Composite sodium source sodium ferric sulfate positive electrode material as well as preparation method and application thereof

Abstract

The invention provides a composite sodium source sodium ferric sulfate positive electrode material and a preparation method and application thereof, the chemical formula of the composite sodium source sodium ferric sulfate positive electrode material is NaxFey (SO4) n, 2 < = x < = 3, 1 < = y < = 2, 1 < = x / y < = 2, 2 < = n < = 3; a composite sodium source, an iron source, a carbon source, an antioxidant and a dispersion medium are mixed according to a stoichiometric ratio, and the mixture is subjected to heat treatment to obtain the composite sodium source sodium ferric sulfate positive electrode material. The composite sodium source is a mixture of reducing sodium salt and non-reducing sodium salt. According to the preparation method provided by the invention, the reductive sodium salt in the composite sodium source is promoted to be oxidized through solid-phase mixing, and a reductive environment is provided; and meanwhile, the time of converting the raw materials from simple mixing to a target crystal form is shortened by utilizing the synergistic effect of friction heat of solid-phase mixing and oxidation heat release of reducing sodium salt, and the method has the advantage of remarkably reducing energy consumption. The synthesized composite sodium source sodium ferric sulfate positive electrode material has the advantages of relatively high capacity, relatively low impurity content and relatively high crystallinity.

Description

Technical Field

[0001] The present invention relates to the technical field of battery cathode materials, and particularly relates to a composite sodium source sodium iron sulfate cathode material, a preparation method thereof, and an application thereof. Background Art

[0002] With the continuous advancement of the global energy structure transformation, sodium-ion batteries, as a new energy storage technology, are gradually becoming an important part of the future sustainable energy system. Sodium-ion cathode materials are a key component of sodium-ion batteries, and they directly affect key performance such as the energy density, cycle performance, and rate performance of the batteries. Polyanion-type cathode materials have better cycle stability and safety, as well as higher working voltages, compared with layered oxides and Prussian blue compounds. According to different anion groups, polyanion-type materials are mainly divided into phosphates, fluorophosphates, pyrophosphates, sulfates, silicates, etc. Among them, sodium iron sulfate has a higher working voltage and an extreme cost advantage. However, the sodium iron sulfate cathode material has problems of poor thermal stability and poor conductivity. At the same time, during the synthesis process, divalent iron is easily oxidized to trivalent iron, resulting in a deviation in the sodium / iron ratio, increasing the content of iron-containing oxides, and affecting the crystallinity and purity of the material.

[0003] How to inhibit the oxidation of divalent iron has become an important issue for sodium iron sulfate cathode materials.

[0004] Jiangsu Zhongna Energy Technology Co., Ltd. has disclosed a preparation method of a sodium iron sulfate / carbon composite cathode material, a cathode material, a cathode electrode sheet, and a sodium-ion battery (CN117154062A). During the preparation of the precursor, a reducing metal element is introduced to increase the proportion of reversible redox ions.

[0005] Shenzhen Institute of Advanced Technology has disclosed sodium iron sulfate and a preparation method thereof (CN116553621B). In the preparation method, sodium sulfite or sodium bisulfite is proposed to be added as an antioxidant to the microwave solvothermal reaction to prepare sodium iron sulfate.

[0006] The above preparation method proves that during the preparation of the cathode material for sodium-ion batteries, adding reducing substances (such as metallic elements) can effectively inhibit the oxidation of divalent iron (Fe²⁺), thereby increasing the proportion of reversible redox active ions in the material. However, the introduced metallic elements may participate in the electrochemical reaction, change the electronic conduction characteristics of the material or occupy the sodium-ion diffusion channels, which may have an adverse effect on the discharge platform voltage and electrochemical stability. On the other hand, if only sodium sulfite or sodium bisulfite is used as the antioxidant, the additional sodium ions generated by its decomposition will interfere with the sodium / iron (Na / Fe) stoichiometry in the material, possibly leading to the formation of lattice defects or unexpected phases, thus reducing the specific capacity or cycle stability of the material. Therefore, the selection of reducing substances needs to balance the accuracy of iron valence regulation and the stoichiometry to avoid negative impacts on the electrochemical performance of the material. Summary of the Invention

[0007] The purpose of the present invention is to provide a composite sodium source sodium iron sulfate cathode material, its preparation method and application. The composite sodium source sodium iron sulfate cathode material synthesized by the present invention has the advantages of high capacity, high crystallinity and purity, and a simple synthesis method.

[0008] To solve the above technical problems, the technical solution adopted by the present invention is: A composite sodium source sodium iron sulfate cathode material, the chemical formula of the composite sodium source sodium iron sulfate cathode material is Na x Fe y (SO4) n , where 2 ≤ x ≤ 3, 1 ≤ y ≤ 2, 1 ≤ x / y ≤ 2, 2 ≤ n ≤ 3; The composite sodium source sodium iron sulfate cathode material is obtained by mixing a composite sodium source, an iron source, a carbon source, an antioxidant and a dispersion medium according to the stoichiometric ratio and heat-treating the mixture.

[0009] Among them, the composite sodium source is a mixture of a reducing sodium salt and a non-reducing sodium salt.

[0010] Furthermore, the reducing sodium salt is one or a combination of sodium sulfide, sodium hydrosulfide, sodium thiosulfate, sodium sulfite, sodium bisulfite, sodium dithionate, sodium hydroxymethanesulfonate, sodium sulfonate, sodium oxalate, etc.; the non-reducing sodium salt is sodium sulfate in sodium sulfate and industrial raw materials.

[0011] Furthermore, the molar ratio of the reducing sodium salt to the non-reducing sodium salt is (0.01~1):1.

[0012] Among them, The iron source is iron sulfate in iron sulfate and industrial raw materials; Na x Fe y Mz (SO4) n It belongs to the Alluaudite-type structure, usually monoclinic system, and the space group is C2 / c. Its three-dimensional framework is formed by the vertex or edge sharing connection of FeO6 octahedra and SO4 tetrahedra, and Na⁺ ions are located in the void channels of the framework.

[0013] Introducing a small amount of sulfite during the solid-phase mixing process can reduce the divalent iron ions oxidized due to heat generation, promote the uniformity of sodium and iron distribution, and promote the crystal structure transformation. The content of the reducing sodium source required increases with the increase of the mixing time and mixing intensity. However, if the addition amount does not match the mixing time and intensity, resulting in excessive addition of reducing sodium salts such as sodium sulfite, the excessive sulfite replacing sulfate will cause local lattice distortion and reduce the structural stability; it may introduce additional redox activity, resulting in the oxidation of SO3²⁻ to SO4²⁻ during the electrochemical process, releasing gas or triggering side reactions, leading to the attenuation of the material capacity; the valence state difference of S may force the adjustment of the oxidation state of Fe, introducing mixed valence states or vacancy defects, significantly affecting the stability and electrochemical performance of the material.

[0014] Among them, The mixing methods include one or a combination of more of oscillation, grinding, ball milling, and sand milling in solid-phase mixing.

[0015] Furthermore, the ratio of the raw material to the ball weight is 1:(25~100), and the mixing time is 0.5~8h.

[0016] Further defined, the carbon source includes one or a combination of more of carbon nanotubes, expanded graphite, graphite oxide, reduced graphite oxide, conductive carbon black, activated carbon, polyaniline, polyvinylpyrrolidone, polypyrrole, glucose, and starch; the antioxidant includes one or a combination of more of ascorbic acid, citric acid, and hydroquinone.

[0017] Furthermore, the total molar ratio of sodium ions to ferrous ions in the composite sodium source and iron source is (2~1):1; the carbon source accounts for 1wt%~15wt% of the total mass of the composite sodium source, iron source, and carbon source; the antioxidant accounts for 1wt%~5wt% of the mass of the iron source. Limiting the ratio of the sodium source / iron source within this range is to synthesize a material belonging to the Alluaudite-type structure, and the material of this structure has a relatively high theoretical specific capacity (100~120mAh / g), which can provide a higher energy density.

[0018] The present invention also discloses a preparation method for a composite sodium source sodium iron sulfate cathode material, and the specific preparation method is as follows: Mix the composite sodium source, iron source, carbon source, antioxidant and dispersion medium according to the stoichiometric ratio. After fully grinding in a ball mill, sinter at a high temperature of 300°C to 500°C for 6h to 24h in a protective atmosphere. After heat preservation, cool naturally and then grind.

[0019] The present invention also discloses an application of the composite sodium source sodium iron sulfate cathode material, and the composite sodium source sodium iron sulfate cathode material is used to prepare a sodium ion battery.

[0020] Compared with the prior art, the present invention has the following beneficial effects: In the present invention, a reducing sodium salt with a lower standard electrode potential is used as a sacrificial agent to construct a composite sodium source. Utilizing the frictional heat generated by the high-speed rotation of the equipment during the solid-phase mixing process, the reducing sodium salt is oxidized to a non-reducing sodium salt, providing a more reducing environment and hindering the oxidation of divalent iron accelerated by heat. At the same time, the synergistic effect of the frictional heat generated by mechanical movement and the exothermic oxidation of the reducing sodium salt improves the uniformity of the distribution of sodium and iron in the precursor, promotes the transformation of the material into the target crystal form, and has the advantages of shortening the mixing time and reducing energy consumption; since the sodium ions directly come from the preset composite sodium source, the sodium-iron ratio (Na / Fe) of the product can be accurately regulated, avoiding the interference of external sodium sources, thereby improving the batch consistency of the material. This method has the advantages of process controllability and economy while ensuring the electrochemical performance of the material. Description of the Drawings

[0021] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for use in the embodiments. It should be understood that the following drawings only show some embodiments of the present invention, so they should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can also be obtained based on these drawings without creative efforts.

[0022] Figure 1 It is the X-ray diffraction (XRD) pattern of the precursor and the sintered sample of the composite sodium source sodium iron sulfate cathode material in Example 1 and Comparative Example 1 of the present invention.

[0023] Figure 2 It is the scanning electron microscope (SEM) image of the composite sodium source sodium iron sulfate cathode material in Example 1 of the present invention.

[0024] Figure 3 It is the scanning electron microscope (SEM) image of the composite sodium source sodium iron sulfate cathode material in Comparative Example 1 of the present invention.

[0025] Figure 4 It is the 0.1C first-cycle charge-discharge curve graph of the button cells made of the composite sodium source sodium iron sulfate cathode material in Examples 1, 2, 3 and Comparative Examples 1, 2 of the present invention. Detailed Embodiments

[0026] In the following, only some exemplary embodiments are briefly described. As those skilled in the art can recognize, the described embodiments can be modified in various different ways without departing from the spirit or scope of the embodiments of the present invention. Therefore, the accompanying drawings and the description are considered to be exemplary in nature rather than restrictive. The embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0027] Embodiment 1 In this embodiment, 12 mmol of sodium sulfate, 1 mmol of sodium sulfite, 17 mmol of ferrous sulfate heptahydrate, 1.66 g of graphite oxide, 0.477 g of ascorbic acid, and 40 g of ethanol were added to a ball milling tank, argon was introduced, the ball milling speed was set at 800 r / min, and ball milling was carried out for 2 h to obtain a black precursor.

[0028] The black precursor was placed in a corundum boat and heated to 400 °C at a rate of 5 °C / min, held for 8 h, and then naturally cooled to room temperature and ground into a powder to obtain a composite sodium source sodium iron sulfate cathode material. The chemical formula of the cathode material is Na 2.6 Fe 1.7 (SO4)3.

[0029] Embodiment 2 The difference between this embodiment and Embodiment 1 is that the ball milling time was adjusted to 4 h.

[0030] Embodiment 3 The difference between this embodiment and Embodiment 1 is that the ratio of the reducing sodium salt to the non-reducing sodium salt in the composite sodium source was adjusted from 0.083:1 to 0.04:1, that is, 12.5 mmol of sodium sulfate and 0.5 mmol of sodium sulfite were added.

[0031] Comparative Example 1 The difference between this Comparative Example 1 and Embodiment 1 is that no reducing sodium salt was added and only a single sodium source was used. At this time, the ratio of the reducing sodium salt to the non-reducing sodium salt was 0.

[0032] Comparative Example 2 The difference between this Comparative Example 2 and Comparative Example 1 is that the ball milling time was adjusted to 4 h.

[0033] Embodiment 4 This embodiment mainly discloses the application of a composite sodium source sodium iron sulfate cathode material. The composite sodium source sodium iron sulfate cathode material is used to prepare a sodium ion battery.

[0034] The preparation method of the sodium-ion battery includes: assembling the prepared composite sodium source sodium iron sulfate cathode material into a sodium-ion coin cell. The assembly method is as follows: mixing and grinding the composite sodium source sodium iron sulfate cathode material with conductive carbon black and binder PVDF, adding N-methylpyrrolidone and shaking evenly to prepare the cathode slurry; coating the cathode slurry on aluminum foil, drying for 12 h, and obtaining the cathode electrode sheet through rolling and cutting; using a sodium sheet as the counter electrode, glass fiber as the separator, and 1M NaClO4 in EC:PC = 1:1 Vol% with 5% FEC solution as the electrolyte, and assembling in a glove box filled with argon. The test conditions of the sodium-ion battery are: the test temperature is 30 °C, the working voltage is 2.0 - 4.5 V, and the current density is as marked in the following table.

[0035] The electrochemical test results of the sodium-ion batteries of different examples and comparative examples are as follows in the table See Figure 1 , the X-ray diffraction (XRD) patterns of the composite sodium source sodium iron sulfate cathode materials in Example 1 and Comparative Example 1. For the precursor, after 2 h of ball milling in Comparative Example 1 without adding a reducing sodium salt, there were still relatively strong peaks of ferrous sulfate heptahydrate and sodium sulfate (20°, 23°, 28°, 34°, etc.). At this time, the solid-phase mixing process only showed a simple mixing of raw materials and did not result in a crystal structure transformation; in Example 1 and Example 2 using the composite sodium source, a pre-reaction occurred during the short-time (2 h, 4 h) solid-phase mixing to form intermediate products. For the sintered samples, under the same heat treatment conditions, Example 1 using the composite sodium source had higher crystallinity and fewer impurity phases compared to Comparative Example 1.

[0036] See Figure 2 , Figure 3 , the scanning electron microscope (SEM) images of Example 1 and Comparative Example 1. At the same ball milling time, after adding the composite sodium source, the fine particles on the material surface decreased, the phase separation phenomenon was greatly weakened, and good connections were formed between the particles, which was beneficial to the transmission of sodium ions. In Comparative Example 1 using a single sodium source, due to the lack of a reducing environment and insufficient solid-phase mixing time, the distribution of iron and sodium was uneven, resulting in sodium-rich phases and iron-containing oxides, leading to a significant reduction in material performance.

[0037] See Figure 4, the first charge-discharge curves of the button cells made of the sodium iron sulfate cathode material with the composite sodium source in Examples 1, 2, 3 and Comparative Examples 1, 2. After adding the composite sodium source, the first charge-discharge specific capacities of the synthesized sodium iron sulfate cathode material in this scheme have all increased, from 53.07 mAh / g of the single sodium source to 92.78 mAh / g of the composite sodium source, approaching the theoretical capacity. Comparative Example 1 and Comparative Example 2 with a single sodium source show that when the reducing sodium source is not added, longer solid-phase mixing time is required to bring about performance improvement, but such improvement is still less than that of Examples 1, 2, 3.

[0038] The present invention adds a reducing sodium salt to construct a composite sodium source, which can use the reducing sodium salt with a lower standard electrode potential as a sacrificial agent. Utilizing the frictional heat generated by the high-speed operation of the equipment during the solid-phase mixing process, it promotes the oxidation of the reducing sodium salt to a non-reducing sodium salt, provides a more reducing environment, and hinders the oxidation of divalent iron accelerated by heat. At the same time, the synergistic effect of the frictional heat generated by mechanical movement and the exothermic oxidation of the reducing sodium salt improves the uniformity of sodium and iron distribution in the precursor, promotes the transformation of the material into the target crystal form, and has the advantages of shortening the mixing time and reducing energy consumption; since the sodium ions directly come from the preset composite sodium source, the sodium-iron ratio (Na / Fe) of the product can be accurately controlled, avoiding the interference of external sodium sources, thereby improving the batch consistency of the material. This method has the advantages of process controllability and economy while ensuring the electrochemical performance of the material.

[0039] Although the preferred embodiments of the present invention have been described, those skilled in the art can make additional changes and modifications once they know the basic creative concept. Therefore, the appended claims are intended to be construed to include the preferred embodiments and all changes and modifications falling within the scope of the present invention.

[0040] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. It should be noted that any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A composite sodium source sodium iron sulfate positive electrode material, characterized in that: The chemical formula of the composite sodium source sodium iron sulfate positive electrode material is Na x Fe y (SO4) n , where 2≤x≤3, 1≤y≤2, 1≤x / y≤2, 2≤n≤3; The composite sodium source, iron source, carbon source, antioxidant and dispersion medium are mixed according to a stoichiometric ratio, and the mixture is heat-treated to obtain a composite sodium source sodium iron sulfate positive electrode material.

2. A composite sodium source sodium iron sulfate positive electrode material according to claim 1, characterized in that: The composite sodium source is a mixture of a reducing sodium salt and a non-reducing sodium salt.

3. A composite sodium source sodium iron sulfate positive electrode material according to claim 2, characterized in that: The reducing sodium salt is one or more combinations of sodium sulfide, sodium hydrosulfide, sodium thiosulfate, sodium sulfite, sodium bisulfite, sodium dithionite, sodium hydroxymethane sulfonate, and sodium sulfonate; the non-reducing sodium salt is sodium sulfate and sodium sulfate contained in industrial raw materials.

4. A composite sodium source sodium iron sulfate positive electrode material according to claim 2, characterized in that: The molar ratio of reducing sodium salt to non-reducing sodium salt is (0.01~1):

1.

5. The composite sodium source sodium iron sulfate positive electrode material according to claim 1, characterized in that: The solid phase mixing method includes one or more combinations of oscillation, grinding, ball milling, and sand milling; the weight ratio of raw materials to balls is 1: (25~100), and the mixing time is 0.5~8h.

6. The composite sodium source sodium iron sulfate positive electrode material according to claim 1, characterized in that: The carbon source includes one or more combinations of carbon nanotubes, expanded graphite, graphite oxide, reduced graphite oxide, conductive carbon black, activated carbon, polyaniline, polyvinyl pyrrolidone, polypyrrole, glucose, and starch; the antioxidant includes one or more combinations of ascorbic acid, citric acid, and hydroquinone.

7. The composite sodium source sodium iron sulfate positive electrode material according to claim 1, characterized in that: The composite sodium source and the iron source satisfy a total molar ratio of sodium ions to ferrous ions of (2-1):1; the carbon source accounts for 1wt%-15wt% of the total mass of the composite sodium source, the iron source and the carbon source; and the antioxidant accounts for 1wt%-5wt% of the mass of the iron source.

8. A method for preparing a composite sodium source sodium iron sulfate positive electrode material, characterized in that: The preparation method is used to prepare the composite sodium source sodium iron sulfate positive electrode material according to any one of claims 1 to 7, and the specific preparation method is as follows: The composite sodium source, iron source, carbon source, antioxidant and dispersion medium are mixed according to a stoichiometric ratio, fully ground in a ball mill, sintered at a high temperature of 300°C to 500°C for 6h to 24h in a protective atmosphere, and then naturally cooled and ground after heat preservation.

9. An application of a composite sodium source sodium iron sulfate positive electrode material, characterized in that: The composite sodium source sodium iron sulfate positive electrode material is the composite sodium source sodium iron sulfate positive electrode material according to any one of claims 1 to 7, and is used to prepare a sodium ion battery.

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