Manganese iron phosphate precursor, preparation method therefor, lithium manganese iron phosphate, positive electrode sheet containing same, and secondary battery

Abstract

Disclosed in the present invention are a manganese iron phosphate precursor, a preparation method therefor, lithium manganese iron phosphate, a positive electrode sheet containing same, and a secondary battery. The preparation method for the manganese iron phosphate precursor comprises the following steps: carrying out a first reaction on a reactant A and a reactant B to prepare the manganese iron phosphate precursor, the reactant A comprising an iron source, a manganese source and water, the reactant B comprising a phosphorus source and water, and at least one of the reactant A and the reactant B comprising a first organic acid. The preparation method provided by the present invention involves a simple process; the manganese phase and the iron phase in a prepared lithium manganese iron phosphate product are uniformly distributed, so that said lithium manganese iron phosphate product has good electrochemical properties when being applied to secondary batteries.

Description

Manganese iron phosphate precursor, preparation method thereof, lithium manganese iron phosphate, positive plate containing same, and secondary battery

[0001] This application claims priority to Chinese patent application 202410628466X with a filing date of 2024 / 5 / 21. This application incorporates the entirety of the aforementioned Chinese patent application. TECHNICAL FIELD

[0002] The present application relates to a manganese iron phosphate precursor, a preparation method thereof, lithium manganese iron phosphate, a positive plate containing same, and a secondary battery. BACKGROUND

[0003] Among phosphate positive electrode material systems, LiFePO4 has achieved large-scale production and is widely used in power batteries and energy storage batteries. However, the voltage platform of LiFePO4 is 3.4 V, and the overall energy density of the battery is low, which limits its development in electric vehicles. If LiMnPO4 can obtain a specific capacity comparable to LiFePO4, it means that the energy density will be 35% higher than LiFePO4. Lithium manganese iron phosphate (LiMnPO4) positive material has the stability of lithium iron phosphate and can improve the energy density of the battery. x Fe (1-x) PO4) positive material has the stability of lithium iron phosphate and can improve the energy density of the battery.

[0004] Currently, there are some problems in the development of lithium manganese iron phosphate material. First, the electrical conductivity of lithium manganese iron phosphate is poorer than that of lithium iron phosphate. Second, in the process of electrochemical reaction, the Jahn-Teller effect leads to poor rate performance and low discharge specific capacity of the material. The current means to solve the above technical problems is to improve the ion and electron transport capacity, and at the same time, introduce other metal elements to change the crystal structure of lithium manganese iron phosphate, thereby improving its stability. The improvement of electron and ion transport can be realized by reducing the particle size, coating and doping, etc. At the same time, by doping other elements, the crystal form of lithium manganese iron phosphate is improved, the Jahn-Teller effect is relieved, and its stability is improved.

[0005] In the prior art, the methods for preparing lithium manganese iron phosphate mainly include liquid phase method and high-temperature solid phase method. The liquid phase method can prepare lithium manganese iron phosphate with high purity, but the carbon coating effect is poor, the tap density of the prepared lithium manganese iron phosphate powder is low, and it needs to be prepared under high temperature and high pressure conditions, the production process is complex, and a large amount of three wastes are generated in the production process, polluting the environment. The high-temperature solid phase method has a simple production process, but there is a problem of uneven distribution of manganese iron phase, poor stability of the material, and poor rate performance and cycle performance. SUMMARY

[0006] The present application mainly aims to overcome the defects that the method for preparing lithium manganese iron phosphate in the prior art is difficult to realize uniform distribution of manganese iron phases and simple process, and provides a lithium manganese iron phosphate precursor and a preparation method thereof, lithium manganese iron phosphate, a positive plate containing the same, and a secondary battery. The preparation method provided by the present application is simple in process, the prepared lithium manganese iron phosphate product has uniform distribution of manganese phases and iron phases, and has good electrochemical performance when applied in a secondary battery.

[0007] To solve the above technical problems, the present application provides the following technical solutions.

[0008] In a first aspect, the present application provides a preparation method of a lithium manganese iron phosphate precursor, comprising the following steps:

[0009] The reactant A and the reactant B are subjected to a first reaction to prepare the lithium manganese iron phosphate precursor; the reactant A comprises an iron source, a manganese source and water; the reactant B comprises a phosphorus source and water, and at least one of the reactant A and the reactant B contains a first organic acid.

[0010] In the present application, the iron source can be a conventional iron-containing substance in the art, for example, at least one of iron powder, ferric oxide, diiron trioxide and iron oxide, preferably iron powder.

[0011] Preferably, the purity of the iron powder is greater than 97wt%.

[0012] Preferably, the iron powder is one or more of primary reduced iron powder, secondary reduced iron powder, carbonyl iron powder, electrolytic iron powder and water-atomized iron powder.

[0013] Preferably, the particle size of the iron powder is 250-600 mesh, for example 250 mesh. The iron powder with this particle size has better reactivity, which is beneficial to improve the uniform distribution of iron phases.

[0014] In the present application, the manganese source can be a conventional manganese-containing substance in the art, for example, at least one of manganese powder, diiron trioxide, ferric oxide and manganese oxide.

[0015] Preferably, the purity of the manganese powder is greater than 95wt%.

[0016] Preferably, the manganese powder is one or more of primary reduced manganese powder, secondary reduced manganese iron powder, carbonyl manganese powder, electrolytic manganese powder and water-atomized manganese powder.

[0017] Preferably, the particle size of the manganese powder is 200-400 mesh, for example 200 mesh. Since the reactivity of manganese is higher than that of iron, the manganese powder with this particle size is more beneficial to form uniform manganese phases.

[0018] In the present application, the phosphorus source can be a conventional phosphorus-containing compound in the art, such as phosphoric acid and / or phosphate, preferably food-grade phosphoric acid, electrical-grade phosphoric acid or industrial-grade phosphoric acid; the phosphoric acid is preferably added in the form of a phosphoric acid solution, and the concentration of phosphoric acid in the phosphoric acid solution is greater than 40%.

[0019] In the present application, the first organic acid is an organic compound containing a carboxyl group and having acidity, and the organic acid can be at least one of monobasic carboxylic acid, dibasic carboxylic acid and tribasic carboxylic acid; the monobasic carboxylic acid is preferably one or more of acetic acid, propionic acid, lactic acid and benzoic acid; the dibasic carboxylic acid is preferably one or more of oxalic acid, benzene dicarboxylic acid, maleic acid, adipic acid, azelaic acid, isophthalic acid and terephthalic acid; and the tribasic carboxylic acid is preferably one or more of trimesic acid, trimellitic acid, citric acid and tartaric acid.

[0020] In some preferred embodiments, the first organic acid is monobasic carboxylic acid. Since there is only one carboxyl group in one organic acid molecule, the probability of forming a ferric phosphate or manganese phosphate compound gel due to the bridging effect of the carboxyl group is reduced, which is conducive to the reaction.

[0021] In the present application, the molar ratio of iron provided by the iron source to phosphorus provided by the phosphorus source can be (0.1-0.8):1, such as 0.21:1 or 0.41:1.

[0022] In the present application, the molar ratio of manganese provided by the manganese source to phosphorus provided by the phosphorus source can be (0.2-0.8):1, such as 0.57:1 or 0.77:1.

[0023] In the present application, the molar ratio of carboxyl provided by the first organic acid to phosphorus provided by the phosphorus source can be (0.75-2.0):1, such as 0.78:1.

[0024] In the present application, the molar ratio of carboxyl provided by the first organic acid to phosphorus provided by the phosphorus source can be (0.7-2.0):1.

[0025] In the present application, the reactant A can further include a first dopant containing at least one of magnesium, cobalt, titanium, vanadium, nickel, niobium, copper and tungsten.

[0026] Preferably, the first dopant is one or more of magnesium oxide, titanium dioxide and niobium dioxide.

[0027] Preferably, the molar ratio of the dopant element provided by the first dopant to the phosphorus provided by the phosphorus source is (0.01-0.05):1, such as 0.03:1.

[0028] In the present application, the first reaction can be carried out by dropping the reactant A into the reactant B; the dropping time is preferably 8-15 min, for example 10 min.

[0029] In the present application, the first reaction can be carried out under stirring, and the stirring speed is preferably 300-800 rpm, for example 500 rpm.

[0030] In the present application, the first reaction can be carried out under inert atmosphere, and the gas of the inert atmosphere is preferably nitrogen, and the flow rate of the nitrogen is preferably 15 L / min-30 L / min, for example 15 L / min.

[0031] In the present application, the temperature of the first reaction is preferably 80℃-95℃, for example 85℃.

[0032] In the present application, the time of the first reaction is preferably 2h-7h, for example 5.5h.

[0033] In the present application, after the first reaction, post-treatment is further carried out, and the post-treatment includes washing and drying.

[0034] In the present application, the preparation method of the manganese iron phosphate precursor, for example, includes the following steps:

[0035] The reactant A and the reactant B are subjected to the first reaction to prepare the manganese iron phosphate precursor; the reactant A includes an iron source, a manganese source and water; the reactant B includes a phosphorus source and water, and the reactant B contains a first organic acid, and the first organic acid is a monobasic carboxylic acid;

[0036] In the present application, the iron source is iron powder; the manganese source is manganese powder; and the phosphorus source is phosphoric acid.

[0037] In the third aspect, the present application provides a preparation method of manganese iron lithium phosphate, which comprises the following steps:

[0038] In the present application, the manganese iron phosphate precursor is an amorphous substance or a substance with low crystallinity, which is a mixture of various iron phosphate salts and manganese phosphate salts.

[0039] In the third aspect, the present application provides a preparation method of manganese iron lithium phosphate, which comprises the following steps:

[0040] In the present application, the reactant C can further comprise a second dopant comprising at least one of magnesium, cobalt, titanium, vanadium, nickel, niobium, copper and tungsten.

[0041] Preferably, the second dopant is one or more of magnesium oxide, titanium dioxide and niobium dioxide.

[0042] Preferably, the molar ratio of the dopant element provided by the second dopant to the phosphorus element provided by the phosphorus source is (0.01-0.05):1.

[0043] In the present application, the lithium source can be a conventional lithium-containing compound in the art. For example, it is at least one of lithium phosphate compounds, lithium carbonate and lithium hydroxide; preferably, it is lithium carbonate.

[0044] Preferably, the lithium phosphate compound is one or more of lithium dihydrogen phosphate, lithium hydrogen phosphate and lithium phosphate.

[0045] In the present application, the second organic acid is an organic compound having carboxyl group and having acidity. The second organic acid can be at least one of monobasic carboxylic acid, dibasic carboxylic acid and tribasic carboxylic acid. Preferably, the monobasic carboxylic acid is one or more of acetic acid, propionic acid, lactic acid and benzoic acid. Preferably, the dibasic carboxylic acid is one or more of oxalic acid, benzene dicarboxylic acid, maleic acid, adipic acid, azelaic acid, isophthalic acid and terephthalic acid. Preferably, the tribasic carboxylic acid is one or more of trimesic acid, trimellitic acid, citric acid and tartaric acid.

[0046] In the present application, the molar ratio of the carboxyl group provided by the second organic acid to the phosphorus element provided by the phosphorus source can be (0.7-2.0):1, for example, 0.78:1.

[0047] In some preferred embodiments, the second organic acid is monobasic carboxylic acid. Since there is only one carboxyl group in one organic acid molecule, the probability of the lithium iron phosphate compound gel is not increased due to the bridging effect of the carboxyl group.

[0048] In the present application, the first organic acid and the second organic acid can be the same in kind.

[0049] In the present application, the carbon source can be a conventional carbon-containing substance, preferably one or more of carbon material, metal compound and conductive polymer. Preferably, the carbon material is glucose, glucose derivative, organic acid, organic acid derivative, phenol formaldehyde resin, polyethylene, polyethylene glycol, polyvinyl alcohol, polyvinyl alcohol derivative, polyacrylic acid, polyacrylic acid derivative, heterocyclic polymer with N or O element or polycondensate.

[0050] The glucose derivative preferably includes at least one of glucose, sucrose, starch and cyclodextrin.

[0051] The organic acid preferably includes at least one of formic acid, acetic acid, oxalic acid, citric acid, tartaric acid and malic acid.

[0052] The polyacrylic acid derivative preferably includes polyacrylate.

[0053] The heterocyclic polymer with N or O element preferably includes polyvinylpyrrolidone.

[0054] The metal compound preferably includes aluminum oxide and / or zinc stannate.

[0055] The conductive polymer preferably includes one or more of polyaniline, polystyrene mercaptan, polyacetylene, lithium carbonate and polycarbonate.

[0056] In the present application, the molar ratio of carbon element provided by the carbon source to phosphorus element provided by the phosphorus source can be (0.05-0.6):1.

[0057] In the present application, the molar ratio of lithium element provided by the lithium source to phosphorus element provided by the phosphorus source can be (1.01-1.05):1, for example 1.04:1.

[0058] In the present application, the particle size of the lithium iron manganese phosphate precursor can be 100nm-800nm, for example 200nm.

[0059] In the present application, the lithium iron manganese phosphate precursor can participate in subsequent reactions in the form of a solution.

[0060] The solid content of the lithium iron manganese phosphate precursor in the solution is preferably 30-70%, for example 44%, 45% or 46%.

[0061] The pH of the solution is preferably 3-8, for example 5.87, 5.94, 5.99 or 6.01.

[0062] The viscosity of the solution is preferably 1000mPa·s-20000mPa·s, for example 8000mPa·s, 7600mPa·s, 12000mPa·s, 9050mPa·s, 10050mPa·s, 8900mPa·s, 7200mPa·s, 8600mPa·s or 9700.

[0063] In the present application, after the second reaction, grinding and drying are further performed before sintering.

[0064] The drying method is preferably spray drying.

[0065] The temperature parameters of the spray drying are preferably: inlet temperature: 220℃, outlet temperature: 110℃.

[0066] The average particle size of the product obtained after drying can be 10μm-150μm.

[0067] In the present application, the sintering temperature can be 600℃-760℃, for example 660℃, 680℃ or 700℃.

[0068] In the present application, the sintering time can be 5h-20h, for example 15h.

[0069] In the present application, the sintering rate can be 3-10℃ / min, for example 5℃ / min.

[0070] In the present application, after the second sintering, crushing is further performed.

[0071] In the present application, the preparation method of the lithium manganese iron phosphate can comprise: the lithium manganese iron phosphate precursor as described above is subjected to a second reaction with a reactant C to obtain a lithium manganese iron phosphate precursor, and after sintering, the lithium manganese iron phosphate is obtained; the reactant C comprises a lithium source, a carbon source, a second organic acid and water; the second organic acid is a monocarboxylic acid; the reactant C further comprises a dopant.

[0072] In a fourth aspect, the present application provides a lithium manganese iron phosphate prepared by the preparation method as described above.

[0073] In the present application, the lithium manganese iron phosphate comprises secondary particles and free primary particles; the number of the free primary particles preferably accounts for 0.1%-5% of the total number of the secondary particles and the free primary particles. The secondary particles are obtained by agglomeration of the primary particles with carbon coating layers. The primary particles are connected by the carbon coating layers to form a loose secondary particle structure.

[0074] In the present application, the manganese phase and the iron phase in the lithium manganese iron phosphate are uniformly distributed. The uniform distribution means that, in the same field of view, the manganese element and the iron element in the SEM image of the lithium manganese iron phosphate are uniformly distributed in the field of view after EDS scanning.

[0075] In the present application, the carbon content in the lithium manganese iron phosphate can be 1.0-2.0%, for example 2.044%, 2.140%, 1.988%, 2.021%, 1.913%, 2.098%, 2.077%, 1.991% or 2.098%, and the percentage is the mass percentage of the carbon element in the total mass of the lithium manganese iron phosphate.

[0076] In the present application, the molecular formula of the lithium manganese iron phosphate material can be Li aMn b Fe c M d PO4@C, wherein "@C represents a carbon coating layer";

[0077] wherein, 1.01≤a≤1.07, 0.0

[0078] wherein, M is selected from at least one of magnesium, cobalt, titanium, vanadium, nickel, niobium, copper and tungsten.

[0079] In the present application, the particle size D10 of the secondary particles in the lithium iron manganese phosphate can be 0.4-0.5 μm, for example 0.41 μm, 0.42 μm, 0.44 μm, 0.45 μm or 0.46 μm.

[0080] In the present application, the particle size D50 of the secondary particles in the lithium iron manganese phosphate can be 1.2-3 μm, for example 1.53 μm, 1.47 μm, 1.42 μm, 1.68 μm, 1.31 μm, 1.35 μm, 1.28 μm, 1.36 μm or 1.24 μm.

[0081] In the present application, the particle size D90 of the secondary particles in the lithium iron manganese phosphate can be 5.0-7.5 μm, for example 6.3 μm, 5.93 μm, 5.43 μm, 6.3 μm, 6.07 μm, 7.26 μm, 6.44 μm, 6.6 μm or 6.06 μm.

[0082] In the present application, the specific surface area of the lithium iron manganese phosphate can be 16-19 m 2 / g, for example 18.1 m 2 / g, 20.5 m 2 / g, 18.2 m 2 / g, 17.9 m 2 / g, 17.8 m 2 / g, 17.6 m 2 / g, 17.5 m 2 / g, 17.3 m 2 / g, 17.1 m 2 / g, 16.8 m 2 / g or 16.5 m 2 / g.

[0083] In the present application, the compacted density of the lithium iron manganese phosphate can be 2.28 g / cm 3 -2.39 g / cm 3 , for example 2.39 g / cm 3 , 2.36 g / cm 3 , 2.34 g / cm 32.33 g / cm 3 2.31 g / cm 3 2.30 g / cm 3 .

[0084] In a fifth aspect, the present application provides a positive electrode sheet, comprising an active material layer and a positive electrode current collector; the active material layer comprises the lithium iron manganese phosphate as described above.

[0085] In a sixth aspect, the present application provides a secondary battery, comprising the positive electrode sheet as described above.

[0086] In the present application, the secondary battery is preferably a lithium ion battery. In the lithium ion battery, the positive electrode sheet, a negative electrode sheet, a separator and an electrolyte can be included.

[0087] On the basis of common general knowledge in the art, the above-mentioned preferred conditions can be combined arbitrarily, thereby obtaining preferred examples of the present application.

[0088] The reagents and raw materials used in the present application are commercially available.

[0089] The positive progress effect of the present application is that:

[0090] The preparation method provided by the present application has the advantages of simple process,

[0091] The lithium iron manganese phosphate prepared by the preparation method of the present application has the advantages of uniform distribution of manganese phase and iron phase, small specific surface area and high compaction density. When it is applied to a secondary battery, it is beneficial to improve the electrochemical performance of the battery, and the capacity retention rate at 1C / 0.1C can be up to 86.6% or more; the cycle number when the capacity decays to 70% of the initial capacity is greater than 245 cycles.

[0092] In some preferred embodiments, the capacity retention rate at 1C / 0.1C is as high as 92.2%. In other preferred embodiments, the cycle number when the capacity decays to 70% of the initial capacity is as high as 475 cycles. BRIEF DESCRIPTION OF DRAWINGS

[0093] FIG. 1 is an SEM image of the lithium iron manganese phosphate prepared in Example 1.

[0094] FIG. 2 is a TEM image of the lithium iron manganese phosphate prepared in Example 1.

[0095] FIG. 3 is a rate performance effect diagram of the lithium iron manganese phosphate prepared in Example 1.

[0096] FIG. 4 is an SEM image of the lithium iron manganese phosphate prepared in Example 1.

[0097] FIG. 5 is an EDS image of the manganese element of the lithium iron manganese phosphate prepared in Example 1.

[0098] Figure 6 is an EDS diagram of the iron element of the lithium manganese iron phosphate prepared in Example 1.

[0099] Figure 7 is an EDS diagram of the phosphorus element of the lithium manganese iron phosphate prepared in Example 1. DETAILED DESCRIPTION

[0100] The application will be further described in the following examples without limiting the application to the examples described. The experimental methods in the following examples, if not specified, are selected according to the conventional methods and conditions, or according to the product instructions.

[0101] In each of the examples, the types and amounts of the first organic acid and the second organic acid are the same.

[0102] Example 1

[0103] (1) Preparation of manganese iron phosphate precursor:

[0104] Preparation of reactant A: 920.83 g of iron powder and 1251.46 g of manganese powder were added to 4 kg of deionized water according to a molar ratio of manganese element to iron element of 0.57:0.4, and stirred uniformly.

[0105] Preparation of reactant B: 2810 g of lactic acid was dissolved in 10 kg of deionized water at 60°C, and then 4598 g of phosphoric acid with a concentration of 85 wt% was added and stirred uniformly.

[0106] Reactant A was added dropwise to reactant B under nitrogen protection within 10 minutes, and stirring was performed while dropping, the nitrogen flow rate was set to 15 L / min, the stirring speed was 500 rpm, the stirring speed could be adjusted according to the state of the solution to prevent the solution from splashing, and the reaction was performed at a temperature of 85°C for 5.5 h to obtain a solution containing a manganese iron phosphate precursor.

[0107] (2) 6.5 kg of deionized water was weighed, 147.5 g of glucose, 1525.29 g of lithium carbonate, and 2810 g of lactic acid were added, 1525.29 g of lithium carbonate was added, and stirring was performed until no bubbles were generated to obtain reactant C; then the solution containing the manganese iron phosphate precursor prepared in step (1) was added to 60°C, reactant C was added within 30 minutes, and 48.66 g of magnesium oxide was added while stirring, the temperature of the solution was controlled to be 60°C, and stirring was performed for 2 h, then a sand mill was started, and the slurry was sand milled to an average particle size of 200 nm to obtain a lithium manganese iron phosphate precursor slurry; the pH of the slurry was 5.87, the solid content was 45%, the viscosity was 8000 mPa·s, the particle size D10 was 0.134 μm, the particle size D50 was 0.206 μm, and the particle size D99 was 1.053 μm.

[0108] (3) grinding, drying, sintering and crushing the lithium manganese iron phosphate precursor slurry prepared in step (2) in sequence to obtain lithium manganese iron phosphate;

[0109] The drying condition is: spray drying, the inlet temperature is 220°C, and the outlet temperature is 110°C. The sintering condition is: gradually increasing the temperature from room temperature to 680°C at a rate of 5°C / min under a nitrogen atmosphere with a purity of 99.999%, and keeping the temperature at 680°C for 15 hours, and then cooling to obtain the sintered product. The crushing condition is: treating the sintered product by a gas flow crushing device to obtain the lithium manganese iron phosphate product, which contains secondary particles and free primary particles. The particle size D10 of the secondary particles is 0.41 μm, the particle size D50 is 1.53 μm, and the particle size D99 is 6.3 μm. The proportion of the free primary particles in the total number of the secondary particles and the free primary particles is 2.5%. The carbon content is 2.044% based on the total mass of the lithium manganese iron phosphate.

[0110] Example 2

[0111] Compared with Example 1, the difference lies in that:

[0112] The molar ratio of manganese element to iron element in step (1) is 0.77:0.20.

[0113] In step (2), the pH of the obtained lithium manganese iron phosphate precursor slurry is 5.94, the solid content is 44%, and the viscosity is 7600 mPa·s.

[0114] The lithium manganese iron phosphate product obtained in step (3) contains secondary particles and free primary particles. The particle size D10 of the secondary particles is 0.44 μm, the particle size D50 is 1.47 μm, the particle size D99 is 5.93 μm, and the proportion of the free primary particles in the total number of the secondary particles and the free primary particles is 1.5%. The carbon content is 2.140% based on the total mass of the lithium manganese iron phosphate.

[0115] Example 3

[0116] Compared with Example 1, the difference lies in that:

[0117] In step (2), the dopant is titanium dioxide, and the amount is 96.08 g. The pH of the obtained lithium manganese iron phosphate precursor slurry is 5.99, the solid content is 46%, and the viscosity is 12000 mPa·s.

[0118] The lithium manganese iron phosphate product obtained in step (3) contains secondary particles and free primary particles, wherein the particle size D10 of the secondary particles is 0.45 μm, the particle size D50 is 1.42 μm, the particle size D99 is 5.43 μm, and the proportion of the free primary particles in the total number of the secondary particles and the free primary particles is 1.8%; the carbon content is 1.988% based on the total mass of the lithium manganese iron phosphate.

[0119] Example 4

[0120] Compared with Example 1, the difference lies in that:

[0121] In step (2), the dopant is niobium dioxide, and the amount thereof is 149.47 g; the pH of the lithium manganese iron phosphate precursor slurry obtained is 6.01, the solid content is 44%, and the viscosity is 9050 mPa·s.

[0122] The lithium manganese iron phosphate product obtained in step (3) contains secondary particles and free primary particles, wherein the particle size D10 of the secondary particles is 0.46 μm, the particle size D50 is 1.68 μm, the particle size D99 is 6.3 μm, and the proportion of the free primary particles in the total number of the secondary particles and the free primary particles is 3%; the carbon content is 2.021% based on the total mass of the lithium manganese iron phosphate.

[0123] Example 5

[0124] Compared with Example 1, the difference lies in that:

[0125] In step (3), the sintering condition is that the temperature is gradually increased to 660 ℃ at a temperature increasing rate of 5 ℃ / min from room temperature under a nitrogen atmosphere with a purity of 99.999%, and then the temperature is kept constant at 660 ℃ for 15 h, and then the sintered product is obtained by cooling.

[0126] The lithium manganese iron phosphate product obtained finally contains secondary particles and free primary particles, wherein the particle size D10 of the secondary particles is 0.41 μm, the particle size D50 is 1.31 μm, the particle size D99 is 6.07 μm, and the proportion of the free primary particles in the total number of the secondary particles and the free primary particles is 1.5%; the carbon content is 1.913% based on the total mass of the lithium manganese iron phosphate.

[0127] Example 6

[0128] Compared with Example 1, the difference lies in that:

[0129] In step (3), the sintering condition is that the temperature is gradually increased to 700 ℃ at a temperature increasing rate of 5 ℃ / min from room temperature under a nitrogen atmosphere with a purity of 99.999%, and then the temperature is kept constant at 700 ℃ for 15 h, and then the sintered product is obtained by cooling.

[0130] The finally obtained lithium manganese iron phosphate product contains secondary particles and free primary particles, wherein the particle size D10 of the secondary particles is 0.41 μm, the particle size D50 is 1.35 μm, the particle size D99 is 7.26 μm, and the proportion of the free primary particles in the total number of the secondary particles and the free primary particles is 2.7%; the carbon content is 2.098% based on the total mass of the lithium manganese iron phosphate.

[0131] Example 7

[0132] Compared with Example 1, the difference lies in that:

[0133] The organic acid in step (1) is propionic acid, and the amount used is 2308 g.

[0134] The viscosity of the lithium manganese iron phosphate precursor slurry obtained in step (2) is 7200 mPa·s.

[0135] The finally obtained lithium manganese iron phosphate product contains secondary particles and free primary particles, wherein the particle size D10 of the secondary particles is 0.46 μm, the particle size D50 is 1.28 μm, the particle size D99 is 6.44 μm, and the proportion of the free primary particles in the total number of the secondary particles and the free primary particles is 3.2%; the carbon content is 2.077% based on the total mass of the lithium manganese iron phosphate.

[0136] Example 8

[0137] Compared with Example 1, the difference lies in that:

[0138] The organic acid in step (1) is oxalic acid, and the amount used is 2279 g.

[0139] The solid content of the lithium manganese iron phosphate precursor slurry obtained in step (2) is 44%, and the viscosity is 8600 mPa·s.

[0140] The finally obtained lithium manganese iron phosphate product contains secondary particles and free primary particles, wherein the particle size D10 of the secondary particles is 0.42 μm, the particle size D50 is 1.36 μm, the particle size D99 is 6.6 μm, and the proportion of the free primary particles in the total number of the secondary particles and the free primary particles is 3.5%; the carbon content is 1.991% based on the total mass of the lithium manganese iron phosphate.

[0141] Example 9

[0142] Compared with Example 1, the difference lies in that:

[0143] The organic acid in step (1) is tartaric acid, and the amount used is 1560 g.

[0144] The solid content of the lithium iron manganese phosphate precursor slurry obtained in step (2) is 46%, and the viscosity is 9700 mPa·s.

[0145] The lithium iron manganese phosphate product obtained in step (3) contains secondary particles and free primary particles, wherein the particle size D10 of the secondary particles is 0.45 μm, the particle size D50 is 1.24 μm, the particle size D99 is 6.06 μm, and the proportion of the free primary particles in the total number of "secondary particles and free primary particles" is 2.7%; the carbon content is 2.098% based on the total mass of the lithium iron manganese phosphate.

[0146] Example 10

[0147] Compared with Example 1, the difference is that:

[0148] The organic acid in step (1) is added in reactant A, and no reactant B is added.

[0149] The carbon content in the final lithium iron manganese phosphate is 1.987% based on the total mass of the lithium iron manganese phosphate.

[0150] Example 11

[0151] Compared with Example 1, the difference is that:

[0152] The total amount of the organic acid in step (1) is unchanged, and half of it is added in each of reactant A and reactant B.

[0153] The carbon content in the final lithium iron manganese phosphate is 2.041% based on the total mass of the lithium iron manganese phosphate.

[0154] Example 12

[0155] Compared with Example 1, the difference is that:

[0156] No dopant is added in step (1).

[0157] The carbon content in the final lithium iron manganese phosphate is 1.998% based on the total mass of the lithium iron manganese phosphate.

[0158] Comparative Example 1

[0159] Compared with Example 1, the difference is that:

[0160] No organic acid is added in step (1).

[0161] Without adding the first organic acid, the viscosity is too high when preparing the lithium iron manganese phosphate precursor, and the reaction cannot be carried out.

[0162] Effect Example

[0163] 1. Resistivity

[0164] The test method refers to the national standard GBT 33822-2017, the equipment model is ST2742B, and the manufacturer is Suzhou Jingge Electronics Co., Ltd. The test results are shown in Table 1.

[0165] 2, Specific surface area

[0166] The test method refers to the standard YS / T1027-2015, the equipment model is TB400, and the manufacturer is Beijing Jingwei Gaobo Science and Technology Co., Ltd. The test results are shown in Table 1.

[0167] 3, Compaction density

[0168] The test is performed in the equipment model UTM7305Z09, and the manufacturer is Shenzhen Sansi Test Instrument Co., Ltd. The test conditions are as follows: the powder compaction pressure is set to 3T, the corresponding powder compaction density is tested, and the test results are shown in Table 1.

[0169] 4, Rate performance

[0170] The lithium manganese iron phosphate powder prepared in the examples is mixed with carbon black and polyvinylidene fluoride (PVDF) in a mass ratio of 95:5:5, respectively, and then coated on an aluminum foil. After drying, appropriate positive electrode test pieces are prepared, and CR2032 button type batteries are formed with lithium metal. The charge-discharge machine (Lantian CT2001A) is used to perform charge-discharge cycle test in the charge-discharge range of 2.5V-4.5V at 25℃, and the discharge specific capacity under 1C and 0.1C is recorded. The rate performance is reflected by the ratio of the two values. The test results are shown in Table 1.

[0171] 5, Cycle performance

[0172] The lithium manganese iron phosphate powder prepared in the examples is mixed with carbon black and polyvinylidene fluoride (PVDF) in a mass ratio of 95:5:5, respectively, and then coated on an aluminum foil. After drying, appropriate positive electrode test pieces are prepared, and CR2032 button type batteries are formed with lithium metal. The charge-discharge machine (Lantian CT2001A) is used to perform charge-discharge cycle test in the charge-discharge range of 2.5V-4.5V at 25℃, and the discharge specific capacity under 1C and 0.1C is recorded. The rate performance is reflected by the ratio of the two values. The test results are shown in Table 1.

[0173] 6, Element distribution characterization

[0174] EDS area scanning test is performed by SEM equipment, the equipment model is ProX, and the manufacturer is FEI.

[0175] Table 1

[0176] As can be seen from Table 1, the lithium manganese iron phosphate prepared by the preparation method provided by the application has the advantages of low resistivity (less than 247.4 Ω·cm), small specific surface area (less than 20.5 m 2 / g) and high compactness (greater than 2.31 g / cm 3 ). When the lithium manganese iron phosphate is applied to a battery, it has the advantages of good rate performance (the capacity retention rate at 1C / 0.1C is greater than 86.6%) and good cycle performance (the cycle number when the capacity decays to 70% of the initial capacity is greater than 245 cycles).

[0177] FIG. 1 is an SEM image of the lithium manganese iron phosphate prepared in Example 1, and the specific conditions are SEM HV: 5.0 kV; SEM MAG: 100 kx; Det: SE; WD: 9.03 mm; View field: 2.54 μm; Bl: 10. FIG. 2 is a TEM image of the lithium manganese iron phosphate prepared in Example 1. The lithium manganese iron phosphate of the application is a secondary particle structure formed by agglomeration of primary particles, and the primary particles are partially connected by a carbon coating layer, and the connection is relatively loose.

[0178] FIG. 3 is a test effect diagram of the rate performance of the lithium manganese iron phosphate prepared in Example 1 when applied to a battery. It can be seen that there are two power platforms, which indicates that the lithium manganese iron phosphate material is formed.

[0179] FIGS. 4-7 are EDS spectra of the lithium manganese iron phosphate prepared in Example 1. FIG. 4 is an SEM image of the lithium manganese iron phosphate prepared in Example 1, and the EDS area scanning test is performed on the boxed area to obtain the distribution diagram of each element. FIG. 5 is an EDS diagram of the manganese element of the lithium manganese iron phosphate prepared in Example 1. FIG. 6 is an EDS diagram of the iron element of the lithium manganese iron phosphate prepared in Example 1. FIG. 7 is an EDS diagram of the phosphorus element of the lithium manganese iron phosphate prepared in Example 1. It can be seen that the iron phase and the manganese phase are uniformly distributed in the lithium manganese iron phosphate prepared by the application.

[0180] Compared with Example 2, the molar ratio of manganese element to iron element is adjusted in Example 1. It can be seen that as the proportion of manganese increases, the resistivity increases, and the rate performance decreases. This may be because the electrical conductivity of lithium manganese phosphate is worse than that of lithium iron phosphate, resulting in an increase in resistivity, an increase in the proportion of manganese ions, and a more easily dissolved manganese ion structure of lithium manganese iron phosphate, resulting in a decrease in rate performance.

[0181] Compared with Examples 1, 3, 4 and 12, the difference lies in whether a dopant is added and the type of the dopant is adjusted. It can be seen that the addition of the dopant can effectively improve the rate performance of the lithium manganese iron phosphate product, and the discharge specific capacity of Examples 1, 3 and 4 at 0.1C and 1C is higher than that of Example 12.

[0182] The difference between Examples 1, 5 and 6 is the sintering temperature. It can be seen that changing the sintering temperature has an effect on the resistivity and rate performance of the lithium manganese iron phosphate, which may be because the sintering temperature is different, the carbon-coated structure and the carbon content are different, thereby affecting the resistivity and rate performance.

[0183] The difference between Example 1 and Examples 7-9 is the type of the first organic acid and the second organic acid. Example 1 and Example 7 are both monobasic carboxylic acids, Example 8 is a dibasic carboxylic acid, and Example 9 is a tribasic carboxylic acid. It can be seen that the addition of monobasic carboxylic acid is more conducive to reducing the resistivity, which may be because monobasic carboxylic acid can be used as a carbon source when it is decomposed at high temperature, and its carbon-coated effect is better.

[0184] The difference between Example 1 and Examples 10 and 11 is the timing of the addition of the first organic acid. It can be seen that the timing of the addition of different first organic acids within the scope of the application can achieve good rate performance and cycle performance, but there is a certain difference in resistivity.

Claims

1. A method for preparing a manganese iron phosphate precursor, characterized in that, It comprises the following steps: The first reaction of the reactant A with the reactant B to prepare the manganese iron phosphate precursor; the reactant A comprises an iron source, a manganese source and water; the reactant B comprises a phosphorus source and water, and the reactant B comprises a first organic acid, which is a monobasic carboxylic acid; Wherein, the iron source is iron powder; the manganese source is manganese powder; the phosphorus source is phosphoric acid.

2. The method of claim 1, wherein the manganese iron phosphate precursor is prepared by the steps of: It satisfies one or more of the following conditions: ​ (1) the first organic acid is one or more of acetic acid, propionic acid, lactic acid and benzoic acid; (2) the molar ratio of the carboxyl provided by the first organic acid to the phosphorus element provided by the phosphorus source is (0.7-2.0):1; (3) the molar ratio of the iron element provided by the iron source to the phosphorus element provided by the phosphorus source is (0.1-0.8):1; (4) the molar ratio of the manganese element provided by the manganese source to the phosphorus element provided by the phosphorus source is (0.2-0.8):

1.

3. The method for preparing the manganese iron phosphate precursor as described in claim 1, characterized in that, It satisfies one or more of the following conditions: (1) the first reaction is carried out under stirring, and the stirring speed is 300-800 rpm; (2) the first reaction is carried out by dropping the reactant A into the reactant B; (3) the first reaction is carried out under an inert atmosphere; (4) the temperature of the first reaction is 80-95℃; (5) the time of the first reaction is 2-7h.

4. A manganese iron phosphate precursor, characterized in that, It is prepared by the preparation method of any one of claims 1-3.

5. A method for producing lithium iron manganese phosphate, characterized by, It carries out the second reaction of the manganese iron phosphate precursor of claim 4 with the reactant C to obtain a manganese iron phosphate lithium precursor, and then sintering to obtain the manganese iron phosphate lithium; the reactant C comprises a lithium source, a carbon source, a second organic acid and water; the second organic acid is a monobasic carboxylic acid; the reactant C further comprises a dopant.

6. The method of claim 5, wherein the lithium iron manganese phosphate is prepared by the steps of: mixing lithium carbonate, manganese carbonate, and iron carbonate; and heating the mixture at a temperature of 600 to 800°C for 6 to 10 hours in an atmosphere of nitrogen gas. It satisfies one or more of the following conditions: (1) the molar ratio of the lithium element provided by the lithium source to the phosphorus element provided by the phosphorus source is (1.01-1.05):1; (2) the molar ratio of the carboxyl provided by the second organic acid to the phosphorus element provided by the phosphorus source is (0.7-2.0):1; (3) the molar ratio of the carbon element provided by the carbon source to the phosphorus element provided by the phosphorus source is (0.05-0.6):

1.

7. The method of claim 5, wherein the lithium iron manganese phosphate is prepared by the steps of: It satisfies one or more of the following conditions: ​ (1) after the second reaction and before sintering, grinding and drying are further carried out; wherein the drying is carried out by spray drying; the temperature parameters of the spray drying are: inlet temperature: 220℃, outlet temperature: 110℃; the average particle size of the product obtained after drying is 10-150μm; (2) the sintering temperature is 600-760℃; (3) the sintering time is 5-20h; (4) the sintering heating rate is 3-10℃ / min; (5) the dopant comprises at least one of magnesium element, cobalt element, titanium element, vanadium element, nickel element, niobium element, copper element and tungsten element; the molar ratio of the doping element provided by the dopant to the phosphorus element provided by the phosphorus source is (0.01-0.05):1; (6) the lithium source is at least one of lithium phosphate-based compounds, lithium carbonate and lithium hydroxide; (7) the second organic acid is one or more of acetic acid, propionic acid, lactic acid and benzoic acid; (8) the carbon source is one or more of carbon materials and conductive polymers; wherein the carbon material is one or more of glucose, glucose derivatives, organic acids, organic acid derivatives, phenol-formaldehyde resin, polyethylene, polyethylene glycol, polyvinyl alcohol, polyvinyl alcohol derivatives, polyacrylic acid, polyacrylic acid derivatives and heterocyclic polymers or condensates with N or O elements; wherein the glucose derivatives include at least one of glucose, sucrose, starch and cyclodextrin; wherein the organic acids in the carbon material include at least one of formic acid, acetic acid, oxalic acid, citric acid, tartaric acid and malic acid; wherein the polyacrylic acid derivatives include polyacrylate; and wherein the heterocyclic polymers with N or O elements include polyvinylpyrrolidone.

8. A lithium iron manganese phosphate characterized by, which is prepared by the method for preparing lithium iron manganese phosphate according to any one of claims 5-7.

9. A positive electrode sheet characterized by comprising: which comprises an active material layer and a positive electrode current collector; the active material layer comprises the lithium iron manganese phosphate according to claim 8.

10. A secondary battery characterized by comprising: which comprises the positive electrode sheet according to claim 9.

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