Electrochemical device and electronic device

Abstract

An electrochemical device and an electronic device. The electrochemical device comprises a negative electrode sheet (10); the negative electrode sheet (10) comprises a negative electrode current collector (13), a first negative electrode material layer (11) and a second negative electrode material layer (12); and the first negative electrode material layer (11) is provided between the negative electrode current collector (13) and the second negative electrode material layer (12). When the electrochemical device is charged to an upper limit cut-off voltage at a constant current at a highest lithium non-precipitation rate, in X-ray diffraction patterns of the first negative electrode material layer (11) and the second negative electrode material layer (12), the characteristic peak of LiC12 satisfies at least one of the following features (1) and (2): (1) in the X-ray diffraction pattern of the second negative electrode material layer (12), the characteristic peak of LiC6 does not appear, 2θ at the maximum intensity in the characteristic peak of LiC12 is A2°, and 25.15≤A2°≤25.30; and (2) in the X-ray diffraction pattern of the first negative electrode material layer (11), 2θ at the maximum intensity in the characteristic peak of LiC12 is A1°, and in the X-ray diffraction pattern of the second negative electrode material layer (12), 2θ at the maximum intensity in the characteristic peak of LiC12 is A2°, and 0≤A1°-A2°≤0.15.

Description

Electrochemical device and electronic device

[0001] This application claims priority to the Chinese patent application No. 202410479582.X, filed on April 19, 2024, and entitled “Electrochemical device and electronic device”, the entire content of which is incorporated herein by reference. TECHNICAL FIELD

[0002] The present application relates to the technical field of electrochemistry, and in particular, to an electrochemical device and an electronic device. BACKGROUND

[0003] With the rapid development of society, higher requirements are put forward for the energy density of electrochemical devices (such as lithium ion batteries). Since silicon-based materials have extremely high theoretical specific capacity, mixing silicon-based materials with graphite as negative electrode active materials can significantly improve the energy density of lithium ion batteries. However, mixing silicon-based materials into graphite will deteriorate the kinetic ability of the negative electrode sheet, lithium is easily deposited on the surface of the negative electrode sheet, affecting the cycle performance of the lithium ion battery and bringing safety hazards. Other designs are the same, the higher the proportion of silicon-based materials, the more serious the kinetic deterioration, and the more lithium is deposited on the surface of the negative electrode sheet under the same charging system. Therefore, how to alleviate the phenomenon of lithium deposition on the surface of the negative electrode sheet in the electrochemical device has become a technical problem to be solved by those skilled in the art. SUMMARY

[0004] The purpose of the present application is to provide an electrochemical device and an electronic device to alleviate the problem of lithium deposition on the surface of the negative electrode sheet in the electrochemical device.

[0005] It should be noted that the present application is explained in the summary of the application by taking a lithium ion battery as an example of an electrochemical device, but the electrochemical device of the present application is not limited to a lithium ion battery. The specific technical solutions are as follows:

[0006] The first aspect of the present application provides an electrochemical device, the electrochemical device comprising a negative electrode sheet, a positive electrode sheet, and a separator disposed between the negative electrode sheet and the positive electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a first negative electrode material layer and a second negative electrode material layer disposed on at least one surface of the negative electrode current collector, the first negative electrode material layer being disposed between the negative electrode current collector and the second negative electrode material layer; when the electrochemical device is charged at a highest non-lithium deposition rate to an upper limit cutoff voltage, the characteristic peak of LiC6 in the X-ray diffraction pattern of the first negative electrode material layer and the second negative electrode material layer satisfies at least one of the following characteristics (1) or (2): (1) the characteristic peak of LiC6 does not appear in the X-ray diffraction pattern of the second negative electrode material layer, and the characteristic peak of LiC12 appears in the X-ray diffraction pattern of the first negative electrode material layer; or (2) the characteristic peak of LiC6 appears in the X-ray diffraction pattern of the second negative electrode material layer, and the characteristic peak of LiC12 does not appear in the X-ray diffraction pattern of the first negative electrode material layer. 12 12 ​The 2θ with the maximum intensity in the characteristic peak is A2°, 25.15≤A2≤25.30; (2) In the X-ray diffraction pattern of the first negative electrode material layer, LiC 12 The 2θ with the maximum intensity in the characteristic peak of LiC is A1°. In the X-ray diffraction pattern of the second negative electrode material layer, LiC 12 The 2θ value at the maximum intensity of the characteristic peak is A2°, and 0≤A1-A2≤0.15. Through the above configuration, the lithium insertion degree of the surface of the negative electrode plate is low, and / or the lithium insertion of the negative electrode plate is uniform. Localized lithium insertion is not observed on the surface of the negative electrode plate, and the problem of lithium deposition on the surface of the negative electrode plate in the electrochemical device is alleviated.

[0007] In some embodiments of the present application, the first negative electrode material layer includes a first negative electrode active material, which includes a first graphite; the second negative electrode material layer includes a second negative electrode active material, which includes a silicon-carbon material and a second graphite; the weight percentage of the silicon-carbon material in the second negative electrode active material is 15% to 50%, and the weight percentage of the silicon element in the silicon-carbon material is 30% to 60%. The first negative electrode material layer at the bottom of the negative electrode plate includes the first graphite, and the second negative electrode material layer at the surface includes the second graphite and the silicon-carbon material. The weight percentage of the silicon-carbon material and the weight percentage of the silicon element in the silicon-carbon material are controlled within the above ranges. This allows the negative electrode plate to provide sufficient active material and also helps to increase the number of lithium-inserting sites on the surface of the negative electrode plate while reducing the probability of volume expansion of the negative electrode plate caused by the silicon element. This can alleviate the problem of lithium deposition on the surface of the negative electrode plate in the electrochemical device, and also enable the electrochemical device to have a higher energy density and a smaller volume expansion rate.

[0008] In some embodiments of the present application, the mass percentage of silicon-carbon material in the second negative electrode active material is 30% to 50%, and the mass percentage of silicon element in the silicon-carbon material is 50% to 60%, which can further improve the problem of lithium deposition on the surface of the negative electrode.

[0009] In some embodiments of the present application, the particle size Dv50 of the first graphite -1 The particle size of the first graphite is Dv50 -1 When regulated within the above range, the electrochemical device has a higher capacity retention rate based on a lower probability of lithium deposition on the surface of the negative electrode.

[0010] In some embodiments of the present application, the particle size Dv50 of the second graphite -2 With the first graphite particle size Dv50 -1 Meet: 1μm≤Dv50 -1 -Dv50 -2 ≤5μm. The particle size of the second graphite is Dv50-2 Dv50 of the first graphite -1 Dv50 of the first graphite -1 Dv50 of the first graphite -2 The values of Dv10, Dv50 and Dv90 of the silicon-carbon material are regulated within the above ranges, which can make the electrochemical device further reduce the probability of lithium precipitation on the surface of the negative electrode tab while having a high capacity retention rate.

[0011] In some embodiments of the present application, the particle size Dv10, Dv50 and Dv90 of the silicon-carbon material satisfy: 5 μm≤Dv10≤6 μm, 8 μm≤Dv50≤10.5 μm, and 13 μm≤Dv90≤17 μm. Regulating the particle size Dv10, Dv50 and Dv90 of the silicon-carbon material within the above ranges can make the electrochemical device have a higher capacity retention rate on the basis of alleviating the problem of lithium precipitation on the surface of the negative electrode tab. -3 -3 -4

[0012] In some embodiments of the present application, the particle size Dv10 and Dv90 of the silicon-carbon material satisfy: 8 μm≤Dv90-Dv10≤10.5 μm. Regulating the value of Dv90-Dv10 within the above range can make the electrochemical device improve its kinetic ability on the basis of having a higher energy density.

[0013] In some embodiments of the present application, the specific surface area of the silicon-carbon material is 0.5 m2 / g to 3.5 m2 / g. Regulating the specific surface area of the silicon-carbon material within the above range can make the electrochemical device have a higher capacity retention rate on the basis of alleviating the problem of lithium precipitation on the surface of the negative electrode tab. 2 2

[0014] In some embodiments of the present application, the specific surface area of the silicon-carbon material is 1.0 m2 / g to 2.0 m2 / g. This is conducive to making the electrochemical device further alleviate the problem of lithium precipitation on the surface of the negative electrode tab while having a higher capacity retention rate. 2 2

[0015] In some embodiments of the present application, the compactness density C1 of the first negative electrode material layer and the compactness density C2 of the second negative electrode material layer satisfy: 1.5 g / cm3≤C1≤1.65 g / cm3, 1.35 g / cm3≤C2≤1.5 g / cm3, and 0.05 g / cm3≤C1-C2≤0.3 g / cm3. Regulating the compactness density C1 of the first negative electrode material layer and the compactness density C2 of the second negative electrode material layer within the above ranges can make the electrochemical device have a higher capacity retention rate on the basis of alleviating the problem of lithium precipitation on the surface of the negative electrode tab. 3 3 3 3 3 3 ​​​​​​​​​​​​The compaction densities C1 of the first negative electrode material layer, C2 of the second negative electrode material layer and the value C1-C2 are regulated within the scope of the present application, and the problem of lithium precipitation on the surface of the negative electrode sheet of the electrochemical device is alleviated, and the electrochemical device has a high energy density.

[0016] In some embodiments of the present application, 0.1 g / cm 3 ≤C1-C2≤0.2 g / cm 3 .

[0017] The second aspect of the present application provides an electronic device comprising the electrochemical device of any of the preceding embodiments. Therefore, the electronic device has good use performance.

[0018] The beneficial effects of the present application are as follows:

[0019] The present application provides an electrochemical device and an electronic device, the electrochemical device comprising a negative electrode sheet, a positive electrode sheet and a separator arranged between the negative electrode sheet and the positive electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a first negative electrode material layer and a second negative electrode material layer arranged on at least one surface of the negative electrode current collector, the first negative electrode material layer being arranged between the negative electrode current collector and the second negative electrode material layer; when the electrochemical device is charged at a highest non-lithium precipitation rate to an upper limit cutoff voltage, the characteristic peaks of LiC 12 in the X-ray diffraction pattern of the first negative electrode material layer and the second negative electrode material layer satisfy at least one of the following characteristics (1) or (2): (1) no characteristic peak of LiC6 appears in the X-ray diffraction pattern of the second negative electrode material layer, and the 2θ at the maximum intensity of the characteristic peak of LiC 12 is A2°, and 25.15≤A2≤25.30; (2) the 2θ at the maximum intensity of the characteristic peak of LiC 12 in the X-ray diffraction pattern of the first negative electrode material layer is A1°, and the 2θ at the maximum intensity of the characteristic peak of LiC 12 in the X-ray diffraction pattern of the second negative electrode material layer is A2°, and 0≤A1-A2≤0.15. Through the above arrangement, the lithium intercalation degree of the surface layer of the negative electrode sheet is low, and / or the lithium intercalation uniformity of the negative electrode sheet is good, and the phenomenon of local increase in lithium intercalation does not appear on the surface of the negative electrode sheet. Thus, the problem of lithium precipitation on the surface of the negative electrode sheet in the electrochemical device is alleviated, and the cycle performance of the electrochemical device is improved.

[0020] Of course, implementing any product or method of the present application does not necessarily require achieving all the advantages described above at the same time. BRIEF DESCRIPTION OF DRAWINGS

[0021] In order to more clearly illustrate the technical solutions of the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed in the embodiment or prior art description. Obviously, the drawings in the following description only represent some of the embodiments of the present application, and all other embodiments obtained by those skilled in the art based on these drawings also belong to the scope of protection of the present application.

[0022] FIG. 1 is a schematic diagram of the cross-sectional structure of the negative electrode tab along the length direction and the thickness direction of some embodiments of the present application;

[0023] FIG. 2 is an X-ray diffraction spectrum of the negative electrode tab of Example 1-1 of the present application;

[0024] FIG. 3 is an X-ray diffraction spectrum of the negative electrode tab of Comparative Example 1 of the present application.

[0025] Reference signs: 10-negative electrode tab; 11-first negative electrode material layer; 12-second negative electrode material layer; 13-negative electrode current collector; 131-first surface; 132-second surface. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solutions, and advantages of the present application more clear, the following will further describe the present application in detail with reference to the drawings and examples. Obviously, the described examples are only some of the embodiments of the present application, not all. All other embodiments obtained by those skilled in the art based on the present application also belong to the scope of protection of the present application.

[0027] It should be noted that in the specific embodiments of the present application, the present application is explained by taking lithium-ion batteries as examples of electrochemical devices, but the electrochemical devices of the present application are not limited to lithium-ion batteries.

[0028] The first aspect of the present application provides an electrochemical device, the electrochemical device comprising a negative electrode tab, a positive electrode tab, and a separator disposed between the negative electrode tab and the positive electrode tab, the negative electrode tab comprising a negative electrode current collector and a first negative electrode material layer and a second negative electrode material layer disposed on at least one surface of the negative electrode current collector, the first negative electrode material layer being disposed between the negative electrode current collector and the second negative electrode material layer. When the electrochemical device is charged at a highest lithium non-extraction rate to an upper limit cutoff voltage, the characteristic peaks of LiC6 in the X-ray diffraction spectrum of the first negative electrode material layer and the second negative electrode material layer satisfy at least one of the following characteristics (1) or (2): (1) the characteristic peaks of LiC6 do not appear in the X-ray diffraction spectrum of the second negative electrode material layer, and the 2θ at the maximum intensity of the characteristic peaks of LiC12 is A2°, 25.15≤A2≤25.30; (2) the characteristic peaks of LiC6 do not appear in the X-ray diffraction spectrum of the first negative electrode material layer, and the 2θ at the maximum intensity of the characteristic peaks of LiC12 is A2°, 25.15≤A2≤25.30. 12 12 12 ​​A1°, and the maximum intensity of the characteristic peak of LiC6 in the X-ray diffraction spectrum of the second negative electrode material layer is A2°, 0≤A1-A2≤0.15. In some embodiments, when the electrochemical device is charged at the highest non-lithium precipitation rate to the upper limit cutoff voltage, the characteristic peak of LiC6 does not appear in the X-ray diffraction spectrum of the first negative electrode material layer, and the maximum intensity of the characteristic peak of LiC6 in the X-ray diffraction spectrum of the second negative electrode material layer is A2°, 25.15≤A2≤25.30. 12 A1°, and the maximum intensity of the characteristic peak of LiC6 in the X-ray diffraction spectrum of the second negative electrode material layer is A2°, 0≤A1-A2≤0.15. In some embodiments, when the electrochemical device is charged at the highest non-lithium precipitation rate to the upper limit cutoff voltage, the characteristic peak of LiC6 does not appear in the X-ray diffraction spectrum of the first negative electrode material layer, and the maximum intensity of the characteristic peak of LiC6 in the X-ray diffraction spectrum of the second negative electrode material layer is A2°, 25.15≤A2≤25.30. 12 A1°, and the maximum intensity of the characteristic peak of LiC6 in the X-ray diffraction spectrum of the second negative electrode material layer is A2°, 0≤A1-A2≤0.15. In some embodiments, when the electrochemical device is charged at the highest non-lithium precipitation rate to the upper limit cutoff voltage, the characteristic peak of LiC6 does not appear in the X-ray diffraction spectrum of the first negative electrode material layer, and the maximum intensity of the characteristic peak of LiC6 in the X-ray diffraction spectrum of the second negative electrode material layer is A2°, 25.15≤A2≤25.30. 12 A1°, and the maximum intensity of the characteristic peak of LiC6 in the X-ray diffraction spectrum of the second negative electrode material layer is A2°, 0≤A1-A2≤0.15. In some embodiments, when the electrochemical device is charged at the highest non-lithium precipitation rate to the upper limit cutoff voltage, the characteristic peak of LiC6 does not appear in the X-ray diffraction spectrum of the first negative electrode material layer, and the maximum intensity of the characteristic peak of LiC6 in the X-ray diffraction spectrum of the second negative electrode material layer is A2°, 25.15≤A2≤25.30. 12 A1°, and the maximum intensity of the characteristic peak of LiC6 in the X-ray diffraction spectrum of the second negative electrode material layer is A2°, 0≤A1-A2≤0.15. In some embodiments, when the electrochemical device is charged at the highest non-lithium precipitation rate to the upper limit cutoff voltage, the characteristic peak of LiC6 does not appear in the X-ray diffraction spectrum of the first negative electrode material layer, and the maximum intensity of the characteristic peak of LiC6 in the X-ray diffraction spectrum of the second negative electrode material layer is A2°, 25.15≤A2≤25.30. 12 A1°, and the maximum intensity of the characteristic peak of LiC6 in the X-ray diffraction spectrum of the second negative electrode material layer is A2°, 0≤A1-A2≤0.15. In some embodiments, when the electrochemical device is charged at the highest non-lithium precipitation rate to the upper limit cutoff voltage, the characteristic peak of LiC6 does not appear in the X-ray diffraction spectrum of the first negative electrode material layer, and the maximum intensity of the characteristic peak of LiC6 in the X-ray diffraction spectrum of the second negative electrode material layer is A2°, 25.15≤A2≤25.30. 12 A1°, and the maximum intensity of the characteristic peak of LiC6 in the X-ray diffraction spectrum of the second negative electrode material layer is A2°, 0≤A1-A2≤0.15, 25.15≤A2≤25.30.

[0029] For the convenience of understanding, in the present application, the length direction of the negative electrode tab itself is defined as X, and the thickness direction of the negative electrode tab itself is defined as Z. It should be understood that the above definition of the direction is for the purpose of facilitating the description of the present application, and the direction defined in the present application can be understood according to the relative position of the actual product elements and the drawings. It can be understood that the length direction and the thickness direction of the negative electrode current collector, the first negative electrode material layer, and the second negative electrode material layer are the same as those of the negative electrode tab. FIG. 1 shows the cross-sectional structure of the negative electrode tab along the length direction X and the thickness direction Z in some embodiments of the present application. As shown in FIG. 1, the negative electrode tab 10 includes a negative electrode current collector 13, a first negative electrode material layer 11, and a second negative electrode material layer 12. The negative electrode current collector 13 includes a first surface 131 and a second surface 132 arranged along the thickness direction Z. The first negative electrode material layer 11 and the second negative electrode material layer 12 are arranged on the two surfaces of the negative electrode current collector 13, i.e., the first surface 131 and the second surface 132. The first negative electrode material layer 11 is arranged between the negative electrode current collector 13 and the second negative electrode material layer 12. It can be understood that in another embodiment of the present application, the first negative electrode material layer 11 and the second negative electrode material layer 12 can be arranged on one surface of the negative electrode current collector 13, i.e., the first surface 131 or the second surface 132.

[0030] In the present application, the "maximum non-lithium precipitation rate" refers to the maximum charging rate of the negative electrode sheet of the electrochemical device under the condition of no lithium precipitation, specifically the maximum non-lithium precipitation rate measured according to the test method in the "test of maximum non-lithium precipitation rate". In the present application, the upper cut-off voltage of the electrochemical device is 4.5 V to 4.55 V. When the electrochemical device is charged at the maximum non-lithium precipitation rate to the upper cut-off voltage, there is no characteristic peak of LiC6 in the X-ray diffraction pattern of the second negative electrode material layer, indicating that the lithium intercalation degree in the second negative electrode material layer is low, and the lithium intercalation degrees of the surface first negative electrode material layer and the bottom second negative electrode material layer are relatively uniform; the characteristic peak of LiC 12 has a maximum intensity at 2θ of A2°, and 25.15≤A2≤25.30. For example, A2 is 25.15, 25.18, 25.20, 25.22, 25.24, 25.25, 25.27, 25.28, 25.29, 25.30, or any value within a range between any two of the above values. A2 within the above range indicates that when the electrochemical device is charged at the maximum non-lithium precipitation rate to the upper cut-off voltage, the lithium intercalation degree in the second negative electrode material layer on the surface of the negative electrode sheet is low, and the precipitation of lithium on the surface of the negative electrode sheet is less. When the electrochemical device is charged at the maximum non-lithium precipitation rate to the upper cut-off voltage, the first negative electrode material layer has an X-ray diffraction pattern, and the characteristic peak of LiC 12 has a maximum intensity at 2θ of A1°, and the second negative electrode material layer has an X-ray diffraction pattern, and the characteristic peak of LiC 12 has a maximum intensity at 2θ of A2°, and 0≤A1-A2≤0.15. For example, A1-A2 is 0.00, 0.01, 0.02, 0.03, 0.04, 0.07, 0.09, 0.10, 0.12, 0.15, or any value within a range between any two of the above values. A1-A2 within the above range indicates that when the electrochemical device is charged at the maximum non-lithium precipitation rate, the lithium intercalation degrees of the first negative electrode material layer and the second negative electrode material layer in the negative electrode sheet are consistent under the condition of incomplete charging, indicating that the lithium intercalation in the negative electrode sheet is uniform, and there is no local increase in lithium intercalation, thereby reducing the probability of lithium precipitation on the surface of the negative electrode sheet. Through the above settings, the lithium intercalation degree on the surface of the negative electrode sheet is low, and / or the lithium intercalation uniformity of the negative electrode sheet is good, and there is no local increase in lithium intercalation on the surface of the negative electrode sheet, and the problem of lithium precipitation on the surface of the negative electrode sheet in the electrochemical device is alleviated.

[0031] In the present application, the range A1° of 2θ at the maximum intensity of the characteristic peak of LiC 12 in the X-ray diffraction pattern of the first negative electrode material layer is not particularly limited, as long as the purpose of the present application can be achieved. For example, 25.27°≤A1≤25.5°.

[0032] In the present application, the range A1° of 2θ at the maximum intensity of the characteristic peak of LiC12 There is no particular limitation on the range A1° of 2θ at the maximum intensity of the characteristic peak, as long as the purpose of the present application can be achieved. For example, this can be achieved by adjusting the thickness of the negative electrode sheet.

[0033] The present invention relates to the X-ray diffraction pattern of LiC in the second negative electrode material layer. 12 There is no particular limitation on the manner in which the 2θ range A2° at the maximum intensity of the characteristic peak of the second negative electrode active material is controlled, as long as the objectives of the present application can be achieved. For example, this can be achieved by controlling at least one of the mass percentage of the silicon-carbon material in the second negative electrode active material or the mass percentage of the silicon element in the silicon-carbon material.

[0034] In some embodiments of the present application, the first negative electrode material layer includes a first negative electrode active material, which includes a first graphite; the second negative electrode material layer includes a second negative electrode active material, which includes a silicon-carbon material and a second graphite; the mass percentage of the silicon-carbon material in the second negative electrode active material is 15% to 50%, and the mass percentage of silicon in the silicon-carbon material is 30% to 60%. For example, the mass percentage of the silicon-carbon material in the second negative electrode active material is 15%, 20%, 26%, 32%, 40%, 46%, 50%, or any value between any two of the above numerical ranges. For example, the mass percentage of silicon in the silicon-carbon material is 30%, 34%, 39%, 42%, 50%, 53%, 60%, or any value between any two of the above numerical ranges. Adding silicon-carbon material to the second negative electrode material layer provided on the surface of the negative electrode plate can increase the gram capacity of the surface of the negative electrode plate, increase the number of lithium insertion sites, and thus reduce the degree of lithium insertion in the second negative electrode material layer, which is beneficial to alleviating the problem of lithium deposition on the surface of the negative electrode plate in the electrochemical device. The first negative electrode material layer at the bottom of the negative electrode plate includes a first graphite, and the second negative electrode material layer on the surface includes a second graphite and a silicon-carbon material. The mass percentage of the silicon-carbon material and the mass percentage of the silicon element in the silicon-carbon material are controlled within the above range. The negative electrode plate can provide sufficient active material, and is also beneficial to the silicon element in the surface of the negative electrode plate. On the basis of increasing the number of lithium insertion sites on the surface of the negative electrode plate, the probability of volume expansion of the negative electrode plate caused by the silicon element is reduced. As a result, the problem of lithium deposition on the surface of the negative electrode plate in the electrochemical device is alleviated, and the electrochemical device has a higher energy density and a smaller volume expansion rate.

[0035] The present application does not impose any particular restrictions on the method for regulating the mass percentage of silicon in the silicon-carbon material, as long as the purpose of the present application can be achieved. For example, when preparing a silicon-carbon material by depositing silicon in porous carbon, the mass percentage of silicon in the silicon-carbon material can be changed by regulating the deposition content of silicon during the preparation process. It is also possible to directly purchase silicon-carbon materials with different silicon content, and in combination with the "Test of the Mass Percentage of Silicon in Silicon-carbon Materials" provided in this application, select the silicon-carbon material with the required silicon content.

[0036] In some embodiments of the present application, the mass percentage of the second graphite in the second negative electrode active material is 50% to 85%.

[0037] In the present application, the first graphite and the second graphite are each independently selected from natural graphite or artificial graphite.

[0038] In some embodiments of the present application, the particle size Dv50 of the first graphite -1 For example, the particle size Dv50 of the first graphite is 11.0 μm to 13.5 μm. -1 The particle size Dv50 of the first graphite is 11.0 μm, 11.3 μm, 11.7 μm, 12.0 μm, 12.8 μm, 13.0 μm, 13.5 μm or any value between any two of the above numerical ranges. -1 By regulating within the above range, the first graphite is less likely to agglomerate, allowing it to be evenly distributed throughout the first negative electrode material layer. Furthermore, the surface area of ​​the first graphite allows for sufficient contact with the electrolyte, allowing the first graphite to fully exert its activity and resulting in a higher capacity for the first negative electrode material layer. As a result, the electrochemical device has a higher energy density while minimizing the probability of lithium deposition on the surface of the negative electrode sheet. This also results in a higher capacity retention rate for the electrochemical device.

[0039] In some embodiments of the present application, the particle size Dv50 of the second graphite -2 With the first graphite particle size Dv50 -1 Meet: 1μm≤Dv50 -1 -Dv50 -2 ≤5μm. For example, Dv50 -1 -Dv50 -2 The value of is 1 μm, 2 μm, 3 μm, 4 μm, 5 μm or any value between any two of the above numerical ranges. -2 With the first graphite particle size Dv50 -1 The difference Dv50 -1 -Dv50 -2The values of the particle size Dv10, Dv50 and Dv90 of the silicon-carbon material are controlled within the above ranges, the silicon-carbon material has a particle size of a suitable size and a small size difference between particles, and when the silicon-carbon material is distributed in the second negative electrode material layer, the probability of agglomeration is small, which is beneficial to uniform distribution in the second negative electrode material layer, thereby improving the wettability of the electrolyte to the negative electrode sheet, and improving the wettability and liquid retention capacity of the electrolyte to the negative electrode sheet. In this way, the utilization rate of the silicon-carbon material is improved, the problem of lithium precipitation on the surface of the negative electrode sheet in the electrochemical device is alleviated, the energy density is improved, and the capacity retention rate is also high.

[0040] The particle size Dv50 of the second graphite in the present application is controlled to be 8.5 μm to 13.5 μm. -2 There is no particular limitation as long as the purpose of the present application can be achieved. For example, the particle size Dv50 of the second graphite is controlled to be 8.5 μm to 13.5 μm. -2

[0041] In some embodiments of the present application, the particle size Dv10, Dv50 and Dv90 of the silicon-carbon material satisfy: 5 μm≤Dv10≤6 μm, 8 μm≤Dv50≤10.5 μm, and 13 μm≤Dv90≤17 μm. -3 -3 For example, the particle size Dv10 of the silicon-carbon material is 5 μm, 5.2 μm, 5.4 μm, 5.7 μm, 5.8 μm, 6 μm, or any value within any two of the above numerical ranges. For example, the particle size Dv50 of the silicon-carbon material is 8 μm, 8.2 μm, 8.7 μm, 9.1 μm, 9.8 μm, 10.0 μm, 10.5 μm, or any value within any two of the above numerical ranges. For example, the particle size Dv90 of the silicon-carbon material is 13 μm, 13.6 μm, 14.4 μm, 14.7 μm, 15.3 μm, 16 μm, 16.5 μm, 17 μm, or any value within any two of the above numerical ranges. -3 -4 The particle size Dv10, Dv50 and Dv90 of the silicon-carbon material are controlled within the above ranges, the silicon-carbon material has a particle size of a suitable size and a small size difference between particles, and when the silicon-carbon material is distributed in the second negative electrode material layer, the probability of agglomeration is small, which is beneficial to uniform distribution in the second negative electrode material layer, thereby improving the wettability of the electrolyte to the negative electrode sheet, and improving the wettability and liquid retention capacity of the electrolyte to the negative electrode sheet. In this way, the utilization rate of the silicon-carbon material is improved, the problem of lithium precipitation on the surface of the negative electrode sheet in the electrochemical device is alleviated, the energy density is improved, and the capacity retention rate is also high.

[0042] ​​​In some embodiments of the present application, the particle sizes Dv10 and Dv90 of the silicon-carbon material satisfy the following conditions: 8μm≤Dv90-Dv10≤10.5μm. For example, the value of Dv90-Dv10 is 8.0μm, 9.7μm, 10.2μm, 10.5μm, or any value between any two of the above numerical ranges. By regulating the value of Dv90-Dv10 within the above range, the size difference between large and small particles in the silicon-carbon material is small. When the silicon-carbon material is distributed in the second negative electrode material layer, silicon-carbon materials of different particle sizes are distributed in a suitable gradient, which is conducive to the close and uniform distribution of the silicon-carbon material in the second negative electrode material layer, and enhances the rapid lithium insertion ability of the silicon-carbon material, so that the electrochemical device has improved its kinetics on the basis of having a higher energy density, and has a higher minimum charge rate and capacity retention rate.

[0043] In this application, Dv10 refers to the particle size at which the volume-based particle size distribution reaches 10% of the cumulative volume from the smallest particle size; Dv50 refers to the particle size at which the volume-based particle size distribution reaches 50% of the cumulative volume from the smallest particle size; and Dv90 refers to the particle size at which the volume-based particle size distribution reaches 90% of the cumulative volume from the smallest particle size. The aforementioned "particles" may be a silicon-carbon material, a first graphite, or a second graphite.

[0044] The present application does not particularly limit the method for controlling the particle size of the silicon-carbon material, the first graphite, and the second graphite, as long as the purpose of the present application can be achieved. For example, it can be achieved by crushing or screening. Alternatively, the particle size Dv50 of the first graphite can be determined by purchasing commercially available silicon-carbon material, the first graphite, and the second graphite, and combining the test method of "Particle Size Test" in this application. -1 , the particle size of the second graphite Dv50 -2 , Particle size of silicon carbon material Dv10, Dv50 -3 and Dv90, and select the required material.

[0045] In some embodiments of the present application, the specific surface area of ​​the silicon-carbon material is 0.5 m 2 / g to 3.5m 2 / g. For example, the specific surface area of ​​silicon carbon material is 0.5m 2 / g, 0.7m 2 / g, 1.0m 2 / g, 1.5m 2 / g, 2.0m 2 / g, 2.5m 2 / g, 3.0m 2 / g, 3.5m 2 / g or any value between any two of the above-mentioned numerical ranges. By adjusting the specific surface area of the silicon-carbon material within the above-mentioned range, the surface of the silicon-carbon material can have more lithium intercalation sites, which is conducive to reducing the probability of lithium precipitation on the negative electrode sheet, and the surface of the silicon-carbon material can fully contact the electrolyte, so that the silicon-carbon material can exhibit its high capacity characteristics, and the negative electrode sheet has a high capacity. Thus, the electrochemical device has a high energy density and capacity retention rate on the basis of alleviating the problem of lithium precipitation on the surface of the negative electrode.

[0046] In some embodiments of the present application, the tap density of the silicon-carbon material is 0.8 g / cm 2 to 1.05 g / cm 2 . For example, the tap density of the silicon-carbon material is 0.8 g / cm 2 , 0.85 g / cm 2 , 0.87 g / cm 2 , 0.91 g / cm 2 , 0.96 g / cm 2 , 1.00 g / cm 2 , 1.05 g / cm 3 .

[0047] The present application does not have a particular limitation on the adjustment method of the specific surface area of the silicon-carbon material, as long as the purpose of the present application can be achieved. For example, it can be achieved by adjusting the particle size of the silicon-carbon material or adjusting the pore size distribution of the porous carbon when the silicon-carbon material is prepared by a deposition method, or it can also be achieved by directly purchasing a silicon-carbon material with a specific surface area within the range of the present application, and combining the test method of "test method of the specific surface area of the silicon-carbon material" in the present application to determine the specific surface area of the silicon-carbon material, and selecting a silicon-carbon material with the desired specific surface area.

[0048] In some embodiments of the present application, the tap density of the silicon-carbon material is 0.8 g / cm 3 to 1.05 g / cm 3 . For example, the tap density of the silicon-carbon material is 0.8 g / cm 3 , 0.85 g / cm 3 , 0.87 g / cm 3 , 0.91 g / cm 3 , 0.96 g / cm 3 , 1.00 g / cm 3 , 1.05 g / cm 3or any value between any two of the above-mentioned numerical ranges. Adjusting the tap density of the silicon-carbon material within the above-mentioned range is beneficial for the silicon-carbon material to be tightly packed in the second negative electrode material layer and have a suitable porosity, so that the electrochemical device has a lower volume expansion rate on the basis of having a higher energy density.

[0049] In some embodiments of the present application, the tap density C1 of the first negative electrode material layer and the tap density C2 of the second negative electrode material layer satisfy: 1.5 g / cm 3 ≤ C1 ≤ 1.65 g / cm 3 , 1.35 g / cm 3 ≤ C2 ≤ 1.5 g / cm 3 , 0.05 g / cm 3 ≤ C1 - C2 ≤ 0.3 g / cm 3 . For example, the tap density C1 of the first negative electrode material layer is 1.5 g / cm 3 , 1.52 g / cm 3 , 1.55 g / cm 3 , 1.57 g / cm 3 , 1.60 g / cm 3 , 1.62 g / cm 3 , 1.65 g / cm 3 or any value between any two of the above-mentioned numerical ranges. For example, the tap density C2 of the second negative electrode material layer is 1.35 g / cm 3 , 1.37 g / cm 3 , 1.40 g / cm 3 , 1.43 g / cm 3 , 1.47 g / cm 3 , 1.5 g / cm 3 or any value between any two of the above-mentioned numerical ranges. For example, the value of C1 - C2 is 0.05 g / cm 3 , 0.1 g / cm 3 , 0.15 g / cm 3 , 0.2 g / cm 3 , 0.25 g / cm 3 , 0.3 g / cm 3or any value between any two of the above-mentioned numerical ranges. By regulating the compaction density C1 of the first negative electrode material layer, the compaction density C2 of the second negative electrode material layer, and the value C1-C2 within the scope of the present application, the gaps between the particles in the first negative electrode material layer and the second negative electrode material layer can be increased when the first negative electrode material layer and the second negative electrode material layer have a higher capacity, so that the electrolyte has a shorter transmission path and a larger number of transmission channels in the first negative electrode material layer and the second negative electrode material layer. In this way, the electrolyte can be uniformly distributed in the first negative electrode material layer and the second negative electrode material layer, the transmission rate of lithium ions and electrons is improved, the probability of lithium precipitation on the surface of the negative electrode sheet is reduced, and the negative electrode sheet has a higher capacity. Therefore, the electrochemical device has a higher energy density while the problem of lithium precipitation on the surface of the negative electrode sheet is alleviated.

[0050] In some embodiments of the present application, 0.1 g / cm 3 ≤ C1-C2 ≤ 0.2 g / cm 3 For example, the value of C1-C2 is 0.1 g / cm 3 , 0.12 g / cm 3 , 0.14 g / cm 3 , 0.17 g / cm 3 , 0.2 g / cm 3 or any value between any two of the above-mentioned numerical ranges. By regulating the difference C1-C2 between the compaction density C1 of the first negative electrode material layer and the compaction density C2 of the second negative electrode material layer within the scope of the present application, the rapid transmission of the electrolyte in the pores of the negative electrode sheet can be accelerated, and therefore the rate performance of the electrochemical device can be further improved.

[0051] The present application does not have a particular limitation on the regulation method of the compaction density of the first negative electrode material layer, as long as the purpose of the present application can be achieved. For example, it can be achieved by regulating the particle size of the first negative electrode active material in the first negative electrode material layer, or it can be achieved by regulating the pressure during the cold pressing process of the negative electrode sheet. The present application does not have a particular limitation on the regulation method of the compaction density of the second negative electrode material layer, as long as the purpose of the present application can be achieved. For example, it can be achieved by regulating the particle size of the second negative electrode active material in the second negative electrode material layer, or it can be achieved by regulating the pressure during the cold pressing process of the negative electrode sheet.

[0052] In some embodiments of the present application, the mass percentage content of the first negative active material is 90% to 98.5% based on the mass of the first negative material layer. Optionally, the first negative material layer further comprises a conductive agent, a thickening agent, and a binder. The present application does not have a particular limitation on the mass percentage content of the conductive agent, the thickening agent, and the binder in the first negative material layer as long as the purpose of the present application can be achieved. For example, the mass percentage content of the conductive agent is 0% to 0.5%, the mass percentage content of the thickening agent is 0.05% to 0.5%, and the mass percentage content of the binder is 1% to 5% based on the mass of the first negative material layer.

[0053] In some embodiments of the present application, the mass percentage content of the second negative active material is 89.8% to 98.5% based on the mass of the second negative material layer. Optionally, the second negative material layer further comprises a conductive agent, a thickening agent, and a binder. The present application does not have a particular limitation on the mass percentage content of the conductive agent, the thickening agent, and the binder in the second negative material layer as long as the purpose of the present application can be achieved. For example, the mass percentage content of the conductive agent is 0.1% to 2%, the mass percentage content of the thickening agent is 0.05% to 0.5%, and the mass percentage content of the binder is 1% to 10% based on the mass of the second negative material layer.

[0054] The present application does not have a particular limitation on the type of the conductive agent, the thickening agent, and the binder, and any conductive agent, thickening agent, and binder known in the art can be used as long as the purpose of the present application can be achieved.

[0055] The present application does not have a particular limitation on the negative current collector as long as the purpose of the present application can be achieved. For example, the negative current collector includes, but is not limited to, a copper foil, a copper alloy foil, a nickel foil, a titanium foil, a nickel foam, or a copper foam, etc. In the present application, the thickness of the negative current collector is not particularly limited as long as the purpose of the present application can be achieved. For example, the thickness of the negative current collector is 4 μm to 20 μm. The present application does not have a particular limitation on the thickness of the first negative material layer and the second negative material layer as long as the purpose of the present application can be achieved. For example, the thickness of the first negative material layer is 20 μm to 50 μm, and the thickness of the second negative material layer is 20 μm to 50 μm.

[0056] The positive electrode tab is not particularly limited in the present application, as long as the object of the present application can be achieved. In some embodiments, the positive electrode tab includes a positive electrode current collector and a positive electrode active material layer, the positive electrode active material layer being disposed on one surface or both surfaces of the positive electrode current collector, the above-mentioned "surface" can be a partial surface of the positive electrode current collector, or can be the entire surface of the positive electrode current collector. The positive electrode current collector is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the positive electrode current collector can include an aluminum foil or an aluminum alloy foil, etc. The positive electrode active material layer of the present application includes a positive electrode active material. The type of positive electrode active material is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the positive electrode active material can include at least one of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based material, lithium cobalt oxide, lithium manganese oxide, lithium manganese iron phosphate, or lithium titanate, etc. In the present application, the positive electrode active material can also include a non-metallic element, which can include at least one of fluorine, phosphorus, boron, chlorine, silicon, or sulfur, which can further improve the stability of the positive electrode active material. In the present application, the thickness of the positive electrode current collector and the positive electrode active material layer is not particularly limited, as long as the object of the present application can be achieved. For example, the thickness of the positive electrode current collector is 5 μm to 20 μm. The thickness of the single-layer positive electrode active material layer is 30 μm to 120 μm. Optionally, the positive electrode active material layer can also include at least one of a conductive agent or a binder. The type of conductive agent and binder in the positive electrode active material layer is not particularly limited in the present application, as long as the object of the present application can be achieved. The mass ratio of the positive electrode active material, the conductive agent, and the binder in the positive electrode active material layer is not particularly limited in the present application, and can be selected by a person skilled in the art according to actual needs, as long as the object of the present application can be achieved.

[0057] The separator is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the material of the separator can include, but is not limited to, at least one of polyethylene (PE), polyolefin (PO) based on polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The type of separator can include at least one of a woven film, a non-woven film, a microporous film, a composite film, a calendered film, or a spunlaid film. The thickness of the separator is not particularly limited in the present application, as long as the object of the present application can be achieved.

[0058] The electrochemical device of the present application also includes an electrolyte. The electrolyte is not particularly limited in the present application as long as the object of the present application can be achieved. For example, in some embodiments, the electrolyte includes a lithium salt and a non-aqueous solvent. The lithium salt can include at least one of LiPF6, LiNO3, LiBF4, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), or lithium difluorophosphate. The content of the lithium salt in the electrolyte is not particularly limited in the present application as long as the object of the present application can be achieved. The non-aqueous solvent is not particularly limited in the present application as long as the object of the present application can be achieved. For example, the non-aqueous solvent can include, but is not limited to, at least one of a carbonate compound, a carboxylic acid ester compound, an ether compound, or other organic solvents. The carbonate compound can include, but is not limited to, at least one of a chain carbonate compound, a cyclic carbonate compound, or a fluorinated carbonate compound. The chain carbonate compound can include, but is not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, or methyl ethyl carbonate. The cyclic carbonate can include, but is not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, or vinyl ethylene carbonate. The fluorinated carbonate compound can include, but is not limited to, at least one of fluorinated ethylene carbonate, 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methyl ethylene carbonate, 1-fluoro-1-methyl ethylene carbonate, 1,2-difluoro-1-methyl ethylene carbonate, 1,1,2-trifluoro-2-methyl ethylene carbonate, or trifluoromethyl ethylene carbonate. The carboxylic acid ester compound can include, but is not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valerolactone, or caprolactone. The ether compound can include, but is not limited to, at least one of dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The other organic solvents can include, but is not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidinone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate.

[0059] The electrochemical device of the present application also includes a housing for containing the positive electrode sheet, the negative electrode sheet, the separator, and the electrolyte, as well as other components in an electrochemical device known in the art, which are not limited by the present application. The housing is not particularly limited by the present application and can be any housing known in the art as long as the purpose of the present application can be achieved.

[0060] The electrochemical device is not particularly limited by the present application and can include any device in which an electrochemical reaction occurs. For example, the electrochemical device can include, but is not limited to, a lithium ion secondary battery (lithium ion battery), a lithium polymer secondary battery, or a lithium ion polymer secondary battery, etc.

[0061] The method for preparing the negative electrode sheet is not particularly limited by the present application as long as the purpose of the present application can be achieved. For example, in some embodiments, the method for preparing the negative electrode sheet includes, but is not limited to, the following steps: (1) mixing the first negative electrode active material, the conductive agent, the thickening agent, and the binder uniformly, then adding the solvent and stirring uniformly to obtain the first negative electrode slurry; (2) mixing the second negative electrode active material, the conductive agent, the thickening agent, and the binder uniformly, then adding the solvent and stirring uniformly to obtain the second negative electrode slurry; (3) coating the first negative electrode slurry on both surfaces of the negative electrode current collector, drying to form the first negative electrode material layer, then coating the second negative electrode slurry on the surface of the first negative electrode material layer away from the negative electrode current collector, drying to form the second negative electrode material layer, and then cold-pressing and slitting to obtain the negative electrode sheet with the first negative electrode material layer and the second negative electrode material layer on both sides. In other embodiments, the method for preparing the negative electrode sheet includes, but is not limited to, the following steps: (1) mixing the first negative electrode active material, the conductive agent, the thickening agent, and the binder uniformly, then adding the solvent and stirring uniformly to obtain the first negative electrode slurry; (2) mixing the second negative electrode active material, the conductive agent, the thickening agent, and the binder uniformly, then adding the solvent and stirring uniformly to obtain the second negative electrode slurry; (3) coating the first negative electrode slurry on one surface of the negative electrode current collector, drying to form the first negative electrode material layer, then coating the second negative electrode slurry on the surface of the first negative electrode material layer away from the negative electrode current collector, and then cold-pressing and slitting to obtain the negative electrode sheet with the first negative electrode material layer and the second negative electrode material layer on one side. The type of the solvent in the above steps (1) and (2) is not particularly limited by the present application as long as the purpose of the present application can be achieved. The solid content of the above first negative electrode slurry and second negative electrode slurry is not particularly limited by the present application as long as the purpose of the present application can be achieved. The temperature and time of drying in the above step (3) are not particularly limited by the present application and can be selected by a person skilled in the art according to the actual situation as long as the purpose of the present application can be achieved.

[0062] The preparation process of the electrochemical device of the present application is well known to those skilled in the art and is not particularly limited in the present application. For example, it may include but is not limited to the following steps: stacking the diaphragm, positive electrode sheet, diaphragm and negative electrode sheet in sequence, and winding, folding and other operations as needed to obtain an electrode assembly with a wound structure, placing the electrode assembly in a packaging bag, injecting the electrolyte into the packaging bag and sealing it, and obtaining the electrochemical device through process flows such as formation, degassing and shaping; or stacking the diaphragm, positive electrode sheet, diaphragm and negative electrode sheet in sequence, and then fixing the four corners of the entire stacked structure with tape to obtain an electrode assembly with a stacked structure, placing the electrode assembly in a packaging bag, injecting the electrolyte into the packaging bag and sealing it, and obtaining the electrochemical device through process flows such as formation, degassing and shaping.

[0063] A second aspect of the present application provides an electronic device, which includes the electrochemical device described in any one of the aforementioned embodiments. Therefore, the electronic device has good performance.

[0064] The electronic device of the present application is not particularly limited and can be any electronic device known in the art. For example, the electronic device can include, but is not limited to, a laptop computer, a pen-type computer, a mobile computer, an electronic book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a headset, a video recorder, an LCD television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flashlight, a camera, a large household battery, and a lithium-ion capacitor.

[0065] Example

[0066] Hereinafter, the embodiments of the present application will be described in more detail with reference to Examples and Comparative Examples. Various tests and evaluations were performed according to the following methods.

[0067] Test methods and equipment:

[0068] Particle size test:

[0069] The particle size distribution was tested using a laser particle size analyzer MasterSizer 2000 to obtain the particle sizes Dv10 and Dv50 of the silicon-carbon material. -3 , Dv90, the particle size of the first graphite Dv50 -1 , the second graphite Dv50 -2 .

[0070] Test of mass percentage of silicon element in silicon-carbon material:

[0071] The silicon element content in the silicon-carbon material was measured using an inductively coupled plasma emission spectrometer (ICP). The silicon-carbon material (0.05-0.2 g) was placed in a nickel crucible, 1.5 g of KOH (potassium hydroxide) was added for alkali fusion (heated at 400℃ for 45 min), and then 50 ml of boiling water was used for leaching for 60 min, and then placed in a 100 ml volumetric flask for constant volume, and part of the diluted solution was taken out for ICP test.

[0072] Test of specific surface area of the silicon-carbon material:

[0073] The specific surface area of the silicon-carbon material of each example and the comparative example was tested by nitrogen adsorption method using a specific surface area analyzer (American Micromeritics Corporation, Tristar II 3020M). The specific test was performed in accordance with the national standard GB / T 19587-2017 "Gas adsorption BET method for determination of specific surface area of solid substances".

[0074] Test of compacted density:

[0075] After the voltage of the lithium ion battery was controlled at 3.95 V (if the voltage of the lithium ion battery was lower than 3.95 V, the lithium ion battery was charged to 3.95 V at 0.2 C; if the voltage of the lithium ion battery was higher than 3.95 V, the lithium ion battery was discharged to 3.95 V at 0.2 C), the negative electrode sheet was disassembled. A small disc with an area of S was punched out on the negative electrode sheet to obtain the mass and thickness of the small disc, which were denoted as m and h, respectively; the second negative electrode material layer on the small disc was removed by etching to obtain the mass and thickness of the small disc after the second negative electrode material layer was removed, which were denoted as m1 and h1, respectively; the first negative electrode material layer on the small disc was removed by etching to obtain the mass and thickness of the negative electrode current collector, which were denoted as m2 and h2, respectively.

[0076] The compacted density C1 of the first negative electrode material layer was (m1-m2) / (h1-h2) / S.

[0077] The compacted density C2 of the second negative electrode material layer was (m-m1) / (h-h1) / S.

[0078] Test of maximum non-lithium precipitation charge rate:

[0079] The lithium ion battery of each example and the comparative example was subjected to charge and discharge at 25℃ according to the following steps: standing for 30 min, discharging to 3.0 V at a constant current of 0.2 C; standing for 30 min; charging to 4.5 V at a charge rate of 0.5 C, and then charging at a constant voltage of 4.5 V until the charging stopped at 0.05 C (at this time, the battery was in a full charge state); this was one charge and discharge cycle. After 50 cycles of the above charge and discharge cycle, the lithium ion battery in the full charge state was disassembled, and whether the negative electrode sheet precipitated lithium was observed: if white or gray lithium appeared on the surface of the negative electrode sheet, it was determined that lithium precipitation occurred; otherwise, it was determined that lithium precipitation did not occur.

[0080] If lithium precipitation does not occur, the lithium ion battery prepared in the same example or comparative example is subjected to the above steps with the charge rate being increased by 0.1 C successively until lithium precipitation occurs on the negative electrode plate.

[0081] The charge rate at which the negative electrode plate does not precipitate lithium for the last time is defined as the highest non-lithium precipitation rate of the lithium ion battery. The higher the highest non-lithium precipitation rate, the more difficult it is for the lithium ion battery to precipitate lithium, i.e., the more fully the problem of lithium precipitation on the surface of the negative electrode plate in the lithium ion battery is alleviated, and the better the cycle performance of the lithium ion battery.

[0082] X-ray diffraction (XRD) test:

[0083] The lithium ion battery is placed in an environment of 25±3°C for 30 min, discharged to 3.0 V at a current of 0.2 C, and then charged to 4.5 V at the highest non-lithium precipitation rate. The lithium ion battery is disassembled to obtain a negative electrode plate, the surface second negative electrode material layer is adhered using adhesive tape, and the second graphite lithium intercalation degree of the second negative electrode material layer is tested. The negative electrode plate after removal of the second negative electrode material layer is used for the first graphite lithium intercalation degree test of the first negative electrode material layer, and the graphite of the second negative electrode material layer and the first negative electrode material layer is characterized by XRD, respectively.

[0084] For the distinction between the second negative electrode material layer and the first negative electrode material layer, the following is specifically performed: the cross section of the electrode plate is characterized by SEM / EDS, the color of the Si element region in the EDS image is different from that of the region without Si element, and there is an obvious boundary between the two regions. A straight line is drawn at this position to obtain the boundary line of the first negative electrode material layer and the second negative electrode material layer. For each example and comparative example 2, the surface of the negative electrode plate is first peeled off using adhesive tape, and the second graphite lithium intercalation degree of the second negative electrode material layer is tested. Then, the negative electrode plate is repeatedly peeled off using adhesive tape until the thickness of the remaining negative electrode plate is first less than the sum of the thickness of the first negative electrode material layer and the thickness of the negative electrode current collector, and the first graphite lithium intercalation degree of the first negative electrode material layer is tested. For comparative example 1, only the first negative electrode material layer is present, and the surface layer and the bottom layer are different in position in the thickness direction of the negative electrode plate. The surface layer of the negative electrode plate is first peeled off using adhesive tape, and the surface layer graphite lithium intercalation degree is tested. Then, the negative electrode plate is repeatedly peeled off using adhesive tape until the thickness of the remaining negative electrode plate is first less than the sum of the thickness of the first negative electrode material layer and the thickness of the negative electrode current collector, and the bottom layer graphite lithium intercalation degree is tested.

[0085] wherein the highest non-lithium precipitation rate is obtained according to the test method in the test of the highest non-lithium precipitation charge rate, and the maximum charge rate at which the negative electrode plate does not precipitate lithium is defined as the highest non-lithium precipitation rate of the lithium ion battery.

[0086] Test of capacity retention:

[0087] The lithium ion battery was rested for 30 min in an environment of 25±3℃, and then charged at the highest non-dissolving rate to 4.5V, and stopped charging when the lithium ion battery was charged at 4.5V constant voltage to 0.05C; the lithium ion battery was rested for 30 min; the lithium ion battery was discharged to 3.0V at a current of 0.5C, and rested for 30 min; the same charging and discharging process was repeated for 300 cycles (cls), and the discharge capacity of the 300th cycle was recorded as C. 300 The discharge capacity of the 2nd cycle was taken as 100% capacity reference value, and the discharge capacity of the 2nd cycle was recorded as C2. Capacity retention (%) = C 300 / C2x 100%.

[0088] Test of energy density:

[0089] First, the lithium ion battery was charged according to the following operation process, and then discharged to obtain the discharge capacity of the lithium ion battery.

[0090] Charging: charged at 0.2C constant current to 4.5V, and then charged at 4.5V constant voltage to 0.05C;

[0091] Discharge: discharged at 0.2C constant current to 3.0V to obtain the discharge energy E1;

[0092] After the charging step of the lithium ion battery was completed, the length L, width W, and height H of the lithium ion battery were tested by a laser thickness gauge to obtain the volume V = LxWxH of the lithium ion battery. The volume energy density (ED) can be calculated by the following formula: ED (Wh / L) = E1 / V.

[0093] Example 1-1

[0094] Preparation of negative electrode sheet

[0095] The first negative active material first graphite (artificial graphite, graphitization degree 95%), the binder polyacrylic acid (weight average molecular weight Mw=3500), and the conductive agent single-walled carbon nanotube were mixed according to a mass ratio of 97:2.8:0.2, deionized water was added as a solvent, and stirring was performed to prepare a first negative electrode slurry with a solid content of 42 wt%, the first negative electrode slurry was coated on both surfaces of a negative electrode current collector copper foil with a thickness of 6 μm, and drying was performed at 100 °C to form a first negative electrode material layer. The second negative active material second graphite (artificial graphite, graphitization degree 95%) and silicon-carbon material, the binder polyacrylic acid (Mw=3500), and the conductive agent single-walled carbon nanotube were mixed according to a mass ratio of 58.2:38.8:2.8:0.2, deionized water was added and stirring was performed to prepare a second negative electrode slurry with a solid content of 42 wt%, the second negative electrode slurry was coated on the surfaces of the two first negative electrode material layers away from the copper foil, and drying was performed at 100 °C to form a second negative electrode material layer. Cold pressing, slitting, and welding of the negative electrode tabs nickel tabs were performed to obtain a negative electrode sheet with a specification of 1000 mm x 80 mm.

[0096] In the second negative active material, the mass percentage content W1 of the silicon-carbon material is 40%, and the mass percentage content W2 of the silicon element in the silicon-carbon material is 50%. The particle size Dv10 of the silicon-carbon material is 5.5 μm, the particle size Dv50 is 9.1 μm, and the particle size Dv90 is 15 μm. The specific surface area of the silicon-carbon material is 1.4 m2 / g. The particle size Dv50 of the first graphite is 12 μm, and the particle size Dv50 of the second graphite is 10 μm. The coating weight per unit area of the first negative electrode material layer CW1 is 90 mg / 1540.25 mm2, the coating weight per unit area of the second negative electrode material layer CW2 is 30 mg / 1540.25 mm2, and the coating weight per unit area of the negative electrode sheet CW is CW1+CW2=120 mg / 1540.25 mm2. The proportion P of the coating weight per unit area of the second negative electrode material layer to the coating weight per unit area of the negative electrode sheet is 1 / 4. The compacted density of the first negative electrode material layer C1 is 1.6 g / cm3, and the compacted density of the second negative electrode material layer C2 is 1.4 g / cm3. -4 2 -1 - 2 2 12 2 3 3

[0097] <Preparation of a positive electrode sheet>

[0098] ​​​​​​​​​​The positive electrode active material lithium cobaltate, the conductive agent conductive carbon black (Super P), and the binder polyvinylidene fluoride (PVDF, Mw = 600000) were mixed in a mass ratio of 97.8:1.4:0.8, N-methyl pyrrolidone (NMP) was added as a solvent, and the positive electrode slurry was stirred in a vacuum stirrer until the solid content was 72wt% and the system was uniform. The positive electrode slurry was uniformly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 10 μm, dried at 85°C, and a single-sided coated positive electrode active material layer (thickness 90 μm) was obtained. The positive electrode tab was obtained by repeating the above steps on the other surface of the aluminum foil, cold pressing, cutting, and welding the positive electrode tab aluminum tab. The specification of the positive electrode tab was 990 mm x 76 mm.

[0099] <Preparation of the separator>

[0100] A polyethylene (PE) porous film with a thickness of 8 μm was used as the separator.

[0101] <Preparation of the electrolyte>

[0102] In a dry argon atmosphere, the non-aqueous solvent ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a mass ratio of 1:1, and the lithium salt lithium hexafluorophosphate was added and stirred to obtain the electrolyte. The lithium salt concentration was 1 mol / L.

[0103] <Preparation of the lithium ion battery>

[0104] The separator, the positive electrode tab, the separator, and the negative electrode tab were sequentially stacked and wound to obtain an electrode assembly. The electrode assembly was placed in a packaging bag of aluminum plastic film, and after removing the water at 80°C, the electrolyte was injected and packaged. After the processes of formation, degassing, and shaping, the lithium ion battery was obtained.

[0105] Examples 1-2 to 1-13

[0106] Except for adjusting the relevant preparation parameters according to Table 1, the rest was the same as Example 1-1.

[0107] Examples 2-1 to 2-6

[0108] Except for adjusting the relevant preparation parameters according to Table 2, the rest was the same as Example 1-3.

[0109] Examples 3-1 to 3-8

[0110] Except for adjusting the relevant preparation parameters according to Table 3, the rest was the same as Example 2-2.

[0111] Examples 4-1 to 4-11

[0112] The rest is the same as Example 3-1 except that the relevant preparation parameters are adjusted according to Table 4.

[0113] Comparative Example 1

[0114] Preparation of the negative electrode tab

[0115] The artificial graphite (graphitization degree 95%), silicon-carbon material (Dv50 = 8.6 μm), binder polyacrylic acid (weight average molecular weight Mw = 3500), and conductive agent single-walled carbon nanotube were mixed in a mass ratio of 87.3:9.7:2.8:0.2, deionized water was added as a solvent, and stirring was performed to prepare a negative electrode slurry with a solid content of 42 wt%. The negative electrode slurry was coated on both surfaces of a negative electrode current collector copper foil, and the first negative electrode material layer was formed by drying at 100°C. The negative electrode tab was obtained by cold pressing, slitting, and welding the nickel tab, and the specification was 1000 mm x 80 mm. The coating weight of the first negative electrode material layer was 150 mg / 1540.25 mm 2 .

[0116] Preparation of the positive electrode tab, Preparation of the separator, Preparation of the electrolyte, and Preparation of the lithium ion battery were the same as in Example 1-1.

[0117] Comparative Example 2

[0118] The rest was the same as in Example 1-1 except that the first negative electrode material layer and the second negative electrode material layer in Preparation of the negative electrode tab were exchanged in position.

[0119] The preparation parameters and performance data of each example and comparative example are shown in Tables 1 to 4.

[0120] Table 1 Note: In Table 1, “P” represents the proportion of the coating weight per unit area of the second negative electrode material layer to the coating weight per unit area of the negative electrode tab; “\” represents that the relevant parameter does not exist.

[0121] As can be seen from Example 1-1 to Example 1-11, Comparative Example 1, and Comparative Example 2, in the electrochemical device of the present application, the first negative electrode material layer located at the bottom layer and the second negative electrode material layer located at the surface layer are simultaneously provided in the negative electrode tab, and when the electrochemical device is charged at the highest lithium non-extraction rate to 4.5 V, the characteristic peak of LiC6 does not appear in the X-ray diffraction pattern of the second negative electrode material layer, the 2θ at the maximum intensity of the characteristic peak of LiC12 is A2°, and 25.15 ≤ A2 ≤ 25.30; and / or, the 2θ at the maximum intensity of the characteristic peak of LiC6 in the X-ray diffraction pattern of the first negative electrode material layer is A1°, and the 2θ at the maximum intensity of the characteristic peak of LiC12 in the X-ray diffraction pattern of the second negative electrode material layer is A2°, and 25.15 ≤ A2 ≤ 25.30. 12 12 12 ​​A2°, 0≤A1-A2≤0.15. The larger maximum lithium precipitation free rate of the electrochemical device of the embodiment indicates that the surface of the negative electrode plate in the electrochemical device is less prone to lithium precipitation, and the problem of lithium precipitation on the surface of the negative electrode plate in the electrochemical device is alleviated, and the electrochemical device has better cycle performance. The electrochemical device of the comparative example has a smaller maximum lithium precipitation free rate, indicating that the surface of the negative electrode plate in the electrochemical device of the comparative example is more prone to lithium precipitation.

[0122] Figure 2 shows the X-ray diffraction pattern of the negative electrode plate of Example 1-1, and Figure 3 shows the X-ray diffraction pattern of the negative electrode plate of Comparative Example 1. As can be seen from Figure 2, in the X-ray diffraction pattern of the second negative electrode material layer of the negative electrode plate of Example 1-1, no characteristic peak of LiC6 appears, and the characteristic peak of LiC 12 The 2θ at the maximum intensity of the characteristic peak of the first negative electrode material layer is 25.24°, and the characteristic peak of LiC 12 The 2θ at the maximum intensity of the characteristic peak of the first negative electrode material layer is 25.24°, and the characteristic peak of LiC 12 The 2θ at the maximum intensity of the characteristic peak of the first negative electrode material layer is 25.24°, and the characteristic peak of LiC 12 The 2θ at the maximum intensity of the characteristic peak of the first negative electrode material layer is 25.24°, and the characteristic peak of LiC 12 The 2θ at the maximum intensity of the characteristic peak of the first negative electrode material layer is 25.24°, and the characteristic peak of LiC

[0123] The mass percentage of the silicon-carbon material in the second negative electrode active material and the mass percentage of silicon in the silicon-carbon material will also generally affect the lithium precipitation on the surface of the negative electrode plate in the electrochemical device. As can be seen from Example 1-1 to Example 1-13, the electrochemical device selected with the mass percentage of the silicon-carbon material in the second negative electrode active material and the mass percentage of silicon in the silicon-carbon material within the range of the present application has a larger maximum lithium precipitation free rate, indicating that the problem of lithium precipitation on the surface of the negative electrode plate in the electrochemical device is alleviated, and the electrochemical device has good cycle performance.

[0124] Table 2

[0125] The particle size Dv50 of the first graphite -1 and the particle size Dv50 of the second graphite -2 The difference Dv50 -1 The difference Dv50 -2 will also generally affect the lithium precipitation on the surface of the negative electrode plate in the electrochemical device and the capacity retention rate. As can be seen from Example 1-3, Example 2-1 to Example 2-6, the electrochemical device selected with the particle size Dv50 of the first graphite -1and Dv50 of the particle size of the second graphite -2 Difference Dv50 - 1-Dv50 -2 The electrochemical device within the scope of this application has a larger maximum non-lithium deposition rate on the basis of a higher capacity retention rate, indicating that the electrochemical device has alleviated the problem of lithium deposition on the surface of the negative electrode plate on the basis of a longer cycle life.

[0126] Table 3

[0127] Particle size of silicon carbon material Dv10, Dv50 -3 , Dv90 and Dv90-Dv10 values ​​usually affect the lithium deposition and capacity retention of the negative electrode surface in the electrochemical device. -3 The electrochemical device whose values ​​of Dv90, Dv90 and Dv90-Dv10 are within the scope of the present application has a larger maximum non-lithium deposition rate on the basis of a higher capacity retention rate, indicating that the electrochemical device has alleviated the problem of lithium deposition on the surface of the negative electrode plate on the basis of a longer cycle life.

[0128] The specific surface area of ​​silicon-carbon materials typically affects lithium deposition on the surface of the negative electrode sheet and capacity retention in electrochemical devices. As can be seen from Examples 2-2, 3-5, and 3-8, electrochemical devices using silicon-carbon materials with specific surface areas within the range of this application exhibit a high maximum non-lithium deposition rate, while maintaining a high capacity retention rate. This indicates that the problem of lithium deposition on the surface of the negative electrode sheet is alleviated while maintaining a long cycle life.

[0129] Table 4

[0130] The compacted density C1 of the first negative electrode material layer, the compacted density C2 of the second negative electrode material layer, and the difference C1-C2 between the two usually affect the lithium deposition on the surface of the negative electrode plate in the electrochemical device and the energy density of the electrochemical device. From Examples 3-1, 4-1 to 4-11, it can be seen that the energy density of the electrochemical device in which the compacted density C1 of the first negative electrode material layer, the compacted density C2 of the second negative electrode material layer, and the difference C1-C2 between the two are within the scope of this application is not changed much, and the maximum non-lithium deposition rate is large, indicating that the problem of lithium deposition on the surface of the negative electrode plate in the electrochemical device is alleviated, and the electrochemical device has a higher energy density.

[0131] It is to be noted that, in the present document, relational terms such as first and second and the like can be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Moreover, the terms "comprises", "comprising", or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but can include other elements not expressly listed or inherent to such process, method, article, or apparatus.

[0132] Each of the embodiments in the present specification is described in a related manner, and the same or similar parts among the embodiments can be referred to each other. Each of the embodiments focuses on the difference from other embodiments.

[0133] The above only describes the preferred embodiments of the present application, and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. An electrochemical device, comprising a negative electrode sheet, a positive electrode sheet, and a separator disposed between the negative electrode sheet and the positive electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a first negative electrode material layer and a second negative electrode material layer disposed on at least one surface of the negative electrode current collector, the first negative electrode material layer being disposed between the negative electrode current collector and the second negative electrode material layer; When the electrochemical device is charged at a constant current at a highest lithium-non- dissolution rate to an upper limit cutoff voltage, characteristic peaks of LiC 12 in X-ray diffraction patterns of the first negative electrode material layer and the second negative electrode material layer satisfy at least one of the following characteristics (1) or (2): (1) The characteristic peaks of LiC (1) the X-ray diffraction pattern of the second negative electrode material layer does not have a characteristic peak of LiC6, and the maximum intensity at the characteristic peak of LiC 12 A2°, 25.15≤A2≤25.30; (2) the maximum intensity at 2θ of a characteristic peak of LiC in the X-ray diffraction pattern of the first negative electrode material layer is A1°, the maximum intensity at 2θ of a characteristic peak of LiC in the X-ray diffraction pattern of the second negative electrode material layer is A2°, and 0≤A1-A2≤0.

15. 12 (2) the maximum intensity at 2θ of a characteristic peak of LiC in the X-ray diffraction pattern of the first negative electrode material layer is A1°, the maximum intensity at 2θ of a characteristic peak of LiC in the X-ray diffraction pattern of the second negative electrode material layer is A2°, and 0≤A1-A2≤0.

15. 12 ​ 2. The electrochemical device of claim 1, wherein, the first negative electrode material layer comprising a first negative electrode active material, the first negative electrode active material comprising a first graphite; the second negative electrode material layer comprising a second negative electrode active material, the second negative electrode active material comprising a silicon-carbon material and a second graphite; a mass percentage content of the silicon-carbon material in the second negative electrode active material being 15% to 50%, and a mass percentage content of silicon in the silicon-carbon material being 30% to 60%.

3. The electrochemical device of claim 2, wherein, a mass percentage content of the silicon-carbon material in the second negative electrode active material being 30% to 50%, and a mass percentage content of silicon in the silicon-carbon material being 50% to 60%.

4. The electrochemical device of claim 2, wherein, The particle size Dv50 of the first graphite is 11.0 pm to 13.5 pm. -1 is 11.0 pm to 13.5 pm.

5. The electrochemical device of claim 4, wherein, The particle diameter Dv50 of the second graphite -2 The particle diameter Dv50 of the first graphite -1 1 pm ≤ Dv50 -1 - Dv50 -2 ≤ 5 pm.

6. The electrochemical device according to claim 2 or 3, wherein The particle size Dv10, Dv50 and Dv90 of the silicon-carbon material satisfy: 5 μm ≤ Dv10 ≤ 6 μm, 8 μm ≤ Dv50 ≤ 10.5 μm, 13 μm ≤ Dv90 ≤ 17 μm. -3 The particle size Dv10, Dv50 and Dv90 of the silicon-carbon material satisfy: 5 μm ≤ Dv10 ≤ 6 μm, 8 μm ≤ Dv50 ≤ 10.5 μm, 13 μm ≤ Dv90 ≤ 17 μm. -3 The particle size Dv10, D 7. The electrochemical device of claim 6, wherein, a particle size Dv10 and Dv90 of the silicon-carbon material satisfying 8 μm ≤ Dv90-Dv10 ≤ 10.5 μm.

8. The electrochemical device according to claim 2 or 3, wherein The silicon-carbon material has a specific surface area of 0.5 m 2 / g to 3.5 m 2 / g.

9. The electrochemical device of claim 8, wherein, The silicon-carbon material has a specific surface area of 1.0 m 2 / g to 2.0 m 2 / g.

10. The electrochemical device of claim 6, wherein, The compacted density C1 of the first negative electrode material layer and the compacted density C2 of the second negative electrode material layer satisfy: 1.5 g / cm 3 ≤ C1 ≤ 1.65 g / cm 3 , 1.35 g / cm 3 ≤ C2 ≤ 1.5 g / cm 3 , 0.05 g / cm 3 ≤ C1 - C2 ≤ 0.3 g / cm 3 .

11. The electrochemical device of claim 10, wherein, 0.1 g / cm 3 ≤ C1-C2 ≤ 0.2 g / cm 3 .

12. An electronic device, wherein, the electronic device comprising the electrochemical device of any one of claims 1 to 11.

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