CN113991070A - Lithium iron phosphate composite material and preparation method and application thereof - Google Patents
Lithium iron phosphate composite material and preparation method and application thereof Download PDFInfo
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Abstract
The application discloses a lithium iron phosphate composite material, a preparation method and an application thereof, wherein the lithium iron phosphate composite material comprises lithium iron phosphate containing an out-phase doping substance; the heterogeneous doping material is lithium iron borate; and carbon is coated outside the lithium iron phosphate. The lithium iron phosphate composite material provided by the application adopts the lithium iron phosphate doped with the lithium iron borate, and can not cause obvious influence on the structural stability of the lithium iron phosphate material in the charging and discharging process while the problem of poor low-temperature discharge performance of the lithium iron phosphate anode material is solved. Compared with a solid phase method, the method for preparing the lithium iron phosphate composite material by adopting the sol-gel method has the characteristics of low energy consumption, good covering property of the synthesized lithium iron phosphate composite material, uniform doping, and good granularity and granularity distribution.
Description
Technical Field
The application relates to a lithium iron phosphate composite material, a preparation method and application thereof, and belongs to the technical field of battery materials.
Background
The performance of power lithium ion batteries depends to a large extent on the performance of the positive electrode material. Among the positive electrode materials that have been developed, phosphate type compounds (Li)yMx(PO4)z) The lithium ion battery cathode material has good safety and good lithium intercalation/deintercalation properties, and becomes the most promising cathode material of a power lithium ion battery. In which LiFePO is used4The material is most concerned and is the focus of research. LiFePO4The material has the advantages of rich raw material resources, environmental friendliness, long cycle life, excellent safety performance and the like, but the inherent low electronic conductivity and lithium ion conductivity cause the problems of low discharge voltage, poor large-current charge and discharge performance, difficult low-temperature charge and discharge and the like, and limit the LiFePO4The materials are used more widely.
For LiFePO at present4The modification (c) is mainly focused on: 1. coating with a high conductive agent, namely, improving the lithium iron phosphate material through carbon coating, conductive metal particles and metal oxides; 2. ion doping, namely improving the intrinsic conductivity of the lithium iron phosphate material by means of cation doping, anion and cation complex doping and the like; 3. the process is optimized, and the morphology and granularity of the lithium iron phosphate are improved by improving the technical processes of sanding, spraying, sintering and the like, so that the electrochemical performance of the lithium iron phosphate is improved.
At present, reports on heterogeneous doping of lithium iron phosphate are few, and the electrochemical performance of the lithium iron phosphate can be effectively improved through doping of a heterostructure. The low temperature of lithium vanadium phosphate (three-dimensional lithium ion channel) which is also a phosphate anode material is superior to that of lithium iron phosphate. Lithium vanadium phosphate and preparation and electrochemical performance research (Harbin industry university, Chengyu) of lithium iron phosphate composite material, disclose that prepare lithium vanadium phosphate and mix lithium iron phosphate positive pole material through solid phase sintering reaction method, lithium vanadium phosphate mixes lithium iron phosphate and improves lithium iron phosphate discharge performance at low temperature to a certain extent, but the material homogeneity that the high temperature solid phase method that adopts synthesizes is poor, the granularity is partial to be big, the carbon cladding is inhomogeneous, lead to some discharge capacities of compound lithium iron phosphate to reduce; secondly, the vanadium lithium phosphate has a plurality of discharging platforms, so that the doped lithium iron phosphate also has a plurality of corresponding discharging platforms, and the plurality of discharging platforms can cause the doped lithium iron phosphate material to have multiphase conversion in the charging and discharging process, so that the structural stability of the material can be reduced in the charging and discharging process.
Chinese patent "a carbon/lithium iron silicate/lithium iron phosphate composite material and a preparation method thereof" (application No. CN201910358483.5, published 2019, 7, 23), discloses a carbon/lithium iron silicate/lithium iron phosphate composite positive electrode material prepared by a solid phase method, and lithium iron silicate doped with lithium iron phosphate makes a lithium iron phosphate discharge curve slope, so that a simple and fast method is provided for accurately measuring and calculating the remaining capacity SOC of a lithium iron phosphate battery. However, the theoretical capacity, the discharge plateau voltage, the electron conductivity and the ion conductivity of the lithium iron silicate are all lower than those of the lithium iron phosphate, so that the conductivity of the lithium iron phosphate after rechecking is poor due to the doping of the lithium iron silicate, and the inherent defect of the solid phase method further limits the exertion of the electrical property of the lithium iron phosphate anode material.
The prior art does not well solve the problem of low-temperature conductivity of the lithium iron phosphate cathode material.
Disclosure of Invention
According to one aspect of the application, a lithium iron phosphate composite material is provided, and the composite material is used as a lithium battery anode and can effectively improve the low-temperature conductivity of the lithium battery.
The lithium iron phosphate composite material comprises lithium iron phosphate containing a heterogeneous doping substance;
the heterogeneous doping material is lithium iron borate;
and carbon is coated outside the lithium iron phosphate.
Optionally, the composite material has a microscopic size of 0.2-0.3 μm;
in the composite material, the content of lithium iron phosphate containing heterogeneous doping substances is 97.5-99 wt%, and the content of carbon is 1-2.5 wt%;
wherein the content of the heterogeneous doping material in the lithium iron phosphate containing the heterogeneous doping material is 0.1-10 wt%, and the content of the lithium iron phosphate is 90-99.9 wt%.
According to still another aspect of the present application, there is provided a method for preparing the above lithium iron phosphate composite, the method comprising at least the steps of:
step 1, heating a mixture containing a lithium source, a boron source, an iron source, a phosphorus source, a carbon source and a complexing agent for reaction to obtain precursor gel;
and 2, drying and sintering the precursor gel to obtain the lithium iron phosphate composite material.
Optionally, the lithium source is selected from at least one of lithium hydroxide, LiOH, lithium carbonate, and lithium oxalate;
the boron source is selected from boric acid H3BO3At least one of borate and boron oxide;
the iron source is selected from ferric phosphate FePO4At least one of ferrous oxalate;
the phosphorus source is selected from ferric phosphate FePO4At least one of ammonium dihydrogen phosphate;
the carbon source is at least one of glucose and carbon nano tubes;
the complexing agent is at least one selected from citric acid and ethylene diammonium tetraacetic acid;
the mixture also comprises a solvent;
the solvent is at least one of water and glycol.
Optionally, the molar ratio of the lithium element, the boron element, the iron element and the phosphorus element in the mixture is 0.95-1.0: 0.01-0.1: 0.95-1.05;
the molar ratio of the complexing agent to the lithium source is 3-4: 0.95-1.0;
the dosage of the carbon source is 1.0-2.5% of the total mass of the mixture.
Optionally, a dispersant is also included in the mixture;
the dispersant comprises polyethylene glycol;
the dosage of the dispersant is 5-10% of the total mass of the mixture.
Specifically, the amount of dispersant used may be independently selected from 5%, 6%, 7%, 8%, 9%, 10% of the total mass of the mixture, or any value therebetween.
Optionally, the conditions of the heating reaction are:
heating and reacting at the temperature of 60-90 ℃ for 10-20 h;
the drying conditions are as follows:
the drying temperature is 70-80 ℃, and the drying time is 6-8 h.
Specifically, the heating reaction temperature may be independently selected from 60 ℃, 65 ℃, 70 ℃, 75 ℃, 80 ℃, 85 ℃, 90 ℃, or any value between the two values.
Specifically, the heating reaction time may be independently selected from 10h, 11h, 12h, 13h, 15h, 16h, 17h, 18h, 19h, 20h, or any value therebetween.
Specifically, the drying temperature may be independently selected from 70 ℃, 72 ℃, 75 ℃, 77 ℃, 80 ℃, or any value between the two.
Specifically, the drying time may be independently selected from 6h, 6.5h, 7h, 7.5h, 8h, or any value therebetween.
Optionally, the sintering comprises presintering and temperature-rising sintering;
the presintering conditions are as follows: the pre-sintering temperature is 300-350 ℃, and the pre-sintering time is 3-5 h;
the conditions of temperature-rising sintering are as follows: raising the temperature to 700-800 ℃, and pre-sintering for 6-8 h;
preferably, before the temperature-rising sintering, the presintered product is subjected to ball milling.
Specifically, the pre-sintering temperature may be independently selected from 300 ℃, 310 ℃, 320 ℃, 330 ℃, 340 ℃, 350 ℃, or any value between the two values.
Specifically, the pre-sintering time may be independently selected from 3h, 3.5h, 4h, 4.5h, 5h, or any value therebetween.
Specifically, the elevated sintering temperature may be independently selected from 700 ℃, 710 ℃, 730 ℃, 750 ℃, 770 ℃, 790 ℃, 800 ℃, or any value between the two values.
Specifically, the temperature-rising sintering time can be independently selected from 6h, 6.5h, 7h, 7.5h and 8h, or any value between the two values.
Optionally, the ball milling parameters for the pre-sintered product are: the frequency is 20-40 Hz, and the ball-to-material ratio is 1.5-3: 1.
Optionally, the dried product is ball milled prior to sintering.
Optionally, the ball milling parameters for the dried product are: the frequency is 20-40 Hz, and the ball-to-material ratio is 1.5-3: 1.
Optionally, the sintering is performed under an inert atmosphere.
Specifically, the inert atmosphere is selected from nitrogen or argon.
Optionally, the step 1 includes:
adding a lithium source, a boron source, an iron source and a phosphorus source into a solvent, and then adding a carbon source, a complexing agent and a dispersing agent to obtain a mixture;
and heating and stirring the mixture for reaction to form gel, and cooling to room temperature to obtain precursor gel.
Specifically, the adding amount of the lithium source, the boron source, the iron source and the phosphorus source is added according to the amount of the lithium iron borate to be doped and the metering ratio requirement of the lithium iron borate and the lithium iron phosphate, and the adding amount of the carbon source is added according to the content requirement of carbon.
According to another aspect of the application, the lithium iron phosphate composite material prepared by any one of the methods and the application of the lithium iron phosphate composite material as a lithium battery cathode material are provided.
The beneficial effects that this application can produce include:
1) the lithium iron phosphate composite material provided by the application adopts lithium iron borate doped lithium iron phosphate, because LiFeBO3Stable structure, high theoretical specific capacity (220mAh/g), good ionic and electronic conductivity, and LiFeBO3The volume change is very small in the charging and discharging process, the problem of poor low-temperature discharging performance of the lithium iron phosphate anode material is solved, and the structure of the lithium iron phosphate material is not stable in the charging and discharging processSex has a significant impact.
2) Compared with a solid phase method, the method provided by the application has the advantages of low energy consumption, good coating property of the synthesized lithium iron phosphate composite material, uniform doping, and good granularity and granularity distribution.
3) According to the preparation method of the lithium iron phosphate composite material, citric acid is used as a complexing agent and a carbon source, gas generated during pyrolysis is beneficial to formation of fine and porous products, the specific surface area is large, and the electronic conductivity of the material can be improved by pyrolysis residual carbon.
Drawings
FIG. 1 is a scanning electron micrograph of sample 1 of the present application;
FIG. 2 is a scanning electron micrograph of sample 2 of the present application;
FIG. 3 is a scanning electron micrograph of a composite obtained in sample 3 of the present application;
FIG. 4 is a scanning electron micrograph of a control sample of the present application;
fig. 5 is a first charge-discharge curve of the resulting lithium battery of sample 1 of the present application;
fig. 6 is a charge and discharge curve of the resulting lithium battery of sample 1 of the present application;
FIG. 7 is a 0.1C charge-discharge curve of lithium batteries obtained from samples 1-3 of the present application and a control sample;
FIG. 8 is a 0.3C discharge curve of lithium batteries obtained from samples 1-3 of the present application and a control sample;
FIG. 9 is a 0.5C discharge curve of lithium batteries obtained from samples 1-3 of the present application and a control sample;
FIG. 10 is a graph showing 1.0C discharge curves of lithium batteries obtained from samples 1 to 3 of the present application and a control sample;
FIG. 11 is a 2.0C discharge curve of lithium batteries obtained from samples 1-3 of the present application and a control sample;
FIG. 12 is a 3.0C discharge curve of lithium batteries obtained from samples 1-3 of the present application and a control sample;
FIG. 13 is a graph showing the rate cycles at 60 ℃ of 1.5 ℃ for lithium batteries obtained from samples 1 to 3 of the present application and a control sample.
Detailed Description
The present application will be described in detail with reference to examples, but the present application is not limited to these examples.
The raw materials in the examples of the present application were all purchased commercially, unless otherwise specified.
The analysis method in the examples of the present application is as follows:
and (4) carrying out microstructure analysis on the compound by using a scanning electron microscope.
Analysis of the substance content of the complex was performed using ICP.
And (4) carrying out performance test on the battery by using a blue test system.
Control group
1. A precursor synthesis stage: preparation of carbon-coated LiFePO by sol-gel method4Positive electrode material
According to LiOH, FePO4Mixing a lithium source, a boron source, an iron source and a phosphorus source in deionized water (the dosage of the deionized water is 3 times of the total mass of all the raw materials), adding citric acid, polyethylene glycol (the dosage of the polyethylene glycol is 1/10 of the total mass of all the raw materials) and glucose accounting for 2.0 percent of the total mass of all the raw materials, stirring for 20 hours in a water bath kettle to form gel, and cooling to room temperature to obtain viscous precursor gel;
2. putting the obtained precursor gel into a vacuum drying oven at 70 ℃ for drying for 8h, then carrying out ball milling for 2h (the ball milling parameter is 20Hz, and the ball-to-material ratio is 2:1), and finally putting the obtained powder into a tube furnace protected by inert atmosphere for sintering, wherein the method specifically comprises the following steps: presintering for 5h at 300 ℃, then ball-milling for 4h, taking out and drying, and sintering for 7h at 750 ℃ under the protection of inert atmosphere to finally obtain the carbon-coated lithium iron phosphate composite material. The control group samples were recorded.
Example 1
1. A precursor synthesis stage: preparation of carbon-coated LiFePO4 positive electrode material by sol-gel method
According to LiOH, H3BO3、FePO4The molar ratio of the lithium source to the citric acid is 0.97:0.03:0.97:3, and the lithium source, the boron source, the iron source and the phosphorus source are added into deionized water (the dosage of the deionized water is the total raw materials)3 times of the total mass of all the raw materials), adding citric acid, polyethylene glycol (the dosage of the polyethylene glycol is 1/10 of the total mass of all the raw materials) and glucose accounting for 2.0 percent of the total mass of all the raw materials, stirring for 20 hours in a water bath to form gel, and cooling to room temperature to obtain viscous precursor gel;
2. putting the obtained precursor gel into a vacuum drying oven at 70 ℃ for drying for 8h, then carrying out ball milling for 2h (ball milling frequency is 30Hz, ball-to-material ratio is 2:1), and finally putting the obtained powder into a tube furnace protected by inert atmosphere for sintering, wherein the steps are as follows: presintering for 5h at 300 ℃, then ball-milling for 4h, taking out and drying, and sintering for 7h at 750 ℃ under the protection of inert atmosphere to finally obtain the carbon-coated lithium iron borate doped lithium iron phosphate composite material. Designated sample 1.
Example 2
1. A precursor synthesis stage: preparation of carbon-coated LiFePO by sol-gel method4Positive electrode material
According to LiOH, H3BO3、FePO4Mixing a lithium source, a boron source, an iron source and a phosphorus source in deionized water (the dosage of the deionized water is 3 times of the total mass of all the raw materials), adding citric acid, polyethylene glycol (the dosage of the polyethylene glycol is 1/10 times of the total mass of all the raw materials) and glucose accounting for 2 percent of the total mass of all the raw materials, stirring for 15 hours in a water bath kettle at 70 ℃ to form gel, and cooling to room temperature to obtain viscous precursor gel;
2. putting the obtained precursor gel into a vacuum drying oven at 75 ℃ for drying for 7h, then carrying out ball milling for 3h (ball milling frequency is 30Hz, ball-to-material ratio is 2:1), and finally putting the obtained powder into a tube furnace protected by inert atmosphere for sintering, wherein the steps are as follows: presintering for 4h at 320 ℃, then ball-milling for 4.5h, taking out, drying, and sintering for 7h at 750 ℃ under the protection of inert atmosphere to finally obtain the carbon-coated lithium iron borate doped lithium iron phosphate composite material. Designated sample 2.
Example 3
1. A precursor synthesis stage: preparation of carbon-coated LiFePO by sol-gel method4Positive electrode material
According to LiOH, H3BO3、FePO4Mixing a lithium source, a boron source, an iron source and a phosphorus source in deionized water (the dosage of the deionized water is 3 times of the total mass of all the raw materials), adding citric acid, polyethylene glycol (the dosage of the polyethylene glycol is 1/10 times of the total mass of all the raw materials) and glucose accounting for 2 percent of the total mass of all the raw materials, stirring for 10 hours at 90 ℃ in a water bath kettle to form gel, and cooling to room temperature to obtain viscous precursor gel;
2. putting the obtained precursor gel into a vacuum drying oven at 80 ℃ for drying for 6h, then carrying out ball milling for 4h (ball milling frequency is 30Hz, ball-to-material ratio is 2:1), and finally putting the obtained powder into a tube furnace protected by inert atmosphere for sintering, wherein the steps are as follows: presintering for 3h at 350 ℃, then carrying out ball milling for 5h, taking out and drying, and sintering for 7h at 800 ℃ under the protection of inert atmosphere to finally obtain the carbon-coated lithium iron borate doped lithium iron phosphate composite material. And recorded as sample 3.
Performing scanning electron microscope analysis on the samples 1-3 and the control group sample, and as shown in fig. 1-4, the microscopic size of the lithium iron phosphate composite material obtained by the method is 0.2-0.3 mu m; . In the composite material of sample 1, the content of lithium iron phosphate containing the heterogeneous doping material is 98 wt%, and the content of carbon is 2 wt%; wherein, in the lithium iron phosphate containing the heterogeneous doping material, the content of the lithium iron borate is 13 wt%, and the content of the lithium iron phosphate is 87 wt%.
The electrochemical energy test was performed on samples 1-3 and the control sample.
Respectively taking samples 1-3 and a control group sample as positive electrode materials, uniformly mixing the positive electrode materials with Super P and PVDF according to a mass ratio of 90:5:5, preparing slurry by taking NMP as a solvent, coating and drying to obtain a positive electrode plate, taking metal lithium as a counter electrode, using a special electrolyte for lithium iron phosphate, assembling a battery, and respectively carrying out electrochemical performance tests under the conditions of 25 ℃, 0.1C, 0.3C, 0.5C, 1C, 2C and 3C (2.5-4.2V).
The test result curve diagrams of the batteries obtained by the comparison group, the sample 1, the sample 2 and the sample 3 are shown in fig. 5-12, and the comparison shows that after the lithium iron borate is compounded with the lithium iron phosphate, the charging and discharging capacity of the battery is obviously improved, the battery shows different electrochemical performances at different sintering temperatures, the specific capacity of the sample two is optimal at a low multiplying power, and the specific capacity of the sample 1 is optimal at a high multiplying power; comparing with the cycle curve diagram 13, it can be found that the lithium iron borate composite lithium iron phosphate has no obvious influence on the cycle performance of the lithium iron phosphate. The performance of the battery obtained by combining the samples is taken as the positive electrode, the charge and discharge performance of the lithium battery is obviously improved, and the capacity of more than 93 percent is still maintained after 300 cycles of charge and discharge, which shows that the battery has good charge and discharge stability.
Although the present application has been described with reference to a few embodiments, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the application as defined by the appended claims.
Claims (10)
1. A lithium iron phosphate composite material is characterized in that the composite material comprises lithium iron phosphate containing heterogeneous doping substances;
the heterogeneous doping material is lithium iron borate;
and carbon is coated outside the lithium iron phosphate.
2. The composite material of claim 1, wherein the composite material has a microscopic dimension of 0.2 to 0.3 μm;
in the composite material, the content of lithium iron phosphate containing heterogeneous doping substances is 97.5-99 wt%, and the content of carbon is 1-2.5 wt%;
wherein the content of the heterogeneous doping material in the lithium iron phosphate containing the heterogeneous doping material is 0.1-10 wt%, and the content of the lithium iron phosphate is 90-99.9 wt%.
3. The method for preparing the lithium iron phosphate composite material according to claim 1 or 2, wherein the method comprises at least the steps of:
step 1, heating a mixture containing a lithium source, a boron source, an iron source, a phosphorus source, a carbon source and a complexing agent for reaction to obtain precursor gel;
and 2, drying and sintering the precursor gel to obtain the lithium iron phosphate composite material.
4. The production method according to claim 3, wherein the lithium source is at least one selected from the group consisting of lithium hydroxide, lithium carbonate, and lithium oxalate;
the boron source is at least one of boric acid, borate and boron oxide;
the iron source is at least one selected from iron phosphate and ferrous oxalate;
the phosphorus source is selected from at least one of iron phosphate and ammonium dihydrogen phosphate;
the carbon source is at least one of glucose and carbon nano tubes;
the complexing agent is at least one selected from citric acid and ethylene diammonium tetraacetic acid;
the mixture also comprises a solvent;
the solvent is at least one selected from pure water and ethylene glycol.
5. The method according to claim 3, wherein the molar ratio of the lithium element, the boron element, the iron element and the phosphorus element in the mixture is 0.95-1.0: 0.01-0.1: 0.95-1.05;
the molar ratio of the complexing agent to the lithium source is 3-4: 0.95-1.0;
the dosage of the carbon source is 1.0-2.5% of the total mass of the mixture.
6. The method of claim 3, wherein the mixture further comprises a dispersant;
the dispersant comprises polyethylene glycol;
the dosage of the dispersing agent is 2-10% of the total mass of the mixture.
7. The production method according to claim 3, wherein the heating reaction conditions are:
heating and reacting at the temperature of 60-90 ℃ for 10-20 h;
the drying conditions are as follows:
the drying temperature is 70-80 ℃, and the drying time is 6-8 h.
8. The production method according to claim 3, wherein the sintering includes pre-sintering and elevated-temperature sintering;
the presintering conditions are as follows: the pre-sintering temperature is 300-350 ℃, and the pre-sintering time is 3-5 h;
the conditions of temperature-rising sintering are as follows: raising the temperature to 700-800 ℃, and pre-sintering for 6-8 h;
preferably, before the temperature-rising sintering, the presintered product is subjected to ball milling.
9. The method of claim 3, wherein the dried product is ball milled prior to sintering.
10. The tube/lithium iron phosphate composite material according to claim 1 or 2, the lithium iron phosphate composite material prepared by the method according to any one of claims 3 to 9, and the application of the composite material as a positive electrode material of a lithium battery.
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN115744858A (en) * | 2022-10-25 | 2023-03-07 | 广西华友新材料有限公司 | Lithium iron phosphate positive electrode material, preparation method thereof, electrode and battery |
| CN116216686A (en) * | 2023-03-28 | 2023-06-06 | 陕西创普斯新能源科技有限公司 | Preparation method of lithium iron phosphate anode material |
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