Method for producing carbon-coated sodium fluoride iron phosphate using waste lithium iron phosphate, and use thereof

Abstract

The present invention relates to the technical field of sodium ion batteries, and in particular to a method for producing carbon-coated sodium fluoroiron phosphate using waste lithium iron phosphate, and its use. The method for producing carbon-coated sodium fluoroiron phosphate using waste lithium iron phosphate includes the steps of: mixing waste lithium iron phosphate material with an alkaline solution to react, then performing solid-liquid separation to obtain an aluminum-containing filtrate and lithium iron phosphate filtrate; homogeneously mixing the lithium iron phosphate filtrate, aluminum chloride, and sodium chloride, and then performing vacuum roasting to obtain a roasted material; homogeneously mixing the roasted material with at least one of a sodium source, an iron source, and a phosphorus source to obtain a mixed material, adding a fluorine source, a carbon source, and a solvent thereto, homogeneously mixing, and then drying and calcining in sequence to obtain carbon-coated sodium fluoroiron phosphate. The method has the advantages of low cost, high added value, short process, high recovery rate, and excellent electrochemical properties of the produced carbon-coated sodium fluoroiron phosphate.

Description

[Technical Field]

[0001] This application claims priority to a Chinese patent application filed on December 13, 2022, bearing application number CN2022116029466 and titled "Method for producing carbon-coated sodium fluoroferric phosphate using waste lithium iron phosphate, and use thereof."

[0002] The present invention relates to the technical field of sodium ion batteries, and in particular to a method for producing carbon-coated sodium fluoroiron phosphate using waste lithium iron phosphate, and its use. [Background technology]

[0003] Due to abundant sodium resources and a similar operating principle to lithium-ion batteries, research on sodium-ion batteries has been attracting attention in recent years as a new energy storage technology that is expected to promote the low-cost use and sustainable development of energy storage technology. Polyanionic compounds have been widely studied as cathode materials with high performance and safety, and the strong covalent bonds in their structure are advantageous for stabilizing the structural framework, effectively improving the safety of electrodes during charging and discharging.

[0004] Sodium iron phosphate is generally considered to have poor electrochemical activity, but the introduction of fluorine to form sodium iron fluorophosphate can increase the electrochemical activity and improve the voltage platform, making this modification a desirable modification for sodium battery cathode materials.

[0005] Lithium iron phosphate scrap is mainly waste electrode plates generated during the battery manufacturing process, i.e., electrode plates generated when batteries are dismantled after disposal. According to statistics, the amount of lithium iron phosphate generated in 2021 was 480,000 tons, and is expected to reach 1 million tons in 2022. How to recycle and utilize these resources is a focus of research.

[0006] In particular, with the gradual rise in the price of lithium, sodium batteries are becoming increasingly important as a future development trend, so it is very important to utilize discarded lithium battery resources to produce sodium battery materials. With this in mind, the present invention is proposed. Summary of the Invention

[0007] A first object of the present invention is to provide a method for producing carbon-coated sodium fluoroiron phosphate using waste lithium iron phosphate, which method has the following characteristics: low cost, high added value, a short process, a high recovery rate, and the produced carbon-coated sodium fluoroiron phosphate has excellent electrochemical properties.

[0008] A second object of the present invention is to provide a positive pole piece.

[0009] A third object of the present invention is to provide a sodium ion battery.

[0010] In order to achieve the above object of the present invention, the following technical solutions are particularly used:

[0011] In a first aspect, the present invention provides a method for producing carbon-coated sodium fluoroiron phosphate using waste lithium iron phosphate, comprising the steps of:

[0012] Step (a): Mixing and reacting the waste lithium iron phosphate material with an alkaline solution, followed by solid-liquid separation to obtain an aluminum-containing filtrate and a lithium iron phosphate filtrate.

[0013] Here, the waste lithium iron phosphate material includes electrode plates produced by dismantling discarded batteries, and its main components include lithium iron phosphate and aluminum.

[0014] In step (a), the waste lithium iron phosphate material is reacted with an alkaline solution, with the purpose of reacting with aluminum in the waste lithium iron phosphate material and dissolving the aluminum.

[0015] In some specific embodiments of the present invention, the reaction in step (a) can be carried out at room temperature and does not require heating, since aluminum reacts with sodium hydroxide to generate a large amount of heat.

[0016] In some specific embodiments of the present invention, in step (a), the reaction is carried out until the bubbles disappear, i.e., no bubbles are generated. Preferably, in step (a), the reaction time is 1 to 10 hours, including but not limited to 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, or any range therebetween.

[0017] The main component of the aluminum-containing filtrate is metaaluminate ions.

[0018] The main component of the lithium iron phosphate filtration residue is lithium iron phosphate.

[0019] (b) The lithium iron phosphate filter residue, aluminum chloride, and sodium chloride are uniformly mixed, and then vacuum roasted to obtain a roasted material.

[0020] The present invention not only uses aluminum chloride and sodium chloride as roasting additives to achieve chlorination roasting, but also reacts aluminum chloride with lithium to form lithium tetrachloroaluminate. Taking advantage of the relatively low boiling point of lithium tetrachloroaluminate, the lithium is evaporated while leaving behind sodium, and most of the other elements, such as phosphorus and iron, are also left behind.

[0021] That is, the main component of the roasted material is sodium iron phosphate.

[0022] During the vacuum roasting process, lithium tetrachloroaluminate escapes with the exhaust gases.

[0023] The vacuum roasting is performed in a vacuum atmosphere, and the vacuum roasting can be used to lower the boiling point of lithium tetrachloroaluminate, lower the roasting temperature, and sufficiently separate lithium tetrachloroaluminate while reducing energy consumption.

[0024] (c) The roasted material is uniformly mixed with at least one of a sodium source, an iron source, and a phosphorus source to obtain a mixed material, to which a fluorine source, a carbon source, and a solvent are added, followed by uniform mixing, followed by drying and calcination to obtain carbon-coated sodium iron fluorophosphate.

[0025] Among them, the sodium source, iron source and phosphorus source are used for sodium supplementation, iron supplementation and phosphorus supplementation, respectively.

[0026] The carbon source acts as a reducing agent to reduce valent iron to trivalent iron, and as a coating agent, it is coated on the surface of sodium iron fluorophosphate to enhance electrical conductivity.

[0027] That is, the carbon-coated sodium iron fluorophosphate obtained in step (c) of the present invention comprises sodium iron fluorophosphate and a carbon coating layer that coats at least a portion of the surface of the sodium iron fluorophosphate.

[0028] The method for producing carbon-coated sodium iron fluorophosphate using waste lithium iron phosphate according to the present invention is a short process, low-cost, environmentally friendly, and has a high recovery rate of iron and phosphorus, making it possible to produce a high-performance sodium battery cathode material - carbon-coated sodium iron fluorophosphate.

[0029] In some specific embodiments of the present invention, the mass percentage of carbon in the carbon-coated sodium iron fluorophosphate is 1% to 5%, including, but not limited to, 2%, 3%, 4%, or any range therebetween.

[0030] Preferably, in step (a), the mass ratio of the waste lithium iron phosphate material to the alkaline solution is 1:5-15, including but not limited to 1:6, 1:7, 1:8, 1:9, 1:10, 1:11, 1:12, 1:13, 1:14, or any range therebetween.

[0031] Preferably, the alkaline solution comprises sodium hydroxide solution, which is used to produce sodium carbonate for easy subsequent recovery.

[0032] Preferably, the molar concentration of the alkaline solution is 3 to 5 mol / L, including, but not limited to, any of 3.5 mol / L, 4 mol / L, 4.5 mol / L, or any range therebetween.

[0033] Preferably, in step (a), the aluminum-containing filtrate is repeatedly utilized until the molar concentration of hydroxide ions therein is less than 0.2 mol / L (including, but not limited to, any of 0.15 mol / L, 0.1 mol / L, 0.05 mol / L, 0.01 mol / L, or any range therebetween), to obtain an aluminum-containing waste liquid.

[0034] Preferably, step (a) further comprises recovering and treating the aluminum-containing waste liquid to obtain aluminum chloride and sodium carbonate.

[0035] Preferably, the recovery treatment method includes introducing carbon dioxide into the aluminum-containing waste liquid to lower its pH to 9 to 11 (including, but not limited to, any value of 9.3, 9.5, 10, 10.5, and 10.8, or any value in a range between these values), mixing the aluminum hydroxide precipitate with hydrochloric acid to react with each other, and then concentrating and crystallizing the mixture to obtain aluminum chloride, and concentrating and crystallizing the sodium carbonate filtrate to obtain sodium carbonate.

[0036] In some specific embodiments of the present invention, during the process of introducing carbon dioxide, the temperature of the mixed material is 40 to 60°C, including but not limited to 42°C, 45°C, 50°C, 55°C, 58°C, or any range therebetween.

[0037] The aluminum chloride obtained above can be used in step (b).

[0038] The sodium carbonate obtained above can be used as the sodium source in step (c).

[0039] The present invention fully utilizes all components in waste lithium iron phosphate material, including aluminum (aluminum foil), which is used to prepare aluminum chloride and sodium hydroxide solution (alkali solution) to prepare sodium carbonate, thereby reducing waste generation and saving costs.

[0040] Preferably, in step (b), the molar ratio of lithium element in the lithium iron phosphate filtrate to the aluminum chloride to the sodium chloride is 1:1.2-1.5:1.02-1.05, for example, 1:1.2:1.02, 1:1.2:1.03, 1:1.2:1.04, 1:1.2:1.05, 1:1.3:1.02, 1:1.3:1.03, 1:1.3:1.04, 1:1.3:1.05, 1:1.4:1.02, 1:1.4:1.03, 1:1.4:1.04, 1:1.4:1.05, 1:1.5:1.02, 1:1.5:1.03, 1:1.5:1.04 or 1:1.5:1.05.

[0041] Preferably, in step (b), the vacuum roasting temperature is 400-600°C, including but not limited to, 420°C, 450°C, 500°C, 550°C, 580°C, or any range therebetween, and the vacuum roasting time is 4-6 hours, including but not limited to, 4.5 hours, 5 hours, 5.5 hours, or any range therebetween.

[0042] The vacuum degree of the vacuum roasting is −0.04 to −0.08 MPa, including, but not limited to, any value among −0.05 MPa, −0.06 MPa, and −0.07 MPa, or any value in the range between these values.

[0043] Preferably, in step (b), exhaust gas from the vacuum roasting is collected and mixed with aqueous ammonia to carry out an aluminum precipitation reaction (i.e., aluminum hydroxide precipitation reaction), followed by solid-liquid separation; the solid material obtained by the solid-liquid separation is then sintered to obtain alumina; the liquid material obtained by the solid-liquid separation is mixed with sodium carbonate to carry out a lithium precipitation reaction (i.e., sodium carbonate precipitation reaction), followed by solid-liquid separation to obtain lithium carbonate.

[0044] The main component of the exhaust gas from vacuum roasting is lithium tetrachloroaluminate. When this is mixed with ammonia water, the aluminum ions react with the ammonia water to form an aluminum hydroxide precipitate. The solid material obtained after solid-liquid separation is the aluminum hydroxide precipitate, which can be calcined to obtain alumina.

[0045] In some specific embodiments of the present invention, the alumina prepared above can be reacted with hydrochloric acid to prepare aluminum chloride, which is used in the subsequent step (b).

[0046] At the same time, the liquid material obtained after the solid-liquid separation contains lithium ions, which can be mixed with sodium carbonate and reacted to obtain lithium carbonate.

[0047] Preferably, the molar concentration of the aqueous ammonia is 0.01 to 0.1 mol / L, including, but not limited to, any of the following values ​​or any range between any of 0.02 mol / L, 0.03 mol / L, 0.04 mol / L, 0.05 mol / L, 0.06 mol / L, 0.07 mol / L, 0.08 mol / L, and 0.09 mol / L.

[0048] In some specific embodiments of the present invention, the molar ratio of nitrogen element in the ammonia water to aluminum element in the exhaust gas is 3.1 to 6:1, including, but not limited to, any of 3.2:1, 3.3:1, 3.4:1, 3.5:1, 3.7:1, 3.9:1, 4:1, 4.5:1, 5:1, and 5.5:1, or any range therebetween.

[0049] In some specific embodiments of the present invention, during the vacuum roasting, the exhaust gas from the roasting is sucked in using an induced draft fan, condensed in a condenser to a temperature of 150°C or less, passed through a dust collection bag, and then absorbed with a water spray to obtain the exhaust gas.

[0050] Currently, the price of lithium carbonate is about 500,000 yuan / ton, and the price of sodium iron fluorophosphate is about 85,000 yuan / ton.

[0051] In this way, the present invention can achieve complete recovery of each component (including each element, phosphorus, iron, and lithium), recover lithium, and not only obtain lithium carbonate, but also produce sodium iron fluorophosphate using elements such as iron and phosphorus, resulting in high added value of the product.

[0052] Preferably, in the aluminum precipitation reaction, the temperature of the mixed material is 40 to 80°C, including but not limited to, any of 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, and 75°C, or any range therebetween. The time for the aluminum precipitation reaction is 1 to 3 hours, including but not limited to, any of 1.5 hours, 2 hours, and 2.5 hours, or any range therebetween.

[0053] Preferably, the temperature of the mixed material in the lithium precipitation reaction is 60 to 90°C, including but not limited to, 65°C, 70°C, 75°C, 80°C, 85°C, or any range therebetween. The time for the lithium precipitation reaction is 1 to 3 hours, including but not limited to, 1.5 hours, 2 hours, 2.5 hours, or any range therebetween.

[0054] Preferably, in step (c), before mixing the roasted material with at least one of a sodium source, an iron source, and a phosphorus source, the contents of Na, Fe, and P in the roasted material are detected, and the roasted material and the phosphorus source are mixed so that the molar ratio of Na, Fe, and P in the mixed material is 0.95-0.98:1:1.02-1.05. Among them, the molar ratio is, for example, 0.95:1:1.02, 0.96:1:1.02, 0.97:1:1.02, 0.98:1:1.02, 0.95:1:1.03, 0.96:1:1.03, 0.97:1:1.03, 0.98:1:1.03, 0.95:1:1.04, 0.96:1:1.04, 0.97:1:1.04, 0.98:1:1.04, 0.95:1:1.05, 0.96:1:1.05, 0.97:1:1.05 or 0.98:1:1.05.

[0055] That is, the roasted material is sampled and detected, the contents of Na, Fe, and P elements therein are detected, the molar ratio of Na:Fe:P in the roasted material is calculated, and at least one raw material selected from a sodium source, an iron source, and a phosphorus source (one, two, or three of which can be added based on the calculation result) is added so that the molar ratio of Na:Fe:P in the roasted material is 0.95 to 0.98:1:1.02 to 1.05.

[0056] Preferably, the sodium source includes at least one of sodium carbonate, sodium bicarbonate, and sodium acetate, the iron source includes at least one of red iron oxide, iron (II) oxalate, and iron acetate, and the phosphorus source includes at least one of phosphoric acid, ammonium monohydrogen phosphate, and ammonium dihydrogen phosphate.

[0057] Preferably, in step (c), the fluorine source comprises sodium fluoride.

[0058] Preferably, the molar ratio of the fluorine source to the Fe element in the mixed material is 0.95 to 0.98:1, including but not limited to 0.95:1, 0.96:1, 0.97:1, 0.98:1, or any value in between.

[0059] Preferably, in step (c), the carbon source comprises at least one of glucose, sucrose, polyethylene glycol, and starch.

[0060] Preferably, the mass ratio of the carbon source to the roasted material is 0.2 to 0.3:1, including, but not limited to, any of the following values: 0.21:1, 0.22:1, 0.23:1, 0.24:1, 0.25:1, 0.26:1, 0.27:1, 0.28:1, and 0.29:1, or any range therebetween.

[0061] Preferably, in step (c), the solvent comprises water and / or an organic solvent, more preferably water.

[0062] In some specific embodiments of the present invention, the organic solvent can be any conventional volatile solvent, such as, but not limited to, ethanol, propanol, and ethylene glycol.

[0063] Preferably, in step (c), the fluorine source, carbon source, and solvent are added, then crushed and mixed uniformly.

[0064] More preferably, the solid particles in the mixed slurry are crushed until their particle diameter is 200 to 400 nm, which includes, but is not limited to, any one of 220 nm, 250 nm, 300 nm, 350 nm, and 380 nm, or any value in a range between these two values.

[0065] Preferably, in step (c), said drying comprises spray drying.

[0066] More preferably, the particle size of the drying material obtained by the spray drying is 10 to 30 μm, including, but not limited to, any of the values ​​of 12 μm, 15 μm, 20 μm, 25 μm, and 28 μm, or any value in a range between these two values.

[0067] In some specific embodiments of the present invention, the moisture content in the dried material obtained by spray drying is less than 0.5 wt%, including, but not limited to, any of the following values: 0.4 wt%, 0.3 wt%, 0.2 wt%, 0.1 wt%, or any range therebetween.

[0068] Preferably, in step (c), the calcination temperature is 550-650°C, including but not limited to, 560°C, 570°C, 580°C, 590°C, 600°C, 610°C, 620°C, 630°C, 640°C, or any range therebetween. The calcination incubation time is 4-6 hours, including but not limited to, 4.5 hours, 5 hours, 5.5 hours, or any range therebetween.

[0069] In some specific embodiments of the present invention, in step (c), the heating rate in the calcination is 100 to 150°C / h, including, but not limited to, 110°C / h, 120°C / h, 130°C / h, 140°C / h, or any value in a range therebetween.

[0070] In some specific embodiments of the present invention, in step (c), after the calcination is completed, the material is cooled to a temperature of 100° C. or less and then discharged.

[0071] In some specific embodiments of the present invention, in step (c), the calcination is carried out under an inert atmosphere, such as, but not limited to, a nitrogen atmosphere and / or an argon atmosphere. Preferably, the oxygen content of the inert atmosphere is less than 5 ppm, including, but not limited to, 4 ppm, 3 ppm, 2 ppm, 1 ppm, or any range therebetween. Preferably, the humidity of the inert atmosphere is less than 3%, including, but not limited to, 2%, 1%, 0.5%, or any range therebetween.

[0072] In some specific examples of the present invention, in step (c), an induced draft fan is connected to the temperature-raising zone of the calcination to exhaust the exhaust gas in the temperature-raising zone.

[0073] Preferably, step (c) further comprises the steps of crushing, sieving and removing iron sequentially after the calcination.

[0074] In some specific embodiments of the present invention, the grinding can be performed using any conventional grinding method, such as, but not limited to, airflow grinding.

[0075] In some specific embodiments of the present invention, the sieving can be performed by any conventional sieving method, for example, but not limited to, using an ultrasonic vibrating screen.

[0076] In some specific embodiments of the present invention, the iron removal can be performed by any conventional iron removal method, for example, but not limited to, an electromagnetic iron remover, and preferably, the iron removal is stopped when the magnetic material in the material is less than 1 ppm, including, but not limited to, 0.8 ppm, 0.5 ppm, 0.3 ppm, 0.1 ppm, or any range therebetween.

[0077] In some specific embodiments of the present invention, after the iron removal, the method further comprises a step of vacuum packaging in a temperature and humidity controlled chamber.

[0078] Preferably, the carbon-coated sodium iron fluorophosphate that has been subjected to the pulverization and sieving has a D50 particle size of 0.5 to 2 μm.

[0079] In a second aspect, the present invention provides a positive electrode piece made of carbon-coated sodium iron fluorophosphate produced primarily by the above-described method for producing carbon-coated sodium iron fluorophosphate using waste lithium iron phosphate.

[0080] In a third aspect, the present invention provides a sodium-ion battery comprising the above-described positive electrode piece.

[0081] The sodium ion battery is low cost and has excellent electrochemical properties. [Effects of the Invention]

[0082] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0083] (1) The method for producing carbon-coated sodium iron fluorophosphate using waste lithium iron phosphate according to the present invention is a short process, low cost, has a high recovery rate of iron and phosphorus, has a low impurity content in the produced carbon-coated sodium iron fluorophosphate, and has excellent electrochemical properties.

[0084] (2) The method for producing carbon-coated sodium iron fluorophosphate using waste lithium iron phosphate according to the present invention can completely recover each element of phosphorus, iron, and lithium, recover lithium to obtain lithium carbonate, and produce sodium iron fluorophosphate using elements such as iron and phosphorus, resulting in a high added value product.

[0085] (3) The method for producing carbon-coated sodium fluoroiron phosphate using waste lithium iron phosphate according to the present invention can fully utilize the exhaust gas and waste liquid generated in the recovery process, thereby reducing waste generation, reducing costs, and avoiding waste of resources. [Brief explanation of the drawings]

[0086] In order to more clearly describe the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly describe the drawings used to describe the specific embodiments or the prior art. Obviously, the drawings described below are some embodiments of the present invention, and those skilled in the art can obtain other drawings based on these drawings without any creative efforts. [Figure 1] FIG. 2 is a 0.1C charge / discharge curve diagram of the carbon-coated sodium iron fluorophosphate prepared in Example 1 according to the present invention. [Figure 2] 1 is an SEM image of carbon-coated sodium iron fluorophosphate produced in Example 1 according to the present invention. [Figure 3] 1 is an XRD image of carbon-coated sodium iron fluorophosphate produced in Example 1 according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0087] The technical solutions of the present invention will be described clearly and completely below with reference to the drawings and specific embodiments. However, those skilled in the art should understand that the examples described below are only some examples of the present invention, not all examples, and are intended to illustrate the present invention and not to limit the scope of the present invention. All other examples obtained by those skilled in the art based on the examples of the present invention without requiring creative work fall within the scope of the present invention. Unless specific conditions are specified in the examples, the procedures are carried out according to standard conditions or conditions recommended by the manufacturers. Unless the manufacturers of the reagents or equipment used are specified, they are all common products purchased commercially. [Example]

[0088] Example 1 The method for producing carbon-coated sodium fluoroiron phosphate using waste lithium iron phosphate according to this embodiment includes the following steps: (1) 10 kg of waste lithium iron phosphate material is mixed with 100 kg of sodium hydroxide solution with a molar concentration of 4 mol / L to react, dissolve the aluminum, and react until the bubbles disappear, at which point the reaction is terminated (reaction time: 3.5 h), and then filtered to obtain a first aluminum-containing filtrate and a lithium iron phosphate filtrate. Here, the impurities in the waste lithium iron phosphate material are mainly aluminum foil, and the proportion is 20.5 mass%. The detection results of the content of each element in the first aluminum-containing filtrate (volume: 80.05 L) produced above are shown in Table 1 below.

[0089] [Table 1]

[0090] (2) The aluminum-containing filtrate obtained in step (1) was reused and further mixed with 10 kg of waste lithium iron phosphate material to react. When hydroxide ions were consumed until the molar concentration was less than 0.18 mol / L, the mixture was filtered. The obtained second aluminum-containing filtrate (volume: 76.46 L) was sampled and detected. The results are shown in Table 2.

[0091] [Table 2]

[0092] Carbon dioxide was introduced into the second aluminum-containing filtrate at a temperature of 50°C, and the pH of the filtrate was then measured online. When the pH dropped to 10.1, the introduction of carbon dioxide gas was stopped and the mixture was stirred for an additional 50 minutes to allow the reaction to proceed, after which it was filtered to obtain an aluminum hydroxide precipitate and a sodium carbonate filtrate.

[0093] The aluminum hydroxide precipitate was dissolved in hydrochloric acid and the final pH was maintained at 1.3 to obtain an aluminum chloride solution, which was then concentrated and crystallized to obtain aluminum chloride crystals with a measured purity of 99.4 wt %, which were then ready for use (in step (3) below).

[0094] The sodium carbonate filtrate obtained above is concentrated and crystallized to obtain sodium carbonate. The sodium carbonate is concentrated until the Baume degree reaches 52, and then cooled for 12 hours to crystallize. When the end temperature reaches 25°C, the crystal is centrifuged and baked. The purity is measured and found to be 99.2 wt%. This is used as the sodium source (to be used in step (4) below).

[0095] (3) Add aluminum chloride and sodium chloride to the lithium iron phosphate filter residue obtained in step (1) so that the molar ratio of lithium element to aluminum chloride to sodium chloride in the lithium iron phosphate filter residue is 1:1.35:1.045, mix uniformly, and then roast at a vacuum of -0.065 MPa and a temperature of 500°C for 5 hours to react, thereby obtaining a roasted material.

[0096] During roasting, the exhaust gas from roasting is sucked in using an induced draft fan, condensed in a condenser to a temperature of 135°C, passed through a dust collection bag, and then absorbed with a water spray to obtain the exhaust gas.

[0097] Then, 0.08 mol / L ammonia water solution (the molar ratio of nitrogen in the ammonia water to aluminum in the exhaust gas is 3.5:1) is added to the above exhaust gas, and after stirring, the mixture is reacted at 60°C for 1.5 hours, the final pH is controlled to 7.9, and then filtered and washed. The obtained precipitate is sintered at 650°C for 7 hours to obtain alumina. The detection data of alumina are shown in Table 3.

[0098] Sodium carbonate was then added to the filtrate obtained by the filtration, and the final pH was adjusted to 10.5, the reaction temperature to 75°C, and the reaction time to 1 hour. The mixture was then filtered and washed to obtain industrial-grade lithium carbonate. Its purity was determined to be 99.35%. The analytical data for industrial-grade lithium carbonate are shown in Table 4.

[0099] [Table 3]

[0100] [Table 4]

[0101] (4) The content of each element in the roasted material obtained in step (3) was detected, and the detection results are shown in Table 5 below.

[0102] [Table 5]

[0103] Calculations revealed that the molar ratio of Na:Fe:P in the roasted material was 0.954:1:0.989. Therefore, a sodium source and a phosphorus source needed to be added. Sodium carbonate and ammonium dihydrogen phosphate were added to the roasted material to maintain a molar ratio of Na:Fe:P of 0.975:1:1.04, and the mixture was homogeneously mixed to obtain a mixed material. Sodium fluoride, a carbon source, and pure water were then added to the mixed material and stirred to form a slurry. Glucose and sucrose were used as carbon sources in a mass ratio of 1:0.3, and the mass of the carbon source (the sum of the masses of glucose and sucrose) was 0.25 times the mass of the roasted material. The molar ratio of sodium fluoride to iron in the mixed material was 0.97:1.

[0104] The slurry material thus stirred was then crushed until the particle diameter of the solid particles in the slurry was 285 nm, and then spray-dried to obtain a spray-dried material with a particle diameter of 21.7 μm and a moisture content of less than 0.5 wt%.

[0105] The spray-dried material was then calcined to obtain a calcined material. The temperature was increased at a rate of 120°C / h. The material was then maintained at 600°C for 5 hours, and then cooled to less than 100°C before being discharged. Nitrogen gas was introduced during calcination to maintain the oxygen content in the calciner at less than 5 ppm and the humidity at less than 3%. An induced draft fan was connected to the temperature-raising zone to exhaust the exhaust gases in the temperature-raising zone.

[0106] The calcined material was jet-milled until the particle size was 1.4 μm. It was then sieved using a 100-mesh ultrasonic vibrating screen. Iron was then removed using an electromagnetic iron remover. When the magnetic content of the material reached less than 1 ppm, iron removal was stopped. The material was then vacuum-packaged in a temperature- and humidity-controlled chamber to obtain carbon-coated sodium iron fluorophosphate, which can be used as a positive electrode material for sodium batteries.

[0107] The detection data for each item of the carbon-coated sodium iron fluorophosphate produced in this example are shown in Table 6 below.

[0108] [Table 6]

[0109] Here, the compressed density is measured at 3T pressure.

[0110] Powder resistivity was measured using the four-point probe method at a pressure of 10 MPa.

[0111] For the electrical specific measurements, a 0.3 Ah soft-pack battery was used. The positive electrode current collector was carbon-coated aluminum foil. The positive electrode composition was a 88:7:5 mass ratio of substrate, SP, and PVDF. The electrode plate was compressed to 2.35 g / mL. The electrolyte was sodium hexafluorophosphate. The negative electrode current collector was aluminum foil. The negative electrode was hard carbon, obtained by pretreating asphalt and then calcining it at high temperature. The measurement voltage range was 2.0 to 4.0 V.

[0112] Free sodium was measured by adding 100 g of pure water to 10 g of the material, stirring at 25°C for 30 minutes, filtering, and measuring sodium ions in the filtrate using ICP.

[0113] Fluorine was measured using a fluoride ion selective electrode.

[0114] To measure iron elution, 100 mL of hydrochloric acid with a molar concentration of 0.05 mol / L is added to 10 g of material, and the material is immersed at 40°C for 30 minutes, then filtered and the iron content of the filtrate is measured.

[0115] The 0.1C charge-discharge curve is shown in Figure 1. From Figure 1, it can be seen that the carbon-coated sodium iron fluorophosphate prepared in this example has a clear discharge platform and small polarization.

[0116] Figure 2 shows an SEM image of the carbon-coated sodium iron fluorophosphate produced above.

[0117] The XRD image of the carbon-coated sodium iron fluorophosphate prepared above is shown in FIG.

[0118] Example 2 The method for producing carbon-coated sodium fluoroiron phosphate using waste lithium iron phosphate according to this example was the same as that of Example 1, except that in step (4), the carbon source was changed to polyethylene glycol and the mass of the polyethylene glycol was 0.3 times the mass of the roasted material.

[0119] Example 3 The method for producing carbon-coated sodium fluoroiron phosphate using waste lithium iron phosphate according to this example was the same as that of Example 1, except that in step (4), the carbon source was changed to starch and the mass of the starch was 0.2 times the mass of the roasted material.

[0120] Comparative Example 1 The method for producing carbon-coated sodium iron phosphate using waste lithium iron phosphate according to this comparative example was the same as in Example 1, except that sodium fluoride was not added in step (4). That is, the product produced in this comparative example was carbon-coated sodium iron phosphate.

[0121] Comparative Example 2 The method for producing sodium iron fluorophosphate using waste lithium iron phosphate according to this comparative example was the same as that of Example 1, except that in step (4), instead of adding a carbon source, a N2+H2 mixed gas was introduced during calcination for reduction. That is, the product produced in this comparative example is sodium iron fluorophosphate with a small carbon coating layer on the surface. The electrical properties of the carbon-coated sodium iron phosphate produced in Comparative Example 1 and the fluorosodium iron phosphate produced in Comparative Example 2 were measured (the detection method was the same as in Example 1), and the results are shown in Table 7.

[0122] [Table 7]

[0123] As a result of comparing the test results of the electrical properties of Example 1 and Comparative Examples 1 and 2, it was found that Example 1 was superior in capacity, initial discharge efficiency, and cycle characteristics.

[0124] Specifically, compared to Comparative Example 1, Example 1 introduces fluorine ions, thereby increasing the capacity. Furthermore, the introduction of fluorine effectively forms doping, forming a eutectic effect, improving ionic conductivity and increasing the capacity. Sodium iron fluorophosphate has a higher sodium content than sodium iron phosphate, and therefore also increases the theoretical capacity.

[0125] Compared with Comparative Example 2, the polyanionic sodium battery material itself has poor electrical conductivity. Therefore, the appropriate carbon coating in Example 1 is effective in improving the material's electronic conductivity. In addition, the appropriate carbon coating can prevent particle growth, make the primary particle size of the particles more uniform, effectively avoid particle growth, and improve the capacity and cycle characteristics.

[0126] As described above, the carbon-coated sodium iron fluorophosphate produced by the method for producing carbon-coated sodium iron fluorophosphate using waste lithium iron phosphate according to the present invention has excellent electrochemical properties and is useful as a positive electrode material for sodium batteries.

[0127] Although the present invention has been illustrated and described through specific embodiments, it should be recognized that the above embodiments are only used to explain the technical means of the present invention and are not limited thereto. Those skilled in the art can modify the technical means described in the above embodiments or replace some or all of the technical features with equivalents without departing from the spirit and scope of the present invention, and it should be understood that these modifications or replacements do not cause the essence of the corresponding technical means to depart from the scope of the technical means of the embodiments of the present invention, and this means that the appended claims include all these replacements and modifications that belong to the scope of the present invention.

Claims

1. A method for producing carbon-coated sodium fluoroiron phosphate using waste lithium iron phosphate, comprising: Step (a) of mixing and reacting the waste lithium iron phosphate material with an alkaline solution, followed by solid-liquid separation to obtain an aluminum-containing filtrate and a lithium iron phosphate filtrate; (b) uniformly mixing the lithium iron phosphate filter residue, aluminum chloride, and sodium chloride, and then vacuum roasting the mixture to obtain a roasted material; and (c) uniformly mixing the roasted material with at least one of a sodium source, an iron source, and a phosphorus source to obtain a mixed material, to which a fluorine source, a carbon source, and a solvent are added, followed by uniform mixing, followed by drying and calcination to obtain carbon-coated sodium iron fluorophosphate.

2. In step (a), the mass ratio of the waste lithium iron phosphate material to the alkaline solution is 1:5-15; 2. The method for producing carbon-coated sodium fluoroiron phosphate using waste lithium iron phosphate according to claim 1, wherein in step (a), the aluminum-containing filtrate is repeatedly used until the molar concentration of hydroxide ions therein becomes less than 0.2 mol / L, thereby obtaining an aluminum-containing waste liquid.

3. In step (a), the alkaline liquid comprises a sodium hydroxide solution; 2. The method for producing carbon-coated sodium fluoroiron phosphate using waste lithium iron phosphate according to claim 1, wherein the molar concentration of the alkaline solution is 3 to 5 mol / L.

4. Step (a) further comprises recovering and treating the aluminum-containing waste liquid to obtain aluminum chloride and sodium carbonate; 3. The method for producing carbon-coated sodium fluoroiron phosphate using waste lithium iron phosphate according to claim 2, wherein the recovery treatment method includes: introducing carbon dioxide into the aluminum-containing waste liquid to lower the pH to 9 to 11, followed by solid-liquid separation to obtain an aluminum hydroxide precipitate and a sodium carbonate filtrate; mixing the aluminum hydroxide precipitate with hydrochloric acid to cause a reaction, followed by concentration and crystallization to obtain aluminum chloride; and concentrating the sodium carbonate filtrate to crystallize to obtain sodium carbonate.

5. 2. The method for producing carbon-coated sodium fluoroiron phosphate using waste lithium iron phosphate according to claim 1, wherein in step (b), a molar ratio of lithium element in the lithium iron phosphate filtration residue, the aluminum chloride, and the sodium chloride is 1:1.2-1.5:1.02-1.

05.

6. In step (b), the temperature of the vacuum roasting is 400 to 600°C, and the time of the vacuum roasting is 4 to 6 hours; 2. The method for producing carbon-coated sodium fluoroiron phosphate using waste lithium iron phosphate according to claim 1, wherein the vacuum roasting is carried out at a vacuum degree of −0.04 to −0.08 MPa.

7. In step (b), exhaust gas from the vacuum roasting is collected and mixed with aqueous ammonia to carry out an aluminum precipitation reaction, followed by solid-liquid separation; the solid material obtained by the solid-liquid separation is then sintered to obtain alumina; the liquid material obtained by the solid-liquid separation is mixed with sodium carbonate to carry out a lithium precipitation reaction, followed by solid-liquid separation to obtain lithium carbonate; the molar concentration of the ammonia water is 0.01 to 0.1 mol / L; In the aluminum precipitation reaction, the temperature of the mixed material is 40-80°C, and the time of the aluminum precipitation reaction is 1-3 hours; 2. The method for producing carbon-coated sodium fluoroiron phosphate using waste lithium iron phosphate according to claim 1, wherein the temperature of the mixed material in the lithium precipitation reaction is 60 to 90°C, and the lithium precipitation reaction time is 1 to 3 hours.

8. 2. The method for producing carbon-coated sodium fluoroiron phosphate using waste lithium iron phosphate according to claim 1, wherein in step (c), before mixing the roasted material with at least one of a sodium source, an iron source, and a phosphorus source, contents of Na, Fe, and P in the roasted material are detected, and the roasted material and the phosphorus source are mixed so that a molar ratio of Na, Fe, and P in the mixed material is 0.95 to 0.98:1:1.02 to 1.

05.

9. The sodium source comprises at least one of sodium carbonate, sodium bicarbonate, and sodium acetate; the iron source includes at least one of red iron, iron (II) oxalate, and iron acetate; the phosphorus source comprises at least one of phosphoric acid, ammonium monohydrogen phosphate, and ammonium dihydrogen phosphate; the fluorine source comprises sodium fluoride; the carbon source comprises at least one of glucose, sucrose, polyethylene glycol, and starch; 2. The method for producing carbon-coated sodium fluoroiron phosphate using waste lithium iron phosphate according to claim 1, wherein the solvent contains water and / or an organic solvent.

10. The molar ratio of the fluorine source to the Fe element in the mixed material is 0.95 to 0.98:1; 2. The method for producing carbon-coated sodium fluoroiron phosphate using waste lithium iron phosphate according to claim 1, wherein a mass ratio of the carbon source to the roasted material is 0.2 to 0.3:

1.

11. 2. The method for producing carbon-coated sodium fluoroiron phosphate using waste lithium iron phosphate according to claim 1, wherein in step (c), the fluorine source, the carbon source, and the solvent are added, then crushed and mixed uniformly, and crushed until the particle diameter of the solid particles in the mixed slurry is 200 to 400 nm.

12. In step (c), the drying includes spray drying, and the particle size of the dried material obtained by the spray drying is 10 to 30 μm; 2. The method for producing carbon-coated sodium fluoroiron phosphate using waste lithium iron phosphate according to claim 1, wherein in step (c), the calcination temperature is 550 to 650°C, and the calcination temperature-retention time is 4 to 6 hours.

13. Step (c) includes the steps of sequentially crushing, sieving, and removing iron after the calcination; The method for producing carbon-coated sodium fluoroiron phosphate using waste lithium iron phosphate according to claim 1, characterized in that the carbon-coated sodium fluoroiron phosphate that has been subjected to the pulverization and sieving has a D50 particle size of 0.5 to 2 μm.

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