CN114512644A - 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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CN114512644A
CN114512644A CN202011286744.6A CN202011286744A CN114512644A CN 114512644 A CN114512644 A CN 114512644A CN 202011286744 A CN202011286744 A CN 202011286744A CN 114512644 A CN114512644 A CN 114512644A
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positive electrode
manganese oxide
lithium nickel
nickel manganese
active material
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CN114512644B (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
    • 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
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    • 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
    • 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
    • H01M2004/021—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

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 inorganic compound; 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, generates a large amount of side reaction products, and is not favorable for 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 inorganic compound, the inorganic compound is selected from any one or more of oxide, fluoride, phosphide and boride, the carbon and the inorganic compound are uniformly distributed in the coating layer, or the carbon and the inorganic compound are distributed in a layered mode, a carbon layer is distributed close to the lithium nickel manganese oxide modified material, and an inorganic 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 to form a phosphorus gradient doped layer.
In one embodiment, the mass ratio of carbon to the inorganic compound in the coating layer is 1: (0.1-10).
In one embodiment, the inorganic compound includes an oxide, fluoride, phosphide or boride of any of Li, Mg, Zn, Ni, Mn, Fe, Co, Ti, Y, Sc, Ru, Cu, Mo, Ge, W, Zr, Ca, Ta, Al, Nb, B, Si, F, S, P and Sr.
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 the 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 ratio of the lithium nickel manganese oxide modified material coating layer 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, the concentration of the phosphorus element in the primary particles is gradually decreased from the outer surface toward the inner surface.
In one embodiment, the thickness of the phosphorus gradient doped layer is 0.5nm to 40 nm.
A preparation method of the positive active material comprises the following steps:
a1, mixing an inorganic compound or an inorganic compound precursor, a carbon source, the lithium nickel manganese oxide modified material and a solvent to obtain a coating mixture;
b1, drying the coating mixture, and heating at 180-550 ℃ for 0.2-24 hours;
alternatively, the first and second electrodes may be,
a2, mixing a lithium nickel manganese oxide modified material, a carbon source and a solvent to obtain a first coating mixture;
2, drying the first coating mixture, and sintering at 180-550 ℃ for 0.5-20 hours to obtain a carbon-coated lithium nickel manganese oxide modified material;
c2, mixing the carbon-coated lithium nickel manganese oxide modified material, an inorganic compound or an inorganic compound precursor and a solvent to obtain a second coating mixture;
d2, drying the second coating mixture, and heating at 180-550 ℃ for 0.2-24 hours.
In one embodiment, the step of providing 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 inorganic 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 inorganic compound are coated together, and the inorganic compound and the carbon are uniformly distributed or the inorganic 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. In addition, the inorganic compound coating layer can improve the interface stability of the lithium nickel manganese oxide modified material.
The surface of the lithium nickel manganese oxide modified material is of a rock-like salt phase structure, a rock-like salt phase surface layer is beneficial to gradient doping of phosphorus elements on the surface of a spinel anode, and the surface doping of P elements can obviously improve the electrochemical performance of the spinel anode active material, including first effect, average efficiency and circulation stability. Meanwhile, the surface gradient P doping of the lithium nickel manganese oxide modified material can improve the electronic conductivity of the surface of the positive active material, the P element doped modified material is coated in the carbon, 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. In addition, a rock-like salt phase structure is introduced on the surface of the lithium nickel manganese oxide while P is doped, the structure is more matched with the lattice constant of an inorganic compound, especially an oxide, and is favorable for the close combination of a coating and the surface of a lithium nickel manganese oxide modified material, so that the coating is more uniform and firmer, and the modification effect is better.
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 an XPS plot of a coated lithium nickel manganese oxide positive active material prepared in example 1 of the present invention;
FIG. 5 is a STEM of a phosphorus-doped lithium nickel manganese oxide modified material obtained in example 2 of the present invention;
fig. 6 shows the relative content change of surface phosphorus element obtained by XPS characterization of the phosphorus-doped lithium nickel manganese oxide modified material prepared in example 2 of the present invention 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 inorganic compound, the inorganic compound is selected from any one or more of oxides, fluorides, phosphides and borides, the carbon and the inorganic compound are uniformly distributed in the coating layer, or the carbon and the inorganic compound are distributed in a layered manner, a carbon layer is distributed close to the lithium nickel manganese oxide modified material, and an inorganic 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 to form a phosphorus gradient doped layer.
The invention firstly carries out P element gradient doping on the surface of a lithium nickel manganese oxide spinel structure to obtain a lithium nickel manganese oxide modified material, and then carries out carbon and inorganic compound co-coating on the basis of the material to obtain the cathode 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 inorganic compound are coated together, and the inorganic compound and the carbon are uniformly distributed or the inorganic 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. In addition, the inorganic compound coating layer can improve the interface stability of the lithium nickel manganese oxide modified material.
The surface of the lithium nickel manganese oxide modified material is of a rock-like salt phase structure, a rock-like salt phase surface layer is beneficial to gradient doping of phosphorus elements on the surface of a spinel anode, and the surface doping of P elements can obviously improve the electrochemical performance of the spinel anode active material, including first effect, average efficiency and circulation stability. Meanwhile, the surface gradient P doping of the lithium nickel manganese oxide modified material can improve the electronic conductivity of the surface of the positive active material, the P element doped modified material is coated in the carbon, 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. In addition, a rock-like salt phase structure is introduced on the surface of the lithium nickel manganese oxide while P is doped, the structure is more matched with the lattice constant of an inorganic compound, especially an oxide, and is favorable for the close combination of a coating and the surface of a lithium nickel manganese oxide modified material, so that the coating is more uniform and firmer, and the modification effect is better.
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 can exhibit a spinel structure.
In some embodiments, the bulk phase of the lithium nickel manganese oxide spinel structure, which may have the formula Li, is uniformly doped with an element that facilitates P doping1+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 oxide coating process, so that the stability and the electronic conductivity 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, and 29 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 commonly used characterization method 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 the STEM linear 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 carbon coating layer and the oxide coating layer on the surface of the anode material provided by the invention can also be characterized by characterization tools such as a high-resolution TEM, an XPS, an infrared analyzer, a Raman analyzer and a sulfur carbon analyzer, wherein the TEM can determine the coating thickness of the surface carbon layer and the components of the oxide, the XPS can determine the type of the surface metal oxide, the sulfur carbon analyzer can determine the quality of the surface coating carbon layer, and the infrared analyzer and the Raman analyzer can determine the chemical bond of the surface coating 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 inorganic compound may be selected from any one or more of oxides, fluorides, phosphides, borides. Preferably, the inorganic compound is selected from oxides, the surface lattice matching of the oxide structure and the rock salt phase structure is higher, the surface combination of the coating layer and the nickel lithium manganate modified material is facilitated, the loss of the coating layer under high pressure is reduced, and the surface stability of the positive active material is improved. The inorganic compound is an inorganic compound.
In some embodiments, the inorganic compound may include an oxide, fluoride, phosphide or boride of any of Li, Mg, Zn, Ni, Mn, Fe, Co, Ti, Y, Sc, Ru, Cu, Mo, Ge, W, Zr, Ca, Ta, Al, Nb, B, Si, F, S, P and Sr. Preferably, the inorganic compound is an oxide, fluoride, phosphide or boride of a metal element among the above elements.
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 both high activity and high stability.
In some embodiments, the mass ratio of carbon to inorganic compound in the coating layer is 1: (0.1 to 1). Under the proportion, P doping, carbon coating and inorganic compound coating are matched with each other, so that the positive active material has high electronic conductivity and interface stability.
The embodiment of the invention also provides a preparation method of the positive active material, which comprises the following steps:
a1, mixing an inorganic compound or an inorganic compound precursor, a carbon source, the lithium nickel manganese oxide modified material and a solvent to obtain a coating mixture;
b1, drying the coating mixture, and heating for 0.2-24 hours at 200-550 ℃ in a non-oxidizing atmosphere.
Alternatively, the first and second liquid crystal display panels may be,
a2, mixing a lithium nickel manganese oxide modified material, a carbon source and a solvent to obtain a first coating mixture;
2, 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;
c2, mixing the carbon-coated lithium nickel manganese oxide modified material, an inorganic compound or an inorganic compound precursor and a solvent to obtain a second coating mixture;
d2, drying the second coating mixture, and heating at 200-550 ℃ for 0.2-24 hours in a non-oxidizing atmosphere.
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 halite-like phase inducer may also be an oxide of the placeholder element, a salt, or a combination thereof.
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, ultrasonic, ball milling, etc.
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 can 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.
The invention can adopt the form of uniformly coating (a1, b1) by carbon and inorganic compounds, thus utilizing the advantages of the inorganic compounds, improving the carbon coating effect and leading the combination of carbon and the lithium nickel manganese oxide modified material to be firmer; the carbon coating and the inorganic compound coating (a2, b2, c2 and d2) can also be adopted, namely, the inorganic compound is coated on the outer layer of the carbon, so that the carbon can be further protected, and the decomposition of the carbon layer and the reaction with the electrolyte can be avoided.
In the step b1, 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, and the method adopts low-temperature carbon coating to avoid the decomposition of the active material. The heating temperature may 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. Similarly, the steps b2 and d2 adopt low-temperature heating.
In some embodiments, the inorganic compound precursor is selected from any one or more of an oxide, an organic, an inorganic fluoride, an inorganic phosphide, an inorganic boride. The organic material forms an oxide by sintering. The organic matter is selected from tetrabutyl titanate, tantalum ethoxide, niobium ethoxide and PVDF in any one or more combinations.
In some embodiments, the carbon source may be selected from one or more of 9, 10-dibromoanthracene, 10 ' -dibromo-9, 9 ' -bianthracene, 6, 11-dibromo-1, 2,3, 4-tetraphenyltriphenylene, 1,3, 5-tris (4 "-iodo-2 ' -biphenyl) benzene, cyclic olefins, citric acid, ascorbic acid, sucrose, polyvinyl alcohol, polyethylene glycol, glucose. In some embodiments, the cyclic olefin is a cyclic olefin having at least two double bonds, selected from the group consisting of cyclopentadiene, benzene, cycloheptatriene, cyclooctatetraene, and combinations of any one or more of the foregoing cyclic olefin derivatives.
In some embodiments, the solvent used in the preparation of the cathode active material may be selected from any one or a combination of more of deionized water, ethanol, tetrahydrofuran, or acetone.
In some embodiments, the mass ratio of the sum of the mass of the carbon source, the inorganic compound and the inorganic compound precursor added in the preparation process to the lithium nickel manganese oxide modified material is (0.1-0.5): 1. Specifically, the ratio may be 0.1:1, 0.2:1, 0.3:1, 0.4:1 or 0.5: 1.
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 corporation was used to study the types of surface elements and chemical environments of powder samples, wherein the X-ray radiation source was Mg K α.
Example 1
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 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 0.5g of C source 10,10 '-dibromo-9, 9' -bianthracene in ethanol to obtain a clear solution, dissolving 0.2g of tetrabutyl titanate in the clear solution to obtain a solution, adding 5g of the phosphorus element gradient doped lithium nickel manganese oxide modified material into the solution, uniformly stirring, heating to 80 ℃ to completely volatilize ethanol to obtain a coating mixture, heating the coating mixture in argon at 400 ℃, preserving heat for 2 hours to obtain a surface P doped lithium nickel manganese oxide positive electrode active material, and then co-coating titanium oxide and carbon.
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 is a TEM image of the coated lithium nickel manganese oxide positive electrode active material of the example, and it can be seen that 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 to 7 nm.
Fig. 4 is an XPS chart of the coated lithium nickel manganese oxide positive electrode active material of the example, and it can be seen that the surface of the lithium nickel manganese oxide modified material has titanium element.
Example 2
180g of LiNi0.4Mn1.6O4Material (Shandong Qixing energy Material Co., Ltd.), 5.4gH3PO4And uniformly mixing the mixture with 10g of oxalic acid, and 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 0.5g of C-source glucose in ethanol to obtain a clear solution, dissolving 1.5g of boron oxide in the clear solution, adding 5g of the phosphorus element gradient doped lithium nickel manganese oxide modified material into the solution, uniformly stirring, heating to 80 ℃ to completely volatilize ethanol to obtain a coating mixture, heating the coating mixture in argon at 300 ℃, preserving heat for 2 hours to obtain surface P doped lithium nickel manganese oxide and carbon co-coated lithium nickel manganese oxide positive active material.
Fig. 5 shows STEM graphs of phosphorus-doped lithium nickel manganese oxide modified materials prepared in example 2, wherein (a) and (b) are images at different magnifications, respectively. As can be seen from FIG. 5, 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. 6 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 LiNi0.5Mn1.5O4Material (Shandong Qixing energy Material Co., Ltd.), 5.4g Cr2O3And 2.67g (NH)4)2HPO420 ml of deionized water was added to the beaker and mixed uniformly, and the beaker was placed in an oil bath pan at 120 ℃ and heated with stirring for 5 hours to obtain a dry mixture. Calcining the obtained mixture in air at 725 ℃ for 5h, wherein the heating rate is 3 ℃/min, and the cooling rate is 5 ℃/minAnd obtaining the phosphorus element gradient doped lithium nickel manganese oxide modified material.
Dissolving 0.5g of C source 1,3, 5-tris (4 '-iodo-2' -biphenyl) benzene in ethanol to obtain a clear solution, dissolving 1g of niobium ethoxide in the clear solution, adding 5g of the phosphorus element gradient doped lithium nickel manganese oxide modified material into the solution, uniformly stirring, heating to 80 ℃ to completely volatilize ethanol to obtain a coated mixture, heating the coated mixture in vacuum at 550 ℃, and preserving heat for 2 hours to obtain a lithium nickel manganese oxide positive electrode active material with a surface P doped, and then co-coating fluoride and carbon.
Example 4
The same phosphorus-doped lithium nickel manganese oxide modified material of example 1 was prepared.
Dissolving 0.5g of C source 10,10 '-dibromo-9, 9' -bianthracene in ethanol to obtain a clear solution, adding 5g of the phosphorus element gradient-doped lithium nickel manganese oxide modified material into the 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 ℃, preserving heat for 2 hours to obtain a surface P-doped lithium nickel manganese oxide material, and then coating carbon on the surface P-doped lithium nickel manganese oxide material.
Dissolving 0.2g of tetrabutyl titanate in the clear solution, adding the obtained carbon-coated lithium nickel manganese oxide material into the tetrabutyl titanate solution, uniformly stirring, heating to 80 ℃ to completely volatilize ethanol to obtain a coated mixture, heating the coated mixture in argon at 400 ℃, preserving heat for 2 hours to obtain a surface P-doped lithium nickel manganese oxide positive active material, and then firstly coating carbon and then coating titanium oxide.
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 titanium oxide.
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 titanium oxide 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 titanium oxide 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 titanium oxide 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(coating the surface of the nickel lithium manganate not doped with phosphorus).
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 boron oxide.
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 boron oxide and is not coated with 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 boron oxide 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 boron oxide and carbon.
The positive active materials prepared in examples 1 to 4 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 BDA0002782572240000161
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 with two sides coated with alumina as a diaphragm, putting the diaphragm between the positive electrode and the negative electrode, dropwise adding conventional carbonate electrolyte, taking the positive electrode piece prepared in the step (1) as the positive electrode, and assembling into 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 BDA0002782572240000171
Figure BDA0002782572240000181
TABLE 2 electrochemical performance of positive active materials of various comparative examples of the present invention
Figure BDA0002782572240000182
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 inorganic compound, the carbon and inorganic compound co-coating can reduce the decomposition of the carbon coated on the surface and the reaction with the electrolyte, and the phosphorus doping, the carbon and the inorganic compound co-coating 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.
The technical features of the embodiments described above may be arbitrarily combined, and for the sake of brevity, all possible combinations of the technical features in the embodiments described above are not described, but should be considered as being within the scope of the present specification 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 more specific and detailed, but not construed 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 (15)

1. A positive electrode 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 inorganic compound, the inorganic compound is selected from any one or more of oxide, fluoride, phosphide and boride, the carbon and the inorganic compound are uniformly distributed in the coating layer, or the carbon and the inorganic compound are distributed in layers, a carbon layer is distributed close to the lithium nickel manganese oxide modified material, and an inorganic 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 to form a phosphorus gradient doped layer.
2. The positive electrode active material according to claim 1, wherein a mass ratio of carbon to the inorganic compound in the coating layer is 1: (0.1-10).
3. The positive electrode active material according to claim 1, wherein the inorganic compound comprises an oxide, fluoride, phosphide or boride of any of Li, Mg, Zn, Ni, Mn, Fe, Co, Ti, Y, Sc, Ru, Cu, Mo, Ge, W, Zr, Ca, Ta, Al, Nb, B, Si, F, S, P and Sr.
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 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, 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 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 is gradually decreased from the outer surface toward the inner surface.
10. The positive electrode active material according to any one of claims 1 to 5, wherein the thickness of the phosphorus gradient doped layer is 0.5nm to 40 nm.
11. A method for producing a positive electrode active material according to any one of claims 1 to 10, comprising the steps of:
a1, mixing the inorganic compound or the precursor of the inorganic compound, a carbon source, the lithium nickel manganese oxide modified material and a solvent to obtain a coating mixture;
b1, drying the coating mixture, and heating at 180-550 ℃ for 0.2-24 hours;
alternatively, the first and second electrodes may be,
a2, mixing a lithium nickel manganese oxide modified material, a carbon source and a solvent to obtain a first coating mixture;
2, drying the first coating mixture, and sintering at 180-550 ℃ for 0.5-20 hours to obtain a carbon-coated lithium nickel manganese oxide modified material;
c2, mixing the carbon-coated lithium nickel manganese oxide modified material, the inorganic compound or the precursor of the inorganic compound and a solvent to obtain a second coating mixture;
d2, drying the second coating mixture, and heating at 180-550 ℃ for 0.2-24 hours.
12. The method of preparing a positive electrode active material according to claim 11, wherein the step of providing 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.
13. The method for preparing a positive electrode active material according to claim 12, 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.
14. 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 10.
15. A lithium-ion secondary battery characterized by comprising:
the positive electrode according to claim 14;
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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