Composite positive electrode material and preparation method thereof

Abstract

According to the composite positive electrode material and the preparation method thereof, compared with the prior art, the composite positive electrode material is prepared in a grain inlaying mode, the material is stable in structure, high in crystallinity and uniform in particle size distribution, and the low-temperature performance and the rate performance of a lithium iron phosphate positive electrode material are improved. According to the invention, a precursor is prepared by using a microwave hydrothermal method, and a surfactant is added at the same time, so that the dispersibility of lithium ions, vanadium ions and phosphate radicals in a solution can be effectively improved, the particle size of the material can be reduced, the agglomeration phenomenon can be reduced, and the composite positive electrode material with small particles and uniform particle size distribution can be formed. In addition, the raw materials are wide in source and low in price, the preparation method is simple, the reaction process is easy to control, the requirements of green chemistry are met, and the method is suitable for industrial large-scale production.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular to a composite positive electrode material and a preparation method thereof. Background Art

[0002] As the scope of application of lithium-ion batteries continues to expand, their performance and safety issues are becoming increasingly important. Although lithium iron phosphate has occupied an increasingly large share of the market due to its advantages such as low cost and long cycle life, its inherent structural characteristics have also led to defects such as limited high-current charge and discharge capabilities and poor low-temperature performance, which has restricted its application in specific application scenarios. In contrast, lithium vanadium phosphate exhibits a higher voltage platform. As a fast ion conductor, it has more lithium ions that can be intercalated and deintercalated, and the three-dimensional diffusion channels of lithium ions in it are more unobstructed, so its conductivity is better than that of lithium iron phosphate. However, the high cost has become a bottleneck for the large-scale industrial application of lithium vanadium phosphate. The combination of lithium iron phosphate and lithium vanadium phosphate is expected to improve the performance of lithium iron phosphate under high-rate and low-temperature conditions.

[0003] Chinese patent CN105870428A discloses a preparation process for a lithium iron phosphate-lithium vanadium phosphate composite positive electrode material precursor, which uses ferrous gluconate and sodium metavanadate as raw materials, with a molar ratio of 1:2, and synthesizes rod-shaped ferrous metavanadate as a precursor by hydrothermal method. The rod-shaped structure increases the contact area with the electrolyte, effectively shortens the diffusion path of lithium ions, and thus improves the rate performance of the positive electrode material. Unfortunately, the prepared positive electrode material has poor consistency in overall morphology, and the improvement in low-temperature performance is not significant.

[0004] Another Chinese patent, CN104269530A, introduces 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 suspended slurry by precisely controlling the stirring temperature and speed. Subsequently, a lithium source, a phosphorus source, and a composite carbon source are added. After high-pressure stirring, reaction, washing, filtering, and freeze-drying, a lithium iron phosphate-lithium vanadium phosphate composite material powder with uniform morphology is obtained. Although the composite material prepared by this method has good morphological consistency, the synthesis steps are cumbersome, and the freeze-drying process consumes a high amount of energy, which affects the economic viability of its industrial application.

[0005] In view of the above-mentioned problems existing in current positive electrode materials, it is urgent to propose a technical solution that can effectively solve the above-mentioned defects. Summary of the Invention

[0006] The object of the present invention is to provide a method for preparing a composite positive electrode material, by which a composite positive electrode material with stable structure, high crystallinity and uniform particle size distribution can be prepared.

[0007] In order to achieve this object, the present invention provides the following scheme:

[0008] A method for preparing a composite positive electrode material comprises the following steps:

[0009] Step 1: dissolving a first lithium source, a vanadium source, and a first phosphorus source in deionized water, adding a first surfactant, and ultrasonically mixing for 1 to 3 hours to obtain a solution A;

[0010] Step 2: Solution A is reacted by microwave hydrothermal method at a temperature of 75 to 150° C. for 10 to 40 minutes, and then dried to obtain a lithium vanadium phosphate precursor;

[0011] Step 3: sintering the lithium vanadium phosphate precursor at 650-850° C. for 6-12 hours and cooling to obtain a lithium vanadium phosphate positive electrode material;

[0012] Step 4: dissolving the second lithium source, the iron source, the second phosphorus source and the 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, mix, and ultrasonically disperse for 12 to 72 hours, then react at a temperature of 90 to 180° C. for 15 to 60 minutes using a microwave hydrothermal method, and dry to obtain a composite cathode material precursor;

[0014] Step 6: Sintering the composite cathode material precursor at 600-800° C. for 6-12 hours to obtain a composite cathode material.

[0015] Preferably, in step 1, the first surfactant is at least one of oleic acid, sodium lauryl sulfate, polyvinyl pyrrolidone, stearic acid and fatty acid glyceride; and 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 hexadecylbenzenesulfonate, 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 1, the molar ratio of the lithium element of the first lithium source, the vanadium element of the vanadium source and the phosphorus element of the first phosphorus source is (1.5-1.7): (1-1.2): (1.5-1.7).

[0018] Preferably, in step 1, 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 1, the vanadium source is at least one of vanadium pentoxide, ammonium metavanadate, sodium vanadate, sodium metavanadate vanadium trioxide, vanadyl oxalate and vanadyl sulfate.

[0020] Preferably, in step four, the molar ratio of the lithium element of the second lithium source to the iron element of the iron source is 1.018-1.028:1, the molar ratio of the lithium element of the second lithium source to the phosphorus element of the second phosphorus source is 0.965-0.985, and the mass of the carbon source is 5-15% 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 positive electrode material, which is prepared by the above-mentioned preparation method of the composite positive electrode material, and includes lithium vanadium phosphate and lithium iron phosphate coated on the surface of the lithium vanadium phosphate, and the lithium vanadium phosphate is embedded in the lattice of the lithium iron phosphate.

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

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

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

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

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

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] (1) The present invention prepares a composite positive electrode 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 the lithium iron phosphate positive electrode material.

[0029] (2) The present invention utilizes a microwave hydrothermal method to prepare a precursor, and the addition of a surfactant can effectively improve the dispersibility of lithium ions, vanadium ions, and phosphate radicals in the solution, thereby reducing the particle size of the material, reducing agglomeration, and facilitating the formation of a composite positive electrode material with smaller particles and uniform particle size distribution.

[0030] (3) The raw materials involved in the present 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 industrial large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is one of the SEM test images of the composite positive electrode material of Example 1 of the present invention;

[0032] Figure 2 This is the second SEM test image of the composite positive electrode material of Example 1 of the present invention;

[0033] Figure 3 It is a graph showing EIS test data of the lithium-ion batteries of the embodiment and the comparative example. DETAILED DESCRIPTION

[0034] To make the technical solutions and advantages of the present invention more clear, 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 part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

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

[0036] Step 1: dissolving a first lithium source, a vanadium source, and a first phosphorus source in deionized water, adding a first surfactant, and ultrasonically mixing for 1 to 3 hours to obtain a solution A;

[0037] Step 2: Solution A is reacted by microwave hydrothermal method at a temperature of 75 to 150° C. for 10 to 40 minutes, and then dried to obtain a lithium vanadium phosphate precursor;

[0038] Step 3: sintering the lithium vanadium phosphate precursor at 650-850° C. for 6-12 hours and cooling to obtain a lithium vanadium phosphate positive electrode material;

[0039] Step 4: dissolving the second lithium source, the iron source, the second phosphorus source and the 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, mix, and ultrasonically disperse for 12 to 72 hours, then use a microwave hydrothermal method to react at a temperature of 90 to 180° C. for 15 to 60 minutes, and dry to obtain a composite cathode material precursor;

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

[0042] The present invention prepares a composite positive electrode material by a grain embedding method. The material has a stable structure, high crystallinity, and uniform particle size distribution, thereby improving the low-temperature performance and rate performance of the lithium iron phosphate positive electrode material.

[0043] The present invention utilizes a microwave hydrothermal method to prepare a precursor, and the addition of a surfactant can effectively improve the dispersibility of lithium ions, vanadium ions, and phosphate radicals in the solution, thereby helping to reduce the particle size of the material, reduce agglomeration, and facilitate the formation of a composite positive electrode material with smaller particles and uniform particle size distribution.

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

[0045] In one embodiment of the present invention, in step 1, the first surfactant is at least one of oleic acid, sodium lauryl sulfate, polyvinyl pyrrolidone, stearic acid, and fatty acid glycerides; and the molar ratio of the first surfactant to the vanadium source is 0.03 to 0.1:1. The addition of the surfactant can effectively improve the dispersibility of lithium ions, vanadium ions, and phosphate radicals in the solution, thereby reducing the particle size of the material and reducing agglomeration, thereby facilitating the formation of a composite cathode material with smaller particles and a uniform particle size distribution.

[0046] In one embodiment according to the present invention, in step five, the second surfactant is at least one of sodium dodecyl sulfonate, sodium hexadecylbenzenesulfonate, 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.

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

[0048] When the molar ratio of the first surfactant to the vanadium source or the second surfactant to the phosphorus source falls below the lower limit, the surfactant is insufficient to completely cover the particle surface, leading to localized agglomeration. When the molar ratio exceeds the upper limit, the particles may reaggregate due to an imbalance in intermolecular forces. This range requires less surfactant than conventional surfactants, resulting in lower costs and a significant reduction in particle size.

[0049] In one embodiment according to the present invention, in step 1, the molar ratio of the lithium element of the first lithium source, the vanadium element of the vanadium source, and the phosphorus element of 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 1, 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 4, 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 1, the vanadium source is at least one of vanadium pentoxide, ammonium metavanadate, sodium vanadate, sodium metavanadate, vanadium trioxide, vanadyl oxalate, and vanadyl sulfate.

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

[0056] In one embodiment according to the present invention, in step 4, the carbon source is at least one of citric acid, glucose, lactic acid, malic acid, sucrose, phenolic resin, and polyethylene glycol.

[0057] In one embodiment according to the present invention, in step 4, the iron source is at least one of ferrous sulfate, ferrous oxalate, ferrous nitrate, ferric nitrate, and ferric chloride.

[0058] In the second aspect of the present invention, the present invention also provides a composite positive electrode material, which is prepared by the above-mentioned preparation method of the composite positive electrode material, including lithium vanadium phosphate and lithium iron phosphate coated on the surface of the lithium vanadium phosphate, and the lithium vanadium phosphate is embedded in the lattice of the lithium iron phosphate.

[0059] The composite cathode material of the present invention features an internally embedded lithium vanadium phosphate primary particle structure surrounded by externally grown lithium iron phosphate primary particles. Within this structure, the lithium vanadium phosphate primary particles are small, while lithium iron phosphate crystals grow on the lithium vanadium phosphate via heterogeneous nucleation. The lattice mismatch at the interface enhances reactivity, reduces lithium ion solid-phase conduction and deintercalation displacement, and thus reduces lithium ion conduction and deintercalation resistance 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, 30 nm; the particle size of lithium iron phosphate is 0.5-3 μm, for example, The particle sizes of lithium vanadium phosphate and lithium iron phosphate need to be limited to this range so that lithium iron phosphate crystals can grow on lithium vanadium phosphate in the form of heterogeneous nucleation, obtaining a composite positive electrode material with a structure of lithium vanadium phosphate primary particles embedded inside and lithium iron phosphate primary particles wrapped and grown outside.

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

[0062] In one embodiment of the present invention, the molar ratio of the vanadium in the lithium vanadium phosphate to the 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. Mutual doping occurs at the interface between the internally embedded lithium vanadium phosphate and the externally encapsulated lithium iron phosphate. The V in the lithium vanadium phosphate is doped into the lithium iron phosphate crystal, and the Fe in the lithium iron phosphate is doped into the lithium vanadium phosphate, which can improve the reversibility of the positive electrode material. At the same time, due to the substitution of Fe and V ions of different valence states in the structure, a p-type or n-type conductive mechanism is introduced into the structure, affecting the ion coordination in the material and improving the lithium ion diffusion rate of the material.

[0063] In the third aspect of the present invention, the present invention further provides a negative electrode sheet, wherein the positive electrode material is the above-mentioned composite positive electrode material.

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

[0065] Among them, the negative electrode plate includes a negative electrode current collector and a negative electrode active material layer coated on at least one surface of the negative electrode current collector. The negative electrode active material layer can be one or more of, 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] Graphite can be selected from one or more of artificial graphite, natural graphite, and modified graphite; silicon-based materials can be selected from one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, and silicon alloys; and tin-based materials 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 secondary batteries. For example, the negative electrode current collector can include, but is not limited to, metal foil, and more specifically, can include, 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, as used in high-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, and LiPF6, as used in low-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, LiPF6, and LiTFSI, as used in overcharge-preventing 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; a chain carbonate, including DFC, DMC, or EMC; or a carboxylic acid ester, including MF, MA, EA, and MP. Additives include, but are not limited to, at least one of a film-forming additive, a conductive additive, a flame retardant additive, an overcharge prevention additive, an additive for controlling the H2O and HF content in the electrolyte, an additive for improving low-temperature performance, and a multifunctional additive.

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

[0069] Example 1

[0070] Preparation of composite cathode materials:

[0071] (1) 1.5 mmol of lithium carbonate, 1 mmol of vanadium pentoxide, and 1.5 mmol of phosphoric acid were weighed separately and dissolved in 500 ml of deionized water. 0.03 mmol of oleic acid was added and ultrasonically mixed for 1 hour to prepare solution A.

[0072] (2) Solution A was subjected to microwave hydrothermal method with a reaction time of 10 min and a reaction temperature of 150° C., and then dried to obtain a lithium vanadium phosphate precursor.

[0073] (3) Sintering the lithium vanadium phosphate precursor in argon and obtaining the lithium vanadium phosphate positive electrode material after cooling, wherein the sintering temperature is 650° C. and the sintering time is 12 hours.

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

[0075] (5) 1 mmol of polyvinyl pyrrolidone and lithium vanadium phosphate positive electrode material were added to solution B, ultrasonically dispersed for 72 hours, synthesized by microwave hydrothermal method with a reaction time of 15 minutes and a reaction temperature of 180°C, and dried to obtain a composite positive electrode material precursor.

[0076] (6) Sintering the composite cathode material precursor in argon and cooling it to obtain the composite cathode material. The sintering temperature is 600° C. and the sintering time is 12 hours.

[0077] Preparation method of secondary battery:

[0078] The negative electrode sheet, positive electrode sheet, and separator are interlaced and wound, then encapsulated with aluminum-plastic film to form a battery cell. This is then placed in a battery case, electrolyte is added, and the battery is sealed to produce a lithium-ion battery. The active material of the negative electrode sheet is graphite, while the active material of the positive electrode sheet is the composite positive electrode material described above.

[0079] Example 2

[0080] Preparation of composite cathode materials

[0081] (1) 1.55 mmol of lithium carbonate, 1.05 mmol of vanadium pentoxide, and 1.55 mmol of phosphoric acid were weighed separately and dissolved in 500 ml of deionized water. 0.053 mmol of oleic acid was added and ultrasonically mixed for 2 hours to prepare solution A.

[0082] (2) Solution A was subjected to microwave hydrothermal method with a reaction time of 20 min and a reaction temperature of 125° C., and then dried to obtain a lithium vanadium phosphate precursor.

[0083] (3) Sintering the lithium vanadium phosphate precursor in argon and obtaining the lithium vanadium phosphate positive electrode material after cooling, wherein the sintering temperature is 700° C. and the sintering time is 10 hours.

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

[0085] (5) 1.05 mmol of polyvinyl pyrrolidone and lithium vanadium phosphate positive electrode material were added to solution B, ultrasonically dispersed for 48 hours, synthesized by microwave hydrothermal method with a reaction time of 30 minutes and a reaction temperature of 150°C, and dried to obtain a composite positive electrode material precursor.

[0086] (6) Sintering the composite cathode material precursor in argon and cooling it to obtain the composite cathode material. The sintering temperature is 650° C. and the sintering time is 10 hours.

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

[0088] Example 3

[0089] Preparation of composite cathode materials

[0090] (1) 1.65 mmol of lithium carbonate, 1.15 mmol of vanadium pentoxide, and 1.65 mmol of phosphoric acid were weighed separately and dissolved in 500 ml of deionized water. 0.081 mmol of oleic acid was added and ultrasonically mixed for 2 hours to prepare solution A.

[0091] (2) Solution A was subjected to microwave hydrothermal method with a reaction time of 30 min and a reaction temperature of 100° C., and then dried to obtain a lithium vanadium phosphate precursor.

[0092] (3) Sintering the lithium vanadium phosphate precursor in argon and obtaining the lithium vanadium phosphate positive electrode material after cooling, wherein the sintering temperature is 800° C. and the sintering time is 8 hours.

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

[0094] (5) 1.15 mmol of polyvinyl pyrrolidone and lithium vanadium phosphate positive electrode material were added to solution B, ultrasonically dispersed for 36 hours, synthesized by microwave hydrothermal method with a reaction time of 30 minutes and a reaction temperature of 150°C, and dried to obtain a composite positive electrode material precursor.

[0095] (6) Sintering the composite cathode material precursor in argon and cooling it to obtain the composite cathode material. The sintering temperature is 650° C. and the sintering time is 10 hours.

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

[0097] Example 4

[0098] Preparation of composite cathode materials

[0099] (1) 1.7 mmol of lithium carbonate, 1.2 mmol of vanadium pentoxide, and 1.7 mmol of phosphoric acid were weighed separately and dissolved in 500 ml of deionized water. 0.12 mmol of oleic acid was added and ultrasonically mixed for 3 hours to prepare solution A.

[0100] (2) Solution A was subjected to microwave hydrothermal method with a reaction time of 40 min and a reaction temperature of 75° C., and then dried to obtain a lithium vanadium phosphate precursor.

[0101] (3) Sintering the lithium vanadium phosphate precursor in argon and obtaining the lithium vanadium phosphate positive electrode material after cooling, wherein the sintering temperature is 850° C. and the sintering time is 6 hours.

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

[0103] (5) 1.368 mmol of polyvinyl pyrrolidone and lithium vanadium phosphate positive electrode material were added to solution B, ultrasonically dispersed for 12 hours, synthesized by microwave hydrothermal method with a reaction time of 60 minutes and a reaction temperature of 90°C, and dried to obtain a composite positive electrode material precursor.

[0104] (6) Sintering the composite cathode material precursor in argon and cooling it to obtain the composite cathode material. The sintering temperature is 800° C. and the sintering time is 6 hours.

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

[0106] Comparative Example 1

[0107] Preparation method of composite positive electrode material:

[0108] (1) 1.5 mmol of lithium carbonate, 1 mmol of vanadium pentoxide, 1.5 mmol of phosphoric acid, 102.3 mmol of lithium carbonate, 100.0 mmol of ferrous oxalate, and 103.1 mmol of ammonium dihydrogen phosphate were weighed and dissolved in 500 mL of deionized water. 5.074 g of glucose was added and stirred 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°C, and a composite positive electrode material precursor was obtained after drying.

[0110] (3) Sintering the composite cathode material precursor in argon and cooling it to obtain the composite cathode material. The sintering temperature is 600° C. and the sintering time is 12 hours.

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

[0112] Comparative Example 2

[0113] Preparation method of lithium iron phosphate:

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

[0115] (2) The lithium vanadium phosphate-lithium iron phosphate composite positive electrode material turbid solution is synthesized by microwave hydrothermal method with a reaction time of 60 min and a reaction temperature of 90° C., and then dried to obtain a lithium vanadium phosphate-lithium iron phosphate positive electrode material precursor.

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

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

[0118] Performance testing:

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

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

[0121] Table 1

[0122]

[0123]

[0124] Table 2

[0125]

[0126] As shown in Table 1 and Figure 3 It can be seen from the test data shown that the electrochemical performance data of Examples 1 to 4 are better than those of Comparative Examples 1 to 2. The discharge specific capacity of the secondary batteries prepared in Examples 1 to 4 is higher, which is more obvious under high rate conditions, indicating that the composite positive electrode material prepared by the grain embedding method of the present invention has a stable structure, high crystallinity, and uniform particle size distribution, which improves the rate performance of the lithium iron phosphate positive electrode material.

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

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

[0129] Based on the disclosure and teachings of the above description, those skilled in the art will be able to make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the above specific embodiments. Any obvious improvements, substitutions, or modifications made by those skilled in the art based on the present invention fall within the scope of protection of the present invention. In addition, although certain specific terms are used in this description, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims

1. A method for preparing a composite positive electrode material, characterized in that: The following steps are involved: Step 1: dissolving a first lithium source, a vanadium source, and a first phosphorus source in deionized water, adding a first surfactant, and ultrasonically mixing for 1 to 3 hours to obtain a solution A; Step 2: Solution A is reacted by microwave hydrothermal method at a temperature of 75 to 150° C. for 10 to 40 minutes, and then dried to obtain a lithium vanadium phosphate precursor; Step 3: sintering the lithium vanadium phosphate precursor at 650-850° C. for 6-12 hours and cooling to obtain a lithium vanadium phosphate positive electrode material; Step 4: dissolving the second lithium source, the iron source, the second phosphorus source and the 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, mix, and ultrasonically disperse for 12 to 72 hours, then react at a temperature of 90 to 180° C. for 15 to 60 minutes using a microwave hydrothermal method, and dry to obtain a composite cathode material precursor; Step 6: Sintering the composite cathode material precursor at 600-800° C. for 6-12 hours to obtain a composite cathode material.

2. The method for preparing a composite positive electrode material according to claim 1, wherein: In the step 1, the first surfactant is at least one of oleic acid, sodium lauryl sulfate, polyvinyl pyrrolidone, stearic acid and fatty acid glyceride; the molar ratio of the first surfactant to the vanadium source is 0.03 to 0.1:

1.

3. The method for preparing a composite positive electrode material according to claim 1, wherein: In the step 5, the second surfactant is at least one of sodium dodecyl sulfonate, sodium hexadecylbenzenesulfonate, 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.

4. The method for preparing a composite cathode material according to claim 1, wherein: In the step 1, the molar ratio of the lithium element of the first lithium source, the vanadium element of the vanadium source, and the phosphorus element of the first phosphorus source is (1.5-1.7): (1-1.2): (1.5-1.7).

5. The method for preparing a composite positive electrode material according to claim 1, wherein: In step 1, the first lithium source is at least one of lithium sulfate, lithium chloride, lithium nitrate, lithium oxalate, lithium acetate and lithium carbonate.

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

7. A composite positive electrode material, characterized in that The composite positive electrode material is prepared by the preparation method of any one of claims 1 to 6, comprising lithium vanadium phosphate and lithium iron phosphate coated on the surface of the lithium vanadium phosphate, and the lithium vanadium phosphate is embedded in the lattice of the lithium iron phosphate.

8. The composite cathode material according to claim 7, characterized in that The particle size of the lithium vanadium phosphate is 10-30 nm; the particle size of the lithium iron phosphate is 0.3-0.9 μm.

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

4.

10. The composite cathode material according to claim 7, characterized in that The molar ratio of the vanadium element in the lithium vanadium phosphate to the iron element in the lithium iron phosphate is 0.01 to 0.07:1.

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