Positive electrode sheet, secondary battery, and electric device

Abstract

This application provides a positive electrode, a secondary battery, and an electrical device. The secondary battery includes a casing and an electrode assembly housed inside the casing. The electrode assembly includes a positive electrode and a negative electrode. The positive electrode extends along a first direction, and its length in the first direction is greater than its width in a second direction, which is perpendicular to the first direction. The negative electrode 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 silicon. 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 includes a first region and a second region located on both sides of the first region in a second direction. The capacitance per unit area of ​​the first region is less than that of the second region. The secondary battery of this application can achieve a balance between energy density and cycle performance.

Description

[0001] Cross-references to related applications

[0002] This disclosure is based on and claims priority to Chinese Patent Application No. 202411754620.4, filed on December 2, 2024, entitled "Positive Electrode Sheet, Secondary Battery and Electrical Device", the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of lithium battery technology, and in particular to a positive electrode, a secondary battery, and an electrical device. Background Technology

[0004] In recent years, as the application scope of secondary batteries has become increasingly wide, they 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, and aerospace.

[0005] Due to the significant advancements in secondary batteries, higher requirements have been placed on their energy density and cycle performance. Summary of the Invention

[0006] This application was made in view of the above-mentioned problems, and its purpose is to provide a positive electrode, a secondary battery, and an electrical device. The secondary battery of this application can achieve a balance between energy density and cycle performance.

[0007] To achieve the above objectives, a first aspect of this application provides a secondary battery comprising: a housing and an electrode assembly housed within the housing, the electrode assembly comprising a positive electrode and a negative electrode, wherein the positive electrode extends along a first direction, the length of the positive electrode in the first direction is greater than its width in a second direction, and the second direction is perpendicular to the first direction; the negative electrode comprises a negative current collector and a negative electrode film layer disposed on at least one side surface of the negative current collector, the negative electrode film layer comprising silicon; the positive electrode comprises a positive current collector and a positive electrode film layer disposed on at least one side surface of the positive current collector, the positive electrode film layer comprising a first region and a second region located on both sides of the first region in the second direction; the capacitance per unit area of ​​the first region is less than the capacitance per unit area of ​​the second region.

[0008] In this application, silicon is added to the negative electrode film layer to improve the energy density of the secondary battery. Furthermore, by making the capacity per unit area of ​​the first region (middle region) in the positive electrode film layer lower than that of the second region (both ends), the number of lithium ions released from the first region is reduced, thereby decreasing the lithium ion insertion amount in the middle region of the negative electrode film layer and reducing volume expansion in the middle region, thus improving the cycle performance of the secondary battery. Therefore, the secondary battery of this application possesses both high energy density and high cycle performance.

[0009] In some embodiments, the electrode assembly is wound to form a wound structure, the winding axis of which is parallel to the second direction. This is beneficial for further improving the volumetric energy density of the secondary battery.

[0010] In some embodiments, the tensile strength of the casing is 350 MPa or higher. Optionally, the casing material includes at least one selected from aluminum alloy, steel, copper alloy, titanium alloy, and nickel alloy. The electrode assembly of this application has low expansion force, resulting in small volume changes during battery operation. Consequently, the pressure exerted by the electrode assembly on the casing is small, which helps to reduce the restriction of the casing on the expansion of the electrode assembly. Using a casing with a tensile strength exceeding 350 MPa can further improve the safety performance of the battery.

[0011] In some embodiments, the silicon element in the negative electrode film layer accounts for 3% to 70% of the total mass. This is beneficial for further improving the energy density of the secondary battery.

[0012] In some embodiments, the negative electrode film layer comprises a silicon-based material, including elemental silicon, silicon-oxygen composites, and silicon-carbon composites. In some embodiments, the silicon-carbon composite further includes a carbon-containing coating layer located on the surface of the porous carbon and / or silicon material. Optionally, the silicon-based material may include at least the silicon-carbon composite. This helps to improve the charging capability of the battery cell.

[0013] In some embodiments, the silicon-oxygen complex includes at least one of unpre-lithium silicon-oxygen compound, pre-lithium silicon-oxygen compound, unpre-magnesium silicon-oxygen compound, and pre-magnesium silicon-oxygen compound.

[0014] In some embodiments, the silicon-carbon composite comprises porous carbon and silicon-containing material dispersed in the pores of the porous carbon, optionally the porous carbon being hard carbon. This helps to improve the charging capability of a single battery cell.

[0015] In some embodiments, the silicon-carbon composite further includes a carbon-containing coating layer located on the surface of the porous carbon and / or silicon material. This can improve the conductivity of the silicon-carbon composite, reduce the internal impedance of the secondary battery, and effectively reduce the probability of direct contact between the silicon-containing material in the porous carbon channels and the external environment, thereby improving the chemical stability of the silicon-carbon composite.

[0016] In some embodiments, the silicon content in the silicon-carbon composite is 30% to 70% by mass. This approach, while maximizing the specific capacity of the negative electrode active material by utilizing silicon, also facilitates the full dispersion of silicon in the carbon-containing porous material and helps control the expansion of silicon during charging.

[0017] In some embodiments, the average particle size of the silicon-carbon composite is from 2 μm to 15 μm, optionally from 7 μm to 11 μm. This makes it easier to increase the compactness of the negative electrode active layer by utilizing the interparticle gaps, thereby further improving the energy density of the battery cell.

[0018] In some embodiments, the silicon-carbon composite has a powder resistivity of 4 Ω·cm to 17 Ω·cm at 8 MPa; the control of the powder resistivity improves the conductivity of the silicon-carbon composite, thereby increasing the charging rate of the battery cell.

[0019] In some embodiments, the BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g to 6.7m 2 / g. This is beneficial for balancing energy density and fast charging performance.

[0020] In some embodiments, the negative electrode film layer includes a first film layer and a second film layer stacked on the side of the first film layer facing away from the negative electrode current collector; the mass percentage of silicon in the second film layer is greater than the mass percentage of silicon in the first film layer. This is beneficial for enhancing the fast-charging performance of the battery.

[0021] In some embodiments, the silicon element in the first film layer accounts for 0 to 20% of the mass, which helps to increase the ion transport channels in the second film layer, thereby further improving the fast charging performance of the secondary battery.

[0022] In some embodiments, the silicon element in the second film layer accounts for 5% to 70% by mass. This is beneficial for further improving the fast-charging performance of the secondary battery.

[0023] In some embodiments, the volumetric energy density of the secondary battery is between 600 Wh / L and 850 Wh / L. This is beneficial for further improving the volumetric energy density of the secondary battery.

[0024] In some embodiments, the width of the positive electrode film layer in the second direction is L, the width of the first region in the second direction is L1, and the total width of the second region in the second direction is L2. The L and L1 satisfy the following relationship: 10% ≤ L1 / L ≤ 40%. This is beneficial for balancing the energy density and cycle performance of the battery.

[0025] In some implementations, L and L2 satisfy the following relationship: 60% ≤ L2 / L ≤ 90%. This is beneficial for balancing the battery's energy density and cycle performance.

[0026] In some embodiments, the ratio of the capacitance per unit area of ​​the first region to the capacitance per unit area of ​​the second region is 60% to 98%. This is beneficial for further improving the energy density and cycle performance of the battery.

[0027] In some embodiments, the capacitance per unit area of ​​the first region is 15mAh / 1540.25mm². 2 Up to 98mAh / 1540.25mm 2 This, on the one hand, helps reduce the volume expansion in the middle region of the negative electrode, thereby improving the cycle performance and safety performance of the secondary battery. On the other hand, it helps the secondary battery achieve high energy density. Optionally, the unit area capacity of the first region is 15mAh / 1540.25mm². 2 Up to 68.4mAh / 1540.25mm 2 .

[0028] In some embodiments, the capacitance per unit area of ​​the second region is 25mAh / 1540.25mm². 2 Up to 100mAh / 1540.25mm 2 This, on the one hand, helps reduce the volume expansion in the middle region of the negative electrode, improving the cycle performance and safety of the secondary battery. On the other hand, it helps the secondary battery achieve high energy density. Optionally, the capacity per unit area of ​​the second region is 25mAh / 1540.25mm². 2 Up to 77mAh / 1540.25mm 2 .

[0029] In some implementations, the thickness of the first region is less than or equal to the thickness of the second region. This allows the first region to achieve a low capacity per unit area, reducing the number of lithium ions released per unit area during charging and discharging. This helps reduce volume expansion in the middle region of the negative electrode, thereby improving the battery's cycle performance.

[0030] In some embodiments, the thickness of the first region of a single layer is 0.005 mm to 0.15 mm, which on the one hand helps to reduce the volume expansion of the middle region of the negative electrode sheet, and on the other hand helps the secondary battery achieve a high energy density.

[0031] In some embodiments, the thickness of the single-layer second region is 0.0185 mm to 0.155 mm. This is beneficial in two ways: firstly, it reduces the volume expansion in the middle region of the negative electrode sheet, and secondly, it helps the secondary battery achieve a high energy density.

[0032] In some embodiments, the compaction density of the second region is greater than or equal to that of the first region. This facilitates a higher compaction density in the second region, allowing it to hold more lithium ions per unit volume, thereby increasing the energy density of the secondary battery.

[0033] In some embodiments, the compaction density of the first region is 3.0 g / cm³. 3 Up to 4g / cm 3 This is beneficial for improving the energy density of secondary batteries.

[0034] In some embodiments, the compaction density of the second region is 3.3 g / cm³. 3 Up to 4.4 g / cm 3 This is beneficial for improving the energy density of secondary batteries.

[0035] In some embodiments, the second region includes a lithium replenishing agent. This is beneficial for improving the energy density of the secondary battery.

[0036] In some embodiments, the mass percentage of lithium replenishment agent in the second region is less than 3%. This helps to improve the energy density of the secondary battery while maintaining low production costs.

[0037] In some embodiments, the positive electrode further includes a lithium strip located on the side of at least one of the second regions facing away from the positive current collector. This is beneficial for improving the energy density of the secondary battery.

[0038] In some embodiments, both the first region and the second region include a positive electrode active material, and the areal density of the positive electrode active material in the first region is less than or equal to the areal density of the positive electrode active material in the second region. This helps to reduce the number of lithium ions released per unit area in the first region, thereby reducing the volume expansion in the middle region of the negative electrode and improving the cycle performance of the battery.

[0039] In some embodiments, the areal density of the positive electrode active material in the first region is 0.15 g / 1540.25 mm². 2Up to 0.4g / 1540.25mm 2 This helps reduce the volume expansion in the middle region of the negative electrode, thereby improving the cycle performance of the battery.

[0040] In some embodiments, the areal density of the positive electrode active material in the second region is 0.18 g / 1540.25 mm². 2 Up to 0.4g / 1540.25mm 2 .

[0041] In some embodiments, both the first and second regions include positive electrode active material, and the coating weight per unit area of ​​the positive electrode active material in the first region is less than or equal to the coating weight per unit area of ​​the positive electrode active material in the second region. This helps to reduce volume expansion in the middle region of the negative electrode sheet, thereby improving the battery's cycle performance and safety performance.

[0042] In some embodiments, the coating weight per unit area of ​​the positive electrode active material in the first region is 0.15 g / 1540.25 mm. 2 Up to 0.4g / 1540.25mm 2 。 This helps reduce the volume expansion in the middle region of the negative electrode, thereby improving the cycle performance of the battery.

[0043] In some embodiments, the coating weight per unit area of ​​the positive electrode active material in the second region is 0.18 g / 1540.25 mm. 2 Up to 0.4g / 1540.25mm 2 This helps reduce the volume expansion in the middle region of the negative electrode, thereby improving the battery's cycle performance.

[0044] In some embodiments, the porosity of the first region is greater than that of the second region. This helps to reduce volume expansion in the middle region of the negative electrode, thereby improving the cycle performance of the battery.

[0045] In some embodiments, both the first region and the second region comprise a lithium transition metal oxide, which includes nickel and lithium. The nickel molar percentage is 50% or more, based on the total molar amount of elements other than lithium in the lithium transition metal oxide. This is beneficial for further improving the energy density of the secondary battery. Optionally, the nickel molar percentage is 60% or more. More preferably, the nickel molar percentage is 80% or more. Even more preferably, the nickel molar percentage is 90% or more.

[0046] In some embodiments, the molar percentage of nickel in the first region is less than that in the second region. This is beneficial for further improving the battery's cycle performance and energy density.

[0047] In some embodiments, the molar percentage of nickel in the first region is 0.6 to 0.97, which is beneficial for further improving the energy density of the secondary battery.

[0048] In some embodiments, the molar percentage of nickel in the second region is 0.7 to 0.97. This is beneficial for further improving the energy density of the secondary battery.

[0049] In some embodiments, at least one of the second regions includes N sub-regions arranged sequentially in the second direction, and the capacity per unit area of ​​the N sub-regions increases sequentially in the direction away from the first region. This improves the cycle performance and safety performance of the battery.

[0050] In some embodiments, the N sub-regions include a first sub-region and a second sub-region, with the second sub-region located between the first sub-region and the first sub-region, and the capacity per unit area of ​​the second sub-region being smaller than that of the first sub-region. This is beneficial for improving the battery's cycle performance and safety performance.

[0051] In some embodiments, the capacitance per unit area of ​​the first sub-region is 54 / 1540.25 mm². 2 Up to 100mAh / 1540.25mm 2 This improves the battery's cycle performance and safety performance.

[0052] In some embodiments, the capacitance per unit area of ​​the second sub-region is 25 / 1540.25 mm². 2 Up to 70mAh / 1540.25mm 2 This is beneficial for further improving battery energy density. On the one hand, it helps the battery achieve high energy density; on the other hand, it helps reduce stress concentration on the negative electrode, thereby improving the battery's cycle performance and safety performance.

[0053] In some embodiments, the width of the first sub-region in the second direction is L21, and the width of the second sub-region in the second direction is L22, wherein L21 and L22 satisfy: 0.1 ≤ L21 / L22 ≤ 10. This helps to further improve the battery's energy density, cycle performance, and safety performance.

[0054] In some implementations, the first region includes: Li a1 Ni x1 Coy1 Mn 1-x1-y1 M1 z1 O2, 0.7≤a1≤1.1, 0.6≤x1≤0.97, 0≤z1≤0.01, and / or, the second sub-region includes: Li a2 Ni x2 Co y2 Mn 1-x2- y2 M2 z3 O2, 0.7≤a2≤1.1, 0.7≤x2≤0.97, 0≤z2≤0.01, and / or, the first sub-region includes: Li a3 Ni x3 Co y3 Mn 1-x3-y3 M3 z3 O2, 0.7≤a3≤1.1, 0.7≤x3≤0.97, 0≤z3≤0.01, wherein M1, M2, and M3 each independently include one or more of Mg, Al, Ca, Ti, V, Cr, Fe, Cu, Zr, Nb, W, and Sr. This is beneficial for further improving the energy density of secondary batteries.

[0055] A second aspect of this application provides a positive electrode sheet extending along a first direction, wherein the length of the positive electrode sheet in the first direction is greater than its width in a second direction, and the second direction is perpendicular to the first direction; and the positive electrode sheet includes a positive current collector and a positive electrode film layer disposed on at least one side surface of the positive current collector, the positive electrode film layer including a first region and a second region located on both sides of the first region in the second direction; the capacitance per unit area of ​​the first region is less than the capacitance per unit area of ​​the second region.

[0056] In some embodiments, the width of the positive electrode film layer in the second direction is L, the width of the first region in the second direction is L1, and the width of the second region in the second direction is L2, wherein L and L1 satisfy the following relationship: 10% ≤ L1 / L ≤ 40%.

[0057] In some implementations, L and L2 satisfy the following relationship: 60% ≤ L2 / L ≤ 90%.

[0058] In some implementations, the ratio of the capacitance per unit area of ​​the first region to the capacitance per unit area of ​​the second region is 60% to 98%.

[0059] In some embodiments, the capacitance per unit area of ​​the first region is 15mAh / 1540.25mm². 2 Up to 98mAh / 1540.25mm 2 ,

[0060] In some embodiments, the capacitance per unit area of ​​the second region is 25mAh / 1540.25mm². 2 Up to 100mAh / 1540.25mm 2 .

[0061] A third aspect of this application provides an electrical device comprising a secondary battery as described in the first aspect of this application, or a positive electrode as described in the second aspect. Therefore, it possesses at least the same advantages as the secondary battery. Attached Figure Description

[0062] Figure 1 This is a schematic diagram of the positive electrode sheet of one embodiment of this application on the XY plane.

[0063] Figure 2 This is a schematic diagram of the positive electrode sheet of one embodiment of this application on the YZ plane.

[0064] Figure 3 This is a schematic diagram of the positive electrode sheet in the YZ plane according to another embodiment of this application.

[0065] Figure 4 This is a schematic diagram of the negative electrode sheet in the YZ plane according to another embodiment of this application.

[0066] Figure 5 This is a schematic diagram of a battery cell according to one embodiment of this application.

[0067] Figure 6 yes Figure 5 An exploded view of a battery cell according to one embodiment of this application is shown.

[0068] Figure 7 This is a schematic diagram of a battery module according to one embodiment of this application.

[0069] Figure 8 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0070] Figure 9 yes Figure 8 An exploded view of a battery pack according to one embodiment of this application is shown.

[0071] Figure 10 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application.

[0072] Explanation of reference numerals in the attached figures:

[0073] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Battery cell; 51 Housing; 52 Electrode assembly; 53 Top cover assembly; 10 Positive electrode sheet; 11 Positive current collector; 12 Positive film layer; 121 First region; 122 Second region; 1221 First sub-region; 1222 Second sub-region; 20 Negative electrode sheet; 21 Negative current collector; 22 Negative film layer; 221 First film layer; 222 Second film layer. Detailed Implementation

[0074] The following detailed description, with appropriate reference to the accompanying drawings, specifically discloses embodiments of the positive electrode, secondary battery, and power-consuming device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions 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.

[0075] 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.

[0076] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0077] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0078] 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.

[0079] Silicon-carbon composites possess high specific capacity, and the industry has begun to improve the energy density of secondary batteries by incorporating them into the negative electrode. However, silicon-carbon composites suffer from high expansion force. Excessive expansion force can restrict electrolyte wetting, leading to lithium plating on the negative electrode and shortening the battery's cycle life. Furthermore, excessive expansion force may cause cracks in the casing welds, resulting in leakage and affecting the battery's safety performance.

[0080] In view of the above, this application proposes a positive electrode, a secondary battery, and an electrical device. This secondary battery achieves a balance between energy density and cycle performance. The invention and its optional embodiments are described in more detail below.

[0081] Secondary batteries

[0082] This application provides a secondary battery, including a housing and an electrode assembly housed inside the housing. The electrode assembly includes a positive electrode and a negative electrode. The positive electrode extends along a first direction, and its length in the first direction is greater than its width in a second direction, which is perpendicular to the first direction. The negative electrode includes a negative current collector and a negative electrode film disposed on at least one surface of the negative current collector, the negative electrode film comprising silicon. The positive electrode includes a positive current collector and a positive electrode film disposed on at least one surface of the positive current collector, the positive electrode film including a first region and a second region located on both sides of the first region in the second direction. The capacitance per unit area of ​​the first region is less than the capacitance per unit area of ​​the second region.

[0083] The inventors conducted expansion force tests on a large number of silicon anode batteries and found that the expansion force of the silicon anode battery during charging is not linearly correlated with its state of charge (SOC). The expansion force increases rapidly when the SOC reaches above 80%, resulting in a significant increase in the expansion force of the silicon anode battery. It is speculated that continuous lithium intercalation into the anode sheet triggers the formation of Li₂ from lithium and silicon. x Si yA phase transition occurs, resulting in a significant increase in the expansion force of the electrode. The inventors further disassembled silicon anode batteries that had reached the end of their cycle life (EOL) and observed the anode electrodes. They found that lithium plating was particularly severe in the middle region of these anode electrodes. They speculated that the large expansion force in the middle region of the anode electrodes caused the electrolyte in that region to be squeezed out, resulting in more severe lithium plating and deteriorating the cycle performance of the battery.

[0084] Based on the above findings, this application proposes a secondary battery that improves energy density by adding silicon to the negative electrode film. Furthermore, by making the capacity per unit area of ​​the first region (middle region) in the positive electrode film lower than that of the second region (both ends), the number of lithium ions released from the first region is reduced, thereby decreasing the lithium ion insertion amount in the middle region of the negative electrode film and reducing volume expansion in the middle region, thus improving the cycle performance of the secondary battery. Therefore, the secondary battery of this application possesses both high energy density and high cycle performance.

[0085] See Figure 1 and Figure 2 In this application, the positive electrode 10 extends along a first direction, and the positive electrode can be divided into a first region 121 and a second region 122 located at both ends of the first region 121 in a second direction.

[0086] In this application, the capacitance per unit area of ​​the positive electrode film layer (first region, second region) is tested using methods known in the art. The positive electrode sheet to be tested can be a pre-prepared positive electrode sheet or a positive electrode sheet obtained by disassembling the battery. The latter will be used as an example to illustrate the testing process below. Specifically, the positive electrode sheet is obtained by disassembling the battery, and the positive electrode sheet is placed along the boundary between the first region and the second region (e.g., ...). Figure 1 Cut along the dotted lines in the diagram to obtain a positive electrode sheet with a central region and two positive electrode sheets with two end regions. Soak the positive electrode sheet with the central region in dimethylsilane (DMC) for 30 minutes, then punch it into a circular sheet with a unit area of ​​1540.25 mm². 2 The wafers were assembled into a lithium half-cell, and the cells were subjected to charge-discharge cycles at a constant current of 0.1C, and the capacity was tested. The test results were compared with those obtained from a 1540.25mm² lithium-ion battery. 2 The ratio of the capacitance per unit area in the first region is used. Similarly, the positive electrode sheets in both regions are soaked in DMC for 30 minutes and then cut into circular sheets of unit area. The circular sheets are assembled into a lithium half-cell, and the resulting lithium half-cell is subjected to charge-discharge cycles at a constant current of 0.1C. At the same time, the capacitance per unit area is measured, and the result is used as the capacitance per unit area in the second region.

[0087] In some embodiments, the electrode assembly is wound to form a wound structure, with the winding axis of the wound structure parallel to the second direction. The wound structure is compact and can accommodate more active materials (positive electrode active material and negative electrode active material) in the same space, which is beneficial to improving the energy density of the secondary battery.

[0088] In some embodiments, the tensile strength of the casing is 350 MPa or higher. High tensile strength of the casing may significantly restrict the expansion of the electrode assembly (especially the negative electrode). This restriction may exacerbate internal pressure, thereby affecting battery performance and safety. The electrode assembly of this application has low expansion force, resulting in minimal volume change during battery operation. Consequently, the electrode assembly exerts less pressure on the casing, helping to alleviate the restriction of the casing on the expansion of the electrode assembly. Using a tensile strength of 350 MPa or higher in conjunction with the casing further optimizes battery safety performance. For example, the tensile strength of the casing is a value between 350 MPa, 400 MPa, 500 MPa, 600 MPa, 700 MPa, 800 MPa, 900 MPa, 1000 MPa, 350 MPa, or any two of these values. Optionally, the casing material includes at least one of aluminum alloy, steel, copper alloy, titanium alloy, and nickel alloy.

[0089] In some embodiments, the volumetric energy density of the secondary battery is between 600 Wh / L and 850 Wh / L. This is beneficial for further improving the volumetric energy density of the secondary battery. For example, the volumetric energy density of the secondary battery is a value between 600 Wh / L, 700 Wh / L, 800 Wh / L, 850 Wh / L, or any two of these values. Optionally, the volumetric energy density of the secondary battery is between 700 Wh / L and 800 Wh / L.

[0090] Positive electrode sheet

[0091] like Figures 1 to 3 As shown, in this application, the positive electrode 10 includes a positive current collector 11 and a positive electrode film layer 12 disposed on at least one surface of the positive current collector 11. The length of the positive electrode 10 in the first direction X is greater than its length in the second direction Y.

[0092] In this application, the first direction can also be referred to as the extension direction of the positive electrode sheet or the coating direction of the positive electrode slurry during the preparation of the positive electrode sheet.

[0093] In this application, the second direction Y is perpendicular to the first direction X. In embodiments where the electrode assembly is a wound structure, the second direction Y is parallel to the winding axis of the wound structure.

[0094] In some embodiments, the positive current collector has two surfaces opposite each other in its own thickness direction (third direction), and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0095] like Figures 1 to 3 As shown, the positive electrode film layer 12 includes a first region 121 and a second region 122 located on both sides of the first region 121 in a second direction.

[0096] like Figures 1 to 3 In some embodiments, the width of the positive electrode film 10 in the second direction is L, and the width of the first region 121 in the second direction is L1, where L and L1 satisfy the following relationship: 10% ≤ L1 / L ≤ 40%. By keeping L1 / L within the above range, it is beneficial to balance the energy density and cycle performance of the battery. Exemplarily, L1 / L is a value between 10%, 20%, 30%, 40%, or any two of these values. Optionally, L1 / L is 20% to 30%.

[0097] See also Figure 2 The width of the second region 122 on one side in the second direction is L2a, and the width of the second region 122 on the other side in the second direction is L2b. The total width of the second regions 122 in the second direction is L2, where L2 = L2a + L2b. L and L2 satisfy the following relationship: 60% ≤ L2 / L ≤ 90%. By keeping L2 / L within the above range, it is beneficial to balance the energy density and cycle performance of the battery. For example, L2 / L is a value between 60%, 70%, 80%, 90%, or any two of these values. Optionally, L2 / L is 60% to 70%.

[0098] In some embodiments, the ratio of the capacitance per unit area of ​​the first region to the capacitance per unit area of ​​the second region is 60% to 98%. By keeping the ratio of the capacitance per unit area of ​​the first region to the capacitance per unit area of ​​the second region within this range, it is advantageous to balance the battery's energy density and cycle performance. Exemplarily, the ratio of the capacitance per unit area of ​​the first region to the capacitance per unit area of ​​the second region is a value between 60%, 70%, 80%, 90%, 98%, or any two of these values. Optionally, the ratio of the capacitance per unit area of ​​the first region to the capacitance per unit area of ​​the second region is 70% to 85%.

[0099] In some implementations, the capacitance per unit area of ​​the first region is 15mAh / 1540.25mm². 2 Up to 98mAh / 1540.25mm 2The capacitance per unit area of ​​the first region falls within the aforementioned range. This, on the one hand, helps reduce volume expansion in the middle region of the negative electrode, thereby improving the cycle performance and safety of the secondary battery. On the other hand, it facilitates achieving high energy density in the secondary battery. For example, the capacitance per unit area of ​​the first region is 15mAh / 1540.25mm². 2 20mAh / 1540.25mm 2 30mAh / 1540.25mm 2 40mAh / 1540.25mm 2 50mAh / 1540.25mm 2 60mAh / 1540.25mm 2 70mAh / 1540.25mm 2 80mAh / 1540.25mm 2 90mAh / 1540.25mm 2 98mAh / 1540.25mm 2 Or a value between any two values ​​within a range. Optionally, the capacitance per unit area of ​​the first region is 15mAh / 1540.25mm². 2 Up to 68.4mAh / 1540.25mm 2 .

[0100] In some implementations, the capacitance per unit area of ​​the second region is 25mAh / 1540.25mm². 2 Up to 100mAh / 1540.25mm 2 The capacitance per unit area of ​​the second region falls within the aforementioned range. This moderate capacitance per unit area is beneficial in two ways: firstly, it reduces volume expansion in the central area of ​​the negative electrode, improving the cycle performance and safety of the secondary battery; secondly, it facilitates achieving high energy density in the secondary battery. For example, the capacitance per unit area of ​​the second region is 25mAh / 1540.25mm². 2 30mAh / 1540.25mm 2 40mAh / 1540.25mm 2 50mAh / 1540.25mm 2 60mAh / 1540.25mm 2 70mAh / 1540.25mm 2 80mAh / 1540.25mm 2 90mAh / 1540.25mm 2 100mAh / 1540.25mm 2Or a value between any two values ​​within a range. Optionally, the capacitance per unit area of ​​the second region is 25mAh / 1540.25mm². 2 Up to 77mAh / 1540.25mm 2 .

[0101] In some implementations, the thickness of the first region is less than or equal to the thickness of the second region. This allows the first region to achieve a low capacity per unit area, reducing the number of lithium ions released per unit area during charging and discharging. This helps reduce volume expansion in the middle region of the negative electrode, thereby improving the battery's cycle performance.

[0102] In some embodiments, the thickness of the single-layer first region is 0.005 mm to 0.15 mm. By keeping the thickness of the single-layer first region within this range, it is beneficial to reduce the volume expansion of the middle region of the negative electrode sheet and to achieve a high energy density in the secondary battery. For example, the thickness of the single-layer first region is 0.005 mm, 0.06 mm, 0.08 mm, 0.1 mm, 0.15 mm, or any value within a range of two such values. Optionally, the thickness of the single-layer first region is 0.02 mm to 0.1 mm.

[0103] In some embodiments, the thickness of the monolayer second region is from 0.0185 mm to 0.155 mm. By keeping the thickness of the monolayer second region within the above range, the number of lithium ions released per unit area of ​​the second region is moderate, which is beneficial for reducing the volume expansion of the middle region of the negative electrode sheet and for achieving a high energy density in the secondary battery. For example, the thickness of the monolayer second region is 0.0185 mm, 0.02 mm, 0.04 mm, 0.06 mm, 0.08 mm, 0.1 mm, 0.155 mm, or a value within a range of any two of these values. Optionally, the thickness of the monolayer second region is from 0.12 mm to 0.141 mm.

[0104] In this application, the thickness of the positive electrode film layer (first region, second region) has a meaning known in the art and can be tested using methods known in the art, such as using a micrometer (e.g., Mitutoyo 293-100 type, with an accuracy of 0.1 μm).

[0105] In some implementations, the compaction density of the second region is greater than or equal to that of the first region. This facilitates a higher compaction density in the second region, allowing it to hold more lithium ions per unit volume, thereby increasing the energy density of the secondary battery.

[0106] In some embodiments, the compaction density of the first region is 3.0 g / cm³. 3Up to 4.0 g / cm 3 By ensuring the compaction density of the first region is within the aforementioned range, it is beneficial to improve the energy density of the secondary battery. For example, the compaction density of the first region is 3 g / cm³. 3 3.3g / cm 3 3.4g / cm 3 3.5g / cm 3 3.6g / cm 3 3.7g / cm 3 3.8g / cm 3 3.9g / cm 3 4g / cm 3 Or the value between any two values ​​within a range.

[0107] In some embodiments, the compaction density of the second region is 3.3 g / cm³. 3 Up to 4.4 g / cm 3 By maintaining the compaction density of the second region within the aforementioned range, it is beneficial to improve the energy density of the secondary battery. For example, the compaction density of the second region is 3.3 g / cm³. 3 3.4g / cm 3 3.5g / cm 3 3.6g / cm 3 3.7g / cm 3 3.8g / cm 3 3.9g / cm 3 4g / cm 3 4.4 g / cm 3 Or the value between any two values ​​within a range.

[0108] In some implementations, the second region includes a lithium replenishing agent. By adding a lithium replenishing agent to the second region, the number of lithium ions released per unit area of ​​the second region can be increased, thereby improving the energy density of the secondary battery.

[0109] In some embodiments, the mass percentage of the lithium replenishing agent in the second region is 3% or less. By keeping the mass percentage of the lithium replenishing agent in the second region within the aforementioned range, it is beneficial to improve the energy density of the secondary battery while maintaining low production costs. For example, the mass percentage of the lithium replenishing agent in the second region is 0%, 1%, 2%, 3%, or a value within a range of any two of these values. Optionally, the mass percentage of the lithium replenishing agent in the second region is 1% to 3%.

[0110] In some embodiments, the positive electrode further includes a lithium strip located on the side of at least one second region facing away from the positive current collector. By providing the lithium strip on the side of the second region facing away from the positive current collector, the number of lithium ions released per unit area of ​​the second region can be increased, thereby improving the energy density of the secondary battery.

[0111] In some embodiments, both the first region and the second region include positive electrode active material, and the areal density of the positive electrode active material in the first region is less than or equal to the areal density of the positive electrode active material in the second region. This helps to reduce the number of lithium ions released per unit area in the first region, thereby reducing the volume expansion in the middle region of the negative electrode and improving the cycle performance of the battery.

[0112] In some embodiments, the areal density of the positive electrode active material in the first region is 0.15 g / 1540.25 mm². 2 Up to 0.4g / 1540.25mm 2 By ensuring the areal density of the positive electrode active material in the first region is within the aforementioned range, the number of lithium ions released per unit area in the second region can be reduced, thereby reducing the volume expansion in the middle region of the negative electrode and improving the battery's cycle performance. For example, the areal density of the positive electrode active material in the first region is 0.15 g / 1540.25 mm². 2 0.2g / 1540.25mm 2 0.3g / 1540.25mm 2 0.4g / 1540.25mm 2 Or the value between any two values ​​within a range.

[0113] In some embodiments, the areal density of the positive electrode active material in the second region is 0.18 g / 1540.25 mm². 2 Up to 0.4g / 1540.25mm 2 The areal density of the positive electrode active material in the second region is within the aforementioned range, which increases the number of lithium ions released per unit area in the second region, thereby improving the energy density of the secondary battery. For example, the areal density of the positive electrode active material in the second region is 0.18 g / 1540.25 mm². 2 0.2g / 1540.25mm 2 0.3g / 1540.25mm 2 0.4g / 1540.25mm 2 Or a value within a range formed by any two values. Optionally, the areal density of the positive electrode active material in the second region is 0.2 g / 1540.25 mm². 2 Up to 0.4g / 1540.25mm 2 .

[0114] In some embodiments, the coating weight per unit area of ​​the positive electrode active material in the first region is less than or equal to the coating weight per unit area of ​​the positive electrode active material in the second region. This design reduces the number of lithium ions released per unit area in the first region, which helps reduce volume expansion in the middle region of the negative electrode, thereby improving the battery's cycle performance and safety.

[0115] In some embodiments, the coating weight per unit area of ​​the positive electrode active material in the first region is 0.15 g / 1540.25 mm. 2 Up to 0.4g / 1540.25mm 2 By controlling the coating weight per unit area of ​​the positive electrode active material in the first region within the aforementioned range, the number of lithium ions released per unit area in the first region can be reduced. This helps to reduce the volume expansion in the middle region of the negative electrode, thereby improving the cycle performance and safety performance of the battery.

[0116] In some embodiments, the coating mass per unit area of ​​the second region is 0.18 g / 1540.25 mm. 2 Up to 0.4g / 1540.25mm 2 By controlling the coating weight per unit area of ​​the positive electrode active material in the second region within the aforementioned range, the number of lithium ions released per unit area in the second region can be increased, which helps to improve the energy density of the battery.

[0117] In some implementations, the porosity of the first region is greater than that of the second region. This allows the first region to achieve a lower capacity per unit area, reducing the number of lithium ions released per unit area during charge and discharge. This helps reduce volume expansion in the middle region of the negative electrode, thereby improving the battery's cycle performance.

[0118] In some embodiments, both the first and second regions include a lithium transition metal oxide, which comprises nickel and lithium, wherein the molar percentage of nickel is 50% or more based on the total molar amount of elements other than lithium in the lithium transition metal oxide. Since lithium transition metal oxides have high capacity, using them as the positive electrode active material is beneficial for further improving the energy density of the secondary battery. Exemplarily, the molar percentage of nickel is 50%, 60%, 70%, 80%, 90%, 98%, or any value within a range of two such values. Optionally, the molar percentage of nickel is 60% or more. Further, the molar percentage of nickel is 80% or more. Even more preferably, the molar percentage of nickel is 90% or more.

[0119] In some implementations, the molar percentage of nickel in the first region is less than or equal to the molar percentage of nickel in the second region. A higher molar percentage of nickel results in a greater number of lithium ions released per unit area of ​​the positive electrode film, and vice versa. By ensuring that the molar percentage of nickel in the first region is less than or equal to that in the second region, it is possible to reduce the number of lithium ions released per unit area in the first region, thereby reducing volume expansion in the middle region of the negative electrode and reducing stress between the first and second regions, thus improving the battery's cycle performance and safety. Conversely, it is also possible to increase the number of lithium ions released per unit area in the first region, thereby increasing the battery's energy density.

[0120] In some embodiments, the molar percentage of nickel in the first region is between 0.6 and 0.97. By keeping the molar percentage of nickel in the first region within the above range, it is beneficial to further improve the energy density of the secondary battery.

[0121] In some embodiments, the molar percentage of nickel in the second region is between 0.7 and 0.97. By keeping the molar percentage of nickel in the second region within the above range, it is beneficial to further improve the energy density of the secondary battery.

[0122] In some embodiments, at least one second region includes N sub-regions arranged sequentially in a second direction, and the capacitance per unit area of ​​the N sub-regions increases sequentially in the direction away from the first region. By setting N sub-regions with sequentially increasing capacitance per unit area in the second region, the number of lithium ions released in the second region changes gradient, causing the volume expansion of the negative electrode sheet to also show a gradient change, thereby effectively reducing stress concentration on the negative electrode sheet and improving the cycle performance and safety performance of the battery.

[0123] See Figure 3 In some embodiments, the N sub-regions include a first sub-region 1221 and a second sub-region 1222, with the second sub-region 1222 located between the first sub-region 1221 and the first sub-region 1222. The capacity per unit area of ​​the second sub-region is less than that of the first sub-region. By controlling the capacity per unit area of ​​the second sub-region to be less than that of the first sub-region, a gradient in the number of lithium ions released within the second and first sub-regions is achieved. This helps reduce stress concentration on the negative electrode, thereby improving the battery's cycle performance and safety performance.

[0124] In some implementations, the capacitance per unit area of ​​the first sub-region is 54 / 1540.25 mm². 2 Up to 100mAh / 1540.25mm 2By controlling the capacity per unit area of ​​the first sub-region within the aforementioned range, the number of lithium ions released per unit area of ​​the first sub-region is moderate. This is beneficial for the battery to achieve high energy density on the one hand, and for reducing stress concentration on the negative electrode sheet on the other hand, thereby improving the battery's cycle performance and safety performance.

[0125] In some implementations, the capacitance per unit area of ​​the second sub-region is 25 / 1540.25 mm². 2 Up to 70mAh / 1540.25mm 2 By controlling the capacitance per unit area of ​​the second sub-region within the aforementioned range, it is beneficial for the battery to achieve high energy density and for reducing stress concentration on the negative electrode, thereby improving the battery's cycle performance and safety performance.

[0126] In some implementations, the width of the first sub-region in the second direction is L21, and the width of the second sub-region in the second direction is L22, where L21 and L22 satisfy: 0.1 ≤ L21 / L22 ≤ 10. By keeping L21 / L22 within the above range, it is helpful to further improve the energy density, cycle performance, and safety performance of the battery.

[0127] In some implementations, the first region includes: Li a1 Ni x1 Co y1 Mn 1-x1-y1 M1 z1 O2, 0.7≤a1≤1.1, 0.6≤x1≤0.97, 0≤z1≤0.01, wherein M1, M2, and M3 each independently include one or more of Mg, Al, Ca, Ti, V, Cr, Fe, Cu, Zr, Nb, W, and Sr. In some embodiments, the second sub-region includes: Li a2 Ni x2 Co y2 Mn 1-x2-y2 M2 z3 O2, 0.7≤a2≤1.1, 0.7≤x2≤0.97, 0≤z2≤0.01. In some implementations, the first sub-region includes: Li a3 Ni x3 Co y3 Mn 1-x3-y3 M3 z3 O2, 0.7≤a3≤1.1, 0.7≤x3≤0.97, 0≤z3≤0.01. This is beneficial for further improving the energy density of secondary batteries.

[0128] 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.).

[0129] In some embodiments, when the secondary battery is a secondary battery, the positive electrode active material may be a positive electrode active material known in the art for use in secondary 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 battery positive electrode active materials 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 NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.1 Al 0.05At least one of O2 and its modified compounds. Examples of lithium 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 manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0130] During the charging and discharging process of a battery, Li undergoes insertion / extraction and consumption, resulting in varying molar Li content at different discharge states. In the examples of positive electrode active materials in this application, the molar Li content refers to the initial state of the material, i.e., before feeding. When the positive electrode active material is applied to the battery system, the molar Li content changes after charge-discharge cycles.

[0131] In the examples of positive electrode active materials in this application, the molar content of oxygen is only a theoretical value. Oxygen release from the crystal lattice will cause changes in the molar content of oxygen, and the actual molar content of oxygen will fluctuate.

[0132] In some embodiments, the positive electrode film layer may optionally include 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.

[0133] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers.

[0134] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0135] Negative electrode sheet

[0136] See Figure 4 In this application, the negative electrode 20 includes a negative current collector 21 and a negative electrode film 22 disposed on at least one surface of the negative current collector 21, the negative electrode film including silicon.

[0137] In some embodiments, the mass percentage of silicon in the negative electrode film is between 3% and 70%. Maintaining the mass percentage of silicon in the negative electrode film within this range is beneficial for further improving the energy density of the secondary battery. For example, the mass percentage of silicon in the negative electrode film is 3%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, or any value between two of these.

[0138] 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.

[0139] 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 material substrate and a metal layer formed on at least one surface of the polymer material 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 material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0140] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include a silicon-based material, comprising one or more of elemental silicon, silicon-oxygen composites, and silicon-carbon composites; optionally, the silicon-based material may include at least a silicon-carbon composite. This helps to improve the charging capability of the battery cell.

[0141] In some embodiments, the silicon-oxygen complex includes at least one of unpre-lithium silicon-oxygen compound, pre-lithium silicon-oxygen compound, unpre-magnesium silicon-oxygen compound, and pre-magnesium silicon-oxygen compound.

[0142] The silicon-carbon composite of this application can be prepared using conventional silicon-carbon composites or conventional preparation methods, such as depositing nano-silicon materials on porous carbon by chemical vapor deposition, and further carbon coating, such as using amorphous carbon coating.

[0143] In some embodiments, the silicon-carbon composite includes porous carbon and silicon-containing material dispersed in the pores of the porous carbon. The porous carbon, acting as a carrier for the silicon-containing material, provides support while its pores offer expansion space, effectively mitigating stress caused by expansion during charging. Especially when the silicon-containing material has a nanometer-scale particle size, it exhibits higher specific capacity and is better dispersed within the pores of the porous carbon. Furthermore, it allows for more efficient utilization of the porous carbon's buffering effect on expansion. When this silicon-carbon composite is used in wound electrode assemblies, it can significantly alleviate the stretching of the outer negative electrode sheet caused by silicon expansion.

[0144] In some embodiments, the porous carbon may optionally be hard carbon. When the porous carbon is hard carbon, it has stronger support, a more stable pore structure, and is harder, thus providing better porosity for the negative electrode active layer, providing a smoother path for active ion transport, and improving the charging capability of the secondary battery.

[0145] In some embodiments, the silicon-containing material includes at least one of elemental silicon, silicon oxides, silicon nitrides, and silicon alloys. In some embodiments, the silicon-containing material includes crystalline silicon, thereby further improving the structural stability of the silicon-containing material and the energy density of the secondary battery.

[0146] In some embodiments, the silicon-carbon composite further includes a carbon-containing coating layer located on the surface of porous carbon and / or silicon-containing materials. This can improve the conductivity of the silicon-carbon composite, reduce the internal impedance of the secondary battery, and effectively reduce the probability of direct contact between the silicon-containing materials in the porous carbon channels and the external environment, thereby improving the chemical stability of the silicon-carbon composite.

[0147] In some embodiments, the silicon content in the silicon-carbon composite is 30% to 70% by mass. This approach, while maximizing the specific capacity of the negative electrode active material by utilizing silicon, also facilitates the full dispersion of silicon in the carbon-containing porous material and helps control the expansion of silicon during charging.

[0148] In this application, the method for testing the silicon content in the silicon-carbon composite can be a method known in the art. As an example, the following method can be used for testing: a certain amount of silicon-carbon composite is taken, and the mass of silicon element in the silicon-carbon composite is obtained by inductively coupled plasma optical emission spectrometry (ICP-OES). The mass percentage of silicon element in the silicon-carbon composite can be calculated.

[0149] In addition to providing structural support and buffering for the expansion of silicon materials, the pores in the silicon-carbon composite also form between the particles. To further improve the flow of lithium ions through the intraparticle and interparticle pores, in some embodiments, the average particle size of the silicon-carbon composite is 2 μm-15 μm. Optionally, the average particle size of the silicon-carbon composite is 7 μm-11 μm, or 5 μm-10 μm. This creates a particle size distribution between the average particle size of the silicon-carbon composite and the average particle size of the graphite material, which is more conducive to increasing the compaction of the negative electrode active layer by utilizing the interparticle gaps, thereby further improving the energy density of the secondary battery.

[0150] The number-average particle size of the aforementioned silicon-carbon composite can be tested using equipment and methods known in the art. For example, a scanning electron microscope (SEM) (e.g., ZEISS Sigma 300) can be used, referring to JY / T010-1996, to obtain SEM images of the negative electrode sheet. As an example, the test can be performed as follows: Randomly select a test sample of length × width = 50 mm × 100 mm on the negative electrode sheet. Randomly select multiple test areas (e.g., 5 areas) within the test sample, and at a certain magnification (e.g., 1000x when measuring silicon-carbon composites), read the particle size of each silicon-carbon composite particle in each test area (i.e., take the distance between the two farthest points on the silicon-carbon composite particle as the particle size). Count the number and particle size values ​​of silicon-carbon composite particles in each test area, and take the arithmetic mean of the silicon-carbon composite particles in each test area, which is the number-average particle size of the silicon-carbon composite particles in the test sample. To ensure the accuracy of the test results, multiple test samples (e.g., 10) can be taken and the above test can be repeated. The average value of each test sample can be taken as the final test result.

[0151] In some embodiments, the powder resistivity of the silicon-carbon composite at 8 MPa is 4 Ω·cm to 17 Ω·cm. Exemplarily, it is a value within a range of 4 Ω·cm, 5 Ω·cm, 6 Ω·cm, 7 Ω·cm, 8 Ω·cm, 9 Ω·cm, 10 Ω·cm, 11 Ω·cm, 12 Ω·cm, 13 Ω·cm, 14 Ω·cm, 15 Ω·cm, 16 Ω·cm, 17 Ω·cm, or any combination thereof. By controlling the powder resistivity as described above, the conductivity of the silicon-carbon composite is improved, thereby increasing the charging rate of the secondary battery.

[0152] In this application, the powder resistivity of silicon-carbon composites can be determined using methods known in the art. As an example, a four-probe method can be used, where two probes apply voltage and the other two probes measure current. The powder resistivity can be calculated by measuring the resistance value. Models of four-probe semiconductor powder resistivity testers include the ST-2722.

[0153] In some embodiments, the BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g-6.7m 2 / g.

[0154] In this application, the method for testing the BET specific surface area of ​​the silicon-carbon composite can be a method known in the art. As an example, referring to GB / T 19587-2017, a nitrogen adsorption specific surface area analysis method can be used. The sample tube containing the first graphite material sample is immersed in liquid nitrogen at -196℃, and the amount of nitrogen adsorbed on the surface of the solid sample at different pressures of 0.05 to 0.30 is measured. Based on the BET multilayer adsorption theory and calculation formula, the amount of monolayer adsorption of the sample is obtained, and thus the BET specific surface area is obtained. This test can be performed using a Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, USA.

[0155] Please continue reading Figure 4 In some embodiments, the negative electrode film 22 includes a first film layer 221 and a second film layer 222 stacked on the side of the first film layer 221 facing away from the negative electrode current collector. The mass percentage of silicon in the second film layer is greater than that in the first film layer. By designing the negative electrode film as a first film layer and a second film layer with different mass percentages of silicon, more silicon can be concentrated in the second film layer when the total amount of silicon is constant. Due to the volume expansion characteristics of silicon, the second film layer with a higher silicon content can provide more transport channels for ion transport. The second film layer is located on top of the anode electrode, and the lithium intercalation potential of the silicon particles is higher, preferably for lithium intercalation. Placing silicon on top can improve the lithium ion migration path at low SOC, thereby enhancing the fast charging performance of the battery.

[0156] In this application, the morphology of the negative electrode film can be tested using methods known in the art. The negative electrode film to be tested can be a prepared negative electrode film or a negative electrode film obtained by disassembling the battery. The latter will be used as an example to illustrate the testing process below. Specifically, the negative electrode sheet is obtained by disassembling the battery, and then placed and locked in a sample holder. An argon ion cross-section polisher (such as the IB-09010CP argon ion cross-section polisher from JEOL Corporation of Japan) is used to cut a cross-section of the negative electrode sheet. A scanning electron microscope (HR-TEM Talos F200) is used to acquire a SEM image of the cross-section of the negative electrode sheet. From the cross-section SEM image, it can be seen that the negative electrode film includes a first film layer and a second film layer stacked together.

[0157] In some embodiments, the mass percentage of silicon in the first film layer is between 0% and 20%. A mass percentage of silicon in the first film layer within this range indicates that the mass percentage of silicon in the first film layer is controlled at a low level. When the total amount of silicon is constant, the silicon is mainly concentrated in the second film layer. This is beneficial for increasing the ion transport channels within the second film layer, thereby further improving the fast-charging performance of the secondary battery. For example, the mass percentage of silicon in the first film layer is 0%, 0.5%, 1%, 1.5%, 2%, 2.5%, 10%, 20%, or a value within a range of any two values.

[0158] In some embodiments, the mass percentage of silicon in the second film layer is between 5% and 70%. A mass percentage of silicon in the second film layer within this range indicates a high mass percentage, which is beneficial for increasing ion transport channels within the second film layer, thereby further improving the fast-charging performance of the secondary battery. Exemplarily, the mass percentage of silicon in the second film layer is 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, or a value within a range of any two of these values. Optionally, the mass percentage of silicon in the second film layer is between 20% and 70%.

[0159] In some embodiments, the negative electrode active material further includes artificial graphite, natural graphite, soft carbon, hard carbon, silicon-carbon composites, tin-based materials, and lithium titanate, etc. The silicon-carbon composite is selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material is 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.

[0160] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may 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).

[0161] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0162] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0163] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0164] electrolytes

[0165] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0166] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0167] In some embodiments, 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.

[0168] In some embodiments, 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.

[0169] In some embodiments, 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.

[0170] Separating membrane

[0171] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0172] In some embodiments, 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.

[0173] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0174] In some embodiments, the battery cell may include an outer packaging (casing). This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0175] In some embodiments, the outer packaging of the battery cell can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the battery cell can also be a flexible package, such as a pouch. The material of the flexible package can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0176] This application does not impose any particular limitation on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 5 The example shown is a square-structured battery cell 5.

[0177] In some implementations, refer to Figure 6 The outer packaging may include a housing 51 and a top cover assembly 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can cover the opening to close the receiving cavity. The positive electrode sheet, negative electrode sheet, and separator can be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the battery cell 5 can be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0178] In some implementations, individual battery cells can be assembled into a battery module. The number of individual battery cells contained in a battery module can be one or more, and the specific number can be selected by those skilled in the art based on the application and capacity of the battery module.

[0179] Figure 7 This is battery module 4, used as an example. (See reference...) Figure 7 In battery module 4, multiple battery cells 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple battery cells 5 can be fixed in place using fasteners.

[0180] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0181] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0182] Figure 8 and Figure 9 This is battery pack 1 as an example. (See reference...) Figure 8 and Figure 9 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0183] Electrical appliances

[0184] In addition, this application also provides an electrical device, which includes the secondary battery provided in this application. The secondary battery can be used as the power source of the electrical device or as the energy storage unit of the electrical device. The electrical device may include, but is not limited to, mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0185] As an electrical device, you can choose individual battery cells, battery modules, or battery packs according to your usage requirements.

[0186] Figure 10 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0187] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a single battery cell as their power source.

[0188] Example

[0189] 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.

[0190] Example 1

[0191] (1) Preparation of positive electrode sheet

[0192] ① Preparation of positive electrode slurry:

[0193] The positive electrode active material (LiNi) 0.8 Co 0.1 Mn 0.1 O2), conductive agent (acetylene black), and binder (polyvinylidene fluoride) are added to the solvent N-methylpyrrolidone (NMP) in a mass ratio of 97:2:1 and stirred evenly to prepare the first positive electrode slurry.

[0194] The positive electrode active material (LiNi) 0.9 Co 0.05 Mn 0.05 O2), conductive agent (acetylene black), and binder (polyvinylidene fluoride) are added to the solvent N-methylpyrrolidone (NMP) in a mass ratio of 97:2:1 and stirred evenly to prepare the second positive electrode slurry.

[0195] ② Coating:

[0196] Two extrusion heads were used to uniformly coat the first and second positive electrode slurries onto the two sides of a 13 μm thick positive electrode current collector (aluminum foil) in separate areas. After drying and cold pressing, the negative electrode sheet was obtained. The coating weight of the positive electrode active material in the first positive electrode slurry was 0.3 g / 1540.25 mm. 2 The coating thickness is 0.12 mm, and the coating width of the first positive electrode slurry is 30 mm. The coating weight of the positive electrode active material in the second positive electrode slurry is 0.3 g / 1540.25 mm. 2 The coating thickness is 0.12 mm, and the coating width of the second positive electrode slurry on one side is 35 mm.

[0197] A first positive electrode slurry forms a first region, and a second positive electrode slurry is coated on both sides of the first positive electrode slurry perpendicular to the coating direction to form a second region. The areal density of the positive electrode active material in both the first and second regions is 0.3 g / 1540.25 mm². 2 .

[0198] (2) Preparation of negative electrode sheet

[0199] A silicon-carbon composite (Si element content 50% by mass), graphite, conductive carbon, thickener (sodium carboxymethyl cellulose), and binder (styrene-butadiene rubber, SBR) were mixed with a certain amount of water at a mass ratio of 40:57:1:1:1 to prepare a negative electrode slurry. The negative electrode slurry was then uniformly coated onto both sides of a 6μm thick negative electrode current collector (copper foil). After drying in an oven, it was cold-pressed and die-cut to a width of 105mm to obtain the negative electrode sheet. The areal density of the negative electrode sheet was 0.12g / 1540.25mm². 2 .

[0200] (3) Preparation of the separating membrane

[0201] A 1μm thick polyethylene film was used as the base film, and a 1μm thick aluminum oxide film was coated on the base film to obtain the separator film.

[0202] (4) Preparation of electrolyte

[0203] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. LiPF6 was then dissolved in the organic solvent to prepare an electrolyte with a LiPF6 concentration of 1 mol / L.

[0204] (5) Preparation of secondary batteries

[0205] The prepared positive and negative electrode sheets are arranged in order, with the separator placed between them to provide isolation. The electrode assembly is then wound up. The electrode assembly is placed in an outer shell (tensile strength of 300 MPa), dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a secondary battery is obtained.

[0206] Parameter testing of positive electrode sheet

[0207] (1) Test of capacitance per unit area:

[0208] The positive electrode sheet is cut along the boundary between the first and second regions to obtain a positive electrode sheet with a middle region and two positive electrode sheets with end regions. The positive electrode sheet with the middle region is soaked in dimethylsilane (DMC) for 30 minutes and then punched into circular wafers with a unit area of ​​1540.25 mm². 2 The wafers were assembled into a lithium half-cell, and charged and discharged at a constant current of 0.1C, while the capacity was measured. The test results were compared with those of a 1540.25mm² lithium-ion battery. 2 The ratio is used as the unit area capacitance of the first region.

[0209] A positive electrode sheet with both ends was soaked in DMC for 30 minutes and then punched into circular wafers of unit area. These wafers were then assembled into a lithium-ion half-cell. The resulting lithium-ion half-cell was subjected to charge-discharge cycles at a constant current of 0.1C, and the capacity was measured simultaneously. The test results were compared with those obtained at 1540.25 mm². 2 The ratio is used as the unit area capacitance of the second region.

[0210] (2) Test of compaction density:

[0211] The electrode sheet is punched to a size of 1540.25 mm². 2 The discs were then measured and weighed. The thickness was recorded as H1 and the mass as W1. The active material on the electrodes was removed using a solvent (DMC solvent for the positive electrode and deionized water for the negative electrode). After removal, the thickness was measured and weighed. The thickness of the empty substrate was recorded as H2 and the mass as W2. The compaction density was calculated as (W1-W2) / ((H1-H2)*1540.25mm). 2 ).

[0212] Battery performance testing

[0213] (1) Test of expansion force:

[0214] ① Place the secondary battery in a steel plate clamp with a pressure sensor and let it stand for 5 minutes;

[0215] ② Charge at a constant current of 0.33C to 4.25V, then charge at a constant voltage of 4.25V to a current of 0.05C, and record the first full charge expansion force F1;

[0216] ③ Let stand for 5 minutes;

[0217] ④ Discharge to 2.5V with a constant current of 0.33C;

[0218] ⑤ Repeat steps ① to ④ above 100 times, and record the full-charge expansion force F2 for 100 cycles.

[0219] (2) Cyclic performance testing:

[0220] ① Let stand for 5 minutes;

[0221] ② Charge at a constant current of 0.33C to 4.25V, then charge at a constant voltage of 4.25V to a current of 0.05C;

[0222] ③ Let stand for 5 minutes;

[0223] ④ Discharge at a constant current of 0.33C to 2.5V, and record the discharge capacity D1 of the first cycle;

[0224] ⑤ Repeat steps ① to ④ above 300 times and record the discharge capacity Dn for 300 cycles;

[0225] 300-cycle capacity retention rate (%) = discharge capacity Dn of the 300th cycle / discharge capacity D1 of the first cycle * 100%.

[0226] (3) Volumetric energy density test:

[0227] ① At 25℃, the secondary battery is charged at a constant current of 0.33C to the charging cutoff voltage of 4.25V, and then charged at a constant voltage of 4.25V until the current is 0.05C. At this point, the secondary battery is fully charged. After the fully charged secondary battery is left to stand for 5 minutes, it is discharged at a constant current of 0.33C to 2.5V. The discharge capacity at this point is the actual capacity of the secondary battery at 0.33C, denoted as C0.

[0228] ②Then, the secondary battery is charged at a constant current of 0.33C0 to the cutoff voltage of 4.25V, and then charged at a constant voltage until the current reaches 0.04C. At this point, the secondary battery is fully charged. After the fully charged secondary battery is left to stand for 5 minutes, it is discharged at a constant current of 0.33C0 to 2.5V, and the discharge energy Q of the secondary battery is obtained. The volumetric energy density of the secondary battery (Wh / L) = discharge energy Q of the secondary battery / volume V of the secondary battery.

[0229] Example 2

[0230] The battery preparation method in Example 2 is similar to that in Example 1. The difference is that in the step of preparing the positive electrode sheet, the coating thickness of the first positive electrode slurry is adjusted to 0.11 mm.

[0231] The positive electrode of Example 2 was tested in the same manner as in Example 1, and the test results are recorded in Table 1-2.

[0232] The performance of the battery in Example 2 was tested in the same manner as in Example 1, and the test results are recorded in Tables 1-3.

[0233] Comparative Example 1

[0234] The battery preparation method of Comparative Example 1 is similar to that of Example 1. The difference is:

[0235] In the preparation of the positive electrode sheet: the negative electrode active material (graphite), conductive agent (conductive carbon), thickener (sodium carboxymethyl cellulose), and binder (SBR) are added to a certain amount of water in a mass ratio of 97:1:1:1, and stirred evenly to obtain a negative electrode slurry. The negative electrode slurry is then evenly coated on both sides of a 6μm thick negative electrode current collector (copper foil), dried in an oven, cold-pressed, and die-cut with a slit width of 105mm to obtain the negative electrode sheet. The areal density of the negative electrode sheet is 0.240g / 1540.25mm. 2 .

[0236] Comparative Example 2

[0237] The battery preparation method of Comparative Example 2 is similar to that of Example 1. The difference is:

[0238] In the process of preparing the positive electrode sheet: a first positive electrode slurry is coated on both sides of a 13μm thick positive electrode current collector (aluminum foil), and after drying and cold pressing, a positive electrode sheet is obtained. The coating width of the first positive electrode slurry is 100mm, and the coating thickness is 0.12mm.

[0239] Comparative Example 3

[0240] The battery preparation method of Comparative Example 3 is similar to that of Example 1. The difference is:

[0241] In the process of preparing the positive electrode sheet: two extrusion heads are used to uniformly coat the first and second positive electrode slurries onto the two sides of a 13μm thick positive electrode current collector (aluminum foil) in separate areas. After drying and cold pressing, the positive electrode sheet is obtained. The coating width of the second positive electrode slurry is 30mm, and the coating thickness is 0.12mm. The first positive electrode slurry is coated on both sides of the second positive electrode slurry in a second direction (perpendicular to the coating direction), with a coating width of 35mm and a coating thickness of 0.12mm on each side.

[0242] The second positive electrode slurry forms the first region, and the first positive electrode slurry is coated on both sides of the second positive electrode slurry to form the second region.

[0243] The positive electrode plates of Comparative Examples 1 to 3 were tested in the same manner as in Example 1, and the test results are recorded in Tables 1-2.

[0244] The performance of the batteries in Comparative Examples 1 to 3 was tested in the same manner as in Example 1, and the test results are recorded in Tables 1-3.

[0245] Table 1-1

[0246]

[0247] The “coating weight” refers to the coating weight per unit area of ​​the positive electrode active material.

[0248] Table 1-2

[0249]

[0250] Among them, "area density" refers to "the area density of the positive electrode active material".

[0251] Table 1-3

[0252]

[0253]

[0254] As can be seen from the data in Tables 1-1 to 1-3, compared with Comparative Example 1 (the negative electrode active material does not contain silicon), Comparative Example 2 (the positive electrode film layer is not divided into regions), and Comparative Example 3 (the unit area capacity of the positive electrode film layer in the middle region is greater than that in the two end regions), the batteries of Example 1 and Example 2 can achieve a balance of high energy density, low expansion force, and high cycle performance.

[0255] Example 3

[0256] The battery preparation method in Example 3 is similar to that in Example 1. The difference lies in the step of preparing the negative electrode sheet:

[0257] ① Preparation of negative electrode slurry

[0258] The first negative electrode slurry is prepared by adding a certain amount of water to graphite, conductive carbon, thickener (such as sodium carboxymethyl cellulose) and binder (SBR) in a mass ratio of 97:1:1:1 and stirring evenly.

[0259] The silicon-carbon composite, graphite, conductive carbon, thickener (such as sodium carboxymethyl cellulose), and binder (SBR) are added to a certain amount of water in a mass ratio of 25:72:1:1:1, and the mixture is stirred evenly to obtain the second negative electrode slurry.

[0260] ② Coating

[0261] The first negative electrode slurry and the second negative electrode slurry are extruded simultaneously using a dual-cavity coating device, wherein the mass ratio of the first negative electrode slurry to the second negative electrode slurry is 1:1. The first negative electrode slurry is coated on the negative electrode current collector to form a first film layer, and the second negative electrode slurry is coated on the side of the first negative electrode slurry away from the negative electrode current collector to form a second film layer. After drying and cold pressing, the negative electrode sheet is obtained.

[0262] Example 4

[0263] The battery preparation method in Example 4 is similar to that in Example 3. The difference lies in the step of preparing the negative electrode sheet:

[0264] ① Preparation of negative electrode slurry

[0265] The first negative electrode slurry is prepared by adding a certain amount of water to silicon-carbon composite (Si content is 20% by mass), graphite, conductive carbon, thickener (such as sodium carboxymethyl cellulose), and binder (SBR) in a mass ratio of 15:82:1:1:1 and stirring evenly.

[0266] A certain amount of water was added to a silicon-carbon composite (Si content of 50%), graphite, conductive carbon, thickener (such as sodium carboxymethyl cellulose), and binder (SBR) in a mass ratio of 40:57:1:1:1, and the mixture was stirred evenly to obtain the second negative electrode slurry.

[0267] Example 5

[0268] The battery preparation method in Example 5 is similar to that in Example 3. The difference lies in the step of preparing the negative electrode sheet:

[0269] ① Negative electrode slurry

[0270] The first negative electrode slurry was prepared by adding a certain amount of water to silicon-carbon composite (Si content of 57%), graphite, conductive carbon, thickener (such as sodium carboxymethyl cellulose), and binder (SBR) in a mass ratio of 35:62:1:1:1 and stirring evenly.

[0271] A silicon-carbon composite (Si content 70% by mass), conductive carbon, a thickener (such as sodium carboxymethyl cellulose), and a binder (SBR) were added to a certain amount of water at a mass ratio of 97:1:1:1. After uniform stirring, the second negative electrode slurry was prepared. The density ratio of the upper and lower layers was 3:7.

[0272] The performance of the batteries in Examples 3 to 5 was tested in the same manner as in Example 1, and the test results are recorded in Table 2.

[0273] Table 2

[0274]

[0275] The data in Table 2 shows that when the negative electrode film includes a first film layer and a second film layer, and the mass percentage of silicon in the second film layer is greater than that in the first film layer, the battery has low expansion force, and the battery can balance high energy density, low expansion force and high cycle performance.

[0276] Examples 6 to 9

[0277] The battery preparation methods in Examples 6 to 9 are similar to those in Example 3. The difference lies in the step of preparing the positive electrode sheet:

[0278] As shown in Table 3-1, adjust at least one of the coating width of the first positive electrode slurry, the coating thickness of the first positive electrode slurry, the coating width of the second positive electrode slurry, and the coating thickness of the second positive electrode slurry.

[0279] The performance of the first and second regions of Examples 6 to 9 was tested in the same manner as in Example 1, and the test results are recorded in Table 3-2.

[0280] The performance of the batteries in Examples 6 to 9 was tested in the same manner as in Example 1, and the test results are recorded in Table 3-3.

[0281] Table 3-1

[0282]

[0283]

[0284] Table 3-2

[0285]

[0286] Table 3-3

[0287]

[0288] The data in Tables 3-1 to 3-3 show that when the ratio of the width of the first region to the width of the positive electrode film is 10% to 40%, and the ratio of the width of the second region to the width of the positive electrode film is 60% to 90%, the battery can achieve both high energy density and high cycle performance.

[0289] Example 10

[0290] The battery preparation method in Example 10 is similar to that in Example 3. The difference lies in the step of preparing the positive electrode sheet:

[0291] The positive electrode active material (LiNi) 0.8 Co 0.1 Mn 0.1 O2), conductive agent (acetylene black), and binder (polyvinylidene fluoride) are added to the solvent N-methylpyrrolidone (NMP) in a mass ratio of 97:2:1 and stirred evenly to prepare the first positive electrode slurry.

[0292] The positive electrode active material (LiNi) 0.9 Co 0.05 Mn 0.05 O2), conductive agent (acetylene black), binder (polyvinylidene fluoride), and lithium supplementer (Li2NiO2) are added to the solvent N-methylpyrrolidone (NMP) in a mass ratio of 96:2:1:1 and stirred evenly to prepare the second positive electrode slurry.

[0293] Example 11

[0294] The battery preparation method in Example 11 is similar to that in Example 3. The difference lies in the step of preparing the positive electrode sheet:

[0295] The positive electrode active material (LiNi) 0.8 Co 0.1 Mn 0.1 O2), conductive agent (acetylene black), binder (polyvinylidene fluoride), and lithium supplementer (Li2NiO2) are added to the solvent N-methylpyrrolidone (NMP) in a mass ratio of 94:2:1:3 and stirred evenly to prepare the first positive electrode slurry.

[0296] The performance of the batteries in Examples 10 and 11 was tested in the same manner as in Example 1, and the test results are recorded in Table 4.

[0297] Table 4

[0298]

[0299] The data in Table 4 show that adding lithium replenishment agent in the second region can further improve the cycle performance of the battery.

[0300] Example 12

[0301] The battery preparation method in Example 12 is similar to that in Example 3. The difference lies in the step of preparing the positive electrode sheet:

[0302] The positive electrode active material (LiNi) 0.93 Co 0.1 Mn 0.1 O2), conductive agent (acetylene black), and binder (polyvinylidene fluoride) are added to the solvent N-methylpyrrolidone (NMP) in a mass ratio of 97:2:1 and stirred evenly to prepare the third positive electrode slurry.

[0303] Three extrusion heads are used to uniformly coat the first, second, and third positive electrode slurries onto both sides of a 13μm thick positive electrode current collector (aluminum foil) in distinct regions. After drying and cold pressing, a positive electrode sheet is obtained. The first positive electrode slurry forms the first region, the second positive electrode slurry is coated on both sides of the first positive electrode slurry to form the second sub-region, and the third positive electrode slurry is coated on both sides of the second positive electrode slurry to form two more first sub-regions.

[0304] The coating width of the first positive electrode slurry is 30 mm. The coating width of the second positive electrode slurry on one side is 10 mm, and the total width is 20 mm. The coating width of the first positive electrode slurry on one side is 25 mm, and the total width is 50 mm. The coating thickness of the first, second, and third positive electrode slurries is 0.12 mm.

[0305] The parameters of each region of the positive electrode sheet of Example 12 were tested in the same manner as in Example 1, and the test results are recorded in Table 5-2.

[0306] The performance of the battery in Example 12 was tested in the same manner as in Example 1, and the test results are recorded in Table 5-3.

[0307] Table 5-1

[0308]

[0309] Table 5-2

[0310]

[0311] Table 5-3

[0312]

[0313] The data in Tables 5-1 to 5-3 show that by sequentially setting a second sub-region and a first sub-region on both sides of the first region, and ensuring that the capacity per unit area of ​​the second sub-region is less than that of the first sub-region, internal stress between different regions can be reduced, and electrolyte wetting between regions (first region, second sub-region, and first sub-region) can be improved. This is beneficial for further improving the cycle life and stress concentration of the battery. Furthermore, since the first sub-region uses a high-capacity positive electrode active material, the volumetric energy density of the battery is also improved accordingly.

[0314] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A secondary battery, characterized in that, include: A housing and an electrode assembly housed inside the housing, the electrode assembly including a positive electrode and a negative electrode, wherein the positive electrode extends along a first direction, the length of the positive electrode in the first direction is greater than its width in a second direction, and the second direction is perpendicular to the first direction; The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one side surface of the negative current collector, the negative electrode film layer including silicon. The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one side surface of the positive current collector. The positive electrode film layer includes a first region and a second region located on both sides of the first region in a second direction. The capacitance per unit area of ​​the first region is less than that of the second region.

2. The secondary battery according to claim 1, characterized in that, The electrode assembly is wound to form a wound structure, and the winding axis of the wound structure is parallel to the second direction.

3. The secondary battery according to claim 1 or 2, characterized in that, The tensile strength of the shell is above 350 MPa.

4. The secondary battery according to any one of claims 1 to 3, characterized in that, The material of the housing includes at least one of aluminum alloy, steel, copper alloy, titanium alloy, and nickel alloy.

5. The secondary battery according to any one of claims 1 to 4, characterized in that, The silicon element in the negative electrode film layer accounts for 3% to 70% of the total mass.

6. The secondary battery according to any one of claims 1 to 5, characterized in that, The negative electrode film layer includes a silicon-based material, which includes one or more of elemental silicon, silicon-oxygen composites, and silicon-carbon composites.

7. The secondary battery according to claim 6, characterized in that, The silicon-based material includes at least the silicon-carbon composite.

8. The secondary battery according to claim 7, characterized in that, The silicon-carbon composite satisfies one or more of the following characteristics: (1) The silicon-carbon composite comprises porous carbon and silicon-containing material dispersed in the pores of the porous carbon. (2) The silicon-carbon composite further includes a carbon-containing coating layer, which is located on the surface of the porous carbon and / or silicon material; (3) The silicon content in the silicon-carbon composite is 30% to 70% by mass; (4) The average particle size of the silicon-carbon composite is 2 μm to 15 μm; (5) The silicon-carbon composite has a powder resistivity of 4 Ω·cm to 17 Ω·cm at 8 MPa; (6) The BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g to 6.7m 2 / g.

9. The secondary battery according to any one of claims 1 to 8, characterized in that, The negative electrode film layer includes a first film layer and a second film layer stacked on the side of the first film layer away from the negative electrode current collector; The mass percentage of silicon in the second film layer is greater than that in the first film layer.

10. The secondary battery according to claim 9, characterized in that, The mass percentage of silicon in the first film layer is 0% to 20%. And / or, The silicon element in the second film layer accounts for 5% to 70% of the total mass.

11. The secondary battery according to any one of claims 1 to 10, characterized in that, The volumetric energy density of the secondary battery is between 600Wh / L and 850Wh / L.

12. The secondary battery according to any one of claims 1 to 11, characterized in that, The width of the positive electrode film layer in the second direction is L, the width of the first region in the second direction is L1, and the total width of the second region in the second direction is L2. The values ​​of L and L1 satisfy the following relationship: 10% ≤ L1 / L ≤ 40%. and / or, The L and L2 satisfy the following relationship: 60% ≤ L2 / L ≤ 90%.

13. The secondary battery according to any one of claims 1 to 12, characterized in that, The ratio of the capacitance per unit area of ​​the first region to the capacitance per unit area of ​​the second region is 60% to 98%.

14. The secondary battery according to any one of claims 1 to 13, characterized in that, The capacitance per unit area of ​​the first region is 15mAh / 1540.25mm². 2 Up to 98mAh / 1540.25mm 2 , And / or, The capacitance per unit area of ​​the second region is 25mAh / 1540.25mm². 2 Up to 100mAh / 1540.25mm 2 .

15. The secondary battery according to any one of claims 1 to 14, characterized in that, The capacitance per unit area of ​​the first region is 15mAh / 1540.25mm². 2 Up to 68.4mAh / 1540.25mm 2 , And / or, The capacitance per unit area of ​​the second region is 25mAh / 1540.25mm². 2 Up to 77mAh / 1540.25mm 2 .

16. The secondary battery according to any one of claims 1 to 15, characterized in that, The thickness of the first region is less than or equal to the thickness of the second region.

17. The secondary battery according to any one of claims 1 to 16, characterized in that, The thickness of a single layer in the first region is 0.005 mm to 0.15 mm. And / or, The thickness of a single layer of the second region is 0.0185 mm to 0.155 mm.

18. The secondary battery according to any one of claims 1 to 17, characterized in that, The compaction density of the second region is greater than or equal to the compaction density of the first region.

19. The secondary battery according to any one of claims 1 to 18, characterized in that, The compaction density of the first region is 3.0 g / cm³. 3 Up to 4.0 g / cm 3 ; And / or, The compaction density of the second region is 3.3 g / cm³. 3 Up to 4.4 g / cm 3 .

20. The secondary battery according to any one of claims 1 to 19, characterized in that, The second region includes a lithium replenisher.

21. The secondary battery according to claim 20, characterized in that, The mass percentage of lithium supplement in the second region is less than 3%.

22. The secondary battery according to any one of claims 1 to 21, characterized in that, The positive electrode also includes a lithium strip located on the side of at least one of the second regions away from the positive current collector.

23. The secondary battery according to any one of claims 1 to 22, characterized in that, Both the first region and the second region include positive electrode active material, and the areal density of the positive electrode active material in the first region is less than or equal to the areal density of the positive electrode active material in the second region.

24. The secondary battery according to claim 23, characterized in that, The areal density of the positive electrode active material in the first region is 0.15 g / 1540.25 mm². 2 Up to 0.4g / 1540.25mm 2 , And / or, The areal density of the positive electrode active material in the second region is 0.18 g / 1540.25 mm². 2 Up to 0.4g / 1540.25mm 2 .

25. The secondary battery according to any one of claims 1 to 24, characterized in that, Both the first region and the second region include positive electrode active material, and the coating weight per unit area of ​​the positive electrode active material in the first region is less than or equal to the coating weight per unit area of ​​the positive electrode active material in the second region.

26. The secondary battery according to any one of claims 1 to 25, characterized in that, The coating weight of the positive electrode active material in the first region is 0.15 g / 1540.25 mm. 2 Up to 0.4g / 1540.25mm 2 , And / or, The coating weight of the positive electrode active material in the second region is 0.18 g / 1540.25 mm. 2 Up to 0.4g / 1540.25mm 2 .

27. The secondary battery according to any one of claims 1 to 26, characterized in that, The porosity of the first region is greater than that of the second region.

28. The secondary battery according to any one of claims 1 to 27, characterized in that, Both the first region and the second region include lithium transition metal oxide, which includes nickel and lithium. The molar percentage of nickel is more than 50% based on the total molar amount of elements other than lithium in the lithium transition metal oxide.

29. The secondary battery according to claim 28, characterized in that, The molar percentage of nickel is 60% or more.

30. The secondary battery according to claim 28 or 29, characterized in that, The molar percentage of nickel is 80% or more.

31. The secondary battery according to any one of claims 28 to 30, characterized in that, The molar percentage of nickel is over 90%.

32. The secondary battery according to any one of claims 28 to 31, characterized in that, The molar percentage of nickel in the first region is less than that in the second region.

33. The secondary battery according to any one of claims 28 to 32, characterized in that, The molar percentage of nickel in the first region is between 0.6 and 0.

97. And / or, The molar percentage of nickel in the second region is 0.7 to 0.

97.

34. The secondary battery according to any one of claims 1 to 33, characterized in that, At least one of the second regions includes N sub-regions arranged sequentially in the second direction, and the capacitance per unit area of ​​the N sub-regions increases sequentially in the direction away from the first region.

35. The secondary battery according to claim 34, characterized in that, The N sub-regions include a first sub-region and a second sub-region. The second sub-region is located between the first sub-region and the first region, and the capacitance per unit area of ​​the second sub-region is less than that of the first sub-region.

36. The secondary battery according to claim 35, characterized in that, The capacitance per unit area of ​​the first sub-region is 54 / 1540.25 mm². 2 Up to 100mAh / 1540.25mm 2 , And / or, The capacitance per unit area of ​​the second sub-region is 25 / 1540.25 mm². 2 Up to 70mAh / 1540.25mm 2 .

37. The secondary battery according to claim 35 or 36, characterized in that, The width of the first sub-region in the second direction is L21, and the width of the second sub-region in the second direction is L22. L21 and L22 satisfy: 0.1≤L21 / L22≤10.

38. The secondary battery according to any one of claims 35 to 37, characterized in that, The first region includes: Li a1 Ni x1 Co y1 Mn 1-x1-y1 M1 z1 O2, 0.7≤a1≤1.1, 0.6≤x1≤0.97, 0≤z1≤0.01, And / or, The second sub-region includes: Li a2 Ni x2 Co y2 Mn 1-x2-y2 M2 z3 O2, 0.7≤a2≤1.1, 0.7≤x2≤0.97, 0≤z2≤0.01, And / or, The first sub-region includes: Li a3 Ni x3 Co y3 Mn 1-x3-y3 M3 z3 O2, 0.7≤a3≤1.1, 0.7≤x3≤0.97, 0≤z3≤0.01, Among them, M1, M2, and M3 each independently include one or more of the following: Mg, Al, Ca, Ti, V, Cr, Fe, Cu, Zr, Nb, W, and Sr.

39. A positive electrode plate, characterized in that, The positive electrode sheet extends along a first direction, and the length of the positive electrode sheet in the first direction is greater than its width in the second direction, wherein the second direction is perpendicular to the first direction; Furthermore, the positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one side surface of the positive current collector. The positive electrode film layer includes a first region and a second region located on both sides of the first region in a second direction. The capacitance per unit area of ​​the first region is less than the capacitance per unit area of ​​the second region.

40. The positive electrode sheet according to claim 39, characterized in that, The width of the positive electrode film layer in the second direction is L, the width of the first region in the second direction is L1, and the width of the second region in the second direction is L2. The L and L1 satisfy the following relationship: 10% ≤ L1 / L ≤ 40%. and / or, The L and L2 satisfy the following relationship: 60% ≤ L2 / L ≤ 90%.

41. The positive electrode sheet according to claim 39 or 40, characterized in that, The ratio of the capacitance per unit area of ​​the first region to the capacitance per unit area of ​​the second region is 60% to 98%.

42. The positive electrode sheet according to any one of claims 39 to 41, characterized in that, The capacitance per unit area of ​​the first region is 15mAh / 1540.25mm². 2 Up to 98mAh / 1540.25mm 2 , And / or, The capacitance per unit area of ​​the second region is 25mAh / 1540.25mm². 2 Up to 100mAh / 1540.25mm 2 .

43. An electrical device, characterized in that, It includes the secondary battery according to any one of claims 1 to 38 or the positive electrode sheet according to any one of claims 39 to 42.

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