Lithium iron phosphate positive electrode material and preparation method therefor, and lithium battery
Patent Information
- Application Number
- PCT/CN2025/130599
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2025-03-28
- Filing Date
- 2025-10-28
- Publication Date
- 2026-10-01
Smart Images
Figure CN2025130599_01102026_PF_FP_ABST
Abstract
Description
A lithium iron phosphate cathode material, its preparation method and lithium battery
[0001] Cross-reference to related applications
[0002] This disclosure claims priority to Chinese Patent Application No. 2025103767850, filed on March 28, 2025, entitled "A Lithium Iron Phosphate Cathode Material, Its Preparation Method and Lithium Battery", the entire contents of which are incorporated herein by reference. Technical Field
[0003] This disclosure relates to the field of lithium-ion battery technology, and more specifically, to a lithium iron phosphate cathode material, its preparation method, and a lithium battery. Background Technology
[0004] Lithium iron phosphate (LFP) is one of the most competitive cathode active materials for lithium-ion batteries currently on the market. Compared with lithium cobalt oxide and ternary cathode materials, it has a longer lifespan and better safety performance. Furthermore, LFP has a 170mAh / g capacity. -1 With its theoretical specific capacity and a plateau discharge voltage of 3.4V, it possesses considerable energy density.
[0005] Tap density and compaction density (collectively referred to as bulk density) are important indicators for evaluating cathode materials. Tap density refers to the bulk density of powder material after tapping, while compaction density refers to the bulk density of powder material after it has been stabilized under pressure. Therefore, both reflect the mass of cathode material per unit volume, which directly affects the compaction density of the prepared electrode sheet, and thus the energy density of the battery. Due to its crystal structure, lithium iron phosphate cathode material has a low ion diffusion rate and low conductivity. Therefore, particle size distribution is typically used to improve the tap density and compaction density of lithium iron phosphate cathode material, achieving a balance between good bulk density and Li-energy efficiency. + Diffusion rate. However, the tap density and compaction density of cathode materials are not limited to particle size distribution, but are also affected by many other factors.
[0006] In view of this, this disclosure is made. Summary of the Invention
[0007] The purpose of this disclosure is to provide a lithium iron phosphate cathode material, its preparation method, and a lithium battery, with the aim of providing a lithium iron phosphate cathode material with high solid density and good cycle performance.
[0008] This disclosure is implemented as follows:
[0009] In a first aspect, this disclosure provides a lithium iron phosphate cathode material, including a lithium iron phosphate active material having a carbon coating layer on the surface of the lithium iron phosphate active material;
[0010] The average sphericity of lithium iron phosphate cathode materials in the Dv50-Dv100 range is greater than or equal to 0.6;
[0011] Suppression index of lithium iron phosphate cathode material The range is 0.4-5.0, where the suppression index is obtained through the following method:
[0012] Density-pressure relationship curves of lithium iron phosphate cathode material at different pressing speeds were obtained. The portion of the density-pressure relationship curve above 100 MPa was linearly fitted and the corresponding slope was obtained. The relationship curve between pressing speed and slope was plotted. The slope of the fitted line obtained by linearly fitting the relationship curve between pressing speed and slope is the pressing exponent.
[0013] In an optional embodiment, the particle strength of the lithium iron phosphate cathode material is 40 MPa-150 MPa.
[0014] In an optional implementation, the lithium iron phosphate cathode material satisfies at least one of the following characteristics:
[0015] Feature A1: Particle strength is 60MPa-120MPa;
[0016] Feature A2: The average roundness of particles in the Dv50-Dv100 range is 0.65-0.80;
[0017] Feature A3: Compacted density is 1.5 g / cm³ 3 -3.0g / cm 3 ;
[0018] Feature A4: The carbon coating layer accounts for 0.5%-5% of the mass of the lithium iron phosphate cathode material.
[0019] In an optional implementation, the following is defined:
[0020] Lithium iron phosphate cathode materials meet the compressibility factor of 120MPa-300MPa.
[0021] Secondly, this disclosure provides a method for preparing any of the lithium iron phosphate cathode materials in the foregoing embodiments, comprising: mixing iron phosphate and a lithium-ion-containing polymer solution, and then mixing with a curing agent to carry out a curing reaction to obtain a gel; wherein the lithium-ion-containing polymer solution is obtained by dissolving a soluble lithium salt and a polymer in a solvent;
[0022] The gel was broken down and dried to obtain the precursor;
[0023] The precursor was calcined to prepare carbon-coated lithium iron phosphate;
[0024] Lithium iron phosphate coated with carbon of different particle sizes was formulated.
[0025] In an optional embodiment, the process for preparing the gel has at least one of the following characteristics:
[0026] Feature B1: The polymer is a soluble polymer containing at least one of hydroxyl, vinyl, and epoxy groups; the curing agent is selected according to the active groups contained in the polymer. When the polymer contains hydroxyl groups, the curing agent is glycidyl ether; when the polymer contains vinyl groups, the curing agent is a soluble photoinitiator; when the polymer contains epoxy groups, the curing agent is a polyol.
[0027] Feature B2: The soluble lithium salt is selected from at least one of lithium chloride, lithium hydroxide, lithium acetate, and lithium oxalate;
[0028] Feature B3: The solvent is selected from at least one of water, ethanol, methanol, and isopropanol;
[0029] Feature B4: The concentration of lithium ions in the lithium-ion-containing polymer solution is 0.2M-0.8M, the mass percentage of polymer in the lithium-ion-containing polymer solution is 1%-3%, and the mass ratio of curing agent to polymer is (1-5):100;
[0030] Feature B5: Anhydrous iron phosphate, a dopant, and a lithium-ion-containing polymer solution are mixed, and then mixed with a curing agent to carry out a curing reaction. The dopant is a soluble compound containing a dopant element, and the dopant element is selected from at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn, Y, Mg, Na, N, S, and F; the total molar ratio of iron to dopant element to lithium in the anhydrous iron phosphate is controlled to be 1:(1.01-1.10).
[0031] Feature B6: The curing reaction method is selected from at least one of heating and stirring reaction, ultraviolet irradiation and stirring reaction, and microwave irradiation and stirring reaction; when the curing reaction is carried out by heating and stirring reaction, the reaction temperature is controlled at 50℃-80℃; when the curing reaction is carried out by ultraviolet irradiation and stirring reaction, the light wavelength is controlled at 200nm-400nm; when the curing reaction is carried out by microwave irradiation and stirring reaction, the microwave irradiation power is controlled at 200W-500W.
[0032] Feature B7: The solid content of the prepared gel is 30%-50%.
[0033] In an optional embodiment, the gel is fragmented and then fluidized and dried;
[0034] Fluidized bed drying refers to drying the fragmented gel in a fluidized bed, controlling the drying temperature at 80℃-120℃ and the air flow rate at 50mL / min-400mL / min.
[0035] In an optional embodiment, the process for preparing carbon-coated lithium iron phosphate from the precursor has at least one of the following characteristics:
[0036] Characteristic C1: Calcination temperature is 550℃-650℃, and calcination time is 2h-6h;
[0037] Characteristic C2: Calcination is carried out under an inert atmosphere;
[0038] Feature C3: The precursor is preheated in a flash dryer before calcination. The inlet air temperature of the flash dryer is controlled at 300℃-500℃, the outlet air temperature is controlled at 100℃-150℃, the rotation speed is controlled at 10Hz-30Hz, and the feeding frequency is controlled at 10Hz-30Hz.
[0039] Feature C4: After the precursor is calcined, it is then crushed and demagnetized to obtain carbon-coated lithium iron phosphate.
[0040] In an optional embodiment, the method of proportioning carbon-coated lithium iron phosphate with different particle sizes is selected from either the first method or the second method.
[0041] The process of proportioning using the first method includes: mixing two types of carbon-coated lithium iron phosphate with different particle size distributions in a certain proportion, wherein the Dv50 of the two types of carbon-coated lithium iron phosphate with different particle size ranges are Dv50 and Dv50 respectively. a Dv50 b , of which 15μm≥Dv50 a ≥2.4Dv50 b ≥1μm; with Dv50 a and Dv50 b The mass ratio of the two carbon-coated lithium iron phosphates is (20%–35%) : (65%–80%).
[0042] The process of proportioning using the second method includes: mixing three types of carbon-coated lithium iron phosphate with different particle size distributions in a certain proportion, wherein the Dv50 of the three types of carbon-coated lithium iron phosphate with different particle size ranges are Dv50, Dv50, and Dv50, respectively. a Dv50 b Dv50 c , of which 15μm≥Dv50 a ≥2.4Dv50 b ≥2.4 2 Dv50 c ≥2μm; with Dv50 a Dv50 b and Dv50c The mass ratio of the three carbon-coated lithium iron phosphates is (2%–20%): (40%–60%): (30%–40%).
[0043] Thirdly, this disclosure provides a lithium battery, including any of the lithium iron phosphate cathode materials in the foregoing embodiments or lithium iron phosphate cathode materials prepared by any of the preparation methods in the foregoing embodiments.
[0044] This disclosure has the following beneficial effects: This disclosure uses a specific method to test the suppression index of lithium iron phosphate cathode materials. By ensuring the compression index meets a specific range, and the average sphericity of larger particles within the Dv50-Dv100 range meets a suitable range, this type of lithium iron phosphate cathode material exhibits superior particle mobility. During compression, the particles can move and rearrange at a faster speed, which is beneficial for obtaining a cathode sheet with high compaction density and improving the charge and discharge capacity of the battery. Simultaneously, it helps to prevent particle breakage under pressure, giving it good compressive strength. Using the lithium iron phosphate cathode material provided in this disclosure enables lithium batteries to achieve both high energy density and capacity retention. Attached Figure Description
[0045] To more clearly illustrate the technical solutions of the embodiments of this disclosure, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of this disclosure and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.
[0046] Figure 1 shows the density-pressure relationship curve;
[0047] Figure 2 is a schematic diagram of the curve fitting in Figure 1;
[0048] Figure 3 shows the curve of the relationship between pressing speed and slope;
[0049] Figure 4 shows the TEM test results of Comparative Example 1 and Example 3;
[0050] Figure 5 is a SEM image of the lithium iron phosphate cathode material prepared in Example 1;
[0051] Figure 6 is a SEM image of the lithium iron phosphate cathode material prepared in Example 3. Detailed Implementation
[0052] To make the objectives, technical solutions, and advantages of the embodiments of this disclosure clearer, the technical solutions in the embodiments of this disclosure will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.
[0053] This disclosure provides a lithium iron phosphate cathode material, including a lithium iron phosphate active material, and a carbon coating layer on the surface of the lithium iron phosphate active material to form a carbon-coated lithium iron phosphate cathode material.
[0054] The average roundness of the particles in the lithium iron phosphate cathode material within the range of Dv50-Dv100 provided in this embodiment is greater than or equal to 0.6. These particles are considered relatively large and have a significant impact on the overall mobility of the powder material. The average roundness of these particles can be 0.60, 0.65, 0.70, 0.80, 0.90, 1.00, etc.
[0055] This disclosure provides an embodiment that creatively defines the suppression index of lithium iron phosphate cathode materials. The compression index is controlled to be between 0.4 and 5.0, such as 0.40, 0.50, 0.80, 1.00, 1.50, 2.00, 2.50, 3.00, 3.50, 4.00, 4.50, and 5.00. The compression index is obtained by: obtaining the density-pressure relationship curves of the lithium iron phosphate cathode material at different compression rates; and performing linear fitting on the portion of the density-pressure relationship curve above 100 MPa (the linear growth portion is usually above 100 MPa) (requiring a goodness of fit r). 2 (≥0.99) and obtain the corresponding slope, plot the relationship curve between pressing speed and slope, and perform linear fitting on the relationship curve between pressing speed and slope (requiring a goodness of fit r). 2 The slope of the fitted line (≥0.98) is the compression index. The specific test method refers to GB / T11106-2022 "Method for Determining the Compressibility Strength of Cylindrical Compactors for Metal Powders," and then the external tangent line of the relationship curve is fitted using software such as Origin. Compression Index This reflects the growth rate of the pressure increase rate of lithium iron phosphate cathode material under different pressing speeds, and reflects the mobility of lithium iron phosphate cathode material particles.
[0056] It should be noted that the pressure increase rate of powder materials varies under different pressing speeds; the faster the pressing speed, the faster the pressure increase. This is because when pressure is applied, powder particles dissipate pressure through repositioning. However, if the pressing speed is too fast, stress cannot be dissipated quickly, leading to a rapid pressure increase. Therefore, the mobility of powder particles affects their pressure dissipation capability. The quality of particle mobility, i.e., stress dissipation capability, directly affects its tap density or compaction density, thus affecting the energy density of the battery. Furthermore, it directly affects the stability of the cathode material during cycling, thereby affecting cycle life and safety. Through extensive experiments, this disclosure has found that the particle mobility of lithium iron phosphate cathode materials is related to their particle size distribution, particle morphology, and particle surface properties. Specifically, when lithium iron phosphate cathode material particles have good particle size distribution, the particles only need to fill the pores and dissipate stress through a shorter movement path, thus exhibiting high mobility; when lithium iron phosphate cathode material particles have high sphericity, the contact area between particles is the lowest, resulting in relatively high mobility; and when the carbon coating layer of lithium iron phosphate cathode material particles is relatively uniform, the friction between particles is low, thus exhibiting relatively high mobility.
[0057] The embodiments disclosed herein provide a suppression index Lithium iron phosphate cathode materials with a compaction index of 0.4–5 exhibit superior particle mobility. During compaction, the particles can rearrange themselves at a faster speed, which is beneficial for obtaining cathode sheets with high compaction density, thereby improving the charge and discharge capacity of the battery. It also helps to prevent particle breakage under pressure. Generally, broken particles may not be able to maintain their original conductive network structure, making the electron conduction path more tortuous, increasing internal resistance, and affecting the battery's high-current discharge performance and efficiency. Typically, the compaction index... The smaller the particle, the better its mobility.
[0058] In some embodiments, the particle sphericity of the lithium iron phosphate cathode material is 0.5-1.0, such as 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, etc.
[0059] In some embodiments, the particle strength of the lithium iron phosphate cathode material is 40 MPa-150 MPa, such as 40 MPa, 50 MPa, 60 MPa, 70 MPa, 80 MPa, 90 MPa, 100 MPa, 110 MPa, 120 MPa, 130 MPa, 140 MPa, 150 MPa, etc. When the lithium iron phosphate cathode material has a pressing index... When the particle strength is 0.4-5 and the particle strength is 40-150 MPa, the lithium iron phosphate cathode material has good particle mobility and high particle mechanical strength, and is not easily broken under pressure. Therefore, the lithium iron phosphate cathode material has a high compaction density. The cathode sheet containing this lithium iron phosphate cathode material has a high compaction density and low internal resistance. The battery containing this cathode sheet has good charge and discharge capacity and cycle stability.
[0060] In a further embodiment, the particle strength of the lithium iron phosphate cathode material is 60 MPa-120 MPa, and the average sphericity of the particles within the Dv50-Dv100 range is 0.65-0.80. The particle strength and average sphericity of the lithium iron phosphate cathode material are preferably within the above range, which is beneficial for further improving the material's mobility and compaction density. The compaction density (test condition: 3T) is 1.5 g / cm³. 3 -3.0g / cm 3 For example, it can be 1.5g / cm 3 2.0g / cm 3 2.5g / cm 3 3.0g / cm 3 The carbon coating layer accounts for 0.5%-5% of the mass of lithium iron phosphate cathode material, such as 0.5%, 1.0%, 2.0%, 3.0%, 4.0%, 5.0%, etc.
[0061] Furthermore, define: Lithium iron phosphate cathode materials meet a compressibility factor of 120MPa-300MPa, such as 120MPa, 150MPa, 180MPa, 200MPa, 230MPa, 250MPa, 300MPa, etc. The higher the compressibility factor, the stronger the compressibility of the lithium iron phosphate cathode material, resulting in a higher compaction density of the cathode sheet, thus leading to higher battery capacity and better cycle stability.
[0062] This disclosure also provides a method for preparing lithium iron phosphate cathode material. A carbon-coated lithium iron phosphate with a uniform carbon coating layer and high sphericity is prepared by combining the sol-gel method and the carbothermal reduction method. The lithium iron phosphate cathode material is further prepared by particle size distribution. The steps are as follows:
[0063] S1. Preparation of gel
[0064] A lithium-ion-containing polymer solution is obtained by dissolving a soluble lithium salt and a polymer in the same solvent. Then, ferric phosphate (such as anhydrous ferric phosphate) is mixed with the lithium-ion-containing polymer solution and then mixed with a curing agent to carry out a curing reaction to obtain a gel.
[0065] It should be noted that by dispersing anhydrous ferric phosphate in a polymer solution containing lithium ions to achieve mixing of anhydrous ferric phosphate and lithium ions, and then allowing the polymer to slowly crosslink and form a gel under stirring conditions, the amount of ferric phosphate, lithium ions, and polymer per unit volume is fixed. This is beneficial for preparing lithium iron phosphate with uniform shape and similar surface properties, thereby improving product uniformity and reducing resistance. At the same time, the polymer serves as a carbon source for subsequent calcination, uniformly coating the surface of the ferric phosphate solid, which not only helps to form a uniform carbon coating layer but also improves the sphericity of the obtained lithium iron phosphate.
[0066] In some embodiments, the soluble lithium salt is selected from at least one of lithium chloride, lithium hydroxide, lithium acetate, and lithium oxalate, and the soluble lithium salt can be any one or more of the above. The solvent is selected from at least one of water, ethanol, methanol, and isopropanol, and the solvent can be any one or more of the above.
[0067] In some embodiments, the polymer is a soluble polymer containing at least one of hydroxyl, vinyl, and epoxy groups, and the polymer contains any one or more of the above active groups. The curing agent is selected according to the active groups contained in the polymer. When the polymer contains hydroxyl groups, the curing agent is glycidyl ether, such as ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, etc.; when the polymer contains vinyl groups, the curing agent is a soluble photoinitiator, such as photoinitiator 2959, but not limited to this; when the polymer contains epoxy groups, the curing agent is a polyol (such as ethylene glycol, glycerol, etc.), and the gel is prepared by reacting the polyol with the epoxy group.
[0068] Specifically, the polymer can be hyaluronic acid, vinyl-grafted hydroxymethyl cellulose, agar, guar gum, alginate, aliphatic epoxy resin, etc.
[0069] Furthermore, the concentration of lithium ions in the lithium-ion-containing polymer solution is 0.2M-0.8M, such as 0.2M, 0.3M, 0.4M, 0.5M, 0.6M, 0.7M, 0.8M, etc. The mass percentage of the polymer in the lithium-ion-containing polymer solution is 1%-3%, such as 1%, 2%, 3%, etc. The amount of curing agent is determined according to the amount of polymer, and the mass ratio of the added curing agent to the polymer is controlled to be (1-5):100, such as 1:100, 2:100, 3:100, 4:100, 5:100, etc.
[0070] In some embodiments, the curing reaction method is selected from at least one of heating and stirring reaction, ultraviolet irradiation and stirring reaction, and microwave irradiation and stirring reaction. The curing reaction method can be any one or more of the above, and can be selected according to the type of polymer. When the curing reaction is carried out by heating and stirring reaction, the reaction temperature is controlled at 50℃-80℃, such as 50℃, 55℃, 60℃, 65℃, 70℃, 75℃, 80℃, etc. When the curing reaction is carried out by ultraviolet irradiation and stirring reaction, the light wavelength is controlled at 200nm-400nm, such as 200nm, 250nm, 300nm, 350nm, 400nm, etc. When the curing reaction is carried out by microwave irradiation and stirring reaction, the microwave irradiation power is controlled at 200W-500W, such as 200W, 300W, 400W, 500W, etc. By adjusting the reaction time, the solid content of the prepared gel can be 30%-50%, such as 30%, 35%, 40%, 45%, 50%, etc.
[0071] In some embodiments, introducing a dopant element into the gel can be achieved by mixing anhydrous iron phosphate, a dopant, and a lithium-ion-containing polymer solution, stirring to disperse the mixture evenly, and then adding a curing agent while stirring. After the addition is complete, a curing reaction is carried out. The dopant is a soluble compound containing a dopant element, and the dopant element is selected from at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn, Y, Mg, Na, N, S, and F. The dopant element can be any one or more of these. The molar ratio of the total molar amount of iron and the dopant element in the anhydrous iron phosphate to the molar amount of lithium is controlled to be 1:(1.01-1.10), such as 1:1.01, 1:1.02, 1:1.03, 1:1.04, 1:1.05, 1:1.06, 1:1.07, 1:1.08, 1:1.09, 1:1.10, etc.
[0072] S2, crushing, drying
[0073] The gel is broken down and dried to remove surface solvents and other impurities, thus obtaining the precursor.
[0074] In some embodiments, the crushing method can be sieving, crushing, grinding, etc., to prevent particle agglomeration.
[0075] In some embodiments, fluidized bed drying can be used to avoid agglomeration. Fluidized bed drying refers to drying the fragmented gel in a fluidized bed, controlling the drying temperature at 80℃-120℃ and the air flow rate at 50mL / min-400mL / min, to obtain precursor particles with relatively uniform particle size. Specifically, the drying temperature can be 80℃, 90℃, 100℃, 110℃, 120℃, etc., and the air flow rate can be 50mL / min, 100mL / min, 200mL / min, 300mL / min, 400mL / min, etc.
[0076] S3, calcination
[0077] Carbon-coated lithium iron phosphate is prepared by calcining the precursor and carbonizing the polymer.
[0078] In some embodiments, the calcination temperature is controlled at 550℃-650℃, such as 550℃, 580℃, 600℃, 630℃, 650℃, etc.; the calcination time is 2h-6h, such as 2h, 3h, 4h, 5h, 6h, etc. The calcination process can be carried out under an inert atmosphere, such as nitrogen or argon.
[0079] In some embodiments, the precursor is preheated in a flash dryer before calcination. The inlet air temperature of the flash dryer is controlled at 300℃-500℃, the outlet air temperature at 100℃-150℃, the rotation speed at 10Hz-30Hz, and the feed frequency at 10Hz-30Hz. Pre-calcination using a flash dryer facilitates rapid shrinkage of the particles at high temperatures, improving the internal bonding strength of the material particles and thus enhancing the mechanical strength characteristics of the material.
[0080] In some embodiments, after the precursor is calcined, it is crushed and demagnetized to obtain carbon-coated lithium iron phosphate. Crushing disperses the calcined agglomerated particles, and demagnetization removes magnetic impurities.
[0081] S4, particle size distribution
[0082] By proportioning carbon-coated lithium iron phosphate particles of different sizes, the gradation effect of the lithium iron phosphate cathode material can be improved through a gradation method, which is beneficial to increasing its tap density and compaction density, and thus improving the battery capacity.
[0083] In some embodiments, the method of proportioning carbon-coated lithium iron phosphate with different particle sizes is selected from the first method or the second method. The first method is to use two carbon-coated lithium iron phosphates with different particle size distributions for gradation, and the second method is to use three carbon-coated lithium iron phosphates with different particle size distributions for gradation.
[0084] Furthermore, the process of proportioning using the first method includes: mixing two types of carbon-coated lithium iron phosphate with different particle size distributions in a certain proportion, wherein the Dv50 of the two types of carbon-coated lithium iron phosphate with different particle size ranges are Dv50 and Dv50, respectively. a Dv50 b , of which 15μm≥Dv50 a ≥2.4Dv50 b ≥1μm, meaning large-particle Dv50 a Greater than or equal to 2.4 times the size of small particles (Dv50) b And Dv50 a And 2.4 times the Dv50 b The values range from 1μm to 15μm, such as 1μm, 3μm, 5μm, 8μm, 10μm, 13μm, 15μm, etc. It has Dv50... a and Dv50 b The mass ratio of the two types of carbon-coated lithium iron phosphate is (20%–35%): (65%–80%). The mass ratio of large-particle carbon-coated lithium iron phosphate to small-particle carbon-coated lithium iron phosphate can be 20%:80%, 25%:75%, 30%:70%, 35%:65%, etc.
[0085] The process of proportioning using the second method includes: mixing three types of carbon-coated lithium iron phosphate with different particle size distributions in a certain proportion, with the Dv50 of the three different particle size ranges of large, medium and small carbon-coated lithium iron phosphate being Dv50 respectively. a Dv50 b Dv50 c , of which 15μm≥Dv50 a ≥2.4Dv50 b ≥2.4 2 Dv50 c ≥2μm, meaning large-particle Dv50 a Greater than or equal to 2.4 times the Dv50 of medium-sized particles b It is also greater than or equal to 2.4 for small particles. 2 Dv50 c And Dv50 a And 2.4 times the Dv50 b 2.4 2 Dv50 c The values range from 2μm to 15μm, such as 2μm, 5μm, 8μm, 10μm, 13μm, 15μm, etc. It has Dv50... a Dv50 b and Dv50 cThe mass ratio of large, medium, and small carbon-coated lithium iron phosphate particles is (2%–20%):(40%–60%):(30%–40%), such as 2%:60%:38%, 5%:55%:40%, 8%:57%:35%, 10%:53%:37%, 13%:57%:30%, 15%:45%:40%, 18%:43%:39%, 20%:40%:40%, etc.
[0086] This disclosure also provides a lithium battery positive electrode sheet, including the lithium iron phosphate positive electrode material provided in this disclosure, which can improve the compaction density of the positive electrode sheet. The lithium battery positive electrode sheet may further include a positive current collector, and a positive active coating is formed on at least one surface of the positive current collector, wherein the lithium iron phosphate positive electrode material exists as a positive active material in the positive active coating.
[0087] This disclosure also provides a lithium battery, including the positive electrode provided in this embodiment, which can improve the charge and discharge capacity of the battery. The lithium battery may also include a negative electrode, electrolyte, separator, etc., forming a complete battery structure. The specific types of the negative electrode, electrolyte, and separator are not limited. During the charge and discharge process of the lithium battery, active ions repeatedly insert and remove themselves between the positive and negative electrode, and the electrolyte plays a role in conducting ions between the positive and negative electrode.
[0088] This disclosure also provides an apparatus comprising the aforementioned lithium battery (secondary battery), battery module, or battery pack. The secondary battery, battery module, or battery pack can serve as a power source for the apparatus or as an energy storage unit. This apparatus can be, but is not limited to, mobile devices (e.g., mobile phones, laptops), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.
[0089] The features and performance of this disclosure will be further described in detail below with reference to embodiments.
[0090] This disclosure provides lithium iron phosphate cathode materials as shown in Table 1, and uses lithium iron phosphate cathode materials to prepare cathode sheets and batteries. The performance of the cathode sheets and batteries is tested, as shown in Table 2.
[0091] Performance testing methods and conditions:
[0092] Electrode resistance: Tested using Yuaneng Technology BER2100 multi-functional electrode resistance meter at 10 different locations, and the average value was calculated.
[0093] Tap density: determined in accordance with GB / T30835-2014.
[0094] Cathode material compaction density: The compaction density of the cathode material was determined according to GB / T30835-2014.
[0095] Compression Index: 1 ± 0.01 g of lithium iron phosphate cathode material is loaded into a steel mold. The powder is continuously compressed in the mold using a Xin Sansi CMT5000 universal mechanical testing machine. The compression speed (moving speed of the indenter) is selected from any three speeds between 0.2 and 15 mm / min, for example, set to 0.2 mm / min, 1 mm / min, 10 mm / min, and 15 mm / min respectively. The mechanical testing machine automatically records the pressure and crossbeam displacement during compression, and the compression index is determined based on the specifications of the steel mold and the mass of the powder. Calculate powder density and pressure to obtain the relationship curves between powder density and pressure at different pressing speeds. Perform linear fitting on the linearly increasing portion of the density-pressure curve (usually at pressures above 100 MPa) (requiring a goodness-of-fit r² ≥ 0.99) and obtain the corresponding slope 1. Measure each sample at each speed at least three times and calculate the average slope 1. Then, plot the relationship curve between pressing speed and the average slope 1. The slope 2 obtained by linearly fitting this curve is the pressing exponent, and the goodness-of-fit r² is used. 2 A value ≥0.98 is considered valid. The test method refers to GB / T 11106-2022 "Method for Compression Determination of Cylindrical Compactor Strength for Metal Powders". Software such as Origin was used to perform linear fitting on the relationship curve. Before linear fitting, the curve could be appropriately smoothed. For example, Origin's smoothing tool was used, selecting the Lowess model with a span parameter of 0–0.5. Example 5 provides the test process for the compression index, as shown in Figures 1, 2, and 3.
[0096] Carbon content percentage: determined in accordance with GB / T30835-2014.
[0097] Chemical composition analysis: Inductively coupled plasma optical emission spectrometry (ICP-OES) was performed using a PE Avio200 model.
[0098] Particle size: The Dv50 and Dv100 of lithium iron phosphate cathode materials, as well as the Dv10, Dv50, Dv90 and broadening coefficient Span of carbon-coated lithium iron phosphate, were tested using an MS3000 laser particle size analyzer in accordance with GB / T 19077.1, where Span = (Dv90 - Dv10) / Dv50.
[0099] Particle strength: An MCT series micro compression tester was used. The indenter applied a test force to a single particle at a constant rate of increase and fixed it between the flat indenter and the lower platen. The deformation of the sample was then automatically measured. A sudden drop in pressure was used to determine if the particle had been crushed. The particle strength was calculated using the maximum pressure F before the sudden drop in pressure according to formula (1). At least 10 particles were taken for each sample and the average value was calculated. Particle strength = 2.8 × F / (πd 2 ) Formula (1)
[0100] F represents the maximum pressure value before the pressure drop, in mN; d represents the particle diameter, in μm.
[0101] Electrode compaction density: The mass ratio of lithium iron phosphate material, binder, and conductive carbon black is 95:3:2. First, PVDF is dissolved in NMP to obtain a 5wt% binder solution. Then, lithium iron phosphate material and conductive carbon black are added and stirred to obtain a slurry. The slurry is coated onto a 20μm thick aluminum foil using a 300μm coating applicator and dried at 100℃ for 8 hours, controlling the single-sided surface density to be 400±1g / m². 2 Then, the dried electrode sheet is pressed into a sheet with a roller temperature of 40℃ and a pressure of 6MPa to obtain a positive electrode sheet. The thickness of the positive electrode sheet is measured using a spiral micrometer with an accuracy of 0.5μm, and the compaction density of the positive electrode sheet PD = surface density of the positive electrode sheet ÷ (thickness of the positive electrode sheet - 20μm) is calculated.
[0102] Battery energy density: The prepared positive electrode sheets were assembled into stacked batteries, and the volumetric energy density and gravimetric energy density of the batteries were tested. The negative electrode sheet preparation method is as follows: the mass ratio of graphite, conductive carbon black, CMC, and styrene-butadiene rubber is 95.5:1:1.5:2; first, CMC is dissolved in water, then graphite and conductive carbon black are added and stirred; finally, styrene-butadiene rubber is added, vacuum stirring is completed, and the material is sieved to obtain a negative electrode slurry with a solid content of 50%. The negative electrode slurry is coated onto copper foil, cold-pressed, and slit to obtain the negative electrode sheet. The compacted density of the negative electrode sheet is 1.55 g / cm³. 3 The active material layer thickness is 110μm; the electrolyte is EC / DMC / EMC+LiPF6+additive, with a LiPF6 concentration of 1M; the separator is a 20μm PE ion exchange membrane; an aluminum-plastic film heat-sealed shell is used; the assembled battery is formed using a Lanqi charging cabinet under the following conditions: 0.1C constant current charging to 3.8V, and 0.1C discharging to 2.0V; the energy density of the formed battery is then tested.
[0103] Cyclic capacity retention: After 1000 cycles at 1C rate, the method is to charge at 1C constant current to 3.8V and then discharge at 1C constant current to 2.0V. The capacity retention is calculated as: discharge capacity in the 1000th cycle ÷ discharge capacity in the 1st cycle × 100%.
[0104] TEM: A FEI Titan Cubed G2 60-300 transmission electron microscope was used.
[0105] SEM: Nova NanoSEM 450 was used. Circularity test method: Based on the acquired SEM images, ImageJ image analysis software was used to analyze the data. ImageJ was used to identify analyzable individual particles, and then ImageJ was used to calculate the projected area and circularity of each particle (C=(4×π×A) / G). 2 Where A is the area of the projected surface, G is the perimeter of the projected surface, and C is the particle roundness. In Examples 2-3, 6, and 8-10, the particle roundness of the lithium iron phosphate cathode materials is 0.5-1.0. The Dv50 and Dv100 of the lithium iron phosphate cathode materials were tested using an MS3000 laser particle size analyzer according to GB / T 19077.1. Particles with a projected area greater than or equal to π(Dv50 / 2) were selected from the SEM images. 2 Using particles as samples, the average roundness of the samples is statistically analyzed. The sample size is no less than 30, which yields the average roundness of particles in the range of Dv50-Dv100.
[0106] Table 1 shows the parameter detection results of the products provided in each embodiment.
[0107] Table 2. Electrode and Battery Performance Test Results
[0108] Analysis of Tables 1 and 2 shows that the lithium iron phosphate cathode material provided in this disclosure balances high compaction density and cycle stability, and the prepared battery has both high energy density and capacity retention.
[0109] Example 1
[0110] This embodiment provides a method for preparing lithium iron phosphate cathode material, the steps of which are as follows:
[0111] (1) Dissolve lithium chloride, titanium sulfate and hyaluronic acid in deionized water to obtain a polymer solution containing lithium ions. Add anhydrous ferric phosphate and stir to keep it evenly dispersed. Then add ethylene glycol diglycidyl ether and heat to 60°C while continuing to stir. Stop the reaction when the viscosity is too high and it is difficult to stir. A gel with a solid content of 48.2% is obtained.
[0112] The lithium ion concentration in the polymer solution containing lithium ions is 0.2M, and the mass percentage of the polymer in the polymer solution containing lithium ions is 1%. The amount of curing agent added is 3% of the polymer (mass fraction, the same below). Anhydrous iron phosphate and titanium oxysulfate are added according to the molar ratio of the total molar amount of Fe and Ti elements to the molar ratio of Li elements of 1:1.05, and the molar ratio of Fe to Ti is 0.95:0.05.
[0113] (2) The gel is crushed by passing it through an 80-mesh sieve under pressure, and then sent to a fluidized bed to dry to constant weight to obtain the precursor.
[0114] The fluidized bed temperature was 120℃ and the air flow rate was 100mL / min.
[0115] (3) The precursor is placed in a flash dryer for pre-calcination to obtain a pre-calcined precursor.
[0116] The flash dryer has an inlet air temperature of 500℃, an outlet air temperature of 100℃, a rotation speed of 30Hz, and a feeding frequency of 30Hz.
[0117] (4) The pre-calcined precursor is calcined, crushed and demagnetized to obtain carbon-coated lithium iron phosphate.
[0118] The calcination process involves an inert atmosphere (nitrogen) at a temperature of 650℃ for 5 hours.
[0119] (5) Carbon-coated lithium iron phosphate prepared with anhydrous iron phosphate of different particle sizes is mixed and proportioned to obtain lithium iron phosphate cathode material.
[0120] Table 3 Grading scheme for Example 1
[0121] Example 2
[0122] This embodiment provides a method for preparing lithium iron phosphate cathode material, the steps of which are as follows:
[0123] (1) Lithium hydroxide and vinyl-grafted hydroxymethyl cellulose were dissolved in deionized water to obtain a polymer solution containing lithium ions. Anhydrous ferric phosphate was added and stirred to keep the dispersion uniform. Then photoinitiator 2959 was added, and ultraviolet radiation was turned on and stirring was continued (ultraviolet wavelength was 254nm). The reaction was stopped when the viscosity was too high and it was difficult to stir, and a gel with a solid content of 30.5% was obtained.
[0124] The lithium ion concentration in the lithium-ion polymer solution is 0.8M, and the mass percentage of the polymer in the lithium-ion polymer solution is 3%. The amount of curing agent added is 3% of the polymer. Anhydrous ferric phosphate is added according to the molar ratio of Fe to Li of 1:1.1.
[0125] (2) The gel is crushed by passing it through an 80-mesh sieve under pressure, and then sent to a fluidized bed to dry to constant weight to obtain the precursor.
[0126] The fluidized bed temperature was 80℃ and the air flow rate was 400mL / min.
[0127] (3) The precursor is calcined, crushed and demagnetized to obtain carbon-coated lithium iron phosphate.
[0128] The calcination process involved an inert gas atmosphere, a temperature of 550℃, and a time of 6 hours.
[0129] (4) Carbon-coated lithium iron phosphate prepared with anhydrous iron phosphate of different particle sizes is mixed and proportioned to obtain lithium iron phosphate cathode material.
[0130] Table 4. Grading Scheme for Example 2
[0131] Example 3
[0132] This embodiment provides a method for preparing lithium iron phosphate cathode material, the steps of which are as follows:
[0133] (1) Dissolve lithium acetate and agar in deionized water at 50°C to obtain a polymer solution containing lithium ions. Add anhydrous ferric phosphate and stir to keep the dispersion uniform. Then add polyethylene glycol diglycidyl ether (Mn 500) and heat to 80°C while continuing to stir. Stop the reaction when the viscosity is too high and it is difficult to stir. A gel with a solid content of 42.7% is obtained.
[0134] The lithium ion concentration in the lithium-ion polymer solution is 0.5M, and the mass percentage of the polymer in the lithium-ion polymer solution is 2%; the amount of curing agent added is 2.5% of the polymer; and anhydrous ferric phosphate is added according to a molar ratio of Fe to Li of 1:1.01.
[0135] (2) The gel is crushed by passing it through an 80-mesh sieve under pressure, and then sent to a fluidized bed to dry to constant weight to obtain the precursor.
[0136] The fluidized bed temperature was 100℃ and the air flow rate was 200mL / min.
[0137] (3) The precursor is placed in a flash dryer for pre-calcination to obtain a pre-calcined precursor.
[0138] The flash dryer has an inlet air temperature of 400℃, an outlet air temperature of 150℃, a rotation speed of 30Hz, and a feeding frequency of 20Hz.
[0139] (4) The pre-calcined precursor is calcined, crushed and demagnetized to obtain carbon-coated lithium iron phosphate.
[0140] The calcination process involves an inert gas atmosphere, a temperature of 600℃, and a time of 3 hours.
[0141] (5) Carbon-coated lithium iron phosphate prepared with anhydrous iron phosphate of different particle sizes is mixed and proportioned to obtain lithium iron phosphate cathode material.
[0142] Table 5. Graded Scheme of Example 3
[0143] Example 4
[0144] The difference from Example 3 is that the gel was dried using ordinary drying, that is, the gel was dried in a 100°C forced-air oven until constant weight;
[0145] Table 6. Graded Scheme of Example 4
[0146] Example 5
[0147] The difference from Example 3 is that in step (1), the polymer in the polymer solution containing lithium ions is 3% by mass; and the amount of curing agent added is 5% of the polymer.
[0148] Table 7. Graded Scheme of Example 5
[0149] Example 6
[0150] (1) Lithium oxalate, zirconium acetate and guar gum were dissolved in deionized water at 50°C to obtain a polymer solution containing lithium ions. Anhydrous ferric phosphate was added and stirred to keep the dispersion uniform. Then ethylene glycol diglycidyl ether was added and heated to 80°C and stirred continuously until the viscosity was too high to stir. The reaction was stopped to obtain a gel with a solid content of 34.9%.
[0151] The lithium ion concentration in the lithium-ion polymer solution is 0.5M, and the mass percentage of the polymer in the lithium-ion polymer solution is 2%. The amount of curing agent added is 1% of the polymer. Anhydrous iron phosphate and dopant are added according to the molar ratio of the total molar amount of Fe and dopant metal elements to Li elements of 1:1.1, and the molar ratio of iron to zirconium is 0.98:0.02.
[0152] (2) The gel is crushed by passing it through an 80-mesh sieve under pressure, and then sent to a fluidized bed to dry to constant weight to obtain the precursor.
[0153] The fluidized bed temperature was 100℃ and the air flow rate was 200mL / min.
[0154] (3) The precursor is placed in a flash dryer for pre-calcination to obtain a pre-calcined precursor.
[0155] The flash dryer has an inlet air temperature of 500℃, an outlet air temperature of 150℃, a rotation speed of 10Hz, and a feeding frequency of 30Hz.
[0156] (4) The pre-calcined precursor is calcined, crushed and demagnetized to obtain carbon-coated lithium iron phosphate.
[0157] The calcination process involved an inert gas atmosphere, a temperature of 550℃, and a time of 4 hours.
[0158] (5) Carbon-coated lithium iron phosphate prepared with anhydrous iron phosphate of different particle sizes is mixed and proportioned to obtain lithium iron phosphate cathode material.
[0159] Table 8. Graded Scheme of Example 6
[0160] Example 7
[0161] The difference from Example 3 is that in step (3), the inlet air temperature of the flash evaporator is 300°C and the outlet air temperature is 100°C.
[0162] Table 9. Graded Scheme of Example 7
[0163] Example 8
[0164] The difference from Example 3 is that the amount of curing agent added in step (1) is 1% of the polymer.
[0165] Table 10. Graded Scheme of Example 8
[0166] Example 9
[0167] The difference from Example 3 is that the amount of curing agent added in step (1) is 5% of the polymer.
[0168] Table 11 Graded Scheme of Example 9
[0169] Example 10
[0170] The difference from Example 3 is that the flash evaporator conditions in step (3) are as follows: the inlet air temperature is 500℃, the outlet air temperature is 150℃, the rotation speed is 30Hz, and the feeding frequency is 10Hz.
[0171] Table 12 Graded Scheme of Example 10
[0172] Comparative Example 1
[0173] The difference from Example 3 is that no curing agent is added in step (1). The steps are as follows:
[0174] Lithium acetate and agar were dissolved in deionized water at 50°C to obtain a polymer solution containing lithium ions. Anhydrous ferric phosphate was added and stirred to maintain uniform dispersion. The solution was then heated to 80°C and stirred continuously until the solvent was completely evaporated to obtain precursor 1.
[0175] The lithium ion concentration in the polymer solution containing lithium ions is 0.5M, and the mass percentage of the polymer in the polymer solution containing lithium ions is 2%. Anhydrous ferric phosphate is fed according to a molar ratio of Fe to Li of 1:1.01.
[0176] (2) Precursor 1 is fed into a fluidized bed and dried to constant weight to obtain precursor 2.
[0177] The fluidized bed temperature was 100℃ and the air flow rate was 200mL / min.
[0178] (3) Precursor 2 is placed in a flash dryer for pre-calcination to obtain pre-calcined precursor.
[0179] The flash dryer has an inlet air temperature of 400℃, an outlet air temperature of 150℃, a rotation speed of 30Hz, and a feeding frequency of 20Hz.
[0180] (4) The pre-calcined precursor is calcined, crushed and demagnetized to obtain carbon-coated lithium iron phosphate.
[0181] The calcination process involves an inert gas atmosphere, a temperature of 600℃, and a time of 3 hours.
[0182] (5) Carbon-coated lithium iron phosphate prepared with anhydrous iron phosphate of different particle sizes is mixed and proportioned to obtain lithium iron phosphate cathode material.
[0183] Table 13 Comparative Example 1 Grading Scheme
[0184] Comparative Example 2
[0185] Anhydrous ferric phosphate, lithium acetate, and agar powder were ball-milled and mixed, then calcined at 600°C for 6 hours in a rotary kiln. After crushing and demagnetization, carbon-coated lithium iron phosphate was obtained. Anhydrous ferric phosphate and lithium acetate were mixed at a molar ratio of n(Fe):n(Li) = 1:1.01. The mixing ratio of anhydrous ferric phosphate, lithium acetate, and agar powder was the same as in Example 3.
[0186] Table 14 Comparative Example 2 Grading Scheme
[0187] Comparative Example 3
[0188] Example 4, No. 1: Carbon-coated lithium iron phosphate.
[0189] Performance characterization:
[0190] Figure 4 shows TEM images of the lithium iron phosphate cathode materials prepared in Comparative Example 1 and Example 3. It can be seen that the lithium iron phosphate cathode material prepared in Example 3 has better uniformity.
[0191] Figure 5 shows the SEM image of the lithium iron phosphate cathode material prepared in Example 1, and Figure 6 shows the SEM image of the lithium iron phosphate cathode material prepared in Example 3. It can be seen that the prepared lithium iron phosphate cathode material contains both large and small particles. The average sphericity of the large particles (Dv50 and above) in Example 3 is higher than that in Example 1.
[0192] Based on Tables 1 and 2 and the preparation methods, the various examples and comparative examples are analyzed:
[0193] Specifically, the excessively high compression index in Comparative Example 1 resulted in a low compaction density, leading to a higher electrode resistance and consequently a lower energy density. This was because the absence of a curing agent significantly reduced the uniformity of the carbon coating layer (see TEM image), resulting in a higher compression index.
[0194] Example 1 Suppression Index The compressibility index meets the requirements of 0.4-5.0, but the particle strength does not meet the requirements of 40MPa-150MPa. Example 1 has a higher particle strength. Compared with Example 5, the two have similar compressibility indices. However, due to the excessively high particle strength of Example 1, its compaction density is lower. The reason is that the particle strength of Example 5 is lower. When calculating the compaction density, the particles will be crushed to release stress and thus increase the compaction density. However, the fragmentation of the particles will lead to a longer electron / ion transport path, which will lead to an increase in the resistance of the electrode. The fragmentation of the particles will also lead to a decrease in cycle performance.
[0195] Example 2 Suppression Index The compressibility factor meets the requirements of 0.4-5.0, the particle strength meets the requirements of 40MPa-150MPa, but does not meet the requirements of 60MPa-120MPa, and the compressibility factor does not meet the requirements of 120MPa-300MPa. In Example 2, due to the large number of small-sized lithium iron phosphate particles, it has a lower compressibility index and therefore a higher compaction density, but also a higher electrode resistance. In addition, the particle strength of Example 2 is lower, so the cycle capacity retention rate is lower.
[0196] Examples 3-5 have similar particle strength but different compaction indices. Considering the percentage content of the carbon coating, average roundness, and gradation scheme, the differences in compaction indices stem from the percentage content of the carbon coating and the average roundness. Performance testing shows that as the compaction index increases, the powder compaction density decreases, thus decreasing the electrode compaction density. The electrode resistance increases slightly because a certain increase in carbon content improves conductivity, but the decrease in roundness leads to an increase in particle interfacial resistance. Therefore, the change in electrode resistance under these two forces is not significant.
[0197] Examples 3, 6, and 7 have similar compression indices but different particle strengths. The greater the particle strength, the lower the compaction density of the powder and the electrode, but the lower the electrode resistance and the higher the cycle capacity retention. Although the average roundness of Examples 6 and 7 is relatively small, their gradation effect is better and there are more small-diameter lithium iron phosphate particles. Therefore, their compression indices are similar to those of Example 3, indicating that the powder movement ability of lithium iron phosphate cathode material can be controlled by adjusting the process parameters of the preparation method disclosed in this paper.
[0198] Examples 3, 6, 8, and 10 have compressibility factors ranging from 120 MPa to 300 MPa. Therefore, compared to other examples, they have the best overall performance, balancing good compaction density, electrode conductivity, and capacity retention.
[0199] In the preparation processes of Examples 3, 8, and 9, different amounts of curing agent were added. The results showed that a larger amount of curing agent resulted in a higher degree of gel cross-linking, leading to a higher compression ratio (CPR) and slightly increased particle strength in the resulting lithium iron phosphate cathode material. Further analysis revealed little difference in the roundness, carbon coating content, and gradation scheme among the three examples. However, when no curing agent was added (Comparative Example 1), the CPR of the lithium iron phosphate material was actually higher than that of Example 9. This is because without the curing agent, a gel cannot be formed, making it impossible to fix the quantity of iron phosphate, lithium ions, and polymers per unit volume. Consequently, the resulting lithium iron phosphate material exhibits severe agglomeration, resulting in a larger Dv50 and a larger span. Furthermore, TEM images of Examples 3 and 1 show that Comparative Example 1, without the addition of a curing agent, exhibited poorer carbon coating uniformity than Example 3, thus resulting in a larger CPR. This indicates that the addition and amount of curing agent significantly affect the uniformity of the carbon coating. Neglecting the curing agent or using excessive curing agent is detrimental to the formation of a uniform coating layer, leading to an increased CPR.
[0200] In the preparation process of Examples 3, 7, and 10, different flash evaporation conditions were used. Specifically, the higher the inlet air temperature during flash evaporation, the higher the particle strength and the better the roundness of the material. However, since the gradation scheme of Example 7 contains more small-diameter lithium iron phosphate particles, Examples 3 and 7 have similar compaction indices. Since the gradation scheme of Example 10 is similar to that of Example 3, Example 10 has a lower compaction index due to its higher average roundness. Although its hardness is higher, its compaction index is lower, so the compaction density of the powder is similar to that of Example 3.
[0201] Comparative Example 2 uses a solid-state method to mix polymer and lithium salt with anhydrous iron phosphate. The resulting lithium iron phosphate cathode material has a larger compression index, resulting in lower compaction density of both powder and electrode, and higher electrode resistance.
[0202] Comparative Example 3 did not undergo gradation compared to Example 4, therefore its compression index was relatively large.
[0203] The above are merely preferred embodiments of this disclosure and are not intended to limit this disclosure. Various modifications and variations can be made to this disclosure by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this disclosure should be included within the scope of protection of this disclosure. Industrial applicability
[0204] This disclosure employs a specific method to test the suppression index of lithium iron phosphate cathode materials. By ensuring the compression index meets a specific range, and the average sphericity of larger particles within the Dv50-Dv100 range meets a suitable range, this type of lithium iron phosphate cathode material exhibits superior particle mobility. During compression, the particles can move and rearrange at a faster speed, which is beneficial for obtaining a cathode sheet with high compaction density and improving the charge and discharge capacity of the battery. It also helps to prevent particle breakage under pressure, giving it good compressive strength. The preparation method of this lithium iron phosphate cathode material is simple and easy to implement, facilitating industrial application and possessing good industrial practicality.
Claims
1. A lithium iron phosphate cathode material, characterized in that, The active material includes lithium iron phosphate, and the surface of the active material has a carbon coating layer. The average sphericity of the particles in the lithium iron phosphate cathode material within the Dv50-Dv100 range is greater than or equal to 0.6; The suppression index of the lithium iron phosphate cathode material The value is 0.4-5.0, wherein the suppression index is obtained by the following method: The density-pressure relationship curves of the lithium iron phosphate cathode material at different pressing speeds are obtained. The portion of the density-pressure relationship curve above 100 MPa is linearly fitted to obtain the corresponding slope. The relationship curve between pressing speed and slope is plotted. The slope of the fitted line obtained by linearly fitting the relationship curve between pressing speed and slope is the pressing exponent.
2. The lithium iron phosphate cathode material according to claim 1, characterized in that, The particle strength of the lithium iron phosphate cathode material is 40MPa-150MPa.
3. The lithium iron phosphate cathode material according to claim 2, characterized in that, The lithium iron phosphate cathode material satisfies at least one of the following characteristics: Feature A1: Particle strength is 60MPa-120MPa; Feature A2: The average roundness of particles in the Dv50-Dv100 range is 0.65-0.80; Feature A3: Compacted density is 1.5 g / cm³ 3 -3.0g / cm 3 ; Feature A4: The carbon coating layer accounts for 0.5%-5% of the mass of the lithium iron phosphate cathode material.
4. The lithium iron phosphate cathode material according to any one of claims 1-3, characterized in that, definition: The lithium iron phosphate cathode material has a compressibility factor of 120MPa-300MPa.
5. A method for preparing the lithium iron phosphate cathode material according to any one of claims 1-4, characterized in that, include: Ferric phosphate and a lithium-ion-containing polymer solution are mixed, and then mixed with a curing agent to carry out a curing reaction to obtain a gel; wherein, the lithium-ion-containing polymer solution is obtained by dissolving a soluble lithium salt and a polymer in a solvent; The gel was broken down and dried to obtain the precursor. The precursor was calcined to prepare carbon-coated lithium iron phosphate; The carbon-coated lithium iron phosphate particles of different sizes were mixed in a specific ratio.
6. The preparation method according to claim 5, characterized in that, The process for preparing the gel has at least one of the following characteristics: Feature B1: The polymer is a soluble polymer containing at least one of hydroxyl, vinyl, and epoxy groups; the curing agent is selected according to the active groups contained in the polymer; when the polymer contains hydroxyl groups, the curing agent is glycidyl ether; when the polymer contains vinyl groups, the curing agent is a soluble photoinitiator; when the polymer contains epoxy groups, the curing agent is a polyol. Feature B2: The soluble lithium salt is selected from at least one of lithium chloride, lithium hydroxide, lithium acetate, and lithium oxalate; Feature B3: The solvent is selected from at least one of water, ethanol, methanol, and isopropanol; Feature B4: The concentration of lithium ions in the lithium-ion-containing polymer solution is 0.2M-0.8M, the mass percentage of polymer in the lithium-ion-containing polymer solution is 1%-3%, and the mass ratio of curing agent to polymer is (1-5):100; Feature B5: Anhydrous iron phosphate, a dopant, and the lithium-ion-containing polymer solution are mixed, and then mixed with a curing agent to carry out a curing reaction. The dopant is a soluble compound containing a dopant element, and the dopant element is selected from at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn, Y, Mg, Na, N, S, and F; the molar ratio of the total molar amount of iron in the anhydrous iron phosphate to the total molar amount of the dopant element to the molar amount of lithium is controlled to be 1:(1.01-1.10). Feature B6: The curing reaction method is selected from at least one of heating and stirring reaction, ultraviolet irradiation and stirring reaction, and microwave irradiation and stirring reaction; when the curing reaction is carried out by heating and stirring reaction, the reaction temperature is controlled at 50℃-80℃; when the curing reaction is carried out by ultraviolet irradiation and stirring reaction, the light wavelength is controlled at 200nm-400nm; when the curing reaction is carried out by microwave irradiation and stirring reaction, the microwave irradiation power is controlled at 200W-500W. Feature B7: The solid content of the prepared gel is 30%-50%.
7. The preparation method according to claim 5, characterized in that, The gel was broken down and then fluidized and dried. Fluidized drying refers to drying the fragmented gel in a fluidized bed, controlling the drying temperature at 80℃-120℃ and the air flow rate at 50mL / min-400mL / min.
8. The preparation method according to claim 5, characterized in that, The process of preparing the carbon-coated lithium iron phosphate from the precursor has at least one of the following characteristics: Characteristic C1: Calcination temperature is 550℃-650℃, and calcination time is 2h-6h; Characteristic C2: Calcination is carried out under an inert atmosphere; Feature C3: The precursor is preheated in a flash dryer before calcination. The inlet air temperature of the flash dryer is controlled at 300℃-500℃, the outlet air temperature is controlled at 100℃-150℃, the rotation speed is controlled at 10Hz-30Hz, and the feeding frequency is controlled at 10Hz-30Hz. Feature C4: After the precursor is calcined, it is then crushed and demagnetized to obtain the carbon-coated lithium iron phosphate.
9. The preparation method according to claim 5, characterized in that, The method of proportioning the carbon-coated lithium iron phosphate with different particle sizes is selected from either the first method or the second method; The process of proportioning using the first method includes: mixing two types of carbon-coated lithium iron phosphate with different particle size distributions in a certain proportion, wherein the Dv50 of the two types of carbon-coated lithium iron phosphate with different particle size ranges are Dv50 and Dv50 respectively. a Dv50 b , of which 15μm≥Dv50 a ≥2.4Dv50 b ≥1μm; with Dv50 a and Dv50 b The mass ratio of the two carbon-coated lithium iron phosphates is (20%–35%) : (65%–80%). The process of proportioning using the second method includes: mixing three types of carbon-coated lithium iron phosphate with different particle size distributions in a certain proportion, wherein the Dv50 of the three types of carbon-coated lithium iron phosphate with different particle size ranges are respectively Dv50. a Dv50 b Dv50 c , of which 15μm≥Dv50 a ≥2.4Dv50 b ≥2.4 2 Dv50 c ≥2μm; with Dv50 a Dv50 b and Dv50 c The mass ratio of the three carbon-coated lithium iron phosphates is (2%–20%): (40%–60%): (30%–40%).
10. A lithium battery, characterized in that, The lithium iron phosphate cathode material includes any one of claims 1-4 or any one of claims 5-9.