Double-layer carbon-coated lithium iron phosphate positive electrode material, preparation method thereof and battery

Abstract

The preparation method comprises the following steps: mixing a lithium source, an iron source, a phosphorus source, a first carbon source and a first metal doping agent according to a preset proportion, carrying out primary high-temperature sintering, carrying out first-layer carbon coating and metal doping, and obtaining a primary sintered material; mixing the primary sintered material with a second carbon source and a second metal doping agent by a solid phase dry method, performing secondary sintering, performing secondary carbon coating and metal doping to obtain a target product, and finally crushing, sieving and deironing the target product to obtain the double-layer carbon-coated lithium iron phosphate material. A lithium source, an iron source, a phosphorus source, a carbon source and a metal doping agent are mixed and sintered by adopting a solid-phase mixing method and a simple carbon coating process to obtain a target product. The double-layer carbon-coated lithium iron phosphate material prepared by the method has high compaction density and high capacity, the coating uniformity of the carbon layer on the surface of the material is improved, and the conductivity is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion battery material preparation, and in particular to a double-layer carbon-coated lithium iron phosphate positive electrode material, a preparation method thereof, and a battery. Background Art

[0002] Lithium iron phosphate cathode material is a commonly used lithium-ion battery cathode material due to its advantages such as high theoretical capacity, long cycle life, good thermal stability and environmental friendliness.

[0003] However, lithium iron phosphate materials themselves have some inherent defects, such as poor electronic conductivity, low ion diffusion coefficient, and low compaction density. These factors seriously restrict their application in high-performance batteries.

[0004] The traditional method to improve the compaction density of lithium iron phosphate is to increase the sintering temperature, but excessively high temperatures will cause abnormal growth of the material, leading to deterioration of the material's electrochemical performance. Existing preparation methods make it difficult to simultaneously improve the conductivity, compaction density, and electrochemical performance of lithium iron phosphate materials. In particular, how to achieve high capacity and good rate performance while maintaining a high compaction density has become an urgent problem to be solved in existing technologies. Carbon coating is a commonly used modification method. Although it can improve the conductivity and cycle stability of the material, the carbon layer formed by its single sintering process often has problems such as insufficient uniformity and limited conductivity improvement. Summary of the Invention

[0005] The purpose of the present invention is to provide a double-layer carbon-coated lithium iron phosphate positive electrode material with the characteristics of high compaction density, high conductivity, and excellent electrochemical performance, and to provide a process method suitable for preparing the above-mentioned high-performance lithium iron phosphate positive electrode material, so that lithium iron phosphate is easier to produce on a large scale.

[0006] The object of the present invention is to provide a method for preparing a double-layer carbon-coated lithium manganese iron phosphate positive electrode material, the method comprising the following steps: Step (1) mixing a lithium source, an iron source, a phosphorus source, a first carbon source, and a first metal dopant in a predetermined ratio to form a mixed material, refining the mixed material, and then sintering the mixed material once to obtain a primary sintered material; the primary sintering conditions are: heating to 800°C to 850°C at a heating rate of 2°C / min to 3°C / min in an inert atmosphere, and keeping the temperature for 6h to 8h; Step (2) solid-phase mixing the primary sintered material with a second carbon source and a second metal dopant, and then performing secondary sintering to obtain a secondary sintered material; the secondary sintering conditions are: heating to 700°C to 800°C at a heating rate of 2°C / min to 3°C / min under an inert atmosphere, and keeping the temperature for 6h to 8h; Step (3) crushes, sieves, and removes iron from the secondary sintered material to obtain the double-layer carbon-coated lithium iron phosphate positive electrode material.

[0007] In some embodiments of the present invention, in step (1), the phosphorus source, the iron source, and the lithium source are mixed in a molar ratio of Li:Fe:P of (1.0-1.04):1:(1.015-1.036).

[0008] In some embodiments of the present invention, in step (1), the mass of the first carbon source accounts for (5.0-10.0)% of the mass of the iron source.

[0009] In some embodiments of the present invention, in step (1), the mass of the metal element in the first metal dopant accounts for (0.15-0.45)% of the mass of the mixed material.

[0010] In some embodiments of the present invention, in step (1), the refinement includes a first grinding and a second grinding, wherein the first grinding uses zirconium beads with a diameter of 0.6 mm to 0.8 mm to grind the mixture to a particle size D50 of 1.8 μm to 2.0 μm; and the second grinding uses zirconium beads with a diameter of 0.3 mm to 0.5 mm to grind the mixture after the first grinding to a particle size D50 of 0.4 μm to 0.6 μm.

[0011] In some embodiments of the present invention, in step (2), the mass of the second carbon source accounts for (0.4-1.0)% of the mass of the primary sintering material.

[0012] In some embodiments of the present invention, in step (2), the mass of the metal element in the second metal dopant accounts for (0.05-0.30)% of the mass of the primary sintered material.

[0013] In some embodiments of the present invention, in step (1) and step (2), the first carbon source and the second carbon source respectively include at least one of glucose, sucrose, starch and polyethylene glycol; the first metal dopant and the second metal dopant respectively include at least one of titanium dioxide, tetrabutyl titanate, magnesium oxide, magnesium acetate, magnesium hydroxide, magnesium nitrate, zirconium nitrate, zirconium hydroxide, zirconium oxide and niobium pentoxide.

[0014] Another object of the present invention is to provide a double-layer carbon-coated lithium manganese iron phosphate positive electrode material, comprising a core, a first coating layer and a second coating layer, wherein the first coating layer coats the core, and the second coating layer coats the side of the first coating layer facing away from the core, the core is lithium iron phosphate material, and the first coating layer and the second coating layer are respectively metal-doped carbon layers.

[0015] The present invention also provides a battery comprising the above-mentioned double-layer carbon-coated lithium iron phosphate positive electrode material.

[0016] Advantages of the present invention: (1) The present invention performs a first layer of carbon coating and metal element doping through a single high-temperature sintering, thereby improving the conductivity of lithium iron phosphate and solving the problems of poor conductivity and poor ion diffusion coefficient of traditional lithium iron phosphate materials; and performs a secondary carbon coating and metal element doping through a secondary sintering, thereby further improving the conductivity of the lithium iron phosphate material, overcoming the disadvantage of insufficient uniformity of the carbon layer coating, and improving the charge and discharge capacity of the material.

[0017] (2) The preparation process of this process method is simple and easy to operate. The solid phase mixing of the material sintered at a high temperature once with the carbon source and metal oxide is carried out through high-efficiency mixing equipment. Since complex and high-energy-consuming process points such as wet mixing, sand milling, and spray drying are omitted, the production cost is greatly reduced and the production efficiency is improved.

[0018] (3) The double-layer carbon-coated lithium iron phosphate positive electrode material structure disclosed in the present invention includes a uniform double-layer carbon coating layer and is doped with metal elements, which improves the conductivity of traditional lithium iron phosphate materials and has electrochemical properties of high compaction density and high capacity. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying any creative work.

[0020] Figure 1 This is a schematic flow chart of an embodiment of a method for preparing a lithium iron phosphate positive electrode material according to the present application; Figure 2 This is a SEM image of the lithium iron phosphate positive electrode material prepared in Example 1 of the present application; Figure 3 This is an SEM image of the lithium iron phosphate positive electrode material prepared in Comparative Example 1 of this application; Figure 4 TEM image of the lithium iron phosphate positive electrode material prepared in Example 1 of the present application; Figure 5 This is a TEM image of the lithium iron phosphate positive electrode material prepared in Comparative Example 1 of this application; Figure 6 These are the charge and discharge curves of the lithium iron phosphate positive electrode materials prepared in Example 1 and Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0021] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0022] As a cathode material for lithium-ion batteries, lithium iron phosphate (LiFePO4) excels in safety, lifespan, and high-temperature stability. However, LiFePO4 has inherent drawbacks such as low compaction density and poor electrical conductivity. Compaction density is closely related to the energy density of a battery, making increasing the compaction density of LiFePO4 a key factor in boosting its energy density. Traditionally, increasing the compaction density of LiFePO4 is achieved by increasing the sintering temperature. However, sintering at excessively high temperatures results in poor electronic conductivity and electrochemical performance.

[0023] In order to overcome the above problems, the conductivity is often improved by surface doping or carbon coating. The surface doping method is adopted, for example, by adding doping elements to the surface of lithium iron phosphate, but the type and amount of doping elements will affect the electronic conductivity and ion mobility of lithium iron phosphate. The common carbon coating method for improving the conductivity of lithium iron phosphate has problems such as the existing carbon coating process is complicated, the cost is high, and the performance improvement effect is limited. For example, the surface of the material is modified by using a mixed gas containing hydrocarbons combined with sintering in three temperature sections, and the growth of the material is regulated by controlling the intake volume of the mixed gas to achieve the effect of improving the electrochemical performance of the lithium iron phosphate positive electrode material, but the obtained material has a lower compaction density.

[0024] like Figure 1 As shown, one embodiment of the present invention provides a method for preparing a double-layer carbon-coated lithium iron phosphate positive electrode material, comprising the following steps: Step (1) mixing a lithium source, an iron source, a phosphorus source, a first carbon source and a first metal dopant in a predetermined ratio, performing a high-temperature sintering, performing a first layer of carbon coating and metal doping, and obtaining a primary sintered material; Step (2) solid-phase mixing the primary sintered material with a second carbon source and a second metal dopant, performing secondary sintering, secondary carbon coating and metal doping to obtain a secondary sintered material; Step (3) crushes, sieves, and removes iron from the secondary sintered material to obtain a double-layer carbon-coated lithium iron phosphate positive electrode material.

[0025] In step (1) of this embodiment, the specific steps of preparing the primary sintered material are as follows: a lithium source, an iron source, a phosphorus source, a first carbon source, and a first metal dopant are mixed in a predetermined ratio to form a mixed material and then refined. The refinement includes a first grinding and a second grinding. In the first grinding, the mixed material is introduced into a grinder for coarse grinding to initially break up large particles of raw materials, thereby preventing equipment blockage during the second grinding and improving production efficiency. In the first grinding, zirconium beads with a particle size of 0.6 mm to 0.8 mm are used to grind the mixed material to a particle size D50 of 1.8 μm to 2.0 μm. Then, a second refinement is performed, in which the mixed material with a particle size D50 of 1.8 μm to 2.0 μm is introduced into a grinder and ground using zirconium beads with a particle size of 0.3 mm to 0.5 mm to a particle size D50 of 0.4 μm to 0.6 μm. The ground mixture is then spray-dried, and the dried material is heated to 800°C to 850°C at a heating rate of 2°C / min to 3°C / min under inert atmosphere protection conditions, held for 6 to 8 hours for primary sintering, and cooled to room temperature to obtain a primary sintered material. Heating at a heating rate of 2°C / min to 3°C / min is beneficial for temperature control of the equipment and for the carbon thermal reduction process of lithium iron phosphate. The holding temperature of the sintering process is set at 800°C to 850°C because this temperature range promotes particle fusion growth and increases the compaction density of the product. Holding for 6 to 8 hours is beneficial to the growth of lithium iron phosphate crystals, improves crystallinity, and balances compaction density and electrical properties. If the holding time is too long, the generated particles will be too large, resulting in a decrease in electrical properties. If the holding time is too short, the compaction density of the material will be low.

[0026] In step (1), the phosphorus source, the iron source, and the lithium source are mixed in a molar ratio of Li:Fe:P of (1.0-1.04):1:(1.015-1.036).

[0027] The phosphorus source may be selected from at least one of ferric phosphate, phosphoric acid, lithium dihydrogen phosphate, monoammonium phosphate and ammonium dihydrogen phosphate.

[0028] The iron source may be selected from at least one of ferric phosphate, ferric oxide, ferroferric oxide and ferric hydroxide.

[0029] The lithium source may be selected from at least one of lithium carbonate, lithium hydroxide and lithium dihydrogen phosphate.

[0030] The first carbon source may be selected from at least one of glucose, sucrose, starch, and polyethylene glycol. The mass of the first carbon source accounts for (5.0-10.0)% of the mass of the iron source. The first carbon source can serve as a raw material for carbothermal reduction and can also form a carbon layer structure of the first coating layer, which is beneficial for improving the conductivity of the lithium iron phosphate.

[0031] The first metal dopant may be selected from at least one of titanium dioxide, tetrabutyl titanate, magnesium oxide, magnesium acetate, magnesium hydroxide, magnesium nitrate, zirconium nitrate, zirconium hydroxide, zirconium oxide, and niobium pentoxide. The amount of the metal element in the first metal dopant accounts for (0.15-0.45)% of the mass of the mixed material. The appropriate addition of the first metal dopant can improve the conductivity of the lithium iron phosphate and improve the electrochemical performance. As the amount of the metal element doping increases, the compaction density of the lithium iron phosphate material decreases.

[0032] The inert atmosphere can be selected from nitrogen and argon.

[0033] In step (2) of this embodiment, the specific steps for preparing the secondary sintered material are as follows: the primary sintered material is mixed with the second carbon source and the second metal dopant by a solid phase dry method, and then secondary sintered to obtain the secondary sintered material; in step (2), the secondary sintering conditions are: heating to 700°C to 800°C at a heating rate of 2°C / min to 3°C / min under an inert atmosphere, and keeping the temperature for 6h to 8h. Selecting the above-mentioned temperature range is conducive to the decomposition and coating of the second carbon source. If the holding temperature of the sintering reaction is too low, the coating effect will be poor. If the temperature is too high, the secondary growth of lithium iron phosphate crystals will occur, resulting in deterioration of electrochemical performance and the formation of foreign matter ferrous phosphide.

[0034] In step (2), the primary sintered material is solid-phase mixed with the second carbon source and the second metal dopant by a mixing device, without the need for complicated processes such as wet mixing, sand milling, and spray drying, which greatly reduces production costs, improves production efficiency, and is conducive to large-scale production.

[0035] The second carbon source may be selected from at least one of glucose, sucrose, starch and polyethylene glycol. The mass of the second carbon source accounts for (0.4-1.0)% of the mass of the primary sintering material.

[0036] The second metal dopant can be selected from at least one of titanium dioxide, tetrabutyl titanate, magnesium oxide, magnesium acetate, magnesium hydroxide, magnesium nitrate, zirconium nitrate, zirconium hydroxide, zirconium oxide, and niobium pentoxide. The amount of the metal element in the second metal dopant is (0.05-0.30)% of the mass of the primary sintered material. Experiments have shown that if the addition amount of the second metal dopant is too high, the compaction density of the lithium iron phosphate will decrease accordingly.

[0037] The inert atmosphere in step (2) can be nitrogen or argon.

[0038] The equipment for solid-phase dry mixing in step (2) can be one of an air flow mixer, a high-pressure mixer, a plowshare mixer and a ribbon mixer.

[0039] The equipment for sintering in step (2) can be one of a pusher kiln, a roller kiln and a rotary kiln; preferably, it is a rotary kiln.

[0040] In step (3) of this embodiment, the specific steps of crushing, screening and removing iron from the secondary sintered material are as follows: crushing the secondary sintered material with an air flow mill, the crushing equipment nozzle is 4mm, the induced draft frequency is 40Hz, the intake pressure is 0.3Mpa, and the grading frequency is 100Hz; then the crushed material is screened with a screen mesh of 80 mesh; the screened material is removed iron with a magnetic field removal device, and the magnetic field strength is ≥900GS, and finally the double-layer carbon-coated lithium iron phosphate positive electrode material of the present invention is obtained.

[0041] In the present invention, a double-layer carbon coating layer with metal doping is formed on the surface of the material through two carbon coatings and metal element doping. During the first sintering process for the first layer of carbon coating and metal element doping, the high temperature promotes the fusion growth of the material particles, significantly improving the compaction density of the material. At the same time, the carbon coating and metal doping achieve a significant improvement in the conductivity of the lithium iron phosphate material. A mixing device is used to solid-phase mix the primary sintered material with the carbon source and the metal oxide without the need for a complicated process. The secondary carbon coating and metal element doping are performed through a secondary sintering process, which improves the integrity and uniformity of the carbon layer and further improves the conductivity of the material. The compaction density is improved by controlling the particle size through two sinterings, and the uniformity and integrity of the carbon layer coating are improved by using a carbon coating process, and the conductivity of the material is improved by combining metal element doping, while taking into account the improvement of the conductivity, compaction density and electrochemical properties of the lithium iron phosphate material, and achieving high capacity while maintaining the high compaction density of the material.

[0042] The above-mentioned positive electrode material, preparation method thereof, and battery are further described below through specific examples.

[0043] Example 1 A method for preparing a lithium iron phosphate positive electrode material comprises the following steps: (1) 8.5 kg of glucose, 2.0 kg of polyethylene glycol (molecular weight 6000), 0.61 kg of titanium dioxide, 24.8 kg of lithium carbonate, 100 kg of ferric phosphate and 204 kg of water were mixed at a molar ratio of Li:Fe:P of 1.030:1:1.027, and stirred evenly to obtain a mixture; the mixture was introduced into a sand mill for the first grinding, using zirconium beads with a diameter of 0.6 mm as the grinding medium, and ground to a particle size D50 of 1.86 μm; The coarsely ground mixture was then introduced into a sand mill for a second grinding, using zirconium beads with a diameter of 0.3 mm as a grinding medium, and ground to a particle size D50 of 0.45 μm. The second-ground material was spray-dried with an air inlet temperature of 260°C and an air outlet temperature of 90°C. The dried material was heated to 810°C at a rate of 2.5°C / min under nitrogen protective gas conditions, kept at this temperature for 7.5 hours, and sintered once, and then naturally cooled to room temperature to obtain a primary sintered material. (2) 100 kg of primary sintered material, 1.0 kg of polyethylene glycol (molecular weight 6000) and 0.167 kg of titanium dioxide were mixed by a high-speed mixer; the effective volume of the high-speed mixer was 300 L, and the mixing frequency was adjusted from 25 Hz to 50 Hz for 10 minutes; the secondary mixed material was subjected to secondary sintering in a rotary kiln under nitrogen protective gas conditions to obtain secondary sintered material; the main temperature zone temperature of the rotary kiln was 780 ° C, and the main temperature zone was kept warm for 7 hours; (3) The cooled material is crushed by a jet mill; the crushed material is sieved through an 80-mesh standard sieve; the sieved material is de-ironed by an electromagnetic de-ironing device and a permanent magnetic de-ironing device to obtain a lithium iron phosphate positive electrode material.

[0044] Figure 2 This is the SEM image of the lithium iron phosphate positive electrode material prepared in Example 1.

[0045] Figure 4 This is a TEM image of the lithium iron phosphate positive electrode material prepared in Example 1.

[0046] Example 2 The only difference from Example 1 is step (2): (2) 100 kg of primary sintered material, 1.0 kg of polyethylene glycol (molecular weight 6000) and 0.334 kg of titanium dioxide were mixed by a high-speed mixer; the effective volume of the high-speed mixer was 300 L, and the mixing frequency was 25 Hz during mixing and then adjusted to 50 Hz for 10 minutes; the secondary mixed material was subjected to secondary sintering in a rotary kiln under nitrogen protective gas conditions to obtain secondary sintered material; the main temperature zone temperature of the rotary kiln was 780 ° C, and the main temperature zone insulation time was 7 hours.

[0047] Example 3 The only difference from Example 1 is step (2): (2) 100 kg of primary sintered material, 1.0 kg of polyethylene glycol (molecular weight 6000) and 0.0835 kg of titanium dioxide were mixed by a high-speed mixer; the effective volume of the high-speed mixer was 300 L, and the mixing frequency was 25 Hz for 5 minutes and then adjusted to 50 Hz for 10 minutes; the secondary mixed material was subjected to secondary sintering in a rotary kiln under nitrogen protective gas conditions to obtain secondary sintered material; the main temperature zone temperature of the rotary kiln was 780 ° C, and the main temperature zone insulation time was 7 hours.

[0048] Example 4 The only difference from Example 1 is step (2): (2) 100 kg of primary sintered material, 1.0 kg of polyethylene glycol (molecular weight 6000) and 0.143 kg of niobium pentoxide were mixed by a high-speed mixer; the effective volume of the high-speed mixer was 300 L, and the mixing frequency was 25 Hz for 5 minutes and then adjusted to 50 Hz for 10 minutes; the secondary mixed material was subjected to secondary sintering in a rotary kiln under nitrogen protective gas conditions to obtain secondary sintered material; the temperature of the main temperature zone of the rotary kiln was 780 ° C, and the insulation time of the main temperature zone was 7 hours.

[0049] Example 5 The only difference from Example 1 is step (2): (2) 100 kg of primary sintered material, 1.0 kg of polyethylene glycol (molecular weight 6000) and 0.179 kg of niobium pentoxide were mixed by a high-speed mixer; the effective volume of the high-speed mixer was 300 L, and the mixing frequency was 25 Hz for 5 minutes and then adjusted to 50 Hz for 10 minutes; the secondary mixed material was subjected to secondary sintering in a rotary kiln under nitrogen protective gas conditions to obtain secondary sintered material; the main temperature zone temperature of the rotary kiln was 780 ° C, and the main temperature zone insulation time was 7 hours.

[0050] Example 6 The only difference from Example 1 is step (2); (2) 100 kg of primary sintered material, 0.4 kg of glucose and 0.167 kg of titanium dioxide were mixed by a high-speed mixer; the effective volume of the high-speed mixer was 300 L, and the mixing frequency was 25 Hz for 5 minutes and then adjusted to 50 Hz for 10 minutes; the secondary mixed material was subjected to secondary sintering in a rotary kiln under nitrogen protective gas conditions to obtain secondary sintered material; the main temperature zone temperature of the rotary kiln was 780 ° C, and the main temperature zone insulation time was 7 hours.

[0051] Example 7 The only difference from Example 1 is step (2): (2) 100 kg of primary sintered material, 0.4 kg of sucrose and 0.167 kg of titanium dioxide were mixed by a high-speed mixer; the effective volume of the high-speed mixer was 300 L, and the mixing frequency was 25 Hz for 5 minutes and then adjusted to 50 Hz for 10 minutes; the secondary mixed material was subjected to secondary sintering in a rotary kiln under nitrogen protective gas conditions to obtain secondary sintered material; the main temperature zone temperature of the rotary kiln was 780 ° C, and the main temperature zone insulation time was 7 hours.

[0052] Example 8 The only difference from Example 1 is step (2).

[0053] (2) 100 kg of primary sintered material, 0.4 kg of starch and 0.167 kg of titanium dioxide were mixed by a high-speed mixer; the effective volume of the high-speed mixer was 300 L, and the mixing frequency was 25 Hz during mixing and mixed for 5 minutes, and then the frequency was adjusted to 50 Hz for 10 minutes; the secondary mixed material was subjected to secondary sintering in a rotary kiln under nitrogen protective gas conditions to obtain secondary sintered material; the main temperature zone temperature of the rotary kiln equipment was 780 ° C, and the main temperature zone insulation time was 7 hours.

[0054] Comparative Example 1 (1) 9.5 kg of glucose, 2.0 kg of polyethylene glycol (molecular weight 6000), 0.784 kg of titanium dioxide, 24.8 kg of lithium carbonate, 100 kg of ferric phosphate and 204 kg of water were mixed in a molar ratio of Li:Fe:P of 1.030:1:1.027, and stirred evenly to obtain a mixed material; the mixed material was introduced into a sand mill for a first grinding, using zirconium beads with a diameter of 0.6 mm as a grinding medium, and grinding to a particle size D50 of 1.86 μm; the coarsely ground material was introduced into a sand mill for a second grinding, using zirconium beads with a diameter of 0.3 mm as a grinding medium, and grinding to a particle size D50 of 0.45 μm; the material after the second grinding was spray dried, with an air inlet temperature of 260 ° C and an air outlet temperature of 90 ° C; the dried material was heated to 810 ° C at a rate of 2.5 ° C / min under nitrogen protective gas conditions, kept warm for 7.5 h for a first sintering, and cooled to room temperature to obtain a first sintered material; (2) The cooled material is crushed by a jet mill; the crushed material is sieved through an 80-mesh standard sieve; the sieved material is de-ironed by an electromagnetic de-ironing device and a permanent magnetic de-ironing device to obtain a lithium iron phosphate positive electrode material.

[0055] Figure 3 This is the SEM image of the lithium iron phosphate positive electrode material prepared in Comparative Example 1.

[0056] Figure 5 This is the TEM image of the lithium iron phosphate positive electrode material prepared in Comparative Example 1.

[0057] Comparative Example 2 (1) 9.5 kg of glucose, 2.0 kg of polyethylene glycol (molecular weight 6000), 0.784 kg of titanium dioxide, 24.8 kg of lithium carbonate, 100 kg of ferric phosphate and 204 kg of water were mixed in a molar ratio of Li:Fe:P of 1.030:1:1.027, and stirred evenly to obtain a mixed material; the mixed material was introduced into a sand mill for a first grinding, using zirconium beads with a diameter of 0.6 mm as a grinding medium, and grinding to a particle size D50 of 1.86 μm; the coarsely ground material was introduced into a sand mill for a second grinding, using zirconium beads with a diameter of 0.3 mm as a grinding medium, and grinding to a particle size D50 of 0.45 μm; the material after the second grinding was spray dried, with an inlet temperature of 260 ° C and an outlet temperature of 90 ° C; the dried material was heated to 780 ° C at a rate of 2.5 ° C / min under nitrogen protective gas conditions, kept warm for 7.5 h for a first sintering, and cooled to room temperature to obtain a first sintered material; (2) The cooled material is crushed by a jet mill; the crushed material is sieved through an 80-mesh standard sieve; the sieved material is de-ironed by an electromagnetic de-ironing device and a permanent magnetic de-ironing device to obtain a lithium iron phosphate positive electrode material.

[0058] 1. The main parameter differences of the positive electrode material preparation methods in Examples 1-8 are shown in Table 1.

[0059] Table 1 2. The main differences in the preparation parameters of the positive electrode materials of Example 1 and Comparative Examples 1 and 2 are shown in Table 1.

[0060] Table 2 Physicochemical properties of lithium iron phosphate cathode materials of Examples 1-8 and Comparative Examples 1-2 (1) Carbon content The infrared analysis method is used to test the carbon content of the LFP positive electrode material. A carbon-sulfur analyzer is used to burn the sample under high temperature and oxygen-rich conditions. The carbon element it contains is oxidized into carbon dioxide and enters the infrared detector with the carrier gas. The carbon content is calculated by quantitatively analyzing the changes in the infrared absorption wavelength intensity of the carbon dioxide signal.

[0061] (2) Specific surface area The nitrogen adsorption-desorption method is used. At liquid nitrogen temperature, the equilibrium adsorption amount of nitrogen on the surface of an object is related to its specific surface area and other characteristics. Combined with the law of change of adsorption amount with relative pressure during the adsorption process, the specific surface area test is carried out.

[0062] (3) Field emission scanning electron microscopy (SEM) test Thermo Fisher's AxiaChemiSEMHiVac scanning electron microscope was used. This instrument has high-resolution imaging capabilities and can clearly observe the microscopic morphology and structural characteristics of the positive electrode material. The acceleration voltage is 10.00kV, the working distance is 10mm, and the magnification is 10,000 times.

[0063] (4) Powder compaction density The test was conducted with reference to the method specified in the standard drafted by the National Technical Committee for Standardization of Nonferrous Metals, "Determination of Compacted Density of Lithium-ion Battery Cathode Material Powder".

[0064] (5) Powder resistivity The test was conducted with reference to the method specified in the standard drafted by the National Technical Committee for Standardization of Nonferrous Metals, "Determination of Resistivity of Lithium-ion Battery Cathode Material Powder".

[0065] (6) Power-off performance The electrical performance test was carried out using button cells. The positive electrode materials prepared in the above embodiments and comparative examples were used as active materials, polyvinylidene fluoride (PVDF) as a binder, and conductive carbon black as a conductive agent in a mass ratio of 90:5:5 to form a mixed powder. N-methylpyrrolidone (NMP): polyvinylidene fluoride (PVDF): conductive carbon black (SP) were then mixed in a weight ratio of 159:6:6 to form a conductive glue. 3.6g of the mixed powder and 5.7g of the conductive glue were placed in a beaker and stirred evenly with a stirrer to form a slurry. The above slurry was evenly coated on an aluminum foil with a thickness of 10um to prepare an electrode, and baked at 100°C for 2h. The roller press pressure was set to 3.5MPa, and the compacted density of the electrode was 2.0g / cm 3 to 2.2g / cm 3 , and then place the electrode in a vacuum drying oven at 95℃ and bake for 2 hours; then punch the electrode into a disc with a diameter of 156mm to obtain the positive electrode.

[0066] Place the positive electrode sheet in the positive electrode shell, and add 2 to 3 drops of electrolyte on the positive electrode sheet with a dropper. The electrolyte is prepared according to the mass ratio of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) of 30:50:20. Place the diaphragm flat on the positive electrode sheet. The isolation membrane is a polyethylene (PE) film with a thickness of 6μm. Add 2 drops of electrolyte on the diaphragm, and place the negative electrode material metal lithium sheet flat on the diaphragm. The diameter of the metal lithium sheet is 15.8mm and the thickness is 0.58mm. Place nickel foam on the lithium sheet. The purity of the nickel foam is greater than 99.5%, the diameter is 16mm, the thickness is 0.5mm, and the porosity is 95%. Cover with the negative electrode shell, package it on the sealing machine, and obtain a button battery for testing.

[0067] (7) Rate performance test At 25°C, the assembled button cell was left to rest for 2 hours. The cell was then charged at 0.1C to 3.75V and switched to constant voltage charging. Constant voltage charging ended when the current decreased to 0.05C, yielding the charge capacity at 0.1C. After 5 minutes of rest, the cell was discharged at 0.1C to 2.0V, yielding the discharge capacity at 0.1C. The cell was left to rest for 5 minutes. The cell was then charged at 0.2C to 3.75V and switched to constant voltage charging. Constant voltage charging ended when the current decreased to 0.05C. After 5 minutes of rest, the cell was discharged at 0.2C to 2.0V, yielding the discharge capacity at 1.0C. The test concluded.

[0068] 3. The following tests were performed on the positive electrode materials obtained in Examples 1-8 and Comparative Examples 1 and 2.

[0069] Table 3 The above results show that: Combined with the information in Table 3 Figure 6 Comparing the performance data of the positive electrode materials of Examples 1-8 with those of Comparative Examples 1 and 2, the compaction of the lithium iron phosphate positive electrode materials prepared by the present invention is above 2.62 g / cc, and the 0.1C discharge capacity and 1C discharge capacity of the prepared materials of Examples 1-8 are improved compared with the comparative examples.

[0070] Combined with the information in Table 3, comparing the materials prepared in Comparative Example 1 and Comparative Example 2, it can be seen that the compaction density of the lithium iron phosphate positive electrode material prepared in Comparative Example 1 is greatly improved compared with the compaction density of Comparative Example 2, but the withholding performance is deteriorated, indicating that the compaction density can be improved by increasing the primary sintering temperature but it will have an adverse effect on the withholding performance. This is because high-temperature sintering can improve the compaction density through particle densification, but it will cause problems such as abnormal growth of material particles, resulting in impaired electrochemical performance.

[0071] Figure 4 This is a TEM image of the lithium iron phosphate cathode material of Example 1. Figure 5 TEM image of the lithium iron phosphate cathode material of Comparative Example 1, Figure 4 The carbon coating layer on the surface of the lithium iron phosphate cathode material of Example 1 has good uniformity, which plays an important role in stabilizing the performance. Figure 5 The thickness of the surface carbon coating layer of the lithium iron phosphate positive electrode material of Comparative Example 1 is quite different. Combined with the information in Table 3, compared with Comparative Example 1, the powder resistivity of the lithium iron phosphate positive electrode material of Example 1 is reduced, and the buckling performance is greatly improved, indicating that the secondary carbon layer coating improves the problem of uneven thickness of the carbon layer of the carbon coating. The uniform carbon coating forms a continuous conductive network, reduces the contact resistance between particles, and promotes rapid electron transmission; combined with the metal element doping on the surface of the material, the conductivity of the material can be optimized by the conductivity of the metal element, and the contact between the material particles can be improved by the carbon layer, thereby achieving a dual improvement in the conductivity and ion conductivity of the lithium iron phosphate positive electrode material.

[0072] Combining the information in Table 3, it can be seen from the comparison of Example 1, Example 2 and Example 3 that as the amount of metal element doping increases, the electrical performance improves, but the material compaction density decreases, such as Example 3. The comprehensive performance is the best in Example 1.

[0073] Combining the information in Table 3 and comparing Example 1, Example 4 and Example 5, it can be seen that the electrochemical performance of the positive electrode material can be improved by surface doping with different metal elements.

[0074] Combining the information in Table 3, a comparison of Examples 1, 6, 7, and 8 shows that while different carbon sources can reduce the powder resistivity of the cathode material, the polyethylene glycol (molecular weight 6000) used in Example 1 exhibits a greater improvement. This is because, compared to carbon sources such as glucose, sucrose, and starch, polyethylene glycol, as an organic carbon source, decomposes during the sintering process to produce organic gases that act on the surface of the primary sintered material, forming a vapor-phase coating that results in a more uniform carbon layer on the material surface.

[0075] The above description is only a preferred embodiment of the present invention and does not limit the patent scope of the present invention. All equivalent structural transformations made by using the contents of the present invention description and drawings under the inventive concept of the present invention, or direct / indirect application in other related technical fields are included in the patent protection scope of the present invention.

Claims

1. A method for preparing a double-layer carbon-coated lithium iron phosphate positive electrode material, characterized in that: The following steps are involved: Step (1) mixing a lithium source, an iron source, a phosphorus source, a first carbon source, and a first metal dopant in a predetermined ratio to form a mixed material, refining the mixed material, and then performing a primary sintering to obtain a primary sintered material; the primary sintering conditions are: heating to 800° C. to 850° C. at a heating rate of 2° C. / min to 3° C. / min in an inert atmosphere, and keeping the temperature for reaction for 6 h to 8 h; Step (2) solid-phase mixing the primary sintered material with a second carbon source and a second metal dopant, and then performing secondary sintering to obtain a secondary sintered material; the secondary sintering conditions are: heating to 700°C to 800°C at a heating rate of 2°C / min to 3°C / min under an inert atmosphere, and keeping the temperature for 6h to 8h; Step (3) crushes, sieves, and removes iron from the secondary sintered material to obtain the double-layer carbon-coated lithium iron phosphate positive electrode material.

2. The method for preparing a double-layer carbon-coated lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step (1), the phosphorus source, the iron source, and the lithium source are mixed in a molar ratio of Li:Fe:P of (1.0-1.04):1:(1.015-1.036).

3. The method for preparing a double-layer carbon-coated lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step (1), the mass of the first carbon source accounts for (5.0-10.0)% of the mass of the iron source.

4. The method for preparing a double-layer carbon-coated lithium iron phosphate cathode material according to claim 2, wherein: In step (1), the amount of the metal element in the first metal dopant accounts for (0.15-0.45)% of the mass of the mixed material.

5. The method for preparing a double-layer carbon-coated lithium iron phosphate cathode material according to claim 2, characterized in that: In step (1), the refinement includes a first grinding and a second grinding. The first grinding uses zirconium beads with a diameter of 0.6 mm to 0.8 mm to grind the mixture to a particle size D50 of 1.8 μm to 2.0 μm; the second grinding uses zirconium beads with a diameter of 0.3 mm to 0.5 mm to grind the mixture after the first grinding to a particle size D50 of 0.4 μm to 0.6 μm.

6. The method for preparing a double-layer carbon-coated lithium iron phosphate cathode material according to claim 1, characterized in that: In step (2), the mass of the second carbon source accounts for (0.4-1.0)% of the mass of the primary sintering material.

7. The method for preparing a double-layer carbon-coated lithium iron phosphate cathode material according to claim 1, characterized in that: In step (2), the amount of the metal element in the second metal dopant accounts for (0.05-0.30)% of the mass of the primary sintered material.

8. The method for preparing a double-layer carbon-coated lithium iron phosphate cathode material according to claim 1, wherein: In step (1) and step (2), the first carbon source and the second carbon source respectively include at least one of glucose, sucrose, starch and polyethylene glycol; the first metal dopant and the second metal dopant respectively include at least one of titanium dioxide, tetrabutyl titanate, magnesium oxide, magnesium acetate, magnesium hydroxide, magnesium nitrate, zirconium nitrate, zirconium hydroxide, zirconium oxide and niobium pentoxide.

9. A double-layer carbon-coated lithium iron phosphate positive electrode material, characterized in that: It includes a core, a first coating layer and a second coating layer, wherein the first coating layer covers the core, and the second coating layer covers the side of the first coating layer away from the core. The core is made of lithium iron phosphate material, and the first coating layer and the second coating layer are respectively metal-doped carbon layers.

10. A battery, characterized in that: It comprises the double-layer carbon-coated lithium iron phosphate positive electrode material as claimed in claim 9.

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