High-stability sulfate-based sodium-ion battery cathode material and preparation method thereof

Abstract

The application discloses a high-stability sulfated sodium ion battery positive electrode material and a preparation method thereof. 2+2x M 2‑x (SO4)3; the sulfated material core is Na 2+2x Fe 2‑x (SO4)3; wherein M is one or two or more of alkaline earth metal elements Ca, Ba and Sr; and the value range of x is 1.5<=x<=2.0. The high-stability sulfated sodium ion battery positive electrode material and the preparation method thereof have the characteristics of high air stability, good structural stability and excellent electrochemical performance.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of sodium ion batteries, in particular to a high-stability sulfate sodium ion battery positive electrode material and a preparation method thereof. BACKGROUND

[0002] The positive electrode material has a great influence on the energy density and electrochemical performance of a sodium ion battery system, and a suitable sodium ion battery positive electrode material needs to meet the requirements of low cost, high capacity, high redox potential, high structural stability, air stability and simple and reliable processing or manufacturing process. However, the performances of currently marketed layered oxides, prussian blue and analogues and polyanions are quite different, and there are very few materials with excellent comprehensive performance.

[0003] As one of the main material types of polyanion-type sodium ion battery positive electrode materials, the sulfate material is rich in resource reserves, has a simple preparation process, a high voltage platform and a high system energy density, and is currently positioned in the low-end power field in the market. However, the sintering temperature of the sulfate material is low, and the bonding energy between the transition metal and the anion group in the crystal structure is low, so that the structural stability and the sensitivity to water are high, and the material is easy to absorb water and convert into a hydrated phase or interface cracking to generate a rock-rock phase in the humid air and during repeated sodium extraction and embedding, and then the material is deactivated.

[0004] At present, the researches on the sulfate material mainly focus on the synthesis method and the improvement of the electronic conductivity, and there are few related researches on the structural stability, which seriously restricts the commercialization process. Therefore, it is of great significance to find a new interface modification process to improve the structural stability of the sulfate material for the practical application of the material. SUMMARY

[0005] The application aims to provide a high-stability sulfate sodium ion battery positive electrode material and a preparation method thereof, which have the characteristics of high air stability, good structural stability and excellent electrochemical performance.

[0006] The application can be implemented by the following technical scheme:

[0007] The application discloses a high-stability sulfate sodium ion battery positive electrode material, which has a core-shell structure and comprises an interface coating layer and a sulfate material inner core. 2+2x M 2-x (SO4)3; the sulfate material inner core is Na 2+2x Fe 2-x (SO4)3; wherein M is one or two or more of alkaline earth metal elements Ca, Ba and Sr; and the value range of x is 1.5<=x<=2.0.

[0008] Another aspect of the present application is to protect the preparation method of the above-mentioned high-stability sodium sulfate-based sodium-ion battery cathode material, which comprises the following steps:

[0009] S1, preparation of precursor mixed solution: wet mixing of iron source, sulfuric acid, antioxidant, sodium source, carbon source and dispersant to obtain uniform precursor mixed solution;

[0010] S2, separation of precursor powder: drying the precursor mixed solution under a protective atmosphere to obtain a precursor powder;

[0011] S3, calcination of sulfate-based material core: calcining the precursor powder in a protective atmosphere to obtain a high-crystallinity sulfate-based material core;

[0012] S4, preparation of pre-coated precursor: wet mixing of the sulfate-based material core and metal M source to promote displacement reaction of material interface atoms, and filtering to obtain a pre-coated precursor;

[0013] S5, high-temperature sintering of interface coating layer: high-temperature sintering of the pre-coated precursor under a protective atmosphere to promote crystalline growth of the material interface coating layer, thereby obtaining a structure-stable sulfate-based sodium-ion battery cathode material.

[0014] The core of improving the structural stability of the sulfate-based material lies in its interface stability. The present application improves the structural stability of the sulfate-based material through in-situ interface modification, which includes an interface coating layer and a sulfate-based material core. The present application process uses liquid phase process to first synthesize the sulfate-based material core, and then dissolves a small amount of alkaline earth metal compounds such as Ca, Ba and Sr in the mixed system. The exchange of the dissolved alkaline earth metal compounds with the transition metal iron element on the surface of the sulfate-based material realizes the coating of the interface inert layer. The coating layer has good modification effect on the sensitivity of the solution system and the reinforcement of the structure, thereby achieving the purpose of improving the structural stability of the sulfate-based material. The material prepared by the process has high air stability, good crystallinity, dense interface coating layer and excellent electrochemical cycle stability.

[0015] Further, in step S3, the temperature of the precursor powder calcination is 350-450°C. When the temperature is lower than 350°C, the sulfate-based material cannot be fully melted and crystallized to form larger grain size, which affects the subsequent interface reaction. When the temperature is higher than 450°C, the sulfate-based material will decompose into iron oxide and sulfur oxide gas.

[0016] Further, in step S4, the solvent system of wet mixing is a mixture of water and organic solvent, and the mass ratio of water to organic solvent is 0.1-5%. If the water content is too high, the amount of dissolved core of the sulfate material is too large, which is not conducive to the uniform reaction of the interface in the subsequent process. If the water content is too low, the M source cannot be effectively dissolved, which causes insufficient ion exchange between the M source and the surface of the core of the sulfate material, and reduces the uniformity of coating.

[0017] Further, in step S5, the high-temperature sintering temperature of the pre-coated precursor powder is 400-450°C, which is aimed at promoting the melting growth between sodium ions, metal M ions, sulfate and other elements at the interface of the sulfate material, and forming a uniform and dense coating layer.

[0018] Further, in step S4, the solvent system of wet mixing is a mixture of water and organic solvent, and the mass ratio of water to organic solvent is 0.1-5%. If the water content is too high, the amount of dissolved core of the sulfate material is too large, which is not conducive to the uniform reaction of the interface in the subsequent process. If the water content is too low, the M source cannot be effectively dissolved, which causes insufficient ion exchange between the M source and the surface of the core of the sulfate material, and reduces the uniformity of coating.

[0019] Further, in step S2, the drying method is one or more of vacuum drying, freeze drying, air blowing drying, spray drying, flash drying, vacuum drying, and natural evaporation, which can realize solid-liquid separation.

[0020] Further, in step S4, the filtration method is one or more of pressure filtration, vacuum filtration, and normal pressure filtration, which realizes solid-liquid separation and removes excess non-sulfate anions and excess metal ions in the solution.

[0021] Further, in steps S2, S3 and S5, the protective atmosphere is one or more of non-oxygen-containing gases such as nitrogen, argon, nitrogen-hydrogen and argon-hydrogen.

[0022] Further, the antioxidant is one or more of ascorbic acid and its derivatives, tea polyphenols, butylated hydroxyanisole, tert-butyl hydroquinone, phytic acid and its derivatives, butylated hydroxytoluene, ethoxyquin, and the like, to prevent oxidation of the divalent iron; and the dispersant is one or more of polyacrylic acid series, polyurethane compounds, polyethylene glycol series, methyl cellulose and its derivatives, polyvinyl alcohol series, and polyvinylpyrrolidone series, which have high dispersion capacity for inorganic carbon in aqueous solution.

[0023] Further, the metal M source is one or more of calcium-containing compounds, barium-containing compounds, and strontium-containing compounds, the calcium-containing compounds are one or more of calcium chloride, calcium chromate, calcium nitrate, and calcium sulfite, the barium-containing compounds are one or more of barium chloride, barium nitrate, barium hydroxide, and barium bicarbonate, and the strontium-containing compounds are one or more of strontium chloride, strontium nitrate, and strontium perchlorate. The three types of metal M ions can form precipitates that are insoluble in water with sulfate ions, while other types of metal ions tend to form sulfates that are highly soluble in water. Therefore, the type of metal M source reacts on the surface of the core of the sulfate material to form a Na 2+2x M 2-x (SO4)3coating layer, in which the metal M ions and sulfate ions have stronger binding energy and are less likely to be hydrolyzed and dissociated by water, thereby effectively protecting the structural stability of the core.

[0024] Further, the iron source is one or more of elemental iron powder, ferrous oxide, ferric oxide, magnetite, hydroxyl ferric oxide, and iron hydroxide, the sodium source is one or more of sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium citrate, sodium oxalate, sodium formate, sodium acetate, sodium sulfate, and sodium nitrate, and the carbon source is one or more of graphite and its derivatives, carbon nanotubes and its slurry, graphene and its slurry, and carbon black and its derivatives, which have inorganic conductive carbon.

[0025] The present application discloses a high-stability sulfate sodium-ion battery cathode material and a preparation method thereof, which has the following beneficial effects:

[0026] First, the air stability is high: Na 2+2x M 2-x (SO4)3coating layer, in which the metal M ions and sulfate ions have stronger binding energy and are less likely to be hydrolyzed and dissociated by water, thereby effectively protecting the structural stability of the core. 2- The electrostatic attraction between the M-S-O and SO4 2+2x Fe 2-x (SO4)3is lower in solubility in water and stronger in stability. Therefore, H2O is less likely to form a hydrogen bond (M-S-O--H-O-H) with O in the M-S-O bond in the humid air, effectively avoiding moisture absorption of the material. In addition, Na 2+2x M 2-xDue to the difference in the types of metal ions in the crystal structure of the (SO4)3 coating and the core of the sulfate material, the ion diffusion channels are locally distorted. This discontinuity in the diffusion channels helps to prevent water molecules from migrating into the core of the sulfate material through the interfacial channels, thereby improving the air stability of the sulfate material.

[0027] Secondly, it has good structural stability: Sulfate materials are often obtained through low-temperature sintering, and the interfacial pulverization problem caused by volume expansion during the sodium insertion / extraction process is quite serious. 2+2x M 2-x The (SO4)3 coating layer is an electrochemically inert material that is uniformly attached to the surface of sulfate materials with a thickness of about a few nm. Due to the existence of ion tunneling, it hardly affects the diffusion of sodium ions. However, the coating layers interlock to form a cross-linked crystal coating layer, which can effectively suppress the phenomena of interface pulverization and lithification caused by attraction during crystal expansion / contraction, thereby improving the structural stability of the material.

[0028] Third, excellent electrochemical performance: sulfate materials are free from SO4 2- The inductive effect of the functional groups results in a high redox potential for this material; however, at high voltages, the iron ions in its structure readily react with SO42-. 2- When the functional groups break their bonds and become free from the structure, the free iron ions are deposited at the negative electrode, which further catalyzes the reaction of the electrolyte, leading to severe degradation of the cell performance. 2+2x M 2-x The construction of the (SO4)3 coating layer can effectively slow down the dissolution of iron ions from the core of sulfate materials, thereby preventing them from catalyzing electrolyte reactions and improving the cycle stability of the battery. Attached Figure Description

[0029] Figure 1 For the application of Na in Example 1 2.8 Fe 1.6 (SO4)3 / Na 2.8 Ba 1.6 TEM of (SO4)3 core-shell material;

[0030] Figure 2 For Na in Comparative Example 1 2.8 Fe 1.6 TEM of (SO4)3 material. Detailed Implementation

[0031] To enable those skilled in the art to better understand the technical solution of the present invention, the product of the present invention will be further described in detail below with reference to embodiments.

[0032] The application discloses a high-stability sulfated sodium ion battery positive electrode material. 2+2x M 2-x (SO4)3; the sulfated material core is Na 2+2x Fe 2-x (SO4)3; wherein M is one or more than two of alkaline earth metal elements Ca, Ba and Sr; and the value range of x is 1.5<=x<=2.0.

[0033] Another aspect of the application is to protect the preparation method of the high-stability sulfated sodium ion battery positive electrode material, which comprises the following steps:

[0034] S1, preparation of a precursor mixed solution: iron source, sulfuric acid, antioxidant, sodium source, carbon source and dispersant are mixed wetly to obtain a uniform precursor mixed solution;

[0035] S2, separation of the precursor powder: the precursor mixed solution is dried to obtain a precursor powder under a protective atmosphere;

[0036] S3, calcination of the sulfated material core: the precursor powder is calcined to obtain a high-crystallinity sulfated material core under a protective atmosphere;

[0037] S4, preparation of a pre-coated precursor: the sulfated material core and metal M source are mixed wetly, and the pre-coated precursor is obtained by filtration;

[0038] S5, high-temperature sintering of the interface coating layer: the pre-coated precursor is high-temperature sintered under a protective atmosphere to obtain a structure-stable sulfated sodium ion battery positive electrode material.

[0039] Further, in step S3, the temperature of the precursor powder calcination is 350-450°C.

[0040] Further, in step S4, the solvent system of the wet mixing is a mixture of water and an organic solvent, and the mass ratio of water to the organic solvent is 0.1-5%.

[0041] Further, in step S5, the high-temperature sintering temperature of the pre-coated precursor powder is 400-450°C.

[0042] Further, in step S4, the solvent system of wet mixing is a mixture of water and an organic solvent, and the organic solvent is one or more of alcohol, ether, aromatic hydrocarbon, aliphatic hydrocarbon, ester, cyclic hydrocarbon, halogenated hydrocarbon, ketone, glycol derivative, acetonitrile, pyridine, and phenol; the alcohol is one or more of methanol, ethanol, and isopropanol; the ether is one or more of diethyl ether and propylene oxide; the aromatic hydrocarbon is one or more of benzene, toluene, and xylene; the ester is one or more of methyl acetate, ethyl acetate, and propyl acetate; the aliphatic hydrocarbon is one or more of pentane, hexane, and octane; the cyclic hydrocarbon is one or more of cyclohexane, cyclohexanone, and toluene cyclohexanone; the halogenated hydrocarbon is one or more of chlorobenzene, dichlorobenzene, and dichloromethane; the ketone is one or more of acetone, methyl butanone, and methyl isobutyl ketone; and the glycol derivative is one or more of ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, and ethylene glycol monobutyl ether.

[0043] Further, in step S2, the drying method is one or more of vacuum drying, freeze drying, air blowing drying, spray drying, flash drying, vacuum drying, and natural evaporation.

[0044] Further, in step S4, the filtering method is one or more of pressure filtration, vacuum suction filtration, and normal pressure filtration.

[0045] Further, in steps S2, S3, and S5, the protective atmosphere is one or more of non-oxygen-containing gases, nitrogen, argon, nitrogen-hydrogen, and argon-hydrogen.

[0046] Further, the antioxidant is one or more of ascorbic acid and its derivatives, tea polyphenol, butylated hydroxyl anisole, tertiary butylated hydroquinone, phytic acid and its derivatives, butylated hydroxyl toluene, butylated hydroxyl toluene, and ethoxyquinoline; and the dispersant is one or more of polyacrylic acid series, polyurethane compound, polyethylene glycol series, methyl cellulose and its derivatives, polyvinyl alcohol series, and polyvinyl pyrrolidone series.

[0047] Further, the metal M source is one or more of calcium-containing compound, barium-containing compound, and strontium-containing compound; the calcium-containing compound is one or more of calcium chloride, calcium chromate, calcium nitrate, and calcium sulfite; the barium-containing compound is one or more of barium chloride, barium nitrate, barium hydroxide, and barium bicarbonate; and the strontium-containing compound is one or more of strontium chloride, strontium nitrate, and strontium perchlorate.

[0048] Further, the iron source is one or two or more of elemental iron powder, ferrous oxide, ferric oxide, magnetite, hydroxyl ferric oxide, and ferric hydroxide, the sodium source is one or two or more of sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium citrate, sodium oxalate, sodium formate, sodium acetate, sodium sulfate, and sodium nitrate, and the carbon source is one or two or more of graphite and its derivatives, carbon nanotubes and its slurry, graphene and its slurry, carbon black and its derivatives, and the like.

[0049] Application Example 1 Na 2.8 Fe 1.6 (SO4)3 / Na 2.8 Ba 1.6 Synthesis of (SO4)3 shell core material and its electrochemical performance

[0050] Na 2.8 Fe 1.6 (SO4)3 / Na 2.8 Ba 1.6 The method for preparing (SO4)3 shell core material comprises the following steps:

[0051] Step 1: Dissolve elemental iron powder, sulfuric acid, and sodium carbonate in water according to a molar ratio of 1.6:3:1.4, add ascorbic acid (3% by weight of elemental iron powder), polyethylene glycol 400 (5% by weight of elemental iron powder), and a small amount of carbon nanotubes as a conductive agent (7% by weight of elemental iron powder), and generate a uniform black slurry after sufficient reaction;

[0052] Step 2: Spray dry the slurry under nitrogen protection, with an outlet temperature controlled at ≥80°C, to obtain a uniform precursor powder;

[0053] Step 3: Calcine the precursor powder at 380°C for 10H under nitrogen protection to obtain a high-crystallinity Na 2.8 Fe 1.6 (SO4)3 material;

[0054] Step 4: Add the Na 2.8 Fe 1.6 (SO4)3 material core and barium chloride (added in an amount of 5% by weight of Na 2.8 Fe 1.6 (SO4)3) to a mixed solution of water and ethanol (water:ethanol weight ratio of 0.5:99.5) to cause a displacement reaction between the iron and barium elements at the material interface, and filter to obtain a pre-coated precursor;

[0055] Step 5: Further calcine the pre-coated precursor at 420°C under nitrogen protection to promote the Na 2.8 Ba 1.6The (SO4)3 coating layer melts and grows, resulting in Na with high structural stability. 2.8 Fe 1.6 (SO4)3 / Na 2.8 Ba 1.6 (SO4)3 core-shell structured material.

[0056] Figure 1 for Na 2.8 Fe 1.6 (SO4)3 / Na 2.8 Ba 1.6 TEM image of the (SO4)3 material interface, showing a uniform layer of Na on its surface. 2.8 Ba 1.6 The (SO4)3 coating layer, due to subtle differences in the core and shell structures, exhibits certain grain boundaries, making it clearly visible in Na. 2.8 Fe 1.6 (SO4)3 material core surface. Na was analyzed using a specific surface area analyzer. 2.8 Fe 1.6 (SO4)3 / Na 2.8 Ba 1.6 The (SO4)3 material test results in Table 1 show that the specific surface area of ​​this material is only 5.8 m². 2 / g, slightly lower than comparative example 1, indicating that Na 2.8 Ba 1.6 The formation of the (SO4)3 coating layer creates a pore-filling effect on the material surface, effectively reducing surface porosity and area. This reduces the probability of the material core contacting moisture in the air, significantly improving its air stability. Simultaneously, the lower specific surface area also helps reduce side reactions between the material interface and the electrolyte under high voltage, improving the stability of the material interface structure. Refined XRD curves revealed that Na… 2.8 Fe 1.6 (SO4)3 / Na 2.8 Ba 1.6 The (SO4)3 material has a phase purity of up to 98.7%, exhibiting an almost perfect crystal structure. Furthermore, after being stored at 80% humidity for one week, its phase purity remained almost unchanged, indicating that moisture in the air did not participate in the structural transformation reaction. In contrast, the uncoated material in Example 1 had largely transformed into a hydrated phase, indicating that Na... 2.8 Ba 1.6 The presence of the (SO4)3 coating layer effectively isolates the material from the interaction between the material and moisture to a certain extent, thereby improving the material's air stability.

[0057] Will Na 2.8 Fe 1.6 (SO4)3 / Na 2.8 Ba 1.6The (SO4)3 material, AB, PVDF were mixed in a mass ratio of 9.2:0.4:0.4 to prepare slurry, and then the black slurry was coated on the aluminum foil using a 150um four-side coater, and then the film was dried in a vacuum drying oven at 100°C for 2 hours. The electrode film was punched into a circular sheet with a radius of 0.6mm using a puncher, and then a CR2016 button cell was assembled in a glove box using sodium metal as the counter electrode, 1mol / L NaClO4 EC+DEC (1:1 vol%)+5% FEC as the electrolyte, and PP / PE / PP three-layer separator.

[0058] The button cell was subjected to constant current charge-discharge test, and the current density was 0.1C (1C=110mAh / g). The results in Table 1 show that the reversible specific capacity of the electrode is 98.7mAh / g in the voltage range of 2.0-4.5V, and the capacity utilization rate is high, which is related to the high purity of the material. The higher the phase purity of the material, the higher the crystallinity, the more the number of effective sodium ions, and the higher the capacity. Compared with Comparative Example 1, the capacity of the electrode does not change much, which shows that the existence of the interface coating layer has little effect on the capacity, which is related to the fact that Na 2.8 Ba 1.6 The results in Table 1 show that in the voltage range of 2.0-4.3V, the capacity retention rate of the electrode is as high as 98.2% after 500 cycles at 1C rate, which shows excellent structural stability and cycle stability compared with Comparative Example 1 (78.9%), which shows that the existence of the interface coating layer effectively reduces the structural cracking of the material due to the volume expansion of the deintercalated sodium, and the coating layer reduces the specific surface area of the material, which to some extent weakens the side reaction between the material and the electrolyte, effectively inhibits the dissolution of the material interface, and thus improves the cycle stability. Further, we tested the Na 2.8 Fe 1.6 (SO4)3 / Na 2.8 Ba 1.6 The air stability of the (SO4)3 material was characterized, and it was found that the reversible specific capacity of the material was 97.5mAh / g after being stored in an 80% humidity environment for one week, and the capacity attenuation was small, which was consistent with the phase purity after storage in humid air. The integrity of the material structure was maintained, and its electrochemical performance was almost not attenuated, while the capacity of the material without coating in Comparative Example 1 was only 43.1mAh / g, which was severely attenuated, which showed that the Na 2.8 Ba 1.6 The coating layer of the (SO4)3 material can effectively isolate the reaction between the moisture in the air and the core of the material, thereby avoiding the decomposition of the core of the sulfate material, greatly improving the storage stability of the material, and being conducive to its large-scale preparation and promotion.

[0059] Application Example 2 Na2.4 Fe 1.8 (SO4)3 / Na 2.4 Ca 1.8 Synthesis of (SO4)3 shell-core material and its electrochemical performance

[0060] Na 2.4 Fe 1.8 (SO4)3 / Na 2.4 Ca 1.8 The method for preparing (SO4)3 shell-core material comprises the following steps:

[0061] Step 1: Dissolve ferric sesquioxide, sulfuric acid and sodium hydroxide in water according to the molar ratio of 0.9:3:2.4, add butyl hydroxy anisole (7% of the weight of ferric sesquioxide), polyacrylic acid (6% of the weight of ferric sesquioxide), and a small amount of graphene as a conductive agent (3% of the weight of ferric sesquioxide), and generate a uniform black slurry after sufficient reaction;

[0062] Step 2: Spray dry the slurry under nitrogen protection, and control the outlet air temperature to be ≥100°C, so as to obtain a uniform precursor powder;

[0063] Step 3: Under nitrogen protection, calcine the precursor powder at 400°C for 8H to obtain a high-crystallinity Na 2.4 Fe 1.8 (SO4)3 material;

[0064] Step 4: Add the Na 2.4 Fe 1.8 (SO4)3 material core and calcium chloride (added in an amount of 3% of the weight of Na 2.4 Fe 1.8 (SO4)3) into a mixed solution of water and diethyl ether (water:diethyl ether weight ratio is 0.7:99.3), so that the material interface iron element and strontium element undergo a displacement reaction, and a pre-coated precursor is obtained by filtration;

[0065] Step 5: Further calcine the pre-coated precursor under nitrogen protection at 440°C, so as to promote the Na 2.8 Ca 1.6 (SO4)3 coating layer to grow by melting, thereby obtaining a Na 2.4 Fe 1.8 (SO4)3 / Na 2.4 Ca 1.8 (SO4)3 shell-core structure material.

[0066] Use a specific surface area analyzer to analyze the Na 2.4 Fe 1.8 (SO4)3 / Na2.4 Ca 1.8 The results of the (SO4)3material test, Table 1, show that the specific surface area of the material is only 4.6m 2 / g, lower than the result in Comparative Example 2, indicating that the Na 2.4 Ca 1.8 The (SO4)3coating layer is more fully ion-exchanged with the Na 2.4 Fe 1.8 The ion exchange of the (SO4)3material core surface is more complete, which forms a certain hole-filling effect on the material surface, making the material surface more dense, and the porosity and surface area of the material are correspondingly reduced, which is conducive to weakening the contact between the material and moisture in the air, and greatly helps to improve the air stability of the material. In addition, the lower specific surface area of the material is also conducive to reducing the side reaction of the material interface with the electrolyte under high pressure, and improving the interface structure stability of the material. XRD curve refinement found that the Na 2.4 Fe 1.8 (SO4)3 / Na 2.4 Ca 1.8 The phase purity of the (SO4)3material is as high as 99.2%, the crystallinity is higher, and the crystal form is more complete. At the same time, after the material is stored in an 80% humidity environment for one week, the phase purity changes little, and no obvious hydration impurity phase is generated, indicating that the existence of the interface coating layer effectively blocks the erosion of moisture in the air on the material structure, thereby improving the air stability of the material.

[0067] The Na 2.4 Fe 1.8 (SO4)3 / Na 2.4 Ca 1.8 The (SO4)3material, AB, and PVDF are mixed at a mass ratio of 9.2:0.4:0.4 to prepare a slurry, and after grinding uniformly, the black slurry is coated on an aluminum foil using a 150um four-side coater, and then the film is dried in a 100°C vacuum drying oven for 2 hours. The electrode film is punched into a circular sheet with a radius of 0.6mm using a sheet puncher, and a metal sodium is used as a counter electrode, 1mol / L NaClO4EC+DEC(1:1vol%)+5%FEC is used as an electrolyte, and a PP / PE / PP three-layer separator is used as a separator, to assemble a CR2016 type button cell in a glove box.

[0068] The constant current charge-discharge test was carried out on the above-mentioned button cell, and the current density was 0.1C (1C = 110 mAh / g). The results in Table 1 show that the reversible specific capacity of the electrode is 103.5 mAh / g in the voltage range of 2.0-4.5V, and the capacity is high, which is related to the high phase purity of the material. The higher the completeness of the crystal structure, the more conducive to the extraction of sodium ions in the structure, and the higher the capacity. Compared with Comparative Example 2, the capacity of the electrode does not change much, which shows that the existence of the interface coating layer has little effect on the capacity. This is related to the fact that Na 2.4 Ca 1.8 The thickness of the (SO4)3coating layer itself is thin. In addition, the results in Table 1 show that in the voltage range of 2.0-4.3V, the capacity retention rate of the electrode is as high as 98.8% after 500 cycles at 1C rate, which shows excellent cycle stability compared with Comparative Example 2 (80.1%). On the one hand, the coating layer inhibits the volume expansion of the core material during the extraction and insertion of sodium, reducing the risk of material cracking. On the other hand, the existence of the interface energy coating layer reduces the specific surface area of the material, to some extent, weakens the side reaction between the material and the electrolyte, effectively inhibits the dissolution of the material interface, and thus improves the cycle stability. Further, we tested the Na 2.4 Fe 1.8 (SO4)3 / Na 2.4 Ca 1.8 The air stability of the Na 2.4 Ca 1.8 The (SO4)3coating layer avoids the decomposition of the sulfate material core in the air and improves the storage stability.

[0069] Comparative Example 1 Na 2.8 Fe 1.6 Synthesis of the Na

[0070] The Na 2.8 Fe 1.6 The preparation method of the Na

[0071] Step 1: Dissolve elemental iron powder, sulfuric acid, and sodium carbonate in water according to the molar ratio of 1.6:3:1.4, add ascorbic acid (3% by weight of elemental iron powder), polyethylene glycol 400 (5% by weight of elemental iron powder), and a small amount of carbon nanotubes as a conductive agent (7% by weight of elemental iron powder), and generate a uniform black slurry after sufficient reaction;

[0072] Step 2: Spray dry the slurry under nitrogen protection, with an outlet temperature control of ≥80°C, to obtain a uniform precursor powder; Step 3: Calcine the precursor powder at 420°C under nitrogen protection to promote material melting and crystalline growth, and naturally cool to obtain Na 2.8 Fe 1.6 (SO4)3material.

[0073] Figure 2 Na 2.8 Fe 1.6 (SO4)3material interface TEM image, which is smooth on the surface and does not have any coating layer. The specific surface area analyzer is used to test the Na 2.8 Fe 1.6 (SO4)3material, and the results in Table 1 show that the specific surface area of the material is 7.4 m 2 / g, which is larger than that of Application Example 1, indicating that there is a certain pore structure in the interface of the material during sintering. The existence of this pore will increase the specific surface area of the material, resulting in a larger viscosity during processes such as uniform slurry coating, which affects processing. At the same time, a larger specific surface area will also increase the side reaction between the material and the electrolyte under high pressure, causing the decomposition of the material interface structure and the rock-rockization, affecting the cycle stability of the material. XRD curve refinement found that the phase purity of the Na 2.8 Fe 1.6 (SO4)3material is as high as 98.9%, and the crystal form is relatively complete and consistent with Application Example 1. In addition, after storing the material in an 80% humidity environment for one week, the phase purity changes greatly, only 42.3%, and the high deterioration of phase purity is related to the structure transformation into a hydrated phase, indicating that the uncoated material in the humid air will spontaneously penetrate the water into the bulk phase inside the material through the ion diffusion channel in the structure, and then cause the generation of hydrated phase, which seriously affects the storage and batch industrial application of the material.

[0074] Na 2.8 Fe 1.6The (SO4)3material, AB, and PVDF were mixed in a mass ratio of 9.2:0.4:0.4 to prepare a slurry, which was uniformly ground and then coated on an aluminum foil using a 150-um four-side coater. The film was then dried in a vacuum drying oven at 100°C for 2 hours. The electrode film was punched into a circular sheet with a radius of 0.6 mm using a puncher, and a CR2016 button cell was assembled in a glove box using metallic sodium as the counter electrode, 1 mol / L NaClO4EC+DEC (1:1 vol%)+5% FEC as the electrolyte, and a PP / PE / PP three-layer separator.

[0075] The button cell was subjected to constant current charge-discharge testing at a current density of 0.1C (1C=110mAh / g). The results in Table 1 show that the reversible specific capacity of the electrode was 99.2mAh / g in the voltage range of 2.0-4.5V, and the capacity was close to that of Application Example 1, which was related to the high phase purity and crystallinity of the material. In addition, the results in Table 1 show that the capacity retention rate of the electrode was only 78.9% after 500 cycles at a 1C rate in the voltage range of 2.0-4.3V, which was a large decrease in cycle stability compared to Application Example 1, which was possibly related to the structural cracking caused by volume expansion during the sodium extraction process of the material. In addition, the material had a larger specific surface area than Application Example 1, which to some extent increased the side reactions between the material interface and the electrolyte, resulting in an increase in the solubility of the material interface and a decrease in the cycle stability. Further, we tested the Na 2.8 Fe 1.6 The air stability of the (SO4)3material was characterized, and it was found that the reversible specific capacity of the material was only 43.1mAh / g after being stored in an 80% humidity environment for one week, which was a serious performance degradation. This was consistent with the above result that the phase purity of the material decreased after being stored in humid air, indicating that the material had strong water absorption, and water molecules in the air could be inserted into the material structure along the ion diffusion channel, thereby causing the material to be converted into a hydrated phase, the sodium storage sites were deactivated, the redox properties of the transition metal were reduced, and the performance was greatly reduced.

[0076] The Na 2.4 Fe 1.8 Synthesis of (SO4)3material and its electrochemical performance

[0077] The Na 2.4 Fe 1.8 The preparation method of the (SO4)3material includes the following steps:

[0078] Step 1: Dissolve ferric oxide, sulfuric acid, and sodium hydroxide in water according to the molar ratio of 0.9:3:2.4, add butyl hydroxy anisole (7% of ferric oxide by weight), polyacrylic acid (6% of ferric oxide by weight), and a small amount of graphene as a conductive agent (3% of ferric oxide by weight), and generate a uniform black slurry after sufficient reaction;

[0079] Step 2: Spray dry the slurry under nitrogen protection, with an outlet temperature control of ≥100°C, to obtain a uniform precursor powder; 2.4 Fe 1.8 (SO4)3material.

[0080] The specific surface area analyzer is used to test the Na 2.4 Fe 1.8 (SO4)3material, and the results in Table 1 show that the specific surface area of the material is 7.1 m 2 / g, which is larger than that of Application Example 2, indicating that the material has more interfacial pores and a larger specific surface area, which will have a greater impact on the storage, processing, and structural stability of the material during the cycling process. At the same time, a larger specific surface area will increase the side reactions between the material and the electrolyte under high pressure, causing the decomposition of the material interface structure and the rock-rockization, affecting the cycling stability of the material. XRD curve refinement found that the phase purity of the Na 2.4 Fe 1.8 (SO4)3material is as high as 99.4%, with high crystallinity and complete crystal form, which is consistent with Application Example 2, indicating that the bulk structure is the same. In addition, after storing the material in an 80% humidity environment for a week, the phase purity changes greatly, only 45.3%, and the degradation of phase purity indicates that the structure has undergone irreversible changes, which is related to the intrusion of water in the humid air, causing the material to be converted into a hydrated phase, which seriously affects the storage and batch industrial application of the material.

[0081] The Na 2.4 Fe 1.8 (SO4)3material, AB, and PVDF are mixed according to a mass ratio of 9.2:0.4:0.4 to prepare a slurry, which is ground uniformly and then coated on an aluminum foil using a 150um four-side preparation device. The film is then dried in a vacuum drying oven at 100°C for 2 hours. The electrode film is punched into a circular sheet with a radius of 0.6mm using a sheet punching machine, and a metal sodium is used as the counter electrode, 1 mol / L NaClO4 EC+DEC (1:1 vol%)+5% FEC is used as the electrolyte, and a PP / PE / PP three-layer separator is used as the separator. A CR2016 type button cell is assembled in a glove box.

[0082] The constant current charge-discharge test was performed on the above-mentioned button cell, and the current density was 0.1C (1C = 110 mAh / g). The results in Table 1 show that the reversible specific capacity of the electrode is 104.3 mAh / g in the voltage range of 2.0-4.5V, and the capacity development is close to that of application example 2, which is consistent with the high purity of the material. It is shown that under the same phase purity, the capacity development of the material is small. In addition, the results in Table 1 show that in the voltage range of 2.0-4.3V, the capacity retention rate of the electrode is only 80.1% after 500 cycles at 1C rate, which is larger than that of application example 2. The stability decreases, which may be related to the volume change caused by the structure expansion / contraction during the sodium extraction process of the material, resulting in the cracking of the crystal. The cracking of the crystal will cause the exposure of the new interface, and then accelerate the side reaction of the interface and the electrolyte, resulting in continuous deterioration of the cycle. In addition, the material has a larger specific surface area than application example 2, which will increase the side reaction of the material interface and the electrolyte to a certain extent, resulting in the increase of the solubility of the material interface and the decrease of the cycle stability. Further, the air stability of the Na 2.4 Fe 1.8 The air stability of the (SO4)3 material was characterized, and it was found that the reversible specific capacity of the material was only 52.1 mAh / g after storing the material in 80% humidity environment for one week, and the performance decayed obviously, which was consistent with the above-mentioned result that the phase purity of the material decreased after storing in humid air. It is shown that the material has poor stability, and in humid air, water molecules will slowly diffuse into the material structure along the ion diffusion channel, and then cause the material to be converted into a hydrated phase, resulting in the loss of sodium storage sites and a significant decrease in performance.

[0083] Table 1 performance test results

[0084]

[0085] The above examples are only specific embodiments of the present application, which are described in detail, but they cannot be interpreted as limiting the scope of the present patent. It should be noted that for ordinary skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, and these obvious alternative forms all belong to the protection scope of the present application.

Claims

1. A high-stability sodium-ion battery cathode material of a sulfate salt, characterized in that: The positive electrode material is a core-shell structure, comprising an interface coating layer and a sulfate material core, the interface coating layer is Na 2+2x M 2-x (SO4)3; the sulfate material core is Na 2+2x Fe 2-x (SO4)3; wherein M is one or two or more of alkaline earth metal elements Ca, Ba, and Sr; and x is in the range of 0.2 or 0.

4.

2. A method for preparing the high-stability positive electrode material of a sodium ion battery of the sulfate salt according to claim 1, characterized in that The method comprises the following steps: S1, preparation of a precursor mixture: wet mixing of an iron source, sulfuric acid, an antioxidant, a sodium source, a carbon source, and a dispersant to obtain a uniform precursor mixture; the antioxidant is one or more than two of ascorbic acid and its derivatives, tea polyphenols, butylated hydroxyanisole, tert-butyl hydroquinone, phytic acid and its derivatives, butylated hydroxytoluene, and ethoxyquinoline; the dispersant is one or more than two of polyacrylic acid series, polyurethane compounds, polyethylene glycol series, methyl cellulose and its derivatives, polyvinyl alcohol series, and polyvinylpyrrolidone series S2, separation of the precursor powder: drying the precursor mixture under a protective atmosphere to obtain a precursor powder; S3, calcination of the sulfate material core: calcining the precursor powder in a protective atmosphere to obtain a high-crystallinity sulfate material core, the calcination temperature being 350-450℃; S4, preparation of a pre-coated precursor: wet mixing of the sulfate material core and a metal M source, and filtering to obtain a pre-coated precursor; the solvent system for wet mixing is a mixture of water and an organic solvent, the mass ratio of water to the organic solvent being 0.1-5%; S5, high-temperature sintering of the interface coating layer: high-temperature sintering of the pre-coated precursor under a protective atmosphere to obtain a sulfate sodium-ion battery cathode material with stable structure, the high-temperature sintering temperature being 400-450℃.

3. The method of claim 2, wherein the method further comprises: adding a sodium source to the mixture to form a sodium-containing mixture; and heating the sodium-containing mixture to form the high-stability sodium-ion battery cathode material. In step S4, the solvent system for wet mixing is a mixture of water and an organic solvent, the organic solvent being one or more than two of alcohol compounds, ether compounds, aromatic hydrocarbon compounds, aliphatic hydrocarbon compounds, ester compounds, cyclic hydrocarbon compounds, halogenated hydrocarbon compounds, ketone compounds, glycol derivatives, acetonitrile, pyridine, and phenol; the alcohol compounds are one or more than two of methanol, ethanol, and isopropyl alcohol, the ether compounds are one or more than two of diethyl ether and propylene oxide, the aromatic hydrocarbon compounds are one or more than two of benzene, toluene, and xylene, the ester compounds are one or more than two of methyl acetate, ethyl acetate, and propyl acetate, the aliphatic hydrocarbon compounds are one or more than two of pentane, hexane, and octane, the cyclic hydrocarbon compounds are one or more than two of cyclohexane, cyclohexanone, and toluene cyclohexanone, the halogenated hydrocarbon compounds are one or more than two of chlorobenzene, dichlorobenzene, and dichloromethane, the ketone compounds are one or more than two of acetone, methyl butanone, and methyl isobutyl ketone, and the glycol derivatives are one or more than two of ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, and ethylene glycol monobutyl ether.

4. The method of claim 2, wherein the method is characterized by: In step S2, the drying method is one or more than two of vacuum drying, freeze drying, air blowing drying, spray drying, flash drying, vacuum drying, and natural evaporation; In step S4, the filtering method is one or more than two of pressure filtration, vacuum suction filtration, and normal pressure filtration; In steps S2, S3, and S5, the protective atmosphere is one or more than two of nitrogen, argon, nitrogen-hydrogen, and argon-hydrogen.

5. The method of claim 2, wherein the method further comprises: adding a sodium source to the mixture of step (a) to form a mixture; and heating the mixture to form the high-stability sodium-ion battery cathode material. The metal M source is one or more of a calcium-containing compound, a barium-containing compound, and a strontium-containing compound, the calcium-containing compound is one or more of calcium chloride, calcium chromate, calcium nitrate, and calcium sulfite, the barium-containing compound is one or more of barium chloride, barium nitrate, barium hydroxide, and barium bicarbonate, and the strontium-containing compound is one or more of strontium chloride, strontium nitrate, and strontium perchlorate.

6. The process for the preparation of a high-stability sodium-ion battery cathode material of the sulfated class according to claim 5, characterized in that: The iron source is one or more of elemental iron powder, ferrous oxide, ferric oxide, magnetite, hydroxyl ferric oxide, and iron hydroxide, the sodium source is one or more of sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium citrate, sodium oxalate, sodium formate, sodium acetate, sodium sulfate, and sodium nitrate, and the carbon source is one or more of graphite and derivatives thereof, carbon nanotubes and slurry thereof, graphene and slurry thereof, and carbon black and derivatives thereof.

Images