A lithium iron phosphate positive electrode material and its preparation method and application

Abstract

The present invention provides a lithium iron phosphate cathode material and a preparation method and application thereof. The preparation method comprises the following steps: (1) providing carbon nanosheets; (2) mixing the carbon nanosheets, an iron source, a phosphorus source, and an oxidant, reacting the mixture, and calcining the mixture once to obtain a precursor; and (3) mixing the precursor, a lithium source, and a reducing agent, and calcining the mixture twice to obtain the lithium iron phosphate cathode material. The present invention prepares a lithium iron phosphate cathode material having a good conductive network by a coprecipitation method and a thermal reduction method, greatly enhancing the conductivity of the lithium iron phosphate cathode material, reducing the impedance of charge transfer, and obtaining nano-scale lithium iron phosphate particles while achieving carbon coating, thereby increasing the diffusion rate of lithium ions and significantly improving the rate performance of the lithium iron phosphate cathode material.

Description

Technical Field

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

[0002] At present, the commonly used positive electrode materials in the lithium-ion battery industry mainly include lithium iron phosphate and ternary materials. Among them, lithium iron phosphate has the advantages of high energy density, low cost, stable charge and discharge platform, environmental friendliness and high safety, while ternary materials have the advantages of high energy density, good power performance, low temperature performance, etc. In the application of lithium iron phosphate materials, the application of lithium iron phosphate is seriously limited due to its inherent problems such as low ion diffusion rate, poor conductivity, and low tap density. In order to expand the application range of lithium iron phosphate materials, make up for its defects in conductivity, and use low-cost modification strategies to meet the needs of industrialization. Researchers have carried out a series of modification measures on lithium iron phosphate materials, and the common modification methods are mainly based on three methods: doping, coating and material nano-sizing.

[0003] Doping refers to doping metal ions into the lithium iron phosphate lattice to create lattice defects to improve electronic conductivity and lithium ion diffusion rate. Material nano-sizing refers to reducing the particle size of lithium iron phosphate particles, increasing the specific surface area of ​​the material, thereby providing more diffusion channels. At the same time, the small size effect of nanoparticles can reduce the depth and range of lithium ion insertion and extraction. Both methods can improve the capacity retention rate and specific capacity of the battery. However, the preparation process of these two methods is complex, the precursor raw materials are expensive, the manufacturing cost is high, and the output is low, which makes it difficult to meet the needs of industrialization.

[0004] Regarding the coating method of lithium iron phosphate materials, CN114335517A discloses a carbon-composite lithium iron phosphate positive electrode material, its preparation method and application. The preparation method includes: subjecting a lithium source, a first phosphorus source, a first ferrous salt, nano-scale carbon-coated ferrous phosphate and a first solvent to a hydrothermal synthesis reaction to obtain a carbon-composite lithium iron phosphate precursor; calcining the carbon-composite lithium iron phosphate precursor under an inert atmosphere or a reducing atmosphere to obtain a carbon-composite lithium iron phosphate positive electrode material. CN111430687A discloses a method for preparing carbon-coated lithium iron phosphate, including the steps of: mixing a lithium source, iron phosphate, a carbon source and water, grinding and granulating to obtain a first powder; obtaining an iron-based metal-organic framework, mixing the iron-based metal-organic framework with the first powder, and grinding to obtain a second powder; calcining the second powder under a protective gas atmosphere to obtain a carbon-coated lithium iron phosphate composite material. CN107240696A discloses a preparation method of carbon-coated lithium iron phosphate, carbon-coated lithium iron phosphate and lithium-ion battery. The preparation method comprises the following steps: firstly mixing a portion of a carbon source with iron phosphate, grinding and drying, and then pre-sintering; then mixing the pre-sintered material, a lithium source and a portion of the carbon source, grinding and drying, and then sintering again to obtain the carbon-coated lithium iron phosphate.

[0005] Although the above coating method improves the conductivity of lithium iron phosphate positive electrode materials, the improvement effect is limited and it is difficult to meet the needs of industrialization.

[0006] Therefore, providing a novel method for preparing a lithium iron phosphate positive electrode material so as to significantly improve the conductivity of the prepared lithium iron phosphate positive electrode material is a technical problem that needs to be solved urgently. Summary of the Invention

[0007] In response to the shortcomings of the prior art, the present invention aims to provide a lithium iron phosphate cathode material, its preparation method, and its application. The present invention prepares a lithium iron phosphate cathode material with a well-conductive network through a coprecipitation and thermal reduction method. This greatly enhances the conductivity of the lithium iron phosphate cathode material, reduces the impedance of charge transfer, and simultaneously obtains nanoscale lithium iron phosphate particles while achieving carbon coating, thereby increasing the diffusion rate of lithium ions and significantly improving the rate performance of the lithium iron phosphate cathode material.

[0008] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0009] In a first aspect, the present invention provides a method for preparing a lithium iron phosphate positive electrode material, the preparation method comprising the following steps:

[0010] (1) providing carbon nanosheets;

[0011] (2) mixing the carbon nanosheets, iron source, phosphorus source and oxidant described in step (1), reacting them, and calcining them once to obtain a precursor;

[0012] (3) Mixing the precursor in step (2) with a lithium source and a reducing agent, and calcining the mixture twice to obtain the lithium iron phosphate positive electrode material.

[0013] The present invention adopts a co-precipitation method to obtain an iron phosphate / carbon nanosheet composite precursor, wherein the iron phosphate is adsorbed on the carbon nanosheet, limiting the growth of the iron phosphate, so that small-sized nano-scale lithium iron phosphate particles can be obtained after thermal reduction. At the same time, the lithium iron phosphate nanoparticles grow uniformly on the carbon nanosheet, have good contact conditions, and can form a good conductive network, greatly enhancing the conductivity of the lithium iron phosphate material and reducing the impedance of charge transfer. The rate performance of the lithium iron phosphate positive electrode material prepared by this preparation method is significantly improved.

[0014] Preferably, the carbon nanosheets in step (1) are nitrogen-doped carbon nanosheets.

[0015] Preferably, the carbon nanosheets are modified before use to obtain modified carbon nanosheets.

[0016] In the present invention, the carbon nanosheets are modified to improve the dispersibility of the carbon nanosheets.

[0017] Preferably, the modification method is:

[0018] The carbon nanosheets are mixed with a modifier to react to obtain the modified carbon nanosheets.

[0019] Preferably, the modifier comprises nitric acid. It should be noted that the mass content of the solute in the nitric acid is 10%.

[0020] Preferably, the mass volume ratio of the carbon nanosheets and the modifier is 1 mg:(8-12) mL, for example, 1 mg:8 mL, 1 mg:9 mL, 1 mg:10 mL, 1 mg:11 mL or 1 mg:12 mL.

[0021] In the present invention, if the mass-to-volume ratio of the carbon nanosheets and the modifier is too small, that is, the amount of the modifier is too large, it will cause waste of resources, increase costs, and may cause excessive surface particles after subsequent compounding with lithium iron phosphate nanoparticles, agglomeration, and reduced conductivity; if the mass-to-volume ratio of the carbon nanosheets and the modifier is too large, that is, the amount of the modifier is too small, the modification effect is not obvious, the lithium iron phosphate nanoparticles compounded on the subsequent carbon nanosheets are limited, and the conductivity is insufficiently improved.

[0022] Preferably, the method for preparing the carbon nanosheets in step (1) comprises a co-precipitation method.

[0023] Preferably, the specific steps of the co-precipitation method include: dissolving the metal nitrate and the organic ligand in a solvent and mixing them, reacting to obtain ZIF-8, and sintering the ZIF-8 to obtain the carbon nanosheets.

[0024] In the present invention, the co-precipitation method is used to synthesize carbon nanosheets to form uniform ZIF-8 nanosheets, wherein ZIF-8 is a zeolite imidazolate framework material having the advantages of regular pore structure, high specific surface area and good conductivity.

[0025] Preferably, the metal nitrate comprises zinc nitrate hexahydrate.

[0026] Preferably, the organic ligand comprises 2-methylimidazole.

[0027] Preferably, the solvent comprises water.

[0028] Preferably, the mass ratio of the metal nitrate to the organic ligand is 1:(2.5-3.5), for example, it can be 1:2.5, 1:2.6, 1:2.7, 1:2.8, 1:2.9, 1:3, 1:3.1, 1:3.2, 1:3.3, 1:3.4 or 1:3.5, etc.

[0029] In the present invention, ZIF-8 nanosheets cannot be formed if the mass ratio of the metal nitrate to the organic ligand is too large or too small.

[0030] Preferably, the sintering temperature is 900-930°C, for example, it can be 900°C, 905°C, 910°C, 915°C, 920°C, 925°C or 930°C.

[0031] In the present invention, if the sintering temperature is too high, carbon nanosheets cannot be formed.

[0032] Preferably, the sintering time is 1.5-2.5 h, for example, 1.5 h, 1.6 h, 1.7 h, 1.8 h, 1.9 h, 2 h, 2.1 h, 2.2 h, 2.3 h, 2.4 h or 2.5 h.

[0033] Preferably, the iron source in step (2) comprises ferrous sulfate heptahydrate.

[0034] Preferably, the phosphorus source in step (2) comprises phosphoric acid.

[0035] Preferably, the oxidant in step (2) comprises hydrogen peroxide, wherein the mass fraction of hydrogen peroxide in the hydrogen peroxide is 36%.

[0036] Preferably, the mixing method in step (2) is:

[0037] (a) mixing an iron source and a phosphorus source to obtain a mixture;

[0038] (b) mixing the mixture of step (a) and the carbon nanosheets for a second time to obtain a solution A;

[0039] (c) adding an oxidant to the solution A in step (b) and mixing three times to obtain a solution B.

[0040] Preferably, the phosphorus source in step (a) is phosphoric acid, the mixture is a mixed solution, and the molar concentration of the iron source in the mixed solution is 0.8-1.2 mol / L, for example, 0.8 mol / L, 0.9 mol / L, 1 mol / L, 1.1 mol / L or 1.2 mol / L.

[0041] Preferably, the mass volume ratio of the carbon nanosheets to the mixed solution in step (b) is 1 mg:(8-12) mL, for example, 1 mg:8 mL, 1 mg:9 mL, 1 mg:10 mL, 1 mg:11 mL or 1 mg:12 mL.

[0042] In the present invention, if the mass-to-volume ratio of the carbon nanosheets and the mixed solution is too large, that is, the amount of carbon nanosheets used is too much, it will lead to excessive particles on the carbon nanosheets, the particles will pile up together, and the optimal conductive network cannot be formed; if the mass-to-volume ratio of the carbon nanosheets and the mixed solution is too small, that is, the amount of carbon nanosheets used is too small, it will lead to insufficient particles and reduced conductivity.

[0043] Preferably, the secondary mixing process in step (b) is accompanied by stirring, and the stirring time is 0.8-1.2 h, for example, 0.8 h, 0.9 h, 1 h, 1.1 h or 1.2 h.

[0044] Preferably, alkali solution is also added during the secondary mixing in step (b).

[0045] In the present invention, the purpose of adding alkali solution during the secondary mixing process is to adjust the pH value of the solution.

[0046] Preferably, the alkali solution comprises aqueous ammonia.

[0047] Preferably, the molar concentration of the alkali solution is 2-3 mol / L, for example, 2 mol / L, 2.2 mol / L, 2.4 mol / L, 2.6 mol / L, 2.8 mol / L or 3 mol / L.

[0048] As a preferred technical solution of the present invention, the pH of the mixed solution in step (2) is 2-5, for example, it can be 2, 2.5, 3, 3.5, 4, 4.5 or 5.

[0049] Preferably, the reaction time in step (2) is 3-8 min, for example, 3 min, 4 min, 5 min, 6 min, 7 min or 8 min.

[0050] Preferably, the reaction temperature in step (2) is 30-80°C, for example, it can be 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, 80°C, etc.

[0051] In the present invention, if the temperature of the reaction in step (2) is too low, it will lead to insufficient oxidation and insufficient formation of lithium iron phosphate, thereby reducing the conductivity of the material.

[0052] Preferably, the atmosphere of the primary calcination in step (2) is an inert atmosphere, and the gas in the inert atmosphere includes nitrogen and / or argon.

[0053] Preferably, the temperature of the primary calcination in step (2) is 500-700°C, for example, 500°C, 550°C, 600°C, 650°C or 700°C.

[0054] In the present invention, the purpose of performing one calcination is to remove crystal water and obtain a precursor with stable composition.

[0055] Preferably, the time of the first calcination in step (2) is 1-3 h, for example, it can be 1 h, 1.2 h, 1.4 h, 1.6 h, 1.8 h, 2 h, 2.2 h, 2.4 h, 2.6 h, 2.8 h or 3 h.

[0056] Preferably, the lithium source in step (3) includes lithium carbonate and / or lithium titanate.

[0057] Preferably, the reducing agent in step (3) comprises an organic carbon source.

[0058] Preferably, the organic carbon source comprises glucose.

[0059] Preferably, the mixing process in step (3) is carried out in a solvent, and the solvent includes ethanol.

[0060] Preferably, the mixing method in step (3) is ball milling.

[0061] Preferably, the ball milling time is 18-30 h, for example, 18 h, 20 h, 22 h, 24 h, 26 h, 28 h or 30 h.

[0062] Preferably, the atmosphere of the secondary calcination in step (3) is an inert atmosphere, and the gas in the inert atmosphere includes nitrogen and / or argon.

[0063] Preferably, the temperature of the secondary calcination in step (3) is 600-800°C, for example, 600°C, 650°C, 700°C, 750°C or 800°C.

[0064] Preferably, the secondary calcination time in step (3) is 5-15 h, for example, it can be 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h, 13 h, 14 h or 15 h.

[0065] As a preferred technical solution, the preparation method comprises the following steps:

[0066] (I) preparing nitrogen-doped carbon nanosheets by a coprecipitation method, and mixing and reacting with a modifier to obtain modified carbon nanosheets;

[0067] (II) mixing the iron source and the phosphorus source once to obtain a mixed solution having an iron source concentration of 0.8-1.2 mol / L;

[0068] (III) mixing the mixed solution of step (II) and the modified carbon nanosheets of step (I) for a second time to obtain solution A;

[0069] Wherein, the mass volume ratio of the modified carbon nanosheets to the mixed solution is 1 mg: (8-12) mL;

[0070] (IV) adding an alkali solution and an oxidant having a molar concentration of 2-3 mol / L to the solution A of step (III) and mixing them three times to obtain a solution B having a pH of 2-5;

[0071] The solution B is reacted at 30-80° C. for 3-8 minutes, and then calcined in an inert atmosphere at 500-700° C. for 1-3 hours to obtain a precursor;

[0072] (V) dispersing the precursor in step (IV) with a lithium source and an organic carbon source in a solvent, ball milling and mixing for 18-30 hours, and then secondary calcining in an inert atmosphere at 600-800° C. for 5-15 hours to obtain the lithium iron phosphate positive electrode material.

[0073] In a second aspect, the present invention provides a lithium iron phosphate positive electrode material prepared by the preparation method described in the first aspect, wherein the lithium iron phosphate positive electrode material comprises a lithium iron phosphate core and a carbon coating layer located on the surface of the core.

[0074] Preferably, the average particle size of the lithium iron phosphate positive electrode material is 200-300 nm, for example, 200 nm, 220 nm, 240 nm, 260 nm, 280 nm or 300 nm.

[0075] The lithium iron phosphate positive electrode material prepared by the preparation method provided by the present invention obtains nano-scale lithium iron phosphate particles with an average particle size of 200-300nm while achieving carbon coating, effectively increasing the transmission rate of lithium ions and improving the electrochemical performance of the material.

[0076] In a third aspect, the present invention provides a lithium-ion battery, wherein the positive electrode of the lithium-ion battery includes the lithium iron phosphate positive electrode material as described in the second aspect.

[0077] The numerical range described in the present invention includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0078] Compared with the prior art, the present invention has the following beneficial effects:

[0079] (1) The present invention prepares a lithium iron phosphate cathode material with a good conductive network by a coprecipitation method and a thermal reduction method, greatly enhancing the conductivity of the lithium iron phosphate cathode material, reducing the impedance of charge transfer, and obtaining nano-scale lithium iron phosphate particles while achieving carbon coating, thereby increasing the diffusion rate of lithium ions and improving the rate performance of the lithium iron phosphate cathode material;

[0080] (2) The preparation method provided by the present invention has lower cost compared with metal doping modification and material nano-modification. BRIEF DESCRIPTION OF THE DRAWINGS

[0081] Figure 1 This is a scan image of the carbon nanosheet in Example 1 of the present invention.

[0082] Figure 2 This is a transmission image of the carbon nanosheet in Example 1 of the present invention.

[0083] Figure 3 This is a scan image of the lithium iron phosphate positive electrode material prepared in Example 1 of the present invention.

[0084] Figure 4 This is a transmission image of the lithium iron phosphate positive electrode material prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0085] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0086] Example 1

[0087] This embodiment provides a method for preparing a lithium iron phosphate positive electrode material, the preparation method comprising the following steps:

[0088] (1) 1.32 g of Zn(NO3)2·6H2O and 3.936 g of 2-methylimidazole were dissolved in 360 mL of deionized water and mixed. The mixture was allowed to stand for 24 h and then centrifuged to obtain white ZIF-8. The nitrogen-doped carbon nanosheets were obtained after calcination at 930 °C for 2 h.

[0089] Wherein, the mass ratio of Zn(NO3)2·6H2O and 2-methylimidazole is 1:2.98;

[0090] (2) mixing the nitrogen-doped carbon nanosheets of step (1) with nitric acid at a mass volume ratio of 1 mg:10 mL to obtain modified carbon nanosheets;

[0091] (3) mixing ferrous sulfate heptahydrate and phosphoric acid once to obtain a mixed solution having a ferrous sulfate heptahydrate concentration of 1 mol / L;

[0092] (4) mixing the mixed solution of step (3) and the modified carbon nanosheets of step (2) for a second time and stirring for 1 hour to obtain solution A;

[0093] Wherein, the mass volume ratio of the modified carbon nanosheets and the mixed solution is 1 mg:10 mL;

[0094] (5) adding ammonia water with a molar concentration of 2.5 mol / L and hydrogen peroxide with a mass content of 36% to the solution A described in step (4) and mixing them three times to obtain a solution B with a pH of 3.5;

[0095] The solution B was reacted at 50° C. for 5 minutes and then calcined in a nitrogen atmosphere at 600° C. for 2 hours to obtain a precursor;

[0096] (6) The precursor described in step (5) is dispersed with lithium carbonate and glucose in ethanol, and the mixture is ball-milled for 24 hours. After mixing, the mixture is dried and calcined for a second time in a nitrogen atmosphere at 700°C for 10 hours to obtain a lithium iron phosphate positive electrode material with a particle size of 250 nm.

[0097] Figure 1 The scanning image of the carbon nanosheets in this embodiment is shown. As can be seen from the figure, the thickness of the carbon nanosheets obtained by calcining ZIF-8 is about 100 nm, the size is uniform, the surface is smooth, and the morphology characteristics of the precursor are well maintained.

[0098] Figure 2 The projection image of the carbon nanosheet in this embodiment is shown. As can be seen from the figure, the thickness of the carbon nanosheet is very uniform and the structure is complete, which is conducive to the recombination of lithium iron phosphate nanoparticles.

[0099] Figure 3A scanning image of the lithium iron phosphate positive electrode material prepared in this embodiment is shown. From the image, it can be seen that there are uniform lithium iron phosphate nanoparticles on the surface of the carbon nanosheets, there is no obvious agglomeration between the particles, and the particles are tightly compounded with the carbon nanosheets, which improves the conductivity of the positive electrode material.

[0100] Figure 4 The transmission image of the lithium iron phosphate positive electrode material prepared in this embodiment is shown. After calcination, the positive electrode material and the carbon nanosheets are closely compounded, and there is no agglomeration of large particles.

[0101] Example 2

[0102] This embodiment provides a method for preparing a lithium iron phosphate positive electrode material, the preparation method comprising the following steps:

[0103] (1) 1.32 g of Zn(NO3)2·6H2O and 3.30 g of 2-methylimidazole were dissolved in 360 mL of deionized water and mixed. The mixture was allowed to stand for 24 h and then centrifuged to obtain white ZIF-8. The nitrogen-doped carbon nanosheets were obtained after calcination at 900 °C for 2.5 h.

[0104] Wherein, the mass ratio of Zn(NO3)2·6H2O and 2-methylimidazole is 1:2.5;

[0105] (2) mixing the nitrogen-doped carbon nanosheets of step (1) with nitric acid at a mass volume ratio of 1 mg:8 mL to obtain modified carbon nanosheets;

[0106] (3) mixing ferrous sulfate heptahydrate and phosphoric acid once to obtain a mixed solution having a ferrous sulfate heptahydrate concentration of 0.8 mol / L;

[0107] (4) mixing the mixed solution of step (3) and the modified carbon nanosheets of step (2) for a second time and stirring for 0.8 h to obtain solution A;

[0108] Wherein, the mass volume ratio of the modified carbon nanosheets to the mixed solution is 1 mg:8 mL;

[0109] (5) adding 2 mol / L ammonia water and 36% by mass hydrogen peroxide to the solution A in step (4) and mixing them three times to obtain a solution B with a pH of 2;

[0110] The solution B was reacted at 30° C. for 8 minutes and then calcined in an argon atmosphere at 500° C. for 3 hours to obtain a precursor;

[0111] (6) The precursor described in step (5) is dispersed with lithium carbonate and citric acid in ethanol, and the mixture is ball-milled for 18 hours. After mixing, the mixture is dried and calcined for a second time in an argon atmosphere at 600°C for 15 hours to obtain a lithium iron phosphate positive electrode material with a particle size of 200 nm.

[0112] Example 3

[0113] This embodiment provides a method for preparing a lithium iron phosphate positive electrode material, the preparation method comprising the following steps:

[0114] (1) 1.32 g of Zn(NO3)2·6H2O and 4.62 g of 2-methylimidazole were dissolved in 360 mL of deionized water and mixed. After standing for 24 h, the mixture was centrifuged to obtain white ZIF-8, which was then calcined at 915 °C for 1.5 h to obtain nitrogen-doped carbon nanosheets.

[0115] Wherein, the mass ratio of Zn(NO3)2·6H2O and 2-methylimidazole is 1:3.5;

[0116] (2) mixing the nitrogen-doped carbon nanosheets described in step (1) with nitric acid at a mass volume ratio of 1 mg:12 mL to obtain modified carbon nanosheets;

[0117] (3) mixing ferrous sulfate heptahydrate and phosphoric acid once to obtain a mixed solution having a ferrous sulfate heptahydrate concentration of 1.2 mol / L;

[0118] (4) mixing the mixed solution of step (3) and the modified carbon nanosheets of step (2) for a second time and stirring for 1.2 h to obtain solution A;

[0119] Wherein, the mass volume ratio of the modified carbon nanosheets and the mixed solution is 1 mg:12 mL;

[0120] (5) adding 3 mol / L ammonia water and 36% by mass hydrogen peroxide to the solution A in step (4) and mixing them three times to obtain a solution B with a pH of 5;

[0121] The solution B was reacted at 80° C. for 3 minutes and then calcined in a nitrogen atmosphere at 700° C. for 1 hour to obtain a precursor;

[0122] (6) The precursor described in step (5) is dispersed with lithium carbonate and methanol in ethanol, and the mixture is ball-milled for 30 hours. After mixing, the mixture is dried and calcined for a second time in a nitrogen atmosphere at 800° C. for 5 hours to obtain a lithium iron phosphate positive electrode material with a particle size of 300 nm.

[0123] Example 4

[0124] The difference between this embodiment and embodiment 1 is that the mass volume ratio of nitrogen-doped carbon nanosheets and nitric acid in step (2) is 1 mg:5 mL.

[0125] The rest of the preparation methods and parameters remained the same as in Example 1.

[0126] Example 5

[0127] The difference between this embodiment and embodiment 1 is that the mass volume ratio of nitrogen-doped carbon nanosheets and nitric acid in step (2) is 1 mg:15 mL.

[0128] The rest of the preparation methods and parameters remained the same as in Example 1.

[0129] Example 6

[0130] The difference between this embodiment and embodiment 1 is that the nitrogen-doped carbon nanosheets are not modified, that is, step (2) is not performed, but the nitrogen-doped carbon nanosheets described in step (1) and the mixed solution described in step (3) are mixed.

[0131] The rest of the preparation methods and parameters remained the same as in Example 1.

[0132] Example 7

[0133] The difference between this embodiment and embodiment 1 is that the mass volume ratio of the modified carbon nanosheets and the mixed solution is 1 mg:5 mL.

[0134] The rest of the preparation methods and parameters remained the same as in Example 1.

[0135] Example 8

[0136] The difference between this embodiment and embodiment 1 is that the mass volume ratio of the modified carbon nanosheets and the mixed solution is 1 mg:15 mL.

[0137] The rest of the preparation methods and parameters remained the same as in Example 1.

[0138] Example 9

[0139] The difference between this embodiment and embodiment 1 is that the reaction temperature in step (5) is 20°C.

[0140] The rest of the preparation methods and parameters remained the same as in Example 1.

[0141] Example 10

[0142] The difference between this embodiment and embodiment 1 is that steps (3) and (4) are not performed. Instead, ferrous sulfate heptahydrate, phosphoric acid, modified carbon nanosheets, hydrogen peroxide and ammonia water are directly mixed, reacted and then calcined to obtain a precursor.

[0143] The rest of the preparation methods and parameters remained the same as in Example 1.

[0144] Example 11

[0145] The difference between this embodiment and embodiment 1 is that steps (1) to (5) are not performed, but iron phosphate, lithium carbonate and glucose are directly mixed.

[0146] The rest of the preparation methods and parameters remained the same as in Example 1.

[0147] Comparative Example 1

[0148] The difference between this comparative example and Example 1 is that the carbon nanosheets are replaced by graphene.

[0149] The rest of the preparation methods and parameters remained the same as in Example 1.

[0150] Performance Testing

[0151] The lithium iron phosphate positive electrode materials provided in Examples 1-11 and Comparative Example 1 were prepared into positive electrode sheets, and assembled with graphite negative electrode sheets, non-woven fabric separators and electrolytes (including ethylene carbonate and propylene carbonate) to obtain lithium-ion button batteries, and electrochemical performance tests were performed.

[0152] The preparation method of lithium-ion button battery is as follows:

[0153] (1) Slurry preparation: The prepared lithium iron phosphate cathode material, binder (PVDF), and acetylene black were mixed in a mass ratio of 8:1:1, and then a certain amount of N-methylpyrrolidone (NMP) was added dropwise and stirred evenly for 3 hours;

[0154] (2) Coating: Wipe the aluminum foil with anhydrous ethanol, then evenly apply the slurry on the surface of the aluminum foil, transfer the electrode to a vacuum drying oven, and vacuum dry at 120°C for 12 hours;

[0155] (3) Punching: The dried electrode is lightly cold pressed and then punched to obtain electrode discs with a diameter of 16 mm;

[0156] (4) Assembly: Weigh each electrode and assemble the half-cell in an argon-protected glove box.

[0157] The electrochemical performance tests of lithium-ion batteries are as follows:

[0158] The battery was subjected to electrochemical impedance spectroscopy (EIS) testing on an electrochemical workstation with a test frequency range of 0.01 Hz to 100 kHz. The testing machine was Shanghai Chenhua 760E.

[0159] The test results are shown in Table 1.

[0160] Table 1

[0161]

[0162]

[0163] analyze:

[0164] It can be seen from the data results in the above table that within the scope of protection of the present invention, a lithium iron phosphate positive electrode material with a good conductive network can be obtained, its charge transfer impedance is significantly reduced, and the rate performance of the lithium iron phosphate positive electrode material is significantly improved.

[0165] Comparison of the data results of Example 1 with those of Examples 4-5 shows that if the nitric acid content is too high, it will lead to waste of resources and may cause excessive surface particles after compounding, resulting in agglomeration, and the conductivity will be reduced; if the nitric acid content is too low, the modification will be insufficient, the number of lithium iron phosphate nanoparticles compounded on the N-doped carbon nanosheets will be limited, and the conductivity will not be sufficiently improved.

[0166] Comparison of the data results of Example 1 and Example 6 shows that if the nitrogen-doped carbon nanosheets are not modified, the lithium iron phosphate nanoparticles cannot be composited on the carbon nanosheets, the carbon nanosheets and the lithium iron phosphate nanoparticles are separated from each other, the conductivity is greatly reduced, and the rate performance of the positive electrode material is also significantly reduced.

[0167] A comparison of the data results of Example 1 and Examples 7-8 shows that if the mass-to-volume ratio of the carbon nanosheets to the mixed solution is too large, there will be too many particles on the carbon nanosheets, and the particles will pile up together, unable to form an optimal conductive network, resulting in poor conductivity of the material; if the mass-to-volume ratio of the carbon nanosheets to the mixed solution is too small, there will be insufficient particles and reduced conductivity.

[0168] Comparison of the data results of Example 1 and Example 9 shows that a too low reaction temperature will lead to insufficient oxidation and insufficient formation of lithium iron phosphate, thereby reducing the conductivity of the material.

[0169] Comparison of the data results of Example 1 and Example 10 shows that direct mixing will cause the nitrogen-doped carbon nanosheets to no longer serve as the main site of crystallization. At this time, crystallization mainly occurs in the solution, which has no effect on the modification of the nitrogen-doped carbon nanosheets. The conductivity of lithium iron phosphate is not improved, the conductivity is poor, the capacity retention rate is low, and the rate performance is poor.

[0170] Comparison of the data results of Example 1 and Example 11 shows that directly mixing iron phosphate, lithium titanate and glucose to prepare lithium iron phosphate particles results in a material with poor conductivity, low capacity retention and poor rate performance.

[0171] Comparing the data results of Example 1 and Comparative Example 1 shows that Comparative Example 1 has the highest transfer impedance and the lowest conductivity, proving that graphene is not suitable. Furthermore, due to its two-dimensional sheet structure, graphene hinders the diffusion of lithium ions. Due to the high impedance, polarization is greater during cycling, resulting in reduced capacity retention and poor rate performance.

[0172] The applicant states that while the above-described embodiments illustrate the process of the present invention, the present invention is not limited to the above-described process steps, nor does it imply that the present invention must rely on the above-described process steps for implementation. Those skilled in the art will appreciate that any improvements to the present invention, equivalent substitutions for the raw materials used, additions of auxiliary components, and selection of specific methods, etc., fall within the scope of protection and disclosure of the present invention.

Claims

1. A method for preparing a lithium iron phosphate positive electrode material, characterized in that: The preparation method comprises the following steps: (1) preparing nitrogen-doped carbon nanosheets by a coprecipitation method, and mixing and reacting with a modifier to obtain modified carbon nanosheets; the modifier includes nitric acid; The mass volume ratio of the carbon nanosheets and the modifier is 1 mg: (8-12) mL; (2) mixing the modified carbon nanosheets, iron source, phosphorus source and oxidant described in step (1), reacting them, and calcining them once to obtain a precursor; The mixing method described in step (2) is: (a) mixing an iron source and a phosphorus source to obtain a mixture; (b) mixing the mixture of step (a) and the carbon nanosheets for a second time to obtain solution A; (c) adding an oxidant to the solution A described in step (b) three times and mixing them to obtain a solution B; (3) mixing the precursor described in step (2) with a lithium source and a reducing agent, and calcining them twice to obtain the lithium iron phosphate positive electrode material; the average particle size of the lithium iron phosphate positive electrode material is 200-300 nm.

2. The preparation method according to claim 1, characterized in that The specific steps of the co-precipitation method include: dissolving metal nitrate and organic ligand in a solvent and mixing them, reacting to obtain ZIF-8, and sintering the ZIF-8 to obtain the carbon nanosheet.

3. The preparation method according to claim 2, characterized in that The metal nitrate includes zinc nitrate hexahydrate.

4. The preparation method according to claim 2, characterized in that The organic ligand includes 2-methylimidazole.

5. The preparation method according to claim 2, characterized in that The solvent includes water.

6. The preparation method according to claim 2, characterized in that The mass ratio of the metal nitrate to the organic ligand is 1:(2.5-3.5).

7. The preparation method according to claim 2, characterized in that The sintering temperature is 900-930°C.

8. The preparation method according to claim 2, characterized in that The sintering time is 1.5-2.5h.

9. The preparation method according to claim 1, characterized in that The iron source in step (2) includes ferrous sulfate heptahydrate.

10. The preparation method according to claim 1, characterized in that The phosphorus source in step (2) includes phosphoric acid.

11. The preparation method according to claim 1, characterized in that The oxidant in step (2) includes hydrogen peroxide.

12. The preparation method according to claim 1, characterized in that In step (a), the phosphorus source is phosphoric acid, the mixture is a mixed solution, and the molar concentration of the iron source in the mixed solution is 0.8-1.2 mol / L.

13. The preparation method according to claim 12, characterized in that The mass volume ratio of the carbon nanosheets to the mixed solution in step (b) is 1 mg:(8-12) mL.

14. The preparation method according to claim 1, characterized in that The secondary mixing process in step (b) is accompanied by stirring, and the stirring time is 0.8-1.2h.

15. The preparation method according to claim 1, characterized in that During the secondary mixing in step (b), alkali solution is also added.

16. The preparation method according to claim 15, characterized in that The alkali solution includes aqueous ammonia.

17. The preparation method according to claim 15, characterized in that The molar concentration of the alkali solution is 2-3 mol / L.

18. The preparation method according to claim 1, characterized in that The pH of the mixed solution in step (2) is 2-5.

19. The preparation method according to claim 1, characterized in that The reaction time in step (2) is 3-8 minutes.

20. The preparation method according to claim 1, characterized in that The reaction temperature in step (2) is 30-80°C.

21. The preparation method according to claim 1, characterized in that The atmosphere of the primary calcination in step (2) is an inert atmosphere, and the gas in the inert atmosphere includes nitrogen and / or argon.

22. The preparation method according to claim 1, characterized in that The temperature of the primary calcination in step (2) is 500-700°C.

23. The preparation method according to claim 1, characterized in that The time of the first calcination in step (2) is 1-3 hours.

24. The preparation method according to claim 1, characterized in that The lithium source in step (3) includes lithium carbonate and / or lithium titanate.

25. The preparation method according to claim 1, characterized in that The reducing agent in step (3) includes an organic carbon source.

26. The preparation method according to claim 25, characterized in that The organic carbon source includes glucose.

27. The preparation method according to claim 1, characterized in that The mixing process in step (3) is carried out in a solvent, and the solvent includes ethanol.

28. The preparation method according to claim 1, characterized in that The mixing method in step (3) is ball milling.

29. The preparation method according to claim 28, characterized in that The ball milling time is 18-30 hours.

30. The preparation method according to claim 1, characterized in that The atmosphere of the secondary calcination in step (3) is an inert atmosphere, and the gas in the inert atmosphere includes nitrogen and / or argon.

31. The preparation method according to claim 1, characterized in that The temperature of the secondary calcination in step (3) is 600-800°C.

32. The preparation method according to claim 1, characterized in that The time of the secondary calcination in step (3) is 5-15h.

33. The preparation method according to claim 1, characterized in that The preparation method comprises the following steps: (I) preparing nitrogen-doped carbon nanosheets by a coprecipitation method, and mixing and reacting with a modifier to obtain modified carbon nanosheets; (II) mixing the iron source and the phosphorus source once to obtain a mixed solution having an iron source concentration of 0.8-1.2 mol / L; (III) mixing the mixed solution of step (II) and the modified carbon nanosheets of step (I) for a second time and stirring for 0.8-1.2 hours to obtain solution A; Wherein, the mass volume ratio of the modified carbon nanosheets to the mixed solution is 1 mg: (8-12) mL; (IV) adding an alkali solution and an oxidant having a molar concentration of 2-3 mol / L to the solution A of step (III) and mixing them three times to obtain a solution B having a pH of 2-5; The solution B is reacted at 30-80° C. for 3-8 minutes and then calcined in an inert atmosphere at 500-700° C. for 1-3 hours to obtain a precursor; (V) dispersing the precursor in step (IV) with a lithium source and an organic carbon source in a solvent, ball milling and mixing for 18-30 hours, and then secondary calcining in an inert atmosphere at 600-800° C. for 5-15 hours to obtain the lithium iron phosphate positive electrode material.

34. A lithium iron phosphate positive electrode material prepared by the preparation method according to any one of claims 1 to 33, characterized in that: The lithium iron phosphate positive electrode material includes a lithium iron phosphate core and a carbon coating layer located on the surface of the core.

35. The lithium iron phosphate cathode material according to claim 34, characterized in that The average particle size of the lithium iron phosphate positive electrode material is 200-300 nm.

36. A lithium ion battery, characterized in that The positive electrode of the lithium-ion battery includes the lithium iron phosphate positive electrode material as described in claim 34.

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