Positive electrode material, secondary battery, and electric device

Abstract

The present application relates to the technical field of batteries, and discloses a positive electrode material, a secondary battery, and an electric device. The positive electrode material comprises a core and a coating layer provided on the outer surface of the core; the core comprises a manganese-iron-lithium oxide; and the coating layer comprises crystalline carbon nitride. According to the present application, the manganese-iron-lithium oxide is coated with the crystalline carbon nitride, so that the electrical conductivity and the cycle stability of the positive electrode material can be significantly improved.

Description

Positive electrode material, secondary battery and electrical device Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a positive electrode material, a secondary battery, and an electrical device. Background Art

[0002] The research and development of positive electrode materials plays a vital role in improving the performance and application of secondary batteries. Manganese iron lithium oxide (LiFeO) offers high stability, safety, and energy density, but its poor conductivity results in poor charge and discharge performance. Furthermore, due to the Jahn-Teller effect, manganese in the LiFeO structure leaks out during battery cycling, causing structural damage. Consequently, secondary batteries made with this material exhibit poor cycling performance.

[0003] Therefore, how to improve the conductivity and cycle stability of manganese iron lithium oxide has become a technical problem that needs to be solved urgently by those skilled in the art.

[0004] Summary of the Invention

[0005] The purpose of this application is to overcome the shortcomings of the existing technology and provide a positive electrode material, a secondary battery and an electrical device, wherein the positive electrode material has excellent conductivity and cycle stability.

[0006] To achieve the above objectives, the first aspect of the present application provides a positive electrode material, comprising a core and a coating layer arranged on the outer surface of the core, the core comprising manganese iron lithium oxide, and the coating layer comprising crystalline carbon nitride.

[0007] As an embodiment of the present application, the cathode material of the present application further satisfies at least one of the following (a)-(d):

[0008] (a) The positive electrode material has an X-ray diffraction pattern having a first diffraction peak in the range of 27.5-29.0°;

[0009] (b) the X-ray diffraction pattern of the positive electrode material has a second diffraction peak in the range of 7.5-9.0°;

[0010] (c) The positive electrode material has a peak in the nitrogen 1s spectrum within the range of 396.5-398.0 eV;

[0011] (d) The carbon 13 solid nuclear magnetic resonance spectrum of the positive electrode material has a peak in the range of 118-126 ppm.

[0012] As an embodiment of the present application, the ratio of the peak intensity of the first diffraction peak to the peak intensity of the strongest peak of the X-ray diffraction pattern of the positive electrode material is 1:(3-15); and / or

[0013] The ratio of the peak intensity of the second diffraction peak to that of the first diffraction peak is 1:(1.5 - 15).

[0014] As an embodiment of the present application, the thickness of the coating layer is 10 - 150 nm. Preferably, the thickness of the coating layer is 50 - 80 nm.

[0015] As an embodiment of the present application, the coating layer accounts for 0.2 - 5 wt% of the total mass of the positive electrode material.

[0016] As an embodiment of the present application, the positive electrode material contains nitrogen element, and the atomic percentage content of the nitrogen element in the positive electrode material is 1 - 7 at%.

[0017] As an embodiment of the present application, the positive electrode material contains an alkali metal element. The alkali metal includes at least one of K and Na, and the atomic percentage content of the alkali metal element in the positive electrode material is 0.1 - 1.2 at%.

[0018] As an embodiment of the present application, the lithium manganese iron oxide includes a compound with the molecular formula Li a Mn x Fe 1-x M 1-a PO4, where 0 < x < 1, 0.9 ≤ a ≤ 1, and M includes at least one of In, La, Zr, Ce, W, Al, Ti, Sr, Mg, Sb, V, Zn, Cu, and Cr.

[0019] The second aspect of the present application provides a method for preparing a positive electrode material, including the following steps: providing a mixture including a nitrogen-rich carbon source and a salt; mixing the mixture, a lithium source, a manganese source, an iron source, a phosphorus source, and a solvent evenly, and sintering to obtain the positive electrode material.

[0020] As an embodiment of the present application, the method for preparing the positive electrode material further satisfies at least one of the following (I)-(VI):

[0021] (I) The nitrogen-rich carbon source includes at least one of ammeline, dicyandiamide, melamine, and 5-aminotetrazole;

[0022] (II) The salt includes at least one of potassium chloride, sodium chloride, potassium bromide, sodium bromide, potassium thiocyanate, and sodium thiocyanate;

[0023] (III) The lithium source includes at least one of lithium oxide, lithium hydroxide, and lithium carbonate;

[0024] (IV) The manganese source includes at least one of manganese oxide, manganese sulfate tetrahydrate, manganese acetate tetrahydrate, and manganese oxalate;

[0025] (V) the iron source comprises at least one of iron flakes, ferrous oxide, ferrous oxide, ferrous sulfate heptahydrate, ferrous acetate, and ferrous oxalate tetrahydrate;

[0026] (VI) The phosphorus source includes at least one of phosphoric acid, ammonium phosphate, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate.

[0027] As an embodiment of the present application, the mass ratio of the nitrogen-rich carbon source to the salt is 1:(2-10); and / or the mass ratio of the total mass of the lithium source, manganese source, iron source, and phosphorus source to the nitrogen-rich carbon source is 100:(5-30).

[0028] A third aspect of the present application provides a secondary battery, comprising a positive electrode plate, wherein the positive electrode plate comprises the above-mentioned positive electrode material.

[0029] A fourth aspect of the present application provides an electrical device, comprising the secondary battery described above, wherein the secondary battery serves as a power supply for the electrical device.

[0030] The beneficial effects of the present application are as follows: the present application coats crystalline carbon nitride on the surface of manganese iron lithium oxide, and the crystalline carbon nitride has a high degree of crystallinity and a structure similar to graphite. Since the π electrons in the crystalline carbon nitride structure can form a delocalized large π bond, the crystalline carbon nitride has a higher degree of conjugation. Therefore, compared with doped carbon and amorphous (non-crystalline) carbon nitride, crystalline carbon nitride has more excellent electrical conductivity, can effectively reduce the resistance of manganese iron lithium oxide, and greatly improve electrical conductivity. At the same time, there are more micropores and mesopores in the crystalline carbon nitride structure, and there are certain gaps between its atomic layers, so these pores and gaps can become lithium ion transmission channels, improving the low-temperature conductivity of the material. In addition, there are many negatively charged nitrogen defects in the crystalline carbon nitride structure. These nitrogen defects can not only make the crystalline carbon nitride more firmly connected to the matrix material, but also can adsorb positively charged particles, helping to suppress the leakage of manganese elements during the cycle. Therefore, the use of crystalline carbon nitride to coat manganese iron lithium oxide can significantly improve its electrical conductivity and cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIG1 is a schematic structural diagram of the positive electrode material described in this application.

[0032] FIG2 is a schematic diagram of the atomic structure of crystalline carbon nitride.

[0033] FIG3 is an X-ray diffraction (XRD) pattern of the positive electrode material of Comparative Example 3.

[0034] FIG4 is an X-ray diffraction (XRD) pattern of the positive electrode material of Example 1.

[0035] FIG5 is a high-resolution X-ray photoelectron spectroscopy (XPS) nitrogen spectrum of the positive electrode material of Comparative Example 3.

[0036] FIG6 is a high-resolution X-ray photoelectron spectroscopy (XPS) nitrogen spectrum of the positive electrode material of Example 1.

[0037] FIG7 is a carbon 13 solid-state nuclear magnetic resonance spectrum of the positive electrode material of Comparative Example 3.

[0038] FIG8 is a carbon 13 solid-state nuclear magnetic resonance spectrum of the positive electrode material of Example 1. DETAILED DESCRIPTION

[0039] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of this application, not all of them. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0040] In this application, the technical features expressed in an open description manner include closed technical solutions composed of the listed features, and also include open technical solutions containing the listed features.

[0041] In this application, when referring to numerical ranges, unless otherwise specified, the numerical ranges are considered continuous and include the minimum and maximum values ​​of the range, as well as every value between such minimum and maximum values. Further, when a range refers to an integer, every integer between the minimum and maximum values ​​of the range is included. In addition, when multiple ranges are provided to describe a feature or characteristic, the ranges can be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges subsumed therein.

[0042] An embodiment of the present application provides a positive electrode material, comprising a core and a coating layer disposed on the outer surface of the core, wherein the core comprises manganese iron lithium oxide, and the coating layer comprises crystalline carbon nitride.

[0043] The inventors of this application have found that the conductivity, lithium ion conductivity and ability to inhibit manganese element leakage of manganese iron lithium oxide positive electrode materials with a coating layer are closely related to the material and state of the coating layer. When nitrogen-doped carbon or amorphous (non-crystalline) carbon nitride is used as the coating layer, since nitrogen-doped carbon and amorphous (non-crystalline) carbon nitride are both amorphous (non-crystalline) substances, their internal long-range disorder will easily hinder the transport process of electrons and lithium ions, thereby reducing the material's conductivity and lithium ion conductivity. The inventors also found that carbon nitride is a two-dimensional material with a graphite-like structure. Compared with nitrogen-doped carbon materials, carbon nitride has a higher nitrogen content, and the basic structure of carbon nitride is a triazine heterocycle composed of carbon atoms and nitrogen atoms, while the basic structure of nitrogen-doped carbon materials, such as nitrogen-doped graphene, is still a six-membered carbon ring, except that some carbon atoms are replaced by nitrogen atoms. Moreover, compared with carbon materials, the structure composed of carbon-nitrogen heterocycles makes the electronic structure of carbon nitride more special.

[0044] The present application is creatively coated with crystalline carbon nitride in manganese iron lithium oxide, and the crystalline carbon nitride has a high degree of crystallinity and a structure similar to graphite. Because the π electrons in the crystalline carbon nitride structure can form a delocalized large π bond, the crystalline carbon nitride has a higher degree of conjugation, so as to have more excellent electrical conductivity (electrical conductivity is better than nitrogen-doped carbon and amorphous carbon nitride), it is possible to effectively reduce the resistance of manganese iron lithium oxide, and significantly improve electrical conductivity. At the same time, there are more micropores and mesopores in the crystalline carbon nitride structure, and there is a certain gap between its atomic layers, so these holes and gaps can become lithium ion transmission channels, improving the low-temperature conductivity of the material. Moreover, there are many negatively charged nitrogen defects in the crystalline carbon nitride structure, and these nitrogen defects can not only make the crystalline carbon nitride more firmly connected to the matrix material, but also can adsorb positively charged particles, assisting in suppressing the seepage of manganese element during the cycle. Therefore, using crystalline carbon nitride to coat manganese iron lithium oxide, it is possible to significantly improve its electrical conductivity and cycle stability.

[0045] In one embodiment, the positive electrode material has a first diffraction peak in the X-ray diffraction pattern within the range of 27.5-29.0°.

[0046] In one embodiment, the positive electrode material has a second diffraction peak in the range of 7.5-9.0° in the X-ray diffraction pattern.

[0047] In one embodiment, the positive electrode material has a peak in the nitrogen 1s spectrum in the range of 396.5-398.0 eV.

[0048] In one embodiment, the positive electrode material has a peak in the carbon 13 solid nuclear magnetic resonance spectrum within the range of 118-126 ppm.

[0049] The inventors of this application have found that two low-intensity and wide diffraction peaks are observed at 13.2° and 27.3° for the amorphous (non-crystalline) carbon nitride-wrapped positive electrode material, indicating that its crystallinity is low; the positive electrode material described in this application has two diffraction peaks in the range of 27.5-29.0° and 7.5-9.0°, which are higher in intensity and narrower than the two diffraction peaks of the amorphous (non-crystalline) carbon nitride-wrapped positive electrode material, indicating that its crystallinity is higher; and the peak of 13.2° shifts to 7.5-9.0°, indicating that the in-plane structure of the material has changed, which can be more conducive to lithium ion transmission, thereby improving the low-temperature conductivity of the material.

[0050] In the X-ray photoelectron spectroscopy nitrogen 1s spectrum of the positive electrode material described in the present application, there is a peak in the range of 396.5-398.0eV, and the peak represents negatively charged nitrogen; in the carbon 13 solid nuclear magnetic resonance spectrum of the positive electrode material, there is a peak in the range of 118-126ppm, and the peak represents a cyanide group, indicating that there are many negatively charged nitrogen defects in the crystalline carbon nitride structure, which can effectively improve the wettability of the coating layer, so that it can be more firmly connected to the manganese iron lithium oxide; at the same time, the groups represented by these peaks can become negatively charged centers, and thus can adsorb positively charged particles, thereby helping to inhibit the leakage of manganese elements during the cycle of the positive electrode material, thereby significantly improving its conductivity and cycle stability.

[0051] In one embodiment, the ratio of the peak intensity of the first diffraction peak to the strongest peak of the X-ray diffraction pattern of the positive electrode material is 1:(3-15), for example, it can be 1:3, 1:4, 1:6, 1:8, 1:10, 1:12, 1:15 or a range consisting of any two values ​​therein.

[0052] In one embodiment, the ratio of the peak intensity of the second diffraction peak to the first diffraction peak is 1:(1.5-15), for example, it can be 1:1.5, 1:3, 1:6, 1:8, 1:10, 1:12, 1:15 or a range consisting of any two values ​​therein.

[0053] The ratio of the peak intensity of the first diffraction peak of the positive electrode material described in the present application to the strongest peak of the X-ray diffraction pattern of the positive electrode material is 1:(3-15), and / or the ratio of the peak intensity of the second diffraction peak to the first diffraction peak is 1:(1.5-15), indicating that the peak intensity of the first diffraction peak of the present application is high, indicating that it has high crystallinity and can efficiently transport electrons and lithium ions.

[0054] In one embodiment, the thickness of the coating layer is 10-150 nm, for example, 10 nm, 20 nm, 30 nm, 50 nm, 60 nm, 80 nm, 100 nm, 120 nm, 150 nm, or a range consisting of any two of these values. The inventors of the present application have found that by controlling the thickness of the coating layer to 10-150 nm, the coating layer can have sufficient active material, thereby effectively providing a lithium ion transmission channel; the nitrogen defect content of the coating layer can also be high, thereby effectively forming a manganese element shielding layer; and at this thickness, the lithium ion transmission distance is short, which is conducive to improving the diffusion efficiency of lithium ions between the positive and negative electrodes, thereby effectively improving the conductivity and cycle performance.

[0055] In one embodiment, the thickness of the coating layer is 50-100 nm. By controlling the thickness of the coating layer within this range, the electrical conductivity and cycle performance can be further improved.

[0056] In one embodiment, the coating layer accounts for 0.2-5wt% of the total mass of the positive electrode material, for example, 0.2wt%, 0.5wt%, 1wt%, 2wt%, 3wt%, 4wt%, 5wt%, or a range consisting of any two of these values. By controlling the coating layer to account for 0.2-5wt% of the total mass of the positive electrode material, the present application can provide sufficient active material and lithium ion transmission channels, and can also improve the diffusion efficiency of lithium ions between the positive and negative electrodes, thereby effectively improving conductivity and cycle performance.

[0057] In one embodiment, the coating layer accounts for 1-2 wt % of the total mass of the positive electrode material. By controlling the coating layer's proportion of the total mass of the positive electrode material within this range, the conductivity and cycle performance can be further improved.

[0058] In one embodiment, the positive electrode material contains nitrogen, and the atomic percentage of nitrogen in the positive electrode material is 1-7 at%, for example, 1 at%, 2 at%, 3 at%, 4 at%, 5 at%, 6 at%, 7 at%, or a range consisting of any two of these values. The high nitrogen content of the positive electrode material of the present application indicates a more complete structure of the carbon nitride and a high coating layer content, which can provide sufficient active material, thereby improving conductivity and cycle performance.

[0059] In one embodiment, the atomic percentage of nitrogen in the positive electrode material is 3-5 at %.

[0060] In one embodiment, the positive electrode material contains an alkali metal element, the alkali metal includes at least one of K and Na, and the atomic percentage content of the alkali metal element in the positive electrode material is 0.1-1.2 at%, for example, it can be 0.1 at%, 0.2 at%, 0.4 at%, 0.5 at%, 0.6 at%, 0.8 at%, 1 at%, 1.2 at% or a range composed of any two of these values. By controlling the alkali metal content within this range in the present application, sufficient active sites can be provided, thereby improving the conductivity and cycle performance.

[0061] In one embodiment, the atomic percentage content of the alkali metal element in the positive electrode material is 0.5-1 at%.

[0062] In one embodiment, the positive electrode material contains carbon element, and the atomic percentage content of the carbon element in the positive electrode material is 2-11 at%, for example, it can be 2 at%, 3 at%, 4 at%, 5 at%, 6 at%, 7 at%, 8 at%, 9 at%, 10 at%, 11 at% or a range composed of any two of these values. By controlling the carbon element content within this range in the present application, the integrity of the crystalline carbon nitride structure can be ensured, and sufficient active substances can be provided, thereby improving the conductivity and cycle performance.

[0063] In one embodiment, the atomic percentage content of the carbon element in the positive electrode material is 6-8 at%.

[0064] In one embodiment, the lithium manganese iron oxide includes a compound with the molecular formula Li a Mn x Fe 1-x M 1-a PO4, where 0 < x < 1, 0.9 ≤ a ≤ 1, and M includes at least one of In, La, Zr, Ce, W, Al, Ti, Sr, Mg, Sb, V, Zn, Cu, Cr.

[0065] In one embodiment, the mentioned M element can be incorporated into the lithium manganese iron oxide by doping and / or surface coating.

[0066] One embodiment of the present application provides a method for preparing a positive electrode material, including the following steps: providing a mixture including a nitrogen-rich carbon source and a salt; mixing the mixture, a lithium source, a manganese source, an iron source, a phosphorus source, and a solvent evenly, and sintering to obtain the positive electrode material.

[0067] In the present application, by mixing evenly a mixture including a nitrogen-rich carbon source and a salt, a lithium source, a manganese source, an iron source, a phosphorus source, and a solvent, and then sintering, the above-mentioned positive electrode material is obtained.

[0068] The mixture comprises a nitrogen-rich carbon source and a salt, and the mixture comprising a nitrogen-rich carbon source and a salt is obtained by uniformly mixing the nitrogen-rich carbon source and the salt.

[0069] It should be noted that those skilled in the art can use conventional methods to mix the nitrogen-rich carbon source and the salt to obtain a mixture, as long as the purpose of mixing is achieved. For example, the nitrogen-rich carbon source and the salt can be uniformly mixed by mechanical stirring, air flow mixing, liquid mixing, and spray drying.

[0070] In one embodiment, the nitrogen-rich carbon source and the salt described in the present application are uniformly mixed by a liquid mixing method to obtain a mixture.

[0071] More specifically, a nitrogen-rich carbon source and salt are added to deionized water, completely dissolved by magnetic stirring in a water bath, dried, and then ground into powder to obtain a mixture including the nitrogen-rich carbon source and salt.

[0072] The temperature of the water bath is 60-80°C, for example, 60°C, 65°C, 70°C, 75°C, 80°C or a range consisting of any two values ​​therein.

[0073] In one embodiment, the drying method includes vacuum drying.

[0074] In one embodiment, the vacuum drying temperature is 60-80°C, for example, 60°C, 65°C, 70°C, 75°C, 80°C or a range consisting of any two values ​​therein.

[0075] In one embodiment, the vacuum drying time is 12-18 hours, for example, it can be 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours or a range consisting of any two values ​​therein.

[0076] In one embodiment, the sintering is carried out under a protective atmosphere and / or a reducing atmosphere, the protective atmosphere is nitrogen or argon, the reducing atmosphere is hydrogen, and the gas flow rate is 50-100 mL / min, for example, it can be 50 mL / min, 60 mL / min, 70 mL / min, 80 mL / min, 90 mL / min, 100 mL / min or a range consisting of any two values ​​therein.

[0077] In one embodiment, the sintering temperature is 500-650°C, for example, 550°C, 560°C, 570°C, 580°C, 600°C, 620°C, 650°C or a range consisting of any two values ​​therein.

[0078] In one embodiment, the sintering time is 2-8 hours, for example, 2 hours, 3 hours, 5 hours, 6 hours, 8 hours or a range consisting of any two values ​​therein.

[0079] In one embodiment, the nitrogen-rich carbon source includes at least one of cyanamide, dicyandiamide, melamine, and 5-aminotetrazole.

[0080] In one embodiment, the salt includes at least one of potassium chloride, sodium chloride, potassium bromide, sodium bromide, potassium thiocyanate, and sodium thiocyanate.

[0081] In one embodiment, the lithium source includes at least one of lithium oxide, lithium hydroxide, and lithium carbonate.

[0082] In one embodiment, the manganese source includes at least one of manganese oxide, manganese sulfate tetrahydrate, manganese acetate tetrahydrate, and manganese oxalate.

[0083] In one embodiment, the iron source includes at least one of iron flakes, ferrous oxide, ferrous oxide, ferrous sulfate heptahydrate, ferrous acetate, and ferrous oxalate tetrahydrate.

[0084] In one embodiment, the phosphorus source includes at least one of phosphoric acid, ammonium phosphate, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate.

[0085] In one embodiment, the mass ratio of the nitrogen-rich carbon source to the salt is 1:(2-10), for example, 1:2, 1:3, 1:5, 1:8, 1:10, or a range consisting of any two values ​​therein. By controlling the mass ratio of the nitrogen-rich carbon source to the salt, the thickness of the coating layer and the content of the coating layer in the positive electrode material can be effectively controlled.

[0086] In one embodiment, the mass ratio of the total mass of the lithium source, manganese source, iron source, and phosphorus source to the nitrogen-rich carbon source is 100:(5-30), for example, it can be 100:5, 100:10, 100:15, 100:20, 100:25, 100:30 or a range consisting of any two values ​​therein.

[0087] One embodiment of the present application provides a secondary battery, including a positive electrode plate, wherein the positive electrode plate includes the positive electrode material described above.

[0088] More specifically, the positive electrode sheet includes a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector, and the positive electrode material layer includes the positive electrode material described above.

[0089] In this application, the type of positive electrode current collector is not particularly limited and can be any known material suitable for use as a positive electrode current collector. In one embodiment, the positive electrode current collector comprises a metal material such as aluminum, stainless steel, nickel plating, titanium, tantalum, or a carbon material such as carbon cloth or carbon paper. In one embodiment, the positive electrode current collector is a metal material. In one embodiment, the positive electrode current collector is aluminum.

[0090] In this application, there is no particular limitation on the form of the positive electrode current collector. When the positive electrode current collector is a metal material, the positive electrode current collector may be in the form of metal foil, metal cylinder, metal coil, metal plate, metal foil, expanded metal, stamped metal, foamed metal, etc. When the positive electrode current collector is a carbon material, the positive electrode current collector may be in the form of, but not limited to, carbon plate, carbon film, carbon cylinder, etc.

[0091] In one embodiment, the positive electrode material layer further includes a conductive agent and a binder.

[0092] In one embodiment, the secondary battery further includes a negative electrode plate, wherein the negative electrode plate includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer includes a negative electrode active material.

[0093] In the present application, there is no particular limitation on the negative electrode current collector, as long as it can achieve the purpose of the present application. For example, it can be copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foam nickel, foam copper or a composite current collector.

[0094] In one embodiment, the negative electrode active material can be natural graphite, artificial graphite, mesophase microcarbon beads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel structure lithium titanate Li4Ti5O 12 , at least one of Li-Al alloy and metallic lithium.

[0095] In one embodiment, the negative electrode active material layer further includes a conductive agent and a binder.

[0096] In one embodiment, the present application has no limitation on the type of the conductive agent mentioned, and any known conductive agent can be used.

[0097] In one embodiment, the conductive agent includes at least one carbon material such as acetylene black, needle coke, carbon nanotubes, and graphene.

[0098] In one embodiment, the present application has no limitation on the type of the binder mentioned, and any known positive electrode binder can be used.

[0099] In one embodiment, the binder includes at least one of polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, nitrocellulose, styrene-butadiene rubber, nitrile rubber, fluororubber, isoprene rubber, polybutadiene rubber, ethylene-propylene rubber, styrene-butadiene-styrene block copolymer or its hydrogenated product, ethylene-propylene-diene terpolymer, styrene-ethylene-butadiene-ethylene copolymer, styrene-isoprene-styrene block copolymer, syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymer, propylene-α-olefin copolymer, polyvinylidene fluoride, polytetrafluoroethylene, fluorinated polyvinylidene fluoride, and polytetrafluoroethylene-ethylene copolymer.

[0100] In the secondary battery mentioned in this application, a separator is usually provided between the positive electrode and the negative electrode to prevent short circuit. There is no particular limitation on the material and shape of the separator, as long as it does not significantly impair the effect of this application.

[0101] In one embodiment, the separator comprises a porous sheet material or non-woven fabric material with excellent liquid retention. Materials of the resin or glass fiber separator include, but are not limited to, polyolefin, aromatic polyamide, polytetrafluoroethylene, polyethersulfone, and the like.

[0102] In one embodiment, the polyolefin is polyethylene or polypropylene. In some embodiments, the polyolefin is polypropylene. The materials of the above-mentioned diaphragm can be used alone or in any combination.

[0103] In some embodiments, the secondary battery may include an outer package, which may be used to encapsulate the electrode assembly and the electrolyte.

[0104] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery can also be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0105] In some embodiments, the type of the electrolyte is also not specifically limited. The electrolyte comprises an electrolyte salt and an organic solvent. The specific types of the electrolyte salt and organic solvent are not specifically limited and can be selected based on actual needs. The electrolyte may also include additives, the types of which are not particularly limited. These additives may be film-forming additives for the positive and / or negative electrodes, or additives that improve certain battery properties, such as high- or low-temperature performance.

[0106] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square, or any other shape.

[0107] An embodiment of the present application provides an electric device, comprising the secondary battery described above, wherein the secondary battery serves as a power supply for the electric device.

[0108] Exemplarily, the above-mentioned electrical devices may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited to these.

[0109] The present application is further described below with specific examples:

[0110] Example 1

[0111] A method for preparing a positive electrode material comprises the following steps:

[0112] 7.5 g of melamine and 37.5 g of potassium chloride were added to 200 mL of deionized water, and the mixture was completely dissolved by magnetic stirring at 300 rpm in an 80°C water bath, followed by vacuum drying at 80°C for 12 h and grinding into powder to obtain a mixture including a nitrogen-rich carbon source and a salt;

[0113] The mixture of the nitrogen-rich carbon source and salt was ball-milled with 6.6 g of lithium carbonate, 11.9 g of manganese sulfate tetrahydrate, 9.9 g of ferrous sulfate heptahydrate, 21.8 g of 40% phosphoric acid, and 50.2 g of deionized water at 300 rpm for 2 h to obtain a uniform pretreated slurry;

[0114] The pretreated slurry was spray dried, heated to 600°C at a rate of 5°C / min in a flowing nitrogen atmosphere of 100 mL / min and sintered for 4 hours. After cooling, it was washed three times with deionized water, vacuum dried at 60°C for 12 hours, and then ground into powder to obtain the crystalline carbon nitride-coated lithium manganese iron phosphate positive electrode material.

[0115] Example 2

[0116] The difference between Example 2 and Example 1 is that the amounts of melamine and potassium chloride used in Example 2 are different from those in Example 1, and all other aspects are the same.

[0117] The amount of melamine used in this embodiment is 2.5 g, and the amount of potassium chloride used is 12.5 g.

[0118] Example 3

[0119] The difference between Example 3 and Example 1 is that the amounts of melamine and potassium chloride used in Example 3 are different from those in Example 1, and all other aspects are the same.

[0120] The amount of melamine used in this embodiment is 1 g, and the amount of potassium chloride used is 5 g.

[0121] Example 4

[0122] The difference between Example 4 and Example 1 is that the amounts of melamine and potassium chloride used in Example 4 are different from those in Example 1, and all other aspects are the same.

[0123] The amount of melamine used in this embodiment is 10 g, and the amount of potassium chloride used is 50 g.

[0124] Example 5

[0125] The difference between Example 5 and Example 1 is that the amounts of melamine and potassium chloride used in Example 5 are different from those in Example 1, and all other aspects are the same.

[0126] The amount of melamine used in this embodiment is 15 g, and the amount of potassium chloride used is 75 g.

[0127] Example 6

[0128] The difference between Example 6 and Example 1 is that the amounts of melamine and potassium chloride used in Example 6 are different from those in Example 1, and all other aspects are the same.

[0129] The amount of melamine used in this embodiment is 25 g, and the amount of potassium chloride used is 125 g.

[0130] Example 7

[0131] The difference between Example 7 and Example 1 is that the amounts of melamine and potassium chloride used in Example 7 are different from those in Example 1, and all other aspects are the same.

[0132] The amount of melamine used in this embodiment is 37.5 g, and the amount of potassium chloride used is 187.5 g.

[0133] Example 8

[0134] The difference between Example 8 and Example 1 is that the amount of potassium chloride used in Example 8 is different from that in Example 1, and all other aspects are the same.

[0135] The consumption of present embodiment potassium chloride is 15g.

[0136] Example 9

[0137] The difference between Example 9 and Example 1 is that the amount of potassium chloride used in Example 9 is different from that in Example 1, and all other aspects are the same.

[0138] The consumption of present embodiment potassium chloride is 150g.

[0139] Example 10

[0140] The difference between Example 10 and Example 1 is that Example 10 uses an equal amount of 5-aminotetrazole to replace melamine, and all other aspects are the same.

[0141] Example 11

[0142] The difference between Example 11 and Example 1 is that Example 11 uses an equal amount of sodium chloride to replace potassium chloride, and all other aspects are the same.

[0143] Example 12

[0144] The difference between Example 12 and Example 1 is that Example 12 uses an equal amount of potassium thiocyanate to replace potassium chloride, and all other aspects are the same.

[0145] Example 13

[0146] The difference between Example 13 and Example 1 is that the ball milling speed of Example 13 is 800 rpm, and the other parts are the same.

[0147] Example 14

[0148] The difference between Example 14 and Example 1 is that the ball milling time of Example 14 is 4 hours, and the other aspects are the same.

[0149] Example 15

[0150] The difference between Example 15 and Example 1 is that the sintering temperature of Example 15 is 550° C., and the other aspects are the same.

[0151] Example 16

[0152] The difference between Example 16 and Example 1 is that the sintering time of Example 16 is 8 hours, and the other aspects are the same.

[0153] Example 17

[0154] The difference between Example 17 and Example 1 is that the sintering temperature of Example 17 is 650° C., and the other aspects are the same.

[0155] Example 18

[0156] The difference between Example 18 and Example 1 is that the sintering time of Example 18 is 2 hours, and the others are the same.

[0157] Comparative Example 1

[0158] The difference between Comparative Example 1 and Example 1 is that the preparation method of the positive electrode material is different.

[0159] The preparation method of the positive electrode material of this comparative example 1 comprises the following steps:

[0160] 6.6 g of lithium carbonate, 11.9 g of manganese sulfate tetrahydrate, 9.9 g of ferrous sulfate heptahydrate, 40% phosphoric acid, and 50.2 g of deionized water were ball-milled at 300 rpm for 2 h to obtain a uniform pretreated slurry;

[0161] The pretreated slurry was spray-dried, heated to 600° C. at a rate of 5° C. / min in a flowing nitrogen atmosphere at 100 mL / min, and sintered for 4 h. After cooling, it was ground into powder to obtain the lithium manganese iron phosphate positive electrode material.

[0162] Comparative Example 2

[0163] The difference between Comparative Example 2 and Comparative Example 1 is that 7.5 g of sucrose was added during ball milling in Comparative Example 2, and the other conditions were the same.

[0164] The preparation method of the positive electrode material of this comparative example 2 comprises the following steps:

[0165] 6.6 g of lithium carbonate, 11.9 g of manganese sulfate tetrahydrate, 9.9 g of ferrous sulfate heptahydrate, 40% phosphoric acid, 7.5 g of sucrose, and 50.2 g of deionized water were ball-milled at 300 rpm for 2 h to obtain a uniform pretreated slurry;

[0166] The pretreated slurry was spray-dried, heated to 600° C. at a rate of 5° C. / min in a flowing nitrogen atmosphere at 100 mL / min, and sintered for 4 h. After cooling, it was ground into powder to obtain the lithium manganese iron phosphate positive electrode material.

[0167] Comparative Example 3

[0168] The difference between Comparative Example 3 and Comparative Example 1 is that Comparative Example 3 uses an equal amount of melamine to replace sucrose, and all other conditions are the same.

[0169] Test Example 1

[0170] Please refer to Figures 1 and 2. Figure 1 is a schematic diagram of the morphology of the positive electrode material described in this application. Manganese iron lithium oxide is a spherical particle with many rod-shaped substances growing on its outer layer. These rod-shaped substances are coated crystalline carbon nitride. Figure 2 shows the atomic structure of crystalline carbon nitride. As shown in the figure, the structure of crystalline carbon nitride is a planar network structure composed of carbon and nitrogen six-membered heterocycles. Its structure contains pores that can serve as channels for lithium ion transmission. In addition, its structure also contains some negatively charged nitrogen and cyanide defects. These negatively charged nitrogen defects can serve as manganese ion adsorption sites, inhibiting the leakage of manganese elements.

[0171] Test Example 2

[0172] The crystalline carbon nitride-coated lithium manganese iron phosphate material prepared in Example 1 and the amorphous (non-crystalline) carbon nitride-coated lithium manganese iron phosphate material prepared in Comparative Example 3 were subjected to XRD, XPS and carbon 13 solid nuclear magnetic resonance spectroscopy tests, and the XPS elemental composition, coating layer thickness and coating layer weight ratio of all examples and comparative examples were measured.

[0173] Figures 3 and 4 are the XRD patterns of Comparative Example 3 and Example 1, respectively. Two diffraction peaks of lower intensity and wider width can be observed at 13.2° and 27.3° for the positive electrode material of Comparative Example 3, indicating that its crystallinity is lower; while the positive electrode material of Example 1 has two diffraction peaks at 8.1° and 28.0°. Compared with Comparative Example 3, the two diffraction peaks are more intense and narrower, indicating that its crystallinity is higher, and the peak at 13.2° shifts to 8.1°, indicating that the in-plane structure of the material changes, forming a large ring as shown in Figure 2; wherein, the ratio of the diffraction peak intensity of Example 1 at 28.0° to the peak intensity of the strongest peak in the X-ray diffraction pattern of Example 1 is 1:8, and the ratio of the diffraction peak intensity of Example 1 at 8.1° to the diffraction peak intensity of Example 1 at 28.0° is 1:1.5.

[0174] Figures 5 and 6 are the XPS nitrogen spectra of Comparative Example 3 and Example 1, respectively. As shown in the figures, the spectra can be divided into 4 or 5 peaks, and the crystalline carbon nitride coating material has an additional peak at 397.7 eV compared to the amorphous (non-crystalline) carbon nitride coating material, which represents the negatively charged nitrogen defect described in Figure 2.

[0175] 7 and 8 are the NMR spectra of Comparative Example 3 and Example 1, respectively. Similarly, Example 1 has an additional peak at 121 ppm compared to Comparative Example 3, which represents the cyano defect described in FIG. 2 .

[0176] Table 1 shows the elemental composition of the prepared positive electrode materials. Comparative Example 3 contains more carbon and nitrogen elements than Comparative Example 1, which proves the presence of carbon nitride. On this basis, Examples 1-18 contain more potassium or sodium elements than Comparative Example 3, which is consistent with the elemental composition of crystalline carbon nitride.

[0177] Table 1 Elemental composition of the positive electrode materials prepared in Examples 1-18 and Comparative Examples 1-3

[0178] Table 2 shows the thickness and mass ratio of the prepared positive electrode material coating layer.

[0179] Table 2 Thickness and mass ratio of the positive electrode material coating layer prepared in Examples 1-18 and Comparative Examples 1-3

[0180] It can be seen from Examples 1-7 that the thickness and content of the coating layer are positively correlated with the amount of nitrogen-rich carbon source. However, it can be seen from Examples 1, 8 and 9 that the thickness and content of the coating layer are approximately negatively correlated with the amount of salt used. It can be seen from Examples 1 and 10 that changing the nitrogen-rich carbon source has no obvious effect on the content of the coating layer. It can be seen from Examples 1, 11 and 12 that changing the type of salt has a certain effect on the thickness and content of the coating layer. It can be seen from Examples 1 and 13-18 that changing the preparation process will affect the thickness and content of the coating layer, and increasing the sintering temperature and extending the sintering time will both lead to a decrease in thickness and content.

[0181] Test Example 3

[0182] The lithium manganese iron phosphate cathode materials prepared in the examples and comparative examples were assembled into button batteries. The preparation method is as follows:

[0183] The positive electrode material, conductive carbon black, binder polyvinylidene fluoride, and solvent N-methylpyrrolidone in a mass ratio of 90:5:5 were mixed to form a uniform electrode slurry. The slurry was then evenly coated on aluminum foil, rolled after drying, and vacuum dried at 120°C for 12 hours to obtain a positive electrode sheet.

[0184] The negative electrode is made of pure lithium strip;

[0185] The diaphragm is made of polypropylene porous membrane;

[0186] Dimethyl carbonate and ethylene carbonate are mixed together in a ratio of 4:6, and lithium hexafluorophosphate is added thereto to serve as an electrolyte solution, wherein the molar concentration of the lithium hexafluorophosphate is 1 mol / L.

[0187] The positive electrode sheet, the negative electrode sheet, the separator and the electrolyte solution are assembled into a button cell to obtain a secondary battery.

[0188] The secondary battery's impedance and electrochemical performance were tested at room temperature, with a test voltage of 2.5-4.3V. Cycling tests were performed at a charge / discharge current rate of 0.1C for 300 cycles. The manganese content in the solution was measured after each cycle. The battery performance is shown in Table 3.

[0189] Table 3 Performance of secondary batteries assembled with the positive electrode materials prepared in Examples 1-18 and Comparative Examples 1-3

[0190] As can be seen from Table 3, from the comparison of Comparative Examples 1 and 2, it can be seen that constructing a carbon coating film through the carbonization reaction of the carbon source in the synthesis of the positive electrode material can improve its conductivity and also inhibit the leakage of manganese elements; and from Comparative Examples 2 and 3, it can be seen that after replacing sucrose with nitrogen-rich carbon source melamine, since the latter can be thermally polymerized under high temperature to form amorphous (non-crystalline) carbon nitride, the presence of nitrogen makes it have higher conductivity and higher affinity for the matrix material, and its graphite-like structure can better inhibit the leakage of manganese elements, so the electrochemical performance of the material is further improved.

[0191] By comparing Example 1 and Comparative Example 3, it can be seen that after salt is added to the raw material, crystalline carbon nitride is generated due to the influence of salt on the thermal polymerization process of the nitrogen-rich carbon source. This coating film has a significantly improved conductivity of the material due to the increase in crystallinity and conjugation degree; in addition, the presence of negative electrical defects and the special structure of crystalline carbon nitride enable it to better inhibit the leakage of manganese elements than amorphous (non-crystalline) carbon nitride, thereby improving the cycle stability of the material.

[0192] Comparing Examples 1 to 2, 4 to 6, and 8 to 18 with Examples 3 and 7, it can be seen that the present application further improves the electrochemical performance by setting the thickness of the coating layer to 10-150 nm and the coating layer accounting for 0.2-5 wt % of the total mass of the positive electrode material.

[0193] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application and are not intended to limit the scope of protection of the present application. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present application.

Claims

1. A positive electrode material, comprising a core and a coating layer disposed on the outer surface of the core, wherein the core comprises manganese iron lithium oxide, and the coating layer comprises crystalline carbon nitride.

2. The positive electrode material according to claim 1, further satisfying at least one of the following (a)-(d): (a) In the X-ray diffraction pattern of the positive electrode material, there is a first diffraction peak in the range of 27.5 - 29.0°; (b) In the X-ray diffraction pattern of the positive electrode material, there is a second diffraction peak in the range of 7.5 - 9.0°; (c) In the X-ray photoelectron spectroscopy nitrogen 1s spectrum of the positive electrode material, there is a spectral peak in the range of 396.5 - 398.0 eV; (d) In the carbon-13 solid nuclear magnetic resonance spectrum of the positive electrode material, there is a spectral peak in the range of 118 - 126 ppm.

3. The positive electrode material according to claim 2, wherein the ratio of the peak intensity of the first diffraction peak to the peak intensity of the strongest peak in the X-ray diffraction pattern of the positive electrode material is 1:(3 - 15); and / or the ratio of the peak intensity of the second diffraction peak to the peak intensity of the first diffraction peak is 1:(1.5 - 15).

4. The positive electrode material according to claim 1, wherein the thickness of the coating layer is 10 - 150 nm.

5. The positive electrode material according to claim 4, wherein the thickness of the coating layer is 50 - 80 nm.

6. The positive electrode material according to claim 1, wherein the coating layer accounts for 0.2 - 5 wt% of the total mass of the positive electrode material.

7. The positive electrode material according to claim 1, wherein the positive electrode material contains nitrogen element, and the atomic percentage content of the nitrogen element in the positive electrode material is 1 - 7 at%; and / or the positive electrode material contains an alkali metal element, the alkali metal comprises at least one of K and Na, and the atomic percentage content of the alkali metal element in the positive electrode material is 0.1 - 1.2 at%.

8. The positive electrode material according to claim 1, wherein the lithium manganese iron oxide comprises a compound with the molecular formula Li a Mn x Fe 1-x M 1-a PO4, where 0 < x < 1, 0.9 ≤ a ≤ 1, and M comprises at least one of In, La, Zr, Ce, W, Al, Ti, Sr, Mg, Sb, V, Zn, Cu, Cr.

9. A method for preparing a positive electrode material according to any one of claims 1 - 8, comprising the following steps: Providing a mixture comprising a nitrogen-rich carbon source and a salt; Mixing the mixture, a lithium source, a manganese source, an iron source, a phosphorus source, and a solvent uniformly, and sintering to obtain the positive electrode material.

10. The preparation method according to claim 9, the preparation method further satisfying at least one of the following (I)-(VI): (I) The nitrogen-rich carbon source comprises at least one of aminocyanide, dicyandiamide, melamine, 5-aminotetrazole; (II) The salt comprises at least one of potassium chloride, sodium chloride, potassium bromide, sodium bromide, potassium thiocyanate, sodium thiocyanate; (III) The lithium source comprises at least one of lithium oxide, lithium hydroxide, lithium carbonate; (IV) The manganese source comprises at least one of manganese oxide, manganese sulfate tetrahydrate, manganese acetate tetrahydrate, manganese oxalate; (V) The iron source comprises at least one of iron sheet, ferrous oxide, ferroferric oxide, ferrous sulfate heptahydrate, ferrous acetate, ferrous oxalate tetrahydrate; (VI) The phosphorus source comprises at least one of phosphoric acid, ammonium phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate.

11. The preparation method according to claim 9, wherein the mass ratio of the nitrogen-rich carbon source to the salt is 1:(2 - 10); and / or The mass ratio of the total mass of the lithium source, manganese source, iron source, and phosphorus source to the mass of the nitrogen-rich carbon source is 100:(5 - 30).

12. A secondary battery, comprising a positive electrode plate, wherein the positive electrode plate contains the positive electrode material according to any one of claims 1 - 8.

13. An electrical device, comprising the secondary battery according to claim 12, wherein the secondary battery serves as a power supply for the electrical device.

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