Ferromanganese oxalate precursor, preparation method and lithium manganese iron phosphate positive electrode material

Abstract

The invention relates to the technical field of preparation of new energy battery positive electrode material precursors, in particular to a ferromanganese oxalate precursor, a preparation method and a lithium manganese iron phosphate positive electrode material.The method comprises the steps that a solution A is prepared from water, ferric salt, manganese salt, metal M salt and an antioxidant; then, preparing a solution B from water, ammonium oxalate and an antioxidant; and finally, under the protection of inert gas, dropwise adding the solution B into the solution A to obtain the oxalate ferromanganese precursor. According to the method, the alpha-crystal-form ferromanganese oxalate is successfully synthesized, the yield of the beta-crystal-form ferromanganese oxalate in the synthesis process is greatly reduced, the ferromanganese and the ferromanganese can be mixed at the atomic level, and the dissolution of manganese and the Jahn-Teller effect are inhibited; in-situ metal element doping improves the electronic conductivity of the material; the problems that an existing lithium iron manganese phosphate positive electrode material prepared from ferromanganese oxalate is poor in electronic conductivity and electrochemical performance and cannot meet the use requirement of a lithium ion battery are solved.

Description

Technical Field

[0001] The present invention relates to the technical field of preparation of precursors for positive electrode materials for new energy batteries, and in particular to a manganese iron oxalate precursor, a preparation method and a lithium manganese iron phosphate positive electrode material. Background Art

[0002] After years of technical research and development, lithium iron phosphate batteries still have bottlenecks in energy density. The theoretical capacity of lithium iron manganese phosphate is the same as that of lithium iron phosphate (about 170mAh / g), but lithium iron manganese phosphate has a higher voltage platform (up to 4.1V). When its actual capacity is the same as that of lithium iron phosphate, the energy density can be increased by 15% to 20%.

[0003] Lithium iron manganese phosphate cathode material has a series of performance advantages such as low cost, high safety, and good thermal stability. The current reasons that limit its large-scale application are mainly concentrated on the preparation process. The addition of manganese elements makes it difficult to achieve atomic-level mixing during the preparation process using the solid-phase method, especially when the manganese content is high, and the uneven mixing of manganese and iron can easily lead to problems such as poor cycle stability and reduced capacity of the material. Therefore, the solid-phase method is obviously not the best choice for preparing high-performance lithium iron manganese phosphate materials, and the liquid-phase method for synthesizing lithium iron manganese phosphate is more technologically advanced.

[0004] At present, the selection of lithium manganese iron phosphate material precursors is still in the exploratory stage. sp =10 -36 , K of Mn3(PO4)2 sp =1.3×10 -32 , two metal divalent ions Fe 2+ and Mn 2+ With PO4 3- The solubility product constants of Fe 2+ and Mn 2+When the electrolyte is dissolved, it is difficult for ferromanganese to form a coprecipitate, which will lead to differences in the ratio of manganese and iron, uneven atomic mixing and other problems. The dissolved manganese ions will also diffuse through the electrolyte to the surface of the negative electrode for reduction reaction to generate metallic manganese or manganese compounds, destroying the negative electrode solid electrolyte interface (Solid Electrolyte Interface membrane, SEI) membrane, resulting in continuous growth of the SEI membrane, leading to loss of active lithium and increase in battery impedance; the dissolved manganese ions will also react with the electrolyte. Manganese has poor conductivity and is prone to polarization, which will also lead to a decrease in cycle life. In addition, manganese in ferromanganese phosphate mostly exists in divalent form, and direct use of trivalent manganese is prone to disproportionation reaction in the solution, affecting the purity of the product. Therefore, it is difficult for lithium manganese phosphate to directly use ferromanganese phosphate as a precursor like lithium iron phosphate. At present, among the common manganese iron salt precursors, ferromanganese oxalate has the advantages of low cost and simple process, and has better application prospects.

[0005] Ferromanganese oxalate is divided into two different structures: α crystal and β crystal. The α crystal has more stable thermodynamic properties, and the synthesized lithium iron manganese phosphate positive electrode material has better crystallinity and better electrochemical performance. However, the current preparation process of ferromanganese oxalate usually produces a large amount of β crystal ferromanganese oxalate during the synthesis process, resulting in poor electrochemical performance of the post-processed synthesized lithium iron manganese phosphate. In addition, ferromanganese oxalate is prone to uneven mixing of manganese and iron, and metal doping during the post-processing process will aggravate the instability of the material structure, resulting in the synthesized lithium iron manganese phosphate having low electronic conductivity and poor electrochemical performance, which cannot meet the use requirements of lithium-ion batteries. Summary of the invention

[0006] In view of the problem that β-crystalline ferromanganese oxalate is easily produced in the preparation process of ferromanganese oxalate in the prior art, resulting in poor electronic conductivity and electrochemical performance of the lithium ferromanganese phosphate positive electrode material prepared using the ferromanganese oxalate, and failure to meet the use requirements of lithium ion batteries, the present invention provides a ferromanganese oxalate precursor, a preparation method and a lithium ferromanganese phosphate positive electrode material.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions: The present invention provides a method for preparing a ferromanganese oxalate precursor, comprising: Solution A is prepared by using water, iron salt, manganese salt, metal M salt and antioxidant; Solution B is prepared using water, ammonium oxalate and an antioxidant; Under the protection of inert gas, solution B is added dropwise to solution A to obtain a manganese iron oxalate precursor.

[0008] Preferably, the metal M salt is one or more of Mg salt, Cr salt, Co salt, Al salt, Cu salt, Zn salt, Ni salt and V salt, and the metal M salt is a non-crystalline metal salt or a crystalline metal salt.

[0009] Preferably, the iron salt is one or more of ferrous nitrate, ferrous sulfate, ferrous acetate and ferrous chloride; the ferrous sulfate is one or more of anhydrous ferrous sulfate, ferrous sulfate monohydrate and ferrous sulfate heptahydrate.

[0010] Preferably, the manganese salt is one or more of manganous nitrate, manganous sulfate, manganous acetate and manganous chloride; the manganous sulfate is one or more of anhydrous manganous sulfate, manganous sulfate monohydrate and manganous sulfate tetrahydrate.

[0011] Preferably, the antioxidant is ascorbic acid and / or citric acid.

[0012] Preferably, in the solution A, the concentration of iron salt is 0.1-0.5 mol / L, the concentration of manganese salt is 0.15-0.75 mol / L, the concentration of metal M salt is 0-0.05 mol / L, and the concentration of antioxidant is 0.01-0.05 mol / L; wherein the molar ratio of manganese ions to iron ions is (1-2.5):1.

[0013] Preferably, the concentration of ammonium oxalate in the solution B is 0.2-0.5 mol / L, and the concentration of the antioxidant is 0-3 mol / L.

[0014] Preferably, the molar ratio of the ammonium oxalate to the total amount of the manganese salt and the iron salt is (1.2-1.4):1.

[0015] The present invention provides a ferromanganese oxalate precursor prepared by the above method, the general chemical formula of which is: Mn x Fe y M 1-x-y C2O4, wherein M is a doped metal element, x is 0.5 to 0.7, and y is 0.3 to 0.5.

[0016] The present invention provides a lithium manganese iron phosphate positive electrode material, which is prepared by using the above-mentioned manganese iron oxalate precursor.

[0017] Compared with the prior art, the present invention has the following beneficial effects: The present invention discloses a ferromanganese oxalate precursor and a preparation method thereof. The method uses water, iron salt, manganese salt, metal M salt and antioxidant to prepare solution A; then, uses water, ammonium oxalate and antioxidant to prepare solution B; finally, solution B is dropped into solution A for mixing, and heated under the protection of inert gas to successfully prepare the ferromanganese oxalate precursor. The present invention uses coprecipitation to synthesize α-crystalline ferromanganese oxalate, which greatly reduces the yield of β-crystalline ferromanganese oxalate during the synthesis process. Ferromanganese can achieve atomic-level mixing, inhibiting manganese dissolution and Jahn-Teller effect; in-situ metal element doping improves the electronic conductivity of the material; the lithium iron manganese phosphate positive electrode material prepared by using this ferromanganese oxalate as a precursor has excellent rate performance, high reversible specific capacity and capacity retention rate, and the preparation method is simple, the raw materials are cheap and easy to obtain, the reaction is easy to control, the conditions are mild, and it is suitable for industrialization.

[0018] The present invention provides a ferromanganese oxalate precursor prepared by the above method, the general chemical formula of which is: Mn x Fe y M 1-x-y C2O4, wherein M is a doped metal element, x is 0.5 to 0.7, and y is 0.3 to 0.5. The α-crystalline ferromanganese oxalate precursor has a higher purity and more stable thermodynamic properties.

[0019] The present invention also provides a lithium iron manganese phosphate positive electrode material, which is prepared using the above-mentioned manganese iron oxalate precursor. The lithium iron manganese phosphate positive electrode material has better crystallinity, excellent electronic conductivity, better electrochemical performance, and can be better suitable for the use requirements of lithium batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The present invention is a schematic flow chart of a method for preparing a ferromanganese oxalate precursor.

[0021] Figure 2 An X-ray diffraction (XRD) diagram of a ferromanganese oxalate precursor prepared in an embodiment of the present invention compared with α-crystalline ferromanganese oxalate.

[0022] Figure 3 This is a SEM image of a ferromanganese oxalate precursor prepared in an embodiment of the present invention.

[0023] Figure 4 The discharge specific capacity at 1C of a lithium iron manganese phosphate positive electrode material prepared using a manganese ferrooxalate precursor prepared in an embodiment of the present invention and a manganese ferrooxalate prepared in a comparative example.

[0024] Figure 5 The 100-cycle cycle stability and coulombic efficiency of lithium iron manganese phosphate positive electrode materials prepared using the manganese iron oxalate precursors prepared in the embodiments and comparative examples of the present invention. DETAILED DESCRIPTION

[0025] In order to enable those skilled in the art to understand the characteristics and effects of the present invention, the following is a general description and definition of the terms and expressions mentioned in the specification and claims. Unless otherwise specified, all technical and scientific terms used in the text are the common meanings understood by those skilled in the art for the present invention. In the event of a conflict, the definition in this specification shall prevail.

[0026] The theories or mechanisms described and disclosed herein, whether correct or incorrect, should not limit the scope of the present invention in any way, that is, the present invention can be implemented without being limited by any specific theory or mechanism.

[0027] In this article, all features such as values, quantities, contents and concentrations defined in the form of numerical ranges or percentage ranges are for simplicity and convenience only. Accordingly, the description of numerical ranges or percentage ranges should be considered to have included and specifically disclosed all possible secondary ranges and individual values ​​within the range (including integers and fractions).

[0028] In this document, unless otherwise specified, “includes,” “including,” “contains,” “has,” or similar terms cover the meanings of “consisting of” and “mainly consisting of,” for example, “A includes a” covers the meanings of “A includes a and other” and “A only includes a.”

[0029] In this document, in order to make the description concise, not all possible combinations of various technical features in various embodiments or examples are described. Therefore, as long as there is no contradiction in the combination of these technical features, the various technical features in various embodiments or examples can be combined arbitrarily, and all possible combinations should be considered to be within the scope of this specification.

[0030] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0031] The following examples use conventional instruments and equipment in the art. The experimental methods in the following examples where specific conditions are not specified are usually carried out under conventional conditions or under conditions recommended by the manufacturer. The various raw materials used in the following examples are conventional commercial products unless otherwise specified, and their specifications are conventional specifications in the art. In the specification of the present invention and the following examples, unless otherwise specified, "%" means weight percentage, "part" means weight part, and ratio means weight ratio.

[0032] The present invention is further described in detail below in conjunction with specific embodiments, which are intended to explain the present invention rather than to limit it.

[0033] Example 1 See also Figure 1 The present invention discloses a method for preparing a ferromanganese oxalate precursor, comprising: S1: preparing solution A using water, iron salt, manganese salt, metal M salt and antioxidant; S2: preparing solution B using water, ammonium oxalate and an antioxidant; S3: Under the protection of inert gas, solution B is added dropwise into solution A to obtain a manganese iron oxalate precursor.

[0034] The method uses co-precipitation to synthesize α-crystalline ferromanganese oxalate, which greatly reduces the yield of β-crystalline ferromanganese oxalate during the synthesis process. Ferromanganese can achieve atomic-level mixing, inhibiting manganese dissolution and Jahn-Teller effect; in-situ metal element doping improves the electronic conductivity of the material; the lithium iron manganese phosphate positive electrode material prepared using this ferromanganese oxalate as a precursor has excellent rate performance, high reversible specific capacity and capacity retention rate, and the preparation method is simple, the raw materials are cheap and easy to obtain, the reaction is easy to control, the conditions are mild, and it is suitable for industrialization.

[0035] Example 2 See also Figure 1 The present invention provides a method for preparing a ferromanganese oxalate precursor, comprising: S1: Solution A is prepared using water, iron salt, manganese salt, metal M salt and antioxidant; wherein the metal M salt is one or more of Mg salt, Cr salt, Co salt, Al salt, Cu salt, Zn salt, Ni salt and V salt; the doped metal M salt may be a non-crystalline metal salt or a crystalline metal salt; the Mg salt may be one or more of anhydrous magnesium sulfate, magnesium sulfate heptahydrate and magnesium acetate tetrahydrate; the Ni salt may be one or more of anhydrous nickel sulfate, nickel sulfate heptahydrate and nickel acetate tetrahydrate; the iron salt may be one or more of ferrous nitrate, ferrous sulfate, ferrous acetate and ferrous chloride; the ferrous sulfate may be one or more of anhydrous ferrous sulfate, ferrous sulfate monohydrate and ferrous sulfate heptahydrate; the manganese salt may be nitrate One or more of manganous acid, manganous sulfate, manganous acetate and manganous chloride, manganous sulfate is one or more of anhydrous manganous sulfate, manganous sulfate monohydrate and manganous sulfate tetrahydrate; the antioxidant is ascorbic acid and / or citric acid; in the solution A, the concentration of iron salt is 0.1-0.5 mol / L, the concentration of manganese salt is 0.15-0.75 mol / L, the concentration of metal M salt is 0-0.05 mol / L, and the concentration of antioxidant is 0.01-0.05 mol / L; wherein the molar ratio of manganese ions to iron ions is (1-2.5):1, and the specific method is: According to the above concentrations and proportions, iron salt, manganese salt, metal M salt and antioxidant are mixed with deionized water, nitrogen is continuously passed through to remove oxygen by bubbling, and the mixture is stirred until uniformly mixed to form solution A. The reaction conditions are 20°C to 60°C.

[0036] S2: Prepare solution B using water, ammonium oxalate and an antioxidant; wherein the concentration of ammonium oxalate is 0.2-0.5 mol / L, the concentration of the antioxidant is 0-3 mol / L, the molar ratio of the ammonium oxalate to the total amount of manganese salt and iron salt is (1.2-1.4):1, and the antioxidant is ascorbic acid and / or citric acid. The specific method is: According to the above concentrations and proportions, ammonium oxalate and antioxidants were added to deionized water, nitrogen was continuously introduced to remove oxygen by bubbling, and the mixture was stirred until uniformly mixed to form solution B.

[0037] S3: Under the protection of inert gas, solution B is added dropwise to solution A to obtain a manganese iron oxalate precursor. The specific method is as follows: The obtained solution B is added dropwise to solution A at a speed of 10 to 50 ml / min by a peristaltic pump, and under the protection of nitrogen, the temperature of solution A is stabilized at 20°C to 60°C while stirring and heating. After all the solution B is added dropwise, the reaction is continued in a nitrogen atmosphere until it is complete to obtain a mixed solution, and the mixed solution is statically aged for 3 to 24 hours, centrifuged, washed, and vacuum dried to obtain a manganese ferrooxalate precursor. Preferably, the stirring speed is 300 to 500 r / min, the centrifugal speed is 7000 to 9000 r / min, the drying temperature is 45°C to 80°C, and the drying time is 18 to 72 hours.

[0038] Example 3 150 mL of deionized water was measured, and 13.9 g of ferrous sulfate heptahydrate, 12.68 g of manganese sulfate monohydrate and 0.5 g of ascorbic acid were added. Nitrogen was continuously bubbled into the solution to deoxygenate it. The solution was heated to 60° C. in a water bath and stirred magnetically for 60 min to obtain solution A. 550 mL of deionized water was measured, and 21.32 g of ammonium oxalate monohydrate and 17 g of ascorbic acid were added. Nitrogen was continuously bubbled into the water to deoxygenate the solution. The solution was heated to 60° C. in a water bath and stirred magnetically for 60 min to obtain a solution B. Solution B was added to solution A by a peristaltic pump. The dropping speed of solution B was set to 15 mL / min. The stirring speed of solution A was set to 300 r / min. The temperature of solution A was maintained at 60°C. After the addition of solution B was completed, solution A was stirred for 1 h under the continuous protection of nitrogen. The reaction solution was allowed to stand for 15 h, centrifuged, washed with deionized water three times, and then dried in a vacuum oven at 60 °C for 48 h to obtain a manganese ferrooxalate precursor, denoted as MFC-1, with a molecular formula of Mn 0.6 Fe0.4 C2O4·2H2O.

[0039] Example 4 150 mL of deionized water was measured, and 13.9 g of ferrous sulfate heptahydrate, 12.68 g of manganese sulfate monohydrate and 0.5 g of ascorbic acid were added. Nitrogen was continuously bubbled into the solution to deoxygenate it. The solution was heated to 60° C. in a water bath and stirred magnetically for 60 min to obtain solution A. 550 mL of deionized water was measured, and 21.32 g of ammonium oxalate monohydrate and 5.5 g of ascorbic acid were added. Nitrogen was continuously bubbled into the water to deoxygenate the solution. The solution was heated to 60° C. in a water bath and stirred magnetically for 60 min to obtain a solution B. Solution B was added to solution A by a peristaltic pump, the dripping speed of solution B was set to 15 mL / min, the stirring speed of solution A was set to 300 r / min, and the temperature of solution A was maintained at 60°C. After the addition of solution B was completed, solution A was stirred for 1 hour under the continuous protection of nitrogen; The reaction solution was allowed to stand for 15 h, centrifuged, washed three times with deionized water, and then dried in a vacuum oven at 60 °C for 48 h to obtain a manganese ferrooxalate precursor, denoted as MFC-2, with a molecular formula of Mn 0.6 Fe 0.4 C2O4·2H2O.

[0040] Example 5 150 mL of deionized water was measured, and 13.76 g of ferrous sulfate heptahydrate, 12.55 g of manganese sulfate monohydrate, 0.2 g of zinc sulfate, and 0.5 g of ascorbic acid were added. Nitrogen was continuously bubbled into the solution to deoxygenate it. The solution was heated to 30° C. in a water bath and stirred magnetically for 60 min to obtain solution A. 550 mL of deionized water was measured, and 21.32 g of ammonium oxalate monohydrate and 11 g of ascorbic acid were added. Nitrogen was continuously bubbled into the water to deoxygenate the solution. The solution was heated to 30° C. in a water bath and stirred magnetically for 60 min to obtain a solution B. Solution B was added to solution A by a peristaltic pump, the dripping speed of solution B was set to 15 mL / min, the stirring speed of solution A was set to 300 r / min, and the temperature of solution A was maintained at 30°C. After the addition of solution B was completed, solution A was stirred for 1 hour under the continuous protection of nitrogen; The reaction solution was allowed to stand for 15 h, centrifuged, washed three times with deionized water, and then dried in a vacuum oven at 60 °C for 48 h to obtain a manganese ferrooxalate precursor, denoted as MFC-3, with a molecular formula of Mn 0.594 Fe 0.396 Zn 0.01 C2O4·2H2O.

[0041] Example 6 150 mL of deionized water was measured, and 13.76 g of ferrous sulfate heptahydrate, 12.55 g of manganese sulfate monohydrate, 0.33 g of nickel sulfate, and 0.5 g of ascorbic acid were added. Nitrogen was continuously bubbled into the solution to deoxygenate it. The solution was heated to 30° C. in a water bath and stirred magnetically for 60 min to obtain solution A. 550 mL of deionized water was measured, and 21.32 g of ammonium oxalate monohydrate and 11 g of ascorbic acid were added. Nitrogen was continuously bubbled into the water to deoxygenate the solution. The solution was heated to 30° C. in a water bath and stirred magnetically for 60 min to obtain a solution B. Solution B was added to solution A by a peristaltic pump, the dripping speed of solution B was set to 15 mL / min, the stirring speed of solution A was set to 300 r / min, and the temperature of solution A was maintained at 30°C. After the addition of solution B was completed, solution A was stirred for 1 hour under the continuous protection of nitrogen; The reaction solution was allowed to stand for 15 h, centrifuged, washed with deionized water three times, and then dried in a vacuum oven at 60 °C for 48 h to obtain a manganese ferrooxalate precursor, denoted as MFC-4, with a molecular formula of Mn 0.594 Fe 0.396 Ni 0.01 C2O4·2H2O.

[0042] Example 7 150 mL of deionized water was measured, and 13.76 g of ferrous sulfate heptahydrate, 12.55 g of manganese sulfate monohydrate, 0.15 g of magnesium sulfate, and 0.5 g of ascorbic acid were added. Nitrogen was continuously bubbled into the solution to deoxygenate it. The solution was heated to 30° C. in a water bath and stirred magnetically for 60 min to obtain solution A. 550 mL of deionized water was measured, and 21.32 g of ammonium oxalate monohydrate and 11 g of ascorbic acid were added. Nitrogen was continuously bubbled into the water to deoxygenate the solution. The solution was heated to 30° C. in a water bath and stirred magnetically for 60 min to obtain a solution B. Solution B was added to solution A by a peristaltic pump, the dropping speed of solution B was set to 15 mL / min, the stirring speed of solution A was set to 300 r / min, and the temperature of solution A was maintained at 30°C. After the addition of solution B was completed, solution A was stirred for 1 h under the continuous protection of nitrogen; The reaction solution was allowed to stand for 15 h, centrifuged, washed with deionized water three times, and then dried in a vacuum oven at 60 °C for 48 h to obtain a manganese ferrooxalate precursor, denoted as MFC-5, with a molecular formula of Mn 0.594 Fe 0.396 Mg 0.01 C2O4·2H2O.

[0043] Example 8 150 mL of deionized water was measured, and 13.76 g of ferrous sulfate heptahydrate, 12.55 g of manganese sulfate monohydrate, 0.15 g of magnesium sulfate, and 0.5 g of ascorbic acid were added. Nitrogen was continuously bubbled into the solution to deoxygenate it. The solution was heated to 30° C. in a water bath and stirred magnetically for 60 min to obtain solution A. 550 mL of deionized water was measured, and 21.32 g of ammonium oxalate monohydrate and 17 g of ascorbic acid were added. Nitrogen was continuously bubbled into the water to deoxygenate the solution. The solution was heated to 30° C. in a water bath and stirred magnetically for 60 min to obtain a solution B. Solution B was added to solution A by a peristaltic pump, the dripping speed of solution B was set to 25 mL / min, the stirring speed of solution A was set to 500 r / min, and the temperature of solution A was maintained at 30°C. After the addition of solution B was completed, solution A was stirred for 1 h under the continuous protection of nitrogen; The reaction solution was allowed to stand for 15 h, centrifuged, washed with deionized water three times, and then dried in a vacuum oven at 60 °C for 48 h to obtain a manganese ferrooxalate precursor, denoted as MFC-6, with a molecular formula of Mn 0.594 Fe 0.396 Mg 0.01 C2O4·2H2O.

[0044] Example 9 150 mL of deionized water was measured, and 14.4 g of ferrous nitrate hexahydrate, 15.6 g of manganese nitrate tetrahydrate, and 0.5 g of ascorbic acid were added. Nitrogen was continuously bubbled into the water to deoxygenate it. The solution was heated to 60° C. in a water bath and stirred magnetically for 60 min to obtain solution A. 550 mL of deionized water was measured, and 21.32 g of ammonium oxalate monohydrate and 17 g of ascorbic acid were added. Nitrogen was continuously bubbled into the water to deoxygenate the solution. The solution was heated to 60° C. in a water bath and stirred magnetically for 60 min to obtain a solution B. Solution B was added to solution A by a peristaltic pump, the dripping speed of solution B was set to 8 mL / min, the stirring speed of solution A was set to 300 r / min, and the temperature of solution A was maintained at 60°C. After the addition of solution B was completed, solution A was stirred for 1 h under the continuous protection of nitrogen; The reaction solution was allowed to stand for 18 h, centrifuged, washed with deionized water three times, and then dried in a vacuum oven at 60 °C for 48 h to obtain a manganese ferrooxalate precursor, denoted as MFC-7, with a molecular formula of Mn 0.6 Fe 0.4 C2O4·2H2O.

[0045] Example 10 150 mL of deionized water was measured, and 9.94 g of ferrous chloride tetrahydrate, 12.68 g of manganese sulfate monohydrate, and 0.5 g of ascorbic acid were added. Nitrogen was continuously bubbled into the water to deoxygenate. The solution was heated to 60° C. in a water bath and stirred magnetically for 60 min to obtain solution A. 550 mL of deionized water was measured, and 21.32 g of ammonium oxalate monohydrate and 17 g of ascorbic acid were added. Nitrogen was continuously bubbled into the water to deoxygenate the solution. The solution was heated to 60° C. in a water bath and stirred magnetically for 60 min to obtain a solution B. Solution B was added to solution A by a peristaltic pump, the dripping speed of solution B was set to 15 mL / min, the stirring speed of solution A was set to 300 r / min, and the temperature of solution A was maintained at 60°C. After the addition of solution B was completed, solution A was stirred for 1 hour under the continuous protection of nitrogen; The reaction solution was allowed to stand for 3 h, centrifuged, washed with deionized water for 3 times, and then vacuum dried in a vacuum oven at 60 °C for 48 h to obtain a manganese ferrooxalate precursor, denoted as MFC-8, with a molecular formula of Mn 0.6 Fe 0.4 C2O4·2H2O.

[0046] Comparative Example 1 150 mL of deionized water was measured, and 13.9 g of ferrous sulfate heptahydrate, 12.68 g of manganese sulfate monohydrate, and 0.5 g of ascorbic acid were added. Nitrogen was continuously bubbled into the solution to deoxygenate it. The solution was heated to 60° C. in a water bath and stirred magnetically for 60 min to obtain solution A. 550 mL of deionized water was measured, and 13.5 g of anhydrous oxalic acid was added. Nitrogen was continuously bubbled into the water to deoxygenate. The water bath was heated to 60° C. and magnetically stirred for 60 min to obtain solution B. Solution B was added to solution A by a peristaltic pump, the dripping speed of solution B was set to 15 mL / min, the stirring speed of solution A was set to 300 r / min, and the temperature of solution A was maintained at 60°C. After the addition of solution B was completed, solution A was stirred for 1 hour under the continuous protection of nitrogen; The reaction solution was allowed to stand for 15 h, centrifuged, washed three times with deionized water, and then dried in a vacuum oven at 50 °C for 48 h to obtain a manganese ferrooxalate precursor, denoted as MFC-9, with a molecular formula of Mn 0.6 Fe 0.4 C2O4·2H2O.

[0047] Comparative Example 2 150 mL of deionized water was measured, and 13.9 g of ferrous sulfate heptahydrate, 12.68 g of manganese sulfate monohydrate, and 0.5 g of ascorbic acid were added. Nitrogen was continuously bubbled into the solution to deoxygenate it. The solution was heated to 60° C. in a water bath and stirred magnetically for 60 min to obtain solution A. 550 mL of deionized water was measured, and 21.32 g of ammonium oxalate monohydrate and 17 g of ascorbic acid were added. Nitrogen was continuously bubbled into the water to deoxygenate the solution. The solution was heated to 60° C. in a water bath and stirred magnetically for 60 min to obtain a solution B. Solution A was added dropwise to solution B obtained in step 2 by a peristaltic pump, the dripping speed of solution A was set to 15 mL / min, the stirring speed of solution B was set to 300 r / min, and the temperature of solution B was maintained at 60°C. After the addition of solution A was completed, solution B was stirred for 1 hour under the continuous protection of nitrogen; The reaction solution was allowed to stand for 15 h, centrifuged, washed three times with deionized water, and then dried in a vacuum oven at 50 °C for 48 h to obtain a manganese ferrooxalate precursor, denoted as MFC-10, with a molecular formula of Mn 0.6 Fe 0.4 C2O4·2H2O.

[0048] Comparative Example 3 150 mL of deionized water was measured, and 13.9 g of ferrous sulfate heptahydrate, 12.68 g of manganese sulfate monohydrate, and 0.5 g of ascorbic acid were added. Nitrogen was continuously bubbled into the solution to deoxygenate it. The solution was heated to 60° C. in a water bath and stirred magnetically for 60 min to obtain solution A. 550 mL of deionized water was measured, and 21.32 g of ammonium oxalate monohydrate and 17 g of ascorbic acid were added. Nitrogen was continuously bubbled into the water to deoxygenate the solution. The solution was heated to 60° C. in a water bath and stirred magnetically for 60 min to obtain a solution B. Pour solution B directly into solution A, and keep the temperature of solution A at 60°C. After all solution B is poured in, solution A is stirred for 1 hour under the continuous protection of nitrogen; The reaction solution was allowed to stand for 15 h, centrifuged, washed with deionized water three times, and then dried in a vacuum oven at 60 °C for 48 h to obtain a manganese ferrooxalate precursor, denoted as MFC-11, with a molecular formula of Mn 0.6 Fe 0.4 C2O4·2H2O.

[0049] See also Figure 2 The ferromanganese oxalate precursor prepared in Example 3 was subjected to X-ray diffraction. The results showed that the ferromanganese oxalate precursor prepared in Example 3 of the present invention was highly consistent with the XRD image of α-crystalline ferromanganese oxalate, indicating that the ferromanganese oxalate precursor prepared by this method was α-crystalline ferromanganese oxalate with high purity.

[0050] See also Figure 3, which is a SEM image of the ferromanganese oxalate precursor prepared in Example 3 of the present invention, it can be seen that the particles of the prepared ferromanganese oxalate precursor have obvious cubic phase, a smooth surface, and a complete morphology and structure of the particles. This further proves that the prepared ferromanganese oxalate precursor is an α-crystalline ferromanganese oxalate with more stable thermodynamic properties.

[0051] In order to further verify the beneficial effects of the ferromanganese oxalate precursor prepared by the method provided by the present invention, the ferromanganese oxalate products MFC-1 to MFC-11 obtained in Examples 3-10 and Comparative Examples 1-3 were mixed with lithium carbonate, diammonium phosphate, and citric acid, respectively, according to a molar ratio of (Fe+Mn): Li: phosphorus source: carbon source = 1:1.06:1:0.45, and 40% of the total mass of deionized water was added. After mixing evenly, ball milling was performed; under the protection of inert gas, pre-sintering was performed at 400°C for 1h, calcining was performed at 750°C for 4h, and naturally cooling was performed to room temperature to obtain a finished lithium ferromanganese phosphate positive electrode material.

[0052] The lithium iron manganese phosphate positive electrode material obtained in the embodiment and the comparative example, the conductive carbon black SuperP as the conductive agent, the polyvinylidene fluoride (PVDF) as the binder, are mixed at a mass ratio of 8:1:1, and a certain amount of organic solvent N-methylpyrrolidone (NMP) is added, and then coated on the carbon-coated aluminum foil after stirring, and vacuum dried in a vacuum oven at 110°C for 12 hours to obtain the positive electrode plate, and the negative electrode uses a metal lithium plate; the diaphragm is GradeGF / A glass fiber membrane; the electrolyte is Duoduo Chemical LB-878 lithium iron manganese phosphate electrolyte; 2032 button cells are assembled in a glove box. The battery is tested for charge and discharge cycle performance, and the discharge specific capacity at 0.1C / 1C and the cycle stability of 100 cycles at 1C / 1C are tested within the cut-off voltage range of 2.5 to 4.5V.

[0053] See also Figure 4 The discharge specific capacity of lithium manganese iron phosphate positive electrode materials prepared by using the manganese iron oxalate precursors prepared in Examples 3-10 and Comparative Examples 1-3 is marked as LMFP-1 to LMFP-11 at 1C. As can be seen from the figure, the MFC prepared in the embodiment synthesizes LMFP-1 to LMFP-8 through post-treatment, and its discharge specific capacity at 1C exceeds 137 mAh / g, and the optimal performance reaches 149.02 mAh / g; all embodiments and comparative examples have two characteristic discharge platforms of 3.5V and 3.9V, and the discharge platform of 3.9V is longer, which shows that the capacity of lithium manganese iron phosphate prepared by the product of the present invention is fully utilized and the Jahn-Teller effect is effectively suppressed.

[0054] See also Figure 5At 1C rate, as the number of cycles increases, the capacity decay of lithium manganese iron phosphate LMFP-1~LMFP-11 is not obvious. This shows that the manganese dissolution of lithium manganese iron phosphate positive electrode material prepared by in-situ metal-doped α-crystalline manganese iron oxalate prepared by co-precipitation method is effectively suppressed, so the capacity is high and the electrode cycle stability is good.

[0055] The present invention also provides a ferromanganese oxalate precursor prepared by the above method, the general chemical formula of which is: Mn x Fe y M 1-x-y C2O4, wherein M is a doped metal element, x is 0.5 to 0.7, and y is 0.3 to 0.5. The α-crystalline ferromanganese oxalate precursor has a higher purity and more stable thermodynamic properties.

[0056] A lithium iron manganese phosphate positive electrode material is prepared using the above-mentioned manganese iron oxalate precursor. The lithium iron manganese phosphate positive electrode material has better crystallinity, excellent electronic conductivity, better electrochemical performance, and can be better adapted to the use requirements of lithium batteries.

[0057] In summary, the present invention provides a ferromanganese oxalate precursor, a preparation method and a lithium iron manganese phosphate positive electrode material. The method adopts coprecipitation to synthesize α-crystalline ferromanganese oxalate, which greatly reduces the yield of β-crystalline ferromanganese oxalate during the synthesis process. Ferromanganese can achieve atomic-level mixing, inhibiting manganese dissolution and Jahn-Teller effect; in-situ metal element doping improves the electronic conductivity of the material; the lithium iron manganese phosphate positive electrode material prepared by using this ferromanganese oxalate as a precursor has excellent rate performance, high reversible specific capacity and capacity retention rate, and the preparation method is simple, the raw materials are cheap and easy to obtain, the reaction is easy to control, the conditions are mild, and it is suitable for industrialization.

[0058] The above description is only a preferred embodiment of the present invention and is not intended to impose any limitation on the technical solution of the present invention. Those skilled in the art should understand that, without departing from the spirit and principles of the present invention, the technical solution can also be subjected to several simple modifications and substitutions, and these modifications and substitutions are also within the scope of protection covered by the claims.

Claims

1. A method for preparing a ferromanganese oxalate precursor, characterized in that: include: Solution A is prepared by using water, iron salt, manganese salt, metal M salt and antioxidant; Solution B is prepared using water, ammonium oxalate and an antioxidant; Under the protection of inert gas, solution B is added dropwise to solution A to obtain a manganese iron oxalate precursor.

2. The method for preparing the ferromanganese oxalate precursor according to claim 1, characterized in that: The metal M salt is one or more of Mg salt, Cr salt, Co salt, Al salt, Cu salt, Zn salt, Ni salt and V salt, and the metal M salt is a non-crystalline metal salt or a crystalline metal salt.

3. The method for preparing the ferromanganese oxalate precursor according to claim 1, characterized in that: The iron salt is one or more of ferrous nitrate, ferrous sulfate, ferrous acetate and ferrous chloride; the ferrous sulfate is one or more of anhydrous ferrous sulfate, ferrous sulfate monohydrate and ferrous sulfate heptahydrate.

4. The method for preparing the ferromanganese oxalate precursor according to claim 1, characterized in that: The manganese salt is one or more of manganous nitrate, manganous sulfate, manganous acetate and manganous chloride; the manganous sulfate is one or more of anhydrous manganous sulfate, manganous sulfate monohydrate and manganous sulfate tetrahydrate.

5. The method for preparing the ferromanganese oxalate precursor according to claim 1, characterized in that: The antioxidant is ascorbic acid and / or citric acid.

6. The method for preparing the ferromanganese oxalate precursor according to claim 1, characterized in that: In the solution A, the concentration of iron salt is 0.1-0.5 mol / L, the concentration of manganese salt is 0.15-0.75 mol / L, the concentration of metal M salt is 0-0.05 mol / L, and the concentration of antioxidant is 0.01-0.05 mol / L; wherein the molar ratio of manganese ions to iron ions is (1-2.5):

1.

7. The method for preparing the ferromanganese oxalate precursor according to claim 1, characterized in that: The concentration of ammonium oxalate in the solution B is 0.2-0.5 mol / L, and the concentration of the antioxidant is 0-3 mol / L.

8. The method for preparing the ferromanganese oxalate precursor according to any one of claims 1 to 7, characterized in that: The molar ratio of the ammonium oxalate to the total amount of the manganese salt and the iron salt is (1.2-1.4):

1.

9. A ferromanganese oxalate precursor prepared by the method according to any one of claims 1 to 8, characterized in that: Its chemical formula is: Mn x Fe y M 1-x-y C2O4, wherein M is a doped metal element, x is 0.5 to 0.7, and y is 0.3 to 0.

5.

10. A lithium manganese iron phosphate positive electrode material, characterized in that: Prepared using the ferromanganese oxalate precursor described in claim 9.

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