Lithium manganese iron phosphate positive electrode material and preparation method therefor

Abstract

A preparation method for a lithium manganese iron phosphate positive electrode material, comprising the following steps: (1) uniformly stirring an iron source, a phosphorus source, a lithium source and a dispersing agent in water, adding a manganese source, and uniformly mixing same to obtain a precursor pre-slurry; and (2) adjusting the solid content of the precursor pre-slurry to reach a target solid content A, the target solid content A satisfying 30 wt% ≤ A ≤ 60 wt%; (3) carrying out primary grinding treatment on the precursor slurry obtained in step (2); and (4) adding an organic phosphine scale and corrosion inhibitor to the precursor slurry, dissolving same, then drying the obtained slurry, and performing sintering and crushing to obtain the lithium manganese iron phosphate positive electrode material. Also provided is a positive electrode material prepared by the method.

Description

A lithium iron manganese phosphate positive electrode material and preparation method thereof Technical Field

[0001] The present invention belongs to the field of preparation of lithium ion battery positive electrode materials, and in particular relates to a preparation method and application of a high-capacity and high-density lithium manganese iron phosphate positive electrode material. Background Art

[0002] Lithium-ion batteries have become the most widely used energy storage method in today's society due to their high energy density, environmental friendliness, and long cycle life. They are widely used in electric vehicles, consumer electronics, and energy storage systems. However, manufacturing low-cost, high-energy-density, and long-cycle-life lithium-ion batteries remains a challenge. The performance of a battery depends primarily on the cathode material. The cathode materials currently widely used in the market include lithium cobalt oxide, lithium manganese oxide, nickel-cobalt-manganese ternary materials, and lithium iron phosphate. Among them, lithium iron phosphate cathode materials are gaining an increasing market share, especially in electric vehicles and energy storage systems, due to their long cycle life, high stability, low cost, and abundant raw material sources.

[0003] However, a major problem with lithium iron phosphate is its relatively low discharge voltage, which limits the energy density of the battery. In contrast, lithium manganese phosphate has a higher discharge voltage platform at the same specific capacity, and therefore has a higher theoretical energy density. However, the electronic and ionic conductivity of lithium manganese phosphate is extremely poor. At the same time, due to the Günther effect, the capacity will gradually decay during the battery charge and discharge cycle, resulting in a short cycle life and cannot be used in practical applications. The manganese and iron elements in lithium manganese iron phosphate can be mutually dissolved in any proportion to form an olivine structure solid solution. It combines the structural stability of lithium iron phosphate with the high voltage of lithium manganese phosphate, has a higher voltage platform than lithium iron phosphate, and has excellent low-temperature performance. It has the potential to be used as a higher energy density positive electrode material in lithium-ion batteries. However, compared with lithium iron phosphate, lithium manganese iron phosphate currently still has problems such as low electronic / ionic conductivity, low discharge capacity, and rapid cycle decay that need to be solved urgently. During the production practice, it was found that the main factors currently limiting the performance of lithium manganese iron phosphate are the poor uniformity of the precursor slurry system and particle size, which easily leads to system inhomogeneity, resulting in uneven distribution of the precursor components obtained by spraying and poor capacity of the lithium manganese iron phosphate obtained by sintering.

[0004] Summary of the Invention

[0005] To solve the above problems, the present invention provides a lithium manganese iron phosphate positive electrode material and a preparation method thereof, which solves the problems of low electronic conductivity and ion conductivity, low discharge capacity and rapid cycle decay of lithium manganese iron phosphate materials.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] A method for preparing a lithium manganese iron phosphate positive electrode material comprises the following steps:

[0008] (1) After uniformly stirring an iron source, a phosphorus source, a lithium source, and a dispersant in water, a manganese source is added and mixed uniformly to obtain a precursor pre-made slurry; specifically, for example, step (1) is performed in a stirring tank, water is added first, and then other materials are added;

[0009] (2) adjusting the solid content of the precursor pre-made slurry to reach a target solid content A, wherein the target solid content A satisfies 30%≤A≤60%, preferably 35%≤A≤50%; preferably, the solid content of the precursor pre-made slurry is adjusted by adding water; preferably, the amount of water used in step (1) accounts for 85-99% by volume of the total amount of water used in step (1) and step (2), for example, 85%, 88%, 90%, 92%, 94%, 97%, 99%, etc., for example, 85-97%;

[0010] (3) grinding the precursor slurry obtained in step (2) under stirring;

[0011] (4) adding an organic phosphine scale and corrosion inhibitor to the precursor slurry obtained in step (3) to dissolve the precursor slurry, preferably performing a secondary grinding treatment, and then drying the obtained slurry, sintering, and pulverizing to obtain a lithium manganese iron phosphate positive electrode material;

[0012] The carbon source is added and dissolved in step (1), or the carbon source is added to the precursor slurry and dissolved in step (4). That is, the carbon source can be added in step (1) or in step (4). For example, in step (4), the carbon source and the organic phosphine scale and corrosion inhibitor are added to the precursor slurry and stirred until completely dissolved.

[0013] In the present invention, by controlling the solid content of the precursor slurry and performing a primary grinding process before adding the organophosphine scale and corrosion inhibitor, and following a specific addition sequence, a slurry with improved stability is obtained, which facilitates obtaining a positive electrode material with a higher compaction density, and simultaneously, a positive electrode material with improved electrochemical properties such as discharge capacity and rate capability. More preferably, a secondary grinding process is performed after the addition of the organophosphine scale and corrosion inhibitor. The organophosphine scale and corrosion inhibitor's complexing, solubilizing, and dispersing properties can form stable, soluble complexes with metal ions or oxides, further enhancing slurry stability and inhibiting slurry precipitation and precipitation. This can further improve the positive electrode material's compaction density and electrochemical properties such as discharge capacity and rate capability.

[0014] The present inventors have found that adding the organic phosphine scale and corrosion inhibitor after the first grinding, compared with adding it before the first grinding (for example, in step (1)), can prevent the organic phosphine scale and corrosion inhibitor from reacting with the alkaline substances in the raw materials and becoming inactivated, and effectively exert its complexing, solubilizing and dispersing effects.

[0015] Preferably, in step (1) of the present invention, the iron source is selected from one or more of iron oxide, ferroferric oxide, ferrous phosphate, ferrous oxalate, and ammonium ferrous phosphate; the phosphorus source is one or more of ferric phosphate, phosphoric acid, lithium dihydrogen phosphate, manganese hydrogen phosphate, and ammonium dihydrogen phosphate; the lithium source is one or more of lithium carbonate, lithium hydroxide, lithium phosphate, and lithium dihydrogen phosphate; the dispersant is one or more of polyethylene glycol 800, polyethylene glycol 6000, methyl cellulose, polyvinyl alcohol, and hexenyl bisstearamide; the manganese source is one or more of manganese dioxide, manganese carbonate, manganese oxalate, manganese sulfate, manganese tetraoxide, and manganese hydrogen phosphate;

[0016] Preferably, the molar ratio of iron element and manganese element added is 0.11-1:1, and the ratio of the molar amount of lithium element added to the sum of the molar amounts of iron element and manganese element added is 0.9-1.1:1; the above iron element, manganese element and lithium element refer to the corresponding elements contained in the raw materials added during the preparation of the slurry.

[0017] Preferably, based on the volume of water added in step (1), the amount of the dispersant added is 5-50 g / L, for example, 5 g / L, 10 g / L, 20 g / L, 30 g / L, 40 g / L, 50 g / L, etc., preferably 10-35 g / L; the preferred dispersant concentration is used to further improve the uniformity of the precursor slurry, so that the slurry is evenly distributed and the particles are of uniform size, which is conducive to ultimately obtaining a lithium manganese iron phosphate positive electrode material with better electrochemical performance.

[0018] Preferably, in step (3) of the present invention, the precursor slurry is subjected to the primary grinding treatment to a particle size D50 of 0.35-0.55 nm, preferably 0.4-0.45 μm; the particle size of the slurry after the primary grinding is controlled within the preferred range, which is conducive to further improving the uniformity of the precursor slurry, making the slurry evenly distributed and the particles of uniform size, and ultimately obtaining a lithium manganese iron phosphate positive electrode material with better electrochemical performance.

[0019] Preferably, the ratio of the sum of the molar amounts of phosphorus in the phosphorus source used in step (1) and the organic phosphine scale and corrosion inhibitor used in step (4) of the present invention to the sum of the molar amounts of iron and manganese in the manganese source and the iron source is p (i.e., P element: (Fe element + Mn element), molar ratio), 0.99≤p≤1.05;

[0020] Preferably, based on the volume of water used in step (1), the amount of the organic phosphine scale inhibitor added is 0.02-0.4 mol / L, preferably 0.04-0.2 mol / L. The preferred amount of organic phosphine scale inhibitor added is beneficial to further improve the uniformity of the slurry and further improve the electrochemical performance of the lithium manganese iron phosphate cathode material.

[0021] The organic phosphine scale inhibitor and corrosion inhibitor contains two or more phosphonic acid groups, and the phosphorus atom is directly connected to the carbon atom. Preferably, the organic phosphine scale inhibitor and corrosion inhibitor includes one or more of hydroxyethylidene diphosphonic acid, aminotrimethylene phosphonic acid, diethylenetriamine penta (methylene phosphonic acid), and ethylenediamine tetramethylene phosphonic acid.

[0022] Preferably, the carbon source includes one or more of glucose, sucrose, ascorbic acid, and starch;

[0023] Preferably, based on the volume of water added in step (1), the amount of the carbon source added is 10-80 g / L, preferably 20-50 g / L;

[0024] Preferably, in step (4) of the present invention, after the organic phosphine scale and corrosion inhibitor is added and dissolved, the resulting slurry has a viscosity of 100-1000 Pa·s, preferably 200-600 Pa·s. The slurry having the above-mentioned preferred viscosity obtained by the preparation method of the present invention has good uniformity and is not prone to sedimentation.

[0025] Preferably, in step (4) of the present invention, the secondary grinding treatment is performed, and the particle distribution uniformity coefficient b in the obtained slurry is less than 0.3, preferably less than 0.2;

[0026] Preferably, in step (4) of the present invention, the drying is carried out by spray drying, and the spray drying air inlet temperature is controlled to be 240-280°C, preferably 250-270°C; the air outlet temperature is controlled to be 85-105°C, preferably 90-100°C;

[0027] Preferably, in step (4) of the present invention, the dried precursor is sintered in a nitrogen atmosphere. The sintering conditions are preferably as follows: heating to 250-350°C at a heating rate of 2-5°C / min, holding for 1-3 hours, then heating to 400-550°C at a heating rate of 2-5°C / min, holding for 1-3 hours, then heating to the target temperature at a heating rate of 2-5°C / min, holding for 5-12 hours. The target temperature is set to 600-800°C, preferably 650-750°C.

[0028] The present invention also provides a lithium iron manganese phosphate positive electrode material prepared by the preparation method of the lithium iron manganese phosphate positive electrode material; the chemical formula of the lithium iron manganese phosphate positive electrode material is preferably Li x Mny Fe z (PO4) p , where 0.9≤x≤1.1, 0.5≤y≤0.9, 0.1≤z≤0.5, 0.99≤p≤1.05, and y+z=1.

[0029] In the present invention, the lithium manganese iron phosphate positive electrode material prepared by the above preparation method has the characteristics of high electronic conductivity and ionic conductivity, small and uniform particle size, low manganese dissolution and iron dissolution rates, and stable structure. The application of this positive electrode material in lithium-ion batteries can improve discharge capacity, rate performance and long cycle life.

[0030] In the present invention, an organic phosphine scale inhibitor is added to the slurry after the first grinding, and preferably a secondary sand grinding treatment is performed. The organic phosphine scale inhibitor can form a stable soluble complex with the metal ions or oxides by utilizing the complexing, solubilizing and dispersing effects of the organic phosphine scale inhibitor, thereby increasing the slurry stability and inhibiting the precipitation and precipitation of the slurry.

[0031] During the high-temperature sintering process, the phosphorus-oxygen groups of the organophosphine scale and corrosion inhibitor provide additional phosphate groups, which combine with Mn and Fe ions to form lithium iron manganese phosphate. Simultaneously, due to the chelation effect between the organophosphine scale and corrosion inhibitor and the metal cations, and the in-situ carbon generated directly on the phosphorus-oxygen tetrahedrons after high-temperature decomposition, the mass transfer process of the lithium iron manganese phosphate during the sintering process is hindered, inhibiting the irregular growth of the lithium iron manganese phosphate and forming small particles of lithium iron manganese phosphate. The graphitized carbon layer produced by these decompositions causes the small particles of lithium iron manganese phosphate to bond into secondary agglomerates. The lithium iron manganese phosphate obtained by this method has a small and uniform particle size. At the same time, the presence of secondary agglomerates increases the compaction density, resulting in high-capacity and high-density lithium iron manganese phosphate. The resulting lithium iron manganese phosphate cathode material has excellent electrochemical properties, with improved discharge capacity and rate performance.

[0032] The present invention facilitates obtaining a precursor slurry with better uniformity by controlling the solid content and addition sequence of the precursor slurry, adding an organophosphine scale and corrosion inhibitor after the primary grinding process, and preferably continuing with a secondary grinding process. More preferably, further controlling the dispersant concentration, slurry viscosity, and sanding particle size within a preferred range further improves the uniformity of the precursor slurry, resulting in a uniform slurry distribution and consistent particle size. The preparation method of the present invention can produce a positive electrode material with a high compaction density and improved electrochemical properties such as discharge capacity and rate capability. DETAILED DESCRIPTION

[0033] The technical solution of the present invention is further described below with reference to specific embodiments. The embodiments provide specific implementation methods and specific operating processes, but the protection scope of the present invention is not limited to the following embodiments.

[0034] Ingredients Description:

[0035] Iron oxide (purity ≥98%), manganese dioxide (purity ≥98%), lithium hydroxide (purity ≥99%), phosphoric acid (purity ≥85%), ammonium dihydrogen phosphate (analytical grade), polyethylene glycol 800, polyvinyl alcohol, polyethylene glycol 6000, sucrose (purity 99%), ascorbic acid (purity 99%), and starch: Manufacturer: Beijing Yinuokai Technology Co., Ltd.

[0036] Ferric phosphate (purity ≥99%), manganese carbonate (analytical grade), methylcellulose, glucose, diethylenetriaminepenta(methylenephosphonic acid) (50 wt% aqueous solution), hydroxyethylidenediphosphonic acid (60 wt% aqueous solution), ethylenediaminetetramethylenephosphonic acid (purity 95%): Manufacturer: Aladdin Biochemical Technology Co., Ltd.

[0037] Ferrous oxalate (purity ≥96%), manganese oxalate (purity ≥99%): Manufacturer: Shanghai MacLean Biochemical Technology Co., Ltd.

[0038] Lithium carbonate and lithium dihydrogen phosphate: analytical grade, Sinopharm Chemical Reagent Co., Ltd.

[0039] Ferrous ammonium phosphate: homemade;

[0040] Aminotrimethylenephosphonic acid: purity 99%, Jining Huipeng Chemical Co., Ltd.

[0041] Description of the detection method:

[0042] Sand mill slurry particle size D50 and particle distribution uniformity coefficient b in the sand mill slurry: The material discharged from the sand mill outlet was tested using a Malvern 3000 laser particle size analyzer.

[0043] Slurry viscosity: Take the slurry at the sand mill outlet for viscosity test using a Brookfield DV2T viscometer.

[0044] Powder compaction density: The finished product after air flow grinding is tested for powder compaction density using Sansi UTM7305 compaction density meter with a test pressure of 3T.

[0045] The contents of each element in the prepared positive electrode material were determined by ICP testing.

[0046] Electrochemical performance testing: The sample material was mixed with a binder, PTFE (polytetrafluoroethylene), and conductive carbon black in a mass ratio of 9:0.5:0.5. The mixture was ground evenly in an agate mortar to obtain a slurry. The slurry was evenly coated onto aluminum foil, vacuum-dried, and cut into pieces to obtain the positive electrode. A lithium sheet was used as the negative electrode, Celgard 2400 as the separator, and a 1 mol / L LiPF6 solution (the solvent was EC (ethylene carbonate), DEC (diethyl carbonate), and DMC (dimethyl carbonate) in a mass ratio of 1:1:1) was used as the electrolyte. CR2032 button cells were assembled in a glove box for electrochemical performance testing. The test voltage range was 2.5-4.5V, and the discharge capacity at 0.1C and 1C rates was measured.

[0047] Example 1

[0048] (1) Add 5L of pure water to a stirring tank in advance (the amount of each raw material added in each subsequent step is based on the 5L of pure water added in step (1) as a benchmark for proportioning). Add iron oxide, ammonium dihydrogen phosphate, lithium hydroxide, and dispersant polyethylene glycol 800 while stirring. The amount of iron oxide, ammonium dihydrogen phosphate, and lithium hydroxide added is 0.8 mol / L, 3.9 mol / L, and 4.12 mol / L, respectively, and the amount of polyethylene glycol 800 added is 30 g / L. After stirring evenly, add 0.8 mol / L of manganese tetraoxide and mix evenly to obtain a precursor pre-made slurry;

[0049] (2) Adding some pure water to the precursor pre-made slurry obtained in step (1) to make the solid content of the slurry reach 45 wt%; the total amount of pure water used in step (1) and step (2) is 5.5 L.

[0050] (3) Grinding the precursor slurry obtained in step (2) under stirring until the particle size D50 reaches 0.4-0.45 μm to obtain a suspension containing lithium manganese iron phosphate precursor, transferring the suspension to another slurry barrel, and at the same time, rinsing the precipitated material at the bottom of the stirring tank with a small amount of pure water and transferring all of it to the slurry barrel;

[0051] (4) Add sucrose and an organic phosphine scale and corrosion inhibitor, hydroxyethylidene diphosphonic acid, to the slurry in the slurry barrel of step (3). Based on the volume of pure water in step (1), the amount of sucrose added is 46 g / L, and the amount of hydroxyethylidene diphosphonic acid added is 0.12 mol / L. Stir for 30 minutes until completely dispersed and dissolved. The viscosity of the resulting slurry is 220 Pa·s. Then, the slurry is transferred to a sand mill stirring tank for secondary grinding. The particle size D50 is controlled to be between 0.3 and 0.32 μm. At this time, the particle distribution uniformity coefficient b in the sand mill slurry is less than 0.2. After grinding, the slurry is spray dried. The spray drying air inlet temperature is controlled to be 260°C and the air outlet temperature is controlled to be 95°C to obtain a lithium manganese iron phosphate precursor.

[0052] (5) Nitrogen atmosphere sintering: The lithium manganese iron phosphate precursor was transferred to a tube furnace, and after nitrogen atmosphere replacement, nitrogen atmosphere sintering was performed. The temperature was increased to 300°C at a heating rate of 2°C / min and kept at this temperature for 1 hour. The temperature was then increased to 500°C at a heating rate of 2°C / min and kept at this temperature for 1 hour. The temperature was then increased to 650°C at a heating rate of 2°C / min and kept at this temperature for 8 hours. After cooling, the product was pulverized by air flow to obtain a carbon-coated lithium manganese iron phosphate sample.

[0053] The lithium manganese iron phosphate cathode material prepared by the above method has the chemical formula Li 1.03 Mn 0.6 Fe 0.4 (PO4) 1.035 , the powder compaction density is 2.38g / cm 3 The first discharge capacity at a 0.1C rate was 155.6 mAh / g; at a 1C rate, the discharge capacity was 145.5 mAh / g. The capacity retention rate after 100 cycles at a 1C charge-discharge rate was 98.6%.

[0054] Example 2

[0055] (1) Add 5L of pure water to a stirring tank in advance (the amount of each raw material added in each subsequent step is based on the 5L of pure water added in step (1) as a benchmark for proportioning). Add ferric phosphate, phosphoric acid, lithium carbonate, and dispersant polyvinyl alcohol under stirring. The amount of ferric phosphate, phosphoric acid, and lithium carbonate added is 1.2mol / L, 2.2mol / L, and 2.02mol / L, respectively, and the amount of polyvinyl alcohol added is 23g / L. After stirring evenly, add 2.8mol / L of manganese oxalate and mix evenly to obtain a precursor pre-made slurry;

[0056] (2) Adding some pure water to the precursor prefabricated slurry obtained in step (1) to make the solid content of the slurry reach 50 wt%; the total amount of pure water used in step (1) and step (2) is 5.35 L.

[0057] (3) Grinding the precursor slurry obtained in step (2) under stirring until the particle size D50 reaches 0.4-0.45 μm to obtain a suspension containing lithium manganese iron phosphate precursor, transferring the suspension to another slurry barrel, and at the same time, rinsing the precipitated material at the bottom of the stirring tank with a small amount of pure water and transferring all of it to the slurry barrel;

[0058] (4) Glucose and amino tris(methylene phosphonic acid) as an organic phosphine scale inhibitor and corrosion inhibitor are added to the slurry in the slurry barrel of step (3). Based on the volume of pure water in step (1), the amount of glucose added is 40 g / L and the amount of amino tris(methylene phosphonic acid) added is 0.2 mol / L. After stirring until completely dispersed and dissolved, the viscosity of the resulting slurry is 560 Pa·s. The slurry is then transferred to a sand mill stirring tank for secondary grinding, and the particle size D50 is controlled to be between 0.3 and 0.32 μm. At this time, the particle distribution uniformity coefficient b in the sand milled slurry is less than 0.2. After sand milling, the slurry is spray dried, and the spray drying air inlet temperature is controlled to be 250°C and the air outlet temperature is controlled to be 90°C to obtain a lithium manganese iron phosphate precursor.

[0059] (5) Nitrogen atmosphere sintering: The lithium manganese iron phosphate precursor was transferred to a tube furnace, and after nitrogen atmosphere replacement, nitrogen atmosphere sintering was performed. The temperature was increased to 250°C at a heating rate of 3°C / min and kept at this temperature for 2 hours. The temperature was then increased to 450°C at a heating rate of 3°C / min and kept at this temperature for 2 hours. The temperature was then increased to 680°C at a heating rate of 3°C / min and kept at this temperature for 7 hours. After cooling, the carbon-coated lithium manganese iron phosphate sample was obtained by air flow pulverization.

[0060] The lithium manganese iron phosphate cathode material prepared by the above method has the chemical formula Li 1.01 Mn 0.7 Fe 0.3 PO4, powder compaction density is 2.4g / cm 3 The first discharge capacity at a rate of 0.1C is 154.8mAh / g; at a current rate of 1C, the discharge capacity is 144.5mAh / g; and the capacity retention rate after 100 cycles at a charge and discharge rate of 1C is 98.4%.

[0061] Example 3

[0062] (1) Add 5L of pure water to a stirring tank in advance (the amount of each raw material added in each subsequent step is based on the 5L of pure water added in step (1) as a benchmark for proportioning). Add ferrous oxalate, lithium dihydrogen phosphate, lithium carbonate, and dispersant polyethylene glycol 6000 under stirring. The amount of ferrous oxalate, lithium dihydrogen phosphate, and lithium carbonate added is 1.2mol / L, 2.2mol / L, and 0.16mol / L, respectively, and the amount of dispersant added is 14g / L. After stirring evenly, add 1.2mol / L of manganese carbonate and mix evenly to obtain a precursor pre-made slurry;

[0063] (2) Adding some pure water to the precursor prefabricated slurry obtained in step (1) to make the solid content of the slurry reach 35 wt%; the total amount of pure water used in step (1) and step (2) is 5.65 L.

[0064] (3) Grinding the precursor slurry obtained in step (2) under stirring until the particle size D50 reaches 0.4-0.45 μm to obtain a suspension containing lithium manganese iron phosphate precursor, transferring the suspension to another slurry barrel, and at the same time, rinsing the precipitated material at the bottom of the stirring tank with a small amount of pure water and transferring all of it to the slurry barrel;

[0065] (4) Add ascorbic acid and an organic phosphine scale and corrosion inhibitor, ethylenediaminetetramethylenephosphonic acid, to the slurry in the slurry barrel of step (3). Based on the volume of pure water in step (1), the amount of ascorbic acid added is 24 g / L, and the amount of ethylenediaminetetramethylenephosphonic acid added is 0.044 mol / L. After stirring until completely dispersed and dissolved, the viscosity of the resulting slurry is 450 Pa·s; then the slurry is transferred to a sand mill stirring tank for secondary grinding, and the particle size D50 is controlled to be between 0.3-0.32 μm. At this time, the particle distribution uniformity coefficient b in the sand mill slurry is less than 0.2. After grinding, the slurry is spray dried, and the spray drying air inlet temperature is controlled to 270°C and the air outlet temperature is controlled to 100°C to obtain a lithium manganese iron phosphate precursor.

[0066] (5) Nitrogen atmosphere sintering: The lithium manganese iron phosphate precursor was transferred to a tube furnace, and after nitrogen atmosphere replacement, nitrogen atmosphere sintering was performed. The temperature was increased to 350°C at a heating rate of 5°C / min and kept at this temperature for 3 hours. The temperature was then increased to 550°C at a heating rate of 5°C / min and kept at this temperature for 3 hours. The temperature was then increased to 730°C at a heating rate of 5°C / min and kept at this temperature for 5 hours. After cooling, the carbon-coated lithium manganese iron phosphate sample was obtained by air flow pulverization.

[0067] The lithium manganese iron phosphate cathode material prepared by the above method has the chemical formula Li 1.05 Mn 0.5 Fe 0.5 (PO4) 0.99 , the powder compaction density is 2.43g / cm 3 The first discharge capacity at a rate of 0.1C is 154.4mAh / g; at a current rate of 1C, the discharge capacity is 144.3mAh / g; and the capacity retention rate after 100 cycles at a charge and discharge rate of 1C is 99.1%.

[0068] Example 4

[0069] (1) Add 5L of pure water to a stirring tank in advance (the amount of each raw material added in each subsequent step is based on the 5L of pure water added in step (1) as a benchmark for proportioning), and add ammonium ferrous phosphate, phosphoric acid, lithium hydroxide, and dispersant polyethylene glycol 6000 under stirring. The amount of ammonium ferrous phosphate, phosphoric acid, and lithium hydroxide added is 1 mol / L, 1.13 mol / L, and 2.48 mol / L, respectively, and the amount of dispersant added is 31 g / L. After stirring evenly, add 1.5 mol / L of manganese carbonate and mix evenly to obtain a precursor pre-made slurry;

[0070] (2) Adding some pure water to the precursor prefabricated slurry obtained in step (1) to make the solid content of the slurry reach 35 wt%; the total amount of pure water used in step (1) and step (2) is 5.05 L.

[0071] (3) Grinding the precursor slurry obtained in step (2) under stirring until the particle size D50 reaches 0.4-0.45 μm to obtain a suspension containing lithium manganese iron phosphate precursor, transferring the suspension to another slurry barrel, and at the same time, rinsing the precipitated material at the bottom of the stirring tank with a small amount of pure water and transferring all of it to the slurry barrel;

[0072] (4) Add starch and diethylenetriamine penta (methylene phosphonic acid), an organic phosphine scale inhibitor and corrosion inhibitor, to the slurry in the slurry barrel of step (3). Based on the volume of pure water in step (1), the amount of starch added is 44 g / L, and the amount of diethylenetriamine penta (methylene phosphonic acid) added is 0.074 mol / L. After stirring until completely dispersed and dissolved, the viscosity of the resulting slurry is 330 Pa·s. The slurry is then transferred to a sand mill stirring tank for secondary grinding, and the particle size D50 is controlled to be between 0.3 and 0.32 μm. At this time, the particle distribution uniformity coefficient b in the sand mill slurry is less than 0.2. After grinding, the slurry is spray dried, and the spray drying air inlet temperature is controlled to be 260°C and the air outlet temperature is controlled to be 95°C to obtain a lithium manganese iron phosphate precursor.

[0073] (5) Nitrogen atmosphere sintering: The lithium manganese iron phosphate precursor was transferred to a tube furnace, and after nitrogen atmosphere replacement, nitrogen atmosphere sintering was performed. The temperature was increased to 300°C at a heating rate of 3°C / min and kept at this temperature for 2 hours. The temperature was then increased to 500°C at a heating rate of 3°C / min and kept at this temperature for 2 hours. The temperature was then increased to 720°C at a heating rate of 3°C / min and kept at this temperature for 7 hours. After cooling, the product was pulverized by air flow to obtain a carbon-coated lithium manganese iron phosphate sample.

[0074] The lithium manganese iron phosphate cathode material prepared by the above method has the chemical formula Li 0.99 Mn 0.6 Fe 0.4 PO4, powder compaction density is 2.41g / cm 3The first discharge capacity at a rate of 0.1C is 153.7mAh / g; at a current rate of 1C, the discharge capacity is 143.5mAh / g; the capacity retention rate after 100 cycles at a charge and discharge rate of 1C is 98.5%.

[0075] Comparative Example 1

[0076] The lithium manganese iron phosphate positive electrode material was prepared by the same method as in Example 1, with the main difference being that no organic phosphorus scale inhibitor was added in step (4), and additional ammonium dihydrogen phosphate was used instead, and the molar amount of phosphorus in the added ammonium dihydrogen phosphate was consistent with the molar amount of phosphorus in the organic phosphorus scale inhibitor in Example 1.

[0077] The lithium manganese iron phosphate cathode material prepared by the above method has the chemical formula Li 1.03 Mn 0.6 Fe 0.4 (PO4) 1.035 , the powder compaction density is 2.31g / cm 3 The first discharge capacity at a rate of 0.1C is 145.8mAh / g; at a current rate of 1C, the discharge capacity is 134.8mAh / g; the capacity retention rate after 100 cycles at a charge and discharge rate of 1C is 95.6%.

[0078] Comparative Example 2

[0079] The lithium manganese iron phosphate cathode material was prepared by the same method as in Example 2, with the main difference being that the organophosphorus scale and corrosion inhibitor aminotrimethylenephosphonic acid was added and mixed uniformly in step (1), and only the carbon source glucose was added in step (4).

[0080] The lithium manganese iron phosphate cathode material prepared by the above method has the chemical formula Li 1.01 Mn 0.7 Fe 0.3 PO4, powder compaction density is 2.38g / cm 3 The first discharge capacity at a rate of 0.1C is 149.1mAh / g; at a current rate of 1C, the discharge capacity is 139.8mAh / g; and the capacity retention rate after 100 cycles at a charge and discharge rate of 1C is 97.2%.

Claims

1. A method for preparing a lithium manganese iron phosphate positive electrode material, characterized in that: The following steps are involved: (1) After the iron source, phosphorus source, lithium source and dispersant are uniformly stirred in water, a manganese source is added, and the mixture is uniformly mixed to obtain a precursor pre-slurry; (2) adjusting the solid content of the precursor pre-slurry to reach a target solid content A, wherein the target solid content A satisfies 30wt%≤A≤60wt%, preferably 35wt%≤A≤50wt%; preferably, the solid content of the precursor pre-slurry is adjusted by adding water; preferably, the amount of water used in step (1) accounts for 85-99% by volume of the total amount of water used in step (1) and step (2); (3) grinding the precursor slurry obtained in step (2) under stirring; (4) adding an organic phosphine scale inhibitor to the precursor slurry obtained in step (3) to dissolve the precursor slurry, preferably performing a secondary grinding treatment, and then drying the obtained slurry, sintering and pulverizing to obtain a lithium manganese iron phosphate positive electrode material; Wherein, a carbon source is added and dissolved in step (1), or a carbon source is added to the precursor slurry in step (4) and dissolved.

2. The preparation method according to claim 1, characterized in that: In step (1), the iron source is selected from one or more of ferric oxide, ferroferric oxide, ferrous phosphate, ferrous oxalate, and ammonium ferrous phosphate; The phosphorus source is one or more of iron phosphate, phosphoric acid, lithium dihydrogen phosphate, manganese hydrogen phosphate, and ammonium dihydrogen phosphate; The lithium source is one or more of lithium carbonate, lithium hydroxide, lithium phosphate and lithium dihydrogen phosphate; The dispersant is one or more of polyethylene glycol 800, polyethylene glycol 6000, methyl cellulose, polyvinyl alcohol and hexenyl bisstearamide; The manganese source is one or more of manganese dioxide, manganese carbonate, manganese oxalate, manganese sulfate, trimanganese tetraoxide, and manganese hydrogen phosphate.

3. The preparation method according to claim 1 or 2, characterized in that: The molar ratio of the added iron element and the added manganese element is 0.11-1:1, and the ratio of the added molar amount of the lithium element to the sum of the added molar amounts of the iron element and the added manganese element is 0.9-1.1:1; Preferably, based on the volume of water added in step (1), the amount of the dispersant added is 5-50 g / L, preferably 10-35 g / L.

4. The preparation method according to any one of claims 1 to 3, characterized in that In step (3), the precursor slurry is subjected to the primary grinding treatment to a particle size D50 of 0.35-0.55 nm, preferably 0.4-0.45 μm.

5. The preparation method according to any one of claims 1 to 4, characterized in that: The ratio of the sum of the molar amounts of phosphorus in the phosphorus source used in step (1) and the organic phosphine scale and corrosion inhibitor used in step (4) to the sum of the molar amounts of iron and manganese in the manganese source and the iron source is p, 0.99≤p≤1.05; Preferably, based on the volume of water used in step (1), the amount of the organic phosphine scale inhibitor added is 0.02-0.4 mol / L, preferably 0.04-0.2 mol / L.

6. The preparation method according to any one of claims 1 to 5, characterized in that: The organic phosphine scale inhibitor contains more than two phosphonic acid groups, and the phosphorus atom is directly connected to the carbon atom. Preferably, the organic phosphine scale inhibitor includes one or more of hydroxyethylidene diphosphonic acid, aminotrimethylene phosphonic acid, diethylenetriamine penta (methylene phosphonic acid), and ethylenediaminetetramethylene phosphonic acid.

7. The preparation method according to any one of claims 1 to 6, characterized in that: The carbon source includes one or more of glucose, sucrose, ascorbic acid, and starch; Preferably, based on the volume of water added in step (1), the amount of the carbon source added is 10-80 g / L, preferably 20-50 g / L.

8. The preparation method according to any one of claims 1 to 7, characterized in that: In step (4), after the organic phosphine scale inhibitor is added and dissolved, the viscosity of the resulting slurry is 100-1000 Pa·s, preferably 200-600 Pa·s; Preferably, in step (4), the secondary grinding treatment is performed and the particle distribution uniformity coefficient b in the obtained slurry is less than 0.3, preferably less than 0.

2.

9. The preparation method according to any one of claims 1 to 8, characterized in that: In step (4), the drying is carried out by spray drying, and the spray drying air inlet temperature is controlled to be 240-280°C, preferably 250-270°C; the air outlet temperature is controlled to be 85-105°C, preferably 90-100°C; Preferably, in step (4), the dried precursor is sintered in a nitrogen atmosphere, and the sintering conditions are preferably: heating to 250-350°C at a heating rate of 2-5°C / min, keeping warm for 1-3h, then heating to 400-550°C at a heating rate of 2-5°C / min, keeping warm for 1-3h, then heating to the target temperature at a heating rate of 2-5°C / min, and keeping warm for 5-12h; Preferably, the target temperature is 600-800°C, preferably 650-750°C.

10. A lithium iron manganese phosphate positive electrode material prepared by the preparation method according to any one of claims 1 to 9; preferably, the chemical formula of the lithium iron manganese phosphate positive electrode material is Li x Mn y Fe z (PO4) p , where 0.9≤x≤1.1, 0.5≤y≤0.9, 0.1≤z≤0.5, 0.99≤p≤1.05, and y+z=1.