A Lithium Iron Manganese Phosphate Cathode Material, Its Preparation Method and Application

Abstract

The present invention discloses a lithium iron manganese phosphate cathode material, a preparation method thereof, and an application thereof. Using the lithium iron manganese phosphate material as a matrix, defects are created on the surface of the matrix material by plasma treatment, and then a non-metallic compound is used as a modifier, and plasma treatment is used to fill, dope, and coat the surface defects of the material with non-metallic elements, thereby preparing a high-stability lithium iron manganese phosphate cathode material. After the best-prepared material, LiFe<subgt;0.5< / subgt;Mn<subgt;0.5< / subgt;PO<subgt;4‑&< / subgt;S@C, is cycled 300 times at 25 °C and a rate of 1.0C, the discharge specific capacity of the material is still as high as 153.8 mAh g<supgt;‑1< / supgt>, and the capacity retention rate is 96.4%. The discharge capacity of the blank material is only 138.5 mAh g<supgt;‑1< / supgt>, and the corresponding capacity retention rate is 87.9%. In addition, the electrochemical cycling performance of the lithium iron manganese phosphate material modified by the present invention at high rates is also improved.

Description

Technical Field

[0001] The present invention relates to the technical field of high-energy battery materials, and particularly relates to a lithium iron manganese phosphate cathode material, a preparation method thereof and an application thereof. Background Art

[0002] Vigorously developing the new energy vehicle industry can effectively reduce environmental pollution and fossil fuel consumption, and is an important way to achieve "green development" of mankind. With the increase in the sales volume of new energy vehicles, the corresponding demand for lithium-ion power batteries is also increasing continuously. As an important component of lithium-ion batteries, the cathode material is a key factor affecting the battery performance. At present, commercial cathode materials include ternary cathode materials and lithium iron phosphate cathode materials, each with its own advantages. For example, ternary cathode materials have excellent ionic conductivity, high discharge capacity and working voltage, etc., while lithium iron phosphate materials have good structural stability, excellent safety performance and low price, etc. However, the above two types of cathode materials also have their own defects. For example, ternary cathode materials are expensive and have poor thermal stability; lithium iron phosphate materials have low discharge capacity and poor ionic conductivity, etc., which all restrict the further development of the two cathode materials. Therefore, developing new cathode materials with high capacity and high safety is of great significance for promoting the further development of the new energy vehicle industry.

[0003] Lithium iron manganese phosphate material LiFe x Mn 1-x PO4(0 < x < 1, LFMP), as a derivative material of lithium iron phosphate, inherits the characteristics of high structural stability and excellent thermal stability of lithium iron phosphate material. At the same time, the introduction of Mn element can increase the voltage platform of the material, thereby increasing the energy density of the material. Compared with lithium iron phosphate, the energy density of lithium iron manganese phosphate material can be increased by about 20%, and it has good application potential. However, problems such as low ionic conductivity, poor rate performance and rapid attenuation of battery performance at high rates also hinder the large-scale popularization of LFMP materials. By optimizing the structure of LFMP materials and coating LFMP with materials having excellent conductivity, the lithium ion diffusion ability and rate performance of LFMP can be effectively improved, thereby further improving the electrochemical performance of the materials. Summary of the Invention

[0004] In order to overcome the existing defects and deficiencies of the above-mentioned lithium iron manganese phosphate cathode material, the purpose of the present invention is to provide a lithium iron manganese phosphate cathode material with high stability, especially a lithium iron manganese phosphate cathode material, its preparation method and application. By using plasma-assisted treatment, the present invention first constructs abundant defects on the surface of the lithium iron manganese phosphate material. Then, through element doping and atomic deposition techniques, inert non-metallic elements are doped and filled into the defects to improve the crystal structure of the material surface layer. At the same time, through the atomic deposition technique, a nanoscale coating layer can be deposited on the surface of the lithium iron manganese phosphate material to improve the interfacial conductivity of the material. Therefore, through the method of the present invention, the structure and interface of the lithium iron manganese phosphate cathode material can be modified simultaneously, and the modified material has excellent electrochemical performance.

[0005] The purpose of the present invention is achieved by the following technical solutions:

[0006] A preparation method of a lithium iron manganese phosphate cathode material, comprising the following steps:

[0007] Using the lithium iron manganese phosphate material as a matrix, plasma treatment is used to create defects on the surface of the matrix material, and then a non-metallic compound is used as a modifier, and plasma treatment is used to fill and dope non-metallic elements into the surface defects of the material and coat it to prepare the lithium iron manganese phosphate cathode material.

[0008] In the above method, preferably, the lithium iron manganese phosphate material without doping and coating modification is used as the matrix, and the lithium iron manganese phosphate material is LiFe x Mn 1-x PO4, 0 < x < 1.

[0009] Further preferably, the lithium iron manganese phosphate material includes at least one of LiFe 0.5 Mn 0.5 PO4, LiFe 0.6 Mn 0.4 PO4 and LiFe 0.2 Mn 0.8 PO4.

[0010] In the above method, the plasma treatment process is as follows: the matrix material is placed in a plasma device, an inert gas is introduced, the vacuum degree and temperature in the device are adjusted, and the plasma device is turned on for treatment to obtain a lithium iron manganese phosphate material rich in oxygen anion defects.

[0011] Further, the lithium iron manganese phosphate material rich in oxygen anion defects is LiFe x Mn 1-x PO 4-& , 0 < x < 1, 0 < & < 0.5 (& represents that the lithium iron manganese phosphate material has oxygen anion defects).

[0012] Further, after the plasma device finishes the treatment, turn off the heating device and the plasma emission device, and cool to room temperature to obtain the lithium iron manganese phosphate material rich in oxygen anion defects.

[0013] Further, the inert gas includes at least one of Ar and N2.

[0014] By virtue of the effects of the inert gas, vacuum degree control and high-temperature plasma treatment, promote the occurrence of ionization, so that the generated ion fragments bombard the oxygen atoms on the surface layer of the material, causing oxygen atom loss and generating vacancy sites, thereby constructing oxygen anion defects on the surface layer of the material.

[0015] Further, the vacuum degree in the device is controlled below 50 Pa, preferably 5 - 20 Pa; the treatment temperature is 200 - 500 °C, preferably 300 - 400 °C.

[0016] Further, the treatment power of the plasma device is not less than 100 W, preferably 140 - 180 W; the treatment time does not exceed 120 min, preferably 30 - 80 min.

[0017] The vacuum degree control is to make the concentration of the inert gas and reduce the energy barrier required for gas ionization; the high-temperature treatment is to make the gas fully ionize and generate a relatively high kinetic energy to bombard the surface layer of the material; the plasma treatment is to ionize the inert gas by means of methods such as discharge, high-frequency electromagnetic oscillation, shock wave and high-energy radiation to generate plasma and ion fragments.

[0018] In the described method, when using a non-metallic compound as a modifier for the reaction, adjust the temperature in the device and the plasma emission power, replace the inert atmosphere with the non-metallic compound for treatment, and obtain the lithium iron manganese phosphate cathode material filled, doped and coated with non-metallic elements.

[0019] The present invention realizes the simultaneous regulation of the structure and interface of the material. The structure regulation is non-metallic element doping, and the non-metallic elements include at least one of S, N, F, and Cl, and the doping amount of the non-metallic elements does not exceed 2 wt%; the interface regulation is surface coating, and the coating includes at least one of B and C compounds, and the thickness of the generated coating layer is 2 - 8 nm.

[0020] Further preferably, the B compound includes at least one of Li2B4O7 and LiBO2.

[0021] In the described method, when using H2S and NH3 as modifiers, replace the inert gas with H2S and NH3, and then replace it with C2H2 or CH3Cl to continue the treatment; when using BF3 as a modifier, it is not necessary to replace it with C2H2 or CH3Cl.

[0022] Further, when using H2S and NH3 as modifiers, the processing power of the plasma equipment is 150 - 300 W, preferably 200 - 250 W; the processing temperature is 100 - 300 °C, preferably 150 - 250 °C; after replacing the inert gas with H2S and NH3, the processing time is 1 - 3 h, and after replacing it with C2H2 or CH3Cl, the processing time does not exceed 2 h.

[0023] Further, when using BF3 as a modifier, the processing power of the plasma equipment is 150 - 300 W, preferably 200 - 250 W; the processing temperature is 100 - 300 °C, preferably 150 - 250 °C; after replacing the inert gas with BF3, the processing time does not exceed 5 h, preferably 2 - 3 h.

[0024] In the present invention, under the action of high temperature and plasma, the gas is ionized by the non - metal compound. On the one hand, the ionized non - metal elements are doped at the material defect sites. On the other hand, the modifier can be deposited on the material surface and form a coating layer to simultaneously modify the structure and interface of the lithium iron manganese phosphate material.

[0025] In the present invention, the surface defects of lithium iron manganese phosphate are mainly caused by the loss of O anions, and the doping of non - metal elements S, N, F or Cl is also mainly doped at the oxygen anion sites. By using the relatively high electronegativity and large ionic radius of S, N, F or Cl, their doping can significantly improve the material lattice environment, enhance the structural stability and increase the surface lithium - ion diffusion rate; C has excellent electrical conductivity, and Li2B4O7 belongs to a fast ion conductor. Coating with C or Li2B4O7 can not only inhibit the side reactions at the material interface, but also greatly improve the lithium - ion transport kinetics at the material interface. Therefore, the lithium iron manganese phosphate material modified by the present invention can overcome the defect of poor conductivity of the material itself, improve the rate performance of the material, and has excellent electrochemical performance and a super - stable structure.

[0026] The present invention also provides a lithium iron manganese phosphate cathode material prepared by the above - mentioned method.

[0027] The present invention also provides the application of the above - mentioned lithium iron manganese phosphate cathode material in the preparation of lithium - ion batteries.

[0028] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0029] First, through plasma treatment, a large number of defects are created on the surface layer of the lithium iron manganese phosphate material. Then, using a non-metallic compound as a modifier and combining plasma treatment and atomic deposition technology, on the one hand, anion doping is achieved at the defect sites on the material surface layer to improve the bulk structure of the material; on the other hand, a fast ion conductor boride or highly conductive carbon is deposited on the material surface to achieve surface coating of the material and improve the poor conductivity problem of the lithium iron manganese phosphate cathode material. Therefore, the present invention can modify both the surface structure and interface of the lithium iron manganese phosphate cathode material, thereby greatly improving the electrochemical performance of the material. Description of the Drawings

[0030] Figure 1 : XRD pattern of the LiFe 0.5 Mn 0.5 PO 4-& S@C sample prepared in Example 1.

[0031] Figure 2 : XRD pattern of the LiFe 0.5 Mn 0.5 PO 4-& TEM image of the LiFe

[0032] Figure 3 : FIB-SEM image of the LiFe 0.5 Mn 0.5 PO 4-& S@C sample prepared in Example 1.

[0033] Figure 4 : ESR spectrum of the LiFe 0.5 Mn 0.5 PO 4-& 、LiFe 0.5 Mn 0.5 PO 4-& S sample prepared in Example 1.

[0034] Figure 5 : TEM image of the LiFe 0.6 Mn 0.4 PO 4-& N@C sample prepared in Example 2.

[0035] Figure 6 : TEM image of the LiFe 0.5 Mn 0.5 PO 4-& F@Li2B4O7 sample prepared in Example 3. Detailed Embodiments

[0036] The following examples are intended to further illustrate the present invention and are not intended to limit the scope of protection of the present invention.

[0037] The materials involved in the following examples are all commercially available.

[0038] The lithium iron manganese phosphate cathode material matrix used in the following examples is a self-prepared cathode material, and the specific preparation process is as follows:

[0039] According to the stoichiometric ratio, a certain amount of FeSO4·7H2O, MnSO4·H2O, and NH4H2PO4 were dissolved in a certain volume of distilled water, and a mixed solution of 0.5M FeSO4·7H2O, 0.5M MnSO4·H2O, and 1.0M NH4H2PO4 was formed under magnetic stirring. Then, the same volume of DMSO solution was added to the solution and magnetic stirring was continued for 1 h. Subsequently, a 3M LiOH·H2O solution was slowly added to the above solution under magnetic stirring, ammonia water was added to adjust the pH to 10, and the mixture was heated in an oil bath to 130 °C under an Ar atmosphere and kept at a constant temperature for 6 h. After cooling to room temperature and washing with distilled water multiple times, the sample was placed in a tube furnace and sintered at 600 °C under an Ar atmosphere for 10 h to obtain the lithium iron manganese phosphate cathode material LiFe 0.5 Mn 0.5 PO4 matrix.

[0040] According to the same material preparation method as above, by adjusting the dosages of FeSO4·7H2O and MnSO4·H2O, the lithium iron manganese phosphate cathode material LiFe 0.6 Mn 0.4 PO4 matrix was prepared.

[0041] Example 1

[0042] (1) A certain amount of the phosphate matrix LiFe 0.5 Mn 0.5 PO4 material was placed in a plasma device, Ar was introduced, and the vacuum pump was turned on to adjust the vacuum degree in the tube to 15 Pa. After turning on the heating device and heating the temperature in the tube to 350 °C, the plasma device was started, the output power was adjusted to 150 W, and the treatment was carried out for 45 min. Then, the heating device and the plasma treatment device were turned off, and after cooling to room temperature, the lithium iron manganese phosphate cathode material with a surface rich in defects was obtained (marked as: LiFe 0.5 Mn 0.5 PO 4-& ).

[0043] (2) Without disassembling and sampling the above treatment, the heater and the plasma emitter were moved to the blank area at the front end of the sample. The heating device was turned on to heat the temperature in the tube to 200 °C, the plasma device was turned on to adjust the power to 250 W, and then Ar was replaced with H2S gas and the treatment was carried out for 2 h. After that, the plasma device was turned off to obtain the S-doped lithium iron manganese phosphate cathode material (marked as: LiFe 0.5 Mn0.5 PO 4-& S). Then, replace the H2S gas with C2H2 and continue the treatment for 1 h to deposit a carbon layer on the material surface. Finally, a lithium iron manganese phosphate cathode material modified by C coating and S doping simultaneously is obtained (marked as: LiFe 0.5 Mn 0.5 PO 4-& S@C).

[0044] (3) Use X-ray diffraction (XRD) to test and analyze the LiFe 0.5 Mn 0.5 PO 4-& S@C obtained in the example, and obtain its XRD pattern, as Figure 1 shown.

[0045] (4) Use transmission electron microscopy (TEM) to test and analyze the LiFe 0.5 Mn 0.5 PO 4-& S@C obtained in Example 1, and obtain its TEM image, as Figure 2 shown.

[0046] (5) Assemble the LiFe 0.5 Mn 0.5 PO4, LiFe 0.5 Mn 0.5 PO 4-& 、LiFe 0.5 Mn 0.5 PO 4-& S and LiFe 0.5 Mn 0.5 PO 4-& S@C obtained in Example 1 into 2016-type button cells and perform charge-discharge tests in the voltage range of 3.0 - 4.3 V.

[0047] (6) The test temperature is 25 °C. The battery is first activated for one cycle at 0.2C and 0.5C, and then cycled at 1.0C and 3C. The test results are shown in Table 1 and Table 2.

[0048] (7) Use dual-beam electron microscopy (FIB-SEM) to perform interface scanning and energy spectrum analysis on the LiFe 0.5 Mn 0.5 PO 4-& S@C sample in Example 1, and obtain its electron microscope image, as Figure 3 shown.

[0049] (8) Use electron paramagnetic resonance (ESR) to analyze the LiFe 0.5 Mn 0.5 PO 4-& 、LiFe 0.5Mn 0.5 PO 4-& The oxygen vacancy test was carried out on the S sample, and its ESR diagram was obtained, as Figure 4 shown.

[0050] Example 2

[0051] (1) A certain amount of phosphate matrix LiFe 0.6 Mn 0.4 PO4 material was placed in a plasma device, Ar was introduced, and the vacuum pump was turned on to adjust the vacuum degree in the tube to 15 Pa. After turning on the heating device and heating the temperature in the tube to 350 °C, the plasma device was started. After adjusting the output power to 150 W and treating for 45 min, the heating device and the plasma treatment device were turned off and cooled to room temperature to obtain a lithium iron manganese phosphate cathode material with a surface rich in defects (marked as: LiFe 0.6 Mn 0.4 PO 4-& ).

[0052] (2) Without disassembling and sampling the above treatment, the heater and the plasma emitter were moved to the blank at the front end of the sample. The heating device was turned on to heat the temperature in the tube to 200 °C, the plasma device was turned on to adjust the power to 250 W, and then Ar was replaced with NH3 gas and treated for 2 h. After that, the plasma device was turned off to obtain an N-doped lithium iron manganese phosphate cathode material (marked as: LiFe 0.6 Mn 0.4 PO 4-& N). Then, NH3 gas was replaced with C2H2 and the treatment was continued for 1 h to deposit a carbon layer on the material surface. Finally, a lithium iron manganese phosphate cathode material modified by C coating and S doping at the same time was obtained (marked as: LiFe 0.6 Mn 0.4 PO 4-& N@C).

[0053] (3) The LiFe 0.6 Mn 0.4 PO 4-& N@C obtained in Example 2 was tested and analyzed by transmission electron microscopy (TEM), and its TEM diagram was obtained, as Figure 5 shown.

[0054] (4) The LiFe 0.6 Mn 0.4 PO 4-& N@C obtained in Example 2 was assembled into a 2016-type button battery and subjected to charge and discharge tests in the voltage range of 3.0 - 4.3 V.

[0055] (5) The test temperatures were 25°C. The battery was first activated for one cycle at 0.2C and 0.5C, and then cycled at 1.0C and 3.0C. The test results are shown in Tables 1 and 2.

[0056] Example 3

[0057] (1) A certain amount of phosphate matrix LiFe 0.5 Mn 0.5 PO4 material was placed in a plasma device. Ar was introduced and the vacuum pump was turned on to adjust the vacuum degree in the tube to 15 Pa. After turning on the heating device and heating the temperature in the tube to 350°C, the plasma device was started. After adjusting the output power to 150 W and treating for 45 min, the heating device and the plasma treatment device were turned off and cooled to room temperature to obtain a lithium iron manganese phosphate cathode material with a surface rich in defects (marked as: LiFe 0.5 Mn 0.5 PO 4-& ).

[0058] (2) Without disassembling and sampling in the above treatment, the heater and the plasma emitter were moved to the blank at the front end of the sample. The heating device was turned on to heat the temperature in the tube to 250°C, and the plasma device was turned on to adjust the power to 230 W. Subsequently, Ar was replaced with BF3 gas and treated for 3 h. Then the plasma device was turned off to obtain a lithium iron manganese phosphate cathode material modified by F-doped Li2B4O7 coating (marked as: LiFe 0.5 Mn 0.5 PO 4-& F@Li2B4O7).

[0059] (3) Transmission electron microscopy (TEM) was used to test and analyze LiFe 0.5 Mn 0.5 PO 4-& F@Li2B4O7 obtained in Example 3, and its TEM image was obtained as shown in Figure 6 .

[0060] (4) LiFe 0.5 Mn 0.5 PO 4-& F@Li2B4O7 obtained in Example 3 was assembled into a 2016-type button battery and subjected to charge and discharge tests in the voltage range of 3.0 - 4.3 V. The test conditions were at 25°C. First, it was activated for one cycle at 0.2C and 0.5C, and then cycled at 1.0C and 3.0C, as shown in Tables 1 and 2.

[0061] Comparative Example 1

[0062] (1) According to Example 1, a lithium iron manganese phosphate cathode material with a surface rich in defects was prepared (marked as: LiFe 0.5 Mn 0.5PO 4-& )。

[0063] (2) In the case where the above treatment does not disassemble and sample, move the heater and plasma emitter to the blank at the front end of the sample, turn on the heating device to heat the temperature inside the tube to 200 °C, replace Ar with H2S gas and treat for 2 h, then turn off the plasma equipment to obtain the modified lithium iron manganese phosphate cathode material (marked as: LiFe 0.5 Mn 0.5 PO 4-& S-1). Then, replace the H2S gas with C2H2 and continue to treat for 1 h to deposit a carbon layer on the material surface. Finally, obtain the C-coated and S-doped modified lithium iron manganese phosphate cathode material (marked as: LiFe 0.5 Mn 0.5 PO 4-& S-1@C).

[0064] (3) The difference between this comparative example and Example 1 is that the power of the plasma device is not adjusted to 250 W. Assemble the LiFe 0.5 Mn 0.5 PO 4-& S-1@C obtained in Comparative Example 1 into a 2016-type button battery and perform charge-discharge tests in the voltage range of 3.0 - 4.3 V. The test conditions are at 25 °C. First, activate one cycle at 0.2C and 0.5C, and then cycle at 1.0C and 3.0C, as shown in Table 1 and Table 2.

[0065] Comparative Example 2

[0066] (1) Obtain the lithium iron manganese phosphate cathode material with rich surface defects (marked as: LiFe 0.6 Mn 0.4 PO 4-& ) according to the method of Example 2.

[0067] (2) In the case where the above treatment does not disassemble and sample, move the heater and plasma emitter to the blank at the front end of the sample, turn on the heating device to heat the temperature inside the tube to 200 °C, turn on the plasma device and adjust the power to 250 W, then replace Ar with NH3 gas and treat for 2 h, then turn off the plasma equipment to obtain the N-doped lithium iron manganese phosphate cathode material (marked as: LiFe 0.6 Mn 0.4 PO 4-& N).

[0068] (3) The difference between this comparative example and Example 2 is that NH3 gas is not replaced with C2H2 and continue to treat for 1 h to deposit a carbon layer on the material surface. Assemble the LiFe 0.6 Mn 0.4 PO 4-&The N was assembled into a 2016-type button battery and charged and discharged within the voltage range of 3.0 - 4.3V. The test conditions were at 25°C. First, it was activated for one cycle at 0.2C and 0.5C, and then cycled at 1.0C and 3.0C, as shown in Table 1 and Table 2.

[0069] Comparative Example 3

[0070] (1) According to the method of Example 1, a lithium iron manganese phosphate cathode material with a surface rich in defects was obtained (marked as: LiFe 0.5 Mn 0.5 PO 4-& ).

[0071] (2) Without disassembling and sampling in the above treatment, move the heater and plasma emitter to the blank at the front end of the sample. Open the heating device to heat the temperature inside the tube to 250°C. Open the plasma device, adjust the power to 230W, and after treating for 3h, turn off the plasma equipment to obtain the cathode material (marked as: LiFe 0.5 Mn 0.5 PO 4-& -1).

[0072] (3) The difference between this comparative example and Example 1 is that Ar was not replaced with H2S gas for 2h, and at the same time, H2S gas was not replaced with C2H2 for further treatment for 1h to deposit a carbon layer on the material surface. Assemble the LiFe 0.5 Mn 0.5 PO 4-& -1 obtained in Comparative Example 3 into a 2016-type button battery and perform charge and discharge tests within the voltage range of 3.0 - 4.3V. The test conditions were at 25°C. First, it was activated for one cycle at 0.2C and 0.5C, and then cycled at 1.0C and 3.0C, as shown in Table 1 and Table 2.

[0073] Table 1 Comparison of electrical properties of materials in examples and comparative examples (tested at 25°C, cycled at 1.0C)

[0074]

[0075] Table 2 Comparison of electrical properties of materials in examples and comparative examples (tested at 25°C, cycled at 3.0C)

[0076]

[0077] For the material LiFe 0.5 Mn 0.5 PO 4-& S@C obtained in Example 1 of the present invention, it can be seen from XRD Figure 1 that the characteristic peaks of lithium iron manganese phosphate in the material are obvious, indicating that the lithium iron manganese phosphate material was successfully prepared in this experiment. In addition, from Figure 2 andFigure 5 It can be seen that after atomic deposition of the material, the carbon coating layer is successfully coated on the surface of the material, which is beneficial to improving the lithium ion diffusion coefficient of the material.

[0078] For the material LiFe 0.5 Mn 0.5 PO 4-& S@C obtained in Example 1 of the present invention, from Figure 3 it can be seen that S is mainly doped in the surface layer of the material. For Figure 4 , LiFe 0.5 Mn 0.5 PO 4-& and LiFe 0.5 Mn 0.5 PO 4-& S, by comparing the ESR diagrams of LiFe 0.5 Mn 0.5 PO 4-& and LiFe 0.5 Mn 0.5 PO 4-& S, the change in the oxygen vacancy concentration in the plasma-treated sample can be found. It can be seen from the figure that both samples have symmetric peaks, and the central site of the peak g = 2.007, which corresponds to oxygen vacancies. The peak intensity in the ESR spectrum is proportional to the concentration of oxygen vacancies. The peak intensity of the LiFe 0.5 Mn 0.5 PO 4-& sample is four times that of the LiFe 0.5 Mn 0.5 PO 4-& S sample, indicating that there are a large number of oxygen vacancies in LiFe 0.5 Mn 0.5 PO 4-& after plasma treatment. After S doping, the oxygen vacancies are significantly reduced, further indicating that S is doped into the oxygen vacancies of the material.

[0079] For the material LiFe 0.5 Mn 0.5 PO 4-& F@Li2B4O7 obtained in Example 3 of the present invention, from its TEM Figure 6 it can be seen that there is an obvious coating layer on the surface of the material, and the crystal lattice in the coating layer is obvious. By measuring and analyzing the lattice spacing of the material, the lattice spacing is 0.348 nm, which belongs to the (202) characteristic crystal plane of the fast ion conductor Li2B4O7, proving that the coating layer on the surface of the material is mainly Li2B4O7. The Li in Li2B4O7 comes from the residual lithium on the surface of the material.

[0080] As can be seen from Table 1 and Table 2, for the modified lithium iron manganese phosphate cathode material obtained by the present method, through plasma high-temperature treatment, oxygen defects can be constructed on the material surface. Through element doping and atomic deposition techniques, highly stable atoms such as F, S, Cl, and N are doped into the surface defect sites of the material to achieve anion surface doping, improve the chemical environment of lattice oxygen atoms, increase the metal-nonmetal bond energy, and inhibit the dissolution of transition metal Mn elements during charge and discharge. At the same time, the atomic deposition technique can improve the material interface environment and construct a highly conductive carbon or Li2B4O7 coating layer at the material interface, which can not only enhance the lithium-ion transport kinetics at the material interface but also effectively isolate the direct contact between the electrolyte and the material, reducing the interfacial side reactions. Generally, the method of the present invention can simultaneously improve the surface structure and interface of the material through simple treatment, enhancing the electrochemical performance of the material.

[0081] Using H2S and C2H2 as modifiers and LiFe 0.5 Mn 0.5 PO4 as the matrix material, the modified lithium iron manganese phosphate cathode material LiFe 0.5 Mn 0.5 PO 4-& S@C with S surface doping and C surface coating modification simultaneously is obtained through treatment. After the material is cycled 100 times and 300 times at 1.0C, the discharge capacities are still as high as 157.2 mAh g -1 and 153.8 mAh g -1 , significantly higher than 146.8 mAh g -1 and 138.5 mAh g -1 of the blank sample. In addition, the modified material also exhibits excellent electrochemical performance at high rates. After the material is cycled 100 times and 200 times at 3.0C, the discharge capacities are 138.2 mAh g -1 and 128.7 mAh g -1 respectively, while the blank sample is only 130.6 mAh g -1 and 119.7 mAh g -1 . It should be noted that if the material is only manufactured with defects through plasma treatment without further doping and coating modification, these defects will rapidly deteriorate the electrochemical performance of the material.

[0082] The above embodiments are the preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent substitution methods and are all included in the protection scope of the present invention.

Claims

1. A preparation method of a lithium iron manganese phosphate cathode material, characterized in that: Using lithium iron manganese phosphate material as the matrix, defects are created on the surface of the matrix material by plasma treatment. Then, using a non-metallic compound as a modifier, plasma treatment is used to fill, dope, and coat the surface defects of the material with non-metallic elements, and a lithium iron manganese phosphate cathode material is prepared. The lithium iron manganese phosphate material includes LiFe 0.5 Mn 0.5 PO4, LiFe 0.6 Mn 0.4 PO4 and LiFe 0.2 Mn 0.8 PO4, at least one of them; When using plasma treatment, the matrix material is placed in a plasma device, an inert gas is introduced, the vacuum degree and temperature inside the device are adjusted, and the plasma device is turned on for treatment to obtain a lithium iron manganese phosphate material rich in oxygen anion defects. When using plasma treatment to create defects on the surface of the matrix material, the inert gas includes at least one of Ar and N2, the vacuum degree of the device is controlled below 50 Pa, the treatment temperature is 200 - 500 °C, the treatment power of the plasma device is not less than 100 W, and the treatment time does not exceed 120 min. When reacting with a non-metallic compound as a modifier, the temperature and plasma emission power inside the device are adjusted, the inert atmosphere is replaced with the non-metallic compound for treatment, and a lithium iron manganese phosphate cathode material filled, doped, and coated with non-metallic elements is obtained. The non-metallic elements filled and doped include at least one of S, N, F, and Cl; the coating includes at least one of B and C compounds. When using plasma treatment to fill, dope, and coat the surface defects of the material with non-metallic elements, the treatment power of the plasma device is 250 - 300 W.

2. The method according to claim 1, wherein: When using plasma treatment to create defects on the surface of the matrix material, the inert gas includes at least one of Ar and N2, the vacuum degree of the device is controlled at 5 - 20 Pa, the treatment temperature is 300 - 400 °C, the treatment power of the plasma device is 140 - 180 W, and the treatment time is 30 - 80 min.

3. The method according to claim 1, wherein: When the modifier does not contain B or C, after doping, at least one of the B and C compounds is replaced for coating.

4. The method according to claim 3, wherein: When using H2S and NH3 as modifiers, the inert gas is replaced with H2S and NH3 to complete doping, and then replaced with C2H2 or CH3Cl to continue the treatment to complete coating; when using BF3 as a modifier, there is no need to replace it with C2H2 or CH3Cl.

5. The method according to claim 3, wherein: When using H2S and NH3 as modifiers, the treatment power of the plasma device is 250 - 300 W; the treatment temperature is 100 - 300 °C; after replacing the inert gas with H2S and NH3, the treatment time is 1 - 3 h, and after replacing it with C2H2 or CH3Cl, the treatment time does not exceed 2 h; when using BF3 as a modifier, the treatment power of the plasma device is 250 - 300 W; the treatment temperature is 100 - 300 °C; after replacing the inert gas with BF3, the treatment time does not exceed 5 h.

6. The method according to claim 5, wherein: When using H2S and NH3 as modifiers, the treatment power of the plasma equipment is 250 W; the treatment temperature is 150 - 250 °C; after replacing the inert gas with H2S and NH3, the treatment time is 1 - 3 h, and after replacing it with C2H2 or CH3Cl, the treatment time does not exceed 2 h; when using BF3 as a modifier, the treatment power of the plasma equipment is 250 W; the treatment temperature is 150 - 250 °C; after replacing the inert gas with BF3, the treatment time is 2 - 3 h.

7. A lithium iron manganese phosphate cathode material prepared by the method according to any one of claims 1 to 6.

8. Use of the lithium iron manganese phosphate cathode material according to claim 7 in the preparation of a lithium ion battery.

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