Doping modified lithium iron phosphate material, preparation method thereof and lithium ion battery

Abstract

The application belongs to the technical field of lithium ion batteries, and particularly relates to a doped modified lithium iron phosphate material, a preparation method thereof and a lithium ion battery. The doped modified lithium iron phosphate material comprises: a doped lithium iron phosphate core; and a carbon coating layer coated on the surface of the doped lithium iron phosphate core. The doped lithium iron phosphate core is B and M co-doped lithium iron phosphate, wherein M represents a doped metal element, and is selected from at least one of Ti, Nb and Al, and B represents a boron element. The B element and other elements (at least one of Ti, Nb and Al) are co-doped into the lithium iron phosphate, so that the rate performance of the carbon-coated lithium iron phosphate material is improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion batteries, and particularly relates to a doped modified lithium iron phosphate material and a preparation method thereof and a lithium ion battery. BACKGROUND

[0002] With the development of clean energy vehicles and energy storage markets, terminal users will have higher requirements for charging, cost and full life cycle life, and fast-charging batteries will play a role in energy security and economy. Among various lithium ion battery positive materials, lithium iron phosphate has the advantages of long cycle life, low cost advantage and high safety performance, and occupies a certain advantage in the market. The electronic conductivity of lithium iron phosphate material itself is extremely poor (10 -9 S / cm), and Li ions are one-dimensional diffusion paths, which results in low ionic conductivity. The above two factors jointly determine that the lithium iron phosphate is limited in terms of large rate performance and fast charging capacity. In view of the low electronic conductivity and ionic conductivity of lithium iron phosphate, the main solution is to use carbon material to coat lithium iron phosphate, but the rate performance of the carbon-coated lithium iron phosphate material obtained is still poor. In addition, the patent with publication number CN111362246A discloses a boron-doped lithium iron phosphate material, and the rate performance of the material still needs to be further improved.

[0003] Therefore, it is of great significance to provide a modified lithium iron phosphate material with good rate performance. SUMMARY

[0004] In view of the above problems in the prior art, the purpose of the present application is to provide a doped modified lithium iron phosphate material and a preparation method thereof and a lithium ion battery. The present application improves the rate performance of the carbon-coated lithium iron phosphate material by co-doping B elements and other elements (at least one of Ti, Nb and Al) into lithium iron phosphate.

[0005] In order to achieve the above purpose, the present application adopts the following technical solutions:

[0006] In the first aspect, the present application provides a doped modified lithium iron phosphate material, which comprises:

[0007] a doped lithium iron phosphate core; and

[0008] a carbon coating layer coated on the surface of the doped lithium iron phosphate core.

[0009] The doped lithium iron phosphate core is B and M co-doped lithium iron phosphate, wherein M represents a doped metal element, and is selected from at least one of Ti, Nb and Al, and B represents a boron element.

[0010] The present application improves the rate performance of carbon-coated lithium iron phosphate material by co-doping B element and other elements (at least one of Ti, Nb, Al) into lithium iron phosphate.

[0011] In the above-mentioned doped modified lithium iron phosphate material, as a preferred embodiment, the chemical formula of the doped lithium iron phosphate core is: x M y FeP m B n O 4-δ wherein 0

[0012] Here, δ represents the number of oxygen defects in a single unit cell, 0≤δ<0.1.

[0013] In the above-mentioned doped modified lithium iron phosphate material, as a preferred embodiment, the chemical formula of the doped lithium iron phosphate core is: x M y FeP m B n O4(such as LiAl 0.02 FeP 0.97 B 0.03 O4, Li 0.96 Nb 0.02 FeP 0.97 B 0.03 O4, Li 0.98 Ti 0.02 FeP 0.97 B 0.03 O4, Li 0.94 Al 0.04 FeP 0.97 B 0.03 O4or Li 0.76 Nb 0.06 FeP 0.97 B 0.03 O4, etc.), wherein 0.7

[0014] In the above-mentioned doped modified lithium iron phosphate material, as a preferred embodiment, the valence of Ti is +4, the valence of Nb is +5, the valence of Al is +3, and the valence of B is +3.

[0015] As a preferred embodiment of the above-mentioned doped modified lithium iron phosphate material, 0.02≤y≤0.04, 0.02≤n≤0.04. By limiting 0.02≤y≤0.04, 0.02≤n≤0.04, the B and M have a relatively better synergistic effect, so that the low rate (for example, 0.1C rate and 1C rate) performance of the carbon-coated lithium iron phosphate material can be significantly improved, thereby significantly improving the discharge specific capacity at low rate.

[0016] In a second aspect, the present application provides a preparation method of the doped modified lithium iron phosphate material according to the first aspect, comprising the following steps:

[0017] S1, mixing a lithium source, an iron source, a phosphorus source, a doping metal source and a boron source in a solvent, and then drying and calcining to obtain a precursor;

[0018] S2, adding a carbon source to the precursor, and then mixing in a solvent, and drying to obtain a mixture;

[0019] S3, calcining the mixture to obtain the doped modified lithium iron phosphate material.

[0020] The preparation method provided by the present application is simple to operate and suitable for mass production.

[0021] As a preferred embodiment of the above-mentioned preparation method of the doped modified lithium iron phosphate material, the lithium source includes at least one of lithium hydroxide, lithium acetate and lithium carbonate.

[0022] As a preferred embodiment of the above-mentioned preparation method of the doped modified lithium iron phosphate material, the iron source includes at least one of ferrous oxalate, ferrous acetate, FeSO4 and organic ferrous salt.

[0023] As a preferred embodiment of the above-mentioned preparation method of the doped modified lithium iron phosphate material, the phosphorus source includes at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid and lithium dihydrogen phosphate.

[0024] As a preferred embodiment of the above-mentioned preparation method of the doped modified lithium iron phosphate material, the doping metal source includes at least one of an aluminum source, a niobium (Nb) source and a titanium (Ti) source.

[0025] As a preferred embodiment of the above-mentioned preparation method of the doped modified lithium iron phosphate material, the aluminum source includes at least one of Al2O3 and Al(NO3)3·9H2O.

[0026] As a preferred embodiment of the above-mentioned preparation method of the doped modified lithium iron phosphate material, the niobium (Nb) source includes at least one of Nb2O5 and Nb(NO3)5.

[0027] In the preparation method of the doped modified lithium iron phosphate material, as a preferred embodiment, the titanium (Ti) source includes at least one of Ti(SO4)2 and TiO2.

[0028] In the preparation method of the doped modified lithium iron phosphate material, as a preferred embodiment, the boron source includes H3BO3.

[0029] In the preparation method of the doped modified lithium iron phosphate material, as a preferred embodiment, the solvent includes at least one of ethanol and ethylene glycol.

[0030] In the preparation method of the doped modified lithium iron phosphate material, as a preferred embodiment, in step S1, the atmosphere of the calcination is an inert atmosphere, for example, N2 atmosphere, the temperature of the calcination is 200-300°C (for example, 200°C, 220°C, 240°C, 260°C, 280°C or 300°C, etc.), and the time of the calcination is 4-6h.

[0031] In the preparation method of the doped modified lithium iron phosphate material, as a preferred embodiment, in step S1, the mixing method is ball milling, and the time of the mixing is 4-8h (for example, 4h, 6h or 8h, etc.).

[0032] In the preparation method of the doped modified lithium iron phosphate material, as a preferred embodiment, the carbon source includes at least one of citric acid, glucose and sucrose.

[0033] In the preparation method of the doped modified lithium iron phosphate material, as a preferred embodiment, in step S2, the mass ratio of the carbon source to the precursor is (2-10):100, for example, 2:100, 4:100, 6:100, 8:100 or 10:100, etc.

[0034] In the preparation method of the doped modified lithium iron phosphate material, as a preferred embodiment, in step S2, the mixing method is ball milling, and the time of the mixing is 4-8h (for example, 4h, 6h or 8h, etc.).

[0035] In the preparation method of the doped modified lithium iron phosphate material, as a preferred embodiment, in step S2, the temperature of the drying is 80-90°C, for example, 80°C, 82°C, 85°C or 90°C, etc.

[0036] In the preparation method of the doped modified lithium iron phosphate material, as a preferred embodiment, in step S3, the atmosphere of the calcination is an inert atmosphere, for example, N2 atmosphere.

[0037] In the preparation method of the doped modified lithium iron phosphate material, as a preferred embodiment, in step S3, the calcination treatment comprises first calcining at 350-500℃ (for example, it can be 350℃, 400℃, 450℃ or 500℃, etc.) for 5-8h (for example, it can be 6h, 7h, 8h or 9h, etc.), and then calcining at 600-800℃ (for example, it can be 600℃, 650℃, 700℃, 750℃ or 800℃, etc.) for 10-14h (for example, it can be 10h, 13h or 15h, etc.). When the sintering temperature is too low, the solid phase reaction is not complete, impurities Fe 3+ are contained in the product, the degree of crystallization is not high, small particles are easily agglomerated together, the specific discharge capacity of the material is low, and the cycle performance is poor; when the sintering temperature is too high, secondary particles are generated in the material, agglomeration occurs, the specific surface area is small, the grain is easily grown with the increase of the temperature, the lithium ion diffusion path is increased, and the performance of the material is reduced.

[0038] In a third aspect, the present application provides a lithium ion battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, the active material of the positive electrode sheet comprising the doped modified lithium iron phosphate material provided in the first aspect or the doped modified lithium iron phosphate material prepared by the preparation method of the doped modified lithium iron phosphate material provided in the second aspect.

[0039] Compared with the prior art, the present application has at least one of the following beneficial effects:

[0040] (1) The present application improves the rate performance of the carbon-coated lithium iron phosphate material by co-doping B element and other elements (at least one of Ti, Nb and Al) into lithium iron phosphate, replacing part of P element or part of P element and part of Li element.

[0041] (2) Compared with P atoms, B atoms have a smaller radius, and a small amount of B doping into LiFePO4 will cause the unit cell volume to shrink, thereby shortening the lithium ion diffusion distance, and B replacing P can enhance the electronic conduction ability between FeO6 octahedral layers, thereby helping to improve the electrochemical performance.

[0042] (3) The incorporation of Ti, Nb, Al and other atoms not only can improve the lithium vacancy concentration in the unit cell and improve the lithium ion transmission rate, but also can balance the charge and reduce the generation of oxygen defects after B doping, further improving the lithium ion transmission rate. BRIEF DESCRIPTION OF DRAWINGS

[0043] Figure 1 XRD pattern of the lithium iron phosphate material provided for the present application comparative example 1;

[0044] Figure 2The impedance test results of the batteries assembled from the lithium iron phosphate materials provided by Example 3 and Comparative Example 1 are compared in the following table. DETAILED DESCRIPTION

[0045] For a further understanding of the present application, preferred embodiments thereof will be described in conjunction with examples, the scope of protection of the present application including but not limited to the following examples. The following examples are only used to illustrate the advantages and effects of the technical scheme of the present application, and do not constitute a limitation on the scope of protection of the present application. Equivalent replacements made by those skilled in the art based on the present application are within the scope of protection of the present application.

[0046] Unless otherwise defined, the technical terms used in the following examples have the same meanings as generally understood by those skilled in the art to which the present application belongs. The experimental reagents used in the following examples, unless otherwise specified, are conventional biochemical reagents; the amounts of the experimental reagents used, unless otherwise specified, are the amounts of reagents used in conventional experimental operations; and the experimental methods, unless otherwise specified, are conventional methods.

[0047] In a first aspect, the embodiments of the present application provide a doped modified lithium iron phosphate material, which comprises: a doped lithium iron phosphate core; and a carbon coating layer coated on the surface of the doped lithium iron phosphate core; the doped lithium iron phosphate core is a B and M co-doped lithium iron phosphate, and the chemical formula is: Li x M y FeP m B n O 4-δ wherein M represents a doped metal element selected from at least one of Ti, Nb and Al, B represents a boron element, 0 x M y FeP m B n 04, wherein 0.7

[0048] In view of the deficiencies of the prior art, the embodiment of the present application provides a high-rate lithium iron phosphate material, the conductivity of the material is improved by co-doping, B doping P sites, Ti, Nb, Al and the like doping Li sites, thereby improving the rate performance of the battery. Ti, Nb and Al can not only be doped into Li sites to generate more Li vacancies and promote the transmission rate of lithium ions, but also balance the charge in the crystal and reduce the generation of oxygen defects after B doping, thereby further improving the transmission rate of lithium ions in the lithium iron phosphate material and the battery capacity, and the doping of Ti, Nb and Al can also effectively reduce the charge transfer resistance of the material and increase the diffusion coefficient of lithium ions.

[0049] In a second aspect, the embodiment of the present application provides a preparation method of the doped modified lithium iron phosphate material according to the first aspect, comprising the following steps:

[0050] S1, mixing a lithium source, an iron source, a phosphorus source, a doping metal source and a boron source in a solvent, and then drying and calcining to obtain a precursor, wherein the lithium source includes at least one of lithium hydroxide, lithium acetate and lithium carbonate, the iron source includes at least one of ferrous oxalate, ferrous acetate, FeSO4 and organic ferrous salt, the phosphorus source includes at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid and lithium dihydrogen phosphate, the doping metal source includes at least one of an aluminum source, a niobium (Nb) source and a titanium (Ti) source, the aluminum source includes at least one of Al2O3 and Al(NO3)3·9H2O, the niobium (Nb) source includes at least one of Nb2O5 and Nb(NO3)5, the titanium (Ti) source includes at least one of Ti(SO4)2 and TiO2, the boron source includes H3BO3, the solvent includes at least one of ethanol and ethylene glycol, the calcination atmosphere is an inert atmosphere, for example, N2 atmosphere, the calcination temperature is 200-300 DEG C, the calcination time is 4-6h, and the mixing mode is ball milling mixing, and the mixing time is 4-8h.

[0051] S2, adding a carbon source to the precursor, and then mixing in a solvent to obtain a mixture after drying, wherein the carbon source includes at least one of citric acid, glucose and sucrose, the mass ratio of the carbon source to the precursor is (2-10):100, the mixing mode is ball milling mixing, the mixing time is 4-8h, and the drying temperature is 80-90 DEG C.

[0052] S3, performing calcination treatment on the mixture to obtain the doped modified lithium iron phosphate material, wherein the calcination atmosphere is an inert atmosphere, for example, N2 atmosphere, and the calcination treatment includes first calcining at 350-500 DEG C for 5-8h, and then calcining at 600-800 DEG C for 10-14h.

[0053] Thirdly, the present invention provides a lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the active material of the positive electrode comprises a doped and modified lithium iron phosphate material provided in the first aspect or a doped and modified lithium iron phosphate material prepared by the preparation method of the doped and modified lithium iron phosphate material provided in the second aspect.

[0054] To further understand the present invention, the following detailed description of the doped and modified lithium iron phosphate material, its preparation method, and lithium-ion battery provided by the present invention is provided in conjunction with embodiments. The scope of protection of the present invention is not limited by the following embodiments.

[0055] Example 1

[0056] The doped and modified lithium iron phosphate material provided in this embodiment includes: a doped lithium iron phosphate core with the chemical formula LiAl. 0.02 FeP 0.97 B 0.03 O4; and a carbon coating layer, which coats the surface of the doped lithium iron phosphate core.

[0057] The method for preparing doped and modified lithium iron phosphate material provided in this embodiment includes the following steps:

[0058] S1. First, according to the molar ratio of Li, Al, Fe, P, and B elements in the chemical formula of the doped lithium iron phosphate core provided in this embodiment (1:0.02:1:0.97:0.03), weigh Li2CO3, Al2O3, FeSO4, NH4H2PO4, and H3BO3, mix them to obtain a mixture, add it to anhydrous ethanol, and wet ball mill for 6 hours. Then, dry it in an oven at 80°C under N2 atmosphere until the anhydrous ethanol is completely evaporated. Grind it thoroughly again to obtain a dry mixture. Calcinate the dry mixture at 200°C under N2 atmosphere for 5 hours, and then cool it to room temperature to obtain the precursor.

[0059] S2. Add citric acid (the mass ratio of citric acid to precursor is 2:100) to the precursor prepared in step S1, then ball mill in anhydrous ethanol for 8 hours, and then dry in an oven at 80°C under N2 atmosphere until the anhydrous ethanol is completely evaporated. Grind thoroughly again to obtain a dry mixture.

[0060] S3. Place the dried mixture prepared in step S2 into a tube furnace filled with purified N2 gas, first calcine it at 350°C for 8 hours, then calcine it at 700°C for 14 hours, and cool it to room temperature to obtain the doped and modified lithium iron phosphate material.

[0061] Example 2

[0062] The doped and modified lithium iron phosphate material provided in this embodiment includes: a doped lithium iron phosphate core with the chemical formula Li. 0.96 Nb 0.02 FeP 0.97 B 0.03 O4; and a carbon coating layer, which coats the surface of the doped lithium iron phosphate core.

[0063] The method for preparing doped and modified lithium iron phosphate material provided in this embodiment includes the following steps:

[0064] S1. First, according to the molar ratio of Li, Nb, Fe, P, and B elements in the chemical formula of the doped lithium iron phosphate core provided in this embodiment (0.96:0.02:1:0.97:0.03), weigh Li2CO3, Nb2O5, FeSO4, NH4H2PO4, and H3BO3, mix them to obtain a mixture, add it to anhydrous ethanol, and wet ball mill for 6 hours. Then, dry it in an oven at 80°C under N2 atmosphere until the anhydrous ethanol is completely evaporated. Grind it thoroughly again to obtain a dry mixture. Calcinate the dry mixture at 200°C under N2 atmosphere for 5 hours, and then cool it to room temperature to obtain the precursor.

[0065] S2. Add citric acid (the mass ratio of citric acid to precursor is 2:100) to the precursor prepared in step S1, then ball mill in anhydrous ethanol for 8 hours, and then dry in an oven at 80°C under N2 atmosphere until the anhydrous ethanol is completely evaporated. Grind thoroughly again to obtain a dry mixture.

[0066] S3. Place the dried mixture prepared in step S2 into a tube furnace filled with purified N2 gas, first calcine it at 350°C for 8 hours, then calcine it at 700°C for 14 hours, and cool it to room temperature to obtain the doped and modified lithium iron phosphate material.

[0067] Example 3

[0068] The doped and modified lithium iron phosphate material provided in this embodiment includes: a doped lithium iron phosphate core with the chemical formula Li. 0.98 Ti 0.02 FeP 0.97 B 0.03 O4; and a carbon coating layer, which coats the surface of the doped lithium iron phosphate core.

[0069] The method for preparing doped and modified lithium iron phosphate material provided in this embodiment includes the following steps:

[0070] S1. First, according to the molar ratio of Li, Ti, Fe, P, and B elements in the chemical formula of the doped lithium iron phosphate core provided in this embodiment (0.98:0.02:1:0.97:0.03), weigh Li2CO3, TiO2, FeSO4, NH4H2PO4, and H3BO3, mix them to obtain a mixture, add anhydrous ethanol, and wet ball mill for 6 hours. Then, dry it in an oven at 80°C under N2 atmosphere until the anhydrous ethanol is completely evaporated. Grind it thoroughly again to obtain a dry mixture. Calcinate the dry mixture at 200°C under N2 atmosphere for 5 hours, and then cool it to room temperature to obtain the precursor.

[0071] S2. Add citric acid (the mass ratio of citric acid to precursor is 2:100) to the precursor prepared in step S1, then ball mill in anhydrous ethanol for 8 hours, and then dry in an oven at 80°C under N2 atmosphere until the anhydrous ethanol is completely evaporated. Grind thoroughly again to obtain a dry mixture.

[0072] S3. Place the dried mixture prepared in step S2 into a tube furnace filled with purified N2 gas, first calcine it at 350°C for 8 hours, then calcine it at 700°C for 14 hours, and cool it to room temperature to obtain the doped and modified lithium iron phosphate material.

[0073] Example 4

[0074] The doped and modified lithium iron phosphate material provided in this embodiment includes: a doped lithium iron phosphate core with the chemical formula Li. 0.94 Al 0.04 FeP 0.97 B 0.03 O4; and a carbon coating layer, which coats the surface of the doped lithium iron phosphate core.

[0075] The method for preparing doped and modified lithium iron phosphate material provided in this embodiment includes the following steps:

[0076] S1. First, according to the molar ratio of Li, Al, Fe, P, and B elements in the chemical formula of the doped lithium iron phosphate core provided in this embodiment (0.94:0.04:1:0.97:0.03), weigh Li2CO3, Al2O3, FeSO4, NH4H2PO4, and H3BO3, mix them to obtain a mixture, add it to anhydrous ethanol, and wet ball mill for 6 hours. Then, dry it in an oven at 80°C under N2 atmosphere until the anhydrous ethanol is completely evaporated. Grind it thoroughly again to obtain a dry mixture. Calcinate the dry mixture at 200°C under N2 atmosphere for 5 hours, and then cool it to room temperature to obtain the precursor.

[0077] S2. Add citric acid (the mass ratio of citric acid to precursor is 2:100) to the precursor prepared in step S1, then ball mill in anhydrous ethanol for 8 hours, and then dry in an oven at 80°C under N2 atmosphere until the anhydrous ethanol is completely evaporated. Grind thoroughly again to obtain a dry mixture.

[0078] S3. Place the dried mixture prepared in step S2 into a tube furnace filled with purified N2 gas, first calcine it at 350°C for 8 hours, then calcine it at 700°C for 14 hours, and cool it to room temperature to obtain the doped and modified lithium iron phosphate material.

[0079] Example 5

[0080] The doped and modified lithium iron phosphate material provided in this embodiment includes: a doped lithium iron phosphate core with the chemical formula Li. 0.76 Nb 0.06 FeP 0.97 B 0.03 O4; and a carbon coating layer, which coats the surface of the doped lithium iron phosphate core.

[0081] The method for preparing doped and modified lithium iron phosphate material provided in this embodiment includes the following steps:

[0082] S1. First, according to the molar ratio of Li, Nb, Fe, P, and B elements in the chemical formula of the doped lithium iron phosphate core provided in this embodiment (0.76:0.06:1:0.97:0.03), weigh Li2CO3, Nb2O5, FeSO4, NH4H2PO4, and H3BO3, mix them to obtain a mixture, add it to anhydrous ethanol, and wet ball mill for 6 hours. Then, dry it in an oven at 80°C under N2 atmosphere until the anhydrous ethanol is completely evaporated. Grind it thoroughly again to obtain a dry mixture. Calcinate the dry mixture at 200°C under N2 atmosphere for 5 hours, and then cool it to room temperature to obtain the precursor.

[0083] S2. Add citric acid (the mass ratio of citric acid to precursor is 2:100) to the precursor prepared in step S1, then ball mill in anhydrous ethanol for 8 hours, and then dry in an oven at 80°C under N2 atmosphere until the anhydrous ethanol is completely evaporated. Grind thoroughly again to obtain a dry mixture.

[0084] S3. Place the dried mixture prepared in step S2 into a tube furnace filled with purified N2 gas, first calcine it at 350°C for 8 hours, then calcine it at 700°C for 14 hours, and cool it to room temperature to obtain the doped and modified lithium iron phosphate material.

[0085] Example 6

[0086] The doped and modified lithium iron phosphate material provided in this embodiment includes: a doped lithium iron phosphate core with the chemical formula LiNb. 0.02 FeP 0.94 B 0.06 O 4-δ ; and a carbon coating layer, which coats the surface of the doped lithium iron phosphate core.

[0087] The method for preparing doped and modified lithium iron phosphate material provided in this embodiment includes the following steps:

[0088] S1. First, according to the molar ratio of Li, Nb, Fe, P, and B elements in the chemical formula of the doped lithium iron phosphate core provided in this embodiment (1:0.02:1:0.94:0.06), weigh Li2CO3, Nb2O5, FeSO4, NH4H2PO4, and H3BO3, mix them to obtain a mixture, add it to anhydrous ethanol, and wet ball mill for 6 hours. Then, dry it in an oven at 80°C under N2 atmosphere until the anhydrous ethanol is completely evaporated. Grind it thoroughly again to obtain a dry mixture. Calcinate the dry mixture at 200°C under N2 atmosphere for 5 hours, and then cool it to room temperature to obtain the precursor.

[0089] S2. Add citric acid (the mass ratio of citric acid to precursor is 2:100) to the precursor prepared in step S1, then ball mill in anhydrous ethanol for 8 hours, and then dry in an oven at 80°C under N2 atmosphere until the anhydrous ethanol is completely evaporated. Grind thoroughly again to obtain a dry mixture.

[0090] S3. Place the dried mixture prepared in step S2 into a tube furnace filled with purified N2 gas, first calcine it at 350°C for 8 hours, then calcine it at 700°C for 14 hours, and cool it to room temperature to obtain the doped and modified lithium iron phosphate material.

[0091] Comparative Example 1

[0092] The lithium iron phosphate material provided in this comparative example includes: a lithium iron phosphate core with the chemical formula LiFePO4; and a carbon coating layer covering the surface of the lithium iron phosphate core.

[0093] The preparation method of the lithium iron phosphate material provided in this comparative example is basically the same as that in Example 1, except that H3BO3 and Al2O3 are not added. The specific steps are as follows:

[0094] S1. First, according to the molar ratio (1:1:1) of Li, Fe, and P elements in the lithium iron phosphate core provided in this comparative example, weigh Li2CO3, FeSO4, and NH4H2PO4, mix them to obtain a mixture, add it to anhydrous ethanol, and wet ball mill for 6 hours. Then, dry it in an oven at 80°C under N2 atmosphere until the anhydrous ethanol is completely evaporated. Grind it thoroughly again to obtain a dry mixture. Calcinate the dry mixture at 200°C under N2 atmosphere for 5 hours, and then cool it to room temperature to obtain the precursor.

[0095] S2. Add citric acid (the mass ratio of citric acid to precursor is 2:100) to the precursor prepared in step S1, then ball mill in anhydrous ethanol for 8 hours, and then dry in an oven at 80°C under N2 atmosphere until the anhydrous ethanol is completely evaporated. Grind thoroughly again to obtain a dry mixture.

[0096] S3. The dried mixture prepared in step S2 is placed in a tube furnace filled with purified N2 gas. First, it is calcined at 350°C for 8 hours, then calcined at 700°C for 14 hours. After cooling to room temperature, lithium iron phosphate material is obtained. The XRD pattern of this lithium iron phosphate material is shown below. Figure 1 As shown.

[0097] Comparative Example 2

[0098] The doped and modified lithium iron phosphate material provided in this comparative example includes: a doped lithium iron phosphate core with the chemical formula LiFeP 0.98 B 0.02 O 4-δ ; and a carbon coating layer, which coats the surface of the doped lithium iron phosphate core.

[0099] The preparation method of the doped and modified lithium iron phosphate material provided in this comparative example includes the following steps:

[0100] S1. First, according to the molar ratio of Li, Fe, P, and B elements in the chemical formula of the doped and modified lithium iron phosphate material provided in this comparative example (1:1:0.98:0.02), weigh FeSO4, Li2CO3, NH4H2PO4, and H3BO3, mix them to obtain a mixture, add it to anhydrous ethanol, and wet ball mill for 6 hours. Then, dry it in an oven at 80°C under N2 atmosphere until the anhydrous ethanol is completely evaporated. Grind it thoroughly again to obtain a dry mixture. Calcine the dry mixture at 200°C under N2 atmosphere for 5 hours, and then cool it to room temperature to obtain the precursor.

[0101] S2. Add citric acid (the mass ratio of citric acid to precursor is 2:100) to the precursor prepared in step S1, then ball mill in anhydrous ethanol for 8 hours, and then dry in an oven at 80°C under N2 atmosphere until the anhydrous ethanol is completely evaporated. Grind thoroughly again to obtain a dry mixture.

[0102] S3. Place the dried mixture prepared in step S2 into a tube furnace filled with purified N2 gas, first calcine it at a low temperature of 350°C for 8 hours, then calcine it at 700°C for 14 hours, and cool it to room temperature to obtain the doped and modified lithium iron phosphate material.

[0103] Comparative Example 3

[0104] The doped and modified lithium iron phosphate material provided in this comparative example includes: a doped lithium iron phosphate core with the chemical formula LiFeP 0.97 B 0.03 O 4-δ ; and a carbon coating layer, which coats the surface of the doped lithium iron phosphate core.

[0105] The preparation method of the doped modified lithium iron phosphate material provided in this comparative example is basically the same as that of comparative example 2. The difference is that FeSO4, Li2CO3, NH4H2PO4 and H3BO3 are weighed according to the molar ratio of Li, Fe, P and B elements in the chemical formula of the doped modified lithium iron phosphate material provided in this comparative example (1:1:0.97:0.03).

[0106] Comparative Example 4

[0107] The doped and modified lithium iron phosphate material provided in this comparative example includes: a doped lithium iron phosphate core with the chemical formula LiFeP 0.94 B 0.06 O 4-δ ; and a carbon coating layer, which coats the surface of the doped lithium iron phosphate core.

[0108] The preparation method of the doped modified lithium iron phosphate material provided in this comparative example is basically the same as that of comparative example 2, except that FeSO4, Li2CO3, NH4H2PO4 and H3BO3 are weighed according to the molar ratio of Li, Fe, P and B elements in the chemical formula of the doped modified lithium iron phosphate material provided in this comparative example (1:1:0.94:0.06).

[0109] Comparative Example 5

[0110] The doped and modified lithium iron phosphate material provided in this comparative example includes: a doped lithium iron phosphate core with the chemical formula Li. 0.94 Al 0.02 FePO4; and a carbon coating layer, which coats the surface of the doped lithium iron phosphate core.

[0111] The preparation method of the doped and modified lithium iron phosphate material provided in this comparative example includes the following steps:

[0112] S1. First, according to the molar ratio of Li, Fe, P, and Al elements in the chemical formula of the doped lithium iron phosphate core provided in this comparative example (0.94:1:1:0.02), weigh FeSO4, NH4H2PO4, Li2CO3, and Al2O3, mix them to obtain a mixture, add it to anhydrous ethanol, and wet ball mill for 6 hours. Then, dry it in an oven at 80°C under N2 atmosphere until the anhydrous ethanol is completely evaporated. Grind it thoroughly again to obtain a dry mixture. Calcinate the dry mixture at 200°C for 5 hours under N2 atmosphere, and then cool it to room temperature to obtain the precursor.

[0113] S2. Add citric acid (the mass ratio of citric acid to precursor is 2:100) to the precursor prepared in step S1, then ball mill in anhydrous ethanol for 8 hours, and then dry in an oven at 80°C under N2 atmosphere until the anhydrous ethanol is completely evaporated. Grind thoroughly again to obtain a dry mixture.

[0114] S3. Place the dried mixture prepared in step S2 into a tube furnace filled with purified N2 gas, first calcine it at 350°C for 8 hours, then calcine it at 700°C for 14 hours, and cool it to room temperature to obtain the doped and modified lithium iron phosphate material.

[0115] Comparative Example 6

[0116] The doped and modified lithium iron phosphate material provided in this comparative example includes: a doped lithium iron phosphate core with the structural formula Li 0.9 Nb 0.02 FePO4; and a carbon coating layer, which coats the surface of the doped lithium iron phosphate core.

[0117] The preparation method of the doped modified lithium iron phosphate material provided in this comparative example is basically the same as that of comparative example 5. The difference is that FeSO4, NH4H2PO4, Li2CO3 and Nb2O5 are weighed according to the molar ratio of Li, Fe, P and Nb elements in the chemical formula of the doped lithium iron phosphate core in the doped modified lithium iron phosphate material provided in this comparative example (0.9:1:1:0.02).

[0118] Comparative Example 7

[0119] The doped and modified lithium iron phosphate material provided in this comparative example includes: a doped lithium iron phosphate core with the chemical formula Li. 0.92 Ti 0.02 FePO4; and a carbon coating layer, which coats the surface of the doped lithium iron phosphate core.

[0120] The preparation method of the doped modified lithium iron phosphate material provided in this comparative example is basically the same as that of comparative example 5. The difference is that FeSO4, NH4H2PO4, Li2CO3 and TiO2 are weighed according to the molar ratio of Li, Fe, P and Ti elements in the chemical formula of the doped lithium iron phosphate core in the doped modified lithium iron phosphate material provided in this comparative example (0.92:1:1:0.02).

[0121] Comparative Example 8

[0122] The doped and modified lithium iron phosphate material provided in this comparative example includes: a doped lithium iron phosphate core with the chemical formula Li. 0.88 Al 0.04 FePO4; and a carbon coating layer, which coats the surface of the doped lithium iron phosphate core.

[0123] The preparation method of the doped modified lithium iron phosphate material provided in this comparative example is basically the same as that of comparative example 5. The difference is that FeSO4, NH4H2PO4, Li2CO3 and Al2O3 are weighed according to the molar ratio of Li, Fe, P and Al elements in the chemical formula of the doped lithium iron phosphate core in the doped modified lithium iron phosphate material provided in this comparative example (0.88:1:1:0.04).

[0124] Comparative Example 9

[0125] The doped and modified lithium iron phosphate material provided in this comparative example includes: a doped lithium iron phosphate core with the structural formula Li 0.7 Nb 0.06 FePO4; and a carbon coating layer, which coats the surface of the doped lithium iron phosphate core.

[0126] The preparation method of the doped modified lithium iron phosphate material provided in this comparative example is basically the same as that of comparative example 5. The difference is that FeSO4, NH4H2PO4, Li2CO3 and Nb2O5 are weighed according to the molar ratio of Li, Fe, P and Nb elements in the chemical formula of the doped lithium iron phosphate core in the doped modified lithium iron phosphate material provided in this comparative example (0.7:1:1:0.06).

[0127] Performance testing

[0128] The lithium iron phosphate materials prepared in Examples 1-6 and Comparative Examples 1-9 were used as positive electrode active materials. They were mixed with Surpe-P and PVDF in a mass ratio of 96:2:2 and NMP was used as solvent to prepare a slurry. The slurry was coated on aluminum foil to prepare the positive electrode. Finally, the positive electrode was cut into a circular electrode sheet with a diameter of 12 mm using a punch. In a clean glove box filled with Ar (O2 content less than 0.1 ppm, H2O content less than 0.1 ppm), a lithium sheet was used as the negative electrode, a polypropylene microporous membrane was used as the separator, and 1 mol / L lithium hexafluorophosphide (LiPF6) (EC:DEC = 1:1) was used as the electrolyte. The CR2032 button battery was prepared according to a certain assembly process. After completion, it was allowed to stand for 24 hours to allow the electrolyte and electrode materials to be fully wetted. At room temperature (25℃±1), charge and discharge tests were conducted at different rates within a voltage range of 2.0-3.7V (charge and discharge tests were conducted sequentially at rates of 0.1C, 1C, 2C, and 5C). The test results are shown in Table 1.

[0129] Table 1

[0130]

[0131]

[0132] As shown in Table 1, the batteries prepared from the lithium iron phosphate materials provided in Examples 1-6 all have higher discharge specific capacities at 0.1C, 1C, 2C, and 5C rates than Comparative Example 1. This indicates that the present invention improves the rate performance of carbon-coated lithium iron phosphate materials by co-doping B with other elements (at least one of Ti, Nb, and Al) into lithium iron phosphate.

[0133] As can be seen from Examples 1-6 and Comparative Examples 2-9, when 0.02≤y≤0.04 and 0.02≤n≤0.06, the lithium iron phosphate materials prepared by co-doping have better discharge specific capacity at high rates (2C, 5C) than the lithium iron phosphate materials prepared by single doping.

[0134] As can be seen from Examples 1-6 and Comparative Examples 2-9, the lithium iron phosphate materials prepared by co-doping in this invention, by limiting 0.02≤y≤0.04 and 0.02≤n≤0.04, have higher discharge specific capacities at 0.1C, 1C, 2C, and 5C rates than lithium iron phosphate materials prepared by single doping with the same proportion. Especially at low rates (e.g., 0.1C and 1C), single-doping with B cannot improve the rate performance of carbon-coated lithium iron phosphate materials. However, co-doping with B and M can significantly improve the low-rate performance of carbon-coated lithium iron phosphate materials, thereby significantly increasing the discharge specific capacity at low rates. The discharge specific capacity is superior to that of single-doped lithium iron phosphate materials in the same proportion, indicating that B and M have a synergistic effect in improving the low-rate performance of carbon-coated lithium iron phosphate materials. The applicant speculates that the reason may be that B doping will form oxygen defects in the crystal lattice, which may cause lithium ions to remain at the oxygen defect positions during transport, reducing the lithium ion transport rate at low rates or preventing lithium ions from being extracted, thereby reducing its discharge specific capacity. However, after co-doping, the charge imbalance after B doping can be neutralized by the doped cations, reducing the number of oxygen defects in the crystal, thereby increasing its discharge specific capacity.

[0135] As shown in Comparative Examples 1-4, single-doping with B cannot improve performance at low rates (e.g., 0.1C and 1C), but it can improve performance at high rates (2C and 5C). The applicant speculates that the reason may be that B doping creates oxygen defects in the crystal lattice, which may cause lithium ions to remain at the oxygen defect sites during transport, reducing the lithium ion transport rate or preventing lithium ions from being extracted, thus reducing its capacity. At high rates, due to the increased conductivity, lithium ions are more easily extracted compared to undoped samples, thereby compensating for the portion of lithium ions that remain, resulting in increased capacity at high rates.

[0136] As shown in Comparative Examples 2-4, when the amount of single-doped B is too large, the rate performance shows a downward trend. The applicant speculates that the reason may be that: doping B ions at P sites can promote the transport capability of lithium ions, but B atoms have a +3 valence while P atoms have a +5 valence. After B doping, oxygen defects will be formed in the crystal lattice. This may cause lithium ions to stay at oxygen defects during transport, reducing the lithium ion transport rate or causing lithium ions to be unable to escape. If the doping amount is too large, the number of oxygen defects will increase.

[0137] As can be seen from Comparative Examples 6 and 9, when the amount of single doped M is too large, the rate performance shows a downward trend. The applicant speculates that the reason may be that doping with M (at least one of Ti, Nb, and Al) can also improve the electrochemical performance of the material. Using high valence state and small radius cation doping can increase the concentration of Li holes in lithium iron phosphate, thereby increasing the lithium ion transport rate and having a positive effect on improving electrochemical performance. However, excessive M doping will reduce the concentration of lithium ions and increase the migration resistance of lithium ions, thereby reducing the electrochemical performance of the material.

[0138] Figure 2 The graph shows a comparison of impedance test results for batteries assembled from lithium iron phosphate materials provided in Example 3 and Comparative Example 1. Figure 2 As shown, the horizontal axis represents the real impedance, the vertical axis represents the imaginary impedance, and the horizontal axis of the intersection of the first half of the arc with the horizontal axis is the charge transfer impedance. Comparing the impedance test results of Example 3 and Comparative Example 1, the charge transfer impedance of the co-doped sample is significantly reduced compared with the undoped sample, and the lithium ion diffusion coefficient is significantly improved. After co-doping, the conductivity of lithium iron phosphate is significantly improved.

[0139] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A doped and modified lithium iron phosphate material, characterized in that, The doped and modified lithium iron phosphate material includes: Doped lithium iron phosphate core; and A carbon coating layer is applied to the surface of the doped lithium iron phosphate core. The doped lithium iron phosphate core is a lithium iron phosphate co-doped with B and M, where M represents the doping metal element, which is Nb, and B represents boron. The valence state of Nb is +5, and the valence state of B is +3; The chemical formula of the doped lithium iron phosphate core is: Li x M y FeP m B n O 4-δ Where, 0.02≤y≤0.04, 0.02≤n≤0.04, m+n=1, 0<x≤1, x+Ay+5m+3n≤6, A is the valence state of M, B is doped with P sites, Nb is doped with Li sites, δ represents the number of oxygen defects in a single unit cell, 0≤δ<0.

1.

2. The doped and modified lithium iron phosphate material according to claim 1, characterized in that, The chemical formula of the doped lithium iron phosphate core is: Li x M y FeP m B n O4, where 0.7 < x ≤ 1, x + Ay + 5m + 3n = 6.

3. A method for preparing a doped and modified lithium iron phosphate material according to any one of claims 1-2, characterized in that, Includes the following steps: S1. Lithium source, iron source, phosphorus source, doped metal source and boron source are mixed in a solvent, and then dried and calcined to obtain the precursor; S2. Add a carbon source to the precursor, then mix it in a solvent, and dry it to obtain a mixture; S3. The mixture is calcined to obtain the doped and modified lithium iron phosphate material.

4. The method for preparing doped and modified lithium iron phosphate material according to claim 3, characterized in that, The lithium source includes at least one of lithium hydroxide, lithium acetate, and lithium carbonate; And / or, the iron source includes at least one of ferrous oxalate, ferrous acetate, FeSO4, and organic ferrous salts; And / or, the phosphorus source includes at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, and lithium dihydrogen phosphate; And / or, the doped metal source includes a niobium source; And / or, the boron source includes H3BO3; And / or, the carbon source includes at least one of citric acid, glucose, and sucrose.

5. The method for preparing the doped and modified lithium iron phosphate material according to claim 4, characterized in that, The niobium source includes at least one of Nb2O5 and Nb(NO3)5.

6. The method for preparing the doped and modified lithium iron phosphate material according to claim 3, characterized in that, The solvent includes at least one of ethanol and ethylene glycol; And / or, in step S1, the calcination atmosphere is an inert atmosphere, the calcination temperature is 200-300℃, and the calcination time is 4-6h; And / or, in step S1, the mixing method is ball milling, and the mixing time is 4~8 hours; And / or, in step S2, the mass ratio of the carbon source to the precursor is (2-10):100; And / or, in step S2, the mixing method is ball milling, and the mixing time is 4~8 hours; And / or, in step S2, the drying temperature is 80~90°C; And / or, in step S3, the calcination atmosphere is an inert atmosphere.

7. The method for preparing the doped and modified lithium iron phosphate material according to claim 3, characterized in that, In step S3, the calcination treatment includes calcining at 350~500℃ for 5~8h, followed by calcination at 600~800℃ for 10~14h.

8. A lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The active material of the positive electrode includes the doped and modified lithium iron phosphate material according to any one of claims 1-2 or the doped and modified lithium iron phosphate material prepared by the preparation method of the doped and modified lithium iron phosphate material according to any one of claims 3-7.

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