Positive electrode material, secondary battery, and electric device

Abstract

This application discloses a positive electrode material, a secondary battery, and an electrical device, belonging to the field of battery technology. The positive electrode material includes a core and a coating layer disposed on the outer surface of the core. The core includes lithium iron manganese oxide, and the coating layer includes crystalline carbon nitride. This application uses crystalline carbon nitride to coat lithium iron manganese oxide, which can significantly improve its conductivity and cycle stability.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to a positive electrode material, a secondary battery, and an electrical device. Background Technology

[0002] The research and development of cathode materials plays a crucial role in improving the performance and application of secondary batteries. Lithium iron manganese oxide (LME) possesses high stability, safety, and energy density, but its poor conductivity results in poor charge-discharge performance. Furthermore, due to the Jahn-Teller effect, manganese leaks out from the LME structure during battery cycling, causing structural damage. Consequently, secondary batteries made from this material exhibit poor cycle performance.

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

[0004] 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.

[0005] To achieve the above objectives, a first aspect of this application provides a cathode material comprising a core and a coating layer disposed on the outer surface of the core, wherein the core comprises lithium iron manganese oxide and the coating layer comprises crystalline carbon nitride.

[0006] As an implementation scheme of this application, at least one of the following (a)-(d) is satisfied:

[0007] (a) The X-ray diffraction pattern of the cathode material has a first diffraction peak in the range of 27.5-29.0°;

[0008] (b) The X-ray diffraction pattern of the cathode material has a second diffraction peak in the range of 7.5-9.0°;

[0009] (c) The nitrogen 1s X-ray photoelectron spectroscopy spectrum of the cathode material has a peak in the range of 396.5-398.0 eV;

[0010] (d) The carbon-13 solid nuclear magnetic resonance spectrum of the cathode material has a peak in the range of 118-126 ppm.

[0011] As an embodiment of this application, 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 cathode material is 1:(3-15); and / or

[0012] The ratio of the peak intensity of the second diffraction peak to that of the first diffraction peak is 1:(1.5 - 15).

[0013] As an embodiment of the present application, the thickness of the coating layer is 10 - 150 nm.

[0014] As an embodiment of the present application, the coating layer accounts for 0.2 - 5 wt% of the total mass of the cathode material.

[0015] As an embodiment of the present application, the cathode material contains nitrogen element, and the atomic percentage content of the nitrogen element in the cathode material is 1 - 7 at%.

[0016] As an embodiment of the present application, the cathode 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 cathode material is 0.1 - 1.2 at%.

[0017] As an embodiment of the present application, the lithium manganese iron phosphate oxide includes a compound with the molecular formula Li

[0018] ,

[0017] Mn x Fe 1-x M 1-a PO4, where 0 < x < 1, 0.9 ≤ a ≤ 1.1, and M includes at least one of In, La, Zr, Ce, W, Al, Ti, Sr, Mg, Sb, V, Zn, Cu, Cr.

[0018] The second aspect of the present application provides a method for preparing a cathode 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 uniformly, and sintering to obtain the cathode material.

[0019] As an embodiment of the present application, at least one of the following (I)-(VI) is satisfied:

[0020] (I) The nitrogen-rich carbon source includes at least one of ammeline, dicyandiamide, melamine, 5-aminotetrazole;

[0021] (II) The salt includes at least one of potassium chloride, sodium chloride, potassium bromide, sodium bromide, potassium thiocyanate, sodium thiocyanate;

[0022] ](III) The lithium source includes at least one of lithium oxide, lithium hydroxide, lithium carbonate;

[0023] (IV) The manganese source includes at least one of manganese oxide, manganese sulfate tetrahydrate, manganese acetate tetrahydrate, manganese oxalate;

[0024] (V) The iron source includes at least one of iron sheet, iron oxide, magnetite, ferrous sulfate heptahydrate, ferrous acetate, ferrous oxalate tetrahydrate;

[0025] (VI) The phosphorus source includes at least one of phosphoric acid, ammonium phosphate, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate.

[0026] As an embodiment of this application, the mass ratio of the nitrogen-rich carbon source to the salt is 1:(2-10).

[0027] As an implementation scheme of this application, the total mass ratio 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 this application provides a secondary battery including a positive electrode sheet comprising the aforementioned positive electrode material.

[0029] A fourth aspect of this application provides an electrical device including the aforementioned secondary battery, wherein the secondary battery serves as the power supply for the electrical device.

[0030] The beneficial effects of this application are as follows: This application coats crystalline carbon nitride onto the surface of lithium iron manganese oxide. The crystalline carbon nitride has high crystallinity, similar to the structure of graphite. Its π electrons can form delocalized large π bonds, possessing a higher degree of conjugation. Compared with doped carbon and amorphous (non-crystalline) carbon nitride, it has superior conductivity, effectively reducing the resistance of lithium iron manganese oxide and significantly improving conductivity. At the same time, the crystalline carbon nitride structure has a large number of micropores and mesopores, and there are certain gaps between its atomic layers. These pores and gaps can become lithium-ion transport channels, improving the low-temperature conductivity of the material. In addition, the crystalline carbon nitride structure has many negatively charged nitrogen defects. These nitrogen defects can make the crystalline carbon nitride more firmly connected to the matrix material, and can also adsorb positively charged particles, helping to suppress the leakage of manganese elements during cycling. Therefore, coating lithium iron manganese oxide with crystalline carbon nitride can significantly improve its conductivity and cycling stability. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the structure of the cathode material described in this application.

[0032] Figure 2 This is a schematic diagram of the atomic structure of crystalline carbon nitride.

[0033] Figure 3 The image shows the X-ray diffraction (XRD) pattern of the cathode material in Comparative Example 3.

[0034] Figure 4 The image shows the X-ray diffraction (XRD) pattern of the cathode material in Example 1.

[0035] Figure 5The high-resolution X-ray photoelectron spectroscopy (XPS) nitrogen spectrum of the cathode material in Comparative Example 3 is shown.

[0036] Figure 6 The high-resolution X-ray photoelectron spectroscopy (XPS) nitrogen spectrum of the cathode material in Example 1.

[0037] Figure 7 The image shows the carbon-13 solid-state nuclear magnetic resonance spectrum of the cathode material in Comparative Example 3.

[0038] Figure 8 The image shows the carbon-13 solid-state nuclear magnetic resonance spectrum of the cathode material in Example 1. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0040] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0041] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0042] This application provides a cathode material, including a core and a coating layer disposed on the outer surface of the core. The core includes lithium iron manganese oxide, and the coating layer includes crystalline carbon nitride.

[0043] The inventors of this application have discovered that the conductivity, lithium-ion conductance, and ability to suppress manganese leaching of coated lithium iron phosphate oxide cathode materials are closely related to the material and state of the coating layer. Using nitrogen-doped carbon or amorphous (non-crystalline) carbon nitride as the coating layer, both nitrogen-doped carbon and amorphous (non-crystalline) carbon nitride are amorphous (non-crystalline) materials. Their long-range disorder makes the transport of electrons and lithium ions easily hindered, thus reducing the material's conductivity and lithium-ion conductance. Furthermore, the inventors found that carbon nitride is a two-dimensional material with a graphite-like structure. Unlike nitrogen-doped carbon materials, carbon nitride has a higher nitrogen content, and its basic structure is a triazine heterocycle composed of carbon and nitrogen atoms. In contrast, nitrogen-doped carbon materials, such as nitrogen-doped graphene, still have a six-membered carbon ring structure, but some carbon atoms are replaced by nitrogen atoms. This carbon-nitrogen heterocycle structure makes the electronic structure of carbon nitride more unique compared to carbon materials.

[0044] This application innovatively coats crystalline carbon nitride with lithium iron manganese oxide. The crystalline carbon nitride has high crystallinity, similar to the structure of graphite, and its π electrons can form delocalized large π bonds, possessing a higher degree of conjugation, thus exhibiting superior conductivity (superior to nitrogen-doped carbon and non-crystalline carbon nitride). This effectively reduces the resistance of lithium iron manganese oxide and significantly improves conductivity. Furthermore, the crystalline carbon nitride structure contains numerous micropores and mesopores, along with certain gaps between its atomic layers. These pores and gaps can serve as lithium-ion transport channels, enhancing the material's low-temperature conductivity. Additionally, the crystalline carbon nitride structure contains many negatively charged nitrogen defects, which allow the crystalline carbon nitride to bond more firmly to the matrix material and also adsorb positively charged particles, helping to suppress the leakage of manganese during cycling. Therefore, coating lithium iron manganese oxide with crystalline carbon nitride can significantly improve its conductivity and cycling stability.

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

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

[0047] In one embodiment, the nitrogen 1s X-ray photoelectron spectroscopy (XPS) spectrum of the cathode material has a peak in the range of 396.5-398.0 eV.

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

[0049] The inventors of this application discovered that amorphous (non-crystalline) carbon nitride-coated cathode materials exhibit two low-intensity and relatively broad diffraction peaks at 13.2° and 27.3°, indicating low crystallinity. The cathode material described in this application has two diffraction peaks in the ranges of 27.5-29.0° and 7.5-9.0°, which are stronger and narrower than the two diffraction peaks of amorphous (non-crystalline) carbon nitride-coated cathode materials, indicating higher crystallinity. The shift of the 13.1° peak to 7.5-9.0° indicates a change in the in-plane structure of the material, which is more conducive to lithium-ion transport and improves the low-temperature conductivity of the material.

[0050] In the X-ray photoelectron spectroscopy (XPS) 1s spectrum of the cathode material described in this application, there are peaks in the range of 396.5-398.0 eV, representing negatively charged nitrogen. In the carbon-13 solid-state NMR spectrum of the cathode material, there are peaks in the range of 118-126 ppm, representing cyano groups. This indicates that there are many negatively charged nitrogen defects in the crystalline carbon nitride structure, which can effectively improve the wettability of the coating layer and make it more firmly connected to lithium iron manganese oxide. At the same time, the groups represented by these peaks can become negatively charged centers, which can adsorb positively charged particles, thereby helping to suppress the leakage of manganese during the cycling process of the cathode material, and thus 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 cathode 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 any two of these values.

[0052] In one embodiment, the ratio of the peak intensity of the second diffraction peak to that of 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 any range of two of these values.

[0053] The ratio of the peak intensity of the first diffraction peak of the cathode material described in this application to the peak intensity of the strongest peak in the X-ray diffraction pattern of the cathode material is 1:(3-15), and 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). This indicates that the peak intensity of the first diffraction peak of this application is high, which indicates 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, it can be 10 nm, 20 nm, 30 nm, 50 nm, 60 nm, 80 nm, 100 nm, 120 nm, 150 nm or any two of these values. The inventors of this application have found that by controlling the thickness of the coating layer to 10-150 nm, the coating layer has sufficient active material, which can effectively provide lithium-ion transport channels. At the same time, the high nitrogen defect content can effectively form a manganese element shielding layer, and the lithium-ion transport distance is relatively short, which is conducive to improving the diffusion efficiency of lithium ions between the positive and negative electrodes, thereby effectively improving 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, conductivity and cycle performance can be further improved.

[0056] In one embodiment, the coating layer accounts for 0.2-5 wt% of the total mass of the cathode material, for example, it can be 0.2 wt%, 0.5 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, or any two of these values. By controlling the coating layer to account for 0.2-5 wt% of the total mass of the cathode material, this application can provide sufficient active material, provide lithium-ion transport channels, improve the diffusion efficiency of lithium ions between the positive and negative electrodes, and thus effectively improve conductivity and cycle performance.

[0057] In one embodiment, the coating layer accounts for 1-2 wt% of the total mass of the cathode material. By controlling the proportion of the coating layer to the total mass of the cathode material within this range, 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, it can be 1 at%, 2 at%, 3 at%, 4 at%, 5 at%, 6 at%, 7 at%, or any two of these values. The high nitrogen content of the positive electrode material of this application indicates that the structure of carbon nitride is more complete, and at the same time, it indicates that the coating layer content is higher, which can provide sufficient active material and improve conductivity and cycle performance.

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

[0060] In one embodiment, the cathode 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 cathode 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 cathode material is 0.5-1 at%.

[0062] In one embodiment, the cathode material contains carbon element, and the atomic percentage content of the carbon element in the cathode 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, while sufficient active substances are provided, improving the conductivity and cycle performance.

[0063] In one embodiment, the atomic percentage content of the carbon element in the cathode 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 ≤ 1.1, 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 cathode 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 uniformly, and sintering to obtain the cathode material.

[0067] In the present application, by uniformly mixing 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 cathode material is obtained.

[0068] The mixture includes a nitrogen-rich carbon source and a salt. By mixing the nitrogen-rich carbon source and the salt evenly, a mixture including a nitrogen-rich carbon source and a salt is obtained.

[0069] It should be noted that those skilled in the art can use conventional methods to mix nitrogen-rich carbon sources and salts to obtain mixtures, as long as the purpose of mixing is achieved. For example, the nitrogen-rich carbon sources and salts can be mixed evenly by mechanical stirring, airflow mixing, liquid mixing, and spray drying.

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

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

[0072] The water bath temperature is 60-80℃, for example, it can be 60℃, 65℃, 70℃, 75℃, 80℃ or any two of these values.

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

[0074] In one embodiment, the vacuum drying temperature is 60-80°C, for example, it can be 60°C, 65°C, 70°C, 75°C, 80°C or any two of these values.

[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 any two of these values.

[0076] In one embodiment, the sintering is carried out under a protective atmosphere and / or a reducing atmosphere, wherein the protective atmosphere is nitrogen or argon and 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 any two of these values.

[0077] In one embodiment, the sintering temperature is 500-650°C, for example, it can be 550°C, 560°C, 570°C, 580°C, 600°C, 620°C, 650°C or any two of these values.

[0078] In one embodiment, the sintering time is 2-8 hours, for example, it can be 2 hours, 3 hours, 5 hours, 6 hours, 8 hours or any two of these values.

[0079] In one embodiment, the nitrogen-rich carbon source includes at least one of ammonia nitrile, 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 sheet, ferrous oxide, iron(II) 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, it can be 1:2, 1:3, 1:5, 1:8, 1:10 or any two of these values. 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 cathode material can be effectively controlled.

[0086] In one embodiment, the total mass ratio 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 any two of these values.

[0087] One embodiment of this application provides a secondary battery, including a positive electrode sheet, wherein the positive electrode sheet comprises the positive electrode material described above.

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

[0089] In this application, there is no particular limitation on the type of positive electrode current collector; it can be any known material suitable for use as a positive electrode current collector. In one embodiment, the positive electrode current collector includes metallic materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum, as well as carbon materials such as carbon cloth and carbon paper. In one embodiment, the positive electrode current collector is a metallic material. In one embodiment, the positive electrode current collector is aluminum.

[0090] There are no particular restrictions on the form of the positive electrode current collector. When the positive electrode current collector is a metallic material, it can be in the form of metal foil, metal cylinder, metal strip, metal plate, metal foil, metal mesh, stamped metal, foamed metal, etc. When the positive electrode current collector is a carbon material, it can be in the form of 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 sheet, the negative electrode sheet including a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector, the negative active material layer including a negative active material.

[0093] In this application, there are no particular restrictions on the negative electrode current collector, as long as it can achieve the purpose of this application. For example, it can be copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, foamed copper, or composite current collector, etc.

[0094] In one embodiment, the negative electrode active material can be natural graphite, artificial graphite, mesophase microcarbon spheres (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, or spinel-structured lithium titanate Li4Ti5O. 12 At least one of Li-Al alloys 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, there is no limitation on the type of conductive agent mentioned in this application, and any known conductive agent may be used.

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

[0098] In one embodiment, there is no limitation on the type of adhesive mentioned in this application, and any known positive electrode adhesive can be used.

[0099] In one embodiment, the adhesive 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 hydrogenation, 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 and negative electrodes to prevent short circuits. There are no particular restrictions on the material and shape of the separator, as long as it does not significantly impair the effectiveness of this application.

[0101] In one embodiment, the diaphragm comprises a porous sheet-like or nonwoven material with excellent liquid retention properties. Materials for resin or glass fiber diaphragms include, but are not limited to, polyolefins, aromatic polyamides, polytetrafluoroethylene, and polyethersulfone.

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

[0103] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the aforementioned electrode assembly and 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, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

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

[0106] This application does not impose any particular restrictions on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape.

[0107] One embodiment of this application provides an electrical device including the secondary battery described above, wherein the secondary battery serves as the power supply for the electrical device.

[0108] For example, the aforementioned 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 thereto.

[0109] The present application is further illustrated below with specific embodiments:

[0110] Example 1

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

[0112] Add 7.5g of melamine and 37.5g of potassium chloride to 200mL of deionized water, and stir magnetically at 300rpm in an 80℃ water bath until completely dissolved. Then, vacuum dry at 80℃ for 12h and grind into powder to obtain a mixture including nitrogen-rich carbon source and salt.

[0113] The above-mentioned mixture of nitrogen-rich carbon source and salt was ball-milled with 6.6g lithium carbonate, 11.9g manganese sulfate tetrahydrate, 9.9g ferrous sulfate heptahydrate, 21.8g phosphoric acid with a mass concentration of 40%, and 50.2g deionized water at 300rpm for 2h 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 h. After cooling, it was washed three times with deionized water, vacuum dried at 60°C for 12 h, and then ground into powder to obtain the crystalline carbon nitride-coated lithium manganese iron phosphate cathode 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, but everything else is the same.

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

[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, but everything else is the same.

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

[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, but everything else is the same.

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

[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, but everything else is the same.

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

[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, but everything else is the same.

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

[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, but everything else is the same.

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

[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, but everything else is the same.

[0135] In this example, the amount of potassium chloride used 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, but everything else is the same.

[0138] In this example, the amount of potassium chloride used is 150g.

[0139] Example 10

[0140] Example 10 differs from Example 1 in that an equal amount of 5-aminotetrazole is used to replace melamine in Example 10, while all other aspects are the same.

[0141] Example 11

[0142] The difference between Example 11 and Example 1 is that in Example 11, an equal amount of sodium chloride is used to replace potassium chloride, while everything else is the same.

[0143] Example 12

[0144] The difference between Example 12 and Example 1 is that in Example 12, potassium chloride is replaced with an equal amount of potassium thiocyanate, while everything else is the same.

[0145] Example 13

[0146] The difference between Example 13 and Example 1 is that the ball milling speed in Example 13 is 800 rpm, while all other aspects are the same.

[0147] Example 14

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

[0157] Comparative Example 1

[0158] The difference between Comparative Example 1 and Example 1 is that the preparation methods of the cathode material are different.

[0159] The preparation method of the cathode material in this comparative example includes the following steps:

[0160] 6.6g lithium carbonate, 11.9g manganese sulfate tetrahydrate, 9.9g ferrous sulfate heptahydrate, 40% phosphoric acid, and 50.2g deionized water were ball-milled at 300 rpm for 2 hours 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 of 100 mL / min, and sintered for 4 h. After cooling, it was ground into powder to obtain the lithium manganese iron phosphate cathode material.

[0162] Comparative Example 2

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

[0164] The preparation method of the cathode material in this comparative example includes the following steps:

[0165] 6.6g lithium carbonate, 11.9g manganese sulfate tetrahydrate, 9.9g ferrous sulfate heptahydrate, 40% phosphoric acid, 7.5g sucrose, and 50.2g deionized water were ball-milled at 300 rpm for 2 hours 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 of 100 mL / min, and sintered for 4 h. After cooling, it was ground into powder to obtain the lithium manganese iron phosphate cathode 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, while everything else is the same.

[0169] Test Example 1

[0170] Please see the appendix Figure 1 Appendix Figure 2 , attached Figure 1 This is a schematic diagram of the morphology of the cathode material described in this application. The lithium iron manganese oxide is a spherical particle with many rod-shaped substances grown on its outer layer; these rod-shaped substances are the coated crystalline carbon nitride. (Attached) Figure 2 The atomic structure of crystalline carbon nitride is shown in the figure. The structure of crystalline carbon nitride is a planar network structure composed of six-membered carbon-nitrogen heterocycles. Its structure contains pores, which can serve as channels for lithium-ion transport. In addition, there are some negatively charged nitrogen and cyano defects in its structure. These negatively charged nitrogen defects can serve as manganese ion adsorption sites and inhibit the leakage of manganese.

[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 C13 solid-state nuclear magnetic resonance spectroscopy tests. At the same time, the XPS elemental composition, coating thickness and coating weight ratio of all examples and comparative examples were measured.

[0173] Appendix Figure 3 and 4 The images shown are the XRD patterns of Example 1 and Comparative Example 3, respectively. The cathode material of Comparative Example 3 exhibits two relatively weak and broad diffraction peaks at 13.2° and 27.3°, indicating lower crystallinity. In contrast, the cathode material of Example 1 shows two diffraction peaks at 8.1° and 28.0°. Compared to the comparative example, these two peaks are stronger and narrower, indicating higher crystallinity. The shift of the peak from 13.1° to 8.1° indicates a change in the in-plane structure of the material, forming the structure shown in the attached image. Figure 2 The large ring shown, wherein the ratio of the intensity of the diffraction peak at 28.0° in Example 1 to the intensity of the strongest peak in the X-ray diffraction pattern of Example 1 is 1:8, and the ratio of the intensity of the diffraction peak at 8.1° in Example 1 to the intensity of the diffraction peak at 28.0° in Example 1 is 1:1.5.

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

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

[0176] Table 1 shows the elemental composition of the prepared cathode materials. Compared with Comparative Example 1, Comparative Example 3 has more carbon and nitrogen elements, which proves the existence of carbon nitride. Examples 1-18 have more potassium or sodium elements compared with Comparative Example 1, which is consistent with the elemental composition of crystalline carbon nitride.

[0177] Table 1

[0178]

[0179] Table 2 shows the thickness and mass percentage of the coating layer of the prepared cathode material.

[0180] Table 2

[0181]

[0182]

[0183] Examples 1-7 show that the thickness and content of the coating layer are positively correlated with the amount of nitrogen-rich carbon source. Examples 1, 8, and 9 show that the thickness and content of the coating layer are approximately negatively correlated with the amount of salt. Examples 1 and 10 show that changing the nitrogen-rich carbon source has no significant effect on the thickness and content of the coating layer. Comparative Examples 1, 11, and 12 show that changing the type of salt has a certain impact on the thickness and content of the coating layer. Examples 1 and 13-18 show that changing the preparation process affects the thickness and content of the coating layer; increasing the sintering temperature and extending the sintering time both lead to a decrease in thickness and content.

[0184] Test Example 3

[0185] The lithium iron phosphate cathode materials prepared in the various embodiments and comparative examples were assembled into coin cells. The preparation methods are as follows:

[0186] A uniform electrode slurry was prepared by mixing positive electrode material, conductive carbon black, and binder polyvinylidene fluoride in a mass ratio of 90:5:5, and solvent N-methylpyrrolidone. The slurry was then uniformly coated onto aluminum foil, dried, rolled, and vacuum dried at 120°C for 12 hours to obtain the positive electrode sheet.

[0187] The negative electrode sheet uses pure lithium strip;

[0188] The diaphragm is made of porous polypropylene.

[0189] Dimethyl carbonate and ethylene carbonate were mixed together in a 4:6 ratio, and lithium hexafluorophosphate was added to form an electrolyte solution, wherein the molar concentration of lithium hexafluorophosphate was 1 mol / L.

[0190] A coin cell is assembled from a positive electrode, a negative electrode, a separator, and an electrolyte solution to obtain a secondary battery.

[0191] The impedance and electrochemical performance of the secondary battery were tested at room temperature, with a test voltage of 2.5-4.3V. Cyclic testing was conducted with a charge / discharge current rate of 0.1C for 300 cycles, and the manganese content in the solution was measured after each cycle. Battery performance is shown in Table 3.

[0192] Table 3

[0193] Impedance Rs (Ω) Manganese content (ppm) Capacity retention Example 1 8.6 49 96.0％ Example 2 9.7 83 93.1％ Example 3 11.7 159 88.9％ Example 4 7.0 / 99.1％ Example 5 7.9 / 98.3％ Example 6 9.5 35 93.0％ Example 7 12.1 29 92.2％ Example 8 9.2 54 95.4％ Example 9 10.7 146 93.9％ Example 10 8.9 24 96.1％ Example 11 7.3 / 99.0％ Example 12 8.4 / 97.8％ Example 13 8.4 53 96.1％ Example 14 9.1 84 95.5％ Example 15 8.0 / 97.9％ Example 16 10.6 162 94.1％ Example 17 8.2 86 95.9％ Example 18 8.1 / 98.2％ Comparative Example 1 14.7 384 85.7％ Comparative Example 2 12.2 215 87.3％ Comparative Example 3 11.3 183 89.6％

[0194] As shown in Table 3, the comparison between Comparative Examples 1 and 2 indicates that constructing a carbon coating film through the carbonization reaction of a carbon source in the synthesis of cathode materials can improve their conductivity and also inhibit the leakage of manganese. However, as shown in Comparative Examples 2 and 3, after replacing sucrose with nitrogen-rich carbon source melamine, the latter can be thermally polymerized at high temperatures to form amorphous (non-crystalline) carbon nitride. The presence of nitrogen gives it higher conductivity and a higher affinity for the matrix material. Furthermore, its graphite-like structure can better inhibit the leakage of manganese, thus further improving the electrochemical performance of the material.

[0195] The comparison between Example 1 and Comparative Example 3 shows that after adding salt to the raw materials, crystalline carbon nitride is generated due to the influence of salt on the thermal polymerization process of nitrogen-rich carbon source. This coating film has a significantly improved conductivity due to the increased crystallinity and conjugation degree. In addition, the presence of negatively charged defects and the special structure of crystalline carbon nitride enable it to better suppress the leaching of manganese elements compared to amorphous (non-crystalline) carbon nitride, thereby improving the cycle stability of the material.

[0196] Comparing Examples 1-2, 4-6, 8-18 with Examples 3 and 7, it can be seen that the present invention further improves the electrochemical performance by having a coating layer thickness of 10-150 nm and a coating layer accounting for 0.2-5 wt% of the total mass of the cathode material.

[0197] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A positive electrode material, characterized in that, It includes a core and a coating layer disposed on the outer surface of the core. The core includes lithium iron manganese oxide, and the coating layer is crystalline carbon nitride. The thickness of the coating layer is 10-150 nm. The X-ray photoelectron spectroscopy (XPS) spectrum of the cathode material shows a peak in the range of 396.5-398.0 eV; the carbon-13 solid-state nuclear magnetic resonance (NMR) spectrum of the cathode material shows a peak in the range of 118-126 ppm. The coating layer accounts for 0.2-5 wt% of the total mass of the cathode material; The crystalline carbon nitride structure contains numerous micropores and mesopores, with certain gaps existing between its atomic layers.

2. The cathode material according to claim 1, characterized in that, Satisfying at least one of the following (a)-(b): (a) The X-ray diffraction pattern of the cathode material has a first diffraction peak in the range of 27.5-29.0°; (b) The X-ray diffraction pattern of the cathode material has a second diffraction peak in the range of 7.5-9.0°.

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

4. The cathode material according to claim 1, characterized in that, The manganese iron lithium 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 ≤ 1.1, and M includes at least one of In, La, Zr, Ce, W, Al, Ti, Sr, Mg, Sb, V, Zn, Cu, Cr.

5. A method for preparing a positive electrode material as described in any one of claims 1-4, characterized in that, Includes the following steps: Provides a mixture including nitrogen-rich carbon sources and salts; The mixture, lithium source, manganese source, iron source, phosphorus source, and solvent are mixed evenly and sintered to obtain the cathode material.

6. The method for preparing the cathode material according to claim 5, characterized in that, Satisfy at least one of the following (I)-(VI): (I) The nitrogen-rich carbon source includes at least one of ammonia nitrile, dicyandiamide, melamine, and 5-aminotetrazole; (II) The salt includes at least one of potassium chloride, sodium chloride, potassium bromide, sodium bromide, potassium thiocyanate, and sodium thiocyanate; (III) The lithium source includes at least one of lithium oxide, lithium hydroxide, and lithium carbonate; (IV) The manganese source includes at least one of manganese oxide, manganese sulfate tetrahydrate, manganese acetate tetrahydrate, and manganese oxalate; (V) The iron source includes at least one of iron sheet, ferrous oxide, iron(II) oxide, ferrous sulfate heptahydrate, ferrous acetate, and ferrous oxalate tetrahydrate; (VI) The phosphorus source includes at least one of phosphoric acid, ammonium phosphate, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate.

7. The method for preparing the cathode material according to claim 5, characterized in that, The mass ratio of the nitrogen-rich carbon source to the salt is 1:(2-10); and / or The total mass ratio of the lithium source, manganese source, iron source, and phosphorus source to the nitrogen-rich carbon source is 100:(5-30).

8. A secondary battery, characterized in that, It includes a positive electrode sheet, wherein the positive electrode sheet comprises the positive electrode material according to any one of claims 1-4.

9. An electrical device, characterized in that, It includes the secondary battery as described in claim 8, wherein the secondary battery serves as the power supply for the electrical device.

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