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 positive pole piece comprises a first lithium iron phosphate particle and a second lithium iron phosphate particle;the particle diameter of the first lithium iron phosphate particle is greater than or equal to 1 micrometer, and the particle diameter of the second lithium iron phosphate particle is less than or equal to 0.9 micrometer;the particle diameter D n50 of the first lithium iron phosphate particle is 1.1-3.0 micrometers, and the particle diameter D n50 of the second lithium iron phosphate particle is 0.25-0.65 micrometers;the positive pole piece satisfies 2<=b<=9;the b is the ratio of the areas of the exothermic characteristic peaks of different positions of the positive pole piece in the DSC test.The two lithium iron phosphate particles with different sizes are introduced to be compounded, and the thermal decomposition characteristic parameters of the positive pole piece are simultaneously controlled, so that the low-temperature energy retention rate of the secondary battery is improved, the thermal stability is good, and the ideal high-temperature cycle stability performance is simultaneously achieved.

Description

TECHNICAL FIELD

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

[0002] The lithium iron phosphate system battery has high safety and can achieve good cycle performance at high temperature. However, the performance of the battery at low temperature still needs to be improved, which results in a low energy retention rate of the battery at low temperature. The particle size of the lithium iron phosphate active material in the battery is refined to shorten the lithium ion transmission path to some extent, improve the kinetic performance, and thus improve the energy retention rate at low temperature. However, this will increase the side reaction between the lithium iron phosphate particles and the electrolyte in the battery at high temperature, which will result in poor cycle performance of the battery at high temperature. SUMMARY

[0003] The present application aims to overcome the deficiencies in the prior art and provides a positive electrode sheet. By introducing two different sizes of lithium iron phosphate particles for compounding and simultaneously controlling the thermal decomposition characteristic parameters of the positive electrode sheet, the low-temperature energy retention rate of the secondary battery can be effectively improved, the thermal stability is good, and the ideal cycle stability at high temperature is achieved.

[0004] To achieve the above-mentioned purpose, in the first aspect of the present application, the present application provides a positive electrode sheet, the positive electrode sheet comprises a positive electrode material layer, the positive electrode material layer comprises a positive electrode active material, the positive electrode active material comprises first lithium iron phosphate particles and second lithium iron phosphate particles;

[0005] The particle diameter of the first lithium iron phosphate particles is greater than or equal to 1 μm, and the particle diameter of the second lithium iron phosphate particles is less than or equal to 0.9 μm.

[0006] The particle diameter D n50 of the first lithium iron phosphate particles is 1.1-3 μm, and the particle diameter D n50 of the second lithium iron phosphate particles is 0.25-0.65 μm.

[0007] The positive electrode sheet satisfies 2≤b≤9.

[0008] The b is the area ratio of the second exothermic characteristic peak to the first exothermic characteristic peak in the curve obtained by DSC (differential scanning calorimetry) test of the positive electrode sheet, wherein the peak position of the first exothermic characteristic peak is 200-250℃, and the peak position of the second exothermic characteristic peak is 250-350℃.

[0009] The present application has the following beneficial effects:

[0010] The application provides a positive electrode sheet, which is prepared by introducing two kinds of lithium iron phosphate particles with different sizes for compounding, and simultaneously regulating the thermal decomposition characteristic parameters of the positive electrode sheet, so that the low-temperature energy retention rate of the secondary battery can be effectively improved, and the cycle stability at high temperature can be simultaneously realized. DETAILED DESCRIPTION

[0011] To make the objectives, technical solutions, and advantages of the embodiments of the application clearer, the technical solutions in the embodiments of the application will be described below clearly and completely. Obviously, the described embodiments are only some, but not all, of the embodiments of the application. Based on the embodiments in the application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the application.

[0012] In the application, the technical features described in an open way include a closed technical solution composed of listed features, and also include an open technical solution containing listed features.

[0013] In the application, if no special description is provided, the numerical range is regarded as continuous and includes the minimum value and the maximum value of the range and each value between the minimum value and the maximum value. Further, when the range refers to an integer, each integer between the minimum value and the maximum value of the range is included. In addition, when multiple ranges are provided to describe a feature or a characteristic, the ranges can be combined. In other words, unless otherwise specified, all the ranges disclosed in the application should be understood as including any and all sub-ranges.

[0014] The application will be further described below with specific embodiments:

[0015] A positive electrode sheet, comprising a positive electrode material layer, the positive electrode material layer comprising a positive electrode active material, the positive electrode active material comprising first lithium iron phosphate particles and second lithium iron phosphate particles;

[0016] The particle diameter of the first lithium iron phosphate particles is greater than or equal to 1 μm, and the particle diameter of the second lithium iron phosphate particles is less than or equal to 0.9 μm;

[0017] The particle diameter D n50 of the first lithium iron phosphate particles is 1.1-3 μm, and the particle diameter D n50 of the second lithium iron phosphate particles is 0.25-0.65 μm;

[0018] The positive electrode sheet satisfies 2≤b≤9;

[0019] The b is the ratio of the area of the second exothermic characteristic peak to the area of the first exothermic characteristic peak in the curve obtained by performing DSC (differential scanning calorimetry) test on the positive electrode sheet, wherein the first exothermic characteristic peak has a peak position of 200-250°C, and the second exothermic characteristic peak has a peak position of 250-350°C.

[0020] In the technical solution of the present application, in order to balance the low-temperature performance and high-temperature stability of the lithium iron phosphate system positive electrode sheet when applied to a secondary battery, the positive electrode sheet is compounded with large and small size ranges of lithium iron phosphate particles, and the particle sizes D n50 The two particles can be stacked in the electrode sheet, the compaction density is balanced, the transmission path of lithium ions is shortened by the small particles, the low-temperature performance of the material can be effectively improved, and the lithium ion transmission efficiency is improved. On the other hand, the positive electrode sheet further regulates the ratio of the areas of the two exothermic characteristic peaks in the DSC test, wherein the first exothermic characteristic peak corresponds to the thermal decomposition reaction of lithium salt in the electrolyte after the positive electrode sheet is soaked in the electrolyte, and the second exothermic characteristic peak corresponds to the chemical reaction of the thermal decomposition reaction product generated by the thermal decomposition of lithium salt with the solvent and the positive electrode material particles. By regulating the area ratio of the two characteristic peaks corresponding to the two reactions, the thermal stability of the positive electrode sheet can be effectively improved, the side reaction activity between the positive electrode sheet and the electrolyte is reduced, and the cycle stability of the battery at high temperature is improved. At the same time, the lithium iron phosphate system material can also balance the low-temperature kinetic performance, and lithium ions can still be effectively deintercalated and transported under fast charging conditions, achieving ideal use performance.

[0021] It should be noted that the area of the first exothermic characteristic peak in the curve obtained by performing DSC test on the positive electrode sheet according to the present application refers to the total area of all peaks corresponding to the first exothermic characteristic peak in the peak position range of 200-250°C, and the area of the second exothermic characteristic peak in the curve obtained by performing DSC test on the positive electrode sheet according to the present application refers to the total area of all peaks corresponding to the second exothermic characteristic peak in the peak position range of 250-350°C.

[0022] In some embodiments, the b is one or a range value of any two of 2, 3, 3.5, 4, 4.5, 5, 6, 6.5, 7, 7.5, 8, and 9.

[0023] Further preferably, the b is 4-7.

[0024] As described above, when the positive electrode sheet is subjected to the DSC test, the first exothermic characteristic peak corresponds to the thermal decomposition reaction of the lithium salt, and the second exothermic characteristic peak corresponds to the exothermic reaction between the product after thermal decomposition and the solvent and the lithium iron phosphate particles. The area ratio b of the two is small, the thermal stability of the overall material is high, but it also means that the passivation effect of the positive electrode sheet is too good, the lithium ion transmission efficiency is low, the low-temperature kinetic performance is poor, especially the energy retention rate at low temperature is low. When b is large, the exothermic reaction activity between the reaction product and the solvent and the lithium iron phosphate particles is high, the heat release is large, the material cannot guarantee the cycle performance at high temperature, the reaction activity is high under high temperature conditions, the side reaction between the electrolyte increases, causing the cycle performance of the secondary battery to deteriorate. Therefore, the area ratio b needs to be limited in a specific range of 2-9, and when it is further preferably in the range of 4-7, the material can further optimize the balance of thermal stability and low-temperature kinetic performance, so that the secondary battery prepared by the positive electrode sheet can achieve better low-temperature performance and high-temperature performance. The exothermic characteristic peak area can be adjusted by the amount of carbon source added during the preparation of the positive electrode material, the doping elements, the preparation process such as grinding, calcination, crushing and the like, and can also be comprehensively controlled by the sheet level such as the positive electrode sheet area density, the compacted density, the porosity and the like.

[0025] It should be noted that the area ratio b of the two exothermic characteristic peaks in the scheme of the present application can be tested by the following test method, but is not limited to the following test method: disassemble the battery at 100% SOC, cut the positive electrode sheet into a test piece with a diameter of 4 mm, then composite the test piece with the electrolyte into a platinum crucible to assemble a battery. The amount of electrolyte used is 2 μL. The electrolyte includes a solvent and a lithium salt. The solvent is an organic solvent compounded by EC (ethylene carbonate), EMC (methyl ethyl carbonate) and DMC (dimethyl carbonate) in a mass ratio of 1:1:2. The lithium salt is lithium hexafluorophosphate and lithium bisfluorosulfonylimide (mass ratio 8:2), and the concentration in the electrolyte is 1 mol / L. The battery is placed in a DSC test cavity in a nitrogen atmosphere at a rate of 5 K / min, and the test temperature range is 25-500°C. Specifically, the positive electrode sheet is in a secondary battery, which needs to be charged to 100% SOC. Specifically, the secondary battery is discharged to 0% SOC at 0.33C, then charged to 3.65V at 0.33C, and the cutoff current is less than 0.05C, i.e. the battery is charged to 100% SOC. Then the secondary battery is disassembled, the obtained positive electrode sheet is soaked in dimethyl carbonate for 2h, dried, and then tested according to the above method.

[0026] In some embodiments, the second exothermic characteristic peak includes a third exothermic characteristic peak and a fourth exothermic characteristic peak, the third exothermic characteristic peak has a peak position of 250-300°C, and the fourth exothermic characteristic peak has a peak position of 300-350°C.

[0027] Further preferably, the difference between the temperature corresponding to the highest value of the third exothermic characteristic peak and the temperature corresponding to the highest value of the fourth exothermic characteristic peak is 30-60°C.

[0028] In the second exothermic characteristic peak, two sub-peaks are included, which correspond to some products produced by thermal decomposition of lithium salt after the tab is soaked in electrolyte (for example, lithium hexafluorophosphate produces reaction products such as PF5, LiF, and HF after thermal decomposition). These products not only react with the vaporized solvent, but also react with lithium iron phosphate particles, thereby producing different exothermic stages, mainly corresponding to two sub-peaks. When the temperature difference corresponding to the highest values of the two sub-peaks is preferably within the above range, not only can the low-temperature kinetic performance of the material be guaranteed, but the thermal stability of the overall material can also be controlled at a relatively optimal level, so that the positive electrode tab applied to the secondary battery has better compatibility of low-temperature performance and high-temperature performance.

[0029] Further preferably, the temperature corresponding to the highest value of the third exothermic characteristic peak is 255-295°C, and the temperature corresponding to the highest value of the fourth exothermic characteristic peak is 300-345°C.

[0030] In some embodiments, the particle diameter of the first lithium iron phosphate particles is 1.1-3 μm, and the particle diameter of the second lithium iron phosphate particles is 0.25-0.65 μm.

[0031] In some embodiments, the particle diameter D n50 of the first lithium iron phosphate particles is one of 1.1 μm, 1.2 μm, 1.3 μm, 1.35 μm, 1.4 μm, 1.5 μm, 1.6 μm, 2 μm, 2.2 μm, 2.5 μm, 2.8 μm, 3 μm, or a range value of any two thereof.

[0032] Further preferably, the particle diameter D n50 of the first lithium iron phosphate particles is 1.2-2.5 μm.

[0033] In the present application, the first lithium iron phosphate particles with a larger size in the positive electrode tab mainly serve as a skeleton support, and the particle diameter D n50 of the first lithium iron phosphate particles is related to the kinetics of the material and the compaction density of the tab. When the particle diameter D n50 of the first lithium iron phosphate particles is within the above range, the compaction density of the positive electrode tab is higher, and the degree of side reaction between the positive electrode tab and the electrolyte can be reduced, thereby improving the cycle stability of the battery under high-temperature environment.

[0034] In some embodiments, the particle diameter D n50is one of 0.25 μm, 0.3 μm, 0.35 μm, 0.4 μm, 0.45 μm, 0.5 μm, 0.55 μm, 0.6 μm, 0.65 μm, or a range value of any two of them.

[0035] Further preferably, the particle diameter D of the second lithium iron phosphate particles is in the range of 0.1-0.2 μm. n50 is in the range of 0.35-0.5 μm.

[0036] In the technical solution of the present application, the particle diameter D of the second lithium iron phosphate particles with smaller size in the positive electrode tab is in the range of 0.1-0.2 μm. n50 will have a certain impact on the overall performance of the material, when the particle diameter D n50 increases, the size difference of the overall lithium iron phosphate particles becomes smaller, the classification gap becomes smaller, the path of lithium ion transmission becomes longer, and thus the low-temperature kinetic performance of the material is affected, and if the particle diameter D n50 decreases, the specific surface area of the overall material increases, the by-products of the electrolyte are more likely to react with it, and the chemical stability of the material at high temperature decreases, and when the particle diameter D of the second lithium iron phosphate particles is preferably in the above range, the low-temperature performance and thermal stability performance of the positive electrode tab prepared from the material can be further improved. n50

[0037] In some embodiments, the particle diameter D of the first lithium iron phosphate particles is in the range of 1-2 μm, and the particle diameter D of the second lithium iron phosphate particles is in the range of 2.5-3.8 μm. n10 n90

[0038] In some embodiments, the particle diameter D of the second lithium iron phosphate particles is in the range of 0.1-0.2 μm, and the particle diameter D of the second lithium iron phosphate particles is in the range of 0.28-0.63 μm. n10 n90

[0039] In some embodiments, the first lithium iron phosphate particles are at least one of spherical particles, spheroidal particles, and ellipsoidal particles.

[0040] It should be noted that in the technical solution of the present application, the particle diameters D n50 , D n10 , and D n90 ​​​​​The test method can be, but is not limited to, the following: the secondary battery in an empty state is disassembled in advance, the positive electrode sheet obtained is soaked in dimethyl carbonate (DMC) at room temperature 25℃ for 2h, dried, then the active material layer is scraped off as a sample, the sample is fixed with conductive glue for CP argon ion polishing, plated with a conductive film, placed on a sample stage and observed under a scanning electron microscope (SEM), adjusted to 10Kx magnification to select three areas for photography, then the first lithium iron phosphate particles and the second lithium iron phosphate particles are determined by particle diameter size, wherein the first lithium iron phosphate particles are larger in size, and the second lithium iron phosphate particles are rod-shaped particles and smaller in size, then the particle size of all particles in each area is measured, and the statistical results of the first lithium iron phosphate particles and the second lithium iron phosphate particles are classified, and the D n50 , the D n10 , and the D n90 of the number distribution of 90% are calculated.

[0041] In some embodiments, the positive electrode active material is further provided with a carbon coating layer.

[0042] Further preferably, the carbon source of the carbon coating layer can be at least one of glucose, sucrose, microcrystalline sugar, and organic polymers.

[0043] Further preferably, the thickness of the carbon coating layer is 1-10nm.

[0044] By coating the two different sizes of lithium iron phosphate particles with a carbon-containing shell, the conductivity of the overall material can be effectively improved, the conductivity of the overall material can be improved, and the energy retention of the battery in a low-temperature environment can be improved. However, the introduction of the carbon shell also increases the specific surface area of the material, and the contact area between the by-products generated by the electrolyte and the lithium iron phosphate particles increases in a high-temperature environment, and the degree of side reaction increases (for example, lithium hexafluorophosphate contained in the electrolyte will further contact and react with the carbon shell after pyrolysis to generate secondary by-products such as carbon fluoride), thereby affecting the cycle stability of the battery at high temperature. When the thickness of the carbon shell coating layer in the positive electrode active material is preferably within the above range, the overall material can balance the conductivity and thermal stability, especially the thermal stability at high temperature.

[0045] In some embodiments, the positive electrode active material further comprises a doping element, and the doping element comprises at least one of Ti, V, and Al.

[0046] Further preferably, the concentration of the doping element is 1500-5500ppm.

[0047] The doping of the above-mentioned doping elements to the lithium iron phosphate particles can improve the kinetic performance of the two kinds of lithium iron phosphate particles, thereby improving the low-temperature energy retention rate of the corresponding secondary battery. However, in a high-temperature environment, the ordered olivine structure of lithium iron phosphate tends to transform into a disordered defect structure, and the excessive introduction of doping elements will exacerbate the lattice distortion of the material and release lattice energy. In addition, the doping elements will also migrate, which will also release heat, ultimately reducing the thermal stability of the material and the high-temperature cycle stability of the battery. When the concentration of the doping elements is preferably within the above-mentioned range, the lithium ion transmission performance of the overall material can be improved, and the thermal stability is not reduced, thereby achieving better electrochemical performance.

[0048] In some embodiments, the compaction density of the positive electrode sheet is 2.45-2.75 g / cm 3 .

[0049] In some embodiments, the areal density of the positive electrode sheet is 300-600 g / m 2 . In the technical solution of the present application, the compaction density of the positive electrode sheet affects the wetting effect of the positive electrode active material and the electrolyte, thereby changing the reaction activity of the positive electrode active material and the electrolyte at high temperature. When the compaction density of the positive electrode sheet is preferably within the above-mentioned range, the positive electrode material can reduce the reaction activity of the electrolyte by-product at high temperature while maintaining high lithium ion transmission efficiency, thereby achieving higher thermal stability and better cycle stability of the battery in a high-temperature environment.

[0050] It should be noted that the positive electrode sheet of the present application can be tested for compaction density by the following method, but is not limited to the following method. The specific method is as follows: disassemble the secondary battery in an empty state to obtain a positive electrode sheet, soak the positive electrode sheet in DMC at room temperature (25℃) for 60 min, take out and dry; use a punching machine to punch the pretreated positive electrode sheet into a circular sheet with a fixed area, the area is denoted as S0, take three circular sheets as parallel samples, and use an electronic balance to weigh the mass of the three circular sheets, take the average value and denote it as M1; use a micrometer to test the thickness of the active material layer (i.e. the thickness after removing the current collector) in the three circular sheets, take the average value and denote it as H, and finally add an appropriate amount of deionized water on the three circular sheets, and gently wipe off the coating on the circular sheet with a dust-free paper to expose the copper foil, and stand at room temperature (or dry) for 10 min, then weigh the mass of the three copper foils, take the average value and denote it as M0, and calculate the compaction density A of the positive electrode sheet according to the following formula: A= (M1-M0) / (H*S0), and the areal density B= (M1-M0) / S0.

[0051] In some embodiments, the mass ratio of the first lithium iron phosphate particles to the second lithium iron phosphate particles is (1:9) to (9:1).

[0052] In some embodiments, the first lithium iron phosphate particles and / or the second lithium iron phosphate particles comprise LiFePO4.

[0053] Further preferably, the first lithium iron phosphate particles and / or the second lithium iron phosphate particles can further comprise a doping element, the doping element comprising at least one of Ti, V, Al, and the concentration of the doping element in the first lithium iron phosphate particles and / or the second lithium iron phosphate particles is 1500-5500 ppm.

[0054] In some embodiments, the first lithium iron phosphate particles can be commercially available products or obtained by self-made methods, and specifically, the first lithium iron phosphate particles can be prepared by a solid phase method, and specifically, the first lithium iron phosphate particles can be prepared by the following method:

[0055] The carbon source, the iron source, the phosphorus source, and the lithium source are mixed in a solvent, the obtained mixture is ground, and then spray dried, the obtained powder particles are calcined, impurities are removed, and the powder is crushed, thereby obtaining the first lithium iron phosphate particles;

[0056] In some embodiments, the solvent comprises water.

[0057] In some embodiments, the lithium source comprises at least one of lithium carbonate, lithium hydroxide, lithium carbonate, lithium nitrate, lithium oxalate, lithium dihydrogen phosphate, lithium citrate, and lithium acetate.

[0058] The phosphorus source comprises at least one of ammonium dihydrogen phosphate, lithium dihydrogen phosphate, and ammonium phosphate.

[0059] Further preferably, the phosphorus source and the lithium source can be the same, such as lithium dihydrogen phosphate.

[0060] The iron source comprises at least one of ferrous oxalate, iron hydroxide, ferrous hydroxide, iron phosphate, ferrous phosphate, iron acetate, ferrous acetate, iron carbonate, ferrous carbonate, diiron trioxide, triiron tetroxide, and iron oxalate.

[0061] The carbon source comprises at least one of glucose, sucrose, microcrystalline sugar, and an organic polymer.

[0062] In some embodiments, the grinding time is 1.5-2.5 h, and the rotation speed is 400-500 r / min.

[0063] In some embodiments, the spray drying pressure is 0.4-0.7 MPa, the inlet temperature is 200-290°C, and the outlet temperature is 80-120°C.

[0064] In some embodiments, the calcination temperature is 730-820°C, the heating rate is 3-5°C / min, and the calcination time is 6-10 h.

[0065] In some embodiments, the pulverization is performed by air flow pulverization, and the pressure during the air flow pulverization is 300-500 KPa.

[0066] It should be noted that in the present application, the particle size of the first lithium iron phosphate particles and the thermal decomposition characteristic parameters of the positive electrode sheet can be controlled by parameter setting during grinding, calcination and pulverization in the preparation process, but are not limited thereto.

[0067] In some embodiments, the second lithium iron phosphate particles can be a commercially available product or obtained by a self-made method. Specifically, the second lithium iron phosphate particles can be obtained by a hydrothermal method, and are prepared by the following method:

[0068] The phosphorus source and the lithium source are mixed and reacted, the obtained product is uniformly mixed with the iron source, and then hydrothermal reaction is performed to obtain iron phosphate particles. Subsequently, the iron phosphate particles are mixed with a carbon source, calcined, ground, spray dried, impurity-removed, and pulverized to obtain the second lithium iron phosphate particles.

[0069] In some embodiments, the lithium source includes at least one of lithium hydroxide and lithium carbonate; and the phosphorus source includes phosphoric acid.

[0070] In some embodiments, the pressure during the hydrothermal reaction is 1.5-2.5 MPa, the temperature is 180-220℃, the time is 0.2-1 h, and the pH of the system is 5-7.

[0071] In some embodiments, the temperature during the calcination is 600-720℃, and the time is 2-4 h.

[0072] In some embodiments, the time during the grinding is 1.5-2.5 h, and the rate is 400-500 r / min.

[0073] In some embodiments, the pressure during the spray drying is 0.4-0.7 MPa, the inlet temperature is 200-290℃, and the outlet temperature is 80-120℃.

[0074] In some embodiments, the pulverization is performed by air flow pulverization, and the frequency during the air flow pulverization is 40-65 Hz.

[0075] It should be noted that in the present application, the particle size of the second lithium iron phosphate particles and the thermal decomposition characteristic parameters of the positive electrode sheet can be controlled by parameter setting during the hydrothermal reaction, grinding, calcination and pulverization in the preparation process, but are not limited thereto.

[0076] In some embodiments, the positive electrode material layer in the positive electrode tab includes a positive electrode active material, a binder, and a conductive agent, the positive electrode active material includes first lithium iron phosphate particles and second lithium iron phosphate particles, and the mass percentage of the positive electrode active material in the positive electrode material layer is 94.5-98.5%.

[0077] In some embodiments, the binder is used to improve the adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector, and any binder can be used without particular limitation as long as it has suitable binder properties and does not significantly cause adverse chemical changes in the battery. For example, the binder includes fluorine-containing polyolefin-based binders, which include but are not limited to polyvinylidene fluoride (PVDF), a polyvinylidene fluoride copolymer, or a modified (e.g., carboxylic acid, acrylic acid, acrylonitrile, etc.) derivative thereof.

[0078] In particular, the binder is selected from polytetrafluoroethylene or polyvinylidene fluoride.

[0079] In some of the embodiments, the mass percentage of the binder in the positive electrode material layer is 1.3-3%, such as 1.3%, 1.5%, 1.8%, 2.0%, 2.2%, 2.5%, 2.8%, 3%, or a range formed by any two of the above values.

[0080] In some embodiments, the conductive agent is used to provide electrical conductivity, and any conductive agent can be used without particular limitation as long as it has suitable electronic conductivity and does not significantly cause adverse chemical changes in the battery. Exemplarily, the conductive agent in the positive electrode active material layer includes but is not limited to at least one of carbon nanotubes, carbon black, graphite, carbon fibers, activated carbon, mesoporous carbon, and fullerenes, wherein the carbon fibers are, for example, carbon nanofibers, etc.; the carbon black is, for example, SP (Super P), acetylene black, Ketjen black, etc.

[0081] In some of the embodiments, the mass percentage of the conductive agent in the positive electrode material layer is 0.5-2.5%, such as 0.5%, 0.8%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 2%, 2.2%, 2.5%, or a range formed by any two of the above values.

[0082] In some embodiments, the porosity of the positive electrode tab is 20-30%.

[0083] It should be noted that the porosity of the positive electrode tab described in the present application can be confirmed by, but not limited to, the following method: disassemble the secondary battery under open circuit to obtain the positive electrode tab, soak the positive electrode tab in DMC at room temperature for 60 min, take out, dry, and then use the tab punching machine to cut the tab into a circular piece with a diameter of 12 mm, and at the same time, use a thickness gauge to measure the thickness of the tab and the current collector, respectively, h1 and h2; weigh the mass of the tab with a precision of 0.00001 g, denoted as m1; calculate the volume v of the cut tab according to the formula v = πr 2 (h1-h2); soak the tab in a closed container with a certain volume of hexadecane for 1 h (the volume of hexadecane in the closed solution is not required, but the required amount must be able to ensure that the tab is completely immersed therein); after 1 h, take out the tab with tweezers and place it on filter paper to dry to a constant weight (generally, the time to dry to a constant weight is 1 h); weigh the mass of the tab with a balance, denoted as m2. The porosity is calculated according to the formula X / v, where X = (m2-m1) / p, p is the density of hexadecane 0.7734 g / cm 3 .

[0084] The porosity of the positive electrode tab is mainly determined by the positive electrode material layer, which has an effect on the wettability of the electrolyte, the ion / electron conduction efficiency, and further affects the low-temperature energy and high-temperature stability of the secondary battery. When the porosity of the material layer is preferably within the above range, the energy retention rate of the secondary battery under low-temperature environment is better, and the thermal stability is also better. Those skilled in the art can adjust the low-temperature performance and high-temperature performance of the secondary battery by adjusting the size of the porosity according to actual needs, or other means can be used for adjustment, which is not limited herein.

[0085] The present application also discloses a secondary battery comprising the positive electrode tab described in the present application.

[0086] In some embodiments, the secondary battery further comprises a negative electrode tab and an electrolyte.

[0087] In some embodiments, the electrolyte comprises an additive, a solvent and a lithium salt.

[0088] In some embodiments, the solvent comprises at least one of a carbonate solvent, a carboxylic acid ester solvent, an ether solvent, a sulfone solvent, a nitrile solvent, and a phosphate solvent.

[0089] Exemplarily, the carbonate-based solvent includes, but is not limited to, at least one of propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC); the carboxylic acid ester-based solvent includes, but is not limited to, at least one of ethyl acetate, methyl formate, and 1,4-butyrolactone; the ether-based solvent includes at least one of dimethyl tetrahydrofuran, tetrahydrofuran, and 1,2-dimethoxyethane; the sulfone-based solvent includes at least one of methyl sulfone and dimethyl sulfoxide; the nitrile-based solvent includes at least one of propionitrile, butyronitrile, 1-(2-cyanoethyl)pyrrole, and 1,3,6-hexanetricarbonitrile; and the phosphate-based solvent includes at least one of trimethyl phosphate and triethyl phosphate.

[0090] Further preferably, the electrolyte solution includes a carboxylic acid ester-based solvent.

[0091] Further preferably, the mass percentage content of the carboxylic acid ester-based solvent in the electrolyte solution is 20-70%.

[0092] The use of the electrolyte solution with the carboxylic acid ester-based solvent in the above preferred content to compound the positive electrode sheet of the application can ensure that the viscosity of the electrolyte solution is moderate, which is conducive to keeping the transmission rate of lithium ions at a high level, so that the kinetics of the secondary battery at low temperature is more optimal. Meanwhile, after infiltrating the positive electrode sheet, the electrolyte solution has a smaller degree of side reaction with the positive electrode sheet and high thermal stability.

[0093] In some embodiments, the lithium salt includes at least one of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetrafluoroborate, lithium bisoxalate borate, lithium difluorobisoxalate borate, lithium triflate, lithium bisfluoromethanesulfonimide, lithium bis-trifluoromethanesulfonimide, lithium difluorobisoxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0094] In some embodiments, the additive includes, but is not limited to, at least one of vinylene carbonate, fluoroethylene carbonate, ethylene sulfate, lithium difluorobisoxalate borate, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)borate, and lithium difluorobisoxalate phosphorus phosphate.

[0095] In some embodiments, the negative electrode sheet includes a negative electrode active material layer, and the negative electrode material layer includes a negative electrode active material, which includes at least one of natural graphite, artificial graphite, mesocarbon microbeads, hard carbon, soft carbon, elemental silicon, silicon monoxide, silicon-carbon composite material, and lithium titanate.

[0096] Further preferably, the negative electrode active material includes at least one of artificial graphite and natural graphite, and the particle size D50 of the negative electrode active material is 5-15 μm. v50

[0097] ​The graphite is used as the negative active material, and the particle size of the graphite is preferably in the above range. The use of the graphite can effectively improve the efficiency of lithium ion conduction, improve the energy retention rate of the battery in a low temperature environment, and avoid excessive side reactions between the negative electrode and the electrolyte, thereby improving the cycle stability of the battery in a high temperature environment.

[0098] The negative material layer can further include a conductive agent and / or a binder.

[0099] The conductive agent in the negative active material layer is used to provide conductivity. Any conductive agent can be used without particular limitation, as long as it has suitable electronic conductivity and does not significantly cause adverse chemical changes in the battery. Illustratively, the conductive agent includes, but is not limited to, at least one of carbon nanotubes, carbon black, graphite, carbon fibers, activated carbon, mesoporous carbon, and fullerenes, wherein the carbon fibers are, for example, carbon nanofibers and the like; the carbon black is, for example, SP, acetylene black, Ketjen black, and the like.

[0100] In some embodiments, the mass percentage of the conductive agent in the negative material layer is 0.4% to 2%, such as 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, or a range formed by any two of the above values.

[0101] The binder in the negative active material layer is used to improve the adhesion between the negative active material particles and the adhesion between the negative active material and the negative current collector. Any binder can be used without particular limitation, as long as it has suitable binder properties and does not significantly cause adverse chemical changes in the battery. Illustratively, the binder includes, but is not limited to, at least one of carboxymethyl cellulose (CMC), styrene butadiene rubber, polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl butyral, and aqueous acrylic resin.

[0102] In some embodiments, the mass percentage of the binder in the negative material layer is 1.0% to 4.5%, such as 1.0%, 1.2%, 1.5%, 1.8%, 2.0%, 2.2%, 2.5%, 2.7%, 3.0%, 3.2%, 3.5%, 3.8%, 4.0%, 4.2%, 4.5%, or a range formed by any two of the above values.

[0103] In some embodiments, the negative active material layer in the negative electrode sheet includes a negative active material, a binder, and a conductive agent, and the mass percentage of the negative active material in the negative active material layer is 70% to 99%.

[0104] The application is further described below with specific examples, which cannot be understood as limiting the scope of the application claimed:

[0105] Example 1

[0106] A positive electrode tab, a secondary battery, and a preparation method of the secondary battery, comprising the following steps:

[0107] (1.1) Preparation of the first lithium iron phosphate particles: a mixture of lithium dihydrogen phosphate, ferrous oxalate, glucose and polyethylene glycol (10.6wt% of the total mass of lithium dihydrogen phosphate and ferrous oxalate, the mass ratio of glucose to polyethylene glycol is 1:2) is mixed in water, an oxide of a doping element is added, and then wet grinding is carried out at a speed of 450 r / min for 1.8 h to a suitable particle size, followed by atomization in a spray drying device (pressure 0.5 MPa, inlet temperature 280℃, outlet temperature 90℃), the obtained particles are calcined in a tube furnace under nitrogen atmosphere at a temperature rising rate of 5℃ / min to 790℃ for 8 h, magnetic separation is carried out, and air crushing is carried out at a pressure of 420 KPa to obtain the first lithium iron phosphate particles; the particle diameter of the first lithium iron phosphate particles ranges from 1 to 4 μm; the average thickness of the carbon coating layer on the particles is 5 nm;

[0108] (1.2) Preparation of the second lithium iron phosphate particles: lithium hydroxide and phosphoric acid are mixed according to the metering ratio, the obtained lithium phosphate is mixed and dispersed in water according to a molar ratio of iron to phosphorus of 1.01:1, a water-soluble salt of a doping element is added, the obtained material is subjected to hydrothermal reaction at 200℃ and 2 MPa for 0.5 h by adjusting the pH, lithium iron phosphate particles are obtained, then glucose (9.3wt% of the mass of lithium iron phosphate particles) is added, grinding is carried out at 480 r / min for 2 h to a suitable particle size, then spray drying is carried out (pressure 0.5 MPa, inlet temperature 280℃, outlet temperature 90℃), and the temperature is raised to 670℃ at a rate of 5℃ / min in a tube furnace under nitrogen atmosphere for calcination for 2.5 h, and air flow crushing is carried out at 65 Hz to obtain the rod-shaped second lithium iron phosphate particles; the particle diameter of the second lithium iron phosphate particles ranges from 0.1 to 0.9 μm, and the average thickness of the carbon coating layer on the particles is 3 nm;

[0109] (1.3) Preparation of the positive electrode tab: the first lithium iron phosphate particles and the second lithium iron phosphate particles are mixed according to a certain mass ratio as the positive electrode material, then the positive electrode material, the conductive agent SP, the conductive agent carbon nanotube and the binder polyvinylidene fluoride are dispersed in N-methylpyrrolidone according to a mass ratio of 96.5:0.65:0.35:2.5, a slurry is prepared by vacuum stirring, then the slurry is coated on the current collector aluminum foil, the coating surface density is set to 550 g / m 2 , and after drying, cold pressing and slitting, the positive electrode tab is obtained by roll pressing at a compaction density of 2.45 g / cm 3 ;

[0110] (2) Preparation of the negative electrode sheet: the negative electrode material artificial graphite, the conductive agent SP, the thickening agent sodium carboxymethyl cellulose, and the binder styrene-butadiene rubber were dispersed in water according to a mass ratio of 96.5:1:1:1.5, and a slurry was prepared by vacuum stirring, followed by coating on a current collector copper foil, with a coating surface density set to 230 g / m 2 , and the negative electrode sheet was obtained by drying, cold pressing, slitting, and roll pressing at a compacted density of 1.6 g / cm 3 ;

[0111] (3) Preparation of the electrolyte: EC (ethylene carbonate), EMC (ethyl methyl carbonate), and DEC (diethyl carbonate) were compounded as solvents according to a mass ratio of 1:1:1, and then lithium hexafluorophosphate was added to prepare an electrolyte with a lithium hexafluorophosphate concentration of 1 mol / L based on the total mass of the electrolyte;

[0112] (4) The positive electrode sheet, a commercially available PP separator, and the negative electrode sheet were stacked in sequence to assemble an electric core, the electric core was placed in an outer packaging shell, electrolyte was injected after drying, and the lithium ion secondary battery was obtained after vacuum packaging, standing, formation, and constant volume.

[0113] Examples 2-26 and Comparative Examples 1-11

[0114] A positive electrode sheet and a secondary battery, which differ from Example 1 only in that the preparation process of the positive electrode sheet is different,

[0115] The parameters of each positive electrode sheet during preparation, the results obtained by testing the products, and the parameters are shown in Tables 1-3, and the test method of each product parameter is as described above. It should be noted that in the technical solution of the present application, the total amount of the conductive agent and the binder in the positive electrode material layer of the positive electrode sheet is constant. When the content of the conductive agent is adjusted, the content of the binder is also adjusted. The ratio between SP and carbon nanotubes in the conductive agent is constant, but the present application does not limit the conductive agent to this limitation. That is, when the content of the conductive agent is adjusted by the person skilled in the art, the content of the positive active material can also be adjusted, the type of the conductive agent can also be adjusted, and the ratio of different substances in the conductive agent can also be adjusted.

[0116] Table 1

[0117]

[0118] Continued Table 1

[0119]

[0120] Table 2

[0121]

[0122] Continued Table 2

[0123]

[0124] Table 3

[0125]

[0126] Table 3 (continued)

[0127]

[0128] Effect Examples

[0129] The lithium ion batteries obtained in each example and comparative example were subjected to the following tests:

[0130] (1) Low-temperature cycle energy retention rate test:

[0131] Each secondary battery obtained in each example and comparative example was pre-stored at 45°C for 24 h after injection, fixed by a glass clamp at a pressure of 0.5 MPa, and stored for 10 min, then charged at a rate of 0.02 C for 300 min, stored for 10 min, and then charged at a rate of 0.1 C for 300 min, stored for 24 h, then charged at a rate of 0.33 C to 3.65 V (cut-off 0.05 C) at 25°C, and then discharged at a rate of 0.33 C to 2.5 V, repeated twice to constant volume, and the discharge energy W1 of the third cycle was recorded. The secondary battery was charged at a rate of 0.33 C to the upper limit voltage 3.65 V, and cut-off 0.05 C; then the secondary battery was placed in a -10°C incubator and stored for 4 h, and finally discharged at a rate of 0.33 C to 2 V, and the discharge energy W2 at this time was recorded. The low-temperature energy retention rate of the secondary battery was calculated by: energy retention rate = 100% x W2 / W1.

[0132] (2) High-temperature cycle performance test:

[0133] Each secondary battery obtained in each example and comparative example was pre-stored at 45°C for 24 h after injection, fixed by a glass clamp at a pressure of 0.5 MPa, and stored for 10 min, then charged at a rate of 0.02 C for 300 min, stored for 10 min, and then charged at a rate of 0.1 C for 300 min, stored for 24 h, then pre-stored at 60°C for 2 h, and then charged at a rate of 0.33 C to 3.65 V (cut-off 0.05 C), and then discharged at a rate of 0.33 C to 2.5 V, and the initial capacity A0 of the battery was recorded. The battery was charged at a rate of 1 C to the upper limit voltage 3.65 V, and then charged at constant voltage to cut-off 0.05 C, stored for 30 min, and then discharged at a rate of 1 C to 2.5 V, and stored for 30 min, which was one cycle. The foregoing cycle was repeated, and the discharge capacity A1 was recorded after each cycle until A1≤80% x A0, and the cycle number was recorded.

[0134] The test results of each secondary battery are shown in Table 4.

[0135] Table 4

[0136]

[0137] According to Table 4, it can be seen that:

[0138] (1) The secondary battery described in the present application is based on the specific compounding of two kinds of lithium iron phosphate particles in the positive electrode sheet and the setting of the heat release parameter b. The large and small size range of lithium iron phosphate particles in the positive electrode sheet are compounded to achieve ideal hierarchical stacking, taking into account the compaction density. By setting small particles, the transmission path of lithium ions is shortened, which not only realizes ideal low temperature performance, but also enables each secondary battery to achieve a low temperature energy retention rate of more than 70%. At the same time, the regulation of parameter b also enables the secondary battery to take into account the cycle stability at high temperature, and the test cycle number can reach more than 600 cycles, with a long cycle life at high temperature. In contrast, Comparative Examples 1 and 7 do not use two size particle compounding as the positive active material, and obviously both cannot achieve good low temperature performance, and the high temperature cycle number of the product described in Comparative Example 7 is less; although the active material in Comparative Examples 2-6 and Comparative Examples 9-11 sets a specific two-particle compounding, the size of the particles in the material is not properly set, or the parameter b is not properly regulated, and the secondary battery cannot take into account both low temperature performance and high temperature performance.

[0139] (2) Parameter b is related to the thermal stability of the positive electrode sheet after being soaked in electrolyte. If parameter b is small, the thermal stability of the overall material is good, but the passivation activity is high, which leads to a lower transmission efficiency of lithium ions. If b is large, although the transmission efficiency of lithium ions is high, the side reaction activity of the positive electrode sheet and the electrolyte in the secondary battery is also large, which makes it difficult to guarantee good high temperature performance, so b needs to be set in the specific range of 2-9, and if it is further optimized in the range of 4-7, the balance between the low temperature and high temperature performance of the secondary battery can be further optimized. On the other hand, in the positive electrode sheet, the first lithium iron phosphate particles of large size mainly act as a skeleton, and when the particle size D n50 Preferably, when it is in the range of 1.2-2.5 μm, the bulk density and structural stability of the overall material are better, and the size of the second lithium iron phosphate particles also affects the performance of the product. Based on the hierarchical gap, the reaction activity of the particles, especially the side reaction activity of the particles and the electrolyte, is different, so the particle size D n50After the preferred process, especially after the preferred process at 0.35-0.5 μm, the high-temperature performance and low-temperature performance of the secondary battery are further improved. As can be seen from Examples 1-22, when the positive electrode sheet in the secondary battery is subjected to the above preferred process, the electrochemical performance is further improved, the number of cycles under high-temperature cycling can reach about 1200 cycles at most, and the energy retention rate under low temperature can reach about 85%.

[0140] (3) In the parameter b, the first exothermic characteristic peak corresponds to the thermal decomposition reaction of lithium salt, the second exothermic characteristic peak corresponds to the exothermic reaction between the product after thermal decomposition and the solvent and lithium iron phosphate particles, and two sub-peaks are included in the second characteristic peak, which respectively correspond to some products produced by the thermal decomposition of lithium salt after the electrode sheet is soaked in electrolyte (for example, lithium hexafluorophosphate produces reaction products such as PF5, LiF and HF after thermal decomposition), these products will not only react with the vaporized solvent, but also react with lithium iron phosphate particles, thereby producing different exothermic stages, mainly corresponding to two sub-peaks. As can be seen from the comparison of the examples, when the temperature difference corresponding to the highest peaks of the two sub-peaks is preferably in the range of 30-60°C, not only can the low-temperature kinetic performance of the material be guaranteed, but also the thermal stability of the overall material can be controlled at a relatively optimal level, so that the positive electrode sheet applied to the secondary battery can balance the low-temperature performance and high-temperature performance.

Claims

1. A positive electrode plate, characterized in that, The positive electrode sheet includes a positive electrode material layer, the positive electrode material layer includes a positive electrode active material, and the positive electrode active material includes first lithium iron phosphate particles and second lithium iron phosphate particles; The diameter of the first lithium iron phosphate particle is ≥1μm, and the diameter of the second lithium iron phosphate particle is ≤0.9μm; The particle diameter D of the first lithium iron phosphate particle n50 The particle diameter D of the second lithium iron phosphate particles is 1.1~3.0μm. n50 The thickness is 0.25~0.65μm; The positive electrode plate satisfies: 2≤b≤9; b is the ratio of the area of ​​the second exothermic characteristic peak to the area of ​​the first exothermic characteristic peak in the curve obtained by DSC testing of the positive electrode, wherein the peak position of the first exothermic characteristic peak is 200~250℃ and the peak position of the second exothermic characteristic peak is 250~350℃.

2. The positive electrode sheet as described in claim 1, characterized in that, The value of b is 4 to 7.

3. The positive electrode sheet as described in claim 1, characterized in that, The second exothermic characteristic peak includes a third exothermic characteristic peak and a fourth exothermic characteristic peak. The peak position of the third exothermic characteristic peak is 250~300℃, and the peak position of the fourth exothermic characteristic peak is 300~350℃.

4. The positive electrode sheet as described in claim 3, characterized in that, The temperature difference between the highest value of the third exothermic characteristic peak and the highest value of the fourth exothermic characteristic peak is 30~60℃.

5. The positive electrode sheet as described in claim 4, characterized in that, The highest value of the third exothermic characteristic peak corresponds to a temperature of 255~295℃, and the highest value of the fourth exothermic characteristic peak corresponds to a temperature of 300~345℃.

6. The positive electrode sheet as described in claim 1, characterized in that, The particle diameter D of the first lithium iron phosphate particle n50 The particle diameter D of the second lithium iron phosphate particle is 1.2~2.5μm, and / or... n50 The thickness is 0.35~0.5μm.

7. The positive electrode sheet as described in claim 1, characterized in that, The positive electrode active material is also provided with a carbon coating layer.

8. The positive electrode sheet as described in claim 7, characterized in that, The thickness of the carbon coating layer is 1 nm to 10 nm.

9. The positive electrode sheet as described in claim 1, characterized in that, The positive electrode active material also includes doping elements, which include at least one of Ti, V, and Al.

10. The positive electrode sheet as described in claim 9, characterized in that, The concentration of the dopant element is 1500~5500ppm.

11. The positive electrode sheet as described in claim 1, characterized in that, The compaction density of the positive electrode sheet is 2.45~2.75 g / cm³. 3 .

12. The positive electrode sheet as described in claim 1, characterized in that, The mass ratio of the first lithium iron phosphate particle to the second lithium iron phosphate particle is (1:9) to (9:1).

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

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 an electrolyte, which comprises a solvent, a lithium salt, and additives; the solvent comprises at least one of carbonate solvents, carboxylic acid ester solvents, ether solvents, sulfone solvents, nitrile solvents, and phosphate ester solvents, and / or the lithium salt comprises 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, and / or the additives comprise at least one of vinylene carbonate, fluoroethylene carbonate, vinyl sulfate, lithium difluorooxalate borate, tris(trimethylsilane) phosphate, tris(trimethylsilane) borate, and lithium difluorooxalate phosphate.

16. The secondary battery as described in claim 15, characterized in that, The electrolyte contains 20-70% by mass of carboxylic acid ester solvents.

17. The secondary battery as described in claim 14, characterized in that, The secondary battery further includes a negative electrode sheet, which comprises a negative electrode active material, including at least one of artificial graphite and natural graphite, wherein the particle size D of the negative electrode active material is... v50 The value is 5~15μm.

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