Sodium ferrous sulfate material and preparation method thereof, positive pole piece and secondary battery

Abstract

The invention provides a sodium ferrous sulfate material and a preparation method thereof, a positive pole piece and a secondary battery, and belongs to the technical field of sodium ion batteries, the method comprises the following steps: mixing ferrite and sodium salt, and grinding to form a precursor; ball-milling and mixing the precursor, a carbon source and water in a protective gas atmosphere to obtain slurry; carrying out drying treatment on the slurry to obtain powder; in a protective gas atmosphere, sintering the powder in stages to obtain a sodium ferrous sulfate material; wherein the step-by-step sintering comprises the substep of sequentially carrying out first sintering, second sintering and third sintering on the powder, the temperature of the first sintering is 140-160 DEG C, the temperature of the second sintering is 290-310 DEG C, and the temperature of the third sintering is 370-390 DEG C. The invention aims to solve the technical problem that the existing sodium ferrous sulfate material has many impure phases.

Description

TECHNICAL FIELD

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

[0002] With the rapid increase in global demand for lithium resources and the continuous rise in the price of lithium ore, sodium ion batteries (SIBs) have attracted much attention due to the abundant sodium ion resources in the earth's crust and low prices. The polyanion-type sodium ferrous sulfate material has the advantages of high theoretical capacity, high voltage platform and low cost, and is suitable as a positive electrode material for sodium ion batteries.

[0003] At present, the commonly used preparation method of sodium ferrous sulfate material is to mix raw materials first and then sinter at high temperature. However, the sodium ferrous sulfate material prepared based on this method has the problem of many impurities. SUMMARY

[0004] In view of the technical problems in the background art, the present application provides a sodium ferrous sulfate material, a preparation method thereof, a positive electrode sheet and a secondary battery, aiming to solve the technical problem of many impurities in the existing sodium ferrous sulfate material.

[0005] In a first aspect, the present application provides a preparation method of a sodium ferrous sulfate material, comprising the following steps: mixing and grinding a ferrous salt and a sodium salt to form a precursor; ball-milling the precursor, a carbon source and water in a protective gas atmosphere to obtain a slurry; drying the slurry to obtain a powder; sintering the powder in a protective gas atmosphere in sections to obtain a sodium ferrous sulfate material; wherein the sintering in sections comprises sequentially performing first sintering, second sintering and third sintering on the powder, the temperature of the first sintering is 140-160℃, the temperature of the second sintering is 290-310℃, and the temperature of the third sintering is 370-390℃.

[0006] In the technical scheme of the embodiment of the present application, the high-temperature sintering process is divided into three-stage processes at specific sintering temperatures, which can separate the dehydration process and the carbonization and crystallization process, remove the crystal water before crystallization, ensure that the crystal water can be smoothly and fully discharged, thereby reducing the impurity phase in the sodium ferrous sulfate material, reducing the powder internal resistance, and improving the electrical conductivity of the material. At the same time, by optimizing the sintering temperature, the removal speed of the crystal water can also be controlled, thereby helping to reduce the pore size and make the pore distribution more uniform in the sodium ferrous sulfate material, thereby improving the green density of the material. On the other hand, by optimizing the sintering process and the sintering temperature of each stage, the sintering temperature gradually increases, which can accurately control the carbonization process, improve the crystallization effect, and improve the crystallinity, thereby better optimizing the electrical conductivity of the material. Overall, by improving the sintering process, the effects of reducing the impurity phase, reducing the powder internal resistance, improving the crystal integrity, and improving the electrical conductivity of the material are achieved. In this way, when the material is used to prepare the positive electrode sheet of the sodium-ion battery, the rate performance and the cycle performance of the battery can be improved.

[0007] In some embodiments, the first sintering time is 3-5h, the second sintering time is 3-6h, and the third sintering time is 8-12h.

[0008] In this embodiment, by optimizing the sintering time of each sintering process and controlling it within the above range, the sintering time can be shortened while ensuring the reaction of each sintering process, thereby improving the production efficiency.

[0009] In some embodiments, the carbon source includes ascorbic acid, graphene oxide, and carbon nanotubes.

[0010] In this embodiment, by optimizing the carbon source and adopting the strategy of three specific carbon sources for collaborative coating, the carbon layer structure of the sodium ferrous sulfate material is optimized, and the electrical conductivity of the material is greatly improved.

[0011] In some embodiments, the mass ratio of the precursor to the ascorbic acid, the graphene oxide, and the carbon nanotubes is 100:(0.3-0.7):(5-8):(2-4).

[0012] In this embodiment, by optimizing the amount of ascorbic acid (VC), graphene oxide (GO), and carbon nanotubes (CNT) and controlling it within the above ratio range, the effects of the three can be balanced, the effect of the synergistic effect can be improved, and the electrical conductivity and the green density of the material can be balancedly improved.

[0013] In some embodiments, the D50 particle size of the precursor is less than or equal to 2μm.

[0014] In the embodiment, the size of the precursor formed after grinding is controlled, so that the precursor is mixed more uniformly, and a better crystal structure is formed.

[0015] In some embodiments, the ferrous salt comprises one or more of ferrous sulfate, ferrous sulfate monohydrate, ferrous sulfate heptahydrate, ferrous oxalate, ferrous chloride, ferrous acetate, ferrous nitrate; and the sodium salt comprises at least one of anhydrous sodium sulfate, anhydrous sodium acetate, sodium oxalate, sodium thiosulfate.

[0016] In the embodiment, the above-mentioned compound is widely available and has good matching with other components, and is suitable for use as an iron source and a sodium source.

[0017] In some embodiments, the molar ratio of the iron element in the ferrous salt to the sodium element in the sodium salt is 1:(1-2).

[0018] In the embodiment, controlling the molar ratio of the iron element to the sodium element within the above range can promote the reaction to proceed fully and synthesize a sodium ferrous sulfate material of a better phase.

[0019] In some embodiments, in the step of mixing the ferrous salt and the sodium salt and grinding to form a precursor, a jet mill is used for grinding, the pressure of the jet mill is 0.2-0.5 MPa, and the grinding time is 0.3-0.6 h.

[0020] In the embodiment, the raw materials are ground and mixed by using a jet mill, and the conditions of the jet mill are optimized, which helps to promote the better dispersion and uniform mixing of the precursor.

[0021] In some embodiments, in the step of ball-milling the precursor, a carbon source, and water in a protective gas atmosphere to obtain a slurry, the ball-milling time is 4-8 h.

[0022] In the embodiment, the ball-milling time is controlled within the above range, which can ensure that the precursor and the carbon source are mixed uniformly.

[0023] In some embodiments, the drying method is freeze-drying.

[0024] In the embodiment, the freeze-drying method is used for drying, which can avoid the adverse effects of high temperature on the material, inhibit the oxidation of ferrous ions, and reduce the content of impurities.

[0025] In some embodiments, the freeze-drying comprises: pre-freezing at -70℃ to -50℃, and then drying at a temperature of -45℃ to -35℃ and a vacuum degree of less than or equal to 20 Pa for 18-24 h.

[0026] In this embodiment, the conditions for freeze-drying are optimized to be controlled within the above range, which can ensure that the slurry is fully dried while avoiding the temperature affecting the material.

[0027] In a second aspect, the embodiments of the present application provide a sodium ferrous sulfate material prepared by the above preparation method, the sodium ferrous sulfate material comprising an inner core and a carbon layer coated outside the inner core, and the material of the inner core comprising sodium ferrous sulfate.

[0028] In the technical solutions of the embodiments of the present application, the sodium ferrous sulfate material prepared by the above preparation method has a high phase purity, a small amount of impurity phase, a low powder internal resistance, and a high conductivity.

[0029] In some embodiments, the mass percentage of ferrous ions in the sodium ferrous sulfate material is greater than or equal to 98%.

[0030] In this embodiment, the sodium ferrous sulfate material has a high content of ferrous ions and a small amount of impurities.

[0031] In some embodiments, the mass percentage of the carbon layer is 2-4.3%.

[0032] In this embodiment, the content of the carbon layer is controlled within this range, which can effectively coat the surface of the inner core, prevent the inner core from being oxidized by the external environment, improve the phase purity and cycle stability, and avoid hindering the diffusion of sodium ions in the deintercalation process due to the excessive thickness of the carbon layer.

[0033] In some embodiments, the sodium ferrous sulfate material has a porous structure with a pore size of 100-300 nm.

[0034] In this embodiment, the sodium ferrous sulfate material has uniformly distributed and small pores, which not only helps to increase the contact area of the electrolyte and improve the efficiency of sodium ion transmission, but also has a high tap density.

[0035] In some embodiments, the raw material of the carbon layer comprises ascorbic acid, graphene oxide, and carbon nanotubes.

[0036] In this embodiment, the carbon layer formed based on the above raw materials has a continuous conductive network, has good conductivity, and has a high tap density.

[0037] In a third aspect, the embodiments of the present application provide a positive electrode sheet comprising the above sodium ferrous sulfate material or comprising the sodium ferrous sulfate material prepared by the above preparation method.

[0038] In this embodiment, the positive electrode sheet contains the above sodium ferrous sulfate material, and thus has the advantages of high rate performance and high cycle performance. In this embodiment, the positive electrode sheet contains the above sodium ferrous sulfate material, and thus has the advantages of high rate performance and high cycle performance. In this embodiment, the positive electrode sheet contains the above sodium ferrous sulfate material, and thus has the advantages of high rate performance and high cycle performance.

[0039] In a fourth aspect, the embodiments of the present application provide a secondary battery, which comprises the positive electrode plate as described above.

[0040] In this embodiment, the secondary battery comprises the positive electrode plate as described above, and thus has the advantages of high rate performance and high cycle performance.

[0041] In a fifth aspect, the embodiments of the present application provide a power consumption device, which comprises the secondary battery as described above.

[0042] In this embodiment, the power consumption device comprises the secondary battery as described above, and thus has the advantages of high rate performance and high cycle performance.

[0043] The above description is only a summary of the technical solutions of the present application. In order to make the technical means of the present application more clear, the embodiments of the present application can be implemented according to the content of the description, and in order to make the above and other purposes, characteristics and advantages of the present application more obvious and easy to understand, the following specific embodiments of the present application are described. BRIEF DESCRIPTION OF DRAWINGS

[0044] In order to more clearly illustrate the technical solutions of the present application, the following will briefly introduce the drawings used in the present application. Obviously, the drawings described below are only some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained without creative labor on the basis of these drawings.

[0045] Figure 1 The flowchart of the preparation method of the sodium ferrous sulfate material proposed in the embodiments of the present application; Figure 2 The CP-SEM image of the sodium ferrous sulfate material prepared in Example 1; Figure 3 The charge-discharge curve of the battery prepared by using the sodium ferrous sulfate material of Example 1; Figure 4 The rate curve of the battery prepared by using the sodium ferrous sulfate material of Example 1; Figure 5 The 1C cycle curve of the battery prepared by using the sodium ferrous sulfate material of Example 1. DETAILED DESCRIPTION

[0046] The embodiments of the technical solutions of the present application will be described in detail below with reference to the drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present application, and therefore only serve as examples, and cannot limit the protection scope of the present application.

[0047] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of this application; the use of the terms "including," "comprising," or "having" and variations thereof herein is intended to be broad and encompass the terms "consisting of" and "consisting essentially of" and variations thereof. Unless otherwise required by context, singular terms shall include pluralities and vice versa. Unless otherwise required by context, the use herein of the singular is also to be construed as a reference to the plural and vice versa.

[0048] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "multiple" is more than two, unless otherwise explicitly and specifically limited.

[0049] Reference herein to "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the application. The appearance of the phrase in various places in the specification is not necessarily all referring to the same embodiment, or to a particular embodiment, or to a particular set of embodiments, and is not necessarily mutually exclusive of other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0050] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of the associated objects, which means that there can be three relationships, for example, A and / or B, which means that there are three cases of A alone, A and B together, and B alone. In addition, the character " / " in this paper generally represents that the front and rear associated objects are a "or" relationship.

[0051] In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two), and similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

[0052] In the description of the embodiments of the present application, the technical terms "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc. The orientation or positional relationship indicated is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the embodiments of the present application and simplifying the description, and therefore cannot be understood as limiting the embodiments of the present application. The device or element indicated must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the embodiments of the present application.

[0053] In the description of the embodiments of the present application, unless otherwise explicitly specified and limited, the technical terms "mounting", "connecting", "connecting", "fixing" and other terms should be understood in a broad sense, for example, it can be fixedly connected, or it can be detachably connected, or it can be integrated; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium; it can be the internal communication of two elements or the interaction relationship between two elements. For those skilled in the art, the specific meanings of the above terms in the embodiments of the present application can be understood according to the specific circumstances.

[0054] At present, the commonly used preparation method of ferrous sodium sulfate material is to mix raw materials first and then high-temperature sintering. However, the ferrous sodium sulfate material prepared based on this method has the problem of many impurities.

[0055] Therefore, the present application provides a preparation method of ferrous sodium sulfate material, a positive electrode sheet and a secondary battery, wherein by adopting the way of segmented sintering, the dehydration and carbonization process is accurately controlled, so as to achieve the technical effect of reducing impurities and improving the conductivity of the material, thereby improving the rate performance of the positive electrode sheet, the secondary battery and the electric device.

[0056] In a first aspect, the embodiments of the present application provide a preparation method of ferrous sodium sulfate material, please refer to Figure 1 , the preparation method comprises the following steps: S10, mixing and grinding ferrous salt and sodium salt to form a precursor; S20, ball-milling mixing the precursor, a carbon source and water in a protective gas atmosphere to obtain a slurry; S30, drying the slurry to obtain a powder; S40, segmentally sintering the powder in a protective gas atmosphere to obtain a ferrous sodium sulfate material; Wherein, the segmental sintering includes sequentially performing first sintering, second sintering and third sintering on the powder. The temperature of the first sintering is 140℃-160℃, for example, it can be 140℃, 145℃, 150℃, 155℃, 160℃ and a value between any two of the above values; the temperature of the second sintering is 290℃-310℃, for example, it can be 290℃, 295℃, 300℃, 305℃, 310℃ and a value between any two of the above values; the temperature of the third sintering is 370℃-390℃, for example, it can be 370℃, 375℃, 380℃, 385℃, 390℃ and a value between any two of the above values.

[0057] In the high-temperature sintering process, the raw materials react and fuse with each other and crystallize to form crystals, and at the same time, the free water and crystal water present in the raw materials are also removed at this stage. However, in actual production, especially in batch production, due to factors such as production equipment, raw material quantity, reaction time, etc., the following problems often occur: the removal effect of crystal water is not good, or the water removed cannot be discharged in time, resulting in crystal water remaining in the crystal, increasing the impurity phase, and the purity of the crystal phase is insufficient, thereby affecting the performance of the material.

[0058] In the technical scheme of the embodiments of the present application, the high-temperature sintering process is divided into three processes at specific sintering temperatures. On the one hand, the dehydration process and the carbonization and crystallization process can be separated, and the crystal water is removed before crystallization to ensure that the crystal water can be smoothly and fully discharged, thereby reducing the impurity phase in the sodium ferrous sulfate material, reducing the powder internal resistance, and improving the electrical conductivity of the material. At the same time, by optimizing the sintering temperature, the speed of removing crystal water can also be controlled, thereby helping to reduce the pore size and make the pore distribution more uniform in the sodium ferrous sulfate material, thereby improving the green density of the material. On the other hand, by optimizing the sintering process and the sintering temperature of each stage, the sintering temperature gradually increases, which can accurately control the carbonization process, improve the crystallization effect, and improve the crystallinity, thereby better optimizing the electrical conductivity of the material. Overall, through the improvement of the sintering process, the effects of reducing the impurity phase of the material, reducing the powder internal resistance, improving the integrity of the crystal, and improving the electrical conductivity of the material are achieved. In this way, when the material is used to prepare the positive electrode sheet of the sodium-ion battery, it helps to improve the rate performance and cycle performance of the battery.

[0059] In addition, the segmented sintering method can also realize continuous production, which is more suitable for mass production.

[0060] Further, in some embodiments, the first sintering time is 3-5 hours, for example, it can be 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, and a value between any two of the above values; the second sintering time is 3-6 hours, for example, it can be 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours, 6 hours, and a value between any two of the above values; the third sintering time is 8-12 hours, for example, it can be 8 hours, 8.5 hours, 9 hours, 9.5 hours, 10 hours, 10.5 hours, 11 hours, 11.5 hours, 12 hours, and a value between any two of the above values.

[0061] In this embodiment, the sintering time of each sintering process is optimized and controlled within the above range, which can ensure sufficient reaction of each sintering process while shortening the sintering time and improving the production efficiency.

[0062] Further, in some embodiments, in step S10, the ferrous salt can include, but is not limited to, one or more of ferrous sulfate, ferrous sulfate monohydrate, ferrous sulfate heptahydrate, ferrous oxalate, ferrous chloride, ferrous acetate, ferrous nitrate. The sodium salt can include, but is not limited to, at least one of anhydrous sodium sulfate, anhydrous sodium acetate, sodium oxalate, sodium thiosulfate.

[0063] In this embodiment, the above-mentioned compounds are widely available and have good matching with other components, and are suitable as iron sources and sodium sources.

[0064] Further, in some embodiments, when the ferrous salt is ferrous sulfate heptahydrate, before step S10, the following step can also be included: vacuum drying the ferrous sulfate heptahydrate to obtain ferrous sulfate monohydrate.

[0065] In this embodiment, when ferrous sulfate heptahydrate is used as the iron source, vacuum drying it first can reduce the proportion of crystal water and reduce impurities.

[0066] Further, in some embodiments, in step S10, the feeding amount of the ferrous salt and the sodium salt satisfies the following condition: the molar ratio of iron elements in the ferrous salt to sodium elements in the sodium salt is 1:(1-2); for example, it can be 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, 1:1.529, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2, and a value between any two of the above values.

[0067] In this embodiment, controlling the molar ratio of iron elements to sodium elements within the above range can promote the reaction to proceed fully and synthesize a better phase of ferrous sulfate sodium material.

[0068] Further, in some embodiments, in step S10, the D50 particle size of the precursor is less than or equal to 2 μm.

[0069] In this embodiment, controlling the size of the precursor formed after grinding can make the precursor mix more uniformly and better fuse to form a better crystal structure. Grinding the sodium salt and the ferrous salt to a suitable size first and then mixing with the carbon source can also better promote the uniform mixing of the raw material components.

[0070] Further, in some embodiments, in step S10, the grinding can be performed by using an air jet mill, and the air jet mill can be operated under the following conditions: the pressure is 0.2-0.5 MPa, for example, 0.2 MPa, 0.3 MPa, 0.4 MPa, 0.5 MPa, or a value between any two of the above values; and the grinding time is 0.3-0.6 h, for example, 0.3 h, 0.35 h, 0.4 h, 0.45 h, 0.5 h, 0.55 h, 0.6 h, or a value between any two of the above values.

[0071] In this embodiment, the raw materials are ground and mixed by using an air jet mill, and the conditions of the air jet mill are optimized, which helps to promote the better dispersion and uniform mixing of the precursor.

[0072] Further, in some embodiments, in step S20, the carbon source includes ascorbic acid, graphene oxide, and carbon nanotubes.

[0073] In this embodiment, the carbon source is optimized, and a strategy of using three specific carbon sources for synergistic coating is used to optimize the carbon layer structure of the sodium ferrous sulfate material, which greatly improves the electrical conductivity of the material. Specifically, ascorbic acid (VC) not only acts as a reducing protective agent, effectively inhibits the oxidation of ferrous ions, and reduces the content of impurities in the sodium ferrous sulfate material, but also decomposes during the first and second sintering processes, which helps to form a porous structure in the sodium ferrous sulfate material, increases the contact area of the electrolyte, and improves the ion transport efficiency of sodium ions; graphene oxide (GO) is reduced during the second sintering process, and the various functional groups on the surface of GO are released by thermal decomposition, which greatly improves the electrical conductivity of the reduced product of GO, and helps to improve the electrical conductivity of the material. At the same time, the sheet structure of GO helps to improve the compaction density of the material; carbon nanotubes (CNT) have good electrical conductivity, which helps to improve the electrical conductivity of the material; when the three are used together, GO provides an in-plane conductive channel, CNT acts as a "conductive bridge" to penetrate the GO layers, reduces the interface impedance, and CNT and GO together construct a three-dimensional continuous conductive network, and the "amorphous carbon" formed by the carbonization of VC can be filled in the network gap to enhance the contact between particles. Under the synergistic action of the three, the electrical conductivity of the material can be greatly improved.

[0074] Further, in some embodiments, the mass ratio of the precursor to the ascorbic acid, the graphene oxide and the carbon nanotube is 100:(0.3-0.7):(5-8):(2-4); for example, the mass ratio can be controlled to be 100:(0.3-0.5):(5-8):(2-4), 100:(0.45-0.7):(5-8):(2-4), 100:(0.4-0.5):(5-8):(2-4), 100:(0.3-0.7):(5-7):(2-4), 100:(0.3-0.7):(6-8):(2-4), 100:(0.3-0.7):(6-7):(2-4), 100:(0.3-0.7):(5-8):(2-3), 100:(0.3-0.7):(5-8):(3-4), 100:(0.3-0.7):(5-8):(2.5-3.5), 100:(0.4-0.5):(6-7):(2.5-3.5), and the like.

[0075] In this embodiment, the feeding amount of VC, GO and CNT is optimized and controlled within the above range, so as to balance the effects of the three and improve the effect of synergistic action, and balance the improvement of the conductivity and the compaction density of the material. If the proportion of VC is too small, it is difficult to sufficiently inhibit the oxidation of ferrous ions, and the filling effect of the gap of the conductive network is limited, which is easy to cause the increase of impurities and the decrease of the conductivity of the material; if the proportion of VC is too large, too many holes are generated, which leads to the increase of the internal resistance of the powder, the decrease of the capacity and the decrease of the compaction density. If the proportion of GO is too large, the space of CNT and VC is easily occupied, which is not conducive to the construction of a more perfect conductive network, and is easy to cause the decrease of the conductivity; if the proportion of GO is too small, it is difficult to improve the problem of too many pores caused by CNT, and is easy to cause the decrease of the compaction density. If the proportion of CNT is too large, the compaction density of the material is easily decreased due to the tubular structure of CNT; if the proportion of CNT is too small, the internal resistance of the powder is easily increased, which affects the conductivity of the material.

[0076] Further, in some embodiments, in step S20, the protective gas can be at least one of argon and nitrogen.

[0077] In this embodiment, the inert gas such as argon and nitrogen is used to create a mixed environment, which can avoid the oxidation of raw materials and help to improve the purity of the product.

[0078] Further, in some embodiments, in step S20, the water can be deionized water, and the amount of water added can be 100:(20-40) of the mass ratio of the precursor to the water.

[0079] In this embodiment, water is added when mixing the precursor and the carbon source, and wet grinding is performed, which is beneficial to uniform mixing. Further, the GO and the CNT, which are not easy to disperse, can be dispersed in water respectively to form dispersions, and then the two dispersions are mixed with the VC and the precursor, which is more beneficial to uniform mixing.

[0080] Further, in some embodiments, in step S20, the ball milling time is 4-8h, for example, can be 4h, 5h, 6h, 7h, 8h and a value between any two of the above values.

[0081] In this embodiment, the ball milling time is controlled within the above range, which can ensure uniform mixing of the precursor and the carbon source.

[0082] Further, in some embodiments, in step S30, the drying method is freeze drying.

[0083] In this embodiment, the freeze drying method is used for drying, which can avoid the adverse effects of high temperature on the material, inhibit the oxidation of ferrous ions, and reduce the content of impurities.

[0084] Further, in some embodiments, the freeze drying includes: first pre-freezing at-70℃ to-50℃, and then drying at a temperature of-45℃ to-35℃ and a vacuum degree less than or equal to 20Pa for 18-24h. The pre-freezing temperature can be-70℃, -65℃, -60℃, -55℃, -50℃ and a value between any two of the above values; the vacuum drying temperature can be-45℃, -44℃, -43℃, -42℃, -41℃, -40℃, -39℃, -38℃, -37℃, -36℃, -35℃ and a value between any two of the above values; and the drying time can be 18h, 19h, 20h, 21h, 22h, 23h, 24h and a value between any two of the above values.

[0085] In this embodiment, the conditions for freeze drying are optimized and controlled within the above range, which can ensure that the slurry is fully dried, while avoiding the influence of temperature on the material.

[0086] In a second aspect, the embodiments of the present application provide a sodium ferrous sulfate material, which can be prepared by the preparation method described above. The sodium ferrous sulfate material includes an inner core and a carbon layer coated outside the inner core, and the material of the inner core includes sodium ferrous sulfate.

[0087] In the technical solution of the embodiments of the present application, the sodium ferrous sulfate material prepared by the preparation method described above has high phase purity, low impurity content, low powder resistance and high conductivity.

[0088] In some embodiments, the sodium ferrous sulfate can have a general formula of Na 2+2x Fe 2-x (SO4)3, and x is any number in a range of 0-1, for example, the sodium ferrous sulfate can have a chemical formula of Na 2.6 Fe 1.7 (SO4)3.

[0089] In some embodiments, the mass percentage of the ferrous ions is greater than or equal to 98%, the content of the ferrous ions is high, and the material has less impurities.

[0090] In some embodiments, the mass percentage of the carbon layer is in a range of 2-4.3%.

[0091] In this embodiment, the content of the carbon layer is controlled in the range, which can effectively coat the surface of the core, prevent the core from being oxidized by the external environment, improve the phase purity and the cycle stability, and avoid hindering the diffusion of sodium ions in the deintercalation process due to the too thick carbon layer.

[0092] In some embodiments, the raw materials of the carbon layer include ascorbic acid, graphene oxide, and carbon nanotubes. Based on the above-mentioned raw materials, a continuous conductive network is formed in the carbon layer, which has better conductivity and higher compaction density.

[0093] In some embodiments, the sodium ferrous sulfate material has a plurality of pores, and the pore size of the pores is in a range of 100-300 nm. The sodium ferrous sulfate material provided in the embodiments has uniformly distributed and small pores, which not only helps to increase the contact area of the electrolyte and improve the ion transport efficiency of sodium ions, but also has a high compaction density.

[0094] In a third aspect, the embodiments of the present application provide a positive electrode sheet, which includes the sodium ferrous sulfate material described above or the sodium ferrous sulfate material prepared by the preparation method described above.

[0095] In this embodiment, the positive electrode sheet contains the sodium ferrous sulfate material described above, and thus has the advantages of high cycle performance and high rate performance.

[0096] In a fourth aspect, the embodiments of the present application provide a secondary battery, which includes the positive electrode sheet described above.

[0097] In this embodiment, the secondary battery contains the positive electrode sheet described above, and thus has the advantages of high cycle performance and high rate performance.

[0098] In a fifth aspect, the embodiments of the present application further provide an electric device, which includes the secondary battery described above.

[0099] The power utilization device provided by the embodiments of the present application can be, but is not limited to, a mobile phone, a tablet computer, a notebook computer, an electric toy, an electric tool, an electric vehicle, an electric automobile, a ship, a spacecraft, etc. The electric toy can include a fixed or mobile electric toy, such as a game console, an electric automobile toy, an electric ship toy, an electric aircraft toy, etc. The spacecraft can include an airplane, a rocket, a space shuttle, a spacecraft, etc.

[0100] Some specific embodiments are listed below. It should be noted that the embodiments described below are exemplary and are used only to explain the present application, and cannot be understood as a limitation of the present application. If a specific technology or condition is not specified in the embodiments, the technology or condition described in the literature in the art or according to the product manual is used. If the reagent or instrument is not specified by the manufacturer, it is a conventional product that can be obtained from the market.

[0101] I. Preparation method Embodiment 1 (1) According to the molar ratio of iron element to sodium element of 1:1.529, ferrous sulfate heptahydrate and anhydrous sodium sulfate were weighed and prepared. The ferrous sulfate heptahydrate was vacuum dried to obtain ferrous sulfate monohydrate.

[0102] (2) The ferrous sulfate monohydrate and the anhydrous sodium sulfate were mixed by airflow milling, the pressure of the airflow mill was 0.3 MPa, and the time was 0.4 h, to obtain a precursor with a D50 of 1.92 μm.

[0103] (3) According to the mass ratio of the precursor to ascorbic acid, graphene oxide, carbon nanotubes, and deionized water of 100:0.64:6.5:2.1:30, ascorbic acid, graphene oxide, carbon nanotubes, and deionized water were weighed. The deionized water was divided into two parts, the graphene oxide was dispersed in one part of the deionized water to obtain a graphene oxide suspension, and the carbon nanotubes were dispersed in the other part of the deionized water to obtain a carbon nanotube dispersion. In a nitrogen atmosphere, the precursor, ascorbic acid, graphene oxide suspension, and carbon nanotube dispersion were ball-mixed, the ball-milling speed was 400 rpm, and the time was 6 h to obtain a slurry.

[0104] (4) The slurry was first pre-frozen at -60°C, and then dried at a temperature of -40°C and a vacuum degree of 10 Pa for 24 h to obtain a powder.

[0105] (5) In a nitrogen atmosphere, the powder was sintered in sections to obtain a ferrous sodium sulfate material. The first section sintering temperature was 150°C, and the holding time was 4 h; the second section sintering temperature was 300°C, and the holding time was 4 h; the third section sintering temperature was 380°C, and the holding time was 10 h. Embodiment 2 (1) According to the molar ratio of iron element to sodium element being 1:1, ferrous sulfate heptahydrate and anhydrous sodium acetate are weighed for standby. The ferrous sulfate heptahydrate is vacuum dried to prepare ferrous sulfate monohydrate.

[0106] (2) The ferrous sulfate monohydrate and the anhydrous sodium acetate are mixed by airflow milling, the pressure of the airflow mill is 0.5 MPa, and the time is 0.3 h, to prepare a precursor with a D50 of 2 μm.

[0107] (3) According to the mass ratio of the precursor to ascorbic acid, graphene oxide, carbon nanotube and deionized water being 100:0.64:6.5:2.1:30, ascorbic acid, graphene oxide, carbon nanotube and deionized water are weighed. The deionized water is divided into two parts, the graphene oxide is dispersed in one part of the deionized water to prepare a graphene oxide suspension, and the carbon nanotube is dispersed in the other part of the deionized water to prepare a carbon nanotube dispersion. In a nitrogen atmosphere, the precursor, ascorbic acid, graphene oxide suspension and carbon nanotube dispersion are ball-mixed, the ball-milling speed is 400 rpm, and the time is 4 h to obtain a slurry.

[0108] (4) The slurry is first pre-frozen at -70°C, and then dried at a temperature of -45°C and a vacuum degree of 10 Pa for 18 h to obtain a powder.

[0109] (5) In a nitrogen atmosphere, the powder is sintered in sections to obtain a ferrous sodium sulfate material. The first section sintering temperature is 140°C, the holding time is 5 h; the second section sintering temperature is 290°C, the holding time is 6 h; the third section sintering temperature is 370°C, and the holding time is 12 h.

[0110] Example 3 (1) According to the molar ratio of iron element to sodium element being 1:1.2, ferrous sulfate heptahydrate and anhydrous sodium sulfate are weighed for standby. The ferrous sulfate heptahydrate is vacuum dried to prepare ferrous sulfate monohydrate.

[0111] (2) The ferrous sulfate monohydrate and the anhydrous sodium sulfate are mixed by airflow milling, the pressure of the airflow mill is 0.2 MPa, and the time is 0.6 h, to prepare a precursor with a D50 of 1.95 μm.

[0112] (3) According to the mass ratio of the precursor to ascorbic acid, graphene oxide, carbon nanotube, and deionized water being 100:0.64:6.5:2.1:30, ascorbic acid, graphene oxide, carbon nanotube, and deionized water are weighed. The deionized water is divided into two parts, graphene oxide is dispersed in one part of the deionized water to prepare a graphene oxide suspension, and carbon nanotube is dispersed in the other part of the deionized water to prepare a carbon nanotube dispersion. In a nitrogen atmosphere, the precursor, ascorbic acid, graphene oxide suspension, and carbon nanotube dispersion are ball-milled at a rotation speed of 400 rpm for 8 h to obtain a slurry.

[0113] (4) The slurry is first pre-frozen at -50°C, and then dried at a temperature of -35°C and a vacuum degree of 10 Pa for 23 h to obtain a powder.

[0114] (5) The powder is sintered in stages in a nitrogen atmosphere to obtain a sodium ferrous sulfate material. The first-stage sintering temperature is 160°C, and the holding time is 3 h; the second-stage sintering temperature is 310°C, and the holding time is 3 h; the third-stage sintering temperature is 390°C, and the holding time is 8 h.

[0115] Example 4 This example is basically the same as Example 1, except that in this example, the first-stage sintering temperature is 130°C. Except for this, the other steps and conditions remain unchanged.

[0116] Example 5 This example is basically the same as Example 1, except that in this example, the first-stage sintering temperature is 170°C. Except for this, the other steps and conditions remain unchanged.

[0117] Example 6 This example is basically the same as Example 1, except that in this example, the second-stage sintering temperature is 280°C. Except for this, the other steps and conditions remain unchanged.

[0118] Example 7 This example is basically the same as Example 1, except that in this example, the second-stage sintering temperature is 320°C. Except for this, the other steps and conditions remain unchanged.

[0119] Example 8 This example is basically the same as Example 1, except that in this example, the mass ratio of the precursor to ascorbic acid is 100:0.2. Except for this, the other steps and conditions remain unchanged.

[0120] Example 9 This example is basically the same as Example 1, except that in this example, the mass ratio of the precursor to ascorbic acid is 100:0.3. Other than that, the steps and conditions are the same.

[0121] Example 10 This example is basically the same as Example 1, except that in this example, the mass ratio of the precursor to ascorbic acid is 100:0.5. Other than that, the steps and conditions are the same.

[0122] Example 11 This example is basically the same as Example 1, except that in this example, the mass ratio of the precursor to ascorbic acid is 100:1. Other than that, the steps and conditions are the same.

[0123] Example 12 This example is basically the same as Example 1, except that in this example, the mass ratio of the precursor to graphene oxide is 100:5. Other than that, the steps and conditions are the same.

[0124] Example 13 This example is basically the same as Example 1, except that in this example, the mass ratio of the precursor to graphene oxide is 100:8. Other than that, the steps and conditions are the same.

[0125] Example 14 This example is basically the same as Example 1, except that in this example, the mass ratio of the precursor to graphene oxide is 100:10. Other than that, the steps and conditions are the same.

[0126] Example 15 This example is basically the same as Example 1, except that in this example, the mass ratio of the precursor to carbon nanotubes is 100:3. Other than that, the steps and conditions are the same.

[0127] Example 16 This example is basically the same as Example 1, except that in this example, the mass ratio of the precursor to carbon nanotubes is 100:4. Other than that, the steps and conditions are the same.

[0128] Example 17 This example is basically the same as Example 1, except that in this example, the mass ratio of the precursor to carbon nanotubes is 100:5. Other than that, the steps and conditions are the same.

[0129] Comparative Example 1 The counter example scheme is basically the same as Example 1, the only difference is that in the counter example step (5), only the third sintering section. In addition, other steps and conditions remain unchanged.

[0130] Counter Example 2 The counter example scheme is basically the same as Example 1, the only difference is that in the counter example step (5), only the second sintering section and the third sintering section. In addition, other steps and conditions remain unchanged.

[0131] Counter Example 3 The counter example scheme is basically the same as Example 1, the only difference is that in the counter example, ascorbic acid is not added. In addition, other steps and conditions remain unchanged.

[0132] Counter Example 4 The counter example scheme is basically the same as Example 1, the only difference is that in the counter example, graphene oxide is not added. In addition, other steps and conditions remain unchanged.

[0133] Counter Example 5 The counter example scheme is basically the same as Example 1, the only difference is that in the counter example, graphene oxide is replaced by conductive carbon black. In addition, other steps and conditions remain unchanged.

[0134] Counter Example 6 The counter example scheme is basically the same as Example 1, the only difference is that in the counter example, step (4) is changed to vacuum drying, vacuum degree 10 Pa, temperature 90 ℃, time 24 h. In addition, other steps and conditions remain unchanged.

[0135] II. Test method (1) Performance test of ferrous sulfate sodium material 1. Element content: The ferrous sulfate sodium material was tested by inductively coupled plasma optical emission spectrometer (ICP-OES), the element content in the material was detected, and the carbon content was tested by infrared carbon and sulfur instrument, and the test results are shown in Table 1.

[0136] 2. Micro-morphology and pore size test of ferrous sulfate sodium material: cross-section polishing-scanning electron microscope (CP-SEM) was used for test. The test results are shown in Figure 2 and Table 1.

[0137] 3. Impurity phase content: Japan Rigaku X-ray diffractometer (XRD) was used for detection, and the test results are shown in Table 1.

[0138] 4. Ferrous ion content: detected by oxidation-reduction titration method.

[0139] 5. Compacted density (PD): The compacted density was tested by UTM7305 battery powder compacted density instrument provided by Shenzhen Sanechips Technology Co., Ltd., the test pressure was 3T, the pressing time was 30s, and the test results are shown in Table 2.

[0140] 6. Powder resistivity: The four-probe method was used for testing, the test pressure was 8MPa, and the test results are shown in Table 2.

[0141] (2) Performance test of secondary battery The sodium ferrous sulfate material prepared in each example and the comparative example was mixed with super fine carbon powder (Super P, SP) and polyvinylidene fluoride according to a mass ratio of 90:5:5, then N-methyl pyrrolidone (NMP) was added for slurry, then coated on an aluminum foil, then dried and pressed into a sheet, then a button-type half battery was prepared with the sodium sheet as the negative electrode, glass fiber as the separator, and sodium hexafluorophosphate (NaPF6) propylene carbonate (PC) solution as the electrolyte, wherein the concentration of NaPF6 in the electrolyte was 1mol / L.

[0142] The battery performance test system (model: CT3002A) of Wuhan Blue Electric Technology Co., Ltd. was used to test the battery, the test temperature was 25±1℃, the voltage range was 2-4.5V, the rate range was 0.1C-10C, and the test results are shown in Table 2 and Figures 3 to 5 .

[0143] III. Analysis of test results of each example and the comparative example Table 1

[0144] Table 2

[0145] Result analysis: Figure 2 It is shown that the sodium ferrous sulfate material prepared in the present application is spherical particles and has a porous structure, and the internal pore size is small and uniformly distributed.

[0146] Table 1 and Table 2 experimental data and Figures 3 to 5 show: Examples 1 to 17 all exhibit high ferrous ion content and compacted density, low average pore size, impurity content and powder resistivity, and the corresponding batteries exhibit high first efficiency, 0.1C first discharge specific capacity, 5C capacity retention rate, and 1C cycle 100 times capacity retention rate, indicating that the sodium ferrous sulfate material prepared by the method of the present application has low impurity phase, high compaction and high conductivity, and the use of the material to prepare the positive electrode sheet helps to obtain a battery with high capacity, high rate performance and high cycle performance; Further, compared with Comparative Examples 1 and 2, Example 1 has lower average pore size, impurity content and powder resistivity, higher compaction density, first efficiency, discharge capacity and capacity retention rate, which indicates that by adopting the way of segmented sintering, the powder is sequentially subjected to first sintering (140-160°C), second sintering (290-310°C) and third sintering (370-390°C), which helps to promote the full discharge of crystallization water, reduce the impurity phase in the sodium ferrous sulfate material, reduce the internal resistance of the powder, improve the crystallization effect of the material, improve the electrical conductivity of the material, at the same time, regulate the pore distribution in the material, and improve the compaction density of the material, so that the battery has high capacity, high rate performance and high cycle performance; Comparative Example 1, Comparative Examples 3 to 5, it can be seen that Example 1 has higher ferrous ion content, compaction density, first efficiency, discharge capacity and capacity retention rate, and lower average pore size, impurity content and powder resistivity, while Comparative Example 3 shows lower ferrous ion content, higher powder resistivity and impurity phase content, Comparative Example 4 shows higher average pore size and powder resistivity, lower compaction density, and Comparative Example 5 shows high pore size, low compaction and extremely high powder resistivity, which indicates that using a specific ratio (VC:GO:CNT=(0.3-0.7):(5-8):(2-4)) of VC+GO+CNT as a carbon source helps to achieve a synergistically coated carbon layer structure, balancedly improve the electrical conductivity and compaction density of the material, and improve the electrochemical performance of the material.

[0147] In addition, Example 1 has higher ferrous ion content and lower impurity phase content than Comparative Example 6, which indicates that using freeze-drying for drying treatment can avoid the adverse effects of high temperature on the material, inhibit the oxidation of ferrous ions, and reduce the impurity phase content.

[0148] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and embodiments having the same technical idea and playing the same role and effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, within the scope of the main idea of the present application, various modifications of the embodiments that can be thought of by those skilled in the art, and other ways constructed by combining part of the components of the embodiments are also included in the scope of the present application.

Claims

1. A method for preparing a sodium ferrous sulfate material, characterized in that, The method comprises the following steps: mixing and grinding a ferrous salt and a sodium salt to form a precursor; ball-milling the precursor, a carbon source, and water in a protective gas atmosphere to obtain a slurry; drying the slurry to obtain a powder; sintering the powder in a protective gas atmosphere in sections to obtain a sodium ferrous sulfate material; wherein the sintering in sections comprises sequentially performing first sintering, second sintering, and third sintering on the powder, the temperature of the first sintering is 140-160°C, the temperature of the second sintering is 290-310°C, and the temperature of the third sintering is 370-390°C.

2. The production method according to claim 1, characterized by, The time of the first sintering is 3-5h, the time of the second sintering is 3-6h, and the time of the third sintering is 8-12h.

3. The production method according to claim 1 or 2, characterized by, The carbon source comprises ascorbic acid, graphene oxide, and carbon nanotubes.

4. The production method according to claim 3, characterized by, The mass ratio of the precursor to the ascorbic acid, the graphene oxide, and the carbon nanotubes is 100:(0.3-0.7):(5-8):(2-4).

5. The preparation method according to claim 1, characterized in that, The D50 particle size of the precursor is less than or equal to 2μm; and / or, The drying method is freeze-drying; and / or, The ferrous salt comprises one or more of ferrous sulfate, ferrous sulfate monohydrate, ferrous sulfate heptahydrate, ferrous oxalate, ferrous chloride, ferrous acetate, and ferrous nitrate; and / or, The sodium salt comprises at least one of anhydrous sodium sulfate, anhydrous sodium acetate, sodium oxalate, and sodium thiosulfate; and / or, The molar ratio of iron in the ferrous salt to sodium in the sodium salt is 1:(1-2); and / or, In the step of ball-milling the precursor, the carbon source, and water in a protective gas atmosphere to obtain a slurry, the ball-milling time is 4-8h.

6. The production method according to claim 5, characterized by, In the step of mixing and grinding a ferrous salt and a sodium salt to form a precursor, a jet mill is used for grinding, the pressure of the jet mill is 0.2-0.5MPa, and the grinding time is 0.3-0.6h; and / or, The freeze-drying comprises pre-freezing at-70°C to-50°C, and then drying at a temperature of-45°C to-35°C and a vacuum degree of less than or equal to 20Pa for 18-24h. The sodium ferrous sulfate material comprises a core and a carbon layer coated outside the core, and the material of the core comprises sodium ferrous sulfate.

7. A sodium ferrous sulfate material produced by the process of any one of claims 1 to 6, characterized in that, In the sodium ferrous sulfate material, the mass percentage of ferrous ions is greater than or equal to 98%; and / or, 8. The sodium ferrous sulfate material of claim 7, wherein, The mass percentage of the carbon layer is 2-4.3%; and / or, The sodium ferrous sulfate material has a porous structure, and the pore size is 100-300nm; and / or, The raw material of the carbon layer comprises ascorbic acid, graphene oxide, and carbon nanotubes. The sodium ferrous sulfate material is prepared by the preparation method of any one of claims 1-6, or the sodium ferrous sulfate material of claim 7 or 8.

9. A positive electrode sheet characterized by comprising: The positive electrode tab of claim 9.

10. A secondary battery characterized by comprising: ​

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