Secondary battery, method for manufacturing the same, and device containing the same

Abstract

The application discloses a secondary battery, a preparation method thereof and a device containing the secondary battery. The secondary battery comprises a negative electrode sheet, the negative electrode sheet comprises a negative electrode current collector and a negative electrode film sheet; the negative electrode film sheet comprises a first negative electrode film layer and a second negative electrode film layer; the first negative electrode film layer is arranged on at least one surface of the negative electrode current collector and comprises a first negative electrode active material, the first negative electrode active material comprises graphite, and the first negative electrode active material comprises or does not comprise a silicon-based material; the second negative electrode film layer is arranged on a surface of the first negative electrode film layer and comprises a second negative electrode active material, the second negative electrode active material comprises artificial graphite and a silicon-based material; the mass proportion of the silicon-based material in the first negative electrode active material is denoted as W1, the mass proportion of the silicon-based material in the second negative electrode active material is denoted as W2, and the negative electrode sheet satisfies W2 >= W1.

Description

Technical Field

[0001] This application belongs to the field of secondary battery technology, specifically relating to a secondary battery, its preparation method, and an apparatus containing the secondary battery. Background Technology

[0002] Rechargeable batteries, represented by lithium-ion batteries, have advantages such as high specific energy, long lifespan, and low cost, and are therefore widely used in the new energy industry. For example, with the increasing prominence of environmental and energy issues, there is an urgent need for the development of new energy electric vehicles, which brings opportunities to the rechargeable battery industry while also placing higher demands on rechargeable batteries.

[0003] The key challenge in the field of rechargeable batteries is to achieve both high energy density and good electrochemical and safety performance. Summary of the Invention

[0004] To achieve the above objectives, a first aspect of this application provides a secondary battery comprising a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative electrode film; the negative electrode film including a first negative electrode layer and a second negative electrode layer; the first negative electrode layer being disposed on at least one surface of the negative current collector and including a first negative electrode active material, the first negative electrode active material including graphite, and the first negative electrode active material including or excluding a silicon-based material; the second negative electrode film layer being disposed on the surface of the first negative electrode film layer and including a second negative electrode active material, the second negative electrode active material including artificial graphite and a silicon-based material; the mass percentage of the silicon-based material in the first negative electrode active material is denoted as W1, the mass percentage of the silicon-based material in the second negative electrode active material is denoted as W2, and the negative electrode sheet satisfies: W2≥W1, and W1+W2≤6%; the silicon-based material including a first silicon-based material and a second silicon-based material, and the particle size of the first silicon-based material being smaller than the particle size of the second silicon-based material; the particle size of the first silicon-based material is 0.05μm~ 7μm; the particle size of the second silicon-based material is 7μm ~ 15μm.

[0005] In the secondary battery of this application, by making the negative electrode sheet include a double-film structure (i.e., a first negative electrode film layer near the negative electrode current collector and a second negative electrode film layer disposed on the first negative electrode film layer), and both the first and second negative electrode film layers include specific negative electrode active materials, the secondary battery can achieve both high energy density and good fast charging capability and cycle performance. More preferably, the secondary battery also has high safety performance. The device of this application includes the aforementioned secondary battery, and therefore has at least the same advantages as the aforementioned secondary battery.

[0006] In any embodiment of the first aspect of this application, 0% < W2 ≤ 6%, and optionally, 1% ≤ W2 ≤ 3%. The second negative electrode film layer uses artificial graphite combined with an appropriate amount of silicon-based material, which can improve the rapid lithium intercalation performance of the negative electrode film while reducing the volume expansion of the battery during the cycle charge and discharge process and reducing the side reactions at the negative electrode interface. This can further improve the energy density and cycle performance of the battery, thereby enabling the secondary battery to better balance energy density, fast charging capability and cycle performance.

[0007] In any embodiment of the first aspect of this application, 0%≤W1≤5%, optionally, 0%≤W1≤3%, and more preferably, W1=0%. When the first negative electrode film layer uses graphite combined with an appropriate amount of silicon-based material, although the energy density of the battery can be further improved, the improvement on fast charging capability is not significant, and it will affect the cycle performance of the battery to a certain extent.

[0008] In any embodiment of the first aspect of this application, W1+W2≤6%, and optionally, W1+W2≤3%. Within the given range, W1+W2 can further balance the energy density, fast charging capability, and cycle performance of the secondary battery.

[0009] In any embodiment of the first aspect of this application, the silicon-based material includes a first silicon-based material and a second silicon-based material, wherein the particle size of the first silicon-based material is smaller than that of the second silicon-based material. The relatively small particle size of the first silicon-based material and the relatively large particle size of the second silicon-based material work together to provide more active sites for the intercalation of active ions and shorten the diffusion path of active ions in the particles. At the same time, the silicon-based materials working together generate fewer new interfaces due to volume changes, reducing the problem of excessively thick solid electrolyte interphase (SEI) film and / or electrolyte consumption caused by electrolyte reactions at new interfaces, thereby further improving the fast charging capability, cycle performance, and storage performance of the secondary battery.

[0010] In any embodiment of the first aspect of this application, the particle size of the first silicon-based material is 0.05 μm to 7 μm. The particle size of the first silicon-based material within this range can further improve the fast charging capability of the secondary battery.

[0011] In any embodiment of the first aspect of this application, the particle size of the second silicon-based material is 7 μm to 15 μm. A particle size within this range can further improve the cycle performance and storage performance of the secondary battery.

[0012] In any embodiment of the first aspect of this application, the first negative electrode active material includes one or more of artificial graphite and natural graphite; optionally, the first negative electrode active material includes artificial graphite.

[0013] In any embodiment of the first aspect of this application, the mass percentage of artificial graphite in the first negative electrode active material is ≥50%; optionally, the mass percentage of artificial graphite in the first negative electrode active material is 60% to 100%. When the mass percentage of artificial graphite in the second negative electrode active material is within the given range, the battery can have higher cycle performance.

[0014] In any embodiment of the first aspect of this application, the first negative electrode active material includes primary particles, and the number of primary particles in the first negative electrode active material accounts for ≥60%, optionally 80% to 100%. The first negative electrode active material may contain a majority of primary particles, in which case the battery capacity is higher; at the same time, increasing the proportion of primary particles can enhance the adhesion between the first negative electrode film layer and the current collector, thereby further improving the cycle performance of the battery.

[0015] In any embodiment of the first aspect of this application, the second negative electrode active material includes secondary particles, and the secondary particles account for ≥50% of the total amount of the second negative electrode active material, optionally ≥70%. When the second negative electrode active material contains a certain amount of secondary particles, it can increase the intercalation / deintercalation active ion channels in the second negative electrode film layer and further reduce the risk of active ion reduction and precipitation, thereby further improving the fast charging performance and cycle performance of the battery.

[0016] In any embodiment of the first aspect of this application, the volume average particle size D of the first negative electrode active material v 50 is greater than the volume average particle size D of the second negative electrode active material. v 50. When the D of the first negative electrode active material v 50 is greater than the D of the second negative electrode active material v At 50°C, it is beneficial to optimize the pore structure of the first and second negative electrode films, thereby further improving the fast charging performance of the battery; at the same time, the optimization of the pore structure helps to reduce the local polarization of the electrodes, thereby further improving the cycle performance of the battery.

[0017] In any embodiment of the first aspect of this application, the volume average particle size D of the first negative electrode active material v 50 is 12μm ~ 18μm, and can be selected as 13μm ~ 17μm.

[0018] In any embodiment of the first aspect of this application, the volume average particle size D of the second negative electrode active material v 50 is 9μm ~ 15μm, and can be selected as 10μm ~ 13μm.

[0019] In any embodiment of the first aspect of this application, the silicon-based material includes one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen compounds, and silicon alloys; optionally, the silicon-based material includes silicon oxide compounds.

[0020] In any embodiment of the first aspect of this application, the areal density of the negative electrode film is 8 mg / cm³. 2 ~13mg / cm 2 9mg / cm 2 ~12.5mg / cm 2 .

[0021] In any embodiment of the first aspect of this application, the compaction density of the negative electrode film is 1.4 g / cm³. 3 ~ 1.7g / cm 3 1.5 g / cm³ is an optional value. 3 ~ 1.65 g / cm 3 Within a given range, the compaction density of the negative electrode film enables the negative electrode to possess both high reversible capacity and good low-cycle expansion performance and kinetic performance, thereby further improving the battery's energy density, fast charging capability, and cycle performance.

[0022] In any embodiment of the first aspect of this application, the porosity of the second negative electrode film is greater than that of the first negative electrode film. By combining the second negative electrode film with its greater porosity and the first negative electrode film with its smaller porosity, the fast charging performance and cycle performance of the battery can be further improved.

[0023] In any embodiment of the first aspect of this application, the areal density ratio of the first negative electrode film layer to the second negative electrode film layer is 2:3 to 3:2. When the areal density ratio of the first negative electrode film layer to the second negative electrode film layer is within the given range, the secondary battery can better balance energy density, fast charging capability, and cycle performance.

[0024] In any embodiment of the first aspect of this application, the thickness ratio of the first negative electrode film layer to the second negative electrode film layer is 4:6 to 7:3. Within this given range, the thickness ratio of the first negative electrode film layer to the second negative electrode film layer enables the secondary battery to better balance energy density, fast charging capability, and cycle performance.

[0025] In any embodiment of the first aspect of this application, the secondary battery includes a positive electrode sheet, the positive electrode sheet includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector and including a positive electrode active material, the positive electrode active material including one or more of lithium transition metal oxides, lithium phosphates with olivine structure and their respective modified compounds.

[0026] Optionally, the positive electrode active material includes one or more of the lithium transition metal oxides and their modified compounds shown in Formula 1. Li a Ni b Co c M d O e A f Formula 1, In Formula 1, 0.8≤a≤1.2, 0.5≤b<1, 0<c<1, 0<d<1, 1≤e≤2, 0≤f≤1, M is selected from one or more of Mn, Al, Zr, Zn, Cu, Cr, Mg, Fe, V, Ti and B, and A is selected from one or more of N, F, S and Cl.

[0027] A second aspect of this application provides a method for preparing a secondary battery, comprising preparing the negative electrode sheet of the secondary battery through the following steps: A first negative electrode film layer comprising a first negative electrode active material is formed on at least one surface of the negative electrode current collector. The first negative electrode active material includes graphite and may or may not include silicon-based materials. A second negative electrode film layer comprising a second negative electrode active material is formed on the surface of the first negative electrode film layer. The second negative electrode active material comprises artificial graphite and silicon-based materials. Wherein, the mass percentage of the silicon-based material in the first negative electrode active material is denoted as W1, and the mass percentage of the silicon-based material in the second negative electrode active material is denoted as W2. The negative electrode sheet satisfies: W2≥W1, and W1+W2≤6%. The silicon-based material includes a first silicon-based material and a second silicon-based material, and the particle size of the first silicon-based material is smaller than the particle size of the second silicon-based material. The particle size of the first silicon-based material is 0.05μm ~ 7μm, and the particle size of the second silicon-based material is 7μm ~ 15μm.

[0028] A third aspect of this application provides an apparatus comprising a secondary battery prepared according to a first aspect of this application or a secondary battery prepared according to a second aspect of this application. Attached Figure Description

[0029] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments of this application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the drawings without creative effort.

[0030] Figure 1 This is a schematic diagram of one embodiment of the secondary battery of this application.

[0031] Figure 2 This is a schematic diagram of one embodiment of the negative electrode sheet in the secondary battery of this application.

[0032] Figure 3 This is a schematic diagram of another embodiment of the negative electrode sheet in the secondary battery of this application.

[0033] Figure 4 This is an exploded view of one embodiment of the secondary battery of this application.

[0034] Figure 5 This is a schematic diagram of one embodiment of the battery module of this application.

[0035] Figure 6 This is a schematic diagram of one embodiment of the battery pack of this application.

[0036] Figure 7 yes Figure 6 The exploded diagram.

[0037] Figure 8 This is a schematic diagram of one embodiment of the device for using a secondary battery as a power source according to this application. Detailed Implementation

[0038] To make the inventive objectives, technical solutions, and beneficial technical effects of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the embodiments described in this specification are merely illustrative and not intended to limit the scope of this application.

[0039] 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.

[0040] In the description of this article, it should be noted that, unless otherwise stated, "above" and "below" include the number itself, and "several" in "one or several" means two or more.

[0041] The foregoing description of this invention is not intended to describe every disclosed embodiment or implementation. Instead, the following description provides more specific examples of exemplary embodiments. Throughout the application, guidance is provided through a series of embodiments that can be used in various combinations. The examples listed are merely representative and should not be construed as exhaustive.

[0042] Secondary batteries An embodiment of the first aspect of this application provides a secondary battery. The secondary battery includes a positive electrode, a negative electrode, and an electrolyte. During charging and discharging, active ions repeatedly insert and extract between the positive and negative electrode. The electrolyte acts as a conductor of ions between the positive and negative electrode.

[0043] [Negative electrode plate] The negative electrode sheet includes a negative current collector and a negative electrode film; the negative electrode film includes a first negative electrode film layer and a second negative electrode film layer; the first negative electrode film layer is disposed on at least one surface of the negative current collector and includes a first negative electrode active material, the first negative electrode active material including graphite, and the first negative electrode active material may or may not include silicon-based materials; the second negative electrode film layer is disposed on the surface of the first negative electrode film layer and includes a second negative electrode active material, the second negative electrode active material including artificial graphite and silicon-based materials; the mass percentage of the silicon-based material in the first negative electrode active material is denoted as W1, the mass percentage of the silicon-based material in the second negative electrode active material is denoted as W2, and the negative electrode sheet satisfies: W2≥W1.

[0044] Through extensive research, the inventors discovered that when the negative electrode of a secondary battery includes a double-layer structure, and the first and second negative electrode layers contain specific negative electrode active materials (the first negative electrode active material includes graphite and may or may not include silicon-based materials, the second negative electrode active material includes artificial graphite and silicon-based materials, and the negative electrode satisfies W2≥W1), the negative electrode film can have a high reversible capacity while reducing the overall thickness of the negative electrode film and shortening the transport path of active ions in the negative electrode film. At the same time, the silicon-based material can play a good role in channel support during the cold pressing of the electrode or the battery cycle expansion, and the volume change of the silicon-based material during the charge and discharge process can further improve the channel structure of the outer layer of the negative electrode film. This enables the negative electrode to have and maintain a good channel structure for electrolyte wetting and reflux, thereby improving the fast charging capability of the secondary battery and effectively improving the cycle performance. Moreover, the first and second negative electrode film layers contain specific negative electrode active materials. During the battery fast charging process, the negative electrode can fully receive active ions from the positive electrode, reducing the phenomenon of active ions being reduced and deposited on the negative electrode surface (such as lithium plating), which is beneficial to improving the battery's fast charging performance and giving the battery higher cycle performance and safety performance.

[0045] Therefore, by optimizing the negative electrode as described above, the secondary battery using it can have a high energy density while also possessing good fast charging capability, cycle performance, and safety performance.

[0046] In some optional embodiments, the mass percentage W2 of the silicon-based material in the second negative electrode active material satisfies 0% < W2 ≤ 6%, for example, it can be 0% < W2 ≤ 5%, 0% < W2 ≤ 4%, 0% < W2 ≤ 3%, 0% < W2 ≤ 2%, etc. Specifically, W2 can be 0.5%, 1.0%, 1.2%, 1.4%, 1.5%, 1.6%, 1.8%, 2.0%, 2.5%, 3%, or 5%, etc. The second negative electrode film layer uses artificial graphite combined with an appropriate amount of silicon-based material, which can improve the rapid lithium intercalation performance of the negative electrode film while minimizing the volume expansion of the battery during charge-discharge cycles and reducing side reactions at the negative electrode interface. This further improves the energy density and cycle performance of the battery, enabling the secondary battery to better balance energy density, fast charging capability, and cycle performance. More preferably, 1% ≤ W2 ≤ 3%. Most preferably, 1% ≤ W2 ≤ 2%.

[0047] In some optional embodiments, the mass percentage of silicon-based material in the first negative electrode active material is 0% ≤ W1 ≤ 5%. For example, it can be 0% ≤ W1 ≤ 4%, 0% ≤ W1 ≤ 3%, 0% ≤ W1 ≤ 2%, 0% ≤ W1 ≤ 1%, etc. Specifically, W1 can be 0%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, or 4%, etc. The inventors have found that when the first negative electrode film layer uses graphite combined with an appropriate amount of silicon-based material, although the energy density of the battery can be further improved, the improvement in fast charging capability is not significant, and it will affect the cycle performance of the battery to some extent. More preferably, 0% ≤ W1 ≤ 3%. Particularly preferably, 0% ≤ W1 ≤ 2%. Most preferably, W1 = 0% (i.e., the first negative electrode film layer does not include silicon-based material).

[0048] In some optional embodiments, the mass percentage W1 of the silicon-based material in the first negative electrode active material and the mass percentage W2 of the silicon-based material in the second negative electrode active material satisfy W1+W2≤10%, optionally W1+W2≤6%. More preferably, W1+W2≤3%. Particularly preferably, 1%≤W1+W2≤3%. This ensures that W1+W2 is within the given range, further balancing the energy density, fast charging capability, and cycle performance of the secondary battery.

[0049] Through in-depth research, the inventors have discovered that, under the premise that the negative electrode sheet of the secondary battery described in this application meets the above design requirements, if the negative electrode active material can optionally meet one or more of the following design conditions, the performance of the secondary battery can be further improved.

[0050] The inventors have discovered that using silicon-based materials with different particle sizes can further improve battery performance. Specifically, silicon-based materials with different particle sizes can be included in the first negative electrode active material, and / or, silicon-based materials with different particle sizes can be included in the second negative electrode active material. In some optional embodiments, the silicon-based materials with different particle sizes include a first silicon-based material and a second silicon-based material, and the particle size of the first silicon-based material is smaller than that of the second silicon-based material. The relatively smaller particle size of the first silicon-based material and the relatively larger particle size of the second silicon-based material work together to provide more active sites for the intercalation of active ions and shorten the diffusion path of active ions in the particles; at the same time, the silicon-based materials with these two materials generate fewer new interfaces due to volume changes, reducing the problem of excessively thick solid electrolyte interphase (SEI) film and / or electrolyte consumption caused by electrolyte reactions at new interfaces, thereby further improving the fast charging capability, cycle performance, and storage performance of the secondary battery.

[0051] Optionally, the particle size of the first silicon-based material is 0.05 μm to 7 μm, and more preferably 0.1 μm to 5 μm. A particle size within the given range can further improve the fast-charging capability of the secondary battery.

[0052] Optionally, the particle size of the second silicon-based material is 7 μm to 15 μm, and more preferably 8 μm to 12 μm. Within the given range, the particle size of the second silicon-based material can further improve the cycle performance and storage performance of the secondary battery.

[0053] In some embodiments, the weight ratio of the first silicon-based material to the second silicon-based material can be 5:95 to 30:70, or optionally 10:90 to 20:80.

[0054] In some embodiments, the silicon-based material may include one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Optionally, the silicon-based material includes silicon oxide compounds. Silicon oxide compounds may have the chemical formula SiO. x 0.6 ≤ x < 2. Alternatively, 0.9 ≤ x ≤ 1.2.

[0055] When both the first negative electrode active material and the second negative electrode active material include silicon-based materials, the silicon-based materials in the first negative electrode active material and the silicon-based materials in the second negative electrode active material can be the same or different.

[0056] When the silicon-based material includes the first silicon-based material and the second silicon-based material described above, the first silicon-based material and the second silicon-based material are each independently selected from one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, and silicon alloys. Optionally, the first silicon-based material and the second silicon-based material each independently include silicon oxide compounds (such as SiO2 described above). x ).

[0057] In some optional embodiments, the mass percentage of artificial graphite in the second negative electrode active material is ≥90%, more preferably ≥95%, even more preferably ≥97%, and particularly preferably ≥98%. When the mass percentage of artificial graphite in the second negative electrode active material is within the given range, the battery can have higher cycle performance.

[0058] In some optional embodiments, the second negative electrode active material includes secondary particles, and the secondary particles account for ≥50% of the total amount of the second negative electrode active material. For example, the percentage of secondary particles in the second negative electrode active material can be 50%~95%, 60%~90%, 70%~95%, or 80%~90%, etc. Specifically, the percentage of secondary particles in the second negative electrode active material can be 50%, 60%, 70%, 80%, or 90%, etc. When the second negative electrode active material contains a certain amount of secondary particles, it can increase the intercalation / deintercalation active ion channels in the second negative electrode film layer and further reduce the risk of active ion reduction and precipitation, thereby further improving the fast charging performance and cycle performance of the battery. More preferably, the percentage of secondary particles in the second negative electrode active material is ≥70%.

[0059] Optionally, in the second negative electrode active material, the artificial graphite includes secondary particles, while the silicon-based material is mainly composed of primary particles. The proportion of primary particles in the silicon-based material can be 80%~100%, 90%~100%, 95%~100%, or 98%~100%, etc.

[0060] In some embodiments, the graphite of the first negative electrode active material may include one or more of artificial graphite and natural graphite. Optionally, the graphite of the first negative electrode active material may include artificial graphite.

[0061] In some optional embodiments, the mass percentage of artificial graphite in the first negative electrode active material is ≥50%, and more preferably 60% to 100%. When the second negative electrode active material includes artificial graphite and silicon-based materials, and the first negative electrode active material of the first negative electrode film contains a large amount of artificial graphite, the volume expansion of the negative electrode sheet during cycling can be further reduced, thereby further improving the cycle performance of the battery; at the same time, artificial graphite has good thermal stability at high temperatures, which can effectively improve the safety performance of the battery during use. Specifically, the mass percentage of artificial graphite in the first negative electrode active material can be 60%, 65%, 70%, 75%, 80%, 85%, 90%, or 100%.

[0062] In some optional embodiments, the first negative electrode active material comprises primary particles, and the primary particles account for ≥60% of the total amount of the first negative electrode active material. For example, the percentage of primary particles in the first negative electrode active material can be 60%~100%, 70%~95%, 80%~100%, 90%~100%, etc. Specifically, the percentage of primary particles in the first negative electrode active material can be 70%, 75%, 80%, 85%, 90%, 95%, or 100%, etc. The first negative electrode active material may contain a majority of primary particles, resulting in a higher battery capacity; simultaneously, increasing the percentage of primary particles can enhance the adhesion between the first negative electrode film and the current collector, thereby further improving the battery's cycle performance. More preferably, the percentage of primary particles in the first negative electrode active material is 80%~100%.

[0063] In some optional embodiments, the volume average particle size D of the first negative electrode active material v 50 is greater than the volume average particle size D of the second negative electrode active material. v 50. The inventors discovered that when the D of the first negative electrode active material... v 50 is greater than the D of the second negative electrode active material v At 50°C, it is beneficial to optimize the pore structure of the first and second negative electrode films, thereby further improving the fast charging performance of the battery; at the same time, the optimization of the pore structure helps to reduce the local polarization of the electrodes, thereby further improving the cycle performance of the battery.

[0064] In some alternative embodiments, the D of the first negative electrode active material v 50 has a thickness of 12μm to 18μm, and can also be selected as 13μm to 17μm.

[0065] In some alternative embodiments, the D of the second negative electrode active material v 50 has a thickness of 9μm to 15μm, and can also be selected as 10μm to 13μm.

[0066] In the secondary battery of this application, the first negative electrode active material and / or the second negative electrode active material may optionally include, in addition to the negative electrode active materials described above, a certain amount of other commonly used negative electrode active materials, such as one or more of natural graphite, soft carbon, hard carbon, tin-based materials, and lithium titanate. The tin-based material may be selected from one or more of elemental tin, tin oxides, and tin alloys.

[0067] All negative electrode active materials used in this application are commercially available. Those skilled in the art can make appropriate selections based on the actual application environment.

[0068] In the secondary battery of this application, the negative electrode film typically comprises a negative electrode active material and optional binders, optional conductive agents, and other optional additives, and is usually formed by coating and drying a negative electrode slurry. The negative electrode slurry coating is typically formed by dispersing the negative electrode active material, optional conductive agents, and binders in a solvent and stirring until homogeneous. The solvent can be N-methylpyrrolidone (NMP) or deionized water.

[0069] As an example, conductive agents may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0070] As an example, the adhesive may include one or more of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, waterborne acrylic resin, polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS).

[0071] Other optional additives include thickeners (such as sodium carboxymethyl cellulose, CMC-Na) and PTC thermistor materials.

[0072] Through in-depth research, the inventors have also discovered that, in addition to meeting the above-mentioned design conditions, if the negative electrode film of the secondary battery described in this application can also optionally meet one or more of the following design conditions, the performance of the secondary battery can be further improved.

[0073] In some optional embodiments, the areal density of the negative electrode film is 8 mg / cm³. 2 ~13mg / cm 2 Alternatively, a 9mg / cm³ option can be selected. 2 ~12.5mg / cm 2 It should be noted that the areal density of the negative electrode film refers to the areal density of the entire negative electrode film (i.e., the sum of the areal densities of the first negative electrode film layer and the second negative electrode film layer).

[0074] In some optional embodiments, the areal density ratio of the first negative electrode film layer to the second negative electrode film layer is 2:3 to 3:2, such as 4:5 to 5:4. When the areal density ratio of the first negative electrode film layer to the second negative electrode film layer is within the given range, the secondary battery can better balance energy density, fast charging capability, and cycle performance.

[0075] In some alternative embodiments, the compaction density of the negative electrode film is 1.4 g / cm³. 3 ~ 1.7 g / cm 3 Alternatively, the compaction density of the negative electrode film is 1.5 g / cm³. 3 ~ 1.65 g / cm 3 It should be noted that the compaction density of the negative electrode film refers to the overall compaction density of the negative electrode film (i.e., the ratio of the areal density to the thickness of the negative electrode film). Within the given range, the compaction density of the negative electrode film enables the negative electrode to possess both high reversible capacity and good low-cycle expansion performance and kinetic performance, thereby further improving the battery's energy density, fast-charging capability, and cycle performance.

[0076] In some alternative embodiments, the porosity of the second negative electrode film is greater than that of the first negative electrode film. By combining the second negative electrode film with its greater porosity with the first negative electrode film with its smaller porosity, the fast charging performance and cycle performance of the battery can be further improved.

[0077] In some alternative embodiments, the porosity of the first negative electrode film layer may be selected as 15% to 35%, or more preferably 20% to 30%.

[0078] In some alternative embodiments, the porosity of the second negative electrode film layer may be selected as 20% to 40%, or more preferably 25% to 35%.

[0079] In some optional embodiments, the thickness ratio of the first negative electrode film layer to the second negative electrode film layer is 4:6 to 7:3, and more preferably 5:5 to 5:3. Within this given range, the thickness ratio of the first negative electrode film layer to the second negative electrode film layer allows the secondary battery to better balance energy density, fast charging capability, and cycle performance.

[0080] In some optional embodiments, the thickness of the negative electrode film can be 60 μm to 90 μm, and more preferably 65 μm to 80 μm. The thickness of the negative electrode film is the sum of the thicknesses of the first negative electrode film layer and the second negative electrode film layer.

[0081] In the secondary battery of this application, the negative electrode current collector can be a metal foil or a composite current collector (a composite current collector can be formed by setting a metal material on a polymer substrate). As an example, the negative electrode current collector can be a copper foil.

[0082] In the secondary battery of this application, the negative electrode film can be disposed on one surface of the negative electrode current collector, or it can be disposed on both surfaces of the negative electrode current collector simultaneously. For example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film is disposed on either or both of the opposite surfaces of the negative electrode current collector.

[0083] Figure 2 A schematic diagram of one embodiment of the negative electrode 10 of this application is shown. The negative electrode 10 is composed of a negative current collector 101, a first negative electrode film layer 103 disposed on two surfaces of the negative current collector 101, and a second negative electrode film layer 102 disposed on the first negative electrode film layer 103.

[0084] Figure 3 A schematic diagram of another embodiment of the negative electrode 10 of this application is shown. The negative electrode 10 is composed of a negative current collector 101, a first negative electrode film layer 103 disposed on one surface of the negative current collector, and a second negative electrode film layer 102 disposed on the first negative electrode film layer 103.

[0085] It should be noted that the parameters of each negative electrode film given in this application (such as film thickness, porosity, compaction density, areal density, etc.) refer to the parameter range of a single-sided film. When the negative electrode film is disposed on both surfaces of the negative electrode current collector, the film parameters on either surface that meet the requirements of this application are considered to fall within the protection scope of this application. Furthermore, the film thickness, porosity, compaction density, areal density, etc. ranges mentioned in this application refer to the film parameters after cold pressing and compaction and used for assembling the battery.

[0086] Furthermore, in the secondary battery of this application, the negative electrode sheet does not exclude other additional functional layers besides the negative electrode film. For example, in some embodiments, the negative electrode sheet of this application further includes a conductive undercoating layer (e.g., composed of a conductive agent and a binder) sandwiched between the negative electrode current collector and the first negative electrode film layer and disposed on the surface of the negative electrode current collector. In other embodiments, the negative electrode sheet of this application further includes a protective covering layer covering the surface of the second negative electrode film layer.

[0087] In this application, the mass percentage of silicon-based materials in the negative electrode active material has a meaning known in the art and can be tested using methods known in the art. For example, a scanning electron microscope (e.g., a ZEISS Sigma 300) can be used to observe the cross-section of the negative electrode film (e.g., a cross-section along the film thickness direction), and energy dispersive X-ray analysis (e.g., the energy dispersive spectrometer integrated with the ZEISS Sigma 300) can be used for quantitative mapping. Thus, the elemental content of the negative electrode film can be determined by scanning the cross-section, and the elemental distribution can be calculated. Within a region of the cross-section, the Si content in that region can be determined by integrating the Si elemental distribution. To improve the accuracy of the test, multiple regions (e.g., 10) can be statistically analyzed, and the average value can be taken as the test result.

[0088] In this application, the particle size of the silicon-based material has a meaning known in the art and can be tested using methods known in the art, such as scanning electron microscopy (e.g., ZEISS Sigma 300). Sample preparation is as follows: First, the negative electrode sheet is cut into a sample of a certain size (e.g., 2cm × 2cm), and fixed to the sample stage with paraffin wax. Then, the sample stage is installed in the sample holder and locked in place. The power of the argon ion cross-section polisher (e.g., IB-19500CP) is turned on and a vacuum is applied (e.g., 10...). -4 Set the argon flow rate (e.g., 0.15 MPa), voltage (e.g., 8 kV), and polishing time (e.g., 2 hours), and adjust the sample stage to swing mode to begin polishing. Sample testing can refer to JY / T010-1996. To ensure the accuracy of the test results, multiple (e.g., 10) different regions can be randomly selected from the sample to be tested for scanning, and the particle size of all silicon-based materials in the scale test area can be read at a certain magnification (e.g., 500x). To further improve the accuracy of the test, the test results of multiple regions can be statistically analyzed.

[0089] In this application, primary particles and secondary particles have the meanings known in the art. Primary particles refer to particles that have not formed aggregates. Secondary particles refer to particles that are aggregated from two or more primary particles. Primary and secondary particles can be easily distinguished by taking SEM images using a scanning electron microscope.

[0090] The proportion of primary or secondary particles in the negative electrode active material can be determined using instruments and methods known in the art, such as scanning electron microscopy. The method for testing the proportion of secondary particles can be as follows: the negative electrode active material is laid and adhered to conductive adhesive to form a sample 6 cm long × 1.1 cm wide; the particle morphology is tested using a scanning electron microscope (such as a ZEISS Sigma 300). The test can be referenced in JY / T010-1996. To ensure the accuracy of the test results, multiple (e.g., 20) different regions can be randomly selected from the sample for scanning tests, and at a certain magnification (e.g., 1000x), the percentage of secondary particles in each region relative to the total number of particles is calculated; this is the proportion of secondary particles in that region. The average of the test results from multiple test regions is taken as the test result. Similarly, the proportion of primary particles in the first negative electrode active material can also be tested.

[0091] In this application, the D of the negative electrode active material v 50 has a well-known meaning in the art and can be determined using instruments and methods known in the art. For example, it can be determined using a laser particle size analyzer (such as the MalvernMaster Size 3000) according to standard GB / T 19077.1-2016. Where D... v 50 is the particle size corresponding to the cumulative volume distribution percentage of the negative electrode active material reaching 50%.

[0092] In this application, the thickness of the negative electrode film has a meaning known in the art and can be tested using methods known in the art, such as a micrometer (e.g., Mitutoyo 293-100, with an accuracy of 0.1 μm).

[0093] In this application, the thicknesses of the first and second negative electrode films can be measured using a scanning electron microscope (e.g., ZEISS Sigma300). Sample preparation is as follows: First, the negative electrode sheet is cut into a sample of a certain size (e.g., 2cm × 2cm), and fixed to the sample stage with paraffin wax. Then, the sample stage is installed in the sample holder and locked in place. The argon ion cross-section polisher (e.g., IB-19500CP) is powered on and a vacuum is applied (e.g., 10...). -4Set the argon flow rate (e.g., 0.15 MPa), voltage (e.g., 8 kV), and polishing time (e.g., 2 hours), and adjust the sample stage to swing mode to begin polishing. Sample testing can refer to JY / T010-1996. To ensure the accuracy of the test results, multiple (e.g., 10) different regions can be randomly selected from the sample to be tested for scanning. At a certain magnification (e.g., 500x), the thicknesses of the first and second negative electrode films in the scale test areas are read, and the average value of multiple test areas is taken as the test result.

[0094] In this application, the areal density of the negative electrode film has a meaning known in the art and can be tested using methods known in the art. For example, take a negative electrode sheet that has been coated on one side and cold-pressed (if it is a negative electrode sheet coated on both sides, the negative electrode film on one side can be wiped off first), cut it into a small circular piece with an area of ​​S1, weigh it, and record its weight as M1. Then wipe off the negative electrode film of the above-weighed negative electrode sheet, weigh the negative current collector, and record it as M0. The areal density of the negative electrode film = (weight of the negative electrode sheet M1 - weight of the negative current collector M0) / S1.

[0095] In this application, the compaction density of the negative electrode film has a meaning known in the art and can be tested using methods known in the art. The compaction density of the negative electrode film = areal density of the negative electrode film / thickness of the negative electrode film.

[0096] It should be noted that the above-mentioned tests on various parameters of the negative electrode active material can be conducted by sampling before coating or by sampling from the negative electrode film layer after cold pressing.

[0097] When the test sample is taken from the cold-pressed negative electrode film, as an example, the sampling can be performed according to the following steps: (1) First, select any cold-pressed negative electrode film and take a sample of the second negative electrode active material (you can use a blade to scrape the powder for sampling). The scraping depth should not exceed the boundary between the first negative electrode film layer and the second negative electrode film layer.

[0098] (2) Next, sample the first negative electrode active material. Because during the cold pressing process of the negative electrode film, there may be an interfusion layer in the boundary area between the first negative electrode film layer and the second negative electrode film layer (that is, the first negative electrode active material and the second negative electrode active material are present in the interfusion layer at the same time), in order to ensure the accuracy of the test, when sampling the first negative electrode active material, the interfusion layer can be scraped off first, and then the powder of the first negative electrode active material can be scraped off and sampled.

[0099] (3) The first and second negative electrode active materials collected above are placed in deionized water, and the first and second negative electrode active materials are filtered, dried, and then each negative electrode active material after drying is sintered at a certain temperature and time (e.g., 400℃, 2h) to remove the binder and conductive carbon, thus obtaining the test samples of the first and second negative electrode active materials.

[0100] During the above sampling process, an optical microscope or a scanning electron microscope can be used to help determine the location of the boundary between the first negative electrode film layer and the second negative electrode film layer.

[0101] [Positive electrode plate] In the secondary battery of this application, the positive electrode includes a positive current collector and a positive electrode film disposed on at least one surface of the positive current collector and comprising a positive active material. For example, the positive current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film is disposed on either or both of the two opposite surfaces of the positive current collector.

[0102] In the secondary battery of this application, the positive electrode active material may be any positive electrode active material known in the art for use in secondary batteries. For example, the positive electrode active material may include one or more of lithium transition metal oxides, lithium-containing phosphates with an olivine structure, and their respective modified compounds. Examples of lithium transition metal oxides may include, but are not limited to, one or more of lithium cobalt oxides, lithium nickel oxides, lithium manganese oxides, lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, lithium nickel cobalt manganese oxides, lithium nickel cobalt aluminum oxides, and their modified compounds. Examples of lithium-containing phosphates with an olivine structure may include, but are not limited to, lithium iron phosphate, lithium iron phosphate and carbon composites, lithium manganese phosphate, lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, lithium manganese iron phosphate and carbon composites, and their respective modified compounds. This application is not limited to these materials, and other conventionally known materials that can be used as positive electrode active materials for secondary batteries may also be used.

[0103] In some alternative embodiments, to further improve the energy density of the battery, the positive electrode active material may include one or more of the lithium transition metal oxides and their modified compounds shown in Formula 1. Li a Ni b Co c M d O e A f Formula 1, In Formula 1, 0.8≤a≤1.2, 0.5≤b<1, 0<c<1, 0<d<1, 1≤e≤2, 0≤f≤1, M is selected from one or more of Mn, Al, Zr, Zn, Cu, Cr, Mg, Fe, V, Ti and B, and A is selected from one or more of N, F, S and Cl.

[0104] In this application, the modified compounds of the above-mentioned materials may be doping modifications and / or surface coating modifications of the positive electrode active materials.

[0105] In the secondary battery of this application, the positive electrode film typically comprises a positive electrode active material, an optional binder, and an optional conductive agent. It is usually formed by coating a positive electrode slurry and then drying and cold pressing it. The positive electrode slurry is typically formed by dispersing the positive electrode active material, an optional conductive agent, and a binder in a solvent and stirring until homogeneous. The solvent may be N-methylpyrrolidone (NMP).

[0106] As an example, the binder used for the positive electrode membrane may include one or more of polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE).

[0107] As an example, the conductive agent used for the positive electrode film may include one or more of graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0108] In the secondary battery of this application, the positive electrode current collector can be a metal foil or a composite current collector (a composite current collector can be formed by setting metal material on a polymer substrate). As an example, the positive electrode current collector can be aluminum foil.

[0109] [Electrolytes] The secondary battery of this application does not have specific restrictions on the type of electrolyte, and can be selected according to needs. For example, the electrolyte can be selected from at least one of solid electrolytes and liquid electrolytes (i.e., electrolyte solutions).

[0110] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0111] In some embodiments, the electrolyte salt may be selected from one or more of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiAsF6 (lithium hexafluoroarsenate), LiFSI (lithium bis(fluorosulfonyl)imide), LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluorooxalate borate), LiBOB (lithium dioxalate borate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluorodioxalate phosphate), and LiTFOP (lithium tetrafluorooxalate phosphate).

[0112] In some embodiments, the solvent may be selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butyl carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0113] 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 additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature performance, additives that improve battery low-temperature performance, etc.

[0114] [Isolation membrane] Secondary batteries using electrolytes, and some secondary batteries using solid electrolytes, also include a separator. The separator is positioned between the positive and negative electrodes, serving a separating function. 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. In some embodiments, the separator material can be selected from one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of each layer may be the same or different.

[0115] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0116] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the aforementioned electrode assembly and electrolyte.

[0117] In some implementations, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, aluminum shell, or 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, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0118] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. Figure 1 This is an example of a square-structured secondary battery 5.

[0119] In some implementations, refer to Figure 4 The outer packaging may include a housing 51 and a cover plate 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 cover plate 53 is used to cover the opening to close the receiving cavity. Positive electrode sheets, negative electrode sheets, and a separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated in the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be adjusted according to requirements.

[0120] In some implementations, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be multiple, the specific number of which can be adjusted according to the application and capacity of the battery module.

[0121] Figure 5 This is battery module 4, used as an example. (See reference...) Figure 5 In battery module 4, multiple secondary batteries 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 secondary batteries 5 can be fixed in place using fasteners.

[0122] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0123] 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 adjusted according to the application and capacity of the battery pack.

[0124] Figure 6 and Figure 7 This is battery pack 1 as an example. (See reference...) Figure 6 and Figure 7 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3. The upper body 2 covers the lower body 3, forming a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0125] Preparation method The second aspect of this application provides a method for preparing a secondary battery, comprising preparing the negative electrode sheet of the secondary battery through the following steps: S10, a first negative electrode film layer comprising a first negative electrode active material is formed on at least one surface of the negative electrode current collector, wherein the first negative electrode active material comprises graphite, and the first negative electrode active material may or may not include a silicon-based material. S20, a second negative electrode film layer comprising a second negative electrode active material is formed on the surface of the first negative electrode film layer facing away from the negative electrode current collector, wherein the second negative electrode active material comprises artificial graphite and silicon-based materials. Let W1 be the mass percentage of the silicon-based material in the first negative electrode active material, and W2 be the mass percentage of the silicon-based material in the second negative electrode active material. The negative electrode sheet satisfies: W2≥W1.

[0126] In the preparation method of the secondary battery of this application, the first negative electrode active material slurry and the second negative electrode active material slurry can be coated simultaneously at one time or coated in two separate times.

[0127] Optionally, the first negative electrode active material slurry and the second negative electrode active material slurry are coated simultaneously in one step. Simultaneous coating in one step can improve the adhesion between the first negative electrode film layer and the second negative electrode film layer, which helps to further improve the cycle performance of the battery.

[0128] Besides the method for preparing the negative electrode sheet of this application, other structures and preparation methods of the secondary battery of this application are known. For example, the positive electrode sheet of this application can be prepared by the following method: the positive active material and optional conductive agent and binder are mixed and dispersed in a solvent (such as NMP), stirred evenly, coated on the positive current collector, dried, and cold-pressed to obtain the positive electrode sheet.

[0129] In some embodiments, a negative electrode, a positive electrode, a separator, and an electrolyte can be assembled to form a secondary battery. As an example, the positive electrode, separator, and negative electrode can be formed into an electrode assembly through a winding or stacking process; the electrode assembly is placed in an outer package, an electrolyte is injected, and the secondary battery is obtained through processes such as vacuum sealing, settling, formation, and shaping.

[0130] The optional technical features of the secondary battery in this application are also applicable to the preparation method of this application.

[0131] Device An embodiment of the third aspect of this application provides an apparatus comprising a secondary battery according to the first aspect of this application and / or a secondary battery prepared by the method of the second aspect of this application. The secondary battery can be used as a power source for the apparatus or as an energy storage unit for the apparatus. The apparatus may be, 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.

[0132] The device can be configured to use a secondary battery, battery module, or battery pack, depending on its usage requirements.

[0133] Figure 8 This is an example device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the device's requirements for high power and high energy density, a battery pack or battery module can be used.

[0134] 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.

[0135] Example The following embodiments describe the disclosure of this application in more detail. These embodiments are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of the disclosure of this application. Unless otherwise stated, all parts, percentages, and ratios reported in the following embodiments are based on weight, and all reagents used in the embodiments are commercially available or synthesized by conventional methods and can be used directly without further processing, and the instruments used in the embodiments are commercially available.

[0136] Example 1 Preparation of negative electrode sheet The first negative electrode active material, artificial graphite, conductive agent, carbon black (Super P), binder, styrene-butadiene rubber (SBR), and thickener, sodium carboxymethyl cellulose (CMC-Na), were thoroughly mixed in an appropriate amount of deionized water at a mass ratio of 96:1.5:1.5:1 to form a uniform first negative electrode slurry; wherein, the volume average particle size D of the first negative electrode active material was... v The particle size is 15.7 μm, and the primary particles account for 95% of the total number of particles in the first negative electrode active material.

[0137] The second negative electrode active material, artificial graphite, and silicon-based material SiOx (x≈1.05) were mixed at a mass ratio of 97:3. Then, the mixture was further mixed with a conductive agent (Super P), a binder (styrene-butadiene rubber, SBR), and a thickener (sodium carboxymethyl cellulose, CMC-Na) at a mass ratio of 96:1.5:1.5:1 in an appropriate amount of deionized water to form a uniform second negative electrode slurry. The volume average particle size D of the second negative electrode active material was... v The particle size is 11.5 μm, and the secondary particles account for 90% of the total number of particles in the second negative electrode active material.

[0138] A first negative electrode slurry and a second negative electrode slurry are simultaneously extruded using a dual-cavity coating device. The first negative electrode slurry is coated onto the current collector to form a first negative electrode film, and the second negative electrode slurry is coated onto the first negative electrode film to form a second negative electrode film. After drying and cold pressing, a negative electrode sheet is obtained. The areal density of the negative electrode sheet is 11 mg / cm³. 2 The compacted density is 1.65 g / cm³. 3 The areal density ratio of the first negative electrode film layer to the second negative electrode film layer is 5:5.

[0139] Preparation of positive electrode sheet LiNi, the positive electrode active material 0.8 Co 0.1 Mn 0.1 O2 (NCM811), conductive agent Super P, and binder polyvinylidene fluoride (PVDF) were mixed thoroughly in an appropriate amount of solvent N-methylpyrrolidone (NMP) at a mass ratio of 97:1.5:1.5 to form a uniform positive electrode slurry. The positive electrode slurry was then coated onto the surface of a positive electrode current collector aluminum foil, and after drying and cold pressing, a positive electrode sheet was obtained. The areal density of the positive electrode sheet was 17.5 mg / cm³. 2 The compacted density is 3.4 g / cm³. 3 .

[0140] Preparation of electrolyte Ethyl carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 4:3:3. LiPF6 was then uniformly dissolved in the above solution to obtain an electrolyte, wherein the concentration of LiPF6 was 1 mol / L.

[0141] Separating membrane PE release film is used.

[0142] Preparation of secondary batteries The positive electrode, separator, and negative electrode are stacked and wound in sequence to obtain an electrode assembly. The electrode assembly is then placed in an outer package, and the electrolyte is added. After processes such as encapsulation, settling, formation, and aging, a secondary battery is obtained.

[0143] Examples 2-27 and Comparative Examples 1-6 The preparation method is similar to that in Example 1, except that the relevant parameters in the negative electrode preparation step are adjusted to obtain the corresponding secondary battery, as detailed in Table 1.

[0144] Test section 1) Battery fast charging capability test At 25°C, the batteries of the above embodiments and comparative examples were charged and discharged for the first time at a current of 1C (i.e., the current value at which the theoretical capacity is completely discharged within 1 hour). Specifically, the batteries were charged at a constant current rate of 1C to a voltage of 4.25V, then charged at a constant voltage rate to a current ≤0.05C, left to stand for 5 minutes, and then discharged at a constant current rate of 0.33C to a voltage of 2.8V. The actual capacity was recorded as C0.

[0145] Then, the battery is sequentially charged at constant current rates of 1.0C0, 1.3C0, 1.5C0, 1.8C0, 2.0C0, 2.3C0, 2.5C0, and 3.0C0 until the full battery charging cutoff voltage of 4.25V or the negative terminal cutoff potential (whichever comes first). After each charge, it is discharged at 1C0 until the full battery discharge cutoff voltage of 2.8V. The charging rate is recorded at different rates until 10%, 20%, 30%, ..., 80% SOC (State of Charge). By plotting the negative electrode potential corresponding to the state of charge (SOC), the charging rate-negative electrode potential curves under different SOC states are obtained. After linear fitting, the charging rate corresponding to the negative electrode potential of 0V under different SOC states is obtained. This charging rate is the charging window under that SOC state, denoted as C10%SOC, C20%SOC, C30%SOC, C40%SOC, C50%SOC, C60%SOC, C70%SOC, and C80%SOC. The charging time T (assuming no lithium plating) from 10%SOC to 80%SOC is calculated according to the formula (60 / C20%SOC + 60 / C30%SOC + 60 / C40%SOC + 60 / C50%SOC + 60 / C60%SOC + 60 / C70%SOC + 60 / C80%SOC) × 10%, in minutes. The shorter the charging time, the better the battery's fast charging performance.

[0146] 2) Battery cycle performance test At 25°C, the secondary batteries prepared in each embodiment and comparative example were charged at a constant current rate of 1 C to the charging cutoff voltage of 4.25V, then charged at a constant voltage until the current ≤0.05C, allowed to stand for 5 minutes, and then discharged at a constant current rate of 1 C to the discharge cutoff voltage of 2.8V, allowed to stand for 5 minutes. This constitutes one charge-discharge cycle. The batteries were subjected to cyclic charge-discharge tests according to this method until the battery capacity decreased to 80%. The number of cycles at this point is the cycle life of the battery at 25°C.

[0147] Table 1

[0148]

[0149]

[0150]

[0151] Table 1 (continued)

[0152]

[0153]

[0154] As can be seen from the results in the table, in the secondary battery of this application embodiment, the negative electrode sheet includes a double film layer structure, and both the upper and lower negative electrode film layers include specific negative electrode active materials, so that the secondary battery has a high energy density while also taking into account good fast charging capability and cycle performance.

[0155] Comparative Examples 1-6 show poor fast charging capability and cycle life of the secondary batteries because W2≥W1 is not satisfied.

[0156] Furthermore, as can be seen from Examples 6, 9-11, and Examples 8, 12-14, when the first negative electrode film layer also includes silicon-based materials and satisfies W2≥W1, although the energy density of the battery will increase, the fast charging performance and cycle performance of the battery will be slightly affected.

[0157] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in this application, and these modifications or substitutions should all be covered within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A secondary battery, comprising a negative electrode sheet, wherein the negative electrode sheet comprises a negative current collector and a negative electrode film; The negative electrode film includes a first negative electrode film layer and a second negative electrode film layer; The first negative electrode film layer is disposed on at least one surface of the negative electrode current collector and includes a first negative electrode active material, the first negative electrode active material including graphite, and the first negative electrode active material may or may not include silicon-based materials; The second negative electrode film layer is disposed on the surface of the first negative electrode film layer and includes a second negative electrode active material, the second negative electrode active material including artificial graphite and silicon-based materials; The mass percentage of the silicon-based material in the first negative electrode active material is denoted as W1, and the mass percentage of the silicon-based material in the second negative electrode active material is denoted as W2. The negative electrode sheet satisfies: W2≥W1, and W1+W2≤6%; The silicon-based material includes a first silicon-based material and a second silicon-based material, wherein the particle size of the first silicon-based material is smaller than the particle size of the second silicon-based material; The particle size of the first silicon-based material is 0.05 μm to 7 μm; the particle size of the second silicon-based material is 7 μm to 15 μm.

2. The secondary battery according to claim 1, wherein, 0% < W2 ≤ 6%, optionally, 1% ≤ W2 ≤ 3%.

3. The secondary battery according to any one of claims 1 to 2, wherein, 0%≤W1≤5%, optionally, 0%≤W1≤3%, and even more optionally, W1=0%.

4. The secondary battery according to any one of claims 1 to 3, wherein, W1+W2≤3%.

5. The secondary battery according to any one of claims 1 to 4, wherein, The first negative electrode active material includes one or more of artificial graphite and natural graphite; optionally, the first negative electrode active material includes artificial graphite.

6. The secondary battery according to claim 5, wherein, The mass percentage of the artificial graphite in the first negative electrode active material is ≥50%; optionally, the mass percentage of the artificial graphite in the first negative electrode active material is 60%~100%.

7. The secondary battery according to any one of claims 1 to 6, wherein, The first negative electrode active material includes primary particles, and the primary particles account for ≥60% of the total amount in the first negative electrode active material, which can be selected as 80%~100%.

8. The secondary battery according to any one of claims 1 to 7, wherein, The second negative electrode active material includes secondary particles, and the secondary particles account for ≥50% of the total amount of the second negative electrode active material, which can be ≥70%.

9. The secondary battery according to any one of claims 1 to 8, wherein, The volume average particle size D of the first negative electrode active material v 50 is greater than the volume average particle size D of the second negative electrode active material. v 50.

10. The secondary battery according to any one of claims 1 to 9, wherein, The volume average particle size D of the first negative electrode active material v 50 is 12μm ~ 18μm, optionally 13μm ~ 17μm; and / or, The volume average particle size D of the second negative electrode active material v 50 is 9μm ~ 15μm, and can be selected as 10μm ~ 13μm.

11. The secondary battery according to any one of claims 1 to 10, wherein, The silicon-based material includes one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen compounds, and silicon alloys; optionally, the silicon-based material includes silicon oxide compounds.

12. The secondary battery according to any one of claims 1 to 11, wherein, The negative electrode film also satisfies one or more of the following (1) to (5): (1) The areal density of the negative electrode film is 8 mg / cm³. 2 ~13mg / cm 2 9mg / cm 2 ~12.5mg / cm 2 ; (2) The compaction density of the negative electrode film is 1.4 g / cm³. 3 ~ 1.7 g / cm 3 1.5 g / cm³ is an optional value. 3 ~ 1.65 g / cm 3 ; (3) The porosity of the second negative electrode film is greater than that of the first negative electrode film; (4) The areal density ratio of the first negative electrode film layer to the second negative electrode film layer is 2:3 ~ 3:2; (5) The thickness ratio of the first negative electrode film layer to the second negative electrode film layer is 4:6 ~ 7:

3.

13. The secondary battery according to any one of claims 1 to 12, wherein, The secondary battery includes a positive electrode sheet, which includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector and including a positive electrode active material. The positive electrode active material includes one or more of lithium transition metal oxides, lithium phosphates with olivine structures, and their respective modified compounds. Optionally, the positive electrode active material includes one or more of the lithium transition metal oxides and their modified compounds represented by Formula 1. Li a Ni b Co c M d O e A f Formula 1, In Formula 1, 0.8≤a≤1.2, 0.5≤b<1, 0<c<1, 0<d<1, 1≤e≤2, 0≤f≤1, M is selected from one or more of Mn, Al, Zr, Zn, Cu, Cr, Mg, Fe, V, Ti and B, and A is selected from one or more of N, F, S and Cl.

14. A method for preparing a secondary battery, comprising preparing the negative electrode sheet of the secondary battery by the following steps: A first negative electrode film layer comprising a first negative electrode active material is formed on at least one surface of the negative electrode current collector. The first negative electrode active material includes graphite and may or may not include silicon-based materials. A second negative electrode film layer comprising a second negative electrode active material is formed on the surface of the first negative electrode film layer. The second negative electrode active material comprises artificial graphite and silicon-based materials. in, The mass percentage of the silicon-based material in the first negative electrode active material is denoted as W1, and the mass percentage of the silicon-based material in the second negative electrode active material is denoted as W2. The negative electrode sheet satisfies: W2≥W1, and W1+W2≤6%; The silicon-based material includes a first silicon-based material and a second silicon-based material, wherein the particle size of the first silicon-based material is smaller than the particle size of the second silicon-based material; The particle size of the first silicon-based material is 0.05 μm to 7 μm; The particle size of the second silicon-based material is 7 μm to 15 μm.

15. An apparatus comprising a secondary battery according to any one of claims 1 to 13 or a secondary battery prepared by the method according to claim 14.

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