Method for preparing lithium iron manganese phosphate from waste lithium iron phosphate and application

Abstract

The invention provides a method for preparing lithium iron manganese phosphate from waste lithium iron phosphate and application, and relates to the technical field of lithium ion batteries. Specifically, the method comprises the following steps: fully mixing a positive electrode of waste lithium iron phosphate, a manganese source, a phosphorus source, a lithium source and a carbon source in water, and sequentially grinding, drying and sintering to obtain the lithium iron manganese phosphate. According to the invention, waste lithium iron phosphate is taken as a raw material, lithium, manganese and phosphorus elements are supplemented, and lithium iron manganese phosphate with excellent performance is synthesized by using a high-temperature solid-phase method; according to the method, the waste lithium iron phosphate does not need to be subjected to any heat treatment or chemical treatment, the synthesis process is simple, operation is easy, the production cost is low, the method efficiency is high, the recovery rate of waste materials is high, meanwhile, high-value battery-grade lithium iron manganese phosphate can be obtained, and good application prospects are achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a method for preparing lithium manganese iron phosphate using waste lithium iron phosphate and its application. Background Art

[0002] Lithium iron phosphate (LFP) has excellent energy density, safety and service life performance. With the rapid development of electric energy equipment, LFP has a very high share in new energy vehicles, energy storage systems and consumer electronics. As a mainstream battery positive electrode material, its market demand is still in a growing stage. However, the recycling of waste LFP positive electrode materials has become increasingly prominent. The recycling of waste LFP positive electrode materials can not only reduce environmental pollution, but also save resources and reduce production costs. Therefore, it is of great significance to research and develop economical, efficient and environmentally friendly waste LFP recycling technology.

[0003] As an upgraded version of lithium iron phosphate, lithium manganese iron phosphate (LMFP) not only inherits the high safety performance, wide source of raw materials and low cost of lithium iron phosphate, but also improves the low temperature performance of the material. In addition, due to the effect of Mn, lithium manganese iron phosphate has a higher discharge voltage and energy density. Therefore, in the research of lithium-ion batteries, the development of lithium manganese iron phosphate with excellent electrochemical performance has become one of the hot spots.

[0004] In view of this, the present invention is proposed. Summary of the invention

[0005] The first purpose of the present invention is to provide a method for preparing lithium iron manganese phosphate from waste lithium iron phosphate, by recycling cheap waste lithium iron phosphate battery positive electrodes to produce high-value battery-grade lithium iron manganese phosphate materials, which can effectively expand the recycling technology route of waste lithium iron phosphate batteries.

[0006] The second object of the present invention is to provide a lithium manganese iron phosphate having the advantages of low cost and high purity.

[0007] A third object of the present invention is to provide a secondary battery.

[0008] A fourth object of the present invention is to provide an electrical device.

[0009] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are particularly adopted:

[0010] A method for preparing lithium iron manganese phosphate from waste lithium iron phosphate comprises the following steps:

[0011] The positive electrode of waste lithium iron phosphate, a manganese source, a phosphorus source, a lithium source and a carbon source are fully mixed in water, and then ground, dried and sintered in sequence to obtain lithium manganese iron phosphate.

[0012] A lithium iron manganese phosphate is prepared by adopting the method for preparing lithium iron manganese phosphate with waste lithium iron phosphate.

[0013] A secondary battery comprises the lithium manganese iron phosphate.

[0014] An electrical device comprises the secondary battery.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] (1) The present invention uses waste lithium iron phosphate as raw material, supplements lithium, manganese and phosphorus elements, and synthesizes lithium iron manganese phosphate with excellent performance by a high-temperature solid phase method; compared with traditional lithium iron phosphate recovery (preparation by recovery of lithium carbonate and iron phosphate), the present invention not only provides a lithium iron phosphate recycling scheme with higher efficiency and higher recovery rate, but also provides a synthesis process of lithium iron manganese phosphate, which has good practicality and economy.

[0017] (2) Since waste lithium iron phosphate contains carbon black, carbon nanotubes, binders, etc., and the present invention does not remove them through high-temperature treatment or chemical treatment, the lithium manganese iron phosphate prepared in this way has better carbon coating, better material conductivity, and more excellent electrochemistry; at the same time, waste lithium iron phosphate itself contains a carbon source, so the amount of carbon source used in the synthesis process is also reduced.

[0018] (3) Compared with the traditional resource recovery method of lithium iron phosphate, the present invention does not require oxidation calcination or other chemical treatment of waste lithium iron phosphate. The entire synthesis process is simple, easy to operate and has low production cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0020] Figure 1 The XRD spectrum of lithium manganese iron phosphate prepared in Example 1 of the present invention is provided;

[0021] Figure 2 The SEM spectrum of lithium manganese iron phosphate prepared in Example 1 of the present invention is provided;

[0022] Figure 3 The charge and discharge curves of the test examples of the present invention are provided. DETAILED DESCRIPTION

[0023] The technical scheme of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and specific embodiments, but those skilled in the art will appreciate that the following described embodiments are part of embodiments of the present invention, rather than all embodiments, and are only used to illustrate the present invention, and should not be considered as limiting the scope of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present invention. If specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer is not specified in the reagents or instruments used, they are all conventional products that can be purchased commercially. In addition, the terms "first", "second" and "third" are only used for descriptive purposes and cannot be interpreted as indicating or implying relative importance.

[0024] A first aspect of the present invention is to provide a method for preparing lithium iron manganese phosphate from waste lithium iron phosphate.

[0025] There are already many methods for recycling waste lithium iron phosphate positive electrode materials in the prior art; however, on the one hand, waste lithium iron phosphate is often used to recover battery-grade lithium iron phosphate, and on the other hand, it is usually necessary to perform pre-operations such as high-temperature heat treatment and acid treatment on the waste lithium iron phosphate positive electrode, which limits the application of waste lithium iron phosphate and increases the resource cost of lithium iron phosphate.

[0026] The method of the present invention mainly comprises the following steps: fully mixing the positive electrode of waste lithium iron phosphate, a manganese source, a phosphorus source, a lithium source and a carbon source in water, and then grinding, drying and sintering in sequence to obtain lithium manganese iron phosphate.

[0027] It is understandable that the positive electrode of the waste lithium iron phosphate of the present invention is obtained by disassembling and separating the discarded lithium iron phosphate battery, or the positive electrode of the waste lithium iron phosphate can also be qualified or unqualified finished lithium iron phosphate. As a preferred embodiment, the present invention also includes: crushing the positive electrode of the waste lithium iron phosphate, and the crushing treatment includes one of mechanical crushing, low-temperature crushing or ultrasonic crushing, but it is worth noting that the positive electrode of the waste lithium iron phosphate or the waste lithium iron phosphate battery has not undergone any heat treatment or chemical treatment (such as acid treatment, etc.).

[0028] As a preferred embodiment, the manganese source includes at least one of trimanganese tetroxide, manganese dioxide, manganese hydroxide or manganese carbonate.

[0029] As a preferred embodiment, the phosphorus source includes at least one of lithium hydrogen phosphate, lithium dihydrogen phosphate, ammonium phosphate, monoammonium phosphate or diammonium phosphate.

[0030] As a preferred embodiment, the lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, dilithium hydrogen phosphate or lithium phosphate.

[0031] As a preferred embodiment, the carbon source includes at least one of glucose, sucrose, maltose, starch, cellulose, citric acid or ascorbic acid.

[0032] As a preferred embodiment, the mixed slurry also includes additives, and the additives include one or two of organic dispersants or dopants; wherein the organic dispersant includes at least one of polyethylene glycol, polyvinyl alcohol, polyvinyl pyrrolidone, methyl pentanol or a cellulose derivative, and the dopant includes a compound containing at least one element of magnesium, titanium, aluminum, niobium, copper, zinc or ruthenium; it can be understood that each compound selected in the dopant usually contains only one of the above elements, and several compound additives can be selected to provide the above elements.

[0033] As an optional embodiment, when the element contained in the dopant is magnesium (Mg), the dopant includes magnesium oxide, magnesium hydroxide, magnesium chloride, magnesium carbonate or magnesium nitrate, etc.; when the element contained in the dopant is titanium (Ti), the dopant includes titanate, titanium dioxide or titanium tetrachloride, etc.; when the element contained in the dopant is aluminum (Al), the dopant includes aluminum oxide, aluminum hydroxide, aluminum chloride, aluminum nitrate, aluminate, etc.; when the element contained in the dopant is niobium (Nb), the dopant includes niobium oxide, niobium chloride or niobium acetate, etc.; when the element contained in the dopant is copper (Cu), the dopant includes copper oxide, copper hydroxide, cuprous oxide, copper carbonate, copper nitrate, etc.; when the element contained in the dopant is zinc (Zn), the dopant includes zinc oxide, zinc hydroxide, zinc nitrate, zinc chloride, etc.; when the element contained in the dopant is ruthenium (Ru), the dopant includes ruthenium oxide, ruthenium oxalate, ruthenium nitrate, etc.

[0034] As a preferred embodiment, in the mixed slurry, the molar ratio of lithium, manganese, iron and phosphorus should be controlled within the range of x:(1-y):y:1, wherein x=1.0-1.2 and y=0.4-0.6. In some optional embodiments, x includes but is not limited to any one of 1, 1.02, 1.05, 1.08, 1.1, 1.12, 1.15, 1.18 and 1.2 or any two thereof, and y includes but is not limited to any one of 0.4, 0.42, 0.45, 0.48, 0.5, 0.52, 0.55, 0.58 and 0.6 or any two thereof.

[0035] It is understandable that the present invention does not directly limit the amount of manganese source, phosphorus source, lithium source and carbon source. This is because the molar amount of the elements contained in the positive electrode of the waste lithium iron phosphate is uncertain. In the actual resource recovery process, it is necessary to adaptively adjust the content of the additional added materials according to the molar amount contained in the waste.

[0036] As a preferred embodiment, the amount of the carbon source added is 0-30% based on the mass of the positive electrode of the waste lithium iron phosphate. Those skilled in the art should adaptively adjust the amount of the carbon source added according to the newness or degree of breakage of the positive electrode of the waste lithium iron phosphate. In some optional embodiments, when the positive electrode of the waste lithium iron phosphate is recovered from complete waste, the amount of the carbon source added is 0-5%; when the positive electrode of the waste lithium iron phosphate is derived from lithium iron phosphate finished material, the amount of the carbon source added is 10%-30%.

[0037] As a preferred embodiment, the solid content of the slurry obtained by mixing is 25% to 50%, including but not limited to any one of 25%, 28%, 30%, 32%, 35%, 38%, 40%, 45%, and 50%, or a numerical range consisting of any two of them.

[0038] As a preferred embodiment, after the grinding, the particle size of the slurry D50 is 0.35μm to 0.45μm, including but not limited to any one of 0.35, 0.36, 0.37, 0.38, 0.39, 0.4, 0.41, 0.42, 0.43, 0.44, 0.45 (μm) or a numerical range consisting of any two of them.

[0039] As an optional embodiment, the mixing may be assisted by oscillation, stirring, centrifugation, ultrasound, etc. to obtain a uniformly dispersed slurry system.

[0040] As a preferred embodiment, the grinding includes mechanical grinding methods such as ball milling or sand milling. Mixing the recycled waste materials with the exogenous materials by grinding can not only play a role in mixing the reaction raw materials evenly, but also play a role in refining the particle size of the raw materials, thereby facilitating the uniformity of the lithium manganese iron phosphate particles and the uniform dispersion of the modifying elements in the additives in the material.

[0041] As a preferred embodiment, the drying is carried out by spray drying; the air inlet temperature of the spray drying is 180°C to 250°C, the air outlet temperature of the spray drying is 80°C to 110°C, and the sprayer frequency is 35Hz to 50Hz.

[0042] As a preferred embodiment, the sintering is carried out in a protective atmosphere, and the protective gas used includes but is not limited to nitrogen, helium, neon or argon.

[0043] As a preferred embodiment, the sintering includes two methods such as (a) or (b), both of which belong to programmed temperature rise, but there are differences in the insulation period; specifically:

[0044] (a) heating to 400°C to 500°C at a first rate, keeping the temperature for 2h to 4h, and then heating to 600°C to 800°C at the first rate again, and keeping the temperature for 8h to 10h again;

[0045] (b) heating to 350° C. to 450° C. at a first rate, then heating to 500° C. to 700° C. at a second rate, then heating to 600° C. to 800° C. at the first rate, and then keeping the temperature for 8 h to 10 h;

[0046] Wherein, the first rate is 1°C / min to 3°C / min, and the second rate is 0.5°C / min to 0.9°C / min.

[0047] It can be understood that in the present invention, by means of programmed temperature increase, by controlling and adopting different temperature increase rates in different temperature ranges, sufficient sintering and crystallization reactions can be achieved in a specific temperature range.

[0048] In the present invention, the sintering temperature and time are limited to the above ranges respectively. On the one hand, when the temperature is too low, the molecular activity and molecular thermal motion are not obvious enough, so they can only be compensated by the sintering time, which easily leads to a waste of time and energy costs, and the degree of mixing of waste materials and exogenous materials is insufficient, making it difficult to form uniform grains; on the other hand, the molecular activity and molecular thermal motion will be too high, and side reactions of additive elements with positive electrode materials or overburning of lithium manganese iron phosphate may occur; and the above sintering treatment can obtain lithium manganese iron phosphate positive electrode materials with appropriate compaction density and electrochemical properties.

[0049] The second aspect of the present invention is to provide lithium manganese iron phosphate obtained by the method for preparing lithium manganese iron phosphate using waste lithium iron phosphate as described in the first aspect.

[0050] A third aspect of the present invention is to provide a secondary battery.

[0051] The secondary battery includes the lithium iron manganese phosphate as described in the second aspect. It is understandable that the secondary battery should include a positive electrode, a negative electrode, an electrolyte and a separator, as well as other necessary or unnecessary functional elements or packaging components, etc., and those skilled in the art can make any selection and combination thereof; and the active ingredients of the positive electrode should at least include the lithium iron manganese phosphate, but the selection of other active ingredients in the positive electrode is not excluded. It is understandable that when the lithium iron manganese phosphate is included in the secondary battery, it can be regarded as an embodiment of the present invention.

[0052] A fourth aspect of the present invention is to provide an electrical device.

[0053] The electrical equipment includes the secondary battery. The electrical equipment can be any equipment or device that relies on electric energy to work or operate, including but not limited to new energy vehicles, building electrical equipment, industrial electrical appliances, household and agricultural electrical appliances, etc. When the lithium-ion battery is included, any electrical equipment equipped with the lithium-ion battery can belong to an embodiment of the present invention.

[0054] Example 1

[0055] (1) Weigh 400 g of waste lithium iron phosphate (mainly samples from dismantled 3Ah lithium iron phosphate batteries, the main components of which are shown in Table 1 below), 284.21 g of manganese tetraoxide, 364.58 g of lithium dihydrogen phosphate, 0.32 g of lithium carbonate, 26.24 g of glucose, and 1614 g of water, and mix them. After testing, the molar ratio of Li, Mn, Fe and P elements in the mixed slurry is 1.02:0.6:0.4:1, and the solid content of the slurry is 40%. The mixed slurry is sand-milled by a sand mill to control the slurry D50 particle size to 0.41 μm.

[0056] (2) The sand-milled slurry was spray-dried at an inlet air temperature of 220° C., an outlet air temperature of 90° C., and an atomizer frequency of 50 Hz to obtain a lithium manganese iron phosphate precursor.

[0057] (3) The lithium iron manganese phosphate precursor is placed in a box-type atmosphere furnace, and under the protection of a nitrogen atmosphere, the temperature is first increased to 450° C. at a rate of 2° C. / min and kept at that temperature for 3 hours, and then the temperature is increased to 700° C. at a rate of 2° C. / min and kept at that temperature for 9 hours; after natural cooling, the lithium iron manganese phosphate of this embodiment can be obtained by crushing.

[0058] The lithium manganese iron phosphate of this embodiment was sampled and subjected to XRD and SEM tests. The results are as follows: Figure 1 and Figure 2As shown. The XRD results show that the diffraction peak of the synthesized material corresponds to lithium manganese iron phosphate, and the peak is sharp and the crystallinity is good. The SEM results show that the primary particles of lithium manganese iron phosphate are spherical, and the primary particle size is 80nm to 200nm.

[0059] Example 2

[0060] It is basically the same as Example 1, except that:

[0061] (1) Weigh 400 g of finished lithium iron phosphate (commercially available finished lithium iron phosphate, the main components of which are shown in Table 1 below), 292.55 g of manganese tetraoxide, 374.39 g of lithium dihydrogen phosphate, 0.45 g of lithium carbonate, 106.73 g of glucose, and 1761 g of water respectively, and mix them. After testing, the molar ratio of Li, Mn, Fe and P elements in the mixed slurry is 1.02:0.6:0.4:1, and the solid content of the slurry is 40%. The mixed slurry is sand-milled by a sand mill to control the slurry D50 particle size to be 0.41 μm.

[0062] Table 1

[0063]

[0064] Example 3

[0065] It is basically the same as Example 1, except that:

[0066] (3) The lithium iron manganese phosphate precursor is placed in a box-type atmosphere furnace, and under the protection of a nitrogen atmosphere, the temperature is raised to 400°C at 2°C / min, then raised to 600°C at 0.7°C / min, and then raised to 750°C at 2°C / min, and kept at this temperature for 8 hours. After natural cooling, the lithium iron manganese phosphate can be obtained by crushing.

[0067] Example 4

[0068] It is basically the same as Example 1, except that:

[0069] (1) 400 g of waste lithium iron phosphate, 318.83 g of manganese dioxide, 364.58 g of lithium dihydrogen phosphate, 0.20 g of lithium hydroxide, 25.46 g of citric acid, and 2587.8 g of water were weighed and mixed. After testing, the molar ratio of Li, Mn, Fe and P elements in the mixed slurry was 1.02:0.6:0.4:1, and the solid content of the slurry was 30%.

[0070] Example 5

[0071] It is basically the same as Example 1, except that:

[0072] Also added in step (1) was 1.96 g of magnesium dioxide.

[0073] Example 6

[0074] It is basically the same as Example 1, except that:

[0075] (2) The sand-milled slurry was spray-dried at an inlet air temperature of 250° C., an outlet air temperature of 110° C., and an atomizer frequency of 50 Hz to obtain a lithium manganese iron phosphate precursor.

[0076] Example 7

[0077] It is basically the same as Example 1, except that:

[0078] (2) The sand-milled slurry was spray-dried at an inlet air temperature of 180° C., an outlet air temperature of 80° C., and an atomizer frequency of 50 Hz to obtain a lithium manganese iron phosphate precursor.

[0079] Comparative Example

[0080] It is basically the same as Example 1, except that:

[0081] (3) The lithium iron manganese phosphate precursor is placed in a box-type atmosphere furnace, and sintered and kept warm at 700° C. for 12 hours under the protection of a nitrogen atmosphere; after natural cooling, the lithium iron manganese phosphate of this embodiment is obtained by crushing.

[0082] Test example

[0083] The lithium manganese iron phosphate of the above-mentioned embodiments and comparative examples are used to prepare corresponding test batteries. The specific steps are as follows: the lithium manganese iron phosphate positive electrode materials, conductive agent (Super-P), and binder (PVDF) of each embodiment and comparative example are mixed evenly in a mass ratio of 94:3:3 as solutes, and N-methylpyrrolidone (NMP) is used as solvent. A centrifugal degassing machine is used to fully mix and prepare positive electrode slurry. The slurry is coated on aluminum foil and dried in a vacuum drying oven at 100°C for 12 hours to make a positive electrode sheet. In an argon atmosphere glove box, the positive electrode sheet, the negative electrode sheet (metal lithium), the diaphragm (U.S. Celgard2325) and the electrolyte (1 mol / LLiPF 6 The ethylene carbonate / diethyl carbonate (volume ratio 1:1) solution) was assembled into a CR2032 button battery.

[0084] The charging and discharging test was conducted by using a blue battery tester; at room temperature (25°C) and a charge and discharge cut-off voltage of 2.5V to 4.5V, the battery was charged at 0.1C constant current and constant voltage (cut-off current 0.02C), discharged at 0.1C constant current (cycled twice); charged at 0.2C constant current and constant voltage, discharged at 0.2C constant current; and finally charged at 0.2C constant current and constant voltage, and discharged at 1C constant current to test the performance of lithium manganese iron phosphate. During this test, the following parameter characteristics were recorded and recorded in Table 2.

[0085] (1) 0.1C discharge specific capacity: the specific capacity of the first cycle of lithium manganese iron phosphate discharge.

[0086] (2) First efficiency: 0.1C first cycle discharge specific capacity / 0.1C first cycle charge specific capacity, which is the coulombic efficiency of the first cycle lithium manganese iron phosphate.

[0087] (3) 1C discharge specific capacity: that is, the specific capacity of lithium manganese iron phosphate discharge in the fourth cycle.

[0088] Furthermore, if Figure 3 The figure shows the charge and discharge curves of the battery prepared in Example 1 at 0.1C, 0.5C and 1C, which can reflect that the lithium manganese iron phosphate battery prepared by the method of the present invention has good charge and discharge performance.

[0089] Table 2

[0090]

[0091] Although the present invention has been illustrated and described with specific embodiments, it should be appreciated that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Those skilled in the art should understand that the technical solutions described in the above embodiments may be modified, or some or all of the technical features thereof may be replaced by equivalents without departing from the spirit and scope of the present invention. However, these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention. Therefore, this means that all such replacements and modifications within the scope of the present invention are included in the appended claims.

Claims

1. A method for preparing lithium iron manganese phosphate from waste lithium iron phosphate, characterized in that: The steps include: The positive electrode of waste lithium iron phosphate, a manganese source, a phosphorus source, a lithium source and a carbon source are fully mixed in water, and then ground, dried and sintered in sequence to obtain lithium manganese iron phosphate.

2. The method for preparing lithium iron manganese phosphate from waste lithium iron phosphate according to claim 1, characterized in that: The manganese source includes at least one of manganese tetraoxide, manganese dioxide, manganese hydroxide or manganese carbonate; and / or, the phosphorus source comprises at least one of lithium hydrogen phosphate, lithium dihydrogen phosphate, ammonium phosphate, monoammonium phosphate or diammonium phosphate; and / or, the lithium source comprises at least one of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, dilithium hydrogen phosphate or lithium phosphate; And / or, the carbon source comprises at least one of glucose, sucrose, maltose, starch, cellulose, citric acid or ascorbic acid.

3. The method for preparing lithium iron manganese phosphate from waste lithium iron phosphate according to claim 1, characterized in that: The mixed slurry further includes an additive, wherein the additive includes one or both of an organic dispersant and a dopant; Wherein, the organic dispersant includes at least one of polyethylene glycol, polyvinyl alcohol, polyvinyl pyrrolidone, methyl amyl alcohol or a cellulose derivative, and the dopant includes at least one element of magnesium, titanium, aluminum, niobium, copper, zinc or ruthenium.

4. The method for preparing lithium iron manganese phosphate from waste lithium iron phosphate according to claim 1, characterized in that: In the mixed slurry, the molar ratio of lithium element, manganese element, iron element and phosphorus element should be controlled within the range of x:(1-y):y:1, wherein x=1.0-1.2 and y=0.4-0.

6.

5. The method for preparing lithium iron manganese phosphate from waste lithium iron phosphate according to claim 1, characterized in that: The solid content of the slurry obtained by mixing is 25% to 50%; And / or, after the grinding, the particle size of the slurry D50 is 0.35 μm to 0.45 μm.

6. The method for preparing lithium iron manganese phosphate from waste lithium iron phosphate according to claim 1, characterized in that: The drying is carried out by spray drying; The inlet air temperature of the spray drying is 180°C to 250°C, the outlet air temperature of the spray drying is 80°C to 110°C, and the sprayer frequency is 35Hz to 50Hz.

7. The method for preparing lithium iron manganese phosphate from waste lithium iron phosphate according to claim 1, characterized in that: The sintering includes one of the following features (a) or (b): (a) heating to 400°C to 500°C at a first rate, keeping the temperature for 2h to 4h, and then heating to 600°C to 800°C at the first rate again, and keeping the temperature for 8h to 10h again; (b) heating to 350° C. to 450° C. at a first rate, then heating to 500° C. to 700° C. at a second rate, then heating to 600° C. to 800° C. at the first rate, and then keeping the temperature for 8 h to 10 h; The first rate is 1°C / min to 3°C / min, and the second rate is 0.5°C / min to 0.9°C / min.

8. Lithium manganese phosphate obtained by the method for preparing lithium manganese phosphate from waste lithium iron phosphate as claimed in any one of claims 1 to 7.

9. A secondary battery, characterized in that: Comprising the lithium manganese iron phosphate as described in claim 8.

10. An electrical device, characterized in that: Comprising the secondary battery as claimed in claim 9.

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