Lithium-ion secondary battery, battery device and electric device

Abstract

Provided in the present application are a lithium-ion secondary battery, a battery device and an electric device. The lithium-ion secondary battery comprises: a positive electrode sheet, wherein a positive electrode active material comprises a lithium-containing phosphate; and a negative electrode sheet, wherein a negative electrode film layer comprises a first negative electrode active layer and a second negative electrode active layer, the first negative electrode active layer comprises a first graphite material, the negative electrode film layer comprises a silicon-based material, the silicon-based material has a specific capacity of 800 mAh / g to 2300 mAh / g, the mass proportion of the silicon-based material in the second negative electrode active layer is 2 wt% to 30 wt%, the mass proportion of the silicon-based material in the first negative electrode active layer is less than that in the second negative electrode active layer, and in the second negative electrode active layer, the delithiation amount in the voltage range of 0.5 V to 2.0 V vs. Li / Li⁺ accounts for 5% to 20% of the total delithiation amount. The technical solution of the present application is beneficial for improving the energy density of the secondary battery without compromising the cycle life.

Description

Lithium ion secondary battery, battery device, and electric device Cross-reference to related applications

[0001] This patent document claims priority to and the benefit of Chinese Patent Application No. 202510322933.0, filed March 18, 2025, entitled “Lithium ion secondary battery, battery device, and electric device.” The entire contents of the aforementioned patent application are incorporated by reference as part of the disclosure of this patent document. TECHNICAL FIELD

[0002] The present application relates to the field of batteries, and more particularly, to a lithium ion secondary battery, a battery device, and an electric device. BACKGROUND

[0003] With the increasing environmental pollution, new energy industry is attracting more and more attention. In the new energy industry, battery technology is an important factor for its development.

[0004] The development of battery technology needs to consider many design factors, such as energy density, cycle life, capacity, reliability, etc. Therefore, how to improve the energy density of lithium ion secondary battery while considering the cycle life is a problem to be solved. SUMMARY

[0005] The present application is made in view of the above problem, and aims to provide a lithium ion secondary battery to improve the energy density while considering the cycle performance of the lithium secondary battery.

[0006] To achieve the above-mentioned purpose, the present application provides a lithium ion secondary battery, a battery device, and an electric device.

[0007] In a first aspect, a lithium ion secondary battery is provided, comprising: a positive electrode sheet, the positive electrode sheet comprising a positive electrode active material, the positive electrode active material comprising a lithium-containing phosphate; a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode film layer, the negative electrode film layer comprising a first negative electrode active layer and a second negative electrode active layer, the second negative electrode active layer being disposed between the negative electrode current collector and the first negative electrode active layer; the first negative electrode active layer comprising a first graphite material; the negative electrode film layer comprising a silicon-based material; wherein the specific capacity of the silicon-based material is 800 mAh / g-2300 mAh / g, the mass fraction of the silicon-based material in the second negative electrode active layer based on the total mass of the second negative electrode active layer is 2wt%-30wt%, and the mass fraction of the silicon-based material in the first negative electrode active layer based on the total mass of the negative electrode film layer is less than the mass fraction of the silicon-based material in the second negative electrode active layer; and the amount of lithium in the voltage interval of 0.5V-2V in the second negative electrode active layer accounts for 5%-20% of the total amount of lithium.

[0008] In the embodiment of the present application, the lithium ion secondary battery includes a positive electrode sheet with a positive active material being a lithium-containing phosphate, which can provide a long cycle life and good stability; and a negative electrode sheet with a double-layer structure design, by adding more silicon-based materials in the negative active layer close to the current collector side, the overall energy density of the lithium secondary battery can be improved, while the upper layer of graphite material and less silicon-based material can reduce the local stress caused by silicon particles, thereby reducing the risk of negative electrode sheet expansion, and the cycle performance of the lithium secondary battery is considered; further, in the voltage range of 0.5V-2V, the silicon element in the second negative active layer mainly affects the delithiation amount, by keeping the gram capacity and mass ratio of the silicon-based material within the above range, the delithiation amount of the negative active material can be improved and the cycle stability is considered, in the voltage range of 0.5V-2V, the delithiation capacity ratio is not less than 5%, the secondary battery has high energy density, and the delithiation capacity ratio is not more than 20%, the lithium secondary battery can maintain a long cycle life. Therefore, the embodiment of the present application can improve the energy density of the lithium secondary battery, and the cycle performance is considered.

[0009] In a possible implementation, based on the total mass of the negative electrode film layer, the mass ratio A of the silicon-based material in the first negative active layer to the mass ratio B of the silicon-based material in the second negative active layer satisfies: 0≤A / B≤0.5.

[0010] In the embodiment of the present application, when A / B is greater than or equal to 0, the mass ratio A of the silicon-based material in the first negative active layer is greater than or equal to 0, which can improve the energy density of the secondary battery, and when the mass ratio A of the silicon-based material in the first negative active layer is less than or equal to 50% of the mass ratio of the silicon-based material in the second active layer, the risk of expansion caused by the silicon-based material can be reduced, and the cycle performance is considered.

[0011] In a possible implementation, the active material of the first negative active layer is artificial graphite.

[0012] In the embodiment of the present application, the active material of the first negative active layer is pure graphite, which can further reduce the risk of expansion caused by the silicon-based material, and the first graphite material has high conductivity and stability, which is beneficial to improve the cycle performance and energy density of the secondary battery.

[0013] In a possible implementation, the second negative active layer includes a second graphite material; the aspect ratio of the first graphite material is less than 2; and the aspect ratio of the second graphite material is 2-4.

[0014] In the embodiments of the present application, the aspect ratio of the first graphite material is less than the aspect ratio of the second graphite material, the second graphite material has a larger aspect ratio, reduces active sites, reduces irreversible reactions of lithium ions on the surface of graphite, reduces consumption of active lithium, improves the life performance of the secondary battery, and can improve the energy density of the battery; in the case that the aspect ratio of the second graphite material is large, the first graphite material with a smaller aspect ratio can have more lithium ion insertion sites, which helps to improve the kinetic performance of the secondary battery, thereby improving the fast charging capability, reducing lithium precipitation, and improving the cycle life of the lithium ion secondary battery.

[0015] In a possible implementation, the volume average particle size of the first graphite material is 6 μm to 11 μm; and the volume average particle size of the second graphite material is 13 μm to 20 μm.

[0016] In the embodiments of the present application, the volume average particle size of the first graphite material is less than that of the second graphite material, which can increase the lithium insertion sites on the surface of the first negative electrode active layer and improve the porosity, which helps the rapid insertion and extraction of lithium ions, thereby improving the kinetic performance of the secondary battery, reducing lithium consumption in the kinetic process of the lithium ion secondary battery during the cycle process, improving the cycle life of the secondary battery, and reducing local stress concentration of the negative electrode sheet and expansion, thereby further improving the cycle life; the larger volume average particle size of the second graphite material can reduce active sites, reduce irreversible reactions of lithium ions on the surface of the negative electrode active material, and improve the life performance of the secondary battery.

[0017] In a possible implementation, the silicon-based material includes at least one of silicon, silicon-oxygen material, and silicon-carbon material. In the embodiments of the present application, the use of the above silicon-based material can provide a higher specific capacity and improve the energy density of the secondary battery.

[0018] In a possible implementation, the silicon-based material includes silicon-carbon material, and the silicon-carbon material includes porous carbon and silicon material deposited in the pores of the porous carbon.

[0019] In the embodiments of the present application, the silicon-carbon material combines the high specific capacity characteristics of silicon material and the high conductivity and stability of carbon material, and by embedding nano-silicon particles in the porous carbon structure, the energy density of the secondary battery can be effectively improved, and the porous carbon structure provides a buffer space for the silicon particles, which can effectively alleviate the volume expansion of silicon during the charging and discharging process of the battery, and improve the cycle stability of the secondary battery.

[0020] In a possible implementation, the surface of the silicon-carbon material has a coating layer, and the coating layer includes carbon elements.

[0021] In the embodiments of the present application, the coating layer has a protective effect on the silicon-carbon particles, reduces the contact between water and pure silicon during preparation, reduces the risk of reaction, and improves the stability of the slurry. At the same time, the coating layer can prevent the pure silicon from contacting the electrolyte, reduce the side reaction, and improve the cycle life of the secondary battery.

[0022] In a possible implementation, the gram capacity of the silicon-carbon material is 1400 mAh / g-2300 mAh / g. Optionally, the gram capacity of the silicon-carbon material is 1600 mAh / g-2000 mAh / g.

[0023] In the embodiments of the present application, the gram capacity of the silicon-carbon material is greater than or equal to 1400 mAh / g, which is conducive to improving the energy density of the secondary battery, and the silicon-carbon material has a suitable strength, which is conducive to reducing the risk of exacerbation of side reactions caused by the crushing of the silicon-carbon material, thereby improving the cycle life of the secondary battery. In the case where the gram capacity of the silicon-carbon material is less than or equal to 2300 mAh / g, it is conducive to reducing the risk of increased swelling of the negative electrode sheet caused by the exposure of silicon on the surface of the porous carbon, reducing the swelling of the negative electrode sheet, and improving the cycle life of the secondary battery.

[0024] In a possible implementation, the volume average particle size of the silicon-carbon material is 3 μm-15 μm. Optionally, the volume average particle size of the silicon-carbon material is 6 μm-10 μm.

[0025] In the embodiments of the present application, when the volume average particle size of the silicon-carbon material is greater than or equal to 3 μm, the specific surface area of the silicon-carbon material is in a suitable range, reducing the occurrence of side reactions of the electrode sheet and improving the cycle life of the secondary battery. When the volume average particle size of the silicon-carbon material is less than or equal to 15 μm, it is conducive to reducing the risk of swelling of the silicon-carbon material and further improving the cycle performance of the battery.

[0026] In a possible implementation, the lithium-containing phosphate includes one or more of lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate, and modified substances thereof.

[0027] In the embodiments of the present application, the lithium-containing phosphate has high structural stability, which is conducive to improving the cycle performance of the secondary battery.

[0028] In a second aspect, a battery device is provided, including the battery cell in the first aspect and any possible implementation thereof. The battery device includes at least one of a battery module and a battery pack.

[0029] In a third aspect, a power utilization device is provided, including the battery device of the third aspect. BRIEF DESCRIPTION OF DRAWINGS

[0030] FIG. 1 is a structural schematic diagram of a negative electrode sheet according to an embodiment of the present application.

[0031] FIG. 2 is a schematic view of a structure of a battery cell according to an embodiment of the present application.

[0032] FIG. 3 is a schematic view of a structure of a battery device according to an embodiment of the present application.

[0033] FIG. 4 is a schematic view of an electric device according to an embodiment of the present application.

[0034] FIG. 5 is a SEM image of a negative electrode according to an embodiment of the present application.

[0035] BRIEF DESCRIPTION OF DRAWINGS 1: negative electrode; 10: negative current collector; 11: first negative active layer; 12: second negative active layer; 3: battery cell; 31: case; 32: end cap assembly; 33: electrode assembly; 34: connecting member; 322: electrode terminal; 330: electrode assembly main body; 331: tab; 4: battery device; 5: electric device. DETAILED DESCRIPTION

[0036] Hereinafter, embodiments of a battery cell and a manufacturing method thereof, a battery, and an electric device according to the present application will be specifically described with appropriate reference to the accompanying drawings. However, there will be cases where unnecessary detailed description is omitted. For example, there will be cases where detailed description of matters known well and repeated description of substantially the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided so that those skilled in the art can fully understand the present application, and are not intended to limit the subject matter recited in the claims.

[0037] The ranges disclosed herein are defined by their lower and upper limits. Ranges created by the upper and lower limits are inclusive of the endpoints. Ranges created by the upper and lower limits are also inclusive of any and all sub-ranges subsumed therein. For example, a range of "60% to 120%" or "60% to 120%" is inclusive of from 60% to 120% by 1% increments. Likewise, a range created by the lower limit of "60% to 120%" is inclusive of from 60% to 120% by 1% increments. That is, every percentage between the lower limit of 60% and the upper limit of 120%, i.e. percentages like 61%, 62%, 63%, and so on, and also including the endpoints, are included in the ranges created by the lower limit of 60% and the upper limit of 120%. The same principle applies to the range created by the upper limit of 60% and 120%. It also applies to ranges created by a minimum of 1 and a maximum of 3. In this instance, every integer that is 1 or more but less than 3 is included in the range created by the minimum of 1 and the maximum of 3, that is, the range "1 to 3" is the same as "1, 2, and 3."

[0038] If not particularly specified, all the embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0039] If not particularly specified, all the technical features and optional technical features of the present application can be combined with each other to form new technical solutions.

[0040] If not particularly specified, all the steps of the present application can be performed in sequence or randomly, and preferably in sequence. For example, the method comprises steps (a) and (b), which means that the method can comprise steps (a) and (b) performed in sequence, or steps (b) and (a) performed in sequence. For example, the method also comprises step (c), which means that step (c) can be added to the method in any order, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0041] In recent years, secondary batteries have been widely used in electric tools, electronic products, electric vehicles, aerospace and other fields due to their high energy density and long service life, and have made great progress.

[0042] The development of battery technology needs to consider many design factors, such as energy density, cycle life, discharge capacity, charge-discharge rate, fast charging ability, reliability, first circle charging capacity, etc. With the wide use of batteries, the requirements for battery energy density and cycle performance are also gradually increasing. Among the positive active materials, lithium-containing phosphates have high structural stability and cycle performance. In order to further improve the energy density of the secondary battery including lithium-containing phosphates, a positive electrode sheet with high coating weight and high compaction density is used.

[0043] However, as the coating weight of the positive active material increases, the coating weight of the negative active material also needs to increase. In the case of increasing the coating thickness of the negative electrode sheet, it will lead to the decrease of the kinetic performance of the secondary battery and the increase of the risk of lithium precipitation. In the case of increasing the compaction density of the negative electrode sheet, it will lead to the decrease of the porosity of the electrode sheet, further reducing the kinetic performance, and the increase of the compaction density will cause a large number of negative active particles to be broken, the reaction sites increase, and the cycle performance decreases.

[0044] In view of this, in one embodiment of the present application, the present application provides a lithium ion secondary battery, comprising: a positive electrode sheet, the positive electrode sheet comprising a positive electrode active material, the positive electrode active material comprising a lithium-containing phosphate; a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode film layer; the negative electrode film layer comprising a first negative electrode active layer and a second negative electrode active layer, the second negative electrode active layer being arranged between the negative electrode current collector and the first negative electrode active layer; the first negative electrode active layer comprising a first graphite material; the negative electrode film layer comprising a silicon-based material; wherein the silicon-based material has a specific capacity of 800 mAh / g-2300 mAh / g, the mass fraction of the silicon-based material in the second negative electrode active layer is 2wt%-30wt% based on the total mass of the second negative electrode active layer, the mass fraction of the silicon-based material in the first negative electrode active layer is less than the mass fraction of the silicon-based material in the second negative electrode active layer based on the total mass of the negative electrode film layer, and the de-lithiation amount of lithium potential in the voltage interval of 0.5V-2V in the second negative electrode active layer accounts for 5%-20% of the total de-lithiation amount. By the above arrangement, the addition of silicon elements in the negative electrode active material can reduce the coating weight of the negative electrode to improve the energy density of the secondary battery, and most of the silicon elements are concentrated in the second negative electrode active layer close to the negative electrode current collector, avoiding direct contact of the silicon elements with the separator. The stress generated by the silicon particles can be homogenized by the graphite material in the first negative electrode active layer in the form of sliding, thereby reducing the risk of lithium precipitation failure caused by the closure of the separator and improving the cycle performance of the secondary battery. In the voltage interval of 0.5V-2V, the de-lithiation capacity accounts for not less than 5%, the secondary battery has a high energy density, the de-lithiation capacity accounts for not more than 20%, and the lithium secondary battery can maintain a long cycle life. By controlling the specific capacity and mass fraction of the silicon-based material in the second negative electrode active layer within the above range, the de-lithiation amount of lithium potential in the voltage interval of 0.5V-2V in the second negative electrode active layer accounts for 5%-20% of the total de-lithiation amount. Therefore, the embodiments of the present application can improve the energy density of the secondary battery while taking into account the cycle performance.

[0045] In the charging process of the lithium ion secondary battery, lithium ions are deintercalated from the positive electrode active material, move and intercalate into the negative electrode active material; and in the discharging process, lithium ions are deintercalated from the negative electrode active material, move and intercalate into the positive electrode active material.

[0046] It should be understood that the "intercalation" process described in the present application refers to the process of lithium ions intercalating into the positive electrode active material or the negative electrode due to electrochemical reaction, and the "deintercalation" process described in the present application refers to the process of lithium ions deintercalating from the positive electrode active material or the negative electrode due to electrochemical reaction.

[0047] It should be understood that the secondary battery in the present application can be a battery monomer. Next, the secondary battery provided by the present application and each part in the secondary battery will be introduced.

[0048] [Secondary lithium ion battery]

[0049] Generally, a secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator. During the charging and discharging of the battery, active ions are inserted and extracted between the positive electrode sheet and the negative electrode sheet. The electrolyte plays a role of conducting ions between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet, mainly playing a role of preventing the short circuit of the positive and negative electrodes, while allowing the ions to pass through.

[0050] In one embodiment of the present application, a lithium secondary battery is provided, which includes a positive electrode sheet and a negative electrode sheet.

[0051] The positive electrode sheet includes a positive current collector and a positive film layer arranged on at least one side of the positive current collector.

[0052] The positive current collector has two opposite sides along its own thickness direction, wherein the positive film layer can be arranged on one side of the positive current collector or on both sides of the positive current collector.

[0053] The positive electrode sheet includes a positive active material, and the positive active material includes a lithium-containing phosphate;

[0054] The lithium-containing phosphate refers to a phosphate having an olivine structure, such as lithium iron phosphate, lithium manganese iron phosphate, lithium manganese phosphate, etc. The lithium-containing phosphate includes transition metals, such as iron and manganese. The lithium-containing phosphate can also be modified by doping or surface coating to improve the corresponding performance. For example, the surface of the lithium-containing phosphate is coated with a carbon material, thereby improving the conductivity of the positive active material.

[0055] The lithium-containing phosphate has relatively high structural stability, and the secondary battery including the lithium-containing phosphate has relatively high cycle life.

[0056] The negative electrode sheet includes a negative current collector and a negative film layer, wherein the negative film layer includes a first negative active layer and a second negative active layer;

[0057] FIG. 1 is a structural schematic diagram of a negative electrode sheet according to an embodiment of the present application. For example, as shown in FIG. 1, the negative electrode sheet 1 includes a negative current collector 10, a second negative active layer 12 arranged on at least one side surface of the negative current collector 10, and a first negative active layer 11 arranged on the surface of the second negative active layer 12.

[0058] The negative current collector 10 has two opposite side surfaces along its own thickness direction. The second negative active layer 12 can be arranged on one side surface of the negative current collector 10 or on both side surfaces of the negative current collector 10. As an example, as shown in FIG. 1, the second negative active layer 12 is arranged on both side surfaces of the negative current collector 10.

[0059] The first negative active layer 11 includes a first graphite material; the negative film layer includes a silicon-based material, wherein the silicon-based material has a specific capacity of 800 mAh / g-2300 mAh / g, the mass percentage of the silicon-based material in the second negative active layer 12 is 2wt%-30wt% based on the total mass of the second negative active layer 12; the mass percentage of the silicon-based material in the first negative active layer 11 is less than the mass percentage of the silicon-based material in the second negative active layer 12 based on the total mass of the negative film layer; and the de-lithiation amount of the second negative active layer 12 in the voltage interval of 0.5V-2V of lithium potential accounts for 5%-20% of the total de-lithiation amount.

[0060] The graphite can include one or both of natural graphite and artificial graphite. The graphite can be primary particles, secondary particles, or mixed particles of primary particles and secondary particles.

[0061] The silicon-based material can include one or more of silicon element, silicon-oxygen material, and silicon-carbon material.

[0062] The silicon element is mostly concentrated in the second negative active layer 12 close to the negative current collector 10, avoiding direct contact of the silicon particles with the separator, and the stress generated by the silicon particles is homogenized in the form of sliding by the first graphite material of the first negative active layer 11, thereby reducing the risk of lithium precipitation failure caused by the closure of the separator and improving the cycle performance of the secondary battery.

[0063] The specific capacity of the silicon-based material can be 800 mAh / g, 900 mAh / g, 1000 mAh / g, 1200 mAh / g, 1500 mAh / g, 1700 mAh / g, 1900 mAh / g, 2000 mAh / g, 2100 mAh / g, 2200 mAh / g, 2300 mAh / g, or any value within the above range.

[0064] The mass percentage of the silicon-based material can be 2wt%, 5wt%, 7wt%, 9wt%, 10wt%, 15wt%, 19wt%, 20wt%, 22wt%, 25wt%, 27wt%, 30wt%, or any value within the above range.

[0065] When the mass percentage of the silicon-based material is greater than 2wt%, the energy density of the secondary battery can be improved; when the mass percentage of the silicon-based material is less than 30wt%, the risk of expansion of the silicon-based material can be avoided, and the cycle performance of the battery is considered. The specific capacity of the silicon-based material is greater than that of the graphite, and by adding the silicon-based material in the negative electrode sheet, the energy density of the secondary battery can be improved; and in the case of a certain capacity of the negative electrode sheet, the addition of the silicon-based material is also beneficial to reducing the thickness of the negative electrode sheet, thereby further improving the energy density of the secondary battery.

[0066] The lithium potential can refer to the potential relative to a lithium electrode. For example, in a coin half-cell consisting of a negative electrode tab and a lithium tab, the lithium potential refers to the potential of the negative electrode tab relative to the lithium tab. The negative electrode tab has different lithium potentials at different charging and discharging stages.

[0067] The amount of lithium extracted from the second negative active layer can refer to the content of lithium extracted from the second negative active layer of the negative electrode tab. As an example, the amount of lithium extracted from the second negative active layer can be measured in the following way.

[0068] For example, after discharging the secondary battery at a rate of 0.05C to 2V at 25°C, the negative electrode tab is obtained by disassembling the secondary battery, the first negative active layer is peeled off by fixing the tab on the adhesive tape using the adhesion of the adhesive tape, and then the negative electrode tab is cut into a specific size and assembled with a lithium tab to form a coin half-cell. Discharge at 200μA to 0.005V, then charge at 2.0V. During this process, the charge and discharge capacity of the lithium potential in the range of 0.5V-2V is recorded, and is denoted as the amount of lithium extracted from the lithium potential in the range of 0.5V-2V. The total amount of lithium extracted is the discharge capacity of the lithium potential in the range of 0V-2V.

[0069] The ratio of the amount of lithium extracted from the second negative active layer to the total amount of lithium extracted in the range of 0.5V-2V of the lithium potential can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 11.5%, 12%, 12.5%, 13%, 13.5%, 14%, 14.5%, 15%, 15.5%, 16%, 16.5%, 17%, 17.5%, 18%, 18.5%, 19%, 19.5%, 20% or any value within the above range.

[0070] When the ratio of the amount of lithium extracted from the second negative active layer to the total amount of lithium extracted in the range of the lithium potential of 0.5V-2V is greater than or equal to 5%, the secondary battery has a high energy density; the higher the negative potential, the higher the risk of oxidation of the solid electrolyte interphase (SEI) film during lithium extraction, and the higher the amount of lithium extracted at high potential, the higher the risk of oxidation and decomposition of the SEI, and more than 20% will cause the cycle to deteriorate sharply, and when the ratio of the amount of lithium extracted from the second negative active layer to the total amount of extracted lithium in the range of 0.5V-2V of the lithium potential is less than or equal to 20%, it is beneficial to reduce the expansion of the negative electrode tab and the expansion force in the secondary battery, and the secondary battery has a long cycle life. Therefore, by setting the specific capacity of the silicon-based material in the range of 800mAh / g-2300mAh / g and maintaining the mass fraction of the silicon-based material in the range of 2wt%-30wt%, the ratio of the amount of lithium extracted from the second negative active layer to the total amount of the lithium potential in the range of 0.5V-2V is 5%-20%, the secondary battery has a high energy density and a long cycle life.

[0071] In some embodiments, the mass ratio A of the silicon-based material in the first negative active layer and the mass ratio B of the silicon-based material in the second negative active layer satisfy: 0≤A / B≤0.5, based on the total mass of the negative electrode film layer.

[0072] A / B can be 0, 0.1, 0.2, 0.3, 0.4, 0.5, or any value within the above range.

[0073] In the above scheme, the ratio of the mass ratio A of the silicon-based material in the first negative active layer and the mass ratio B of the silicon-based material in the second negative active layer is greater than or equal to 0, that is, the silicon-based material in the first negative active layer is greater than or equal to 0wt%, which can improve the energy density of the lithium secondary battery; and when the silicon-based material in the first negative active layer is less than or equal to 50wt% of the mass ratio of the silicon-based material in the second negative active layer, the risk of expansion caused by the silicon-based material can be reduced, and the energy density and cycle performance of the secondary battery can be considered.

[0074] In some embodiments, the active material in the first negative active layer is artificial graphite. In the above scheme, the active material in the first negative active layer is pure graphite, which can further reduce the risk of expansion caused by the lower layer silicon-based material and improve the cycle performance of the secondary battery.

[0075] In some embodiments, the second negative active layer 12 includes a second graphite material; the aspect ratio of the first graphite material is less than 2; and the aspect ratio of the second graphite material is 2-4.

[0076] The aspect ratio of the first graphite material can be 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, or any value within the above range, and the aspect ratio of the second graphite material can be 2, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, or any value within the above range.

[0077] The aspect ratio is a well-known concept in the art and has a meaning known in the art, and can be measured by a test method and instrument known in the art. The aspect ratio can refer to the ratio of the length to the diameter of the first graphite material and the second graphite material.

[0078] In the case that the aspect ratio of the first graphite material is less than 2 and the aspect ratio of the second graphite material is 2-4, the aspect ratio of the second graphite material is greater than that of the first graphite material, so that the second graphite material reduces active sites and reduces the consumption of lithium ions, while having good stability and structural integrity. The second graphite material particles are more likely to form directional arrangement in the negative electrode sheet, thereby improving the conductivity and structural stability. The first graphite material has relatively smaller graphite particles, higher specific surface area and more active sites, which can better contact the electrolyte when placed in the first negative electrode active layer, thereby improving the deintercalation efficiency of lithium ions, improving the rate performance and rapid charging capacity of the battery, reducing lithium precipitation, and improving the cycle life of the secondary battery.

[0079] In some embodiments, the volume average particle size of the first graphite material is 6-11 μm, and the volume average particle size of the second graphite material is 13-20 μm. FIG. 5 is a SEM image of a negative electrode sheet according to an embodiment of the present application. In some embodiments, for example, as shown in FIG. 5, the volume average particle size of the first graphite material is less than that of the second graphite material, and the aspect ratio of the first graphite material is also less than that of the second graphite material.

[0080] The volume average particle size of the first graphite material can be 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, or any value within the above range, and the volume average particle size of the second graphite material can be 13, 13.5, 14, 14.5, 15, 15.5, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5, 20, or any value within the above range.

[0081] In the case that the volume average particle size of the second graphite material is greater than that of the first graphite material, the second graphite material with a larger particle size has less lithium consumption, which can improve the energy density while taking into account the cycle performance; the first graphite material with a smaller particle size has a higher specific surface area and more active sites, which is conducive to the intercalation and deintercalation of lithium ions, thereby improving the kinetic performance of the secondary battery, improving the fast charging capability, and the contact area between smaller graphite particles is smaller, the stress distribution is more uniform, which is conducive to reducing local stress concentration, thereby improving the cycle stability of the secondary battery.

[0082] In one possible implementation, the silicon-based material includes at least one of elemental silicon, silicon-oxygen material, silicon-carbon material, and optionally at least one of nano-silicon, silicon monoxide, and silicon-carbon in gas phase.

[0083] In some embodiments, the silicon-carbon material includes porous carbon and silicon deposited in the pores of the porous carbon. The porous carbon can be one or more of porous hard carbon and porous soft carbon. As an example, the porous carbon is porous hard carbon.

[0084] The porous carbon can refer to a material with a carbon matrix and a pore structure in the matrix. The silicon element in the silicon-carbon material is deposited in the pore structure of the porous carbon.

[0085] As an example, the silicon-carbon material is prepared by a gas phase deposition method.

[0086] The porous carbon has a certain protective effect on the silicon, which can reduce the expansion degree of the silicon and improve the particle integrity during the cycle process. In this way, the expansion of the negative electrode sheet is reduced, thereby improving the cycle life of the secondary battery.

[0087] In some embodiments, the surface of the silicon-carbon material has a coating layer, and the coating layer includes carbon elements. When the silicon-carbon material is observed (for example, by SEM), the coating layer on the surface of the silicon-carbon material can be observed. At the same time, combined with EDS analysis, it can be tested that the coating layer includes carbon elements. The coating layer can avoid the exposure of silicon particles on the surface of the silicon-carbon particles, reduce the expansion risk of the silicon-carbon particles, reduce the expansion of the negative electrode sheet, and reduce the side reaction between the silicon material and the electrolyte, thereby improving the cycle life of the secondary battery.

[0088] In some embodiments, the silicon-carbon material has a specific capacity of 1400 mAh / g to 2300 mAh / g.

[0089] The specific capacity of the silicon-carbon material can be 1400 mAh / g, 1500 mAh / g, 1600 mAh / g, 1700 mAh / g, 1800 mAh / g, 1900 mAh / g, 2000 mAh / g, 2100 mAh / g, 2200 mAh / g, 2300 mAh / g, or any value within the above range.

[0090] The specific capacity of the silicon-carbon material can be measured by the following method. For example, the silicon-carbon material is obtained during the preparation of the negative electrode sheet, the silicon-carbon material is prepared into a slurry, and then the slurry is coated on a copper foil to obtain a negative electrode sheet; the negative electrode sheet and a lithium sheet are combined to form a coin-type half battery. The coin-type half battery is discharged at a rate of 0.05C to 0.005V, and then charged at a constant current to 2.0V, to measure the first cycle capacity of the coin-type half battery. The specific capacity of the silicon-carbon material = the first cycle capacity of the coin-type half battery / the mass of the silicon-carbon material.

[0091] The specific capacity of the silicon-carbon material is related to the mass content of the silicon element in the silicon-carbon material. The higher the mass content of the silicon element, the greater the specific capacity of the silicon-carbon material. Within a certain range, the higher the mass content of the silicon element in the silicon-carbon material, the more the pores of the porous carbon are filled with silicon, and the silicon-carbon material has high strength and is not easy to be crushed. However, when the mass content of the silicon element in the silicon-carbon material is too high, the silicon element is distributed on the outer surface of the porous carbon, and the expansion degree of the silicon-carbon material becomes large.

[0092] In the case that the gram capacity of the silicon-carbon material is greater than or equal to 1400 mAh / g, the deposition amount of the silicon material inside the porous carbon is relatively large, which is beneficial to improve the energy density of the secondary battery, and the silicon-carbon material has a relatively suitable strength, which is beneficial to reduce the risk of exacerbation of side reactions caused by the crushing of the silicon-carbon material, reduce the consumption of lithium ions, and thus improve the cycle life of the secondary battery; in the case that the gram capacity of the silicon-carbon material is less than or equal to 2300 mAh / g, it is beneficial to reduce the risk of increased expansion of the negative electrode sheet caused by the exposure of silicon on the surface of the porous carbon, and it is beneficial to reduce the expansion of the negative electrode sheet, thereby improving the cycle life of the secondary battery.

[0093] In some embodiments, the gram capacity of the silicon-carbon material is 1600 mAh / g to 2000 mAh / g. In this way, it is beneficial to further improve the cycle life of the secondary battery.

[0094] In some embodiments, the volume average particle size of the silicon-carbon material is 3 μm to 15 μm.

[0095] The volume average particle size Dv50 of the material represents the particle size corresponding to the cumulative volume distribution percentage of 50%, which can be determined by instruments and methods known in the art. For example, GB / T 19077-2016 Particle Size Distribution Laser Diffraction Method can be referred to for convenient determination by using a laser particle size analyzer. The test instrument can be a Mastersizer 3000 laser particle size analyzer of Malvern Instruments Ltd., UK.

[0096] The volume average particle size of the silicon-carbon material can be 3 μm, 4 μm, 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 within the above range.

[0097] In the case that the volume average particle size of the silicon-carbon material is not less than 3 μm, the active specific surface area of the particles is relatively large, the active lithium consumed by the side reaction is reduced, and thus the cycle life of the secondary battery is improved; in the case that the volume average particle size of the carbon-silicon material is not greater than 15 μm, it is beneficial to solid-phase diffusion, the internal resistance is reduced, and the cycle performance of the secondary battery is improved.

[0098] [Positive electrode sheet]

[0099] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, and the positive electrode film layer includes a positive electrode active material.

[0100] As an example, the positive electrode current collector has two opposite surfaces in the thickness direction thereof, and the positive electrode film layer is disposed on any one or both of the two opposite surfaces of the positive electrode current collector.

[0101] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0102] In some embodiments, the positive electrode active material in the positive electrode film layer includes a lithium phosphate with an olivine structure. Examples of lithium phosphate with an olivine structure may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0103] The positive electrode film may optionally include at least one of the positive electrode active materials known in the art for batteries: lithium transition metal oxides and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811lithium nickel cobalt aluminum oxide (e.g., LiNi 0.85 Co 0.15 Al 0.05 O2) and modified compounds thereof.

[0104] In some embodiments, the positive electrode film layer further optionally includes a binder. As an example, the binder can include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylic ester resin.

[0105] In some embodiments, the positive electrode film layer further optionally includes a conductive agent. As an example, the conductive agent can include at least one of super P, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0106] In some embodiments, the positive electrode tab can be prepared by dispersing the above-mentioned components for preparing the positive electrode tab, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on a positive electrode current collector, and after processes such as drying, cold pressing, etc., obtaining the positive electrode tab.

[0107] [Positive electrode tab]

[0108] As described above, the negative electrode current collector has two side surfaces opposite along its own thickness direction. The negative electrode film layer can be disposed on one side surface of the negative electrode current collector, or on both side surfaces of the negative electrode current collector.

[0109] In some embodiments, the negative electrode film layer includes a first negative electrode active layer and a second negative electrode active layer, the second negative electrode active layer is disposed between the negative electrode current collector and the first negative electrode active layer; the first negative electrode active layer includes a first graphite material; the negative electrode film layer includes a silicon-based material; wherein the silicon-based material has a specific capacity of 800 mAh / g to 2300 mAh / g, the mass fraction of the silicon-based material in the second negative electrode active layer is 2 wt% to 36 wt% based on the total mass of the second negative electrode active layer, the mass fraction of the silicon-based material in the first negative electrode active layer is less than the mass fraction of the silicon-based material in the second negative electrode active layer based on the total mass of the negative electrode film layer, and the de-lithiation amount of the second negative electrode active layer in a voltage interval of 0.5 V to 2 V of lithium potential accounts for 5% to 20% of the total de-lithiation amount.

[0110] In some embodiments, the negative current collector can employ a metal foil or a composite current collector. For example, as a metal foil, a copper foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base layer (e.g., a base layer of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0111] In some embodiments, the negative active material can employ a negative active material for a battery known in the art. As an example, the negative active material can include at least one of artificial graphite, natural graphite, soft carbon, hard carbon, a silicon-based material, a tin-based material, and lithium titanate, etc. The silicon-based material can be selected from at least one of elemental silicon, a silicon oxide compound, a silicon-carbon composite, a silicon-nitrogen composite, and a silicon alloy. The tin-based material can be selected from at least one of elemental tin, a tin oxide compound, and a tin alloy. However, the present application is not limited to these materials, and other conventional materials that can be used as a negative active material for a battery can also be used. These negative active materials can be used alone or in combination of two or more.

[0112] In some embodiments, the negative film layer can further optionally include a binder. The binder can be selected from at least one of styrene butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0113] As described above, in some embodiments, the negative film layer can further optionally include conductive carbon. The conductive carbon can be selected from at least one of super P, acetylene black, conductive carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0114] In some embodiments, the negative film layer can further optionally include other additives, such as a thickening agent (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0115] In some embodiments, the negative electrode sheet can be prepared by dispersing the components described above for preparing the negative electrode sheet, such as the negative active material, the conductive carbon, the binder, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry, coating the negative electrode slurry on a negative current collector, and drying, cold-pressing, etc., to obtain the negative electrode sheet.

[0116] [Electrolyte]

[0117] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0118] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0119] For lithium-ion battery cells, the electrolyte salt may include one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0120] For lithium-ion battery cells, the solvent may include one or more of the following: ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0121] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0122] [Isolation membrane]

[0123] This application does not impose any particular restrictions on the type of separator membrane; any known porous separator membrane with good chemical and mechanical stability can be selected.

[0124] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0125] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0126] [Battery cell]

[0127] In some embodiments, the secondary battery is a single battery cell, which can be the smallest structural unit of the battery.

[0128] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0129] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0130] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 2 shows a square-structured battery cell 3 as an example.

[0131] In some embodiments, referring to FIG2, the battery cell 3 includes a housing 31, an end cap assembly 32, and an electrode assembly 33 disposed in the housing 31. The housing 31 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 31 has an opening communicating with the receiving cavity, and the end cap assembly 32 can be closed by covering the opening. The end cap assembly 32 includes electrode terminals 322, as shown in FIG3, the end cap assembly 32 includes two electrode terminals 322, one of which is a positive electrode terminal and the other is a negative electrode terminal. The electrode assembly 33 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 33. The positive electrode sheet, negative electrode sheet, and separator can be formed into the electrode assembly 33 by a winding process or a stacking process. The electrode assembly 33 includes an electrode assembly body 330 and tabs 331 extending from the electrode assembly body 330. The battery cell 3 also includes a connecting member 34, which is used to connect the tabs 331 and the electrode terminals 322 of the electrode assembly 33. The battery cell 3 may contain one or more electrode assemblies 33, which can be selected by those skilled in the art according to specific practical needs.

[0132] In some implementations, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0133] [Battery Device]

[0134] In some embodiments, the battery device includes at least one of a battery module and a battery pack.

[0135] Figure 3 is a schematic diagram of the structure of a battery device according to an embodiment of this application. For example, as shown in Figure 3, the battery device 4 is a battery pack. The battery pack may include a housing and multiple battery cells 3 housed in the housing. The multiple battery cells 3 are connected in series, parallel, or mixed connections. The battery cells 3 can directly form a battery pack, or they can first form battery modules, and then multiple battery modules form a battery pack. Referring to Figure 3, in the battery device 4, the multiple battery cells 3 can be arranged sequentially along the length of the battery device 4. Of course, they can also be arranged in any other arbitrary manner. Furthermore, the multiple battery cells 3 can be fixed using fasteners.

[0136] Optionally, the battery device 4 may also include a housing with a receiving space in which multiple battery cells 3 are received.

[0137] [Electrical appliances]

[0138] In addition, this application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in this application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0139] As the electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0140] Figure 4 shows an example of an electrical device 5. This electrical device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the secondary battery for this electrical device, a battery pack or battery module can be used.

[0141] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0142] [Example]

[0143] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0144] Example 1

[0145] (1) Preparation of positive electrode sheet

[0146] Lithium iron phosphate (LiFePO4), the positive electrode active material, acetylene black, and polyvinylidene fluoride (PVDF), were dissolved in N-methylpyrrolidone at a mass ratio of 97.9:0.5:1.6 and mixed thoroughly to obtain a positive electrode slurry. The positive electrode slurry was then uniformly coated onto aluminum foil used as the positive electrode current collector. After drying, rolling, and slitting, a coating weight of 23.4 mg / cm³ was obtained. 2 The compacted density is 2.7 g / cm³. 3 The positive electrode sheet.

[0147] (2) Preparation of negative electrode sheet

[0148] Preparation of fumed silicon-carbon materials: Porous carbon was weighed and placed in a rotary kiln, heated to 500℃ under argon protection, and held for 2 hours to desorb the air adsorbed within the porous carbon. A mixture of silane and argon gas was introduced into the rotary kiln at a flow rate of 4 L / min, with a volume ratio of silane to argon of 1:4, maintaining a slight positive pressure of 200 Pa. The rotary kiln was rotated at a frequency of 20 Hz to obtain silicon-carbon particles. The temperature of the rotary kiln was raised to 600℃, and a mixture of acetylene and argon gas was introduced at a flow rate of 3 L / min. The rotary kiln was then rotated at a frequency of 20 Hz to coat the silicon-carbon particles. After natural cooling, the particles were passed through a 200-mesh sieve to obtain the silicon-carbon material.

[0149] The second negative electrode active material is obtained by mixing silicon-carbon material with artificial graphite, the second graphite material. The second negative electrode active material, conductive carbon black, thickener CMC, binder SBR, and single-arm carbon nanotubes were mixed in a weight ratio of 96.4:0.5:1:2:0.1, deionized water was added, and the mixture was prepared into a second negative electrode active layer slurry with a solid content of 52% under vacuum stirring. The first graphite material, artificial graphite, conductive carbon black, thickener CMC, and binder SBR were mixed in a weight ratio of 97.9:0.5:1:0.6, deionized water was added, and the mixture was prepared into a first negative electrode active layer slurry with a solid content of 52% under vacuum stirring. The second negative electrode active layer slurry was then coated onto the negative electrode current collector copper foil using a double-layer extrusion coating machine to obtain the second negative electrode active layer. The first negative electrode active layer slurry was then coated onto the second negative electrode active layer to obtain the first negative electrode active layer. After drying, the mixture was cold-pressed, trimmed, cut into sheets, and slit to obtain the negative electrode sheet. The coating weight of both the first and second negative electrode active layers is 5.12 mg / cm³. 2 The compaction density of the negative electrode sheet is 1.65 g / cm³. 3Based on the total mass of the second negative electrode active layer, the mass content of the fumed silicon carbide material is 2wt%, the volume average particle size of the second graphite material artificial graphite is 16μm, the volume average particle size of the first graphite material artificial graphite is 9μm, the volume average particle size of the fumed silicon carbide material is 9μm, and the specific capacity of the fumed silicon carbide material is 1700mAh / g.

[0150] (3) Separating membrane

[0151] Porous polyethylene film is used as the separator.

[0152] (4) Preparation of electrolyte

[0153] The organic solvents ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 3:3:4 to obtain a mixed solvent. Then, fully dried lithium salt (LiPF6) was dissolved in the above mixed solvent at a ratio of 1 mol / L, and 3% fluoroethylene carbonate (FEC) was added to obtain the electrolyte.

[0154] (5) Preparation of secondary batteries

[0155] The positive electrode, separator, and negative electrode are prepared in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The electrodes are then wound, hot-pressed, shaped, and welded to obtain the electrode assembly. The electrode assembly is placed in a square aluminum shell, vacuum-dried, and then injected with electrolyte. After standing, formation testing, aging, and capacity testing, a single battery cell with a volume of 0.411L is finally obtained.

[0156] Example 2

[0157] Compared to Example 1, in Example 2, based on the total mass of the second negative electrode active layer, the mass content of the fumed silicon-carbon material is 6 wt%, and the coating weight of both the first and second negative electrode active layers is 4.80 mg / cm³. 2 .

[0158] Example 3

[0159] Compared to Example 1, in Example 3, based on the total mass of the second negative electrode active layer, the mass content of the fumed silicon-carbon material is 10 wt%, and the coating weight of both the first and second negative electrode active layers is 4.48 mg / cm³. 2 .

[0160] Example 4

[0161] Compared to Example 1, in Example 4, based on the total mass of the second negative electrode active layer, the mass content of the fumed silicon-carbon material is 14 wt%, and the coating weight of both the first and second negative electrode active layers is 4.22 mg / cm³. 2 .

[0162] Example 5

[0163] Compared to Example 1, in Example 5, based on the total mass of the second negative electrode active layer, the mass content of the fumed silicon-carbon material is 18 wt%, and the coating weights of both the first and second negative electrode active layers are 3.96 mg / cm³. 2 .

[0164] Example 6

[0165] Compared with Example 2, with the same coating weights for the first and second negative electrode active layers as in Example 2, in Example 6, based on the total mass of the second negative electrode active layer, the mass content of the fumed silicon carbide material is 12wt%, and the specific capacity of the fumed silicon carbide material is 800mAh / g.

[0166] Example 7

[0167] Compared with Example 2, with the same coating weights for the first and second negative electrode active layers as in Example 2, in Example 7, based on the total mass of the second negative electrode active layer, the mass content of the fumed silicon carbide material is 4.5 wt%, and the specific capacity of the fumed silicon carbide material is 2300 mAh / g.

[0168] Example 8

[0169] Compared with Example 2, with the same coating weights for the first and second negative electrode active layers as in Example 2, in Example 8, based on the total mass of the second negative electrode active layer, the mass content of silicon-based material is 8 wt%, the capacity of silicon-based material is 1200 mAh / g, and the silicon-based material in Example 8 is silicon suboxide.

[0170] Example 9

[0171] Compared with Example 2, with the same coating weights for the first and second negative electrode active layers as in Example 2, in Example 9, based on the total mass of the second negative electrode active layer, the mass content of silicon-based material is 4.5 wt%, the specific capacity of silicon-based material is 2300 mAh / g, and the silicon-based material in Example 9 is nano-silicon.

[0172] Example 10

[0173] Compared to Example 2, the volume average particle size of the silicon-based material in Example 10 is 3 μm.

[0174] Example 11

[0175] Compared to Example 2, the volume average particle size of the silicon-based material in Example 11 is 15 μm.

[0176] Example 12

[0177] Compared with Example 2, the volume average particle size of the first graphite material in Example 12 is 6 μm, and the volume average particle size of the second graphite material is 16 μm.

[0178] Example 13

[0179] Compared with Example 2, the volume average particle size of the first graphite material in Example 13 is 11 μm, and the volume average particle size of the second graphite material is 16 μm.

[0180] Example 14

[0181] Compared with Example 2, the volume average particle size of the first graphite material in Example 14 is 9 μm, and the volume average particle size of the second graphite material is 13 μm.

[0182] Example 15

[0183] Compared with Example 2, the volume average particle size of the first graphite material in Example 15 is 9 μm, and the volume average particle size of the second graphite material is 20 μm.

[0184] Example 16

[0185] Compared to Example 2, in Example 16, the mass content of fumed silicon carbide material based on the total mass of the second negative electrode active layer is 5 wt%. The fumed silicon carbide material is mixed with the first graphite material, artificial graphite, to obtain the first negative electrode active material. The first negative electrode active material, conductive agent carbon black, thickener CMC, and binder SBR are mixed in a weight ratio of 97.9:0.5:1:0.6, deionized water is added, and a solid content of 52% is set. The mixture is then stirred in a vacuum mixer to prepare the first negative electrode active layer slurry. Based on the total mass of the first negative electrode active layer, the mass content of fumed silicon carbide material is 1 wt%.

[0186] Comparative Example 1

[0187] Compared with Example 2, with the same coating weight of the first negative electrode active layer and the second negative electrode active layer as in Example 2, the first negative electrode active layer and the second negative electrode active layer in Comparative Example 1 each contain 3 wt% silicon-carbon material.

[0188] Comparative Example 2

[0189] Compared with Example 2, the first negative electrode active layer in Comparative Example 2 contains 4 wt% silicon-carbon material and the second negative electrode active layer contains 2 wt% silicon-carbon material.

[0190] Comparative Example 3

[0191] Compared with Example 2, the second negative electrode active layer in Comparative Example 3 contains 20 wt% silicon-carbon material, and the delithiation rate of the second negative electrode active layer from 0.5V to 2V is 21.5%.

[0192] The secondary batteries obtained in Examples 1-16 and Comparative Examples 1-3 were subjected to parameter and performance tests. The test results are shown in Table 1 below.

[0193] Table 1: Specific parameters of Examples 1-16 and Comparative Examples 1-3

[0194] (1) Energy density test

[0195] The volumetric energy density of a single battery cell = the energy of the single battery cell / the volume of the single battery cell.

[0196] The energy of a single battery cell is measured as follows: At 25°C, the battery cell is charged and discharged within a voltage range of 2V-3.65V using a current density of 0.33C. After three cycles, the energy of the third discharge cycle is recorded as the energy of the battery cell. The unit for battery cell energy is Wh, and the unit for battery cell capacity is Ah.

[0197] (2) Cyclic life test

[0198] At 25°C, the battery cells were discharged to 10% SOC using a current density of 0.33C, and then charged to 100% SOC at a constant current of 0.33C. This 10% SOC-100% SOC cycle was repeated 9 times. On the 10th cycle, the cells were discharged to 0% SOC, and capacity decay was calculated starting from the discharge capacity of the 10th cycle. This process was repeated 1000 times, and the capacity retention rate of the battery cells after 1000 cycles was recorded.

[0199] Among them, the higher the capacity retention rate, the longer the cycle life of the battery cell.

[0200] (3) Test method for expansion force

[0201] The secondary battery is placed between the first and second plates of a three-piece rigid clamp, and a pressure sensor is placed between the second and third plates. The sensor is connected to a computer to monitor the expansion force in the planar area of ​​the secondary battery online.

[0202] (4) Lithium removal percentage (ρ) test from 0.5V to 2V

[0203] At 25°C, the secondary battery was discharged to 2V at a rate of 0.05C. The battery was then disassembled to obtain the negative electrode. The electrode was fixed to adhesive tape, and the first negative electrode active layer was peeled off using the tape's adhesive properties. The negative electrode was then cut to a specific size and assembled with a lithium sheet to form a coin cell. It was then discharged at a constant current of 200µA to 0.005V, followed by constant current charging to 2.0V. During this process, the charging capacity within the lithium potential range of 0.5V to 2V was recorded, denoted as the amount of lithium delithiation (C1) within this range. The total amount of lithium delithiation was the charging capacity within the 0.005V to 2V range (Ctotal), where ρ = C1 / Ctotal.

[0204] (5) Powder particle size test

[0205] The fully loaded negative electrode sheet is placed in DI water to separate it from the negative electrode current collector. After washing and filtering three times, the active material particles are dried. The particle size distribution of the powder is then tested using the laser particle size distribution method, referring to GB / T 19077-2016 / ISO 13320:2009.

[0206] (6) Aspect Ratio Test

[0207] Disassemble the battery cell fully discharged, remove the negative electrode, and perform a CP test, referring to document JY / T010-1996. According to the CP interface diagram, connect the two ends of the particle (red line) as the major axis, and then draw a perpendicular line from the midpoint of the major axis to the edge of the particle (green line) as the minor axis. The major-to-minor ratio = length of the major axis / length of the minor axis.

[0208] The test results for the above parameters are shown in Table 2.

[0209] Table 2: Test results of Examples 1-16 and Comparative Examples 1-3

[0210] According to Examples 1-16 and Comparative Examples 1-3, when the positive electrode includes lithium iron phosphate, by setting a double-layer active layer for the negative electrode, the second negative electrode active layer near the negative electrode current collector side includes silicon-based material and graphite, the first negative electrode active layer on the surface of the negative electrode film layer includes graphite material, the specific capacity of the silicon-based material is 800mAh / g to 2300mAh / g, the mass percentage of the silicon-based material in the second negative electrode active layer is 2wt% to 30wt%, and the mass percentage of the silicon-based material in the first negative electrode active layer is less than the mass percentage of the silicon-based material in the second negative electrode active layer; in the second negative electrode active layer, the amount of lithium delithiation in the voltage range of 0.5V to 2V accounts for 5% to 20% of the total delithiation amount, the lithium secondary battery has a high energy density, while also taking into account the cycle performance of the secondary battery.

[0211] As can be seen from Examples 1-5, by maintaining the mass ratio of silicon-based material in the second negative electrode active layer within the range of 2 to 30, the delithiation ratio of the second negative electrode active layer at 0.5V to 2V can be maintained at 5% to 20%, resulting in a high energy density for the lithium secondary battery. At the same time, it avoids electrode expansion caused by an excessively high proportion of silicon-based material, takes into account the cycle performance of the battery, and has a high capacity retention rate.

[0212] As can be seen from Examples 2 and 6-7, by controlling the specific capacity of the silicon-based material within the range of 800mAh / g to 2300mAh / g, and ensuring that the lithium delithiation rate of the second negative electrode active layer (0.5V-2V) is 5% to 20%, the lithium secondary battery has a high cycle life.

[0213] Combining Examples 2 and 8-9, different types of silicon-based materials can improve the energy density of secondary batteries while maintaining cycle life.

[0214] Combining Examples 2 and 10-11, when the volume average particle size of silicon-based materials is between 3 μm and 15 μm, the secondary battery has a high cycle life. As the volume average particle size of silicon-based materials increases, the energy density increases, the expansion force increases, and the cycle performance decreases.

[0215] In conjunction with Examples 2 and 12-15, by controlling the volume average particle size of the first graphite material to be 6μm to 11μm and the volume average particle size of the second graphite material to be 13μm to 20μm, the energy density can be improved, local stress concentration can be reduced, and the cycle performance of the secondary battery can be improved.

[0216] In conjunction with Examples 2, 16 and Comparative Examples 1-2, the cycle performance of the secondary battery can be improved by controlling the mass content of silicon-based material in the first negative electrode active layer to be less than the mass content of silicon-based material in the second negative electrode active layer.

[0217] Combining Example 2 and Comparative Example 3, by controlling the mass ratio of silicon-based materials and maintaining the delithiation ratio of the second negative electrode active layer in the range of 5% to 20% at 0.5V to 5V, the energy density of the secondary battery can be improved while reducing electrode expansion and taking into account cycle performance.

[0218] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A lithium-ion secondary battery, characterized in that, include: A positive electrode sheet, wherein the positive electrode sheet includes a positive active material, and the positive active material includes a lithium phosphate; The negative electrode sheet includes a negative current collector and a negative electrode film layer; The negative electrode film layer includes a first negative electrode active layer and a second negative electrode active layer, wherein the second negative electrode active layer is disposed between the negative electrode current collector and the first negative electrode active layer; The first negative electrode active layer includes a first graphite material; The negative electrode film layer comprises a silicon-based material; The specific capacity of the silicon-based material is 800mAh / g to 2300mAh / g, and the mass percentage of the silicon-based material in the second negative electrode active layer is 2wt% to 30wt% based on the total mass of the second negative electrode active layer. Based on the total mass of the negative electrode film layers, the mass percentage of the silicon-based material in the first negative electrode active layer is less than the mass percentage of the silicon-based material in the second negative electrode active layer; In the second negative electrode active layer, the amount of lithium delithiation within the voltage range of 0.5V to 2V accounts for 5% to 20% of the total lithium delithiation.

2. The lithium-ion secondary battery according to claim 1, characterized in that, Based on the total mass of the negative electrode film, the mass ratio A of the silicon-based material in the first negative electrode active layer and the mass ratio B of the silicon-based material in the second negative electrode active layer satisfy: 0 ≤ A / B ≤ 0.

5.

3. The lithium-ion secondary battery according to claim 1 or 2, characterized in that, The active material in the first negative electrode active layer is artificial graphite.

4. The lithium-ion secondary battery according to any one of claims 1 to 3, characterized in that, The second negative electrode active layer includes a second graphite material; The aspect ratio of the first graphite material is less than 2; The aspect ratio of the second graphite material is 2 to 4.

5. The lithium-ion secondary battery according to claim 4, characterized in that, The volume average particle size of the first graphite material is 6 μm to 11 μm; The volume average particle size of the second graphite material is 13 μm to 20 μm.

6. The lithium-ion secondary battery according to any one of claims 1 to 5, characterized in that, The silicon-based material includes at least one of elemental silicon, silicon-oxygen materials, and silicon-carbon materials.

7. The lithium-ion secondary battery according to claim 6, characterized in that, The silicon-carbon material comprises porous carbon and silicon material deposited in the pores of the porous carbon.

8. The lithium-ion secondary battery according to claim 6 or 7, characterized in that, The surface of the silicon-carbon material has a coating layer, which includes carbon elements.

9. The lithium-ion secondary battery according to any one of claims 6 to 8, characterized in that, The specific capacity of the silicon-carbon material is 1400mAh / g to 2300mAh / g.

10. The lithium-ion secondary battery according to claim 9, characterized in that, The specific capacity of the silicon-carbon material is 1600mAh / g to 2000mAh / g.

11. The lithium-ion secondary battery according to any one of claims 6 to 10, characterized in that, The volume average particle size of the silicon-carbon material is 3 μm to 15 μm.

12. The lithium-ion secondary battery according to claim 11, characterized in that, The volume average particle size of the silicon-carbon material is 6 μm to 10 μm.

13. The lithium-ion secondary battery according to any one of claims 1 to 12, characterized in that, The lithium-containing phosphate includes one or more of lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate, and their modified forms.

14. A battery device, characterized in that, The battery device includes a lithium-ion secondary battery as described in any one of claims 1-13, wherein the battery device comprises at least one of a battery module and a battery pack.

15. An electrical appliance, characterized in that, Includes the battery device as described in claim 14.

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