Lithium ion secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a lithium ion secondary battery. Comprising a positive plate and a negative plate, the positive plate comprises a positive active material, the positive active material comprises lithium iron phosphate, and the lithium iron phosphate comprises secondary particles formed by a plurality of primary particles; the lithium iron phosphate comprises an element V, and the mass content a1 of the element V in the lithium iron phosphate is 1000 ppm to 20000 ppm; the mass content a2 of the element Fe in the lithium iron phosphate is 150000 ppm to 350000 ppm; the ratio of a1 to a2 is 1: (8-280); the negative plate comprises a negative active material, the negative active material comprises graphite, and the average particle size D1 of the secondary particles and the average particle size D2 of the graphite meet the condition that D1: D2 is 1: (0.5-15). The battery disclosed by the invention has relatively high energy density and excellent cycle performance, rate capability and low-temperature performance.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and particularly to a lithium-ion secondary battery. Background Art

[0002] As a secondary battery with high energy density, lithium-ion batteries have been widely used in fields such as mobile terminal devices, power tools, and new energy vehicles. Among them, lithium iron phosphate cathode materials have become an ideal choice for lithium-ion battery cathode materials due to their advantages such as high safety, long cycle life, and low cost. However, lithium iron phosphate cathode materials have defects such as poor electrical conductivity and low ion diffusion ability, resulting in their actual capacity being much lower than the theoretical value, serious capacity attenuation during high-rate charge and discharge, and it is difficult to meet the usage requirements of the battery.

[0003] The prior art has found that doping element V in lithium iron phosphate cathode materials can improve the electrical conductivity of lithium iron phosphate cathode materials, but the improvement effect on the migration and diffusion of lithium ions is limited. At the same time, when lithium iron phosphate batteries discharge at high current in a low-temperature environment, there are problems such as large battery polarization, rapid reduction of the discharge platform voltage, and too little battery discharge capacity, which directly affect the performance and normal use of the battery and hinder the rapid development of the battery industry.

[0004] Therefore, it is very important to invent a lithium-ion secondary battery that can balance high energy density and excellent cycle performance, rate performance, and low-temperature performance. Summary of the Invention

[0005] The purpose of the present invention is to overcome the above problems existing in the prior art, and provide a lithium-ion secondary battery. The lithium-ion secondary battery of the present invention (hereinafter simply referred to as the battery) not only ensures high energy density, but also has excellent cycle performance, rate performance, and low-temperature performance, can meet the application requirements of high-rate discharge, and has broad application prospects.

[0006] In the related art, doping element V in lithium iron phosphate cathode materials can improve the electrical conductivity of lithium iron phosphate cathode materials, but the improvement effect on the migration and diffusion of lithium ions is limited. The reason is that: the doping of element V can partially replace the lithium site or iron site in the lithium iron phosphate crystal, affect the electron distribution and transmission path in the material, optimize the electronic structure of lithium iron phosphate, reduce its band gap, and thus improve the transmission efficiency and electrical conductivity of lithium ions. However, due to the certain limitation of the crystal structure of lithium iron phosphate itself on the migration of lithium ions, although the doping of element V reduces the lithium ion diffusion energy barrier to a certain extent, the increase amplitude of the diffusion coefficient is still limited and cannot meet the application scenarios with extremely high requirements for the rapid migration and diffusion of lithium ions such as high-rate charge and discharge.

[0007] For the above reasons, the inventors conducted a large number of targeted studies and found that doping element V in the lithium iron phosphate cathode material in combination with other modification strategies can synergistically optimize the overall performance of the battery, specifically manifested in:

[0008] First, by regulating the content ratio of element V and element Fe in lithium iron phosphate, the energy density and rate performance of lithium-ion batteries can be effectively improved. The reason is that doping an appropriate amount of element V in the lithium iron phosphate cathode material can change the electronic structure of lithium iron phosphate, improve the lithium-ion transport efficiency and redox potential, thereby increasing the plateau voltage and improving its energy density. However, excessive doping may cause the lithium-ion transport channels to be blocked. Therefore, reasonably regulating the content ratio of element V and element Fe can avoid problems such as low lithium-ion transport rate and unstable structure caused by excessive doping of element V, ensure the structural integrity of the cathode active material during charge and discharge cycles, and maintain the stability of battery performance. At the same time, it can also optimize the crystal structure and surface properties of lithium iron phosphate, create more channels and sites conducive to the rapid migration and diffusion of lithium ions, reduce the lithium-ion diffusion resistance, and thus improve the rate performance and energy density of the battery. When the content ratio of element V and element Fe in lithium iron phosphate is too small (e.g., <1:280), the low content of element V is difficult to occupy iron or lithium vacancies after entering the lithium iron phosphate crystal, and cannot effectively reduce the energy barrier of lithium-ion diffusion; while the excessive content of element Fe will also block the lithium-ion diffusion path, affecting the cold start performance of the battery. At the same time, the excessive content of element Fe will also cause irreversible phase changes in lithium iron phosphate during charge and discharge, accelerating the battery capacity decay and reducing the lithium-ion capacity retention rate. When the content ratio of element V and element Fe in lithium iron phosphate is too large (e.g., >1:8), doping a high content of element V will cause the particle size of lithium iron phosphate to become smaller and agglomerate, blocking the lithium-ion diffusion path and increasing the transport resistance of lithium-ion batteries, thereby reducing the cold start performance of lithium-ion batteries; the low content of element Fe will cause the crystal structure of lithium iron phosphate to be incomplete, reduce the conductivity of the cathode active material, increase the charge transfer resistance, and further deteriorate the cold start ability of the battery. In addition, the low content of element Fe is more likely to undergo structural changes during charge and discharge, thus affecting the cycle stability of the battery.

[0009] Second, by regulating the ratio of the average particle size of lithium iron phosphate secondary particles to the average particle size of graphite, the energy density, cycling performance, and low-temperature performance of lithium-ion batteries can be further improved. The reason is as follows: When the ratio of the average particle size of lithium iron phosphate secondary particles to the average particle size of graphite is regulated within an appropriate range, the diffusion path of lithium ions in the active material can be shortened, and the active sites of the electrochemical reaction can be increased, thereby improving the rate performance and low-temperature performance of the battery. At the same time, an appropriate particle size ratio can also optimize the porosity of the electrode sheet, improve the wettability of the electrolyte and the lithium-ion transport efficiency, and further enhance the energy density and overall performance of the battery. When the ratio of the average particle size of lithium iron phosphate secondary particles to the average particle size of graphite is too small (for example, <1:15), the lithium iron phosphate with too small a particle size may cause particle agglomeration, reduce the solid-phase diffusion coefficient of lithium ions, increase the internal resistance of the battery. During high-rate charge and discharge, more current is converted into heat, resulting in increased battery heat loss, affecting the battery capacity and discharge performance, and reducing the energy density of the battery; while the graphite particles with too large a particle size will make the channels for lithium ions to embed and deembed longer, reduce the lithiation active sites, and cause difficulties in lithium ion intercalation, seriously affecting the cold start performance of the battery. When the ratio of the average particle size of lithium iron phosphate secondary particles to the average particle size of graphite is too large (for example, >1:0.5), the lithium iron phosphate secondary particles with too large a particle size are prone to fracture and crack during high-temperature or deep cycling, reducing the structural stability of the positive electrode active material, accelerating battery capacity decay, and affecting the battery cycling performance; the graphite with too small a particle size is prone to pulverization during charge and discharge, resulting in the shedding of active substances, further deteriorating the cycling ability. In addition, the specific surface area of the small particle size graphite in contact with the electrolyte becomes larger, and the solid electrolyte interface (SEI) film formed during charge and discharge will consume more charge, increasing the irreversible capacity loss of the battery, and reducing the energy density and storage capacity retention rate of the battery.

[0010] Based on this, the inventors of the present invention proposed the following solution:

[0011] The present invention provides a lithium-ion secondary battery, including a positive electrode sheet and a negative electrode sheet; the positive electrode sheet includes a positive electrode active material, the positive electrode active material includes lithium iron phosphate, and the lithium iron phosphate includes secondary particles formed by a plurality of primary particles; the lithium iron phosphate includes element V, and the mass content a1 of element V in the lithium iron phosphate is 1000 ppm - 20000 ppm; the mass content a2 of element Fe in the lithium iron phosphate is 150000 ppm - 350000 ppm; a1:a2 is 1:(8 - 280); the negative electrode sheet includes a negative electrode active material, the negative electrode active material includes graphite, and the average particle size D1 of the secondary particles and the average particle size D2 of the graphite satisfy: D1:D2 is 1:(0.5 - 15).

[0012] Through the above technical solution, the present invention has at least the following advantages compared with the prior art:

[0013] The lithium-ion secondary battery of the present invention has a high energy density, as well as excellent cycle performance, rate performance, and low-temperature performance.

[0014] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein. Brief Description of the Drawings

[0015] Figure 1 Shown is a scanning electron microscope (SEM) image of natural graphite according to an embodiment of the present invention. Detailed Description of the Invention

[0016] The following provides a detailed description of the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0017] The present invention provides a lithium-ion secondary battery, the lithium-ion secondary battery includes a positive electrode sheet and a negative electrode sheet; the positive electrode sheet includes a positive electrode active material, the positive electrode active material includes lithium iron phosphate, the lithium iron phosphate includes secondary particles formed by a number of primary particles; the lithium iron phosphate includes element V, and the mass content a1 of element V in the lithium iron phosphate is 1000 ppm - 20000 ppm, for example, 1000 ppm, 5000 ppm, 10000 ppm, 15000 ppm or 20000 ppm; the mass content a2 of element Fe in the lithium iron phosphate is 150000 ppm - 350000 ppm, for example, 150000 ppm, 200000 ppm, 250000 ppm, 300000 ppm or 350000 ppm; a1:a2 is 1:(8 - 280), for example, 1:8, 1:10, 1:20, 1:50, 1:100, 1:150, 1:200, 1:250 or 1:280.

[0018] In one example, a1 is 1500 ppm - 18000 ppm; a2 is 180000 ppm - 300000 ppm.

[0019] In one example, a1:a2 is 1:(10 - 200).

[0020] In the present invention, the term "a plurality" means that the number of the lithium iron phosphate primary particles is greater than or equal to 2, such as 2, 3, 4, 5, 6, 7, 8, 9, 10, etc.

[0021] In the present invention, the mass contents of element V and element Fe in the lithium iron phosphate can be measured by conventional methods in the art. For example, the battery is discharged to 0% SOC, the positive electrode plate is disassembled and taken out, and after the cross-section of the positive electrode plate is polished by an argon ion milling machine, it is measured by an energy dispersive spectrometer (EDS).

[0022] In the present invention, the negative electrode plate includes a negative electrode active material, the negative electrode active material includes graphite, and the average particle size D1 of the secondary particles and the average particle size D2 of the graphite satisfy: D1:D2 is 1:(0.5 - 15), such as 1:0.5, 1:1, 1:3, 1:5, 1:7, 1:10 or 1:15.

[0023] In one example, D1 is 1000 nm - 6000 nm, such as 1000 nm, 2000 nm, 3000 nm, 4000 nm, 5000 nm or 6000 nm.

[0024] In one example, D1 is 1600 nm - 5800 nm.

[0025] In one example, D2 ≤ 20 μm, such as 20 μm, 18 μm, 16 μm, 14 μm, 12 μm, 10 μm, 8 μm, 6 μm, 4 μm, 3 μm or 2 μm.

[0026] In one example, D2 is 3.5 μm - 16 μm.

[0027] In one example, D1:D2 is 1:(1 - 9).

[0028] In the present invention, the average particle size D3 of the primary particles is 30 nm - 200 nm, such as 30 nm, 50 nm, 100 nm, 150 nm or 200 nm.

[0029] In one example, D3 is 38.7 nm - 160 nm.

[0030] In one example, D3:D1 is 1:(8 - 180), such as 1:8, 1:10, 1:20, 1:50, 1:100, 1:150 or 1:180.

[0031] In one example, D3:D1 is 1:(12 - 120).

[0032] When the average particle size ratio of the lithium iron phosphate primary particles to the secondary particles is too small (e.g., <1:180), the primary particle size is too small or the secondary particle size is too large, and the voids between the lithium iron phosphate particles are too large. The too low bulk density reduces the compaction density of the positive electrode sheet, thereby reducing the energy density of the battery. When the average particle size ratio of the lithium iron phosphate primary particles to the secondary particles is too large (e.g., >1:8), the primary particle size is too large or the secondary particle size is too small, the lithium ion transmission path increases, resulting in an increase in the internal resistance of the lithium ion battery, and thus affecting the low-temperature cold start performance of the battery. Therefore, adjusting the average particle size ratio of the lithium iron phosphate primary particles to the secondary particles within a suitable range can effectively improve the energy density of the battery and improve the low-temperature performance of the battery.

[0033] In the present invention, the average particle size of the secondary particles and the average particle size of the graphite can be measured by conventional methods in the art. For example, through SEM, at least 20 secondary particles / graphite particles are selected in the electron micrograph, and the particle size of each secondary particle / graphite particle is measured, and the average value is taken. If the secondary particle / graphite particle is a regular circle in the micrograph, then the particle size of the particle is the diameter of the circle; if the secondary particle / graphite particle is not a "regular circle" in the micrograph, then the particle size of the particle is the average value of the sizes measured at multiple different angles.

[0034] In the present invention, the positive electrode sheet further includes a positive electrode conductive agent, and the positive electrode conductive agent includes first carbon black and / or acetylene black.

[0035] In one example, the positive electrode conductive agent includes first carbon black, and the average particle size b1 of the first carbon black is 30 nm - 300 nm, such as 30 nm, 50 nm, 100 nm, 150 nm, 200 nm, 250 nm or 300 nm.

[0036] In one example, the positive electrode conductive agent does not include carbon nanotubes and graphene.

[0037] In one example, b1:D1 is 1:(5 - 160), such as 1:5, 1:20, 1:50, 1:100, 1:150 or 1:160.

[0038] In one example, b1:D1 is 1:(12 - 90).

[0039] The positive electrode conductive agent does not include carbon nanotubes and graphene, and first carbon black or acetylene black is selected, which can ensure sufficient contact with the lithium iron phosphate particles and effectively improve the conductivity of the positive electrode sheet.

[0040] In addition, by controlling the average particle size ratio of the first carbon black to the lithium iron phosphate secondary particles within an appropriate range, the rate performance, low-temperature performance, and energy density of the battery can be effectively improved. When the average particle size ratio of the first carbon black to the lithium iron phosphate secondary particles is too small (e.g., <1:160), the first carbon black with too small a particle size will cause agglomeration between the lithium iron phosphate particles, reducing the structural stability of the positive electrode active material, resulting in uneven electrochemical reactions during battery cycling, reducing the cycle life of the battery. In addition, the first carbon black with too small a particle size will also increase the hindrance effect of lithium ions on the surface of the positive electrode active material, reducing the diffusion rate of lithium ions, and thus affecting the cold start performance of the battery; while the lithium iron phosphate secondary particles with too large a particle size will increase the diffusion path of lithium ions, increasing the transport resistance of lithium ions during charge and discharge, further reducing the discharge capacity of the lithium ion battery. At the same time, it also reduces the packing efficiency between the lithium iron phosphate particles, thereby reducing the energy density of the lithium ion battery. When the average particle size ratio of the first carbon black to the lithium iron phosphate secondary particles is too large (e.g., >1:5), the specific surface area of the first carbon black with too large a particle size is too small to provide sufficient reaction sites and effective adsorption positions for the positive electrode active material, resulting in a decrease in the conductivity of the battery and an increase in the internal resistance, thus reducing the discharge capacity of the battery; while the specific surface area of the lithium iron phosphate with too small a particle size is too large, and forming a CEI film during charge and discharge will consume more charges, resulting in an increase in the irreversible capacity loss of the battery and a decrease in the capacity retention rate during storage.

[0041] In the present invention, the average particle size of the first carbon black can be obtained by testing using conventional methods in the art. For example, through SEM, at least 20 first carbon blacks are selected in the electron microscope image, the particle size of each first carbon black is measured, and the average value is taken. If the first carbon black is a regular circle in the microscope image, then the particle size of the first carbon black is the diameter of the circle; if the first carbon black is not a "regular circle" in the microscope image, then the particle size of the first carbon black is the average value of the sizes measured at multiple different angles.

[0042] In the present invention, the lithium ion secondary battery further includes an electrolyte, and the electrolyte includes at least one of cyclic carbonate solvents, linear carbonate solvents, and linear carboxylate solvents.

[0043] In one example, the cyclic carbonate solvent includes ethylene carbonate (EC) and / or propylene carbonate (PC).

[0044] In one example, the mass content c1 of the cyclic carbonate solvent in the electrolyte is 25%-50%, for example, 25%, 30%, 35%, 40%, 45%, or 50%.

[0045] In one example, the linear carbonate solvent includes at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC).

[0046] In one example, the mass content c2 of the linear carbonate solvent in the electrolyte is 40% - 60%, such as 40%, 45%, 50%, 55%, or 60%.

[0047] In one example, the linear carboxylate solvent includes ethyl propionate (EP) and / or propyl propionate (PP).

[0048] In one example, the mass content c3 of the linear carboxylate solvent in the electrolyte is 5% - 30%, such as 5%, 10%, 15%, 20%, 25%, or 30%.

[0049] In one example, (c2 + c3) / c1 is 1:(0.33 - 1), such as 1:0.33, 1:0.4, 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9, or 1:1.

[0050] By regulating the contents of the linear carbonate solvent, the linear carboxylate solvent, and the cyclic carbonate solvent in the electrolyte to satisfy a specific relationship, the present invention can ensure that the electrolyte has low viscosity and high ionic conductivity while effectively improving the interfacial compatibility between the electrolyte and the electrode material. It is beneficial to the rapid migration of lithium ions in the electrolyte, improves the transmission efficiency of lithium ions, reduces the interfacial impedance, and further enhances the rate performance and low-temperature performance of the battery. When (c2 + c3) / c1 < 1:1, the low contents of the linear carbonate solvent and the linear carboxylate solvent will reduce the ionic transport ability of the electrolyte, resulting in an increase in internal resistance and a significant decrease in the charge-discharge efficiency at high rates. Moreover, the high content of the cyclic carbonate solvent will further increase the viscosity of the electrolyte, inhibit the migration of lithium ions, exacerbate the decrease in ionic conductivity, and deteriorate the rate performance of the battery; when (c2 + c3) / c1 > 1:0.33, the low content of the cyclic carbonate will cause the SEI film to be unable to form fully or have a fragile structure, resulting in the continuous penetration of the electrolyte into the negative electrode to react with it, continuously consuming lithium ions and active substances, and exacerbating the loss of battery capacity. Moreover, the high contents of the linear carbonate solvent and the linear carboxylate solvent will further dilute the cyclic carbonate solvent, resulting in the more ineffective formation of the SEI film, seriously affecting the energy density and cycle life of the battery.

[0051] In the present invention, the graphite includes natural graphite; the sphericity S of the natural graphite is 0.8 - 1, such as 0.8, 0.85, 0.9, 0.95, or 1.

[0052] In one example, S is 0.85 - 0.95.

[0053] In the present invention, the OI value of the negative electrode sheet is 6 - 30, such as 6, 10, 15, 20, 25 or 30.

[0054] In one example, the OI value of the negative electrode sheet is 10 - 25.

[0055] In one example, the ratio of the sphericity of the natural graphite to the OI value of the negative electrode sheet is 1:(8 - 35), such as 1:8, 1:10, 1:15, 1:20, 1:25, 1:30, 1:35.

[0056] In one example, the ratio of the sphericity of the natural graphite to the OI value of the negative electrode sheet is 1:(11 - 28).

[0057] The discharge power of natural graphite is more excellent than that of artificial graphite. On the one hand, the crystal structure of natural graphite is complete and has fewer defects, which makes the resistance of lithium ions during insertion and extraction smaller and the transmission rate faster. On the other hand, the average particle size of natural graphite is less than 20 μm. The smaller particle size means a larger specific surface area, increasing the contact area between the negative electrode and the electrolyte, shortening the lithium ion transmission path, and further improving the discharge power of the battery. At the same time, natural graphite has good sphericity, good anisotropy, and strong lithium intercalation ability. It can be better stacked during the electrode preparation process to form a uniform and stable electrode structure, reduce the voids and stress concentration inside the electrode, improve the mechanical stability of the electrode, and thus enhance the cycle performance of the battery. It can also store more lithium ions, providing higher capacity and energy density for the battery, helping to maintain the stability of the battery during charge and discharge processes. Especially in a low-temperature environment, it can ensure the smooth insertion and extraction of lithium ions, thereby improving the cold start performance of the battery.

[0058] Adjusting the ratio of the sphericity of natural graphite to the OI value of the negative electrode sheet within an appropriate range can further improve the cycle performance, rate performance, low-temperature performance, and energy density of the battery. When the ratio of sphericity to OI value is too small (e.g., <1:35), the small sphericity means that the shape of natural graphite particles is irregular, which will lead to an increase in the contact area between the electrolyte and the surface of natural graphite, thereby exacerbating the formation of the SEI film, consuming more lithium ions, and affecting the capacity retention rate during battery storage. During charge and discharge, problems such as graphite particle fragmentation or delamination of the layered structure may occur in irregular particles, resulting in poor cycle stability of the battery; while too large an OI value means that the layered structure of natural graphite is more parallel to the surface of the electrode current collector. During charge and discharge, the parallel arrangement of the graphite layered structure is likely to cause more severe electrode swelling, further deteriorating the cycle life of the battery. At the same time, too large an OI value of the negative electrode sheet will also make the stacking of graphite particles less compact, resulting in a decrease in the tap density of the electrode and affecting the energy density of the battery. When the ratio of sphericity to OI value is too large (e.g., >1:8), the large sphericity will increase the lithium ion diffusion path, reduce the rate performance of the battery, and affect the cold start performance of the battery. At the same time, the surface characteristics of spherical graphite particles will cause irreversible decomposition of the electrolyte at low potentials, thereby affecting the interface stability and cycle life of the battery; at high current densities, too small an OI value of the negative electrode sheet will lead to a significant decrease in the stability of the graphite layered structure, accelerating the attenuation of the battery cycle life.

[0059] In the present invention, the sphericity of the natural graphite can be obtained by testing with conventional methods in the art. For example, the battery is discharged to 0% SOC, the negative electrode sheet is disassembled and taken out, the negative electrode sheet is cleaned with dimethyl carbonate (DMC) and dried, and the negative electrode sheet is polished along the thickness direction by an argon ion milling instrument. The images of each particle in the SEM image of natural graphite at a certain magnification (e.g., 2500 times) are analyzed by image processing software (such as Image ProPlus) to obtain the perimeter and area of each particle. The perimeter equivalent radius r1 and area equivalent radius r2 of each particle are calculated respectively. Then, the sphericity S of each particle = r2 / r1, and the sphericities of each particle are weighted and averaged by quantity to obtain the sphericity of natural graphite.

[0060] In the present invention, the OI value of the negative electrode sheet can be obtained by testing with conventional methods in the art. For example, X-ray diffraction (XRD) technology is used to analyze the crystal structure of natural graphite materials to evaluate their orientation, and the intensity ratio of the diffraction peak of the (004) crystal plane to the diffraction peak of the (110) crystal plane is measured. According to OI = I(004) / I(110), the OI value of the negative electrode sheet can be calculated.

[0061] In the present invention, the lithium-ion secondary battery further includes a separator, and the porosity ε1 of the separator is 30% - 80%, such as 30%, 40%, 50%, 60%, 70% or 80%.

[0062] In the present invention, the porosity ε2 of the positive electrode sheet is 25% - 60%, such as 25%, 30%, 35%, 40%, 45%, 50%, 55% or 60%.

[0063] In one example, ε1 is 40% - 70%; ε2 is 28% - 40%.

[0064] In one example, ε2:ε1 is 1:(0.6 - 3), such as 1:0.6, 1:0.8, 1:1, 1:1.2, 1:1.4, 1:1.6, 1:1.8, 1:2, 1:2.2, 1:2.4, 1:2.6, 1:2.8 or 1:3.

[0065] In one example, ε2:ε1 is 1:(1.2 - 2.4).

[0066] In the present invention, the separator adopts a high-porosity base film, which can provide more channels and shorter diffusion paths for the migration of lithium ions between the positive and negative electrodes. When the battery is charged and discharged at a high rate, a large number of lithium ions need to quickly pass through the separator in a short time. The abundant pores of the high-porosity base film can greatly reduce the transmission resistance of lithium ions, further promoting the rapid conduction of lithium ions and improving the rate performance of the battery.

[0067] Regulating the ratio of the porosity of the positive electrode sheet to the porosity of the separator within a suitable range can effectively improve the rate performance, low-temperature performance and cycle performance of the battery. When the ratio of the porosity of the positive electrode sheet to the porosity of the separator is too small (e.g., <1:3), the too small porosity of the positive electrode sheet will limit the wettability of the electrolyte, reduce the transmission channels of lithium ions in the positive electrode sheet, resulting in a decrease in the diffusion efficiency of lithium ions and affecting the cold start ability of the battery. In addition, the too small porosity of the positive electrode sheet will also lead to an increase in the internal stress of the positive electrode sheet, and the volume expansion of the positive active material during charge and discharge cannot be buffered, thus accelerating the rupture and shedding of the positive active material particles and ultimately shortening the cycle life of the battery; while the too large porosity of the separator will reduce its thermal stability, thereby reducing the high-temperature storage and cycle performance of the battery. When the ratio of the porosity of the positive electrode sheet to the porosity of the separator is too large (e.g., >1:0.6), the too large porosity of the positive electrode sheet causes the electron conduction path to become longer, increasing the electron transmission resistance of the electrode, thereby increasing the internal resistance of the battery; the too small porosity of the separator will limit the transmission channels of lithium ions, increasing the resistance of lithium ion diffusion and further increasing the internal resistance of the battery, seriously affecting the discharge rate performance of the battery.

[0068] In the present invention, the porosity of the positive electrode sheet and the separator can be obtained by testing through conventional methods in the art, such as measuring by mercury intrusion method using a Micromeritics AutoPore V 9600 mercury porosimeter.

[0069] In the present invention, the negative electrode sheet further includes a negative electrode conductive agent, and the negative electrode conductive agent includes carbon nanotubes and a second carbon black; the particle size of the second carbon black is b2, and the length of the carbon nanotubes is b3.

[0070] In one example, b2 is 10 nm - 300 nm, such as 10 nm, 50 nm, 100 nm, 150 nm, 200 nm, 250 nm, or 300 nm.

[0071] In one example, b3 is 10 μm - 80 μm, such as 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, or 80 μm.

[0072] In one example, b2:b3 is 1:(40 - 5000), such as 1:40, 1:70, 1:100, 1:500, 1:1000, 1:2000, 1:3000, 1:4000, 1:5000.

[0073] In one example, b2:b3 is 1:(200 - 3000).

[0074] When the negative electrode conductive agent adopts a composite material of carbon nanotubes and carbon black, the conductivity of the negative electrode can be further improved, and the rate performance and cycling performance of the battery can be enhanced.

[0075] Adjusting the ratio of the second carbon black particle size to the carbon nanotube length within an appropriate range can effectively improve the electrical conductivity, low-temperature performance, and energy density of the battery. When the ratio of the second carbon black particle size to the carbon nanotube length is too small (e.g., <1:5000), the carbon black with too small particle size is prone to agglomeration, reducing the continuity and uniformity of the conductive network, thus affecting the electrical conductivity of the battery; moreover, the too-long carbon nanotubes will also increase the packing density of the negative electrode active material, affect the diffusion path of lithium ions, increase the polarization of the battery, and further deteriorate the cold start performance of the battery. When the ratio of the negative electrode carbon black particle size to the carbon nanotube length is too large (e.g., >1:40), the carbon black with too large particle size will limit the diffusion path of lithium ions, resulting in a decrease in the transport efficiency of lithium ions in the electrode, thus affecting the cold start performance of the battery. At the same time, the large-sized carbon black particles will also cause an increase in the packing density of the electrode material, local stress concentration, and accelerate the destruction of the electrode structure, thereby reducing the cycle life of the battery; moreover, the mechanical support ability of the too-short carbon nanotubes is weak, and it is difficult to effectively inhibit the volume change of the electrode material during charge and discharge, resulting in a decrease in the electrode structure stability and further deteriorating the cycle performance of the battery. In addition, the too-short carbon nanotubes will reduce the lithium ion storage space, shorten the diffusion path of lithium ions therein, resulting in a decrease in the insertion and extraction efficiency of lithium ions, thus reducing the overall capacity and energy density of the battery.

[0076] In the present invention, the particle size of the second carbon black can be obtained by testing through conventional methods in the art. For example, through SEM, at least 20 second carbon blacks are selected in the electron micrograph, the particle size of each second carbon black is measured, and the average value is taken. If the second carbon black is a regular circle in the micrograph, then the particle size of the second carbon black is the diameter of the circle; if the second carbon black is not a "regular circle" in the micrograph, then the particle size of the second carbon black is the average value of the sizes measured at multiple different angles.

[0077] In the present invention, the length of the carbon nanotube can be obtained by testing through conventional methods in the art. For example, through SEM, at least 20 carbon nanotubes are selected in the electron micrograph, the length of each carbon nanotube is measured, and the average value is taken.

[0078] In the present invention, D2:b3 is 1:(0.6 - 24), for example, 1:0.6, 1:1, 1:5, 1:10, 1:15, 1:20, or 1:24.

[0079] In one example, D2:b3 is 1:(1.5 - 15).

[0080] Adjusting the ratio of the average particle size of natural graphite to the length of carbon nanotubes within an appropriate range can effectively improve the rate performance and cycling performance of the battery. When the ratio of the average particle size of natural graphite to the length of carbon nanotubes is too small (e.g., <1:24), the too-small average particle size of natural graphite will increase its specific surface area, leading to an increase in side reactions with the electrolyte, thereby consuming more lithium ions, reducing the charge-discharge efficiency, and affecting the discharge capacity of the battery; while the too-long carbon nanotubes will cause the accumulation of conductive agents in the negative electrode, increasing the internal resistance and further reducing the discharge capacity of the battery, deteriorating the cold start ability of the battery. When the ratio of the average particle size of natural graphite to the length of carbon nanotubes is too large (e.g., >1:0.6), the too-large average particle size of natural graphite reduces the active sites and diffusion channels for lithium ion insertion, and the diffusion path of lithium ions becomes longer, resulting in poor rate performance of the battery. At the same time, during the charge-discharge process, graphite is more likely to experience volume expansion and structural instability problems, leading to the peeling and shedding of graphite sheets, thereby affecting the cycle life of the battery; while the relatively weaker structural stability of the too-short carbon nanotubes will cause the fracture of the conductive network during the charge-discharge process, further deteriorating the cycling performance of the battery.

[0081] In the present invention, the positive electrode sheet may further include a positive electrode binder. The positive electrode binder may include at least one of polyvinylidene fluoride, carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, and polytetrafluoroethylene.

[0082] In the present invention, the negative electrode sheet may further include a negative electrode binder. The negative electrode binder may include at least one of polyvinylidene fluoride, carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, and polytetrafluoroethylene.

[0083] In the present invention, the electrolyte further includes an electrolyte salt, and the electrolyte salt may be selected from at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPF2O2), and lithium bis(fluorosulfonyl)imide (LiFSI).

[0084] In the present invention, the battery may further include a separator. The separator may be a conventional choice in the art. For example, the separator includes at least one of polyethylene and polypropylene.

[0085] The assembly method of the battery can be carried out in a conventional manner in the art.

[0086] In the present invention, the battery includes a wound lithium ion battery or a stacked lithium ion battery.

[0087] In the present invention, the charging cut-off voltage of the lithium ion secondary battery is 3.6V - 3.8V; the discharging cut-off voltage is 2.0V - 2.5V.

[0088] It should be noted that in the present invention, the numerical representations such as "first", "second", etc. are only used to distinguish different substances or usage methods, and do not represent the difference in sequence.

[0089] The present invention will be described in detail below through embodiments. The embodiments described in the present invention are only a part of the embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0090] In the following examples, unless otherwise specified, the materials used are commercially available analytical pure.

[0091] Example 1

[0092] Prepare a battery according to the following method:

[0093] (1) Prepare a positive electrode sheet

[0094] Mix lithium iron phosphate (a1 is 9755 ppm, a2 is 239985 ppm, D1 is 3755 nm, D3 is 99.4 nm, a1:a2 is 1:24.6, D3:D1 is 1:37.7), first carbon black (b1 is 165 nm), and polyvinylidene fluoride in a mass ratio of 93.5:3:3.5, add N-methylpyrrolidone, and stir under a vacuum mixer until evenly mixed to obtain a positive electrode slurry with a solid content of 60%; coat the positive electrode slurry evenly on both surfaces of the aluminum foil, and after baking, rolling, and slitting, obtain a positive electrode sheet. Among them, b1:D1 is 1:22.7; ε2 is 34%.

[0095] (2) Prepare a negative electrode sheet

[0096] Mix natural graphite (D2 is 9.7 μm, S is 0.9), second carbon black (b2 is 155 nm), carbon nanotubes (b3 is 45 μm), and styrene-butadiene rubber in a mass ratio of 95:2:0.5:2.5, add ethylene carbonate accounting for 1% of the total mass of the above materials, and then add deionized water to obtain a negative electrode slurry with a solid content of 46%; coat the negative electrode slurry evenly on a high-strength carbon-coated copper foil with a thickness of 6 μm, dry, roll, die-cut, and cut into pieces to obtain a negative electrode sheet. Among them, D1:D2 is 1:2.58; b2:b3 is 1:291; D2:b3 is 1:4.6; the OI value of the negative electrode sheet is 17.5, and the ratio of the sphericity of the natural graphite to the OI value of the negative electrode sheet is 1:19.6.

[0097] (3) Prepare a battery

[0098] Stack the positive electrode sheet prepared in step (1), the separator (ε1 is 55%), and the negative electrode sheet prepared in step (2) in sequence, ensuring that the separator is between the positive and negative electrode sheets to play an isolation role, and then obtain a bare battery through winding / lamination; after hot pressing the bare battery, weld the aluminum pole ear and the nickel-plated copper pole ear, and after encapsulation, bake it in a vacuum at 95 °C for 24 hours. Inject the electrolyte (lithium hexafluorophosphate dissolved in a mixed solution composed of DMC / EMC / EP / EC (mass ratio 1:0.4:0.4:1.2), and the concentration of lithium hexafluorophosphate is 1 mol / L) into the dried bare battery, and obtain a lithium-ion battery through processes such as formation, secondary sealing, sorting, and OCV testing. Among them, ε2:ε1 is 1:1.618.

[0099] Example 2

[0100] Prepare the battery according to the following method:

[0101] (1) Prepare the positive electrode sheet

[0102] Mix lithium iron phosphate (a1 is 1512 ppm, a2 is 299886 ppm, D1 is 1606 nm, D3 is 133.7 nm, a1:a2 is 1:198, D3:D1 is 1:12), the first carbon black (b1 is 134 nm), and polyvinylidene fluoride in a mass ratio of 93.5:3:3.5, add N-methylpyrrolidone, and stir under a vacuum mixer until evenly mixed to obtain a positive electrode slurry with a solid content of 60%; evenly coat the positive electrode slurry on both surfaces of the aluminum foil, and obtain the positive electrode sheet after baking, rolling, and slitting. Among them, b1:D1 is 1:12; ε2 is 33.3%.

[0103] (2) Prepare the negative electrode sheet

[0104] Mix natural graphite (D2 is 14.6 μm, S is 0.85), the second carbon black (b2 is 109 nm), carbon nanotubes (b3 is 22 μm), and styrene-butadiene rubber evenly in a mass ratio of 95:2:0.5:2.5, add ethylene carbonate accounting for 1% of the total mass of the above materials, and then add deionized water to obtain a negative electrode slurry with a solid content of 46%; evenly coat the negative electrode slurry on a high-strength carbon-coated copper foil with a thickness of 6 μm, and obtain the negative electrode sheet after drying, rolling, die-cutting, and sheet-making. Among them, D1:D2 is 1:9; b2:b3 is 1:200; D2:b3 is 1:1.5; the OI value of the negative electrode sheet is 10, and the ratio of the sphericity of the natural graphite to the OI value of the negative electrode sheet is 1:11.8.

[0105] (3) Prepare the battery

[0106] Stack the positive electrode sheet prepared in step (1), the separator (ε1 is 40%), and the negative electrode sheet prepared in step (2) in sequence, ensuring that the separator is between the positive and negative electrode sheets to play an insulating role, and then wind to obtain a bare battery; after hot pressing the bare battery, weld the aluminum electrode tab and the nickel-plated copper electrode tab, and after encapsulation, bake in a vacuum at 95 °C for 24 hours. Inject the electrolyte (lithium hexafluorophosphate dissolved in a mixed solution composed of DMC / EMC / EP / EC (mass ratio 1:1:0.2:0.8), and the concentration of lithium hexafluorophosphate is 1 mol / L) into the dried bare battery, and after processes such as formation, secondary sealing, sorting, and OCV testing, obtain a lithium-ion battery. Among them, ε2:ε1 is 1:1.2.

[0107] Example 3

[0108] Prepare the battery according to the following method:

[0109] (1) Prepare the positive electrode sheet

[0110] Mix lithium iron phosphate (a1 is 17985 ppm, a2 is 180015 ppm, D1 is 4502 nm, D3 is 38.7 nm, a1:a2 is 1:10, D3:D1 is 1:116), the first carbon black (b1 is 50 nm), and polyvinylidene fluoride in a mass ratio of 93.5:3:3.5, add N-methylpyrrolidone, and stir under a vacuum mixer until evenly mixed to obtain a positive electrode slurry with a solid content of 60%; evenly coat the positive electrode slurry on both side surfaces of the aluminum foil, and after baking, rolling, and slitting, obtain the positive electrode sheet. Among them, b1:D1 is 1:90; ε2 is 29.1%.

[0111] (2) Prepare the negative electrode sheet

[0112] Mix natural graphite (D2 is 4.5 μm, S is 0.95), the second carbon black (b2 is 23 nm), carbon nanotubes (b3 is 68 μm), and styrene-butadiene rubber evenly in a mass ratio of 95:2:0.5:2.5, add ethylene carbonate accounting for 1% of the total mass of the above materials, and then add deionized water to obtain a negative electrode slurry with a solid content of 46%; evenly coat the negative electrode slurry on a high-strength carbon-coated copper foil with a thickness of 6 μm, dry, roll, die-cut, and cut into pieces to obtain the negative electrode sheet. Among them, D1:D2 is 1:1; b2:b3 is 1:2941; D2:b3 is 1:15; the OI value of the negative electrode sheet is 25, and the ratio of the sphericity of the natural graphite to the OI value of the negative electrode sheet is 1:26.3.

[0113] (3) Prepare the battery

[0114] Stack the positive electrode sheet prepared in step (1), the separator (ε1 is 70%), and the negative electrode sheet prepared in step (2) in sequence, ensuring that the separator is between the positive and negative electrode sheets to play an isolation role, and then obtain a bare battery by winding; after hot pressing the bare battery, weld the aluminum electrode tab and the nickel-plated copper electrode tab, and after encapsulation, bake it in a vacuum at 95 °C for 24 hours. Inject the electrolyte (lithium hexafluorophosphate dissolved in a mixed solution composed of DMC / EMC / EP / EC (mass ratio 0.6:0.2:0.8:1.4), and the concentration of lithium hexafluorophosphate is 1 mol / L) into the dried bare battery, and obtain a lithium-ion battery through processes such as formation, secondary sealing, sorting, and OCV testing. Among them, ε2:ε1 is 1:2.4.

[0115] Example 4

[0116] Used to verify the influence brought by the change of "the mass content a1 of element V in lithium iron phosphate".

[0117] This group of examples is carried out with reference to Example 1. The difference is that the mass content a1 of element V in lithium iron phosphate is changed, specifically as follows:

[0118] Example 4a, a1 is 1200 ppm; among them, a1:a2 is 1:200;

[0119] Example 4b, a1 is 19988 ppm; among them, a1:a2 is 1:12.

[0120] Example 5

[0121] Used to verify the influence brought by the change of "the mass content a2 of element Fe in lithium iron phosphate".

[0122] This group of examples is carried out with reference to Example 1. The difference is that the mass content a2 of element Fe in lithium iron phosphate is changed, specifically as follows:

[0123] Example 5a, a2 is 150012 ppm; among them, a1:a2 is 1:15;

[0124] Example 5b, a2 is 349995 ppm; among them, a1:a2 is 1:36.

[0125] Example 6

[0126] Used to verify the influence brought by the change of "a1:a2".

[0127] This group of examples is carried out with reference to Example 1. The difference is that the mass content a1 of element V in lithium iron phosphate and the mass content a2 of element Fe are changed, specifically as follows:

[0128] Example 6a, a1 is 1250 ppm; a2 is 349995 ppm; a1:a2 is 1:280;

[0129] Example 6b, a1 is 18752 ppm; a2 is 150012 ppm; a1:a2 is 1:8.

[0130] Example 7

[0131] Used to verify the impact brought about by the change of "D1:D2".

[0132] This group of examples is carried out with reference to Example 1. The difference is that the average particle size D1 of the lithium iron phosphate secondary particles and the average particle size D2 of the graphite are changed, specifically as follows:

[0133] Example 7a, D1 is 1332 nm; D2 is 20 μm; D1:D2 is 1:15; among them, D2:b3 is 1:2.3; D3:D1 is 1:13.3; b1:D1 is 1:8;

[0134] Example 7b, D1 is 5998 nm; D2 is 3 μm, D1:D2 is 1:0.5; among them, D2:b3 is 1:15; D3:D1 is 1:59; b1:D1 is 1:36.

[0135] Example 8

[0136] Used to verify the impact brought about by the change of "D3:D1".

[0137] This group of examples is carried out with reference to Example 1 or Example 2. The difference is that the average particle size D1 of the lithium iron phosphate secondary particles and the average particle size D3 of the lithium iron phosphate primary particles are changed, specifically as follows:

[0138] Example 8a, carried out with reference to Example 1. The difference is that D1 is 5798 nm; D3 is 33 nm; D3:D1 is 1:176; among them, D1:D2 is 1:1.7; b1:D1 is 1:35.7;

[0139] Example 8b, carried out with reference to Example 2. The difference is that D3 is 200 nm; D3:D1 is 1:8.

[0140] Example 9

[0141] Used to verify the impact brought about by the change of "the positive electrode conductive agent".

[0142] This group of embodiments is carried out with reference to Embodiment 1. The difference is that the positive electrode conductive agent is carbon nanotubes. Specifically as follows: Lithium iron phosphate (a1 is 9755 ppm, a2 is 239985 ppm, D1 is 3755 nm, D3 is 99.4 nm, a1:a2 is 1:24.4, D3:D1 is 1:38.5), carbon nanotubes and polyvinylidene fluoride are mixed according to a mass ratio of 93.5:3:3.5, N-methylpyrrolidone is added, and stirring is carried out under the action of a vacuum mixer until evenly mixed to obtain a positive electrode slurry with a solid content of 60%; the positive electrode slurry is evenly coated on both side surfaces of the aluminum foil, and after baking, rolling and slitting, a positive electrode sheet is obtained.

[0143] Example 10

[0144] Used to verify the influence brought by the change of "b1:D1".

[0145] This group of embodiments is carried out with reference to Embodiment 1 or Embodiment 2. The difference is that the average particle size b1 of the first carbon black and the average particle size D1 of the secondary particles of lithium iron phosphate are changed. Specifically as follows:

[0146] Example 10a, carried out with reference to Embodiment 1. The difference is that D1 is 5798 nm; b1 is 31 nm; b1:D1 is 1:200; among them, D1:D2 is 1:1.7; D3:D1 is 1:59;

[0147] Example 10b, carried out with reference to Embodiment 2. The difference is that b1 is 295 nm and b1:D1 is 1:5.4.

[0148] Example 11

[0149] Used to verify the influence brought by the change of "negative electrode conductive agent".

[0150] This group of embodiments is carried out with reference to Embodiment 1. The difference is that the negative electrode conductive agent is carbon nanotubes. Specifically as follows: Natural graphite (D2 is 9.7 μm, S is 0.9), carbon nanotubes (b3 is 45 μm) and styrene-butadiene rubber are mixed evenly according to a mass ratio of 95:2.5:2.5, ethylene carbonate accounting for 1% of the total mass of the above materials is added, and then deionized water is added to obtain a negative electrode slurry with a solid content of 46%; the negative electrode slurry is evenly coated on a high-strength carbon-coated copper foil with a thickness of 6 μm, and after drying, rolling, die-cutting and sheet-making, a negative electrode sheet is obtained.

[0151] Example 12

[0152] Used to verify the influence brought by the change of "b2:b3".

[0153] This group of embodiments is carried out with reference to Embodiment 1. The difference is that the average particle size b2 of the second carbon black and the length b3 of the carbon nanotubes are changed. Specifically as follows:

[0154] Example 12a, b2 is 11 nm; b3 is 79 μm; b2:b3 is 1:7143;

[0155] Example 12b, b2 is 298 nm; b3 is 12 μm; b2:b3 is 1:40.

[0156] Example 13

[0157] Used to verify the impact brought by the change of "D2:b3".

[0158] This group of examples is carried out with reference to Example 2 or Example 3. The difference is that the length b3 of the carbon nanotubes is changed, specifically as follows:

[0159] Example 13a, carried out with reference to Example 3. The difference is that b3 is 79 μm; D2:b3 is 1:18; among them, b2:b3 is 1:3448;

[0160] Example 13b, carried out with reference to Example 2. The difference is that b3 is 12 μm; D2:b3 is 1:0.8; among them, b2:b3 is 1:110.

[0161] Example 14

[0162] Used to verify the impact brought by the change of "negative electrode active material".

[0163] This group of examples is carried out with reference to Example 1. The difference is that the negative electrode active material is changed, specifically as follows:

[0164] Example 14a, the negative electrode active material is artificial graphite;

[0165] Example 14b, the negative electrode active material is a mixture of artificial graphite and natural graphite (mass ratio 5:5).

[0166] Example 15

[0167] Used to verify the impact brought by the change of "the ratio of the sphericity of natural graphite to the OI value of the negative electrode sheet".

[0168] This group of examples is carried out with reference to Example 1. The difference is that the sphericity of natural graphite and the intensity ratio of the (004) crystal plane diffraction peak to the (110) crystal plane diffraction peak are changed, specifically as follows:

[0169] Example 15a, S is 0.8, the OI value of the negative electrode sheet is 28, and the ratio of the sphericity of natural graphite to the OI value of the negative electrode sheet is 1:34.5;

[0170] Example 15b, S is 0.95, the OI value of the negative electrode sheet is 7.6, and the ratio of the sphericity of natural graphite to the OI value of the negative electrode sheet is 1:8.

[0171] Example 16

[0172] To verify the influence brought by the change of "ε2:ε1".

[0173] This group of examples is carried out with reference to Example 1. The difference is that the porosity ε2 of the positive electrode sheet and the porosity ε1 of the separator are changed, specifically as follows:

[0174] Example 16a, ε1 is 70%, ε2 is 24%, and ε2:ε1 is 1:3;

[0175] Example 16b, ε1 is 40%, ε2 is 66%, and ε2:ε1 is 1:0.6.

[0176] Comparative Example 1

[0177] Carried out with reference to Example 1. The difference is that the mass content a1 of element V in lithium iron phosphate is changed, specifically as follows:

[0178] Comparative Example 1, a1 is 39755 ppm, and a1:a2 is 1:6.

[0179] Comparative Example 2

[0180] Carried out with reference to Example 1. The difference is that lithium iron phosphate does not include element V.

[0181] Comparative Example 3

[0182] Carried out with reference to Example 1. The difference is that the mass content a1 of element V and the mass content a2 of element Fe in lithium iron phosphate are changed, specifically as follows:

[0183] Comparative Example 3a, a1 is 885 ppm, a2 is 380085 ppm, and a1:a2 is 1:500;

[0184] Comparative Example 3b, a1 is 29988 ppm, a2 is 130012 ppm, and a1:a2 is 1:4.3.

[0185] Comparative Example 4

[0186] Carried out with reference to Example 1. The difference is that the average particle size D1 of the secondary particles of lithium iron phosphate and the average particle size D2 of graphite are changed, specifically as follows:

[0187] Comparative Example 4a, D1 is 995 nm, D2 is 20 μm, and D1:D2 is 1:20;

[0188] Comparative Example 4b, D1 is 5998 nm, D2 is 2 μm, and D1:D2 is 1:0.33.

[0189] Test Example

[0190] (1) Energy density test

[0191] The lithium-ion batteries prepared in the examples and comparative examples were subjected to an energy density test. The specific test method is as follows:

[0192] The weight of the lithium-ion battery after the second sealing process was recorded as the battery weight. In the sorting process, the lithium-ion battery was charged at a constant current and constant voltage of 1C to the charging cut-off voltage of 3.65V, with a cut-off current of 0.05C. After standing for 5 minutes, the energy discharged at a constant current of 1C to the discharge cut-off voltage of 2.2V was recorded as the discharge energy. Then, the ratio of the discharge energy to the battery weight was the energy density, with the unit of Wh / kg. The results were recorded in Table 1.

[0193] (2) Cold start test

[0194] The lithium-ion batteries prepared in the examples and comparative examples were subjected to a cold start test. The specific test method is as follows:

[0195] In an environment of (25±2)°C, it was discharged at a constant current of 1C to the discharge cut-off voltage of 2.2V and left standing for 30 minutes; then it was charged at a constant current and constant voltage of 1C to the charging cut-off voltage of 3.65V, with a cut-off current of 0.05C, and left standing for 30 minutes; it was discharged at a constant current of 1C to the discharge cut-off voltage of 2.2V to obtain the actual battery capacity C0; left standing for 30 minutes; charged at a constant current and constant voltage of 1C to the charging cut-off voltage of 3.65V, with a cut-off current of 0.05C, and discharged at 1C0 for 30 minutes, which was 50% SOC; after standing still for 2 hours at (25±2)°C, the battery was placed in a -30°C constant temperature oven and kept at a constant temperature for 4 hours; the terminal voltage value of the 10C constant current discharge for 2 seconds was measured as the cold start terminal voltage of the battery, with the unit of V. The results were recorded in Table 1.

[0196] (3) Discharge rate test

[0197] The lithium-ion batteries prepared in the examples and comparative examples were subjected to a discharge rate test. The specific test method is as follows:

[0198] At (25 ± 2) °C, discharge at a constant current of 1C to the discharge cut-off voltage of 2.2V, and set aside for 30 min; then charge at a constant current and constant voltage of 1C to the charge cut-off voltage of 3.65V, with a cut-off current of 0.05C, and set aside for 30 min; discharge at a constant current of 1C to the discharge cut-off voltage of 2.2V to obtain the actual capacity C0 of the battery, and set aside for 30 min; charge at a constant current and constant voltage of 1C to the charge cut-off voltage of 3.65V, with a cut-off current of 0.05C, and discharge at 1C0 for 30 min, which is 50% SOC; after standing at (25 ± 2) °C for 2 h, discharge at a constant current at a certain rate for 10 s to the discharge cut-off voltage of 2.2V, and this rate is the discharge rate of the battery, and record the results in Table 1.

[0199] (4) Cycle test

[0200] Perform cycle tests on the lithium-ion batteries prepared in the examples and comparative examples. The specific test methods are as follows:

[0201] At 55 °C, discharge at a constant current of 1C to the discharge cut-off voltage of 2.2V, and set aside for 30 min; then charge at a constant current and constant voltage of 1C to the charge cut-off voltage of 3.65V, with a cut-off current of 0.05C, and set aside for 30 min; discharge at a constant current of 1C to the discharge cut-off voltage of 2.2V, and set aside for 30 min; repeat the above full charge and discharge steps until the capacity decays to 80% and stop. The number of repeated cycles is the cycle life, and use this to evaluate the cycle performance of the battery after aging, and record the results in Table 1.

[0202] Table 1

[0203]

[0204]

[0205] It can be seen from Table 1 that compared with the comparative examples, the battery of the present invention has a higher energy density, a high discharge rate, and significantly improves the cold start voltage and cycle life of the battery.

[0206] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A lithium ion secondary battery, characterized in that, The lithium ion secondary battery includes a positive electrode sheet and a negative electrode sheet; The positive electrode sheet includes a positive electrode active material, the positive electrode active material includes lithium iron phosphate, and the lithium iron phosphate includes secondary particles formed by a number of primary particles; the lithium iron phosphate includes element V, and the mass content a1 of element V in the lithium iron phosphate is 1000 ppm - 20000 ppm; the mass content a2 of element Fe in the lithium iron phosphate is 150000 ppm - 350000 ppm; a1:a2 is 1:(8 - 280); The negative electrode sheet includes a negative electrode active material, the negative electrode active material includes graphite, and the average particle size D1 of the secondary particles and the average particle size D2 of the graphite satisfy: D1:D2 is 1:(0.5 - 15).

2. The lithium ion secondary battery according to claim 1, wherein The graphite includes natural graphite; And / or, a1:a2 is 1:(10 - 200); And / or, D1:D2 is 1:(1 - 9); And / or, a1 is 1500 ppm - 18000 ppm; And / or, a2 is 180000 ppm - 300000 ppm.

3. The lithium ion secondary battery according to claim 1 or 2, wherein D1 is 1000 nm - 6000 nm; preferably 1600 nm - 5800 nm; And / or, the average particle size D3 of the primary particles is 30 nm - ​ 4. The lithium ion secondary battery according to claim 1 or 2, wherein, ​ ​ 5. The lithium ion secondary battery according to claim 1 or 2, wherein, ​ ​ ​ 6. The lithium ion secondary battery according to claim 1 or 2, wherein, ​ ​ ​ ​ 7. The lithium ion secondary battery according to claim 2, wherein, ​ ​ Preferably, the ratio of the sphericity of the natural graphite to the OI value of the negative electrode sheet is 1:(8-35); more preferably 1:(11-28).

8. The lithium ion secondary battery according to claim 1 or 2, wherein The lithium-ion secondary battery further comprises a diaphragm, wherein the porosity ε1 of the diaphragm is 30%-80%, preferably 40%-70%. And / or, the porosity ε2 of the positive electrode sheet is 25%-60%; preferably 28%-40%; Preferably, ε2:ε1 is 1:(0.6-3); more preferably, it is 1:(1.2-2.4).

9. The lithium ion secondary battery according to claim 1 or 2, wherein The negative electrode sheet further includes a negative electrode conductive agent, and the negative electrode conductive agent includes carbon nanotubes and a second carbon black; Preferably, the particle size of the second carbon black is b2, the length of the carbon nanotube is b3, and the ratio of b2:b3 is 1:(40-5000); more preferably 1:(200-3000); More preferably, b2 is 10 nm-300 nm; More preferably, b3 is 10 μm-80 μm.

10. The lithium ion secondary battery according to claim 9, wherein, D2:b3 is 1:(0.6-24); preferably 1:(1.5-15).

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