Negative electrode and preparation method therefor, secondary battery and electric device

Abstract

Provided in the present application are a negative electrode and a preparation method therefor, a secondary battery and an electric device. The negative electrode comprises a negative electrode material layer, and the negative electrode material layer comprises graphite. The negative electrode material layer satisfies: OI<70, and Ra≥0.65μm, and upon calculation according to the OI value and the Ra numerical value, the OI value / Ra≤80, 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, and Ra is the surface roughness of the negative electrode material layer, with the unit thereof being μm.

Description

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

[0001] The present application claims priority to the Chinese patent application No. 202410458776.1, filed on April 15, 2024, entitled "Negative electrode and preparation method thereof, secondary battery and electric device", the whole content of which is incorporated herein by reference. TECHNICAL FIELD

[0002] The present application relates to the technical field of battery, in particular to a negative electrode and preparation method thereof, a secondary battery and an electric device. BACKGROUND

[0003] With the popularity of consumer electronics and new energy vehicles, consumers have increasingly high requirements for the charging rate of the above-mentioned electric devices. The charging rate of the electric device is largely dependent on the performance of the negative electrode of the secondary battery carried by the electric device. At present, graphite is often selected as the active material of the negative electrode of the secondary battery. However, the layered structure of graphite makes the active ions need to be deintercalated / intercalated from the end face, so the industry tries to reduce the orientation degree of the graphite negative electrode to improve the charging kinetics of the secondary battery, so as to realize the better fast charging performance of the secondary battery. However, in some cases, simply reducing the orientation degree of the graphite negative electrode has limited improvement on the rate performance of the negative electrode, therefore, it is urgent to provide a brand-new negative electrode solution. SUMMARY

[0004] In view of this, the embodiments of the present application provide a negative electrode and a preparation method thereof, a secondary battery and an electric device. The special parameter design of the negative electrode makes it possible to provide a secondary battery with better rate performance and better fast charging performance.

[0005] The first aspect of the present application provides a negative electrode, comprising a negative electrode material layer, the negative electrode material layer comprising graphite; the negative electrode material layer satisfies: an OI value < 70, a Ra≥0.65 μm, and the OI value / Ra≤80 calculated by the numerical values of the OI value and the Ra; 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, unit: μm.

[0006] The above OI value reflects the orientation degree of the negative electrode material layer. The negative electrode provided in the embodiments has a specific quantitative relationship between the orientation degree and the surface roughness of the negative electrode material layer, which can improve the ion deintercalation / exintercalation speed of the graphite, improve the electrolyte wetting performance and liquid retention capacity of the negative electrode, and thus optimize the reaction kinetics of the negative electrode. Based on the above quantitative relationship, the orientation degree of the negative electrode material layer is controlled in the range of less than 70, which can expose more end faces of the graphite, facilitating the deintercalation / exintercalation of active ions. The Ra is controlled in the range of greater than or equal to 0.65 μm, which can make the surface of the negative electrode material layer have a relatively rich pore structure, facilitating the wetting and retention of the electrolyte. Therefore, the above negative electrode can be used to provide a secondary battery with relatively optimal rate performance and good fast charging performance.

[0007] The second aspect of the present application provides a preparation method of a negative electrode, which can be used to prepare the negative electrode provided in the first aspect of the embodiments, and the preparation method comprises:

[0008] The raw material including graphite is calendered to form a negative electrode material layer, and a negative electrode is obtained; the negative electrode material layer satisfies: OI value < 70, Ra≥0.65 μm, and OI value / Ra≤80 calculated by the numerical values of the OI value and the Ra; wherein the OI value is the intensity ratio 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, with the unit of μm.

[0009] The above preparation method is simple and easy to implement, has strong controllability, and is high in production efficiency, and is suitable for large-scale industrial production.

[0010] The third aspect of the present application provides a secondary battery comprising the negative electrode provided in the first aspect of the present application. Since the negative electrode provided in the embodiments is used, the secondary battery has relatively optimal rate performance and good fast charging performance.

[0011] The fourth aspect of the present application provides an electric equipment comprising the secondary battery provided in the embodiments. Since the secondary battery provided in the embodiments is used, the electric equipment has good market prospects. DETAILED DESCRIPTION

[0012] Graphite is widely used as a negative active material due to its low cost and high energy density, but due to its layered structure, graphite does not have the ability to quickly deintercalate / exintercalate active ions, which results in that the battery basically cannot achieve fast charging performance, and limits the application scenarios of the battery. In order to solve the above technical problem, the industry controls 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 beneficial to building more pore structures on the surface layer of the negative electrode material layer, thereby facilitating the wetting of the electrolyte.

[0013] But the applicant found that if simply improving the surface roughness of the negative electrode material layer, or, reducing the orientation degree of the graphite negative electrode alone, may cause the negative electrode material layer to have performance short board, resulting in limited improvement of the rate performance and fast charging performance of the final battery. Based on the above findings, when the applicant further controls the orientation degree of the negative electrode and the surface roughness of the negative electrode within a suitable range, and the two satisfy a certain mathematical relationship, a better synergistic effect can be achieved, 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 application provides a negative electrode, which comprises a negative electrode material layer, the negative electrode material layer comprises graphite; the negative electrode material layer satisfies: OI value < 70, Ra≥0.65μm, and the ratio of OI value and Ra satisfies: 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 Ra is the surface roughness of the negative electrode material layer, unit: μm.

[0014] The above OI value reflects the orientation degree of the negative electrode material layer. The ratio of the OI value and the surface roughness Ra of the negative electrode material layer in units of 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 and 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 improve the de- / intercalation speed of active ions (such as 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 the range of less than 70, which is easy to obtain and can make the graphite expose its end face more, which is beneficial to the de- / 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 rich pore structure, which is more conducive to the infiltration and retention of electrolyte. Therefore, the above negative electrode can be used to provide a secondary battery with better rate performance and fast charging performance.

[0015] Exemplarily, 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.

[0016] In the embodiments 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 X-rays are incident from the surface of the negative electrode material layer, and an XRD spectrum of the negative electrode material layer is obtained, 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 the negative electrode with graphite as the main active material, the orientation degree of graphite in the negative electrode material layer is reflected, and 3 different positions of each negative electrode are sampled and tested, and the average value is taken. Specifically, taking the copper target as an example for XRD testing, the 2θ of the (004) characteristic peak of graphite is in the range of 53°-56°, and the 2θ of the (110) characteristic peak of graphite is in the range of 76°-79°; if other X-ray sources are used, the 2θ range of the (004) and (110) characteristic peaks of graphite under the corresponding X-ray source can be obtained by substituting the wavelength of the X-ray source into the Bragg diffraction formula.

[0017] Exemplarily, 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, the ion transmission is not conducive, and the functional performance of the negative electrode is too poor.

[0018] In the embodiments of the present application, the surface roughness Ra of the negative electrode material layer specifically refers to the profile arithmetic mean deviation, which is the arithmetic mean of the absolute values of the profile deviation in the sampling length; the surface roughness Ra of the negative electrode material layer is measured by the stylus method. Specifically, the sample is fixed on a metal plate, and the surface of the negative electrode material layer to be tested is arranged away from the metal plate, and the surface roughness of the negative electrode material layer is tested according to GB / T 1031-2009. It can be understood that in actual application, the negative electrode generally includes a negative electrode current collector and a negative electrode material layer arranged on the surface of the negative electrode current collector, at this time, the electrolyte mainly enters and diffuses into the negative electrode material layer from the pore structure of the surface of the negative electrode material layer away from the current collector to achieve the effect of wetting 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 is specifically the surface roughness of the surface of the negative electrode material layer away from the current collector.

[0019] Suitable surface roughness Ra not only facilitates the infiltration of electrolyte, but also can improve the liquid retention capacity of the electrolyte by the negative electrode, thereby improving the reaction kinetics and cycle performance of the negative electrode. If Ra is too small, the negative electrode material layer surface has too few pore structures, which is not conducive to the infiltration of 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, 0.65 μm, 0.8 μm, 1.0 μm, 1.2 μm, 1.5 μm, 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 side is 20%-50%. In the embodiments of the present application, the surface porosity of the negative electrode material layer can be tested by using a focused ion beam scanning electron microscope (Focused Ion Beam-Scanning Electron Microscope, FIB-SEM). Specifically, first, the surface of the negative electrode material layer is polished by using argon ions, and then the negative electrode material layer is continuously cut and imaged by FIB; along the thickness direction, the negative electrode material layer is cut every 10 nm, and after cutting, the cross section of the exposed negative electrode material layer is photographed by SEM, which is repeated more than 1000 times to obtain the SEM photos of the cross sections of the negative electrode material layer at different thicknesses, and the porosity of the negative electrode material layer within 10 μm thickness from the surface of the negative electrode material layer away from the negative electrode current collector to the negative electrode current collector is obtained by using software for three-dimensional reconstruction, 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%, 50%.

[0020] Considering different preparation processes, and in order to improve the adhesion between the negative electrode material layer and the current collector, in some embodiments, there is also a glue layer 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 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. The material of the above-mentioned glue layer is not specifically limited, and the material of the above-mentioned glue layer can be any material known in the art, which is conductive, for example, the glue layer includes conductive carbon and a binder.

[0021] In the embodiments of the present application, the negative electrode current collector can be any current collector known in the art. Exemplarily, the negative electrode current collector can be a copper foil, a carbon-coated copper foil, a stainless steel foil, etc.

[0022] 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. In this way, the negative electrode is easy to prepare, and the rate performance and fast charging capability can be better improved. 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.

[0023] In some embodiments of the present application, the D50 of the graphite in the negative electrode is 5 μm-25 μm. Controlling the D50 of the graphite in the above range is conducive to achieving a larger compaction density, and at the same time, the length of the de-intercalation / intercalation path of the active ions can be controlled in a suitable range, which is conducive to the transmission of ions and electrons, so that the fast charging performance of the final negative electrode can be further improved. For example, the D50 of the graphite can be, but is not limited to, 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 18 μm, 20 μm, 22 μm, 25 μm, etc.

[0024] In some embodiments of the present application, the graphitization degree of the graphite in the negative electrode is ≥90%; for example, 90%-98%. In this way, the graphite has fewer defects (defect structures caused by having heteroatoms or lacking carbon atoms in the carbon six-membered ring structure), high structural stability, better electrical conductivity, and is also more conducive to the conduction of active ions, and can further improve its specific capacity. In some cases, the graphitization degree of the 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 modified graphite includes but is not limited to graphite modified by oxidation and graphite modified by halogenation; wherein the graphite modified by halogenation can include graphite modified by fluorine element, chlorine element, bromine element and iodine element, and is preferably graphite modified by fluorination.

[0025] In the embodiments of the present application, the graphitization degree of the graphite can be measured by XRD. Specifically, after the XRD spectrum of the graphite is measured, the diffraction angle 2θ of its (002) crystal face is obtained, and the interplanar spacing d of the (002) crystal face of the graphite is calculated according to the Bragg formula (2dsinθ=nλ) d 002 , unit: nm; wherein λ is the wavelength of the X-Ray used in the XRD test, and n is the diffraction order, which is n=1 here. 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 ideal graphite crystal.

[0026] It can be understood that the negative electrode material layer includes graphite, and the negative electrode active material can be only graphite, or the negative electrode material layer further includes 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, and the mass fraction of graphite in the negative electrode material layer is 80%-100%. In this way, there is enough negative electrode active material in the negative electrode material layer to ensure the performance of the negative electrode, and there is enough graphite to improve the energy density of the final battery. For example, the mass fraction of graphite in the negative electrode material layer can be, but is not limited to, 80%, 82%, 85%, 88%, 90%, 92%, 95%, 98%, or 100%.

[0027] In some other embodiments of the present application, the negative electrode material layer further 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, mesocarbon microbeads, hard carbon, and soft carbon. At this time, the mass fraction of graphite in the negative electrode material layer is 50%-90%. In this way, it is helpful to balance the ion / electron diffusion rate of the negative electrode and the electrolyte retention capacity while taking into account the energy density of the final battery, thereby ensuring that the fast charging performance, rate performance, and cycle performance of the final battery are all good. Specifically, when other negative electrode active materials are included, the mass fraction of graphite in the negative electrode material can be, but is not limited to, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, or 90%.

[0028] Generally, in order to improve the structural stability and conductivity of the negative electrode material layer, the negative electrode material layer can further 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 fraction of the binder in the negative electrode material layer is 1%-5%. A suitable binder content is helpful to improve the structural stability of the negative electrode, and does not occupy the content of other components such as negative electrode active materials, so that the comprehensive performance of the negative electrode is good. For example, the mass fraction 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%, or 5.0%.

[0029] In the embodiments of the present application, the binder can be selected from any known binder suitable for the negative electrode; illustratively, the binder includes, but is not limited to, at least one of tetrafluoroethylene and copolymers thereof, polyvinylidene fluoride and copolymers thereof, polyolefins and copolymers thereof (e.g., polyethylene-polyethylene glycol block copolymer, etc.), polyethers and copolymers thereof (e.g., polyethylene oxide, etc.), polyphenylene ether and copolymers thereof, polysiloxanes and copolymers thereof (e.g., polydimethylsiloxane, poly(dimethylsiloxane-co-alkylmethylsiloxane), etc.), polyesters and copolymers thereof (e.g., polyvinyl ester, polyvinyl acetate, polyacrylate, etc.), carboxymethyl cellulose, butyl rubber, and nitrile rubber. Specifically, the polyolefins include one or more of polyethylene, polypropylene, ethylene / propylene copolymer, ethylene / polyvinylidene fluoride copolymer, propylene / polyvinylidene fluoride copolymer; the polytetrafluoroethylene and copolymers thereof can be at least one of tetrafluoroethylene / ethylene copolymer, tetrafluoroethylene / propylene copolymer, tetrafluoroethylene / polyvinylidene fluoride copolymer, tetrafluoroethylene / ether copolymer, tetrafluoroethylene / branched polyether copolymer, tetrafluoroethylene / vinyl ether copolymer, tetrafluoroethylene / branched polyether / vinyl ether copolymer, tetrafluoroethylene / siloxane copolymer.

[0030] In some embodiments of the present application, the negative electrode material layer further includes a conductive agent, and the mass ratio of the conductive agent in the negative electrode material layer is 0.3% to 2%. Controlling the content of the conductive agent within the above range ensures that there is sufficient conductive agent to participate in the construction of the conductive network in the negative electrode material layer, which guarantees good electronic conductivity of the negative electrode and helps to exert the power performance of the final battery; at the same time, it also does not occupy the proportion of other components such as the negative electrode active material, which helps to exert the performance of the battery. Illustratively, the mass ratio 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%, or 2.0%. In the embodiments of the present application, the conductive agent can be selected from any conductive agent known in the art; illustratively, the conductive agent includes, but is not limited to, at least one of acetylene black, ketjen black, Super-P, carbon nanotube, carbon nanofiber, activated carbon, and graphene.

[0031] In some embodiments of the present application, the double surface area density of the negative electrode is 100 g / m 2 -500 g / m 2 . Understandably, the double surface area density refers to the sum of the surface area densities of the two surfaces of the negative electrode, for the negative electrode in which the negative electrode material layer is provided on both surfaces of the negative electrode current collector. Controlling the double surface area density of the negative electrode within the above range helps to achieve a higher energy density and helps to exert the performance of the negative electrode. Illustratively, the double surface area density of the negative electrode can be, but is not limited to, 100 g / m 2 , 150 g / m 2 , 180 g / m 2 , 200 g / m2 220 g / m 2 250 g / m 2 280 g / m 2 300 g / m 2 320 g / m 2 350 g / m 2 380 g / m 2 400 g / m 2 420 g / m 2 450 g / m 2 500 g / m 2 and the like.

[0032] In some embodiments of the present application, the compaction density of the negative electrode is 1.1 g / cm 3 1.8 g / cm 3 Controlling the compaction density of the negative electrode within the above range is conducive to maintaining the structural stability of the negative electrode material layer and good wettability of the electrode sheet, thereby being conducive to the rate performance and cycle performance of the battery; it is also conducive to achieving a higher battery energy density and a lower internal resistance. Exemplarily, the compaction density of the negative electrode can be, but is not limited to, 1.10 g / cm 3 1.20 g / cm 3 1.30 g / cm 3 1.40 g / cm 3 1.42 g / cm 3 1.45 g / cm 3 1.48 g / cm 3 1.50 g / cm 3 1.52 g / cm 3 1.55 g / cm 3 1.5 g / cm 3 1.60 g / cm 3 1.70 g / cm 3 1.80 g / cm 3 and the like.

[0033] In the embodiments of the present application, the thickness of the negative electrode current collector and the negative electrode material layer is not specifically limited, and those skilled in the art can select according to actual needs. When there is a glue layer 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 the structural stability of the negative electrode current collector and the negative electrode material layer is good, and the performance of the negative electrode is not affected.

[0034] The embodiments of the present application also provide a preparation method of a negative electrode, comprising:

[0035] The raw material including graphite is calendered to form a negative electrode material layer, and a negative electrode is obtained; the negative electrode material layer satisfies: an OI value < 70, Ra≥0.65 μm, and the OI value / Ra≤80 in the numerical value of the OI value and Ra; 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 units of μm.

[0036] The preparation method is simple and easy to implement, has high production efficiency, and is suitable for large-scale industrial production.

[0037] In some embodiments of the present application, the raw material further includes a binder and a conductive agent.

[0038] In some specific embodiments, the raw material further includes other negative electrode active materials in addition to graphite. For details, please refer to the foregoing description.

[0039] In some embodiments of the present application, the corresponding proportion of the negative electrode active material (including graphite), the binder and the conductive agent are weighed as needed, mixed in a mixer (for example, a V-type mixer), and the raw material is obtained. In some specific embodiments, the mixed material is further put into a pulverizer (for example, an air flow pulverizer) for grinding and mixing to obtain the raw material. In some specific embodiments, the crushing gas pressure of the air flow pulverizer is 0.1-0.8 MPa.

[0040] In the industry, the general preparation process of the negative electrode generally includes a dry process and a wet process. The dry preparation process does not need to use a solvent, and does not need to be dried, is more environmentally friendly, and is also conducive to reducing costs.

[0041] In some embodiments of the present application, the dry method is used to prepare the negative electrode, and at this time, the preparation method of the negative electrode includes the following step S01: the raw material is first calendered to obtain a negative electrode material layer (i.e., a self-supporting film); wherein the first calendering includes: passing the raw material through the first roller and the second roller arranged oppositely. In some specific embodiments, the first roller and the second roller are arranged oppositely in the horizontal direction, and the raw material passes between the first roller and the second roller to form a self-supporting film from the powder-like raw material.

[0042] In some embodiments, the pressure of the first roller and the second roller in step S01 is independently 1 t-20 t. In some embodiments, the pressure of the first roller and the second roller is independently 1 t-20 t. In this way, the negative electrode material layer can be formed with a suitable surface density and a suitable compaction density, the orientation of the graphite can be guided, a suitable ion diffusion channel can be built 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 embodiments, the pressure of the first roller and the second roller is the same, so that the wear of the equipment can be reduced and the quality of the negative electrode can be ensured to be better. For example, the pressure of the first roller and the second roller in step S01 can be independently 0.1 t, 0.5 t, 1 t, 2 t, 5 t, 8 t, 10 t, 12 t, 15 t, 18 t, 20 t, etc. It can be understood that there must be a certain gap between the first roller and the second roller so that the raw material can pass through and form a film. In some embodiments of the present application, the size of the gap between the first roller and the second roller in the direction from the first roller to the second roller in step S01 is 70 μm-150 μm. The size of the above-mentioned gap refers to the minimum vertical distance between the surface of the first roller and the surface of the second roller, that is, the commonly known roll gap size is 70 μm-150 μm. Adjusting any one of the roll gap size and the pressure of the first roller and the second roller, or adjusting the above-mentioned parameters at the same time, can adjust the pressure received by the raw material, so as to adjust the OI value and the surface roughness Ra of the negative electrode material layer. Similarly, controlling the roll gap size within the above-mentioned range is beneficial to the formation of the negative electrode material layer and to the adjustment of the OI value / Ra to be better. Specifically, the roll gap size between the first roller and the second roller can be, but is not limited to, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm.

[0043] In some embodiments, in order to obtain negative electrode material layers with different OI values / Ra, or in order to adjust the compaction density, surface density, etc. of the negative electrode material layer, the pressure of the first roller and the second roller on the production line can be kept unchanged, and the roll gap size of the first roller and the second roller can be adjusted when producing different negative electrode material layers.

[0044] It is well known that in the self-supporting film, generally the binder is fiberized and forms a binder network to ensure the structural stability of the self-supporting film. In some embodiments, the raw material further includes a binder, and the surface temperature of the first roller and the second roller in step S01 is independently 100-250°C. In this way, the requirement for rolling force can be reduced, the calendering effect can be improved, and the fiberization of the binder can be further promoted, the formation of the binder network is promoted, thereby ensuring the structural stability of the self-supporting film; at the same time, the risk of affecting the properties of the binder or other components due to overheating and increasing the deformation amount of the roller can be sufficiently reduced, thereby being not conducive to the formation of the self-supporting film. In some embodiments, the surface temperature of the first roller and the second roller is the same, so that the quality of the negative electrode can be improved while reducing the wear of the equipment. For example, the surface temperature of the first roller and the second roller can be independently 100°C, 120°C, 150°C, 180°C, 200°C, 220°C, 250°C, etc.

[0045] In some embodiments, in step S01, the linear speed ratio of the first roller and the second roller is 1:(1-3). In this way, the material between the two rollers is subjected to a shear force, which not only makes the binder further fiberized and prepared into a self-supporting film, but also reduces the dependence on pressure and roll gap, increases the calendering amount while improving the calendering effect. More importantly, controlling the linear speed ratio of the two rollers within the above range is also conducive to ensuring that the finally prepared negative electrode material layer has good mechanical properties, and is conducive to reducing the areal density of the negative electrode material layer, and facilitating the performance of the final negative electrode electrochemical performance. Specifically, in step S01, the linear speed ratio of the first roller and the second roller can be but 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.

[0046] In some embodiments of the present application, in order to reduce the thickness of the negative electrode material layer, improve the compaction density, and reduce the areal density of the negative electrode, a step S02 of second calendering the material obtained by the first calendering in step S01 is further included. In other words, the second calendering can be a thinning treatment on the self-supporting film obtained by the first calendering.

[0047] In some embodiments, step S02 includes passing the material obtained by the first calendering through the third roller and the fourth roller arranged oppositely to obtain the negative electrode material layer.

[0048] In some embodiments, the pressure of the third roller and the fourth roller in step S02 is independently 1 t-20 t. For example, the linear pressure of the third roller and the fourth roller in step S02 can be independently 0.1 t, 0.5 t, 1 t, 2 t, 5 t, 8 t, 10 t, 12 t, 15 t, 18 t, 20 t, etc. In this way, not only the areal density of the negative electrode material layer is reduced, but also the orientation degree of graphite in the negative electrode material layer is reduced, and the Ra of the negative electrode material layer is improved. In some embodiments, the linear pressure of the third roller and the fourth roller is equal.

[0049] In some embodiments of the present application, the gap between the third roller and the fourth roller in step S02 is 50 μm-120 μm from the direction of the third roller to the fourth roller. The size of the above-mentioned gap refers to the minimum vertical distance between the surface of the third roller and the surface of the fourth roller, that is, the commonly known roll gap size is 50 μm-120 μm. Similarly, in some 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 be constant, and the roll gap size of the third roller and the fourth roller is adjusted. Specifically, the roll gap size between the first roller and the second roller can be but is not limited to 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm.

[0050] In some embodiments, the surface temperature of the third roller and the fourth roller in step S02 is independently 100°C-250°C. For example, the surface temperature of the third roller and the fourth roller in step S02 can be independently 100°C, 120°C, 150°C, 180°C, 200°C, 220°C, 250°C, etc. In this way, the orientation degree of graphite in the negative electrode material layer is low, and the surface roughness Ra is high. Similarly, in some embodiments, the surface temperature of the third roller and the fourth roller is the same.

[0051] In some embodiments, the linear speed ratio of the third roller and the fourth roller in step S02 is 1:(1-3). For example, the rotation rate ratio 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, in combination with the pressure and surface temperature of the third roller and the fourth roller, the linear speed ratio of the third roller and the fourth roller is controlled to be in the above range, which is more conducive to the comprehensive electrochemical performance of the finally prepared negative electrode.

[0052] In the embodiments of the present application, the parameters of the first calendering and the parameters of the second calendering can be the same or different.

[0053] In some other embodiments, to obtain the negative electrode material layer with the target compaction density and the areal density, the second calendering is followed by S03: third calendering. The third calendering comprises: passing the material obtained after the second calendering through a fifth roller and a sixth roller oppositely arranged to obtain the negative electrode material layer; wherein the linear pressure of the fifth roller and the sixth roller is independently 1 t-20 t; the surface temperature of the fifth roller and the sixth roller is 100°C-250°C; and the ratio of the linear speed of the fifth roller and the sixth roller is 1:(1-3). Similarly, the pressure of the fifth roller and the sixth roller is equal, and the surface temperature of the fifth roller and the sixth roller is equal.

[0054] 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 30 μm-100 μm. The size of the above-mentioned gap refers to the minimum vertical distance between the surface of the fifth roller and the surface of the sixth roller, that is, the commonly known roll gap size is 30 μm-100 μm. Similarly, in some specific embodiments of the present application, when producing different negative electrode material layers, the linear pressure of the fifth roller and the sixth roller in the direction from the fifth roller to the sixth roller is kept unchanged, and the roll gap size of the fifth roller and the sixth roller in the direction from the fifth roller to the sixth roller is adjusted. Specifically, the roll gap size between the fifth roller and the sixth roller can be but is not limited to 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, or 100 μm.

[0055] In some embodiments of the present application, the negative electrode material layer (self-supporting film) prepared in the above-mentioned step S01, or the negative electrode material layer prepared in the above-mentioned step S02, or the negative electrode material layer prepared in the above-mentioned step S03 is placed on the surface of the current collector, and then heat pressing is performed to obtain a negative electrode. In this way, the negative electrode material layer can be fixed on the current collector, and the negative electrode material layer can be further pressed to further control the compaction density of the negative electrode material layer.

[0056] It can be understood that the surface of the current collector is further provided with a glue layer. In some specific embodiments, when the opposite surfaces of the current collector are provided with the negative electrode material layer, two pieces of the negative electrode material layer can be respectively attached to the opposite surfaces of the current collector provided with the glue layer, and heat pressing is performed together.

[0057] In some embodiments of the present application, the pressure of the hot-pressing is 1t-20t, and the temperature of the hot-pressing is 100℃-250℃. Exemplarily, the linear pressure of the hot-pressing can be, but is not limited to, 0.1t, 0.5t, 1t, 2t, 5t, 8t, 10t, 12t, 15t, 18t, or 20t. Exemplarily, the temperature of the hot-pressing can be, but is not limited to, 100℃, 120℃, 150℃, 180℃, 200℃, 220℃, or 250℃. In this way, the orientation degree of the graphite and the surface roughness Ra of the negative electrode material layer can be further adjusted within a more optimal range. It should be noted that the hot-pressing is also performed by using two oppositely arranged rollers to hot-press the material, and the pressure and the surface temperature of the two rollers are equal.

[0058] The embodiments of the present application also provide a secondary battery including the negative electrode provided by the embodiments of the present application. Since the negative electrode provided by the embodiments of the present application is used, the secondary battery has a more optimal power performance, a good fast-charging performance, and a good long-cycle performance.

[0059] In some embodiments of the present application, the secondary battery is a liquid battery using a liquid electrolyte. In some embodiments, the secondary battery includes a positive electrode, the aforementioned negative electrode, and an electrolyte and a separator arranged between the positive electrode and the negative electrode.

[0060] In the embodiments of the present application, the positive electrode can be any positive electrode known in the art. The separator and the electrolyte can be any electrolyte and separator known in the art.

[0061] In other embodiments, the secondary battery is a semi-solid battery. In other embodiments, the secondary battery is a solid-state battery using a solid-state electrolyte.

[0062] In the embodiments of the present application, the secondary battery can be a lithium ion battery, a sodium ion battery, or other alkali metal ion battery.

[0063] The embodiments of the present application also provide an electrical equipment including the secondary battery provided by the embodiments of the present application. Since the electrical equipment is provided with the secondary battery provided by the embodiments of the present application, the electrical equipment has a good market prospect.

[0064] In some embodiments of the present application, the electrical equipment includes, but is not limited to, a vehicle, a consumer electronic product, etc. The vehicle includes, but is not limited to, a new energy vehicle, a power-assisted bicycle, etc.

[0065] The technical solutions of the present application are further described in the following embodiments.

[0066] Embodiment 1

[0067] The raw material was first calendered, wherein the raw material was a binder (specifically polyvinylidene fluoride, PTFE), a conductive agent (specifically carbon black) and graphite (D50 of the graphite was 13.7 μm, and the graphitization degree was 94%) in a mass ratio of 3:1:96, the pressure of the first roller and the second roller was both 10 t, the surface temperature of the first roller and the second roller was both 150 °C, the roller gap of the first roller and the second roller was 100 μm, and the ratio of the linear speed of the first roller to the second roller was 1:1.3, to obtain a first material; the first material was second calendered: the pressure of the third roller and the fourth roller was both 10 t, the temperature of the third roller and the fourth roller was both 150 °C, the ratio of the linear speed of the third roller to the fourth roller was 1:1, and the roller gap was 65 μm, to obtain a second material;

[0068] The second material was placed on both sides of the current collector (specifically copper foil) for hot-pressing composite treatment, wherein the pressure of the hot-pressing composite treatment was 10 t, the roller gap width relative to the two rollers was 90 μm, and the surface temperature of the two rollers was both 150 °C, to obtain a negative electrode. The OI value of the negative electrode was 49, the Ra was 0.72 μm, the OI value / Ra was 68.0, and the double-sided area density of the negative electrode material layer was 200 g / m 2 , and the compacted density was 1.5 g / cm 3 .

[0069] Example 2

[0070] The difference from Example 1 was that in the first calendering, the roller gap of the first roller and the second roller was 150 μm, and the ratio of the linear speed of the first roller to the second roller was 1:2.5; in the second calendering, the ratio of the linear speed of the third roller to the fourth roller was 1:1.2, and the roller gap was 80 μm; and in the hot-pressing composite process, the roller gap width relative to the two rollers was 70 μm. The OI value of the negative electrode finally prepared in this example was 28, the Ra was 0.75 μm, and the OI value / Ra was 37.3.

[0071] Example 3

[0072] The difference from Example 1 was that in the first calendering, the roller gap of the first roller and the second roller was 120 μm, and the ratio of the linear speed of the first roller to the second roller was 1:1.6; in the second calendering, the ratio of the linear speed of the third roller to the fourth roller was 1:1.2, and the roller gap was 70 μm; and in the hot-pressing composite process, the roller gap width relative to the two rollers was 80 μm. The OI value of the negative electrode finally prepared in this example was 38, the Ra was 0.78 μm, and the OI value / Ra was 48.7.

[0073] Example 4

[0074] The difference from Example 1 is that in the first calendering, the roll gap of the first and second rollers is 130 μm, and the linear speed ratio of the first and second rollers is 1:1.5; in the second calendering, the linear speed ratio of the third and fourth rollers is 1:1.5, and the roll gap is 80 μm; in the hot-pressing process, the roll gap width of the two rollers is 75 μm. The OI value of the negative electrode finally prepared in this example is 54, the Ra is 0.85 μm, and the OI value / Ra is 63.5.

[0075] Example 5

[0076] The difference from Example 1 is that in the first calendering, the roll gap of the first and second rollers is 80 μm, and the linear speed ratio of the first and second rollers is 1:1.2; in the second calendering, the linear speed ratio of the third and fourth rollers is 1:1.6, and the roll gap is 90 μm; in the hot-pressing process, the roll gap width of the two rollers is 70 μm. The OI value of the negative electrode finally prepared in this example is 24, the Ra is 0.98 μm, and the OI value / Ra is 24.4.

[0077] Example 6

[0078] The difference from Example 1 is that in the first calendering, the roll gap of the first and second rollers is 120 μm, and the linear speed ratio of the first and second rollers is 1:1.8; in the second calendering, the linear speed ratio of the third and fourth rollers is 1:1.4, and the roll gap is 90 μm; in the hot-pressing process, the roll gap width of the two rollers is 80 μm. The OI value of the negative electrode finally prepared in this example is 16, the Ra is 1.05 μm, and the OI value / Ra is 15.2.

[0079] Example 7

[0080] The difference from Example 1 is that in the first calendering, the roll gap of the first and second rollers is 80 μm, and the linear speed ratio of the first and second rollers is 1:1.2; in the second calendering, the linear speed ratio of the third and fourth rollers is 1:1.2, and the roll gap is 70 μm; in the hot-pressing process, the roll gap width of the two rollers is 70 μm. The OI value of the negative electrode finally prepared in this example is 41, the Ra is 0.68 μm, and the OI value / Ra is 60.2.

[0081] Example 8

[0082] The difference from Example 1 is that in the first calendering, the roll gap of the first and second rollers is 110 μm, and the linear speed ratio of the first and second rollers is 1:1.4; in the second calendering, the linear speed ratio of the third and fourth rollers is 1:1.4, and the roll gap is 85 μm; in the hot-pressing process, the roll gap width of the two rollers is 75 μm. The OI value of the negative electrode finally prepared in this example is 66, the Ra is 0.87 μm, and the OI value / Ra is 75.8.

[0083] Example 9

[0084] The difference from Example 1 is that in the first calendering, the roll gap of the first roll and the second roll is 90 μm, and the linear speed ratio of the first roll and the second roll is 1:1.3; in the second calendering, the linear speed ratio of the third roll and the fourth roll is 1:1.3, and the roll gap is 80 μm; in the hot-pressing process, the roll gap width of the two opposite rolls is 75 μm. The OI value of the negative electrode finally prepared in this example is 42, the Ra is 0.82 μm, and the OI value / Ra is 51.2.

[0085] Example 10

[0086] The difference from Example 1 is that in the first calendering, the roll gap of the first roll and the second roll is 130 μm, and the linear speed ratio of the first roll and the second roll is 1:1.6; in the second calendering, the linear speed ratio of the third roll and the fourth roll is 1:1.5, and the roll gap is 90 μm; in the hot-pressing process, the roll gap width of the two opposite rolls is 80 μm. The OI value of the negative electrode finally prepared in this example is 47, the Ra is 0.98 μm, and the OI value / Ra is 47.9.

[0087] Example 11

[0088] The difference from Example 1 is that in the first calendering, the roll gap of the first roll and the second roll is 120 μm, and the linear speed ratio of the first roll and the second roll is 1:3; in the second calendering, the linear speed ratio of the third roll and the fourth roll is 1:1.2, and the roll gap is 110 μm; in the hot-pressing process, the roll gap width of the two opposite rolls is 80 μm. The OI value of the negative electrode finally prepared in this example is 12, the Ra is 1.25 μm, and the OI value / Ra is 9.6.

[0089] To highlight the beneficial effects of the embodiments of the present application, the following comparative examples are provided.

[0090] Comparative Example 1

[0091] A negative electrode, which is different from the negative electrode provided in Example 1 only in that the OI value of the negative electrode of Comparative Example 1 is 42, the Ra is 0.63 μm, and the OI value / Ra is 66.6.

[0092] Comparative Example 2

[0093] A negative electrode, which is different from the negative electrode provided in Example 1 only in that the OI value of the negative electrode of Comparative Example 1 is 72, the Ra is 0.95 μm, and the OI value / Ra is 75.7.

[0094] Comparative Example 3

[0095] A negative electrode, which is different from the negative electrode provided in Example 1 only in that the OI value of the negative electrode of Comparative Example 1 is 62, and Ra is 0.69 μm, and the OI value / Ra is 89.9.

[0096] Electrochemical performance test:

[0097] ①The negative electrode, the separator (polyethylene / polypropylene composite film), and the lithium iron phosphate positive electrode prepared in each of the above examples and comparative examples were stacked, and an electrolyte was injected to prepare a soft package battery.

[0098] The mass ratio of the electrolyte was LiPF6:EC:DEC:VC = 12:26:60:2.

[0099] The preparation method of the lithium iron phosphate positive electrode comprises the following steps: mixing the positive electrode active material lithium iron phosphate, the binder (specifically, polyvinylidene fluoride), and the conductive agent (specifically, conductive carbon black) according to a mass ratio of 90:5:5, and dispersing them in a solvent (specifically, N-methyl pyrrolidone) to obtain a positive electrode slurry; coating the positive electrode slurry on the opposite two side surfaces of the positive electrode current collector (specifically, a carbon-coated aluminum foil), and drying and rolling the positive electrode slurry. 3

[0100] Preparation of the battery: the negative electrode, the positive electrode, and the electrolyte of each example and comparative example were assembled to obtain a battery, and the battery was subjected to formation.

[0101] ②Rate performance test: the change in the discharge capacity of each battery at different rates of 0.33 C and 3 C was tested at 25°C, and the voltage range was 2 V-3.8 V. The ratio of the discharge capacity at 3 C to the discharge capacity at 0.33 C was the capacity retention rate at the rate, which reflected the rate performance. Table 2 summarizes the ratio (3C / 0.33C) of the first cycle discharge capacity at 3 C to the first cycle discharge capacity at 0.33 C of each battery.

[0102] ​③ fast charging performance test: using Yuan energy in-situ expansion analyzer to evaluate the expansion thickness of the battery under different charging rates, which can determine the lithium precipitation voltage and SOC window of the battery under different charging rates, and is a non-destructive lithium precipitation detection method; specifically, the SOC before the thickness inflection point appears in the process of 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 precipitation starts at this rate; the fast charging process is to start charging at a high rate (such as 4.5C), when the SOC at which lithium precipitation starts at this rate is reached, jump to the next rate (such as 4C) to continue charging the SOC at which lithium precipitation starts at this rate, then jump to the next rate (such as 3.5C) to continue charging the SOC at which lithium precipitation starts at this rate until charging to 80% SOC of the battery, and the sum of the charging time at each rate is the 0-80% SOC fast charging time of the battery. The time required for the battery to charge from 10% SOC to 80% SOC is compared as a parameter for comparing fast charging performance.

[0103] Table 1

[0104] From the data in Table 1, it can be found that the battery using the negative electrode provided by the embodiments of the present application has obvious improvement in fast charging performance and capacity retention rate under high rate. Further, when the OI value, Ra and OI value / Ra of the negative electrode are all controlled within the more recommended range of the embodiments of the present application (Examples 2, 3, 5, 6, 11), it is more beneficial to optimize the rate performance and fast charging performance of the battery.

[0105] The above is an exemplary embodiment of the present application, it should be noted that for ordinary skilled in the art, without departing from the principles of the present application, can make some improvements and refinements, these improvements and refinements are also considered as the protection scope of the present application.

Claims

1. A negative electrode, characterized by comprising: The negative electrode material layer comprises graphite; The negative electrode material layer satisfies: OI value < 70, Ra≥0.65μm, and OI value / Ra≤80 calculated by the numerical values of the OI value and the Ra. 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, with the unit of μm.

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

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

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

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

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

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

8. The negative electrode according to any one of claims 1 to 7, characterized by, 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 by, The negative electrode further comprises 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 away from the side of the current collector is 20%-50%.

10. A method for producing the negative electrode according to any one of claims 1 to 9, characterized by, The negative electrode comprises: The raw material comprising graphite is calendered to form a negative electrode material layer, thereby obtaining a negative electrode; the negative electrode material layer satisfies: OI value < 70, Ra≥0.65μm, and OI value / Ra≤80 calculated by the numerical values of the OI value and the Ra; 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, with the unit of μm.

11. The method of claim 10, wherein, The preparation of the negative electrode material layer comprises: The first calendering of the raw material; The first calendering comprises: passing the raw material through the first roller and the second roller arranged oppositely, the pressure of the first roller and the second roller is independently 1t-20t, and the surface temperature is independently 100℃-250℃; the linear speed ratio of the first roller and the second roller is 1:(1-3); The size of the gap between the first roller and the second roller in the direction from the first roller to the second roller is 70μm-150μm.

12. The method of claim 11, wherein, The second calendering of the material obtained by the first calendering is further included; The second calendering comprises: passing the material obtained by the first calendering through the third roller and the fourth roller arranged oppositely, the pressure of the third roller and the fourth roller is independently 1t-20t, and the surface temperature is independently 100℃-250℃; the linear speed ratio of the third roller and the fourth roller is 1:(1-3); 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 30μm-150μm.

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

14. The method of claim 12, wherein, The second calendering is followed by placing the obtained material on the surface of a current collector, and performing a hot-pressing process to obtain the negative electrode; wherein the pressure of the hot-pressing process is 1 t-20 t, and the temperature of the hot-pressing process is 100 ℃-250 ℃.

15. A secondary battery characterized by comprising: The secondary battery comprises the negative electrode according to any one of claims 1-9, or the negative electrode prepared according to any one of claims 10-14.

16. An electrical device, characterized by The electric device comprises the secondary battery according to claim 15.

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