Method for improving ferric sodium phosphate pyrophosphate by using multifunctional conductive carbon layer

Abstract

The invention discloses a method for improving sodium ferric phosphate pyrophosphate by using a multifunctional conductive carbon layer, which comprises the following steps: S1, sequentially adding a sodium source, a phosphorus source and an iron source into a solvent, carrying out ultrasonic dispersion, and fully stirring until the raw materials are completely dissolved to obtain a yellow solution A; s2, adding a carbon source into the yellow solution A, and continuously stirring until all the raw materials are completely dissolved to obtain a solution B; s3, carrying out sol-gel treatment on the solution B until the solution completely becomes gel, so as to obtain a precursor C; s4, performing vacuum drying treatment on the precursor C, and taking out the precursor C after the precursor C is dried to obtain a product D; and S5, carrying out high-temperature calcination on the product D to obtain the sodium ferric pyrophosphate positive electrode material. By introducing the mushroom dreg modified sodium ferric pyrophosphate positive electrode material, the conductivity of the material can be improved, the diffusion kinetics of sodium ions can be improved at the same time, and excellent electrochemical performance such as rate, cycle and charge and discharge can be realized.

Description

Technical Field

[0001] The invention belongs to the field of sodium ion batteries and relates to a method for preparing sodium ferric pyrophosphate, and particularly relates to a method for improving sodium ferric pyrophosphate materials with a multifunctional conductive carbon layer. Background Art

[0002] Sodium-ion batteries have become the best alternative to lithium-ion batteries due to their advantages such as rich storage, low cost, long cycle life, good structural stability and simple preparation process. In recent years, due to the price fluctuations of lithium salts, the research and development of new sodium-ion batteries has attracted much attention. At present, the mainstream sodium-ion batteries can be divided into the following three types: Prussian blue, layered oxides and polyanions. Polyanion-type sodium-ion battery sodium iron pyrophosphate (NFPP) has a stable voltage platform, good thermal stability, and small volume change during cycling. It is a very promising sodium-ion battery positive electrode material. Among them, the sodium iron pyrophosphate positive electrode material of the sodium iron pyrophosphate phase contains the characteristics of phosphate and pyrophosphate, and has a high working voltage (3.1V) and theoretical capacity (129mAh g -1 Furthermore, the open skeleton composed of pyrophosphate and phosphate groups minimizes its volume change, ensuring high structural and thermal stability. Furthermore, sodium ferric pyrophosphate has three-dimensional ion diffusion channels, thus reducing the energy barrier for sodium ion migration.

[0003] However, the synthesis of sodium ferric pyrophosphate is difficult, and problems such as multiple impurities and impure phases are more prominent. The impurities are mainly sodium pyrophosphate and sodium ferric phosphate, which may be caused by the synthesis temperature and the reactants used, potentially limiting the electrochemical performance of NFPP. The current synthesis methods of sodium ferric pyrophosphate include: solid phase method, sol-gel method, carbon thermal reduction method, spray drying method, etc. Among them, spray drying and combustion methods can be considered as practical technologies for obtaining high-quality products for large-scale production and commercialization. In addition, (PO4) in NFPP 3- The intrinsic electronic insulating properties of the groups may lead to reduced electronic conductivity and slow ion diffusion, which hinders their practical application in SIBs. Summary of the Invention

[0004] In order to solve the problems of low electronic conductivity and low lithium ion diffusion rate of sodium ferric pyrophosphate positive electrode materials, the present invention provides a method for improving sodium ferric pyrophosphate with a multifunctional conductive carbon layer. This method uses discarded microbial residue rich in nitrogen and fluorine as a biomass carbon source, and adopts a sol-gel method to coordinate high-temperature solid-phase sintering to synthesize the sodium ferric pyrophosphate positive electrode material. During the high-temperature sintering process, the residue will be converted into a carbon layer coated on the surface of the sodium ferric pyrophosphate positive electrode material, greatly improving the electrical conductivity and ion diffusion kinetics of the sodium ferric pyrophosphate positive electrode material. In addition, the sodium ferric pyrophosphate positive electrode material prepared by the present invention exhibits excellent long-cycle stability and excellent rate performance.

[0005] The purpose of the present invention is achieved through the following technical solutions:

[0006] A method for improving sodium ferric pyrophosphate with a multifunctional conductive carbon layer comprises the following steps:

[0007] Step S1: Add a sodium source, a phosphorus source, and an iron source to a solvent in sequence, disperse them ultrasonically, and stir them thoroughly until the raw materials are completely dissolved to obtain a yellow solution A, wherein:

[0008] The sodium source includes one of sodium fluoride (NaF), sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3), sodium hydroxide (NaOH), sodium phosphate (Na3PO4), sodium acetate (CHCOONa), sodium oxalate (Na2C2O4), sodium oxide (Na2O), sodium pyrophosphate (Na4P2O7), and disodium hydrogen phosphate (Na2HPO4);

[0009] The iron source includes one of ferrous oxalate dihydrate (FeC2O4·2H2O), ferric phosphate (FePO4), ferric nitrate (Fe(NO3)3), ferric acetate (Fe(CH3COO)3), ferric oxide (Fe2O3), and ferrosoferric oxide (Fe3O4);

[0010] The phosphorus source includes one of ammonium dihydrogen phosphate (NH4H2PO4), diammonium hydrogen phosphate ((NH4)2HPO4), phosphoric acid (H3PO4), and pyrophosphoric acid (H4P2O7);

[0011] The solvent is one of soluble solvents such as anhydrous ethanol, acetone, DMF (N, N-dimethylformamide);

[0012] The ultrasonic dispersion time is 5 to 10 hours and the power is 50 to 150W;

[0013] The stirring time is 2 to 10 hours;

[0014] Step S2: Add the carbon source to the yellow solution A and continue stirring until all the raw materials are completely dissolved to obtain solution B, wherein:

[0015] The molar ratio of the sodium source:phosphorus source:iron source:carbon source=4:3:4:0.1;

[0016] The carbon source is one of fungus residue, glucose, asphalt, and citric acid;

[0017] The bacterial residue is a discarded microbial bacterial residue, and the specific preparation method is as follows: first, the feces of animals such as cattle, horses, and sheep are fermented and treated, wherein: the fermentation needs to be carried out in an anaerobic environment, and the humidity is preferably 50-60%. Too high humidity is prone to anaerobic odor, and too low humidity inhibits microbial activity. The temperature needs to be controlled at 45-60°C and should not be too high, and the fermentation time is about 15-20 days; then, hydrofluoric acid and sodium hydroxide are used to acid-soak the fermentation product, wherein: during acid leaching, hydrofluoric acid with a mass fraction of 40wt% is used to fully soak for 6-10h and then filter, and then the filter residue is soaked in a sodium hydroxide solution with a solubility of 30% for 10-15h, stirred and filtered; finally, after drying and crushing, the discarded microbial bacterial residue is converted into a biomass carbon source, wherein: the drying time is 8-12h, and the temperature is 80-100°C;

[0018] The stirring speed is 50-80 rpm, the temperature is 75-85°C, and the time is 2-10 hours;

[0019] Step S3: subjecting solution B to a sol-gel process until the solution completely turns into a gel to obtain a precursor C, wherein:

[0020] The sol-gel treatment temperature is 80-90°C, the stirring time is 45-50 hours, the solution is completely dry, and the precursor is a spherical gel jelly with uniform color;

[0021] Step S4: vacuum-dry the precursor C, and take it out after drying to obtain a product D, wherein:

[0022] The vacuum drying time is 10 to 15 hours, the temperature is 80 to 100°C, and the heating rate is 5 to 10°C / min;

[0023] Step S5: calcining the product D at a high temperature to obtain a sodium iron pyrophosphate positive electrode material, wherein:

[0024] The high temperature calcination atmosphere is one of inert gases such as argon, nitrogen, and argon-hydrogen mixture;

[0025] The high-temperature calcination time is 10 to 12 hours, the sintering temperature is 500 to 550° C., and the heating rate is 3 to 5° C. / min.

[0026] The sodium ferric pyrophosphate obtained by the above method can be used as a positive electrode material for a sodium ion battery. The preparation method of the sodium ion battery is as follows: sodium ferric pyrophosphate, SP conductive agent, and PVDF binder are uniformly mixed in NMP (N-methylpyrrolidone) in a mass ratio of 94-96:3-2:3-2, and then coated on an aluminum foil current collector. After high-temperature drying, the mixture is rolled and cut into positive electrode sheets, which are then assembled with die-cut negative electrode sheets and separators for battery cells, shelled, injected with liquid, and sealed to form a sodium ion battery.

[0027] Compared with the prior art, the present invention has the following advantages:

[0028] 1. The present invention can improve the sodium ion diffusion kinetics while improving the conductivity of the material itself by introducing fungus residue-modified sodium iron pyrophosphate positive electrode material, thereby achieving excellent electrochemical properties such as rate, cycle, and charge and discharge.

[0029] 2. The present invention utilizes discarded microbial residue as a biomass carbon source, which has a wide range of sources and abundant reserves, low processing costs, and is environmentally friendly.

[0030] 3. The sodium salt, iron salt, and phosphate provided by the present invention are easily soluble in organic solvents such as anhydrous ethanol and acetone, and the samples obtained by high-temperature sintering have high phase purity, a simple preparation process, and are convenient for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 The XRD patterns of the sodium iron pyrophosphate materials of Example 1, Example 2 and Comparative Example 1 are shown;

[0032] Figure 2 This is a scanning electron microscope image of the sodium iron pyrophosphate material of Example 1;

[0033] Figure 3 This is a scanning electron microscope image of the sodium iron pyrophosphate material of Comparative Example 1;

[0034] Figure 4 This is the X-ray photoelectron spectrum of the sodium iron pyrophosphate material of Example 1;

[0035] Figure 5 The figure is a rate performance diagram of the sodium iron pyrophosphate material of Example 1, Example 2, Example 3 and Comparative Example 1;

[0036] Figure 6 These are the charge and discharge diagrams of the sodium iron pyrophosphate materials of Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0037] The technical solution of the present invention is further described below with reference to the accompanying drawings, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention that does not depart from the spirit and scope of the technical solution of the present invention should be included in the scope of protection of the present invention.

[0038] Example 1

[0039] This embodiment provides a method for preparing a modified sodium iron pyrophosphate positive electrode material, and the specific steps are as follows:

[0040] (1) 4.24 g of sodium carbonate (Na2CO3), 3.45 g of ammonium dihydrogen phosphate (NH4H2PO4), and 7.20 g of ferrous oxalate dihydrate (FeC2O4·2H2O) were weighed and dissolved in 50 mL of anhydrous ethanol. The mixture was ultrasonically treated at 25°C for 8 h at a power of 50 W to obtain solution A. Subsequently, 2 g of fungus residue was introduced into solution A and stirred until it was completely dissolved to obtain mixed solution B. The temperature was then raised to 85°C and the above solution was subjected to a sol-gel treatment until the solution was completely evaporated and transformed into a jelly-like gel to obtain product C. The obtained product C was placed in a vacuum drying oven and dried at 85°C for 12 h to obtain product D. The preparation method of the microbial residue is as follows: first, fermenting cattle feces and other treatments, wherein: the fermentation is carried out in an anaerobic environment, the humidity is 50-60%, the temperature needs to be controlled within 60°C, and the time is 20 days; then, the fermentation product is subjected to acid leaching and alkaline soaking treatment using hydrofluoric acid and sodium hydroxide, wherein: during acid leaching, the fermentation product is fully soaked in hydrofluoric acid with a mass fraction of 40wt% for 8h and then filtered, and then the filter residue is soaked in sodium hydroxide solution with a solubility of 30% for 12h, stirred and then filtered; finally, the microbial residue is obtained by drying and crushing, and the drying time is 10h and the temperature is 85°C.

[0041] (2) After drying, 13.85 g of product D was fully ground and poured into a crucible, and then placed in a tubular furnace for high-temperature sintering treatment. The sintering temperature was 550 ° C (heating rate 5 ° C / min), and the time was 12 h. Argon was introduced as a protective gas. After the reaction was completed, the product E obtained was the sodium iron pyrophosphate positive electrode material.

[0042] (3) The prepared sodium iron pyrophosphate positive electrode material, SP conductive agent, and PVDF binder were weighed and uniformly mixed in NMP (N-methylpyrrolidone) in a mass ratio of 95:3:2. After coating, rolling, and slicing, the battery was assembled (components include: positive electrode sheet, negative electrode sheet, diaphragm, gasket, spring sheet, electrolyte) and tested.

[0043] Example 2

[0044] This embodiment provides a method for preparing a modified sodium iron pyrophosphate positive electrode material, and the specific steps are as follows:

[0045] (1) 1.60 g of sodium hydroxide (NaOH), 3.45 g of ammonium dihydrogen phosphate (NH4H2PO4), and 7.20 g of ferrous oxalate dihydrate (FeC2O4·2H2O) were weighed and dissolved in 50 mL of anhydrous ethanol. The mixture was ultrasonically treated at 25°C for 8 h at a power of 50 W to obtain solution A. Subsequently, 2 g of fungus residue was introduced into solution A and stirred until it was completely dissolved to obtain a mixed solution B. The temperature was then raised to 85°C and the above solution was subjected to a sol-gel treatment until the solution was completely evaporated and transformed into a jelly-like gel to obtain product C. The obtained product C was then placed in a vacuum drying oven and dried at 85°C for 12 h to obtain product D. The preparation method of the microbial residue is as follows: first, horse feces is fermented and treated, wherein: the fermentation is carried out in an anaerobic environment, the humidity is 50-60%, the temperature needs to be controlled within 60°C, and the time is 20 days; then, the fermentation product is subjected to acid leaching and alkaline soaking treatment using hydrofluoric acid and sodium hydroxide, wherein: during acid leaching, hydrofluoric acid with a mass fraction of 40wt% is used for full soaking for 8h and then filtered, and then the filter residue is soaked in a sodium hydroxide solution with a solubility of 30% for 12h, stirred and then filtered; finally, the microbial residue is obtained by drying and crushing, and the drying time is 10h and the temperature is 85°C.

[0046] (2) After drying, 12.25 g of product D was fully ground and poured into a crucible, and then placed in a tubular furnace for high-temperature sintering treatment. The sintering temperature was 550 ° C (heating rate 5 ° C / min), the time was 12 h, and argon was introduced as a protective gas. After the reaction was completed, the product E obtained was the sodium iron pyrophosphate positive electrode material.

[0047] (3) The prepared sodium iron pyrophosphate positive electrode material, SP conductive agent, and PVDF binder were weighed and uniformly mixed in NMP (N-methylpyrrolidone) in a mass ratio of 95:3:2. After coating, rolling, and slicing, the battery was assembled (components include: positive electrode sheet, negative electrode sheet, diaphragm, gasket, spring sheet, electrolyte) and tested.

[0048] Example 3

[0049] This embodiment provides a method for preparing a modified sodium iron pyrophosphate positive electrode material, and the specific steps are as follows:

[0050] (1) 3.28 g of sodium acetate (CH3COONa), 3.45 g of ammonium dihydrogen phosphate (NH4H2PO4), and 7.20 g of ferrous oxalate dihydrate (FeC2O4·2H2O) were weighed and dissolved in 50 mL of anhydrous ethanol. The mixture was ultrasonicated at 25°C for 8 h and the ultrasonic power was 50 W to obtain solution A. Subsequently, 2 g of fungus residue was introduced into solution A and stirred until it was completely dissolved to obtain mixed solution B. The temperature was then raised to 85°C and the above solution was subjected to sol-gel treatment until the solution was completely evaporated and transformed into a jelly-like gel to obtain product C. The obtained product C was then placed in a vacuum drying oven and dried at 85°C for 12 h to obtain product D. The preparation method of the microbial residue is as follows: first, sheep feces are fermented and treated, wherein: the fermentation is carried out in an anaerobic environment, the humidity is 50-60%, the temperature needs to be controlled within 60°C, and the time is 20 days; then, the fermentation product is subjected to acid leaching and alkaline soaking treatment using hydrofluoric acid and sodium hydroxide, wherein: during acid leaching, hydrofluoric acid with a mass fraction of 40wt% is used for full soaking for 8h and then filtered, and then the filter residue is soaked in a sodium hydroxide solution with a solubility of 30% for 12h, stirred and then filtered; finally, the microbial residue is dried and crushed to obtain the microbial residue, and the drying time is 10h and the temperature is 85°C.

[0051] (2) After drying, 12.93 g of product D was fully ground and poured into a crucible, and then placed in a tubular furnace for high-temperature sintering treatment. The sintering temperature was 550 ° C (heating rate 5 ° C / min), and the time was 12 h. Argon was introduced as a protective gas. After the reaction was completed, the product E obtained was the sodium iron pyrophosphate positive electrode material.

[0052] (3) The prepared sodium iron pyrophosphate positive electrode material, SP conductive agent, and PVDF binder were weighed and uniformly mixed in NMP (N-methylpyrrolidone) in a mass ratio of 95:3:2. After coating, rolling, and slicing, the battery was assembled (components include: positive electrode sheet, negative electrode sheet, diaphragm, gasket, spring sheet, electrolyte) and tested.

[0053] Example 4

[0054] This embodiment provides a method for preparing a modified sodium iron pyrophosphate positive electrode material, and the specific steps are as follows:

[0055] (1) 1.68 g of sodium fluoride (NaF), 3.45 g of ammonium dihydrogen phosphate (NH4H2PO4), and 7.20 g of ferrous oxalate dihydrate (FeC2O4·2H2O) were weighed and dissolved in 50 mL of anhydrous ethanol. The mixture was ultrasonicated at 25°C for 8 h and the ultrasonic power was 50 W to obtain solution A. Subsequently, 2 g of fungus residue was introduced into solution A and stirred until it was completely dissolved to obtain mixed solution B. The temperature was then raised to 85°C and the above solution was subjected to sol-gel treatment until the solution was completely evaporated and transformed into a jelly-like gel to obtain product C. The obtained product C was then placed in a vacuum drying oven and dried at 85°C for 12 h to obtain product D. The preparation method of the microbial residue is as follows: first, fermenting cattle feces and other treatments, wherein: the fermentation is carried out in an anaerobic environment, the humidity is 50-60%, the temperature needs to be controlled within 60°C, and the time is 20 days; then, the fermentation product is subjected to acid leaching and alkaline soaking treatment using hydrofluoric acid and sodium hydroxide, wherein: during acid leaching, the fermentation product is fully soaked in hydrofluoric acid with a mass fraction of 40wt% for 8h and then filtered, and then the filter residue is soaked in sodium hydroxide solution with a solubility of 30% for 12h, stirred and then filtered; finally, the microbial residue is obtained by drying and crushing, and the drying time is 10h and the temperature is 85°C.

[0056] (2) After drying, 11.23 g of product D was fully ground and poured into a crucible, and then placed in a tubular furnace for high-temperature sintering treatment. The sintering temperature was 550 ° C (heating rate 5 ° C / min), the time was 12 h, and argon was introduced as a protective gas. After the reaction was completed, the product E obtained was the sodium iron pyrophosphate positive electrode material.

[0057] (3) The prepared sodium iron pyrophosphate positive electrode material, SP conductive agent, and PVDF binder were weighed and uniformly mixed in NMP (N-methylpyrrolidone) in a mass ratio of 95:3:2. After coating, rolling, and slicing, the battery was assembled (components include: positive electrode sheet, negative electrode sheet, diaphragm, gasket, spring sheet, electrolyte) and tested.

[0058] Example 5

[0059] This embodiment provides a method for preparing a modified sodium iron pyrophosphate positive electrode material, and the specific steps are as follows:

[0060] (1) 4.24 g of sodium carbonate (Na2CO3), 3.45 g of diammonium hydrogen phosphate ((NH4)2HPO4), and 7.20 g of ferrous oxalate dihydrate (FeC2O4·2H2O) were weighed and dissolved in 50 mL of anhydrous ethanol. The mixture was ultrasonicated at 25°C for 8 h and the ultrasonic power was 50 W to obtain solution A. Subsequently, 2 g of fungus residue was introduced into solution A and stirred until it was completely dissolved to obtain mixed solution B. The temperature was then raised to 85°C and the above solution was subjected to sol-gel treatment until the solution was completely evaporated and transformed into a jelly-like gel to obtain product C. The obtained product C was then placed in a vacuum drying oven and dried at 85°C for 12 h to obtain product D. The preparation method of the microbial residue is as follows: first, fermenting cattle feces and other treatments, wherein: the fermentation is carried out in an anaerobic environment, the humidity is 50-60%, the temperature needs to be controlled within 60°C, and the time is 20 days; then, the fermentation product is subjected to acid leaching and alkaline soaking treatment using hydrofluoric acid and sodium hydroxide, wherein: during acid leaching, the fermentation product is fully soaked in hydrofluoric acid with a mass fraction of 40wt% for 8h and then filtered, and then the filter residue is soaked in sodium hydroxide solution with a solubility of 30% for 12h, stirred and then filtered; finally, the microbial residue is obtained by drying and crushing, and the drying time is 10h and the temperature is 85°C.

[0061] (2) After drying, 13.28 g of product D was fully ground and poured into a crucible, and then placed in a tubular furnace for high-temperature sintering treatment. The sintering temperature was 550 ° C (heating rate 5 ° C / min), the time was 12 h, and argon was introduced as a protective gas. After the reaction was completed, the product E obtained was the sodium iron pyrophosphate positive electrode material.

[0062] (3) The prepared sodium iron pyrophosphate positive electrode material, SP conductive agent, and PVDF binder were weighed and uniformly mixed in NMP (N-methylpyrrolidone) in a mass ratio of 95:3:2. After coating, rolling, and slicing, the battery was assembled (components include: positive electrode sheet, negative electrode sheet, diaphragm, gasket, spring sheet, electrolyte) and tested.

[0063] Example 6

[0064] This embodiment provides a method for preparing a modified sodium iron pyrophosphate positive electrode material, and the specific steps are as follows:

[0065] (1) 4.24 g of sodium carbonate (Na2CO3), 5.34 g of pyrophosphate (H4P2O7), and 7.20 g of ferrous oxalate dihydrate (FeC2O4·2H2O) were weighed and dissolved in 50 mL of anhydrous ethanol. The mixture was ultrasonicated at 25°C for 8 h and the ultrasonic power was 50 W to obtain solution A. Subsequently, 2 g of fungus residue was introduced into solution A and stirred until it was completely dissolved to obtain mixed solution B. The temperature was then raised to 85°C and the above solution was subjected to sol-gel treatment until the solution was completely evaporated and transformed into a jelly-like gel to obtain product C. The obtained product C was then placed in a vacuum drying oven and dried at 85°C for 12 h to obtain product D. The preparation method of the microbial residue is as follows: first, fermenting cattle feces and other treatments, wherein: the fermentation is carried out in an anaerobic environment, the humidity is 50-60%, the temperature needs to be controlled within 60°C, and the time is 20 days; then, the fermentation product is subjected to acid leaching and alkaline soaking treatment using hydrofluoric acid and sodium hydroxide, wherein: during acid leaching, the fermentation product is fully soaked in hydrofluoric acid with a mass fraction of 40wt% for 8h and then filtered, and then the filter residue is soaked in sodium hydroxide solution with a solubility of 30% for 12h, stirred and then filtered; finally, the microbial residue is obtained by drying and crushing, and the drying time is 10h and the temperature is 85°C.

[0066] (2) After drying, 14.78 g of product D was fully ground and poured into a crucible, and then placed in a tubular furnace for high-temperature sintering treatment. The sintering temperature was 550 ° C (heating rate 5 ° C / min), the time was 12 h, and argon was introduced as a protective gas. After the reaction was completed, the product E obtained was the sodium iron pyrophosphate positive electrode material.

[0067] (3) The prepared sodium iron pyrophosphate positive electrode material, SP conductive agent, and PVDF binder were weighed and uniformly mixed in NMP (N-methylpyrrolidone) in a mass ratio of 95:3:2. After coating, rolling, and slicing, the battery was assembled (components include: positive electrode sheet, negative electrode sheet, diaphragm, gasket, spring sheet, electrolyte) and tested.

[0068] Example 7

[0069] This embodiment provides a method for preparing a modified sodium iron pyrophosphate positive electrode material, and the specific steps are as follows:

[0070] (1) 4.24 g of sodium carbonate (Na2CO3), 3.45 g of ammonium dihydrogen phosphate (NH4H2PO4), and 16.16 g of ferric nitrate (Fe(NO3)3) were weighed and dissolved in 50 mL of anhydrous ethanol. The mixture was ultrasonically treated at 25°C for 8 h and an ultrasonic power of 50 W to obtain solution A. Subsequently, 2 g of fungus residue was introduced into solution A and stirred until it was completely dissolved to obtain a mixed solution B. The temperature was then raised to 85°C and the above solution was subjected to a sol-gel treatment until the solution was completely evaporated and transformed into a jelly-like gel to obtain product C. The obtained product C was then placed in a vacuum drying oven and dried at 85°C for 12 h to obtain product D. The preparation method of the microbial residue is as follows: first, fermenting cattle feces and other treatments, wherein: the fermentation is carried out in an anaerobic environment, the humidity is 50-60%, the temperature needs to be controlled within 60°C, and the time is 20 days; then, the fermentation product is subjected to acid leaching and alkaline soaking treatment using hydrofluoric acid and sodium hydroxide, wherein: during acid leaching, the fermentation product is fully soaked in hydrofluoric acid with a mass fraction of 40wt% for 8h and then filtered, and then the filter residue is soaked in sodium hydroxide solution with a solubility of 30% for 12h, stirred and then filtered; finally, the microbial residue is obtained by drying and crushing, and the drying time is 10h and the temperature is 85°C.

[0071] (2) After drying, 16.25 g of product D was fully ground and poured into a crucible, and then placed in a tubular furnace for high-temperature sintering treatment. The sintering temperature was 550 ° C (heating rate 5 ° C / min), the time was 12 h, and argon was introduced as a protective gas. After the reaction was completed, the product E obtained was the sodium iron pyrophosphate positive electrode material.

[0072] (3) The prepared sodium iron pyrophosphate positive electrode material, SP conductive agent, and PVDF binder were weighed and uniformly mixed in NMP (N-methylpyrrolidone) in a mass ratio of 95:3:2. After coating, rolling, and slicing, the battery was assembled (components include: positive electrode sheet, negative electrode sheet, diaphragm, gasket, spring sheet, electrolyte) and tested.

[0073] Example 8

[0074] This embodiment provides a method for preparing a modified sodium iron pyrophosphate positive electrode material, and the specific steps are as follows:

[0075] (1) 4.24 g of sodium carbonate (Na2CO3), 3.45 g of ammonium dihydrogen phosphate (NH4H2PO4), and 7.64 g of ferric acetate (Fe(CH3COO)3) were weighed and dissolved in 50 mL of anhydrous ethanol. The mixture was ultrasonically treated at 25°C for 8 h and the ultrasonic power was 50 W to obtain solution A. Subsequently, 2 g of fungus residue was introduced into solution A and stirred until it was completely dissolved to obtain mixed solution B. The temperature was then raised to 85°C and the above solution was subjected to sol-gel treatment until the solution was completely evaporated and transformed into a jelly-like gel to obtain product C. The obtained product C was then placed in a vacuum drying oven and dried at 85°C for 12 h to obtain product D. The preparation method of the microbial residue is as follows: first, fermenting cattle feces and other treatments, wherein: the fermentation is carried out in an anaerobic environment, the humidity is 50-60%, the temperature needs to be controlled within 60°C, and the time is 20 days; then, the fermentation product is subjected to acid leaching and alkaline soaking treatment using hydrofluoric acid and sodium hydroxide, wherein: during acid leaching, the fermentation product is fully soaked in hydrofluoric acid with a mass fraction of 40wt% for 8h and then filtered, and then the filter residue is soaked in sodium hydroxide solution with a solubility of 30% for 12h, stirred and then filtered; finally, the microbial residue is obtained by drying and crushing, and the drying time is 10h and the temperature is 85°C.

[0076] (2) After drying, 13.23 g of product D was fully ground and poured into a crucible, and then placed in a tubular furnace for high-temperature sintering treatment. The sintering temperature was 550 ° C (heating rate 5 ° C / min), the time was 12 h, and argon was introduced as a protective gas. After the reaction was completed, the product E obtained was the sodium iron pyrophosphate positive electrode material.

[0077] (3) The prepared sodium iron pyrophosphate positive electrode material, SP conductive agent, and PVDF binder were weighed and uniformly mixed in NMP (N-methylpyrrolidone) in a mass ratio of 95:3:2. After coating, rolling, and slicing, the battery was assembled (components include: positive electrode sheet, negative electrode sheet, diaphragm, gasket, spring sheet, electrolyte) and tested.

[0078] Comparative Example 1

[0079] This comparative example provides a method for preparing a modified sodium iron pyrophosphate positive electrode material, and the specific steps are as follows:

[0080] (1) 4.24 g of sodium carbonate (Na2CO3), 3.45 g of ammonium dihydrogen phosphate (NH4H2PO4), and 7.20 g of ferrous oxalate dihydrate (FeC2O4·2H2O) were weighed and dissolved in 50 mL of anhydrous ethanol. The mixture was ultrasonicated at 25°C for 8 h and the ultrasonic power was 50 W to obtain solution A. Subsequently, 2 g of glucose was introduced into solution A and stirred until it was completely dissolved to obtain a mixed solution B. The temperature was then raised to 85°C and the above solution was subjected to a sol-gel treatment until the solution was completely evaporated and transformed into a jelly-like gel to obtain product C. The obtained product C was then placed in a vacuum drying oven and dried at 85°C for 12 h to obtain product D.

[0081] (2) After drying, 13.28 g of product D was fully ground and poured into a crucible, and then placed in a tubular furnace for high-temperature sintering treatment. The sintering temperature was 550 ° C (heating rate 5 ° C / min), the time was 12 h, and argon was introduced as a protective gas. After the reaction was completed, the product E obtained was the sodium iron pyrophosphate positive electrode material.

[0082] (3) The prepared sodium iron pyrophosphate positive electrode material, SP conductive agent, and PVDF binder were weighed and uniformly mixed in NMP (N-methylpyrrolidone) in a mass ratio of 95:3:2. After coating, rolling, and slicing, the battery was assembled (components include: positive electrode sheet, negative electrode sheet, diaphragm, gasket, spring sheet, electrolyte) and tested.

[0083] Comparative Example 2

[0084] This embodiment provides a method for preparing a modified sodium iron pyrophosphate positive electrode material, and the specific steps are as follows:

[0085] (1) 4.24 g of sodium carbonate (Na2CO3), 3.45 g of ammonium dihydrogen phosphate (NH4H2PO4), and 7.20 g of ferrous oxalate dihydrate (FeC2O4·2H2O) were weighed and dissolved in 50 mL of anhydrous ethanol. The mixture was ultrasonicated at 25°C for 8 h and the ultrasonic power was 50 W to obtain solution A. Subsequently, 2 g of citric acid was introduced into solution A and stirred until it was completely dissolved to obtain a mixed solution B. The temperature was then raised to 85°C and the above solution was subjected to a sol-gel treatment until the solution was completely evaporated and transformed into a jelly-like gel to obtain product C. The obtained product C was then placed in a vacuum drying oven and dried at 85°C for 12 h to obtain product D.

[0086] (2) After drying, 13.89 g of product D was fully ground and poured into a crucible, and then placed in a tubular furnace for high-temperature sintering treatment. The sintering temperature was 550 ° C (heating rate 5 ° C / min), the time was 12 h, and argon was introduced as a protective gas. After the reaction was completed, the product E obtained was the sodium iron pyrophosphate positive electrode material.

[0087] (3) The prepared sodium iron pyrophosphate positive electrode material, SP conductive agent, and PVDF binder were weighed and uniformly mixed in NMP (N-methylpyrrolidone) in a mass ratio of 95:3:2. After coating, rolling, and slicing, the battery was assembled (components include: positive electrode sheet, negative electrode sheet, diaphragm, gasket, spring sheet, electrolyte) and tested.

[0088] Comparative Example 3

[0089] This embodiment provides a method for preparing a modified sodium iron pyrophosphate positive electrode material, and the specific steps are as follows:

[0090] (1) 4.24 g of sodium carbonate (Na2CO3), 3.45 g of ammonium dihydrogen phosphate (NH4H2PO4), and 7.20 g of ferrous oxalate dihydrate (FeC2O4·2H2O) were weighed and dissolved in 50 mL of anhydrous ethanol. The mixture was ultrasonically treated at 25°C for 8 h and a power of 50 W to obtain solution A. Subsequently, 2 g of asphalt was introduced into solution A and stirred until it was completely dissolved to obtain a mixed solution B. The temperature was then raised to 85°C, and the above solution was subjected to a sol-gel process until the solution was completely evaporated to dryness and transformed into a jelly-like gel to obtain product C. The obtained product C was then dried in a vacuum drying oven at 85°C for 12 h to obtain product D.

[0091] (2) After drying, 11.42 g of product D was fully ground and poured into a crucible, and then placed in a tubular furnace for high-temperature sintering treatment. The sintering temperature was 550 ° C (heating rate 5 ° C / min), the time was 12 h, and argon was introduced as a protective gas. After the reaction was completed, the product E obtained was the sodium iron pyrophosphate positive electrode material.

[0092] (3) The prepared sodium iron pyrophosphate positive electrode material, SP conductive agent, and PVDF binder were weighed and uniformly mixed in NMP (N-methylpyrrolidone) in a mass ratio of 95:3:2. After coating, rolling, and slicing, the battery was assembled (components include: positive electrode sheet, negative electrode sheet, diaphragm, gasket, spring sheet, electrolyte) and tested.

[0093] Performance test: The positive electrode materials of the embodiment and the comparative example were made into button batteries and the electrochemical performance was tested on a high-performance battery testing system, Blue Electric. The test conditions were: voltage range: 2.0-3.8V, temperature: 25°C, current size: 0.1C. The results are shown in Table 1:

[0094] Table 1

[0095]

[0096]

[0097] It can be found from Table 1 that by comparing Example 1 with Examples 2 to 4, Example 1 with Examples 5 to 6, and Example 1 with Examples 7 to 8, it is proved that sodium carbonate, ammonium dihydrogen phosphate, and ferrous oxalate dihydrate are the best sodium source, phosphorus source, and iron source for the sodium ferric pyrophosphate positive electrode material, showing the best cycle stability and rate performance.

[0098] The synthesized sodium iron pyrophosphate cathode material was tested by XRD analysis. Figure 1 The peak intensity of the crystal diffraction peak of the sodium ferric pyrophosphate of Example 1 is sharper, indicating that the crystal has high crystallinity, good structural stability and less impurity components.

[0099] The samples of Example 1 and Comparative Example 1 were tested by scanning electron microscopy. The results are as follows: Figure 2 (Example 1) and Figure 3 (Comparative Example 1) shows that the sodium iron pyrophosphate positive electrode material particles synthesized in Example 1 are uniformly dispersed and not agglomerated. At the same time, analysis found that coating the surface of the sodium iron pyrophosphate material with a conductive carbon coating can greatly improve the material conductivity. Figure 3 Scanning electron microscopy tests show that the morphology of the sodium iron pyrophosphate material of Comparative Example 1 is uneven, and the agglomeration phenomenon is more obvious, which is not conducive to sufficient infiltration of the electrolyte, resulting in slow sodium ion insertion and extraction kinetics, leading to poor electrochemical reaction and poor discharge capacity.

[0100] In addition, XPS analysis was performed on the prepared sodium iron pyrophosphate cathode material to detect the chemical composition and element valence state of the material surface. Figure 4 The XPS spectra of nitrogen and fluorine in the sodium ferric pyrophosphate material of Example 1 are shown, proving that nitrogen and fluorine have been successfully doped into the sodium ferric pyrophosphate material. Nitrogen doping can provide a fast charge transfer network, facilitating electron transfer, while fluorine doping can improve the deintercalation and deintercalation of sodium ions and maintain the stability of the sodium ferric pyrophosphate crystal structure over a long period. Using fungus residue as a biomass carbon source to modify the sodium ferric pyrophosphate material can establish an ultra-high electronic conductive network, which is beneficial for rapid electron transfer throughout the electrode and promotes rapid reaction kinetics. Figure 5 The rate spectra of Example 1 and Comparative Examples 1 to 3 are shown. It can be found that the sodium iron pyrophosphate material prepared in Example 1 exhibits excellent rate performance at a current density of 0.1 to 2C. This is mainly attributed to the fact that microbial residue as a biomass carbon source improves the electronic conductivity and sodium ion transfer rate of the sodium iron pyrophosphate material during the charge and discharge process, thereby greatly improving the capacity and rate performance of the battery. Figure 6 The charge-discharge curves of the sodium ferric pyrophosphate materials of Example 1 and Comparative Example 1 at 0.5C are shown. During charge and discharge, the sodium ferric pyrophosphate material of Example 1 exhibits lower polarization and a higher charge-discharge voltage plateau than that of Comparative Example 1, also demonstrating its high discharge specific capacity. This is mainly attributed to the fact that the fungus residue, as a carbon source, improves the electronic conductivity and ion transport rate of the sodium ferric pyrophosphate material, thereby enhancing its electrochemical reaction activity.

Claims

1. A method for improving sodium iron pyrophosphate with a multifunctional conductive carbon layer, characterized in that The method comprises the following steps: Step S1, sequentially adding a sodium source, a phosphorus source, and an iron source to a solvent, ultrasonically dispersing the mixture and stirring the mixture until the raw materials are completely dissolved to obtain a yellow solution A; Step S2, adding a carbon source to the yellow solution A and continuously stirring until all the raw materials are completely dissolved, thereby obtaining a solution B, wherein the molar ratio of the sodium source:phosphorus source:iron source:carbon source is 4:3:4:0.1; Step S3, subjecting solution B to a sol-gel process until the solution completely turns into a gel to obtain precursor C; Step S4, vacuum drying the precursor C, and taking it out after drying to obtain product D; Step S5: calcining the product D at a high temperature to obtain a sodium iron pyrophosphate positive electrode material.

2. The method for improving sodium ferric pyrophosphate with a multifunctional conductive carbon layer according to claim 1, characterized in that The sodium source includes one of sodium fluoride, sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium phosphate, sodium acetate, sodium oxalate, sodium oxide, sodium pyrophosphate, and disodium hydrogen phosphate; the iron source includes one of ferrous oxalate dihydrate, ferric phosphate, ferric nitrate, ferric acetate, ferric oxide, and ferroferric oxide; the phosphorus source includes one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, and pyrophosphoric acid; the carbon source is one of mushroom residue, glucose, asphalt, and citric acid; and the solvent is one of anhydrous ethanol, acetone, and DMF.

3. The method for improving sodium iron pyrophosphate with a multifunctional conductive carbon layer according to claim 1, characterized in that In the step S1, the ultrasonic dispersion time is 5 to 10 hours, the power is 50 to 150 W, and the stirring time is 2 to 10 hours.

4. The method for improving sodium iron pyrophosphate with a multifunctional conductive carbon layer according to claim 1, characterized in that The bacterial residue is discarded microbial residue, and the specific preparation method is as follows: first, the animal feces is fermented; then, the fermentation product is acid-leached and alkaline-soaked with hydrofluoric acid and sodium hydroxide; finally, after drying, the discarded microbial residue is crushed and sintered to convert it into a biomass carbon source.

5. The method for improving sodium ferric pyrophosphate with a multifunctional conductive carbon layer according to claim 4, characterized in that The fermentation is carried out in an anaerobic environment with a humidity of 50-60%, a temperature of 45-60° C., and a time of 15-20 days. During the acid leaching, hydrofluoric acid with a mass fraction of 40wt% is used for full immersion for 6-10 hours, and then filtered. Then, the filter residue is soaked in a sodium hydroxide solution with a solubility of 30% for 10-15 hours, stirred, and then filtered. The drying time is 8-12 hours, and the temperature is 80-100° C.

6. The method for improving sodium iron pyrophosphate with a multifunctional conductive carbon layer according to claim 5, characterized in that In step S2, the stirring speed is 50-80 rpm, the temperature is 75-85° C., and the stirring time is 2-10 hours.

7. The method for improving sodium iron pyrophosphate with a multifunctional conductive carbon layer according to claim 1, characterized in that The temperature of the sol-gel treatment is 80-90° C., and the stirring time is 45-50 hours.

8. The method for improving sodium iron pyrophosphate with a multifunctional conductive carbon layer according to claim 1, characterized in that The vacuum drying time is 10 to 15 hours, the temperature is 80 to 100° C., and the heating rate is 5 to 10° C. / min.

9. The method for improving sodium ferric pyrophosphate with a multifunctional conductive carbon layer according to claim 1, characterized in that The high-temperature calcination atmosphere is one of argon, nitrogen, and argon-hydrogen mixed inert gas; the high-temperature calcination time is 10 to 12 hours, the sintering temperature is 500 to 550° C., and the heating rate is 3 to 5° C. / min.

10. Use of sodium ferric pyrophosphate prepared by the method according to any one of claims 1 to 9 in a positive electrode material for a sodium ion battery.

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