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

Abstract

The application provides a lithium iron phosphate borate positive electrode material, a preparation method thereof, a positive electrode sheet and a secondary battery, and belongs to the technical field of battery positive electrode materials. x FeTi y P z B w O (4z+3w+2y) / C, wherein x, y, z and w are non-zero positive numbers, 1.02<=x<=1.05, 0.01<=y<=0.05, 0.92<=z<=0.95, and 0.05<=w<=0.15, and the mass content of C is 1.00% to 1.35% based on 100% of the mass of the lithium iron phosphate borate positive electrode material. The lithium iron phosphate borate positive electrode material provided by the application is not only low in price and excellent in ion conductivity, but also high in capacity, excellent in rate performance, and further improved in cycle stability, and has a wider application prospect.

Description

Technical Field

[0001] This invention relates to the field of battery cathode material technology, specifically to a lithium iron phosphate borate cathode material and its preparation method, cathode sheet, and secondary battery. Background Technology

[0002] With the development of new energy vehicles, competition is becoming increasingly fierce, and the requirements for cost-effectiveness are also getting higher and higher. New energy vehicles are required to have both long driving range and low price. Batteries generally account for more than 40% of the cost of new energy vehicles, and their cost-effectiveness directly determines the cost-effectiveness of new energy vehicles. Cathode materials account for more than 30% of the cost of batteries. Therefore, reducing the cost of cathode materials and improving their performance can effectively enhance the competitiveness of new energy vehicles.

[0003] Lithium iron phosphate (LFP) is increasingly being used as the cathode material in batteries for new energy vehicles due to its advantages such as low cost and high safety. The key to the steady development of LFP lies in how to continuously improve its performance and reduce its production cost.

[0004] In view of this, the present invention is hereby proposed. Summary of the Invention

[0005] In view of the technical problems existing in the background art, this application provides a lithium iron phosphate cathode material and its preparation method, cathode sheet and secondary battery, aiming to solve the problems of low performance and high production cost of existing lithium iron phosphate cathode materials.

[0006] In a first aspect, embodiments of this application provide a lithium iron phosphate cathode material, the general chemical formula of which is Li. x FeTi y P z B w O (4z+3w+2y) / C, where x, y, z, and w are all non-zero positive numbers, 1.02≤x≤1.05, 0.01≤y≤0.05, 0.92≤z≤0.95, and 0.05≤w≤0.15. Based on the mass of the phosphate boric acid cathode material as 100%, the mass fraction of carbon element is 1.00% to 1.35%.

[0007] The lithium iron phosphate cathode material provided in this application uses trivalent titanium for cation doping and boron for anion doping, while also being coated with carbon. Trivalent titanium has a larger ionic radius and forms more defects, resulting in better ionic conductivity and higher capacity for the lithium iron phosphate cathode material. Boron is doped in the form of borate ions, and the molar mass of borate is less than that of phosphate, which is beneficial to improving the capacity of the lithium iron phosphate cathode material. Carbon, as a coating layer, not only improves the conductivity of the lithium iron phosphate cathode material but also enhances its cycle stability.

[0008] In some embodiments, the D10 particle size of the lithium iron phosphate borate material is ≤0.5μm, the D50 particle size is ≤1.60μm, and the D90 particle size is ≤19.00μm.

[0009] In this embodiment, the lithium iron phosphate cathode material has a more uniform particle size, which is more conducive to improving the capacity and cycle stability of the battery prepared by using it as a cathode material.

[0010] In some embodiments, the pH value of the lithium iron phosphate borate cathode material is 8.50 to 9.00, which makes it easier to homogenize during the subsequent preparation of cathode slurry and also makes it easier to prepare cathode sheets with uniform dispersion and stable performance.

[0011] In some embodiments, the lithium iron phosphate borate cathode material has a compaction density of 2.35 g / mL to 2.50 g / mL and a specific surface area of ​​10.1 m². 2 / g~14.3m 2 / g, the powder resistivity is 4.0Ω.cm~13.0Ω.cm.

[0012] In this embodiment, the lithium iron phosphate cathode material has a higher compaction density, a larger specific surface area, and a lower powder resistivity, which is more conducive to improving the capacity of the battery formed using this lithium iron phosphate cathode material.

[0013] In some embodiments, the lithium iron phosphate borate cathode material has an iron leaching content of 3.0 ppm to 10.0 ppm, an elemental iron content of 0.01 ppm to 0.05 ppm, a chloride ion content of 40.0 ppm to 80.0 ppm, a water content of 280 ppm to 350 ppm, a free lithium content of 140 ppm to 200 ppm, and a magnetic impurity content of ≤1 ppm.

[0014] In this embodiment, the lithium iron phosphate cathode material has iron leaching ≤10.0ppm, elemental iron ≤0.05ppm, chloride ion ≤80.0ppm, water content ≤350ppm, free lithium ≤200ppm, and magnetic impurity content ≤1ppm. Its structure is stable and the impurity content is low, which makes it have better ionic conductivity and cycle stability.

[0015] Secondly, embodiments of this application provide a method for preparing lithium iron phosphate borate cathode material, the method comprising the following steps: uniformly mixing ferrous source, trivalent titanium source, carbon source, phosphoric acid, boric acid and solvent to obtain a mixed slurry; preheating and drying the mixed slurry to obtain a pre-dried material; and calcining the pre-dried material to obtain lithium iron phosphate borate cathode material.

[0016] In the technical solution of this application embodiment, lithium source, ferrous source, trivalent titanium source, carbon source, phosphoric acid, boric acid, and solvent are used as raw materials, which is more conducive to enhancing the driving force of solid-phase reaction and more likely to form a eutectic. Compared with the dry mixing and calcination process, the reaction temperature is further reduced, the particles are further refined, the capacity is further increased, and the production cost is further reduced. At the same time, the trivalent titanium ions have a larger ionic radius and form more defects, thereby making the cathode material more conductive and with a higher capacity. Boron is doped in the form of borate ions, and the molar mass of borate is less than that of phosphate, which is beneficial to improving capacity. Carbon, as a coating layer, not only helps to improve conductivity but also helps to improve cycle stability. In addition, compared with the traditional process, the above technical solution reduces the steps of water slurrying, grinding, and spray drying. According to the statistics of actual production, the increased cost (including labor, equipment depreciation, energy consumption, environmental protection, etc.) is approximately 2,500 yuan / ton. The preparation method of lithium iron phosphate cathode material provided in this application can save 2,500 yuan / ton in this step alone. Based on an annual production of 100,000 tons of lithium iron phosphate cathode material, the cost can be saved by 250 million yuan / year.

[0017] In some embodiments, the molar ratio of iron in the ferrous source, titanium in the trivalent titanium source, lithium in the lithium source, carbon in the carbon source, phosphorus in the phosphoric acid solution, and boron in the boric acid is 1:(0.01-0.05):(1.02-1.05):(0.36-1.32):(0.92-0.95):(0.05-0.15).

[0018] In this embodiment, the lithium iron phosphate cathode material obtained by the above molar ratio uses trivalent titanium ions for cation doping and boron for anion doping, while also being coated with carbon. The trivalent titanium ions have a larger ionic radius and form more defects, thereby improving the ionic conductivity and increasing the capacity of the lithium iron phosphate cathode material. The boron is doped in the form of borate ions, and the molar mass of borate is less than that of phosphate, which is beneficial to improving the capacity. Furthermore, boric acid has a fluxing effect, which is beneficial to obtaining lithium iron phosphate cathode material with increased compaction density at a lower calcination temperature. The carbon coating layer not only improves the ionic conductivity of the lithium iron phosphate cathode material but also improves the cycle stability.

[0019] In some specific embodiments, the step of mixing the lithium source, ferrous source, trivalent titanium source, carbon source, phosphoric acid, boric acid and solvent includes: mixing phosphoric acid with solvent to form a phosphoric acid solution, and then mixing the phosphoric acid solution with the lithium source, ferrous source, trivalent titanium source and boric acid. Preferably, the mass fraction of the phosphoric acid solution is 70%-80% to improve the efficiency of uniform mixing of the phosphoric acid solution and other raw materials.

[0020] In some embodiments, the ferrous source is a water-soluble ferrous source, comprising at least one of ferrous acetate and / or ferrous chloride. The lithium source is a water-soluble lithium source, comprising at least one of lithium chloride and / or lithium acetate. At high temperatures, volatile anions combine with hydrogen ions in the acid to form volatile low-boiling-point acids. Following the principle of preparing low-boiling-point acids from high-boiling-point acids, high-boiling-point acids such as boric acid and phosphoric acid are mixed with chloride ions, acetate ions, etc., to form low-boiling-point hydrogen chloride and acetic acid, while other ions combine to form lithium iron borate phosphate. During calcination, the driving force of the solid-phase reaction is stronger, making it easier to form a eutectic, which helps to lower the calcination temperature, shorten the sintering time, and thus reduce energy consumption and cost. Therefore, the lithium iron borate phosphate cathode material provided in this application not only has a lower cost and shorter process, but also finer particles and higher capacity. Meanwhile, the introduction of boric acid in this application can serve as both a dopant and a flux, which can further reduce the calcination temperature and avoid the problems of increased particle size, iron phosphide content, and magnetic material content in lithium iron phosphate cathode materials caused by high-temperature calcination. This is beneficial to improving the electrochemical performance of lithium iron phosphate cathode materials in terms of capacity, cycle performance, and rate performance.

[0021] In this application, a trivalent titanium source is selected to provide titanium element. Trivalent titanium salts have a lower melting point (decomposition point) and can melt at a lower temperature, enhancing the dispersion effect and making the trivalent titanium doped more uniformly in lithium iron borate phosphate. In some embodiments, the trivalent titanium source includes titanium trihalide, preferably titanium trichloride. This can utilize the principle of preparing low-boiling-point acids from high-boiling-point acids to lower the calcination reaction temperature, and can also utilize the larger ionic radius of trivalent titanium ions, which leads to more defects formed during the doping process, which is beneficial to enhancing the ionic conductivity of lithium ions, thereby improving the electrochemical performance of lithium iron borate phosphate cathode material in terms of charge specific capacity, discharge specific capacity, etc.

[0022] In some embodiments, the carbon source is a water-soluble carbon source, which includes any one or more of ascorbic acid and citric acid. Preferably, the water-soluble carbon source is a mixture of ascorbic acid and citric acid, and the mass ratio of ascorbic acid to citric acid is (0.05-0.2):(0.92-0.95), which is more conducive to improving the conductivity and cycle stability of the lithium iron phosphate borate cathode material.

[0023] In some embodiments, the mixed slurry is preheated and dried at 100°C-150°C to obtain a pre-dried material with an H2O mass fraction of less than or equal to 0.5%. That is, during the preheating and drying process, the moisture content of the dried material is controlled to ≤0.5% after moisture removal, which is beneficial for producing a more dense and uniform product during subsequent calcination, thereby further improving the density of the lithium iron phosphate cathode material and improving its compaction density and capacity.

[0024] In some embodiments, the step of calcining the pre-dried material to obtain lithium iron phosphate cathode material includes: under a protective atmosphere, the pre-dried material is subjected to a first calcination temperature for a first holding time, and then subjected to a second calcination temperature for a second holding time to obtain lithium iron phosphate cathode material; wherein the first calcination temperature is 120℃-150℃, the first holding time is 2h-4h, the second calcination temperature is 600℃-700℃, and the second holding time is 4h-6h.

[0025] In this embodiment, controlling the first calcination temperature, the first holding time, the second calcination temperature, and the second holding time makes it easier to prepare lithium iron phosphate cathode materials with finer and more uniform particles, more regular morphology, and higher density, which is beneficial to improving the compaction density and capacity of lithium iron phosphate cathode materials.

[0026] Thirdly, embodiments of this application provide a positive electrode sheet, which includes the lithium iron phosphate borate positive electrode material provided in the first aspect.

[0027] In this embodiment, the positive electrode sheet contains the aforementioned lithium iron phosphate borate positive electrode material, which not only has higher compaction density and capacity, but also superior ionic conductivity and cycle stability, and has a broader application prospect in the field of secondary batteries.

[0028] Fourthly, embodiments of this application provide a secondary battery, which includes the lithium iron phosphate borate cathode material provided in the first aspect or the cathode sheet provided in the third aspect.

[0029] In this embodiment, the secondary battery includes the aforementioned positive electrode sheet, which not only has high capacity but also superior ionic conductivity and cycle stability.

[0030] Fifthly, embodiments of this application provide an electrical device, which includes the lithium iron phosphate borate cathode material provided in the first aspect, the cathode sheet provided in the third aspect, or the secondary battery provided in the fourth aspect.

[0031] In this embodiment, the electrical device includes the aforementioned secondary battery, thus possessing the advantage of excellent cycle stability.

[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 solution 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 flowchart illustrating the preparation process of the lithium iron phosphate borate cathode material provided in Example 1.

[0035] Figure 2 The image shows the scanning electron microscope (SEM) results of the lithium iron phosphate borate cathode material provided in Example 1.

[0036] Figure 3 SEM image of the lithium iron phosphate borate cathode material provided in Example 2;

[0037] Figure 4 SEM image of the lithium iron phosphate borate cathode material provided in Example 3;

[0038] Figure 5 The image shows the SEM results of the lithium iron phosphate borate cathode material provided for Comparative Example 2. Detailed Implementation

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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).

[0045] In the description of the embodiments of this application, the technical terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0046] In the description of the embodiments of this application, unless otherwise expressly specified and limited, technical terms such as "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. For those skilled in the art, the specific meaning of the above terms in the embodiments of this application can be understood according to the specific circumstances.

[0047] With the development of new energy vehicles, lithium iron phosphate cathode materials are becoming increasingly prevalent in batteries as cathode materials due to their low cost and high safety. However, existing lithium iron phosphate cathode materials still suffer from problems such as low performance and high production costs.

[0048] To address the technical problems of low performance and high production cost of existing lithium iron phosphate cathode materials, this application provides a lithium iron phosphate cathode material, its preparation method, cathode electrode, secondary battery, and power supply device. The lithium iron phosphate cathode material employs trivalent titanium for cation doping and boron for anion doping, while also being coated with carbon. The larger ionic radius of trivalent titanium results in more defects, leading to better ionic conductivity and higher capacity. Boron, compared to phosphorus, has a smaller ionic radius, which is more conducive to increasing the compaction density of the cathode material, thereby improving capacity. The carbon coating layer not only improves ionic conductivity but also enhances cycle stability. Consequently, the performance of the cathode electrode, secondary battery, and power supply device incorporating this lithium iron phosphate cathode material is also improved.

[0049] The electrical devices provided in this application embodiment can be, but are not limited to, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, spacecraft, etc. Among them, electric toys can include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc., and spacecraft can include airplanes, rockets, space shuttles, and spacecraft, etc.

[0050] In a first aspect, embodiments of this application provide a lithium iron phosphate borate cathode material, the chemical formula of which is Li. x FeTi y P z B w O (4z+3w+2y) / C, where x, y, z, and w are all non-zero positive numbers, 1.02≤x≤1.05, 0.01≤y≤0.05, 0.92≤z≤0.95, 0.05≤w≤0.15, and the mass fraction of carbon element is 1.00% to 1.35% based on the mass of boric acid phosphate material as 100%.

[0051] In this application, the lithium iron phosphate cathode material includes a core and a coating layer covering the outer surface of the core, wherein the chemical formula of the core material is: Li x FeTi y P z B w O (4z+3w+2y) The coating material is carbon material.

[0052] The lithium iron phosphate cathode material provided in this application uses trivalent titanium for cation doping and boron for anion doping, while also being coated with carbon. Trivalent titanium has a larger ionic radius and forms more defects, resulting in better ionic conductivity and higher capacity for the lithium iron phosphate cathode material. Boron is doped in the form of borate ions, and the molar mass of borate is less than that of phosphate, which is beneficial to improving the capacity of the lithium iron phosphate cathode material. Carbon, as a coating layer, not only improves the conductivity of the lithium iron phosphate cathode material but also enhances its cycle stability.

[0053] Typical, but not limiting, in the chemical formula of the above-mentioned lithium iron borate phosphate cathode material, x is such as 1.02, 1.03, 1.04, 1.05 or any two values; y is such as 0.01, 0.02, 0.03, 0.04, 0.05 or any two values; z is such as 0.92, 0.93, 0.94, 0.95 or any two values; and the mass fraction of C, based on 100% of the lithium iron borate phosphate cathode material, is such as 1.00%, 1.02%, 1.05%, 1.08%, 1.10%, 1.12%, 1.15%, 1.18%, 1.20%, 1.25%, 1.30%, 1.35% or any two values.

[0054] Furthermore, in some embodiments, the D10 particle size of the above-mentioned lithium iron phosphate borate cathode material is ≤0.5μm, the D50 particle size is ≤1.60μm, and the D90 particle size is ≤19.00μm.

[0055] In this application, D10 particle size refers to the particle size corresponding to the cumulative particle size distribution of lithium iron phosphate material reaching 10%, which means that particles smaller than this value account for 10% of the total particles; D50 particle size refers to the particle size corresponding to the cumulative particle size distribution of lithium iron phosphate material reaching 50%, which means that particles smaller than this value account for 50% of the total particles; D90 particle size refers to the particle size corresponding to the cumulative particle size distribution of lithium iron phosphate material reaching 90%, which means that particles smaller than this value account for 90% of the total particles.

[0056] In the technical solution of this application embodiment, the particle size of lithium iron phosphate borate cathode material is more uniform and finer, which is more conducive to improving the capacity and cycle stability of the battery prepared by it as cathode material.

[0057] Furthermore, in some embodiments, the pH value of the above-mentioned lithium iron phosphate borate cathode material is 8.50 to 9.00, such as 8.50, 8.60, 8.70, 8.80, 8.90, 9.00, etc., which makes it easier to homogenize during the subsequent preparation of cathode slurry, and also makes it easier to prepare cathode sheets with uniform dispersion and stable performance.

[0058] Furthermore, in some embodiments, the lithium iron phosphate borate cathode material has a compaction density of 2.35 g / mL to 2.50 g / mL and a specific surface area of ​​10.1 m². 2 / g-14.3m 2 With a powder resistivity of 4.0 Ω·cm to 13.0 Ω·cm, it has a higher compaction density, a larger specific surface area, and a lower powder resistivity, which is more conducive to improving the capacity of batteries formed using this cathode material.

[0059] Typical, but not limiting, compaction densities of lithium iron phosphate borate cathode materials include 2.35 g / mL, 2.40 g / mL, 2.45 g / mL, 2.50 g / mL, or any combination of two values; and specific surface areas include, for example, 10.1 m². 2 / g, 10.5m 2 / g, 10.8m 2 / g, 11.0 m 2 / g, 11.2m 2 / g, 11.5m 2 / g, 11.8m 2 / g, 12.0m 2 / g, 12.5m 2 / g, 12.8m 2 / g, 13.0m 2 / g, 13.2m 2 / g, 13.5m 2 / g, 13.8m 2 / g, 14.0m 2 / g, 14.2m 2 / g, 14.3m 2 / g or any range of two values; its powder resistivity is 4.0Ω.cm, 4.5Ω.cm, 5.0Ω.cm, 5.5Ω.cm, 6.0Ω.cm, 8.0Ω.cm, 9.0Ω.cm, 10.0Ω.cm, 11.0 Ω.cm, 12.0Ω.cm, 13.0 Ω.cm or any range of two values.

[0060] Furthermore, in some embodiments, the lithium iron phosphate cathode material has an iron leaching content of 3.0 ppm to 10.0 ppm, an elemental iron content of 0.01 ppm to 0.05 ppm, a chloride ion content of 40.0 ppm to 80.0 ppm, a water content of 140 ppm to 200 ppm, a free lithium content of 140 ppm to 200 ppm, and a magnetic impurity content ≤ 1 ppm. That is, the iron leaching content, elemental iron content, chloride ion content, water content, and free lithium content of the lithium iron phosphate cathode material provided in this application are all in the ppm level in terms of mass concentration. Its structure is more stable and the impurity content is lower, thereby giving it better ionic conductivity and cycle stability.

[0061] Secondly, embodiments of this application provide a method for preparing a lithium iron phosphate borate cathode material, such as... Figure 1 As shown, the preparation method includes the following steps: mixing ferrous source, trivalent titanium source, carbon source, phosphoric acid, boric acid and solvent to obtain a mixed slurry; preheating and drying the mixed slurry to obtain a pre-dried material; calcining the pre-dried material to obtain lithium iron phosphate borate cathode material.

[0062] The method for preparing lithium iron phosphate cathode material provided in this application uses lithium source, ferrous source, trivalent titanium source, carbon source, phosphoric acid, and boric acid as raw materials, which is more conducive to enhancing the driving force of solid-phase reaction and more likely to form a eutectic. Compared with the dry mixing and calcination process, the reaction temperature is further reduced, the particles are further refined, the capacity is further improved, and the production cost is further reduced. At the same time, the trivalent titanium ions have a larger ionic radius and form more defects, thus making the lithium iron phosphate cathode material have better ionic conductivity and higher capacity. Boron is doped in the form of borate ions, and the molar mass of borate is less than that of phosphate, which is beneficial to improving the capacity of lithium iron phosphate cathode material. Carbon, as a coating layer, not only helps to improve the conductivity of lithium iron phosphate cathode material, but also helps to improve cycle stability.

[0063] Specifically, the preparation method of lithium iron phosphate cathode material provided in this application introduces boric acid, which can be used as both a dopant and a flux, to further reduce the calcination temperature and avoid the problems of increased particle size, iron phosphate, and magnetic material content in lithium iron phosphate cathode material caused by high-temperature calcination.

[0064] Furthermore, the preparation method of lithium iron phosphate cathode material provided in this application reduces steps such as water addition for slurry formation, grinding, and spray drying compared to traditional processes. According to actual production statistics, the increased cost (including labor, equipment depreciation, energy consumption, and environmental protection costs) is approximately 2,500 yuan per ton. The preparation method of lithium iron phosphate cathode material provided in this application can save 2,500 yuan per ton in this step alone. Based on an annual production of 100,000 tons of lithium iron phosphate cathode material, this translates to cost savings of 250 million yuan per year.

[0065] Furthermore, in some embodiments, the aforementioned ferrous source is an aqueous ferrous source, and the water-soluble ferrous source is an anionic volatile ferrous salt; the lithium source is a water-soluble lithium source, and the water-soluble lithium source is an anionic volatile lithium salt; the trivalent titanium source is an anionic volatile titanium salt. For example, the water-soluble ferrous source includes, but is not limited to, one or more of ferrous acetate and / or ferrous chloride; the water-soluble lithium salt includes, but is not limited to, one or more of lithium acetate and / or lithium chloride; and the water-soluble titanium source is, for example, titanium halide, to facilitate thorough mixing of the water-soluble ferrous source, the water-soluble lithium salt, and the water-soluble trivalent titanium source with the acid. Meanwhile, at high temperatures, volatile anions combine with hydrogen ions in the acid to form volatile low-boiling-point acids. This allows for the preparation of low-boiling-point acids from high-boiling-point acids using the same principle as high-boiling-point acids such as boric acid and phosphoric acid, mixed with chloride and acetate ions to form low-boiling-point hydrogen chloride and acetic acid. Other ions combine to form a composite lithium iron phosphate borate. This solid-phase reaction has a stronger driving force and facilitates the formation of a eutectic. Compared to the dry mixing and calcination process, the reaction temperature is further reduced, the particles are further refined, the capacity is further increased, and the production cost is further reduced.

[0066] Specifically, the method of dry mixing and calcining of iron phosphate, lithium carbonate and carbon source is a completely solid-state sintering method. It does not have the driving force of this application (the principle of preparing low-boiling acid from high-boiling acid). At the same time, the raw material particles are large and the ion diffusion distance is long. High-temperature calcination requires a higher reaction temperature to achieve diffusion. However, a higher calcination temperature will cause the particles to become larger and the capacity to be further reduced.

[0067] The method for preparing lithium iron phosphate (LFP) cathode material provided in this application introduces boric acid, which can act as both a dopant and a flux, further reducing the calcination temperature and avoiding the problems of particle size increase and increased iron phosphide and magnetic material content caused by high-temperature calcination. By using volatile anionic ferrous salts, volatile anionic lithium salts, and volatile anionic titanium salts as raw materials, only a lower temperature is needed (20°C-30°C lower than other dry mixing methods). The higher volatility of low-boiling-point acids promotes better material fusion and finer particle size, thereby further improving capacity.

[0068] Furthermore, in some embodiments, the trivalent titanium source used is anionic trivalent titanium salt, which has a lower melting point (decomposition point) and can melt at a lower temperature, enhancing the dispersion effect and making the trivalent titanium more uniformly doped in lithium iron phosphate borate. Trivalent titanium sources include, but are not limited to, titanium trihalides, such as titanium trifluoride, titanium trichloride, and titanium tribromide. In particular, when the trivalent titanium source is titanium trichloride, it can utilize the principle of preparing low-boiling-point acids from high-boiling-point acids to lower the calcination reaction temperature. It can also utilize the larger ionic radius of trivalent titanium ions, which leads to more defects formed during the doping process, thus enhancing the ionic conductivity of lithium ions. Therefore, its doping effect is superior to that of tetravalent titanium (such as titanium dioxide). Furthermore, titanium trichloride has a lower melting point (decomposition point) of only 440℃. Compared to the high melting point of other traditional dopants such as titanium dioxide, titanium trichloride can melt at a lower temperature, resulting in better dispersion and more uniform doping. This is more conducive to improving the electrochemical performance of lithium iron phosphate borate cathode materials in terms of charge specific capacity and discharge specific capacity.

[0069] Furthermore, in some embodiments, the carbon source can be any carbon source commonly used in the art. To facilitate improved uniformity of mixing and conductivity of the raw materials, a water-soluble carbon source is preferred, including any one or more of ascorbic acid and / or citric acid, more preferably a mixture of ascorbic acid and citric acid. Preferably, the mass ratio of ascorbic acid to citric acid is (0.05-0.2):(0.92-0.95), such as 0.05:0.92, 0.08:0.92, 0.10:0.92, 0.15:0.92, 0.2:0.92, 0.05:0.95, 0.08:0.95, 0.10:0.95, 0.15:0.95, 0.2:0.95, or any range of two values.

[0070] Furthermore, in some embodiments, the solvent is selected from at least one of pure water, deionized water, or distilled water.

[0071] Furthermore, in some embodiments, during the preparation of lithium iron phosphate borate cathode material, the molar ratio of iron in the ferrous source, titanium in the trivalent titanium source, lithium in the lithium source, carbon in the carbon source, phosphorus in the phosphoric acid solution, and boron in the boric acid is 1:(0.01-0.05):(1.02-1.05):(0.36-1.32):(0.92-0.95):(0.05-0.15), which is beneficial for improving the capacity and cycle stability of the cathode material. If the amount of iron in the ferrous source is 1 mol, the amount of trivalent titanium in the trivalent titanium source is 0.01 mol, 0.02 mol, 0.03 mol, 0.04 mol, 0.05 mol, or any two of these values; the amount of lithium in the lithium source is 1.02 mol, 1.03 mol, 1.04 mol, 1.05 mol, or any two of these values; and the amount of carbon in the carbon source is 0.36 mol, 0.48 mol, 0.60 mol, or any two of these values. The amounts of phosphorus in phosphoric acid are 0.72 mol, 0.90 mol, 1.08 mol, 1.20 mol, 1.32 mol, or any two of these values; the amounts of phosphorus in phosphoric acid are 0.92 mol, 0.93 mol, 0.94 mol, 0.95 mol, or any two of these values; the amounts of boron in boric acid are 0.05 mol, 0.08 mol, 0.10 mol, 0.12 mol, 0.15 mol, or any two of these values.

[0072] Furthermore, in some embodiments, to facilitate uniform mixing of phosphoric acid and other raw materials, phosphoric acid is first mixed with a solvent to form a phosphoric acid solution, and then the phosphoric acid solution is mixed with a lithium source, a ferrous source, a trivalent titanium source, and boric acid to form a mixed slurry. Alternatively, a phosphoric acid solution can be used to simultaneously provide phosphoric acid and a solvent. Further, it is preferred that the mass fraction of the phosphoric acid solution is 70%-80%, such as 70%, 72%, 75%, 78%, 80%, or any range of two values.

[0073] Furthermore, in some embodiments, the preparation method of lithium iron phosphate cathode material involves loading, preheating and drying, furnace feeding, furnace discharge, and unloading using a sagger and a return line, repeating this process sequentially to obtain the lithium iron phosphate cathode material. During the loading process, a metering module is installed below the loading station. This module is electrically connected to and controls the drive motor of the feeder, achieving precise feeding. The sagger can be a graphite sagger.

[0074] Furthermore, in some embodiments, the mixed slurry is preheated and dried at 100°C-150°C to obtain a pre-dried material with an H2O mass fraction of less than or equal to 0.5%, which facilitates thorough drying during the preheating and drying process and avoids reactions between the raw materials. Simultaneously, the moisture content of the dried material is controlled to ≤0.5wt%, which helps to produce a more dense and uniform product during subsequent calcination, thereby further improving the density of the lithium iron phosphate borate cathode material.

[0075] Further, in some embodiments, the mixed slurry is prepared according to the following steps: phosphoric acid is mixed with a solvent to form a phosphoric acid solution with a mass fraction of 70% to 80%, which is then set aside. A carbon source is first added to a mixture of ferrous source, trivalent titanium source, and lithium source, followed by the addition of the phosphoric acid solution and boric acid. The mixture is then placed in a double-cone mixer for mixing to obtain the mixed slurry.

[0076] In some embodiments, during the preheating and drying process, an exhaust fan is turned on to remove the exhaust gas. The extracted exhaust gas is absorbed by spraying with lithium hydroxide solution, and the resulting absorbed waste liquid is returned to the mixed slurry preparation process for recycling, which helps to reduce environmental pollution and energy consumption.

[0077] In some embodiments, the step of calcining the pre-dried material to obtain lithium iron phosphate cathode material includes: under a protective atmosphere, the pre-dried material is subjected to a first calcination temperature and a first holding time, followed by a second calcination temperature and a second holding time to obtain the lithium iron phosphate cathode material; wherein the first calcination temperature is 120℃-150℃, the first holding time is 2h-4h, the second calcination temperature is 600℃-700℃, and the second holding time is 4h-6h. By controlling the first calcination temperature, the first holding time, the second calcination temperature, and the second holding time, it is more beneficial to prepare lithium iron phosphate cathode material with finer and more uniform particles, more regular morphology, and higher density, while also helping to further save energy and reduce production costs.

[0078] Typically, but not limitingly, the first calcination temperature is such as 120℃, 130℃, 140℃, 150℃ or any range of two values; the first holding time is such as 2h, 2.5h, 3h, 3.5h, 4h or any range of two values; the second calcination temperature is such as 600℃, 620℃, 650℃, 680℃, 700℃ or any range of two values; and the second holding time is such as 4h, 4.5h, 5h, 5.5h, 6h or any range of two values.

[0079] In some embodiments, during the calcination process described above, a protective gas is introduced to form a protective atmosphere, maintaining the oxygen volume content below 5 ppm during calcination to further reduce the introduction of impurity phases. This protective gas includes, but is not limited to, any one of nitrogen, argon, or helium.

[0080] In some embodiments, the calcination process described above is carried out in a roller furnace. During the calcination process, a protective gas is introduced to form a protective atmosphere, maintaining the oxygen volume content in the roller furnace below 5 ppm, in order to further reduce the introduction of impurity phases.

[0081] In some embodiments, the pre-dried material is heated from room temperature to a first calcination temperature at a first heating rate and held for a first holding time, then heated from the first calcination temperature to a second calcination temperature at a second heating rate and held for a second holding time, and then cooled to a material temperature less than or equal to 100°C before being discharged to obtain lithium iron phosphate borate cathode material; wherein the first heating rate is 20°C / h-30°C / h, and the second heating rate is 50°C / h-100°C / h, so as to facilitate a more dense cathode material with fewer defects and a more regular morphology.

[0082] Typical, but not limiting, the first heating rate is 20℃ / h, 22℃ / h, 25℃ / h, 28℃ / h, 30℃ / h, or a range of any two values, and the second heating rate is 50℃ / h, 55℃ / h, 60℃ / h, 70℃ / h, 80℃ / h, 90℃ / h, 95℃ / h, 100℃ / h, or a range of any two values.

[0083] In some embodiments, the calcined feedstock is crushed, sieved, iron-removed, and packaged to obtain lithium iron borate phosphate cathode material. The sieving process uses a 100-200 mesh sieve to remove iron until the magnetic impurity content of anhydrous iron phosphate is below 1 ppm, after which it is vacuum-packaged under constant temperature (25±5℃) and constant humidity (≤10%) conditions.

[0084] In some embodiments, the calcined feedstock is transported via a pipeline system to an air jet mill, then sieved using an ultrasonic vibrating screen, de-ironed using an electromagnetic separator, and vacuum-packed in a constant temperature and humidity chamber to obtain lithium iron phosphate cathode material. The constant temperature and humidity chamber is maintained at 25±5℃ and ≤10% humidity.

[0085] To further reduce environmental pollution, in some embodiments, the waste gas generated during the calcination process is introduced into a spray absorption tower by an induced draft fan and absorbed by pure water. The absorbent reacts with iron powder, and the resulting ferrous solution is concentrated and crystallized and then returned to the preparation step of the mixed slurry as a raw material to further reduce energy consumption.

[0086] Thirdly, embodiments of this application provide a positive electrode sheet, which includes the lithium iron phosphate borate positive electrode material provided in the first aspect.

[0087] In this embodiment, the positive electrode sheet contains the aforementioned lithium iron phosphate borate positive electrode material, which not only has higher compaction density and capacity, but also superior ionic conductivity and cycle stability, and has a broader application prospect in the field of secondary batteries.

[0088] Fourthly, embodiments of this application provide a secondary battery, which includes the lithium iron phosphate borate cathode material provided in the first aspect or the cathode sheet provided in the third aspect.

[0089] In this embodiment, the secondary battery includes the aforementioned positive electrode, thus not only having high capacity but also superior ionic conductivity and cycle stability.

[0090] Fifthly, embodiments of this application provide an electrical device, such as a vehicle, including the lithium iron phosphate borate cathode material provided in the first aspect, the cathode sheet provided in the third aspect, or the secondary battery provided in the fourth aspect.

[0091] In this embodiment, the electrical device includes the aforementioned secondary battery, thus exhibiting excellent cycle stability. Some specific embodiments are listed below. 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 used without specified manufacturers are all conventional products that can be obtained commercially.

[0092] Example 1

[0093] This embodiment provides a lithium iron phosphate borate cathode material, which is prepared according to the following steps:

[0094] (1) Ferrous acetate, titanium trichloride, lithium acetate, ascorbic acid, citric acid, 80% phosphoric acid solution and boric acid are put into an inclined mixer and mixed to obtain a mixed slurry; wherein, the molar ratio of ferrous acetate, titanium trichloride, lithium acetate, ascorbic acid, citric acid, phosphoric acid and boric acid is 1:0.03:1.04:0.015:0.12:0.94:0.1, and during the mixing process, grinding balls are added into the inclined mixer. The grinding balls are made of stainless steel and coated with polyurethane. The diameter of the grinding balls is 5cm, the rotation speed is 8r / min, the filling amount is 60% of the effective volume of the mixer, and the mixing time is 50min.

[0095] (2) The mixed slurry is fed into the graphite sagger by a screw feeder and then preheated and dried until the free water content of the mixed slurry is less than 0.5wt% to obtain the pre-dried material. The temperature during preheating and drying is 120℃, and the exhaust fan is turned on at the same time to remove the exhaust gas. The exhaust gas is absorbed by spraying with lithium hydroxide solution, and the resulting absorption waste liquid is returned to step (1) for recycling.

[0096] (3) The pre-dried material is placed in a roller furnace for calcination. During the calcination process, the temperature is first raised to 135℃ at a rate of 25℃ / h and held at this temperature for 4 hours. Then, the temperature is raised to 650℃ at a rate of 80℃ / h and held at this temperature for 5 hours. The material is then cooled down to a temperature ≤100℃ and discharged. During the calcination process, nitrogen is introduced into the roller furnace to maintain the oxygen volume content in the roller furnace below 5ppm. The waste gas generated during calcination is introduced into a spray absorption tower by an induced draft fan and absorbed by pure water. Iron powder is added to the absorption liquid for reaction. The resulting ferrous solution is concentrated and crystallized and then returned to step (1) for recycling. The calcined material is transported to an airflow pulverizer through a pipeline conveying system. After being screened by an ultrasonic vibrating screen and vacuum-packed in a constant temperature and humidity chamber, lithium iron phosphate cathode material is obtained. The screening is carried out using a 150-mesh ultrasonic vibrating screen. The temperature in the constant temperature and humidity chamber is 25±5℃ and the humidity is ≤10%.

[0097] In this embodiment, the feeding, preheating and drying, furnace feeding, furnace discharging and unloading are carried out in a cyclical manner via a return line. During the feeding process, a metering module is installed below the feeding station. The metering module has an accuracy of 0.5%. The metering module is electrically connected to the drive motor of the feeder and controls the drive motor to achieve precise feeding.

[0098] Example 2

[0099] This embodiment provides a lithium iron phosphate borate cathode material, which is prepared according to the following steps:

[0100] (1) Ferrous chloride, titanium trichloride, lithium chloride, ascorbic acid, citric acid, 70% phosphoric acid solution and boric acid are put into a slant mixer and mixed to obtain a mixed slurry; wherein, the molar ratio of ferrous chloride, titanium trichloride, lithium chloride, ascorbic acid, citric acid, phosphoric acid and boric acid is 1:0.05:1.05:0.02:0.2:0.95:0.15, and during the mixing process, grinding balls are added into the double cone mixer. The grinding balls are made of stainless steel and coated with polyurethane. The diameter of the grinding balls is 3cm, the rotation speed is 5r / min, the filling amount is 50% of the effective volume of the mixer, and the mixing time is 30min.

[0101] (2) The mixed slurry is fed into the graphite sagger by a screw feeder and then dried by preheating until the free water content of the paper mixed slurry is less than 0.5wt%, and the drying is stopped to obtain the pre-dried material. The temperature during preheating is 150℃, and the exhaust fan is turned on at the same time to remove the exhaust gas. The exhaust gas is absorbed by spraying with lithium hydroxide solution, and the resulting absorption waste liquid is returned to step (1) for recycling.

[0102] (3) The pre-dried material is placed in a roller furnace for calcination. During the calcination process, the temperature is first raised to 150°C at a rate of 20°C / h and held at this temperature for 4 hours. Then, the temperature is raised to 700°C at a rate of 100°C / h and held at this temperature for 4 hours. Finally, the temperature is lowered to ≤100°C before discharge. During the calcination process, nitrogen is introduced into the roller furnace to maintain the oxygen volume content in the roller furnace below 5 ppm. The waste gas generated during calcination is introduced into a spray absorption tower by an induced draft fan and absorbed by pure water. Iron powder is added to the absorbent liquid for reaction. The resulting ferrous solution is concentrated and crystallized and then returned to step (1) for recycling. After the calcined material is transported to the air jet mill via a pipeline system, it is screened by an ultrasonic vibrating screen and vacuum-packed in a constant temperature and humidity chamber to obtain lithium iron borate phosphate cathode material. The screening is carried out using a 100-mesh ultrasonic vibrating screen to remove iron until the content of magnetic impurities in anhydrous iron phosphate is less than 1 ppm before vacuum packaging. The temperature in the constant temperature and humidity chamber is 25±5℃ and the humidity is ≤10%.

[0103] In this embodiment, the feeding, preheating and drying, furnace feeding, furnace discharge and unloading are carried out in a cyclical manner through a return line. During the feeding process, a metering module is set below the feeding station. The metering module is electrically connected to the drive motor of the feeder and controls the drive motor to achieve precise feeding.

[0104] Example 3

[0105] This embodiment provides a lithium iron phosphate borate cathode material, which is prepared according to the following steps:

[0106] (1) Ferrous acetate, titanium trichloride, lithium chloride, ascorbic acid, citric acid, 80% phosphoric acid solution and boric acid are put into a slant mixer and mixed to obtain a mixed slurry. The molar ratio of ferrous acetate, titanium trichloride, lithium chloride, ascorbic acid, citric acid, phosphoric acid and boric acid is 1:0.01:1.02:0.01:0.05:0.92:0.1. Then it is put into a double cone mixer for mixing. During the mixing process, grinding balls are added to the double cone mixer. The grinding balls are made of stainless steel and coated with polyurethane. The diameter of the grinding balls is 3cm, the rotation speed is 5r / min, the filling amount is 50% of the effective volume of the mixer, and the mixing time is 30min.

[0107] (2) The mixed slurry is fed into the graphite sagger by a screw feeder and then dried by preheating until the free water content of the paper mixed slurry is less than 0.5wt%, and the drying is stopped to obtain the pre-dried material. The temperature during preheating is 100℃, and the exhaust fan is turned on at the same time to remove the exhaust gas. The exhaust gas is absorbed by spraying with lithium hydroxide solution, and the resulting absorption waste liquid is returned to step (1) for recycling.

[0108] (3) The pre-dried material is placed in a roller furnace for calcination. During the calcination process, the temperature is first raised to 120℃ at a rate of 30℃ / h and held at this temperature for 2 hours. Then, the temperature is raised to 600℃ at a rate of 50℃ / h and held at this temperature for 6 hours. Then, the temperature is lowered to ≤100℃ and the material is discharged. During the calcination process, nitrogen is introduced into the roller furnace to maintain the oxygen volume content in the roller furnace below 5ppm. The waste gas generated during calcination is introduced into the spray absorption tower by an induced draft fan and absorbed by pure water. Iron powder is added to the absorption liquid for reaction. The resulting ferrous solution is concentrated and crystallized and then returned to step (1) for recycling. The calcined material is transported to the air jet mill through a pipeline conveying system. After being screened by an ultrasonic vibrating screen and vacuum-packed in a constant temperature and humidity chamber, lithium iron phosphate cathode material is obtained. Among them, the screening is carried out by a 200-mesh ultrasonic vibrating screen. After removing iron until the magnetic impurity content of anhydrous iron phosphate is below 1ppm, it is vacuum-packed. The temperature in the constant temperature and humidity chamber is 25±5℃ and the humidity is ≤10%.

[0109] In this embodiment, the feeding, preheating and drying, furnace feeding, furnace discharge and unloading are carried out in a cyclical manner through a return line. During the feeding process, a metering module is set below the feeding station. The metering module is electrically connected to the drive motor of the feeder and controls the drive motor to achieve precise feeding.

[0110] Example 4

[0111] This embodiment provides a lithium iron phosphate borate cathode material, which is prepared according to the following steps:

[0112] (1) Ferrous chloride, titanium trichloride, lithium acetate, ascorbic acid, citric acid, 77% phosphoric acid solution and boric acid are mixed in an inclined mixer to obtain a mixed slurry; wherein, the molar ratio of ferrous chloride, titanium trichloride, lithium acetate, ascorbic acid, citric acid, phosphoric acid and boric acid is 1:0.03:1.02:0.02:0.1:0.95:0.1, and during the mixing process, grinding balls are added to the inclined mixer. The grinding balls are made of stainless steel and coated with polyurethane. The diameter of the grinding balls is 6 cm, the rotation speed is 8 r / min, the filling amount is 55% of the effective volume of the mixer, and the mixing time is 40 min.

[0113] (2) The mixed slurry is fed into the graphite sagger by a screw feeder and then preheated and dried until the free water content of the mixed slurry is less than 0.5wt% to obtain the pre-dried material. The temperature during preheating and drying is 140℃, and the exhaust fan is turned on at the same time to remove the exhaust gas. The exhaust gas is absorbed by spraying with lithium hydroxide solution, and the resulting absorption waste liquid is returned to step (1) for recycling.

[0114] (4) The pre-dried material is placed in a roller furnace for calcination. During the calcination process, the temperature is first raised to 130℃ at a rate of 22℃ / h and held at this temperature for 3 hours. Then, the temperature is raised to 640℃ at a rate of 65℃ / h and held at this temperature for 5 hours. The material is then cooled to a temperature ≤100℃ before being discharged. During the calcination process, nitrogen is introduced into the roller furnace to maintain the oxygen volume content in the roller furnace below 5ppm. The waste gas generated during calcination is introduced into a spray absorption tower by an induced draft fan and absorbed by pure water. Iron powder is added to the absorption liquid for reaction. The resulting ferrous solution is concentrated and crystallized and then returned to step (1) for recycling. The calcined material is transported to an airflow pulverizer through a pipeline conveying system. After being screened by an ultrasonic vibrating screen and vacuum-packed in a constant temperature and humidity chamber, lithium iron phosphate cathode material is obtained. Among them, the screening is carried out by a 150-mesh screen to remove iron until the magnetic impurity content of anhydrous iron phosphate is less than 1ppm before vacuum packaging. The temperature in the constant temperature and humidity chamber is 25±5℃ and the humidity is ≤10%.

[0115] Example 5

[0116] The difference between this embodiment and Embodiment 1 is that ferrous sulfate is used instead of ferrous acetate as the water-soluble ferrous source, and lithium nitrate is used instead of lithium acetate as the water-soluble lithium source.

[0117] Example 6

[0118] The difference between this embodiment and Example 1 is that ascorbic acid was not added, but the number of moles of citric acid is the same as the sum of the number of moles of ascorbic acid and citric acid in Example 1.

[0119] Example 7

[0120] The difference between this embodiment and Example 1 is that citric acid was not added, but the number of moles of ascorbic acid is the same as the sum of the number of moles of ascorbic acid and citric acid in Example 1.

[0121] Example 8

[0122] The difference between this embodiment and Embodiment 1 is that titanium tribromide is used instead of titanium trichloride as the titanium source.

[0123] Example 9

[0124] The difference between this embodiment and Embodiment 1 is that the molar ratio of ferrous acetate, titanium trichloride, lithium acetate, ascorbic acid, citric acid, phosphoric acid, and boric acid is 1:0.01:1.02:0.01:0.05:0.92:0.05.

[0125] Example 10

[0126] The difference between this comparative example and Example 1 is that the molar ratio of ferrous acetate, titanium trichloride, lithium acetate, ascorbic acid, citric acid, phosphoric acid, and boric acid is 1:0.03:1.04:0.015:0.12:0.84:0.2.

[0127] Example 11

[0128] The difference between this comparative example and Example 1 is that the molar ratio of ferrous acetate, titanium trichloride, lithium acetate, ascorbic acid, citric acid, phosphoric acid, and boric acid is 1:0.15:1.04:0.015:0.12:0.84:0.2.

[0129] Comparative Example 1

[0130] The difference between this comparative example and Example 1 is that boric acid was not added, but the number of moles of phosphoric acid is the same as the sum of the number of moles of phosphoric acid and boric acid in Example 1.

[0131] Comparative Example 2

[0132] The difference between this comparative example and Example 1 is that ferrous acetate is replaced with an equimolar amount of ferric phosphate, lithium acetate is replaced with an equimolar amount of lithium carbonate, and phosphoric acid is not added.

[0133] Comparative Example 3

[0134] The difference between this comparative example and Example 1 is that titanium dioxide is used instead of titanium trichloride as the titanium source, and the titanium dioxide is in the rutile phase with a primary particle size of about 30 nm and a BET of 30 nm. 2 / g.

[0135] Comparative Example 4

[0136] The difference between this comparative example and Example 1 is that titanium trichloride was not added as a dopant.

[0137] Test methods

[0138] 1. The cathode materials provided in the above embodiments and comparative examples were subjected to morphological inspection using scanning electron microscopy.

[0139] 2. The content of each element, compaction density, BET, D10, D50, D90, powder resistivity, iron dissolution, elemental iron content, magnetic foreign matter content, chloride ion content, moisture content, free lithium content, and pH of the cathode materials provided in the examples and comparative examples were measured respectively. The results are shown in Table 1 and Table 2 below.

[0140] Among them, (1) the content of each element in the cathode material was determined by an ICP-OES spectrometer;

[0141] (2) The test method for compaction density is as follows: refer to GB / T 24533, determination of powder compaction density;

[0142] (3) The test method for BET is as follows: Refer to GB / T 19587, gas adsorption BET method for determining the specific surface area of ​​solid substances.

[0143] (4) The determination methods for D10, D50 and D90 are as follows: refer to GB / T 19077, particle size analysis laser diffraction method;

[0144] (5) The method for determining the resistivity of powder is as follows: the four-probe method is used, and the measurement is carried out under a pressure of 10 MPa.

[0145] (6) The method for determining iron dissolution is as follows: 1g of the test sample is added to 100mL of 0.1mol / L hydrogen fluoride-ethanol solution, stirred and dissolved at 45℃ for 30min, then filtered, and the iron content in the filtrate is measured, which is the amount of iron dissolution.

[0146] (7) The method for determining the content of elemental iron is as follows: refer to the potassium dichromate titration method in GB / T 223.7-2002;

[0147] (8) The method for determining the content of magnetic foreign matter is as follows: Weigh 100g of material and pour it into a plastic bottle with a cap. Add 1000g of pure water and then add an 8000GS magnetic rod. Protect the outside of the magnetic rod with polytetrafluoroethylene. Then tighten the cap, place the plastic bottle horizontally and rotate it at a speed of 10-20r / min. After rotating for 30-45min, stop rotating and remove the magnetic rod. Add 1000g of pure water and place it into the plastic bottle. Tighten the cap, place the plastic bottle horizontally and rotate it at a speed of 10-20r / min. After rotating for 5-10min, stop rotating and remove the magnetic rod. Dissolve the magnetic rod in aqua regia and 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 it by the weight of the material to obtain the content of magnetic foreign matter in the material.

[0148] (9) Method for determining moisture content: GB / T 6283, Determination of moisture content in chemical products by Karl Fischer method (general method);

[0149] (10) Method for determining chloride ion content: GB / T23273.6-2009, Ion-selective electrode method;

[0150] (11) Method for determining free lithium content: SJT 11794-2021, automatic potentiometric titration method;

[0151] (12) The method for pH determination is as follows: refer to GB / T 9724, General Rules for pH Determination of Chemical Reagents.

[0152] Figure 2 SEM image of the lithium iron phosphate borate cathode material provided in Example 1; Figure 3 SEM image of the lithium iron phosphate borate cathode material provided in Example 2; Figure 4 SEM image of the lithium iron phosphate borate cathode material provided in Example 3; Figure 5 SEM image of the lithium iron phosphate borate cathode material provided for Comparative Example 2. From... Figures 2-5 It can be seen that, compared with the cathode material formed by the traditional dry mixing and calcination process in Comparative Example 2, the lithium iron phosphate cathode material particles provided in Examples 1-3 of this application have better dispersibility, higher sphericity, more regular morphology, higher particle size consistency, and reasonable particle size matching, resulting in better compaction density of the product.

[0153] Table 1

[0154] Li (%) Fe (%) P(％) B C(％) Ti (ppm) D10(μm) D50(μm) D90(μm) Example 1 4.45 34.68 18.44 0.69％ 1.32 7435 0.31 1.32 9.46 Example 2 4.42 34.54 18.32 0.98％ 1.31 7378 0.35 1.29 11.24 Example 3 4.39 34.73 18.37 0.76％ 1.01 1759 0.48 1.59 18.45 Example 4 4.35 34.69 18.54 0.73％ 1.21 5325 0.42 1.51 9.21 Example 5 4.44 34.65 18.47 0.68％ 1.31 7421 0.32 1.35 9.41 Example 6 4.44 34.64 18.43 0.67％ 1.39 7412 0.28 1.32 9.38 Example 7 4.46 34.69 18.49 0.68％ 1.27 7448 0.29 1.38 9.78 Example 8 4.44 34.56 18.37 0.64％ 1.31 7328 0.32 1.28 9.38 Example 9 4.42 34.75 18.52 0.35％ 1.31 7511 0.32 1.26 9.98 Example 10 4.46 34.43 18.12 1.45％ 1.31 7413 0.35 1.39 9.59 Example 11 4.35 34.43 18.12 1.47％ 1.32 36574 0.52 1.52 19.45 Comparative Example 1 4.42 34.26 19.12 2.1ppm 1.30 7235 0.33 1.25 9.98 Comparative Example 2 4.48 34.89 18.15 0.65％ 1.35 7498 0.36 1.21 13.56 Comparative Example 3 4.42 34.35 18.27 0.67％ 1.30 7449 0.32 1.39 9.59 Comparative Example 4 4.48 34.97 18.65 0.71％ 1.34 13.8 0.76 1.76 21.78

[0155] As can be seen from Table 1, the lithium iron phosphate cathode materials provided in Examples 1-11 of this application have D10 particle size ≤ 0.5 μm, D50 particle size ≤ 1.60 μm, and D90 particle size ≤ 19.45 μm. The lithium iron phosphate cathode materials provided in Examples 1-11 of this application have more uniform particle size by using titanium for cation doping and boron for anion doping, which is more conducive to improving the compaction density of the cathode sheet prepared by using it as a cathode material.

[0156] Table 2

[0157]

[0158]

[0159] As can be seen from Table 2, the compaction density of the lithium iron phosphate borate cathode material provided in Examples 1-11 of this application is 2.35–2.52 m³ / s. 2 / g, with a specific surface area of ​​8.6m² 2 / g-14.3m 2With a powder resistivity of 4.0 Ω·cm to 13.0 Ω·cm, it is more conducive to improving the capacity of batteries formed using this lithium iron phosphate borate cathode material.

[0160] Furthermore, as can be seen from Table 2, the lithium iron phosphate cathode materials provided in Examples 1-9 of this application have the molecular formula satisfying Li x FeTi y P z B w O (4z+3w+2y) / C, where x, y, z, and w are all non-zero positive numbers, and when 1.02≤x≤1.05, 0.01≤y≤0.05, 0.92≤z≤0.95, and 0.05≤w≤0.15, the iron leaching is 3.0ppm~10.0ppm, the elemental iron is 0.01ppm~0.05ppm, the chloride ion is 40.0ppm~80.0ppm, the water content is 280ppm~350ppm, the free lithium is 140ppm~200ppm, and the magnetic impurity content is ≤1ppm. The low magnetic material content is more conducive to improving the capacity and rate performance of lithium iron phosphate cathode material.

[0161] The lithium iron phosphate cathode material provided in Examples 10-11 of this application has an excessively high boron content, which in turn results in an excessively high elemental iron content. As a result, the self-discharge of the battery cell made from the lithium iron phosphate cathode material is large, and the K value is too high, which affects the performance of the secondary battery product.

[0162] Performance testing of positive electrode sheet and secondary battery

[0163] The positive electrode materials provided in the examples and comparative examples were respectively prepared into positive electrode sheets and assembled into secondary batteries. The specific process is as follows: The positive electrode materials provided in the examples and comparative examples were mixed evenly with conductive carbon black (SP) conductive agent, polyvinylidene fluoride (PVDF), and N-methylpyrrolidone (NMP) using a high-speed mixer. The mass ratio of the positive electrode material, SP conductive agent, and PVDF provided in the examples or comparative examples was 90:5:5. Then, an automatic coating machine was used to coat aluminum foil. The coated aluminum foil was dried in an oven and rolled according to the required compaction density. It was then cut into round pieces of the required size, weighed, and dried again to obtain the positive electrode sheet. Using lithium sheet as the negative electrode sheet, the positive electrode shell, negative electrode shell, positive electrode sheet, lithium sheet, separator, and electrolyte were assembled into a button cell according to requirements. The button cell was hung on a battery testing system and tested after standing.

[0164] The assembled button batteries were tested using the Landian Battery Testing System (Wuhan Landian CT2001A Battery Testing System) at a temperature of 25°C, a charging limit voltage of 3.75V, and a discharge termination voltage of 2.0V. Charging and discharging were performed at 0.1C and 10C rates, respectively, to obtain the initial discharge specific capacity at 0.1C, 0.1C, and 10C. Furthermore, the 1C discharge specific capacity of the assembled button batteries at -20°C was tested, and its percentage relative to the 1C discharge specific capacity at 25°C was calculated, thus obtaining the capacity retention rate at -20°C. The results are shown in Table 3 below.

[0165] Table 3

[0166]

[0167]

[0168] As can be seen from the data in Table 3, the lithium iron phosphate cathode material provided in this application uses trivalent titanium for cation doping and boron for anion doping, and is also coated with carbon. This makes the lithium iron phosphate cathode material not only have high charge specific capacity and discharge specific capacity, but also have a high capacity retention rate at -20℃, and has strong low temperature resistance.

[0169] A comparison of Examples 1-9 with Examples 10-11 shows that maintaining the molar ratio of iron, titanium, lithium, carbon, phosphorus, and boron in the lithium iron phosphate cathode material provided in this application at 1:(0.01-0.05):(1.02-1.05):(0.06-0.22):(0.92-0.95):(0.05-0.15) is beneficial to improving the charging specific capacity, discharging specific capacity, and capacity retention rate at -20℃ of the lithium iron phosphate cathode material.

[0170] A comparison of Examples 1-9 with Comparative Example 1 shows that the anion doping of borate is beneficial to improving the electrical performance of lithium iron phosphate cathode materials. A comparison of Examples 1-9 with Comparative Example 2 shows that the electrical performance of lithium iron phosphate cathode materials prepared by replacing ferrous acetate with lithium phosphate and lithium acetate with lithium carbonate is significantly reduced. This indicates that compared with the traditional dry mixing and calcination method, the lithium iron phosphate cathode material prepared by the method provided in this application has significantly improved electrical performance.

[0171] As can be seen from the comparison between Examples 1-9 and Comparative Example 3, compared with the use of tetravalent titanium ions for cation doping, the lithium iron phosphate borate cathode material provided in this application uses trivalent titanium ions for cation doping, which significantly improves the charging specific capacity, discharging specific capacity and capacity retention rate at -20℃.

[0172] As can be seen from the comparison between Examples 1-11 and Comparative Example 4, compared with the lithium iron phosphate cathode material prepared without titanium doping, the lithium iron phosphate cathode material provided in this application uses trivalent titanium ions for cation doping, which significantly improves the charging specific capacity, discharging specific capacity and capacity retention rate at -20℃.

[0173] 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 borate cathode material, characterized in that, The general formula of the lithium iron borate phosphate cathode material is Li x FeTi y P z B w O (4z+3w+2y) / C, where x, y, z and w are all non-zero positive numbers, 1.02≤x≤1.05, 0.01≤y≤0.05, 0.92≤z≤0.95, 0.05≤w≤0.15, and the mass fraction of carbon in the phosphate boric acid cathode material is 1.00%-1.35%; the oxidation state of Ti is +3.

2. The lithium iron phosphate borate cathode material according to claim 1, characterized in that, The lithium iron phosphate borate material has a D10 particle size ≤ 0.50 μm, a D50 particle size ≤ 1.60 μm, and a D90 particle size ≤ 19.00 μm.

3. The lithium iron phosphate borate cathode material according to claim 1, characterized in that, The lithium iron phosphate borate cathode material has a compaction density of 2.35 g / mL to 2.50 g / mL and a specific surface area of ​​10.1 m². 2 / g~14.3 m 2 / g, powder resistivity is 4.0Ω.cm~13.0 Ω.cm.

4. The lithium iron phosphate borate cathode material according to any one of claims 1 to 3, characterized in that, The lithium iron phosphate borate cathode material has the following characteristics: iron leaching content is 3.0ppm~10.0ppm, elemental iron content is 0.01ppm~0.05ppm, chloride ion content is 40.0ppm~80.0ppm, water content is 280ppm~350ppm, free lithium content is 140ppm~200ppm, and magnetic impurity content is ≤1ppm.

5. A method for preparing lithium iron phosphate borate cathode material, characterized in that, The preparation method includes the following steps: A mixed slurry is obtained by mixing lithium source, ferrous source, trivalent titanium source, carbon source, phosphoric acid, boric acid and solvent; The mixed slurry is preheated and dried to obtain a pre-dried material; The pre-dried material is calcined to obtain the lithium iron phosphate borate cathode material.

6. The method for preparing lithium iron phosphate borate cathode material according to claim 5, characterized in that, The molar ratio of iron in the ferrous source, titanium in the trivalent titanium source, lithium in the lithium source, carbon in the carbon source, phosphorus in the phosphoric acid, and boron in the boric acid is 1:(0.01-0.05):(1.02-1.05):(0.36-1.32):(0.92-0.95):(0.05-0.15).

7. The method for preparing lithium iron phosphate borate cathode material according to claim 6, characterized in that, The ferrous source includes at least one of ferrous acetate and / or ferrous chloride. The lithium source includes at least one of lithium chloride and / or lithium acetate; The trivalent titanium source includes titanium trihalides; The step of mixing lithium source, ferrous source, trivalent titanium source, carbon source, phosphoric acid, boric acid and solvent includes: mixing the phosphoric acid with the solvent to form a phosphoric acid solution, and then mixing the phosphoric acid solution with the lithium source, the ferrous source, the trivalent titanium source and the boric acid; The carbon source includes at least one of ascorbic acid and / or citric acid.

8. The method for preparing lithium iron phosphate borate cathode material according to claim 7, characterized in that, The trivalent titanium source is titanium trichloride; The phosphoric acid solution contains 70% to 80% phosphoric acid by mass. The carbon source is a mixture of ascorbic acid and citric acid, and the mass ratio of ascorbic acid to citric acid is (0.05-0.2):(0.92-0.95).

9. The method for preparing lithium iron phosphate borate cathode material according to claim 5, characterized in that, The mixed slurry is preheated and dried at 100℃-150℃ to obtain a pre-dried material with an H2O mass fraction of less than or equal to 0.5%. And / or, the step of calcining the pre-dried material to obtain lithium iron phosphate borate cathode material includes: In a protective atmosphere, the pre-dried material is subjected to a first calcination temperature and a first holding time, and then subjected to a second calcination temperature and a second holding time to obtain the lithium iron phosphate cathode material. Wherein, the first calcination temperature is 120℃-150℃, and the first heat preservation time is 2h-4h; The second calcination temperature is 600℃-700℃, and the second holding time is 4h-6h.

10. A positive electrode plate, characterized in that, The positive electrode includes the lithium iron phosphate positive electrode material according to any one of claims 1 to 4 or the lithium iron phosphate positive electrode material obtained by the preparation method of the lithium iron phosphate positive electrode material according to any one of claims 5 to 9.

11. A secondary battery, characterized in that, The secondary battery comprises the lithium iron phosphate borate cathode material as described in any one of claims 1 to 4 or the cathode sheet as described in claim 10.

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