Positive electrode material, positive electrode plate, lithium secondary battery, battery module, battery pack and device

Abstract

The invention relates to a positive electrode material, a positive electrode plate, a lithium secondary battery, a battery module, a battery pack and a device. The positive electrode material provided by the invention comprises lithium iron phosphate single-crystal particles and lithium iron phosphate secondary particles. Wherein the lithium iron phosphate secondary particles have a low specific surface area, so that the consumption of a binder and a solvent in the pulping process can be reduced, the solid content of slurry is increased, and the processing performance of the thick coated pole piece is improved. And meanwhile, gaps among the lithium iron phosphate secondary particles are fully filled with the lithium iron phosphate single crystals, so that the compaction density of the pole piece can be further improved, and the diaphragm resistance of the positive pole piece is reduced, thereby improving the gram volume exertion and cycle performance of the lithium secondary battery.

Description

[0001] This application is a divisional application of the invention patent with the application number 202080104510.7 and the invention name of "Positive Electrode Material, Positive Electrode Sheet, Lithium Secondary Battery, Battery Module, Battery Pack and Device", which was filed on September 3, 2020. Technical Field

[0002] This application relates to the field of batteries, and more specifically, to a positive electrode material, a positive electrode sheet, a lithium secondary battery, a battery module, a battery pack and a device. Background Art

[0003] As a positive electrode material for lithium-ion batteries, lithium iron phosphate exhibits advantages in terms of safety performance, cycle performance and cost, and is widely used in the fields of buses, passenger cars and large-scale energy storage. However, the voltage platform, capacity utilization and electrode compaction density of lithium iron phosphate full batteries are all at relatively low levels. Therefore, compared with ternary positive electrode material batteries, the energy density of lithium iron phosphate batteries is not satisfactory.

[0004] Through cell design calculations, it can be found that increasing the coating weight per unit area of the active material on the positive electrode sheet, that is, the mass surface density of the active material, can significantly improve the energy density of lithium-ion batteries, while increasing the utilization efficiency of other materials (current collector, housing, electrolyte and separator), and reducing the comprehensive cost of the cell. However, when the positive electrode material in a lithium-ion battery is lithium iron phosphate, during the actual manufacturing process or use, the positive electrode sheet is extremely prone to cold pressing breakage or film peeling, and it is difficult to achieve high energy density.

[0005] Therefore, how to prepare a lithium-ion battery that is compatible with high safety, long cycle life and high energy density has become an urgent problem to be solved in the industry. Summary of the Invention

[0006] In view of the problems in the background art, this application provides a positive electrode material, a positive electrode sheet, a lithium secondary battery, a battery module, a battery pack and a device.

[0007] In a first aspect, this application provides a positive electrode material, which includes a first positive electrode active material represented by formula (I) and a second positive electrode active material represented by formula (II).

[0008] LiFe 1-x1 M1 x1 PO 4 (I)

[0009] LiFe 1-x2 M2 x2 PO 4 (II)

[0010] Wherein, 0 ≤ x1 ≤ 0.1, 0 ≤ x2 ≤ 0.1; M1 and M2 are each independently selected from at least one of Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr or Ti; the first positive electrode active material is a single crystal particle, and the second positive electrode active material is a secondary particle.

[0011] Compared with the prior art, the present application has at least the following advantages:

[0012] The positive electrode material provided by the present application includes a first positive electrode active material and a second positive electrode active material. The first positive electrode active material is a single crystal particle of lithium iron phosphate, and the second positive electrode active material is a secondary particle of lithium iron phosphate. Among them, the secondary particle of lithium iron phosphate has a low specific surface area, which can reduce the consumption of the binder and solvent during the pulping process and increase the solid content of the slurry. The increase in the solid content of the slurry can reduce the processing difficulty of the thick-coated electrode sheet: when the active material layer of the thick-coated electrode sheet is dried, the evaporation amount of the dispersed solvent decreases, the volume strain of the film layer decreases, and the segregation effect of the binder in the direction perpendicular to the surface of the electrode sheet is suppressed, and the electrode sheet is not likely to have phenomena such as powder falling, cracking, and film peeling. At the same time, the single crystal particles of lithium iron phosphate are fully filled into the particle gaps between the secondary particles of lithium iron phosphate, increasing the adhesion force between the secondary particles of lithium iron phosphate, further improving the compaction density of the electrode sheet; it can also improve the flatness of the contact surface between the active material and the current collector, which helps to slow down the damage to the surface of the current collector during the cold pressing process of the secondary particles of lithium iron phosphate on the electrode sheet, enhance the flexibility of the electrode sheet, and reduce the probability of electrode sheet fracture during the processing. In addition, since the production cost of the secondary particles of lithium iron phosphate is relatively low, the technical solution of the present application can also reduce the comprehensive cost of the battery cell.

[0013] In the positive electrode material provided by the present application, the secondary particle is a pomegranate-like morphology formed by the aggregation of a plurality of primary particles. Optionally, the primary particle is a single crystal nanoparticle. Optionally, the average particle size d of the primary particle is in the range of 20 nm - 500 nm. At this time, the particle size of the primary particle is within a suitable range, and both the side reaction rate between the positive electrode material and the electrolyte and the kinetic performance of the battery positive electrode are at a reasonable level, and the capacity performance and cycle performance of the battery are relatively good.

[0014] In the positive electrode material provided by the present application, the second positive electrode active material satisfies at least one of the following conditions: (1) The median particle size Dv50 of the second positive electrode active material is 2.5 μm - 10.5 μm; optionally, the median particle size Dv50 of the second positive electrode active material is 4.5 μm - 8.5 μm; further optionally, the median particle size Dv50 of the second positive electrode active material is 6 μm - 7 μm; (2) The specific surface area of the second positive electrode active material is 3.5 m 2 / g - 10.5 m 2 / g. Optionally, the specific surface area of the second positive electrode active material is 5 m 2 / g - 9 m 2 / g; Further optionally, the specific surface area of the second positive electrode active material is 6.5 m 2 / g - 7.5 m 2 / g.

[0015] When the median particle size Dv50 and the specific surface area BET of the second positive electrode active material are within the ranges provided in this application, its particle size and specific surface area are appropriate, which is conducive to the sufficient filling of the first positive electrode active material in the particle gaps thereof, resulting in a higher compaction density of the electrode, a better flatness of the contact layer between the active material and the current collector, and good processability of the electrode. At the same time, when the solid content of the slurry of the positive electrode material mixture system is within a suitable range, it is conducive to improving the capacity performance and cycling performance of the battery.

[0016] In the positive electrode material provided in this application, the single crystal particles are independent particles with continuous internal lattices and almost no grain boundary separation.

[0017] In the positive electrode material provided in this application, the powder resistivity of the first positive electrode active material at 12 Mpa is 150 Ω · cm or less. Optionally, the powder resistivity of the first positive electrode active material at 12 Mpa is 80 Ω · cm or less. Further optionally, the powder resistivity of the first positive electrode active material at 12 Mpa is 30 Ω · cm or less.

[0018] As the powder resistivity of the first positive electrode active material decreases, its conductivity is improved, which is conducive to enhancing the electron transport performance between the positive electrode active materials, reducing the film resistivity of the positive electrode sheet, and promoting the capacity performance and cycling performance of the battery.

[0019] In the positive electrode material provided in this application, the first positive electrode active material satisfies at least one of the following conditions: (1) the median particle size Dv50 of the first positive electrode active material is 0.5 μm - 2.0 μm, (2) the specific surface area of the first positive electrode active material is 6.0 m 2 / g - 20 m 2 / g.

[0020] When the median particle size Dv50 of the first positive electrode active material is within the range of 0.5 μm - 2.0 μm, it is conducive to its sufficient filling into the particle gaps of the secondary particles of lithium iron phosphate of the second positive electrode active material, and better improves the compaction density of the electrode and the processability of the electrode. When the specific surface area of the first positive electrode active material is 6.0 m 2 / g - 20 m 2Within the range of / g, it is beneficial to further reduce the consumption of the binder during the pulping process, increase the solid content of the slurry, and at the same time ensure a large specific surface area for the electrochemical reaction activity.

[0021] In the positive electrode material provided by the present application, the mass percentage content of the second positive electrode active material is greater than that of the first positive electrode active material. Optionally, the mass percentage content of the second positive electrode active material is 55%-90%; further optionally, the mass percentage content of the second positive electrode active material is 60%-85%; still further optionally, the mass percentage content of the second positive electrode active material is 65%-80%.

[0022] In the embodiment of the present application, the second active material lithium iron phosphate secondary particles have a low specific surface area, which can increase the solid content of the slurry, reduce the processing difficulty of the thick-coated electrode sheet, and reduce the production cost of the positive electrode material; while the first active material lithium iron phosphate single crystal is filled in the particle gaps of the lithium iron phosphate secondary particles, further improving the compaction density of the electrode sheet and the processing performance of the electrode sheet. When the mass percentage content of the second positive electrode active material is greater than that of the first positive electrode active material, that is, when the mass percentage content of the second positive electrode active material is greater than 50%, a positive electrode sheet with improved performance and lower cost can be obtained. With the further increase of the content of the second positive electrode active material, the compaction density and processing performance of the electrode sheet are further improved; at the same time, an appropriate amount of the first positive electrode active material needs to be added to fill the gaps between the lithium iron phosphate secondary particles to increase the adhesion between the lithium iron phosphate secondary particles, improve the compaction density and processing performance of the electrode sheet, improve the electron transport performance and reduce the film resistance. When the mass percentage content of the second positive electrode active material is 60%-85%; or further 65%-80%, the first positive electrode active material and the second positive electrode active material act synergistically to improve the comprehensive processing performance of the slurry and the electrical performance of the electrode sheet.

[0023] In a second aspect, the present application provides a positive electrode sheet, including a positive electrode active material layer, and the positive electrode active material layer contains the positive electrode material of the first aspect of the present application.

[0024] The coating weight per unit area on one side of the positive electrode sheet provided by the present application is 140 g / m 2 -390 g / m 2 ; optionally, the coating weight per unit area on one side is 190 g / m 2 -320 g / m 2 ; further optionally, the coating weight per unit area on one side is 230 g / m 2 -280 g / m 2 .

[0025] In the positive electrode sheet provided in the present application, the limit solid content of the positive electrode slurry is increased, so that a higher coating weight and a larger coating thickness can be achieved; with the increase of coating weight and coating thickness, the battery volume utilization rate is improved, which helps to improve the energy density, but the lithium ion migration path is also longer, the diffusion impedance is increased, the lithium ion concentration polarization of the battery becomes larger during the discharge process, and the battery capacity is reduced. When the coating weight is 230g / m 2 -280g / m 2 When the battery is charged, the battery energy density and cycle performance are optimal.

[0026] The membrane resistance of the positive electrode sheet provided in the present application is less than 2000mΩ; optionally, the membrane resistance is less than 800mΩ; further optionally, the membrane resistance is less than 200mΩ. The positive electrode sheet of the present application has a lower membrane resistance, which can promote the energy density and cycle performance of the battery.

[0027] In a third aspect, the present application provides a lithium secondary battery, which includes the positive electrode sheet described in the second aspect of the present application.

[0028] In a fourth aspect, the present application provides a battery module, which includes the lithium secondary battery described in the third aspect of the present application.

[0029] In a fifth aspect, the present application provides a battery pack, which includes the battery module described in the fourth aspect of the present application.

[0030] In a sixth aspect, the present application provides a device comprising the lithium secondary battery described in the third aspect of the present application or the battery module described in the fourth aspect of the present application or the battery pack described in the fifth aspect of the present application, wherein the lithium secondary battery or the battery module or the battery pack is used as a power source or energy storage unit for the device.

[0031] Compared with traditional lithium iron phosphate materials, the positive electrode material provided by the present application has a significantly improved slurry limit solid content during the slurry making process, the cracking of the electrode sheet after thick coating is improved, the bonding force between the diaphragm and the current collector is improved, and the diaphragm resistance is reduced. The use of the positive electrode material of the present application to prepare positive electrode sheets and apply them to lithium secondary batteries can significantly improve the energy density, kinetic performance and cycle performance of lithium secondary batteries. At the same time, the device using the lithium secondary battery or battery module or battery pack of the present application as a power source or energy storage unit also has good power performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 1 is a scanning electron microscope image of the second positive electrode active material at different magnifications according to a specific embodiment of the present application: wherein the magnification of 1A is 1000X times, the magnification of 1B is 5000X times, and the magnification of 1C is 10000X times;

[0033] Figure 2 Scanning electron microscope images of the first positive electrode active material according to a specific embodiment of the present application at different magnifications: Among them, the magnification of 2A is 10,000X, and the magnification of 2B is 30,000X;

[0034] Figure 3 Scanning electron microscope image of the cross-section of the positive electrode sheet according to a specific embodiment of the present application;

[0035] Figure 4 Stereogram of a lithium secondary battery according to a specific embodiment of the present application;

[0036] Figure 5 is Figure 4 Exploded view of the lithium secondary battery shown;

[0037] Figure 6 Stereogram of a battery module according to a specific embodiment of the present application;

[0038] Figure 7 Stereogram of a battery pack according to a specific embodiment of the present application;

[0039] Figure 8 is Figure 7 Exploded view of the battery pack shown;

[0040] Figure 9 Schematic diagram of a device according to a specific embodiment of the present application.

[0041] Among them, the reference numerals are explained as follows:

[0042] 1 Battery pack

[0043] 2 Upper box body

[0044] 3 Lower box body

[0045] 4 Battery module

[0046] 5 Lithium secondary battery

[0047] 51 Housing

[0048] 52 Electrode assembly

[0049] 53 Top cover assembly Specific embodiment

[0050] The present application will be further described below in conjunction with specific embodiments. It should be understood that these specific embodiments are only used to illustrate the present application and not to limit the scope of the present application.

[0051] [Positive electrode material]

[0052] The first aspect of the present application relates to a cathode material, which comprises a first cathode active material represented by formula (I) and a second cathode active material represented by formula (II).

[0053] LiFe 1-x1 M1 x1 PO 4 (I)

[0054] LiFe 1-x2 M2 x2 PO 4 (II)

[0055] Wherein, 0≤x1≤0.1, 0≤x2≤0.1; M1 and M2 are each independently selected from at least one of Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr or Ti; the first cathode active material is single crystal particles, and the second cathode active material is secondary particles.

[0056] The inventors of the present application have found through research that: in the production process of lithium iron phosphate cathode materials, in order to improve the capacity performance and kinetic performance of the cathode materials, sufficient nanocrystallization is generally required. However, the nanocrystallization process makes the lithium iron phosphate material have a high specific surface energy, resulting in difficult processing of the material. When the coating thickness of the cathode is increased, the phenomena of powder shedding, cracking and film peeling of the electrode sheet will be aggravated. At the same time, to achieve the same electrode sheet compaction density, a higher surface density electrode sheet requires a greater cold pressing roller pressure, and the cathode material particles will cause damage to the metal substrate during this process. The defects generated in the electrode sheet during the above cold pressing process will cause the electrode sheet to break during subsequent winding and the bare battery cell during hot pressing.

[0057] The inventors of the present application have further found that: in the embodiments of the present application, the lithium iron phosphate secondary particles of the second cathode active material have a low specific surface area, which can reduce the consumption of the binder and solvent during the pulping process and increase the solid content of the slurry. The increase in the solid content of the slurry can reduce the processing difficulty of the thick-coated electrode sheet: when the active material layer of the thick-coated electrode sheet is dried, the volatilization amount of the dispersant is reduced, the volume strain of the film layer is reduced, and the segregation effect of the binder in the direction perpendicular to the surface of the electrode sheet is suppressed, and the electrode sheet is not likely to occur phenomena such as powder shedding, cracking and film peeling. At the same time, the lithium iron phosphate single crystal of the first cathode active material is fully filled into the particle gaps of the lithium iron phosphate secondary particles, increasing the adhesion force between the lithium iron phosphate secondary particles, further improving the compaction density of the electrode sheet; it can also improve the flatness of the contact surface between the active material and the current collector, helping to slow down the damage to the surface of the current collector by the lithium iron phosphate secondary particles during the cold pressing process of the electrode sheet, enhancing the flexibility of the electrode sheet, and reducing the probability of electrode sheet breakage during the processing.

[0058] In some embodiments of the present application, the lithium iron phosphate secondary particles serving as the second positive electrode active material have a pomegranate-like morphology formed by the aggregation of a plurality of lithium iron phosphate primary particles. The lithium iron phosphate primary particles are the smallest structural units constituting the hierarchical structure of the secondary particles, referring to independent particles with low porosity, which can be observed by an electron microscope.

[0059] In some embodiments of the present application, the average particle size d of the primary particles is in the range of 20 nm to 500 nm. In the present application, when the average particle size d of the primary particles is within the above range, the side reaction rate between the formed positive electrode material and the electrolyte and the kinetic performance of the battery positive electrode are both at a reasonable level, and the capacity performance and cycle performance of the battery are relatively excellent. The average particle size d of the primary particles is obtained by statistically measuring the particle size of the primary particles using the major axis statistical method. Specifically: in the SEM image, measure the diameters in the major axis direction of a plurality of primary particles and take the average value.

[0060] In some embodiments of the present application, the lithium iron phosphate primary particles that aggregate to form the lithium iron phosphate secondary particles are single crystal nanoparticles. Figure 1 Scanning electron microscope images of the second positive electrode active material at different magnifications are shown, where the magnification of 1A is 1000X, the magnification of 1B is 5000X, and the magnification of 1C is 10000X.

[0061] In some embodiments of the present application, the median particle size Dv50 of the second positive electrode active material is 2.5 μm to 10.5 μm. In some embodiments of the present application, the median particle size Dv50 of the second positive electrode active material is 4.5 μm to 8.5 μm. In some embodiments of the present application, the median particle size Dv50 of the second positive electrode active material is 6 μm to 7 μm. Among them, the meaning of the median particle size Dv50 value is: in the powder sample, the particle volumes larger and smaller than this value each account for 50% of the total volume.

[0062] In some embodiments of the present application, the specific surface area of the second positive electrode active material is 3.5 m 2 / g - 10.5 m 2 / g. In some embodiments of the present application, the specific surface area of the second positive electrode active material is 5 m 2 / g - 9 m 2 / g. In some embodiments of the present application, the specific surface area of the second positive electrode active material is 6.5 m 2 / g - 7.5 m 2 / g.

[0063] When the median particle size Dv50 and the specific surface area BET of the second positive electrode active material are within the above ranges, its particle size and specific surface area are appropriate, which is conducive to the sufficiency of the filling of the first positive electrode active material in the particle gaps thereof, resulting in a higher tap density of the electrode sheet, a better flatness of the contact layer between the active material and the current collector, and good processability of the electrode sheet. At the same time, when the solid content of the slurry of the positive electrode material mixture system is within a suitable range, it is conducive to the capacity performance and cycle performance of the battery.

[0064] In some embodiments of the present application, the single crystal lithium iron phosphate particles as the first positive electrode active material are independent lithium iron phosphate particles with continuous internal lattices and almost no grain boundary separation. Figure 2 The scanning electron microscope images of the first positive electrode active material at different magnifications are shown. Among them, the magnification of 2A is 10,000X, and the magnification of 2B is 30,000X.

[0065] In some embodiments of the present application, the powder resistivity of the first positive electrode active material at 12 Mpa is 150 Ω · cm or less. In some embodiments of the present application, the powder resistivity of the first positive electrode active material at 12 Mpa is 80 Ω · cm or less. In some embodiments of the present application, the powder resistivity of the first positive electrode active material at 12 Mpa is 30 Ω · cm or less.

[0066] The first positive electrode active material has a lower powder resistivity and higher conductivity, which is conducive to improving the electron transport performance between the positive electrode active materials, reducing the sheet resistivity of the positive electrode sheet, and promoting the capacity performance and cycle performance of the battery.

[0067] In some embodiments of the present application, the median particle size Dv50 of the first positive electrode active material is 0.5 μm - 2.0 μm, which is conducive to its sufficient filling into the particle gaps of the secondary particles of the second positive electrode active material, lithium iron phosphate, and better improving the tap density of the electrode sheet and the processability of the electrode sheet.

[0068] In some embodiments of the present application, the specific surface area of the single crystal lithium iron phosphate particles of the first positive electrode active material is 6.0 m 2 / g - 20 m 2 / g, which is conducive to further reducing the consumption of the binder during the pulping process and increasing the solid content of the slurry.

[0069] In some embodiments of the present application, the mass percentage content of the second positive electrode active material is greater than that of the first positive electrode active material. In some embodiments of the present application, the mass percentage content of the second positive electrode active material is 55%-90%; in some embodiments of the present application, the mass percentage content of the second positive electrode active material is 60%-85%; in some embodiments of the present application, the mass percentage content of the second positive electrode active material is 65%-80%.

[0070] When the mass percentage content of the second positive electrode active material is greater than that of the first positive electrode active material, that is, when the mass percentage content of the second positive electrode active material is greater than 50%, a positive electrode sheet with improved performance and lower cost can be obtained. As the content of the second positive electrode active material further increases, the compaction density and processing performance of the sheet also further improve; at the same time, an appropriate amount of the first positive electrode active material needs to be added to fill the gaps between the lithium iron phosphate secondary particles, so as to increase the adhesion between the lithium iron phosphate secondary particles, improve the compaction density and processing performance of the sheet, improve the electron transport performance and reduce the film resistance. When the mass percentage content of the second positive electrode active material is 60%-85%; or further 65%-80%, the first positive electrode active material and the second positive electrode active material interact synergistically to improve the comprehensive processing performance of the slurry and the electrical performance of the sheet.

[0071] [Positive electrode sheet]

[0072] The second aspect of the present application relates to a positive electrode sheet, including a positive electrode active material layer, and the positive electrode active material layer contains the positive electrode material of the first aspect of the present application.

[0073] Take the positive electrode sheet of some embodiments of the present application, and use an argon ion beam to cut the sheet perpendicular to the surface of the sheet to expose the cross-section of the positive electrode sheet. Figure 3 The scanning electron microscope image of the cross-section of the positive electrode sheet is shown. From Figure 3 it can be seen that the single crystal particles of lithium iron phosphate of the first positive electrode active material are filled in the particle gaps of the lithium iron phosphate secondary particles of the second positive electrode active material.

[0074] In some embodiments of the present application, the coating weight per unit area on one side of the positive electrode sheet is 140 g / m 2 -390 g / m 2 . In some embodiments of the present application, the coating weight per unit area on one side of the positive electrode sheet is 190 g / m 2 -320 g / m 2 . In some embodiments of the present application, the coating weight per unit area on one side of the positive electrode sheet is 230 g / m 2 -280 g / m 2 .

[0075] The positive electrode sheet provided by the embodiments of the present application has an increased limit solid content of the positive electrode paste, so that a higher coating weight and a larger coating thickness can be achieved. With the increase of the coating weight and the coating thickness, the battery volume utilization rate is improved, which helps to improve the energy density. However, the lithium ion migration path also becomes longer, the diffusion impedance increases, the lithium ion concentration polarization becomes larger during the discharge process of the battery, and the battery capacity performance decreases instead. Therefore, the coating weight can affect the comprehensive performance of the battery energy density and cycle performance.

[0076] In some embodiments of the present application, the sheet resistance of the positive electrode sheet is less than 2000 mΩ. In some embodiments of the present application, the sheet resistance of the positive electrode sheet is less than 800 mΩ. In some embodiments of the present application, the sheet resistance of the positive electrode sheet is less than 200 mΩ. The sheet resistance of the positive electrode sheet refers to the overall resistance of the positive electrode sheet after the positive electrode active material layer is provided on both the upper and lower surfaces of the positive electrode current collector at room temperature. The positive electrode sheet provided by the embodiments of the present application has the advantages of low sheet resistance, high energy density of the battery cell, and good cycle performance.

[0077] In some embodiments of the present application, the positive electrode sheet includes a positive electrode current collector and the positive electrode active material layer provided on at least one surface of the positive electrode current collector. The positive electrode active material layer can be provided on one surface of the positive electrode current collector or on both surfaces of the positive electrode current collector. The positive electrode active material layer contains the positive electrode material of the first aspect of the present application.

[0078] In some embodiments of the present application, the positive electrode active material layer may further include a conductive agent and a binder, where the types and contents of the conductive agent and the binder are not specifically limited and can be selected according to actual needs. The binder generally includes fluorinated polyolefin binders. Relative to the fluorinated polyolefin binders, water is usually a good solvent, that is, the fluorinated polyolefin binders usually have good solubility in water. For example, the fluorinated polyolefin binders may include, but are not limited to, polyvinylidene fluoride (PVDF), vinylidene fluoride copolymers, or their modified (such as carboxylic acid, acrylic acid, acrylonitrile, etc.) derivatives. In the positive electrode active material layer, since the binder itself has poor conductivity, the amount of the binder cannot be too high. Optionally, the mass percentage content of the binder in the positive electrode active material layer is less than or equal to 2 wt% to obtain a lower sheet resistance of the electrode. The conductive agent of the positive electrode sheet can be various conductive agents applicable to lithium ion (secondary) batteries in the art. For example, it can be a combination of one or more of acetylene black, conductive carbon black, carbon fiber (VGCF), carbon nanotube (CNT), Ketjen black, etc. The weight of the conductive agent can account for 1 wt% - 10 wt% of the total mass of the positive electrode material layer. Optionally, the weight ratio of the conductive agent to the positive electrode active material in the positive electrode sheet is greater than or equal to 1.5:95.5.

[0079] In some embodiments of the present application, the type of the positive current collector is not specifically limited and can be selected according to actual needs. The positive current collector is usually a laminate, which is usually a structure or component that can collect current. The positive current collector can be various materials in the art suitable for use as the positive current collector of an electrochemical energy storage device. For example, the positive current collector can include but is not limited to metal foils, and more specifically can include but is not limited to nickel foils and aluminum foils.

[0080] Those skilled in the art can select a suitable method to prepare the positive electrode sheet. For example, it can include the following steps: After mixing the positive electrode active material, binder, and conductive agent of the first aspect of the present application to form a slurry, it is coated on the positive current collector.

[0081] [Lithium secondary battery]

[0082] The third aspect of the present application provides a lithium secondary battery, which includes the positive electrode sheet described in the second aspect of the present application.

[0083] In some embodiments of the present application, the lithium secondary battery may include a positive electrode sheet, a negative electrode sheet, a separator interposed between the positive electrode sheet and the negative electrode sheet, and an electrolyte. Among them, the positive electrode sheet is the positive electrode sheet described in the second aspect of the present application.

[0084] Except for using the positive electrode sheet described in the second aspect of the present application, the structure and preparation method of the lithium secondary battery of the present application are known per se.

[0085] In some embodiments of the present application, the negative electrode sheet of the lithium secondary battery generally includes a negative current collector and a negative electrode active material layer located on the surface of the negative current collector. The negative electrode active material layer generally includes a negative electrode active material. The negative electrode active material can be various materials in the art suitable for use as the negative electrode active material of a lithium secondary battery. For example, it can include but is not limited to graphite, soft carbon, hard carbon, carbon fiber, mesophase carbon microspheres, silicon-based materials, tin-based materials, lithium titanate, or a combination of one or more of other metals that can form an alloy with lithium. Among them, the graphite can be selected from a combination of artificial graphite, natural graphite, and modified graphite; the silicon-based materials can be selected from a combination of elemental silicon, silicon oxides, silicon-carbon composites, and silicon alloys; the tin-based materials can be selected from a combination of elemental tin, tin oxides, and tin alloys. The negative current collector is usually a structure or component that collects current. The negative current collector can be various materials in the art suitable for use as the negative current collector of a lithium secondary battery. For example, the negative current collector can include but is not limited to metal foils, and more specifically can include but is not limited to copper foils. In addition, the negative electrode sheet can also be a lithium sheet.

[0086] In some embodiments of the present application, the separator of the lithium secondary battery can be various materials applicable to the separator of lithium secondary batteries in the art. For example, it can be a combination of one or more of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fiber, including but not limited to these.

[0087] In some embodiments of the present application, the electrolyte of the lithium secondary battery can be various electrolytes applicable to lithium secondary batteries in the art. For example, the electrolyte generally includes an electrolyte and a solvent. The electrolyte usually can include a lithium salt. More specifically, the lithium salt can be an inorganic lithium salt and / or an organic lithium salt. Specifically, it can be a combination of one or more of LiPF 6 、LiBF 4 、LiN(SO 2 F) 2 (abbreviated as LiFSI), LiN(CF 3 SO 2 ) 2 (abbreviated as LiTFSI), LiClO 4 、LiAsF 6 、LiB(C 2 O 4 ) 2 (abbreviated as LiBOB), LiBF 2 C 2 O 4 (abbreviated as LiDFOB), etc. For another example, the concentration of the electrolyte can be 0.8 mol / L to 1.5 mol / L. The solvent can be various solvents applicable to the electrolyte of lithium secondary batteries in the art. The solvent of the electrolyte is usually a non-aqueous solvent, preferably an organic solvent, specifically a combination of one or more of ethylene carbonate, propylene carbonate, butylene carbonate, pentylene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, or their halogenated derivatives, including but not limited to these.

[0088] In some embodiments of the present application, the method for preparing the lithium secondary battery should be known to those skilled in the art. For example, the positive electrode sheet, the separator, and the negative electrode sheet can each be a laminate, so that they can be cut into a target size and then stacked in sequence, or can be wound to a target size to form an electric core, and can be further combined with an electrolyte to form a lithium secondary battery.

[0089] Figure 4 FIG. shows a perspective view of a lithium secondary battery according to a specific embodiment of the present invention. Figure 5 is Figure 4 the exploded view of the lithium ion secondary battery shown. Refer to Figure 4And Figure 5 , the lithium secondary battery 5 according to the present application (hereinafter referred to as the battery cell 5) includes an outer package 51, an electrode assembly 52, a top cover assembly 53, and an electrolyte (not shown). The electrode assembly 52 is housed in the housing 51, and the number of the electrode assemblies 52 is not limited and may be one or more.

[0090] It should be noted that Figure 4 the shown battery cell 5 is a can-type battery, but the present application is not limited thereto. The battery cell 5 may be a pouch-type battery, that is, the housing 51 is replaced by a metal plastic film and the top cover assembly 53 is cancelled.

[0091] [Battery module]

[0092] The fourth aspect of the present application provides a battery module, which includes the lithium secondary battery described in the third aspect of the present application. In some embodiments, the lithium secondary batteries may be assembled into a battery module, and the number of the lithium secondary batteries included in the battery module may be multiple, and the specific number may be adjusted according to the application and capacity of the battery module. Figure 6 is a perspective view of a battery module 4 as an example. Refer to Figure 6 , in the battery module 4, multiple lithium secondary batteries 5 may be arranged in sequence along the length direction of the battery module 4. Of course, they may also be arranged in any other manner. Further, the multiple lithium secondary batteries 5 may be fixed by fasteners. Optionally, the battery module 4 may further include a housing having an accommodation space, and the multiple lithium secondary batteries 5 are accommodated in the accommodation space.

[0093] [Battery pack]

[0094] The fifth aspect of the present application provides a battery pack, which includes the battery module described in the fourth aspect of the present application. In some embodiments, the above battery modules may be assembled into a battery pack, and the number of the battery modules included in the battery pack may be adjusted according to the application and capacity of the battery pack. Figure 7 is a perspective view of a battery pack 1 as an example, Figure 8 is Figure 7 the exploded view of the shown battery pack. Refer to Figure 7 and Figure 8 , in the battery pack 1, a battery box and multiple battery modules 4 arranged in the battery box may be included. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form a closed space for accommodating the battery modules 4. The multiple battery modules 4 may be arranged in the battery box in any manner.

[0095] [Device]

[0096] The sixth aspect of the present application provides a device, which includes the lithium secondary battery described in the third aspect of the present application, or the battery module described in the fourth aspect of the present application, or the battery pack described in the fifth aspect of the present application. The lithium secondary battery, the battery module, or the battery pack is used as the power source or the energy storage unit of the device. The device can be, but is not limited to, a mobile device (such as a mobile phone, a laptop computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship, a satellite, an energy storage system, etc.

[0097] The device can select a lithium secondary battery, a battery module, or a battery pack according to its usage requirements.

[0098] Figure 9 The schematic diagram of the device according to a specific embodiment of the present application is shown. The device can be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the high power and high energy density requirements of the device for the lithium-ion secondary battery (i.e., the secondary battery of the present application), a battery pack or a battery module can be adopted.

[0099] Another example of the device can be a mobile phone, a tablet computer, a laptop computer, etc. The device usually requires thin and light design, and a lithium-ion secondary battery (i.e., the secondary battery of the present application) can be used as the power source.

[0100] Those skilled in the art can understand that: in the different embodiments of the present application mentioned above, the various limitations or preferred ranges of the component selection, component content, and material physical and chemical property parameters in the electrochemically active material can be arbitrarily combined, and the various embodiments obtained by such combination are still within the scope of the present application and are regarded as part of the disclosure content of this specification.

[0101] Unless otherwise specified, the various parameters involved in this specification have the general meanings known in the art and can be measured by the methods known in the art. For example, they can be tested according to the methods given in the embodiments of the present application. In addition, the preferred ranges and options of the various different parameters given in the various preferred embodiments can be arbitrarily combined, and the various combinations thus obtained are all regarded as being within the disclosure scope of the present application.

[0102] The advantages of the present application are further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present application and not to limit the scope of the present application.

[0103] (1) The influence of the parameters of the first active substance and the second active substance on the technical effects of the present application

[0104] Examples 1 to 36, Comparative Examples 1 to 2

[0105] 1. Preparation of the positive electrode material:

[0106] Mix the first positive electrode active material, single-crystal lithium iron phosphate, and the second positive electrode active material, secondary particle lithium iron phosphate, according to the mass percentage ratio to obtain the positive electrode material.

[0107] 2. Preparation of the positive electrode plate:

[0108] Dry-mix the positive electrode material, the binder polyvinylidene fluoride (PVDF), and the conductive carbon according to a weight ratio of 96.5:2.0:1.5, add an appropriate amount of N-methylpyrrolidone (NMP) solvent, and stir and mix well to form a uniform positive electrode slurry; coat this slurry on a carbon-coated aluminum foil with a thickness of 13 μm as the positive electrode current collector, then dry and cold-press it, cut it into strips for standby to obtain the positive electrode plate.

[0109] 3. Obtain the negative electrode plate, separator, electrolyte, and housing according to the conventional methods in the art:

[0110] Negative electrode plate: Stir and mix the negative electrode active material, graphite, the conductive carbon, and the binder polyvinylidene fluoride (PVDF) in an appropriate amount of water solvent according to a weight ratio of 95:3:2 to form a uniform negative electrode slurry; coat this slurry on a Cu foil as the negative electrode current collector, and after drying, cold-press the electrode plate to the designed density, cut it into strips for standby.

[0111] Separator: Use a 12-μm PP separator.

[0112] Electrolyte: Use the general lithium iron phosphate electrolyte.

[0113] Housing: The housing material uses an aluminum-plastic film.

[0114] 4. Preparation of the lithium secondary battery:

[0115] Wind the positive electrode plate, the separator, and the negative electrode plate to form a bare battery core, encapsulate the bare battery core with an aluminum-plastic film, inject the electrolyte, and obtain the lithium secondary battery after formation, degassing, and high-temperature aging.

[0116] For the relevant parameters of the positive electrode materials, the performance of the positive electrode plates and the lithium secondary batteries in Examples 1 to 36 and Comparative Examples 1 to 2, they can be detected by the conventional methods in the art. The following are examples of the detection methods:

[0117] 1. BET specific surface area of the first positive electrode active material:

[0118] Detection is carried out using a specific surface area tester, and the model of the testing instrument is: tristar 3020. The detection steps are briefly described as follows: Take 2.0 g - 4 g of the first positive electrode active material sample; put the sample into the sample tube, place it in the heating pack, and perform degassing treatment (degassing conditions: 200 °C, 2 hours); after degassing is completed, when the temperature of the degassing station drops to room temperature, remove the sample tube from the degassing station, weigh and calculate the total mass of the sample; put the sample tube into the test filling rod and perform BET test to obtain the detection result.

[0119] 2. Median particle size Dv50 of the first positive electrode active material:

[0120] Measuring instrument: Laser particle size analyzer; Equipment model: Malvern Mastersizer 2000E or Mastersizer 3000. Since the particles of the positive electrode material, under the irradiation of the laser beam, the angle of the scattered light is inversely proportional to the particle diameter and the scattered light intensity decays logarithmically with the increase of the angle, and the energy distribution of the scattered light is directly related to the particle diameter distribution, the particle size distribution characteristics can be obtained by receiving and measuring the energy distribution of the scattered light. The solvent used for testing can be water or other organic solvents used for preparing the slurry, and the sample is dispersed by ultrasonic. The obtained analysis result Dv50 means that 50% of the total volume of the particles have a diameter greater than this value, and another 50% of the total volume of the particles have a diameter less than this value.

[0121] 3. Powder resistivity of the first positive electrode active material under 12 Mpa:

[0122] Dry the first positive electrode active material powder, weigh an appropriate amount of powder, and use a powder resistivity tester, with the equipment model being Suzhou Jingge ST2722 or Sansi Zongheng UTM7305. Place the dried powder sample in the mold / sample chamber of the resistivity tester, with the depth of the sample chamber being 20 mm and the cross-sectional area being 1 cm 2 , then slowly apply pressure from small to large, manually collect data, and record the powder resistivity test results corresponding to different pressure points.

[0123] 4. Median particle size Dv50 of the second positive electrode active material:

[0124] Detect according to the detection method of the median particle size Dv50 of the first positive electrode active material.

[0125] 5. BET specific surface area of the second positive electrode active material:

[0126] Detect according to the detection method of the BET specific surface area of the first positive electrode active material.

[0127] 6. Membrane resistance of the positive electrode sheet:

[0128] Using the ACCFLM film resistor test system of Hangzhou Chuanyuan Technology Co., Ltd. and adopting the double-probe method, the film resistance of the entire positive electrode plate (with a positive active material layer provided on both sides) in the implementation mode of this application is detected.

[0129] 7. Detection of the bonding force between the film and the current collector:

[0130] Take the positive electrode plate to be tested, and use a blade to cut a sample with a width of 30 mm and a length of 100 - 160 mm; at the same time, stick double-sided tape on a steel plate, with the tape width of 20 mm and a length of 90 - 150 mm. Stick the cut electrode plate sample on the double-sided tape with the test surface facing down. Insert a piece of paper tape with the same width as the electrode plate and a length greater than the sample length by 80 - 200 mm under the electrode plate, and fix it with crepe tape. Then use a Sansi tensile testing machine to detect the bonding force between the film and the current collector.

[0131] 8. Specific capacity performance:

[0132] The specific capacity at 1 / 3C at 25°C is used to characterize the capacity performance of the battery. The specific detection process is briefly described as follows: (1) Place the battery in an oven at 25°C and let it stand for 2 h until the battery temperature remains at 25°C; (2) Charge at 1 / 3C CC to 2.0V; (3) Pause for 5 min; (4) Charge at 1 / 3C CC to 3.65V and then CV until I ≤ 0.05C; (5) Pause for 5 min; (6) Discharge at 1 / 3C DC to 2.0V. This step is the actual test of the cell capacity. Combining with the mass of the positive active material, the specific capacity performance can be calculated.

[0133] 9. Cycle performance test:

[0134] The cycle at 1C / 1C at 60°C is adopted to detect the number of cycles when the battery capacity decays to 80% of the initial value. The specific process is briefly described as follows: (1) Place the battery in an oven at 45°C and let it stand for 2 h until the battery temperature remains at 25°C; (2) Charge at a constant current of 1C to 3.65V, and continue constant voltage charging until the charging current is less than 0.05C and then stop; (3) Pause for 5 min; (4) Discharge at a constant current of 1C to 2.5V; (5) Pause for 5 min. Steps (2) to (6) are one charge-discharge cycle of the battery. Continuously repeat steps (2) to (6) until the number of cycles when the battery capacity decays to 80% of the initial value.

[0135] The relevant parameters of the positive electrode materials in Examples 1 - 36 and Comparative Examples 1 - 2 are shown in Table 1.

[0136] Table 1

[0137]

[0138] The performance test results of Examples 1 to 36 and Comparative Examples 1 to 2 are shown in Table 2.

[0139] Table 2

[0140]

[0141] Data discussion:

[0142] (1) Examples 1 to 36, Comparative Examples 1 to 2:

[0143] The positive electrode material of Comparative Example 1 only contains single-crystal lithium iron phosphate particles. It can be seen from the test data in Table 2 that the processing performance of the positive electrode sheet of Comparative Example 1 is poor, specifically manifested in that the ultimate solid content of the positive electrode material slurry is very low, the cracking of the coated electrode sheet is serious, and the bonding force between the film and the current collector is low. At the same time, the capacity performance and cycling performance of the lithium secondary battery of Comparative Example 1 are not good.

[0144] The positive electrode material of Comparative Example 2 only contains secondary lithium iron phosphate particles. It can be seen from the data in Table 2 that for the positive electrode sheet of Comparative Example 2, although the ultimate solid content of the slurry has been greatly improved and the cracking of the coated electrode sheet has been improved, the bonding force between the film and the current collector is very low, and the film resistance of the positive electrode sheet is relatively high, and the capacity performance and cycling performance of the battery are also not ideal.

[0145] The positive electrode materials of Examples 1 to 36 contain single-crystal lithium iron phosphate particles and secondary lithium iron phosphate particles. Compared with Comparative Example 1 and Comparative Example 2, the processing performance of the positive electrode sheets of Examples 1 to 36 has been generally improved. The ultimate solid content of the positive electrode slurry is relatively high, the cracking of the thick-coated electrode sheet has been improved, the bonding force between the film and the current collector has been increased, the film resistance has been reduced, and the battery capacity performance and cycling performance are better than those of Comparative Example 1 and Comparative Example 2.

[0146] In addition, it can be seen from Examples 1 to 5 that by using secondary lithium iron phosphate particles and single-crystal lithium iron phosphate particles doped with different elements, the positive electrode materials obtained after mixing can all achieve the effects of improving the electrode sheet processing performance, increasing the battery capacity performance and cycling performance; among them, the doping of Ti element has the most significant improvement on the capacity performance and cycling performance of the battery cell, and the preferred doping amount of Ti element in the positive electrode material is 2000 ppm.

[0147] (2) Examples 5 to 9:

[0148] Examples 5 to 9 show the change of the powder resistivity of single-crystal lithium iron phosphate particles at 12 Mpa in the positive electrode material provided by the present application, and its influence on the electrode sheet processing performance and battery performance.

[0149] The powder resistivity of the first positive electrode active material, single-crystal lithium iron phosphate particles, at 12 Mpa is 150 Ω ·less than cm, or 80 Ω · less than cm, or further 30 Ω · less than cm. As the powder resistivity of the lithium iron phosphate single crystal particles decreases, their conductivity increases, the electron transport performance between the positive active materials is better, the sheet resistivity of the positive electrode sheet decreases accordingly, which is beneficial to the capacity and cycle performance of the battery.

[0150] (3) Examples 5, 10 to 17:

[0151] Examples 5, 10 to 17 show the influence of the change of the median particle size Dv50 of the lithium iron phosphate secondary particles in the positive electrode material provided by the present application on the electrode sheet processing performance and battery performance.

[0152] When the median particle size Dv50 of the lithium iron phosphate secondary particles is less than 2.5 μm, the specific surface area of the secondary particles is relatively large, resulting in enhanced liquid absorption, which may cause slight cracking during the drying process of the sheet; at the same time, the uniformity of the conductive carbon distribution in the active material layer after coating is insufficient, increasing the sheet resistance. When the median particle size Dv50 of the lithium iron phosphate secondary particles is greater than 10.5 μm, the filling sufficiency of the first positive active material in the gaps between its particles is weakened, which is not conducive to improving the compaction density of the electrode sheet, and also affects the flatness of the contact layer between the active material and the current collector, resulting in an impact on the contact between the active material and the current collector, and instead increasing the probability of electrode sheet fracture during the processing. When the Dv50 of the lithium iron phosphate secondary particles is 6 μm - 7 μm, the electrode sheet processing performance is better, and the bonding force between the sheet and the current collector is also better, which is conducive to the battery capacity and cycle performance reaching the best state.

[0153] (4) Examples 5, 18 to 25:

[0154] Examples 5, 18 to 25 show the influence of the change of the specific surface area BET of the lithium iron phosphate secondary particles in the positive electrode material provided by the present application on the electrode sheet processing performance and battery performance.

[0155] When the specific surface area BET of the lithium iron phosphate secondary particles is lower than 3.5 m 2 / g, the electrochemical reaction activity of the lithium secondary battery is relatively low, affecting the cycle performance of the battery to a certain extent. When the specific surface area BET of the lithium iron phosphate secondary particles is higher than 10.5 m 2 / g, the improvement effect on the solid content of the positive electrode slurry and the electrode sheet processing performance becomes less obvious. When the specific surface area BET of the lithium iron phosphate secondary particles is 6.5 m 2 / g - 7.5 m 2When it is [specific value] / g, it can not only significantly improve the solid content of the positive electrode slurry and the processing performance of the electrode sheet, but also ensure the electrochemical reaction activity of the battery, making the improvement effect of the battery capacity and cycle performance more obvious.

[0156] (5) Examples 5, 26 - 30:

[0157] Examples 5, 26 - 30 show the influence of the change in the average particle size of the primary particles that agglomerate to form lithium iron phosphate secondary particles in the positive electrode material provided by this application on the processing performance of the electrode sheet and the battery performance.

[0158] When the average particle size d of the primary particles is in the range of 20nm - 500nm, the particle size of the primary particles is within a suitable range, and both the side reaction rate between the positive electrode material and the electrolyte and the kinetic performance of the battery positive electrode are at a reasonable level, and the battery capacity and cycle performance are relatively good. When the average particle size d of the primary particles is less than 20nm, the side reaction rate between the positive electrode material and the electrolyte is relatively large. Although it is beneficial to the initial capacity of the positive electrode material, the cycle performance of the battery is not high. When the average particle size d of the primary particles is greater than 500nm, although the processability of the positive electrode slurry and the compaction density of the positive electrode sheet are improved, the kinetic performance of the battery positive electrode decreases, resulting in a decrease in the battery capacity.

[0159] (6) Examples 5, 31 - 36:

[0160] Examples 5, 31 - 36 show the influence of the change in the mass percentage content of lithium iron phosphate secondary particles and lithium iron phosphate single crystal particles in the positive electrode material provided by this application on the processing performance of the electrode sheet and the battery performance.

[0161] The lithium iron phosphate secondary particles have a low specific surface area, which can increase the solid content of the slurry, reduce the processing difficulty of thick-coated electrode sheets, and reduce the production cost of the cathode material. The lithium iron phosphate single crystal is filled in the particle gaps of the lithium iron phosphate secondary particles, further increasing the compaction density of the electrode sheet and improving the processing performance of the electrode sheet. When the mass percentage content of the second cathode active material is greater than that of the first cathode active material, that is, when the mass percentage content of the second cathode active material is greater than 50%, a cathode electrode sheet with improved performance and lower cost can be obtained. As the content of the second cathode active material further increases, the compaction density and processing performance of the electrode sheet are further improved. At the same time, an appropriate amount of the first cathode active material needs to be added to fill the gaps between the lithium iron phosphate secondary particles to increase the adhesion between the lithium iron phosphate secondary particles, improve the compaction density and processing performance of the electrode sheet, improve the electron transport performance and reduce the film resistance. When the mass percentage content of the second cathode active material is 60%-85%; or further 65%-80%, the first cathode active material and the second cathode active material interact synergistically to improve the comprehensive processing performance of the slurry and the electrical performance of the electrode sheet.

[0162] (2) Influence of the coating weight per unit area on one side of the cathode electrode sheet on the technical effect of the present application

[0163] Examples 37-44

[0164] In Examples 37-44, the method for preparing the cathode material and the lithium secondary battery is basically the same as that in Example 5, except that the coating weight per unit area on one side of the cathode electrode sheet is changed.

[0165] In addition, for the cathode electrode sheets and lithium secondary batteries in Examples 37-44, in addition to performing the same performance tests as in Examples 1-36, the energy density of the lithium secondary battery was also detected.

[0166] The detection method for the energy density of the lithium secondary battery is as follows: (1) Place the battery in an oven environment at 25°C and let it stand for 2 h until the battery temperature remains at 25°C; (2) 1 / 3C DC to 2.0V; (3) Pause for 5 min; (4) 1 / 3C CC to 3.65V and CV to I≤0.05C; (5) Pause for 5 min; (6) 1 / 3C DC to 2.0V. This step is to actually test the capacity of the battery cell. Combining with the mass of the cathode active material, the specific capacity can be calculated. The energy of the battery cell can be calculated by integrating the voltage-capacity curve. Energy value / battery cell weight value = energy density.

[0167] The relevant parameters of the cathode materials in Examples 37-44 are shown in Table 3.

[0168] Table 3

[0169]

[0170] The performance test results of the positive electrode sheets and lithium secondary batteries of Examples 37 to 44 are shown in Table 4 below.

[0171] Table 4

[0172]

[0173] Data discussion:

[0174] Examples 37 to 44 show the influence of the coating weight per unit area on one side of the positive electrode sheet on the technical effects of the present application.

[0175] The positive electrode paste with a high solid content can be prepared by using the positive electrode material provided in the present application, and further a positive electrode sheet with a high coating weight and a high coating thickness can be obtained. With the increase of the coating weight and the coating thickness, the battery volume utilization rate is improved, which helps to improve the energy density. However, the lithium ion migration path also becomes longer, the diffusion impedance increases, and the lithium ion concentration polarization becomes larger during the discharge process of the battery, resulting in a decrease. When the coating weight is 230 g / m 2 -280 g / m 2 , the comprehensive performance of the battery energy density and cycle performance is the best.

[0176] Those skilled in the art can also make changes and modifications to the above-mentioned embodiments according to the disclosure and teachings of the above specification. Therefore, the present application is not limited to the specific embodiments disclosed and described above, and some modifications and changes to the present application should also fall within the protection scope of the claims of the present application. In addition, although some specific terms are used in this specification, these terms are only for convenience of description and do not constitute any limitation to the present application.

Claims

1. A battery cell, characterized in that, it includes a positive electrode plate; the positive electrode plate includes a first positive active material and a second positive active material; the first positive active material is single crystal particles; the second positive active material is secondary particles; the secondary particles are in a pomegranate-like morphology formed by agglomeration of multiple primary particles; the primary particles are single crystal nanoparticles.

2. The battery cell according to claim 1, characterized in that, the particle size d of the primary particles is 700 nm or less.

3. The battery cell according to claim 1, characterized in that, the particle size d of the primary particles is in the range of 20 nm - 500 nm.

4. The battery cell according to claim 1, characterized in that, The coating weight per unit area on one side of the positive electrode sheet is 100 g / m 2 -450 g / m 2 .

5. The battery cell according to claim 1, characterized in that, The coating weight per unit area on one side of the positive electrode sheet is 140 g / m 2 - 390 g / m 2 .

6. The battery cell according to claim 1, characterized in that, The coating weight per unit area on one side of the positive electrode sheet is 190 g / m 2 - 320 g / m 2 .

7. The battery cell according to claim 1, characterized in that, The coating weight per unit area on one side of the positive electrode sheet is 230 g / m 2 -280 g / m 2 .

8. The battery cell according to claim 1, characterized in that, the sheet resistance of the positive electrode plate is less than 2000 mΩ.

9. The battery cell according to claim 1, characterized in that, the sheet resistance of the positive electrode plate is less than 800 mΩ.

10. The battery cell according to claim 1, characterized in that, the sheet resistance of the positive electrode plate is less than 200 mΩ.

11. The battery cell according to any one of claims 1 - 10, characterized in that, the single crystal particles are independent particles with continuous internal lattice and almost no grain boundary separation.

12. The battery cell according to any one of claims 1 - 10, characterized in that, the second positive active material is a lithium iron phosphate cathode material.

13. The battery cell according to any one of claims 1 - 10, characterized in that, the second positive active material contains the substance shown in formula (II); LiFe 1-x2 M2 x2 PO 4 (II); wherein, 0 ≤ x1 ≤ 0.1, 0 ≤ x2 ≤ 0.1, and M1 and M2 are each independently selected from at least one of Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr or Ti.

14. The battery cell according to any one of claims 1 - 10, characterized in that, the first positive active material is a lithium iron phosphate cathode material.

15. The battery cell according to any one of claims 1 - 10, characterized in that, the first positive active material contains the substance shown in formula (I); LiFe 1-x1 M1 x1 PO 4 (I); wherein, 0 ≤ x1 ≤ 0.1, 0 ≤ x2 ≤ 0.1, and M1 and M2 are each independently selected from at least one of Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr or Ti.

16. A lithium secondary battery, characterized in that, it includes the battery cell according to any one of claims 1 - 15.

17. A battery module, characterized in that, it includes the lithium secondary battery according to claim 16.

18. A battery pack, characterized in that, it includes the battery module according to claim 17.

19. An electrical device, characterized in that, Including the lithium secondary battery according to claim 16, or the battery module according to claim 17, or the battery pack according to claim 18, where the lithium secondary battery, the battery module, or the battery pack is used as a power source or an energy storage unit of the electrical device.

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