A method for preparing a cathode material and its application

Abstract

This invention discloses a method for preparing a cathode material and its application, comprising the following steps: S1. Dispersing manganese, iron, lithium, and phosphorus sources, then crushing and drying them; S2. Performing microwave plasma chemical vapor deposition (MSCVD) on the powder obtained after drying in step S1; the heat treatment yields lithium iron manganese phosphate material; S3. Coating the powder obtained after plasma treatment in step S2 with carbon and then pulverizing it. This invention can prepare Li0.05 cathode materials that can improve the performance of lithium iron manganese phosphate. + Positive electrode materials with high diffusion rate and electronic conductivity.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of new energy materials, and particularly relates to a preparation method of a positive electrode material and application thereof. BACKGROUND

[0002] Lithium ion batteries are widely used in the fields of new energy vehicles, mobile devices, energy storage power stations and the like due to their high working voltage and excellent cycle performance. With the continuous development of technology, people have put forward higher requirements for lithium ion batteries, and designing and developing electrode materials with higher capacity, higher power, higher energy density and better cycle stability have become one of the research hotspots in the field of new energy.

[0003] Lithium iron phosphate (LFP) with an olivine structure is mainly used in lithium ion batteries, batteries of electric / hybrid vehicles or energy storage power stations. However, the low charge / discharge voltage platform (3.4 V) leads to low energy density, which limits its development in the field of energy storage. Lithium manganese iron phosphate (LMFP) has a high discharge voltage (3.4 V and 4.1 V two platforms) compared with LFP, and its energy density is 20% higher than that of LFP. The crystal structure of LMFP is very similar to that of LFP, which is an olivine structure, and the theoretical specific capacity is 170 mAh g -1 . The electrochemical performance of LMFP is limited by electron transport and ion diffusion. The strong P-O covalent bond in PO4 3- stabilizes the oxygen atom, ensuring that Li + can be embedded / extracted in a stable crystal structure. The olivine LMFP structure exhibits good safety and cycle stability. However, LMFP is a semiconductor compound with extremely low electronic conductivity, because the M-O (M = Fe, Mn) octahedron is isolated by the P-O tetrahedron, and there is no continuous M-O network in its crystal structure. In addition, the strong P-O covalent bond also prevents Li + from being transported through the PO4 3- tetrahedron. The transport of Li + can only be one-dimensional diffusion along the b-axis, which reduces the diffusion rate of Li + and leads to poor conductivity.

[0004] Therefore, how to further improve the diffusion rate of Li + and the electronic conductivity of lithium manganese iron phosphate is currently an urgent task. SUMMARY

[0005] The present application aims to at least solve one of the problems in the prior art. To this end, the present application provides a preparation method of a positive electrode material, which can prepare a positive electrode material that improves the diffusion rate of Li + and the electronic conductivity of lithium manganese iron phosphate.

[0006] The application further provides application of the positive electrode material prepared by the preparation method of the positive electrode material in preparation of a secondary battery.

[0007] The application provides a preparation method of a positive electrode material according to a first aspect of the application, which comprises the following steps:

[0008] S1. mixing and dispersing a manganese source, an iron source, a lithium source and a phosphorus source, crushing and drying;

[0009] S2. performing heat treatment on the powder obtained after drying in step S1, wherein the heat treatment is performed in a microwave plasma environment;

[0010] The lithium manganese iron phosphate material is obtained after the gas deposition treatment;

[0011] S3. crushing the powder obtained in step S2 after carbon coating; the method of carbon coating is microwave plasma chemical vapor deposition.

[0012] The application provides a preparation method of a positive electrode material according to an embodiment of the application, which has at least the following beneficial effects:

[0013] 1. The carbon layer deposited by the microwave plasma chemical vapor deposition method (MPCVD) in step S3 not only protects the positive electrode sheet after carbon coating from electrolyte corrosion, but also improves the electronic conductivity due to the high graphitization degree of the positive electrode material, and finally improves the electrochemical performance of the obtained positive electrode material.

[0014] 2. The carbon-coated lithium manganese iron phosphate synthesized by the MPCVD method has controllable particle size, and compared with conventional nitrogen atmosphere sintering, the growth and agglomeration of particles under high-temperature calcination are overcome, and the specific capacity and cycle stability of the material are improved.

[0015] 3. Coating a layer of carbon on the surface of the lithium manganese iron phosphate material is an effective means to improve the electrochemical performance of the electrode material. On the one hand, the carbon layer exists between the particles, which can enhance the conductivity of the material and weaken the polarization effect; on the other hand, the carbon layer can provide more fast tunnels for electron transmission, and carbon coating can inhibit the growth of crystal grains, which is beneficial to shorten the lithium ion movement distance, thereby improving the rate performance of the lithium manganese iron phosphate active material.

[0016] According to some embodiments of the application, the manganese source comprises at least one of manganese oxalate, manganese monoxide, trimanganese tetroxide, dimanganese trioxide, manganous phosphate and manganous hydrogen phosphate.

[0017] According to some embodiments of the application, the iron source is at least one of iron phosphate, iron oxide, ferrous oxalate and iron powder.

[0018] According to some embodiments of the present application, the phosphorus source is at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, lithium dihydrogen phosphate, lithium phosphate and phosphoric acid.

[0019] According to some embodiments of the present application, the lithium source is at least one of lithium phosphate, lithium dihydrogen phosphate, lithium carbonate and lithium hydroxide.

[0020] According to some embodiments of the present application, before the dispersing in step S1, a dopant is added.

[0021] According to some embodiments of the present application, the dopant includes at least one of Zn dopant, Mg dopant, Ti dopant, Al dopant, Cr dopant, Zr dopant, Ni dopant and Co dopant.

[0022] The lithium ion transmission path is along the

[010] crystal plane direction; the doping of the metal ions of the above-mentioned doping elements shortens the bond length in the olivine structure MO6 (M = Mn, Fe, Mg) octahedron, and the Li-O bond in the LiO6 octahedron becomes longer, so that the lithium ion diffusion channel is wider and easier to migrate, which is beneficial to the improvement of the electrochemical performance. Therefore, the lattice distortion caused by the doping of the above-mentioned metal elements can reduce the surface energy of the crystal of the positive electrode material, and can also inhibit the growth of the crystal, so that the nanoscale lithium iron manganese phosphate is synthesized.

[0023] The doping of the metal elements can improve the lithium ion diffusion rate of the lithium iron manganese phosphate material, so that the lithium ion diffusion channel is wider and easier to migrate, so as to improve the electrochemical performance of the material. Different metal elements do not exist charge compensation defects (for example, Mg 2+ on the M 2+ site, Ni 2+ ), due to the charge compensation mechanism, equivalent doping can produce vacancies and the energy of homovalent 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 it difficult for heterovalent doping.

[0024] According to some embodiments of the present application, in step S1, the particle size of the crushed material is D 50 0.3-1.0 μm.

[0025] The lithium iron manganese phosphate material can accommodate a large amount of Li + reversible deintercalation without causing its own structure change under the above-mentioned particle size. Under the above-mentioned particle size, the diffusion path of Li + is avoided to be increased due to the too large particle size of the material, the migration rate of Li + is reduced, and the rate performance of the material is deteriorated; at the same time, the surface energy is avoided to be increased due to the too small particle size of the material, and the phenomenon of easy agglomeration is avoided.

[0026] According to some embodiments of the present application, the crushing includes grinding.

[0027] According to some embodiments of the present application, the grinding time is 1-10 hours.

[0028] According to some embodiments of the present application, in step S1, the drying comprises spray drying.

[0029] According to some embodiments of the present application, the temperature of the inlet of the drying process is 180-250℃.

[0030] According to some embodiments of the present application, the temperature of the outlet of the drying process is 90-140℃.

[0031] According to some embodiments of the present application, in step S1, the spray drying comprises constant-rate spray drying and decreasing-rate spray drying.

[0032] According to some embodiments of the present application, the feeding speed of the spray drying is 5-10 L / min.

[0033] In the constant-rate drying stage, the temperature of the droplet remains unchanged, the moisture on the surface continuously evaporates, the moisture inside the droplet migrates to the surface, the drying air continuously transfers heat to the droplet, and the temperature of the carrier gas decreases; in the decreasing-rate drying stage, the surface of the droplet has started to solidify, the temperature gradually decreases from the outside to the inside, and the water content of the particle gradually decreases. When the inlet / outlet air temperature is lower than 180 / 90℃, the time required for the surface of the droplet to solidify is longer, and the particles are more likely to agglomerate in the continuous collision process. However, when the temperature is too high, higher than 250 / 140℃, the chemical properties of the precursor are changed, Fe and Mn in the material are easily oxidized to high valence state, and the energy consumption and cost are increased.

[0034] The present application can realize rapid transfer of heat and mass by combining spray drying technology, so that the material is quickly dried and regular nanoparticles are formed, and the uniformity is good. Then, using the MPCVD method, a uniform carbon layer can be better coated on the surface of the particles.

[0035] According to some embodiments of the present application, in step S2, the temperature of the heat treatment is 600-800℃.

[0036] According to some embodiments of the present application, in step S2, the time of the heat treatment is 10-60 min.

[0037] Microwave plasma chemical vapor deposition (MPCVD) can synthesize nanoparticles in a very short time, which can avoid the grain growth caused by traditional long-time high-temperature heat treatment. Small particle size is beneficial to shorten the migration path of lithium ions in the deintercalation process, and effectively improve the electrochemical performance of lithium manganese iron phosphate.

[0038] According to some embodiments of the present application, in step S2, the plasma used in the microwave plasma includes hydrogen plasma.

[0039] According to some embodiments of the present application, the flow rate of the hydrogen plasma is 10-100 sccm.

[0040] In MPCVD, the above-mentioned temperature, time and methane flow rate are parameters for adjusting the speed of methane cracking, which affect the carbon-coating effect, ensuring the carbon atoms obtained by decomposition of methane per unit time, while avoiding the formation of a large number of carbon particles due to the too fast cracking speed of methane.

[0041] According to some preferred embodiments of the present application, in step S2, the heat treatment is performed in a microwave plasma chemical vapor deposition (MPCVD) reaction tank.

[0042] According to some embodiments of the present application, in step S2, the lithium iron manganese phosphate powder material is obtained after the plasma treatment.

[0043] According to some embodiments of the present application, in step S3, the carbon-coated carbon source includes methane.

[0044] According to some embodiments of the present application, the flow rate of the methane is 10-100 sccm.

[0045] According to some embodiments of the present application, the pulverizing method includes air jet milling.

[0046] According to some embodiments of the present application, the classification frequency of the air jet milling is 150-260 Hz.

[0047] According to some embodiments of the present application, the air pressure of the air jet milling is 0.3-0.6 MPa.

[0048] According to some embodiments of the present application, the positive electrode material is LiMn x Fe y M z PO4 / C, 0.59≤x≤0.61, 0.36≤y≤0.38, 0.02≤z≤0.04, x+y+z=1, and M is at least one of Zn, Mg, V, Ti, Al, Cr, Zr, Ni and Co.

[0049] The lithium iron manganese phosphate is not a simple physical mixture of lithium manganese phosphate and lithium iron phosphate, and since the iron and manganese ions in the material have similar radii, a solid solution is easily formed, thus realizing atomic-level mixing. When the Mn content is too high, the elongation of the Mn-O bond caused by the Jahn-Teller distortion of Mn 3+ in the charging and discharging process, and the elongation of the Mn-O bond caused by the Jahn-Teller distortion of Mn 3-The edge sharing of tetrahedron is responsible for the activation energy of the charge carrier migration, which brings the intrinsic kinetics of the LMFP with high Mn content to be slow. Therefore, the best performance can be achieved when the Mn:Fe ratio is about 6:4.

[0050] According to the second aspect of the present application, the application of the positive electrode material prepared by the above preparation method in a secondary battery is provided.

[0051] Other features and advantages of the present application will be set forth in the following description, and in part will become apparent from the description, or can be learned by practice of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0052] The above and / or additional aspects and advantages of the present application will become apparent and be readily understood from the following description, taken in conjunction with the accompanying drawings, in which:

[0053] Figure 1 is the XRD pattern of Example 3 in the present application;

[0054] Figure 2 is the SEM pattern of Example 3 in the present application;

[0055] Figure 3 is the electrochemical performance pattern of Example 3;

[0056] Figure 4 is the electrochemical performance pattern of Comparative Example 1;

[0057] Figure 5 is the electrochemical performance pattern of Example 3 and Comparative Example 1. DETAILED DESCRIPTION

[0058] Embodiments of the present application are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference signs represent the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by reference to the accompanying drawings are exemplary and are only used to explain the present application, and cannot be understood as a limitation of the present application.

[0059] Example 1

[0060] The present embodiment discloses a preparation method of a positive electrode material, LiMn 0.8 Fe 0.18 M 0.02 PO4 / C, M is Zn, and the preparation method is as follows:

[0061] S1: weigh 1790g of manganese oxalate, 405g of ferrous oxalate, 1303g of lithium dihydrogen phosphate, and 21g of zinc oxide, mix them in 10L of deionized water, and use a sand mill to sand mill after fully stirring and uniformizing, slurry D50 0.35 pm;

[0062] S2: The sand-milled slurry is centrifugal spray dried, with an inlet temperature of 210 DEG C, an outlet temperature of 110 DEG C, and a feeding speed of 5 L / min, to obtain dried precursor powder;

[0063] S3: The spray-dried precursor powder is placed on a substrate in an MPCVD reaction tank, and hydrogen plasma is introduced for treatment, at a reaction temperature of 600 DEG C, for a treatment time of 15 min, at a hydrogen flow rate of 10 sccm, to obtain LiMn 0.8 Fe 0.18 Mg 0.02 PO4nanoparticles;

[0064] S4: Methane gas is introduced to achieve carbon coating on the surface of the LiMn 0.8 Fe 0.18 Mg 0.02 PO4nanoparticles, at a reaction temperature of 700 DEG C, for a treatment time of 10 min, at a hydrogen flow rate of 100 sccm, and a methane flow rate of 10 sccm, to obtain LiMn 0.8 Fe 0.18 Mg 0.02 PO4 / C;

[0065] S5: The LiMn 0.8 Fe 0.18 Mg 0.02 PO4 / C is pulverized by using an airflow mill, at a classification frequency of 220 Hz and an air pressure of 0.5 MPa, to obtain a final product.

[0066] Example 2

[0067] The present example discloses a preparation method of a positive electrode material, in which LiMn 0.7 Fe 0.29 M 0.01 PO4 / C is synthesized according to a ratio of x:y:z = 0.7:0.29:0.01, and the preparation method is as follows:

[0068] S1: 994.5 g of manganese hydrogen phosphate, 895 g of manganese oxalate, 626.5 g of iron phosphate, 523.5 g of lithium carbonate, and 11 g of nickel oxide are weighed and mixed in 8 L of deionized water, and then uniformly stirred and sand-milled by using a sand mill to obtain a slurry D 50 0.4 pm;

[0069] S2: The sand-milled slurry is centrifugal spray dried, with an inlet temperature of 220 DEG C, an outlet temperature of 100 DEG C, and a feeding speed of 10 L / min, to obtain dried precursor powder;

[0070] S3: The spray-dried precursor powder is placed on the substrate in the MPCVD reaction tank, hydrogen plasma is introduced, the reaction temperature is 700℃, the treatment time is 10 min, the hydrogen flow rate is 20 sccm, and LiMn 0.7 Fe 0.29 Ni 0.01 PO4 nanoparticles are obtained.

[0071] S4: Methane gas is introduced to the surface of the LiMn 0.7 Fe 0.29 Ni 0.01 PO4 nanoparticles to achieve carbon coating, the reaction temperature is 650℃, the treatment time is 20 min, the hydrogen flow rate is 90 sccm, and the methane flow rate is 20 sccm, and LiMn 0.7 Fe 0.29 Ni 0.01 PO4 / C is obtained.

[0072] S5: The LiMn 0.7 Fe 0.29 Ni 0.01 PO4 / C is pulverized by using an air flow mill, wherein the classification frequency is 200 Hz and the air pressure is 0.55 MPa, and the final product is obtained.

[0073] Example 3

[0074] The embodiment discloses a preparation method of a positive electrode material, LiMn 0.6 Fe 0.37 M 0.03 PO4 / C is synthesized in a proportion of x:y:z=0.6:0.37:0.03, and the preparation method is as follows:

[0075] S1: 1580g of manganese sesquioxide, 936g of iron phosphate, 1092g of lithium dihydrogen phosphate, 230g of lithium carbonate and 20g of magnesium oxide are weighed and mixed in 10L of deionized water, and then the mixture is uniformly stirred and sand-milled to slurry D 50 is 0.55μm;

[0076] S2: The sand-milled slurry is centrifugally spray-dried, the drying conditions are that the inlet temperature is 200℃, the outlet temperature is 120℃, and the feeding speed is 8L / min, and the precursor dry powder is obtained.

[0077] S3: The spray-dried precursor powder is placed on the substrate in the MPCVD reaction tank, hydrogen plasma is introduced, the reaction temperature is 650℃, the treatment time is 10 min, the hydrogen flow rate is 50 sccm, and LiMn 0.6 Fe 0.37 Mg 0.03 PO4 nanoparticles are obtained.

[0078] S4: Introducing methane gas into LiMn 0.6 Fe 0.37 Mg 0.03 PO4 nanoparticles to realize carbon coating, the reaction temperature is 800℃, the treatment time is 5min, the hydrogen flow rate is 50sccm, and the methane flow rate is 50sccm, so that LiMn 0.6 Fe 0.37 Mg 0.03 PO4 / C is obtained.

[0079] S5: The LiMn 0.6 Fe 0.37 Mg 0.03 PO4 / C is pulverized by using an airflow mill, wherein the classification frequency is 180Hz, and the air pressure is 0.6MPa, so that the final product is obtained.

[0080] The XRD pattern of the product prepared in Example 3 is shown in Figure 1 The results show that the diffraction peaks of the sample belong to the orthorhombic olivine-type crystal structure, and the diffraction peaks can be matched with the standard card diffraction peaks of LiMnPO4(PFD#77-0178), and the X-ray diffraction peak high angle is offset, because the Mn 2+ radius ratio Fe 2+ is large, and the lattice spacing of the positive electrode material is reduced; the obtained peak intensity is equivalent, and the crystallinity is good, so that the pure lithium manganese iron phosphate is synthesized, the carbon exists in an amorphous form, which cannot be detected by XRD, so the existence of carbon does not affect the crystal structure of the material.

[0081] The SEM pattern of the product prepared in Example 3 is shown in Figure 2 The results show that the carbon-coated lithium manganese iron phosphate has a uniform particle size distribution, which indicates that MPCVD is a very effective way to control the particle size, because MPCVD can synthesize nanoparticles in a very short time, and can avoid the grain growth caused by traditional long-time high-temperature heat treatment. The small particle size obtained is beneficial to shorten the migration path of lithium ions in the deintercalation process, and can effectively improve the electrochemical performance of lithium manganese iron phosphate.

[0082] Example 4

[0083] The embodiment discloses a preparation method of a positive electrode material, and LiMn 0.5 Fe 0.48 M 0.02 PO4 / C is synthesized according to the ratio of x:y:z=0.5:0.48:0.02, and the preparation method is as follows:

[0084] S1: 790 g of manganese trioxide, 768 g of iron oxide, 1040 g of lithium dihydrogen phosphate, and 16 g of titanium dioxide are weighed out and mixed in 8 L of deionized water, and after being uniformly stirred, sand milling is performed using a sand mill to obtain slurry D 50 is 0.4 μm;

[0085] S2: The sand-milled slurry is spray-dried under the following conditions: inlet temperature 210°C, outlet temperature 110°C, and feed rate 6 L / min, to obtain a precursor dried powder;

[0086] S3: The precursor powder obtained by spray-drying is placed on a substrate in an MPCVD reaction tank, and hydrogen plasma treatment is performed at a reaction temperature of 700°C for a treatment time of 10 min at a hydrogen flow rate of 20 seem to obtain LiMn 0.5 Fe 0.48 M 0.02 PO4;

[0087] S4: Methane gas is introduced to achieve carbon coating on the surface of the LiMn 0.6 Fe 0.37 Mg 0.03 PO4nanoparticles at a reaction temperature of 800°C for a treatment time of 5 min at a hydrogen flow rate of 50 seem and a methane flow rate of 50 seem to obtain LiMn 0.6 Fe 0.37 Mg 0.03 PO4 / C;

[0088] S5: Methane gas is introduced to achieve carbon coating on the surface of the LMFP nanoparticles at a reaction temperature of 800°C for a treatment time of 25 min at a hydrogen flow rate of 100 seem and a methane flow rate of 10 seem to obtain LiMn 0.5 Fe 0.48 M 0.02 PO4 / C.

[0089] Example 5

[0090] This example discloses a preparation method of a positive electrode material, and differs from Example 3 in that no dopant containing a doping element is added, and LiMn 0.6 Fe 0.4 PO4 / C nanoparticles are synthesized in a ratio of x:y:z = 0.6:0.4:0. The remaining conditions are the same.

[0091] Example 6

[0092] This example discloses a preparation method of a positive electrode material, and differs from Example 3 in that the precursor powder obtained by spray-drying is placed on a substrate in an MPCVD reaction tank, and hydrogen plasma treatment is performed at a reaction temperature of 700°C for a treatment time of 20 min at a hydrogen flow rate of 80 seem.

[0093] Example 7

[0094] This example discloses a preparation method of a positive electrode material, and the difference from Example 3 is that the doped element-containing dopant added is Ni, and the rest of the conditions are the same.

[0095] Example 8

[0096] This example discloses a preparation method of a positive electrode material, and the difference from Example 3 is that in step S1, sand milling is performed using a sand mill to a slurry D 50 0.8 μm.

[0097] Example 9

[0098] This example discloses a preparation method of a positive electrode material, and the difference from Example 3 is that in step S3, the reaction temperature is 750°C.

[0099] Example 10

[0100] This example discloses a preparation method of a positive electrode material, and the difference from Example 3 is that in step S3, the treatment time is 25 min.

[0101] Example 11

[0102] This example discloses a preparation method of a positive electrode material, and the difference from Example 3 is that in step S3, the hydrogen flow rate is 100 sccm.

[0103] Example 12

[0104] This example discloses a preparation method of a positive electrode material, and the difference from Example 3 is that in step S4, the reaction temperature is 600°C.

[0105] Example 13

[0106] This example discloses a preparation method of a positive electrode material, and the difference from Example 3 is that in step S3, the treatment time is 25 min.

[0107] Example 14

[0108] This example discloses a preparation method of a positive electrode material, and the difference from Example 3 is that in step S3, the methane flow rate is 100 sccm.

[0109] Comparative Example 1

[0110] This comparative example discloses a preparation method of a positive electrode material, and the difference from Example 3 is that a soluble organic carbon source (glucose) is used as the carbon source, mixed in a slurry for sand milling, spray drying, and then synthesizing LiMn 0.6 Fe0.37 M 0.03 PO4 / C nanoparticles.

[0111] The coin cell performance diagrams of the products prepared in Example 3 and Comparative Example 1 are shown below. Figure 3 and 4 As shown. Figure 3 The charge-discharge curve of the sample prepared in Example 3 at 0.1C shows that the specific capacity at 0.1C discharge is 155 mAh / g. Figure 4 The charge-discharge curve of the sample prepared for Comparative Example 1 is shown at 0.1C, with a discharge specific capacity of 142 mAh / g at 0.1C. The results indicate that the lithium iron manganese phosphate material coated with carbon by MPCVD has better performance.

[0112] The coin cell performance diagrams of the products prepared in Example 3 and Comparative Example 1 are shown below. Figure 4 and 5 As shown. Figure 5 The impedance spectra of Example 3 and Comparative Example 1 are shown. The impedance spectra consist of a high-frequency region and a low-frequency region. The semicircle diameter of the high-frequency region represents the electrochemical transfer impedance R. ct The straight line in the low-frequency region represents Li + The diffusion rate has the largest R value compared to ratio 1. ct The value indicates that the sample experiences significant resistance and has low electronic conductivity, resulting in greater polarization and lower specific capacity during charging and discharging.

[0113] Test Example 1

[0114] This test example tested the charge-discharge test results of the positive electrode materials of Examples 1-14 and Comparative Example 1. The test results are shown in Table 1. Examples 1-14 and Comparative Example 1 used equal volumes of LiPF6 and diethyl carbonate (DEC) electrolyte, with a LiPF6 concentration of 1M. A coin cell was prepared using a lithium metal sheet as the negative electrode. The rate performance of the battery was tested using a LAND battery programmable tester (LAND CT2001A).

[0115] Table 1 Performance Test Results

[0116]

[0117]

[0118] In this invention, the carbon layers deposited by microwave plasma chemical vapor deposition (MPCVD) in Examples 1-14 not only protect the carbon-coated positive electrode from electrolyte corrosion, but also improve electronic / ionic conductivity and electrochemical performance due to the high degree of graphitization of the positive electrode material.

[0119] The embodiments of the present application are described in detail above with reference to the drawings, but the present application is not limited to the above-described embodiments, and various changes can be made within the knowledge of those skilled in the art without departing from the spirit of the present application.

Claims

1. A method for producing a positive electrode material, characterized by, The method comprises the following steps: S1. mixing and dispersing a manganese source, an iron source, a lithium source and a phosphorus source, crushing, and drying; S2. performing heat treatment on the powder obtained after drying in step S1, wherein the heat treatment is performed in a microwave plasma environment; a lithium manganese iron phosphate material is obtained after the heat treatment, in step S2, the plasma used in the microwave plasma comprises hydrogen plasma, the flow rate of the hydrogen plasma is 10-100 sccm; in step S2, the temperature of the heat treatment is 600-800℃; in step S2, the time of the heat treatment is 10-60 min; S3. crushing the powder obtained in step S2 after carbon coating; the method of carbon coating is microwave plasma chemical vapor deposition; in step S1, before the dispersing, a dopant is added; the dopant comprises at least one of Zn dopant, Mg dopant, Ti dopant, Al dopant, Cr dopant, Zr dopant, Ni dopant and Co dopant.

2. The method of claim 1, wherein the method further comprises: In step S1, the D50 of the broken material is 0.3 to 1.0 μm. 50 0.3 to 1.0 μm.

3. The method for preparing the cathode material according to claim 1, characterized in that, in step S1, the drying comprises spray drying; the temperature of the air inlet of the spray drying process is 180-250℃, and the temperature of the air outlet of the spray drying process is 90-140℃.

4. The method of claim 1, wherein the method further comprises: in step S3, the carbon source for carbon coating comprises methane.

5. The method for preparing the cathode material according to claim 4, characterized in that, the flow rate of the methane is 10-100 sccm.

6. The method of claim 1, wherein the method further comprises: The positive electrode material is LiMn x Fe y M z PO4 / C, 0 < x < 1, 0 < y < 0.95, 0 < z < 0.05, x + y + z = 1, M is at least one of Zn, Mg, V, Ti, Al, Cr, Zr, Ni and Co.

7. Use of a positive electrode material prepared by the preparation method in any one of claims 1-6 in the preparation of a secondary battery.

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