A negative electrode sheet and a lithium ion battery

Abstract

The application provides a negative electrode sheet and a lithium ion battery. The first aspect of the application provides a negative electrode sheet, which comprises a negative electrode current collector and a negative electrode active layer arranged on at least one functional surface of the negative electrode current collector, wherein the negative electrode active layer comprises a first negative electrode active layer, and the first negative electrode active layer comprises a silicon material and a carbon material; in the thickness direction of the first negative electrode active layer, the silicon material is distributed in the first negative electrode active layer in N linear arrangements, and the average particle number of the silicon material in each linear arrangement is 1.5-5.5; and the particle number of the silicon material in a 50μm*50μm area is 5-50. The negative electrode sheet provided by the application can effectively improve the cycle performance of the lithium ion battery.

Description

[0001] This disclosure pertains to application number 202111052450.1 filed with the Chinese Patent Office, with an application date of [date missing].

[0002] On September 8, 2021, a divisional application was filed for the invention patent application entitled "A negative electrode sheet and a lithium-ion battery". Technical Field

[0003] This invention relates to a negative electrode and a lithium-ion battery, and relates to the field of lithium-ion battery technology. Background Technology

[0004] Lithium-ion batteries have become the mainstream energy storage device for electronic products. As people's demands for longer battery life and faster charging capabilities continue to increase, higher requirements are being placed on the energy density of lithium-ion batteries. Silicon materials have a high specific capacity, and using a mixture of silicon and carbon materials as the negative electrode active material to improve the energy density of lithium-ion batteries has become an industry trend.

[0005] However, silicon materials are prone to volume expansion, causing the silicon-containing negative electrode to expand excessively during charging and discharging, which in turn affects the cycle performance of lithium-ion batteries. Furthermore, as the silicon content in the negative electrode gradually increases, the expansion rate is higher and the cycle performance is worse. Therefore, how to improve the problem of silicon material volume expansion and improve the cycle performance of lithium-ion batteries has received increasing attention. Summary of the Invention

[0006] This invention provides a negative electrode sheet to improve the problem of easy volume expansion of silicon materials and improve the cycle performance of lithium-ion batteries.

[0007] The present invention also provides a lithium-ion battery with good cycle performance.

[0008] The first aspect of the present invention provides a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode active layer disposed on at least one functional surface of the negative electrode current collector, the negative electrode active layer comprising a first negative electrode active layer comprising a silicon material and a carbon material;

[0009] In the thickness direction of the first negative electrode active layer, the silicon material is distributed in the first negative electrode active layer in N linear arrangements, and the average number of silicon material particles in each linear arrangement is 1.5-5.5; the number of silicon material particles in a 50μm*50μm region is 5-50.

[0010] This invention provides a negative electrode sheet. Based on current negative electrode sheet structures, it includes a negative electrode current collector and a negative electrode active layer disposed on at least one functional surface of the negative electrode current collector. It is understood that the negative electrode current collector is sheet-like, and its functional surfaces refer to two relatively large surfaces in the negative electrode current collector used to load the negative electrode active layer. Specifically, these are the upper and lower surfaces formed by the length and width directions of the negative electrode current collector. The negative electrode active layer is disposed on at least one functional surface of the negative electrode current collector, and the negative electrode active layer includes a first negative electrode active layer, for example... Figure 1 This is a schematic diagram of the structure of a negative electrode sheet provided in an embodiment of the present invention, as shown below. Figure 1 As shown, the negative electrode sheet includes a negative electrode current collector 100 and a first negative electrode active layer 201, which is disposed on the upper and lower functional surfaces of the negative electrode current collector 100. It is understood that the first negative electrode active layer includes silicon material. To improve the problem of easy volume expansion of silicon material, this invention limits the number of silicon material particles in the first negative electrode active layer. Specifically, in the thickness direction of the first negative electrode active layer, the silicon material is distributed in the first negative electrode active layer in a linear arrangement of N, that is, starting from the negative electrode current collector side and extending along the thickness direction of the first negative electrode active layer towards the separator side, N equidistant straight lines are drawn, with a distance of 10 μm between the lines. The total number of silicon material particles on the N equidistant straight lines is counted, and the total number of particles / N = 1.5-5.5; simultaneously, the number of silicon material particles in a 50 μm * 50 μm region is... The number of silicon particles falling within a 50μm x 50μm sample area in the plane formed by the length and thickness of the first negative electrode active layer is 5-50. It should be noted that when sampling the first negative electrode active layer in a 50μm x 50μm size, the number of silicon particles within the sample area will vary, but should remain within the aforementioned range. Furthermore, depending on the thickness of the first negative electrode active layer, those skilled in the art can use different sample sizes and then perform proportional conversion. The number of silicon particles can be obtained through scanning electron microscopy, for example... Figure 2 This is a cross-sectional SEM image of the negative electrode sheet provided in an embodiment of the present invention. Figure 3 A cross-sectional SEM image of the negative electrode sheet provided in another embodiment of the present invention, as shown below. Figure 2-3 As shown, Figure 2 The bright white area in the middle is the negative electrode current collector 100, and the left and right sides are the first negative electrode active layer 201, which includes silicon material. Figure 2The arrow below points in the thickness direction of the first negative electrode active layer. The average number of silicon particles in the thickness direction of the first negative electrode active layer is 1.5-5.5, and the number of silicon particles in a 50μm*50μm square region is 5-50. In summary, the negative electrode sheet provided by this invention includes a first negative electrode active layer, and silicon material is distributed in N linear arrangements in the thickness direction of the first negative electrode active layer, with an average number of silicon particles of 1.5-5.5 in each linear arrangement; the number of silicon particles in a 50μm*50μm region is 5-50. By controlling the number of silicon particles within the above range, the volume expansion of the silicon material can be effectively controlled, improving the cycle performance of the lithium-ion battery.

[0011] In one specific embodiment, since the surface potential of the negative electrode is lower during cycling and the reaction of the surface particles is more intense, and silicon particles are more easily broken than graphite particles during cycling, in order to further reduce the cycling expansion of the negative electrode sheet and improve the cycling performance of the lithium-ion battery, the content of silicon material on the surface of the negative electrode active layer can be reduced to further reduce the impact of silicon material volume expansion on the negative electrode sheet. Specifically, the negative electrode active layer includes a second negative electrode active layer, which is disposed on the surface of the first negative electrode active layer away from the negative electrode current collector, and the number of silicon material particles in the first negative electrode active layer is greater than the number of silicon material particles in the second negative electrode active layer.

[0012] Figure 4 This is a schematic diagram of the structure of a negative electrode sheet provided in another embodiment of the present invention, as shown below. Figure 4 As shown, the negative electrode sheet includes a negative electrode current collector 100, a first negative electrode active layer 201, and a second negative electrode active layer 202. The first negative electrode active layer 201 is disposed on the upper surface of the negative electrode current collector 100, and the second negative electrode active layer 202 is disposed on the upper surface of the first negative electrode active layer 201 away from the negative electrode current collector 100. That is, the first negative electrode active layer 201 and the second negative electrode active layer 201 are stacked sequentially on the upper surface of the negative electrode current collector 100, and the number of silicon material particles in the first negative electrode active layer is greater than the number of silicon material particles in the second negative electrode active layer. The number of particles in this case is the same as the aforementioned method for counting particles, and will not be repeated here.

[0013] When the negative electrode sheet includes a second negative electrode active layer, due to the different number of silicon material particles, the first negative electrode active layer slurry and the second negative electrode active layer slurry need to be prepared separately during the preparation process, and then coated sequentially or simultaneously on the surface of the negative electrode current collector. In order to further improve the cycle expansion of the negative electrode sheet, enhance the cycle performance of the lithium-ion battery, and simplify the negative electrode sheet preparation process, the second negative electrode active layer includes carbon material, that is, the second negative electrode active layer does not include silicon material.

[0014] To further improve the energy density while enhancing the cycle performance of lithium-ion batteries, the D50 of the carbon material... 碳 The D50 of the silicon material 硅 The mass m1 of the carbon material, the mass m2 of the silicon material, and the thickness H of the negative electrode active layer satisfy the relationship 1:

[0015]

[0016] D50 碳 D50 硅 And H has the same unit, and m1 and m2 have the same unit;

[0017] For ease of explanation, this invention defines the values ​​obtained by calculating the particle size and mass ratio of carbon and silicon materials and the thickness of the negative electrode active layer according to the formula shown in Equation 1 as the M value. The M value reflects the ratio of the number of silicon material particles to the number of carbon material particles in the negative electrode active layer. Specifically, D50 碳 and D50 硅 These refer to the particle size values ​​corresponding to a cumulative distribution of 50% in carbon and silicon materials, respectively. The units for both are the same, such as μm. The particle size can be measured by a laser particle size analyzer. The mass ratio of carbon to silicon materials refers to the ratio of the mass of carbon materials to the mass of silicon materials in the negative electrode active layer. The units for both are the same, such as grams. The thickness H of the negative electrode active layer refers to the total thickness of the negative electrode active layer located on one functional surface of the negative electrode current collector. Its unit is the same as that of D50.

[0018] When the negative electrode active layer includes only the first negative electrode active layer, the D50 of the carbon material, the D50 of the silicon material, the mass of the carbon material and the silicon material, and the thickness of the first negative electrode active layer are substituted into the formula shown in Equation 1 for calculation. When the negative electrode active layer includes the second negative electrode active layer, in the formula shown in Equation 1, if the D50 of the carbon material in the first negative electrode active layer and the second negative electrode active layer are the same, then the D50 is directly substituted into Equation 1 for calculation. If the D50 of the carbon material in the first negative electrode active layer and the second negative electrode active layer are different, then the average D50 (the calculation formula is D50A*A%+D50B*B%, where A% and B% are the proportions of the two different carbon materials in the negative electrode active layer) is substituted into Equation 1 for calculation. The thickness H of the negative electrode active layer is the total thickness of the first negative electrode active layer and the second negative electrode active layer. The mass m1 of the carbon material is the total mass of the carbon material in the first negative electrode active layer and the second negative electrode active layer. The mass m2 of the silicon material is the total mass of the silicon material in the first negative electrode active layer and the second negative electrode active layer.

[0019] The silicon and carbon materials used in this invention are conventional materials in the art. For example, the silicon material is one or more of silicon, silicon oxide, and silicon carbon, and the carbon material is graphite. In the actual production process of the negative electrode, the number of silicon particles can be controlled by adjusting the mass fraction and particle size of the silicon material, as well as the thickness of the negative electrode active layer. Specifically,

[0020] The mass of the silicon material is 1%-15% of the total mass of the silicon and carbon materials.

[0021] When the particle size of silicon or carbon materials is too large, the number of silicon particles per unit area will decrease. Therefore, it is necessary to limit the particle size of silicon and carbon materials. Specifically, the D50 of silicon materials is 5-8 μm and the D50 of carbon materials is 10-18 μm.

[0022] When the negative electrode active layer is too thick, the silicon material is sparsely dispersed within it, which is detrimental to improving the energy density of lithium-ion batteries and the fabrication of the negative electrode sheet. When the negative electrode active layer is too thin, the silicon material is too densely distributed, leading to severe expansion of the negative electrode sheet. Therefore, to further balance the energy density and cycle performance of lithium-ion batteries, the thickness of the negative electrode active layer is 40-80 μm. It can be understood that when the negative electrode active layer only includes the first negative electrode active layer, the thickness of the first negative electrode active layer is 40-80 μm. When the negative electrode active layer includes both the first and second negative electrode active layers, the total thickness of the first and second negative electrode active layers is 40-80 μm. It should be noted that the thickness here refers to the thickness of the negative electrode active layer on one functional surface of the negative electrode current collector. The thickness of the negative electrode active layer on the other functional surface of the negative electrode current collector is also within the above range, but the thickness values ​​of the negative electrode active layers on the two functional surfaces can be the same or different.

[0023] In addition to silicon and carbon materials, the negative electrode active layer also includes conductive agents, binders, and dispersants. The inventors have discovered that PAA-type (polyacrylic acid) binders help alleviate the volume expansion of silicon materials. Therefore, when the negative electrode active layer includes silicon materials, PAA-type compounds are preferred as binders. Furthermore, when the negative electrode sheet includes a first negative electrode active layer and a second negative electrode active layer, since the silicon content in the first negative electrode active layer is greater than that in the second negative electrode active layer, the corresponding content of PAA-type binders in the first negative electrode active layer is greater than that in the second negative electrode active layer.

[0024] The PAA-type adhesive has a molecular weight of 1 million to 2 million, including one or more of -CH3, -CH2, -CH=, -OR, -CHO, -Li, and -Na.

[0025] When the second negative electrode active layer includes a carbon material, the binder can be a conventional binder in the art, for example, an SBR binder.

[0026] Furthermore, since silicon materials have poor conductivity, when the first negative electrode active layer includes silicon materials, the conductivity of the active layer should be appropriately improved. Since the conductivity of carbon nanotubes in conventional conductive agents in the art is much greater than that of conventional carbon black conductive agents, the first negative electrode active layer also includes carbon nanotubes. When the negative electrode active layer includes a first negative electrode active layer and a second negative electrode active layer, the content of carbon nanotubes in the first negative electrode active layer is greater than the content of carbon nanotubes in the second negative electrode active layer.

[0027] Furthermore, the inventors discovered that when the carbon nanotube content in the negative electrode active layer is too high, lithium-ion batteries have a risk of bulging. In order to balance the conductivity of silicon materials and the safety of lithium-ion batteries, carbon nanotubes and carbon black can be mixed as conductive agents. That is, the first negative electrode active layer also includes carbon black, and the mass ratio of the carbon black to the carbon nanotube is (10:1)-(1:1). When the silicon material content in the first negative electrode active layer increases, the carbon nanotube content can be appropriately increased.

[0028] When the second negative electrode active layer includes carbon material, the conductive agent can be carbon nanotubes or carbon black. However, it should be noted that the conductivity of the first negative electrode active layer and the second negative electrode active layer is equivalent.

[0029] The negative electrode sheet can be prepared using conventional techniques in this field. First, the negative electrode active material, conductive agent, binder, and dispersant are mixed to prepare a negative electrode active layer slurry. Then, the prepared negative electrode active layer slurry is coated onto at least one functional surface of the negative electrode current collector to obtain the negative electrode sheet. Considering the requirements for the content of each component in the negative electrode active layer, the mass percentage of the negative electrode active material in the first negative electrode active layer is 95%-97.5%, the mass percentage of the conductive agent is 0.5%-2.5%, the mass percentage of the binder is 1.5%-2.5%, and the mass percentage of the dispersant is 0.5%-1.5%. The mass percentage of the negative electrode active material in the second negative electrode active layer is 96%-98%, the mass percentage of the conductive agent is 0%-1.5%, the mass percentage of the binder is 1%-2%, and the mass percentage of the dispersant is 1%-2%.

[0030] In summary, the negative electrode sheet provided by the present invention includes a first negative electrode active layer. In the thickness direction of the first negative electrode active layer, silicon material is distributed in the first negative electrode active layer in N linear arrangements, and the average number of silicon material particles in each linear arrangement is 1.5-5.5. The number of silicon material particles in a 50μm*50μm area is 5-50, which can effectively control the volume expansion of silicon material and improve the cycle performance of lithium-ion batteries.

[0031] A second aspect of the present invention provides a lithium-ion battery, the lithium-ion battery comprising any of the negative electrode sheets described above.

[0032] This invention provides a lithium-ion battery, which is prepared by combining a positive electrode, a separator, and an electrolyte with the aforementioned negative electrode using conventional techniques in the art. The lithium-ion battery provided by this invention exhibits good cycle performance.

[0033] The implementation of this invention has at least the following advantages:

[0034] 1. The negative electrode sheet provided by the present invention includes a first negative electrode active layer. In the thickness direction of the first negative electrode active layer, the silicon material is distributed in the first negative electrode active layer in N linear arrangements, and the average number of silicon material particles in each linear arrangement is 1.5-5.5. The number of silicon material particles in a 50μm*50μm area is 5-50, which can effectively control the volume expansion of silicon material and improve the cycle performance of lithium-ion battery.

[0035] 2. The lithium-ion battery provided by this invention has good cycle performance. Attached Figure Description

[0036] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0037] Figure 1 This is a schematic diagram of the structure of a negative electrode sheet provided in an embodiment of the present invention;

[0038] Figure 2 A cross-sectional SEM image of a negative electrode sheet provided in an embodiment of the present invention;

[0039] Figure 3 A cross-sectional SEM image of the negative electrode sheet provided in another embodiment of the present invention;

[0040] Figure 4 This is a schematic diagram of the negative electrode sheet provided in another embodiment of the present invention.

[0041] Explanation of reference numerals in the attached figures:

[0042] 100-Negative current collector;

[0043] 201 - First negative electrode active layer;

[0044] 202 - Second negative electrode active layer. Detailed Implementation

[0045] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0046] Example 1

[0047] The negative electrode sheet provided in this embodiment includes a negative electrode current collector copper foil and a first negative electrode active layer, wherein,

[0048] The first negative electrode active layer comprises 96.5 parts by mass of negative electrode active material, 1 part by mass of conductive agent, 1.5 parts by mass of PAA binder and 1 part by mass of dispersant CMC.

[0049] The negative electrode active materials include graphite and SiO2. 1.2 Graphite and SiO 1.2 The mass ratio is 85:15;

[0050] Graphite has a D10 of 6 μm, a D50 of 13 μm, and a D90 of 29 μm.

[0051] SiO 1.2 The D10 is 0.6 μm, the D50 is 6.7 μm, and the D90 is 12 μm;

[0052] The conductive agent includes carbon nanotubes and carbon black, and the mass ratio of carbon nanotubes to carbon black is 1:1.

[0053] The copper foil has a thickness of 6 μm, and the first negative electrode active layer has a thickness of 60 μm;

[0054] The method for preparing the negative electrode sheet provided in this embodiment includes the following steps:

[0055] Graphite, SiO 1.2 Carbon nanotubes, carbon black, PAA binder, and CMC dispersant are dispersed in deionized water to prepare a negative electrode active layer slurry. This slurry is then uniformly coated onto the two functional surfaces of a copper foil to obtain the first negative electrode active layer, thus obtaining the negative electrode sheet.

[0056] A flat cross-section of the first negative electrode active layer was prepared using an argon ion milling apparatus (CP), and the SiO₂ in the negative electrode active layer was examined using an electron scanning microscope. 1.2 The number of SiO particles was counted. Specifically, starting from the copper foil, ten equidistant lines were drawn perpendicular to the copper foil and extending to the diaphragm. The spacing between each line was 10 μm. The number of SiO particles on the lines was counted. 1.2The number of particles was calculated, and the average value was taken. Ten square regions (50μm x 50μm) were sampled from the negative electrode active layer, and the SiO₂ content within each region was statistically analyzed. 1.2 The number of particles was calculated, and the average value was taken. Statistically, in the first negative electrode active layer provided in this embodiment, SiO... 1.2 The average number of particles in the thickness direction of the first negative electrode active layer is 5.5, and the average number of particles in each 50μm*50μm region is 50.

[0057] Substituting the parameters of the negative electrode active layer into Equation 1 for calculation, the calculated value of M is 3.57.

[0058] Example 2

[0059] The negative electrode sheet and its preparation method provided in this embodiment are the same as those in Embodiment 1, except that the negative electrode active material includes graphite and SiO2. 1.2 Graphite and SiO 1.2 The mass ratio is 93:7;

[0060] Using the same method as in Example 1, the SiO content in the first negative electrode active layer provided in this example was statistically analyzed. 1.2 The average number of particles in the thickness direction of the first negative electrode active layer is 3.5, and the average number of particles in each 50μm*50μm region is 25.

[0061] The calculated value of M in this embodiment is 1.98.

[0062] Example 3

[0063] The negative electrode sheet and its preparation method provided in this embodiment are the same as those in Embodiment 1, except that the negative electrode active material includes graphite and SiO2. 1.2 Graphite and SiO 1.2 The mass ratio is 99:1;

[0064] Statistical analysis was performed using the same method as in Example 1. The SiO₂ content in the first negative electrode active layer provided in this example was statistically analyzed. 1.2 The average number of particles in the thickness direction of the first negative electrode active layer is 1.5, and the average number of particles in each 50μm*50μm region is 5.

[0065] The calculated value of M in this embodiment is 0.33.

[0066] Example 4

[0067] The negative electrode sheet provided in this embodiment includes a negative electrode current collector copper foil and a first negative electrode active layer and a second negative electrode active layer sequentially stacked on the surface of the copper foil, wherein...

[0068] The first negative electrode active layer comprises 96.5 parts by mass of negative electrode active material, 1 part by mass of conductive agent, 1.5 parts by mass of PAA binder and 1 part by mass of dispersant CMC.

[0069] Negative electrode active materials include graphite and SiO2. 1.2 Graphite and SiO 1.2 The mass ratio is 86:14;

[0070] Graphite has a D10 of 6 μm, a D50 of 13 μm, and a D90 of 29 μm.

[0071] SiO 1.2 The D10 is 0.6 μm, the D50 is 6.7 μm, and the D90 is 12 μm;

[0072] The conductive agent includes carbon nanotubes and carbon black, and the mass ratio of carbon nanotubes to carbon black is 1:1.

[0073] The copper foil has a thickness of 6 μm, the first negative electrode active layer has a thickness of 30 μm, and the second negative electrode active layer has a thickness of 30 μm.

[0074] The second negative electrode active layer comprises 97.5 parts by mass of graphite (with the same particle size as the graphite in the first negative electrode active layer), 0.5 parts by mass of carbon black, 1 part by mass of binder SBR, and 1 part by mass of dispersant CMC.

[0075] The method for preparing the negative electrode sheet provided in this embodiment includes the following steps:

[0076] Graphite, SiO 1.2 Carbon nanotubes, conductive carbon black, PAA binder, and CMC dispersant are dispersed in deionized water to prepare a first negative electrode active layer slurry. Graphite, conductive carbon black, SBR binder, and CMC dispersant are dispersed in deionized water to prepare a second negative electrode active layer slurry. Subsequently, the first and second negative electrode active layer slurries are uniformly coated onto the surface of copper foil to obtain a negative electrode sheet.

[0077] Statistical analysis was performed using the same method as in Example 1. The SiO₂ content in the first negative electrode active layer provided in this example was statistically analyzed. 1.2 The average number of particles in the thickness direction of the first negative electrode active layer is 3.5, and the average number of particles in each 50μm*50μm region is 25.

[0078] The calculated value of M in this embodiment is 1.98.

[0079] Comparative Example 1

[0080] The negative electrode sheet provided in this comparative example includes a negative electrode current collector copper foil and a negative electrode active layer. The negative electrode active layer includes 97.5 parts by mass of graphite, 0.5 parts by mass of carbon black, 1 part by mass of SBR binder and 1 part by mass of dispersant CMC. The thickness of the negative electrode active layer is 60 μm.

[0081] Comparative Example 2

[0082] The negative electrode sheet and its preparation method provided in this comparative example are the same as those in Example 1, except that the negative electrode active material includes graphite and SiO2. 1.2 Graphite and SiO 1.2 The mass ratio is 4:1;

[0083] Statistical analysis was performed using the same method as in Example 1. The SiO content in the first negative electrode active layer provided in this comparative example was statistically analyzed. 1.2 The average number of particles in the thickness direction of the first negative electrode active layer is 6.5, and the average number of particles in each 50μm*50μm region is 68.

[0084] The M value calculated in this comparative example is 4.34.

[0085] The present invention further describes the preparation of lithium-ion batteries using the negative electrode sheets provided in Examples 1-4 and Comparative Examples 1-2 in combination with positive electrode sheets, and tests the cycle performance and energy density of the lithium-ion batteries. The test results are shown in Table 1.

[0086] The positive electrode sheet includes a positive current collector aluminum foil and a positive active layer. The positive active layer includes 97.6 parts by mass of lithium cobalt oxide, 1.3 parts by mass of conductive agent and 1.1 parts by mass of binder PVDF. The conductive agent includes conductive carbon black and carbon nanotubes, and the mass ratio of conductive carbon black to carbon nanotubes is 4:1.

[0087] The diaphragm is an Asahi Kasei 5+2+2 oil-based diaphragm.

[0088] The energy density testing method includes: measuring the capacity of lithium-ion batteries at 25°C using a charge-discharge regime of 0.2C charging, 0.5C discharging, and 0.025C cutoff; the plateau voltage of the lithium-ion battery is the plateau voltage under 0.2C discharge rate.

[0089] Energy density (ED) is calculated using the following formula: ED (Wh / L) = Capacity * Platform voltage / (Length * Width * Thickness).

[0090] The 25℃ cycle capacity retention rate test and cycle expansion rate test methods include: at 25℃, the lithium-ion battery is cycled for 500T at a cycle regime of 2C charging, 0.5C discharging, and 0.025C cutoff; capacity retention rate = discharge capacity (per cycle) / initial capacity; cycle expansion rate = (thickness after cycle - initial thickness) / initial thickness.

[0091] The test methods for 45℃ cycle capacity retention and cycle expansion rate include: at 45℃, the lithium-ion battery is cycled for 300T at a cycle regime of 1C charging, 0.5C discharging, and 0.025C cutoff; capacity retention rate = discharge capacity (per cycle) / initial capacity; cycle expansion rate = (thickness after cycle - initial thickness) / initial thickness.

[0092] Table 1. Energy density and cycle performance test results of the lithium-ion batteries provided in Examples 1-4 and Comparative Examples 1-2

[0093]

[0094] According to the data provided in Comparative Examples 1-2, when the negative electrode includes silicon material, the energy density of the lithium-ion battery increases, but the cycle capacity retention decreases and the expansion rate increases, resulting in poorer cycle performance of the lithium-ion battery. Comparing the data provided in Examples 1-4 with those in Comparative Example 2, the energy density of the lithium-ion battery decreases slightly, but the cycle capacity retention rate increases significantly and the expansion rate decreases significantly. Therefore, the negative electrode provided by this invention can effectively improve the cycle performance of lithium-ion batteries. According to the data provided in Examples 1-4, the performance of the negative electrode provided in Example 4 is better than that in Examples 1-3, indicating that the double-layer structure of the negative electrode helps to further improve the cycle performance of lithium-ion batteries. According to the data provided in Examples 1-3, when the D50 of the carbon material in the negative electrode active layer... 碳 D50 of silicon materials 硅 The mass m1 of the carbon material, the mass m2 of the silicon material, and the thickness H of the negative electrode active layer are calculated according to Equation 1. The value M is in the range of 0.5-12, which helps to further balance the energy density and cycle performance of lithium-ion batteries.

[0095] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A negative electrode sheet characterized by comprising: The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on at least one functional surface of the negative electrode current collector. The negative electrode active layer includes a first negative electrode active layer, which includes silicon material and carbon material. Along the thickness direction of the first negative electrode active layer, the silicon material is distributed in N linear arrangements within the first negative electrode active layer, and the average number of silicon material particles in each linear arrangement is 1.5-5.5; the silicon material has a thickness of 50 μm. The number of particles in a 50μm region is 5-50; The silicon material includes one or more of silicon, silicon oxide, and silicon carbon; The mass of the silicon material is 1%-15% of the total mass of the silicon and carbon materials; The total thickness of the negative electrode active layer is 40-80 μm.

2. The negative electrode sheet according to claim 1, characterized by The silicon material includes silicon oxide.

3. The negative electrode sheet according to claim 1, characterized by The carbon material D50 碳 The D50 of the silicon material 硅 The mass m1 of the carbon material, the mass m2 of the silicon material, and the thickness H of the negative electrode active layer satisfy the relationship 1: Formula 1 D50 碳 D50 硅 and H are the same unit, m1 and m2 are the same unit.

4. The negative electrode sheet according to any one of claims 1 to 3, characterized by, The negative electrode active layer includes a second negative electrode active layer, which is disposed on the surface of the first negative electrode active layer away from the negative electrode current collector, and the number of silicon material particles in the first negative electrode active layer is greater than the number of silicon material particles in the second negative electrode active layer.

5. The negative electrode sheet according to claim 4, characterized by The second negative electrode active layer comprises a carbon material.

6. The negative electrode sheet according to claim 5, characterized by The silicon material has a D50 of 5-8 μm, and the carbon material has a D50 of 10-18 μm.

7. The negative electrode sheet according to claim 5, wherein The first negative electrode active layer also includes a PAA-type binder.

8. The negative electrode sheet according to claim 4, characterized by The content of PAA-type binder in the first negative electrode active layer is greater than the content of PAA-type binder in the second negative electrode active layer.

9. The negative electrode sheet according to claim 5 or 6, characterized in that, The first negative electrode active layer also includes carbon nanotubes.

10. The negative electrode sheet according to claim 4, characterized by The carbon nanotube content in the first negative electrode active layer is greater than the carbon nanotube content in the second negative electrode active layer.

11. The negative electrode sheet according to claim 9, wherein The first negative electrode active layer also includes carbon black, and the mass ratio of the carbon black to the carbon nanotube is (10:1)-(1:1).

12. A lithium-ion battery, characterized by, The lithium-ion battery includes the negative electrode sheet as described in any one of claims 1-11.

Images