Secondary battery cell, secondary battery, and electric device

Abstract

This application provides a secondary battery cell, a secondary battery, and an electrical device. The secondary battery cell includes a negative electrode sheet; the negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector. The negative electrode film layer includes a negative electrode active material and a binder; the negative electrode active material includes a silicon-containing material, and the binder includes a styrene-acrylic polymer; based on the total mass of the negative electrode film layer, the mass fraction w1 of the binder satisfies: 3% ≤ w1 ≤ 10%, and the porosity p of the negative electrode sheet satisfies: 25% ≤ p ≤ 50%. This secondary battery cell has good cycle performance.

Description

Technical Field

[0001] This application relates to the field of batteries, and more specifically, to a secondary battery cell, a secondary battery, and an electrical device. Background Technology

[0002] In recent years, secondary batteries, mainly lithium-ion batteries, have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power plants, as well as in many fields such as power tools, electric bicycles, electric motorcycles, electric cars, military equipment, and aerospace, thus achieving great development.

[0003] With the development and application of secondary batteries, higher requirements are being placed on their capacity. Silicon-containing anodes represent an important development direction for high-capacity secondary batteries; however, the expansion and contraction of silicon-containing materials severely impacts the cycle performance of these batteries. Therefore, improving the cycle performance of secondary batteries is a pressing technical problem that needs to be solved. Summary of the Invention

[0004] This application is made in view of the above-mentioned technical problems, and its purpose is to provide a secondary battery cell, a secondary battery and an electrical device, wherein the expansion problem of the silicon-containing negative electrode in the secondary battery cell is improved and it has good cycle performance.

[0005] In a first aspect, a secondary battery cell is provided, the secondary battery cell comprising a negative electrode sheet; the negative electrode sheet comprising a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer comprising a negative electrode active material and a binder; the negative electrode active material comprising a silicon-containing material, the binder comprising a styrene-acrylic polymer; based on the total mass of the negative electrode film layer, the mass fraction w1 of the binder satisfies: 3% ≤ w1 ≤ 10%, and the porosity p of the negative electrode sheet satisfies: 25% ≤ p ≤ 50%.

[0006] Provided that the active material includes silicon-containing materials, the secondary battery cell of this application, by controlling the porosity of the negative electrode sheet within the range of 25%-50% and selecting styrene-acrylic polymer as a binder and controlling its mass fraction within the range of 3%-10%, can improve the problem of powder shedding of the negative electrode film due to the expansion and contraction of silicon-containing materials during cycling and reduce the expansion of the negative electrode sheet. Therefore, the secondary battery cell of this application has good cycle performance.

[0007] In this application, the term "styrene-acrylate polymer" refers to styrene-acrylate copolymers, including but not limited to styrene-methyl methacrylate copolymers, styrene-methyl acrylate copolymers, styrene-butyl acrylate copolymers, and styrene-ethyl acrylate copolymers.

[0008] In some embodiments, 3% ≤ w1 ≤ 8%.

[0009] In some embodiments, the negative electrode sheet satisfies at least one of the following conditions (1)-(3): (1) In the secondary battery cell that has not been cycled, the porosity p of the negative electrode sheet satisfies: 40% ≤ p ≤ 50%; (2) In the secondary battery cell with a SOC of 100%, the porosity p of the negative electrode sheet satisfies: 25% ≤ p ≤ 30%; (3) In the secondary battery cell with a SOC of 0%, the porosity p of the negative electrode sheet satisfies: 30% ≤ p ≤ 40%.

[0010] In some embodiments, the compaction density ρ of the negative electrode sheet satisfies: 0.34 g / cm³ 3 ≤ρ≤1.5g / cm 3 .

[0011] In some embodiments, the mass fraction w2 of the silicon-containing material, based on the total mass of the negative electrode film, satisfies: 80% ≤ w2 ≤ 100%.

[0012] In some embodiments, the mass content w3 of silicon element, based on the total mass of the negative electrode film, satisfies: 32% ≤ w3 ≤ 60%.

[0013] In some embodiments, the silicon-containing material comprises a porous silicon-carbon material, wherein the specific surface area BET1 of the porous silicon-carbon material satisfies: 2m² 2 / g≤BET1≤3.5m 2 / g.

[0014] In some embodiments, the molar ratio n1:n2 of silicon to carbon in the porous silicon-carbon material satisfies: (4:6)≤(n1:n2)≤(6:4).

[0015] In some embodiments, the average sphericity of the silicon-carbon material particles satisfy:

[0016] In some embodiments, the average particle size D of the styrene-acrylic polymer particles satisfies: 400nm ≤ D ≤ 600nm.

[0017] In some embodiments, the negative electrode film layer includes a conductive agent having a sheet structure.

[0018] In some embodiments, the conductive agent includes at least one of graphene and flake graphite.

[0019] In some embodiments, the mass fraction w4 of the conductive agent, based on the total mass of the negative electrode film, satisfies: 1% ≤ w4 ≤ 10%.

[0020] In some embodiments, 3% ≤ w4 ≤ 8%.

[0021] In some embodiments, the specific surface area BET2 of the conductive agent satisfies: 20m² 2 / g≤BET2≤800m 2 / g.

[0022] In some embodiments, the thickness h of one sheet of the conductive agent satisfies: 1nm ≤ h ≤ 200nm.

[0023] In a second aspect, a secondary battery is provided, the secondary battery comprising a battery cell in any of the implementable embodiments of the first aspect.

[0024] Thirdly, an electrical device is provided, the electrical device comprising a battery cell in any of the feasible embodiments of the first aspect, and / or a secondary battery in the second aspect. Attached Figure Description

[0025] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments of this application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the drawings without creative effort.

[0026] Figure 1 This is a schematic diagram of a secondary battery.

[0027] Figure 2 This is a schematic diagram of a secondary battery cell. Detailed Implementation

[0028] The embodiments of this application are hereby disclosed in detail with appropriate reference to the accompanying drawings. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of actually identical structures may be omitted. This is to avoid making the following description unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0029] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0030] In the description of this application, it should be noted that, unless otherwise stated, "a plurality of" means two or more; the terms "upper," "lower," "left," "right," "inner," "outer," etc., indicating orientation or positional relationships are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this application. Furthermore, the terms "first," "second," "third," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0031] Unless otherwise specified, in this application, the phrase "A and / or B" means "A, B, or both A and B". More specifically, the condition "A and / or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0032] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0033] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0034] Unless otherwise specified, any undefined terms shall have their technically accepted meanings.

[0035] The embodiments of this application will be described next.

[0036] In recent years, rechargeable batteries have seen significant development due to their high energy density and long lifespan, finding widespread application in power tools, electronic products, electric vehicles, aerospace, and other fields. Typically, a rechargeable battery cell consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During the charging and discharging process, active ions repeatedly insert and extract between the positive and negative electrodes. The electrolyte acts as a conductor of these active ions between the electrodes. The separator, positioned between the positive and negative electrodes, prevents short circuits while allowing active ions to pass through, ensuring the normal electrochemical reactions within the battery cell.

[0037] With the development of rechargeable batteries and the continuous expansion of their application scenarios, higher requirements are being placed on the energy density of individual battery cells. Active materials are one of the direct factors affecting the energy density of individual battery cells. Among them, silicon-containing materials have received widespread attention for their high specific capacity, which can improve the energy density of individual battery cells. However, the volume expansion problem of silicon-containing materials is particularly serious, reaching up to 300%. Therefore, for the negative electrode sheet, there is a severe expansion and contraction process during battery cycling. During repeated expansion and contraction, the negative electrode active material and negative electrode film are prone to detaching from the negative electrode sheet, leading to rapid capacity decay and poor cycle performance in rechargeable battery cells.

[0038] In view of this, embodiments of this application provide a secondary battery cell, wherein the negative electrode active material of the secondary battery cell includes silicon-containing materials, and the secondary battery cell has good cycle performance.

[0039] Next, the secondary battery cell provided in this application will be introduced.

[0040] [Secondary battery cell]

[0041] Firstly, a secondary battery cell is provided, comprising a negative electrode sheet, which includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector. The negative electrode film layer includes a negative electrode active material and a binder. The negative electrode active material includes a silicon-containing material, and the binder includes a styrene-acrylic polymer. Based on the total mass of the negative electrode film layer, the mass fraction w1 of the binder satisfies: 3% ≤ w1 ≤ 10%; optionally, 3% ≤ w1 ≤ 8%; the porosity p of the negative electrode sheet satisfies: 25% ≤ p ≤ 50%.

[0042] When the mass fraction of styrene-acrylic polymer is controlled within the range of 3%-10%, and the porosity of the negative electrode sheet is controlled within the range of 25%-50%, the detachment of the negative electrode active material or negative electrode film layer, including silicon-containing materials, from the negative electrode sheet during cycling can be improved, thereby enhancing the cycle performance of the secondary battery cell. This is likely because styrene-acrylic polymer, as a binder, has good elasticity. A styrene-acrylic polymer binder with a mass fraction in the range of 3%-10% can maintain its adhesiveness during the continuous expansion and contraction of the negative electrode active material, reducing the risk of detachment. Simultaneously, controlling the porosity of the negative electrode sheet within the range of 25%-50% provides sufficient space within the negative electrode sheet for the expansion of silicon-containing materials, thereby reducing the expansion of the negative electrode sheet during the cycling process of the secondary battery cell, reducing friction between the negative electrode sheet and other components in the secondary battery, and further reducing the risk of detachment of the negative electrode active material or negative electrode film layer. Therefore, the secondary battery cell of this application can improve the expansion problem of the negative electrode sheet containing silicon material during cycling and improve the cycle performance of the secondary battery cell.

[0043] Furthermore, if the porosity of the negative electrode sheet is too large, for example, above 50%, it will affect the energy density of the secondary battery cell on the one hand; on the other hand, it will cause the transport particle size of active lithium ions to become longer, affecting lithium ion dynamics. In more serious cases, the electrolyte may break due to excessive porosity, and the lithium ion transport at that location will be interrupted.

[0044] In this application, the term "styrene-acrylate polymer" refers to styrene-acrylate copolymers, including but not limited to styrene-methyl methacrylate copolymers, styrene-methyl acrylate copolymers, styrene-butyl acrylate copolymers, and styrene-ethyl acrylate copolymers.

[0045] Silicon-containing materials can include at least one of elemental silicon, silicon carbide, and silicon oxide. For example, elemental silicon can be silicon nanoparticles, silicon nanowires, or silicon nanotubes. Silicon carbide can be carbon-coated silicon materials with a core-shell structure, silicon-carbon nanotube composites, silicon-amorphous carbon composites, etc. Silicon oxide can be silicon suboxide, etc.

[0046] w1 can be 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a value within the range obtained by any combination of two of the above values. p can be 25%, 26%, 27%, 28%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, or a value within the range obtained by any combination of two of the above values.

[0047] In some embodiments, in a non-cycled secondary battery cell, the porosity p of the negative electrode sheet satisfies: 40% ≤ p ≤ 50%.

[0048] In some embodiments, in a secondary battery cell with a SOC of 100%, the porosity p of the negative electrode sheet satisfies: 25% ≤ p ≤ 30%.

[0049] In some embodiments, in a secondary battery cell with a SOC of 0%, the porosity p of the negative electrode sheet satisfies: 30% ≤ p ≤ 40%.

[0050] SOC (State of charge) refers to the state of charge of a single secondary battery cell. When the SOC of a single secondary battery cell is 100%, it means that the cell is fully charged; when the SOC of a single secondary battery cell is 0%, it means that the cell is fully discharged.

[0051] In uncycled rechargeable battery cells, the porosity of the negative electrode sheet is in the range of 40%-50%, indicating a relatively high porosity within the negative electrode sheet. This porosity provides space for the expansion of silicon-containing materials during cycling. In other words, it allows the silicon-containing materials to expand inwards during the expansion process, thereby reducing the overall expansion of the negative electrode sheet. Therefore, it can improve the problem of negative electrode active material and negative electrode film shedding caused by negative electrode sheet expansion, helping to improve the cycle performance of the rechargeable battery cell.

[0052] In some examples, uncycled secondary battery cells can also be referred to as pre-formation secondary battery cells. It should be understood that after formation and aging processes, the porosity of the negative electrode plate in a secondary battery cell decreases to some extent due to the formation of the SEI film. The greater the porosity of the pre-formation secondary battery cell, the greater the porosity of the post-formation secondary battery cell will be.

[0053] During the charging process of a secondary battery cell, active lithium ions are extracted from the positive electrode active material, move to the negative electrode, and embed themselves in the negative electrode active material. The silicon-containing material undergoes volume expansion during this process, and the pores in the negative electrode sheet are reduced as the negative electrode active material occupies them. During the discharging process of the secondary battery cell, re-extracting lithium ions are extracted from the negative electrode active material, move to the positive electrode active material, and embed themselves in the positive electrode active material. The silicon-containing material undergoes volume contraction during this process, and the pores in the negative electrode sheet increase. Therefore, the porosity of the negative electrode sheet of the secondary battery cell of this application is in the range of 25%-30% under full charge and in the range of 30%-40% under full discharge.

[0054] In some embodiments, the compaction density ρ of the negative electrode sheet satisfies: 0.34 g / cm³ 3 ≤ρ≤1.5g / cm 3 .

[0055] The porosity of the negative electrode sheet is affected by its compaction density. The higher the compaction density, the lower the porosity. Therefore, provided the binder meets the aforementioned design requirements, the porosity of the negative electrode sheet can be controlled at 0.34 g / cm³. 3 -1.5g / cm 3 Within this range, the porosity of the negative electrode sheet can be kept within the range of 40%-50%, which helps to reduce the expansion of the negative electrode sheet during cycling, further improves the problem of easy shedding of negative electrode active material and negative electrode film, and improves the cycle performance of secondary battery cells.

[0056] ρ can be 0.34 g / cm³ 3 0.35g / cm 3 0.36g / cm 3 0.37g / cm 3 0.38g / cm 3 0.39g / cm 3 0.4g / cm 3 0.5g / cm 3 0.6g / cm 3 0.7g / cm 3 0.8g / cm 3 0.9g / cm 3 1.0g / cm 31.1g / cm 3 1.2g / cm 3 1.3g / cm 3 1.4g / cm 3 1.5g / cm 3 , or its value is within the range obtained by combining any two of the above values.

[0057] In some embodiments, the mass fraction w2 of silicon-containing material, based on the total mass of the negative electrode film, satisfies: 80% ≤ w2 ≤ 100%.

[0058] The higher the mass fraction of silicon-containing material in the negative electrode film, the better it is for the energy density of the secondary battery cell, and the greater the expansion of the negative electrode sheet. Therefore, when the porosity of the negative electrode sheet and the binder meet the design requirements of the aforementioned embodiments, the mass fraction of silicon-containing material can reach the range of 80%-100%, thereby improving both the cycle performance and energy density of the secondary battery cell.

[0059] w2 can be 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, 100%, or a value within the range obtained by any combination of the above two values.

[0060] In some embodiments, the mass content of silicon element w3, based on the total mass of the negative electrode film, satisfies: 32% ≤ w3 ≤ 60%.

[0061] The higher the mass fraction of silicon in the negative electrode film, the better it is for the energy density of the secondary battery cell, and the greater the expansion of the negative electrode sheet. Therefore, when the porosity of the negative electrode sheet and the binder meet the design requirements of the aforementioned embodiments, the mass fraction of silicon can reach the range of 32%-60%, in other words, enabling the high-silicon system secondary battery cell to have good cycle performance.

[0062] w3 can be 32%, 34%, 36%, 38%, 40%, 42%, 44%, 46%, 48%, 50%, 52%, 54%, 56%, 58%, 60%, or a value within the range obtained by any combination of the above two values.

[0063] In some embodiments, the silicon-containing material includes a porous silicon-carbon material, wherein the specific surface area BET1 of the porous silicon-carbon material satisfies: 2m² 2 / g≤BET1≤3.5m 2 / g.

[0064] For example, the silicon-carbon material can be a porous silicon-carbon material formed by porous carbon and nano-silicon. That is, the silicon-carbon material particles have a porous structure inside. The more pores inside the material, the larger its specific surface area. Compared to other types of silicon-carbon materials, porous silicon-carbon materials, with their inherent porous structure, can provide space for silicon element expansion, helping to reduce the expansion of the silicon-carbon material itself during cycling. Therefore, provided that the porosity of the negative electrode and the binder meet the aforementioned design requirements, by selecting a porous silicon-carbon material with a high specific surface area, the expansion of the negative electrode can be further reduced, improving the cycle performance of the secondary battery cell.

[0065] BET1 can be 2m 2 / g、2.1m 2 / g, 2.2m 2 / g, 2.3m 2 / g, 2.4m 2 / g, 2.5m 2 / g, 2.6m 2 / g, 2.7m 2 / g, 2.8m 2 / g, 2.9m 2 / g、3m 2 / g, 3.1m 2 / g, 3.2m 2 / g, 3.3m 2 / g, 3.4m 2 / g, 3.5m 2 / g, or its value is within the range obtained by combining any two of the above values.

[0066] In some embodiments, the molar ratio of silicon to carbon in the porous silicon-carbon material, n1:n2, satisfies: (4:6)≤(n1:n2)≤(6:4).

[0067] (n1:n2) could be, for example, (4:6), (4.5:5.5), (5:5), (5.5:4.5), (6:4), or its value could be within the range obtained by any combination of the two values ​​mentioned above.

[0068] In some embodiments, the average sphericity of the porous silicon carbon material particles satisfy:

[0069] Silicon-carbon materials typically have high hardness. If the particles have many sharp edges, they are prone to friction during expansion and contraction, which can damage the SEI film on the surface of the negative electrode. This necessitates the continuous formation of a new SEI film on the negative electrode surface, consuming active lithium ions and affecting the cycle performance of the secondary battery cell. Silicon-carbon materials with higher sphericity exhibit less friction between particles. For secondary battery cells with negative electrode porosity and binder properties meeting the designs in the aforementioned embodiments, using porous silicon-carbon materials with a sphericity of 0.85-1 can further improve the cycle performance of the secondary battery cell.

[0070] On the other hand, silicon-carbon materials with higher sphericity have more complex manufacturing processes, are more difficult to prepare, and are more challenging to industrialize. However, for secondary battery cells with negative electrode porosity and binder properties that meet the designs in the aforementioned embodiments, secondary battery cells with good cycle performance can also be obtained using silicon-carbon materials with a sphericity of less than 0.85. This may be because the higher porosity of the negative electrode reduces friction between silicon-carbon particles, thereby reducing the risk of SEI film damage during cycling and giving the secondary battery cell good cycle performance.

[0071] It can be 0.85, 0.86, 0.87, 0.88, 0.90, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1.0, or its value is within the range obtained by any combination of the above two values.

[0072] In some embodiments, the average particle size D of the styrene-acrylic polymer particles satisfies: 400nm ≤ D ≤ 600nm.

[0073] By controlling the average particle size of the styrene-acrylic polymer (SAP) particles within the range of 400 nm to 600 nm, it can be matched with the porosity of the negative electrode sheet, further improving the expansion problem of the negative electrode sheet during cycling. The reason for this is that if the average particle size of the SAP particles is too small, it is not conducive to the adhesion between active particles, affecting the adhesive performance of the binder; if the average particle size of the SAP particles is too large, it may swell and absorb liquid, increasing the pore size while decreasing the adhesive performance. This reduces the bonding force between the negative electrode active materials, weakening the internal binding force of the electrode sheet during expansion and causing it to expand outwards, thus exacerbating the expansion of the negative electrode sheet. Therefore, when the porosity of the negative electrode sheet is within the range of 25% to 50%, controlling the average particle size of the SAP within this range can further reduce the expansion of the negative electrode sheet during cycling while ensuring the excellent adhesive performance of the SAP.

[0074] For example, D can be 400nm, 410nm, 420nm, 430nm, 440nm, 450nm, 460nm, 470nm, 480nm, 490nm, 500nm, 510nm, 520nm, 530nm, 540nm, 550nm, 560nm, 570nm, 580nm, 590nm, 600nm, or a value within the range obtained by combining any two of the above values.

[0075] In some embodiments, the negative electrode film layer includes a conductive agent having a sheet structure. In some embodiments, the conductive agent includes at least one of graphene and sheet graphite.

[0076] Conductive agents with a layered structure, also known as two-dimensional conductive agents, differ from common conductive agents such as carbon black and carbon nanotubes. These layered conductive agents encapsulate the negative electrode active material, reducing the internal impedance of the secondary battery cell and improving cycle performance. When the negative electrode active material includes silicon-containing materials with high hardness, using conductive agents with a layered structure, such as graphene or sheet graphite, can, on the one hand, protect the silicon-containing material by encapsulating it, reducing material damage caused by friction during cycling and lowering the risk of SEI film breakage during cycling; on the other hand, the encapsulation of the conductive agent can improve the conductivity of the silicon-containing material, thereby reducing the internal impedance of the secondary battery cell. Therefore, the secondary battery cell of this application, by using a conductive agent with a layered structure, can further improve the cycle performance of the secondary battery cell.

[0077] In some embodiments, the mass fraction w4 of the conductive agent, based on the total mass of the negative electrode film, satisfies: 1% ≤ w4 ≤ 10%; optionally, 3% ≤ w4 ≤ 8%.

[0078] A higher mass fraction of conductive agent in the negative electrode film layer is more beneficial to improving the cycle life of the secondary battery cell, but it also affects the energy density of the secondary battery cell. By controlling the mass fraction of conductive agent within the range of 1%-10%, it is possible to improve the cycle performance of the secondary battery cell while also taking into account its energy density.

[0079] w4 can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a value within the range obtained by any combination of the above two values.

[0080] In some embodiments, the specific surface area BET2 of the conductive agent satisfies: 20m² 2 / g≤BET2≤800m 2 / g.

[0081] In some embodiments, the thickness h of a conductive agent sheet satisfies: 1nm ≤ h ≤ 200nm.

[0082] Thickness in the range of 1nm-200nm, BET in the range of 20m 2 / g-800m 2 Conductive agents within the / g range possess a two-dimensional or similar two-dimensional layered structure. On one hand, the layered structure of the conductive agent provides stronger encapsulation of the negative electrode active material, resulting in a larger contact area between the conductive agent and the particles of the negative electrode active material, thus improving conductivity. On the other hand, silicon-containing materials typically have high hardness, making them prone to damaging the current collector during expansion and contraction, and also detrimental to the structural stability of the silicon-containing material itself. The encapsulation effect of the layered structure of the conductive agent can provide protection and lubrication between the negative electrode active material and the current collector.

[0083] Conductive agents with thicknesses and BET values ​​outside this range may suffer from poor contact between the conductive agent and the particles of the negative electrode active material, or be prone to agglomeration, and be difficult to prepare. For example, conductive agents with excessively large thicknesses and BET values ​​may have a near-three-dimensional structure, failing to effectively encapsulate the negative electrode active material. This results in poor contact between the silicon-containing material and the conductive agent, easily damaging the current collector and affecting the power performance of the secondary battery cell. Conversely, conductive agents with insufficient thickness are prone to agglomeration, also causing poor contact with the negative electrode active material and affecting the power performance of the secondary battery cell. Next, using a lithium-ion battery as a specific example, a detailed description of the positive electrode, negative electrode, separator, and electrolyte in a battery cell will be provided. It should be understood that the lithium-ion battery is only an example; the solution provided in this application can also be applied to other types of secondary batteries, such as sodium-ion batteries and magnesium-ion batteries.

[0084] [Negative electrode plate]

[0085] A negative electrode typically includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector. The negative electrode film layer includes a negative electrode active material.

[0086] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0087] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0088] In one embodiment, in addition to the negative electrode active material already mentioned, the negative electrode active material may also be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, tin-based materials, and lithium titanate, etc. Tin-based materials may be selected from at least one of elemental tin, tin oxides, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as battery negative electrode active materials may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0089] In one embodiment, in addition to the styrene-acrylic polymer adhesive already mentioned above, the adhesive may also be selected from at least one of 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).

[0090] In one embodiment, in addition to the conductive agent already mentioned above, the conductive agent may also be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0091] In one embodiment, the negative electrode film layer also includes other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0092] In one embodiment, the negative electrode sheet can be prepared by forming a negative electrode slurry using the components described above. For example, the negative electrode active material, conductive agent, binder, and any other components are dispersed in a solvent (e.g., N-methylpyrrolidone) to form the negative electrode slurry. The negative electrode slurry is then coated onto a negative electrode current collector, and after drying, cold pressing, and other processes, the negative electrode sheet is obtained.

[0093] [Positive electrode plate]

[0094] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including a positive electrode active material.

[0095] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0096] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0097] In one embodiment, the positive electrode active material may be a known positive electrode active material for batteries. As an example, the positive electrode active material may include at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also known as NCM333), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM523), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM211), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM811), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 Al 0.05At least one of O2 and its modified compounds. Examples of lithium-containing phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium iron manganese phosphate, and lithium iron manganese phosphate and carbon composites. During the charging and discharging process, Li undergoes insertion / extraction and consumption, resulting in different molar contents of Li in the positive electrode active material when the battery is discharged to different states. In the examples of positive electrode active materials in this application, the molar content of Li refers to the initial state of the material, i.e., the state before feeding. After charge-discharge cycles, the molar content of Li changes when the positive electrode active material is applied to the battery system. In the examples of positive electrode active materials in this application, the molar content of O is only an ideal value; lattice oxygen release causes changes in the molar content of O, and the actual molar content of O will fluctuate.

[0098] In one embodiment, the positive electrode film layer further includes a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0099] In one embodiment, the positive electrode film layer further includes a conductive agent. As an example, the conductive agent may also include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, graphene, and carbon nanofibers.

[0100] In one embodiment, the positive electrode sheet can be prepared by forming a positive electrode slurry from the components described above. For example, the positive electrode active material, conductive agent, binder, and any other components are dispersed in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry. The positive electrode slurry is then coated onto a positive electrode current collector, and after drying, cold pressing, and other processes, the positive electrode sheet is obtained.

[0101] Electrolyte

[0102] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not specify any particular type of electrolyte; it can be selected according to requirements. The electrolyte includes electrolyte salts and solvents.

[0103] In one embodiment, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0104] In one embodiment, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0105] In one embodiment, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0106] [Isolation Component]

[0107] This application does not impose any particular restrictions on the type of separator. For example, any known porous membrane with good chemical and mechanical stability can be selected.

[0108] In one embodiment, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0109] [Rechargeable Battery]

[0110] This application also provides a secondary battery, including the secondary battery cell described in the above embodiments. A secondary battery can be a single physical module comprising one or more secondary battery cells to provide higher voltage and capacity. When there are multiple secondary battery cells, the multiple secondary battery cells are connected in series, parallel, or mixed via a busbar.

[0111] In some embodiments, the secondary battery can be a battery pack, which includes a housing and individual secondary battery cells, with the individual secondary battery cells or battery modules housed in the housing.

[0112] In some embodiments, the housing may be part of the vehicle's chassis structure. For example, a portion of the housing may be at least a part of the vehicle's floor, or a portion of the housing may be at least a part of the vehicle's crossbeams and longitudinal beams.

[0113] In some embodiments, the secondary battery may be located in the energy storage device. The energy storage device includes energy storage containers, energy storage cabinets, etc.

[0114] Figure 1 This is a schematic diagram of a secondary battery according to this application. Figure 2This is a schematic diagram of a secondary battery cell according to this application. Figure 1-2 As shown, the secondary battery 10 may include multiple secondary battery cells 20 to meet different power usage needs.

[0115] The secondary battery 10 may further include a housing with a hollow interior, housing multiple secondary battery cells 20. For example, multiple secondary battery cells 20 may be connected in parallel, series, or a mixed configuration and then placed inside the housing. The housing may include a first housing portion 101 and a second housing portion 102, which are fitted together to form the housing. The shapes of the first housing portion 101 and the second housing portion 102 may be determined by the shape of the components housed within, for example, by the shape of the combination of the multiple secondary battery cells 20 housed within. At least one of the first housing portion 101 and the second housing portion 102 may have an opening. For example, as... Figure 1 As shown, only one of the first housing portion 101 and the second housing portion 102 may be a hollow cuboid with an opening, while the other may be plate-shaped to cover the opening. Taking the second housing portion 102 as a hollow cuboid with one opening and the first housing portion 101 as plate-shaped as an example, the first housing portion 101 covers the opening of the second housing portion 102 to form a housing with a closed chamber, which can be used to accommodate multiple secondary battery cells 20.

[0116] In addition, this application also provides an electrical device that includes the secondary battery described in the foregoing embodiments.

[0117] For example, unlike Figure 1 As shown, the first housing portion 101 and the second housing portion 102 can both be hollow cuboids with one open side each. The openings of the first housing portion 101 and the second housing portion 102 are opposite to each other, and the first housing portion 101 and the second housing portion 102 are interlocked to form a housing with a closed chamber. This chamber can accommodate multiple secondary battery cells 20. The multiple secondary battery cells 20 are connected in parallel, series, or mixed and placed inside the housing formed by the interlocking of the first housing portion 101 and the second housing portion 102.

[0118] In some embodiments, the secondary battery may further include other components. For example, the secondary battery may further include a busbar component, which can be used to realize electrical connections between multiple secondary battery cells 20, such as in parallel, series, or mixed connections. Specifically, the busbar component can realize electrical connections between secondary battery cells 20 by connecting to the electrode terminals of the secondary battery cells 20; or, the busbar component can also realize electrical connections between secondary battery cells 20 by connecting to other components of the secondary battery cells 20. The busbar component can be fixed to corresponding components of the secondary battery cells 20 by welding, for example, by welding to electrode terminals, sealing structures, or housings, etc., and the embodiments of this application are not limited thereto.

[0119] The secondary battery cells 20 can be directly assembled into secondary battery 10, or they can be first assembled into battery modules, and then multiple battery modules can be assembled into secondary battery 10.

[0120] [Electrical appliances]

[0121] This application provides an electrical device, including the battery described in the above embodiments.

[0122] Electrical devices can include vehicles, mobile phones, portable devices, laptops, ships, spacecraft, electric toys, and power tools, etc. Vehicles can be gasoline-powered cars, natural gas-powered cars, or new energy vehicles; new energy vehicles can be pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles, etc. Spacecraft include airplanes, rockets, space shuttles, and spacecraft, etc. Electric toys include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc. 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, etc. This application does not impose any special limitations on the above-mentioned electrical devices.

[0123] This application provides an electrical device, which is a vehicle.

[0124] The vehicle can be a gasoline-powered vehicle, a natural gas-powered vehicle, or a new energy vehicle. New energy vehicles can be pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles, etc. The vehicle's interior can house a motor, a controller, and a secondary battery 10. The controller is used to control the secondary battery 10 to supply power to the motor. For example, the secondary battery 10 can be located at the bottom, front, or rear of the vehicle. The secondary battery 10 can be used for vehicle power supply; for example, it can serve as the vehicle's operating power source for the vehicle's electrical system, such as for the power needs of starting, navigation, and operation. In another embodiment of this application, the secondary battery 10 can not only serve as the vehicle's operating power source but also as the vehicle's drive power source, replacing or partially replacing gasoline or natural gas to provide driving power to the vehicle.

[0125] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0126] [Examples and Comparative Examples]

[0127] Example 1

[0128] (1) Preparation of negative electrode sheet

[0129] Silicon-carbon material (molar ratio of silicon to carbon is 5:5), sheet-like conductive agent, and styrene-acrylic polymer binder are uniformly mixed in water at a mass ratio of 8:1:1. The mixture is then coated on both sides of a copper foil. After drying, cold pressing, and cutting, the negative electrode sheet is obtained.

[0130] (2) Preparation of positive electrode sheet

[0131] LiN, the positive electrode active material i0.8 Co 0.1 Mn 0.1 O2 (NCM811), conductive carbon black, and PVDF binder are mixed evenly in N-methylpyrrolidone solvent at a mass ratio of 8:1:1. The mixture is then coated on both sides of aluminum foil. After drying, cold pressing, and cutting, the positive electrode sheet is obtained.

[0132] (3) Preparation of battery cells

[0133] The positive electrode, separator, and negative electrode are stacked in sequence and wound to obtain an electrode assembly. The electrode assembly is placed in a housing, an electrolyte is injected, and then it is encapsulated to obtain a single battery cell. The electrolyte is a 1M lithium salt (LiPF6) electrolyte with ethylene carbonate, diethyl carbonate, and dimethyl carbonate as solvents (volume ratio 1:1:1).

[0134] In Example 1, the porosity of the negative electrode sheet in the uncycled secondary battery cell was 45%, the mass fraction of the binder in the negative electrode film was 5%, and the mass fraction of silicon was 54%. The Dv50 of the binder particles was 500 nm, and the specific surface area of ​​the silicon-carbon material was 3.2 m². 2 / g.

[0135] Examples 2-15 and Comparative Examples 1-3

[0136] Compared with Example 1, the differences are as follows: the parameters of the negative electrode active material, the binder parameters, and the negative electrode sheet parameters are different from those in Example 1, as detailed in Table 1.

[0137] Product parameters and performance data of Examples 1-15 and Comparative Examples 1-3.

[0138] Table 1: Product parameters and performance parameters of Examples 1-15 and Comparative Examples 1-3

[0139] In Table 1, "Active Material" represents the negative electrode active material in the secondary battery cell; "BET1" represents the specific surface area of ​​the negative electrode active material; "Silicon-to-Carbon Ratio" represents the molar ratio of silicon and carbon elements in the silicon-carbon material; "Type" represents the type of binder used in the preparation of the negative electrode sheet; "w1" represents the mass fraction of the binder in the negative electrode film; "D" represents the average particle size of the binder particles; "p Before Cycle" represents the porosity of the negative electrode sheet in the secondary battery cell that has not undergone cycling; "w3" represents the mass fraction of silicon in the negative electrode film; "p 0% SOC" represents the porosity of the negative electrode sheet in the secondary battery cell with a SOC of 0%; "p 100% SOC" represents the porosity of the negative electrode sheet in the secondary battery cell with a SOC of 100%; "Expansion Force Growth Rate" represents the expansion force growth rate of the surface with the largest area of ​​the secondary battery cell when the secondary battery cell is cycled to 80% SOH; "Electrode Condition After Cycle" represents whether the negative electrode sheet of the secondary battery cell has cracked after cycling to 80% SOH.

[0140] In Comparative Example 1, the binder used in preparing the negative electrode sheet was sodium polyacrylate (PAA), a linear polymer compound with a molecular weight of approximately 800,000 (with an error of no more than 5%). In Comparative Example 2, the binder used in preparing the negative electrode sheet was polyvinyl alcohol (PVA), also a linear polymer compound with a molecular weight of approximately 800,000 (with an error of no more than 5%).

[0141] As shown in the embodiments and comparative examples, the negative electrode sheets of the secondary battery cells in the embodiments showed no cracking after cycling, while cracking occurred in the comparative examples. The cyclic expansion force of the secondary battery cells in the embodiments was much smaller than that in the comparative examples. This indicates that the expansion problem of the negative electrode sheet containing silicon-carbon material in the secondary battery cells of the embodiments was improved during cycling, and the negative electrode sheet showed no cracking after cycling, thus improving the cycle performance of the secondary battery cells. Therefore, it is demonstrated that, provided the negative electrode active material includes silicon-containing materials, by controlling the porosity of the negative electrode sheet within the range of 25%-50% and controlling the mass fraction of the styrene-acrylic polymer binder within the range of 3%-10%, the expansion problem of the negative electrode sheet caused by silicon-containing materials during cycling can be improved, thereby enhancing the cycle performance of the secondary battery cells.

[0142] Based on Example 4 in Table 1, by adjusting the parameters of the conductive agent, data for Examples 16-22 as shown in Table 2 were obtained.

[0143] Table 2: Product parameters and performance data for Examples 16-22 Types of conductive agents w4 h(nm) <![CDATA[BET2(m 2 / g)]]> Capacity retention Example 16 flake graphite 0.50％ 200nm 22 84.3％ Example 17 flake graphite 1％ 200nm 22 87.5％ Example 18 flake graphite 5％ 200nm 22 91.8％ Example 19 flake graphite 10％ 200nm 22 93.9％ Example 20 flake graphite 5％ 100nm 22 94.1％ Example 21 graphene 5％ 4nm 1000 95.0％ Example 22 Super P 5％ / 450 80.6％

[0144] In Table 2, "Type of Conductive Agent" indicates the type of conductive agent used in preparing the negative electrode sheet; "w4" indicates the mass fraction of the conductive agent in the negative electrode film; "h" indicates the thickness of one conductive agent in the layered structure; "BET2" indicates the specific surface area of ​​the conductive agent; and "Capacity Retention Rate" indicates the capacity retention rate of the secondary battery cell after 200 cycles.

[0145] Comparative analysis of Examples 16-22 shows that the capacity retention of the secondary battery cells in Examples 16-21 is superior to that in Example 22. This indicates that the conductive agents with two-position layered structures, such as sheet graphite and graphene, further improve the cycle performance of the secondary battery cells compared to Super P. A possible reason is that the layered conductive agents have a coating effect on the silicon-carbon material, which can reduce the internal impedance of the secondary battery cell, protect the silicon-carbon material and the current collector, reduce damage to the current collector from the silicon-carbon material during cycling, and reduce the risk of SEI film damage, thereby improving the cycle performance of the secondary battery cells.

[0146] According to Examples 1-4 in Table 1, the DC impedance of the secondary battery cells in Examples 1-4 was tested, and the measured DC internal resistance data are shown in Table 3.

[0147] Table 3: DCR performance data of Examples 1-4

[0148] In Table 3, "w1" represents the mass fraction of the binder in the negative electrode film; "DCR" represents the DC internal resistance value measured in the DC internal resistance test of the secondary battery cell; and "capacity retention rate" represents the capacity retention rate of the secondary battery cell after 200 cycles.

[0149] Comparative analysis of Examples 1-4 in Tables 1 and 3 shows that the mass fraction of the binder in the negative electrode film also affects the DCR of the secondary battery cell. A higher mass fraction of binder in the negative electrode film results in a higher DCR for the secondary battery cell. The capacity retention of the secondary battery cell is affected by both electrode expansion and DCR. It can be seen that in Examples 1-2, although the DCR of the secondary battery cell increases, the rate of increase in expansion force decreases, and the capacity retention of Example 1 is better than that of Example 2. In Examples 1 and 3-4, the rate of increase in expansion force decreases, but the rate of decrease is slower, while the DCR of the secondary battery cell further increases, and the rate of increase is faster, resulting in a decreasing trend in capacity retention. Therefore, secondary battery cells with a binder mass fraction of 3%-8% can achieve both low expansion force and low DCR, allowing the secondary battery cell to balance good cycle performance and capacity retention.

[0150] The following is a brief description of the testing methods for the physicochemical and performance parameters involved in the embodiments of this application. It should be understood that the following testing methods are only examples, and other testing methods known in the art can also be used for testing.

[0151] 1. Test methods for electrode porosity and specific surface area of ​​materials.

[0152] The electrode sheet and powder material to be tested are subjected to dehydration and degassing treatment.

[0153] At liquid nitrogen temperature, the treated electrode or powder material is placed in a gas adsorption apparatus, and then the pressure of nitrogen gas is increased to allow nitrogen to be adsorbed into the pores of the sample. The adsorption amount at different pressures is measured using the gas adsorption apparatus to obtain adsorption isotherms.

[0154] The specific surface area and porosity of the electrode or powder material under test are calculated using the BET model based on the isotherms.

[0155] 2. Test method for adhesive mass fraction

[0156] With the SOC of the secondary battery cell at 0%, the secondary battery cell is disassembled, and an appropriate amount of negative electrode sheet is placed in a thermogravimetric analysis (TGA) balance. The negative electrode sheet is subjected to thermogravimetric analysis at a heating rate of 10℃ / min and a temperature range of 0℃-1000℃. The weight loss (%)-temperature (℃) curve of the negative electrode sheet is obtained by TG-MS. Based on this curve, the mass fraction of binder in the negative electrode sheet can be calculated.

[0157] 3. Test method for the mass fraction of silicon in the negative electrode film layer

[0158] The negative electrode film layer on the negative electrode sheet was scraped off, digested in concentrated nitric acid and hydrofluoric acid, and then the inductively coupled plasma emission spectrum was measured to obtain the mass fraction of silicon in the negative electrode film layer.

[0159] Test equipment: Inductively Coupled Plasma Emission Spectrometer (ICP).

[0160] Test method: Digestion: 1+1 aqua regia, digestion method: plate digestion / acid-removing digestion / microwave digestion (high temperature and high pressure ~200℃), and the mass fraction of silicon element is measured.

[0161] 4. Test method for the molar ratio of silicon to carbon in porous silicon-carbon materials

[0162] The negative electrode active material was digested in concentrated nitric acid and hydrofluoric acid, and then the inductively coupled plasma emission spectrum was measured to obtain the molar ratio of silicon to carbon in the negative electrode active material.

[0163] Test equipment: Inductively Coupled Plasma Emission Spectrometer (ICP).

[0164] Test method: Digestion: 1+1 aqua regia, digestion method: plate digestion / acid-removing digestion / microwave digestion (high temperature and high pressure ~200℃). The mass fraction of silicon and carbon elements was measured, and the molar amount was calculated based on the molar mass of each element. Finally, the molar ratio of silicon and carbon elements was normalized to obtain the molar ratio.

[0165] 5. Test method for electrode compaction density

[0166] Thirty unit areas are randomly selected from the electrode to be tested. For each unit area, the mass m1 of the material (excluding the current collector) on that unit area electrode is measured. The electrode thickness H1 and the current collector thickness H0 are also measured. The compaction density of each unit area electrode is calculated as m1 / (H1-H0). The sum of the results from these 30 randomly selected unit areas on the electrode to be tested is then divided by 30 to obtain the compaction density of the electrode to be tested.

[0167] 6. Test method for the average sphericity of silicon-carbon material particles

[0168] Laser particle size analysis measures the overall particle size distribution. The particle size under two conditions, static and free sedimentation, is expressed as dl and ds, respectively. The sphericity of the particles is ds / dl.

[0169] The testing procedure for particle size distribution can be referenced in standard GB / T19077-2016.

[0170] For example, take a clean beaker, add an appropriate amount of the sample to be tested, add a surfactant, and then add a dispersant. Sonicate at 120W / 5min to ensure that the sample is completely dispersed in the dispersant. After pouring the sample into the injection tower, it circulates with the solution to the test optical path system. Under the irradiation of the laser beam, the particle size distribution characteristics (shading degree: 8-12%) can be obtained by receiving and measuring the energy distribution of the scattered light.

[0171] 7. Test method for the average particle size of styrene-acrylic polymer particles

[0172] The "LIBMAS Lithium-ion Battery Material Microscopic Intelligent Analysis System" software was used for automatic AI identification of primary particles, drawing particle outlines, and obtaining particle quantity, number, area, and maximum caliper diameter. 2000 particles were collected, and the average was calculated. The average particle diameter = the sum of all measured particle dimensions (longest diameter) / the sum of all measured particle numbers.

[0173] 8. Test method for the growth rate of expansion force of secondary battery cells

[0174] For a secondary battery cell with dimensions of 28mm × 148mm × 97mm, the cell was adjusted to a 30% SOC state, which was taken as the initial state. An aluminum plate clamp was applied to the surface of the battery cell with the largest area, using a clamping force of 0.4MPa, ensuring the surface was completely enclosed by the clamp. Full charge-discharge cycles were performed at 0.33C / 0.33C at room temperature until the capacity decayed to 80% of the initial value, which was taken as the post-cycle state. The maximum expansion force was measured using a pressure sensor, and the expansion force growth rate was calculated by comparing the maximum expansion force measured in the initial state and the post-cycle state.

[0175] 9. Test method for whether the electrode cracks after cycling.

[0176] At 45℃, charge at 0.5C to 100% SOC, let stand for 30 minutes, then discharge at 1C to 0% SOC, and let stand for another 30 minutes. This constitutes one cycle. After cycling to 80% SOH, determine whether the negative electrode is cracked by taking a CT scan or disassembling the secondary battery cell.

[0177] 10. Test method for capacity retention

[0178] At 45℃, the battery cell is charged at 0.5C to 100% SOC, allowed to rest for 30 minutes, then discharged at 1C to 0% SOC, and allowed to rest for another 30 minutes. This constitutes one cycle. The discharge capacity of the first cycle is recorded as Cap1. The test cell is then subjected to the same charge-discharge cycle, and the discharge capacity of the 200th cycle is recorded as Cap200. The capacity retention rate of the test cell after 200 cycles is C = [Cap200 / Cap1] × 100%.

[0179] 11. Test method for the thickness of conductive agent

[0180] The particle morphology and size of the sample to be tested were obtained by SEM. 200 conductive agent particles were selected in the field of view, and the thickness and number of layers were measured. The average value was calculated to obtain the thickness of each conductive agent layer. The thickness of each conductive agent layer was used to represent the thickness of "one piece of conductive agent".

[0181] 12. Test method for DC resistance (DCR) of a secondary battery cell

[0182] At 25℃, a single secondary battery cell is charged at a constant current of 0.33C to 4.25V, and then charged at a constant voltage until the current is 0.05C. The cell is then discharged at a constant current of 0.2C for 4 hours to adjust the battery to 20% SOC; the voltage of the cell at this point is recorded as U1. Finally, the cell is discharged at a constant current of 2C for 30 seconds, with a sampling time of 0.1 seconds; the voltage at the end of the discharge is recorded as U2. The discharge DCR of the cell at 20% SOC is used to represent the cell's DCR. The cell's DCR = (U1 - U2) / 4C.

[0183] Although this application has been described with reference to preferred embodiments, various modifications can be made thereto and components can be replaced with equivalents without departing from the scope of this application. In particular, the technical features mentioned in the various embodiments can be combined in any manner, provided there is no structural conflict. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A secondary battery cell, characterized in that, The secondary battery cell includes a negative electrode sheet; The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, the negative electrode film layer including a negative electrode active material and a binder; The negative electrode active material includes a silicon-containing material, and the binder includes a styrene-acrylic polymer; Based on the total mass of the negative electrode film, the mass fraction w1 of the binder satisfies: 3% ≤ w1 ≤ 10%, and the porosity p of the negative electrode sheet satisfies: 25% ≤ p ≤ 50%.

2. The secondary battery cell according to claim 1, characterized in that, 3％≤w1≤8％。 3. The secondary battery cell according to claim 1 or 2, characterized in that, The negative electrode sheet satisfies at least one of the following conditions (1)-(3): (1) In the secondary battery cell that has not undergone cycling, the porosity p of the negative electrode sheet satisfies: 40% ≤ p ≤ 50%; (2) In a secondary battery cell with a SOC of 100%, the porosity p of the negative electrode sheet satisfies: 25% ≤ p ≤ 30%; (3) In a secondary battery cell with a SOC of 0%, the porosity p of the negative electrode sheet satisfies: 30% ≤ p ≤ 40%.

4. The secondary battery cell according to any one of claims 1-3, characterized in that, The compacted density p of the negative electrode sheet satisfies: 0.34 g / cm 3 ≤ p ≤ 1.5 g / cm 3 .

5. The secondary battery cell according to any one of claims 1-4, characterized in that, Based on the total mass of the negative electrode film, the mass fraction w2 of the silicon-containing material satisfies: 80% ≤ w2 ≤ 100%.

6. The secondary battery cell according to any one of claims 1-5, characterized in that, Based on the total mass of the negative electrode film, the mass content of silicon element w3 satisfies: 32% ≤ w3 ≤ 60%.

7. The secondary battery cell according to any one of claims 1-6, characterized in that, The silicon-containing material includes porous silicon-carbon material, and the specific surface area BET1 of the porous silicon-carbon material satisfies: 2m². 2 / g≤BET1≤3.5m 2 / g.

8. The secondary battery cell according to claim 7, characterized in that, In the porous silicon-carbon material, the molar ratio of silicon to carbon, n1:n2, satisfies: (4:6)≤(n1:n2)≤(6:4).

9. The secondary battery cell according to any one of claims 1-8, characterized in that, The average sphericity φ of the porous silicon-carbon material particles satisfies: 0.85≤φ≤1.

10. The secondary battery cell according to claim 8 or 9, characterized in that, The average particle size D of the styrene-acrylic polymer particles satisfies: 400nm≤D≤600nm.

11. The secondary battery cell according to any one of claims 1-10, characterized in that, The negative electrode film layer includes a conductive agent, which has a sheet structure.

12. The secondary battery cell according to claim 11, characterized in that, The conductive agent includes at least one of graphene and flake graphite.

13. The secondary battery cell according to claim 11 or 12, characterized in that, Based on the total mass of the negative electrode film, the mass fraction w4 of the conductive agent satisfies: 1% ≤ w4 ≤ 10%.

14. The secondary battery cell according to claim 13, characterized in that, 3％≤w4≤8％。 15. The secondary battery cell according to any one of claims 11-14, characterized in that, The specific surface area BET2 of the conductive agent satisfies: 20m² 2 / g≤BET2≤800m 2 / g.

16. The secondary battery cell according to any one of claims 11-15, characterized in that, The thickness h of one sheet of the conductive agent satisfies: 1nm≤h≤200nm.

17. A secondary battery, characterized in that, The secondary battery comprises any one of the secondary battery cells according to claims 1-16.

18. An electrical appliance, characterized in that, The electrical device includes a secondary battery cell as described in any one of claims 1-16, and / or the secondary battery as described in claim 17.

Images