A method for preparing carbon-coated lithium iron phosphate positive electrode material by recycling struvite waste liquid in porous Si production process

Abstract

The application belongs to the technical field of wastewater treatment and relates to a method for preparing carbon-coated lithium iron phosphate positive electrode material by recycling struvite waste liquid in the production process of porous Si, which comprises the following steps: step 1, preparing a nitridation reaction product one; step 2, preparing an ant nest-like porous silicon recovery filtrate one; step 3, preparing struvite filter cake and struvite filtrate; step 4, preparing a precursor; step 5, preparing a powder sample; and step 6, preparing carbon-coated lithium iron phosphate positive electrode material. The application adopts a coprecipitation method to prepare ant nest-like porous silicon with high purity and struvite with high crystallinity. The carbon-coated lithium iron phosphate positive electrode material is generated by recycling struvite waste liquid by using a carbothermal reduction method, and has commercial value. The waste liquid is fully recycled and reused, which effectively reduces the consumption of natural resources and waste emissions, reduces the negative impact on the environment, improves economic benefits, realizes the sustainable development of resources, and has low raw material prices, simple synthesis, short process flow, environmental friendliness and can be mass-produced.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment technology, and in particular relates to a method for preparing carbon-coated lithium iron phosphate cathode material by recovering struvite waste liquid during porous Si production. Background Technology

[0002] With rapid industrial development, the types and quantities of waste liquids have increased dramatically, leading to increasingly widespread and severe water pollution that threatens human health and safety. The generation of waste liquids is unavoidable in industrial production, and how to reduce environmental pollution and treatment costs through management and treatment is a problem we need to continuously explore. Classifying and treating industrial wastewater, recovering useful substances, and recycling it are of great significance for reducing toxic and harmful substances entering the ecological environment, lowering water costs for production, and promoting the achievement of carbon neutrality goals.

[0003] In recent years, LFP (lithium iron phosphate, LiFePO4) cathode materials have been widely studied due to their simple production process and good safety. Wu et al. synthesized well-crystallized (NH4)[Fe2(OH)(PO4)2]·2H2O particles using Fe(NO3)3·9H2O, (NH4)2HPO4, and NH4H2PO4 as raw materials via ultrasonic chemistry, and used these particles as precursors to prepare the lithium-ion battery cathode material LiFePO4 / C. Liu Xiaoyu et al. used phosphoric acid waste liquid with FeSO4 and H2O2, heated and stirred to obtain FeSO4·2H2O precipitate, added a lithium source (LiOH or Li2CO3) and a carbon source (sucrose, glucose, or citric acid), ball-milled it, and calcined it to obtain lithium iron phosphate cathode material. Therefore, a reasonable experimental design can recover and reuse waste liquid to generate the desired LFP / C material. How to utilize struvite waste liquid to recover and generate LiFePO4 / C cathode material remains to be studied. Summary of the Invention

[0004] This invention addresses the aforementioned prior art by providing a method for preparing carbon-coated lithium iron phosphate cathode materials by recovering struvite wastewater during porous Si production. The specific technical solution is as follows:

[0005] A method for preparing carbon-coated lithium iron phosphate cathode material by recovering struvite waste liquid during porous Si production includes the following steps:

[0006] Step 1: Grind magnesium silicide evenly and place it in a reaction vessel. Then place the reaction vessel in an inert atmosphere tube furnace, heat it to 750-780℃, start ammonia gas and keep it at that temperature for 5-7 hours for nitriding. After the holding time is over, stop the ammonia gas and remove the furnace as it cools down to obtain reaction product one. Keep the inert gas flowing during the reaction process.

[0007] Step 2: Grind the reaction product obtained in Step 1 into powder, add it to phosphoric acid solution and wash it with acid for 3-5 hours, filter and separate the first filtrate for recovery, then filter until neutral, and freeze dry under vacuum to obtain ant nest-like porous silica.

[0008] Step 3: Add ammonia to the first filtrate recovered in Step 2 to adjust the pH value to 9.5-11, then stir for more than 2 hours, let stand for 9-11 hours and filter to obtain struvite filter cake and struvite filtrate.

[0009] Step 4: After adjusting the pH of the struvite waste liquid to a slightly acidic level, add FeCl3, heat and stir in a water bath at 70-90℃ for 4-8 hours, and obtain the precursor after centrifugation;

[0010] Step 5: Mix the precursor, lithium carbonate, and glucose and add them to a ball mill jar containing ethanol. Ball mill for 5-8 hours, then filter and freeze dry for 10-14 hours to obtain a powder sample.

[0011] Step 6: Seal the powder sample in a reactor and place it in an inert atmosphere tube furnace. Under the protection of inert gas, keep it at 400-500℃ for 2-5 hours; then keep it at 600-700℃ for 6-8 hours. After natural cooling, carbon-coated lithium iron phosphate cathode material is obtained.

[0012] The ant-nest porous silicon successfully separated by the co-precipitation method of this invention can be used as a commercial anode material and can also yield highly crystalline struvite, which has commercial value. The waste liquid of the ant-nest porous silicon of this invention does not contain Ca. 2 +,Zn 2+ This process, without interfering with the crystallization of struvite, results in more stable struvite. The present invention further involves adjusting the pH of the struvite waste liquid to a weakly acidic state, then adding anhydrous ferric chloride to generate a precursor (NH4)[Fe2(OH)(PO4)2]·2H2O. The precursor is then mixed with glucose and lithium carbonate in ethanol solvent, ball-milled until homogeneous, and heat-treated to obtain a highly crystalline LFP / C cathode material with commercial value.

[0013] Furthermore, in steps 1 and 6, the inert gas is argon or nitrogen.

[0014] Furthermore, in steps 1 and 6, the heating rate of the inert atmosphere tube furnace is 2-20℃ / min, preferably 5℃ / min.

[0015] The inert atmosphere tube furnace heats up at a rate of 5℃ / min to prevent damage to the furnace resistance wire due to excessive heating and to precisely control the temperature of the inert atmosphere tube furnace.

[0016] Furthermore, in step 2, the molar ratio of P to Mg is greater than or equal to 2:1, preferably 2:1.

[0017] In step 2, if the amount of phosphoric acid added is insufficient, other impurities will be generated in the product, and silicon cannot be successfully separated.

[0018] Furthermore, in step 3, the pH value is 9.5-11, preferably 10.

[0019] In step 3, a pH value that is too high or too low is not conducive to the formation of struvite and may also lead to the formation of byproducts.

[0020] Furthermore, in step 4, H3PO4 is added to the struvite waste liquid to adjust the pH value to 5.5-6.5, preferably pH=6.

[0021] Further, in step 5, the molar ratio of the precursor, lithium carbonate and glucose is 1:(0.5-2):(0.05-0.5).

[0022] The glucose acts as a reducing agent to prevent the oxidation of ferrous iron. At the same time, it can form a carbon film on the surface of lithium iron phosphate particles, which enhances conductivity and prevents particles from agglomerating and growing.

[0023] Furthermore, in step 5, the mass ratio of the material balls in the ball mill jar is 1:10, and the rotation speed is 400-600 r / min.

[0024] The beneficial effects of this invention are as follows:

[0025] The ant-nest porous silicon successfully separated by the co-precipitation method of this invention can be used as a commercial anode material and can also yield highly crystalline struvite, which has commercial value. The waste liquid of the ant-nest porous silicon of this invention does not contain Ca. 2 +,Zn 2+ This method does not interfere with the crystallization of struvite, resulting in more stable struvite. Furthermore, this invention successfully recycles struvite waste liquid using a carbothermal reduction method to generate carbon-coated lithium iron phosphate cathode material, which has commercial value. The carbothermal reduction method directly uses iron (III) compounds as iron precursors, which are relatively cheaper, more readily available, and chemically stable than iron (II) compounds.

[0026] This invention fully recycles and reuses waste liquid, effectively reducing the consumption of natural resources and the discharge of waste, minimizing negative environmental impacts, improving economic efficiency, and achieving sustainable resource development. The raw materials used in this invention are inexpensive, the synthesis is simple, the process is short, and it is environmentally friendly, making it suitable for mass production. Attached Figure Description

[0027] Figure 1 The image shown is a scanning electron microscope (SEM) image of the porous silicon described in Example 1 of this invention.

[0028] Figure 2 The image shows the XRD pattern of the ant-nest porous silicon obtained in Example 1 of this invention.

[0029] Figure 3 This is the XRD pattern of the struvite obtained in Example 1 of this invention;

[0030] Figure 4 The image shown is the XRD pattern of the precursor obtained in Embodiment 1 of the present invention.

[0031] Figure 5 The image shown is the XRD pattern of LFP / C obtained in Embodiment 1 of the present invention;

[0032] Figure 6 This is a scanning electron microscope image of the LFP / C obtained in Example 1 of the present invention;

[0033] Figure 7 The Raman plot of LFP / C obtained in Embodiment 1 of the present invention;

[0034] Figure 8 This is a graph showing the electrochemical performance of LFP / C obtained in Example 1 of this invention;

[0035] Figure 9 The image shown is the XRD pattern of product A obtained in Comparative Example 1 of this invention.

[0036] Figure 10 The image shows the XRD pattern of LFP / C obtained in Comparative Example 1 of this invention;

[0037] Figure 11 The image shown is the XRD pattern of product B obtained in Comparative Example 2 of this invention. Detailed Implementation

[0038] The principles and features of the present invention are described below with reference to examples. The examples are only used to explain the present invention and are not intended to limit the scope of the present invention.

[0039] Example 1:

[0040] A method for preparing carbon-coated lithium iron phosphate cathode material by recovering struvite waste liquid during porous Si production includes the following steps:

[0041] Step 1: Grind 3g of magnesium silicide evenly and place it in a reaction vessel. Then place the reaction vessel in an argon atmosphere tube furnace and heat it to 750-780℃ at a heating rate of 5℃ / min. Then start to introduce ammonia gas and keep it at this temperature for 6 hours for nitriding. During nitriding, NH3:Ar = 3:1, NH3: 90ml / min, Ar: 30ml / min. After the holding time is over, stop the ammonia gas supply and take it out with the furnace to cool it down. The reaction product 1 is obtained. Argon gas is kept flowing during the reaction.

[0042] Step 2: Grind the reaction product obtained in Step 1 into powder. Grinding is to prevent the formation of clumps of magnesium nitride after nitriding, making it easier to react completely with the phosphoric acid solution. Then, add it to the phosphoric acid solution and wash for 4 hours to dissolve the reaction product in the acid solution (Mg:P = 1:2). Filter and separate the first filtrate for recovery. Then filter until neutral to remove any residual impurities. Then transfer the obtained sample to a freeze dryer and freeze dry (cold trap temperature -40℃) for 24 hours to obtain ant nest porous silicon.

[0043] The results were observed using a NovaNano SEM 230 scanning electron microscope from FEI Corporation, USA. Figure 1 As shown, porous silicon with an ant-nest-like structure; the results obtained from analysis using a Rigaku D / max-2500 X-ray diffractometer are as follows. Figure 2 As shown, this indicates that silicon was successfully separated;

[0044] Step 3: Add 25% ammonia to the first filtrate recovered in Step 2 to adjust the pH to 10, then stir for more than 2 hours and let stand for 10 hours to promote struvite crystallization. Filter to obtain struvite filter cake and struvite filtrate. Rinse the struvite filter cake with deionized water to remove any residual impurities. Filter to recover the white solid and dry in a vacuum oven at 55°C for 24 hours to obtain struvite.

[0045] The XRD pattern of the obtained struvite is shown below. Figure 3 As shown, its diffraction peaks all correspond to Mg(NH4)PO4·6H2O, and no other impurity peaks were found, indicating that the material has high purity, and the strong peak intensity indicates that the material has good crystallinity.

[0046] Step 4: Take 150ml of struvite waste liquid, add an appropriate amount of H3PO4 to adjust the pH of the struvite waste liquid to 6, add 1g of FeCl3, heat and stir in an 80℃ water bath for 8h, the solution is light yellow; remove impurities by centrifugation with deionized water multiple times at a speed of 3000r / min, freeze-dry the precipitate overnight to obtain the precursor (NH4)[Fe2(OH)(PO4)2]·2H2O;

[0047] The precursor was analyzed using a Rigaku D / max-2500 X-ray diffractometer (Japan), and the results are as follows: Figure 4 As shown;

[0048] Step 5: Mix 8.02g of precursor, 1.59g of lithium carbonate and 0.43g of glucose into a ball mill jar containing 400ml of ethanol, ball mill at 500r / min for 6h, filter, freeze dry for 12h to obtain powder sample;

[0049] Step 6: The powder sample is sealed in a crucible in a glove box and heated to 450°C in a tube furnace under the protective gas Ar, at a heating rate of 5°C / min. The temperature is held for 5 hours, and then the temperature is increased to 650°C and held for 8 hours. After natural cooling, the LFP / C cathode material is obtained.

[0050] The LFP / C cathode material was analyzed using a Rigaku D / max-2500 X-ray diffraction analyzer (Japan), and the results are as follows: Figure 5 As shown, all diffraction peaks of the final product correspond to the LFP phase, and no other impurity peaks were found, indicating high material purity. The strong peak intensities indicate good crystallinity. Due to the amorphous structure and low content of carbon, no obvious diffraction peaks were observed.

[0051] The LFP / C cathode material was observed using a NovaNano SEM 230 scanning electron microscope from FEI Corporation, USA. Figure 6 As shown, LiFePO4 particles can be clearly seen; its Raman spectrum is as follows. Figure 7 As shown in the figure, 1324 and 1586cm -1 The two broad peaks at the specified location belong to the d-band (disordered carbon) and g-band (graphitic carbon), respectively, confirming the presence of the carbon layer; its electrochemical properties, such as Figure 8 As shown.

[0052] Comparative Example 1:

[0053] Steps 1-3: Same as in Example 1, and will not be repeated here;

[0054] Step 4: Take 150ml of struvite waste liquid, add an appropriate amount of H3PO4 to adjust the pH of the struvite waste liquid to 6, add 1g of FeCl3, heat and stir in an 80℃ water bath for 8h, the solution is light yellow; after only one centrifugation at 3000r / min, freeze dry the precipitate overnight to obtain product A;

[0055] Product A was analyzed using a Rigaku D / max-2500 X-ray diffractometer (Japan). The results are as follows: Figure 9 As shown, this indicates that NH4H2PO4 impurities remain in the obtained product A.

[0056] Step 5: Same as in Example 1, and will not be repeated here;

[0057] Step 6: Same as in Example 1, and will not be repeated here;

[0058] The sample was analyzed using a Rigaku D / max-2500 X-ray diffractometer (Japan), and the results are as follows: Figure 10As shown, the presence of impurity Li4P2O7 in LFP indicates that the impurity NH4H2PO4 in the precursor can affect the purity of LFP and may involve side reactions: 2Li2CO3 + 2NH4H2PO4 → Li4P2O7 + 2NH3↑ + 2CO2↑ + 3H2O↑.

[0059] Comparative Example 2:

[0060] Steps 1-3: Same as in Example 1, and will not be repeated here;

[0061] Step 4: Take 150ml of struvite waste liquid, add an appropriate amount of H3PO4 to adjust the pH of the struvite waste liquid to 6, add 1g of FeCl3, stir in a water bath at room temperature for 8 hours, the solution is dark yellow; remove impurities by centrifugation with deionized water multiple times at a speed of 3000r / min, freeze-dry the precipitate overnight to obtain product B;

[0062] Product B was analyzed using a Rigaku D / max-2500 X-ray diffractometer (Japan). The results are as follows: Figure 11 As shown, the product (NH4)Fe(HPO4)2 was obtained, indicating that a certain amount of energy (temperature) is required to promote the reaction in the process of obtaining the precursor.

[0063] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. 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 carbon-coated lithium iron phosphate cathode material by recovering struvite waste liquid during porous Si production, characterized in that, Includes the following steps: Step 1: Grind magnesium silicide evenly and place it in a reaction vessel. Then place the reaction vessel in an inert atmosphere tube furnace, heat it to 750-780℃, start ammonia gas and keep it at that temperature for 5-7 hours for nitriding. After the holding time is over, stop the ammonia gas and remove the furnace as it cools down to obtain reaction product one. Keep the inert gas flowing during the reaction process. Step 2: Grind the reaction product obtained in Step 1 into powder, add it to phosphoric acid solution and wash it with acid for 3-5 hours, filter and separate the first filtrate for recovery, then filter until neutral, and freeze dry under vacuum to obtain ant nest-like porous silica. Step 3: Add ammonia to the first filtrate recovered in Step 2 to adjust the pH value to 9.5-11, then stir for more than 2 hours, let stand for 9-11 hours and filter to obtain struvite filter cake and struvite filtrate. Step 4: Adjust the pH of the struvite waste liquid to 5.5-6.5, add FeCl3, heat and stir in a water bath at 70-90℃ for 4-8 hours, and obtain the precursor after centrifugation; Step 5: Mix the precursor, lithium carbonate, and glucose and add them to a ball mill jar containing ethanol. Ball mill for 5-8 hours, filter, and freeze dry for 10-14 hours to obtain a powder sample. The molar ratio of the precursor, lithium carbonate, and glucose is 1:(0.5-2):(0.05-0.5). Step 6: Seal the powder sample in a reactor and place it in an inert atmosphere tube furnace. Under the protection of inert gas, keep it at 400-500℃ for 2-5 hours; then keep it at 600-700℃ for 6-8 hours. After natural cooling, carbon-coated lithium iron phosphate cathode material is obtained.

2. The method for preparing carbon-coated lithium iron phosphate cathode material by recovering struvite wastewater during porous Si production according to claim 1, characterized in that, In steps 1 and 6, the inert gas is argon or nitrogen.

3. The method for preparing carbon-coated lithium iron phosphate cathode material by recovering struvite waste liquid during porous Si production according to claim 2, characterized in that, In steps 1 and 6, the heating rate of the inert atmosphere tube furnace is 2-20℃ / min.

4. The method for preparing carbon-coated lithium iron phosphate cathode material by recovering struvite wastewater during porous Si production according to claim 1, characterized in that, In step 2, the molar ratio of P:Mg is greater than or equal to 2:

1.

5. The method for preparing carbon-coated lithium iron phosphate cathode material by recovering struvite wastewater during porous Si production according to claim 1, characterized in that, In step 5, the mass ratio of the ball to the milling jar is 1:10.

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