A composite lithium iron manganese phosphate positive electrode material and its preparation method and application

Abstract

The present disclosure provides a composite lithium iron manganese phosphate cathode material and its preparation method and application. The preparation method comprises the following steps: (1) mixing a lithium iron manganese phosphate core material with a surfactant solution, ultrasonically dispersing to obtain composite micelles of the lithium iron manganese phosphate, and adding a swelling agent while heating and stirring to obtain rod-shaped micelles; (2) mixing the rod-shaped micelles with a phosphorus source, an iron source, and a lithium source, and heating and reacting to obtain a precursor; (3) sintering the precursor to obtain the composite lithium iron manganese phosphate cathode material. The composite lithium iron manganese phosphate cathode material disclosed herein has lithium iron manganese phosphate as a core and lithium iron phosphate with a spatial gradient pore structure as a shell. The lithium iron phosphate with spatial gradient pores is coated on the outside of the lithium iron manganese phosphate to inhibit the dissolution of manganese, while increasing the specific surface area of ​​the material, shortening the lithium ion transmission distance, and giving full play to the electrical properties of the material.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries and relates to a composite manganese iron phosphate lithium cathode material and a preparation method and application thereof. Background Art

[0002] At present, the most commonly used positive electrode materials in lithium batteries are lithium iron phosphate and ternary materials. Compared with ternary materials, lithium iron phosphate (LFP) has better safety performance and cycle stability, and its raw materials are widely available and low in cost. However, the energy density of lithium iron phosphate is low, which makes it unable to meet the specific needs of some products.

[0003] Lithium manganese iron phosphate (LMFP) is an olivine-type phosphate cathode material obtained by doping manganese (Mn) on the basis of LFP. It inherits the excellent cycle performance and safety of LFP, and the high voltage characteristics of manganese give LMFP a higher voltage platform and energy density, which can take into account both high safety and high energy density.

[0004] CN116873892A improves the manganese dissolution problem of lithium iron manganese phosphate by doping elements and changing the ratio of the reaction solutions. The molar content of manganese decreases gradually from the center to the surface of the positive electrode material, while the molar content of iron and aluminum increases gradually. However, this method is complicated to operate and requires continuous calculation of the concentrations of various reaction solutions.

[0005] CN111900344A discloses a method for preparing a carbon-coated lithium manganese iron phosphate positive electrode material. First, a transition metal salt solution A, a phosphorus solution B, and an ammonia solution C, which are configured according to the molar ratio of Mn and Fe, are simultaneously added dropwise to a reactor to prepare a lithium manganese iron phosphate positive electrode material precursor; then the precursor is prepared with a lithium source according to a molar ratio, and a coating carbon source and a doping metal compound are added, and the carbon-coated lithium manganese iron phosphate positive electrode material is calcined under inert atmosphere protection.

[0006] The aforementioned method produces lithium manganese iron phosphate, which can dissolve manganese during charge and discharge due to the Jan-Teller effect of manganese ions. This can severely damage the electrolyte structure and reduce battery cycle performance. Therefore, it is crucial to find a method to address manganese dissolution during charge and discharge while also improving lithium ion transfer rates. Summary of the Invention

[0007] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0008] The purpose of the present disclosure is to provide a composite lithium iron manganese phosphate positive electrode material, a preparation method and application thereof. The present disclosure designs a core-shell structured lithium iron manganese phosphate / lithium iron phosphate material, and prepares a composite lithium iron manganese phosphate positive electrode material with lithium iron manganese phosphate as the core and lithium iron phosphate with a spatial gradient pore structure as the shell through soft template micelles. The lithium iron phosphate with spatial gradient pores is coated on the outside of the lithium iron manganese phosphate to inhibit the dissolution of manganese, and the lithium iron phosphate with spatial gradient pores as the shell can increase the specific surface area of ​​the material, shorten the lithium ion transmission distance, and give full play to the electrical properties of the material.

[0009] To achieve this purpose, the present disclosure adopts the following technical solutions:

[0010] In a first aspect, the present disclosure provides a method for preparing a composite lithium iron manganese phosphate positive electrode material, the preparation method comprising the following steps:

[0011] (1) mixing the lithium manganese iron phosphate core material with a surfactant solution, ultrasonically dispersing to obtain composite micelles of the lithium manganese iron phosphate, and adding a swelling agent while heating and stirring to obtain rod-shaped micelles;

[0012] (2) mixing the rod-shaped micelles with a phosphorus source, an iron source, and a lithium source, and heating the mixture to react to obtain a precursor;

[0013] (3) Sintering the precursor to obtain the composite manganese iron phosphate lithium positive electrode material.

[0014] The present invention adopts soft template micelles to prepare porous materials. The micelles formed by the surfactant are fixed on the lithium iron manganese phosphate in the inner core due to attraction to form composite spherical micelles. The swelling agent is introduced at different positions near the micelle core to play a pore expansion role. The formed micelles change dynamically with the introduction of the swelling agent, gradually changing from spherical micelles to gradient rod-shaped micelles that fuse together. Then, phosphorus source, iron source and lithium source are added and adsorbed on the spatial gradient micelles by electrostatic interaction. The lithium iron phosphate is hydrothermally synthesized and calcined at high temperature to decompose and remove the micelle template, thereby obtaining a composite lithium iron manganese phosphate positive electrode material with lithium iron phosphate coated with lithium iron manganese phosphate having spatial gradient pores. The presence of gradient pores in lithium iron phosphate can increase the specific surface area of ​​the material, shorten the lithium ion transmission path, and allow lithium ions in the inner core lithium iron manganese phosphate to be more quickly released and embedded.

[0015] In one embodiment, the lithium iron manganese phosphate core material in step (1) includes a lithium iron manganese phosphate core and a carbon coating layer disposed on the surface of the lithium iron manganese phosphate core.

[0016] In one embodiment, based on 100% of the mass of the lithium manganese iron phosphate core material, the mass of the carbon coating layer is 0.1-3%, for example, 0.1%, 1%, 1.5%, 2% or 3%.

[0017] In one embodiment, the lithium manganese iron phosphate core material is prepared by the following method:

[0018] An iron source, a manganese source, a phosphorus source, a lithium source and a solvent are mixed, and a solid phase precursor is obtained by solvent thermal drying. The solid phase precursor is mixed with a carbon source and calcined to obtain the lithium iron manganese phosphate core material.

[0019] In one embodiment, the iron source includes any one of ferrous phosphate, ferrous oxalate, ferrous sulfate or ferrous chloride, or a combination of at least two thereof.

[0020] In one embodiment, the manganese source includes any one of manganese sulfate, manganese chloride, manganese nitrate, or manganese phosphate, or a combination of at least two thereof.

[0021] In one embodiment, the phosphorus source includes any one of ammonium dihydrogen phosphate, ammonium phosphate, ammonium dihydrogen phosphate, lithium dihydrogen phosphate or phosphoric acid, or a combination of at least two thereof.

[0022] In one embodiment, the lithium source includes any one of lithium carbonate, lithium hydroxide, or lithium acetate, or a combination of at least two thereof.

[0023] In one embodiment, the carbon source includes any one of glucose, starch, sucrose or cellulose, or a combination of at least two thereof.

[0024] In one embodiment, the calcination temperature is 600-850°C, for example, 600°C, 650°C, 700°C, 750°C or 850°C.

[0025] In one embodiment, the calcination treatment time is 6 to 8 hours, for example: 6 hours, 6.5 hours, 7 hours, 7.5 hours or 8 hours.

[0026] In one embodiment, the mass concentration of the surfactant solution in step (1) is 0.1-8%, for example, 0.1%, 1%, 2%, 5%, 6% or 8%, and can be optionally 2-6%.

[0027] In one embodiment, the surfactant comprises cetyltrimethylammonium bromide (CTAB).

[0028] In one embodiment, the molar ratio of the lithium iron manganese phosphate core material to the surfactant is 1:(0.01-0.2), for example: 1:0.01, 1:0.05, 1:0.1, 1:0.15 or 1:0.2.

[0029] In one embodiment, the ultrasonic dispersion time is 1 to 3 hours, for example, 1 hour, 1.5 hours, 2 hours, 2.5 hours or 3 hours.

[0030] In one embodiment, the temperature of the heating and stirring in step (1) is 50-70°C, for example, 50°C, 55°C, 60°C, 65°C or 70°C.

[0031] During the heating and stirring process, the swelling agent is slowly added, and the size of the spherical composite micelles increases dynamically with the slow introduction of the swelling agent, and the spherical micelles gradually fuse into gradient rod-shaped micelles.

[0032] In one embodiment, the swelling agent comprises an alkane.

[0033] In one embodiment, the alkane includes any one or a combination of at least two of n-hexane, n-heptane, n-octane, n-decane or cyclohexane.

[0034] In one embodiment, the molar ratio of the surfactant to the swelling agent is 1:(0.01-0.3), for example: 1:0.01, 1:0.05, 1:0.1, 1:0.2 or 1:0.3, etc., and can be optionally 1:(0.05-0.2), and further can be 1:(0.08-0.15).

[0035] In one embodiment, the reaction is carried out for 0.5 to 1 hour after the swelling agent is added.

[0036] In one embodiment, the molar ratio of phosphorus in the phosphorus source, iron in the iron source and lithium in the lithium source in step (2) is 1:1:(1-1.02), for example: 1:1:1, 1:1:1.005, 1:1:1.01, 1:1:1.015 or 1:1:1.02, etc.

[0037] In one embodiment, the temperature of the heating reaction is 120-180°C, for example, 120°C, 140°C, 150°C, 160°C or 180°C.

[0038] In one embodiment, the heating reaction time is 5 to 8 hours, for example, 5 hours, 5.5 hours, 6 hours, 7 hours or 8 hours.

[0039] In one embodiment, the sintering treatment in step (3) includes pre-firing and calcining.

[0040] During the calcination process, the lithium iron phosphate serving as the shell structure decomposes the CTAB template at high temperature to leave a gradient pore structure, and acts as a carbon source to form an internal conductive network channel in the positive electrode material.

[0041] In one embodiment, the pre-calcination temperature is 300-450°C, for example, 300°C, 320°C, 350°C, 400°C or 450°C.

[0042] In one embodiment, the pre-burning time is 2 to 4 hours, for example, 2 hours, 2.5 hours, 3 hours, 3.5 hours or 4 hours.

[0043] In one embodiment, the calcination temperature is 550-850°C, for example, 550°C, 600°C, 700°C, 750°C or 850°C.

[0044] In one embodiment, the calcination time is 4 to 10 hours, for example, 4 hours, 5 hours, 6 hours, 8 hours or 10 hours.

[0045] In a second aspect, the present disclosure provides a composite lithium manganese iron phosphate positive electrode material, which is prepared by the method described in the first aspect.

[0046] In one embodiment, the composite lithium iron manganese phosphate positive electrode material includes a lithium iron manganese phosphate core and a lithium iron phosphate coating layer disposed on the surface of the lithium iron manganese phosphate core.

[0047] In one embodiment, the lithium iron phosphate coating layer comprises a spatially gradient pore structure.

[0048] In the composite lithium iron manganese phosphate cathode material disclosed herein, lithium iron phosphate acts as a shell coating the exterior of the lithium iron manganese phosphate. Although the outer shell of the lithium iron phosphate is porous, because it exists in a gradient pore pattern, the pore size of the lithium iron phosphate near the inner core of the lithium iron manganese phosphate is small and less distributed. This portion of the lithium iron phosphate can completely coat the lithium iron manganese phosphate to prevent manganese dissolution. Furthermore, as organic entities, the micelles contain a large amount of carbon, which, after sintering, can serve as a carbon source to form a conductive network channel within the cathode material, thereby improving the conductivity of the cathode material.

[0049] In a third aspect, the present disclosure provides a positive electrode plate, which comprises the composite manganese iron phosphate lithium positive electrode material as described in the second aspect.

[0050] In a fourth aspect, the present disclosure provides a lithium-ion battery, comprising the positive electrode sheet as described in the third aspect.

[0051] Compared with the prior art, the present disclosure has the following beneficial effects:

[0052] (1) The present invention discloses a composite lithium iron manganese phosphate positive electrode material with a core-shell structure of lithium iron phosphate / lithium iron phosphate, which is prepared by soft template micelles. The material has lithium iron phosphate as the core and lithium iron phosphate with a spatial gradient pore structure as the shell. The lithium iron phosphate with spatial gradient pores is coated on the outside of the lithium iron manganese phosphate to inhibit the dissolution of manganese, and the lithium iron phosphate with spatial gradient pores as the shell can increase the specific surface area of ​​the material, shorten the lithium ion transmission distance, and give full play to the electrical properties of the material.

[0053] (2) The composite manganese iron phosphate lithium cathode material prepared by the method disclosed herein has a first charge capacity of over 161.5 mAh / g and an initial efficiency of over 96.5%. The battery exhibits excellent rate performance, with a discharge capacity of over 156.2 mAh / g at 0.1C, over 145.2 mAh / g at 1C, and over 132.9 mAh / g at 3C.

[0054] Still other aspects will become apparent upon reading and understanding the accompanying drawings and detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] The accompanying drawings are used to provide a further understanding of the technical solution of this article and constitute a part of the specification. Together with the embodiments of this application, they are used to explain the technical solution of this article and do not constitute a limitation on the technical solution of this article.

[0056] Figure 1 This is a schematic diagram of the 3D structure of a composite lithium iron manganese phosphate positive electrode material obtained according to an embodiment of the present disclosure, 1-lithium iron manganese phosphate core, 2-lithium iron phosphate shell with spatially gradient pores.

[0057] Figure 2 This is a scanning electron microscope image of the cross section of the composite manganese iron phosphate lithium positive electrode material prepared in Example 1 of the present disclosure. DETAILED DESCRIPTION

[0058] The technical solution of the present disclosure is further described below through specific implementation methods. Those skilled in the art should understand that the embodiments are only used to help understand the present disclosure and should not be regarded as specific limitations of the present disclosure.

[0059] The lithium manganese iron phosphate core material used in the embodiments and comparative examples of the present disclosure was prepared by the following method:

[0060] According to the iron-manganese molar ratio (6:4), different masses of lithium carbonate (Li2CO3), manganese carbonate (MnCO3), ferrous oxalate (FeC2O4·2H2O), and diammonium hydrogen phosphate ((NH4)2HPO4) were weighed as raw materials to make the molar ratio of lithium source, iron-manganese source, and phosphorus source be 1:1:1. The above-weighed materials were mixed and dispersed in a first solution (a mixture of deionized water and ethanol, with a volume ratio of 1:3), and then subjected to solvent thermal drying at 140°C to obtain LiFe 0.6 Mn 0.4 PO4 solid phase precursor. After the precursor is crushed, add a little glucose. Glucose accounts for 1% of LiFe 0.6 Mn 0.4 After ball milling and mixing, the mixture was heated to 700° C. at a heating rate of 5° C. / min in an argon atmosphere and calcined for 8 h, and then cooled naturally to obtain the lithium manganese iron phosphate core material.

[0061] Example 1

[0062] This embodiment provides a composite lithium manganese iron phosphate positive electrode material, and the preparation method of the composite lithium manganese iron phosphate positive electrode material is as follows:

[0063] (1) CTAB is mixed with water to obtain a dispersed solution with a CTAB mass concentration of 4%, and the lithium iron manganese phosphate core material is mixed with the dispersed solution according to a molar ratio of lithium iron manganese phosphate: CTAB = 1:0.15, and ultrasonic dispersion treatment is performed for 2 hours to obtain spherical composite micelles of lithium iron manganese phosphate, and then heated to 65° C. with stirring, and cyclohexane is slowly added, with a molar ratio of CTAB to cyclohexane of 1:0.1. After the addition is completed, stirring is performed for 1 hour to obtain rod-shaped micelles;

[0064] (2) (NH4)2HPO4, FeC2O4·2H2O and Li2CO3 were added in sequence to make the molar ratio of lithium, iron and phosphorus 1:1:1, and the temperature was continued to be raised to 140°C for hydrothermal reaction for 6 hours. The product was collected by centrifugation, washed alternately with deionized water and ethanol several times, and then dried in vacuum at 90°C for 2 hours to obtain a precursor;

[0065] (3) The precursor was ground and heated to 350°C at a heating rate of 5°C / min in an argon atmosphere for pre-calcination for 3.5 hours, and then heated to 680°C at a heating rate of 10°C / min for calcination for 7 hours to obtain the composite manganese iron phosphate lithium positive electrode material.

[0066] The 3D structural diagram of the composite lithium iron manganese phosphate positive electrode material is as follows Figure 1 As shown, 1 is the lithium iron manganese phosphate core and 2 is the lithium iron phosphate shell with spatial gradient pores. The cross-sectional scanning electron microscope image of the composite lithium iron manganese phosphate positive electrode material is shown in FIG. Figure 2 As shown by Figure 2 It can be seen that the composite manganese iron lithium phosphate described in the present disclosure is a core-shell structure, the core is solid manganese iron lithium phosphate, and the shell is lithium iron phosphate with holes. The shell has a gradient hole structure characteristic, radiating radially from the inside to the outside along the core.

[0067] Example 2

[0068] This embodiment provides a composite lithium manganese iron phosphate positive electrode material, and the preparation method of the composite lithium manganese iron phosphate positive electrode material is as follows:

[0069] (1) CTAB is mixed with water to obtain a dispersed solution with a CTAB mass concentration of 6%, and the lithium iron manganese phosphate core material is mixed with the dispersed solution according to a molar ratio of lithium iron manganese phosphate: CTAB = 1:0.2, and ultrasonic dispersion treatment is performed for 3 hours to obtain spherical composite micelles of lithium iron manganese phosphate, and then heated to 70° C. with stirring, and cyclohexane is slowly added with a molar ratio of CTAB to n-heptane of 1:0.08. After the addition is completed, stirring is performed for 0.8 hours to obtain rod-shaped micelles;

[0070] (2) adding (NH4)2HPO4, FeC2O4·2H2O and Li2CO3 in sequence to make the molar ratio of lithium, iron and phosphorus 1:1:1, and continuing to heat to 120℃ for hydrothermal reaction for 8h. The product was collected by centrifugation, washed alternately with deionized water and ethanol several times, and then dried in vacuum at 100℃ for 2h to obtain a precursor;

[0071] (3) The precursor was ground and heated to 300°C at a heating rate of 5°C / min in an argon atmosphere for pre-calcination for 4 hours, and then heated to 550°C at a heating rate of 10°C / min for calcination for 10 hours to obtain the composite manganese iron phosphate lithium positive electrode material.

[0072] The 3D structural diagram of the composite lithium iron manganese phosphate positive electrode material is as follows Figure 1 As shown, 1 is the lithium iron manganese phosphate core and 2 is the lithium iron phosphate shell with spatial gradient pores.

[0073] Example 3

[0074] This embodiment provides a composite lithium manganese iron phosphate positive electrode material, and the preparation method of the composite lithium manganese iron phosphate positive electrode material is as follows:

[0075] (1) CTAB is mixed with water to obtain a dispersion solution with a CTAB mass concentration of 2%, and the lithium iron manganese phosphate core material is mixed with the dispersion solution according to a molar ratio of lithium iron manganese phosphate: CTAB = 1:0.01, and ultrasonic dispersion treatment is performed for 1 hour to obtain spherical composite micelles of lithium iron manganese phosphate, and then heated to 70° C. with stirring, and cyclohexane is slowly added with a molar ratio of CTAB to n-heptane of 1:0.15. After the addition is completed, stirring is performed for 0.5 hour to obtain rod-shaped micelles;

[0076] (2) adding (NH4)2HPO4, FeC2O4·2H2O and Li2CO3 in sequence to make the molar ratio of lithium, iron and phosphorus 1:1:1, and continuing to heat to 120℃ for hydrothermal reaction for 8h. The product was collected by centrifugation, washed alternately with deionized water and ethanol several times, and then dried in vacuum at 100℃ for 2h to obtain a precursor;

[0077] (3) The precursor was ground and heated to 450°C at a heating rate of 5°C / min in an argon atmosphere for pre-calcination for 2 hours, and then heated to 850°C at a heating rate of 10°C / min and calcined for 4 hours to obtain the composite manganese iron phosphate lithium positive electrode material.

[0078] The 3D structural diagram of the composite lithium iron manganese phosphate positive electrode material is as follows Figure 1 As shown, 1 is the lithium iron manganese phosphate core and 2 is the lithium iron phosphate shell with spatial gradient pores.

[0079] Example 4

[0080] The only difference between this embodiment and embodiment 1 is that the molar ratio of the surfactant to the swelling agent is 1:0.05, and the other conditions and parameters are exactly the same as those in embodiment 1.

[0081] Example 5

[0082] The only difference between this embodiment and embodiment 1 is that the molar ratio of the surfactant to the swelling agent is 1:0.2, and the other conditions and parameters are exactly the same as those in embodiment 1.

[0083] Example 6

[0084] The only difference between this embodiment and embodiment 1 is that the temperature of heating and stirring is 40° C., and the other conditions and parameters are exactly the same as those in embodiment 1.

[0085] Example 7

[0086] The only difference between this embodiment and embodiment 1 is that the temperature of heating and stirring is 80° C., and the other conditions and parameters are exactly the same as those in embodiment 1.

[0087] Comparative Example 1

[0088] This comparative example provides a composite lithium manganese iron phosphate positive electrode material, and the preparation method of the composite lithium manganese iron phosphate positive electrode material is as follows:

[0089] (1) The lithium iron manganese phosphate core material was dispersed in deionized water, and (NH4)2HPO4, FeC2O4·2H2O and Li2CO3 were added in sequence so that the molar ratio of lithium, iron and phosphorus was 1:1:1. The temperature was continued to be raised to 140℃ for hydrothermal reaction for 6h. The product was collected by centrifugation, washed alternately with deionized water and ethanol several times, and then dried in vacuum at 90℃ for 2h to obtain a core-shell structure precursor.

[0090] (2) The precursor with a core-shell structure was ground, and in an argon atmosphere, the temperature was raised to 350°C at a heating rate of 5°C / min for pre-calcination for 3.5 hours, and then the temperature was raised to 680°C at a heating rate of 10°C / min for calcination for 7 hours to obtain the composite manganese iron phosphate lithium positive electrode material.

[0091] Comparative Example 2

[0092] The only difference between this comparative example and comparative example 1 is that glucose is added in an amount of 5% by mass of the lithium iron phosphate as the shell in step (3), and then calcined at high temperature. Other conditions and parameters are exactly the same as those in comparative example 1.

[0093] Comparative Example 3

[0094] The only difference between this comparative example and Example 1 is that no surfactant is added, and other conditions and parameters are exactly the same as those in Example 1.

[0095] Comparative Example 4

[0096] The only difference between this comparative example and Example 1 is that no swelling agent is added, and other conditions and parameters are exactly the same as those in Example 1.

[0097] Performance testing:

[0098] The composite manganese iron phosphate lithium cathode material prepared in the Examples and Comparative Examples was uniformly mixed with acetylene black and polyvinylidene fluoride in a mass ratio of 90:5:5 in N-methylpyrrolidone, then coated on aluminum foil and dried in a vacuum drying oven. After drying, the battery was assembled in an argon glove box and pressed into a positive electrode sheet using a tablet press. The negative electrode was a metal lithium sheet. The electrolyte was 1M LiPF6-EC:DMC (volume ratio 1:1), and a polypropylene porous membrane was used as a separator. The electrochemical performance was tested, and the test results are shown in Table 1:

[0099] Table 1

[0100]

[0101]

[0102] As can be seen from Table 1, it can be obtained from Examples 1-3 that the first charge of the battery made of the composite manganese iron phosphate lithium positive electrode material prepared by the method described in the present disclosure can reach more than 161.5 mAh / g, the first efficiency can reach more than 96.5%, the 0.1C discharge capacity can reach more than 156.2 mAh / g, the 1C discharge capacity can reach more than 145.2 mAh / g, and the 3C discharge capacity can reach more than 132.9 mAh / g.

[0103] By comparing Example 1 and Example 4-5, it can be seen that in the preparation process of the composite manganese iron phosphate lithium positive electrode material described in the present invention, the molar ratio of the surfactant and the swelling agent will affect its performance. The molar ratio of the surfactant and the swelling agent is controlled at 1: (0.08~0.15), and the performance of the composite manganese iron phosphate lithium positive electrode material obtained is better. If the amount of swelling agent added is too large, the pore size of the rod-shaped micelles becomes larger, and the positive electrode material raw material adsorbed externally becomes less, resulting in a decrease in the active substance of the lithium iron phosphate as the shell in the core-shell structure, affecting the electrical properties of the material; if the amount of swelling agent added is too small, the micelles fail to form suitable rod-shaped gradient micelles, and the lithium iron phosphate as the shell in the core-shell structure does not have enough pores to accelerate the shortening of lithium ion transmission distance and improve the rate performance.

[0104] By comparing Example 1 and Examples 6-7, it can be seen that during the preparation process of the composite manganese iron phosphate lithium positive electrode material described in the present disclosure, the heating and stirring temperature when adding the swelling agent will affect its performance. The heating and stirring temperature is controlled at 50-70°C, and the performance of the composite manganese iron phosphate lithium positive electrode material is better. If the heating and stirring temperature is too high, the micelles will have a large dynamic change range during the swelling process, and the pore size difference at both ends will be large. The pore size close to the inside of the manganese iron phosphate lithium core is smaller, and lithium ions cannot be transmitted quickly and effectively. If the heating and stirring temperature is too low, the micelles will have a small dynamic change range during the swelling process, and the gradient rod-shaped micelle morphology cannot be achieved.

[0105] From the comparison between Example 1 and Comparative Example 1, it can be seen that the composite manganese iron phosphate lithium positive electrode material disclosed in the present invention has a gradient pore structure in the lithium iron phosphate shell, which is more conducive to the rapid transmission of lithium ions in the lithium iron manganese phosphate in the core, thereby improving the rate performance of the battery.

[0106] From the comparison between Example 1 and Comparative Example 2, it can be seen that when synthesizing lithium iron phosphate as a shell, adding a carbon source as a conductive material can improve the electrical properties of the positive electrode material.

[0107] By comparing Example 1 with Comparative Examples 3-4, it can be seen that the present disclosure uses soft template micelles to prepare porous materials. The micelles formed by the surfactant are fixed on the lithium iron manganese phosphate in the inner core due to attraction to form composite spherical micelles. The swelling agent is introduced at different positions near the micelle core to play a pore expansion role. The formed micelles change dynamically with the introduction of the swelling agent, gradually changing from spherical micelles to gradient rod-shaped micelles that fuse together. The phosphorus source, iron source and lithium source are adsorbed on the spatial gradient micelles by electrostatic interaction, and then the lithium iron phosphate is hydrothermally synthesized and calcined at high temperature to decompose and remove the micelle template, thereby obtaining a composite lithium iron manganese phosphate positive electrode material with lithium iron phosphate coated with lithium iron manganese phosphate having spatial gradient pores. The presence of gradient pores in lithium iron phosphate can increase the specific surface area of ​​the material, shorten the lithium ion transmission path, and allow lithium ions in the inner core lithium iron manganese phosphate to be more quickly released and embedded.

Claims

1. A method for preparing a composite lithium iron manganese phosphate positive electrode material, comprising the following steps: (1) mixing the lithium manganese iron phosphate core material with a surfactant solution, ultrasonically dispersing to obtain composite micelles of the lithium manganese iron phosphate, and adding a swelling agent while heating and stirring to obtain rod-shaped micelles; (2) mixing the rod-shaped micelles with a phosphorus source, an iron source, and a lithium source, and heating the mixture to react to obtain a precursor; (3) sintering the precursor to obtain the composite manganese iron phosphate lithium positive electrode material; The surfactant comprises cetyltrimethylammonium bromide, the swelling agent comprises an alkane, and the alkane comprises any one or a combination of at least two of n-hexane, n-heptane, n-octane, n-decane or cyclohexane; The temperature of the heating reaction is 120-180° C., and the time of the heating reaction is 5-8 hours.

2. The preparation method according to claim 1, wherein The lithium iron manganese phosphate core material in step (1) includes a lithium iron manganese phosphate core and a carbon coating layer arranged on the surface of the lithium iron manganese phosphate core.

3. The preparation method according to claim 2, wherein Based on the mass of the lithium manganese iron phosphate core material being 100%, the mass of the carbon coating layer is 0.1-3%.

4. The preparation method according to claim 1, wherein The lithium manganese iron phosphate core material is prepared by the following method: An iron source, a manganese source, a phosphorus source, a lithium source and a solvent are mixed, and a solid phase precursor is obtained by solvent thermal drying. The solid phase precursor is mixed with a carbon source and calcined to obtain the lithium iron manganese phosphate core material.

5. The preparation method according to claim 4, wherein The iron source includes any one of ferrous phosphate, ferrous oxalate, ferrous sulfate or ferrous chloride, or a combination of at least two of them.

6. The preparation method according to claim 4, wherein The manganese source includes any one of manganese sulfate, manganese chloride, manganese nitrate or manganese phosphate, or a combination of at least two of them.

7. The preparation method according to claim 4, wherein The phosphorus source includes any one of ammonium dihydrogen phosphate, ammonium phosphate, ammonium dihydrogen phosphate, lithium dihydrogen phosphate or phosphoric acid, or a combination of at least two of them.

8. The preparation method according to claim 4, wherein The lithium source includes any one of lithium carbonate, lithium hydroxide or lithium acetate, or a combination of at least two of them.

9. The preparation method according to claim 4, wherein The carbon source includes any one of glucose, starch, sucrose or cellulose, or a combination of at least two of them.

10. The preparation method according to claim 4, wherein The calcination temperature is 600-850°C.

11. The preparation method according to claim 4, wherein The calcination treatment time is 6 to 8 hours.

12. The preparation method according to claim 1, wherein The mass concentration of the surfactant solution in step (1) is 0.1-8%.

13. The preparation method according to claim 1, wherein The mass concentration of the surfactant solution in step (1) is 2-6%.

14. The preparation method according to claim 1, wherein The molar ratio of the lithium iron manganese phosphate core material to the surfactant is 1:(0.01-0.2).

15. The preparation method according to claim 1, wherein The ultrasonic dispersion time is 1 to 3 hours.

16. The preparation method according to claim 1, wherein The temperature of the heating and stirring in step (1) is 50-70°C.

17. The preparation method according to claim 1, wherein The molar ratio of the surfactant to the swelling agent is 1:(0.01-0.3).

18. The preparation method according to claim 1, wherein The molar ratio of the surfactant to the swelling agent is 1:(0.05-0.2).

19. The preparation method according to claim 1, wherein The molar ratio of the surfactant to the swelling agent is 1:(0.08-0.15).

20. The preparation method according to claim 1, wherein The swelling agent is added and reacted for 0.5 to 1 hour.

21. The preparation method according to claim 1, wherein The molar ratio of phosphorus in the phosphorus source, iron in the iron source and lithium in the lithium source in step (2) is 1:1:(1-1.02).

22. The preparation method according to claim 1, wherein The sintering treatment in step (3) includes pre-firing and calcining.

23. The preparation method according to claim 22, wherein The pre-firing temperature is 300-450°C.

24. The preparation method according to claim 22, wherein The pre-burning time is 2 to 4 hours.

25. The preparation method according to claim 22, wherein The calcination temperature is 550-850°C.

26. The preparation method according to claim 22, wherein The calcination time is 4 to 10 hours.

27. A composite lithium iron manganese phosphate positive electrode material prepared by the method according to any one of claims 1 to 26.

28. The composite lithium iron manganese phosphate positive electrode material according to claim 27, wherein: The composite lithium iron manganese phosphate positive electrode material includes a lithium iron manganese phosphate core and a lithium iron phosphate coating layer arranged on the surface of the lithium iron manganese phosphate core.

29. The composite lithium manganese iron phosphate positive electrode material according to claim 28, wherein The lithium iron phosphate coating layer comprises a spatial gradient pore structure.

30. A positive electrode sheet comprising the composite manganese iron phosphate lithium positive electrode material according to claim 27. A lithium ion battery comprising the positive electrode sheet according to claim 30 .

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