Preparation method of boronized carbon nitride composite lithium manganese iron phosphate positive electrode material

Abstract

The invention relates to the technical field of lithium batteries, and provides a preparation method of boronized carbon nitride composite lithium iron manganese phosphate, which comprises the following steps: adding a nitrogen source and a boron source into lithium iron manganese phosphate core particles, grinding, spray-drying, and carrying out secondary calcination to obtain the boronized carbon nitride composite lithium iron manganese phosphate positive electrode material. According to the method, secondary high-temperature calcination is assisted by spray drying, so that the lithium manganese iron phosphate can be modified by the boronized carbon nitride. Firstly, carbon nitride has high conductivity, boron atoms on a BNC structure are embedded into a carbon nitride skeleton in a substitution or interstitial form, and an sp2 hybrid conductive network is optimized; secondly, the BNC layer is combined with the inner core through a Mn-B-N bond, so that the dissolution of manganese is inhibited; 3, the thickness of the coating layer is accurately regulated and controlled (such as 3-5nm), and the electronic conductivity (10 <-2 >-10 <-3 > S / cm) and the lithium ion diffusion rate (10 <-10 >-10 <-11 > cm < 2 > / s) are balanced; and finally, the hydrophobic surface of the BNC reduces the side reaction of the electrolyte and improves the high-temperature cycling stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of cathode materials for lithium-ion batteries, and particularly relates to a composite material of lithium manganese iron phosphate (LiMn x Fe 1-x PO4, 0 < x < 1) coated with boron-modified carbon nitride (BNC), a preparation method thereof, and an application of the material in high-performance lithium-ion batteries. Background Art

[0002] Lithium manganese iron phosphate (LiMn x Fe 1-x PO4), as an upgraded material of lithium iron phosphate (LiFePO4), can improve the working voltage (about 4.1 V vs. Li+ / Li) and energy density (theoretical specific capacity 170 mAh / g) of the material by introducing manganese element (Mn) to partially replace iron (Fe), while retaining the high safety and cycle stability of the phosphate system. However, LiMn x Fe 1-x PO4, 0 < x < 1 still has the following key problems: poor electronic conductivity: the intrinsic electronic conductivity is only 10 -12 ~10 -13 S / cm, which limits the high-rate charge and discharge performance; manganese dissolution problem: during long-term cycling, Mn 3+ is prone to disproportionation reaction to generate Mn 2+ and dissolve in the electrolyte, resulting in capacity decay; low ion diffusion rate: the one-dimensional diffusion channels of lithium ions in the olivine structure are easily blocked, especially the performance decreases significantly under high voltage and low temperature conditions.

[0003] Existing technical improvement schemes include: (1) Carbon coating technology. Traditional carbon coatings (such as graphite, carbon nanotubes, amorphous carbon) improve the electron transport efficiency through a surface conductive layer. For example, patent CN201510001234.5 discloses a preparation method of carbon-coated lithium manganese iron phosphate, but the binding force between its carbon layer and the material interface is weak, it is easy to fall off during cycling, and the carbon material has insufficient ability to inhibit side reactions of electrolyte decomposition. (2) Carbon nitride coating technology. Carbon nitride (g-C3N4) has advantages such as high chemical stability and corrosion resistance to electrolyte. Patent US20200091456A1 proposes to use g-C3N4 to coat LiFePO4, but its two-dimensional layered structure significantly hinders the diffusion of lithium ions and does not solve the manganese dissolution problem. (3) Boron doping modification technology. Boron atom doping can adjust the electronic structure of carbon materials and enhance interfacial charge transport. For example, patent JP2020156789A discloses a boron-doped carbon-coated lithium iron phosphate material, but it does not combine the framework stability of carbon nitride, resulting in insufficient high-temperature performance.

[0004] Therefore, there is an urgent need to develop modified lithium iron manganese phosphate materials to comprehensively improve the above problems. SUMMARY OF THE INVENTION

[0005] Based on this, in view of the technical problems that the introduction of some current carbon sources does not significantly improve the conductivity of lithium iron manganese phosphate, and the Jahn-Teller effect and the improvement effect of the cycling performance of lithium iron manganese phosphate are also not obvious, it is necessary to provide a boron carbonitride composite lithium iron manganese phosphate cathode material, a preparation method thereof, and an application. 3+ To achieve this object, the present invention adopts the following technical solutions:

[0006] In a first aspect, the present invention provides a boron carbonitride composite lithium iron manganese phosphate, which comprises lithium iron manganese phosphate core particles and a boron carbonitride material coated on the outer side of the lithium iron manganese phosphate core particles.

[0007] The boron carbonitride composite lithium iron manganese phosphate provided by the present invention coats boron carbonitride (BNC) on the surface of the lithium iron manganese phosphate core particles, and the chemical formula is B a C b N c (0.1 < a < 1.0, 6 < b < 8, 7 < c < 9), wherein carbon nitride (g-C3N4) itself has high conductivity, and boron atoms in the BNC structure are embedded in the carbon nitride skeleton in a substitution or interstitial form, optimizing the sp 2 hybrid conductive network. The BNC layer is combined with the core through Mn-B-N bonds to inhibit manganese dissolution; the thickness of the coating layer is precisely controlled (such as 3-5 nm) to balance the electronic conductivity (10 -2 ~10 -3 S / cm) and the lithium ion diffusion rate (10 -10 ~10 -11 cm 2 / s); the hydrophobic surface of BNC reduces the side reaction of the electrolyte and improves the high-temperature cycling stability.

[0008] Preferably, the molar ratio of Mn:Fe in the boron carbonitride composite lithium iron manganese phosphate is (1-4):1, for example, it can be 1:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1 or 4:1, etc., but is not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0009] Preferably, the molar ratio of Li:(Mn + Fe) in the boron carbonitride composite lithium iron manganese phosphate is (1-1.08):1, for example, it can be 1:1, 1.02:1, 1.04:1, 1.06:1 or 1.08:1, etc., but is not limited to the listed values, and other unlisted values within this numerical range are equally applicable.​

[0010] Preferably, the D50 of the lithium manganese iron phosphate matrix core is 50~650nm, such as 50nm, 100nm, 150nm, 200nm, 250nm, 300nm, 350nm, 400nm, 450nm, 500nm, 550nm, 600nm or 650nm, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0011] Preferably, the thickness of the BNC layer is 1 to 20 nm, such as 2 nm, 5 nm, 10 nm, 15 nm or 20 nm, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0012] In a second aspect, the present invention provides a method for preparing the boronized carbon nitride composite lithium manganese iron phosphate as described in the first aspect, the method comprising the following steps: Step 1: Mix lithium source, iron source, manganese source, phosphorus source and carbon source and put them into a sand mill. Add ultrapure water and zirconium oxide grinding balls for sand milling to obtain a mixed slurry with a particle size of 300~600nm. Step 2: Spray dry the above-mentioned mixed slurry to obtain a powdered mixture. Then, calcine the powdered mixture at 380℃~850℃ for 2~8 hours under inert gas protection to obtain lithium manganese iron phosphate core particles.

[0013] Step 3: The lithium manganese iron phosphate core particles, boron source, and nitrogen source are ball-milled in ultrapure water. Then, the resulting mixture is spray-dried and transferred to a tube furnace for secondary calcination at 600~900℃ for 5~10 hours to obtain the boronized carbon nitride composite lithium manganese iron phosphate cathode material.

[0014] Preferably, in step one, the lithium source is one or more of lithium hydroxide, anhydrous lithium carbonate, lithium acetate, lithium nitrate, lithium oxalate, lithium phosphate, and lithium dihydrogen phosphate.

[0015] Preferably, in step one, the phosphorus source is one or more of ammonium dihydrogen phosphate, ammonium phosphate, ammonium hypophosphite, ammonium polyphosphate, lithium dihydrogen phosphate, lithium phosphate, and diammonium hydrogen phosphate.

[0016] Preferably, the iron source in step one is one or more of ferric phosphate, iron oxide red, iron tetroxide, ferric oxalate, ferric sulfate, or ferric chloride.

[0017] Preferably, the manganese source in step one is one or more of manganese dioxide, manganese pentoxide, manganese tetroxide, manganese carbonate, manganese acetate, manganese oxalate, or manganese hydrogen phosphate.

[0018] Preferably, in step one, the mixing ratio of the iron source and the manganese source is such that the total molar amount of iron is mixed with the molar ratio of manganese to one of the following: (1-4):1.

[0019] Preferably, in step one, the mixing ratio of lithium source, iron source and manganese source is: mixed according to one of the following: the molar ratio of lithium to total iron and manganese is (1~1.08):1.

[0020] Preferably, in step one, the carbon source is one or more of glucose, sucrose, citric acid, maltose, starch, polyethylene glycol (PEG), citric acid, or oxalic acid, and the amount used is 15% to 25% of the total mass of the lithium source, manganese source, and iron source.

[0021] Preferably, in step one, the mass of ultrapure water is 100% to 300% of the total mass of lithium source, anhydrous iron phosphate, carbon source, and titanium dioxide; the mass ratio of zirconia grinding beads to the total mass of raw materials is 0.3 to 6:1; the grinding time is 0.5 to 10 hours; the grinding speed is 1500 to 3000 rpm; and the particle size range of the mixed slurry is D50: 300 to 600 nm.

[0022] Preferably, in step two, the spray drying inlet temperature is 180℃~240℃, the outlet temperature is set to 70℃~110℃, the spray drying spray pressure is 0.2MPa~0.3MPa, and the feed rate is 1000~2000ml / h; the calcination conditions are: after introducing nitrogen gas at room temperature for 20~60min, the temperature is raised to 380℃~850℃ at a heating rate of 2~8℃ / min, and held for 2~8h.

[0023] Preferably, in step three, the nitrogen source is one or more of dicyandiamide, urea, melamine, and polyacetonitrile, and the amount used is 1% to 10% of the total mass of the raw materials.

[0024] Preferably, in step three, the boron source is one or more of boric acid and boron oxide, and the amount used is 0.01% to 0.1% of the mass of the nitrogen source.

[0025] Preferably, in step three, the ratio of the mass of the zirconia grinding beads to the total mass of the raw materials is 0.3~6:1, the ball milling time is 2~6 hours, and the sand milling speed is 300~1000 rpm.

[0026] Preferably, in step three, the secondary calcination conditions are as follows: after passing nitrogen gas through the furnace at room temperature for 20-60 minutes, the temperature is increased to 600-900℃ at a heating rate of 2-8℃ / min, and held for 5-10 hours.

[0027] The method for preparing boronized carbon nitride composite lithium manganese iron phosphate provided by this invention involves adding carbon nitride material during the mixing of lithium manganese iron phosphate raw materials, enabling thorough coating of the lithium manganese iron phosphate core particles. The outer coating layer improves the conductivity and electrochemical activity of lithium manganese iron phosphate, thus enhancing the performance of the electrode material, including higher discharge capacity and better cycle stability. Furthermore, the outer boronized carbon nitride layer provides an additional protective layer, preventing direct contact between lithium manganese iron phosphate and the electrolyte, reducing material oxidation and electrolyte corrosion, thereby extending the lifespan of the electrode material. Moreover, carbon nitride, as a highly conductive material, effectively increases the diffusion rate of lithium ions in the electrode material. This helps improve the reaction kinetics of the electrode material, reducing resistance during battery charging and discharging, thereby increasing the battery's energy density and power density. Furthermore, the boron (B) on the boronized g-C3N4 structure can effectively anchor to transition metal elements in the bulk phase of lithium manganese iron phosphate, significantly enhancing electron transport between them and inhibiting Mn dissolution.

[0028] In this application, carbon nitride itself has high electrical conductivity, and boron atoms on the BNC structure are embedded in the carbon nitride framework in a substituted or interstitial manner, optimizing sp 2 Hybrid conductive network; BNC layer binds to the core via Mn-BN bonds to suppress manganese leaching; coating layer thickness is precisely controlled (e.g., 3~5nm) to balance electronic conductivity (10). -2 ~10 -3 S / cm) and lithium-ion diffusion rate (10⁻¹⁰~10⁻¹¹ cm⁻¹) 2 / s); The hydrophobic surface of BNC reduces electrolyte side reactions and improves high-temperature cycling stability. Attached Figure Description

[0029] Figure 1 This is a SEM image of the material obtained in Embodiment 1 of this application.

[0030] Figure 2 This is a graph showing the change in charge and discharge capacity after 500 cycles at 1C in Embodiment 1 of this application. Detailed Implementation

[0031] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0032] To better understand the present invention, the following description, in conjunction with embodiments and related drawings, further illustrates the content of the present invention, but the content of the present invention is not limited to the following embodiments.

[0033] Unless otherwise specified, the reagents, methods and equipment used in this invention are conventional reagents, methods and equipment in this technical field.

[0034] Unless otherwise specified, all reagents and materials used in the following examples are commercially available.

[0035] Example 1 A method for preparing a boronized carbon nitride composite lithium manganese iron phosphate cathode material, wherein the lithium manganese iron phosphate is LiMn 0.6 Fe 0.4 PO4 includes the following steps: (1) First, take 1413ml of deionized water and add it to the premixing tank. Add 156.71g of lithium carbonate (Li2CO3) and 304.12g of phosphoric acid (H3PO4) to the premixing tank and stir for 20min to obtain lithium phosphate solution. Then weigh 250g, 180g, 30g, 5g and 20g of iron phosphate, manganese tetroxide, glucose, titanium dioxide and PEG respectively. Stir at 20℃ for 1h to obtain solid-liquid mixture. Then transfer it to a sand mill for grinding. Grind at 2500rpm for 1.5h until the particle size D50 of the mixed slurry reaches 360nm. (2) The mixed slurry was spray-dried. The spray drying inlet temperature was 220℃, the outlet temperature was 85℃, the spray pressure was 0.25MPa, and the feed rate was 1500ml / h to obtain a precursor powder. (3) Then place it in a box furnace filled with nitrogen and heat treat it at 700°C for 6 hours to obtain lithium manganese iron phosphate particles; (4) Next, weigh 400g, 50g and 5g of lithium manganese iron phosphate particles, dicyandiamide and boric acid respectively and add them to 735ml of ultrapure water. After premixing evenly, transfer to a ball mill for grinding. Grind at 2500rpm for 5h until a nano slurry with D50 of 400nm is formed. (5) The nano slurry is spray-dried. The spray drying inlet temperature is 220℃, the outlet temperature is 85℃, the spray pressure is 0.25MPa, and the feed rate is 1500ml / h. (6) The material is then placed in a nitrogen-filled box furnace and nitrogen is introduced at room temperature for 30 minutes. The temperature is then increased to 650°C at a rate of 3°C / min, and heat-treated at 650°C for 5 hours. Finally, it is crushed into powder with a D50 of approximately 0.5 μm, which is the boronized carbon nitride composite lithium manganese iron phosphate cathode material. The SEM image of the obtained material is shown below. Figure 1 As shown.

[0036] Example 2 The only difference between this embodiment and embodiment 1 is that the molar ratio of manganese tetroxide and ferric phosphate in step (1) is 6.5:3.5. All other conditions and parameters are exactly the same as in embodiment 1.

[0037] Example 3 The only difference between this embodiment and embodiment 1 is that the boric acid in step (4) is replaced with boron oxide, while the other conditions and parameters are exactly the same as in embodiment 1.

[0038] Comparative Example 1 The only difference between this comparative example and Example 1 is that: it does not coat the carbon nitride with boride, and dicyandiamide and boric acid are not added in step (4). Other conditions and parameters are exactly the same as in Example 1.

[0039] Comparative Example 2 The only difference between this comparative example and Example 1 is that it is not coated with carbon nitride, and 5g of boric acid is added in step (1). Other conditions and parameters are exactly the same as those in Comparative Example 1.

[0040] Comparative Example 3 The only difference between this comparative example and Example 1 is that the nitrided carbon boron was not used, and step (4) does not contain boric acid. All other conditions and parameters are exactly the same as in Example 1.

[0041] Physicochemical performance testing: The composite materials prepared in each embodiment and comparative example were subjected to physical and electrochemical performance tests. The carbon content was defined as the percentage of carbon per unit mass of the material, and was detected using an infrared carbon-sulfur analyzer. The electrochemical performance of each embodiment and comparative example was experimentally studied using CR2032 button half-cells. The specific preparation process of the positive electrode is as follows: The composite materials prepared using the various embodiments and comparative examples were assembled into button cells as follows: The composite materials, acetylene black, and binder PVDF (polyvinylidene fluoride) were weighed in a mass ratio of 80:10:10, added to an agate mortar, and ground thoroughly until uniformly mixed; an appropriate amount of N-methylpyrrolidone was added dropwise to the above mixture, and the mixture was ground again until a uniform slurry of a certain viscosity was formed; the slurry was coated onto dry carbon-coated aluminum foil using a scraper, and first dried in a forced-air drying oven at 80°C for 3 hours, and then dried in a vacuum environment at 120°C for 12 hours. After the temperature dropped to room temperature, the electrode sheet was removed; the removed electrode sheet was compacted using a roller press, and then cut into circular positive electrode sheets with a diameter of 14 mm using a punching machine, ready for use.

[0042] Lithium foil was selected as the negative electrode, and the electrolyte was 1.0 mol / L LiPF6 / (EC+EMC+DMC) (where LiPF6 is lithium hexafluorophosphate, EC is ethylene carbonate, EMC is ethyl methyl carbonate, and DMC is dimethyl carbonate, with a volume ratio of EC, EMC, and DMC of 1:1:1). A Celgard 2400 polypropylene microporous membrane was used as the separator. All materials, including the positive electrode, negative electrode, electrolyte, and separator, were assembled into a CR2032 coin cell in a glove box filled with high-purity argon gas (H2O < 0.1 ppm, O2 < 0.1 ppm). The cells were then sealed using a sealing machine. Finally, the assembled CR2032 coin cells were allowed to stand at room temperature for 12 hours for activation before use. The test results are shown in Table 1. The battery assembled from the boronized, nitrided, carbon phosphate, and lithium manganese iron phosphate prepared in Example 1 exhibits the following 1C cycle performance: Figure 2 As shown, from Figure 2 It can be seen that the cathode material obtained in Example 1 has excellent cycle performance, and the discharge specific capacity hardly decreases within 500 cycles.

[0043] The test results of the above embodiments and comparative examples are shown in Table 1. Table 1 Electrochemical performance test results

[0044] As shown in Table 1, the modified lithium manganese iron phosphate material prepared by this invention exhibits excellent capacity and cycle performance. Furthermore, a comparison between Example 1 and Comparative Examples 1-3 demonstrates that this invention, by employing a carbon boride-nitrogenide composite lithium manganese iron phosphate, can significantly improve the rate performance and cycle performance of the cathode material, as well as increase the discharge specific capacity. This is because carbon nitride (g-C3N4) itself has high conductivity, and boron atoms in the BNC structure are embedded in the carbon nitride framework in a substituted or interstitial manner, optimizing the sp² hybrid conductive network. The BNC layer is bonded to the core through Mn-BN bonds, suppressing manganese dissolution; the hydrophobic surface of BNC reduces electrolyte side reactions and improves cycle stability.

[0045] The above embodiments are merely illustrative examples and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations; therefore, any obvious variations or modifications derived therefrom remain within the scope of protection of this invention.

Claims

1. A boronized carbon nitride composite lithium iron phosphate cathode material, characterized in that, The modified lithium iron manganese phosphate material includes lithium iron manganese phosphate and a carbon coating layer coated on the surface of the lithium iron manganese phosphate; wherein, the chemical formula of the lithium iron manganese phosphate is LiFe x Mn 1-x PO4, 0 < x < 1; the coating layer is boron nitride carbon, and the chemical general formula is B a C b N c (0.1 < a < 1.0, 6 < b < 8, 7 < c < 9).

2. The boronized carbon nitride composite lithium manganese iron phosphate according to claim 1, characterized in that... ； The molar ratio of Mn:Fe in the boronized carbon nitride composite lithium manganese iron phosphate is (1-4):1; The molar ratio of Li:(Mn+Fe) in the boronized carbon nitride composite lithium manganese iron phosphate is (1~1.08):1; The D50 of the lithium manganese iron phosphate matrix core is 50~650nm; The thickness of the BNC layer is 1–20 nm.

3. A method for preparing the boronized carbon nitride-coated composite lithium manganese iron phosphate according to claim 1, characterized in that, The preparation method includes the following steps: Step 1: Mix lithium source, iron source, manganese source, phosphorus source and carbon source and put them into a sand mill. Add ultrapure water and zirconium oxide grinding balls for sand milling to obtain a mixed slurry with a particle size of 300~600nm. Step 2: Spray dry the above mixed slurry to obtain a powdered mixture. Then, calcine the powdered mixture at 380℃~850℃ for 2~8 hours under inert gas protection to obtain lithium manganese iron phosphate core particles. Step 3: Place the lithium manganese iron phosphate core particles, boron source, and nitrogen source into ultrapure water for ball milling. Then, spray dry the resulting mixture and transfer it to a tube furnace for secondary calcination at a temperature of 600~900℃ for 5~10 hours to obtain the boronized carbon nitride composite lithium manganese iron phosphate cathode material.

4. The preparation method of the boronized and nitrided carbon-coated composite lithium manganese iron phosphate cathode material as described in claim 3, characterized in that, The lithium source is one or more of lithium hydroxide, anhydrous lithium carbonate, lithium acetate, lithium nitrate, lithium oxalate, lithium phosphate, and lithium dihydrogen phosphate. The phosphorus source is one or more of the following: ammonium dihydrogen phosphate, ammonium phosphate, ammonium hypophosphite, ammonium polyphosphate, lithium dihydrogen phosphate, lithium phosphate, and diammonium hydrogen phosphate; The iron source is one or more of the following: iron phosphate, iron oxide red, iron tetroxide, iron oxalate, iron sulfate, or iron chloride. The manganese source is one or more of manganese dioxide, manganese pentoxide, manganese tetroxide, manganese carbonate, manganese acetate, manganese oxalate, or manganese hydrogen phosphate. The carbon source is one or more of glucose, sucrose, citric acid, maltose, starch, polyethylene glycol (PEG), citric acid or oxalic acid, and the amount used is 15% to 25% of the total mass of lithium source, manganese source and iron source.

5. The preparation method of the boron nitride carbon-coated composite lithium manganese iron phosphate cathode material as described in claim 3, characterized in that, In step one, the mass of ultrapure water is 100% to 300% of the total mass of lithium source, anhydrous iron phosphate, carbon source, and titanium dioxide; the mass ratio of zirconia grinding beads to the total mass of raw materials is 0.3 to 6:1; the grinding time is 0.5 to 10 hours; the grinding speed is 1500 to 3000 rpm; and the particle size range of the mixed slurry is D50: 300 to 600 nm.

6. The preparation method of the boronized and nitrided carbon-coated composite lithium manganese iron phosphate cathode material as described in claim 3, characterized in that, In step two, the spray drying inlet temperature is 180℃~240℃, the outlet temperature is set to 70℃~110℃, the spray drying pressure is 0.2MPa~0.3MPa, and the feed rate is 1000~2000ml / h; the calcination conditions are: after introducing nitrogen gas at room temperature for 20~60min, the temperature is raised to 380℃~850℃ at a heating rate of 2~8℃ / min, and held for 2~8h.

7. The preparation method of the boron nitride carbon-coated composite lithium manganese iron phosphate cathode material as described in claim 3, characterized in that, The nitrogen source is one or more of dicyandiamide, urea, melamine, and polyacetonitrile, and the amount used is 1% to 10% of the total mass of the raw materials; The boron source is one or more of boric acid and boron oxide, and the amount used is 0.01% to 0.1% of the mass of the nitrogen source.

8. The preparation method of the boron nitride carbon-coated composite lithium manganese iron phosphate cathode material as described in claim 3, characterized in that, In step three, the ratio of the mass of the zirconia grinding beads to the total mass of the raw materials is 0.3~6:1, the ball milling time is 2~6 hours, and the sand milling speed is 300~1000 rpm.

9. The preparation method of the boron nitride carbon-coated composite lithium manganese iron phosphate cathode material as described in claim 3, characterized in that, In step three, the secondary calcination conditions are as follows: after passing nitrogen gas through the furnace at room temperature for 20-60 minutes, the temperature is increased to 600-900℃ at a heating rate of 2-8℃ / min, and held for 5-10 hours.

10. A boronized and nitride-coated carbon-encapsulated composite lithium manganese iron phosphate cathode material, characterized in that, It is prepared by the method for preparing boronized carbon nitride-coated composite lithium manganese iron phosphate cathode material according to any one of claims 1-9.

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