Lithium manganese iron phosphate active material and preparation method thereof, lithium ion battery positive electrode and preparation method thereof, and lithium ion battery

Abstract

The invention discloses a lithium manganese iron phosphate active material and a preparation method thereof, a lithium ion battery positive electrode and a preparation method thereof, and a lithium ion battery, and belongs to the technical field of lithium batteries. The lithium manganese iron phosphate active material comprises a plurality of modified lithium manganese iron phosphate particles, conductive carbon black particles, carbon nanotubes and a porous carbon network; the preparation method comprises the following steps: doping elements in a lithium manganese iron phosphate core to form metal sites, coating a fast ion conductor on the surface of the core, forming a porous carbon network structure after a heating reaction of a metal-organic framework material formed by imidazole, Fe and Mn, and forming a conductive network with conductive carbon black and CNT, so that the conductive carbon black and the CNT are fully and uniformly dispersed at the periphery of the modified lithium manganese iron phosphate; according to the invention, the electron conductivity and the ion conductivity can be improved, the side reaction of electrolyte can be reduced, the homogenate production time can be shortened, the coating effect can be improved, and the compaction density of the pole piece can be improved.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, and in particular to a lithium manganese iron phosphate active material and its preparation method, a lithium-ion battery cathode and its preparation method, and a lithium-ion battery. Background Technology

[0002] An ideal cathode material must simultaneously meet five core requirements: high energy density, high power density, long cycle life, high safety, and low cost. Lithium manganese iron phosphate (LMFP) perfectly meets most of these requirements: it has a stable olivine phase crystal structure, and its constituent elements (iron, manganese, and phosphorus) are all elements with high abundance in the Earth's crust, possessing a relatively ideal 4.0V voltage platform. In terms of performance parameters, the theoretical energy density at the material level reaches 431Wh / kg, and the cell-level energy density can be stably maintained above 210Wh / kg, demonstrating significant comprehensive advantages.

[0003] However, LMFPs currently face three key limitations that hinder their further application: First, their low ion diffusion coefficient and electronic conductivity result in insufficient reaction kinetics under high-rate charge-discharge conditions; second, the small particle size design and high carbon content modification used to improve conductivity directly lead to low material compaction density, while also causing difficulties in electrode coating, rolling, and other processing steps; and third, the presence of manganese easily triggers a significant Jamie-Taylor effect, causing distortion of the crystal structure during cycling, which in turn affects the material's cycling stability and lifespan. Summary of the Invention

[0004] To address the aforementioned issues, this invention proposes a lithium manganese iron phosphate active material and a lithium-ion battery. The lithium manganese iron phosphate core is doped with elements to form metal sites, and the core surface is coated with a fast ion conductor. Furthermore, a metal-organic framework material formed by imidazole and Fe / Mn, after heating, forms a porous carbon network structure, which is then combined with conductive carbon black and CNTs to form a conductive network. This ensures that the conductive carbon black and CNTs are fully and uniformly dispersed around the modified lithium manganese iron phosphate, thereby improving electronic and ionic conductivity, reducing electrolyte side reactions, shortening homogenization production time, improving coating effect, and increasing electrode compaction density.

[0005] In a first aspect, the present invention provides a lithium manganese iron phosphate active material, comprising a plurality of modified lithium manganese iron phosphate particles, conductive carbon black particles, carbon nanotubes and porous carbon networks; wherein the mass ratio of the modified lithium manganese iron phosphate particles, conductive carbon black, carbon nanotubes and porous carbon networks is (95.8~98.9):(0.2~0.7):(0.1~0.5):(0.5~3.0); The modified lithium manganese iron phosphate particles consist of a lithium manganese iron phosphate core doped with metal element M and sequentially coated with a fast ion conductor coating layer and a carbon coating layer. The modified lithium manganese iron phosphate particles, conductive carbon black particles, carbon nanotubes, and porous carbon networks are assembled into spherical or near-spherical secondary particles; in the secondary particles, the porous carbon network is cross-linked on the outer surface of each modified lithium manganese iron phosphate particle; the conductive carbon black particles and carbon nanotubes are dispersed around the modified lithium manganese iron phosphate particles or embedded in the porous carbon network.

[0006] The modified lithium manganese iron phosphate particles, conductive carbon black particles, carbon nanotubes, and porous carbon network structures are intercalated and / or bonded together to tightly bind and assemble into spherical or near-spherical secondary particles.

[0007] In a preferred embodiment, in the lithium manganese iron phosphate core, the molar ratio of Mn:Fe is (0.0001~0.9999):(0.9999~0.0001); the particle size of a single modified lithium manganese iron phosphate particle is 20nm~300nm; and the molar ratio of lithium manganese iron phosphate, metal element M, and fast ion conductor in the modified lithium manganese iron phosphate particle is (97~99.997):(1~0.001):( 1~0.001): (1~0.001); the mass ratio of the fast ion conductor coating layer to the carbon coating layer is (0.5~1.0): (1.0~2.0); the thickness of the fast ion conductor coating layer is 1nm~9nm; the carbon coating layer is an amorphous carbon layer with a thickness of 1nm~15nm; in the lithium manganese iron phosphate core doped with metal element M, the doped metal element is selected from at least one of calcium, molybdenum, zinc, nickel, tungsten, magnesium, titanium, chromium, zirconium, vanadium and cobalt.

[0008] In a preferred embodiment, the fast ion conductor is selected from at least one of the LISICON system, NASICON structure, perovskite structure and Li3N derivatives.

[0009] The fast ion conductor is preferably Li. 1.3 Al 0.3 Ti 1.7 (PO4)3, Na3Zr2Si2PO 12 Li 0.33 La 0.56 TiO3, Li 14 ZnGeO4.

[0010] In a preferred embodiment, the thickness of the porous carbon network is 10 nm to 50 nm; the size of the secondary particles is 0.5 μm to 5 μm.

[0011] In a second aspect, the present invention provides a method for preparing the lithium manganese iron phosphate active material described in the first aspect, comprising the steps of: (1) The phosphorus source, manganese source, iron source, lithium source, doped metal element, fast ion conductor and carbon source II are wet ground for 1~5h to obtain a mixture, and then carbon source I is added and stirred to obtain the first slurry; (2) The conductive carbon black, carbon nanotubes and dispersant are mixed to prepare the second slurry; (3) The first slurry and the second slurry are heated and reacted, and then alternately sprayed and granulated. The mixture is heated and reacted in an inert atmosphere to obtain the lithium manganese iron phosphate active material.

[0012] Preferably, the wet grinding time is 2 hours.

[0013] Preferably, the inert atmosphere is selected from at least one of nitrogen atmosphere, argon atmosphere or helium atmosphere.

[0014] Preferably, the first and second slurries are sprayed and granulated sequentially from nozzles A and B at flow rates of 38 L / h and 10 L / h, respectively, with alternating spraying and layered granulation.

[0015] Preferably, after spray granulation, the temperature is raised to 600-900℃ (preferably 800℃) at a rate of 2-10℃ / min (preferably 3℃ / min) in a protective atmosphere and held for 3-7h (preferably 5h). High-temperature treatment can obtain a stable lithium manganese iron phosphate crystal structure.

[0016] Preferably, the inlet air temperature of the spray granulation is 180~260℃ (preferably 230℃), and the outlet air temperature is 80~150℃ (preferably 110℃).

[0017] Preferably, the product obtained after the heating reaction is pulverized and sieved.

[0018] In a preferred embodiment, the molar ratio of the phosphorus source, manganese source, iron source, and lithium source is Li:(Mn+Fe):P=(0.99~1.05):(0.98~1.01):1, wherein the molar ratio of Mn to Fe is 0.0001~0.9999:0.9999~0.0001.

[0019] In a preferred embodiment, the phosphorus source is selected from at least one of phosphoric acid, ammonium dihydrogen phosphate, lithium dihydrogen phosphate, diammonium hydrogen phosphate, and ammonium phosphate.

[0020] In a preferred embodiment, the manganese source is selected from at least one of manganese sulfate, manganese chloride, manganese acetate, and manganese oxalate.

[0021] In a preferred embodiment, the iron source is selected from at least one of ferrous oxide, ferric oxide, ferric oxide, ferric nitrate, ferrous nitrate, and ferrous oxalate.

[0022] In a preferred embodiment, the lithium source is selected from at least one of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate.

[0023] In a preferred embodiment, the heating reaction conditions in (3) include: temperature 600~800℃, time 3~15h; heating rate 2~20℃ / min.

[0024] In a preferred embodiment, the spraying conditions in (3) include a temperature of 100–400°C.

[0025] In a preferred embodiment, the carbon source I is selected from at least one of glucose, sucrose, polyethylene glycol or polymethyl methacrylate; the mass fraction of the carbon source I in the mixture is 1% to 5%; the carbon source I is carbonized by heating to form a carbon coating layer.

[0026] In a preferred embodiment, the carbon source II is selected from imidazole compounds; the molar ratio of carbon source II to Mn in the manganese source is (1-5):(50-90); carbon source II is carbonized by heating reaction to form a porous carbon network.

[0027] In a preferred embodiment, the imidazole compound is selected from at least one of 2-methylimidazole, 1,2-dimethylimidazole, 1H-1,2,3-triazole, or a derivative thereof.

[0028] In a preferred embodiment, the carbon nanotube is one or more of single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes.

[0029] Thirdly, the present invention provides a positive electrode for a lithium-ion battery, comprising the lithium manganese iron phosphate active material as described in any of the first aspects, or the lithium manganese iron phosphate active material prepared by any of the methods described in the second aspect.

[0030] Fourthly, the present invention provides a method for preparing the positive electrode of the lithium-ion battery described in the third aspect, characterized in that the lithium manganese iron phosphate active material, binder, dispersant and solvent are mixed to prepare a positive electrode slurry, the slurry is coated on a current collector aluminum foil, dried, rolled and die-cut to obtain a positive electrode sheet.

[0031] Fifthly, the present invention provides a method for preparing the positive electrode of the lithium-ion battery, wherein the mass ratio of the lithium manganese iron phosphate active material, binder, and dispersant is (97.1~98.9):(1.0~2.5):(0.1~0.4).

[0032] In a preferred embodiment, the adhesive is polyvinylidene fluoride (PVDF), and the PVDF is selected from one or more of HSV900, 5130, and 6020.

[0033] In a preferred embodiment, the dispersant is selected from one or more of polyvinylpyrrolidone, sodium dodecyl sulfate, fatty alcohol polyoxyethylene ether, Tween-60, and dispersant KD-1.

[0034] In a preferred embodiment, the solvent is N. Methylpyrrolidone.

[0035] In a preferred embodiment, the compaction density of the positive electrode sheet is 2.0~2.6 g / cm³. 3 .

[0036] In a sixth aspect, the present invention provides a lithium-ion battery containing lithium manganese iron phosphate active material as described in any of the first aspects, or containing lithium manganese iron phosphate active material prepared by any of the methods described in the second aspect, or containing a positive electrode as described in the third aspect.

[0037] This invention proposes a lithium manganese iron phosphate active material and a lithium-ion battery. The core is doped with elements to form metal sites, and the core surface is coated with a fast ion conductor. A metal-organic framework material formed by imidazole and Fe / Mn, after heating, forms a porous carbon network structure. The core doping elements prevent manganese iron enrichment and phase separation from affecting electrical performance, forming metal sites and enhancing the conductivity of the main material. The carbon coating significantly improves the conductivity, rate performance, and cycle life of lithium manganese iron phosphate by constructing an electronic conductive network, inhibiting particle growth, blocking ion corrosion, reducing Mn dissolution, and improving structural stability. The fast ion conductor layer on the core surface serves two purposes: firstly, it acts as a protective layer, inhibiting manganese dissolution and side reactions between the cathode material and the electrolyte, thereby improving the material's cycle stability; secondly, it acts as a lithium-ion conductor, enhancing the material's ionic conductivity, reducing charge transfer resistance during charging and discharging, effectively improving the interfacial impedance and charge between the electrolyte and the cathode material, and improving rate performance. The porous carbon network structure enhances the conductivity of the material while preventing the agglomeration of conductive carbon black, CNTs, and lithium manganese iron phosphate. This allows the porous carbon network to intercalate and / or adhere to the conductive carbon black, CNTs, and lithium manganese iron phosphate to form spherical or near-spherical secondary particles. The resulting lithium manganese iron phosphate active material can shorten the homogenization time and reduce the amount of adhesive used, enabling continuous and automated production. It also improves the electrode slurry preparation and coating problems caused by the small particle size and large specific surface area of ​​pure lithium manganese iron phosphate. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the structure of the lithium manganese iron phosphate active material prepared in Example 1; Figure 2 This is a schematic diagram of the structure of the lithium manganese iron phosphate active material prepared in Comparative Example 1. Detailed Implementation

[0039] Example 1 A lithium manganese iron phosphate active material includes the following steps: (1) Weigh the solid raw materials (the four solid raw materials, lithium source, phosphorus source, manganese source and iron source, are in the molar ratio of Li:P:Mn:Fe of 1:1:0.6:0.4), that is, the target chemical composition of the lithium manganese iron phosphate core is LiMn. 0.6 Fe 0.4 PO4 (LM6F4P). Also take Li. 1.3 Al 0.3 Ti 1.7 (PO4)3, C3H4N2, and MgCO3. Among them, LM6F4P:MgCO3:Li 1.3 Al 0.3 Ti 1.7 The molar ratio of (PO4)3:C3H4N2 is 99.25:0.2:0.25:0.3. After adding a mixed solvent, the mixture is wet-milled for 2 hours. Finally, glucose (LM6F4P to glucose mass ratio is 99:1) is added and stirred evenly to obtain the first slurry. The weight ratio of solid raw material to mixed solvent is 65:35. The mixed solvent is composed of N,N-dimethylformamide (DMF), anhydrous ethanol, water and ammonium hydroxide (25wt%) in a volume ratio of 10:10:15:5.

[0040] (2) The conductive carbon black, CNT, dispersant and ethanol are mixed evenly to obtain the second slurry; wherein the dispersant is polyvinylpyrrolidone, the weight ratio of solid raw materials (conductive carbon black, CNT, dispersant) to ethanol is 60:40, and the mass ratio of conductive carbon black, CNT, dispersant to LM6F4P in the first slurry is 0.5:0.3:0.1:97.

[0041] (3) The first and second slurries were sprayed from nozzles A and B at flow rates of 38 L / h and 10 L / h, respectively, for granulation. The inlet air temperature was 230℃, the outlet air temperature was 110℃, and the atomizing disc rotation speed was 13500 rpm. The resulting particles were heated to 800℃ in an N2 atmosphere at a rate of 3℃ / min and held at that temperature for 5 hours. Glucose decomposed into a carbon layer at the high temperature, and C3H4N2 transformed into a porous carbon network structure, encapsulating LM6F4P and conductive carbon black. Finally, the particles were naturally cooled to room temperature to obtain lithium manganese iron phosphate active material. Figure 1 As shown, the active material of lithium manganese iron phosphate includes modified lithium manganese iron phosphate, conductive carbon black, CNTs and porous carbon network structure. The modified lithium manganese iron phosphate particles, conductive carbon black particles, carbon nanotubes and porous carbon network structure are intercalated and / or bonded to tightly bind and assemble into spherical secondary particles with a particle size D50 of 2 μm.

[0042] Preparation of soft-pack lithium-ion batteries: In step (3), the mass ratio of lithium manganese iron phosphate active material, polyvinylidene fluoride (PVDF), and dispersant (polyvinylpyrrolidone) is 97.9:2.0:0.1. Solvent is added, and the mixture is stirred and dispersed evenly in one step to obtain a positive electrode slurry. The positive electrode slurry is coated on the current collector aluminum foil, dried, rolled, and die-cut to obtain a positive electrode sheet. It is then assembled with a commercial negative electrode sheet and a commercial PP separator, and injected with commercial lithium manganese iron phosphate electrolyte to obtain a soft-pack lithium-ion battery. The electrolyte is 1 mol LiPF6 EC:EMC:DEC (3:4:3). The negative electrode material is commercial graphite. The negative electrode sheet is made of PP separator. The process involves winding → hot pressing → encapsulation → baking → liquid injection → formation → degassing → capacity testing.

[0043] Example 2 Same as Example 1, except that step (2) does not include 0.1 wt% dispersant. All other steps are the same as in Example 1, specifically including: (1) Weigh the solid raw materials (four solid raw materials: lithium source, phosphorus source, manganese source and iron source, in a Li:P:Mn:Fe molar ratio of 1:1:0.6:0.4), that is, the target chemical composition of the lithium manganese iron phosphate core is LiMn0.6Fe0.4PO4 (LM6F4P), and separately take LM6F4P:MgCO3:Li 1.3 Al 0.3 Ti 1.7 The molar percentages of MgCO3 and Li in (PO4)3:C3H4N2 were 99.25:0.2:0.25:0.3. 1.3 Al 0.3 Ti 1.7 (PO4)3 and C3H4N2 were then wet-milled for 2 hours with a mixed solvent. Glucose (LM6F4P to glucose mass ratio of 99:1) was then added and stirred until homogeneous to obtain the first slurry. The weight ratio of solid raw materials to mixed solvent was 65:35. The mixed solvent consisted of N,N-dimethylformamide (DMF), anhydrous ethanol, water and ammonium hydroxide (25wt%) in a volume ratio of 10:10:15:5.

[0044] (2) The conductive carbon black, CNT and ethanol are mixed evenly to obtain the second slurry; wherein the conductive carbon black, CNT and LM6F4P in the first slurry are in a mass ratio of 0.5:0.3:97; the weight ratio of solid raw materials (conductive carbon black, CNT) to ethanol is 60:40.

[0045] (3) The first and second slurries were sprayed from nozzles A and B at flow rates of 38 L / h and 10 L / h, respectively, with an inlet air temperature of 230°C, an outlet air temperature of 110°C, and an atomizing disc rotation speed of 13500 rpm. The resulting particles were heated to 800°C at a rate of 3°C / min in an N2 atmosphere and held at that temperature for 5 hours. Glucose decomposed into a carbon layer at the high temperature, and C3H4N2 transformed into a porous carbon network structure, encapsulating LM6F4P and conductive carbon black. Finally, the particles were naturally cooled to room temperature to obtain lithium manganese iron phosphate active material with a particle size D50 of 2 μm.

[0046] Preparation of soft-pack lithium-ion batteries: The mass ratio of lithium manganese iron phosphate active material, polyvinylidene fluoride binder (PVDF), and dispersant is 97.8:2:0.2. Solvent is added, and the mixture is stirred and dispersed evenly in one step to obtain a positive electrode slurry. The positive electrode slurry is coated onto a current collector aluminum foil, dried, rolled, and die-cut to obtain a positive electrode sheet. This sheet is then assembled with a commercial negative electrode sheet and a commercial PP separator, and injected with commercial lithium manganese iron phosphate electrolyte to obtain a soft-pack lithium-ion battery.

[0047] Example 3 Similar to Example 1, except that in step (2), the first and second slurries are mixed evenly before spray granulation. All other steps are the same as in Example 1, specifically including: (1) Weigh the solid raw materials (the four solid raw materials, lithium source, phosphorus source, manganese source and iron source, are in the molar ratio of Li:P:Mn:Fe of 1:1:0.6:0.4), that is, the target chemical composition of the lithium manganese iron phosphate core is LiMn 0.6 Fe 0.4 PO4(LM6F4P), with additional LM6F4P:MgCO3:Li 1.3 Al 0.3 Ti 1.7 The molar percentages of MgCO3 and Li1.3Al in (PO4)3:C3H4N2 were 99.25:0.2:0.25:0.3. 0.3 Ti 1.7 (PO4)3 and C3H4N2 were then added to a mixed solvent and wet-milled for 2 hours. Finally, glucose (LM6F4P to glucose mass ratio of 99:1) was added and stirred evenly to obtain the first slurry. The weight ratio of solid raw materials to mixed solvent was 65:35. The mixed solvent was composed of N,N-dimethylformamide (DMF), anhydrous ethanol, water and ammonium hydroxide (25wt%) in a volume ratio of 10:10:15:5.

[0048] (2) The conductive carbon black, CNT, dispersant and ethanol are mixed evenly to obtain the second slurry; wherein the dispersant is polyvinylpyrrolidone, the weight ratio of solid raw materials (conductive carbon black, CNT, dispersant) to ethanol is 60:40, and the mass ratio of conductive carbon black, CNT, dispersant to LM6F4P in the first slurry is 0.5:0.3:0.1:97.

[0049] (3) The first and second slurries were mixed evenly and sprayed at a flow rate of 24 L / h for granulation. The inlet air temperature was 230℃, the outlet air temperature was 110℃, and the atomizing disc rotation speed was 13500 rpm. The resulting particles were heated to 800℃ in an N2 atmosphere at a rate of 3℃ / min and held at that temperature for 5h. Glucose decomposed into a carbon layer at high temperature, and C3H4N2 was transformed into a porous carbon network structure, which encapsulated lithium manganese iron phosphate and conductive carbon black. Finally, the particles were naturally cooled to room temperature to obtain lithium manganese iron phosphate active material with a particle size D50 of 2μm.

[0050] Preparation of soft-pack lithium-ion batteries: The mass ratio of lithium manganese iron phosphate active material, polyvinylidene fluoride (PVDF), and dispersant is 97.9:2.0:0.1. Solvent is added, and the mixture is stirred and dispersed evenly in one step to obtain a positive electrode slurry. The positive electrode slurry is coated on a current collector aluminum foil, dried, rolled, and die-cut to obtain a positive electrode sheet. This positive electrode sheet is then assembled with a commercial negative electrode sheet and a commercial PP separator, and injected with commercial lithium manganese iron phosphate electrolyte to obtain a soft-pack lithium-ion battery.

[0051] Comparative Example 1 A lithium manganese iron phosphate active material includes the following steps: Same as Example 1, except that step (1) does not include 0.3% imidazole. All other steps are the same as in Example 1, specifically including: (1) Weigh the solid raw materials (the four solid raw materials, lithium source, phosphorus source, manganese source and iron source, are weighed according to the molar ratio of Li:P:Mn:Fe of 1:1:0.6:0.4), that is, the target chemical composition of the lithium manganese iron phosphate core is LiMn 0.6 Fe 0.4 PO4(LM6F4P), with additional LM6F4P:MgCO3:Li 1.3 Al 0.3 Ti 1.7 The molar percentages of MgCO3 and Li3 were 99.55:0.2:0.25. 1.3 Al 0.3 Ti 1.7(PO4)3 was then added to a mixed solvent and wet-milled for 2 hours. Finally, glucose (LM6F4P to glucose mass ratio of 99:1) was added and stirred evenly to obtain the first slurry. The weight ratio of solid raw material to mixed solvent was 65:35. The mixed solvent was composed of N,N-dimethylformamide (DMF), anhydrous ethanol, water and ammonium hydroxide (25wt%) in a volume ratio of 10:10:15:5.

[0052] (2) The conductive carbon black, CNT, dispersant and ethanol are mixed evenly to obtain the second slurry; wherein the dispersant is polyvinylpyrrolidone, the weight ratio of solid raw materials (conductive carbon black, CNT, dispersant) to ethanol is 60:40, and the mass ratio of conductive carbon black, CNT, dispersant to LM6F4P in the first slurry is 0.5:0.3:0.1:97.

[0053] (3) The first and second slurries were sprayed from nozzles A and B at flow rates of 38 L / h and 10 L / h, respectively, for granulation. The inlet air temperature was 230℃, the outlet air temperature was 110℃, and the atomizing disc rotation speed was 13500 rpm. The resulting particles were heated to 800℃ in an N2 atmosphere at a rate of 3℃ / min and held at that temperature for 5 hours. Glucose decomposed and transformed into a carbon layer at the high temperature. Finally, the particles were naturally cooled to room temperature to obtain lithium manganese iron phosphate active material with a particle size D50 of 2 μm. Figure 2 As shown, the active material of lithium manganese iron phosphate consists of conductive carbon black particles and carbon nanotubes dispersed around the modified lithium manganese iron phosphate particles.

[0054] Preparation of a pouch lithium-ion battery: The active material (lithium manganese iron phosphate), polyvinylidene fluoride (PVDF), and dispersant are mixed in a mass ratio of 97.9:2.0:0.1. A solvent is added, and the mixture is stirred and dispersed evenly in one step to obtain a positive electrode slurry. The positive electrode slurry is coated onto a current collector aluminum foil, dried, rolled, and die-cut to obtain a positive electrode sheet. This sheet is then assembled with a commercially available negative electrode sheet and a commercially available PP separator, and finally injected with commercially available lithium manganese iron phosphate electrolyte to produce a pouch lithium-ion battery.

[0055] Comparative Example 2 A lithium manganese iron phosphate active material includes the following steps: Unlike Example 1, the lithium manganese iron phosphate active material in step (2) does not include conductive carbon black, CNTs, or other components. Specifically, it includes: (1) Weigh the solid raw materials (the four solid raw materials, lithium source, phosphorus source, manganese source and iron source, are weighed according to the molar ratio of Li:P:Mn:Fe of 1:1:0.6:0.4), that is, the target chemical composition of the lithium manganese iron phosphate core is LiMn 0.6 Fe 0.4 PO4(LM6F4P), with additional LM6F4P:MgCO3:Li 1.3 Al0.3 Ti 1.7 The molar percentages of MgCO3 and Li in (PO4)3:C3H4N2 were 99.25:0.2:0.25:0.3. 1.3 Al 0.3 Ti 1.7 (PO4)3 and C3H4N2 were then added to a mixed solvent and wet-milled for 2 hours. Finally, glucose (LM6F4P to glucose mass ratio of 99:1) was added and stirred evenly to obtain the first slurry. The weight ratio of solid raw materials to mixed solvent was 65:35. The mixed solvent was composed of N,N-dimethylformamide (DMF), anhydrous ethanol, water and ammonium hydroxide (25wt%) in a volume ratio of 10:10:15:5.

[0056] (2) Subsequently, the first slurry was spray-granulated at a flow rate of 38 L / h, with an inlet air temperature of 230℃, an outlet air temperature of 110℃, and an atomizing disc rotation speed of 13500 rpm. The resulting particles were heated to 800℃ in an N2 atmosphere at a rate of 3℃ / min and held at that temperature for 5 h. Glucose decomposed into a carbon layer at high temperature, and C3H4N2 transformed into a porous carbon network structure, which encapsulated lithium manganese iron phosphate and conductive carbon black. Finally, the particles were naturally cooled to room temperature to obtain lithium manganese iron phosphate active material with a particle size D50 of 1 μm.

[0057] Preparation of a soft-pack lithium-ion battery: The mass ratio of lithium manganese iron phosphate active material, polyvinylidene fluoride (PVDF), dispersant, conductive carbon black, and CNT is 97:2.0:0.2:0.5:0.3. According to the above ratio, the lithium manganese iron phosphate active material and conductive carbon black are dry-mixed, then the PVDF binder and solvent are added and kneaded. CNTs and dispersant are then added, and the mixture is stirred and dispersed in multiple steps to obtain a positive electrode slurry. The positive electrode slurry is coated onto a current collector aluminum foil, dried, rolled, and die-cut to obtain a positive electrode sheet. This positive electrode sheet is then assembled with a commercial negative electrode sheet and a commercial PP separator, and injected with commercial lithium manganese iron phosphate electrolyte to obtain a soft-pack lithium-ion battery.

[0058] Comparative Example 3 Similar to Example 1, except that step (1) does not include a dopant element with a molar percentage of 0.2%. All other steps are the same as in Example 1, specifically including: (1) Weigh the solid raw materials (the four solid raw materials, lithium source, phosphorus source, manganese source and iron source, are in the molar ratio of Li:P:Mn:Fe of 1:1:0.6:0.4), that is, the target chemical composition of the lithium manganese iron phosphate core is LiMn. 0.6 Fe 0.4 PO4 (LM6F4P), also taken from LM6F4P: Li 1.3 Al 0.3 Ti 1.7The mixture of Li1.3Al0.3Ti1.7(PO4)3 and C3H4N2 with a molar percentage of (PO4)3:C3H4N2 of 99.45:0.25:0.3 was wet-milled for 2 hours with a mixed solvent. Finally, glucose (LM6F4P to glucose mass ratio of 99:1) was added and stirred evenly to obtain the first slurry. The weight ratio of solid raw materials to mixed solvent was 65:35. The mixed solvent was composed of N,N-dimethylformamide (DMF), anhydrous ethanol, water and ammonium hydroxide (25wt%) in a volume ratio of 10:10:15:5.

[0059] (2) The conductive carbon black, CNT, dispersant and ethanol are mixed evenly to obtain the second slurry; wherein the dispersant is polyvinylpyrrolidone, the weight ratio of solid raw materials (conductive carbon black, CNT, dispersant) to ethanol is 60:40, and the mass ratio of conductive carbon black, CNT, dispersant to LM6F4P in the first slurry is 0.5:0.3:0.1:97.

[0060] (3) The first and second slurries were sprayed from nozzles A and B at flow rates of 38 L / h and 10 L / h, respectively, with an inlet air temperature of 230°C, an outlet air temperature of 110°C, and an atomizing disc rotation speed of 13500 rpm. The resulting particles were heated to 800°C at a rate of 3°C / min in an N2 atmosphere and held at that temperature for 5 hours. Glucose decomposed into a carbon layer at the high temperature, and C3H4N2 transformed into a porous carbon network structure, which encapsulated lithium manganese iron phosphate and conductive carbon black. Finally, the particles were naturally cooled to room temperature to obtain lithium manganese iron phosphate active material with a particle size D50 of 2 μm.

[0061] Preparation of soft-pack lithium-ion batteries: The mass ratio of lithium manganese iron phosphate active material, polyvinylidene fluoride (PVDF), and dispersant is 97.8:2.0:0.2. Solvent is added, and the mixture is stirred and dispersed evenly in one step to obtain a positive electrode slurry. The positive electrode slurry is coated on a current collector aluminum foil, dried, rolled, and die-cut to obtain a positive electrode sheet. This positive electrode sheet is then assembled with a commercial negative electrode sheet and a commercial PP separator, and injected with commercial lithium manganese iron phosphate electrolyte to obtain a soft-pack lithium-ion battery.

[0062] Comparative Example 4 Same as Example 1, except that step (1) does not include 0.25% of a fast ion conductor. All other steps are the same as in Example 1, specifically including: (1) Weigh the lithium source, phosphorus source, manganese source and iron source in a molar ratio of 1:1:0.6:0.4, that is, the target chemical composition of the lithium manganese iron phosphate core is LiMn. 0.6 Fe0.4 PO4 (LM6F4P), MgCO3 and C3H4N2 with a molar percentage of LM6F4P:MgCO3:C3H4N2 of 99.5:0.2:0.3, are added and wet-milled for 2 hours with a mixed solvent. Finally, glucose (LM6F4P to glucose mass ratio of 99:1) is added and stirred evenly to obtain the first slurry. The weight ratio of solid raw material to mixed solvent is 65:35. The mixed solvent is composed of N,N-dimethylformamide (DMF), anhydrous ethanol, water and ammonium hydroxide (25wt%) in a volume ratio of 10:10:15:5.

[0063] (2) The conductive carbon black, CNT, dispersant and ethanol are mixed evenly to obtain the second slurry; wherein the dispersant is polyvinylpyrrolidone, the weight ratio of solid raw materials (conductive carbon black, CNT, dispersant) to ethanol is 60:40, and the mass ratio of conductive carbon black, CNT, dispersant to LM6F4P in the first slurry is 0.5:0.3:0.1:97.

[0064] (3) The first and second slurries were sprayed from nozzles A and B at flow rates of 38 L / h and 10 L / h, respectively, with an inlet air temperature of 230℃, an outlet air temperature of 110℃, and an atomizing disc rotation speed of 13500 rpm. The resulting particles were heated to 800℃ in an N2 atmosphere at a rate of 3℃ / min and held at that temperature for 5 hours. Glucose decomposed into a carbon layer at the high temperature, and C3H4N2 transformed into a porous carbon network structure, encapsulating lithium manganese iron phosphate and conductive carbon black. Finally, the particles were naturally cooled to room temperature to obtain lithium manganese iron phosphate active material with a particle size D50 of 2 μm. Figure 1 As shown, the active material of lithium manganese iron phosphate includes modified lithium manganese iron phosphate, conductive carbon black, CNTs, and a porous carbon network structure.

[0065] Preparation of soft-pack lithium-ion batteries: The mass ratio of lithium manganese iron phosphate active material, polyvinylidene fluoride (PVDF), and dispersant is 97.8:2.0:0.2. Solvent is added, and the mixture is stirred and dispersed evenly in one step to obtain a positive electrode slurry. The positive electrode slurry is coated on a current collector aluminum foil, dried, rolled, and die-cut to obtain a positive electrode sheet. This positive electrode sheet is then assembled with a commercial negative electrode sheet and a commercial PP separator, and injected with commercial lithium manganese iron phosphate electrolyte to obtain a soft-pack lithium-ion battery.

[0066] Test case Test instruments and parameters: Mark an initial interval length of 100 cm (denoted as C0) on the coated positive electrode sheet. When the electrode sheet compaction density reaches 2.35 g / cm³, measure the actual length of this interval (denoted as C1). The calculation formula is: (C1-C0) / C0*100%.

[0067] Initial Coulombic Efficiency: The assembled soft-pack lithium-ion battery was charged and discharged using the Blue Electric testing system. The charging and discharging conditions were as follows: at 45±0.5℃, it was charged at a constant current of 0.1C to a cutoff voltage of 3.2V, and the capacity C0 was recorded. After resting for 30 minutes, it was charged at a constant current of 0.2C to a cutoff voltage of 3.4V, and the capacity C1 was recorded. The temperature was adjusted, and at 25±0.5℃, it was charged at a constant current of 0.5C to a cutoff voltage of 3.65V, followed by constant voltage charging to a cutoff current of 0.05C, and the capacity C2 was recorded. After resting for 30 minutes, it was discharged at a constant current of 0.5C to a cutoff voltage of 2.50V, and the capacity C3 was recorded. The calculation formula was: Initial Coulombic Efficiency = C3 / (C0+C1+C2)*100%.

[0068] 3C rate discharge capacity retention: The assembled soft-pack lithium-ion batteries were subjected to charge-discharge tests using the Blue Electric testing system. The charge-discharge conditions were as follows: at 25±0.5℃, constant current charging at 0.33C to a cutoff voltage of 3.65V, followed by constant voltage charging to a cutoff current of 0.05C, and the capacity C4 was recorded; after resting for 30 minutes, constant current discharge at 0.33C to a cutoff voltage of 2.50V was recorded, and the capacity C5 was recorded; after resting for 30 minutes, constant current charging at 0.33C to a cutoff voltage of 3.65V was recorded, followed by constant voltage charging to a cutoff current of 0.05C, and the capacity C6 was recorded; after resting for 30 minutes, constant current discharge at 3C to a cutoff voltage of 2.50V was recorded, and the capacity C7 was recorded. Calculation method: 3C rate discharge capacity retention = C7 / C5*100%.

[0069] Capacity retention rate after 1000cls at 25℃@1C: The assembled soft-pack lithium-ion battery was charged and discharged using the Blue Electric test system. The charging and discharging conditions were as follows: under 25±0.5℃, 1C constant current charging was performed to the cutoff voltage of 3.65V, followed by constant voltage charging to the cutoff current of 0.05C, and the capacity C8 was recorded; after resting for 30min, 1C constant current discharging was performed to the cutoff voltage of 2.50V, and the capacity C9 was recorded; then 1000 charge and discharge cycles were repeated to obtain the discharge capacity Cn; calculation method: capacity retention rate after 1000cls at 25℃@1C = Cn / C9*100%.

[0070] Table 1 Electrical Performance Tests of Pouch Lithium-ion Batteries

Claims

1. A lithium manganese iron phosphate active material, characterized in that, The lithium manganese iron phosphate active material comprises several modified lithium manganese iron phosphate particles, conductive carbon black particles, carbon nanotubes, and a porous carbon network; the mass ratio of the modified lithium manganese iron phosphate particles, conductive carbon black, carbon nanotubes, and porous carbon network is (95.8~98.9):(0.2~0.7):(0.1~0.5):(0.5~3.0). The modified lithium manganese iron phosphate particles consist of a lithium manganese iron phosphate core doped with metal element M and sequentially coated with a fast ion conductor coating layer and a carbon coating layer. The modified lithium manganese iron phosphate particles, conductive carbon black particles, carbon nanotubes, and porous carbon networks are assembled into spherical or near-spherical secondary particles; in the secondary particles, the porous carbon network is cross-linked on the outer surface of each modified lithium manganese iron phosphate particle; the conductive carbon black particles and carbon nanotubes are dispersed around the modified lithium manganese iron phosphate particles or embedded in the porous carbon network.

2. The lithium manganese iron phosphate active material according to claim 1, characterized in that, In the lithium iron phosphate core, in molar ratio, Mn: Fe=(0.0001~0.9999):(0.9999~0.0001); And / or, the particle size of a single modified lithium manganese iron phosphate particle is 20 nm to 300 nm; And / or, in the modified lithium manganese iron phosphate particles, the molar ratio of lithium manganese iron phosphate, metal element M, and fast ion conductor is (97~99.997):(1~0.001):(1~0.001):(1~0.001). And / or, the mass ratio of the fast ion conductor coating layer to the carbon coating layer is (0.5~1.0):(1.0~2.0); And / or, the thickness of the fast ion conductor coating layer is 1 nm to 9 nm; And / or, the thickness of the carbon coating layer is 1 nm to 15 nm; And / or, in the lithium manganese iron phosphate core doped with metal element M, the doped metal element is selected from at least one of calcium, molybdenum, zinc, nickel, tungsten, magnesium, titanium, chromium, zirconium, vanadium and cobalt; And / or, the fast ion conductor is selected from at least one of the LISICON system, NASICON structure, perovskite structure and Li3N derivatives; And / or, the thickness of the porous carbon network is 10 nm to 50 nm; And / or, the size of the secondary particles is 0.5μm~5μm.

3. The method for preparing the lithium manganese iron phosphate active material according to claim 1 or 2, characterized in that, Including the following steps: (1) The phosphorus source, manganese source, iron source, lithium source, doped metal element, fast ion conductor and carbon source II are wet ground for 1~5h to obtain a mixture, and then carbon source I is added and stirred to obtain the first slurry; (2) The conductive carbon black, carbon nanotubes and dispersant are mixed to prepare the second slurry; (3) The first slurry and the second slurry are heated and reacted, and then alternately sprayed and granulated. The mixture is heated and reacted in an inert atmosphere to obtain the lithium manganese iron phosphate active material.

4. The preparation method according to claim 3, characterized in that, The molar ratio of the phosphorus source, manganese source, iron source, and lithium source is Li:(Mn+Fe):P=(0.99~1.05):(0.98~1.01):1, wherein the molar ratio of Mn to Fe is (0.0001~0.9999) :(0.9999~0.0001). And / or, the phosphorus source is selected from at least one of phosphoric acid, ammonium dihydrogen phosphate, lithium dihydrogen phosphate, diammonium hydrogen phosphate, and ammonium phosphate; And / or, the manganese source is selected from at least one of manganese sulfate, manganese chloride, manganese acetate and manganese oxalate; And / or, the iron source is selected from at least one of ferrous oxide, magnetite, ferric oxide, ferric nitrate, ferrous nitrate and ferrous oxalate; And / or, the lithium source is selected from at least one of lithium carbonate, lithium hydroxide and lithium dihydrogen phosphate; And / or, in (3), the conditions for the heating reaction include: temperature 600~800℃, time 3~15h; heating rate 2~20℃ / min; And / or, in (3), the spraying conditions include: a temperature of 100–400°C; And / or, the imidazole compound is selected from at least one of 2-methylimidazole, 1,2-dimethylimidazole, 1H-1,2,3-triazole, or a derivative thereof; The carbon nanotubes are one or more of single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes.

5. The preparation method according to claim 3, characterized in that, The carbon source I is carbonized by heating and reaction to form the carbon coating layer; The carbon source II is carbonized by heating to form the porous carbon network.

6. The preparation method according to claim 3, characterized in that, The carbon source I is selected from at least one of glucose, sucrose, polyethylene glycol, or polymethyl methacrylate; and / or, the mass fraction of the carbon source I in the mixture is 1% to 5%; And / or, the carbon source II is selected from imidazole compounds; and / or, the molar ratio of the carbon source II to Mn in the manganese source is (1-5):(50-90).

7. The positive electrode of a lithium-ion battery, characterized in that... ， The active material contains lithium manganese iron phosphate as described in claim 1 or 2, or contains lithium manganese iron phosphate active material prepared by any one of claims 3-6.

8. A method for preparing a positive electrode for a lithium-ion battery, characterized in that, The active material of lithium manganese iron phosphate as described in any one of claims 1-4 or the active material of lithium manganese iron phosphate prepared by the method described in any one of claims 5-13, a binder, a dispersant and a solvent are mixed to prepare a positive electrode slurry. The slurry is coated on a current collector aluminum foil, dried, rolled and die-cut to obtain a positive electrode sheet.

9. The method for preparing a lithium-ion battery cathode according to claim 8, characterized in that, The mass ratio of the lithium manganese iron phosphate active material, binder, and dispersant is (97.1~98.9):(1.0~2.5):(0.1~0.4); And / or, the adhesive is polyvinylidene fluoride (PVDF), wherein the PVDF is selected from one or more of HSV900, 5130, and 6020; And / or, the dispersant is selected from one or more of polyvinylpyrrolidone, sodium dodecyl sulfate and fatty alcohol polyoxyethylene ether, Tween-60, and dispersant KD-1; And / or, the solvent is N Methylpyrrolidone; And / or, the compaction density of the positive electrode sheet is 2.0~2.6 g / cm³. 3 .

10. A lithium-ion battery, characterized in that, The active material contains lithium manganese iron phosphate as described in claim 1 or 2, or contains lithium manganese iron phosphate active material prepared by any one of claims 3-6, or contains the positive electrode as described in claim 7.

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