Modified lithium manganese iron phosphate materials and their preparation methods, cathode materials and lithium-ion batteries

Abstract

This invention provides a modified lithium manganese iron phosphate material, its preparation method, a cathode material, and a lithium-ion battery. The modified lithium manganese iron phosphate material comprises a core and a hydrophobic carbon layer coating the surface of the core. The core comprises lithium manganese iron phosphate material, doped with niobium and / or aluminum. The raw material for the hydrophobic carbon layer is a carbon source modified with a specific type of hydrophobic agent. This invention uses a hydrophobic agent to modify and graft the carbon source, which can form a stable core-shell structure and effectively improve the stability of the lithium manganese iron phosphate structure. The hydrophobic agent grafted onto the carbon source surface and the carbon chain structure provided by the carbon source are beneficial to improving the electronic conductivity and electrical conductivity of the lithium manganese iron phosphate material. The doping of specific elements can effectively suppress the collapse of the lithium manganese iron phosphate crystal structure and improve its electronic conductivity. The specific type of hydrophobic agent significantly enhances the hydrophobicity of the lithium manganese iron phosphate material, which is beneficial to improving the electrochemical performance of the material.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and more specifically, to a modified lithium manganese iron phosphate material, its preparation method, a cathode material, and a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries possess advantages such as high conversion efficiency, high energy density and power density, no memory effect, and long lifespan, making them the most mature and widely used battery technology. There is a growing expectation for lithium-ion battery materials to achieve lower costs, higher energy density, and enhanced safety performance.

[0003] Lithium manganese iron phosphate (LMFP) is obtained by doping lithium iron phosphate (LFP) with Mn ions. Because LMFP has a higher operating voltage (up to 4.1V), it has a potential energy density that is about 20% higher than LFP. The low cost, high stability, and environmental friendliness of LMFP also give it broad development prospects.

[0004] However, lithium manganese iron phosphate materials have some inherent drawbacks, such as: Mn during charging and discharging... 3+ The JT effect causes lattice distortion, which can disrupt structural stability; the electronic conductivity is low (approximately 1 × 10⁻⁶). -9 (s / cm); the lithium-ion diffusion coefficient is relatively low (approximately 5.1 × 10⁻⁶). -14 cm 2 Issues such as excessive moisture content due to contact with air during processing and preparation mean that the electrochemical performance of lithium-ion batteries, including charge / discharge performance and cycle efficiency, still need to be improved. Summary of the Invention

[0005] The main objective of this invention is to provide a modified lithium manganese iron phosphate material and its preparation method, a cathode material, and a lithium-ion battery, in order to solve the problems of poor structural stability, low electronic conductivity, low lithium-ion diffusion coefficient, and easy excessive moisture content during the preparation process in the existing lithium manganese iron phosphate material, which leads to poor electrochemical performance.

[0006] To achieve the above objectives, according to one aspect of the present invention, a modified lithium manganese iron phosphate material is provided, comprising a core and a hydrophobic carbon layer coated on the surface of the core, wherein the core comprises lithium manganese iron phosphate material with the general formula Li. x Fe y Mn 1-y- z M zPO4, 1.01≤x≤1.02, 0<y≤0.4, 0.002≤z≤0.005; M is niobium and / or aluminum; the raw material for the hydrophobic carbon layer is a carbon source modified with a hydrophobic agent; the hydrophobic agent includes one or more of octadecyltrichlorosilane, dodecanoic acid, oleic acid, tridecafluorooctyltriethoxysilane, tetraethoxysilane, and polyacrylamide. This invention modifies and grafts the carbon source using a hydrophobic agent. During preparation, the hydrophobic carbon source exhibits a coiled shape, tightly encapsulating the precursor of lithium manganese iron phosphate material to form a stable core-shell structure. The hydrophobic carbon layer supports the overall structure of lithium manganese iron phosphate, maintaining its structural stability and preventing damage, thus improving the structural stability of lithium manganese iron phosphate and ensuring its stability during subsequent pulverization. The carbon links of the hydrophobic agent branch onto the surface of the carbon source, forming organic side chains. During the preparation of lithium manganese iron phosphate, the carbon source and these side chains undergo carbonization. The rich carbon chain structure is beneficial to improving electron transport efficiency, thereby improving the electronic conductivity and electrical conductivity of the lithium manganese iron phosphate material. The modified lithium manganese iron phosphate material of this invention contains specific doping elements niobium and / or aluminum. On the one hand, the doping elements can expand the lithium-ion diffusion channels and reduce the energy barrier of lithium-ion transport; on the other hand, the addition of doping elements can make the lithium manganese iron phosphate material highly oriented, with fine grains, and form an ordered superlattice structure. The highly oriented grains can effectively suppress the increase of internal resistance and improve the capacity retention of the material. The superlattice structure can also effectively suppress the collapse of the crystal structure during charging and discharging, forming defects and holes inside the crystal, thereby changing the electronic structure of the material and improving the electronic conductivity of the material. Modifying lithium manganese iron phosphate materials with hydrophobic agents can form hydrophobic structures on their surfaces, significantly improving the hydrophobicity of lithium manganese iron phosphate materials. This helps to solve the problem of excessive moisture content in the preparation process of lithium manganese iron phosphate materials, and can further effectively improve the electrochemical performance of lithium manganese iron phosphate materials.

[0007] Furthermore, the D50 particle size of the modified lithium manganese iron phosphate material is 350–420 nm. Because the modified lithium manganese iron phosphate material of this invention includes a hydrophobic carbon layer, the carbon source molecular chains tightly encapsulate the precursor of the lithium manganese iron phosphate material, resulting in smaller particle size and a more compact carbon layer. While maintaining the hydrophobicity of the lithium manganese iron phosphate material, by controlling the particle size within the aforementioned small particle size range, the lithium-ion transfer path can be shortened more effectively, and its transfer rate can be increased, thereby further improving the electrochemical performance of the lithium manganese iron phosphate material.

[0008] Furthermore, the mass ratio of the core to the carbon source is 100:(5.2–6.2). Under these conditions, the core and the carbon source can work synergistically. Under reduction reaction conditions, without affecting lithium-ion insertion / extraction, the carbon source can tightly encapsulate the core, stably supporting the overall structure of lithium manganese iron phosphate, providing more sufficient conductive channels, and further improving the conductivity of lithium manganese iron phosphate materials. This results in better stability, hydrophobicity, and electrochemical performance of lithium manganese iron phosphate materials.

[0009] According to another aspect of the present invention, a method for preparing modified lithium manganese iron phosphate material is provided, comprising the following steps: Step S1, pretreating a carbon source to obtain a pretreated carbon source, and subjecting the pretreated carbon source to a first stirring and photochemical reaction with a first solvent and a hydrophobic agent to obtain a first mixed solution containing the modified carbon source; Step S2, sequentially subjecting a lithium source, an iron source, a manganese source, a phosphorus source, and a second solvent to a second stirring and a first grinding to obtain a second mixed solution; Step S3, sequentially subjecting the first mixed solution, the second mixed solution, and a dopant containing M to a third stirring to obtain a third mixed solution, and sequentially subjecting it to a second grinding, drying, and sintering to obtain the modified lithium manganese iron phosphate material; wherein the hydrophobic agent includes one or more of octadecyltrichlorosilane, dodecanoic acid, oleic acid, tridecafluorooctyltriethoxysilane, tetraethoxysilane, and polyacrylamide; and M is niobium and / or aluminum. The modified lithium manganese iron phosphate material obtained by the above preparation method contains hydrophobic groups, has good hydrophobic properties, can effectively avoid the problem of excessive moisture caused by contact with air during subsequent processing, and has a stable structure and high cycle stability. Furthermore, the formation of the carbon layer and the doping of M ions can effectively improve the lithium-ion diffusion coefficient and electronic conductivity.

[0010] Further, in step S1, the pretreatment is carried out in an air and / or nitrogen atmosphere; more preferably, the gas flow rate in the atmosphere is 0.3-0.5 L / min; and / or the pretreatment time is 30-40 s; and / or the first stirring includes the following steps: firstly, a first solvent is added to the pretreated carbon source for first sub-stirring, and then a hydrophobic agent is added for second sub-stirring; preferably, the first sub-stirring time is 2-4 h; and / or the second sub-stirring time is 3-4 h; and / or the stirring speed is 200-300 rpm; and / or the photochemical reaction time is 0.5-2 h, and the temperature is 40-60 °C; preferably, the photochemical reaction method includes one or more of infrared radiation, electromagnetic radiation, and infrared laser methods. Under the above conditions, the carbon source surface has more active groups, the modification and grafting reaction of the carbon source proceeds more fully, and more organic carbon chains are grafted onto the modified carbon source. On the one hand, this is more conducive to tightly coating the lithium manganese iron phosphate precursor, which further helps to improve the stability of the modified lithium manganese iron phosphate material structure and reduce the material particle size. On the other hand, it is more conducive to improving the hydrophobicity of the modified lithium manganese iron phosphate material. In addition, the richer branched structure can further improve the electronic conductivity of lithium manganese iron phosphate.

[0011] Further, in step S1, the carbon source includes one or more of glucose, polyacrylate, starch, PVA, and PVDF; and / or the first solvent includes one or more of water, ethylene glycol, propanol, acetone, and N-methylpyrrolidone; preferably, the first solvent includes water and ethylene glycol, wherein the volume ratio of water to ethylene glycol is 1:(0.5-1), more preferably 1:(0.5-0.6); and / or the hydrophobic agent includes octadecyltrichlorosilane and / or dodecanoic acid; preferably, when the hydrophobic agent is a mixture of octadecyltrichlorosilane and dodecanoic acid, the mass ratio of the two is (1-2):1; and / or the mass ratio of the hydrophobic agent to the carbon source is (4-7):100; and / or the mass ratio of the first solvent to the carbon source is 100:(42-65). Under the above conditions, the grafting effect of the carbon source is better, and the hydrophobicity of the lithium manganese iron phosphate material is better.

[0012] Further, in step S2, lithium source, iron source, manganese source, phosphorus source, and dopant are weighed according to the stoichiometric ratio of Li:Fe:Mn:M:P as x:y:(1-yz):z:1, wherein 1.01≤x≤1.02, 0<y≤0.4, 0.002≤z≤0.005; and / or the lithium source includes one or more of lithium carbonate, lithium dihydrogen phosphate, lithium dihydrogen phosphate, and lithium hydroxide; and / or the iron source includes one or more of ferric oxide, ferric phosphate, ferrous oxalate, ferrous oxide, and ferromanganese phosphate; and / or The manganese source includes one or more of manganese iron phosphate, manganese carbonate, and manganese tetroxide; and / or the phosphorus source includes one or more of lithium dihydrogen phosphate, ammonium dihydrogen phosphate, iron phosphate, and manganese iron phosphate; and / or the second solvent includes a polar solvent; and / or the mass ratio of the total lithium source, manganese source, iron source, and phosphorus source to the second solvent is (44-66):100; and / or the second stirring speed is 200-300 rpm for 2-3 hours; and / or the first grinding is performed until the particle size of the solid particles in the second mixed solution is 800-1000 nm. Under the above conditions, the compatibility of the raw materials is better, and the raw materials can synergistically improve each other, which is more conducive to improving the electrochemical performance of lithium manganese iron phosphate.

[0013] Further, in step S3, the dopant includes niobium pentoxide and / or aluminum oxide; and / or the third stirring speed is 200-300 rpm for 1-2 h; and / or the second grinding is performed until the particle size of the solid particles in the third mixed solution is 350-420 nm; and / or the drying temperature is 290-310 °C; and / or the sintering includes a first sintering and a second sintering performed sequentially; preferably, the temperature of the first sintering is 750-790 °C for 3-5 h; the temperature of the second sintering is 670-710 °C for 12-16 h; preferably, the sintering atmosphere is argon and / or nitrogen, and the oxygen content in the atmosphere is less than 20 ppm. Under the above conditions, it is more advantageous to prepare modified lithium manganese iron phosphate materials with fine particle size and uniform distribution, and the modified lithium manganese iron phosphate has better electrochemical performance.

[0014] According to another aspect of the present invention, a cathode material is provided, comprising the above-described modified lithium manganese iron phosphate material, or comprising the modified lithium manganese iron phosphate material prepared according to the above-described preparation method. The cathode material of the present invention has a stable structure, strong hydrophobicity, excellent electronic conductivity and lithium-ion diffusion coefficient, good charge-discharge performance, and long cycle life.

[0015] According to another aspect of the present invention, a lithium-ion battery is provided, comprising the above-described positive electrode material. The lithium-ion battery of the present invention has a stable structure, strong hydrophobicity, excellent electronic conductivity and lithium-ion diffusion coefficient, good charge-discharge performance, and long cycle life.

[0016] Compared with the prior art, the technical solution of the present invention has the following effects:

[0017] (1) This invention modifies and grafts carbon sources by using hydrophobic agents. The hydrophobic carbon source is curled during the preparation process, tightly wrapping the precursor of lithium manganese iron phosphate material to form a stable core-shell structure. The hydrophobic carbon layer can support the overall structure of lithium manganese iron phosphate, maintain its own structural stability and not be easily damaged, which is conducive to improving the stability of the lithium manganese iron phosphate structure. It can also stabilize its own structure and not be easily damaged during the subsequent crushing process.

[0018] (2) The carbon links of the hydrophobic agent are branched on the surface of the carbon source to form organic branches. During the preparation of lithium manganese iron phosphate, the carbon source and the branches are carbonized. The rich carbon chain structure is conducive to improving the electron transport efficiency, thereby improving the electronic conductivity and electrical conductivity of lithium manganese iron phosphate material.

[0019] (3) The modified lithium manganese iron phosphate material of the present invention contains specific doping elements niobium and / or aluminum. On the one hand, the doping elements can expand the lithium ion diffusion channels and reduce the energy barrier of lithium ion transport. On the other hand, the addition of doping elements can make the lithium manganese iron phosphate material highly oriented, with fine grains, and form an ordered superlattice structure. The highly oriented grains can effectively suppress the increase of internal resistance and improve the capacity retention rate of the material. The superlattice structure can also effectively suppress the collapse of the crystal structure during charging and discharging, forming defects and holes inside the crystal, thereby changing the electronic structure of the material and improving the electronic conductivity of the material.

[0020] (4) The hydrophobic agent is used to modify the lithium manganese iron phosphate material, which can form a hydrophobic structure on its surface, thus significantly improving the hydrophobicity of the lithium manganese iron phosphate material. This helps to solve the problem of excessive moisture in the preparation process of lithium manganese iron phosphate material, and can further effectively improve the electrochemical performance of lithium manganese iron phosphate material. Attached Figure Description

[0021] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0022] Figure 1 A TEM image of the modified lithium manganese iron phosphate material according to Example 1 of the present invention is shown. Detailed Implementation

[0023] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0024] As described in the background section of this invention, existing technologies suffer from problems such as poor structural stability, low electronic conductivity, low lithium-ion diffusion coefficient, and easy excess moisture during preparation, leading to poor electrochemical performance. To address these issues, a modified lithium manganese iron phosphate material is provided in a typical embodiment of this invention. The modified lithium manganese iron phosphate material of this invention comprises a core and a hydrophobic carbon layer coating the surface of the core. The core comprises lithium manganese iron phosphate material with the general formula Li. x Fe y Mn 1-y-z M z PO4, 1.01≤x≤1.02, 0<y≤0.4, 0.002≤z≤0.005; M is niobium and / or aluminum; the raw material for the hydrophobic carbon layer is a carbon source modified with a hydrophobic agent; the hydrophobic agent includes one or more of octadecyltrichlorosilane, dodecanoic acid, oleic acid, tridecafluorooctyltriethoxysilane, tetraethoxysilane and polyacrylamide.

[0025] In the existing technology, during the preparation of lithium manganese iron phosphate, the carbon source exists in an extended conformation under the condition of a dispersant (such as glucose in water as a dispersant). The polar groups are uniformly distributed along the molecular chain, resulting in a loose carbon layer after carbonization. This leads to poor encapsulation of lithium manganese iron phosphate particles. During charging and discharging, as lithium ions are extracted and inserted, the volume of lithium manganese iron phosphate crystals changes continuously, and the structural integrity is destroyed. With the progress of cycling, the electrical performance of lithium manganese iron phosphate deteriorates severely.

[0026] The modified lithium manganese iron phosphate material of this invention comprises a core and a hydrophobic carbon layer coating the surface of the core. The core comprises lithium manganese iron phosphate material, and the raw material for the hydrophobic carbon layer is a carbon source modified with a specific type of hydrophobic agent according to this invention. The inventors unexpectedly discovered in numerous experiments that modifying and grafting the carbon source with a hydrophobic agent, grafting hydrophobic groups onto the carbon source, causes the extended carbon source molecular chains in the hydrophobic carbon source to coil up during the preparation process, tightly encapsulating the precursor of the lithium manganese iron phosphate material, forming a stable core-shell structure. The hydrophobic carbon layer can support the overall structure of the lithium manganese iron phosphate, maintaining its structural stability and preventing structural collapse during charge-discharge cycles, thereby effectively improving the cycle efficiency of lithium manganese iron phosphate. Furthermore, the tight encapsulation of the precursor of the lithium manganese iron phosphate material by the carbon source molecular chains also results in a smaller particle size of the modified lithium manganese iron phosphate and a denser carbon layer, thus effectively improving the conductivity of the lithium manganese iron phosphate.

[0027] In addition, the carbon links of the hydrophobic agent are branched on the surface of the carbon source to form organic branches. During the preparation of lithium manganese iron phosphate, the carbon source and the branches undergo carbonization. The rich carbon chain structure is beneficial to improving the electron transport efficiency, thereby improving the electronic conductivity and electrical conductivity of lithium manganese iron phosphate materials.

[0028] It should be noted that the present invention specifically uses the above-mentioned hydrophobic agents to modify the carbon source, so that the surface of the modified lithium manganese iron phosphate material contains a hydrophobic structure, which significantly improves the hydrophobicity of the lithium manganese iron phosphate material. This helps to solve the problem of excessive moisture content in the preparation process of lithium manganese iron phosphate material, and can further effectively improve the electrochemical performance of lithium manganese iron phosphate material.

[0029] Furthermore, the modified lithium manganese iron phosphate material of this invention contains specific doping elements niobium and / or aluminum. On the one hand, the doping elements can expand the lithium-ion diffusion channels and reduce the energy barrier for lithium-ion transport, thereby effectively improving the lithium-ion diffusion coefficient. On the other hand, the addition of doping elements can make the lithium manganese iron phosphate material highly oriented, with fine grains, and form an ordered superlattice structure. The highly oriented grains can effectively suppress the increase in internal resistance and improve the capacity retention of the material. The superlattice structure can also effectively suppress the collapse of the crystal structure during charging and discharging, forming defects and holes inside the crystal, thereby changing the electronic structure of the material and improving the electronic conductivity of the material.

[0030] In existing technologies, the smaller the particle size of lithium manganese iron phosphate (LFP), the shorter the path and the faster the lithium ion transfer from the LFP crystal, resulting in better electrochemical performance. However, smaller particle sizes also lead to a larger specific surface area and increased water absorption. To avoid water absorption, the particle size of LFP is generally limited to a relatively large range (e.g., 480–600 nm). Because the modified LFP material of this invention includes a hydrophobic carbon layer, the carbon source molecular chains tightly encapsulate the precursor of the LFP material, resulting in smaller particle sizes and a more compact carbon layer. In a preferred embodiment, the D50 particle size of the modified LFP material is 350–420 nm. By controlling the particle size within the aforementioned small range while maintaining the hydrophobicity of the LFP material, the lithium ion transfer path can be shortened more effectively, and the transfer speed increased, thereby further improving the electrochemical performance of the LFP material.

[0031] In a preferred embodiment, the mass ratio of the core to the carbon source is 100:(5.2-6.2). Under these conditions, the core and the carbon source can work synergistically. Under reduction reaction conditions, without affecting lithium-ion insertion / extraction, the carbon source can tightly encapsulate the core, stably supporting the overall structure of lithium manganese iron phosphate, providing more sufficient conductive channels, and further improving the conductivity of lithium manganese iron phosphate materials. This results in better stability, hydrophobicity, and electrochemical performance of lithium manganese iron phosphate materials.

[0032] In another typical embodiment of the present invention, a method for preparing the above-mentioned modified lithium manganese iron phosphate material is also provided, comprising the following steps: Step S1, pretreating a carbon source to obtain a pretreated carbon source, and subjecting the pretreated carbon source to a first stirring and photochemical reaction with a first solvent and a hydrophobic agent to obtain a first mixed solution containing the modified carbon source; Step S2, subjecting a lithium source, an iron source, a manganese source, a phosphorus source, and a second solvent to a second stirring and a first grinding to obtain a second mixed solution; Step S3, subjecting the first mixed solution, the second mixed solution, and a dopant containing M to a third stirring to obtain a third mixed solution, and subjecting the third grinding, drying, and sintering to obtain the modified lithium manganese iron phosphate material; wherein, the hydrophobic agent includes one or more of octadecyltrichlorosilane, dodecanoic acid, oleic acid, tridecafluorooctyltriethoxysilane, tetraethoxysilane, and polyacrylamide; M is niobium and / or aluminum.

[0033] In the preparation method of this invention, the carbon source is first pretreated to increase the active groups in the carbon source, creating conditions for the introduction of hydrophobic groups. Then, the pretreated carbon source, solvent, and hydrophobic agent are stirred and subjected to a photochemical reaction. During this process, the hydrophobic agent decomposes, and the hydrophobic groups in the hydrophobic agent are grafted onto the side chains of the carbon source, resulting in a first mixed solution containing the modified carbon source. Next, the lithium source, iron source, manganese source, and phosphorus source are stirred and ground with a second solvent to ensure uniform distribution of the lithium source, iron source, manganese source, and phosphorus source. The resulting second mixed solution is stirred and ground with the dopant and the first mixed solution to ensure uniform dispersion and full contact of the materials. In the third mixed solution, the modified carbon source exhibits a coiled shape, tightly encapsulating the precursor of the lithium manganese iron phosphate material. The core-shell structure is stable and not easily damaged, which is beneficial to improving the stability of the lithium manganese iron phosphate structure and further improving the cycle life of the lithium manganese iron phosphate material. Finally, the mixture is dried, granulated, and sintered. During the sintering process, based on the principle of high-valence metal doping, the doping of M effectively reduces the diffusion barrier of lithium ions, which is conducive to promoting the migration of lithium ions, reducing the band gap energy, improving electronic conductivity, and promoting electron-hole recombination. The resulting modified lithium manganese iron phosphate material includes a lithium manganese iron phosphate core and a carbon layer coated on the outside of the core. The carbon layer contains hydrophobic groups.

[0034] The modified lithium manganese iron phosphate material obtained by the above preparation method contains hydrophobic groups and has good hydrophobic properties. This effectively avoids the problem of excessive moisture content caused by contact with air during subsequent processing. Furthermore, it exhibits structural stability and high cycle stability. In addition, the formation of the carbon layer and the doping of M ions can effectively improve the lithium-ion diffusion coefficient and electronic conductivity.

[0035] In a preferred embodiment, step S1 includes pretreatment with plasma treatment; preferably, the plasma treatment is carried out in an air and / or nitrogen atmosphere; more preferably, the gas flow rate in the atmosphere is 0.3–0.5 L / min; and / or the plasma treatment power is 50–70 W; and / or the plasma treatment time is 30–40 s. Under the above conditions, the carbon source surface has a greater number of active groups, which is more conducive to grafting reactions with hydrophobic agents. More organic carbon chains are grafted onto the modified carbon source. On the one hand, this is more conducive to tightly coating the lithium manganese iron phosphate precursor, further improving the stability of the modified lithium manganese iron phosphate material structure and reducing the material particle size. On the other hand, it is more conducive to improving the hydrophobicity of the modified lithium manganese iron phosphate material. In addition, the richer branched structure can further improve the electronic conductivity of lithium manganese iron phosphate.

[0036] In a preferred embodiment, step S1, the first stirring includes the following steps: first, adding a first solvent to the pretreated carbon source for a first sub-stirring, and then adding a hydrophobic agent for a second sub-stirring; preferably, the first sub-stirring time is 2-4 hours; and / or the second sub-stirring time is 3-4 hours. Under the above conditions, first stirring the pretreated carbon source with the solvent to dissolve the carbon source in the solvent, and then stirring with the hydrophobic agent to dissolve the hydrophobic agent, can make the modification and grafting reaction of the carbon source proceed more fully.

[0037] In a preferred embodiment, in step S1, the first stirring speed is 200–300 rpm. Under these conditions, the distribution of the pretreated carbon source, hydrophobic agent, and first solvent is more uniform, and the contact is more sufficient, which is more conducive to the modification and grafting reaction.

[0038] To facilitate the modification and grafting reaction, in a preferred embodiment, the photochemical reaction time in step S1 is 0.5–2 hours, and the temperature is 40–60°C. Preferably, the photochemical reaction method includes one or more of infrared radiation, electromagnetic radiation, and infrared laser methods. The above-mentioned photochemical reaction conditions are simple, easy to control, and have a short process. If the reaction time is too long, the solvent may evaporate rapidly, causing abnormal feeding during the spraying process. Moreover, with the evaporation of the solvent, some hydrophobic agents may precipitate abnormally, which may easily lead to incomplete reaction. If the reaction time is too short, the grafting may be incomplete, and some groups may not be completely detached, thus failing to effectively improve the hydrophobic properties of the carbon source.

[0039] To enable the carbon source to more tightly encapsulate the lithium manganese iron phosphate precursor in the solvent, thus facilitating the preparation of structurally stable modified lithium manganese iron phosphate materials, in a preferred embodiment, in step S1, the carbon source includes one or more of glucose, polyacrylate, starch, PVA, and PVDF; and / or the first solvent includes one or more of water, ethylene glycol, propanol, acetone, and N-methylpyrrolidone; preferably, the first solvent includes water and ethylene glycol, wherein the volume ratio of water to ethylene glycol is 1:(0.5-1), more preferably 1:(0.5-0.6), and more preferably, when the first solvent includes water and ethylene glycol, in step S1, water is added first, followed by ethylene glycol. The solvent components under the above conditions act as dispersants, further improving the coiling degree of the carbon chain of the carbon source, allowing the lithium manganese iron phosphate precursor and carbon source to more fully form a core-shell structure, thereby more effectively improving the structural stability of the modified lithium manganese iron phosphate and reducing its particle size.

[0040] In order to further improve the grafting effect of carbon source and thus further improve the hydrophobicity of lithium manganese iron phosphate material, in a preferred embodiment, in step S1, the hydrophobic agent includes octadecyltrichlorosilane and / or dodecanoic acid; preferably, when the hydrophobic agent is a mixture of octadecyltrichlorosilane and dodecanoic acid, the mass ratio of the two is (1~2):1.

[0041] In order to ensure that the raw materials are in full contact and react, thereby improving the effect of carbon source grafting modification, in a preferred embodiment, in step S1, the mass ratio of hydrophobic agent to carbon source is (4-7):100; and / or the mass ratio of first solvent to carbon source is 100:(42-65).

[0042] In a preferred embodiment, in step S2, lithium source, iron source, manganese source, phosphorus source, and dopant are weighed according to the stoichiometric ratio of Li:Fe:Mn:M:P as x:y:(1-yz):z:1, wherein 1.01≤x≤1.02, 0<y≤0.4, and 0.002≤z≤0.005; and / or the lithium source includes one or more of lithium carbonate, lithium dihydrogen phosphate, lithium dihydrogen phosphate, and lithium hydroxide; and / or the iron source includes one or more of ferric oxide, ferric phosphate, ferrous oxalate, ferrous oxide, and ferromanganese phosphate; and / or the manganese source includes one or more of ferromanganese phosphate, manganese carbonate, and manganese tetroxide; and / or the phosphorus source includes one or more of lithium dihydrogen phosphate, ammonium dihydrogen phosphate, ferric phosphate, and ferromanganese phosphate. The above raw materials are inexpensive and widely available, and their synergistic effects further enhance the electrochemical performance of lithium manganese iron phosphate.

[0043] In order to make the contact between the lithium source, iron source, manganese source and phosphorus source more sufficient and the solubility in the solvent better, in a preferred embodiment, in step S2, the second solvent includes a polar solvent, preferably water; and / or the sum of the lithium source, manganese source, iron source and phosphorus source, in a mass ratio of (44-66):100 to the second solvent.

[0044] For similar reasons, in a preferred embodiment, in step S2, the second stirring speed is 200-300 rpm and the time is 2-3 hours.

[0045] In order to make the modified lithium manganese iron phosphate material smaller in particle size and better in electrochemical performance, in a preferred embodiment, the particle size of the solid particles in the second mixed solution is 800-1000 nm during the first grinding process.

[0046] In a preferred embodiment, in step S3, the dopant includes niobium pentoxide and / or aluminum oxide. Under the above conditions, it is more advantageous to improve the lithium-ion diffusion coefficient of the modified lithium manganese iron phosphate, and further more advantageous to improve the electrochemical performance of the modified lithium manganese iron phosphate.

[0047] In a preferred embodiment, in step S3, the third stirring speed is 200–300 rpm for 1–2 hours; and / or the second grinding is performed until the particle size of the solid particles in the third mixed solution is 350–420 nm. Under these conditions, the materials can be in more thorough contact, which is more conducive to preparing modified lithium manganese iron phosphate materials with fine particle size and uniform distribution.

[0048] In order to allow the solvent to evaporate more fully and the solid materials to be more fully dispersed and evenly distributed, in a preferred embodiment, the drying temperature in step S3 is 290-310°C.

[0049] In a preferred embodiment, step S3, sintering includes a first sintering and a second sintering performed sequentially; preferably, the temperature of the first sintering is 750–790°C, and the time is 3–5 hours; the temperature of the second sintering is 670–710°C, and the time is 12–16 hours; preferably, the sintering atmosphere is argon and / or nitrogen, with an oxygen content of less than 20 ppm. During the relatively high-temperature first sintering process, crystal nuclei can be formed, while the relatively low-temperature second sintering process is conducive to grain growth. If the temperature of the first sintering is too low or the time is too short, it is not conducive to crystal nuclei growth; if the temperature of the first sintering is too high or the time is too long, it easily leads to excessive crystal nuclei growth, resulting in a large particle size of the modified lithium manganese iron phosphate material. If the temperature of the second sintering is too low or the time is too short, it is not conducive to grain formation; if the temperature of the second sintering is too high or the time is too long, the grain size grows excessively, resulting in an excessively large particle size.

[0050] In another typical embodiment of the present invention, a cathode material is also provided, comprising the above-described modified lithium manganese iron phosphate material, or comprising the modified lithium manganese iron phosphate material prepared according to the above-described preparation method. The cathode material of the present invention has a stable structure, strong hydrophobicity, excellent electronic conductivity and lithium-ion diffusion coefficient, good charge-discharge performance, and long cycle life.

[0051] In another typical embodiment of the present invention, a lithium-ion battery is also provided, comprising the above-mentioned positive electrode material. The lithium-ion battery of the present invention has a stable structure, strong hydrophobicity, excellent electronic conductivity and lithium-ion diffusion coefficient, good charge-discharge performance, and long cycle life.

[0052] In typical, but not limiting, modified lithium manganese iron phosphate materials, the mass ratio of the core to the carbon source is 100:5.2, 100:5.5, 100:6, 100:6.2, or any two of these values.

[0053] Typical, but not limiting, the volume ratio of water to ethylene glycol in the first solvent is 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9, 1:1, or any two of these values.

[0054] Typical, but not limiting, mass ratios of hydrophobic agent to carbon source are 4:100, 5:100, 6:100, 7:100, or any two of these values.

[0055] Typical, but not limiting, mass ratios of the first solvent to the carbon source are 100:42, 100:45, 100:50, 100:55, 100:60, 100:65, or any two of these values.

[0056] Typically, but not limitingly, the first sintering temperature is 750℃, 770℃, 790℃ or any two of these values, and the time is 3h, 4h, 5h or any two of these values; the second sintering temperature is 670℃, 690℃, 710℃ or any two of these values, and the time is 12h, 14h, 16h or any two of these values, with an oxygen content of less than 20ppm in the sintering atmosphere.

[0057] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0058] Example 1

[0059] The hydrophobic agent is octadecyltrichlorosilane and dodecanoic acid (the mass ratio of octadecyltrichlorosilane and dodecanoic acid is 1.5:1); the mass ratio of the total amount of hydrophobic agent to the carbon source is 5.5:100.

[0060] Step S1: Using glucose as the carbon source, the carbon source is treated in a plasma analyzer for 30 seconds under air atmosphere, gas flow rate of 0.4 L / min, and plasma treatment power of 60 W to obtain a pretreated carbon source. The pretreated carbon source is added to the first portion of water and stirred at 300 rpm for 2 hours to dissolve it. Ethylene glycol (the mass ratio of the first portion of water to ethylene glycol to the carbon source is 100:53, and the volume ratio of the first portion of water to ethylene glycol is 1:0.75) is added and stirred for 60 minutes. Octadecyltrichlorosilane is then added and the mixture is treated with infrared light at 50°C for 30 minutes. Dodecanoic acid is then added, and the mixture is stirred for 2 hours and maintained at 45°C for 2 hours to obtain a first mixed solution containing the modified carbon source.

[0061] Step S2: Weigh out lithium source (lithium dihydrogen phosphate), iron source (iron phosphate), manganese source (manganese tetroxide), dopant (Nb2O5), and phosphorus source (iron phosphate) according to the stoichiometric ratio Li:Fe:Mn:M:P = 1.01:0.3:0.696:0.004:1. Add the lithium source, iron source, manganese source, and phosphorus source to the second solvent (the second part is water, the total of lithium source, manganese source, iron source, and phosphorus source, with a mass ratio of 55:100 to the second part of water). Stir at 300 rpm for 3 hours. After homogenization, perform ultrafine grinding in an ultrafine grinder at 1900 rpm to obtain the second mixed solution, wherein the particle size of the solid particles is 1000 nm.

[0062] Step S3: Add the dopant and the first mixed solution (the mass percentage of the first mixed solution in the third mixed solution is 15%) to the second mixed solution, continue stirring at 300 rpm for 2 hours, perform ultrafine grinding in a sand mill at 1900 rpm, the particle size of the solid particles in the third mixed solution is 400 nm, spray dry at 300℃, and then sinter under argon conditions (oxygen content less than 20 ppm). During the sintering process, the first sintering temperature is 770℃ for 5 hours, and the second sintering temperature is 690℃ for 16 hours to obtain the modified lithium manganese iron phosphate material.

[0063] The modified lithium manganese iron phosphate material consists of a lithium manganese iron phosphate core and a carbon layer containing hydrophobic groups. These hydrophobic groups are reactive silanols and carbon chains. A TEM image of the modified lithium manganese iron phosphate material is shown below. Figure 1 .Depend on Figure 1 It can be seen that lithium manganese iron phosphate particles are round and smooth, without sharp edges, and the carbon layer is tightly wrapped, with a carbon layer thickness between 2-5 nm.

[0064] Example 2

[0065] The only difference from Example 1 is:

[0066] In the hydrophobic agent, the mass ratio of octadecyltrichlorosilane to dodecanoic acid is 2:1.

[0067] Example 3

[0068] The only difference from Example 1 is:

[0069] In the hydrophobic agent, the mass ratio of octadecyltrichlorosilane to dodecanoic acid is 1:1.

[0070] Example 4

[0071] The only difference from Example 1 is:

[0072] The hydrophobic agent is octadecyltrichlorosilane.

[0073] Example 5

[0074] The only difference from Example 1 is:

[0075] The hydrophobic agent is dodecanoic acid.

[0076] Example 6

[0077] The only difference from Example 1 is:

[0078] The hydrophobic agent is oleic acid.

[0079] Example 7

[0080] The only difference from Example 1 is:

[0081] The hydrophobic agent is tridecafluorooctyltriethoxysilane.

[0082] Example 8

[0083] The only difference from Example 1 is:

[0084] The mass ratio of the total amount of hydrophobic agent to the carbon source is 4:100.

[0085] Example 9

[0086] The only difference from Example 1 is:

[0087] The mass ratio of the total amount of hydrophobic agent to the carbon source is 7:100.

[0088] Example 10

[0089] The only difference from Example 1 is:

[0090] In step S1, the types of carbon sources and the parameters in the pretreatment process vary, specifically as follows:

[0091] Using glucose as a carbon source, the carbon source was treated in a plasma analyzer for 30 seconds under nitrogen atmosphere, gas flow rate of 0.5 L / min, and plasma processing power of 70 W to obtain a pretreated carbon source.

[0092] Example 11

[0093] The only difference from Example 1 is:

[0094] In step S1, the types of carbon sources and the parameters in the pretreatment process vary, specifically as follows:

[0095] Using glucose as a carbon source, the carbon source was treated in a plasma analyzer for 40 seconds under air atmosphere, gas flow rate of 0.3 L / min, and plasma processing power of 50 W to obtain a pretreated carbon source.

[0096] Example 12

[0097] The only difference from Example 1 is:

[0098] In step S1, the parameters during the photochemical reaction differ depending on the type of the first solvent. Specifically:

[0099] The pretreated carbon source was added to the first portion of water and stirred at 300 rpm for 2 hours to dissolve it. Then, N-methylpyrrolidone (the sum of the first portion of water and N-methylpyrrolidone, with a mass ratio of 100:42 to the carbon source and a volume ratio of 1:0.5 to the first portion of water) was added and stirred for 60 minutes. Octadecyltrichlorosilane was then added and the mixture was treated with infrared radiation at 40°C for 60 minutes. Dodecanoic acid was then added and the mixture was stirred for 4 hours and maintained at 60°C for 2 hours to obtain the first mixed solution containing the modified carbon source.

[0100] Example 13

[0101] The only difference from Example 1 is:

[0102] In step S1, the parameters during the photochemical reaction differ depending on the type of the first solvent. Specifically:

[0103] The pretreated carbon source was added to the first portion of water and stirred at 300 rpm for 2 hours to dissolve it. Propanol (the content of which is the sum of the first portion of water and N-methylpyrrolidone, with a mass ratio of 100:65 to the carbon source and a volume ratio of 1:0.6 between the first portion of water and propanol) was added and stirred for 60 minutes. Octadecyltrichlorosilane was then added and the mixture was treated with electromagnetic radiation at 50°C for 30 minutes. Dodecanoic acid was then added and the mixture was stirred for 2 hours and maintained at 45°C for 2 hours to obtain the first mixed solution containing the modified carbon source.

[0104] Example 14

[0105] The only difference from Example 1 is:

[0106] In step S2, the types and contents of the lithium source, iron source, manganese source, phosphorus source, and second solvent are different, specifically as follows:

[0107] In step S2, according to the stoichiometric ratio of Li:Fe:Mn:M:P = 1.01:0.1:0.898:0.002:1, weigh out the lithium source (lithium carbonate), iron source (ferrous oxalate), manganese source (manganese carbonate), dopant (Nb2O5), and phosphorus source (ammonium dihydrogen phosphate), and the second solvent is water. The total mass ratio of the lithium source, manganese source, iron source, and phosphorus source to the second part of water is 66:100.

[0108] Example 15

[0109] The only difference from Example 1 is:

[0110] In step S2, the types and contents of the lithium source, iron source, manganese source, phosphorus source, and second solvent are different, specifically as follows:

[0111] In step S2, according to the stoichiometric ratio of Li:Fe:Mn:M:P = 1.02:0.4:0.595:0.005:1, weigh out the lithium source (lithium hydroxide), iron source (ferrous oxide), manganese source (manganese carbonate), dopant (Nb2O5), and phosphorus source (ammonium dihydrogen phosphate), and the second solvent is water (the second part). The total mass ratio of the lithium source, manganese source, iron source, and phosphorus source to the second part of water is 44:100.

[0112] Example 16

[0113] The only difference from Example 1 is:

[0114] In step S3, the dopant is aluminum oxide.

[0115] Example 17

[0116] The only difference from Example 1 is:

[0117] In step S3, the dopant is niobium pentoxide.

[0118] Example 18

[0119] The only difference from Example 1 is:

[0120] Step S1: Using glucose as the carbon source, the carbon source is treated in a plasma analyzer for 30 seconds under air atmosphere, gas flow rate of 0.4 L / min, and plasma treatment power of 60 W to obtain a pretreated carbon source. The pretreated carbon source is added to the first portion of water and stirred at 200 rpm for 2 hours to dissolve it. Ethylene glycol (the mass ratio of the first portion of water to ethylene glycol to the carbon source is 1:0.53, and the volume ratio of the first portion of water to ethylene glycol is 1:0.75) is added and stirred for 60 minutes. Octadecyltrichlorosilane is then added and the mixture is treated with infrared light at 60°C for 30 minutes. Dodecanoic acid is then added, and the mixture is stirred for 3 hours and maintained at 45°C for 2 hours to obtain a first mixed solution containing the modified carbon source.

[0121] In step S2, according to the stoichiometric ratio of Li:Fe:Mn:M:P = 1.01:0.3:0.696:0.004:1, weigh out the lithium source (lithium dihydrogen phosphate), iron source (iron phosphate), manganese source (manganese tetroxide), dopant (Nb2O5), and phosphorus source (iron phosphate). Add the lithium source, iron source, manganese source, and phosphorus source to the second solvent (the second part is water, and the total mass ratio of the lithium source, manganese source, iron source, and phosphorus source to the second part of water is 55:100). Stir at 200 rpm for 3 hours. After homogenization, perform ultrafine grinding in a sand mill to obtain the second mixed solution, wherein the particle size of the solid particles is 800 nm.

[0122] Step S3: Add the dopant and the first mixed solution to the second mixed solution, continue stirring at 200 rpm for 1 hour, and then perform ultrafine grinding. The solid particles in the third mixed solution have a particle size of 420 nm. Spray dry under air intake at 290°C, and then sinter under argon conditions (oxygen content less than 20 ppm). During the sintering process, the first sintering temperature is 780°C for 5 hours, and the second sintering temperature is 700°C for 16 hours to obtain the modified lithium manganese iron phosphate material.

[0123] Example 19

[0124] The only difference from Example 1 is:

[0125] Step S1: Using glucose as the carbon source, the carbon source is treated in a plasma analyzer for 30 seconds under air atmosphere, gas flow rate of 0.4 L / min, and plasma treatment power of 60 W to obtain a pretreated carbon source. The pretreated carbon source is added to the first portion of water and stirred at 300 rpm for 4 hours to dissolve it. Ethylene glycol (the mass ratio of the first portion of water to ethylene glycol to the carbon source is 1:0.53, and the volume ratio of the first portion of water to ethylene glycol is 1:0.75) is added and stirred for 60 minutes. Octadecyltrichlorosilane is then added and the mixture is treated with infrared light at 40°C for 30 minutes. Dodecanoic acid is then added, and the mixture is stirred for 4 hours and maintained at 45°C for 2 hours to obtain a first mixed solution containing the modified carbon source.

[0126] In step S2, according to the stoichiometric ratio of Li:Fe:Mn:M:P = 1.01:0.3:0.696:0.004:1, weigh out the lithium source (lithium dihydrogen phosphate), iron source (iron phosphate), manganese source (manganese tetroxide), dopant (Nb2O5), and phosphorus source (iron phosphate). Add the lithium source, iron source, manganese source, and phosphorus source to the second solvent (the second part is water, the total of the lithium source, manganese source, iron source, and phosphorus source, and the mass ratio of the second part of water to the second part of water is 55:100). Stir at 300 rpm for 2 hours. After homogenization, perform ultrafine grinding in a sand mill to obtain the second mixed solution, wherein the particle size of the solid particles is 1000 nm.

[0127] Step S3: Add the dopant and the first mixed solution to the second mixed solution, continue stirring at 300 rpm for 2 hours, and then perform ultrafine grinding. The solid particles in the third mixed solution have a particle size of 350 nm. Spray dry at an inlet air temperature of 310°C, and then sinter under argon conditions (oxygen content less than 20 ppm). During the sintering process, the first sintering temperature is 780°C for 5 hours, and the second sintering temperature is 700°C for 16 hours to obtain the modified lithium manganese iron phosphate material.

[0128] Example 20

[0129] The only difference from Example 1 is:

[0130] In step S3, the first sintering temperature during the sintering process is 790℃ for 5 hours, and the second sintering temperature is 710℃ for 16 hours, thereby obtaining the modified lithium manganese iron phosphate material.

[0131] Example 21

[0132] The only difference from Example 1 is:

[0133] In step S3, the first sintering temperature during the sintering process is 750℃ for 3 hours, and the second sintering temperature is 670℃ for 16 hours, thereby obtaining the modified lithium manganese iron phosphate material.

[0134] Example 22

[0135] The hydrophobic agent is octadecyltrichlorosilane and dodecanoic acid (the mass ratio of octadecyltrichlorosilane and dodecanoic acid is 2:1); the mass ratio of the total amount of hydrophobic agent to the carbon source is 5:100.

[0136] Step S1: Using glucose as the carbon source, the carbon source is treated in a plasma analyzer for 40 seconds under air atmosphere, gas flow rate of 0.4 L / min, and plasma treatment power of 60 W to obtain a pretreated carbon source. The pretreated carbon source is added to water and stirred at 300 rpm for 3 hours to dissolve it. Ethylene glycol (the mass ratio of the first part of water and ethylene glycol to the carbon source is 100:42, and the volume ratio of the first part of water to ethylene glycol is 1:0.6) is added and stirred for 30 minutes. Octadecyltrichlorosilane is then added and the mixture is treated with infrared light at 50°C for 60 minutes. Dodecanoic acid is then added, and the mixture is stirred for 1 hour and maintained at 45°C for 2 hours to obtain a first mixed solution containing the modified carbon source.

[0137] Step S2: Weigh out lithium source (lithium dihydrogen phosphate), iron source (iron phosphate), manganese source (manganese tetroxide), dopant (Nb2O5), and phosphorus source (iron phosphate) according to the stoichiometric ratio Li:Fe:Mn:M:P = 1.01:0.3:0.696:0.004:1. Add the lithium source, iron source, manganese source, and phosphorus source to the second solvent (the second part is water, the total of lithium source, manganese source, iron source, and phosphorus source, with a mass ratio of 55:100 to the second part of water). Stir at 300 rpm for 2 hours. After homogenization, perform ultrafine grinding in an ultrafine grinder at 1900 rpm to obtain the second mixed solution, wherein the particle size of the solid particles is 1000 nm.

[0138] Step S3: Add the dopant and the first mixed solution to the second mixed solution, continue stirring at 300 rpm for 2 hours, and then perform ultrafine grinding. The solid particles in the third mixed solution have a particle size of 400 nm. Spray dry at an inlet air temperature of 300℃, and then sinter under argon conditions (oxygen content less than 20 ppm). During the sintering process, the first sintering temperature is 780℃ for 5 hours, and the second sintering temperature is 670℃ for 16 hours to obtain the modified lithium manganese iron phosphate material.

[0139] Example 23

[0140] The hydrophobic agents are octadecyltrichlorosilane and dodecanoic acid (the mass ratio of octadecyltrichlorosilane and dodecanoic acid is 1:1); the mass ratio of the total amount of hydrophobic agent to the carbon source is 4:100.

[0141] Step S1: Using glucose as the carbon source, the carbon source is treated in a plasma analyzer for 30 seconds under air atmosphere, gas flow rate of 0.4 L / min, and plasma treatment power of 60 W to obtain a pretreated carbon source. The pretreated carbon source is added to the first portion of water and stirred at 300 rpm for 2 hours to dissolve it. Ethylene glycol (the mass ratio of the first portion of water to ethylene glycol to the carbon source is 100:50, and the volume ratio of the first portion of water to ethylene glycol is 1:0.5) is added and stirred for 60 minutes. Octadecyltrichlorosilane is then added and the mixture is treated with infrared light at 50°C for 30 minutes. Dodecanoic acid is then added and stirred for 2 hours. The mixture is then kept at 60°C for 2 hours to obtain a first mixed solution containing the modified carbon source.

[0142] Step S2: Weigh out lithium source (lithium hydroxide), iron source (iron phosphate), manganese source (manganese tetroxide), dopant (Nb2O5), and phosphorus source (iron phosphate) according to the stoichiometric ratio Li:Fe:Mn:M:P = 1.01:0.3:0.698:0.002:1. Add the lithium source, iron source, manganese source, and phosphorus source to the second solvent (the second part is water, and the total mass ratio of the lithium source, manganese source, iron source, and phosphorus source to the second part of water is 66:100). Stir at 300 rpm for 3 hours. After homogenization, perform ultrafine grinding in an ultrafine grinder to obtain the second mixed solution, wherein the particle size of the solid particles is 1000 nm.

[0143] Step S3: Add the dopant and the first mixed solution to the second mixed solution, continue stirring at 300 rpm for 2 hours, and then perform ultrafine grinding. The solid particles in the third mixed solution have a particle size of 420 nm. Spray dry at an inlet air temperature of 310℃, and then granulate and sinter. During the sintering process under argon conditions (oxygen content less than 20 ppm), the first sintering temperature is 750℃ for 3 hours, and the second sintering temperature is 670℃ for 12 hours to obtain the modified lithium manganese iron phosphate material.

[0144] Example 24

[0145] The hydrophobic agents are octadecyltrichlorosilane and dodecanoic acid (the mass ratio of octadecyltrichlorosilane and dodecanoic acid is 1:1); the mass ratio of the total amount of hydrophobic agent to the carbon source is 6:100.

[0146] Step S1: Using glucose as the carbon source, the carbon source is treated in a plasma analyzer for 40 seconds under air atmosphere, gas flow rate of 0.4 L / min, and plasma treatment power of 60 W to obtain a pretreated carbon source. The pretreated carbon source is added to the first portion of water and stirred at 200 rpm for 2 hours to dissolve it. Ethylene glycol (the mass ratio of the first portion of water to ethylene glycol to the carbon source is 100:65, and the volume ratio of the first portion of water to ethylene glycol is 1:0.8) is added and stirred for 60 minutes. Octadecyltrichlorosilane is then added and the mixture is treated with infrared light at 50°C for 60 minutes. Dodecanoic acid is then added, and the mixture is stirred for 1 hour and maintained at 45°C for 4 hours to obtain a first mixed solution containing the modified carbon source.

[0147] In step S2, according to the stoichiometric ratio Li:Fe:Mn:M:P = 1.02:0.1:0.898:0.002:1, weigh out the lithium source (lithium dihydrogen phosphate), iron source (iron phosphate), manganese source (manganese tetroxide), dopant (Nb2O5), and phosphorus source (iron phosphate). Add the lithium source, iron source, manganese source, and phosphorus source to the second solvent (the second part is water, and the total mass ratio of the lithium source, manganese source, iron source, and phosphorus source to the second part of water is 44:100). Stir at 200 rpm for 3 hours. After homogenization, perform ultrafine grinding in an ultrafine grinder to obtain the second mixed solution, wherein the particle size of the solid particles is 800 nm.

[0148] Step S3: Add the dopant and the first mixed solution to the second mixed solution, continue stirring at 200 rpm for 2 hours, and then perform ultrafine grinding. The solid particles in the third mixed solution have a particle size of 420 nm. Spray dry at an inlet air temperature of 300℃, then granulate and sinter. During the sintering process under argon conditions (oxygen content less than 20 ppm), the first sintering temperature is 750℃ for 3 hours, and the second sintering temperature is 710℃ for 12 hours to obtain the modified lithium manganese iron phosphate material.

[0149] In a glove box filled with argon atmosphere, the prepared modified lithium manganese iron phosphate material was used as the positive electrode material, graphite was used as the negative electrode, and the battery was encapsulated in a battery case to isolate it from air, thus assembling a lithium-ion battery.

[0150] Example 25

[0151] The hydrophobic agent is dodecanoic acid; the mass ratio of the total amount of hydrophobic agent to the carbon source is 5:100.

[0152] Step S1: Using glucose as the carbon source, the carbon source is treated in a plasma analyzer for 40 seconds under air atmosphere, gas flow rate of 0.4 L / min, and plasma treatment power of 60 W to obtain a pretreated carbon source. The pretreated carbon source is added to the first portion of water and stirred at 200 rpm for 3 hours to dissolve it. Ethylene glycol (the mass ratio of the first portion of water to ethylene glycol to the carbon source is 100:60, and the volume ratio of the first portion of water to ethylene glycol is 1:1) is added and stirred for 30 minutes. Octadecyltrichlorosilane is then added, and the mixture is treated with infrared light at 50°C for 60 minutes. Dodecanoic acid is then added, and the mixture is stirred for 1 hour and maintained at 60°C for 2 hours to obtain a first mixed solution containing the modified carbon source.

[0153] In step S2, according to the stoichiometric ratio of Li:Fe:Mn:M:P = 1.02:0.2:0.795:0.005:1, weigh out the lithium source (lithium carbonate), iron source (iron phosphate), manganese source (manganese tetroxide), dopant (Nb2O5), and phosphorus source (iron manganese phosphate), and add them to the second solvent (the second part is water, the sum of lithium source, manganese source, iron source, and phosphorus source, with a mass ratio of 45:100 to the second part of water). Stir at 200 rpm for 2 hours. After homogenization, perform ultrafine grinding in an ultrafine grinder at 1800 rpm to obtain the second mixed solution, wherein the particle size of the solid particles is 800 nm.

[0154] Step S3: Add dopant to the second mixed solution. Continue stirring the first mixed solution at 200 rpm for 2 hours, then perform ultrafine grinding at 1800 rpm. The solid particles in the third mixed solution have a particle size of 400 nm. Spray dry at an inlet air temperature of 310℃, then granulate and sinter. During the sintering process under argon conditions (oxygen content less than 20 ppm), the first sintering temperature is 750℃ for 3 hours, and the second sintering temperature is 710℃ for 12 hours to obtain the modified lithium manganese iron phosphate material.

[0155] Comparative Example 1

[0156] The only difference from Example 1 is:

[0157] No pretreatment or modification is performed on the carbon source.

[0158] Comparative Example 2

[0159] The only difference from Example 1 is:

[0160] The mass ratio of hydrophobic agent to carbon source is 9:100.

[0161] Comparative Example 3

[0162] The only difference from Example 1 is:

[0163] The mass ratio of hydrophobic agent to carbon source is 3:100.

[0164] Comparative Example 4

[0165] The only difference from Example 1 is:

[0166] Replace the hydrophobic agent with water.

[0167] Comparative Example 5

[0168] The only difference from Example 1 is:

[0169] In step S1, the carbon source is treated in a plasma analyzer for 40 seconds; then treated at 45°C for 60 minutes under infrared radiation; and finally, dodecanoic acid is added and stirred for 1 hour.

[0170] Comparative Example 6

[0171] CN 116354328 A.

[0172] Comparative Example 7

[0173] The only difference from Example 1 is:

[0174] The dopant is vanadium pentoxide.

[0175] In a glove box filled with argon atmosphere, the prepared modified lithium manganese iron phosphate material was used as the positive electrode material, graphite was used as the negative electrode, and the battery was encapsulated in a battery case to isolate it from air, thus assembling a lithium-ion battery.

[0176] The D50 particle size, electronic conductivity, lithium-ion diffusion coefficient, and electrochemical performance test results of the modified lithium manganese iron phosphate prepared in the above embodiments and comparative examples are shown in Table 1.

[0177] Test method:

[0178] D50 particle size: Tested using a laser particle size analyzer, ultrasonic time 20-60s;

[0179] Lithium-ion diffusion coefficient: tested by constant current intermittent titration under the conditions of current 0.2C, voltage 2.0~4.5V, temperature 25℃, and humidity 40RH%.

[0180] Charge and discharge performance test: The 0.2C discharge specific capacity and 1C discharge specific capacity were tested at 2.0~4.5V and 25℃.

[0181] Cyclic performance: Capacity retention after 200 cycles at 25°C, 2.0–4.5V, and 1C.

[0182] Table 1

[0183]

[0184]

[0185] Compared with the unmodified carbon source in Comparative Example 1, the capacity of lithium manganese iron phosphate material using the modified carbon source of this invention is slightly improved, while the cycle retention rate and lithium-ion diffusion coefficient are significantly improved. Compared with Comparative Example 8, the addition of specific doping elements in this invention significantly improves the lithium-ion diffusion effect.

[0186] As can be seen from the above, compared with the comparative example, the embodiments of the present invention can effectively solve the problems of poor structural stability, low electronic conductivity, low lithium-ion diffusion coefficient, and easy excessive moisture content during the preparation process of lithium manganese iron phosphate materials by controlling the raw materials and doping elements of the hydrophobic carbon layer in the modified lithium manganese iron phosphate materials, which lead to poor electrochemical performance.

[0187] Specifically, this invention modifies and grafts a carbon source using a hydrophobic agent. The hydrophobic carbon source is curled up during the preparation process, tightly wrapping the precursor of lithium manganese iron phosphate material to form a stable core-shell structure. The hydrophobic carbon layer can support the overall structure of lithium manganese iron phosphate, maintaining its own structural stability and making it less susceptible to damage. This is beneficial to improving the stability of the lithium manganese iron phosphate structure and ensuring that its structure remains stable and not easily damaged during subsequent pulverization.

[0188] Secondly, the carbon links of the hydrophobic agent are branched on the surface of the carbon source to form organic branches. During the preparation of lithium manganese iron phosphate, the carbon source and the branches undergo carbonization. The rich carbon chain structure is beneficial to improving the electron transport efficiency, thereby improving the electronic conductivity and electrical conductivity of lithium manganese iron phosphate materials.

[0189] In addition, the modified lithium manganese iron phosphate material of the present invention contains specific doping elements niobium and / or aluminum. On the one hand, the doping elements can expand the lithium-ion diffusion channels and reduce the energy barrier of lithium-ion transport. On the other hand, the addition of doping elements can make the lithium manganese iron phosphate material highly oriented, with fine grains, and form an ordered superlattice structure. The highly oriented grains can effectively suppress the increase of internal resistance and improve the capacity retention of the material. The superlattice structure can also effectively suppress the collapse of the crystal structure during charging and discharging, forming defects and holes inside the crystal, thereby changing the electronic structure of the material and improving the electronic conductivity of the material.

[0190] Furthermore, modifying lithium manganese iron phosphate materials with hydrophobic agents can form hydrophobic structures on their surfaces, significantly improving the hydrophobicity of lithium manganese iron phosphate materials. This helps to solve the problem of excessive moisture content in the preparation process of lithium manganese iron phosphate materials, and can further effectively improve the electrochemical performance of lithium manganese iron phosphate materials.

[0191] Furthermore, it can be seen that when all process parameters are within the preferred range of this invention, the modified lithium manganese iron phosphate material has a smaller particle size and better electrochemical performance of the lithium-ion battery.

[0192] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A modified lithium manganese iron phosphate material, characterized in that, comprise a core and a hydrophobic carbon layer coated on the surface of the core, The inner core comprises a lithium iron manganese phosphate material with a general formula of Li x Fe y Mn 1-y-z M z PO4, 1.01≤x≤1.02, 0 The hydrophobic carbon layer is prepared by modifying a carbon source with a hydrophobic agent; the hydrophobic agent comprises one or more of octadecyltrichlorosilane, dodecanoic acid, oleic acid, tridecafluorooctyltriethoxysilane, tetraethoxysilane and polyacrylamide; the mass ratio of the inner core to the carbon source is 100: (5.2~6.2); The preparation method of the modified manganese iron lithium phosphate material comprises the following steps: In step S1, the carbon source is pretreated to obtain a pretreated carbon source, and the pretreated carbon source is subjected to first stirring and photochemical reaction with a first solvent and a hydrophobic agent to obtain a first mixed solution containing a modified carbon source; the pretreatment comprises plasma treatment; the first stirring comprises the following steps: first, the first solvent is added to the pretreated carbon source for first sub-stirring, and then the hydrophobic agent is added for second sub-stirring, the time of the second sub-stirring being 3-4 h; the mass ratio of the hydrophobic agent to the carbon source is (4-7):100; In step S2, a lithium source, an iron source, a manganese source and a phosphorus source are sequentially subjected to second stirring and first grinding with a second solvent to obtain a second mixed solution; In step S3, the first mixed solution, the second mixed solution and a doping agent containing M are subjected to third stirring to obtain a third mixed solution, and sequentially subjected to second grinding, drying and sintering to obtain the modified manganese iron lithium phosphate material; the sintering comprises first sintering and second sintering which are sequentially performed; the temperature of the first sintering is 750-790 ℃, and the time is 3-5 h; the temperature of the second sintering is 670-710 ℃, and the time is 12-16 h. 2.The modified lithium iron manganese phosphate material of claim 1, characterized in that, The D50 particle size of the modified manganese iron lithium phosphate material is 350-420 nm. 3.The modified lithium iron manganese phosphate material of claim 1 or 2, characterized in that, In the step S1, The pretreatment is performed in an air and / or nitrogen gas atmosphere, and the flow rate of the gas in the gas atmosphere is 0.3-0.5 L / min; and / or The time of the pretreatment is 30-40 s; and / or The time of the first sub-stirring is 2-4 h; and / or the stirring speed is 200-300 rpm; and / or The time of the photochemical reaction is 0.5-2 h, and the temperature is 40-60 ℃, and the mode of the photochemical reaction comprises one or more of infrared radiation, electromagnetic radiation and infrared laser. 4.The modified lithium iron manganese phosphate material of claim 1 or 2, characterized in that, In the step S1, The carbon source comprises one or more of glucose, polyacrylate, starch, PVA and PVDF; and / or The first solvent comprises one or more of water, ethylene glycol, propanol, acetone and N-methyl pyrrolidone; and / or The hydrophobic agent comprises octadecyltrichlorosilane and / or dodecanoic acid; and / or The mass ratio of the first solvent to the carbon source is 100:(42-65).

5. The modified lithium iron manganese phosphate material of claim 4, wherein, In the step S1, The first solvent comprises water and ethylene glycol, and the volume ratio of water to ethylene glycol is 1:(0.5-1); and / or When the hydrophobic agent is a mixture of octadecyltrichlorosilane and dodecanoic acid, the mass ratio of the two is (1-2):

1. 6.The modified lithium iron manganese phosphate material of claim 1 or 2, characterized in that, In the step S2, The lithium source, the iron source, the manganese source, the phosphorus source and the doping agent are weighed according to the stoichiometric ratio Li:Fe:Mn:M:P as x:y:(1-y-z):z:1, wherein 1.01≤x≤1.02, 0 The lithium source comprises one or more of lithium carbonate, lithium dihydrogen phosphate, lithium hydrogen phosphate and lithium hydroxide; and / or The iron source comprises one or more of ferrous oxide, iron phosphate, ferrous oxalate, ferrous oxide and manganese iron phosphate; and / or The manganese source comprises one or more of manganese iron phosphate, manganese carbonate and trimanganese tetraoxide; and / or The phosphorus source comprises one or more of lithium dihydrogen phosphate, ammonium dihydrogen phosphate, iron phosphate and manganese iron phosphate; and / or The second solvent comprises a polar solvent; and / or The mass ratio of the sum of the lithium source, the manganese source, the iron source and the phosphorus source to the second solvent is (44~66):100; and / or The speed of the second stirring is 200~300rpm and the time is 2~3h; and / or The first grinding is performed to a solid particle size of 800~1000nm in the second mixed solution. 7.The modified lithium iron manganese phosphate material of claim 1 or 2, characterized in that, In the step S3, The dopant comprises niobium pentoxide and / or aluminum oxide; and / or The speed of the third stirring is 200~300rpm and the time is 1~2h; and / or The second grinding is performed to a solid particle size of 350~420nm in the third mixed solution; and / or The drying temperature is 290~310℃; and / or The sintering atmosphere is argon and / or nitrogen, and the oxygen content in the atmosphere is less than 20ppm.

8. A positive electrode material, characterized by, The modified manganese iron lithium phosphate material of claim 1 or 2.

9. A lithium-ion battery, characterized by The positive electrode material of claim 8.

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