A lithium iron phosphate material and its preparation method, positive electrode sheet and secondary battery

Abstract

The present invention provides a lithium iron phosphate material and a preparation method thereof, a positive electrode plate and a secondary battery, and relates to the field of positive electrode materials. The lithium iron phosphate material of the present invention satisfies 1.10≤σ b / σ f ≤5.00,σ b is the sample standard deviation of the circularity C of large-particle lithium iron phosphate, σ f The sample standard deviation of the circularity C of small-particle lithium iron phosphate is obtained by the difference in the sample standard deviation of the circularity of large and small particles, so that large-particle lithium iron phosphate and small-particle lithium iron phosphate with different morphologies are coordinated with each other, thereby making the lithium iron phosphate material have a good tap density and specific surface area. The method for preparing the lithium iron phosphate material of the present invention vacuum dehydrates large-particle iron phosphate seeds partially coated with polydopamine to obtain surface-dehydrated modified iron phosphate seeds, thereby affecting the morphology of large-particle iron phosphate obtained by precipitation reaction, and using the large and small seeds through a co-precipitation method to obtain an iron phosphate material, ultimately producing a lithium iron phosphate material with a good tap density and specific surface area.

Description

Technical Field

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

[0002] Lithium iron phosphate (LiFePO4) with olivine structure is a type of positive electrode material for secondary batteries. It has the characteristics of wide raw material sources, low price, good thermal stability, environmental friendliness and stable discharge platform, making it one of the most promising positive electrode materials.

[0003] However, the low tap density and low actual specific capacity of lithium iron phosphate materials limit their development. There is a close relationship between the tap density of lithium iron phosphate materials and the particle size. If the powder composed of particles has an ideal particle size distribution, so that small particles can fill the gaps between large particles as much as possible, its tap density can be further increased, thereby improving the volumetric specific capacity of secondary batteries.

[0004] In this field, large and small particle grading is usually used to increase the tap density of lithium iron phosphate materials. However, since lithium ions in lithium iron phosphate materials mainly diffuse along the one-dimensional direction, the transmission speed of lithium ions inside the material is slow. The grading method still cannot effectively alleviate the problem of reduced rate performance caused by the slow ion transmission rate in lithium iron phosphate materials.

[0005] In view of this, the present invention is proposed. Summary of the Invention

[0006] The purpose of the present invention includes providing a lithium iron phosphate material and a preparation method thereof, a positive electrode plate and a secondary battery. The particle size distribution curve of the lithium iron phosphate material is a bimodal distribution, and the standard deviation of the circularity of large-particle lithium iron phosphate is greater than the standard deviation of the circularity of small-particle lithium iron phosphate. Large-particle lithium iron phosphate and small-particle lithium iron phosphate with different morphologies cooperate with each other, so that the lithium iron phosphate material has good tap density and specific surface area, and thus has better specific capacity and rate performance.

[0007] In order to achieve the above object, the present invention provides a lithium iron phosphate material in the first aspect. The particle size distribution curve of the lithium iron phosphate material is a bimodal distribution. The particle size distribution curve includes a first peak and a second peak in the positive direction of the horizontal coordinate. The particle sizes corresponding to the first peak and the second peak are D f and D b , the unit is μm; the corresponding volume percentage is V f and V b ;

[0008] Lithium iron phosphate material satisfies 1.10≤σ b / σ f ≤5.00, where σb is the sample standard deviation of the circularity C of large-particle lithium iron phosphate, σ f is the sample standard deviation of the circularity C of small-particle lithium iron phosphate, circularity C=(4×π×A) / P 2 , A is the projected area of ​​the lithium iron phosphate particle, and P is the perimeter of the projected area;

[0009] Large particle lithium iron phosphate has a projected area greater than or equal to S max =1 / 4×π×D b 2 The projected area of ​​lithium iron phosphate particles is less than or equal to S min =1 / 4×π×D f 2 of lithium iron phosphate particles.

[0010] In some embodiments, the lithium iron phosphate material satisfies at least one of the following features 1 and 2:

[0011] Feature 1: σ b Satisfy 0.05≤σ b ≤0.15;

[0012] Feature 2: σ f Satisfying 0.02≤σ f ≤0.09.

[0013] In some embodiments, the lithium iron phosphate material satisfies at least one of the following characteristics ad:

[0014] Feature a: 0.3μm≤D f ≤1.0μm and 1.2μm≤D b ≤10μm;

[0015] Feature b: 3%≤V f ≤10% and 3%≤V b ≤10%;

[0016] Characteristic c: 0.2≤V b / V f ≤2.5;

[0017] Characteristic d: 2.5≤D b / D f ≤12.

[0018] In some embodiments, the lithium iron phosphate material satisfies: ; Among them, span satisfies 1.5≤span≤5, span=(Dv90-Dv10) / Dv50, Dv90, Dv50 and Dv10 are the particle sizes corresponding to when the cumulative volume percentage of lithium iron phosphate material reaches 90%, 50% and 10%, respectively, and the unit is μm.

[0019] In a second aspect, the present invention provides a method for preparing the lithium iron phosphate material according to the first aspect, comprising:

[0020] Coating the large-size iron phosphate seed crystals with polydopamine to obtain the large-size iron phosphate seed crystals partially coated with polydopamine;

[0021] vacuum dehydrating the large-particle iron phosphate seed crystals partially coated with polydopamine to obtain modified iron phosphate seed crystals;

[0022] The modified ferric phosphate seed crystals and the small-particle ferric phosphate seed crystals are mixed, and an iron salt and a phosphorus source are added to carry out a precipitation reaction in water to obtain a ferric phosphate material;

[0023] The iron phosphate material, lithium source and carbon source are mixed and then calcined to obtain lithium iron phosphate material.

[0024] In some embodiments, the preparation method of large-particle iron phosphate seeds partially coated with polydopamine includes: mixing and dispersing large-particle iron phosphate seeds and a surfactant in a Tris-HCl buffer solution containing dopamine hydrochloride, stirring the reaction, filtering, washing, and drying to obtain large-particle iron phosphate seeds partially coated with polydopamine.

[0025] In some embodiments, the method for preparing large-particle iron phosphate seeds partially coated with polydopamine includes at least one of the following features 1 to 6:

[0026] Feature 1: The surfactant is an anionic surfactant, and the concentration of the surfactant in the Tris-HCl buffer solution is 0.1-3 g / L;

[0027] Feature 2: The mass ratio of large-particle iron phosphate seeds to surfactant is 50:1~5;

[0028] Feature 3: The particle size of large-particle iron phosphate seeds is 400-1500nm;

[0029] Feature 4: The pH of Tris-HCl buffer solution is 8-8.5;

[0030] Feature 5: The concentration of dopamine hydrochloride in Tris-HCl buffer solution is 0.1-0.3 mg / mL;

[0031] Feature 6: The stirring rate of the stirring reaction is 500-1000 rpm and the time is 20-30 hours.

[0032] In some embodiments, the method for preparing the lithium iron phosphate material includes at least one of the following features A to G:

[0033] Feature A: The vacuum dehydration temperature is 160-190°C and the time is 1-1.5 hours;

[0034] Feature B: The particle size of the small-particle iron phosphate seed crystals is 200-600 nm;

[0035] Characteristic C: The volume ratio of the modified ferric phosphate seed crystals to the small-particle ferric phosphate seed crystals is 0.9-1.5:1;

[0036] Feature D: The volume ratio of the total volume of the modified ferric phosphate seed crystals and the small-particle ferric phosphate seed crystals to the volume of water is 1:4-12;

[0037] Characteristic E: The concentration of iron salt in water is 0.5-1.2 mol / L;

[0038] Feature F: The molar ratio of the iron element of the iron salt to the phosphorus element of the phosphorus source is 1:1-1.5;

[0039] Feature G: The stirring rate of the precipitation reaction is 350-750 rpm.

[0040] In a third aspect, the present invention provides a positive electrode plate, comprising the lithium iron phosphate material according to the first aspect of the present invention or the lithium iron phosphate material obtained by the preparation method provided by the second aspect of the present invention.

[0041] In a fourth aspect, the present invention provides a secondary battery, comprising the positive electrode sheet provided in the third aspect.

[0042] The beneficial effects of the present invention include:

[0043] The lithium iron phosphate material provided by the present invention satisfies 1.10≤σ b / σ f ≤5.00, where σ b is the sample standard deviation of the circularity C of large-particle lithium iron phosphate, σ f is the sample standard deviation of the circularity C of small-particle lithium iron phosphate. Through the difference in the sample standard deviation of the circularity of large and small particles of lithium iron phosphate material, large-particle lithium iron phosphate and small-particle lithium iron phosphate with different morphologies are matched with each other, so that the lithium iron phosphate material has good tap density and specific surface area.

[0044] The present invention provides a method for preparing a lithium iron phosphate material. The method comprises vacuum dehydrating large-particle iron phosphate seeds partially coated with polydopamine to obtain surface-dehydrated modified iron phosphate seeds, thereby affecting the morphology of large-particle iron phosphate obtained by a precipitation reaction. The large and small seeds are then co-precipitated to obtain an iron phosphate material, ultimately producing a lithium iron phosphate material having good tap density and specific surface area. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without paying any creative work.

[0046] Figure 1 This is a SEM image of the lithium iron phosphate material provided in Example 1 of the present invention;

[0047] Figure 2 This is a SEM image of the lithium iron phosphate material provided in Comparative Example 2 of the present invention;

[0048] Figure 3 This is the particle size distribution curve of the lithium iron phosphate material provided in Examples 4-5 of the present invention. DETAILED DESCRIPTION

[0049] Below, with appropriate reference to the accompanying drawings, a detailed description of a lithium iron phosphate material and its preparation method, a positive electrode plate and a secondary battery of the present invention is specifically disclosed. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0050] The "ranges" disclosed herein are defined in terms of lower and upper limits. A given range is defined by selecting a lower limit and an upper limit, and the selected lower and upper limits define the boundaries of the particular range. In particular, "(", ")", "[", and "]" denote intervals, where "(" or ")" denotes an open interval, i.e., the endpoints of the interval are not included; and "[" and "]" denote a closed interval, i.e., the endpoints of the interval are included. Ranges defined in this manner can be inclusive or exclusive of the endpoints and can be combined arbitrarily, i.e., any lower limit can be combined with any upper limit to form a range.

[0051] Specifically, for example, if ranges of 60-120 and 80-110 are listed for a particular parameter, it is understood that ranges of 60-110 and 80-120 are also contemplated. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise indicated, the numerical range "ab" is an abbreviation for any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed herein, and "0-5" is merely an abbreviation for these numerical combinations. Additionally, when a parameter is stated as an integer greater than or equal to 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc. If (10, 20) is listed, it is understood as any value between 10 and 20, excluding 10 and 20; (10, 20] is understood as any value between 10 and 20, excluding 10.

[0052] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0053] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0054] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0055] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0056] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0057] Lithium iron phosphate materials have an inherent problem of low ion transport rates. Particle nano-sizing is an effective means of improving lithium ion transport rates. However, while particle nano-sizing shortens the transport path and improves rate performance, it also leads to low energy density due to low tap density. Therefore, grading is often used in this field to increase the material's tap density by utilizing the filling effect between large and small particles. However, due to the lower specific surface area and longer ion diffusion paths of large particles, grading increases the material's tap density but reduces the material's specific surface area, resulting in little significant improvement in the material's rate performance.

[0058] The first aspect of the present invention provides a lithium iron phosphate material, wherein the particle size distribution curve of the lithium iron phosphate material is a bimodal distribution, wherein the particle size distribution curve includes a first peak and a second peak in the positive direction of the horizontal coordinate, and the particle sizes corresponding to the first peak and the second peak are D f and D b , the unit is μm; the corresponding volume percentage is V f and V b ;

[0059] Lithium iron phosphate material satisfies 1.10≤σ b / σ f ≤5.00, where σ b is the sample standard deviation of the circularity C of large-particle lithium iron phosphate, σ f is the sample standard deviation of the circularity C of small-particle lithium iron phosphate, circularity C=(4×π×A) / P 2 , A is the projected area of ​​the lithium iron phosphate particle, and P is the perimeter of the projected area;

[0060] Large particle lithium iron phosphate has a projected area greater than or equal to S max =1 / 4×π×D b 2 The projected area of ​​lithium iron phosphate particles is less than or equal to S min =1 / 4×π×D f 2 To address the problem that conventional particle size grading can increase tap density but reduce the specific surface area of ​​lithium iron phosphate cathode materials, the present invention provides a lithium iron phosphate material in which the standard deviation of sphericity of large-particle lithium iron phosphate is greater than that of small-particle lithium iron phosphate. This lithium iron phosphate material achieves both good tap density and good specific surface area.

[0061] The circularity C of the projection surface of lithium iron phosphate particles can reflect the sphericity of lithium iron phosphate particles. The closer it is to 1, the better the sphericity of the particles. b and σ f They represent the sample standard deviations of the circularity C of large and small lithium iron phosphate particles, respectively. The larger the sample standard deviation, the higher the dispersion of the circularity C value, which is manifested as a greater difference in the sphericity of the particles; the smaller the sample standard deviation, the higher the circularity The lower the degree of discreteness, the smaller the difference in sphericity of the particles. The lithium iron phosphate material in the present invention satisfies 1.10≤σ b / σ f ≤5.00 (σb / σf can be 1.10, 1.15, 1.20, 1.25, 1.30, 1.35, 1.40, 1.45, 1.50, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0 or any value between 1.10 and 5.00). The lithium iron phosphate material within this range has good tap density and specific surface area.

[0062] In some embodiments, the σ of the lithium iron phosphate material b Satisfy 0.05≤σ b ≤0.15(σ b It can be 0.05, 0.075, 0.08, 0.085, 0.09, 0.095, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15 or any value between 0.05 and 0.15).

[0063] In some embodiments, the σ of the lithium iron phosphate material f Satisfying 0.02≤σ f ≤0.09(σ f It can be 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09 or any value between 0.02 and 0.09).

[0064] Because a standard sphere has the smallest surface area under the same volume, the sphericity of lithium iron phosphate materials affects their specific surface area and tap density, ultimately affecting their electrochemical performance. Specifically, the higher the sphericity, the smaller the specific surface area of ​​the lithium iron phosphate material, reducing the contact area between the material and the electrolyte and lowering the efficiency of lithium ion transmission. The lower the sphericity, the lower the tap density of the lithium iron phosphate material, ultimately reducing the material's volumetric energy density.

[0065] Generally, the sphericity of lithium iron phosphate particles is relatively uniform, which is beneficial to improving the tap density and reducing the internal resistance. However, the higher the sphericity, the smaller the surface area of ​​the particles, which is not conducive to improving the specific surface area. The present invention provides a lithium iron phosphate material with a bimodal particle size distribution curve, thereby distinguishing large lithium iron phosphate particles from small lithium iron phosphate particles in the lithium iron phosphate material, wherein the standard deviation of the circularity of the large lithium iron phosphate particles (σ b ) is larger, while the standard deviation of circularity of small-particle lithium iron phosphate (σ f ) is smaller.

[0066] If the uniformity of the sphericity of large-particle lithium iron phosphate is too high or too low, it will not be conducive to improving the overall performance of the lithium iron phosphate material. Specifically, if the sphericity difference of large-particle lithium iron phosphate is too large, it may be manifested as particles with too low sphericity in the large particles, which will cause the pores formed by the overlap between the particles to be too large, and the material's tap density will be reduced. When the sphericity difference of large-particle lithium iron phosphate is too small, it may be manifested as all high sphericity, all low sphericity, or other sphericities of the same degree. When all high sphericity is manifested, the total specific surface area of ​​the material will decrease. When all low sphericity is manifested, the material's tap density will decrease. If it is other spherical shapes of the same degree, it will be difficult for the material to have both good tap density and specific surface area. Small-particle lithium iron phosphate fills the gaps between large-particle lithium iron phosphate, plays a role in improving the tap density, and satisfies 0.02≤σ f Small lithium iron phosphate particles with a density of ≤0.09 can well fill the gaps in large lithium iron phosphate particles and improve the tap density of lithium iron phosphate materials.

[0067] In some embodiments, the lithium iron phosphate material satisfies 0.3 μm ≤ D f ≤1.0μm and 1.2μm≤D b ≤10μm (where D f It can be any value among 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1.0 μm or 0.3-1.0 μm; wherein D b It can be 1.2 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm or any value between 1.2 and 10 μm).

[0068] In some embodiments, the lithium iron phosphate material satisfies 3%≤V f ≤10% and 3%≤V b ≤10% (V f or V b It can be 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or any value between 3% and 10%).

[0069] In some embodiments, the lithium iron phosphate material satisfies 0.2≤V b / V f ≤2.5 (where V b / V f It can be 0.2, 0.5, 1.0, 1.5, 2.0, 2.5 or any value between 0.2 and 2.5).

[0070] V b / V f Reflects the difference in volume fractions of the two largest particles in the lithium iron phosphate material, that is, the ratio of the number of particles of the two sizes. The larger the value, the greater the difference in volume occupied by the two particles, and the smaller the value, the smaller the difference in volume occupied by the two particles. b / V f Too large or too small can not effectively improve the tap density of the positive electrode material. b / V f Too large indicates that the particle size is D f The number of small lithium iron phosphate particles is compared with the particle size D b Too many large lithium iron phosphate particles will reduce the overall specific surface area of ​​the lithium iron phosphate material; b / V f Too small indicates that the particle size is D b The number of large lithium iron phosphate particles is compared with the particle size D f There are too many small particles of lithium iron phosphate. Since the diffusion path formed by the accumulation of small particles of lithium iron phosphate is relatively tortuous and long, it will limit the diffusion of lithium ions and ultimately reduce the rate performance and specific capacity.

[0071] In some embodiments, the lithium iron phosphate material satisfies 2.5≤D b / D f ≤12 (where D b / D f It can be 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 7.5, 8.0, 8.5, 9.0, 9.5, 10.0, 10.5, 11.0, 11.5, 12.0 or any value between 2.5 and 12).

[0072] D b / D f Reflects the size difference between the two largest particle sizes in the lithium iron phosphate material. The larger the value, the greater the difference between the two particle sizes. The smaller the value, the smaller the difference between the two particle sizes. b / D f If the particle size is too small, the grading effect of the particles cannot be fully utilized, resulting in an inability to effectively improve the tap density of the positive electrode material; D b / D fToo much will cause the small particles of lithium iron phosphate to be "overburned" during the roasting process.

[0073] In some embodiments, the lithium iron phosphate material satisfies: ; Wherein, span = (Dv90-Dv10) / Dv50, satisfying 1.5≤span≤5, and the Dv90, Dv50 and Dv10 are the particle sizes corresponding to when the cumulative volume percentage of the lithium iron phosphate material reaches 90%, 50% and 10%, respectively, and the unit is μm.

[0074] Span reflects the overall particle size distribution width of the lithium iron phosphate material. The larger the value, the wider the particle size distribution, and the smaller the value, the narrower the particle size distribution.

[0075] Lithium iron phosphate material meets ( It can be 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70 or any value in the range of 2-70). b / D f 、V b / V f , span, and σ b / σ f By coordinating with each other, lithium iron phosphate material has better tap density and specific surface area, and therefore has better specific capacity and rate performance.

[0076] A second aspect of the present invention provides a method for preparing a lithium iron phosphate material, the method comprising:

[0077] Coating the large-size iron phosphate seed crystals with polydopamine to obtain the large-size iron phosphate seed crystals partially coated with polydopamine;

[0078] vacuum dehydrating the large-particle iron phosphate seed crystals partially coated with polydopamine to obtain modified iron phosphate seed crystals;

[0079] The modified ferric phosphate seed crystals and the small-particle ferric phosphate seed crystals are mixed, and an iron salt and a phosphorus source are added to carry out a precipitation reaction in water to obtain a ferric phosphate material;

[0080] The iron phosphate material, the lithium source and the carbon source are mixed and calcined to obtain the lithium iron phosphate material.

[0081] The present invention uses vacuum dehydration treatment to dehydrate the exposed surface layer of large-particle iron phosphate seed crystals partially coated with polydopamine into anhydrous iron phosphate, thereby obtaining modified iron phosphate seed crystals having a portion of the surface coated with polydopamine and a portion of the surface as anhydrous iron phosphate. Since anhydrous iron phosphate is easily soluble in an acid solution, during a precipitation reaction, the anhydrous iron phosphate on the surface of the modified iron phosphate seed crystal is dissolved by the acid to expose the iron phosphate dihydrate, which serves as a site for the continued growth of iron phosphate crystals. However, the surface coated with polydopamine is effectively closed, making it difficult to serve as a site for the growth of iron phosphate. Since the growth rates of the polydopamine-coated surface and the surface of the anhydrous iron phosphate in the modified iron phosphate seed crystal are inconsistent, the resulting iron phosphate particles have poor sphericity.

[0082] During the precipitation reaction process, the present invention simultaneously adds modified iron phosphate seed crystals and small-particle iron phosphate seed crystals, causing them to grow in a mixed solution of a phosphorus source and an iron salt to form large-particle iron phosphate and small-particle iron phosphate. The modified iron phosphate seed crystals grow into large-particle iron phosphate with low sphericity uniformity, and the small-particle iron phosphate seed crystals grow into iron phosphate with high sphericity uniformity due to being unmodified, thereby obtaining an iron phosphate material with a large and small particle size distribution using a one-step method. The iron phosphate material is further mixed and calcined with a lithium source to obtain a lithium iron phosphate material with a bimodal distribution and in which the sphericity difference of the large-particle lithium iron phosphate is greater than the sphericity difference of the small-particle lithium iron phosphate.

[0083] In some embodiments, the preparation method of large-particle iron phosphate seeds partially coated with polydopamine includes: mixing and dispersing large-particle iron phosphate seeds and a surfactant in a Tris-HCl buffer solution containing dopamine hydrochloride, stirring the reaction, filtering, washing, and drying to obtain large-particle iron phosphate seeds partially coated with polydopamine.

[0084] The present invention adopts the above method to prepare large-particle iron phosphate seeds partially coated with polydopamine, and obtains large-particle iron phosphate seeds partially covered with polydopamine on the surface of the large-particle iron phosphate seeds by controlling the concentration of components.

[0085] In some embodiments, the phosphorus source includes at least one of ammonium phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, sodium monohydrogen phosphate, and sodium dihydrogen phosphate.

[0086] In some embodiments, the iron salt includes at least one of ferric chloride, ferric nitrate, and ferric sulfate.

[0087] In some embodiments, the carbon source includes at least one of sucrose, glucose, starch, dopamine, and polyethylene glycol.

[0088] In some embodiments, the lithium source is at least one of lithium hydroxide, lithium carbonate, and lithium nitrate.

[0089] In some embodiments, the mass ratio of the carbon source content to the ferric phosphate material is 0.05-0.15:1 (the mass ratio of the carbon source content to the ferric phosphate material can be 0.05, 0.08, 0.10, 0.12, 0.14, 0.15 or any value between 0.05 and 0.15).

[0090] In some embodiments, the calcination temperature is 700-850°C (the calcination temperature can be 700°C, 725°C, 750°C, 775°C, 800°C, 825°C, 850°C or any value between 700-850°C), and the time is 20-35h (the time can be 20h, 22h, 25h, 27h, 30h, 32h, 35h or any value between 20-35h).

[0091] In some embodiments, the surfactant is at least one of anionic surfactants, including at least one of sodium dodecylbenzenesulfonate (SDBS), sodium fatty alcohol ether sulfate (AES) and sodium stearate methyl polyoxyethylene ether sulfonate (FMES); the addition of the surfactant can prevent the agglomeration of large-particle iron phosphate seeds, which is conducive to the generation of large-particle iron phosphate seeds partially coated with polydopamine with a target coating morphology.

[0092] In some embodiments, the concentration of the surfactant in the Tris-HCl buffer solution is 0.1-3 g / L (the concentration of the surfactant in the Tris-HCl buffer solution can be 0.1 g / L, 0.5 g / L, 1.0 g / L, 1.5 g / L, 2.0 g / L, 2.5 g / L, 3.0 g / L, or any value between 0.1 and 3 g / L).

[0093] In some embodiments, the mass ratio of large particle size ferric phosphate seeds to surfactant is 50:1-5 (the mass ratio of large particle size ferric phosphate seeds to surfactant can be 50:1, 50:2, 50:3, 50:4, 50:5 or any value in the range of 50:1-5).

[0094] In some embodiments, the particle size of the large-particle ferric phosphate seed crystals is 400-1500 nm (the particle size of the large-particle ferric phosphate seed crystals can be any value in the range of 400-500 nm, 600-700 nm, 700-800 nm, 800-900 nm, 900-1000 nm, 1000-1100 nm, 1100-1200 nm, 1200-1300 nm, 1300-1400 nm, 1400-1500 nm or 400-1500 nm); by controlling the particle size of the large-particle ferric phosphate seed crystals, D b The value of D b The value of varies with the particle size of the large-particle iron phosphate seeds.

[0095] In some embodiments, the particle size of the small-particle iron phosphate seed crystals is 200-600 nm (the particle size of the small-particle iron phosphate seed crystals can be any value in the range of 200-300 nm, 300-400 nm, 400-500 nm, 500-600 nm or 200-600 nm); the D f The value of D f The value of varies with the particle size of the small-particle iron phosphate seeds.

[0096] In some embodiments, the pH of the Tris-HCl buffer solution is 8-8.5 (the pH of the Tris-HCl buffer solution may be 8, 8.1, 8.2, 8.3, 8.4, 8.5, or any value between 8 and 8.5).

[0097] In some embodiments, the concentration of dopamine hydrochloride in Tris-HCl buffer solution is 0.1-0.3 mg / mL (eg, 0.1 mg / mL, 0.15 mg / mL, 0.2 mg / mL, 0.25 mg / mL, 0.3 mg / mL, or any value between 0.1-0.3 mg / mL).

[0098] The present invention controls the concentration of dopamine hydrochloride to control the coating amount of polydopamine on the large-particle iron phosphate seed crystals partially coated with polydopamine, thereby affecting the sample standard deviation σ of the circularity C of the large-particle lithium iron phosphate. b Specifically, within the specified range, as the concentration of dopamine hydrochloride increases σ b The bigger.

[0099] In some embodiments, the temperature of vacuum dehydration is 160-190°C (the temperature of vacuum dehydration can be 160°C, 165°C, 170°C, 175°C, 180°C, 185°C, 190°C or any value between 160-190°C), and the time is 1-1.5h (the time can be 1h, 1.1h, 1.2h, 1.3h, 1.4h, 1.5h or any value between 1-1.5h).

[0100] At this temperature, ferric phosphate dihydrate can remove crystal water to obtain anhydrous ferric phosphate. The present invention controls the vacuum dehydration time and temperature to control the thickness of the anhydrous ferric phosphate formed by dehydration of the surface layer of the modified ferric phosphate seed crystals, thereby affecting the sphericity of the obtained large-particle ferric phosphate. Specifically, the higher the vacuum dehydration temperature and the longer the time, the thicker the anhydrous ferric phosphate, the lower the circularity of the large-particle ferric phosphate obtained by the precipitation reaction, and the σ of the large-particle lithium iron phosphate obtained by subsequent calcination with the lithium source. b On the contrary, the lower the vacuum dehydration temperature and the shorter the time, the larger the σ of the obtained large-particle lithium iron phosphate b The smaller.

[0101] In some embodiments, the volume ratio of modified ferric phosphate seeds to small particle size ferric phosphate seeds is 0.9-1.5:1 (the volume ratio of modified ferric phosphate seeds to small particle size ferric phosphate seeds can be 0.9:1, 1.0:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1 or any value in the range of 0.9-1.5:1).

[0102] In some embodiments, the volume ratio of the total volume of the modified ferric phosphate seeds and the small-particle ferric phosphate seeds to water is 1:4-12 (the volume ratio of the total volume of the modified ferric phosphate seeds and the small-particle ferric phosphate seeds to water can be 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, 1:11, 1:12 or any value between 1:4 and 12).

[0103] The present invention affects V by controlling the volume ratio of modified ferric phosphate seeds to small-particle ferric phosphate seeds. b / V f , V b / V f It increases with the increase of the volume ratio of modified ferric phosphate seeds to small-particle ferric phosphate seeds.

[0104] In some embodiments, the concentration of the iron salt in water is 0.5-1.2 mol / L (the concentration of the iron salt in water can be 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1.0 mol / L, 1.1 mol / L, 1.2 mol / L, or any value between 0.5-1.2 mol / L).

[0105] In some embodiments, the molar ratio of the iron element of the iron salt to the phosphorus element of the phosphorus source is 1:1-1.5 (the molar ratio of the iron element of the iron salt to the phosphorus element of the phosphorus source can be 1:1, 1:1.2, 1:1.3, 1:1.4, 1:1.5 or any value of 1:1-1.5).

[0106] The molar ratio of iron and phosphorus in the liquid phase during the precipitation reaction will affect the particle size distribution (span) of the lithium iron phosphate material. As the iron-phosphorus ratio decreases, the particle size distribution becomes wider.

[0107] In some embodiments, the stirring rate of the precipitation reaction is 350-750 rpm (the stirring rate of the precipitation reaction can be 350 rpm, 400 rpm, 450 rpm, 500 rpm, 550 rpm, 600 rpm, 650 rpm, 700 rpm, 750 rpm or any value between 350-750 rpm); as the stirring rate increases, the particle size of the lithium iron phosphate material becomes smaller.

[0108] The third aspect of the present invention provides a positive electrode plate, comprising a positive electrode collector and a positive electrode material layer disposed on the positive electrode collector and comprising a positive electrode active material. The positive electrode material layer can be disposed on one surface of the positive electrode collector or on both surfaces of the positive electrode collector.

[0109] Wherein, the positive electrode active material includes the lithium iron phosphate material provided by the first aspect of the present invention, or the lithium iron phosphate material obtained by the preparation method provided by the second aspect of the present invention.

[0110] The present invention further provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet and a separator, wherein the positive electrode sheet comprises the positive electrode sheet provided in the third aspect of the present invention.

[0111] [Negative electrode]

[0112] In the secondary battery of the present invention, the negative electrode plate may include a negative electrode current collector and a negative electrode material layer disposed on the negative electrode current collector and including a negative electrode active material. The negative electrode material layer may be disposed on one surface of the negative electrode current collector or on both surfaces of the negative electrode current collector.

[0113] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0114] In some embodiments, the negative electrode active material may adopt the negative electrode active material for secondary batteries known in the art. As an example, the negative electrode active material may include at least one of the following materials: graphite (such as artificial graphite, natural graphite), soft carbon, hard carbon, mesophase carbon microbeads, carbon fibers, carbon nanotubes, silicon-based materials, tin-based materials and lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxides and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for secondary batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0115] [Electrolytes]

[0116] The electrolyte conducts ions between the positive and negative electrodes. The present invention does not specifically limit the type of electrolyte, and the electrolyte can be selected based on needs. For example, the electrolyte can be selected from an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0117] In some embodiments, the electrolyte salt can be selected from one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), lithium dioxalatoborate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorobis(oxalatophosphate) (LiDFOP) and lithium tetrafluorooxalatophosphate (LiTFOP).

[0118] In some embodiments, the solvent can be selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS) and diethyl sulfone (ESE).

[0119] In some embodiments, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, or additives capable of improving certain properties of the secondary battery, such as additives that improve the overcharge performance of the secondary battery, additives that improve the high-temperature performance of the secondary battery, and additives that improve the low-temperature performance of the secondary battery.

[0120] [Isolation film]

[0121] In the secondary battery of the present invention, a separator is disposed between the positive electrode and the negative electrode to provide isolation. The type of separator is not specifically limited, and any known porous separator with good chemical and mechanical stability may be selected. In some embodiments, the separator may be made of at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of the layers may be the same or different, without particular limitation.

[0122] The positive electrode sheet, the negative electrode sheet and the separator can be made into an electrode assembly through a lamination process or a winding process, so that the separator is placed between the positive electrode sheet and the negative electrode sheet to play an isolation role; the electrode assembly is placed in an outer package, the electrolyte is injected and the package is sealed to obtain a lithium-ion battery.

[0123] The outer packaging of a lithium-ion battery is used to encapsulate the electrode assembly and electrolyte. In some embodiments, the outer packaging of the lithium-ion battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the lithium-ion battery can also be a soft package, such as a bag-type soft package. The material of the soft package can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), polybutylene succinate (PBS), etc.

[0124] The present invention has no particular limitation on the shape of the lithium-ion battery, which may be cylindrical, square, or any other shape.

[0125] In some embodiments, lithium-ion batteries can be assembled into a secondary battery module. The secondary battery module can contain multiple lithium-ion batteries, and the specific number can be adjusted according to the application and capacity of the secondary battery module.

[0126] In some embodiments, the secondary battery modules may be assembled into a secondary battery pack. The number of secondary battery modules contained in the secondary battery pack may be adjusted according to the application and capacity of the secondary battery pack.

[0127] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0128] The features and properties of the present invention are further described in detail below with reference to the examples. It should be understood that these examples are only used to illustrate the present invention and are not intended to limit the scope of the present invention.

[0129] Example 1:

[0130] (1) Preparation of large-particle iron phosphate seeds partially coated with polydopamine: Large-particle iron phosphate seeds with a particle size of 800-900 nm and sodium dodecylbenzenesulfonate were dispersed in a Tris-HCl buffer solution with a dopamine hydrochloride concentration of 0.2 mg / mL using ultrasonic treatment. After continuous stirring at 750 rpm for 24 h, the mixture was filtered, washed, and dried to obtain large-particle iron phosphate seeds partially coated with polydopamine; the concentration of sodium dodecylbenzenesulfonate in the buffer solution was 1.8 g / L, and the pH value of the Tris-HCl buffer solution was 8.5; and drying was performed at 80°C for 6 h.

[0131] (2) Preparation of modified iron phosphate seed crystals: The large-particle iron phosphate seed crystals partially coated with polydopamine were vacuum dehydrated at 185°C for 1.5 h to obtain modified iron phosphate seed crystals.

[0132] (3) Preparation of iron phosphate material: modified iron phosphate seeds and small-particle iron phosphate seeds with a particle size of 200-300 nm were dispersed in deionized water at a volume ratio of 1.2:1, and then ferric chloride solution and diammonium hydrogen phosphate solution were added. The mixture was precipitated at a rotation speed of 650 rpm for 6 h, and then transferred to a phosphoric acid solution for aging for 4 h. After filtration, washing, drying, and calcination, the iron phosphate material was obtained.

[0133] The volume ratio of the total volume of the modified ferric phosphate seeds and the small-particle ferric phosphate seeds to deionized water is 1:8; during the precipitation reaction, the amount of ferric chloride solution added is controlled to maintain the concentration of iron ions in the reaction solution at 1 mol / L; during the precipitation reaction, the amount of diammonium hydrogen phosphate solution added is controlled to maintain the concentration of phosphorus in the reaction solution at 1.1 mol / L; and ammonia water is used to control the pH of the reaction solution to 1.5.

[0134] (4) Preparation of lithium iron phosphate material: The iron phosphate material, lithium hydroxide, glucose and ethanol are mixed and ground to obtain a slurry; the slurry is dried and ground, and then calcined at 750°C for 20 hours to obtain a lithium iron phosphate material; the molar ratio of iron element to lithium element in the slurry is 1:1.03, the mass ratio of glucose to iron phosphate material is 0.05:1, and the solid content of the slurry is 35wt%.

[0135] Example 2:

[0136] The only difference between this embodiment and embodiment 1 is that:

[0137] In step (1) of preparing large-particle iron phosphate seed crystals partially coated with polydopamine: replace "the concentration of dopamine hydrochloride is 0.2 mg / mL" with "the concentration of dopamine hydrochloride is 0.1 mg / mL";

[0138] In step (2) of preparing modified iron phosphate seed crystals, replace “dehydrating the large-particle iron phosphate seed crystals partially coated with polydopamine at 185° C. under vacuum for 1.5 h to obtain modified iron phosphate seed crystals” with “dehydrating the large-particle iron phosphate seed crystals partially coated with polydopamine at 160° C. under vacuum for 1 h to obtain modified iron phosphate seed crystals”;

[0139] In step (3) of preparing the iron phosphate material, replace “transfer to a phosphoric acid solution and age for 4 h” with “transfer to a phosphoric acid solution and age for 6 h”.

[0140] Example 3:

[0141] The only difference between this embodiment and embodiment 1 is that:

[0142] In step (1) of preparing large-particle iron phosphate seed crystals partially coated with polydopamine: replace "the concentration of dopamine hydrochloride is 0.2 mg / mL" with "the concentration of dopamine hydrochloride is 0.3 mg / mL";

[0143] In step (2) of preparing modified iron phosphate seed crystals, replace “dehydrating the large-particle iron phosphate seed crystals partially coated with polydopamine at 185° C. under vacuum for 1.5 h to obtain modified iron phosphate seed crystals” with “dehydrating the large-particle iron phosphate seed crystals partially coated with polydopamine at 190° C. under vacuum for 1.5 h to obtain modified iron phosphate seed crystals”;

[0144] In the process of preparing the iron phosphate material in step (3): replace “transfer to a phosphoric acid solution and age for 4 hours” with “transfer to a phosphoric acid solution and age for 3 hours”.

[0145] Example 4:

[0146] The only difference between this embodiment and embodiment 1 is that:

[0147] Replace the process of “step (3) preparing the iron phosphate material” with “(3) preparing the iron phosphate material: dispersing the modified iron phosphate seed crystals and the small-particle iron phosphate seed crystals with a particle size of 200-300 nm in deionized water at a volume ratio of 0.9:1, then adding ferric chloride solution and diammonium hydrogen phosphate solution, and carrying out precipitation reaction at a rotation speed of 750 rpm for 6 h, then transferring to a phosphoric acid solution for aging for 5 h, filtering, washing, drying, and calcining to obtain the iron phosphate material;

[0148] The volume ratio of the total volume of the modified ferric phosphate seed crystals and the small-particle ferric phosphate seed crystals with a particle size of 200-300 nm to the volume of deionized water is 1:10;

[0149] During the precipitation reaction, the amount of ferric chloride solution added was controlled to maintain the iron ion concentration at 1.1 mol / L; the amount of diammonium hydrogen phosphate solution added was controlled to maintain the phosphorus concentration in the solution at 1.43 mol / L.

[0150] Example 5:

[0151] (1) Preparation of large-particle iron phosphate seeds partially coated with polydopamine: Large-particle iron phosphate seeds with a particle size of 500-600 nm and sodium dodecylbenzenesulfonate were dispersed in a Tris-HCl buffer solution containing 0.1 mg / mL dopamine hydrochloride by ultrasonic treatment at a mass ratio of 25:1. After continuous stirring at a speed of 1000 rpm for 24 hours, the mixture was filtered, washed, and dried to obtain large-particle iron phosphate seeds partially coated with polydopamine; the concentration of sodium dodecylbenzenesulfonate in the buffer solution was 2.2 g / L, and the pH value of the Tris-HCl buffer solution was 8.5; and drying was performed at 80°C for 6 hours.

[0152] (2) Preparation of modified iron phosphate seed crystals: The large-particle iron phosphate seed crystals partially coated with polydopamine were vacuum dehydrated at 185°C for 1.5 h to obtain modified iron phosphate seed crystals.

[0153] (3) Preparation of iron phosphate material: modified iron phosphate seeds and small-particle iron phosphate seeds with a particle size of 200-300 nm were dispersed in deionized water at a volume ratio of 1.2:1, and then ferric chloride solution and diammonium hydrogen phosphate solution were added. The mixture was precipitated at a speed of 600 rpm for 6 h, and then transferred to a phosphoric acid solution for aging for 6 h. After filtration, washing, drying, and calcination, the iron phosphate material was obtained.

[0154] The volume ratio of the total volume of the modified ferric phosphate seed crystals and the small-particle ferric phosphate seed crystals with a particle size of 200-300 nm to the volume of deionized water is 1:8;

[0155] During the precipitation reaction, the amount of ferric chloride solution added was controlled to maintain the concentration of iron ions at 0.9 mol / L; and the amount of diammonium hydrogen phosphate solution added was controlled to maintain the concentration of phosphorus in the solution at 0.99 mol / L.

[0156] (4) Preparation of lithium iron phosphate material: The iron phosphate material, lithium hydroxide, glucose and ethanol are mixed and ground to obtain a slurry; the slurry is dried and ground, and then calcined at 750°C for 20 hours to obtain a lithium iron phosphate material; the molar ratio of iron element to lithium element in the slurry is 1:1.03, the mass ratio of glucose to iron phosphate material is 0.05:1, and the solid content of the slurry is 35wt%.

[0157] Example 6:

[0158] The only difference between this embodiment and embodiment 1 is that:

[0159] In step (3) of preparing the ferric phosphate material, the following sentence is replaced with “dispersing the modified ferric phosphate seed crystals and the small-particle ferric phosphate seed crystals with a particle size of 200-300 nm in deionized water at a volume ratio of 1.2:1”: “dispersing the modified ferric phosphate seed crystals and the small-particle ferric phosphate seed crystals with a particle size of 200-300 nm in deionized water at a volume ratio of 1.5:1”;

[0160] Replace "precipitation reaction at a rotation speed of 650 rpm for 6 h" with "precipitation reaction at a rotation speed of 400 rpm for 6 h";

[0161] Replace “During the precipitation reaction, the amount of diammonium hydrogen phosphate solution added was controlled to maintain the concentration of phosphorus in the solution at 1.1 mol / L” with “During the precipitation reaction, the amount of diammonium hydrogen phosphate solution added was controlled to maintain the concentration of phosphorus in the solution at 1.5 mol / L”.

[0162] Example 7:

[0163] The only difference between this embodiment and embodiment 1 is that:

[0164] In step (3) of preparing the ferric phosphate material, the following sentence is replaced with “dispersing the modified ferric phosphate seed crystals and the small-particle ferric phosphate seed crystals with a particle size of 200-300 nm in deionized water at a volume ratio of 1.2:1”: “dispersing the modified ferric phosphate seed crystals and the small-particle ferric phosphate seed crystals with a particle size of 200-300 nm in deionized water at a volume ratio of 0.9:1”;

[0165] Replace “precipitation reaction at a rotation speed of 650 rpm for 6 h” with “precipitation reaction at a rotation speed of 750 rpm for 6 h”;

[0166] Replace “During the precipitation reaction, the amount of diammonium hydrogen phosphate solution added was controlled to maintain the concentration of phosphorus in the solution at 1.1 mol / L” with “During the precipitation reaction, the amount of diammonium hydrogen phosphate solution added was controlled to maintain the concentration of phosphorus in the solution at 1.3 mol / L”.

[0167] Example 8:

[0168] The only difference between this embodiment and embodiment 1 is that:

[0169] In the process of preparing large-particle iron phosphate seeds partially coated with polydopamine in step (1): "Using ultrasonic treatment, large-particle iron phosphate seeds with a particle size of 800-900 nm and sodium dodecylbenzenesulfonate are dispersed in a Tris-HCl buffer solution with a dopamine hydrochloride concentration of 0.2 mg / mL at a mass ratio of 25:1, and after continuous stirring at a speed of 750 rpm for 24 hours, the large-particle iron phosphate seeds partially coated with polydopamine are obtained by filtering, washing, and drying; sodium dodecylbenzenesulfonate is dispersed in a buffer solution with a particle size of 800-900 nm and a mass ratio of 25:1, and the large-particle iron phosphate seeds are obtained by filtering, washing, and drying; The phrase "large-particle iron phosphate seeds with a particle size of 1300-1400 nm and sodium dodecylbenzenesulfonate are dispersed in a Tris-HCl buffer solution with a dopamine hydrochloride concentration of 0.2 mg / mL using ultrasonic treatment, and the mixture is stirred at 750 rpm for 24 hours, filtered, washed, and dried to obtain large-particle iron phosphate seeds partially coated with polydopamine; the concentration of sodium dodecylbenzenesulfonate in the buffer solution is 0.9 g / L" is replaced with "large-particle iron phosphate seeds with a particle size of 1300-1400 nm and sodium dodecylbenzenesulfonate in a mass ratio of 25:1 are dispersed in a Tris-HCl buffer solution with a dopamine hydrochloride concentration of 0.2 mg / mL using ultrasonic treatment, and the mixture is stirred at 750 rpm for 24 hours, and the mixture is filtered, washed, and dried to obtain large-particle iron phosphate seeds partially coated with polydopamine; the concentration of sodium dodecylbenzenesulfonate in the buffer solution is 0.9 g / L";

[0170] In the process of preparing the iron phosphate material in step (3): replace “precipitation reaction at a rotation speed of 650 rpm for 6 hours, and then transferred to a phosphoric acid solution for aging for 4 hours” with “precipitation reaction at a rotation speed of 500 rpm for 6 hours, and then transferred to a phosphoric acid solution for aging for 7 hours”;

[0171] Replace “During the precipitation reaction, the amount of diammonium hydrogen phosphate solution added was controlled to maintain the concentration of phosphorus in the solution at 1.1 mol / L” with “During the precipitation reaction, the amount of diammonium hydrogen phosphate solution added was controlled to maintain the concentration of phosphorus in the solution at 1.5 mol / L”.

[0172] Example 9:

[0173] The only difference between this embodiment and embodiment 1 is that:

[0174] Replace “step (3) preparing the iron phosphate material” with “dispersing the modified iron phosphate seed crystals and the small-particle iron phosphate seed crystals with a particle size of 500-600 nm in deionized water at a volume ratio of 1.2:1, then adding ferric chloride solution and diammonium hydrogen phosphate solution, and carrying out precipitation reaction at a rotation speed of 700 rpm for 6 hours, then transferring to a phosphoric acid solution for aging for 4 hours, filtering, washing, drying, and calcining to obtain the iron phosphate material;

[0175] The volume ratio of the total volume of the modified ferric phosphate seed crystals and the small-particle ferric phosphate seed crystals with a particle size of 500-600 nm to the volume of deionized water is 1:8;

[0176] During the precipitation reaction, the amount of ferric chloride solution added was controlled to maintain the iron ion concentration at 1 mol / L; the amount of diammonium hydrogen phosphate solution added was controlled to maintain the phosphorus concentration in the solution at 1.3 mol / L.

[0177] Example 10:

[0178] The only difference between this embodiment and embodiment 1 is that:

[0179] In step (1) of preparing large-particle iron phosphate seed crystals partially coated with polydopamine: replace "sodium dodecylbenzenesulfonate" with "sodium fatty alcohol ether sulfate";

[0180] Replace the process of “step (3) preparing the iron phosphate material” with “(3) preparing the iron phosphate material: dispersing the modified iron phosphate seed crystals and the small-particle iron phosphate seed crystals with a particle size of 200-300 nm in deionized water at a volume ratio of 1.4:1, then adding ferric chloride solution and diammonium hydrogen phosphate solution, and carrying out precipitation reaction at a rotation speed of 750 rpm for 6 hours, then transferring to a phosphoric acid solution for aging for 6 hours, filtering, washing, drying, and calcining to obtain the iron phosphate material;

[0181] The volume ratio of the total volume of the modified ferric phosphate seed crystals and the small-particle ferric phosphate seed crystals with a particle size of 200-300 nm to the volume of deionized water is 1:12;

[0182] During the precipitation reaction, the amount of ferric chloride solution added was controlled to maintain the iron ion concentration at 1.2 mol / L; the amount of diammonium hydrogen phosphate solution added was controlled to maintain the phosphorus concentration in the solution at 1.68 mol / L.

[0183] Example 11:

[0184] The only difference between this embodiment and embodiment 1 is that:

[0185] In step (1) of preparing large-particle iron phosphate seed crystals partially coated with polydopamine: replace "sodium dodecylbenzenesulfonate" with "sodium fatty alcohol ether sulfate";

[0186] Replace the process of “step (3) preparing the iron phosphate material” with “(3) preparing the iron phosphate material: dispersing the modified iron phosphate seed crystals and the small-particle iron phosphate seed crystals with a particle size of 200-300 nm in deionized water at a volume ratio of 1.1:1, then adding ferric chloride solution and diammonium hydrogen phosphate solution, and carrying out precipitation reaction at a rotation speed of 600 rpm for 6 hours, then transferring to a phosphoric acid solution for aging for 5 hours, filtering, washing, drying, and roasting to obtain the iron phosphate material;

[0187] The volume ratio of the total volume of the modified ferric phosphate seed crystals and the small-particle ferric phosphate seed crystals with a particle size of 200-300 nm to the volume of deionized water is 1:7;

[0188] During the precipitation reaction, the amount of ferric chloride solution added was controlled to maintain the iron ion concentration at 0.8 mol / L; the amount of diammonium hydrogen phosphate solution added was controlled to maintain the phosphorus concentration in the solution at 0.88 mol / L.

[0189] Example 12:

[0190] (1) Preparation of large-particle iron phosphate seeds partially coated with polydopamine: Large-particle iron phosphate seeds with a particle size of 1200-1300 nm and sodium dodecylbenzenesulfonate were dispersed in a Tris-HCl buffer solution with a dopamine hydrochloride concentration of 0.12 mg / mL by ultrasonic treatment. After continuous stirring at 650 rpm for 24 h, the mixture was filtered, washed, and dried to obtain large-particle iron phosphate seeds partially coated with polydopamine; the concentration of sodium dodecylbenzenesulfonate in the buffer solution was 2.6 g / L, and the pH value of the Tris-HCl buffer solution was 8.0; and drying was performed at 80°C for 6 h.

[0191] (2) Preparation of modified iron phosphate seed crystals: The large-particle iron phosphate seed crystals partially coated with polydopamine were vacuum dehydrated at 185°C for 1.5 h to obtain modified iron phosphate seed crystals.

[0192] (3) Preparation of iron phosphate material: modified iron phosphate seed crystals and small-particle iron phosphate seed crystals with a particle size of 400-500 nm were dispersed in deionized water at a volume ratio of 1:1, and then ferric chloride solution and diammonium hydrogen phosphate solution were added. The mixture was precipitated at a rotation speed of 400 rpm for 6 h, and then transferred to a phosphoric acid solution for aging for 4 h. After filtration, washing, drying, and calcination, the iron phosphate material was obtained.

[0193] The volume ratio of the total volume of the modified ferric phosphate seed crystals and the small-particle ferric phosphate seed crystals with a particle size of 400-500 nm to the volume of deionized water is 1:6;

[0194] During the precipitation reaction, the amount of ferric chloride solution added was controlled to maintain the concentration of iron ions at 0.6 mol / L; during the precipitation reaction, the amount of diammonium hydrogen phosphate solution added was controlled to maintain the concentration of phosphorus in the solution at 0.66 mol / L.

[0195] (4) Preparation of lithium iron phosphate material: The iron phosphate material, lithium hydroxide, glucose and ethanol are mixed and ground to obtain a slurry; the slurry is dried and ground, and then calcined at 750°C for 20 hours to obtain a lithium iron phosphate material; the molar ratio of iron element to lithium element in the slurry is 1:1.01, the mass ratio of glucose to iron phosphate material is 0.05:1, and the solid content of the slurry is 35wt%.

[0196] Example 13:

[0197] (1) Preparation of large-particle iron phosphate seeds partially coated with polydopamine: Using ultrasonic treatment, large-particle iron phosphate seeds with a particle size of 800-900 nm and sodium dodecylbenzenesulfonate were dispersed in a Tris-HCl buffer solution with a dopamine hydrochloride concentration of 0.2 mg / mL at a mass ratio of 35:1. After continuous stirring at a speed of 500 rpm for 24 hours, the mixture was filtered, washed, and dried to obtain large-particle iron phosphate seeds partially coated with polydopamine; the concentration of sodium dodecylbenzenesulfonate in the buffer solution was 2.6 g / L, and the pH value of the Tris-HCl buffer solution was 8.5; and drying was performed at 80°C for 6 hours.

[0198] (2) Preparation of modified iron phosphate seed crystals: The large-particle iron phosphate seed crystals partially coated with polydopamine were vacuum dehydrated at 185°C for 1.5 h to obtain modified iron phosphate seed crystals.

[0199] (3) Preparation of iron phosphate material: modified iron phosphate seeds and small-particle iron phosphate seeds with a particle size of 200-300 nm were dispersed in deionized water at a volume ratio of 0.9:1, and then ferric chloride solution and diammonium hydrogen phosphate solution were added. The mixture was precipitated at a rotation speed of 750 rpm for 6 h, and then transferred to a phosphoric acid solution for aging for 4 h. After filtration, washing, drying, and roasting, the iron phosphate material was obtained;

[0200] The volume ratio of the total volume of the modified ferric phosphate seed crystals and the small-particle ferric phosphate seed crystals with a particle size of 200-300 nm to the volume of deionized water is 1:12;

[0201] During the precipitation reaction, the amount of ferric chloride solution added was controlled to maintain the concentration of iron ions at 1.2 mol / L; during the precipitation reaction, the amount of diammonium hydrogen phosphate solution added was controlled to maintain the concentration of phosphorus in the solution at 1.56 mol / L.

[0202] (4) Preparation of lithium iron phosphate material: The iron phosphate material, lithium hydroxide, glucose and ethanol are mixed and ground to obtain a slurry; the slurry is dried and ground, and then calcined at 750°C for 20 hours to obtain a lithium iron phosphate material; the molar ratio of iron element to lithium element in the slurry is 1:1.01, the mass ratio of glucose to iron phosphate material is 0.05:1, and the solid content of the slurry is 35wt%.

[0203] Example 14:

[0204] (1) Preparation of large-particle iron phosphate seeds partially coated with polydopamine: Large-particle iron phosphate seeds with a particle size of 500-600 nm and sodium dodecylbenzenesulfonate were dispersed in a Tris-HCl buffer solution with a dopamine hydrochloride concentration of 0.1 mg / mL using ultrasonic treatment. After continuous stirring at 750 rpm for 24 h, the mixture was filtered, washed, and dried to obtain large-particle iron phosphate seeds partially coated with polydopamine; the concentration of sodium dodecylbenzenesulfonate in the buffer solution was 2.0 g / L, and the pH value of the Tris-HCl buffer solution was 8.5; and drying was performed at 80°C for 6 h.

[0205] (2) Preparation of modified iron phosphate seed crystals: The large-particle iron phosphate seed crystals partially coated with polydopamine were vacuum dehydrated at 185°C for 1.5 h to obtain modified iron phosphate seed crystals.

[0206] (3) Preparation of iron phosphate material: modified iron phosphate seeds and small-particle iron phosphate seeds with a particle size of 200-300 nm were dispersed in deionized water at a volume ratio of 1.5:1, and then ferric chloride solution and diammonium hydrogen phosphate solution were added. The mixture was precipitated at a rotation speed of 500 rpm for 6 h, and then transferred to a phosphoric acid solution for aging for 4 h. After filtration, washing, drying, and calcination, the iron phosphate material was obtained;

[0207] The volume ratio of the total volume of the modified ferric phosphate seed crystals and the small-particle ferric phosphate seed crystals with a particle size of 200-300 nm to the volume of deionized water is 1:5;

[0208] During the precipitation reaction, the amount of ferric chloride solution added was controlled to maintain the concentration of iron ions at 0.7 mol / L; during the precipitation reaction, the amount of diammonium hydrogen phosphate solution added was controlled to maintain the concentration of phosphorus in the solution at 0.77 mol / L.

[0209] (4) Preparation of lithium iron phosphate material: The iron phosphate material, lithium hydroxide, glucose and ethanol are mixed and ground to obtain a slurry; the slurry is dried and ground, and then calcined at 750°C for 20 hours to obtain a lithium iron phosphate material; the molar ratio of iron element to lithium element in the slurry is 1:1.02, the mass ratio of glucose to iron phosphate material is 0.05:1, and the solid content of the slurry is 35wt%.

[0210] Example 15:

[0211] (1) Preparation of large-particle iron phosphate seeds partially coated with polydopamine: Large-particle iron phosphate seeds with a particle size of 1100-1200 nm and sodium dodecylbenzenesulfonate were dispersed in a Tris-HCl buffer solution with a dopamine hydrochloride concentration of 0.2 mg / mL by ultrasonic treatment. After continuous stirring at 600 rpm for 24 h, the mixture was filtered, washed, and dried to obtain large-particle iron phosphate seeds partially coated with polydopamine; the concentration of sodium dodecylbenzenesulfonate in the buffer solution was 1.2 g / L, and the pH value of the Tris-HCl buffer solution was 8.5; and drying was performed at 80°C for 6 h.

[0212] (2) Preparation of modified iron phosphate seed crystals: The large-particle iron phosphate seed crystals partially coated with polydopamine were vacuum dehydrated at 185°C for 1.5 h to obtain modified iron phosphate seed crystals.

[0213] (3) Preparation of iron phosphate material: modified iron phosphate seed crystals and small-particle iron phosphate seed crystals with a particle size of 500-600 nm were dispersed in deionized water at a volume ratio of 1.4:1, and then ferric chloride solution and diammonium hydrogen phosphate solution were added. The mixture was precipitated at a speed of 600 rpm for 6 h, and then transferred to a phosphoric acid solution for aging for 6 h. After filtration, washing, drying, and roasting, the iron phosphate material was obtained;

[0214] The volume ratio of the total volume of the modified ferric phosphate seed crystals and the small-particle ferric phosphate seed crystals with a particle size of 500-600 nm to the volume of deionized water is 1:7;

[0215] During the precipitation reaction, the amount of ferric chloride solution added was controlled to maintain the concentration of iron ions at 0.8 mol / L; and the amount of diammonium hydrogen phosphate solution added was controlled to maintain the concentration of phosphorus in the solution at 1.12 mol / L.

[0216] (4) Preparation of lithium iron phosphate material: The iron phosphate material, lithium hydroxide, glucose and ethanol are mixed and ground to obtain a slurry; the slurry is dried and ground, and then calcined at 750°C for 20 hours to obtain a lithium iron phosphate material; the molar ratio of iron element to lithium element in the slurry is 1:1.01, the mass ratio of glucose to iron phosphate material is 0.05:1, and the solid content of the slurry is 35wt%.

[0217] Comparative Example 1:

[0218] The only difference between this comparative example and Example 1 is that:

[0219] In step (1) of preparing large-particle iron phosphate seed crystals partially coated with polydopamine: replace "the concentration of dopamine hydrochloride is 0.2 mg / mL" with "the concentration of dopamine hydrochloride is 0.4 mg / mL";

[0220] Replace “after continuous stirring at a speed of 750 rpm for 24 hours” with “after continuous stirring at a speed of 500 rpm for 24 hours”.

[0221] In step (2) of preparing modified iron phosphate seed crystals, the following is replaced with “dehydrating the large-particle iron phosphate seed crystals partially coated with polydopamine at 185° C. under vacuum for 1.5 h to obtain modified iron phosphate seed crystals”: ​​“dehydrating the large-particle iron phosphate seed crystals partially coated with polydopamine at 195° C. under vacuum for 2 h to obtain modified iron phosphate seed crystals”;

[0222] Replace the process of “step (3) preparing the iron phosphate material” with “(3) preparing the iron phosphate material: dispersing the modified iron phosphate seed crystals and the small-particle iron phosphate seed crystals with a particle size of 200-300 nm in deionized water at a volume ratio of 1.3:1, then adding ferric chloride solution and diammonium hydrogen phosphate solution, and carrying out precipitation reaction at a rotation speed of 400 rpm for 6 hours, then transferring to a phosphoric acid solution for aging for 3 hours, filtering, washing, drying, and calcining to obtain the iron phosphate material;

[0223] The volume ratio of the total volume of the modified ferric phosphate seed crystals and the small-particle ferric phosphate seed crystals with a particle size of 200-300 nm to the volume of deionized water is 1:8;

[0224] During the precipitation reaction, the amount of ferric chloride solution added was controlled to maintain the iron ion concentration at 1 mol / L; the amount of diammonium hydrogen phosphate solution added was controlled to maintain the phosphorus concentration in the solution at 1.1 mol / L.

[0225] Comparative Example 2:

[0226] (1) Preparation of modified iron phosphate seed crystals: Large-particle iron phosphate seed crystals with a particle size of 800-900 nm were dried at 80°C for 5 h, and then vacuum-dehydrated at 150°C for 1 h to obtain modified iron phosphate seed crystals.

[0227] (2) Preparation of iron phosphate material: modified iron phosphate seeds and small-particle iron phosphate seeds with a particle size of 200-300 nm were dispersed in deionized water at a volume ratio of 1.2:1, and then ferric chloride solution and diammonium hydrogen phosphate solution were added. The mixture was precipitated at a rotation speed of 650 rpm for 6 h, and then transferred to a phosphoric acid solution for aging for 4 h. After filtration, washing, drying, and roasting, the iron phosphate material was obtained;

[0228] The volume ratio of the total volume of the modified ferric phosphate seeds and the small-particle ferric phosphate seeds to deionized water is 1:8; during the precipitation reaction, the amount of ferric chloride solution added is controlled to maintain the concentration of iron ions in the reaction solution at 1 mol / L; during the precipitation reaction, the amount of diammonium hydrogen phosphate solution added is controlled to maintain the concentration of phosphorus in the reaction solution at 1.1 mol / L; and ammonia water is used to control the pH of the reaction solution to 1.5.

[0229] (3) Preparation of lithium iron phosphate material: The iron phosphate material, lithium hydroxide, glucose and ethanol are mixed and ground to obtain a slurry; the slurry is dried and ground, and then calcined at 750°C for 20 hours to obtain a lithium iron phosphate material; the molar ratio of iron element to lithium element in the slurry is 1:1.03, the mass ratio of glucose to iron phosphate material is 0.05:1, and the solid content of the slurry is 35wt%.

[0230] Comparative Example 3:

[0231] The difference between this comparative example and comparative example 2 is only that:

[0232] In the process of preparing modified ferric phosphate seed crystals in step (1), “further dehydrated in vacuum at 150° C. for 1 h” is replaced with “further dehydrated in vacuum at 200° C. for 1 h”.

[0233] Comparative Example 4:

[0234] (1) Preparation of iron phosphate material: large-particle iron phosphate seeds with a particle size of 800-900 nm and small-particle iron phosphate seeds with a particle size of 200-300 nm were dispersed in deionized water at a volume ratio of 1.2:1, and then ferric chloride solution and diammonium hydrogen phosphate solution were added. The mixture was precipitated at a rotation speed of 650 rpm for 6 h, and then transferred to a phosphoric acid solution for aging for 4 h. After filtering, washing, drying, and calcining, the iron phosphate material was obtained.

[0235] The volume ratio of the total volume of the large-particle ferric phosphate seed crystals and the small-particle ferric phosphate seed crystals to deionized water is 1:8; during the precipitation reaction, the amount of the added ferric chloride solution is controlled to maintain the concentration of iron ions in the reaction solution at 1 mol / L; during the precipitation reaction, the amount of the added diammonium hydrogen phosphate solution is controlled to maintain the concentration of phosphorus in the reaction solution at 1.1 mol / L; and ammonia water is used to control the pH of the reaction solution to 1.5.

[0236] (2) Preparation of lithium iron phosphate material: The iron phosphate material, lithium hydroxide, glucose and ethanol are mixed and ground to obtain a slurry; the slurry is dried and ground, and then calcined at 750°C for 20 hours to obtain a lithium iron phosphate material; the molar ratio of iron element to lithium element in the slurry is 1:1.03, the mass ratio of glucose to iron phosphate material is 0.05:1, and the solid content of the slurry is 35wt%.

[0237] Comparative Example 5:

[0238] The only difference between this comparative example and Example 1 is that:

[0239] In step (1) of preparing large-particle iron phosphate seed crystals partially coated with polydopamine: replace "the concentration of dopamine hydrochloride is 0.2 mg / mL" with "the concentration of dopamine hydrochloride is 0.4 mg / mL";

[0240] Replace “after stirring continuously at a rotation speed of 750 rpm for 24 hours,” with “after stirring continuously at a rotation speed of 400 rpm for 24 hours,”.

[0241] In the process of preparing modified ferric phosphate seed crystals in step (2), “dehydrating the large-particle ferric phosphate seed crystals partially coated with polydopamine in a vacuum at 185°C for 1.5 h” is replaced with “dehydrating the large-particle ferric phosphate seed crystals partially coated with polydopamine in a vacuum at 205°C for 1 h”.

[0242] The lithium iron phosphate materials prepared in the above examples and comparative examples were tested, and the specific items and methods are as follows:

[0243] 1. Particle Size Distribution: According to GB / T 19077-2016, the particle size distribution of the lithium iron phosphate materials prepared in each Example and Comparative Example was measured using a laser particle size analyzer. The test results for each Example and Comparative Example are shown in Table 1.

[0244] Figure 3 The particle size distribution diagram of the lithium iron phosphate material provided in Examples 4 and 5. It can be seen from the figure that the particle size distribution of the lithium iron phosphate material obtained in Example 5 is larger than that in Example 4. b and D f Smaller; V of Example 5 f Smaller, V b This indicates that the lithium iron phosphate material with target characteristics can be obtained by adjusting the preparation process parameters according to the method of the present invention.

[0245] 2. Morphology characterization: The lithium iron phosphate samples of each embodiment and comparative example were tested using a scanning electron microscope. Figure 1 The SEM image of the lithium iron phosphate material provided in Example 1 shows the coexistence of lithium iron phosphate materials with relatively good and poor circularity and larger particle size. Figure 2 The SEM image of the lithium iron phosphate material provided in Comparative Example 2 shows that the lithium iron phosphate material with large particle size has better sphericity.

[0246] 3. Circularity calculation method: Analyze SEM images using ImageJ. The specific steps are as follows:

[0247] 1) Obtaining an SEM image of an analyzable lithium iron phosphate material;

[0248] 2) Using the “magic wand” tool in ImageJ, the boundaries of the particles in the SEM image were identified and the projection surface of each particle in the SEM image was independently selected;

[0249] 3) The area, perimeter, and circularity C of the projected surface of each lithium iron phosphate particle in the SEM image were obtained by ImageJ analysis;

[0250] 4) Obtain D according to the particle size distribution curve f and D b , and calculate the area of ​​the corresponding standard circle respectively to determine the standard for identifying large-particle lithium iron phosphate and small-particle lithium iron phosphate; the formula for the area of ​​the standard circle is: S=1 / 4×π×d 2 , where d is D f or D b ; Large particle lithium iron phosphate has a projected area greater than or equal to S max =1 / 4×π×D b 2 The projected area of ​​lithium iron phosphate particles is less than or equal to S min =1 / 4×π×D f 2 lithium iron phosphate particles;

[0251] 5) screening the data obtained in step 3) based on the criteria for large-particle lithium iron phosphate and small-particle lithium iron phosphate;

[0252] 6) The data obtained by screening according to step 5) are calculated and the sample standard deviation of the circularity C of the large-particle lithium iron phosphate and the small-particle lithium iron phosphate is calculated.

[0253] The circularity calculation formula is: C = (4 × π × A) / P 2 , where A is the projected area of ​​the lithium iron phosphate particles and P is the perimeter of the projected surface; the formula for the sample standard deviation is: , when calculating the sample standard deviation, the number of samples N must be at least 30, where C i is the circularity C of the i-th lithium iron phosphate particle, where 0.15≤C≤1.00, is the average circularity of N lithium iron phosphate particles.

[0254] The test results of each embodiment and comparative example are shown in Table 1.

[0255] 4. Tap density test:

[0256] The lithium iron phosphate materials of the embodiments and comparative examples were tested using a tap density tester BT-302 according to GB / T 5162-2006. The test results of the embodiments and comparative examples are shown in Table 2.

[0257] 5. Specific surface area test:

[0258] According to GB / T 19587-2004, the specific surface area of ​​the lithium iron phosphate material of each embodiment and comparative example was measured using a physical adsorption instrument. The test results of each embodiment and comparative example are shown in Table 2.

[0259] 6. Electrochemical performance test:

[0260] Battery Assembly:

[0261] A uniform slurry of cathode material: acetylene black: PVDF (75:15:10 by mass) was prepared and evenly coated onto an aluminum foil substrate to serve as the battery's positive electrode. The simulated battery used a lithium sheet as the negative electrode, a polypropylene porous membrane as the separator, and an electrolyte consisting of 1 mol LiPF₆ dissolved in 1 L of a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 by volume). The positive electrode, negative electrode, electrolyte, and separator were assembled into a battery in an argon-protected glove box.

[0262] Steps for simulating battery rate testing:

[0263] First, charge the battery to 4.2V at a constant current, then discharge it to 2.0V at a rate current. The capacity released is the discharge capacity at that rate. After discharge, discharge it again at a constant current to 2.0V, and then test it at the next rate. The battery's discharge capacity at 0.1C, 1C, and 5C was tested, and the test results are shown in Table 2.

[0264] Table 1 Parameters of lithium iron phosphate materials

[0265]

[0266] Table 2 Physical and electrochemical properties of lithium iron phosphate materials

[0267]

[0268] It can be seen from Table 1-2 that the lithium iron phosphate materials of Examples 1-3 and Comparative Examples 1-5 have different σ b / σ f , as σ b / σ f As the tap density of the lithium iron phosphate material increases, the larger the BET is, the discharge capacity at 0.1C increases first and then decreases, and the capacity at 0.1C to 5C increases first and then decreases. Specifically, from Example 1, Example 2 and Comparative Example 2, as σ b / σ f The increase indicates that the morphological uniformity of large-particle lithium iron phosphate is reduced, which reduces the grading effect of lithium iron phosphate, so the tap density decreases. However, at the same time, the irregular large particles increase the BET, which increases the specific surface area of ​​the lithium iron phosphate material as a whole, which is beneficial to Li + The deintercalation of σ increases the specific capacity and rate. b / σ fFurther increase, due to the low morphology uniformity of large-particle lithium iron phosphate, the tap density further decreases; at the same time, these irregular large particles further increase BET, but the irregular large particles lead to increased internal resistance of the battery, reducing the specific capacity and the capacity retention rate from 0.1C to 5C. b / σ f Lithium iron phosphate materials in the range of ≤5.00 have good specific capacity and rate performance.

[0269] From the examples 1, 4-5, and 13-14, the In the preferred scope of the present invention, the Outside the preferred range of the present invention, it can be seen that The lithium iron phosphate material within the preferred range has a higher tap density and specific surface area, and therefore has a better specific capacity and a capacity retention rate of 0.1C to 5C.

[0270] The lithium iron phosphate materials of Examples 1, 6 and 7 are only V b / V f The difference is large, as V b / V f As the tap density of the lithium iron phosphate material increases, it first increases and then decreases, the BET gradually decreases, the discharge specific capacity at 0.1C first increases and then decreases, and the capacity retention at 0.1C to 5C first increases and then decreases. Specifically, from Example 1 and Example 7, V b / V f Increase, indicating that the particle size is D b The proportion of large-particle lithium iron phosphate increases relatively, or the particle size is D f The proportion of small-particle lithium iron phosphate is relatively reduced, so the tap density is increased and the specific surface area is reduced. However, due to the reduction in the proportion of small-particle lithium iron phosphate, small particles are accumulated, thereby reducing the tortuous diffusion path formed by the accumulation, which is beneficial to Li + The diffusion of V ultimately leads to an increase in specific capacity and capacity retention from 0.1C to 5C. b / V f As the density of lithium iron phosphate increases, the BET decreases, which reduces the contact area of ​​the lithium iron phosphate material. At the same time, large particles increase the transmission path of lithium ions, so the specific capacity and the capacity retention rate from 0.1C to 5C decrease.

[0271] The lithium iron phosphate materials of Examples 1, 8 and 9 are only D b / D f The difference is large, with D b / Df As the tap density increases, the BET increases first and then decreases, the specific capacity increases first and then decreases, and the capacity from 0.1C to 5C increases first and then decreases. Specifically, from Example 1 and Example 9, D b / D f As the density of lithium iron phosphate increases, the grading effect of lithium iron phosphate is better and the tap density increases, but the particle size of small particles of lithium iron phosphate decreases, which has a greater impact on the increase of lithium iron phosphate BET. Therefore, the overall BET of lithium iron phosphate material increases, and thus the specific capacity and the capacity retention rate from 0.1C to 5C increase. From Example 8 and Example 1, D b / D f However, the increase in the particle size of large-particle lithium iron phosphate has a greater impact on the reduction of lithium iron phosphate BET, so the overall BET of the lithium iron phosphate material is reduced, and thus the specific capacity and the capacity retention rate from 0.1C to 5C are reduced.

[0272] Increasing or decreasing span affects the tap density, BET, specific capacity, and capacity retention rate from 0.1C to 5C of lithium iron phosphate materials. However, the impact trend depends not only on the particle size of Dv10, Dv50, and Dv90, but also on their quantity. From Examples 10-11 of the present invention and Example 1, only the span differs significantly. As span increases, the tap density and BET first increase and then decrease. Consequently, the specific capacity and capacity retention rate from 0.1C to 5C also first increase and then decrease.

[0273] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A lithium iron phosphate material, characterized in that: The particle size distribution curve of the lithium iron phosphate material is a bimodal distribution, and the particle size distribution curve includes a first peak and a second peak in the positive direction of the horizontal coordinate, and the particle sizes corresponding to the first peak and the second peak are D f and D b , the unit is μm; the corresponding volume percentage is V f and V b ; The lithium iron phosphate material satisfies 1.10≤σ b / σ f ≤5.00, where σ b is the sample standard deviation of the circularity C of large-particle lithium iron phosphate, σ f is the sample standard deviation of the circularity C of small-particle lithium iron phosphate, the circularity C=(4×π×A) / P 2 , A is the projected area of ​​the lithium iron phosphate particle, and P is the perimeter of the projected area; The large-particle lithium iron phosphate has a projected area greater than or equal to S max =1 / 4×π×D b 2 The lithium iron phosphate particles, wherein the small particles of lithium iron phosphate have a projected area less than or equal to S min =1 / 4×π×D f 2 lithium iron phosphate particles; The lithium iron phosphate material satisfies 0.2≤V b / V f ≤2.5; The lithium iron phosphate material satisfies 2.5≤D b / D f ≤12; The lithium iron phosphate material meets the following requirements: ; Wherein, span satisfies 1.5≤span≤5, and the span=(Dv90-Dv10) / Dv50, Dv90, Dv50 and Dv10 are the particle sizes corresponding to when the cumulative volume percentage of the lithium iron phosphate material reaches 90%, 50% and 10%, respectively, and the unit is μm; The lithium iron phosphate material satisfies a tap density of 0.75 g / mL ≤ 1.28 g / mL; The lithium iron phosphate material satisfies 0.3 μm≤D f ≤1.0μm and 1.2μm≤D b ≤10μm; The lithium iron phosphate material satisfies 3%≤V f ≤10% and 3%≤V b ≤10%.

2. The lithium iron phosphate material according to claim 1, characterized in that The lithium iron phosphate material satisfies at least one of the following characteristics 1 and 2: Feature 1: The σ b Satisfy 0.05≤σ b ≤0.15; Feature 2: The σ f Satisfying 0.02≤σ f ≤0.

09.

3. A method for preparing the lithium iron phosphate material according to any one of claims 1-2, characterized in that: The preparation method comprises: coating large-particle iron phosphate seed crystals with polydopamine to obtain large-particle iron phosphate seed crystals partially coated with polydopamine; vacuum dehydrating the large-particle iron phosphate seed crystals partially coated with polydopamine to obtain modified iron phosphate seed crystals; mixing the modified iron phosphate seed crystals with small-particle iron phosphate seed crystals, adding iron salt and phosphorus source to carry out precipitation reaction in water to obtain iron phosphate material; and mixing the iron phosphate material, lithium source and carbon source and then roasting to obtain lithium iron phosphate material.

4. The preparation method according to claim 3, characterized in that The preparation method of the partially polydopamine-coated large-particle iron phosphate seed crystals comprises: mixing and dispersing the large-particle iron phosphate seed crystals and a surfactant in a Tris-HCl buffer solution containing dopamine hydrochloride, stirring for reaction, filtering, washing, and drying to obtain the partially polydopamine-coated large-particle iron phosphate seed crystals.

5. The preparation method according to claim 4, characterized in that The method for preparing the large-particle iron phosphate seed crystals partially coated with polydopamine includes at least one of the following features 1 to 6: Feature 1: The surfactant is an anionic surfactant, and the concentration of the surfactant in the Tris-HCl buffer solution is 0.1-3 g / L; Feature 2: The mass ratio of the large-particle iron phosphate seed crystals to the surfactant is 50:1-5; Feature 3: The particle size of the large-particle iron phosphate seed crystals is 400-1500 nm; Feature 4: The pH of the Tris-HCl buffer solution is 8-8.5; Feature 5: The concentration of the dopamine hydrochloride in the Tris-HCl buffer solution is 0.1-0.3 mg / mL; Feature 6: The stirring reaction is carried out at a stirring rate of 500-1000 rpm and for a time of 20-30 hours.

6. The preparation method according to claim 3, characterized in that The preparation method includes at least one of the following features A to G: Feature A: The vacuum dehydration temperature is 160-190°C and the time is 1-1.5 hours; Feature B: The particle size of the small-particle iron phosphate seed crystals is 200-600 nm; Feature C: The volume ratio of the modified ferric phosphate seed crystals to the small-particle ferric phosphate seed crystals is 0.9-1.5:1; Feature D: The volume ratio of the total volume of the modified ferric phosphate seed crystals and the small-particle ferric phosphate seed crystals to the water is 1:4-12; Feature E: The concentration of the iron salt in the water is 0.5-1.2 mol / L; Feature F: The molar ratio of the iron element of the iron salt to the phosphorus element of the phosphorus source is 1:1-1.5; Feature G: The stirring rate of the precipitation reaction is 350-750 rpm.

7. A positive electrode plate, characterized in that: The positive electrode plate comprises the lithium iron phosphate material according to any one of claims 1 to 2, or the lithium iron phosphate material obtained by the preparation method according to any one of claims 3 to 6.

8. A secondary battery, characterized in that: Including the positive electrode sheet according to claim 7.

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