Carbon-coated lithium manganese iron phosphate, and preparation method and use thereof

Abstract

The application provides a carbon-coated lithium manganese iron phosphate and a preparation method and application thereof, wherein the preparation method optimizes the carbon-coating process of lithium manganese iron phosphate by using PMMA, and the lithium manganese iron phosphate is obtained by multiple carbon-coating through multiple addition of carbon sources and PMMA sintering; PMMA can inhibit the growth of nascent particles, so that smaller particle size microparticle is obtained, and PMMA can make a large number of nanopores in the obtained material, effectively increase the specific surface area, and small particle size and high specific surface area are beneficial to increase the contact area with electrolyte, reduce the diffusion distance of lithium ions and weaken the electrochemical polarization in subsequent application of the obtained material; through multiple sintering coating of carbon sources and PMMA, the uniformity of coating is greatly improved, the crystallinity of the product is improved, the structure of the obtained LMFP material is compact, the structural stability is enhanced, the product density is improved, and the LMFP material with high purity and excellent crystallinity can be prepared.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of batteries, and relates to carbon-coated lithium manganese iron phosphate and a preparation method and application thereof. BACKGROUND

[0002] In order to replace fossil fuels and alleviate environmental problems, lithium ion batteries, especially their application and development in hybrid and electric vehicles, have attracted widespread attention from academia and industry.

[0003] The positive electrode material is an important part affecting the performance of lithium ion batteries. Among numerous positive electrode materials, lithium manganese iron phosphate (LMFP) has been growing in popularity in recent years due to its low price, environmental friendliness, and high theoretical specific capacity (170 mAh / g). In particular, the discharge voltage of this material is 4.1V, which is more ideal than that of lithium iron phosphate materials with the same olivine structure. It has a higher theoretical specific energy of 697 Wh / kg (about 20% higher than the specific energy of 595 Wh / kg of lithium iron phosphate), and can adapt to popular electrolytes, making it a positive electrode material of interest.

[0004] However, due to the characteristics of the olivine structure of lithium manganese iron phosphate material, the material has low electronic and ionic conductivity, which results in poor conductivity and poor rate characteristics, hindering the application of the material in actual production and life. At the same time, during the charging and discharging process, manganese ions are dissolved and deposited on the negative electrode surface, thereby destroying the SEI film, causing the SEI film to continuously regenerate and repair, consuming a large amount of active lithium, and easily causing capacity loss.

[0005] Carbon coating is an important means to effectively enhance the electronic conductivity of lithium manganese iron phosphate, promote the migration of lithium ions, improve manganese dissolution, and improve stability. Usually, it is obtained by directly sintering a carbon source with a lithium manganese phosphate material, but this method often produces some problems that affect the performance of the carbon-coated material, such as uneven coating, excessive particle growth, poor crystallinity, etc. To solve the above problems, the prior art uses a method of adding carbon source multiple times and sintering, but this multiple carbon coating method has limited uniformity improvement effect and low limitation on particle growth, and the density of the material is difficult to guarantee.

[0006] Therefore, in order to realize the commercial mass production and application of lithium manganese iron phosphate positive electrode material, it is necessary to optimize the carbon coating method to gradually build high-purity, well-crystallized, small-particle-size, and high-density lithium manganese iron phosphate products in a comprehensive consideration. SUMMARY

[0007] In view of the problems in the prior art, the purpose of the present application is to provide a carbon-coated lithium manganese iron phosphate and a preparation method and use thereof, wherein the preparation method optimizes the carbon-coating process of lithium manganese iron phosphate by PMMA, and obtains the lithium manganese iron phosphate with multiple carbon coatings by multiple additions of carbon sources and PMMA sintering; PMMA is beneficial to inhibit the growth of nascent particles and make the material produce a large number of nanopores, greatly improving the uniformity of coating; and the preparation method can prepare the LMFP material with high purity and excellent crystallization.

[0008] To achieve this purpose, the present application adopts the following technical solutions:

[0009] In the first aspect, the present application provides a preparation method of carbon-coated lithium manganese iron phosphate, comprising:

[0010] Mixing lithium manganese iron phosphate raw materials, carbon sources and PMMA, and sintering to obtain sintered materials;

[0011] Re-mixing and re-sintering the obtained sintered materials with carbon sources and PMMA, repeating the re-mixing and re-sintering processes to obtain carbon-coated lithium manganese iron phosphate.

[0012] The following is a preferred technical solution of the present application, but not as a limitation of the technical solutions provided by the present application, through the following technical solutions, the technical purpose and beneficial effects of the present application can be better achieved and realized.

[0013] As a preferred technical solution of the present application, in the preparation method, the process of mixing the carbon sources and PMMA and sintering is a total of three times.

[0014] Preferably, the temperature of the last sintering is higher than that of the previous sintering.

[0015] Preferably, the temperature of the first sintering is 200-230℃.

[0016] Preferably, the temperature of the last sintering is 600-650℃.

[0017] Preferably, the time of the last sintering is longer than that of the previous sintering.

[0018] Preferably, the time of the first sintering is 3-5h.

[0019] Preferably, the heating rate of the sintering and the re-sintering is 2-10℃ / min.

[0020] Preferably, the sintering and the re-sintering are both carried out in the atmosphere of a protective gas.

[0021] As a preferred technical scheme of the present application, the first amount of the carbon source is 1% to 15% of the total mass of the lithium manganese iron phosphate raw material; and the amount of the carbon source when it is mixed with the sintering material again is 1% to 15% of the mass of the sintering material.

[0022] Preferably, the amount of the carbon source remains the same each time.

[0023] As a preferred technical scheme of the present application, the amount of the PMMA each time is 30% to 70% of the total mass of the lithium manganese iron phosphate raw material.

[0024] Preferably, the amount of the PMMA remains the same each time.

[0025] As a preferred technical scheme of the present application, the mixing and the re-mixing include grinding and / or stirring.

[0026] Preferably, the grinding includes ball milling.

[0027] Preferably, the ball milling uses a solvent, which includes at least one of acetone, ethanol or ethylene glycol.

[0028] Preferably, the solvent is volatilized and removed during the ball milling.

[0029] Preferably, the grinding medium of the ball milling includes zirconium oxide medium and / or agate medium.

[0030] Preferably, the rotation speed of the ball milling is 400 to 600 r / min, and the time is 8 to 14 h.

[0031] As a preferred technical scheme of the present application, the lithium manganese iron phosphate raw material includes a lithium source, a manganese source, an iron source and a phosphorus source.

[0032] Preferably, the amounts of the lithium source, the manganese source, the iron source and the phosphorus source are controlled according to the chemical formula of the lithium manganese iron phosphate Li a Mn b Fe c PO4, wherein a = 0.4 to 1.1, b = 0.5 to 0.9, c = 0.2 to 0.5, and b + c = 1.

[0033] As a preferred technical scheme of the present application, the lithium source includes lithium hydroxide and / or lithium carbonate.

[0034] Preferably, the manganese source includes manganese oxalate and / or manganese carbonate.

[0035] Preferably, the iron source includes ferrous sulfate and / or ferrous oxalate.

[0036] Preferably, the phosphorus source includes at least one of phosphorus, ammonium dihydrogen phosphate or di-ammonium hydrogen phosphate.

[0037] Preferably, the carbon source comprises at least one of glucose, sucrose or maltose.

[0038] As a preferred technical solution of the present application, the preparation method comprises the following steps:

[0039] The lithium source, the manganese source, the iron source, the phosphorus source, the carbon source and PMMA are mixed, the molar ratio of lithium, manganese, iron and phosphate is controlled to be (0.4-1.0):(0.5-0.9):(0.2-0.5):1, the amount of the carbon source is controlled to be 1%-15% of the total mass of the lithium source, the manganese source, the iron source and the phosphorus source, the amount of PMMA is controlled to be 30%-70% of the total mass of the lithium source, the manganese source, the iron source and the phosphorus source, the mixture is dissolved in a solvent acetone, a zirconium oxide medium is used, ball milling is carried out at a rotating speed of 400-600 r / min for 8-14 h, after the solvent is volatilized and removed, grinding is carried out using a maroon mortar, a ground material is obtained, then the first sintering is carried out at a temperature rising rate of 2-10 ℃ / min to 200-230 ℃ for 3-5 h under a mixed gas atmosphere with a volume fraction of 5% of hydrogen and nitrogen, and a first sintered material is obtained;

[0040] The first sintered material is mixed with the carbon source and PMMA again, the amount of the carbon source and PMMA is not changed, the temperature rising rate and the mixed gas atmosphere are not changed, the second sintering is carried out at 400-450 ℃ for 4-6 h, and a second sintered material is obtained;

[0041] The second sintered material is mixed with the carbon source and PMMA again, the amount of the carbon source and PMMA is not changed, the third sintering is carried out at 600-650 ℃ for 10-15 h, and a carbon-coated manganese iron lithium phosphate is obtained.

[0042] In the second aspect, the present application provides a carbon-coated manganese iron lithium phosphate, which is obtained by the preparation method in the first aspect, and comprises a manganese iron lithium phosphate and a carbon material coated on the surface of the manganese iron lithium phosphate.

[0043] Preferably, the average particle size of the carbon-coated manganese iron lithium phosphate is 100-200 nm.

[0044] In the third aspect, the present application provides a lithium ion battery containing the carbon-coated manganese iron lithium phosphate in the second aspect.

[0045] Compared with the prior art, the present application has at least the following beneficial effects:

[0046] The preparation method provided by the present application optimizes the carbon coating process of lithium manganese iron phosphate by using PMMA, and obtains lithium manganese iron phosphate (LMFP) coated with carbon multiple times by adding carbon source and PMMA multiple times for sintering; PMMA and carbon source synergize, PMMA acts as an auxiliary additive, which can inhibit the growth of primary particles, so that smaller particle size microparticle groups are obtained, and PMMA can produce a large number of nanopores in the obtained material, effectively increasing the specific surface area, and small particle size and high specific surface area are beneficial to increasing the contact area with electrolyte, reducing the diffusion distance of lithium ions, and weakening the electrochemical polarization of the obtained material in subsequent application; after multiple sintering coating of carbon source and PMMA, the uniformity of coating is greatly improved, the crystallinity of the product is improved, the NPMA structure is compact, the structural stability is enhanced, and the product density is improved; the preparation method can prepare LMFP material with excellent purity and crystallinity. BRIEF DESCRIPTION OF DRAWINGS

[0047] Figure 1 is an SEM image of the carbon-coated lithium manganese iron phosphate obtained in Example 1;

[0048] Figure 2 is a first charge-discharge capacity test diagram of the battery made of the carbon-coated lithium manganese iron phosphate obtained in Example 1. DETAILED DESCRIPTION

[0049] The technical solutions of the present application will be further described through specific embodiments.

[0050] Those skilled in the art should understand that the embodiments are only to help understand the present application, and should not be regarded as specific limitations of the present application.

[0051] In some specific embodiments, the present application provides a preparation method of carbon-coated lithium manganese iron phosphate, which comprises:

[0052] Mixing lithium manganese iron phosphate raw material, carbon source and PMMA, sintering to obtain sintered material;

[0053] Re-mixing and re-sintering the obtained sintered material with carbon source and PMMA, repeating the re-mixing and re-sintering process to obtain carbon-coated lithium manganese iron phosphate.

[0054] The preparation method of the application first uniformly mixes PMMA, a carbon source and a precursor (lithium manganese iron phosphate raw material), then performs sintering under suitable sintering conditions, and then repeatedly sintering the obtained coated material, and adding the carbon source and PMMA each time. Finally, a carbon-coated lithium manganese iron phosphate positive electrode material is obtained, which is a porous nanoparticle micro-agglomerate (NPMA) coated with a single layer of carbon through multiple carbon source coatings. The preparation method realizes multiple carbon coatings of lithium manganese iron phosphate through multiple mixing and sintering of the lithium manganese iron phosphate raw material, the carbon source and PMMA, which is simple in operation, good in coating effect, and can improve the problem of uneven coating layer in the traditional mechanical mixing method. And a material with good purity and good crystallinity can be prepared, and the uniformly coated porous nanoparticle micro-agglomerate (NPMA) can maximize the improvement of the cycle reversibility and rate capability of the LMFP cathode.

[0055] Specifically, in the process of sintering pyrolysis, the decomposition products of the carbon source are coated on the surface of the reaction precursor, which helps to establish an ultra-thin innermost carbon coating layer in the interior of each particle. At the same time, due to the rich polar groups of PMMA, it can tightly gather around the particles and maintain a stable micellar structure, thereby greatly limiting the growth of primary particles. In addition, PMMA also evaporates and decomposes at the sintering temperature, and its evaporation and pyrolysis process can produce a large number of nanoscale pores in the entire agglomerated particles, which is beneficial to increase the specific surface area of the active material. The suitable sintering temperature can finally effectively improve the crystallinity of the product prepared by the preparation method of the application, and the multiple sintering and gradual increase of the temperature to high-temperature sintering can further increase the density of the material, so that the structure of the NPMA material formed is dense, which is beneficial to enhance the structural stability and improve the product density. And the multiple re-introduction of PMMA and sintering is beneficial to further inhibit the growth of primary particles and reduce the particle size, thereby increasing the contact area of the particles with the electrolyte, reducing the diffusion distance of Li + , and weakening the electrochemical polarization.

[0056] It should be noted that the PMMA of the application is different from the prior art which only uses PMMA as a carbon source to form a carbon coating layer. The PMMA in the application is used for particle regulation of lithium manganese iron phosphate, and the purpose is to optimize the crystal structure. PMMA has rich polar groups, which can tightly gather around the particles to limit the growth of the particles, so that the structure of lithium manganese iron phosphate is densified, which is beneficial to improve the stability, and the introduction of PMMA can increase the void density to inhibit the growth of primary particles, and can make the slurry and coating of the material uniform, which is beneficial to increase the contact area with the electrolyte and reduce the diffusion distance of Li +The diffusion distance of the carbon source weakens the electrochemical polarization. The particle regulation effect on the lithium manganese iron phosphate is not achieved by using a conventional carbon source such as glucose alone or PMMA alone, and if PMMA is used alone, it has a great impact on the performance of the material, so it is not suitable to use it as a sole basic carbon source.

[0057] In one embodiment, the lithium manganese iron phosphate raw material is first mixed with the carbon source uniformly, and then sintering is started, and then PMMA is added during the sintering process and sintering is completed. The process of completing the mixing and sintering or the process of completing the re-mixing and re-sintering according to this process can make the lithium manganese iron phosphate raw material and the carbon source more fully mixed, and improve the coating effect.

[0058] In one embodiment, the process of mixing the carbon source and PMMA and sintering is repeated three times.

[0059] The present application preferably obtains the carbon-coated lithium manganese iron phosphate by repeating the addition of the carbon source and PMMA and sintering at least three times, and matches the preferred sintering process. Specifically, the sintering temperature is gradually increased from low to high, and the sintering time is gradually extended, so as to combine the best routes of low temperature and high temperature. For the formation of lithium manganese iron phosphate, the three sintering stages are respectively "pyrolysis", "formation" and "growth" stages, aiming to improve the reaction efficiency, improve the purity of the product and form a nano-particle micro-agglomerate structure. "Formation" and "growth" require more reaction time than "pyrolysis". Specifically, the low-temperature synthesis route corresponding to the "pyrolysis" process (the first sintering) can promote the preliminary formation of the lithium manganese iron phosphate product (inorganic precursor), minimize the particle size, and be beneficial to increase the contact area with the electrolyte. At this time, the carbon source is partially pyrolyzed, and the small particles are preliminarily coated, which will help to establish an ultra-high density innermost carbon coating layer in the interior of each particle, and also be beneficial to improve the conductivity of the material. At this time, the main role of PMMA is to pre-densify the inorganic precursor. When the temperature is gradually increased to the temperature of "formation", low-crystallinity LMFP is generated, and the complete carbonization of the carbon source is completed, and the uniformity, integrity and density of the coating layer are consolidated. The high-temperature synthesis route corresponding to the "growth" stage (the last sintering) is beneficial to improve the crystallinity, avoid arsenic defects, and increase the electrode density and the material density. It should be noted that due to the low diffusion rate of carbon in the temperature range of LMFP formation and the rapid grain growth, the traditional solid-state synthesis route of doping carbon before and / or after the entire reaction usually cannot guarantee the uniformity of the carbon coating layer on the surface and the entire interior of each particle, while the present application can solve the problem of the traditional solid-state synthesis route by multiple addition of the carbon source and PMMA and through a specific sintering process.

[0060] In one embodiment, the temperature of the last sintering is greater than the temperature of the previous sintering.

[0061] When the sintering temperature can make the carbon source decompose to form carbon coating and the manganese iron lithium phosphate can be generated and grown, the lower the temperature is, the more conducive to controlling the particle size. Therefore, sintering at low temperature first, and then increasing the sintering temperature each time, can inhibit the particle size, is conducive to obtaining small particle size particles, and the subsequent increase in temperature can ensure a high degree of crystallinity.

[0062] In an embodiment, the temperature of the first sintering is 200-230°C, such as 200°C, 205°C, 210°C, 215°C, 220°C, 225°C or 230°C, but not limited to the listed values, and other values not listed in the above value range are also applicable.

[0063] The temperature of the first sintering is preferably 200-230°C, which can make the carbon source melt and coat on the surface of the material, and this temperature range is also a suitable generation reaction temperature for manganese iron lithium phosphate.

[0064] In an embodiment, the temperature of the last sintering is 600-650°C, such as 600°C, 605°C, 610°C, 615°C, 620°C, 625°C, 630°C, 635°C, 640°C, 645°C or 650°C, but not limited to the listed values, and other values not listed in the above value range are also applicable.

[0065] The temperature of the last sintering is 600-650°C, which is conducive to improving the crystallinity of the material, avoiding reaction defects, and improving the electrode density.

[0066] In an embodiment, the time of the last sintering is greater than the time of the previous sintering.

[0067] In an embodiment, the time of the first sintering is 3-5h, such as 3h, 3.5h, 4h, 4.5h or 5h, but not limited to the listed values, and other values not listed in the above value range are also applicable.

[0068] In an embodiment, the heating rate of the sintering and the re-sintering is 2-10°C / min, such as 2°C / min, 4°C / min, 6°C / min, 8°C / min or 10°C / min, but not limited to the listed values, and other values not listed in the above value range are also applicable.

[0069] During the sintering process, controlling a slow heating rate is conducive to avoiding structure collapse caused by rapid crystallization and improving the stability of the material structure.

[0070] In one embodiment, the sintering and the re-sintering are both performed in an atmosphere of a protective gas; the protective gas can be hydrogen and / or an inert gas; wherein the inert gas can be at least one of nitrogen, helium and argon, for example, can be performed in a mixed atmosphere of hydrogen and an inert gas; further, the volume fraction of hydrogen in the mixed atmosphere is 1% to 10%, for example, 1%, 3%, 5%, 8% or 10%, etc., but not limited to the listed values, other values not listed in the above numerical range are also applicable.

[0071] In one embodiment, the amount of the carbon source each time is 1% to 15% of the total mass of the lithium manganese iron phosphate raw material, for example, 1%, 3%, 5%, 7%, 9%, 11%, 13% or 15%, etc., but not limited to the listed values, other values not listed in the above numerical range are also applicable.

[0072] In one embodiment, the amount of the carbon source each time is kept the same.

[0073] Keeping the amount of the carbon source and the PMMA the same each time is conducive to uniform and dense results each time.

[0074] In one embodiment, the amount of the PMMA each time is 30% to 70% of the total mass of the lithium source, the manganese source, the iron source and the phosphorus source, for example, 30%, 32%, 34%, 36%, 38%, 40%, 42%, 44%, 46%, 48%, 50%, 52%, 54%, 56%, 58%, 60%, 62%, 64%, 66%, 68% or 70%, etc., preferably 40% to 60%, further preferably 47% to 51%, but not limited to the listed values, other values not listed in the above numerical range are also applicable.

[0075] In one embodiment, the amount of the PMMA each time is kept the same.

[0076] In one embodiment, the mixing and the re-mixing include grinding and / or stirring.

[0077] In one embodiment, the grinding includes ball milling.

[0078] In one embodiment, the ball milling uses a solvent, and the solvent includes acetone.

[0079] In one embodiment, the solvent is volatilized and removed during the ball milling.

[0080] In one embodiment, the grinding medium of the ball milling includes a zirconia medium and / or an agate medium.

[0081] In one embodiment, the rotation speed of the ball milling is 400-600 r / min, such as 400 r / min, 440 r / min, 480 r / min, 520 r / min, 560 r / min or 600 r / min, etc., and the time is 8-14 h, such as 8 h, 9 h, 10 h, 11 h, 12 h, 13 h or 14 h, etc., but not limited to the listed values, and other values not listed in the above value range are also applicable.

[0082] Exemplarily, before the last sintering, the mixing method is ball milling, and in order to obtain a sintered material with a suitable particle size for making electrode sheets, the sintered material obtained by the last sintering can be first ball milled and then finely ground by using agate grinding to obtain a powder with a suitable particle size.

[0083] In one embodiment, the chemical formula of the formed lithium manganese iron phosphate is Li a Mn b Fe c PO4, and the amount of the lithium source, the manganese source, the iron source and the phosphorus source is controlled, wherein a = 0.4-1.1, such as a can be 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 or 1.1, etc., b = 0.5-0.9, such as b can be 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85 or 0.9, etc., c = 0.2-0.5, such as c can be 0.2, 0.25, 0.3, 0.35, 0.4, 0.45 or 0.5, etc., and b + c = 1, but not limited to the listed values, and other values not listed in the above value range are also applicable.

[0084] In one embodiment, the lithium source includes lithium hydroxide and / or lithium carbonate.

[0085] In one embodiment, the manganese source includes manganese oxalate and / or manganese carbonate.

[0086] In one embodiment, the iron source includes ferrous sulfate and / or ferrous oxalate.

[0087] In one embodiment, the phosphorus source includes at least one of phosphoric acid, ammonium dihydrogen phosphate or di-ammonium hydrogen phosphate, and exemplary but non-limiting combinations can be a combination of phosphoric acid and ammonium dihydrogen phosphate, a combination of phosphoric acid and di-ammonium hydrogen phosphate, or a combination of ammonium dihydrogen phosphate and di-ammonium hydrogen phosphate, etc.

[0088] In one embodiment, the carbon source includes at least one of glucose, sucrose or maltose, and exemplary but non-limiting combinations can be a combination of glucose and sucrose, a combination of glucose and maltose, or a combination of sucrose and maltose, etc.

[0089] In one embodiment, the present application provides a preparation method of carbon-coated lithium manganese iron phosphate, comprising the following steps:

[0090] The lithium source, manganese source, iron source, phosphorus source, carbon source and PMMA are mixed, the molar ratio of lithium, manganese, iron and phosphate is controlled to be (0.4-1.0):(0.5-0.9):(0.2-0.5):1, the amount of the carbon source is controlled to be 1%-15% of the total mass of the lithium source, manganese source, iron source and phosphorus source, the amount of the PMMA is controlled to be 30%-70% of the total mass of the lithium source, manganese source, iron source and phosphorus source, the mixture is dissolved in a solvent acetone, a zirconium oxide medium is used, ball milling is performed at a rotating speed of 400-600 r / min for 8-14 h, after the solvent is volatilized and removed, a grinding mortar is used for grinding to obtain a grinding material, then the grinding material is subjected to first sintering at a temperature of 200-230 °C at a temperature increasing rate of 2-10 °C / min for 3-5 h under a mixed gas atmosphere of hydrogen and nitrogen with a volume fraction of 5% to obtain a first sintered material;

[0091] The first sintered material is mixed with the carbon source and PMMA again, the amount of the carbon source and PMMA is not changed, the temperature increasing rate and mixed gas atmosphere are not changed, and the second sintering is performed at a temperature of 400-450 °C for 4-6 h to obtain a second sintered material;

[0092] The second sintered material is mixed with the carbon source and PMMA again, the amount of the carbon source and PMMA is not changed, the temperature increasing rate and mixed gas atmosphere are not changed, and the third sintering is performed at a temperature of 600-650 °C for 10-15 h to obtain the carbon-coated lithium manganese iron phosphate.

[0093] In some specific embodiments, the present application provides a carbon-coated lithium manganese iron phosphate, which is obtained by the preparation method provided in one of the above specific embodiments.

[0094] The obtained carbon-coated lithium manganese iron phosphate is a carbon-coated porous nanoparticle micromass (NPMA) with an average particle size of 100-200 nm.

[0095] In some specific embodiments, the present application provides a lithium ion battery containing the carbon-coated lithium manganese iron phosphate provided in one of the above specific embodiments.

[0096] Example 1

[0097] The present embodiment provides a preparation method of lithium manganese iron phosphate, comprising the following steps:

[0098] Li2CO3 (purity 99.5%), 0.016 mol of manganese oxalate (MnC2O4·2H2O, Mn content 30 wt%), 0.004 mol of ferrous oxalate (FeC2O4·2H2O, purity 98.5%), and 0.02 mol of ammonium dihydrogen phosphate (NH4H2PO4, purity 99%) were thoroughly mixed with 0.5 g of sucrose and 3.2 g of PMMA in acetone, and ball-milling was performed at a rotation speed of 500 r / min using a zirconia grinding medium for 10 h. After the acetone was volatilized at room temperature, the mixture was further ground in an agate mortar, and then pyrolysis of the mixture was performed by first sintering at 220°C (heating rate 2°C / min) under an N2 / H2 atmosphere with a partial pressure ratio of 95 / 5 for 4 h to obtain a first sintered material;

[0099] Then, 3.20 g of PMMA and 0.5 g of sucrose were further mixed with the obtained first sintered material, and after thorough mixing, second sintering was performed at 450°C (heating rate 2°C / min) under the same atmosphere for 5 h to obtain a second sintered material.

[0100] Finally, 3.20 g of PMMA and 0.5 g of sucrose were further mixed with the obtained second sintered material, and after thorough mixing, third sintering was performed at 600°C (heating rate 2°C / min) under the same atmosphere for 10 h to obtain a third sintered material.

[0101] After the sintering was completed, the product was cooled under a protective atmosphere (pure nitrogen), and the cooled product was ground and pulverized to a powder, which was sieved using a 400-mesh sieve, and the undersize material was collected to obtain carbon-coated manganese iron lithium phosphate.

[0102] Example 2

[0103] The present example provides a method for preparing manganese iron lithium phosphate, the method comprising the following steps:

[0104] LiOH (purity 99.9%), 0.01 mol of manganese carbonate (MnCO3, purity 99.9%), 0.006 mol of ferrous sulfate (FeSO4·7H2O, purity 99.5%), and 0.02 mol of diammonium hydrogen phosphate ((NH4)2HPO4, purity 99.9%) were thoroughly mixed with 0.1 g of glucose and 3 g of PMMA in acetone, and ball-milling was performed at a rotation speed of 600 r / min using a zirconia grinding medium for 8 h. After the acetone was volatilized at room temperature, the mixture was further ground in an agate mortar, and then pyrolysis of the mixture was performed by first sintering at 200°C (heating rate 3.5°C / min) under an N2 / H2 atmosphere with a partial pressure ratio of 95 / 5 for 5 h to obtain a first sintered material.

[0105] Then, 3g of PMMA and 0.1g of sucrose were mixed again into the obtained first sintered material, and after being mixed again thoroughly, the second sintering was performed at 400℃ for 6h at the same heating rate of 3.5℃ / min under the same atmosphere, to obtain a second sintered material;

[0106] Finally, 3g of PMMA and 0.1g of sucrose were mixed again into the obtained second sintered material, and after being mixed again thoroughly, the third sintering was performed at 630℃ for 8h at the same heating rate of 3.5℃ / min under the same atmosphere, to obtain a third sintered material;

[0107] After the sintering was completed, the product after cooling was ground into powder, and sieved using a 400-mesh screen, and the undersize was collected, to obtain the carbon-coated lithium manganese iron phosphate.

[0108] Example 3

[0109] The present embodiment provides a preparation method of lithium manganese iron phosphate, which comprises the following steps:

[0110] 0.02mol of lithium carbonate (Li2CO3, purity 99.5%), 0.018mol of manganese carbonate (MnCO3, purity 99.9%), 0.01mol of ferrous sulfate (FeSO4·7H2O, purity 99.5%), and 0.02mol of ammonium dihydrogen phosphate (NH4H2PO4, purity 99%) were mixed with 1.2g of maltose and 3.6g of PMMA in acetone, and ball-milling was performed at a rotation speed of 400r / min for 14h using zirconium oxide grinding media; after the acetone was volatilized at room temperature, the mixture was further ground in an agate mortar, and then the first sintering was performed at 230℃ (heating rate 5℃ / min) for 3h under an N2 / H2 atmosphere with a partial pressure ratio of 95 / 5, to pyrolyze the mixture, and obtain a first sintered material;

[0111] Then, 3.6g of PMMA and 1.2g of sucrose were mixed again into the obtained first sintered material, and after being mixed again thoroughly, the second sintering was performed at 430℃ for 7h at the same heating rate of 5℃ / min under the same atmosphere, to obtain a second sintered material;

[0112] Finally, 3.6g of PMMA and 1.2g of sucrose were mixed again into the obtained second sintered material, and after being mixed again thoroughly, the third sintering was performed at 650℃ for 9h at the same heating rate of 5℃ / min under the same atmosphere, to obtain a third sintered material;

[0113] After sintering is completed, the product after cooling is ground into powder, sieved with a 400-mesh screen, and the undersize is collected to obtain the carbon-coated lithium manganese iron phosphate.

[0114] Example 4

[0115] This example provides a preparation method of lithium manganese iron phosphate, in which the amount of the carbon source sucrose is adjusted from 0.5 g to 0.05 g each time, and other conditions are the same as in Example 1.

[0116] Example 5

[0117] This example provides a preparation method of lithium manganese iron phosphate, in which the amount of the carbon source sucrose is adjusted from 0.5 g to 0.07 g each time, and other conditions are the same as in Example 1.

[0118] Example 6

[0119] This example provides a preparation method of lithium manganese iron phosphate, in which the amount of the carbon source sucrose is adjusted from 0.5 g to 0.25 g each time, and other conditions are the same as in Example 1.

[0120] Example 7

[0121] This example provides a preparation method of lithium manganese iron phosphate, in which the amount of the carbon source sucrose is adjusted from 0.5 g to 0.75 g each time, and other conditions are the same as in Example 1.

[0122] Example 8

[0123] This example provides a preparation method of lithium manganese iron phosphate, in which the amount of the carbon source sucrose is adjusted from 0.5 g to 1 g each time, and other conditions are the same as in Example 1.

[0124] Example 9

[0125] This example provides a preparation method of lithium manganese iron phosphate, in which the amount of the carbon source sucrose is adjusted from 0.5 g to 1.2 g each time, and other conditions are the same as in Example 1.

[0126] Example 10

[0127] This example provides a preparation method of lithium manganese iron phosphate, in which the amount of PMMA is adjusted from 3.2 g to 1.79 g each time, and other conditions are the same as in Example 1.

[0128] Comparative Example 1

[0129] The comparative example 1 provides a preparation method of lithium manganese iron phosphate, which only performs one sintering, and the total amount of the carbon source and PMMA is kept unchanged, that is, the total amount of the carbon source sucrose is 1.5 g, the total amount of PMMA is 9.6 g, the sintering temperature is set to 220°C, and the time is 19 h, and other conditions are completely the same as those in the example 1.

[0130] The comparative example 2

[0131] The comparative example 3 provides a preparation method of lithium manganese iron phosphate, which adjusts the sintering temperature from 220°C to 600°C, and other conditions are completely the same as those in the comparative example 2.

[0132] The comparative example 3

[0133] The comparative example 3 provides a preparation method of lithium manganese iron phosphate, which adjusts the sintering temperature from 220°C to 600°C, and other conditions are completely the same as those in the comparative example 2.

[0134] The comparative example 4

[0135] The comparative example 4 provides a preparation method of lithium manganese iron phosphate, which does not add PMMA in the first sintering, but keeps the total amount of PMMA unchanged, that is, the amount of PMMA added in the second sintering and the third sintering is adjusted from 3.2 g to 4.8 g, and other conditions are completely the same as those in the example 1.

[0136] The comparative example 5

[0137] The comparative example 5 provides a preparation method of lithium manganese iron phosphate, which does not add PMMA in the second sintering, but keeps the total amount of PMMA unchanged, that is, the amount of PMMA added in the first sintering and the third sintering is adjusted from 3.2 g to 4.8 g, and other conditions are completely the same as those in the example 1.

[0138] The comparative example 6

[0139] The comparative example 6 provides a preparation method of lithium manganese iron phosphate, which does not add PMMA in the third sintering, but keeps the total amount of PMMA unchanged, that is, the amount of PMMA added in the first sintering and the second sintering is adjusted from 3.2 g to 4.8 g, and other conditions are completely the same as those in the example 1.

[0140] The comparative example 7

[0141] The comparative example 1 provides a preparation method of lithium manganese iron phosphate, which does not add PMMA in the first sintering and the second sintering, but keeps the total amount of PMMA unchanged, i.e. the amount of PMMA added in the third sintering is adjusted from 3.2 g to 9.6 g, and other conditions are completely same as those in the example 1.

[0142] Comparative example 8

[0143] The comparative example 1 provides a preparation method of lithium manganese iron phosphate, which does not use PMMA, and other conditions are completely same as those in the example 1.

[0144] Comparative example 9

[0145] The comparative example 1 provides a preparation method of lithium manganese iron phosphate, which does not use PMMA, and uses an equal amount of carbon source sucrose to replace the amount of PMMA, and other conditions are completely same as those in the example 1.

[0146] I. Characterization test:

[0147] Figure 1 The SEM image of the carbon-coated lithium manganese iron phosphate anode material prepared by the preparation method of the example 1 is shown in the figure, and it can be seen from the figure that the obtained carbon-coated lithium manganese iron phosphate is a carbon-coated porous nanoparticle microparticle (NPMA) with an average particle size of 100-200 nm.

[0148] II. Performance test:

[0149] The carbon-coated lithium manganese iron material obtained in the example and the comparative example is used as an anode material, the anode material, the conductive active substance Super P and the binder PVDF are mixed in a mass ratio of 95:3:2, N-methyl pyrrolidone is used as a dispersant, the mixed slurry is uniformly coated on an aluminum foil, and the aluminum foil is dried in a vacuum drying box at 120°C for 6h, and then cut into an electrode sheet with a diameter of 12mm; using a 2032 battery shell, a PP separator and a commercialized LB315 electrolyte, the prepared electrode sheet is used as a negative electrode, a lithium sheet is used as a counter electrode to assemble a half battery, and the assembled battery needs to be placed for 12h before electrochemical test. The test voltage range is 2.0V-4.4V, the charging is carried out in a constant current and constant voltage mode at a rate of 0.1 times to 4.4V, the cutoff current is 0.02C, and the discharging is carried out in a constant current mode to 2.0V. The test results are recorded in Table 1.

[0150] Table 1

[0151] Group Initial charge capacity (mAh / g) Initial discharge capacity (mAh / g) First efficiency (%) Example 1 163.0 157.3 96.5 Example 2 161.6 153.1 94.7 Example 3 161.8 152.1 94.0 Example 4 160.9 150.8 93.7 Example 5 162.8 155.5 95.5 Example 6 163.5 154.3 94.3 Example 7 162.5 152.7 93.9 Example 8 162.1 153.5 94.6 Example 9 161.8 151.3 93.5 Example 10 162.4 154.5 95.1 Comparative Example 1 151.9 136.1 89.5 Comparative Example 2 154.3 138.7 89.8 Comparative Example 3 152.6 137.2 89.9 Comparative Example 4 153.2 138,4 90.3 Comparative Example 5 148.9 135.2 90.7 Comparative Example 6 147.8 136.5 92.3 Comparative Example 7 151.5 137.8 90.9 Comparative Example 8 152.8 129.4 84.8 Comparative Example 9 153.2 128.8 84.1

[0152] Figure 2is a first charge-discharge capacity test chart of the battery made of the carbon-coated lithium manganese iron phosphate obtained in Example 1. As can be seen in combination with Table 1, the carbon-coated lithium manganese iron phosphate obtained in Example 1 can provide a first charge capacity of 163.0 mAh / g and a first discharge capacity of 157.3 mAh / g for the battery when used as a positive electrode material, with a first efficiency of 96.5%.

[0153] Comparative Examples 1-3 were compared with Example 1, and it was found that when the amount of carbon source and PMMA was kept unchanged, but only one sintering was performed, the performance of the obtained carbon-coated lithium manganese iron phosphate was greatly affected, regardless of whether the sintering temperature was low or high. When the three sintering processes were kept unchanged, as in Comparative Examples 4-7, but PMMA was not added in one or two of the sintering processes, the performance of the obtained carbon-coated lithium manganese iron phosphate was also affected. As in Comparative Examples 8 and 9, the carbon-coated lithium manganese iron phosphate obtained without using PMMA had the worst performance.

[0154] The above describes preferred embodiments of the present application, but the present application is not limited to the specific details in the above-described embodiments. Within the technical concept of the present application, various simple modifications can be made to the technical solutions of the present application, and these simple modifications all belong to the protection scope of the present application.

[0155] In addition, it should be noted that each specific technical feature described in the above-described specific embodiments can be combined in any appropriate manner without contradiction. In order to avoid unnecessary repetition, the present application will not further describe various possible combinations.

[0156] Furthermore, various different embodiments of the present application can also be combined in any appropriate manner, as long as it does not deviate from the technical concept of the present application, and it should be considered as disclosed content of the present application.

Claims

1. A method for preparing carbon-coated lithium manganese iron phosphate, characterized in that, The preparation method includes: Lithium manganese iron phosphate raw material, carbon source and PMMA are mixed and sintered to obtain sintered material; The obtained sintered material is remixed with carbon source and PMMA and re-sintered. The remixing and re-sintering process is repeated three times in total. Furthermore, the temperature of each subsequent sintering is higher than that of the previous sintering; the temperature of the first sintering is 200~230℃; and the temperature of the last sintering is 600~650℃. Furthermore, the time for the subsequent sintering is longer than the time for the previous sintering; the time for the first sintering is 3-5 hours. Furthermore, the heating rate for the sintering and re-sintering is 2~10℃ / min; Furthermore, both the sintering and the re-sintering are carried out under a protective gas atmosphere; Carbon-coated lithium manganese iron phosphate was obtained.

2. The preparation method according to claim 1, characterized in that, The initial amount of carbon source used is 1% to 15% of the total mass of lithium manganese iron phosphate raw material; the amount of carbon source used when mixing carbon source with sintering material is 1% to 15% of the mass of sintering material.

3. The preparation method according to claim 1 or 2, characterized in that, The amount of carbon source used each time remains the same.

4. The preparation method according to claim 1, characterized in that, The amount of PMMA used each time is 30% to 70% of the total mass of the lithium manganese iron phosphate raw material.

5. The preparation method according to claim 1 or 4, characterized in that, The amount of PMMA used each time remains the same.

6. The preparation method according to claim 1, characterized in that, The mixing and remixing methods include grinding and / or stirring.

7. The preparation method according to claim 6, characterized in that, The grinding includes ball milling.

8. The preparation method according to claim 7, characterized in that, The ball milling uses a solvent, which includes at least one of acetone, ethanol, or ethylene glycol.

9. The preparation method according to claim 8, characterized in that, The solvent is removed by evaporation during ball milling.

10. The preparation method according to claim 7, characterized in that, The grinding media of the ball mill include zirconium oxide media and / or agate media.

11. The preparation method according to claim 7, characterized in that, The ball milling speed is 400~600 r / min, and the time is 8~14 h.

12. The preparation method according to claim 1, characterized in that, The lithium manganese iron phosphate raw material includes lithium source, manganese source, iron source and phosphorus source.

13. The preparation method according to claim 12, characterized in that, According to the chemical formula of the formed lithium manganese iron phosphate, Li a Mn b Fe c The amounts of lithium, manganese, iron and phosphorus sources in PO4 are controlled, where a = 0.4~1.1, b = 0.5~0.9, c = 0.2~0.5, and b+c = 1.

14. The preparation method according to claim 12, characterized in that, The lithium source includes lithium hydroxide and / or lithium carbonate.

15. The preparation method according to claim 12, characterized in that, The manganese source includes manganese oxalate and / or manganese carbonate.

16. The preparation method according to claim 12, characterized in that, The iron source includes ferrous sulfate and / or ferrous oxalate.

17. The preparation method according to claim 12, characterized in that, The phosphorus source includes at least one of phosphorus, ammonium dihydrogen phosphate, or diammonium hydrogen phosphate.

18. The preparation method according to claim 1, characterized in that, The carbon source includes at least one of glucose, sucrose, or maltose.

19. The preparation method according to claim 1, characterized in that, The preparation method includes the following steps: A mixture of lithium, manganese, iron, phosphorus, carbon, and PMMA is prepared, with the molar ratio of lithium, manganese, iron, and phosphate controlled at (0.4~1.0):(0.5~0.9):(0.2~0.5):

1. The amount of carbon source is controlled at 1%~15% of the total mass of lithium, manganese, iron, and phosphorus sources, and the amount of PMMA is controlled at 30%~70% of the total mass of lithium, manganese, iron, and phosphorus sources. The mixture is dissolved in acetone and ball-milled for 8~14 hours using zirconium oxide media at a speed of 400~600 r / min. After the solvent evaporates, the mixture is ground with agate slurry to obtain abrasive material. Then, under a mixed atmosphere of 5% hydrogen and nitrogen by volume, the mixture is sintered at 200~230℃ at a rate of 2~10℃ / min for 3~5 hours to obtain the first sintered material. The first sintering material is mixed again with carbon source and PMMA, while keeping the amount of carbon source and PMMA constant, and keeping the heating rate and mixing atmosphere constant. The second sintering is carried out at 400~450℃ for 4~6 hours to obtain the second sintering material. The second sintering material is mixed again with the carbon source and PMMA, and the amount of carbon source and PMMA is kept constant. The third sintering is carried out at 600~650℃ for 10~15h to obtain carbon-coated lithium manganese iron phosphate.

20. A carbon-coated lithium manganese iron phosphate, characterized in that, The carbon-coated lithium manganese iron phosphate obtained by the preparation method according to any one of claims 1-19 comprises lithium manganese iron phosphate and carbon material coated on the surface of the lithium manganese iron phosphate.

21. A lithium-ion battery, characterized in that, The lithium-ion battery contains carbon-coated lithium manganese iron phosphate as described in claim 20.

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