Preparation method of lithium manganese iron phosphate

Abstract

The invention discloses a preparation method of lithium manganese iron phosphate, which comprises the following steps: (1) taking ferromanganese hydroxide FexMn1-x (OH) 2, wherein X is more than or equal to 0.1 and less than or equal to 0.9; the preparation method comprises the following steps: preparing manganese and ferrous into slurry, adding phosphoric acid with the same molar amount of manganese and ferrous, stirring at 50-90 DEG C until ferromanganese hydrogen phosphate Fe < x > Mn < 1-x > HPO4. 3H2O is formed, and separating solids; (2) the solid obtained in the step (1) is divided into two parts, the first part is sintered at the temperature of 380-500 DEG C, and ferromanganese pyrophosphate is formed; mixing the second part with a lithium source, and sintering for the first time to form a lithium iron manganese phosphate precursor; and (3) mixing ferromanganese pyrophosphate, the lithium ferromanganese phosphate precursor, a carbon source and a supplementary lithium source, and carrying out secondary sintering to obtain a lithium ferromanganese phosphate finished product. A large-particle precursor generated after the fine-particle ferromanganese pyrophosphate and the ferromanganese hydrogen phosphate are subjected to one-time sintering forms particle gradation, the compaction density is improved, only the ferromanganese hydrogen phosphate process parameters are controlled, and the process is simplified.

Description

Technical Field

[0001] This invention relates to a method for synthesizing electrode materials, and more particularly to a method for preparing lithium manganese iron phosphate. Background Technology

[0002] Lithium manganese iron phosphate (LiFe) x Mn y Compared to lithium iron phosphate (LiFePO4), lithium iron phosphate (LiFePO4) has a higher high-voltage platform and theoretically higher energy density, but it suffers from lower electronic conductivity, lower lithium-ion diffusion rate, lower compaction density, and lower Mn content after charging. 2+ Become Mn 3+ Mn 3+ Unstable towards Mn 2+ Transformation leads to changes in crystal structure, resulting in reduced capacity and other issues. Furthermore, during high-temperature charging and discharging, Mn... 3+ The instability is particularly severe.

[0003] CN 119764397 A discloses a composite-coated modified lithium manganese iron phosphate cathode material and its preparation method. During secondary sintering, a silicon-containing resin is introduced as a carbon source to block subsequent adverse chain reactions and ensure cycle stability, but the compaction density problem remains unresolved. CN 118183679 A discloses a high-pressure, high-density, long-cycle lithium manganese iron phosphate cathode material and its preparation method. Metal-doped flake-like ferromanganese pyrophosphate and ferromanganese phosphate are prepared using a liquid-phase method. The primary particle size distribution of the metal-doped ferromanganese pyrophosphate is approximately 200-500 nm, and that of the metal-doped ferromanganese phosphate is approximately 100-300 nm. By combining ferromanganese pyrophosphate and ferromanganese phosphate, and through post-sintering modification, the compaction density of lithium manganese iron phosphate is increased to 2.42 g / cc, improving cycle stability. However, the process route is lengthy and complex. Summary of the Invention

[0004] Purpose of the invention: The purpose of this invention is to provide a method for preparing lithium manganese iron phosphate, so as to improve the compaction density of lithium manganese iron phosphate and simplify the preparation process.

[0005] Technical solution: The preparation method of lithium manganese iron phosphate according to the present invention includes the following steps:

[0006] (1) Take manganese iron hydroxide Fe x Mn 1-x (OH)₂, wherein 0.1 ≤ X ≤ 0.9, is prepared into a slurry, and phosphoric acid, in an amount equal to the total molar amount of manganese and iron, is added. The mixture is stirred at 50 ~ 90 ℃ until ferric manganese hydrogen phosphate (Fe) is formed. x Mn 1-x HPO4·3H2O, used to separate solids;

[0007] (2) The solid obtained in step (1) is divided into two parts. The first part is sintered at 380~500℃ to form manganese iron pyrophosphate; the second part is mixed with lithium source and sintered for the first time to form lithium manganese iron phosphate precursor.

[0008] (3) Mix manganese iron pyrophosphate, lithium manganese iron phosphate precursor, carbon source, and supplementary lithium source, and perform a second sintering to obtain lithium manganese iron phosphate finished product.

[0009] Preferably, in step (1), the manganese iron hydroxide Fe x Mn 1-x The preparation method of (OH)2 is as follows: under the protection of inert gas, a mixed solution of manganese salt and ferrous salt is prepared, an alkali is added, the pH of the mixed solution is adjusted to 8.5~10.5, and the solid is separated; the concentration of the mixed solution of manganese salt and ferrous salt is 0.8~2 mol / L, and the alkali is ammonia water.

[0010] Preferably, in step (1), the sodium content in the ferromanganese hydroxide is less than 20 ppm, the sulfur content is less than 80 ppm, and the pH is 3-4 after the addition of phosphoric acid. By controlling the impurity content of the ferromanganese hydroxide and the formation environment of ferromanganese hydrogen phosphate in advance, the particle size of ferromanganese hydrogen phosphate is ensured to be within the target range, thereby ensuring that the particle size of ferromanganese pyrophosphate is within the target range, which is beneficial to improving the compaction density through gradation.

[0011] Preferably, in step (1), the slurry has a solid content of 8-15%, a phosphoric acid mass fraction of 85%, and a reaction time of 1-2 hours. A reducing agent of 0.1-0.3% (molar ratio) of manganese ferric hydroxide, specifically ascorbic acid, is added to the slurry to prevent oxidation of the manganese ferric hydroxide.

[0012] Preferably, in steps (2) and (3), the molar ratio of Li : (Mn+Fe) : P is 0.98~1.05 : 0.96~1 : 1.

[0013] Preferably, in step (2), the second part of manganese iron hydrogen phosphate, lithium source and water are mixed to form a slurry with a solid content of 20~40%, ground to D50=0.3~0.5μm, spray dried and then sintered for the first time.

[0014] Preferably, in step (3), the ratio of manganese iron pyrophosphate to lithium manganese iron phosphate precursor is 3:7 to 1:9.

[0015] Preferably, in step (3), manganese iron pyrophosphate, lithium manganese iron phosphate precursor, carbon source, supplementary lithium source and water are mixed to prepare a slurry with a solid content of 40~50%, ground to D50=0.2~0.8μm, spray dried and then sintered for the second time.

[0016] Preferably, in step (3), the carbon source is a mixture of sucrose and hydrophilic polyether siloxane, and the amount of carbon source added is 7-12% of the total mass of manganese iron pyrophosphate, lithium manganese iron phosphate precursor, and supplementary lithium source. The mass ratio of polyether siloxane to sucrose is 6:4 to 8:2, and the polyether siloxane is polyether-modified polydimethylsiloxane.

[0017] Preferably, the manganese salt is at least one of manganese sulfate, manganese acetate, manganese nitrate, manganese chloride, and manganese citrate; the iron salt is at least one of ferrous sulfate, ferrous acetate, ferrous nitrate, and ferrous chloride; and the lithium source and supplementary lithium source are lithium dihydrogen phosphate and / or lithium carbonate.

[0018] Preferably, the first sintering is carried out under inert gas protection, with a sintering temperature of 550~650℃, a sintering time of 4~8 hours, and a heating rate ≤10℃ / min. Under inert gas protection, the oxygen content is 20~100ppm.

[0019] Preferably, the second sintering is carried out under inert gas protection, with a sintering temperature of 700~800℃, a sintering time of 8~14 hours, and a heating rate of ≤10℃ / min.

[0020] Beneficial Effects: Compared with the prior art, the present invention has the following significant advantages: 1. Improves the compaction density of lithium manganese iron phosphate and simplifies the preparation process: After calcination to remove crystal water, a portion of ferromanganese hydrogen phosphate will generate ferromanganese pyrophosphate with a primary particle size of 20-50 nm. This fine particle is uniformly mixed with the large-particle precursor generated after the first sintering of ferromanganese hydrogen phosphate to form a particle size distribution, thereby improving the compaction density of lithium manganese iron phosphate. The process parameters for preparing ferromanganese hydrogen phosphate are controlled only once, greatly simplifying the process; 2. Ensures long-term, high-temperature storage capacity retention. Efficiency: Phosphorus, iron, and manganese sources are prepared by liquid-phase method to achieve atomic-level uniform mixing and form a uniform distribution at the raw material end. After two-step sintering, a long-range ordered solid solution is formed. Polyether silane is used as a carbon source and compounded with sucrose. After high-temperature sintering, a 5-8 nm thick Si-C coating layer can be formed on the surface of lithium manganese iron phosphate particles, avoiding manganese dissolution that may be caused by small particle size of manganese iron pyrophosphate. At the same time, it can adsorb HF generated by electrolyte decomposition, isolate HF from the corrosion of lithium manganese iron phosphate particles, inhibit manganese dissolution, and improve high-temperature storage and cycle performance. Attached Figure Description

[0021] Figure 1 This is a process flow diagram of the synthesis of manganese phosphate in Example 1 of the present invention;

[0022] Figure 2 This is a process flow diagram of the synthesis of lithium manganese iron phosphate in Example 1 of the present invention;

[0023] Figure 3The image shows the morphological electron microscope image of the manganese iron hydrogen phosphate synthesized in Example 1 of this invention.

[0024] Figure 4 The image shows the morphology of manganese iron pyrophosphate synthesized in Example 1 of this invention.

[0025] Figure 5 This is a schematic diagram of the structure of lithium manganese iron phosphate co-coated with Si and C. Detailed Implementation

[0026] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0027] Example 1: This example provides a lithium manganese iron phosphate (LiFe) 0.4 Mn 0.6 The preparation method of PO4 is as follows:

[0028] 1. Preparation of ferric manganese hydrogen phosphate and ferric manganese pyrophosphate:

[0029] (1) Under nitrogen protection, manganese sulfate and ferrous sulfate (Mn:Fe=0.6:0.4, x=0.4) were dissolved in deionized water to prepare a 1 mol / L manganese-iron salt mixed solution. Then, ammonia was used as a precipitant and pH adjuster to adjust the pH of the mixed solution to 8.5, precipitating manganese-iron hydroxide ((Fe) 0.4 Mn 0.6 (OH)2).

[0030] (2) The manganese ferrohydride was defiltered and washed with pure water until the sodium content was below 20 ppm and the sulfur content was below 80 ppm. Water was added to adjust the slurry to a solid content of 15%. Ascorbic acid with a molar ratio of 0.2% of the manganese ferrohydride was added to prevent the oxidation of manganese ferrohydride. 85% phosphoric acid was slowly added while stirring. The molar ratio of P to (Mn+Fe) in the phosphoric acid was 1:1. After the phosphoric acid was added, the pH was 3~4. The mixture was stirred at 90℃ for 1 hour, then defiltered, washed with pure water, and dried at 80℃ to obtain manganese ferrophosphate (Fe). 0.4 Mn 0.6 HPO4·3H2O).

[0031] (3) Take a portion of ferromanganese hydrogen phosphate and sinter it at 400°C in air atmosphere to obtain ferromanganese pyrophosphate (Fe). 0.4 Mn 0.6 )2P2O7, such as Figure 4 As shown, manganese ferric pyrophosphate is in granular form with a particle size of 20~50nm.

[0032] 2. Preparation of lithium manganese iron phosphate:

[0033] Step 1: Add 700g of 0.6~0.8mm zirconium beads, 364.8g of pure water, and 205.3g of Fe to a 2L PTFE ball mill jar.0.4 Mn 0.6 HPO4·3H2O (homemade), 37.9 g of Li2CO3 (battery grade), were mixed to form a slurry with a solid content of approximately 40%. Two batches were prepared following the steps above. The slurry was ball-milled in a planetary ball mill until the particle size D50 = 0.3-0.4 μm (Malvin 3000 laser particle size analyzer, refractive index 2.6). The milled slurry was then spray-dried (inlet air temperature approximately 230℃, outlet air temperature approximately 105℃), and then sintered at 650℃ for 6 h to obtain the lithium manganese iron phosphate precursor.

[0034] Step 2: Add 700g of 0.6~0.8mm zirconium beads, 130g of lithium manganese iron phosphate precursor, and 63.4g of Fe to a 2L polytetrafluoroethylene ball milling jar. 0.4 Mn 0.6 A slurry with a solid content of approximately 40% was prepared by mixing 2P2O7, 16.5 g Li2CO3 (battery grade), 6 g sucrose (food grade), 32 g polyether siloxane (50% solid content), and 371.8 g pure water. The slurry was ball-milled in a planetary ball mill to a particle size D50 of 0.3~0.4 μm (Malvin laser particle size analyzer, refractive index 1.692). The milled slurry was then spray-dried (inlet air temperature approximately 200℃, outlet air temperature approximately 100℃) and sintered at 730℃ for 10 h under a nitrogen atmosphere to finally obtain SiO2 and carbon-coated lithium manganese iron phosphate Si-C-LFMP-A.

[0035] Comparative Example 1: This comparative example provides a method for preparing lithium manganese iron phosphate, which differs from Example 1 mainly in that it uses a solid-state method to prepare lithium manganese iron phosphate, and the steps are as follows:

[0036] Step 1: Add 700g of 0.6~0.8mm zirconium beads, 107.94g of MnC2O4·2H2O (battery grade), 62.48g of LiH2PO4 (battery grade), 60.32g of FePO4 (battery grade), 14.85g of Li2CO3 (battery grade), and 368.9g of pure water to a slurry with a solid content of 40% in a 2L polytetrafluoroethylene ball milling jar. Prepare two jars according to the above steps. Ball mill the slurry on a planetary ball mill until the particle size D50 = 0.3~0.4μm (Malvin 3000 laser particle size analyzer, refractive index 2.6). Spray dry the milled slurry (inlet air temperature around 230℃, outlet air temperature around 105℃), and then sinter at 650℃ for 6h to obtain lithium manganese iron phosphate primary sintered material powder.

[0037] Step 2: Add 700g of 0.6~0.8mm zirconium beads, 200g of lithium manganese iron phosphate primary sintering material powder, 6g of sucrose (food grade), 32g of polyether siloxane (50% solid content), and 330g of pure water to a 2L polytetrafluoroethylene ball milling jar to prepare a slurry with a solid content of 40%. The slurry is ball-milled on a planetary ball mill until the particle size D50 = 0.3~0.4μm (Malvin laser particle size analyzer, refractive index 1.692). The milled slurry is spray-dried (inlet air temperature around 200℃, outlet air temperature around 100℃), and then sintered at 730℃ for 10h under a nitrogen atmosphere to finally obtain carbon-coated lithium manganese iron phosphate Si-C-LFMP-B.

[0038] Comparative Example 2: This comparative example provides a method for preparing lithium manganese iron phosphate, the main difference from Example 1 being that ferromanganese hydrogen phosphate is replaced with ferromanganese carbonate (Fe). 0.4 Mn 0.6 CO3 was used, and the carbon source was changed to sucrose + PEG4000. Ferric manganese pyrophosphate was not added in step 2. Details are as follows:

[0039] Step 1: Add 700g of 0.6~0.8mm zirconium beads, 332.2g of pure water, and 115.3g of Fe to a 2L PTFE ball mill jar. 0.4 Mn 0.6 CO3 (battery grade) and 106.2 g of LiH2PO4 (battery grade) were mixed to form a slurry with a solid content of 40%. Two batches were prepared according to the above steps. The slurry was ball-milled in a planetary ball mill until the particle size D50 = 0.3~0.4 μm (Malvin 3000 laser particle size analyzer, refractive index 2.6). The milled slurry was spray-dried (inlet air temperature around 230℃, outlet air temperature around 105℃), and then sintered at 650℃ for 6 hours to obtain lithium manganese iron phosphate primary sintered material powder.

[0040] Step 2: Add 700g of 0.6~0.8mm zirconium beads, 200g of lithium manganese iron phosphate primary sintering material powder, 6g of sucrose (food grade), 14g of PEG4000, and 330g of pure water to a 2L polytetrafluoroethylene ball milling jar to prepare a slurry with a solid content of 40%. The slurry is ball-milled on a planetary ball mill until the particle size D50 = 0.3~0.4μm (Malvin laser particle size analyzer, refractive index 1.692). The milled slurry is spray-dried (inlet air temperature around 200℃, outlet air temperature around 100℃), and then sintered at 730℃ for 10h under a nitrogen atmosphere to finally obtain carbon-coated lithium manganese iron phosphate C-LFMP-2.

[0041] Comparative Example 3: This comparative example uses the same raw material Fe as Example 1. 0.4 Mn 0.6 HPO4·3H2O, step 1 is the same, the difference is that manganese ferric pyrophosphate ((Fe) is not added in step 2. 0.4 Mn0.6 )2P2O7).

[0042] Comparative Example 4: The main difference between this comparative example and Example 1 is that the carbon source is replaced with non-water-soluble polyethyl silicone resin.

[0043] Comparative Example 5: This comparative example provides a method for preparing lithium manganese iron phosphate. The main difference from Example 1 is that the carbon source is changed to sucrose + PEG4000, and manganese iron pyrophosphate is not added in step 2.

[0044] Example 1: Samples prepared according to Examples 1-4 were tested using the following methods:

[0045] 1. First, the lithium manganese iron phosphate material from Example 1 and Comparative Examples 1 to 5 is made into a soft-pack battery.

[0046] 2. After the soft-pack battery is formed and capacity-tested, its capacity when fully charged at room temperature (25℃) is recorded as C1, and its capacity when fully discharged is recorded as D1.

[0047] 3. The battery in step 2 above is recharged at 25°C, placed in a high-temperature chamber with the temperature set at 60°C, and left for 14 days. After that, it is taken out and discharged at room temperature (25°C). The capacity is recorded as D2. (D2 / D1)*100% is the storage capacity retention rate at 60°C.

[0048] 4. Then repeat step 3 and conduct tests for 3 to 6 consecutive months. Based on the obtained data, draw a standard fitting curve and obtain the trend of change. The trend results are shown in Table 1.

[0049] Table 1: Detection Data

[0050]

[0051] Comparative Example 1 used a solid-phase method to prepare lithium manganese iron phosphate. The uneven distribution of manganese and iron elements in the precursor led to increased manganese dissolution, resulting in poor capacity retention during long-term storage at 60°C. Comparative Example 2 used ferromanganese carbonate, a precursor in which manganese and iron had already formed a solid solution, with sucrose and PEG4000 as the carbon source. Conventional carbon source coating did not significantly improve the high-temperature storage performance of lithium manganese iron phosphate. Comparative Example 5 used ferromanganese hydrogen phosphate, which had already formed a solid solution, under similar conditions to Comparative Example 2, but with improved compaction density. Comparative Example 3 did not add ferromanganese pyrophosphate for particle size distribution, resulting in a lower compaction density than Example 1. Comparative Example 4 changed the carbon source, leading to a decrease in cycle stability. Example 1 synthesized ferromanganese hydrogen phosphate first using a liquid-phase co-precipitation method. Manganese and iron were evenly distributed, and since both manganese and iron are divalent, lithium manganese iron phosphate precursors could be directly prepared without reduction, avoiding the capacity reduction problem caused by insufficient reduction. A portion of the lithium iron manganese phosphate and lithium source are sintered once to form primary lithium iron manganese phosphate precursor particles with larger particle sizes. Another portion of the lithium iron manganese phosphate undergoes a calcination process to remove crystal water, resulting in primary lithium iron manganese pyrophosphate particles with even finer particle sizes. During secondary grinding, the lithium iron manganese phosphate precursor and lithium iron manganese pyrophosphate are particle size-graded to ensure compaction density. During secondary sintering, polyether siloxane acts as both a carbon source and a good surfactant, playing a role in dispersion and defoaming. After high-temperature sintering, polyether siloxane forms C and Si, simultaneously coating the lithium iron manganese phosphate with C and Si. The Si reacts with HF produced by electrolyte decomposition to form SiF4, preventing HF from corroding the lithium iron manganese phosphate, inhibiting manganese dissolution, and ensuring cycle stability.

Claims

1. A method for preparing lithium manganese iron phosphate, characterized in that, Includes the following steps: (1) Take manganese iron hydroxide Fe x Mn 1-x (OH)₂, wherein 0.1 ≤ X ≤ 0.9, is prepared into a slurry, and phosphoric acid of equal molar amount to the total molar amount of manganese and iron is added. The mixture is stirred at 50 ~ 90 ℃ until ferric manganese hydrogen phosphate (Fe) is formed. x Mn 1-x HPO4·3H2O, used to separate solids; (2) The solid obtained in step (1) is divided into two parts. The first part is sintered at 380~500℃ to form manganese iron pyrophosphate; the second part is mixed with lithium source and sintered for the first time to form lithium manganese iron phosphate precursor. (3) Mix manganese iron pyrophosphate, lithium manganese iron phosphate precursor, carbon source, and supplementary lithium source, and perform a second sintering to obtain lithium manganese iron phosphate finished product.

2. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that, In step (1), manganese iron hydroxide Fe x Mn 1-x The preparation method of (OH)2 is as follows: under the protection of inert gas, a mixed solution of manganese salt and ferrous salt is prepared, an alkali is added, the pH of the mixed solution is adjusted to 8.5~10.5, and the solid is separated; the concentration of the mixed solution of manganese salt and ferrous salt is 0.8~2 mol / L, and the alkali is ammonia water.

3. The method for preparing lithium manganese iron phosphate according to claim 2, characterized in that, In step (1), the sodium content in the manganese iron hydroxide is less than 20 ppm, the sulfur content is less than 80 ppm, and after adding phosphoric acid, the pH is 3~4.

4. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that, In step (2), the second part of manganese iron hydrogen phosphate, lithium source and water are mixed to form a slurry with a solid content of 20~40%, which is ground to D50=0.3~0.5μm, spray dried and then sintered for the first time.

5. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that, In steps (2) and (3), the molar ratio of Li: (Mn+Fe): P is 0.98~1.05: 0.96~1:

1.

6. The method for preparing lithium manganese iron phosphate according to any one of claims 1 to 5, characterized in that, In step (3), the ratio of manganese iron pyrophosphate to lithium manganese iron phosphate precursor is 3:7 to 1:

9.

7. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that, In step (3), the carbon source is a mixture of sucrose and polyether siloxane. The amount of carbon source added is 7-12% of the total mass of manganese iron pyrophosphate, lithium manganese iron phosphate precursor, and supplementary lithium source. The mass ratio of polyether siloxane to sucrose is 6:4 to 8:

2.

8. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that, In step (4), manganese iron pyrophosphate, lithium manganese iron phosphate precursor, carbon source, supplementary lithium source and water are mixed to form a slurry with a solid content of 40~50%, which is ground to D50=0.2~0.8μm, spray dried and then sintered for the second time.

9. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that, The manganese salt is at least one of manganese sulfate, manganese acetate, manganese nitrate, manganese chloride, and manganese citrate; the iron salt is at least one of ferrous sulfate, ferrous acetate, ferrous nitrate, and ferrous chloride; and the lithium source and supplementary lithium source are lithium dihydrogen phosphate and / or lithium carbonate.

10. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that, The first sintering temperature is 550~650℃ and the time is 4~8 hours. The second sintering temperature is 700~800℃ and the time is 8~14 hours.

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