Lithium iron phosphate material, preparation method thereof and lithium ion battery

Abstract

The invention relates to a lithium iron phosphate material, a preparation method thereof and a lithium ion battery, a ternary grading system is constructed through mutual matching of particles with different particle sizes, Fe site and PO4 site co-doping is carried out on large particles and medium particles, the electronic conductivity and the lithium ion transmission efficiency of the lithium iron phosphate material are improved, Li site doping is carried out on small particles, and the lithium ion transmission efficiency of the lithium iron phosphate material is improved. The continuous side reaction of the lithium iron phosphate material in the long-term cycle process is inhibited, the compaction density of the lithium iron phosphate material is effectively improved, and the capacity exertion and cycle stability are optimized.

Description

Technical Field

[0001] This disclosure relates to a lithium iron phosphate material and its preparation method, and a lithium-ion battery. Background Technology

[0002] Lithium iron phosphate (LiFePO4) is a lithium-ion battery cathode material with an olivine structure. Due to its high theoretical capacity, excellent thermal stability, and low cost, it is widely used in power batteries and energy storage. To meet market demands for longer driving range and higher energy density, improving the volumetric energy density of lithium-ion batteries has become a core issue in the industry. Increasing the compaction density of the cathode material is one effective way to achieve this goal. Traditional preparation processes mainly synthesize high-compact-density lithium iron phosphate materials through solid-state methods, liquid-state methods, and carbothermal reduction methods. However, the compaction density of lithium iron phosphate materials prepared by existing processes still has room for improvement, and their cycle life is relatively poor. Summary of the Invention

[0003] The purpose of this disclosure is to provide a lithium iron phosphate material with improved electrical performance, a method for preparing the same, and a lithium-ion battery.

[0004] To achieve the above objectives, the first aspect of this disclosure provides a method for preparing lithium iron phosphate material, the method comprising: The first mixture containing a phosphorus iron source, a first lithium source, a first dopant and a first carbon source is wet-milled, dried and then sintered to obtain a first precursor. The second mixture containing an iron source, a phosphorus source, a second lithium source, a second dopant, and a second carbon source is wet-milled, dried, and then subjected to a second sintering to obtain a second precursor. The third mixture containing the first precursor and the first carbon source composition is wet-milled to obtain a first slurry with a first particle size; The fourth mixture containing the first precursor and the second carbon source composition is wet-milled to obtain a second slurry with a second particle size. The fifth mixture containing the second precursor and the third carbon source composition is wet-milled to obtain a third slurry with a third particle size. The sixth mixture containing the first slurry, the second slurry, and the third slurry is dried and then subjected to a third sintering to obtain lithium iron phosphate material; The first dopant includes Fe-site dopant and PO4-site dopant, the second dopant includes Li-site dopant, and the first particle size, the second particle size, and the third particle size decrease sequentially.

[0005] Optionally, the Fe-site dopant includes at least one of NiO, TiO2, Mn2O5, NH4VO3, CeO2, and Cr2O3; Optionally, based on the total weight of the lithium iron phosphate material, the content of Fe-doped elements in the lithium iron phosphate material is 0.4~0.6 wt%.

[0006] Optionally, the PO4-site dopant includes at least one of SiO2, LiF, H3BO3, and MgO; Optionally, based on the total weight of the lithium iron phosphate material, the content of the PO4 doping element in the lithium iron phosphate material is 0.1~0.3wt%.

[0007] Optionally, the Li-site dopant includes at least one of Na2CO3, K2CO3, ZrO2, Nb2O5, and NH4VO3; Optionally, based on the total weight of the lithium iron phosphate material, the content of the Li-site doping element in the lithium iron phosphate material is 0.1~0.5wt%.

[0008] Optionally, the iron phosphorus source includes at least one of iron phosphate, hydroxyferric phosphate, and ferrous phosphate; Optionally, the iron-phosphorus molar ratio of the phosphorus-iron source is 0.96~0.97, and the BET specific surface area is 8~16 m². 2 / g, with a primary particle size of 80~150nm.

[0009] Optionally, the iron source includes iron oxide and / or ferrous oxalate.

[0010] Optionally, the first lithium source includes at least one of lithium carbonate, lithium hydroxide, and lithium oxalate; The second lithium source includes at least one of lithium hydroxide, lithium carbonate, lithium dihydrogen phosphate, lithium phosphate, lithium hydrogen phosphate, and lithium oxalate; The phosphorus source includes at least one of phosphoric acid, phosphorus pentoxide, ammonium dihydrogen phosphate, lithium dihydrogen phosphate, lithium phosphate, and lithium hydrogen phosphate.

[0011] Optionally, the first carbon source and the second carbon source each independently include at least one of glucose, sucrose, polyethylene glycol, citric acid, polyvinylpyrrolidone, and starch; Optionally, based on the total weight of the first precursor, the carbon content of the first precursor is 0.1~0.5 wt%; Optionally, based on the total weight of the second precursor, the carbon content of the second precursor is 0.1 to 0.5 wt%.

[0012] Optionally, the first carbon source composition, the second carbon source composition, and the third carbon source composition each independently include a third carbon source, a fourth carbon source, and a fifth carbon source; the third carbon source includes at least one of graphene, carbon nanotubes, and carbon nanoparticles; the fourth carbon source includes at least one of epoxy resin, acrylic resin, cellulose acetate resin, polyurethane resin, phenolic resin, and polyamide resin; and the fifth carbon source includes at least one of glucose, sucrose, polyethylene glycol, citric acid, and starch. Optionally, in the third mixture, the weight ratio of the first precursor, the third carbon source, the fourth carbon source, and the fifth carbon source is 1000:(0.1~0.5):(5~30):(20~50). Optionally, in the fourth mixture, the weight ratio of the first precursor, the third carbon source, the fourth carbon source, and the fifth carbon source is 1000:(0.1~0.5):(5~30):(20~50). Optionally, in the fifth mixture, the weight ratio of the second precursor, the third carbon source, the fourth carbon source and the fifth carbon source is 1000:(0.1~0.5):(5~20):(20~50).

[0013] Optionally, the first particle size is 1.0~2.0 μm, the second particle size is 0.3~1.0 μm, and the third particle size is 0.1~0.4 μm; and / or, In the sixth mixture, the weight ratio of the first slurry, the second slurry, and the third slurry is 1:(0.3~1):(0.3~0.8).

[0014] Optionally, the conditions for the first sintering include: a temperature of 790~810℃ and a time of 6~10h; The conditions for the second sintering include: a temperature of 500~700℃ and a time of 3~7h; The conditions for the third sintering include: a temperature of 700~760℃ and a time of 4~8h.

[0015] A second aspect of this disclosure provides a lithium iron phosphate material prepared by the method described in the first aspect of this disclosure.

[0016] In a third aspect, this disclosure provides a lithium-ion battery comprising the lithium iron phosphate material described in the second aspect of this disclosure.

[0017] Through the above technical solution, this disclosure constructs a ternary gradation system by mixing particles of different sizes. Large and medium-sized particles are co-doped at Fe and PO4 sites to improve the electronic conductivity and lithium-ion transport efficiency of the lithium iron phosphate material, while small particles are doped at Li sites to suppress continuous side reactions during long-term cycling. This effectively improves the compaction density of the lithium iron phosphate material and optimizes its capacity utilization and cycle stability. Lithium-ion batteries prepared using this lithium iron phosphate material exhibit significantly improved electrical performance and have broad market application prospects.

[0018] Other features and advantages of this disclosure will be described in detail in the following detailed description section. Attached Figure Description

[0019] The accompanying drawings are provided to further illustrate the present disclosure and form part of the specification. They are used together with the following detailed description to explain the present disclosure, but do not constitute a limitation thereof. In the drawings: Figure 1 This is an electron microscope image of the lithium iron phosphate material prepared in Example 1. Detailed Implementation

[0020] The specific embodiments of this disclosure will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit this disclosure.

[0021] The first aspect of this disclosure provides a method for preparing lithium iron phosphate material, the method comprising the following steps S1 to S6: S1. The first mixture containing phosphorus iron source, first lithium source, first dopant and first carbon source is wet-milled and dried, and then subjected to first sintering to obtain the first precursor; S2. The second mixture containing iron source, phosphorus source, second lithium source, second dopant and second carbon source is wet-milled and dried, and then subjected to second sintering to obtain the second precursor. S3. The third mixture containing the first precursor and the first carbon source composition is wet-milled to obtain a first slurry with a first particle size; S4. The fourth mixture containing the first precursor and the second carbon source composition is wet-milled to obtain a second slurry with a second particle size. S5. The fifth mixture containing the second precursor and the third carbon source composition is wet-milled to obtain a third slurry with a third particle size. S6. The sixth mixture containing the first slurry, the second slurry and the third slurry is dried, and then subjected to a third sintering to obtain lithium iron phosphate material; The first dopant includes Fe-site dopant and PO4-site dopant, the second dopant includes Li-site dopant, and the first particle size, the second particle size, and the third particle size decrease sequentially.

[0022] In step S1, a first precursor is prepared using a first mixture containing a phosphorus iron source, a first lithium source, a first dopant, and a first carbon source. This first precursor is used in subsequent steps to prepare a first slurry with the largest particle size (i.e., the first particle size) and a second slurry with a medium particle size (i.e., the second particle size). Co-doping of large and medium-sized particles, which occupy the main volume in lithium iron phosphate materials but have long ion / electron transport paths, with Fe and PO4 sites is beneficial for widening lattice channels, introducing lattice vacancy defects, improving electronic conductivity and lithium-ion transport efficiency, enhancing the electrochemical activity of lithium iron phosphate materials, and optimizing their capacity utilization.

[0023] In one specific embodiment, the iron phosphorus source may include iron phosphate (FePO4) or ferric hydroxyphosphate (Fe2+). x At least one of (PO4)(OH)·nH2O, x being 1.4~1.5, and ferrous phosphate (Fe3(PO4)2), preferably including at least one of ferric phosphate and hydroxyferric phosphate. Further, the iron-to-phosphorus molar ratio (Fe / P) of the phosphorus source can be 0.96~0.97, and the BET specific surface area can be 8~16 m². 2 / g, the primary particle size can be 80~150nm. Using the above-mentioned phosphorus iron source as raw material is beneficial for preparing particles with regular morphology and achieving precise control of particle size, thereby improving the compaction density of lithium iron phosphate materials.

[0024] The first lithium source may include at least one of lithium carbonate (Li2CO3), lithium hydroxide (LiOH), and lithium oxalate (Li2C2O4), preferably at least one of lithium carbonate and lithium hydroxide. The first lithium source preferably has a high purity (e.g., above 99.5%) to improve the quality of the lithium iron phosphate material. The amount of the first lithium source can be adjusted within a certain range. Specifically, the molar ratio of the first lithium source (calculated as Li) to the iron phosphate source (calculated as Fe) can be 1.0~1.1, preferably 1.01~1.06, and the resulting first precursor's iron phosphate molar ratio (P / Fe / Li) can be 1:(0.960~0.970):(0.970~1.028).

[0025] The Fe site dopant refers to one or more cations that can occupy the original Fe in the lithium iron phosphate lattice through substitution during the sintering process. 2+The metal compound at the ionic site can be at least one of Ni, Ti, Mn, V, Ce, and Cr (i.e., Fe-site dopant elements). In one specific embodiment, the Fe-site dopant can include at least one of NiO, TiO2, Mn2O5, NH4VO3, CeO2, and Cr2O3, preferably at least one of TiO2 and NH4VO3. The amount of Fe-site dopant can be adjusted within a certain range to ensure that the lithium iron phosphate material has a suitable Fe-site dopant element content. Specifically, based on the total weight of the lithium iron phosphate material, the Fe-site dopant element content of the lithium iron phosphate material is 0.4~0.6 wt%.

[0026] The PO4-site dopant refers to one or more of its anionic groups or their decomposition products that can replace the original phosphate (PO4) in lithium iron phosphate through substitution during the sintering process. 3- Compounds containing a portion of the anionic framework (such as a central P atom or a coordinating O atom) can be at least one of compounds containing Si, F, B, and Mg (i.e., PO4-site dopant). In one specific embodiment, the PO4-site dopant may include at least one of SiO2, LiF, H3BO3, and MgO, preferably at least one of H3BO3 and MgO. The amount of PO4-site dopant can be adjusted within a certain range to ensure that the lithium iron phosphate material has a suitable PO4-site dopant content. Specifically, based on the total weight of the lithium iron phosphate material, the PO4-site dopant content of the lithium iron phosphate material is 0.1~0.3 wt%.

[0027] The first carbon source may include at least one selected from glucose, sucrose, polyethylene glycol (PEG), citric acid, polyvinylpyrrolidone (PVP), and starch, preferably at least one selected from glucose, sucrose, polyethylene glycol, and citric acid. Using a lower amount of the first carbon source is beneficial for obtaining a high-quality precursor with higher crystallinity, more regular morphology, and more concentrated size distribution. The resulting first precursor has a lower carbon content; specifically, based on the total weight of the first precursor, the carbon content of the first precursor can be 0.1~0.5 wt%, preferably 0.2~0.4 wt%.

[0028] The liquid medium used in wet grinding can include at least one of deionized water and anhydrous ethanol. Specifically, the conditions for wet grinding can include: a temperature of 10~45℃, a grinding particle size D50 of 0.3~0.7μm, and a solid content of 30~50wt%.

[0029] Drying can be spray drying. Specifically, drying conditions may include: inlet temperature of 180~240℃, outlet temperature of 80~140℃, particle size D50 of 20~60μm, and moisture content ≤1.5wt%.

[0030] The first sintering can be carried out in the presence of a protective gas, which may include at least one of nitrogen and argon. Specifically, the conditions for the first sintering may include a temperature of 790~810℃ and a time of 6~10h. Further, after the first sintering, first precursor particles can be obtained by an air jet milling step, and the particle size D50 of the first precursor particles may be 1.8~2.3μm.

[0031] In step S2, a second precursor is prepared using a second mixture containing an iron source, a phosphorus source, a second lithium source, a second dopant, and a second carbon source. This second precursor is used in subsequent steps to prepare a third slurry with the smallest particle size (i.e., the third particle size). Li-site doping of the smallest particles in lithium iron phosphate materials, which have the largest specific surface area and are most prone to side reactions in electrochemical reactions, can enhance their mechanical strength and structural stability, suppress pulverization and continuous side reactions during long-term cycling, and improve the cycle life of lithium iron phosphate materials.

[0032] The raw materials for preparing the second precursor are different from those for preparing the first precursor. In one specific embodiment, the iron source may include at least one of iron oxide (Fe2O3) and ferrous oxalate (FeC2O4). Using the above-mentioned iron source as a raw material is beneficial for preparing uniform small particles and improving the accuracy of the small particle ratio in lithium iron phosphate materials.

[0033] The phosphorus source may include at least one of phosphoric acid (H3PO4), phosphorus pentoxide (P2O5), ammonium dihydrogen phosphate (NH4H2PO4), lithium dihydrogen phosphate (LiH2PO4), lithium phosphate (Li3PO4), and lithium hydrogen phosphate (Li2HPO4), preferably at least one of ammonium dihydrogen phosphate, lithium dihydrogen phosphate, and phosphoric acid. The second lithium source may include at least one of lithium hydroxide, lithium carbonate, lithium dihydrogen phosphate, lithium phosphate, lithium hydrogen phosphate, and lithium oxalate, preferably at least one of lithium hydroxide, lithium carbonate, and lithium dihydrogen phosphate. The amounts of the phosphorus source and the second lithium source can be adjusted within a certain range to ensure that the resulting second precursor has a suitable phosphorus-iron-lithium molar ratio. Specifically, the phosphorus-iron-lithium molar ratio (P / Fe / Li) of the second precursor can be (1.01~1.03):1:(1.01~1.03).

[0034] The Li-site dopant refers to one or more cations that can occupy the original Li in the lithium iron phosphate lattice through substitution during the sintering process. +The metal compound at the ionic site can be at least one of Na, K, Zr, Nb, and V (i.e., Li-site dopant elements). In one specific embodiment, the Li-site dopant can include at least one of Na₂CO₃, K₂CO₃, ZrO₂, Nb₂O₅, and NH₄VO₃, preferably at least one of Nb₂O₅ and NH₄VO₃. The amount of Li-site dopant can be adjusted within a certain range to give the lithium iron phosphate material a suitable Li-site dopant element content. Specifically, based on the total weight of the lithium iron phosphate material, the Li-site dopant element content of the lithium iron phosphate material can be 0.1~0.5wt%, preferably 0.1~0.3wt%.

[0035] The second carbon source may include at least one selected from glucose, sucrose, polyethylene glycol, citric acid, polyvinylpyrrolidone, and starch; preferably, it includes at least one selected from glucose, sucrose, polyethylene glycol, and citric acid. The amount of the second carbon source can be adjusted within a low range to give the resulting second precursor a low carbon content. Specifically, based on the total weight of the second precursor, the carbon content of the second precursor can be 0.1 to 0.5 wt%, preferably 0.2 to 0.4 wt%.

[0036] The liquid medium used in wet grinding can include at least one of deionized water and anhydrous ethanol. Specifically, the conditions for wet grinding can include: a temperature of 10~40℃, a rotation speed of 500~1000 rpm, a solid content of 40~60 wt%, and a time of 0.5~1 h.

[0037] Drying can be spray drying. Specifically, drying conditions may include: inlet temperature of 180~240℃, outlet temperature of 80~140℃, particle size D50 of 20~60μm, and moisture content ≤1.5wt%.

[0038] The second sintering can be carried out in the presence of a protective gas, which may include at least one of nitrogen and argon. Specifically, the conditions for the second sintering may include a temperature of 500~700℃ and a time of 3~7h. Further, after the second sintering, second precursor particles can be obtained by an air jet milling step, and the particle size D50 of the second precursor particles may be 0.3~0.5μm.

[0039] In steps S3 to S5, the two precursors obtained in the above steps are wet-milled with a carbon source composition to prepare three slurries with specific target particle sizes.

[0040] In one specific embodiment, the first carbon source composition, the second carbon source composition, and the third carbon source composition may each independently include a third carbon source, a fourth carbon source, and a fifth carbon source. The third carbon source may include at least one of graphene, carbon nanotubes, and carbon nanoparticles, preferably at least one of graphene and carbon nanotubes, wherein the graphene may be an aqueous solution of redox graphene with a concentration of 5-15 mg / mL. The fourth carbon source may include at least one of epoxy resin, acrylic resin, cellulose acetate resin, polyurethane resin, phenolic resin, and polyamide resin, preferably at least one of epoxy resin and polyurethane resin, wherein the above resins may be soluble in organic solvents (such as ethanol, dimethyl sulfoxide, N-methylpyrrolidone, etc.). The fifth carbon source may include at least one of glucose, sucrose, polyethylene glycol, citric acid, and starch, preferably at least one of glucose, polyethylene glycol, and citric acid. By adding the above-mentioned third and fourth carbon sources during slurry preparation, it is beneficial to form a dense carbon layer during secondary sintering, further suppressing side reactions of lithium iron phosphate materials and improving their cycle performance.

[0041] The amount of each carbon source composition (i.e., the first carbon source composition to the third carbon source composition) can be higher than that used when preparing the precursor. This not only forms a complete and dense carbon coating layer during final sintering, but also helps to maintain particle size distribution, improve compaction density and capacity, construct a multi-dimensional composite conductive network, and reduce internal resistance.

[0042] Specifically, in the third mixture, the weight ratio of the first precursor, the third carbon source, the fourth carbon source and the fifth carbon source can be 1000:(0.1~0.5):(5~30):(20~50), preferably 1000:(0.1~0.2):(10~20):(30~40).

[0043] In the fourth mixture, the weight ratio of the first precursor, the third carbon source, the fourth carbon source and the fifth carbon source can be 1000:(0.1~0.5):(5~30):(20~50), preferably 1000:(0.1~0.2):(10~20):(30~40).

[0044] In the fifth mixture, the weight ratio of the second precursor, the third carbon source, the fourth carbon source and the fifth carbon source can be 1000:(0.1~0.5):(5~30):(20~50), preferably 1000:(0.1~0.2):(10~20):(30~40).

[0045] The liquid medium used in wet grinding can include at least one of deionized water and anhydrous ethanol. Specifically, the conditions for wet grinding can include a temperature of 10~45℃ and a solid content of 30~50wt%.

[0046] The particle size of the solid phase material in the first, second, third, and fourth slurries decreases sequentially. In one specific embodiment, the first particle size can be 1.0~2.0μm, preferably 1.1~1.5μm; the second particle size can be 0.3~1.0μm, preferably 0.4~0.7μm; and the third particle size can be 0.1~0.4μm, preferably 0.1~0.25μm.

[0047] In step S6, the ratio of the first slurry, the second slurry, and the third slurry can be adjusted within a certain range. In one specific embodiment, the weight ratio of the first slurry, the second slurry, and the third slurry in the sixth mixture can be 1:(0.3~1):(0.3~0.8), preferably 1:(0.4~0.8):(0.4~0.6. The above slurry ratio is beneficial for achieving continuous gradation from small particles to large particles, improving the compaction density of lithium iron phosphate material, effectively controlling particle ratio error, widening the process window, and improving mass production yield.

[0048] Drying can be spray drying. Specifically, drying conditions may include: inlet temperature of 180~240℃, outlet temperature of 80~140℃, particle size D50 of 20~60μm, and moisture content ≤1.5wt%.

[0049] The third sintering can be carried out in the presence of a protective gas, which may include at least one of nitrogen and argon. Specifically, the conditions for the third sintering may include a temperature of 700~760℃ and a time of 4~8h. Further, after the third sintering, lithium iron phosphate material can be obtained by an air jet milling step, and the particle size D50 of the lithium iron phosphate material can be 0.8~1.2μm.

[0050] A second aspect of this disclosure provides lithium iron phosphate materials prepared by the method of the first aspect of this disclosure.

[0051] This lithium iron phosphate material comprises three particle sizes: large, medium, and small, exhibiting a relatively continuous particle size distribution. Specifically, particles with a diameter greater than 700 nm account for 1-10%, particles with a diameter between 250 and 700 nm account for 18-30%, and particles with a diameter less than 250 nm account for 60-80%. It also possesses high compaction density and specific capacity; specifically, the compaction density of this lithium iron phosphate material is 2.64-2.70 g / cm³. 3 .

[0052] Lithium-ion batteries made using this lithium iron phosphate material exhibit significantly improved electrical performance and have broad market application prospects.

[0053] A third aspect of this disclosure provides a lithium-ion battery, including the lithium iron phosphate material of the second aspect of this disclosure.

[0054] This disclosure does not impose any special limitations on the specific structure, packaging form, and manufacturing method of lithium-ion batteries. For example, a lithium-ion battery may include a positive electrode, a negative electrode, an electrolyte, and a separator. The positive electrode may include a positive current collector and a positive electrode material, and the positive electrode material may include the lithium iron phosphate material of the second aspect of this disclosure. The negative electrode may include a negative current collector and a negative electrode material (such as graphite). The electrolyte may be a carbonate-based electrolyte, and the separator may be a polyolefin separator. The packaging form of the lithium-ion battery may be a pouch battery, a square aluminum-cased battery, a blade aluminum-cased battery, etc. This lithium-ion battery has the same beneficial effects as described above, and will not be repeated here.

[0055] The present disclosure is further described in detail below through examples, but is not intended to limit the present disclosure. Unless otherwise specified, all raw materials used in the examples are commercially available.

[0056] The microstructure of lithium iron phosphate materials was examined using a Zeiss Sigma 360 scanning electron microscope under the following conditions: voltage 10 kV, magnification 20 kV. Particle size distribution was statistically analyzed using a Nano Measurer; carbon content and metal element content were determined using a carbon-sulfur analyzer.

[0057] Example 1 S1. 4000g of ferric phosphate (Fe / P = 0.965, BET specific surface area = 12m²) 2 1022.64g of lithium carbonate (P / Fe / Li ratio of 1:0.965:0.999, carbon content of 0.3wt%, and deionized water) were mixed in a ball mill to form a dispersion. The dispersion was then transferred to a sand mill and ground at 20℃, with a particle size D50 of 0.5μm and a solid content of 40wt%. The resulting slurry was spray-dried, with the inlet air temperature controlled at 210℃ and the outlet air temperature at 110℃, resulting in a granulation D50 of 40μm and a moisture content ≤1.5wt%. The resulting precursor powder was calcined at 800℃ for 8 hours under a nitrogen atmosphere. The sintered material was then pulverized to obtain the first precursor (P / Fe / Li ratio of 1:0.965:0.999, carbon content of 0.3wt%, and D50 of 2.0μm).

[0058] S2. Mix 2000g of iron oxide and 2655.31g of lithium dihydrogen phosphate, and simultaneously add 169g of sucrose, 8.1g of niobium pentoxide and 4655g of deionized water in a ball mill to form a dispersion. Grind at 800rpm for 40min at 20℃, dry, and calcine the obtained precursor powder at 600℃ for 6h under nitrogen atmosphere. Crush the sintered material to obtain the second precursor (P / Fe / Li ratio of 1.02:1:1.02, carbon content of 0.3wt%, D50 of 0.4μm).

[0059] S3. Mix 4000g of the first precursor, 100mL of redox graphene aqueous solution (concentration of 5mg / mL), 60g of epoxy resin dissolved in 500mL of ethanol, 120g of polyethylene glycol and 6000g of deionized water to form a dispersion, grind at 20℃, and control the solid content at 40wt% to obtain the first slurry with a particle size of 1.3μm.

[0060] S4. Mix 4000g of the first precursor, 100mL of redox graphene aqueous solution (concentration of 5mg / mL), 60g of epoxy resin dissolved in 500mL of ethanol, 120g of polyethylene glycol and 6000g of deionized water to form a dispersion, grind at 20℃, and control the solid content at 40wt% to obtain a second slurry with a particle size of 0.5μm.

[0061] S5. Mix 4000g of the second precursor, 100mL of redox graphene aqueous solution (concentration of 5mg / mL), 60g of epoxy resin dissolved in 500mL of ethanol, 120g of polyethylene glycol and 6000g of deionized water to form a dispersion, grind at 20℃, and control the solid content at 40wt% to obtain a third slurry with a particle size of 0.18μm.

[0062] S6. Mix the first slurry, second slurry, and third slurry at a weight ratio of 5:3:2 for 1 hour. Spray dry the resulting slurry, controlling the inlet air temperature at 210℃ and the outlet air temperature at 110℃, achieving a particle size D50 of 40μm and a moisture content ≤1.5wt%. Calcinate the resulting powder at 730℃ for 6 hours under a nitrogen atmosphere. Crush the sintered material to obtain lithium iron phosphate material with a particle size D50 of 1.0μm, a carbon content of 1.3wt%, a titanium content of 0.5wt%, a magnesium content of 0.2wt%, and a niobium content of 0.2wt%. See electron micrographs for details. Figure 1 As shown, the lithium iron phosphate material has a dense packing structure formed by triple-sized particles, of which 3% have a particle size distribution of 700 nm or more, 24% have a particle size distribution of 250-700 nm, and 73% have a particle size distribution of less than 200 nm.

[0063] Example 2 S1. 4000g of ferric phosphate (Fe / P = 0.960, BET specific surface area = 8m²) 2 1032.48 g of lithium carbonate (P / Fe / Li ratio 1:0.960:1.018, carbon content 0.2 wt%, and deionized water were mixed in a ball mill to form a dispersion. The dispersion was then transferred to a sand mill and ground at 30°C, with a particle size D50 of 0.6 μm and a solid content of 50 wt%. The resulting slurry was spray-dried, with the inlet air temperature controlled at 240°C and the outlet air temperature at 140°C, resulting in a particle size D50 of 60 μm and a moisture content ≤1.5 wt%. The resulting precursor powder was calcined at 790°C for 10 h under a nitrogen atmosphere. The sintered material was then pulverized to obtain the first precursor (P / Fe / Li ratio 1:0.960:1.018, carbon content 0.2 wt%, D50 2.3 μm).

[0064] S2. Mix 2000g of iron oxide and 2681.34g of lithium dihydrogen phosphate, and simultaneously add 169g of sucrose, 10.8g of niobium pentoxide and 4655g of deionized water in a ball mill to form a dispersion. Grind at 1000rpm for 30min at 30℃, dry, and calcine the obtained precursor powder at 700℃ for 5h under nitrogen atmosphere. Crush the sintered material to obtain the second precursor (P / Fe / Li ratio of 1.01:1:1.01, carbon content of 0.2wt%, D50 of 0.5μm).

[0065] S3. Mix 4000g of the first precursor, 100mL of redox graphene aqueous solution (concentration of 5mg / mL), 60g of epoxy resin dissolved in 500mL of ethanol, 120g of glucose and 4000g of deionized water to form a dispersion, grind at 30℃, and control the solid content at 50wt% to obtain the first slurry with a particle size of 1.4μm.

[0066] S4. Mix 4000g of the first precursor, 100mL of redox graphene aqueous solution (concentration of 5mg / mL), 60g of epoxy resin dissolved in 500mL of ethanol, 120g of glucose and 4000g of deionized water to form a dispersion, grind at 30℃, and control the solid content at 50wt% to obtain a second slurry with a particle size of 0.7μm.

[0067] S5. Mix 4000g of the second precursor, 100mL of redox graphene aqueous solution (concentration of 5mg / mL), 60g of epoxy resin dissolved in 500mL of ethanol, 120g of glucose and 4000g of deionized water to form a dispersion, grind at 30℃, and control the solid content at 50wt% to obtain a third slurry with a particle size of 0.25μm.

[0068] S6. Mix the first slurry, second slurry, and third slurry at a weight ratio of 5:2:3 for 1 hour. Spray dry the resulting slurry, controlling the inlet air temperature at 240℃ and the outlet air temperature at 140℃, achieving a granulation D50 of 60μm and a moisture content ≤1.5wt%. Calcinate the resulting powder at 710℃ for 8 hours under a nitrogen atmosphere. Crush the sintered material to obtain lithium iron phosphate material with a particle size D50 of 1.2μm, a carbon content of 1.5wt%, a titanium content of 0.6wt%, a magnesium content of 0.3wt%, and a niobium content of 0.3wt%. Electron micrographs and... Figure 1 Similarly, the number of particles with a diameter greater than 700 nm accounted for 2%, the number of particles with a diameter between 250 and 700 nm accounted for 28%, and the number of particles with a diameter less than 250 nm accounted for 69%.

[0069] Example 3 S1. 4000g of ferric phosphate (Fe / P = 0.970, BET specific surface area = 16m²) 2 A mixture of 993.2 g of lithium carbonate (P / Fe / Li ratio of 1:0.970:0.980, carbon content of 0.4 wt%, and deionized water was added to a ball mill to form a dispersion. The dispersion was then transferred to a sand mill and ground at 10 °C, with a particle size D50 of 0.4 μm and a solid content of 30% wt%. The resulting slurry was spray-dried, with the inlet air temperature controlled at 180 °C and the outlet air temperature at 80 °C, resulting in a granulation D50 of 20 μm and a moisture content ≤1.5 wt%. The resulting precursor powder was calcined at 810 °C for 6 h under a nitrogen atmosphere. The sintered material was then pulverized to obtain the first precursor (P / Fe / Li ratio of 1:0.970:0.980, carbon content of 0.4 wt%, D50 of 1.8 μm).

[0070] S2. Mix 2000g of iron oxide and 2629.28g of lithium dihydrogen phosphate, and simultaneously add 189g of sucrose, 5.4g of niobium pentoxide and 3100g of deionized water in a ball mill to form a dispersion. Grind at 500rpm for 60min at 10℃, dry, and calcine the obtained precursor powder at 500℃ for 7h under nitrogen atmosphere. Crush the sintered material to obtain the second precursor (P / Fe / Li ratio of 1.03:1:1.03, carbon content of 0.4wt%, D50 of 0.3μm).

[0071] S3. Mix 4000g of the first precursor, 100mL of redox graphene aqueous solution (concentration of 5mg / mL), 60g of epoxy resin dissolved in 500mL of ethanol, 140g of citric acid and 9333g of deionized water to form a dispersion, grind at 10℃, and control the solid content at 30wt% to obtain the first slurry with a particle size of 1.1μm.

[0072] S4. Mix 4000g of the first precursor, 100mL of redox graphene aqueous solution (concentration of 5mg / mL), 60g of epoxy resin dissolved in 500mL of ethanol, 140g of citric acid and 9333g of deionized water to form a dispersion, grind at 10℃, and control the solid content at 30wt% to obtain a second slurry with a particle size of 0.4μm.

[0073] S5. Mix 4000g of the second precursor, 100mL of redox graphene aqueous solution (concentration of 5mg / mL), 60g of epoxy resin dissolved in 500mL of ethanol, 140g of citric acid and 9333g of deionized water to form a dispersion, grind at 10℃, and control the solid content at 30wt% to obtain a third slurry with a particle size of 0.1μm.

[0074] S6. Mix the first slurry, second slurry, and third slurry at a weight ratio of 6:2:2 for 1 hour. Spray dry the resulting slurry, controlling the inlet air temperature at 180℃ and the outlet air temperature at 80℃, achieving a granulation D50 of 20μm and a moisture content ≤1.5wt%. Calcinate the resulting powder at 760℃ for 4 hours under a nitrogen atmosphere. Crush the sintered material to obtain lithium iron phosphate material with a particle size D50 of 0.8μm, a carbon content of 1.1wt%, a titanium content of 0.4wt%, a magnesium content of 0.1wt%, and a niobium content of 0.1wt%. Electron micrographs and... Figure 1 Similarly, the number of particles with a diameter greater than 700nm accounted for 6%, the number of particles with a diameter between 250nm and 700nm accounted for 19%, and the number of particles with a diameter less than 250nm accounted for 75%.

[0075] Example 4 The lithium iron phosphate material was prepared according to the method of Example 1, except that in steps S3 to S5, 160g of glucose and 160g of polyethylene glycol were used as carbon sources. In the prepared lithium iron phosphate material, the proportion of particles with a particle size distribution of more than 700nm was 4.5%, the proportion of particles with a particle size distribution of 250~700nm was 22.5%, and the proportion of particles with a particle size distribution of less than 250nm was 73%.

[0076] Example 5 Lithium iron phosphate material was prepared according to the method of Example 1, except that in step S1, the carbon content of the first precursor was 0.5 wt%, and in step S2, the carbon content of the second precursor was 0.5 wt%. In the prepared lithium iron phosphate material, the proportion of particles with a particle size distribution of more than 700 nm was 3%, the proportion of particles with a particle size distribution of 250~700 nm was 28%, and the proportion of particles with a particle size distribution of less than 250 nm was 69%.

[0077] Example 6 The lithium iron phosphate material was prepared according to the method of Example 1, except that in steps S3 to S5, the weight ratio of the first or second precursor, graphene oxide, epoxy resin and polyethylene glycol was 1000:0.1:5:20. In the prepared lithium iron phosphate material, the number of particles with a particle size distribution of more than 700 nm accounted for 5%, the number of particles with a particle size distribution of 250 to 700 nm accounted for 29%, and the number of particles with a particle size distribution of less than 250 nm accounted for 66%.

[0078] Comparative Example 1 Lithium iron phosphate material was prepared according to the method of Example 1, except that in step S6, the first slurry and the third slurry were mixed in a weight ratio of 6:4, i.e., the second slurry was not added. In the prepared lithium iron phosphate material, the number of particles with a particle size distribution of 700 nm or larger accounted for 6%, the number of particles with a particle size distribution of 250-700 nm accounted for 15%, and the number of particles with a particle size distribution of less than 250 nm accounted for 79%.

[0079] Comparative Example 2 Lithium iron phosphate material was prepared according to the method of Example 1, except that in step S1, magnesium oxide (PO4 site dopant) was not used; only titanium dioxide was used. In the prepared lithium iron phosphate material, particles with a diameter greater than 700 nm accounted for 3.5%, particles with a diameter between 250 and 700 nm accounted for 25%, and particles with a diameter of 250 nm accounted for 71.5%.

[0080] Comparative Example 3 Lithium iron phosphate material was prepared according to the method of Example 1, except that in step S1, magnesium oxide was used instead of titanium dioxide (Fe-site dopant). In the prepared lithium iron phosphate material, particles with a diameter greater than 700 nm accounted for 5%, particles with a diameter between 250 and 700 nm accounted for 38%, and particles with a diameter less than 250 nm accounted for 57%.

[0081] Comparative Example 4 Lithium iron phosphate material was prepared according to the method of Example 1, except that in step S1, magnesium oxide (PO4 site dopant) was not used, and only titanium dioxide was used; in step S2, 12.77g of titanium dioxide was used as the dopant to replace niobium pentoxide. In the prepared lithium iron phosphate material, particles with a diameter greater than 700nm accounted for 1%, particles with a diameter between 250 and 700nm accounted for 17%, and particles with a diameter less than 250nm accounted for 82%.

[0082] Test case The lithium iron phosphate materials of the examples and comparative examples were subjected to compaction density tests and assembled into batteries to test cycle performance. The results are listed in Table 1.

[0083] The test method for compaction density is as follows: Weigh 1g of lithium iron phosphate material sample into a compaction mold, place it into the compaction equipment and start the test. The powder compaction density result is taken as the result under 30KN pressure.

[0084] The battery assembly method is as follows: Lithium iron phosphate material, Super-P and PVDF are dispersed in NMP at a weight ratio of 80:10:10. After ball milling and uniform dispersion, the mixture is coated on aluminum foil and vacuum dried to obtain the positive electrode sheet. The electrolyte is 1 mol / L LiPF6, with a solvent volume ratio of EC:DMC:EMC = 1:1:1. The separator is Celgard polypropylene membrane, and the lithium metal sheet is the negative electrode. Together, they are assembled into a coin cell.

[0085] The cycle performance test method is as follows: the test voltage range is 2.0~3.75V. Charge to 3.75V using constant current and constant voltage charging method, and discharge to 2.0V using constant current discharging method. The charge and discharge current is 0.1C for 1 cycle. Then, cycle 2000 times with 1C charge and discharge current. The cutoff voltage condition is the same as 0.1C.

[0086] Table 1

[0087] As can be seen from Table 1, the lithium iron phosphate material prepared in the examples has a higher compaction density and exhibits better cycle performance in lithium-ion batteries.

[0088] The preferred embodiments of this disclosure have been described in detail above with reference to the accompanying drawings. However, this disclosure is not limited to the specific details of the above embodiments. Within the scope of the technical concept of this disclosure, various simple modifications can be made to the technical solutions of this disclosure, and these simple modifications all fall within the protection scope of this disclosure.

[0089] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, this disclosure will not describe the various possible combinations separately.

[0090] Furthermore, various different embodiments of this disclosure can be combined in any way, as long as they do not violate the spirit of this disclosure, they should also be regarded as the content disclosed in this disclosure.

Claims

1. A method for preparing lithium iron phosphate material, characterized in that, The method includes: The first mixture containing a phosphorus iron source, a first lithium source, a first dopant and a first carbon source is wet-milled, dried and then sintered to obtain a first precursor. The second mixture containing an iron source, a phosphorus source, a second lithium source, a second dopant, and a second carbon source is wet-milled, dried, and then subjected to a second sintering to obtain a second precursor. The third mixture containing the first precursor and the first carbon source composition is wet-milled to obtain a first slurry with a first particle size; The fourth mixture containing the first precursor and the second carbon source composition is wet-milled to obtain a second slurry with a second particle size. The fifth mixture containing the second precursor and the third carbon source composition is wet-milled to obtain a third slurry with a third particle size. The sixth mixture containing the first slurry, the second slurry, and the third slurry is dried and then subjected to a third sintering to obtain lithium iron phosphate material; The first dopant includes Fe-site dopant and PO4-site dopant, the second dopant includes Li-site dopant, and the first particle size, the second particle size, and the third particle size decrease sequentially.

2. The method according to claim 1, wherein, The Fe-site dopant includes at least one of NiO, TiO2, Mn2O5, NH4VO3, CeO2, and Cr2O3; Optionally, based on the total weight of the lithium iron phosphate material, the content of Fe-doped elements in the lithium iron phosphate material is 0.4~0.6 wt%.

3. The method according to claim 1, wherein, The PO4-site dopant includes at least one of SiO2, LiF, H3BO3, and MgO; Optionally, based on the total weight of the lithium iron phosphate material, the content of the PO4 doping element in the lithium iron phosphate material is 0.1~0.3wt%.

4. The method according to claim 1, wherein, The Li-site dopant includes at least one of Na2CO3, K2CO3, ZrO2, Nb2O5, and NH4VO3; Optionally, based on the total weight of the lithium iron phosphate material, the content of the Li-site doping element in the lithium iron phosphate material is 0.1~0.5wt%.

5. The method according to claim 1, wherein, The iron source of phosphorus includes at least one of iron phosphate, hydroxyferric phosphate and ferrous phosphate; and / or, the iron source includes iron oxide and / or ferrous oxalate. Optionally, the iron-phosphorus molar ratio of the phosphorus-iron source is 0.96~0.97, and the BET specific surface area is 8~16 m². 2 / g, with a primary particle size of 80~150nm.

6. The method according to claim 1, wherein, The first lithium source includes at least one of lithium carbonate, lithium hydroxide, and lithium oxalate; The second lithium source includes at least one of lithium hydroxide, lithium carbonate, lithium dihydrogen phosphate, lithium phosphate, lithium hydrogen phosphate, and lithium oxalate; The phosphorus source includes at least one of phosphoric acid, phosphorus pentoxide, ammonium dihydrogen phosphate, lithium dihydrogen phosphate, lithium phosphate, and lithium hydrogen phosphate.

7. The method according to claim 1, wherein, The first carbon source and the second carbon source each independently include at least one of glucose, sucrose, polyethylene glycol, citric acid, polyvinylpyrrolidone, and starch; Optionally, based on the total weight of the first precursor, the carbon content of the first precursor is 0.1~0.5 wt%; Optionally, based on the total weight of the second precursor, the carbon content of the second precursor is 0.1 to 0.5 wt%.

8. The method according to claim 1, wherein, The first carbon source composition, the second carbon source composition, and the third carbon source composition each independently include a third carbon source, a fourth carbon source, and a fifth carbon source; the third carbon source includes at least one of graphene, carbon nanotubes, and carbon nanoparticles; the fourth carbon source includes at least one of epoxy resin, acrylic resin, cellulose acetate resin, polyurethane resin, phenolic resin, and polyamide resin; and the fifth carbon source includes at least one of glucose, sucrose, polyethylene glycol, citric acid, and starch. Optionally, in the third mixture, the weight ratio of the first precursor, the third carbon source, the fourth carbon source, and the fifth carbon source is 1000:(0.1~0.5):(5~30):(20~50). Optionally, in the fourth mixture, the weight ratio of the first precursor, the third carbon source, the fourth carbon source, and the fifth carbon source is 1000:(0.1~0.5):(5~30):(20~50). Optionally, in the fifth mixture, the weight ratio of the second precursor, the third carbon source, the fourth carbon source and the fifth carbon source is 1000:(0.1~0.5):(5~30):(20~50).

9. The method according to claim 1, wherein, The first particle size is 1.0~2.0 μm, the second particle size is 0.3~1.0 μm, and the third particle size is 0.1~0.4 μm; and / or, In the sixth mixture, the weight ratio of the first slurry, the second slurry, and the third slurry is 1:(0.3~1):(0.3~0.8).

10. The method according to claim 1, wherein, The conditions for the first sintering include: a temperature of 790~810℃ and a time of 6~10h; The conditions for the second sintering include: a temperature of 500~700℃ and a time of 3~7h; The conditions for the third sintering include: a temperature of 700~760℃ and a time of 4~8h.

11. The lithium iron phosphate material prepared by the method according to any one of claims 1 to 10.

12. A lithium-ion battery, characterized in that, Including the lithium iron phosphate material as described in claim 11.

Images