Composite cathode material and preparation method thereof

Abstract

The application discloses a kind of composite positive electrode material and preparation method thereof, compared with prior art, composite positive electrode material is prepared by the way of grain mosaic, the material structure is stable, crystallinity is high, particle size distribution is uniform, improve the low temperature performance and rate capability of lithium iron phosphate positive electrode material.The application also uses microwave hydrothermal method to prepare precursor, while adding surfactant can effectively improve the dispersity of lithium ion, vanadium ion and phosphate in solution, which is beneficial to reduce the particle size of the material and reduce the agglomeration phenomenon, and is beneficial to form composite positive electrode material with smaller particles and uniform particle size.In addition, the raw materials involved in the application are widely available, low in price, and the preparation method is simple and the reaction process is easy to control, meeting the requirements of green chemistry and suitable for industrial large-scale production.

Description

Technical Field

[0001] This invention relates to the field of secondary battery technology, and in particular to a composite cathode material and its preparation method. Background Technology

[0002] As the application scope of lithium-ion batteries continues to expand, their performance and safety are becoming increasingly important. Although lithium iron phosphate (LFP) has gained a larger market share due to its low cost and long cycle life, its inherent structural characteristics also lead to limitations such as restricted high-current charge / discharge capabilities and poor low-temperature performance, thus limiting its application in specific scenarios. In contrast, lithium vanadium phosphate (LFP) exhibits a higher voltage platform, and as a fast ion conductor, it possesses more lithium-ion intercalation / deintercalation capabilities, with more unobstructed three-dimensional diffusion channels, resulting in superior conductivity compared to LFP. However, its high cost has become a bottleneck for the large-scale industrial application of LFP. Combining LFP and LFP may improve the performance of LFP under high-rate and low-temperature conditions.

[0003] Chinese patent CN105870428A discloses a preparation process for a lithium iron phosphate-lithium vanadium phosphate composite cathode material precursor. This process uses ferrous gluconate and sodium metavanadate as raw materials in a 1:2 molar ratio, synthesizing rod-shaped ferrous metavanadate as a precursor via a hydrothermal method. The rod-shaped structure increases the contact area with the electrolyte, effectively shortening the lithium-ion diffusion path and thus improving the rate performance of the cathode material. Unfortunately, the prepared cathode material exhibits poor overall morphological uniformity, and the improvement in low-temperature performance is not significant.

[0004] Another Chinese patent, CN104269530A, describes a method for hydrothermal synthesis of lithium vanadium phosphate-lithium iron phosphate composite cathode materials. This method involves adding an iron source solution and a vanadium source solution to a high-pressure stirred reactor, adding an appropriate amount of urea, and obtaining a suspension slurry by precisely controlling the stirring temperature and speed. Subsequently, a lithium source, a phosphorus source, and a composite carbon source are added, followed by high-pressure stirring, washing, filtration, and freeze-drying to obtain a uniformly morphological lithium iron phosphate-lithium vanadium phosphate composite powder. Although the composite material prepared by this method has good morphological consistency, the synthesis steps are cumbersome, and the freeze-drying process has high energy consumption, affecting the economic viability for its industrial application.

[0005] Given the aforementioned problems with current cathode materials, there is an urgent need to propose a technical solution that can effectively address these deficiencies. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing composite cathode materials, which can produce composite cathode materials with stable structure, high crystallinity, and uniform particle size distribution.

[0007] To achieve this objective, the present invention provides the following solution:

[0008] A method for preparing a composite cathode material includes the following steps:

[0009] Step 1: Dissolve the first lithium source, vanadium source and the first phosphorus source in deionized water, add the first surfactant, and sonicate for 1 to 3 hours to obtain solution A;

[0010] Step 2: React solution A using a microwave hydrothermal method at a temperature of 75–150℃ for 10–40 min, then dry to obtain the lithium vanadium phosphate precursor;

[0011] Step 3: Sinter the lithium vanadium phosphate precursor at 650-850°C for 6-12 hours, then cool to obtain the lithium vanadium phosphate cathode material;

[0012] Step 4: Dissolve the second lithium source, iron source, second phosphorus source and carbon source in deionized water to obtain solution B;

[0013] Step 5: Pour the lithium vanadium phosphate cathode material and the second surfactant into solution B and mix. Disperse by ultrasonication for 12-72 hours, then react at 90-180°C for 15-60 minutes using microwave hydrothermal method. Dry to obtain the composite cathode material precursor.

[0014] Step 6: Sinter the composite cathode material precursor at 600-800℃ for 6-12 hours to obtain the composite cathode material.

[0015] Preferably, in step one, the first surfactant is at least one of oleic acid, sodium dodecyl sulfate, polyvinylpyrrolidone, stearic acid, and fatty acid glycerides; the molar ratio of the first surfactant to the vanadium source is 0.03 to 0.1:1.

[0016] Preferably, in step five, the second surfactant is at least one of sodium dodecyl sulfonate, sodium hexadecylbenzene sulfonate, hexadecyltrimethylammonium bromide, polyvinylpyrrolidone, and tetrabutylammonium bromide; and the molar ratio of the second surfactant to the phosphorus source is 0.01 to 0.08:1.

[0017] Preferably, in step one, the molar ratio of lithium in the first lithium source, vanadium in the vanadium source, and phosphorus in the first phosphorus source is (1.5-1.7):(1-1.2):(1.5-1.7).

[0018] Preferably, in step one, the first lithium source is at least one of lithium sulfate, lithium chloride, lithium nitrate, lithium oxalate, lithium acetate, and lithium carbonate.

[0019] Preferably, in step one, the vanadium source is at least one selected from vanadium pentoxide, ammonium metavanadate, sodium vanadate, sodium metavanadate vanadium trioxide, vanadium oxalate, and vanadium oxysulfate.

[0020] Preferably, in step four, the molar ratio of lithium in the second lithium source to iron in the iron source is 1.018 to 1.028:1, the molar ratio of lithium in the second lithium source to phosphorus in the second phosphorus source is 0.965 to 0.985, and the mass of the carbon source is 5 to 15% of the sum of the masses of the second lithium source, the iron source, and the second phosphorus source.

[0021] The present invention also provides a composite cathode material, which is prepared by the above-mentioned method for preparing composite cathode material, comprising lithium vanadium phosphate and lithium iron phosphate coated on the surface of lithium vanadium phosphate, wherein the lithium vanadium phosphate is embedded in the lattice of the lithium iron phosphate.

[0022] Preferably, the lithium vanadium phosphate has a particle size of 10-30 nm; the lithium iron phosphate has a particle size of 0.3-0.9 μm.

[0023] Preferably, the ratio of the composite cathode material (D90-D10) to D50 is 1.9 to 2.4.

[0024] Preferably, the molar ratio of vanadium in the lithium vanadium phosphate to iron in the lithium iron phosphate is 0.01 to 0.07:1.

[0025] The present invention also provides a negative electrode sheet, wherein the positive electrode material is the above-mentioned composite positive electrode material.

[0026] The present invention also provides a secondary battery, comprising a cell formed by the above-mentioned positive electrode, separator and negative electrode, an electrolyte, and a battery casing for encapsulating the cell and electrolyte.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0028] (1) The present invention prepares a composite cathode material by grain embedding. The material has a stable structure, high crystallinity and uniform particle size distribution, which improves the low temperature performance and rate performance of lithium iron phosphate cathode material.

[0029] (2) The present invention uses microwave hydrothermal method to prepare precursor. At the same time, the addition of surfactant can effectively improve the dispersibility of lithium ions, vanadium ions and phosphate ions in solution, which is conducive to reducing the particle size of the material, reducing agglomeration, and forming a composite cathode material with smaller particles and uniform particle size distribution.

[0030] (3) The raw materials involved in this invention are widely available and inexpensive. The preparation method is simple and the reaction process is easy to control, which meets the requirements of green chemistry and is suitable for large-scale industrial production. Attached Figure Description

[0031] Figure 1 This is one of the SEM images of the composite cathode material in Embodiment 1 of the present invention;

[0032] Figure 2 This is the second SEM image of the composite cathode material of Embodiment 1 of the present invention;

[0033] Figure 3 The graphs show the EIS test data of the lithium-ion batteries in the examples and comparative examples. Detailed Implementation

[0034] To make the technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] According to a first aspect of the present invention, a method for preparing a composite cathode material is provided, comprising the following steps:

[0036] Step 1: Dissolve the first lithium source, vanadium source and the first phosphorus source in deionized water, add the first surfactant, and sonicate for 1 to 3 hours to obtain solution A;

[0037] Step 2: React solution A using a microwave hydrothermal method at a temperature of 75–150℃ for 10–40 min, then dry to obtain the lithium vanadium phosphate precursor;

[0038] Step 3: Sinter the lithium vanadium phosphate precursor at 650-850℃ for 6-12 hours, then cool to obtain the lithium vanadium phosphate cathode material;

[0039] Step 4: Dissolve the second lithium source, iron source, second phosphorus source and carbon source in deionized water to obtain solution B;

[0040] Step 5: Pour the lithium vanadium phosphate cathode material and the second surfactant into solution B and mix. Disperse by ultrasonication for 12-72 hours, then react at 90-180℃ for 15-60 minutes using microwave hydrothermal method. Dry to obtain the composite cathode material precursor.

[0041] Step 6: Sinter the composite cathode material precursor at 600-800℃ for 6-12 hours to obtain the composite cathode material.

[0042] This invention prepares a composite cathode material through a grain embedding method. The material has a stable structure, high crystallinity, and uniform particle size distribution, which improves the low-temperature performance and rate performance of lithium iron phosphate cathode materials.

[0043] This invention utilizes a microwave hydrothermal method to prepare precursors. The addition of surfactants can effectively improve the dispersibility of lithium ions, vanadium ions, and phosphate ions in the solution, which is beneficial for reducing the particle size of the material, reducing agglomeration, and forming composite cathode materials with smaller particles and uniform particle size distribution.

[0044] The raw materials involved in this invention are widely available and inexpensive. The preparation method is simple and the reaction process is easy to control, which meets the requirements of green chemistry and is suitable for large-scale industrial production.

[0045] In one embodiment of the present invention, in step one, the first surfactant is at least one selected from oleic acid, sodium dodecyl sulfate, polyvinylpyrrolidone, stearic acid, and fatty acid glycerides; the molar ratio of the first surfactant to the vanadium source is 0.03 to 0.1:1. Adding a surfactant can effectively improve the dispersibility of lithium ions, vanadium ions, and phosphate ions in the solution, which is beneficial for reducing the particle size of the material, reducing agglomeration, and forming a composite cathode material with smaller particles and a uniform particle size distribution.

[0046] In one embodiment of the present invention, in step five, the second surfactant is at least one selected from sodium dodecyl sulfonate, sodium hexadecylbenzene sulfonate, hexadecyltrimethylammonium bromide, polyvinylpyrrolidone, and tetrabutylammonium bromide; the molar ratio of the second surfactant to the phosphorus source is 0.01 to 0.08:1.

[0047] The hydrophilic and hydrophobic groups of the first and second surfactants can adsorb onto the particle surface, reducing the interfacial tension between the particles and the solvent, and decreasing the van der Waals forces between particles, thereby inhibiting aggregation. They can also selectively adsorb onto specific crystal faces, inhibiting crystal growth along certain directions and guiding the formation of smaller and more isotropic particles.

[0048] When the molar ratio of the first surfactant to the vanadium source or the second surfactant to the phosphorus source is below the lower limit, the surfactant is insufficient to completely cover the particle surface, leading to local agglomeration; when it is above the upper limit, the particles may re-agglomerate due to an imbalance of intermolecular forces. This range requires less surfactant than conventional surfactants, resulting in lower cost while significantly reducing particle size.

[0049] In one embodiment of the present invention, in step one, the molar ratio of lithium in the first lithium source, vanadium in the vanadium source, and phosphorus in the first phosphorus source is (1.5-1.7):(1-1.2):(1.5-1.7).

[0050] In one embodiment of the present invention, in step one, the first lithium source is at least one of lithium sulfate, lithium chloride, lithium nitrate, lithium oxalate, lithium acetate, and lithium carbonate.

[0051] In one embodiment of the present invention, in step four, the second lithium source is at least one of lithium sulfate, lithium chloride, lithium nitrate, lithium oxalate, lithium acetate, and lithium carbonate.

[0052] In one embodiment of the present invention, in step one, the vanadium source is at least one selected from vanadium pentoxide, ammonium metavanadate, sodium vanadate, sodium metavanadate vanadium trioxide, vanadium oxalate, and vanadium oxysulfate.

[0053] In one embodiment of the present invention, in step four, the molar ratio of lithium in the second lithium source to iron in the iron source is 1.018 to 1.028:1, the molar ratio of lithium in the second lithium source to phosphorus in the second phosphorus source is 0.965 to 0.985, and the mass of the carbon source is 5 to 15% of the sum of the masses of the second lithium source, the iron source, and the second phosphorus source.

[0054] In one embodiment of the present invention, in step one, the phosphorus source is one or more of ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, ammonium phosphate, sodium dihydrogen phosphate, sodium monohydrogen phosphate, and sodium phosphate.

[0055] In one embodiment of the present invention, in step three, the inert gas during sintering is either nitrogen or argon.

[0056] In one embodiment of the present invention, in step four, the carbon source is at least one selected from citric acid, glucose, lactic acid, malic acid, sucrose, phenolic resin, and polyethylene glycol.

[0057] In one embodiment of the present invention, in step four, the iron source is at least one of ferrous sulfate, ferrous oxalate, ferrous nitrate, ferric nitrate, and ferric chloride.

[0058] In a second aspect, the present invention also provides a composite cathode material, prepared by the above-described method for preparing composite cathode materials, comprising lithium vanadium phosphate and lithium iron phosphate coated on the surface of lithium vanadium phosphate, wherein the lithium vanadium phosphate is embedded in the lattice of lithium iron phosphate.

[0059] The composite cathode material of this invention has the characteristics of internally embedded lithium vanadium phosphate primary particles and externally encapsulated lithium iron phosphate primary particles. The internal lithium vanadium phosphate primary particles are small, and lithium iron phosphate crystals grow on the lithium vanadium phosphate in a heterogeneous nucleation manner. Due to lattice mismatch at the phase interface, the reactivity is improved, and the lithium-ion solid-phase conduction and deintercalation displacement are reduced, thereby reducing the resistance to lithium-ion conduction and deintercalation within the particles.

[0060] In one embodiment of the present invention, the particle size of lithium vanadium phosphate is 10-30 nm; for example, it can be 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, 16 nm, 17 nm, 18 nm, 19 nm, 20 nm, 21 nm, 22 nm, 23 nm, 24 nm, 25 nm, 26 nm, 27 nm, 28 nm, 29 nm, or 30 nm; the particle size of lithium iron phosphate is 0.5-3 μm, for example, it can be... The particle sizes of lithium vanadium phosphate (LNP) and lithium iron phosphate (LFP) are 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2.0 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm, 2.6 μm, 2.7 μm, 2.8 μm, 2.9 μm, and 3.0 μm. The particle sizes of LNP and LFP must be limited to this range so that LFP crystals can grow on LNP in a heterogeneous nucleation manner, resulting in a composite cathode material with an internally embedded LNP primary particle and an externally encapsulated LFP primary particle structure.

[0061] In one embodiment of the invention, the ratio of the composite cathode material (D90-D10) to D50 is 1.9 to 2.4. This parameter range indicates that the material particles have good uniformity.

[0062] In one embodiment of the present invention, the molar ratio of vanadium in the lithium vanadium phosphate to iron in the lithium iron phosphate is 0.01 to 0.07:1; for example, it can be 0.02:1; 0.03:1; 0.04:1; 0.05:1; 0.06:1; 0.07:1. Interdoping occurs at the interface between the internally embedded lithium vanadium phosphate and the externally grown lithium iron phosphate. The V doping of lithium vanadium phosphate enters the lithium iron phosphate crystal, and the Fe doping of lithium iron phosphate enters the lithium vanadium phosphate, which can improve the reversibility of the cathode material. Simultaneously, due to the substitution of Fe and V ions of different valence states in the structure, a p-type or n-type conductivity mechanism is introduced into the structure, affecting the ion coordination in the material and increasing the lithium-ion diffusion rate.

[0063] In a third aspect, the present invention also provides a negative electrode sheet, wherein the positive electrode material is the aforementioned composite positive electrode material.

[0064] In a fourth aspect, the present invention also provides a secondary battery comprising a cell formed by the above-described positive electrode, separator, and negative electrode, an electrolyte, and a battery casing for encapsulating the cell and electrolyte.

[0065] The negative electrode includes a negative current collector and a negative active material layer coated on at least one surface of the negative current collector. The negative active material layer may be one or more of the following, including but not limited to graphite, soft carbon, hard carbon, carbon fiber, mesophase carbon microspheres, silicon-based materials, tin-based materials, lithium titanate, or other metals that can form alloys with lithium.

[0066] The graphite can be selected from one or more of artificial graphite, natural graphite, and modified graphite; the silicon-based material can be selected from one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, and silicon alloys; the tin-based material can be selected from one or more of elemental tin, tin oxide compounds, and tin alloys. The negative electrode current collector is typically a structure or component that collects current. The negative electrode current collector can be any material suitable for use as a negative electrode current collector in a secondary battery, for example, it can be, but is not limited to, metal foil, and more specifically, it can be, but is not limited to, copper foil.

[0067] The electrolyte of this secondary battery includes an organic solvent, an electrolyte lithium salt, and additives. The electrolyte lithium salt can be LiPF6 and / or LiBOB used in high-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, and LiPF6 used in low-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, LiPF6, and LiTFSI used in overcharge-resistant electrolytes; or it can be at least one of LiClO4, LiAsF6, LiCF3SO3, and LiN(CF3SO2)2. The organic solvent can be a cyclic carbonate, including PC and EC; it can also be a chain carbonate, including DFC, DMC, or EMC; or it can be a carboxylic acid ester, including MF, MA, EA, MP, etc. The additives include, but are not limited to, at least one of film-forming additives, conductive additives, flame-retardant additives, overcharge-resistant additives, additives for controlling the H2O and HF content in the electrolyte, additives for improving low-temperature performance, and multifunctional additives.

[0068] The present invention will be further described below through specific embodiments.

[0069] Example 1

[0070] Preparation of composite cathode materials:

[0071] (1) Weigh 1.5 mmol lithium carbonate, 1 mmol vanadium pentoxide and 1.5 mmol phosphoric acid respectively, dissolve them in 500 ml deionized water, add 0.03 mmol oleic acid, sonicate for 1 hour to prepare solution A.

[0072] (2) Solution A was subjected to microwave hydrothermal method for 10 min at 150 °C and dried to obtain lithium vanadium phosphate precursor.

[0073] (3) The lithium vanadium phosphate precursor was sintered in argon gas and cooled to obtain the lithium vanadium phosphate cathode material. The sintering temperature was 650℃ and the sintering time was 12 hours.

[0074] (4) Weigh 102.3 mmol lithium carbonate, 100.0 mmol ferrous oxalate and 103.1 mmol ammonium dihydrogen phosphate and dissolve them in 500 mL deionized water. Add 5.074 g glucose and stir to obtain solution B.

[0075] (5) Add 1 mmol of polyvinylpyrrolidone and lithium vanadium phosphate cathode material to solution B, disperse ultrasonically for 72 hours, synthesize by microwave hydrothermal method, reaction time 15 min, reaction temperature 180℃, and dry to obtain composite cathode material precursor.

[0076] (6) The composite cathode material precursor is sintered in argon gas and cooled to obtain the composite cathode material. The sintering temperature is 600℃ and the sintering time is 12 hours.

[0077] Methods for preparing secondary batteries:

[0078] A lithium-ion battery is prepared by interleaving and winding a negative electrode, a positive electrode, and a separator, then encapsulating them with an aluminum-plastic film. The cell is placed in a battery casing, electrolyte is added, and then the casing is sealed. The active material of the negative electrode is graphite, and the active material of the positive electrode is the aforementioned composite positive electrode material.

[0079] Example 2

[0080] Preparation of composite cathode materials

[0081] (1) Weigh 1.55 mmol lithium carbonate, 1.05 mmol vanadium pentoxide and 1.55 mmol phosphoric acid respectively, dissolve them in 500 ml deionized water, add 0.053 mmol oleic acid, and sonicate for 2 hours to prepare solution A.

[0082] (2) Solution A was subjected to microwave hydrothermal method for 20 min at 125 °C and dried to obtain lithium vanadium phosphate precursor.

[0083] (3) The lithium vanadium phosphate precursor was sintered in argon gas and cooled to obtain the lithium vanadium phosphate cathode material. The sintering temperature was 700℃ and the sintering time was 10 hours.

[0084] (4) Weigh 35.8 mmol lithium carbonate, 35 mmol ferrous oxalate and 36.1 mmol ammonium dihydrogen phosphate and dissolve them in 500 mL deionized water. Add 1.775 g glucose and stir to obtain solution B.

[0085] (5) Add 1.05 mmol of polyvinylpyrrolidone and lithium vanadium phosphate cathode material to solution B, disperse ultrasonically for 48 hours, synthesize by microwave hydrothermal method, reaction time 30 min, reaction temperature 150℃, and dry to obtain composite cathode material precursor.

[0086] (6) The composite cathode material precursor is sintered in argon gas and cooled to obtain the composite cathode material. The sintering temperature is 650℃ and the sintering time is 10 hours.

[0087] The rest are the same as in Implementation 1, and will not be repeated here.

[0088] Example 3

[0089] Preparation of composite cathode materials

[0090] (1) Weigh 1.65 mmol lithium carbonate, 1.15 mmol vanadium pentoxide and 1.65 mmol phosphoric acid respectively, dissolve them in 500 ml deionized water, add 0.081 mmol oleic acid, and sonicate for 2 hours to prepare solution A.

[0091] (2) Solution A was subjected to microwave hydrothermal method for 30 min at 100 °C and dried to obtain lithium vanadium phosphate precursor.

[0092] (3) The lithium vanadium phosphate precursor was sintered in argon gas and cooled to obtain the lithium vanadium phosphate cathode material. The sintering temperature was 800℃ and the sintering time was 8 hours.

[0093] (4) Weigh 23.5 mmol lithium carbonate, 23.0 mmol ferrous oxalate and 36.1 mmol ammonium dihydrogen phosphate and dissolve them in 500 mL deionized water. Add 1.014 g glucose and stir to obtain solution B.

[0094] (5) Add 1.15 mmol of polyvinylpyrrolidone and lithium vanadium phosphate cathode material to solution B, disperse ultrasonically for 36 hours, synthesize by microwave hydrothermal method, reaction time 30 min, reaction temperature 150℃, and dry to obtain composite cathode material precursor.

[0095] (6) The composite cathode material precursor is sintered in argon gas and cooled to obtain the composite cathode material. The sintering temperature is 650℃ and the sintering time is 10 hours.

[0096] The rest are the same as in Implementation 1, and will not be repeated here.

[0097] Example 4

[0098] Preparation of composite cathode materials

[0099] (1) Weigh 1.7 mmol lithium carbonate, 1.2 mmol vanadium pentoxide and 1.7 mmol phosphoric acid respectively, dissolve them in 500 ml deionized water, add 0.12 mmol oleic acid, and sonicate for 3 hours to prepare solution A.

[0100] (2) Solution A was subjected to microwave hydrothermal method for 40 min at 75 °C and dried to obtain lithium vanadium phosphate precursor.

[0101] (3) The lithium vanadium phosphate precursor was sintered in argon gas and cooled to obtain the lithium vanadium phosphate cathode material. The sintering temperature was 850℃ and the sintering time was 6 hours.

[0102] (4) Weigh 17.5 mmol lithium carbonate, 17.1 mmol ferrous oxalate and 17.7 mmol ammonium dihydrogen phosphate and dissolve them in 500 mL deionized water. Add 0.869 g glucose and stir to obtain solution B.

[0103] (5) Add 1.368 mmol of polyvinylpyrrolidone and lithium vanadium phosphate cathode material to solution B, disperse ultrasonically for 12 hours, synthesize by microwave hydrothermal method, reaction time 60 min, reaction temperature 90℃, and dry to obtain composite cathode material precursor.

[0104] (6) The composite cathode material precursor is sintered in argon gas and cooled to obtain the composite cathode material. The sintering temperature is 800℃ and the sintering time is 6 hours.

[0105] The rest are the same as in Implementation 1, and will not be repeated here.

[0106] Comparative Example 1

[0107] Preparation method of composite cathode material:

[0108] (1) Weigh 1.5 mmol lithium carbonate, 1 mmol vanadium pentoxide, 1.5 mmol phosphoric acid, 102.3 mmol lithium carbonate, 100.0 mmol ferrous oxalate and 103.1 mmol ammonium dihydrogen phosphate and dissolve them in 500 mL deionized water. Add 5.074 g glucose and stir to obtain a mixed solution.

[0109] (2) The mixed solution was synthesized by microwave hydrothermal method, with a reaction time of 15 min and a reaction temperature of 180℃. After drying, the composite cathode material precursor was obtained.

[0110] (3) The composite cathode material precursor is sintered in argon gas and cooled to obtain the composite cathode material. The sintering temperature is 600℃ and the sintering time is 12 hours.

[0111] The rest are the same as in Implementation 1, and will not be repeated here.

[0112] Comparative Example 2

[0113] Preparation method of lithium iron phosphate:

[0114] (1) Weigh 1.7 mmol lithium carbonate, 1.2 mmol vanadium pentoxide, 1.7 mmol phosphoric acid, 17.5 mmol lithium carbonate, 17.1 mmol ferrous oxalate and 17.7 mmol ammonium dihydrogen phosphate and dissolve them in 500 mL deionized water. Add 0.869 g glucose and stir.

[0115] (2) The turbid liquid of the lithium vanadium phosphate-lithium iron phosphate composite cathode material was synthesized by microwave hydrothermal method. The reaction time was 60 min and the reaction temperature was 90℃. After drying, the precursor of lithium vanadium phosphate-lithium iron phosphate cathode material was obtained.

[0116] (3) The precursor was sintered in argon gas and cooled to obtain lithium vanadium phosphate-lithium iron phosphate composite cathode material. The sintering temperature was 800℃ and the sintering time was 6 hours.

[0117] The rest are the same as in Implementation 1, and will not be repeated here.

[0118] Performance testing:

[0119] The electrochemical performance of the secondary batteries from Examples 1-4 and Comparative Examples 1-2 was tested, and the test results are shown in Tables 1 and 2. Figure 3 ;

[0120] The composite cathode material of Example 1 was tested using electron microscopy. The test results are shown in [Figure 1]. Figure 1 .

[0121] Table 1

[0122]

[0123]

[0124] Table 2

[0125]

[0126] As shown in Table 1 and Figure 3 The test data shown indicates that the electrochemical performance data of Examples 1-4 are better than those of Comparative Examples 1-2. The secondary batteries prepared in Examples 1-4 have higher discharge specific capacity, which is more obvious under high rate conditions. This shows that the composite cathode material prepared by the present invention through grain embedding has a stable structure, high crystallinity, and uniform particle size distribution, which improves the rate performance of lithium iron phosphate cathode material.

[0127] As shown in Table 2, the electrochemical performance data of Examples 1-4 are better than those of Comparative Examples 1-2. Compared with Comparative Examples 1 and 2, the low-temperature performance of the secondary batteries prepared in Examples 1-4 is better, indicating that the composite cathode material prepared by the present invention through grain embedding has a stable structure, high crystallinity, and uniform particle size distribution, which improves the low-temperature performance of lithium iron phosphate cathode material.

[0128] like Figure 1 and Figure 2 It can be seen that the composite cathode material of the present invention has high crystallinity and uniform particle size distribution.

[0129] Based on the disclosure and teachings of the foregoing specification, those skilled in the art can make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments described above, and any obvious improvements, substitutions, or modifications made by those skilled in the art based on the present invention are within the scope of protection of the present invention. Furthermore, although some specific terms are used in this specification, these terms are only for convenience of explanation and do not constitute any limitation on the present invention.

Claims

1. A method for preparing a composite cathode material, characterized in that, Includes the following steps: Step 1: Dissolve the first lithium source, vanadium source and the first phosphorus source in deionized water, add the first surfactant, and sonicate for 1 to 3 hours to obtain solution A; the first surfactant is at least one of oleic acid, sodium dodecyl sulfate, polyvinylpyrrolidone, stearic acid and fatty acid glycerides; the molar ratio of the first surfactant to the vanadium source is 0.03 to 0.1:1; Step 2: React solution A using microwave hydrothermal method at a temperature of 75~150℃ for 10~40 min, then dry to obtain lithium vanadium phosphate precursor; Step 3: Sinter the lithium vanadium phosphate precursor at 650~850℃ for 6~12 hours, then cool to obtain the lithium vanadium phosphate cathode material; Step 4: Dissolve the second lithium source, iron source, second phosphorus source and carbon source in deionized water to obtain solution B; Step 5: Pour the lithium vanadium phosphate cathode material and the second surfactant into solution B and mix. Disperse ultrasonically for 12-72 hours, then react using a microwave hydrothermal method at 90-180°C for 15-60 minutes. Dry to obtain the composite cathode material precursor. The second surfactant is at least one of sodium dodecyl sulfonate, sodium hexadecylbenzene sulfonate, hexadecyltrimethylammonium bromide, polyvinylpyrrolidone, and tetrabutylammonium bromide. The molar ratio of the second surfactant to the phosphorus source is 0.01-0.08:

1. Step 6: Sinter the composite cathode material precursor at 600~800℃ for 6~12 hours to obtain the composite cathode material; The composite cathode material includes lithium vanadium phosphate and lithium iron phosphate coated on the surface of lithium vanadium phosphate, and the lithium vanadium phosphate is embedded in the crystal lattice of lithium iron phosphate.

2. The method for preparing the composite cathode material according to claim 1, characterized in that, In step one, the molar ratio of lithium in the first lithium source, vanadium in the vanadium source, and phosphorus in the first phosphorus source is (1.5~1.7):(1~1.2):(1.5~1.7).

3. The method for preparing the composite cathode material according to claim 1, characterized in that, In step one, the first lithium source is at least one of lithium sulfate, lithium chloride, lithium nitrate, lithium oxalate, lithium acetate, and lithium carbonate.

4. The method for preparing the composite cathode material according to claim 1, characterized in that, In step four, the molar ratio of lithium in the second lithium source to iron in the iron source is 1.018 to 1.028:1, the molar ratio of lithium in the second lithium source to phosphorus in the second phosphorus source is 0.965 to 0.985, and the mass of the carbon source is 5 to 15% of the sum of the masses of the second lithium source, the iron source, and the second phosphorus source.

5. A composite cathode material, characterized in that, It is prepared by the method for preparing the composite cathode material according to any one of claims 1 to 4.

6. The composite cathode material according to claim 5, characterized in that, The lithium vanadium phosphate has a particle size of 10-30 nm; the lithium iron phosphate has a particle size of 0.3-0.9 μm.

7. The composite cathode material according to claim 5, characterized in that, The ratio of the composite cathode material (D90-D10) to D50 is 1.9 to 2.

4.

8. The composite cathode material according to claim 5, characterized in that, The molar ratio of vanadium in the lithium vanadium phosphate to iron in the lithium iron phosphate is 0.01~0.07:1.

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