Surface-functionalized lithium manganese iron phosphate, positive pole piece containing surface-functionalized lithium manganese iron phosphate and battery containing surface-functionalized lithium manganese iron phosphate

Abstract

The invention belongs to the technical field of batteries, and provides surface-functionalized lithium manganese iron phosphate, a positive pole piece containing the surface-functionalized lithium manganese iron phosphate and a battery, the surface-functionalized lithium manganese iron phosphate comprises carbon-coated lithium manganese iron phosphate, and the surface of the carbon-coated lithium manganese iron phosphate has oxygen-containing groups and nitrogen-containing groups. The transmission efficiency of lithium ions can be improved through oxygen-containing groups, electrolyte decomposition is inhibited, and side reactions are reduced; meanwhile, the electron conductivity of the surface of the material can be improved through nitrogen-containing groups, the structural stability of the material is enhanced, dissolution of manganese is inhibited, and then the capacity and the cycling stability are effectively improved. Furthermore, the surface-functionalized lithium manganese iron phosphate is matched with brown alginate as a binder to prepare a positive pole piece, so that alginate can be chelated with Mn < 2 + >, active substances are more effectively prevented from being lost into an electrolyte, and the improvement of the cycle performance is further ensured.

Description

Technical Field

[0001] This invention belongs to the field of battery technology and relates to a surface-functionalized lithium manganese iron phosphate, a positive electrode containing the same, and a battery. Background Technology

[0002] In the selection of cathode materials for lithium-ion batteries, lithium iron phosphate (LiFePO4) with an olivine structure is widely used due to its excellent safety, high cycle stability, and environmental friendliness. However, the one-dimensional lithium-ion conduction path of this material limits the diffusion rate of lithium ions, thus affecting the overall performance of the battery. Especially under fast charging or high-current discharging conditions, the transport of lithium ions within the material and at the interface is hindered, leading to a significant increase in interfacial impedance. Furthermore, lithium iron phosphate itself possesses semiconductor properties, with a conductivity of approximately 1 × 10⁻⁶. -9 The S / cm ratio further limits its potential for application in high-performance batteries.

[0003] To overcome these problems, researchers turned their attention to lithium manganese iron phosphate (LMFP), a novel cathode material obtained by partially replacing iron in lithium iron phosphate with manganese. Theoretically, LMFP combines the advantages of both lithium iron phosphate and lithium manganese phosphate, retaining good thermal stability and safety, and promising to optimize battery energy density by adjusting the manganese-to-iron ratio. However, in practical applications, it was found that LMFP has a lower conductivity than lithium iron phosphate, only about 1 × 10⁻⁶. -13 The low conductivity (S / cm) significantly limits its commercial application potential. Low conductivity means that electrons cannot move efficiently within the material, thus affecting the battery's charge / discharge efficiency and power output.

[0004] Besides conductivity issues, LMFPs also face the risk of manganese dissolution under high temperature or overcharge conditions. Manganese dissolution leads to damage to the surface structure of the cathode material and triggers a series of side reactions. These side reactions exacerbate the increase in interfacial impedance, severely affecting the long-term stability and lifespan of the battery. Especially in applications such as electric vehicles that require frequent fast charging, effectively suppressing manganese dissolution while improving the conductivity of LMFPs has become a critical technical challenge that urgently needs to be addressed.

[0005] Besides the inherent potential and properties of the cathode material itself, its performance in the cathode electrode sheet is also related to other materials such as binders, conductive agents, and the electrode sheet manufacturing process. Among these, commonly used binders such as polyvinylidene fluoride (PVDF) are not conductive materials themselves; their dispersion state and their interaction with the cathode material and electrolyte have a significant impact on electron and ion conduction. Furthermore, the use of PVDF requires dissolving it in N-methylpyrrolidone (NMP), a toxic and expensive organic solvent that readily releases toxic gases during battery thermal runaway. Its use and disposal also increase equipment investment and wastewater treatment costs. Summary of the Invention

[0006] In view of the problems existing in the prior art, the purpose of this invention is to provide a surface-functionalized lithium manganese iron phosphate, a positive electrode containing the same, and a battery. The surface-functionalized lithium manganese iron phosphate includes carbon-coated lithium manganese iron phosphate, the surface of which has oxygen-containing groups and nitrogen-containing groups. The oxygen-containing groups can improve lithium-ion transport efficiency, inhibit electrolyte decomposition, and reduce side reactions; simultaneously, the nitrogen-containing groups can improve the electronic conductivity of the material surface, enhance the structural stability of the material, and inhibit manganese dissolution, thereby effectively improving capacity and cycle stability.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a surface-functionalized lithium manganese iron phosphate, including carbon-coated lithium manganese iron phosphate, wherein the surface of the carbon-coated lithium manganese iron phosphate has oxygen-containing groups and nitrogen-containing groups.

[0009] In this invention, oxygen-containing groups and nitrogen-containing groups are introduced onto the surface of carbon-coated lithium manganese iron phosphate. The oxygen-containing groups can improve the lithium-ion transport efficiency, inhibit electrolyte decomposition, and reduce side reactions; the nitrogen-containing groups can improve the electronic conductivity of the material surface, enhance the structural stability of the material, and inhibit the dissolution of manganese. Therefore, the presence of oxygen-containing groups and nitrogen-containing groups can effectively improve the electrochemical performance of the material, such as capacity and cycle stability.

[0010] It should be noted that the term "surface" refers to the exposed outer surface of carbon-coated lithium manganese iron phosphate as a whole material. In the area covered by the carbon coating layer, it refers to the carbon surface covered by the carbon coating layer. If there is an area not covered by the carbon coating layer, it refers to the surface of the lithium manganese iron phosphate component.

[0011] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. The technical objectives and beneficial effects of the present invention can be better achieved and realized through the following technical solutions.

[0012] As a preferred embodiment of the present invention, in the carbon-coated lithium manganese iron phosphate, the chemical formula of lithium manganese iron phosphate includes LiMn. x Fe 1-x PO4, 0.5 < x ≤ 0.8, for example, x can be 0.52, 0.55, 0.58, 0.6, 0.63, 0.65, 0.68, 0.7, 0.72, 0.75, 0.78 or 0.8, etc., preferably x = 0.6.

[0013] Preferably, the particle size D of the carbon-coated lithium manganese iron phosphate is... 50 The wavelength range is 550~950nm, such as 550nm, 580nm, 600nm, 630nm, 650nm, 680nm, 700nm, 720nm, 750nm, 780nm, 800nm, 830nm, 850nm, 880nm, 900nm, 920nm or 950nm, etc., and the carbon coating thickness is 15~30nm, such as 15nm, 18nm, 20nm, 23nm, 25nm, 28nm or 30nm, etc.

[0014] In a second aspect, the present invention provides a method for preparing surface-functionalized lithium manganese iron phosphate as described in the first aspect, comprising the following steps:

[0015] Provides carbon-coated lithium manganese iron phosphate;

[0016] Carbon-coated lithium manganese iron phosphate is mixed with an ammonifying agent and subjected to ammonification treatment. Then it is mixed with an oxidizing agent and subjected to oxidation treatment. Finally, it is subjected to heat treatment to obtain surface-functionalized lithium manganese iron phosphate.

[0017] This invention directly uses pre-formed carbon-coated lithium manganese iron phosphate (LMFP). For example, the carbon coating layer is formed in situ during the carbothermic reduction process of LMFP synthesis. The surface is then subjected to ammoniation and oxidation treatments using an ammonifying agent and an oxidizing agent, thereby introducing nitrogen-containing and oxygen-containing groups onto the LMFP surface. It should be noted that, for optimal results, this invention performs ammoniation treatment before oxidation treatment.

[0018] As a preferred embodiment of the present invention, the ammoniating agent in the ammoniating agent solution includes at least one of ammonia, urea, ammonium nitrate, pyridine, or polyaniline.

[0019] Preferably, the mass percentage concentration of the ammoniating agent solution is 5% to 15%, such as 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, or 15%.

[0020] Preferably, in the ammoniation treatment, the mass ratio of the carbon-coated lithium manganese iron phosphate to the ammoniation agent is 1:(10~15), for example, 1:10, 1:10.5, 1:11, 1:11.5, 1:12, 1:12.5, 1:13, 1:13.5, 1:14, 1:14.5 or 1:15, etc.

[0021] Preferably, the temperature of the ammoniation treatment is 30~40℃, such as 30℃, 32℃, 34℃, 36℃, 38℃ or 40℃; the stirring speed is 400~800rpm, such as 400rpm, 430rpm, 450rpm, 480rpm, 500rpm, 530rpm, 550rpm, 580rpm, 600rpm, 630rpm, 650rpm, 680rpm, 700rpm, 720rpm, 750rpm, 780rpm or 800rpm; the time is 2~6h, such as 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h or 6h; the pH is 8.8~9.2, such as 8.8, 8.85, 8.9, 8.95, 9, 9.05, 9.1, 9.15 or 9.2.

[0022] Preferably, the oxidant in the oxidant solution includes at least one of hydrogen peroxide, sodium hypochlorite, ammonium persulfate, or potassium permanganate.

[0023] Preferably, the mass percentage concentration of the oxidant solution is 10% to 40%, for example, 10%, 13%, 15%, 18%, 20%, 23%, 25%, 28%, 30%, 32%, 35%, 38%, or 40%.

[0024] Preferably, in the oxidation treatment, the volume ratio of the oxidant solution to the ammoniating agent solution is (0.8~1.2):1, for example, 0.8:1, 0.83:1, 0.85:1, 0.9:1, 0.92:1, 0.95:1, 0.98:1, 1:1, 1.05:1, 1.08:1, 1.1:1, 1.13:1, 1.16:1, or 1.2:1, etc.

[0025] Preferably, in the oxidation treatment, the mass ratio of the carbon-coated lithium manganese iron phosphate to the oxidant solution is 1:(10~15), for example, 1:10, 1:10.5, 1:11, 1:11.5, 1:12, 1:12.5, 1:13, 1:13.5, 1:14, 1:14.5 or 1:15, etc.

[0026] Preferably, after the oxidation treatment, the parts are first washed, dried and ground in sequence, and then subjected to the heat treatment.

[0027] Preferably, the detergent used for washing includes ethanol and deionized water, and the washing process ends when the wash water is neutral.

[0028] Preferably, the drying temperature is 90~110℃, such as 90℃, 95℃, 98℃, 100℃, 102℃, 105℃, 108℃ or 110℃, and the time is 8~12h, such as 8h, 8.5h, 9h, 9.5h, 10h, 10.5h, 11h, 11.5h or 12h.

[0029] Preferably, the heat treatment is carried out in an inert atmosphere, and the temperature of the heat treatment is 300~600℃, such as 300℃, 330℃, 350℃, 380℃, 400℃, 420℃, 450℃, 480℃, 500℃, 530℃, 550℃, 580℃ or 600℃, etc., and the time is 2~6h, such as 2h, 2.3h, 2.5h, 2.8h, 3h, 3.3h, 3.6h, 4h, 4.5h, 4.8h, 5h, 5.3h, 5.5h, 5.8h or 6h, etc.

[0030] As a preferred embodiment of the present invention, the method for preparing the carbon-coated lithium manganese iron phosphate includes:

[0031] A lithium source, phosphorus source, manganese source and iron source are mixed, and after adjusting the pH, a hydrothermal reaction is carried out to obtain a precursor; the precursor is then subjected to chemical vapor deposition using a gaseous carbon source to obtain carbon-coated lithium manganese iron phosphate.

[0032] Preferably, the lithium source and the phosphorus source are premixed to obtain a lithium phosphate solution, and the manganese source and the iron source are premixed to obtain a metal salt solution. After the lithium phosphate solution and the metal salt solution are mixed, the pH is adjusted.

[0033] Preferably, the pH adjuster used to adjust the pH includes a sodium hydroxide solution with a pH range of 5 to 7, such as 5, 5.3, 5.5, 5.8, 6, 6.2, 6.5, 6.8 or 7.

[0034] Preferably, the hydrothermal reaction is carried out in a high-pressure reactor at a temperature of 160~200℃, such as 160℃, 170℃, 180℃, 190℃ or 200℃, for a time of 6~10h, such as 6h, 6.5h, 7h, 7.5h, 8h, 8.5h, 9h, 9.5h or 10h.

[0035] Preferably, the gaseous carbon source includes acetylene.

[0036] Preferably, the temperature of the chemical vapor deposition is 700~800℃, such as 700℃, 710℃, 720℃, 730℃, 740℃, 750℃, 760℃, 770℃, 780℃, 790℃ or 800℃, and the time is 4~8h, such as 4h, 4.3h, 4.5h, 4.8h, 5h, 5.5h, 5.8h, 6h, 6.3h, 6.8h, 7h, 7.2h, 7.5h, 7.8h or 8h.

[0037] Thirdly, the present invention provides a positive electrode sheet, wherein the positive electrode active material in the positive electrode sheet includes the surface-functionalized lithium manganese iron phosphate described in the first aspect.

[0038] As a preferred embodiment of the present invention, the binder in the positive electrode sheet includes alginate.

[0039] Preferably, the alginate includes sodium alginate.

[0040] Alginate, derived from marine brown algae (such as kelp and giant kelp), is a renewable resource, aligning with the green development trend of new energy materials. Alginate is non-toxic and harmless, preventing the release of toxic gases during battery thermal runaway. Alginate can be used in water-based dispersion systems to replace organic solvents, reducing equipment investment and wastewater treatment costs. Furthermore, when combined with conductive agents as a binder, alginate can form a three-dimensional conductive network, further improving the electron transport efficiency of the cathode material. More importantly, as a binder in the cathode sheet, alginate can interact with Mn... 2+ Chelation prevents the loss of positive electrode active material into the electrolyte, which helps improve cycle performance.

[0041] Preferably, based on the total mass of the positive electrode active material, binder, and conductive agent as 100%, the lithium manganese iron phosphate accounts for 93% to 97%, such as 93%, 93.5%, 94%, 94.5%, 95%, 95.5%, 95.8%, 96%, 96.3%, 96.8%, or 97%, etc., and the alginate accounts for 1% to 3%, such as 1%, 1.3%, 1.5%, 1.8%, 2%, 2.3%, 2.5%, 2.8%, or 3%, etc.

[0042] Preferably, the compaction density of the positive electrode sheet is 2.1~2.4 mg / cm³. 3 For example, 2.1 mg / cm 3 2.15 mg / cm 3 2.2 mg / cm 3 2.25 mg / cm 3 2.3 mg / cm 3 2.33 mg / cm 3 2.35 mg / cm 32.38 mg / cm 3 Or 2.4 mg / cm 3 wait.

[0043] Fourthly, the present invention provides a method for manufacturing the positive electrode sheet described in the third aspect, the method comprising:

[0044] Alginate and water are mixed to form an alginate binder solution. The alginate binder solution is then mixed with the positive electrode active material and a conductive agent and ball-milled to obtain a slurry. The slurry is coated onto a current collector, dried, and then rolled and die-cut to obtain the positive electrode sheet.

[0045] As a preferred technical solution of the present invention, the mass percentage concentration of the alginate binder solution is 2% to 4%, such as 2%, 2.3%, 2.5%, 2.8%, 3%, 3.3%, 3.5%, 3.8% or 4%.

[0046] Preferably, the ball milling time is 5 to 20 minutes, such as 5 minutes, 8 minutes, 10 minutes, 13 minutes, 15 minutes, 18 minutes, or 20 minutes.

[0047] Preferably, the drying temperature is 100~110℃, such as 100℃, 102℃, 104℃, 106℃, 108℃ or 110℃, and the time is 3~5h, such as 3h, 3.3h, 3.5h, 3.8h, 4h, 4.2h, 4.5h, 4.8h or 5h.

[0048] Fifthly, the present invention provides a battery comprising the positive electrode sheet described in the third aspect.

[0049] It should be noted that, due to space limitations and to avoid redundancy, this invention does not exhaustively list all point values ​​within the above numerical range, but it is not limited to the listed values ​​either; other unlisted values ​​within the above numerical range are also applicable.

[0050] Compared with existing technical solutions, the present invention has at least the following beneficial effects:

[0051] In the surface-functionalized lithium manganese iron phosphate of the present invention, oxygen-containing groups and nitrogen-containing groups are introduced on the surface. The oxygen-containing groups can improve the lithium ion transport efficiency, inhibit electrolyte decomposition, and reduce side reactions; the nitrogen-containing groups can improve the electronic conductivity of the material surface, enhance the structural stability of the material, and inhibit the dissolution of manganese, which is beneficial to improving the capacity and cycle performance of the material.

[0052] Furthermore, this invention uses surface-functionalized lithium manganese iron phosphate as the positive electrode material and incorporates alginate extracted from brown algae as a binder to obtain the positive electrode sheet. The alginate can react with Mn...2+ Chelation prevents the loss of active substances into the electrolyte, further ensuring improved cycle performance. Detailed Implementation

[0053] The technical solution of the present invention will be further illustrated below through specific embodiments.

[0054] Those skilled in the art will understand that the embodiments described are merely illustrative of the invention and should not be construed as limiting the invention.

[0055] Example 1

[0056] This embodiment provides a surface-functionalized lithium manganese iron phosphate, including carbon-coated lithium manganese iron phosphate, wherein the surface of the carbon-coated lithium manganese iron phosphate has oxygen-containing groups and nitrogen-containing groups; wherein the carbon-coated lithium manganese iron phosphate has the chemical formula LiMn. 0.6 Fe 0.4 PO4; the particle size D of the carbon-coated lithium manganese iron phosphate 50 The wavelength is 720nm, and the carbon coating thickness is 22nm.

[0057] The preparation method of the surface-functionalized lithium manganese iron phosphate includes:

[0058] S1. At room temperature, add 0.5 mol of lithium oxalate (Li2C2O4) and 1 mol of phosphoric acid (H3PO4) to 300 mL of deionized water and stir with a magnetic stirrer for 20 min to obtain a lithium phosphate solution. Add 0.6 mol of manganese chloride (MnCl2) and 0.4 mol of ferrous chloride (FeCl2) to 100 mL of deionized water and stir with a magnetic stirrer for 20 min to obtain a metal salt solution. Finally, combine the lithium phosphate solution with the metal salt solution. The solutions were mixed and the pH was adjusted to 6.5 with sodium hydroxide solution. Then, the mixture was transferred to a high-pressure reactor and subjected to hydrothermal reaction at 180°C for 10 hours. After the reaction was completed, the suspension was cooled to room temperature and the supernatant was removed. The precipitate was centrifuged at 5000 rpm for 10 minutes and washed three times with water and ethanol alternately. The centrifuged precipitate was then dried in a drying oven at 60°C for 5 hours and then in a vacuum drying oven at 80°C for 4 hours to obtain the precursor of lithium manganese iron phosphate.

[0059] S2. After grinding the dried precursor, place it in a chemical vapor deposition (CVD) apparatus. Under the protective gas argon, introduce the organic carbon source acetylene into the CVD apparatus and perform CVD at 800°C for 8 hours to obtain carbon-coated lithium manganese iron phosphate.

[0060] S3. Prepare a 30% hydrogen peroxide solution as the oxidant solution and a 10% ammonia solution as the ammonifying agent solution. First, weigh and mix the carbon-coated lithium manganese iron phosphate with the ammonia solution at a mass ratio of 1:15. Then, stir at 600 rpm for 4 hours at 35°C to perform ammonification treatment and adjust the pH to around 9 to obtain a mixed solution. Then, add the hydrogen peroxide solution and the previously added ammonia solution to the above mixed solution at a volume ratio of 1:1 and stir together to perform oxidation treatment. After stirring, wash with ethanol and deionized water until the solution is neutral. Then, dry in an oven at 100°C for 10 hours. After drying, grind the material into uniform particles. Finally, heat treat at 450°C for 4 hours in an inert atmosphere to obtain surface-functionalized lithium manganese iron phosphate.

[0061] Example 2

[0062] This embodiment refers to Example 1 for preparing surface-functionalized lithium iron manganese phosphate. The difference between this embodiment and Example 1 is that in step S3 of this embodiment, the mass ratio of carbon-coated lithium iron manganese phosphate to ammonia solution is adjusted from 1:15 to 1:5. Apart from the above differences, the other materials, processes and conditions used are strictly consistent with those in Example 1.

[0063] Example 3

[0064] This embodiment refers to Example 1 for preparing surface-functionalized lithium iron manganese phosphate. The difference between this embodiment and Example 1 is that in step S3 of this embodiment, the amount of hydrogen peroxide solution is adjusted, and the mass ratio of hydrogen peroxide solution to the added ammonia solution is adjusted from 1:1 to 1:0.33, so that the mass ratio of the carbon-coated lithium iron manganese phosphate to the oxidant solution is adjusted from 1:15 to 1:5. Apart from the above differences, the other materials, process operations and conditions used are strictly consistent with those of Example 1.

[0065] Example 4

[0066] This embodiment refers to Example 1 for preparing surface-functionalized lithium iron manganese phosphate. The difference between this embodiment and Example 1 is that in step S3 of this embodiment, the amount of hydrogen peroxide solution is adjusted, and the mass ratio of hydrogen peroxide solution to the added ammonia solution is adjusted from 1:1 to 1.33, so that the mass ratio of the carbon-coated lithium iron manganese phosphate to the oxidant solution is adjusted from 1:15 to 1:20. Apart from the above differences, the other materials, process operations and conditions used are strictly consistent with those of Example 1.

[0067] Comparative Example 1

[0068] This comparative example prepares surface-functionalized lithium iron manganese phosphate according to Example 1. The difference between this comparative example and Example 1 is that in step S3 of this comparative example, no ammonifying agent solution is used and no ammonification treatment is performed. Instead, the carbon-coated lithium iron manganese phosphate is directly oxidized. Apart from the above differences, the other materials, process operations and conditions used are strictly consistent with those of Example 1.

[0069] Comparative Example 2

[0070] This comparative example prepares surface-functionalized lithium iron manganese phosphate according to Example 1. The difference between this comparative example and Example 1 is that in step S3 of this comparative example, no oxidant solution is used and no oxidation treatment is performed. Instead, only the carbon-coated lithium iron manganese phosphate is aminated. Apart from the above differences, the other materials, process operations and conditions used are strictly consistent with those of Example 1.

[0071] Comparative Example 3

[0072] This comparative example prepares surface-functionalized lithium iron manganese phosphate according to Example 1. The difference between this comparative example and Example 1 is that this comparative example does not use any ammonifying agent solution or oxidizing agent solution, and does not perform ammonification and oxidation treatment, that is, step S3 is omitted. The carbon-coated lithium iron manganese phosphate obtained in step S2 is used for subsequent testing. Apart from the above differences, the other materials, process operations and conditions used are strictly consistent with those in Example 1.

[0073] Application Example 1

[0074] This application example provides a positive electrode sheet and a battery containing the same. The positive electrode material of the positive electrode sheet is surface-functionalized lithium iron manganese phosphate obtained in any one of Examples 1-10 or Comparative Examples 1-2, or carbon-coated lithium iron manganese phosphate in Comparative Example 3. The binder in the positive electrode sheet is alginate. The manufacturing method of the positive electrode sheet and the assembly method of the battery include:

[0075] Sodium alginate powder was weighed and dissolved in deionized water at a ratio of 3%. The solution was heated and stirred at 35°C until dissolved to obtain an alginate binder solution. A total of 10g of lithium manganese iron phosphate, alginate, carbon black (SP), and carbon nanotubes (CNT) was weighed and poured into a ball mill jar. The mixture was stirred for 10 minutes using a high-speed vibrating ball mill to obtain a uniformly mixed slurry. This slurry was then evenly coated onto an aluminum current collector foil and dried in a vacuum drying oven at 105°C for 4 hours. Finally, it was rolled to a density of 2.35 mg / cm³. 3After compaction, the material is die-cut and weighed to obtain the positive electrode sheet. After drying in a vacuum drying oven at 105℃ for 2 hours, it is placed in a glove box for assembly into a half-cell. Using lithium metal as the negative electrode sheet, the button cell is assembled in an argon-filled glove box according to the following order: negative electrode shell, spring plate, gasket, negative electrode sheet, separator, electrolyte, positive electrode sheet, and positive electrode shell. Finally, the battery is sealed using a battery sealing machine to obtain the final battery.

[0076] Application Comparative Example 1

[0077] This application compares the positive electrode sheet and the battery containing it obtained with reference to Application Example 1. The difference between this application example and Application Example 1 is that the binder is changed from alginate to PVDF in this application example, and PVDF is mixed with solvent NMP to prepare a 3% adhesive solution for use. Apart from the above differences, the other materials, process operations and conditions used are strictly consistent with Application Example 1.

[0078] Characterization and testing:

[0079] I. Resistivity

[0080] At 25°C, a four-probe powder resistivity meter was used to test the surface-functionalized lithium iron manganese phosphate obtained in Examples 1-10 or Comparative Examples 1-2, or the carbon-coated lithium iron manganese phosphate in Comparative Example 3. The resistivity values ​​were recorded when the pressure reached 20 MPa. The results are shown in Table 1.

[0081] Table 1

[0082]

[0083] As shown in Table 1, the resistivity of lithium manganese iron phosphate after dual modification is significantly reduced, indicating that the oxygen-containing and nitrogen-containing groups on its surface improve its conductivity. Comparative Examples 1 and 2 also showed a decrease in resistivity after surface functionalization with a single modifier, but the combined effect of the two modifiers resulted in a more significant decrease, indicating that both can reduce the material resistance.

[0084] II. Charge / discharge capacity

[0085] At 25°C, the lithium-ion batteries obtained in Application Examples 1-4 and Comparative Example 1 were left to stand for 12 hours, then charged at a constant current of 1C to a voltage of 4.2V, left to stand for 5 minutes, and then discharged at a 1C rate to 2.5V, left to stand for 5 minutes. The first charge-discharge capacity was recorded; the results are shown in Table 2.

[0086] III. Cyclic Performance

[0087] At 25°C, the lithium-ion batteries obtained in Application Examples 1-4 and Comparative Example 1 were charged at a constant current rate of 1C to a voltage of 4.2V, allowed to stand for 5 minutes, and then discharged at a rate of 1C to 2.5V, and allowed to stand for 5 minutes. This constituted one charge-discharge cycle. The lithium-ion batteries were subjected to 300 charge-discharge cycles at 25°C and 55°C under the above conditions, and the charge-discharge capacity and capacity retention were recorded. The results are shown in Table 2.

[0088] Table 2

[0089]

[0090] Table 2 shows that the battery prepared with alginate binder has a higher cycle capacity retention rate at 25°C and 55°C than that of PVDF adhesive, indicating that alginate binder has a beneficial effect on the cycle performance of the battery. As can be seen from Examples 1-4, Example 4, with too low a solid-liquid ratio (i.e., too much oxidant), may lead to local over-oxidation of the material, reducing the stability of the material and its cycle stability. Example 2, with too low a solid-liquid ratio (i.e., too little oxidant), may lead to solution dilution, resulting in poor surface functionalization of LMFP and failure to uniformly coat the surface.

[0091] IV. Rate Performance

[0092] At 25°C, the lithium-ion batteries obtained in Application Examples 1-4 and Comparative Example 1 were charged and discharged at current rates of 0.1C, 0.33C, 1C, and 5C, respectively, and the discharge specific capacity at each rate was recorded. The results are recorded in Table 3.

[0093] Table 3

[0094]

[0095] As shown in Table 3, the discharge capacity of the alginate binder is higher than that of the PVDF binder at several rate ranges for both binders, indicating that the alginate binder can improve the rate performance of the battery.

[0096] In summary, this invention utilizes lithium manganese iron phosphate with dual surface functionalization treatment. The oxygen-containing groups improve lithium-ion transport efficiency, while the nitrogen-containing groups enhance the electronic conductivity of the material surface and inhibit manganese dissolution. Further, the alginate binder and Mn... 2+ Chelation further inhibits manganese dissolution, and the three can work synergistically to further reduce the interfacial impedance of LMFP, suppress the Ginger-Taylor effect, and effectively improve capacity and cycling stability.

[0097] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0098] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0099] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A surface-functionalized lithium manganese iron phosphate, characterized in that, This includes carbon-coated lithium manganese iron phosphate, wherein the surface of the carbon-coated lithium manganese iron phosphate has oxygen-containing groups and nitrogen-containing groups.

2. The surface-functionalized lithium manganese iron phosphate according to claim 1, characterized in that, In the carbon-coated lithium manganese iron phosphate, the chemical formula of lithium manganese iron phosphate includes LiMn. x Fe 1-x PO4, 0.5 < x ≤ 0.8, preferably x = 0.6; Preferably, the particle size D of the carbon-coated lithium manganese iron phosphate is... 50 The wavelength ranges from 550 to 950 nm, and the carbon coating thickness is 15 to 30 nm.

3. A method for preparing surface-functionalized lithium manganese iron phosphate as described in claim 1 or 2, characterized in that, Includes the following steps: Provides carbon-coated lithium manganese iron phosphate; Carbon-coated lithium manganese iron phosphate is mixed with an ammonifying agent solution for ammonification, then mixed with an oxidizing agent solution for oxidation, and finally heat-treated to obtain surface-functionalized lithium manganese iron phosphate.

4. The method for preparing surface-functionalized lithium manganese iron phosphate according to claim 3, characterized in that, The ammoniating agent in the ammoniating agent solution includes at least one of ammonia, urea, ammonium nitrate, pyridine, or polyaniline; Preferably, the mass percentage concentration of the ammoniating agent solution is 5% to 15%; Preferably, in the ammoniation treatment, the mass ratio of the carbon-coated lithium manganese iron phosphate to the ammoniation agent solution is 1:(10~15); Preferably, the ammoniation treatment is carried out at a temperature of 30-40°C, a stirring speed of 400-800 rpm, a time of 2-6 hours, and a pH of 8.8-9.

2. Preferably, the oxidant in the oxidant solution includes at least one of hydrogen peroxide, sodium hypochlorite, ammonium persulfate, or potassium permanganate; Preferably, the mass percentage concentration of the oxidant solution is 10% to 40%. Preferably, in the oxidation treatment, the volume ratio of the oxidant solution to the ammoniating agent solution is (0.8~1.2):1; Preferably, after the oxidation treatment, the parts are first washed, dried and ground in sequence, and then subjected to the heat treatment. Preferably, the detergent used for washing includes ethanol and deionized water, and the washing process ends when the wash water is neutral. Preferably, the drying temperature is 90~110℃ and the time is 8~12h; Preferably, the heat treatment is carried out in an inert atmosphere, and the temperature of the heat treatment is 300~600℃, and the time is 2~6h.

5. The method for preparing surface-functionalized lithium manganese iron phosphate according to claim 3 or 4, characterized in that, The method for preparing the carbon-coated lithium manganese iron phosphate includes: A lithium source, phosphorus source, manganese source and iron source are mixed, and after adjusting the pH, a hydrothermal reaction is carried out to obtain a precursor; the precursor is then subjected to chemical vapor deposition using a gaseous carbon source to obtain carbon-coated lithium manganese iron phosphate. Preferably, the lithium source and the phosphorus source are premixed to obtain a lithium phosphate solution, and the manganese source and the iron source are premixed to obtain a metal salt solution. After the lithium phosphate solution and the metal salt solution are mixed, the pH is adjusted. Preferably, the pH adjuster used for adjusting the pH includes a sodium hydroxide solution with a pH range of 5 to 7; Preferably, the hydrothermal reaction is carried out in a high-pressure reactor at a temperature of 160-200°C for 6-10 hours. Preferably, the gaseous carbon source includes acetylene; Preferably, the chemical vapor deposition temperature is 700~800℃ and the time is 4~8h.

6. A positive electrode sheet, characterized in that, The positive electrode active material in the positive electrode sheet includes the surface-functionalized lithium manganese iron phosphate as described in claim 1 or 2.

7. The positive electrode sheet according to claim 6, characterized in that, The binder in the positive electrode sheet includes alginate; Preferably, the alginate includes sodium alginate; Preferably, based on the total mass of the positive electrode active material, binder, and conductive agent as 100%, the lithium manganese iron phosphate accounts for 93% to 97%, and the alginate accounts for 1% to 3%. Preferably, the compaction density of the positive electrode sheet is 2.1~2.4 mg / cm3.

8. A method for manufacturing a positive electrode sheet as described in claim 6 or 7, characterized in that, The manufacturing method includes: Alginate and water are mixed to form an alginate binder solution. The alginate binder solution is then mixed with the positive electrode active material and a conductive agent and ball-milled to obtain a slurry. The slurry is coated onto a current collector, dried, and then rolled and die-cut to obtain the positive electrode sheet.

9. The method for manufacturing the positive electrode sheet according to claim 8, characterized in that, The mass percentage concentration of the alginate binder solution is 2%~4%; Preferably, the ball milling time is 5-20 minutes; Preferably, the drying temperature is 100~110℃ and the time is 3~5h.

10. A battery, characterized in that, It contains the positive electrode sheet as described in claim 6 or 7.

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