A lithium iron manganese phosphate material, its precursor, preparation method, and lithium-ion battery

Abstract

A manganese iron phosphate precursor, the manganese iron phosphate precursor having a surface rich in Fe, and in the manganese iron phosphate precursor, the content of Fe increases in a gradient from the core center of the precursor to the core surface layer of the precursor, the manganese iron phosphate precursor being doped with a non-metallic element and not doped with a metallic element; a method for preparing the manganese iron phosphate precursor, and a method for preparing a lithium manganese iron phosphate cathode material using the above manganese iron phosphate precursor and its product, including a lithium ion battery comprising the cathode material.

Description

Technical Field

[0001] This application belongs to the technical field of cathode materials for lithium-ion batteries. Specifically, it relates to a lithium iron manganese phosphate material, its precursor, a preparation method thereof, and a lithium-ion battery. Background Art

[0002] Lithium-ion batteries have the advantages of high specific capacity, long cycle life, no memory effect, fast charging rate, high rate performance, green environmental protection, safety and non-toxicity when applied to portable electronic devices and electric vehicles. Among them, the lithium iron phosphate cathode material has attracted much attention in lithium-ion batteries due to its low price, strong safety, and high cycle stability. However, its low energy density restricts its specific applications. By adding manganese to the lithium iron phosphate material to synthesize a lithium iron manganese phosphate solid solution material, the energy density of lithium iron phosphate can be increased, making lithium iron manganese phosphate an excellent energy storage material with high energy density and high safety.

[0003] However, for lithium iron manganese phosphate with an olivine structure, FeO6 and MnO6 are located on octahedrons and are cross-linked by PO4 tetrahedrons, and there is no continuous FeO6 (MnO6) co-edge octahedron network, which makes its conductivity very poor. At the same time, the PO4 tetrahedron is located between the FeO6 (MnO6) octahedrons, blocking the lithium-ion diffusion channels and restricting its movement to only one-dimensional channels, resulting in a relatively low lithium-ion diffusion rate and poor rate performance. During the charge and discharge process, the Jahn-Teller effect occurs during the phase transformation between LiMnPO4 and MnPO4, promoting the precipitation of manganese and reducing the cycle stability.

[0004] Currently, the commonly used methods for synthesizing lithium iron manganese phosphate cathode materials usually have the following defects:

[0005] The prepared material has an uneven particle size distribution, and the synthesized lithium iron manganese phosphate material has poor electronic conductivity and lithium-ion diffusivity, and poor rate performance.

[0006] In view of this, this application is specifically proposed. Summary of the Invention

[0007] One of the purposes of this application is to provide a lithium iron manganese phosphate precursor, which is beneficial to alleviating the interface deterioration of the lithium iron manganese phosphate material, preventing electrolyte erosion, reducing the dissolution of Mn, and is beneficial to improving the lithium-ion diffusion rate, conductivity and rate performance of the lithium iron manganese phosphate material.

[0008] The second purpose of this application is to provide a preparation method of the above-mentioned lithium iron manganese phosphate precursor.

[0009] The third purpose of this application is to provide a preparation method of a lithium iron manganese phosphate material.

[0010] A fourth object of the present application is to provide a lithium iron manganese phosphate material obtained by the above preparation method.

[0011] A fifth object of the present application is to provide a lithium ion battery using the above lithium iron manganese phosphate material as a cathode material.

[0012] In order to achieve at least one of the above objects of the present application, the following technical solutions can be adopted:

[0013] The first solution of the present application includes providing a manganese iron phosphate precursor, which has a surface rich in Fe, and in the manganese iron phosphate precursor, the content of Fe increases in a gradient from the core center of the precursor to the core surface layer of the precursor; the manganese iron phosphate precursor is doped with non-metal elements and not doped with metal elements.

[0014] In some embodiments of the present application, the manganese iron phosphate precursor is spherical particles.

[0015] In some embodiments of the present application, the D 50 of the manganese iron phosphate precursor is 0.5 - 1 μm.

[0016] The second solution of the present application includes providing a preparation method for the above manganese iron phosphate precursor, which includes the following steps: heating the raw materials for preparing the manganese iron phosphate precursor to obtain a viscous material; sintering the viscous material.

[0017] In some embodiments of the present application, the raw materials for preparing the manganese iron phosphate precursor include a phosphorus source, an insoluble manganese source, a soluble iron source, a first carbon source, and a dopant.

[0018] In some embodiments of the present application, the heating method is oil bath heating.

[0019] In some embodiments of the present application, the soluble iron source is first prepared into an iron solution, and then added dropwise to be mixed with the remaining raw materials for preparing the manganese iron phosphate precursor and then subjected to oil bath heating.

[0020] In some embodiments of the present application, the dropping rate of the soluble iron source is 10 - 100 mL / min.

[0021] In some embodiments of the present application, the dropping rate of the soluble iron source is 50 - 80 mL / min.

[0022] In some embodiments of the present application, the phosphorus source includes at least one of ammonium dihydrogen phosphate, phosphoric acid, ammonium phosphate, pyrophosphoric acid, and metaphosphoric acid.

[0023] In some embodiments of the present application, the insoluble manganese source includes at least one of manganese oxalate, manganese carbonate, manganese tetroxide, manganese dioxide, and manganese monoxide.

[0024] In some embodiments of the present application, the soluble iron source includes at least one of iron chloride, iron sulfate, iron nitrate, ferrous nitrate, and iron bromide.

[0025] In some embodiments of the present application, the first carbon source includes at least one of glucose, citric acid, sucrose, polyethylene glycol, β-cyclodextrin, polyacrylamide, Tween-80, and polyvinylpyrrolidone.

[0026] In some embodiments of the present application, the addition amount of the first carbon source is 1.5-4 wt% of the total mass of the soluble iron source and the insoluble manganese source.

[0027] In some embodiments of the present application, the dopant includes at least one of tetrabutylammonium fluoride, boric acid, ammonium nitrate, and trinitrotoluene.

[0028] In some embodiments of the present application, the addition amount of the dopant is 0.5-10 wt% of the iron manganese phosphate precursor.

[0029] In some embodiments of the present application, the addition amount of the dopant is 1-6 wt% of the iron manganese phosphate precursor.

[0030] In some embodiments of the present application, the oil bath heating is carried out at 150-200 °C for 5-30 h.

[0031] In some embodiments of the present application, the oil bath heating is carried out at 170-190 °C for 12-24 h.

[0032] In some embodiments of the present application, the oil bath heating is carried out under stirring conditions.

[0033] In some embodiments of the present application, the stirring speed is 300-500 r / min.

[0034] In some embodiments of the present application, the oil bath heating is carried out under a protective atmosphere.

[0035] In some embodiments of the present application, the protective atmosphere includes a nitrogen atmosphere, an argon atmosphere, or a helium atmosphere.

[0036] In some embodiments of the present application, the sintering is carried out at 450-600 °C for 5-10 h.

[0037] In some embodiments of the present application, the sintering is carried out under a protective atmosphere.

[0038] In some embodiments of the present application, the protective atmosphere includes a nitrogen atmosphere, an argon atmosphere, or a helium atmosphere.

[0039] The third solution of the present application includes providing a method for preparing lithium iron manganese phosphate material, comprising the following steps: co-high temperature sintering a manganese iron phosphite precursor with a lithium source and a second carbon source to obtain a doped carbon-coated lithium iron manganese phosphate material.

[0040] In some embodiments of the present application, the lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium acetate, and lithium oxalate.

[0041] In some embodiments of the present application, the second carbon source includes at least one of glucose, citric acid, sucrose, polyethylene glycol, β-cyclodextrin, polyacrylamide, Tween-80, and polyvinylpyrrolidone.

[0042] In some embodiments of the present application, the molar ratio of Li in the lithium source to Mn+Fe in the manganese iron phosphite precursor is 1:1 to 1.05:1.

[0043] In some embodiments of the present application, the molar ratio of Li in the lithium source to Mn+Fe in the manganese iron phosphite precursor is 1.01:1 to 1.03:1.

[0044] In some embodiments of the present application, the addition amount of the second carbon source is 2-10 wt% of the manganese iron phosphite precursor.

[0045] In some embodiments of the present application, the addition amount of the second carbon source is 4-6 wt% of the manganese iron phosphite precursor.

[0046] In some embodiments of the present application, the high temperature sintering is carried out at 650-800 °C for 10-24 h.

[0047] In some embodiments of the present application, before co-high temperature sintering with the lithium source and the second carbon source, it may further include crushing the manganese iron phosphite precursor to reach a preset particle size.

[0048] In some embodiments of the present application, the crushing treatment is carried out by using a jet mill crushing method.

[0049] The fourth solution of the present application includes providing a lithium iron manganese phosphate material prepared by the preparation method of the foregoing embodiments.

[0050] In some embodiments of the present application, the chemical formula of the lithium iron manganese phosphate material is LiMn 0.6 Fe 0.4 PO 4-x M x / C;

[0051] wherein, M is selected from any one of F, B, and N; 0 < x ≤ 0.2.

[0052] The fifth solution of this application includes providing a lithium-ion battery, the cathode material of which is the lithium iron manganese phosphate material of the foregoing embodiment.

[0053] The lithium iron manganese phosphate precursor provided by this application has a surface rich in Fe. In the lithium iron manganese phosphate precursor, the content of Fe increases in a gradient from the core center of the precursor to the core surface layer of the precursor, which can effectively alleviate the interface deterioration, prevent the electrolyte erosion, and significantly reduce the dissolution of Mn. It is doped with a non-metallic dopant, and the conduction band of the non-metallic atom can become the carrier of electrons, improving the electron transport ability on the surface of the cathode material. And it can stabilize the internal structure of lithium iron manganese phosphate and will not block the Li + transport channel, thus playing a role in improving the rate performance of the material.

[0054] The preparation method of this precursor is simple, easy to operate and easy to control. The lithium iron manganese phosphate material further prepared from this precursor has the characteristics of high material homogeneity, fast lithium ion diffusion rate, strong conductivity and high rate, and can be used to prepare lithium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] In order to more clearly illustrate the technical solutions of the embodiments of this application, the drawings required to be used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this application, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can be obtained based on these drawings without creative efforts.

[0056] Figure 1 It is the XRD pattern of the lithium iron manganese phosphate cathode material of Example 1 in the test examples of this application;

[0057] Figure 2 It is the SEM images of the lithium iron manganese phosphate cathode materials of Example 1 and Comparative Example 1 in the test examples of this application;

[0058] Figure 3 It is the coin cell performance graph of the lithium iron manganese phosphate cathode materials of Example 1 and Comparative Example 1 in the test examples of this application. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0059] The implementation solutions of this application will be described in detail below in conjunction with the embodiments. However, those skilled in the art will understand that the following embodiments are only used to illustrate this application and should not be regarded as limiting the scope of this application. For those not specified in the embodiments, they are carried out according to the conventional conditions or the conditions recommended by the manufacturer. For the reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0060] The endpoints and any values within the ranges disclosed in this application are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0061] This application provides a precursor of iron manganese phosphate. The precursor of iron manganese phosphate has a surface rich in Fe, and in the precursor of iron manganese phosphate, the content of Fe increases in a gradient from the core center of the precursor to the core surface layer of the precursor; the precursor of iron manganese phosphate is doped with non-metal elements and not doped with metal elements.

[0062] The precursor of iron manganese phosphate provided by this application has a surface rich in Fe, and in the precursor of iron manganese phosphate, the content of Fe increases in a gradient from the core center of the precursor to the core surface layer of the precursor, which can effectively alleviate the interface deterioration, prevent the electrolyte erosion, and significantly reduce the dissolution of Mn. It is doped with a non-metal dopant. The conduction band of the non-metal atom can become the carrier of electrons, improve the electron transport ability on the surface of the cathode material, and it can stabilize the internal structure of lithium iron manganese phosphate and will not block the Li + transport channel, thereby playing a role in improving the rate performance of the material.

[0063] It should be noted that generally, the doping elements in the cathode material are metal elements. After research and practice, the inventor creatively proposes that doping with non-metal elements can improve the electron transport ability and rate performance of lithium iron manganese phosphate.

[0064] Specifically, by doping the oxygen site with non-metal elements (specifically the O site in the P-O tetrahedron in the olivine structure), the conduction band of the non-metal element can become the carrier of electrons, which can improve the electron transport ability on the surface of the cathode material, stabilize the internal structure of lithium iron manganese phosphate, and will not block the Li + transport channel, thereby improving the rate performance of the material.

[0065] It should be further noted that the existing technology often uses transition metal element doping, such as doping with Mg, Al, Cr, Mo, Zr, etc. mainly by weakening the Li-O bond, increasing the lattice volume, obtaining higher ion mobility and diffusion coefficient, reducing lattice distortion, and at the same time, metal site doping can inhibit the generation of Li / M (M = Fe, Mn) anti-site defects, and Li in the diffusion channel +It is easy to migrate without being blocked, which helps to improve the electrochemical performance. In this application, the doping of non-metal elements can, compared with the doping of metal elements, also weaken the Li-O bond, making it easier for lithium ions to be inserted and extracted. Due to the induction of non-metal elements, the electron cloud of the phosphate group is rearranged, and the distance between P-O bonds is shortened, making the structure more stable, thus affecting the conductivity and electrochemical performance of the material.

[0066] In addition, due to the different doping sites, there will also be differences in the selection of doping elements. In the doping of metal elements, due to the charge compensation mechanism, equivalent doping can generate vacancies and the energy of isovalent substitution is the lowest. In addition, the greater the charge difference between the dopant and the host ion, the higher the doping energy of the dopant, which makes hetero-valent doping difficult, and non-metal elements are more likely to enter the doping sites.

[0067] In some alternative embodiments, the iron manganese phosphate precursor is spherical particles. Exemplarily, the D 50 of the iron manganese phosphate precursor can be 0.5 - 1 μm. In addition, it is not excluded that the iron manganese phosphate precursor is of an irregular shape.

[0068] For reference, the preparation of the above-mentioned iron manganese phosphate precursor may include the steps of: heating the raw materials for preparing the iron manganese phosphate precursor to obtain a viscous material; sintering the viscous material (which can be referred to as "low-temperature pre-sintering").

[0069] In some alternative embodiments, the raw materials for preparing the iron manganese phosphate precursor include a phosphorus source, an insoluble manganese source, a soluble iron source, a first carbon source, and a dopant.

[0070] The above heating method can be oil bath heating. That is, the corresponding preparation process includes: subjecting the phosphorus source, the insoluble manganese source, the soluble iron source, the first carbon source, and the dopant to oil bath heating to obtain a viscous material; pre-sintering the viscous material.

[0071] Preparing the doped iron manganese phosphate precursor by the oil bath liquid phase method is beneficial to making the insoluble raw materials and the dopant etc. mix evenly, achieving a nano-scale or even atomic-scale mixing between the reactants other than the iron source, with few impurities, and is beneficial to obtaining a precursor with better performance.

[0072] In some embodiments, the soluble iron source is first prepared into an iron solution, and then added dropwise to be mixed with the remaining raw materials for preparing the iron manganese phosphate precursor and then subjected to oil bath heating.

[0073] This method can make the insoluble raw materials and the dopant etc. mix evenly before adding the soluble iron source, achieving a nano-scale or even atomic-scale mixing between the reactants other than the iron source, with few impurities; on this basis, adding the soluble iron source dropwise and controlling the dropping rate to control the crystallization precipitation rate is more beneficial to obtaining a precursor rich in iron concentration gradient.

[0074] The dropping rate of the above-mentioned soluble iron source can be 10 - 100 mL / min, such as 10 mL / min, 20 mL / min, 30 mL / min, 40 mL / min, 50 mL / min, 60 mL / min, 70 mL / min, 80 mL / min, 90 mL / min or 100 mL / min, etc., and can also be any other value within the range of 10 - 100 mL / min. In some preferred embodiments, the dropping rate of the soluble iron source is 50 - 80 mL / min.

[0075] In the above method, if the dropping rate of the soluble iron source is too slow, incomplete nucleation of the precursor will occur; if the dropping rate of the soluble iron source is too fast, the iron element will crystallize too fast on the particle surface, resulting in uneven distribution.

[0076] By way of reference, the phosphorus source may exemplarily include at least one of ammonium dihydrogen phosphate, phosphoric acid, ammonium phosphate, pyrophosphoric acid, and metaphosphoric acid.

[0077] The insoluble manganese source may exemplarily include at least one of manganese oxalate, manganese carbonate, manganese tetraoxide, manganese dioxide, and manganese monoxide.

[0078] The soluble iron source may exemplarily include at least one of ferric chloride, ferric sulfate, ferric nitrate, ferrous nitrate, and ferric bromide.

[0079] In this application, the dosages of the above phosphorus source, insoluble manganese source, and soluble iron source are set according to the molar ratios of Mn in the insoluble manganese source, Fe in the soluble iron source, and P in the phosphorus source corresponding to the molar ratios of Mn, Fe, and P in the chemical formula of the lithium iron manganese phosphate material preset. For example, the molar ratio of Mn in the insoluble manganese source to Fe in the soluble iron source is 6:4.

[0080] The first carbon source may exemplarily include at least one of glucose, citric acid, sucrose, polyethylene glycol, β-cyclodextrin, polyacrylamide, Tween-80, and PVP.

[0081] Preferably, the addition amount of the first carbon source is 1.5 - 4 wt% of the total mass of the soluble iron source and the insoluble manganese source, such as 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt% or 4 wt%, etc., and can also be any other value within the range of 1.5 - 4 wt%.

[0082] In this application, on the one hand, the first carbon source acts as a reducing agent to reduce some high-valent metals (for example, reducing Fe 3+ to Fe 2+ ), and on the other hand, it can also play the role of the first layer of carbon coating.

[0083] If the addition amount of the first carbon source is too small, the high-valent metal elements cannot be completely reduced; if the addition amount of the first carbon source is too large, the carbon coating layer is too thick, and the path for Li + to be deintercalated from the material becomes longer, which instead deteriorates the electrochemical performance.

[0084] The dopant may exemplarily include at least one of tetrabutylammonium fluoride, boric acid, ammonium nitrate, and trinitrotoluene.

[0085] Preferably, the addition amount of the dopant can be 0.5-10 wt% of the lithium iron manganese phosphate precursor, such as 0.5 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, or 10 wt%, etc., or any other value within the range of 0.5-10 wt%, and preferably 1-6 wt%.

[0086] If the dosage of the dopant is too small, its effect of weakening the Li-O bond and shortening the distance between P-O bonds is weak, and it cannot achieve the acceleration of Li + transport; if the dosage of the dopant is too large, the doped elements will not completely enter the lattice of lithium iron manganese phosphate, and the ions that do not enter the lattice will block the diffusion of Li + in the channels.

[0087] As a reference, the oil bath heating can be carried out at 150-200 °C for 5-30 h.

[0088] Specifically, the temperature of the oil bath heating can be 150 °C, 155 °C, 160 °C, 165 °C, 170 °C, 175 °C, 180 °C, 185 °C, 190 °C, 195 °C, or 200 °C, etc., or any other value within the range of 150-200 °C.

[0089] The time of the oil bath heating can be 5 h, 10 h, 15 h, 20 h, 25 h, or 30 h, etc., or any other value within the range of 5-30 h.

[0090] In some preferred embodiments, the oil bath heating is carried out at 170-190 °C (such as 170 °C, 175 °C, 180 °C, 185 °C, or 190 °C, etc.) for 12-24 h (such as 12 h, 15 h, 18 h, 20 h, 22 h, or 24 h, etc.).

[0091] More preferably, the above oil bath heating process is carried out under stirring conditions. The stirring speed can be 300-500 r / min (such as 300 r / min, 350 r / min, 400 r / min, 450 r / min, or 500 r / min, etc.).

[0092] It should be noted that during ordinary water bath heating for long-term constant temperature heating, when the temperature exceeds 80°C, it will gradually evaporate and needs to be replenished irregularly, resulting in uneven heating during the synthesis process and poor performance of the synthesized finished product. Moreover, water bath heating cannot reach temperatures above 150°C.

[0093] Preferably, the above oil bath heating process is carried out under a protective atmosphere condition. The protective atmosphere can be, by way of example but not limitation, a nitrogen atmosphere, or it can also be a helium atmosphere or an argon atmosphere, etc. Nitrogen, for example, can maintain an in-and-out dynamic balance.

[0094] Through stirring and the dynamic balance of the protective gas, it is beneficial to carry out the water evaporated during the oil bath process through the flowing gas, thereby obtaining a viscous material.

[0095] Furthermore, the low-temperature pre-sintering of the viscous material can be carried out at 450 - 600°C for 5 - 10 h.

[0096] Specifically, the pre-sintering temperature can be 450°C, 500°C, 550°C, or 600°C, etc., or it can also be any other value within the range of 450 - 600°C.

[0097] The pre-sintering time can be 5 h, 5.5 h, 6 h, 6.5 h, 7 h, 7.5 h, 8 h, 8.5 h, 9 h, 9.5 h, or 10 h, etc., or it can also be any other value within the range of 5 - 10 h.

[0098] It should be noted that if the pre-sintering temperature is lower than 450°C or the time is shorter than 5 h, it is likely to cause insufficient reaction of the material during the sintering process; if the pre-sintering temperature is higher than 600°C or the time is longer than 10 h, it is likely to cause the formation of other impurity phases.

[0099] Preferably, the above pre-sintering process is also carried out under a protective atmosphere (such as a nitrogen atmosphere) condition.

[0100] The obtained iron manganese phosphate precursor can be further used to prepare a lithium iron manganese phosphate material with fast lithium ion diffusion rate, strong conductivity, and high rate performance.

[0101] Furthermore, the present application also proposes a lithium iron manganese phosphate material further prepared from the above iron manganese phosphate precursor.

[0102] For reference, the chemical formula of the lithium iron manganese phosphate material can be LiMn 0.6 Fe 0.4 PO 4-x M x / C; wherein, M is selected from any one of F, B, and N; 0 < x ≤ 0.2.

[0103] That is, the value of x can be 0.05, 0.1, 0.15, 0.2, etc., or any other value within the range of 0 (excluding) to 0.2.

[0104] The lithium iron manganese phosphate material has high conductivity, capacity, and rate performance.

[0105] As a reference, the preparation method of the lithium iron manganese phosphate material may include the following steps:

[0106] Co-sinter the iron manganese phosphate precursor with a lithium source and a second carbon source at high temperature to obtain a doped carbon-coated lithium iron manganese phosphate material;

[0107] Among them, the doping element in the iron manganese phosphate precursor is a non-metal element.

[0108] In some preferred embodiments, before co-sintering with the lithium source and the second carbon source, the iron manganese phosphate precursor may also be pulverized to reach a preset particle size.

[0109] Exemplarily, the pulverization treatment may be carried out by using a jet mill pulverization method.

[0110] By pulverizing the iron manganese phosphate precursor into small particle sizes through pulverization treatment (such as jet pulverization treatment), the small particle-sized iron manganese phosphate can better synthesize high-crystallinity lithium iron manganese phosphate with the lithium salt, which is beneficial to improving the uniformity of carbon coating and the ionic conductivity.

[0111] As a reference, the lithium source used in this application may exemplarily include at least one of lithium carbonate, lithium hydroxide, lithium acetate, and lithium oxalate.

[0112] The second carbon source may exemplarily include at least one of glucose, citric acid, sucrose, polyethylene glycol, β-cyclodextrin, polyacrylamide, Tween-80, and polyvinylpyrrolidone (PVP).

[0113] Among them, the molar ratio of Li in the lithium source to Mn + Fe in the iron manganese phosphate precursor can be 1 - 1.05:1, such as 1:1, 1.01:1, 1.02:1, 1.03:1, 1.04:1, or 1.05:1, etc., and preferably 1.01 - 1.03:1.

[0114] The addition amount of the second carbon source can be 2 - 10 wt% of the iron manganese phosphate precursor (such as 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, or 10 wt%, etc.), and preferably 4 - 6 wt%.

[0115] The above-mentioned second carbon source mainly plays the role of the second layer of coating, and the C coating in this application is mainly the coating of the second carbon source.

[0116] If the addition amount of the second carbon source is too small, the carbon coating is uneven, resulting in poor conductivity of the material; if the addition amount of the second carbon source is too large, it is easy to cause the carbon coating layer to be too thick, and the path for Li+ to be deintercalated from the material becomes longer, which instead deteriorates the electrochemical performance.

[0117] In this application, the lithium iron manganese phosphate powder can be first mixed with a lithium source and a second carbon source to obtain a mixture, and then the mixture is subjected to high-temperature sintering in a nitrogen atmosphere.

[0118] By carrying out the process in a nitrogen atmosphere during the formation of lithium iron manganese phosphate and lithium manganese iron phosphate, the oxidation of Mn 2+ to Mn 3+ / Mn 4+ can be effectively reduced.

[0119] As a reference, the high-temperature sintering can be carried out at 650 - 800 °C for 10 - 24 h.

[0120] Specifically, the temperature of the high-temperature sintering can be 650 °C, 700 °C, 750 °C or 800 °C, etc., or any other value within the range of 650 - 800 °C.

[0121] The time of the high-temperature sintering can be 10 h, 12 h, 15 h, 18 h, 20 h or 24 h, etc., or any other value within the range of 10 - 24 h.

[0122] If the temperature of the high-temperature sintering is lower than 650 °C or the sintering time is shorter than 10 h, the reaction of the materials during sintering is insufficient; if the temperature of the high-temperature sintering is higher than 800 °C or the sintering time is longer than 24 h, other impurity phases are generated.

[0123] In addition, this application also provides a lithium-ion battery, the positive electrode material of which is the above-mentioned lithium manganese iron phosphate material.

[0124] The features and properties of the present invention will be further described in detail below in conjunction with the embodiments.

[0125] Example 1

[0126] This example provides a lithium manganese iron phosphate material, the molecular formula of which is LiMn 0.6 Fe 0.4 PO 3.85 B 0.15 / C.

[0127] The lithium manganese iron phosphate material is prepared by the following method:

[0128] S1: Prepare ferric chloride into an aqueous solution of ferric chloride; Add 15 g of manganese carbonate, 33.3 g of phosphoric acid, 0.67 g of boric acid (doping amount 0.15 mol, accounting for 1.7% of the mass of the iron manganese phosphate precursor), and the first carbon source (composed of glucose and polyethylene glycol, where the mass of grape carbon is 1.5% of the total mass of manganese carbonate and ferric chloride, and the mass of polyethylene glycol is 1.5% of the total mass of manganese carbonate and ferric chloride) into 100 mL of deionized water to obtain a mixed solution; Dropwise add the aqueous solution of ferric chloride (dropwise addition rate is 60 mL / min) into the above mixed solution (finally, 14.1 g of ferric chloride is contained in the 100 mL of deionized water), stir evenly to obtain a mixed material; Stir the mixed material (stirring speed is 400 r / min) in an oil bath at 180 °C for 12 h, introduce nitrogen during the heating process (the inlet and outlet nitrogen achieve dynamic balance), and the evaporated water is carried out by the flowing gas to obtain a viscous material.

[0129] S2: Sinter the viscous material in a nitrogen atmosphere at 500 °C for 6 h to obtain a doped iron manganese phosphate precursor.

[0130] S3: Crush the doped iron manganese phosphate precursor by a jet mill, and the D50 of the obtained iron manganese phosphate precursor after crushing is 0.6 μm.

[0131] S4: Mix the crushed iron manganese phosphate precursor with 23 g of lithium dihydrogen phosphate (the molar ratio of Li to (Mn + Fe) is 1.02:1) and the second carbon source (composed of glucose and polyethylene glycol, where the mass of grape carbon is 2.5% of the mass of the iron manganese phosphate precursor, and the mass of polyethylene glycol is 2.5% of the mass of the iron manganese phosphate precursor) evenly in a high-speed mixer to obtain a mixed material; Sinter the mixed material in a nitrogen atmosphere at 700 °C for 10 h to obtain LiMn 0.6 Fe 0.4 PO 3.85 B 0.15 / C.

[0132] Example 2

[0133] This example provides a lithium iron manganese phosphate material, and its molecular formula is LiMn 0.6 Fe 0.4 PO 3.9 F 0.1 / C.

[0134] The lithium iron manganese phosphate material is prepared by the following method:

[0135] S1: Prepare an aqueous solution of iron nitrate; add 30 g of manganese oxalate, 54 g of ammonium dihydrogen phosphate, 3.13 g of tetrabutylammonium fluoride (doping amount 0.1 mol, accounting for 5% of the mass of the manganese iron phosphate precursor), and a first carbon source (composed of citric acid and PVP, where the mass of citric acid is 1% of the total mass of manganese oxalate and iron nitrate, and the mass of PVP is 1% of the total mass of manganese oxalate and iron nitrate) to 180 mL of deionized water to obtain a mixed solution; dropwise add the aqueous solution of iron nitrate (dropwise addition rate is 70 mL / min) to the above mixed solution (finally, 33.9 g of iron nitrate is contained in the 180 mL of deionized water), stir evenly to obtain a mixed material; stir the mixed material (stirring speed is 500 r / min) in an oil bath at 190 °C for 20 h, and introduce nitrogen during the heating process (dynamic balance of the inlet and outlet nitrogen), and the evaporated water is carried out by the flowing gas to obtain a viscous material.

[0136] S2: Sinter the viscous material in a nitrogen atmosphere at 550 °C for 8 h to obtain a doped manganese iron phosphate precursor.

[0137] S3: Crush the doped manganese iron phosphate precursor by a jet mill, and the D50 of the crushed manganese iron phosphate precursor is 0.8 μm;

[0138] S4: Mix the crushed manganese iron phosphate precursor with 12.8 g of lithium dihydrogen phosphate (the molar ratio of Li to (Mn + Fe) is 1.03:1) and a second carbon source (composed of citric acid and PVP, where the mass of citric acid is 2% of the mass of the manganese iron phosphate precursor, and the mass of PVP is 2% of the mass of the manganese iron phosphate precursor) evenly in a high-speed mixer to obtain a mixed material; sinter the mixed material in a nitrogen atmosphere at 750 °C for 18 h to obtain LiMn 0.6 Fe 0.4 PO 3.9 F 0.1 / C.

[0139] Example 3

[0140] This example provides a lithium manganese iron phosphate material, and its molecular formula is LiMn 0.6 Fe 0.4 PO 3.8 N 0.2 / C.

[0141] The lithium manganese iron phosphate material is prepared by the following method:

[0142] S1: Prepare ferric sulfate into an aqueous solution of ferric sulfate; Add 20 g of manganese monoxide, 100.3 g of ammonium dihydrogen phosphate, 3.5 g of ammonium nitrate (doping amount 0.2 mol, accounting for 3% of the mass of the iron manganese phosphate precursor), and a first carbon source (composed of sucrose and β-cyclodextrin, wherein the mass of sucrose is 0.8% of the total mass of manganese monoxide and ferric sulfate, and the mass of β-cyclodextrin is 0.8% of the total mass of manganese monoxide and ferric sulfate) into 400 mL of deionized water to obtain a mixed solution; Dropwise add (dropwise addition rate is 55 mL / min) the aqueous solution of ferric sulfate into the above mixed solution (finally, 37.4 g of ferric sulfate is contained in the 400 mL of deionized water), stir evenly to obtain a mixed material; Stir (stirring speed is 350 r / min) the mixed material in an oil bath at 185 °C for 16 h, and introduce nitrogen during the heating process (dynamic balance of the incoming and outgoing nitrogen), and the evaporated water is carried out by the flowing gas to obtain a viscous material.

[0143] S2: Sinter the viscous material in a nitrogen atmosphere at 450 °C for 10 h to obtain a doped iron manganese phosphate precursor.

[0144] S3: Crush the doped iron manganese phosphate precursor by a jet mill, and the D50 of the obtained iron manganese phosphate precursor after crushing is 0.9 μm;

[0145] S4: Mix the crushed iron manganese phosphate precursor evenly with 24.4 g of lithium carbonate (the molar ratio of Li to (Mn + Fe) is 1.01:1) and a second carbon source (composed of sucrose and β-cyclodextrin, wherein the mass of sucrose is 2.2% of the mass of the iron manganese phosphate precursor, and the mass of β-cyclodextrin is 2.2% of the mass of the iron manganese phosphate precursor) in a high-speed mixer to obtain a mixed material; Sinter the mixed material in a nitrogen atmosphere at 730 °C for 12 h to obtain LiMn 0.6 Fe 0.4 PO 3.8 N 0.2 / C.

[0146] Example 4

[0147] The difference between this example and Example 1 is that the amount of boric acid is 0.45 g (doping amount 0.1 mol, accounting for 1.1% of the mass of the iron manganese phosphate precursor), and the corresponding molecular formula of the obtained lithium iron manganese phosphate material is LiMn 0.6 Fe 0.4 PO 3.9 B 0.1 / C.

[0148] Example 5

[0149] The difference between this example and Example 1 is that the dopant is trinitrotoluene.

[0150] Example 6

[0151] The difference between this example and Example 1 is that the oil bath temperature is 150 °C.

[0152] Example 7

[0153] The difference between this example and Example 1 is that the oil bath temperature is 200 °C.

[0154] Example 8

[0155] The difference between this example and Example 1 is that the addition amount of the second carbon source is 3 wt% of the iron manganese phosphate precursor (the second carbon source is composed of glucose and polyethylene glycol with a mass ratio of 1:1).

[0156] Example 9

[0157] The difference between this example and Example 1 is that the addition amount of the second carbon source is 4 wt% of the iron manganese phosphate precursor (the second carbon source is composed of glucose and polyethylene glycol with a mass ratio of 1:1).

[0158] Comparative Example 1

[0159] The difference between this comparative example and Example 1 is that in S1, instead of performing the oil bath heating step in nitrogen, the components are directly stirred evenly in 100 mL of deionized water and then dried in a vacuum drying oven (the drying temperature is 180 °C and the drying time is 20 h).

[0160] Comparative Example 2

[0161] The difference between this comparative example and Example 1 is that in S1, there is no stirring during the oil bath process.

[0162] ]>Comparative Example 3

[0163] The difference between this comparative example and Example 1 is that in S1, the first carbon source is not added.

[0164] Comparative Example 4

[0165] The difference between this comparative example and Example 1 is that in S1, the addition amount of the first carbon source is 1 wt% of the total mass of the soluble iron source and the insoluble manganese source (the first carbon source is composed of glucose and polyethylene glycol with a mass ratio of 1:1).

[0166] Comparative Example 5

[0167] The difference between this comparative example and Example 1 is that in S1, the addition amount of the first carbon source is 6 wt% of the total mass of the soluble iron source and the insoluble manganese source (the first carbon source is composed of glucose and polyethylene glycol with a mass ratio of 1:1).

[0168] Comparative Example 6

[0169] The difference between this comparative example and Example 1 is that in S2, the temperature of low-temperature pre-sintering is 400 °C.

[0170] Comparative Example 7

[0171] The difference between this comparative example and Example 1 is that in S2, the temperature of low-temperature pre-sintering is 650 °C.

[0172] Comparative Example 8

[0173] The difference between this comparative example and Example 1 is that there is no step S3, that is, the precursor is not pulverized.

[0174] Comparative Example 9

[0175] The difference between this comparative example and Example 1 is that the D50 of the obtained lithium iron manganese phosphate precursor powder after pulverization is 1.5 μm.

[0176] Comparative Example 10

[0177] The difference between this comparative example and Example 1 is that in S4, no second carbon source is added.

[0178] Comparative Example 11

[0179] The difference between this comparative example and Example 1 is that in S4, the addition amount of the second carbon source is 1 wt% of the lithium iron manganese phosphate precursor (the second carbon source is composed of glucose and polyethylene glycol with a mass ratio of 1:1).

[0180] Comparative Example 12

[0181] The difference between this comparative example and Example 1 is that in S4, the addition amount of the second carbon source is 15 wt% of the lithium iron manganese phosphate precursor (the second carbon source is composed of glucose and polyethylene glycol with a mass ratio of 1:1).

[0182] Comparative Example 13

[0183] The difference between this comparative example and Example 1 is that in S4, the temperature of high-temperature sintering is 550 °C.

[0184] Comparative Example 14

[0185] The difference between this comparative example and Example 1 is that in S4, the temperature of high-temperature sintering is 850 °C.

[0186] Test Example

[0187] ①. Perform X-ray powder diffraction test on the obtained lithium iron manganese phosphate cathode material of Example 1, and the results are as Figure 1 shown.

[0188] From Figure 1It can be seen that the diffraction peaks of the sample belong to the orthorhombic olivine-type crystalline structure, and all diffraction peaks can match the diffraction peaks of the LiMnPO4 (PFD#77-0178) standard card. The high-angle shift of the X-ray diffraction peaks is because the Mn of 2+ has a larger radius ratio than that of Fe of 2+ , resulting in a decrease in the lattice spacing of the cathode material. The obtained peak intensities are comparable and the crystallinity is good, indicating that pure lithium iron manganese phosphate has been synthesized.

[0189] ②. Field emission scanning electron microscopy tests were carried out on the lithium iron manganese phosphate cathode materials obtained in Example 1 and Comparative Example 1, and the results are as Figure 2 shown.

[0190] It can be seen from Figure 2 that the lithium iron manganese phosphate particles synthesized by the oil bath liquid phase method are smaller, the particle size distribution is more uniform, and the dispersibility is better. The small particle size is beneficial to increasing the contact area between the active material and the electrolyte, shortening the migration path of lithium ions during the insertion and extraction process, and effectively improving the electrochemical performance of lithium iron manganese phosphate. The particles of lithium iron manganese phosphate in Comparative Example 1 are larger, and the agglomeration is more serious, and the dispersibility is also poor.

[0191] ③. Charge and discharge tests were carried out on the lithium iron manganese phosphate cathode materials obtained in Examples 1-9 and Comparative Examples 1-14, and the results are shown in Table 1. Among them, the charge-discharge performance diagrams of the lithium iron manganese phosphate cathode materials in Example 1 and Comparative Example 1 are as Figure 3 shown.

[0192] Specifically, equal volumes of LiPF6 and diethyl carbonate (DEC) were used as the electrolyte, the concentration of LiPF6 was 1 mol / L, and a lithium metal sheet was used as the negative electrode to prepare a button-type half-cell. The battery rate performance was detected using a LAND battery program control tester (LANDCT2001A). The test conditions were: three cycles at 0.1C and three cycles at 1C were tested at room temperature of 25°C, and the voltage range was 2-4.3V.

[0193] Table 1 Performance test results

[0194] 0.1C discharge specific capacity (mAh / g) Initial efficiency (%) 1C discharge specific capacity (mAh / g) Example 1 160.0 97.5 146 Example 2 157.6 98 143 Example 3 158.8 97.4 144 Example 4 155.1 96.5 140 Example 5 161.2 97 144 Example 6 154.6 95 139 Example 7 153 92.1 138 Example 8 152.6 93 140 Example 9 151.5 94.8 139 Comparative Example 1 146.8 90 136 Comparative Example 2 148 88.8 129 Comparative Example 3 137.9 86.2 121 Comparative Example 4 145.5 91 129 Comparative Example 5 150.3 89.9 135 Comparative Example 6 149.9 90.1 133 Comparative Example 7 144 90.3 134 Comparative Example 8 147.9 90.5 135 Comparative Example 9 148 89.3 134 Comparative Example 10 136.3 76.6 117 Comparative Example 11 149.6 90.8 136 Comparative Example 12 137 88 126 Comparative Example 13 149.8 86 132 Comparative Example 14 139.3 85.2 121

[0195] It can be seen from Table 1 that the lithium iron manganese phosphate cathode materials provided in the embodiments of the present application are better than the comparative examples in terms of discharge specific capacity and initial efficiency.

[0196] By comparing Example 1 and Comparative Examples 1-14, it is found that when the preparation conditions change, the performance of the lithium iron manganese phosphate cathode material will decrease.

[0197] In summary, in the present application, by doping oxygen sites with non-metal atoms, the conduction band of the non-metal atoms can become the carrier of electrons, improving the electron transport ability on the surface of the cathode material and stabilizing the internal structure of lithium iron manganese phosphate, without blocking the Li + transport channels, thereby improving the rate performance of the material. The low-temperature oil bath liquid phase method at least achieves the mixing of insoluble reactants at the nanometer or even atomic level. On this basis, a soluble iron source is added dropwise, so that the spherical surface of the synthesized product is rich in Fe, which can effectively alleviate the interface deterioration, prevent the electrolyte erosion, and significantly reduce the dissolution of Mn. The obtained lithium iron manganese phosphate material has a fast lithium ion diffusion rate, strong conductivity, and high rate, and can be used for preparing lithium ion batteries.

[0198] The above are only the preferred embodiments of the present application and are not intended to limit the present application. For those skilled in the art, the present application can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

[0199] Industrial Applicability

[0200] The lithium iron manganese phosphate precursor provided by the present application has a surface rich in Fe. In the lithium iron manganese phosphate precursor, the content of Fe increases in a gradient from the core center of the precursor to the core surface layer of the precursor, which can effectively alleviate the interface deterioration, prevent the electrolyte erosion, and significantly reduce the dissolution of Mn. It is doped with a non-metal dopant, and the conduction band of the non-metal atoms can become the carrier of electrons, improving the electron transport ability on the surface of the cathode material. Moreover, it can stabilize the internal structure of lithium iron manganese phosphate and does not block the Li + transport channels, thereby playing a role in improving the rate performance of the material. The corresponding preparation method is simple, easy to operate, and easy to control. The obtained lithium iron manganese phosphate material has a fast lithium ion diffusion rate, strong conductivity, and high rate, and can be used for preparing lithium ion batteries.

Claims

1. A preparation method of an iron manganese phosphite precursor, characterized in that, It includes the following steps: heating the raw materials for preparing the iron manganese phosphate precursor to obtain a viscous material; sintering the viscous material; The heating method is oil bath heating; the oil bath heating is carried out under stirring conditions; the oil bath heating is carried out under a protective atmosphere condition; The raw materials for preparing the iron manganese phosphate precursor include a phosphorus source, an insoluble manganese source, a soluble iron source, a first carbon source and a dopant; the soluble iron source is first prepared into an iron solution, and then added dropwise to be mixed with the rest of the raw materials for preparing the iron manganese phosphate precursor and then subjected to oil bath heating; the dropping rate of the soluble iron source is 10 - 100 mL / min; the addition amount of the first carbon source is 1.5 - 4 wt% of the total mass of the soluble iron source and the insoluble manganese source; the oil bath heating is carried out at 150 - 200 °C for 5 - 30 h; the stirring speed is 300 - 500 r / min; The sintering is carried out at 450 - 600 °C for 5 - 10 h.

2. The preparation method according to claim 1, characterized in that, The dropping rate of the soluble iron source is 50 - 80 mL / min.

3. The preparation method according to claim 1, characterized in that, The phosphorus source includes at least one of ammonium dihydrogen phosphate, phosphoric acid, ammonium phosphate, pyrophosphoric acid and metaphosphoric acid.

4. The preparation method according to claim 1, wherein, The insoluble manganese source includes at least one of manganese oxalate, manganese carbonate, manganese tetraoxide, manganese dioxide and manganese monoxide.

5. The preparation method according to claim 1, wherein The soluble iron source includes at least one of ferric chloride, ferric sulfate, ferric nitrate, ferrous nitrate and ferric bromide.

6. The preparation method according to claim 1, characterized in that, The first carbon source includes at least one of glucose, citric acid, sucrose, polyethylene glycol, β-cyclodextrin, polyacrylamide, Tween-80 and polyvinylpyrrolidone.

7. The preparation method according to claim 1, wherein The dopant includes at least one of tetrabutylammonium fluoride, boric acid, ammonium nitrate and trinitrotoluene.

8. The preparation method according to claim 7, wherein, The addition amount of the dopant is 0.5 - 10 wt% of the iron manganese phosphate precursor.

9. The preparation method according to claim 8, characterized in that, The addition amount of the dopant is 1 - 6 wt% of the iron manganese phosphate precursor.

10. The preparation method according to claim 1, characterized in that, The oil bath heating is carried out at 170 - 190 °C for 12 - 24 h.

11. The preparation method according to claim 1, characterized in that, The protective atmosphere includes a nitrogen atmosphere, an argon atmosphere or a helium atmosphere.

12. According to the preparation method described in claim 1, characterized in that, The sintering is carried out under a protective atmosphere condition.

13. The preparation method according to claim 12, wherein The protective atmosphere during the sintering process includes a nitrogen atmosphere, an argon atmosphere or a helium atmosphere.

14. A manganese iron phosphite precursor, characterized in that, The iron manganese phosphate precursor has a surface rich in Fe, and in the iron manganese phosphate precursor, the content of Fe increases in a gradient from the core center of the precursor to the core surface layer of the precursor; the iron manganese phosphate precursor is doped with non-metal elements and not doped with metal elements; The iron manganese phosphate precursor is prepared by the preparation method according to any one of claims 1 to 13.

15. The iron manganese phosphate precursor according to claim 14, characterized in that, The iron manganese phosphate precursor is spherical particles.

16. The iron manganese phosphate precursor according to claim 15, characterized in that, The D of the iron manganese phosphate precursor 50 is 0.5 - 1 μm.

17. A preparation method of lithium iron manganese phosphate material, characterized in that It includes the following steps: co-high temperature sintering the iron manganese phosphate precursor according to any one of claims 14 to 16 with a lithium source and a second carbon source to obtain a doped carbon-coated lithium iron manganese phosphate material; Before co-high temperature sintering with the lithium source and the second carbon source, it further includes crushing the iron manganese phosphate precursor to reach a preset particle size; the D of the crushed iron manganese phosphate precursor 50 is 0.5 - 1 μm; The addition amount of the second carbon source is 2 - 10 wt% of the iron manganese phosphate precursor. The high temperature sintering is carried out at 650 - 800 °C for 10 - 24 h.

18. The preparation method according to claim 17, wherein, The lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium acetate and lithium oxalate.

19. The preparation method according to claim 17, characterized in that, The second carbon source includes at least one of glucose, citric acid, sucrose, polyethylene glycol, β-cyclodextrin, polyacrylamide, Tween-80, and polyvinylpyrrolidone.

20. The preparation method according to claim 17, characterized in that, The molar ratio of Li in the lithium source to Mn+Fe in the lithium iron manganese phosphate precursor is 1:1 to 1.05:

1.

21. The preparation method according to claim 20, characterized in that, The molar ratio of Li in the lithium source to Mn+Fe in the lithium iron manganese phosphate precursor is 1.01:1 to 1.03:

1.

22. The preparation method according to claim 17, wherein, The addition amount of the second carbon source is 4-6 wt% of the lithium iron manganese phosphate precursor.

23. The preparation method according to claim 17, characterized in that, The comminution treatment is carried out by using a jet mill for comminution.

24. A lithium iron manganese phosphate material, characterized in that, The lithium iron manganese phosphate material is prepared by the preparation method described in any one of claims 17 to 23.

25. The lithium iron manganese phosphate material according to claim 24, wherein The chemical formula of the lithium manganese iron phosphate material is LiMn 0.6 Fe 0.4 PO 4-x M x / C; Wherein, M is selected from any one of F, B, and N; 0 < x ≤ 0.

2.

26. A lithium-ion battery, characterized in that, The positive electrode material of the lithium ion battery is the lithium iron manganese phosphate material described in claim 24 or 25.

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