Preparation method of carbon nanotube composite lithium iron phosphate positive electrode material

Abstract

The application discloses a preparation method of a carbon nanotube composite lithium iron phosphate positive electrode material, which comprises the following steps: (1) weighing anhydrous iron phosphate, iron powder and anhydrous lithium phosphate according to a stoichiometric ratio, adding an organic carbon source and an organic solvent A, fully sand milling the above raw materials, uniformly mixing the raw materials to form slurry A, and stirring and ultrasonic dispersing carbon nanotubes in an organic solvent B to form slurry B; (2) simultaneously injecting slurry A and slurry B into a drying cavity of a spray drying device in two ways, uniformly mixing slurry A and slurry B in a spraying process and removing solvents; and (3) sintering the spray drying product to obtain the carbon nanotube composite lithium iron phosphate positive electrode material. The preparation process is simple and controllable, the atomic utilization rate is close to 100% and no pollution is caused, the solvent can be recycled, the preparation efficiency is high, the preparation cost is low, the obtained material has high tap density and conductivity, and the cycle stability and rate performance of the lithium iron phosphate positive electrode material can be improved.

Description

TECHNICAL FIELD

[0001] The application relates to a preparation method of a carbon nanotube composite lithium iron phosphate positive electrode material. TECHNICAL BACKGROUND

[0002] The lithium iron phosphate battery has the advantages of safety, cycle life and price, and its share in the electric vehicle power battery has exceeded 50%, and in the field of electrochemical energy storage, it has exceeded 90%, and its shipment growth rate in 2021 has exceeded 80%, showing strong competitiveness and bright prospects.

[0003] From the perspective of materials, the LiFePO4 positive electrode material has a high theoretical capacity (170 mAh g -1 ) and a stable working voltage (3.4 V vs. Li + / Li), good structural stability, and cheap and pollution-free raw materials, which make it have the potential for large-scale application. However, since there is no continuous FeO6 shared edge octahedron in the structure of LiFePO4, the electron conduction can only occur in the Fe-O-Fe direction, and a good conductive network cannot be formed, so the electrical conductivity is low. The PO4 tetrahedron between the FeO6 octahedron and the LiO6 octahedron restricts the change of the lattice volume, which hinders the deintercalation / intercalation process of Li + , so that the lithium ion diffusion rate of LiFePO4 is low. The low lithium ion diffusion coefficient (about 10 -15 cm 2 S -1 at 298 K) and the low electronic conductivity (about 10 -9 S cm -1 at 298 K) cause the serious decay of the discharge specific capacity of the lithium iron phosphate material at a rate higher than 10C, which hinders the further development and application of lithium iron phosphate.

[0004] In synthesis, lithium iron phosphate has high-temperature solid-phase method, carbon thermal reduction method, hydrothermal method, liquid-phase coprecipitation method and sol-gel method and the like. The solid-phase method is easy to industrialize production and is the main synthesis route at present. The high-temperature solid-phase method often introduces ball milling or sand milling and the like in the first step to reduce the particle size of raw materials, and then granulation and heat treatment are carried out. For the synthesis path in which the precursor will decompose, two-step heat treatment is usually adopted, the first step is at about 350 DEG C to discharge the gas generated by decomposition, to provide a better basis for the second heat treatment crystallization molding. The carbon thermal reduction method has low raw material cost, can realize in-situ formation and coating of carbon, and improves the conductivity. However, the material prepared by the method is poorer than the traditional high-temperature solid-phase method in capacity and rate performance, has higher requirements for iron source, has relatively long reaction time, the temperature is difficult to control, and the consistency of the product is not high. The coprecipitation method, hydrothermal method and sol-gel method have the advantages of small product particle size and controllable distribution, and have good performance in capacity performance, but the process flow is complex and is not easy to scale production.

[0005] In order to improve the poor electronic conductivity, slow lithium ion diffusion and the like of LiFePO4, the currently widely used strategies include coating a conductive network on the surface of LiFePO4, doping with metal or non-metal elements, and optimizing the size and morphology of LiFePO4 material. Among all the modification methods, the carbon material coating / composite technology is considered to be the most effective method for enhancing the electrochemical performance of LiFePO4. SUMMARY

[0006] In view of the above status, the purpose of the present application is to provide a preparation method of carbon nanotube composite lithium iron phosphate positive electrode material, which has abundant raw material sources, high preparation efficiency, good controllability, low preparation cost, high atomic utilization rate, no pollution, safe and reliable operation, high tap density and conductivity of the prepared carbon nanotube composite lithium iron phosphate positive electrode material, and can improve the cycle stability and rate performance of lithium iron phosphate material.

[0007] The technical solutions of the present application are described below.

[0008] The present application provides a preparation method of carbon nanotube composite lithium iron phosphate positive electrode material, comprising the following steps:

[0009] (1) according to the design of LiFePO4, take the stoichiometric ratio of anhydrous iron phosphate, iron powder, anhydrous lithium phosphate; add organic carbon source and organic solvent A, the organic solvent A is used to dissolve the organic carbon source and as the dispersion medium of sanding, the above raw materials are sanding to make the raw materials mixed uniformly to form slurry A; a certain amount of carbon nanotube is stirred and ultrasonic dispersed in organic solvent B to form slurry B; the organic carbon source is glucose or sucrose, the addition amount of the organic carbon source is 5%-15% (preferably 10%) of the total mass of the raw materials, the volume of the organic solvent A is 0.6-1.0 mL / g (preferably 0.8-0.9 mL / g) of the total mass of the raw materials; the volume of the organic solvent B is 5-30 mL / g (preferably 20-30 mL / g) of the mass of the carbon nanotube; the total mass of the raw materials is the total mass of the anhydrous iron phosphate, the iron powder, the anhydrous lithium phosphate and the organic carbon source;

[0010] (2) then the slurry A and the slurry B are injected into the drying cavity of the spray drying device through two independent feeding ports 1 and 2, the feeding ports 1 and 2 are connected with the nozzles 1 and 2 arranged in the drying cavity respectively, the nozzles 1 and 2 are oppositely arranged in the drying cavity and the distance between the nozzles 1 and 2 is controlled to be 20-35 cm, wherein the slurry A is pumped into the feeding port 1 at a rate of 80-180 mL / min and sprayed out from the corresponding nozzle 1, the slurry B is pumped into the feeding port 2 at a rate of 40-100 mL / min and sprayed out from the corresponding nozzle 2, and the feeding rate of the slurry A is at least 1.3 times of the feeding rate of the slurry B, the materials sprayed out from the two nozzles are sufficiently mixed due to the opposite arrangement of the nozzles, so that the slurry A and the slurry B are uniformly mixed and the solvent is removed during the spraying process;

[0011] (3) the spray drying product obtained in the step (2) is put into a high temperature furnace, sintered at 650-750 ℃ for 4-12 h, the sintering atmosphere is one of nitrogen, argon, hydrogen-nitrogen mixed gas or hydrogen-argon mixed gas, and then naturally reduced to room temperature to obtain the carbon nanotube composite lithium iron phosphate positive electrode material.

[0012] In the application, the average particle size of the iron powder is 2-3 μm. Research results show that the iron powder with small particle size can reduce the grinding time and improve the electrochemical performance of the product, and considering the production method and cost of the iron powder, the iron powder with the particle size is preferred.

[0013] In the application, the organic solvent A and the organic solvent B are independently ethanol, methanol or acetone. Considering the volatility and toxicity of the solvents, the organic solvent A and the organic solvent B are both ethanol. In the application, in order to grind the materials sufficiently and mix them uniformly, the sanding speed is set to 1500-3000 rpm and the sanding time is 3-6 h. Research results show that higher sanding speed and longer sanding time can improve the electrochemical performance of the product.

[0014] In the present application, the inlet temperature of the spray dryer is 180-250℃, and the outlet temperature is 100-140℃, preferably the inlet temperature is 220℃, and the outlet temperature is 110℃.

[0015] In the present application, in order to ensure good crystallinity and the sintering rate of lithium iron phosphate, the sintering temperature is preferably 700℃, and the sintering time is 4-12h.

[0016] In the present application, the sintering atmosphere used is nitrogen, argon, hydrogen-nitrogen mixed gas or hydrogen-argon mixed gas. Hydrogen gas is used as a reducing gas, which can reduce trivalent iron to divalent iron and inhibit the oxidation of residual oxygen in the system. Therefore, hydrogen-nitrogen mixed gas (such as hydrogen-nitrogen mixed gas with a hydrogen volume fraction of 5-10%) or hydrogen-argon mixed gas (such as hydrogen-argon mixed gas with a hydrogen volume fraction of 5-10%) is preferably used.

[0017] In the present application, the carbon nanotubes used are multi-walled carbon nanotubes with an average diameter of 10-50nm and an average length of 100nm-10μm. In order to optimize the tap density and electrical conductivity, carbon nanotubes with an average diameter of 30nm and an average length of 3μm are preferably used.

[0018] The present application provides the application of the prepared carbon nanotube composite lithium iron phosphate material as a positive electrode material for lithium ion batteries.

[0019] Compared with the prior art, the present application has the following beneficial effects:

[0020] (1) The present application uses low-cost raw materials to produce lithium iron phosphate. The preparation process is simple and controllable, the atomic utilization rate is close to 100%, and there is no pollution. The solvent can be recycled. Through sanding, spray drying and high-temperature calcination, a lithium iron phosphate positive electrode material with good performance can be prepared. The particle size can be controlled by adjusting the technical parameters of spray drying. The preparation efficiency is high, and the preparation cost is low.

[0021] (2) The carbon nanotube composite lithium iron phosphate material prepared in the present application adds carbon nanotubes in the spray process, which avoids the destruction of the structure of carbon nanotubes in the sanding process, fully utilizes the high conductivity of carbon nanotubes, and effectively improves the cycle stability and rate performance of the material when used as a lithium ion battery positive electrode material. BRIEF DESCRIPTION OF DRAWINGS

[0022] The brief description of the drawings is provided to provide a further understanding of the embodiments of the present application, constitutes a part of the present application, and does not constitute a limitation on the embodiments of the present application.

[0023] Figure 1 SEM image of CNT used in Examples 1-4;

[0024] Figure 2SEM images of CNT used in Examples 5-10;

[0025] Figure 3 (a) and (b) are XRD images of LiFePO4 / CNT composite materials in Examples 1-10;

[0026] Figure 4 is an SEM image of LiFePO4 / CNT composite material in Example 6;

[0027] Figure 5 (a), (b) and (c) are cycle performance comparison images of LiFePO4 / CNT composite materials in Examples 1-7;

[0028] Figure 6 is a cycle performance comparison image of LiFePO4 / CNT composite materials in Examples 7 and 8;

[0029] Figure 7 is a cycle performance comparison image of LiFePO4 / CNT composite materials in Examples 7, 9 and 10.

[0030] Figure 8 is a cycle performance comparison image of LiFePO4 / CNT composite materials in Examples 7 and 11. DETAILED DESCRIPTION

[0031] The technical solutions of the present application are further illustrated by specific examples below, but the protection scope of the present application is not limited thereto:

[0032] The spray dryer used in the examples of the present application is provided with two feeding ports 1 and 2, which are connected with spray ports 1 and 2 arranged in the drying chamber, respectively. The spray ports 1 and 2 are oppositely arranged in the drying chamber and the distance between them is 28 cm. Other components and connection relationship are the same as those of a conventional spray dryer.

[0033] Example 1:

[0034] Take stoichiometric ratio of average particle size of 2 μm iron powder, anhydrous iron phosphate, anhydrous lithium phosphate, a total of 1800 g, 200 g of glucose, 1500 mL of ethanol together into the sand mill, the sand mill at 1500 rpm ball milling 6 h to form slurry A. 20 g of average diameter 20 nm, average length 5 μm carbon nanotubes are stirred and ultrasonically dispersed in 500 mL of ethanol to form slurry B. Set the inlet temperature of the spray dryer to 220°C, the outlet temperature to 110°C, slurry A is pumped into inlet 1 by diaphragm pump 1 at a rate of 100 mL / min, slurry B is pumped into inlet 2 by diaphragm pump 2 at a rate of 50 mL / min, and the slurry is sprayed into the drying chamber of the spray dryer through the oppositely arranged nozzles 1 and 2 for spray drying, so that the slurry A and the slurry B are uniformly mixed during the spraying process and the solvent is removed. The dried precursor is calcined, under the condition that the protective gas is nitrogen-hydrogen mixed gas (hydrogen gas volume fraction is 5%), at a rate of 5°C / min -1 From room temperature to 700°C, keep for 8h, after natural cooling, get LiFePO4 / C composite material, recorded as LFP / CNT-1.

[0035] Example 2:

[0036] Take stoichiometric ratio of average particle size of 2 μm iron powder, anhydrous iron phosphate, anhydrous lithium phosphate, a total of 1800 g, 200 g of glucose, 1500 mL of ethanol together into the sand mill, the sand mill at 1500 rpm ball milling 6 h to form slurry A. 20 g of average diameter 20 nm, average length 5 μm carbon nanotubes are stirred and ultrasonically dispersed in 500 mL of ethanol to form slurry B. Set the inlet temperature of the spray dryer to 220°C, the outlet temperature to 110°C, slurry A is pumped into inlet 1 by diaphragm pump 1 at a rate of 100 mL / min, slurry B is pumped into inlet 2 by diaphragm pump 2 at a rate of 50 mL / min, and the slurry is sprayed into the drying chamber of the spray dryer through the oppositely arranged nozzles 1 and 2 for spray drying, so that the slurry A and the slurry B are uniformly mixed during the spraying process and the solvent is removed. The dried precursor is calcined, under the condition that the protective gas is nitrogen-hydrogen mixed gas (hydrogen gas volume fraction is 5%), at a rate of 5°C / min -1 From room temperature to 700°C, keep for 8h, after natural cooling, get LiFePO4 / C composite material, recorded as LFP / CNT-2.

[0037] Example 3:

[0038] The stoichiometric ratio of the average particle size of 2 μm iron powder, anhydrous iron phosphate, and anhydrous lithium phosphate is 1800 g, 200 g of glucose, and 1500 mL of ethanol are added together into a sand mill, and the sand mill is ball milled at 3000 rpm for 6 h to form slurry A. 40 g of carbon nanotubes with an average diameter of 25 nm and an average length of 300 nm are stirred and ultrasonically dispersed in 500 mL of ethanol to form slurry B. The inlet temperature of the spray dryer is set to 220°C, the outlet temperature is set to 110°C, slurry A is pumped into inlet 1 at a rate of 100 mL / min by a diaphragm pump 1, and slurry B is pumped into inlet 2 at a rate of 50 mL / min by a diaphragm pump 2. The slurry A and slurry B are uniformly mixed and the solvent is removed during the spraying process by injecting the slurry into the drying chamber of the spray dryer through the oppositely arranged nozzles 1 and 2. The dried precursor is calcined under the condition that the protective gas is a nitrogen-hydrogen mixed gas (hydrogen volume fraction is 5%) at a rate of 5°C / min -1 The LiFePO4 / C composite material is obtained after natural cooling from room temperature to 700°C for 8 h, and is recorded as LFP / CNT-3.

[0039] Example 4:

[0040] The stoichiometric ratio of the average particle size of 2 μm iron powder, anhydrous iron phosphate, and anhydrous lithium phosphate is 1800 g, 200 g of glucose, and 1500 mL of ethanol are added together into a sand mill, and the sand mill is ball milled at 3000 rpm for 6 h to form slurry A. 40 g of carbon nanotubes with an average diameter of 25 nm and an average length of 300 nm are stirred and ultrasonically dispersed in 500 mL of ethanol to form slurry B. The inlet temperature of the spray dryer is set to 220°C, the outlet temperature is set to 110°C, slurry A is pumped into inlet 1 at a rate of 100 mL / min by a diaphragm pump 1, and slurry B is pumped into inlet 2 at a rate of 50 mL / min by a diaphragm pump 2. The slurry A and slurry B are uniformly mixed and the solvent is removed during the spraying process by injecting the slurry into the drying chamber of the spray dryer through the oppositely arranged nozzles 1 and 2. The dried precursor is calcined under the condition that the protective gas is a nitrogen-hydrogen mixed gas (hydrogen volume fraction is 5%) at a rate of 5°C / min -1 The LiFePO4 / C composite material is obtained after natural cooling from room temperature to 700°C for 8 h, and is recorded as LFP / CNT-3.

[0041] Example 5:

[0042] Take the stoichiometric ratio of the average particle size of 2 μm iron powder, anhydrous iron phosphate, anhydrous lithium phosphate, a total of 1800 g, 200 g of glucose, 1500 mL of ethanol together into the sand mill, the sand mill at 3000 rpm ball milling 6 h to form slurry A. 20 g of average diameter 30 nm, average length 3 μm carbon nanotubes were stirred and ultrasonically dispersed in 500 mL of ethanol to form slurry B. Set the inlet temperature of the spray dryer to 220°C, the outlet temperature to 110°C, slurry A is pumped into inlet 1 by diaphragm pump 1 at a rate of 100 mL / min, slurry B is pumped into inlet 2 by diaphragm pump 2 at a rate of 50 mL / min, and the dry cavity of the spray dryer is injected through the opposite spray ports 1 and 2 for spray drying, so that the slurry A and the slurry B are uniformly mixed and the solvent is removed during the spraying process. The dried precursor is calcined, under the condition that the protective gas is nitrogen-hydrogen mixed gas (hydrogen volume fraction is 5%), at a rate of 5°C / min -1 From room temperature to 700°C, keep for 8h, after natural cooling, get LiFePO4 / C composite material, recorded as LFP / CNT-5.

[0043] Example 6:

[0044] Take the stoichiometric ratio of the average particle size of 2 μm iron powder, anhydrous iron phosphate, anhydrous lithium phosphate, a total of 1800 g, 200 g of glucose, 1500 mL of ethanol together into the sand mill, the sand mill at 3000 rpm ball milling 6 h to form slurry A. 20 g of average diameter 30 nm, average length 3 μm carbon nanotubes were stirred and ultrasonically dispersed in 500 mL of ethanol to form slurry B. Set the inlet temperature of the spray dryer to 220°C, the outlet temperature to 110°C, slurry A is pumped into inlet 1 by diaphragm pump 1 at a rate of 100 mL / min, slurry B is pumped into inlet 2 by diaphragm pump 2 at a rate of 50 mL / min, and the dry cavity of the spray dryer is injected through the opposite spray ports 1 and 2 for spray drying, so that the slurry A and the slurry B are uniformly mixed and the solvent is removed during the spraying process. The dried precursor is calcined, under the condition that the protective gas is nitrogen-hydrogen mixed gas (hydrogen volume fraction is 5%), at a rate of 5°C / min -1 From room temperature to 700°C, keep for 8h, after natural cooling, get LiFePO4 / C composite material, recorded as LFP / CNT-5.

[0045] Example 7:

[0046] The stoichiometric ratio of the average particle size of 2 μm iron powder, anhydrous iron phosphate, and anhydrous lithium phosphate is 1800 g, 200 g of glucose, and 1500 mL of ethanol are added together into a sand mill, and the sand mill is ball milled at 3000 rpm for 6 h to form slurry A. 60 g of carbon nanotubes with an average diameter of 30 nm and an average length of 3 μm are stirred and ultrasonically dispersed in 500 mL of ethanol to form slurry B. The inlet temperature of the spray dryer is set to 220°C, the outlet temperature is set to 110°C, slurry A is pumped into inlet 1 at a rate of 100 mL / min by diaphragm pump 1, and slurry B is pumped into inlet 2 at a rate of 50 mL / min by diaphragm pump 2, and the two are injected into the drying chamber of the spray dryer through the oppositely arranged nozzles 1 and 2 for spray drying, so that the slurry A and the slurry B are uniformly mixed during the spraying process and the solvent is removed. The dried precursor is calcined under the condition that the protective gas is a nitrogen-hydrogen mixed gas (hydrogen volume fraction is 5%) at a rate of 5°C / min -1 The LiFePO4 / C composite material is obtained after natural cooling from room temperature to 700°C for 8 h, and is recorded as LFP / CNT-7.

[0047] Example 8

[0048] The stoichiometric ratio of the average particle size of 2 μm iron powder, anhydrous iron phosphate, and anhydrous lithium phosphate is 1800 g, 200 g of glucose, and 1500 mL of ethanol are added together into a sand mill, and the sand mill is ball milled at 3000 rpm for 6 h to form slurry A. 60 g of carbon nanotubes with an average diameter of 30 nm and an average length of 3 μm are stirred and ultrasonically dispersed in 500 mL of ethanol to form slurry B. The inlet temperature of the spray dryer is set to 220°C, the outlet temperature is set to 110°C, slurry A is pumped into inlet 1 at a rate of 100 mL / min by diaphragm pump 1, and slurry B is pumped into inlet 2 at a rate of 50 mL / min by diaphragm pump 2, and the two are injected into the drying chamber of the spray dryer through the oppositely arranged nozzles 1 and 2 for spray drying, so that the slurry A and the slurry B are uniformly mixed during the spraying process and the solvent is removed. The dried precursor is calcined under the condition that the protective gas is a nitrogen-hydrogen mixed gas (hydrogen volume fraction is 5%) at a rate of 5°C / min -1 The LiFePO4 / C composite material is obtained after natural cooling from room temperature to 700°C for 8 h, and is recorded as LFP / CNT-7.

[0049] Example 9

[0050] The stoichiometric ratio of the average particle size of 2 μm iron powder, anhydrous iron phosphate, and anhydrous lithium phosphate was weighed according to the design of LiFePO4, 1800 g of glucose and 1500 mL of ethanol were added into a sand mill, and the sand mill was ball milled at 3000 rpm for 6 h to form slurry A. 60 g of carbon nanotubes with an average diameter of 30 nm and an average length of 3 μm were stirred and ultrasonically dispersed in 500 mL of ethanol to form slurry B. The inlet temperature of the spray dryer was set to 220°C, the outlet temperature was set to 110°C, slurry A was pumped into inlet 1 at a rate of 150 mL / min by diaphragm pump 1, and slurry B was pumped into inlet 2 at a rate of 75 mL / min by diaphragm pump 2. Simultaneously, the dry chamber of the spray dryer was injected through the oppositely arranged nozzles 1 and 2 to perform spray drying, so that the slurry A and the slurry B were uniformly mixed and the solvent was removed during the spraying process. The dried precursor was calcined under the condition that the protective gas was nitrogen-hydrogen mixed gas (hydrogen volume fraction was 5%) at a rate of 5°C / min -1 The LiFePO4 / C composite material was obtained after natural cooling from room temperature to 700°C for 8 h, and was recorded as LFP / CNT-9.

[0051] Example 10

[0052] The stoichiometric ratio of the average particle size of 2 μm iron powder, anhydrous iron phosphate, and anhydrous lithium phosphate was weighed according to the design of LiFePO4, 1800 g of glucose and 1500 mL of ethanol were added into a sand mill, and the sand mill was ball milled at 3000 rpm for 6 h to form slurry A. 60 g of carbon nanotubes with an average diameter of 30 nm and an average length of 3 μm were stirred and ultrasonically dispersed in 500 mL of ethanol to form slurry B. The inlet temperature of the spray dryer was set to 220°C, the outlet temperature was set to 110°C, slurry A was pumped into inlet 1 at a rate of 80 mL / min by diaphragm pump 1, and slurry B was pumped into inlet 2 at a rate of 40 mL / min by diaphragm pump 2. Simultaneously, the dry chamber of the spray dryer was injected through the oppositely arranged nozzles 1 and 2 to perform spray drying, so that the slurry A and the slurry B were uniformly mixed and the solvent was removed during the spraying process. The dried precursor was calcined under the condition that the protective gas was nitrogen-hydrogen mixed gas (hydrogen volume fraction was 5%) at a rate of 5°C / min -1 The LiFePO4 / C composite material was obtained after natural cooling from room temperature to 700°C for 8 h, and was recorded as LFP / CNT-10.

[0053] Example 11

[0054] According to the design of LiFePO4, 1800g of iron powder with an average particle size of 2μm, anhydrous iron phosphate, and anhydrous lithium phosphate, along with 200g of glucose and 1500mL of ethanol, were weighed and added to a sand mill. Then, 60g of carbon nanotubes with an average diameter of 30nm and an average length of 3μm were added. The sand mill was ball-milled at 3000rpm for 6 hours to form a slurry. The inlet temperature of the spray dryer was set to 220℃ and the outlet temperature to 110℃. The slurry was pumped into the feed inlet 1 by diaphragm pump 1 at a rate of 100mL / min, and then injected into the drying chamber of the spray dryer through nozzle 1 for spray drying, thereby uniformly mixing the slurry and removing the solvent during the spraying process. The dried precursor was calcined and, under the protective gas of a nitrogen-hydrogen mixture (hydrogen integral of 5%), was sprayed at 5℃ for 1 minute. -1 The LiFePO4 / C composite material was obtained by heating from room temperature to 700℃ and holding for 8 hours, followed by natural cooling. It is denoted as LFP / CNT-11.

[0055] 0.32 g of the composite material prepared in Examples 1-11 above, 0.04 g of the conductive agent acetylene black, and 0.04 g of the binder PVDF were weighed and placed in a mortar. After grinding evenly, 3 mL of NMP solvent was added, and the mixture was stirred evenly. The mixture was then coated onto the surface of a 6 cm × 10 cm aluminum foil and vacuum dried at 120 °C for 12 h. The foil was then removed and assembled into a coin cell using lithium metal as the counter electrode. The coin cell was tested on a lithium-ion battery charge-discharge system (LAND CT2001A), and the results are shown in the attached figure. Figures 5-8 As shown.

[0056] As attached Figure 5 As shown in (a), the carbon nanotube composite lithium iron phosphate LFP / CNT-1 and LFP / CNT-2 prepared by sand milling at 1500 rpm and 3000 rpm for 6 h have discharge specific capacities of 133 and 145 mAh g at 0.1C and 1C, respectively. -1 and 142, 153mAhg -1 .

[0057] As attached Figure 5 As shown in (b), the carbon nanotube composite lithium iron phosphate LFP / CNT-2 / 3 / 4 prepared with carbon nanotubes of 25 nm diameter and 300 nm length added at amounts of 20 / 60 / 100 g exhibited discharge specific capacities of 153, 151, and 148 mAh g at 0.1 C and 1 C, respectively. -1 and 142, 141, 139mAh g -1 .

[0058] As attached Figure 5As shown in (c), the LFP / CNT-5 / 6 / 7 composites with different contents of carbon nanotubes with a diameter of 30 nm and a length of 3 μm prepared in Examples 5 / 6 / 7 had discharge specific capacities of 155, 154, and 152 mAh g at 0.1C and 1C, respectively. -1 and 144, 146, 142 mAh g -1 .

[0059] As attached Figure 6 As shown, the LFP / CNT-7 / 8 prepared in Examples 7 and 8 with different feeding methods had discharge specific capacities of 152 and 150 mAh g at 0.1C and 1C, respectively. -1 and 146, 132mAh g -1 .

[0060] As attached Figure 7 As shown, the LFP / CNT-7 / 9 / 10 samples prepared in Examples 7, 9, and 10 with different pumping rates exhibited discharge specific capacities of 152, 150, and 153 mAh g at 0.1C and 1C, respectively. -1 and 146, 138, 135mAh g -1 .

[0061] As attached Figure 8 As shown, the LFP / CNT-7 / 11 prepared by different composite processes in Examples 7 and 11 have discharge specific capacities of 152 and 155 mAh g at 0.1C and 1C, respectively. -1 and 146, 135mAh g -1 .

[0062] The results show that a higher milling speed leads to better mixing of iron powder, anhydrous iron phosphate, anhydrous lithium phosphate, and glucose, resulting in improved performance of the final product. Furthermore, using carbon nanotubes of appropriate size and in suitable amounts to composite with lithium iron phosphate significantly enhances the product's performance under both 0.1C and 1C discharge conditions. In addition, employing a multi-inlet feeding method allows for better uniform mixing of carbon nanotubes and lithium iron phosphate, positively contributing to improved material properties.

[0063] The above description, in conjunction with embodiments showing good results, provides a detailed explanation of the present invention, but it should not be construed as limiting the specific implementation of the invention to the described embodiments. Those skilled in the art will recognize that various modifications and substitutions can be made without departing from the research approach of the invention, and all such modifications and substitutions are included within the scope defined by the claims of the present invention.

Claims

1. A method for preparing a carbon nanotube composite lithium iron phosphate cathode material, characterized in that... The preparation method comprises the following steps: (1) The chemical stoichiometric ratio of anhydrous iron phosphate, iron powder and anhydrous lithium phosphate is weighed according to the design of LiFePO4, an organic carbon source and an organic solvent A are added, the above raw materials are fully sand-milled to uniformly mix the raw materials to form a slurry A; a certain amount of carbon nanotubes is stirred and ultrasonically dispersed in an organic solvent B to form a slurry B; the organic carbon source is glucose or sucrose, the addition amount of the organic carbon source is 5%-15% of the total mass of the raw materials, the volume amount of the organic solvent A is 0.6-1.0 mL / g based on the total mass of the raw materials; the volume amount of the organic solvent B is 5-30 mL / g based on the mass of the carbon nanotubes; the total mass of the raw materials is the total mass of the anhydrous iron phosphate, the iron powder, the anhydrous lithium phosphate and the organic carbon source; (2) Then the slurry A and the slurry B are simultaneously injected into a drying cavity of a spray drying device through two independent feeding ports 1 and 2, the feeding ports 1 and 2 are connected with nozzles 1 and 2 arranged in the drying cavity respectively, the nozzles 1 and 2 are oppositely arranged in the drying cavity and the distance between the nozzles 1 and 2 is controlled to be 20-35 cm, wherein the slurry A is pumped into the feeding port 1 at a rate of 80-180 mL / min and sprayed out from the corresponding nozzle 1, the slurry B is pumped into the feeding port 2 at a rate of 40-100 mL / min and sprayed out from the corresponding nozzle 2, and the feeding rate of the slurry A is at least 1.3 times of the feeding rate of the slurry B, so that the slurry A and the slurry B are uniformly mixed and the solvent is removed in the spraying process; (3) The spray drying product obtained in the step (2) is placed into a high-temperature furnace, sintered at 650-750 ℃ for 4-12 h in a sintering atmosphere of one of nitrogen, argon, hydrogen-nitrogen mixed gas or hydrogen-argon mixed gas, and then naturally reduced to room temperature to obtain the carbon nanotube composite lithium iron phosphate positive electrode material.

2. The production method according to claim 1, characterized by: The average particle size of the iron powder is 2-3 μm.

3. The production method according to claim 1, wherein: The organic solvent A and the organic solvent B are each independently ethanol, methanol or acetone.

4. The production method according to claim 1, wherein: In the step (1), the volume amount of the organic solvent A is 0.8-0.9 mL / g based on the total mass of the raw materials.

5. The production method according to claim 1, wherein: In the step (1), the volume amount of the organic solvent B is 20-30 mL / g based on the mass of the carbon nanotubes.

6. The production method according to claim 1, wherein: In the step (1), the sand-milling rotation speed is set to 1500-3000 rpm and the sand-milling time is 3-6 h.

7. The production method according to claim 1, wherein: The inlet temperature of the spray drying device is 180-250 ℃ and the outlet temperature is 100-140 ℃.

8. The production method according to claim 1, wherein: The inlet temperature of the spray drying device is 220 ℃ and the outlet temperature is 110 ℃.

9. The production method according to claim 1, wherein: In the step (3), the sintering temperature is 700 ℃ and the sintering time is 4-12 h.

10. The production method according to claim 1, wherein: The hydrogen volume fraction in the hydrogen-nitrogen mixed gas is 5-10% and the hydrogen volume fraction in the hydrogen-argon mixed gas is 5-10%.

11. The production method according to claim 1, wherein: The carbon nanotubes are multi-walled carbon nanotubes with an average diameter of 10-50 nm and an average length of 100 nm-10 μm.

12. The production method according to claim 11, characterized by: The average diameter of the carbon nanotubes is 30 nm and the average length is 3 μm.

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