CN114122333A - Nano onion carbon composite lithium iron phosphate cathode material and preparation method and application thereof - Google Patents

Nano onion carbon composite lithium iron phosphate cathode material and preparation method and application thereof Download PDF

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CN114122333A
CN114122333A CN202111413962.6A CN202111413962A CN114122333A CN 114122333 A CN114122333 A CN 114122333A CN 202111413962 A CN202111413962 A CN 202111413962A CN 114122333 A CN114122333 A CN 114122333A
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carbon
iron phosphate
lithium iron
nano onion
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CN114122333B (en
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张法明
邓传迅
商彩云
熊义峰
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Southeast University
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
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    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
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    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/139—Processes of manufacture
    • H01M4/1397—Processes of manufacture of electrodes based on inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/52—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron
    • H01M4/525—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron of mixed oxides or hydroxides containing iron, cobalt or nickel for inserting or intercalating light metals, e.g. LiNiO2, LiCoO2 or LiCoOxFy
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    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
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    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
    • H01M4/5825—Oxygenated metallic salts or polyanionic structures, e.g. borates, phosphates, silicates, olivines
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Abstract

The invention discloses a nano onion carbon composite lithium iron phosphate cathode material and a preparation method and application thereof. The nano onion carbon material and the coating carbon are in good contact, and a lithium iron phosphate composite carbon conductive double network is formed together, so that the conductive performance of the lithium iron phosphate is improved. The invention adopts a pressure sintering method to tightly combine the interface of the nano onion carbon and the lithium iron phosphate particles, and overcomes the problems of poor conductivity of the conventional conductive carbon black conductive agent and untight contact with the positive active material after the conductive agent is added. The composite material is compounded by adding the nano onion carbon on the basis of the conventional carbon-coated lithium iron phosphate powder, so that the conductivity of the active substance of the positive electrode is improved, the capacity and the rate charge and discharge performance of the positive electrode material are improved, and the composite material can be applied to the fields of lithium ion batteries, energy storage and the like.

Description

Nano onion carbon composite lithium iron phosphate cathode material and preparation method and application thereof
Technical Field
The invention belongs to the field of electrode materials and the field of electrochemical energy storage, and particularly relates to a nano onion carbon composite lithium iron phosphate cathode material and a preparation method and application thereof.
Background
Lithium iron phosphate (LiFePO)4) The material is discovered by Goodenough et al in 1997, and is a widely used lithium ion battery anode material due to environmental friendliness, stable structure and low price. The olivine-structured lithium iron phosphate has a one-dimensional lithium ion transmission channel, and the conductivity of the olivine-structured lithium iron phosphate is only 10-8S/cm, so that the capacity of the olivine-structured lithium iron phosphate is limited. At present, the conductivity of the lithium iron phosphate particles is improved mainly through two ways, the conductivity of the lithium iron phosphate particles can be greatly improved through a surface carbon coating process, and the conductivity of the lithium iron phosphate particles is improved by coating organic carbon or inorganic carbon on the lithium iron phosphate particles. In addition, the other way of improving the conductivity of the lithium iron phosphate is based on the de-intercalation mechanism of lithium ions in the lithium iron phosphate, and the lithium iron phosphate particles which cannot participate in electrochemical reaction are reduced by reducing the particle size to the nanometer level to reduce polarization, so that the capacity and the rate capability of the lithium iron phosphate in practical use are improved.
The performance of lithium iron phosphate is determined by the particle size and the electrical conductivity, the particle size mainly influences the conduction of lithium ions, namely the insertion and extraction of the lithium ions, and the interconversion between FePO4 and LiFePO4 is limited by the particle size in the charging and discharging process. Conductivity here mainly refers to the transport of electrons in the lithium iron phosphate particles. The amorphous carbon is coated on the surface of the lithium iron phosphate to form a carbon layer, so that the wetting of the electrolyte to particles is enhanced, the carbon layer is good in conductivity, and the concentration polarization is reduced. Generally, when a positive active material is coated on a current collector, a binder and a conductive additive are added, wherein the binder mainly plays a role in binding, and meanwhile, the binder also has higher ionic conductivity and electronic conductivity, the conductive additive is beneficial to improving the conductivity of the positive electrode, and a conductive network is constructed on the current collector to improve the contact between the active material and the current collector. At present, the mainstream conductive agents are conductive carbon black conductive agents with simple process and low price, namely granular Super-P and chain Ketjen black. With the development of material preparation technology, sp2Hybrid carbon materials are gradually obtained and widely researched, wherein the electrical conductivity of the carbon nano tube and the graphene is excellent and far higher than sp3The hybridized amorphous carbon conductive agent has the components of the conductive agent playing a vital role in the performance of the final positive pole piece and even the battery. Thus, a series of additives taking a novel nano carbon material as a main body, such as a carbon nano tube conductive agent, a graphene conductive agent and the like, and a multi-element conductive additive forming a cross-linked composite conductive network are derived. The graphene conductive additive is high in price, industrial production is not realized, the carbon nano tube realizes small-scale production, but the single-walled carbon nano tube is easy to agglomerate, the problem of dispersion is solved at present mainly by preparing a conductive agent into slurry, and obtaining a pole piece after subsequent active substances are mixed and dried, but the effect of the conductive agent can be reduced if the conductive agent is not fully contacted with the active substances when the positive electrode slurry is prepared.
Disclosure of Invention
The purpose of the invention is as follows: aiming at the problems in the prior art, the invention provides a nano onion carbon composite lithium iron phosphate anode material, wherein a nano onion carbon material is inserted between carbon-coated lithium iron phosphate particles, because of excellent conductivity, the contact between an anode and a current collector is improved, the ion and electron exchange capacity of an anode active substance and electrolyte is improved, and the problem that the capacity of lithium iron phosphate is limited under high multiplying power due to the conductivity of a carbon coating layer is solved.
The invention also provides a preparation method of the nano onion carbon composite lithium iron phosphate cathode material, wherein a nano onion carbon material additive is mixed into carbon-coated lithium iron phosphate particles in an ultrasonic dispersion mixing stirring mode, and the nano onion carbon and the carbon-coated lithium iron phosphate form a composite material with tightly combined interfaces through discharge plasma sintering or vacuum hot pressing sintering.
The third purpose of the invention is to provide the application of the nano onion carbon composite lithium iron phosphate cathode material. The method is mainly applied to the manufacture of the anode piece of the lithium ion battery and the lithium ion battery.
The technical scheme is as follows: in order to achieve the above purpose, the nano onion carbon composite lithium iron phosphate cathode material provided by the invention uses carbon-coated lithium iron phosphate powder as a matrix, nano onion carbon as an additive, and nano onion carbon is uniformly dispersed in the carbon-coated lithium iron phosphate.
Wherein the nano onion carbon is in a hollow spherical multilayer fullerene carbon structure, and the diameter of the nano onion carbon is 3-100 nm. The diameter is the diameter of the original nano onion carbon. Because of the good structural stability of the nano onion carbon, the diameter of the nano onion carbon is basically consistent with the size of the originally purchased nano onion carbon after the material is formed.
Wherein the carbon-coated lithium iron phosphate powder has a particle size of 5nm to 20 μm, wherein the carbon content is 1 to 3% by mass, and the carbon has an amorphous structure. The particle size is the size of the original carbon-coated lithium iron phosphate particles, and after the composite material is formed by processing, the particle size is aggregated and enlarged under the action of pressure sintering as seen from an electrode scanning electron microscope picture, and is also enlarged compared with the original particles after grinding.
The preparation method of the nano onion-carbon composite lithium iron phosphate anode material comprises the following steps of:
(1) liquid phase chemical mixing: adding nano onion carbon into a solvent for ultrasonic dispersion, adding carbon-coated lithium iron phosphate powder, continuing dispersion, and then stirring;
(2) ball milling: ball milling the stirred and mixed liquid obtained in the step (1);
(3) and (3) drying: filtering the liquid after ball milling in the step (2), drying and sieving;
(4) sintering and forming: taking the product obtained in the step (3), and performing spark plasma sintering or hot-pressing sintering to obtain a block composite material;
(5) grinding and sieving the block composite material obtained in the step (4) to obtain a nano onion carbon composite lithium iron phosphate anode material with uniform particles; or slicing the block composite material to obtain the flaky nano onion carbon composite lithium iron phosphate anode material.
Wherein, in the step (1), the adding proportion of the nano onion carbon material is 0.1-5% of the mass of the carbon-coated lithium iron phosphate; preferably 3%.
Wherein, absolute ethyl alcohol is adopted as a dispersion solution of the nano onion carbon in the step (1), and ultrasonic dispersion is carried out for 0.5-2 h.
Wherein, the carbon-coated lithium iron phosphate powder is added in the step (1), the ultrasonic dispersion is continued for 0.5-2h, the ultrasonic power is 100-200W, and then the stirring is magnetic stirring for 10-24h, and the stirring speed is 500-800 rpm.
Wherein, the ball milling mode in the step (2) is high-energy ball milling, the rotating speed of the planetary ball mill is 200-1000rpm, and the ball milling time is 10-20 h.
Wherein, the air is dried for 10 to 20 hours after the filtration in the step (3), the drying temperature is 60 to 120 ℃, and the 200-mesh sieve is screened after the complete drying.
Wherein, the conditions of the spark plasma sintering in the step (4) are as follows: the pressure is 5-200MPa, the temperature is 500-900 ℃, the heat preservation time is 1-30min at the highest sintering temperature, and the sintering atmosphere is high-purity argon or vacuum; the hot-pressing sintering conditions are as follows: under the protection of vacuum or inert atmosphere, the pressure is 5-80MPa, the temperature is 500-900 ℃, and the heat preservation time at the highest sintering temperature is 10-180 min.
Wherein the slicing mode in the step (5) is diamond wire cutting, the processing speed is 10mm-500mm/min, and the thickness of the processed composite anode is 20-200 μm.
Preferably, the preparation method comprises the following steps:
(1) calculating and weighing the original powder: weighing carbon-coated lithium iron phosphate powder and nano onion carbon powder with corresponding mass, wherein the mass fractions of the nano onion carbon powder relative to the carbon-coated lithium iron phosphate are respectively 0.1-5 wt.%,
(2) liquid phase chemical mixing: adding anhydrous ethanol into nano onion carbon, performing ultrasonic dispersion for 0.5-2h, adding carbon-coated lithium iron phosphate powder, continuing to disperse for 0.5-2h, and then performing magnetic stirring for 10-24 h; ball milling the stirred and mixed liquid;
(3) and (3) drying: filtering the stirred and mixed liquid by using filter paper, drying for 10-20h by blowing, wherein the drying temperature is 60-120 ℃, and sieving by using a 100-mesh sieve with 400 meshes after completely drying;
(4) and (3) pressure sintering: weighing a certain mass of the product obtained in the step (3), and manufacturing by adopting a forming mode such as spark plasma sintering and the like.
a. The spark plasma sintering method comprises the following steps: and (3) pouring the powder in the step (4) into a graphite mold, assembling an upper pressure head and a lower pressure head, placing the graphite mold into a discharge plasma sintering furnace for sintering and molding, under the protection of vacuum or high-purity argon, at the pressure of 5-200MPa and the temperature of 500 plus materials of 900 ℃, measuring the temperature by a thermocouple or infrared temperature, wherein the heating speed is 30-100 ℃/min, and the heat preservation time at the highest sintering temperature is 1-60 min. Naturally furnace cooling, and demoulding to obtain the block product.
Preferably, a hot press sintering method may also be employed:
b. the hot-pressing sintering method comprises the following steps: and (3) pouring the powder in the step (4) into a graphite mold, assembling an upper pressure head and a lower pressure head, placing the graphite mold into a hot-pressing sintering furnace for sintering and molding, wherein the pressure is 5-80MPa, the temperature is 500-900 ℃, the heating speed is 10-30 ℃/min, and the heat preservation time is 10-180min at the highest sintering temperature under the protection of vacuum or inert atmosphere. Naturally furnace cooling, and demoulding to obtain the block product.
(5) And (4) mechanically grinding the product obtained in the step (4), and sieving the product with a 100-mesh and 400-mesh sieve to obtain the nano onion carbon composite lithium iron phosphate material with uniform particles.
Ultrasonically dispersing 0.1-5% by mass of a nano onion carbon material into absolute ethyl alcohol, adding carbon-coated lithium iron phosphate particles with the particle size of 1-5 mu m, continuously performing ultrasonic dispersion, magnetically stirring the dispersed mixture, and filtering and drying the mixed slurry to obtain nano onion carbon mixed lithium iron phosphate; and (3) performing pressure sintering on the mixed lithium iron phosphate material to obtain the nano onion carbon composite lithium iron phosphate anode material.
The nano onion carbon composite lithium iron phosphate material adopts a nano onion carbon material with the particle size smaller than that of a lithium iron phosphate raw material, is uniformly dispersed by ultrasonic and magnetic stirring, is pressed and sintered to enable a nano onion carbon conductive agent to be tightly combined with carbon-coated lithium iron phosphate particles to form a secondary-coated cross-linked network structure, and is coated with particles obtained by grinding and crushing to form a lithium ion battery positive pole piece, so that the conductivity of the lithium ion battery positive pole piece is improved. Now thatThe technology for preparing the positive pole piece mainly comprises the steps of mixing a positive active substance, a conductive agent, a binder and a solvent, fully stirring to obtain electrode slurry with proper viscosity, and then coating the electrode slurry on an aluminum foil current collector. In the positive pole piece obtained by the method, the conductivity cannot be fully exerted due to poor contact between the conductive agent and the active substance, and the capacity rate performance of the positive active substance cannot be fully exerted by using the conductive carbon black Super-P. The nano onion carbon used in the invention is SP2The hybrid carbon material has excellent conductivity, can improve the contact between a positive mixture and a current collector, improve the contact between a positive active substance and a conductive agent, enhance the conductivity of a pole piece, simultaneously enable the interface of micron particles and onion carbon to be fully contacted and combined by using a pressure sintering mode, improve the density of the particles, and enable single particles to be mixed with the nano onion carbon material after grinding and crushing. The small amount of nano onion carbon material can greatly improve the conductivity of the active particles of the positive electrode, thereby improving the capacity and the rate charge and discharge performance of the lithium ion battery manufactured based on the positive electrode material.
The nano onion carbon composite lithium iron phosphate material has the characteristics of large specific surface area and good conductivity, and can effectively improve the electrochemical performance of a positive pole piece. The electrochemical performance of the lithium ion battery is improved because, on one hand, the contact between the electrolyte and the active substance is improved due to the addition of the additional conductive nano carbon, the concentration polarization is reduced, and meanwhile, the impedance between the current collector and the active substance is reduced due to the addition of the conductive nano carbon material, so that the lithium ion battery is favorable for high-rate charge and discharge. On the other hand, the conductive carbon material sintered by SPS is tightly combined with the active substance, which is beneficial to the rapid conduction of electrons. The performance of the lithium iron phosphate material compounded by adding the nano onion carbon is equivalent to that of the lithium iron phosphate compounded by the graphene, but considering the high price of the graphene, the nano onion carbon can replace the graphene to a certain extent to be used as a conductive agent, the nano onion carbon is added by using an SPS (liquid phase plasma deposition) method to obtain composite material particles with higher density, the exertion of the capacity of the lithium iron phosphate material is improved, and the application value is realized in the aspect of improving the exertion of the performance of the existing micron-sized lithium iron phosphate.
The composite material provided by the invention mainly takes conventional carbon-coated lithium iron phosphate powder sold in the market as a matrix, the nano onion carbon as an additive, the nano onion carbon is uniformly dispersed in the carbon-coated lithium iron phosphate in the composite positive electrode structure, and the nano onion carbon material and the coated carbon are in good contact to jointly form a lithium iron phosphate composite carbon conductive double network, so that the conductive performance of the lithium iron phosphate is improved. According to the invention, the nano onion carbon is added to mainly play a role of a conductive agent, the nano onion carbon is tightly combined with the lithium iron phosphate particle interface by adopting a pressure sintering method, the problems of poor conductivity of the conventional conductive carbon black conductive agent and untight contact between the conductive agent and an anode active material after being added are solved, and the nano onion carbon composite lithium iron phosphate anode material is obtained through pressure sintering. The composite material is compounded by adding the nano onion carbon on the basis of the conventional carbon-coated lithium iron phosphate powder, so that the conductivity of the active substance of the positive electrode is improved, the capacity and the rate charge and discharge performance of the positive electrode material are improved, and the composite material can be applied to the fields of lithium ion batteries, energy storage and the like.
The invention selects the nano onion carbon composite carbon-coated lithium iron phosphate, which is different from the mode of directly preparing slurry after the traditional conductive agent is mixed with the anode material. After filtering and drying, sintering the carbon-coated lithium iron phosphate and the nano onion carbon, increasing the transmission speed of electrons by utilizing the conductivity of the nano onion carbon material, reducing electrochemical polarization, reducing the interface mismatch of the lithium iron phosphate composite material by utilizing sintering heat treatment, promoting lithium ion diffusion, improving the conductivity, increasing the tap density by adopting pressurization treatment, and improving the volume energy density of the electrode. Finally, the charge-discharge capacity and the rate capability of the lithium iron phosphate composite anode are improved. The preparation process of the invention has the following advantages: 1. the mixing treatment of the lithium iron phosphate and the nano onion carbon is to increase the transmission speed of electrons by utilizing the conductivity of the carbon so as to reduce electrochemical polarization. 2. The pressure treatment may increase the tap density. 3. The sintering heat treatment can reduce LiFePO4the/C interface is mismatched, the lithium ion diffusion is promoted, and the conductivity is improved. 4. The block body is ground and crushed, and then the mixture is put into a furnace,the effect is that small particles facilitate the fabrication and coating of the cathode slurry in a conventional manner.
Has the advantages that: compared with the prior art, the invention has the following advantages:
the invention provides a combination of a nano onion carbon composite lithium iron phosphate anode material for the first time. Compared with the existing lithium iron phosphate anode piece preparation technology, the method disclosed by the invention has the advantages that the nano onion carbon material is uniformly dispersed into lithium iron phosphate particles by an ultrasonic dispersion magnetic stirring method on the basis of the existing commercially available lithium iron phosphate powder, and the nano onion carbon is fully contacted and combined with the interfaces of the carbon-coated lithium iron phosphate particles in a pressure sintering manner, so that the impedance among the particle interfaces is favorably reduced, and the charging and discharging capacity of the original commercially available lithium iron phosphate material under the 2C multiplying power is remarkably improved.
On the basis of the commercially available carbon-coated lithium iron phosphate powder which is industrially produced at present, a small amount of nano onion carbon is added for a series of treatments, so that the performance of the original lithium iron phosphate material can be obviously improved. Compared with the defect that the performance is reduced due to the fact that carbon nano tubes and graphene are easy to wind or fold in application, the onion-shaped stable structure and the intrinsic property of the nano onion carbon are less affected by industrial processing such as dispersion ball milling and the like, and the industrial product is better in consistency. At present, the price of nano onion carbon is lower than that of graphene, and compared with graphene, the nano onion carbon is easier to obtain and has the potential of being widely applied to lithium ion battery conductive agents.
Drawings
FIG. 1 is an X-ray diffraction pattern before and after discharge plasma sintering of nano onion-carbon compounded lithium iron phosphate, with a graphene addition group as a control;
FIG. 2 shows a positive electrode plate prepared by a coating process before and after discharge plasma sintering of nano onion-carbon composite lithium iron phosphate;
FIG. 3 shows the transmission profiles of nano-onion-carbon and nano-onion-carbon composite lithium iron phosphate;
FIG. 4 is an electrochemical impedance spectrum of a lithium ion battery composed of nano onion-carbon compounded lithium iron phosphate;
FIG. 5 is a charge-discharge curve of a lithium ion battery assembled by positive pole pieces before and after discharge plasma sintering of nano onion-carbon compounded lithium iron phosphate at a 2C rate;
fig. 6 is a rate performance diagram of a lithium ion battery composed of nano onion-carbon compounded lithium iron phosphate, and the charge and discharge rates are consistent.
Detailed Description
The invention is further illustrated by the following figures and examples.
Materials, reagents and the like used in examples are commercially available unless otherwise specified.
Nano onion carbon, manufacturer: jiangsu Nanjing Mingchang New Material science and technology Co., Ltd, type: 99% of nano onion carbon.
Carbon-coated lithium iron phosphate, manufacturer: shandong Gelin lithium industry, type: lithium iron phosphate LFPLiFePO4The carbon content of the lithium iron phosphate is 1.5%, and commercially available lithium iron phosphate is coated with carbon (no C in the model), and the lithium iron phosphate is not conductive and cannot be directly used for batteries.
In the embodiment, nano onion carbon with various sizes and carbon-coated lithium iron phosphate can be sold in the market or customized according to needs.
Example 1
The nano onion carbon composite lithium iron phosphate carbon composite material is produced by sintering through a discharge plasma technology, wherein the diameter of the nano onion carbon is 8-10nm, the onion carbon is of a hollow structure, the hollow size is 2-4nm, the number of carbon layers is about 10, the median particle size of carbon-coated lithium iron phosphate powder is 1.1 mu m, and 90% of particles are smaller than 10 mu m and are respectively nano powder and micron powder. A cylindrical composite sintered body having a diameter of 20mm and a height of 3mm was produced. After being ground and crushed, the material is screened to be used as a composite active material for electrode coating and lithium ion battery manufacturing.
The method comprises the following specific steps:
(1) weighing 0.3g of nano onion carbon powder (the median particle size is about 9.2nm), putting the nano onion carbon powder into a beaker, adding 100g of absolute ethyl alcohol, and ultrasonically dispersing for 2 hours at the temperature of 25 ℃ and under the power of 180W; continuously adding 10g of carbon-coated lithium iron phosphate powder with a carbon content of 1.5% (purity of 99%, median particle diameter D50 ═ 1.1 μm, D90<10 μm), and magnetically stirring at 500rpm for 10 h; milling was carried out for 5h using a planetary ball mill at 600 rpm.
(2) And (3) filtering the nano onion carbon and carbon-coated lithium iron phosphate mixed liquid by using filter paper, drying the filtered nano onion carbon and carbon-coated lithium iron phosphate mixed liquid for 10 hours in a vacuum drying oven at the temperature of 80 ℃, and sieving the dried nano onion carbon and carbon-coated lithium iron phosphate mixed liquid by using a 200-mesh sieve after complete drying.
(3) Determining the size parameters of the sintered product: phi is 20mm, h is 3 mm. The mass of the powder required was calculated to be 3.395g according to the mass density formula m ═ ρ V.
Pouring the powder obtained in the step (2) into a graphite mold, assembling an upper pressure head and a lower pressure head, and placing the graphite mold into a spark plasma sintering furnace, wherein SPS sintering parameters are as follows: and measuring the temperature in Ar atmosphere and infrared at the heating speed of 100 ℃/min, heating to 800 ℃, keeping the temperature under the pressure of 20MPa for 20min to compact and form the material, and then cooling in a furnace to room temperature.
(4) And (4) grinding and crushing the block in the step (3), and sieving with a 200-mesh sieve to obtain the nano onion-carbon composite lithium iron phosphate material.
(5) And (3) taking 160mg of the composite material in the step (4) as a positive electrode active substance, dissolving polyvinylidene fluoride and conductive carbon black in 1mL of N-methyl pyrrolidone according to the mass ratio of 8:1:1, fully stirring and mixing the mixture until slurry with proper viscosity is obtained, coating the slurry on an aluminum foil current collector, and drying, cutting and rolling the slurry to obtain the positive electrode piece.
(6) Taking the prepared electrode plate as a positive electrode, taking a metal lithium plate as a negative electrode, taking Celgard 2400 as a diaphragm to separate the positive electrode from the negative electrode, taking 1M LiPF6 dissolved in a mixed solution of ethylene carbonate and diethyl carbonate as an electrolyte, using a 2032 button battery case in a glove box filled with argon gas, assembling an inner shell, the positive electrode plate, the diaphragm, the negative electrode lithium plate, a gasket, an elastic sheet and an outer shell in sequence, dripping 0.25mL of electrolyte, and finishing battery packaging by using a sealing machine.
Phase structure analysis and electrochemical performance test: the method comprises the steps of respectively carrying out phase analysis on a nano onion-carbon composite lithium iron phosphate sample by using an X-ray diffractometer, photographing the appearance of a pole piece by using an electron scanning microscope after coating the pole piece, manufacturing a button 2032 battery in a glove box, testing the electrochemical impedance spectrum of the battery by using an electrochemical platform, and testing the charge-discharge curve and the multiplying power performance of the battery by using a battery performance testing system.
Fig. 1 is an XRD spectrum of the nano onion carbon composite lithium iron phosphate material obtained by adding nano onion carbon and performing SPS sintering in this example, it can be seen that 3 wt% of the nano onion carbon is added, the crystal structure of the lithium iron phosphate is not changed, and the peak of the nano onion carbon cannot be detected due to too small addition amount, and a lithium iron phosphate group to which the nano onion carbon is not added and a graphene addition group having the same mass to which the nano onion carbon is replaced are used for comparison (both comparisons are used in subsequent experiments).
Fig. 2 is an electron microscope microscopic image of a positive plate made by coating nano onion carbon compounded lithium iron phosphate, (a) 1.5% carbon-coated lithium iron phosphate is directly subjected to SPS sintering; (b) adding nano onion carbon to perform SPS sintering; (c) and adding graphene to perform SPS sintering. The conductive carbon black and binder used in the fabrication of the positive electrode sheet, and graphene, which is intercalated in the active particles, are clearly observed, and nano onion carbon, which has particles of about 10nm too small, is not observed. After SPS sintering, the aggregation degree of the particles is improved, and graphene and active substances are combined more tightly.
FIG. 3(a) is a transmission profile of nano onion carbon with a particle diameter of about 10nm, and selected area electron diffraction showing a typical diffraction pattern for carbon. (b) The existence of carbon-coated lithium iron phosphate and nano onion carbon is confirmed, but the nano onion carbon in the gaps of the carbon-coated lithium iron phosphate is aggregated to a certain extent because the transmission sample needs ethanol ultrasonic dispersion treatment, and the comparison between the graph in fig. 2 and fig. 3 and the graph in the graph can simultaneously prove that the nano onion carbon is inserted into the active particles and is more tightly combined with the active material.
Fig. 4 is an electrochemical impedance spectrum (0.1-10kHz, 5mV) of a button 2032 battery manufactured by a nano onion-carbon composite lithium iron phosphate material after an anode coating process, it can be seen that after nano onion-carbon is added for SPS sintering, the charge transfer impedance of the lithium iron phosphate composite material is reduced from more than 200 ohms to about 110 ohms, and the addition of nano onion-carbon is not obviously different from that of graphene, which shows that in the aspect of improving impedance, nano onion-carbon can play a similar effect to graphene, further proving that the conductivity of the material of the invention is excellent.
Fig. 5 is a charge-discharge curve of a button 2032 battery manufactured by the nano onion-carbon composite lithium iron phosphate material after the anode coating process at a 2C rate. The electrode sheet was weighed, and a test current was set by converting the actual active material to 80% at a 1C magnification of 170 mA/g. The SPS sintering improves the charge and discharge platform of the lithium iron phosphate composite material, the charge and discharge platform is smoother, the modified material is more beneficial to charge and discharge with large multiplying power, the discharge capacity 116mAh/g of the nano onion carbon composite lithium iron phosphate material under the multiplying power of 2C is 91.3% of that of the graphene added composite lithium iron phosphate material 127mAh/g, and the capacities of the lithium iron phosphate composite material added with 3 wt% of nano onion carbon and 3 wt% of graphene are respectively improved by 18.4% and 29.6% compared with 98mAh/g of the original carbon-coated lithium iron phosphate material with 1.5% of carbon content. The performance of the lithium iron phosphate composite material added with the nano onion carbon is greatly improved compared with that of the lithium iron phosphate material not added, but the performance of the lithium iron phosphate composite material is equivalent to that of the graphene with the best performance at present. The price of the nano onion carbon adopted by the invention is lower than that of graphene, meanwhile, the nano onion carbon has better tolerance in industrial processing, the graphene has large surface area and is easy to wrinkle after being treated, and the structure of the nano onion carbon is very stable.
Fig. 6 shows the rate capability (0.2C-20C, 1C rate 170mA/g) of a button 2032 battery manufactured by the nano onion-carbon composite lithium iron phosphate material after the anode coating process, the capacity of the nano onion-carbon composite lithium iron phosphate material is increased by 0% -20% compared with that of the original lithium iron phosphate at low rate, the capacity is increased by 40.6% and 121.1% at high rate 5C and 10C discharge rate, and is slightly lower than 62.5% and 156% of the graphene group, but the capacity close to 40mAh/g can be exerted at 20C rate, which shows that the composite anode material prepared by adding nano onion-carbon can improve the conductivity and electrochemical polarization of active substances, improve the electrochemical performance of micron-sized carbon-coated lithium iron phosphate, and has a certain application value in the field of lithium ion batteries.
Example 2
The nano onion carbon composite lithium iron phosphate carbon composite material is produced by sintering through a hot pressing technology, wherein the diameter of nano onion carbon is 50nm, the median particle size of carbon-coated lithium iron phosphate powder is 5nm, and 90% of particles with the size smaller than 7nm are all nano powder. A cylindrical composite sintered body having a diameter of 20mm and a height of 3mm was produced. After being ground and crushed, the material is screened to be used as a composite active material for electrode coating and lithium ion battery manufacturing.
The method comprises the following specific steps:
(1) weighing 0.3g of nano onion carbon powder (with the particle size of 50nm) and placing the nano onion carbon powder into a beaker, adding 100g of absolute ethyl alcohol, and dispersing for 2 hours at 25 ℃ by using ultrasonic 180W power; continuously adding 10g of carbon-coated lithium iron phosphate powder (the purity is 99 percent and the D50 particle size is 5nm) with the carbon content of 1.5 percent, and magnetically stirring for 10 hours at the rotating speed of 800 rpm; milling was carried out using a planetary ball mill at 500rpm for 8 h.
(2) And filtering the mixed solution of the nano onion carbon and the carbon-coated lithium iron phosphate by using filter paper, drying the mixed solution for 10 hours in a vacuum drying oven at the temperature of 80 ℃, and sieving the dried mixed solution by using a 200-mesh sieve after complete drying.
(3) Determining the size parameters of the sintered product: phi is 20mm, h is 3 mm. The mass of the powder required was calculated to be 3.395g according to the mass density formula m ═ ρ V.
Pouring the powder obtained in the step (2) into a graphite mold, assembling an upper pressure head and a lower pressure head, and placing the graphite mold into a vacuum hot-pressing furnace, wherein the sintering parameters are as follows: measuring the temperature in vacuum atmosphere and a thermocouple, wherein the temperature rise speed is 10 ℃/min. Heating to 800 deg.C, maintaining at 20MPa for 30min to compact the material, and cooling to room temperature.
(4) And (4) grinding and crushing the block in the step (3), and sieving with a 200-mesh sieve to obtain the nano onion-carbon composite lithium iron phosphate material.
Example 3
The nano onion carbon composite lithium iron phosphate carbon composite material is produced by sintering through a discharge plasma technology, wherein the diameter of nano onion carbon is 100nm, and the median particle size of carbon-coated lithium iron phosphate powder is 100nm and is all nano powder. A cylindrical composite sintered body having a diameter of 20mm and a height of 3mm was produced. After being ground and crushed, the material is screened to be used as a composite active material for electrode coating and lithium ion battery manufacturing.
The method comprises the following specific steps:
(1) weighing 0.3g of nano onion carbon powder (with the particle size of 100nm) and placing the nano onion carbon powder into a beaker, adding 100g of absolute ethyl alcohol, and dispersing for 2 hours at 25 ℃ by using ultrasonic 180W power; continuously adding 10g of carbon-coated lithium iron phosphate powder (the purity is 99 percent and the D50 particle size is 100nm) with the carbon content of 1.5 percent, and magnetically stirring for 10 hours at the rotating speed of 800 rpm; milling was carried out using a planetary ball mill at 400rpm for 10 h.
(2) Taking out the mixed solution of the nano onion carbon and the carbon-coated lithium iron phosphate, filtering by using filter paper, putting into a vacuum drying oven, drying at 80 ℃ for 10h, and sieving by using a 200-mesh sieve after complete drying.
(3) Determining the size parameters of the sintered product: phi is 20mm, h is 3 mm. The mass of the powder required was calculated to be 3.395g according to the mass density formula.
Pouring the powder obtained in the step (2) into a graphite mold, assembling an upper pressure head and a lower pressure head, and placing the graphite mold into a spark plasma sintering furnace, wherein SPS sintering parameters are as follows: and measuring the temperature in Ar atmosphere and infrared, wherein the temperature rise speed is 100 ℃/min. Heating to 800 deg.C, maintaining at 20MPa for 20min to compact the material, and cooling to room temperature.
(4) And (4) grinding and crushing the block in the step (3), and sieving with a 200-mesh sieve to obtain the nano onion-carbon composite lithium iron phosphate material.
Example 4
The nano onion carbon composite lithium iron phosphate carbon composite material is produced by sintering through a hot pressing technology, wherein the diameter of the nano onion carbon is 30nm, the diameter of the conductive carbon black SP is 40nm, and the particle size of the carbon-coated lithium iron phosphate powder is 20 mu m. A cylindrical composite sintered body having a diameter of 20mm and a height of 3mm was produced. After being ground and crushed, the material is screened to be used as a composite active material for electrode coating and lithium ion battery manufacturing.
The method comprises the following specific steps:
(1) respectively weighing 0.3g of nano onion carbon powder (with the particle size of 30nm) and 0.3g of conductive carbon black SP, placing the nano onion carbon powder and the conductive carbon black SP into a beaker, adding 100g of absolute ethyl alcohol, and dispersing for 2 hours at 25 ℃ by using ultrasonic 180W power; continuously adding 10g of carbon-coated lithium iron phosphate powder (the purity is 99 percent and the D50 particle size is 20 mu m) with the carbon content of 1.5 percent, and magnetically stirring for 10 hours at the rotating speed of 600 rpm; milling was carried out using a planetary ball mill at 800rpm for 3 h.
(2) Filtering the mixed solution of nano onion carbon and carbon-coated lithium iron phosphate by using filter paper, drying the mixed solution for 10 hours in a vacuum drying oven at the temperature of 80 ℃, and sieving the dried mixed solution by using a 200-mesh sieve after complete drying.
(3) Determining the size parameters of the sintered product: phi is 20mm, h is 3 mm. The mass of the powder required was calculated to be 3.395g according to the mass density formula.
Pouring the powder obtained in the step (2) into a graphite mold, assembling an upper pressure head and a lower pressure head, and placing the graphite mold into a vacuum hot-pressing furnace, wherein the sintering parameters are as follows: measuring the temperature in vacuum atmosphere and a thermocouple, wherein the temperature rise speed is 10 ℃/min. Heating to 800 deg.C, maintaining at 20MPa for 120min to compact the material, and cooling to room temperature.
(4) And (4) grinding and crushing the block in the step (3), and sieving with a 200-mesh sieve to obtain the nano onion-carbon composite lithium iron phosphate material.
(5) And (3) taking 160mg of the composite material in the step (4) as a positive electrode active substance, dissolving 95%: 2.5%: 2.5% of the composite material, polyvinylidene fluoride and conductive carbon black in a proper amount of 1mL of N-methyl pyrrolidone, fully stirring and mixing the mixture until slurry with proper viscosity is obtained, coating the slurry on an aluminum foil current collector, and obtaining a positive electrode piece after drying, cutting and rolling.
(6) Taking the prepared electrode plate as a positive electrode, taking a metal lithium plate as a negative electrode, taking Celgard 2400 as a diaphragm to separate the positive electrode from the negative electrode, taking 1M LiPF6 dissolved in a mixed solution of ethylene carbonate and diethyl carbonate as an electrolyte, using a 2032 button battery case in a glove box filled with argon, dropping a proper amount of 0.25mL electrolyte into the battery case according to the sequence of an inner shell, the positive electrode plate, the diaphragm, the negative electrode lithium plate, a gasket, an elastic sheet and an outer shell, and finishing battery packaging by using a sealing machine.
Example 5
The nano onion carbon composite lithium iron phosphate carbon composite material is produced by sintering through a discharge plasma sintering technology, wherein the diameter of the nano onion carbon is 80nm, and the median particle size of carbon-coated lithium iron phosphate powder is 10 mu m. And sintering the aluminum sheet current collector together to prepare a cylindrical composite cathode material sintered body with the diameter of 10mm and the height of 1 mm. The slice is thinned, and one side of the aluminum sheet is reserved to obtain the positive plate which can be directly used for manufacturing the lithium ion battery.
The method comprises the following specific steps:
(1) weighing 0.5g of nano onion carbon powder (with the particle size of 80nm), putting the nano onion carbon powder into a beaker, adding 100g of absolute ethyl alcohol, and dispersing for 2 hours at 25 ℃ by using ultrasonic 180W power; continuously adding 10g of carbon-coated lithium iron phosphate powder (with the purity of 99 percent and the D50 particle size of 10 microns) containing 1.5 percent, and magnetically stirring at the rotating speed of 700rpm for 10 hours; milling was carried out using a planetary ball mill at 500rpm for 6 h.
(2) And filtering the mixed solution of the nano onion carbon and the carbon-coated lithium iron phosphate by using filter paper, drying the mixed solution for 10 hours in a vacuum drying oven at the temperature of 80 ℃, and sieving the dried mixed solution by using a 200-mesh sieve after complete drying.
(3) Determining the size parameters of the sintered product: phi is 10mm, h is 1 mm. The mass of the powder required was calculated to be 1.131g according to the mass density formula.
Placing one layer of aluminum foil at the bottom of the graphite mold, pouring the powder of (2) into the graphite mold, assembling an upper pressing head and a lower pressing head, and placing the powder into a discharge plasma sintering furnace, wherein the sintering parameters are as follows: and measuring the temperature in Ar atmosphere and infrared, wherein the temperature rise speed is 100 ℃/min. Heating to 800 deg.C, maintaining at 20MPa for 10min to compact the material, and cooling to room temperature.
(4) And (4) cutting the block body in the step (3) to 100 micrometers by using a diamond wire, and reserving one side of an aluminum foil to obtain the nano onion carbon composite lithium iron phosphate material positive pole piece.
(5) Taking the prepared electrode plate as a positive electrode, taking a metal lithium plate as a negative electrode, taking Celgard 2400 as a diaphragm to separate the positive electrode from the negative electrode, taking 1M LiPF6 dissolved in a mixed solution of ethylene carbonate and diethyl carbonate as an electrolyte, using a 2032 button battery case in a glove box filled with argon, dropping 0.25mL of electrolyte into the electrode plate, the diaphragm, the negative electrode lithium plate, a gasket, an elastic sheet and an outer shell according to the sequence of an inner shell, the positive electrode plate, the diaphragm, the negative electrode lithium plate, the gasket, the elastic sheet and the outer shell, and finishing battery packaging by using a sealing machine.
Example 6
Example 6 was prepared identically to example 1, except that: the adding proportion of the nano onion carbon material in the step (1) is 0.1 percent of the mass of the carbon-coated lithium iron phosphate, the material is dispersed for 0.5 hour by ultrasonic 200W, and then the material is magnetically stirred for 24 hours at 600 rpm. And (3) in the step (2), the drying mode is air blast drying, the drying time is 10 hours, the drying temperature is 120 ℃, and the dried product is sieved by a 200-mesh sieve after being completely dried. The conditions of spark plasma sintering in the step (3) are as follows: the pressure is 5MPa, the temperature is 500 ℃, and the heat preservation time at the highest sintering temperature is 30 min.
Example 7
Example 7 differs from example 1 in that: the conditions of spark plasma sintering are as follows: the pressure is 200MPa, the temperature is 900 ℃, and the heat preservation time at the highest sintering temperature is 1 min.
Example 8
Example 8 was prepared in the same manner as example 1, except that: the adding proportion of the nano onion carbon material in the step (1) is 5% of the mass of the carbon-coated lithium iron phosphate, the material is dispersed for 2 hours by ultrasonic 100W, and then the material is magnetically stirred for 10 hours at 800 rpm. And (3) drying in the step (2) in a vacuum manner, wherein the drying time is 20h, the drying temperature is 60 ℃, and the dried product is sieved by a 200-mesh sieve after being completely dried. And (3) hot-pressing sintering under the conditions of argon atmosphere, pressure of 5MPa, temperature of 500 ℃ and heat preservation time of 10 min.
Example 9
Example 9 differs from example 1 in that: and (3) hot-pressing sintering under the conditions of argon atmosphere, pressure of 5MPa, temperature of 500 ℃ and heat preservation time of 180 min.

Claims (10)

1. The nano onion carbon composite lithium iron phosphate cathode material is characterized in that carbon-coated lithium iron phosphate powder is used as a matrix, nano onion carbon is used as an additive, and the nano onion carbon is dispersed in gaps among the carbon-coated lithium iron phosphate particles.
2. The nano onion carbon composite lithium iron phosphate cathode material of claim 1, wherein the nano onion carbon is in a hollow spherical multi-layer fullerene carbon structure with a diameter of 3-100 nm.
3. The nano onion-carbon composite lithium iron phosphate cathode material as claimed in claim 1, wherein the carbon-coated lithium iron phosphate powder has a particle size of 5nm to 20 μm, a carbon content of 1 to 3%, and carbon has an amorphous structure.
4. The preparation method of the nano onion-carbon composite lithium iron phosphate cathode material as claimed in claim 1, comprising the following steps:
(1) liquid phase chemical mixing: adding nano onion carbon into a solvent for ultrasonic dispersion, adding carbon-coated lithium iron phosphate powder, continuing dispersion, and then stirring;
(2) ball milling: ball milling the stirred and mixed liquid obtained in the step (1);
(3) and (3) drying: filtering the liquid after ball milling in the step (2), drying and sieving;
(4) sintering and forming: taking the product obtained in the step (3), and performing spark plasma sintering or hot-pressing sintering to obtain a block composite material;
(5) grinding and sieving the block composite material obtained in the step (4) to obtain a nano onion carbon composite lithium iron phosphate anode material with uniform particles; or slicing the block composite material to obtain the flaky nano onion carbon composite lithium iron phosphate anode material.
5. The method for preparing the nano onion-carbon composite lithium iron phosphate cathode material as claimed in claim 4, wherein the nano onion-carbon material is preferably added in a proportion of 0.1-5% by mass of the carbon-coated lithium iron phosphate in the step (1).
6. The method for preparing the nano onion-carbon composite lithium iron phosphate cathode material as claimed in claim 4, wherein absolute ethyl alcohol is adopted as a dispersion solution of nano onion-carbon in the step (1), and ultrasonic dispersion is carried out for 0.5-2 h.
7. The method for preparing the anode material of nano onion-carbon composite lithium iron phosphate as claimed in claim 4, wherein the carbon-coated lithium iron phosphate powder is added in the step (1), the ultrasonic dispersion is continued for 0.5-2h, the ultrasonic power is 100-200W, and then the stirring is magnetic stirring for 10-24h, wherein the stirring speed is 500-800 rpm.
8. The method for preparing the nano onion-carbon composite lithium iron phosphate cathode material as claimed in claim 4, wherein the ball milling in step (2) is high energy ball milling at a rotation speed of 200-1000rpm using a planetary ball mill for 10-20 h; and (3) drying in a blowing drying mode or a vacuum drying mode for 10-20h at 60-120 ℃, and sieving with a 200-mesh sieve after complete drying.
9. The method for preparing the nano onion-carbon composite lithium iron phosphate cathode material as claimed in claim 4, wherein the discharge plasma sintering conditions in step (4) are as follows: the pressure is 5-200MPa, the temperature is 500-900 ℃, the heat preservation time is 1-30min at the highest sintering temperature, and the sintering atmosphere is high-purity argon or vacuum; the hot-pressing sintering conditions are as follows: under the protection of vacuum or inert atmosphere, the pressure is 5-80MPa, the temperature is 500-900 ℃, and the heat preservation time is 10-180min at the highest sintering temperature; the slicing mode in the step (5) is diamond wire cutting, the processing speed is 10mm-500mm/min, and the thickness of the processed composite anode is 20-200 mu m.
10. The application of the nano onion-carbon composite lithium iron phosphate positive electrode material disclosed by claim 1 as a lithium ion battery positive electrode piece.
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