Preparation method of high-compaction and high-multiplying lithium manganese iron phosphate positive electrode material

Abstract

The application discloses a preparation method of a high-compaction high-multiplying lithium manganese iron phosphate positive material, which comprises the following steps: dissolving lithium dihydrogen phosphate in water, adding iron phosphate to uniformly disperse, and obtaining iron phosphate slurry with surface enrichment of lithium dihydrogen phosphate; mixing the iron phosphate slurry, a manganese source and water to uniformly form a slurry; and adding a lithium source, a carbon source and double dopants, uniformly stirring, and then performing spray drying and sintering to obtain the lithium manganese iron phosphate positive material. The lithium manganese iron phosphate prepared by the method has high compaction performance, and the compaction density can reach 2.34 g / cm 3 The manganese dissolution phenomenon is reduced, the highest manganese dissolution is not more than 83 ppm after 1C cycle for 1000 times, the electrochemical performance is effectively improved, and the discharge can reach more than 132 mAh / g after 1C cycle for 1000 times.

Description

Technical Field

[0001] This invention belongs to the field of lithium manganese iron phosphate cathode material preparation, and particularly relates to a method for preparing high-compact, high-rate lithium manganese iron phosphate cathode material. Background Technology

[0002] Lithium iron phosphate (LiFePO4, LFP) and ternary lithium (LiNi) x CoyMn 1-x-y O2 (NCM) is currently the mainstream cathode material in the new energy battery market. This is not only due to current environmental protection policies, but also an inevitable trend given the scarcity of mineral energy resources. However, under the current conditions of commercial production, low cost, high energy density, and high safety are the goals pursued by the market.

[0003] Among mainstream materials, although NCM has high energy density, its stability and cycle life are relatively poor, and it also has the disadvantage of high cost. LFP, based on its olivine structure, has higher stability and better cycle life, but its energy density and voltage platform are slightly insufficient to meet the requirements of high-end batteries.

[0004] Given the market's ever-increasing demands for energy density, safety, and rate performance, the industry eagerly anticipates designing a cathode material that combines the characteristics of both LFP and ternary lithium. Against this backdrop, lithium manganese iron phosphate (LiMnFePO4) has emerged as a promising candidate. x Fe 1-x PO4 (LMFP) is gradually coming into the researchers' view. In a sense, LMFP is a modified version of LFP, achieved by doping Mn ions at the Fe sites of LFP. 3+ With Mn 2+ By adjusting the valence state between these states, the operating voltage can be increased to approximately 4.0V, enabling the realization of Li + The insertion and extraction process of LMFP improves the energy density of LFP under the same conditions, while also taking into account the advantages of LFP's safety performance and cycle stability.

[0005] However, in practical applications, LMFP has drawbacks such as low electrical conductivity and poor rate performance. Moreover, the compaction density of LMFP in industrial production is mostly between 2.2 and 2.3 g / cm³. 3 Compared to the current industrial-scale compaction of lithium iron phosphate, which has achieved 2.6-2.65 g / cm³, this method achieves a compaction density of 2.6-2.65 g / cm³. 3 There is a significant gap, resulting in limited effectiveness in improving bulk density. The shortcomings in ratio and compaction greatly restrict the application of the material in practice. Summary of the Invention

[0006] Purpose of the invention: This invention provides a method for preparing lithium manganese iron phosphate cathode material with improved compaction and rate performance.

[0007] Technical solution: The preparation method of the high-compact, high-rate lithium manganese iron phosphate cathode material of the present invention includes the following steps:

[0008] (1) After dissolving lithium dihydrogen phosphate in water, add ferric phosphate and disperse evenly to obtain ferric phosphate slurry with lithium dihydrogen phosphate enriched on the surface;

[0009] (2) Iron phosphate slurry, manganese source and water are mixed evenly to form a slurry; lithium source, carbon source and dual dopant are added, and after stirring evenly, the lithium manganese iron phosphate cathode material is obtained by spray drying and sintering.

[0010] Furthermore, the mass of lithium dihydrogen phosphate added during the preparation of this lithium manganese iron phosphate cathode material is 1-3% of the mass of iron phosphate.

[0011] Furthermore, the lithium manganese iron phosphate cathode material LiMn used in the preparation of this lithium manganese iron phosphate cathode material (1-x) Fe x In PO4, the molar ratio of lithium, manganese, iron and phosphorus is (1.03-1.08):(1-x):x:1, where 0.3≤x≤0.8.

[0012] Furthermore, the iron phosphate used in the preparation of this lithium manganese iron phosphate cathode material is a compound of type A iron phosphate and type B iron phosphate in a mass ratio of (1-3):1; and the iron-to-phosphorus ratio of type A iron phosphate is 0.975-0.985, while that of type B iron phosphate is 0.96-0.965.

[0013] Furthermore, the lithium source used in the preparation of this lithium manganese iron phosphate cathode material is one or more of lithium carbonate, lithium hydroxide, lithium phosphate, and lithium sulfate.

[0014] Furthermore, the amount of carbon source added during the preparation of this lithium manganese iron phosphate cathode material is 10-14% of the mass of iron phosphate.

[0015] Furthermore, the carbon source used in the preparation of this lithium manganese iron phosphate cathode material is any two of glucose, sucrose, straw powder, polyethylene glycol (PEG), polyvinylpyrrolidone, and Ketjen black, and the mass ratio of the two is 1:(2-4).

[0016] Furthermore, the manganese source used in the preparation of this lithium manganese iron phosphate cathode material is one or more of manganese sulfate, manganese oxalate, manganese nitrate, manganese tetroxide, manganese carbonate, and manganese dioxide.

[0017] Furthermore, the dual dopants used in the preparation of this lithium manganese iron phosphate cathode material are aluminum oxide and titanium dioxide. The molar amount of aluminum oxide is 3-6% of the molar amount of iron phosphate, and the molar amount of titanium dioxide is 0.5-2% of the molar amount of iron phosphate.

[0018] Furthermore, the sintering temperature for preparing this lithium manganese iron phosphate cathode material is 700-750℃, and the holding time is 9-10h.

[0019] Beneficial effects: Compared with the prior art, the significant advantage of this invention is that the lithium manganese iron phosphate prepared by this method not only has high compaction performance, but also achieves a compaction density of 2.34 g / cm³. 3 The above measures can reduce manganese leaching, with the highest manganese leaching not exceeding 83 ppm after 1000 cycles at 1C; they also effectively improve electrochemical performance, with a discharge capacity of over 132 mAh / g after 1000 cycles at 1C. Attached Figure Description

[0020] Figure 1 This is a SEM image of the lithium iron phosphate cathode material at 20k in Example 2 of the present invention;

[0021] Figure 2 This is a 3k SEM image of the lithium iron phosphate cathode material in Example 2 of the present invention;

[0022] Figure 3 This is a 3k SEM image of the lithium iron phosphate cathode material of Comparative Example 2 of this invention. Detailed Implementation

[0023] The technical solution of the present invention will be further described in detail below with reference to the embodiments and accompanying drawings.

[0024] It should be noted that all raw materials used in this invention are commercially available. The ferric phosphate used is an AB composite type, wherein the iron-to-phosphorus ratio of type A ferric phosphate is 0.975-0.985, and the iron-to-phosphorus ratio of type B ferric phosphate is 0.96-0.965.

[0025] In the following embodiments and comparative examples of the present invention, the dual dopants are alumina and titanium dioxide, with the molar amount of alumina being 4% of the molar amount of iron phosphate and the molar amount of titanium dioxide being 1.5% of the molar amount of iron phosphate. The composite carbon source used in the following embodiments and comparative examples of the present invention is a mixture of polyvinylpyrrolidone and sucrose in a mass ratio of 1:2.

[0026] Example 1

[0027] The method for preparing lithium manganese iron phosphate cathode material in Example 1 includes the following steps:

[0028] (1) When type A ferric phosphate and type B ferric phosphate are mixed in a mass ratio of 2:1, an iron-to-phosphorus ratio of 0.975 and a specific surface area of ​​6.5 m² are formed.2 / g, D50 < 5μm ferric phosphate. 1% (by mass) of lithium dihydrogen phosphate was dissolved in water and stirred to form a homogeneous aqueous solution. Ferric phosphate was then added and stirred for 30 min to obtain a ferric phosphate slurry with lithium dihydrogen phosphate enriched on the surface.

[0029] (2) Lithium carbonate and manganese oxalate were taken according to the molar ratio of lithium, manganese, iron and phosphorus 1.03:0.7:0.3:1. First, manganese oxalate was added to the iron phosphate slurry and ultrasonically dispersed for 30 min. Then, lithium carbonate, 10% of the mass of iron phosphate composite carbon source, alumina and titanium dioxide were added. The precursor slurry was stirred and dispersed for 45 min to obtain the precursor slurry. The precursor slurry was ground in a sand mill at a speed of 360 rpm for 120 min to obtain a slurry with a particle size D50 of 0.485 μm. The slurry was then transported to a spray dryer with an air inlet temperature of 250℃ and an atomizing wheel speed of 12000 rpm to obtain precursor particles with a particle size D50 of 34.1 μm.

[0030] (3) The precursor particles are prepared by granulation equipment to produce precursor particles with a particle size of about 3 mm. They are then loaded into a double-layer graphite sagger and conveyed into a roller kiln for calcination. The temperature is increased by gradient, the heating rate is 10℃ / h, the calcination temperature is 720℃, and the calcination time is 10h. The resulting material is lithium manganese iron phosphate.

[0031] Comparative Example 1 - Blank Control Group

[0032] The preparation method of the lithium manganese iron phosphate cathode material in Comparative Example 1 is basically the same as that in Example 1, except that lithium dihydrogen phosphate is not added, and includes the following steps:

[0033] (1) When type A ferric phosphate and type B ferric phosphate are mixed in a mass ratio of 2:1, an iron-to-phosphorus ratio of 0.975 and a specific surface area of ​​6.5 m² are formed. 2 / g, D50<5μm iron phosphate.

[0034] (2) Lithium carbonate, manganese oxalate and iron phosphate were taken according to the molar ratio of lithium, manganese, iron and phosphorus 1.03:0.7:0.3:1. At the same time, 10% of the composite carbon source of iron phosphate, as well as alumina and titanium dioxide were weighed and added to water. The mixture was stirred and dispersed for 45 min to obtain a precursor slurry. The precursor slurry was ground in a sand mill at a speed of 360 rpm for 120 min to obtain a slurry with a particle size D50 of 0.483 μm. The slurry was then transported to a spray dryer with an air inlet temperature of 250℃ and an atomizing wheel speed of 12000 rpm to obtain precursor particles with a particle size D50 of 34.3 μm.

[0035] (3) The precursor particles are prepared by granulation equipment to produce precursor particles with a particle size of about 3 mm. They are then loaded into a double-layer graphite sagger and conveyed into a roller kiln for calcination. The temperature is increased by gradient, the heating rate is 10℃ / h, the calcination temperature is 720℃, and the calcination time is 10h. The resulting material is lithium manganese iron phosphate.

[0036] Example 2

[0037] The method for preparing lithium iron manganese phosphate cathode material in Example 2 includes the following steps:

[0038] (1) When type A ferric phosphate and type B ferric phosphate are mixed in a mass ratio of 3:1, an iron-to-phosphorus ratio of 0.976 and a specific surface area of ​​6.5 m² are formed. 2 / g, D50 < 5μm ferric phosphate. 3% (by mass) of lithium dihydrogen phosphate was dissolved in water and stirred to form a homogeneous aqueous solution. Ferric phosphate was then added and stirred for 30 min to obtain a ferric phosphate slurry enriched with lithium dihydrogen phosphate on its surface.

[0039] (2) Lithium carbonate and manganese oxalate were taken according to the molar ratio of lithium, manganese, iron and phosphorus 1.08:0.2:0.8:1. First, manganese oxalate was added to the iron phosphate slurry and ultrasonically dispersed for 30 min. Then, lithium carbonate, 12% of the mass of iron phosphate composite carbon source, alumina and titanium dioxide were added. After stirring and dispersing for 45 min, the precursor slurry was obtained. The precursor slurry was ground at a sand mill speed of 360 rpm for 120 min to obtain a slurry with a particle size D50 of 0.481 μm. The slurry was then transported to a spray dryer with an air inlet temperature of 250℃ and an atomizing wheel speed of 12000 rpm to obtain precursor particles with a particle size D50 of 32.5 μm.

[0040] (3) The precursor particles are prepared by granulation equipment to produce precursor particles with a particle size of about 3 mm. They are then loaded into a double-layer graphite sagger and conveyed into a roller kiln for calcination. The temperature is increased by gradient, the heating rate is 10℃ / h, the calcination temperature is 720℃, and the calcination time is 10h. The resulting material is lithium manganese iron phosphate.

[0041] Comparative Example 2 - Blank Control Group

[0042] The preparation method of the lithium manganese iron phosphate cathode material in Comparative Example 2 is basically the same as that in Example 2, except that lithium dihydrogen phosphate is not added, and includes the following steps:

[0043] (1) When type A ferric phosphate and type B ferric phosphate are mixed in a mass ratio of 3:1, an iron-to-phosphorus ratio of 0.976 and a specific surface area of ​​6.5 m² are formed. 2 / g, D50<5μm iron phosphate.

[0044] (2) Lithium carbonate, manganese oxalate and iron phosphate were taken according to the molar ratio of lithium, manganese, iron and phosphorus 1.08:0.2:0.8:1. At the same time, 12% of the composite carbon source of iron phosphate, as well as alumina and titanium dioxide were weighed and added to water. The mixture was stirred and dispersed for 45 min to obtain a precursor slurry. The precursor slurry was ground in a sand mill at a speed of 360 rpm for 120 min to obtain a slurry with a particle size D50 of 0.48 μm. The slurry was then transported to a spray dryer with an air inlet temperature of 250℃ and an atomizing wheel speed of 12000 rpm to obtain precursor particles with a particle size D50 of 33.2 μm.

[0045] (3) The precursor particles are prepared by granulation equipment to produce precursor particles with a particle size of about 3 mm. They are then loaded into a double-layer graphite sagger and conveyed into a roller kiln for calcination. The temperature is increased by gradient, the heating rate is 10℃ / h, the calcination temperature is 720℃, and the calcination time is 10h. The resulting material is lithium manganese iron phosphate.

[0046] Example 3

[0047] The method for preparing lithium iron manganese phosphate cathode material in Example 3 includes the following steps:

[0048] (1) When type A ferric phosphate and type B ferric phosphate are mixed in a mass ratio of 1:1, an iron-to-phosphorus ratio of 0.976 and a specific surface area of ​​6.5 m² are formed. 2 / g, D50 < 5μm ferric phosphate. 2% (by mass) of lithium dihydrogen phosphate was dissolved in water and stirred to form a homogeneous aqueous solution. Ferric phosphate was then added and stirred for 30 min to obtain ferric phosphate slurry with lithium dihydrogen phosphate enriched on the surface.

[0049] (2) Lithium carbonate and manganese oxalate were taken according to the molar ratio of lithium, manganese, iron and phosphorus 1.05:0.5:0.5:1. First, manganese oxalate was added to the iron phosphate slurry and ultrasonically dispersed for 30 min. Then, lithium carbonate, 14% of the mass of iron phosphate composite carbon source, alumina and titanium dioxide were added. After stirring and dispersing for 45 min, the precursor slurry was obtained. The precursor slurry was ground at a sand mill speed of 360 rpm for 120 min to obtain a slurry with a particle size D50 of 0.487 μm. The slurry was then transported to a spray dryer with an air inlet temperature of 250℃ and an atomizing wheel speed of 12000 rpm to obtain precursor particles with a particle size D50 of 34.5 μm.

[0050] (3) The precursor particles are prepared by granulation equipment to produce precursor particles with a particle size of about 3 mm. They are then loaded into a double-layer graphite sagger and conveyed into a roller kiln for calcination. The temperature is increased by gradient, the heating rate is 10℃ / h, the calcination temperature is 720℃, and the calcination time is 10h. The resulting material is lithium manganese iron phosphate.

[0051] Performance testing

[0052] The lithium manganese iron phosphate cathode materials obtained in Examples 1 to 3 and Comparative Examples 1 and 2 were subjected to air jet milling to obtain powder with a particle size D50 of 1.01 μm. After sieving and demagnetization for 30 min, the lithium manganese iron phosphate cathode material was obtained. Its compaction density was measured, and the results are shown in Table 1 below.

[0053] Coin cells were fabricated and their electrochemical performance was evaluated using the following method: A core-shell structured lithium manganese iron phosphate cathode material, Super P, and PVDF were uniformly mixed in an NMP solution at a ratio of 8:1:1 to obtain a slurry. The slurry was then applied to bright aluminum foil by hand and dried at 100°C. After the NMP had completely evaporated, the electrode was cut into 13mm diameter electrode sheets. These sheets were then dried overnight at 105°C in a vacuum oven. After weighing, the sheets were quickly transferred to a glove box. Lithium metal was used as the counter electrode, Celgard 2400 as the separator, and 1 mol / L LiPF6 dissolved in a mixed solvent of EC / DMC / EMC (volume ratio 1:1:1) was used as the electrolyte. The assembled batteries were then subjected to electrochemical performance testing using a Xinwei 4008 constant current test cabinet with a test voltage range of 2.0-4.35V. The electrochemical performance results are shown in Table 1.

[0054] Table 1 Electrochemical performance

[0055]

[0056] Meanwhile, the structure of the cathode material in Example 2 was characterized, and the results are as follows: Figure 1-2 As shown in the figure, the lithium manganese iron phosphate cathode material prepared in the example has a uniform carbon coating layer on its surface, and the particles have regular morphology and size gradation, with larger particles dominating and smaller particles assisting to improve the compaction density. This indicates that lithium dihydrogen phosphate plays a fluxing role, allowing the smaller particles to grow further. In contrast, Comparative Example 2 shows a small particle distribution and a more uniform overall particle size distribution.

[0057] Furthermore, by comparing the examples and comparative data in Table 1, it can be seen that the preparation process of the present invention, by first pretreating iron phosphate with lithium dihydrogen phosphate and mixing it with the Mn source, not only effectively increases the compaction density with the addition of lithium dihydrogen phosphate, but also effectively reduces the dissolution of Mn, thereby improving the rate performance and cycle stability.

[0058] Based on the experimental results, the mechanism can be further deduced as follows: When preparing lithium manganese iron phosphate cathode material in this invention, lithium dihydrogen phosphate and iron phosphate are first compounded to form iron phosphate with lithium dihydrogen phosphate attached to the surface. On this basis, it is first compounded with a manganese source, and then compounded with a lithium source and a carbon source dual dopant. After spray drying and calcination, lithium manganese iron phosphate cathode material is obtained.

[0059] First, based on the lithium dihydrogen phosphate enriched on the surface of iron phosphate, the negatively charged phosphate ions enhance the adsorption capacity of iron phosphate and positively charged manganese, improving their synergistic reaction and inhibiting the dissolution of Mn ions, thus improving rate performance. Simultaneously, the glassy state formed by the melting of lithium dihydrogen phosphate during the calcination process promotes the rapid migration, rearrangement, and entry of manganese and iron ions into the crystal lattice in the liquid phase, inhibiting the formation and separation of the manganese-rich phase and forming a uniform lithium iron manganese phosphate solid solution, further suppressing the dissolution of Mn ions. Furthermore, the glassy state formed by the melting of lithium dihydrogen phosphate can lock in iron and manganese; even after cooling, manganese remains anchored around iron. Therefore, by adding lithium dihydrogen phosphate to "lock in" free manganese ions, the pathway for Mn dissolution is effectively cut off, fundamentally and effectively inhibiting Mn dissolution.

[0060] Secondly, the liquid-phase sintering environment provided by the glassy state of molten lithium dihydrogen phosphate effectively fills the pores inside the precursor, making the surface of the cathode material particles more regular and improving the compaction of the prepared lithium manganese iron phosphate cathode material.

[0061] Furthermore, this invention employs Al and Ti oxides for dual doping at Li and Fe sites to adjust the crystal structure and increase the Li vacancy concentration, thereby enhancing the Li content. + The transport rate is improved, and the ionic conductivity of lithium manganese iron phosphate is enhanced. In addition, the dual doping forms strong Ti-O bonds and lattice distortion inside lithium manganese iron phosphate, increases the van der Waals forces between lattices, and promotes the structural stability and compaction density of lithium manganese iron phosphate materials, resulting in lithium manganese iron phosphate materials with high compaction density and high rate performance.

[0062] In addition to the above embodiments, it should be noted that the technical effects claimed by the present invention can be achieved by using the preparation process and the limited parameter range of the present invention, and therefore no further examples will be given to support these claims.

Claims

1. A method for preparing a high-compact, high-rate lithium manganese iron phosphate cathode material, characterized in that, Includes the following steps: (1) After dissolving lithium dihydrogen phosphate in water, add ferric phosphate and disperse evenly to obtain ferric phosphate slurry with lithium dihydrogen phosphate enriched on the surface; (2) Iron phosphate slurry, manganese source and water are mixed evenly to form a slurry; lithium source, carbon source and dual dopant are added, and after stirring evenly, the lithium manganese iron phosphate cathode material is obtained by spray drying and sintering.

2. The method for preparing high-compact, high-rate lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The mass of lithium dihydrogen phosphate added is 1-3% of the mass of iron phosphate.

3. The method for preparing high-density, high-rate lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The lithium manganese iron phosphate cathode material LiMn (1-x) Fe x In PO4, the molar ratio of lithium, manganese, iron and phosphorus is (1.03-1.08):(1-x):x:1, where 0.3≤x≤0.

8.

4. The method for preparing high-compact, high-rate lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The ferric phosphate is composed of type A ferric phosphate and type B ferric phosphate in a mass ratio of (1-3):1; and the iron-to-phosphorus ratio of type A ferric phosphate is 0.975-0.985, while that of type B ferric phosphate is 0.96-0.

965.

5. The method for preparing high-compact, high-rate lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The lithium source is one or more of lithium carbonate, lithium hydroxide, lithium phosphate, and lithium sulfate.

6. The method for preparing high-compact, high-rate lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The amount of carbon source added is 10-14% of the mass of iron phosphate.

7. The method for preparing high-compact, high-rate lithium manganese iron phosphate cathode material according to claim 6, characterized in that, The carbon source is any two of glucose, sucrose, straw powder, polyethylene glycol, polyvinylpyrrolidone, and Ketjen black, and the mass ratio of the two is 1:(2-4).

8. The method for preparing high-compact, high-rate lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The manganese source is one or more of manganese sulfate, manganese oxalate, manganese nitrate, manganese tetroxide, manganese carbonate, and manganese dioxide.

9. The method for preparing high-compact, high-rate lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The dual dopant is aluminum oxide and titanium dioxide, with the molar amount of aluminum oxide being 3-6% of the molar amount of iron phosphate, and the molar amount of titanium dioxide being 0.5-2% of the molar amount of iron phosphate.

10. The method for preparing high-compact, high-rate lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The sintering temperature is 700-750℃, and the holding time is 9-10h.

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