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

Abstract

The present application relates to the technical field of positive electrode materials of batteries, and provides a lithium iron borate phosphate positive electrode material and a preparation method therefor, a positive electrode sheet, and a secondary battery. The chemical general formula of the lithium iron borate phosphate positive electrode material is LixFeTiyPzBwO(4z+3w+2y) / C, wherein 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, and on the basis of the mass of the lithium iron borate phosphate positive electrode material being 100%, the mass content of C is 1.00% to 1.35%. The lithium iron borate phosphate positive electrode material provided by the present application has a low price, excellent ionic conductivity, high capacity, excellent rate performance, and improved cycling stability, and thus has wide prospects for application.

Description

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

[0001] The present application relates to the technical field of battery positive electrode materials, in particular to a lithium iron phosphate borate positive electrode material, a preparation method thereof, a positive electrode sheet and a secondary battery. BACKGROUND

[0002] With the development of new energy vehicles, the competition is becoming more and more fierce, and the cost performance is also becoming higher and higher, both requiring high endurance mileage of new energy vehicles and low price of new energy vehicles. The cost of batteries in new energy vehicles accounts for more than 40%, and the cost performance of new energy vehicles is directly determined by the cost performance of batteries. The cost of positive electrode materials in batteries accounts for more than 30%, therefore, reducing the cost of positive electrode materials and improving their performance can effectively improve the competitiveness of new energy vehicles.

[0003] Lithium iron phosphate is increasingly used as a positive electrode material in new energy vehicles due to its low cost, high safety and other advantages. How to continuously improve the performance of lithium iron phosphate and continuously reduce its production cost is the key to the stable development of lithium iron phosphate.

[0004] Therefore, the present application is proposed.

[0005] SUMMARY

[0006] In view of the technical problems in the background art, the present application provides a lithium iron phosphate borate positive electrode material, a preparation method thereof, a positive electrode sheet and a secondary battery, aiming to solve the problems of low performance and high production cost of existing lithium iron phosphate positive electrode materials.

[0007] In a first aspect, the present application provides a lithium iron phosphate borate positive electrode material, the chemical formula of which is Li 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, 0.05≤w≤0.15, and the mass fraction of carbon element is 1.00%-1.35% based on 100% of the mass of the lithium iron phosphate borate positive electrode material.

[0008] The lithium iron phosphate borate positive electrode material provided by the embodiment of the present application is doped with trivalent titanium as cations, doped with boron as anions, and coated with carbon. The ion radius of trivalent titanium is larger, and more defects are formed, so that the lithium iron phosphate borate positive electrode material has better ion conductivity and higher capacity. The boron is doped in the form of borate ions, and the molar mass of the borate ions is smaller than that of the phosphate ions, which is conducive to improving the capacity of the lithium iron phosphate borate positive electrode material. The carbon coating layer is conducive to improving the conductivity and cycle stability of the lithium iron phosphate borate positive electrode material.

[0009] In some embodiments, the D10 particle size of the lithium iron phosphate borate material is ≤0.5 μm, the D50 particle size is ≤1.60 μm, and the D90 particle size is ≤19.00 μm.

[0010] In the embodiment, the particle size of the lithium iron phosphate borate positive electrode material is more uniform, so that the capacity and cycle stability of the battery prepared by using the lithium iron phosphate borate positive electrode material as the positive electrode material are improved.

[0011] In some embodiments, the pH value of the lithium iron phosphate borate positive electrode material is 8.50-9.00, so that the subsequent slurry preparation process is more conducive to uniform slurry, and the positive electrode material prepared has uniform dispersion and stable performance.

[0012] In some embodiments, the lithium iron phosphate borate positive electrode material has a tap density of 2.35 g / mL-2.50 g / mL, a specific surface area of 10.1 m 2 / g-14.3 m 2 / g, and a powder resistivity of 4.0 Ω·cm-13.0 Ω·cm.

[0013] In the embodiment, the lithium iron phosphate borate positive electrode material has a higher tap density, a larger specific surface area, and a lower powder resistivity, so that the capacity of the battery formed by using the lithium iron phosphate borate positive electrode material is improved.

[0014] In some embodiments, the lithium iron phosphate borate positive electrode material has iron elution of 3.0 ppm-10.0 ppm, elemental iron of 0.01 ppm-0.05 ppm, chloride ions of 40.0 ppm-80.0 ppm, water content of 280 ppm-350 ppm, free lithium of 140 ppm-200 ppm, and magnetic impurity content of ≤1 ppm.

[0015] In the embodiment, the lithium iron phosphate borate positive electrode material has iron elution of ≤10.0 ppm, elemental iron of ≤0.05 ppm, chloride ions of ≤80.0 ppm, water content of ≤350 ppm, free lithium of ≤200 ppm, and magnetic impurity content of ≤1 ppm. The structure of the lithium iron phosphate borate positive electrode material is stable, the impurity content is low, and the lithium iron phosphate borate positive electrode material has excellent ion conductivity and cycle stability.

[0016] In a second aspect, the embodiments of the present application provide a preparation method of a lithium iron borophosphate positive electrode material, which comprises the following steps: uniformly mixing a ferrous source, a trivalent titanium source, a carbon source, phosphoric acid, boric acid and a 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 the lithium iron borophosphate positive electrode material.

[0017] In the technical scheme of the embodiments of the present application, the lithium source, the ferrous source, the trivalent titanium source, the carbon source, the phosphoric acid, the boric acid and the solvent are used as raw materials, which is more conducive to enhancing the driving force of the solid-phase reaction, more conducive to forming a co-melt, the reaction temperature is further reduced, the particles are further refined, the capacity is further improved, and the production cost is further reduced compared with the dry mixing and calcination process. Meanwhile, the ionic radius of the trivalent titanium ion is larger, and more defects are formed, so that the ion conductivity of the positive electrode material is better, and the capacity is higher; the boron element is doped in the form of borate ion, and the molar mass of the borate is smaller than that of the phosphate, which is conducive to improving the capacity; the carbon serves as a coating layer, which is conducive to improving the conductivity and the cycle stability. In addition, compared with the traditional process, the above technical scheme reduces the steps of water addition, slurry, grinding and spray drying. According to the actual production statistics, the cost (including labor, equipment depreciation, energy consumption, environmental protection, etc.) is increased by about 2500 yuan / ton. The preparation method of the lithium iron borophosphate positive electrode material provided by the present application can save 2500 yuan / ton only in this step, and according to the annual production of 100,000 tons of lithium iron borophosphate positive electrode material, the cost can be saved by 250 million yuan / year.

[0018] In some embodiments, the molar ratio of the iron element in the ferrous source, the titanium element in the trivalent titanium source, the lithium element in the lithium source, the carbon element in the carbon source, the phosphorus element in the phosphoric acid solution and the boron element 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).

[0019] In this embodiment, the lithium iron borophosphate positive electrode material obtained by the above molar ratio is doped with trivalent titanium ions, doped with boron elements, and coated with carbon. The ionic radius of the trivalent titanium ion is larger, and more defects are formed, so that the ion conductivity of the lithium iron borophosphate positive electrode material is better, and the capacity is higher; the boron element is doped in the form of borate ion, and the molar mass of the borate is smaller than that of the phosphate, which is conducive to improving the capacity, and the boric acid has a fluxing effect, which is conducive to obtaining a lithium iron borophosphate positive electrode material with improved compaction density at a lower calcination temperature; the carbon serves as a coating layer, which is conducive to improving the ion conductivity of the lithium iron borophosphate positive electrode material, and is conducive to improving the cycle stability.

[0020] In some embodiments, the step of mixing the lithium source, the ferrous source, the trivalent titanium source, the carbon source, the phosphoric acid, the boric acid and the solvent includes: mixing the phosphoric acid and 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. Preferably, the mass fraction of the phosphoric acid solution is 70%-80%, so as to improve the mixing efficiency of the phosphoric acid solution and other raw materials.

[0021] In some embodiments, the ferrous source is a water-soluble ferrous source, which includes at least one of ferrous acetate and / or ferrous chloride. The lithium source is a water-soluble lithium source, which includes at least one of lithium chloride and / or lithium acetate. At high temperatures, the volatile anion combines with the hydrogen ion in the acid to form a volatile low-boiling-point acid, and then the principle of preparing a low-boiling-point acid by using a high-boiling-point acid is adopted. The high-boiling-point acids such as boric acid and phosphoric acid are mixed with the ions such as chloride and acetate to form low-boiling-point hydrogen chloride and acetic acid, and other ions are combined together to form lithium iron phosphate boric acid. In the calcination process, the driving force of the solid-phase reaction is stronger, and it is easier to form a co-melt. Therefore, it is beneficial to reduce the calcination temperature, shorten the sintering time, reduce the energy consumption, and reduce the cost. The lithium iron phosphate boric acid positive electrode material provided by the present application not only has a lower cost and a shorter process, but also has more refined particles and higher capacity. At the same time, the boric acid introduced in the present application can not only act as a dopant but also as a fluxing agent, which can further reduce the calcination temperature, avoid the problem of particle size increase of the lithium iron phosphate boric acid positive electrode material, and the increase of the content of iron phosphide and magnetic substances caused by high-temperature calcination, and is beneficial to the improvement of the electrochemical performance of the lithium iron phosphate boric acid positive electrode material in terms of capacity, cycle performance and rate performance.

[0022] In the present application, the trivalent titanium source is selected to provide titanium elements. The melting point (decomposition point) of the trivalent titanium salt is lower, and it can melt at a lower temperature, which enhances the dispersion effect and makes the trivalent titanium more uniformly doped in the lithium iron phosphate boric acid. In some embodiments, the trivalent titanium source includes titanium trihalide, preferably titanium trichloride. The principle of preparing a low-boiling-point acid by using a high-boiling-point acid can be used to reduce the calcination reaction temperature, and the larger ionic radius of the trivalent titanium ion can be used to form more defects in the doping process, which is beneficial to enhancing the ionic conductivity of lithium ions, thereby improving the electrochemical performance of the lithium iron phosphate boric acid positive electrode material in terms of charge specific capacity and discharge specific capacity.

[0023] 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 beneficial to improving the conductivity and cycle stability of the lithium iron phosphate boric acid positive electrode material.

[0024] In some embodiments, the mixed slurry is preheated and dried at 100-150 DEG C to obtain a pre-dried material with a mass fraction of H2O less than or equal to 0.5%. That is, in the preheating and drying process, the water content of the dried material is controlled to be ≤0.5% after the water is removed, so that the generated product is more compact and uniform in the subsequent calcination process, thereby further improving the compactness of the lithium iron borophosphate positive electrode material, and facilitating the improvement of the tap density and capacity of the lithium iron borophosphate positive electrode material.

[0025] In some embodiments, the step of calcining the pre-dried material to obtain the lithium iron borophosphate positive electrode material comprises: under a protective atmosphere, pre-drying the material at a first calcination temperature for a first holding time, and then at a second calcination temperature for a second holding time to obtain the lithium iron borophosphate positive electrode material; wherein the first calcination temperature is 120-150 DEG C, the first holding time is 2-4 h, the second calcination temperature is 600-700 DEG C, and the second holding time is 4-6 h.

[0026] In this embodiment, by controlling the first calcination temperature, the first holding time, the second calcination temperature and the second holding time, the lithium iron borophosphate positive electrode material with more refined and uniform particles, more regular morphology and higher compactness can be prepared, which is beneficial to improve the tap density and capacity of the lithium iron borophosphate positive electrode material.

[0027] In a third aspect, the embodiments of the present application provide a positive electrode sheet, which comprises the lithium iron borophosphate positive electrode material provided in the first aspect.

[0028] In this embodiment, the positive electrode sheet comprises the lithium iron borophosphate positive electrode material described above, which not only has higher tap density and capacity, but also has more excellent ion conductivity and cycle stability, and has a broader application prospect in the field of secondary batteries.

[0029] In a fourth aspect, the embodiments of the present application provide a secondary battery, which comprises the lithium iron borophosphate positive electrode material provided in the first aspect or the positive electrode sheet provided in the third aspect.

[0030] In this embodiment, the secondary battery comprises the positive electrode sheet described above, which not only has high capacity, but also has more excellent ion conductivity and cycle stability.

[0031] In a fifth aspect, the embodiments of the present application provide an electric device, which comprises the lithium iron borophosphate positive electrode material provided in the first aspect, the positive electrode sheet provided in the third aspect or the secondary battery provided in the fourth aspect.

[0032] In this embodiment, the electric device comprises the secondary battery described above, and thus has the advantage of excellent cycle stability.

[0033] The above description is only a summary of the technical solutions of the present application. In order to make the technical means of the present application more clearly understood and can be implemented according to the content of the specification, and in order to make the above and other purposes, characteristics and advantages of the present application more obvious and easy to understand, the following specific embodiments of the present application are described. BRIEF DESCRIPTION OF DRAWINGS

[0034] In order to more clearly illustrate the technical solutions of the present application, the drawings used in the present application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creating laborious work.

[0035] Figure 1 is a preparation flow chart of the lithium iron phosphate borate positive electrode material provided in Example 1;

[0036] Figure 2 is a scanning electron microscope (SEM) result graph of the lithium iron phosphate borate positive electrode material provided in Example 1;

[0037] Figure 3 is a SEM result graph of the lithium iron phosphate borate positive electrode material provided in Example 2;

[0038] Figure 4 is a SEM result graph of the lithium iron phosphate borate positive electrode material provided in Example 3;

[0039] Figure 5 is a SEM result graph of the lithium iron phosphate borate positive electrode material provided in Comparative Example 2. DETAILED DESCRIPTION

[0040] The embodiments of the technical solutions of the present application will be described in detail below with reference to the drawings. The following examples are only used to more clearly illustrate the technical solutions of the present application, therefore only as an example, and cannot limit the protection scope of the present application.

[0041] 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 the present application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the present application; the terms "include" and "have" and any variations thereof in the specification and claims of the present application and the above description of drawings are intended to cover non-exclusive inclusion.

[0042] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "a plurality of" is two or more, unless otherwise specifically limited.

[0043] Reference to“an embodiment” herein means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the application. The appearances of the phrase“in an embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are they necessarily mutually exclusive of one another. It is expressly understood that any of the embodiments described herein can be incorporated into any other embodiment in a manner known to those of ordinary skill in the art.

[0044] In the description of the embodiments of the application, the term“and / or” only means an association relationship of the associated objects, which means that there can be three relationships, for example, A and / or B, which can mean that A exists alone, A and B exist together, and B exists alone. In addition, the character“ / ” herein generally means that the front and rear associated objects are in an“or” relationship.

[0045] In the description of the embodiments of the application, the term“a plurality of” means two or more (including two), and similarly, “a plurality of groups” means two or more groups (including two groups), and “a plurality of pieces” means two or more pieces (including two pieces).

[0046] In the description of the embodiments of the application, the technical terms“center”,“longitudinal”,“transverse”,“length”,“width”,“thickness”,“upper”,“lower”,“front”,“rear”,“left”,“right”,“vertical”,“horizontal”,“top”,“bottom”,“inner”,“outer”,“clockwise”,“counterclockwise”,“axial”,“radial”,“circumferential” and the like indicate the orientation or positional relationship shown in the drawings, which is only for the convenience of describing the embodiments of the application and simplifying the description, and does not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the embodiments of the application.

[0047] In the description of the embodiments of the application, unless otherwise explicitly specified and limited, the technical terms“mounting”,“connection”,“connection”,“fixing” and the like should be understood in a broad sense, for example, it can be fixedly connected, or it can be detachably connected, or it can be integrated; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium; it can be the internal communication of two elements or the interaction relationship of two elements. For those of ordinary skill in the art, the specific meaning of the above terms in the embodiments of the application can be understood according to the specific circumstances.

[0048] With the development of new energy vehicles, the lithium iron phosphate positive electrode material has a higher and higher proportion in the new energy vehicle field as the positive electrode material of the battery due to its low cost and high safety, but the existing lithium iron phosphate positive electrode material still has the problems of low performance and high production cost.

[0049] To solve the technical problems of low performance and high production cost of the existing lithium iron phosphate positive electrode material, the application provides a lithium iron borophosphate positive electrode material, a preparation method thereof, a positive electrode sheet, a secondary battery and an electric device, wherein the lithium iron borophosphate positive electrode material is doped with trivalent titanium as cations and doped with boron as anions, and is coated with carbon, the ionic radius of trivalent titanium is larger, and more defects are formed, so that the ionic conductivity of the positive electrode material is better, and the capacity is higher; compared with phosphorus, the ionic radius of boron is smaller, which is more conducive to improving the compaction density of the positive electrode material, and thus the capacity is improved; as a coating layer, carbon is not only conducive to improving the ionic conductivity, but also conducive to improving the cycle stability. Therefore, the corresponding performance of the positive electrode sheet, the secondary battery and the electric device containing the lithium iron borophosphate positive electrode material is also improved.

[0050] The electric device provided by the embodiments of the application can be, but is not limited to, a mobile phone, a tablet computer, a notebook computer, an electric toy, an electric tool, an electric car, an electric vehicle, a ship, a spacecraft, etc. The electric toy can include a fixed or mobile electric toy, such as a game console, an electric car toy, an electric ship toy and an electric aircraft toy, etc. The spacecraft can include an airplane, a rocket, a space shuttle and a spacecraft, etc.

[0051] In a first aspect, the embodiments of the application provide a lithium iron borophosphate positive electrode material, which has a chemical general formula of Li 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 fraction of carbon is 1.00% to 1.35% based on 100% of the mass of the lithium iron borophosphate material.

[0052] In the application, the lithium iron borophosphate positive electrode material includes a core and a coating layer coated on the outer surface of the core, wherein the material of the core has a chemical general formula of Li x FeTi y P z B w O (4z+3w+2y) , and the material of the coating layer is a carbon material.

[0053] The lithium iron phosphate borate positive electrode material provided in the embodiments of the present application is doped with trivalent titanium as cations, doped with boron as anions, and coated with carbon. The ion radius of trivalent titanium is larger, and more defects are formed, so that the lithium iron phosphate borate positive electrode material has better ion conductivity and higher capacity. The boron is doped in the form of borate ions, and the molar mass of the borate ions is smaller than that of the phosphate ions, which is conducive to improving the capacity of the lithium iron phosphate borate positive electrode material. The carbon coating layer is conducive to improving the conductivity and cycle stability of the lithium iron phosphate borate positive electrode material.

[0054] Typically but not limitedly, in the general chemical formula of the lithium iron phosphate borate positive electrode material, x is, for example, 1.02, 1.03, 1.04, 1.05, or a range value formed by any two of the values; y is, for example, 0.01, 0.02, 0.03, 0.04, 0.05, or a range value formed by any two of the values; z is, for example, 0.92, 0.93, 0.94, 0.95, or a range value formed by any two of the values; and the mass fraction of C is, for example, 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 a range value formed by any two of the values, based on 100% of the mass of the lithium iron phosphate borate positive electrode material.

[0055] Further, in some embodiments, the D10 particle size of the lithium iron phosphate borate positive electrode material is ≤0.5 μm, the D50 particle size is ≤1.60 μm, and the D90 particle size is ≤19.00 μm.

[0056] In the present application, the D10 particle size refers to the particle size corresponding to the cumulative particle size distribution of 10% of the lithium iron phosphate borate material, and its physical meaning is that the particles with a particle size less than the value account for 10% of the total particles; the D50 particle size refers to the particle size corresponding to the cumulative particle size distribution of 50% of the lithium iron phosphate borate material, and its physical meaning is that the particles with a particle size less than the value account for 50% of the total particles; and the D90 particle size refers to the particle size corresponding to the cumulative particle size distribution of 90% of the lithium iron phosphate borate material, and its physical meaning is that the particles with a particle size less than the value account for 90% of the total particles.

[0057] In the technical solution of the embodiments of the present application, the particle size of the lithium iron phosphate borate positive electrode material is more uniform and more refined, so that the capacity and cycle stability of the battery prepared by using the lithium iron phosphate borate positive electrode material as a positive electrode material are improved.

[0058] Further, in some embodiments, the pH value of the above-mentioned lithium iron borophosphate positive electrode material is 8.50-9.00, such as 8.50, 8.60, 8.70, 8.80, 8.90, 9.00, etc., which is further conducive to uniform slurry in the subsequent process of preparing the positive electrode slurry, and is also conducive to preparing the positive electrode material with uniform dispersion and stable performance.

[0059] Further, in some embodiments, the compaction density of the lithium iron borophosphate positive electrode material is 2.35 g / mL-2.50 g / mL, the specific surface area is 10.1 m 2 / g-14.3 m 2 / g, and the powder resistivity is 4.0 Ω·cm-13.0 Ω·cm, which is further conducive to improving the capacity of the battery formed by using the positive electrode material.

[0060] Typically but not limitedly, the compaction density of the lithium iron borophosphate positive electrode material is, for example, 2.35 g / mL, 2.40 g / mL, 2.45 g / mL, 2.50 g / mL, or a range value formed by any two numerical values; the specific surface area is, for example, 10.1 m 2 / g, 10.5 m 2 / g, 10.8 m 2 / g, 11.0 m 2 / g, 11.2 m 2 / g, 11.5 m 2 / g, 11.8 m 2 / g, 12.0 m 2 / g, 12.5 m 2 / g, 12.8 m 2 / g, 13.0 m 2 / g, 13.2 m 2 / g, 13.5 m 2 / g, 13.8 m 2 / g, 14.0 m 2 / g, 14.2 m 2 / g, 14.3 m 2 / g, or a range value formed by any two numerical values; and the powder resistivity is, for example, 4.0 Ω·cm, 4.5 Ω·cm, 5.0 Ω·cm, 5.5 Ω·cm, 6.0 Ω·cm, 8.0 Ω·cm, 9.0 Ω·cm, 10.0 Ω·cm, 11.0 Ω·cm, 12.0 Ω·cm, 13.0 Ω·cm, or a range value formed by any two numerical values.

[0061] Further, in some embodiments, the lithium iron borophosphate positive electrode material has iron elution of 3.0 ppm to 10.0 ppm, elemental iron of 0.01 ppm to 0.05 ppm, chloride ions of 40.0 ppm to 80.0 ppm, water content of 140 ppm to 200 ppm, free lithium of 140 ppm to 200 ppm, and magnetic impurity content of ≤1 ppm, that is, the iron elution, elemental iron, chloride ion content, water content, and free lithium content of the lithium iron borophosphate positive electrode material provided by the present application are all in ppm, the structure is more stable, the impurity content is lower, and thus the lithium iron borophosphate positive electrode material has more excellent ion conductivity and cycle stability.

[0062] In a second aspect, the present application provides a preparation method of the lithium iron borophosphate positive electrode material, as shown in FIG. 1, which comprises the following steps: mixing a ferrous source, a trivalent titanium source, a carbon source, phosphoric acid, boric acid, and a 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 the lithium iron borophosphate positive electrode material.

[0063] The preparation method of the lithium iron borophosphate positive electrode material provided by the present 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 easily forming a eutectic body. 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. Meanwhile, the larger ionic radius of the trivalent titanium ion forms more defects, so that the lithium iron borophosphate positive electrode material has better ion conductivity and higher capacity; the boron element is doped in the form of borate ion, and the molar mass of the borate ion is smaller than that of the phosphate ion, which is conducive to improving the capacity of the lithium iron borophosphate positive electrode material; and the carbon as a coating layer is not only conducive to improving the conductivity of the lithium iron borophosphate positive electrode material, but also conducive to improving the cycle stability.

[0064] Specifically, the preparation method of the lithium iron borophosphate positive electrode material provided by the present application introduces boric acid as a dopant and a fluxing agent, which can further reduce the calcination temperature and avoid the problems of particle size increase, iron phosphide and magnetic substance content increase, and the like of the lithium iron borophosphate positive electrode material caused by high-temperature calcination.

[0065] Further, the preparation method of the lithium iron borophosphate positive electrode material provided by the present application reduces the steps of water addition, slurry, grinding, and spray drying, and the like compared with the traditional process. According to the actual production statistics, the cost (including labor, equipment depreciation, energy consumption, environmental protection, and the like) is increased by about 2500 yuan / ton. The preparation method of the lithium iron borophosphate positive electrode material provided by the present application can save 2500 yuan / ton only in this step, and according to the annual production of 100,000 tons of the lithium iron borophosphate positive electrode material, the cost can be saved by 250 million yuan / year.

[0066] Further, in some embodiments, the above-mentioned ferrous source is a water-soluble ferrous source, the water-soluble ferrous source is an anion volatile ferrous salt; the lithium source is a water-soluble lithium source, the water-soluble lithium source is an anion volatile lithium salt, and the trivalent titanium source is an anion 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 or the like, so as to facilitate the sufficient mixing of the water-soluble ferrous source, the water-soluble lithium salt, and the water-soluble trivalent titanium source with the acid. At the same time, at high temperature, the anion volatile anion combines with the hydrogen ion in the acid to form a volatile low-boiling-point acid, and then the principle of preparing a low-boiling-point acid with a high-boiling-point acid is used to mix the high-boiling-point acid such as boric acid and phosphoric acid with the anion such as chloride and acetate to form low-boiling-point hydrogen chloride and acetic acid, and other ions are combined together to form a composite lithium iron phosphate borate, and the driving force of the solid-phase reaction is stronger, and it is easier to form a co-melt. 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.

[0067] Specifically, the dry mixing and calcination method using iron phosphate, lithium carbonate and carbon source to sinter completely in solid phase does not have the driving force of the present application (the principle of preparing a low-boiling-point acid with a high-boiling-point acid), and the particle size of the raw material is large, the ion diffusion distance is long, and a higher reaction temperature is required for diffusion at high temperature calcination. A higher calcination temperature will cause the particle size to increase and the capacity to further decrease.

[0068] The preparation method of the lithium iron phosphate borate positive electrode material provided by the present application introduces boric acid as a dopant and a fluxing agent, which can further reduce the calcination temperature and avoid the problems of particle size increase and iron phosphide and magnetic material content increase of the lithium iron phosphate borate positive electrode material caused by high-temperature calcination. By using volatile anion ferrous salt, volatile anion lithium salt and volatile anion titanium salt as raw materials, only at a lower temperature (20-30℃ lower than other dry mixing methods), the more volatile low-boiling-point acid is used to promote better material fusion and further refine the particles, thereby further improving the capacity.

[0069] Further, in some embodiments, the trivalent titanium source adopted is an anion volatile trivalent titanium salt, which has a lower melting point (decomposition point) and can be melted at a lower temperature, thereby enhancing the dispersion effect and making the doping of trivalent titanium in the lithium iron phosphate borate more uniform. The trivalent titanium source includes but is not limited to titanium trihalides, such as titanium trifluoride, titanium trichloride, titanium tribromide, etc. When the trivalent titanium source is titanium trichloride, it can not only take advantage of the principle of preparing low-boiling-point acid with high-boiling-point acid to reduce the calcination reaction temperature, but also take advantage of the larger ionic radius of titanium ions in the trivalent state, which forms more defects in the doping process, thereby enhancing the ionic conductivity of lithium ions. Therefore, the doping effect of titanium trichloride is more excellent than that of tetravalent titanium (titanium dioxide, etc.). Moreover, titanium trichloride has a lower melting point (decomposition point) of only 440°C, compared with the high melting point of other traditional doping agents such as titanium dioxide. Therefore, the use of titanium trichloride can melt at a lower temperature, has a better dispersion effect, and is more uniform in doping, which is more conducive to improving the electrochemical performance of the lithium iron phosphate borate cathode material in terms of charge specific capacity, discharge specific capacity, etc.

[0070] Further, in some embodiments, the carbon source can be any commonly used carbon source in the art. In order to improve the uniformity of the mixture of raw materials and the conductivity, the carbon source is preferably a water-soluble carbon source, which includes any one or more of ascorbic acid and / or citric acid, and is further 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 value composed of two numerical values.

[0071] Further, in some embodiments, the solvent is selected from at least one of pure water, deionized water, or distilled water.

[0072] Further, in some embodiments, the molar ratio of iron element in ferrous source, titanium element in trivalent titanium source, lithium element in lithium source, carbon element in carbon source, phosphorus element in phosphoric acid solution and boron element in boric acid is 1:(0.01-0.05):(1.02-1.05):(0.36-1.32):(0.92-0.95):(0.05-0.15) during the preparation of the lithium iron borophosphate positive electrode material, so as to facilitate the preparation of the positive electrode material with improved capacity and cycle stability. For example, the amount of iron element in the ferrous source is 1 mol, the amount of titanium element in the trivalent titanium source is 0.01 mol, 0.02 mol, 0.03 mol, 0.04 mol, 0.05 mol or any range formed by two of the values; the amount of lithium element in the lithium source is 1.02 mol, 1.03 mol, 1.04 mol, 1.05 mol or any range formed by two of the values; the amount of carbon element in the carbon source is 0.36 mol, 0.48 mol, 0.60 mol, 0.72 mol, 0.90 mol, 1.08 mol, 1.20 mol, 1.32 mol or any range formed by two of the values; the amount of phosphorus element in the phosphoric acid is 0.92 mol, 0.93 mol, 0.94 mol, 0.95 mol or any range formed by two of the values; and the amount of boron element in the boric acid is 0.05 mol, 0.08 mol, 0.10 mol, 0.12 mol, 0.15 mol or any range formed by two of the values.

[0073] Further, in some embodiments, in order to facilitate the uniform mixing of phosphoric acid and other raw materials, the phosphoric acid is first mixed with a solvent to form a phosphoric acid solution, and then the phosphoric acid solution is mixed with the lithium source, the ferrous source, the trivalent titanium source and the boric acid to form a mixed slurry. Alternatively, the phosphoric acid solution is used to provide both the phosphoric acid and the solvent. Further, the mass fraction of the phosphoric acid solution is preferably 70%-80%, such as 70%, 72%, 75%, 78%, 80% or any range formed by two of the values.

[0074] Further, in some embodiments, in the preparation method of the lithium iron borophosphate positive electrode material, the loading, preheating and drying, furnace charging, furnace discharging and unloading are sequentially and cyclically performed by the sagger and the use of the return line, so as to obtain the lithium iron borophosphate positive electrode material. During the loading process, a metering module is arranged below the loading station, and the metering module is electrically connected with and controls the driving motor of the feeding machine to realize accurate feeding. The sagger can be a graphite sagger.

[0075] Further, in some embodiments, the mixed slurry is preheated and dried at 100-150°C to obtain a pre-dried material with a mass fraction of H2O less than or equal to 0.5%, so as to facilitate sufficient drying in the preheating and drying process and avoid reactions between the raw materials. At the same time, the water content of the dried material is controlled to be ≤0.5wt%, so as to facilitate the generation of more dense and uniform products in the subsequent calcination process, thereby further improving the density of the lithium iron borophosphate positive electrode material.

[0076] Further, in some embodiments, the mixed slurry is prepared according to the following steps: mixing phosphoric acid with a solvent to form a phosphoric acid solution with a mass fraction of 70%-80%, ready for use. First, add the carbon source to the mixture of ferrous source, trivalent titanium source and lithium source, then add the phosphoric acid solution and boric acid, and then put it into a double-cone mixer to mix, to obtain the mixed slurry.

[0077] In some embodiments, during the preheating and drying process, the exhaust gas is removed by opening the exhaust fan, and the extracted exhaust gas is absorbed by spraying lithium hydroxide solution, and the obtained absorption waste liquid is returned to the mixed slurry preparation process for recycling, so as to reduce environmental pollution while reducing energy consumption.

[0078] In some embodiments, the step of calcining the pre-dried material to obtain the lithium iron borophosphate positive electrode material includes: under a protective atmosphere, pre-drying the material at a first calcination temperature for a first holding time, then at a second calcination temperature for a second holding time, to obtain the lithium iron borophosphate positive electrode material; wherein the first calcination temperature is 120-150°C, the first holding time is 2-4h, the second calcination temperature is 600-700°C, and the second holding time is 4-6h. By controlling the first calcination temperature, the first holding time, the second calcination temperature and the second holding time, it is more conducive to preparing a lithium iron borophosphate positive electrode material with more refined and uniform particles, more regular morphology and higher density, while it is also conducive to further saving energy and reducing production costs.

[0079] Typically but not limitedly, the first calcination temperature is, for example, 120°C, 130°C, 140°C, 150°C or a range value composed of any two of the above values; the first holding time is, for example, 2h, 2.5h, 3h, 3.5h, 4h or a range value composed of any two of the above values; the second calcination temperature is, for example, 600°C, 620°C, 650°C, 680°C, 700°C or a range value composed of any two of the above values; and the second holding time is, for example, 4h, 4.5h, 5h, 5.5h, 6h or a range value composed of any two of the above values.

[0080] In some embodiments, a protective gas is introduced to form a protective atmosphere during the calcination process to maintain the oxygen volume content below 5 ppm, so as to further reduce the introduction of impurities.

[0081] In some embodiments, the calcination process is performed in a roller furnace. A protective gas is introduced to form a protective atmosphere during the calcination process to maintain the oxygen volume content below 5 ppm in the roller furnace, so as to further reduce the introduction of impurities.

[0082] In some embodiments, the pre-dried material is heated from room temperature to a first calcination temperature at a first heating rate, and then heated from the first calcination temperature to a second calcination temperature at a second heating rate, and then cooled to a material temperature of less than or equal to 100°C for discharge, to obtain the lithium iron borophosphate positive electrode material; wherein the first heating rate is 20-30°C / h, and the second heating rate is 50-100°C / h, so as to obtain a more compact, less defective, and more regular morphology positive electrode material.

[0083] Typically but not limitedly, the first heating rate is, for example, 20°C / h, 22°C / h, 25°C / h, 28°C / h, 30°C / h, or a range value formed by any two of the above values, and the second heating rate is, for example, 50°C / h, 55°C / h, 60°C / h, 70°C / h, 80°C / h, 90°C / h, 95°C / h, 100°C / h, or a range value formed by any two of the above values.

[0084] In some embodiments, the calcined material is obtained after crushing, screening, iron removal, and packaging to obtain the lithium iron borophosphate positive electrode material. The screening is performed using a 100-200 mesh screen, and the vacuum packaging is performed after the iron removal to obtain a water-free iron phosphate with a magnetic impurity content of less than 1 ppm. The vacuum packaging is performed under constant temperature (25±5°C) and constant humidity (≤10%).

[0085] In some embodiments, the calcined material is transported to an air jet mill through a pipeline conveying system, and then screened by an ultrasonic vibrating screen, removed by an electromagnetic iron remover, and vacuum packaged in a constant temperature and humidity room to obtain the lithium iron borophosphate positive electrode material. The temperature in the constant temperature and humidity room is 25±5°C, and the humidity is ≤10%.

[0086] In order 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 absorption liquid is reacted with iron powder to obtain a ferrous solution, which is concentrated and crystallized and then returned to the preparation step of obtaining the mixed slurry as raw material, so as to further reduce energy consumption.

[0087] In a third aspect, the embodiments of the present application provide a positive electrode sheet, which comprises the lithium iron borophosphate positive electrode material provided in the first aspect.

[0088] In this embodiment, the positive electrode sheet comprises the lithium iron borophosphate positive electrode material described above, and has higher compaction density and capacity, and has more excellent ion conductivity and cycle stability, and has a broader application prospect in the field of secondary batteries.

[0089] In a fourth aspect, the embodiments of the present application provide a secondary battery, which comprises the lithium iron borophosphate positive electrode material provided in the first aspect or the positive electrode sheet provided in the third aspect.

[0090] In this embodiment, the secondary battery comprises the positive electrode sheet described above, and has higher capacity, and has more excellent ion conductivity and cycle stability.

[0091] In a fifth aspect, the embodiments of the present application provide a power utilization device, such as a vehicle, which comprises the lithium iron borophosphate positive electrode material provided in the first aspect, the positive electrode sheet provided in the third aspect, or the secondary battery provided in the fourth aspect.

[0092] In this embodiment, the power utilization device comprises the secondary battery described above, and has excellent cycle stability. Some specific embodiments are listed below, and it should be noted that the embodiments described below are exemplary and are used to explain the present application, and cannot be understood as a limitation on the present application. If a specific technology or condition is not specified in the embodiments, the technology or condition described in the literature in the art or according to the product manual is used. If the manufacturer of the reagent or instrument is not specified, it is a conventional product that can be obtained on the market.

[0093] Embodiment 1

[0094] This embodiment provides a lithium iron borophosphate positive electrode material, which is prepared according to the following steps:

[0095] (1) Put ferrous acetate, titanium trichloride, lithium acetate, ascorbic acid, citric acid, a phosphoric acid solution with a mass fraction of 80%, and boric acid into a conical mixer to mix, 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 to the conical mixer, the inside of the grinding balls is stainless steel, the outside is wrapped with polyurethane, the diameter of the grinding balls is 5 cm, the rotation speed is 8 r / min, the filling amount is 60% of the effective volume of the mixer, and the mixing time is 50 min.

[0096] (2) The mixed slurry is added into the graphite crucible through the feeder using a screw feeder, and then preheating and drying is performed until the free water content of the mixed slurry is less than 0.5wt% to stop drying, to obtain a pre-dried material; wherein the temperature during preheating and drying is 120°C, and at the same time, an exhaust fan is started to remove the exhaust gas, and the removed exhaust gas is absorbed by spraying lithium hydroxide solution, and the obtained absorption waste liquid is returned to step (1) for recycling.

[0097] (3) The pre-dried material is placed into a roller furnace for calcination. During calcination, the temperature is first increased to 135°C at a rate of 25°C / h, and then kept at this temperature for 4h, and then increased to 650°C at a rate of 80°C / h, and then kept at this temperature for 5h, and then discharged when the material temperature is ≤100°C, and during calcination, nitrogen is introduced into the roller furnace to maintain the oxygen volume content in the roller furnace below 5ppm. The exhaust gas generated during calcination is introduced into a spray absorption tower for absorption using pure water, and the absorption liquid is reacted with iron powder, and the obtained ferrous solution is concentrated and crystallized to return to step (1) for recycling. The calcined material is transported to an air jet mill through a pipeline conveying system, and then screened through an ultrasonic vibrating screen, and vacuum packaged in a constant temperature and humidity chamber to obtain a lithium iron borophosphate positive electrode material; wherein the screening is performed using a 150-mesh ultrasonic vibrating screen, and the temperature in the constant temperature and humidity chamber is 25±5°C, and the humidity is ≤10%.

[0098] In this embodiment, the loading, preheating and drying, furnace charging, furnace discharging, and material pouring are sequentially and cyclically performed through a return line. During the loading process, a metering module is arranged below the loading station, the accuracy of the metering module is 0.5%, and the metering module is electrically connected with and controls the driving motor of the feeder to realize accurate feeding.

[0099] Example 2

[0100] This embodiment provides a lithium iron borophosphate positive electrode material, which is prepared according to the following steps:

[0101] (1) Iron (II) chloride, titanium trichloride, lithium chloride, ascorbic acid, citric acid, a 70wt% phosphoric acid solution, and boric acid are put into a conical mixer to obtain a mixed slurry; wherein the molar ratio of iron (II) 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 conical mixer, the inside of the grinding ball is stainless steel, the outside is wrapped with polyurethane, the diameter of the grinding ball is 3cm, the rotating speed is 5r / min, the filling amount is 50% of the effective volume of the mixer, and the mixing time is 30min.

[0102] (2) The mixed slurry is added into the graphite crucible through the feeder using a screw feeder, and then the free water content of the mixed slurry is dried to less than 0.5wt% by preheating, and the drying is stopped, to obtain a pre-dried material; wherein the temperature during preheating and drying is 150°C, and at the same time, the exhaust fan is started to remove the exhaust gas, and the exhaust gas is absorbed by spraying lithium hydroxide solution, and the absorption waste liquid is returned to step (1) for recycling.

[0103] (3) The pre-dried material is placed into a roller furnace for calcination. During the calcination process, the temperature is first increased to 150°C at a rate of 20°C / h, and then kept at this temperature for 4h. Then the temperature is increased to 700°C at a rate of 100°C / h, and then kept at this temperature for 4h. Then the temperature is decreased to ≤100°C for discharging. During the calcination process, nitrogen gas is introduced into the roller furnace to maintain the oxygen volume content in the roller furnace below 5ppm. The exhaust gas generated during calcination is introduced into a spray absorption tower for absorption using pure water, and the absorption liquid is reacted with iron powder. The obtained ferrous solution is concentrated and crystallized, and then returned to step (1) for recycling. The calcined material is transported to an air jet mill through a pipeline conveying system, then screened by an ultrasonic vibrating screen, and vacuum packaged in a constant temperature and humidity room to obtain the lithium iron borophosphate positive electrode material; wherein the screening is performed using a 100-mesh ultrasonic vibrating screen, and the material is vacuum packaged after the content of magnetic impurities of the iron-free ferrous phosphate is reduced to below 1ppm. The temperature in the constant temperature and humidity room is 25±5°C, and the humidity is ≤10%.

[0104] In this embodiment, the loading, preheating and drying, furnace entry, furnace discharge, and material pouring are sequentially and cyclically performed through a return line. During the loading process, a metering module is arranged below the loading station, and the metering module is electrically connected with and controls the driving motor of the feeder to realize accurate loading.

[0105] Example 3

[0106] This embodiment provides a lithium iron borophosphate positive electrode material, which is prepared according to the following steps:

[0107] (1) Ferrous acetate, titanium trichloride, lithium chloride, ascorbic acid, citric acid, a phosphoric acid solution with a mass fraction of 80%, and boric acid are put into a conical mixer to obtain a mixed slurry, wherein 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, and then put into a double-cone mixer; and during the mixing process, grinding balls are added into the double-cone mixer, the inside of the grinding balls is stainless steel, the outside is wrapped with polyurethane, the diameter of the grinding balls is 3cm, the rotating speed is 5r / min, the filling amount is 50% of the effective volume of the mixer, and the mixing time is 30min.

[0108] (2) The mixed slurry is added into the graphite crucible through the feeder using a screw feeder, and then the free water content of the mixed slurry is dried to less than 0.5wt% by preheating, and the drying is stopped, to obtain a pre-dried material; wherein the temperature during preheating and drying is 100°C, and at the same time, an exhaust fan is started to remove the exhaust gas, and the exhaust gas is absorbed by spraying lithium hydroxide solution, and the absorption waste liquid is returned to step (1) for recycling.

[0109] (3) The pre-dried material is placed into a roller furnace for calcination. During the calcination process, the temperature is first increased to 120°C at a rate of 30°C / h, and then kept at this temperature for 2h, and then increased to 600°C at a rate of 50°C / h, and then kept at this temperature for 6h, and then decreased to a material temperature of ≤100°C for discharging. During the calcination process, nitrogen is introduced into the roller furnace to maintain the oxygen volume content in the roller furnace below 5ppm. The exhaust gas generated during calcination is introduced into a spray absorption tower for absorption using pure water, and the absorption liquid is reacted with iron powder to obtain a ferrous solution, which is concentrated and crystallized and then returned to step (1) for recycling. The calcined material is transported to an air jet mill through a pipeline conveying system, and then screened through an ultrasonic vibrating screen, and vacuum packaged in a constant temperature and humidity chamber to obtain a lithium iron borophosphate positive electrode material; wherein the screening is performed using a 200-mesh ultrasonic vibrating screen, and the iron content is reduced to below 1ppm of the magnetic impurities of anhydrous iron phosphate before vacuum packaging, and the temperature in the constant temperature and humidity chamber is 25±5°C and the humidity is ≤10%.

[0110] In this embodiment, the loading, preheating and drying, furnace charging, furnace discharging and material discharging are sequentially and cyclically performed through a return line. During the loading process, a metering module is arranged below the loading station, and the metering module is electrically connected with and controls the driving motor of the feeder to realize accurate loading.

[0111] Example 4

[0112] This embodiment provides a lithium iron borophosphate positive electrode material, which is prepared according to the following steps:

[0113] (1) Iron (II) chloride, titanium trichloride, lithium acetate, ascorbic acid, citric acid, a 77wt% phosphoric acid solution and boric acid are put into a conical mixer to obtain a mixed slurry; wherein the molar ratio of iron (II) 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 into the conical mixer, the inside of the grinding balls is stainless steel, the outside is wrapped with polyurethane, the diameter of the grinding balls is 6cm, the rotating speed is 8r / min, the filling amount is 55% of the effective volume of the mixer, and the mixing time is 40min.

[0114] (2) The mixed slurry is added into the graphite crucible through the feeder by using a screw feeder, and then pre-dried to stop drying when the free water content of the mixed slurry is less than 0.5wt%, to obtain a pre-dried material; wherein the temperature during pre-drying is 140℃, and at the same time, an exhaust fan is started to exhaust the waste gas, and the exhausted waste gas is absorbed by spraying lithium hydroxide solution, and the absorption waste liquid is returned to step (1) for recycling.

[0115] (4) The pre-dried material is placed into a roller furnace for calcination. During the calcination process, the temperature is first increased to 130℃ at a rate of 22℃ / h, and then kept at this temperature for 3h. Then the temperature is increased to 640℃ at a rate of 65℃ / h, and kept at this temperature for 5h. Then the temperature is decreased to ≤100℃ for discharging. 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. The absorption liquid is reacted by adding iron powder, and the obtained ferrous solution is concentrated and crystallized to return to step (1) for recycling. The calcined material is transported to an air jet mill through a pipeline conveying system, then screened by an ultrasonic vibrating screen, and vacuum packaged in a constant temperature and humidity room to obtain the lithium iron borophosphate positive electrode material; wherein the screening is performed using a 150 mesh screen, and the vacuum packaging is performed after the iron content is reduced to below 1ppm of the magnetic impurities of anhydrous iron phosphate. The temperature in the constant temperature and humidity room is 25±5℃, and the humidity is ≤10%.

[0116] Example 5

[0117] The difference between this example and Example 1 is that ferrous sulfate is used to replace ferrous acetate as the water-soluble ferrous source, and lithium nitrate is used to replace lithium acetate as the water-soluble lithium source.

[0118] Example 6

[0119] The difference between this example and Example 1 is that ascorbic acid is not added, but the molar number of citric acid is the same as the sum of the molar numbers of ascorbic acid and citric acid in Example 1.

[0120] Example 7

[0121] The difference between this example and Example 1 is that citric acid is not added, but the molar number of ascorbic acid is the same as the sum of the molar numbers of ascorbic acid and citric acid in Example 1.

[0122] Example 8

[0123] The difference between this example and Example 1 is that titanium tribromide is used to replace titanium trichloride as the titanium source.

[0124] Example 9

[0125] The difference between this 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 1:0.01:1.02:0.01:0.05:0.92:0.05.

[0126] Example 10

[0127] The difference between this 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.

[0128] Example 11

[0129] The difference between this 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.

[0130] Comparative Example 1

[0131] The difference between this example and Example 1 is that no boric acid is 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.

[0132] Comparative Example 2

[0133] The difference between this example and Example 1 is that equimolar amounts of iron phosphate are used to replace ferrous acetate, equimolar amounts of lithium carbonate are used to replace lithium acetate, and no phosphoric acid is added.

[0134] Comparative Example 3

[0135] The difference between this example and Example 1 is that titanium dioxide is used to replace 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 m 2 / g.

[0136] Comparative Example 4

[0137] The difference between this example and Example 1 is that no titanium trichloride doping is added.

[0138] Test Method

[0139] 1. The positive electrode materials provided in the above examples and comparative examples are respectively subjected to morphology detection using a scanning electron microscope.

[0140] 2. The content of each element, the compaction density, BET, D10, D50, D90, powder resistivity, iron leaching, elemental iron content, magnetic foreign matter content, chloride ion content, moisture content, free lithium content, and pH of the positive electrode material provided by the examples and comparative examples were measured, respectively, and the results are shown in Tables 1 and 2 below.

[0141] (1) The content of each element in the positive electrode material was measured by ICP-OES spectrometer;

[0142] (2) The test method for compaction density was: reference GB / T 24533, determination of powder compaction density;

[0143] (3) The test method for BET was: reference GB / T 19587, gas adsorption BET method for determination of specific surface area of solid substances;

[0144] (4) The determination method for D10, D50 and D90 was: reference GB / T 19077, particle size analysis laser diffraction method;

[0145] (5) The determination method for powder resistivity was: four-probe method, measured under a pressure of 10 MPa;

[0146] (6) The determination method for iron leaching was: 1 g of the sample to be tested was added to 100 mL of a hydrogen fluoride-ethanol solution with a concentration of 0.1 mol / L, stirred and dissolved at a temperature of 45°C for 30 min, then filtered, and the content of iron element in the filtrate was measured, which was the iron leaching amount;

[0147] (7) The determination method for elemental iron content was: reference GB / T 223.7-2002 potassium dichromate titration method;

[0148] (8) The determination method for magnetic foreign matter content was: 100 g of the material was weighed and poured into a plastic bottle with a cover, 1000 g of pure water was added, then a 8000 GS magnetic sub was added, the outside of the magnetic sub was protected by polytetrafluoroethylene, then the cover was tightly covered, the plastic bottle was placed horizontally and rotated, the rotation speed was 10-20 r / min, after rotating for 30-45 min, the rotation was stopped, the magnetic sub was taken out, 1000 g of pure water was added and put into the plastic bottle, the cover was tightly covered, the plastic bottle was placed horizontally and rotated, the rotation speed was 10-20 r / min, after rotating for 5-10 min, the rotation was stopped, the magnetic sub was taken out, aqua regia was added for dissolution, and the content of nickel, chromium, copper, zinc and iron in the dissolution solution was measured by ICP, and the total mass of nickel, chromium, copper, zinc and iron was calculated, then divided by the weight of the material, to obtain the magnetic foreign matter content of the material.

[0149] (9) The determination method for moisture content was: GB / T 6283, determination of moisture content in chemical products by Karl Fischer method (general method).

[0150] (10) The determination method of chloride ion content: GB / T 23273.6-2009, ion selective electrode method;

[0151] (11) The determination method of free lithium content: SJT 11794-2021, automatic potentiometric titration method;

[0152] (12) The determination method of pH is: refer to GB / T 9724, general rules for pH determination of chemical reagents.

[0153] Figure 2 is an SEM image of the lithium iron phosphate borate positive electrode material provided in Example 1; Figure 3 is an SEM image of the lithium iron phosphate borate positive electrode material provided in Example 2; Figure 4 is an SEM image of the lithium iron phosphate borate positive electrode material provided in Example 3; and Figure 5 is an SEM image of the lithium iron phosphate borate positive electrode material provided in Comparative Example 2. As can be seen from Figures 2-5, compared with the positive electrode material formed by the traditional dry mixing and calcination process of Comparative Example 2, the lithium iron phosphate borate positive electrode material provided in Examples 1-3 has better dispersibility, higher sphericity, and more regular morphology, higher particle size consistency and reasonable size distribution, so that the product has good compaction density.

[0154] Table 1

[0155] As can be seen from Table 1, the D10 particle size of the lithium iron phosphate borate positive electrode material provided in Examples 1-11 is ≤0.5 μm, the D50 particle size is ≤1.60 μm, and the D90 particle size is ≤19.45 μm. The lithium iron phosphate borate positive electrode material provided in Examples 1-11 is doped with titanium as a cation and boron as an anion, so that the particle size is more uniform and the compaction density of the positive electrode sheet prepared by using the lithium iron phosphate borate positive electrode material as a positive electrode material is improved.

[0156] Table 2

[0157] As can be seen from Table 2, the compaction density of the lithium iron phosphate borate positive electrode material provided in Examples 1-11 is 2.35-2.52 m 2 / g, the specific surface area is 8.6 m 2 / g-14.3 m 2 / g, and the powder resistivity is 4.0 Ω.cm-13.0 Ω.cm, which is more conducive to improving the capacity of the battery formed by using the lithium iron phosphate borate positive electrode material.

[0158] In addition, as can be seen from Table 2, the lithium iron phosphate borate positive electrode material provided in Examples 1-9 has a molecular formula of Li x FeTi y Pz B w O (4z+3w+2y) / C, wherein 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, the iron dissolution 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, the content of magnetic impurities is ≤1ppm, the content of magnetic substances is low, which is more conducive to improving the capacity and rate performance of the lithium iron borophosphate positive electrode material.

[0159] The lithium iron borophosphate positive electrode material provided by the embodiments 10-11 has too high content of boron element, which causes too high content of elemental iron in the lithium iron borophosphate positive electrode material, and the self-discharge of the electric core made of the lithium iron borophosphate positive electrode material is large, the K value is high, and the product performance of the secondary battery made is affected.

[0160] Performance test of the positive electrode sheet and the secondary battery

[0161] The positive electrode materials provided by the embodiments and the comparative examples were respectively prepared into positive electrode sheets and assembled into secondary batteries, and the specific process was as follows: the positive electrode materials provided by the embodiments and the comparative examples were respectively mixed with conductive carbon black (SP) conductive agent, polyvinylidene fluoride (PVDF) and N-methyl pyrrolidone (NMP) by a high-speed mixer, wherein the mass ratio of the positive electrode material provided by the embodiments or the comparative examples, the SP conductive agent and the PVDF was 90:5:5. Then, an automatic coating machine was used to coat on an aluminum foil, the coated aluminum foil was baked and dried in an oven and was rolled according to the required compaction density, was cut into a circular sheet with a required size and was weighed, and then was dried again to obtain a positive electrode sheet; and a lithium sheet was used as a negative electrode sheet, a positive electrode shell, a negative electrode shell, the positive electrode sheet, the lithium sheet, a separator and an electrolyte were assembled into a button cell according to requirements; and the button cell was hung on a battery test system for standing and then was tested.

[0162] The button cell assembled above was placed and tested by using a blue battery test system (Wuhan blue battery CT2001A type battery test system), the test temperature was 25℃, the charge limit voltage was 3.75V, the discharge termination voltage was 2.0V, the charging and discharging were respectively carried out at 0.1C and 10C rate currents to obtain the 0.1C initial specific discharge capacity, the 0.1C initial specific discharge capacity and the 10C initial specific discharge capacity; the 1C discharge specific capacity of the button cell assembled above under the condition of-20℃ was further tested and the percentage of the 1C discharge specific capacity under the condition of-20℃ to the 1C discharge specific capacity under the condition of 25℃ was calculated to obtain the capacity retention rate at-20℃, and the results are shown in Table 3.

[0163] Table 3

[0164] As can be seen from the data in Table 3, the lithium iron borophosphate positive electrode material provided by the embodiments of the present application is doped with trivalent titanium as cations and doped with boron as anions, and is coated with carbon, so that the lithium iron borophosphate positive electrode material not only has high charge specific capacity and discharge specific capacity, but also has high capacity retention rate at -20°C and strong low-temperature resistance.

[0165] As can be seen from the comparison between Examples 1-9 and Examples 10-11, the molar ratio of iron, titanium, lithium, carbon, phosphorus and boron in the lithium iron borophosphate positive electrode material provided by the embodiments of the present application is 1:(0.01-0.05):(1.02-1.05):(0.06-0.22):(0.92-0.95):(0.05-0.15), which is beneficial to the improvement of the charge specific capacity, discharge specific capacity and -20°C capacity retention rate of the lithium iron borophosphate positive electrode material.

[0166] As can be seen from the comparison between Examples 1-9 and Comparative Example 1, the anion doping of borate is beneficial to the improvement of the electrical properties of the lithium iron borophosphate positive electrode material; as can be seen from the comparison between Examples 1-9 and Comparative Example 2, the electrical properties of the lithium iron borophosphate positive electrode material prepared by replacing ferrous acetate with lithium phosphate and replacing lithium acetate with lithium carbonate are significantly decreased, indicating that compared with the traditional dry mixing and calcination, the preparation method of the lithium iron borophosphate positive electrode material provided by the present application significantly improves the electrical properties of the prepared lithium iron borophosphate positive electrode material.

[0167] 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 borophosphate positive electrode material provided by the present application uses trivalent titanium ions for cation doping, which significantly improves the charge specific capacity, discharge specific capacity and -20°C capacity retention rate.

[0168] As can be seen from the comparison between Examples 1-11 and Comparative Example 4, compared with the lithium iron borophosphate positive electrode material prepared without titanium doping, the lithium iron borophosphate positive electrode material provided by the present application uses trivalent titanium ions for cation doping, which significantly improves the charge specific capacity, discharge specific capacity and -20°C capacity retention rate.

[0169] Note that the present application is not limited to the above-described embodiments. The above-described embodiments are merely examples, and embodiments having substantially the same configuration, function, and effect as the technical idea of the present application are included in the technical scope of the present application. Furthermore, other modes constructed by applying various modifications to the embodiments, or by combining part of the configurations of the embodiments, which can be conceived by those skilled in the art, without departing from the spirit of the present application, are also included in the scope of the present application.

Claims

1. A lithium iron borophosphate positive electrode material, characterized in that, The general formula of the lithium iron borophosphate positive electrode material is Li 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, 0.05≤w≤0.15, and the mass fraction of carbon in the lithium iron borophosphate positive electrode material is 1.00%-1.35%.

2. The lithium iron borophosphate positive electrode material of claim 1, wherein, The D10 particle size of the lithium iron phosphate borate material is less than or equal to 0.50 μm, the D50 particle size is less than or equal to 1.60 μm, and the D90 particle size is less than or equal to 19.00 μm.

3. The lithium iron borophosphate cathode material of claim 1, wherein, The compaction density of the lithium iron borophosphate positive electrode material is 2.35 g / mL-2.50 g / mL, the specific surface area is 10.1 m 2 / g-14.3 m 2 / g, and the powder resistivity is 4.0 Ω.cm-13.0 Ω.cm.

4. The lithium iron borophosphate positive electrode material of any one of claims 1 to 3, wherein, The lithium iron phosphate borate positive electrode material has iron elution of 3.0 ppm to 10.0 ppm, elemental iron of 0.01 ppm to 0.05 ppm, chloride ions of 40.0 ppm to 80.0 ppm, water content of 280 ppm to 350 ppm, free lithium of 140 ppm to 200 ppm, and magnetic impurity content of less than or equal to 1 ppm.

5. A method for preparing a lithium iron borophosphate positive electrode material, characterized in that, The preparation method comprises the following steps: mixing a lithium source, a ferrous source, a trivalent titanium source, a carbon source, phosphoric acid, boric acid and a 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 the lithium iron phosphate borate positive electrode material.

6. The method of claim 5, wherein the lithium iron borophosphate cathode material is prepared by the steps of: mixing a lithium source, an iron source, a boron source, and a phosphorus source to form a mixture; and heating the mixture to form the lithium iron borophosphate 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 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 comprises at least one of ferrous acetate and / or ferrous chloride; and / or, the lithium source comprises at least one of lithium chloride and / or lithium acetate; and / or, the trivalent titanium source comprises titanium trihalide, preferably titanium trichloride; and / or, the step of mixing the lithium source, the ferrous source, the trivalent titanium source, the carbon source, the phosphoric acid, the boric acid and the solvent comprises 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, preferably the mass fraction of the phosphoric acid in the phosphoric acid solution is 70%-80%; and / or, the carbon source comprises at least one of ascorbic acid and / or citric acid, and the carbon source is preferably 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).

8. The preparation method of the lithium iron phosphate borate positive electrode material according to claim 5, wherein: the mixed slurry is preheated and dried at 100°C-150°C to obtain a pre-dried material with a mass fraction of H2O less than or equal to 0.5%; and / or, the step of calcining the pre-dried material to obtain the lithium iron phosphate borate positive electrode material comprises: in a protective atmosphere, the pre-dried material is calcined at a first calcination temperature for a first holding time, and then calcined at a second calcination temperature for a second holding time to obtain the lithium iron phosphate borate positive electrode material; wherein the first calcination temperature is 120°C-150°C, and the first holding time is 2h-4h; the second calcination temperature is 600°C-700°C, and the second holding time is 4h-6h.

9. A positive electrode sheet characterized by comprising: The positive electrode sheet includes the lithium iron borophosphate positive electrode material of any one of claims 1 to 4 or the lithium iron borophosphate positive electrode material obtained from the preparation method of any one of claims 5 to 8.

10. A secondary battery characterized by comprising: The secondary battery includes the lithium iron borophosphate positive electrode material of any one of claims 1 to 4 or the positive electrode sheet of claim 9.

Images