A positive electrode sheet, a secondary battery

Abstract

The application discloses a positive pole piece and a secondary battery, and belongs to the technical field of batteries. The pole piece is prepared by using two kinds of lithium iron phosphate particles as active materials and simultaneously using a lithium supplementing agent. The two components are synchronously controlled. When the positive pole piece is applied to the secondary battery, the lithium ion transmission efficiency of the active material in the pole piece can be effectively improved, the kinetic performance of the secondary battery is improved, the fast charging performance is good, the active lithium can be effectively supplemented by the lithium supplementing agent, the accumulation degree of the active material is high, and the secondary battery has high volume energy density.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to a positive electrode sheet and a secondary battery. Background Technology

[0002] Lithium iron phosphate (LFP) cathode materials possess high safety and theoretical cycle stability; however, their energy density is relatively low, and they are prone to some loss of active lithium during initial charge and discharge, resulting in an actual energy density lower than the theoretical value. Furthermore, to improve the fast-charging performance of LFP cathode materials, their kinetic properties are often enhanced through cathode material modification. However, such approaches can lead to a decrease in the density of the cathode material, further reducing the energy density of the secondary battery. Summary of the Invention

[0003] The purpose of this application is to overcome the shortcomings of the existing technology and provide a positive electrode sheet. This electrode sheet uses two types of lithium iron phosphate particles as active materials and a lithium replenishing agent as well, and the two components are simultaneously regulated. When this positive electrode sheet is applied to a secondary battery, it can not only effectively improve the lithium-ion transport efficiency of the active material in the electrode sheet and improve the dynamic performance of the secondary battery, but also improve the fast charging performance. At the same time, the lithium replenishing agent can effectively replenish the active lithium, and the active material has a high degree of stacking, so the secondary battery also has a high volumetric energy density.

[0004] To achieve the above objectives, in the first aspect of this application, this application provides A positive electrode sheet, the positive electrode sheet comprising a positive electrode active material layer, the positive electrode active material layer comprising a positive electrode active material and a lithium supplementing agent; The positive electrode active material includes a first material and a second material; The first material includes lithium iron phosphate particles, wherein the particle size of the lithium iron phosphate is ≥0.9μm; The second material comprises lithium iron phosphate particles, wherein the particle size of the lithium iron phosphate is <0.9 μm. The lithium iron phosphate particles in the second material are ball-and-stick shaped and satisfy a≥1.05, where a is the average ratio of the length a1 of the lithium iron phosphate particle in the long axis direction to the length a2 of the short axis at the middle position of the long axis in the direction perpendicular to the long axis. The lithium-containing compound includes lithium phosphate; The positive electrode plate satisfies a / (100×b×c)=0.296~200; Where b is the mass percentage of lithium supplementation agent in the positive electrode active material layer, and cμm is the cumulative particle size D of the second material. n50 .

[0005] The beneficial effects of this application are as follows: This application provides a positive electrode sheet, which uses two types of lithium iron phosphate particles as active materials and a lithium replenishing agent as a compound. The two components are simultaneously regulated. When this positive electrode sheet is applied to a secondary battery, it can not only effectively improve the lithium-ion transport efficiency of the active material in the electrode sheet and improve the dynamic performance of the secondary battery, but also improve the fast charging performance. At the same time, the lithium replenishing agent can effectively replenish the active lithium and the active material has a high degree of stacking, so the secondary battery also has a high volumetric energy density. Attached Figure Description

[0006] Figure 1 This is a schematic diagram showing the length D1 of the short axis of the lithium iron phosphate particles in the second material of this application at a position 1 / 4 of the length in the direction perpendicular to the long axis, and the length D2 of the short axis at a position in the middle of the direction perpendicular to the long axis. Detailed Implementation

[0007] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0008] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0009] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0010] The present application is further illustrated below with specific embodiments: A positive electrode sheet, the positive electrode sheet comprising a positive electrode active material layer, the positive electrode active material layer comprising a positive electrode active material and a lithium supplementing agent; The positive electrode active material includes a first material and a second material; The first material includes lithium iron phosphate particles, wherein the particle size of the lithium iron phosphate is ≥0.9μm; The second material comprises lithium iron phosphate particles, wherein the particle size of the lithium iron phosphate is <0.9 μm. The lithium iron phosphate particles in the second material are ball-and-stick shaped and satisfy a≥1.05, where a is the average ratio of the length a1 of the lithium iron phosphate particle in the long axis direction to the length a2 of the short axis at the middle position of the long axis in the direction perpendicular to the long axis. The lithium supplement includes lithium phosphate; The positive electrode plate satisfies a / (100×b×c)=0.296~200; Where b is the mass percentage of lithium supplementation agent in the positive electrode active material layer, and cμm is the cumulative particle size D of the second material. n50 .

[0011] In this application, to balance the volumetric energy density and fast-charging performance of the positive electrode when applied to a secondary battery, two types of lithium iron phosphate particles with different particle sizes are blended as active materials. Simultaneously, lithium phosphate, a lithium replenishing agent, is introduced. Furthermore, the aspect ratio, particle size, and lithium compound content of the small-sized lithium iron phosphate rod-shaped particles are simultaneously adjusted: small-sized spherical-rod-shaped lithium iron phosphate particles are blended with large-sized lithium iron phosphate particles. Controlling the aspect ratio of these particles directly affects the particle configuration. By increasing the aspect ratio of the small-sized lithium iron phosphate particles, the lithium-ion transport radius can be shortened through the short axis of the particles, thereby improving the material's kinetic transport performance and enhancing the fast-charging performance of the electrode when applied to a secondary battery. However, this adjustment affects the compactness of the particle packing, thus impacting the volumetric energy density of the positive electrode when applied to a battery. Therefore, in this application, the small-sized particles... The aspect ratio of the particles cannot be arbitrarily controlled; the particle size of small-sized lithium iron phosphate particles also needs to be controlled simultaneously to allow small-sized particles to effectively intercalate among large-sized lithium iron phosphate particles, optimizing the particle stacking effect. Furthermore, the positive electrode sheet described in this application further introduces lithium phosphate as a lithium replenishing agent and controls its content. Since lithium phosphate has a suitable delithiation potential (mainly between 3.7 and 4V), which is higher than the working potential of most negative electrode materials, lithium phosphate will preferentially undergo delithiation reaction during charging to replenish appropriate active lithium and form an SEI film, compensating for the loss of active lithium in lithium iron phosphate and improving the lithium insertion / extraction capacity of the battery. At the same time, this component has good stability in air, which helps to reduce the production difficulty and safety risks of the battery. Its substance also has high chemical stability and is chemically inert with most electrode slurries (such as N-methylpyrrolidone), ultimately enabling the resulting secondary battery to achieve a good volumetric energy density level.

[0012] In some embodiments, the positive electrode sheet satisfies the following range: a / (100×b×c) = 0.296, 0.5, 0.8, 1, 1.2, 1.5, 1.8, 1.810, 1.825, 2, 3, 5, 10, 20, 50, 60, 66, 66.6, 66.680, 67, 70, 100, 120, 150, 180, 200.

[0013] More preferably, the positive electrode sheet satisfies: a / (100×b×c)=1.8~67.

[0014] As mentioned above, for the positive electrode sheet described in this application, the relationship between the three parameters a / (100×b×c) needs to be controlled within the range of 0.296 to 200. If the relationship is lower than the lower limit of 0.296, the kinetic performance of the positive electrode sheet during lithium-ion transport will deteriorate, making it difficult to achieve ideal fast-charging performance. However, if the relationship is higher than the upper limit of 200, the degree of material stacking and active lithium replenishment in the positive electrode sheet will be difficult to balance, which will also make it difficult to achieve ideal volumetric energy density when applied to secondary batteries. When the relationship is controlled within the limited range and further optimized to the above-mentioned preferred range, the positive electrode sheet can achieve the expected level in both fast-charging performance and volumetric energy density when applied to secondary batteries, and can achieve further improvement.

[0015] In some embodiments, the cumulative particle size D of the first material is... n50 The thickness is 1.0~1.5μm.

[0016] In some embodiments, the cumulative particle size D of the first material is... n50 The range is one or any two of the following: 1μm, 1.1μm, 1.2μm, 1.25μm, 1.3μm, and 1.5μm.

[0017] In some embodiments, the cumulative particle size D of the second material is... n50 c is 0.2~0.75μm.

[0018] In some implementations, c is a range of one or any two of 0.2μm, 0.25μm, 0.3μm, 0.35μm, 0.4μm, 0.45μm, 0.5μm, 0.55μm, 0.6μm, 0.65μm, 0.7μm, and 0.75μm.

[0019] More preferably, c is 0.3~0.55μm.

[0020] When two different sizes of lithium iron phosphate particles are used as active particles, the size setting of the smaller lithium iron phosphate particles will lead to changes in the overall particle packing density and the specific surface area of ​​the smaller particles. When the D of the smaller second lithium iron phosphate particle... n50 When the particle size is less than 0.2 μm, the specific surface area of ​​small particles is large, which increases the degree of side reactions between the electrolyte and lithium iron phosphate particles, degrades lithium intercalation / deintercalation efficiency and capacity utilization, and lowers the volumetric energy density of the battery; while if D n50 When the particle size is greater than 0.75 μm, the overall particle density decreases, the lithium-ion transport path increases, and the fast-charging performance of the battery decreases. Therefore, when the particle size is preferably controlled within 0.2~0.75 μm, or even more preferably within the range of 0.3~0.55 μm, the lithium insertion / extraction activity and efficiency of the active particles in the positive electrode can be greatly improved, the side reactions between the particles and the electrolyte are reduced, and the corresponding fast-charging performance and volumetric energy density of the secondary battery are better.

[0021] It should be noted that in the technical solution of this application, the cumulative particle size D of the first material is... n50 The cumulative distribution of the second material's particle size D n50 Specifically, this can be confirmed through, but is not limited to, the following methods: The secondary battery was discharged to the lower limit of 2.5V at 0.33C for venting treatment, then disassembled. The obtained positive electrode sheet was soaked in dimethyl carbonate (DMC) at 25°C for 2 hours, dried, and then the positive electrode active material layer was scraped off as a sample. The sample was fixed with conductive adhesive, CP argon ion polished, and a conductive film was deposited. It was placed on a sample stage and observed under a scanning electron microscope (SEM). Three areas were photographed at 10Kx magnification. Then, the lithium iron phosphate particles in the first material and the lithium iron phosphate particles in the second material were determined by the particle diameter. The lithium iron phosphate particles in the first material were... The first material has a relatively large particle size, ≥0.9μm. The second material contains lithium iron phosphate particles that are spherical and rod-shaped with a smaller size, <0.9μm. The particle size is measured using measurement software. During the measurement of the lithium iron phosphate particles in the second material, the length a1 along the major axis and the length a2 along the minor axis at the midpoint of the major axis are measured beforehand. The particle size is then calculated as (a1+a2) / 2. The particle size of 200 particles in this region is measured, and the particle size distribution representing 50% of the total particle size is calculated, thus obtaining the cumulative particle size distribution D of the second material. n50 c); Subsequently, lithium iron phosphate particles in the first material were located within this area, and particle size measurements were performed on 200 particles. The particle size distribution representing 50% of the total particle size distribution was calculated, thus obtaining the cumulative distribution and particle size distribution D of the lithium iron phosphate particles in the first material. n50 .

[0022] More preferably, the cumulative particle size D of the first material and the second material is... n50 The ratio is 2.6 to 5.65.

[0023] By combining two types of lithium iron phosphate particles with different particle sizes, the kinetic performance and volumetric energy density of the cathode active material can be improved. When the particle size ratio of the two is preferably within the above range, the combination effect between the particles is better, the density is higher, the lithium ion transport sites are more concentrated, and the overall transport path is shorter.

[0024] In some implementations, a = 1.05~4.

[0025] In some implementations, a is a range of one or any two of the following: 1.05, 1.2, 1.5, 1.8, 2, 2.2, 2.5, 2.8, 3, 3.2, 3.5, 3.8, 4.

[0026] More preferably, a = 1.5~2.5.

[0027] Because lithium iron phosphate has an olivine structure, which consists of FeO6 octahedra, LiO6 octahedra, and PO4 tetrahedra, with O atoms arranged in a hexagonal close-packed manner, in the b-axis direction, the LiO6 octahedra form a linear chain structure through shared edges, providing a continuous transport channel for lithium ions. This one-dimensional channel allows lithium ions to be rapidly inserted and extracted during charging and discharging. In other directions (such as the a-axis or c-axis), the transport rate is lower due to the obstruction of the PO4 tetrahedra. In this application, the short axis length of the lithium iron phosphate particles in the second material corresponds to the b-axis. Therefore, the larger the a, the shorter the transport path of lithium ions during transport, which is beneficial to improving the transport rate of lithium ions. When the aspect ratio of the lithium iron phosphate particles is preferably within the above range, it can not only effectively utilize the active capacity of the particles and improve the dynamic performance of the secondary battery, but also ensure that the compaction density of the positive electrode sheet is within a moderate range, thereby ensuring a higher energy density and correspondingly higher lithium ion insertion / extraction efficiency and extraction rate.

[0028] More preferably, a1 = 0.1~0.6μm and a2 = 0.05~0.2μm.

[0029] It should be noted that, in this application, the average value 'a' of the ratio of the length a1 of the second material in the major axis direction to the length a2 of the minor axis at the midpoint of the major axis in the direction perpendicular to the major axis can be determined by, but is not limited to, the following methods: The secondary battery was discharged at a rate of 0.33C to the lower limit of 2.5V for venting treatment, then disassembled. The obtained positive electrode sheet was soaked in dimethyl carbonate (DMC) at 25°C for 2 hours, dried, and then the positive electrode active material layer was scraped off as a sample. The sample was fixed with conductive adhesive, CP argon ion polished, and a conductive film was deposited. It was placed on a sample stage and observed under a scanning electron microscope (SEM). Three areas were photographed at a magnification of 10Kx. Then, the lithium iron phosphate particles of the first material and the second material were determined by the particle diameter. The materials consist of lithium iron phosphate particles. In the first material, the lithium iron phosphate particles are larger, with a particle size ≥ 0.9 μm. In the second material, the lithium iron phosphate particles are spherical and smaller, with a particle size < 0.9 μm. After selecting the lithium iron phosphate particles from the second material, the length a1 of the particle along its long axis and the length a2 of its short axis at the midpoint of the long axis are measured using measurement software. The ratio of these two lengths for the particle is a1 / a2. The ratio of these two lengths is measured for 200 lithium iron phosphate particles from each material in this region, and the average value is calculated as a.

[0030] In some embodiments, the cross-section of the plane containing the minor axis at the midpoint of the major axis of the lithium iron phosphate particles in the second material, perpendicular to the major axis, is at least one of a circle, a near-circular shape, or an ellipse.

[0031] In this application's technical solution, small-sized ball-and-stick shaped lithium iron phosphate particles are combined with large particles as active materials. This not only effectively optimizes the overall material stacking effect, but also helps the overall material achieve better kinetic lithium-ion transport performance, resulting in a faster transport rate and superior fast-charging performance of the secondary battery corresponding to the positive electrode sheet.

[0032] In some implementations, b = 0.05~5%.

[0033] It should be noted that 'b' in the positive electrode sheet described in this application can be confirmed by, but is not limited to, the following methods: The secondary battery was discharged at 0.33C to the lower limit voltage of 2.5V for venting treatment. Then, the positive electrode was disassembled and soaked in DMC solution at 25℃ for 4 hours, vacuum dried, and the positive electrode active material layer powder was scraped off, ground, sieved, and a 320-mesh powder sample was placed in a high-resolution X-ray diffractometer (XRD, model Rigaku UItima IV). The test conditions were set as follows: copper target, scanning voltage of 40KV, current of 40mA, scanning range of 5~90°, and scanning rate of 2° / min. The XRD was calibrated using the silicon internal standard method. After the test, the test data was exported. Peak area refinement was performed in the XRD software: the main phase lithium iron phosphate and iron phosphate structure files were imported, the atomic parameter information of lithium iron phosphate / iron phosphate was compared, and the lithium iron phosphate structure card was selected for fitting. The fitting was repeated multiple times until the fitting converged and the Rwp value tended to stabilize, and the content of b was determined.

[0034] In some implementations, b is a range of one or any two of the following: 0.05%, 0.1%, 0.2%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 3%, 4%, 4.5%, and 5%.

[0035] More preferably, the lithium supplement further includes lithium carbonate.

[0036] In some embodiments, b = b1 + b2, where b1 is the mass percentage of lithium carbonate in the positive electrode active material layer, and b2 is the mass percentage of lithium phosphate in the positive electrode active material layer.

[0037] More preferably, b1 is greater than 0 and less than or equal to 1.5%; More preferably, b2 = 0.05~5%.

[0038] More preferably, b = 0.1~2%.

[0039] Lithium phosphate, as a lithium source, when compounded with lithium iron phosphate particles, has a delithiation potential of around 3.7V. During the first charge of a secondary battery, lithium phosphate undergoes an irreversible electrochemical decomposition reaction at its operating potential, releasing lithium ions and electrons to provide additional lithium ions for the battery. The released lithium ions migrate through the electrolyte to the negative electrode surface, participating in the formation of the SEI film, which can compensate for the lithium source, increase battery capacity, and thus improve the battery's volumetric energy density. Secondly, lithium phosphate allows ions (here, lithium ions Li⁺) to move rapidly within its crystal lattice structure, thereby achieving efficient ion conduction. Therefore, it can act as a fast ion conductor, providing additional lithium ion diffusion paths, improving lithium ion insertion / extraction capabilities, and enhancing the battery's kinetic performance. In addition, when lithium phosphate is present in the lithium replenishing agent, lithium carbonate can also be further introduced. This substance also has the effect of replenishing active lithium, increasing the abundance of the lithium replenishing agent.

[0040] In the positive electrode sheet described in this application, the total amount b added should not be too small. If the amount added is too small, the capacity improvement of the lithium supplementation agent on the composite lithium iron phosphate particles will not be significant. However, if the amount added is too large, in addition to affecting the overall compaction density of the positive electrode sheet, there is also a risk of generating additional impurity phases, which weakens the electrochemical performance of the secondary battery. When the total content of the two substances is preferably within the above range, the synergy between the two substances and the two types of lithium iron phosphate particles is higher, which further improves the lithium-ion transport efficiency and maintains the chemical stability of the overall system, ultimately improving the fast charging performance and energy density of the secondary battery.

[0041] In some embodiments, the positive electrode active material and the lithium replenishing agent are disposed in the positive electrode active material layer, and the volume ratio of the second material in the total volume of the first material and the second material is 50-80%.

[0042] To further optimize the lithium-ion conduction efficiency and overall material stability of the positive electrode active material layer in the positive electrode sheet, the volume ratio of the large-sized first material and the small-sized ball-and-stick-shaped second material in the positive electrode active material layer of this application is preferably within the above-mentioned range. Under this setting, the small-sized ball-and-stick-shaped lithium iron phosphate particles have a better stacking effect after filling the gaps between the large-sized lithium iron phosphate particles, the lithium-ion transport distance is shortened, the kinetic performance is better, and the compaction density can also be further optimized, resulting in better capacity utilization.

[0043] It should be noted that the volume ratio of the second material in the positive electrode active material layer in the total volume of the first material and the second material in the technical solution of this application can be limited by, but is not limited to, the following methods: The secondary battery was discharged at 0.33C to the lower limit voltage of 2.5V for discharge treatment. Then the positive electrode was disassembled and soaked in DMC solution at 25°C for 4 hours, vacuum dried, and a 1cm×1cm area in the middle region of the electrode was cut as a test piece. Focused ion beam scanning electron microscopy (SEM-FIB) was used, with the following test conditions: cutting voltage 30 kV, cutting beam current 3 nA, cutting range 70 × 50 × 9 μm, interlayer spacing 20 mm, imaging voltage 2 kV, imaging beam current 0.8 nA, resolution 3072 × 2048, and working distance 3.5 mm. Avizo was used to calculate the number and size of particles within the 1 / 3 region, specifically the 23.98 × 23.98 × 7.96 μm area. The particles were categorized according to their size in the first material and the second material. The first material contained larger lithium iron phosphate particles (≥ 0.9 μm), while the second material contained smaller, rod-shaped particles (< 0.9 μm). The volume percentage of the second material in the total volume of the first and second materials was calculated.

[0044] In some embodiments, the surface of the second material is further provided with a carbon coating layer.

[0045] More preferably, the average thickness of the carbon coating layer on the surface of the second material is 3~10 nm.

[0046] When the surface of small-sized ball-and-stick shaped lithium iron phosphate particles is further optimized with a carbon coating layer, not only can the overall conductivity of the material be effectively improved, but the uniformity of the particles dispersed in the gaps between the lithium iron phosphate particles in the first material can also be improved. When the thickness of the carbon coating layer is further preferably within the above-mentioned range, the conductivity of the particles is better and the lithium-ion transport efficiency can reach a higher level.

[0047] It should be noted that the average thickness of the carbon coating layer on the surface of the second lithium iron phosphate particles in this application can be confirmed by, but is not limited to, the following methods: The secondary battery was discharged and disassembled. The resulting positive electrode sheet was soaked in dimethyl carbonate (DMC) at 25°C for 2 hours and dried. The positive electrode active material layer was then scraped off as a sample. The powder sample was uniformly dispersed in ethanol and tested by TEM (transmission electron microscopy). The magnification was adjusted to 300KX. The particles were screened by the ball-and-stick morphology of the lithium iron phosphate particles in the second material. Three different positions were selected and magnified to 500KX to measure the thickness of the coating layer on the outer surface of the core. The number of particles was counted as 200, and the average value was calculated to obtain the coating layer thickness of the second material.

[0048] In some embodiments, the porosity of the positive electrode sheet is 20-30%.

[0049] The porosity of the positive electrode sheet is mainly determined by the positive electrode active material layer it contains. Especially when the positive electrode active material layer contains particles of different sizes, it is preferable that its porosity is within the above range. This can not only improve the lithium ion insertion / extraction efficiency, but also avoid significant side reactions between the electrolyte and the active material layer after the electrolyte wets the positive electrode sheet, thus reducing the capacity and stability of the secondary battery. The secondary battery has better volumetric energy density and fast charging performance.

[0050] It should be noted that the porosity of the positive electrode sheet described in this application can be confirmed by, but is not limited to, the following methods: The secondary battery is discharged to the lower limit of 2.5V at a rate of 0.33C for decomposition treatment. The positive electrode sheet is obtained by disassembling the decomposed secondary battery. The positive electrode sheet is immersed in DMC at room temperature for 60 minutes, removed, and dried. Subsequently, the electrode sheet is cut into circular pieces with a diameter of 12mm using an electrode sheet punching machine. Simultaneously, the thickness of the electrode sheet and the current collector is measured using a thickness gauge, denoted as h1 and h2 respectively. The mass is weighed using a balance with an accuracy of 0.00001g and recorded as m1. According to the formula v=πr 2 Calculate the volume v of the cut electrode sheet (h1-h2); immerse the electrode sheet in a sealed container with a certain volume of hexadecane for 1 hour (the volume of hexadecane in the sealed solution is not required, but the amount must be sufficient to completely submerge the electrode sheet); after 1 hour, remove the electrode sheet with tweezers and place it on filter paper to absorb dry until constant weight (generally, 1 hour is sufficient to absorb dry to constant weight). Weigh it using a balance and record the mass as m2. Calculate the porosity using the formula X / v, where X = (m2-m1) / ρ, and ρ is the density of hexadecane, 0.7734 g / cm³. 3 .

[0051] In some embodiments, the positive electrode active material layer further includes a conductive agent, a binder, and a dispersant.

[0052] More preferably, the conductive agent includes at least one of carbon black, acetylene black, artificial graphite, natural graphite, carbon nanotubes, and graphene; the binder includes at least one of polytetrafluoroethylene and polyvinylidene fluoride; and the dispersant includes at least one of polyvinylpyrrolidone, polyethylene glycol, sodium carboxymethyl cellulose, and polyacrylic acid.

[0053] More preferably, in the positive electrode active material layer, the mass ratio of the positive electrode active material, the conductive agent, the binder and the dispersant is (94.5~98.5):(0.5~1.5):(1.3~2.2):(0.05~2).

[0054] In some embodiments, the lithium iron phosphate has the structural formula Li. xFeMPO4, wherein x = 0.9~1.1, and M includes at least one of Mn, Ni, Co, Cr, Cu, Bi, Sb, Ti, and V.

[0055] More preferably, the content of M in the positive electrode active material layer is 500~5000ppm.

[0056] In some embodiments, the average ratio d of the length D1 of the minor axis at a position 1 / 4 of the length in the direction perpendicular to the major axis of the lithium iron phosphate particles in the second material to the length D2 of the minor axis at a position midway in the direction perpendicular to the major axis is 0.55~1. Figure 1 As shown.

[0057] More preferably, the positive electrode sheet satisfies: d / b = 20~1600.

[0058] When the lithium iron phosphate particles in the second material are arranged in a ball-and-stick configuration, the ratio d of the length of the short axis at the middle position of the long axis to the length of the short axis at the 1 / 4 position of the long axis is further studied. As the ratio d increases, the curvature of the particle length direction gradually decreases, and the ball-and-stick particles gradually transform into rectangular particles, which cannot effectively shorten the lithium-ion transport path and the kinetic improvement is not obvious. If the ratio d decreases, the configuration will tend to transform into a spindle shape. Although the lithium-ion transport path is shortened, the degree of stacking will also decrease, affecting the energy density of the battery. In addition, the polarization effect will be improved, and the degree of side reactions between the material and the electrolyte will increase to a certain extent. When the ratio d is preferably in the range of 0.55 to 1, the ion conduction rate of the particles can be further improved, and the degree of side reactions with the electrolyte is lower, corresponding to the kinetic performance and volumetric energy density of the secondary battery.

[0059] Furthermore, when the ratio d to the content b of the lithium-containing compound is preferably in the range of 20 to 1600, the active lithium replenishment of the lithium-containing compound is higher, and the secondary battery, with better kinetic performance, further improves the lithium intercalation / deintercalation capacity and has a higher volumetric energy density.

[0060] This application also provides a secondary battery, including the aforementioned positive electrode.

[0061] In some embodiments, the areal density of the positive electrode sheet is 400~500 g / m³. 2 The compacted density is 2.45~2.8 g / cm³. 3 .

[0062] It should be noted that the areal density and compacted density of the positive electrode sheet described in this application can be confirmed by, but not limited to, the following methods: The positive electrode sheet is obtained by disassembling a secondary battery in its empty state; the positive electrode sheet is immersed in DMC at room temperature (25°C) for 60 minutes, removed, and dried; the pretreated positive electrode sheet is punched into circular sheets of a fixed area using a punching machine, the area of ​​which is denoted as S0; three circular sheets are taken as parallel samples, and the mass of each of the three circular sheets is weighed using an electronic balance, the average value is taken and denoted as M1; the density is measured using a micrometer. The average thickness of the active material layer in the three discs (i.e., the total thickness minus the current collector) is recorded as H. Finally, an appropriate amount of deionized water is dropped onto each of the three discs, and the coating on the discs is gently wiped off with lint-free paper to expose the copper foil. The discs are left to stand at room temperature (or dried) for 10 minutes. After the copper foil is dry, the mass of the three copper foils is weighed and the average mass is recorded as M0. The compaction density of the positive electrode is calculated according to the following formulas: A = (M1-M0) / (H*S0), and the areal density is B = (M1-M0) / S0.

[0063] In some embodiments, the secondary battery further includes a negative electrode.

[0064] In some embodiments, the negative electrode sheet includes a negative electrode active material layer, which includes a negative electrode active material, including at least one of artificial graphite, natural graphite, silicon-carbon composite material, and lithium titanate.

[0065] In some embodiments, the negative electrode active material layer further includes a conductive agent, a thickener, and a binder.

[0066] More preferably, the conductive agent includes at least one of carbon black, acetylene black, artificial graphite, natural graphite, carbon nanotubes, and graphene; the thickener includes at least one of sodium carboxymethyl cellulose, polyvinylpyrrolidone, polyethylene glycol, and sodium alginate; and the binder includes at least one of polyvinylidene fluoride, styrene-butadiene rubber, polyacrylic acid, polyimide, and polyurethane.

[0067] In some embodiments, the negative electrode active material includes at least one of natural graphite and artificial graphite.

[0068] More preferably, the average particle size of the negative electrode active material is 5~25μm.

[0069] It should be noted that the average particle size of the negative electrode active material described in this application can be confirmed by, but is not limited to, the following methods: Discharge the secondary battery at a rate of 0.33C to the lower limit of 2.5V for venting treatment; disassemble the negative electrode sheet of the secondary battery and immerse it in DMC for 2 hours; dry it; scrape off the negative electrode active material layer; use a laser particle size distribution measuring instrument (Mastersizer 3000) to measure the particle size distribution according to the laser diffraction method (specific steps refer to GB / T19077-2016); and determine the particle size D corresponding to a cumulative particle size distribution percentage reaching 50%. V50 This refers to the average particle size of the negative electrode active material.

[0070] More preferably, the average particle size of the negative electrode active material is 6.5~12μm.

[0071] When the negative electrode active material is mainly graphite, further optimizing the particle size of the graphite material within the above-mentioned range can not only improve the efficiency of lithium ion insertion and extraction, but also reduce the polarization effect inside the electrode during lithium ion insertion and extraction. When lithium ions are transported to the negative electrode, the negative electrode active material is more resistant to ion impact, thus achieving better fast charging performance.

[0072] In some embodiments, the negative electrode active material includes a silicon-carbon composite material.

[0073] More preferably, b = 0.09~4%.

[0074] While silicon-carbon composite materials have a high theoretical capacity during lithium-ion insertion and extraction, they also cause volume expansion, which can lead to the rupture of the interfacial film and the risk of loss of active lithium. Therefore, when the negative electrode active material contains silicon-carbon composite materials, further optimizing the content of lithium compounds in the positive electrode within the above range can effectively compensate for the loss of active lithium caused by the secondary battery in the early stage of cycling, and improve the volumetric energy density and fast charging performance of the secondary battery.

[0075] In some embodiments, the areal density of the negative electrode sheet is 180~230 g / m³. 2 The compacted density is 1.5~1.65 g / cm³. 3 .

[0076] The areal density and compaction density of the negative electrode sheet described in this application can be tested and confirmed using the same test methods as those used for the areal density and compaction density of the positive electrode sheet, but not limited to those described above.

[0077] In some embodiments, the secondary battery further includes an electrolyte comprising a solvent and a lithium salt.

[0078] In some embodiments, the solvent includes at least one of carbonate solvents, carboxylic acid ester solvents, ether solvents, sulfone solvents, nitrile solvents, and phosphate ester solvents.

[0079] Exemplary examples include, but are not limited to, at least one of propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC); carboxylic acid ester solvents include, but are not limited to, at least one of ethyl acetate (EA), ethyl propionate (EP), methyl formate, methyl acetate (MA), and 1,4-butyrolactone; ether solvents include at least one of dimethyltetrahydrofuran, tetrahydrofuran, and 1,2-dimethoxyethane; sulfone solvents include at least one of methyl sulfone and dimethyl sulfoxide; nitrile solvents include at least one of propionitrile, butyronitrile, 1-(2-cyanoethyl)pyrrole, and 1,3,6-hexanetrionitrile; and phosphate ester solvents include at least one of trimethyl triphosphate and triethyl phosphate.

[0080] More preferably, the solvent may also include, but is not limited to, at least one of carbonate solvent fluorinated derivatives, carboxylic acid ester solvent fluorinated derivatives, ether solvent fluorinated derivatives, sulfone solvent fluorinated derivatives, nitrile solvent fluorinated derivatives, and phosphate ester solvent fluorinated derivatives.

[0081] In some embodiments, the lithium salt includes at least one of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetrafluoroborate, lithium dioxalate borate, lithium difluorooxalate borate, lithium trifluoromethanesulfonate, lithium difluoromethanesulfonylimide, lithium ditrifluoromethanesulfonylimide, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0082] More preferably, the concentration of lithium salt in the electrolyte is 0.6~1.8 mol / L.

[0083] More preferably, the concentration of lithium salt in the electrolyte is one or any two of the following: 0.6 mol / L, 0.8 mol / L, 1 mol / L, 1.2 mol / L, 1.5 mol / L, and 1.8 mol / L.

[0084] The secondary battery described in this application is formulated by compounding various suitable solvents and lithium salts according to actual needs, and the concentration of lithium salts is adjusted as long as normal use effect can be achieved, and is not limited to the above-described scheme.

[0085] In some embodiments, the electrolyte further contains additives, including at least one of vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethylene sulfate (DTD), lithium difluorooxalate borate (LiODFB), tri(trimethylsilane) phosphate (TMSP), tri(trimethylsilane) borate (TMSB), and lithium difluorooxalate phosphate (LiODFP).

[0086] More preferably, the solvent of the electrolyte includes EA, EP and MA.

[0087] More preferably, the volume ratio of EA, EP and MA is (10~60):(20~40):(20~50).

[0088] By using the preferred solvents described above to prepare the electrolyte, the viscosity of the electrolyte can be controlled at a low level. This not only improves the contact between the electrolyte and the positive electrode, but also enhances the migration efficiency of lithium ions, thereby improving the fast-charging performance of the secondary battery.

[0089] In some embodiments, the first material can be a commercially available product, or it can be prepared by, but is not limited to, the following methods: The lithium source, iron source, and phosphorus source precursors, as well as element M precursor (if element M is present), and carbon source are mixed evenly to obtain a mixture that is dispersed in a solvent and wet-milled, spray-dried, and the resulting powder is calcined once, pulverized, sintered a second time, and pulverized again to obtain the first material. In some embodiments, a lithium supplement is also added to the mixture, and / or, a lithium supplement is added to the powder before secondary sintering, wherein the lithium supplement may be lithium phosphate; In some embodiments, a lithium supplement agent is added to the powder before secondary sintering, and the lithium supplement agent may be lithium carbonate; It should be noted that the timing of adding lithium supplementer during the preparation of the first material in this application is not unique. Those skilled in the art can add it in either of the two situations mentioned above, or in both process stages, or not add it in either process stage and introduce lithium supplementer in other processes.

[0090] In some embodiments, the solvent includes at least one of water, methanol, and ethanol.

[0091] In some embodiments, the lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium nitrate, lithium oxalate, lithium dihydrogen phosphate, lithium citrate, and lithium acetate. The phosphorus source includes at least one of ammonium dihydrogen phosphate, ammonium phosphate, and lithium dihydrogen phosphate. The iron source includes at least one of ferrous oxalate, ferric hydroxide, ferrous hydroxide, ferric phosphate, ferrous phosphate, ferric acetate, ferrous acetate, ferric carbonate, ferrous carbonate, ferric oxide, ferric oxide, and ferric oxalate. It should be noted that the phosphorus source and the iron source can be the same, such as iron phosphate.

[0092] The carbon source includes at least one of glucose, sucrose, and polyethylene glycol.

[0093] The precursor of element M can be at least one of the oxides, compounds, or salts of element M, and there is no specific limitation thereto. For example, when M is Ti, the precursor of element M can be at least one of titanium dioxide or tetrabutyl titanate.

[0094] In some embodiments, the wet grinding is wet ball milling, the ball milling time is 2-4 hours, and the rotation speed is 1000-1500 r / min.

[0095] In some embodiments, the spray drying pressure is 0.4~0.7 MPa, the atomizer speed is 8000~15000 r / min, the inlet temperature is 180~250℃, and the outlet temperature is 70~100℃.

[0096] In some embodiments, the primary calcination temperature is 780~810℃, the heating rate is 4~6℃ / min, and the holding time is 8~12h; the secondary calcination temperature is 760~790℃, the heating rate is 4~6℃ / min, and the holding time is 6~8h.

[0097] In some embodiments, the pulverization is performed using airflow pulverization at a pressure of 0.5~1.2 MPa.

[0098] In the technical solution of this application, the particle size of the first material can be controlled by setting parameters during wet grinding and pulverization, but it is not limited to this. Those skilled in the art can also use other means to control the particle size of the component according to the actual situation.

[0099] In some embodiments, the second material can be commercially available or can be prepared by, but not limited to, the following methods: pre-mixing lithium source and phosphorus source to prepare lithium phosphate, then mixing and emulsifying lithium phosphate with iron source in a solvent, and subjecting the resulting slurry to high-pressure hydrothermal reaction and washing treatment to obtain lithium iron phosphate slurry; taking the lithium iron phosphate slurry and spray drying it, then mixing it with carbon source and lithium supplementer, grinding it, and then spray drying it to obtain mixed dry powder; subjecting the mixed dry powder to high-temperature sintering and pulverizing, and dispersing the resulting particulate powder in a solvent, sand milling, spray drying, and secondary pulverization to obtain the second material.

[0100] In some embodiments, the amount of lithium supplementer added during the preparation of the second material is 0.1-3.5% based on the mass of lithium iron phosphate particles obtained after spray drying of the lithium iron phosphate slurry. Those skilled in the art may also omit the lithium supplementer during the preparation of the second material depending on the actual situation.

[0101] In some embodiments, the lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium nitrate, lithium oxalate, lithium dihydrogen phosphate, lithium citrate, and lithium acetate. The phosphorus source includes at least one of ammonium dihydrogen phosphate, ammonium phosphate, and lithium dihydrogen phosphate. The iron source includes at least one of ferrous oxalate, ferric hydroxide, ferrous hydroxide, ferric phosphate, ferrous phosphate, ferric acetate, ferrous acetate, ferric carbonate, ferrous carbonate, ferric oxide, ferric oxide, and ferric oxalate. The carbon source includes at least one of glucose, sucrose, and polyethylene glycol.

[0102] In some embodiments, the high-pressure hydrothermal reaction is carried out at a temperature of 150°C to 250°C, a pressure of 1 to 3 MPa, and a reaction time of 1 to 5 hours. In some embodiments, the milling speed is 600~1000 rpm and the time is 3~6 hours. Alternatively, the milling process can be replaced by airflow pulverizer. In some embodiments, the high-temperature sintering temperature is 650~750℃ and the time is 8~14h; In some embodiments, the spray drying pressure is 0.4~0.7 MPa, the atomizer speed is 8000~15000 r / min, the inlet temperature is 180~250℃, and the outlet temperature is 70~100℃.

[0103] In some embodiments, the pulverization is performed by airflow pulverization, the pressure of the primary airflow pulverization is 400~500 kPa, the pressure of the secondary airflow pulverization is 0.6~1 MPa, and the grading frequency is 30~50 Hz.

[0104] In the scheme described in this application, the shape and aspect ratio of the second material are achieved by adjusting the condition parameters during the high-pressure reaction process and the condition parameters during sand milling and pulverization. However, it is not limited to this. Those skilled in the art can also use other control methods to control the shape, aspect ratio and particle size of the second material according to the actual production situation.

[0105] It should be noted that, as mentioned above, in this application, the lithium replenishing agent in the positive electrode active material layer can be introduced in the form of raw materials or substances obtained from the simultaneous reaction of the lithium iron phosphate particles of the first and second materials during preparation, and the number of times it is added during the preparation process is not unique (such as the lithium replenishing agent being introduced twice during the preparation of the first material mentioned above), and no specific requirements are made.

[0106] The present invention is further illustrated below with specific embodiments, which should not be construed as limiting the scope of protection claimed by the present invention: In the technical solution of this application, the preparation method of the positive electrode sheet is as follows: the positive electrode active material, lithium supplement, conductive agent, binder and dispersant are dispersed in N-methylpyrrolidone (NMP) according to the mass ratio to obtain a positive electrode slurry, the positive electrode slurry is coated on aluminum foil to obtain an aluminum foil with the positive electrode slurry coated on the surface; after drying, it is rolled and cut to obtain a positive electrode sheet.

[0107] The preparation method of the negative electrode is as follows: The negative electrode material, binder, conductive agent, and thickener are dispersed in deionized water according to the mass ratio to obtain a negative electrode slurry. The negative electrode slurry is coated on copper foil. After drying, cold pressing, and slitting, the negative electrode sheet is obtained.

[0108] The electrolyte is prepared as follows: The solvent, lithium salt, and additives are mixed in a certain mass ratio to obtain the electrolyte.

[0109] Example 1 A positive electrode sheet and a secondary battery, wherein the preparation method of the secondary battery includes the following steps: (1) Preparation of positive electrode active material: (1.1) Preparation of the first material: The precursors—lithium carbonate (lithium source), iron phosphate (phosphorus-iron ratio 0.96), a mixture of glucose and PEG (carbon source added at 8% of the total precursor mass), titanium dioxide (element M precursor, 2500 ppm based on the total precursor mass), and lithium phosphate (if present)—were dispersed in pure water according to the stoichiometric ratio, controlling the solid content to 45%. After mixing and stirring for 1 hour, the resulting precursor slurry was circulated into a sand mill. The zirconium ball filling rate in the sand mill was 70%, with a zirconium ball size of 0.3~0.4μm. The milling process was carried out at 1300rpm for 3.5h, followed by spray drying at 11000r / min and 0.5MPa (inlet air temperature 220℃, outlet air temperature 75℃). The resulting powder was calcined for 10h at 800℃ under a nitrogen atmosphere at a rate of 5℃ / min, and then subjected to a first air jet milling at a grinding gas pressure of 480KPa and a classification frequency of 41Hz. Subsequently, lithium carbonate and lithium phosphate (if present) were added during the pre-dispersion process, and after high-speed mixing, the mixture was calcined again at 760℃ under a nitrogen atmosphere at a rate of 5℃ / min for 8h. A second air jet milling was then performed at a pressure of 0.8MPa to obtain the first material. (1.2) Preparation of the second material: Lithium hydroxide (lithium source), phosphoric acid (phosphorus source), and ferrous sulfate (iron source) were dispersed in pure water and emulsified. The resulting slurry was transferred to a high-pressure reactor and reacted under high temperature and high pressure conditions of 200℃ and 2MPa for 2 hours to obtain lithium iron phosphate slurry. The obtained slurry was spray-dried at 11000r / min and 0.5MPa (inlet air temperature 220℃, outlet air temperature 75℃). Then, glucose (10wt% of glucose in the total mass of particles and glucose) was added and mixed. The mixture was subjected to dynamic grinding, calcined at 700℃ for 10 hours under a nitrogen atmosphere, spray-dried at 11000 r / min and 0.5 MPa (inlet air temperature 220℃, outlet air temperature 80℃), and then subjected to a first air jet milling at 50 kPa pressure. The resulting mixed dry powder was then mixed with lithium phosphate and lithium carbonate and dispersed in ethanol, followed by sand milling, spray drying at 11000 r / min and 0.5 MPa (inlet air temperature 220℃, outlet air temperature 80℃), and a second air jet milling at 1 MPa to obtain the final product, which is the second material. (2) Preparation of the positive electrode sheet: The first material and the second material were mixed at a mass ratio of 0.4:1 to form the positive electrode material. Then, the positive electrode material, conductive agent SP, conductive agent CNT (carbon nanotubes), and binder polyvinylidene fluoride were dispersed in N-methylpyrrolidone at a mass ratio of 96.5:1:0.5:2. The slurry was prepared by vacuum stirring and then coated onto the current collector aluminum foil. The coating surface density was set to 450 g / cm³. 2 After drying, cold pressing, slitting, and roller pressing, the positive electrode sheet is obtained, with a compacted density of 2.6 g / cm³. 3 ; (3) Preparation of negative electrode sheet: The negative electrode active material artificial graphite, conductive agent acetylene black, thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber are dispersed in water at a mass ratio of 96.4:1:1.2:1.4. The mixture is then vacuum stirred to prepare a slurry, which is then coated on both sides of the current collector copper foil. After drying, cold pressing, and slitting, the negative electrode sheet is obtained. The negative electrode material is graphite. The areal density of the electrode sheet is 200 g / m³. 2 Compacted density 1.6 g / cm³ 3 .

[0110] (4) Selection of diaphragm: Select commercially available polyethylene diaphragm with a thickness of 15μm; (5) Preparation of electrolyte: Ethyl carbonate, methyl ethyl carbonate, and diethyl carbonate were mixed in a mass ratio of 1:1:1 to obtain an organic solvent. Lithium salt LiPF6 was added and dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L. (6) The positive electrode, separator and negative electrode are wound and assembled into a cell in sequence. The cell is placed in the outer packaging shell, dried and injected with electrolyte. After vacuum sealing, standing for 24 hours and formation, the secondary battery is obtained.

[0111] The formation steps are as follows: After placing the secondary battery at a high temperature of 45°C for 24 hours, place it on a glass clamp with a clamp pressure of 0.5MPa, let it stand for 10 minutes, charge it at a rate of 0.02C for 300 minutes and stop, let it stand for 10 minutes, charge it at a rate of 0.1C for 300 minutes and stop, and repeat the cycle three times.

[0112] The parameters of the secondary battery were statistically analyzed, and the results are shown in Tables 1-3.

[0113] Examples 2-34 A secondary battery differs from Example 1 only in that the preparation process parameters of the positive electrode active material and the characteristic parameters of the obtained positive electrode material are different, as shown in Tables 1-3.

[0114] Comparative Examples 1-4 A secondary battery differs from Example 1 only in that the preparation process parameters of the positive electrode active material and the characteristic parameters of the obtained positive electrode material are different, as shown in Tables 1-3.

[0115] Table 1 Continued from Table 1 Continued from Table 1 Continued from Table 1 Table 2 Continued from Table 2 Example of effect The cathode materials and secondary batteries obtained in each embodiment and comparative example were tested as follows: (1) Fast charging performance test: The secondary batteries obtained in each embodiment and comparative example were charged at a constant current rate of 0.3C to 3.65V, and then charged at a constant voltage rate until the current dropped to 0.05C (cutoff); after standing for 30 minutes, they were discharged at a rate of 0.3C to 2.5V, cyclically for three cycles, and the discharge capacity Q1 of the third cycle was recorded; they were charged at 0.3C to 10% SOC of Q1 (cutoff); charged at 3C to 20% SOC of Q1 (cutoff); charged at 2.8C to 40% SOC of Q1 (cutoff); and charged at 2.4C to Q1... The charging time was recorded as follows: 50% SOC cutoff; 2.2C charging to 60% SOC cutoff; 1.8C charging to 70% SOC cutoff; 1.6C charging to 80% SOC cutoff; 1.0C charging to 90% SOC cutoff; 0.5C charging to 95% SOC cutoff; 0.33C charging to 3.65V, 0.05C cutoff; after full charge, discharge at 1C to 2.5V, and record the charging time for each secondary battery in the 10%~80% SOC range. (2) Volumetric energy density test method: Measure the volume V of the secondary battery, place the battery in the fixture, apply a force of 3000N, charge it with a constant current of 0.33C to the upper limit voltage of 3.65V, let it rest for 30min, discharge it with a constant current of 0.33C to the lower limit voltage of 2.5V, let it rest for 30min, and repeat the cycle 3 times; obtain the discharge capacity (in Ah) and energy E (average of three cycles), discharge energy density = E / V (in Wh / L).

[0116] The test results are shown in Table 3.

[0117] Table 3 As can be seen from Table 3: (1) In the secondary battery described in this application, by using two types of lithium iron phosphate particles with different particle size ranges as active materials in the positive electrode sheet and introducing lithium-containing compounds, and simultaneously coordinating the aspect ratio, particle size and lithium-containing compound content of small-sized particles, the secondary battery can meet the range of a / (100×b×c)=0.296~200. This not only shortens the path of lithium ions during transport and improves the dynamic performance, but also improves the fast charging performance of the battery. The fast charging time of the secondary battery obtained in each embodiment is within 26 minutes. At the same time, the particle stacking effect is optimized, and the lithium-containing compounds can effectively supplement the active lithium, resulting in a high volumetric energy density of the battery, which can reach 378Wh / kg and above, and excellent overall performance.

[0118] (2) When the positive electrode is synchronously regulated by a, b and c, if a / (100×b×c) is too high, the stacking effect of the positive electrode and the active lithium replenishment effect are not good, and the volumetric energy density of the battery cannot meet the standard. However, if it is too low, the fast charging performance of the battery will be poor due to the poor kinetic performance of lithium ion transport in the positive electrode. Therefore, it is necessary to limit the ratio of the three. At the same time, if a / (100×b×c) = 0.18~67 is further optimized, the secondary battery will have a better balance between volumetric energy density and fast charging performance, and the two performances can reach a higher level.

[0119] (3) During the synergistic regulation of the three, the aspect ratio and particle size range of small-sized lithium iron phosphate particles will have a certain impact on the overall filling and stacking effect of the particles, and will also affect the deintercalation rate of active lithium and the degree of side reaction between the particles and the electrolyte. When the particle size c is further preferably in the range of 0.3~0.55μm, or the aspect ratio a is further preferably in the range of 1.5~2.5, the overall deintercalation effect of lithium iron phosphate particles is better, and the fast charging performance and volumetric energy density of the battery are better. On the other hand, the content of lithium-containing compounds in the active material layer of the electrode is related to the replenishment of active lithium, and it can also provide an additional lithium ion diffusion path for lithium ions. When its content is further preferably in the range of 0.1~2%, the addition of lithium-containing compounds will not affect the compaction density of the electrode too much, the probability of impurity phase formation is lower, and the performance of the secondary battery is better.

Claims

1. A positive electrode plate, characterized in that, The positive electrode sheet includes a positive active material layer, which includes a positive active material and a lithium supplement agent; The positive electrode active material includes a first material and a second material; The first material includes lithium iron phosphate particles, wherein the particle size of the lithium iron phosphate is ≥0.9μm; The second material comprises lithium iron phosphate particles, wherein the particle size of the lithium iron phosphate is <0.9 μm. The lithium iron phosphate particles in the second material are ball-and-stick shaped and satisfy a≥1.05, where a is the average ratio of the length a1 of the lithium iron phosphate particle in the long axis direction to the length a2 of the short axis at the middle position of the long axis in the direction perpendicular to the long axis, and a=1.05~4; The lithium supplement includes lithium phosphate; The positive electrode plate satisfies a / (100×b×c)=0.8~200; Where b is the mass percentage of lithium supplementation agent in the positive electrode active material layer, b = 0.1~2%; cμm is the cumulative particle size D of the second material. n50 c = 0.2 ~ 0.75; The average ratio of the length of the minor axis of the lithium iron phosphate particles in the second material at a position 1 / 4 of the length in the direction perpendicular to the major axis to the length of the minor axis at a position in the middle of the direction perpendicular to the major axis is d = 0.55~0.

95. The positive electrode sheet satisfies: d / b = 39.5~840.

2. The positive electrode sheet as described in claim 1, characterized in that, The cumulative particle size D of the first material n50 =1~1.5μm.

3. The positive electrode sheet as described in claim 1, characterized in that, The value of a is 1.5 to 2.

5.

4. The positive electrode sheet as described in claim 1, characterized in that, The positive electrode sheet satisfies a / (100×b×c)=0.8~67.

5. The positive electrode sheet as described in claim 1, characterized in that, The cumulative particle size D of the first material and the second material n50 The ratio is 2.6 to 5.

65.

6. The positive electrode sheet as described in claim 1, characterized in that, a1 = 0.1~0.6μm, and / or a2 = 0.05~0.2μm.

7. The positive electrode sheet as described in claim 1, characterized in that, In the second material, the cross-section of the plane containing the minor axis at the midpoint of the major axis of the lithium iron phosphate particles in the direction perpendicular to the major axis is at least one of the following: circular, near-circular, or elliptical.

8. The positive electrode sheet as described in claim 1, characterized in that, The lithium supplement also includes lithium carbonate, wherein b = b1 + b2, where b1 is the mass percentage of lithium carbonate in the positive electrode active material layer, and b2 is the mass percentage of lithium phosphate in the positive electrode active material layer; b1 is greater than 0 and less than or equal to 1.5%, and / or b2 = 0.05~1%.

9. The positive electrode sheet as described in claim 1, characterized in that, In the positive electrode active material layer, the volume ratio of the second material in the total volume of the first material and the second material is 50-80%.

10. The positive electrode sheet as described in claim 1, characterized in that, The second material has a carbon coating layer on its surface; the average thickness of the carbon coating layer on the surface of the second material is 3~10 nm.

11. The positive electrode sheet as described in claim 1, characterized in that, The porosity of the positive electrode sheet is 20-30%.

12. The positive electrode sheet as described in claim 1, characterized in that, The lithium iron phosphate has the structural formula Li. x FeMPO4, wherein x = 0.9~1.1, and M includes at least one of Mn, Ni, Co, Cr, Cu, Bi, Sb, Ti, and V.

13. The positive electrode sheet as described in claim 12, characterized in that, The content of M in the positive electrode active material layer is 500~5000ppm.

14. A secondary battery, characterized in that, Includes the positive electrode sheet as described in any one of claims 1 to 13.

15. The secondary battery as described in claim 14, characterized in that, The secondary battery further includes a negative electrode sheet, which includes a negative electrode active material layer. The negative electrode active material layer includes a negative electrode active material, which includes at least one of artificial graphite, natural graphite, silicon-carbon composite material, and lithium titanate.

16. The secondary battery as described in claim 15, characterized in that, The negative electrode active material includes at least one of natural graphite and artificial graphite, and the average particle size of the negative electrode active material is 5~25μm.

17. The secondary battery as described in claim 15, characterized in that, The negative electrode active material includes a silicon-carbon composite material.

18. The secondary battery as described in claim 14, characterized in that, The secondary battery further includes an electrolyte, which comprises a solvent and a lithium salt; the solvent includes at least one of carbonate solvents, carboxylic acid ester solvents, ether solvents, sulfone solvents, nitrile solvents, and phosphate ester solvents.

19. The secondary battery as described in claim 18, characterized in that, The carbonate solvents include at least one of propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate, and / or the carboxylic acid ester solvents include at least one of ethyl acetate, methyl formate, and 1,4-butyrolactone, and / or the ether solvents include at least one of dimethyltetrahydrofuran, tetrahydrofuran, and 1,2-dimethoxyethane, and / or the sulfone solvents include at least one of methyl sulfone and dimethyl sulfoxide, and / or the nitrile solvents include at least one of propionitrile, butyronitrile, 1-(2-cyanoethyl)pyrrole, and 1,3,6-hexanetrionitrile, and / or the phosphate ester solvents include at least one of trimethyl triphosphate and triethyl phosphate.

20. The secondary battery as described in claim 19, characterized in that, The solvents in the electrolyte include ethyl acetate, ethyl propionate, and methyl acetate.

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