Preparation method and application of iron manganese phosphate precursor

Abstract

This invention discloses a method for preparing and applying a manganese iron phosphate precursor. The method involves sequentially adding metal liquid A and metal liquid B to an organic solution at a preset flow rate. After the reaction is complete, the resulting liquid is subjected to solid-liquid separation and dehydration to obtain manganese iron phosphate. Both metal liquid A and metal liquid B contain iron and manganese salts. The concentration of manganese ions in metal liquid A is greater than that in metal liquid B, and the concentration of iron ions in metal liquid B is greater than that in metal liquid A. This application utilizes an organic solution-water system to adjust the system environment, allowing Fe and Mn to synthesize a homogeneous solid solution without adjusting the pH with an alkaline solution. Furthermore, it allows for the construction of a manganese iron phosphate precursor with a concentration gradient and a core-shell structure, reducing the distribution of manganese on the surface of the cathode material, thereby alleviating manganese leaching problems and improving the material's cycle performance and discharge capacity.

Description

Technical Field

[0001] The present invention relates to the technical field of ion battery electrode materials, and in particular to a preparation method and application of an iron manganese phosphate precursor. Background Art

[0002] Lithium manganese phosphate has a similar olivine structure to lithium iron phosphate, with high safety, good stability, higher working potential, and higher theoretical energy density than lithium iron phosphate, making it an important upgrade direction for lithium iron phosphate cathode materials. However, lithium manganese phosphate also has disadvantages such as low conductivity, side reactions with electrolytes, and Jahn Teller effect. Since the radius of Mn ions and Fe ions is close, they can be dissolved in any proportion. By adjusting the iron-manganese ratio in the system, the synthesized lithium iron manganese phosphate has the advantages of both and fully combines the advantages of both. Although excessive Mn content will improve the dual voltage problem, it will also make the material discharge specific capacity low and decay rapidly due to the Jahn Teller effect, and the low Mn content cannot increase the platform voltage and thus cause low specific capacity. Therefore, reducing the dissolution of Mn and stabilizing the structure are one of the problems faced by lithium manganese iron phosphate cathode materials.

[0003] The difficulties in synthesizing high-quality lithium iron manganese phosphate are mainly due to: (1) 3+ , Mn 3+

[0004] The precipitation pH in aqueous solution varies greatly, FePO 4 ·2H 2 O and MnPO 4 ·H 2 It is difficult to form a uniform solid solution when O is precipitated and exists as a single phase, making it difficult to synthesize high-quality lithium iron manganese phosphate;

[0005] (2) The Mn element in lithium manganese iron phosphate has more contact with the electrolyte on the surface of the positive electrode material, resulting in Mn dissolution.

[0006] In view of this, the present invention is proposed. Summary of the invention

[0007] The purpose of the present invention is to provide a preparation method and application of a manganese iron phosphate precursor.

[0008] The present invention is achieved in that:

[0009] In a first aspect, the present invention provides a method for preparing a manganese iron phosphate precursor, comprising the following steps:

[0010] Prepare liquids, preparing metal liquid A, metal liquid B and an organic solution, wherein both metal liquid A and metal liquid B contain iron salt and manganese salt, the concentration of manganese ions in the metal liquid A is greater than the concentration of manganese ions in the metal liquid B, and the concentration of iron ions in the metal liquid B is greater than the concentration of iron ions in the metal liquid A; the organic solution contains phosphoric acid or phosphate;

[0011] Co-precipitation: metal liquid A and metal liquid B are added to the organic solution at a preset flow rate. After the reaction is completed, the liquid after the reaction is separated into solid and liquid and dehydrated to obtain manganese iron phosphate.

[0012] In an optional embodiment, the molar ratio of Mn:Fe in the molten metal A is (0.5-1):(0-0.5); the molar ratio of Mn:Fe in the molten metal B is (0-0.5):(0.5-1).

[0013] In an optional embodiment, the iron salt is at least one of ferrous sulfate, ferric sulfate, ferric chloride, ferric oxalate, and ferric nitrate;

[0014] and / or the manganese salt is at least one of manganese nitrate, manganese oxalate, manganese sulfate, manganese chloride and manganese acetate;

[0015] and / or the phosphate is at least one of ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, sodium phosphate, sodium dihydrogen phosphate, sodium monohydrogen phosphate and potassium phosphate;

[0016] And / or the organic solvent is at least one of methanol, ethanol, ethylene glycol, propanol, n-butanol, acetonitrile, pyridine and phenol.

[0017] In an optional embodiment, the organic solution further contains an oxidant, and the oxidant is at least one of nitric acid, sodium nitrate, ammonium nitrate, ferric acid, perchloric acid, sodium ferrate, sodium bismuthate, sodium persulfate, hypochlorous acid, and ozone;

[0018] and / or the organic solution further contains a stabilizer, wherein the stabilizer is one of Schiff base, sodium pyrophosphate or pyrophosphoric acid;

[0019] Preferably, the amount of the oxidant added is to oxidize Fe in the metal liquid A and the metal liquid B. 2+ 120-150% of the theoretical molar amount;

[0020] Preferably, the molar ratio of the amount of the stabilizer added to the manganese ion is (1-5):20;

[0021] Preferably, the Schiff base is one of dihydroxyacetone phosphate, 3-phosphoglyceraldehyde and formylformic acid thiosemicarbazide Schiff base.

[0022] In an optional embodiment, the mass ratio of water to organic solution in the reaction solution is 1:(3-10), and the molar ratio of the total amount of manganese and iron to phosphorus is 1:(1-1.5);

[0023] Preferably, the total amount of manganese and iron per liter of the reaction solution is 1.0×10 -2 -1.5×10 -2 mol;

[0024] Preferably, the flow rate of the molten metal A and / or the molten metal B is 0.8-4 g / min per liter of organic solution.

[0025] In an optional embodiment, the reaction temperature of the coprecipitation step is 30-90° C., the stirring rate during the reaction is 25-45 Hz, and the reaction solution is aged for 0.5-5 h after the addition of the metal liquid B is stopped.

[0026] In an optional embodiment, after the coprecipitation step, the solid phase obtained by solid-liquid separation is sequentially washed, dried, and dehydrated to obtain anhydrous manganese iron phosphate powder.

[0027] In a second aspect, the present invention provides a method for preparing lithium manganese iron phosphate, wherein the manganese iron phosphate described in any one of the aforementioned embodiments is mixed with water, a lithium source, a carbon source, and a surfactant and ground to obtain a slurry, and the slurry is spray-dried and sintered to obtain a carbon-coated lithium manganese iron phosphate.

[0028] In an optional embodiment, the molar ratio of the lithium source, ferromanganese phosphate, carbon source, and surfactant is (1.02-1.04):1.00:(0.03-0.06):(0.03-0.06).

[0029] In a third aspect, the present invention provides an application of lithium iron manganese phosphate obtained by the preparation method of lithium iron manganese phosphate described in the aforementioned embodiment in a positive electrode material.

[0030] The present invention has the following beneficial effects:

[0031] The present application uses an organic solution-water system to adjust the system environment. Fe and Mn can synthesize a uniform solid solution under the action of the organic solution without adjusting the pH of the alkaline solution, which is helpful for synthesizing high-performance lithium manganese iron phosphate.

[0032] Through process control, a manganese iron phosphate precursor with a concentration gradient and a core-shell structure can be constructed as needed, reducing the distribution of manganese on the surface of the positive electrode material and thereby alleviating the manganese dissolution problem, thereby improving the material's cycle performance and discharge capacity. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0034] Figure 1 This is a SEM image of the anhydrous manganese iron phosphate prepared in Example 1;

[0035] Figure 2 This is the XRD pattern of the anhydrous manganese iron phosphate prepared in Example 1;

[0036] Figure 3 This is a SEM image of the lithium iron manganese phosphate prepared in Example 1;

[0037] Figure 4 This is the XRD pattern of lithium iron manganese phosphate prepared in Example 1. DETAILED DESCRIPTION

[0038] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the 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 of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0039] The present invention provides a method for preparing a manganese iron phosphate precursor, comprising the following steps:

[0040] Prepare liquids, preparing metal liquid A, metal liquid B and an organic solution, wherein both metal liquid A and metal liquid B contain iron salt and manganese salt, the concentration of manganese ions in the metal liquid A is greater than the concentration of manganese ions in the metal liquid B, and the concentration of iron ions in the metal liquid B is greater than the concentration of iron ions in the metal liquid A; the organic solution contains phosphoric acid or phosphate;

[0041] Co-precipitation: metal liquid A and metal liquid B are added to the organic solution at a preset flow rate. After the reaction is completed, the liquid after the reaction is separated into solid and liquid and dehydrated to obtain manganese iron phosphate.

[0042] On the one hand, in this embodiment, two kinds of metal liquids, metal liquid A and metal liquid B, with different iron ion and manganese ion concentrations are used, and the metal liquid A is first added to the organic solution, so that the molar ratio of iron and manganese in the manganese phosphate or iron manganese phosphate precipitated first is consistent with that of the metal liquid A, and a core rich in manganese and poor in iron or rich in manganese and free of iron can be obtained; then the metal liquid B is added, and the iron manganese phosphate continues to precipitate on the core, and the molar ratio of iron and manganese in the precipitate gradually approaches the molar ratio of iron and manganese in the metal liquid B, and a transition layer with a concentration gradient can be obtained at this time; when the amount of the metal liquid B added is sufficient, an iron manganese phosphate shell layer consistent with the molar ratio of iron and manganese in the metal liquid B can be precipitated on the transition layer eventually, and the general formula of the precipitate in this embodiment is Fe a Mn b PO 4 ·(2a+b)H 2 O.

[0043] Those skilled in the art can adjust the thickness and composition of the core, transition layer and shell layer by adjusting the timing of adding the metal liquid B and the concentration and flow rate of the metal liquid A and the metal liquid B as needed, assuming that the time taken for the injection of the metal liquid A is T. The obtained transition layer with a concentration gradient has a stable concentration gradient and better controllability in industrial applications.

[0044] In this embodiment, a core-shell structure with a concentration gradient can be obtained according to the timing of adding the metal liquid B, and an outer core rich in Fe and poor in Mn and an inner core rich in Mn and poor in Fe can be directionally constructed. The concentration gradient and the core-shell structure as lithium iron manganese phosphate precursors can effectively reduce the contact area between Mn and the electrolyte, reduce the dissolution of Mn, and improve the performance of the synthesized lithium iron manganese phosphate.

[0045] On the other hand, due to Fe 3+ , Mn 3+ The precipitation pH in aqueous solution varies greatly, so FePO 4 ·2H 2 O and MnPO 4 ·H 2 When O is precipitated in water, it is difficult to form a uniform solid solution and exists as a single phase, making it difficult to synthesize high-quality lithium iron manganese phosphate; the process of the present invention adjusts the system environment by using an organic solvent-water system, so that Fe and Mn can form a uniform solid solution under the action of an organic solvent without adding an alkali solution, and can synthesize a high-performance lithium iron manganese phosphate positive electrode material.

[0046] In other optional embodiments of the present application, the molar ratio of Mn:Fe in the molten metal A is (0.5-1):(0-0.5); the molar ratio of Mn:Fe in the molten metal B is (0-0.5):(0.5-1).

[0047] In this embodiment, the molar ratio of Mn:Fe in the molten metal A and the molten metal B can be adjusted to directionally construct manganese iron phosphate with an outer core rich in Fe and poor in Mn, and an inner core rich in Mn and poor in Fe. The concentrations of iron and manganese in the manganese iron phosphate are controlled within a certain range, which can effectively improve the platform voltage while taking into account the conductive properties.

[0048] In other optional embodiments of the present application, the iron salt is at least one of ferrous sulfate, ferric sulfate, ferric chloride, ferric oxalate, and ferric nitrate;

[0049] and / or the manganese salt is at least one of manganese nitrate, manganese oxalate, manganese sulfate, manganese chloride and manganese acetate;

[0050] and / or the phosphate is at least one of ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, sodium phosphate, sodium dihydrogen phosphate, sodium monohydrogen phosphate and potassium phosphate;

[0051] And / or the organic solvent is at least one of methanol, ethanol, ethylene glycol, propanol, n-butanol, acetonitrile, pyridine and phenol.

[0052] In other optional embodiments of the present application, the organic solution further contains an oxidant, and the oxidant is at least one of nitric acid, sodium nitrate, ammonium nitrate, ferric acid, perchloric acid, sodium ferrate, sodium bismuthate, sodium persulfate, hypochlorous acid, and ozone;

[0053] and / or the organic solution further contains a stabilizer, wherein the stabilizer is one of Schiff base, sodium pyrophosphate or pyrophosphoric acid;

[0054] Preferably, the amount of the oxidant added is to oxidize Fe in the metal liquid A and the metal liquid B. 2+ 120-150% of the theoretical molar amount;

[0055] Preferably, the molar ratio of the amount of the stabilizer added to the manganese ion is (1-5):20;

[0056] Preferably, the Schiff base is one of dihydroxyacetone phosphate, 3-phosphoglyceraldehyde and formylformic acid thiosemicarbazide Schiff base.

[0057] In this embodiment, the oxidant is when there is Fe 2+ ions, Fe 2+ Oxidized to Fe 3+ On the other hand, when there is no Fe 2+ When adding Fe3+ On this basis, when nitrate and phosphoric acid are added to the reaction solution of this embodiment, no additional oxidant may be added.

[0058] In this embodiment, a stabilizer is added during the precursor synthesis process. Since Mn 3+ Prone to disproportionation reaction to generate Mn 2+ and MnO 2 , cannot exist stably, so additives are needed to stabilize Mn 3+ Schiff bases are easily soluble in alcohol solvents and can react with Mn 3+ Forming Mn with good stability 3+ Complex, stabilizes Mn in alcohol-water system 3+ In addition, the oxidant in this embodiment is preferably nitric acid or ammonium nitrate. In the phosphoric acid solution, Mn 3+ The dismutation of nitrate can be greatly inhibited when nitrate is used as a strong oxidant.

[0059] In other optional embodiments of the present application, the mass ratio of water to organic solution in the reaction solution is 1:(3-10), and the molar ratio of the total amount of manganese and iron to phosphorus is 1:(1-1.5);

[0060] Preferably, the total amount of manganese and iron per liter of the reaction solution is 1.0×10 -2 -1.5×10 -2 mol;

[0061] Preferably, the flow rate of the molten metal A and / or the molten metal B is 0.8-4 g / min per liter of organic solution. For example, when the volume of the organic solution is 1 L, the flow rate of the molten metal A and / or the molten metal B is 0.8-4 g / min; when the volume of the organic solution is 10 L, the flow rate of the molten metal A and / or the molten metal B is 8-40 g / min, and so on.

[0062] The concentrations of iron, manganese and phosphorus in the reaction system are controlled to regulate the precipitation rate and concentration gradient of lithium iron phosphate.

[0063] In other optional embodiments of the present application, the reaction temperature of the coprecipitation step is 30-90° C., the stirring frequency during the reaction is 25-45 Hz, and the reaction solution is aged for 0.5-5 h after the addition of the metal liquid B is stopped.

[0064] In other optional embodiments of the present application, after the coprecipitation step, the solid phase obtained by solid-liquid separation is sequentially washed, dried, and dehydrated to obtain anhydrous manganese iron phosphate powder. Specifically, the solid phase can be washed with ethanol, with a mass ratio of ethanol to solid phase of 1:3 in each washing step, washed 3 times, then dried at 100°C for 15h, and then dehydrated at 645°C for 2.5h to obtain anhydrous manganese iron phosphate powder.

[0065] The present application also provides a method for preparing lithium iron manganese phosphate in an embodiment, wherein the iron manganese phosphate, water, a lithium source, a carbon source, and a surfactant described in any of the above embodiments are mixed and ground to obtain a slurry, and the slurry is spray-dried and sintered to obtain carbon-coated lithium iron manganese phosphate.

[0066] By selecting iron manganese phosphate precursors with different structures, the contact between Mn ions and the electrolyte is reduced during the synthesis of lithium iron manganese phosphate.

[0067] In other optional embodiments of the present application, the molar ratio of the lithium source, ferromanganese phosphate, carbon source, and surfactant is (1.02-1.04):1.00:(0.03-0.06):(0.03-0.06).

[0068] The present application also provides an embodiment of the use of the lithium iron manganese phosphate described in the above embodiment in a positive electrode material.

[0069] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0070] Embodiment 1:

[0071] This embodiment provides a method for preparing lithium manganese iron phosphate, comprising the following specific steps:

[0072] (1) 1.12 kg of ferrous sulfate heptahydrate and 1.53 kg of manganese sulfate heptahydrate were dissolved in 2 L of pure water to obtain a manganese-rich and iron-poor (Fe:Mn=42:58) metal liquid A with Fe and Mn concentrations of 45.28 g / L and 60.78 g / L, respectively; 2.25 kg of ferrous sulfate heptahydrate and 0.71 kg of manganese sulfate heptahydrate were dissolved in 2 L of pure water to obtain an iron-rich and manganese-poor (Fe:Mn=75:25) metal liquid B with Fe and Mn concentrations of 90.60 g / L and 28.03 g / L, respectively; 1905 g of phosphoric acid, 1.1 kg of nitric acid, and 0.3 kg of dihydroxyacetone phosphate were dissolved in 13 L of ethanol to obtain a solution C;

[0073] (2) adding liquid metal A to the reactor containing solution C at a rate of 45 g / min, stirring at a frequency of 35 Hz, and heating to 60° C. When liquid metal A is just added, liquid metal B is controlled to be added to the container containing liquid metal A at a rate of 45 g / min. 0.5 h after the addition of liquid metal B is completed, solid-liquid separation is performed to obtain ferromanganese phosphate filter cake D and mother liquor;

[0074] (3) After washing, drying and dehydration, filter cake D is obtained to obtain anhydrous manganese iron phosphate powder with a core-shell structure. The product has a core-shell structure consisting of manganese iron phosphate with an internal ratio of Fe:Mn=42:58 and manganese iron phosphate with an external ratio of Fe:Mn=75:25;

[0075] (4) The obtained 1000g of manganese iron phosphate, 3300g of pure water, 250g of lithium carbonate, 65g of glucose, and 45g of PEG were prepared into a slurry with a solid content of 35%, and finely ground to a D50 of about 0.45-0.50um. The slurry was spray-dried and sintered at 750°C for 8h to obtain a carbon-coated lithium manganese iron phosphate with an iron-manganese ratio of 6:4.

[0076] Depend on Figure 1 It can be seen that the synthesized iron manganese phosphate precursor is spherical particles, and the secondary particles are soft agglomerated; Figure 2 It can be seen that the manganese iron phosphate precursor is mainly 29-0715 crystal form, Mn ions enter the Fe lattice site, and no new crystal form and impurity peaks are generated; Figure 3 The lithium iron manganese phosphate cathode material synthesized in the process is nanoscale, with good dispersion between particles and no adhesion. Figure 4 The XRD of the synthesized lithium iron manganese phosphate positive electrode material shows that the crystal form of the synthesized product is olivine crystal form, and there is no impurity phase.

[0077] Embodiment 2:

[0078] This embodiment provides a method for preparing lithium manganese iron phosphate, comprising the following specific steps:

[0079] (1) 0.748 kg of ferrous sulfate heptahydrate and 1.64 kg of manganese sulfate heptahydrate were dissolved in 2 L of pure water to obtain a manganese-rich and iron-poor (Fe:Mn=31:69) metal liquid A with Fe and Mn concentrations of 30.15 g / L and 65.21 g / L, respectively; 1.94 kg of ferrous sulfate heptahydrate and 0.98 kg of manganese sulfate heptahydrate were dissolved in 2 L of pure water to obtain an iron-rich and manganese-poor (Fe:Mn=66:34) metal liquid B with Fe and Mn concentrations of 78.15 g / L and 39.11 g / L, respectively; 1985 g of phosphoric acid, 0.96 kg of nitric acid, and 0.25 kg of dihydroxyacetone phosphate were dissolved in 13 L of butanol to obtain a solution C;

[0080] (2) Liquid metal A was added dropwise to the reactor containing solution C at a rate of 75 g / min, with a stirring rate of 45 Hz and heated to 50° C. When the liquid metal A was added to 50%, liquid metal B was controlled to be added dropwise to the container containing liquid metal A at a rate of 75 g / min. After 1.5 hours after the addition of liquid metal B was completed, solid-liquid separation was performed to obtain ferromanganese phosphate filter cake D and mother liquor;

[0081] (3) After washing, drying and dehydration, filter cake D is obtained to obtain anhydrous manganese iron phosphate powder with a core-shell structure, wherein Fe:Mn=5:5, the interior of the product is manganese iron phosphate with Fe:Mn=31:69, the exterior is manganese iron phosphate with Fe:Mn=66:34, and there is a transition layer in the middle from Fe:Mn=31:69 to Fe:Mn=66:34, the transition layer has a radially uniform distribution, and the Fe concentration increases and the Mn concentration decreases along the radial direction;

[0082] (4) The obtained 1000g manganese iron phosphate, 4500g pure water, 240g lithium carbonate, 60g glucose, and 50g PEG are prepared into a slurry with a solid content of 30%, and finely ground to a D50 of about 0.58-0.63um. The slurry is spray-dried and sintered at 720°C for 10h to obtain a carbon-coated lithium manganese iron phosphate with an iron-manganese ratio of 5:5.

[0083] Embodiment 3:

[0084] This embodiment provides a method for preparing lithium manganese iron phosphate, comprising the following specific steps:

[0085] (1) 1.12 kg of ferrous sulfate heptahydrate and 1.53 kg of manganese sulfate heptahydrate were dissolved in 2 L of pure water to obtain a manganese-rich and iron-poor (Fe:Mn=42:58) metal liquid A with Fe and Mn concentrations of 45.28 g / L and 60.78 g / L, respectively; 2.25 kg of ferrous sulfate heptahydrate and 0.71 kg of manganese sulfate heptahydrate were dissolved in 2 L of pure water to obtain an iron-rich and manganese-poor (Fe:Mn=75:25) metal liquid B with Fe and Mn concentrations of 90.60 g / L and 28.03 g / L, respectively; 1877 g of phosphoric acid, 2000 g of 27.5% hydrogen peroxide, and 0.3 kg of dihydroxyacetone phosphate were dissolved in 13 L of ethylene glycol to obtain solution C;

[0086] (2) Liquid metal A was added dropwise to the reactor containing solution C at a rate of 45 g / min, with a stirring rate of 35 Hz and heated to 60° C. When liquid metal A began to be added dropwise, liquid metal B was controlled to be added dropwise to the container containing liquid metal A at a rate of 45 g / min. One hour after the addition of liquid metal B was completed, solid-liquid separation was performed to obtain ferromanganese phosphate filter cake D and mother liquor;

[0087] (3) After washing, drying and dehydration, filter cake D is obtained to obtain anhydrous manganese iron phosphate powder with radially uniform distribution. The product has a radially uniformly distributed concentration gradient structure composed of manganese iron phosphate with an internal Fe:Mn=42:58 gradually changing to Fe:Mn=75:25 along the radial direction;

[0088] (4) The obtained 1000g manganese iron phosphate, 3300g pure water, 250g lithium carbonate, 75g sucrose, and 63g PEG are used to obtain a slurry with a solid content of 35%, which is ground to a D50 of about 0.60-0.65um, spray-dried, and sintered at 730°C for 12h to obtain carbon-coated lithium manganese iron phosphate.

[0089] Example 4

[0090] This embodiment provides a method for preparing lithium manganese iron phosphate, which differs from the preparation method in Embodiment 1 in that:

[0091] (1) 3.37 kg of ferrous sulfate heptahydrate was dissolved in 2 L of pure water to obtain a metal solution A with an Fe concentration of 135.54 g / L, and 2.27 kg of manganese sulfate heptahydrate was dissolved in 2 L of pure water to obtain a metal solution B with a Mn concentration of 90.14 g / L; 1050 g of phosphoric acid, 1.1 kg of nitric acid and 0.3 kg of phosphoglyceraldehyde were dissolved in 13 L of ethanol to obtain a solution C;

[0092] (2) The metal liquid B was added dropwise to the reactor containing the solution C at a rate of 45 g / min, with a stirring rate of 35 Hz and heated to 60° C. After the addition of the metal liquid B was completed, the metal liquid A was controlled to be added dropwise to the container containing the metal liquid B at a rate of 45 g / min. 0.5 h after the addition of the metal liquid A was completed, solid-liquid separation was performed to obtain ferromanganese phosphate filter cake D and mother liquor;

[0093] (3) After the filter cake D is washed, dried and dehydrated, anhydrous manganese iron phosphate powder is obtained. The obtained manganese iron phosphate has a core-shell structure composed of internal manganese phosphate and external iron phosphate;

[0094] (4) The obtained 1000g manganese iron phosphate, 3300g pure water, 250g lithium carbonate, 65g glucose, and 45g PEG were prepared into a slurry with a solid content of 35%, and finely ground to a D50 of about 0.45-0.50um. The slurry was spray-dried and sintered at 750°C for 10h to obtain a carbon-coated lithium manganese iron phosphate with an iron-manganese ratio of 6:4.

[0095] Comparative Example 1

[0096] The preparation method of lithium iron phosphate in this comparative example comprises the following steps:

[0097] (1) 2.33 kg of ferrous sulfate heptahydrate was dissolved in 2 L of pure water to obtain a metal solution A with an Fe concentration of 93.71 g / L; 790 g of phosphoric acid and 500 g of hydrogen peroxide were dissolved in 13 L of ethanol to obtain a solution B;

[0098] (2) adding the molten metal A to the reactor containing the solution B at a rate of 45 g / min, stirring at a rate of 35 Hz, and heating to 70°C. After the addition of the molten metal A was completed, the molten metal B was controlled to be added to the container containing the molten metal A at a rate of 45 g / min. After 3 hours of the addition of the molten metal B, solid-liquid separation was performed to obtain the iron phosphate filter cake D and the mother liquor;

[0099] (3) After the filter cake D is washed, dried and dehydrated, an anhydrous iron phosphate powder with a core-shell structure is obtained;

[0100] (4) The obtained 1000g iron phosphate, 3300g pure water, 250g lithium carbonate, 65g glucose, and 45g PEG are used to prepare a slurry with a solid content of 35%, and the slurry is finely ground to a D50 of about 0.45-0.50um. The slurry is spray-dried and sintered at 750°C for 8h to obtain carbon-coated lithium iron phosphate.

[0101] Comparative Example 2

[0102] The preparation method of lithium manganese phosphate in this comparative example comprises the following steps:

[0103] (1) 1.50 kg of manganese sulfate heptahydrate was dissolved in 2 L of pure water to obtain a metal solution A with a Mn concentration of 65.31 g / L; 595 g of phosphoric acid, 0.28 kg of nitric acid, and 0.10 kg of Schiff base were dissolved in 3 L of ethanol to obtain a solution B;

[0104] (2) The metal liquid A was added dropwise to the reactor containing the solution B at a rate of 45 g / min, the stirring rate was 45 Hz, and the temperature was heated to 60° C. After 1 hour of the completion of the metal liquid addition, the solid-liquid separation was performed to obtain a dark green manganese phosphate filter cake C and a mother liquor;

[0105] (3) After the filter cake C is washed, dried and dehydrated, anhydrous manganese phosphate powder is obtained;

[0106] (4) The obtained 1000g manganese phosphate, 3300g pure water, 250g lithium carbonate, 65g glucose, and 45g PEG were used to prepare a slurry with a solid content of 35%, and the slurry was finely ground to a D50 of about 0.45-0.50um. The slurry was spray-dried and sintered at 750°C for 8h to obtain carbon-coated lithium manganese phosphate.

[0107] Comparative Example 3

[0108] This comparative example provides a method for preparing lithium manganese iron phosphate, which differs from that of Example 1 in that:

[0109] In step (2), the metal liquid B is added dropwise to the reactor containing the solution C at a rate of 45 g / min. After the addition of the metal liquid B is completed, the metal liquid A is controlled to be added dropwise to the container containing the metal liquid B at a rate of 45 g / min. The core-shell structure of the manganese iron phosphate is formed by an internal manganese iron phosphate with Fe: Mn = 75:25 and an external manganese iron phosphate with Fe: Mn = 42:58. The electrochemical performance of the manganese iron phosphate formed is reduced. The manganese iron phosphate prepared in this comparative example is used as a precursor of the lithium manganese iron phosphate positive electrode material, with Fe: Mn = 60:40. Due to the high proportion of Mn on the outside, the contact of Mn with the electrolyte is not effectively improved. The prepared lithium manganese iron phosphate has more Mn dissolved and the performance is slightly worse.

[0110] Comparative Example 4

[0111] The difference between Comparative Example 4 and Example 1 is that in step (1) of Comparative Example 4, ethanol is not used as the solvent, but pure water is used as the solvent;

[0112] After the reaction, no dark green precipitate was generated, but a white precipitate was obtained. After the reaction, the solid-liquid separation was performed, and the Fe and Mn concentrations of the mother liquor were tested to be approximately 0.1 g / L and 15.17 g / L, respectively, proving that Fe and Mn did not co-precipitate, and only Fe in the system participated in the reaction, and no uniform iron-manganese phosphate precursor was formed.

[0113] Physical and chemical indicators of manganese iron phosphate:

[0114] Table 1 shows the physical and chemical indicators of the iron manganese phosphate products prepared in Examples 1, 2, 3, 4 and Comparative Examples 1, 2, 3, 4. As shown in Table 1, the Fe / (Fe+Mn) ratio of the iron manganese phosphate products prepared in the examples meets the designed ratio, (Fe+Mn) / P meets the performance requirements of high-performance lithium iron manganese phosphate for precursors, and the BET and TD of the iron manganese phosphate synthesized in the examples are higher than those in the comparative examples, which is beneficial to improving the activity of the synthesized lithium iron manganese phosphate and improving the compaction performance of the product.

[0115]

[0116] Electrochemical performance:

[0117] Table 2 shows the electrochemical performance of the positive electrode materials synthesized by using manganese iron phosphate as raw materials prepared in Examples 1, 2, 3, 4 and Comparative Examples 1, 2, 3, 4. The specific data are obtained by testing with equipment such as an electrochemical workstation. At room temperature of 25°C, the first discharge capacity and first efficiency were tested at a charge and discharge voltage of 2.0-3.65V and an initial charge and discharge rate of 0.1C; at room temperature of 25°C, the cycle performance of 200 weeks and the powder compaction performance were tested at a charge and discharge voltage of 2.0-3.65V and a charge and discharge rate of 1C. It can be seen from Table 2 that the electrochemical performance of the lithium iron phosphate product prepared in the embodiment is significantly better than that of Comparative Examples 1, 2, and 3, especially Example 1.

[0118] Table 2 Comparison of performance indicators of lithium iron manganese phosphate

[0119]

[0120]

[0121] As can be seen from Table 1, the electrical properties of the lithium iron manganese phosphate positive electrode material prepared by the present invention show that by dissolving Fe and Mn in any proportion, it is possible to construct an iron manganese phosphate core-shell structure with a concentration gradient, improve the powder characteristics, and increase the compaction density of the synthesized lithium iron manganese phosphate. However, at the same time, simply increasing the Mn content in the iron manganese phosphate will increase the polarization in the lithium iron manganese phosphate battery and reduce the discharge performance. By constructing a core-shell structure with a higher Fe concentration in the outer layer, the dissolution of Mn can be effectively reduced, the polarization phenomenon can be reduced, and the effect of inhibiting the reduction of discharge performance can be achieved.

[0122] Comparison between Examples 1 and 3 and Example 4 shows that when Fe / (Fe+Mn) is similar, adjusting the Mn content in the shell can adjust the position of the impurity energy level of the lithium iron phosphate, making the impurity energy level closer to the bottom of the conduction band and reducing the distance for electrons to transition to the conduction band, making it easier for electrons to transition, thereby increasing the conductivity of the doped lithium iron manganese phosphate and enhancing the conductive properties, which is beneficial to improving the performance of the lithium iron phosphate.

[0123] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a manganese iron phosphate precursor, characterized in that: The steps include: Prepare liquids, preparing metal liquid A, metal liquid B and an organic solution, wherein both metal liquid A and metal liquid B contain iron salt and manganese salt, the concentration of manganese ions in the metal liquid A is greater than the concentration of manganese ions in the metal liquid B, and the concentration of iron ions in the metal liquid B is greater than the concentration of iron ions in the metal liquid A; the organic solution contains phosphoric acid or phosphate; Co-precipitation: metal liquid A and metal liquid B are added to the organic solution at a preset flow rate. After the reaction is completed, the liquid after the reaction is separated into solid and liquid and dehydrated to obtain manganese iron phosphate.

2. The method for preparing the iron manganese phosphate precursor according to claim 1, characterized in that: The molar ratio of Mn:Fe in the molten metal A is (0.5-1):(0-0.5); the molar ratio of Mn:Fe in the molten metal B is (0-0.5):(0.5-1).

3. The method for preparing the iron manganese phosphate precursor according to claim 1, characterized in that: The iron salt is at least one of ferrous sulfate, ferric sulfate, ferric chloride, ferric oxalate, and ferric nitrate; and / or the manganese salt is at least one of manganese nitrate, manganese oxalate, manganese sulfate, manganese chloride and manganese acetate; and / or the phosphate is at least one of diammonium phosphate, monoammonium phosphate, sodium phosphate, sodium dihydrogen phosphate, sodium monohydrogen phosphate and potassium phosphate; And / or the organic solvent is at least one of methanol, ethanol, ethylene glycol, propanol, n-butanol, acetonitrile, pyridine and phenol.

4. The method for preparing the iron manganese phosphate precursor according to claim 1, characterized in that: The organic solution further contains an oxidant, which is at least one of nitric acid, sodium nitrate, ammonium nitrate, ferric acid, perchloric acid, sodium ferrate, sodium bismuthate, sodium persulfate, hypochlorous acid, and ozone; And / or the organic solution further contains a stabilizer, and the stabilizer is one of Schiff base, sodium pyrophosphate or pyrophosphoric acid.

5. The method for preparing the iron manganese phosphate precursor according to claim 4, characterized in that: The amount of the oxidant added is to oxidize the Fe in the metal liquid A and the metal liquid B. 2+ 120-150% of the theoretical molar amount.

6. The method for preparing the iron manganese phosphate precursor according to claim 4, characterized in that: The molar ratio of the amount of the stabilizer added to the manganese ion is (1-5):

20.

7. The method for preparing the iron manganese phosphate precursor according to claim 4, characterized in that: The Schiff base is one of dihydroxyacetone phosphate, 3-phosphoglyceraldehyde and formylformic acid thiosemicarbazide Schiff base.

8. The method for preparing the iron manganese phosphate precursor according to claim 1, characterized in that: The mass ratio of water to organic solution in the liquid after the reaction is 1:(3-10), and the molar ratio of the total amount of manganese and iron to phosphorus is 1:(1-1.5).

9. The method for preparing the iron manganese phosphate precursor according to claim 1, characterized in that: The total amount of manganese and iron in the reaction solution is 1.0×10 -2 -1.5×10 -2 mol.

10. The method for preparing the iron manganese phosphate precursor according to claim 1, characterized in that: The flow rate of the metal liquid A and / or the metal liquid B is 0.8-4 g / min per liter of organic solution.

11. The method for preparing the iron manganese phosphate precursor according to claim 1, characterized in that: The reaction temperature of the coprecipitation step is 30-90° C., the stirring rate during the reaction is 25-45 Hz, and the metal liquid B is aged for 0.5-5 hours after the addition is stopped to obtain the post-reaction liquid.

12. The method for preparing the iron manganese phosphate precursor according to claim 1, characterized in that: After the coprecipitation step, the solid phase obtained by solid-liquid separation is washed, dried and dehydrated in sequence to obtain anhydrous manganese iron phosphate powder.

13. A method for preparing lithium manganese iron phosphate, characterized in that: The manganese iron phosphate obtained by the preparation method of the manganese iron phosphate precursor according to any one of claims 1 to 12 is mixed with water, a lithium source, a carbon source, and a surfactant and ground to obtain a slurry, and the slurry is spray-dried and sintered to obtain a carbon-coated lithium manganese iron phosphate.

14. The method for preparing lithium manganese iron phosphate according to claim 13, characterized in that: The molar ratio of the lithium source, manganese iron phosphate, carbon source and surfactant is (1.02-1.04):1.00:(0.03-0.06):(0.03-0.06).

15. Use of the lithium iron manganese phosphate obtained by the preparation method of the lithium iron manganese phosphate according to claim 13 or 14 in positive electrode materials.

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