A positive electrode material, a preparation method thereof and a lithium ion battery

Abstract

The application provides a positive electrode material and a preparation method thereof and a lithium ion battery, and belongs to the technical field of battery materials. The application comprises the following steps: acid treatment of iron oxide to obtain acid-modified iron oxide; mixing of the acid-modified iron oxide, a second iron source, a lithium source and a phosphorus source to obtain a mixture, and then roasting treatment of the mixture to obtain a positive electrode material. Through the acid treatment of the iron oxide, on the one hand, the impurities and loose oxide layers on the surface of the iron oxide can be removed, more active sites are exposed, the reactivity of the iron oxide and the second iron source is matched, the reaction process is cooperatively carried out, the formation of regular and uniform crystal structures is promoted, and a foundation for obtaining the positive electrode material with excellent electrochemical performance is laid; on the other hand, the surface of the iron oxide particles can be regulated, so that the agglomeration between the particles can be reduced when the particles are mixed with the second iron source particles, the dispersibility of the particles is improved, and the mixture can form an excellent gradation.

Description

Technical Field

[0001] This invention belongs to the field of battery materials technology, specifically relating to a cathode material, its preparation method, and a lithium-ion battery. Background Technology

[0002] Lithium iron phosphate (LiFePO4) material, as the cathode material of lithium-ion batteries, has become the mainstream choice for power batteries and energy storage systems due to its high safety, long cycle life, low cost and environmental friendliness.

[0003] However, lithium iron phosphate materials have poor intrinsic conductivity, which leads to severe polarization during high-current charge and discharge, limiting their rate performance; insufficient capacity performance restricts the improvement of energy density; and the capacity decays rapidly in the later stages of cycling, so the lifetime still needs to be optimized.

[0004] To address these issues, researchers have conducted extensive studies on material structure, preparation processes, and interface control. For example, carbon coating has been introduced to improve the conductivity of lithium iron phosphate materials and suppress particle agglomeration. However, uneven or excessive carbon coating thickness can increase interfacial impedance, leading to increased polarization voltage at high rates. Sol-gel or solvothermal methods have been used to prepare nanoparticles, shortening the lithium-ion diffusion path and increasing the electrolyte contact area. However, nanoparticles are prone to agglomeration during cycling, resulting in decreased utilization of active materials. Spray drying has been used to prepare secondary particle structures to improve the capacity performance of lithium iron phosphate materials. Although secondary particle structures can alleviate early capacity loss, capacity retention still decreases with increasing cycle count due to lithium dendrite growth and interfacial side reactions in the later stages of cycling, especially under high temperature (>45℃) conditions. Furthermore, this process exhibits significant batch-to-batch differences in particle size distribution, which can lead to increased battery performance dispersion and is detrimental to industrial production.

[0005] Therefore, how to further optimize the structure of lithium iron phosphate materials and improve their electrochemical performance is a technical problem that urgently needs to be solved. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a cathode material, its preparation method, and a lithium-ion battery. This invention achieves surface modification of iron oxide through acid treatment. On one hand, it removes impurities and loose oxide layers from the iron oxide surface, exposing more active sites and significantly improving the reactivity of the iron oxide. This allows the reactivity of the iron oxide to match that of the second iron source, achieving synergistic reaction and promoting the formation of a regular, uniformly sized crystal structure, laying the foundation for obtaining a cathode material with excellent electrochemical performance. On the other hand, it also regulates the surface of the iron oxide particles. The acid-modified iron oxide particles have a more uniform surface charge distribution, reducing particle agglomeration and improving particle dispersibility when mixed with the second iron source particles, resulting in a better particle size distribution in the mixture. The cathode material prepared using this method exhibits excellent electrochemical performance.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a method for preparing a cathode material, the method comprising the following steps:

[0009] Iron oxide is acid-treated to obtain acid-modified iron oxide.

[0010] The acid-modified iron oxide, the second iron source, the lithium source, and the phosphorus source are mixed to obtain a mixture, and then the mixture is calcined to obtain the cathode material.

[0011] This invention achieves surface modification of iron oxide through acid treatment. On one hand, it removes impurities and loose oxide layers from the iron oxide surface, exposing more active sites and significantly improving the reactivity of the iron oxide. This allows the reactivity of the iron oxide to match that of the second iron source, achieving synergistic reaction and promoting the formation of a regular, uniformly sized crystal structure. This lays the foundation for obtaining a cathode material with excellent electrochemical performance. On the other hand, it also regulates the surface of the iron oxide particles. The acid-modified iron oxide particles have a more uniform surface charge distribution, reducing particle agglomeration and improving particle dispersibility when mixed with the second iron source particles, resulting in a better particle size distribution in the mixture. The cathode material prepared by this method exhibits excellent electrochemical performance.

[0012] This invention alters the surface properties of iron oxide by acid treatment, making it easier to refine during the mixing process.

[0013] It should be noted that if iron oxide is not acid-treated, the untreated iron oxide will have low activity due to the presence of surface impurities and oxide layers. It will be difficult to play a full role in the crystal growth reaction, and its participation will be low, which will easily lead to an increase in crystal structure defects and affect the electrochemical performance of the cathode material. The surface properties of iron oxide particles are uneven, which makes them prone to agglomeration with the second iron source particles, resulting in unreasonable particle size distribution and making it difficult to achieve the ideal size distribution effect. The reaction process between untreated iron oxide and the second iron source is not synchronized (for example, there may be excessive reaction of iron phosphate while the reaction of iron oxide is incomplete), which can easily lead to side reactions or uneven composition, affecting the electrochemical performance of the cathode material.

[0014] Preferably, the acid solution used in the acid treatment process includes inorganic acids and / or organic acids.

[0015] Preferably, the inorganic acid includes any one or a combination of at least two of phosphoric acid, nitric acid, hydrochloric acid, and sulfuric acid.

[0016] Preferably, the organic acid includes any one or a combination of at least two of oxalic acid, citric acid, malic acid, tartaric acid, and acetic acid.

[0017] Preferably, the concentration of the acid solution is 0.05-0.5 mol / L, for example, it can be 0.05 mol / L, 0.1 mol / L, 0.15 mol / L, 0.2 mol / L, 0.25 mol / L, 0.3 mol / L, 0.35 mol / L, 0.4 mol / L, 0.45 mol / L, or 0.5 mol / L, etc.

[0018] In this invention, the appropriate concentration of acid solution is beneficial for dissolving the oxide layer on the surface of iron oxide while maintaining its nanoparticle structure, inhibiting the dissolution of the matrix iron, and achieving only surface reaction without destroying the structure of the iron oxide itself.

[0019] Preferably, the solid-liquid ratio of the iron oxide and the acid solution is 1g:(2-4)mL, for example, it can be 1g:2mL, 1g:3mL or 1g:4mL, etc.

[0020] This invention employs a suitable solid-liquid ratio, which helps to efficiently remove the surface oxide layer while ensuring the structural integrity of iron oxide particles. This achieves surface activation and efficient removal of impurities, while avoiding excessive dissolution of the matrix structure, thus providing an ideal precursor for subsequent lithium iron phosphate synthesis.

[0021] It should be noted that the solid-liquid ratio refers to the ratio of the mass of iron oxide to the volume of the acid solution.

[0022] Preferably, the acid treatment is a staged acid treatment, which includes a first stage and a second stage performed sequentially, wherein the temperature of the first stage is lower than the temperature of the second stage.

[0023] In this invention, a staged acid treatment using a gradient heating method helps to balance the dissolution efficiency of the oxide layer with the protection of the iron oxide particle structure, avoiding explosive dissolution caused by single-stage high-temperature treatment.

[0024] Preferably, the acid used in the first stage is the same as or different from the acid used in the second stage.

[0025] Preferably, the temperature of the first stage is 30-60℃, for example, it can be 30℃, 40℃, 50℃ or 60℃, and the heat preservation time is 3-7h, for example, it can be 3h, 4h, 5h, 6h or 7h.

[0026] Preferably, the temperature of the second stage is 50-80℃, for example, 50℃, 60℃, 70℃ or 80℃, and the heat preservation time is 0.5-3h, for example, 0.5h, 1h, 2h or 3h.

[0027] Preferably, the heating rate in the first stage is 0.2-0.6℃ / min, for example, it can be 0.2℃ / min, 0.3℃ / min, 0.4℃ / min, 0.5℃ / min or 0.6℃ / min, etc.

[0028] Preferably, the heating rate in the second stage is 0.2-0.6℃ / min, for example, it can be 0.2℃ / min, 0.3℃ / min, 0.4℃ / min, 0.5℃ / min or 0.6℃ / min, etc.

[0029] Preferably, the D50 of the acid-modified iron oxide is 0.1-1.5 μm, for example, it can be 0.1 μm, 0.3 μm, 0.5 μm, 0.7 μm, 0.9 μm, 1.1 μm, 1.3 μm or 1.5 μm, etc.

[0030] It should be noted that the D50 of acid-modified iron oxide refers to the particle size corresponding to a cumulative particle size distribution percentage of 50% for a sample. The same applies to the following.

[0031] Preferably, the second iron source includes any one or a combination of at least two of ferrous oxalate, ferric phosphate dihydrate, and anhydrous ferric phosphate.

[0032] Preferably, the D50 of the second iron source is 2-15μm, for example, it can be 2μm, 4μm, 6μm, 8μm, 10μm, 12μm, 14μm or 15μm, etc.

[0033] Preferably, the molar ratio of iron in the acid-modified iron oxide to iron in the second iron source is (0.05-0.5):1, for example, it can be 0.05:1, 0.1:1, 0.2:1, 0.3:1, 0.4:1 or 0.5:1, etc.

[0034] In this invention, the acid-modified iron oxide and the second iron source, with a suitable molar ratio, work synergistically to better exert their synergistic effect, fill the pores between particles, optimize the particle packing density, and achieve a superior matching effect.

[0035] Preferably, the lithium source includes any one or a combination of at least two of lithium carbonate, lithium hydroxide, lithium acetate, lithium acetate, lithium citrate, and lithium dihydrogen phosphate.

[0036] Preferably, the phosphorus source includes any one or a combination of at least two of the following: ammonium dihydrogen phosphate, anhydrous ferric phosphate, ferric phosphate dihydrate, phosphoric acid, and lithium dihydrogen phosphate.

[0037] Preferably, the molar ratio of Fe:Li:P in the mixture is 1:(1-1.07):(1.01-1.05), wherein the range of Li "1-1.07" can be, for example, 1, 1.01, 1.02, 1.03, 1.04 or 1.05, and the range of P "1.01-1.05" can be, for example, 1.01, 1.02, 1.03, 1.04 or 1.05.

[0038] Preferably, the mixing process is accompanied by grinding.

[0039] Preferably, the grinding method includes ball milling or sand milling.

[0040] Preferably, the grinding rate is 500-1200 rpm, for example, it can be 500 rpm, 600 rpm, 700 rpm, 800 rpm, 900 rpm, 1000 rpm, 1100 rpm or 1200 rpm, etc.

[0041] Preferably, the D50 of the mixture is 0.35-0.85μm, for example, it can be 0.35μm, 0.45μm, 0.55μm, 0.65μm, 0.75μm or 0.85μm, etc.

[0042] Preferably, the roasting process includes a first-stage roasting and a second-stage roasting, wherein the temperature of the first-stage roasting is lower than the temperature of the second-stage roasting.

[0043] This invention employs a multi-stage calcination method with gradient heating, which helps to form a "dense nucleus-porous carbon shell" structure in the cathode material particles, avoiding problems such as uneven carbon coating and LiFePO4 lattice defects caused by excessive temperature span in traditional single-stage calcination.

[0044] Preferably, the temperature of the first-stage roasting is 300-700℃, for example, 300℃, 400℃, 500℃, 600℃ or 700℃, and the holding time is 4-9h, for example, 4h, 5h, 6h, 7h, 8h or 9h.

[0045] Preferably, the temperature of the secondary calcination is 650-850℃, for example, 650℃, 700℃, 750℃, 800℃ or 850℃, and the holding time is 8-20h, for example, 8h, 10h, 12h, 14h, 16h, 18h or 20h.

[0046] Preferably, the atmosphere for the calcination treatment is an inert atmosphere, such as nitrogen or argon.

[0047] In a second aspect, the present invention provides a cathode material, which is prepared by the preparation method described in the first aspect.

[0048] Thirdly, the present invention provides a lithium-ion battery, wherein the positive electrode of the lithium-ion battery includes the positive electrode material as described in the second aspect.

[0049] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0050] Compared with the prior art, the present invention has the following beneficial effects:

[0051] (1) This invention achieves surface modification of iron oxide by acid treatment. On the one hand, it can remove impurities and loose oxide layer on the surface of iron oxide, expose more active sites, significantly improve the reactivity of iron oxide, make the reactivity of iron oxide and the second iron source match, realize the synergistic reaction process, promote the formation of regular and uniform crystal structure, and lay the foundation for obtaining positive electrode material with excellent electrochemical performance. On the other hand, it can also regulate the surface properties of iron oxide particles. The surface charge distribution of acid-modified iron oxide particles is more uniform. When mixed with the second iron source particles, it can reduce the agglomeration between particles, improve the dispersibility of particles, and make the mixture form a better particle size distribution.

[0052] (2) The cathode material prepared by this method has excellent electrochemical performance. Attached Figure Description

[0053] Figure 1 This is a process flow diagram provided for Embodiment 1 of the present invention. Detailed Implementation

[0054] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0055] Example 1

[0056] This embodiment provides a method for preparing a cathode material, the process flow diagram of which is shown below. Figure 1 As shown, the preparation method includes the following steps:

[0057] (1) Iron oxide with a D50 of 0.9 μm was mixed with acid solution and heated to 45 °C at a heating rate of 0.3 °C / min for the first stage of acid treatment. Then, the temperature was increased to 65 °C at a heating rate of 0.3 °C / min for the second stage of acid treatment to obtain acid-modified iron oxide with a D50 of 0.88 μm.

[0058] The acid solution is a phosphoric acid solution with a concentration of 0.15 mol / L; the solid-liquid ratio of iron oxide to acid solution is 1 g: 3 mL.

[0059] (2) The acid-modified iron oxide, the second iron source, the lithium source and the phosphorus source were ball-milled and mixed to obtain a mixture with a D50 of 0.45 μm.

[0060] The second iron source is iron phosphate dihydrate with a D50 of 4.31 μm, the lithium source is lithium carbonate, and the phosphorus source is ammonium dihydrogen phosphate. The molar ratio of iron in the acid-modified iron oxide to that in the second iron source is 0.15:1. The molar ratio of Fe:Li:P in the mixture is 1:1.03:1.04. The ball milling speed is 1000 rpm.

[0061] (3) Under a nitrogen atmosphere, the mixture is subjected to a first-stage calcination and a second-stage calcination in sequence. After the calcination is completed, it is cooled, washed and dried to obtain the positive electrode material.

[0062] The first-stage roasting temperature is 400℃ and the holding time is 6 hours; the second-stage roasting temperature is 800℃ and the holding time is 10 hours.

[0063] Example 2

[0064] This embodiment provides a method for preparing a cathode material, the method comprising the following steps:

[0065] (1) Iron oxide with a D50 of 1.2 μm was mixed with acid solution and heated to 30 °C at a heating rate of 0.3 °C / min for the first stage of acid treatment. Then, the temperature was increased to 50 °C at a heating rate of 0.3 °C / min for the second stage of acid treatment to obtain acid-modified iron oxide with a D50 of 1.15 μm.

[0066] The acid solution is a nitric acid solution with a concentration of 0.3 mol / L; the solid-liquid ratio of iron oxide to acid solution is 1 g: 2 mL.

[0067] (2) The acid-modified iron oxide, the second iron source, the lithium source and the phosphorus source were ball-milled and mixed to obtain a mixture with a D50 of 0.4 μm.

[0068] The second iron source is iron phosphate dihydrate with a D50 of 5.58 μm, the lithium source is lithium carbonate, the phosphorus source is ammonium dihydrogen phosphate, the molar ratio of iron in the acid-modified iron oxide to iron in the second iron source is 0.5:1, the molar ratio of Fe:Li:P in the mixture is 1:1.03:1.04, and the ball milling speed is 500 rpm.

[0069] (3) Under a nitrogen atmosphere, the mixture is subjected to a first-stage calcination and a second-stage calcination in sequence. After the calcination is completed, it is cooled, washed and dried to obtain the positive electrode material.

[0070] The first-stage roasting temperature is 300℃ and the holding time is 9 hours; the second-stage roasting temperature is 750℃ and the holding time is 20 hours.

[0071] Example 3

[0072] This embodiment provides a method for preparing a cathode material, the method comprising the following steps:

[0073] (1) Iron oxide with a D50 of 1.2 μm was mixed with acid solution and heated to 60 °C at a heating rate of 0.5 °C / min for the first stage of acid treatment. Then, the temperature was increased to 80 °C at a heating rate of 0.5 °C / min for the second stage of acid treatment to obtain acid-modified iron oxide with a D50 of 1 μm.

[0074] The acid solution is an oxalic acid solution with a concentration of 0.5 mol / L; the solid-liquid ratio of iron oxide to acid solution is 1 g: 4 mL.

[0075] (2) The acid-modified iron oxide, the second iron source, the lithium source and the phosphorus source were ball-milled and mixed to obtain a mixture with a D50 of 0.38 μm.

[0076] The second iron source is ferrous oxalate with a D50 of 3.2 μm, the lithium source is lithium carbonate, the phosphorus source is ammonium dihydrogen phosphate, the molar ratio of iron in the acid-modified iron oxide to iron in the second iron source is 0.25:1, the molar ratio of Fe:Li:P in the mixture is 1:1.05:1.03, and the ball milling speed is 1200 rpm.

[0077] (3) Under a nitrogen atmosphere, the mixture is subjected to a first-stage calcination and a second-stage calcination in sequence. After the calcination is completed, it is cooled, washed and dried to obtain the positive electrode material.

[0078] The first-stage roasting temperature is 350℃, and the holding time is 8 hours; the second-stage roasting temperature is 830℃, and the holding time is 8 hours.

[0079] Example 4

[0080] The difference between this embodiment and Embodiment 1 is that after the acid treatment in the first stage, filtration and washing are performed, and then the washed iron oxide is placed in a sulfuric acid solution with a concentration of 0.15 mol / L, and heated to 65°C at a heating rate of 0.3°C / min for the second stage of acid treatment.

[0081] The remaining preparation methods and parameters are consistent with those in Example 1.

[0082] Example 5

[0083] The difference between this embodiment and Embodiment 1 is that the concentration of the acid solution in step (1) is 0.01 mol / L.

[0084] The remaining preparation methods and parameters are consistent with those in Example 1.

[0085] Example 6

[0086] The difference between this embodiment and Embodiment 1 is that the concentration of the acid solution in step (1) is 1.5 mol / L.

[0087] The remaining preparation methods and parameters are consistent with those in Example 1.

[0088] Example 7

[0089] The difference between this embodiment and embodiment 1 is that the solid-liquid ratio of iron oxide and acid in step (1) is 1g:6mL.

[0090] The remaining preparation methods and parameters are consistent with those in Example 1.

[0091] Example 8

[0092] The difference between this embodiment and embodiment 1 is that the solid-liquid ratio of iron oxide and acid in step (1) is 1g:1mL.

[0093] The remaining preparation methods and parameters are consistent with those in Example 1.

[0094] Example 9

[0095] The difference between this embodiment and Embodiment 1 is that the first stage of acid treatment is not performed.

[0096] The remaining preparation methods and parameters are consistent with those in Example 1.

[0097] Example 10

[0098] The difference between this embodiment and Embodiment 1 is that the second stage of acid treatment is not performed.

[0099] The remaining preparation methods and parameters are consistent with those in Example 1.

[0100] Example 11

[0101] The difference between this embodiment and Embodiment 1 is that the temperature of the acid treatment in the first stage is 80°C.

[0102] The remaining preparation methods and parameters are consistent with those in Example 1.

[0103] Example 12

[0104] The difference between this embodiment and Embodiment 1 is that the temperature of the acid treatment in the second stage is 30°C.

[0105] The remaining preparation methods and parameters are consistent with those in Example 1.

[0106] Example 13

[0107] The difference between this embodiment and embodiment 1 is that the molar ratio of iron in the acid-modified iron oxide and the iron in the second iron source in step (2) is 0.02:1.

[0108] The remaining preparation methods and parameters are consistent with those in Example 1.

[0109] Example 14

[0110] The difference between this embodiment and embodiment 1 is that the molar ratio of iron in the acid-modified iron oxide and the iron in the second iron source in step (2) is 0.6:1.

[0111] The remaining preparation methods and parameters are consistent with those in Example 1.

[0112] Comparative Example 1

[0113] The difference between this comparative example and Example 1 is that step (1) is omitted, that is, the unmodified iron oxide, the second iron source, the lithium source and the phosphorus source are directly ball-milled and mixed.

[0114] The remaining preparation methods and parameters are consistent with those in Example 1.

[0115] Comparative Example 2

[0116] The difference between this comparative example and Example 1 is that no second iron source is added in step (2).

[0117] The remaining preparation methods and parameters are consistent with those in Example 1.

[0118] Performance testing

[0119] The lithium-ion battery is fabricated based on the cathode material provided in the above embodiments and comparative examples, and the specific steps include:

[0120] The above-mentioned positive electrode material, conductive agent (acetylene black), and binder (polyvinylidene fluoride) are mixed in a mass ratio of 8:1:1 and added to N-methylpyrrolidone solvent to obtain a positive electrode slurry. This slurry is then coated onto aluminum foil and dried to obtain a positive electrode sheet. Artificial graphite is used as the negative electrode sheet. The electrolyte is a 1 mol / L lithium hexafluorophosphate solution (the solvent includes ethylene carbonate, dimethyl carbonate, and diethyl carbonate in a volume ratio of 1:1:1), and the separator is a polyethylene separator. The above-mentioned positive electrode sheet, negative electrode sheet, and separator are assembled, and then the electrolyte is injected to obtain a CR2032 coin cell.

[0121] The capacity and cycle performance of the CR2032 button cell were tested under the following conditions: the first discharge capacity was tested at room temperature of 25°C with a charge / discharge voltage of 2.0-3.75V and an initial charge / discharge rate of 0.1C; the cycle performance was tested for 200 cycles at room temperature of 25°C with a charge / discharge voltage of 2.0-3.75V and a charge / discharge rate of 1C.

[0122] The 5C rate performance test was conducted under the following conditions: at room temperature of 25℃, the charge / discharge voltage was 2.0-3.75V, and the discharge specific capacity was 5C.

[0123] The test results are shown in Table 1.

[0124] Table 1

[0125]

[0126]

[0127] analyze:

[0128] As shown in Table 1, this invention achieves surface modification of iron oxide through acid treatment. On the one hand, it removes impurities and loose oxide layers from the iron oxide surface, exposing more active sites and significantly improving the reactivity of the iron oxide. This allows the reactivity of the iron oxide to match that of the second iron source, achieving synergistic reaction and promoting the formation of a regular, uniformly sized crystal structure, laying the foundation for obtaining a cathode material with excellent electrochemical performance. On the other hand, it also regulates the surface properties of the iron oxide particles. The surface charge distribution of the acid-modified iron oxide particles is more uniform, reducing particle agglomeration and improving particle dispersibility when mixed with the second iron source particles, resulting in a better particle size distribution in the mixture. The cathode material prepared by this method exhibits excellent electrochemical performance.

[0129] As can be seen from Examples 1 and 4, the acid treatment in the first stage and the acid treatment in the second stage use different acid solutions in synergy, which is beneficial to dissolving the oxide layer and impurities on the surface of iron oxide while controlling the surface roughness and enhancing the chemical bonding with the second iron source. This allows for a more complete chemical reaction with lithium source, phosphorus source, etc., laying the foundation for obtaining a cathode material with better gradation effect in the future.

[0130] As can be seen from the comparison between Example 1 and Examples 5-6, if the concentration of acid is too low, the surface oxide layer will not dissolve completely, leaving residual impurities and insufficient exposure of active sites, which will not be able to fully synergize with the second carbon source, resulting in poor electrical performance of the cathode material; if the concentration of acid is too high, over-dissolution is likely to occur, resulting in extremely low yield and failure to meet process requirements.

[0131] As can be seen from the comparison between Example 1 and Examples 7-8, if the solid-liquid ratio of iron oxide and acid is too small, although it does not affect the dissolution of the oxide layer and impurities, and the electrical performance does not deteriorate significantly, the excessive amount will increase the cost and is not conducive to cost reduction and efficiency improvement. If the solid-liquid ratio of iron oxide and acid is too large, the uniformity of acid wetting with iron oxide will be poor, the surface etching will be uneven, and the unmodified area will have poor reaction activity during sintering, which will affect the performance of lithium iron phosphate batteries.

[0132] As can be seen from the comparison between Example 1 and Examples 9-10, if the first stage of acid treatment is not performed, the untreated oxide layer needs to be dissolved in the second stage of acid treatment, which cannot fully activate the iron oxide, resulting in a deterioration in the electrical performance of the synthesized lithium iron phosphate cathode material; if the second stage of acid treatment is not performed, only part of the oxide layer and impurities are dissolved, and the iron oxide is not fully activated, which affects the performance of the material's electrical properties.

[0133] As can be seen from the comparison between Example 1 and Examples 11-12, if the temperature of the acid treatment in the first stage is too high, excessive dissolution will occur, which will not only dissolve the surface oxide layer and impurities, but also dissolve the particle structure of iron oxide, affecting the yield of acid-modified iron oxide; if the temperature of the acid treatment in the second stage is too low, the iron oxide will not be fully activated, affecting the performance of electrical properties.

[0134] A comparison of Examples 1 and 13-14 shows that if the molar ratio of iron in the acid-modified iron oxide to the iron in the second iron source is too small, the synergistic filling effect will be insufficient, affecting the electrical properties of the material; if the molar ratio of iron in the acid-modified iron oxide to the iron in the second iron source is too large, there will be too many small particles, resulting in low material bulk density and affecting the material energy density.

[0135] As can be seen from the comparison between Example 1 and Comparative Example 1, if step (1) is not performed, that is, the untreated iron oxide, the second iron source, the lithium source and the phosphorus source are directly ball-milled and mixed, the untreated iron oxide has low activity due to the presence of surface impurities and oxide layer. It is difficult to play a full role in the crystal growth reaction process, and the participation is low. This can easily lead to an increase in crystal structure defects and affect the electrochemical performance of the cathode material. The surface properties of the iron oxide particles are uneven, and they are easy to agglomerate with the second iron source particles, resulting in unreasonable particle size distribution and difficulty in achieving the ideal size distribution effect. The reaction process of the untreated iron oxide and the second iron source is not synchronized (for example, the iron phosphate reacts excessively while the iron oxide reacts incompletely), which can easily produce side reactions or uneven composition, affecting the electrochemical performance of the cathode material.

[0136] As can be seen from the comparison between Example 1 and Comparative Example 2, if the second iron source is not added in step (2) to work synergistically with the acid-modified iron oxide, although the 0.1C discharge does not deteriorate significantly, the discharge performance at high rates deteriorates, indicating that the removal of the second iron source is not conducive to the improvement of rate performance.

[0137] It should be noted that the present invention is illustrated through the above embodiments, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials used in the present invention, additions of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for preparing a lithium iron phosphate cathode material, characterized in that, The preparation method includes the following steps: Iron oxide is subjected to acid treatment to obtain acid-modified iron oxide; the concentration of the acid solution used in the acid treatment process is 0.05-0.5 mol / L; the solid-liquid ratio of the iron oxide to the acid solution is 1 g:(2-4) mL; the acid treatment is a staged acid treatment, which includes a first stage and a second stage performed sequentially; the temperature of the first stage is 30-60℃, and the holding time is 3-7 h; the temperature of the second stage is 50-80℃, and the holding time is 0.5-3 h; The acid-modified iron oxide, the second iron source, the lithium source and the phosphorus source are mixed to obtain a mixture, and then the mixture is calcined to obtain the lithium iron phosphate cathode material. The second iron source includes any one or a combination of two of ferrous oxalate, ferric phosphate dihydrate, and anhydrous ferric phosphate; the molar ratio of iron in the acid-modified iron oxide to iron in the second iron source is (0.05-0.5):

1.

2. The preparation method according to claim 1, characterized in that, The acid solution used in the acid treatment process includes inorganic acids and / or organic acids.

3. The preparation method according to claim 2, characterized in that, The inorganic acid includes any one or a combination of at least two of phosphoric acid, nitric acid, hydrochloric acid, and sulfuric acid.

4. The preparation method according to claim 2, characterized in that, The organic acid includes any one or a combination of at least two of oxalic acid, citric acid, malic acid, tartaric acid, and acetic acid.

5. The preparation method according to claim 1, characterized in that, The temperature in the first stage is lower than the temperature in the second stage.

6. The preparation method according to claim 1, characterized in that, The acid used in the first stage may be the same as or different from the acid used in the second stage.

7. The preparation method according to claim 1, characterized in that, The heating rate in the first stage is 0.2-0.6℃ / min.

8. The preparation method according to claim 1, characterized in that, The heating rate in the second stage is 0.2-0.6℃ / min.

9. The preparation method according to claim 1, characterized in that, The D50 of the acid-modified iron oxide is 0.1-1.5 μm.

10. The preparation method according to claim 1, characterized in that, The D50 of the second iron source is 2-15 μm.

11. The preparation method according to claim 1, characterized in that, The lithium source includes any one or a combination of at least two of lithium carbonate, lithium hydroxide, lithium acetate, lithium citrate, and lithium dihydrogen phosphate.

12. The preparation method according to claim 1, characterized in that, The phosphorus source includes any one or a combination of at least two of the following: ammonium dihydrogen phosphate, anhydrous ferric phosphate, ferric phosphate dihydrate, phosphoric acid, and lithium dihydrogen phosphate.

13. The preparation method according to claim 1, characterized in that, The molar ratio of Fe:Li:P in the mixture is 1:(1-1.07):(1.01-1.05).

14. The preparation method according to claim 1, characterized in that, The mixing process is accompanied by grinding.

15. The preparation method according to claim 14, characterized in that, The grinding method includes ball milling or sand milling.

16. The preparation method according to claim 14, characterized in that, The grinding rate is 500-1200 rpm.

17. The preparation method according to claim 1, characterized in that, The D50 of the mixture is 0.35-0.85 μm.

18. The preparation method according to claim 1, characterized in that, The roasting process includes primary roasting and secondary roasting, wherein the temperature of primary roasting is lower than that of secondary roasting.

19. The preparation method according to claim 18, characterized in that, The temperature of the first-stage roasting is 300-700℃, and the holding time is 4-9 hours.

20. The preparation method according to claim 18, characterized in that, The secondary roasting temperature is 650-850℃, and the holding time is 8-20h.

21. The preparation method according to claim 1, characterized in that, The atmosphere for the roasting process is an inert atmosphere.

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