CN114512641A - Positive electrode active material, preparation method thereof, positive electrode and lithium ion secondary battery - Google Patents

Positive electrode active material, preparation method thereof, positive electrode and lithium ion secondary battery Download PDF

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CN114512641A
CN114512641A CN202011284599.8A CN202011284599A CN114512641A CN 114512641 A CN114512641 A CN 114512641A CN 202011284599 A CN202011284599 A CN 202011284599A CN 114512641 A CN114512641 A CN 114512641A
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positive electrode
active material
manganese oxide
nickel manganese
lithium nickel
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CN114512641B (en
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武怿达
黄学杰
詹元杰
马晓威
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Institute of Physics of CAS
Songshan Lake Materials Laboratory
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Songshan Lake Materials Laboratory
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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
    • H01M4/362—Composites
    • H01M4/366—Composites as layered products
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G53/00—Compounds of nickel
    • C01G53/40—Nickelates
    • C01G53/42—Nickelates containing alkali metals, e.g. LiNiO2
    • C01G53/44—Nickelates containing alkali metals, e.g. LiNiO2 containing manganese
    • C01G53/50—Nickelates containing alkali metals, e.g. LiNiO2 containing manganese of the type [MnO2]n-, e.g. Li(NixMn1-x)O2, Li(MyNixMn1-x-y)O2
    • 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
    • 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
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/131—Electrodes based on mixed oxides or hydroxides, or on mixtures of oxides or hydroxides, e.g. LiCoOx
    • 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
    • H01M4/50—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese
    • H01M4/505—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese of mixed oxides or hydroxides containing manganese for inserting or intercalating light metals, e.g. LiMn2O4 or LiMn2OxFy
    • 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
    • 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
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H01M4/624—Electric conductive fillers
    • 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
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H01M4/624—Electric conductive fillers
    • H01M4/625—Carbon or graphite
    • 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
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H01M4/628—Inhibitors, e.g. gassing inhibitors, corrosion inhibitors
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/01—Particle morphology depicted by an image
    • C01P2004/04—Particle morphology depicted by an image obtained by TEM, STEM, STM or AFM
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/60—Particles characterised by their size
    • C01P2004/64—Nanometer sized, i.e. from 1-100 nanometer
    • H—ELECTRICITY
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    • 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
    • H01M2004/021—Physical characteristics, e.g. porosity, surface area
    • 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
    • H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
    • H01M2004/028—Positive electrodes
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Abstract

The invention discloses a positive active material, which comprises a lithium nickel manganese oxide modified material and a coating layer on the surface of the lithium nickel manganese oxide modified material, wherein the coating layer comprises carbon and an organic compound, the organic compound has-N bond or-COO bond, a carbon layer is distributed close to the lithium nickel manganese oxide modified material, and the organic compound layer is superposed on the carbon layer; the lithium nickel manganese oxide modified material comprises primary particles with a core spinel phase and a rock-salt-like phase as a shell; the rock-like salt phase contains at least one space occupying element of Mg, Zn, Ni, Mn, Fe, Co, Ti, Cr, Y, Sc, Ru, Cu, Mo, Ge, W, Zr, Ca, Ta, Sr, Al, Nb, B, Si, F and S, and the space occupying element is positioned at the 16c or 8a position of the spinel phase; the rock-like salt phase is also doped with phosphorus elements, and the phosphorus elements are distributed in a gradient manner from the outer surface to the inner part of the rock-like salt phase. The invention also discloses a preparation method of the positive active material, a positive electrode of the lithium ion secondary battery containing the positive active material and the lithium ion secondary battery.

Description

Positive electrode active material, preparation method thereof, positive electrode and lithium ion secondary battery
Technical Field
The invention relates to the technical field of positive electrode materials, in particular to a positive electrode active material, a preparation method, a positive electrode and a lithium ion secondary battery.
Background
Compared with other rechargeable battery systems, the lithium ion secondary battery has the advantages of high working voltage, light weight, small volume, no memory effect, low self-discharge rate, long cycle life, high energy density and the like, and is widely applied to mobile terminal products such as mobile phones, notebook computers, tablet computers and the like. In practical applications, how to improve the electronic conductance of the material is crucial for further commercial applications.
The use of carbon to coat and compound lithium nickel manganese oxide materials has been proposed to improve the electronic conductivity of the materials. However, the conventional carbon layer is decomposed violently under high voltage to generate a large amount of side reaction products, which is not beneficial to the stable circulation of a high-voltage battery system.
Disclosure of Invention
Based on the above, it is necessary to provide a positive active material, a preparation method thereof, a positive electrode and a lithium ion secondary battery aiming at the problem that the improvement of the electron conductivity of the lithium nickel manganese oxide positive electrode material is not ideal.
A positive electrode active material comprises a lithium nickel manganese oxide modified material and a coating layer on the surface of the lithium nickel manganese oxide modified material, wherein the coating layer comprises carbon and an organic compound, the organic compound has NH groups, the carbon and the organic compound are distributed in a layered mode, a carbon layer is distributed close to the lithium nickel manganese oxide modified material, and an organic compound layer is superposed on the carbon layer;
the lithium nickel manganese oxide modified material comprises primary particles of a spinel phase and a rock-like salt phase, wherein the spinel phase is a core, and the rock-like salt phase is distributed on the surface of the spinel phase to form a shell;
the spinel phase is of a lithium nickel manganese oxide spinel structure;
the rock-salt-like phase is induced by the lithium nickel manganese oxide spinel structure and comprises at least one space occupying element of Mg, Zn, Ni, Mn, Fe, Co, Ti, Cr, Y, Sc, Ru, Cu, Mo, Ge, W, Zr, Ca, Ta, Sr, Al, Nb, B, Si, F and S, and the space occupying element is positioned at the 16c or 8a position of the spinel phase;
the rock-like salt phase is also doped with phosphorus elements, and the phosphorus elements are distributed in a gradient manner from the outer surface to the inner part of the rock-like salt phase.
In one embodiment, the mass ratio of carbon to organic compound in the coating layer is 1: (0.1-10).
In one embodiment, the organic compound is selected from any one or more of polyimide, polypyrrole, polyaniline, polymethyl methacrylate, and polycarbazole.
In one embodiment, the lithium nickel manganese oxide spinel structure has a chemical formula of Li1+xNi0.5-yMn1.5- zOuWherein x is more than or equal to 0.2 and less than or equal to 0.2, y is more than or equal to 0.2 and less than or equal to 0.2, z is more than or equal to 0.2 and u is more than or equal to 3.8 and less than or equal to 4.2.
In one embodiment, the lithium nickel manganese oxide spinel structure has a chemical formula of Li1+xNi0.5-yMn1.5- zMsOuWherein M is at least one of Mg, Zn, Ni, Mn, Fe, Co, Ti, Cr, Y, Sc, Ru, Cu, Mo, Ge, W, Zr, Ca, Ta, Sr, Al, Nb, B, Si, F and S, x is more than or equal to-0.2 and less than or equal to-0.2, z is more than or equal to-0.2 and less than or equal to-0.2, S is more than 0 and less than or equal to-0.2, and u is more than or equal to 3.8 and less than or equal to 4.2.
In one embodiment, the thickness of the coating layer of the lithium nickel manganese oxide modified material is 2 nm-20 nm.
In one embodiment, the spinel phase has a thickness of 0.1 μm to 30 μm.
In one embodiment, the thickness of the rock-salt-like phase is 0.5nm to 50 nm.
In one embodiment, in the primary particles, the concentration of the phosphorus element gradually decreases from the outer surface to the inner surface, the phosphorus element forms a phosphorus gradient doped layer, and the thickness of the phosphorus gradient doped layer is 0.5 nm-40 nm.
A preparation method of the positive active material comprises the following steps:
a. mixing the lithium nickel manganese oxide modified material with a carbon source and a solvent to obtain a first coating mixture;
b. drying the first coating mixture, and sintering the mixture for 0.5 to 20 hours at the temperature of between 200 and 550 ℃ in a non-oxidizing atmosphere to obtain a carbon-coated lithium nickel manganese oxide modified material;
c. mixing the carbon-coated lithium nickel manganese oxide modified material, an organic compound and a solvent to obtain a second coating mixture, or mixing the carbon-coated lithium nickel manganese oxide modified material, an organic compound precursor and a solvent to obtain a second coating mixture; and
d. and heating the second coating mixture at 120-400 ℃ for reaction for 2-20 hours.
In one embodiment, the organic compound precursor is imidized or polymerized to obtain the organic compound.
In one embodiment, the organic compound precursor is selected from any one or more of polyamic acid, pyrrole, aniline, methyl methacrylate, and carbazole.
In one embodiment, the polyamic acid is selected from quaternary polyamic acids.
In one embodiment, the quaternary polyamic acid is selected from any one or more of pyromellitic dianhydride, biphenyl dianhydride, phenylenediamine, and oxydianiline.
In one embodiment, the heating reaction process of step c is as follows: heating the second coating mixture to 55-65 ℃ for 20-40 minutes, continuously heating to 115-125 ℃ for 20-40 minutes, continuously heating to 195-205 ℃ for 50-70 minutes, continuously heating to 290-310 ℃ for 50-70 minutes, and continuously heating to 390-400 ℃ for 8-15 minutes.
In one embodiment, the preparation method of the lithium nickel manganese oxide modified material comprises the following steps:
mixing a phosphorus source, a rock-like salt phase inducer and a lithium nickel manganese oxide spinel structure material to obtain a doped mixture; and
sintering the doped mixture at 600-1200 ℃ for 0.5-20 hours.
In one embodiment, the sintering process of the dopant mixture is: heating to 600-1200 ℃ at the heating rate of 0.5-10 ℃/min, then sintering for 0.5-20 hours, and then cooling to room temperature at the cooling rate of 0.5-10 ℃/min.
A positive electrode of a lithium ion secondary battery comprises a positive electrode current collector and a positive electrode active material layer positioned on the positive electrode current collector, wherein the positive electrode active material layer comprises the positive electrode active material.
A lithium ion secondary battery comprising:
the positive electrode;
a negative electrode including a negative electrode current collector and a negative electrode active material layer on the negative electrode current collector;
a separator and an electrolyte.
The P element gradient doping is carried out on the surface of the lithium nickel manganese oxide spinel structure to obtain the lithium nickel manganese oxide modified material, and the carbon and organic compound coating is carried out on the modified material to obtain the anode active material. The carbon coating can enhance the electronic conductivity of the lithium nickel manganese oxide material, and the use of conductive additive carbon is reduced in the subsequent process of preparing the positive plate; however, carbon is easily decomposed under high pressure and reacts with electrolyte to generate water, carbon dioxide and other harmful substances to cause the decomposition of the electrolyte of a battery system, and in order to further protect the material coated with carbon, the carbon and the organic compound are coated together, and the organic compound is distributed on the outer layer of the carbon layer, so that the carbon layer can be protected, the decomposition of the carbon layer is reduced, and the coating effect of the carbon layer is improved.
The organic compound of the present invention has an-N bond or-COO bond, can generate unshared electrons by an NH group and H in a COOH group, and lithium ions migrating in the secondary battery during charge and discharge are captured by the unshared electrons on the organic compound in the positive electrode active material to have lithium ion conductivity. Therefore, the coating having the-N bond or the-COO bond on the surface of the positive electrode active material can protect the carbon layer, protect the interface stability of the positive electrode active material, and improve the lithium ion conductivity of the positive electrode active material, thereby facilitating the migration of lithium ions and electrons. However, the electron conductivity of the traditional organic compound is not high, which can cause the electron conductivity of the surface of the anode active material to be reduced, and the carbon layer and the surface of the nickel lithium manganate modified material of the invention are doped with gradient P, which can increase the electron conductivity. And the surface doping of the P element can obviously improve the electrochemical properties of the spinel cathode active material, including first effect, average efficiency and cycling stability. The P element doped modified material is coated in the carbon, so that the electronic conductivity enhancement effect of the surface carbon coating is enhanced, and even if the surface carbon is decomposed, the doped phosphorus in the internally coated modified material can still play a role in enhancing the electronic conductivity. The surface gradient P doping of the lithium nickel manganese oxide modified material is combined with the coating of surface carbon and organic compounds, so that the interface stability of the positive active material is improved, and the positive active material has higher electronic conductivity and lithium ion conductivity.
Drawings
FIG. 1 is a STEM of a phosphorus-doped lithium nickel manganese oxide modified material prepared in example 1 of the present invention;
FIG. 2 is a STEM line scan of a phosphorus-doped lithium nickel manganese oxide modified material prepared in example 1 of the present invention;
FIG. 3 is a TEM image of a coated lithium nickel manganese oxide positive active material obtained in example 1 of the present invention;
FIG. 4 is a STEM chart of a phosphorus-doped lithium nickel manganese oxide modified material obtained in example 2 of the present invention;
fig. 5 shows the relative content change of phosphorus element on the surface of the phosphorus-doped lithium nickel manganese oxide modified material prepared in example 2 of the present invention, which is characterized by XPS at different etching depths.
Detailed Description
To facilitate an understanding of the invention, the invention will now be described more fully with reference to the accompanying drawings. Preferred embodiments of the present invention are shown in the drawings. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.
Other than as shown in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients, physical and chemical properties, and so forth used in the specification and claims are to be understood as being modified in all instances by the term "about". For example, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached claims are approximations that can be suitably varied by those skilled in the art in seeking to obtain the desired properties utilizing the teachings disclosed herein. The use of numerical ranges by endpoints includes all numbers within that range and any range within that range, for example, 1 to 5 includes 1, 1.1, 1.3, 1.5, 2, 2.75, 3, 3.80, 4, and 5, and the like.
The embodiment of the invention provides a positive electrode active material which comprises a lithium nickel manganese oxide modified material and a coating layer on the surface of the lithium nickel manganese oxide modified material, wherein the coating layer comprises carbon and an organic compound, the organic compound has a nitrogen bond (-N) or an ester bond (-COO), the carbon and the organic compound are distributed in a layered mode, a carbon layer is distributed close to the lithium nickel manganese oxide modified material, and an organic compound layer is superposed on the carbon layer.
The lithium nickel manganese oxide modified material comprises primary particles of a spinel phase and a rock-like salt phase, wherein the spinel phase is a core, and the rock-like salt phase is distributed on the surface of the spinel phase to form a shell;
the spinel phase is of a lithium nickel manganese oxide spinel structure;
the rock-salt-like phase is induced by the lithium nickel manganese oxide spinel structure and comprises at least one space occupying element of Mg, Zn, Ni, Mn, Fe, Co, Ti, Cr, Y, Sc, Ru, Cu, Mo, Ge, W, Zr, Ca, Ta, Sr, Al, Nb, B, Si, F and S, and the space occupying element is positioned at the 16c or 8a position of the spinel phase;
the rock-like salt phase is also doped with phosphorus elements, and the phosphorus elements are distributed in a gradient manner from the outer surface to the inner part of the rock-like salt phase.
The P element gradient doping is carried out on the surface of the lithium nickel manganese oxide spinel structure to obtain the lithium nickel manganese oxide modified material, and the carbon and organic compound coating is carried out on the modified material to obtain the anode active material. The carbon coating can enhance the electronic conductivity of the lithium nickel manganese oxide material, and the use of conductive additive carbon is reduced in the subsequent process of preparing the positive plate; however, carbon is easily decomposed under high pressure and reacts with electrolyte to generate water, carbon dioxide and other harmful substances to cause the decomposition of the electrolyte of a battery system, and in order to further protect the material coated with carbon, the carbon and the organic compound are coated together, and the organic compound is distributed on the outer layer of the carbon layer, so that the carbon layer can be protected, the decomposition of the carbon layer is reduced, and the coating effect of the carbon layer is improved.
The organic compound of the present invention has an-N bond or-COO bond, can generate unshared electrons by an NH group and H in a COOH group, and lithium ions migrating in the secondary battery during charge and discharge are captured by the unshared electrons on the organic compound in the positive electrode active material to have lithium ion conductivity. Therefore, the organic compound having a group of a-N bond or a-COO bond is coated on the surface of the positive electrode active material, so that not only can the carbon layer be protected and the interface stability of the positive electrode active material be protected, but also the lithium ion conductivity of the positive electrode active material can be improved, thereby facilitating the migration of lithium ions and electrons. However, the electron conductivity of the traditional organic compound is not high, which can cause the electron conductivity of the surface of the anode active material to be reduced, and the carbon layer and the surface of the nickel lithium manganate modified material of the invention are doped with gradient P, which can increase the electron conductivity. And the surface doping of the P element can obviously improve the electrochemical properties of the spinel cathode active material, including first effect, average efficiency and cycling stability. The P element doped modified material is coated in the carbon, so that the electronic conductivity enhancement effect of the surface carbon coating is enhanced, and even if the surface carbon is decomposed, the phosphorus doped in the internally coated modified material can still play a role in enhancing the electronic conductivity. The surface gradient P doping of the lithium nickel manganese oxide modified material is combined with the coating of surface carbon and organic compounds, so that the interface stability of the positive active material is improved, and the positive active material has higher electronic conductivity and lithium ion conductivity.
A core-shell structure is generally defined as an ordered assembly of one material coated with another material by chemical bonding or other forces. The primary particles in the present invention have a core-shell-like structure, and the defined core-shell-like structure "core" and "shell" are actually integrated. The "shell" is a rock-salt phase structure formed by surface reconstruction of a spinel structure. The structure of the lithium nickel manganese oxide modified material comprises two phases, so that the microstructure of a surface layer is different from the microstructure of the interior of the material, the interior of the material formed in the way is called a core, the surface layer is called a shell, and the material with the structure is defined as a material with a core-shell-like structure.
The primary particles refer to the smallest units constituting the lithium nickel manganese oxide-modified material, and specifically refer to the smallest units that can be determined based on the geometric configuration of appearance. Aggregates of primary particles are secondary particles. The primary particles have a core-shell-like structure in which a spinel phase core and a rock-salt-like phase shell are integrated, no grain boundary exists at a boundary between the spinel phase and the rock-salt-like phase, and the spinel phase and the rock-salt-like phase cannot be separated from each other by oxygen bonding.
In some embodiments, the lithium nickel manganese oxide spinel structure has the formula Li1+xNi0.5-yMn1.5-zOuWherein x is more than or equal to 0.2 and less than or equal to 0.2, y is more than or equal to 0.2 and less than or equal to 0.2, z is more than or equal to 0.2 and u is more than or equal to 3.8 and less than or equal to 4.2. The values of x, y and z may vary depending on the ratio between the elements, but are set within a range such that the compound represented by the formula may exhibit a spinel structure.
At one endIn some embodiments, the bulk phase of the lithium nickel manganese oxide spinel structure is uniformly doped with an element that facilitates P doping, and the chemical formula of the lithium nickel manganese oxide spinel structure can be Li1+xNi0.5-yMn1.5-zMsOuWherein M is at least one of Mg, Zn, Ni, Mn, Fe, Co, Ti, Cr, Y, Sc, Ru, Cu, Mo, Ge, W, Zr, Ca, Ta, Sr, Al, Nb, B, Si, F and S, x is more than or equal to-0.2 and less than or equal to-0.2, z is more than or equal to-0.2 and less than or equal to-0.2, S is more than 0 and less than or equal to-0.2, and u is more than or equal to 3.8 and less than or equal to 4.2. The values of x, y, z, s, u may vary depending on the ratio between the elements, but are set within a range such that the compound represented by the formula may exhibit a spinel structure.
The preferred occupying element is Al, and the Al element is more favorable for improving the structural stability of the positive active material and reducing the barrier of doping phosphorus element into a spinel structure.
In some embodiments, the thickness of the spinel phase may be any value between 0.1 μm and 30 μm (where spinel phase refers to the core), and may for example further comprise 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm, 26 μm, 27 μm, 28 μm, 29 μm.
In some embodiments, the thickness of the rock-salt-like phase may be any value between 0.5nm and 50nm, and may further include, for example, 0.5nm, 1nm, 2nm, 3nm, 4nm, 5nm, 6nm, 7nm, 8nm, 9nm, 10nm, 11nm, 12nm, 13nm, 14nm, 15nm, 16nm, 17nm, 18nm, 19nm, 20nm, 21nm, 22nm, 23nm, 24nm, 25nm, 26nm, 27nm, 28nm, 29nm, 30nm, 31nm, 32nm, 33nm, 34nm, 35nm, 36nm, 37nm, 38nm, 39nm, 40nm, 41nm, 42nm, 43nm, 44nm, 45nm, 46nm, 47nm, 48nm, 49nm, 50 nm.
The lithium nickel manganese oxide modified material provided by the invention is doped with phosphorus, but is different from a phosphate-coated positive electrode active material in the prior art. The phosphate-coated positive electrode active material is a material formed by covering a spinel positive electrode material with phosphate crystal structure or amorphous phosphate, and a coating layer can be seen on the surface of the material through a transmission electron microscope. In the modified cathode active material provided by the invention, phosphorus is doped in the primary particles, and the phosphorus is doped in the spinel structure in a gradient manner from the surface to the inside of the primary particle particles.
The spinel phase and the rock-salt-like phase of the primary particles may both be doped with the phosphorus element, but the phosphorus element is preferentially doped in the rock-salt-like phase. The doping amount (concentration) of the phosphorus element in the primary particles is gradually reduced from outside to inside. The surface is doped with phosphorus in a gradient manner, the concentration gradient of the doped elements is reduced from outside to inside, the doping amount can be reduced, the material interface in contact with the electrolyte is ensured to have higher doping concentration and higher structural stability, and meanwhile, the surface is doped in a gradient manner, so that the structural stress generated in the de-intercalation process of lithium ions can be well relieved. Among high-valence elements, phosphorus doping can obviously improve the stability of the surface of the material and increase the stability of an interface, and meanwhile, the phosphorus doping on the surface is combined with an organic compound coating process, so that the stability and the electronic conductance of the material can be obviously improved.
The structure of the primary particles with the gradient distribution of the phosphorus element can be defined as a phosphorus gradient doping layer. In some embodiments, the thickness of the phosphorus gradient doped layer may be any value between 0.5nm and 40nm, and may further include, for example, 0.5nm, 1nm, 2nm, 3nm, 4nm, 5nm, 6nm, 7nm, 8nm, 9nm, 10nm, 11nm, 12nm, 13nm, 14nm, 15nm, 16nm, 17nm, 18nm, 19nm, 20nm, 21nm, 22nm, 23nm, 24nm, 25nm, 26nm, 27nm, 28nm, 29nm, 31nm, 32nm, 33nm, 34nm, 35nm, 36nm, 37nm, 38nm, 39nm, 40 nm.
The cathode material, the rock-like salt phase surface layer and the phosphorus gradient doped layer provided by the invention can be characterized by a characterization method commonly used in the field, for example, a Scanning Transmission Electron Microscope (STEM) and an X-ray photoelectron spectroscopy microscope (XPS) can be used for characterization, wherein the STEM can be used for accurately seeing the rock-like salt phase distribution generated on the surface due to partial occupying elements occupying 16c or 8a positions of spinel octahedrons, and STEM line scanning can also prove the gradient distribution of phosphorus elements. Meanwhile, the gradient distribution of the phosphorus element in the phosphorus gradient doped layer can be proved by etching analysis of X-ray photoelectron spectroscopy. The organic coating layer on the surface can be characterized by XPS, TEM, infrared spectrum and Raman spectrum, wherein the XPS can be used for representing elements of the organic matter coated on the surface, the TEM can see the surface coating layer, and the infrared spectrum and the Raman spectrum can be used for detecting-N bonds or-COO bonds on the surface of the material. Whether the cathode material is in the scope of the patent can be determined by the above characterization but not limited thereto.
In some embodiments, the organic compound may be selected from any one or more of polyimide, polypyrrole, polyaniline, polymethyl methacrylate, and polycarbazole. Preferably, the organic compound is selected from polyimides. Preferably, the organic compound layer is obtained by in-situ synthesis of polyimide on the surface of the carbon-coated lithium nickel manganese oxide modified material, and the in-situ synthesis is favorable for the combination firmness of the lithium nickel manganese oxide modified material and the carbon coating layer and the stability of the coating layer under high pressure.
In some embodiments, the thickness of the lithium nickel manganese oxide modified material coating layer is 2-20 nm. Under the thickness proportion, the positive active material has high stability, high electronic conductivity and high lithium ion conductivity. Different from the traditional positive active material, the positive active material has high electronic conductivity and lithium ion conductivity, and the transfer of electrons is realized while the lithium is released and inserted.
In some embodiments, the mass ratio of carbon to organic compound in the coating layer is 1: (0.1-10). Under the proportion, P doping, carbon coating and organic compound coating are matched with each other, so that the anode active material has high electronic conductivity, lithium ion conductivity and interface stability.
The coating layer of the invention is a carbon layer coated firstly, and then an organic compound layer is coated on the carbon layer. Because of the particularity of organic matter reaction, the invention separately carries out carbon coating and organic compound coating, and can avoid the hardness of the organic compound heating process to the carbon coating.
The embodiment of the invention also provides a preparation method of the positive active material, which comprises the following steps:
a. mixing the lithium nickel manganese oxide modified material with a carbon source and a solvent to obtain a first coating mixture;
b. drying the first coating mixture, and sintering at 120-550 ℃ for 0.5-20 hours in a non-oxidizing atmosphere to obtain a carbon-coated lithium nickel manganese oxide modified material;
c. mixing the carbon-coated lithium nickel manganese oxide modified material, an organic compound and a solvent to obtain a second coating mixture, or mixing the carbon-coated lithium nickel manganese oxide modified material, an organic compound precursor and a solvent to obtain a second coating mixture; and
d. and heating the second coating mixture at 120-400 ℃ for reaction for 2-20 hours.
In some embodiments, the preparation step of the lithium nickel manganese oxide modified material comprises the following steps:
m1, mixing a phosphorus source, a rock-like salt phase inducer and a lithium nickel manganese oxide spinel structure material to obtain a doped mixture; and
m2, sintering the doped mixture at 600-1200 ℃ for 0.5-20 hours.
The lithium nickel manganese oxide spinel structure material can be prepared by methods known to those skilled in the art. For example, it can be prepared by a low-temperature solid phase method. Specifically, nickel salt, manganese salt, lithium hydroxide and oxalic acid can be mixed and ball-milled to prepare a precursor, and then the precursor is calcined at high temperature to obtain the lithium nickel manganese oxide spinel structure material.
The phosphorus source may be selected from one or more of nickel phosphate, cobalt phosphate, manganese phosphate, magnesium phosphate, calcium phosphate, iron phosphate, copper phosphate, zinc phosphate, titanium phosphate, zirconium phosphate, lithium phosphate, cobalt pyrophosphate, nickel pyrophosphate, manganese pyrophosphate, magnesium pyrophosphate, calcium pyrophosphate, iron pyrophosphate, copper pyrophosphate, zinc pyrophosphate, titanium pyrophosphate, zirconium pyrophosphate, ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, lithium dihydrogen phosphate, dilithium hydrogen phosphate, lithium pyrophosphate, pyrophosphoric acid, phosphoric acid, and phosphorus pentoxide.
The halite-like phase inducer may include one or more of an oxide, a salt of the placeholder element. For example, MgO, ZnO, Fe2O3、CoO、TiO、Cr2O3、Y2O3、Sc2O3、RuO2、CuO、MoO3、GeO2、WO3、ZrO2、CaO、Ta2O5、Al2O3、Nb2O、Nb2O5、B2O3、SiO2、Al(OH)3、H3BO3、NaAlO2、Na2SiO3And the like.
The rock-like salt phase inducer may include one or more of organic or inorganic acids, such as hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, acetic acid, formic acid, oxalic acid, citric acid, and the like. The organic or inorganic acid may promote the production of the rock-like salt phase. The rock-like salt phase inducer can be one or more of organic acid or inorganic acid, and can also comprise one or more of oxides and salts of the space-occupying elements. In some cases, the lithomimetic salt phase inducing agent may also be an oxide of the placeholder element, alone, or in combination with one or more of the salts.
Specifically, the placeholder elements are composed of elements other than Ni, Mn, such as one or more of Mg, Zn, Fe, Co, Ti, Cr, Y, Sc, Ru, Cu, Mo, Ge, W, Zr, Ca, Ta, Sr, Al, Nb, B, Si, F, S, etc., in which case the rock salt phase inducer is one or more of oxides and salts of Mg, Zn, Fe, Co, Ti, Cr, Y, Sc, Ru, Cu, Mo, Ge, W, Zr, Ca, Ta, Sr, Al, Nb, B, Si, F and S, or further including organic or inorganic acids. The placeholder elements consist of Ni, Mn, in which case the rock salt phase inducer may consist of a metal-free acidic compound, or one or more of the oxides or salts of Ni and Mn and one or more of organic acids, inorganic acids.
The mass ratio of the phosphorus source, the rock-like salt phase inducer and the lithium nickel manganese oxide spinel structural material can be 1: the ratio before (20-400) may be, for example, 1:1:50, 1:1:80, 1:1:100, 1:1:150, 1:1:200, 1: 1:250,1:1:300,1:1:350.
In step m1, the phosphorus source, the rock-salt-like phase inducer, and the lithium nickel manganese oxide spinel structure material may be mixed by methods known to those skilled in the art, such as mechanical mixing, ultrasonication, ball milling, and the like.
Preferably, the sintering process of the doping mixture of step m2 is: heating to 600-1200 ℃ at the heating rate of 0.5-10 ℃/min, then sintering for 0.5-20 hours, and then cooling to room temperature at the cooling rate of 0.5-10 ℃/min. The sintering temperature may be 600 deg.C, 650 deg.C, 700 deg.C, 800 deg.C, 900 deg.C, 1000 deg.C, 1100 deg.C, 1200 deg.C.
The sintering in step m2 may be performed under oxygen, air, an atmosphere containing a reducing gas such as hydrogen, or an atmosphere containing oxygen and an atmosphere containing a reducing gas such as nitrogen or argon.
In some embodiments, a precursor of the organic compound is imidized or polymerized to obtain the organic compound. Imidization, namely, carrying out imidization on the surface of the carbon-coated lithium nickel manganese oxide modified material by an organic compound precursor to synthesize the organic compound with an imine group in situ.
In some embodiments, the organic compound precursor may be selected from any one or more of polyamic acid, pyrrole, aniline, methyl methacrylate, and carbazole.
In some embodiments, the organic compound precursor can be selected from polyamic acids, which can be selected from quaternary polyamic acids. In some embodiments, the quaternary polyamic acid can be selected from any one or more of pyromellitic dianhydride, biphenyl dianhydride, phenylenediamine, and oxydianiline.
Wherein the polyamic acid can be prepared by reacting an aromatic anhydride with a diamine at the same equivalent weight.
In some embodiments, the aromatic anhydride may be selected from any one of the following or a mixture of two or more of the following: phthalic anhydride, pyromellitic dianhydride, 3',4,4' -biphenyltetracarboxylic dianhydride, 4', 4-oxydiphthalic anhydride, 3',4,4' -benzophenonetetracarboxylic dianhydride, trimellitic glycol, 4,4' - (4', 4-isopropylbisphenoxy) diphthalic anhydride and trimellitic anhydride.
In some embodiments, the diamine may be selected from any one of the following or a mixture of two or more of the following: 4,4 '-oxydianiline, p-phenylenediamine, 2-bis (4- (4-aminophenoxy) -phenyl) propane, p-methylenedianiline, propyltetramethyldisiloxane, polyarylamine, 4' -diaminodiphenylsulfone, 2 '-bis (trifluoromethyl) -4,4' -diaminobiphenyl, and 3, 5-diamino-1, 2, 4-triazole.
In some embodiments, the mass ratio of the organic compound precursor to the lithium nickel manganese oxide modified material is (0.5-20): 1000. Specifically, the mass ratio of the organic compound precursor to the lithium nickel manganese oxide-modified material is within the above range, but not limited to, 0.5:1000, 1:1000, 2:1000, 3:1000, 4:1000, 5:1000, 10:1000, 15:1000, and 20: 1000.
Preferably, the step of obtaining the first coating mixture in step b comprises: dissolving a carbon source in a solvent to obtain a carbon source solution, and then mixing the carbon source solution with the lithium nickel manganese oxide modified material. Through dilution, the uniformity of carbon coating on the surface of the lithium nickel manganese oxide modified material is improved. In some embodiments, the solvent may be selected from one or more of the following solvents: deionized water, ethanol, tetrahydrofuran, or acetone.
Preferably, the first coating mixture is evaporated to dryness at low temperature under vacuum before sintering in step b. The vacuum evaporation temperature may be, for example, 20 ℃ to 50 ℃. The sintering temperature can be 200 deg.C, 250 deg.C, 300 deg.C, 350 deg.C, 400 deg.C, 450 deg.C, 500 deg.C, and 550 deg.C. In a non-oxidizing atmosphere (such as inert gas or vacuum gas), the lithium nickel manganese oxide modified material is easily decomposed at high temperature.
The step of obtaining a second coating mixture of step c comprises: and dissolving an organic compound in a solvent to obtain an organic compound solution, and then mixing the organic compound solution with the lithium nickel manganese oxide modified material. Through dilution, the uniformity of the organic compound coated on the surface of the carbon-coated lithium nickel manganese oxide modified material is improved. In some embodiments, the solvent may be selected from one or more of the following solvents: cyclohexane, carbon tetrachloride, chloroform, dichloromethane, dimethylformamide, dimethylacetamide and N-methylpyrrolidone.
In the step d, the heating temperature is below 400 ℃, the heating temperature is not too high, and the decomposition of organic compounds is avoided. Preferably, the coating mixture is stirred during heating. The heating temperature may be 200 deg.C, 220 deg.C, 250 deg.C, 280 deg.C, 300 deg.C, 330 deg.C, 360 deg.C, 380 deg.C, 400 deg.C. Preferably, the heating reaction process of step d is as follows: heating the second coating mixture to 55-65 ℃ for 20-40 minutes, continuously heating to 115-125 ℃ for 20-40 minutes, continuously heating to 195-205 ℃ for 50-70 minutes, continuously heating to 290-310 ℃ for 50-70 minutes, and continuously heating to 390-400 ℃ for 8-15 minutes. By maintaining for a certain period of time at a specific temperature interval, the coating uniformity and bonding firmness of the in-situ synthesized organic compound layer can be improved.
The invention also provides a positive electrode of the lithium ion secondary battery, which comprises a positive electrode current collector and a positive electrode active material layer positioned on the positive electrode current collector, wherein the positive electrode active material layer comprises the positive electrode active material.
As the positive electrode current collector, a conductive member formed of a highly conductive metal as used in a positive electrode of a lithium ion secondary battery of the related art is preferable. For example, aluminum or an alloy including aluminum as a main component may be used. The shape of the positive electrode current collector is not particularly limited, since it may vary depending on the shape of the lithium ion secondary battery, etc. For example, the positive electrode collector may have various shapes such as a rod shape, a plate shape, a sheet shape, and a foil shape.
The positive active material layer further includes a conductive additive and a binder.
The conductive additive may be a conductive additive that is conventional in the art, and the present invention is not particularly limited thereto. For example, in some embodiments, the conductive additive is carbon black (e.g., acetylene black or Ketjen black).
The binder may be a binder conventional in the art, and the present invention is not particularly limited thereto, and may be composed of polyvinylidene fluoride (PVDF), or carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR). In some embodiments, the binder is polyvinylidene fluoride (PVDF).
The present invention also provides a lithium ion secondary battery comprising:
the positive electrode as described above;
a negative electrode including a negative electrode current collector and a negative electrode active material layer on the negative electrode current collector;
a separator and an electrolyte.
As a current collector of the negative electrode,
the negative electrode, separator and electrolyte may employ negative electrode current collectors, separators and electrolyte materials that are conventional in the art, and the present invention is not particularly limited thereto.
The negative electrode current collector may be copper, and the shape of the negative electrode current collector is also not particularly limited, and may be rod-shaped, plate-shaped, sheet-shaped, and foil-shaped, and may vary depending on the shape of the lithium ion secondary battery, and the like. The negative active material layer includes a negative active material, a conductive additive, and a binder. The negative active material, conductive additive and binder are also conventional in the art. In some embodiments, the negative active material is metallic lithium. The conductive additives and binders are as described above and will not be described in detail here.
The separator may be a separator used in a general lithium ion secondary battery, such as a polyolefin-based film, for example, a microporous film made of polyethylene or polypropylene; a multi-layer film of a porous polyethylene film and polypropylene; nonwoven fabrics formed of polyester fibers, aramid fibers, glass fibers, and the like; and a base film formed by adhering ceramic fine particles such as silica, alumina, and titania to the surfaces thereof. In some embodiments, the separator is a three layer film of PP/PE/PP coated on both sides with alumina.
The electrolyte may include an electrolyte and a non-aqueous organic solvent. The electrolyte is preferably LiPF6、LiBF4、LiSbF6、LiAsF6. The non-aqueous organic solvent can be carbonate, ester and ether. Among them, carbonates such as Ethylene Carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC) and ethylmethyl carbonate (EMC) can be preferably used. In some embodiments, the electrolyte is LiPF6The non-aqueous electrolyte of Ethylene Carbonate (EC)/dimethyl carbonate (DMC) with the concentration of 1mol/L, wherein the volume ratio of EC to DMC is 1: 1.
The following are specific examples, which are intended to provide further detailed description of the present invention and to assist those skilled in the art and researchers in understanding the present invention, and the technical conditions and the like are not intended to limit the present invention. Any modification made within the scope of the claims of the present invention is within the scope of the claims of the present invention.
In the following examples, STEM was performed using a spherical aberration correcting scanning transmission microscope model JEM ARM200F (JEOL, Tokyo, Japan); x-ray photoelectron Spectroscopy (XPS) an ESCALB 250 model X-ray photoelectron spectrometer manufactured by Thermo Fisher was used to study the types of surface elements and chemical environment of powder samples, wherein the X-ray radiation source was Mg K alpha.
Example 1
18g of LiNi0.5Mn1.5O4Materials (Shandong Qixing energy materials Co., Ltd.), 0.54g CuO and 0.267g (NH)4)2HPO4And uniformly mixing, calcining the obtained mixture in oxygen at 600 ℃ for 5h, wherein the heating rate is 3 ℃/min, and the cooling rate is 5 ℃/min, so as to obtain the phosphorus element gradient doped lithium nickel manganese oxide modified material.
Dissolving 0.8g of C source 10,10 '-dibromo-9, 9' -bianthracene in ethanol to obtain a clear solution, adding 12g of the phosphorus element gradient-doped lithium nickel manganese oxide modified material into the clear solution, uniformly stirring, heating to 80 ℃ to completely volatilize ethanol to obtain a first coating mixture, heating the first coating mixture in argon at 400 ℃, and preserving heat for 2 hours to obtain a surface P-doped carbon-coated lithium nickel manganese oxide modified material.
Weighing pyromellitic dianhydride and 4,4' -diaminodiphenyl ether in a molar ratio of 1:1, taking dimethylacetamide as a solvent, synthesizing a polyamic acid solution with the concentration of 10%, preparing a 1% polyamic acid solution, taking 8mL of the polyamic acid solution, stirring for 1h in an argon atmosphere, adding 10g of the carbon-coated lithium nickel manganese oxide modified material obtained in the previous step to obtain a second coating mixture, heating and stirring, keeping at 60 ℃ for 30 minutes, keeping at 120 ℃ for 30 minutes, keeping at 200 ℃ for 60 minutes, keeping at 300 ℃ for 60 minutes and keeping at 400 ℃ for 10 minutes, and carrying out imidization to finally obtain the carbon and polyimide jointly coated lithium nickel manganese oxide positive electrode active material.
FIG. 1 shows a STEM diagram of a phosphorus-doped lithium nickel manganese oxide modified material prepared in example 1. As can be seen from FIG. 1, the surface of the material has a rock-like salt phase generated by occupying 16c atoms of spinel octahedrons, and the thickness of the rock-like salt phase is about 12 nm.
Fig. 2 is a STEM line scan diagram of the phosphorus content of the surface of the lithium nickel manganese oxide modified material prepared in example 1, and it can be seen from fig. 2 that the surface of the doped lithium nickel manganese oxide has no coating layer, and it can be seen from fig. 1 that the phosphorus element is distributed in the rock-like salt phase, and the content of the phosphorus element gradually decreases from the surface to the inside.
FIG. 3 shows a TEM of the coated lithium nickel manganese oxide positive electrode active material of the example, in which a coating layer is formed on the surface of the lithium nickel manganese oxide modified material, and the thickness of the coating layer is about 6 nm.
Example 2
18g of LiNi0.4Mn1.6O4Materials (Shandong Qixing energy materials Co., Ltd.), 0.54gH3PO4And 1g of oxalic acid are uniformly mixed, and the obtained mixture is calcined for 5 hours at the temperature of 600 ℃ in oxygen, the heating rate is 3 ℃/minute, and the cooling rate is 5 ℃/minute, so that the phosphorus element gradient doped lithium nickel manganese oxide modified material is obtained.
Dissolving 1g of C-source glucose in ethanol to obtain a clear solution, adding 12g of the phosphorus element gradient doped lithium nickel manganese oxide modified material into the clear solution, uniformly stirring, heating to 80 ℃ to completely volatilize ethanol to obtain a first coating mixture, heating the first coating mixture in argon at 350 ℃, and preserving heat for 5 hours to obtain a surface P-doped carbon-coated lithium nickel manganese oxide modified material.
Preparing a 4% polyamide acid solution by taking N-methylpyrrolidone as a solvent, then preparing a 1% polyamide acid solution, taking 8mL of the polyamide acid solution, stirring for 1h in an argon atmosphere, adding 10g of the carbon-coated lithium nickel manganese oxide modified material obtained in the previous step, heating and stirring, keeping at 55 ℃ for 30 minutes, keeping at 125 ℃ for 30 minutes, keeping at 205 ℃ for 60 minutes, keeping at 305 ℃ for 60 minutes and keeping at 390 ℃ for 10 minutes to implement imidization, and finally obtaining the carbon and polyimide jointly-coated lithium nickel manganese oxide positive electrode active material.
Fig. 4 shows STEM graphs of the phosphorus-doped lithium nickel manganese oxide modified material prepared in example 2, wherein (a) and (b) are images at different magnifications, respectively. As can be seen from fig. 4, the surface of the material has a rock-like salt phase generated by occupying spinel octahedron 8a atoms, and the thickness of the rock-like salt phase is about 10 nm.
FIG. 5 shows the relative content change of phosphorus on the surface of the lithium nickel manganese oxide modified material obtained in example 2 by XPS characterization at different etching depths, and we can see that the content of phosphorus decreases from the surface to the inside with the increase of the etching depth.
Example 3
180g of LiNi0.5Mn1.5O4Materials (Shandong Qixing energy materials Co., Ltd.), 5.4g of CuO and 2.67g of (NH)4)2HPO4And uniformly mixing, calcining the obtained mixture in oxygen at 625 ℃ for 5h, wherein the heating rate is 3 ℃/min, and the cooling rate is 5 ℃/min, so as to obtain the phosphorus element gradient doped lithium nickel manganese oxide modified material.
Dissolving 8g of C source 10,10 '-dibromo-9, 9' -bianthracene in ethanol to obtain a clear solution, adding 120g of the phosphorus element gradient-doped lithium nickel manganese oxide modified material into the clear solution, uniformly stirring, heating to 80 ℃ to completely volatilize ethanol to obtain a first coating mixture, heating the first coating mixture in argon at 550 ℃, and preserving heat for 1h to obtain a surface P-doped carbon-coated lithium nickel manganese oxide modified material.
Preparing a pyrrole solution with the mass concentration of 1% by taking ethanol as a solvent, taking 80mL of the pyrrole solution, stirring for 1h in an argon atmosphere, adding 100g of the lithium nickel manganese oxide modified material obtained in the step, heating and stirring, keeping at 55 ℃ for 30 min, keeping at 100 ℃ for 30 min, and keeping at 150 ℃ for 60 min to finally obtain the surface gradient P-doped lithium nickel manganese oxide positive electrode active material coated with carbon and polypyrrole.
Comparative example 1
Comparative example 1 is substantially the same as example 1 except that the lithium nickel manganese oxide-modified material is not doped with phosphorus, i.e., no phosphorus source (NH) is added during the preparation process4)2HPO4(coating the surface of the nickel lithium manganate not doped with phosphorus).
Comparative example 2
Comparative example 2 is substantially the same as example 1 except that the phosphorus-doped lithium nickel manganese oxide modified material is coated with only carbon and not with polyimide.
Comparative example 3
Comparative example 3 is substantially the same as example 1 except that the phosphorus-doped lithium nickel manganese oxide modified material is coated with only polyimide and is not coated with carbon.
Comparative example 4
Comparative example 4 is substantially the same as example 1 except that the phosphorus-doped lithium nickel manganese oxide modified material is not coated with polyimide and carbon.
Comparative example 5
Comparative example 5 is substantially the same as example 1 except that the lithium nickel manganese oxide-modified material is not doped with phosphorus and is not further coated with polyimide and carbon.
Comparative example 6
Comparative example 6 is substantially the same as example 2 except that the lithium nickel manganese oxide-modified material is not doped with phosphorus, i.e., no phosphorus source (NH) is added during the preparation process4)2HPO4(in the absence of phosphorus doping)Surface coating of lithium nickel manganese oxide).
Comparative example 7
Comparative example 7 is substantially the same as example 2 except that the phosphorus-doped lithium nickel manganese oxide modified material is coated with only carbon and not with polyimide.
Comparative example 8
Comparative example 8 is substantially the same as example 2 except that the phosphorus-doped lithium nickel manganese oxide modified material is coated with only polyimide and not carbon.
Comparative example 9
Comparative example 9 is substantially the same as example 2 except that the phosphorus-doped lithium nickel manganese oxide modified material is not coated with polyimide and carbon.
Comparative example 10
Comparative example 10 is substantially the same as example 2 except that the lithium nickel manganese oxide-modified material is not doped with phosphorus and is not further coated with polyimide and carbon.
The positive active materials prepared in examples 1 to 3 and comparative examples 1 to 10 were assembled into a button cell according to the following procedure.
(1) Preparation of Positive electrode sheet
The positive electrode active materials prepared in examples and comparative examples, carbon black as a conductive additive, and polyvinylidene fluoride (PVDF) as a binder were dispersed in N-methylpyrrolidone (NMP) at a weight ratio of 80:8:10, and mixed uniformly to prepare a uniform positive electrode slurry. Uniformly coating the uniform positive electrode slurry on an aluminum foil current collector with the thickness of 15 mu m, drying at 55 ℃ to form a pole piece with the thickness of 100 mu m, and rolling the pole piece under a roller press (the pressure is about 1MPa multiplied by 1.5 cm)2) Cut into the diameter of
Figure BDA0002781922790000171
Then the round piece is placed in a vacuum oven to be dried for 6 hours at the temperature of 120 ℃, and after natural cooling, the round piece is taken out and placed in a glove box to be used as a positive pole piece.
(2) Assembling lithium ion secondary battery
And (2) in a glove box filled with inert atmosphere, taking metal lithium as the negative electrode of the battery, taking a PP/PE/PP three-layer film coated with alumina on two sides as a diaphragm, placing the diaphragm between the positive electrode and the negative electrode, dropwise adding the diaphragm into conventional carbonate electrolyte, taking the positive electrode piece prepared in the step (1) as the positive electrode, and assembling the button battery with the model number of CR 2032.
Cycle testing
(1) High-temperature circulation:
and standing the prepared button cell for 10 hours at room temperature (25 ℃), then carrying out charge-discharge activation on the button cell, and then carrying out charge-discharge cycle test on the prepared button cell by adopting a blue cell charge-discharge tester. The method comprises the steps of firstly cycling at a rate of 0.1C for 1 week under the condition of room temperature (25 ℃), and then continuing cycling at a rate of 0.2C for 4 weeks, wherein the charging and discharging voltage range of the battery is controlled to be 3.5V-4.9V. Then, the button cell is transferred to a high-temperature environment of 55 ℃, the circulation is continued for 50 weeks at the multiplying power of 0.2C, and the charging and discharging voltage range of the cell is still controlled to be 3.5V-4.9V.
(2) And (3) room temperature circulation:
and standing the prepared button cell for 10 hours at room temperature (25 ℃), then carrying out charge-discharge activation on the button cell, and then carrying out charge-discharge cycle test on the prepared button cell by adopting a blue cell charge-discharge tester. The method comprises the steps of firstly cycling at a rate of 0.1C for 1 week under the condition of room temperature (25 ℃), and then continuing cycling at a rate of 0.2C for 200 weeks, wherein the charging and discharging voltage range of the battery is controlled to be 3.5V-4.9V.
The experimental data are shown in tables 1 and 2.
TABLE 1 electrochemical Properties of Positive electrode active Material of examples of the present invention
Figure BDA0002781922790000181
Figure BDA0002781922790000191
TABLE 2 electrochemical Properties of the positive electrode active materials of the present invention in respective comparative examples
Figure BDA0002781922790000192
The result shows that compared with the nickel lithium manganate active material not doped with phosphorus, the capacity retention rate, the cycle performance and the electronic conductance of the battery are greatly improved after surface phosphorus doping, because harmful side reactions between the positive active material and the electrolyte are relieved after phosphorus gradient doping, the decomposition of the electrolyte and the dissolution of Mn/Ni are inhibited, and the cycle stability and the activity of the battery are improved.
Compared with the material without the organic compound, the co-coating of the carbon and the organic compound can reduce the decomposition of the carbon coated on the surface and the reaction with the electrolyte, and the co-coating of the phosphorus doping, the carbon and the organic compound are matched with each other, so that the coating effect can be improved, and the surface stability and the electronic conductivity of the cathode active material after the surface coating are further improved. Also, the coating of the organic compound improves the lithium ion conductivity of the positive electrode active material.
All possible combinations of the technical features of the above embodiments may not be described for the sake of brevity, but should be considered as within the scope of the present disclosure as long as there is no contradiction between the combinations of the technical features.
The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is specific and detailed, but not to be understood as limiting the scope of the invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.

Claims (19)

1. The positive active material is characterized by comprising a lithium nickel manganese oxide modified material and a coating layer on the surface of the lithium nickel manganese oxide modified material, wherein the coating layer comprises carbon and an organic compound, the organic compound has-N bond or-COO bond, the carbon and the organic compound are distributed in a layered mode, a carbon layer is distributed close to the lithium nickel manganese oxide modified material, and an organic compound layer is superposed on the carbon layer;
the lithium nickel manganese oxide modified material comprises primary particles of a spinel phase and a rock-like salt phase, wherein the spinel phase is a core, and the rock-like salt phase is distributed on the surface of the spinel phase to form a shell;
the spinel phase is of a lithium nickel manganese oxide spinel structure;
the rock-salt-like phase is induced by the lithium nickel manganese oxide spinel structure and comprises at least one space occupying element of Mg, Zn, Ni, Mn, Fe, Co, Ti, Cr, Y, Sc, Ru, Cu, Mo, Ge, W, Zr, Ca, Ta, Sr, Al, Nb, B, Si, F and S, and the space occupying element is positioned at the 16c or 8a position of the spinel phase;
the rock-like salt phase is also doped with phosphorus elements, and the phosphorus elements are distributed in a gradient manner from the outer surface to the inner part of the rock-like salt phase.
2. The positive electrode active material according to claim 1, wherein a mass ratio of carbon to the organic compound in the coating layer is 1: (0.1-10).
3. The positive electrode active material according to claim 1, wherein the organic compound is one or more selected from the group consisting of polyimide, polypyrrole, polyaniline, polymethyl methacrylate, and polycarbazole.
4. The positive electrode active material according to claim 1, wherein the lithium nickel manganese oxide spinel structure has a chemical formula of Li1+xNi0.5-yMn1.5-zOuWherein x is more than or equal to 0.2 and less than or equal to 0.2, y is more than or equal to 0.2 and less than or equal to 0.2, z is more than or equal to 0.2 and u is more than or equal to 3.8 and less than or equal to 4.2.
5. The positive electrode active material according to claim 1, wherein the nickel isThe chemical formula of the lithium manganate spinel structure is Li1+xNi0.5-yMn1.5-zMsOuWherein M is at least one of Mg, Zn, Ni, Mn, Fe, Co, Ti, Cr, Y, Sc, Ru, Cu, Mo, Ge, W, Zr, Ca, Ta, Sr, Al, Nb, B, Si, F and S, x is more than or equal to-0.2 and less than or equal to-0.2, z is more than or equal to-0.2 and less than or equal to-0.2, S is more than 0 and less than or equal to-0.2, and u is more than or equal to 3.8 and less than or equal to 4.2.
6. The positive electrode active material according to any one of claims 1 to 5, wherein the coating layer of the lithium nickel manganese oxide-modified material has a thickness of 2nm to 20 nm.
7. The positive electrode active material according to any one of claims 1 to 5, wherein the spinel phase has a thickness of 0.1 to 30 μm.
8. The positive electrode active material according to any one of claims 1 to 5, wherein the thickness of the rock-salt-like phase is 0.5nm to 50 nm.
9. The positive electrode active material according to any one of claims 1 to 5, wherein the concentration of the phosphorus element in the primary particles gradually decreases from the outer surface toward the inner surface, the phosphorus element forms a phosphorus gradient doped layer, and the thickness of the phosphorus gradient doped layer is 0.5nm to 40 nm.
10. A method for producing a positive electrode active material according to any one of claims 1 to 9, comprising the steps of:
a. mixing the lithium nickel manganese oxide modified material with a carbon source and a solvent to obtain a first coating mixture;
b. drying the first coating mixture, and sintering the mixture for 0.5 to 20 hours at the temperature of between 200 and 550 ℃ in a non-oxidizing atmosphere to obtain a carbon-coated lithium nickel manganese oxide modified material;
c. mixing the carbon-coated lithium nickel manganese oxide modified material, the organic compound and a solvent to obtain a second coating mixture, or mixing the carbon-coated lithium nickel manganese oxide modified material, a precursor of the organic compound and the solvent to obtain the second coating mixture; and
d. and heating the second coating mixture at 120-400 ℃ for reaction for 2-20 hours.
11. The method for producing a positive electrode active material according to claim 10, wherein a precursor of the organic compound is imidized or polymerized to obtain the organic compound.
12. The method for preparing a positive electrode active material according to claim 10, wherein the precursor of the organic compound is one or more selected from polyamic acids, azoles, anilines, methyl methacrylates, and carbazoles.
13. The method for producing a positive electrode active material according to claim 12, wherein the polyamic acid is selected from quaternary polyamic acids.
14. The method for preparing a positive electrode active material according to claim 13, wherein the quaternary polyamic acid is selected from any one or more of pyromellitic dianhydride, biphenyl dianhydride, phenylenediamine, and oxydianiline.
15. The method for preparing a positive electrode active material according to claim 10, wherein the heating reaction process of the step c is: heating the second coating mixture to 55-65 ℃ for 20-40 minutes, continuously heating to 115-125 ℃ for 20-40 minutes, continuously heating to 195-205 ℃ for 50-70 minutes, continuously heating to 290-310 ℃ for 50-70 minutes, and continuously heating to 390-400 ℃ for 8-15 minutes.
16. The method for preparing the positive electrode active material according to claim 10, wherein the step of preparing the lithium nickel manganese oxide modified material comprises:
mixing a phosphorus source, a rock-like salt phase inducer and a lithium nickel manganese oxide spinel structure material to obtain a doped mixture; and
sintering the doped mixture at 600-1200 ℃ for 0.5-20 hours.
17. The method for preparing a positive electrode active material according to claim 16, wherein the sintering process of the doping mixture is: heating to 600-1200 ℃ at the heating rate of 0.5-10 ℃/min, then sintering for 0.5-20 hours, and then cooling to room temperature at the cooling rate of 0.5-10 ℃/min.
18. A positive electrode for a lithium ion secondary battery, comprising a positive electrode current collector and a positive electrode active material layer on the positive electrode current collector, wherein the positive electrode active material layer comprises the positive electrode active material according to any one of claims 1 to 9.
19. A lithium-ion secondary battery characterized by comprising:
the positive electrode according to claim 18;
a negative electrode including a negative electrode current collector and a negative electrode active material layer on the negative electrode current collector;
a separator and an electrolyte.
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Citations (2)

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Publication number Priority date Publication date Assignee Title
JP2015099662A (en) * 2013-11-18 2015-05-28 旭化成株式会社 Positive electrode active material, positive electrode, and nonaqueous electrolyte secondary battery
CN108400291A (en) * 2018-01-16 2018-08-14 浙江衡远新能源科技有限公司 A kind of lithium ion battery composite cathode material and preparation method thereof

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2015099662A (en) * 2013-11-18 2015-05-28 旭化成株式会社 Positive electrode active material, positive electrode, and nonaqueous electrolyte secondary battery
CN108400291A (en) * 2018-01-16 2018-08-14 浙江衡远新能源科技有限公司 A kind of lithium ion battery composite cathode material and preparation method thereof

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