Electrochemical devices and electronic devices

Abstract

The application provides an electrochemical device and an electronic device, the electrochemical device comprising a negative electrode tab, the negative electrode tab comprising a negative electrode current collector, a first negative electrode material layer and a second negative electrode material layer, 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 lithium-unprecipitating rate to an upper limit cutoff voltage, the characteristic peaks of LiC 12 in the 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 LiC6 do not appear in the X-ray diffraction pattern of the second negative electrode material layer, the 2θ at the maximum intensity of the characteristic peaks of LiC 12 is A2°, and 25.15≤A2≤25.30; (2) the 2θ at the maximum intensity of the characteristic peaks of LiC 12 in the X-ray diffraction pattern of the first negative electrode material layer is A1°, the 2θ at the maximum intensity of the characteristic peaks of LiC 12 in the X-ray diffraction pattern of the second negative electrode material layer is A2°, and 0≤A1-A2≤0.15.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrochemistry, and in particular to an electrochemical device and an electronic device. BACKGROUND

[0002] 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. 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. 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

[0003] 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.

[0004] It should be noted that the present application is explained by taking lithium ion batteries as an example of electrochemical devices in the summary of the application, but the electrochemical device of the present application is not limited to lithium ion batteries. The specific technical solutions are as follows:

[0005] The first aspect of the present application provides an electrochemical device, which comprises 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 comprises 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 is arranged between the negative electrode current collector and the second negative electrode material layer; when the electrochemical device is charged at the highest non-lithium deposition rate to the 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, 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 X-ray diffraction pattern of the second negative electrode material layer has no characteristic peak of LiC 12A2°, 0≤A1-A2≤0.15. Through the above setting, the lithium intercalation degree of the surface layer of the negative pole piece is low, and / or the lithium intercalation uniformity of the negative pole piece is good, and the phenomenon of local increase of lithium intercalation on the surface of the negative pole piece does not occur, and the problem of lithium precipitation on the surface of the negative pole piece in the electrochemical device is alleviated.

[0006] In some embodiments of the present application, the first negative material layer includes a first negative active material, the first negative active material includes a first graphite; the second negative material layer includes a second negative active material, the second negative active material includes a silicon-carbon material and a second graphite; the mass percentage of the silicon-carbon material in the second negative active material is 15% to 50%, and the mass percentage of silicon in the silicon-carbon material is 30% to 60%. The first negative material layer at the bottom of the negative pole piece includes the first graphite, and the second negative material layer at the surface layer includes the second graphite and the silicon-carbon material, and the mass percentage of the silicon-carbon material and the mass percentage of silicon in the silicon-carbon material are controlled within the above ranges, so that the negative pole piece can provide sufficient active material, and it is also beneficial to make the silicon element in the surface layer of the negative pole piece reduce the probability of volume expansion of the negative pole piece on the basis of improving the lithium intercalation site in the surface layer of the negative pole piece. Therefore, the problem of lithium precipitation on the surface of the negative pole piece in the electrochemical device can be alleviated, and the electrochemical device has a high energy density and a small volume expansion rate.

[0007] In some embodiments of the present application, the mass percentage of the silicon-carbon material in the second negative active material is 30% to 50%, and the mass percentage of silicon in the silicon-carbon material is 50% to 60%, which can further improve the problem of lithium precipitation on the surface of the negative pole piece.

[0008] In some embodiments of the present application, the particle size Dv50 -1 of the first graphite is 11.0 μm to 13.5 μm. The particle size Dv50 -1 of the first graphite is controlled within the above range, and the electrochemical device has a high capacity retention rate on the basis of having a low probability of lithium precipitation on the surface of the negative pole piece.

[0009] In some embodiments of the present application, the particle size Dv50 -2 of the second graphite is different from the particle size Dv50 -1 of the first graphite, and satisfies: 1 μm≤Dv50 -1 -Dv50 -2 ≤5 μm. The particle size Dv50 -2 of the second graphite is different from the particle size Dv50 -1 of the first graphite, and the difference Dv50 -1 -Dv50 -2By controlling the value within the above range, the electrochemical device can maintain a high capacity retention rate while further reducing the probability of surface degradation on the negative electrode.

[0010] In some embodiments of this application, the particle size of the silicon-carbon material is Dv10, Dv50. -3 Dv90 satisfies: 5μm≤Dv10≤6μm, 8μm≤Dv50 -3 ≤10.5μm, 13μm≤Dv90≤17μm. The particle size of silicon-carbon materials is Dv10, Dv50. -4 With Dv90 controlled within the above range, the problem of lithium deposition on the surface of the negative electrode in the electrochemical device is alleviated, while maintaining a high capacity retention rate.

[0011] In some embodiments of this application, the particle sizes Dv10 and Dv90 of the silicon-carbon material satisfy the following: 8 μm ≤ Dv90 - Dv10 ≤ 10.5 μm. By controlling the values ​​of Dv90-Dv10 within the above range, the electrochemical device can improve its kinetic capabilities while maintaining a high energy density.

[0012] In some embodiments of this application, the specific surface area of ​​the silicon-carbon material is 0.5 m². 2 / g to 3.5m 2 / g. By controlling the specific surface area of ​​silicon-carbon materials within the above range, the electrochemical device can achieve a high capacity retention rate while alleviating the lithium plating problem on the negative electrode surface.

[0013] In some embodiments of this application, the specific surface area of ​​the silicon-carbon material is 1.0 m². 2 / g to 2.0m 2 / g. This helps to further alleviate the problem of lithium deposition on the surface of the negative electrode while maintaining a high capacity retention rate in electrochemical devices.

[0014] In some embodiments of this application, the compaction density C1 of the first negative electrode material layer and the compaction density C2 of the second negative electrode material layer satisfy: 1.5 g / cm³ 3 ≤C1≤1.65g / cm 3 1.35g / cm 3 ≤C2≤1.5g / cm 3 0.05g / cm 3 ≤C1-C2≤0.3g / cm 3 By controlling 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 of C1-C2 within the range of this application, the electrochemical device exhibits a high energy density while mitigating the lithium plating problem on the surface of the negative electrode sheet.

[0015] In some embodiments of this application, 0.1 g / cm 3 ≤C1-C2≤0.2g / cm 3 .

[0016] A second aspect of this application provides an electronic device comprising the electrochemical device described in any of the foregoing embodiments. Therefore, the electronic device exhibits good performance in use.

[0017] The beneficial effects of this application are:

[0018] This application provides an electrochemical device and an electronic device. The electrochemical device includes a negative electrode, a positive electrode, and a separator disposed between the negative electrode and the positive electrode. The negative electrode includes a negative 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 current collector. The first negative electrode material layer is disposed between the negative current collector and the second negative electrode layer. When the electrochemical device is charged to the upper limit cutoff voltage at the highest non-lithium deposition rate constant current, the X-ray diffraction patterns of the first negative electrode material layer and the second negative electrode material layer show that LiC... 12 The characteristic peaks satisfy at least one of the following characteristics (1) or (2): (1) No characteristic peaks of LiC6 appear in the X-ray diffraction pattern of the second negative electrode material layer. 12 The 2θ at the maximum intensity of 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 at the maximum intensity of the characteristic peak is A1°. In the X-ray diffraction pattern of the second negative electrode material layer, LiC 12 The 2θ at the maximum intensity of the characteristic peak is A2°, and 0≤A1-A2≤0.15. With the above settings, the lithium intercalation degree on the surface of the negative electrode is low, and / or the lithium intercalation uniformity of the negative electrode is good, with no localized increase in lithium intercalation on the surface of the negative electrode. Therefore, the problem of lithium deposition on the surface of the negative electrode in electrochemical devices can be alleviated, and the cycle performance of the electrochemical device can be improved.

[0019] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of this application 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 only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these drawings.

[0021] Figure 1This is a schematic diagram of the cross-sectional structure of the negative electrode sheet along its length and thickness directions in some embodiments of this application;

[0022] Figure 2 The X-ray diffraction pattern of the negative electrode sheet of Embodiment 1-1 of this application;

[0023] Figure 3 The X-ray diffraction pattern is that of the negative electrode sheet of Comparative Example 1 of this application.

[0024] Figure label:

[0025] 10-Negative electrode sheet; 11-First negative electrode material layer; 12-Second negative electrode material layer; 13-Negative electrode current collector; 131-First surface; 132-Second surface. Detailed Implementation

[0026] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0027] It should be noted that, in the specific embodiments of this application, lithium-ion batteries are used as an example of electrochemical devices to explain this application, but the electrochemical devices of this application are not limited to lithium-ion batteries.

[0028] The first aspect of this application provides an electrochemical device comprising a negative electrode, a positive electrode, and a separator disposed between the negative electrode and the positive electrode. The negative electrode includes a negative 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 current collector, with the first negative electrode material layer disposed between the negative current collector and the second negative electrode material layer. When the electrochemical device is charged to the upper limit cutoff voltage at a constant current with the highest non-lithium deposition rate, the X-ray diffraction patterns of the first and second negative electrode material layers show that LiC... 12 The characteristic peaks satisfy at least one of the following characteristics (1) or (2): (1) No characteristic peaks of LiC6 appear in the X-ray diffraction pattern of the second negative electrode material layer. 12 The 2θ at the maximum intensity of 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 at the maximum intensity of the characteristic peak is A1°. In the X-ray diffraction pattern of the second negative electrode material layer, LiC 12The 2θ at the maximum intensity of the characteristic peak is A2°, and 0 ≤ A1 - A2 ≤ 0.15. In some embodiments, when the electrochemical device is charged to the upper limit cutoff voltage at the highest non-lithiation rate using a constant current, no characteristic peak of LiC6 appears in the X-ray diffraction pattern of the second negative electrode material layer. 12 The 2θ at the maximum intensity of the characteristic peak is A2°, 25.15≤A2≤25.30. In other embodiments, when the electrochemical device is charged at a constant current to the upper limit cutoff voltage at the highest non-lithiation rate, the X-ray diffraction pattern of the first negative electrode material layer shows that LiC 12 The 2θ at the maximum intensity of the characteristic peak is A1°. In the X-ray diffraction pattern of the second negative electrode material layer, LiC 12 The 2θ at the maximum intensity of the characteristic peak is A2°, 0≤A1-A2≤0.15. In some further embodiments, when the electrochemical device is charged at a constant current to the upper limit cutoff voltage at the highest non-lithiation rate, the X-ray diffraction pattern of the first negative electrode material layer shows that LiC 12 The 2θ at the maximum intensity of the characteristic peak is A1°. In the X-ray diffraction pattern of the second negative electrode material layer, LiC 12 The 2θ at the maximum intensity of the characteristic peak is A2°, 0≤A1-A2≤0.15, 25.15≤A2≤25.30.

[0029] For ease of understanding, in this application, the length direction of the negative electrode sheet is defined as X, and the thickness direction is defined as Z. It should be understood that the above definitions of direction are for the purpose of describing this application, and the directions defined in this application can be understood based on the relative positions of the elements in the accompanying drawings and the actual product. It is understood that the length and thickness directions 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 sheet. Figure 1 The following are schematic cross-sectional views of the negative electrode sheet along its length direction X and thickness direction Z in some embodiments of this application, such as... Figure 1 As shown, the negative electrode sheet 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 disposed along its thickness direction Z. The first negative electrode material layer 11 and the second negative electrode material layer 12 are disposed on both surfaces of the negative electrode current collector 13, i.e., on the first surface 131 and the second surface 132, and the first negative electrode material layer 11 is disposed between the negative electrode current collector 13 and the second negative electrode material layer 12. It is understood that in some other embodiments of this application, the first negative electrode material layer 11 and the second negative electrode material layer 12 may be disposed on one surface of the negative electrode current collector 13, i.e., on the first surface 131 or the second surface 132.

[0030] In this application, "maximum non-lithiation rate" refers to the maximum charging rate of the electrochemical device under non-lithiation conditions on the negative electrode, specifically the maximum non-lithiation rate measured according to the test method in "Test of Maximum Non-lithiation Charging Rate". In this application, the upper limit cutoff voltage of the electrochemical device is 4.5V to 4.55V. When the electrochemical device is charged at a constant current to the upper limit cutoff voltage at the maximum non-lithiation rate, no characteristic peaks of LiC6 appear 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 degree of the surface first negative electrode material layer and the bottom second negative electrode material layer is relatively uniform; LiC 12 The maximum intensity of 2θ in the characteristic peak is A2°, 25.15≤A2≤25.30. For example, A2 can be 25.15, 25.18, 25.20, 25.22, 25.24, 25.25, 25.27, 25.28, 25.29, 25.30, or any value between any two of the above ranges. A2 falling within the above range indicates that when the electrochemical device is charged at a constant current to the upper cutoff voltage at the highest non-lithiation rate, the lithium intercalation degree in the second negative electrode material layer on the surface of the negative electrode is low, and there is less lithium deposition on the surface of the negative electrode. When the electrochemical device is charged at a constant current to the upper cutoff voltage at the highest non-lithiation rate, the X-ray diffraction pattern of the first negative electrode material layer shows that LiC... 12 The 2θ at the maximum intensity of the characteristic peak is A1°. In the X-ray diffraction pattern of the second negative electrode material layer, LiC 12 The 2θ at the maximum intensity of the characteristic peak is A2°, where 0 ≤ A1-A2 ≤ 0.15. For example, A1-A2 can be 0.00, 0.01, 0.02, 0.03, 0.04, 0.07, 0.09, 0.10, 0.12, 0.15, or any value between any two of the above ranges. A1-A2 values ​​within the above range indicate that when the electrochemical device is charged at the highest non-lithiation rate, the degree of lithium intercalation in the first and second negative electrode material layers of the negative electrode is relatively consistent in the incompletely charged state. This indicates good uniformity of lithium intercalation in the negative electrode, without any localized increase in lithium intercalation, thus reducing the probability of lithium deposition on the surface of the negative electrode. Through the above settings, the degree of lithium intercalation on the surface of the negative electrode is low, and / or the uniformity of lithium intercalation is good, with no localized increase in lithium intercalation on the surface of the negative electrode, thus alleviating the problem of lithium deposition on the surface of the negative electrode in the electrochemical device.

[0031] This application examines the X-ray diffraction pattern of the first negative electrode material layer, LiC. 12 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 this application can be achieved. For example, 25.27°≤A1≤25.5°.

[0032] This application examines the X-ray diffraction pattern of LiC in the first anode material layer.12 There are no particular restrictions on the method of adjusting the range A1° of 2θ at the maximum intensity of the characteristic peak, as long as the purpose of this application can be achieved. For example, it can be achieved by adjusting the thickness of the negative electrode sheet.

[0033] This application examines the X-ray diffraction pattern of LiC in the second anode material layer. 12 There are no particular restrictions on the method of controlling the range A2° of 2θ at the maximum intensity of the characteristic peak, as long as the purpose of this application can be achieved. For example, it can be achieved by controlling at least one of the mass percentage of silicon-carbon material in the second negative electrode active material or the mass percentage of silicon element in the silicon-carbon material.

[0034] In some embodiments of this 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 silicon-carbon material in the second negative electrode active material is 15% to 50%, and the mass percentage of silicon element in the silicon-carbon material is 30% to 60%. For example, the mass percentage of 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 ranges. For example, the mass percentage of silicon element in the silicon-carbon material is 30%, 34%, 39%, 42%, 50%, 53%, 60%, or any value between any two of the above ranges. Adding silicon-carbon material to the second negative electrode material layer on the surface of the negative electrode sheet can increase the specific capacity of the negative electrode sheet surface and increase the number of lithium intercalation sites, thereby reducing the degree of lithium intercalation in the second negative electrode material layer. This helps alleviate the problem of lithium plating on the surface of the negative electrode sheet in electrochemical devices. The bottom first negative electrode material layer of the negative electrode sheet includes a first graphite, and the top second negative electrode material layer includes a second graphite and silicon-carbon material. By controlling the mass percentage of silicon-carbon material and the mass percentage of silicon element in the silicon-carbon material within the aforementioned range, the negative electrode sheet can provide sufficient active material. This also helps to reduce the probability of volume expansion of the negative electrode sheet caused by silicon element, while increasing the number of lithium intercalation sites. Therefore, it helps to alleviate the problem of lithium plating on the surface of the negative electrode sheet in electrochemical devices and also enables electrochemical devices to have higher energy density and lower volume expansion rate.

[0035] This application does not impose any particular restrictions on the method of controlling the mass percentage of silicon in silicon-carbon materials, as long as the purpose of this application can be achieved. For example, when preparing silicon-carbon materials by depositing silicon in porous carbon, the mass percentage of silicon in the silicon-carbon material can be changed by controlling the amount of silicon deposited during the preparation process. Alternatively, silicon-carbon materials with different silicon contents can be purchased directly, and the desired silicon content can be selected by referring to the "testing of the mass percentage of silicon in silicon-carbon materials" provided in this application.

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

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

[0038] In some embodiments of this application, the particle size of the first graphite is Dv50. -1 The particle size ranges from 11.0 μm to 13.5 μm. For example, the first graphite has a particle size Dv50. -1 The particle size 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 ranges. The particle size Dv50 of the first graphite is... -1 Within the aforementioned range, the likelihood of agglomeration of the first graphite is low, allowing it to be uniformly distributed within the first negative electrode material layer. Furthermore, the surface area of ​​the first graphite ensures sufficient contact with the electrolyte, enabling it to fully exert its activity and resulting in a high capacity for the first negative electrode material layer. Consequently, the electrochemical device exhibits high energy density while having a low probability of lithium plating on the negative electrode surface. This also contributes to a high capacity retention rate in the electrochemical device.

[0039] In some embodiments of this application, the particle size of the second graphite is Dv50. -2 With the first graphite particle size Dv50 -1 Satisfies: 1μm≤Dv50 -1 -Dv50 -2 ≤5μm. For example, Dv50 -1 -Dv50 -2 The value is 1μm, 2μm, 3μm, 4μm, 5μm, or any value between any two of the above ranges. The particle size Dv50 of the second graphite... -2 With the first graphite particle size Dv50 -1 The difference Dv50 -1 -Dv50 -2With the value controlled within the above range, the first graphite and the second graphite have a suitable difference in kinetic capabilities, which matches the difference in lithium insertion rate required by the first negative electrode material layer and the second negative electrode material layer. This is beneficial for the electrochemical device to have high energy density while further reducing the probability of surface cleavage of the negative electrode sheet, and also enables the electrochemical device to have a high capacity retention rate.

[0040] This application addresses the particle size Dv50 of the second graphite. -2 There are no particular restrictions, as long as the purpose of this application can be achieved. For example, the particle size of the second graphite is Dv50. -2 The range is from 8.5 μm to 13.5 μm.

[0041] In some embodiments of this application, the particle size of the silicon-carbon material is Dv10, Dv50. -3 Dv90 satisfies: 5μm≤Dv10≤6μm, 8μm≤Dv50 -3 ≤10.5μm, 13μm≤Dv90≤17μm. For example, the particle size Dv10 of silicon carbide materials is 5μm, 5.2μm, 5.4μm, 5.7μm, 5.8μm, 6μm, or any value between any two of the above ranges. For example, the particle size Dv50 of silicon carbide materials... -3 The particle size Dv90 of silicon carbide material is 8μm, 8.2μm, 8.7μm, 9.1μm, 9.8μm, 10.0μm, 10.5μm, or any value between any two of the above ranges. For example, the particle size Dv90 of silicon carbide material is 13μm, 13.6μm, 14.4μm, 14.7μm, 15.3μm, 16μm, 16.5μm, 17μm, or any value between any two of the above ranges. The particle size Dv10 and Dv50 of silicon carbide material are also considered. -4 When Dv90 is controlled within the aforementioned range, silicon-carbon materials with suitable particle sizes and minimal inter-particle size differences are less likely to agglomerate when distributed in the second negative electrode material layer. This facilitates uniform distribution within the second negative electrode material layer, thereby improving the wettability of the electrolyte on the negative electrode sheet and enhancing its wettability and liquid retention capacity. Consequently, the utilization rate of silicon-carbon materials is improved, the problem of lithium plating on the surface of the negative electrode sheet in electrochemical devices is alleviated, its energy density is increased, and it also exhibits a high capacity retention rate.

[0042] In some embodiments of this application, the particle sizes Dv10 and Dv90 of the silicon-carbon material satisfy the following condition: 8 μm ≤ Dv90 - Dv10 ≤ 10.5 μm. For example, the values ​​of Dv90 - Dv10 are 8.0 μm, 9.7 μm, 10.2 μm, 10.5 μm, or any value between any two of the above ranges. By controlling the values ​​of Dv90 - Dv10 within the above range, the size difference between large and small particles in the silicon-carbon material is smaller. When the silicon-carbon material is distributed in the second anode material layer, silicon-carbon materials of different particle sizes are distributed in a suitable gradient, which is beneficial for the close and uniform distribution of silicon-carbon material in the second anode material layer and improves the rapid lithium intercalation capability of silicon-carbon material. This allows the electrochemical device to improve its kinetics while maintaining a high energy density, resulting in a higher minimum charge rate and capacity retention rate.

[0043] In this application, Dv10 refers to the particle size that, in a volumetric particle size distribution, reaches 10% of the cumulative volume from the smallest particle size side; Dv50 refers to the particle size that, in a volumetric particle size distribution, reaches 50% of the cumulative volume from the smallest particle size side; and Dv90 refers to the particle size that, in a volumetric particle size distribution, reaches 90% of the cumulative volume from the smallest particle size side. The aforementioned "particles" can be silicon carbide materials, first graphite, or second graphite.

[0044] This application does not impose any particular restrictions on the method of controlling the particle size of silicon carbide material, first graphite, and second graphite, as long as the purpose of this application can be achieved. For example, it can be achieved through crushing or sieving. Alternatively, commercially available silicon carbide material, first graphite, and second graphite can be purchased, and the particle size Dv50 of the first graphite can be determined by combining the test method for "particle size testing" in this application. -1 The particle size of the second graphite is Dv50. -2 Silicon-carbon materials with particle sizes Dv10 and Dv50 -3 Simply select Dv90 and choose the required materials.

[0045] In some embodiments of this application, the specific surface area of ​​the silicon-carbon material is 0.5 m². 2 / g to 3.5m 2 / g. For example, the specific surface area of ​​silicon-carbon materials is 0.5m². 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 ranges. By controlling the specific surface area of ​​the silicon-carbon material within the above range, the surface of the silicon-carbon material can have more lithium intercalation sites, which helps to reduce the probability of lithium plating on the negative electrode. Furthermore, the surface of the silicon-carbon material can have sufficient contact with the electrolyte, allowing the silicon-carbon material to exert its high capacity characteristics, resulting in a high capacity negative electrode. Therefore, the electrochemical device, while mitigating the lithium plating problem on the negative electrode surface, has high energy density and capacity retention.

[0046] In some embodiments of this application, the specific surface area of ​​the silicon-carbon material is 1.0 m². 2 / g to 2.0m 2 / g. For example, the specific surface area of ​​silicon-carbon materials is 1.0 m². 2 / g, 1.2m 2 / g, 1.3m 2 / g, 1.5m 2 / g, 1.7m 2 / g, 2.0m 2 / g or any value between any two of the above ranges. Controlling the specific surface area of ​​silicon-carbon materials within the above range helps to further alleviate the problem of lithium deposition on the negative electrode surface while maintaining high energy density and capacity retention in electrochemical devices.

[0047] This application does not impose any particular restrictions on the method of controlling the specific surface area of ​​silicon-carbon materials, as long as it achieves the purpose of this application. For example, it can be achieved by controlling the particle size of silicon-carbon materials or by controlling the pore size distribution of porous carbon when preparing silicon-carbon materials using a deposition method. Alternatively, it can be achieved by directly purchasing silicon-carbon materials with a specific surface area within the range of this application and combining it with the test method for "testing the specific surface area of ​​silicon-carbon materials" in this application to determine the specific surface area of ​​the silicon-carbon materials and select silicon-carbon materials with the desired specific surface area.

[0048] In some embodiments of this application, the tap density of the silicon carbide material is 0.8 g / cm³. 3 Up to 1.05 g / cm 3 For example, the tap density of silicon carbide materials is 0.8 g / cm³. 3 0.85g / cm 3 0.87g / cm 3 0.91g / cm 3 0.96g / cm 3 1.00g / cm 3 1.05g / cm 3Or any value between any two of the above ranges. Controlling the tap density of silicon-carbon material within the above range is beneficial for the silicon-carbon material to be densely packed in the second negative electrode material layer and to have a suitable porosity, so that the electrochemical device has a low volume expansion rate while having a high energy density.

[0049] In some embodiments of this application, the compaction density C1 of the first negative electrode material layer and the compaction density C2 of the second negative electrode material layer satisfy: 1.5 g / cm³ 3 ≤C1≤1.65g / cm 3 1.35g / cm 3 ≤C2≤1.5g / cm 3 0.05g / cm 3 ≤C1-C2≤0.3g / cm 3 For example, the compaction density C1 of the first negative electrode material layer is 1.5 g / cm³. 3 1.52g / cm 3 1.55g / cm 3 1.57g / cm 3 1.60g / cm 3 1.62g / cm 3 1.65g / cm 3 Or any value between any two of the above ranges. For example, the compaction density C2 of the second negative electrode material layer is 1.35 g / cm³. 3 1.37g / cm 3 1.40g / cm 3 1.43 g / cm 3 1.47 g / cm 3 1.5g / cm 3 Or any value between any two of the above ranges. For example, the value of C1-C2 is 0.05 g / cm³. 3 0.1g / cm 3 0.15g / cm 3 0.2g / cm 3 0.25g / cm 3 0.3g / cm 3Or any value between any two of the above-mentioned numerical ranges. By adjusting the compaction density C1 of the first negative electrode material layer, the compaction density C2 of the second negative electrode material layer, and the values ​​of C1-C2 within the range of this application, when the first and second negative electrode material layers have high capacity, the gaps between the particles in the first and second negative electrode material layers allow the electrolyte to have shorter transport paths and a greater number of transport channels in the first and second negative electrode material layers. In this way, the electrolyte can be uniformly distributed in the first and second negative electrode material layers, the transport rate of lithium ions and electrons is improved, the probability of lithium plating on the surface of the negative electrode is reduced, and the negative electrode also has a high capacity. Thus, the electrochemical device has a high energy density while alleviating the lithium plating problem on the surface of the negative electrode.

[0050] In some embodiments of this application, 0.1 g / cm 3 ≤C1-C2≤0.2g / cm 3 For example, the value of C1-C2 is 0.1 g / cm³. 3 0.12g / cm 3 0.14 g / cm 3 0.17g / cm 3 0.2g / cm 3 Or any value between any two of the above-mentioned numerical ranges. By adjusting 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 range of this application, the rapid transport of electrolyte within the pores of the negative electrode sheet can be accelerated, thereby further improving the rate performance of the electrochemical device.

[0051] This application does not specifically limit the method for controlling the compaction density of the first negative electrode material layer, as long as it achieves the purpose of this application. For example, it can be achieved by controlling the particle size of the first negative electrode active material in the first negative electrode material layer, or by controlling the pressure during the cold pressing process of the negative electrode sheet. This application also does not specifically limit the method for controlling the compaction density of the second negative electrode material layer, as long as it achieves the purpose of this application. For example, it can be achieved by controlling the particle size of the second negative electrode active material in the second negative electrode material layer, or by controlling the pressure during the cold pressing process of the negative electrode sheet.

[0052] In some embodiments of this application, the mass percentage of the first negative electrode active material is 90% to 98.5% based on the mass of the first negative electrode material layer. Optionally, the first negative electrode material layer further includes a conductive agent, a thickener, and a binder. This application does not impose any particular limitation on the mass percentage of the conductive agent, thickener, and binder in the first negative electrode material layer, as long as the purpose of this application is achieved. For example, based on the mass of the first negative electrode material layer, the mass percentage of the conductive agent is 0% to 0.5%, the mass percentage of the thickener is 0.05% to 0.5%, and the mass percentage of the binder is 1% to 5%.

[0053] In some embodiments of this application, the mass percentage of the second negative electrode active material is 89.8% to 98.5% based on the mass of the second negative electrode material layer. Optionally, the second negative electrode material layer further includes a conductive agent, a thickener, and a binder. This application does not impose any particular limitation on the mass percentage of the conductive agent, thickener, and binder in the second negative electrode material layer, as long as the purpose of this application is achieved. For example, based on the mass of the second negative electrode material layer, the mass percentage of the conductive agent is 0.1% to 2%, the mass percentage of the thickener is 0.05% to 0.5%, and the mass percentage of the binder is 1% to 10%.

[0054] This application does not impose any particular restrictions on the types of conductive agents, thickeners, and binders. Conductive agents, thickeners, and binders known in the art can be selected, as long as they can achieve the purpose of this application.

[0055] This application does not impose any particular limitation on the negative electrode current collector, as long as it achieves the purpose of this application. For example, the negative electrode current collector includes, but is not limited to, copper foil, copper alloy foil, nickel foil, titanium foil, nickel foam, or copper foam. In this application, there is no particular limitation on the thickness of the negative electrode current collector, as long as it achieves the purpose of this application. For example, the thickness of the negative electrode current collector is 4 μm to 20 μm. This application also does not impose any particular limitation on the thickness of the first negative electrode material layer and the second negative electrode material layer, as long as they achieve the purpose of this application. For example, the thickness of the first negative electrode material layer is 20 μm to 50 μm, and the thickness of the second negative electrode material layer is 20 μm to 50 μm.

[0056] This application does not impose any particular limitation on the positive electrode sheet, as long as it achieves the purpose of this application. In some embodiments, the positive electrode sheet includes a positive current collector and a positive active material layer. The positive active material layer is disposed on one or both surfaces of the positive current collector. The aforementioned "surface" can be a portion of the surface of the positive current collector or the entire surface of the positive current collector. This application does not impose any particular limitation on the positive current collector, as long as it achieves the purpose of this application. For example, the positive current collector can contain aluminum foil or aluminum alloy foil, etc. The positive active material layer of this application contains a positive active material. This application does not impose any particular limitation on the type of positive active material, as long as it achieves the purpose of this application. For example, the positive active material can contain at least one of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide, lithium manganese oxide, lithium manganese iron phosphate, or lithium titanate, etc. In this application, the positive active material can also contain non-metallic elements, which can include at least one of fluorine, phosphorus, boron, chlorine, silicon, or sulfur. These elements can further improve the stability of the positive active material. In this application, there are no particular limitations on the thickness of the positive electrode current collector and the positive electrode active material layer, as long as the purpose of this 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 may also include at least one of a conductive agent or a binder. This application does not particularly limit the types of conductive agents and binders in the positive electrode active material layer, as long as the purpose of this application can be achieved. This application does not particularly limit the mass ratio of the positive electrode active material, conductive agent, and binder in the positive electrode active material layer; those skilled in the art can choose according to actual needs, as long as the purpose of this application can be achieved.

[0057] This application does not impose any particular limitation on the diaphragm, as long as it achieves the purpose of this application. For example, the diaphragm material may include, but is not limited to, at least one of polyethylene (PE), polyolefins (PO) primarily composed of polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The type of diaphragm may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane, or spun membrane. This application does not impose any particular limitation on the thickness of the diaphragm, as long as it achieves the purpose of this application.

[0058] The electrochemical device of this application also includes an electrolyte. This application does not impose any particular limitation on the electrolyte, as long as it achieves the purpose of this application. For example, in some embodiments, the electrolyte includes a lithium salt and a non-aqueous solvent. The lithium salt may 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 difluoroborate. This application does not limit the content of lithium salt in the electrolyte, as long as it achieves the purpose of this application. This application does not impose any particular limitation on the non-aqueous solvent, as long as it achieves the purpose of this application. For example, the non-aqueous solvent may include, but is not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents. The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorocarbonate compounds. The aforementioned chain carbonate compounds may include, but are 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 aforementioned cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, or vinylene carbonate. Fluorinated carbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate, 1,2-difluoroethylene carbonate, 1,1,2-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The aforementioned carboxylic acid ester compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valproic acid lactone, or caprolactone. The aforementioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The aforementioned other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate.

[0059] The electrochemical device of this application also includes a housing for accommodating the positive electrode, the negative electrode, the separator, and the electrolyte, as well as other components known in electrochemical devices in the art. This application does not limit the scope of these other components. This application does not impose any particular limitation on the housing; it can be any housing known in the art, as long as it achieves the purpose of this application.

[0060] This application does not impose any particular limitation on the type of electrochemical device, which may include any device in which an electrochemical reaction occurs. For example, electrochemical devices may include, but are not limited to, lithium-ion secondary batteries, lithium polymer secondary batteries, or lithium-ion polymer secondary batteries.

[0061] This application does not impose any particular restrictions on the preparation method of the negative electrode sheet, as long as it can achieve the purpose of this application. For example, in some embodiments, the preparation method of the negative electrode sheet includes, but is not limited to, the following steps: (1) after mixing the first negative electrode active material, conductive agent, thickener and binder evenly, add solvent and stir evenly to obtain the first negative electrode slurry; (2) after mixing the second negative electrode active material, conductive agent, thickener and binder evenly, add solvent and stir evenly to obtain the second negative electrode slurry; (3) after coating the first negative electrode slurry on the two surfaces of the negative electrode current collector, after drying, a first negative electrode material layer is formed, and then, the second negative electrode slurry is coated on the surfaces of the two first negative electrode material layers away from the negative electrode current collector, after drying, a second negative electrode material layer is formed, and after cold pressing and cutting, a negative electrode sheet with a first negative electrode material layer and a second negative electrode material layer on both sides is obtained. In other embodiments, the preparation method of the negative electrode sheet includes, but is not limited to, the following steps: (1) mixing the first negative electrode active material, conductive agent, thickener, and binder evenly, adding solvent, and stirring evenly to obtain a first negative electrode slurry; (2) mixing the second negative electrode active material, conductive agent, thickener, and binder evenly, adding solvent, and stirring evenly to obtain a second negative electrode slurry; (3) coating the first negative electrode slurry on one surface of the negative electrode current collector, drying it, forming a first negative electrode material layer on the surface of the negative electrode current collector, coating the second negative electrode slurry on the surface of the first negative electrode material layer away from the negative electrode current collector, drying it, and then cold pressing and slitting to obtain a negative electrode sheet with a first negative electrode material layer and a second negative electrode material layer on one side. This application does not have any particular restrictions on the type of solvent in steps (1) and (2) above, as long as it can achieve the purpose of this application. This application does not have any particular restrictions on the solid content of the first and second negative electrode slurries above, as long as it can achieve the purpose of this application. This application does not impose any particular restrictions on the drying temperature and time in step (3) above. Those skilled in the art can choose according to the actual situation, as long as the purpose of this application can be achieved.

[0062] The preparation process of the electrochemical device in this application is well known to those skilled in the art, and this application has no particular limitations. For example, it may include, but is not limited to, the following steps: stacking the diaphragm, positive electrode, diaphragm and negative electrode in sequence, and winding, folding or other operations as needed to obtain a wound electrode assembly; placing the electrode assembly in a packaging bag; injecting electrolyte into the packaging bag and sealing it; and obtaining the electrochemical device through processes such as formation, degassing and shaping. Alternatively, stacking the diaphragm, positive electrode, diaphragm and negative electrode in sequence, and then fixing the four corners of the entire stacked structure with tape to obtain a stacked electrode assembly; placing the electrode assembly in a packaging bag; injecting electrolyte into the packaging bag and sealing it; and obtaining the electrochemical device through processes such as formation, degassing and shaping.

[0063] A second aspect of this application provides an electronic device comprising the electrochemical device described in any of the foregoing embodiments. Therefore, the electronic device exhibits good performance in use.

[0064] The electronic device described in this application is not particularly limited and can be any electronic device known in the prior art. For example, the electronic device may include, but is not limited to: laptop computers, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, household large-capacity batteries, and lithium-ion capacitors.

[0065] Example

[0066] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below.

[0067] Test method and equipment:

[0068] Particle size testing:

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

[0070] Test of the mass percentage of silicon in silicon-carbon materials:

[0071] The silicon content in silicon-carbon materials was measured using inductively coupled plasma atomic emission spectrometry (ICP). The silicon-carbon material was dissolved in nitric acid, and the solution was diluted to volume using a volumetric flask. A portion of the diluted solution was then used for ICP testing.

[0072] Testing the specific surface area of ​​silicon-carbon materials:

[0073] The specific surface area of ​​the silicon-carbon materials in each embodiment and comparative example was measured using a specific surface area analyzer (Tristar II 3020M, Micron Instruments, USA) via nitrogen adsorption. The specific tests were conducted according to the national standard GB / T19587-2017, "Determination of Specific Surface Area of ​​Solid Substances by Gas Adsorption BET Method".

[0074] Test of compaction density:

[0075] After controlling the lithium-ion battery voltage to 3.95V (if the lithium-ion battery voltage is lower than 3.95V, charge it to 3.95V at 0.2C; if the lithium-ion battery voltage is higher than 3.95V, discharge it to 3.95V at 0.2C), disassemble it to obtain the negative electrode sheet. Punch a small disc with an area of ​​S onto the negative electrode sheet, and obtain the mass and thickness of the small disc, denoted as m and h, respectively. Remove the second negative electrode material layer on the small disc by etching, and obtain the mass and thickness of the small disc with the second negative electrode material layer removed, denoted as m1 and h1, respectively. Remove the first negative electrode material layer on the small disc by etching, and obtain the mass and thickness of the negative current collector, denoted as m2 and h2, respectively.

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

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

[0078] Test of the highest non-lithium-deposition charging rate:

[0079] The lithium-ion batteries of each embodiment and comparative example were charged and discharged at 25°C according to the following steps: allowed to stand for 30 minutes, then discharged at a constant current of 0.2C to 3.0V; allowed to stand for 30 minutes; then charged at a constant current of 0.5C to 4.5V, followed by charging at a constant voltage of 4.5V to 0.05C, at which point charging was stopped (this is the fully charged state); this constitutes one charge-discharge cycle. After 50 cycles of the above charge-discharge cycle, the fully charged lithium-ion batteries were disassembled, and the negative electrode was observed for lithium plating: if white or gray lithium appeared on the surface of the negative electrode, it was determined to be lithium plating; otherwise, it was determined not to be lithium plating.

[0080] If lithium plating does not occur, take a lithium-ion battery prepared in the same embodiment or comparative example, and increase the charging rate by 0.1C each time while keeping other charging and discharging parameters unchanged. Repeat the above steps until lithium plating occurs on the negative electrode.

[0081] The charging rate at which the negative electrode last exhibits no lithium plating is defined as the highest non-lithiation rate for that lithium-ion battery. A higher highest non-lithiation rate indicates that the lithium-ion battery is less prone to lithium plating, meaning the problem of lithium plating on the negative electrode surface is more effectively mitigated, resulting in better cycle performance.

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

[0083] The lithium-ion battery was left to stand at 25±3℃ for 30 minutes, then discharged to 3.0V at a current of 0.2C, and then charged to 4.5V at a constant current with the highest non-lithiation rate. The lithium-ion battery was then disassembled to obtain the negative electrode sheet. The second negative electrode material layer on the surface was removed with tape for the second graphite lithium intercalation test of the second negative electrode material layer. The negative electrode sheet after removing the second negative electrode material layer was used for the first graphite lithium intercalation test of the first negative electrode material layer. The graphite of the second negative electrode material layer and the first negative electrode material layer were characterized by XRD.

[0084] The distinction between the second and first negative electrode material layers is as follows: A cross-section of the electrode is subjected to SEM / EDS characterization. In the EDS image, the color of the Si element region differs from the color of the region without Si elements, and there is a clear boundary between the two regions. A straight line drawn at this point yields the boundary line between the first and second negative electrode material layers. For each embodiment and Comparative Example 2, the surface layer of the negative electrode is first peeled off with tape for the second graphite lithium intercalation test of the second negative electrode material layer. Then, tape is repeatedly used to peel off portions of the negative electrode until the remaining negative electrode thickness is first less than the sum of the thicknesses of the first negative electrode material layer and the negative current collector, for the first graphite lithium intercalation test of the first negative electrode material layer. For Comparative Example 1, there is only the first negative electrode material layer. The difference between the surface layer and the bottom layer is due to the different positions of the negative electrode sheet thickness direction. It is not necessary to use different negative electrode sheet compositions as a distinguishing condition. First, the surface layer of the negative electrode sheet is peeled off with tape for the surface graphite lithium intercalation test. Then, tape is repeatedly used to peel off part of the negative electrode sheet until the thickness of the remaining negative electrode sheet is less than 1 / 2 of the total thickness of the first negative electrode material layer and the thickness of the negative electrode current collector for the bottom graphite lithium intercalation test.

[0085] The highest non-lithiation rate is obtained according to the test method in "Test of the highest non-lithiation charging rate". The maximum charging rate under the condition that the negative electrode does not precipitate lithium is defined as the highest non-lithiation rate of the lithium-ion battery.

[0086] Capacity retention testing:

[0087] The lithium-ion battery was left to stand at 25±3℃ for 30 minutes, then charged at the highest rational rate with a constant current to 4.5V. Charging was then stopped at a constant voltage of 4.5V to 0.05C. The battery was left to stand for 30 minutes. It was then discharged at 0.5C to 3.0V and left to stand for 30 minutes. This charge-discharge cycle was repeated 300 times (cls), and the discharge capacity on the 300th cycle was recorded as C. 300 Using the discharge capacity of the second cycle as the 100% capacity baseline, the discharge capacity of the second cycle is recorded as C2. Capacity retention rate (%) = C 300 / C2×100%.

[0088] Energy density testing:

[0089] First, charge the lithium-ion battery according to the following procedure, then discharge it to obtain the discharge capacity of the lithium-ion battery.

[0090] Charging: Charge at a constant current of 0.2C to 4.5V, then charge at a constant voltage of 4.5V to 0.05C;

[0091] Discharge: Discharge at a constant current of 0.2C to 3.0V, and obtain the discharge energy E1;

[0092] After the lithium-ion battery charging process is completed, the length L, width W, and height H of the lithium-ion battery are measured using a laser thickness gauge to obtain the volume V = L × W × H. Its volumetric energy density (ED) can be calculated using the following formula: ED (Wh / L) = E1 / V.

[0093] Example 1-1

[0094] <Preparation of Negative Electrode Sheets>

[0095] The first negative electrode active material, first graphite (artificial graphite, graphitization degree 95%), binder polyacrylic acid (weight average molecular weight Mw = 3500), and conductive agent single-walled carbon nanotubes were mixed at a mass ratio of 97:2.8:0.2. Deionized water was added as a solvent, and the mixture was stirred to prepare a first negative electrode slurry with a solid content of 42wt%. The first negative electrode slurry was coated onto both surfaces of a 6μm thick copper foil negative electrode current collector and dried at 100℃ to form the first negative electrode material layer. The second negative electrode active material, second graphite (artificial graphite, graphitization degree 95%), silicon carbon material, binder polyacrylic acid (Mw = 3500), and conductive agent single-walled carbon nanotubes were thoroughly mixed at a mass ratio of 77.6:19.4:2.8:0.2. Deionized water was added, and the mixture was stirred to prepare a second negative electrode slurry with a solid content of 42wt%. This slurry was coated onto the surfaces of the two first negative electrode material layers away from the copper foil and dried at 100℃ to form the second negative electrode material layer. Cold pressing and slitting yield negative electrode sheets with dimensions of 1000mm × 80mm.

[0096] In the second negative electrode active material, the mass percentage of silicon-carbon material W1 is 40%, and the mass percentage of silicon element W2 in the silicon-carbon material is 50%. The particle size of the silicon-carbon material is Dv10 = 5.5 μm and Dv50 = 5.5 μm. -4 =9.1μm, Dv90=15μm. The specific surface area of ​​silicon-carbon materials is 1.2m². 2 / g. The particle size of the first graphite is Dv50. -2 =12μm, the particle size of the second graphite is Dv50 -2 =10μm. The coating weight per unit area of ​​the first negative electrode material layer, CW1, is 90mg / 1540.25mm. 2 The coating weight per unit area of ​​the second negative electrode material layer, CW2, is 30 mg / 1540.25 mm. 2 The coating weight per unit area of ​​the negative electrode sheet (CW) 12 =CW1 + CW2 = 120mg / 1540.25mm 2 The weight per unit area of ​​the second negative electrode material layer accounts for 1 / 4 of the weight per unit area of ​​the negative electrode sheet. The compaction density of the first negative electrode material layer is C1 = 1.6 g / cm³. 3 The compaction density of the second negative electrode material layer, C2, is 1.4 g / cm³. 3 .

[0097] <Preparation of the positive electrode>

[0098] Lithium cobalt oxide (CCO) as the positive electrode active material, conductive carbon black (Super P) as the conductive agent, and polyvinylidene fluoride (PVDF, Mw = 600000) as the binder were mixed in a mass ratio of 97.8:1.4:0.8. N-methylpyrrolidone (NMP) was added as a solvent, and the mixture was stirred under vacuum until a homogeneous positive electrode slurry with a solid content of 72 wt% was obtained. The positive electrode slurry was uniformly coated onto one surface of a 10 μm thick aluminum foil for the positive electrode current collector and dried at 85°C to obtain a positive electrode sheet with a single-sided coating of positive electrode active material (90 μm thick). The above steps were then repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive electrode active material. After cold pressing, cutting, and welding of positive electrode tabs, a positive electrode sheet with a size of 990 mm × 76 mm was obtained for use.

[0099] <Preparation of the diaphragm>

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

[0101] <Preparation of Electrolyte>

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

[0103] <Preparation of Lithium-ion Batteries>

[0104] The separator, positive electrode, separator, and negative electrode are stacked in sequence and wound to obtain the electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag, and after dehydration at 80°C, electrolyte is injected and the battery is sealed. After processes such as formation, degassing, and shaping, a lithium-ion battery is obtained.

[0105] Examples 1-2 to Examples 1-13

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

[0107] Examples 2-1 to 2-6

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

[0109] Examples 3-1 to 3-8

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

[0111] Examples 4-1 to 4-11

[0112] Except for adjusting the relevant preparation parameters according to Table 4, the rest is the same as in Example 3-1.

[0113] Comparative Example 1

[0114] <Preparation of Negative Electrode Sheets>

[0115] Artificial graphite (95% graphitization), silicon carbon material (Dv50 = 8.6 μm), polyacrylic acid binder (weight average molecular weight Mw = 3500), and single-walled carbon nanotubes conductive agent were mixed in a mass ratio of 87.3:9.7:2.8:0.2. Deionized water was added as a solvent, and the mixture was stirred to prepare a negative electrode slurry with a solid content of 42 wt%. The negative electrode slurry was coated onto both surfaces of the copper foil current collector and dried at 100°C to form the first negative electrode material layer. Cold pressing and slitting were then performed to obtain negative electrode sheets with a size of 1000 mm × 80 mm. The coating weight of the first negative electrode material layer was 150 mg / 1540.25 mm. 2 .

[0116] The preparation of the positive electrode, the separator, the electrolyte, and the lithium-ion battery are the same as in Examples 1-1.

[0117] Comparative Example 2

[0118] Except for swapping the positions of the first and second negative electrode material layers in the <Preparation of Negative Electrode Sheet>, the rest is the same as in Example 1-1.

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

[0120] Table 1

[0121]

[0122] Note: In Table 1, “P” represents the proportion of the unit area coating weight of the second negative electrode material layer to the unit area coating weight of the negative electrode sheet; “\” indicates that there is no relevant parameter.

[0123] As can be seen from Examples 1-1 to 1-11, Comparative Example 1, and Comparative Example 2, the electrochemical device of this application, by simultaneously providing a first negative electrode material layer at the bottom and a second negative electrode material layer at the surface in the negative electrode sheet, and when the electrochemical device is charged to 4.5V at the highest non-lithiation rate constant current, no characteristic peak of LiC6 appears in the X-ray diffraction pattern of the second negative electrode material layer. 12 The 2θ at the maximum intensity of the characteristic peak is A2°, 25.15≤A2≤25.30; and / or, in the X-ray diffraction pattern of the first negative electrode material layer, LiC 12The 2θ at the maximum intensity of the characteristic peak is A1°. In the X-ray diffraction pattern of the second negative electrode material layer, LiC 12 The 2θ at the maximum intensity of the characteristic peak is A2°, 0≤A1-A2≤0.15. This results in a higher maximum non-lithiation rate for the electrochemical device in the embodiment, indicating that lithium deposition is less likely to occur on the surface of the negative electrode in the electrochemical device, thus alleviating the problem of lithium deposition on the surface of the negative electrode and improving the cycle performance of the electrochemical device. In contrast, the maximum non-lithiation rate of the electrochemical device in the comparative example is smaller, indicating that lithium deposition is more likely to occur on the surface of the negative electrode in the comparative example.

[0124] Figure 2 The X-ray diffraction pattern of the negative electrode sheet of Example 1-1 is shown; Figure 3 The X-ray diffraction pattern of the negative electrode sheet in Comparative Example 1 is shown. From Figure 2 It can be seen that in the negative electrode sheet of Example 1-1, the characteristic peaks of LiC6 were not observed in the X-ray diffraction pattern of the second negative electrode material layer. 12 The 2θ at the maximum intensity of the characteristic peak is 25.24°. In the X-ray diffraction pattern of the first negative electrode material layer, LiC... 12 The 2θ at the maximum intensity of the characteristic peak of LiC is 25.26°. 12 The 2θ values ​​at the maximum intensity of the characteristic peaks are quite close, indicating good uniformity of lithium intercalation in the negative electrode. From Figure 3 As can be seen from the X-ray diffraction pattern of the surface layer of the negative electrode in Comparative Example 1, characteristic peaks of LiC6 appear. 12 The 2θ at the maximum intensity of the characteristic peak is 25.12°. In the underlying X-ray diffraction pattern, LiC... 12 The 2θ at the point of maximum intensity in the characteristic peak is 25.35°.

[0125] The mass percentage of silicon-carbon material in the second negative electrode active material and the mass percentage of silicon in the silicon-carbon material generally affect the lithium deposition on the surface of the negative electrode in the electrochemical device. As can be seen from Examples 1-1 to 1-13, electrochemical devices using silicon-carbon material in the second negative electrode active material and silicon in the silicon-carbon material within the scope of this application exhibit a larger maximum non-lithium deposition rate, indicating that the problem of lithium deposition on the surface of the negative electrode in the electrochemical device is alleviated, and the electrochemical device has good cycle performance.

[0126] Table 2

[0127]

[0128] The first graphite has a particle size of Dv50. -1 and its Dv50 with the particle size of the second graphite -2Difference Dv50 -1 -Dv50 -2 This also typically affects lithium plating on the surface of the negative electrode in electrochemical devices and capacity retention. As can be seen from Examples 1-3 and Examples 2-1 to 2-6, the selected first graphite with a particle size Dv50... -1 and its Dv50 with the particle size of the second graphite -2 Difference Dv50 -1 -Dv50 -2 The electrochemical device within the scope of this application has a high maximum non-lithiation rate while maintaining a high capacity retention rate, indicating that the electrochemical device alleviates the problem of lithium deposition on the surface of the negative electrode while having a long cycle life.

[0129] Table 3

[0130]

[0131] Silicon-carbon materials with particle sizes Dv10 and Dv50 -3 The values ​​of Dv90 and Dv90-Dv10 typically affect lithium plating and capacity retention on the surface of the negative electrode in electrochemical devices. As can be seen from Examples 2-2, 3-1 to 3-4, the particle sizes Dv10 and Dv50 of the selected silicon-carbon material... -3 Electrochemical devices with values ​​of Dv90 and Dv90-Dv10 within the scope of this application have a large maximum non-lithiation rate while maintaining a high capacity retention rate, indicating that the electrochemical device alleviates the problem of lithium deposition on the surface of the negative electrode while having a long cycle life.

[0132] The specific surface area of ​​silicon-carbon materials typically affects lithium deposition on the surface of the negative electrode and capacity retention in electrochemical devices. As can be seen from Examples 2-2, 3-5 to 3-8, electrochemical devices using silicon-carbon materials with a specific surface area within the range of this application exhibit both high capacity retention and a large maximum non-lithiation rate, indicating that the electrochemical device alleviates the problem of lithium deposition on the negative electrode surface while maintaining a long cycle life.

[0133] Table 4

[0134]

[0135]

[0136] The compaction density C1 of the first negative electrode material layer, the compaction density C2 of the second negative electrode material layer, and the difference between them C1-C2 typically affect the lithium deposition on the surface of the negative electrode in an electrochemical device and the energy density of the electrochemical device. As can be seen from Examples 3-1, 4-1 to 4-11, electrochemical devices using compaction densities C1 and C2 of the first and second negative electrode material layers, and the difference between them C1-C2, within the scope of this application, exhibit little change in energy density and possess a relatively large maximum non-lithiation rate. This indicates that the problem of lithium deposition on the surface of the negative electrode in the electrochemical device is alleviated, and the electrochemical device has a high energy density.

[0137] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, or article.

[0138] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0139] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. An electrochemical device, comprising a negative electrode, a positive electrode, and a separator disposed between the negative electrode and the positive electrode, wherein the negative electrode comprises a negative 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 current collector, the first negative electrode material layer being disposed between the negative current collector and the second negative electrode material layer; When the electrochemical device is charged to the upper limit cutoff voltage at a constant current rate with the highest non-lithium deposition rate, the X-ray diffraction patterns of the first and second negative electrode material layers show that LiC 12 The characteristic peaks satisfy the following characteristics: （ 1) The X-ray diffraction pattern of the second negative electrode material layer did not show the characteristic peaks of LiC6. 12 The 2θ at the maximum intensity of the characteristic peak is A2°, and 25.15≤A2≤25.30; (2) In the X-ray diffraction pattern of the first negative electrode material layer, LiC 12 The 2θ at the maximum intensity of the characteristic peak is A1°. In the X-ray diffraction pattern of the second negative electrode material layer, LiC 12 The 2θ at the maximum intensity of the characteristic peak is A2°, and 0≤A1-A2≤0.

15.

2. The electrochemical device according to claim 1, wherein, The first negative electrode material layer includes a first negative electrode active material, and the first negative electrode active material includes a first graphite; The second negative electrode material layer includes a second negative electrode active material, which includes silicon-carbon material and second graphite. The silicon-carbon material in the second negative electrode active material has a mass percentage content of 15% to 50%, and the silicon element in the silicon-carbon material has a mass percentage content of 30% to 60%.

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

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

5. The electrochemical device according to claim 4, wherein, The particle size of the second graphite is Dv50 -2 With the first graphite having a particle size Dv50 -1 Satisfies: 1μm≤Dv50 -1 -Dv50 -2 ≤5μm.

6. The electrochemical device according to claim 2 or 3, wherein, The silicon-carbon material has particle sizes Dv10 and Dv50. -3 Dv90 satisfies: 5μm≤Dv10≤6μm, 8μm≤Dv50 -3 ≤10.5μm, 13μm≤Dv90≤17μm.

7. The electrochemical device according to claim 6, wherein, The particle sizes Dv10 and Dv90 of the silicon-carbon material satisfy the following: 8μm≤Dv90-Dv10≤10.5μm.

8. The electrochemical device according to claim 2 or 3, wherein, The specific surface area of ​​the silicon-carbon material is 0.5 m². 2 / g to 3.5m 2 / g.

9. The electrochemical device according to claim 8, wherein, The specific surface area of ​​the silicon-carbon material is 1.0 m². 2 / g to 2.0m 2 / g.

10. The electrochemical device according to claim 6, wherein, The compaction density C1 of the first negative electrode material layer and the compaction density C2 of the second negative electrode material layer satisfy: 1.5 g / cm³ 3 ≤C1≤1.65g / cm 3 1.35g / cm 3 ≤C2≤1.5g / cm 3 0.05g / cm 3 ≤C1-C2≤0.3g / cm 3 .

11. The electrochemical device according to claim 10, wherein, 0.1g / cm 3 ≤C1-C2≤0.2g / cm 3 。 12. An electronic device, wherein, The electronic device includes the electrochemical device according to any one of claims 1 to 11.

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