Salt-free wastewater preparation method of sodium ion battery prussian blue type positive electrode material

Abstract

This invention discloses a method for preparing Prussian blue-based cathode materials without saline wastewater. The method employs a combined controlled crystallization and vacuum distillation process to synthesize Prussian blue-based materials. The entire preparation process is free of saline wastewater discharge, and the mother liquor generated from each batch can be recycled after filtration. Therefore, except for the first batch which requires the addition of sodium sulfate and a complexing agent to the base liquid, subsequent processes can directly utilize the same concentration of sodium sulfate and complexing agent in the mother liquor, avoiding fluctuations in salt concentration and saving raw material costs. The cathode materials synthesized using this method maintain consistent physicochemical properties such as particle size, tap density, and material composition between batches, as well as the electrochemical performance of the assembled batteries, indicating that the reused mother liquor as a base liquid does not affect the performance of the cathode material. This invention significantly simplifies the wastewater treatment facilities and process flow, eliminating the need for large-scale equipment such as MVR (Mechanical Vacuum Resiner) systems and effectively reducing material production costs.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of battery materials, and relates to a salt-containing wastewater-free preparation method of a Prussian blue type positive electrode material, in particular to a salt-containing wastewater-free coprecipitation preparation method of a Prussian blue type positive electrode material for a sodium ion battery. BACKGROUND

[0002] Compared with O3 or P2 type sodium ion layered transition metal oxide positive electrode materials, the Prussian blue type positive electrode material for the sodium ion battery has the advantages of simple synthesis process, high reversible capacity, controllable voltage platform, stable cycle performance, good rate performance, and low raw material cost. At present, the commonly used method for synthesizing the Prussian blue type material in the academic and industrial circles is a simple liquid phase deposition method. The patent document with the application number 2022103693251 proposes a control crystallization concentration technology to replace the conventional one-pot method for coprecipitation, so that the material particle size and tap density are controllable by prolonging the precipitation period, and the reaction kettle utilization rate and yield are improved. However, whether it is intermittent precipitation, continuous precipitation or one-pot precipitation, a large amount of salt-containing wastewater will inevitably be produced, so corresponding filtration and evaporation equipment are needed to recover metal salts in the wastewater, so that the wastewater discharge meets the corresponding national standard requirements, but the initial investment cost of the wastewater treatment equipment is huge, the treatment steps are complicated, and a large amount of auxiliary materials such as electric power and steam are consumed in the treatment process, thereby increasing the generation cost of the Prussian blue type material. At the same time, the highest content of salt in the wastewater is sodium sulfate and other sodium salts, which are difficult to reduce. The reason is that in order to increase the sodium content in the Prussian blue analog, thereby increasing the specific capacity of the positive electrode material and reducing the defects in the material (if the sodium content is reduced, in order to maintain the charge balance of the material, the iron cyanide Fe(CN)6 3-The sodium content in the Prussian blue precipitate can also be increased by adding a large amount of sodium sulfate to the reaction mother liquor and raw materials to create a liquid phase environment with a sodium content close to saturation. However, the high solubility of sodium sulfate in aqueous solution (19.5 g / 100 g H2O at 20°C and 42.7 g / H2O at 90°C) means that the mother liquor will contain a large amount of nearly saturated sodium sulfate, which needs to be treated. The discharge of sodium-containing salt wastewater can cause land salinization and damage the ecological environment, and therefore the sodium sulfate in the wastewater needs to be evaporated and recovered in the domestic market, especially in inland areas. The current industry generally uses a mechanical vapor recompression (MVR) device to recover and utilize the secondary steam to evaporate and crystallize the sodium salt wastewater, while achieving cyclic evaporation to reduce energy consumption. Referring to the cost of treating domestic cobalt carbonate or ternary precursor wastewater, for a 20,000-ton-per-year precursor project, the one-time investment in an MVR device is about 15-20 million yuan, and the operating cost per ton of additional steam, electricity, and other operating costs is about 300-700 yuan. In addition, the mother liquor in the Prussian blue production process also contains impurities such as excess Fe, Co, Ni, Mn, and cyanide-based Fe(CN)6 3- In addition to the above, small particle impurities with a particle size of less than 0.5 microns that pass through the filter cloth or microporous rod concentration equipment also need to be thoroughly recovered by multiple filtering devices such as safety filters, plate and frame filter presses, and microporous filters. Another possible impurity in the mother liquor is a complexing agent, and whether it contains a complexing agent and its type depend on the adopted precipitation process. The most common complexing agents are sodium citrate and ethylenediaminetetraacetic acid disodium, which are organic metal salts with high COD (chemical oxygen demand) and are difficult to biodegrade, and therefore need to be removed by chemical oxidation, membrane method, evaporation crystallization, etc. As can be seen from the above, the composition of the mother liquor in the Prussian blue precipitation is complex, and therefore a variety of evaporation, sedimentation, and filtration devices need to be matched to recover the impurities, so that the wastewater can meet the Inorganic Chemical Industry Pollutant Discharge Standard (Ni <0.5 mg / L, Co, Mn <1 mg / L, COD <50 mg / L, total cyanide <0.5 mg / L, GB31573-2015). Therefore, it is crucial to seek a simple and efficient mother liquor recovery process to reduce the production cost of Prussian blue-based positive electrode materials. SUMMARY

[0003] In order to improve the above technical problems, the present application provides a salt-free wastewater preparation method of a sodium-ion battery Prussian blue type positive electrode material. By using a combination of a coprecipitation reactor and a vacuum distillation device, solid-liquid concentration is carried out by the vacuum distillation device, thereby realizing long-period intermittent precipitation. Unlike traditional filter cloth filtration or micropore filtration for solid-liquid separation, the vacuum distillation only evaporates the water in the reactor, and the sodium salt, complexing agent, and excess metal ions dissolved in the aqueous solution do not enter the mother liquor system. Since the sodium salt and the complexing agent are not lost, the Na ion concentration and the complexing agent concentration in the precipitation reactor can remain relatively stable, so that the raw materials do not need to be added with sodium salt and complexing agent in the subsequent continuous feeding process. After each batch of precipitation is qualified, only pure water is discharged as waste liquid, and then the mother liquor and the precipitation slurry in the reactor are separated in the discharging process. The separated mother liquor can be directly used as the bottom liquid for the next batch of precipitation synthesis.

[0004] In order to achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0005] A salt-free wastewater preparation method of a Prussian blue type positive electrode material, comprising: carrying out intermittent precipitation of a reaction liquid composed of a bottom liquid, a raw material 1 and a raw material 2 in a reactor provided with a concentration device, to obtain the Prussian blue type positive electrode material.

[0006] According to an embodiment of the present application, the bottom liquid is a saturated or near-saturated sodium sulfate or sodium chloride solution, to provide a sodium-rich environment for the precipitation process. Preferably, the concentration of the bottom liquid is 200-450 g / L, and examples include 200 g / L, 300 g / L, 400 g / L and 450 g / L.

[0007] According to an embodiment of the present application, the temperature of the precipitation is room temperature to 100°C, and examples include 25°C, 40°C, 60°C, 80°C and 100°C.

[0008] According to an embodiment of the present application, the reaction liquid further contains a complexing agent. Preferably, the concentration of the complexing agent is 0.1-15 g / L, and examples include 0.1 g / L, 0.5 g / L, 1 g / L, 2 g / L, 5 g / L, 8 g / L, 10 g / L, 12 g / L and 15 g / L.

[0009] Preferably, the complexing agent can be provided by the raw material 1 or by salt-free wastewater.

[0010] According to an embodiment of the present application, the complexing agent is at least one of trisodium citrate, ammonia, sodium oxalate, sodium phosphate, sodium pyrophosphate, sodium acetate, ethylenediaminetetraacetic acid, disodium ethylenediaminetetraacetate, sodium manganese ethylenediaminetetraacetate, triethanol ethylenediamine, urea, sodium succinate, sodium tartrate, polyvinylpyrrolidone and polyacrylamide.

[0011] According to the embodiments of the present application, the raw material 1 is an aqueous solution of sodium ferrocyanide. Preferably, the concentration of the aqueous solution of sodium ferrocyanide is 0.1-2 M, exemplarily 0.1 M, 0.5 M, 1 M, 2 M.

[0012] According to the embodiments of the present application, sodium sulfate or sodium chloride is also optionally added to the raw material 1 to a saturated or near-saturated concentration.

[0013] According to the embodiments of the present application, the raw material 2 is an aqueous solution of at least one of manganese, iron, cobalt, nickel sulfate, nitrate, oxalate, acetate, chloride, etc. Preferably, the concentration of the aqueous solution of at least one of manganese, iron, cobalt, nickel sulfate, nitrate, oxalate, acetate, chloride, etc. is 0.1-3 M, exemplarily 0.1 M, 0.5 M, 1 M, 2 M, 3 M.

[0014] According to the embodiments of the present application, the concentration device used in the intermittent precipitation is a reduced pressure distillation device. For example, the reduced pressure distillation device can be a straight condenser, a coiled condenser, a spherical condenser, a rotary evaporator or a reduced pressure distillation column, etc. with a corresponding size vacuum pump.

[0015] According to the embodiments of the present application, the solid content of the slurry in the intermittent precipitation process is controlled at 0.2%-60%, preferably 3-20%.

[0016] According to the embodiments of the present application, the period of the intermittent precipitation is 1-720 h, preferably 4-60 h, exemplarily 1 h, 4 h, 10 h, 30 h, 80 h, 100 h, 200 h, 400 h, 500 h, 600 h, 720 h.

[0017] According to the embodiments of the present application, the raw material 1 and the raw material 2 are added to the reactor by means of quantitative dropping, and the adding device is a peristaltic pump, a metering pump, a screw pump or by means of a high tank gravity flow; and the flow control device is an electromagnetic flowmeter, a rotor flowmeter, a valve, etc.

[0018] According to the embodiments of the present application, the high limit and the low limit of the liquid level in the reactor are set by a liquid level meter or a liquid level switch. Preferably, the difference between the high limit and the low limit of the liquid level does not exceed 20% of the height of the reactor.

[0019] Preferably, when the slurry in the reactor reaches the high limit of the liquid level, the feeding of the raw material 1 and the raw material 2 is suspended, and the reduced pressure distillation device is started.

[0020] Preferably, the vacuum pump connected to the vacuum distillation apparatus draws the internal pressure of the reactor to -0.04 to -0.1 MPa and maintains a constant pressure. Because the pressure is lower than atmospheric pressure, the slurry inside the reactor boils at a temperature below 100°C. The water in the mother liquor evaporates and then recondenses back into water through the condenser of the vacuum distillation apparatus, entering the wastewater collection bottle, causing the liquid level inside the reactor to drop. When the lower limit of the liquid level is reached, the vacuum pump connected to the vacuum distillation apparatus is shut off, the vacuum distillation apparatus stops operating, nitrogen is introduced into the reactor to restore atmospheric pressure, and the feed is restarted. This process is repeated until the liquid level reaches the upper limit again.

[0021] According to an embodiment of the present invention, the wastewater distilled by the vacuum distillation apparatus is salt-free pure water (containing no salts, and the generated wastewater is all ultrapure salt water after distillation), while various salts dissolved in the reactor aqueous solution remain in the reactor.

[0022] According to an embodiment of the present invention, when the liquid level in the reactor reaches the upper limit for the first time, no further bottom liquid needs to be added to the raw material 1.

[0023] According to an embodiment of the present invention, the reactor is a reaction vessel. Preferably, the volume of the reaction vessel is 100 mL to 10 mL. 3 Preferably 1L to 1m 3 .

[0024] According to an embodiment of the present invention, the reactor is further provided with a stirring device, such as a stirring paddle or a magnetic rotor stirrer.

[0025] Preferably, the stirring paddle can be a single layer or a double layer.

[0026] Preferably, the impeller blades can be one of the following: paddle mixer, anchor mixer, frame mixer, propeller mixer, open turbine, disc turbine, etc.

[0027] Preferably, the stirring speed during the reaction process is 10 rpm to 1000 rpm, for example 10 rpm, 40 rpm, 100 rpm, 200 rpm, 500 rpm, and 1000 rpm.

[0028] According to an embodiment of the present invention, the intermittent precipitation process also requires the reactor, raw material 1, and raw material 2 to be protected with an inert atmosphere or a reducing atmosphere to prevent oxidation of divalent transition metal ions. Preferably, the inert atmosphere includes nitrogen or argon; the reducing atmosphere can be hydrogen, carbon monoxide, or a mixture of the above gases.

[0029] According to an embodiment of the present invention, during the intermittent sedimentation process, an online dissolved oxygen meter is used to monitor the oxygen content of the slurry in the reactor in real time, and the oxygen content is controlled between 0% and 40%.

[0030] According to an embodiment of the present invention, the feeding can be stopped once all indicators in the intermittent sedimentation process meet the requirements, and the sedimentation of that batch is completed.

[0031] According to an embodiment of the present invention, after the intermittent precipitation is completed, the slurry in the reactor needs to be separated into solid and liquid. The separated mother liquor can be returned to the reactor and used directly as the base liquid for the next batch of precipitation reaction. After heating and passing nitrogen gas to the required temperature, the next batch of production can begin. The separated filter cake is washed, dried and sieved to obtain Prussian blue cathode material.

[0032] Preferably, the drying is carried out under vacuum or a protective atmosphere; further, the drying temperature is 100-200°C, exemplarily 140°C; the drying time is 6-48 hours, exemplarily 24 hours.

[0033] The present invention also provides a Prussian blue-based cathode material prepared by the above preparation method.

[0034] According to an embodiment of the present invention, the chemical formula of the Prussian blue-based cathode material is (Na). x (M) y (Fe(CN)6) z ·mH2O, where: M represents one or more of Mn, Fe, Co, and Ni, 1≤x≤2, 0.8≤y≤1, 0.8≤z≤1, and 0≤m≤2.

[0035] The present invention also provides the application of the above-mentioned Prussian blue cathode material as a cathode material for sodium-ion batteries.

[0036] The present invention also provides a sodium-ion battery, wherein the positive electrode of the sodium-ion battery comprises the above-mentioned Prussian blue-type positive electrode material.

[0037] The beneficial effects of this invention:

[0038] (1) This invention synthesizes Prussian blue materials by using a combination of controlled crystallization precipitation and vacuum distillation. The mother liquor is concentrated using vacuum distillation, which extends the precipitation cycle and allows for precise control of the particle size and tap density of the Prussian blue materials. This invention allows for in-situ concentration of the slurry inside the reactor, eliminating the need for conventional intermittent precipitation systems such as transfer tanks, overflow tanks, and concentration tanks, and also eliminating the need to place filtration equipment inside the reactor body. This simplifies the precipitation process. Furthermore, since the slurry is entirely concentrated in the reactor, the solid content of the precipitation process is increased, which helps improve the morphology of the precipitated particles. Simultaneously, the wastewater generated throughout the process is deionized and contains no other salts.

[0039] (2) This invention uses vacuum distillation for concentration, and the waste liquid contains only pure water. The precipitate in the reaction vessel and the sodium salt, complexing agent, metal ions, etc. dissolved in the aqueous solution are all retained in the reaction system and will not be lost with the mother liquor. Moreover, the sodium ion content and complexing agent concentration in the reaction system can remain stable during the precipitation process of this invention, and there is no need to continuously replenish the raw materials in the later stage, thereby reducing the large consumption of raw materials, especially sodium salt.

[0040] (3) The process of the present invention simplifies the mother liquor treatment process during precipitation, eliminating the need for MVR or multi-effect evaporation devices (for recovering sodium salts, etc.), plate and frame filter presses, microporous filters, security filters, etc. (for metal ion precipitation and filtration separation) required in traditional mother liquor treatment processes. This greatly simplifies the waste liquid treatment process and equipment investment. At the same time, the heat generated by water bath or oil bath heating during the high-temperature precipitation process is used for distillation, reducing operating costs such as electricity consumption, and thus reducing material processing costs.

[0041] (4) This invention provides a precipitation synthesis process for Prussian blue cathode materials without saline wastewater. The concentration is carried out by a matching vacuum distillation device. Only pure water is discharged as waste liquid during the precipitation synthesis process, while other metal salts remain in the reactor. Furthermore, the residual mother liquor in each batch of reactor can be recycled as the bottom liquid for the next batch of precipitation synthesis. In addition, since the sodium salt and complexing agent in the bottom liquid of the reactor are not discharged with the wastewater, it is not necessary to continuously add a large amount of sodium sulfate and complexing agent in the subsequent batch precipitation process, thereby greatly reducing the cost and steps of mother liquor treatment. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of the synthesis apparatus for Example 1.

[0043] Figure 2 This is a photograph of the saline-free wastewater collected during the sedimentation process in Example 1.

[0044] Figure 3 The image shows an electron microscope image of the Prussian blue derivative product prepared in Example 1.

[0045] Figure 4 The graph shows the 2C cycle performance of sodium-ion half-cells assembled from the Prussian blue cathode materials prepared in Examples 1, 2, and 3.

[0046] Figure 5 The image shows an electron microscope image of the Prussian blue derivative product prepared in Example 2.

[0047] Figure 6 The image shows an electron microscope image of the Prussian blue derivative product prepared in Example 3. Detailed Implementation

[0048] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are merely illustrative and explanatory of the present invention, and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0049] Unless otherwise stated, the raw materials and reagents used in the following examples are commercially available products or can be prepared by known methods.

[0050] Example 1

[0051] A method for preparing Prussian blue-based cathode materials for sodium-ion batteries without saline wastewater includes the following steps:

[0052] (1) Prepare a 1M sodium ferrocyanide (Na₄Fe(CN)₆) solution and add 0.05M trisodium citrate complexing agent as raw material 1. Prepare a 2M MnSO₄ solution as raw material 2. Add saturated Na₂SO₄ solution as a bottom liquid to a 1L reactor, with the bottom liquid volume being 40% of the full reactor volume. Use an oil bath for heating, with a precipitation temperature of 80℃. After purging with nitrogen for 2 hours, start feeding raw material 1 and raw material 2 separately. The hourly feed rates for raw material 1 and raw material 2 are 1 / 60 and 1 / 30 of the reactor volume, respectively. The stirring speed is 40 rpm, and the pH value of the reactor is controlled between 5 and 6. The synthesis apparatus is as follows: Figure 1As shown, when the liquid level in the reactor reaches the upper limit (the reaction liquid volume is 80% of the full reactor volume), feeding is stopped, the vacuum pump connected to the reduced pressure distillation unit is turned on, and the vacuum pump pressure is adjusted (fluctuating within the range of -0.06 to 0.07 MPa) until the liquid level is stable without boiling over, and slurry concentration is carried out (the water in the mother liquor in the reactor is cooled through the condenser and enters the mother liquor collection bottle, while the dissolved salts in the slurry and mother liquor remain in the reactor). When the liquid level in the reactor continuously drops and reaches the lower limit (the reaction liquid volume is 60% of the full reactor volume), the vacuum pump stops working, the concentration ends, and raw material 1 and raw material 2 are fed again to restart the reaction. This process is repeated until the sedimentation time reaches 30 hours. The particle size of the Prussian blue material is tested to be D50 = 13 μm, and the tap density is 0.72 g / cm³. 3 (If the requirements are met), the reaction stops, and the saline-free wastewater generated during the reaction is collected in a round-bottom flask, such as... Figure 2 As shown, it is transparent and colorless. After aging in the reactor for 12 hours, the mother liquor was separated by vacuum filtration and stored separately under nitrogen for use in the next batch synthesis. The filter cake was washed with 80°C ultrapure water, and then vacuum dried in a vacuum oven at 140°C for 48 hours. After grinding, the Prussian blue derivative product was obtained. The chemical formula of the Prussian blue derivative is Na. 1.8 Mn((Fe(CN)6).

[0053] The SEM image of the Prussian blue derivative product prepared in this embodiment is shown below. Figure 3 As shown in the figure, the morphology of the Prussian blue derivative is an aggregate of cubic blocks.

[0054] The Prussian blue derivative prepared in this embodiment was weighed according to the mass ratio of Prussian blue derivative: carbon black (Super-P): PVDF = 85:10:5, and NMP solvent was added to prepare a slurry. The slurry was prepared according to a concentration of 2 mg / cm³. 2 The amount of material was evenly coated onto carbon-coated aluminum foil, and after being rolled and vacuum dried at 140℃ for 12 hours, it was cut into electrode sheets with a diameter of 10 mm. The prepared electrode sheets were assembled into 2016 button half-cells with GF / D separator (purchased from Duoduo Reagent), sodium sheet (self-made) counter electrode, 1M NaPF6 electrolyte, and EC / DEC + 5% FEC in a volume ratio of 1:1.

[0055] The half-cell assembled in this embodiment was charged and discharged at 2C (300mA / g) within the charge-discharge range of 2–4.2V. The cycle capacity was tested, and the results are as follows. Figure 4As shown in the figure, the half-cell assembled from the Prussian blue derivative cathode material synthesized in this embodiment has a first-cycle discharge capacity of 129.1 mAh / g and a capacity of 105.4 mAh / g after 150 cycles, with a capacity retention rate of 80.3%. This indicates that the Prussian blue derivative cathode material prepared in this invention can maintain good cycling performance at high rates.

[0056] Example 2

[0057] A method for preparing Prussian blue-based cathode materials for sodium-ion batteries without saline wastewater includes the following steps:

[0058] (1) Prepare raw material 1 (but without adding the complexing agent trisodium citrate, as the mother liquor base liquid of this batch already contains the complexing agent used in the previous batch) and raw material 2 with the same concentration as in Example 1. Add all the mother liquor preserved after filtration of the finished product from Example 1 to a 1L reactor as the base liquid. Heat the mother liquor to 80℃ and purge with nitrogen for 2 hours. Then, feed raw material 1 and raw material 2 separately. The precipitation process is the same as in Example 1, except that the reactor in this example is started at full capacity. The hourly feed rates of raw material 1 and raw material 2 are 1 / 60 and 1 / 30 of the reactor volume, respectively. The stirring speed is 40 rpm, and the pH value of the reactor is controlled between 5 and 6. Concentrate using a vacuum distillation apparatus. After 30 hours of precipitation, the particle size of the Prussian blue material is tested to be D50 = 12.5 μm, and the tap density is 0.70 g / cm³. 3 (If the requirements are met), the reaction is stopped. After aging for 12 hours, the mother liquor is separated by vacuum filtration and stored separately under nitrogen gas for use in the next batch synthesis. The washing and drying process is the same as in Example 1.

[0059] The SEM image of the Prussian blue derivative product prepared in this embodiment is shown below. Figure 5 As shown in the figure, the particle morphology and size of the Prussian blue derivative product prepared in this embodiment are similar to those in Example 1.

[0060] The chemical formula of the Prussian blue derivative prepared in this embodiment is Na. 1.82 Mn((Fe(CN)6), the battery electrode and assembly process are the same as in Example 1.

[0061] The half-cell assembled in this embodiment was charged and discharged at 2C (300mA / g) to test its cycle capacity. The results are as follows: Figure 4 As shown in the figure, the half-cell assembled from the Prussian blue cathode material synthesized in this embodiment has a first-cycle discharge capacity of 129.8 mAh / g and a capacity of 103.4 mAh / g after 150 cycles, with a capacity retention rate of 79.7%. This indicates that using the mother liquor from Example 1 as the starting material for the next cycle, the battery cycle performance is close to that of Example 1.

[0062] Example 3

[0063] A method for preparing Prussian blue-based cathode materials for sodium-ion batteries without saline wastewater includes the following steps:

[0064] (1) Prepare raw materials 1 and 2 with the same concentration as in Example 2. Add the mother liquor (preserved after filtration of the finished product from Example 2) as a base liquid to a 1L reactor. Heat the mother liquor to 80℃ and purge with nitrogen for 2 hours. Then, feed raw materials 1 and 2 separately. The precipitation process is the same as in Examples 1 and 2. The hourly feed rates of raw materials 1 and 2 are 1 / 60 and 1 / 30 of the reactor volume, respectively. The stirring speed is 40 rpm, and the pH of the reactor is controlled between 5 and 6. Concentrate using a vacuum distillation apparatus. After 30 hours of precipitation, the particle size of the Prussian blue material is measured to be D50 = 12.8 μm, and the tap density is 0.72 g / cm³. 3 Once the requirements are met, the reaction stops. After aging for 12 hours, the material in the reactor is separated by vacuum filtration and stored separately under nitrogen for use in the next batch synthesis. The washing and drying process is the same as in Examples 1 and 2.

[0065] The SEM image of the Prussian blue derivative product prepared in this embodiment is shown below. Figure 6 As shown in the figure, the particle morphology and size of the Prussian blue derivative product prepared in this embodiment are similar to those in Examples 1 and 2.

[0066] The chemical formula of the Prussian blue derivative prepared in this embodiment is Na. 1.81 Mn((Fe(CN)6), the battery electrode and assembly process are the same as in Examples 1 and 2.

[0067] The half-cell assembled in this embodiment was charged and discharged at 2C (300mA / g) to test its cycle capacity. The results are as follows: Figure 4 As shown in the figure, the half-cell assembled from the Prussian blue cathode material synthesized in this embodiment has a first-cycle discharge capacity of 129.8 mAh / g and a capacity of 104.9 mAh / g after 150 cycles, with a capacity retention rate of 80.9%. This indicates that the mother liquor in Example 2 can be used for the next batch of precipitation synthesis, and further shows that the mother liquor produced in Example 1 can be continuously recycled. The physicochemical and electrochemical properties of the cathode material synthesized in this way are comparable to those of Example 1. Moreover, only salt-free pure water is discharged during the entire precipitation process. The conductivity of this wastewater is tested to be 5.6 μS / cm, which is close to the conductivity of the salt-free water added in Example 1, indicating that no other salts are added to the wastewater.

[0068] The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiments. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing saline-free wastewater for Prussian blue-based cathode materials, characterized in that, The method involves intermittent precipitation of a reaction solution consisting of a base liquid, raw material 1, and raw material 2 in a reactor equipped with a concentration device to prepare Prussian blue-based cathode materials. The concentration device used in the intermittent precipitation is a vacuum distillation device; The solid content of the slurry during the intermittent sedimentation process is controlled at 3-20%. The intermittent precipitation cycle is 4~60h, and the vacuum pump connected to the vacuum distillation device draws the internal pressure of the reactor to -0.04~-0.1MPa and maintains a constant pressure. After the intermittent precipitation is completed, the slurry in the reactor needs to be separated into solid and liquid components. The separated mother liquor can be returned to the reactor and used directly as the base liquid for the next batch of precipitation reaction. After heating and purging with nitrogen to the required temperature, the next batch of production can begin. The separated filter cake is washed, dried, and sieved to obtain Prussian blue cathode material. The base liquid is a saturated or nearly saturated sodium sulfate or sodium chloride solution with a concentration of 200~450 g / L. The reaction solution also contains a complexing agent with a concentration of 0.1~15 g / L. The complexing agent is trisodium citrate; The raw material 1 is an aqueous solution of sodium ferrocyanide, and the concentration of the aqueous solution of sodium ferrocyanide is 0.1~2M; The raw material 2 is an aqueous solution of at least one of the sulfates, nitrates, oxalates, acetates, and chlorides of manganese, iron, cobalt, and nickel, and the concentration of the aqueous solution of at least one of the sulfates, nitrates, oxalates, acetates, and chlorides of manganese, iron, cobalt, and nickel is 0.1~3M.

2. The preparation method according to claim 1, characterized in that, The reactor is also equipped with a stirring device.

3. The preparation method according to claim 2, characterized in that, The stirring device is a stirring paddle or a magnetic rotor.

4. The preparation method according to claim 3, characterized in that, The stirring paddle can be single-layered or double-layered.

5. The preparation method according to any one of claims 1-4, characterized in that, The stirring speed during the reaction process is 10 rpm to 1000 rpm.

6. The Prussian blue cathode material prepared by the preparation method according to any one of claims 1-5.

7. The Prussian blue-based cathode material as described in claim 6, characterized in that, The chemical formula of the Prussian blue-based cathode material is (Na). x (M) y (Fe(CN)6) z ·mH2O, where: M represents one or more of Mn, Fe, Co, and Ni, 1≤x≤2, 0.8≤y≤1, 0.8≤z≤1, and 0≤m≤2.

8. The application of the Prussian blue cathode material as described in claim 6 or 7 as a cathode material for sodium-ion batteries.

9. A sodium-ion battery, characterized in that, The positive electrode of the sodium-ion battery comprises the Prussian blue-type positive electrode material as described in claim 6 or 7.

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