Secondary battery and electronic device

Abstract

The invention provides a secondary battery and an electronic device, a negative electrode piece of the secondary battery comprises a negative electrode current collector and a negative electrode active material layer, the negative electrode active material layer comprises a first negative electrode material layer and a second negative electrode material layer, the first negative electrode material layer is arranged between the negative electrode current collector and the second negative electrode material layer, and the second negative electrode material layer is arranged between the first negative electrode material layer and the second negative electrode material layer. The second negative electrode material layer contains a silicon element; the first negative electrode material layer comprises lithium titanate and a first active material, and the first active material comprises a first silicon-containing material; after the secondary battery is formed, a solid electrolyte interface film is formed on the surface of lithium titanate, the solid electrolyte interface film comprises Li2O and ROCO2Li, and R is selected from C1 to C5 alkyl groups; the mass ratio of Li2O in the solid electrolyte interface film is a, the mass ratio of ROCO2Li in the solid electrolyte interface film is b, and b / a is larger than or equal to 8 and smaller than or equal to 24. By means of the arrangement, the secondary battery can give consideration to energy density, cycle life and expansion rate.

Description

Technical Field

[0001] The present application relates to the field of electrochemical technologies, and particularly to 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 material has a higher energy density and a higher theoretical specific capacity compared with the traditional graphite negative electrode. However, during the cycling process of lithium-ion batteries, the volume of silicon material changes greatly. Affected by the alternating expansion stress of silicon material particles, the negative electrode current collector is prone to extension along the length direction and the width direction, resulting in purple spots at the head and tail of the negative electrode pole piece and puncturing the outer package, which may cause safety problems.

[0003] In order to overcome the above problems, the prior art often coats lithium titanate (LTO) on the surface of the negative electrode current collector to improve its extension. However, due to the relatively high required LTO content, the first Coulombic efficiency of LTO in lithium-ion batteries is only about 20% at 3V to 4.53V, resulting in a reduction in the first-cycle charge-discharge efficiency of lithium-ion batteries and serious irreversible capacity loss, causing a huge loss of energy density. In order to solve the problems of low first-cycle charge-discharge efficiency and serious irreversible capacity loss of lithium-ion batteries, lithium supplementation is an effective solution idea. However, after lithium supplementation, the stability of the positive electrode pole piece and the negative electrode pole piece is relatively low, which is likely to affect the cycling performance of lithium-ion batteries. Based on this, how to balance the energy density and cycling performance of lithium-ion batteries has become a technical problem that needs to be solved urgently by those skilled in the art. Summary of the Invention

[0004] The purpose of the present application is to provide a secondary battery and an electronic device to balance the energy density, cycle life, and expansion rate 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 secondary battery. The secondary battery includes a negative electrode plate, a positive electrode plate, and a separator disposed between the negative electrode plate and the positive electrode plate. 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 negative electrode material layer and a second negative electrode material layer. The first negative electrode material layer is disposed between the negative electrode current collector and the second negative electrode material layer, and the second negative electrode material layer contains silicon elements. The first negative electrode material layer includes lithium titanate and a first active material, and the first active material includes a first silicon-containing material. A solid electrolyte interface (SEI) film is formed on the surface of the lithium titanate. The solid electrolyte interface film includes Li2O and ROCO2Li, where R is selected from C1 to C5 alkyl groups. The mass ratio of Li2O in the solid electrolyte interface film is a, and the mass ratio of ROCO2Li in the solid electrolyte interface film is b, and 8 ≤ b / a ≤ 24. In the secondary battery provided by the first aspect of the present application, in the negative electrode plate, by disposing the first negative electrode material layer including lithium titanate and the first active material and the second negative electrode material layer containing silicon elements on the surface of the negative electrode current collector as the negative electrode active material layer, 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 direction and the width direction (hereinafter referred to as XY extension), 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 plate and piercing the outer package to cause liquid leakage is reduced. The fact that lithium titanate is only disposed in the first negative electrode material layer also reduces the amount of lithium titanate used compared with the prior art, thereby reducing the energy density loss brought by lithium titanate to the secondary battery. During the charge and discharge cycle of the secondary battery, the silicon material in the negative electrode active material layer continuously expands and contracts, resulting in a decrease in the contact tightness between the silicon material particles. The poor contact between the particles of the first active material in the first negative electrode material layer and the negative electrode current collector causes the problem of lithium ion conduction inactivation, and the capacity of the secondary battery decays rapidly. 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 the inactivated particles of the first active material, improving the electrical contact between the particles of the first active material in the first negative electrode material layer. In this way, applying the negative electrode plate to the secondary battery can improve the problem of "lithium ion conduction inactivation" 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.After the secondary battery is formed, a solid electrolyte interface film is formed on the surface of lithium titanate, and the ratio b / a of the mass percentages of ROCO2Li and Li2O in the solid electrolyte interface film is controlled within the scope of this application. This indicates that the content of the inorganic component Li2O in the solid electrolyte interface film on the surface of lithium titanate is relatively high, but the gas generation during the formation of the solid electrolyte interface film is less. In this way, the formed solid electrolyte interface film has good mechanical and chemical stability, which can reduce the erosion of the electrolyte on lithium titanate during the cycling process of the secondary battery, improve the stability of lithium titanate particles, and play a good role in reducing the XY extension of the negative electrode current collector by lithium titanate during the cycling process of the entire life cycle of the secondary battery. The good mechanical and chemical stability of the solid electrolyte interface film is also conducive to reducing the swelling rate during the charge and discharge cycling process of the secondary battery, and the negative electrode sheet has a stable structure that enables lithium ions to be smoothly inserted. Therefore, through the above settings, the secondary battery can take into account energy density, cycle life, and swelling rate.

[0007] In some embodiments of this application, 13 ≤ b / a ≤ 17. By controlling the value of b / a within the above range, the secondary battery can further reduce the swelling rate while having a relatively high energy density and a long cycle life.

[0008] In some embodiments of this application, based on the mass of the negative electrode active material layer, the mass percentage of lithium titanate is W1, and the mass percentage 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. By controlling the mass percentage of lithium titanate in the negative electrode active material layer and the ratio of its mass percentage to the mass percentage of silicon element in the first active material within the scope of this application, the secondary battery has a relatively high energy density and a long cycle life on the basis of having a relatively small swelling rate.

[0009] In some embodiments of this application, the separator includes a polymer layer and a prelithiation layer provided on one surface of the polymer layer, and the prelithiation layer includes a lithium supplement agent; based on the mass of the prelithiation layer, the mass percentage of the delithiation product of the lithium supplement agent is 0.5% to 2.5%. By selecting the above-mentioned separator containing a lithium supplement agent and controlling the content of the delithiation product of the lithium supplement agent in the prelithiation layer within the above range, the separator can provide active lithium during the charge and discharge cycling process of the secondary battery, and the secondary battery has a relatively high energy density on the basis of having a long cycle life and a low swelling rate.

[0010] In some embodiments of this application, the thickness of the polymer layer is 4 μm to 5 μm, and the thickness of the prelithiation layer is 5 μm to 15 μm. By controlling the thicknesses of the polymer layer and the prelithiation layer within the above range, the secondary battery has a relatively high energy density on the basis of having a long cycle life and a low swelling rate.

[0011] In some embodiments of the present application, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a first positive electrode material layer and a second positive electrode material layer, and the first positive electrode material layer is provided between the positive electrode current collector and the second positive electrode material layer; the first positive electrode material layer includes a lithium supplement agent, and based on the mass of the positive electrode active material layer, the mass percentage content of the de-lithiated product of the lithium supplement agent is 0.5% to 2.0%. By providing the first positive electrode material layer containing the lithium supplement agent in the positive electrode sheet and controlling the content of the de-lithiated product of the lithium supplement agent within the above range, the positive electrode sheet can provide active lithium during the charge and discharge cycle of the secondary battery. On the basis of the secondary battery having a long cycle life and a low expansion rate, it has a high energy density.

[0012] In some embodiments of the present application, the separator includes a polymer layer and a prelithiated layer provided on one surface of the polymer layer. The prelithiated layer includes a lithium supplement agent; based on the mass of the prelithiated layer, the mass percentage content of the de-lithiated product of the lithium supplement agent is 0.5% to 2.5%. The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a first positive electrode material layer and a second positive electrode material layer, and the first positive electrode material layer is provided between the positive electrode current collector and the second positive electrode material layer; the first positive electrode material layer includes a lithium supplement agent, and based on the mass of the positive electrode active material layer, the mass percentage content of the de-lithiated product of the lithium supplement agent is 0.5% to 2.0%.

[0013] In some embodiments of the present application, the de-lithiated products in the separator and the positive electrode sheet independently include Li x FeO 0.5(2+x) 、Li 2-z MnO2、Li 1.2-r Ni 0.13 Fe 0.13 Mn 0.54 O2 or Li 1-t FePO4, and 1≤x≤6, 1.6≤z≤2, 1≤r≤1.2, 0.8≤t≤1. The de-lithiated products of the above types indicate that the lithium supplement agent releases oxygen during the formation process of the secondary battery, forming a solid electrolyte interface film rich in inorganic component Li2O with good mechanical and chemical stability on the surface of lithium titanate.

[0014] In some embodiments of the present application, the particle size Dv50 of the lithium supplement agent is 1.8 μm to 2.5 μm. By controlling the particle size of the lithium supplement agent within the above range, the secondary battery has a high energy density on the basis of having a long cycle life and a low expansion rate.

[0015] In some embodiments of the present application, the thickness of the first negative electrode material layer is T1, where 10 μm ≤ T1 ≤ 12 μm. By controlling the single-layer thickness of the first negative electrode material layer within the above range, the secondary battery can have a relatively high energy density while having a long cycle life.

[0016] 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%. By controlling the value of T1 / T2 × 100% within the above range, the secondary battery can have a relatively high energy density while having a long cycle life.

[0017] The second 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 performance in use.

[0018] Advantages of the present application:

[0019] The present application provides a secondary battery and an electronic device. The secondary battery includes a negative electrode plate, a positive electrode plate, and a separator disposed between the negative electrode plate and the positive electrode plate. 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 negative electrode material layer and a second negative electrode material layer. The first negative electrode material layer is disposed between the negative electrode current collector and the second negative electrode material layer, and the second negative electrode material layer contains silicon. The first negative electrode material layer includes lithium titanate and a first active material, and the first active material includes a first silicon-containing material. A solid electrolyte interface film is formed on the surface of the lithium titanate, and the solid electrolyte interface film includes Li2O and ROCO2Li, where R is selected from C1 to C5 alkyl groups. The mass ratio of Li2O in the solid electrolyte interface film is a, and the mass ratio of ROCO2Li in the solid electrolyte interface film is b, and 8 ≤ b / a ≤ 24. In the negative electrode plate, by disposing the first negative electrode material layer including lithium titanate and the first active material and the second negative electrode material layer including a silicon-containing material on the surface of the negative electrode current collector as the negative electrode active material layer, the 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, making the XY extension of the negative electrode current collector smaller, and reducing the probability of purple spots appearing at the head and tail of the negative electrode plate and piercing the outer package to cause liquid leakage. The fact that the lithium titanate is only disposed in the first negative electrode material layer also reduces the amount of lithium titanate used compared with the prior art, thereby reducing the energy density loss brought by the lithium titanate to the secondary battery. During the charge and discharge cycle of the secondary battery, the silicon material in the negative electrode active material layer continuously expands and contracts, resulting in a decrease in the contact tightness between the silicon material particles. The poor contact between the particles of the first active material in the first negative electrode material layer and the negative electrode current collector causes the problem of lithium ion conduction inactivation, and the capacity of the secondary battery decays rapidly. The blending of lithium titanate with the first active material can play an effect of "filling gaps" between the first active materials. The lithium titanate bridges the inactivated particles of the first active material, improving the electrical contact between the particles of the first active material in the first negative electrode material layer. In this way, applying the negative electrode plate to the 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 decay of the secondary battery capacity, and thus improve the cycle life of the secondary battery.A solid electrolyte interface film is formed on the surface of lithium titanate, and the ratio b / a of the mass ratios of ROCO2Li and Li2O in the solid electrolyte interface film is regulated within the scope of this application, indicating that the content of the inorganic component Li2O in the solid electrolyte interface film on the surface of lithium titanate is relatively high but the gas generation during the formation of the solid electrolyte interface film is less. In this way, the formed solid electrolyte interface film has good mechanical and chemical stability, which can reduce the erosion of the electrolyte on lithium titanate during the cycling of the secondary battery, improve the stability of lithium titanate particles, and play a good role in reducing the XY extension of the negative electrode current collector of lithium titanate during the entire life cycle of the secondary battery. The good mechanical and chemical stability of the solid electrolyte interface film is also beneficial to reducing the swelling rate during the charge and discharge cycling of the secondary battery. Therefore, through the above settings, the secondary battery can balance the energy density, cycle life, and swelling rate.

[0020] Of course, it is not necessary for any product or method implementing this application to achieve all the above-mentioned advantages simultaneously. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the technical solutions in the embodiments of this application, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of this application, and those of ordinary skill in the art can also obtain other embodiments based on these drawings.

[0022] Figure 1 It is a schematic cross-sectional structure diagram of a negative electrode plate along its thickness direction and length direction for some embodiments of this application;

[0023] Figure 2 It is a schematic cross-sectional structure diagram of a separator along its thickness direction and length direction for some embodiments of this application;

[0024] Figure 3 It is a schematic cross-sectional structure diagram of a positive electrode plate along its thickness direction and length direction for some embodiments of this application.

[0025] Reference numerals: 100 - negative electrode plate; 10 - negative electrode current collector; 11 - negative electrode active material layer; 111 - first negative electrode material layer; 112 - second negative electrode material layer; 200 - separator; 201 - polymer layer; 202 - prelithiation layer; 300 - positive electrode plate; 30 - positive electrode current collector; 31 - positive electrode active material layer; 311 - first positive electrode material layer; 312 - second positive electrode material layer. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0026] 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 of 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.

[0027] It should be noted that in the specific embodiments of the present application, a lithium-ion battery is used 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. The specific technical solutions are as follows:

[0028] In the first aspect of the present application, a secondary battery is provided. The secondary battery includes a negative electrode plate, a positive electrode plate, and a separator disposed between the negative electrode plate and the positive electrode plate. 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 negative electrode material layer and a second negative electrode material layer. The first negative electrode material layer is disposed between the negative electrode current collector and the second negative electrode material layer, and the second negative electrode material layer contains silicon. The first negative electrode material layer includes lithium titanate (LTO) and a first active material, and the first active material includes a first silicon-containing material. After the secondary battery is formed, a solid electrolyte interface (SEI) film is formed on the surface of the lithium titanate. The solid electrolyte interface film includes Li2O and ROCO2Li, where R is selected from C1 to C5 alkyl groups. Further, the above "C1 to C5 alkyl groups" are selected from any one of methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, or n-pentyl groups. The mass ratio of Li2O in the solid electrolyte interface film is a, and the mass ratio of ROCO2Li in the solid electrolyte interface film is b, and 8 ≤ b / a ≤ 24. Further, 32% ≤ b ≤ 79.1%, and 1.2% ≤ a ≤ 6.6%.

[0029] For ease of understanding, in the present application, the self-length direction of the negative electrode plate in the unfolded state 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 the directions defined in the present application can be understood according to the relative positions of the accompanying drawings and the actual product elements. Moreover, the self-length direction and the self-thickness direction of the negative electrode current collector, the first negative electrode material layer, the second negative electrode material layer, the separator, and the positive electrode plate 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, and the above "surface" can be a partial surface or the entire surface of the negative electrode current collector. For example Figure 1As shown, the negative electrode plate 100 includes a negative electrode current collector 10 and a negative electrode active material layer 11. The negative electrode active material layer 11 is disposed on two surfaces of the negative electrode current collector 10. The negative electrode active material layer 11 includes a first negative electrode material layer 111 and a second negative electrode material layer 112. The first negative electrode material layer 111 is disposed between the negative electrode current collector 10 and the second negative electrode material layer 112. The thickness of the first negative electrode material layer 111 is shown as T1, and the thickness of the second negative electrode material layer 112 is shown as T3. The total thickness of the negative electrode active material layer 11 is the sum of the thickness of the first negative electrode material layer 111 and the thickness of the second negative electrode material layer 112. The total thickness of the negative electrode active material layer 11 is shown as T2. It should be noted that T1, T2, and T3 in this application are all the thicknesses on the side of the negative electrode current collector. Of course, it can be understood that in some embodiments of this application, the negative electrode active material layer 11 is disposed on one surface of the negative electrode current collector 10.

[0030] For example, the value of b / a is 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or any value between any two of the above numerical ranges. After the secondary battery is formed, a solid electrolyte interface film is formed on the surface of the lithium titanate. The solid electrolyte interface film contains a stable inorganic component Li2O and an organic component ROCO2Li. The presence of the inorganic component Li2O can improve the mechanical and chemical stability of the solid electrolyte interface film on the surface of the lithium titanate, so as to reduce the erosion of the electrolyte on the lithium titanate during the cycling of the secondary battery. When the value of b / a is less than 8, it indicates that when the total mass of ROCO2Li and Li2O in the solid electrolyte interface film is constant, the relative content of ROCO2Li is too small and the relative content of Li2O is too large. It also means that too much gas (such as oxygen O2) is generated during the formation of the solid electrolyte interface film, which is likely to damage the structure of the negative electrode plate and affect the lithium intercalation reaction, thereby affecting the energy density and cycling performance of the secondary battery. When the value of b / a is greater than 24, it indicates that when the total mass of ROCO2Li and Li2O in the solid electrolyte interface film is constant, the relative content of ROCO2Li is too large and the relative content of Li2O is too small. The mechanical and chemical stability of the solid electrolyte interface film cannot be improved. The solid electrolyte interface film formed on the surface of the lithium titanate during the cycling of the secondary battery is not sufficient to resist the erosion of the electrolyte on the lithium titanate, and the effect of the lithium titanate on improving the extension of the negative electrode current collector cannot be exerted.

[0031] Overall, for the secondary battery provided in the first aspect of the present application, in the negative electrode sheet, a first negative electrode material layer including lithium titanate and a first active material and a second negative electrode material layer including a silicon-containing material are provided on the surface of the negative electrode current collector as the negative electrode active material layer. 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 direction and the width direction (hereinafter referred to as XY extension), 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. The fact that lithium titanate is only provided in the first negative electrode material layer also reduces the amount of lithium titanate compared with the prior art, thereby reducing the energy density loss brought by lithium titanate to the secondary battery. In the negative electrode active material layer, the silicon material expands and contracts continuously during the charge and discharge cycle of the secondary battery, resulting in a decrease in the contact tightness between the silicon material particles. The poor contact between the first active material particles in the first negative electrode material layer and the negative electrode current collector causes the problem of lithium ion conduction deactivation, and the capacity of the secondary battery decays rapidly. The blending of lithium titanate and the first active material can play a role of "filling gaps" between the first active materials. Lithium titanate builds a bridge between the deactivated first active material particles, improving the electrical contact between the first active material particles in the first negative electrode material layer. In this way, applying the negative electrode sheet to the 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 secondary battery capacity, and thus improve the cycle life of the secondary battery. After the secondary battery is formed, a solid electrolyte interface film is formed on the surface of lithium titanate, and the ratio b / a of the mass ratio of ROCO2Li and Li2O in the solid electrolyte interface film is regulated within the scope of the present application, indicating that the content of the inorganic component Li2O in the solid electrolyte interface film on the surface of lithium titanate is relatively high but the gas generation during the formation of the solid electrolyte interface film is less. In this way, the formed solid electrolyte interface film has good mechanical and chemical stability, which can reduce the erosion of the electrolyte on lithium titanate during the cycle of the secondary battery, improve the stability of lithium titanate particles, and play a good role in reducing the XY extension of the negative electrode current collector by lithium titanate during the entire life cycle of the secondary battery. The good mechanical and chemical stability of the solid electrolyte interface film is also beneficial to reducing the expansion rate during the charge and discharge cycle of the secondary battery, and the negative electrode sheet also has a stable structure to enable lithium ions to be smoothly embedded. Therefore, through the above settings, the secondary battery can take into account energy density, cycle life, and expansion rate.

[0032] In some embodiments of the present application, 13 ≤ b / a ≤ 17. For example, the value of b / a is 13, 14, 15, 16, 17, or any value between any two of the above numerical ranges. By controlling the value of b / a within the above range, after the secondary battery is formed, the content of the inorganic component Li2O in the SEI film formed on the surface of lithium titanate is relatively high, but the gas generation during the formation of the solid electrolyte interface film is less. In this way, the formed solid electrolyte interface film has good mechanical and chemical stability, which can reduce the erosion of the electrolyte on lithium titanate during the cycling of the secondary battery, improve the stability of lithium titanate particles, and play a good role in reducing the XY extension of the negative electrode current collector by lithium titanate during the cycling of the entire life cycle of the secondary battery. The good mechanical and chemical stability of the solid electrolyte interface film is also beneficial to reducing the swelling rate during the charge and discharge cycling of the secondary battery, and the negative electrode sheet has a stable structure that enables lithium ions to be smoothly inserted. Therefore, the secondary battery can further reduce the swelling rate while having a high energy density and a long cycle life.

[0033] In the present application, the mass ratio a of Li2O in the solid electrolyte interface film is usually achieved by controlling the content of the lithium supplement in the positive electrode sheet and / or the separator. The greater the content of the lithium supplement, the greater a. The mass ratio b of ROCO2Li in the solid electrolyte interface film is usually achieved by controlling the content of the lithium supplement in the positive electrode sheet and / or the separator. The smaller the content of the lithium supplement, the greater b.

[0034] In some embodiments of the present application, 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 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. For example, the value of W2 / W1 is 1.5, 1.7, 2.0, 2.2, 2.4, 2.6, 2.8, 3.0, 3.1, 3.3, 3.4, 3.7, 3.8, 4.0 or any value between any two of the above numerical ranges. By controlling 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 silicon material particles on the XY extension of the negative electrode current collector, making the XY extension of the negative electrode current collector smaller, and reducing the probability of purple spots appearing at the head and tail of the negative electrode tab and piercing the outer package to cause leakage. It also enables lithium titanate to have a smaller total addition amount in the negative electrode plate, 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. During the charge and discharge cycle of the secondary battery, the silicon material in the negative electrode active material layer continuously expands and contracts, resulting in a decrease in the contact tightness between silicon material particles. The poor contact between each particle of the first active material in the first negative electrode material layer and the negative electrode current collector causes the problem of lithium ion conduction deactivation, and the capacity of the secondary battery decays rapidly. The blending of lithium titanate with the first active material can play an effect of "filling gaps" between the first active materials. Lithium titanate bridges the deactivated particles of the first active material, improving the electrical contact between each particle of the first active material in the first negative electrode 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 secondary battery capacity, and thus improve the cycle life of the secondary battery. Therefore, the secondary battery has a smaller expansion rate, a higher energy density, and a longer cycle life on this basis.

[0035] The present application has no particular limitation on the value of W2, as long as the purpose of the present application can be achieved. For example, 2.25% ≤ W2 ≤ 10%.

[0036] The present application has no particular limitation on the control method of the mass percentage content of silicon element in the first active material, as long as the purpose of the present application can be achieved. For example, it can be achieved by controlling the proportion of the first active material in the negative electrode active material layer or by controlling the type of the first active material.

[0037] 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 negative electrode material layer includes a second active material, and the second active material includes a second silicon-containing material. The present application does not particularly limit the type of the second silicon-containing material, as long as the object of the present application can be achieved. For example, the second 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 active material includes a second silicon-containing material and a carbon material. The present application does not particularly limit the mass ratio of the second silicon-containing material and the carbon material in the second active material, and those skilled in the art can select according to actual needs, as long as the object of the present application can be achieved. In some embodiments of the present application, the first negative electrode material layer includes lithium titanate, a first active material, and a carbon material. The present application does not particularly limit the type of the 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, or soft carbon. 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 SiO y , where 0 < y < 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 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.

[0038] In some embodiments of the present application, the separator includes a polymer layer and a prelithiation layer provided on one surface of the polymer layer, and the prelithiation layer includes a lithium supplement agent. As Figure 2As shown, the separator 200 includes a polymer layer 201 and a prelithiated layer 202, and the prelithiated layer 202 is disposed on one surface of the polymer layer 201. Based on the mass of the prelithiated layer, the mass percentage content of the delithiated product of the lithium supplementing agent is 0.5% to 2.5%. For example, based on the mass of the prelithiated layer, the mass percentage content of the delithiated product of the lithium supplementing agent is 0.5%, 0.6%, 0.9%, 1.1%, 1.3%, 1.5%, 1.7%, 1.8%, 2.0%, 2.1%, 2.2%, 2.3%, 2.5%, or any value between any two of the above numerical ranges. The initial Coulombic efficiency of LTO is only about 20% at 3V to 4.53V of the secondary battery, which easily leads to a reduction in the charge-discharge efficiency of the first cycle of the secondary battery and serious irreversible capacity loss. By selecting the above-mentioned separator containing a lithium supplementing agent and controlling the content of the delithiated product of the lithium supplementing agent in the prelithiated layer within the above range, the separator can provide active lithium during the charge-discharge cycle of the secondary battery to compensate for the loss of active lithium caused by LTO with a low initial Coulombic efficiency in the first negative electrode material layer of the negative electrode sheet, thereby improving the energy density of the secondary battery. Thus, the secondary battery has a high energy density on the basis of having a long cycle life and a low expansion rate.

[0039] The present application has no particular limitation on the composition of the polymer layer as long as the object of the present application can be achieved. For example, the polymer layer includes a polymer. In some embodiments of the present application, the prelithiated layer includes a lithium supplementing agent and a polymer. The present application has no particular limitation on the type of the above polymer as long as the object of the present application can be achieved. For example, the polymer includes at least one of polyolefin polymers, polyacrylonitrile polymers, or polycarboxylate polymers. The present application has no particular limitation on the types of polyolefin polymers, polyacrylonitrile polymers, and polycarboxylate polymers as long as the object of the present application can be achieved. Exemplarily, the polymer includes at least one of polyvinylidene fluoride-hexafluoropropylene copolymer, poly(vinylidene carbonate), polycyanoacrylate, polymethacrylate, polyacrylonitrile, or polymaleic anhydride.

[0040] In some embodiments of the present application, the thickness of the polymer layer is 4μm to 5μm, and the thickness of the prelithiated layer is 5μm to 15μm. As Figure 2 shown, the thickness of the polymer layer 201 is denoted as T 201 shown, the thickness of the prelithiated layer 202 is denoted as T 202As shown. For example, the thickness of the polymer layer is 4 μm, 4.1 μm, 4.3 μm, 4.4 μm, 4.5 μm, 4.7 μm, 4.8 μm, 5 μm, or any value between any two of the above numerical ranges. For example, the thickness of the prelithiation layer is 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, or any value between any two of the above numerical ranges. By controlling the thicknesses of the polymer layer and the prelithiation layer within the above ranges, the separator has a smaller thickness, which reduces the probability of energy density loss of the secondary battery caused by the separator thickness while providing active lithium during the charge and discharge cycles of the secondary battery. Thus, the secondary battery has a higher energy density on the basis of having a longer cycle life and a lower swelling rate.

[0041] In some embodiments of the present application, the positive electrode plate includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a first positive electrode material layer and a second positive electrode material layer, and the first positive electrode material layer is disposed between the positive electrode current collector and the second positive electrode material layer. The above "positive electrode active material layer disposed on at least one surface of the positive electrode current collector" means that the positive electrode active material layer is disposed on one surface or two surfaces of the positive electrode current collector, and the above "surface" may be a partial surface or the entire surface of the positive electrode current collector. As Figure 3 shown, the positive electrode plate 300 includes a positive electrode current collector 30 and a positive electrode active material layer 31. The positive electrode active material layer 31 is disposed on two surfaces of the positive electrode current collector 30. The positive electrode active material layer 31 includes a first positive electrode material layer 311 and a second positive electrode material layer 312, and the first positive electrode material layer 311 is disposed between the positive electrode current collector 30 and the second positive electrode material layer 312. Of course, it can be understood that in some embodiments of the present application, the positive electrode active material layer 31 is disposed on one surface of the positive electrode current collector 30. The first positive electrode material layer includes a lithium supplement agent, and based on the mass of the positive electrode active material layer, the mass percentage content of the delithiated product of the lithium supplement agent is 0.5% to 2.0%. For example, based on the mass of the positive electrode active material layer, the mass percentage content of the delithiated product of the lithium supplement agent is 0.5%, 0.6%, 0.9%, 1.1%, 1.3%, 1.5%, 1.7%, 1.8%, 2.0%, or any value between any two of the above numerical ranges. By disposing the first positive electrode material layer containing a lithium supplement agent in the positive electrode plate and controlling the content of the delithiated product of the lithium supplement agent within the above range, the positive electrode plate can provide active lithium during the charge and discharge cycles of the secondary battery to compensate for the loss of active lithium caused by the low first Coulomb efficiency LTO in the first negative electrode material layer of the negative electrode plate, thereby improving the energy density of the secondary battery. Thus, the secondary battery has a higher energy density on the basis of having a longer cycle life and a lower swelling rate.

[0042] In some embodiments of the present application, the separator includes a polymer layer and a prelithiated layer disposed on one surface of the polymer layer, and the prelithiated layer includes a lithium supplement agent. Based on the mass of the prelithiated layer, the mass percentage content of the delithiated product of the lithium supplement agent is 0.5% to 2.5%. For example, based on the mass of the prelithiated layer, the mass percentage content of the delithiated product of the lithium supplement agent is 0.5%, 0.6%, 0.9%, 1.1%, 1.3%, 1.5%, 1.7%, 1.8%, 2.0%, 2.1%, 2.2%, 2.3%, 2.5% or any value between any two of the above numerical ranges. The positive electrode plate includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a first positive electrode material layer and a second positive electrode material layer, and the first positive electrode material layer is disposed between the positive electrode current collector and the second positive electrode material layer. The first positive electrode material layer includes a lithium supplement agent. Based on the mass of the positive electrode active material layer, the mass percentage content of the delithiated product of the lithium supplement agent is 0.5% to 2.0%. For example, based on the mass of the positive electrode active material layer, the mass percentage content of the delithiated product of the lithium supplement agent is 0.5%, 0.6%, 0.9%, 1.1%, 1.3%, 1.5%, 1.7%, 1.8%, 2.0% or any value between any two of the above numerical ranges. The lithium supplement agent is provided simultaneously in the separator and the positive electrode plate to provide active lithium during the charge and discharge cycles of the secondary battery, so as to make up for the loss of active lithium caused by the low first Coulomb efficiency LTO in the first negative electrode material layer of the negative electrode plate, thereby improving the energy density of the secondary battery.

[0043] It should be noted that more than one delithiated product is often generated after the lithium supplement agent is delithiated. Since the content of some secondary products is very small, the delithiated product in the present application refers to the main product generated after the lithium supplement agent is delithiated. When testing the mass percentage content of the delithiated product in the present application, only the mass percentage content of the main product with the largest content is included in the mass percentage content of the delithiated product, and the content of the secondary product is not included in the mass percentage content of the delithiated product. In some embodiments of the present application, the delithiated products in the separator and the positive electrode plate independently include Li x FeO 0.5(2+x) , Li 2-z MnO2, Li 1.2- r Ni 0.13 Fe 0.13 Mn 0.54 O2 or Li 1-tAt least one of FePO4, where 1 ≤ x ≤ 6, 1.6 ≤ z ≤ 2, 1 ≤ r ≤ 1.2, 0.8 ≤ t ≤ 1. For example, x is 1, 2, 3, 4, 5, 6 or any value between any two of the above numerical ranges. For example, z is 1.6, 1.7, 1.8, 1.9, 2 or any value between any two of the above numerical ranges. For example, r is 1, 1.1, 1.2 or any value between any two of the above numerical ranges. For example, t is 0.8, 0.9, 1 or any value between any two of the above numerical ranges. The delithiated products of the above types indicate that the lithium supplement releases oxygen during the formation process of the secondary battery, forming a solid electrolyte interface film rich in inorganic component Li2O with good mechanical and chemical stability on the surface of lithium titanate.

[0044] The present application does not particularly limit the types of the lithium supplement in the separator and the positive electrode sheet, as long as the object of the present application can be achieved. For example, the lithium supplement in the separator and the lithium supplement in the positive electrode sheet are each independently selected from Li2FeO3, Li2FeO4, Li3FeO4, Li5FeO4, Li2MnO2, LiFePO4 or Li 1.2 Ni 0.13 Fe 0.13 Mn 0.54 At least one of O2.

[0045] In some embodiments of the present application, the particle size Dv50 of the lithium supplement is 1.8 μm to 2.5 μm. For example, the particle size Dv50 of the lithium supplement is 1.8 μm, 1.9 μm, 2.0 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm or any value between any two of the above numerical ranges. Controlling the particle size of the lithium supplement within the above range is beneficial to making the pre-lithiation layer or the first positive electrode material layer containing the lithium supplement have an appropriate thickness, and is also beneficial to the uniform distribution of the lithium supplement in the pre-lithiation layer or the first positive electrode material layer, so as to provide more active lithium during the charge and discharge process of the secondary battery, thereby improving the energy density of the secondary battery. Thus, the secondary battery has a high energy density on the basis of having a long cycle life and a low expansion rate.

[0046] The present application does not particularly limit the particle sizes of lithium titanate and the first active material, as long as the object of the present application can be achieved. For example, the particle size Dv10 of lithium titanate is D 11 、the particle size Dv50 is D 12 、the particle size Dv90 is D 13 , where 0.3 μm ≤ D 11 ≤ 0.4 μm, 0.6 μm ≤ D 12 ≤ 0.8 μm, 1.2 μm ≤ D 13 ≤ 1.5 μm. For example, the particle size of the first active material satisfies: 2.4 μm ≤ D21 ≤4.8 μm, 4.8 μm ≤ D 22 ≤9.6 μm, 9.6 μm ≤ D 23 ≤18 μm.

[0047] In the present application, the particle size Dv10 refers to the particle size at which, in the particle size distribution based on volume, starting from the small particle size side, the cumulative volume reaches 10%. The particle size Dv50 refers to the particle size at which, in the particle size distribution based on volume, starting from the small particle size side, the cumulative volume reaches 50%. The particle size Dv90 refers to the particle size at which, in the particle size distribution based on volume, starting from the small particle size side, the cumulative volume reaches 90%. The above-mentioned "particles" can be any one of the lithium supplement agent, lithium titanate or the first active material.

[0048] In some embodiments of the present application, as Figure 1 shown, the thickness of the first negative electrode material layer 111 is T1, 10 μm ≤ T1 ≤ 12 μm. For example, T1 is 10 μm, 10.4 μm, 10.6 μm, 11 μm, 11.2 μm, 11.5 μm, 11.7 μm, 12 μm or any value between any two of the above numerical ranges. Controlling the thickness of the first negative electrode material layer within the above range is beneficial to reducing the total addition amount of lithium titanate in the negative electrode plate, 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. Thus, the secondary battery can have a high energy density on the basis of having a long cycle life.

[0049] In some embodiments of the present application, as Figure 1 shown, the total thickness of the negative electrode active material layer 11 is T2, 12% ≤ T1 / T2 × 100% ≤ 37%. For example, the value of T1 / T2 × 100% is 12%, 15%, 17%, 20%, 22%, 25%, 26%, 28%, 30%, 32%, 33%, 35%, 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 plate, 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. Thus, the secondary battery can have a high energy density on the basis of having a long cycle life.

[0050] In the present application, there is no particular limitation on the total thickness of the negative electrode active material layer, as long as the object of the present application can be achieved. For example, 27 μm ≤ T2 ≤ 100 μm. There is no particular limitation on the thickness of the second negative electrode material layer in the present application, as long as the object of the present application can be achieved. For example, the thickness of the second negative electrode material layer is from 15 μm to 90 μm.

[0051] The present application has no particular limitation on 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 negative electrode material layer may further include at least one of a negative electrode conductive agent, a dispersant, or a negative electrode binder. The second negative electrode material layer may further include at least one of a negative electrode conductive agent, a dispersant, or a negative electrode binder. The present application has no particular limitation on the types of the negative electrode conductive agent, the dispersant, and the negative electrode binder in the first negative electrode material layer and the second negative electrode material layer, as long as the object of the present application can be achieved. The present application has no particular limitation on the mass ratios of the negative electrode conductive agent, the dispersant, and the negative electrode binder in the first negative electrode material layer and the second negative electrode material layer, as long as the object of the present application can be achieved. The present application has no particular limitation on the content of the second active material, as long as the object of the present application can be achieved. For example, based on the mass of the first negative electrode material layer, the mass percentage content of lithium titanate is 15% to 25%, the mass percentage content of the first active material is 73.7% to 83.3%, the mass percentage content of the carbon material is 0% to 10%, the mass percentage content of the negative electrode conductive agent is 0.2% to 0.3%, the mass percentage content of the dispersant is 0.3% to 0.4%, and the mass percentage content of the negative electrode binder is 0.8% to 1.0%. Based on the mass of the second negative electrode material layer, the mass percentage content of the second active material is 98.3% to 98.7%, the mass percentage content of the negative electrode conductive agent is 0.2% to 0.3%, the mass percentage content of the dispersant is 0.3% to 0.4%, and the mass percentage content of the negative electrode binder is 0.8% to 1.0%.

[0052] The present application has 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 has 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 manganese oxide, lithium nickel cobalt aluminate, lithium iron phosphate, lithium-rich manganese-based material, lithium cobalt oxide, lithium manganese oxide, lithium manganese iron phosphate, or lithium titanate, etc. In the present application, the positive electrode active material may further contain a non-metallic element, and the non-metallic 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 thickness 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. Optionally, the first positive electrode material layer may further include at least one of a positive electrode conductive agent or a positive electrode binder, and the second positive electrode material layer may further include at least one of a positive electrode conductive agent or a positive electrode binder. The present application has no particular limitation on the types of the positive electrode conductive agent and the positive electrode binder in the first positive electrode material layer and the second positive electrode material layer, as long as the object of the present application can be achieved. The present application has 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 first positive electrode material layer and the second positive electrode 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 first positive electrode material layer, the mass percentage content of the positive electrode active material is 72.5% to 82.9%, the mass percentage content of the lithium supplement agent is 15% to 25%, 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 1.0% to 2.0%. Based on the mass of the second positive electrode 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 1.0% to 2.0%.

[0053] The present application does not particularly limit the preparation method of the negative electrode sheet, as long as the purpose of the present application can be achieved. For example, the preparation method of the negative electrode sheet includes but is not limited to the following steps: (1) preparing a first negative electrode material layer slurry and a second negative electrode material layer slurry; (2) uniformly coating the first negative electrode material layer slurry on one surface of the negative electrode current collector, drying to form the first negative electrode material layer, and then uniformly coating the second negative electrode material layer slurry on the surface of the first negative electrode material layer away from the negative electrode current collector, drying to form the second negative electrode material layer, thus obtaining a semi-finished negative electrode sheet 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 sheet with a double-sided coated negative electrode active material layer, and obtaining the negative electrode sheet after cold pressing and slitting. In some other embodiments of the present application, the semi-finished negative electrode sheet can also be cold pressed and slit after the above step (2) to obtain a negative electrode sheet with a single-sided coated negative electrode active material layer.

[0054] The present application does not particularly limit the preparation method of the positive electrode sheet, as long as the purpose of the present application can be achieved. For example, the preparation method of the positive electrode sheet includes but is not limited to the following steps: (1) preparing a first positive electrode material layer slurry and a second positive electrode material layer slurry; (2) uniformly coating the first positive electrode material layer slurry on one surface of the positive electrode current collector, drying to form the first positive electrode material layer, and then uniformly coating the second positive electrode material layer slurry on the surface of the first positive electrode material layer away from the positive electrode current collector, drying to form the second positive electrode material layer, thus obtaining a semi-finished positive electrode sheet with a single-sided coated positive electrode active material layer; (3) repeating step (2) on the other surface of the positive electrode current collector to obtain a semi-finished positive electrode sheet with a double-sided coated positive electrode active material layer, and obtaining the positive electrode sheet after cold pressing and slitting. In some other embodiments of the present application, the semi-finished positive electrode sheet can also be cold pressed and slit after the above step (2) to obtain a positive electrode sheet with a single-sided coated positive electrode active material layer.

[0055] The present application does not particularly limit the preparation method of the separator, as long as the purpose of the present application can be achieved. For example, the preparation method of the separator includes but is not limited to the following steps: (1) adding a lithium supplement agent and a polymer into an organic solvent, stirring to obtain a first spinning solution, performing electrospinning on the first spinning solution, and drying the electrospun product at 60 °C to 70 °C for 0.5 h to 1.5 h to obtain a prelithiated layer; (2) mixing a polymer and an organic solution to obtain a second spinning solution, performing electrospinning on the second spinning solution on one surface of the prelithiated layer to form a polymer layer, and obtaining the separator after drying the product at 60 °C to 70 °C for 0.5 h to 1.5 h. The prelithiated layer faces the positive electrode sheet.

[0056] The present application does not particularly limit the types of the above-mentioned organic solvents, as long as the purpose of the present application can be achieved. For example, the organic solvents include, but are not limited to, at least one of N,N-dimethylformamide (DMF), N,N-dimethylacetamide, acetone, N-methylpyrrolidone, hexafluoroisopropanol, or tetrahydrofuran. The present application does not particularly limit the types of the above-mentioned lithium supplement agents, as long as the purpose of the present application can be achieved. For example, at least one of Li2FeO3, Li2FeO4, Li3FeO4, Li5FeO4, Li2MnO2, LiFePO4, or Li 1.2 Ni 0.13 Fe 0.13 Mn 0.54 at least one of O2. The present application does not particularly limit the types of the polymers in the above-mentioned polymer layer, as long as the purpose of the present application can be achieved. For example, the polymers in the polymer layer include at least one of polyolefin polymers, polyacrylonitrile polymers, or polycarboxylate polymers. The present application does not particularly limit the types of polyolefin polymers, polyacrylonitrile polymers, and polycarboxylate polymers, as long as the purpose of the present application can be achieved. Exemplarily, the polymers include at least one of polyvinylidene fluoride-hexafluoropropylene copolymer, poly(vinylidene carbonate), polycyanoacrylate, polymethacrylate, polyacrylonitrile, or polymaleic anhydride. The present application does not particularly limit the types of the polymers in the above-mentioned prelithiation layer, as long as the purpose of the present application can be achieved. For example, the polymers in the prelithiation layer include at least one of polyolefin polymers, polyacrylonitrile polymers, or polycarboxylate polymers. The present application does not particularly limit the types of polyolefin polymers, polyacrylonitrile polymers, and polycarboxylate polymers, as long as the purpose of the present application can be achieved. Exemplarily, the polymers include at least one of polyvinylidene fluoride-hexafluoropropylene copolymer, poly(vinylidene carbonate), polycyanoacrylate, polymethacrylate, polyacrylonitrile, or polymaleic anhydride. However, the types of the polymers in the polymer layer and the prelithiation layer need to be different.

[0057] The present application does not particularly limit the preparation process parameters of the above-mentioned electrospinning. Those skilled in the art can select according to actual needs, as long as the purpose of the present application can be achieved. For example, the solid content of the first spinning solution is 8 wt% to 15 wt%, and the mass ratio of the main components, the lithium supplement agent to the polymer, is (1.0 to 2.7):(97.3 to 99.0). Based on the mass of the prelithiation layer, the mass percentage of the lithium supplement agent is 1.0% to 2.7%, and the mass percentage of the polymer is 97.3% to 99.0%. The solid content of the second spinning solution is 8 wt% to 15 wt%.

[0058] The secondary battery of the present application is not particularly limited, and it may include any device that undergoes an electrochemical reaction. For example, the secondary battery may include, but is not limited to: lithium metal secondary batteries, lithium ion secondary batteries (lithium ion batteries), lithium polymer secondary batteries, or lithium ion polymer secondary batteries.

[0059] The present application does not particularly limit the manufacturing method of the secondary battery, and a manufacturing method well-known in the art can be selected as long as the object of the present application can be achieved. For example, the manufacturing method of the secondary battery includes, but is not limited to, the following steps: stacking a separator, a positive electrode sheet, a separator, and a negative electrode sheet in sequence, and winding, folding, etc. as needed to obtain a wound electrode assembly, placing the electrode assembly in an outer package, injecting an electrolyte into the outer package and sealing it to obtain a secondary battery; or, stacking a separator, a positive electrode sheet, a separator, and a negative electrode sheet in sequence, and then fixing the four corners of the entire laminated structure to obtain a laminated electrode assembly, placing the electrode assembly in an outer package, injecting an electrolyte into the outer package and sealing it to obtain a secondary battery.

[0060] The second 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.

[0061] The electronic device of the present application is not particularly limited, and it may be any electronic device known in the prior art. For example, the electronic device may include, but is not limited to: laptop computers, pen input computers, mobile computers, e-book players, mobile phones, portable fax machines, portable copiers, portable printers, head-mounted stereo headphones, video recorders, liquid crystal televisions, portable cleaners, portable CD players, mini discs, transceivers, electronic notebooks, calculators, memory cards, portable tape recorders, radios, backup power supplies, motors, automobiles, motorcycles, motorized bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium ion capacitors.

[0062] Examples

[0063] 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.

[0064] Testing methods and equipment:

[0065] Sampling of the negative electrode sheet:

[0066] At 25 °C, the lithium-ion battery is discharged at 0.2C to the discharge cut-off voltage, and the lithium-ion battery is disassembled under an argon atmosphere. The negative electrode plate, the positive electrode plate, and the separator are taken out. After soaking the negative electrode plate, the positive electrode plate, and the separator in dimethyl carbonate (DMC) for 20 min, they are rinsed once with DMC and then once with acetone in sequence. Then, the negative electrode plate, the positive electrode plate, and the separator are placed in an oven and baked at 60 °C for 12 h, and the separator is baked for 6 h to obtain the treated negative electrode plate, positive electrode plate, and separator.

[0067] Among them, the charge cut-off voltage of the lithium-ion battery in the examples and comparative examples of this application is 4.45 V, and the discharge cut-off voltage is 3.0 V. It can be understood that when the voltage range marked on the outer package of the factory battery is 3.0 V to 4.45 V, the charge cut-off voltage is 4.45 V, and the discharge cut-off voltage is 3.0 V.

[0068] Unless otherwise specified, the following test methods are carried out using the negative electrode plate, positive electrode plate, and separator obtained in the above manner.

[0069] Sampling of the first negative electrode material layer:

[0070] Take the negative electrode plate obtained by the aforementioned method, and use plasma polishing technology to polish from the upper layer of the negative electrode plate. After polishing away 90% of the total thickness of the active material layer of the negative electrode plate, the remaining material layer on the surface of the negative electrode current collector is the first negative electrode material layer.

[0071] Determination of the content of each component in the solid electrolyte interface film:

[0072] Take the first negative electrode material layer obtained by the aforementioned method, and use X-ray photoelectron spectroscopy (XPS) to measure the contents of Li2O and ROCO2Li in the solid electrolyte interface film, where R is an alkyl group from C1 to C5. Calculate the ratio of the content b of ROCO2Li to the content a of Li2O, which is the value of b / a.

[0073] Among them, first measure the thickness of the SEI film on the surface of lithium titanate by taking a photo with a scanning electron microscope (SEM). During the test, control the detection depth of the XPS test signal to be less than the thickness of the SEI film. For example, if the thickness of the SEI film is 100 nm, controlling the detection depth of the XPS test signal to be less than 100 nm can make the detection depth of the test signal less than the thickness of the SEI film.

[0074] It can be understood that the thickness of the SEI can be judged by SEM through the following several criteria: ① Surface covering layer: The surface of the particles presents a uniform or non-uniform film-like structure. ② Cracks or wrinkles: After charge and discharge, the SEI may crack due to volume changes. ③ Blurred particle boundaries: The SEI coverage causes the original particle contours to become blurred.

[0075] Testing of content:

[0076] (1) Testing the silicon element content in the first active material:

[0077] Take the first negative electrode material layer obtained by the aforementioned method, scrape the first negative electrode material layer from the surface of the negative electrode current collector, and use inductively coupled plasma mass spectrometry (ICP-MS) for testing, then the mass percentage content A of silicon element in the first negative electrode material layer can be accurately measured. Among them, the test instrument used for ICP-MS testing is the NexlON 350X inductively coupled plasma mass spectrometer of PerkinElmer.

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

[0079] If it is a negative electrode plate with a single-sided coating of the negative electrode active material layer, the coating mass CW of the negative electrode active material layer = m1 - m3, and the coating mass CW1 of the first negative electrode material layer = m2 - m3.

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

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

[0082] (2) Testing the content of lithium titanate:

[0083] Take the negative electrode plate, scrape the negative electrode active material layer from the negative electrode plate and then conduct inductively coupled plasma mass spectrometry (ICP-MS) testing, then the content of titanium element in the negative electrode active material layer can be measured. Then, according to the chemical formula Li4Ti5O of lithium titanate 12 calculate the content of lithium titanate, and the calculation basis of the lithium titanate content is the negative electrode active material layer. Among them, the test instrument used for ICP-MS testing is the NexlON 350X inductively coupled plasma mass spectrometer of PerkinElmer.

[0084] (3) Testing the content of the delithiated product:

[0085] Obtaining delithiated products: (a) Take the above-mentioned positive electrode sheet sample, scrape the entire positive electrode active material layer from the surface of the positive electrode current collector, and scrape the powder of the first positive electrode material layer on the side originally close to the positive electrode current collector; make the powder of the first positive electrode material layer into a suspension with a concentration of 0.1 g / mL. Under the action of gravity or centrifugal force, different substance particles settle, and according to the sedimentation rates of different substance particles, different substances in the first positive electrode material layer are separated. The method for preparing the above suspension: Add 100 g of the substance of the first positive electrode material layer to 1.0 L of deionized water, and use a homogenizer to break up the substance of the first positive electrode material layer under ice bath conditions to form a uniform suspension with a concentration of 0.1 g / mL. After drying the different substances separated above, elemental analysis is respectively carried out. Those containing Fe element, Mn element or Ni element are delithiated products. (b) Take the above-mentioned separator sample, scrape the surface of the prelithiated layer in the separator to obtain the powder of the prelithiated layer. Place the above powder in a muffle furnace and sinter at 500 °C for 6 h, and collect the powder sample after calcination to obtain the delithiated product.

[0086] Perform the following elemental analysis on the delithiated products obtained above. Use X-ray diffraction (XRD) test and Raman test to obtain the compositional structure information of the delithiated products, thereby reverse-deduce their chemical formulas and calculate their mass percentage contents. The steps of elemental analysis are as follows: Take 0.4 g of the powder sample of the above delithiated product, digest it with 10 mL of aqua regia, which is obtained by mixing concentrated nitric acid and concentrated hydrochloric acid in a volume ratio of 1:1. Then make up the volume to 100 mL, and use an inductively coupled plasma spectrometer (ICP) to measure the content of each element in the solution.

[0087] XRD analysis: Use an X-ray diffractometer (instrument model: Bruker D8 ADVANCE) to perform XRD test on the above powder sample to obtain the XRD pattern of the delithiated product. Analyze the XRD pattern, and compare the position, intensity and shape of the diffraction peaks with the standard pattern to obtain the phase composition of the delithiated product. Among them, Cu Kα ray is used.

[0088] Raman spectroscopy analysis: Use a Raman spectrometer (instrument model: LabRAM HR Evolution) to perform Raman test on the above powder sample to obtain the Raman spectrum of the delithiated product. Compare its characteristic peaks with the standard pattern to obtain the information on the metal bond structure of the substance of the delithiated product.

[0089] Measurement of thickness:

[0090] (1) Measurement of the thickness of the negative electrode active material layer and the first negative electrode material layer:

[0091] First, perform cross-sectional ion polishing on the negative electrode sheet, and conduct microscopic analysis on the cross-section through a scanning electron microscope (SEM) to obtain the total thickness T2 of the negative active material layer. After analyzing the titanium element with an energy dispersive spectrometer (EDS), determine the first negative electrode material layer, and then obtain the thickness of the first negative electrode material layer through SEM.

[0092] (2) Measurement of the thickness of the polymer layer and the prelithiation layer:

[0093] First, perform cross-sectional ion polishing on the separator, and then conduct microscopic analysis on the cross-section through SEM to measure the thicknesses of the polymer layer and the prelithiation layer respectively.

[0094] Measurement of particle size:

[0095] It should be noted that whether the lithium supplement agent de-lithiates has little effect on its particle size. Therefore, the particle size of the de-lithiated product is used to equivalently represent the particle size of the lithium supplement agent. The sample acquisition method refers to the method for obtaining the de-lithiated product in the "Measurement of the content of de-lithiated product". 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 size Dv50 of the lithium supplement agent.

[0096] Measurement of the XY elongation rate of the negative electrode current collector:

[0097] Charge the lithium-ion battery at a constant current of 1C to 4.45V in an environment of 25±3°C, then charge it at a constant voltage of 4.45V to 0.025C, and then discharge it at a constant current of 0.7C to 3.0V. This is one charge-discharge cycle. Conduct tests according to the above charge-discharge cycle. After 200 cycles, charge at a constant current of 1C to 4.45V, and then charge at a constant voltage of 4.45V to 0.025C. Disassemble and take out the negative electrode sheet, measure the width of the copper foil in the empty copper foil area as σ0, and measure the widths of the copper foil at the front end (head), middle end (middle part), and tail end (tail) of the area with the negative active material layer as σ1, σ2, and σ3 respectively. It should be noted that when the disassembled negative electrode sheet 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.

[0098] XY elongation rate at the head (%) = (σ1 - σ0) / σ0 × 100%;

[0099] XY elongation rate in the middle (%) = (σ2 - σ0) / σ0 × 100%;

[0100] XY elongation rate at the tail (%) = (σ3 - σ0) / σ0 × 100%;

[0101] Average XY elongation rate (%) = (Head elongation rate + Middle elongation rate + Tail elongation rate) / 3.

[0102] 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.

[0103] Measurement of energy density:

[0104] First, the dimensions of the lithium-ion battery were measured to obtain the thickness T, width W, and length L, and the volume V was calculated through the following formula: V = T × W × L.

[0105] Then, the lithium-ion battery was charged according to the following operation process and then discharged to obtain the discharge capacity E of the lithium-ion battery:

[0106] Charging: Constant current charging at 0.7C until 4.45V, then constant voltage charging at 4.45V until 0.05C;

[0107] Discharging: Constant current discharging at 0.2C until 3.0V to obtain the discharge energy E.

[0108] The energy density (ED) of the lithium-ion battery can be calculated through the following formula: ED (Wh / L) = E / V.

[0109] Measurement of cycle life:

[0110] Take 5 lithium-ion batteries from each example and comparative example, charge them at a constant current of 0.7C to 4.45V in an environment of 25 ± 3°C, then charge them at a constant voltage of 4.45V to 0.05C, and then discharge them at a constant current of 0.5C 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 decay to 80% of the original capacity is used as the cycle life of the lithium-ion battery. The average value of 5 lithium-ion batteries is taken as the final value.

[0111] Measurement of swelling rate:

[0112] 10 lithium-ion batteries were tested for each example or comparative example, and the average value was taken as the final result.

[0113] The lithium-ion battery is charged at a constant current of 0.7C to 4.45V in an environment of 25±3°C, and then charged at a constant voltage of 4.45V to 0.05C. The thickness of the lithium-ion battery is measured here, which is recorded as the thickness of the lithium-ion battery in the first cycle. Then, it is discharged at a constant current of 0.5C to 3.0V. This is one charge-discharge cycle, and the test is carried out according to the above charge-discharge cycle. When the cycle reaches 200 cycles, it is charged at a constant current of 0.7C to 4.45V, and then charged at a constant voltage of 4.45V to 0.05C. The thickness of the lithium-ion battery is measured here, which is recorded as the thickness of the lithium-ion battery in the 200th cycle. Then, the thickness expansion rate of the lithium-ion battery is calculated by the following formula: Thickness expansion rate (%) = [(Thickness of the lithium-ion battery in the 200th cycle / Thickness of the lithium-ion battery in the first cycle) - 1] × 100%.

[0114] Example 1-1

[0115] <Preparation of negative electrode sheet>

[0116] Lithium titanate, the first active material silicon carbide (SiC), carbon material 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 20.5:20.4:54.0:3.4:0.3:1.4, and then deionized water is added and stirred evenly to prepare a first negative electrode slurry with a solid content of 38wt%.

[0117] The second active material artificial graphite and silicon carbide (SiC, Dv50 = 8.6μm), negative electrode binder polyacrylic acid, and negative electrode conductive agent single-walled carbon nanotubes are mixed evenly according to a mass ratio of 77.6:19.4:2.8:0.2, and then deionized water is added and stirred evenly to prepare a second negative electrode slurry with a solid content of 40wt%.

[0118] The first negative electrode slurry is evenly coated on one surface of a negative electrode current collector copper foil with a thickness of 12μm, and dried at 85°C to form a first negative electrode material layer; the second negative electrode slurry is evenly coated on the surface of the first negative electrode material layer away from the copper foil, and dried at 85°C to form a second negative electrode material layer; thus, 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 76mm×856mm for use.

[0119] Among them, based on the mass of the negative electrode active material layer, the mass percentage content W1 of lithium titanate is 2%, and the mass percentage content W2 of silicon element in the first active material is 6%. The thickness T1 of the first negative electrode material layer is 11μm, and the total thickness T2 of the negative electrode active material layer is 46μm.

[0120] <Preparation of positive electrode sheet>

[0121] Mix the lithium supplement Li5FeO4, the cathode active material lithium cobaltate, the cathode conductive agent conductive carbon black, and the cathode binder polyvinylidene fluoride (PVDF, weight average molecular weight of 5×10 5 ) in a mass ratio of 20:77.8:1.4:0.8, add N-methylpyrrolidone (NMP) as a solvent, and stir in a vacuum mixer until a first cathode slurry with a solid content of 75 wt% and a homogeneous system is obtained.

[0122] Mix the cathode active material lithium cobaltate, the cathode conductive agent conductive carbon black, and the cathode binder polyvinylidene fluoride (PVDF, weight average molecular weight of 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 mixer until a second cathode slurry with a solid content of 75 wt% and a homogeneous system is obtained.

[0123] Coat the first cathode slurry evenly on one surface of a 6-μm-thick cathode current collector aluminum foil, and dry it at 90°C to form a first cathode material layer; coat the second cathode slurry evenly on the surface of the first cathode material layer away from the aluminum foil, and dry it at 90°C to form a second cathode material layer; thus, a cathode electrode with a single-sided coated cathode active material layer is obtained. Then, repeat the above steps on the other surface of the aluminum foil to obtain a cathode electrode with a double-sided coated cathode active material layer. After cold pressing and slitting, a cathode electrode with a specification of 74 mm×851 mm is obtained for use.

[0124] <Preparation of separator>

[0125] Use a 8-μm-thick polyethylene (PE) porous film as the separator.

[0126] <Preparation of electrolyte>

[0127] 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, dissolve and mix evenly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0128] <Preparation of lithium-ion battery>

[0129] Stack the above-prepared separator, cathode electrode, separator, and anode electrode in sequence, and wind them to obtain an electrode assembly. Place the electrode assembly in an aluminum-plastic film outer package, dry it, inject the electrolyte, and obtain a lithium-ion battery through processes such as vacuum packaging, standing, formation, degassing, and edge trimming.

[0130] Among them, the parameters of the formation process are as follows: at 70 ± 2 °C, the pressure of a single lithium-ion battery is 200 MPa. The lithium-ion battery is charged at a constant current of 0.3C for 120 s, at a constant current of 1.2C for 180 s, at a constant current of 1.5C for 2000 s, and then left standing for 5 min.

[0131] Examples 1-2 to Examples 1-5

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

[0133] Among them, the change in the value of b / a is achieved by adjusting the addition amount of the lithium supplement in the positive electrode sheet. When the mass percentage content of the lithium supplement changes, the mass percentage content of the positive electrode active material changes accordingly, while the mass percentage contents of the positive electrode conductive agent and the positive electrode binder remain unchanged. The sum of the mass percentage contents of the lithium supplement, the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder is 100%.

[0134] Examples 1-6 to Examples 1-9

[0135] Except for adjusting the relevant preparation parameters according to Table 1, the rest are the same as those in Example 1-1.

[0136] Among them, the change in the value of b / a is caused by the change in the particle size Dv50 of the lithium supplement.

[0137] Examples 1-10 to Examples 1-12

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

[0139] Among them, the change in the types of delithiated products is achieved by adjusting the types of lithium supplements.

[0140] Examples 2-1 to Examples 2-4

[0141] Except for adjusting the relevant preparation parameters according to Table 3, the rest are the same as those in Example 1-1.

[0142] Among them, when the mass percentage content of lithium titanate changes, the mass percentage contents of the first active material and artificial graphite change accordingly, while the mass percentage contents of the negative electrode binder, the negative electrode conductive agent, and the dispersant remain unchanged. The sum of the mass percentage contents of lithium titanate, the first active material, artificial graphite, the negative electrode binder, the negative electrode conductive agent, and the dispersant is 100%. When the mass percentage content of silicon element in the first active material changes, the mass percentage content of silicon element in the negative electrode active material layer is kept unchanged by adjusting the mass percentage contents of artificial graphite and silicon carbide in the second material layer.

[0143] Examples 3-1 to Examples 3-8

[0144] Except for regulating the relevant preparation parameters according to Table 4, the rest is the same as in Example 1-1.

[0145] Among them, in Examples 3-1 to 3-4, when the thickness of the first negative electrode material layer changes, the thickness of the second negative electrode material layer changes accordingly so that the total thickness of the negative electrode active material layer remains unchanged; when the thicknesses of the first negative electrode material layer and the second negative electrode material layer change, the mass percentage content of artificial graphite in the first negative electrode material layer and the second negative electrode material layer is regulated to keep the mass percentage content W1 of lithium titanate and the mass percentage content W2 of silicon element in the first active material unchanged.

[0146] In Examples 3-5 to 3-8, when the total thickness of the negative electrode active material layer changes, the thickness of the first negative electrode material layer remains unchanged, and the thickness of the second negative electrode material layer changes accordingly; when the total thickness of the negative electrode active material layer changes, the content of lithium titanate, the first active material and artificial graphite in the first negative electrode material layer, and the content of artificial graphite and silicon-carbon material in the second negative electrode material layer are regulated to keep the mass percentage content W1 of lithium titanate and the mass percentage content W2 of silicon element in the first active material unchanged.

[0147] Example 4-1

[0148] <Preparation of Positive Electrode Plate>

[0149] 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 of 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 mixer until a positive electrode slurry with a solid content of 75 wt% and a uniform 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 plate 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 plate with a double-sided coated positive electrode active material layer. After cold pressing and slitting, a positive electrode plate with a specification of 74 mm × 851 mm is obtained for use.

[0150] <Preparation of Separator>

[0151] 2.0 g of the lithium supplement Li5FeO4 and 98.0 g of the polymer powder polyvinylidene fluoride - hexafluoropropylene copolymer (PVDF - HFP) were added to 1 L of the organic solvent DMF, and stirred evenly to form a first spinning solution, where the mass concentration of Li5FeO4 was 2.0 g / L and the mass concentration of PVDF - HFP was 98.0 g / L. After electrospinning, it was dried at 65 °C for 1 h to form a prelithiation layer; 100.0 g of the polymer powder polyvinylidene fluoride - hexafluoropropylene copolymer and 1 L of the organic solvent DMF were mixed to prepare a second spinning solution, where the mass concentration of PVDF - HFP was 100.0 g / L. After electrospinning the second spinning solution and drying at 65 °C for 1 h, a polymer layer was formed on one surface of the prelithiation layer. The prelithiation layer faced the positive electrode plate.

[0152] The preparation of the negative electrode plate, the preparation of the electrolyte, and the preparation of the lithium - ion battery were the same as those in Example 1 - 1.

[0153] Examples 4 - 2 to Example 4 - 13

[0154] Except for adjusting the relevant preparation parameters according to Table 5, the rest were the same as those in Example 5 - 1.

[0155] Example 5 - 1

[0156] The preparation of the separator was the same as that in Example 4 - 3.

[0157] The preparation of the negative electrode plate, the preparation of the positive electrode plate, the preparation of the electrolyte, and the preparation of the lithium - ion battery were the same as those in Example 1 - 1.

[0158] Comparative Example 1 and Comparative Example 2

[0159] Except for adjusting the relevant preparation parameters according to Table 1, the rest were the same as those in Example 1 - 1.

[0160] Comparative Example 3

[0161] Except for preparing the negative electrode plate according to the following steps, the rest were the same as those in Example 1 - 1.

[0162] The preparation of the negative electrode plate

[0163] Artificial graphite and silicon carbide compound (Dv50 = 8.6 μm), negative electrode binder polyacrylic acid, negative electrode conductive agent single - wall carbon nanotubes, and dispersant carboxymethyl cellulose were mixed evenly according to a mass ratio of 87.3:9.7:2.8:0.2, and then deionized water was added and stirred evenly to prepare a negative electrode slurry with a solid content of 38 wt%.

[0164] 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 plate with a double-sided coated negative electrode active material layer. Then, it is cold-pressed and slit to obtain a negative electrode plate 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.

[0165] Comparative Example 4

[0166] Except for preparing the negative electrode plate according to the following steps, the rest is the same as in Example 1-1.

[0167] <Preparation of Negative Electrode Plate>

[0168] 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 61.3:30.0:5.7:2.8:0.2, and then deionized water is added and stirred evenly to prepare a first negative electrode material layer slurry with a solid content of 38 wt%.

[0169] 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 negative electrode material layer slurry with a solid content of 38 wt%.

[0170] The first negative electrode material layer slurry is evenly coated on the negative electrode current collector copper foil, and after drying at 85 °C, a first negative electrode material layer is formed; the second negative electrode material layer slurry is evenly coated on the surface of the first negative electrode material layer away from the copper foil, and after drying at 85 °C, a second negative electrode material layer is formed; that is, a semi-finished negative electrode plate 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 plate with a double-sided coated negative electrode active material layer. Then, it is cold-pressed and slit to obtain a negative electrode plate with a specification of 76 mm × 856 mm for standby.

[0171] The preparation parameters and performance data of each example and comparative example are shown in Tables 1 to 7.

[0172] Table 1

[0173]

[0174] Note: In Table 1, the "mass percentage of the delithiated product", "particle size Dv50 of the lithium supplement agent", "type of the lithium supplement agent", and "delithiated product of the lithium supplement agent" are all parameters in the positive electrode sheet. In the negative electrode sheet of "Comparative Example 3" in Table 1, the first negative electrode material layer is not provided, and only the second material layer is provided as the negative electrode active material layer. In the negative electrode sheet of "Comparative Example 4" in Table 1, the first negative electrode material layer does not include the first active material.

[0175] Table 2

[0176]

[0177]

[0178] It can be seen from Examples 1-1 to 1-12 and Comparative Examples 1 to 4 that for the secondary battery of the present application, by providing a first negative electrode material layer containing lithium titanate and a first active material between the negative electrode current collector and the second negative electrode material layer including a silicon-containing material, after the secondary battery is formed, a solid electrolyte interface film is formed on the surface of the lithium titanate, and the ratio b / a of the mass ratio of ROCO2Li and Li2O in the solid electrolyte interface film is regulated within the range of the present application, so that the XY elongation rates of the head, middle, and tail of the negative electrode current collector are small, and the secondary battery has a high energy density, a long cycle life, and a low swelling rate, indicating that the secondary battery of the example can balance XY elongation, energy density, cycle life, and swelling rate. For the secondary battery of the comparative example, the first negative electrode material layer is not provided or the first negative electrode material layer does not contain the first active material, and the ratio b / a of the mass ratio of ROCO2Li and Li2O in the solid electrolyte interface film is not within the range of the present application, and the XY elongation rates of the head, middle, and tail of its negative electrode current collector are large, or the secondary battery has at least one of a low energy density, a short cycle life, or a high swelling rate.

[0179] When a first positive electrode material layer containing a lithium supplement agent is provided in the positive electrode sheet, the content of the delithiated product of the lithium supplement agent usually affects the energy density, cycle life, and swelling rate of the secondary battery. It can be seen from Examples 1-1 to 1-5 that for the secondary battery with the content of the delithiated product of the lithium supplement agent within the range of the present application, the XY elongation rates of the head, middle, and tail of its negative electrode current collector are small, and the secondary battery has a high energy density, a long cycle life, and a low swelling rate, indicating that the secondary battery can balance XY elongation, energy density, cycle life, and swelling rate.

[0180] The particle size Dv50 of the lithium supplement usually affects the energy density, cycle life, and swelling rate of the secondary battery. It can be seen from Examples 1-1, 1-6 to 1-9 that for a secondary battery using a lithium supplement with a particle size Dv50 within the scope of this application, the XY elongation rates of the head, middle, and tail of the negative electrode current collector are small, and the secondary battery has a high energy density, a long cycle life, and a low swelling rate, indicating that the secondary battery can balance XY elongation, energy density, cycle life, and swelling rate.

[0181] The types of lithium-depleted products of the lithium supplement usually affect the energy density, cycle life, and swelling rate of the secondary battery. It can be seen from Examples 1-1, 1-10 to 1-12 that for a secondary battery using a lithium supplement with lithium-depleted product types within the scope of this application, the XY elongation rates of the head, middle, and tail of the negative electrode current collector are small, and the secondary battery has a high energy density, a long cycle life, and a low swelling rate, indicating that the secondary battery can balance XY elongation, energy density, cycle life, and swelling rate.

[0182] Table 3

[0183]

[0184] The mass percentage content W1 of lithium titanate and the value of the ratio W2 / W1 between its mass percentage content W2 of silicon element in the first active material usually affect the energy density, cycle life, and swelling rate of the secondary battery. It can be seen from Examples 1-1, 2-1 to 2-4 that for a secondary battery using a lithium titanate with a mass percentage content W1 and a ratio W2 / W1 between its mass percentage content W2 of silicon element in the first active material within the scope of this application, the XY elongation rates of the head, middle, and tail of the negative electrode current collector are small, and the secondary battery has a high energy density, a long cycle life, and a low swelling rate, indicating that the secondary battery can balance XY elongation, energy density, cycle life, and swelling rate.

[0185] Table 4

[0186]

[0187] The thickness T1 of the first negative electrode material layer usually affects the energy density, cycle life, and swelling rate of the secondary battery. It can be seen from Example 1-1, Example 3-1 to Example 3-4 that for a secondary battery with the thickness T1 of the first negative electrode 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, and the secondary battery has a high energy density, a long cycle life, and a low swelling rate, indicating that the secondary battery can balance XY elongation, energy density, cycle life, and swelling rate. When the mass percentage content W1 of lithium titanate and the mass percentage content W2 of silicon element in the first active material remain unchanged and the total thickness of the negative electrode active material layer remains unchanged, when the first negative electrode material layer becomes thicker, it means that the contents of LTO and Si in the "lower layer (the first negative electrode material layer)" decrease, 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 sheet 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.

[0188] The relationship value of T1 / T2×100% between the thickness T1 of the first negative electrode material layer and the total thickness T2 of the negative electrode active material layer usually affects the energy density, cycle life, and swelling rate of the secondary battery. It can be seen from Example 1-1, Example 3-1 to Example 3-8 that for a secondary battery with the relationship value of T1 / T2×100% between the thickness T1 of the first negative electrode 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, and the secondary battery has a high energy density, a long cycle life, and a low swelling rate, indicating that the secondary battery can balance XY elongation, energy density, cycle life, and swelling rate. When the mass percentage content W1 of lithium titanate and the mass percentage content W2 of silicon element in the first active material remain unchanged, the total thickness of the negative electrode active material layer increases, the thickness of the upper layer (the second negative electrode material layer) increases, and the thickness of the lower layer (the first negative electrode material layer) remains unchanged. LTO in the lower layer becomes more concentrated, the alleviating effect on the swelling of silicon in the lower layer is enhanced, and the XY elongation becomes better; the "interstitial filling" effect of LTO becomes worse, the compaction density of the negative electrode sheet decreases, and the energy density of the secondary battery decreases. However, because the total thickness of the negative electrode active material layer increases, the energy density of the secondary battery generally increases; the effect of the more concentrated LTO on improving the silicon conductive network becomes better, but from the perspective of the entire negative electrode sheet, the absolute content of silicon in the upper layer is too high, which will lead to a shorter cycle life of the secondary battery.

[0189] Table 5

[0190]

[0191] Note: In Table 5, "\\" indicates no corresponding parameter; in Table 5, "mass percentage of the delithiated product", "type of the lithium supplement agent", and "delithiated product of the lithium supplement agent" are all parameters in the separator.

[0192] Table 6

[0193]

[0194] When a pre-lithiation layer containing a lithium supplement agent is provided in the separator, the content of the delithiated product of the lithium supplement agent usually affects the energy density, cycle life, and swelling rate of the secondary battery. It can be seen from Examples 1-1, 4-1 to 4-5 that for a secondary battery with the content of the delithiated product of the lithium supplement agent within the scope of this application, the head, middle, and tail of the negative electrode current collector have a smaller XY elongation rate, and the secondary battery has a higher energy density, a longer cycle life, and a lower swelling rate, indicating that the secondary battery can balance XY elongation, energy density, cycle life, and swelling rate.

[0195] The thickness of the polymer layer in the separator usually affects the energy density, cycle life, and swelling rate of the secondary battery. It can be seen from Examples 4-3, 4-6 to 4-9 that for a secondary battery with the thickness of the polymer layer within the scope of this application, the head, middle, and tail of the negative electrode current collector have a smaller XY elongation rate, and the secondary battery has a higher energy density, a longer cycle life, and a lower swelling rate, indicating that the secondary battery can balance XY elongation, energy density, cycle life, and swelling rate.

[0196] The thickness of the pre-lithiation layer in the separator usually affects the energy density, cycle life, and swelling rate of the secondary battery. It can be seen from Examples 4-3, 4-10 to 4-13 that for a secondary battery with the thickness of the pre-lithiation layer within the scope of this application, the head, middle, and tail of the negative electrode current collector have a smaller XY elongation rate, and the secondary battery has a higher energy density, a longer cycle life, and a lower swelling rate, indicating that the secondary battery can balance XY elongation, energy density, cycle life, and swelling rate.

[0197] Table 7

[0198]

[0199] Note: In Table 7, "W3" represents the mass percentage of the delithiated product in the positive electrode active material layer, and "W4" represents the mass percentage of the delithiated product in the pre-lithiation layer.

[0200] The setting position of the lithium supplement agent usually also affects the energy density, cycle life and swelling rate of the secondary battery. It can be seen from Example 1-1, Example 4-3 and Example 5-1 that for the secondary battery with the setting position of the lithium supplement agent within the scope of this application, the head, middle and tail of the negative electrode current collector have a smaller XY elongation rate, and the secondary battery has a higher energy density, a longer cycle life and a lower swelling rate, indicating that the secondary battery can balance XY elongation, energy density, cycle life and swelling rate.

[0201] It should be noted that in this text, 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 such 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 elements inherent to such process, method, article or device.

[0202] 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, and the key point of each embodiment is to illustrate the differences from other embodiments.

[0203] The above are only the preferred embodiments of this application and are not intended to limit this application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of this application shall be included within the scope of protection of this application.

Claims

1. A secondary battery, comprising a negative electrode sheet, a positive electrode sheet and a separator disposed between the negative electrode sheet and the positive electrode sheet, wherein the negative electrode sheet comprises 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 negative electrode material layer and a second negative electrode material layer, wherein the first negative electrode material layer is disposed between the negative electrode current collector and the second negative electrode material layer, and wherein the second negative electrode material layer contains silicon; The first negative electrode material layer includes lithium titanate and a first active material, and the first active material includes a first silicon-containing material; A solid electrolyte interface film is formed on the surface of the lithium titanate, wherein the solid electrolyte interface film comprises Li2O and ROCO2Li, and R is selected from C1 to C5 alkyl; The mass proportion of Li2O in the solid electrolyte interface membrane is a, the mass proportion of ROCO2Li in the solid electrolyte interface membrane is b, and 8≤b / a≤24.

2. The secondary battery according to claim 1, wherein 13≤b / a≤17.

3. The secondary battery according to claim 1, wherein 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.

4. The secondary battery according to claim 1, wherein The separator includes a polymer layer and a pre-lithiation layer disposed on a surface of the polymer layer, wherein the pre-lithiation layer includes a lithium supplement; Based on the mass of the pre-lithiation layer, the mass percentage of the delithiation product of the lithium supplement agent is 0.5% to 2.5%.

5. The secondary battery according to claim 4, wherein The thickness of the polymer layer is 4 μm to 5 μm, and the thickness of the pre-lithiation layer is 5 μm to 15 μm.

6. The secondary battery according to claim 1 or 4, wherein: The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer includes a first positive electrode material layer and a second positive electrode material layer, wherein the first positive electrode material layer is disposed between the positive electrode current collector and the second positive electrode material layer; The first positive electrode material layer includes a lithium supplement agent, and based on the mass of the positive electrode active material layer, the mass percentage of a delithiation product of the lithium supplement agent is 0.5% to 2.0%.

7. The secondary battery according to claim 6, wherein The lithium de-lithiation products of the lithium supplement agent in the separator and the positive electrode sheet each independently include Li x FeO 0.5(2+x) , Li 2-z MnO2、Li 1.2-r Ni 0.13 Fe 0.13 Mn 0.54 O2 or Li 1-t At least one of FePO4, and 1≤x≤6, 1.6≤z≤2, 1≤r≤1.2, 0.8≤t≤1.

8. The secondary battery according to claim 7, wherein The particle size Dv50 of the lithium supplement agent is 1.8 μm to 2.5 μm.

9. The secondary battery according to claim 1, wherein The thickness of the first negative electrode material layer is T1, 10 μm≤T1≤12 μm.

10. The secondary battery according to claim 9, wherein The total thickness of the negative electrode active material layer is T2, 12%≤T1 / T2×100%≤37%.

11. An electronic device, wherein: The electronic device includes the secondary battery according to any one of claims 1 to 10.

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