Negative electrode and preparation method thereof, secondary battery and electric equipment

Abstract

The invention provides a negative electrode and a preparation method thereof, a secondary battery and electric equipment. The negative electrode comprises a negative electrode material layer, wherein the negative electrode material layer comprises graphite; the negative electrode material layer satisfies: OIlt; 70, Ra > = 0.65 [mu] m, and OI value / Ra < = 80, calculated based on the OI value and the value of Ra; wherein the OI value is the intensity ratio of a (004) characteristic peak to a (110) characteristic peak in an X-ray diffraction pattern of the negative electrode material layer, the Ra is the surface roughness of the negative electrode material layer, and the unit is [mu] m. Due to the special parameter design of the negative electrode, the negative electrode can be used for providing a secondary battery with relatively excellent rate capability and good quick charge performance.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a negative electrode and a preparation method thereof, a secondary battery, and an electrical device. Background Art

[0002] With the popularity of consumer electronic devices and new energy vehicles, consumers have increasingly higher requirements for the charging speed of these electrical devices. The charging speed of electrical devices depends largely on the performance of the negative electrode of the secondary battery they carry. Current secondary batteries often use graphite as the active material for the negative electrode. However, the layered structure of graphite requires active ions to be deintercalated / embedded from its end faces. Therefore, the industry has tried to improve the charging dynamics of secondary batteries by reducing the orientation of the graphite negative electrode to achieve better fast charging performance of secondary batteries. However, in some cases, simply reducing the orientation of the graphite negative electrode has limited improvement on the rate performance of the negative electrode. Therefore, there is an urgent need to provide a new negative electrode solution. Summary of the Invention

[0003] In view of this, the embodiments of the present application provide a negative electrode and a preparation method thereof, a secondary battery and an electrical device. The special parameter design of the negative electrode enables it to be used to provide a secondary battery with both better rate performance and better fast charging performance.

[0004] In a first aspect, the present application provides a negative electrode, comprising a negative electrode material layer, wherein the negative electrode material layer comprises graphite; the negative electrode material layer satisfies: OI value <70, Ra ≥ 0.65 μm, and calculated using the numerical values ​​of the OI value and the Ra, OI value / Ra ≤ 80; wherein the OI value is the ratio of the intensities of the (004) characteristic peak and the (110) characteristic peak in the X-ray diffraction pattern of the negative electrode material layer, and the Ra is the surface roughness of the negative electrode material layer, in μm.

[0005] The above OI value reflects the orientation degree of the negative electrode material layer. The negative electrode provided in the embodiment of the present application satisfies a specific quantitative relationship between its orientation degree and the surface roughness of the negative electrode material layer, which can improve the electrolyte wetting performance and liquid retention capacity of the negative electrode while improving the ion deintercalation / intercalation speed of the graphite, thereby optimizing the reaction kinetics of the negative electrode. Based on the above quantitative relationship, controlling the orientation degree OI value of the negative electrode material layer within a range of less than 70 can allow the graphite to expose more of its end faces, which is beneficial to the deintercalation / intercalation of active ions; controlling Ra within a range of greater than or equal to 0.65μm can allow the surface of the negative electrode material layer to have a richer pore structure, which is more conducive to the wetting and retention of the electrolyte. Therefore, the above negative electrode can be used to provide a secondary battery with both superior rate performance and good fast charging performance.

[0006] A second aspect of the present application provides a method for preparing a negative electrode, which can be used to prepare the negative electrode provided in the first aspect of the embodiment of the present application. The preparation method comprises:

[0007] A raw material including graphite is rolled to form a negative electrode material layer to obtain a negative electrode; the negative electrode material layer satisfies: OI value <70, Ra ≥ 0.65 μm, and calculated based on the numerical values ​​of the OI value and the Ra, OI value / Ra ≤ 80; wherein the OI value is the ratio of the intensity of the (004) characteristic peak and the (110) characteristic peak in the X-ray diffraction spectrum of the negative electrode material layer, and the Ra is the surface roughness of the negative electrode material layer, in μm.

[0008] The preparation method is simple and easy to implement, has strong process controllability, high production efficiency, and is suitable for large-scale industrial production.

[0009] The third aspect of the present application provides a secondary battery, comprising the negative electrode provided in the first aspect of the present application. Due to the use of the negative electrode provided in the embodiment of the present application, the secondary battery has excellent rate performance and good fast charging performance.

[0010] In a fourth aspect, the present application provides an electrical device comprising the secondary battery provided in the embodiments of the present application. Due to the secondary battery provided in the embodiments of the present application, the electrical device has a good market prospect. DETAILED DESCRIPTION

[0011] Graphite is widely used as a negative electrode active material due to its advantages such as low cost and high energy density. However, due to its layered structure, graphite does not have the ability to quickly de- / intercalate active ions, resulting in the battery being unable to achieve fast charging performance, which limits the battery's application scenarios. In order to solve the above technical problems, the industry regulates the orientation of graphite in the negative electrode to expose more end faces to build a fast channel for active ions in the negative electrode. Increasing the surface roughness of the negative electrode material layer is conducive to building more porous structures on the surface of the negative electrode material layer, which is conducive to the infiltration of the electrolyte.

[0012] However, the applicant found that if the surface roughness of the negative electrode material layer is simply improved, or the orientation of the graphite negative electrode is reduced alone, it may cause the negative electrode material layer to have performance shortcomings, resulting in limited improvement in the rate performance and fast charging performance of the final battery. Based on the above findings, when the applicant further regulates the orientation degree of the negative electrode and the surface roughness of the negative electrode within a suitable range, and when the two satisfy a certain mathematical relationship, a better synergistic effect can be exerted, and a better electrolyte diffusion channel and ion diffusion channel can be constructed in the negative electrode material layer at the same time. Therefore, the embodiment of the present application provides a negative electrode, including a negative electrode material layer, the negative electrode material layer includes graphite; the negative electrode material layer satisfies: OI value <70, Ra ≥ 0.65 μm, and calculated based on the numerical values ​​of OI value and Ra, OI value / Ra ≤ 80; wherein the OI value is the ratio of the intensity of the (004) characteristic peak and the (110) characteristic peak in the X-ray diffraction spectrum of the negative electrode material layer, and Ra is the surface roughness of the negative electrode material layer, in μm.

[0013] The above OI value reflects the orientation degree of the negative electrode material layer. The ratio of the OI value to the surface roughness Ra of the negative electrode material layer in μm satisfies a specific quantitative relationship (OI value / Ra≤80), so that the graphite can better expose its end face, thereby constructing a fast channel for active ions in the negative electrode material layer, improving the power performance of the negative electrode; at the same time, the negative electrode material layer also has a suitable surface roughness, which can simultaneously improve the deintercalation / intercalation speed of the active ions (for example, lithium ions) of the graphite and the electrolyte infiltration and liquid retention capacity of the negative electrode, thereby improving the rate performance and fast charging performance of the final battery. Based on the above quantitative relationship, the orientation degree OI value of the negative electrode material layer is controlled within a range of less than 70, which is easy to prepare and can make the graphite expose its end face more, which is beneficial to the deintercalation / intercalation of active ions; controlling Ra to be greater than or equal to 0.65μm can make the surface of the negative electrode material layer have a richer pore structure, which is more conducive to the infiltration and retention of the electrolyte. Therefore, the above-mentioned negative electrode can be used to provide a secondary battery with both better rate performance and better fast charging performance.

[0014] Illustratively, the value of OI value / Ra can be, but is not limited to, 80, 78, 75, 72, 70, 68, 65, 62, 60, 55, 50, 45, 40, 35, 30, 25, 20, 15, 10, 5, etc.

[0015] In the embodiment of the present application, the OI value can be tested by the following method: the negative electrode is placed in an X-ray diffraction (XRD) tester so that the X-ray is incident from the surface of the negative electrode material layer to obtain an XRD spectrum of the negative electrode material layer. The OI value is the ratio of the intensity of the (004) characteristic peak and the (110) characteristic peak in the X-ray diffraction spectrum of the negative electrode material layer, which can be expressed as I (004) / I (110)For negative electrodes with graphite as the main active material, this reflects the orientation of graphite in the negative electrode material layer. Samples are taken from three different locations on each negative electrode for testing, and the results are averaged. Specifically, taking the X-ray source used in the XRD test as an example, the 2θ range of the (004) characteristic peak of graphite is in the range of 53°-56°, and the 2θ range of the (110) characteristic peak of graphite is in the range of 76°-79°. If other X-ray light sources are used, the 2θ range of the (004) and (110) characteristic peaks of graphite under the corresponding X-ray light source can be obtained by substituting the wavelength of the X-ray light source into the Bragg diffraction formula.

[0016] For example, the OI value can be, but is not limited to, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 15, 18, 20, 22, 25, 28, 30, 32, 35, 38, 40, 42, 45, 48, 50, 55, 58, 59, 60, 62, 65, 68, 70, etc. If the OI value is too high, it is not conducive to the transport of ions, resulting in poor functional performance of the negative electrode.

[0017] In the embodiments of the present application, the surface roughness Ra of the negative electrode material layer specifically refers to: the arithmetic mean deviation of the profile, which is the arithmetic mean of the absolute values ​​of the profile deviation within the sampling length; the surface roughness Ra of the negative electrode material layer is measured using a stylus method. Specifically, the sample is fixed on a metal plate, and the surface of the negative electrode material layer to be tested is placed away from the metal plate, and the surface roughness of the negative electrode material layer is tested in accordance with GB / T1031-2009. It can be understood that in actual applications, the negative electrode generally includes a negative electrode current collector and a negative electrode material layer disposed on the surface of the negative electrode current collector. In this case, the electrolyte mainly enters and diffuses into the negative electrode material layer through the pore structure on the surface of the negative electrode material layer away from the current collector to achieve the effect of infiltration and liquid retention. Therefore, when the negative electrode has a negative electrode current collector, the surface of the negative electrode material layer is the surface away from the current collector; the above-mentioned Ra specifically refers to the surface roughness of the side of the negative electrode material layer away from the current collector.

[0018] Appropriate surface roughness Ra is not only conducive to the infiltration of the electrolyte, but also can improve the negative electrode's ability to retain the electrolyte, thereby improving the reaction kinetics and cycle performance of the negative electrode. If Ra is too small, the pore structure on the surface of the negative electrode material layer is too small, which is not conducive to the infiltration of the electrolyte, thereby affecting the rate and fast charging performance of the battery. Exemplarily, the surface roughness Ra of the negative electrode material layer can be, but is not limited to, 0.65μm, 0.8μm, 1.0μm, 1.2μm, 1.5μm, etc. In some embodiments of the present application, the negative electrode material layer is arranged on the surface of the current collector; the surface porosity of the negative electrode material layer away from the current collector is 20%-50%. In the embodiment of the present application, a focused ion beam scanning electron microscope (FIB-SEM) can be used to test the surface porosity of the negative electrode material layer. Specifically, the surface of the negative electrode material layer is first polished using argon ions, and then the negative electrode material layer is continuously cut and imaged by FIB. Cutting is performed every 10 nm along the thickness direction, and SEM photographs of the exposed cross-section of the negative electrode material layer are taken after cutting. This is repeated more than 1,000 times to obtain SEM photographs of cross-sections of the negative electrode material layer at different thicknesses. Three-dimensional reconstruction is performed using software to obtain the porosity of the negative electrode material layer within a thickness of 10 μm, from the surface of the negative electrode material layer facing away from the negative electrode current collector, i.e., the surface porosity of the negative electrode material layer. Specifically, the surface porosity of the negative electrode material layer can be, but is not limited to, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48%, or 50%.

[0019] Taking into account different preparation processes and in order to improve the bonding strength between the negative electrode material layer and the current collector, in some embodiments, a glue layer is further provided between the negative electrode material layer and the current collector. The glue layer is used to fix the negative electrode material layer and improve the structural stability of the negative electrode, thereby being more conducive to the performance of the negative electrode. In the embodiments of the present application, in order to ensure the performance of the negative electrode, the above-mentioned glue layer can be conductive. There is no specific limitation on the material of the above-mentioned glue layer. The material of the above-mentioned glue layer can be any material known in the art as long as it is conductive. For example, the glue layer includes conductive carbon and a binder.

[0020] In the embodiments of the present application, the negative electrode current collector may be any current collector known in the art. For example, the negative electrode current collector may be copper foil, carbon-coated copper foil, stainless steel foil, etc.

[0021] In some embodiments of the present application, the OI value is ≤ 45; and / or 0.75 μm ≤ Ra ≤ 1.5 μm; and / or 3 ≤ OI value / Ra ≤ 50. This facilitates fabrication of the negative electrode and improves rate performance and fast charging capabilities. In some specific embodiments, the OI value is ≤ 43; and / or 0.75 μm ≤ Ra ≤ 1.3 μm; and / or 9 ≤ OI value / Ra ≤ 50.

[0022] In some embodiments of the present application, the D50 of the graphite in the negative electrode is 5μm-25μm. Controlling the D50 of graphite within the above range is conducive to achieving a larger compaction density. At the same time, the length of the extraction / embedding path of the active ions can be controlled within a suitable range, which is conducive to the transmission of ions and electrons, thereby further improving the fast charging performance of the final negative electrode. For example, the D50 of graphite can be, but is not limited to, 5μm, 8μm, 10μm, 12μm, 15μm, 18μm, 20μm, 22μm, 25μm, etc.

[0023] In some embodiments of the present application, the graphite in the negative electrode has a degree of graphitization of ≥90%; for example, 90%-98%. In this way, the graphite has fewer defects (defective structures caused by heteroatoms or missing carbon atoms in the carbon hexagonal ring structure), high structural stability, better electrical conductivity, and is more conducive to the conduction of active ions, and its specific capacity can be further improved. In some cases, the degree of graphitization of graphite is related to its source. In the embodiments of the present application, the graphite can be selected from natural graphite, artificial graphite and modified graphite; wherein the above-mentioned modified graphite includes but is not limited to oxidatively modified graphite and halogenated modified graphite; wherein the halogenated modified graphite can include graphite modified with fluorine, chlorine, bromine and iodine, preferably fluorinated modified graphite.

[0024] In the embodiment of the present application, the graphitization degree of graphite can be measured by XRD. Specifically, after measuring the XRD spectrum of graphite, its (002) crystal plane diffraction angle 2θ is obtained, and the interplanar spacing d of the (002) crystal plane of graphite is calculated according to the Bragg formula (2dsinθ=nλ). 002 , in nm; where λ is the wavelength of X-Ray used in XRD testing, and n is the diffraction order, where n = 1. Then the graphitization degree G is calculated using the Franklin formula: G = (0.344-d 002 ) / (0.344-0.3354)×100%; wherein 0.344 nm is the interlayer spacing of completely non-graphitized carbon, and 0.3345 nm is the interlayer spacing of an ideal graphite crystal.

[0025] It can be understood that the negative electrode material layer includes graphite, and the negative electrode active material may be only graphite, or the negative electrode material layer may also include other negative electrode active materials in addition to graphite. In some specific embodiments of the present application, only graphite is used as the negative electrode active material. In this case, the mass proportion of graphite in the negative electrode material layer is 80%-100%. In this way, there can be a sufficient amount of negative electrode active material in the negative electrode material layer to ensure the performance of the negative electrode, and there is a sufficient amount of graphite to improve the energy density of the final battery. Exemplarily, the mass proportion of graphite in the negative electrode material layer can be, but is not limited to, 80%, 82%, 85%, 88%, 90%, 92%, 95%, 98%, and 100%.

[0026] In some other embodiments of the present application, the negative electrode material layer includes other negative electrode active materials in addition to graphite. In some specific embodiments, the above-mentioned other negative electrode active materials include but are not limited to at least one of silicon-based materials, graphene, mesophase microcarbon beads, hard carbon and soft carbon. At this time, the mass proportion of graphite in the negative electrode material layer is 50%-90%. In this way, it is helpful to take into account the energy density of the final battery while balancing the ion / electron diffusion rate of the negative electrode and the electrolyte retention capacity, thereby ensuring that the fast charging performance, rate performance and cycle performance of the final battery are good. Specifically, when other negative electrode active materials are included, the mass proportion of graphite in the negative electrode material can be, but is not limited to, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%.

[0027] In general, in order to improve the structural stability and conductivity of the negative electrode material layer, the negative electrode material layer may also include a binder and a conductive agent. In some embodiments of the present application, the negative electrode material layer includes a binder, and the mass proportion of the binder in the negative electrode material layer is 1%-5%. A suitable binder ratio is beneficial to improving the structural stability of the negative electrode without crowding out the content of other components such as the negative electrode active material, so that the overall performance of the negative electrode is better. For example, the mass proportion of the binder in the negative electrode material layer can be, but is not limited to, 1%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, etc.

[0028] In the embodiment of the present application, the above-mentioned binder can be selected from any well-known binder suitable for the negative electrode; illustratively, the above-mentioned binder includes but is not limited to tetrafluoroethylene and its copolymers, polyvinylidene fluoride and its copolymers, polyolefins and their copolymers (for example, polyethylene-polyethylene glycol block copolymers, etc.), polyethers and their copolymers (for example, polyethylene oxide, etc.), polyphenylene ethers and their copolymers, polysiloxanes and their copolymers (for example, polydimethylsiloxane, poly(dimethylsiloxane-co-alkylmethylsiloxane)), polyesters and their copolymers (for example, polyvinyl esters, polyvinyl acetate, polyacrylates, etc.), carboxymethyl cellulose, styrene-butadiene latex, and at least one of nitrile rubber. Specifically, the polyolefin includes one or more of polyethylene, polypropylene, ethylene / propylene copolymer, ethylene / vinylidene fluoride copolymer, and propylene / vinylidene fluoride copolymer; polytetrafluoroethylene and its copolymers can be at least one of tetrafluoroethylene / ethylene copolymer, tetrafluoroethylene / propylene copolymer, tetrafluoroethylene / vinylidene fluoride copolymer, tetrafluoroethylene / ether copolymer, tetrafluoroethylene / branched polyether copolymer, tetrafluoroethylene / vinyl ether copolymer, tetrafluoroethylene / branched polyether / vinyl ether copolymer, and tetrafluoroethylene / siloxane copolymer.

[0029] In some embodiments of the present application, the negative electrode material layer also includes a conductive agent, and the weight proportion of the conductive agent in the negative electrode material layer is 0.3%-2%. By controlling the content of the conductive agent within the above range, a sufficient amount of conductive agent is involved in the construction of the conductive network in the negative electrode material layer, ensuring good electronic conductivity of the negative electrode, which is beneficial to the power performance of the final battery; at the same time, it will not squeeze out the proportion of other components such as the negative electrode active material, which is beneficial to the performance of the battery. Exemplarily, the weight proportion of the conductive agent in the negative electrode material layer can be, but is not limited to, 0.3%, 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, 1.8%, and 2.0%. In the embodiments of the present application, the conductive agent can be selected from any conductive agent known in the art; exemplarily, the conductive agent includes, but is not limited to, at least one of acetylene black, Ketjen black, Super-P, carbon nanotubes, carbon nanofibers, activated carbon, and graphene.

[0030] In some embodiments of the present application, the double-sided density of the negative electrode is 100 g / m 2 -500g / m 2 . It can be understood that the double-sided surface density refers to the sum of the surface densities of the negative electrode material layers on both surfaces of the negative electrode current collector for a negative electrode having a negative electrode material layer provided on both surfaces. Controlling the double-sided surface density of the negative electrode within the above range is conducive to achieving a higher energy density and is conducive to the performance of the negative electrode. For example, the double-sided surface density of the negative electrode can be, but is not limited to, 100g / m 2 , 150g / m 2 , 180g / m 2 , 200g / m2 , 220g / m 2 , 250g / m 2 , 280g / m 2 , 300g / m 2 , 320g / m 2 、350g / m 2 , 380g / m 2 , 400g / m 2 , 420g / m 2 , 450g / m 2 , 500g / m 2 wait.

[0031] In some embodiments of the present application, the compaction density of the negative electrode is 1.1 g / cm 3 -1.8g / cm 3 Controlling the compaction density of the negative electrode within the above range is beneficial to maintaining the structural stability of the negative electrode material layer and good electrode wettability, thereby benefiting the rate performance and cycle performance of the battery; it is also beneficial to achieving a higher battery energy density and lower internal resistance. For example, the compaction density of the negative electrode can be, but is not limited to, 1.10 g / cm 3 , 1.20g / cm 3 , 1.30g / cm 3 , 1.40g / cm 3 , 1.42g / cm 3 , 1.45g / cm 3 , 1.48g / cm 3 , 1.50g / cm 3 , 1.52g / cm 3 , 1.55g / cm 3 , 1.5g / cm 3 8. 1.60g / cm 3 , 1.70g / cm 3 , 1.80g / cm 3 wait.

[0032] In the embodiments of this application, the thickness of the negative electrode current collector and the negative electrode material layer are not specifically limited, and those skilled in the art may select thicknesses based on actual needs. When a glue layer is present between the negative electrode current collector and the negative electrode material layer, the thickness of the glue layer is also not limited, as long as it ensures good structural stability of the negative electrode current collector and the negative electrode material layer and does not affect the performance of the negative electrode.

[0033] The present invention also provides a method for preparing a negative electrode, comprising:

[0034] A raw material including graphite is rolled to form a negative electrode material layer to obtain a negative electrode; the negative electrode material layer satisfies: OI value <70, Ra ≥ 0.65 μm, and calculated using the numerical values ​​of OI value and Ra, OI value / Ra ≤ 80; wherein the OI value is the ratio of the intensity of the (004) characteristic peak and the (110) characteristic peak in the X-ray diffraction pattern of the negative electrode material layer, and the Ra is the surface roughness of the negative electrode material layer, in μm.

[0035] The preparation method is simple and easy to implement, has high production efficiency, and is suitable for large-scale industrial production.

[0036] In some embodiments of the present application, the raw materials further include a binder and a conductive agent.

[0037] In some specific embodiments, the above raw materials also include other negative electrode active materials in addition to graphite. For details, please refer to the above description.

[0038] In some embodiments of the present application, the negative electrode active material (including graphite), binder, and conductive agent are weighed in corresponding proportions as needed and placed in a mixer (e.g., a V-type mixer) for mixing to obtain a raw material. In some specific embodiments, the mixed material is further placed in a pulverizer (e.g., a jet mill) for grinding and mixing to obtain a raw material. In some specific embodiments, the crushing pressure of the jet mill is 0.1 MPa-0.8 MPa.

[0039] In the industry, the common anode preparation processes include dry and wet processes. The dry process does not require the use of solvents or drying, is more environmentally friendly, and also helps reduce costs.

[0040] In some embodiments of the present application, the negative electrode is prepared using a dry process. In this case, the negative electrode preparation method includes step S01: performing a first rolling operation on the raw material to obtain a negative electrode material layer (i.e., a self-supporting film); wherein the first rolling operation includes passing the raw material through a first roller and a second roller disposed opposite each other. In some specific embodiments, the first roller and the second roller are disposed horizontally opposite each other, and the raw material passes between the first roller and the second roller, so that the powdered raw material forms a self-supporting film.

[0041] In some specific embodiments, in step S01, the pressure of the first roller and the second roller is independently 1t-20t. In some specific embodiments, the pressure of the first roller and the second roller is independently 1t-20t. In this way, the negative electrode material layer can be formed and have a suitable surface density and compaction density, and the graphite orientation can be guided, a suitable ion diffusion channel can be constructed in the negative electrode material layer, and the surface roughness of the negative electrode material layer can be controlled within a suitable range. In some specific embodiments, the pressure of the first roller and the second roller is the same, which can reduce equipment wear and help ensure the quality of the negative electrode. Exemplarily, in step S01, the pressure of the first roller and the second roller can be independently 0.1t, 0.5t, 1t, 2t, 5t, 8t, 10t, 12t, 15t, 18t, 20t, etc. It is understood that a certain gap must exist between the first and second rollers to allow the raw material to pass through and form a film. In some embodiments of the present application, in step S01, the gap between the first and second rollers, in the direction from the first roller toward the second roller, is 70μm-150μm. This gap size refers to the minimum vertical distance between the surfaces of the first roller and the second roller, which is commonly referred to in the industry as a roll gap size of 70μm-150μm. Adjusting either the roll gap size or the pressure of the first and second rollers, or both, can adjust the pressure on the raw material and thereby adjust the OI value and surface roughness Ra of the negative electrode material layer. Similarly, controlling the roll gap size within the above range not only facilitates the formation of the negative electrode material layer but also helps to achieve a more optimal OI value / Ra. Specifically, the roll gap size between the first and second rollers can be, but is not limited to, 70μm, 80μm, 90μm, 100μm, 110μm, 120μm, 130μm, 140μm, or 150μm.

[0042] In some specific embodiments, in order to obtain negative electrode material layers with different OI values / Ra, or in other words, in order to adjust parameters such as the compaction density and surface density of the negative electrode material layer, when producing different negative electrode material layers, the pressure of the first roller and the second roller on the production line can be kept unchanged, and the roller gap size between the first roller and the second roller can be adjusted.

[0043] As is well known, in self-supporting films, the binder typically undergoes fiberization and forms a binder network to ensure the structural stability of the self-supporting film. In some embodiments, the raw materials also include a binder. In step S01, the surface temperatures of the first and second rollers are independently between 100°C and 250°C. This reduces the rolling force requirement, improves the calendering effect, and further promotes binder fiberization and the formation of a binder network, thereby ensuring the structural stability of the self-supporting film. It also minimizes the risk of overheating that could affect the properties of the binder or other components, increase roller deformation, and thus hinder the formation of the self-supporting film. In some embodiments, the surface temperatures of the first and second rollers are the same, which reduces equipment wear and improves the quality of the negative electrode. For example, the surface temperatures of the first and second rollers can independently be 100°C, 120°C, 150°C, 180°C, 200°C, 220°C, 250°C, etc.

[0044] In some specific embodiments, in step S01, the ratio of the linear speeds of the first roller and the second roller is 1:(1-3). In this way, a shear force is applied to the material between the two rollers, which not only further fiberizes the binder and prepares it into a self-supporting film, but also reduces the dependence on pressure and roller gap, increases the calendering amount and improves the calendering effect. More importantly, controlling the ratio of the linear speeds of the two within the above range is also conducive to ensuring that the final negative electrode material layer has good mechanical properties, and is conducive to reducing the surface density of the negative electrode material layer, which is conducive to the final negative electrode electrochemical performance. Specifically, in step S01, the ratio of the linear speeds of the first roller and the second roller can be, but is not limited to, 1:1, 1:1.2, 1:1.5, 1:1.8, 1:2, 1:2.2, 1:2.5, 1:2.8, 1:3.0, etc.

[0045] In other embodiments of the present application, in order to reduce the thickness of the negative electrode material layer, increase the compaction density, and reduce the surface density of the negative electrode, the method further includes step S02: subjecting the material obtained by the first calendering in step S01 to a second calendering. In other words, the second calendering can be a thinning process for the self-supporting film obtained by the first calendering.

[0046] In some specific embodiments, step S02 includes: passing the material obtained by the first calendering through a third roller and a fourth roller that are oppositely arranged to obtain a negative electrode material layer.

[0047] In some specific embodiments, in step S02, the pressure of the third and fourth rollers is independently 1t-20t. For example, in step S02, the linear pressure of the third and fourth rollers can independently be 0.1t, 0.5t, 1t, 2t, 5t, 8t, 10t, 12t, 15t, 18t, 20t, etc. This not only helps reduce the surface density of the negative electrode material layer and increase its compaction density, but also helps reduce the graphite orientation in the negative electrode material layer and increase the Ra of the negative electrode material layer. In some specific embodiments, the linear pressure of the third and fourth rollers is equal.

[0048] In some embodiments of the present application, in step S02, the size of the gap between the third roller and the fourth roller in the direction from the third roller to the fourth roller is 50μm-120μm. The size of the above gap refers to the minimum vertical distance between the surface of the third roller and the surface of the fourth roller, that is, the roller gap size commonly referred to in the industry is 50μm-120μm. Similarly, in some specific embodiments of the present application, when producing different negative electrode material layers, the linear pressure of the third roller and the fourth roller is controlled to remain unchanged, and the roller gap size of the third roller and the fourth roller is adjusted. Specifically, the roller gap size between the first roller and the second roller can be, but is not limited to, 50μm, 60μm, 70μm, 80μm, 90μm, 100μm, 110μm, and 120μm.

[0049] In some specific embodiments, in step S02, the surface temperatures of the third and fourth rollers are independently between 100°C and 250°C. For example, in step S02, the surface temperatures of the third and fourth rollers can independently be 100°C, 120°C, 150°C, 180°C, 200°C, 220°C, 250°C, etc. This helps to control the graphite in the negative electrode material layer to have a low degree of orientation and a high surface roughness Ra. Similarly, in some specific embodiments, the surface temperatures of the third and fourth rollers are the same.

[0050] In some specific embodiments, in step S02, the ratio of the linear speeds of the third roller and the fourth roller is 1:(1-3). For example, the ratio of the rotational speeds of the third roller and the fourth roller can be, but is not limited to, 1:1, 1:1.2, 1:1.5, 1:1.8, 1:2, 1:2.2, 1:2.5, 1:2.8, 1:3.0, etc. Similarly, under the coordination of the pressure and surface temperature of the third and fourth rollers, controlling the ratio of their linear speeds within the above range is more conducive to better comprehensive electrochemical performance of the final negative electrode.

[0051] In the embodiment of the present application, the parameters of the first calendering may be the same as or different from the parameters of the second calendering.

[0052] In some other embodiments, to obtain a negative electrode material layer having a target compaction density and areal density, the second rolling step further includes S03: a third rolling step. The third rolling step includes passing the material obtained after the second rolling step through a fifth roller and a sixth roller arranged opposite each other to obtain the negative electrode material layer. The linear pressure of the fifth roller and the sixth roller are independently 1t-20t; the surface temperature of the fifth roller and the sixth roller is 100°C-250°C; and the linear speed ratio of the fifth roller and the sixth roller is 1:(1-3). Similarly, the pressure and surface temperature of the fifth roller and the sixth roller are equal.

[0053] In some embodiments of the present application, in step S03, the size of the gap between the fifth roller and the sixth roller in the direction from the fifth roller to the sixth roller is 30μm-100μm. The size of the above gap refers to the minimum vertical distance between the surface of the fifth roller and the surface of the sixth roller, that is, the roller gap size commonly referred to in the industry is 30μm-100μm. Similarly, in some specific embodiments of the present application, when producing different negative electrode material layers, the linear pressure of the fifth roller pointing to the sixth roller is controlled to remain unchanged, and the roller gap size of the fifth roller pointing to the sixth roller is adjusted. Specifically, the roller gap size between the fifth roller and the sixth roller can be, but is not limited to, 30μm, 40μm, 50μm, 60μm, 70μm, 80μm, 90μm, or 100μm.

[0054] In some embodiments of the present application, the negative electrode material layer (self-supporting film) prepared in step S01, or the negative electrode material layer prepared in step S02, is placed on the surface of a current collector, or the negative electrode material layer prepared in step S03 is placed on the surface of a current collector, and a hot pressing composite process is performed to obtain a negative electrode. In this way, the negative electrode material layer can be fixed on the current collector, and further pressure can be applied to the negative electrode material layer to further control the compaction density of the negative electrode material layer.

[0055] It is understood that the surface of the current collector is also provided with an adhesive layer. In some specific embodiments, when the two opposite surfaces of the current collector have negative electrode material layers, two negative electrode material layers can be respectively attached to the two opposite surfaces of the current collector provided with the adhesive layer and hot-pressed together.

[0056] In some specific embodiments of the present application, the pressure of the above-mentioned hot pressing composite is 1t-20t, and the temperature of the hot pressing composite is 100℃-250℃. For example, the linear pressure of hot pressing can be, but is not limited to, 0.1t, 0.5t, 1t, 2t, 5t, 8t, 10t, 12t, 15t, 18t, and 20t. For example, the temperature of hot pressing composite can be, but is not limited to, 100℃, 120℃, 150℃, 180℃, 200℃, 220℃, and 250℃. In this way, the orientation degree of graphite and the surface roughness Ra of the negative electrode material layer can be further regulated within a preferred range. It should be noted that hot pressing composite also uses two rollers arranged opposite to each other to perform hot pressing on the material, and the pressure and surface temperature of the two rollers are equal.

[0057] The present invention also provides a secondary battery comprising the negative electrode provided in the present invention. Due to the use of the negative electrode provided in the present invention, the secondary battery has excellent power performance, good fast charging performance, and good long-cycle performance.

[0058] In some embodiments of the present application, the secondary battery is a liquid battery using a liquid electrolyte. In some specific embodiments, the secondary battery includes a positive electrode, the negative electrode, and an electrolyte and a separator disposed between the positive electrode and the negative electrode.

[0059] In the embodiments of the present application, the positive electrode can be any positive electrode known in the art. The separator and electrolyte can be any electrolyte and separator known in the art.

[0060] In other embodiments, the secondary battery is a semi-solid-state battery. In other embodiments, the secondary battery is a solid-state battery using a solid electrolyte.

[0061] In the embodiment of the present application, the secondary battery may be a lithium-ion battery, a sodium-ion battery, or other alkali metal ion battery.

[0062] The present invention also provides an electric device including the secondary battery provided in the present invention. Due to the inclusion of the secondary battery provided in the present invention, the electric device has a good market prospect.

[0063] In some embodiments of the present application, the above-mentioned electrical equipment includes but is not limited to vehicles, consumer electronic products, etc. Among them, the above-mentioned vehicles include but are not limited to new energy vehicles, power-assisted bicycles, etc.

[0064] The technical solution of this application is further illustrated below with multiple embodiments.

[0065] Example 1

[0066] The raw materials are subjected to a first calendering, wherein the raw materials are a binder (specifically polyvinylidene fluoride, PTEF), a conductive agent (specifically carbon black), and graphite (graphite having a D50 of 13.7 μm and a degree of graphitization of 94%) in a mass ratio of 3:1:96, the pressure of the first roller and the second roller are both 10t, the surface temperature is both 150°C, the gap between the first roller and the second roller is 100 μm, and the ratio of the linear speeds of the first roller and the second roller is 1:1.3, to obtain a first material; the first material is subjected to a second calendering, wherein the pressure of the third roller and the fourth roller are both 10t, the temperature is both 150°C, the linear speed ratio of the third roller and the fourth roller is 1:1, and the gap is 65 μm, to obtain a second material;

[0067] The second material was placed on both sides of the current collector (specifically copper foil) and subjected to hot pressing and laminating. The pressure of the hot pressing and laminating process was 10t, the gap between the two rollers was 90μm, and the surface temperature was 150°C to obtain a negative electrode. The OI value of the negative electrode was 49, Ra was 0.72μm, OI value / Ra was 68.0, and the double-sided density of the negative electrode material layer was 200g / m 2 , compacted density is 1.5g / cm 3 .

[0068] Example 2

[0069] The differences from Example 1 are as follows: during the first calendering, the gap between the first and second rollers was 150 μm, and the linear speed ratio of the first and second rollers was 1:2.5; during the second calendering, the linear speed ratio of the third and fourth rollers was 1:1.2, and the gap was 80 μm; during the hot pressing lamination process, the gap width between the two rollers was 70 μm. The resulting negative electrode had an OI value of 28, an Ra of 0.75 μm, and an OI / Ra ratio of 37.3.

[0070] Example 3

[0071] The differences from Example 1 are as follows: during the first calendering, the gap between the first and second rollers was 120 μm, and the linear speed ratio of the first and second rollers was 1:1.6; during the second calendering, the linear speed ratio of the third and fourth rollers was 1:1.2, and the gap was 70 μm; during the hot pressing and laminating process, the gap width between the two rollers was 80 μm. The resulting negative electrode had an OI value of 38, an Ra of 0.78 μm, and an OI / Ra ratio of 48.7.

[0072] Example 4

[0073] The differences from Example 1 are as follows: during the first calendering, the gap between the first and second rollers was 130 μm, and the linear speed ratio of the first and second rollers was 1:1.5; during the second calendering, the linear speed ratio of the third and fourth rollers was 1:1.5, and the gap was 80 μm; during the hot pressing and laminating process, the gap width between the two rollers was 75 μm. The resulting negative electrode had an OI value of 54, an Ra of 0.85 μm, and an OI / Ra ratio of 63.5.

[0074] Example 5

[0075] The differences from Example 1 are as follows: during the first calendering, the gap between the first and second rollers was 80 μm, and the linear speed ratio of the first and second rollers was 1:1.2; during the second calendering, the linear speed ratio of the third and fourth rollers was 1:1.6, and the gap was 90 μm; during the hot pressing and laminating process, the gap width between the two rollers was 70 μm. The resulting negative electrode had an OI value of 24, an Ra of 0.98 μm, and an OI / Ra ratio of 24.4.

[0076] Example 6

[0077] The differences from Example 1 are as follows: during the first calendering, the gap between the first and second rollers was 120 μm, and the linear speed ratio of the first and second rollers was 1:1.8; during the second calendering, the linear speed ratio of the third and fourth rollers was 1:1.4, and the gap was 90 μm; during the hot pressing and laminating process, the gap width between the two rollers was 80 μm. The resulting negative electrode had an OI value of 16, an Ra of 1.05 μm, and an OI / Ra ratio of 15.2.

[0078] Example 7

[0079] The differences from Example 1 are as follows: during the first calendering, the gap between the first and second rollers was 80 μm, and the linear speed ratio of the first and second rollers was 1:1.2; during the second calendering, the linear speed ratio of the third and fourth rollers was 1:1.2, and the gap was 70 μm; during the hot pressing and laminating process, the gap width between the two rollers was 70 μm. The resulting negative electrode had an OI value of 41, an Ra of 0.68 μm, and an OI / Ra ratio of 60.2.

[0080] Example 8

[0081] The differences from Example 1 are as follows: during the first calendering, the gap between the first and second rollers was 110 μm, and the linear speed ratio of the first and second rollers was 1:1.4; during the second calendering, the linear speed ratio of the third and fourth rollers was 1:1.4, and the gap was 85 μm; during the hot pressing and laminating process, the gap width between the two rollers was 75 μm. The resulting negative electrode had an OI value of 66, an Ra of 0.87 μm, and an OI / Ra ratio of 75.8.

[0082] Example 9

[0083] The differences from Example 1 are as follows: during the first calendering, the gap between the first and second rollers was 90 μm, and the linear speed ratio of the first and second rollers was 1:1.3; during the second calendering, the linear speed ratio of the third and fourth rollers was 1:1.3, and the gap was 80 μm; during the hot pressing and laminating process, the gap width between the two rollers was 75 μm. The resulting negative electrode had an OI value of 42, an Ra of 0.82 μm, and an OI / Ra ratio of 51.2.

[0084] Example 10

[0085] The differences from Example 1 are as follows: during the first calendering, the gap between the first and second rollers was 130 μm, and the linear speed ratio of the first and second rollers was 1:1.6; during the second calendering, the linear speed ratio of the third and fourth rollers was 1:1.5, and the gap was 90 μm; during the hot pressing and laminating process, the gap width between the two rollers was 80 μm. The resulting negative electrode had an OI value of 47, an Ra of 0.98 μm, and an OI / Ra ratio of 47.9.

[0086] Example 11

[0087] The differences from Example 1 are as follows: during the first calendering, the gap between the first and second rollers was 120 μm, and the linear speed ratio of the first and second rollers was 1:3; during the second calendering, the linear speed ratio of the third and fourth rollers was 1:1.2, and the gap was 110 μm; during the hot pressing and laminating process, the gap width between the two rollers was 80 μm. The resulting negative electrode had an OI value of 12, an Ra of 1.25 μm, and an OI / Ra ratio of 9.6.

[0088] In order to highlight the beneficial effects of the embodiments of the present application, the following comparative examples are provided.

[0089] Comparative Example 1

[0090] A negative electrode, the only difference from the negative electrode provided in Example 1 is that the OI value of the negative electrode in Comparative Example 1 is 42, Ra is 0.63 μm, and OI value / Ra is 66.6.

[0091] Comparative Example 2

[0092] A negative electrode, the only difference from the negative electrode provided in Example 1 is that the OI value of the negative electrode in Comparative Example 1 is 72, Ra is 0.95 μm, and OI value / Ra is 75.7.

[0093] Comparative Example 3

[0094] A negative electrode, the only difference from the negative electrode provided in Example 1 is that the OI value of the negative electrode in Comparative Example 1 is 62, Ra is 0.69 μm, and OI value / Ra is 89.9.

[0095] Electrochemical performance test:

[0096] ① The negative electrode, separator (polyethylene / polypropylene composite film), and lithium iron phosphate positive electrode prepared in the above embodiments and comparative examples were stacked, and the electrolyte was injected to prepare a soft-pack battery.

[0097] Among them, the mass ratio of the electrolyte is LiPF6:EC:DEC:VC=12:26:60:2.

[0098] The preparation method of the lithium iron phosphate positive electrode comprises: mixing the positive electrode active material lithium iron phosphate, the binder (specifically polyvinylidene fluoride) and the conductive agent (specifically conductive carbon black) in a mass ratio of 90:5:5, dispersing them in a solvent (specifically N-methylpyrrolidone), and mixing them evenly to obtain a positive electrode slurry; coating the positive electrode slurry on the opposite sides of the positive electrode current collector (specifically carbon-coated aluminum foil) (the double-sided surface density of each embodiment is consistent, which is 440g / cm 3 ), drying and roller pressing.

[0099] Preparation of the battery: The negative electrode, positive electrode and electrolyte of each embodiment and comparative example were assembled to obtain a battery, and then formed.

[0100] ② Rate performance test: The discharge capacity of each battery was tested at different rates, such as 0.33C and 3C, at 25°C, with a voltage range of 2V-3.8V. The ratio of the discharge capacity at 3C to that at 0.33C is the capacity retention rate at that rate, which reflects the quality of the rate performance. Table 2 summarizes the ratio of the first-cycle discharge capacity at 3C to the first-cycle discharge capacity at 0.33C (3C / 0.33C) of each battery.

[0101] ③ Fast charging performance test: The Yuanneng in-situ expansion analyzer is used to evaluate the expansion thickness of the battery cell at different charging rates, which can determine the lithium plating voltage and SOC window of the battery cell at different charging rates. It is a non-destructive lithium plating detection method; specifically, the SOC at which the thickness inflection point appears during the rate charging (4.5C, 4C, 3.5C, 3C, 2.5C, 2C, 1.5C, 1C, 0.5C, 0.33C) is pushed forward by 20% as the SOC at which lithium plating starts at this rate; the fast charging process is to start charging at a high rate (such as 4.5C), and when the SOC at which lithium plating starts at this rate is reached, jump to the next rate (such as 4C) to continue charging at the SOC at which lithium plating starts at this rate, and then jump to the next rate (such as 3.5C) to continue charging at the SOC at which lithium plating starts at this rate until the battery is charged to 80% SOC. The sum of the charging time at each rate is calculated as the fast charging time of the battery 0-80% SOC. The time required to charge the battery from 10% SOC to 80% SOC is used as a parameter for comparing fast charging performance.

[0102] Table 1

[0103] Case 3C capacity retention rate 0-80% SOC fast charging time Example 1 68.6％ 19.3min Example 2 69.8％ 18.1min Example 3 69.5％ 18.4min Example 4 68.9％ 19.1min Example 5 70.2％ 17.9min Example 6 70.4％ 17.6min Example 7 68.1％ 19.9min Example 8 68.4％ 19.4min Example 9 69.3％ 18.6min Example 10 69.2％ 18.8min Example 11 70.8％ 17.3min Comparative Example 1 67.4％ 20.8min Comparative Example 2 67.2％ 20.4min Comparative Example 3 66.7％ 21.5min

[0104] From the data in Table 1, it can be seen that the battery using the negative electrode provided in the examples of this application has significantly improved fast charging performance and capacity retention at high rates. Furthermore, when the OI value, Ra, and OI value / Ra of the negative electrode are all controlled within the ranges recommended by the examples of this application (Examples 2, 3, 5, 6, and 11), it is more conducive to optimizing the battery's rate performance and fast charging performance.

[0105] The above is an exemplary embodiment of the present application. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made thereto without departing from the principles of the present application. These improvements and modifications are also considered to be within the scope of protection of the present application.

Claims

1. A negative electrode, characterized in that comprising a negative electrode material layer, wherein the negative electrode material layer comprises graphite; The negative electrode material layer satisfies: OI value <70, Ra ≥ 0.65 μm, and calculated based on the numerical values ​​of the OI value and the Ra, OI value / Ra ≤ 80; The OI value is the ratio of the intensities of the (004) characteristic peak and the (110) characteristic peak in the X-ray diffraction pattern of the negative electrode material layer, and the Ra is the surface roughness of the negative electrode material layer, in μm.

2. The negative electrode according to claim 1, characterized in that OI value ≤ 45; and / or 0.75 μm ≤ Ra ≤ 1.5 μm; and / or 3 ≤ OI value / Ra ≤ 50.

3. The negative electrode according to claim 1, characterized in that OI value ≤ 43; and / or 0.75 μm ≤ Ra ≤ 1.3 μm; and / or 9 ≤ OI value / Ra ≤ 50.

4. The negative electrode according to any one of claims 1 to 3, characterized in that The D50 of the graphite is 5 μm-25 μm.

5. The negative electrode according to any one of claims 1 to 4, characterized in that The graphitization degree of the graphite is ≥90%.

6. The negative electrode according to any one of claims 1 to 5, characterized in that The double-sided density of the negative electrode material layer is 100 g / m 2 -500g / m 2 .

7. The negative electrode according to any one of claims 1 to 6, characterized in that The compaction density of the negative electrode material layer is 1.1 g / cm 3 -1.8g / cm 3 .

8. The negative electrode according to any one of claims 1 to 7, characterized in that The mass content of the graphite in the negative electrode material layer is ≥80%.

9. The negative electrode according to any one of claims 1 to 8, characterized in that The negative electrode further includes a current collector, and the negative electrode material layer is arranged on the surface of the current collector; the surface porosity of the negative electrode material layer on the side away from the current collector is 20%-50%.

10. A method for preparing a negative electrode according to any one of claims 1 to 9, characterized in that: include: A raw material including graphite is rolled to form a negative electrode material layer to obtain a negative electrode; the negative electrode material layer satisfies: OI value <70, Ra ≥ 0.65 μm, and calculated based on the numerical values ​​of the OI value and the Ra, OI value / Ra ≤ 80; wherein the OI value is the ratio of the intensity of the (004) characteristic peak and the (110) characteristic peak in the X-ray diffraction spectrum of the negative electrode material layer, and the Ra is the surface roughness of the negative electrode material layer, in μm.

11. The preparation method according to claim 10, characterized in that: The preparation of the negative electrode material layer includes: Performing a first calendering on the raw material; The first rolling process includes: passing the raw material through a first roller and a second roller arranged opposite to each other, wherein the pressure of the first roller and the second roller are independently 1t-20t, and the surface temperature is independently 100°C-250°C; and the ratio of the linear speeds of the first roller and the second roller is 1:(1-3); In a direction from the first roller to the second roller, a size of a gap between the first roller and the second roller is 70 μm to 150 μm.

12. The preparation method according to claim 11, characterized in that The method further comprises performing a second calendering on the material obtained by the first calendering; The second calendering comprises: passing the material obtained by the first calendering through a third roller and a fourth roller arranged opposite to each other, wherein the pressure of the third roller and the fourth roller are independently 1t-20t, and the surface temperature is independently 100°C-250°C; and the ratio of the linear speeds of the third roller and the fourth roller is 1:(1-3); In a direction from the third roller to the fourth roller, a size of a gap between the third roller and the fourth roller is 30 μm to 150 μm.

13. The preparation method according to claim 11, characterized in that The method further includes placing the material obtained after the first calendering on the surface of a current collector and performing a hot pressing composite treatment to obtain the negative electrode; wherein the pressure of the hot pressing composite is 1t-20t and the temperature is 100℃-250℃.

14. The preparation method according to claim 12, characterized in that The method further includes placing the material obtained after the second calendering on the surface of the current collector and performing a hot pressing composite treatment to obtain the negative electrode; wherein the pressure of the hot pressing composite is 1t-20t, and the temperature of the hot pressing is 100℃-250℃.

15. A secondary battery, characterized in that: The secondary battery comprises the negative electrode according to any one of claims 1 to 9, or the negative electrode prepared according to the preparation method according to any one of claims 10 to 14.

16. An electrical device, characterized in that: The electric device includes the secondary battery according to claim 15 .