Secondary battery and electrochemical device

Abstract

The invention provides a secondary battery and an electrochemical device, and relates to the technical field of batteries. The negative pole piece in the secondary battery comprises a negative current collector; the first active layer is arranged on the surface of at least one side of the negative electrode current collector, and the first active layer comprises a first silicon-carbon negative electrode material with the sphericity degree of A1; the second active layer is arranged on the surface of the first active layer, and the second active layer comprises a second silicon-carbon negative electrode material with the sphericity degree of A2; a2gt is satisfied; a1; the diaphragm comprises a polymer base membrane and a porous ceramic layer arranged on the surface of one side of the polymer base membrane, and the porous ceramic layer is opposite to the second active layer. The energy density of the secondary battery disclosed by the invention is above 750Wh / kg; the K value is less than 0.05 mV / h; the capacity retention ratio after 500 cycles at the normal temperature of 25 DEG C is 90% or above; and the capacity retention ratio after 500 cycles at the high temperature of 45 DEG C is more than 80%.

Description

Technical Field

[0001] This application relates to the field of battery technology, particularly to secondary batteries and electrochemical devices. Background Technology

[0002] With the increasing demand for energy density in lithium-ion rechargeable batteries, silicon-based anodes have attracted significant attention from researchers due to their high energy density resulting from high specific capacity. High-energy-density batteries typically incorporate silicon-containing materials, such as silicon-carbon materials, to enhance the specific capacity of the anode active material. However, silicon-containing materials are prone to significant volume changes during charge and discharge, leading to poor adhesion of the anode active material, resulting in powder shedding and breakage, which in turn causes battery cycle capacity decay and self-discharge. Therefore, there is a need to develop a rechargeable battery that combines high energy density, low self-discharge, and long cycle life. Summary of the Invention

[0003] The purpose of this application is to design a secondary battery containing a silicon-based anode to solve the above-mentioned problems in the prior art, thereby improving the compatibility of the secondary battery with the silicon-based anode and enabling the secondary battery to maintain high energy density while also having a low self-discharge rate and a long cycle life.

[0004] To achieve the above objectives, a first aspect of this application provides a secondary battery, the secondary battery comprising a positive electrode, a separator, a negative electrode, and an electrolyte, wherein the negative electrode comprises: Negative electrode current collector; A first active layer is disposed on at least one side surface of the negative electrode current collector, and the first active layer includes a first silicon-carbon negative electrode material with a sphericity of A1; A second active layer is disposed on the surface of the first active layer, and the second active layer includes a second silicon-carbon anode material with a sphericity of A2. A2 > A1; The membrane includes a polymer base membrane and a porous ceramic layer disposed on one side surface of the polymer base membrane, wherein the porous ceramic layer is in contact with the second active layer.

[0005] As an embodiment of this application, the secondary battery satisfies: 0.5 ≤ (w² × A² × H) / (10w¹ × A¹ × H) ≤ 4 Wherein, w1 represents the mass percentage content of the first silicon-carbon anode material in the first active layer, in %; w2 represents the mass percentage of the second silicon-carbon anode material in the second active layer, in %; A1 represents the sphericity of the first silicon-carbon anode material; A2 represents the sphericity of the second silicon-carbon anode material; H represents the thickness of the polymer-based film, in μm; h represents the thickness of the porous ceramic layer, in μm.

[0006] As an embodiment of this application, the sphericity of the first silicon-carbon anode material satisfies 0.4≤A1≤0.8.

[0007] As an embodiment of this application, the sphericity of the second silicon-carbon anode material satisfies 0.8≤A2≤1.

[0008] As an embodiment of this application, the mass percentage w1 of the first silicon-carbon anode material in the first active layer satisfies 10% ≤ w1% ≤ 30%.

[0009] As an embodiment of this application, the mass percentage w2 of the second silicon-carbon anode material in the second active layer satisfies 40% ≤ w2% ≤ 60%.

[0010] As an embodiment of this application, in the diaphragm, the thickness of the polymer base film satisfies 5μm≤Hμm≤15μm.

[0011] As an embodiment of this application, in the diaphragm, the thickness of the porous ceramic layer satisfies 1μm≤hμm≤6μm.

[0012] As an embodiment of this application, the diaphragm satisfies: 0.20≤h / H≤0.75.

[0013] As an embodiment of this application, the particle sizes of the first silicon-carbon anode material and the second silicon-carbon anode material independently satisfy: 8μm≤Dv50μm≤20μm; Dv50 represents the particle size corresponding to when the cumulative volume of the first silicon-carbon anode material or the second silicon-carbon anode material reaches 50%.

[0014] As an embodiment of this application, the porous ceramic layer includes a ceramic material, which includes at least one of alumina, boehmite, titanium dioxide, aluminum silicate, silicon carbide, and silicon nitride.

[0015] As an embodiment of this application, the thickness of the first active layer is denoted as H1, with units of μm, and satisfies: 5μm≤H1μm≤30μm.

[0016] As an embodiment of this application, the thickness of the second active layer is denoted as H2, with units of μm, and satisfies: 10μm≤H2μm≤40μm.

[0017] As an embodiment of this application, the thickness ratio of the first active layer to the second active layer is (5-30):(10-40).

[0018] As an embodiment of this application, the porosity of the porous ceramic layer is denoted as φ, with the unit being %, and satisfies the following condition: 40% ≤ φ% ≤ 80%.

[0019] A second aspect of this application provides an electrochemical device comprising the secondary battery described in the first aspect of this application.

[0020] Compared with the prior art, the beneficial effects of this application are: This application designs a secondary battery containing a silicon-based anode by setting two active layers on the anode and designing the sphericity of the silicon-carbon anode material in the two active layers. This, together with the porous ceramic layer set on the separator, improves the compatibility of the secondary battery with the silicon-based anode, enabling the secondary battery to maintain high energy density while also having a low self-discharge rate and a long cycle life.

[0021] The energy density of the secondary battery in this application is above 750Wh / kg; the K value is less than 0.05mV / h; the capacity retention rate is above 90% after 500 cycles at room temperature of 25℃; and the capacity retention rate is above 80% after 500 cycles at high temperature of 45℃. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0023] In this application, the technical features described in an open-ended manner include both closed technical solutions composed of the listed features and open technical solutions composed of the listed features.

[0024] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0025] Unless otherwise specified, all reagents or instruments used in this application are commercially available products.

[0026] A first aspect of this application provides a secondary battery, the secondary battery comprising a positive electrode, a separator, a negative electrode, and an electrolyte, wherein the negative electrode comprises: Negative electrode current collector; A first active layer is disposed on at least one side surface of the negative electrode current collector, and the first active layer includes a first silicon-carbon negative electrode material with a sphericity of A1; A second active layer is disposed on the surface of the first active layer, and the second active layer includes a second silicon-carbon anode material with a sphericity of A2. A2 > A1; The membrane includes a polymer base membrane and a porous ceramic layer disposed on one side surface of the polymer base membrane, wherein the porous ceramic layer is in contact with the second active layer.

[0027] This application constructs a "structure-interface" synergistic system adapted to silicon-based anodes by using a differentiated sphericity design of the double-layer active layer of the anode and precise matching with the porous ceramic layer of the separator. This system not only fully leverages the high capacity advantage of silicon-based materials to improve energy density, but also solves the pain point of cycle volume change of silicon-based anodes through multi-dimensional protection and optimization, ultimately achieving a balance between high energy density, long cycle life and low self-discharge rate in secondary batteries.

[0028] The dramatic volume expansion of silicon-carbon materials during lithium intercalation is the core cause of anode failure. This application divides the anode active layer into a first active layer (near the current collector) and a second active layer (near the separator), forming a "buffer-support" synergistic system through functional differentiation between the two layers. The first active layer directly bonds to the current collector, undertaking the function of "basic conductivity and initial capacity"; the second active layer, as the "interface layer" in contact with the electrolyte and separator, must simultaneously consider capacity contribution and interface stability. This layered design avoids cracking caused by stress concentration from expansion in a single active layer—when the first active layer generates stress due to the expansion of silicon-carbon materials, the second active layer can absorb some of the stress through its own structural deformation, while simultaneously preventing active material particles from detaching towards the separator, ensuring the overall structural integrity of the anode.

[0029] This application explicitly states that "the sphericity A2 of the silicon-carbon material in the second active layer is greater than that in the first active layer": (1) First active layer (lower A1): Low sphericity materials usually have more irregular morphology, and their deposition process will form a richer pore structure. On the one hand, these pores can serve as a "buffer space" for the expansion of silicon-based materials, reducing the damage of expansion stress to the current collector bonding interface. On the other hand, low sphericity materials have a relatively large specific surface area, which can ensure a certain conductive network density while avoiding excessive electrolyte decomposition due to excessive specific surface area. In addition, lower sphericity can further enhance the bonding force between silicon-carbon materials and negative electrode current collectors through mechanical interlocking and friction, reducing the risk of peeling of the active layer and current collector during charge-discharge cycles.

[0030] (2) Second active layer (higher A2): High sphericity materials have excellent flowability and packing density, which can form a denser and more uniform active layer structure. First, dense packing can improve the volumetric energy density of the negative electrode. The high packing density of high sphericity materials can directly reduce the ineffective porosity inside the active layer, increase the content of active material per unit volume, and thus improve the overall energy density of the battery. Second, the surface of high sphericity materials is smoother and has no sharp edges, reducing the risk of puncturing the separator; at the same time, uniform spherical particles can form more regular ion transport channels, reduce the transport resistance of lithium ions in the active layer, and improve the capacity retention rate during battery cycling.

[0031] In this application, the diaphragm adopts a "polymer-based membrane + porous ceramic layer" structure, with the porous ceramic layer facing the second active layer. This design forms the following synergistic effect with the negative electrode double-layer active layer: (1) If tiny particles (such as silicon-carbon powder) detached from the silicon-based anode during cycling enter the pores of the separator, it can cause separator blockage and even trigger micro-short circuits between the positive and negative electrodes, exacerbating self-discharge. The porous ceramic layer can act as a "secondary filter layer" to prevent active material particles detached from the anode from entering the micropores of the polymer-based membrane, while supporting the separator structure. Even if the active layer of the anode undergoes some deformation due to expansion, the rigid structure of the ceramic layer can prevent the separator from being squeezed and deformed, ensuring the smooth flow of ion transport channels, thereby reducing cycle life decay caused by separator failure.

[0032] (2) The high dielectric constant of ceramic particles can weaken the interaction between lithium ions and anions in the electrolyte, promote the dissociation and migration of lithium ions, and improve ionic conductivity. This works in conjunction with the regular ion channels formed by the high sphericity material of the second active layer to further optimize the ion transport efficiency inside the battery and ensure cycle stability under high energy density.

[0033] In some embodiments of this application, the secondary battery satisfies: 0.5 ≤ (w² × A² × H) / (10w¹ × A¹ × H) ≤ 4 Wherein, w1 represents the mass percentage content of the first silicon-carbon anode material in the first active layer, in %; w2 represents the mass percentage of the second silicon-carbon anode material in the second active layer, in %; A1 represents the sphericity of the first silicon-carbon anode material; A2 represents the sphericity of the second silicon-carbon anode material; H represents the thickness of the polymer-based film, in μm; h represents the thickness of the porous ceramic layer, in μm.

[0034] Silicon-carbon materials with low sphericity have sharp edges that can compress or even puncture the separator during expansion, causing micro-short circuits and exacerbating self-discharge. In negative electrode applications with a double active layer, to avoid breakage due to excessive silicon content in the lower active layer, more silicon is placed in the upper active layer (surface layer). However, surface placement can lead to poor K-value. To better prevent powder shedding and puncture of the separator, the ceramic coating on the negative electrode side of the separator needs to be thickened, and / or, preferably, a silicon-carbon negative electrode material with higher sphericity should be placed on the surface. When the parameters of the first active layer, the second active layer, and the separator satisfy the above-mentioned relationship, the secondary battery can maintain a high energy density while further improving cycle capacity retention and reducing self-discharge.

[0035] In some embodiments of this application, the sphericity of the first silicon-carbon anode material satisfies 0.4 ≤ A1 ≤ 0.8. When the sphericity of the first silicon-carbon anode material is within the aforementioned suitable range, it ensures the formation of a richer porous structure during its deposition process, serving as a "buffer space" for the expansion of the silicon-based material and reducing the damage to the current collector interface caused by expansion stress. Furthermore, through mechanical interlocking and friction, it further enhances the bonding force between the silicon-carbon material and the anode current collector, reducing the risk of peeling between the active layer and the current collector during charge-discharge cycles. Specifically, the value of A1 can be any value from 0.4, 0.5, 0.6, 0.7, and 0.8, or fall within the range formed by any two of the aforementioned values.

[0036] In some embodiments of this application, the sphericity of the second silicon-carbon anode material satisfies 0.8 ≤ A2 ≤ 1. Having the sphericity of the second silicon-carbon anode material within this suitable range can reduce the risk of membrane puncture and improve the overall energy density of the battery. The value of A2 can specifically be any one of 0.8, 0.9, 0.98, or 1, or fall within the range formed by any two of the above values.

[0037] In some embodiments of this application, the mass percentage w1 of the first silicon-carbon anode material in the first active layer satisfies 10% ≤ w1% ≤ 30%. Specifically, the mass percentage of the first silicon-carbon anode material in the first active layer can be any value among 10%, 20%, and 30%, or fall within the range formed by any two of the above values.

[0038] In some embodiments of this application, the mass percentage w2 of the second silicon-carbon anode material in the second active layer satisfies 40% ≤ w2% ≤ 60%. Specifically, the mass percentage of the second silicon-carbon anode material in the second active layer can be any value among 40%, 50%, and 60%, or fall within the range formed by any two of the above values.

[0039] To ensure the energy density of the secondary battery, a higher content of silicon-carbon anode material needs to be added to the anode active layer. However, the large volume change rate of the silicon-carbon anode material leads to separation from the current collector, resulting in poor cycle stability. To avoid this problem, this application adds a smaller content of silicon-carbon anode material to the first active layer near the current collector and a higher content of silicon-carbon anode material to the second active layer. This can further improve the energy density of the secondary battery while ensuring its cycle life.

[0040] In some embodiments of this application, the thickness of the polymer base film in the separator satisfies 5μm ≤ Hμm ≤ 15μm. Specifically, the thickness of the polymer base film can be any value among 5μm, 10μm, 12μm, and 15μm, or fall within the range formed by any two of the aforementioned values.

[0041] In some embodiments of this application, the thickness of the porous ceramic layer in the diaphragm satisfies 1μm≤hμm≤6μm. Specifically, the thickness of the porous ceramic layer can be any value among 1μm, 2μm, 3μm, 3.5μm, 3.8μm, 4μm, 5.25μm, and 6μm, or fall within the range formed by any two of the aforementioned values.

[0042] In some embodiments of this application, the diaphragm satisfies: 0.20 ≤ h / H ≤ 0.75. The value of h / H can specifically be any value among 0.20, 0.50, and 0.75, or fall within the range formed by any two of the above values.

[0043] Within the aforementioned suitable range, the thickness and thickness ratio of the porous ceramic layer and the base film in the separator allow for efficient mechanical complementarity. The base film, with its excellent flexibility and tensile strength, provides fundamental support for the overall structure, preventing breakage and wrinkling during processing, winding, and battery assembly. Meanwhile, the porous ceramic layer, with its rigidity, compensates for the base film's deficiencies in puncture and impact resistance, effectively resisting damage from electrode burrs and particulate impurities inside the battery. Through their synergistic effect, the overall tensile strength and puncture resistance of the separator are significantly improved, maintaining a stable structural morphology even under dynamic conditions of long-term charge-discharge cycles, reducing the risk of battery short circuits caused by separator damage.

[0044] In some embodiments of this application, the porosity of the porous ceramic layer is denoted as φ, in percentage (%), and satisfies the following condition: 40% ≤ φ% ≤ 80%. The separator needs to possess both sufficient mechanical strength and flexibility, and a suitable ceramic layer porosity is key to achieving this balance. A suitable porosity allows the ceramic layer to maintain high mechanical strength while retaining a certain degree of flexibility—the pores between ceramic particles can buffer external stress, reducing the probability of breakage during battery assembly. Simultaneously, during the cyclic expansion of the silicon-carbon anode, the pores can adapt to changes in electrode volume through minute deformations, preventing wrinkles or breakage of the separator and ensuring the battery maintains good structural integrity throughout long-term cycling.

[0045] In some embodiments of this application, the porous ceramic layer includes ceramic materials. This application does not limit the types of ceramic materials in the porous ceramic layer. Common membrane ceramic materials in the art can be used in this application. For example, the ceramic material can be at least one of alumina, boehmite, titanium dioxide, aluminum silicate, silicon carbide, and silicon nitride, including but not limited to the listed ceramic materials.

[0046] In some embodiments of this application, the particle sizes of the first silicon-carbon anode material and the second silicon-carbon anode material independently satisfy: 8μm ≤ Dv50μm ≤ 20μm; Dv50 represents the particle size corresponding to when the cumulative volume of the first or second silicon-carbon anode material reaches 50%. Within this suitable range, the particle size of the silicon-carbon anode material has several advantages. First, a suitable particle size can reduce the absolute value of the volume expansion of individual particles, reducing the internal stress caused by expansion. Second, a reasonable particle size allows for a more elastic structure inside the electrode, and the gaps between particles can accommodate some of the volume expansion, preventing structural collapse and active material shedding due to expansion. Simultaneously, this particle size range can reduce defect sites on the particle surface, lower the probability of side reactions between the electrolyte and the material surface, and reduce excessive growth or rupture of the SEI film. These effects collectively improve the cycle stability of the silicon-carbon anode, enabling the battery to maintain a high capacity retention rate after long-term charge-discharge cycles and extending the battery's lifespan. For example, the particle size of the first silicon-carbon anode material and the second silicon-carbon anode material can independently be any value among 8μm, 15μm, and 20μm, or fall within the range formed by any two of the above values.

[0047] In some embodiments of this application, the thickness of the first active layer is denoted as H1, in μm, and satisfies: 5μm≤H1μm≤30μm. Specifically, the thickness of the first active layer can be any value among 5μm, 10μm, 20μm, and 30μm, or fall within the range formed by any two of the aforementioned values.

[0048] In some embodiments of this application, the thickness of the second active layer is denoted as H2, in μm, and satisfies the following condition: 10μm ≤ H2μm ≤ 40μm. Specifically, the thickness of the second active layer can be any value among 10μm, 20μm, 30μm, and 40μm, or fall within the range formed by any two of the aforementioned values.

[0049] In some embodiments of this application, the thickness ratio of the first active layer to the second active layer is (5-30):(10-40).

[0050] When the thicknesses of the first active layer and the second active layer are within the above-mentioned appropriate ranges, on the basis of ensuring the bonding strength between the active layer and the current collector, the energy density of the secondary battery can be further improved. An appropriate thickness ratio can further enhance the cycle stability of the secondary battery and extend its service life. Silicon-carbon particles with a relatively high sphericity have better symmetry, and the stress generated during expansion is more uniform, making it less likely to have particle rupture caused by local stress concentration; while silicon-carbon particles with a relatively low sphericity can further absorb the volume increment generated by expansion through the "elastic deformation space" of their irregular shapes, reducing the overall deformation amplitude of the active layer. An appropriate thickness ratio enables the two active layers to form a structural combination of "rigid support + flexible buffer": the high-sphericity layer provides the basic structural support to inhibit the overall collapse of the active layer; the low-sphericity layer fills the gaps and buffers the expansion stress to avoid interlayer delamination. This synergistic effect can significantly reduce the structural damage of the electrode during cycling, reduce the shedding of active materials and the repeated rupture and regeneration of the SEI film, significantly improve the cycle stability of the battery, and extend its service life.

[0051] In some embodiments of the present application, in the first active layer and the second active layer of the negative electrode sheet, a conductive agent, a thickening agent, and a binder may also be selectively included as needed. In the present application, the types of the conductive agent, the thickening agent, and the binder in the negative electrode sheet are not limited. The conductive agent includes but is not limited to at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon nanotubes, graphene, reduced graphene oxide, and carbon nanofibers. The thickening agent includes but is not limited to sodium carboxymethyl cellulose (CMC). The binder includes but is not limited to polyacrylic acid (PAA).

[0052] In some embodiments of the present application, the positive electrode sheet includes a positive electrode current collector and a positive electrode active layer loaded on at least one surface of the positive electrode current collector and containing a positive electrode active material. The present application does not limit the type of the positive electrode active material, and any positive electrode active material commonly used in the art can be used in the present application to prepare the secondary battery. Exemplarily, in some embodiments of the present application, the positive electrode active material includes but is not limited to at least one of ternary positive electrode materials, lithium iron phosphate, and lithium manganese iron phosphate. The positive electrode active material preferably includes LiNi 0.3 , 0.5 , 0.2 , z , 1-x-y-z , x , y Co y Mn z X 1-x-y-z O2, 0 < x ≤ 1, 0 < y ≤ 1, 0 < z ≤ 1, x + y + z ≤ 1. Among them, X can be selected from one or more of Y, Ti, Na, Fe, Co, Ni, Mn, Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, V. More specifically, the positive electrode active material can be selected from LiNiO2, LiCoO2, LiMnO2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi0.6 Co 0.2 Mn 0.2 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.5 Co 0.2 Al 0.3 One or more of O2.

[0053] The positive electrode active layer may also include conductive agents and binders as needed. This application does not limit the types of conductive agents and binders in the positive electrode active layer. The conductive agents include, but are not limited to, at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon nanotubes, graphene-reduced graphene oxide, and carbon nanofibers. The binders include, but are not limited to, at least one of polyvinylidene fluoride, copolymers of polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0054] This application does not limit the types of positive and negative current collectors. The negative current collector is preferably made of copper foil or carbon-coated copper foil. The positive current collector can be made of metal materials such as aluminum, stainless steel, nickel plating, titanium, or tantalum; carbon materials such as carbon cloth or carbon paper; or composite materials formed by polymers and metal layers. In some embodiments, aluminum foil is preferably used as the positive current collector.

[0055] In some embodiments of this application, there are no restrictions on the type of solvent used to form the positive electrode slurry and / or negative electrode slurry, as long as it is a solvent that can dissolve or disperse the positive electrode active material, the negative electrode active material, the conductive agent, the binder, or the dispersant.

[0056] This application does not impose specific limitations on the electrolyte; any electrolyte commonly used in the art can be used to prepare the secondary battery in this application. The electrolyte includes organic solvents and lithium salts.

[0057] The use of the organic solvent is unrestricted, as long as it can serve as a medium for the movement of ions participating in the electrochemical reaction of the battery. Specifically, the organic solvent can be an ester-based solvent, such as methyl acetate, ethyl acetate, γ-butyrolactone, ε-caprolactone, etc.; an ether-based solvent, such as dibutyl ether, tetrahydrofuran, etc.; a ketone-based solvent, such as cyclohexanone; an aromatic hydrocarbon-based solvent, such as benzene, fluorobenzene, etc.; a carbonate-based solvent, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), etc.; an alcohol-based solvent, such as ethanol, isopropanol, etc.; a nitrile, such as R-CN (where R is a straight-chain, branched, or cyclic C2 to C20 hydrocarbon group, and may include double-bonded aromatic rings or ether bonds); an amide-based solvent, such as dimethylformamide; a dioxolane-based solvent, such as 1,3-dioxolane; or a sulfolane-based solvent, etc. Preferably, carbonate solvents are used, and more preferably, cyclic carbonates with high ionic conductivity and high dielectric constant, such as ethylene carbonate or propylene carbonate, which can improve the charge and discharge performance of the battery, are used in mixtures with low-viscosity linear carbonate compounds (such as ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, etc.). In this case, when cyclic carbonates and chain carbonates are mixed in a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte may be excellent.

[0058] The lithium salt can be any compound capable of providing lithium ions for lithium secondary batteries, without particular limitations. Specifically, the lithium salt can be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, etc. The concentration of the lithium salt is preferably in the range of 8% to 20%, specifically any value from 8%, 10%, 12%, and 20%, or within any two of the above values. When the concentration of the lithium salt is within the above range, the electrolyte has suitable conductivity and viscosity, thereby exhibiting excellent electrolyte performance, and lithium ions can move effectively.

[0059] In addition to the electrolyte components mentioned above, additives can be added to the electrolyte according to performance requirements. These additives may include halogenated alkylene carbonates such as ethylene difluorocarbonate; or pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glycol dimethyl ether, hexaphosphotriamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidinanes, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, aluminum trichloride, etc., to improve battery life characteristics, inhibit battery capacity decline, and increase battery discharge capacity. The content of these additives can be 0.1% to 5% of the total electrolyte weight.

[0060] In some embodiments of this application, the preparation of a secondary battery includes: stacking a positive electrode, a separator, and a negative electrode in sequence, with the separator acting as a separator between the positive and negative electrodes; then winding the cells into a square bare cell; inserting the cell into a battery casing; baking the cell at 65–95°C to remove water; injecting electrolyte; sealing the cell; and then performing processes such as settling, hot and cold pressing, formation, clamping, and capacity testing to obtain the secondary battery.

[0061] In some embodiments of this application, the secondary battery may include an outer packaging, which can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery may also be a soft pack, such as a pouch-type soft pack. The material of the soft pack can be plastic, such as one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate, or an aluminum-plastic film, such as an aluminum-plastic film formed by a composite of a PA layer, an aluminum layer, and a PP layer. The shape of the secondary battery is not particularly limited; it can be cylindrical, square, or any other arbitrary shape.

[0062] A second aspect of this application provides an electrochemical device comprising the secondary battery described in the first aspect of this application. The electrochemical device can be an application device such as a vehicle, mobile phone, portable device, laptop computer, ship, spacecraft, electric toy, and power tool. Vehicles can be new energy vehicles, including pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles; spacecraft include airplanes, rockets, space shuttles, and spacecraft; electric toys include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys; power tools include metal cutting power tools, grinding power tools, assembly power tools, and railway power tools, such as electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers. This application does not impose any special limitations on the aforementioned electrochemical device.

[0063] The following are specific embodiments of this application, and the technical solutions of this application are further described in conjunction with the embodiments. However, this application is not limited to these embodiments. Unless otherwise specified, the reagents, methods, and equipment used in this application are all conventional reagents, methods, and equipment in this technical field.

[0064] Example 1 A secondary battery is provided, and its preparation method includes the following steps: Preparation of positive electrode sheet The positive electrode active material LiCoO2, conductive agent SuperP, and binder polyvinylidene fluoride (PVDF) were thoroughly mixed in N-methylpyrrolidone (NMP) solvent at a mass ratio of 98.5:0.5:1 to obtain a positive electrode slurry. This slurry was then coated onto both sides of an Al foil positive electrode current collector, with the Al foil having a thickness of 12 μm. After drying (drying under vacuum at 85°C for 4 hours), cold pressing, slitting, and welding of electrode tabs, a positive electrode sheet with a positive electrode active layer was obtained (the compacted density of the positive electrode sheet was 4.10 g / cm³). 3 ); Negative electrode preparation The first silicon-carbon anode material (parameters are detailed in Table 1), thickener sodium carboxymethyl cellulose (CMC), binder styrene-butadiene rubber (SBR), and conductive agent carbon black (purchased from Cabot's Vulcan XC-72R, with a particle size of 20-50 nm) were mixed and dispersed in deionized water at a mass ratio of 97.8:0.6:0.6:1 to prepare the first anode slurry. The second silicon-carbon anode material (parameters are detailed in Table 1), thickener sodium carboxymethyl cellulose (CMC), binder styrene-butadiene rubber (SBR), and conductive agent carbon black (purchased from Cabot's Vulcan XC-72R, with a particle size of 20-50 nm) were mixed and dispersed in deionized water at a mass ratio of 97.8:0.6:0.6:1 to prepare the second anode slurry. A double-layer coating process is used to sequentially coat a first slurry and a second slurry onto a 6μm thick copper foil for the negative electrode current collector. The first slurry is coated on the surface of the copper foil, and the second slurry is coated on the surface of the first slurry. The foil is then dried in an oven, rolled, slit, and the tabs are welded to obtain a negative electrode sheet with a negative electrode active layer. The compacted density of the negative electrode sheet is 1.70 g / cm³. 3 ); Diaphragm preparation A 10μm thick polypropylene base membrane was used, and a porous ceramic layer was coated on one side of the base membrane. The slurry forming the porous ceramic layer was a dispersion of polyvinylidene fluoride, inorganic nano-ceramic particles, and hydroxymethyl cellulose in a mass ratio of 94.5:2:3.4. The inorganic nano-ceramic particles were nano-alumina (TX-AL-500 purchased from Jiangsu Tianxing New Materials). The parameters of the diaphragm are detailed in Table 1. Electrolyte preparation At room temperature (25°C), in a glove box filled with argon (H2O < 1 ppm, O2 < 1 ppm), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), propyl propionate (PP), and ethylene carbonate (VC) were mixed uniformly in a mass ratio of 20:30:20:28:2 to obtain a mixed solvent. Molecular sieves were used to remove water, resulting in a mixed solvent. Lithium electrolyte salt LiPF6 was added to the mixed solvent, and the mixture was continuously stirred and cooled with dry ice to ensure that the electrolyte temperature did not rise by more than 2°C. Finally, a colorless and transparent liquid was obtained, and an electrolyte with a lithium salt content of 8 wt% was obtained. Assembly of secondary batteries The prepared positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes. They are then wound into a bare cell and placed into a soft-pack casing. After top-side sealing, electrolyte injection (electrolyte, injection coefficient 1.5 g / Ah), formation, and sorting, a second sealing process (liquid retention coefficient 1.1 g / Ah) is performed to obtain a secondary battery.

[0065] Examples 2-18, Comparative Examples 1-5 A series of secondary batteries are provided, which are prepared according to the method of Example 1, with the differences detailed in Table 1.

[0066] It should be noted that in Table 1: (1) The thickness H1 of the first active layer and the thickness H2 of the second active layer can be obtained by scanning electron microscopy (SEM) to test the cross-section of the negative electrode in the thickness direction; the thickness H of the base film and the thickness h of the porous ceramic layer in the separator can also be obtained by scanning electron microscopy (SEM) to test the cross-section of the separator in the thickness direction. (2) In this application, the sphericity A1 of the first silicon-carbon material and the sphericity A2 of the second silicon-carbon material can be obtained by conventional testing methods in the art. Taking the sphericity of the first silicon-carbon material as an example, it is obtained by the following method: Discharge the battery to 0% SOC, disassemble and remove the negative electrode sheet, or remove the negative electrode sheet directly. Polish the cross-section of the negative electrode sheet with an argon ion mill, and then image the obtained cross-section using a scanning electron microscope (SEM) in backscatter imaging mode. Analyze the SEM image of the negative electrode active layer at a certain magnification (e.g., 2500x) in backscatter mode using image processing software such as Image Pro Plus. First, determine the average particle size to identify the first silicon carbon (conductive carbon black is at the nanometer level, while silicon carbon material is at the micrometer level). Obtain the perimeter and area of ​​each particle, and calculate the perimeter equivalent radius r1 and area equivalent radius r2 of each first silicon carbon particle. Then, the sphericity S of each particle is S = r2 / r1. Finally, perform a weighted average of the sphericity of each particle to obtain the sphericity of the first silicon carbon.

[0067] (3) Test method for porosity of porous ceramic layers: The weighed diaphragm (W) d After soaking in n-butanol for 2 hours, remove the product, gently blot the liquid off the surface with filter paper, and then weigh it (W). w The mass W of n-butanol absorbed by the modified membrane can then be obtained. b =W w -W d The pore volume of the diaphragm can be determined by the mass (W) of n-butanol. b The density of n-butanol (ρ) b Dividing this volume by the dry film volume (V) yields the result. p The ratio of W% to W% is the porosity P of the modified membrane, calculated using the formula: P% = (W / W) w -W d ) / (V p ×ρ b ).

[0068] Table 1 Performance testing The performance of the secondary batteries prepared in the above embodiments and comparative examples was tested as follows, and the test results are shown in Table 2: (1) Energy density (unit: Wh / kg): The batteries prepared in the examples and comparative examples were tested for energy density. The specific testing method is as follows: charge at 0.2C to the upper limit voltage of 4.5V (cut off at 0.02C), discharge at 0.2C to the lower limit voltage of 3V, and repeat 3 times; the discharge energy of the 3rd discharge is recorded as Q; then use a 2.5D microscope to measure the width L and height W of the battery, and use a PPG thickness meter to measure the full-charge thickness T of the battery; then the energy density is Q / (L×W×T), and the unit is Wh / L.

[0069] (2) Self-discharge performance, K value (mV / h): The battery was charged at a constant current density of 0.2C to 4.5V, and then charged at a constant voltage of 4.5V with a cutoff current of 0.02C. After resting for 10 minutes, the open circuit voltage V1 of the battery was recorded. Then, the battery was kept resting for 24 hours, and the open circuit voltage V2 of the battery was recorded after the resting period. The self-discharge rate of the battery k (mV / h) = (V1-V2) / 24 (in mV / h).

[0070] (3) Capacity retention after 500 cycles at 1C / 0.7C: The lithium-ion battery was charged at a constant current to full charge voltage (maximum design voltage of 4.5V) at a charging current of 5C in environments of 25℃ and 45℃ respectively. Then, it was charged at a constant voltage until the current was 0.02C. Then, it was discharged at a constant current at a discharge current of 0.7C until the final voltage was 3.0V. The discharge capacity of the first cycle was recorded. The above steps were repeated for charging and discharging cycles, and the discharge capacity of the lithium-ion battery in each charge-discharge cycle was recorded. The calculation formula is: capacity retention rate after 500 cycles = discharge specific capacity after 500 cycles / discharge specific capacity after the first cycle × 100%.

[0071] Table 2 The results above show that: This application constructs a "structure-interface" synergistic system adapted to silicon-based anodes by using a differentiated sphericity design of the double-layer active layer of the anode and precise matching with the porous ceramic layer of the separator. This system not only fully leverages the high capacity advantage of silicon-based materials to improve energy density, but also solves the pain point of cycle volume change of silicon-based anodes through multi-dimensional protection and optimization, ultimately achieving a balance between high energy density, long cycle life and low self-discharge rate in secondary batteries.

[0072] The closer the surface sphericity is to 1, the lower the K value; the higher the membrane porosity and the thicker the coating, the lower the K value; the thicker the base film and coating of the separator, the more the energy density will be compressed; therefore, within the above-mentioned appropriate range of values ​​for (w2×A2×H) / (10w1×A1×h), the secondary battery can achieve both high volumetric energy density and further improve the battery's high-temperature storage performance, reduce the battery's self-discharge level, and avoid the situation where the separator thickness is too thin, resulting in lower separator strength, making it easier to be punctured by silicon-based particles, which would worsen the battery's self-discharge and storage performance. Furthermore, using an excessively thin separator can easily lead to poor interfacial adhesion with the negative electrode coating, which can easily cause electrode powder shedding.

[0073] The energy density of the secondary battery in this application is above 750Wh / kg; the K value is less than 0.05mV / h; the capacity retention rate is above 90% after 500 cycles at room temperature of 25℃; and the capacity retention rate is above 80% after 500 cycles at high temperature of 45℃.

[0074] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A secondary battery, comprising a positive electrode, a separator, a negative electrode, and an electrolyte, characterized in that, The negative electrode sheet includes: Negative electrode current collector; A first active layer is disposed on at least one side surface of the negative electrode current collector, and the first active layer includes a first silicon-carbon negative electrode material with a sphericity of A1. A second active layer is disposed on the surface of the first active layer, and the second active layer includes a second silicon-carbon anode material with a sphericity of A2. A2 > A1; The membrane includes a polymer base membrane and a porous ceramic layer disposed on one side surface of the polymer base membrane, wherein the porous ceramic layer is in contact with the second active layer.

2. The secondary battery according to claim 1, characterized in that, The secondary battery satisfies: 0.5≤(w²×A²×H) / (10w¹×A¹×h)≤4 Wherein, w1 represents the mass percentage content of the first silicon-carbon anode material in the first active layer, in %; w2 represents the mass percentage of the second silicon-carbon anode material in the second active layer, in %; A1 represents the sphericity of the first silicon-carbon anode material; A2 represents the sphericity of the second silicon-carbon anode material; H represents the thickness of the polymer-based film, in μm; h represents the thickness of the porous ceramic layer, in μm.

3. The secondary battery according to claim 2, characterized in that, The sphericity of the first silicon-carbon anode material satisfies 0.4≤A1≤0.8; and / or, the sphericity of the second silicon-carbon anode material satisfies 0.8≤A2≤1.

4. The secondary battery according to claim 2, characterized in that, The mass percentage w1 of the first silicon-carbon anode material in the first active layer satisfies 10% ≤ w1% ≤ 30%; and / or, the mass percentage w2 of the second silicon-carbon anode material in the second active layer satisfies 40% ≤ w2% ≤ 60%.

5. The secondary battery according to claim 2, characterized in that, In the diaphragm, the thickness of the polymer base film satisfies 5μm≤Hμm≤15μm; and / or, the thickness of the porous ceramic layer satisfies 1μm≤hμm≤6μm.

6. The secondary battery according to claim 5, characterized in that, It satisfies: 0.20≤h / H≤0.

75.

7. The secondary battery according to claim 1, characterized in that, The particle sizes of the first silicon-carbon anode material and the second silicon-carbon anode material independently satisfy: 8μm≤Dv50μm≤20μm; Dv50 represents the particle size corresponding to when the cumulative volume of the first silicon-carbon anode material or the second silicon-carbon anode material reaches 50%.

8. The secondary battery according to claim 1, characterized in that, The porous ceramic layer includes a ceramic material, which includes at least one of alumina, boehmite, titanium dioxide, aluminum silicate, silicon carbide, and silicon nitride.

9. The secondary battery according to claim 1, characterized in that, It satisfies at least one of the following characteristics: (1) The thickness of the first active layer is denoted as H1, and the unit is μm, which satisfies: 5μm≤H1μm≤30μm; (2) The thickness of the second active layer is denoted as H2, in μm, and satisfies: 10μm≤H2μm≤40μm; (3) The thickness ratio of the first active layer to the second active layer is (5-30):(10-40); (4) The porosity of the porous ceramic layer is denoted as φ, and the unit is %, which satisfies: 40% ≤ φ% ≤ 80%.

10. An electrochemical device, characterized in that, The electrochemical device includes a secondary battery as described in any one of claims 1 to 9.

Images