CN111211311B - Preparation method of porous nano lithium iron phosphate composite material - Google Patents
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Abstract
The invention discloses a preparation method of a porous nano lithium iron phosphate composite material. The preparation process comprises the following steps: preparing sulfonated polystyrene microspheres and an alkaline phenolic resin aqueous solution, carrying out chemical reaction, sequentially carrying out reflux reaction and elution, then adding an iron phosphate solution and a nitrogen source, forming hydrogel through hydrothermal reaction, then adding the hydrogel into an inorganic lithium salt solution for soaking, carrying out stirring reaction, filtering, drying at low temperature, carbonizing, and doping with gas to obtain the porous nano lithium iron phosphate composite material. The prepared material utilizes nitrogen doping to improve the specific capacity of the material, and resin coated on the surface of lithium iron phosphate is carbonized to form a porous structure of porous hard carbon and a large interlayer spacing of the hard carbon, so that the liquid absorption and retention capability of the material are improved, and the multiplying power and the low-temperature cycle performance of the material are improved.
Description
Technical Field
The invention belongs to the field of lithium ion battery material preparation, and particularly relates to a porous nano lithium iron phosphate composite material and a preparation method thereof.
Background
With the improvement of the requirements of pure electric vehicles or passenger cars on the low temperature and the rate capability of the lithium iron phosphate, the lithium iron phosphate battery is required to have high low temperature and rate capability while improving the energy density so as to meet the requirements of the quick charging performance and the low temperature charging performance of the lithium iron phosphate. The current lithium iron phosphate has low ion conductivity and electron conductivity, and is only suitable for charging and discharging under low current density, and the specific capacity is reduced during high-rate charging and discharging, so that the application of the material is limited. For example, patent application No. 201810924854.7 discloses a method for preparing a porous lithium iron phosphate electrode material. According to the method, lithium hydroxide and slightly excessive phosphoric acid are used as a lithium source and a phosphorus source, a lithium phosphate precursor is synthesized by a precipitation method under the stirring condition, ethylene glycol is used as a dispersing agent for a solvothermal method and an auxiliary agent for generating air holes, the precursor solution is mixed and uniformly dispersed, ferrous sulfate is used as an iron source, ascorbic acid is used as a reducing agent, and the nano porous lithium iron phosphate electrode material is obtained by high-temperature calcination. Although the specific capacity of the material is improved, the carbon layer is an amorphous carbon layer left by ethylene glycol decomposition, and the carbon layer and lithium iron phosphate have poor binding force, and the carbon layer has small spacing and low lithium ion transmission rate, so that the intercalation and extraction rate of lithium ions in the charging and discharging process is slow, the process is complex, the process is difficult to control, and the consistency is difficult to ensure.
Disclosure of Invention
Aiming at the problems that the low-temperature performance of the existing lithium iron phosphate is poor, the binding force between a carbon layer and the lithium iron phosphate is poor, the distance between the carbon layers is small in the charging and discharging processes, the multiplying power performance of the lithium iron phosphate is influenced, and the like, the invention aims to coat a hard carbon material and a nitrogen-doped element thereof on the surface of the lithium iron phosphate by a hydrothermal method to prepare the nano porous nano lithium iron phosphate, so that the electron transmission rate and the ion transmission rate of the material are improved, and the multiplying power low-temperature performance of the material is improved.
The technical scheme of the invention is realized by the following modes: a preparation method of a porous nano lithium iron phosphate composite material comprises the following steps in percentage by weight:
1) preparation of hard carbon precursor material a: mixing sulfonated polystyrene microspheres and alkaline phenolic resin to prepare aqueous solution with the concentration of 5-20%, and sequentially carrying out reflux reaction and elution to obtain a hard carbon precursor material A;
wherein the weight ratio is as follows: sulfonated polystyrene microspheres: alkaline phenolic resin =1 (0.5-2); the diameter of the sulfonated polystyrene microspheres is (0.1-2) mu m;
2) preparing a lithium iron phosphate composite precursor material B: adding a hard carbon precursor material A, an iron phosphate solution, a nitrogen source and hydrogen peroxide into deionized water to prepare an aqueous solution with the concentration of (1-10)% through ultrasonic dispersion, transferring the aqueous solution into a high-pressure reaction kettle, reacting for (1-12) h at the temperature of (120-200) ° C, naturally cooling to room temperature to obtain blocky hydrogel, drying and crushing the blocky hydrogel, adding the blocky hydrogel into an inorganic lithium salt solution, reacting for (1-12) h at the temperature of (100-200) ° C, filtering, transferring the solution into a vacuum oven, drying for (12-72) h at the low temperature of (30-60) ° C, supplementing water into hydrogel once every 3h, and continuously drying in vacuum to obtain a lithium iron phosphate composite precursor material B;
wherein the weight ratio of the hard carbon precursor material A: iron phosphate solid: nitrogen source: hydrogen peroxide =10 (10-50): (1-5): (0.1-1):
3) preparing the lithium iron phosphate composite microsphere C: transferring the lithium iron phosphate composite precursor material B into a tubular furnace, heating to 200-300 ℃ at a heating rate of 1-5 ℃/min under an inert atmosphere, preserving heat for 1-5 h, heating to 800-1500 ℃ at a heating rate of 1-5 ℃/min, preserving heat for 1-5 h, and then cooling to room temperature under an inert atmosphere to obtain the lithium iron phosphate composite microspheres C;
4) preparing modified lithium iron phosphate composite microspheres: carrying out ball milling and crushing on the lithium iron phosphate composite microspheres C, transferring the lithium iron phosphate composite microspheres C into a tube furnace, firstly removing air in the tube through inert gas, and then modifying mixed gas, wherein the volume ratio of the mixture is as follows: argon gas: and (2) the modified gas is 1: 1-5, the flow rate is (1-10) ml/min, the temperature is increased to (800-1000) DEG C at the temperature increase rate of 1-5 ℃/min, and the surface or the pores of the material are doped to obtain the modified lithium iron phosphate composite microspheres.
The concentration of the ferric phosphate in the step (2) is (1-10)%;
the nitrogen source in the step (2) is one of aniline, urea, melamine, pyrrole and ammonia water;
the modified gas is one of chlorine trifluoride, chlorine pentafluoride, acetylene and ozone;
the inorganic lithium salt is one of lithium carbonate, lithium hydroxide and lithium acetate, and the concentration is (1-10)%.
According to the invention, the sulfonated polystyrene microspheres and the alkaline phenolic resin are subjected to chemical reaction by a chemical method, so that the binding force between the materials is improved, the structure stability of the hard carbon composite material is facilitated, the cycle performance of the hard carbon composite material is improved, the hard carbon composite material is coated on the surface of lithium iron phosphate, and the transmission rate of lithium ions is improved by virtue of the characteristic of large spacing between hard carbon layers. Meanwhile, the nitrogen source is added to improve the electronic conductivity of the carbon material, the surface defect degree of the material is reduced through gas surface modification, the first efficiency of the material and the compatibility of the material and the electrolyte are improved, and the cycle performance of the material is improved. Meanwhile, the iron phosphate precursor is prepared by a hydrothermal method, has uniform and controllable holes, and is more fully reacted with lithium salt later, so that the stable lithium iron phosphate porous material is obtained.
Drawings
Fig. 1 is an SEM image of the porous lithium iron phosphate composite prepared in example 1.
Detailed Description
Example 1:
a preparation method of a porous nano lithium iron phosphate composite material comprises the following steps:
1. preparation of hard carbon precursor material a: mixing 10g of sulfonated polystyrene microspheres (with the diameter of 1 mu m) and 10g of alkaline phenolic resin, adding the mixture into 200g of deionized water, uniformly stirring to obtain 10wt% aqueous solution, adding the aqueous solution into a three-neck flask, adding 2g of sodium carbonate aqueous solution, adjusting the pH value to 8.0, keeping the temperature at 80 ℃ for 60min, and then washing to obtain a hard carbon precursor material A;
2. preparing a lithium iron phosphate composite precursor material B: adding 10g of hard carbon precursor material A, 600ml of hard carbon precursor material A, 5% of iron phosphate solution, 3g of pyrrole and 0.5g of hydrogen peroxide into 1120ml of deionized water to prepare 5wt% of aqueous solution, obtaining uniform solution through ultrasonic dispersion, transferring the uniform solution into a high-pressure reaction kettle, reacting for 6 hours at the temperature of 180 ℃, naturally cooling to room temperature to obtain blocky hydrogel, drying, crushing, adding the blocky hydrogel into 1000ml of 5% of lithium hydroxide solution, reacting for 6 hours at the temperature of 150 ℃, filtering, transferring the blocky hydrogel into a vacuum oven, drying for 48 hours at the low temperature of 50 ℃, supplementing water to the hydrogel once every 3 hours, and continuously drying in vacuum to obtain a lithium iron phosphate composite precursor material B;
3. preparing the lithium iron phosphate composite microsphere C: transferring the lithium iron phosphate composite precursor material B into a tubular furnace, heating to 250 ℃ at a heating rate of 3 ℃/min under an inert atmosphere, preserving heat for 3 hours, heating to 1000 ℃ at a heating rate of 3 ℃/min, preserving heat for 3 hours, and cooling to room temperature under an argon inert atmosphere to obtain the lithium iron phosphate composite microspheres C;
4. preparing modified lithium iron phosphate composite microspheres: carrying out ball milling and crushing on the modified lithium iron phosphate composite microspheres C, transferring the modified lithium iron phosphate composite microspheres C into a tube furnace, firstly removing air in the tube through argon inert gas, and modifying mixed gas through chlorine trifluoride, wherein the volume ratio is as follows: argon gas: chlorine trifluoride =10:3, the flow rate is 5ml/min, and the temperature is increased to 900 ℃ at the temperature increase rate of 3 ℃/min, so as to obtain the modified lithium iron phosphate composite microspheres.
Example 2:
1. preparation of hard carbon precursor material a: mixing 5g of sulfonated polystyrene microspheres (with the diameter of 0.5 mu m) and 10g of alkaline phenolic resin, adding the mixture into 300g of deionized water, uniformly stirring to obtain a 5wt% aqueous solution, adding the aqueous solution into a three-neck flask, adding 2g of sodium carbonate aqueous solution, adjusting the pH value to 8.0, keeping the temperature at 80 ℃ for 60min, and washing to obtain a hard carbon precursor material A;
2. preparing a lithium iron phosphate composite precursor material B: adding 10g of hard carbon precursor material A, 1000ml of a 1% iron phosphate solution, 1g of aniline and 0.1g of hydrogen peroxide into 1089ml of deionized water to prepare a 1wt% aqueous solution, performing ultrasonic dispersion to obtain a uniform solution, transferring the uniform solution into a high-pressure reaction kettle, reacting at 120 ℃ for 12h, naturally cooling to room temperature to obtain a block-shaped hydrogel, drying, crushing, adding the block-shaped hydrogel into 1000ml of a 1% lithium carbonate solution, reacting at 120 ℃ for 12h, filtering, transferring the solution into a vacuum oven, drying at 30 ℃ for 72h, supplementing water to the hydrogel once every 3h, and continuously performing vacuum drying to obtain a lithium iron phosphate composite precursor material B;
3. preparing the lithium iron phosphate composite microsphere C: transferring the lithium iron phosphate composite precursor material B into a tubular furnace, heating to 200 ℃ at the heating rate of 1 ℃/min under the inert atmosphere of argon, preserving heat for 1h, then heating to 800 ℃ at the heating rate of 1 ℃/min, preserving heat for 1h, and then cooling to room temperature under the inert atmosphere of argon to obtain the lithium iron phosphate composite microspheres C;
4. preparing modified lithium iron phosphate composite microspheres: carrying out ball milling and crushing on the modified lithium iron phosphate composite microspheres C, transferring the modified lithium iron phosphate composite microspheres C into a tube furnace, firstly removing air in the tube through argon inert gas, and modifying mixed gas through chlorine pentafluoride, wherein the volume ratio is as follows: argon gas: and (3) chlorine pentafluoride =10:1, the flow rate is 1ml/min, and the temperature is increased to 800 ℃ at the temperature increase rate of 1 ℃/min to obtain the modified lithium iron phosphate composite microspheres.
Example 3:
1. preparation of hard carbon precursor material a: mixing 10g of sulfonated polystyrene microspheres (with the diameter of 2 mu m) and 5g of alkaline phenolic resin, adding the mixture into 75g of deionized water, uniformly stirring to obtain a 20wt% aqueous solution, adding the aqueous solution into a three-neck flask, adding 2g of sodium carbonate aqueous solution, adjusting the pH value to 8.0, keeping the temperature at 80 ℃ for 60min, and then washing to obtain a hard carbon precursor material A;
2. preparing a lithium iron phosphate composite precursor material B: adding 10g of hard carbon precursor material A, 500ml of a hard carbon precursor material A, 10% of an iron phosphate solution, 1g of urea and 1g of hydrogen peroxide into 100ml of deionized water to prepare a 10wt% aqueous solution, performing ultrasonic dispersion to obtain a uniform solution, transferring the uniform solution into a high-pressure reaction kettle, reacting at the temperature of 200 ℃ for 1h, naturally cooling to room temperature to obtain a block-shaped hydrogel, drying and crushing the block-shaped hydrogel, adding the block-shaped hydrogel into 500ml of a 10% lithium acetate solution, reacting at the temperature of 200 ℃ for 1h, filtering, transferring the solution into a vacuum oven, drying at the low temperature of 60 ℃ for 12h, supplementing hydrogel water once every 3h, and then continuing vacuum drying to obtain a lithium iron phosphate composite precursor material B;
3. preparing the lithium iron phosphate composite microsphere C: transferring the lithium iron phosphate composite precursor material B into a tubular furnace, heating to 300 ℃ at the heating rate of 5 ℃/min under the inert atmosphere of argon, preserving heat for 5 hours, heating to 1500 ℃ at the heating rate of 5 ℃/min, preserving heat for 5 hours, and then cooling to room temperature under the inert atmosphere of argon to obtain the lithium iron phosphate composite microspheres C;
4. preparing modified lithium iron phosphate composite microspheres: carrying out ball milling and crushing on the modified lithium iron phosphate composite microspheres C, transferring the modified lithium iron phosphate composite microspheres C into a tube furnace, firstly removing air in the tube through argon inert gas, and then modifying mixed gas through acetylene, wherein the volume ratio of the mixed gas is as follows: argon gas: acetylene =10:5, the flow rate is 10ml/min, and the temperature is raised to 1000 ℃ at a temperature rise rate of 5 ℃/min, so as to obtain the modified lithium iron phosphate composite microsphere.
Comparative example:
adding 10g of phenolic resin and 50g of lithium iron phosphate into 1000ml of deionized water, ultrasonically dispersing uniformly, transferring into a high-pressure reaction kettle, reacting at 150 ℃ and 2Mpa for 3h, naturally cooling to room temperature, filtering, vacuum drying at 50 ℃ for 48h, crushing, transferring into a tube furnace, heating to 800 ℃ at a heating rate of 5 ℃/min under the atmosphere of argon, preserving heat for 3h, and cooling to room temperature under the atmosphere of argon to obtain the lithium iron phosphate composite material.
1) And (4) SEM test:
as can be seen from the SEM image of the lithium iron phosphate composite material prepared in example 1, the material is in a sphere-like shape, and the particle size is (5-20) mum.
2) Physical and chemical property test
The specific surface area, tap density and specific capacity of the material are tested according to the national standard GB/T30835-2014 carbon composite lithium iron phosphate anode material for the lithium ion battery.
The test method comprises the following steps: weighing 2.0000g of each of the four samples in the embodiments 1-3 and the comparative example, respectively preparing each sample into a button cell, namely mixing 2.0000g of the sample with 0.1111g of conductive carbon black and 0.1111g of PVDF (according to the mass ratio of 0.9: 0.05), adding 2.5g of NMP (N-methyl pyrrolidone) as an organic solvent, fully and uniformly mixing, coating a film with the thickness of 140 micrometers on an aluminum foil, drying for 2h in vacuum at 120 ℃, beating into a 5mm wafer by using a puncher, tabletting under 10MPa by using a tabletting machine, keeping the temperature for 12h in vacuum at 120 ℃, and weighing the weight of a positive plate. A button cell is assembled in an argon-protected glove box, a metal lithium sheet is used as a negative electrode, an electrolyte is an EC (ethylene carbonate) and DMC (1, 2-dimethyl carbonate) mixed solvent with the volume ratio of 1:1, an electrolyte LiPF6 is used, and a diaphragm is a Celgard2400 microporous polyethylene film. The assembled cell was tested for electrical performance on a blue tester. And in the voltage range of 2.5V-4.2V, charging/discharging at a constant current of 0.2C to test the specific capacity, and simultaneously carrying out charging at 0.2C and discharging at 10C to test the specific capacity. The results are shown in table 1, in which a1, a2, A3 and B1, B2 represent the button cells prepared by the five samples of examples 1.1 to 1.3 and comparative examples 1 and 2.
TABLE 1 physicochemical Properties
As can be seen from the results in table 1, the discharge capacity and the first efficiency of the lithium iron phosphate cathode material prepared in the first embodiment are significantly higher than those of the lithium iron phosphate cathode material prepared in the comparative example 1, because the hard carbon composite material in which nitrogen is doped on the surface of lithium iron phosphate has the characteristics of large interlayer spacing and high specific surface area, and provides a fast channel for the insertion and extraction of lithium ions during the charge and discharge processes, thereby improving the gram capacity performance and the rate capability of the material. Meanwhile, the freeze-drying method can increase the pores of the material while maintaining the structure of the material, thereby improving the specific surface area of the material.
3) Soft package battery
And (3) electrochemical performance testing: taking the positive electrode materials prepared in the examples 1-3 and the comparative example, carrying out slurry mixing and coating to prepare a negative electrode piece, taking artificial graphite as a negative electrode, taking EC/DEC/PC (EC: DEC: PC =1:1: 1) as an electrolyte and lipF as a solute6And Celgard2400 membrane as a separator, 5Ah soft package batteries C1, C2, C3 and D1 were prepared respectively.
The positive plate was then tested for its liquid absorption capacity and for its first efficiency, cycling performance (1.0C/1.0C). The test method refers to the national standard GB/T30835-2014 carbon composite lithium iron phosphate anode material for the lithium ion battery; and meanwhile, calculating the mass energy density of the soft package lithium ion battery according to the discharge capacity and the mass of the lithium ion battery.
The test results are shown in table 2 below.
TABLE 2 imbibition Capacity of Positive plate
As can be seen from table 2, the liquid absorbing and retaining capabilities of the negative electrode in examples 1 to 3 are all significantly better than those of the comparative example, and the analysis reasons are as follows: the lithium iron phosphate composite material with the shell of the porous hard carbon coating layer prepared by a hydrothermal method has nanoscale holes, and the liquid absorption and retention capacity of the material is improved.
TABLE 3 cycling performance of pouch cells
The cycle performance of the soft package batteries in the table 3 and the examples 1 to 3 is obviously superior to that of the comparative example, and the analysis reason is as follows: the porous lithium iron phosphate material with large specific surface area can be prepared by adopting a freeze drying method, has large specific surface area, and improves the liquid absorption capacity of the material, thereby improving the cycle performance of the material; meanwhile, the hard carbon precursor prepared by a chemical method has the characteristics of stable structure and large interlayer spacing, and is coated on the surface of the lithium iron phosphate to improve the surface structure stability of the material and the cycle performance of the material, and the active points on the surface of the material are reduced by gas surface modification to reduce the occurrence probability of side reactions of the material and further improve the cycle performance of the material.
Claims (5)
1. A preparation method of a porous nano lithium iron phosphate composite material comprises the following steps in percentage by weight:
(1) preparation of hard carbon precursor material a: mixing sulfonated polystyrene microspheres and alkaline phenolic resin to prepare an aqueous solution with the concentration of 5-20%, and sequentially carrying out reflux reaction and elution to obtain a hard carbon precursor material A;
wherein the weight ratio is as follows: sulfonated polystyrene microspheres: alkaline phenolic resin =1: 0.5-2; the diameter of the sulfonated polystyrene microspheres is 0.1-2 mu m;
(2) preparing a lithium iron phosphate composite precursor material B: adding a hard carbon precursor material A, an iron phosphate solution, a nitrogen source and hydrogen peroxide into deionized water to prepare a 1-10% aqueous solution, performing ultrasonic dispersion to obtain a uniform solution, transferring the uniform solution into a high-pressure reaction kettle, reacting at 120-200 ℃ for 1-12 h, naturally cooling to room temperature to obtain a block-shaped hydrogel, drying, crushing, adding the block-shaped hydrogel into an inorganic lithium salt solution, reacting at 100-200 ℃ for 1-12 h, filtering, transferring into a vacuum oven, drying at 30-60 ℃ for 12-72 h, supplementing water to the hydrogel once every 3h, and continuously performing vacuum drying to obtain a lithium iron phosphate composite precursor material B; wherein the weight ratio of the hard carbon precursor material A: iron phosphate solid: nitrogen source: hydrogen peroxide =10: 10-50: 1-5: 0.1-1:
(3) preparing the lithium iron phosphate composite microsphere C: transferring the lithium iron phosphate composite precursor material B into a tubular furnace, heating to 200-300 ℃ at a heating rate of 1-5 ℃/min under an inert atmosphere, preserving heat for 1-5 h, heating to 800-1500 ℃ at a heating rate of 1-5 ℃/min, preserving heat for 1-5 h, and then cooling to room temperature under an inert atmosphere to obtain lithium iron phosphate composite microspheres C;
(4) preparing modified lithium iron phosphate composite microspheres: carrying out ball milling and crushing on the lithium iron phosphate composite microspheres C, transferring the lithium iron phosphate composite microspheres C into a tube furnace, firstly removing air in the tube through inert gas, and then modifying mixed gas, wherein the volume ratio of the mixture is as follows: argon gas: and (3) modified gas =10: 1-5, the flow rate is 1-10 ml/min, the temperature is raised to 800-1000 ℃ at the temperature raising rate of 1-5 ℃/min, and the modified lithium iron phosphate composite microspheres are obtained by doping on the surface or in the pores of the material.
2. The preparation method of the porous nano lithium iron phosphate composite material according to claim 1, wherein the preparation method comprises the following steps: the concentration of the ferric phosphate in the step (2) is 1-10%.
3. The preparation method of the porous nano lithium iron phosphate composite material according to claim 1, wherein the preparation method comprises the following steps: and (3) the nitrogen source in the step (2) is one of aniline, urea, melamine, pyrrole and ammonia water.
4. The preparation method of the porous nano lithium iron phosphate composite material according to claim 1, wherein the preparation method comprises the following steps: the modified gas in the step (4) is one of chlorine trifluoride, chlorine pentafluoride, acetylene and ozone.
5. The preparation method of the porous nano lithium iron phosphate composite material according to claim 1, wherein the preparation method comprises the following steps: the inorganic lithium salt is one of lithium carbonate, lithium hydroxide and lithium acetate, and the concentration is 1-10%.
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