Lithium ferric manganese phosphate positive electrode material coated with carbon layer based on CVD (chemical vapor deposition) method and preparation method of lithium ferric manganese phosphate positive electrode material

Abstract

The invention discloses a lithium manganese iron phosphate positive electrode material coated with a carbon layer based on a CVD (chemical vapor deposition) method and a preparation method, the preparation method comprises the following steps: forming a uniform carbon coating layer on a lithium manganese iron phosphate precursor based on a CVD coating process, mixing the lithium manganese iron phosphate precursor with the uniform carbon coating layer and a lithium salt, and carrying out co-sintering, and the lithium salt can permeate through the carbon layer to form a lithium-doped uniform carbon coating layer. According to the method, the coating effect is improved, the problem that uniform coating cannot be formed on the surface of the precursor in a traditional carbon coating process is effectively solved, precipitation of trivalent manganese ions at a high temperature can be slowed down, the problem that the capacity exertion is reduced due to the fact that an amorphous carbon layer formed by a CVD cracking coating process affects lithium ion deintercalation is solved, and the performance of the lithium ion battery is improved. The safety and the service life of the battery are improved.

Description

Technical Field

[0001] The present invention mainly relates to the technical field of lithium battery manufacturing, and more specifically, to a carbon coating method for improving the high-temperature performance of lithium iron manganese phosphate. Background Art

[0002] With the increasing global demand for environmental protection and energy, the development of electric vehicles has attracted more and more attention. As the main power source of electric vehicles, the performance improvement of lithium-ion batteries is crucial. Among them, lithium iron manganese phosphate is widely used as the cathode material of lithium-ion batteries due to its high energy density, long cycle life, and environmental friendliness. However, lithium iron manganese phosphate has the problem of manganese dissolution at high temperatures, which not only affects the safety of the battery but also reduces its electrochemical performance.

[0003] To solve this problem, the existing solutions in the prior art mainly form a protective layer by carbon coating to prevent manganese dissolution. One is the traditional carbon coating process, that is, a carbon layer is formed by mixing a carbon-containing organic matter and a lithium iron manganese phosphate precursor and then sintering at high temperature. The other is the CVD cracking coating process, that is, a uniform secondary carbon coating layer is formed on the surface of the lithium iron manganese phosphate particles that have already been carbon-coated. However, both of these methods have certain problems. The traditional carbon coating process cannot form a uniform coating on the surface of the precursor, resulting in poor coating effect, and there is a serious manganese dissolution phenomenon in the sintered lithium iron manganese phosphate. Although the CVD cracking coating process can form a uniform carbon coating layer on the surface of the lithium iron manganese phosphate particles to solve the problem of manganese dissolution, the formed amorphous carbon layer will affect the lithium ion deintercalation, resulting in a decrease in capacity and rate performance. Summary of the Invention

[0004] Compared with the prior art, the present invention aims to solve the following problems: the problem that the traditional carbon coating process cannot form a uniform coating on the surface of the precursor; the problem that the amorphous carbon layer formed by the CVD cracking coating process affects the lithium ion deintercalation, resulting in a decrease in capacity performance; the problem that the prior art cannot effectively solve the precipitation problem of trivalent manganese ions at high temperatures, thus seriously affecting the high-temperature cycle and storage performance of the battery.

[0005] To achieve the above object, a preparation method of a lithium iron manganese phosphate cathode material coated with a carbon layer based on the CVD method provided by the present application includes the following steps:

[0006] Step (1) Solid-phase mixing of a manganese source and an iron source to prepare manganese iron oxide by the high-temperature solid-phase sintering method, or adding a divalent manganese ion solution and a ferrous ion solution to a solution containing a precipitating agent to prepare a manganese iron-containing salt by the coprecipitation method;

[0007] Step (2): The manganese iron oxide or the manganese iron-containing salt is used as a manganese iron precursor and placed in a CVD fluidized bed reactor for fluidization. An inert gas is used as the fluidizing gas, and a gaseous carbon source is input for cracking. The cracked gaseous carbon source forms a carbon coating layer on the surface of the manganese iron precursor;

[0008] Step (3): The manganese iron precursor with a carbon coating layer is solid-phase mixed with a phosphorus source and / or a lithium source and subjected to solid-phase sintering to obtain carbon-coated lithium manganese iron phosphate LiMn x Fe y PO 4 ;

[0009] In the said step (1), 0 ≤ x ≤ 1, 0 ≤ y ≤ 1; in the said step (1), the stoichiometric ratio of manganese in the manganese source to iron in the iron source or the stoichiometric ratio of divalent manganese ions to ferrous ions is 1.5 - 1.55.

[0010] Further, the weight of the carbon coating layer obtained in step (2) accounts for 0.5% - 5% of the weight of the manganese iron precursor.

[0011] Further, the manganese source is one or more of manganese sulfate, manganese carbonate, manganese acetate, manganese phosphate, manganese nitrate, manganese oxalate, manganese citrate with or without crystal water; the iron source is one or more of ferrous sulfate, ferrous carbonate, ferrous acetate, ferrous phosphate, ferrous nitrate, ferrous oxalate, ferrous citrate, ferric sulfate, ferric carbonate, ferric acetate, ferric phosphate, ferric nitrate, ferric oxalate, ferric citrate with or without crystal water.

[0012] Further, the lithium source is one or more of lithium carbonate, lithium hydroxide, lithium phosphate, lithium oxalate, lithium acetate, lithium sulfate, lithium nitrate, lithium chloride, and the phosphorus source is one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium tripolyphosphate, phosphoric acid, calcium phosphate, phosphate ester, lithium dihydrogen phosphate, iron phosphate, lithium phosphate, lithium dihydrogen phosphate, manganese phosphate.

[0013] Further, the precipitating agent is one or more of oxalic acid, sodium oxalate, potassium oxalate, ammonium oxalate, phosphoric acid, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, potassium dihydrogen phosphate, ammonium carbonate, sodium carbonate, potassium carbonate, sodium hydroxide, ammonia water.

[0014] Through the above steps, the problem of precipitation of trivalent manganese ions at high temperatures can be effectively solved, thereby significantly improving the high-temperature cycle and storage performance of lithium manganese iron phosphate. The specific operation is to test the high-temperature cycle and storage performance of the prepared large single-crystal lithium manganese iron phosphate at 60°C. The results show that its high-temperature cycle performance and storage performance have been significantly improved.

[0015] A lithium iron manganese phosphate cathode material coated with a carbon layer by CVD method, wherein the lithium iron manganese phosphate cathode material is the carbon-coated lithium manganese iron phosphate obtained by the preparation method, and the particle size of the carbon-coated lithium manganese iron phosphate is 100nm - 2μm.

[0016] The D50 of the carbon-coated lithium manganese iron phosphate is 1.5μm, and the specific surface area is 12m 2 / g, and the tapped density is 1.2g / cm 3 , and the compression density is 2.5g / cm 3 .

[0017] A lithium ion battery, which has the cathode material described above.

[0018] Compared with the existing technology, the beneficial effects of this technical solution are as follows:

[0019] 1. The method of coating the carbon layer by vapor deposition CVD in the present invention can form a uniform carbon coating layer on the precursor of lithium manganese iron phosphate, effectively solving the problem that the traditional carbon coating process cannot form a uniform coating on the surface of the precursor, thereby improving the coating effect, reducing the dissolution of manganese, and improving the safety of the battery.

[0020] 2. After forming the carbon coating layer in the present invention, it is mixed with lithium salt and phosphorus source for co-sintering to form large single crystal lithium manganese iron phosphate. This method can not only further improve the uniformity of the carbon coating layer, but also allow the lithium salt to penetrate through the carbon layer to form a uniformly lithium-doped carbon coating layer. This structure can not only effectively exert the specific capacity, but also slow down the precipitation of trivalent manganese ions at high temperature, thereby improving the high-temperature cycle and storage performance of lithium iron manganese phosphate.

[0021] 3. Compared with the traditional carbon coating process and CVD cracking coating process, the method of the present invention can not only form a uniform coating on the surface of the precursor, but also avoid the problem that the amorphous carbon layer is too thick to hinder the lithium ion transmission. Therefore, the method of the present invention can effectively solve the problem of the precipitation of trivalent manganese ions at high temperature without sacrificing the specific capacity performance and rate performance, thereby improving the high-temperature cycle and storage performance of the battery. Description of the Drawings

[0022] Figure 1 It is the discharge curve of the coin cell of the carbon-coated lithium manganese iron phosphate cathode material prepared in Example 1.

[0023] Figure 2 It is the discharge curve of the coin cell of the carbon-coated lithium manganese iron phosphate cathode material prepared in Example 2.

[0024] Figure 3 It is the discharge curve of the coin cell of the carbon-coated lithium manganese iron phosphate cathode material prepared in Comparative Example 1.

[0025] Figure 4 Discharge curve of the button cell of the lithium iron manganese phosphate cathode material coated with carbon prepared in Comparative Example 2.

[0026] Figure 5 Scanning electron microscope (SEM) image of the lithium iron manganese phosphate material coated with carbon prepared in Example 1.

[0027] Figure 6 Scanning electron microscope (SEM) image of the lithium iron manganese phosphate material coated with carbon prepared in Example 2.

[0028] Figure 7 Scanning electron microscope (SEM) image of the lithium iron manganese phosphate material coated with carbon prepared in Comparative Example 1.

[0029] Figure 8 Scanning electron microscope (SEM) image of the lithium iron manganese phosphate material coated with carbon prepared in Comparative Example 2.

[0030] Figure 9 Raman images of the lithium iron manganese phosphate materials coated with carbon prepared in Example 1, Example 2, Comparative Example 1 and Comparative Example 2 of the present invention.

[0031] Figure 10 X-ray diffraction images of the lithium iron manganese phosphate materials coated with carbon prepared in Example 1, Example 2, Comparative Example 1 and Comparative Example 2 of the present invention.

[0032] Figure 11 Long cycle images of the lithium iron manganese phosphate materials coated with carbon prepared in Example 1, Example 2, Comparative Example 1 and Comparative Example 2 of the present invention. Detailed implementation manners

[0033] In order to make the objectives, technical solutions and advantages of the disclosed embodiments of the present invention clearer, the technical solutions of the disclosed embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings of the disclosed embodiments of the present invention. Obviously, the described embodiments are only a part of the disclosed embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the disclosed embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0034] Example 1

[0035] The preparation method of the lithium iron manganese phosphate cathode material coated with carbon by synthesizing the manganese iron precursor by the solid-phase method, and its preparation steps are as follows:

[0036] 1) Using MnSO 4 ·H 2 O as the manganese source, FeSO 4 ·7H 2 O as the iron source, MnSO4 ·H 2 O and FeSO 4 ·7H 2 O has a molar ratio of 6:4, and then solid-phase mixing is carried out; the uniformly mixed materials are heated to 500 °C for high-temperature solid-phase sintering to obtain a manganese-iron precursor (Mn 0.6 Fe 0.4 ) 2 O 3 .

[0037] 2) Put the manganese-iron precursor (Mn 0.6 Fe 0.4 ) 2 O 3 into a CVD fluidized-bed reactor. The reactor temperature is set at 700 °C, and acetylene is introduced. The reaction time is 2 hours. Acetylene is input, and nitrogen is also input as a fluidizing gas. Under these conditions, acetylene cracks on the surface of the manganese-iron precursor to coat the manganese-iron precursor, forming a uniform amorphous carbon coating layer. The weight of the amorphous carbon coating layer accounts for 0.5%-5% of the weight of the manganese-iron precursor (Mn 0.6 Fe 0.4 ) 2 O 3 particles. 3) Mix the manganese-iron precursor with a uniform amorphous carbon coating layer with a lithium salt and a phosphorus source for co-sintering. The specific operation is to solid-phase mix and grind the carbon-coated manganese-iron precursor (Mn 0.6 Fe 0.4 ) 2 O 3 , lithium carbonate, and ammonium dihydrogen phosphate in a tube furnace filled with argon (purity ≥ 99.999%) at 800 °C for high-temperature solid-phase sintering for 5 hours to obtain large single-crystal lithium manganese iron phosphate LiMn 0.6 Fe 0.4 PO 4 . The carbon-coated manganese-iron precursor and the lithium salt and the phosphorus source are mixed in a ratio of 1:1.05:1 according to the sum of the molar amounts of manganese and iron, the molar amount of lithium atoms, and the molar amount of phosphorus atoms. During this process, the lithium salt will penetrate through the carbon layer to form a uniformly lithium-doped carbon coating layer.

[0038] Test the particle size, specific surface area, and tapped density of the large single-crystal lithium manganese iron phosphate LiMn 0.6 Fe 0.4 PO 4 material obtained in step 3) of this example. The results show that the D50 of this material is 1.5 μm, the specific surface area is 12 m 2 / g, and the tapped density is as high as 1.2 g / cm 3 , and the compacted density of the material can reach 2.5 g / cm 3 , which is much higher than the common 0.8-1.0 g / cm 3The tap density and the compaction density of 2.3 g / cm 3 are much lower than the common 20 m 2 / g specific surface area in current commercial applications. In Example 1, the lithium iron manganese phosphate precursor, manganese iron oxide, was synthesized first. The true density of manganese iron oxide is higher than that of iron oxide but lower than that of manganese oxide. The true density of iron oxide is higher than that of iron salts such as ferrous sulfate. Subsequently, when synthesizing lithium iron manganese phosphate, since the traditional solid-phase method uses iron source, manganese source, lithium source, and phosphorus source to synthesize lithium iron manganese phosphate, due to the existence of low-density iron source, the synthesized lithium iron manganese phosphate material has low tap density and large specific surface area. After making into electrode sheets, it has low compaction density, low energy density, large self-discharge, and slightly poor cycle performance; the traditional liquid-phase method is to co-precipitate manganese iron carbonate or manganese iron hydroxide, with a porous and fluffy morphology and small density. Subsequently, the lithium iron manganese phosphate material synthesized from the co-precipitated manganese iron source, lithium source, and phosphorus source also has low tap density and large specific surface area. After making into electrode sheets, it has low compaction density, low energy density, large self-discharge, and slightly poor cycle performance. A higher compaction density can make the roller compaction density of the electrode sheet high. When the electrode sheet is thin, more electrode sheets can be accommodated in a given battery case, ultimately making the battery have a higher energy density. At the same time, a low specific surface area can reduce the content of the binder, making the proportion of the active material higher, further improving the energy density of the battery; and a low specific surface area makes the side reaction between the material and the electrolyte less, improving the shelf performance and cycle life of the battery.

[0039] Example 2

[0040] A preparation method of a carbon-coated lithium iron manganese phosphate cathode material by synthesizing a manganese iron precursor by the co-precipitation method, and its preparation steps are as follows:

[0041] 1) Prepare the reaction solution:

[0042] Dissolve manganese sulfate in deionized water to prepare a 0.5 mol / L divalent manganese ion solution;

[0043] Dissolve ferrous sulfate in deionized water to prepare a 0.5 mol / L ferrous ion solution;

[0044] Dissolve sodium oxalate in deionized water to prepare a 0.5 mol / L oxalate solution;

[0045] 2) Add the divalent manganese ion solution and the ferrous ion solution to the oxalate solution simultaneously, where the molar ratio of the divalent manganese ion to the ferrous ion is controlled as n(Mn 2+ ):n(Fe 2+ ) = 1.55, and the molar ratio of the oxalate to the added divalent manganese ion and ferrous ion is controlled as n(C 2 O 4 2- ):n(Mn 2+ +Fe 2+) = 4.99, under a nitrogen atmosphere, stir and heat. The heating temperature is 50 - 100 °C, and the reaction time is 5 - 20 h. After washing and drying, manganese iron oxalate is obtained;

[0046] 3) Put the manganese iron oxalate into a CVD fluidized bed reactor. Set the reactor temperature to 700 °C, introduce acetylene, and the reaction time is 2 hours. Input acetylene, and at the same time input nitrogen as the fluidizing gas. Under these conditions, acetylene cracks on the surface of the manganese iron oxalate to coat the manganese iron oxalate, forming a uniform amorphous carbon coating layer. In this step, the weight of the amorphous carbon coating layer accounts for 0.5% - 5% of the weight of the manganese oxalate particles.

[0047] 4) Mix the manganese iron oxalate with a uniform amorphous carbon coating layer with a lithium salt and a phosphorus source for co-sintering. The specific operation is as follows: solid-phase mix and grind the carbon-coated manganese iron oxalate, lithium carbonate, and ammonium dihydrogen phosphate. In a tubular furnace filled with argon (purity ≥ 99.999%), perform high-temperature solid-phase sintering at 800 °C for 5 hours to obtain large single-crystal lithium manganese iron phosphate LiMn 0.6 Fe 0.4 PO 4 . The carbon-coated manganese iron oxalate, lithium salt, and phosphorus source are mixed in a ratio of the sum of the molar amounts of manganese and iron, the molar amount of lithium atoms, and the molar amount of phosphorus atoms of 1:1.2:1. During this process, the lithium salt will penetrate through the carbon layer to form a uniformly lithium-doped amorphous carbon coating layer.

[0048] Comparative Example 1

[0049] A method for preparing a lithium manganese iron phosphate cathode material by synthesizing a manganese iron precursor by coprecipitation and then carbon coating, the preparation steps are as follows:

[0050] 1) Prepare the reaction solution:

[0051] Dissolve manganese sulfate in deionized water to prepare a 0.5 mol / L divalent manganese ion solution;

[0052] Dissolve ferrous sulfate in deionized water to prepare a 0.5 mol / L ferrous ion solution;

[0053] Dissolve sodium oxalate in deionized water to prepare a 0.5 mol / L oxalate solution;

[0054] 2) Add the divalent manganese ion solution and the ferrous ion solution to the oxalate solution simultaneously, where the molar ratio of the divalent manganese ion to the ferrous ion is controlled as n(Mn 2+ ):n(Fe 2+ ) = 1.55, and the molar ratio of the oxalate to the added divalent manganese ion and ferrous ion is controlled as n(C 2 O 4 2- ):n(Mn 2+ +Fe2+ ) = 4.99, under a nitrogen atmosphere, stir and heat, with the heating temperature being 50 - 100 °C and the reaction time being 5 - 20 h. After washing and drying, manganese iron oxalate is obtained;

[0055] 3) Weigh lithium dihydrogen phosphate and the manganese iron oxalate from step 2), mix them in a ratio of 1.2:1:1 according to the molar ratio of the lithium source, phosphorus source, and the sum of the molar amounts of manganese and iron in the manganese iron oxalate. After adding 10 wt% of sucrose based on the total mass of the lithium source, phosphorus source, and manganese iron oxalate, grind the mixture to D 50 = 0.2 μm and D 100 = 10 μm, then under a nitrogen atmosphere, heat to 500 °C and continuously calcine at 500 °C for 2 h, and then cool to room temperature to obtain the lithium manganese iron phosphate precursor;

[0056] 4) Crush the lithium manganese iron phosphate precursor, continue to heat it to 600 °C in a tubular furnace filled with argon (purity ≥ 99.999%), and continuously calcine at 600 °C for 15 h, and then cool to room temperature to obtain the carbon-coated lithium manganese iron phosphate cathode material.

[0057] Comparative Example 2

[0058] A method for preparing a carbon-coated lithium manganese iron phosphate cathode material by synthesizing a manganese iron precursor through a coprecipitation method, and its preparation steps are as follows:

[0059] 1) Prepare the reaction solution:

[0060] Dissolve manganese sulfate in deionized water to prepare a 0.5 mol / L divalent manganese ion solution;

[0061] Dissolve ferrous sulfate in deionized water to prepare a 0.5 mol / L ferrous ion solution;

[0062] Dissolve sodium oxalate in deionized water to prepare a 0.5 mol / L oxalate solution;

[0063] 2) Add the divalent manganese ion and ferrous ion solutions to the oxalate solution simultaneously, where the molar ratio of the divalent manganese ion to the ferrous ion is controlled as n(Mn 2+ ):n(Fe 2+ ) = 1.55, and the molar ratio of oxalate to the added divalent manganese ion and ferrous ion is controlled as n(C 2 O 4 2- ):n(Mn 2+ +Fe 2+ ) = 4.99. Under a nitrogen atmosphere, stir and heat, with the heating temperature being 50 - 100 °C and the reaction time being 5 - 20 h. After washing and drying, manganese iron oxalate is obtained;

[0064] 3) Weigh lithium dihydrogen phosphate and manganese iron oxalate, and mix them in a molar ratio of 1.2:1:1 based on the sum of the molar amounts of lithium source, phosphorus source, manganese, and iron in manganese iron oxalate. Grind the mixture to D 50 = 0.2 μm and D 100 = 8 μm. Then, under a nitrogen atmosphere, heat it to 600 °C and continuously calcine it at 600 °C for 15 h, and then cool it to room temperature to obtain the lithium iron manganese phosphate precursor;

[0065] 4) Put the lithium iron manganese phosphate precursor into a CVD fluidized bed reactor. Set the reactor temperature to 700 °C, introduce acetylene, and the reaction time is 2 hours. Input acetylene and also input nitrogen as the fluidizing gas. Under these conditions, acetylene cracks on the surface of the lithium iron manganese phosphate precursor to coat the lithium iron manganese phosphate precursor, forming a uniform amorphous carbon coating layer. In this step, the weight of the amorphous carbon coating layer accounts for 0.5% - 5% of the weight of the lithium iron manganese phosphate precursor particles.

[0066] As Figures 5 to 8 shown, the particle size of Example 1 is approximately between 200 nm and 1 μm, and the particle sizes of Example 2, Comparative Example 1, and Comparative Example 2 are between 100 nm and 500 nm. The particle size of Example 1 is larger because the precursor in Example 1 is prepared by the solid-phase method. The solid-phase method conducts reactions at high temperatures, and the reactants directly react in the solid state, making it easy to obtain larger particle sizes. After mixing and heat treatment with the lithium source, the particle size of the lithium iron manganese phosphate sample obtained is on the larger side; while the precursors of other samples are prepared by the co-precipitation method. The co-precipitation method conducts reactions in solution, and the combination and precipitation processes between ions are relatively fast, and the particle size is smaller than that obtained by the solid-phase reaction. Therefore, after mixing with the lithium source, the particle size of the lithium iron manganese phosphate prepared from the precursor prepared by the co-precipitation method is smaller than that of the precursor prepared by the solid-phase method.

[0067] Raman spectroscopy is a spectroscopic technique used to study the molecular vibrations, rotations, and other low-frequency modes of materials, Figure 9 in which different curves represent different samples (Example 1, Example 2, Comparative Example 1, and Comparative Example 2), and the value of I D / I G is marked next to each curve. This is the intensity ratio of the D peak (the peak related to the defects of the carbon material) to the G peak (the peak related to the graphitization degree of the carbon material). I D / I G is used to evaluate the degree of disorder or defect density of the carbon material. Generally, a higher I D / I G value means that there are more defects or disordered structures in the material. In this figure, the I D / I GThe value is the lowest (0.98), which may indicate that its carbon coating has a higher degree of graphitization or fewer defects, while the I of Comparative Example 1 D / I G value is the highest (1.15), indicating that its carbon coating may have more defects or a lower degree of graphitization.

[0068] XRD patterns were used to determine the crystal structure and phase composition of the materials. Among them, Figure 10 the bottommost blue vertical line represents the standard diffraction peak position of LiFe 0.5 Mn 0.5 PO 4 (PDF#42-0580). The diffraction peaks of Example 1 are sharper and closer to the standard peaks, which may mean that the samples of Example 1 have better crystal quality and fewer impurity phases.

[0069] The lithium iron manganese phosphate composite materials prepared in Examples 1-2 and Comparative Examples 1-2 were combined with a conductive agent and a binder to prepare a positive electrode sheet, and the prepared positive electrode sheet was made into a button cell, and the button cell was assembled for electrochemical performance test and analysis. The specific steps are as follows:

[0070] (1) Preparation of the positive electrode sheet:

[0071] According to the positive electrode active material: The prepared carbon-coated lithium iron manganese phosphate positive electrode material was mixed with polyvinylidene fluoride and acetylene black in a mass ratio of 80:10:10 in NMP solvent, and then the mixture was coated on aluminum foil and dried in vacuum at 120 °C to make a positive electrode sheet.

[0072] (2) Assembly of the battery:

[0073] The positive electrode sheet, the separator and the negative electrode sheet were successively loaded into the battery case, and during this process, the electrolyte was added. The battery was assembled and tested on a blue battery test system; among them, the negative electrode sheet was polished metallic lithium, the separator was a PP separator, and the electrolyte was 0.25 M LiPF6.

[0074] (3) Electrochemical performance test:

[0075] Under the condition of 55 °C, within the voltage window of 2.5-4.3 V, charge-discharge tests were carried out at a current density of 1C (1C = 170 mAh g -1 ). The obtained results are shown in Table 1.

[0076] Table 1

[0077]

[0078] As can be seen from Table 1, Comparative Example 1 is a traditional carbon coating process, that is, a carbon-containing organic compound, a phosphorus source, a lithium source, and a manganese iron oxalate precursor are mixed and then sintered at high temperature. Under the conditions of solid-phase high-temperature reaction, carbon-coated lithium iron phosphate manganese is produced. Based on Comparative Example 1, Comparative Example 2 further improves the preparation method. In the co-mixing step, no carbon-containing organic compound is added. The manganese iron oxalate precursor is obtained in the co-precipitation step, and then it is further sintered with the lithium source and the phosphorus source at high temperature and ground to obtain a lithium iron phosphate manganese precursor. Finally, an amorphous carbon layer is coated on the surface of the lithium iron phosphate manganese precursor particles by an acetylene CVD process. In contrast, the initial specific capacity of the cathode material in Comparative Example 2 is increased at 55 °C. This is because the carbon layer coated by acetylene CVD in Comparative Example 2 is more uniform than the preparation method of directly adding the carbon source sucrose to the lithium source, the phosphorus source, and the manganese iron oxalate in Comparative Example 1, and a graphitized carbon layer is formed, optimizing the lithium ion migration channel and increasing the lithium ion diffusion coefficient. In addition, since the particle size of the material has no obvious change, the acetylene CVD coating improves the lithium ion transport rate. The specific capacity of the cathode material in Comparative Example 2 is also increased after 100 cycles compared with that in Comparative Example 1 at a high temperature of 55 °C. This is because when the cathode material in Comparative Example 1 is cycled for charge and discharge at a high temperature of 55 °C, trivalent manganese ions are severely dissolved out and contact with the electrolyte, resulting in a fast attenuation rate.

[0079] Based on Comparative Example 2, in Example 2, the amorphous carbon layer is first coated on the manganese iron oxalate and then solid-phase sintered with the lithium source and the phosphorus source. Such a preparation process results in an increase in the initial specific capacity of the cathode material in Example 2 compared with that in Comparative Example 2 at 55 °C. Because Example 2 further overcomes the hindrance of the amorphous carbon layer to lithium ion transport in Comparative Example 2, that is, lithium ions are located outside the carbon layer, and at the same time, lithium ions can penetrate into the carbon layer, reducing the distance of lithium ion insertion and extraction, further increasing the lithium ion diffusion coefficient. At the same time, the carbon coating layer is directly coated on the manganese iron oxalate, which can more directly reduce the dissolution of manganese ions and prevent them from contacting the electrolyte. As a result, the capacity retention rate of the cathode material in Example 2 is higher than that in Comparative Example 2 after multiple cycles of charge and discharge at a high temperature of 55 °C.

[0080] Example 1 further improves the preparation process on the basis of Example 2. In Example 2, manganese iron oxalate is prepared by the coprecipitation method, while in Example 1, the manganese iron precursor is prepared by the solid-phase sintering method. Since the crystallinity of the manganese iron precursor prepared by solid-phase sintering is better than that of the manganese iron oxalate prepared by the coprecipitation method, the crystal structure of lithium iron manganese phosphate prepared based on the manganese iron precursor prepared by the solid-phase method is more complete and has fewer defects. The lattice stability of the positive electrode material in Example 1 is greater during charge and discharge, and the ionic conductivity is higher. In the olivine structure, the lithium ion migration channel is one-dimensional, that is, one. Unless the number of lithium ion migration channels is increased, defects will only hinder the diffusion of lithium ions. The crystal structure of Example 1 is more complete and has fewer defects, so the migration rate of lithium ions in Example 1 is increased. Therefore, the specific capacity and cycle life of the positive electrode material in Example 1 at 55 °C are better than those of other samples.

[0081] The prior art "Research and Application on the Preparation and Electrochemical Properties of Lithium Iron Phosphate and Lithium Iron Manganese Phosphate" discloses a carbon-coated lithium iron manganese phosphate (LMFP-10). Using lithium carbonate, manganese carbonate, ammonium dihydrogen phosphate, and ferrous oxalate as raw materials, where the weighed manganese to iron atomic ratio is 6:4, and then grinding with gelatin. Finally, the mixed slurry is dried in a vacuum oven and in a tube furnace filled with argon (purity ≥ 99.999%), first pre-carbonized at 350 °C for 3 hours and then heated to 650 °C and calcined for 10 hours to obtain the final product (heating rate is 5 °C per minute). The gelatin addition amount is preferably 10 wt%. The particle size of lithium iron manganese phosphate (LMFP-10) is between 100 and 700 nm, and the thickness of the carbon coating layer is about 4 nm. The positive electrode material prepared by the method of Comparative Example 1 is subjected to charge and discharge tests at a current density of 1C at room temperature within a voltage window of 2.5 - 4.3V, and its initial specific capacity and specific capacity after 300 cycles are the same as those of LMFP-10. In Examples 1 and 2 of the present invention, due to the amorphous carbon layer coated by CVD on the outer layer of the lithium iron manganese phosphate precursor particles, the amorphous carbon layer overcomes the dissolution of manganese under high-temperature conditions, and the prepared positive electrode material has a higher capacity retention rate at high temperatures.

[0082] The above examples are only for illustrating the technical concept and characteristics of the present invention. The purpose is to enable ordinary technicians in the field to understand the content of the present invention and implement it accordingly, and it should not be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the essence of the content of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for preparing a lithium iron manganese phosphate positive electrode material coated with a carbon layer based on a CVD method, characterized in that: The following steps are involved: Step (1) solid-phase mixing of a manganese source and an iron source to prepare ferromanganese oxide by a high-temperature solid-phase sintering method, or adding a divalent manganese ion solution and a ferrous ion solution to a solution containing a precipitant to prepare a ferromanganese-containing salt by a co-precipitation method; Step (2) placing ferromanganese oxide or ferromanganese-containing salt as a ferromanganese precursor into a CVD fluidized bed reactor for fluidization, using an inert gas as a fluidizing gas, inputting a gaseous carbon source for cracking, and the cracked gaseous carbon source forming a carbon coating layer on the surface of the ferromanganese precursor; Step (3) solid-phase mixing the ferromanganese precursor having a carbon coating layer with a phosphorus source and / or a lithium source and performing solid-phase sintering to obtain carbon-coated lithium ferromanganese phosphate.

2. The preparation method according to claim 1, characterized in that: In the step (1), the stoichiometric ratio of manganese in the manganese source to iron in the iron source or the stoichiometric ratio of divalent manganese ions to ferrous ions is 1.5-1.

55.

3. The preparation method according to claim 1, characterized in that: The weight of the carbon coating layer obtained in step (2) accounts for 0.5%-5% of the weight of the ferromanganese precursor.

4. The preparation method according to claim 1, characterized in that: The manganese source is one or more of manganese sulfate, manganese carbonate, manganese acetate, manganese phosphate, manganese nitrate, manganese oxalate, and manganese citrate, which contain or do not contain water of crystallization; the iron source is one or more of ferrous sulfate, ferrous carbonate, ferrous acetate, ferrous phosphate, ferrous nitrate, ferrous oxalate, ferrous citrate, ferrous sulfate, ferrous carbonate, ferric acetate, ferric phosphate, ferric nitrate, ferrous oxalate, and ferrous citrate, which contain or do not contain water of crystallization.

5. The preparation method according to claim 1, characterized in that: The lithium source is one or more of lithium carbonate, lithium hydroxide, lithium phosphate, lithium oxalate, lithium acetate, lithium sulfate, lithium nitrate, and lithium chloride; the phosphorus source is one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium tripolyphosphate, phosphoric acid, calcium phosphate, phosphate ester, lithium dihydrogen phosphate, iron phosphate, lithium phosphate, lithium dihydrogen phosphate, and manganese phosphate.

6. The preparation method according to claim 1, characterized in that: The precipitant is one or more of oxalic acid, sodium oxalate, potassium oxalate, ammonium oxalate, phosphoric acid, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, potassium dihydrogen phosphate, ammonium carbonate, sodium carbonate, potassium carbonate, sodium hydroxide, and ammonia water.

7. A lithium iron manganese phosphate positive electrode material coated with a carbon layer based on a CVD method, characterized in that: The lithium iron manganese phosphate positive electrode material is the carbon-coated lithium iron manganese phosphate obtained by the preparation method of any one of claims 1 to 8, and the particle size of the carbon-coated lithium iron manganese phosphate particles is 100 nm to 2 μm.

8. The positive electrode material according to claim 7, characterized in that The carbon-coated lithium manganese iron phosphate has a D50 of 1.5 μm and a specific surface area of ​​12 m 2 / g, tap density is 1.2g / cm 3 , compacted density is 2.5g / cm 3 .

9. A lithium ion battery, characterized in that: The lithium ion battery comprises the positive electrode material according to claim 9.

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