Negative electrode sheet, secondary battery, and electric device

Abstract

The application discloses a negative electrode sheet, which comprises a negative electrode current collector, a first negative electrode active material layer and a second negative electrode active material layer. The first negative electrode active material layer and the second negative electrode active material layer are both coated on the same surface of the negative electrode current collector, the compaction density of the first negative electrode active material layer is less than that of the second negative electrode active material layer, and the coating weight per unit area of the first negative electrode active material layer is less than that of the second negative electrode active material layer. The first negative electrode active material layer comprises a first negative electrode active material, and the second negative electrode active material layer comprises a second negative electrode active material. The gram capacity of the first negative electrode active material is greater than that of the second negative electrode active material. In this way, the application can improve the combination of lithium ions and negative electrode materials, thereby improving the safety performance of the secondary battery.

Description

Technical Field

[0001] This application relates to the field of energy storage technology, and in particular to a negative electrode sheet, a secondary battery, and an electrical device. Background Technology

[0002] The negative electrode, acting as a carrier of lithium ions and electrons, plays a crucial role in energy storage and release during the charging and discharging process of a secondary battery. Besides affecting energy density, the negative electrode in a lithium-ion battery also influences its safety performance. Specifically, the structure of the negative electrode is related to the bonding ability between lithium ions and the negative electrode material. Different negative electrode structures can mitigate internal short circuits caused by lithium metal deposition and reduce the deposition of excess lithium ions during charging, thus preventing the formation of lithium dendrites. Currently, most negative electrodes are made from the same electrode paste. However, this type of electrode has a limited capacity to accommodate lithium ions, which hinders the mitigation of lithium metal deposition and the deposition of excess lithium ions during charging, thus making it difficult to improve the safety performance of the secondary battery. Summary of the Invention

[0003] In view of the above problems, the purpose of this application is to provide a negative electrode sheet, a secondary battery, and an electrical device that can improve the bonding ability between lithium ions and negative electrode materials, thereby improving the safety performance of the secondary battery.

[0004] A first aspect of this application provides a negative electrode sheet, comprising a negative current collector, a first negative active material layer, and a second negative active material layer. Both the first and second negative active material layers are disposed on the same surface of the negative current collector. The compaction density of the first negative active material layer is less than that of the second negative active material layer, and the coating weight per unit area of ​​the first negative active material layer is less than that of the second negative active material layer. The first negative active material layer comprises a first negative active material, and the second negative active material layer comprises a second negative active material. The specific capacity of the first negative active material is greater than that of the second negative active material.

[0005] According to some embodiments of this application, the ratio of the compaction density of the first negative electrode active material layer to the compaction density of the second negative electrode active material layer is 0.75 to 0.97.

[0006] According to some embodiments of this application, the ratio of the compaction density of the first negative electrode active material layer to the compaction density of the second negative electrode active material layer is 0.80 to 0.93.

[0007] According to some embodiments of this application, the ratio of the specific capacity of the first negative electrode active material to the specific capacity of the second negative electrode active material is 1.03 to 1.33.

[0008] According to some embodiments of this application, the ratio of the specific capacity of the first negative electrode active material to the specific capacity of the second negative electrode active material is 1.12 to 1.15.

[0009] According to some embodiments of this application, the compaction density of the first negative electrode active material layer is 1.54 g / cm³. 3 ~1.68g / cm 3 The compaction density of the second negative electrode active material layer is 1.59 g / cm³. 3 ~2.24g / cm 3 .

[0010] According to some embodiments of this application, the specific capacity of the first negative electrode active material is 345 mAh / g to 473 mAh / g, and the specific capacity of the second negative electrode active material is 336 mAh / g to 420 mAh / g.

[0011] According to some embodiments of this application, the first negative electrode active material includes at least one of artificial graphite, natural graphite, mesophase carbon microspheres, soft carbon, or hard carbon.

[0012] According to some embodiments of this application, the second negative electrode active material includes at least one of artificial graphite, natural graphite, mesophase carbon microspheres, soft carbon, or hard carbon.

[0013] According to some embodiments of this application, the first negative electrode active material includes one or more of silicon, silicon alloy, silicon oxide, or silicon-carbon composite.

[0014] According to some embodiments of this application, the second active material includes one or more of silicon, silicon alloys, silicon oxides, or silicon-carbon composites.

[0015] According to some embodiments of this application, the mass percentage content of silicon in the first negative electrode active material layer is 2%-20%, and the mass percentage content of silicon in the second negative electrode active material layer is 0%-10%.

[0016] A second aspect of this application provides a secondary battery, comprising a housing and an electrode assembly housed within the housing. The electrode assembly includes a positive electrode, a negative electrode as described above, and a separator separating the positive and negative electrodes. The positive electrode, separator, and negative electrode are sequentially stacked and wound to form the electrode assembly. The positive electrode includes a positive current collector and a positive active material layer disposed on the surface of the positive current collector, which is connected to a positive electrode tab. The negative current collector is connected to a negative electrode tab.

[0017] According to some embodiments of this application, the negative electrode sheet in the electrode assembly includes a straight section and a bent section; the negative electrode tab is connected to the straight section of the negative electrode sheet, and the active material layer in the straight section of the negative electrode sheet is the first negative electrode active material layer.

[0018] According to some embodiments of this application, the first negative electrode active material layer is disposed on the outermost negative electrode sheet of the electrode assembly.

[0019] A third aspect of this application provides an electrical device, including a secondary battery as described in the second aspect and a load electrically connected to the secondary battery.

[0020] The negative electrode sheet provided in this application embodiment, under the condition that the active material layer thickness in each region of the negative electrode sheet is the same, has a lower compaction density of the first negative electrode active material layer compared to the second negative electrode active material layer, and the coating weight of the first negative electrode active material layer is less than that of the second negative electrode active material layer. The first negative electrode active material layer has a higher specific capacity compared to the second negative electrode active material, which makes the unit volume energy density of the first negative electrode active material approximately the same as that of the second negative electrode active material, thus improving the safety performance of the secondary battery. When the first negative electrode active material layer has a lower compaction density compared to the second negative electrode active material layer, the lower compaction density allows the electrolyte to fully wet around the active material particles, increasing ion transport channels and reducing the solid-phase diffusion resistance of active lithium ions within the material. During high-rate charging, active lithium ions are more easily reduced, and lithium plating is less likely to occur. Simultaneously, the higher compaction density in the second negative electrode active material reduces the spacing between active material particles, resulting in tighter contact, more conductive network contact points, and thus increased conductivity, thereby reducing the internal resistance of the secondary battery to some extent. When the first negative electrode active material layer has a higher specific capacity than the second negative electrode active material layer, the higher specific capacity of the first negative electrode active material layer also makes the first negative electrode active material closer to the complete layered structure of ideal graphite. This results in fewer defects in the crystal structure, fewer side reactions, and less negative electrode active material required for the secondary battery to reach the expected capacity, which is more conducive to improving the volumetric energy density of the secondary battery. Furthermore, fewer defects in the crystal structure indicate reduced surface activity. During cycling, the SEI film will be continuously damaged and repaired, consuming less active lithium in the process, thus delaying the capacity decay of the secondary battery. The lower specific capacity of the second negative electrode active material layer also increases the interplanar spacing of the material, improving the resistance to ions entering the interior of the second active material material caused by the higher compaction density of the second negative electrode active material layer. It also makes it less likely to increase the material's volume expansion rate, and has a smaller impact on the high-rate charge and discharge of the secondary battery and the stability of the second negative electrode active material structure. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0022] Figure 1 This application provides a schematic diagram of the structure of a negative electrode sheet;

[0023] Figure 2 A schematic diagram of a secondary battery without a casing provided in this application;

[0024] Figure 3 for Figure 2 A schematic diagram of the negative electrode plate in the electrode assembly in its unfolded state. Detailed Implementation

[0025] In this document, unless otherwise stated, “a,” “this,” “at least one,” and “one or more,” as well as instances where no quantifier is used, are used interchangeably. Unless otherwise stated herein, the use of the singular form is also intended to include the plural form.

[0026] In this description, it should be noted that, unless otherwise stated, "above" and "below" do not include the number itself, and "multiple" in "one or more" means two or more.

[0027] When a composition is described as including or containing specific components, it is anticipated that optional components not covered in this application are not excluded from the composition, and that the composition may be constituted or composed of the components involved. Similarly, when a method is described as including or containing specific process steps, it is anticipated that optional process steps not covered in this application are not excluded from the method, and that the method may be constituted or composed of the process steps involved.

[0028] For simplicity, this paper only explicitly discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form an undefined range; and any lower limit can be combined with other lower limits to form an undefined range, just as any upper limit can be combined with any other upper limit to form an undefined range. Furthermore, although not explicitly stated, every point or individual value between the endpoints of a range is included within that range. Therefore, each point or individual value can serve as its own lower or upper limit and be combined with any other point or individual value, or with other lower or upper limits, to form an undefined range.

[0029] I. Negative electrode plate

[0030] As mentioned in the background section, if interference from factors such as manufacturing processes and raw materials is excluded, each region on a negative electrode sheet made from the same electrode slurry has approximately the same lithium-ion insertion / extraction capability. However, during cyclic charging and discharging of the secondary battery, factors such as uneven lithium-ion concentration distribution in the external electrolyte, current edge effects, different solid-phase diffusion rates due to varying thicknesses of the active material layer, or low polarization leading to easy lithium alloying can cause lithium deposition on the surface of these regions of the negative electrode sheet, which is detrimental to improving the safety performance of the secondary battery.

[0031] The inventors of this application have discovered that, in response to the above-mentioned situation, the negative electrode sheet can be made of two electrode pastes with different binding capabilities between lithium ions and negative electrode materials. Specifically, an electrode paste with a slightly weaker binding capability between lithium ions and negative electrode materials is coated in areas of the negative electrode sheet less affected by the above-mentioned factors of lithium deposition, while an electrode paste with a slightly stronger binding capability between lithium ions and negative electrode materials is coated in areas of the negative electrode sheet more affected by the above-mentioned factors of lithium deposition. This reduces the occurrence of lithium deposition and thus improves the safety performance of the secondary battery.

[0032] However, the different binding capabilities of the two electrode slurries to lithium ions and anode materials also mean that they have different parameters. These different parameters may include different compaction densities, different coating weights, different specific capacities, and different coating thicknesses. This may result in the coated portions of the two electrode slurries having different volumetric energy densities. These different volumetric energy densities can lead to lithium plating due to overcharging during the recharge-discharge cycle of the secondary battery, which is detrimental to improving the safety performance of the secondary battery.

[0033] Therefore, based on the premise that the energy density per unit volume is consistent across all regions of the aforementioned negative electrode sheet, the negative electrode sheet simultaneously satisfies the following relationships: PD1 / PD2 = M1 / M2 = S2 / S1, and PD1 < PD2, M1 < M2, S1 > S2. Compared to negative electrode sheets made from the same electrode paste, the negative electrode sheet provided in this application can maintain the normal capacity of the negative electrode sheet and improve the safety performance of the secondary battery.

[0034] Wherein, PD1 represents the compaction density of the first negative electrode active material layer, that is, the density of the first negative electrode active material layer after cold pressing, in g / cm³. 3 For example, it can be calculated based on the compaction density PD1 of the first negative electrode active material layer: PD1 = coating surface density CW1 of the first negative electrode active material layer on the negative electrode sheet / coating thickness of the first negative electrode active material layer. Here, CW1 represents the coating surface density of the first negative electrode active material layer on the negative electrode sheet, i.e., the weight of the first negative electrode active material per unit area on the negative electrode sheet after cold pressing, in g / cm³. 2 It should be noted that M1 is the coating weight per unit area of ​​the first negative electrode active material on the negative electrode sheet after cold pressing.

[0035] Similarly, PD2 represents the compaction density of the second negative electrode active material layer, that is, the density of the second negative electrode active material layer after cold pressing, in g / cm³. 3 For example, it can be calculated based on the compaction density PD2 of the second negative electrode active material layer: PD2 = (coating areal density CW2 of the second negative electrode active material layer on the negative electrode sheet) / (coating thickness of the second negative electrode active material layer). CW2 represents the coating areal density of the second negative electrode active material layer on the negative electrode sheet, i.e., the weight of the second negative electrode active material per unit area on the negative electrode sheet after cold pressing, in g / cm³. 2 It should be noted that M2 is the coating weight per unit area of ​​the first negative electrode active material on the negative electrode sheet after cold pressing. It is worth mentioning that CW1 and CW2 can be measured according to the method described in the embodiments of this application, or according to other methods known in the art.

[0036] Wherein, S1 represents the specific capacity of the first negative electrode active material, that is, the ratio of the capacity that the first negative electrode active material can release on the negative electrode plate to the weight of the first negative electrode active material during the first charge and discharge of the secondary battery, in mAh / g. Similarly, S2 represents the specific capacity of the second negative electrode active material, that is, the ratio of the capacity that the second negative electrode active material can release on the negative electrode plate to the weight of the second negative electrode active material during the first charge and discharge of the secondary battery, in mAh / g.

[0037] Figure 1 For a schematic diagram of the structure of a negative electrode 13 provided in an embodiment of this application, please refer to [link / reference]. Figure 1 In the example shown, the negative electrode 13 includes a negative current collector 133, a first negative active material layer 131, and a second negative active material layer 132. The first negative active material layer 131 includes a first negative active material, and the second negative active material layer 132 includes a second negative active material. Both the first negative active material layer 131 and the second negative active material layer 132 are coated on the same surface of the negative current collector 133. Exemplarily, along the thickness direction of the negative current collector 133, the negative current collector 133 has two opposing surfaces, and both the first negative active material layer 131 and the second negative active material layer 132 are coated on either of these two surfaces. Of course, provided that both the first negative active material layer 131 and the second negative active material layer 132 are coated on the same surface of the negative current collector 133, both surfaces can be simultaneously coated with both the first negative active material layer 131 and the second negative active material layer 132.

[0038] The negative electrode 13 provided in this application embodiment, under the condition that the coating thickness of each region on the negative electrode 13 is the same, has a lower compaction density of the first negative electrode active material layer 131 compared to the second negative electrode active material layer 132, and a higher specific capacity compared to the second negative electrode active material. This makes the unit volume energy density of the first negative electrode active material approximately the same as that of the second negative electrode active material, which is beneficial to improving the safety performance of the secondary battery. It should be noted that the aforementioned condition that the coating thickness of each region on the negative electrode 13 is the same does not mean that the coating thickness of each region on the negative electrode 13 is exactly the same, but rather that the coating thickness of each region on the negative electrode 13 is approximately the same within the allowable error range of the manufacturing process. For example, the allowable error range is 0μm-2μm. That is to say, the difference between the coating thickness of the first negative electrode active material layer 131 and the coating thickness of the second negative electrode active material layer 132 can be 0μm-2μm.

[0039] When the first negative electrode active material layer 131 has a lower compaction density than the second negative electrode active material layer 132, the lower compaction density of the first negative electrode active material layer 131 allows the electrolyte to fully wet the active material particles, increasing ion transport channels and reducing the solid-phase diffusion resistance of active lithium ions within the material. During high-rate charging, active lithium ions are more easily reduced, making lithium plating less likely. Simultaneously, the higher compaction density in the second negative electrode active material reduces the spacing between active material particles, resulting in tighter contact, more conductive network contact points, and thus increased conductivity, thereby reducing the internal resistance of the secondary battery to some extent.

[0040] When the first negative electrode active material layer 131 has a higher specific capacity than the second negative electrode active material layer 132, the higher specific capacity of the first negative electrode active material layer 131 also makes the first negative electrode active material closer to the complete layered structure of ideal graphite. This results in fewer defects in the crystal structure, fewer side reactions, and less negative electrode active material required for the secondary battery to reach the expected capacity, which is more conducive to improving the volumetric energy density of the secondary battery. Furthermore, fewer defects in the crystal structure indicate reduced surface activity. During cycling, the SEI film will be continuously damaged and repaired, consuming less active lithium and delaying the capacity decay of the secondary battery. The lower specific capacity of the second negative electrode active material layer 132 also increases the interplanar spacing of the material, improving the resistance to ions entering the interior of the second active material material caused by the higher compaction density of the second negative electrode active material layer 132. It also makes it less likely to increase the material's volume expansion rate, thus having a smaller impact on the high-rate charge and discharge of the secondary battery and the stability of the second negative electrode active material structure.

[0041] According to some embodiments of this application, the ratio of the compaction density PD1 of the first negative electrode active material layer 131 to the compaction density PD2 of the second negative electrode active material layer 132 is 0.75 to 0.97. If the ratio of PD1 / PD2 is less than 0.75, it indicates that the compaction density of the first negative electrode active material layer 131 relative to the second negative electrode active material layer 132 is too large, or the compaction density of the second negative electrode active material layer 132 relative to the first negative electrode active material layer 131 is too small. If the compaction density of the first negative electrode active material layer 131 relative to the second negative electrode active material layer 132 is too large, the electrolyte will have difficulty in fully wetting the first active material particles, resulting in a reduction in electrolyte absorption, a reduction or blockage of ion transport channels, and thus a greater likelihood of lithium plating on the surface of the first negative electrode active material layer 131. In addition, a larger compaction density will also cause the negative electrode current collector 133 to stretch excessively, increasing the probability of the negative electrode sheet 13 becoming brittle. If the compaction density of the second negative electrode active material layer 132 is too low compared to the first negative electrode active material layer 131, although the electrolyte can fully wet the area around the first active material particles, the conductivity is reduced due to fewer contact points in the conductive network, which is detrimental to achieving high-rate charge and discharge of the secondary battery. If the PD1 / PD2 ratio is greater than 0.97, it indicates that the compaction density of the first negative electrode active material layer 131 and the second negative electrode active material layer 132 are not significantly different, almost equivalent to the negative electrode sheet 13 coated with the same electrode slurry. In this case, the negative electrode sheet 13 is less likely to improve the safety performance of the secondary battery. Therefore, limiting PD1 / PD2 to this value range allows for a balance between ion conduction and electronic conduction, thus improving both the safety performance and the battery capacity of the secondary battery. To ensure unimpeded electron transport and increase the number of ion transport channels, thereby enabling high-rate charge and discharge of the secondary battery, the ratio of the compaction density PD1 of the first negative electrode active material layer 131 to the compaction density PD2 of the second negative electrode active material layer 132 is further set to 0.80-0.93. For example, the compaction density PD1 of the first negative electrode active material layer 131 is 1.54-1.68 g / cm³. 3 The compaction density PD2 of the second negative electrode active material layer 132 is 1.59 g / cm³. 3 ~2.24g / cm 3 .

[0042] According to some embodiments of this application, the ratio of the specific capacity S1 of the first negative electrode active material to the specific capacity S2 of the second negative electrode active material is 1.03 to 1.33. If the ratio of S1 / S2 is less than 1.03, it indicates that the specific capacity of the first negative electrode active material relative to the second negative electrode active material is too small, or the specific capacity of the second negative electrode active material relative to the first negative electrode active material is too large. If the specific capacity of the first negative electrode active material relative to the second negative electrode active material layer 132 is too small, according to the above relationship, it will lead to a difference between the unit volume capacity of the first negative electrode active material layer 131 and the unit volume capacity of the second negative electrode active material layer 132. The different unit volume energy densities will cause lithium plating due to overcharging during the cycle charging and discharging of the secondary battery, which is detrimental to improving the safety performance of the secondary battery. If the specific capacity of the second negative electrode active material is too large relative to the first negative electrode active material, it will lead to a decrease in the overall lithium-ion insertion / extraction capability of the negative electrode sheet 13. During high-rate charge and discharge, the lithium ions extracted from the positive electrode will not have enough time to insert into the negative electrode material, resulting in the precipitation of metallic lithium on the electrode surface, triggering side reactions and generating a large number of by-products, which is detrimental to the cycle stability of the secondary battery. If the ratio of S1 / S2 is greater than 1.33, it indicates that the specific capacity of the first negative electrode active material is too large relative to the second negative electrode active material, or the specific capacity of the second negative electrode active material is too small relative to the first negative electrode active material. If the specific capacity of the first negative electrode active material is too large relative to the second negative electrode active material layer 132, the interlayer spacing of the first negative electrode active material material is too small, the solid-phase diffusion resistance of ions inside the material is large, and it is more difficult for active ions to enter the negative electrode active material, which is detrimental to the high-rate charge and discharge of the secondary battery. If the specific capacity of the second negative electrode active material is too small relative to the first negative electrode active material, it is not conducive to improving the overall volumetric energy density of the secondary battery. By limiting the S1 / S2 ratio to this range, both the overall volumetric energy density and cycle stability of the secondary battery can be improved. Further, the S1 / S2 ratio is 1.12 to 1.15. For example, the specific capacity of the first negative electrode active material is 345 mAh / g to 473 mAh / g, and the specific capacity of the second negative electrode active material is 336 mAh / g to 420 mAh / g.

[0043] It is understood that the embodiments of this application do not specifically limit the first and second negative electrode active materials, as long as they are conductive and unlikely to cause chemical changes in the secondary battery. For example, the first and second negative electrode active materials can independently include at least one of artificial graphite, natural graphite, mesophase carbon microspheres, soft carbon, or hard carbon. In addition, the first negative electrode active material layer 131 and / or the second negative electrode active material layer 132 may also include conductive agents, binders, and dispersants. Conductive agents include, but are not limited to, conductive fibers, conductive tubes, metal powders, conductive whiskers, conductive metal oxides, and conductive materials. Conductive fibers can be carbon fibers and metal fibers; conductive tubes can be carbon nanotubes; metal powders can be fluorocarbon powders, aluminum powders, and nickel powders; conductive whiskers can be zinc oxide and potassium titanate; conductive metal oxides can be titanium oxides; and conductive materials can be polyphenylene derivatives, etc. The binder or dispersant may comprise at least one material selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, materials in which hydrogen is replaced by Li, Na, or Ca, and combinations thereof. Furthermore, the post-dispersant of the binder may also comprise various copolymers thereof.

[0044] The embodiments of this application do not specifically limit the structure of the negative electrode current collector 133, as long as it is conductive and unlikely to undergo chemical changes with the first negative electrode active material, the second negative electrode active material, and the electrolyte. For example, copper, stainless steel, aluminum, nickel, titanium, or aluminum or stainless steel treated with one of carbon, nickel, or titanium silver can be used. Exemplarily, transition metals such as copper and nickel, which are good at adsorbing carbon, can be used as the negative electrode current collector 133.

[0045] The negative electrode 13 according to another embodiment of this application is the same as the negative electrode 13 described above, except that the first negative electrode active material layer 131 contains a first negative electrode active material, and the second negative electrode active material layer 132 contains a second negative electrode active material, and the first negative electrode active material layer 131 and / or the second negative electrode active material layer 132 also contain one or more of silicon, silicon alloy, silicon oxide, or silicon-carbon composite. Doping the first negative electrode active material with the aforementioned silicon-containing active material particles and doping the second negative electrode active material with the aforementioned silicon-containing active material particles can improve the conductivity of the first negative electrode active material and / or the second negative electrode active material, thereby increasing the specific capacity of the first negative electrode active material layer 131 and / or the diffusion coefficient of active ions within it, and further improving the volumetric energy density of the negative electrode 13.

[0046] Because the volume of the silicon-containing negative electrode 13 increases excessively during the charge and discharge of the secondary battery, in order to reduce the volume expansion of the secondary battery, the mass percentage of silicon in the first negative electrode active material layer 131 is further set at 2% to 20%, and the mass percentage of silicon in the second negative electrode active material layer 132 is set at 0% to 10%. Maintaining the mass percentage of silicon in the first negative electrode active material within this range results in superior physical strength, chemical stability, and conductivity compared to a silicon-containing first negative electrode active material exceeding 20%. This leads to less volume expansion of the secondary battery during charge and discharge, a lower probability of side reactions with the electrolyte, and better cycle stability. Maintaining the mass percentage of silicon in the second negative electrode active material layer 132 within this range achieves essentially the same effect. It should be noted that, since the mass percentage of silicon is linearly related to the specific capacity of the active material, and the specific capacity in the second negative electrode active material layer 132 is smaller than that in the first negative electrode active material layer 131, the upper limit of the mass percentage of silicon in the second negative electrode active material is lower.

[0047] According to some embodiments of this application, the graphitization degree of both the first negative electrode active material and the second negative electrode active material is greater than 92%. A higher graphitization degree indicates a higher degree of ordering of the active material, fewer defects such as stacking faults and dislocations in the crystal, lower surface activity, and easier formation of a stable and dense SEI film during cycling, which is beneficial for improving cycling performance.

[0048] According to some embodiments of this application, the particle sizes of the first negative electrode active material and the second negative electrode active material both satisfy the following: 2μm < D10, 5μm < D50 < 20μm, and 10μm < D90 < 50μm.

[0049] Secondary batteries

[0050] The secondary battery provided in this application includes a casing (not shown) and an electrode assembly 10 housed within the casing. The electrode assembly 10 includes a positive electrode 11, a negative electrode 13 as described above, and a separator 12 separating the negative electrode 13 from the negative electrode 13. The positive electrode 11, the separator 12, and the negative electrode 13 are sequentially stacked and wound to form a structure as shown in the figure. Figure 2 The electrode assembly 10 is shown. The positive electrode 11 includes a positive current collector and a positive active material layer disposed on the surface of the current collector, with a positive electrode tab 20 connected to the current collector. The negative electrode 13 has a negative current collector 133 connected to a negative electrode tab 30. Furthermore, the secondary battery includes an electrolyte, which is contained within a casing, and the electrode assembly 10 is immersed in the electrolyte. It is understood that the electrode assembly 10 of this application can also be a stacked electrode assembly 10, and no specific limitation is made here.

[0051] According to some embodiments of this application, see also Figure 2 and Figure 3 In this wound electrode assembly 10, the negative electrode sheet 13 includes a straight section 10a and a bent section 10b. A negative electrode tab 30 is connected to the straight section 10a of the negative electrode sheet 13. The active material layer in the straight section 10a of the negative electrode sheet 13 is a first negative electrode active material layer 131. In other words, except for the straight section 10a where the negative electrode tab 30 is located, where the active material layer is the first negative electrode active material layer 131, the active material layers in both the straight section 10a and the bent section 10b where the negative electrode tab 30 is not located are second negative electrode active material layers 132. Generally, the potential around the negative electrode tab 30 is relatively low, making it easier to reach the lithium plating potential and plating lithium. By providing a first negative electrode active material layer 131 with a slightly stronger binding capacity between lithium ions and the negative electrode material around the negative electrode tab 30, lithium plating can be improved, and the capacity of the secondary battery can be maintained at an appropriate level. Of course, the first negative electrode active material layer 131 is not limited to this; it can be applied to any region of the negative electrode 13 that is prone to lithium deposition to improve lithium deposition, depending on actual usage requirements. It should be noted that the straight section 10a in the wound electrode assembly 10 refers to the generally flat extension of the negative electrode 13 after winding, while the bent section 10b in the wound electrode assembly 10 refers to the bent extension of the negative electrode 13 relative to the straight section 10a after winding.

[0052] According to some embodiments of this application, please refer to Figure 3In the electrode assembly 10, the active material layer of the outermost negative electrode 13 is the first negative electrode active material layer 131. In other words, apart from the active material layer of the outermost negative electrode 13 being the first negative electrode active material layer 131, the other parts of the electrode assembly 10 can be the second negative electrode active material layer 132. This is because in the internal structure of the wound secondary battery, the outermost negative electrode 13 is subject to weaker binding force from the winding structure, resulting in more free electrolyte around it and easier electrolyte accumulation. By setting the active material layer of the outermost negative electrode 13 as the first negative electrode active material layer 131, which has a slightly stronger binding ability between lithium ions and negative electrode materials, lithium plating can be improved, and the capacity of the secondary battery can be maintained at an appropriate level. Of course, the first negative electrode active material layer 131 is not limited to this; it can be applied to any area of ​​the negative electrode 13 that is prone to lithium plating to improve lithium plating, depending on the actual application requirements.

[0053] In some embodiments of this application, the type of positive electrode active material in the positive electrode active material layer is not specifically limited. For example, the positive electrode active material can be lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium olivine-structured lithium phosphate, etc. These positive electrode active materials can be used alone or in combination. Exemplarily, the positive electrode active material is selected from LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(NCM 333 LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM 523 LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM 622 LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM 811 LiNi 0.85 Co 0.15 Al 0.05 One or more of O2, LiFePO4 (LFP), and LiMnPO4.

[0054] In some embodiments of this application, the type of separator 12 is not specifically limited, and may be, but is not limited to, polyethylene, polypropylene, polyvinylidene fluoride and their multilayer composite films.

[0055] In some embodiments of this application, the electrolyte includes an electrolyte salt and an organic solvent. The specific types and compositions of the electrolyte salt and organic solvent are not specifically limited and can be selected according to actual needs. The electrolyte may also include additives. There are no particular limitations on the types of additives; they can be negative electrode film-forming additives, positive electrode film-forming additives, or additives that can improve certain battery performance characteristics, such as additives that improve battery overcharge performance, battery high-temperature performance, and battery low-temperature performance.

[0056] III. Electrical Appliances

[0057] This application further provides an electrical device comprising the secondary battery and load described in the second aspect of this application, wherein the secondary battery is electrically connected to the load. The electrical devices of this application include, but are not limited to, laptops, pen-based computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and secondary capacitors, etc.

[0058] The following uses a secondary battery as an example to further illustrate this application through specific embodiments and comparative examples. These embodiments are only for illustrative purposes, and this application is not limited to the following embodiments. Any modifications or equivalent substitutions to the technical solutions of this application that do not depart from the scope of the technical solutions of this invention should be covered within the protection scope of this application.

[0059] The secondary batteries in the examples and comparative examples were prepared according to the following method.

[0060] Preparation of the positive electrode sheet:

[0061] ① Place the conductive agent and lithium cobalt oxide into a planetary high-energy ball mill and dry grind for 10 to 100 minutes;

[0062] ② Transfer the material obtained in ① to a self-rotating and revolution-rotating mixer. Add all the binder and 1 / 3 to 2 / 3 of the dispersion medium according to the formula weight to the mixer. Mix at high speed for 5 to 30 minutes. After mixing, defoam for 2 to 10 minutes.

[0063] ③ Add the remaining 1 / 3 to 2 / 3 of the formulation weight of the dispersion medium to the material obtained in step ②, and stir at high speed for 5 to 30 minutes. After stirring, defoam for 1 to 5 minutes to obtain the positive electrode slurry. The dispersion medium is N-methylpyrrolidone (NMP), the conductive agent is conductive carbon black and carbon nanotubes; the binder is polyvinylidene fluoride; the solid content of the positive electrode slurry is 75%. The positive electrode slurry is coated on aluminum foil, dried at 100℃, and cold-pressed to obtain the positive electrode sheet.

[0064] Preparation of the negative electrode sheet: Artificial graphite (first negative electrode active material) and artificial graphite (second negative electrode active material), acetylene black (conductive agent), sodium carboxymethyl cellulose (CMC) (thickener), and styrene-butadiene rubber (SBR) (binder) were mixed in a weight ratio of 96:1:1.5:1.5. Deionized water was added, and a negative electrode slurry was obtained under vacuum stirring. The solid content of the negative electrode slurry was 54 wt.%. The negative electrode slurry was uniformly coated onto a copper foil current collector. The coated copper foil was dried at 85°C, then cold-pressed, cut, and slit. Finally, it was dried under vacuum at 120°C for 12 hours to obtain the negative electrode sheet. It should be noted that during the coating process, the overlap between the first and second negative electrode active material layers was controlled to 0–0.5 mm to avoid copper or aluminum leakage during coating, which could lead to safety risks.

[0065] Preparation of the separator: PE porous polymer film was used as the separator.

[0066] Preparation of electrolyte: Ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 3:7. Then, fully dried lithium salt LiPF6 was dissolved in the mixed organic solvent at a ratio of mol / L. Based on the above basic electrolyte, 2 wt.% of fluoroethylene carbonate (FEC) was added to prepare the electrolyte.

[0067] Preparation of secondary batteries: The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. Then, the electrode assembly is wound up to obtain the electrode assembly. The electrode assembly is placed in the outer packaging foil, and the prepared electrolyte is injected into the dried battery. After vacuum sealing, standing, formation, and shaping, the preparation of secondary batteries is completed.

[0068] Examples 1-13 and Comparative Examples 1-3 were prepared using a method similar to that of Example 1, with specific differences shown in Table 1. Table 1 shows the parameters for Examples 2-8 and Comparative Examples 1-3.

[0069] The following examples and comparative examples show artificial graphite and SiO2 prepared using different methods. x (0≤X<2) Materials are artificial graphite A, artificial graphite B, artificial graphite C, artificial graphite D, and SiO₂.x Materials with different gram capacities, where the first charge-discharge gram capacity of A is 356 mAh / g, the first charge-discharge gram capacity of B is 346 mAh / g, the first charge-discharge gram capacity of C is 341 mAh / g, the first charge-discharge gram capacity of D is 336 mAh / g, and the first charge-discharge gram capacity of SiO x material is 1600 mAh / g; the particle sizes of the negative electrode materials all satisfy 2 μm < D10 < 15 μm, 5 μm < D50 < 20 μm, and 10 μm < D90 < 50 μm. Among them, the graphitization degree of the graphite material > 92%.

[0070] Table 1

[0071]

[0072] Test methods for parameters of negative electrode sheets and performance parameters of secondary batteries

[0073] The performance parameters involved in each embodiment are measured according to the following methods.

[0074] (1) D10, D50, and D90 of the negative electrode sheet:

[0075] Using a laser diffraction particle size distribution analyzer (Malvern Mastersizer 3000), according to the laser diffraction method for particle size distribution GB / T19077-2016, measure the particle size distribution of the first negative electrode active material and the second negative electrode active material to obtain D90, D50, and D10.

[0076] (2) Coating surface density CW1 of the first negative electrode active material layer and coating surface density CW2 of the second negative electrode active material layer of the negative electrode sheet:

[0077] Weigh the weight of the first negative electrode active material layer using a standard balance and measure the coating area of the first negative electrode active material layer using a ruler, and then calculate the mass per unit area of the first negative electrode active material layer, that is, the coating surface density CW1 (g / cm 2 ).

[0078] Weigh the weight of the second negative electrode active material layer using a standard balance and measure the coating area of the second negative electrode active material layer using a ruler, and then calculate the mass per unit area of the second negative electrode active material layer, that is, the coating surface density CW2 (g / cm 2 ).

[0079] (3) Compaction density PD1 of the first negative electrode active material layer and compaction density PD2 of the second negative electrode active material layer of the negative electrode sheet:

[0080] First, measure the areal density CW1 of the first negative electrode active material layer of the negative electrode sheet as described above. Then, measure the coating thickness of the first negative electrode active material layer using a micrometer (measure at least 5 points and take the average value). Based on the formula: Compacted density of the first negative electrode active material layer = Coating areal density CW1 of the first negative electrode active material layer (g / cm³),... 2 The compaction density PD1 (g / cm) of the first negative electrode active material layer is calculated from the coating thickness (cm) of the first negative electrode active material layer. 3 The calculation method for the compaction density PD2 of the second negative electrode active material layer is the same as above, and will not be repeated here.

[0081] (4) Specific capacity of the first negative electrode active material and the specific capacity of the second negative electrode active material

[0082] The first negative electrode active material is mixed, coated, and dried to form a negative electrode sheet. A lithium sheet is used as the positive electrode, and the sheets are assembled into coin cells for testing. The coin cells are discharged at 0.05C to 5.0mV, discharged at 50μA to 5.0mV, discharged at 10μA to 5.0mV, and charged at 0.1C to 2.0V. The capacity of the coin cells at these times is recorded and denoted as the specific capacity of the first negative electrode active material. The specific capacity of the second negative electrode active material can also be obtained using the aforementioned method.

[0083] Secondary battery related parameter testing

[0084] (5) Lithium plating in secondary batteries

[0085] At 25℃, the lithium-ion battery is charged to 4.45V with a constant current of 1C, then charged to 0.05C with a constant voltage of 4.45V, left to stand for 2 minutes, and then discharged to 3.0V with a constant current of 1C, left to stand for 2 minutes. This constitutes one cycle. After repeating 10 cycles, the lithium-ion battery is disassembled to obtain the electrode assembly. The electrode assembly is laid flat. If any part of the negative electrode sheet is found to be larger than 2mm... 2 The lithium deposition in the area is determined to be lithium deposition on the negative electrode.

[0086] (6) Battery cycle performance of secondary batteries

[0087] Five lithium-ion batteries were prepared using all comparative examples and embodiments, and the average value was taken. The lithium-ion batteries were repeatedly charged and discharged using the following steps, and the cycle capacity retention rate and thickness expansion rate of the lithium-ion batteries were calculated.

[0088] First, the lithium-ion battery was charged and discharged for the first time in an environment of 25°C. Constant current charging was performed at a charging current of 1C until the upper limit voltage of 4.48V was reached, then constant voltage charging was switched to constant voltage charging. Then, constant current discharging was performed at a discharging current of 1C until the final voltage was 3V. The discharge capacity and the thickness of the fully charged lithium-ion battery were recorded for the first cycle. Then, 300, 500, 700 and 900 charge and discharge cycles were performed, and the discharge capacity and the thickness of the fully charged secondary battery were recorded for the 300th, 500th, 700th and 900th cycles.

[0089] Test Results

[0090] Table 2 shows detailed data on the first and second negative electrode active material layers from Examples 1-13 and Comparative Examples 1-3. Table 3 shows the effect of applying the first negative electrode active material layer to the lithium-electrode ...

[0091] Table 2

[0092]

[0093]

[0094] Table 3

[0095]

[0096] Comparing Examples 1-5 with Comparative Example 1, it can be seen that Comparative Example 1 experienced severe lithium plating after 500 cycles. When the compaction density ratio of the first active material layer to the second active material layer is 0.98, the lithium plating phenomenon is significantly improved; when the compaction density ratio is 0.97, the lithium plating phenomenon is further improved. When the compaction density ratio is 0.94, lithium plating does not occur within the cycle life.

[0097] Example 6 is compared with Comparative Example 2. Comparative Example 2 experienced severe lithium plating after 500 cycles, while Example 6 had a compaction density ratio of 0.93, which met the requirement of no lithium plating within the cycle life range.

[0098] Examples 6-13 are compared with Comparative Example 3. Comparative Example 3 experienced severe lithium plating after 500 cycles. As the specific capacity of the first active material layer increased, the compaction density decreased, and the compaction density ratio of the first active material layer to the second active material layer decreased. The number of lithium plating cycles of the first active material layer during cycling was effectively extended, which means that the cycle stability of the secondary battery was improved.

[0099] It should be noted that the examples and comparative examples use artificial graphite and SiO2. x The material explains the technical principles used, showing that reducing the compaction density of the first negative electrode active material layer significantly improves the effect.

[0100] The above description is merely an embodiment of this application and does not limit the patent scope of this application. Any equivalent structural or procedural transformations made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. A negative electrode sheet for winding to form an electrode assembly, the negative electrode sheet comprising a negative electrode current collector, a first negative electrode active material layer, and a second negative electrode active material layer, wherein the first negative electrode active material layer and the second negative electrode active material layer are both disposed on the same surface of the negative electrode current collector, the first negative electrode active material layer comprising a first negative electrode active material, and the second negative electrode active material layer comprising a second negative electrode active material, characterized in that, The negative electrode sheet includes a tab connection area and a non-tab connection area for connecting the negative electrode tab. In the electrode assembly, the negative electrode sheet has a straight section and a bent section. The tab connection area is located in the straight section, a part of the non-tab connection area is located in the straight section, and another part of the non-tab connection area is located in the bent section. The active material layer of the tab connection area is the first negative electrode active material layer, and the active material layer of the non-tab connection area is the second negative electrode active material layer. The thickness of the first negative electrode active material layer is the same as the thickness of the second negative electrode active material layer, the compaction density of the first negative electrode active material layer is less than the compaction density of the second negative electrode active material layer, the coating weight per unit area of ​​the first negative electrode active material layer is less than the coating weight per unit area of ​​the second negative electrode active material layer, and the specific capacity of the first negative electrode active material is greater than the specific capacity of the second negative electrode active material. The ratio of the compaction density of the first negative electrode active material layer to the compaction density of the second negative electrode active material layer is 0.75 to 0.

97. The ratio of the specific capacity of the first negative electrode active material to the specific capacity of the second negative electrode active material is 1.03 to 1.

33.

2. The negative electrode sheet according to claim 1, characterized in that, The ratio of the compaction density of the first negative electrode active material layer to the compaction density of the second negative electrode active material layer is 0.80 to 0.

93.

3. The negative electrode sheet according to claim 1, characterized in that, The ratio of the specific capacity of the first negative electrode active material to the specific capacity of the second negative electrode active material is 1.12 to 1.

15.

4. The negative electrode sheet according to any one of claims 1-3, characterized in that, The negative electrode sheet satisfies at least one of the following conditions: A. The compaction density of the first negative electrode active material layer is 1.54 g / cm³. 3 ~1.68g / cm 3 The compaction density of the second negative electrode active material layer is 1.59 g / cm³. 3 ~2.24g / cm 3 ; B. The specific capacity of the first negative electrode active material is 345 mAh / g to 473 mAh / g, and the specific capacity of the second negative electrode active material is 336 mAh / g to 420 mAh / g.

5. The negative electrode sheet according to claim 1, characterized in that, The first negative electrode active material and the second negative electrode active material independently include at least one of artificial graphite, natural graphite, mesophase carbon microspheres, soft carbon or hard carbon.

6. The negative electrode sheet according to claim 5, characterized in that, The first negative electrode active material and the second active material may also independently include one or more of silicon, silicon alloy, silicon oxide or silicon-carbon composite.

7. The negative electrode sheet according to claim 6, characterized in that, The silicon content in the first negative electrode active material layer is 2%-20% by mass, and the silicon content in the second negative electrode active material layer is 0%-10% by mass.

8. A secondary battery, characterized in that, The device includes a housing and an electrode assembly housed within the housing. The electrode assembly includes a positive electrode, a negative electrode as described in any one of claims 1-3, and a separator separating the positive and negative electrode. The positive electrode contains a positive current collector and a layer of positive active material disposed on the surface of the positive current collector. The positive current collector is connected to a positive electrode tab, and the negative current collector is connected to a negative electrode tab. The positive electrode, the separator, and the negative electrode are sequentially stacked and wound to form the electrode assembly.

9. The secondary battery according to claim 8, characterized in that, In the electrode assembly, the negative electrode sheet includes a straight section and a bent section; the negative electrode tab is connected to the straight section of the negative electrode sheet, and the active material layer in the straight section of the negative electrode sheet is the first negative electrode active material layer.

10. The secondary battery according to claim 8, characterized in that, The first negative electrode active material layer is disposed on the outermost negative electrode sheet of the electrode assembly.

11. An electrical device comprising a secondary battery and a load, wherein the secondary battery is electrically connected to the load, characterized in that, The secondary battery is the secondary battery as described in any one of claims 8-10.

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