A negative electrode sheet, a secondary battery, and an electric device

Abstract

The application discloses a negative pole piece, a secondary battery and an electric device, and belongs to the technical field of batteries.The negative pole piece comprises a negative pole current collector and a negative material layer arranged on at least one surface of the negative pole current collector, the negative material layer comprises a silicon-carbon active material, and the silicon-carbon active material and the negative material layer satisfy a certain relationship formula.The negative pole piece disclosed by the application can reduce the risk of material falling, is applied to a secondary battery, improves the cycle life of the battery, and simultaneously improves the overall kinetic performance of the battery.

Description

Technical Field

[0001] This invention belongs to the field of battery technology and relates to a negative electrode sheet, a secondary battery, and an electrical device. Background Technology

[0002] Currently, with electric vehicle users demanding higher driving ranges, the energy density of lithium batteries, a core component of electric vehicles, needs further improvement. Among battery anode materials, silicon in silicon-carbon anode materials has a theoretical specific capacity far exceeding that of graphite. Therefore, silicon-carbon anodes can store more lithium ions per unit mass, thus significantly improving the overall energy density of the battery.

[0003] In the industry, silicon-carbon anode materials are typically formed by nano-sizing silicon-based materials and combining them with carbon materials. However, in existing silicon-carbon anodes, silicon particles are prone to volume shrinkage and expansion during charging and discharging, which can cause electrode shedding and reduce the cycle life of the battery. Summary of the Invention

[0004] In order to overcome the shortcomings of the existing technology, the purpose of this invention is to provide a negative electrode sheet, a secondary battery and an electrical device. The negative electrode sheet can reduce the risk of material loss. When applied in a secondary battery, it can improve the cycle life of the battery and enhance the overall dynamic performance of the battery.

[0005] A first aspect of the present invention provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector, the negative electrode material layer comprising a silicon-carbon active material, and the silicon-carbon active material and the negative electrode material layer satisfying the following relationship:

[0006] 12≤1000×a×c / b≤265;

[0007] a represents the sphericity of the silicon-carbon active material; b represents the flexibility value of the negative electrode sheet obtained by testing with a flexibility meter, in mN. -1 c represents the porosity of the negative electrode material layer.

[0008] Preferably, the sphericity 'a' of the silicon-carbon active material ranges from 0.56 to 0.97; and the flexibility value 'b' of the negative electrode sheet ranges from 1.24 to 6.5 mN. -1 The porosity c of the negative electrode material layer ranges from 12% to 42%.

[0009] Preferably, the silicon-carbon active material satisfies at least one of the following: particle size D v The range of 50 is 4-10 μm; the powder resistivity is 3-45 mΩ·m; and the degree of graphitization is 75%-96%.

[0010] Preferably, the silicon content in the silicon-carbon active material is 1.5%-45% by mass; and / or, the thickness of the negative electrode current collector is 4-7 μm.

[0011] Preferably, the negative electrode material layer further includes graphite material, and the ratio of the aspect ratio of the graphite material to the aspect ratio of the silicon-carbon active material is 0.6-3.

[0012] A second aspect of the present invention provides a secondary battery, including the negative electrode sheet described above.

[0013] Preferably, the secondary battery includes a positive electrode sheet, and the positive electrode sheet includes a positive electrode active material, satisfying at least one of the following: the positive electrode active material is selected from at least one of lithium iron phosphate (LFP) and ternary positive electrode active material (NCM); the particle size D of the positive electrode active material is... v The thickness of 50 ranges from 2 to 4 μm; the compaction density of the positive electrode sheet is 2.5-4 g / cm³. 3 .

[0014] Furthermore, the ternary cathode active material has the general formula Li. x Ni y Co z Mn t O2; where 0.75≤x≤1.2, y≥0.8, y+z+t=1.

[0015] Preferably, the positive electrode active material contains a dopant element, which is selected from at least one of Al, Ti, La, Y, Sr, Zr, W, Mo, V, F, Nb, and B, and the mass content of the dopant element in the positive electrode active material is 0.05% to 0.5%.

[0016] And / or, the positive electrode active material includes a coating layer with a thickness of 5-40 nm.

[0017] Preferably, the secondary battery further includes an electrolyte with a viscosity of 1.8-4.5 mPa·s at 25°C; the electrolyte preferably also includes additives selected from at least one of carbonates and sulfur-containing organic compounds.

[0018] A third aspect of the present invention provides an electrical device comprising the secondary battery described above.

[0019] The beneficial effects of this invention include:

[0020] This invention provides a negative electrode sheet, a secondary battery including the negative electrode sheet, and an electrical device. By controlling the flexibility of the negative electrode sheet, the porosity of the surface negative electrode material, and the sphericity of the silicon-carbon active material therein, and satisfying certain relational conditions, the risk of material shedding from the negative electrode sheet of this invention is reduced. The secondary battery including this negative electrode sheet has a long cycle life, and the overall kinetic performance of the battery is good. Detailed Implementation

[0021] The technical solutions in the embodiments of this application are described clearly and completely below. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0022] Research has found that silicon-carbon anode materials are core carriers for high energy density. The inherent defects of silicon can be overcome and stress concentration within the material can be alleviated by employing a highly spherical design. However, the higher the sphericity of silicon-carbon particles, the greater their surface curvature and geometric smoothness, and the lack of sharp edges and rough structures, resulting in weaker physical "interlocking" with the binder and poorer adhesion with the current collector. During charging and discharging, the expansion and contraction of silicon particles make them more prone to peeling off from the current collector, leading to insufficient adhesion between particles and between the material layer and the current collector. This causes electrode shedding, which in turn leads to loss of active material, breakage of the conductive network, and exacerbation of interfacial side reactions, ultimately reducing the battery's cycle life.

[0023] By increasing the flexibility of the electrode, such as by using flexible binders or increasing the amount of binder, the bonding strength between the silicon-carbon anode active material and the current collector can be increased, preventing electrode shedding, further improving electrode integrity, and extending battery cycle life. However, if the flexibility of the silicon-carbon electrode is too high, the binder will form an insulating layer between the anode active material particles, hindering lithium ion (Li-) ion exchange. + The migration of electrolyte ions leads to a decrease in the battery's kinetic performance. Further improvements in the porosity of the negative electrode material layer can promote rapid electrolyte wetting, increase the lithium-ion diffusion rate, and thus improve the battery's kinetic performance.

[0024] This invention provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode material layer comprises a silicon-carbon active material, and the silicon-carbon active material and the negative electrode material layer satisfy the following relationship:

[0025] 12≤1000×a×c / b≤265; as a preferred option, 35≤1000×a×c / b≤138;

[0026] a represents the sphericity of the silicon-carbon active material; b represents the flexibility value of the negative electrode sheet obtained by testing with a flexibility meter, in mN. -1 c represents the porosity of the negative electrode material layer.

[0027] Sphericity is a geometric parameter that quantitatively characterizes how close a particle's morphology is to an ideal sphere. Its value ranges from 0 to 1. The closer the value is to 1, the more regular the particle shape.

[0028] The present invention does not limit the test method for the sphericity 'a' of silicon-carbon active materials; for example, the test method for sphericity 'a' is as follows:

[0029] 1) Pretreatment: Discharge the battery to the lower limit voltage at 0.33C (e.g., in the example, it is a ternary system, lithium-ion nickel cobalt oxide (NCM), and the lower limit voltage is 2.5V). Take the empty battery, disassemble the electrode, soak the electrode in dimethyl carbonate (DMC) solution for 4 hours, and then air dry.

[0030] 2) SEM testing: The pre-treated negative electrode sheet was surface polished, and its morphology was observed using a scanning electron microscope (SEM) in backscatter mode. The atomic number contrast was used to distinguish the silicon-carbon phase from the graphite phase (graphite is dark, silicon-carbon is bright), and images were acquired from clear areas. The boundaries of individual silicon-carbon particles were extracted using image analysis software to obtain the projected area, perimeter, area equivalent diameter d1, and perimeter equivalent diameter d2 of a single particle. The sphericity a of a single silicon-carbon particle was calculated: a = d1 / d2. The average sphericity of 100 particles was taken.

[0031] The present invention does not limit the testing method for the flexibility value b of the negative electrode sheet; for example, the testing method for the flexibility value b is as follows:

[0032] Discharge the battery to 2.5V at 0.33C. Disassemble the battery and remove the negative electrode. Cut several rolls of outer electrode sheets (the second outer electrode sheet, including the current collector) into 5cm*5cm pieces and place them in a constant temperature and humidity chamber at 25±3℃ and 35±3% for more than 1 hour. Then, place the electrode sheets on the platform of a microcomputer flexibility tester (model: ITM-RRD01*1, manufacturer: Dongguan Internes Precision Instruments Co., Ltd.), set the gap width to 20mm and the probe descent depth to 8mm, and measure the force value F in mN. The flexibility value b = 1 / F*100 in mN. -1 .

[0033] The present invention does not limit the testing method for the porosity c of the negative electrode sheet; for example, the testing method for the porosity c is as follows:

[0034] 1) Pretreatment: Discharge the battery at 0.33C to the lower limit voltage of 2.5V, take the empty battery, disassemble the negative electrode, soak the electrode in dimethyl carbonate (DMC) solution for 4 hours, and then air dry;

[0035] 2) Inspection: Subsequently, the electrode was cut into round pieces with a diameter of 19mm using an electrode punching machine. Simultaneously, the thickness of the electrode and the current collector was measured using a thickness gauge, denoted as h1 and h2 respectively. The mass was weighed using a balance with an accuracy of 0.00001g and recorded as m1. According to the formula v=πr 2 Calculate the volume v of the material layer on the surface of the cut electrode (h1-h2); immerse the electrode in a sealed container with a certain volume of hexadecane for 1 hour (the volume of hexadecane in the sealed solution is not required, but the amount must be sufficient to completely submerge the electrode); after 1 hour, remove the electrode with tweezers and place it on filter paper to absorb dry until constant weight (generally, 1 hour is sufficient to absorb dry to constant weight), weigh it using a balance, and record the mass as m2. Calculate the porosity c according to the formula c=X / v, where X=(m2-m1) / ρ, and ρ is the density of hexadecane, 0.7734 g / cm³. 3 .

[0036] In some embodiments of the present invention, the sphericity α ranges from 0.56 to 0.97, preferably from 0.72 to 0.92, and further from 0.75 to 0.9.

[0037] By controlling the sphericity 'a' of the silicon-carbon active material within the aforementioned range, this invention enables a more balanced stress distribution within the material during charging and discharging, reducing localized stress concentrations. This effectively suppresses material pulverization and structural collapse, lowers the risk of electrode shedding, and improves battery cycle life. Simultaneously, it helps shorten the diffusion path of lithium ions from the electrolyte into the particle interior. Furthermore, the smoother surface of the spherical particles leads to a more uniform SEI film formation, reducing interfacial impedance and facilitating rapid lithium ion insertion, thus improving the battery's kinetic performance.

[0038] In some embodiments of the present invention, the flexibility value b ranges from 1.24 to 6.5 mN. -1 Preferred range: 1.8-5 mN -1 In this embodiment of the invention, the flexibility value b of the negative electrode sheet is controlled within the above-mentioned range. This allows the electrode sheet to absorb the mechanical stress generated by the expansion and contraction of the silicon material during charging and discharging through deformation, while also helping to maintain the integrity of the electrode structure and the stability of the interface. This prevents the active material from cracking and peeling off, thus improving the cycle life of the battery. It also avoids the problem of excessive binder addition, which can block the micropores and mesopores originally used for electrolyte wetting and lithium ion migration, ensuring smooth lithium ion transport and taking into account the battery's dynamic performance.

[0039] In some embodiments of the present invention, the porosity c ranges from 12% to 42%, preferably from 20% to 35%, and further from 20% to 28%, for example, 20%, 20.5%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, and 28%. By controlling the porosity c of the negative electrode material layer within the above range, the embodiments of the present invention can enhance the interparticle bonding force, improve the mechanical strength of the electrode, effectively disperse internal stress, thereby inhibiting the cracking and peeling of the active material, reducing the risk of electrode shedding, and improving the cycle life of the battery; it can also improve the effective wetting of the negative electrode by the electrolyte, promote the rapid diffusion of lithium ions, and improve the kinetic performance of the battery.

[0040] Therefore, by comprehensively controlling the ratio of a, b, and c to satisfy the above formula, the embodiment of the present invention reduces the risk of electrode shedding of silicon-carbon anode, improves the cycle life of the battery, improves the overall dynamic performance of the battery, shortens the overall charge and discharge time of the battery, and reduces the overall internal resistance of the battery.

[0041] In the relationship described in the embodiments of this invention, if 'a' is too large, the bonding strength between the silicon-carbon active material and the current collector is low, and the electrode is prone to falling off; if 'a' is too small, the expansion of silicon lithium intercalation easily leads to stress concentration in the silicon-carbon material, causing the carbon matrix of the silicon-carbon material to collapse and the surface coating layer to be damaged, thereby reducing the cycle life. If 'b' is too small, the electrode cannot alleviate the stress of silicon volume expansion and contraction through deformation, leading to stress accumulation, making the electrode more prone to falling off, and reducing the cycle life of the battery. If the relationship exceeds the lower limit; if 'b' is too large, the amount of binder increases, which will block the micropores and mesopores originally used for electrolyte wetting and lithium ion migration, hindering lithium ion transport and reducing kinetic performance. If 'c' is too large, the contact between material particles is poor and the mechanical strength is insufficient, making it difficult to maintain structural integrity during the repeated volume expansion and contraction of the silicon material, thereby increasing the risk of delamination between the silicon-carbon material and the current collector; if 'c' is too small, the channels for electrolyte wetting of the negative electrode are blocked, making lithium ion transport difficult.

[0042] In this invention, the formation of the negative electrode material layer is not limited. Those skilled in the art can prepare the negative electrode material layer on the surface of the negative electrode current collector using conventional techniques. For example, silicon-carbon active material, binder, and solvent are mixed to form a negative electrode slurry, which is then coated on at least one surface of the negative electrode current collector and dried to form the negative electrode material layer.

[0043] The negative electrode material layer described in this embodiment of the invention also includes graphite material, such as artificial graphite, which, as an active material, can compensate for the poor conductivity of silicon carbon. In some embodiments of the invention, the aspect ratio of the graphite material to that of silicon carbon is 0.6-3.0, which is beneficial for synergistic optimization of activity. The aspect ratio of a material generally refers to the ratio of the equivalent diameter (or thickness / diameter) of a single particle, fiber, tubular, or sheet-like material in its principal extension direction (length) to the perpendicular direction, and is used to describe the degree of anisotropy of its geometry.

[0044] As some preferred examples, the silicon-carbon active material can be blended with graphite as the negative electrode active component, and mixed with a conductive agent, binder, and dispersant, and a solvent is added to prepare a uniform negative electrode slurry. Based on the total solid mass of the negative electrode slurry, the content percentage of the negative electrode active component is 90-98 wt%, the content percentage of the conductive agent is 0.4-3 wt%, the content percentage of the binder is 1.5-5 wt%, and the content percentage of the dispersant is 0.3-1.5 wt%. Specifically, the negative electrode slurry can be coated on at least one surface of the negative electrode current collector, dried, rolled, and slit to obtain the negative electrode sheet.

[0045] In this invention, the source and preparation method of the silicon-carbon active material are not limited. Those skilled in the art can use conventional techniques to prepare active silicon / carbon composites. For example, a porous carbon matrix and a silicon source are mixed and subjected to high-temperature composite treatment to obtain a silicon / carbon composite, which is the silicon-carbon active material.

[0046] As a preferred example, a porous carbon matrix is ​​placed in a vapor deposition furnace, and silane, dichlorosilane, or trichlorosilane is used as the silicon source. Vapor-phase silicon infiltration is performed under a high-temperature inert atmosphere, allowing elemental silicon to be uniformly deposited within the pores of the porous carbon matrix, thus obtaining a silicon / porous carbon composite. Further, acetylene, pitch, glucose, etc., are used as carbon sources, and the composite is surface-coated with carbon using chemical vapor deposition or liquid-phase coating. This is followed by high-temperature pyrolysis to form a dense, amorphous carbon layer. Subsequently, the coated product is preferably subjected to high-speed mechanical impact shaping, with the shaping linear speed controlled at 5–25 m / s (preferably 10–20 m / s) and the shaping time at 5–60 min (preferably 15–40 min), to obtain a silicon-carbon composite negative electrode active material with high sphericity, excellent tap density, and stable structure, which is the silicon-carbon active material. The higher the forming speed (linear velocity), the greater the sphericity 'a' of the silicon-carbon material. However, too high a speed may cause the material to break, thus destroying the spherical structure. Increasing the forming time will increase the sphericity 'a', but too long a time will also cause the particles to break.

[0047] The graphite material is a carbon material with a certain degree of graphitization, including natural graphite and artificial graphite; artificial graphite can be obtained by conventional high-temperature carbonization of carbon source precursors. As a preferred preparation example, one or more of phenolic resin, epoxy resin, and polyimide resin are selected as resin-based carbon source precursors, and / or one or more of lignin, coconut shell, starch, cotton fiber, and sodium alginate are selected as biomass-based carbon source precursors; after crushing and sieving, high-temperature carbonization is carried out under an inert atmosphere to obtain the graphite material.

[0048] In embodiments of the present invention, the mass content of silicon in the silicon-carbon active material is preferably 1.5%-25%, more preferably 2-15%, for example 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 13%, 14%, and 15%. Excessive silicon content exacerbates the volume expansion effect, increases the risk of material shedding, and reduces cycle life; insufficient silicon content results in a weak expansion effect, making it difficult to reconstruct the electrode microstructure and reducing kinetics.

[0049] In an embodiment of the present invention, the D of the silicon-carbon active material v 50 is preferably 4-10 μm, more preferably 5-8 μm. Its D v If the value is too high (e.g., 50), the lithium-ion transport path is long and the kinetics are poor; if it is too low (e.g., the specific surface area is large), the side reactions with the electrolyte increase, and the cycle life is poor.

[0050] D V 50 represents the particle size corresponding to a 50% cumulative volume distribution percentage of silicon-carbon active material, reflecting the particle size distribution of silicon-carbon active material in the negative electrode. (Regarding D...) V The detection method for 50 is not limited in this invention. Those skilled in the art can use conventional techniques to detect the D of silicon-carbon active materials. V For example, a laser particle size analyzer can be used to measure 50.

[0051] In embodiments of the present invention, the resistivity of the silicon-carbon active material powder is preferably 3~45 mΩ·m, and more preferably 5~35 mΩ·m. If the powder resistivity is too high, electron transport is hindered, internal polarization of the battery increases, and kinetics are poor; if it is too low, side effects such as excessive conductive agent or excessive particle breakage occur, resulting in poor cycle life.

[0052] Powder resistivity refers to the electrical resistance of a powder material per unit volume, usually measured in ohm-meters (Ω·m), and is one of the important indicators for evaluating the electrical conductivity of powders. This invention does not limit the methods for detecting the resistivity of silicon-carbon active material powders; those skilled in the art can measure it using commonly used powder resistivity testers.

[0053] In embodiments of the present invention, the graphitization degree of the silicon-carbon active material is preferably 75%-96%, and more preferably 80-95%. If the graphitization degree is too high, the interlayer bonding force is strong and the flexibility is poor. It is difficult to adapt to volume changes during repeated charge and discharge, and microcracks are easily generated, leading to particle breakage and electrode structure failure, resulting in poor cycle life. If the graphitization degree is too low, the structure is disordered, there are many defects, uneven interlayer spacing, and poor dynamics.

[0054] As a preferred example, the mass ratio of silicon to carbon in the negative electrode material layer is 1.5 to 33. This embodiment of the invention controls this ratio to avoid excessive silicon, which would exacerbate the volume expansion and contraction effect, increase the risk of electrode shedding, and reduce the cycle life of the battery. Furthermore, it utilizes the conductive framework and porous structure of porous carbon to optimize electron / ion transport efficiency and improve the battery's kinetic performance.

[0055] This invention does not limit the testing method for the graphitization degree of the negative electrode material layer. Those skilled in the art can test the graphitization degree of the negative electrode material layer using conventional techniques. For example, the testing method for the graphitization degree of the material is as follows:

[0056] 1) Pretreatment: Discharge the battery to the lower limit voltage of 2.5V at 0.33C. Take the negative electrode sheet of the lithium-ion battery in the empty state and soak it in dimethyl carbonate (DMC) solution at room temperature for 4 hours. After soaking, take out the electrode sheet and dry it in a vacuum environment. Use a ceramic knife to scrape off the material powder on the surface of the negative electrode sheet.

[0057] 2) Detection: Subsequently, X-ray diffraction (XRD) was used to test the characteristic peak spectrum of the powder. The interplanar spacing d(002) of the graphite crystals of the powder was determined based on the spectrum. Then, the degree of graphitization was calculated based on the formula (0.3440-d(002)) / (0.3440-0.3354) to finally obtain the degree of graphitization.

[0058] This invention does not limit the testing method for the silicon content in the negative electrode material layer. Those skilled in the art can detect the silicon content in the negative electrode material layer using conventional techniques. For example, the silicon content is tested as follows:

[0059] 1) Pretreatment: Discharge the battery to the lower limit voltage of 2.5V at 0.33C. Take the negative electrode sheet of the lithium-ion battery in the empty state and soak it in dimethyl carbonate (DMC) solution at room temperature for 4 hours. After soaking, take out the electrode sheet and dry it in a vacuum environment. Use a ceramic knife to scrape off the material powder on the surface of the electrode sheet.

[0060] 2) Detection: The silicon content in the negative electrode was tested using the alkaline dissolution-ICP method. The test sample was the scraped negative electrode active material powder. A sample of powder was weighed and placed in a nickel crucible pre-filled with potassium hydroxide. A small amount of potassium hydroxide was added to cover the sample surface, and two drops of ethanol were added. The mixture was heated on an electric furnace until the potassium hydroxide melted and dehydrated. Then, it was transferred to a muffle furnace at 1100℃ and kept at this molten temperature for 8 hours. The nickel crucible was removed and allowed to cool slightly. The mixture was placed in a 300 mL plastic beaker, and hot water was added for extraction. After the reaction, the crucible was washed out. HCl was added to the extract for acidification, and hydrogen peroxide and hydrochloric acid were added to form a mixed acid to further convert the silicon compounds to silicon ions. After cooling, the extract was washed out with water and transferred to a 100 mL volumetric flask. The volume was adjusted to 100 mL, and the solution was shaken well. After standing, the solution was transferred to another 100 mL volumetric flask, adjusted to 100 mL, shaken well, and allowed to stand until clear to obtain the test solution. Simultaneously, a blank solution was prepared as a control. No powder was added to the blank solution. Following the operating procedure, a blank sample control was prepared to eliminate any potential influences from the operation. ICP testing was performed on the test solution. The elemental detection wavelength was selected, and the experimental conditions were set: based on the sample characteristics and the ICP testing of the test solution, the Si elemental detection wavelength of 288.158 nm was selected. The Si content of the element was determined by ICP testing.

[0061] Furthermore, the negative electrode material layer described in this embodiment of the invention also includes a binder and a dispersant. The binder is primarily a flexible binder, which may be one or more of styrene-butadiene latex (SBR) binders and polyacrylic acid (PAA) binders; the dispersant is, for example, a cellulose ether derivative. In some embodiments, by adjusting the mass ratio of PAA binder to SBR to 1:9~7:3, and further to (2:8)~(5:5), precise control of the flexibility of the negative electrode sheet is achieved; as the proportion of flexible binder increases, b increases. The higher the polyacrylic acid content, the better the rigidity.

[0062] By controlling the roller pressing pressure to 10~95 T and the aspect ratio of the active material to 1.0~3.2, the porosity of the electrode can be adjusted while achieving a compacted density of 1.25~1.65 g / cm³. 3 Furthermore, the rolling pressure is 20~60 T (or 22-40 T), and the aspect ratio of the silicon-carbon active material is 1.3~2.6. The higher the pressure, the smaller the porosity c of the negative electrode sheet; the smaller the aspect ratio of the material, the larger c.

[0063] In this embodiment of the invention, a negative electrode slurry (solvent such as N-methylpyrrolidone, NMP) comprising the above components is coated on at least one surface of the negative electrode current collector; wherein the mass percentage of negative electrode active material: conductive agent: binder: dispersant is (90-98):(0.4-3):(1.5-5.5):(0.3-1.5). Besides metal foil, the current collector can be selected from composite current collectors; composite current collectors can further improve the flexibility of the electrode, alleviate internal stress, and reduce the risk of electrode shedding. The composite current collector can be selected from PET-based composite copper foil or PP-based composite copper foil; the surface of the foil can also be roughened to improve the adhesion between the active material layer and the foil, thereby improving the flexibility of the electrode.

[0064] As an example, the thickness of the negative electrode current collector is preferably 4~7μm, and more preferably 5~6μm. If the current collector is too thick, the lithium ion transport path is prolonged, and the battery's kinetic performance decreases; if the current collector is too thin, its tensile strength and ductility are insufficient, making it difficult to withstand stress changes during charging and discharging, increasing the risk of electrode shedding, and reducing the battery's cycle life.

[0065] As a preferred example, the negative electrode material layer is a double-layer coating (single-sided coating) of negative electrode slurry. Its areal density is 60~230 g / m³. 2 Further, it is 100~180g / m 2 The outer layer (relatively far from the current collector) uses small particles to shorten the lithium-ion diffusion path and improve the battery's kinetic performance; the inner layer uses large particles to alleviate silicon volume expansion, reduce stress concentration, lower the risk of material loss, and improve the battery's cycle life. That is, the negative electrode material layer includes large and small particles, with small particles located in the outer layer and large particles in the inner layer (the electrode sheet can be cut using a CP cutter to obtain the interface in the thickness direction, and the particle size can be identified using SEM). The average particle size of the large particles is 9~15μm, and the average particle size of the small particles is 3~9μm.

[0066] This invention provides a secondary battery including the aforementioned negative electrode sheet, primarily a lithium-ion battery. The secondary battery further includes a positive electrode sheet, which comprises a positive electrode material layer. This positive electrode material layer includes a positive electrode active material, selectable from at least one of lithium iron phosphate (LFP) and ternary positive electrode active material (NCM). Further, the ternary positive electrode active material has the general formula Li x Ni y Co z Mn t O2; where 0.75≤x≤1.2, y≥0.8, y+z+t=1. For example, the positive electrode active material is LiNi. 0.92 Co 0.07 Mn 0.01 O2.

[0067] As a preferred example, the compaction density of the positive electrode sheet is 2.5~4 g / cm³. 3 Further, it is 3~3.5g / cm³. 3 If the compaction density is too high, the electrolyte wetting of the positive electrode will be hindered, resulting in poor kinetics; if it is too low, the side reactions between the positive electrode material and the electrolyte will increase, resulting in poor cycle life.

[0068] In this invention, the formation of the positive electrode material layer is not limited. Those skilled in the art can prepare the positive electrode material layer on the surface of the positive electrode current collector using conventional techniques. For example, a positive electrode active material (commercially available or prepared by conventional methods), a conductive agent, and a binder are mixed in a mass ratio (positive electrode active material: conductive agent: binder mass range 97-98:1-2:0.5-1.5), and a solvent such as NMP is added to prepare a positive electrode slurry. This slurry is then coated on at least one surface of the positive electrode current collector and dried to form the positive electrode material layer.

[0069] As a preferred example, the component mixture of the positive electrode material layer can be stirred in a vacuum mixer until the system is homogeneous to obtain a positive electrode slurry. This positive electrode slurry is then coated onto one surface of the positive electrode current collector aluminum foil, and then sequentially coated onto the other surface. After air drying at room temperature, it is transferred to an oven for further drying. Following cold pressing and slitting, the positive electrode sheet is obtained. Its areal density can range from 220 to 500 g / m³. 2 Further, it is 300~420g / m 2 The preferred compaction density range is 2.5~4 g / cm³. 3 Further 3.5~4g / cm 3 .

[0070] In an embodiment of the invention, the particle size D of the positive electrode active material is... v The particle size D can range from 2 to 4 μm. v If the value is too high (e.g., 50), the lithium-ion transport path is long and the kinetics are poor; if it is too low (e.g., the specific surface area is large), the side reactions with the electrolyte increase, and the cycle life is poor.

[0071] In some embodiments, the positive electrode active material may include a dopant element; the dopant element may be a rare earth element, a transition metal element, or a non-metallic element, mainly selected from at least one of the following: lanthanum (La), yttrium (Y), aluminum (Al), titanium (Ti), strontium (Sr), zirconium (Zr), tungsten (W), molybdenum (Mo), vanadium (V), niobium (Nb), fluorine (F), and boron (B). The mass content of the dopant element in the positive electrode active material is preferably 0.05% to 0.5%, more preferably 0.1% to 0.4%. Excessive dopant content will introduce distortion in the crystal lattice, forming microcracks and affecting cycle life; insufficient content will fail to optimize the lithium-ion transport channels and affect kinetics.

[0072] In some embodiments, a coating layer may be provided on the surface of the positive electrode active material, and the thickness of the coating layer is preferably 5-40 nm. If the coating layer thickness is too large, the lithium-ion transport path is prolonged, affecting the kinetics; if the coating layer thickness is too small, the side reactions with the electrolyte increase, affecting the cycle.

[0073] The positive electrode material layer described in this embodiment of the invention further includes a conductive agent and a binder. The conductive agent is preferably acetylene black, and the binder may be polyvinylidene fluoride (PVDF) or the like.

[0074] The secondary battery provided in this invention also includes an electrolyte as a carrier for ion transport. It is typically prepared by mixing lithium salt and an organic solvent in a specific ratio. The lithium salt can be an inorganic lithium salt and / or an organic lithium salt (e.g., lithium hexafluorophosphate LiPF6 and lithium bis(fluorosulfonyl)imide LiFSI), and the organic solvent includes ethylene carbonate, propylene carbonate, etc. In this invention, the viscosity of the electrolyte at 25°C is preferably 1.8~4.5 mPa·s, more preferably 1.9~3.6 mPa·s. If the electrolyte viscosity is too high, its fluidity is poor, hindering lithium-ion transport and affecting kinetics; if the viscosity is too low, a uniform and stable SEI film cannot be formed, continuously consuming active lithium and affecting cycling.

[0075] Preferably, the electrolyte further includes additives for improving film-forming properties and other functions; the additives are selected from at least one of carbonates and sulfur-containing organic compounds, including but not limited to at least one of fluoroethylene carbonate (FEC), vinylene carbonate (VC), vinylene carbonate (VEC), 1,3-propanesulfonyl lactone (PS), and 1,3,2-dioxothiapentane-2,2-dioxide (DTD), and can be commercially available.

[0076] In some preferred embodiments, the specific mass content of the additives in the electrolyte is: FEC: 1%-15%, VC: 0-1%, PS: 0-1%, VEC: 0-1%, DTD: 0-2%. Further, it is preferred to add at least two of the following: fluoroethylene carbonate (FEC), vinylene carbonate (VC), 1,3-propanesulfonyl lactone (PS), vinylene carbonate (VEC), and 1,3,2-dioxothiapentane-2,2-dioxide (DTD). The total content of the additives can be 2-10 wt% of the total mass of the electrolyte, more preferably 3-8%. The use of certain electrolyte additives in these embodiments of the invention can promote the formation of a uniform and dense SEI film on the negative electrode, reduce interfacial impedance, promote lithium-ion transport, improve the kinetic performance of the battery, and simultaneously reduce side reactions between the negative electrode material and the electrolyte, reduce active lithium consumption, and improve the cycle life of the battery.

[0077] In this embodiment of the invention, a porous separator (also known as a separator membrane) is provided between the positive and negative electrodes of the lithium-ion battery. The main function of the separator is to block the positive and negative electrodes of the battery to prevent direct contact that could lead to a short circuit, while allowing lithium ions in the electrolyte to pass through freely to achieve ion transport.

[0078] Regarding the preparation of the separator, this invention does not limit the scope; those skilled in the art can obtain it using conventional techniques. For example, one or more of ceramic oxides (SiO2, TiO2, Al2O3), aramid, polyvinylidene fluoride, polyimide, polyacrylonitrile, and polyethylene terephthalate can be selected as the base film coating. The base film is selected from polyolefin films (further, it can be commercially available polyethylene PE film). Furthermore, the selection of the separator can be adjusted according to actual needs.

[0079] In a specific embodiment of the present invention, the above-mentioned positive electrode sheet, separator, and negative electrode sheet can be stacked in sequence, so that the separator is placed between the positive and negative electrode sheets to play a role in isolation, and then the bare cell is obtained by winding. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, standing, formation, shaping and other processes, a lithium-ion secondary battery is obtained (the battery preparation process can be adjusted according to the actual situation).

[0080] This invention also provides an electrical device, including the aforementioned secondary battery; the secondary battery serves as its power supply. This invention does not limit the specific type of the electrical device.

[0081] To better understand the technical content of this invention, specific embodiments are provided below to further illustrate the invention. The raw materials used in these embodiments are commercially available. Unless otherwise specified, the preparation and testing procedures described are standard practices in the art.

[0082] Example 1

[0083] This embodiment provides a secondary battery, and the specific preparation method includes the following:

[0084] 1) Preparation of positive electrode sheet:

[0085] LiNi, the positive electrode active material 0.92 Co 0.07 Mn 0.01 O2 (D) vThe positive electrode active material (2.8 μm), conductive agent acetylene black, and binder PVDF were mixed at a mass ratio (positive electrode active material: conductive agent: binder mass ratio 97.5:1.5:1), and solvent NMP was added. The mixture was stirred under vacuum until the system was homogeneous to obtain the positive electrode slurry. The positive electrode slurry was coated onto one surface of the positive electrode current collector aluminum foil, and then coated onto the other surface in sequence. After drying at room temperature, it was transferred to an oven for further drying. After cold pressing and slitting, the positive electrode sheet with an areal density of 382 g / m³ was obtained. 2 Compacted density 3.5 g / cm³ 3 .

[0086] 2) Preparation of negative electrode sheet:

[0087] Phenolic resin was selected as the resin-based carbon source precursor. After crushing and sieving, it was carbonized and activated to create pores under an inert atmosphere to obtain a porous carbon matrix. The porous carbon matrix was placed in a vapor deposition furnace, and silane was used as the silicon source for vapor-phase silicon infiltration under a high-temperature inert atmosphere, so that elemental silicon was uniformly deposited in the pores of the carbon matrix to obtain a silicon / porous carbon composite. Using acetylene as the carbon source, the composite was coated with carbon on its surface by chemical vapor deposition, and a dense amorphous carbon layer was formed by high-temperature pyrolysis. Subsequently, the coated product was subjected to high-speed mechanical impact shaping, with the shaping linear speed controlled at 16 m / s and the shaping time controlled at 25 min, to obtain a silicon-carbon composite material with high sphericity, excellent tap density, and stable structure.

[0088] The silicon-carbon active material contains 45% silicon by mass (10% of the silicon content in the negative electrode); particle size D v The thickness of the powder is 7 μm; the resistivity of the powder is 9.2 mΩ·m; and the degree of graphitization is 78%.

[0089] The silicon-carbon composite material was mixed with artificial graphite as the negative electrode active material, conductive agent, PAA binder, SBR binder, and dispersant. Pure water was added as a solvent to prepare a uniform negative electrode slurry, wherein the mass percentages of negative electrode active material: conductive agent: binder: dispersant were 95.5:0.8:3.5:0.2; and the PAA:SBR binder ratio was 3.6:6.4. The negative electrode slurry was coated onto at least one surface of a negative electrode current collector (5.5 μm thick), dried, rolled, and slit to obtain the negative electrode sheet. The areal density was 125 g / m³. 2 The electrode flexibility was precisely controlled by adjusting the mass ratio of PAA-type binder to SBR to 3.6:6.4. The electrode compaction density of 1.38 g / cm³ was achieved by controlling the roller pressure to 32 T and the aspect ratio of the active material to 1.95. 3 The greater the pressure, the smaller the value of c; the smaller the aspect ratio, the larger the value of c.

[0090] 3) Preparation of electrolyte

[0091] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and fluoroethylene carbonate (FEC) were mixed in a volume ratio of 1:1:1:1 to obtain a mixed organic solvent. Then, thoroughly dried lithium salts LiPF6 and LiFSI were dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1.2 mol / L. Fluoroethylene carbonate (FEC), vinylene carbonate (VC), and 1,3-propanesulfonyl lactone (PS) were added in equal proportions, with the total amount of additives being 8 wt% of the total mass of the electrolyte. The viscosity of the electrolyte at 25°C was 2.3 mPa·s.

[0092] 4) Preparation of the separating membrane

[0093] Al2O3 was chosen as the base film coating, and the base film was selected from PE.

[0094] 5) Assembly and formation

[0095] The positive electrode, separator, and negative electrode are stacked in sequence and then wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a lithium-ion battery is obtained.

[0096] The prepared lithium-ion batteries were subjected to performance testing, including:

[0097] 1. Cycle life test methods;

[0098] Place the test subject in a 25℃ temperature chamber and perform the following operations: charge it to 4.25V at 1 / 3C, then charge it at a constant voltage to the cutoff current of 0.05C; let it stand for 30 minutes, then discharge it at 1 / 3C to the lower discharge limit voltage of 2.5V; repeat the above operation 3 times, and take the discharge capacity of the third cycle as the initial capacity of the battery; then proceed to the following steps.

[0099] 1) Charge at a constant current rate of 0.5C to 4.25V, and then charge at a constant voltage until the current drops to 0.05C;

[0100] 2) Let it stand for 10 minutes;

[0101] 3) Discharge to 2.5V at a rate of 0.5C;

[0102] 4) Let it stand for 10 minutes.

[0103] Perform cycle tests according to steps 1)-4) until the capacity of the lithium-ion battery is less than 80% of the initial capacity, and record the number of cycles (25℃@0.5C@80%SOC).

[0104] 2. Fast charging performance is represented by DCR, and the testing method is as follows:

[0105] 1) Place the battery in a 25°C incubator until thermal equilibrium is reached;

[0106] 2) Charge the battery at a constant current rate of 1 / 3C to 4.25V, charge it at a constant voltage rate until the current is less than 0.05C, and then discharge it at 0.33C to 2.5V. Repeat this process more than 3 times to obtain the battery's discharge capacity and record the battery's capacity C1.

[0107] 3) Let it stand for 5 minutes;

[0108] 4) Adjust the battery charge to 50% SOC at a 1 / 3C discharge rate;

[0109] 5) Discharge at a 1C rate for 18 seconds, record the battery voltage U2, current I before discharge stops, and battery voltage U1 after the battery voltage stabilizes. Calculate the DC internal resistance R1 using the formula R1=(U2-U1) / I. This R1 is the DC discharge rate (DCR) at 25℃ and 1C.

[0110] Examples 2-16 and Comparative Examples 1-4

[0111] Examples 2-16 and Comparative Examples 1-4 each provide a lithium-ion battery. The preparation method is similar to that of Example 1, except that the control of the sphericity a of the silicon-carbon active material, the flexibility value b of the negative electrode, and the porosity c of the negative electrode surface are different.

[0112] See Table 1 for specific settings.

[0113] In some cases, the shaping rotation speed (linear velocity) of the silicon-carbon material was: 16, 20, 25, 15 m / s (Examples 1-4), 8, 22, 7, 21 (Comparative Examples 1-4); the shaping time (min) was: 25, 40, 15, 15 (Examples 1-4), 10, 50, 8, 40 (Comparative Examples 1-4). The binder PAA:SBR ratio was: 3.6:6.4, 5:5, 2:8, 1:9 (Examples 1-4), 1:9, 7:3, 1:9, 6:4 (Comparative Examples 1-4); the total binder ratio was: 3.5, 4.5, 3, 2.5% (Examples 1-4), 2, 6, 2.2, 5.5% (Comparative Examples 1-4). Roller pressure T: 32, 40, 22, 10 (Examples 1-4), 15, 50, 12, 48 (Comparative Examples 1-4); Aspect ratio of negative electrode active material: 1.95, 2.6, 1.7, 1.3 (Examples 1-4), 1.5, 3, 1.4, 3.2 (Comparative Examples 1-4).

[0114] Table 1 Key parameter settings for Examples 1-16 and Comparative Examples 1-4

[0115]

[0116]

[0117]

[0118] According to Table 1, the secondary battery described in this embodiment of the invention has a long cycle life of 800-1800 cycles, and the overall dynamic performance of the battery is good, with a DCR of 1.38-2.06mΩ, which is beneficial for application.

[0119] This article uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only for the purpose of helping to understand the method and core ideas of the present invention. The above descriptions are only preferred embodiments of the present invention. It should be noted that due to the limitations of textual expression, while there are objectively infinite specific structures, those skilled in the art can make several improvements, modifications, or changes without departing from the principles of the present invention, and can also combine the above technical features in an appropriate manner. These improvements, modifications, changes, or combinations, or the direct application of the inventive concept and technical solution to other situations without modification, should all be considered within the scope of protection of the present invention.

Claims

1. A negative electrode sheet, comprising a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector, characterized in that, The negative electrode material layer includes a silicon-carbon active material, and the silicon-carbon active material and the negative electrode material layer satisfy the following relationship: 12≤1000×a×c / b≤265; a represents the sphericity of the silicon-carbon active material; b is the flexibility value of the negative electrode sheet obtained by testing with a flexibility tester, in mN. -1 ; c represents the porosity of the negative electrode material layer.

2. The negative electrode sheet according to claim 1, characterized in that, The sphericity (a) of the silicon-carbon active material ranges from 0.56 to 0.97; the flexibility (b) of the negative electrode sheet ranges from 1.24 to 6.5 mN. -1 The porosity c of the negative electrode material layer ranges from 12% to 42%.

3. The negative electrode sheet according to claim 1, characterized in that, The particle size D of the silicon-carbon active material v The range of 50 is 4-10 μm.

4. The negative electrode sheet according to claim 1, characterized in that, The resistivity of the silicon-carbon active material powder is 3-45 mΩ·m.

5. The negative electrode sheet according to claim 1, characterized in that, The degree of graphitization of the silicon-carbon active material is 75%-96%.

6. The negative electrode sheet according to any one of claims 1-5, characterized in that, The silicon content in the silicon-carbon active material is 1.5%-45% by mass.

7. The negative electrode sheet according to any one of claims 1-5, characterized in that, The thickness of the negative electrode current collector is 4-7 μm.

8. The negative electrode sheet according to any one of claims 1-5, characterized in that, The negative electrode material layer also includes graphite material, and the ratio of the aspect ratio of the graphite material to the aspect ratio of the silicon-carbon active material is 0.6-3.

9. A secondary battery, characterized in that, Includes the negative electrode sheet as described in any one of claims 1-8.

10. The secondary battery according to claim 9, characterized in that, The secondary battery includes a positive electrode sheet, and the positive electrode sheet includes a positive electrode active material, wherein the positive electrode active material is selected from at least one of lithium iron phosphate and ternary positive electrode active materials.

11. The secondary battery according to claim 10, characterized in that, The compaction density of the positive electrode sheet is 2.5-4 g / cm³. 3 .

12. The secondary battery according to claim 10, characterized in that, The ternary cathode active material has the general formula Li. x Ni y Co z Mn t O2; where 0.75≤x≤1.2, y≥0.8, y+z+t=1.

13. The secondary battery according to claim 10, characterized in that, The particle size D of the positive electrode active material v The range of 50 is 2-4 μm.

14. The secondary battery according to claim 10, characterized in that, The positive electrode active material contains doping elements, which are selected from at least one of Al, Ti, La, Y, Sr, Zr, W, Mo, V, F, Nb, and B.

15. The secondary battery according to claim 14, characterized in that, The mass content of the dopant element in the positive electrode active material is 0.05% to 0.5%.

16. The secondary battery according to claim 10, characterized in that, The positive electrode active material includes a coating layer with a thickness of 5-40 nm.

17. The secondary battery according to any one of claims 9-16, characterized in that, The secondary battery also includes an electrolyte, the viscosity of which is 1.8-4.5 mPa·s at 25°C.

18. The secondary battery according to claim 17, characterized in that, The electrolyte also includes additives, which are selected from at least one of carbonates and sulfur-containing organic compounds.

19. An electrical appliance, characterized in that, Includes the secondary battery as described in any one of claims 9-18.

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