A multi-modified lithium manganese iron phosphate cathode material, its preparation method and application

Abstract

This invention provides a multi-modified lithium manganese iron phosphate cathode material, its preparation method, and its application. The preparation method includes the following steps: co-precipitating a manganese-rich, iron-poor solution, an oxidant, a complexing agent solution, a phosphorus source solution, and a doped metal source solution by injecting them concurrently into a base liquid; switching the manganese-rich, iron-poor solution to a manganese-poor, iron-rich solution for coprecipitation to obtain a first precursor material; mixing the first precursor material, a nitrogen-containing carbon source, a cobalt source, and a solvent to obtain a mixed slurry, spray-drying it, and sintering it to obtain a second precursor material; and mixing the second precursor material, a lithium source, and activated porous carbon and sintering them into the multi-modified lithium manganese iron phosphate cathode material. This invention, through multi-modification of the lithium manganese iron phosphate cathode material, not only improves the overall ionic and electronic conductivity of the material and effectively suppresses the dissolution of Mn²⁺ during cycling, but also enhances the structural stability of the material.

Description

Technical Field

[0001] This invention belongs to the field of cathode material technology, and relates to a multi-modified lithium manganese iron phosphate cathode material, its preparation method and application. Background Technology

[0002] In today's lithium-ion battery field, cathode materials play a decisive role in battery performance. Lithium iron phosphate (LFP) is widely used in electric vehicles and energy storage due to its low cost, good safety, and long cycle life; however, its energy density is limited, making it difficult to meet the demands for long driving range and high energy storage. While ternary materials offer high energy density, they suffer from resource scarcity, high cost, and compromised safety. Therefore, developing novel cathode materials that combine high energy density, long cycle life, excellent safety, and low cost has become a key direction for industry development.

[0003] Lithium manganese iron phosphate (LMP) is a promising cathode material. Its crystal structure is similar to that of lithium iron phosphate (LFP), both being olivine-type structures. This structure endows LMP with high stability; even when all lithium ions are released during charging, structural collapse does not occur, ensuring excellent safety performance. Furthermore, LMP has low raw material costs, is environmentally friendly, and its production process is similar to that of LFP. The mature process is suitable for large-scale mass production, making it a promising candidate for applications in mid-to-low-end vehicles and energy storage.

[0004] CN120440862A discloses a method for preparing carbon-coated lithium manganese iron phosphate cathode material, belonging to the field of battery cathode material technology, including the following steps: co-precipitating a mixed solution of iron source, manganese source, and phosphorus source with a precipitant solution, and drying to obtain a precursor of manganese iron phosphate; adding a lithium source, and calcining and grinding in stages under an inert gas atmosphere to obtain lithium manganese iron phosphate powder; ball milling the lithium manganese iron phosphate powder with a flux, dispersing it in a carbon source solution, uniformly coating the carbon source, and then drying, calcining in stages, and ball milling to obtain the final product.

[0005] CN120097305A discloses a method for preparing and applying a lithium manganese iron phosphate cathode material with uniform iron and manganese mixture, comprising: S1: synthesizing a spherical precursor by co-precipitation reaction; S2: dehydrating the precursor under a protective atmosphere and mixing it with a lithium source, a phosphorus source, and water sand mill to obtain a first slurry; S3: adding a carbon source to the first slurry, stirring evenly to obtain a second slurry, and then rapidly drying and granulating to obtain a dry powder; S4: calcining the dry powder under a protective atmosphere to obtain the lithium manganese iron phosphate cathode material.

[0006] The lithium manganese iron phosphate cathode material prepared by the above method is difficult to balance stability and conductivity, and its performance is poor in actual use. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the present invention aims to provide a multi-modified lithium manganese iron phosphate cathode material, its preparation method, and its application. By performing multiple modifications on the lithium manganese iron phosphate cathode material, the present invention can not only improve the overall ionic and electronic conductivity of the material and effectively suppress the dissolution of Mn²⁺ during cycling, but also improve the structural stability of the material.

[0008] To achieve this objective, the present invention employs the following technical solution:

[0009] In a first aspect, the present invention provides a method for preparing a multi-modified lithium manganese iron phosphate cathode material, the method comprising the following steps:

[0010] Manganese-rich and iron-poor solutions and manganese-poor and iron-rich solutions were prepared separately. The manganese-rich and iron-poor solutions, oxidant, complexing agent solution, phosphorus source solution, and doped metal source solution were injected into the bottom liquid in a co-flow manner to carry out the first coprecipitation reaction. The manganese-rich and iron-poor solutions were switched to manganese-poor and iron-rich solutions to carry out the second coprecipitation reaction to obtain the first precursor material.

[0011] A first precursor material, a nitrogen-containing carbon source, a cobalt source, and a solvent are mixed to obtain a mixed slurry. After spray drying the mixed slurry, a second precursor material is obtained by a first sintering treatment.

[0012] The second precursor material, lithium source, and activated porous carbon are mixed and subjected to a second sintering process to obtain the multi-modified lithium manganese iron phosphate cathode material.

[0013] The iron in the manganese-rich iron-poor solution and the manganese-poor iron-rich solution described in this invention is ferrous iron.

[0014] In the initial stage of this invention, a high-manganese solution is used to form a manganese-rich core. Subsequently, a high-iron solution is switched to form a coating layer with high conductivity and structural stability. The internal manganese-rich core provides high voltage, while the external iron-rich shell serves multiple functions as an electronically conductive layer. It improves the overall electronic conductivity of the material and effectively suppresses the dissolution of Mn²⁺ during cycling, enhancing material stability, especially at high temperatures. Simultaneously, the presence of the iron-rich shell alleviates the Jahn-Teller effect and phase separation problems during charge-discharge processes, improving cycle life. Furthermore, the addition of dopant ions during co-precipitation generates more lithium vacancies, widening the lithium-ion migration channels and significantly improving the intrinsic ionic conductivity and crystal structure stability of the material. Subsequently, a nitrogen-doped carbon layer and cobalt doping are introduced onto the precursor surface. The introduction of nitrogen increases the defects and active sites in the carbon layer, further enhancing its electronic conductivity and lithium-ion adsorption capacity. Co can be doped into the coating layer; the Co²⁺ / Co³⁺ redox couple helps improve the electrochemical activity of the material. Finally, by activating porous carbon coating to form a porous carbon network, a composite coating layer structure is formed with an inner cobalt-nitrogen-doped carbon layer and an outer porous carbon layer. This not only improves the ionic conductivity of the material through doping, but also greatly improves the electronic conductivity of the material through the composite carbon coating layer, achieving a "dual conductivity" effect.

[0015] Preferably, with the total molar amount of manganese and iron in the manganese-rich and iron-poor solution being 100%, the molar amount of manganese in the manganese-rich and iron-poor solution is 80% to 95%, for example: 80%, 82%, 85%, 90% or 95%, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0016] Preferably, with the total molar amount of manganese and iron in the manganese-poor and iron-rich solution being 100%, the molar amount of manganese in the manganese-poor and iron-rich solution is 5% to 20%, for example: 5%, 8%, 10%, 15% or 20%, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0017] Preferably, the total molar concentration of manganese and iron in the manganese-rich iron-poor solution and the manganese-poor iron-rich solution is independently 2 mol / L to 8 mol / L, for example: 2 mol / L, 3 mol / L, 5 mol / L, 6 mol / L or 8 mol / L, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0018] Preferably, the phosphorus source solution comprises a phosphoric acid solution.

[0019] Preferably, the molar concentration of the phosphorus source solution is 2 mol / L to 8 mol / L, for example: 2 mol / L, 3 mol / L, 5 mol / L, 6 mol / L or 8 mol / L, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0020] Preferably, the oxidant includes hydrogen peroxide;

[0021] Preferably, the mass concentration of the hydrogen peroxide is 20% to 30%, for example: 20%, 22%, 28%, 28% or 30%, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0022] Preferably, the complexing agent solution comprises ammonia.

[0023] Preferably, the molar concentration of the complexing agent solution is 2 mol / L to 8 mol / L, for example: 2 mol / L, 3 mol / L, 5 mol / L, 6 mol / L or 8 mol / L, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0024] Preferably, the solute in the doped metal source solution includes a titanium source and / or a vanadium source.

[0025] Preferably, the titanium source includes titanium chloride.

[0026] Preferably, the vanadium source includes ammonium vanadate.

[0027] Preferably, the molar concentration of the doped metal source solution is 0.1 mol / L to 0.5 mol / L, for example: 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L or 0.5 mol / L, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0028] Preferably, the base liquid includes a complexing agent and an oxidizing agent.

[0029] Preferably, the pH of the first coprecipitation reaction is 2 to 5, for example: 2, 2.5, 3, 4 or 5, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0030] Preferably, the temperature of the first coprecipitation reaction is 40℃~80℃, for example: 40℃, 50℃, 60℃, 70℃ or 80℃, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0031] Preferably, the time for the first coprecipitation reaction is 3h to 10h, for example: 3h, 5h, 6h, 8h or 10h, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0032] Preferably, the pH of the second coprecipitation reaction is 2 to 5, for example: 2, 2.5, 3, 4 or 5, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0033] Preferably, the temperature of the second coprecipitation reaction is 40℃~80℃, for example: 40℃, 50℃, 60℃, 70℃ or 80℃, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0034] Preferably, the time for the second coprecipitation reaction is 3h to 10h, for example: 3h, 5h, 6h, 8h or 10h, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0035] Preferably, the nitrogen-containing carbon source includes urea.

[0036] Preferably, the cobalt source includes cobalt nitrate.

[0037] Preferably, the solvent includes water.

[0038] Preferably, the mass ratio of the first precursor material, the nitrogen-containing carbon source, and the cobalt source is 100:(0.2~5):(0.001~0.01), for example: 100:0.2:0.001, 100:0.5:0.002, 100:1:0.005, 100:2:0.002, or 100:5:0.01, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0039] Preferably, the solid content of the mixed slurry is 30% to 50%, for example: 30%, 35%, 40%, 45% or 50%, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0040] Preferably, the temperature of the first sintering treatment is 600℃~800℃, for example: 600℃, 650℃, 700℃, 750℃ or 800℃, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0041] Preferably, the activated porous carbon is prepared by the following method:

[0042] Furfural residue is pre-carbonized at 400℃~500℃ (e.g., 400℃, 420℃, 450℃, 480℃, or 500℃, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable) for 0.5h~1.5h (e.g., 0.5h, 0.8h, 1h, 1.2h, or 1.5h, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable). Then, it is activated with steam at 900℃~1000℃ (e.g., 900℃, 920℃, 950℃, 980℃, or 1000℃, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable) for 20min~40min (e.g., 20min, 25min, 30min, 35min, or 40min, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable) to obtain activated porous carbon.

[0043] Preferably, the mass ratio of the second precursor material to the activated porous carbon is 100:(5~15), for example: 100:5, 100:8, 100:10, 100:12 or 100:15, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0044] Preferably, the atmosphere for the second sintering treatment includes nitrogen and / or argon.

[0045] Preferably, the temperature of the second sintering treatment is 600℃~850℃, for example: 600℃, 650℃, 700℃, 750℃, 800℃ or 850℃, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0046] In a second aspect, the present invention provides a multi-modified lithium manganese iron phosphate cathode material, which is prepared by the preparation method described in the first aspect.

[0047] The multi-modified lithium manganese iron phosphate cathode material includes a doped lithium manganese iron phosphate core and a cobalt nitrogen-doped carbon coating layer and a porous carbon coating layer sequentially disposed on the surface of the doped lithium manganese iron phosphate core.

[0048] Thirdly, the present invention provides a positive electrode sheet comprising the multi-modified lithium manganese iron phosphate positive electrode material as described in the second aspect.

[0049] Fourthly, the present invention provides a lithium-ion battery comprising a positive electrode as described in the third aspect.

[0050] Compared with the prior art, the present invention has the following beneficial effects:

[0051] (1) By performing multiple modifications on the lithium manganese iron phosphate cathode material, this invention can not only improve the overall ionic conductivity and electronic conductivity of the material and effectively suppress the dissolution of Mn²⁺ during cycling, but also improve the structural stability of the material.

[0052] (2) The battery made of the multi-modified lithium manganese iron phosphate cathode material of the present invention can achieve a capacity retention rate of over 93.17% after 300 cycles at 2C, and a specific capacity of over 127.2 mAh / g at 5C discharge. After cycling, the Mn 2+ The leaching amount can be up to 159.9 ppm. Detailed Implementation

[0053] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0054] The "range" disclosed in this invention can be defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the specific range. This type of range definition can include or exclude endpoints; any endpoint can be independently included or excluded, and they can be arbitrarily combined, meaning any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60~120 and 80~110 are listed for specific parameters, it is understood that ranges of 60~110 and 80~120 are also expected. Furthermore, if minimum range values ​​1 and 2 are listed, and maximum range values ​​3, 4, and 5 are also listed, then the following ranges are all expected: 1~3, 1~4, 1~5, 2~3, 2~4, and 2~5. In this invention, unless otherwise stated, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0~5" indicates that all real numbers between "0" and "5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is described as an integer ≥2, it is equivalent to listing integers such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc. For instance, when a parameter is described as an integer selected from "2~10", it is equivalent to listing the integers 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0055] In this invention, "a combination of at least two" refers to a quantity greater than or equal to two, unless otherwise specified. For example, "any combination of one or at least two" means one or more or more items. It can be understood that when referring to "a combination of at least two," it refers to any suitable combination of multiple items, that is, a combination of "at least two" items carried out in a manner that does not conflict with and enables the implementation of this invention.

[0056] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0057] The term "embodiment" as used in this invention means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment or implementation of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described in this invention can be combined with other embodiments.

[0058] In this invention, open-ended technical features or solutions described using terms such as "comprising" do not exclude additional members beyond those listed unless otherwise specified. They can be considered as providing both closed-ended features or solutions comprised of the listed members and open-ended features or solutions that include additional members beyond the listed members. For example, A includes a1, a2, and a3. Unless otherwise specified, it may also include other members or exclude additional members. This can be considered as providing both technical features or solutions where "A is composed of a1, a2, and a3" or "A is selected from a1, a2, and a3," and technical features or solutions where "A includes not only a1, a2, and a3, but also other members."

[0059] In this invention, unless otherwise specified, the features or solutions corresponding to "and / or" include any one of two or more of the related listed items, as well as any and all combinations of the related listed items. These arbitrary and all combinations include any two related listed items, any more related listed items, or a combination of all related listed items. For example, "A and / or B" represents a group consisting of A, B, and "a combination of A and B". "Containing A and / or B" can mean "containing A, containing B, and containing A and B", or "containing A, containing B, or containing A and B", and can be appropriately understood according to the context.

[0060] In this invention, the terms "first," "second," "third," "fourth," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features. Moreover, "first," "second," "third," "fourth," etc., serve only as a non-exhaustive enumeration and should be understood not to constitute a closed limitation on quantity.

[0061] The activated porous carbon used in the embodiments and comparative examples of this invention was prepared by the following method:

[0062] The furfural residue was pre-carbonized at 450℃ for 1 hour, and then activated with steam at 950℃ for 30 minutes to obtain the activated porous carbon.

[0063] Example 1

[0064] This embodiment provides a multi-modified lithium manganese iron phosphate cathode material, which is prepared by the following method:

[0065] A manganese-rich, iron-poor sulfate solution (Mn:Fe = 9:1, molar concentration 5 mol / L), a manganese-poor, iron-rich sulfate solution (Mn:Fe = 1:9, molar concentration 5 mol / L), and 5 mol / L ammonia water were prepared as the base solutions. The manganese-rich, iron-poor sulfate solution, 5 mol / L phosphoric acid solution, 25% hydrogen peroxide, 5 mol / L ammonia water, and a mixed solution of ammonium vanadate and titanium chloride solution (V:Ti = 2:1, molar concentration 0.2 mol / L) were injected concurrently into the base solutions. After a first coprecipitation reaction was carried out at pH 3.5 and temperature 60℃ for 5 h, the manganese-rich, iron-poor sulfate solution was switched to a manganese-poor, iron-rich sulfate solution, and a second coprecipitation reaction was carried out at the same conditions for 5 h to obtain the first precursor material.

[0066] The first precursor material, urea, cobalt nitrate and water were mixed in a mass ratio of 100:2:0.05 to obtain a mixed slurry with a solid content of 40%. After spray drying, the mixed slurry was subjected to a first sintering treatment at 700°C under a nitrogen atmosphere to obtain the second precursor material.

[0067] A second precursor material, lithium carbonate, and activated porous carbon were mixed, wherein the mass ratio of the second precursor material to the activated porous carbon was 100:10, and the second precursor material and lithium carbonate were added at a molar ratio of Li:Mn+Fe = 1.05:1. The mixture was then subjected to a second sintering treatment at 700℃ under an argon atmosphere to obtain the multi-modified lithium manganese iron phosphate cathode material.

[0068] Example 2

[0069] This embodiment provides a multi-modified lithium manganese iron phosphate cathode material, which is prepared by the following method:

[0070] A manganese-rich, iron-poor sulfate solution (Mn:Fe = 8:2, molar concentration 2 mol / L), a manganese-poor, iron-rich sulfate solution (Mn:Fe = 2:8, molar concentration 2 mol / L), and 2 mol / L ammonia water were prepared as the base solutions. The manganese-rich, iron-poor sulfate solution, 4 mol / L phosphoric acid solution, 20% hydrogen peroxide, 2 mol / L ammonia water, and a mixed solution of ammonium vanadate and titanium chloride solution (V:Ti = 1:2, molar concentration 0.1 mol / L) were injected concurrently into the base solutions. After a first coprecipitation reaction was carried out at pH 5 and temperature 40℃ for 10 h, the manganese-rich, iron-poor sulfate solution was switched to a manganese-poor, iron-rich sulfate solution, and a second coprecipitation reaction was carried out at the same conditions for 10 h to obtain the first precursor material.

[0071] The first precursor material, urea, cobalt nitrate and water were mixed in a mass ratio of 100:0.2:0.001 to obtain a mixed slurry with a solid content of 50%. After spray drying, the mixed slurry was subjected to a first sintering treatment at 600°C under a nitrogen atmosphere to obtain the second precursor material.

[0072] A second precursor material, lithium carbonate, and activated porous carbon were mixed, wherein the mass ratio of the second precursor material to the activated porous carbon was 100:5, and the second precursor material and lithium carbonate were added at a molar ratio of Li:Mn+Fe = 1.05:1. The mixture was then subjected to a second sintering treatment at 600℃ under an argon atmosphere to obtain the multi-modified lithium manganese iron phosphate cathode material.

[0073] Example 3

[0074] This embodiment provides a multi-modified lithium manganese iron phosphate cathode material, which is prepared by the following method:

[0075] A manganese-rich, iron-poor sulfate solution (Mn:Fe = 95:5, molar concentration 8 mol / L), a manganese-poor, iron-rich sulfate solution (Mn:Fe = 5:95, molar concentration 8 mol / L), and 8 mol / L ammonia water were prepared as the base solutions. The manganese-rich, iron-poor sulfate solution, 8 mol / L phosphoric acid solution, 30% hydrogen peroxide, 8 mol / L ammonia water, and a mixed solution of ammonium vanadate and titanium chloride solution (V:Ti = 1:2, molar concentration 0.5 mol / L) were injected concurrently into the base solutions. After a first coprecipitation reaction was carried out at pH 2 and temperature 80℃ for 3 hours, the manganese-rich, iron-poor sulfate solution was switched to a manganese-poor, iron-rich sulfate solution, and a second coprecipitation reaction was carried out at the same conditions for 3 hours to obtain the first precursor material.

[0076] The first precursor material, urea, cobalt nitrate and water were mixed in a mass ratio of 100:5:0.01 to obtain a mixed slurry with a solid content of 30%. After spray drying, the mixed slurry was subjected to a first sintering treatment at 800℃ under a nitrogen atmosphere to obtain the second precursor material.

[0077] A second precursor material, lithium carbonate, and activated porous carbon were mixed, wherein the mass ratio of the second precursor material to the activated porous carbon was 100:15, and the second precursor material and lithium carbonate were added at a molar ratio of Li:Mn+Fe = 1.05:1. The mixture was then subjected to a second sintering treatment at 850°C under an argon atmosphere to obtain the multi-modified lithium manganese iron phosphate cathode material.

[0078] Example 4

[0079] The only difference between this embodiment and Embodiment 1 is that the mass ratio of the first precursor material to urea is 100:0.1, while the other conditions and parameters are exactly the same as in Embodiment 1.

[0080] Example 5

[0081] The only difference between this embodiment and Embodiment 1 is that the mass ratio of the first precursor material to urea is 100:10; all other conditions and parameters are exactly the same as in Embodiment 1.

[0082] Example 6

[0083] The only difference between this embodiment and Embodiment 1 is that the mass ratio of the first precursor material to cobalt nitrate is 100:0.1. All other conditions and parameters are exactly the same as in Embodiment 1.

[0084] Example 7

[0085] The only difference between this embodiment and Embodiment 1 is that the mass ratio of the second precursor material to the activated porous carbon is 100:2; all other conditions and parameters are exactly the same as in Embodiment 1.

[0086] Example 8

[0087] The only difference between this embodiment and Embodiment 1 is that the mass ratio of the second precursor material to the activated porous carbon is 100:20; all other conditions and parameters are exactly the same as in Embodiment 1.

[0088] Comparative Example 1

[0089] The only difference between this comparative example and Example 1 is that only a sulfate solution with a ratio of Mn:Fe of 1:1 was used for co-precipitation to prepare the precursor; all other conditions and parameters were exactly the same as in Example 1.

[0090] Comparative Example 2

[0091] The only difference between this comparative example and Example 1 is that the urea is replaced with an equal amount of conventional carbon source glucose; all other conditions and parameters are exactly the same as in Example 1.

[0092] Comparative Example 3

[0093] The only difference between this comparative example and Example 1 is that cobalt nitrate is not added; all other conditions and parameters are exactly the same as in Example 1.

[0094] Comparative Example 4

[0095] The only difference between this comparative example and Example 1 is that no doped metal source solution (ammonium vanadate and titanium chloride) is added; all other conditions and parameters are exactly the same as in Example 1.

[0096] Comparative Example 5

[0097] The only difference between this comparative example and Example 1 is that no active porous carbon is added; all other conditions and parameters are exactly the same as in Example 1.

[0098] Performance testing:

[0099] The prepared lithium manganese iron phosphate cathode material, conductive carbon black, and polyvinylidene fluoride were weighed out in a mass ratio of 8:1:1 and added to N-methylpyrrolidone to form a slurry. The slurry was then coated onto aluminum foil, dried, and sliced ​​to obtain the cathode sheet. A lithium metal sheet was used as the counter electrode, a polypropylene microporous membrane as the separator, and 1 mol / L LiPF6 as the electrolyte (wherein the solvent is a 1:1 volume ratio mixture of ethylene carbonate and dimethyl carbonate). The CR2025 button cell was assembled in an argon glove box. Cycle performance and rate performance tests were performed on the CR2025 button cell.

[0100] Cyclic performance test conditions: After the assembled battery is left to stand for 2 hours, it is subjected to 300 cycles at 25℃, voltage range of 2.5-4.35V, and 2C rate.

[0101] Test conditions for rate performance: After the assembled battery is left to stand for 2 hours, it is subjected to 5 cycles at 0.2C, 0.5C, 1C, 2C and 5C respectively under the conditions of 25℃ and voltage range of 2.5-4.35V.

[0102] Mn 2+ Dissolution rate detection method: After soaking the material in electrolyte for 24 hours, filter it and detect the dissolved Mn in the electrolyte by ICP-MS. 2+ concentration.

[0103] The test results are shown in Table 1:

[0104] Table 1

[0105]

[0106] As shown in Table 1, and based on Examples 1-8, the battery made from the multi-modified lithium manganese iron phosphate cathode material of the present invention can achieve a capacity retention rate of over 93.17% after 300 cycles at 2C, and a specific capacity of over 127.2 mAh / g at 5C discharge. After cycling, the Mn... 2+ The leaching amount can be up to 159.9 ppm.

[0107] A comparison of Examples 1 and 4-5 shows that the amount of nitrogen-containing carbon source sodium ethylenediaminetetraacetate added during the preparation of the multi-modified lithium manganese iron phosphate cathode material of the present invention affects its performance. Controlling the mass ratio of the first precursor material to sodium ethylenediaminetetraacetate at 100:0.2~5 results in a multi-modified lithium manganese iron phosphate cathode material with better performance. If the amount of nitrogen-containing carbon source added is too high, the coating layer becomes too thick, which not only fails to further improve conductivity but also hinders lithium-ion diffusion, leading to a decrease in ionic conductivity and consequently impairing rate performance. If the amount of nitrogen-containing carbon source added is too low, a complete and effective conductive network cannot be formed, resulting in limited improvement in the electronic conductivity of the material, leading to poor rate performance and decreased cycle performance of the battery.

[0108] A comparison of Examples 1 and 6 shows that adding trace amounts of cobalt to the surface of the precursor can improve the electrochemical activity and structural stability of the material. However, excessive cobalt doping can cause lattice distortion, which in turn can destroy the stable structure of the precursor and may lead to a decrease in electrochemical performance.

[0109] A comparison of Examples 1 and 7-8 shows that the amount of activated porous carbon added during the preparation of the multi-modified lithium manganese iron phosphate cathode material of the present invention affects its performance. Controlling the mass ratio of the second precursor material to the activated porous carbon at 100:5~15 results in a multi-modified lithium manganese iron phosphate cathode material with better performance. If the amount of activated porous carbon added is too high, the excessive amount of inactive porous carbon will dilute the proportion of active material, significantly reducing the volumetric energy density and gravimetric energy density of the electrode. If the amount of activated porous carbon added is too low, an effective conductive path cannot be established between secondary particles, resulting in high electron transport resistance.

[0110] Comparing Example 1 and Comparative Example 1, it can be seen that the present invention initially uses a high-manganese solution to form a manganese-rich core, and subsequently switches to a high-iron solution to form a coating layer with high conductivity and structural stability. The internal manganese-rich core provides high voltage, while the external iron-rich shell plays multiple roles. As an electronically conductive layer, it can improve the overall electronic conductivity of the material and effectively suppress the dissolution of Mn²⁺ during cycling, thereby improving the material stability, especially its performance at high temperatures. At the same time, the presence of the iron-rich shell can alleviate the Jahn-Teller effect and phase separation problems during charge and discharge, thus improving cycle life.

[0111] As can be seen from the comparison between Example 1 and Comparative Examples 2-3, the present invention uses a nitrogen-containing carbon source and cobalt to coat the precursor in one step. The introduction of nitrogen can increase the defects and active sites of the carbon layer, further improving the electronic conductivity and lithium ion adsorption capacity of the carbon layer. Co can be doped into the coating layer. The redox couple of Co²⁺ / Co³⁺ helps to improve the electrochemical activity of the material and also plays a role in stabilizing the structure.

[0112] As can be seen from the comparison between Example 1 and Comparative Example 4, the addition of dopant ions during the co-precipitation process of the present invention can generate more lithium vacancies, broaden the lithium ion migration channels, and significantly improve the intrinsic ionic conductivity and crystal structure stability of the material.

[0113] As can be seen from the comparison between Example 1 and Comparative Example 5, the present invention forms a porous carbon network by activating porous carbon coating, forming a composite coating layer structure with an inner cobalt nitrogen doped carbon layer and an outer porous carbon layer. This not only improves the ionic conductivity of the material through doping, but also greatly improves the electronic conductivity of the material through the composite carbon coating layer, achieving a "dual conductivity" effect.

[0114] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing a multi-modified lithium manganese iron phosphate cathode material, characterized in that, The preparation method includes the following steps: Manganese-rich and iron-poor solutions and manganese-poor and iron-rich solutions were prepared separately. The manganese-rich and iron-poor solutions, oxidant, complexing agent solution, phosphorus source solution, and doped metal source solution were injected into the bottom liquid in a co-flow manner to carry out the first coprecipitation reaction. The manganese-rich and iron-poor solutions were switched to manganese-poor and iron-rich solutions to carry out the second coprecipitation reaction to obtain the first precursor material. A first precursor material, a nitrogen-containing carbon source, a cobalt source, and a solvent are mixed to obtain a mixed slurry. After spray drying the mixed slurry, a second precursor material is obtained by a first sintering treatment. The second precursor material, lithium source and activated porous carbon are mixed and subjected to a second sintering treatment to obtain the multi-modified lithium manganese iron phosphate cathode material. With the total molar amount of manganese and iron in the manganese-rich and iron-poor solution being 100%, the molar amount of manganese in the manganese-rich and iron-poor solution is 80%~95%. With the total molar amount of manganese and iron in the manganese-poor and iron-rich solution being 100%, the molar amount of manganese in the manganese-poor and iron-rich solution is 5%~20%. The total molar concentration of manganese and iron in the manganese-rich and iron-poor solutions and the manganese-poor and iron-rich solutions are independently 2mol / L~8mol / L.

2. The preparation method according to claim 1, characterized in that, The phosphorus source solution includes a phosphoric acid solution.

3. The preparation method according to claim 1, characterized in that, The molar concentration of the phosphorus source solution is 2 mol / L to 8 mol / L.

4. The preparation method according to claim 1, characterized in that, The oxidizing agent includes hydrogen peroxide.

5. The preparation method according to claim 4, characterized in that, The hydrogen peroxide has a mass concentration of 20% to 30%.

6. The preparation method according to claim 1, characterized in that, The complexing agent solution includes ammonia.

7. The preparation method according to claim 1, characterized in that, The molar concentration of the complexing agent solution is 2 mol / L to 8 mol / L.

8. The preparation method according to claim 1, characterized in that, The solute in the doped metal source solution includes a titanium source and / or a vanadium source.

9. The preparation method according to claim 8, characterized in that, The titanium source includes titanium chloride.

10. The preparation method according to claim 8, characterized in that, The vanadium source includes ammonium vanadate.

11. The preparation method according to claim 1, characterized in that, The molar concentration of the doped metal source solution is 0.1 mol / L to 0.5 mol / L.

12. The preparation method according to claim 1, characterized in that, The base liquid includes a complexing agent and an oxidizing agent.

13. The preparation method according to claim 1, characterized in that, The pH of the first coprecipitation reaction is 2-5.

14. The preparation method according to claim 1, characterized in that, The temperature of the first coprecipitation reaction is 40℃~80℃.

15. The preparation method according to claim 1, characterized in that, The time for the first coprecipitation reaction is 3h to 10h.

16. The preparation method according to claim 1, characterized in that, The pH of the second coprecipitation reaction is 2-5.

17. The preparation method according to claim 1, characterized in that, The temperature for the second coprecipitation reaction is 40℃~80℃.

18. The preparation method according to claim 1, characterized in that, The second coprecipitation reaction takes 3 to 10 hours.

19. The preparation method according to claim 1, characterized in that, The nitrogen-containing carbon source includes urea.

20. The preparation method according to claim 1, characterized in that, The cobalt source includes cobalt nitrate.

21. The preparation method according to claim 1, characterized in that, The solvent includes water.

22. The preparation method according to claim 1, characterized in that, The mass ratio of the first precursor material, the nitrogen-containing carbon source, and the cobalt source is 100:(0.2~5):(0.001~0.01).

23. The preparation method according to claim 1, characterized in that, The solid content of the mixed slurry is 30% to 50%.

24. The preparation method according to claim 1, characterized in that, The temperature of the first sintering treatment is 600℃~800℃.

25. The preparation method according to claim 1, characterized in that, The activated porous carbon is prepared by the following method: After pre-carbonizing furfural residue at 400℃~500℃ for 0.5h~1.5h, it is activated with steam at 900℃~1000℃ for 20min~40min to obtain activated porous carbon.

26. The preparation method according to claim 1, characterized in that, The mass ratio of the second precursor material to activated porous carbon is 100:(5~15).

27. The preparation method according to claim 1, characterized in that, The atmosphere for the second sintering process includes nitrogen and / or argon.

28. The preparation method according to claim 1, characterized in that, The temperature of the second sintering treatment is 600℃~850℃.

29. A multi-modified lithium iron phosphate cathode material, characterized in that, The multiple modified lithium manganese iron phosphate cathode material is prepared by the preparation method according to any one of claims 1-28: The multi-modified lithium manganese iron phosphate cathode material includes a doped lithium manganese iron phosphate core and a cobalt nitrogen-doped carbon coating layer and a porous carbon coating layer sequentially disposed on the surface of the doped lithium manganese iron phosphate core.

30. A positive electrode plate, characterized in that, The positive electrode comprises the multi-modified lithium manganese iron phosphate positive electrode material as described in claim 29.

31. A lithium-ion battery, characterized in that, The lithium-ion battery includes the positive electrode as described in claim 30.

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