Negative pole piece, secondary battery and electronic device

Abstract

The invention provides a negative pole piece, a secondary battery and an electronic device, the negative pole piece comprises a negative current collector and a negative active material layer arranged on at least one surface of the negative current collector, the negative active material layer comprises a first material layer and a second material layer, and the first material layer is arranged between the negative current collector and the second material layer; and the second material layer contains a silicon element. The first material layer comprises lithium titanate and a first active material, and the first active material comprises a first silicon-containing material; based on the mass of the negative electrode active material layer, the mass percentage content of the lithium titanate is W1, the mass percentage content of the silicon element in the first active material is W2, and W1 and W2 meet the conditions that W1 is larger than or equal to 1.5% and smaller than or equal to 2.5%, and W2 / W1 is larger than or equal to 1.5% and smaller than or equal to 4.0. When the negative pole piece is applied to the secondary battery, the cycle life of the secondary battery can be prolonged while the energy density loss of the secondary battery is relieved.

Description

Technical Field

[0001] The present application relates to the field of electrochemical technologies, and particularly to a negative electrode sheet, a secondary battery, and an electronic device. Background Art

[0002] With the popularization and replacement of consumer electronic products such as smart phones, laptops, and smart homes, the market demand for consumer lithium-ion batteries has been continuously increasing. Due to the diversified development of electronic products, the requirements for lithium-ion batteries are also getting higher and higher, and lithium-ion batteries have also developed greatly. As a material for the negative electrode of lithium-ion batteries, silicon-containing materials have a higher energy density and a relatively high theoretical specific capacity compared with traditional graphite negative electrodes. However, during the cycling process of lithium-ion batteries, the volume of silicon-containing materials changes greatly. Affected by the alternating expansion stress of silicon-containing material particles, the negative electrode current collector is prone to extension in the length and width directions, resulting in purple spots at the head and tail of the negative electrode sheet and piercing the outer packaging, causing safety problems.

[0003] In order to overcome the above problems, the prior art often coats lithium titanate on the surface of the negative electrode current collector to improve its extension. However, due to the high content of lithium titanate required, it causes a huge loss of energy density in lithium-ion batteries. Summary of the Invention

[0004] The purpose of the present application is to provide a negative electrode sheet, a secondary battery, and an electronic device to improve the cycle life while alleviating the energy density loss of the secondary battery.

[0005] It should be noted that in the summary of the invention of the present application, lithium-ion batteries are used as examples of secondary batteries to explain the present application. However, the secondary batteries of the present application are not limited to lithium-ion batteries. The specific technical solutions are as follows:

[0006] The first aspect of the present application provides a negative electrode plate, which includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. The negative electrode active material layer includes a first material layer and a second material layer. The first material layer is disposed between the negative electrode current collector and the second material layer; the second material layer contains silicon element; the first material layer includes lithium titanate and a first active material, and the first active material includes a first silicon-containing material; based on the mass of the negative electrode active material layer, the mass percentage content of lithium titanate is W1, and the mass percentage content of silicon element in the first active material is W2. W1 and W2 satisfy: 1.5% ≤ W1 ≤ 2.5%, and 1.5 ≤ W2 / W1 ≤ 4.0. For the negative electrode plate provided by the first aspect of the present application, by disposing a first material layer containing lithium titanate and a first active material and a second material layer containing silicon element on the surface of the negative electrode current collector as the negative electrode active material layer, and regulating the mass percentage content of lithium titanate in the negative electrode active material layer and the ratio of it to the mass percentage content of silicon element in the first active material within the scope of the present application, lithium titanate can better improve the influence of the alternating expansion stress of the silicon-containing material particles on the extension of the negative electrode current collector in the length and width directions (hereinafter referred to as XY extension), making the XY extension of the negative electrode current collector smaller, and reducing the probability of purple spots and puncturing the outer packaging to cause liquid leakage at the head and tail of the negative electrode plate. It also enables lithium titanate to have a relatively small total addition amount in the negative electrode plate. The addition of the first active material including the first silicon-containing material in the first material layer in the present application is beneficial to improving the capacity of the first material layer, so that when the negative electrode plate is applied to a secondary battery, the energy density loss brought by lithium titanate to the secondary battery can be reduced. The silicon-containing material in the negative electrode active material layer expands and contracts continuously during the charge and discharge cycle of the secondary battery, reducing the contact tightness between the silicon-containing material particles. The poor contact between the particles of the first active material in the first material layer and the negative electrode current collector easily causes the problem of lithium ion conduction deactivation, resulting in an accelerated decay of the capacity of the secondary battery. In the present application, the blending of lithium titanate and the first active material can play an effect of "filling gaps" between the first active materials. Lithium titanate bridges between the deactivated particles of the first active material, improving the electrical contact between the particles of the first active material in the first material layer. In this way, applying the negative electrode plate to a secondary battery can improve the problem of "lithium ion conduction deactivation" faced by the secondary battery during the charge and discharge cycle, slow down the decay of the capacity of the secondary battery, and thus improve the cycle life of the secondary battery. Therefore, applying the negative electrode plate of the first aspect of the present application to a silicon-based secondary battery can alleviate the energy density loss of the secondary battery while improving the cycle life of the secondary battery, relieve the purple spot phenomenon at the head and tail of the negative electrode plate, and the secondary battery has good kinetic performance.

[0007] In some embodiments of the present application, the particle size Dv10 of lithium titanate is D 11 、the particle size Dv50 is D 12 、and the particle size Dv90 is D 13, where 0.3 μm ≤ D 11 ≤ 0.4 μm, 0.7 μm ≤ D 12 ≤ 1.0 μm, 1.1 μm ≤ D 13 ≤ 1.5 μm. Adjusting the particle size Dv10, particle size Dv50, and particle size Dv90 of lithium titanate within the above ranges is beneficial to the distribution of each particle of lithium titanate in the gaps formed by other particles in the first material layer, and is beneficial to improving the electrical contact between the particles of the active material in the first material layer.

[0008] In some embodiments of the present application, the particle size Dv10 of the first active material is D 21 , the particle size Dv50 is D 22 , and the particle size Dv90 is D 23 , and the following is satisfied between the particle size of lithium titanate and the particle size of the first active material: 8.0 ≤ D 21 / D 11 ≤ 12.0, 7.3 ≤ D 22 / D 12 ≤ 10.4, 8.6 ≤ D 23 / D 13 ≤ 11.8. Adjusting the value of D 21 / D 11 , the value of D 22 / D 12 , and the value of D 23 / D 13 within the above ranges, the secondary battery using the negative electrode sheet can alleviate the loss of energy density while further improving its cycle life, and is also beneficial to alleviating the purple spot phenomenon at the head and tail of the negative electrode sheet, making the secondary battery have good kinetic performance.

[0009] In some embodiments of the present application, the single-layer thickness of the first material layer is T1, and 10 μm ≤ T1 ≤ 12 μm. Adjusting the single-layer thickness of the first material layer within the above range and simultaneously reducing the total addition amount of lithium titanate in the negative electrode sheet enables the secondary battery to have a high energy density on the basis of having a long cycle life.

[0010] In some embodiments of the present application, the total thickness of the negative electrode active material layer is T2, and 12% ≤ T1 / T2 × 100% ≤ 37%. Adjusting the value of T1 / T2 × 100% within the above range, the secondary battery can have a high energy density on the basis of having a long cycle life.

[0011] In some embodiments of the present application, the negative electrode sheet satisfies at least one of the following characteristics: (1) Based on the mass of the negative electrode active material layer, the mass percentage content of silicon element in the negative electrode active material layer is W3, and 3.5% ≤ W3 ≤ 30%; (2) 2.4 ≤ W2 / W1 ≤ 4.0. This is conducive to enabling the negative electrode sheet to have a higher capacity, and the secondary battery using the negative electrode sheet has a higher energy density and a longer cycle life.

[0012] In some embodiments of the present application, the specific surface area of lithium titanate is 3.1 m 2 / g to 8.2 m 2 / g. By controlling the specific surface area of lithium titanate within the above range, the lithium intercalation and deintercalation capacity of lithium titanate can be improved and the solid-phase transfer impedance can be reduced, which can enhance the capacity and rate performance of the secondary battery.

[0013] In some embodiments of the present application, the powder conductivity of lithium titanate is 10 -6 S / cm to 10 -4 S / cm. By controlling the powder conductivity of lithium titanate within the above range, it is beneficial to improve the rate performance, cycle stability, fast charging performance and mechanical stability of the secondary battery, and at the same time, it also helps to improve the first Coulomb efficiency.

[0014] In some embodiments of the present application, lithium titanate satisfies at least one of the following characteristics: (1) The specific surface area of lithium titanate is 4.2 m 2 / g to 8.2 m 2 / g; (2) The powder conductivity of lithium titanate is 4×10 -5 S / cm to 6×10 -5 μS / cm.

[0015] In some embodiments of the present application, the second material layer includes a second active material, the second active material includes a second silicon-containing material, and the second silicon-containing material includes at least one of silicon carbide, silicon oxide or pure silicon. The second silicon-containing materials of the above types have a higher energy density and theoretical specific capacity. Selecting the second silicon-containing materials of the above types is conducive to enabling the negative electrode sheet to have a higher capacity.

[0016] In some embodiments of the present application, the negative electrode current collector includes any one of a copper current collector, a foam copper current collector, a nickel current collector or a stainless steel current collector. Selecting the negative electrode current collector of the above types is conducive to enabling the secondary battery to have good kinetic performance, a higher energy density and a longer cycle life.

[0017] The second aspect of the present application provides a secondary battery, which includes the negative electrode sheet described in any of the foregoing embodiments. Therefore, the secondary battery has good kinetic performance, a higher energy density and a longer cycle life.

[0018] The third aspect of the present application provides an electronic device, which includes the secondary battery described in any of the foregoing embodiments. Therefore, the electronic device has good performance in use.

[0019] Advantages of the present application:

[0020] The present application provides a negative electrode sheet, a secondary battery, and an electronic device. The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. The negative electrode active material layer includes a first material layer and a second material layer. The first material layer is disposed between the negative electrode current collector and the second material layer; the second material layer contains silicon element. The first material layer includes lithium titanate and a first active material, and the first active material includes a first silicon-containing material; based on the mass of the negative electrode active material layer, the mass percentage content of lithium titanate is W1, and the mass percentage content of silicon element in the first active material is W2. W1 and W2 satisfy: 1.5% ≤ W1 ≤ 2.5%, and 1.5 ≤ W2 / W1 ≤ 4.0. Through the above settings, lithium titanate can better improve the influence of the alternating expansion stress of the silicon-containing material particles on the XY extension of the negative electrode current collector, so that the XY extension of the negative electrode current collector is smaller, and the probability of purple spots appearing at the head and tail of the negative electrode sheet and piercing the outer package to cause liquid leakage is reduced. It also enables lithium titanate to have a relatively small total addition amount in the negative electrode sheet, so that when the negative electrode sheet is applied to a secondary battery, the energy density loss brought by lithium titanate to the secondary battery can be reduced. The blending of lithium titanate and the first active material can play an effect of "filling gaps" between large particle materials, improving the electrical contact between the particles of the active material in the first material layer. Applying the negative electrode sheet to a secondary battery can improve the problem of "lithium ion conduction inactivation" caused by the expansion of the silicon-containing material during the charge and discharge cycle of the secondary battery, thereby improving the cycle life of the secondary battery, alleviating the purple spot phenomenon at the head and tail of the negative electrode sheet, and enabling the secondary battery to have good kinetic performance.

[0021] Of course, it is not necessary for any product or method implementing the present application to achieve all the above-mentioned advantages simultaneously. Description of the Drawings

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, other embodiments can also be obtained based on these drawings.

[0023] Figure 1 It is a schematic cross-sectional structure diagram of the negative electrode sheet of some embodiments of the present application along its thickness direction and length direction.

[0024] Reference numerals: 100 - negative electrode plate; 10 - negative electrode current collector; 11 - first material layer; 12 - second material layer; 20 - negative electrode active material layer. Detailed implementation manners

[0025] Next, the technical solutions in the present application will be clearly and completely described in conjunction with the embodiments of the present application and the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those skilled in the art based on the present application belong to the scope of protection of the present application.

[0026] It should be noted that in the specific implementation manners of the present application, a lithium - ion battery is taken as an example of a secondary battery to explain the present application. However, the secondary battery of the present application is not limited to lithium - ion batteries.

[0027] In the first aspect of the present application, a negative electrode plate is provided. The negative electrode plate includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. The negative electrode active material layer includes a first material layer and a second material layer. The first material layer is disposed between the negative electrode current collector and the second material layer. The second material layer contains silicon. The first material layer includes lithium titanate and a first active material. The first active material includes a first silicon - containing material. Based on the mass of the negative electrode active material layer, the mass percentage content of lithium titanate is W1, and the mass percentage content of silicon in the first active material is W2. W1 and W2 satisfy: 1.5% ≤ W1 ≤ 2.5%, and 1.5 ≤ W2 / W1 ≤ 4.0. Further, 2.4 ≤ W2 / W1 ≤ 4.0.

[0028] For ease of understanding, in the present application, the self - length direction of the negative electrode plate is defined as X, and the self - thickness direction is defined as Z. It should be understood that the above definitions of directions are for the purpose of conveniently describing the present application and can be understood according to the relative positions of the accompanying drawings and the actual product elements. Moreover, the self - length direction and thickness direction of the negative electrode current collector, the first material layer, and the second material layer are the same as those of the negative electrode plate. The above "negative electrode active material layer disposed on at least one surface of the negative electrode current collector" means that the negative electrode active material layer is disposed on one surface or two surfaces of the negative electrode current collector. The above "surface" can be a partial surface or the entire surface of the negative electrode current collector. As Figure 1 shown, the negative electrode plate 100 includes a negative electrode current collector 10 and a negative electrode active material layer 20. The negative electrode active material layer 20 is disposed on two surfaces of the negative electrode current collector 10. The negative electrode active material layer 20 includes a first material layer 11 and a second material layer 12. The first material layer 11 is disposed between the negative electrode current collector 10 and the second material layer 12. Of course, it can be understood that in some embodiments of the present application, the negative electrode active material layer 20 is disposed on one surface of the negative electrode current collector 10.

[0029] For example, W1 is 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, or any value between any two of the above numerical ranges. When lithium titanate (abbreviated as LTO, chemical formula Li4Ti5O 12 ) undergoes spinel structure during lithium ion insertion or extraction, both the lattice constant and volume change are very small, less than 1%. This "zero strain property" can avoid the structural damage caused by the back-and-forth expansion and contraction of the electrode material. Coating it on the surface of the negative electrode current collector can reduce the alternating expansion stress on the negative electrode current collector caused by silicon-containing material particles during the charge and discharge process of the secondary battery, reduce the purple spot problem at the head and tail of the negative electrode sheet interface caused by the XY extension (hereinafter referred to as XY extension) of the negative electrode current collector in the length and width directions, and also reduce the probability of leakage caused by the XY extension of the negative electrode current collector piercing the outer package, resulting in safety problems. However, the specific capacity of lithium titanate is lower than that of conventional carbon active materials, and the discharge voltage platform of lithium titanate to lithium is about 1.6V, corresponding to a full battery voltage of about 2.3V, which is outside the current operating voltage range (3V to 4.45V) of conventional lithium cobalt oxide secondary batteries. Therefore, the use of lithium titanate will cause a loss of volume energy density (VED) in the secondary battery. When W1 is less than 1.5%, the mass percentage content of lithium titanate is too low, and the improvement effect of lithium titanate on the ductility of the negative electrode current collector is not obvious; when W1 is greater than 2.5%, the mass percentage content of lithium titanate is too high, which will cause excessive volume energy density loss in the secondary battery.

[0030] For example, the value of W2 / W1 is 1.5, 1.7, 1.8, 2.0, 2.2, 2.4, 2.5, 2.7, 2.8, 3.0, 3.2, 3.3, 3.6, 3.7, 3.8, 4.0, or any value between any two of the above numerical ranges. The more silicon-containing material particles on the surface of the negative electrode current collector, the greater the XY extension of the negative electrode current collector affected by the alternating expansion stress of the silicon-containing material particles. The introduction of lithium titanate can improve the influence of the alternating expansion stress of the silicon-containing material particles on the XY extension of the negative electrode current collector. The inventors have found through research that when the value of W2 / W1 is less than 1.5, the content of lithium titanate is too high compared to the content of silicon-containing materials. The introduction of excessive lithium titanate will lead to a decrease in the proportion of negative electrode active materials, and will also cause a loss of volume energy density in the secondary battery, resulting in a decrease in the volume energy density and a shortening of the cycle life of the secondary battery; when the value of W2 / W1 is greater than 4.0, the content of lithium titanate is too low compared to the content of silicon-containing materials, and the improvement of lithium titanate on the XY extension of the negative electrode current collector is far from enough to offset the influence of the alternating expansion stress of the silicon-containing material particles on the negative electrode current collector.

[0031] Overall, for the negative electrode sheet provided in the first aspect of the present application, by disposing a first material layer containing lithium titanate and a first active material and a second material layer containing silicon element on the surface of the negative electrode current collector as the negative electrode active material layer, and regulating the mass percentage content of lithium titanate in the negative electrode active material layer and the ratio of its silicon element mass percentage content to that in the first active material within the scope of the present application, lithium titanate can preferably improve the influence of the alternating expansion stress of the silicon-containing material particles on the XY extension of the negative electrode current collector, resulting in a smaller XY extension of the negative electrode current collector and a reduced probability of purple spots appearing at the head and tail of the negative electrode sheet and leakage caused by piercing the outer package. It also enables lithium titanate to have a relatively small total addition amount in the negative electrode sheet. In the present application, the addition of the first active material including the first silicon-containing material to the first material layer is beneficial to improving the capacity of the first material layer, thereby reducing the energy density loss brought by lithium titanate to the secondary battery when the negative electrode sheet is applied to the secondary battery. During the charge and discharge cycle of the secondary battery, the silicon-containing material in the negative electrode active material layer continuously expands and contracts, reducing the contact tightness between the silicon-containing material particles. The poor contact between the particles of the first active material in the first material layer and the negative electrode current collector causes the problem of lithium ion conduction inactivation, leading to an accelerated attenuation of the capacity of the secondary battery. Lithium titanate has good electronic conductivity. In the present application, the blending of lithium titanate with the first active material can achieve an "interstitial filling" effect among the first active materials. Lithium titanate builds a bridge between the inactivated particles of the first active material, improving the electrical contact between the particles of the first active material in the first material layer. In this way, applying the negative electrode sheet to a silicon-based secondary battery can improve the problem of "lithium ion conduction inactivation" faced during the charge and discharge cycle of the secondary battery, slow down the attenuation of the capacity of the secondary battery, and thus improve the cycle life of the secondary battery. Therefore, applying the negative electrode sheet of the first aspect of the present application to the secondary battery can, while alleviating the energy density loss of the secondary battery, improve the cycle life of the secondary battery, relieve the purple spot phenomenon at the head and tail of the negative electrode sheet, and endow the secondary battery with good kinetic performance.

[0032] The present application does not particularly limit the type of the first silicon-containing material, as long as the object of the present application can be achieved. For example, the first silicon-containing material includes, but is not limited to, at least one of silicon carbide compounds, silicon oxide compounds, or pure silicon. In some embodiments of the present application, the second material layer includes a second active material, the second active material includes a second silicon-containing material, and the second silicon-containing material includes at least one of silicon carbide compounds, silicon oxide compounds, or pure silicon. The first silicon-containing material and the second silicon-containing material of the above types have a relatively high energy density and theoretical specific capacity. Selecting the silicon-containing material of the above types is beneficial to making the negative electrode sheet have a relatively high capacity. In the present application, the silicon carbide compound is a silicon-carbon composite material. Based on the mass of the silicon-carbon composite material, the mass percentage content of silicon element is 30% to 70%, and the mass percentage content of carbon element is 30% to 70%. The present application does not particularly limit the silicon-carbon composite material, as long as the object of the present application can be achieved. For example, the silicon-carbon composite material can be a composite material obtained by deposition. Exemplarily, the silicon-carbon composite material can be a composite material with silicon material deposited on a carbon skeleton, or a composite material with carbon material deposited on a silicon skeleton. The silicon oxide compound includes SiOx, where 0 < x < 2. Exemplarily, the silicon oxide compound can include silicon monoxide (SiO, the molar ratio of silicon to oxygen is 1:1). It can be understood that the "silicon-containing material" in the present application can be the first silicon-containing material, or the second silicon-containing material, or both the first silicon-containing material and the second silicon-containing material. In the present application, the first silicon-containing material and the second silicon-containing material can be the same or different.

[0033] The present application does not particularly limit the mass percentage content W2 of silicon element in the first active material, as long as the object of the present application can be achieved. For example, based on the mass of the negative electrode active material layer, the mass percentage content W2 of silicon element in the first active material is 2.25% to 10%.

[0034] The present application does not particularly limit the regulation method of the mass percentage content of silicon element in the first active material, as long as the object of the present application can be achieved. For example, it can be achieved by regulating the proportion of the first active material in the negative electrode active material layer, or by regulating the type of the first active material.

[0035] It can be understood that in some embodiments of the present application, the first active material layer includes lithium titanate, the first active material, and carbon material. In some embodiments of the present application, the second active material includes a second silicon-containing material and carbon material. The present application does not particularly limit the type of carbon material, as long as the object of the present application can be achieved. For example, the carbon material includes, but is not limited to, at least one of artificial graphite, natural graphite, hard carbon, soft carbon, or mesophase carbon microspheres.

[0036] In some embodiments of the present application, the particle size Dv10 of lithium titanate is D 11 and the particle size Dv50 is D 12, the particle size Dv90 is D 13 , where 0.3 μm ≤ D 11 ≤ 0.4 μm, 0.7 μm ≤ D 12 ≤ 1.0 μm, 1.1 μm ≤ D 13 ≤ 1.5 μm. For example, D 11 is 0.3 μm, 0.31 μm, 0.32 μm, 0.33 μm, 0.34 μm, 0.35 μm, 0.36 μm, 0.37 μm, 0.38 μm, 0.39 μm, 0.4 μm or any value between any two of the above numerical ranges. For example, D 12 is 0.7 μm, 0.71 μm, 0.74 μm, 0.76 μm, 0.78 μm, 0.8 μm, 0.82 μm, 0.87 μm, 0.9 μm, 0.93 μm, 1.0 μm or any value between any two of the above numerical ranges. For example, D 13 is 1.1 μm, 1.15 μm, 1.2 μm, 1.23 μm, 1.27 μm, 1.3 μm, 1.32 μm, 1.37 μm, 1.4 μm, 1.45 μm, 1.5 μm or any value between any two of the above numerical ranges. Adjusting the particle size Dv10, particle size Dv50, and particle size Dv90 of lithium titanate within the above ranges is conducive to the distribution of each particle of lithium titanate in the gaps formed by other particles in the first material layer, and is conducive to improving the electrical contact between the particles of the active material in the first material layer. In this way, when the negative electrode sheet is applied to a secondary battery, it is conducive to making the secondary battery have a high energy density while further improving the problem of "lithium ion conduction inactivation" faced during the charge and discharge cycle of the secondary battery, thereby further improving the cycle life of the secondary battery, and is also conducive to alleviating the purple spot phenomenon at the head and tail of the negative electrode sheet, making the secondary battery have good kinetic performance.

[0037] In some embodiments of the present application, the particle size Dv10 of the first active material is D 21 , the particle size Dv50 is D 22 , and the particle size Dv90 is D 23 , and the particle size of lithium titanate and the particle size of the first active material satisfy: 8.0 ≤ D 21 / D 11 ≤ 12.0, 7.3 ≤ D 22 / D 12 ≤ 10.4, 8.6 ≤ D 23 / D 13 ≤ 11.8. For example, the value of D 21 / D 11 is 8.0, 8.3, 8.7, 9.0, 9.2, 9.6, 10.0, 10.5, 11.0, 11.3, 11.6, 12.0 or any value between any two of the above numerical ranges. For example, D22 / D 12 The value of is 7.3, 7.4, 7.7, 8.0, 8.3, 8.6, 9.0, 9.6, 9.8, 10.0, 10.4 or any value between any two of the above numerical ranges. For example, D 23 / D 13 The value of is 8.6, 8.8, 9.0, 9.3, 9.7, 10.0, 10.4, 10.6, 11.0, 11.5, 11.8 or any value between any two of the above numerical ranges. Take D 21 / D 11 The value of, D 22 / D 12 The value of and D 23 / D 13 The value of is regulated within the above range. Lithium titanate has a smaller particle size than the first active material. The small-particle lithium titanate is blended with the large-particle first active material, and the small-particle lithium titanate is fully filled into the voids formed by each particle of the large-particle first active material, which can achieve the effect of "interstitial filling" of small particles between large particles, improve the electrical contact between each particle of the first active material in the first material layer, so as to improve the problem of "lithium ion conduction inactivation" faced by the secondary battery using the negative electrode plate during the charge and discharge cycle, slow down the attenuation of the secondary battery capacity, and thus further improve the cycle life of the secondary battery. Thereby, while alleviating the energy density loss, the secondary battery further improves its cycle life. It is also beneficial to make the lithium titanate in the first material layer improve the influence of the alternating expansion stress of the silicon-containing material particles on the XY extension of the negative electrode current collector, alleviate the purple spot phenomenon at the head and tail of the negative electrode plate, and make the secondary battery have good kinetic performance.

[0038] This application has no particular limitation on the particle size Dv10, particle size Dv50, and particle size Dv90 of the first active material, as long as the purpose of this application can be achieved. For example, the particle size of the first active material satisfies: 2.4μm ≤ D 21 ≤ 4.8μm, 4.9μm ≤ D 22 ≤ 11μm, 9.4μm ≤ D 23 ≤ 18μm.

[0039] In the present application, the "particle size Dv10 of lithium titanate" refers to the particle size of lithium titanate particles at which the cumulative volume reaches 10% starting from the smaller particle size side in the particle size distribution based on volume; the "particle size Dv50 of lithium titanate" refers to the particle size of lithium titanate particles at which the cumulative volume reaches 50% starting from the smaller particle size side in the particle size distribution based on volume; the "particle size Dv90 of lithium titanate" refers to the particle size of lithium titanate particles at which the cumulative volume reaches 90% starting from the smaller particle size side in the particle size distribution based on volume. The "particle size Dv10 of the first active material" refers to the particle size of the first active material particles at which the cumulative volume reaches 10% starting from the smaller particle size side in the particle size distribution based on volume; the "particle size Dv50 of the first active material" refers to the particle size of the first active material particles at which the cumulative volume reaches 50% starting from the smaller particle size side in the particle size distribution based on volume; the "particle size Dv90 of the first active material" refers to the particle size of the first active material particles at which the cumulative volume reaches 90% starting from the smaller particle size side in the particle size distribution based on volume.

[0040] In some embodiments of the present application, the single-layer thickness of the first material layer is T1, and 10 μm ≤ T1 ≤ 12 μm. For example, T1 is 10 μm, 10.5 μm, 11 μm, 11.6 μm, 12 μm, or any value between any two of the above numerical ranges. Controlling the single-layer thickness of the first material layer within the above range is beneficial to reducing the total addition amount of lithium titanate in the negative electrode sheet, so that when the negative electrode sheet is applied to a secondary battery, the energy density loss brought by lithium titanate to the secondary battery can be reduced. Thus, on the basis of having a long cycle life, the secondary battery can have a high energy density, and it is also beneficial to alleviate the purple spot phenomenon at the head and tail of the negative electrode sheet, so that the secondary battery has good kinetic performance. It is also beneficial to enable the lithium titanate in the first material layer to improve the influence of the alternating expansion stress of the silicon-containing material particles on the XY extension of the negative electrode current collector, alleviate the purple spot phenomenon at the head and tail of the negative electrode sheet, and make the secondary battery have good kinetic performance.

[0041] In some embodiments of the present application, the total thickness of the negative electrode active material layer is T2, and 12% ≤ T1 / T2 × 100% ≤ 37%. For example, the value of T1 / T2 × 100% is 12%, 15%, 17%, 20%, 22%, 24%, 27%, 30%, 32%, 37%, or any value between any two of the above numerical ranges. Controlling the value of T1 / T2 × 100% within the above range is beneficial to reducing the total addition amount of lithium titanate in the negative electrode sheet, so that when the negative electrode sheet is applied to a secondary battery, the energy density loss brought by lithium titanate to the secondary battery can be reduced. Thus, on the basis of having a long cycle life, the secondary battery can have a high energy density.

[0042] The present application does not particularly limit the total thickness of the negative electrode active material layer, as long as the object of the present application can be achieved. For example, the total thickness of the negative electrode active material layer is 25 μm to 110 μm. The present application does not particularly limit the single-layer thickness of the second material layer, as long as the object of the present application can be achieved. For example, the single-layer thickness of the second material layer is 15 μm to 90 μm.

[0043] In some embodiments of the present application, based on the mass of the negative electrode active material layer, the mass percentage content of silicon element in the negative electrode active material layer is W3, and 3.5% ≤ W3 ≤ 30%. For example, W3 is 3.5%, 8%, 10%, 12.5%, 15%, 17.6%, 22%, 24%, 26%, 27.7%, 30% or any value between any two of the above numerical ranges. The active material containing silicon element has a high energy density and theoretical specific capacity. Controlling the mass percentage content of silicon element in the negative electrode active material layer within the above range is beneficial to enabling the negative electrode sheet to have a high capacity, and the secondary battery using the negative electrode sheet has a high energy density and a long cycle life.

[0044] The present application does not particularly limit the control method of the mass percentage content W3 of silicon element in the negative electrode active material layer, as long as the object of the present application can be achieved. For example, it can be achieved by controlling at least one of the addition amount of the first active material in the first material layer or the addition amount of the second silicon-containing material in the second material layer.

[0045] In some embodiments of the present application, the specific surface area of lithium titanate is 3.1 m 2 / g to 8.2 m 2 / g. For example, the specific surface area of lithium titanate is 3.1 m 2 / g, 3.5 m 2 / g, 3.9 m 2 / g, 4.2 m 2 / g, 4.7 m 2 / g, 5.1 m 2 / g, 5.7 m 2 / g, 6.0 m 2 / g, 6.7 m 2 / g, 7.0 m 2 / g, 7.5 m 2 / g, 8.2 m 2 / g or any value between any two of the above numerical ranges. Controlling the specific surface area of lithium titanate within the above range, lithium titanate has more active sites during the reaction process, which can improve the lithium deintercalation / insertion capacity of lithium titanate and reduce the solid-phase transport impedance, and improve the capacity and rate performance of the secondary battery.

[0046] In some embodiments of the present application, the specific surface area of lithium titanate is 4.2 m 2from 4.2 m² / g to 7.0 m² / g 2 / g. For example, the specific surface area of lithium titanate is 4.2 m 2 / g, 4.7 m 2 / g, 5.1 m 2 / g, 5.7 m 2 / g, 6.0 m 2 / g, 6.7 m 2 / g, 7.0 m 2 / g or any value between any two of the above numerical ranges. By controlling the specific surface area of lithium titanate within the above range, lithium titanate has more active sites during the reaction, further improving the powder conductivity and lithium ion diffusion coefficient of lithium titanate, increasing the lithium insertion / extraction capacity of the negative electrode sheet, and thus improving the energy density of the secondary battery.

[0047] The present application has no particular limitation on the control method of the particle sizes of lithium titanate and the first active material, as long as the purpose of the present application can be achieved. For example, it can be achieved by crushing and screening. The present application has no particular limitation on the control method of the specific surface area of lithium titanate, as long as the purpose of the present application can be achieved. For example, it can be achieved by controlling the particle size of lithium titanate. Generally speaking, the larger the particle size of lithium titanate, the smaller the specific surface area; the smaller the particle size of lithium titanate, the larger the specific surface area.

[0048] In some embodiments of the present application, the powder conductivity of lithium titanate is 10 -6 S / cm to 10 -4 S / cm. Further, the powder conductivity of lithium titanate is 4×10 -5 S / m to 6×10 -5 S / cm. For example, the powder conductivity of lithium titanate is 10 -6 S / cm, 7×10 -6 S / cm, 10 -5 S / cm, 4×10 -5 S / cm, 5×10 -5 S / cm, 6×10 -5 S / cm, 8×10 -5 S / cm, 10 -4 S / cm or any value between any two of the above numerical ranges. By controlling the powder conductivity of lithium titanate within the above range, it is beneficial to improve the rate performance, cycle stability, fast charging performance and mechanical stability of the secondary battery, and also helps to improve the first Coulomb efficiency.

[0049] The present application has no particular limitation on the control method of the powder conductivity of lithium titanate, as long as the purpose of the present application can be achieved. For example, it can be achieved by purchasing commercially available lithium titanate with a powder conductivity within the scope of the present application.

[0050] In some embodiments of the present application, the negative electrode current collector includes any one of a copper current collector, a copper foam current collector, a nickel current collector, or a stainless steel current collector. The negative electrode current collectors of the above types are applied to the negative electrode plate. The XY extension of the negative electrode current collector can be improved, and the probability of purple spots appearing at the head and tail of the negative electrode plate and leakage caused by puncturing the outer package is reduced. The prepared negative electrode plate is applied to a secondary battery, which can alleviate the loss of energy density of the secondary battery while improving the cycle life of the secondary battery, and is also beneficial to alleviating the purple spot phenomenon at the head and tail of the negative electrode plate, making the secondary battery have good kinetic performance.

[0051] The present application does not particularly limit the thickness of the negative electrode current collector, as long as the object of the present application can be achieved. For example, the thickness of the negative electrode current collector is 6 μm to 10 μm. Optionally, the first material layer may further include at least one of a negative electrode conductive agent, a dispersant, or a negative electrode binder. The second material layer may further include at least one of a negative electrode conductive agent, a dispersant, or a negative electrode binder. The present application does not particularly limit the types of the negative electrode conductive agent, the dispersant, and the negative electrode binder in the first material layer and the second material layer, as long as the object of the present application can be achieved. The present application does not particularly limit the mass ratios of the negative electrode conductive agent, the dispersant, and the negative electrode binder in the first material layer and the second material layer, as long as the object of the present application can be achieved. The present application does not particularly limit the contents of the first active material and the second active material, as long as the object of the present application can be achieved. For example, based on the mass of the negative electrode active material layer, the mass percentage content of lithium titanate is 1.5% to 2.5%, the mass percentage content of the first active material is 5% to 20%, the mass percentage content of the second silicon-containing material is 2% to 40%, the mass percentage content of the carbon material is 35% to 90%, the mass percentage content of the negative electrode conductive agent is 0.2% to 0.8%, the mass percentage content of the dispersant is 0.3% to 1.7%, and the mass percentage content of the negative electrode binder is 0.8% to 1.8%.

[0052] The present application does not particularly limit the preparation method of the negative electrode plate, as long as the object of the present application can be achieved. For example, in some embodiments of the present application, the preparation method of the negative electrode plate includes but is not limited to the following steps: (1) preparing a first material layer slurry and a second material layer slurry; (2) uniformly coating the first material layer slurry on one surface of the negative electrode current collector, drying to form the first material layer, and then uniformly coating the second material layer slurry on the surface of the first material layer away from the negative electrode current collector, drying to form the second material layer, thus obtaining a semi-finished negative electrode plate with a single-sided coated negative electrode active material layer; (3) repeating step (2) on the other surface of the negative electrode current collector to obtain a semi-finished negative electrode plate with a double-sided coated negative electrode active material layer, and then obtaining the negative electrode plate after cold pressing and slitting. In some other embodiments of the present application, the semi-finished negative electrode plate can also be cold pressed and slit after step (2) to obtain a negative electrode plate with a single-sided coated negative electrode active material layer.

[0053] The second aspect of the present application provides a secondary battery, which includes the negative electrode sheet described in any of the foregoing embodiments. Therefore, the secondary battery has good kinetic performance, a relatively high energy density, and a long cycle life.

[0054] In some embodiments of the present application, the secondary battery includes a positive electrode sheet. The present application places no particular limitation on the positive electrode sheet, as long as the object of the present application can be achieved. In one embodiment, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer, and the positive electrode active material layer is disposed on one surface or both surfaces of the positive electrode current collector. The above-mentioned "surface" may be a partial surface or the entire surface of the positive electrode current collector. The present application places no particular limitation on the positive electrode current collector, as long as the object of the present application can be achieved. For example, the positive electrode current collector may include aluminum foil, aluminum alloy foil, etc. The positive electrode active material layer of the present application contains a positive electrode active material. The present application places no particular limitation on the type of the positive electrode active material, as long as the object of the present application can be achieved. For example, the positive electrode active material may include at least one of lithium nickel cobalt manganate, lithium nickel cobalt aluminate, lithium iron phosphate, lithium-rich manganese-based material, lithium cobaltate, lithium manganate, lithium manganese iron phosphate, or lithium titanate. In the present application, the positive electrode active material may further contain a non-metal element, and the non-metal element may include at least one of fluorine, phosphorus, boron, chlorine, silicon, or sulfur, and these elements can further improve the stability of the positive electrode active material. In the present application, there is no particular limitation on the thicknesses of the positive electrode current collector and the positive electrode active material layer, 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 a single-layer positive electrode active material layer is 30 μm to 120 μm. Optionally, the positive electrode active material layer may further include at least one of a positive electrode conductive agent or a positive electrode binder. The present application places no particular limitation on the types of the positive electrode conductive agent and the positive electrode binder in the positive electrode active material layer, as long as the object of the present application can be achieved. The present application places no particular limitation on the mass ratio of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder in the positive electrode active material layer, and those skilled in the art can select according to actual needs as long as the object of the present application can be achieved. For example, based on the mass of the positive electrode active material layer, the mass percentage content of the positive electrode active material is 97.5% to 97.9%, the mass percentage content of the positive electrode conductive agent is 0.8% to 1.7%, and the mass percentage content of the positive electrode binder is 0.6% to 1.7%.

[0055] In some embodiments of the present application, the secondary battery further includes a separator disposed between the positive electrode plate and the negative electrode plate to separate the positive electrode plate and the negative electrode plate, prevent internal short circuit of the secondary battery, allow electrolyte ions to pass freely, and not affect the progress of the electrochemical charge and discharge process. The present application has no particular limitation on the separator as long as the object of the present application can be achieved. For example, the material of the separator may include, but is not limited to, at least one of polyolefins (PO) mainly composed of polyethylene (PE) and polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex or aramid. The type of the separator may include at least one of a woven film, a non-woven film, a microporous film, a composite film, a rolled film or a spun film. The present application has no particular limitation on the thickness of the separator as long as the object of the present application can be achieved.

[0056] In some embodiments of the present application, the secondary battery further includes an electrolyte. The present application has no particular limitation on the electrolyte as long as the object of the present application can be achieved. The secondary battery includes an outer package, and the negative electrode plate, the positive electrode plate, the separator and the electrolyte are accommodated in the outer package. The present application has no particular limitation on the outer package as long as the object of the present application can be achieved.

[0057] The secondary battery of the present application is not particularly limited and may include any device that undergoes an electrochemical reaction. For example, the secondary battery may include, but is not limited to: a lithium metal secondary battery, a lithium ion secondary battery (lithium ion battery), a lithium polymer secondary battery or a lithium ion polymer secondary battery.

[0058] The present application has no particular limitation on the preparation method of the secondary battery, and a preparation method well-known in the art can be selected as long as the object of the present application can be achieved. For example, the preparation method of the secondary battery includes, but is not limited to, the following steps: stacking the separator, the positive electrode plate, the separator and the negative electrode plate in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly, placing the electrode assembly into the outer package, injecting the electrolyte into the outer package and sealing it to obtain the secondary battery; or stacking the separator, the positive electrode plate, the separator and the negative electrode plate in sequence, and then fixing the four corners of the entire laminated structure to obtain a laminated electrode assembly, placing the electrode assembly into the outer package, injecting the electrolyte into the outer package and sealing it to obtain the secondary battery.

[0059] The third aspect of the present application provides an electronic device, which includes the secondary battery described in any one of the foregoing embodiments. Therefore, the electronic device has good use performance.

[0060] The electronic device of the present 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 computer, pen-input computer, mobile computer, e-book player, portable phone, portable fax machine, portable copier, portable printer, head-mounted stereo earphone, video recorder, liquid crystal TV, portable cleaner, portable CD player, minidisc, transceiver, electronic notepad, calculator, memory card, portable recorder, radio, backup power supply, motor, automobile, motorcycle, power-assisted bicycle, bicycle, lighting fixture, toy, game machine, clock, power tool, flashlight, camera, large household battery, and lithium-ion capacitor.

[0061] Examples

[0062] Hereinafter, examples and comparative examples are given to more specifically illustrate the embodiments of the present application. Various tests and evaluations are carried out according to the following methods.

[0063] Testing method and device:

[0064] Obtaining the negative electrode sheet:

[0065] Under the condition of 25 °C, a lithium-ion battery that is discharged at a constant current of 0.2C to a cut-off voltage of 3.0V is disassembled, the negative electrode sheet is taken out, soaked in dimethyl carbonate (DMC) for 20 min, and then rinsed once with DMC and acetone in sequence. Then the negative electrode sheet is placed in an oven and baked at 80 °C for 12 h to obtain a processed negative electrode sheet sample. It can be understood that when the voltage range marked on the outer package of the factory battery is 3.0V to 4.45V, the charging cut-off voltage is 4.45V and the discharging cut-off voltage is 3.0V. Unless otherwise specified, the charging cut-off voltage of the lithium-ion battery as an example in the present application is 4.45V and the discharging cut-off voltage is 3.0V.

[0066] Unless otherwise specified, the negative electrode sheets used in the following test methods are obtained by the above method.

[0067] Obtaining lithium titanate and the first active material:

[0068] Take the negative electrode sheet obtained by the aforementioned method and use plasma polishing technology to polish starting from the upper layer of the negative electrode sheet. After polishing and removing 90% of the total thickness of the active material layer of the negative electrode sheet, the remaining material layer on the surface of the negative electrode current collector is the first material layer. After scraping the substances of the remaining material layer from the negative electrode current collector, lithium titanate and the first active material are obtained by differential centrifugation. Differential centrifugation is a method of separation that utilizes the difference in sedimentation rates of different particles in a centrifugal field. Among them, particles with a larger density will sediment first, while particles with a smaller density require a higher centrifugal force and a longer time to sediment. The specific steps are as follows: Make the substances of the first material layer into a suspension. Under the action of gravity or centrifugal force, different substance particles sediment, and according to the sedimentation rates of different substance particles, different substances in the first material layer are separated. The above method for preparing the suspension: Add 100 g of the substances of the first material layer to 1.0 L of deionized water, and under ice bath conditions, use a homogenizer to break up the substances of the first material layer to form a uniform suspension with a concentration of 0.1 g / mL.

[0069] Elemental analysis is respectively carried out on the different substances obtained above. Those containing titanium element are lithium titanate, those containing silicon element are the first active material, and those containing neither silicon element nor titanium element are carbon materials.

[0070] Unless otherwise specified, the lithium titanate and the first active material used in the following test methods are obtained by the above method.

[0071] Measurement of particle size:

[0072] Referring to the national standard GB / T 19077-2016 (《Laser diffraction method for particle size distribution》), use a laser particle size analyzer (such as Malvern Master Size 3000) to measure the particle sizes Dv10, Dv50, and Dv90 of lithium titanate and the first active material.

[0073] Measurement of thickness:

[0074] First, perform cross-section ion polishing treatment on the negative electrode sheet, conduct microscopic analysis on the cross-section through a scanning electron microscope (SEM) to obtain the total thickness T2 of the negative electrode active material layer. After analyzing the titanium element by an energy dispersive spectrometer (EDS) to determine the first material layer, then obtain the single-layer thickness T1 of the first material layer through SEM.

[0075] Measurement of content:

[0076] (1) Measurement of the silicon element content W2 in the first active material:

[0077] The negative electrode plate is polished starting from the surface of the negative electrode plate using plasma polishing technology. After 90% of the thickness of the negative active material layer is polished off, the remaining part of the negative active material layer is the first material layer. The remaining part is scraped off from the surface of the negative current collector, and the mass percentage content A of silicon element in the first material layer can be accurately measured by inductively coupled plasma mass spectrometry (ICP-MS). Among them, the test instrument used for ICP-MS test is the NexlON 350X inductively coupled plasma mass spectrometer of PerkinElmer Corporation.

[0078] Calculation of coating weight: Cut a sample with an area of 1 cm 2 from the negative electrode plate, weigh it on a balance and record it as m1. Then, after removing the second material layer on the sample, weigh it on a balance and record it as m2. Then, after removing the remaining first material layer completely, weigh the negative current collector on a balance and record it as m3;

[0079] If it is a negative electrode plate with a negative active material layer coated on one side, the coating weight CW of the negative active material layer = m1 - m3, and the coating weight CW1 of the first material layer = m2 - m3.

[0080] If it is a negative electrode plate with a negative active material layer coated on both sides, the coating weight CW of the negative active material layer = (m1 - m3) / 2, and the coating weight CW1 of the first material layer = (m2 - m3) / 2.

[0081] The mass percentage content W2 of silicon element in the first active material = A × CW / CW1.

[0082] (2) Measurement of the silicon element content W3 in the negative active material layer:

[0083] After scraping the negative active material layer from the negative electrode plate, perform inductively coupled plasma mass spectrometry (ICP-MS) test, and the content of silicon element in the negative active material layer can be measured. Among them, the test instrument used for ICP-MS test is the NexlON 350X inductively coupled plasma mass spectrometer of PerkinElmer Corporation.

[0084] (3) Measurement of the lithium titanate content:

[0085] Measure the content of titanium element using the test method in "Measurement of the silicon element content in the negative active material layer", and calculate the content of lithium titanate according to the chemical formula Li4Ti5O 12 of lithium titanate. The calculation basis of the lithium titanate content is the negative active material layer.

[0086] Measurement of the specific surface area of lithium titanate:

[0087] The obtained lithium titanate was dried in a vacuum drying oven at 200 °C for 2 h, and then the specific surface area of lithium titanate was measured by nitrogen adsorption method using a specific surface area analyzer (model TristarⅡ3020M) in accordance with the national standard "Determination of Specific Surface Area of Solid Substances by Gas Adsorption BET Method" (GB / T 19587-2017).

[0088] Measurement of powder conductivity:

[0089] Using an FT-8100 series four-probe powder conductivity tester and adopting the four-probe method measurement mode, the powder conductivity of lithium titanate can be measured by single electricity. The specific steps are as follows:

[0090] The obtained lithium titanate was dried in a vacuum drying oven at 200 °C for 2 h, and then loaded into a sample tube and measured in a four-probe powder conductivity tester to obtain the powder conductivity of lithium titanate.

[0091] Measurement of XY elongation rate of the negative current collector:

[0092] The lithium-ion battery was charged at a constant current of 1C to 4.45V in an environment of 25 ± 3 °C, then charged at a constant voltage of 4.45V to 0.025C, and then discharged at a constant current of 0.7C to 3.0V. This is one charge-discharge cycle. The test was carried out according to the above charge-discharge cycle. After 200 cycles, it was charged at a constant current of 1C to 4.45V, and then charged at a constant voltage of 4.45V to 0.025C. The negative electrode plate was disassembled and taken out, and the width of the copper foil in the empty copper foil area was measured as σ0. The widths of the copper foils at the front end (head), middle end (middle), and tail end (tail) of the area with the negative electrode active material layer were measured as σ1, σ2, and σ3 respectively. It should be noted that when the disassembled negative electrode plate is in the unfolded state, the side close to the winding center is the front end, the side far from the winding center is the tail end, and the middle end is located between the front end and the tail end. It can be understood that the winding center can be understood as the part clamped by the winding needle during the winding process of the lithium-ion battery.

[0093] Head XY elongation rate (%) = (σ1 - σ0) / σ0 × 100%;

[0094] Middle XY elongation rate (%) = (σ2 - σ0) / σ0 × 100%;

[0095] Tail XY elongation rate (%) = (σ3 - σ0) / σ0 × 100%;

[0096] Average XY elongation rate (%) = (Head elongation rate + Middle elongation rate + Tail elongation rate) / 3.

[0097] For each example and comparative example, 5 lithium-ion batteries were tested, and the average value of the test results of 5 lithium-ion batteries was taken as the XY elongation level of the current lithium-ion battery design.

[0098] Judgment of the purple spot situation on the interface:

[0099] Place the lithium-ion battery in an incubator at 25°C and let it stand for 60 minutes to make the lithium-ion battery reach a constant temperature. Charge the lithium-ion battery that has reached a constant temperature at a constant current of 1C to 4.45V at 25°C, then charge it at a constant voltage of 4.45V to 0.025C, let it stand for 5 minutes, and then discharge it at a constant current of 1C to 3.0V; this is one charge-discharge cycle. After 200 charge-discharge cycles, charge it at a constant current of 1C to 4.45V again, and then charge it at a constant voltage of 4.45V to 0.025C to obtain a fully charged battery after 200 cycles. Disassemble the lithium-ion battery in a dry room with a humidity of less than 5%, and take pictures to record whether there are purple spots at the corner interface of the negative electrode sheet. After taking pictures, use 3D MAX to measure the area of the purple spots at the two folds at the head and tail of the negative electrode sheet and the total area of the negative electrode active material layer, and calculate the percentage S of the purple spot area in the area of the negative electrode sheet. If S ≤ 0.01%, record it as "no purple spot"; if 0.01% < S ≤ 0.5%, record it as "slight purple spot"; if 0.5% < S ≤ 1.5%, record it as "moderate purple spot"; if 1.5% < S, record it as "severe purple spot". It can be understood that the meaning of the above "two folds at the head and tail" is the part with two winding layers formed at the head or tail of the negative electrode sheet in the lithium-ion battery with a winding structure. It should be noted that the lithium-ion batteries in the embodiments and comparative examples of the present application are taken as an example of the winding structure, but the present application is not limited thereto.

[0100] Test of energy density:

[0101] First, measure the size of the lithium-ion battery to obtain the thickness T, width W, and length L, and calculate the volume V through the following formula: V = T × W × L.

[0102] Then, charge the lithium-ion battery according to the following operation process and then discharge it to obtain the discharge capacity E of the lithium-ion battery:

[0103] Charging: Charge at a constant current of 1C to 4.45V, and then charge at a constant voltage of 4.45V to 0.025C;

[0104] Discharging: Discharge at a constant current of 0.2C to 3.0V to obtain the discharge energy E.

[0105] The volume energy density (VED) of the lithium-ion battery can be calculated through the following formula: VED (Wh / L) = E / V.

[0106] Test of cycle life:

[0107] Take 5 lithium-ion batteries from each example and comparative example, charge them at a constant current of 1C to 4.45V in an environment of 25±3°C, then charge them at a constant voltage of 4.45V to 0.025C, and then discharge them at a constant current of 0.7C to 3.0V. This is one charge-discharge cycle, and the test is carried out according to the above charge-discharge cycle. The number of cycles required for the lithium-ion battery capacity to decline to 80% of the first-cycle discharge capacity is used as the cycle life of the lithium-ion battery. Take the average value of 5 lithium-ion batteries as the measured value.

[0108] Example 1-1

[0109] <Preparation of negative electrode sheet>

[0110] Lithium titanate, the first active material silicon-carbon compound, carbon material artificial graphite, negative electrode binder polyacrylic acid, negative electrode conductive agent single-walled carbon nanotube, and dispersant carboxymethyl cellulose were mixed evenly according to a mass ratio of 5.7:22.8:64.4:5.4:0.3:1.4, and then deionized water was added and stirred evenly to prepare a first material layer slurry with a solid content of 38wt%.

[0111] The second active material artificial graphite and silicon-carbon compound (Dv50 = 8.6μm), negative electrode binder polyacrylic acid, negative electrode conductive agent single-walled carbon nanotube, and dispersant carboxymethyl cellulose were mixed evenly according to a mass ratio of 77.6:19.4:2.4:0.2:0.4, and then deionized water was added and stirred evenly to prepare a second material layer slurry with a solid content of 40wt%.

[0112] The first material layer slurry was evenly coated on the negative electrode current collector copper foil (copper current collector), dried at 85°C to form a first material layer; the second material layer slurry was evenly coated on the surface of the first material layer away from the copper foil, and dried at 85°C to form a second material layer; thus, a semi-finished negative electrode sheet with a single-sided coated negative electrode active material layer was obtained; the above steps were repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coated negative electrode active material layer. Then, it was cold-pressed and slit into a negative electrode sheet with a specification of 76mm×856mm for use.

[0113] Among them, based on the mass of the negative electrode active material layer, the mass percentage content W1 of lithium titanate, the mass percentage content W2 of silicon element in the first active material, and the mass percentage content W3 of silicon element in the negative electrode active material layer are shown in Table 1. The particle size Dv10 of lithium titanate, i.e., D 11 , particle size Dv50, i.e., D 12 , particle size Dv90, i.e., D 13 , the specific surface area of lithium titanate, the particle size Dv10 of the first active material, i.e., D 21 , particle size Dv50, i.e., D 22 , particle size Dv90, i.e., D 23, as shown in Table 2. The single-layer thickness T1 of the first material layer and the total thickness T2 of the negative electrode active material layer are as shown in Table 4.

[0114] <Preparation of the positive electrode tab>

[0115] Mix the positive electrode active material lithium cobaltate, the positive electrode conductive agent conductive carbon black, and the positive electrode binder polyvinylidene fluoride (PVDF, weight average molecular weight is 5×10 5 ) in a mass ratio of 97.8:1.4:0.8, add N-methylpyrrolidone (NMP) as a solvent, and stir in a vacuum blender until a positive electrode slurry with a solid content of 75 wt% and a homogeneous system is obtained. Coat the positive electrode slurry evenly on one surface of a positive electrode current collector aluminum foil with a thickness of 6 μm, and dry it at 90 °C to obtain a positive electrode tab with a single-sided coated positive electrode active material layer (thickness 80 μm). Then, repeat the above steps on the other surface of the aluminum foil to obtain a positive electrode tab with a double-sided coated positive electrode active material layer. After cold pressing and slitting, a positive electrode tab with a specification of 74 mm×851 mm is obtained for use.

[0116] <Preparation of the separator>

[0117] Use a polyethylene (PE) porous film with a thickness of 8 μm as the separator.

[0118] <Preparation of the electrolyte>

[0119] In a dry argon atmosphere, mix the organic solvents ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a mass ratio of 30:50:20 to obtain a basic electrolyte, and then add lithium hexafluorophosphate as a lithium salt to the basic electrolyte and dissolve and mix evenly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0120] <Preparation of the lithium-ion battery>

[0121] Stack the above-prepared separator, positive electrode tab, separator, and negative electrode tab in sequence and wind them to obtain an electrode assembly. Place the electrode assembly in an aluminum-plastic film outer package, inject the electrolyte after drying, and obtain a lithium-ion battery through processes such as vacuum packaging, standing, formation, degassing, and edge trimming.

[0122] Examples 1-2 to Examples 1-4

[0123] Except for adjusting the relevant preparation parameters according to Table 1, the rest are the same as Example 1-1.

[0124] Among them, when the mass percentage content W1 of lithium titanate changes, the mass percentage content of artificial graphite in the first material layer changes accordingly, and the mass percentage content W2 of the first active material and the mass percentage contents of the negative electrode binder, negative electrode conductive agent, and dispersant remain unchanged.

[0125] The mass percentage content W2 of silicon element in the first active material is achieved by regulating the content of the first active material. When the mass percentage content of the first active material changes, the mass percentage content of artificial graphite in the first material layer changes accordingly, while the mass percentage content W1 of lithium titanate and the mass percentage contents of the negative electrode binder, negative electrode conductive agent and dispersant remain unchanged. The mass percentage contents of artificial graphite and silicon carbide in the second material layer change accordingly, so that the mass percentage content W3 of silicon element in the negative electrode active material layer remains unchanged.

[0126] Examples 1-5 to 1-9

[0127] Except for adjusting the relevant preparation parameters according to Table 1, the rest are the same as in Example 1-1.

[0128] In Examples 1-5 to 1-9, the mass percentage content W3 of silicon element in the negative electrode active material layer is achieved by regulating the content of the second silicon-containing material in the second active material. When the mass percentage content of the second silicon-containing material changes, the mass percentage content of artificial graphite in the second active material changes accordingly, while the mass percentage contents of the negative electrode binder, negative electrode conductive agent and dispersant in the second material layer remain unchanged.

[0129] Examples 2-1 to 2-15

[0130] Except for adjusting the relevant preparation parameters according to Table 2, the rest are the same as in Example 1-2.

[0131] Examples 3-1 to 3-8

[0132] Except for adjusting the relevant preparation parameters according to Table 4, the rest are the same as in Example 1-2.

[0133] Among them, in Examples 3-1 to 3-4, when the single-layer thickness of the first material layer changes, the single-layer thickness of the second material layer changes accordingly so that the total thickness of the negative electrode active material layer remains unchanged; when the single-layer thicknesses of the first material layer and the second material layer change, the mass percentage content W1 of lithium titanate, the mass percentage content W2 of silicon element in the first active material, and the mass percentage content W3 of silicon element in the negative electrode active material layer are made to remain unchanged by regulating the mass percentage contents of artificial graphite in the first material layer and the second material layer.

[0134] In Examples 3-5 to 3-8, when the total thickness of the negative electrode active material layer changes, the single-layer thickness of the first material layer remains unchanged, and the single-layer thickness of the second material layer changes accordingly. When the total thickness of the negative electrode active material layer changes, by adjusting the contents of lithium titanate, the first active material, and artificial graphite in the first material layer, and the contents of artificial graphite and silicon carbide compound in the second material layer, the mass percentage content W1 of lithium titanate, the mass percentage content W2 of silicon element in the first active material, and the mass percentage content W3 of silicon element in the negative electrode active material layer are made to remain unchanged.

[0135] Example 4-1

[0136] Except for adjusting the relevant preparation parameters according to Table 5, the rest are the same as in Example 1-2.

[0137] Comparative Examples 1 to 4

[0138] Except for adjusting the relevant preparation parameters according to Table 1, the rest are the same as in Example 1-1.

[0139] Among them, when the mass percentage content W1 of lithium titanate changes, the mass percentage content of artificial graphite in the first material layer changes accordingly, and the mass percentage content W2 of the first active material and the mass percentage contents of the negative electrode binder, negative electrode conductive agent, and dispersant remain unchanged.

[0140] The mass percentage content W2 of silicon element in the first active material is achieved by adjusting the content of the first active material. When the mass percentage content of the first active material changes, the mass percentage content of artificial graphite in the first material layer changes accordingly, and the mass percentage content W1 of lithium titanate and the mass percentage contents of the negative electrode binder, negative electrode conductive agent, and dispersant remain unchanged. The mass percentage contents of artificial graphite and silicon carbide compound in the second material layer change accordingly to keep the mass percentage content W3 of silicon element in the negative electrode active material layer unchanged.

[0141] Comparative Example 5

[0142] Except for preparing the negative electrode plate according to the following steps, the rest are the same as in Example 1-2.

[0143] <Preparation of Negative Electrode Plate>

[0144] Mix artificial graphite and silicon carbide compound (Dv50 = 8.6 μm), negative electrode binder polyacrylic acid, negative electrode conductive agent single-walled carbon nanotube, and dispersant carboxymethyl cellulose in a mass ratio of 87.3:9.7:2.6:0.2:0.2, and then add deionized water and stir evenly to make a negative electrode slurry with a solid content of 38 wt%.

[0145] The negative electrode slurry is evenly coated on the negative electrode current collector copper foil, and after drying at 85 °C, a negative electrode active material layer is formed; the above steps are repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coated negative electrode active material layer. Then, it is cold-pressed and slit into a negative electrode sheet with a specification of 76 mm × 856 mm for standby. Among them, the total thickness of the negative electrode active material layer is 46 μm.

[0146] Comparative Example 6

[0147] Except for preparing the negative electrode sheet according to the following steps, the rest is the same as in Examples 1-2.

[0148] <Preparation of Negative Electrode Sheet>

[0149] Lithium titanate, artificial graphite, negative electrode binder polyacrylic acid, negative electrode conductive agent single-walled carbon nanotubes, and dispersant carboxymethyl cellulose are mixed evenly according to a mass ratio of 8.1:83.2:5.7:2.8:0.2, and then deionized water is added and stirred evenly to prepare a first material layer slurry with a solid content of 38 wt%.

[0150] The second active material artificial graphite and silicon carbide compound (Dv50 = 8.6 μm), negative electrode binder polyacrylic acid, negative electrode conductive agent single-walled carbon nanotubes, and dispersant carboxymethyl cellulose are mixed evenly according to a mass ratio of 67.3:20:9.7:2.8:0.2, and then deionized water is added and stirred evenly to prepare a second material layer slurry with a solid content of 38 wt%.

[0151] The first material layer slurry is evenly coated on the negative electrode current collector copper foil, and after drying at 85 °C, a first material layer is formed; the second material layer slurry is evenly coated on the surface of the first material layer away from the copper foil, and after drying at 85 °C, a second material layer is formed; that is, a semi-finished negative electrode sheet with a single-sided coated negative electrode active material layer is obtained; the above steps are repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coated negative electrode active material layer. Then, it is cold-pressed and slit into a negative electrode sheet with a specification of 76 mm × 856 mm for standby.

[0152] Among them, based on the mass of the negative electrode active material layer, the mass percentage content W1 of lithium titanate and the mass percentage content W3 of silicon element in the negative electrode active material layer are shown in Table 1. The particle size Dv10 of lithium titanate, that is, D 11 、particle size Dv50, that is, D 12 、particle size Dv90, that is, D 13 are the same as in Examples 1-1, and the specific surface area of lithium titanate is the same as in Examples 1-1. The single-layer thickness T1 of the first material layer, the single-layer thickness of the second material layer, and the total thickness T2 of the negative electrode active material layer are the same as in Examples 1-1.

[0153] Comparative Example 7

[0154] Except for adjusting the relevant preparation parameters according to Table 1, the rest are the same as in Comparative Example 3.

[0155] Among them, the mass percentage content W2 of silicon element in the first active material is achieved by regulating the content of the first active material. When the mass percentage content of the first active material changes, the mass percentage content of artificial graphite in the first material layer changes accordingly, while the mass percentage content W1 of lithium titanate and the mass percentage contents of the negative electrode binder, negative electrode conductive agent, and dispersant remain unchanged. The mass percentage contents of artificial graphite and silicon carbide in the second material layer change accordingly to keep the mass percentage content W3 of silicon element in the negative electrode active material layer unchanged.

[0156] The preparation parameters and performance data of each example and comparative example are shown in Tables 1 to 5.

[0157] Table 1

[0158]

[0159]

[0160] Note: The "\ " in Table 1 indicates no corresponding parameter.

[0161] It can be seen from Examples 1-1 to 1-4 and Comparative Examples 1 to 7 that in this application, a first material layer containing lithium titanate and a first active material is provided between the negative electrode current collector and the second material layer, and the mass percentage content of lithium titanate in the negative electrode active material layer and the ratio of its mass percentage content to the mass percentage content of silicon element in the first active material are regulated within the scope of this application, so that the negative electrode current collector has a small XY elongation rate at the head, middle and tail, the purple spot situation at the head and tail of the negative electrode pole is relatively slight, and the secondary battery has a high energy density and a long cycle life, indicating that the secondary battery of this application can take into account good kinetic performance, high energy density and long cycle life. For the secondary batteries of the comparative examples, the first material layer is not provided or the first material layer does not contain the first active material, the mass percentage content of lithium titanate in the negative electrode active material layer or the ratio of the mass percentage content of lithium titanate in the negative electrode active material layer to the mass percentage content of silicon element in the first active material is not within the scope of this application, the negative electrode current collector has a larger XY elongation rate at the head, middle and tail, the purple spot phenomenon at the head and tail of the negative electrode pole is more serious, or the energy density of the secondary battery is lower, or the cycle life of the secondary battery is shorter, indicating that the secondary battery cannot take into account kinetic performance, energy density and cycle life. Among them, when comparing Comparative Examples 2 and 4 with Example 1-2, the reason for the significant decrease in energy density is that the conductivity of lithium titanate increases with the increase of the lithium intercalation amount. When the secondary battery discharges to a low state of charge (SOC≤15%), the conductivity of lithium titanate decreases significantly, which will affect the conductive network of the silicon-containing material and the carbon material at the end of the secondary battery discharge, resulting in the inability to release the capacity of the silicon-containing material and the carbon material normally. When the content of lithium titanate is too high (such as W1>2.5%), the conductivity of the conductive network of the silicon-containing material and the carbon material at the end of the secondary battery discharge is extremely poor, which will cause the energy density of the secondary battery to decrease. The reason why the cycle life does not decrease is that during the cycle life test of the secondary battery, a relatively large discharge rate of 0.7C is used for discharge, and the polarization (such as charge transfer polarization and concentration polarization) is relatively large during the discharge process, and less lithium ions are deintercalated, so there is no impact on the cycle life.

[0162] The mass percentage content W3 of silicon element in the negative electrode active material layer usually affects the kinetic performance, energy density and cycle life of the secondary battery. It can be seen from Example 1-2 and Examples 1-5 to 1-8 that for the secondary battery with the mass percentage content W3 of silicon element in the negative electrode active material layer within the scope of this application, the negative electrode current collector has a small XY elongation rate at the head, middle and tail, the purple spot situation at the head and tail of the negative electrode pole is relatively slight, and the secondary battery has good kinetic performance, high energy density and long cycle life.

[0163] The type of silicon-containing material usually affects the kinetic performance, energy density and cycle life of the secondary battery. It can be seen from Examples 1-2 and 1-9 that the secondary battery using the type of silicon-containing material within the scope of the present application has a smaller XY elongation at the head, middle and tail of the negative electrode current collector, and the purple spots at the head and tail of the negative electrode pole piece are relatively mild, and the secondary battery has good kinetic performance, higher energy density and longer cycle life.

[0164] Table 2

[0165]

[0166]

[0167] Table 3

[0168]

[0169]

[0170] The particle size Dv10, particle size Dv50, and particle size Dv90 of lithium titanate usually affect the dynamic performance, energy density, and cycle life of the secondary battery. It can be seen from Examples 1-2, 2-1, 2-3 to 2-11, 2-14, and 2-15 that when the particle size Dv10, particle size Dv50, and particle size Dv90 of lithium titanate are within the scope of this application, the specific surface area and powder conductivity of lithium titanate are also within the scope of this application. The secondary battery with the particle size Dv10, particle size Dv50, and particle size Dv90 of lithium titanate, the specific surface area and powder conductivity of lithium titanate within the scope of this application, the head, middle, and tail of the negative electrode current collector have a smaller XY elongation, the purple spots at the head and tail of the negative electrode pole piece are relatively mild, and the secondary battery has good dynamic performance, high energy density, and long cycle life.

[0171] Relationship between the particle size of lithium titanate and the particle size of the first active material D 21 / D 11 , D 22 / D 12 and D 23 / D 13 The value of usually affects the kinetic performance, energy density and cycle life of the secondary battery. 21 / D 11 , D 22 / D 12 and D 23 / D 13The secondary battery with a value within the scope of the present application has a smaller XY elongation rate at the head, middle, and tail of the negative electrode current collector, and the purple spot situation at the head and tail of the negative electrode tab is relatively mild. The secondary battery has good kinetic performance, a high energy density, and a long cycle life.

[0172] Table 4

[0173]

[0174] The single-layer thickness T1 of the first material layer usually affects the kinetic performance, energy density, and cycle life of the secondary battery. It can be seen from Examples 1-2, Examples 3-1 to 3-4 that for the secondary battery with the single-layer thickness T1 of the first material layer within the scope of the present application, the head, middle, and tail of the negative electrode current collector have a smaller XY elongation rate, and the purple spot situation at the head and tail of the negative electrode tab is relatively mild. The secondary battery has good kinetic performance, a high energy density, and a long cycle life. When the mass percentage content W1 of lithium titanate, the mass percentage content W2 of silicon element in the first active material, and the mass percentage content W3 of silicon element in the negative electrode active material layer remain unchanged, and the total thickness of the negative electrode active material layer remains unchanged, when the first material layer becomes thicker, it means that the content of LTO and Si in the "lower layer (the first material layer)" decreases, LTO in the lower layer is more dispersed, the alleviating effect on the swelling of silicon in the lower layer weakens, and the XY elongation becomes worse; the "interstitial filling" effect of LTO becomes better, the compaction density of the negative electrode tab increases, and the energy density of the secondary battery increases; however, the effect of the dispersed LTO on improving the silicon conductive network becomes worse, so the cycle life of the secondary battery becomes shorter.

[0175] The value of T1 / T2×100% representing the relationship between the single-layer thickness T1 of the first material layer and the total thickness T2 of the negative electrode active material layer usually affects the kinetic performance, energy density, and cycle life of the secondary battery. As can be seen from Examples 1-2, Examples 3-1 to Examples 3-8, for a secondary battery with the value of T1 / T2×100% representing the relationship between the single-layer thickness T1 of the first material layer and the total thickness T2 of the negative electrode active material layer within the scope of this application, the XY elongation rates of the head, middle, and tail of the negative electrode current collector are small, the purple spot conditions at the head and tail of the negative electrode tab are relatively mild, and the secondary battery has good kinetic performance, a relatively high energy density, and a long cycle life. With the mass percentage content W1 of lithium titanate, the mass percentage content W2 of silicon element in the first active material, and the mass percentage content W3 of silicon element in the negative electrode active material layer remaining unchanged, when the total thickness of the negative electrode active material layer increases, the thickness of the upper layer (the second material layer) increases while the thickness of the lower layer (the first material layer) remains unchanged. LTO in the lower layer becomes more concentrated, enhancing the alleviating effect on the silicon expansion in the lower layer and improving the XY elongation; the "interstitial" effect of LTO becomes worse, the compaction density of the negative electrode tab decreases, and the energy density of the secondary battery decreases. However, due to the increase in the total thickness of the negative electrode active material layer, the energy density of the secondary battery generally increases; the effect of the more concentrated LTO on improving the silicon conductive network becomes better. However, looking at the entire negative electrode tab, the absolute content of silicon in the upper layer is too high, which will lead to a shorter cycle life of the secondary battery.

[0176] Table 5

[0177]

[0178] The type of the negative electrode current collector usually affects the kinetic performance, energy density, and cycle life of the secondary battery. As can be seen from Examples 1-2 and Examples 4-1, for a secondary battery with the type of the negative electrode current collector within the scope of this application, the XY elongation rates of the head, middle, and tail of the negative electrode current collector are small, the purple spot conditions at the head and tail of the negative electrode tab are relatively mild, and the secondary battery has good kinetic performance, a relatively high energy density, and a long cycle life.

[0179] It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements but also includes other elements not expressly listed, or further includes elements inherent to such process, method, article or device.

[0180] Each embodiment in this specification is described in a related manner. For the same or similar parts among the embodiments, reference can be made to each other. Each embodiment focuses on the differences from other embodiments.

[0181] The above are only the preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. A negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer comprises a first material layer and a second material layer, wherein the first material layer is disposed between the negative electrode current collector and the second material layer; and the second material layer contains silicon element; The first material layer includes lithium titanate and a first active material, and the first active material includes a first silicon-containing material; Based on the mass of the negative electrode active material layer, the mass percentage of the lithium titanate is W1, the mass percentage of silicon in the first active material is W2, and W1 and W2 satisfy: 1.5%≤W1≤2.5%, and 1.5≤W2 / W1≤4.

0.

2. The negative electrode sheet according to claim 1, wherein: The particle size Dv10 of the lithium titanate is D 11 、Dv50 is D 12 、Dv90 is D 13 , where 0.3μm≤D 11 ≤0.4μm, 0.7μm≤D 12 ≤1.0μm, 1.1μm≤D 13 ≤1.5μm.

3. The negative electrode sheet according to claim 2, wherein: The particle size Dv10 of the first active material is D 21 、Dv50 is D 22 、Dv90 is D 23 The particle size of the lithium titanate and the particle size of the first active material satisfy: 8.0≤D 21 / D 11 ≤12.0,7.3≤D 22 / D 12 ≤10.4,8.6≤D 23 / D 13 ≤11.

8.

4. The negative electrode sheet according to claim 1, wherein: The single layer thickness of the first material layer is T1, 10 μm≤T1≤12 μm.

5. The negative electrode sheet according to claim 4, wherein: The total thickness of the negative electrode active material layer is T2, 12%≤T1 / T2×100%≤37%.

6. The negative electrode sheet according to any one of claims 1 to 5, wherein: The negative electrode plate satisfies at least one of the following characteristics: (1) Based on the mass of the negative electrode active material layer, the mass percentage of silicon in the negative electrode active material layer is W3, 3.5%≤W3≤30%; (2)2.4≤W2 / W1≤4.

0.

7. The negative electrode sheet according to any one of claims 1 to 5, wherein: The specific surface area of ​​the lithium titanate is 3.1 m 2 / g to 8.2m 2 / g.

8. The negative electrode sheet according to any one of claims 1 to 5, wherein: The powder conductivity of lithium titanate is 10 - 6 S / m to 10 -4 S / m.

9. The negative electrode sheet according to any one of claims 1 to 5, wherein: The lithium titanate satisfies at least one of the following characteristics: (1) The specific surface area of ​​the lithium titanate is 4.2 m 2 / g to 7.0m 2 / g; (2) The electrical conductivity of the lithium titanate powder is 4×10 -5 S / m to 6×10 -5 S / m.

10. The negative electrode sheet according to any one of claims 1 to 5, wherein: The second material layer includes a second active material, the second active material includes a second silicon-containing material, and the second silicon-containing material includes at least one of a silicon-carbon compound, a silicon-oxygen compound, or pure silicon.

11. The negative electrode sheet according to any one of claims 1 to 5, wherein: The negative electrode current collector includes any one of a copper current collector, a foam copper current collector, a nickel current collector or a stainless steel current collector.

12. A secondary battery, wherein: The secondary battery comprises the negative electrode sheet according to any one of claims 1 to 11.

13. An electronic device, wherein: The electronic device includes the secondary battery according to claim 12.

Images