Lithium iron phosphate material, preparation method thereof, positive electrode sheet and secondary battery

Abstract

The application provides a lithium iron phosphate material, a preparation method thereof, a positive electrode sheet and a secondary battery, wherein the lithium iron phosphate material comprises lithium iron phosphate active particles and a coating layer; the coating layer is coated on the surface of the lithium iron phosphate active particles, the coating layer comprises iron carbide, and the mass fraction of the iron carbide in the lithium iron phosphate material is 0.7% to 3.1%. By arranging the iron carbide in the coating layer on the surface of the lithium iron phosphate active particles and controlling the mass fraction of the iron carbide in the lithium iron phosphate material, the iron elution amount of the lithium iron phosphate material is effectively reduced, the stability of the coating layer is improved, the powder resistivity of the lithium iron phosphate material is reduced, and the conductivity of the lithium iron phosphate material is improved. The positive electrode sheet and / or the secondary battery based on the lithium iron phosphate material have good capacity performance, cycle performance, rate performance, low-temperature performance and safety performance.

Description

Technical Field

[0001] This application belongs to the field of secondary battery technology, and particularly relates to a lithium iron phosphate material and its preparation method, a positive electrode sheet, and a secondary battery. Background Technology

[0002] Lithium iron phosphate (LiFePO4), as the cathode material of lithium-ion batteries, is known for its high voltage, high energy density, environmental friendliness, and good safety performance. It occupies an important position in the electric vehicle market, such as as the cathode active material of secondary batteries in the field of new energy vehicles.

[0003] However, lithium iron phosphate (LFP) has low electronic conductivity, resulting in low discharge capacity and poor rate performance, which limits its application in secondary batteries. To address these issues, existing technologies have proposed surface coating modification methods. Currently, common coating materials are generally carbon materials, but due to their low sintering temperature and low degree of graphitization of the carbon layer, the LFP materials formed by carbon coating have poor conductivity and high powder resistivity.

[0004] Therefore, a new method is needed to prepare lithium iron phosphate materials with low powder resistivity, thereby effectively improving the electronic conductivity of lithium iron phosphate materials, reducing the interface resistance, and thus reducing the temperature rise of secondary batteries containing lithium iron phosphate materials. This would give lithium iron phosphate materials greater advantages in applications such as high-energy-density secondary batteries, quasi-solid-state secondary batteries, and solid-state secondary batteries. Summary of the Invention

[0005] In view of the technical problems existing in the background art, this application provides a lithium iron phosphate material and its preparation method, a positive electrode sheet and a secondary battery, aiming to solve the technical problem of high powder resistivity of existing lithium iron phosphate materials.

[0006] In a first aspect, embodiments of this application provide a lithium iron phosphate material, comprising: lithium iron phosphate active particles and a coating layer; wherein the coating layer coats the surface of the lithium iron phosphate active particles, the coating layer comprises iron carbide, and the mass fraction of iron carbide in the lithium iron phosphate material is 0.7% to 3.1%.

[0007] In the technical solution of this application embodiment, by setting iron carbide in the coating layer on the surface of lithium iron phosphate active particles and controlling the mass fraction of iron carbide in lithium iron phosphate material, the iron dissolution of lithium iron phosphate material is effectively reduced and the stability of the coating layer is improved, while the powder resistivity of lithium iron phosphate material is reduced and the conductivity of lithium iron phosphate material is improved.

[0008] In some embodiments, the average particle size of the primary particles of the lithium iron phosphate material is 100 nm to 400 nm; the mass fraction of carbon in the lithium iron phosphate material is 0.3% to 1.5%; the average particle size of the active lithium iron phosphate particles is 82 nm to 395 nm; and the thickness of the coating layer is 5 nm to 18 nm.

[0009] In the above embodiments, the small average particle size of the primary particles of the lithium iron phosphate material is beneficial for close contact and uniform arrangement between the particles, thereby improving the volumetric energy density of the lithium iron phosphate material. Furthermore, the small particle size of the lithium iron phosphate particles improves the ionic and electronic conductivity of the lithium iron phosphate material, thus enhancing its rate performance and low-temperature performance. When the mass fraction of carbon in the lithium iron phosphate material meets the above-mentioned range, it enhances the electronic conductivity of the lithium iron phosphate material while fully coating the surface of the active particles, improving the structural stability of the lithium iron phosphate material. When the average particle size of the active particles and the thickness of the coating layer meet the above-mentioned ranges, it enhances the overall electronic conductivity and structural stability of the material, improving the charge / discharge efficiency and rate performance of the lithium iron phosphate material.

[0010] In some embodiments, the powder resistivity of the lithium iron phosphate material at 8 MPa pressure is less than 30 Ω·cm; the iron leaching amount of the lithium iron phosphate material is less than 300 ppm; and the BET specific surface area of ​​the lithium iron phosphate material is 7.0 m². 2 / g~15.0m 2 / g; the compaction density of the lithium iron phosphate material under 3T pressure is 2.2g / mL~2.6g / mL; the content of magnetic material in the lithium iron phosphate material is 0.1ppm~210ppm.

[0011] In the above embodiments, the low resistivity of the lithium iron phosphate powder reduces the contact resistance of the lithium iron phosphate material, enhancing its rate performance and low-temperature performance while reducing the heat generated inside the lithium iron phosphate material during charge-discharge, extending its cycle life, and reducing the risk of thermal runaway in secondary batteries using lithium iron phosphate. This is beneficial for the application of lithium iron phosphate in high-energy-density secondary batteries, quasi-solid-state secondary batteries, and solid-state secondary batteries. The low iron dissolution of the lithium iron phosphate material helps to improve the problem of iron deposition at the negative electrode and its impact on electrolyte stability, thereby improving the cycle performance and cycle life of secondary batteries using lithium iron phosphate. The moderate BET specific surface area of ​​the lithium iron phosphate material helps to provide an appropriate number of active sites, improving the electrochemical activity of the positive electrode material. The high compaction density of the lithium iron phosphate material and the greater density between the lithium iron phosphate particles are beneficial to increasing the volumetric energy density of the lithium iron phosphate material. The low content of magnetic materials in the lithium iron phosphate material helps to reduce the possibility of dangers such as short circuits or thermal runaway.

[0012] Secondly, embodiments of this application provide a method for preparing lithium iron phosphate material, comprising the following steps:

[0013] Provides lithium iron phosphate active particles;

[0014] Lithium iron phosphate active particles undergo a first coating treatment to obtain intermediate particles;

[0015] The intermediate particles undergo a second coating treatment to obtain lithium iron phosphate material; wherein,

[0016] The lithium iron phosphate material includes lithium iron phosphate active particles and a coating layer; the coating layer is coated on the surface of the lithium iron phosphate active particles, and the coating layer includes iron carbide, with the mass fraction of iron carbide in the lithium iron phosphate material being 0.7%~3.1%.

[0017] In the technical solution of this application embodiment, the preparation method of this application is simple and easy to implement. Specifically, the lithium iron phosphate active particles are sequentially subjected to a first coating treatment and a second coating treatment to form a coating layer containing iron carbide on the surface of the lithium iron phosphate active particles. The mass fraction of iron carbide in the lithium iron phosphate material is controlled to obtain the lithium iron phosphate material. While effectively reducing the amount of iron leaching from the lithium iron phosphate material and improving the stability of the coating layer, the resistivity of the lithium iron phosphate powder is reduced and the conductivity of the lithium iron phosphate material is improved.

[0018] In some embodiments, the preparation steps of lithium iron phosphate active particles include: mixing a first ferrous source, a phosphorus source, a lithium source, a first alkali, a first reducing agent, a dispersant, and a first solvent to obtain a first mixture; subjecting the first mixture to a first reaction treatment and a first solid-liquid separation treatment to obtain lithium iron phosphate active particles; wherein, the molar ratio of iron in the first ferrous source to phosphorus in the phosphorus source is 1:(1.2~1.5); the molar ratio of iron in the first ferrous source to lithium in the lithium source is 1:(2.1~2.3); the molar ratio of iron in the first ferrous source to lithium in the lithium source is 1:(2.1~2.3); the molar ratio of iron in the first ferrous source to lithium in the lithium source is 1:(2.1~2.3). The molar ratio of iron to the first alkali is 1:(1.2~2); the molar ratio of iron to the first reducing agent in the first ferrous source is 1:(0.1~0.2); the dispersant is at least one of polyethylene glycol, triethylhexyl phosphate, sodium dodecyl sulfate, and methylpentanol; the mass fraction of the dispersant in the first mixture is 0.5%~1%; the density of the first mixture is 1.20 g / mL~1.35 g / mL; the temperature of the first reaction treatment is 180℃~250℃, and the time of the first reaction treatment is 10h~20h.

[0019] In the above embodiments, the first ferrous source, phosphorus source, lithium source, first alkali, first reducing agent, dispersant and first solvent are mixed for a first reaction treatment, followed by a first solid-liquid separation treatment, which is beneficial to obtaining lithium iron phosphate active particles with regular morphology and fine particle size. The introduction of the first reducing agent helps to inhibit the oxidation of ferrous ions, thereby facilitating the full participation of the iron element provided by the first ferrous source in the formation of lithium iron phosphate active particles. The introduction of the dispersant helps to uniformly disperse other raw materials, making the reaction more uniform and complete, thereby facilitating the obtaining of lithium iron phosphate active particles with regular morphology and fine particle size, which is convenient for the full coating in subsequent coating steps.

[0020] In the above embodiments, when the molar ratio of iron in the first ferrous source to phosphorus in the phosphorus source meets the above-mentioned range, it is beneficial for the iron provided by the first ferrous source to fully participate in the reaction to form lithium iron phosphate active particles. When the molar ratio of iron in the first ferrous source to lithium in the lithium source meets the above-mentioned range, the lithium element is in excess, which is beneficial for promoting the complete reaction and compensating for the loss of lithium element caused by high temperature. This helps to obtain lithium iron phosphate material with good crystallinity, high purity, and stable structure, thereby improving the electrochemical performance of lithium iron phosphate material. When the molar ratio of iron in the first ferrous source to the first alkali meets the above-mentioned range, the oxidation of ferrous ions can be avoided, thereby making the uncoated lithium iron phosphate highly crystallizable and avoiding the presence of trivalent iron. When the molar ratio of iron to the first reducing agent in the first ferrous source meets the above-mentioned range, the oxidation of ferrous ions is suppressed while avoiding waste of raw materials. The dispersant has the characteristics of low cost, wide availability, and good solubility, which is conducive to achieving more uniform dispersion of each raw material. When the mass fraction of the dispersant in the first mixture meets the above-mentioned range, it is conducive to achieving sufficient dispersion of each raw material, thereby improving the uniformity of the obtained lithium iron phosphate active particles. When the density of the first mixture meets the above-mentioned range, it is conducive to achieving uniform mixing of each raw material, making the first reaction treatment more uniform and sufficient, and avoiding the problems of large first solvent consumption and low production efficiency. When the temperature and time of the first reaction treatment meet the above-mentioned range, it is conducive to obtaining lithium iron phosphate materials with stable crystal structure, small particle size, and regular morphology.

[0021] In some embodiments, the step of obtaining intermediate particles by first coating of lithium iron phosphate active particles includes: mixing lithium iron phosphate active particles, a second ferrous source and a second solvent to obtain a second mixture; mixing a second alkali and a second reducing agent into the second mixture at a first mixing temperature and a first mixing time to obtain a third mixture; and subjecting the third mixture to a second solid-liquid separation treatment to obtain intermediate particles; wherein the mass ratio of lithium iron phosphate active particles to iron in the second ferrous source is 1:(0.01~0.02); the molar ratio of iron in the second ferrous source to the second reducing agent is 1:(3~6); the first mixing temperature is 25℃~45℃, and the first mixing time is 1h~3h.

[0022] In the above embodiments, the second alkali and the second reducing agent are mixed into the second mixture containing lithium iron phosphate active particles and the second ferrous source at the first mixing temperature and the first mixing time, so that the ferrous ions react with the reducing agent to form elemental iron, and the elemental iron is deposited on the surface of the lithium iron phosphate active particles to initially form a metallic iron coating layer. This application uses a chemical plating method to form a metallic iron coating layer on the surface of lithium iron phosphate active lithium. Compared with other processes, the process of this application is simple and highly controllable.

[0023] In the above embodiments, when the mass ratio of lithium iron phosphate active particles to iron in the second ferrous source meets the above range, it is beneficial for the iron element formed by the reduction of ferrous ions in the second ferrous source to fully coat the lithium iron phosphate active particles, which is beneficial for effectively reducing the powder resistivity of lithium iron phosphate material; when the molar ratio of iron in the second ferrous source to the second reducing agent meets the above range, it is beneficial for fully reducing ferrous ions and avoiding raw material waste; when the first mixing temperature and the first mixing time meet the above range, it is beneficial for controlling the coating speed, uniformly forming the metallic iron coating layer, and making the metallic iron coating layer more complete.

[0024] In some embodiments, the step of obtaining lithium iron phosphate material by second coating treatment of intermediate particles includes: mixing intermediate particles, carbon source and third solvent, drying to obtain dried material; and sintering the dried material to obtain lithium iron phosphate material.

[0025] In the above embodiments, intermediate particles, carbon source and third solvent are mixed and dried to make the carbon source and intermediate particles uniformly mixed to obtain dried material; the dried material is sintered to make the carbon source pyrolyze at high temperature to initially form a carbon coating layer, the carbon coating layer coats the metallic iron coating layer and further reacts with the elemental iron in the metallic iron coating layer to form iron carbide to obtain a coating layer containing iron carbide, thereby avoiding the increase of iron dissolution caused by direct contact between elemental iron and electrolyte, while improving the conductivity of lithium iron phosphate material and the stability of coating layer.

[0026] In some embodiments, the mass ratio of intermediate particles to carbon source is 1:(0.04~0.08); the carbon source is at least one of glucose, sucrose, PEG, and water-soluble starch; the step of obtaining lithium iron phosphate material by sintering the dried material includes: obtaining lithium iron phosphate material by holding the dried material at a sintering temperature of 800℃~900℃ for 5h~10h.

[0027] In the above embodiments, when the mass ratio of intermediate particles to carbon source meets the above range, it is beneficial for the carbon coating layer formed by the carbon source to fully coat the intermediate particles, and for the fully reacted with the metallic iron coating layer after coating, thereby improving the electronic conductivity and structural stability of the lithium iron phosphate material. The above-mentioned carbon source is widely available, low in cost, and suitable for forming a uniform carbon coating layer after high-temperature pyrolysis, thereby reacting with the metallic iron coating layer to form a coating layer, improving the electronic conductivity and structural stability of the lithium iron phosphate material. When the sintering temperature and holding time meet the above range, it is beneficial for the high-temperature pyrolysis of the carbon source and its full reaction with metallic iron to generate iron carbide, thereby forming a coating layer containing iron carbide on the surface of the lithium iron phosphate active particles.

[0028] Thirdly, embodiments of this application provide a positive electrode sheet, which includes lithium iron phosphate material prepared by the preparation method of lithium iron phosphate material provided in the first aspect of this application or lithium iron phosphate material provided in the second aspect of this application.

[0029] In the technical solution of this application embodiment, the positive electrode sheet contains the above-mentioned lithium iron phosphate material, and therefore has good capacity performance, cycle performance, rate performance, low temperature performance and safety performance.

[0030] Fourthly, embodiments of this application provide a secondary battery, which includes the positive electrode sheet provided in the third aspect of this application.

[0031] In the technical solution of this application embodiment, the secondary battery includes the above-mentioned positive electrode sheet, and therefore has good capacity performance, cycle performance, rate performance, low temperature performance and safety performance.

[0032] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0033] To more clearly illustrate the technical solutions of this application, the accompanying drawings used in this application will be briefly described below. Obviously, the drawings described below are merely some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort.

[0034] Figure 1 This is a process flow diagram of one embodiment of the method for preparing lithium iron phosphate material provided in this application;

[0035] Figure 2 This is a scanning electron microscope image of the lithium iron phosphate material provided in Embodiment 1 of this application. Detailed Implementation

[0036] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0037] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0038] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0039] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0040] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0041] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0042] In the description of the embodiments of this application, "ppm" means the mass of the tested element, molecule or ion in parts per million of the sample mass.

[0043] In the description of the embodiments of this application, unless otherwise specified, "content" means the mass percentage of the tested element, molecule or ion in the sample.

[0044] The high resistivity of lithium iron phosphate powder in existing technologies results in low electronic conductivity, poor capacity and rate performance, thus limiting the practical application of lithium iron phosphate in secondary batteries.

[0045] To address the high resistivity of existing lithium iron phosphate (LFP) materials, this application provides a LFP material, its preparation method, a positive electrode, and a secondary battery. This application incorporates iron carbide in a coating layer on the surface of LFP active particles and controls the mass fraction of iron carbide in the LFP material. This effectively reduces iron leaching from the LFP material, improves the stability of the coating layer, lowers the powder resistivity, and enhances the conductivity of the LFP material. The positive electrode and / or secondary battery based on the above LFP material exhibit excellent capacity performance, cycle performance, rate performance, low-temperature performance, and safety performance.

[0046] In a first aspect, embodiments of this application provide a lithium iron phosphate material, comprising: lithium iron phosphate active particles and a coating layer; wherein, the coating layer coats the surface of the lithium iron phosphate active particles, the coating layer comprises iron carbide, and the mass fraction of iron carbide in the lithium iron phosphate material is 0.7% to 3.1%, for example, it can be 0.7%, 0.8%, 1%, 1.2%, 1.5%, 1.7%, 2.0%, 2.2%, 2.5%, 2.7%, 3%, 3.1%, or any value between any two of the above.

[0047] In the technical solution of this application embodiment, by setting iron carbide in the coating layer on the surface of lithium iron phosphate active particles and controlling the mass fraction of iron carbide in the lithium iron phosphate material, the iron dissolution of the lithium iron phosphate material is effectively reduced and the stability of the coating layer is improved, while the powder resistivity of the lithium iron phosphate material is reduced and the conductivity of the lithium iron phosphate material is improved. If the mass fraction of iron carbide is too low, it is easy to lead to insufficient reduction of powder resistivity; if the mass fraction of iron carbide is too high, it is easy to lead to increased iron dissolution. Furthermore, the mass fraction of iron carbide in the lithium iron phosphate material is preferably 1.7% to 2.2%.

[0048] Furthermore, in some embodiments, the average particle size of the primary particles of the lithium iron phosphate material is 100nm~400nm, and further, it can be 280nm~400nm.

[0049] In the technical solution of this application embodiment, the average particle size of the primary particles of lithium iron phosphate material is small, which is conducive to achieving close contact and uniform arrangement between lithium iron phosphate material particles, thereby improving the volumetric energy density of lithium iron phosphate material. In addition, the small particle size of lithium iron phosphate material particles is conducive to improving the ionic conductivity and electronic conductivity of lithium iron phosphate material, thereby improving the rate performance and low temperature performance of lithium iron phosphate material.

[0050] Furthermore, in some embodiments, the mass fraction of carbon in the lithium iron phosphate material is 0.3% to 1.5%, for example, it can be 0.3%, 0.5%, 0.8%, 1.0%, 1.3%, 1.5% and any two of the above values, and is more preferably 0.4% to 1.0%.

[0051] In the technical solution of this application embodiment, when the mass fraction of carbon element in the lithium iron phosphate material meets the above range, it is beneficial to enhance the electronic conductivity of the lithium iron phosphate material while fully coating the surface of the lithium iron phosphate active particles and improving the structural stability of the lithium iron phosphate material.

[0052] In some embodiments, the lithium iron phosphate material proposed in this application has a Li mass percentage content of 4.45-4.57%, preferably 4.47-4.51%; an Fe mass percentage content of 34.65-35.87%, preferably 35.2-35.4%; and a P mass percentage content of 19.38-19.76%, preferably 19.5-19.6%.

[0053] Furthermore, in some embodiments, the average particle size of the lithium iron phosphate active particles is 82nm~395nm, for example, it can be 82nm, 90nm, 100nm, 120nm, 150nm, 180nm, 200nm, 230nm, 250nm, 290nm, 300nm, 330nm, 350nm, 360nm, 390nm, 395nm, or any value between any two of the above, preferably 300nm~350nm. The thickness of the coating layer is 5nm~18nm; for example, it can be 5nm, 8nm, 10nm, 13nm, 15nm, 18nm, or any value between any two of the above, preferably 10nm~15nm.

[0054] In the technical solution of this application embodiment, when the average particle size of lithium iron phosphate active particles and the thickness of the coating layer meet the above range, it is beneficial to enhance the overall electronic conductivity and structural stability of the material, and improve the charge and discharge efficiency and rate performance of lithium iron phosphate material.

[0055] Furthermore, in some embodiments, the powder resistivity of the lithium iron phosphate material at a pressure of 8 MPa is less than 30 Ω·cm, for example 29 Ω·cm, 25 Ω·cm, 20 Ω·cm, 15 Ω·cm, 10 Ω·cm, 5 Ω·cm, 3 Ω·cm, 2 Ω·cm, 1 Ω·cm, etc., further less than or equal to 15 Ω·cm, even further less than or equal to 10 Ω·cm, and even further less than or equal to 5 Ω·cm.

[0056] In the technical solution of this application embodiment, the powder resistivity of lithium iron phosphate material is low, which reduces the contact resistance of lithium iron phosphate material. While enhancing the rate performance and low temperature performance of lithium iron phosphate material, it reduces the heat generated inside lithium iron phosphate material during charge-discharge, extends the cycle life of lithium iron phosphate material, and reduces the risk of thermal runaway of secondary batteries using lithium iron phosphate material. This is beneficial to the application of lithium iron phosphate material in the fields of high energy density secondary batteries, quasi-solid-state secondary batteries, and solid-state secondary batteries.

[0057] Furthermore, in some embodiments, the iron leaching amount of the lithium iron phosphate material is less than 300 ppm, for example, it can be 299 ppm, 250 ppm, 200 ppm, 190 ppm, 150 ppm, 130 ppm, 100 ppm, 80 ppm, 50 ppm, 30 ppm, 20 ppm, 10 ppm, 5 ppm, 1 ppm, etc. Preferably, in some embodiments, the iron leaching amount of the lithium iron phosphate material can be less than 30 ppm.

[0058] In the technical solution of this application embodiment, the iron dissolution of lithium iron phosphate material is low, which is beneficial to improving the problem of iron element precipitation at the negative electrode and affecting the stability of the electrolyte, thereby improving the cycle performance and cycle life of secondary batteries using lithium iron phosphate material.

[0059] Furthermore, in some embodiments, the BET specific surface area of ​​the lithium iron phosphate material is 7.0 m². 2 / g~15.0m 2 / g, for example, can be 7m 2 / g、8m 2 / g, 10m 2 / g、12m 2 / g、14m 2 / g, 15m 2 / g and any value between any two of the above, further, BET can preferably be 7.0m. 2 / g~9m 2 / g.

[0060] In the technical solution of this application embodiment, the lithium iron phosphate material has a moderate BET specific surface area, which helps to provide a suitable number of active sites and improve the electrochemical activity of the cathode material.

[0061] Furthermore, in some embodiments, the loose packing density of the lithium iron phosphate material is 0.5 g / mL to 1 g / mL.

[0062] In the technical solution of this application embodiment, the lithium iron phosphate material has a high loose packing density and the particles of the lithium iron phosphate material are in closer contact, which is beneficial to increasing the volumetric energy density of the lithium iron phosphate material.

[0063] Furthermore, in some embodiments, the tap density of the lithium iron phosphate material is 1.3 g / mL to 1.8 g / mL.

[0064] In the technical solution of this application embodiment, the lithium iron phosphate material has a high tap density and the particles of the lithium iron phosphate material are in closer contact, which is beneficial to increasing the volumetric energy density of the lithium iron phosphate material.

[0065] Furthermore, in some embodiments, the compaction density of the lithium iron phosphate material under 3T pressure is 2.2 g / mL to 2.6 g / mL; for example, it can be 2.2 g / mL, 2.3 g / mL, 2.4 g / mL, 2.5 g / mL, 2.6 g / mL, or any value between two of the above. Preferably, in some embodiments, the compaction density can be controlled within the range of 2.4 g / mL to 2.5 g / mL.

[0066] In the technical solution of this application embodiment, the lithium iron phosphate material has a high compaction density and a higher degree of compactness between lithium iron phosphate material particles, which is beneficial to increasing the volumetric energy density of lithium iron phosphate material.

[0067] Further, in some embodiments, the content of magnetic material in the lithium iron phosphate material is 0.1ppm to 210ppm; for example, it can be 0.1ppm, 0.4ppm, 0.5ppm, 0.8ppm, 0.9ppm, 1ppm, 3ppm, 5ppm, 10ppm, 20ppm, 30ppm, 50ppm, 80ppm, 100ppm, 150ppm, 200ppm, 210ppm, or any value between any two of the above. Further, the content of magnetic material in the lithium iron phosphate material is 0.1ppm to 30ppm. Preferably, in some embodiments, the content of magnetic material in the lithium iron phosphate material is controlled between 0.4ppm and 0.9ppm.

[0068] In the technical solution of this application embodiment, the content of magnetic material in lithium iron phosphate material is low, which helps to reduce the possibility of dangers such as short circuit or thermal runaway.

[0069] Furthermore, in some embodiments, the free lithium content in the lithium iron phosphate material is less than 100 ppm, such as 99 ppm, 90 ppm, 80 ppm, 50 ppm, 40 ppm, 30 ppm, 20 ppm, 10 ppm, 5 ppm, 1 ppm, etc. In some preferred embodiments, the free lithium content in the lithium iron phosphate material can be controlled below 50 ppm.

[0070] In the technical solution of this application embodiment, when the free lithium content in the lithium iron phosphate material meets the above range, the free lithium content is low, which can avoid unnecessary side reactions between the free lithium content and the electrolyte, thus affecting the safety and stability of the battery.

[0071] Furthermore, the lithium iron phosphate material proposed in this application has stable quality and is suitable for production and use. In some embodiments, the pH value of the lithium iron phosphate material can be stably controlled within the range of 8 to 10, and can be more precisely controlled within the range of 8 to 9; the moisture content of the lithium iron phosphate material can be controlled within 400 ppm, and can be controlled within 200 ppm.

[0072] The lithium iron phosphate material proposed in this application exhibits superior performance. In some embodiments, at 25°C, when the lithium iron phosphate material is applied to a secondary battery, the specific capacity at 0.1C initial charge can reach 159.5 mAh / g to 165 mAh / g; the specific capacity at 0.1C initial discharge can reach 154 mAh / g to 162 mAh / g; and the initial discharge efficiency at 0.1C can reach over 96%, further exhibiting a 0.1C initial discharge efficiency of 96% to 9.2%; the specific capacity at 1C initial discharge can reach 139 mAh / g to 150 mAh / g; and the specific capacity at 10C initial discharge can reach 129 mAh / g to 140 mAh / g. At -20°C, when the lithium iron phosphate material is applied to a secondary battery, the specific capacity at 1C initial discharge can reach 80 mAh / g to 106 mAh / g.

[0073] Secondly, embodiments of this application provide a method for preparing lithium iron phosphate material, comprising the following steps:

[0074] Provides lithium iron phosphate active particles;

[0075] Lithium iron phosphate active particles undergo a first coating treatment to obtain intermediate particles;

[0076] The intermediate particles undergo a second coating treatment to obtain lithium iron phosphate material; wherein,

[0077] The lithium iron phosphate material includes lithium iron phosphate active particles and a coating layer; wherein, the coating layer is coated on the surface of the lithium iron phosphate active particles, and the coating layer includes iron carbide, and the mass fraction of iron carbide in the lithium iron phosphate material is 0.7%~3.1%.

[0078] In the technical solution of this application embodiment, the preparation method of this application is simple and easy to implement. Specifically, the lithium iron phosphate active particles are sequentially subjected to a first coating treatment and a second coating treatment to form a coating layer containing iron carbide on the surface of the lithium iron phosphate active particles. The mass fraction of iron carbide in the lithium iron phosphate material is controlled to obtain the lithium iron phosphate material. While effectively reducing the amount of iron leaching from the lithium iron phosphate material and improving the stability of the coating layer, the resistivity of the lithium iron phosphate powder is reduced and the conductivity of the lithium iron phosphate material is improved.

[0079] In some embodiments, the method for preparing lithium iron phosphate material provided in this application is used to prepare the lithium iron phosphate material as described above.

[0080] Furthermore, in some embodiments, the preparation steps of lithium iron phosphate active particles include:

[0081] The first ferrous source, phosphorus source, lithium source, first alkali, first reducing agent, dispersant and first solvent are mixed to obtain the first mixture;

[0082] The first mixture undergoes a first reaction treatment and a first solid-liquid separation treatment to obtain lithium iron phosphate active particles.

[0083] In the technical solution of this application embodiment, a first ferrous source, a phosphorus source, a lithium source, a first alkali, a first reducing agent, a dispersant, and a first solvent are mixed for a first reaction treatment (i.e., a hydrothermal reaction), followed by a first solid-liquid separation treatment. This process is beneficial for obtaining lithium iron phosphate active particles with regular morphology and fine particle size. The introduction of the first reducing agent helps to suppress the oxidation of ferrous ions, thereby facilitating the full participation of the iron element provided by the first ferrous source in the formation of lithium iron phosphate active particles. The introduction of the dispersant helps to uniformly disperse other raw materials, making the reaction more uniform and complete, thus facilitating the obtaining of lithium iron phosphate active particles with regular morphology and fine particle size, which is conducive to the full coating in subsequent coating steps.

[0084] Furthermore, in some embodiments, the first ferrous source is at least one of ferrous sulfate, ferrous chloride, and ferrous acetate.

[0085] Furthermore, in some embodiments, the phosphorus source is at least one of phosphoric acid, ammonium monohydrogen phosphate, and ammonium dihydrogen phosphate.

[0086] Furthermore, in some embodiments, the lithium source is at least one of lithium hydroxide and lithium carbonate.

[0087] Furthermore, in some embodiments, the first alkali is at least one of ammonia, sodium hydroxide, and potassium hydroxide.

[0088] Furthermore, in some embodiments, the first reducing agent is at least one of ascorbic acid and hydrazine hydrate.

[0089] Furthermore, in some embodiments, the dispersant is at least one of polyethylene glycol, triethylhexyl phosphate, sodium dodecyl sulfate, and methylpentanol.

[0090] In the technical solution of this application embodiment, the above-mentioned dispersant has the characteristics of low cost and wide availability, and good solubility, which is conducive to achieving more uniform dispersion of each raw material.

[0091] Furthermore, in some embodiments, the first solvent is selected from at least one of distilled water, deionized water, pure water, and ultrapure water.

[0092] In the technical solution of this application embodiment, the first solvent is characterized by its wide availability and environmental friendliness, and using water as a solvent can effectively reduce production costs.

[0093] Furthermore, in some embodiments, the molar ratio of iron in the first ferrous source to phosphorus in the phosphorus source is 1:(1.2~1.5).

[0094] In the technical solution of this application embodiment, when the molar ratio of iron in the first ferrous source to phosphorus in the phosphorus source meets the above range, it is beneficial to enable the iron provided by the first ferrous source to fully participate in the reaction to form lithium iron phosphate active particles.

[0095] Furthermore, in some embodiments, the molar ratio of iron in the first ferrous source to lithium in the lithium source is 1:(2.1~2.3).

[0096] In the technical solution of this application embodiment, when the molar ratio of iron in the first ferrous source to lithium in the lithium source meets the above range, the lithium element is in excess, which is beneficial to promote the complete reaction and compensate for the loss of lithium element caused by high temperature. This helps to obtain lithium iron phosphate material with good crystallization, high purity and stable structure, thereby improving the electrochemical performance of lithium iron phosphate material.

[0097] Furthermore, in some embodiments, the molar ratio of iron in the first ferrous source to the first base is 1:(1.2~2).

[0098] In the technical solution of this application embodiment, when the molar ratio of iron element in the first ferrous source to the first alkali meets the above range, the oxidation of ferrous ions can be avoided, thereby making the uncoated lithium iron phosphate highly crystalline and avoiding the presence of trivalent iron.

[0099] Furthermore, in some embodiments, the molar ratio of iron in the first ferrous source to the first reducing agent is 1:(0.1~0.2).

[0100] In the technical solution of this application embodiment, when the molar ratio of iron element in the first ferrous source to the first reducing agent meets the above range, the oxidation of ferrous ions is suppressed while the waste of raw materials is avoided.

[0101] Furthermore, in some embodiments, the mass fraction of the dispersant in the first mixture is 0.5% to 1%.

[0102] In the technical solution of this application embodiment, when the mass fraction of the dispersant in the first mixture meets the above range, it is beneficial to achieve sufficient dispersion of each raw material, thereby improving the uniformity of the obtained lithium iron phosphate active particles.

[0103] Furthermore, in some embodiments, the density of the first mixture is 1.20 g / mL to 1.35 g / mL.

[0104] In the technical solution of this application embodiment, when the density of the first mixture meets the above range, it is beneficial to achieve uniform mixing of each raw material, making the first reaction process more uniform and sufficient, and avoiding the problems of large amount of first solvent and low production efficiency.

[0105] Furthermore, in some embodiments, the temperature of the first reaction treatment is 180°C to 250°C, the time of the first reaction treatment is 10h to 20h, and the pressure of the first reaction treatment is 10.0MPa to 20.0MPa.

[0106] In the technical solution of this application embodiment, when the temperature, time, and pressure of the first reaction treatment meet the above-mentioned ranges, it is beneficial to obtain lithium iron phosphate material with stable crystal structure, small particle size, and regular morphology. If the temperature or time of the first reaction treatment is too low, the crystal structure growth will be incomplete, and the small size will easily lead to agglomeration, thereby affecting the subsequent processing performance and the electrochemical performance of the final lithium iron phosphate material. If the temperature or time of the first reaction is too high, it is easy to cause the crystal growth of lithium iron phosphate active particles to run out of control or the appearance of unnecessary by-products, thereby affecting the electrochemical performance of the final lithium iron phosphate material.

[0107] Furthermore, in some embodiments, the first reaction treatment is carried out under stirring conditions, and the stirring rate is 180 r / min to 400 r / min.

[0108] In the technical solution of this application embodiment, the first reaction process is carried out under stirring conditions, and when the stirring rate meets the above range, it is beneficial to achieve uniform mixing of materials, making the reaction more uniform and complete.

[0109] Furthermore, in some embodiments, the atmosphere for the first reaction treatment is a protective atmosphere.

[0110] In the technical solution of this application embodiment, when the atmosphere of the first reaction treatment is a protective atmosphere, the oxidation of ferrous ions can be avoided. This application does not limit the type of protective atmosphere, and those skilled in the art can select it according to the actual situation. For example, the protective atmosphere can be nitrogen, argon, etc.

[0111] Furthermore, in some embodiments, the step of obtaining intermediate particles by first coating treatment of lithium iron phosphate active particles includes:

[0112] Lithium iron phosphate active particles, a second ferrous source, and a second solvent are mixed to obtain a second mixture.

[0113] The second alkali and the second reducing agent are mixed into the second mixture at the first mixing temperature and within the first mixing time to obtain the third mixture;

[0114] The third mixture undergoes a second solid-liquid separation process to obtain intermediate particles.

[0115] In the technical solution of this application embodiment, the second alkali and the second reducing agent are mixed into the second mixture containing lithium iron phosphate active particles and the second ferrous source at the first mixing temperature and the first mixing time, so that the ferrous ions react with the reducing agent to form elemental iron, and the elemental iron is deposited on the surface of the lithium iron phosphate active particles to initially form a metallic iron coating layer. This application uses a chemical plating method to form a metallic iron coating layer on the surface of lithium iron phosphate active lithium. Compared with other processes, the process of this application is simple and highly controllable.

[0116] Furthermore, in some embodiments, the second ferrous source is at least one of ferrous sulfate, ferrous chloride, and ferrous acetate.

[0117] In the technical solution of this application embodiment, the above-mentioned second ferrous source has the characteristics of low cost and wide availability, and good solubility, which can achieve more uniform dispersion.

[0118] Furthermore, in some embodiments, the mass ratio of lithium iron phosphate active particles to iron in the second ferrous source is 1:(0.01~0.02).

[0119] In the technical solution of this application embodiment, when the mass ratio of lithium iron phosphate active particles to iron in the second ferrous source meets the above-mentioned range, it is beneficial for the iron element formed by the reduction of ferrous ions in the second ferrous source to fully coat the lithium iron phosphate active particles, which is beneficial for effectively reducing the powder resistivity of lithium iron phosphate material. If the amount of the second ferrous source added is too low, the iron carbide content formed after the subsequent second coating treatment will be low, the powder resistivity of lithium iron phosphate material will be high, and the electrochemical performance of the battery will not be improved sufficiently. If the amount of the second ferrous source added is too high, although the iron carbide content formed after the subsequent second coating treatment will be high, resulting in a low powder resistivity of lithium iron phosphate material, it will also cause excessive iron dissolution, thereby adversely affecting the electrochemical performance of the secondary battery.

[0120] Furthermore, in some embodiments, the second solvent is water, for example, selected from at least one of distilled water, deionized water, pure water, and ultrapure water.

[0121] In the technical solution of this application embodiment, the second solvent is characterized by its wide availability, low price, and environmental friendliness. Using water as a solvent can effectively reduce production costs.

[0122] Furthermore, in some embodiments, the ratio of the amount of the second ferrous source to the volume of the second solvent is (1~3) mol / L.

[0123] In the technical solution of this application embodiment, when the ratio of the second ferrous source to the second solvent meets the above range, it is beneficial to the uniform mixing of each component, making the reaction more uniform and complete, and avoiding the problem of large amount of second solvent and low production efficiency.

[0124] Furthermore, in some embodiments, the second alkali is at least one of ammonia, sodium hydroxide, and potassium hydroxide.

[0125] In the technical solution of this application embodiment, the second alkali has the characteristics of low cost and wide availability, and good solubility, which is conducive to achieving more uniform dispersion.

[0126] Furthermore, in some embodiments, the second alkali is mixed into the second mixture in the form of a second alkali solution, and the concentration of the second alkali solution is 0.5 mol / L to 2 mol / L.

[0127] In the technical solution of this application embodiment, the second alkali is mixed into the second mixture in the form of a second alkali solution, and when the concentration of the second alkali solution meets the above range, it is beneficial to add the material evenly and avoid local over-concentration, which would affect the uniformity and stability of the third mixture.

[0128] Furthermore, in some embodiments, the second reducing agent is hydrazine hydrate.

[0129] In the technical solution of this application embodiment, the second reducing agent has the characteristic of strong reducing power and can reduce ferrous iron to ferrous metal in an alkaline environment.

[0130] Furthermore, in some embodiments, the second reducing agent is mixed into the second mixture in the form of a second reducing agent solution, and the concentration of the second reducing agent solution, by mass fraction, is 10% to 30%.

[0131] In the technical solution of this application embodiment, the second reducing agent is mixed into the second mixture in the form of a second reducing agent solution, and when the concentration of the second reducing agent solution meets the above range, it is beneficial to add the material evenly and avoid local over-concentration, which would affect the uniformity and stability of the third mixture.

[0132] Furthermore, in some embodiments, the molar ratio of iron in the second ferrous source to the second reducing agent is 1:(3~6).

[0133] In the technical solution of this application embodiment, when the molar ratio of iron element in the second ferrous source to the second reducing agent meets the above range, it is beneficial to fully reduce ferrous ions and avoid waste of raw materials.

[0134] Furthermore, in some embodiments, the first mixing temperature is 25°C to 45°C, and the first mixing time is 1 hour to 3 hours.

[0135] In the technical solution of this application embodiment, when the first mixing temperature and the first mixing time meet the above-mentioned ranges, it is beneficial to control the coating speed, and the metallic iron coating layer is formed uniformly and more complete. If the first mixing temperature is too low or the first mixing time is too short, it is easy to cause incomplete coating, resulting in an incomplete coating layer; if the first mixing temperature is too high, the nucleation rate is too high, and homogeneous nucleation is likely to occur, which is also likely to cause an incomplete coating layer; if the first mixing time is too long, it is easy to reduce production efficiency.

[0136] Furthermore, in some embodiments, the second alkali and the second reducing agent are mixed into the second mixture at a first mixing temperature and a first mixing time, maintaining a pH value of 9-11 to obtain a third mixture.

[0137] In the technical solution of this application embodiment, maintaining a pH value of 9-11 helps to control the coating speed, avoid homogeneous nucleation, and make the coating more complete. If the pH value is too low, the reducing agent has weak reducing power, which makes it difficult to completely reduce iron. If the pH value is too high, the reduction rate is too fast, and homogeneous nucleation of iron occurs, resulting in iron powder being generated separately without being coated on the surface of lithium iron phosphate, leading to an incomplete coating layer.

[0138] In this application, it should be noted that the first solid-liquid separation process includes filtration and washing. In practice, the sulfate content in the washed material can be monitored, and washing can be stopped when the sulfate content is below 50 ppm. The second solid-liquid separation process includes filtration and washing. In practice, pure water can be used for washing, and washing can be stopped when the conductivity of the washed water is ≤200 μS / cm.

[0139] Furthermore, in some embodiments, the step of obtaining lithium iron phosphate material by subjecting the intermediate particles to a second coating treatment includes:

[0140] Intermediate particles, carbon source, and third solvent are mixed and dried to obtain dried material;

[0141] The dried material is sintered to obtain lithium iron phosphate material.

[0142] In the technical solution of this application embodiment, intermediate particles, carbon source and third solvent are mixed and dried to make the carbon source and intermediate particles uniformly mixed to obtain dried material; the dried material is sintered to make the carbon source pyrolyze at high temperature to initially form a carbon coating layer, the carbon coating layer coats the metallic iron coating layer and further reacts with the elemental iron in the metallic iron coating layer to form iron carbide to obtain a coating layer containing iron carbide, thereby avoiding the increase of iron dissolution caused by direct contact between elemental iron and electrolyte, while improving the conductivity of lithium iron phosphate material and the stability of coating layer.

[0143] Furthermore, in some embodiments, the mass ratio of intermediate particles to carbon source is 1:(0.04~0.08).

[0144] In the technical solution of this application embodiment, when the mass ratio of intermediate particles to carbon source meets the above-mentioned range, it is beneficial for the carbon coating layer formed by the carbon source to fully coat the intermediate particles, and for the coated particles to fully react with the metallic iron coating layer, thereby improving the electronic conductivity and structural stability of the lithium iron phosphate material. If the amount of carbon source added is too low, the coating layer is too thin, which easily leads to insufficient electronic conductivity and structural stability of the obtained lithium iron phosphate material, and excessive iron dissolution; if the amount of carbon source added is too high, the coating layer is too thick, which hinders the transport of lithium ions and generates inactive impurities, thereby causing a decrease in the electrochemical performance of the lithium iron phosphate material.

[0145] Furthermore, in some embodiments, the carbon source is at least one of glucose, sucrose, PEG, and water-soluble starch.

[0146] In the technical solution of this application embodiment, the above-mentioned carbon source is widely available and low in cost, and is suitable for forming a uniform carbon coating layer after high-temperature pyrolysis, thereby reacting with the metallic iron coating layer to form a coating layer, improving the electronic conductivity and structural stability of lithium iron phosphate material.

[0147] Furthermore, in some embodiments, the mass of the carbon source accounts for 5% to 10% of the total mass of the carbon source and the third solvent.

[0148] In the technical solution of this application embodiment, when the mass ratio of the carbon source to the total mass of the carbon source and the third solvent meets the above range, it is beneficial to the uniform mixing of each component, making the reaction more uniform and complete.

[0149] Furthermore, in some embodiments, the step of mixing intermediate particles, carbon source and third solvent, and drying them to obtain dried material includes: mixing intermediate particles, carbon source and third solvent, and spray drying them to obtain dried material; wherein the particle size of the dried material is 5μm~15μm, and the moisture content, by mass fraction, is less than 1.5%.

[0150] In the technical solution of this application embodiment, spray drying is used for the drying process. This not only dries the material quickly but also obtains uniform dried material particles with high sphericity and particle size within the aforementioned range. This is beneficial for improving the tap density and bulk density of lithium iron phosphate materials, thereby enhancing their energy density, cycle life, and other electrochemical properties. When the moisture content of the dried material meets the aforementioned range, it helps to achieve a uniform and efficient sintering process, ensuring good contact between particles, thereby improving the overall density and conductivity of lithium iron phosphate materials.

[0151] In some embodiments, the particle size of the dried material is the D50 particle size of the dried material.

[0152] Furthermore, in some embodiments, the step of sintering the dried material to obtain lithium iron phosphate material includes: holding the dried material at a sintering temperature for a specified time to obtain lithium iron phosphate material; wherein the sintering temperature is 800℃~900℃ and the holding time is 5h~10h.

[0153] In the technical solution of this application embodiment, when the sintering temperature and holding time meet the above range, it is beneficial for the carbon source to undergo high-temperature pyrolysis and fully react with metallic iron to generate iron carbide, thereby forming a coating layer containing iron carbide on the surface of lithium iron phosphate active particles.

[0154] In some embodiments, the sintering process is carried out under a protective atmosphere, which may be a nitrogen atmosphere.

[0155] Furthermore, the step of obtaining lithium iron phosphate material by sintering the dried material includes: heating the dried material to a first temperature at a first heating rate under a nitrogen atmosphere, heating it from the first temperature to a sintering temperature at a second heating rate, and holding it at that temperature for a specified time to obtain lithium iron phosphate material; wherein the first heating rate is 1℃ / min~3℃ / min, the first temperature is 400℃~500℃, the second heating rate is 3℃ / min~6℃ / min, the sintering temperature is 800℃~900℃, and the holding time is 5h~10h.

[0156] In the technical solution of this application embodiment, a slower heating rate is first used to ensure that the carbonization process is slow and sufficient, thereby achieving a more complete coating effect. Then, a faster heating rate is used to heat to the sintering temperature and hold it at that temperature. This not only helps carbon and iron to form a eutectic and then iron carbide, thereby reducing the powder resistivity of lithium iron phosphate material and improving the stability of the surface coating layer, effectively reducing the iron leaching content, but also helps to improve production efficiency.

[0157] In some embodiments, after sintering, a cooling process can be performed. After the material temperature drops to less than or equal to 80°C, it can be screened and iron removed before packaging to obtain lithium iron phosphate material products.

[0158] Thirdly, embodiments of this application provide a positive electrode sheet, which includes lithium iron phosphate material prepared by the preparation method of lithium iron phosphate material provided in the first aspect of this application or lithium iron phosphate material provided in the second aspect of this application.

[0159] In the technical solution of this application embodiment, the positive electrode sheet contains the above-mentioned lithium iron phosphate material, and therefore has good capacity performance, cycle performance, rate performance, low temperature performance and safety performance.

[0160] Fourthly, embodiments of this application provide a secondary battery, which includes the positive electrode sheet provided in the third aspect of this application.

[0161] In the technical solution of this application embodiment, the secondary battery includes the above-mentioned positive electrode sheet, and therefore has good capacity performance, cycle performance, rate performance, low temperature performance and safety performance.

[0162] The following are some specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0163] I. Preparation Method

[0164] Example 1

[0165] (1) Ferrous sulfate, phosphoric acid (mass fraction 50%), lithium carbonate, ammonia (mass fraction 30%), ascorbic acid, and polyethylene glycol 2000 were added to pure water to obtain a first mixture with a density of 1.29 g / mL. The first mixture was then added to a hydrothermal reactor, and nitrogen gas was introduced to remove oxygen so that the oxygen content in the reactor was less than 100 ppm. The reaction was carried out at 220°C for 15 h at a reaction pressure of 14 MPa and a stirring speed of 200 r / min. After the reaction was completed, the material temperature was cooled to ≤80°C and the pressure was released. The material was taken out, filtered, and washed until the sulfate content of the material was less than 50 ppm. The washing was then stopped to obtain lithium iron phosphate active particles. The molar ratio of ferrous sulfate, phosphoric acid, lithium carbonate, ammonia, and ascorbic acid was 1:1.35:1.10:1.6:0.15. The mass fraction of polyethylene glycol 2000 in the first mixture was 0.75%.

[0166] (2) Lithium iron phosphate active particles were added to a 2 mol / L ferrous sulfate solution, and the mass ratio of lithium iron phosphate active particles to ferrous ions was controlled at 1:0.015. After stirring and dispersing, a second mixture was obtained. Then, a 20% sodium hydroxide solution and a 30% hydrazine hydrate solution were added to the second mixture. The pH value of the reaction was maintained at 10.5, the feeding temperature was 35℃, and the feeding time was 2h. Then, the mixture was filtered and washed with pure water. The washing was stopped when the conductivity of the washing water was ≤200μS / cm to obtain intermediate particles. The molar ratio of ferrous ions to hydrazine hydrate was 1:4.5.

[0167] (3) The intermediate particles were added to a sucrose solution with a mass fraction of 8%, and the mass ratio of intermediate particles to sucrose was controlled to be 1:0.065. After stirring and dispersing, the mixture was spray-dried to control the particle size of the material to be 12.4 μm and the moisture content to be less than 1.5% (by mass fraction) to obtain the dried material. The dried material was then calcined at high temperature under a nitrogen atmosphere. During the high-temperature calcination process, the temperature was first increased from room temperature to 450℃ at a heating rate of 2℃ / min, and then increased from 450℃ to 850℃ at a heating rate of 4.5℃ / min. The temperature was held at 850℃ for 8 hours. After the high-temperature calcination was completed, the material temperature was cooled to ≤80℃. After sieving, iron removal and packaging, lithium iron phosphate material was obtained. The tap density of the lithium iron phosphate material was measured to be 1.56 g / mL and the loose packing density was 0.65 g / mL. Other characterization data are shown in Tables 1 to 3.

[0168] Example 2

[0169] (1) Ferrous chloride, ammonium hydrogen phosphate, lithium carbonate, sodium hydroxide, ascorbic acid, and triethylhexylphosphoric acid were added to pure water to obtain a first mixture with a density of 1.20 g / mL. The first mixture was then added to a hydrothermal reactor, and nitrogen gas was introduced to remove oxygen. The reaction was carried out at 180°C for 10 h at a reaction pressure of 12 MPa. The stirring speed was controlled at 180 r / min. After the reaction was completed, the material temperature was cooled to ≤80°C and the pressure was released. The material was taken out, filtered, and washed. Washing was stopped when the sulfate content of the material was less than 50 ppm to obtain lithium iron phosphate active particles. The molar ratio of ferrous chloride, ammonium hydrogen phosphate, lithium carbonate, sodium hydroxide, and ascorbic acid was 1:1.2:1.05:1.2:0.1. The mass fraction of triethylhexylphosphoric acid in the first mixture was 0.5%.

[0170] (2) Add lithium iron phosphate active particles to a 1 mol / L ferrous chloride solution, control the mass ratio of lithium iron phosphate active particles to ferrous ions to be 1:0.01, stir and disperse to obtain a second mixture, then add 10% sodium hydroxide solution and 15% hydrazine hydrate solution to the second mixture, maintain the pH value of the reaction at 9, the feeding temperature at 25℃, the feeding time at 1h, then filter and wash to obtain intermediate particles; wherein, the molar ratio of ferrous ions to hydrazine hydrate is 1:3.

[0171] (3) The intermediate particles were added to a 5% water-soluble starch solution, and the mass ratio of the intermediate particles to the water-soluble starch was controlled to be 1:0.04. After stirring and dispersing, the mixture was spray-dried to control the particle size of the material to be 5.2 μm and the moisture content to be less than 1.5% (by mass fraction) to obtain the dried material. The dried material was then calcined at high temperature under a nitrogen atmosphere. During the high-temperature calcination process, the temperature was first raised from room temperature to 400℃ at a heating rate of 1℃ / min, and then raised from 400℃ to 800℃ at a heating rate of 3℃ / min. The temperature was kept at 800℃ for 5 hours. After the high-temperature calcination was completed, the material was cooled to a temperature ≤80℃. After screening, iron removal and packaging, lithium iron phosphate material was obtained.

[0172] Example 3

[0173] (1) Ferrous acetate, ammonium dihydrogen phosphate, lithium hydroxide, potassium hydroxide, hydrazine hydrate, and sodium dodecyl sulfate were added to pure water to obtain a first mixture with a density of 1.35 g / mL. The first mixture was then added to a hydrothermal reactor, and nitrogen gas was introduced to remove oxygen. The reaction was carried out at 250°C for 20 h at a reaction pressure of 16 MPa. The stirring speed was controlled at 400 r / min. After the reaction was completed, the material temperature was cooled to ≤80°C and the pressure was released. The material was taken out, filtered, and washed. Washing was stopped when the sulfate content of the material was less than 50 ppm to obtain lithium iron phosphate active particles. The molar ratio of ferrous acetate, ammonium dihydrogen phosphate, lithium hydroxide, potassium hydroxide, and hydrazine hydrate was 1:1.5:2.3:2:0.2, and the mass fraction of sodium dodecyl sulfate in the first mixture was 1%.

[0174] (2) Lithium iron phosphate active particles were added to a 3 mol / L ferrous acetate solution, and the mass ratio of lithium iron phosphate active particles to ferrous ions was controlled to be 1:0.02. After stirring and dispersing, a second mixture was obtained. Then, a 30% sodium hydroxide solution and a 45% hydrazine hydrate solution were added to the second mixture. The pH value of the reaction was maintained at 11, the feeding temperature was 45℃, and the feeding time was 3h. Then, the mixture was filtered and washed to obtain intermediate particles. The molar ratio of ferrous ions to hydrazine hydrate was 1:6.

[0175] (3) The intermediate particles were added to a glucose solution with a mass fraction of 10%, and the mass ratio of intermediate particles to glucose was controlled to be 1:0.08. After stirring and dispersing, the mixture was spray-dried to control the particle size of the material to be 14.9 μm and the moisture content to be less than 1.5% (by mass fraction) to obtain the dried material. The dried material was then calcined at high temperature under a nitrogen atmosphere. During the high-temperature calcination process, the temperature was first raised from room temperature to 500℃ at a heating rate of 3℃ / min, and then raised from 500℃ to 900℃ at a heating rate of 6℃ / min. The temperature was kept at 900℃ for 10 hours. After the high-temperature calcination was completed, the material temperature was cooled to ≤80℃. After screening, iron removal and packaging, lithium iron phosphate material was obtained.

[0176] Example 4

[0177] Compared with Example 1, the difference is that in step (2), the mass ratio of lithium iron phosphate active particles to ferrous ions is 1:0.005, and the molar ratio of ferrous ions to hydrazine hydrate is 1:2.5.

[0178] Example 5

[0179] Compared with Example 1, the difference is that in step (2), the mass ratio of lithium iron phosphate active particles to ferrous ions is 1:0.025, and the molar ratio of ferrous ions to hydrazine hydrate is 1:7.3.

[0180] Example 6

[0181] Compared with Example 1, the difference is that in step (2), the pH value of the reaction is maintained at 8.

[0182] Example 7

[0183] The difference from Example 1 is that in step (2), the pH value of the reaction is maintained at 12.

[0184] Example 8

[0185] Compared with Example 1, the difference is that in step (2), the feeding temperature is 20°C and the feeding time is 0.5h.

[0186] Example 9

[0187] Compared with Example 1, the difference is that in step (2), the feeding temperature is 50°C and the feeding time is 4h.

[0188] Example 10

[0189] Compared with Example 1, the difference is that in step (3), the mass ratio of intermediate particles to sucrose is controlled to be 1:0.035.

[0190] Example 11

[0191] Compared with Example 1, the difference is that in step (3), the mass ratio of intermediate particles to sucrose is controlled to be 1:0.089.

[0192] Example 12

[0193] Compared with Example 1, the difference is that in step (3), the high-temperature calcination process first raises the temperature from room temperature to 450°C at a heating rate of 2°C / min, then raises the temperature from 450°C to 750°C at a heating rate of 4.5°C / min, and then holds the temperature at 750°C for 4 hours.

[0194] Example 13

[0195] Compared with Example 1, the difference is that in step (3), the high-temperature calcination process first raises the temperature from room temperature to 450°C at a heating rate of 2°C / min, then raises the temperature from 450°C to 950°C at a heating rate of 4.5°C / min, and then holds the temperature at 950°C for 12 hours.

[0196] Comparative Example 1

[0197] (1) Ferrous sulfate, phosphoric acid (mass fraction 50%), lithium carbonate, ammonia (mass fraction 10%), ascorbic acid, and polyethylene glycol 2000 were added to pure water to obtain a first mixture with a density of 1.29 g / mL. The first mixture was then added to a hydrothermal reactor, and nitrogen gas was introduced to remove oxygen. The reaction was carried out at 220°C for 15 h at a reaction pressure of 14 MPa. The stirring speed was controlled at 200 r / min. After the reaction was completed, the material temperature was cooled to ≤80°C and the pressure was released. The material was taken out, filtered, and washed. Washing was stopped when the sulfate content of the material was lower than 50 ppm to obtain lithium iron phosphate active particles. The molar ratio of ferrous sulfate, phosphoric acid, lithium carbonate, ammonia, and ascorbic acid was 1:1.35:1.10:1.6:0.15. The mass fraction of polyethylene glycol 2000 in the first mixture was 0.75%.

[0198] (2) Lithium iron phosphate active particles were added to a sucrose solution with a mass fraction of 8 wt%, and the mass ratio of intermediate particles to sucrose was controlled to be 1:0.065. After stirring and dispersing, the mixture was spray-dried to control the particle size of the material to be 12.4 μm and the moisture content (by mass fraction) to be less than 1.5%. The spray material was then subjected to high-temperature calcination. During the high-temperature calcination process, the temperature was first raised from room temperature to 450℃ at a heating rate of 2℃ / min, and then raised from 450℃ to 850℃ at a heating rate of 4.5℃ / min. The temperature was then maintained at 850℃ for 8 hours. After the high-temperature calcination was completed, the temperature was cooled to ≤80℃. After screening, iron removal and packaging, lithium iron phosphate material was obtained.

[0199] II. Testing Methods

[0200] (1) Performance testing of lithium iron phosphate materials

[0201] 1. Element content: ICP-OES was used for measurement, and the test results are shown in Table 1.

[0202] 2. Mass fraction of iron carbide in lithium iron phosphate materials: XRD was used for testing, and the ratio of the peak height of the main strong peak of iron carbide to that of lithium iron phosphate was calculated. The test results are shown in Table 1.

[0203] 3. Average particle size of primary particles in lithium iron phosphate material: SEM was used for testing, and the results are shown in Table 1 and... Figure 1 .

[0204] 4. Average particle size of lithium iron phosphate active particles: SEM test was used, and the test results are shown in Table 1.

[0205] 5. Thickness of the coating layer: TEM test was used, and the test results are shown in Table 1.

[0206] 6. Powder resistivity: The four-probe method was used for testing at a pressure of 8 MPa. The test results are shown in Table 1.

[0207] 7. BET specific surface area: The BET method was used to determine the specific surface area. The test results are shown in Table 1.

[0208] 8. Loose packing density: The test was conducted using the funnel method according to GB / T 31057.1-2014. The test results are shown in Table 1.

[0209] 9. Tap density: The tap density meter was used for testing. The number of vibrations was 5000. The test results are shown in Table 1.

[0210] 10. Compacted density (PD): Tested using a compaction density meter with a test pressure of 3T and a compaction time of 30S. The test results are shown in Table 1.

[0211] 11. Magnetic Material: Weigh 100g of the material and pour it into a capped plastic bottle. Add 1000g of pure water, then add an 8000GS magnetic magnet. Protect the magnet with PTFE. Tighten the cap, place the plastic bottle horizontally, and rotate it at 10-20 rpm for 30-45 minutes. Stop rotating and remove the magnetic magnet. Add another 1000g of pure water to the plastic bottle, tighten the cap, place the bottle horizontally, and rotate it at 10-20 rpm for 5-10 minutes. Stop rotating and remove the magnetic magnet. Dissolve the magnetic material in aqua regia. Measure the solution using ICP. Make up the volume and measure the content of nickel, chromium, copper, zinc, and iron. Calculate the total mass of nickel, chromium, copper, zinc, and iron, and divide by the weight of the material to obtain the magnetic material content. The test results are shown in Table 1.

[0212] 12. pH value: Refer to "GB / T 9724-2007 General Rules for Determination of pH Value of Chemical Reagents", and the test results are shown in Table 1.

[0213] 13. Iron leaching content: 10g of material was added to 100mL of 0.05mol / L hydrochloric acid and soaked at 40℃ for 30min. Then the mixture was filtered. The iron content in the filtrate was measured by ICP-OES to obtain the iron leaching data. The test results are shown in Table 1.

[0214] 14. Free lithium: Measured by automatic potentiometric titration. The test results are shown in Table 1.

[0215] 15. Moisture content: The Karl Fischer (KF) method was used for testing, and the results are shown in Table 1.

[0216] (2) Performance testing of secondary batteries

[0217] The lithium iron phosphate materials prepared in each embodiment and comparative example were mixed with ultrafine carbon powder (Super P, SP) and polyvinylidene fluoride in a mass ratio of 90:5:5, respectively. Then, N-methylpyrrolidone (NMP) was added to form a slurry, which was then coated onto aluminum foil, dried, and pressed into sheets. Using lithium sheets as the negative electrode, glass fiber as the separator, lithium hexafluorophosphate (LiPF6) as the lithium salt, and ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of 1:1:1 as organic solvents, the lithium salt was dissolved in the organic solvent to prepare an electrolyte with a concentration of 1M. Button batteries were then prepared, and their electrochemical performance was tested at 25°C and -20°C, respectively. The current rates were 0.1C, 1C, and 10C, and the voltage ranged from 2.00V to 3.75V. The test results are shown in Table 1.

[0218] III. Analysis of Test Results for Each Embodiment and Comparative Example

[0219] Table 1

[0220]

[0221] Table 2

[0222]

[0223] Table 3

[0224]

[0225] Please see Figure 1 ,pass Figure 1 As can be seen, the lithium iron phosphate material prepared in Example 1 of this application is spherical with a smooth surface, and the average particle size of the primary particles is approximately 290~390nm.

[0226] Please refer to Table 1. As can be seen from Table 1, the lithium iron phosphate material prepared in the embodiments of this application has a particularly low powder resistivity, which greatly reduces the contact resistance of the material and provides excellent rate and low temperature performance, making it suitable for use in secondary batteries, such as solid-state batteries.

[0227] Compared to Example 1, Comparative Example 1 only used carbon coating, and its iron leaching amount was comparable to that of Example 1, but its powder resistivity was significantly higher than that of Example 1, resulting in significantly worse electrochemical performance. The above results demonstrate that this application can significantly reduce the powder resistivity of lithium iron phosphate materials without affecting the iron leaching amount.

[0228] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A lithium iron phosphate material, characterized in that, The lithium iron phosphate material comprises lithium iron phosphate active particles and a coating layer; the coating layer is coated on the surface of the lithium iron phosphate active particles, the coating layer comprises iron carbide, the mass fraction of iron carbide in the lithium iron phosphate material is 1.98% to 2.2%, the iron elution amount of the lithium iron phosphate material is less than 30 ppm, and the average particle size of primary particles of the lithium iron phosphate material is 100 nm to 400 nm. The mass fraction of carbon elements in the lithium iron phosphate material is 0.3% to 1.5%; and / or, 2. The lithium iron phosphate material of claim 1, wherein, The average particle size of the lithium iron phosphate active particles is 82 nm to 395 nm, and the thickness of the coating layer is 5 nm to 18 nm. The powder resistivity of the lithium iron phosphate material under a pressure of 8 MPa is less than 30 Ω·cm; and / or, 3. The lithium iron phosphate material of claim 1, wherein, The compaction density of the lithium iron phosphate material under a pressure of 3T is 2.2 g / mL to 2.6 g / mL; and / or, The BET specific surface area of the lithium iron phosphate material is 7.0 m 2 / g ~ 15.0 m 2 / g; and / or, The content of magnetic substances in the lithium iron phosphate material is 0.1 ppm to 210 ppm. The method comprises the following steps:

4. A method of producing a lithium iron phosphate material, characterized by, Providing lithium iron phosphate active particles; A metal iron coating layer is formed on the surface of the lithium iron phosphate active particles by using a chemical plating method to obtain intermediate particles; The intermediate particles are subjected to a second coating treatment to obtain the lithium iron phosphate material; wherein, The step of subjecting the intermediate particles to a second coating treatment to obtain the lithium iron phosphate material comprises: mixing the intermediate particles, a carbon source, and a third solvent, and subjecting the mixture to a drying treatment to obtain a dried material; and subjecting the dried material to a sintering treatment to obtain the lithium iron phosphate material; The lithium iron phosphate material comprises lithium iron phosphate active particles and a coating layer; the coating layer is coated on the surface of the lithium iron phosphate active particles, the coating layer comprises iron carbide, the mass fraction of iron carbide in the lithium iron phosphate material is 1.98% to 2.2%, the iron elution amount of the lithium iron phosphate material is less than 30 ppm, and the average particle size of primary particles of the lithium iron phosphate material is 100 nm to 400 nm. The preparation steps of the lithium iron phosphate active particles comprise:

5. The method of claim 4, wherein the lithium iron phosphate material is prepared by the steps of: mixing lithium carbonate, iron oxide, and phosphoric acid; and heating the mixture to a temperature of 600-800°C for 2-10 hours. Mixing a first ferrous source, a phosphorus source, a lithium source, a first alkali, a first reducing agent, a dispersing agent, and a first solvent to obtain a first mixture; The first mixture is subjected to a first reaction treatment and a first solid-liquid separation treatment to obtain the lithium iron phosphate active particles; wherein, The molar ratio of iron elements in the first ferrous source to phosphorus elements in the phosphorus source is 1:(1.2 to 1.5); and / or, The molar ratio of iron elements in the first ferrous source to lithium elements in the lithium source is 1:(2.1 to 2.3); and / or, The molar ratio of iron elements in the first ferrous source to the first alkali is 1:(1.2 to 2); and / or, The molar ratio of iron elements in the first ferrous source to the first reducing agent is 1:(0.1 to 0.2); and / or, The dispersing agent is at least one of polyethylene glycol, triethylhexyl phosphoric acid, sodium dodecyl sulfate, and methyl amyl alcohol; and / or, The mass fraction of the dispersing agent in the first mixture is 0.5% to 1%; and / or, The density of the first mixture is 1.20 g / mL to 1.35 g / mL; and / or, ​ The first reaction treatment is performed at a temperature of 180-250 DEG C for 10-20 hours.

6. The method of claim 4, wherein the lithium iron phosphate material is prepared by the steps of: The step of forming a metal iron coating layer on the surface of the lithium iron phosphate active particles by electroless plating to obtain intermediate particles comprises: ​ The lithium iron phosphate active particles, the second ferrous source and the second solvent are mixed to obtain a second mixture; The second alkali and the second reducing agent are mixed into the second mixture at a first mixing temperature for a first mixing time to obtain a third mixture; The third mixture is subjected to a second solid-liquid separation treatment to obtain intermediate particles; wherein, The mass ratio of the lithium iron phosphate active particles to the iron element in the second ferrous source is 1:(0.01-0.02); and / or, The molar ratio of the iron element in the second ferrous source to the second reducing agent is 1:(3-6); and / or, The first mixing temperature is 25-45 DEG C, and the first mixing time is 1-3 hours.

7. The method for preparing lithium iron phosphate material according to claim 4, characterized in that, The mass ratio of the intermediate particles to the carbon source is 1:(0.04-0.08); and / or, The carbon source is at least one of glucose, sucrose, PEG and water-soluble starch; and / or, The step of subjecting the dried material to a sintering treatment to obtain the lithium iron phosphate material comprises: The dried material is subjected to a sintering treatment at a sintering temperature for a holding time to obtain the lithium iron phosphate material; wherein, The sintering temperature is 800-900 DEG C, and the holding time is 5-10 hours.

8. A positive electrode sheet characterized by comprising: The positive electrode sheet comprises the lithium iron phosphate material of any one of claims 1-3 or the lithium iron phosphate material prepared by the method of any one of claims 4-7.

9. A secondary battery characterized by comprising: The secondary battery comprises the positive electrode sheet of claim 8.

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