to improve the conductivity of the positive electrode. Conductive carbon black has a dot-like structure, which has a limited effect on conductivity but is relatively low-cost. Carbon nanotubes, on the other hand, have a linear structure, entwining with the active material to form a conductive network. However, this is more expensive and difficult to disperse. Combining conductive carbon black with carbon nanotubes not only reduces the difficulty of coating the ingredients but also allows for the overall conductive agent content to be controlled within an appropriate range, preventing excessive concentrations from reducing the active material content.

[0109] The binder PVDF serves to bond the positive electrode active material, conductive agent and current collector together, maintaining the mechanical structure and electrochemical stability of the electrode during the charge and discharge process.

[0110] As an example, the mass proportion of the conductive carbon black can be 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5% or within the range formed by any two of the above values; the mass proportion of the carbon nanotubes can be 0.7%, 0.8%, 0.9%, 1.0%, 1.1% or within the range formed by any two of the above values; the mass proportion of the polyvinylidene fluoride can be 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5% or within the range formed by any two of the above values.

[0111] In an optional embodiment, the compaction density of the positive electrode active layer is 3.2 to 3.4 g / cm 3 .

[0112] The compaction density of the positive electrode active layer is less than 3.2g / cm 3 When the density is higher than 3.4 g / cm, the thickness of the positive electrode sheet may be too large, which is not conducive to the improvement of energy density. In addition, the contact between the positive electrode active material particles and the conductive agent is not close enough, the internal resistance increases, and the polarization increases. If the compaction is higher than 3.4 g / cm 3 When the positive electrode is compacted too high, it may even cause the active material particles to break. The exposure of the new interface will accelerate the consumption of the electrolyte and the occurrence of side reactions, leading to a decrease in cycle life. However, the performance difference is not significant within this range, and verification of different compaction densities will not be carried out in subsequent embodiments.

[0113] As an example, the compaction density of the positive electrode active layer can be 3.2 g / cm 3 , 3.3g / cm 3 、3.4g / cm 3 Or within the range formed by any two of the above values.

[0114] In an optional embodiment, the surface density of the positive electrode active layer is 14 to 27 mg / cm 2 .

[0115] If the positive electrode surface density of the positive electrode active layer is too low or too high, it will increase the difficulty of coating and reduce the coating yield and consistency. At the same time, if the positive electrode surface density is too low, it will not be conducive to improving the battery energy density.

[0116] As an example, the surface density of the positive electrode active layer can be 14 mg / cm 2 、16mg / cm 2 、18mg / cm 2 , 20mg / cm 2 , 22mg / cm 2 , 24mg / cm2 , 26mg / cm 2 , 27mg / cm 2 Or within the range formed by any two of the above values.

[0117] The present invention also provides an electrical device comprising the above-mentioned lithium-ion battery.

[0118] [Electrolyte]

[0119] The electrolyte of the present invention includes a solvent, a lithium salt and an additive. The types and amounts of the solvent, lithium salt and additive are not particularly limited and can be selected from the types and amounts of solvents, lithium salts and additives conventionally used in the art; for example, the solvent is ethylene carbonate, propylene carbonate and ethyl methyl carbonate in a volume ratio of 1:2:7, the lithium salt is lithium hexafluorophosphate (12.5 wt %), and the additives are 10 wt % of fluoroethylene carbonate and 0.5 wt % of difluorophosphate.

[0120] [Diaphragm]

[0121] The diaphragm of the present invention is an oil-based diaphragm, preferably a diaphragm with a ceramic layer; as an example, a diaphragm with a ceramic layer with a total thickness of 11 μm can be selected, wherein the base film is 7 μm thick, one side has a ceramic layer with a thickness of 2 μm, and both sides have glue, each layer of glue is 1 μm thick.

[0122]

Positive electrode

[0123] The positive electrode sheet of the present invention includes a positive electrode current collector and a positive electrode active material layer coated on one side or both sides of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material, a conductive agent and a binder. The positive electrode active material layer includes 95.9 to 97.6 wt% of the positive electrode active material, 1.5 to 2.6 wt% of the conductive agent, and 0.9 to 1.5 wt% of the binder in terms of mass percentage.

[0124] The positive electrode active material of the present invention is a nickel-cobalt-manganese ternary positive electrode material LiNi x Co y Mn 1-x-y O2, x≥0.8, preferably x≥0.92, y≥0.02, 1-xy>0; for nickel-cobalt-manganese ternary positive electrode materials, except LiNi 0.8 Co 0.1 Mn 0.1 In addition to O2 (also referred to as NCM811) high nickel ternary, you can also choose LLiNi 0.9 Co 0.05 Mn 0.05 O2 (also referred to as NCM90), etc.

[0125] The present invention does not particularly limit the conductive agent in the positive electrode sheet, which can be selected from the conductive agents conventionally used in the field, including but not limited to one or more of conductive carbon black, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, carbon nanotubes, and metal powder, preferably including 0.8% to 1.5% by mass of conductive carbon black and 0.7% to 1.1% by mass of carbon nanotubes.

[0126] The present invention does not particularly limit the binder in the positive electrode sheet, which can be selected from the binders commonly used in the art, including but not limited to one or more of styrene-butadiene rubber emulsion, polytetrafluoroethylene emulsion, sodium carboxymethyl cellulose, sodium alginate, polyvinyl alcohol, polyacrylic acid, lithium polyacrylate, sodium polyacrylate, carboxylated chitosan, and polyvinylidene fluoride, preferably including 0.9% to 1.5% by mass of polyvinylidene fluoride.

[0127] In the positive electrode sheet of the present invention, under the same positive electrode active material composition, the content and type of the positive electrode active material, binder and thickener within the above-mentioned ratio range have no significant effect on the effect, and the content and type of the conductive agent and binder will not be verified in subsequent examples.

[0128]

Negative electrode

[0129] The negative electrode sheet of the present invention comprises a negative electrode current collector and a negative electrode active layer coated on one side or both sides of the negative electrode current collector, that is, the double-layer coated negative electrode mentioned above in the present invention.

[0130] The negative electrode active layer includes a first negative electrode active layer and a second negative electrode active layer, both of which include a negative electrode active material, a conductive agent, a binder and a thickener; the negative electrode active material in the first negative electrode active layer is the first negative electrode active material, and the negative electrode active material in the second negative electrode active layer is the second negative electrode active material.

[0131] The first negative electrode active material comprises 10%-30% silicon material and 70%-90% first graphite, wherein the first graphite comprises needle coke secondary graphite particles and petroleum coke single graphite particles in a mass ratio of 1:9 to 9:1. The second negative electrode active material comprises 10%-30% silicon material and 70%-90% second graphite, wherein the second graphite is needle coke secondary graphite particles coated with carbon nanotubes. The carbon nanotube coating amount is 0.05-0.5% based on the total amount of the negative electrode graphite.

[0132] In the first negative electrode active layer, the first negative electrode active material accounts for 93.55% to 95.8% of the mass of the first negative electrode active layer, the conductive agent accounts for 0.55% to 0.95% of the mass of the first negative electrode active layer, the binder accounts for 3.2% to 4.7% of the mass of the first negative electrode active layer, and the thickener accounts for 0.45% to 0.8% of the mass of the first negative electrode active layer; in the second negative electrode active layer, the second negative electrode active material accounts for 93.55% to 95.8% of the mass of the second negative electrode active layer, the conductive agent accounts for 0.55% to 0.95% of the mass of the second negative electrode active layer, the binder accounts for 3.2% to 4.7% of the mass of the second negative electrode active layer, and the thickener accounts for 0.45% to 0.8% of the mass of the second negative electrode active layer.

[0133] The present invention does not particularly limit the types of conductive agent, binder and thickener in the negative electrode sheet. The selection range can refer to the types of conductive agent and binder in the positive electrode sheet. For example, the binder includes one or more of styrene-butadiene rubber (SBR), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyacrylic acid (PAA), polyisobutylene (PIB), polyimide (PI), polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, and polymethyl methacrylate (PMMA); and / or the conductive agent includes carbon nanotubes (CNT), Ketjen black (K B), one or more of mesophase carbon microspheres, vapor deposited grown carbon fibers (VGCF), conductive carbon black (SP), conductive graphite, and acetylene black (AB); preferably, the conductive agent includes conductive carbon black and carbon nanotubes, the binder includes polyacrylic acid (PAA) and styrene-butadiene rubber (SBR), and the thickener includes carboxymethyl cellulose (CMC); more preferably, the content of the conductive carbon black is 0.5% to 0.7%, the content of the carbon nanotubes is 0.05% to 0.25%; the content of the polyacrylic acid is 1.5% to 2.2%, the content of the styrene-butadiene rubber is 1.8% to 2.5%; and the content of the carboxymethyl cellulose is 0.45% to 0.8%.

[0134] In the first and second negative electrode active layers of the present invention, under the same silicon material content, the conductive agent, binder, and thickener contents within the above-mentioned ratio ranges have no significant effect on the performance, and the content and types of the conductive agent, binder, and thickener will not be verified in subsequent examples. Furthermore, under different silicon material contents, the effects of the composition and content settings of the first and second graphites have the same trend; therefore, the trend results of the composition and content settings of the first and second graphites under different silicon material contents will not be verified in subsequent examples.

[0135] The lithium-ion battery provided by the present invention will be further described in detail below through specific examples.

[0136] Unless otherwise specified, the reagents, materials, and instruments used in the following examples are conventional reagents, conventional materials, and conventional instruments in the art and can be obtained commercially. The reagents involved can also be synthesized by conventional methods in the art.

[0137] According to an embodiment of the present invention, on the other hand, the present invention further provides an electrical device comprising the above-mentioned lithium-ion battery.

[0138] Example 1

[0139] A secondary battery, the specific preparation process is as follows:

[0140] 1. Preparation of positive electrode sheet: high nickel ternary positive electrode material LiNi 0.92 Co 0.06 Mn 0.02 O2 (48.5wt% single crystal particles + 48.5wt% polycrystalline particles), a conductive agent (0.9wt% carbon black + 0.9wt% carbon nanotubes), and a binder (1.2wt% PVDF) were mixed with NMP in appropriate proportions and ball-milled to form a uniform slurry to produce the positive electrode slurry. The slurry was evenly coated on the surface of the positive electrode current collector and dried. After roller pressing, slitting, and die-cutting, the positive electrode sheet was obtained.

[0141] 2. Preparation of negative electrode sheet: Mix the negative electrode active material (70.95wt% second graphite + 23.65wt% SiC), conductive agent (0.6wt% carbon black + 0.15wt% carbon nanotubes), binder (1.8wt% PAA + 2.2wt% SBR), thickener (0.65wt%) with water and ball mill to make a uniform slurry; this is the negative electrode surface slurry; at the same time, mix the negative electrode active material (70.95wt% first graphite + 23.65wt% SiC), conductive agent (0.6wt% carbon black + 0.15wt% carbon nanotubes), binder (1.8wt% PAA + 2.2wt% SBR), thickener (0.65wt%) with water and ball mill to make a uniform slurry; this is the negative electrode surface slurry; at the same time, mix the negative electrode active material (70.95wt% first graphite + 23.65wt% SiC), conductive agent (0.6wt% carbon black + 0.1 5wt% carbon nanotubes), a binder (1.8wt% PAA + 2.2wt% SBR), a thickener (0.65wt%) and water were mixed to prepare a negative electrode bottom layer slurry in the same way; the two layers of slurry were sprayed on the surface of the negative electrode collector in sequence and dried, and the negative electrode sheet was obtained after roller pressing, slitting and die cutting; the bottom layer of the negative electrode sheet close to the surface of the negative electrode collector was the first negative electrode active layer, and the surface layer of the negative electrode sheet away from the surface of the negative electrode collector was the second negative electrode active layer, and the surface density of the first negative electrode active layer and the second negative electrode active layer were both 4.5mg / cm 2 The compaction density of the first negative electrode active layer and the second negative electrode active layer is 1.5g / cm 3 ; The cross section of the negative electrode is as follows Figure 1 shown.

[0142] The first graphite includes single-grain graphite and secondary-grain graphite, the mass ratio of the single-grain graphite to the secondary-grain graphite is 3:7, the surface of the first graphite is provided with a soft carbon coating layer, and the coating amount of the soft carbon coating layer is 1.5%; the specific surface area of ​​the first graphite is 0.9m 2 / g, the OI value of the first graphite is 6.5, and the Dv50 of the first graphite is 15 μm.

[0143] The second graphite is a secondary particle graphite with an outer layer coated with carbon nanotubes. The coating amount of carbon nanotubes in the surface layer is 0.2%. The surface of the second graphite is provided with a soft carbon coating layer. The coating amount of the soft carbon coating layer is 1%. The average length of the carbon nanotubes is 10μm and the average diameter is 2nm. The specific surface area of ​​the second graphite is 1.2m 2 / g, the OI value of the second graphite is 5.8, the Dv50 of the second graphite is 10 μm, and the Dv50 of the silicon material SiC is 9 μm.

[0144] The negative electrode current collector includes a negative electrode foil and a carbon coating layer arranged on the surface of the negative electrode foil; the negative electrode foil is a copper foil with a thickness of 5 μm, and the surface density of the carbon coating layer is 0.8 g / m 2 , the thickness of the carbon coating layer is 2.0μm.

[0145] 3. Preparation of electrolyte: The electrolyte includes solvent, lithium salt and additives. The solvent is ethylene carbonate, propylene carbonate and ethyl methyl carbonate in a volume ratio of 1:2:7. The lithium salt is lithium hexafluorophosphate (12.5 wt%). The additives are 10 wt% fluoroethylene carbonate and 0.5 wt% difluorophosphate.

[0146] 4. Diaphragm: The diaphragm is an oil-based diaphragm with a thickness of 11μm, of which the base film is 7μm thick, one side has a ceramic layer with a thickness of 2μm, and both sides have glue, each layer of glue is 1μm thick;

[0147] 5. Preparation of the battery: The positive and negative electrodes are baked for a period of time, and a complete lithium battery is prepared through processes such as lamination, packaging, liquid injection, formation, and sorting.

[0148] Examples 2-7 and Comparative Examples 1-5

[0149] The difference between Examples 2-7 and Comparative Examples 1-5 of the present invention and Example 1 is that the designs of the first graphite and the second graphite are different, and the other parameter conditions are exactly the same as those of Example 1. The specific settings of the differences are shown in Table 1.

[0150] Table 1

[0151]

[0152]

[0153] In the comparative example 4 above, the specific surface area of ​​the first graphite is 1.1 m 2 / g, Dv50 is 11μm; the specific surface area of ​​the second graphite is 1.0m 2 / g, Dv50 is 10μm, and the rest are the same as in Example 1.

[0154] In the above-mentioned comparative example 5, the negative electrode active material (35.475wt% second graphite + 35.475wt% first graphite + 23.65wt% SiC), the conductive agent (0.6wt% carbon black + 0.15wt% carbon nanotubes), the binder (1.8wt% PAA + 2.2wt% SBR), and the thickener (0.65wt%) were mixed with water and dispersed by ball milling to form a uniform slurry. The slurry was sprayed on the surface of the negative electrode current collector and dried. After rolling, slitting, and die-cutting, the negative electrode sheet was obtained. The surface density, compaction density and thickness of the negative electrode sheet were the same as those in Example 1, and the parameter conditions of the second graphite and the first graphite were the same as those in Example 1.

[0155] Example 8-25

[0156] The difference between Examples 8-25 of the present invention and Example 1 is that the specific surface area, OI value and Dv50 of the first graphite and the second graphite are changed, the parameter conditions of the carbon nanotubes in the second negative electrode active layer are changed, the parameter conditions of the first graphite and the second graphite soft carbon coating layer are changed, the surface density of the first negative electrode active layer and the second negative electrode active layer is changed, or the composition of the positive electrode sheet is changed; the other parameter conditions are exactly the same as those in Example 1, and the specific differences are shown in Tables 2 and 3.

[0157] Table 2 Table 3

[0158]

[0159]

[0160] The prepared batteries of the examples and comparative examples were subjected to high temperature cycle, fast charge window, and constant capacity tests. The specific test process is as follows:

[0161] 1. High temperature cycle test

[0162] Carry out 1C / 1C charge and discharge test in 45±2℃ environment with voltage range of 2.75-4.15V. The steps are as follows: 1C constant current charge to the upper limit voltage and then constant voltage charge with cut-off current of 0.05C; let it stand for 30min; 1C discharge to the lower limit voltage; repeat such charge and discharge steps until the capacity retention rate reaches 75% (the capacity retention rate is the ratio of the discharge capacity to the first cycle discharge capacity). The thickness and DCIR of the battery cell are measured every 200T (50% SOC 3C / 10s), and the ratios are made to the thickness and DCIR before the test, which are the rate of change of the battery cell expansion rate and DCIR. If the rate of change of the battery cell expansion rate and DCIR is too large, it means that the high temperature cycle performance of the battery cell has deteriorated.

[0163] 2. Fast charging window test

[0164] The test was conducted at 25±2℃ with a voltage range of 2.5-4.25V. The steps were as follows: charge to the upper limit voltage with nC, and cut off at 0.05C; leave for 30 minutes; discharge to the lower limit voltage with nC; leave for 30 minutes, charge and discharge 20 times, and then charge the battery to the upper limit voltage with nC (n is 1, 2, and 3 respectively). The battery was then disassembled to observe whether there was lithium plating on the negative electrode.

[0165] 3. Constant volume test

[0166] Tested at 25±2°C with a voltage range of 2.5-4.25V, the procedure is as follows: charge at 0.2C to the upper voltage limit, then cut off at 0.05C; rest for 30 minutes; then discharge at 0.2C to the lower voltage limit. The battery's output energy (Wh) and weight (kg) are then converted to WED. In the battery field, WED typically refers to weight energy density (WED), which is the energy output per unit mass of a battery. Its unit is Wh / kg, representing the amount of energy provided by each kilogram of battery.

[0167] Through several groups of test examples, the batteries provided in the embodiments and comparative examples of the present invention were tested and verified, and the verification results are shown in Tables 4 and 5 below.

[0168] Table 4

[0169]

[0170]

[0171] Table 5

[0172] Cycle life Battery cell expansion rate Rate of change of DCIR Is there lithium deposition? WED Example 8 850T 15.70％ 16.10％ 1C does not produce lithium, 2C produces lithium 348.4 Example 9 835T 15.20％ 17.40％ 2C does not deposit lithium 347.6 Example 10 640T 26.40％ 26.40％ 1C lithium plating 348.5 Example 11 635T 27.60％ 26.50％ 1C does not produce lithium, 2C produces lithium 347.6 Example 12 840T 16.90％ 17.20％ 1C does not produce lithium, 2C produces lithium 347.6 Example 13 820T 17.40％ 17.50％ 2C does not deposit lithium 347.4 Example 14 814T 17.50％ 17.60％ 2C does not deposit lithium 347.3 Example 15 803T 17.90％ 17.90％ 2C does not deposit lithium 346.2 Example 16 800T 19.00％ 18.00％ 1C does not produce lithium, 2C produces lithium 345.6 Example 17 785T 20.03％ 18.30％ 1C does not produce lithium, 2C produces lithium 345.3 Example 18 637T 26.10％ 26.00％ 1C lithium plating 346.5 Example 19 678T 24.60％ 24.00％ 1C lithium plating 344.5 Example 20 620T 29.70％ 27.20％ 1C lithium plating 347.6 Example 22 1100T 10.00％ 12.40％ 2C does not deposit lithium 332.5 Example 23 680T 24.00％ 22.07％ 1C does not produce lithium, 2C produces lithium 355.8 Example 24 870T 14.10％ 15.90％ 1C does not produce lithium, 2C produces lithium 349 Example 25 842T 15.00％ 16.10％ 2C does not deposit lithium 350.5

[0173] It can be seen from the results of Table 4 above that the negative electrode sheet is designed to be double-layer coated, that is, secondary particle graphite coated with carbon nanotubes is selected as the second negative electrode active material in the second negative electrode active layer (surface layer) of the negative electrode sheet, and the specific surface area of ​​the second negative electrode active material is optimized to improve the fast charging performance; at the same time, secondary particle graphite and single particle graphite are selected in the first negative electrode active layer (bottom layer) of the negative electrode sheet to form the first negative electrode active material to improve the high temperature performance; by improving the fast charging performance of the second negative electrode active material and cooperating with the first negative electrode active material to improve the high temperature performance, the purpose of comprehensively improving the fast charging and high temperature performance of the battery by the silicon-doped graphite negative electrode sheet is achieved.

[0174] The results in Table 5 show that further optimizing the specific surface area, OI value, and Dv50 of the first graphite and the second graphite, controlling the parameter conditions of the carbon nanotubes in the second negative electrode active layer, the parameter conditions of the first graphite and the second graphite soft carbon coating layer, the surface density of the first negative electrode active layer and the second negative electrode active layer, and the composition of the positive electrode sheet within the preferred range of the present invention can further significantly improve the fast charging and high temperature performance of the battery with the silicon-doped graphite negative electrode sheet.

[0175] Although the embodiments of the present invention have been described with reference to the accompanying drawings, those skilled in the art may make various modifications and variations without departing from the spirit and scope of the present invention. Such modifications and variations are all within the scope defined by the appended claims.

Claims

1. A negative electrode sheet, characterized in that: The negative electrode active layer comprises a negative electrode current collector and a negative electrode active layer arranged on at least one side of the negative electrode current collector, wherein the negative electrode active layer comprises a first negative electrode active layer arranged on the negative electrode current collector and a second negative electrode active layer arranged on a side of the first negative electrode active layer away from the negative electrode current collector; The first negative electrode active layer includes a first negative electrode active material, the first negative electrode active material includes a silicon material and a first graphite, the first graphite includes secondary graphite particles and single graphite particles, and the mass ratio of the secondary graphite particles to the single graphite particles is 1:9 to 9:1; The second negative electrode active layer includes a second negative electrode active material, which includes a silicon material and a second graphite. The second graphite is a secondary particle graphite with an outer layer coated with carbon nanotubes. The specific surface area of ​​the second graphite is greater than that of the first graphite.

2. The negative electrode sheet according to claim 1, characterized in that: The specific surface area of ​​the first graphite is 0.8 to 1.1 m 2 / g, the specific surface area of ​​the second graphite is 1.0 to 1.4 m 2 / g; And / or, the OI value of the first graphite is greater than the OI value of the second graphite; preferably, the OI value of the first graphite is 6.2-6.7, and the OI value of the second graphite is 5.5-6.0; And / or, the Dv50 of the first graphite is greater than the Dv50 of the second graphite; preferably, the Dv50 of the first graphite is 11-20 μm, and the Dv50 of the second graphite is 8-15 μm.

3. The negative electrode sheet according to claim 1 or 2, characterized in that: The coating amount of the carbon nanotubes is 0.05 to 0.5% based on the total amount of graphite in the second negative electrode active layer; and / or, the carbon nanotubes have an average length of 5 to 15 μm and an average diameter of 1 to 5 nm; And / or, in the first negative electrode active layer, the secondary graphite particles further have a soft carbon coating layer, and the coating amount of the soft carbon coating is 0.5% to 2%; And / or, in the second negative electrode active layer, a soft carbon coating layer is further present in the secondary graphite particles, and the coating amount of the soft carbon coating is 0.5% to 2%.

4. The negative electrode sheet according to any one of claims 1 to 3, characterized in that: The compaction density of the negative electrode active layer is 1.4-1.65 g / cm 3 ; And / or, the surface density of the first negative electrode active layer and the second negative electrode active layer is 1.8 to 6.5 mg / cm 2 Preferably, the ratio of the surface density of the first negative electrode active layer to the surface density of the second negative electrode active layer is 3:7 to 7:3; And / or, the Dv50 of the silicon material is 6 to 12 μm.

5. The negative electrode sheet according to any one of claims 1 to 4, characterized in that: The first negative electrode active layer and the second negative electrode active layer also include a conductive agent, a binder and a thickener; Preferably, the conductive agent comprises 0.5% to 0.7% by mass of conductive carbon black and 0.05% to 0.25% by mass of carbon nanotubes; And / or, the binder comprises 1.5% to 2.2% by mass of polyacrylic acid and 1.8% to 2.5% by mass of styrene-butadiene rubber; And / or, the thickener comprises 0.45% to 0.8% by mass of carboxymethyl cellulose; And / or, the mass proportion of silicon material in the first negative electrode active material is 10% to 30%, and the mass proportion of silicon material in the second negative electrode active material is 10% to 30%.

6. The negative electrode sheet according to any one of claims 1 to 5, characterized in that: The negative electrode current collector includes a negative electrode foil and a carbon coating layer arranged on the surface of the negative electrode foil; the surface density of the carbon coating layer is 0.3 to 1.3 g / m 2 , thickness is less than or equal to 3.0μm.

7. The negative electrode sheet according to claim 6, characterized in that: The thickness of the negative electrode foil is 4 to 10 μm; And / or, the tensile strength of the negative electrode foil is TD ≥ 400 MPa, MD ≥ 400 MPa; the elongation of the negative electrode foil is TD ≥ 4.5%, MD ≥ 4.5%.

8. A lithium ion battery, characterized in that: A negative electrode sheet according to any one of claims 1 to 7, further comprising a positive electrode sheet; The positive electrode sheet comprises a positive electrode current collector and a positive electrode active layer, wherein the positive electrode active layer comprises a positive electrode active material, and the positive electrode active material is a ternary material LiNi x Co y Mn 1-x-y O2, wherein x≥0.8, y≥0.02, 1-xy>0; preferably, x≥0.

92.

9. The lithium-ion battery according to claim 8, characterized in that The ternary material LiNi x Co y Mn 1-x-y O2 includes single crystal particles and polycrystalline particles, and the mass ratio of the single crystal particles to the polycrystalline particles is (0.33-3):1; And / or, the positive electrode active layer further includes a conductive agent and a binder; the conductive agent includes 0.8% to 1.5% by weight of conductive carbon black and 0.7% to 1.1% by weight of carbon nanotubes, and the binder is 0.9% to 1.5% by weight of polyvinylidene fluoride; And / or, the compaction density of the positive electrode active layer is 3.2 to 3.4 g / cm 3 ; And / or, the surface density of the positive electrode active layer is 14 to 27 mg / cm 2 .

10. An electrical device, characterized in that: A lithium-ion battery comprising the lithium-ion battery according to any one of claims 8 to 9.

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