Method for regenerating waste lithium iron phosphate into olivine-type sodium iron phosphate and application thereof

Abstract

The application discloses a method for regenerating waste lithium iron phosphate into olivine-type sodium iron phosphate and application thereof, and relates to the technical field of batteries. A carbon source is added into the waste lithium iron phosphate, and the mixture is uniformly obtained through grinding. The mixture is calcined under an inert atmosphere to form an activated material with an initial thin carbon layer. The activated material is mixed with a sodium source, a fluorine source and a magnesium source, and a solvent is added to obtain a mixed slurry. The mixed slurry is heated and stirred to react. The reaction slurry is filtered and spray-dried to obtain a precursor powder. The precursor powder is subjected to staged sintering under a hydrogen atmosphere to form regenerated sodium iron phosphate. The carbon source is used to activate the crystal framework, then the fluorine is used to optimize the sodium ion migration channel, and the magnesium source is uniformly doped into the crystal lattice to inhibit volume expansion. The three are combined to optimize the crystal structure, improve the conductivity of the regenerated material, and ensure the battery performance of the regenerated sodium iron phosphate material.

Description

Technical Field

[0001] This application relates to the technical field of batteries, specifically to a method and application for regenerating waste lithium iron phosphate into olivine-type sodium iron phosphate. Background Technology

[0002] After repeated use, lithium iron phosphate batteries experience internal side reactions such as dissolution of active materials, blockage of separator pores, and electrolyte aging, which can affect their normal operation. Recycling used lithium batteries not only facilitates resource reuse but also helps protect the environment and reduce pollution.

[0003] In some related technologies, the current recycling of lithium iron phosphate from waste batteries usually requires multiple leaching, separation, and purification steps, which is lengthy, energy-intensive, and causes element loss in lithium iron phosphate. Existing short-process technologies mostly use other substances from different sources to treat waste lithium iron phosphate, resulting in insufficient ion exchange during the treatment process. Furthermore, they do not specifically address the pain points of poor conductivity and insufficient lattice stability of sodium iron phosphate itself, resulting in the prepared sodium iron phosphate material still having problems such as poor conductivity, slow sodium ion migration kinetics, and insufficient cycle stability. Summary of the Invention

[0004] In order to solve at least one of the problems mentioned in the background art, this application proposes a method and application for regenerating waste lithium iron phosphate into olivine-type sodium iron phosphate.

[0005] The specific technical solutions provided in this application are as follows: In a first aspect, a method for regenerating waste lithium iron phosphate into olivine-type sodium iron phosphate is provided, the method comprising: A carbon source is added to waste lithium iron phosphate, and the mixture is ground evenly to obtain a mixture. The mixture is then calcined in an inert atmosphere to form an activated material. The activating material is mixed with a sodium source, a fluorine source, and a magnesium source, and a solvent is added to obtain a mixed slurry. The slurry is then heated and stirred to allow it to react. After filtration and reaction, the mixed slurry yields a solid product. The solid product is then spray-dried to obtain a precursor powder. The precursor powder is then sintered in a hydrogen atmosphere to form regenerated sodium iron phosphate.

[0006] In one specific embodiment, the amount of carbon source added is 5% to 10% of the molar amount of the waste lithium iron phosphate; and / or, the carbon source includes one or more of sucrose, glucose, citric acid and polyethylene glycol.

[0007] In one specific embodiment, the carbon source is added to the waste lithium iron phosphate, and the mixture is obtained by grinding. Specifically, the process includes: adding the carbon source to the waste lithium iron phosphate and mixing, and grinding for 30 min to 60 min at a speed of 300 r / min to 400 r / min to obtain the mixture. And / or, calcining the mixture in an inert atmosphere to form an activated material, specifically including: heating the mixture to 250°C to 350°C at a rate of 3°C / min to 7°C / min in an inert atmosphere, holding it at that temperature for 2h to 4h, and then cooling it to room temperature to obtain the activated material.

[0008] In one specific embodiment, the amount of sodium source used is 1.05 to 1.1 times the theoretical amount of sodium used, and the theoretical amount of sodium used is based on the molar amount of the waste lithium iron phosphate. And / or, the amount of the fluorine source used is 0.03% to 0.08% of the molar amount of the waste lithium iron phosphate; And / or, the amount of magnesium source used is 2% to 4% of the molar amount of the waste lithium iron phosphate, based on the molar amount of the waste lithium iron phosphate; And / or, the sodium source includes one or more of sodium acetate, sodium formate, sodium oxalate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium citrate, sodium carbonate, sodium bicarbonate, and sodium hydroxide; And / or, the fluorine source includes one or more of sodium fluoride, ammonium fluoride and ammonium bifluoride; And / or, the magnesium source includes one or more of magnesium acetate, magnesium formate, and magnesium citrate.

[0009] In one specific embodiment, the activating material is mixed with the sodium source, the fluorine source, and the magnesium source, and the solvent is added to obtain the mixed slurry. The slurry is then heated and stirred to allow it to react, specifically including: Add the sodium source to the activated material and stir to dissolve. Then add the fluorine source and the magnesium source in sequence. Then add the solvent at a solid-liquid ratio of 1:(4~8) to prepare the mixed slurry. The mixed slurry is heated in a water bath to 70℃~90℃, stirred evenly at a rate of 300r / min~500r / min, and acid solution is added to adjust the pH of the mixed slurry to the range of 5.5~6.5 and react for 3h~5h.

[0010] In one specific embodiment, the slurry after filtration and reaction is mixed to obtain a solid product, and the solid product is spray-dried to obtain a precursor powder, specifically including: The reaction slurry is centrifuged and filtered to obtain a solid product. The solid product is then spray-dried to obtain the precursor powder with a water content of ≤0.3%. The inlet air temperature of the spray dryer is 120℃~160℃ and the outlet air temperature is 85℃~105℃.

[0011] In one specific embodiment, the precursor powder is sintered in a hydrogen atmosphere to form regenerated sodium iron phosphate, specifically including: During the sintering process, hydrogen gas is continuously introduced and the hydrogen gas pressure in the sintering furnace is maintained at 0.01 MPa to 0.02 MPa. The precursor powder is first pre-calcined at 300°C to 400°C for 5 to 7 hours at a rate of 1°C to 3°C to 3°C to 3°C to 3°C to 3°C to 3°C for 8 to 12 hours to generate the regenerated sodium iron phosphate.

[0012] In one specific embodiment, the regenerated sodium iron phosphate particles are olivine-type; And / or, the crystal structure of the regenerated sodium iron phosphate particles is orthorhombic; And / or, the cell volume of the regenerated sodium iron phosphate particles is greater than or equal to 292.73 ų, and the first cell edge length a of the particles satisfies 10.34 Å to 10.36 Å, the second cell edge length b satisfies 6.0 Å to 6.02 Å, and the third cell edge length c satisfies 4.7 Å to 4.71 Å.

[0013] Secondly, an electrode sheet is provided, which is prepared from sodium iron phosphate obtained by the method described above for recycling waste lithium iron phosphate into olivine-type sodium iron phosphate.

[0014] Thirdly, a sodium-ion battery is provided, the sodium-ion battery comprising the electrode sheets described above.

[0015] The embodiments of this application have the following beneficial effects: The embodiments of this application involve adding a carbon source to waste lithium iron phosphate to remove electrolyte residue and binder from the electrode powder surface, while simultaneously forming an initial thin carbon layer to protect the olivine crystal framework from collapse. This is achieved by replacing lithium with sodium and partially replacing PO4 with fluorine. 3- Sites are used to optimize sodium ion migration channels; magnesium sources, which are readily soluble in water and have small magnesium ion radii, can be uniformly dispersed in the material voids and doped into the lattice to suppress volume expansion. This scheme achieves Li + / Na + Slow and uniform exchange is used to achieve precise doping of fluorine and magnesium ions, followed by segmented sintering to achieve complete crystallization and dense carbon layer coating. The reducing atmosphere ensures the Fe... 2+ The valence state is stable, avoiding the formation of impurity phases. Attached Figure Description

[0016] To more clearly illustrate the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 A schematic diagram of a method for regenerating waste lithium iron phosphate into olivine-type sodium iron phosphate; Figure 2 This is a SEM image of waste lithium iron phosphate in Example 1 of this application; Figure 3 This is a SEM image of the olivine-type regenerated sodium iron phosphate prepared in Example 1 of this application. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0019] The "range" disclosed herein is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also expected. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0020] Unless otherwise specified in this application, all embodiments and preferred embodiments mentioned herein can be combined to form new technical solutions.

[0021] Unless otherwise specified, all technical features and preferred features mentioned herein can be combined to form new technical solutions.

[0022] In this application, unless otherwise specified, all steps mentioned herein may be performed sequentially or randomly, but are preferably performed sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0023] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0024] This application provides a method for regenerating waste lithium iron phosphate into olivine-type sodium iron phosphate, such as... Figure 1 As shown, it includes the following steps: S1. Add a carbon source to waste lithium iron phosphate, grind it evenly to obtain a mixture, and calcine the mixture in an inert atmosphere to form an activated material with an initial thin carbon layer.

[0025] A carbon source was added to waste lithium iron phosphate and mixed. The mixture was then milled using a planetary ball mill at a speed of 300-400 rpm for 30-60 minutes to obtain a homogeneous mixture. This mixture was then calcined in an inert atmosphere to form an activated material. Specifically, the mixture was heated to 250-350°C at a rate of 3-7°C / min under an inert atmosphere, held at that temperature for 2-4 hours, and then cooled to room temperature to obtain the activated material. This calcination process removes electrolyte residue and binder from the electrode powder surface and forms an initial thin carbon layer, protecting the olivine crystal framework from collapse and providing a structural basis for subsequent in-situ ion exchange. After calcination, thermogravimetric analysis showed that the organic residue was ≤0.5%.

[0026] In one specific embodiment, the amount of carbon source added is 5% to 10% of the molar amount of waste lithium iron phosphate; the carbon source includes one or more of sucrose, glucose, citric acid and polyethylene glycol. The above-mentioned amount of carbon source added can ensure that the subsequent carbon coating thickness is appropriate. If the carbon source is too much, it will easily lead to carbon agglomeration. If the carbon source is insufficient, it cannot be ensured that the initial thin carbon layer formed after calcination can effectively protect the crystal skeleton.

[0027] S2. Mix the activating material with sodium source, fluorine source and magnesium source, and add solvent to obtain a mixed slurry. Heat and stir to make the mixed slurry react.

[0028] Specifically, the process includes: adding a sodium source to the activating material and stirring to dissolve it; then adding a fluorine source and a magnesium source in sequence; then adding a solvent at a solid-liquid ratio of 1:(4~8) to prepare a mixed slurry; heating the mixed slurry in a water bath to 70℃~90℃; stirring evenly at a rate of 300r / min~500r / min; and adding an acid solution to adjust the pH of the mixed slurry to the range of 5.5~6.5 and reacting for 3h~5h.

[0029] It should be noted that after adding the sodium source and stirring to dissolve, the fluorine source and magnesium source are added sequentially, with a 30-minute interval between the addition of the sodium, fluorine, and magnesium sources. The water bath temperature is controlled at 70℃~90℃, and the stirring speed at 300r / min~500r / min to ensure the Li... + / Na + The reaction involves slow and uniform exchange, while simultaneously achieving precise doping of fluorine and magnesium ions. During the reaction, dilute acetic acid is used to adjust the pH of the slurry to 5.5-6.5 to maintain the optimal acid-base environment for ion exchange. This reaction does not require complex acid leaching and separation. It directly completes ion exchange and doping by utilizing structural homology. The pretreated and activated crystal framework serves as the basis for in-situ exchange. The simultaneous dual doping and ion exchange avoids the tediousness of secondary modification. The mild reaction conditions ensure structural integrity.

[0030] In this embodiment, the amount of sodium source used is 1.05 to 1.1 times the theoretical amount of sodium. The theoretical amount of sodium is based on the molar amount of waste lithium iron phosphate, where the theoretical amount of sodium used refers to the amount of sodium required for theoretical lithium-sodium exchange. The actual amount of sodium source added is set to be more than the theoretical amount of sodium to compensate for sodium loss during the reaction process. The sodium source includes one or more of sodium acetate, sodium formate, sodium oxalate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium citrate, sodium carbonate, sodium bicarbonate, and sodium hydroxide.

[0031] The amount of fluorine source used is 0.03% to 0.08% of the molar amount of waste lithium iron phosphate. The fluorine source includes one or more of sodium fluoride, ammonium fluoride, and ammonium bifluoride. The compounds included in the above fluorine source have good solubility and no impurity ions are introduced into the reaction system. The fluorine source has high doping efficiency and fluorine replaces part of PO4. 3+Site optimization is used to optimize sodium ion migration channels. Furthermore, a magnesium source is incorporated, with the amount of magnesium source being 2% to 4% of the molar amount of waste lithium iron phosphate, based on the molar amount of waste lithium iron phosphate. The magnesium source includes one or more of magnesium acetate, magnesium formate, and magnesium citrate. The substances in the magnesium source are readily soluble in water, and the small radius of magnesium ions allows for uniform dispersion in the material voids, doping into the crystal lattice to suppress volume expansion. Simultaneously, fluoride ions are combined to optimize the crystal structure and carbon layer coating, synergistically improving conductivity and increasing the sodium ion migration rate.

[0032] Specifically, the amount of fluorine source can be selected as 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, and 0.08%, or any combination of two of the above values, based on the molar amount of waste lithium iron phosphate. If the amount of fluorine is too small, the reaction will be incomplete, leaving a large amount of unreacted LiFePO4 or intermediate FePO4 after the reaction, reducing the crystallinity of the final product, sodium iron phosphate. If the amount of fluorine source exceeds the above range, the excess of fluorine ions will lead to the formation of NaF impurity phase, exacerbating lattice distortion, and coating the particle surface with a NaF layer, inhibiting ion transport. At the same time, NaF is an insulating phase, which will block the electron and ion conduction paths, ultimately leading to a decrease in the specific capacity of the prepared sodium-ion battery and accelerated cycle decay.

[0033] The amount of magnesium source can be selected as 2%, 2.2%, 2.4%, 2.6%, 2.8%, 3%, 3.2%, 3.4%, 3.6%, 3.8%, 4%, or any range of two of the above values. Magnesium ions are used to coordinate with fluoride ions, partially replacing PO4 with fluoride ions. 3 + At the same time, magnesium ions are doped into the crystal lattice. If the magnesium source is excessive, it will lead to the formation of other impurity particles. The presence of impurity particles may cause severe grain agglomeration. If the amount of magnesium is too small, there will not be enough magnesium ions to dop into the crystal lattice and cause distortion. The resulting sodium iron phosphate crystal structure has poor stability.

[0034] S3. After filtering the reaction mixture, a solid product is obtained. The solid product is spray-dried to obtain a precursor powder. The precursor powder is then sintered in a hydrogen atmosphere to form regenerated sodium iron phosphate.

[0035] Specifically, the process includes: centrifuging and filtering the reacted slurry to obtain a solid product; spray drying the solid product to obtain a precursor powder with a moisture content ≤0.3%; the inlet air temperature for spray drying is 120℃~160℃, and the outlet air temperature is 85℃~105℃; this spray drying process ensures uniform precursor particle size and a moisture content ≤0.3%. The hydrogen pressure inside the sintering furnace is maintained at 0.01MPa~0.02MPa, i.e., maintaining a slightly positive pressure of 0.01MPa~0.02MPa; hydrogen is continuously introduced during sintering, and sintering is performed in stages under conditions where the hydrogen purity is ≥99.9%; air is purged for 30 minutes before calcination to prevent Fe... 2+ Oxidation is first performed by heating at 1℃ / min~3℃ / min to 300℃~400℃ for 5h~7h to pre-crystallize the precursor powder, followed by calcination at 530℃~570℃ for 8h~12h to generate regenerated sodium iron phosphate. This further crystallizes the crystals and achieves dense carbon layer coating. The reducing atmosphere ensures the Fe... 2+ The stable valence state avoids the formation of impurity phases, and the segmented heating can prevent the problem of excessively large particle size caused by rapid crystal growth, ensuring that the product is suitable for electrode preparation requirements.

[0036] The regenerated sodium iron phosphate particles prepared using the above method have an olivine-type structure and an orthorhombic crystal system; the unit cell volume of the regenerated sodium iron phosphate particles is greater than or equal to 292.73 Å. 3 Furthermore, the edge length 'a' of the first unit cell of the particles satisfies 10.34 Å to 10.36 Å, the edge length 'b' of the second unit cell satisfies 6.0 Å to 6.02 Å, and the edge length 'c' of the third unit cell satisfies 4.7 Å to 4.71 Å. Based on this embodiment, by adding a carbon source to waste lithium iron phosphate to remove electrolyte residue and binder from the electrode powder surface, an initial thin elastic layer is formed to protect the olivine crystal framework from collapse. Sodium loss is replenished by sodium, and fluorine partially replaces PO4. 3+ Sites are used to optimize sodium ion migration channels; magnesium sources, which are readily soluble in water and have small magnesium ion radii, can be uniformly dispersed in the material voids and doped into the lattice to suppress volume expansion. This scheme achieves Li + / Na + The process involves slow and uniform exchange, precise doping of fluorine and magnesium ions, followed by segmented sintering to achieve complete crystallization and dense carbon layer coating, ensuring stable electrochemical performance when the regenerated sodium iron phosphate is used in batteries.

[0037] Corresponding to the above embodiments, this application provides an electrode sheet prepared from sodium iron phosphate obtained by the method described above for regenerating waste lithium iron phosphate into olivine-type sodium iron phosphate.

[0038] Corresponding to the above embodiments, a sodium-ion battery is provided, which includes the electrode sheets described above.

[0039] Example 1 S1. Pretreatment and activation: Take waste lithium iron phosphate powder and add sucrose at 8% of the powder mass. Mix the powder evenly in a planetary ball mill, then place it in a tube furnace, introduce nitrogen atmosphere, heat to 300℃ at 5℃ / min, keep at the temperature for 3h, and cool naturally to room temperature to obtain activated powder.

[0040] S2. In-situ lithium-sodium exchange and doping: Weigh 20g of activated powder, add sodium acetate (the amount of sodium acetate added is 108% of the theoretical lithium-sodium exchange), add sodium fluoride (the molar amount of sodium fluoride is 0.05% of the molar amount of waste lithium iron phosphate), add magnesium acetate (the molar amount of magnesium acetate is 3% of the molar amount of waste lithium iron phosphate), add deionized water to make a slurry with a solid-liquid mass ratio of 1:6, place the slurry in an 80℃ water bath, adjust the pH of the slurry to 6 using dilute acetic acid, stir at a rate of 400r / min, and continue the reaction for 4h.

[0041] S3. Segmented calcination: The above slurry was transferred to a centrifuge, centrifuged, filtered, and then spray-dried to obtain precursor powder. The precursor powder was placed in a tube furnace, hydrogen gas was introduced, and the temperature was increased to 350℃ at 2℃ / min, held for 6 hours, and then increased to 550℃ and held for 10 hours. After calcination, carbon-coated fluorine-magnesium co-doped olivine-type sodium iron phosphate material was obtained.

[0042] Example 2 S1. Pretreatment activation: Take waste lithium iron phosphate powder and add sucrose at 5% of the powder mass. Mix the powder evenly in a planetary ball mill, then place it in a tube furnace, introduce nitrogen atmosphere, heat to 300℃ at 5℃ / min, keep at the temperature for 2 hours, and cool naturally to room temperature to obtain activated powder.

[0043] S2. In-situ lithium-sodium exchange and doping: Weigh 20g of activated powder and add sodium acetate (105% of the theoretical lithium-sodium exchange), sodium fluoride (0.03% of the molar amount of lithium iron phosphate), and magnesium acetate (2% of the molar amount of lithium iron phosphate).

[0044] Sodium acetate was added at 105% of the theoretical lithium-sodium exchange capacity. Sodium fluoride was added at a molar amount equal to 0.03% of the waste lithium iron phosphate. Magnesium acetate was added at a molar amount equal to 2% of the waste lithium iron phosphate. Deionized water was added to prepare a slurry with a solid-liquid mass ratio of 1:5. The slurry was placed in a water bath at 70°C, and the pH of the slurry was adjusted to 5.5 using dilute acetic acid. The mixture was stirred at a rate of 300 r / min and reacted continuously for 3 hours.

[0045] S3. Segmented calcination: The above slurry was transferred to a centrifuge, centrifuged, filtered, and then spray-dried to obtain precursor powder. The precursor powder was placed in a tube furnace, hydrogen gas was introduced, and the temperature was increased to 350℃ at 2℃ / min, held for 5 hours, and then increased to 530℃ and held for 8 hours. After calcination, carbon-coated fluorine-magnesium co-doped olivine-type sodium iron phosphate material was obtained.

[0046] Example 3 S1. Pretreatment and activation: Take waste lithium iron phosphate powder and add sucrose at 10% of the powder mass. Place it in a planetary ball mill and mill at 400 r / min for 60 min to mix evenly. Then place it in a tube furnace, introduce nitrogen atmosphere, heat to 300℃ at 5℃ / min, keep at the temperature for 4 h, and cool naturally to room temperature to obtain activated powder.

[0047] S2. In-situ lithium-sodium exchange and doping: Weigh 20g of activated powder, add sodium acetate (the amount of sodium acetate added is 110% of the theoretical lithium-sodium exchange), add sodium fluoride (the molar amount of sodium fluoride is 0.08% of the molar amount of waste lithium iron phosphate), add magnesium acetate (the molar amount of magnesium acetate is 4% of the molar amount of waste lithium iron phosphate), and add deionized water to prepare a slurry with a solid-liquid mass ratio of 1:7; place the slurry in a 90℃ water bath, adjust the pH of the slurry to 6 using dilute acetic acid, stir at a rate of 500r / min, and continue the reaction for 5h.

[0048] S3. Segmented calcination: The above slurry was transferred to a centrifuge, centrifuged, filtered, and then spray-dried to obtain precursor powder. The precursor powder was placed in a tube furnace, hydrogen gas was introduced, and the temperature was increased to 350℃ at 2℃ / min, held for 7 hours, and then increased to 570℃ and held for 12 hours. After calcination, carbon-coated fluorine-magnesium co-doped olivine-type sodium iron phosphate material was obtained.

[0049] Example 4 Corresponding to the above embodiments, the difference between this embodiment and Embodiment 1 is that the molar amount of sodium fluoride added is 0.04% of the molar amount of waste lithium iron phosphate powder, while the other sample amounts, process steps and parameter settings are the same.

[0050] Example 5 Corresponding to the above embodiments, the difference between this embodiment and Embodiment 1 is that the molar amount of sodium fluoride added is 0.06% of the molar amount of waste lithium iron phosphate powder, while the remaining sample amount, process steps and parameter settings are the same.

[0051] Example 6 Corresponding to the above embodiments, the difference between this embodiment and Embodiment 1 is that the molar amount of sodium fluoride added is 0.07% of the molar amount of waste lithium iron phosphate powder, while the remaining sample amount, process steps and parameter settings are the same.

[0052] Example 7 Corresponding to the above embodiments, the difference between this embodiment and Embodiment 1 is that the molar amount of sodium fluoride added is 0.02% of the molar amount of waste lithium iron phosphate powder, while the remaining sample amount, process steps and parameter settings are the same.

[0053] Example 8 Corresponding to the above embodiments, the difference between this embodiment and Embodiment 1 is that the molar amount of sodium fluoride added is 0.09% of the molar amount of waste lithium iron phosphate powder, while the other sample amounts, process steps and parameter settings are the same.

[0054] Example 9 Corresponding to the above embodiments, the difference between this embodiment and Embodiment 1 is that the molar amount of magnesium acetate added is 1% of the molar amount of waste lithium iron phosphate powder, while the remaining sample amount, process steps and parameter settings are the same.

[0055] Example 10 Corresponding to the above embodiments, the difference between this embodiment and Embodiment 1 is that the molar amount of magnesium acetate added is 5% of the molar amount of waste lithium iron phosphate powder, while the remaining sample amount, process steps and parameter settings are the same.

[0056] Comparative Example 1 S1. Following the pre-activation steps of Example 1, activated powder is obtained.

[0057] S2. Take 20g of activated powder, add only sodium acetate (the amount added is 108% of the amount of sodium required for theoretical lithium-sodium exchange), add deionized water to make a slurry with a solid-liquid mass ratio of 1:6, and stir in an 80℃ water bath at 400r / min for 4h.

[0058] S3. Following the segmented calcination steps of Example 1, carbon-coated pure-phase sodium iron phosphate material was obtained.

[0059] Comparative Example 2 S1. Following the pre-activation steps of Example 1, activated powder is obtained.

[0060] S2. Take 20g of activated powder, add sodium acetate (the amount added is 108% of the amount of sodium required for theoretical lithium-sodium exchange) and sodium fluoride (the molar ratio is 0.05% of lithium iron phosphate), add deionized water to make a slurry with a solid-liquid mass ratio of 1:6, and stir in an 80℃ water bath at 400r / min for 4h.

[0061] S3. Following the segmented calcination steps of Example 1, carbon-coated pure-phase sodium iron phosphate material was obtained.

[0062] Comparative Example 3 S1. Following the pre-activation steps of Example 1, activated powder is obtained.

[0063] S2. Take 20g of activated powder, add sodium acetate (the amount added is 108% of the amount of sodium required for theoretical lithium-sodium exchange) and magnesium acetate (the molar ratio is 3% of lithium iron phosphate), add deionized water to make a slurry with a solid-liquid mass ratio of 1:6, and stir in an 80℃ water bath at 400r / min for 4h.

[0064] S3. Following the segmented calcination steps of Example 1, carbon-coated pure-phase sodium iron phosphate material was obtained.

[0065] Comparative Example 4 S1. Take 20g of waste lithium iron phosphate electrode powder, add 1mol / L sulfuric acid solution for acid leaching, filter to obtain a leachate containing lithium, iron and phosphorus, remove impurities and concentrate, add sodium carbonate to recover lithium carbonate.

[0066] S2. Add sodium hydroxide to the remaining leachate to adjust the pH, precipitate the ferric phosphate precursor, wash and dry it, and then mix it with sodium acetate and sucrose.

[0067] S3. Following the segmented sintering steps of Example 1, carbon-coated sodium iron phosphate material was obtained.

[0068] The crystal structure and cell parameters of the products in the above embodiments and comparative examples were tested under the following conditions, and the test results are shown in Table 1.

[0069] (1) Crystal structure, cell parameters and cell volume testing Crystal structure, cell parameters, and cell volume were determined using X-ray powder diffraction (XRD) with an EVASTAR instrument and Cu-Kα radiation (λ = 0.15406 nm) as the target source. Before testing, samples were thoroughly ground to a particle size of less than 75 μm and pressed onto the sample stage. Continuous scanning mode was used, with a scanning range of 10°–80°, a step size of 0.02°, and a scanning speed of 2° / min. The raw diffraction data were refined using Topas 4.2 software with Rietveld full-spectrum fitting. Using the standard olivine-type NeFePO4 crystal structure as the initial model, the crystal system, space group, cell parameters (a, b, c), and cell volume λ were refined. All tested samples exhibited an orthorhombic crystal system with space group Pnma (No. 62).

[0070] (2) Testing of regenerated sodium iron phosphate as a cathode material for sodium-ion batteries Using the regenerated sodium iron phosphate from the above examples and comparative examples as the positive electrode material for sodium-ion batteries, the positive electrode material, Super P, and PVDF were mixed in a weight ratio of 8:1:1. An appropriate amount of NMP was added as a solvent, and the mixture was homogenized in a homogenizer to prepare a positive electrode slurry. The positive electrode slurry was uniformly coated onto aluminum foil, dried, and sliced ​​to obtain the positive electrode sheet. Six electrode sheets with a mass difference not exceeding 0.00050 g were used for battery coin-type assembly, with a sodium metal sheet as the negative electrode and a glass fiber separator as the separator. After assembling the positive electrode sheet, negative electrode, and separator into the coin cell, an electrolyte (NaPF6, concentration 1 mol / L) was injected. The battery was pressed to obtain a sodium-ion coin cell half-cell. All 2032 half-cells were assembled and left to stand for 8 hours before charge-discharge testing. The test item was a 0.1C first-cycle charge-discharge test. The test result was the average of the six sample batteries. A difference greater than 5% in the test result was considered an outlier and discarded. The data shown in Table 2 were obtained.

[0071] Table 1. Results of cell parameter testing

[0072] Table 2 Battery performance test results

[0073] Combining the test results in Tables 1 and 2 above, it can be concluded that the method employed, by co-doping waste lithium iron phosphate with fluorine, magnesium, and carbon, achieves the largest particle lattice volume, complete and distortion-free lattice in the prepared regenerated sodium iron phosphate; unobstructed sodium ion channels, strong structural stability, the highest discharge capacity and first-cycle coulombic efficiency across the entire series, and the best matching between structure and electrochemical performance. Furthermore, combined with... Figure 2 and Figure 3 As shown, the particles in waste lithium iron phosphate are irregularly shaped or spherical, and there are a large number of impurities between the particles. Figure 3 The regenerated sodium iron phosphate particles prepared by this method are regularly olive-shaped, and there are almost no impurities between the particles.

[0074] In conjunction with Examples 1-3 and Examples 4-6, when different amounts of fluorine and magnesium are used for doping, and different processing conditions are applied to waste lithium iron phosphate, the cell volume, lattice integrity, and 0.1C charge / discharge specific capacity of the prepared regenerated sodium iron phosphate are slightly lower than those in Example 1. However, the crystal structure of the regenerated sodium iron phosphate remains relatively stable. The amount of fluorine doping is finely adjusted within a defined range, the cell volume changes little, and the crystal state is stable. The electrochemical fluctuations are weak, and the modification effect is stable within this doping range.

[0075] In conjunction with Examples 7 to 10, when the amount of fluorine or magnesium source selected exceeds the range of 0.03% to 0.08% and 2% to 4% in this example, if the doping amount of the two is too small, the doping amount of fluorine ions and magnesium ions is insufficient and the modification effect is not fully utilized. The cell shrinks, the ion channel narrows, and the sodium storage capacity and the reversibility of insertion and extraction decrease slightly in tandem. However, when the doping amount of fluorine or magnesium is excessive, the excessive doping causes lattice distortion, damages the crystal framework, hinders sodium ion transport, increases side reactions, and the capacity and coulombic efficiency decline significantly.

[0076] Combining Example 1 with Comparative Examples 1 to 4, it can be seen that the original lattice has narrow channels, poor structural stability, and a small cell volume; its electrochemical performance is poor. When only fluorine or magnesium is used for doping, the improvement in lattice is limited, and synergistic optimization cannot be achieved, resulting in electrochemical performance weaker than all dual-doped samples. Furthermore, the sodium iron phosphate product prepared by traditional leaching process exhibits the worst lattice structure, poor ion transport conditions, and poor discharge capacity and initial efficiency.

[0077] Although preferred embodiments have been described in this application, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of the embodiments of this application.

[0078] Obviously, those skilled in the art can make various modifications and variations to this application without departing from the spirit and scope of this application. Therefore, if such modifications and variations fall within the scope of the claims of this application and their equivalents, this application also intends to include such modifications and variations.

Claims

1. A method for regenerating waste lithium iron phosphate into olivine-type sodium iron phosphate, characterized in that, The method includes: A carbon source is added to waste lithium iron phosphate, and the mixture is ground evenly to obtain a mixture. The mixture is then calcined in an inert atmosphere to form an activated material. The activating material is mixed with a sodium source, a fluorine source, and a magnesium source, and a solvent is added to obtain a mixed slurry. The slurry is then heated and stirred to allow it to react. After filtration and reaction, the mixed slurry yields a solid product. The solid product is then spray-dried to obtain a precursor powder. The precursor powder is then sintered in a hydrogen atmosphere to form regenerated sodium iron phosphate.

2. The method for regenerating waste lithium iron phosphate into olivine-type sodium iron phosphate according to claim 1, characterized in that, The amount of carbon source added is 5% to 10% of the molar amount of the waste lithium iron phosphate. And / or, the carbon source includes one or more of sucrose, glucose, citric acid, and polyethylene glycol.

3. The method for regenerating waste lithium iron phosphate into olivine-type sodium iron phosphate according to claim 1 or 2, characterized in that, The carbon source is added to the waste lithium iron phosphate, and the mixture is ground until homogeneous to obtain the mixture, specifically comprising: The carbon source is added to the waste lithium iron phosphate and mixed. The mixture is then ground for 30 min to 60 min at a speed of 300 r / min to 400 r / min until homogeneous to obtain the mixture. And / or, calcining the mixture under an inert atmosphere to form an activated material, specifically including: Under an inert atmosphere, the mixture is heated to 250°C to 350°C at a rate of 3°C to 7°C per minute, held at that temperature for 2 to 4 hours, and then cooled to room temperature to obtain the activated material.

4. The method for regenerating waste lithium iron phosphate into olivine-type sodium iron phosphate according to claim 1 or 2, characterized in that, Also includes: The amount of sodium source used is 1.05 to 1.1 times the theoretical amount of sodium used, and the theoretical amount of sodium used is based on the molar amount of the waste lithium iron phosphate. And / or, the amount of the fluorine source used is 0.03% to 0.08% of the molar amount of the waste lithium iron phosphate; And / or, the amount of magnesium source used is 2% to 4% of the molar amount of the waste lithium iron phosphate, based on the molar amount of the waste lithium iron phosphate; And / or, the sodium source includes one or more of sodium acetate, sodium formate, sodium oxalate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium citrate, sodium carbonate, sodium bicarbonate, and sodium hydroxide; And / or, the fluorine source includes one or more of sodium fluoride, ammonium fluoride and ammonium bifluoride; And / or, the magnesium source includes one or more of magnesium acetate, magnesium formate, and magnesium citrate.

5. The method for regenerating waste lithium iron phosphate into olivine-type sodium iron phosphate according to claim 1 or 2, characterized in that, The activating material is mixed with the sodium source, the fluorine source, and the magnesium source, and the solvent is added to obtain the mixed slurry. The slurry is then heated and stirred to allow it to react. Specifically, this includes: Add the sodium source to the activated material and stir to dissolve. Then add the fluorine source and the magnesium source in sequence. Then add the solvent at a solid-liquid ratio of 1:(4~8) to prepare the mixed slurry. The mixed slurry is heated in a water bath to 70℃~90℃, stirred evenly at a rate of 300r / min~500r / min, and acid solution is added to adjust the pH of the mixed slurry to the range of 5.5~6.5 and react for 3h~5h.

6. The method for regenerating waste lithium iron phosphate into olivine-type sodium iron phosphate according to claim 1 or 2, characterized in that, After filtration and reaction, a mixed slurry is obtained as a solid product. The solid product is then spray-dried to obtain a precursor powder, specifically comprising: The reaction slurry is centrifuged and filtered to obtain a solid product. The solid product is then spray-dried to obtain the precursor powder with a water content of ≤0.3%. The inlet air temperature of the spray dryer is 120℃~160℃ and the outlet air temperature is 85℃~105℃.

7. The method for regenerating waste lithium iron phosphate into olivine-type sodium iron phosphate according to claim 6, characterized in that, The precursor powder is sintered in a hydrogen atmosphere to form regenerated sodium iron phosphate, specifically including: During the sintering process, hydrogen gas is continuously introduced, and the hydrogen gas pressure inside the sintering furnace is maintained at 0.01 MPa to 0.02 MPa. First, the precursor powder is pre-calcined at 300℃~400℃ / min at a rate of 1℃ / min~3℃ / min for 5h~7h to allow it to initially crystallize. Then, it is calcined at 530℃~570℃ for 8h~12h to generate the regenerated sodium iron phosphate.

8. The method for regenerating waste lithium iron phosphate into olivine-type sodium iron phosphate according to claim 1 or 2, characterized in that, The regenerated sodium iron phosphate particles are olivine-type; And / or, the crystal structure of the regenerated sodium iron phosphate particles is orthorhombic; And / or, the cell volume of the regenerated sodium iron phosphate particles is greater than or equal to 292.73 Å. 3 Furthermore, the first cell edge length a of the particle satisfies 10.34Å~10.36Å, the second cell edge length b satisfies 6.0Å~6.02Å, and the third cell edge length c satisfies 4.7Å~4.71Å.

9. An electrode sheet, characterized in that, The electrode sheet is prepared from sodium iron phosphate obtained by the method of recycling waste lithium iron phosphate into olivine-type sodium iron phosphate according to any one of claims 1 to 8.

10. A sodium-ion battery, characterized in that, The sodium-ion battery includes the electrode sheet as described in claim 9, and the sodium-ion battery has a 0.1C charging specific capacity ≥ 150.8 mAh / g, a 0.1C discharging specific capacity ≥ 143 mAh / g, and a first-cycle coulombic efficiency > 94%.

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