CN114068920A - Lithium iron phosphate positive electrode active material, preparation method thereof, positive plate and battery - Google Patents

Lithium iron phosphate positive electrode active material, preparation method thereof, positive plate and battery Download PDF

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CN114068920A
CN114068920A CN202010782962.2A CN202010782962A CN114068920A CN 114068920 A CN114068920 A CN 114068920A CN 202010782962 A CN202010782962 A CN 202010782962A CN 114068920 A CN114068920 A CN 114068920A
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iron phosphate
active material
particle
lithium iron
positive electrode
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李二阳
徐茶清
曹文玉
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BYD Co Ltd
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
    • H01M4/5825—Oxygenated metallic salts or polyanionic structures, e.g. borates, phosphates, silicates, olivines
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/04—Processes of manufacture in general
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/136—Electrodes based on inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy
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Abstract

The application provides a lithium iron phosphate positive electrode active material, which has two peaks in a particle size distribution frequency curve counted according to volume percentage, wherein the particle size position of the highest point of a first peak is between 0.3 and 0.6 mu m, the particle size position of the highest point of a second peak is between 1.0 and 3.0 mu m, and the ratio of the integral area of the first peak to the integral area of the second peak is (0.1-1): 1. the application also provides a preparation method of the lithium iron phosphate positive active material, a positive plate and a battery. The lithium iron phosphate positive active material with the particle size in unique bimodal distribution can realize the closest packing, and the compacted density of the pole piece prepared from the lithium iron phosphate positive active material can be 2.7g/cm3The above.

Description

Lithium iron phosphate positive electrode active material, preparation method thereof, positive plate and battery
Technical Field
The invention relates to the technical field of batteries, in particular to a lithium iron phosphate positive active material, a preparation method thereof, a positive plate and a battery.
Background
Lithium ion batteries are a new generation of green high-energy batteries, and increasingly play an important role in various fields. As an important component of the lithium ion battery, the positive electrode material of the lithium ion battery is determinedThe performance, price and development of lithium batteries are in progress. Currently, the most studied positive electrode material is LiCoO2、LiNiO2、LiMn2O4、LiFePO4(lithium iron phosphate). Wherein LiCoO is concentrated in the lithium iron phosphate anode material2、LiNiO2、LiMn2O4The battery has the advantages of high structural stability, good safety performance, moderate working voltage, good platform characteristics, large theoretical capacity and the like, and gradually becomes a hot spot of competitive research of battery workers.
However, the lithium iron phosphate has a relatively obvious defect that the pole piece made of the lithium iron phosphate has low compacted density (2.1-2.3 g/cm)3) Directly results in lower energy density of the battery prepared by the material, and hinders the practical application of the material. Therefore, it is necessary to provide a lithium iron phosphate positive electrode active material having a high compaction density.
Disclosure of Invention
In view of this, the first aspect of the present application provides a lithium iron phosphate positive active material, the particle size of which is uniquely bimodal, the lithium iron phosphate positive active material can realize the closest packing, and the pole piece prepared from the lithium iron phosphate positive active material has a higher compacted density (2.7 g/cm)3Above).
In a first aspect, the present application provides a lithium iron phosphate positive electrode active material, where a particle size distribution frequency curve of the lithium iron phosphate positive electrode active material has two peaks, where the particle size distribution frequency curve is distributed in terms of volume percentage, a particle size position of a highest point of a first peak is between 0.3 μm and 0.6 μm, a particle size position of a highest point of a second peak is between 1.0 μm and 3.0 μm, and a ratio of an integral area of the first peak to an integral area of the second peak is (0.1-1): 1. wherein the particle size distribution map can be measured by a Malvern particle sizer. The abscissa of the particle size distribution frequency curve is the particle size, and the ordinate is the volume differential or volume frequency, which represents the volume percentage of each grade of powder by volume.
In other words, the lithium iron phosphate positive electrode active material of the present application includes, in a mass ratio of (0.1-1): 1, wherein the D50 particle size of the small-particle lithium iron phosphate is in the range of 0.2-0.6 μm, and the D50 particle size of the large-particle lithium iron phosphate is in the range of 1.0-3.0 μm.
The granularity of lithium iron phosphate positive active material is unique bimodal distribution in this application, represent that this lithium iron phosphate positive active material has specific mass ratio and specific granularity's tiny particle lithium iron phosphate and large granule lithium iron phosphate, can make tiny particle lithium iron phosphate fully fill the clearance between large granule lithium iron phosphate, realize the most closely packed, thereby promote the compaction density of the pole piece of being made by this lithium iron phosphate positive active material by a wide margin, and make the battery of being made by this lithium iron phosphate positive active material have higher specific capacity.
Preferably, the ratio of the integrated area of the first peak to the integrated area of the second peak is (0.1-0.8): 1. therefore, the capacity of the battery can be well reduced due to excessive large-particle lithium iron phosphate, and the compaction density can not be effectively improved due to excessive small-particle lithium iron phosphate.
Wherein the compacted density of the pole piece made of the lithium iron phosphate positive active material is 2.65g/cm3The above.
The second aspect of the present application provides a method for preparing a lithium iron phosphate positive electrode active material, including:
the specific surface area is 0.1-1m2Per gram of large-particle iron phosphate and a specific surface area of 15 to 35m2Mixing the small-particle iron phosphate per gram according to the mass ratio of (2-10) to 1, and mixing the obtained mixed iron phosphate with a lithium source, a carbon source and a solvent to obtain mixed slurry; wherein the average value of the primary particle diameters of the large-particle iron phosphate is in the range of 1-3 μm, the average value of the primary particle diameters of the small-particle iron phosphate is in the range of 0.05-0.4 μm, and the coefficient of variation of the primary particle diameters of the large-particle iron phosphate and the small-particle iron phosphate is not more than 60%;
grinding the mixed slurry, then carrying out spray drying, and roasting the obtained powder; and (3) sequentially crushing, refining and sieving the roasted material to obtain the lithium iron phosphate positive active material.
According to the preparation method provided by the second aspect of the application, when the lithium iron phosphate positive active material is prepared, large and small iron phosphates with primary particles having specific particle sizes and uniformity are mixed according to a specific proportion, so that the prepared lithium iron phosphate can inherit the basic morphology and sizes of the two iron phosphate particles during roasting, the lithium iron phosphate positive active material with the particle size in the special bimodal distribution is obtained, the closest packing can be realized, and the compaction density of a pole piece is improved.
In the present application, the "specific surface area" is a specific surface area measured using nitrogen adsorption according to the BET equation. "primary particles" refers to particles having distinct grain boundaries, and the particle size of the primary particles may be measured by Scanning Electron Microscopy (SEM), rather than by malvern granulometry or the like. The "coefficient of variation of primary particle size" is exemplified by large-particle iron phosphate, and the coefficient of variation is
Figure BDA0002620880070000031
Wherein the standard deviation σ of the primary particle diameter1Can be expressed as:
Figure BDA0002620880070000032
wherein σ1In μm, n represents the number of primary particles of large-particle iron phosphate, DiRepresenting the size of each primary particle of the large-particle iron phosphate,
Figure BDA0002620880070000033
represents the average value of the primary particle diameters of large-particle iron phosphate. Sigma1The uniformity of the primary particle size of the large iron phosphate particles can be reflected.
In the application, the variation coefficient of the primary particle size of the large-particle iron phosphate and the small-particle iron phosphate is not more than 60%. At the moment, the primary particles of the two types of iron phosphate have narrow size distribution, and small-particle lithium iron phosphate and large-particle lithium iron phosphate with narrow size distribution can be obtained by using the two types of iron phosphate as raw materials, so that the compaction density is improved to the greatest extent. Optionally, the coefficient of variation of the primary particle diameters of the large-particle lithium iron phosphate and the small-particle lithium iron phosphate is not more than 50%.
In the present application, the ratio of the average primary particle size of the large-particle iron phosphate to the average primary particle size of the small-particle iron phosphate is (2.5 to 60): 1. at this moment, the difference between the primary particle diameters of the large-particle iron phosphate and the small-particle iron phosphate is large, and the particle size difference between the small-particle lithium iron phosphate and the large-particle lithium iron phosphate prepared from the large-particle iron phosphate and the small-particle iron phosphate is also large, so that the compaction density of the pole piece is promoted. Optionally, the ratio of the average primary particle size of the large-particle iron phosphate to the average primary particle size of the small-particle iron phosphate is (5-30): 1.
in the application, the mass ratio of the large-particle iron phosphate to the small-particle iron phosphate is (2-10):1, and based on the large-particle iron phosphate with more mass, large-particle lithium iron phosphate with more mass can be obtained (the preparation ratio of the large-particle lithium iron phosphate to the small-particle lithium iron phosphate is at least 1:1), so that a pole piece made of the lithium iron phosphate positive active material has higher compaction density. Optionally, the mass ratio of the large-particle iron phosphate to the small-particle iron phosphate is (2-6): 1.
Wherein the material of the small-particle iron phosphate and the large-particle iron phosphate is selected from FePO independently4·2H2O and FePO4At least one of (1).
Optionally, the mixed iron phosphate and the lithium source are used in an amount such that the molar ratio of the iron element, the phosphorus element and the lithium element is 1 (1-1.05) to 1-1.05. Optionally, in order to make the lithium iron phosphate positive active material have better conductivity, the amount of the carbon source can ensure that the carbon content in the roasted material is 1% -1.8%. Preferably 1.3% to 1.6%.
In the present application, the lithium source may be at least one selected from the group consisting of lithium hydroxide, lithium oxide, lithium chloride, lithium nitrite, lithium nitrate, lithium oxalate, lithium carbonate, lithium acetate, lithium phosphate, lithium dihydrogen phosphate, and dilithium hydrogen phosphate. The carbon source may be selected from one or more of glucose, sucrose, citric acid, carboxymethyl cellulose, polyethylene glycol, malic acid, tartaric acid, oxalic acid, salicylic acid, succinic acid, glycine, and ethylenediaminetetraacetic acid, but is not limited thereto. The solvent may be selected from one or more of water, ethanol, methanol, N-methylpyrrolidone, acetone, dimethyl carbonate (DMC), Ethyl Methyl Carbonate (EMC), and ethylene carbonate (EMC), but is not limited thereto.
Wherein the solid content of the mixed slurry is 30-60 wt%. Therefore, the high energy consumption of grinding caused by too low solid content can be avoided, and the sand mill is prevented from being blocked easily due to too high solid content.
In the present application, the D50 particle size of the milled mixed slurry was in the range of 300nm to 800 nm. Therefore, the raw materials can be fully mixed, the reaction activity is realized, and the influence on the conductivity of the lithium iron phosphate anode active material due to excessive bond breakage and excessive lattice defects of the raw materials can be avoided. Further, the D50 particle size of the mixed slurry after grinding is in the range of 350nm to 800 nm.
Wherein the outlet temperature of the spray drying is 80-120 ℃. Optionally, the inlet temperature of the spray drying is controlled at 200-280 ℃. Further, the water content of the powder obtained by spray drying is within 3 w%.
Optionally, the temperature of the calcination is 700-. Alternatively, the time for the calcination may be 4 to 12 hours. The proper roasting time can ensure that the lithium iron phosphate is fully crystallized and has high crystallization integrity. Further, the time of the low-temperature roasting can be 8-10 h.
Wherein the D50 particle size of the crushed and refined material is less than 2 μm, and the D99 particle size is less than 10 μm. Therefore, the size of the roasted material is not too large, and the subsequent screening is avoided to be discarded as waste.
In the present application, the above-mentioned sieving is to remove abnormally large finished granules and foreign particles, and optionally, the sieving is performed with a 300-mesh sieve. Further, prior to said screening, comprising: and (4) iron removal treatment. The method specifically comprises the step of enabling the crushed and refined materials to pass through a dry-type iron remover to remove magnetic impurities introduced in the raw materials and the preparation process.
Optionally, the mixed slurry further comprises a doping element source. The doping element of the doping element source is one or more of Mg, Mo, Ti, V, Mn, Nb and the like. The source of doping element may be added in the form of an oxide, hydroxide or salt of the doping element. The introduction of the doping element can improve the discharge efficiency of the lithium iron phosphate anode active material.
Further, the molar weight of the doping element is 0.3% -0.7% of the molar weight of the Fe element in the mixed iron phosphate. The structural formula of the lithium iron phosphate positive active material in the application can be represented as follows: LiFeMxPO4@ C, wherein M represents doping elements, x represents the molar ratio of the doping elements to Fe elements, and x is more than or equal to 0 and less than or equal to 0.7 percent. Furthermore, x is more than or equal to 0.3 percent and less than or equal to 0.7 percent.
The preparation method provided by the second aspect of the application is simple in process and easy to operate.
The third aspect of the present application provides a positive plate, the positive plate includes a current collector and a positive active material layer disposed on the surface of the current collector, the positive active material layer includes a lithium iron phosphate positive active material according to the first aspect of the present application or includes a lithium iron phosphate positive active material prepared by the method according to the second aspect of the present application.
Wherein the positive electrode active material layer further includes a conductive agent and a binder. The conductive agent and the binder are conventional choices in the battery field. For example, the conductive agent may employ one or more of conductive carbon black (such as acetylene black, ketjen black), carbon nanotubes, carbon fibers, graphite, furnace black, and the like. The binder may be one or more of sodium carboxymethylcellulose (CMC), Styrene Butadiene Rubber (SBR), polyvinylidene fluoride (PVDF), Polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), polyolefins (e.g., polyethylene, polypropylene, polystyrene), and the like.
The mass percentage of the lithium iron phosphate positive active material in the positive active material layer is 95-97%.
Wherein the compaction density of the positive plate is 2.7g/cm3The above. The compaction density of the positive plate provided by the application is at least 17% higher than that of the existing positive plate on the market.
A fourth aspect of the present application provides a battery comprising a positive electrode sheet according to the third aspect of the present application. Based on the positive plate, the positive plate has higher compaction density, and the battery has higher specific discharge capacity, energy density and the like.
Drawings
Fig. 1 is a frequency graph of malvern particle size distribution data of lithium iron phosphate positive electrode active materials in examples 1 to 2 of the present application and comparative examples 1 to 2.
Detailed Description
The following are exemplary embodiments of the present application and it should be noted that those skilled in the art can make several improvements and modifications without departing from the principle of the present application, and these improvements and modifications should also be construed as the protection scope of the present application.
The technical solution of the present application is described below with reference to a plurality of specific embodiments.
Example 1
A preparation method of a lithium iron phosphate positive electrode active material comprises the following steps:
(1) selecting two kinds of anhydrous ferric phosphate, wherein the specific area of the small-particle anhydrous ferric phosphate is 30m2(ii)/g, the average value of the primary particle diameters thereof is 0.2 μm, the standard deviation of the primary particle diameters thereof is 0.055 μm, and the coefficient of variation is 27.5%; the specific area of the large-particle anhydrous ferric phosphate is 0.1m2(ii)/g, the average value of the primary particle diameters thereof is 1.8 μm, the standard deviation of the primary particle diameters thereof is 0.78 μm, and the coefficient of variation is 43.3%;
(2) according to the weight ratio of 2.5: 1, weighing large-particle anhydrous iron phosphate and small-particle anhydrous iron phosphate according to the mass ratio of the large-particle anhydrous iron phosphate to obtain mixed iron phosphate, wherein the mixed iron phosphate is obtained according to Li: fe: mixing the P, a lithium source (specifically lithium carbonate) and a carbon source (specifically monohydrate glucose) at a molar ratio of 1:1:1, wherein the adding amount of the carbon source can ensure that the carbon content in the finished lithium iron phosphate cathode active material is 1.3%, adding ethanol in a certain proportion, and uniformly mixing the materials by a high-speed dispersion machine to prepare mixed slurry with the solid content of 50%;
(3) transferring the mixed slurry into a sand mill, grinding the mixed slurry until the particle size of D50 is about 550nm, and then carrying out spray drying on the ground mixed slurry, wherein the inlet temperature of spray drying equipment is 200 ℃, the outlet temperature of the spray drying equipment is 100 ℃, and the water content of powder obtained by spray drying is within 3%;
(4) transferring the powder obtained by spray drying into a sagger, placing into a roasting furnace, roasting at 800 ℃ for 8h, and then carrying out jet milling on the roasted material to ensure that the D50 particle size of the jet milled material is below 2 microns and the D100 particle size is less than 10 microns; and (3) passing the material after jet milling through a dry-type iron remover to remove magnetic impurities introduced in the raw material neutralization preparation process, and then passing through a 300-mesh screen to obtain the lithium iron phosphate anode active material.
A malvern particle size instrument is used to perform a malvern particle size test on the lithium iron phosphate positive active material obtained in example 1, a particle size distribution frequency curve counted according to volume fraction is shown in fig. 1, and fig. 1 also contains particle size test results of other examples and comparative examples.
The particle size distribution frequency curve (in volume percentage) of the lithium iron phosphate positive electrode active material in example 1 had two peaks, in which the highest point of the first peak was at a particle size position of 0.405 μm, the highest point of the second peak was at a particle size position of 1.45 μm, and the ratio of the integrated area of the first peak to the integrated area of the second peak was 0.66: 1.
example 2
The preparation method of the lithium iron phosphate positive electrode active material is different from the embodiment 1 in that: in the step (2), the mass ratio of the large-particle anhydrous iron phosphate to the small-particle anhydrous iron phosphate is 2: 1.
referring to fig. 1, the particle size distribution frequency curve (in percentage by volume) of the lithium iron phosphate cathode active material in example 2 has two peaks, wherein the highest point of the first peak has a particle size position of 0.46 μm, the highest point of the second peak has a particle size position of 1.88 μm, and the ratio of the integrated area of the first peak to the integrated area of the second peak is 0.5: 1.
in order to highlight the beneficial effects brought by the technical scheme of the embodiment of the application, the following comparative examples 1-2 are provided.
Comparative example 1:
referring to fig. 1, the lithium iron phosphate positive electrode active material in comparative example 1 had two peaks in a particle size distribution frequency curve (in terms of volume percentage), in which the highest point of the first peak had a particle size position of 0.357 μm, the highest point of the second peak had a particle size position of 2.42 μm, and the ratio of the integrated area of the first peak to the integrated area of the second peak was 6.8: 1.
the method for preparing the lithium iron phosphate positive active material in comparative example 1 is different from that of example 1 in that: the mass ratio of the small-particle anhydrous iron phosphate to the large-particle anhydrous iron phosphate is 1: 1.
Comparative example 2:
referring to fig. 1, the particle size distribution frequency curve (in volume percentage) of the lithium iron phosphate positive active material of comparative example 2 has only one peak, and the highest point of the peak has a particle size position of 0.405 μm.
The method for preparing the lithium iron phosphate positive active material in comparative example 2 is different from that of example 1 in that: the parameters of two anhydrous iron phosphates were chosen as follows: the average primary particle diameter of the small-particle anhydrous iron phosphate is in the range of 0.05 μm to 0.3 μm (e.g., 0.2 μm); the average of the secondary particle diameters of the large anhydrous iron phosphate particles is in the range of 0.6 to 3 μm (for example, 3 μm), but the primary particle diameter of the large anhydrous iron phosphate particles is close to that of the small anhydrous iron phosphate particles, and the average thereof is also 0.2 μm.
Example 3
A lithium iron phosphate positive electrode active material has two peaks in a particle size distribution frequency curve (counted by volume percentage), wherein the particle size position of the highest point of a first peak is 0.523 mu m, the particle size position of the highest point of a second peak is 2.13 mu m, and the ratio of the integral area of the first peak to the integral area of the second peak is 0.25: 1.
the method for preparing the lithium iron phosphate positive active material in example 3 is different from example 1 in that: the parameters of two selected iron phosphates are as follows: the mass ratio of the large-particle iron phosphate to the small-particle iron phosphate is 4:1, wherein the specific area of the small-particle iron phosphate is 35m2(ii)/g, the average of the primary particle diameters thereof is 0.130 μm, the standard deviation of the primary particle diameters thereof is 0.0548 μm, and the coefficient of variation is 42.15%; large particleSpecific area of iron phosphate is 1m2(g), the average primary particle diameter thereof was 2.01. mu.m, the standard deviation of the primary particle diameter thereof was 0.91. mu.m, and the coefficient of variation was 45.27%.
Example 4
The lithium iron phosphate positive electrode active material has two peaks in a particle size distribution frequency curve (counted by volume percentage), wherein the particle size position of the highest point of a first peak is 0.594 mu m, the particle size position of the highest point of a second peak is 2.75 mu m, and the ratio of the integral area of the first peak to the integral area of the second peak is 0.13: 1.
the method for preparing the lithium iron phosphate positive active material in example 4 is different from example 1 in that: the parameters of the two selected iron phosphates are as follows: the mass ratio of the large-particle iron phosphate to the small-particle iron phosphate is 9:1, wherein the specific area of the small-particle iron phosphate is 15m2(ii)/g, the average value of the primary particle diameters thereof is 0.4 μm, the standard deviation of the primary particle diameters thereof is 0.168 μm, and the coefficient of variation is 42%; the specific area of the large-particle iron phosphate is 0.3m2(ii)/g, the average primary particle diameter thereof is 2.55 μm, the standard deviation of the primary particle diameter thereof is 0.969 μm, and the coefficient of variation is 38%.
In order to strongly support the beneficial effects brought by the technical scheme of the embodiment of the present application, the compacted density of the positive electrode sheet prepared from the lithium iron phosphate positive active material of each embodiment and the comparative example, and the discharge electrochemical performance of the prepared battery were tested, and the test results are shown in table 1.
The positive plate is prepared as follows: according to the mass ratio of 100: 1: 2.5 uniformly mixing the lithium iron phosphate positive active materials of the examples and the comparative examples with a conductive agent (CNT: graphene: 5), a PVDF binder and a certain amount of NMP respectively to obtain a mixed slurry, uniformly coating the prepared slurry on an Al foil by using a 250 μm slit die, coating one surface of the Al foil, drying the Al foil, coating the other surface of the Al foil, drying the Al foil, and rolling the Al foil under a pressure of 10Mpa to obtain a rolled pole piece. Cutting a phi 15mm wafer in the middle area, weighing, measuring the thickness, and calculating the compacted density of the pole piece.
The cell was prepared as follows: according to the mass ratio of 100: 1: 2.5 the lithium iron phosphate positive electrode active materials of the examples and comparative examples were respectively mixed with a conductive materialUniformly mixing the electrical agent (CNT: graphene: 5) and PVDF with a certain amount of NMP to obtain positive electrode slurry, coating the positive electrode slurry on one surface of an Al foil, drying in vacuum at 110 ℃, pressing to obtain the thick positive electrode slurry<A round piece with the diameter of 12mm and the diameter of 0.3mm is used as a positive electrode, a metal lithium piece is used as a negative electrode, a Celgard 2300 microporous membrane is used as a diaphragm, and 1.0mol/L LiPF6Ethylene Carbonate (EC): and (3) taking a solution of dimethyl carbonate (DMC) in a volume ratio of 1: 1-5 as an electrolyte, and assembling the solution into the R2025 button cell in a glove box.
The electrochemical performance test shown in Table 1 was carried out on the above-mentioned battery using a Xinwei 3008 battery test system in which the charge-discharge cutoff voltage was 2.5 to 3.8V (vs. Li/Li)+) Constant temperature at 25 ℃ and charge-discharge multiplying power of 0.1C. Specifically, the method for testing the discharge specific capacity of the battery at the normal temperature of 0.1C comprises the following steps: charging at 25 deg.C with 0.1C constant current until cut-off voltage is 3.8V, charging at 3.8V constant voltage until cut-off current is 0.01C, standing, discharging at 0.1C constant current until 2.5V, and recording discharge capacity. The calculation mode of the volume energy density of the battery at 0.1C is as follows: compacted density discharge specific capacity discharge voltage (discharge voltage calculated as 3.25 v).
TABLE 1 test results of examples and comparative examples
Figure BDA0002620880070000101
As can be seen from the results of the pole piece compaction densities in Table 1, the pole piece compaction densities of examples 1-4 of the present application are superior to the pole piece compaction densities of the comparative examples, and the volumetric energy density of the battery is much greater than that of the comparative examples. In addition, as can be seen from the comparison between example 1 and comparative examples 1 to 2, if the mass of the large-particle iron phosphate is less than or equal to that of the small-particle iron phosphate (comparative example 1), or the primary particle size of the large-particle iron phosphate is very close to that of the small-particle iron phosphate (comparative example 2), the particle size distribution curve of the prepared lithium iron phosphate positive electrode active material is quite different from that of the examples of the present application, and further, the compaction density of the positive electrode sheet prepared therefrom is low, and the volumetric energy density of the battery is also low. The lithium iron phosphate positive active material with unique particle size and bimodal distribution prepared by the method provided by the embodiment of the application can realize the closest packing, so that the compaction density of a pole piece can be greatly improved, the battery has higher volume energy density, and higher specific discharge capacity is considered.
The above-mentioned embodiments only express several embodiments of the present application, and the description thereof is more specific and detailed, but not construed as limiting the scope of the present application. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the concept of the present application, which falls within the scope of protection of the present application. Therefore, the protection scope of the present patent shall be subject to the appended claims.

Claims (10)

1. The lithium iron phosphate positive electrode active material is characterized in that two peaks are arranged in a particle size distribution frequency curve of the lithium iron phosphate positive electrode active material, wherein the particle size distribution frequency curve is distributed according to volume percentage, the particle size position of the highest point of a first peak is 0.3-0.6 μm, the particle size position of the highest point of a second peak is 1.0-3.0 μm, and the ratio of the integral area of the first peak to the integral area of the second peak is (0.1-1): 1.
2. the lithium iron phosphate positive electrode active material according to claim 1, wherein a ratio of an integrated area of the first peak to an integrated area of the second peak is (0.1 to 0.8): 1.
3. the lithium iron phosphate positive active material according to claim 1 or 2, wherein a positive electrode sheet made of the lithium iron phosphate positive active material has a compacted density of 2.65g/cm3The above.
4. A preparation method of a lithium iron phosphate positive electrode active material is characterized by comprising the following steps:
the specific surface area is 0.1-1m2Per gram of large-particle iron phosphate and a specific surface area of 15 to 35m2Per gram of small particle iron phosphate(2-10) mixing the mixed iron phosphate with a lithium source, a carbon source and a solvent according to the mass ratio of 1 to obtain mixed slurry; wherein the average value of the primary particle diameters of the large-particle iron phosphate is in the range of 1-3 μm, the average value of the primary particle diameters of the small-particle iron phosphate is in the range of 0.05-0.4 μm, and the coefficient of variation of the primary particle diameters of the large-particle iron phosphate and the small-particle iron phosphate is not more than 60%;
grinding the mixed slurry, then carrying out spray drying, and roasting the obtained powder; and (3) sequentially crushing, refining and sieving the roasted material to obtain the lithium iron phosphate positive active material.
5. The method according to claim 4, wherein the mixed slurry after grinding has a particle size of 300nm to 800nm as D50.
6. The method as claimed in claim 4, wherein the temperature of the calcination is 700-810 ℃.
7. The method according to claim 4, wherein the mixed iron phosphate and the lithium source are used in such an amount that the molar ratio of the iron element, the phosphorus element and the lithium element is 1 (1-1.05) to 1-1.05.
8. The production method according to any one of claims 4 to 7, wherein the lithium iron phosphate positive active material comprises, in mass ratio (0.1 to 1): 1, wherein the D50 particle size of the small-particle lithium iron phosphate is in the range of 0.2-0.6 μm, and the D50 particle size of the large-particle lithium iron phosphate is in the range of 1.0-3.0 μm.
9. A positive electrode sheet, comprising a current collector and a positive electrode active material layer disposed on the surface of the current collector, wherein the positive electrode active material layer comprises the lithium iron phosphate positive electrode active material according to any one of claims 1 to 3 or the lithium iron phosphate positive electrode active material prepared by the method according to any one of claims 4 to 8.
10. A battery comprising the positive electrode sheet according to claim 9.
CN202010782962.2A 2020-08-06 2020-08-06 Lithium iron phosphate positive electrode active material, preparation method thereof, positive plate and battery Pending CN114068920A (en)

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