Composite positive electrode material and preparation method thereof, positive plate and lithium ion battery

Abstract

The invention provides a composite positive electrode material and a preparation method thereof, a positive plate and a lithium ion battery. The composite positive electrode material comprises an inner core and a shell layer coating at least part of the surface of the inner core, the core comprises lithium manganese iron phosphate and a modification material, and the modification material contains a carbon chain structure; and the shell layer comprises a ternary material and amorphous carbon. According to the composite positive electrode material provided by the invention, on one hand, the lithium manganese iron phosphate is coated with the ternary material composite amorphous carbon, so that the conductivity and the ionic conductivity of the lithium manganese iron phosphate are improved; on the other hand, a modification material is introduced into the lithium manganese iron phosphate material, lithium manganese iron phosphate particles are inhibited and modified through a carbon chain structure in the modification material, and agglomeration of the lithium manganese iron phosphate particles is reduced, so that lithium manganese iron phosphate is coated by the ternary material more uniformly, and the overall electrical performance of the composite positive electrode material is effectively improved; meanwhile, the manganese dissolving phenomenon of lithium manganese iron phosphate is improved, and the overall structural stability of the material is improved.

Description

Technical Field

[0001] The present application belongs to the field of battery materials, and specifically relates to a composite positive electrode material and a preparation method, a positive electrode sheet, and a lithium-ion battery. Background Art

[0002] With the rapid development of modern electronic devices and new energy vehicle industries, the requirements for battery performance are becoming increasingly stringent. Due to its relatively low redox potential (3.4V), lithium iron phosphate batteries have a low energy density. The development of its energy density has almost reached its limit, and there is little room for performance improvement. In order to overcome the limitation of energy density, researchers introduced manganese elements into lithium iron phosphate and replaced part of the Fe in LiFePO4 with Mn to synthesize lithium manganese iron phosphate (LMFP) with a similar structure. This allows LMFP to inherit the high multiplicity and thermal stability of lithium iron phosphate and has the characteristics of high operating voltage. It is a very popular research material in positive electrode materials.

[0003] However, lithium manganese iron phosphate still has some inherent defects. Its electronic conductivity and lithium ion diffusion coefficient are low, which will affect the battery's charge and discharge performance and rate performance; manganese elements may dissolve during the cycle, resulting in a short cycle life.

[0004] Therefore, how to improve lithium manganese iron phosphate materials, enhance their conductivity and cycle performance, and broaden the application scenarios of lithium manganese iron phosphate materials is a problem that needs to be solved at present. Summary of the invention

[0005] One of the purposes of the present application is to provide a composite positive electrode material, aiming to solve the problem that the electrical performance of batteries prepared from existing lithium manganese iron phosphate materials is not ideal, especially the poor cycle performance.

[0006] Another object of the present application is to provide a method for preparing a composite positive electrode material. Another object of the present application is to provide a positive electrode sheet and a lithium-ion battery.

[0007] The first embodiment of the present application provides a composite positive electrode material, comprising a core and a shell layer covering at least a portion of the surface of the core;

[0008] The core comprises lithium iron manganese phosphate and a modification material, wherein the modification material contains a carbon chain structure, and the chemical formula of the lithium iron manganese phosphate is LiMn n Fe 1-n PO4, where 0.2≤n≤0.8;

[0009] The shell layer includes a ternary material and amorphous carbon, and the chemical formula of the ternary material is LiNi x Co y Mn 1-x-yO2, where 0.45≤x≤0.65, 0.1≤y≤0.2.

[0010] In some embodiments, the particle size of the core is 1-5 μm.

[0011] In some embodiments, the shell layer has a thickness of 200-600 nm.

[0012] In some embodiments, the mass ratio of the lithium manganese iron phosphate, the modifying material, the ternary material and the amorphous carbon is 1: (0.01-0.1): (0.1-0.3): (0.05-0.15).

[0013] In some embodiments, the specific surface area of ​​the composite positive electrode material is 12.5 to 25.8 m 2 / g.

[0014] In some embodiments, the compaction density of the composite positive electrode material is 2.25-2.60 g / cm 3 .

[0015] In some embodiments, the modifying material includes at least one of polyvinyl pyrrolidone, polyethylene glycol, polyvinyl alcohol, polyacrylic acid, propylene glycol methyl ether acetate, hexadecyl trimethyl ammonium bromide and sodium dodecyl sulfate.

[0016] In some embodiments, the amorphous carbon includes at least one of carbon nanotubes, graphene, carbon fibers, and conductive carbon black.

[0017] The second embodiment of the present application provides a method for preparing a composite positive electrode material, which is used to prepare the composite positive electrode material in any of the above embodiments, comprising the following steps:

[0018] Providing a first manganese source, an iron source and a first lithium source, mixing them in a phosphoric acid aqueous solution, and reacting to obtain a manganese iron lithium phosphate solution;

[0019] Adding an auxiliary agent to the lithium iron manganese phosphate solution, performing a solvent thermal reaction and a first sintering to obtain a lithium iron manganese phosphate composite material;

[0020] Providing a second lithium source, a nickel source, a cobalt source, and a second manganese source, and mixing them to form a precursor solution;

[0021] Adding lithium manganese iron phosphate composite material and carbon source to the precursor solution to react and obtain a mixed sol;

[0022] The mixed sol is freeze-dried, and subjected to a second sintering and a third sintering to obtain the composite positive electrode material.

[0023] In some embodiments, the first manganese source includes at least one of manganese sulfate, manganese carbonate, manganese acetate, manganese nitrate, manganese phosphate, manganese oxalate, manganese dihydrogen phosphate, and manganese chloride.

[0024] In some embodiments, the iron source includes at least one of ferrous sulfate, ferrous phosphate, ferrous nitrate, ferrous oxalate, ferrous sulfide, ferrous chloride, and ferrous acetate.

[0025] In some embodiments, the first lithium source includes at least one of lithium hydroxide, lithium carbonate, lithium dihydrogen phosphate, lithium oxalate, lithium nitrate, and lithium acetate.

[0026] In some embodiments, the auxiliary agent includes at least one of polyvinyl pyrrolidone, polyethylene glycol, polyvinyl alcohol, polyacrylic acid, propylene glycol methyl ether acetate, hexadecyl trimethyl ammonium bromide and sodium lauryl sulfate.

[0027] In some embodiments, the carbon source includes at least one of carbon nanotubes, graphene, carbon fibers, and conductive carbon black.

[0028] In some embodiments, the second lithium source includes at least one of lithium acetate, lithium carbonate, lithium sulfate, lithium nitrate, lithium oxalate, lithium dihydrogen phosphate, and lithium hydroxide.

[0029] In some embodiments, the nickel source includes at least one of nickel acetate, nickel sulfate, nickel nitrate, nickel chloride, and nickel oxalate.

[0030] In some embodiments, the cobalt source includes at least one of cobalt acetate, cobalt sulfate, cobalt nitrate, cobalt chloride, and cobalt acid.

[0031] In some embodiments, the second manganese source includes at least one of manganese acetate, manganese sulfate, manganese nitrate, manganese chloride, and manganese oxalate.

[0032] In some embodiments, a molar ratio of the first lithium source, the first manganese source, the iron source, and the phosphoric acid is (1-1.05):(0.2-0.8):(0.2-0.8):1.

[0033] In some embodiments, the mass of the auxiliary agent accounts for 1-10% of the sum of the mass of the first manganese source and the iron source.

[0034] In some embodiments, the molar ratio of the second lithium source, the nickel source, the cobalt source and the second manganese source is (0.95-1.05): (0.45-0.65): (0.10-0.20): (0.15-0.35).

[0035] In some embodiments, the mass ratio of the precursor, the lithium manganese iron phosphate composite material and the carbon source is (0.1-0.3):1:(0.05-0.15).

[0036] In some embodiments, the freeze-drying temperature is -60 to -20°C, the vacuum degree is 10 to 100 Pa, and the drying time is 8 to 48 hours.

[0037] In some embodiments, the first sintering is performed at a temperature of 500 to 850° C. and for a time of 2 to 8 hours.

[0038] In some embodiments, the second sintering is performed at a temperature of 350 to 600° C. and for a time of 3 to 8 hours.

[0039] In some embodiments, the temperature of the third sintering is 550-850° C., and the time is 5-10 hours.

[0040] In some embodiments, after providing a second lithium source, a nickel source, a cobalt source, and a second manganese source, and mixing them to form a precursor solution, the method further includes:

[0041] Add complexing agent and stir for a certain period of time.

[0042] In some embodiments, the complexing agent includes at least one of EDTA, citric acid, glycine, tartaric acid, salicylic acid, and ascorbic acid.

[0043] In some embodiments, the molar ratio of the complexing agent to the total concentration of metal ions in the precursor solution is (1-2):1.

[0044] The third embodiment of the present application provides a positive electrode sheet, comprising the composite positive electrode material in any of the above embodiments, or comprising the composite positive electrode material prepared by the preparation method in any of the above embodiments.

[0045] A fourth embodiment of the present application provides a lithium-ion battery, comprising the positive electrode sheet in the above embodiment.

[0046] The present application provides a composite positive electrode material, including a core and a shell layer covering at least a portion of the surface of the core; the core includes lithium manganese iron phosphate and a modifying material, the modifying material contains a carbon chain structure, and the chemical formula of the lithium manganese iron phosphate is LiMn n Fe 1- n PO4, wherein 0.1≤n≤0.9; the shell layer includes a ternary material and amorphous carbon, and the chemical formula of the ternary material is LiNi x Co y Mn 1-x-yO2, wherein 0.1≤x≤0.9, 0.1≤y≤0.9. The composite positive electrode material provided in the present application uses a ternary material composite amorphous carbon to coat lithium iron manganese phosphate, thereby increasing the electrical conductivity and ionic conductivity of lithium iron manganese phosphate; on the other hand, a modifying material is introduced into the lithium iron manganese phosphate material, and the carbon chain structure in the modifying material is used to inhibit and modify the lithium iron manganese phosphate particles, thereby reducing the agglomeration of the lithium iron manganese phosphate particles, thereby making the ternary material more uniformly coated with the lithium iron manganese phosphate, effectively improving the overall electrical properties of the composite positive electrode material, while improving the manganese dissolution phenomenon of lithium iron manganese phosphate, and improving the overall structural stability of the material. DETAILED DESCRIPTION

[0047] The technical solutions in the embodiments of the present application will be clearly and completely described below in combination with the specific implementation methods in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present application.

[0048] In addition, in the description of the present application, the term "including" means "including but not limited to". The terms first, second, third, etc. are used only as labels and do not impose numerical requirements or establish an order. Various embodiments of the present application may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity and should not be understood as a rigid limitation on the scope of the present application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within the range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the numbered range, such as 1, 2, 3, 4, 5 and 6, which applies regardless of the range. In addition, whenever a numerical range is indicated in this article, it is meant to include any cited numbers (fractional or integer) within the indicated range.

[0049] As a new type of battery material, lithium iron manganese phosphate has many advantages, such as high theoretical specific capacity, good thermal stability, relatively low cost and environmental friendliness, but its low electronic conductivity and lithium ion diffusion coefficient will limit the application scenarios of lithium iron manganese phosphate. The inventors found that by compounding with ternary materials, lithium iron manganese phosphate can play its advantages in safety and cost, while absorbing the advantages of ternary materials in energy density and voltage platform to achieve better battery performance. However, due to the fact that lithium iron manganese phosphate particles are prone to agglomeration, ternary materials are prone to uneven material distribution during the process of compounding with lithium iron manganese phosphate, resulting in no obvious improvement in battery performance.

[0050] Therefore, the inventors have found through research that by modifying lithium manganese iron phosphate with modifying materials, a composite positive electrode material uniformly coated with ternary materials can be obtained, thereby effectively improving the conductivity and cycle performance of the battery.

[0051] The first embodiment of the present application provides a composite positive electrode material, including a core and a shell layer covering at least a portion of the surface of the core;

[0052] The core includes lithium iron manganese phosphate and a modification material. The modification material contains a carbon chain structure. The chemical formula of lithium iron manganese phosphate is LiMn n Fe 1-n PO4, where 0.2≤n≤0.8;

[0053] The shell layer includes ternary materials and amorphous carbon. The chemical formula of the ternary material is LiNi x Co y Mn 1-x-y O2, where 0.45≤x≤0.65, 0.1≤y≤0.2.

[0054] By coating lithium iron manganese phosphate with ternary materials, the conductivity and ion conductivity of lithium iron manganese phosphate can be increased. In addition, the combination of the ternary material in the shell and amorphous carbon can also increase the electronic conductivity of the composite material, thereby further improving the overall electrical performance of the composite positive electrode material.

[0055] Furthermore, the modified material is connected to the lithium iron manganese phosphate through chemical bonds, and the carbon chain structure in the modified material can be adsorbed on the surface of the lithium iron manganese phosphate particles in the dispersed system and extend into the surrounding medium, preventing the lithium iron manganese phosphate particles from approaching and aggregating through the steric hindrance effect, thereby keeping the lithium iron manganese phosphate particles dispersed, and further allowing the modified material molecules to form a thin film with a certain elasticity on the surface of the lithium iron manganese phosphate particles, providing a physical barrier between the particles, increasing the effective distance between the lithium iron manganese phosphate particles, making it difficult for the lithium iron manganese phosphate particles to collide and aggregate with each other, thereby reducing the agglomeration of the lithium iron manganese phosphate particles and improving the dispersion stability of the lithium iron manganese phosphate composite material system. On this basis, the lithium iron manganese phosphate core modified by the modified material has a smaller particle size, and the shell formed by the ternary material composite amorphous carbon can uniformly coat the lithium iron manganese phosphate composite material, so that the shell has a more uniform and ideal thickness, thereby effectively inhibiting the manganese dissolution phenomenon of the lithium iron manganese phosphate material and improving the overall structural stability of the material; in addition, the lithium iron manganese phosphate material with more uniform dispersion and smaller particles can also increase the specific surface area of ​​the composite positive electrode material, thereby improving the overall electrical performance of the composite positive electrode material.

[0056] In some embodiments, the particle size of the core is 1-5 μm.

[0057] It is understandable that the value of the particle size of the core (unit: μm) can be any value among 1, 2, 3, 4, 5 or a range between any two values. A smaller particle size can increase the specific surface area of ​​the composite positive electrode material, thereby increasing the contact area with the electrolyte and improving the energy density of the battery; but too small a core particle size may increase the interface resistance and reduce the energy transmission efficiency of the battery. When the core particle size meets the above range, the battery has a higher energy density and a more ideal structural stability.

[0058] In some embodiments, the shell layer has a thickness of 200-600 nm.

[0059] It is understood that the thickness of the shell layer (unit: nm) can be any value among 200, 300, 400, 500, 600 or a range between any two values. The appropriate thickness can effectively inhibit the dissolution of manganese ions and increase the overall conductivity of the composite positive electrode material. When the thickness of the shell layer meets the above range, the battery has a relatively ideal cycle performance.

[0060] In some embodiments, the mass ratio of lithium manganese iron phosphate, the modifying material, the ternary material and the amorphous carbon is 1:(0.01-0.1):(0.1-0.3):(0.05-0.15).

[0061] It is understood that the mass ratio of lithium manganese iron phosphate, modified material, ternary material and amorphous carbon is any value in 1:0.01:0.1:0.05, 1:0.01:0.2:0.05, 1:0.01:0.3:0.05, 1:0.05:0.2:0.05, 1:0.1:0.2:0.05, 1:0.05:0.2:0.1, 1:0.05:0.2:0.15 or the range between any two values. When the mass of lithium manganese iron phosphate, modified material, ternary material and amorphous carbon meets the above ratio, the core particle size and shell thickness of the composite positive electrode material have a relatively ideal ratio, so that the battery has an ideal energy density and cycle performance.

[0062] In some embodiments, the specific surface area of ​​the composite positive electrode material is 12.5 to 25.8 m 2 / g.

[0063] It is understandable that the value of the specific surface area of ​​the composite positive electrode material (unit: m 2 / g) can be any value among 12.5, 13.8, 17.1, 20.4, 23.7, 25.8 or the range between any two values. A larger specific surface area can increase the contact area between the positive electrode material and the electrolyte, making it easier for lithium ions to be embedded and extracted between the electrode material and the electrolyte, thereby improving the charge and discharge efficiency of the battery. In addition, it can provide more reaction sites and faster ion transmission channels, thereby improving the rate performance of the battery. However, too high a specific surface area may cause the stability of the battery to decrease. When the specific surface area of ​​the composite positive electrode material meets the above range of values, the battery has good charge and discharge efficiency, rate performance and stability.

[0064] In some embodiments, the compaction density of the composite cathode material is 2.25 to 2.60 g / cm 3 .

[0065] It is understandable that the compaction density of the composite positive electrode material (unit: g / cm 3 ) can be any value among 2.25, 2.30, 2.35, 2.40, 2.45, 2.50, 2.55, 2.60 or a range between any two values. When the compaction density of the composite positive electrode material meets the above value range, the battery has an ideal energy density and cycle energy.

[0066] In some embodiments, the modification material includes at least one of polyvinyl pyrrolidone (PVP), polyethylene glycol (PEG), polyvinyl alcohol (PVA), polyacrylic acid (PAA), propylene glycol methyl ether acetate (PMA), hexadecyl trimethyl ammonium bromide (CTAB), and sodium dodecyl sulfate (SDS). Taking PVP as an example, PVP forms coordination bonds with metal ions in lithium iron manganese phosphate through carbonyl groups and nitrogen atoms in its molecules, and is combined together through coordination. The large number of pyrrolidone groups contained in the PVP molecular chain have strong polarity and cohesive energy, so that the PVP molecules themselves can be closely gathered together to form a continuous protective film with a certain strength on the surface of the lithium iron manganese phosphate particles, thereby avoiding the agglomeration of the lithium iron manganese phosphate particles.

[0067] In some embodiments, the amorphous carbon includes at least one of carbon nanotubes, graphene, carbon fibers, and conductive carbon black.

[0068] The second embodiment of the present application provides a method for preparing a composite positive electrode material, which is used to prepare the composite positive electrode material in any of the above embodiments, comprising the following steps:

[0069] Providing a first manganese source, an iron source and a first lithium source, mixing them in a phosphoric acid aqueous solution, and reacting to obtain a manganese iron lithium phosphate solution;

[0070] Adding an additive to a lithium iron manganese phosphate solution, performing a solvent thermal reaction and a first sintering to obtain a lithium iron manganese phosphate composite material;

[0071] Providing a second lithium source, a nickel source, a cobalt source, and a second manganese source, and mixing them to form a precursor solution;

[0072] Adding lithium manganese iron phosphate composite material and carbon source to the precursor solution to react and obtain a mixed sol;

[0073] The mixed sol is freeze-dried, and subjected to a second sintering and a third sintering to obtain a composite positive electrode material.

[0074] The inventors of this application have found through research that when a simple stirring method is used to compound lithium iron manganese phosphate and ternary materials, it is difficult to ensure that the two are fully mixed and uniform, which may lead to uneven distribution of materials, and the particle agglomeration of the materials is relatively serious. At the same time, the composite material obtained is only a simple point-to-point contact between the two, which affects the overall performance of the battery. The scheme provided by this application introduces a modified material by adding an auxiliary agent during the synthesis of the lithium iron manganese phosphate composite material, which can ensure that the lithium iron manganese phosphate particles are well modified and inhibited during the formation process, which is conducive to the uniform growth of particles and avoids agglomeration. In addition, the lithium iron manganese phosphate particles are modified preferentially before the lithium iron manganese phosphate is coated with amorphous carbon using a ternary material, which can effectively form a stable protective film on the surface of the lithium iron manganese phosphate particles, improve the stability of the particles, and avoid the difficulty of the auxiliary agent to penetrate into the particles caused by the addition of auxiliary agents in the subsequent process, resulting in the dispersion and modification of the lithium iron manganese phosphate particles. The effect is not obvious.

[0075] Subsequently, the synthesized lithium manganese iron phosphate composite material was added to the ternary material precursor solution containing a carbon source, and the ternary material composite amorphous carbon-coated lithium manganese iron phosphate was obtained by freeze drying and subsequent high-temperature carbonization, which increased the conductivity, ion conductivity and electronic conductivity of lithium manganese iron phosphate and improved the electrochemical performance of the composite positive electrode material. At the same time, the freeze drying method can effectively avoid the superposition and agglomeration of the ternary material during the coating process, and evenly coat the ternary material on the surface of the lithium manganese iron phosphate material, thereby significantly improving the manganese dissolution phenomenon of the lithium manganese iron phosphate material and improving the structural stability and cycle performance of the battery.

[0076] In some embodiments, the first manganese source includes at least one of manganese sulfate, manganese carbonate, manganese acetate, manganese nitrate, manganese phosphate, manganese oxalate, manganese dihydrogen phosphate, and manganese chloride.

[0077] In some embodiments, the iron source includes at least one of ferrous sulfate, ferrous phosphate, ferrous nitrate, ferrous oxalate, ferrous sulfide, ferrous chloride, and ferrous acetate.

[0078] In some embodiments, the first lithium source includes at least one of lithium hydroxide, lithium carbonate, lithium dihydrogen phosphate, lithium oxalate, lithium nitrate, and lithium acetate.

[0079] In some embodiments, the auxiliary agent includes at least one of polyvinyl pyrrolidone (PVP), polyethylene glycol (PEG), polyvinyl alcohol (PVA), polyacrylic acid (PAA), propylene glycol methyl ether acetate (PMA), cetyltrimethylammonium bromide (CTAB), and sodium dodecyl sulfate (SDS).

[0080] In some embodiments, the carbon source includes at least one of carbon nanotubes, graphene, carbon fibers, and conductive carbon black.

[0081] In some embodiments, the second lithium source includes at least one of lithium acetate, lithium carbonate, lithium sulfate, lithium nitrate, lithium oxalate, lithium dihydrogen phosphate, and lithium hydroxide.

[0082] In some embodiments, the nickel source includes at least one of nickel acetate, nickel sulfate, nickel nitrate, nickel chloride, and nickel oxalate.

[0083] In some embodiments, the cobalt source includes at least one of cobalt acetate, cobalt sulfate, cobalt nitrate, cobalt chloride, and cobalt acid.

[0084] In some embodiments, the second manganese source includes at least one of manganese acetate, manganese sulfate, manganese nitrate, manganese chloride, and manganese oxalate.

[0085] In some embodiments, the molar ratio of the first lithium source, the first manganese source, the iron source, and the phosphoric acid is (1-1.05):(0.2-0.8):(0.2-0.8):1.

[0086] It can be understood that the molar ratio of the first lithium source, the first manganese source, the iron source and the phosphoric acid can be any value of 1:0.2:0.8:1, 1.02:0.4:0.6:1, 1.03:0.6:0.4:1, 1.05:0.8:0.2:1 or a range between any two values. The molar ratio of the first lithium source, the first manganese source, the iron source and the phosphoric acid satisfies the above value range, and an ideal manganese iron lithium phosphate can be obtained.

[0087] In some embodiments, the mass of the auxiliary agent accounts for 1-10% of the sum of the mass of the first manganese source and the iron source.

[0088] It is understandable that the percentage of the mass of the auxiliary agent to the sum of the mass of the first manganese source and the iron source can be any value of 1%, 3%, 5%, 7%, 10%, or a range between any two values. When the ratio of the mass of the auxiliary agent to the sum of the mass of the first manganese source and the iron source meets the above value range, the modified material formed by the auxiliary agent can be used to ideally modify and disperse the lithium manganese iron phosphate particles, while avoiding excessive auxiliary agent occupying space to affect the overall energy density of the composite positive electrode material.

[0089] In some embodiments, the molar ratio of the second lithium source, the nickel source, the cobalt source, and the second manganese source is (0.95-1.05):(0.45-0.65):(0.10-0.20):(0.15-0.35).

[0090] It can be understood that the molar ratio of the second lithium source, the nickel source, the cobalt source and the second manganese source is any value or a range between any two values ​​of 0.95:0.45:0.2:0.35, 1:0.5:0.2:0.3, 1:0.55:0.1:0.35, 1:0.6:0.15:0.25, 1.05:0.65:0.1:0.25. When the molar ratio of the second lithium source, the nickel source, the cobalt source and the second manganese source meets the above ratio range, an ideal ternary material precursor can be obtained.

[0091] In some embodiments, the mass ratio of the precursor, the lithium manganese iron phosphate composite material and the carbon source is (0.1-0.3):1:(0.05-0.15).

[0092] It is understood that the mass ratio of the precursor, the lithium iron manganese phosphate composite material and the carbon source can be any value of 0.1:1:0.05, 0.2:1:0.05, 0.3:1:0.05, 0.2:1:0.1, 0.2:1:0.15 or a range between any two values. When the addition ratio of the precursor, the lithium iron manganese phosphate composite material and the carbon source meets the above range, the composite positive electrode material finally prepared has a relatively ideal core particle size and shell thickness, so that the battery has an ideal energy density and cycle performance.

[0093] In some embodiments, the freeze-drying temperature is -60 to -20°C, the vacuum degree is 10 to 100 Pa, and the drying time is 8 to 48 hours.

[0094] It can be understood that the freeze-drying temperature (unit: ℃) can be any value among -60, -50, -40, -30, -20 or a range between any two values; the freeze-drying vacuum degree (unit: Pa) can be any value among 10, 20, 40, 60, 100 or a range between any two values; the freeze-drying drying time (unit: h) can be any value among 8, 12, 16, 24, 36, 48 or a range between any two values. In a low-temperature environment, the thermal motion activity of molecules is greatly reduced. During the freezing process, the molecular kinetic energy in the mixed sol decreases sharply, the Brownian motion becomes very weak, and the collision frequency between particles is greatly reduced, which can effectively inhibit the superposition and agglomeration behavior of the ternary material precursor or the lithium iron manganese phosphate composite material; in addition, in a vacuum environment, the frozen solvent can skip the liquid stage during the freezing process, avoiding the effect of the liquid surface tension on the molecules, so that the ternary material precursor can be evenly distributed around the lithium iron manganese phosphate composite material and amorphous carbon, and will not gather together due to the pull of surface tension, thereby promoting the ternary material precursor to be evenly coated on the surface of the lithium iron manganese phosphate composite material after combining with amorphous carbon. When the freezing temperature, vacuum degree and drying time of the freeze-drying step meet the above range of values, the superposition and agglomeration of the ternary material precursor can be effectively inhibited, while ensuring that the ternary material composite amorphous carbon and the lithium iron manganese phosphate composite material fully react to form a uniform shell layer coated on the surface of the lithium iron manganese phosphate composite material.

[0095] In some embodiments, the first sintering is performed at a temperature of 500 to 850° C. for a time of 2 to 8 hours.

[0096] It can be understood that the value of the temperature of the first sintering (unit: ° C) can be any value of 500, 550, 600, 650, 700, 750, 800, 850 or a range between any two values, and the value of the time of the first sintering (unit: h) can be any value of 2, 3, 4, 5, 6, 7, 8 or a range between any two values. When the temperature and time of the first sintering meet the above ranges, the prepared lithium manganese iron phosphate composite material can have an ideal crystal structure and particle morphology.

[0097] In some embodiments, the second sintering is performed at a temperature of 350-600° C. and for a time of 3-8 hours.

[0098] It can be understood that the temperature of the second sintering (unit: ℃) can be any value among 350, 400, 450, 500, 550, 600 or a range between any two values, and the time of the second sintering (unit: h) can be any value among 3, 4, 5, 6, 7, 8 or a range between any two values.

[0099] In some embodiments, the third sintering temperature is 550-850° C. and the time is 5-10 hours.

[0100] It can be understood that the value of the temperature of the third sintering (unit: ° C) can be any value of 550, 600, 650, 700, 750, 800, 850 or a range between any two values, and the value of the time of the third sintering (unit: h) can be any value of 5, 6, 7, 8, 9, 10 or a range between any two values. After the freeze-drying is completed, the intermediate product is sintered twice with different temperature gradients, which can realize the decomposition, nucleus formation, crystal growth, structure improvement and other processes of the material in stages and steps. When the temperature and time of the second sintering, and the temperature and time of the third sintering meet the above range of values, the final composite positive electrode material reaches a good state in terms of crystal structure, particle morphology, electrochemical performance, etc.

[0101] In some embodiments, after providing a second lithium source, a nickel source, a cobalt source, and a second manganese source, and mixing them to form a precursor solution, the method further includes:

[0102] Add complexing agent and stir for a certain period of time.

[0103] By adding a complexing agent, the metal ions can be dispersed more evenly in the solvent, avoiding the agglomeration or precipitation of metal ions due to excessive local concentration, thereby further promoting the uniform distribution of the ternary material precursor. It also helps in the subsequent stirring and reaction process to allow the metal ions to fully contact and mix with materials such as lithium manganese iron phosphate composite materials and amorphous carbon to form a uniform sol system.

[0104] In some embodiments, the complexing agent includes at least one of EDTA, citric acid, glycine, tartaric acid, salicylic acid, and ascorbic acid.

[0105] In some embodiments, the molar ratio of the complexing agent to the total concentration of metal ions in the precursor solution is (1-2):1.

[0106] It is understood that the molar ratio of the complexing agent to the total concentration of metal ions in the precursor solution can be any value of 1:1, 1.5:1, 2:1, or a range between any two values. When the molar ratio of the complexing agent to the total concentration of metal ions in the precursor solution satisfies the above-mentioned proportional relationship, the complexing agent can make the metal ions uniformly dispersed in the solvent while avoiding the occurrence of side reactions caused by excessive amounts.

[0107] The third embodiment of the present application provides a positive electrode sheet, comprising the composite positive electrode material in any of the above embodiments, or comprising the composite positive electrode material prepared by the preparation method in any of the above embodiments.

[0108] A fourth embodiment of the present application provides a lithium-ion battery, comprising the positive electrode sheet in the above embodiment.

[0109] The following is a description of a composite positive electrode material and preparation method, a positive electrode sheet, and a lithium-ion battery provided by the present application in combination with specific embodiments:

[0110] Example 1

[0111] Example 1 provides a composite positive electrode material, which is prepared by the following method:

[0112] S1: Manganese sulfate monohydrate and ferrous sulfate heptahydrate were dissolved in deionized water, and then a certain amount of phosphoric acid aqueous solution (85wt%) was added. Then, lithium hydroxide aqueous solution was dripped into the mixture under vigorous stirring, and the pH value was adjusted to 7. After fully stirring for half an hour, a certain amount of PVP was added and the mixture was quickly transferred to a high-pressure reactor for solvent thermal reaction at a temperature of 180°C and a duration of 2 hours. After cooling to room temperature, the mixture was washed with deionized water several times and dried to obtain LiMn 0.6 Fe 0.4 PO4 precursor. 0.6 Fe 0.4 The PO4 precursor was ground in a grinder and then sintered at 650°C for 2 hours in a horizontal tube furnace filled with high-purity argon to obtain a lithium manganese iron phosphate composite material.

[0113] S2: Lithium acetate dihydrate, nickel acetate tetrahydrate, cobalt acetate tetrahydrate, and manganese acetate tetrahydrate were dissolved in a mixed solvent of deionized water and ethanol. The total ionic metal concentration of the mixed solution was 0.5 mol / L. EDTA, citric acid, and glycine were added as complexing agents at a fixed ratio of 1:1. After stirring for half an hour, lithium iron manganese phosphate composite materials were added, followed by carbon nanotubes. The mixed solution was stirred at room temperature for 10 hours, and then stirred in an oil bath at 80°C for 4 hours to form a uniform sol. It was then placed in a freeze dryer for vacuum drying, and the resulting powder was finally calcined at 550°C for 6 hours, and then calcined at 800°C to obtain the final sample.

[0114] Embodiments 2 to 4

[0115] The preparation processes of Examples 2 to 4 are consistent with those of Example 1, and the only difference is the adjustment of the process parameters.

[0116] Comparative Example 1

[0117] Comparative Example 1 provides a positive electrode material, which is prepared by the following method:

[0118] Manganese sulfate monohydrate and ferrous sulfate heptahydrate were dissolved in deionized water, and then a certain amount of phosphoric acid aqueous solution (85wt%) was added. Then, lithium hydroxide aqueous solution was dripped into the mixture under vigorous stirring, and the pH value was adjusted to 7. After being fully stirred for half an hour, the mixture was transferred to a high-pressure reactor for solvent thermal reaction at a reaction temperature of 180°C and a duration of 2 hours. After cooling to room temperature, the mixture was washed with deionized water several times and dried to obtain LiMn 0.6 Fe 0.4 PO4 precursor. 0.6 Fe 0.4 The PO4 precursor was ground in a grinder and then sintered at 650°C for 2 hours in a horizontal tube furnace filled with high-purity argon to obtain lithium manganese iron phosphate material.

[0119] Comparative Example 2

[0120] Comparative Example 2 provides a positive electrode material, which is prepared in the following manner:

[0121] S1: Manganese sulfate monohydrate and ferrous sulfate heptahydrate were dissolved in deionized water, and then a certain amount of phosphoric acid aqueous solution (85wt%) was added. Then, lithium hydroxide aqueous solution was dripped into the mixture under vigorous stirring, and the pH value was adjusted to 7. After being fully stirred for half an hour, the mixture was transferred to a high-pressure reactor for solvothermal reaction at a temperature of 180°C for 2 hours. After cooling to room temperature, the mixture was washed with deionized water several times and dried to obtain LiMn 0.6 Fe 0.4 PO4 precursor. 0.6 Fe 0.4 The PO4 precursor was ground in a grinder and then sintered at 650°C for 2 hours in a horizontal tube furnace filled with high-purity argon to obtain a lithium manganese iron phosphate composite material.

[0122] S2: Dissolve lithium acetate dihydrate, nickel acetate tetrahydrate, cobalt acetate tetrahydrate, and manganese acetate tetrahydrate in a mixed solvent of deionized water and ethanol, the total ion metal concentration of the mixed solution is 0.5 mol / L, add EDTA, citric acid, and glycine as complexing agents in a fixed ratio of 1:1, and mix to obtain a ternary material. Mix the lithium manganese iron phosphate composite material with the ternary material, add PVP and mix evenly, and use a ball mill to dry mix for 8 hours to obtain the positive electrode material.

[0123] The composite positive electrode materials obtained in the above examples and comparative examples are further used to prepare lithium manganese iron phosphate batteries. The specific preparation methods are as follows:

[0124] The positive electrode active material, conductive agent acetylene black and binder PVDF are mixed and dispersed in NMP in a mass ratio of 96:2.2:1.8, and the solid content is controlled at about 52%. The mixed black slurry is coated on both sides of the aluminum foil, and then baked, rolled and cut to obtain the positive electrode sheet.

[0125] The obtained positive electrode sheet is matched with the graphite negative electrode sheet, and the lithium manganese iron phosphate battery cell is finally obtained through the steps of slitting, winding, drying, liquefaction, and capacity injection.

[0126] The process parameters of Examples 1 to 4 and Comparative Examples 1 to 2 are shown in Table 1.

[0127] Table 1

[0128]

[0129]

[0130] The performance of the composite positive electrode materials and the battery cells prepared in Examples 1 to 4 and Comparative Examples 1 to 2 was tested as follows:

[0131] 0.1C discharge capacity in grams: Charge the battery to 4.25V at 0.1C constant current and constant voltage at 25±2℃, with a cut-off current of 0.05C. After leaving it for 10 minutes, discharge it to 2.5V at 0.1C constant current and leave it for 10 minutes. Repeat the charge and discharge three times and use the third discharge capacity as the judgment.

[0132] 1C discharge capacity in grams: Charge the battery to 4.25V at 1C constant current and constant voltage at 25±2℃, with a cut-off current of 0.05C. After leaving it for 10 minutes, discharge it to 2.5V at 1C constant current and leave it for 10 minutes. Repeat the charge and discharge three times and use the third discharge capacity as the judgment.

[0133] 100 cycle capacity retention rate: The battery was charged to 4.25V at 0.5C constant current and constant voltage at 25±2℃, with a cut-off current of 0.05C. After standing for 10 min, it was discharged to 2.5V at 0.5C constant current. This cycle was repeated for 100 times and the capacity retention rate was recorded. The 100th cycle capacity retention rate (%) = (100th cycle discharge specific capacity / first cycle discharge specific capacity) * 100%.

[0134] The test results are shown in Table 2.

[0135] Table 2

[0136]

[0137] As can be seen from Table 1 and Table 2, the embodiment of the present invention freeze-dries and composites lithium iron manganese phosphate and ternary materials, so that lithium iron manganese phosphate and ternary material particles can be in good contact and tightly combined, effectively improving the agglomeration of the material, while alleviating the manganese dissolution problem of lithium iron manganese phosphate, and significantly improving the capacity and cycle stability of the battery. According to Comparative Example 1, it can be seen that when the modification of the modifying material and the coating of the ternary material are lost, only lithium iron manganese phosphate is used as the positive electrode material, and the compaction density and specific surface area of ​​the positive electrode material are both low, resulting in unsatisfactory battery capacity and cycle stability. According to Comparative Example 2, it can be seen that in the process of preparing the composite positive electrode material, only the process of first coating the ternary material and then modifying the inner core with an additive is used. Compared with the lithium iron manganese phosphate positive electrode material in Comparative Example 1, the prepared battery performance is slightly improved, but compared with the composite positive electrode materials in Examples 1 to 4, there are still problems of low compaction density and specific surface area, and unsatisfactory battery capacity and cycle performance. Therefore, the process provided in the present application of first adding an additive to modify the lithium iron manganese phosphate and then coating it with a ternary material and amorphous carbon can effectively improve the battery's performance in capacity and cycle performance.

[0138] The above is a detailed introduction to the single crystal silicon rods, preparation methods, and silicon wafers provided in the embodiments of the present application. Specific examples are used in the present application to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the technical solutions and core ideas of the present application. Ordinary technicians in this field should understand that they can still modify the technical solutions recorded in the aforementioned embodiments, or replace some of the technical features therein with equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A composite positive electrode material, characterized in that: It comprises a core and a shell layer covering at least a part of the surface of the core; The core includes lithium iron manganese phosphate and a modification material, wherein the modification material contains a carbon chain structure, and the chemical formula of the lithium iron manganese phosphate is LiMn n Fe 1-n PO4, where 0.2≤n≤0.8; The shell layer includes a ternary material and amorphous carbon, and the chemical formula of the ternary material is LiNi x Co y Mn 1-x-y O2, where 0.45≤x≤0.65, 0.1≤y≤0.

2.

2. A composite positive electrode material according to claim 1, characterized in that: The particle size of the core is 1 to 5 μm; and / or, The thickness of the shell layer is 200 to 600 nm; and / or, The mass ratio of the lithium manganese iron phosphate, the modifying material, the ternary material and the amorphous carbon is 1: (0.01-0.1): (0.1-0.3): (0.05-0.15).

3. A composite positive electrode material according to claim 1, characterized in that: The specific surface area of ​​the composite positive electrode material is 12.5 to 25.8 m 2 / g; and / or, The compaction density of the composite positive electrode material is 2.25-2.60 g / cm 3 .

4. A composite positive electrode material according to claim 1, characterized in that: The modification material comprises at least one of polyvinyl pyrrolidone, polyethylene glycol, polyvinyl alcohol, polyacrylic acid, propylene glycol methyl ether acetate, hexadecyl trimethyl ammonium bromide and sodium lauryl sulfate; and / or, The amorphous carbon includes at least one of carbon nanotubes, graphene, carbon fibers and conductive carbon black.

5. A method for preparing a composite positive electrode material according to any one of claims 1 to 4, characterized in that: The steps include: Providing a first manganese source, an iron source and a first lithium source, mixing them in a phosphoric acid aqueous solution, and reacting to obtain a manganese iron lithium phosphate solution; Adding an auxiliary agent to the lithium iron manganese phosphate solution, performing a solvent thermal reaction and a first sintering to obtain a lithium iron manganese phosphate composite material; Providing a second lithium source, a nickel source, a cobalt source, and a second manganese source, and mixing them to form a precursor solution; Adding lithium manganese iron phosphate composite material and carbon source to the precursor solution to react and obtain a mixed sol; The mixed sol is freeze-dried, and subjected to a second sintering and a third sintering to obtain the composite positive electrode material.

6. The method for preparing a composite positive electrode material according to claim 5, characterized in that: The first manganese source comprises at least one of manganese sulfate, manganese carbonate, manganese acetate, manganese nitrate, manganese phosphate, manganese oxalate, manganese dihydrogen phosphate and manganese chloride; and / or, The iron source comprises at least one of ferrous sulfate, ferrous phosphate, ferrous nitrate, ferrous oxalate, ferrous sulfide, ferrous chloride and ferrous acetate; and / or, The first lithium source includes at least one of lithium hydroxide, lithium carbonate, lithium dihydrogen phosphate, lithium oxalate, lithium nitrate and lithium acetate; and / or, The auxiliary agent comprises at least one of polyvinyl pyrrolidone, polyethylene glycol, polyvinyl alcohol, polyacrylic acid, propylene glycol methyl ether acetate, hexadecyl trimethyl ammonium bromide and sodium lauryl sulfate; and / or, The carbon source comprises at least one of carbon nanotubes, graphene, carbon fibers and conductive carbon black; and / or, The second lithium source comprises at least one of lithium acetate, lithium carbonate, lithium sulfate, lithium nitrate, lithium oxalate, lithium dihydrogen phosphate and lithium hydroxide; and / or, The nickel source comprises at least one of nickel acetate, nickel sulfate, nickel nitrate, nickel chloride and nickel oxalate; and / or, The cobalt source comprises at least one of cobalt acetate, cobalt sulfate, cobalt nitrate, cobalt chloride and cobalt acid; and / or, The second manganese source includes at least one of manganese acetate, manganese sulfate, manganese nitrate, manganese chloride and manganese oxalate.

7. The method for preparing a composite positive electrode material according to claim 5, characterized in that: The molar ratio of the first lithium source, the first manganese source, the iron source and the phosphoric acid is (1-1.05): (0.2-0.8): (0.2-0.8): 1; and / or, The mass of the auxiliary agent accounts for 1 to 10% of the sum of the mass of the first manganese source and the iron source; and / or, The molar ratio of the second lithium source, the nickel source, the cobalt source and the second manganese source is (0.95-1.05): (0.45-0.65): (0.10-0.20): (0.15-0.35); and / or, The mass ratio of the precursor, the lithium manganese iron phosphate composite material and the carbon source is (0.1-0.3):1:(0.05-0.15).

8. The method for preparing a composite positive electrode material according to claim 5, characterized in that: The freeze-drying temperature is -60 to -20°C, the vacuum degree is 10 to 100 Pa, and the drying time is 8 to 48 hours.

9. The method for preparing a composite positive electrode material according to claim 5, characterized in that: The first sintering temperature is 500-850°C and the time is 2-8h; and / or, The second sintering temperature is 350-600°C and the time is 3-8h; and / or, The temperature of the third sintering is 550-850° C. and the time is 5-10 hours.

10. The method for preparing a composite positive electrode material according to claim 5, characterized in that: After providing a second lithium source, a nickel source, a cobalt source and a second manganese source, and mixing them to form a precursor solution, the method further includes: Add complexing agent and stir for a certain period of time.

11. The method for preparing a composite positive electrode material according to claim 10, characterized in that: The complexing agent comprises at least one of EDTA, citric acid, glycine, tartaric acid, salicylic acid and ascorbic acid; and / or, The molar ratio of the complexing agent to the total concentration of metal ions in the precursor solution is (1-2):

1.

12. A positive electrode sheet, characterized in that: The invention comprises the composite positive electrode material as claimed in any one of claims 1 to 4 or the composite positive electrode material prepared by the preparation method as claimed in any one of claims 5 to 11.

13. A lithium ion battery, characterized in that: Comprising the positive electrode sheet as claimed in claim 12.

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