Positive plate, battery and vehicle

Abstract

The invention discloses a positive plate, a battery and a vehicle, relates to the technical field of batteries, and aims to improve the low-temperature DCR performance of the positive plate and the battery so as to improve the cruising ability and the power performance of the vehicle. The positive plate comprises a positive current collector and a positive material layer arranged on at least one side surface of the positive current collector, the positive electrode material layer comprises lithium iron phosphate particles; the lithium iron phosphate particles comprise single crystal particles, and the single crystal particles comprise sphere-like particles; wherein the positive plate meets the following conditions: CR is the compression ratio,% of the positive plate; bET is the specific surface area of the lithium iron phosphate particles, and m < 2 > / g; the WE is the weight elongation percentage,% of the positive plate.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, and in particular to a positive electrode sheet, a battery and a vehicle. BACKGROUND

[0002] As an electrode material of a battery, lithium iron phosphate has the following working principle when it is applied to a battery: during charging, part of lithium ions in lithium iron phosphate are released, transferred to a negative electrode through an electrolyte, and embedded in a carbon material of the negative electrode; meanwhile, electrons are released from a positive electrode and reach the negative electrode from an external circuit to maintain the balance of the chemical reaction. During discharging, lithium ions are released from the negative electrode, reach the positive electrode through the electrolyte, and the negative electrode releases electrons to reach the positive electrode from the external circuit to provide energy to the outside world.

[0003] Although lithium iron phosphate material has the advantages of high stability and long cycle life due to its olivine structure, it also has the characteristics of low intrinsic conductivity and small ion diffusion coefficient. This leads to the problem of insufficient low-temperature DCR (Dynamic Charge Resistance) performance of the battery containing lithium iron phosphate. SUMMARY

[0004] The present application provides a positive electrode sheet, a battery and a vehicle to improve the low-temperature DCR performance of the positive electrode sheet and the battery containing lithium iron phosphate, and further improve the power performance and endurance of the vehicle.

[0005] In a first aspect, the embodiments of the present application provide a positive electrode sheet, comprising a positive electrode current collector, and a positive electrode material layer arranged on at least one side surface of the positive electrode current collector; the positive electrode material layer comprises lithium iron phosphate particles; the lithium iron phosphate particles comprise single crystal particles, and the single crystal particles comprise spherical particles; wherein the positive electrode sheet satisfies:

[0006]

[0007] wherein the CR is the compression ratio of the positive electrode sheet; the BET is the specific surface area of the lithium iron phosphate particles, m 2 / g; and the WE is the weight elongation rate of the positive electrode sheet, %.

[0008] In some embodiments, the lithium iron phosphate particles comprise polycrystalline particles; and / or, the single crystal particles comprise flaky particles.

[0009] In some embodiments, the areal density of the positive electrode sheet is 100-400 g / m 2 ; and / or, the compaction density of the positive electrode sheet ranges from 2.3 to 2.8 g / cm 3 .

[0010] In some embodiments, the CR ranges from 60% to 80%.

[0011] In some embodiments, the BET range is 7 to 14 m 2 / g.

[0012] In some embodiments, the WE ranges from 0.5% to 3%.

[0013] In some embodiments, the surface of the lithium iron phosphate particles has a carbon coating layer; wherein,

[0014] Based on the mass of the lithium iron phosphate particles having the carbon coating layer, the mass fraction of the carbon coating layer is 0.1 to 3 wt %.

[0015] In some embodiments, the positive electrode material layer includes a conductive agent and a binder; wherein, based on the mass of the positive electrode material layer, the mass percentage of the conductive agent is 0.7-1.1 wt%, and the mass fraction of the binder is 1.6-2.0 wt%.

[0016] In some embodiments, the conductive agent includes one or more of conductive carbon black, carbon nanotubes, and carbon nanofibers;

[0017] The binder includes one or more of polyvinylidene fluoride, polyvinylidene difluoride, styrene-butadiene rubber and sodium carboxymethyl cellulose.

[0018] In a second aspect, an embodiment of the present application provides a battery, comprising the positive electrode sheet as described above.

[0019] In a third aspect, an embodiment of the present application provides a vehicle, comprising the battery as described above.

[0020] The embodiments of the present application have the following beneficial effects:

[0021] In the embodiment of the present application, in the positive electrode sheet, since the spherical particles in the single crystal particles contained in the lithium iron phosphate particles are primary particles, there is a gradation between the spherical particles, that is, the spherical particles in the single crystal particles themselves have different particle sizes and a high size distribution SPAN (Span of Particle Size Distribution). Therefore, the gaps between the spherical particles distributed on the positive electrode current collector are small, so the lithium iron phosphate particles in the positive electrode material layer can be relatively evenly distributed, with suitable CR and WE characteristics. In this way, the BET of the lithium iron phosphate particles in the positive electrode sheet, the compression ratio CR and the weight elongation WE of the positive electrode sheet can be synergistically controlled to meet the following requirements: Thus, the low-temperature DCR performance of the positive electrode sheet and the battery in which the positive electrode sheet is located is improved, and the internal resistance loss and voltage drop of the resistance in use are effectively reduced, so that the cruising range and low-temperature power response performance of the vehicle in which the battery is located are significantly enhanced. BRIEF DESCRIPTION OF DRAWINGS

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed to be used in the embodiment description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0023] Figure 1 The SEM test diagram of the spherical-like particles provided by the embodiments of the present application is shown;

[0024] Figure 2 The SEM test diagram of the lithium iron phosphate particles obtained by mixing the spherical-like particles and the polycrystalline particles in Example 3 provided by the embodiments of the present application is shown;

[0025] Figure 3 The SEM test diagram of the polycrystalline particles provided by the embodiments of the present application is shown. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solutions and advantages of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0027] The ranges disclosed herein are defined by their lower and upper endpoints, given that a range is defined by selecting a lower endpoint and an upper endpoint, the selected lower and upper endpoints define the boundaries of the particular range. Ranges defined by endpoints can be inclusive or exclusive of the endpoints, and can be arbitrarily combined, i.e., any lower endpoint can be combined with any upper endpoint to form a range. For example, if ranges of 60-120 and 80-110 are listed, it is understood that ranges of 60-110 and 80-120 are also contemplated. Also, if minimum range values of 1 and 2 are listed, and if maximum range values of 3, 4, and 5 are listed, then the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present disclosure, unless otherwise stated, a numerical range "a-b" indicates a shorthand way of describing each and every

[0028] In the present disclosure, all the embodiments and preferred embodiments mentioned herein can be combined with each other to form new technical solutions, unless otherwise specified.

[0029] In the present disclosure, all the technical features and preferred features mentioned herein can be combined with each other to form new technical solutions, unless otherwise specified.

[0030] In the present disclosure, all the steps mentioned herein can be performed in sequence or randomly, but preferably in sequence. For example, the method comprises steps (a) and (b), which means that the method can comprise steps (a) and (b) performed in sequence, or steps (b) and (a) performed in sequence. For example, the method can further comprise step (c), which means that step (c) can be added to the method in any order, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0031] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0032] To solve the problem of insufficient low-temperature DCR performance of lithium iron phosphate in the positive plate and the battery, the application provides a positive plate, which comprises a positive current collector and a positive material layer arranged on at least one side surface of the positive current collector. The positive material layer comprises lithium iron phosphate particles. The lithium iron phosphate particles comprise single crystal particles, and the single crystal particles comprise spherical particles. The positive plate satisfies the following formula:

[0033]

[0034] wherein CR is the compression ratio of the positive plate, %; BET is the specific surface area of the lithium iron phosphate particles, m 2 / g; and WE is the weight elongation rate of the positive plate, %.

[0035] The compression ratio CR means that the thickness of the positive material layer after rolling in the positive plate accounts for the percentage of the thickness of the positive material layer before rolling (i.e., after coating the positive slurry and drying). The thickness of the positive material layer before rolling in the present application and the thickness of the positive material layer in the positive plate (i.e., the thickness of the positive material layer after rolling) can be measured by a micrometer.

[0036] The weight elongation rate WE represents the extension of the lithium iron phosphate particles in all directions during the rolling process. The calculation method is as follows: the areal density of the positive plate before rolling is subtracted from the areal density of the positive plate (i.e., the areal density after rolling), and then divided by the areal density before rolling to obtain the percentage.

[0037] The areal density of the positive plate before rolling (excluding the positive current collector) and the areal density after rolling (excluding the positive current collector) can be tested by a puncher and an electronic balance.

[0038] It is worth noting that the spherical particles in the single crystal particles themselves have a size distribution with a wide width, so when they are arranged on the positive current collector, the smaller spherical particles can be filled between the larger spherical particles, so the voids are small, thereby exhibiting the characteristics of low CR, suitable WE and lithium ion conduction rate. On this basis, the present application controls BET, CR and WE in coordination to satisfy the ranges listed in the above formula, so that the lithium iron phosphate particles do not appear obvious agglomeration during the homogenization process, reduces the risk of belt breakage during rolling; and avoids continuously increasing the coating thickness in order to load a preset mass of lithium iron phosphate particles per unit area on the positive current collector during the coating process, which leads to serious extrusion between the lithium iron phosphate particles in the positive material layer after rolling, and the lithium ion migration rate is reduced instead; thereby effectively improving the low-temperature DCR performance of the positive plate and the battery.

[0039] Exemplarily, the above formula The value of the formula can be 2.4, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0, 14.0, 15.0, 16.0, 17.0, 18.0, or a range selected from any two of the above values.

[0040] Thus, the embodiments of the present application can control the BET of the lithium iron phosphate particles, the CR and WE of the positive electrode sheet by controlling the value of the above formula, so as to achieve a synergistic control among the three, to ensure that the lithium iron phosphate particles in the positive electrode material layer can be uniformly coated on the positive electrode current collector without agglomeration, and effectively avoid the risk of positive electrode sheet breakage, while effectively improving the low-temperature DCR performance of the positive electrode sheet and the lithium ion battery in which the positive electrode sheet is located.

[0041] Further, the above positive electrode sheet can satisfy:

[0042] Preferably, Exemplarily, the formula The value of the formula can be 3.3, 3.9, 4.6, 5.3, 6.0, 6.7, 7.0, or a range selected from any two of the above values.

[0043] The single crystal particles are primary particles. Understandably, the spherical-like particles of the single crystal particles can be spherical-like particles mainly in spherical shape; which can include spherical-like particles similar to rugby balls; or can be spherical-like particles with most of the surface being round, and only part of the surface having edges, depressions, or protrusions, please refer to Figure 1 .

[0044] Further, since the flaky particles have the characteristic of shorter lithium ion migration channel, in an embodiment, the single crystal particles can also include flaky particles, so as to improve the lithium ion conduction rate of the lithium iron phosphate particles through the flaky particles in the single crystal particles, improve the uniformity of the distribution of the lithium iron phosphate particles on the positive electrode current collector through the spherical-like particles in the single crystal particles, and reduce the risk of breakage of the positive electrode sheet (i.e. the positive electrode sheet is broken and scrapped during the rolling process), so as to improve the low-temperature DCR performance and utilization rate of the positive electrode sheet.

[0045] To further improve the lithium ion migration rate of the lithium iron phosphate particles in the positive electrode sheet, in an embodiment, the lithium iron phosphate particles can also include polycrystalline particles. The polycrystalline particles are secondary particles composed of primary particles. The morphology of the secondary particles is spherical, please refer to Figure 3 .

[0046] Thus, in an embodiment, the lithium iron phosphate particles include single crystal particles and polycrystalline particles.

[0047] Among them, single crystal particles are primary particles, and the morphology of single crystal particles includes spherical particles and flaky particles. Polycrystalline particles are secondary particles composed of primary particles.

[0048] Furthermore, the surface density of the positive electrode sheet is 100 to 400 g / m 2 For example, the surface density of the positive electrode sheet can be 100g / m 2 , 110g / m 2 , 120g / m 2 , 130g / m 2 , 140g / m 2 , 150g / m 2 , 160g / m 2 , 170g / m 2 , 180g / m 2 , 190g / m 2 , 200g / m 2 , 210g / m 2 , 220g / m 2 , 230g / m 2 , 240g / m 2 , 250g / m 2 , 260g / m 2 , 270g / m 2 , 280g / m 2 , 290g / m 2 , 300g / m 2 , 310g / m 2 , 320g / m 2 , 330g / m 2 , 340g / m 2 、350g / m 2 , 360g / m 2 , 370g / m 2 , 380g / m 2 、390g / m 2 , 400g / m 2 Or selected from the range consisting of any two of the above values.

[0049] Preferably, the surface density of the positive electrode sheet is 200 to 300 g / m 2 .

[0050] Furthermore, the compaction density of the positive electrode sheet is in the range of 2.3 to 2.8 g / cm 3 For example, the compaction density of the positive electrode sheet can be set to 2.3 g / cm 3 , 2.4g / cm 3 , 2.5g / cm 3 , 2.6g / cm 32.7 g / cm3 3 2.8 g / cm3 3 or a range between any two of the above values.

[0051] Further, when the lithium iron phosphate particles include the polycrystalline particles and / or the flaky particles in the single-crystalline particles, in the process of coating the positive electrode slurry containing the lithium iron phosphate particles, there are large gaps between the polycrystalline particles on the positive electrode current collector and between the flaky particles in the single-crystalline particles. The spheroidal particles in the single-crystalline particles are primary particles, there is a gradation between the spheroidal particles, and there are small gaps between the spheroidal particles. In the process of coating the electrode sheet, in order to achieve a predetermined compactness and / or area density, when the gaps between the lithium iron phosphate particles become large, it is inevitable that the thickness of the positive electrode material layer after coating increases; this easily causes the lithium iron phosphate particles to be arranged too densely, making it difficult for lithium ions to be extracted and embedded during charging and discharging, and reducing the low-temperature DCR performance. At the same time, the excessive thickness can lead to incomplete solvent volatilization of the positive electrode sheet during the baking stage, resulting in an increase in the weight loss rate of the positive electrode sheet, affecting the normal use of the positive electrode sheet; and the large gaps can consume more active sites in the binder, leading to poor adhesion of the lithium iron phosphate particles to the positive electrode current collector, and there is a risk of delamination, so when the compression ratio CR is selected from 60% to 80%, the risks during coating and baking can be reduced. Illustratively, the value of the CR can be 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, or a range selected from any two of the above values.

[0052] Further, to avoid the difficulty of homogenizing the slurry (i.e., the positive electrode slurry) caused by the agglomeration of lithium iron phosphate particles, and thus leading to uneven distribution of lithium iron phosphate particles in the positive electrode sheet, in an embodiment, the specific surface area BET of the lithium iron phosphate particles is 7 to 14 m2 / g. Illustratively, the specific surface area BET can be set to 7 m2 / g, 8 m2 / g, 9 m2 / g, 10 m2 / g, 11 m2 / g, 12 m2 / g, 13 m2 / g, 14 m2 / g, or a range selected from any two of the above values. 2 2 2 2 2 2 2 2 2 ​​​​​​​​or a range selected from any two of the above. Since the specific surface area is generally in a corresponding relationship with the particle size of lithium iron phosphate, the larger the specific surface area, the smaller the lithium iron phosphate particles, and the lithium iron phosphate particles that are too small will increase the risk of agglomeration between the lithium iron phosphate particles in the homogenization process, resulting in too large a rebound in the standing viscosity of the positive electrode slurry, affecting the coating effect; and the smaller the specific surface area, the larger the lithium iron phosphate particles, and the kinetics of the lithium iron phosphate particles is insufficient, which is not conducive to the migration of lithium ions in the lithium iron phosphate particles, i.e. the polarization will be larger, affecting the development of the charge and discharge capacity of the material, therefore, by controlling the BET of the lithium iron phosphate particles, the risk in the homogenization process is reduced, while the reaction kinetics is ensured.

[0053] The BET of the above lithium iron phosphate particles can be directly tested by the nitrogen adsorption-desorption method, or can be obtained by weighted calculation of the BET of the spherical particles, and the BET of the flaky particles and / or the BET of the polycrystalline particles in the single crystal particles included in the lithium iron phosphate particles. The BET values obtained by the two implementation manners are close. The following takes the weighted calculation of the BET of the lithium iron phosphate particles as an example for description:

[0054] First, the mass ratio a1 of the spherical particles in the single crystal particles to the lithium iron phosphate particles, the mass ratio a2 of the flaky particles in the single crystal particles to the lithium iron phosphate particles, and the mass ratio a3 of the polycrystalline particles to the lithium iron phosphate particles are determined, as well as the BET1 of the spherical particles in the single crystal particles, the BET2 of the flaky particles in the single crystal particles, and the BET3 of the polycrystalline particles. Then, the mass ratios are weighted and summed as the weight values of the corresponding specific surface areas BET, and the BET of the lithium iron phosphate particles is obtained. For details, see the following formula: BET = a1 x BET1 + a2 x BET2 + a3 x BET3.

[0055] Wherein, a1 + a2 + a3 = 1, 0 < a1 ≤ 1, 0 ≤ a2 < 1, 0 ≤ a3 < 1. Preferably, 0.7 ≤ a1 ≤ 0.9, 0.05 ≤ a2 ≤ 0.1, 0.05 ≤ a3 ≤ 0.2.

[0056] For example, when the lithium iron phosphate particles are composed of polycrystalline particles and spherical particles in the single crystal particles. Wherein, the specific surface area of the spherical particles is 12.5 m 2 / g, a1 = 0.8, the specific surface area of the polycrystalline particles is 7.5 m 2 / g, a3 = 0.2, then the specific surface area BET of the lithium iron phosphate particles is 12.5*0.8 + 7.5*0.2 = 11.5 m 2 / g.

[0057] It should be noted that when a2 is set to 0, the positive electrode material layer does not contain flaky particles in the single crystal particles, and no lithium iron phosphate particles are added, that is, the positive electrode sheet only includes spherical particles; since the mixing ratio of spherical particles and polycrystalline particles (i.e., the corresponding a1, a3) will affect the compaction density of the mixed lithium iron phosphate particles, when the a3 of the polycrystalline particles in the positive electrode sheet is relatively high or relatively low, the specific surface area of the lithium iron phosphate particles needs to be controlled within the corresponding range to balance the compaction density of the lithium iron phosphate particles, so as to improve the DCR performance of the battery and avoid the risk of capacity reduction of the battery.

[0058] At the same time, in order to avoid the effective filling and cooperation of the gaps between the lithium iron phosphate particles on the positive electrode current collector in the positive electrode material layer due to the absence of spherical particles, thereby causing the agglomeration of lithium iron phosphate particles, too large CR, and the risk of WE too large, a1≠0 in the embodiments of the present application.

[0059] Understandably, the lithium iron phosphate particles in the embodiments of the present application can be obtained by mixing the corresponding spherical particles, and flaky particles and / or polycrystalline particles according to their respective preset mass ratios a1, a2, a3.

[0060] The BET of the spherical particles in the single crystal particles is 10-14 m 2 / g; the volume median particle size D V 50 is 1.0-1.6 μm. The BET of the flaky particles in the single crystal particles is 14-18 m 2 / g; the volume median particle size D V 50 is 1.0-2.0 μm. The BET of the polycrystalline particles is 6-10 m 2 / g; the volume median particle size D V 50 is 2-8 μm.

[0061] The molecular expression of the lithium iron phosphate particles can be LiFe X Mn 1-X PO4, wherein X ranges from 0.2≤X≤1. Specifically, X can be set to 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1 or a range composed of any two of the above values. In an embodiment, in order to further improve the specific capacity and other performances of the lithium iron phosphate particles, the lithium iron phosphate particles can also contain trace amounts of doping elements. Exemplarily, the doping elements are Ti elements and / or V elements.

[0062] The content of Ti elements in the lithium iron phosphate particles is 2000-6000 ppm, and the content of V elements is less than or equal to 1000 ppm, based on the mass of the lithium iron phosphate particles.

[0063] Further, to improve the utilization of the positive electrode sheet and avoid the risk of belt breakage caused by excessive WE, thereby avoiding the situation of the electrode sheet being scrapped at the corresponding position, in an embodiment, the weight elongation rate WE of the positive electrode sheet is selected from 0.5% to 3%. Exemplarily, the WE can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.8%, 1.9%, 2%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3%, or a range selected from any two of the above. When the single crystal particles include flaky particles, and / or the lithium iron phosphate particles include polycrystalline particles, the gap between the lithium iron phosphate particles in the positive electrode material layer increases, and the lithium iron phosphate particles move a greater distance when rolled. In particular, polycrystalline particles with better sphericity exhibit obvious fluidity, and when they are subjected to the pressure of rolling, the lithium iron phosphate particles in the positive electrode material layer move outward, and the lithium iron phosphate particles in contact with the positive current collector exert a greater force on the positive current collector, thereby easily causing the positive current collector to break: the positive current collector breaks. Understandably, the greater the weight elongation rate WE, the greater the risk of the positive current collector breaking.

[0064] Further, to improve the cycle performance of the positive electrode material layer in the positive electrode sheet, in an embodiment, the surface of the lithium iron phosphate particles has a carbon coating layer. The carbon coating layer can partially coat or completely coat the surface of the lithium iron phosphate particles.

[0065] The mass fraction of the carbon coating layer is 0.1 to 3 wt% based on the mass of the lithium iron phosphate particles with the carbon coating layer. The carbon coating layer can be a coating layer composed of elemental carbon.

[0066] Exemplarily, the mass fraction of the carbon coating layer is 0.1 wt%, 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, or a range selected from any two of the above.

[0067] To further improve the low-temperature DCR performance of the positive electrode sheet and the battery, in an embodiment, the positive electrode material layer further includes a conductive agent, thereby improving the effective conductivity of the positive electrode sheet through the conductive agent and improving the low-temperature DCR performance of the positive electrode sheet and the battery. Specifically, the mass percentage of the conductive agent is 0.7 to 1.1 wt% based on the mass of the positive electrode material layer. Exemplarily, the mass percentage of the conductive agent is 0.7 wt%, 0.8 wt%, 0.9 wt%, 1.0 wt%, 1.1 wt%, or a range selected from any two of the above.

[0068] To further improve the cycle performance and service life of the positive plate and the battery, in an embodiment, the positive electrode material further comprises a binder. The mass percentage of the binder is 1.6-2.0 wt% based on the mass of the positive electrode material layer. For example, the mass percentage of the binder can be 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt%, 2.0 wt%, or a range selected from any two of the above.

[0069] Optionally, the conductive agent includes but is not limited to one or more of conductive carbon black, carbon nanotubes, and carbon nanofibers.

[0070] Optionally, the binder includes but is not limited to one or more of polyvinylidene fluoride, polyvinylidene fluoride, styrene butadiene rubber, and sodium carboxymethyl cellulose.

[0071] Optionally, the positive current collector includes but is not limited to at least one of pure aluminum foil, carbon-coated aluminum foil, and composite aluminum foil. Further, a solvent is also needed in the process of preparing the positive plate, and the solvent configuration in this embodiment can be N-methyl pyrrolidone.

[0072] In this embodiment, by setting the positive plate material to include spherical particles and lithium iron phosphate particles for doping, wherein the lithium iron phosphate particles include polycrystalline particles and / or flaky particles, by controlling the doping, the risk in the coating and baking process is reduced before and after the rolling of the obtained positive plate, the specific surface area of the doping components is controlled to eliminate the risk in the homogenization process, while ensuring the reaction kinetics, the weight elongation before and after rolling is controlled to eliminate the risk of broken strips in the rolling process, and the production efficiency is ensured, by controlling the doping parameters in the above doping process, the prepared battery can have high rate performance, and the problem of difficult processing in the doping process is solved.

[0073] Based on the same inventive concept, the embodiments of the present application provide a battery, which comprises the positive plate as described above.

[0074] Based on the same inventive concept, the embodiments of the present application provide a vehicle, which comprises the battery as described above.

[0075] The following embodiments are provided to explain the above-mentioned positive plate and battery in detail.

[0076] Embodiment 1

[0077] S1, according to the mass ratio of the spherical-like particles a1, the mass ratio of the flaky particles a2, and the mass ratio of the polycrystalline particles a3 in the single-crystal particles, the spherical-like particles, the flaky particles and the polycrystalline particles are taken, please refer to Table 2. And add to the stirring tank, the stirring speed is 300 rpm, the time is 120 min, until the naked eye can not see the color difference between the particles, that is, the lithium iron phosphate particles are obtained.

[0078] The above-mentioned spherical-like particles are single-crystal particles in which the spherical and spherical-like particles account for more than 60% of the total number of particles in the SEM image. The flaky particles have a first distance between two non-joined faces and a second distance between two faces perpendicular to the first distance, and the difference between the first distance and the second distance is greater than a preset threshold value; thus, the flaky particles exhibit a flaky morphology with a small first distance and a large second distance; the flaky particles are also single-crystal particles.

[0079] Step 2, the lithium iron phosphate particles are used as a positive electrode material, acetylene black and carbon nanotubes are used as conductive agents at the same time, wherein the acetylene black is one of conductive carbon blacks, and polyvinylidene fluoride is used as a binder, the four are mixed according to the mass ratio of 97:0.9:0.3:1.8 and a solvent N-methyl pyrrolidone to obtain a positive electrode slurry with a solid content of 55%, and then the positive electrode slurry is coated on both sides of the current collector aluminum foil, so that the surface density of the coated positive electrode is 295 g / m 2 .

[0080] Then, the positive electrode sheet is dried and rolled to obtain a positive electrode sheet with a surface compaction density of 2.7 g / cm 3 , and the roll gap and pressure during rolling are shown in Table 2.

[0081] In the process of preparing the positive electrode sheet, the specific surface area BET of the lithium iron phosphate particles is tested by a nitrogen adsorption specific surface area tester; the thickness of the positive electrode material layer on the positive electrode sheet before rolling and the thickness of the positive electrode material layer on the positive electrode sheet after rolling are measured respectively using a micrometer to calculate the compression ratio CR. The coated intermediate electrode sheet before rolling and the positive electrode sheet obtained by rolling are cut into small round sheets with an area of 50.24 cm 2 using a punch rounder, and weighed using an electronic balance to calculate the weight elongation rate WE. Please refer to Tables 2-3.

[0082] S2, artificial graphite, conductive carbon black, butadiene rubber and lithium carboxymethyl cellulose are mixed according to the mass ratio of 96.6:0.6:1.6:1.2 to prepare a negative electrode slurry with a solid content of 45%; and the negative electrode slurry is coated on the current collector to form a negative electrode sheet with a surface density of 130 g / m 2 .

[0083] S3, the positive electrode sheet, the negative electrode sheet, the separator and the electrolyte are prepared into a stack core through a Z-shaped laminating machine, and then the lithium ion battery is obtained through tab welding, soft package battery side sealing, top sealing, liquid injection and pre-sealing.

[0084] Examples 2-11, Comparative Examples 1-4

[0085] Different from Example 1, the BET of the flaky particles and / or the polycrystal particles contained in the lithium iron phosphate particles in S1 was changed, and / or, the content, please refer to Table 2, Table 3 for details; other aspects consistent with Example 1.

[0086] Further, the compaction density of the aforementioned positive electrode sheet can be calculated by the surface density of the positive electrode sheet (i.e. the roll-pressed electrode sheet) when testing WE, and the thickness of the positive electrode material layer on the positive electrode sheet.

[0087] For the spherical-like particles of the single crystal particles, the flaky particles, and the polycrystal particles of the single crystal particles used in the aforementioned examples and comparative examples, each satisfies the following parameters; please refer to Table 1 for details.

[0088] Table 1

[0089]

[0090] In Table 1, the SPAN is obtained by the following formula: And, the voltage window of the 0.1C charge gram capacity at the time of testing is 2.0-3.75V, and the nominal specific capacity for calculating the 0.1C charge gram capacity is 162mAh / g.

[0091] In addition, the spherical-like particles, the flaky particles, and the polycrystal particles in S1 in the above examples and comparative examples all have a carbon coating layer. Among them, the mass fraction of the carbon coating layer of the spherical-like particles is 1.12wt% based on the mass of the spherical-like particles with a carbon coating layer. The mass fraction of the carbon coating layer of the flaky particles is 1.12wt% based on the mass of the flaky particles with a carbon coating layer. The mass fraction of the carbon coating layer of the polycrystal particles is 1.12wt% based on the mass of the polycrystal particles with a carbon coating layer.

[0092] Further, the batteries prepared in the above groups of examples and comparative examples were subjected to low-temperature discharge DCR tests under the same conditions, wherein the low-temperature discharge DCR test conditions are as follows:

[0093] At 25°C, the battery prepared in the embodiment was first fully charged at 0.33C, then discharged at 0.33C to 90% SOC, and then stood at -20°C for 1h to reach thermal equilibrium, and rapidly discharged at 0.4C for 10s. The voltage difference before and after rapid discharge of the battery was divided by the discharge current to obtain the low-temperature discharge DCR of the battery cell at 90% SOC; and so on, data was recorded every 10% SOC, and the low-temperature discharge DCR at 90% SOC, 80% SOC, 70% SOC, 60% SOC, 50% SOC, 40% SOC, 30% SOC, 20% SOC, and 10% SOC were recorded respectively. The average value of the DCR at each SOC is the average low-temperature discharge DCR in Table 3.

[0094] Table 2

[0095]

[0096]

[0097] Note: The thickness of the positive electrode material layer after rolling in the table is the thickness of the positive electrode material layer in the positive electrode sheet in the embodiment of the present application.

[0098] Table 3

[0099]

[0100] Note: The surface density after rolling in the table is the surface density of the positive electrode material layer in the positive electrode sheet in the embodiment of the present application.

[0101] Combining the data of Examples 1 to 9 in Tables 2 to 3, it can be seen that when When the value range is 3 to 12, the positive electrode sheet is relatively easy to process, and the corresponding low-temperature DCR performance is relatively high. This indicates that the compression ratio CR, specific surface area BET, and weight elongation WE are coordinated and controlled. At this time, the risk of difficulties in homogenization and tape breakage during positive electrode sheet processing is low, and positive electrode sheet production can be efficient.

[0102] It can be further seen from Examples 1 to 4 that as the content of flaky particles or polycrystalline particles other than spherical particles in the lithium iron phosphate particles increases, The value of gradually decreases, and its low-temperature DCR performance is improved. At the same time, due to the appropriate values ​​of BET, CR, and WE, the agglomeration risk and the risk of band breakage during the homogenization process in the preparation of the positive electrode sheet can be controlled, and the processing difficulty is also reduced simultaneously.

[0103] Combining Examples 7 to 9, it can be seen that as the content of flaky particles in the lithium iron phosphate particles increases, the specific surface area and compression ratio of the lithium iron phosphate particles gradually decrease, and the weight elongation of the positive electrode sheet gradually increases. This is mainly because the flaky particles themselves have a small particle size and a high specific surface area. At the same time, The value shows a gradually decreasing trend, and the corresponding low-temperature DCR performance deteriorates; and the corresponding processing difficulty increases simultaneously.

[0104] Continuing to observe the data in Table 3, we can see that when The value of is between 3.3 and 5.5, which shows better low temperature DCR performance. The value of tends to the endpoint value, and The further the value is from 3.3 to 5.5, the more obvious the deterioration of its low-temperature DCR performance is. In addition, it is very easy to agglomerate during the homogenization process and the belt is easily broken during the rolling process, which makes the processing of the positive electrode sheet very difficult.

[0105] Although preferred embodiments of the present invention have been described, those skilled in the art may make additional changes and modifications to these embodiments once they become aware of the basic creative concepts. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the embodiments of the present invention.

[0106] Obviously, those skilled in the art may make various changes and modifications to this application without departing from the spirit and scope of this application. Thus, if these modifications and variations of this application fall within the scope of the claims of this application and their equivalents, this application is intended to include these modifications and variations.

Claims

1. A positive electrode sheet, characterized in that: A positive electrode sheet comprising a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector; the positive electrode material layer comprises lithium iron phosphate particles; the lithium iron phosphate particles comprise single crystal particles, and the single crystal particles comprise spherical particles; wherein the positive electrode sheet satisfies: Wherein, the CR is the compression ratio of the positive electrode sheet, %; the BET is the specific surface area of ​​the lithium iron phosphate particles, m 2 / g; WE is the weight elongation of the positive electrode sheet, %.

2. The positive electrode sheet according to claim 1, characterized in that The lithium iron phosphate particles further include polycrystalline particles; and / or the single crystal particles include flaky particles.

3. The positive electrode sheet according to claim 1, characterized in that The surface density of the positive electrode sheet is in the range of 100 to 400 g / m 2 ; and / or, the compaction density of the positive electrode sheet is in the range of 2.3 to 2.8 g / cm 3 .

4. The positive electrode sheet according to any one of claims 1 to 3, characterized in that: The CR ranges from 60% to 80%.

5. The positive electrode sheet according to any one of claims 1 to 3, characterized in that: The range of BET is 7 to 14 m 2 / g.

6. The positive electrode sheet according to any one of claims 1 to 3, characterized in that: The range of the WE is 0.5% to 3%.

7. The positive electrode sheet according to claim 1, wherein: The surface of the lithium iron phosphate particles has a carbon coating layer; wherein, Based on the mass of the lithium iron phosphate particles having the carbon coating layer, the mass fraction of the carbon coating layer is 0.1 to 3 wt %.

8. The positive electrode sheet according to any one of claims 1 to 3 and 7, characterized in that: The positive electrode material layer further includes a conductive agent and a binder; wherein, based on the mass of the positive electrode material layer, the mass percentage of the conductive agent is 0.7-1.1 wt%, and the mass percentage of the binder is 1.6-2.0 wt%.

9. A battery, characterized in that: The battery comprises the positive electrode sheet according to any one of claims 1 to 8.

10. A vehicle, characterized in that: The vehicle includes the battery described in claim 9.

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