A method for preparing lithium iron phosphate using ferrophosphorus slag and its use

Abstract

The present invention provides a method for preparing lithium iron phosphate by using phosphorus iron slag and its use. The method includes dehydrating and decarbonizing the phosphorus iron slag, then compounding it with battery-grade iron phosphate, and performing a lithiation reaction with a raw material lithium source and a carbon source to generate lithium iron phosphate. The present invention uses phosphorus iron slag as a raw material, which can not only utilize the iron and phosphorus element resources contained therein, but also utilize the beneficial impurity elements contained therein, such as titanium element, so that the phosphorus iron slag is used as a dopant. At the same time, under the compounding of battery-grade lithium phosphate, the influence of other impurity elements that are harmful to doping due to excessive content, such as aluminum element, etc., is reduced to achieve the best doping effect and effectively improve the performance of the obtained lithium iron phosphate. And the method does not need to perform acid leaching or impurity removal on the phosphorus iron slag, is simple to operate, and has a cost advantage.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and relates to a method for preparing lithium iron phosphate using phosphorus iron slag and its uses. Background Art

[0002] The global automotive industry is undergoing a profound transformation, and new energy vehicles are showing a booming development trend. With the increasingly stringent environmental protection policies, continuous breakthroughs in battery technology, and the deepening of consumers' awareness of sustainable travel methods, the electrification transformation has become an irreversible trend in the global automotive industry.

[0003] Currently, the focus of the automotive industry on new energy vehicle power batteries has shifted from the single "energy density theory" to more comprehensive and balanced considerations. That is, on the basis of ensuring a certain energy density, more attention is paid to reducing costs and elevating the safety performance to a crucial position, so as to meet the needs of more mass consumers.

[0004] In recent years, important innovative achievements have also been made in the field of battery technology in terms of cost reduction and safety improvement. Among them, the development of CTP (Cell to Pack) technology enables the direct integration of battery cells into the battery pack, greatly improving the space utilization rate and energy density, and reducing the battery volume. The optimized design of new structure batteries such as blade batteries has elevated the energy density of lithium iron phosphate (LFP) cathode materials to a new level, and the lithium iron phosphate material itself can also take into account high safety performance. Therefore, it effectively solves the problem that ternary lithium batteries have relatively weak safety at high energy density, and provides a practical large-scale commercial support solution for the balance of energy density, cost, and safety.

[0005] With the wide application of lithium iron phosphate batteries in fields such as energy storage and electric vehicles, the problem of the treatment of waste lithium iron phosphate batteries has gradually emerged and has gradually become an important bottleneck restricting the sustainable development of lithium iron phosphate. Compared with other types of lithium-ion batteries such as lithium cobalt oxide and ternary materials, the waste lithium iron phosphate batteries do not contain precious metals such as cobalt, and the economic benefits generated after their recycling are relatively low. Therefore, the industry itself is not very active in the recycling of lithium iron phosphate.

[0006] Moreover, traditionally, the recycling process of waste lithium iron phosphate batteries mainly includes disassembly, crushing, and the extraction of valuable metals from the cathode materials. Existing technologies often adopt selective lithium extraction technology to obtain lithium resources, leaving phosphorus iron slag. The phosphorus iron slag contains relatively high levels of aluminum, copper, and carbon elements, and the effective separation of these elements requires cumbersome and costly impurity removal process operations. More critically, currently, the leaching technology route is often adopted for phosphorus iron slag, which requires a large amount of acidic solution, and this link will lead to the generation of a large amount of wastewater, further increasing the treatment difficulty and treatment cost.

[0007] Therefore, in the current context of pursuing low-carbon environmental protection and resource recycling, how to develop a technical solution that can directly prepare lithium iron phosphate materials with excellent performance from phosphorus iron slag without relying on acid leaching and without repeated impurity removal steps is an urgent problem to be solved in the current battery recycling field. This is not only conducive to reducing the overall recycling cost and improving resource utilization rate, but also a key link in promoting the closed-loop of the new energy industry chain and ensuring the construction of an environment-friendly society. Summary of the Invention

[0008] In view of the problems existing in the prior art, the purpose of the present invention is to provide a method for preparing lithium iron phosphate from phosphorus iron slag and its use. The method includes dehydrating and decarbonizing the phosphorus iron slag, then compounding it with battery-grade iron phosphate, and performing a lithiumation reaction with a raw material lithium source and a carbon source to generate lithium iron phosphate. The present invention uses phosphorus iron slag as a raw material, which can not only utilize the iron and phosphorus element resources contained therein, but also utilize the beneficial impurity elements contained therein, such as titanium element, making the phosphorus iron slag a doping agent. At the same time, with the compounding of battery-grade lithium phosphate, the influence of other impurity elements that are harmful to doping due to excessive content, such as aluminum element, etc., is reduced to achieve the best doping effect and effectively improve the performance of the obtained lithium iron phosphate. And the method does not require acid leaching or impurity removal of the phosphorus iron slag, has simple operation, and has a cost advantage.

[0009] To achieve this purpose, the present invention adopts the following technical solutions:

[0010] In the first aspect, the present invention provides a method for preparing lithium iron phosphate from phosphorus iron slag, the method comprising:

[0011] Dehydrating and decarbonizing the phosphorus iron slag to obtain anhydrous iron phosphate;

[0012] Mixing the obtained anhydrous iron phosphate, battery-grade iron phosphate, a lithium source, a carbon source, and a morphology control agent, wherein the carbon source includes a reducing carbon source and a conductive polymer carbon source, to obtain a mixed material;

[0013] Formulating the mixed material into a slurry and then grinding it to obtain at least three abrasives with different particle sizes, and performing particle size grading on the obtained abrasives to obtain a graded material;

[0014] Performing spray drying on the obtained graded material to obtain a spray material;

[0015] Performing a lithiumation reaction on the obtained spray material to generate lithium iron phosphate.

[0016] In addition to phosphorus and iron elements, the phosphorus-iron slag described in the present invention usually also contains other impurity elements, such as carbon, titanium element, aluminum element and other transition metal elements, as well as a small amount of lithium element. Among them, the transition metal elements will have a doping effect on lithium iron phosphate. For example, the doping of tetravalent titanium ions can effectively improve the tap density and capacity performance of lithium iron phosphate. Therefore, the phosphorus-iron slag can be used as a dopant to achieve in-situ doping while synthesizing lithium iron phosphate. However, since the doping of some impurity elements is not beneficial to the electrochemical performance, such as trivalent aluminum ions, which even play a negative role when doped in excess, when using the phosphorus-iron slag dopant, the content of the impurity elements that play a doping effect also needs to be considered. For this reason, the present invention not only uses phosphorus-iron slag, but also uses battery-grade iron phosphate as a raw material and compound it with the dehydrated and decarbonized phosphorus-iron slag, so as to reduce the doping degree of impurity elements, and then weaken the adverse effects of some bad doping elements, and can exert and utilize the effects brought by beneficial doping elements. Through this compounding method, the phosphorus-iron slag can be reasonably utilized, which can not only save the cost of part of phosphorus and iron raw materials, but also save the cost of part of conventional dopants. More importantly, the method described in the present invention does not require acid leaching or impurity removal treatment, and only through dehydration and decarbonization, subsequent synthesis applications can be carried out, thus greatly simplifying the process and being beneficial to further cost control and production efficiency improvement. It can be seen that the method can effectively realize the high-value resource recovery application of phosphorus-iron slag.

[0017] At the same time, in order to improve the comprehensive performance of lithium iron phosphate synthesized using phosphorus-iron slag as a raw material, the preparation method described in the present invention also uses two carbon sources in combination, uses a morphology control agent to produce spherical particles, and performs at least three levels of particle size grading. These three optimization means are combined with using phosphorus-iron slag as a raw material and dopant to realize the preparation of high-performance lithium iron phosphate.

[0018] The following are the preferred technical solutions of the present invention, but do not limit the technical solutions provided by the present invention. Through the following technical solutions, the technical purposes and beneficial effects of the present invention can be better achieved and realized.

[0019] As a preferred technical solution of the present invention, the phosphorus-iron slag includes the phosphorus-iron slag after lithium extraction from waste lithium iron phosphate materials.

[0020] Preferably, before the dehydration and decarbonization, the phosphorus-iron slag is first dried and crushed.

[0021] Preferably, the D50 particle size achieved by the crushing is 1 to 1.5 μm, such as 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm or 1.5 μm, etc., but not limited to the listed values, and other unlisted values within the above numerical range are equally applicable.

[0022] Preferably, the temperature of the drying is 100 - 150 °C, such as 100 °C, 120 °C, 130 °C, 140 °C or 150 °C, etc., and the time is 12 - 24 h, such as 12 h, 14 h, 16 h, 18 h, 20 h, 22 h or 24 h, etc. However, it is not limited to the listed values, and other unlisted values within the above value range are equally applicable.

[0023] Preferably, the method for dehydration and decarbonization includes subjecting the ferrophosphorus slag to a first sintering in an oxygen-containing atmosphere.

[0024] Preferably, the heating rate of the first sintering is 2 - 10 °C / min, such as 2 °C / min, 5 °C / min, 8 °C / min or 10 °C / min, etc., the heat preservation temperature is 600 - 800 °C, such as 600 °C, 640 °C, 680 °C, 720 °C, 760 °C or 800 °C, etc., and the time is 6 - 8 h, such as 6 h, 6.4 h, 6.8 h, 7.2 h, 7.6 h or 8 h, etc. However, it is not limited to the listed values, and other unlisted values within the above value range are equally applicable.

[0025] As a preferred technical solution of the present invention, the mass ratio of the anhydrous iron phosphate to the battery-grade iron phosphate is 1:(0.1 - 1), such as 1:0.1, 1:0.2, 1:0.3, 1:0.4, 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9 or 1:1, etc. However, it is not limited to the listed values, and other unlisted values within the above value range are equally applicable.

[0026] Preferably, the amounts of the anhydrous iron phosphate, the battery-grade iron phosphate and the lithium source are controlled according to the molar ratio of iron element to lithium element of 1:(1.00 - 1.05), such as 1:1.00, 1:1.01, 1:1.02, 1:1.03, 1:1.04 or 1:1.05, etc. However, it is not limited to the listed values, and other unlisted values within the above value range are equally applicable.

[0027] Preferably, the amounts of the anhydrous iron phosphate, the battery-grade iron phosphate and the carbon source are controlled according to the molar ratio of iron element to carbon element of 1:(0.07 - 0.09), such as 1:0.07, 1:0.075, 1:0.08, 1:0.085 or 1:0.09, etc. However, it is not limited to the listed values, and other unlisted values within the above value range are equally applicable.

[0028] Preferably, the reducing carbon source includes at least one of sucrose, glucose, cyclodextrin, cellulose, tannic acid, polyethylene glycol, vitamin C or pitch.

[0029] Preferably, the conductive polymer carbon source includes a polymer conductive polymer and / or a polymer electrolyte, and the conductive polymer carbon source includes at least one of polyaniline, polypyrrole, polyacrylamide, polystyrene, or polyvinylpyrrolidone.

[0030] Preferably, the mass ratio of the reducing carbon source to the conductive polymer carbon source is 1:(0.5 - 3), such as 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9, 1:1, 1:1.3, 1:1.5, 1:1.8, 1:2, 1:2.1, 1:2.3, 1:2.5, 1:2.8, or 1:3, etc., but is not limited to the listed values, and other unlisted values within the above numerical range are equally applicable.

[0031] Preferably, the combination of the reducing carbon source and the conductive polymer carbon source includes the combination of sucrose and polyaniline, the combination of sucrose and polypyrrole, the combination of sucrose and polyacrylamide, the combination of sucrose and polystyrene, the combination of sucrose and polyvinylpyrrolidone, the combination of glucose and polypyrrole, the combination of cyclodextrin and polyacrylamide, the combination of cellulose and polystyrene, the combination of tannic acid and polyvinylpyrrolidone, the combination of polyethylene glycol and cellulose, the combination of vitamin C and glucose, or the combination of asphalt and polystyrene, etc. The above combinations are all typical but non-limiting examples.

[0032] In the present invention, the carbon source is the simultaneous use of a reducing carbon source and a conductive polymer carbon source, so that the reducing carbon source mainly completes the carbothermal reduction process and the conductive polymer carbon source mainly completes the carbon coating process, preventing the problem of uneven coating of the lithium iron phosphate material caused by the competition between carbon deposition and carbon diffusion when using a single carbon source. Considering that the formation temperature of lithium iron phosphate is about 400°C, therefore, the reducing carbon source is preferably a carbon source with a lower carbonization temperature, so as to complete the carbonization and reduction process before 400°C, and the conductive polymer carbon source is preferably a polymer conductive polymer and / or a polymer electrolyte carbon source with a higher carbonization temperature (>500°C), so as to complete the carbon coating process. In short, by the cooperation of the reducing carbon source and the conductive polymer carbon source, the effect of improving the uniformity of the final carbon coating layer can be achieved and can be reflected in the electrochemical performance of the final product.

[0033] Preferably, the lithium source includes at least one of lithium carbonate, lithium hydroxide, or lithium fluoride.

[0034] As a preferred technical solution of the present invention, the method further includes that the morphology control agent includes at least one of dodecyltrimethylammonium bromide, sodium dodecylbenzenesulfonate, sodium stearate, sodium laurate, or sodium dodecyl sulfate.

[0035] Preferably, the mass of the morphology control agent accounts for 1% to 2% of the total mass of the anhydrous iron phosphate and the battery-grade iron phosphate, such as 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9% or 2%, etc., but is not limited to the listed values, and other unlisted values within the above value range are equally applicable.

[0036] In the present invention, a morphology control agent is used, so that the synthesized lithium iron phosphate is spherical particles, which is beneficial to improving the packing and increasing the tap density.

[0037] Preferably, the method further includes mixing a supplementary dopant with the anhydrous iron phosphate, the battery-grade iron phosphate, the lithium source and the carbon source to obtain the mixture.

[0038] Preferably, the supplementary dopant contains at least one of the elements Ti, Mg, Ni, Co or Mn.

[0039] In the present invention, due to the compounding of phosphorus iron slag and battery-grade iron phosphate, the doping degree of impurity elements in the phosphorus iron slag decreases as a whole, and there may be a phenomenon that the doping degree of some beneficial doping elements is insufficient. At this time, conventional dopants can be further supplemented to supplement beneficial doping elements and achieve the best doping effect with the phosphorus iron slag. The specific usage amount of the supplementary dopant should be reasonably adjusted according to the target doping degree and doping effect.

[0040] As a preferred technical solution of the present invention, the solvent in the slurry includes water.

[0041] Preferably, calculated based on the total mass of the slurry being 100%, the solid content of the slurry is 30% to 50%, such as 30%, 32%, 35%, 37%, 40%, 42%, 45%, 48% or 50%, etc., but is not limited to the listed values, and other unlisted values within the above value range are equally applicable.

[0042] As a preferred technical solution of the present invention, the graded material includes abrasive A with a particle size of 1.2 to 1.5 μm, such as 1.2 μm, 1.25 μm, 1.3 μm, 1.35 μm, 1.4 μm, 1.45 μm or 1.5 μm, etc., abrasive B with a particle size of 0.8 to 1.0 μm, such as 0.8 μm, 0.85 μm, 0.9 μm, 0.95 μm or 1 μm, etc., and abrasive C with a particle size of 0.4 to 0.8 μm, such as 0.4 μm, 0.45 μm, 0.5 μm, 0.55 μm, 0.6 μm, 0.65 μm, 0.7 μm, 0.75 μm or 0.8 μm, etc., but is not limited to the listed values, and other unlisted values within the above value range are equally applicable.

[0043] It should be noted that the particle size of the abrasive refers to the particle size achieved by all components as a whole, rather than the particle size of a single component.

[0044] Preferably, the mass ratio of the abrasive A to the abrasive B is 1:(1 - 3), such as 1:1, 1:1.3, 1:1.5, 1:1.8, 1:2, 1:2.3, 1:2.5, 1:2.8 or 1:3, etc. The mass ratio of the abrasive B to the abrasive C is 1:(1.1 - 2.2), such as 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2, 1:2.1 or 1:2.2, etc. However, it is not limited to the listed values, and other unlisted values within the above numerical range are equally applicable. Further preferably, the mass ratio of the abrasive A, the abrasive B and the abrasive C is 1:2:2.

[0045] The present invention preferably realizes the filling of small particles into the voids between large particles through particle size grading to improve the compaction density. It should be noted that when the material particles are spherical, the combination of particle size grading can achieve the best effect of improving the compaction density.

[0046] Exemplarily, the specific process of grinding the slurry in the method of the present invention includes continuously grinding all the slurry and taking samples from the slurry being ground at least three times to obtain at least three abrasives with different particle sizes. For example, part of the slurry can be taken out as abrasive A when the slurry is ground to 1.2 - 1.5 μm, and the rest is continuously ground until it is ground to 0.8 - 1.0 μm, then another part is taken out as abrasive B, and the rest is continuously ground until it is ground to 0.4 - 0.8 μm, and then taken out as abrasive C, and then each abrasive is mixed by particle size grading to form a graded mixture. Or, the specific process of grinding the slurry in the method of the present invention includes dividing the obtained slurry into at least three parts, grinding each part separately into abrasives with corresponding particle sizes, and then mixing each abrasive by particle size grading to form a graded mixture. For example, the slurry can be divided into three parts, and the three parts of the slurry are separately ground to particle sizes of 1.2 - 1.5 μm, 0.8 - 1.0 μm and 0.4 - 0.8 μm to obtain abrasive A, abrasive B and abrasive C respectively, and then each abrasive is mixed by particle size grading to form a graded mixture.

[0047] As a preferred technical solution of the present invention, the inlet air temperature of the spray drying is 200 - 350°C, such as 200°C, 220°C, 250°C, 270°C, 290°C, 310°C, 330°C or 350°C, etc., the outlet air temperature is 80 - 120°C, such as 80°C, 90°C, 100°C, 110°C or 120°C, etc., and the feeding frequency is 15 - 25Hz, such as 15Hz, 17Hz, 19Hz, 21Hz, 23Hz or 25Hz, etc. However, it is not limited to the listed values, and other unlisted values within the above numerical range are equally applicable.

[0048] The function of the spray drying in the present invention is to control the staged batching into a secondary spherical morphology composed of a primary spherical morphology (controlled by using a morphology control agent and performing three-stage particle size grading grinding) through an atomization disk, so as to ensure the spherical growth of the material during sintering.

[0049] As a preferred technical solution of the present invention, the method of the lithiation reaction includes subjecting the spray material to a second sintering under the protection of an inert atmosphere.

[0050] Preferably, the process of the second sintering includes heating at a heating rate of 3 - 5°C / min, such as 3°C / min, 3.5°C / min, 4°C / min, 4.5°C / min or 5°C / min, etc. First, keep it at 350 - 400°C for 3 - 5h to carry out the carbonization and reduction process of the reducing carbon source. The temperature of carbonization and reduction can be, for example, 350°C, 360°C, 370°C, 380°C, 390°C or 400°C, etc., and the time of carbonization and reduction can be, for example, 3h, 3.5h, 3.8h, 4h, 4.5h or 5h, etc. Then continue to heat up and keep it at 700 - 800°C for 8 - 12h to carry out the carbonization and coating process of the conductive polymer carbon source. The temperature of carbonization and coating can be, for example, 700°C, 720°C, 740°C, 760°C, 780°C or 800°C, etc., and the time of carbonization and coating can be, for example, 8h, 9h, 10h, 11h or 12h, etc. However, it is not limited to the listed values, and other unlisted values within the above numerical range are equally applicable.

[0051] Preferably, the inert atmosphere includes nitrogen.

[0052] In the second aspect, the present invention provides a lithium iron phosphate, which is prepared according to the method described in the first aspect.

[0053] In the third aspect, the present invention provides a lithium ion battery, which contains the lithium iron phosphate described in the second aspect.

[0054] Compared with the prior art solutions, the present invention has at least the following beneficial effects:

[0055] The present invention uses ferrophosphorus slag as a dopant and iron phosphorus raw material, and realizes the synthesis and in-situ doping of lithium iron phosphate by compounding battery-grade iron phosphate. The method can achieve the high-value resource utilization of ferrophosphorus slag without complex processes such as acid leaching and impurity removal. The method is simple to operate and has a cost advantage.

[0056] In the present invention, a spherical product particle is obtained by adding a morphology control agent, and the packing voids are filled through a grading method of three particle sizes, thereby improving the compaction density. In addition, by using a reducing carbon source and a conductive polymer carbon source in combination, as the reducing carbon source and the coating carbon source respectively, the problem of uneven coating carbon layer is ensured to be solved, and the electrochemical performance of the obtained lithium iron phosphate product is further improved. Brief Description of the Drawings

[0057] Figure 1 It is the XRD test pattern of the lithium iron phosphate obtained in Example 1, Example 2 and Comparative Example 1;

[0058] Figure 2 It is the SEM test pattern of the lithium iron phosphate obtained in Example 1;

[0059] Figure 3 It is the SEM test pattern of the lithium iron phosphate obtained in Comparative Example 1;

[0060] Figure 4 It is the first charge-discharge test pattern of the lithium iron phosphate obtained in Example 2 and Comparative Example 1. Detailed Description of the Invention

[0061] The technical solution of the present invention will be further described below through specific embodiments.

[0062] Those skilled in the art should understand that the embodiments are only helpful for understanding the present invention and should not be regarded as specific limitations on the present invention.

[0063] In the following examples and comparative examples of the present invention, the ferrophosphorus slag used is the ferrophosphorus slag after lithium extraction from waste lithium iron phosphate materials, and its main composition is shown in Table 1.

[0064] Table 1

[0065] Component Mass percentage (%) Fe 26.2 Al 0.13 Ti 0.25 C 2.44 Li 0.11 Ni 0.02 Co 0.01 Mn 0.03

[0066] Example 1

[0067] This example provides a method for preparing lithium iron phosphate using ferrophosphorus slag, and the method includes:

[0068] (1) Take the ferrophosphorus slag and dry the surface moisture in a forced-air drying oven at a drying temperature of 110 °C for 24 h. Put the dried ferrophosphorus slag into a muffle furnace, in an air atmosphere, keep the temperature at 700 °C for 6 h, with a heating rate of 5 °C / min, and perform the first sintering to complete dehydration and decarbonization to obtain anhydrous iron phosphate.

[0069] (2) Mix the obtained anhydrous iron phosphate and battery-grade iron phosphate at a mass ratio of 1:1 to form an iron phosphate raw material. Then, add a lithium source and a carbon source to the iron phosphate raw material according to the molar ratio of iron, lithium, and carbon of 1:1.04:0.08. The lithium source is lithium carbonate, and the carbon source is glucose and polyaniline with a mass ratio of 1:2.33. Then, add 1.5% of the morphology control agent CTAB based on the total mass of the iron phosphate raw material to obtain a mixture. Then, add water to the mixture at a solid content of 35% for premixing to prepare a slurry. Grind the obtained slurry to obtain abrasive A with a particle size of 1.2 μm, abrasive B with a particle size of 0.8 μm, and abrasive C with a particle size of 0.4 μm. Perform particle size grading on abrasive A, abrasive B, and abrasive C according to a mass ratio of 1:2:2 to obtain a graded mixture.

[0070] (3) Spray-dry the graded mixture, control the inlet air temperature of the spray drying at 220 °C, the outlet air temperature at 105 °C, and the feeding frequency at 15 Hz to obtain a spray material.

[0071] (4) Put the obtained spray material into a tube furnace in a nitrogen atmosphere, control the heating rate at 5 °C / min, first keep it at 350 °C for 5 h, and then keep it at 760 °C for 10 h to perform the second sintering to complete the lithiation reaction and obtain lithium iron phosphate.

[0072] Example 2

[0073] This example provides a method for preparing lithium iron phosphate using ferrophosphorus slag. In step (2) of this method, the dosage of battery-grade iron phosphate is adjusted so that the mass ratio of anhydrous iron phosphate to battery-grade iron phosphate is adjusted from 1:1 to 1:0.25. Except for the above, other conditions are exactly the same as those in Example 1.

[0074] Example 3

[0075] This example provides a method for preparing lithium iron phosphate using ferrophosphorus slag. In step (2) of this method, the dosage of battery-grade iron phosphate is adjusted so that the mass ratio of anhydrous iron phosphate to battery-grade iron phosphate is adjusted from 1:1 to 1:0.11. Except for the above, other conditions are exactly the same as those in Example 1.

[0076] Example 4

[0077] This embodiment provides a method for preparing lithium iron phosphate using phosphorus iron slag. In step (2) of the method, the total amount of carbon source is kept unchanged, and the mass ratio of glucose to polyaniline is adjusted from 1:2.33 to 1:1.2. Except for the above, other conditions are exactly the same as those in Embodiment 1.

[0078] Embodiment 5

[0079] This embodiment provides a method for preparing lithium iron phosphate using phosphorus iron slag. In step (2) of the method, the total amount of carbon source is kept unchanged, and the mass ratio of glucose to polyaniline is adjusted from 1:2.33 to 1:3. Except for the above, other conditions are exactly the same as those in Embodiment 1.

[0080] Comparative Example 1

[0081] This comparative example provides a method for preparing lithium iron phosphate using phosphorus iron slag. In step (2) of the method, the total amount of carbon source is kept unchanged, and the mass ratio of glucose to polyaniline is adjusted from 1:2.33 to 1:0, that is, no conductive polymer carbon source is used, and an equal amount of reducing carbon source is used to replace the conductive polymer carbon source. Except for the above, other conditions are exactly the same as those in Embodiment 1.

[0082] Comparative Example 2

[0083] This comparative example provides a method for preparing lithium iron phosphate using phosphorus iron slag. In step (2) of the method, the total amount of carbon source is kept unchanged, and the mass ratio of glucose to polyaniline is adjusted from 1:2.33 to 0:1, that is, no reducing carbon source is used, and an equal amount of conductive polymer carbon source is used to replace the reducing carbon source. Except for the above, other conditions are exactly the same as those in Embodiment 1.

[0084] Comparative Example 3

[0085] This comparative example provides a method for preparing lithium iron phosphate using phosphorus iron slag. In step (2) of the method, the obtained slurry is ground to obtain only abrasive A with a particle size of 1.2 μm, without particle size grading, and abrasive A is applied to step (3) for spray drying. Except for the above, other conditions are exactly the same as those in Embodiment 1.

[0086] Comparative Example 4

[0087] This comparative example provides a method for preparing lithium iron phosphate using phosphorus iron slag. In step (2) of the method, the obtained slurry is ground to obtain only abrasive B with a particle size of 0.8 μm, without particle size grading, and abrasive B is applied to step (3) for spray drying. Except for the above, other conditions are exactly the same as those in Embodiment 1.

[0088] Comparative Example 5

[0089] This comparative example provides a method for preparing lithium iron phosphate using phosphorus iron slag. In step (2) of the method, the obtained slurry is ground to obtain only abrasive C with a particle size of 0.4 μm, without particle size grading. The abrasive C is applied to step (3) for spray drying. Except for the above, other conditions are exactly the same as those in Example 1.

[0090] Example 6

[0091] This comparative example provides a method for preparing lithium iron phosphate using phosphorus iron slag. In step (2) of the method, the morphology control agent is not used. Except for the above, other conditions are exactly the same as those in Example 1.

[0092] Comparative Example 7

[0093] This comparative example provides a method for synthesizing lithium iron phosphate. In this method, phosphorus iron slag is not used, that is, step (1) is not carried out. In step (2), an equal amount of battery-grade iron phosphate is used to replace the anhydrous iron phosphate. Since the battery-grade iron phosphate does not contain a titanium source, a titanium source titanium dioxide with the same doping amount is additionally added during mixing. Except for the above, other conditions are exactly the same as those in Example 1.

[0094] Comparative Example 8

[0095] This comparative example provides a method for synthesizing lithium iron phosphate. In this method, phosphorus iron slag is not used, that is, step (1) is not carried out. In step (2), an equal amount of battery-grade iron phosphate is used to replace the anhydrous iron phosphate. Since the battery-grade iron phosphate does not contain a titanium source, a titanium source titanium dioxide with the same doping amount is additionally added during mixing. In step (2), the obtained slurry is ground to obtain only abrasive C with a particle size of 0.4 μm, without particle size grading. The abrasive C is applied to step (3) for spray drying. Except for the above, other conditions are exactly the same as those in Example 1.

[0096] Comparative Example 9

[0097] This comparative example provides a method for synthesizing lithium iron phosphate. In this method, battery-grade iron phosphate is not used, that is, in step (2), an equal amount of anhydrous iron phosphate is used to replace the battery-grade iron phosphate. Except for the above, other conditions are exactly the same as those in Example 1.

[0098] Characterization and testing:

[0099] Ⅰ. Figure 1 This is the XRD test pattern of the lithium iron phosphate obtained in Example 1, Example 2, and Comparative Example 1. Its peak positions and intensities correspond one by one to the standard card, proving that it is lithium iron phosphate.

[0100] Ⅱ. Figure 2 and Figure 3The following are SEM images of the lithium iron phosphate obtained in Example 1 and Comparative Example 3. As can be seen from the images, the lithium iron phosphate obtained in Example 1 has three particle size distributions due to gradation. This allows medium particles to fill the gaps between large particles, and small particles to fill the remaining gaps, thereby increasing the compaction density. Comparative Example 3 fails to achieve this gradation effect.

[0101] III. The lithium iron phosphate obtained in the examples and comparative examples was subjected to compaction density testing. The specific method includes: testing by using a Sansi vertical and horizontal compaction density meter, according to the compaction density = mass / volume principle, and the results are recorded in Table 2.

[0102] IV. The lithium iron phosphate obtained in the Examples and Comparative Examples was used as the positive electrode material to fabricate positive electrode sheets and assemble into batteries. Testing method: (1) Battery assembly: negative electrode shell - lithium sheet - LiPF6 electrolyte - Celgard 2500 separator - LiPF6 electrolyte - positive electrode sheet - gasket - spring - positive electrode shell; (2) Testing conditions: The assembled button-type batteries were allowed to stand in a 25°C constant temperature chamber for 10 h. After standing, electrochemical performance was tested using a LAND battery testing system over a voltage range of 2.0 to 3.75 V. The results are recorded in Table 2.

[0103] Table 2

[0104]

[0105]

[0106] Figure 4 This is a specific capacity test diagram of the lithium iron phosphate obtained in Example 2 and Comparative Example 1. It can be seen from the figure that if only glucose is used as both coating and reducing carbon source, the coating is uneven due to the competition between its reduction and coating, resulting in lower electrochemical performance.

[0107] By comparing Example 1 with Examples 2-3, it can be seen that when the amount of anhydrous ferric phosphate after dehydration and decarbonization of ferrophosphorus slag is small, the content of beneficial ions such as titanium ions is small, and the electrochemical performance is limited. However, if used in excess, the aluminum ion content increases, resulting in poor electrochemical performance.

[0108] Comparison of Example 1 with Examples 4-5 and Comparative Examples 1-2 reveals that glucose serves as both the reducing and coating carbon, leading to a relatively low carbonization temperature. Competition between the reduction and coating processes results in uneven carbon coating, resulting in poor electrochemical performance. High glucose / polyaniline dosages result in a blended carbon layer, and the relatively high price of polyaniline hinders cost reduction and efficiency gains.

[0109] Comparison of Example 1 with Comparative Examples 3-5 shows that the compaction without grading is lower.

[0110] Comparing Example 1 with Comparative Example 6, it can be seen that without adding a morphology control agent, the spherical morphology cannot be controlled, resulting in poor filling effect, low tap density, and particle agglomeration. The particles are large and the lithium ion transmission speed is slow, leading to poor electrochemical performance.

[0111] Comparing Example 1 with Comparative Examples 7-9, it is found that the titanium ions in the phosphoferrous slag can fully meet the effect of the additional titanium source for battery-grade iron phosphate. Moreover, using a graded mixture with a relatively wide three-stage particle size distribution can improve the tap density, enabling small and medium particles to fill the voids between larger particles of the next size, thereby improving the tap density.

[0112] The present invention uses the above embodiments to illustrate the detailed process equipment and process flow of the present invention. However, the present invention is not limited to the above detailed process equipment and process flow, that is, it does not mean that the present invention must rely on the above detailed process equipment and process flow to be implemented. Those skilled in the art should understand that any improvement of the present invention, the equivalent substitution of each raw material of the product of the present invention, the addition of auxiliary components, and the selection of specific methods, etc., all fall within the protection scope and the disclosure scope of the present invention.

[0113] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all belong to the protection scope of the present invention.

[0114] In addition, it should be noted that in the above specific embodiments, the various specific technical features described can be combined in any suitable manner without conflict. To avoid unnecessary repetition, the present invention will not separately describe various possible combination methods.

[0115] In addition, any combination can be made between various different embodiments of the present invention as long as it does not violate the idea of the present invention, and it should also be regarded as the content disclosed by the present invention.

Claims

1. A method for preparing lithium iron phosphate using phosphorus iron slag, characterized in that, The method includes: Dehydrating and decarbonizing the ferrophosphorus slag to obtain anhydrous iron phosphate; the method of dehydrating and decarbonizing is to conduct a first sintering on the ferrophosphorus slag in an oxygen atmosphere, and the heat preservation temperature of the first sintering is 700 - 800 °C; Mixing the obtained anhydrous iron phosphate, battery-grade iron phosphate, lithium source, carbon source and morphology control agent, where the carbon source includes a reducing carbon source and a conductive polymer carbon source, to obtain a mixed material; the mass ratio of the anhydrous iron phosphate to the battery-grade iron phosphate is 1:(0.25 - 1); Formulating the mixed material into a slurry and then grinding it to obtain at least three abrasives with different particle sizes, and performing particle size grading on the obtained abrasives to obtain a graded material; Performing spray drying on the obtained graded material to obtain a spray material; Performing a lithiation reaction on the obtained spray material to generate lithium iron phosphate.

2. The method according to claim 1, characterized in that, The ferrophosphorus slag includes the ferrophosphorus slag after lithium extraction from waste lithium iron phosphate materials.

3. The method according to claim 1, wherein Before performing the dehydration and decarbonization, the ferrophosphorus slag is first dried and crushed.

4. The method according to claim 3, characterized in that, The D50 particle size achieved by the crushing is 1 - 1.5 μm.

5. The method according to claim 3, wherein The temperature of the drying is 100 - 150 °C, and the time is 12 - 24 h.

6. The method according to claim 1, characterized in that, The time of the first sintering is 6 - 8 h.

7. The method according to claim 1, characterized in that, Controlling the dosages of the anhydrous iron phosphate, battery-grade iron phosphate and lithium source according to the molar ratio of iron element to lithium element of 1:(1.00 - 1.05).

8. The method according to claim 1, wherein Controlling the dosages of the anhydrous iron phosphate, battery-grade iron phosphate and carbon source according to the molar ratio of iron element to carbon element of 1:(0.07 - 0.09).

9. The method according to claim 1, wherein The reducing carbon source includes at least one of sucrose, glucose, cyclodextrin, cellulose, tannic acid, polyethylene glycol, vitamin C or pitch.

10. The method according to claim 1, characterized in that, The conductive polymer carbon source includes at least one of polyaniline, polypyrrole, polyacrylamide, polystyrene or polyvinylpyrrolidone.

11. The method according to claim 1, wherein The mass ratio of the reducing carbon source to the conductive polymer carbon source is 1:(0.5 - 3).

12. The method according to claim 1, wherein The lithium source includes at least one of lithium carbonate, lithium hydroxide or lithium fluoride.

13. The method according to claim 1, characterized in that The morphology control agent includes at least one of dodecyltrimethylammonium bromide, sodium dodecylbenzenesulfonate, sodium stearate, sodium laurate or sodium dodecyl sulfate.

14. The method according to claim 1, wherein The mass of the morphology control agent accounts for 1% - 2% of the total mass of the anhydrous iron phosphate and the battery-grade iron phosphate.

15. The method according to claim 1, wherein The method further includes mixing a supplementary dopant with the anhydrous iron phosphate, battery-grade iron phosphate, lithium source and carbon source to obtain the mixed material.

16. The method according to claim 14, characterized in that The supplementary dopant contains at least one of Ti, Mg, Ni, Co or Mn elements.

17. The method according to claim 1, wherein The solvent in the slurry includes water.

18. The method according to claim 1, characterized in that, The solid content of the slurry is 30% - 50%.

19. The method according to claim 1, characterized in that The graded material includes abrasive A with a particle size of 1.2 - 1.5 μm, abrasive B with a particle size of 0.8 - 1.0 μm, and abrasive C with a particle size of 0.4 - 0.8 μm.

20. The method according to claim 19, wherein The mass ratio of abrasive A to abrasive B is 1:(1 - 3), and the mass ratio of abrasive B to abrasive C is 1:(1.1 - 2.2).

21. The method according to claim 1, characterized in that, The inlet air temperature of the spray drying is 200 - 350 °C, the outlet air temperature is 80 - 120 °C, and the feeding frequency is 15 - 25 Hz.

22. The method according to claim 1, wherein The method of the lithiation reaction includes subjecting the spray material to a second sintering under the protection of an inert atmosphere.

23. The method according to claim 22, characterized in that, The process of the second sintering includes first holding at 350 - 400 °C for 3 - 5 h, and then holding at 700 - 800 °C for 8 - 12 h.

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