CN107180956B - Lithium titanate negative electrode material and preparation method thereof - Google Patents
Lithium titanate negative electrode material and preparation method thereof Download PDFInfo
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- CN107180956B CN107180956B CN201710391330.1A CN201710391330A CN107180956B CN 107180956 B CN107180956 B CN 107180956B CN 201710391330 A CN201710391330 A CN 201710391330A CN 107180956 B CN107180956 B CN 107180956B
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- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
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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
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
- H01M10/052—Li-accumulators
- H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
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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
- 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
- H01M4/485—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of mixed oxides or hydroxides for inserting or intercalating light metals, e.g. LiTi2O4 or LiTi2OxFy
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- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
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Abstract
The invention belongs to the field of energy storage research, and particularly relates to a lithium titanate negative electrode material which comprises a core structure and a shell structure, wherein the core structure is a secondary particle structure and comprises a main electric conducting network with a porous structure and primary nano particles filled in the porous main electric conducting network; strong chemical bonds and force action exist among the dominant network structures; the chemical bond tightly locks the nano-sized primary particles in the pore structure of the main conducting network. Thereby ensuring that the lithium titanate negative electrode material has excellent electrochemical performance.
Description
Technical Field
The invention belongs to the technical field of energy storage materials, and particularly relates to a lithium titanate negative electrode material and a preparation method thereof.
Background
Since birth, lithium ion batteries have revolutionary changes in the field of energy storage due to their advantages of rapid charging and discharging, good low-temperature performance, large specific energy, small self-discharge rate, small volume, light weight, and the like, and are widely used in various portable electronic devices and electric vehicles. However, with the improvement of living standard of people, higher user experience puts higher requirements on the lithium ion battery: faster charging and discharging (such as 5C or even 10C), wider temperature range (such as minus thirty degrees centigrade), and the like; in order to solve the above problems, it is necessary to find a new electrode material having more excellent properties.
The current commercialized lithium ion battery cathode material is mainly graphite, but cannot meet the urgent needs of users due to low charge and discharge speed (generally, the charge and discharge speed is within 1C) and poor low-temperature performance (generally, the use temperature is above-10 ℃); therefore, development of an anode material having a higher charge/discharge rate and used in a wider temperature range is urgently needed. As a negative electrode material of a lithium ion battery, lithium titanate has been attracting attention: the charge and discharge speed can be more than 10 ℃, and the material still can exert more ideal capacity at the temperature of minus 30 ℃, so the material is one of the optimal choices of the new generation of fast-charging anode materials.
However, the lithium titanate material particles have poor conductivity, so that the internal resistance of the battery after the battery is assembled is high, and gas is easily generated in the charging and discharging process, so that the use of the battery is influenced, and the wider application of the battery is limited. In order to solve the problems, the prior art mainly comprises the steps of nano-crystallization of lithium titanate particles, addition of a conductive material with excellent conductive performance into lithium titanate material particles and the like so as to improve the conductive performance of the whole particles of the lithium titanate material; meanwhile, the coating technology is adopted, so that the problem of gas generation in the process of using the material after being prepared into a battery is solved.
However, the lithium titanate particles with the nano structure are easy to agglomerate and have high dispersion difficulty; the commonly used conductive agent materials are generally small in size (nanometer), large in specific surface area and difficult to disperse. However, in order to maximize the conductive effect of the conductive agent and to prepare a lithium titanate secondary particle material with better performance, it is necessary to ensure that the nano lithium titanate particles and the conductive agent are uniformly dispersed. Meanwhile, the contact area between the nano-structure lithium titanate material and the conductive agent is small, and the gap is large, so that the contact resistance is relatively large, the nano-structure lithium titanate material is easy to contact with an electrolyte to generate gas, the prepared battery has high internal resistance, and the gas production rate is larger in the using process, so that the electrochemical performance of the lithium titanate negative electrode material is influenced.
In view of the above, there is a need for a lithium titanate negative electrode material and a preparation method thereof, which can uniformly disperse two materials (nano lithium titanate particles and conductive networks) with high dispersion difficulty, and ensure that the two materials are tightly connected together, thereby preparing a lithium titanate negative electrode material with excellent performance.
Disclosure of Invention
The invention aims to: aiming at the defects of the prior art, the lithium titanate anode material comprises a core structure and a shell structure, wherein the core structure is a secondary particle structure and comprises a main electric conduction network with a porous structure and nano primary particles filled in the pore structure of the main electric conduction network; strong chemical bonds and force action exist among the dominant network structures; the chemical bond tightly locks the nano-sized primary particles in the pore structure of the main conducting network. Thereby ensuring that the lithium titanate negative electrode material has excellent electrochemical performance.
In order to achieve the purpose, the invention adopts the following technical scheme:
a lithium titanate negative electrode material comprises a core structure and a shell structure, wherein the core structure is a secondary particle structure and comprises a main electric conducting network with a porous structure and nano primary particles filled in the pore structure of the main electric conducting network; strong chemical bonds and force action exist among the dominant network structures; the chemical bond tightly locks the nano primary particles in the pore structure of the main conducting network, and the nano primary particles contain lithium titanate particles. The shell structure is a coating layer commonly used for cathode materials and is mainly obtained by coating and carbonizing materials such as asphalt and the like, so that the invention is not explained in detail.
As an improvement of the lithium titanate negative electrode material, the bond type providing the strong bonding force is a hydrogen bond or/and a chemical bond; the mass of the oxygen-containing functional groups forming the hydrogen bonds or/and the chemical bonds accounts for 1-40% of the mass of the whole main electric network structure.
As an improvement of the lithium titanate negative electrode material, the main electric conduction network has flexibility, and the inside of the main electric conduction network contains functional groups; the hydrogen bonds or/and chemical bonds result from the reaction of oxygen-containing functional groups within the main conducting network.
As an improvement of the lithium titanate negative electrode material, the main power transmission network structure is at least one of an open graphene structure, an open expanded graphite structure and a worm-like graphene structure; the primary particles comprise nano lithium titanate negative electrode particles; an auxiliary electric conduction network can be distributed between the main electric conduction network and the primary particles and tightly connects the main electric conduction network and the nano primary particles together.
As an improvement of the lithium titanate negative electrode material of the invention, the nano primary particles further comprise non-nano lithium titanate negative electrode particles; the non-nano lithium titanate negative electrode particles are at least one of natural graphite, artificial graphite, mesocarbon microbeads, soft carbon, hard carbon, petroleum coke, carbon fibers, pyrolytic resin carbon, silicon carbon negative electrode particles, tin-based negative electrode materials, transition metal nitrides, tin-based alloys, germanium-based alloys, aluminum-based alloys, antimony-based alloys and magnesium-based alloys; the auxiliary conducting network is obtained by carbonizing a high polymer material; the high polymer material is obtained by in-situ polymerization of a high polymer monomer; the auxiliary conducting network further comprises at least one of conductive carbon black, super conductive carbon, Ketjen black, carbon nanotubes, graphene and acetylene black.
The invention also discloses a preparation method of the lithium titanate negative electrode material, which is characterized by mainly comprising the following steps:
step 1, precursor preparation: uniformly dispersing the primary particles in a solvent to obtain a precursor;
step 2, preparing a modified main power guide network structure: placing the main conducting network structure with the porous structure in an oxidation environment, and grafting a functional group to obtain a modified main conducting network structure;
step 3, filling: filling the precursor prepared in the step 1 into a modified main power transmission network structure;
step 4, closing the opening: placing the porous main electric network structure in a reducing atmosphere to promote functional groups grafted on the main electric network structure to react to generate strong bonding force, and sealing or partially sealing the pore structure in the porous main electric network structure;
and 5, coating and carbonizing the product obtained in the step 4 to obtain the finished lithium titanate negative electrode material.
As an improvement of the preparation method of the lithium titanate negative electrode material, the surface of the primary particles in the step 1 is modified to be functionalized primary particles, and the functional groups are carboxyl groups or/and hydroxyl groups; the grafted functional group in the step 2 comprises at least one of carboxyl, hydroxyl, epoxy, carbonyl, nitro and amino; and 4, adding a reducing agent or/and performing direct hydrothermal reduction on the reducing environment.
As an improvement of the preparation method of the lithium titanate negative electrode material, a polymer monomer can be added in the step 1, namely, the primary particles and the polymer monomer are mixed and kneaded to obtain a precursor in which the polymer monomer is uniformly dispersed on the surface of the primary particles; in this case, it is necessary to perform a polymerization reaction after step 3, in which the product of step 3 is placed in an environment where an initiator is present to promote polymerization of the polymer monomer dispersed on the surface of the primary particles, thereby obtaining a high molecular polymer.
As an improvement of the preparation method of the lithium titanate negative electrode material, the nano primary particles in the step 1 contain nano lithium titanate particles; the nano primary particles can also comprise non-nano lithium titanate negative electrode particles; during kneading reaction, high molecular polymer, carbon source component, conductive agent component and solvent component are also added; in step 1, the kneading process is as follows: kneading the nano primary particles, the surfactant 1, the polymer monomer and the solvent 1 to obtain a mixture 1; kneading the conductive agent component, the surfactant 2 and the solvent 2 to obtain a mixture 2; and then blending the mixture 1 and the mixture 2, and uniformly dispersing to obtain precursor slurry.
As an improvement of the preparation method of the lithium titanate negative electrode material, the filling process in the step 3 is as follows:
pretreating the porous main conducting network structure material, wherein the pretreatment comprises surface activation or/and surfactant addition;
before filling, placing the porous main conducting network structure material in a vacuum environment, vacuumizing, removing air in a pore structure, vacating a space for filling a precursor, and then placing the porous main conducting network structure material in precursor slurry to start filling;
in the filling process, pressure is applied to extrude the precursor into the hole; the temperature is increased, and the viscosity of the precursor is reduced; and (5) adding mechanical disturbance and opening the hole opening.
As an improvement of the preparation method of the lithium titanate negative electrode material, in the step 1, the polymer monomer comprises acrylates, methacrylates, styrene, acrylonitrile, methacrylonitrile, polyethylene glycol dimethacrylate, polyethylene glycol diacrylate, divinylbenzene, trimethylolpropane trimethacrylate, methyl methacrylate, N-dimethylacrylamide, N-acryloylmorpholine, methyl acrylate, ethyl acrylate, butyl acrylate, hexyl N-acrylate, cyclohexyl 2-acrylate, dodecyl acrylate, ethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, neopentyl glycol diacrylate, 1, 6-hexanediol diacrylate, tetraethylene glycol diacrylate, tripropylene glycol diacrylate, ethylene glycol diacrylate, propylene glycol diacrylate, ethylene, At least one of ethoxylated pentaerythritol tetraacrylate, propoxylated pentaerythritol acrylate, bis-trihydroxypropane tetraacrylate, pentaerythritol triacrylate, trimethylolpropane trimethacrylate, propoxylated glycerol triacrylate, tris (2-hydroxyethyl) isocyanurate triacrylate trimethylolpropane triacrylate, propoxylated trimethylolpropane triacrylate, ethoxylated trimethylolpropane triacrylate, pentaerythritol tetraacrylate; and 4, at least one of initiator cumene hydroperoxide, tert-butyl hydroperoxide, dicumyl peroxide, di-tert-butyl peroxide, dibenzoyl peroxide, lauroyl peroxide, tert-butyl peroxybenzoate, tert-butyl peroxypivalate, diisopropyl peroxydicarbonate and dicyclohexyl peroxydicarbonate.
As an improvement of the preparation method of the lithium titanate negative electrode material, the nano primary particles in the step 1 contain nano lithium titanate particles; the nano primary particles can also comprise non-nano lithium titanate negative electrode particles, and the non-nano lithium titanate negative electrode particles are at least one of natural graphite, artificial graphite, mesocarbon microbeads, soft carbon, hard carbon, petroleum coke, carbon fibers, pyrolytic resin carbon, silicon-based negative electrode particles, tin-based negative electrode materials, transition metal nitrides, tin-based alloys, germanium-based alloys, aluminum-based alloys, antimony-based alloys and magnesium-based alloys; during the kneading reaction, a high molecular polymer, a carbon source component, a conductive agent component or/and a solvent component can be added, wherein the high molecular polymer comprises at least one of polymethyl methacrylate (PMMA), polyvinylidene fluoride (PVDF), Styrene Butadiene Rubber (SBR), sodium carboxymethylcellulose (CMC) and polyacrylonitrile, the carbon source component comprises glucose, sucrose, soluble starch, cyclodextrin, furfural, sucrose, glucose, corn starch, tapioca starch, wheat starch, cellulose, polyvinyl alcohol, polyethylene glycol, polyethylene wax, phenolic resin, vinyl pyrrolidone, epoxy resin, polyvinyl chloride, polyalditol, furan resin, urea-formaldehyde resin, polymethyl methacrylate, polyvinylidene fluoride or polyacrylonitrile, petroleum coke, oil-based needle coke and coal-based needle coke, and the conductive agent component comprises at least one of conductive carbon black, conductive carbon, conductive agent component or/and solvent component, At least one of super conductive carbon, Ketjen black, carbon nanotubes, graphene and acetylene black, and at least one of water, alcohols, ketones, alkanes, esters, aromatics, N-methylpyrrolidone, dimethylformamide, diethylformamide, dimethyl sulfoxide and tetrahydrofuran.
As an improvement of the preparation method of the lithium titanate negative electrode material, the preparation process of the main power conducting network structure in the step 2 comprises the following steps: preparing an open graphene structure, an open expanded graphite structure and a vermicular graphene structure: the preparation method comprises the following steps of taking crystalline flake graphite or microcrystalline graphite (vermicular graphene can be prepared, graphene lamella are connected together tightly, developed gap structures are distributed among the lamellae, and primary particles can be conveniently filled, meanwhile, the particle size of the microcrystalline graphene is small, the particle size of the prepared vermicular graphene is about 10 mu m and is matched with the diameter of a lithium titanate negative electrode particle of a final finished product), controlling the degree of oxidation intercalation (mainly moderate in degree of oxidation, too low in degree of oxidation and incapable of forming a porous structure), completely stripping the graphite lamellae in the reduction process and incapable of forming a connected porous structure), and then carrying out heat treatment and expansion to obtain the porous structure with openings among the lamellae connected together and among the same graphite; then, functional groups are grafted in an oxidizing environment to obtain the modified main conducting network structure
As an improvement of the preparation method of the lithium titanate negative electrode material, the surfactant 1 comprises at least one of a wetting agent, a dispersing agent, a penetrating agent, a solubilizer, a cosolvent and a cosolvent; the solvent 1 is at least one of water, alcohols, ketones, alkanes, esters, aromatics, N-methylpyrrolidone, dimethylformamide, diethylformamide, dimethyl sulfoxide and tetrahydrofuran. The surfactant 2 comprises at least one of a wetting agent, a dispersing agent, a penetrating agent, a solubilizer, a cosolvent and a cosolvent; the solvent 2 is at least one of water, alcohols, ketones, alkanes, esters, aromatics, N-methylpyrrolidone, dimethylformamide, diethylformamide, dimethyl sulfoxide and tetrahydrofuran.
The invention has the advantages that:
1. the modified main electric conduction network structure and the modified primary particles have similar functional groups, so that a precursor formed by the primary particles can enter the pore structure of the main electric conduction network more conveniently; the full filling of the pore structure is realized, and the specific gravity of primary particles in the lithium titanate composite material is improved;
2. the strong bonding acting force between the inner sheet layers of the main power transmission network structure can effectively seal primary particles in the main power transmission network structure, and the stability of the primary particles in a finished product secondary particle structure is ensured; meanwhile, after the main power conducting network is sealed, the contact between the electrolyte and primary particles can be effectively blocked, and the generation of gas generation side reactions in the charging and discharging process is reduced or even eliminated, so that the gas generation problem in the circulating process of the lithium titanate battery is thoroughly solved;
3. the auxiliary conducting network structure tightly connects the main conducting network structure and the primary particles, so that the contact area between the primary particles and the main conducting network is increased, the contact resistance is reduced, and the prepared lithium titanate battery has lower impedance;
4. in the preparation process, the polymer monomer with low viscosity and the primary nano-particles are kneaded and dispersed, so that the uniform dispersion of the primary nano-particles can be ensured, and the polymer monomer is uniformly distributed on the surfaces of the primary nano-particles;
5. the precursor with lower viscosity (because the viscosity of the polymer monomer is low) is easier to fill into the pore structure of the main conducting network, and the pores of the porous structure of the main conducting network are all filled with the primary nanoparticles.
Detailed Description
The present invention and its advantageous effects will be described in detail below with reference to specific embodiments, but the embodiments of the present invention are not limited thereto.
Comparative example, a lithium titanate secondary particulate material having a particle diameter of 10 μm was prepared;
step 1, mixing: lithium titanate with the particle size of 100nm, conductive carbon black, sodium dodecyl sulfate, polyvinylpyrrolidone and NMP (solid content is 0.5%) are mixed and stirred for 10 hours according to the mass ratio of the lithium titanate, the conductive carbon black, the sodium dodecyl sulfate and the polyvinylpyrrolidone, wherein the mass ratio of the lithium titanate, the conductive carbon black, the sodium dodecyl sulfate and the polyvinylpyrrolidone is 94:4.9:1:0.1, and the slurry is obtained.
Step 2, preparing secondary particles: adjusting spray drying conditions to prepare lithium titanate secondary particles with the particle diameter of 10 mu m; and then coating and carbonizing to obtain the lithium titanate negative electrode material.
Example 1 is different from the comparative example in that the present example includes the following steps:
step 1, precursor preparation: mixing lithium titanate with the particle size of 100nm, methyl methacrylate, sodium dodecyl sulfate (the mass ratio of lithium titanate to methyl methacrylate to sodium dodecyl sulfate is 95:4:1) and NMP, kneading the mixture (the solid content is 10 percent), revolving the mixture at 30 revolutions per minute and rotating the mixture at 300 revolutions per minute; kneading for 4h to obtain a uniformly dispersed precursor;
step 2, preparing a main conducting network structure of the modified vermicular graphene: selecting microcrystalline graphite as a raw material, adding concentrated sulfuric acid and potassium permanganate to perform oxidation intercalation to obtain graphite oxide, and performing heat treatment to obtain vermicular graphene; putting the vermicular graphene into a mixture of concentrated sulfuric acid, potassium permanganate and sodium nitrate to modify the vermicular graphene to obtain modified vermicular graphene grafted with 1% of functional groups for later use;
step 3, filling: vacuumizing the modified vermicular graphene obtained in the step 2, then placing the modified vermicular graphene in the precursor obtained in the step 1, applying pressure to the precursor, simultaneously performing ultrasonic oscillation to enable the precursor to be filled into the pore structure of the vermicular graphene, and separating to obtain the modified vermicular graphene filled with the precursor;
step 4, polymerization reaction: dissolving tert-butyl peroxybenzoate in NMP to obtain a solution, spraying the solution onto the surface of the modified worm-like graphene filled with the precursor obtained in the step (3), and heating to promote the polymerization of methyl methacrylate dispersed on the surface of lithium titanate particles, so that the lithium titanate particles and the modified worm-like graphene lamellar layer are tightly bonded together;
step 5, closing the opening: carrying out solvothermal reaction on the product obtained in the step (4), promoting functional groups grafted on vermicular graphene sheet layers (between adjacent sheet layers) to react, generating new chemical bonds, and sealing the openings of the vermicular graphene sheet layers;
and 6, coating and carbonizing the product obtained in the step 5 (and carbonizing the coating layer and the polymer at the same time) to obtain the finished lithium titanate negative electrode material.
Embodiment 2 is different from embodiment 1 in that this embodiment includes the following steps:
step 2, preparing a main conducting network structure of the modified vermicular graphene: selecting microcrystalline graphite as a raw material, adding concentrated sulfuric acid and potassium permanganate to perform oxidation intercalation to obtain graphite oxide, and performing heat treatment to obtain vermicular graphene; putting the vermicular graphene into a mixture of concentrated sulfuric acid, potassium permanganate and sodium nitrate to modify the vermicular graphene to obtain modified vermicular graphene grafted with 5% of functional groups for later use;
the rest is the same as the embodiment 1, and the description is omitted.
Embodiment 3 is different from embodiment 1 in that this embodiment includes the following steps:
step 2, preparing a main conducting network structure of the modified vermicular graphene: selecting microcrystalline graphite as a raw material, adding concentrated sulfuric acid and potassium permanganate to perform oxidation intercalation to obtain graphite oxide, and performing heat treatment to obtain vermicular graphene; putting the vermicular graphene into a mixture of concentrated sulfuric acid, potassium permanganate and sodium nitrate to modify the vermicular graphene to obtain modified vermicular graphene grafted with 15% of functional groups for later use;
the rest is the same as the embodiment 1, and the description is omitted.
Embodiment 4 is different from embodiment 1 in that this embodiment includes the following steps:
step 2, preparing a main conducting network structure of the modified vermicular graphene: selecting microcrystalline graphite as a raw material, adding concentrated sulfuric acid and potassium permanganate to perform oxidation intercalation to obtain graphite oxide, and performing heat treatment to obtain vermicular graphene; putting the vermicular graphene into a mixture of concentrated sulfuric acid, potassium permanganate and sodium nitrate to modify the vermicular graphene to obtain modified vermicular graphene grafted with 20% of functional groups for later use;
the rest is the same as the embodiment 1, and the description is omitted.
Embodiment 5 differs from embodiment 1 in that this embodiment includes the following steps:
step 2, preparing a main conducting network structure of the modified vermicular graphene: selecting microcrystalline graphite as a raw material, adding concentrated sulfuric acid and potassium permanganate to perform oxidation intercalation to obtain graphite oxide, and performing heat treatment to obtain vermicular graphene; putting the vermicular graphene into a mixture of concentrated sulfuric acid, potassium permanganate and sodium nitrate to modify the vermicular graphene to obtain modified vermicular graphene grafted with 25% of functional groups for later use;
the rest is the same as the embodiment 1, and the description is omitted.
Embodiment 6 differs from embodiment 1 in that this embodiment includes the following steps:
step 2, preparing a main conducting network structure of the modified vermicular graphene: selecting microcrystalline graphite as a raw material, adding concentrated sulfuric acid and potassium permanganate to perform oxidation intercalation to obtain graphite oxide, and performing heat treatment to obtain vermicular graphene; putting the vermicular graphene into a mixture of concentrated sulfuric acid, potassium permanganate and sodium nitrate to modify the vermicular graphene to obtain modified vermicular graphene grafted with 40% of functional groups for later use;
the rest is the same as the embodiment 1, and the description is omitted.
Embodiment 7 is different from embodiment 1 in that this embodiment includes the following steps:
step 1, precursor preparation: mixing modified lithium titanate (surface hydroxylation) with the particle size of 100nm, a carbon nano tube and NMP (solid content is 10%) and kneading, wherein the revolution is 30 revolutions per minute, and the rotation is 300 revolutions per minute; kneading for 4h to obtain uniformly dispersed precursor slurry;
step 2, preparing a main conducting network structure of the modified vermicular graphene: selecting microcrystalline graphite as a raw material, adding concentrated sulfuric acid and potassium permanganate to perform oxidation intercalation to obtain graphite oxide, and performing heat treatment to obtain vermicular graphene; putting the vermicular graphene into a mixture of concentrated sulfuric acid, potassium permanganate and sodium nitrate to modify the vermicular graphene to obtain modified vermicular graphene grafted with 20% of functional groups for later use;
step 3, filling: vacuumizing the modified vermicular graphene obtained in the step 2, then placing the modified vermicular graphene in the precursor obtained in the step 1, applying pressure to the precursor, simultaneously performing ultrasonic oscillation to enable the precursor to be filled into a vermicular graphene pore structure, and separating to obtain the modified vermicular graphene filled with the precursor;
step 4, closing the opening: carrying out solvothermal reaction on the product obtained in the step (3) to promote functional groups grafted on the vermicular graphene sheet layers (between adjacent sheet layers) to react to generate new chemical bonds, and sealing the openings of the vermicular graphene sheet layers;
and 5, coating and carbonizing the product obtained in the step 4 (and carbonizing the coating layer and the polymer at the same time) to obtain the finished lithium titanate negative electrode material.
The rest is the same as the embodiment 1, and the description is omitted.
Embodiment 8 differs from embodiment 1 in that this embodiment includes the following steps:
step 1, kneading: mixing lithium titanate with the particle size of 100nm and artificial graphite (the mass ratio of the lithium titanate to the artificial graphite is 9:1), trimethylolpropane trimethacrylate, hexadecyl dimethyl allyl ammonium chloride (the mass ratio of the lithium titanate to the artificial graphite is 90:4:1), ethanol, kneading, wherein the revolution is 5 revolutions per minute, and the rotation is 10 revolutions per minute; kneading for 8h to obtain a mixture 1; mixing methyl vinyl dimethoxysilane, graphene, polyoxyethylene alkylphenol ether (mass ratio of methyl vinyl dimethoxysilane to graphene: polyoxyethylene alkylphenol ether is 5:4.9:0.1) and ethanol, kneading (solid content is 4%), revolving for 5 revolutions/min, and rotating for 10 revolutions/min; kneading for 8h to obtain a mixture 2; mixing the mixture 1 and the mixture 2 (the mass ratio of (lithium titanate + artificial graphite): graphene is 90:4.9), kneading continuously, revolving for 5 revolutions per minute, and rotating for 10 revolutions per minute; kneading for 6h to obtain a precursor in which a polymer monomer is uniformly coated on the surface of primary particles (lithium titanate and artificial graphite), the polymer monomer and graphene are uniformly dispersed, and the graphene and the primary particles are uniformly dispersed;
step 2, preparing the main conducting network structure of the modified expanded graphite: flake graphite is selected as a raw material, then concentrated sulfuric acid and potassium permanganate are added for oxidation intercalation to obtain graphite oxide, and then heat treatment is carried out to obtain expanded graphite; placing the expanded graphite in a mixture of concentrated sulfuric acid, potassium permanganate and sodium nitrate to modify the expanded graphite to obtain a modified expanded graphite main power grid structure grafted with 20% of functional groups for later use;
step 3, filling: vacuumizing the main conductive network structure of the modified expanded graphite obtained in the step (2), then placing the vacuumized main conductive network structure of the modified expanded graphite in the precursor obtained in the step (1), applying pressure to the precursor, simultaneously performing ultrasonic oscillation to enable the precursor to be filled into the main conductive network structure of the modified expanded graphite, and separating to obtain the main conductive network structure of the modified expanded graphite filled with the precursor;
step 4, polymerization reaction: dissolving tert-butyl peroxybenzoate in NMP to obtain a solution, spraying the solution on the surface of the modified expanded graphite filled with the precursor obtained in the step (3), and heating to promote the polymerization of methyl methacrylate dispersed on the surface of the primary particles, so that the primary particles and the modified expanded graphite sheet are tightly bonded together;
step 5, closing the opening: adding a reducing agent into the product obtained in the step (4) to promote functional groups grafted on the modified expanded graphite sheet layers (between the adjacent sheet layers) to react to generate new chemical bonds, and sealing the openings of the modified expanded graphite sheet layers;
the rest is the same as the embodiment 1, and the description is omitted.
Assembling the battery: stirring the lithium titanate negative electrode material prepared in comparative example and example 1-8 with a conductive agent, a binder and a solvent to obtain electrode slurry, and then coating the electrode slurry on a current collector to form a negative electrode; assembling the negative electrode, the positive electrode (lithium cobaltate is used as an active substance) and the isolating membrane to obtain a bare cell, and then bagging to perform top side sealing, drying, liquid injection, standing, formation, shaping and degassing to obtain a finished battery.
And (3) testing the material performance:
and (3) gram capacity test: the gram capacity test of the battery cores prepared from the lithium titanate materials in the examples and the comparative examples is carried out in an environment at 25 ℃ according to the following procedures: standing for 3 min; charging to 2.8V at a constant current of 1C and charging to 0.1C at a constant voltage of 2.8V; standing for 3 min; discharging the 1C at constant current to 1.5V to obtain discharge capacity D1; standing for 3 min; charging the 1C to 2.35V by constant current; and (3) standing for 3min, completing the capacity test, and dividing the weight of the lithium titanate material in the cathode electrode piece by D1 to obtain the gram capacity of the cathode, wherein the obtained result is shown in Table 1.
Testing internal resistance: the internal resistance of the battery cells prepared from the lithium titanate materials in the examples and the comparative examples is tested in an environment at 25 ℃ according to the following procedures: standing for 3 min; charging to 2.35V at a constant current of 1C and charging to 0.1C at a constant voltage of 2.35V; standing for 3 min; and testing the DCR value of the battery cell by adopting an electrochemical workstation, wherein the obtained result is shown in table 1.
And (3) rate performance test: the rate capability test of the battery cell prepared from the lithium titanate materials of the examples and the comparative examples is carried out in an environment of 25 ℃ according to the following procedures: standing for 3 min; charging to 2.8V at a constant current of 1C and charging to 0.1C at a constant voltage of 2.8V; standing for 3 min; discharging to 1.5V at constant current of 0.5C to obtain discharge capacity D1; standing for 3 min; charging to 2.8V at a constant current of 1C and charging to 0.1C at a constant voltage of 2.8V; standing for 3 min; discharging at constant current of 5C to 1.5V to obtain discharge capacity D2; standing for 3 min; rate performance testing was then completed and the cell rate performance was D2/D1 x 100% with the results shown in table 1.
And (3) carrying out cycle test on the battery cells prepared from the lithium titanate materials of the examples and the comparative examples in an environment at 25 ℃ according to the following flow: standing for 3 min; charging to 2.8V at a constant current of 1C and charging to 0.1C at a constant voltage of 2.8V; standing for 3 min; discharging the 1C at constant current to 1.5V to obtain discharge capacity D1; standing for 3min, charging to 2.8V at constant current of 1C and charging to 0.1C at constant voltage of 2.8V; standing for 3 min; discharging to 1.5V at constant current at 1C to obtain discharge capacity Di; standing for 3min "and repeating 999 times to obtain D1000, then completing the cycle test, and calculating the capacity retention rate to be D1000/D1 × 100%, and obtaining the results shown in Table 1.
And (3) evaluating the gas production: and observing the appearance of the battery subjected to the cycle test, and judging the gas production amount of the battery. The results are shown in Table 1.
From table 1, the lithium titanate negative electrode material with excellent performance can be prepared, and the battery core assembled by taking the lithium titanate negative electrode material as the negative electrode active substance has excellent electrochemical performance. Specifically, comparing the comparative example with examples 1 to 6, it can be seen that, as the oxygen-containing functional groups on the modified main conducting network structure sheet layer increase, the electrochemical performance of the battery cell becomes better first and then worse, because when the oxygen-containing functional groups are too small, the sealing effect is poor, and the battery cell cannot fully function; when the oxygen-containing functional group is too much, the seal is too tight, and ion diffusion during charge and discharge is hindered. The present invention has general applicability as can be seen from the various embodiments.
TABLE 1 Battery cell performance table prepared from different lithium titanate negative electrode materials
Variations and modifications to the above-described embodiments may also occur to those skilled in the art, which fall within the scope of the invention as disclosed and taught herein. Therefore, the present invention is not limited to the above-mentioned embodiments, and any obvious improvement, replacement or modification made by those skilled in the art based on the present invention is within the protection scope of the present invention. Furthermore, although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.
Claims (6)
1. A lithium titanate negative electrode material comprises a core structure and a shell structure, wherein the core structure is a secondary particle structure and comprises a main electric conduction network with a porous structure and nano primary particles filled in the pore structure of the main electric conduction network; stronger bonding force action exists between the main power supply networks; and tightly locking the nano primary particles in the pore structure of the main conducting network through the bonding force, wherein the nano primary particles contain lithium titanate particles;
the preparation method mainly comprises the following steps:
step 1, precursor preparation: uniformly dispersing the primary particles in a solvent to obtain a precursor; step 2, preparing a modified main power guide network structure: placing the main conducting network structure with the porous structure in an oxidation environment, and grafting a functional group to obtain a modified main conducting network structure; step 3, filling: filling the precursor prepared in the step 1 into a modified main power transmission network structure; step 4, closing the opening: placing the porous main conducting network structure in a reduction environment, and promoting functional groups grafted on the main conducting network structure to react to generate strong bonding force to seal or partially seal the pore structure in the porous main conducting network structure; step 5, coating and carbonizing the product obtained in the step 4 to obtain a finished product lithium titanate cathode material;
the filling process in the step 3 is as follows:
pretreating the porous main conducting network structure material, wherein the pretreatment comprises surface activation or/and surfactant addition;
before filling, placing the porous main conducting network structure material in a vacuum environment for vacuumizing, discharging air in a hole structure to make space for filling a precursor, and then placing the porous main conducting network structure material in precursor slurry for starting filling;
in the filling process, pressure is applied to extrude the precursor into the hole; the temperature is increased, and the viscosity of the precursor is reduced; adding mechanical disturbance and opening a hole;
the category of bonds providing strong bonding forces are hydrogen bonds or/and chemical bonds; the mass of the oxygen-containing functional groups forming the hydrogen bonds or/and the chemical bonds accounts for 1-40% of the mass of the whole main electric network.
2. A lithium titanate negative electrode material according to claim 1, characterized in that the main conductive network has flexibility and contains functional groups inside; the hydrogen bonds or/and chemical bonds result from the reaction of oxygen-containing functional groups within the main conducting network.
3. A lithium titanate negative electrode material according to claim 1, characterized in that the main conductive network is at least one of an open graphene structure, an open expanded graphite structure, a vermicular graphene structure; the nano primary particles comprise nano lithium titanate negative electrode particles; and an auxiliary electric conduction network is also distributed between the main electric conduction network and the primary particles and tightly connects the main electric conduction network and the nano primary particles together.
4. A lithium titanate negative electrode material as claimed in claim 3, wherein said nano-sized primary particles further comprise non-nano lithium titanate negative electrode particles; the non-nano lithium titanate negative electrode particles are at least one of natural graphite, artificial graphite, mesocarbon microbeads, petroleum coke, carbon fibers, pyrolytic resin carbon, silicon carbon negative electrode particles, tin-based negative electrode materials, transition metal nitrides, germanium-based alloys, aluminum-based alloys, antimony-based alloys and magnesium-based alloys; the auxiliary conducting network is obtained by carbonizing a high polymer material; the high polymer material is obtained by in-situ polymerization of a high polymer monomer; the auxiliary conducting network further comprises at least one of super conducting carbon, Ketjen black, carbon nanotubes, graphene and acetylene black.
5. A lithium titanate negative electrode material as claimed in claim 1, characterized in that the surface of the primary particles in step 1 is modified to become functionalized primary particles, and the functional groups are carboxyl groups or/and hydroxyl groups; the grafted functional group in the step 2 comprises at least one of carboxyl, hydroxyl, epoxy, carbonyl and nitro; and 4, adding a reducing agent or/and performing direct hydrothermal reduction on the reducing environment.
6. The lithium titanate negative electrode material as claimed in claim 1, wherein a polymer monomer is further added in step 1, i.e., the primary particles and the polymer monomer are mixed and kneaded to obtain a precursor in which the polymer monomer is uniformly dispersed on the surface of the primary nanoparticles; in this case, it is necessary to perform a polymerization reaction after step 3, in which the product of step 3 is placed in an environment where an initiator is present to promote polymerization of the polymer monomer dispersed on the surface of the primary particles, thereby obtaining a high molecular polymer.
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