Carbon-coated lithium iron phosphate, preparation method thereof, positive plate and lithium ion battery

Abstract

The invention provides carbon-coated lithium iron phosphate and a preparation method thereof, a positive plate and a lithium ion battery. The preparation method comprises the following steps: S1, mixing raw materials including a lithium source, a phosphorus source, an iron source and a first solvent to obtain slurry; s2, sequentially carrying out spray drying and crushing treatment on the slurry to obtain a lithium iron phosphate precursor; s3, carrying out polymerization reaction on raw materials of the lithium iron phosphate precursor, styrene, an initiator and a second solvent in an inert atmosphere, and adding a cationic monomer in the polymerization reaction process to obtain a cationic polystyrene coated lithium iron phosphate precursor; and S4, calcining the cationic polystyrene coated lithium iron phosphate precursor in an inert atmosphere to obtain the carbon coated lithium iron phosphate. According to the preparation method disclosed by the invention, lithium iron phosphate is coated with a layer of porous carbon while being controlled to be nano-sized, so that the electrochemical impedance of the lithium iron phosphate material is reduced, and the rate capability of the lithium iron phosphate material is improved.

Description

Technical Field

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

[0002] Lithium iron phosphate (LiFePO4) with olivine structure is the positive electrode active material of lithium-ion batteries. It has the advantages of high specific capacity, stable operating voltage, low cost and environmental friendliness. Therefore, it is generally regarded as the preferred positive electrode material for power batteries at home and abroad.

[0003] The theoretical capacity of LiFePO4 is 174mAh / g, and the actual discharge capacity can reach more than 160mAh / g, but the performance of LiFePO4 batteries is significantly reduced at low temperatures, limiting its use in winter and high-cold areas. Researchers believe that the intrinsic electronic conductivity and ion diffusion rate of olivine-structured LiFePO4 are low, and the deposition of active lithium at low temperatures and its catalytic growth of the solid electrolyte interface further lead to a decrease in ion conductivity and electron migration rate in the electrolyte, thereby affecting the capacity and power performance of lithium-ion batteries. In addition, with the release of the "Shenxing Supercharge" battery, people's demand for fast charging of lithium-ion batteries has become increasingly enthusiastic. Under high current, the heat generation of LiFePO4 with low conductivity is bound to be serious, and the temperature of the battery will rise accordingly, increasing the safety risk of the battery. In response to this, researchers have modified LiFePO4 materials by nano-sizing, metal doping, and carbon coating, hoping to improve the conductivity and ion diffusion rate of LiFePO4 materials, thereby improving low-temperature performance and fast charging performance.

[0004] The Chinese patent application with the patent application publication number CN102263247A discloses a method for preparing a high-performance doped LiFePO4 carbon-coated positive electrode material: the iron source, lithium source, phosphorus source, liquid water-soluble phenolic resin and doping elements are ball-milled and mixed in deionized water in a certain proportion, then spray-dried, calcined twice under an inert atmosphere, and finally crushed to obtain a doped LiFePO4 carbon-coated positive electrode material with both high capacity and good processing performance. This method adds phenolic resin as a coating carbon source while synthesizing the precursor, and the product particle size will be larger, which is not conducive to fast charging and low temperature performance.

[0005] The Chinese patent with the patent authorization announcement number CN114171740B discloses a method for preparing nano LiFePO4 positive electrode materials: hollow carbon spheres are prepared using nano chitosan, and then the hollow carbon spheres are dispersed in water, and then lithium salts, phosphate salts, and iron salts are added to dissolve, and then a hydrothermal reaction is carried out under nitrogen protection to prepare a LiFePO4 precursor, and finally the precursor is calcined to obtain nano LiFePO4 with high conductivity and high tap density. This method uses a carbon skeleton to grow LiFePO4, and the carbon content may be too high, which may affect the initial efficiency and cycle stability of the battery.

[0006] The Chinese patent with the patent authorization announcement number CN114583159B discloses a method for preparing a low-temperature lithium iron phosphate cathode material: honeycomb iron phosphate, lithium source and sugar substances are mixed with water, and then sand-milled to obtain a dispersion; the dispersion is spray-dried to obtain a precursor; the precursor is sintered under a protective atmosphere, and the layered Li-Al double metal hydroxide is sintered by dry powder spraying to obtain a special interlayer structure of low-temperature lithium iron phosphate cathode material. The honeycomb LiFePO4 prepared by this method has a large specific surface area, which increases the risk of gelation during the slurrying process.

[0007] The existing technology for coating and modifying lithium iron phosphate usually adds a coating carbon source when synthesizing the precursor, which easily leads to a larger particle size of the product, longer electronic conductivity and ion diffusion paths, increased polarization, and reduced rate and low-temperature performance. Under nano-technology, most of the wet grinding methods are used to reduce the particle size of the precursor, but nano lithium iron phosphate is easy to agglomerate during the calcination process.

[0008] Therefore, effectively designing the grain size and coating method of LiFePO4 to improve its structural stability and rate performance is a technical problem that needs to be solved urgently. Summary of the invention

[0009] The main purpose of the present invention is to provide a carbon-coated lithium iron phosphate and a preparation method thereof, a positive electrode sheet and a lithium ion battery, so as to solve the problem of low rate performance of carbon-coated lithium iron phosphate in the prior art.

[0010] In order to achieve the above-mentioned purpose, according to one aspect of the present invention, a method for preparing carbon-coated lithium iron phosphate is provided, and the method comprises: step S1, mixing raw materials including a lithium source, a phosphorus source, an iron source and a first solvent to obtain a slurry; step S2, spray-drying and pulverizing the slurry in sequence to obtain a lithium iron phosphate precursor; step S3, polymerizing the raw materials including the lithium iron phosphate precursor, styrene, an initiator and a second solvent in an inert atmosphere, and adding a cationic monomer during the polymerization reaction to obtain a cationic polystyrene-coated lithium iron phosphate precursor; step S4, calcining the cationic polystyrene-coated lithium iron phosphate precursor in an inert atmosphere to obtain the carbon-coated lithium iron phosphate.

[0011] Furthermore, in the above step S3, in terms of mass fraction, the amount of styrene added is 5 to 15 wt% of the lithium iron phosphate precursor, the amount of the initiator added is 0.8 to 1.2 wt% of styrene, and the amount of the cationic monomer added is 5 to 10 wt% of styrene; and / or the polymerization reaction temperature is 50 to 80°C; and / or the polymerization reaction time is 15 to 20 h; and / or the polymerization reaction is carried out under stirring, and the stirring speed of the polymerization reaction is 300 to 500 rpm; and / or the cationic monomer is benzylethyltrimethylammonium chloride and / or 2-(methacryloyloxy)ethyltrimethylammonium chloride.

[0012] Furthermore, in the above step S3, the cationic monomer is added in batches. Preferably, the polymerization reaction time is recorded as T, and the total amount of cationic monomer added is recorded as M. As the polymerization reaction proceeds, when T reaches 70 to 100 min, the cationic monomer is added to the polymerization reaction system, and is added once every 20 to 30 min, each addition amount is m1, and the addition is stopped after 3 to 5 additions; wherein, m1 / M=1.0% to 1.5%; when T reaches 220 to 240 min, the cationic monomer is continued to be added to the polymerization reaction, and is added once every 40 to 60 min, each addition amount is m2, and the addition is stopped after 1 to 2 additions; wherein, m2 / M=5.0% to 8.0%; when T reaches 300 to 320 min, the cationic monomer is continued to be added to the polymerization reaction, and is added once every 3 to 5 min, each addition amount is m3, until the total mass of the cationic monomer is added; wherein, m3 / M=1.0% to 1.5%.

[0013] Furthermore, in the above step S4, the calcination conditions are: first heating to 200-400°C at a heating rate of 2-10°C / min and keeping warm for 3-6h, then heating to 600-800°C at a heating rate of 2-10°C / min and keeping warm for 8-12h.

[0014] Furthermore, in the above step S2, the solid content of the slurry is 10-40%; and / or the air inlet temperature of the spray drying is 180-240°C; and / or the air outlet temperature of the spray drying is 80-160°C; and / or the average particle size of the lithium iron phosphate precursor is 300-900nm.

[0015] Furthermore, in the above step S1, the molar ratio of lithium in the lithium source, phosphorus in the phosphorus source and iron in the iron source is 1:0.98~1.02:1; and / or, the raw material also includes a nitrogen source, and the mass ratio of the nitrogen source to the lithium source is 1:2~3; preferably, the nitrogen source is selected from any one or more of urea, azobisisobutyronitrile, p-nitroaniline, triethylamine, ethyleneimine and pyrrolidine; and / or, the lithium source is selected from any one or more of lithium carbonate, lithium hydroxide, lithium acetate and lithium oxalate; and / or, the phosphorus source is diammonium phosphate and / or phosphoric acid; and / or, the iron source is selected from any one or more of ferrous oxalate, ferrous chloride and ferric oxide.

[0016] According to another aspect of the present invention, a carbon-coated lithium iron phosphate is provided. The carbon-coated lithium iron phosphate is prepared by the aforementioned preparation method.

[0017] Furthermore, the D50 of the carbon-coated lithium iron phosphate is 0.5-0.9 μm.

[0018] According to another aspect of the present invention, a positive electrode sheet is provided, wherein the positive electrode sheet contains the aforementioned carbon-coated lithium iron phosphate.

[0019] According to another aspect of the present invention, a lithium-ion battery is provided, comprising a positive electrode sheet, an electrolyte and a negative electrode sheet, wherein the positive electrode sheet is the positive electrode sheet mentioned above.

[0020] By applying the technical solution of the present invention, the slurry is spray-dried and crushed in sequence in step S2 of the present application, which helps to control the size of the lithium iron phosphate precursor at the nanometer level, thereby helping to make the size of the carbon-coated lithium iron phosphate finally prepared also at the nanometer level, thereby helping to improve the conductivity of the carbon-coated lithium iron phosphate and shorten the diffusion path of lithium ions. Step S3 is to coat the surface of the nano lithium iron phosphate precursor with a cationic polystyrene layer, which can effectively prevent the agglomeration between the nano lithium iron phosphate precursor particles to form a monodisperse spherical cationic polystyrene-coated lithium iron phosphate precursor; finally, calcined under a protective atmosphere, the cationic polystyrene layer is converted into a porous carbon layer, the carbon layer is conducive to electronic conductivity, and the porous structure is conducive to the shuttling of lithium ions. The preparation method of the present application controls the lithium iron phosphate to be nanometer-sized while coating a layer of porous carbon, which helps to reduce the electrochemical impedance of the lithium iron phosphate material and improve its rate performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The drawings constituting a part of the present application are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0022] Figure 1 shows the XRD diffraction pattern of carbon-coated lithium iron phosphate in Example 1 of the present application;

[0023] Figure 2 The rate performance diagrams of the samples in Example 1, Example 2, Example 3, Comparative Example 1 and Comparative Example 2 of the present application are shown. DETAILED DESCRIPTION

[0024] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0025] As analyzed in the background technology of this application, the carbon-coated lithium iron phosphate in the prior art has the problem of low rate performance. In order to solve the above problem, this application provides a carbon-coated lithium iron phosphate and a preparation method thereof, a positive electrode sheet and a lithium-ion battery.

[0026] In a typical embodiment of the present application, a method for preparing carbon-coated lithium iron phosphate is provided, and the preparation method comprises: step S1, mixing raw materials including a lithium source, a phosphorus source, an iron source and a first solvent to obtain a slurry; step S2, spray-drying and pulverizing the slurry in sequence to obtain a lithium iron phosphate precursor; step S3, polymerizing the raw materials of the lithium iron phosphate precursor, styrene, an initiator and a second solvent in an inert atmosphere, and adding a cationic monomer during the polymerization reaction to obtain a cationic polystyrene-coated lithium iron phosphate precursor; step S4, calcining the cationic polystyrene-coated lithium iron phosphate precursor in an inert atmosphere to obtain a carbon-coated lithium iron phosphate.

[0027] In step S2 of the present application, the slurry is spray-dried and pulverized in sequence, which helps to control the size of the lithium iron phosphate precursor at the nanometer level, thereby helping to make the size of the carbon-coated lithium iron phosphate finally prepared also at the nanometer level, thereby helping to improve the conductivity of the carbon-coated lithium iron phosphate and shorten the diffusion path of lithium ions. Step S3 coats the surface of the nano lithium iron phosphate precursor with a cationic polystyrene layer, which can effectively prevent the agglomeration between the nano lithium iron phosphate precursor particles to form a monodisperse spherical cationic polystyrene-coated lithium iron phosphate precursor; finally, calcination is performed under a protective atmosphere, and the cationic polystyrene layer is converted into a porous carbon layer. The carbon layer is conducive to electronic conductivity, and the porous structure is conducive to the shuttling of lithium ions. The preparation method of the present application controls the lithium iron phosphate to be nanometer-sized while coating a layer of porous carbon, which helps to reduce the electrochemical impedance of the lithium iron phosphate material and improve its rate performance.

[0028] In one embodiment of the present application, in the above step S3, in terms of mass fraction, the amount of styrene added is 5-15wt% of the lithium iron phosphate precursor, the amount of the initiator added is 0.8-1.2wt% of styrene, and the amount of the cationic monomer added is 5-10wt% of styrene; and / or, the polymerization reaction temperature is 50-80°C; and / or, the polymerization reaction time is 15-20h; and / or, the polymerization reaction is carried out under stirring, and the stirring speed of the polymerization reaction is 300-500rpm; and / or, the cationic monomer is benzylethyltrimethylammonium chloride and / or 2-(methacryloyloxy)ethyltrimethylammonium chloride.

[0029] Preferably, the amount of styrene added is controlled within the above range, which helps to control the thickness of the carbon coating layer finally formed. Preferably, the amount of initiator added is controlled within the above range, which helps to improve the efficiency of the polymerization reaction. At the same time, the molecular weight distribution of the polymer is controlled, which is conducive to forming a carbon coating layer with stable structure and good conductivity. Preferably, the amount of cationic monomer added is controlled within the above range, which helps to reduce the probability of agglomeration of the lithium iron phosphate precursor during the polymerization reaction and control the degree of polymerization of cationic polystyrene. Preferably, the temperature and time of the polymerization reaction are controlled within the above range, which helps to improve the efficiency of the polymerization reaction and control the degree of polymerization of cationic polystyrene. Preferably, the stirring speed is controlled within the above range, which helps to improve the dispersion uniformity of the reactants and reduce the probability of particle agglomeration. Preferably, the type of cationic monomer is controlled within the above range, which helps to improve the compatibility with styrene, thereby helping to further control the degree of polymerization of the polymer and reduce the probability of agglomeration of the lithium iron phosphate precursor. The lipophilic groups in benzylethyltrimethylammonium chloride and 2-(methacryloyloxy)ethyltrimethylammonium chloride have unsaturated C=C bonds, which can participate in the end-capping polymerization of styrene. In the aqueous system, the hydrophilic groups of the cationic monomers carry positive charges due to hydrolysis, and the presence of three methyl groups also increases their steric hindrance effect, which helps to avoid complexation with free radicals in the system. Therefore, the surface electrostatic repulsion between the end-capped microspheres is achieved, which helps to improve the monodispersity of the lithium iron phosphate precursor.

[0030] Including but not limited to, the second solvent is water and / or ethanol, and the initiator is potassium persulfate and / or azobisisobutyronitrile. Preferably, when the proportion of water in the second solvent is higher than that of ethanol, the preferred initiator is potassium persulfate, and when the proportion of ethanol in the second solvent is higher than that of water, the preferred initiator is azobisisobutyronitrile.

[0031] In order to fully remove oxygen in the system, in one embodiment of the present application, in the above step S3, an inert gas is passed into the system for 15 to 30 minutes before the polymerization reaction begins.

[0032] In order to further control the degree of polymerization of the polymer and reduce the probability of agglomeration of the lithium iron phosphate precursor, in one embodiment of the present application, it is preferred that in the above step S3, the cationic monomer is added in batches. Preferably, the polymerization reaction time is recorded as T, and the total amount of cationic monomer added is recorded as M. As the polymerization reaction proceeds, when T reaches 70 to 100 minutes, the cationic monomer is added to the polymerization reaction system, and is added once every 20 to 30 minutes, and the amount added each time is m1. After adding 3 to 5 times, the addition is stopped; wherein m1 / M=1% to 1.5%; when T reaches 220 to 240 min, continue to add cationic monomer to the polymerization reaction, and add it once every 40 to 60 min, each addition amount is m2, and stop adding after 1 to 2 additions; wherein m2 / M=5% to 8%; when T reaches 300 to 320 min, continue to add cationic monomer to the polymerization reaction, and add it once every 3 to 5 min, each addition amount is m3, until the total mass of the cationic monomer is added; wherein m3 / M=1% to 1.5%.

[0033] In order to improve the accuracy of the amount of cationic monomer added, in one embodiment of the present application, the cationic monomer and water are prepared into a solution for addition; preferably, the volume ratio of the cationic monomer to water is 1:1.

[0034] In one embodiment of the present application, in the above step S4, the calcination conditions are: first heating to 200-400°C at a heating rate of 2-10°C / min and keeping warm for 3-6 hours, then heating to 600-800°C at a heating rate of 2-10°C / min and keeping warm for 8-12 hours.

[0035] Initially, the temperature is raised to 200-400°C at a heating rate of 2-10°C / min, and kept within this temperature range for 3-6 hours. The main function of this stage is to promote the initial pyrolysis of the cationic polystyrene layer and begin to form a carbonized layer. The lower heating rate and moderate insulation temperature help to make the cationic polystyrene layer pyrolyze evenly, thereby helping to form a primary carbon layer with uniform structure. Subsequently, the temperature is raised to 600-800°C at 2-10°C / min again and kept for 8-12 hours. The long-term insulation at high temperature in this stage promotes further pyrolysis and graphitization of the carbon layer, forming a carbon coating layer with high conductivity and porous structure. High temperature helps to increase the crystallinity of the carbon layer, thereby helping to improve the electronic conductivity of the carbon-coated lithium iron phosphate, while long-term insulation helps to promote the formation of a porous structure.

[0036] In one embodiment of the present application, in the above step S2, the solid content of the slurry is 10-40%; and / or the air inlet temperature of the spray drying is 180-240°C; and / or the air outlet temperature of the spray drying is 80-160°C; and / or the average particle size of the lithium iron phosphate precursor is 300-900nm.

[0037] Controlling the solid content of the slurry within the above range helps to balance the rheological properties and drying efficiency of the slurry. Preferably, the air inlet temperature of the spray drying and the air outlet temperature of the spray drying are controlled within the above range, which helps to improve the drying efficiency while ensuring the stability of the material structure. Preferably, the average particle size of the lithium iron phosphate precursor is controlled within the above range, which helps to control the final carbon-coated lithium iron phosphate to be at the nanometer level, thereby helping to improve the rate performance of the carbon-coated lithium iron phosphate.

[0038] In one embodiment of the present application, in the above step S1, the molar ratio of lithium in the lithium source, phosphorus in the phosphorus source and iron in the iron source is 1:0.98~1.02:1; and / or, the raw material also includes a nitrogen source, and the mass ratio of the nitrogen source to the lithium source is 1:2~3; preferably, the nitrogen source is selected from any one or more of urea, azobisisobutyronitrile, p-nitroaniline, triethylamine, ethyleneimine and pyrrolidine; and / or, the lithium source is selected from any one or more of lithium carbonate, lithium hydroxide, lithium acetate and lithium oxalate; and / or, the phosphorus source is diammonium phosphate and / or phosphoric acid; and / or, the iron source is selected from any one or more of ferrous oxalate, ferrous chloride and ferric oxide.

[0039] The molar ratio of lithium in the lithium source, phosphorus in the phosphorus source and iron in the iron source is preferably controlled within the above range, which helps to reduce the risk of decreased capacity and cycle stability of lithium iron phosphate due to deviation in element ratio. During the calcination process, the nitrogen source is incorporated into the lattice of lithium iron phosphate to form nitrogen-doped LiFePO4, which helps to improve the conductivity, structural stability and thermal stability of lithium iron phosphate. It is preferred to control the types of nitrogen source, lithium source, phosphorus source and iron source within the above range, which helps to further improve the rate performance of carbon-coated lithium iron phosphate.

[0040] In order to improve the uniformity of raw material mixing, in one embodiment of the present application, the mixing in the above step S1 is preferably sand milling mixing, and the sand milling mixing time is 3 to 6 hours.

[0041] Including but not limited to, the gas in the above-mentioned inert atmosphere is selected from any one or more of nitrogen, argon and helium.

[0042] Including but not limited to, the first solvent is water.

[0043] In another typical embodiment of the present application, a carbon-coated lithium iron phosphate is provided. The carbon-coated lithium iron phosphate is prepared by the aforementioned preparation method.

[0044] The carbon-coated lithium iron phosphate has a high rate performance.

[0045] In one embodiment of the present application, the D50 of the carbon-coated lithium iron phosphate is 0.5-0.9 μm.

[0046] The carbon-coated lithium iron phosphate having the above-mentioned particle size helps to reduce the probability of agglomeration of the carbon-coated lithium iron phosphate while shortening the transmission path of lithium ions, thereby helping to further improve the high rate performance of the carbon-coated lithium iron phosphate.

[0047] In another typical embodiment of the present application, a positive electrode sheet is provided, wherein the positive electrode sheet contains the aforementioned carbon-coated lithium iron phosphate.

[0048] The above positive electrode sheet has a high rate performance.

[0049] In another typical embodiment of the present application, a lithium-ion battery is provided, comprising a positive electrode sheet, an electrolyte and a negative electrode sheet, wherein the positive electrode sheet is the positive electrode sheet mentioned above.

[0050] The above lithium-ion battery has high rate performance and safety.

[0051] The beneficial effects of the present application will be further illustrated below in conjunction with embodiments.

[0052] Example 1

[0053] (1) Synthesis of lithium iron phosphate precursor:

[0054] Weigh 369.46g of lithium carbonate, 719.30g of ferrous oxalate, and 573.13g of ammonium dihydrogen phosphate, add them to a sand mill, then add 2176.84g of deionized water for sand milling for 0.5h, then add 150.15g of urea and continue sand milling for 3.5h; add an appropriate amount of deionized water to the sand milled slurry to adjust the solid content of the uniform slurry to 20%; control the air inlet temperature of the spray dryer to 200°C and the air outlet temperature to 100°C, and spray dry the uniform slurry. The obtained dry lithium iron phosphate precursor is crushed, and the average particle size of the lithium iron phosphate precursor is controlled to be 500nm to obtain a lithium iron phosphate precursor.

[0055] (2) Preparation of cationic polystyrene-coated lithium iron phosphate precursor:

[0056] In a 2000ml four-necked flask, 100g of lithium iron phosphate precursor, 5g of styrene, and 0.05g of K2S2O8 were added and dispersed in 900g of deionized water. Under the action of mechanical stirring at 400rpm, nitrogen was introduced for 30min, and then placed in a 70℃ constant temperature water bath. The entire reaction process was carried out under nitrogen protection. 0.375g of 2-(methacryloyloxy)ethyltrimethylammonium chloride (DMC) monomer was weighed and mixed with deionized water in a volume ratio of 1:1 to prepare a mixed solution. The polymerization reaction time was T. When T=90min, the reaction solution was added Cationic monomer solution, the mass of cationic monomer in each added solution is 0.005g; add once every 30min, and stop adding after 4 additions; when T=240min, start adding cationic monomer solution to the reaction solution again, the mass of cationic monomer in each added solution is 0.025g; add once every 60min, and stop adding after 2 additions; when T=303min, start adding cationic monomer to the reaction solution again, the mass of cationic monomer in each added solution is 0.005g; add once every 3min until the cationic monomer solution is added. The polymerization reaction time is 17h. After the polymerization is completed, the product is filtered, washed, separated, and vacuum dried to obtain a cationic polystyrene-coated lithium iron phosphate precursor.

[0057] (3) Preparation of carbon-coated lithium iron phosphate

[0058] The precursor dried in step (2) is first heated to 300°C at a heating rate of 5°C / min in a high-purity argon atmosphere and kept warm for 3 hours, and then heated to 700°C at a heating rate of 5°C / min and kept warm for 10 hours; after cooling, it is pulverized by air flow to finally obtain carbon-coated lithium iron phosphate with a D50 of 0.7μm.

[0059] Example 2

[0060] The difference from Example 1 is that in step (2), the amount of styrene is 10 g, the amount of K2S2O8 is 0.1 g, and the amount of DMC is 0.75 g, and finally carbon-coated lithium iron phosphate is obtained.

[0061] Example 3

[0062] The difference from Example 1 is that in step (2), the amount of styrene is 15 g, the amount of K2S2O8 is 0.15 g, and the amount of DMC is 1.125 g, and finally carbon-coated lithium iron phosphate is obtained.

[0063] Example 4

[0064] The difference from Example 1 is that the amount of styrene is 4 g, and carbon-coated lithium iron phosphate is finally obtained.

[0065] Example 5

[0066] The difference from Example 1 is that the amount of DMC is 0.25 g, and carbon-coated lithium iron phosphate is finally obtained.

[0067] Example 6

[0068] The difference from Example 1 is that the amount of DMC is 0.5 g, and carbon-coated lithium iron phosphate is finally obtained.

[0069] Example 7

[0070] The difference from Example 1 is that the amount of DMC is 0.6 g, and carbon-coated lithium iron phosphate is finally obtained.

[0071] Example 8

[0072] The difference from Example 1 is that the polymerization reaction temperature is 50° C., the polymerization reaction time is 20 h, the stirring speed of the polymerization reaction is 300 rpm, and finally carbon-coated lithium iron phosphate is obtained.

[0073] Example 9

[0074] The difference from Example 1 is that the polymerization reaction temperature is 80° C., the polymerization reaction time is 15 h, the stirring speed of the polymerization reaction is 500 rpm, and finally carbon-coated lithium iron phosphate is obtained.

[0075] Example 10

[0076] The difference from Example 1 is that the polymerization reaction temperature is 90° C., the polymerization reaction time is 12 h, the stirring speed of the polymerization reaction is 600 rpm, and finally carbon-coated lithium iron phosphate is obtained.

[0077] Embodiment 11

[0078] The difference from Example 1 is that the polymerization reaction time is H. When H=70min, the cationic monomer solution is added to the reaction solution, and the mass of the cationic monomer in the solution added each time is 0.006g; it is added once every 30min, and the addition is stopped after 3 additions; when H=240min, the cationic monomer solution is added to the reaction solution again, and the mass of the cationic monomer in the solution added each time is 0.030g; it is added once every 40min, and the addition is stopped after 2 additions; when H=300min, the cationic monomer is added to the reaction solution again, and the mass of the cationic monomer in the solution added each time is 0.006g; it is added once every 5min until the cationic monomer solution is added, and finally carbon-coated lithium iron phosphate is obtained.

[0079] Example 12

[0080] The difference from Example 1 is that the polymerization reaction time is H. When H=100min, the cationic monomer solution is added to the reaction solution, and the mass of the cationic monomer in the solution added each time is 0.004; it is added once every 20min, and the addition is stopped after 5 additions; when H=220min, the cationic monomer solution is added to the reaction solution again, and the mass of the cationic monomer in the solution added each time is 0.019g; it is added once every 60min, and the addition is stopped after 1 addition; when H=320min, the cationic monomer is added to the reaction solution again, and the mass of the cationic monomer in the solution added each time is 0.004g; it is added once every 3min until the cationic monomer solution is added, and finally carbon-coated lithium iron phosphate is obtained.

[0081] Embodiment 13

[0082] The difference from Example 1 is that the polymerization reaction time is H. When H=60min, the cationic monomer solution is added to the reaction solution, and the mass of the cationic monomer in the solution added each time is 0.008; it is added once every 10min, and the addition is stopped after 6 additions; when H=200min, the cationic monomer solution is added to the reaction solution again, and the mass of the cationic monomer in the solution added each time is 0.035; it is added once every 30min, and the addition is stopped after 2 additions; when H=280min, the cationic monomer is added to the reaction solution again, and the mass of the cationic monomer in the solution added each time is 0.002; it is added once every 2min until the cationic monomer solution is added, and finally carbon-coated lithium iron phosphate is obtained.

[0083] Embodiment 14

[0084] The difference from Example 1 is that the average particle size of the lithium iron phosphate precursor is 300 nm, and the carbon-coated lithium iron phosphate with a D50 of 0.5 μm is finally obtained.

[0085] Embodiment 15

[0086] The difference from Example 1 is that the average particle size of the lithium iron phosphate precursor is 900 nm, and the carbon-coated lithium iron phosphate with a D50 of 0.9 μm is finally obtained.

[0087] Example 16

[0088] The difference from Example 1 is that the average particle size of the lithium iron phosphate precursor is 200 nm, and the carbon-coated lithium iron phosphate with a D50 of 0.4 μm is finally obtained.

[0089] Embodiment 17

[0090] The difference from Example 1 is that the mass of urea is 184.73 g, and carbon-coated lithium iron phosphate is finally obtained.

[0091] Embodiment 18

[0092] The difference from Example 1 is that the mass of urea is 123.15 g, and carbon-coated lithium iron phosphate is finally obtained.

[0093] Embodiment 19

[0094] The difference from Example 1 is that the mass of urea is 92.36 g, and carbon-coated lithium iron phosphate is finally obtained.

[0095] Comparative Example 1

[0096] The difference from Example 1 is that step (2) is skipped, and the lithium iron phosphate precursor obtained in step (1) is directly subjected to step (3) to obtain lithium iron phosphate that is not carbon-coated.

[0097] Comparative Example 2

[0098] The difference from Example 1 is that after adding the lithium source, iron source, phosphorus source and deionized water in step (1), sand milling is performed for 0.5 h, and then after adding urea, liquid resol phenolic resin is added according to 10% of the theoretical output of lithium iron phosphate mass, and then sand milling is performed together. The subsequent steps are consistent with step (1) of Example 1; then step (2) is skipped, and the lithium iron phosphate precursor obtained in step (1) is directly subjected to step (3) to obtain carbon-coated lithium iron phosphate.

[0099] Comparative Example 3

[0100] The difference from Example 1 is that 2-(methacryloyloxy)ethyltrimethylammonium chloride is not added in step (2), and carbon-coated lithium iron phosphate is finally obtained.

[0101] Performance Testing

[0102] The samples prepared in the examples and comparative examples were subjected to electrochemical performance tests using button-type lithium batteries (CR2032), and the assembly environment was a glove box filled with argon. The sample, conductive agent (SP) and binder (PVDF) were prepared into positive electrode slurry in a mass ratio of 8:1:1, and the solvent was N-methylpyrrolidone (NMP). The prepared slurry was then coated on aluminum foil with a coating thickness of 50 μm, and then dried at 80°C for 12 hours under vacuum conditions, rolled, and punched. A lithium sheet was used as the negative electrode, the diaphragm was a PE film, and the electrolyte was a solution of LiPF6 (volume ratio EC:DEC=1:1). The prepared button-type lithium battery was left to stand for 12 hours for electrochemical testing. The battery charge and discharge test was tested using a blue electric test cabinet with a voltage range of 2.5 to 3.65V and a test temperature of 25°C. The test results are shown in Table 1.

[0103] Table 1

[0104]

[0105] Figure 1 The XRD diffraction pattern of carbon-coated lithium iron phosphate in Example 1 of the present application is as follows: Figure 1 It can be seen that it is the characteristic peak of LiFePO4.

[0106] Figure 2 The rate performance diagrams of the samples in Example 1, Example 2, Example 3, Comparative Example 1 and Comparative Example 2 of the present application are obtained by testing at 25°C, and the data are tested three times at the rates of 0.1C, 0.33C, 1C, 2C, 3C, 5C and 10C. Figure 2 It can be seen that the gram capacity of Examples 1 to 3 is slightly different at low rates (0.1C, 0.33C, 1C, 2C), but at high rates (3C, 5C, 10C), the gram capacity of Example 2 (10% polystyrene coating) is significantly better than that of Example 1 (5% polystyrene coating) and Example 3 (15% polystyrene coating). The possible reason is that when the polystyrene coating is 5%, the coating layer on the surface of the nano-lithium iron phosphate precursor particles is thin or there is an uncoated area, and the carbon layer formed after calcination may not be completely coated, resulting in large polarization at high rates and low released capacity; and when the polystyrene coating is 15%, the coating layer may be thicker, which has an inhibitory effect on the transmission of lithium ions, thereby affecting its discharge performance under high current. Compared with Comparative Example 1 (nano lithium iron phosphate without carbon coating), Examples 1 to 3 show better rate performance. For uncoated nano lithium iron phosphate, the effect of electron transmission by the active material itself is far inferior to that of carbon-coated nano lithium iron phosphate, because the conductivity of lithium iron phosphate itself is low, so polarization is serious, especially significant under high current. Compared with comparative example 2 (carbon coating is performed by adding an organic carbon source when synthesizing the precursor), the rate performance of embodiments 1 to 3 is relatively good, because the cationic polystyrene layer is coated on the surface of the nano lithium iron phosphate precursor, and the cationic polystyrene layer can effectively prevent the agglomeration between the nano lithium iron phosphate precursor particles, forming a monodisperse phase spherical cationic polystyrene-coated lithium iron phosphate precursor, thereby improving the conductivity and rate performance of the carbon-coated lithium iron phosphate finally formed.

[0107] From the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects:

[0108] In step S2 of the present application, the slurry is spray-dried and pulverized in sequence, which helps to control the size of the lithium iron phosphate precursor at the nanometer level, thereby helping to make the size of the carbon-coated lithium iron phosphate finally prepared also at the nanometer level, thereby helping to improve the conductivity of the carbon-coated lithium iron phosphate and shorten the diffusion path of lithium ions. Step S3 coats the surface of the nano lithium iron phosphate precursor with a cationic polystyrene layer, which can effectively prevent the agglomeration between the nano lithium iron phosphate precursor particles to form a monodisperse spherical cationic polystyrene-coated lithium iron phosphate precursor; finally, calcination is performed under a protective atmosphere, and the cationic polystyrene layer is converted into a porous carbon layer. The carbon layer is conducive to electronic conductivity, and the porous structure is conducive to the shuttling of lithium ions. The preparation method of the present application controls the lithium iron phosphate to be nanometer-sized while coating a layer of porous carbon, which helps to reduce the electrochemical impedance of the lithium iron phosphate material and improve its rate performance.

[0109] The above are only embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing carbon-coated lithium iron phosphate, characterized in that: The preparation method comprises: Step S1, mixing raw materials including a lithium source, a phosphorus source, an iron source and a first solvent to obtain a slurry; Step S2, spray drying and pulverizing the slurry in sequence to obtain a lithium iron phosphate precursor; Step S3, subjecting the raw materials of the lithium iron phosphate precursor, styrene, an initiator and a second solvent to a polymerization reaction in an inert atmosphere, and adding a cationic monomer during the polymerization reaction to obtain a cationic polystyrene-coated lithium iron phosphate precursor; Step S4, calcining the cationic polystyrene-coated lithium iron phosphate precursor in an inert atmosphere to obtain the carbon-coated lithium iron phosphate.

2. The preparation method according to claim 1, characterized in that: In the step S3, the amount of styrene added is 5-15wt% of the lithium iron phosphate precursor, the amount of the initiator added is 0.8-1.2wt% of the styrene, and the amount of the cationic monomer added is 5-10wt% of the styrene. and / or, the polymerization reaction temperature is 50-80° C.; and / or, the polymerization reaction time is 15-20 h; and / or, the polymerization reaction is carried out under stirring, and the stirring speed of the polymerization reaction is 300-500 rpm; And / or, the cationic monomer is benzylethyltrimethylammonium chloride and / or 2-(methacryloyloxy)ethyltrimethylammonium chloride.

3. The preparation method according to claim 1 or 2, characterized in that: In the step S3, the cationic monomer is added in batches. Preferably, the polymerization reaction time is recorded as T, and the total amount of the cationic monomer added is recorded as M. As the polymerization reaction proceeds, When T reaches 70 to 100 min, the cationic monomer is added to the polymerization reaction system, and the addition is performed once every 20 to 30 min, and the amount added each time is m1. The addition is stopped after 3 to 5 additions; wherein, m1 / M=1.0%~1.5%; When T reaches 220-240 min, continue to add the cationic monomer to the polymerization reaction, and add it once every 40-60 min, each addition amount is m2, and stop adding after 1-2 additions; wherein, m2 / M=5.0%~8.0%; When T reaches 300-320 min, continue to add the cationic monomer to the polymerization reaction, and add it once every 3-5 min, with the amount added each time being m3, until the total mass of the cationic monomer is added; wherein, m3 / M=1.0%~1.5%.

4. The preparation method according to any one of claims 1 to 3, characterized in that In step S4, the calcination conditions are: first heating to 200-400°C at a heating rate of 2-10°C / min and keeping warm for 3-6 hours, then heating to 600-800°C at a heating rate of 2-10°C / min and keeping warm for 8-12 hours.

5. The preparation method according to any one of claims 1 to 4, characterized in that: In step S2, the solid content of the slurry is 10-40%; and / or, the air inlet temperature of the spray drying is 180-240°C; and / or, the air outlet temperature of the spray drying is 80-160°C; And / or, the average particle size of the lithium iron phosphate precursor is 300-900 nm.

6. The preparation method according to any one of claims 1 to 5, characterized in that In the step S1, the molar ratio of the lithium element in the lithium source, the phosphorus element in the phosphorus source, and the iron element in the iron source is 1:0.98 to 1.02:1; And / or, the raw material further includes a nitrogen source, and the mass ratio of the nitrogen source to the lithium source is 1:2-3; Preferably, the nitrogen source is selected from any one or more of urea, azobisisobutyronitrile, p-nitroaniline, triethylamine, ethyleneimine and pyrrolidine; And / or, the lithium source is selected from any one or more of lithium carbonate, lithium hydroxide, lithium acetate and lithium oxalate; And / or, the phosphorus source is diammonium phosphate and / or phosphoric acid; And / or, the iron source is selected from any one or more of ferrous oxalate, ferrous chloride and ferric oxide.

7. A carbon-coated lithium iron phosphate, characterized in that: The carbon-coated lithium iron phosphate is prepared by the preparation method described in any one of claims 1 to 6.

8. The carbon-coated lithium iron phosphate according to claim 7, characterized in that: The D50 of the carbon-coated lithium iron phosphate is 0.5-0.9 μm.

9. A positive electrode sheet, characterized in that: The positive electrode sheet contains the carbon-coated lithium iron phosphate according to claim 7 or 8.

10. A lithium ion battery comprising a positive electrode, an electrolyte and a negative electrode, characterized in that: The positive electrode sheet is the positive electrode sheet according to claim 9.

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