Highly compacted low-iron-leaching lithium iron phosphate, preparation method thereof and lithium battery

Abstract

The application discloses high-compaction low-iron-dissolution lithium iron phosphate and a preparation method and a lithium battery thereof, and belongs to the technical field of lithium ion battery cathode materials. The preparation method comprises the following steps: mixing, granulating, drying a phosphoric acid iron precursor, a lithium source and a carbon source, and then sintering in a sintering kiln, wherein a mixed atmosphere mainly composed of nitrogen and containing trace oxygen is used in a high-temperature reaction section, the volume fraction of oxygen is 50ppm-3000ppm, and the temperature of the high-temperature reaction section is 800 DEG C-840 DEG C; and the target product is obtained after cooling, crushing and iron removal. By introducing controlled trace oxygen in the high-temperature crystallization section, the generation of iron phosphide by-products is inhibited, the compaction density of the obtained lithium iron phosphate material powder is not lower than 2.60g / cm 3 , the iron ion dissolution rate is not higher than 20mg / L, and the cycle stability is good. The method has strong process amplification and is suitable for continuous and large-scale production of high-compaction lithium iron phosphate materials for large-capacity energy storage cells.

Description

Technical Field

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

[0002] As the capacity of individual energy storage cells continues to increase, the requirements for energy density per unit volume, manufacturing cost, and safety throughout the entire life cycle are simultaneously rising at the system level. For lithium iron phosphate materials, in order to reduce the proportion of inactive materials in the electrode and improve the volume utilization rate of the cell, the material supply side usually needs to push the compaction density of the cathode powder to 2.60 g / cm³ or even higher than 2.65 g / cm³. One common way to achieve high compaction is to optimize particle size distribution, improve the integrity of secondary particles, and appropriately increase the sintering temperature to obtain a larger particle ratio and higher crystallinity that are more conducive to compaction.

[0003] However, traditional solid-state lithium iron phosphate (LiFePO4) processes typically employ a single-sintering process, where the reduction of ferric iron, lithiation, and carbon coating are simultaneously completed in a single high-temperature sintering operation. This type of process can usually yield relatively pure LiFePO4 phases within the 650℃–760℃ range; however, when the sintering temperature is increased to above 800℃ in pursuit of high compaction, the excessively strong reducing atmosphere created by carbon source decomposition and localized oxygen deficiency within the kiln can easily cause some iron to further transform to lower valence states, thereby inducing the formation of FeP, Fe2P, and even Fe3P phosphide secondary phases.

[0004] The presence of the secondary phase in iron phosphide can have several adverse effects. First, the formation of the secondary phase often indicates a deviation in the composition of the main phase and a deterioration in the local surface chemical state, leading to a decrease in batch consistency of the material. Second, the secondary phase and the accompanying surface defects can amplify the problem of iron dissolution during high-temperature storage and cycling. The dissolved iron ions, after migrating to the negative electrode, can easily lead to an aggravation of side reactions and abnormal thickening of the SEI. In severe cases, it may even induce the precipitation of metallic iron and the risk of internal short circuits. Third, the traditional single-firing process lacks precise control over the oxygen potential in the kiln within the high-temperature range, which can easily lead to a narrow window and difficulty in scale-up due to the coexistence of insufficient reaction in the front stage and excessive reduction in the back stage.

[0005] Therefore, developing a lithium iron phosphate preparation process that is suitable for high-temperature crystallization above 800℃, can suppress excessive iron reduction, and takes into account the stability of continuous mass production has become a key technical issue in the industrialization of high-pressure lithium iron phosphate for high-end energy storage. Summary of the Invention

[0006] To address the aforementioned shortcomings of existing technologies, the present invention aims to provide a high-compaction, low-iron-dissolution lithium iron phosphate, its preparation method, and a lithium battery. This method, while maintaining the smooth reduction and lithiation of iron phosphate to lithium iron phosphate, introduces controlled trace amounts of oxygen during the high-temperature crystallization stage to establish a suitable oxygen potential window, thereby suppressing the formation of secondary phases such as FeP, Fe2P, and Fe3P. This results in a material that simultaneously possesses high compaction, low iron dissolution, good rate performance, and excellent cycle stability.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for preparing high-pressure, low-iron-leaching lithium iron phosphate, comprising the following steps: (1) Mix the iron phosphate precursor, lithium source, carbon source and one or more additives selected from dispersants to obtain a mixture; (2) The mixture is granulated and dried to obtain the precursor to be sintered; (3) The precursor to be sintered is sintered in a sintering kiln. The sintering process includes at least a reduction lithiation section and a high-temperature crystallization section performed sequentially, wherein: the temperature of the reduction lithiation section is 680℃~760℃ and the oxygen volume fraction in the atmosphere is 50ppm~300ppm; the temperature of the high-temperature crystallization section is 800℃~840℃ and the oxygen volume fraction in the atmosphere is 300ppm~3000ppm. (4) Cool, crush and remove iron from the sintered product to obtain lithium iron phosphate material.

[0008] In the above technical solution, trace amounts of oxygen are mainly used to adjust the excessively low oxygen potential caused by carbon cracking, CO accumulation, and localized carbon excess in the high-temperature range. By stabilizing the oxygen potential in the high-temperature range within the lithium iron phosphate phase stability region, Fe can be... 3+ →Fe 2+ Reduction and Li + Diffusion intercalation continues, while reducing the driving force for further decomposition of lithium iron phosphate into the iron phosphide secondary phase.

[0009] In the high-temperature crystallization range of 800℃ to 840℃, the reducing gases (mainly CO and H2) produced by carbon source decomposition and the surface-active carbon atoms have strong reducing properties, which can reduce the Fe in the lithium iron phosphate lattice. 2+ Further reduction to Fe 0The newly formed nano-zero-valent iron readily reacts with neighboring phosphorus elements, successively forming iron phosphide secondary phases such as FeP, Fe2P, and even Fe3P. These side reactions not only consume iron and phosphorus in the main phase, leading to a shift in the stoichiometry of lithium iron phosphate, but also, since the iron phosphide phase typically exhibits metallic conductivity, its precipitation disrupts the integrity of the carbon coating layer and forms localized micro-batteries on the cathode surface, accelerating electrolyte decomposition and iron dissolution. In this invention, after introducing trace amounts of oxygen (50 ppm to 3000 ppm by volume) into the high-temperature crystallization stage, the oxygen preferentially reacts with reducing gases (CO, H2) and surface-active carbon: 2CO + O2 → 2CO2, 2H2 + O2 → 2H2O, C + O2 → CO2, thereby effectively consuming excess reducing species and raising the local oxygen potential to the thermodynamically stable range of lithium iron phosphate (i.e., avoiding Fe…). 2+ (The range of oxygen partial pressures that are further reduced). Within this oxygen potential window, the main reaction Fe... 3+ →Fe 2+ Lithium intercalation can still proceed smoothly, while excessive reduction generates Fe. 0 The side reaction pathway leading to FeP formation is then interrupted. Therefore, the introduction of trace amounts of oxygen is not for oxidizing the already formed lithium iron phosphate, but rather as an "oxygen potential regulator" to precisely suppress the nucleation of the secondary phase, thereby achieving both high compaction (≥2.60 g / cm³) and low iron dissolution (≤20 mg / L).

[0010] Preferably, the volume fraction of oxygen in the high-temperature crystallization section is 500 ppm to 1500 ppm.

[0011] Preferably, the primary particle size (D50) of the iron phosphate precursor is 0.5 μm to 5.0 μm; the lithium source is one or both of lithium carbonate and lithium hydroxide monohydrate; the carbon source is selected from one or more of glucose, sucrose, starch, asphalt, phenolic resin, and polyvinyl alcohol, and the amount of carbon source added is 10 wt% to 25 wt% of the mass of the iron phosphate precursor.

[0012] Preferably, the granulation in step (2) is spray granulation, the solid content of the slurry before granulation is 55wt% to 75wt%, and the D50 particle size of the particles obtained after granulation is 8μm to 30μm.

[0013] Preferably, during the sintering process, an online oxygen content analyzer and a mass flow controller are used to perform closed-loop control of the atmosphere in each temperature zone to maintain a slight positive pressure of 5 Pa to 30 Pa inside the kiln, and the actual oxygen volume fraction fluctuation in adjacent temperature zones does not exceed ±100 ppm.

[0014] To achieve precise and stable control of oxygen content within the kiln, this invention employs the following closed-loop control strategy: Independent air inlets and exhaust gas sampling ports are set up in the reduction lithiation section and high-temperature crystallization section of the roller kiln or pusher kiln, respectively. At least one online zirconia or laser oxygen analyzer is configured for each temperature zone. The analyzer detects the oxygen volume fraction in the exhaust gas of that temperature zone in real time and transmits the detection signal to a programmable logic controller (PLC). The PLC compares the measured value with the target setpoint (e.g., 100–300 ppm for the reduction lithiation section and 500–1500 ppm for the high-temperature crystallization section), calculates the deviation, and outputs adjustment commands to the nitrogen and oxygen mass flow controllers (MFCs) for the corresponding temperature zone. If the measured oxygen content is lower than the lower limit of the target range, the oxygen MFC opening is appropriately increased or the nitrogen MFC opening is decreased; if the measured oxygen content is higher than the upper limit of the target range, the oxygen MFC opening is decreased or the nitrogen MFC opening is increased. Meanwhile, by adjusting the opening of the main exhaust valve in each temperature zone and the amount of nitrogen supplied, a slight positive pressure (5Pa~30Pa) is maintained inside the kiln to prevent sudden changes in oxygen content caused by backflow of external air. Air curtain isolation or differential pressure control can be set between adjacent temperature zones to avoid mutual interference between the atmospheres. The sampling-calculation-adjustment cycle of the entire control loop does not exceed 5 seconds, thereby ensuring that the oxygen volume fraction fluctuation in each temperature zone is controlled within ±100ppm, providing a reliable atmosphere guarantee for the stable mass production of high-pressure lithium iron phosphate.

[0015] Preferably, in step (1), the molar ratio of the lithium source to the iron phosphate precursor is 1.01 to 1.06:1, calculated as the lithium-iron molar ratio.

[0016] Preferably, the sintering time of the high-temperature crystallization section is 2h to 10h, and the sum of the holding times of the reduced lithiation section and the high-temperature crystallization section is 8h to 20h.

[0017] Secondly, the present invention provides a high-compact, low-iron-leaching lithium iron phosphate material prepared by any of the above-described preparation methods. Preferably, the material has a powder compaction density of not less than 2.60 g / cm³, an iron ion dissolution rate of not more than 20 mg / L, a total carbon content of 0.60 wt% to 1.60 wt%, a secondary particle D50 particle size of 0.9 μm to 2 μm, and a specific surface area of ​​4 m² / g to 20 m² / g.

[0018] Thirdly, the present invention provides a lithium-ion battery or energy storage cell, wherein the positive electrode active material comprises the aforementioned lithium iron phosphate material.

[0019] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are: 1. This invention significantly broadens the operable window of high-temperature sintering by introducing controlled trace amounts of oxygen in the high-temperature crystallization section, enabling the material to maintain the stability of the main phase in the temperature range of 800℃~840℃ or even higher, close to the industrial upper limit, thereby inhibiting the formation of excessively reduced secondary phases such as FeP, Fe2P and Fe3P from the source. 2. This invention can balance particle growth and phase purity control, making it particularly suitable for the preparation of high-compact lithium iron phosphate materials for large-capacity energy storage cells; the resulting material is more conducive to forming a powder system with complete secondary particles and reasonable particle size distribution, and the compaction density can stably reach above 2.60 g / cm³. 3. The mixed atmosphere control method adopted in this invention is compatible with continuous roller kilns, pusher kilns or other industrial continuous kilns, and can be used with online oxygen analyzers, mass flow controllers and segmented inlet and outlet structures to achieve closed-loop control. The process has strong scalability and is close to the equipment conditions of existing mainstream lithium iron phosphate mass production lines. Attached Figure Description

[0020] Figure 1 This is a process flow diagram of the preparation method of the present invention; Figure 2 This is a SEM image of the lithium iron phosphate cathode material obtained in Example 1. Detailed Implementation

[0021] The technical solution of the present invention will be further described in detail below through specific embodiments, but the scope of protection of the present invention is not limited thereto. In the following embodiments, unless otherwise specified, the raw materials used are all conventional industrial products in the art, and the percentage content is all by mass percentage.

[0022] The performance testing method is defined as follows: (1) Powder compaction density: Tested according to the method specified in GB / T44330-2024; (2) Iron leaching: The test shall be conducted in accordance with the method specified in Appendix D of GB / T33822-2017 "Nano Lithium Iron Phosphate" for the standard iron ion leaching rate; (3) Cycle retention rate: Graphite was used as the negative electrode, the NP ratio was set to 1.08, and the electrolyte was 1M LiPF6 dissolved in a solution with a volume ratio of EC:DEC:EMC of 1:1:1. The cells were assembled into full cells and cycled at 45°C according to the 1C / 1C regime. The capacity retention rate after the specified number of cycles was recorded.

[0023] Example 1 like Figure 1 As shown, a method for preparing high-pressure, low-iron-leaching lithium iron phosphate includes the following steps: (1) Mix the iron phosphate precursor, lithium source, carbon source and optional additives to obtain a mixture; wherein the iron phosphate precursor is 1000 kg of anhydrous iron phosphate, the lithium source is 252 kg of lithium carbonate, the carbon source is 140 kg of sucrose and 10 kg of phenolic resin, after adding deionized water, it is premixed in a high-speed shear mixer for 30 min, and then dispersed by sand milling to obtain a uniform slurry with a solid content of 68%, which is the mixture.

[0024] (2) Spray granulation of the slurry obtained in step (1), control the air inlet temperature to be 230℃ and the air outlet temperature to be 105℃, and obtain a granulation precursor with D50 of 16.2μm, which is the precursor to be sintered.

[0025] (3) The precursor to be sintered is fed into a continuous roller kiln for sintering: first, it enters the reduction lithiation section at 720°C and oxygen volume fraction of 200ppm and is kept for 2 hours; then it enters the high-temperature crystallization section at 820°C and oxygen volume fraction of 500ppm and is kept for 6 hours, wherein the high-temperature crystallization section is the high-temperature reaction section.

[0026] (4) After cooling the sintered product, it is subjected to iron removal, air jet milling and batch processing to obtain lithium iron phosphate material. Figure 2 The SEM image of the lithium iron phosphate cathode material obtained in this embodiment is shown, which shows that the secondary particles have complete morphology and uniform particle size distribution.

[0027] Example 2 The difference from Example 1 is as follows: the lithium source is 254 kg of lithium carbonate; the carbon source is 160 kg of glucose and 8 kg of phenolic resin; the slurry solid content is 70%; the particle D50 obtained by spray granulation is 15.4 μm; during the sintering process, the temperature of the reduction lithiation section is 730℃ and the oxygen volume fraction is approximately 300 ppm, the temperature of the high-temperature crystallization section is 830℃ and the oxygen volume fraction is approximately 1000 ppm, and the holding time of the high-temperature crystallization section is 7 hours. The remaining steps are the same as in Example 1.

[0028] Example 3 The difference from Example 1 is as follows: the lithium source is 286 kg of lithium hydroxide monohydrate; the carbon source is 120 kg of starch and 12 kg of asphalt; the solid content of the slurry is 67%; the particle D50 obtained by spray granulation is 17.1 μm; during the sintering process, the temperature of the reduction lithiation section is 710°C, the oxygen volume fraction is approximately 50 ppm, and the holding time is 3 h; the temperature of the high-temperature crystallization section is 810°C, the oxygen volume fraction is approximately 1500 ppm, and the holding time is 6 h. The remaining steps are the same as in Example 1.

[0029] Example 4 The difference from Example 1 is that the oxygen volume fraction in the high-temperature crystallization section is 300 ppm.

[0030] Example 5 The difference from Example 1 is that the oxygen volume fraction in the high-temperature crystallization section is 3000 ppm.

[0031] Comparative Example 1 Except for the absence of mixed atmosphere control, the raw material ratios and granulation conditions were basically the same as in Example 1. The first sintering was carried out under a pure nitrogen atmosphere, held at 720°C for 2 hours, followed by a holding at 820°C for 6 hours. The resulting lithium iron phosphate material is designated as Comparative Example 1.

[0032] Comparative Example 2 Except for increasing the oxygen volume fraction to approximately 3500 ppm during the high-temperature crystallization stage, the other conditions were essentially the same as in Example 1. The resulting lithium iron phosphate material is designated as Comparative Example 2.

[0033] Tables 1 and 2 list the comparison of various parameters and performance results between Examples 1-5 and Comparative Examples 1-2, respectively.

[0034] Table 1 is a comparison table of parameters for each embodiment and comparative example.

[0035] Table 2 shows the performance results of each embodiment and each comparative example.

[0036] Results Analysis As shown in Tables 1 and 2, in Examples 1-5, by introducing trace amounts of oxygen in the range of 50 ppm to 3000 ppm within the high-temperature range, the oxygen potential of the sintered material can be stabilized within a window suitable for the existence of the lithium iron phosphate main phase, thereby controlling the iron ion dissolution rate below 20 mg / L even at a high temperature of around 820°C. Simultaneously, the secondary particles of the material remain intact with a concentrated particle size distribution, and the compaction density of the positive electrode sheet stably reaches 2.62 g / cm³. 3 ~2.65g / cm 3 .

[0037] Comparative Example 1 shows that when pure nitrogen is used throughout the high-temperature sintering process and oxygen potential regulation is lacking, although the compaction density of the material can reach a high level, the iron phosphide secondary phase increases significantly, and the iron ion dissolution rate rises markedly. In Comparative Example 1, under high temperature with pure nitrogen, the carbon layer is consumed more intensely due to its participation in side reactions, leading to damage to the integrity of the coating layer, manifested as an abnormal decrease in specific surface area and a decrease in total carbon content, further deteriorating interfacial stability, and a significant decrease in the 45℃ cycle retention rate. This indicates that simply relying on increasing temperature and particle size control cannot fundamentally solve the problem of excessive iron reduction in high-compact materials.

[0038] Comparative Example 2 shows that if the oxygen volume fraction is too high, although the content of the iron phosphide secondary phase can be further reduced, the high local oxygen potential will inhibit sufficient reduction and lithiation, leading to crystal structure instability, decreased carbon content, uneven coating, and easier dissolution of iron ions, which is detrimental to overall electrical performance. Therefore, the low oxygen partial pressure window defined in this invention is clearly necessary.

[0039] The process of this invention is particularly suitable for continuous industrial production. In practice, independent air inlets, exhaust gas sampling ports, and online oxygen content monitoring points can be set in the reduction lithiation section and high-temperature crystallization section of the roller kiln or pusher kiln, respectively. The oxygen and nitrogen flow rates are adjusted by a mass flow controller, and kiln pressure control, exhaust gas recovery, and inter-section isolation structures are used to avoid local high oxygen impacts, thereby obtaining a stable and reproducible mass production process window.

[0040] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art can make various improvements and modifications without departing from the spirit and principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing lithium iron phosphate by high-pressure, low-iron leaching, characterized in that, Includes the following steps: (1) Mix the iron phosphate precursor, lithium source, carbon source and one or more additives selected from dispersants to obtain a mixture; (2) The mixture is granulated and dried to obtain the precursor to be sintered; (3) The precursor to be sintered is sintered in a sintering kiln. The sintering process includes at least a reduction lithiation section and a high-temperature crystallization section performed sequentially. The temperature of the reduction lithiation section is 680℃~760℃, and the oxygen volume fraction in the atmosphere is 50ppm~300ppm. The temperature of the high-temperature crystallization section is 800℃~840℃, and the oxygen volume fraction in the atmosphere is 300ppm~3000ppm. The main atmosphere for sintering is either nitrogen or argon. (4) Cool, crush and remove iron from the sintered product to obtain lithium iron phosphate material.

2. The method for preparing high-pressure, low-iron-leaching lithium iron phosphate according to claim 1, characterized in that, The volume fraction of oxygen in the high-temperature crystallization section is 500 ppm to 1500 ppm.

3. The method for preparing high-pressure, low-iron-leaching lithium iron phosphate according to claim 1, characterized in that, The primary particle size (D50) of the iron phosphate precursor is 0.5 μm to 5.0 μm; the lithium source is one or both of lithium carbonate and lithium hydroxide monohydrate; the carbon source is selected from one or more of glucose, sucrose, starch, asphalt, phenolic resin, and polyvinyl alcohol, and the amount of carbon source added is 10 wt% to 25 wt% of the mass of the iron phosphate precursor.

4. The method for preparing high-pressure, low-iron-leaching lithium iron phosphate according to claim 1, characterized in that, The granulation in step (2) is spray granulation. The solid content of the slurry before granulation is 55wt% to 75wt%, and the D50 particle size of the particles obtained after granulation is 8μm to 30μm.

5. The method for preparing high-pressure, low-iron-leaching lithium iron phosphate according to claim 1, characterized in that, During the sintering process, an online oxygen content analyzer and a mass flow controller are used to perform closed-loop control of the atmosphere in each temperature zone to maintain a slight positive pressure of 5 Pa to 30 Pa inside the kiln, and the actual oxygen volume fraction fluctuation in adjacent temperature zones does not exceed ±100 ppm.

6. The method for preparing high-pressure, low-iron-leaching lithium iron phosphate according to claim 1, characterized in that, In step (1), the molar ratio of the lithium source to the iron phosphate precursor is 1.01 to 1.06:1, calculated as the molar ratio of lithium to iron.

7. The method for preparing high-pressure, low-iron-leaching lithium iron phosphate according to claim 1, characterized in that, The sintering time of the high-temperature crystallization section is 2h to 10h, and the sum of the holding times of the reduced lithiation section and the high-temperature crystallization section is 8h to 20h.

8. A high-pressure, low-iron-leaching lithium iron phosphate method, characterized in that, The material is prepared by the preparation method according to any one of claims 1-7; the powder compaction density is greater than or equal to 2.60 g / cm³, the iron ion dissolution rate is less than or equal to 20 mg / L, the total carbon content is 0.60 wt% to 1.60 wt%, the secondary particle D50 particle size is 0.5 μm to 2 μm, and the specific surface area is 4 m² / g to 20 m² / g.

9. A lithium battery, characterized in that, The positive electrode active material of the lithium battery includes the high-pressure low-iron leached lithium iron phosphate as described in claim 8.

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