Secondary battery and electric device

Abstract

The invention provides a secondary battery and an electric device, and relates to the technical field of lithium ion batteries. The secondary battery comprises a positive pole piece and a negative pole piece, the positive pole piece comprises lithium-containing phosphate, and the negative pole piece comprises a negative pole current collector, and a first negative pole active material layer and a second negative pole active material layer which are arranged on the same side of the negative pole current collector; the first negative electrode active material layer comprises a silicon carbon material and first graphite, and the mass content of the silicon element in the negative electrode plate is 0.5-5%; the mass content A1 of the silicon element in the second negative electrode active material layer is smaller than or equal to 0.5%, the mass content A2 of the silicon element in the first negative electrode active material layer is larger than A1, the thickness ratio of the first negative electrode active material layer is smaller than or equal to 60%, and the thickness of the first negative electrode active material layer is 5-35 microns. According to the arrangement, under the condition that the total silicon element mass content in the negative electrode plate of the lithium-containing phosphate battery system is the same, the expansion of the silicon-containing negative electrode plate can be effectively improved, and the cycle performance of the secondary battery is improved.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, and more particularly to a secondary battery and an electrical device. Background Technology

[0002] The negative electrode active material in lithium-ion batteries is generally graphite. If silicon is added to the negative electrode active material, the theoretical specific capacity and energy density of the negative electrode sheet will be greatly improved. Therefore, lithium-ion batteries with silicon-based negative electrode sheets are the future development direction.

[0003] However, during the charging and discharging process of lithium-ion batteries, silicon particles undergo lithiation and delithiation, resulting in significant expansion and contraction. This not only damages the structural integrity and stability of the negative electrode, but also causes significant expansion forces within the battery layers, thus worsening the battery's cycle life.

[0004] Therefore, effectively reducing the expansion of silicon-containing anodes without reducing the amount of silicon-based materials added is a challenge. Summary of the Invention

[0005] This application is made in view of the above-mentioned problems, and its purpose is to provide a secondary battery and an electrical device that can effectively reduce the expansion of silicon-containing negative electrode sheets and improve battery cycle life while having the same amount of silicon-based material added.

[0006] The first aspect of this application provides a secondary battery, which includes a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes a lithium phosphate, the negative electrode includes a negative current collector, and a negative active material layer disposed on the surface of the negative current collector. The negative active material layer includes a first negative active material layer and a second negative active material layer disposed on the same side of the negative current collector.

[0007] The first negative electrode active material layer comprises silicon-carbon material and first graphite, wherein the silicon-carbon material comprises porous carbon and silicon distributed in the porous carbon; based on the total mass of the negative electrode active material layer, the mass content of silicon element in the negative electrode sheet is 0.5% to 5%; the mass content of silicon element in the second negative electrode active material layer is less than or equal to 0.5%, and the mass content of silicon element in the first negative electrode active material layer is greater than A1; based on the total thickness of the first negative electrode active material layer and the second negative electrode active material layer, the thickness of the first negative electrode active material layer accounts for ≤60%, and the thickness of the first negative electrode active material layer is 5μm-35μm.

[0008] Therefore, this application provides a lithium phosphate battery system. By controlling the total mass of the negative electrode active material layer in the negative electrode sheet, the mass content of silicon in the negative electrode sheet is controlled to be 0.5% to 5%, thus keeping the total silicon mass content of the negative electrode sheet low, making it compatible with the lithium phosphate battery system and effectively reducing secondary battery expansion. Simultaneously, based on the low total silicon mass content, a layered coating method is further adopted, and the mass content of silicon A1 in the second negative electrode active material layer is controlled to be less than or equal to 0.5%, while the mass content of silicon A2 in the first negative electrode active material layer is greater than A1. That is, the second negative electrode active material layer contains very little or no silicon, allowing the silicon-carbon material to be concentrated as much as possible in the second negative electrode active material layer. Using a first negative electrode active material layer as the negative electrode active material facilitates the relatively uniform dispersion of silicon-carbon material within the first negative electrode active material layer. This ensures that the second negative electrode active material layer is largely unaffected by the expansion of silicon-carbon material, thereby effectively reducing the expansion of the negative electrode sheet. Simultaneously, since silicon-carbon material is mainly concentrated in the first negative electrode active material layer, the thickness ratio of the first negative electrode active material layer should not be too high. The thickness ratio of the first negative electrode active material layer in the total thickness of the first and second negative electrode active material layers should be ≤60%, and the thickness of the first negative electrode active material layer should be 5μm-35μm. This further reduces the number of particles disturbed during silicon expansion, further effectively reducing the expansion of the negative electrode sheet.

[0009] In any embodiment, based on the total thickness of the first negative electrode active material layer and the second negative electrode active material layer, the thickness of the first negative electrode active material layer accounts for 24%-50%.

[0010] By controlling the thickness ratio of the first negative electrode active material layer within the above range, not only can it be mass-produced with high yield, but also by controlling the first negative electrode active material layer to not occupy the majority of the total thickness, the number of disturbed particles in the second negative electrode active material layer during silicon expansion is reduced, so that the secondary battery has good energy density while also taking into account good cycle performance.

[0011] In any embodiment, the thickness of the second negative electrode active material is 22μm-52μm.

[0012] By controlling the thickness of the first negative electrode active material layer and / or the thickness of the second negative electrode active material within the above range, the secondary battery can achieve both good energy density and good cycle performance.

[0013] In any embodiment, the volumetric particle size distribution Dv50 of the first graphite is ≤12μm, ≤7μm; and / or, The negative electrode active material of the second negative electrode active material layer includes a second graphite, and the volumetric particle size distribution of the second graphite is 7μm≤Dv50≤15μm.

[0014] By controlling the volumetric particle size distribution Dv50 of the first and second graphite within the above range, the expansion of silicon-carbon materials has more buffer space, which helps to reduce expansion while taking into account kinetics.

[0015] In any embodiment, the number of silicon material particles contained in the second negative electrode active material layer in the complete cross-section of the negative electrode sheet is ≤5.

[0016] The above configuration further illustrates that the second negative electrode active material layer contains very little or no silicon, allowing the silicon to be concentrated in the first negative electrode active material layer. This facilitates the more uniform dispersion of silicon-carbon materials in the first negative electrode active material layer, ensuring that the particles in the second negative electrode active material layer are not disturbed by the expansion of silicon-carbon materials, thereby effectively reducing the expansion of the negative electrode sheet.

[0017] In any implementation, A1 is 0.

[0018] In other words, the second negative electrode active material layer does not contain silicon. That is, the silicon-carbon material is concentrated in the first negative electrode active material layer, which further ensures that the particles in the second negative electrode active material layer are not disturbed by the expansion of the silicon-carbon material, and further effectively reduces the expansion of the negative electrode sheet.

[0019] In any embodiment, the mass content of silicon in the negative electrode is 1.0%-3.5%.

[0020] Within the aforementioned range, secondary batteries exhibit better cycle performance.

[0021] In some embodiments, the silicon-carbon material has an average diameter of 3 μm to 10 μm.

[0022] Controlling the average diameter of silicon-carbon materials within the above-mentioned range can effectively improve the cycle life of secondary batteries.

[0023] In some implementations, the silicon-carbon material has an average diameter of 6 μm to 9 μm.

[0024] Controlling the average diameter of silicon-carbon materials within the aforementioned range can further effectively improve the cycle life of secondary batteries.

[0025] In any embodiment, the positive electrode sheet includes a positive active material layer, and the total coating weight of the positive active material layer is ≤0.3g / 1540.25mm. 2 The first negative electrode active material layer is located between the negative electrode current collector and the second negative electrode active material layer.

[0026] The small total coating weight of the positive electrode active material layer results in a thinner overall thickness of the corresponding negative electrode film layer (the first negative electrode active material layer and the second negative electrode active material layer), and the overall dynamic performance of the negative electrode sheet is good. Therefore, the first negative electrode active material layer is set between the negative electrode current collector and the second negative electrode active material layer to reduce the contact between the first negative electrode active material layer and the electrolyte, reduce the occurrence of side reactions, improve cycle performance, and help the secondary battery to balance rate performance and cycle performance.

[0027] In any embodiment, the positive electrode sheet includes a positive active material layer, and the total coating weight of the positive active material layer is ≥0.4g / 1540.25mm. 2 The second negative electrode active material layer is located between the negative electrode current collector and the first negative electrode active material layer.

[0028] The total coating weight of the positive electrode active material layer is ≥0.4g / 1540.25mm. 2 In other words, a large total coating weight of the positive electrode results in a large overall energy density of the corresponding negative electrode active material layer, but poor kinetic performance. Therefore, the first negative electrode active material layer is set on the side of the second negative electrode active material layer away from the negative electrode current collector, so that the first negative electrode active material layer can fully contact the electrolyte, improve kinetic performance, and help the secondary battery to achieve both good energy density and rate performance.

[0029] In some embodiments, the compacted density of the 3T powder of the positive electrode active material in the positive electrode active material layer is 2.55 g / cm³. 3 -2.70g / cm 3 .

[0030] The 3T powder compaction density of the positive electrode active material is within the above range, which is beneficial to improving the energy density of the secondary battery.

[0031] In some embodiments, the positive electrode includes a positive active material layer, wherein the area particle size distribution Ds50 of the particles in a cross-section along the thickness direction of the positive electrode is 600 nm-900 nm; and / or, In the cross-section of the positive electrode active material layer along the thickness direction of the electrode sheet, the area particle size distribution Ds90 is 1400nm-2100nm; and / or, In the cross-section along the thickness direction of the positive electrode active material layer, the particle size distribution width is 1.855-2.375. The particle size distribution width = (particle area particle size distribution Ds90 - particle area particle size distribution Ds10) / particle area particle size distribution Ds50.

[0032] Among them, the particle area distribution Ds90, particle area distribution Ds50, and particle area distribution Ds10 refer to the particle size corresponding to the cumulative area distribution of particles reaching 90%, 50%, and 10% respectively in the cumulative area distribution curve of particles.

[0033] By controlling the area particle size distribution Ds50 and / or area particle size distribution Ds90 and / or particle size distribution width distribution of the particles in the cross-section along the thickness direction of the positive electrode active material layer to conform to the above range, it is beneficial to improve the compaction density of the positive electrode active material layer and thus improve the energy density of the secondary battery.

[0034] In some embodiments, the lithium-containing phosphate includes at least one of lithium iron phosphate and its modified compounds.

[0035] In some embodiments, either the first negative electrode active material layer or the second negative electrode active material layer serves as the surface layer, and the remaining layer is the inner layer, located between the surface layer and the negative electrode current collector. The volumetric particle size distribution Dv50 of the negative electrode active material in the surface layer is smaller than that in the inner layer.

[0036] The particle size distribution Dv50 of the negative electrode active material in the surface layer is smaller than that in the inner layer, which shortens the ion diffusion path and increases the reactive sites in the surface layer, while relieving the volume expansion stress in the bottom layer. This is beneficial for balancing high energy density and fast charging performance. A second aspect of this application also provides an electrical device comprising the secondary battery of the first aspect of this application. Attached Figure Description

[0037] Figure 1 This is a cross-sectional SEM image of the negative electrode sheet in Example 1.

[0038] Figure 2 This is a schematic diagram of the cross-section of the negative electrode sheet in Comparative Example 1.

[0039] Figure 3 This is a schematic cross-sectional view of the negative electrode sheet in Example 1.

[0040] Figure 4 A schematic cross-sectional view of the negative electrode sheet provided for some implementation methods.

[0041] Figure 5 This is a schematic cross-sectional view of the negative electrode sheet in Comparative Example 1.

[0042] Figure 6 This is a schematic cross-sectional view of the negative electrode sheet in Comparative Example 2.

[0043] Figure 7 This is a schematic cross-sectional view of the negative electrode sheet in Comparative Example 3.

[0044] Figure 8 This is a cross-sectional schematic diagram of the negative electrode sheet in Example 6.

[0045] Figure 9 This is a cross-sectional SEM image of the negative electrode sheet of Example 1 (with the average diameter of the silicon-carbon material annotated).

[0046] Figure 10 This is a schematic diagram of a battery cell according to one embodiment of this application.

[0047] Figure 11 yes Figure 9 An exploded view of a secondary battery according to one embodiment of this application is shown.

[0048] Figure 12 This is a schematic diagram of a battery module according to one embodiment of this application.

[0049] Figure 13 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0050] Figure 14 yes Figure 13 An exploded view of a battery pack according to one embodiment of this application is shown.

[0051] Explanation of reference numerals in the attached figures: 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 - Negative electrode sheet; 100 - Negative electrode current collector; 110 - First negative electrode active material layer; 120 - Second negative electrode active material layer; 130 - Silicon-carbon material; 140 - Graphite. Detailed Implementation

[0052] The embodiments of the secondary battery and power-consuming device of this application are hereby disclosed in detail with appropriate reference to the accompanying drawings. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid making the following description unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0053] 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, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0054] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0055] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0056] Unless otherwise specified, all steps of this application may be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means 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.

[0057] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0058] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0059] During charging and discharging, silicon-carbon and silicon materials in secondary batteries release and insert lithium ions, causing significant volume expansion of the silicon-based materials. This disrupts the structural stability and integrity of the active material layer and damages the solid electrolyte interphase (SEI) membrane, allowing free electrolyte to penetrate into the active material layer. This triggers a series of unstable reactions, including the decomposition of the free electrolyte and repeated damage to the SEI membrane, resulting in a significant decrease in the cycle performance of the secondary battery. Reducing the use of silicon, while minimizing expansion, leads to a decrease in energy density.

[0060] Therefore, effectively reducing the expansion of silicon-containing anodes while maintaining the same amount of silicon-based material added is a challenge.

[0061] Based on this, the first aspect of the present application provides a secondary battery, which includes a positive electrode, a negative electrode, a separator and an electrolyte. The positive electrode includes a lithium phosphate, the negative electrode includes a negative current collector and a negative active material layer disposed on the surface of the negative current collector. The negative active material layer includes a first negative active material layer and a second negative active material layer disposed on the same side of the negative current collector.

[0062] The first negative electrode active material layer includes silicon-carbon material and first graphite, wherein the silicon-carbon material includes porous carbon and silicon distributed in the porous carbon.

[0063] Based on the total mass of the negative electrode active material layer, the silicon content in the negative electrode sheet is 0.5% to 5%. The silicon content A1 in the second negative electrode active material layer is less than or equal to 0.5%, and the silicon content A2 in the first negative electrode active material layer is greater than A1. Based on the total thickness of the first and second negative electrode active material layers, the thickness of the first negative electrode active material layer accounts for ≤60%, and the thickness of the first negative electrode active material layer is 5μm-35μm.

[0064] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode active material layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector. It should be noted that, regardless of how the negative electrode active material layer is disposed, it is sufficient as long as the relevant settings on either side of the two opposite surfaces of the negative electrode current collector meet the requirements of the first negative electrode active material layer and the second negative electrode active material layer described above.

[0065] It is understandable that the first negative electrode active material layer and the second negative electrode active material layer are disposed on the same side of the negative electrode current collector. That is, the first negative electrode active material layer is located between the second negative electrode active material layer and the negative electrode current collector, or the second negative electrode active material layer is located between the first negative electrode active material layer and the negative electrode current collector.

[0066] The first negative electrode active material layer includes silicon carbon material and first graphite, that is, the negative electrode active material in the first negative electrode active material layer includes silicon carbon material and first graphite.

[0067] Based on the total mass of the negative electrode active material layer, the mass content of silicon in the negative electrode sheet is 0.5% to 5%, which means that, taking the total mass of the negative electrode active material layer in the negative electrode sheet as 100%, the mass percentage of the total silicon in the negative electrode sheet is 0.5% to 5%. That is, the mass content of silicon in the negative electrode sheet = total mass of silicon in the negative electrode active material layer / total mass of the negative electrode active material layer * 100%.

[0068] The mass content A1 of silicon in the second negative electrode active material layer is: A1 = silicon in the second negative electrode active material layer / total mass of the second negative electrode active material layer * 100%.

[0069] The mass content of silicon in the first negative electrode active material layer, A2, refers to: A2 = silicon in the first negative electrode active material layer / total mass of the first negative electrode active material layer * 100%.

[0070] Based on the total mass of the negative electrode active material layer, the mass content of silicon in the negative electrode sheet is 0.5% to 5%, and the mass content of silicon in the second negative electrode active material layer, A1, is less than or equal to 0.5%, while the mass content of silicon in the first negative electrode active material layer, A2, is greater than A1. That is, the second negative electrode active material layer contains very little or no silicon (the silicon in the second negative electrode active material layer can be provided by elemental silicon, silicon-carbon materials, etc., and the raw materials for providing silicon are not limited). Silicon-carbon materials are concentrated in the first negative electrode active material layer as much as possible as the negative electrode active material.

[0071] Since the negative electrode sheet includes a first negative electrode active material layer and a second negative electrode active material layer, the thickness of the first negative electrode active material layer cannot be zero. Therefore, based on the total thickness of the first and second negative electrode active material layers, 0 < the thickness percentage of the first negative electrode active material layer ≤ 60%. It can be understood that the negative electrode sheet can be a double-sided coated electrode sheet or a single-sided coated electrode sheet. Here, the thickness percentage of the first negative electrode active material layer refers to: single-sided first negative electrode active material layer / (total thickness of the single-sided first negative electrode active material layer and the second negative electrode active material layer on the same side) * 100%.

[0072] It should be noted that both the thickness of the first negative electrode active material layer and its thickness after liquid injection formation are increased compared to before liquid injection formation. Specifically, before liquid injection formation, the thickness of the first negative electrode active material layer is 5μm-30μm, and after liquid injection formation, the thickness of the first negative electrode active material layer is 6μm-35μm. For example, before liquid injection formation, the thickness of the first negative electrode active material layer is any value of 5μm, 10μm, 13, 15μm, 20μm, 22μm, 25μm, 27μm, and 30μm, or between any two values.

[0073] In this application, regarding the boundary between the first negative electrode active material layer and the second negative electrode active material layer: the microstructure of the cross section of the negative electrode sheet along the thickness direction can be observed first, and the boundary between the first negative electrode active material layer and the second negative electrode active material layer can be confirmed based on the obvious discontinuous changes in the microstructure. When the boundary between the first negative electrode active material layer and the second negative electrode active material layer is not obvious, energy dispersive spectroscopy (EDS) surface scanning can be used to obtain the silicon element distribution map. At this time, there are two situations, the first situation (1): since it is difficult for the particles in each layer to enter the other layer when the double-layer coating is applied, the particles are located on the side opposite to the layer (the layer where the particles originally were). Therefore, when the cross section of the negative electrode sheet is not obvious, the boundary between the first negative electrode active material layer and the second negative electrode active material layer can be confirmed based on the obvious discontinuous changes in the microstructure. Figure 3 As shown, Figure 3 The corresponding silicon element distribution location should correspond to Figure 3 In the case of silicon-carbon material 130, if the silicon element distribution diagram shows no silicon element on the side near the negative electrode current collector 100, it can be assumed that the film layer on the side near the negative electrode current collector does not contain silicon-carbon material. In this case, the region containing all silicon elements is designated as the first negative electrode active material layer 110, and the side without silicon elements is designated as the second negative electrode active material layer 120. Similarly, when the cross-section of the negative electrode sheet is as shown... Figure 8 As shown, Figure 8 The corresponding silicon element distribution location should correspond to Figure 8 In the case of silicon-carbon material 130, when the silicon element distribution diagram shows that the side near the negative electrode current collector 100 contains silicon element and the side away from the negative electrode current collector 100 does not contain silicon element, it can be considered that the film layer on the side near the negative electrode current collector 100 contains silicon-carbon material 130. In this case, the region containing all silicon-carbon material 130 is taken as the first negative electrode active material layer 110, and the side without silicon-carbon material 130 is taken as the second negative electrode active material layer 120. In the above-mentioned case (1), there may be conditions such as... Figure 4 As shown, some silicon-carbon material 130 is located at the interface of the two coating layers, causing it to protrude from the interface of the first negative electrode active material layer 110 and partially enter the second negative electrode active material layer 120. In this case, the area where this part of the silicon-carbon material 130 is located can also be regarded as the first negative electrode active material layer 110, that is, the cross-section at this time is as shown. Figure 4As shown, the boundary line between the first negative electrode active material layer 110 and the second negative electrode active material layer 120 is not a straight line. Case (2): [Example 1] Figure 5 As shown, both the first negative electrode active material layer 110 and the second negative electrode active material layer 120 contain silicon-carbon material 130. Since the content of silicon-carbon material 130 in the two layers is different, the boundary line of the silicon element concentration distribution of silicon-carbon material 130 in the corresponding silicon element distribution diagram can be used as the reference line to distinguish the first negative electrode active material layer 110 and the second negative electrode active material layer 120 with the concentration difference as the baseline. The silicon element concentration in the first negative electrode active material layer 110 is greater than that in the second negative electrode active material layer 120.

[0074] The magnification of the electrode cross-section is not required, as long as the negative electrode current collector and the negative electrode active material layers on both sides are clearly visible. If multiple cross-sections are used, the magnification should be uniform. To ensure more accurate testing of the total silicon content and the silicon content in each layer, three regions can be tested, and the average value obtained can be used as the total silicon content and the silicon content in each layer.

[0075] Non-limitingly, the cross-section of the negative electrode sheet can be obtained using instruments or equipment including, but not limited to, focused electron beam (FIB) microscopes (non-limiting examples such as the FEIScios2HiVac device, etc.) and ion cross-section polishers (non-limiting examples such as the IB-09010CP argon ion cross-section polisher and IB-19500CP ion cross-section polisher from JEOL Corporation, Japan). The cross-section of the negative electrode sheet can also be obtained using plasma quenching. Microscopic morphology observation methods can employ instruments or equipment including, but not limited to, scanning electron microscopy (SEM) technology. Non-limitingly, high-resolution field emission scanning electron microscopes can be used; non-limiting examples of SEM instruments include the Sigma300 scanning electron microscope and the Apreo2SEM field emission scanning electron microscope from ZEISS Corporation, Germany. Understandably, due to the large expansion of silicon-based materials, battery designs typically require more buffer space. However, this reduces the volumetric energy density of the battery. The high energy density of ternary systems, when combined with silicon-based materials, allows them to buffer energy loss caused by the battery's buffer space, maintaining overall battery performance. On the other hand, lithium phosphate battery systems, which use lithium phosphate as the positive electrode active material, have a lower energy density. If space is reserved for silicon expansion, the actual energy density will be even lower, making it difficult to meet the requirements for power batteries. Therefore, a combination of ternary system and silicon-based material anode is usually used.

[0076] In this application, the aforementioned improvements to the silicon-containing anode of the lithium phosphate battery system can effectively reduce the expansion of the anode sheet. Therefore, in batteries of the same specifications, the battery buffer space can be effectively reduced, and the proportion of silicon-containing anode and cathode sheets in the unit space of the battery can be increased as much as possible, thereby improving the volumetric energy density. This allows it to be compatible with lithium iron phosphate and its modified compounds and has a better volumetric energy density, enabling it to be used as a power battery, while also taking into account the high cycle life and high safety performance of the lithium phosphate battery system.

[0077] Specifically, due to the different charging voltages of lithium phosphate and ternary materials, the breathing effect of lithium phosphate is greater than that of ternary materials, resulting in more thorough charging and discharging and thus greater expansion of silicon-carbon materials. Therefore, by controlling the total mass of the negative electrode active material layer in the negative electrode sheet, the mass content of silicon element in the negative electrode sheet is controlled to be 0.5% to 5%. Maintaining a low total silicon element mass content in the negative electrode sheet effectively reduces expansion, making it better suited for lithium phosphate battery systems, thereby obtaining a lithium phosphate-silicon negative electrode battery system with excellent cycle performance.

[0078] It should be noted that for negative electrode sheets with a high total silicon content, regardless of whether the silicon-carbon material is concentrated in the first negative electrode active layer or the second negative electrode active material layer, the high total silicon content leads to the concentration of silicon-carbon material, resulting in stress concentration in the silicon-carbon material layer. Furthermore, the presence of lithium phosphate will exacerbate the expansion and deteriorate the overall performance of the battery.

[0079] In this application, because the total silicon content in the negative electrode active material of the negative electrode sheet is low, a layered coating method is adopted. The mass content of silicon in the second negative electrode active material layer, A1, is controlled to be less than or equal to 0.5%, while the mass content of silicon in the first negative electrode active material layer, A2, is greater than A1. That is, the second negative electrode active material layer contains very little or no silicon, so that the silicon-carbon material is concentrated in the first negative electrode active material layer as the negative electrode active material. This is beneficial for the silicon-carbon material to be more uniformly dispersed in the first negative electrode active material layer, so that the particles in the second negative electrode active material layer are not disturbed by the expansion of the silicon-carbon material, thereby effectively reducing the expansion of the negative electrode sheet. At the same time, since the silicon-carbon material is mainly concentrated in the first negative electrode active material layer, the thickness of the first negative electrode active material layer is controlled to account for a relatively high percentage of the total thickness. The thickness of the first negative electrode active material layer is controlled to account for ≤60% of the total thickness of the first and second negative electrode active material layers. This further helps to reduce the number of non-silicon negative electrode active materials disturbed during silicon expansion, thereby effectively reducing the expansion of the negative electrode sheet. Through the combined effects of the above, under the condition that the total silicon content in the negative electrode of the lithium phosphate battery system is the same, the expansion of the silicon-containing negative electrode can be effectively improved, thereby enhancing the cycle performance of the secondary battery.

[0080] For example, based on the total mass of the negative electrode active material layer in the negative electrode sheet, the mass content of silicon in the negative electrode sheet is any one of 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0% or between any two values.

[0081] For example, based on the total thickness of the first negative electrode active material layer and the second negative electrode active material layer, the thickness percentage of the first negative electrode active material layer is any one of 5%, 9%, 10%, 20%, 24%, 30%, 39%, 40%, 41%, 50%, 59%, 60%, or between any two values.

[0082] In summary, this application provides a lithium phosphate battery system. By controlling the total mass of the negative electrode active material layer, the silicon content in the negative electrode sheet is controlled to be 0.5% to 5%, thus keeping the total silicon content low and making it compatible with the lithium phosphate battery system, effectively reducing secondary battery expansion. Furthermore, based on the low total silicon content, a layered coating method is employed, and the silicon content A1 in the second negative electrode active material layer is controlled to be less than or equal to 0.5%, while the silicon content A2 in the first negative electrode active material layer is greater than A1. That is, the second negative electrode active material layer contains very little or no silicon, allowing the silicon-carbon material to be concentrated as much as possible in the first negative electrode. As the negative electrode active material, the silicon-carbon material in the active material layer is more uniformly dispersed in the first negative electrode active material layer, so that the second negative electrode active material layer is not disturbed by the expansion of the silicon-carbon material, thus effectively reducing the expansion of the negative electrode sheet. At the same time, since the silicon-carbon material is mainly concentrated in the first negative electrode active material layer, the thickness ratio of the first negative electrode active material layer should not be too high. The thickness ratio of the first negative electrode active material layer in the total thickness of the first and second negative electrode active material layers should be ≤60%, and the thickness of the first negative electrode active material layer should be 5μm-35μm. This further reduces the number of particles disturbed during silicon expansion, and further effectively reduces the expansion of the negative electrode sheet.

[0083] In some implementations, the thickness of the first negative electrode active material layer accounts for 24%-50% based on the total thickness of the first negative electrode active material layer and the second negative electrode active material layer.

[0084] By controlling the thickness ratio of the first negative electrode active material layer within the above range, not only can it be mass-produced with high yield, but also by controlling the first negative electrode active material layer to not occupy the majority of the total thickness, the number of disturbed particles in the second negative electrode active material layer during silicon expansion is reduced, so that the secondary battery has good energy density while also taking into account good cycle performance.

[0085] For example, based on the total thickness of the first negative electrode active material layer and the second negative electrode active material layer, the thickness percentage of the first negative electrode active material layer is any one of 24%, 30%, 35%, 39%, 40%, 41%, 45%, 50% or between any two values.

[0086] In some embodiments, the thickness of the second negative electrode active material is 22 μm-52 μm.

[0087] It should be noted that the thickness of the second negative electrode active material layer after liquid injection formation is increased compared to that before liquid injection formation.

[0088] Before liquefaction formation, the thickness of the second negative electrode active material was 22μm-49μm. After liquefaction formation, the thickness of the second negative electrode active material was 25μm-52μm.

[0089] By controlling the thickness of the first negative electrode active material layer and / or the second negative electrode active material layer within the above range, the secondary battery can achieve both good energy density and good cycle performance.

[0090] For example, before liquid injection formation, the thickness of the second negative electrode active material is any value of 22μm, 23μm, 27μm, 30μm, 32μm, 34μm, 40μm, 41μm, 45μm, and 49μm, or between any two values. After liquid injection formation, the thickness of the second negative electrode active material is any value of 25μm, 30μm, 34μm, 40μm, 45μm, 48μm, 50μm, and 52μm, or between any two values.

[0091] It should be noted that the negative electrode active material in the second negative electrode active material layer includes carbon-based materials, wherein the carbon-based materials include at least one of graphite, soft carbon, and hard carbon. Graphite includes at least one of artificial graphite and natural graphite.

[0092] For example, the negative electrode active material in the second negative electrode active material layer includes graphite.

[0093] In some embodiments, the volumetric particle size distribution Dv50 of the first graphite is ≤12μm, ≤7μm; and / or, The negative electrode active material of the second negative electrode active material layer includes a second graphite, and the volumetric particle size distribution of the second graphite is 7μm≤Dv50≤15μm.

[0094] The first graphite and the second graphite include at least one of artificial graphite and natural graphite, respectively. For example, the first graphite and the second graphite are artificial graphite.

[0095] By controlling the volumetric particle size distribution Dv50 of the first and second graphite within the above range, the expansion of silicon-carbon materials has more buffer space, which helps to reduce expansion while taking into account kinetics.

[0096] For example, the first graphite volumetric particle size distribution Dv50 is any value of 7μm, 8μm, 9μm, 10μm, 11μm, 12μm or between any two values.

[0097] For example, the second graphite volumetric particle size distribution Dv50 is any value of 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm or between any two values.

[0098] It should be noted that the volumetric particle size distribution Dv50 of the first and second graphite can be the same or different.

[0099] In some embodiments, the number of silicon material particles contained in the second negative electrode active material layer in the complete cross-section of the negative electrode sheet is ≤5.

[0100] "Complete cross-section of the negative electrode sheet" refers to a cross-section perpendicular to the thickness direction of the negative electrode sheet, and this cross-section includes the negative electrode sheet, the first negative electrode active material layer, and the second negative electrode active material layer. The complete cross-section of the negative electrode sheet can be obtained using the method for obtaining the electrode sheet cross-section as shown above.

[0101] The statement that the number of silicon particles in the second negative electrode active material layer in the complete cross-section of the negative electrode sheet is ≤5 means that the number of silicon particles in the second negative electrode active material layer in the complete cross-section of the negative electrode sheet is 0-5. In other words, the second negative electrode active material layer contains very little or no silicon. It can be understood that the active material in the second negative electrode active material layer is primarily other non-silicon anode active materials, such as graphite.

[0102] Silicon materials refer to all silicon-containing materials, including but not limited to silicon-carbon materials, silicon-oxygen materials, and elemental silicon. It is understood that the silicon material identified in the complete cross-section of the negative electrode is the entire material. For example, when silicon exists as silicon-carbon material in the negative electrode, the identified silicon material refers to the quantity of silicon-carbon material, not the quantity of silicon particles.

[0103] The above configuration further illustrates that the second negative electrode active material layer contains very little or no silicon, allowing the silicon to be concentrated in the first negative electrode active material layer. This facilitates the more uniform dispersion of silicon-carbon materials in the first negative electrode active material layer, ensuring that the particles in the second negative electrode active material layer are not disturbed by the expansion of silicon-carbon materials, thereby effectively reducing the expansion of the negative electrode sheet.

[0104] For example, in the complete cross-section of the negative electrode sheet, the second negative electrode active material layer contains 0-3 silicon materials.

[0105] In some implementations, A1 is 0.

[0106] It is understandable that at this point, in the complete cross-section of the negative electrode sheet, the number of silicon material particles contained in the second negative electrode active material layer is 0.

[0107] With A1 equal to 0, meaning that the second negative electrode active material layer does not contain silicon, the silicon-carbon material is concentrated in the first negative electrode active material layer. This further ensures that the particles in the second negative electrode active material layer are not disturbed by the expansion of the silicon-carbon material, thus effectively reducing the expansion of the negative electrode sheet.

[0108] In some implementations, the silicon content in the negative electrode is 1.0%-3.5% by mass.

[0109] Within the aforementioned range, secondary batteries exhibit better cycle performance.

[0110] For example, based on the total mass of the negative electrode active material layer, the mass content of silicon in the negative electrode sheet is any one of 1.00%, 1.25%, 1.50%, 1.75%, 2.00%, 2.25%, 2.50%, 2.75%, 3.00%, 3.25%, 3.50%, or between any two values.

[0111] It should be noted that the negative electrode active material in this application includes silicon-carbon material, that is, silicon-carbon material is used as the negative electrode active material. Since the silicon in silicon-carbon material is distributed in porous carbon, the porous carbon restricts the expansion of silicon, resulting in low overall expansion of silicon-carbon material, which is beneficial to reduce the expansion of negative electrode sheet and improve the cycle performance of secondary battery.

[0112] Silicon-carbon materials can be purchased commercially or prepared at home.

[0113] In some embodiments, the silicon-carbon material is prepared by vapor deposition. The volume expansion of silicon-carbon material prepared by vapor deposition is relatively low compared to other silicon materials, and it has high initial efficiency. During the cycle, the particle integrity is good, and it will not break or pulverize, resulting in good cycle performance.

[0114] For example, the method of preparing silicon-carbon materials by vapor deposition includes, but is not limited to, placing porous carbon in a reaction atmosphere and reacting at 400°C-550°C for at least 8 hours, wherein the reaction atmosphere is a mixture of a protective gas and a silicon source gas, the gas flow ratio of the protective gas and the silicon source gas is 90:10-70:30, and the protective gas includes at least one of an inert gas and hydrogen.

[0115] For example, the silicon-carbon material may also include a carbon coating layer, wherein porous carbon and silicon particles together serve as the bulk, and the carbon coating layer coats the surface of the bulk.

[0116] The carbon coating layer can be discontinuously coated on the surface of the substrate. The thickness of the carbon coating layer can be selected according to actual needs, such as 1nm-50nm, and its thickness is not limited here.

[0117] In some embodiments, the initial specific capacity of the silicon-carbon material is 1400 mAh / g to 2300 mAh / g.

[0118] When silicon-carbon materials include porous carbon and silicon, with silicon distributed within the porous carbon, the amount of nano-silicon deposition inside the porous carbon is positively correlated with the initial specific capacity of the silicon-carbon material. By controlling the initial specific capacity of the silicon-carbon material within the aforementioned range, the amount of porous carbon deposition is appropriate. The silicon-carbon material not only has a better true density, which is beneficial for improving the energy density of the secondary battery, but it is also less prone to breakage. Furthermore, the surface of the silicon-carbon material is basically free of silicon, resulting in less expansion. Therefore, selecting the aforementioned silicon-carbon material with higher specific capacity not only results in less expansion and less breakage of the silicon-carbon material itself, which is beneficial for improving the cycle performance of the secondary battery, but also, since the total silicon content in the lithium iron phosphate battery system is low, selecting the aforementioned silicon-carbon material with high specific capacity can effectively improve the energy density of the secondary battery.

[0119] For example, the initial specific capacity of the silicon-carbon material is any one of 1400mAh / g, 1450mAh / g, 1500mAh / g, 1550mAh / g, 1600mAh / g, 1650mAh / g, 1700mAh / g, 1750mAh / g, 1800mAh / g, 1850mAh / g, 1900mAh / g, 1950mAh / g, 2000mAh / g, 2050mAh / g, 2100mAh / g, 2150mAh / g, 2200mAh / g, 2250mAh / g, or 2300mAh / g, or between any two of these values.

[0120] In some embodiments, the silicon-carbon material has an average diameter of 3 μm-10 μm.

[0121] In this application, the test of the average diameter of silicon-carbon material refers to cutting the negative electrode sheet perpendicular to the large surface of the negative electrode sheet with an argon ion beam to expose the cross-section. The cross-section is photographed with a scanning electron microscope, and the longest diameter of the silicon-carbon material particles is statistically analyzed using the length-diameter statistical method. Multiple particles are randomly selected from the electron microscope scan image, for example, the average of the longest diameters of 5 silicon-carbon material particles, as the average diameter of the silicon-carbon material.

[0122] If the average diameter of silicon-carbon materials is too small, the active specific surface area will increase. The increased active specific surface area will increase the amount of active lithium required for side reactions, which will directly affect the cycle life of the secondary battery. If the average diameter of silicon-carbon materials is too large, solid-phase diffusion will be difficult, internal resistance will increase, and the temperature rise of the secondary battery will be aggravated, thus deteriorating the cycle performance. At the same time, if the average diameter of silicon-carbon materials is too large, it will also lead to expansion and deterioration, which will further aggravate the deterioration of cycle performance.

[0123] Therefore, controlling the average diameter of silicon-carbon materials within the above range is beneficial to improving the cycle life of secondary batteries.

[0124] For example, the average diameter of the silicon-carbon material is any value of 3.0 μm, 3.5 μm, 4.0 μm, 4.5 μm, 5.0 μm, 5.5 μm, 6.0 μm, 6.5 μm, 7.0 μm, 7.5 μm, 8.0 μm, 8.5 μm, 9.0 μm, 9.5 μm, 10.0 μm or between any two values.

[0125] In some implementations, the silicon-carbon material has an average diameter of 6 μm-9 μm.

[0126] Controlling the average diameter of silicon-carbon materials within the aforementioned range can further effectively improve the cycle life of secondary batteries.

[0127] It is understandable that the first negative electrode active material layer can be disposed between the negative electrode current collector and the second negative electrode active material layer, or the first negative electrode active material layer can be disposed on the side of the second negative electrode active material layer that is away from the negative electrode current collector.

[0128] In some embodiments, the positive electrode includes a positive active material layer, the total coating weight of which is ≤0.3g / 1540.25mm. 2 The first negative electrode active material layer is located between the negative electrode current collector and the second negative electrode active material layer.

[0129] It should be noted that in this application, the total coating weight of the positive electrode active material layer refers to the coating weight on one side only.

[0130] The first negative electrode active material layer is located between the negative electrode current collector and the second negative electrode active material layer. That is, the second negative electrode active material layer is located on the side of the first negative electrode active material layer that is away from the negative electrode current collector. At this time, the second negative electrode active material layer is in direct contact with the electrolyte.

[0131] The total coating weight of the positive electrode active material layer should be controlled to ≤0.3g / 1540.25mm. 2In other words, the small total coating weight of the positive electrode active material layer results in a thinner overall thickness of the corresponding negative electrode film layer (the first negative electrode active material layer and the second negative electrode active material layer), and the overall dynamic performance of the negative electrode sheet is good. Therefore, the first negative electrode active material layer is set between the negative electrode current collector and the second negative electrode active material layer to reduce the contact between the first negative electrode active material layer and the electrolyte, reduce the occurrence of side reactions, improve cycle performance, and help the secondary battery to balance rate performance and cycle performance.

[0132] In some embodiments, the positive electrode includes a positive active material layer, the total coating weight of which is ≥0.4g / 1540.25mm. 2 The second negative electrode active material layer is located between the negative electrode current collector and the first negative electrode active material layer.

[0133] The second negative electrode active material layer is located between the negative electrode current collector and the first negative electrode active material layer. That is, the first negative electrode active material layer is located on the side of the second negative electrode active material layer that is away from the negative electrode current collector. At this time, the first negative electrode active material layer is in direct contact with the electrolyte.

[0134] The total coating weight of the positive electrode active material layer is controlled to be ≥0.4g / 1540.25mm. 2 In other words, a large total coating weight of the positive electrode active material layer results in a large overall energy density of the corresponding negative electrode active material layer but poor kinetic performance. Therefore, the first negative electrode active material layer is placed on the side of the second negative electrode active material layer away from the negative electrode current collector, so that the first negative electrode active material layer can fully contact the electrolyte, improve kinetic performance, and help the secondary battery to achieve both good energy density and rate performance.

[0135] In some implementations, 0.3g / 1540.25mm 2 The total coating weight of the positive electrode active material layer is less than 0.4g / 1540.25mm. 2 In this case, the first negative electrode active material layer can be disposed between the negative electrode current collector and the second negative electrode active material layer, or the first negative electrode active material layer can be disposed on the side of the second negative electrode active material layer away from the negative electrode current collector. There is no limitation here, and the choice can be made according to actual needs.

[0136] In some embodiments, the compacted density of the 3T powder of the positive electrode active material in the positive electrode active material layer is 2.55 g / cm³. 3 -2.70g / cm 3 .

[0137] The test steps for 3T powder compaction density are as follows: Disassemble the fully discharged battery (0V) in a dry glove box, remove the positive electrode sheet, soak the positive electrode sheet in DMC solution for 12 hours to wash away excess electrolyte and lithium salt, then scrape off the powder, soak it in N-methylpyrrolidone (NMP) for 2 days to dissolve the binder (PVDF), then filter and wash to remove residual NMP and impurities, and finally dry to obtain positive electrode active material powder. Then, using a UTM7105 device with a mold inner diameter of 13mm, weigh 2g of positive electrode active material powder and place it in the mold, apply 3T pressure, and record the corresponding compaction density.

[0138] The 3T powder compaction density of the positive electrode active material is within the aforementioned high range, which is beneficial for improving the energy density of the secondary battery.

[0139] For example, the compacted density of the 3T powder of the positive electrode active material is 2.55 g / cm³. 3 2.59g / cm 3 2.63 g / cm 3 2.65g / cm 3 2.70 g / cm 3 It can be any value in the range or any two values ​​in between.

[0140] In some embodiments, the positive electrode includes a positive active material layer, wherein the area particle size distribution Ds50 of the particles in a cross-section along the thickness direction of the positive electrode is 600 nm-900 nm; and / or, In the cross-section of the positive electrode active material layer along the thickness direction of the electrode sheet, the area particle size distribution Ds90 is 1400nm-2100nm; and / or, In the cross-section along the thickness direction of the positive electrode active material layer, the particle size distribution width is 1.855-2.375. The particle size distribution width = (particle area particle size distribution Ds90 - particle area particle size distribution Ds10) / particle area particle size distribution Ds50.

[0141] Among them, the particle area distribution Ds90, particle area distribution Ds50, and particle area distribution Ds10 refer to the particle size corresponding to the cumulative area distribution of particles reaching 90%, 50%, and 10% respectively in the cumulative area distribution curve of particles.

[0142] It should be noted that the particle area size distribution Ds90 can be obtained by observing the microstructure of the complete cross-section of the negative electrode sheet to obtain the particle area size distribution Ds90, particle area size distribution Ds50, and particle area size distribution Ds10. Non-limitingly, the cross-section of the negative electrode sheet can be obtained using instruments or equipment including but not limited to a focused electron beam (FIB) microscope (non-limiting examples such as the FEIScios 2HiVac device, etc.) and an ion cross-section polisher (non-limiting examples such as the IB-09010CP argon ion cross-section polisher and IB-19500CP ion cross-section polisher of JEOL Corporation of Japan, etc.). The cross-section of the negative electrode sheet can also be obtained by plasma quenching. Microscopic morphology observation methods can employ instruments or equipment including but not limited to scanning electron microscopy (SEM) technology. In particular, high-resolution field emission scanning electron microscopes can be used. Examples of non-limiting SEM instruments include the Sigma 300 scanning electron microscope and the Apreo 2 SEM field emission scanning electron microscope from ZEISS GmbH, Germany.

[0143] It should be noted that in the cross-section of the positive electrode active material layer along the thickness direction of the positive electrode sheet, the particles are mainly positive electrode active material, and also include some conductive agents such as conductive carbon black.

[0144] By controlling the area particle size distribution Ds50 and / or area particle size distribution Ds90 and / or particle size distribution width distribution of the particles in the cross-section along the thickness direction of the positive electrode active material layer to conform to the above range, it is beneficial to improve the compaction density of the positive electrode active material layer and thus improve the energy density of the secondary battery.

[0145] For example, in the cross-section of the positive electrode active material layer along the thickness direction of the positive electrode sheet, the area particle size distribution Ds50 of the particles is any value of 600nm, 650nm, 700nm, 750nm, 800nm, 852nm, 900nm or between any two values.

[0146] For example, in the cross section along the thickness direction of the positive electrode active material layer, the area particle size distribution Ds90 of the particles is any value among 1400nm, 1500nm, 1600nm, 1700nm, 1800nm, 1900nm, 2000nm, and 2100nm, or between any two values.

[0147] For example, in the cross-section of the positive electrode active material layer along the thickness direction of the electrode sheet, the particle size distribution width is any value of 1.855, 1.900, 1.925, 1.950, 1.975, 2.000, 2.050, 2.100, 2.150, 2.200, 2.250, 2.300, 2.350, 2.375 or between any two values.

[0148] In some embodiments, the lithium phosphate includes LiMPO4, and M includes Fe and non-Fe elements.

[0149] It should be noted that the above LiMPO4 is not a specific molecular structure formula, but a general expression of lithium iron phosphate.

[0150] In some implementations, the non-Fe element includes one or both of a first doping element and a second doping element, wherein the first doping element is an iron site doping element and the second doping element is a phosphorus site doping element.

[0151] In some embodiments, the first doping element includes one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, and Ge.

[0152] In some embodiments, the first doping element includes at least two of Mn, Ti, V, Ni, Co, and Mg.

[0153] In some implementations, the second doping element includes one or more elements selected from B, S, Si, and N.

[0154] In some embodiments, the lithium-containing phosphate includes Li 1+x Fe 1-y A y P 1-z R z O4, where x is any value in the range of -0.100 to 0.100, y is any value in the range of 0.001 to 0.500, z is any value in the range of 0.001 to 0.100, A includes one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Mn, Ni, Co, Ga, Sn, Sb, Nb and Ge, and R includes one or more elements selected from B, S, Si and N.

[0155] In some embodiments, the lithium-containing phosphate includes Li a A e Fe 1-f B f P 1-g C g O 4-n Dn , wherein, A includes one or more elements of Zn, Al, Na, K, Mg, Nb, Mo, and W; B includes one or more elements of Ti, V, Zr, Mn, Ni, Mg, Co, Ga, Sn, Sb, Nb, and Ge; C includes one or more elements of B, S, Si, and N; D includes one or more elements of S, F, Cl, and Br; a is selected from the range of 0.9 to 1.1, e is selected from the range of 0.001 to 0.1, f is selected from the range of 0.001 to 0.5, g is selected from the range of 0.001 to 0.1, n is selected from the range of 0.001 to 0.1, and the positive electrode active material is electrically neutral.

[0156] It should be noted that Li a A e Fe 1-f B f P 1-g C g O 4-n D n The compound is actually a specific LiMPO4 material.

[0157] In some embodiments, the lithium-containing phosphate includes at least one of lithium iron phosphate and its modified compounds.

[0158] Among lithium iron phosphate and its modified compounds, the modified compounds may include coating, mixing, or doping with other materials or elements. The coating includes, but is not limited to, carbon coating on the surface. Lithium iron phosphate and its modified compounds can be those well-known in the art for lithium-ion batteries.

[0159] In some embodiments, lithium iron phosphate and its modified compounds include at least one of LiFe x M (1-x) PO4 and its coated modified compounds, wherein 0 < x ≤ 1, and M includes at least one of Co, Mn, Ni, Mg, Zn, and Al.

[0160] In some embodiments, the lithium iron phosphate modified compound includes Li 1+x Fe 1-y A y P 1-z R z O4, where x is any value within the range of -0.100 to 0.100, y is any value within the range of 0.001 to 0.500, z is any value within the range of 0.001 to 0.100, A includes one or more elements of Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Mn, Ni, Co, Ga, Sn, Sb, Nb, and Ge, and R includes one or more elements of B, S, Si, and N.

[0161] During the charging and discharging process of a secondary battery, Li undergoes insertion / extraction and consumption, resulting in varying Li molar content at different discharge states. In the examples of positive electrode active materials listed in this application, the Li molar content represents the initial state of the material, i.e., the state before feeding. As the positive electrode material is applied to the battery system, the Li molar content changes after charge-discharge cycles. Similarly, in the examples of positive electrode active materials listed in this application, the O molar content is only a theoretical value. Lattice oxygen release causes changes in the oxygen molar content, leading to fluctuations in the actual O molar content.

[0162] In some embodiments, either the first negative electrode active material layer or the second negative electrode active material layer is used as the surface layer, and the remaining layer is used as the inner layer, which is located between the surface layer and the negative electrode current collector; wherein, the volumetric particle size distribution Dv50 of the negative electrode active material in the surface layer is smaller than the volumetric particle size distribution Dv50 of the negative electrode active material in the inner layer.

[0163] It should be noted that the term "anode active material" in the context of the surface and inner layers does not limit the type of anode active material, but rather refers to the overall volume particle size distribution of the material in that layer that can serve as anode active material.

[0164] The volumetric particle size distribution Dv50 of the negative electrode active material can be tested using a Malvern particle size analyzer (Mastersizer 2000E).

[0165] In other words, regardless of whether the inner layer is the first or the second negative electrode active material layer, according to the positional relationship, the volumetric particle size distribution Dv50 of the negative electrode active material in the surface layer is smaller than that in the inner layer. This shortens the ion diffusion path and increases the reactive sites in the surface layer, while alleviating the volume expansion stress in the bottom layer, which is beneficial for balancing high energy density and fast charging performance. In addition, the secondary battery and power-consuming device of this application will be described below with appropriate reference to the accompanying drawings.

[0166] [Rechargeable Battery] The second aspect of this application provides a secondary battery, which is a lithium-ion battery.

[0167] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0168] [Positive electrode plate] The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive active material layer including a positive active material.

[0169] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive active material layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0170] 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.).

[0171] In some embodiments, the positive electrode active material 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.

[0172] In some embodiments, the positive electrode active material layer 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, and carbon nanofibers.

[0173] 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.

[0174] [Negative electrode] In some embodiments, the negative electrode sheet includes a negative current collector and a first negative active material layer and a second negative active material layer disposed on the same side of the negative current collector, wherein the first negative active material layer and the second negative active material layer serve as negative electrode film layers.

[0175] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, wherein the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0176] 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.).

[0177] In some embodiments, the first negative electrode active material layer and / or the second negative electrode active material 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).

[0178] In some embodiments, the first negative electrode active material layer and / or the second negative electrode active material layer 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.

[0179] In some embodiments, the first negative electrode active material layer and / or the second negative electrode active material layer may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0180] In other embodiments, the current collector of the negative electrode sheet may typically include a current collector body and a base coating. The base coating may be disposed on at least one side of the negative electrode current collector body. The base coating does not contain negative electrode active material, but may include a small amount of carbon material. However, the carbon material forms a thin coating and cannot function as a negative electrode active material.

[0181] [Electrolytes] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not specify any particular type of electrolyte; it can be selected according to requirements.

[0182] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0183] 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.

[0184] 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.

[0185] 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.

[0186] [Isolation film] 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.

[0187] 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.

[0188] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0189] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0190] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0191] In this application, a secondary battery can refer to a single battery cell, or it can refer to a single physical module comprising multiple battery cells to provide higher voltage and capacity, and it can take the form of a battery pack, battery module, etc.

[0192] 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 10 The example shown is a square-structured battery cell 5.

[0193] In some implementations, refer to Figure 11 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 be placed over the opening to close the receiving cavity. The positive electrode sheet, negative electrode sheet, and separator may 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 may be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0194] 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.

[0195] Figure 12 This is battery module 4, used as an example. (See reference...) Figure 12 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.

[0196] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0197] 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.

[0198] Figure 13 and Figure 14 This is battery pack 1 as an example. (See reference...) Figure 13 and Figure 14The 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.

[0199] In addition, this application also provides an electrical device, which includes a secondary battery (at least one of a battery cell, battery module, or battery pack) provided in this application. The secondary battery can be used as a power source for the electrical device or as an energy storage unit of the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., 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.

[0200] As an electrical device, you can choose individual battery cells, battery modules, or battery packs according to your usage requirements.

[0201] As an example, the electrical device is 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 electrical device, a battery pack or battery module can be used.

[0202] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0203] 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.

[0204] Example 1 Preparation of the positive electrode sheet Lithium iron phosphate (LiFePO4) as the positive electrode active material, acetylene black as the conductive agent, and polyvinylidene fluoride (PVDF) as the binder were mixed at a mass ratio of 97.9:0.5:1.6. N-methylpyrrolidone (NMP) as the solvent was added, and the mixture was stirred under vacuum until homogeneous to obtain the positive electrode slurry. The positive electrode slurry was then uniformly coated onto both surfaces of the positive electrode current collector aluminum foil. After drying, the foil underwent cold pressing, edge trimming, cutting, and slitting to obtain the positive electrode sheet. The total coating weight of the positive electrode active material layer (on one side) was 0.35 g / 1540.25 mm. 2 .

[0205] Preparation of the negative electrode sheet First negative electrode active material layer slurry: The negative electrode active material (artificial graphite and silicon carbon material in a mass ratio of 87.5:12.5), conductive agent carbon black, thickener CMC, binder SBR, and single-arm carbon nanotubes are mixed in deionized water in a weight ratio of 96.4:0.5:1:2:0.1 and stirred evenly under vacuum to prepare the first negative electrode active material layer slurry.

[0206] Among them, silicon-carbon materials include porous carbon and silicon distributed in porous carbon, and the average diameter of silicon-carbon materials is 8 μm.

[0207] Second negative electrode active material layer slurry: The negative electrode active material artificial graphite, conductive agent carbon black, thickener CMC and binder SBR are mixed in deionized water at a weight ratio of 96.4:0.5:1.1:2 and stirred evenly under vacuum to prepare the second negative electrode active material slurry.

[0208] Using a double-layer extrusion coating machine, the first negative electrode active material layer slurry and the second negative electrode active material layer slurry are uniformly coated on one surface of a negative electrode current collector copper foil with a thickness of 6μm. The foil is then dried at 110℃ to obtain a negative electrode sheet with a double-layer negative electrode active material layer coated on one side. After drying, the above coating steps are repeated on the other surface of the negative electrode sheet. Then, the negative electrode sheet is dried, cold-pressed, trimmed, cut, and slit to produce a negative electrode sheet for a secondary battery.

[0209] In the negative electrode sheet, the total thickness of the first negative electrode active material layer and the second negative electrode active material layer on one side is 54 μm, the thickness of the first negative electrode active material layer is 22 μm, and the thickness of the second negative electrode active material layer is 32 μm. Based on the total thickness of the first negative electrode active material layer and the second negative electrode active material layer on one side, the thickness of the first negative electrode active material layer accounts for 41%, and the total silicon element mass content in the negative electrode sheet is 2.5%.

[0210] The second negative electrode active material layer is located between the first negative electrode active material layer and the negative electrode current collector, that is, the first negative electrode active material layer serves as the surface layer.

[0211] Electrolyte Preparation of electrolyte: In an argon atmosphere glove box with a water content of <10ppm, ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) were mixed uniformly at a volume ratio of 3:3:4 as an organic solvent. LiPF6 was then uniformly dissolved in the above solution, and fluoroethylene carbonate (FEC) was added to obtain the electrolyte. In this electrolyte, the concentration of LiPF6 was 1mol / L and the mass content of FEC was 3wt%.

[0212]

Isolation Film

[0213]

Battery preparation

[0214] Comparative Examples 1-4 The only difference between Comparative Example 1 and Example 1 is that in the first negative electrode active material layer slurry, artificial graphite and silicon carbide materials with a mass ratio of 89:11 are used together as active materials. In the second negative electrode active material layer slurry, a mixture of artificial graphite and silicon carbide materials with a mass ratio of 99:1 replaces the artificial graphite in the second negative electrode active material layer slurry of Example 1.

[0215] The only difference between Comparative Example 2 and Example 1 is that in the first negative electrode active material layer slurry, artificial graphite and silicon carbide materials with a mass ratio of 92.9:7.1 are used together as active materials. The thickness of the first negative electrode active material layer prepared from the first negative electrode active material layer slurry is 38 μm, and the thickness of the second negative electrode active material layer prepared from the second negative electrode active material layer slurry is 16 μm. Based on the total thickness of the first and second negative electrode active material layers, the thickness of the first negative electrode active material layer accounts for 70%.

[0216] The only difference between Comparative Example 3 and Example 1 is that in the first negative electrode active material layer slurry, artificial graphite and silicon carbon materials with a mass ratio of 95:5 are used as active materials. The first negative electrode active material layer slurry is used to coat one surface of the negative electrode current collector in a single layer (that is, the second negative electrode active material layer slurry is not used for coating). It is dried at 110°C to obtain a negative electrode sheet with a single-sided coating of a negative electrode active material layer. After drying, the above coating steps are repeated on the other surface of the negative electrode sheet. Then, the negative electrode sheet is dried, cold-pressed, trimmed, cut, and slit to produce the negative electrode sheet for the secondary battery. The thickness of the single-sided negative electrode active material layer is 54 μm.

[0217] The only difference between Comparative Example 4 and Example 1 is that artificial graphite and silicon carbide materials with a mass ratio of 70:30 are used together as active materials in the first negative electrode active material layer slurry.

[0218] The following tests were conducted on the negative electrode sheets and secondary batteries prepared in Example 1 and Comparative Examples 1-4: In the following test methods, the cross-section of the negative electrode sheet is obtained as follows: 1. Sample preparation: Cut a sample of about 1 cm × 1 cm from the negative electrode sheet. Cleaning: Clean the sample with deionized water or ethanol to remove surface impurities. Fixing: Fix the sample on a sample stage suitable for ion polishing, usually using conductive glue. 2. Ion polishing: Equipment selection: Use an ion polisher (argon ion polisher). Polishing parameter settings: Ion type: Argon ion (Ar⁺). Polishing voltage: Usually 3kV~5 kV. Polishing current: About 1 mA~5 mA. Polishing time: Adjust according to the characteristics of the sample material, usually 30 min~60 min. Polishing process: Put the sample into the ion polisher. Start the equipment and polish according to the set parameters. After polishing, turn off the equipment and take out the sample. (1) Thickness of the first / second negative electrode active material layer: The cross-section of the polished sample was observed using a scanning electron microscope (SEM). Image processing and analysis: The SEM images were analyzed using image processing software (such as ImageJ, Origin, etc.). The boundary between the first and second negative electrode active material layers was confirmed based on the obvious discontinuous changes in microstructure. When the boundary was not obvious, an energy dispersive spectroscopy (EDS) surface scan, as described in this embodiment, was used to obtain the silicon elemental distribution to determine the boundary between the first and second negative electrode active material layers. Ten locations were randomly selected from each negative electrode active layer to obtain the average thickness, which was taken as the corresponding thickness of the first / second negative electrode active material layer. The thickness percentage of the first negative electrode active material layer = thickness of the first negative electrode active material layer / (thickness of the first negative electrode active material layer + thickness of the second negative electrode active material layer) * 100%.

[0219] (2) Test of silicon content by mass: As shown above, after determining the boundaries of the first and second negative electrode active material layers, an energy dispersive spectroscopy (EDS) instrument is used to perform surface scanning on three regions of each layer to obtain the average silicon content in the negative electrode active material layer, the first negative electrode active material layer, and the second negative electrode active material layer, which is used as the silicon mass content in the corresponding layer.

[0220] (3) Cyclic performance test: The battery cells were discharged to 10% SOC using a current density of 0.33C, and then charged to 100% SOC using a constant current of 0.33C. This 10% SOC-100% SOC cycle was repeated 9 times. On the 10th cycle, the cells were discharged to 0% SOC, and capacity decay was calculated starting from the discharge capacity of the 10th cycle. This process was repeated 1000 times, and the capacity retention rate of the battery cells after 1000 cycles was recorded.

[0221] Among them, the higher the capacity retention rate, the longer the cycle life of the battery cell.

[0222] (4) Expansion force test: The secondary battery is placed between the first and second plates of the three-piece rigid clamp, and a pressure sensor is placed between the second and third plates. The pressure sensor is connected to a computer to monitor the expansion force of the secondary battery's planar area online.

[0223] The test results are shown in Table 1.

[0224] Table 1. Test results of Example 1 and Comparative Examples 1-4

[0225] The cross-sectional SEM image of the negative electrode sheet in Example 1 is shown below. Figure 1 As shown, the cross-sectional SEM image of the negative electrode sheet in Comparative Example 1 is as follows. Figure 2 As shown, Figure 1 as well as Figure 2 The dashed line represents the interface between the first and second negative electrode active material layers. Schematic cross-sectional views of the negative electrode sheets in Examples 1, 1, 2, and 3 are shown below. Figure 3 , Figure 5 , Figure 6 as well as Figure 7 As shown. Graphite 140 is the artificial graphite corresponding to the above embodiments and comparative examples.

[0226] Among them, according to Figures 1 to 3 , Figure 5 As shown in Table 1, compared with Example 1, under the conditions of the same total silicon content in the negative electrode sheet and the same thickness ratio of the first negative electrode active material layer, the silicon content in the second negative electrode active material layer in Comparative Example 1 is too high, which causes more disturbance to the graphite 140 in the second negative electrode active material layer when silicon expands, resulting in a significant increase in expansion force compared with Example 1 and a decrease in capacity retention rate after 1000 cycles.

[0227] according to Figure 3 , Figure 6 As shown in Table 1, compared with Example 1, Comparative Example 2 has the same total silicon content in the negative electrode sheet and the silicon-carbon material is only set on the first negative electrode active material layer. However, due to the excessive thickness of the first negative electrode active material layer 110, which is the main body, the silicon-carbon material 130 disturbs more graphite 140 during expansion, resulting in a significant increase in expansion force compared with Example 1. The capacity retention rate decreases after 1000 cycles, and the cycle performance decreases.

[0228] according to Figure 3 , Figure 7As can be seen from the data in Table 1, compared with Example 1, under the condition that the total silicon content in the negative electrode is the same, Comparative Example 3 has a significantly increased expansion force compared with Example 1 due to the lack of layered coating and the mixing method, which causes more disturbance to the graphite 140 when the silicon-carbon material 130 expands, resulting in a decrease in capacity retention rate after 1000 cycles.

[0229] Compared with Example 1, Comparative Example 4 showed that the total silicon content in the negative electrode was too high and it was not compatible with the lithium iron phosphate system, resulting in a significant increase in expansion force and a decrease in capacity retention after 1000 cycles.

[0230] Examples 2-7 The only difference between Examples 2-5 and Example 1 is that, in the double-layer negative electrode active material layer, the total silicon content in the negative electrode sheet is controlled to be the same as in the example, which is 2.5%. Based on the total thickness (single side) of the first negative electrode active material layer and the second negative electrode active material layer, the ratio of artificial graphite to silicon carbide in the first negative electrode active material layer is adjusted to change the thickness ratio of the first negative electrode active material layer, as shown in Table 2. The difference between Example 6 and Example 1 is only that: in the double-layer negative electrode active material layer, under the condition that the total silicon content in the negative electrode sheet is controlled at the same 2.5% as in Example 1, the surface layer is adjusted to be the second negative electrode active material layer and the inner layer is the first negative electrode active material layer, and the ratio of artificial graphite to silicon carbon material in the first active layer is adjusted so that the thickness ratio of the first negative electrode active material layer is basically the same as in Example 1. The specific thickness distribution is different as shown in Table 2.

[0231] The only difference between Example 7 and Example 1 is that, in the double-layer negative electrode active material layer, under the condition that the total silicon content in the negative electrode sheet is the same as in Example 1 (2.5%), the ratio of artificial graphite to silicon carbon material in the first active layer is adjusted so that the thickness ratio of the first negative electrode active material layer is basically the same as in Example 1. The specific thickness distribution is shown in Table 2.

[0232] It should be noted that in the above embodiments 1-7, the slurry used in the second negative electrode active material layer is the same, that is, the active material of the second negative electrode active material layer is artificial graphite.

[0233] The negative electrode sheets and secondary batteries prepared in Examples 2-7 were tested, and the test results are shown in Table 2.

[0234] Table 2 Test Results

[0235] In Table 2, 87.5% artificial graphite + 12.5% ​​silicon carbon means that the negative electrode active material is composed of artificial graphite and silicon carbon materials with a mass ratio of 87.5:12.5. Similarly, other values ​​can be obtained, which will not be elaborated here. Figure 8 This is a cross-sectional schematic diagram of the negative electrode sheet of Example 6, where graphite 140 is the corresponding artificial graphite in Example 6, and it can be seen that the surface layer is the second negative electrode active material layer 120.

[0236] As shown in Table 2, based on the total thickness of the first and second negative electrode active material layers, the thickness ratio of the first negative electrode active material layer is ≤60%. Controlling the first negative electrode active material layer to not be too high helps to reduce the number of non-silicon negative electrode active materials disturbed during silicon expansion, thereby effectively reducing the expansion of the negative electrode sheet. The combined effect of the two can effectively improve the expansion of silicon-containing negative electrode sheets and enhance the cycle performance of secondary batteries under the condition that the total silicon element mass content in the negative electrode sheet is the same.

[0237] It should be noted that although Example 5 exhibits excellent cycle performance, its manufacturing process is difficult, yield is extremely low, and mass production feasibility is limited. Therefore, based on the total thickness of the first and second negative electrode active material layers, the thickness ratio of the first negative electrode active material layer should be greater than 10% and less than or equal to 50%, ideally between 20% and 50%. This not only allows for industrial production but also effectively reduces the expansion of the silicon-containing negative electrode sheet, thereby improving the cycle performance of the secondary battery.

[0238] Example 8 The difference between Example 8 and Example 1 lies only in that: the composition of the negative electrode active material in the slurry of the first negative electrode active material layer is adjusted to be artificial graphite and silicon-carbon materials with a mass ratio of 88.3:11.7, and the composition of the negative electrode active material in the slurry of the second negative electrode active material layer is adjusted to be artificial graphite and silicon-carbon materials with a mass ratio of 99.5:0.5. This is to ensure that the total silicon content of the negative electrode sheet and the thickness ratio of the first negative electrode active material layer are the same as in Example 1, while also ensuring that the second negative electrode active material layer contains silicon. The performance of the secondary battery prepared in Example 8 was also tested, and the results are shown in Table 3.

[0239] Table 3 Test Results

[0240] As shown in Table 3, the second negative electrode active material layer can contain a very small amount of silicon-carbon material. By controlling the mass content of silicon element A1 in the second negative electrode active material layer to be less than or equal to 0.5%, the secondary battery has low expansion and good cycle performance.

[0241] Examples 9-11 The only difference between Examples 9-11 and Example 1 is that the composition of the negative electrode active material in the first negative electrode active material layer is controlled so that the total silicon content in the negative electrode sheet is different, as shown in Table 4. The performance of the secondary batteries prepared in Examples 9-11 was also tested, and the results are shown in Table 4.

[0242] Table 4

[0243] As shown in Table 4, the mass percentage of silicon in the negative electrode active material of the lithium iron phosphate battery system is controlled to be 0.5% to 5%; furthermore, when the mass content of silicon is 1.0%-3.5%, the secondary battery has low expansion force and good cycle performance.

[0244] Examples 12-14 The only difference between Examples 12-14 and Examples 1, 6, and 7 is that the total coating weight of the first negative electrode active material layer and the second negative electrode active material layer, as well as the positions of the first negative electrode active material layer and the second negative electrode active material layer, were adjusted as shown in Table 4. At the same time, the performance of the secondary batteries prepared in Examples 12-14 was tested.

[0245] The total coating weight test of the positive electrode active material layer involved disassembling a fully charged secondary battery to obtain the positive electrode sheet. After cleaning the surface impurities of the positive electrode sheet with dimethyl formate and drying it, a piece with an area of ​​1540.25 mm² was cut. 2 The positive electrode sample is weighed on a balance and recorded as q1. Then the positive active material layer on the positive electrode is washed off, and the positive current collector is weighed on a balance and recorded as q2. Since the electrode is a positive electrode with a double-sided coating of positive active material layer, the total coating weight of the positive active material layer is (q1-q2) / 2.

[0246] The results are shown in Table 5. Table 5

[0247] As can be seen from Table 5, the first negative electrode active material layer can be located between the negative electrode current collector and the second negative electrode active material layer, or it can be located on the side of the second negative electrode active material layer away from the negative electrode current collector.

[0248] As can be seen from Examples 12 and 6, the total coating weight of the positive electrode active material layer is controlled to be ≤0.3g / 1540.25mm. 2In other words, the small total coating weight of the positive electrode active material layer results in a thinner overall thickness of the corresponding negative electrode film layer (the first negative electrode active material layer and the second negative electrode active material layer), and the overall dynamic performance of the negative electrode sheet is good. Therefore, the first negative electrode active material layer is set between the negative electrode current collector and the second negative electrode active material layer to reduce the contact between the first negative electrode active material layer and the electrolyte, reduce the occurrence of side reactions, improve cycle performance, and help the secondary battery to balance rate performance and cycle performance.

[0249] As can be seen from Examples 13, 14, and 7, the total coating weight of the positive electrode active material layer is controlled to be ≥0.4g / 1540.25mm. 2 In other words, a large total coating weight of the positive electrode active material layer results in a large overall energy density of the corresponding negative electrode active material layer, but poor kinetic performance. Therefore, the first negative electrode active material layer is placed on the side of the second negative electrode active material layer away from the negative electrode current collector, so that the first negative electrode active material layer can fully contact the electrolyte, improve kinetic performance, and help the secondary battery to achieve good energy density and rate performance, as well as good cycle performance.

[0250] Examples 15-16 The only difference between Examples 15-16 and Example 1 is that the selection of silicon-carbon material was adjusted as shown in Table 5. The performance of the secondary batteries prepared in Examples 15-16 was also tested, and the results are shown in Table 5.

[0251] Test of the average diameter of silicon-carbon material: The negative electrode sheet was cut open with an argon ion beam perpendicular to the large surface of the negative electrode sheet to expose the cross-section. The cross-section was photographed with a scanning electron microscope. The longest diameter of the silicon-carbon material particles was statistically analyzed using the length-diameter statistical method. Five silicon-carbon material particles were randomly selected from the electron microscope scan image, and the average of the longest diameters of the five silicon-carbon material particles was taken as the average diameter of the silicon-carbon material.

[0252] The test results are shown in Table 6. Table 6

[0253] in, Figure 9 The image shown is a cross-sectional SEM image of the negative electrode sheet of Example 1 (with the average diameter of the silicon-carbon material marked). According to Table 6, the average particle size of the silicon-carbon material is 8 μm.

[0254] As shown in Table 6, the average diameter of silicon-carbon materials is 3μm-10μm, and the secondary battery has low expansion force and good cycle performance. It can also be seen that when the average diameter of silicon-carbon materials is around 8μm, for example, when the average diameter of silicon-carbon materials is 6μm-9μm, the expansion force and cycle performance of the secondary battery can be further optimized.

[0255] Examples 17-18 The only difference between Examples 17-18 and Example 1 is that the selection of the positive electrode active material was adjusted as shown in Table 7. At the same time, the performance of the secondary batteries prepared in Examples 17-18 was tested, and the results are shown in Table 7.

[0256] In Table 7, the particle area distributions Ds90, Ds50, and Ds10 all refer to the particle sizes when the particle area distribution reaches 90%, 50%, and 10% respectively, in the cumulative area distribution curve of the particles in the cross-section (also called the cross-section) of the positive electrode sheet perpendicular to the large surface of the positive electrode sheet using an argon ion beam and photographed with a scanning electron microscope.

[0257] Table 7

[0258] According to Table 7, the area particle size distribution Ds50 of the particles in the cross-section along the thickness direction of the positive electrode active material layer is 600nm-900nm; and / or, the area particle size distribution Ds90 of the particles in the cross-section along the thickness direction of the positive electrode active material layer is 1400nm-2100nm; and / or the particle size distribution width of the particles in the cross-section along the thickness direction of the positive electrode active material layer is 1.855-2.375. The secondary battery has low expansion force and good cycle performance.

[0259] 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, The device includes a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes a lithium phosphate-containing material, and the negative electrode includes a negative current collector and a negative active material layer disposed on the surface of the negative current collector. The negative active material layer includes a first negative active material layer and a second negative active material layer disposed on the same side of the negative current collector. The first negative electrode active material layer includes a silicon-carbon material and a first graphite, wherein the silicon-carbon material includes porous carbon and silicon distributed in the porous carbon; Based on the total mass of the negative electrode active material layer, the mass content of silicon in the negative electrode sheet is 0.5% to 5%. In the second negative electrode active material layer, the mass content of silicon element A1 is less than or equal to 0.5%, and in the first negative electrode active material layer, the mass content of silicon element A2 is greater than A1. Based on the total thickness of the first negative electrode active material layer and the second negative electrode active material layer, the thickness ratio of the first negative electrode active material layer is ≤60%; the thickness of the first negative electrode active material layer is 5μm-35μm.

2. The secondary battery according to claim 1, characterized in that, Based on the total thickness of the first negative electrode active material layer and the second negative electrode active material layer, the thickness of the first negative electrode active material layer accounts for 24%-50%.

3. The secondary battery according to claim 1, characterized in that, The thickness of the second negative electrode active material is 22μm-52μm.

4. The secondary battery according to claim 1, characterized in that, The volumetric particle size distribution Dv50 of the first graphite is ≤7μm; and / or, The negative electrode active material in the second negative electrode active material layer includes a second graphite, and the volumetric particle size distribution Dv50 of the second graphite is 7μm≤15μm.

5. The secondary battery according to claim 1, characterized in that, In the complete cross-section of the negative electrode sheet, the second negative electrode active material layer contains ≤5 silicon material particles.

6. The secondary battery according to claim 1, characterized in that, A1 is 0.

7. The secondary battery according to any one of claims 1-6, characterized in that, The mass content of silicon in the negative electrode is 1.0%-3.5%.

8. The secondary battery according to any one of claims 1-6, characterized in that, The average diameter of the silicon-carbon material is 3μm-10μm.

9. The secondary battery according to any one of claims 1-6, characterized in that, The average diameter of the silicon-carbon material is 6μm-9μm.

10. The secondary battery according to any one of claims 1-6, characterized in that, The positive electrode sheet includes a positive active material layer, and the total coating weight of the positive active material layer is ≤0.3g / 1540.25mm. 2 The first negative electrode active material layer is located between the negative electrode current collector and the second negative electrode active material layer.

11. The secondary battery according to any one of claims 1-6, characterized in that, The positive electrode sheet includes a positive active material layer, and the total coating weight of the positive active material layer is ≥0.4g / 1540.25mm. 2 The second negative electrode active material layer is located between the negative electrode current collector and the first negative electrode active material layer.

12. The secondary battery according to any one of claims 1-6, characterized in that, The compacted density of the 3T powder of the positive electrode active material in the positive electrode sheet is 2.55 g / cm³. 3 -2.70g / cm 3 .

13. The secondary battery according to any one of claims 1-6, characterized in that, The positive electrode sheet includes a positive active material layer; In the cross-section of the positive electrode active material layer along the thickness direction of the positive electrode sheet, the area particle size distribution Ds50 of the particles is 600nm-900nm; and / or, In the cross-section of the positive electrode active material layer along the thickness direction of the electrode sheet, the area particle size distribution Ds90 of the particles is 1400nm-2100nm; and / or, In the cross-section along the thickness direction of the positive electrode active material layer, the particle size distribution width is 1.855-2.375, and the particle size distribution width = (particle area particle size distribution Ds90 - particle area particle size distribution Ds10) / particle area particle size distribution Ds50. Among them, the particle area distribution Ds90, particle area distribution Ds50, and particle area distribution Ds10 refer to the particle size corresponding to the cumulative area distribution of particles reaching 90%, 50%, and 10% respectively in the cumulative area distribution curve of particles.

14. The secondary battery according to any one of claims 1-6, characterized in that, The lithium-containing phosphate includes at least one of lithium iron phosphate and its modified compounds.

15. The secondary battery according to any one of claims 1-6, characterized in that, Either the first negative electrode active material layer or the second negative electrode active material layer is used as the surface layer, and the remaining layer is used as the inner layer. The inner layer is located between the surface layer and the negative electrode current collector. Wherein, the volumetric particle size distribution Dv50 of the negative electrode active material in the surface layer is smaller than that of the negative electrode active material in the inner layer.

16. An electrical appliance, characterized in that, Includes the secondary battery as described in any one of claims 1-15.

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