Battery

Abstract

The invention provides a battery. The battery comprises a positive plate and a negative plate, the negative plate comprises a negative active material layer, the negative active material layer comprises a negative active material, the negative active material comprises graphite, and the average particle size of the negative active material is 0.8-8 [mu] m; the positive plate comprises a positive active material layer, the positive active material layer comprises a positive active material, the positive active material comprises secondary particles, the secondary particles comprise primary particles, the ratio of the average particle size of the secondary particles to the average particle size of the negative active material is 1: (0.02-10), and the ratio of the average particle size of the secondary particles to the average particle size of the negative active material is 1: (0.02-10). The battery has relatively high energy density, good storage performance and low-temperature performance.

Description

Technical Field

[0001] The present invention relates to the technical field of battery energy, and particularly to a battery. Background Art

[0002] A battery is a common electrochemical device. For example, lithium-ion batteries are currently widely used in industries such as portable electronic devices and electric vehicles. The expansion of the portable electronic product and electric vehicle markets has also put forward higher requirements for the performance of batteries such as lithium-ion batteries. Graphite is a common negative electrode active material. However, traditional batteries with graphite as the negative electrode active material have poor performance in low-temperature environments, which limits the application of batteries in low-temperature environments. At the same time, due to factors such as poor ability of active ions to intercalate and deintercalate in the negative electrode active material and mismatched ability of active ions to intercalate and deintercalate in the positive and negative electrodes, batteries generally have defects such as low energy density, poor low-temperature performance, short cycle life, and poor storage performance. Summary of the Invention

[0003] The present invention provides a battery with high energy density, long cycle life, good storage performance, and low-temperature performance, overcoming the defects existing in the prior art.

[0004] One aspect of the present invention provides a battery, including a positive electrode sheet and a negative electrode sheet; the negative electrode sheet includes a negative electrode active material layer, the negative electrode active material layer includes a negative electrode active material, the negative electrode active material includes graphite, and the average particle size of the negative electrode active material is 0.8 - 8 μm; the positive electrode sheet includes a positive electrode active material layer, the positive electrode active material layer includes a positive electrode active material, the positive electrode active material includes secondary particles, the secondary particles include primary particles, and the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:0.02 - 10.

[0005] According to an embodiment of the present invention, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:0.04 - 6; and / or, the secondary particles are spherical structures.

[0006] According to an embodiment of the present invention, the average particle size of the secondary particles is 0.8 - 40 μm, preferably 1 - 30 μm; and / or, the average particle size of the primary particles is 0.1 - 0.6 μm; and / or, the ratio of the average particle size of the secondary particles to the average particle size of the primary particles is 1:0.0025 - 0.75, preferably 1:0.01 - 0.5.

[0007] According to an embodiment of the present invention, the average particle size of the negative electrode active material is 1.2 - 6 μm.

[0008] According to an embodiment of the present invention, the specific surface area of the negative electrode active material is 1 - 8 m2 / g; and / or, the specific surface area of the secondary particles is 4 to 20 m 2 / g; and / or, the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is 1:0.05 to 2, preferably 1:0.1 to 1.8.

[0009] According to an embodiment of the present invention, the positive electrode active material includes LiMn2O4, Li4Ti5O 12 , Li3V2(PO4)3, LiFe 1-x E x PO4, LiNi x Mn y Co 1-x-y O2 and LiNi x Co y Al 1-x-y O2, and one or more of them, where 0 ≤ x ≤ 1, 0 ≤ y ≤ 1, x + y ≤ 1, and E is selected from one or more of Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, V, and Ti; preferably, the positive electrode active material is LiFePO4.

[0010] According to an embodiment of the present invention, the areal density of the negative electrode active material layer is 1.9 to 10.8 mg / cm²; and / or, the tap density of the negative electrode active material layer is 1.2 to 1.5 g / cm 3 ; and / or, the areal density of the positive electrode active material layer is 4 to 20 mg / cm²; and / or, the tap density of the positive electrode active material layer is 1.7 to 2.0 g / cm 3 .

[0011] According to an embodiment of the present invention, the battery further includes a separator, and the porosity of the separator is 36% to 64%; and / or, the battery further includes an electrolyte; the viscosity of the electrolyte is less than or equal to 5 mPa·s.

[0012] According to an embodiment of the present invention, the electrolyte includes a solvent, and the solvent includes one or more of dimethyl carbonate, methyl acetate, ethyl propionate, 1,1,2,2,3,3,4,4 - octafluoro - 5 - methoxypentane, 1,1,2,2 - tetrafluoroethyl - 2,2,3,3 - tetrafluoropropyl ether, bis(2,2,2 - trifluoroethyl) ether, ethyl methyl carbonate, and 1,2 - propylene carbonate.

[0013] According to an embodiment of the present invention, the positive electrode sheet includes a first lithium supplement agent; preferably, the first lithium supplement agent includes lithium metal and lithium - containing compounds, the lithium metal includes one or more of lithium powder, lithium foil, and lithium strip, and the lithium - containing compounds include Li x Ny , Li x S y , Li x M y O z One or more of them, wherein, for Li x N y in it, x≥1, y>0; for Li x N y in it, x≥1, y>0; for Li x M y O z in it, x≥1, z≥1, y≥0, and M is selected from one or more of Fe, Ni, Si, Co, Mn, and C; preferably, the mass percentage of the first lithium supplement agent in the positive electrode active material layer is 0.2% - 5%.

[0014] The implementation of the present invention has at least the following beneficial effects: The battery of the present invention includes a negative electrode active material, and the negative electrode active material includes graphite. By controlling the average particle size of the negative electrode active material to be 0.8 - 8 μm, and matching it with a positive electrode active material including secondary particles, and controlling the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material to be 1:0.02 - 10, the battery has a high energy density, a long cycle life, good storage performance, and low-temperature performance, enabling the battery to have good electrochemical performance in a wider temperature range. Description of the Drawings

[0015] Figure 1 It is a scanning electron microscope image of the negative electrode active material layer on one side of the negative electrode sheet in Example 1;

[0016] Figure 2 It is a scanning electron microscope image of the negative electrode active material layer on one side of the negative electrode sheet in Comparative Example 2;

[0017] Figure 3 It is a scanning electron microscope image of the positive electrode active material layer on one side of the positive electrode sheet in Example 1. Detailed Embodiments

[0018] To enable those skilled in the art to better understand the solution of the present invention, the present invention will be further described in detail below. The specific embodiments listed below only describe the principles and features of the present invention, and the examples given are only used to explain the present invention and do not limit the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

[0019] Conventional batteries with graphite as the negative electrode active material have poor performance in low-temperature environments, which limits their application in low-temperature environments. At the same time, due to factors such as poor intercalation and deintercalation ability of active ions in the negative electrode active material and mismatched intercalation and deintercalation abilities of active ions in the positive and negative electrodes, batteries generally have defects such as low energy density, poor low-temperature performance, short cycle life, and poor storage performance. For example, when the battery is a lithium-ion battery, the active ion is a lithium ion.

[0020] Based on this, an embodiment of the present invention provides a battery, including a positive electrode sheet and a negative electrode sheet; the negative electrode sheet includes a negative electrode active material layer, the negative electrode active material layer includes a negative electrode active material, the negative electrode active material includes graphite, and the average particle size of the negative electrode active material is 0.8 to 8 μm, such as 0.8 μm, 1 μm, 1.2 μm, 1.5 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, or a range composed of any two of them; the positive electrode sheet includes a positive electrode active material layer, the positive electrode active material layer includes a positive electrode active material, the positive electrode active material includes secondary particles, the secondary particles include primary particles, and the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:0.02 to 10, such as 1:0.02, 1:0.04, 1:0.05, 1:0.1, 1:0.2, 1:0.5, 1:1, 1:2, 1:3, 1:5, 1:6, 1:8, 1:10, or a range composed of any two of them.

[0021] According to the research of the inventors, low-temperature conditions will reduce the diffusion rate of active ions in the positive and negative electrode active materials. This application uses a negative electrode active material with an average particle size of 0.8 - 8 μm, which is beneficial to shortening the diffusion path of active ion intercalation and deintercalation, increasing the active ion migration channels, enhancing the active ion intercalation and deintercalation ability, reducing the internal resistance of the battery, and thus improving the energy density and low-temperature performance of the battery (specifically reflected in the cold start performance). At the same time, the positive electrode active material includes secondary particles formed by primary particles. The porous structure of the secondary particles and the smaller-sized primary particles that make up the secondary particles help to shorten the diffusion path of active ions inside the material, reduce the resistance of active ion diffusion, and accelerate the transmission speed of active ions; at the same time, controlling the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material to be 1:0.02 to 10 is beneficial to matching the active ion intercalation and deintercalation abilities in the positive and negative electrodes while enhancing the active ion intercalation and deintercalation ability in the negative electrode active material, thereby inhibiting the massive accumulation of active ions and the precipitation of active ions on the surface of the negative electrode, which is beneficial to reducing the irreversible loss of active ions and improving the cycle life and storage performance (especially high-temperature storage performance) of the battery.

[0022] When the ratio of the average particle size of the secondary particles of the positive electrode active material to the average particle size of the negative electrode active material is less than 1:10, the particle size of the secondary particles is too small or the particle size of the negative electrode active material is too large; the positive electrode active material with too small a particle size has too large a specific surface area, the surface energy of the particles increases, and they are easy to agglomerate and react with the electrolyte, which increases the internal resistance of the lithium-ion battery and seriously affects the cold start and energy density; the negative electrode active material with too large a particle size has too long a diffusion path for lithium ions to embed and escape inside, and the larger internal resistance affects the cold start capability. When the ratio of the average particle size of the secondary particles to the negative electrode active material is greater than 1:0.02, the particle size of the secondary particles of the positive electrode active material is too large or the particle size of the graphite of the negative electrode active material is too small; the positive electrode secondary particles with too large particle size result in a too long diffusion path for lithium ions during the insertion and extraction process, and the increase in internal resistance affects the energy density of the lithium-ion battery; the graphite particles with too small particle size have too large a specific surface area in contact with the electrolyte, and the SEI film formed during the initial charge and discharge process consumes too much charge, resulting in excessive irreversible capacity loss, which reduces the initial coulombic efficiency, energy density and capacity retention rate of the lithium-ion battery.

[0023] In addition, during the battery cycle, due to the embedding and extraction of active ions, the graphite will expand in volume, causing the positive electrode sheet to be squeezed, and the secondary particles formed by the agglomeration of primary particles will have cracking problems. The use of negative electrode active materials with an average particle size of 0.8~8μm can better withstand the volume changes caused by the embedding and extraction of active ions, reduce the rupture and structural peeling of negative electrode active materials, and inhibit the volume expansion of negative electrode active materials during charging and discharging. By controlling the ratio of the average particle size of secondary particles to the average particle size of negative electrode active materials to 1:0.02~10, it is beneficial to suppress the volume expansion of negative electrode active materials while reducing the extrusion of positive electrode sheets caused by the volume expansion of negative electrode active materials, which can effectively alleviate the rupture of secondary particles and improve battery performance such as stability and cycle life.

[0024] Therefore, under the above system, the negative electrode active material in the present invention includes graphite, and the average particle size of the negative electrode active material is controlled to be 0.8~8 μm, and the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:0.02~10, which is beneficial to improve the deintercalation ability of the active ions in the negative electrode active material, and match the deintercalation ability of the active ions in the positive and negative electrodes, thereby improving the energy density, low temperature performance, cycle life and storage performance of the battery.

[0025] In some embodiments, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:0.04 to 6, such as 1:0.04, 1:0.05, 1:0.1, 1:0.2, 1:0.5, 1:1, 1:2, 1:3, 1:5, 1:6, or the range composed of any two of them. While enhancing the ability of active ions to intercalate and deintercalate in the negative electrode active material, the ability of active ions to intercalate and deintercalate in the positive and negative electrodes is matched, thereby inhibiting the massive accumulation of active ions and the precipitation of active ions on the surface of the negative electrode, which is beneficial to reducing the irreversible loss of active ions and improving the performance of the battery such as cycle life and storage performance. In addition, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material within the above range is beneficial to further reducing the extrusion of the positive electrode sheet caused by the volume expansion of graphite, effectively alleviating the cracking of the secondary particles, and improving the performance of the battery such as stability and cycle life.

[0026] In this application, materials such as negative electrode active materials and secondary particles can be obtained by conventional methods in the art, such as commercially purchased or self-made by conventional methods in the art.

[0027] In some embodiments, the secondary particles are spherical structures, that is, the secondary particles of the present invention are basically spherical, and can also be called secondary spherical particles. Compared with non-spherical structures such as flakes, the spherical structure of the secondary particles is beneficial to reducing the charge transfer impedance and the active ion diffusion impedance, reducing the internal resistance of the battery, and improving the rate performance of the battery. At the same time, the combined use of the secondary spherical particles and the negative electrode active material with an average particle size of 0.8 to 8 μm helps to reduce the interface impedance of the battery, improve the energy density of the battery, and at the same time improve the low-temperature discharge ability of the battery.

[0028] In the embodiments of the present invention, the average particle size of the negative electrode active material can be measured by analyzing the particle sizes of at least 10 negative electrode active material particles through a scanning electron microscope (SEM) or other electron microscopes, and calculating the average value of the particle sizes of at least 10 negative electrode active material particles as the average particle size.

[0029] Specifically, during implementation, the average particle size of the negative electrode active material can be measured through the following process: Perform SEM testing on the negative electrode active material layer on the surface of the negative electrode sheet. At a magnification of 5 K and an image size of 20 μm Under 20 μm, measure the particle sizes of at least 10 negative electrode active material particles. If the number of negative electrode active material particles in one image is less than 10, then select another image for measurement until the particle sizes of at least 10 graphite particles are measured. Then calculate the average value of the measured particle sizes of at least 10 graphite particles, which is the average particle size of graphite. Among them, the particle size of each negative electrode active material particle is the average of its maximum size and its minimum size.

[0030] In the embodiments of the present invention, the average particle size of the secondary particles can be measured by analyzing the particle sizes of at least 30 secondary particles through methods such as scanning electron microscopy (SEM) or other electron microscopes, and calculating the average value of the particle sizes of at least 30 secondary particles as the average particle size.

[0031] Specifically, the average particle size of the secondary particles can be measured through the following process: perform SEM testing on the positive electrode active material layer on the surface of the positive electrode sheet with a magnification of 5 K and an image size of 10 μm. At 10 μm, measure the particle sizes of at least 30 secondary particles. If the number of secondary particles in one image is less than 30, randomly select another image for measurement until the particle sizes of at least 30 secondary particles are measured. Then calculate the average value of the measured particle sizes of at least 30 secondary particles, which is the average particle size of the secondary particles. Among them, the particle size of each secondary particle is the average of its maximum size and its minimum size.

[0032] In the embodiments of the present invention, the average particle size of the primary particles can be measured by analyzing the particle sizes of at least 30 secondary particles through methods such as scanning electron microscopy (SEM) or other electron microscopes, and calculating the average value of the particle sizes of at least 30 primary particles as the average particle size.

[0033] Specifically, the average particle size of the primary particles can be measured through the following process: perform SEM testing on the positive electrode active material layer on the surface of the positive electrode sheet at a magnification of 30 K and an image size of 1 μm. At 1 μm, measure the particle sizes of the primary particles. Measure the particle sizes of at least 30 primary particles. If the number of primary particles in one image is less than 30, randomly select another image for measurement until the particle sizes of at least 30 primary particles are measured. Then calculate the average value of the measured particle sizes of at least 30 primary particles, which is the average particle size of the primary particles. Among them, the particle size of each primary particle is the average of its maximum size and its minimum size.

[0034] Generally, the negative electrode sheet further includes a negative electrode current collector. The negative electrode active material layer is present on at least one side surface of the negative electrode current collector. The negative electrode active material layer further includes a negative electrode conductive agent, a thickening agent, and a negative electrode binder. The negative electrode conductive agent may include one or more of conductive carbon black, carbon nanotubes (CNT), acetylene black, graphene, Ketjen black, and carbon fiber; the thickening agent may include sodium carboxymethyl cellulose; the negative electrode binder may include one or more of carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyvinyl alcohol, and sodium polyacrylate; the negative electrode current collector includes copper foil.

[0035] Generally, the positive electrode plate further includes a positive current collector, and the positive active material layer is present on at least one surface of the positive current collector. The positive active material layer further includes a positive binder and a positive conductive agent. The positive binder includes one or more of polyvinylidene fluoride (PVDF), polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, polyvinyl chloride, carboxylated polyvinyl chloride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, etc.; the positive conductive agent includes one or more of conductive carbon black, carbon nanotubes (CNT), acetylene black, graphene, Ketjen black, carbon fiber; the positive current collector includes aluminum foil.

[0036] In some embodiments, the average particle size of the secondary particles is 0.8 to 40 μm, such as 0.8 μm, 0.9 μm, 1.0 μm, 2.0 μm, 3.0 μm, 5.0 μm, 10.0 μm, 20.0 μm, 30.0 μm, 40.0 μm or the range composed of any two of them. Preferably, it is 1 to 30 μm. When the average particle size of the secondary particles is greater than or equal to 0.8 μm, the specific surface area of the secondary particles is relatively small, the surface energy of the particles is low, it is not easy to agglomerate, and it is beneficial to inhibit the side reaction between the secondary particles and the electrolyte, reduce the internal resistance of the battery, and thus improve the cold start performance and energy density of the battery; at the same time, controlling the average particle size of the secondary particles to be less than or equal to 40 μm is beneficial to shorten the diffusion path of active ions during the process of insertion and extraction, reduce the internal resistance of the battery, and thus improve the energy density and low-temperature cold start performance of the battery.

[0037] In addition, the average particle size of the primary particles is 0.1 to 0.6 μm, such as 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm or the range composed of any two of them, which is beneficial to shorten the active ion transport path, reduce the internal resistance of the battery, and thus improve the energy density and low-temperature cold start performance of the battery.

[0038] In some embodiments, the ratio of the average particle size of the secondary particles to the average particle size of the primary particles is 1:0.0025 to 0.75, such as 1:0.0025, 1:0.005, 1:0.0075, 1:0.01, 1:0.02, 1:0.05, 1:0.1, 1:0.15, 1:0.2, 1:0.3, 1:0.4, 1:0.5, 1:0.6, 1:0.7, 1:0.75 or the range composed of any two of them. When the ratio of the average particle size of the secondary particles to the average particle size of the primary particles is within the above range, it is beneficial to shorten the diffusion path of active ions inside the primary particles and secondary particles, reduce the internal resistance of the battery, thereby improving the rate performance and low-temperature performance of the battery and other performances, and is beneficial to the close arrangement of the primary particles in the secondary particles, reduce the voids between the secondary particles, improve the tap density of the positive active material and the compaction density of the positive active material layer, and further improve the energy density of the battery. Preferably, it is 1:0.01 to 0.5.

[0039] In some embodiments, the average particle size of the negative active material is 1.2 to 6 μm, such as 1.2 μm, 1.5 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm or the range composed of any two of them. When the average particle size of the negative active material is within the above range, it is beneficial to further shorten the diffusion path of active ion insertion and extraction, increase the active ion migration channels, improve the active ion insertion and extraction ability, reduce the internal resistance of the battery, thereby improving the energy density and low-temperature performance of the battery. And when the average particle size of the negative active material is within the above range, it can better withstand the volume change caused by the process of active ion insertion and extraction, reduce the cracking and structural exfoliation of the negative active material, further inhibit the volume expansion of the negative active material during charge and discharge, and further reduce the extrusion of the positive electrode sheet caused by the volume expansion of the negative active material, and can effectively alleviate the cracking of the secondary particles, improve the stability and cycle life of the battery and other performances.

[0040] In some embodiments, the specific surface area of the negative active material is 1 to 8 m 2 / g, such as 1 m 2 / g, 2 m 2 / g, 3m 2 / g, 4 m 2 / g, 5 m 2 / g, 6 m 2 / g, 7 m 2 / g, 8 m 2 / g or a range composed of any two of them is beneficial to improving the intercalation and deintercalation ability of active ions in the negative electrode active material, reducing the internal resistance of the battery, thereby improving the energy density and low-temperature performance of the battery, and is beneficial to reducing the contact area between the negative electrode active material and the electrolyte, thereby reducing the side reactions of the electrolyte and the charge consumed by the solid electrolyte interface film (SEI) formed during the first charge and discharge process of the battery, reducing the irreversible capacity loss, and thus improving the performance such as the first Coulomb efficiency and energy density of the battery.

[0041] In some embodiments, the specific surface area of the secondary particles is 4-20 m 2 / g, such as 4 m 2 / g, 5 m 2 / g, 6 m 2 / g, 7 m 2 / g, 8 m 2 / g, 9 m 2 / g, 10 m 2 / g, 12 m 2 / g, 15 m 2 / g, 18 m 2 / g, 20 m 2 / g or a range composed of any two of them. When the specific surface area of the secondary particles is within the above range, it is beneficial to improving the intercalation and deintercalation ability of active ions in the secondary particles, reducing the internal resistance of the battery, thereby improving the cold start performance and energy density of the battery, and when the specific surface area of the secondary particles is within the above range, its particle surface energy is low, it is not easy to agglomerate, and it is beneficial to suppressing the side reactions between the secondary particles and the electrolyte, thereby improving the performance such as the cycle life of the battery.

[0042] In some embodiments, the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is 1:0.05-2, such as 1:0.05, 1:0.1, 1:0.2, 1:0.5, 1:1, 1:1.5, 1:1.8, 1:2 or a range composed of any two of them, preferably 1:0.1-1.8. When the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is within the above range, the intercalation and deintercalation ability of active ions in the secondary particles and the negative electrode active material is similar, which is beneficial to suppressing the massive accumulation of active ions resulting in the precipitation of active ions on the surface of the negative electrode, reducing the irreversible loss of active ions, and improving the performance such as the cycle life and storage performance of the battery.

[0043] The specific surface area of the material (such as the specific surface area of the secondary particles and the specific surface area of the negative electrode active material) of the present invention can be measured by conventional methods in the art. Specifically, during implementation, an electronic analytical balance is used to weigh the sample, and then the sample is placed in a sample tube for degassing treatment. After the degassed sample tube is installed on the analysis station, TriStar 3020 is used to test the sample to obtain the specific surface area.

[0044] The battery of the present invention may include a lithium-ion battery.

[0045] In some embodiments, the positive electrode active material includes one or more of LiMn2O4, Li4Ti5O 12 , Li3V2(PO4)3, LiFe 1-x E x PO4, LiNi x Mn y Co 1-x-y O2 and LiNi x Co y Al 1-x-y O2, wherein 0 ≤ x ≤ 1, 0 ≤ y ≤ 1, x + y ≤ 1, and E is selected from one or more of at least one of Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, V, and Ti.

[0046] In some embodiments, the positive electrode active material is lithium iron phosphate (LiFePO4), that is, LiFe 1-x E x PO4 is LiFePO4, which can further take into account improving the high-temperature storage performance and cycle life of the battery and other performances.

[0047] Specifically, a battery with lithium iron phosphate as the positive electrode active material and graphite as the negative electrode active material has poor low-temperature performance. By controlling the average particle size of graphite to be 0.8 - 8 μm and controlling the ratio of the average particle size of graphite to the secondary particles of lithium iron phosphate to be 1:0.02 - 10, it is beneficial to shorten the diffusion path of active ions embedding and extracting, increase the migration channels of active ions, improve the ability of active ions to de-embed, reduce the internal resistance of the battery, thereby improving the energy density and low-temperature performance of the battery with lithium iron phosphate as the positive electrode active material. At the same time, making the ability of active ions to de-embed in the positive and negative electrodes match, and then inhibiting the large accumulation of active ions leading to the precipitation of active ions on the surface of the negative electrode, which is beneficial to reducing the irreversible loss of active lithium and improving the cycle life and storage performance of the battery with lithium iron phosphate as the positive electrode active material and other performances. Further improving the cycle life and other performances of the battery.

[0048] Specifically, in LiFe 1-x E x PO4, 0 ≤ x ≤ 1, and E is selected from one or more of at least one of Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, V, and Ti; in LiNi x Mn y Co 1-x-y O2, 0 ≤ x ≤ 1, 0 ≤ y ≤ 1, x + y ≤ 1; in LiNi x Co y Al 1-x-yIn O2, 0 ≤ x ≤ 1, 0 ≤ y ≤ 1, and x + y ≤ 1.

[0049] In the embodiments of the present invention, the areal density of the negative electrode active material layer (or negative electrode paste) is the areal density of single-sided coating. The testing processes of the areal density and the tap density are as follows: Take a negative electrode sheet sample. Specifically, a cutter can be used to cut the negative electrode sheet to obtain a negative electrode sheet sample with appropriate dimensions. Test the total mass m1 of the negative electrode sheet sample, the total thickness T1 of the negative electrode sheet sample, and the single-sided surface area S in the thickness direction of the negative electrode sheet sample. Among them, T1 = the total thickness of the negative electrode active material layer + the thickness of the negative electrode current collector. When negative electrode active material layers are respectively provided on both the front and back surfaces of the negative electrode current collector, the total thickness of the negative electrode active material layer = the thickness of the negative electrode active material layer on one surface of the negative electrode current collector + the thickness of the negative electrode active material layer on the other surface of the negative electrode current collector. Then, scrape off the negative electrode active material layer on the negative electrode sheet sample, and test the mass m2 of the obtained negative electrode current collector and the thickness T2 of the negative electrode current collector. Then, the total thickness of the negative electrode active material layer = T1 - T2, the areal density of the negative electrode active material layer = (m1 - m2) / (2 S), and the tap density of the negative electrode active material layer = the areal density of the negative electrode active material layer ×2 / the total thickness of the negative electrode active material layer = (m1 - m2) / (S×(T1 - T2)).

[0050] In the embodiments of the present invention, the areal density of the positive electrode active material layer (or positive electrode paste) is the areal density of single-sided coating. The testing processes of the areal density and the tap density are as follows: Take a positive electrode sheet sample. Specifically, a cutter can be used to cut the positive electrode sheet to obtain a positive electrode sheet sample with appropriate dimensions. Test the total mass m3 of the positive electrode sheet sample, the total thickness T3 of the positive electrode sheet sample, and the single-sided surface area S1 in the thickness direction of the positive electrode sheet sample. Among them, T3 = the total thickness of the positive electrode active material layer + the thickness of the positive electrode current collector. When positive electrode active material layers are respectively provided on both the front and back surfaces of the positive electrode current collector, the total thickness of the positive electrode active material layer = the thickness of the positive electrode active material layer on one surface of the positive electrode current collector + the thickness of the positive electrode active material layer on the other surface of the positive electrode current collector. Then, scrape off the positive electrode active material layer on the positive electrode sheet sample, and test the mass m4 of the obtained positive electrode current collector and the thickness T4 of the positive electrode current collector. Then, the total thickness of the positive electrode active material layer = T3 - T4, the areal density of the positive electrode active material layer = (m3 - m4) / (2 S1), and the tap density of the positive electrode active material layer = the areal density of the positive electrode active material layer ×2 / the total thickness of the positive electrode active material layer = (m3 - m4) / (S1×(T3 - T4)).

[0051] In some embodiments, the areal density of the negative electrode active material layer is 1.9 to 10.8 mg / cm², such as 1.9 mg / cm², 2.0 mg / cm², 3.0 mg / cm², 4.0 mg / cm², 5.0 mg / cm², 6.0 mg / cm², 7.0 mg / cm², 8.0 mg / cm², 9.0 mg / cm², 10.0 mg / cm², 10.8 mg / cm², or the range composed of any two of them.

[0052] In addition, the tap density of the negative electrode active material layer is 1.2 g / cm 3 ~1.5 g / cm 3 For example, 1.2 g / cm 3 、1.25 g / cm 3 、1.3 g / cm 3 、1.35 g / cm 3 、1.4 g / cm 3 、1.45 g / cm 3 、1.5 g / cm 3 Or the range composed of any two of them. Preferably 1.25 to 1.4 g / cm 3 When the tap density of the negative electrode active material layer is greater than or equal to 1.2 g / cm 3 , the spacing between the negative electrode active materials is small, increasing the contact probability and contact area between the negative electrode active materials, thereby improving the conductivity of the negative electrode sheet, reducing the polarization of the battery, and thus improving the cold start performance of the battery; when the tap density of the negative electrode active material layer is less than or equal to 1.5 g / cm 3 , the contact between the negative electrode active materials is relatively loose, increasing the channels for the transport of active ions, which is beneficial to the rapid movement of active ions, improving the discharge capacity of the battery, and thus improving the discharge capacity, cold start capacity, energy density and other performance of the battery.

[0053] In some embodiments, the areal density of the positive electrode active material layer is 4 to 20 mg / cm², such as 4 mg / cm², 5 mg / cm², 6 mg / cm², 7 mg / cm², 8 mg / cm², 9 mg / cm², 10 mg / cm², 12 mg / cm², 15 mg / cm², 18.0 mg / cm², 20 mg / cm², or the range composed of any two of them.

[0054] In addition, the tap density of the positive electrode active material layer is 1.7 to 2.0 g / cm 3 For example, 1.7 g / cm 3 、1.75 g / cm 3 、1.8 g / cm 3, 1.85 g / cm 3 , 1.9 g / cm 3 , 1.95 g / cm 3 , 2.0 g / cm 3 Or a range composed of any two of them is beneficial to increasing the contact probability and contact area between secondary particles, thereby improving the conductivity of the positive electrode sheet, reducing battery polarization, enhancing the cold start performance of the battery, and increasing the transmission channels for active ions, which is beneficial to promoting the transmission of active ions and improving the performance such as the energy density of the battery.

[0055] In some embodiments, the battery further includes a separator, and the porosity of the separator is 36% - 64%, such as 36%, 38%, 40%, 45%, 50%, 55%, 60%, 64% or a range composed of any two of them. Preferably 40% - 55%, it can provide more transmission channels for active ions, which is beneficial to reducing the internal resistance of the battery and improving the low-temperature performance of the battery. When the porosity of the separator is less than or equal to 64%, it can avoid the problem of micro-short circuit caused by excessive deposition of active ions on the negative electrode and piercing the separator, reducing the self-discharge rate of the battery; at the same time, when the porosity of the separator is greater than or equal to 36%, it is beneficial to the transmission of active ions, reducing the internal resistance of the battery, and improving the energy density and cold start performance of the battery.

[0056] The porosity of the separator in the present invention can be measured by conventional methods in the art. Specifically, when implemented, the porosity of the separator is measured by the mercury intrusion method using a Micromeritics AutoPore V 9600 mercury porosimeter.

[0057] The separator in the present invention is a high-porosity separator, including one or more of a first separator and a second separator. The first separator includes a base film and a coating located on at least one side of the base film, and the second separator includes a base film. Among them, the coating includes one or more of a ceramic layer and an adhesive layer. The ceramic layer includes one or more of alumina, boehmite, magnesia, magnesium hydroxide, and titanium oxide, and the adhesive layer includes polyvinylidene fluoride (PVDF).

[0058] Specifically, when the first separator includes a base film and a ceramic layer located on one side of the base film, the first separator is a separator with a single-sided ceramic, and the ceramic layer is located on the surface of the base film facing the positive electrode sheet or on the surface of the base film facing the negative electrode sheet.

[0059] When the first separator includes a base film and an adhesive layer located on one side of the base film, the first separator is a separator with a single-sided adhesive, and the adhesive layer can be located on the surface of the base film facing the positive electrode sheet or on the surface of the base film facing the negative electrode sheet.

[0060] When the first diaphragm includes a base membrane, a ceramic layer located on one side of the base membrane, and a glue layer located on the side of the base membrane facing away from the ceramic layer, the first diaphragm is a diaphragm using single-sided glue and single-sided ceramic, the glue layer is located on the side of the base membrane facing the positive electrode sheet, and the ceramic layer is located on the side of the base membrane facing the negative electrode sheet, or the glue layer is located on the side of the base membrane facing the negative electrode sheet, and the ceramic layer is located on the side of the base membrane facing the positive electrode sheet.

[0061] The above-mentioned base film can be a conventional membrane material in the field such as a polypropylene (PP) membrane (PP film), a polyethylene (PE) membrane (PE film), a polypropylene / polyethylene (PP / PE) double-layer composite membrane, a polyimide electrospinning membrane (PI), a polypropylene / polyethylene / polypropylene (PP / PE / PP) three-layer composite membrane, or a cellulose non-woven membrane.

[0062] In some embodiments, the battery also includes an electrolyte, and the viscosity of the electrolyte is less than or equal to 5 mPa·s, for example, less than or equal to 5 mPa·s, less than or equal to 4.5 mPa·s, less than or equal to 4 mPa·s, less than or equal to 3.5 mPa·s, less than or equal to 3 mPa·s, less than or equal to 2.5 mPa·s, less than or equal to 2 mPa·s, or a range consisting of any two thereof, preferably less than or equal to 3.5 mPa·s, which is beneficial to increasing the active ion migration rate, reducing concentration polarization, and reducing the internal resistance of the battery, thereby improving the energy density and cold start performance of the battery.

[0063] The viscosity of the electrolyte can be measured by conventional methods in the art. In specific implementation, the viscosity is measured at room temperature using a Fluidicam microfluidic visual rheometer from Formulaction, France.

[0064] In some embodiments, the electrolyte also includes a solvent, and the solvent includes one or more of dimethyl carbonate, methyl acetate, ethyl propionate, 1,1,2,2,3,3,4,4-octafluoro-5-methoxypentane, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, bis(2,2,2-trifluoroethyl) ether, ethyl methyl carbonate, and 1,2-propylene glycol carbonate, which is beneficial to reducing the viscosity of the electrolyte and improving the energy density and cold start performance of the battery.

[0065] Generally, the electrolyte also includes a lithium salt, which may include one or more of lithium hexafluorophosphate (LiPF6) and lithium perchlorate. In addition, the electrolyte may also include an additive, which may include one or more of fluoroethylene carbonate, difluoroethylene carbonate, fluorocyclotriphosphazene, hexafluorocyclotriphosphazene, and vinylene carbonate (VC).

[0066] In some embodiments, the positive electrode sheet includes a first lithium supplementing agent; preferably, the first lithium supplementing agent includes lithium metal and a lithium-containing compound, the lithium metal includes one or more of lithium powder, lithium foil, and lithium tape, and the lithium-containing compound includes Li x N y 、Li x S y 、Li x M y O z one or more of them, wherein, in Li x N y , x≥1, y>0; in Li x N y , x≥1, y>0, in Li x M y O z , x≥1, z≥1, y≥0, and M is selected from one or more of Fe, Ni, Si, Co, Mn, and C.

[0067] Exemplarily, Li x M y O z may include lithium nickelate (LiNiO2).

[0068] When the first lithium supplementing agent is lithium powder, the lithium powder and the binder are coated on the surface of the positive electrode active material layer, which is beneficial to improving the adhesion between the lithium powder and the positive electrode active material layer and the utilization efficiency of the lithium powder.

[0069] In the embodiments of the present application, the first lithium supplementing agent can be coated on the surface of the positive electrode active material layer by using a binder, and the binder used can be a conventional binding material, for example, it can include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), aqueous polyvinylidene fluoride (aqueous PVDF), polyvinyl alcohol (PVA), and polyvinylpyrrolidone (PVP), and no special limitation is made thereto.

[0070] When the first lithium supplementing agent is lithium foil or lithium tape, the surface of the positive electrode sheet is roll-pressed with lithium foil or lithium tape of metal lithium, and the positive electrode active material layer is in direct contact with the low-potential lithium foil or lithium tape of metal lithium. Under the action of the potential difference, electrons spontaneously migrate to the negative electrode sheet, and lithium ions are embedded in the negative electrode active material along with it.

[0071] When the first lithium supplementing agent is a lithium-containing compound, the first lithium supplementing agent exists in the positive electrode active material layer.

[0072] Specifically, the negative electrode sheet includes a second lithium supplementing agent, and the second lithium supplementing agent includes lithium metal, and the lithium metal includes one or more of lithium powder, lithium foil, and lithium tape.

[0073] When the second lithium supplement is lithium powder, the lithium powder and the binder are coated on the surface of the negative electrode active material layer, which is beneficial to improving the adhesion between the lithium powder and the negative electrode active material layer and the utilization efficiency of the lithium powder.

[0074] In the embodiments of the present application, the second lithium supplement can be coated on the surface of the negative electrode active material layer by using a binder. The binder used can be a conventional binding material, for example, it can include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), aqueous polyvinylidene fluoride (aqueous PVDF), polyvinyl alcohol (PVA), and polyvinylpyrrolidone (PVP), and no special limitation is made thereto.

[0075] When the second lithium supplement is a lithium foil or a lithium strip, the metal lithium foil or lithium strip is roll-pressed on the surface of the negative electrode sheet. The negative electrode active material layer is in direct contact with the low-potential metal lithium foil or lithium strip. Under the action of the potential difference, electrons spontaneously migrate to the negative electrode sheet, and lithium ions are embedded in the negative electrode active material along with it.

[0076] In some embodiments, the positive electrode active material layer includes a first lithium supplement, and the mass percentage of the first lithium supplement in the positive electrode active material layer is 0.2% - 5%, for example, 0.2%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5% or the range composed of any two of them. Preferably, it is 0.5% - 3%. It can increase the lithium source, compensate for the irreversible lithium loss caused by the formation of the SEI film, and is beneficial to improving the first Coulomb efficiency of the battery and enhancing the energy density and capacity retention rate of the battery.

[0077] In the embodiments of the present invention, the negative electrode sheet can be prepared by a conventional method in the art, for example, by a coating method. Specifically, components for forming the negative electrode active material layer such as the negative electrode active material, the negative electrode conductive agent, the thickening agent, and the negative electrode binder can be dispersed in a negative electrode solvent. The negative electrode solvent includes, for example, water, and a negative electrode slurry is prepared. Then, it is coated on the surface of the negative electrode current collector, and after processes such as drying and rolling, the negative electrode sheet is obtained. Among them, the processes involved such as coating, drying, and rolling are conventional operations for preparing the negative electrode sheet by the coating method, and no special limitation is made thereto.

[0078] In the embodiments of the present invention, the positive electrode sheet can be prepared by a conventional method in the art, for example, by a coating method. Specifically, components for forming the positive electrode active material layer such as the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder can be dispersed in a positive electrode solvent. The positive electrode solvent includes, for example, N-methylpyrrolidone (NMP), and a positive electrode slurry is prepared. Then, it is coated on the surface of the positive electrode current collector, and after processes such as drying and rolling, the positive electrode sheet is obtained. Among them, the processes involved such as coating, drying, and rolling are conventional operations for preparing the positive electrode sheet by the coating method, and no special limitation is made thereto.

[0079] In specific implementation, when the positive electrode sheet includes a first lithium supplement agent and the first lithium supplement agent is a lithium-containing compound, the positive electrode active material, the positive electrode conductive agent, the positive electrode binder, and the first lithium supplement agent and other components for forming the positive electrode active material layer can be dispersed in a positive electrode solvent. The positive electrode solvent includes, for example, N-methylpyrrolidone (NMP), and a positive electrode slurry is prepared. Then, it is coated on the surface of the positive electrode current collector, and after processes such as drying and rolling, the positive electrode sheet is obtained.

[0080] Generally, a battery includes a battery cell, an electrolyte, and a housing for encapsulating the battery cell. The electrolyte is injected into the battery cell within the housing. The battery cell includes a positive electrode sheet, a negative electrode sheet, and a separator located between the positive electrode sheet and the negative electrode sheet. Among them, the battery cell can be a wound battery cell and / or a stacked battery cell.

[0081] In the embodiments of the present invention, a battery can be prepared by conventional methods in the art. For example, the positive electrode sheet, the separator, and the negative electrode sheet can be stacked alternately to obtain a stacked battery cell; or the positive electrode sheet, the separator, and the negative electrode sheet can be wound in sequence to obtain a wound battery cell. Then, the battery cell is placed in a housing, and after conventional processes such as encapsulation, baking, liquid injection (i.e., injecting the electrolyte), formation, secondary encapsulation, sorting, and open circuit voltage (OCV) testing, the battery is obtained.

[0082] In the embodiments of the present invention, a conventional housing material in the art can be used to encapsulate the battery cell. The housing includes, for example, a soft packaging material such as an aluminum-plastic film (in this case, the battery is a soft-packaged battery), but is not limited thereto.

[0083] The present invention will be further introduced below through specific embodiments.

[0084] In the embodiments of the present invention and the comparative examples, the solid content in the positive electrode slurry is the ratio of the total mass of the positive electrode active material, the positive electrode binder, and the positive electrode conductive agent to the total mass of the positive electrode slurry; the solid content in the negative electrode slurry in the embodiments and the comparative examples is the ratio of the total mass of the negative electrode active material, the negative electrode binder, and the negative electrode conductive agent to the total mass of the negative electrode slurry.

[0085] Example 1

[0086] (1) Preparation of the positive electrode sheet

[0087] Mix lithium iron phosphate (LiFePO4), PVDF, and conductive carbon black to form a uniform and stable positive electrode mixture. In this positive electrode mixture, the mass percentage of lithium iron phosphate is 93.5 wt%, the mass percentage of PVDF is 3.5 wt%, and the mass percentage of conductive carbon black is 3 wt%. Use NMP as the positive electrode solvent to make a positive electrode slurry, and the solid content in the positive electrode slurry is 60 wt%. Coat the positive electrode slurry evenly on both sides of the aluminum foil, dry it, and compact it with a roll press to form a positive electrode active material layer on both the front and back surfaces of the aluminum foil, obtaining a positive electrode sheet. Conduct an SEM test on the positive electrode active material layer of the positive electrode sheet, and the results are as Figure 3 shown. Among them, the average particle size of the secondary particles (i.e., lithium iron phosphate secondary particles) is 5 μm, the average particle size of the primary particles (i.e., lithium iron phosphate primary particles) is 0.2 μm, and the ratio of the average particle size of the secondary particles to the average particle size of the primary particles is 1:0.04. In addition, the specific surface area of the secondary particles is 10 m 2 / g, the surface density of the positive electrode active material layer is 10 mg / cm², and the tap density of the positive electrode active material layer is 1.85 g / cm 3 .

[0088] (2) Preparation of the negative electrode sheet

[0089] Mix graphite (negative electrode active material), SBR, and conductive carbon black, and form a uniform and stable negative electrode mixture through stirring and mixing. In this negative electrode mixture, the mass percentage of graphite is 95 wt%, the mass percentage of SBR is 3 wt%, and the mass percentage of conductive carbon black is 2 wt%. Use water as the negative electrode solvent to make a negative electrode slurry, and the solid content in the negative electrode slurry is 46 wt%. Coat the negative electrode slurry evenly on both sides of the copper foil, dry it, and compact it with a roll press to form a negative electrode active material layer on both the front and back surfaces of the copper foil, obtaining a negative electrode sheet. Conduct an SEM test on the negative electrode active material layer of the negative electrode sheet, and the results are as Figure 1 shown. Among them, the average particle size of the negative electrode active material is 3.5 μm. In addition, the specific surface area of the negative electrode active material is 2.8 m 2 / g, the surface density of the negative electrode active material layer is 4.7 mg / cm², and the tap density of the negative electrode active material layer is 1.33 g / cm 3 , and the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:0.7, and the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is 1:0.28.

[0090] (3) Preparation of the battery

[0091] The positive electrode sheet and the negative electrode sheet are punched, and wound in the order of the positive electrode sheet, the separator, and the negative electrode sheet to form a bare battery cell. After hot pressing, the aluminum electrode tab and the nickel-plated copper electrode tab are welded and the primary encapsulation is carried out. After encapsulation, it is baked in a high-temperature vacuum at 95 °C for 24 hours. Then, the electrolyte is injected, and the battery is formed, secondarily encapsulated, sorted, and the open circuit voltage (OCV) is tested to obtain the battery.

[0092] Among them, the electrolyte is composed of ethylene carbonate, dimethyl carbonate, propylene carbonate, and lithium hexafluorophosphate. Among them, the volume ratio of ethylene carbonate, dimethyl carbonate, and propylene carbonate is 1:1:1, the concentration of lithium hexafluorophosphate is 1 mol / L, and the viscosity of the electrolyte is 3.0 mPa·s.

[0093] Among them, the base film of the separator is a polypropylene / polyethylene (PP / PE) double-layer composite film. There are coatings on both sides of the polypropylene / polyethylene (PP / PE) double-layer composite film. One side is an adhesive layer (PVDF), and the other side is a ceramic layer (aluminum oxide). The side with the ceramic layer (aluminum oxide) coating faces the positive electrode sheet, and the porosity of the separator is 46%.

[0094] Example 2

[0095] The difference from Example 1 is that in the preparation process of the positive electrode sheet: lithium iron phosphate (LiFePO4), PVDF, conductive carbon black, and lithium nickelate are stirred and mixed to form a uniform and stable positive electrode mixture. In this positive electrode mixture, the mass percentage content of lithium iron phosphate is 90.7 wt%, the mass percentage content of PVDF is 3.5 wt%, the mass percentage content of conductive carbon black is 3 wt%, and the mass percentage content of lithium nickelate is 2.8 wt%. The remaining steps and conditions are the same as those in Example 1.

[0096] Example 3

[0097] The difference from Example 1 is that the average particle size of the negative electrode active material is 0.8 μm, the specific surface area of the negative electrode active material is 8 m 2 / g, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:0.16, and the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is 1:0.8. The remaining steps and conditions are the same as those in Example 1.

[0098] Example 4

[0099] The difference from Example 1 is that the average particle size of the negative electrode active material is 1.2 μm, the specific surface area of the negative electrode active material is 7.6 m 2 / g, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:0.24, and the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is 1:0.76. The remaining steps and conditions are the same as those in Example 1.

[0100] Example 5

[0101] It is different from Example 1 in that the average particle size of the negative electrode active material is 4 μm, the specific surface area of the negative electrode active material is 2.6 m 2 / g, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:0.8, the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is 1:0.26, and the remaining steps and conditions are the same as those in Example 1.

[0102] Example 6

[0103] It is different from Example 1 in that the average particle size of the negative electrode active material is 6 μm, the specific surface area of the negative electrode active material is 2 m 2 / g, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:1.2, the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is 1:0.2, and the remaining steps and conditions are the same as those in Example 1.

[0104] Example 7

[0105] It is different from Example 1 in that the average particle size of the negative electrode active material is 8 μm, the specific surface area of the negative electrode active material is 1 m 2 / g, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:1.6, the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is 1:0.1, and the remaining steps and conditions are the same as those in Example 1.

[0106] Example 8

[0107] It is different from Example 1 in that the average particle size of the negative electrode active material is 0.8 μm, the specific surface area of the negative electrode active material is 8 m 2 / g, the average particle size of the secondary particles is 40 μm, the specific surface area of the secondary particles is 4 m 2 / g, the average particle size of the primary particles is 0.6 μm, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:0.02, the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is 1:2, and the ratio of the average particle size of the secondary particles to the average particle size of the primary particles is 1:0.015. The remaining steps and conditions are the same as those in Example 1.

[0108] Example 9

[0109] It is different from Example 1 in that the average particle size of the negative electrode active material is 1.2 μm, the specific surface area of the negative electrode active material is 7.8 m 2 / g, the average particle size of the secondary particles is 30 μm, and the specific surface area of the secondary particles is 5 m 2 / g, the average particle size of the primary particles is 0.5 μm, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:0.04, the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is 1:1.56, and the ratio of the average particle size of the secondary particles to the average particle size of the primary particles is 1:0.017. The remaining steps and conditions are the same as those in Example 1.

[0110] Example 10

[0111] The difference from Example 1 is that the average particle size of the negative electrode active material is 5.2 μm, and the specific surface area of the negative electrode active material is 2.4 m 2 / g, the average particle size of the secondary particles is 1.3 μm, and the specific surface area of the secondary particles is 15 m 2 / g, the average particle size of the primary particles is 0.2 μm, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:4, the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is 1:0.16, and the ratio of the average particle size of the secondary particles to the average particle size of the primary particles is 1:0.154. The remaining steps and conditions are the same as those in Example 1.

[0112] Example 11

[0113] The difference from Example 1 is that the average particle size of the negative electrode active material is 6 μm, and the specific surface area of the negative electrode active material is 2 m 2 / g, the average particle size of the secondary particles is 1 μm, and the specific surface area of the secondary particles is 18 m 2 / g, the average particle size of the primary particles is 0.15 μm, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:6, the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is 1:0.11, and the ratio of the average particle size of the secondary particles to the average particle size of the primary particles is 1:0.15. The remaining steps and conditions are the same as those in Example 1.

[0114] Example 12

[0115] The difference from Example 1 is that the average particle size of the negative electrode active material is 8 μm, and the specific surface area of the negative electrode active material is 1 m 2 / g, the average particle size of the secondary particles is 0.8 μm, and the specific surface area of the secondary particles is 20 m 2 / g, the average particle size of the primary particles is 0.1 μm, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:10, the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is 1:0.05, the ratio of the average particle size of the secondary particles to the average particle size of the primary particles is 1:0.125, and the remaining steps and conditions are the same as those in Example 1.

[0116] Example 13

[0117] The difference from Example 1 is that the average particle size of the secondary particles is 60 μm, the average particle size of the primary particles is 0.012 μm, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:0.058, the ratio of the average particle size of the secondary particles to the average particle size of the primary particles is 1:0.0002, and the remaining steps and conditions are the same as those in Example 1.

[0118] Example 14

[0119] The difference from Example 1 is that the average particle size of the secondary particles is 0.4 μm, the average particle size of the primary particles is 0.3 μm, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:8.75, the ratio of the average particle size of the secondary particles to the average particle size of the primary particles is 1:0.755, and the remaining steps and conditions are the same as those in Example 1.

[0120] Example 15

[0121] The difference from Example 1 is that the average particle size of the secondary particles is 40 μm, the specific surface area of the secondary particles is 4m 2 / g, the average particle size of the primary particles is 0.1 μm, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:0.09, the ratio of the average particle size of the secondary particles to the average particle size of the primary particles is 1:0.0025, the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is 1:0.7, and the remaining steps and conditions are the same as those in Example 1.

[0122] Example 16

[0123] The difference from Example 1 is that the average particle size of the secondary particles is 30 μm, the specific surface area of the secondary particles is 5m 2 / g, the average particle size of the primary particles is 0.3 μm, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:0.12, the ratio of the average particle size of the secondary particles to the average particle size of the primary particles is 1:0.01, the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is 1:0.56, and the remaining steps and conditions are the same as those in Example 1.

[0124] Example 17

[0125] The difference from Example 1 is that the average particle size of the secondary particles is 2 μm, and the specific surface area of the secondary particles is 13 m 2 / g, the average particle size of the primary particles is 0.5 μm, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:1.75, the ratio of the average particle size of the secondary particles to the average particle size of the primary particles is 1:0.25, and the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is 1:0.22. The remaining steps and conditions are the same as those in Example 1.

[0126] Example 18

[0127] The difference from Example 1 is that the average particle size of the secondary particles is 1 μm, and the specific surface area of the secondary particles is 18 m 2 / g, the average particle size of the primary particles is 0.5 μm, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:3.5, the ratio of the average particle size of the secondary particles to the average particle size of the primary particles is 1:0.5, and the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is 1:0.16. The remaining steps and conditions are the same as those in Example 1.

[0128] Example 19

[0129] The difference from Example 1 is that the average particle size of the secondary particles is 0.8 μm, and the specific surface area of the secondary particles is 20 m 2 / g, the average particle size of the primary particles is 0.6 μm, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:4.38, the ratio of the average particle size of the secondary particles to the average particle size of the primary particles is 1:0.75, and the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is 1:0.14. The remaining steps and conditions are the same as those in Example 1.

[0130] Example 20

[0131] The difference from Example 1 is that the areal density of the negative electrode active material layer is 1.9 mg / cm², and the remaining steps and conditions are the same as those in Example 1.

[0132] Example 21

[0133] The difference from Example 1 is that the areal density of the negative electrode active material layer is 10.8 mg / cm², and the remaining steps and conditions are the same as those in Example 1.

[0134] Example 22

[0135] The difference from Example 1 is that the areal density of the positive electrode active material layer is 4 mg / cm², and the remaining steps and conditions are the same as those in Example 1.

[0136] Example 23

[0137] It is different from Example 1 in that the areal density of the positive electrode active material layer is 20 mg / cm², and the remaining steps and conditions are the same as those in Example 1.

[0138] Comparative Example 1

[0139] It is different from Example 1 in that the average particle size of the negative electrode active material is 15 μm. In addition, the specific surface area of the negative electrode active material is 0.84 m 2 / g, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:3, and the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is 1:0.084. The remaining steps and conditions are the same as those in Example 1.

[0140] Comparative Example 2

[0141] It is different from Example 1 in that graphite and hard carbon are used as the negative electrode active material. In the negative electrode mixture, the mass percentage content of graphite is 66.5 wt%, and the mass percentage content of hard carbon is 28.5%. SEM testing was performed on the negative electrode active material layer of the negative electrode sheet in Comparative Example 2, and the results are as Figure 2 shown. Among them, the average particle size of the negative electrode active material is 10 μm. The specific surface area of the negative electrode active material is 0.96 m 2 / g, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:2, and the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is 1:0.096. The remaining steps and conditions are the same as those in Example 1.

[0142] Comparative Example 3

[0143] It is different from Example 1 in that the average particle size of the negative electrode active material is 0.5 μm, and the specific surface area of the negative electrode active material is 9 m 2 / g, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:0.1, and the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is 1:0.9. The remaining steps and conditions are the same as those in Example 1.

[0144] Comparative Example 4

[0145] It is different from Example 1 in that the average particle size of the negative electrode active material is 0.6 μm, and the specific surface area of the negative electrode active material is 8.1 m 2 / g, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:0.01, the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is 1:2.08, the average particle size of the secondary particles is 60 μm, and the specific surface area of the secondary particles is 3.9 m 2 / g, the average particle size of the primary particles is 1 μm, the ratio of the average particle size of the secondary particles to the average particle size of the primary particles is 1:0.017, and the remaining steps and conditions are the same as those in Example 1.

[0146] Comparative Example 5

[0147] The difference from Example 1 is that the average particle size of the negative electrode active material is 12 μm, and the specific surface area of the negative electrode active material is 0.9 m 2 / g, the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:20, the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material is 1:0.04, the average particle size of the secondary particles is 0.6 μm, and the specific surface area of the secondary particles is 21 m 2 / g, the average particle size of the primary particles is 0.05 μm, the ratio of the average particle size of the secondary particles to the average particle size of the primary particles is 1:0.083, and the remaining steps and conditions are the same as those in Example 1.

[0148] The types of negative electrode active materials, the content of negative electrode active materials (i.e., the mass percentage of negative electrode active materials in the negative electrode mixture), the average particle size of negative electrode active materials, the specific surface area of negative electrode active materials, the areal density of the negative electrode active material layer, the average particle size of secondary particles, the average particle size of primary particles, and the specific surface area of secondary particles in each example and comparative example are summarized in Table 1. The areal density of the positive electrode active material layer, the content of positive electrode active materials (i.e., the mass percentage of positive electrode active materials in the positive electrode mixture), the content of lithium supplement agent (i.e., the mass percentage of lithium supplement agent in the positive electrode mixture), the ratio of the average particle size of secondary particles to the average particle size of primary particles, the ratio of the average particle size of secondary particles to the average particle size of negative electrode active materials, and the ratio of the specific surface area of secondary particles to the specific surface area of negative electrode active materials in each example and comparative example are summarized in Table 2.

[0149] The electrochemical performance of the batteries in each example and comparative example was tested through the following processes respectively, and the results are shown in Table 3:

[0150] (1)Energy density: Denote the weight of the battery after the second sealing process as the battery weight. During the sorting process, charge the battery at a constant current and constant voltage of 1C until the charging upper limit voltage (3.65V when the positive electrode active material layer contains lithium iron phosphate, and 4.0V when the positive electrode active material layer contains lithium iron phosphate and the first lithium supplement agent (lithium nickelate)), with a cut-off current of 0.05C. After standing for 5 minutes, discharge it at a constant current of 1C to 2.2V, and record the discharge energy. Then, the ratio of the discharge energy to the battery weight is the energy density, with the unit of Wh / kg;

[0151] (2)Cold start performance: The cold start performance is reflected by the terminal voltage at the end of battery cold start. Under the environment of (25±2)°C, discharge the battery at a constant current of 1C to 2.2V and stand for 30 minutes; then charge it at a constant current and constant voltage of 1C to 3.65V, with a cut-off current of 0.05C, and stand for 30 minutes; discharge it at a constant current of 1C to 2.2V to obtain the actual discharge capacity C0 of the battery; stand for 30 minutes; charge it at a constant current and constant voltage of 1C to 3.65V, with a cut-off current of 0.05C, and discharge it at 1C0 for 30 minutes, which is 50% SOC; stand still at (25±2)°C for 2 hours; then put the battery into a -30°C constant temperature box and keep it at a constant temperature for 4 hours; then test the terminal voltage value of discharging at a constant current of 6C for 1 second, which is the terminal voltage at the end of battery cold start, with the unit of V;

[0152] (3)Storage capacity retention rate: The storage performance of the battery is reflected by the storage capacity retention rate;

[0153] Under the environment of (25±2)°C, discharge the battery at a constant current of 1C to the discharge cut-off voltage of 2.2V and stand for 30 minutes; then charge it at a constant current and constant voltage of 1C to the charging limit voltage of 3.65V, with a cut-off current of 0.05C, and stand for 30 minutes; discharge it at a constant current of 1C to the discharge cut-off voltage of 2.2V to obtain the actual discharge capacity C0 of the battery, and stand for 30 minutes; charge it at a constant current and constant voltage of 1C to the charging limit voltage of 3.65V, with a cut-off current of 0.05C, and then open-circuit and stand for 30 days under the environment of 60°C; then stand still at (25±2)°C for 1 hour, discharge it at a constant current of 1C to the discharge cut-off voltage of 2.2V, and then charge it at a constant current and constant voltage of 1C to the charging limit voltage of 3.65V, with a cut-off current of 0.05C, and then discharge it at a constant current of 1C to the discharge cut-off voltage of 2.2V to obtain the restored capacity C1; among them, the storage capacity retention rate (or restored capacity retention rate) of the battery is C1 / C0 100%.

[0154] (4) Initial Coulombic efficiency: In the formation process, the battery is charged at a constant current of 0.2C to 3.0V at 65 ± 5°C, and the charging capacity C2 is recorded. Then it is left standing for 1 min. Then it is charged at a constant current of 0.5C to the upper charging voltage (3.65V when the positive electrode active material layer contains lithium iron phosphate, and 4.0V when the positive electrode active material layer contains lithium iron phosphate and the first lithium supplement agent), and the charging capacity C3 is recorded. In the sorting process, it is charged at a constant current and constant voltage of 1C to the upper charging voltage (3.65V for lithium iron phosphate, 4.0V for lithium iron phosphate blended with a lithium supplement agent) at 25 ± 5°C, and the charging capacity C4 is recorded. Then it is left standing for 5 min. Then it is discharged at a constant current of 1C to the lower discharge voltage of 2.2V, and the discharge capacity C5 is recorded. The initial Coulombic efficiency of the battery cell = C5 / (C2 + C3 + C4) 100%.

[0155] (5) Self-discharge rate: In an environment of (25 ± 2)°C, the battery is discharged at a standard constant current of 1C to the discharge cut-off voltage of 2.2V, and then left standing for 30 min. Then it is charged at a standard constant current and constant voltage of 1C to the charging limit voltage of 3.65V, with a cut-off current of 0.05C, and left standing for 30 min. It is discharged at a standard constant current of 1C to the discharge cut-off voltage of 2.2V, and the actual discharge capacity C0 of the battery cell is recorded. Then it is left standing for 30 min. It is charged at a constant current of 1C0 to 36% SOC, and then 3% SOC is discharged at 1C0. At this time, it is 33% SOC, and left standing for 30 min. The initial voltage OCV1 is recorded. After storing at room temperature for 30 days, the terminal voltage OCV2 is recorded. The self-discharge rate of the battery cell = (OCV1 - OCV2) / storage time (30 24h), with the unit of mV / h.

[0156] Table 1

[0157]

[0158]

[0159] Table 2

[0160]

[0161] Table 3

[0162]

[0163] As can be seen from Table 1, compared with Comparative Examples 1 - 5, the negative electrode active materials in Examples 1 - 23 include graphite, the average particle size of the negative electrode active material is 0.8 - 8 μm, and the ratio of the average particle size of the secondary particles to the average particle size is 1:0.02 - 10, which is beneficial to reducing the self-discharge rate of the battery cell, and improving the performance of the battery such as energy density, initial Coulombic efficiency, low-temperature performance, and storage performance.

[0164] Furthermore, compared with Examples 13 to 14, Examples 1 to 12, Examples 15 to 23 further control the secondary particles to be 0.8 to 40 μm, and the ratio of the average particle size of the secondary particles to the particle size of the primary particles is 0.0025 to 0.75, further reducing the self-discharge rate of the battery cell and improving the performance of the battery such as energy density, first Coulomb efficiency, low-temperature performance, and storage performance.

[0165] Furthermore, compared with Examples 3, 7, 8, 12, 12, 15, and 19, Examples 1, 2, 4 to 6, 9 to 11, 16 to 18, and 20 to 23 further control at least one of the average particle size of the negative electrode active material to be 1.2 to 6 μm, the average particle size of the secondary particles to be 1 to 30 μm, the ratio of the average particle size of the secondary particles to the average particle size of the primary particles to be 1:0.01 to 0.05, and the ratio of the specific surface area of the secondary particles to the specific surface area of the negative electrode active material to be 1:0.1 to 1.8, further reducing the self-discharge rate of the battery cell and improving the performance of the battery such as energy density, first Coulomb efficiency, low-temperature performance, and storage performance.

[0166] Furthermore, compared with Example 1, the positive electrode active material layer in Example 2 includes a first lithium supplement agent (lithium nickelate), and the mass percentage of the first lithium supplement agent in the positive electrode active material layer is 0.2% to 5%, further reducing the self-discharge rate of the battery cell and improving the performance of the battery such as energy density, first Coulomb efficiency, low-temperature performance, and storage performance.

[0167] Finally, it should be noted that: After considering the specification and practicing the invention disclosed herein, those skilled in the art will readily think of other embodiments of the present invention. The present invention is intended to cover any variations, uses, or adaptations of the present invention, which follow the general principles of the present invention and include common general knowledge or conventional technical means in the technical field not disclosed in the present invention. It is not limited to the exact structures described above and shown in the drawings, and various modifications and changes can be made without departing from its scope. The scope of the present invention is only limited by the appended claims.

Claims

1. A battery, characterized in that: Including positive electrode sheet and negative electrode sheet; The negative electrode sheet comprises a negative electrode active material layer, the negative electrode active material layer comprises a negative electrode active material, the negative electrode active material comprises graphite, and the average particle size of the negative electrode active material is 0.8-8 μm; The positive electrode sheet includes a positive electrode active material layer, the positive electrode active material layer includes a positive electrode active material, the positive electrode active material includes secondary particles, the secondary particles include primary particles, and the ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:0.02~10.

2. The battery according to claim 1, characterized in that The ratio of the average particle size of the secondary particles to the average particle size of the negative electrode active material is 1:0.04-6; And / or, the secondary particles are spherical in structure.

3. The battery according to claim 1, characterized in that The average particle size of the secondary particles is 0.8 to 40 μm, preferably 1 to 30 μm; And / or, the average particle size of the primary particles is 0.1-0.6 μm; And / or, the ratio of the average particle size of the secondary particles to the average particle size of the primary particles is 1:0.0025-0.75, preferably 1:0.01-0.

5.

4. The battery according to claim 1, characterized in that The average particle size of the negative electrode active material is 1.2-6 μm.

5. The battery according to claim 1, characterized in that The specific surface area of ​​the negative electrode active material is 1-8 m 2 / g; And / or, the specific surface area of ​​the secondary particles is 4 to 20 m 2 / g; And / or, the ratio of the specific surface area of ​​the secondary particles to the specific surface area of ​​the negative electrode active material is 1:0.05-2, preferably 1:0.1-1.

8.

6. The battery according to claim 1, characterized in that The positive electrode active material includes LiMn2O4, Li4Ti5O 12 、Li3V2(PO4)3、LiFe 1-x E x PO4、LiNi x Mn y Co 1-x-y O2 and LiNi x Co y Al 1-x-y O2, wherein 0≤x≤1, 0≤y≤1, x+y≤1, and E is selected from one or more of Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, V and Ti; Preferably, the positive electrode active material is LiFePO4.

7. The battery according to claim 1, characterized in that The surface density of the negative electrode active material layer is 1.9-10.8 mg / cm²; And / or, the compaction density of the negative electrode active material layer is 1.2-1.5 g / cm 3 ; And / or, the surface density of the positive electrode active material layer is 4-20 mg / cm²; And / or, the compaction density of the positive electrode active material layer is 1.7-2.0 g / cm 3 .

8. The battery according to claim 1, characterized in that The battery also includes a diaphragm, the porosity of the diaphragm is 36% to 64%; And / or, the battery further includes an electrolyte, and the viscosity of the electrolyte is less than or equal to 5 mPa·s.

9. The battery according to claim 8, characterized in that The electrolyte includes a solvent, and the solvent includes one or more of dimethyl carbonate, methyl acetate, ethyl propionate, 1,1,2,2,3,3,4,4-octafluoro-5-methoxypentane, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, bis(2,2,2-trifluoroethyl) ether, ethyl methyl carbonate, and 1,2-propylene glycol carbonate.

10. The battery according to claim 1, characterized in that The positive electrode sheet includes a first lithium supplement; Preferably, the first lithium supplement comprises lithium metal and a lithium-containing compound, wherein the lithium metal comprises one or more of lithium powder, lithium foil, and lithium strip, and the lithium-containing compound comprises Li x N y , Li x S y , Li x M y O z One or more of, wherein Li x N y x≥1, y>0; Li x N y x≥1, y>0, Li x M y O z Where x≥1, z≥1, y≥0, M is selected from one or more of Fe, Ni, Si, Co, Mn, and C; Preferably, the positive electrode active material layer includes the first lithium supplement agent, and the mass percentage of the first lithium supplement agent in the positive electrode active material layer is 0.2% to 5%.

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