Sodium ion positive electrode material and preparation method and application thereof

Abstract

The invention provides a sodium ion positive electrode material and a preparation method and application, and belongs to the field of sodium ion batteries, the sodium ion positive electrode material comprises an inner core and a coating layer coating the inner core, the inner core is ferric sodium phosphate pyrophosphate, the coating layer is a carbon coating layer, the carbon coating layer comprises an organic carbon source, an inorganic carbon source and MXene, and the organic carbon source and the inorganic carbon source are mutually connected. And the thickness of the carbon coating layer is 0.2 [mu] m-0. 4 [mu] m. In the carbon coating layer, inorganic and organic carbon are compounded and MXene is doped, and meanwhile, the thickness of the carbon coating layer is controlled, so that the compaction density, the conductivity and the structural stability of the sodium ion positive electrode material are favorably improved, and the rate capability and the cycling stability of the sodium ion positive electrode material are further improved.

Description

Technical Field

[0001] The present invention relates to the field of sodium ion batteries, and specifically relates to a sodium ion positive electrode material, a preparation method and an application thereof. Background Art

[0002] At present, sodium-ion batteries (SIBs) have attracted much attention due to their outstanding advantages such as abundant sodium ion resources in the earth's crust, low price, and similar working principles to mainstream lithium-ion batteries (LIBs). They have become the most likely alternative to lithium-ion batteries in grid-scale energy storage and new energy vehicles. However, the electrode materials of SIBs still face some huge challenges, such as difficult-to-overcome structural instability, slow ion diffusion, low operating voltage and low energy / power density. In order to solve these problems, scientists studying SIBs are mainly committed to designing and preparing new electrode materials with high adaptability and reversibility in the process of large sodium ion intercalation / extraction. Among them, for the cathode material layer of SIBs, transition metal oxides (TMOs) and polyanionic compounds (PACs) are the two most promising candidate materials. Although the theoretical capacity of PAC is slightly lower than that of TMO (such as NaMnO2, P2-Na 2 / 3 Fe 1 / 2 Mn 1 / 2 While PACs typically exhibit superior cycle life compared to TMOs, they are notoriously difficult to manufacture. A major potential application for SIBs is grid-scale energy storage, for which ultra-long cycle life is a crucial requirement. Therefore, it can be inferred that if the comprehensive electrochemical performance of PACs (including high-rate capability, specific capacity (Cs), and operating voltage) is improved, their application in SIBs will be promising.

[0003] Sodium superionic conductor (NASICON) is one of the representative PAC materials. Its microstructure is a stable three-dimensional (3D) open framework composed of PO tetrahedron and MO octahedron (M represents transition metal), which can realize Na + Among polyanion cathode materials, iron-based sodium cathode materials have the advantages of lowest cost and non-toxicity. Among them, sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) (NFPP) has the advantages of low raw material cost, environmental friendliness and non-toxicity, high specific capacity (129mAh / g), excellent cycle performance and small cycle volume expansion. As the most promising iron-based sodium cathode material, it has been widely studied.

[0004] In the existing technology, most iron-based sodium phosphate cathode materials are synthesized using a solid-phase method, that is, using iron phosphate as the raw material to prepare them. The prepared iron-based sodium phosphate cathode materials have good performance. However, with the increasing demand for sodium energy density, the current compaction density and electrochemical performance of NFPP need to be further improved. However, the use of iron phosphate as the raw material to prepare NFPP is limited by the influence of the iron phosphate morphology, making it difficult to increase the compaction density of NFPP. The use of other phosphorus or iron sources will reduce the conductivity or structural stability of NFPP. Therefore, it is urgent to solve the problem of difficulty in further improving the conductivity, structural stability and compaction density of NFPP at the same time. Summary of the Invention

[0005] In view of the technical problems existing in the background technology, the present application provides a sodium ion positive electrode material and a preparation method and application, aiming to solve the technical problem that the compaction density, conductivity and structural stability of the sodium ion positive electrode material NFPP are difficult to further improve at the same time.

[0006] In a first aspect, an embodiment of the present application provides a sodium ion positive electrode material, which includes a core and a coating layer coating the core, wherein the core is sodium ferric pyrophosphate, the coating layer is a carbon coating layer, the material of the carbon coating layer includes an organic carbon source, an inorganic carbon source and MXene, and the thickness of the carbon coating layer is 0.2μm~0.4μm.

[0007] In the technical solution of the embodiment of the present application, a carbon coating layer is prepared from a composite carbon material and serves as an outer shell, forming a core-shell structure with the sodium ferric pyrophosphate core. The resulting sodium ion positive electrode material has a high compaction density. The carbon coating layer comprises an organic carbon source, an inorganic carbon source, and MXene. On the one hand, the present application utilizes the excellent conductivity and mechanical properties of the inorganic carbon source to improve the electrochemical performance and compaction density of the resulting sodium ion positive electrode material. On the other hand, MXene is utilized to stabilize the carrier and reduce its volume change. After sintering, the organic carbon source forms a conductive network carbon layer and is coated on the MXene surface, thereby simultaneously improving the conductivity and mechanical stability of the sodium ion positive electrode material.

[0008] Furthermore, the present application also ensures the electronic conductivity of the obtained sodium ion positive electrode material by controlling the thickness of the carbon coating layer. The present application ensures that the thickness of the carbon coating layer is appropriate and uniform, and that the distance of the ion transmission path is appropriate to improve the charge and discharge efficiency; therefore, the present application uses inorganic and organic carbon composites, incorporates MXene and controls the thickness of the carbon coating layer to simultaneously improve the compaction density, conductivity and structural stability of the obtained sodium ion positive electrode material, thereby improving its rate performance and cycle stability.

[0009] In some embodiments, the secondary particle size of the sodium ion positive electrode material is 4 μm to 8 μm, and the compaction density is 2.05 g / cm3 ~2.13g / cm 3 , with a specific surface area of 10m 2 / g~11m 2 / g.

[0010] In this embodiment, the provided sodium ion positive electrode material has uniform secondary particle size, high compaction density, and large specific surface area, which is beneficial to rapid sodium ion transport and subsequent secondary battery preparation, and is beneficial to improving the rate performance of the secondary battery.

[0011] In some embodiments, the organic carbon source includes at least one of glucose and PEG-2000; and / or, the inorganic carbon source includes at least one of graphene oxide, carbon nanotubes, and aqueous carbon nanotubes; and / or, the MXene includes MXene-V2C nanosheets.

[0012] In this embodiment, graphene oxide has excellent conductivity and mechanical properties that can improve the electrochemical properties of the material, and its excellent ductility can effectively increase the compaction density; carbon nanotubes and / or water-based carbon nanotubes can form channels, which can improve the conductivity of the material and the number of ion transport channels; MXene-V2C nanosheets have a nano-multilayer stable structure, which can effectively improve the rate performance and cycle stability of the resulting sodium ion positive electrode material.

[0013] In a second aspect, an embodiment of the present application provides a method for preparing a sodium ion positive electrode material, comprising the following steps: mixing a ferrous source solution, a phosphorus source solution and an oxidant under acidic conditions, and then adding an inorganic carbon source, aging, washing and drying to obtain a precursor material; mixing the precursor material with a sodium source, an organic carbon source and MXene, and dispersing the mixture to obtain a mixed slurry; and drying and sintering the mixed slurry to obtain a sodium ion positive electrode material.

[0014] In the technical solution of the embodiment of the present application, under acidic conditions, a ferrous source, a phosphorus source and an oxidant are used as raw materials to prepare a ferric hydrogen phosphate precursor material by a co-precipitation method. The iron-phosphorus ratio (0.73) of the ferric hydrogen phosphate precursor is closer to the iron-phosphorus ratio in NFPP than that of ferric phosphate (0.98), and the morphology is denser, so that the compaction density of the prepared NFPP product will be significantly improved, and the BET will also decrease to a certain extent; at the same time, the present application adds an inorganic carbon source in the precursor material synthesis stage, changes its morphology in the precursor stage and utilizes the high conductivity of the inorganic carbon source, so as to improve the electronic conductivity of the precursor material; thereafter, by adding an organic carbon source and MXene for secondary carbon source coating, the conductivity and mechanical stability of the prepared sodium ion positive electrode material can be improved, so that the conductivity is further improved, thereby achieving the purpose of improving rate performance and cycle stability.

[0015] In some embodiments, the molar ratio of the ferrous source, the phosphorus source, and the oxidant is 1:(1.2-1.8):(0.2-1.2); and / or, the amount of the inorganic carbon source is 1% to 3% by mass of the ferrous source; and / or, the molar ratio of the precursor material, the sodium source, and the organic carbon source is 1:(3.5-4.5):(0.02-0.04); and / or, the amount of MXene added is 0.01% to 0.03% by mass of the precursor material.

[0016] In this embodiment, by controlling the amount of each raw material, especially the addition amount of inorganic carbon source, organic carbon source and MXene, a uniform carbon coating layer is formed, which is beneficial to improving the conductivity and structural stability of the obtained sodium ion positive electrode material.

[0017] In some embodiments, the ferrous source includes at least one of ferrous sulfate and ferrous phosphate; and / or the phosphorus source includes at least one of monoammonium phosphate and diammonium phosphate; and / or the oxidant includes at least one of hydrogen peroxide and peracetic acid; and / or the inorganic carbon source includes at least one of graphene oxide, carbon nanotubes and aqueous carbon nanotubes; and / or the sodium source includes at least one of sodium bicarbonate and sodium carbonate.

[0018] In this embodiment, to improve the dispersibility of the inorganic carbon source, the inorganic carbon source can be added in the form of a slurry. The method of addition can be adapted to the actual situation and is not limited here. The mass concentration of the graphene oxide slurry or the aqueous carbon nanotube slurry is 4-6%. It should be noted that the amount of the inorganic carbon source is 1% to 3% of the mass of the ferrous source, calculated based on the solid content of the inorganic carbon source.

[0019] In some embodiments, the pH value of the acidic condition is 1.8 to 2.2; and / or the aging time is 3 hours to 12 hours; and / or the washing is filter pressing until the conductivity of the filter press filtrate is less than 150 μs and the pH is less than 5; and / or the drying method includes forced air drying or vacuum drying.

[0020] In this embodiment, the pH value of the acidic conditions affects the speed of precipitation formation, and if the pH value is too high, iron hydroxide precipitation will be formed, and if the pH value is too low, iron phosphate will be formed. Therefore, the pH value in this application is preferably 1.8 to 2.2. Specifically, the pH value can be lowered by an acid solution such as a phosphoric acid solution, and the pH value can be increased by an alkaline solution such as ammonia water. At the same time, a reaction temperature that is too high or too low, and an aging time that is too long or too short are not conducive to the generation of precursor materials; the drying method has an effect on the microscopic morphology of the generated precursor material, and air drying or vacuum drying are both beneficial to improving the compaction density of the obtained precursor material, especially the vacuum drying method is more favorable.

[0021] In some embodiments, the dispersion treatment includes grinding, and the D50 particle size of the solid phase particles in the mixed slurry after the dispersion treatment is less than 0.6 μm; and / or, the drying includes spray drying, the inlet air temperature of the spray drying is 220°C to 260°C, and the outlet air temperature of the spray drying is 85°C to 95°C; and / or, the sintering conditions include: sintering at a temperature of 500°C to 550°C for 16h to 24h under a protective atmosphere.

[0022] In this embodiment, by controlling the particle size of the solid phase particles in the slurry, spray drying and subsequent sintering are facilitated to form a uniform carbon-coated sodium iron pyrophosphate positive electrode material.

[0023] In some embodiments, after the sintering treatment, the sintered product is further subjected to the steps of crushing, screening and iron removal to obtain a sodium ion positive electrode material.

[0024] Among them, the environmental humidity of crushing and screening is controlled below 10%, and the particle size range of the sodium ion positive electrode material after crushing is controlled by screening, which is conducive to the rapid transmission of sodium ions and subsequent battery preparation.

[0025] In a third aspect, an embodiment of the present application provides a positive electrode plate, which includes the sodium ion positive electrode material provided in the first aspect of the present application, or includes the sodium ion positive electrode material prepared by the preparation method provided in the second aspect of the present application.

[0026] In this embodiment, the positive electrode plate includes the above-mentioned sodium ion positive electrode material, and thus has the advantages of good rate performance and good cycle performance, and can maintain high capacity at high current density.

[0027] In a fourth aspect, an embodiment of the present application provides a secondary battery, comprising the positive electrode sheet provided in the third aspect of the present application.

[0028] In this embodiment, the secondary battery of the present application includes the above-mentioned positive electrode plate, and thus has the advantages of good rate performance and good cycle performance, and can maintain high capacity at high current density.

[0029] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] To more clearly illustrate the technical solution of this application, the following is a brief introduction to the drawings used in this application. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be derived from these drawings without inventive effort.

[0031] Figure 1 This is a SEM image of the precursor material obtained in Comparative Example 1 of this application; Figure 2 This is a SEM image of the precursor material obtained in Comparative Example 2 of this application; Figure 3 This is a SEM image of the precursor material obtained in Example 1 of the present application; Figure 4 This is a SEM image of the precursor material obtained in Example 2 of the present application; Figure 5 This is the XRD pattern of the precursor material obtained in Example 1 of the present application; Figure 6 This is a SEM image of the sodium ion positive electrode material obtained in Example 1 of the present application; Figure 7 This is the XRD pattern of the sodium ion positive electrode material obtained in Example 1 of the present application; Figure 8 The rate performance of the battery assembled with the sodium ion positive electrode material obtained in Example 1 of the present application. DETAILED DESCRIPTION

[0032] The following embodiments of the technical solution of the present application will be described in detail with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application and are therefore only examples and are not intended to limit the scope of protection of the present application.

[0033] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in the specification and claims of this application and the above-mentioned figure descriptions are intended to cover non-exclusive inclusions.

[0034] In the description of the embodiments of this application, the technical terms "first" and "second" are used only to distinguish different objects and should not be understood to indicate or imply relative importance or implicitly specify the quantity, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, the meaning of "plurality" is more than two, unless otherwise clearly and specifically defined.

[0035] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0036] In the description of the embodiments of this application, the term "and / or" is simply a description of the association relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent the following three situations: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in this document generally indicates that the associated objects are in an "or" relationship.

[0037] In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

[0038] In the description of the embodiments of the present application, the technical terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the embodiments of the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the embodiments of the present application.

[0039] In the description of the embodiments of the present application, unless otherwise expressly specified or limited, technical terms such as "installed," "connected," "connected," and "fixed" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integration; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; internal connections between two components or interactions between two components. Those skilled in the art can understand the specific meanings of the above terms in the embodiments of the present application based on specific circumstances.

[0040] In the existing technology, sodium iron pyrophosphate (NFPP) is widely studied as the most promising iron-based sodium battery positive electrode material. With the increasingly higher requirements for sodium battery energy density, the compaction density and electrical performance of NFPP need to be further improved. When using common iron phosphate to prepare NFPP, the influence of the iron phosphate morphology will make it difficult to increase the compaction density of NFPP. The use of other phosphorus sources or iron sources may reduce the conductivity and structural stability of NFPP, and lead to poor rate performance and cycle stability of the assembled sodium battery.

[0041] In order to solve the technical problem that the compaction density, conductivity and structural stability of the sodium ion positive electrode material NFPP cannot be taken into account at the same time, the present application provides a sodium ion positive electrode material and a preparation method and application, wherein the sodium ion positive electrode material uses sodium ferric pyrophosphate as the core and a carbon coating layer as the outer shell to form a core-shell structure, and the obtained material has a high compaction density; the material of the carbon coating layer includes an organic carbon source, an inorganic carbon source and MXene, and the conductivity and structural stability of the material are improved by inorganic-organic carbon composite and doping with MXene, thereby achieving the technical effect of improving the rate performance and cycle stability, and thus the rate performance and cycle stability performance of the positive electrode sheet and the secondary battery are also improved.

[0042] In a first aspect, an embodiment of the present application provides a sodium ion positive electrode material, which includes a core and a coating layer coating the core, wherein the core is sodium ferric pyrophosphate, the coating layer is a carbon coating layer, the material of the carbon coating layer includes an organic carbon source, an inorganic carbon source and MXene, and the thickness of the carbon coating layer is 0.2μm~0.4μm.

[0043] In the present application, a carbon coating layer is prepared from a composite carbon material and used as an outer shell to form a core-shell structure with a sodium iron pyrophosphate core, wherein the material of the carbon coating layer includes an organic carbon source, an inorganic carbon source, and MXene. On the one hand, the present application utilizes the excellent electrical conductivity and mechanical properties of the inorganic carbon source to improve the electrochemical performance and compaction density of the obtained sodium ion positive electrode material; on the other hand, MXene is used to stabilize the carrier and reduce its volume change. After sintering, the organic carbon source will form a conductive network carbon layer and be coated on the surface of the MXene, thereby simultaneously improving the electrical conductivity and mechanical stability of the sodium ion positive electrode material. The present application also needs to control the thickness of the carbon coating layer. If the carbon coating layer is too thick, it will increase the internal stress and thus affect its long-term cycle stability, and reduce the sodium ion diffusion rate; if it is too thin, it will be uneven and easily lead to poor interface stability, and will also affect the electronic conductivity of the positive electrode material. The present application ensures that the thickness of the carbon coating layer is appropriate and uniform, and that the distance of the ion transmission path is appropriate to improve the charge and discharge efficiency. Therefore, this application uses inorganic and organic carbon composites, incorporating MXene and controlling the thickness of the carbon coating layer to simultaneously improve the compaction density, conductivity and structural stability of the positive electrode material, thereby improving its rate performance and cycle stability.

[0044] Furthermore, in some embodiments, the secondary particle size of the sodium ion positive electrode material is 4 μm to 8 μm, and the compaction density is 2.05 g / cm 3 ~2.13g / cm 3 , with a specific surface area of 10m 2 / g~11m 2 / g.

[0045] In the present application, the provided sodium ion positive electrode material has uniform secondary particle size, high compaction density, and large specific surface area, which is beneficial to the rapid transport of sodium ions and subsequent battery preparation, and is beneficial to improving the battery's rate performance.

[0046] Furthermore, in some embodiments, the organic carbon source includes at least one of glucose and PEG-2000; and / or, the inorganic carbon source includes at least one of graphene oxide, carbon nanotubes, and aqueous carbon nanotubes; and / or, the MXene includes MXene-V2C nanosheets.

[0047] In the present application, graphene oxide has excellent conductivity and mechanical properties that can improve the electrochemical properties of the material, and its excellent ductility can effectively increase the compaction density; carbon nanotubes and / or water-based carbon nanotubes can form channels, which can improve the conductivity of the material and the number of ion transport channels; MXene-V2C nanosheets have a nano-multilayer stable structure, which can effectively improve the rate performance and cycle stability of the resulting sodium ion positive electrode material.

[0048] In a second aspect, an embodiment of the present application provides a method for preparing a sodium ion positive electrode material, comprising the following steps: mixing a ferrous source solution, a phosphorus source solution and an oxidant under acidic conditions, and then adding an inorganic carbon source, aging, washing and drying to obtain a precursor material; mixing the precursor material with a sodium source, an organic carbon source and MXene, and dispersing the mixture to obtain a mixed slurry; and drying and sintering the mixed slurry to obtain a sodium ion positive electrode material.

[0049] In the present application, under acidic conditions, a ferrous source, a phosphorus source and an oxidant are used as raw materials to prepare a ferric hydrogen phosphate precursor material by a co-precipitation method. The iron-phosphorus ratio (0.73) of the ferric hydrogen phosphate precursor is closer to the iron-phosphorus ratio in NFPP than that of ferric phosphate (0.98), and the morphology is denser, so that the compaction density of the NFPP product will be significantly improved and the BET will also be reduced to a certain extent; at the same time, the present application adds an inorganic carbon source in the precursor material synthesis stage, changes its morphology in the precursor stage and utilizes the high conductivity of the inorganic carbon source to further improve the electronic conductivity of the precursor; thereafter, by adding an organic carbon source and MXene for secondary carbon source coating, the conductivity and mechanical stability of the material are improved at the same time, so that the conductivity is further improved, thereby achieving the purpose of improving the rate performance and cycle stability.

[0050] Furthermore, in some embodiments, the molar ratio of the ferrous source, the phosphorus source and the oxidant is 1: (1.2-1.8): (0.2-1.2), including but not limited to 1:1.2:0.2, 1:1.4:0.2, 1:1.4:0.2, 1:1.5:0.2, 1:1.6:0.2, 1:1.8:0.2, 1:1.2:0.4, 1:1.5:0.5, 1:1.5:0.6, 1:1.8:1.0, 1:1.8:1.2 or any value within the range of 1: (1.2-1.8): (0.2-1.2); the amount of the inorganic carbon source is 1% to 3% of the mass of the ferrous source, including but not limited to 1%, 2% and 3%. %, 3% or any value within the range of 1% to 3%; the molar ratio of the precursor material, the sodium source and the organic carbon source is 1:(3.5-4.5):(0.02-0.04), including but not limited to 1:3.5:0.02, 1:3.5:0.03, 1:3.5:0.04, 1:4:0.03, 1:4.5:0.03 or any value within the range of 1:(3.5-4.5):(0.02-0.04); the addition amount of MXene is 0.01% to 0.03% of the mass of the precursor material, including but not limited to 0.01%, 0.02%, 0.03% or any value within the range of 0.01% to 0.03%.

[0051] In the present application, by controlling the amount of each raw material, especially the addition amount of inorganic carbon source, organic carbon source and MXene, a uniform carbon coating layer is formed, which is more conducive to improving the conductivity and structural stability of the obtained positive electrode material.

[0052] Furthermore, in some embodiments, the concentration of the ferrous source solution is 0.5 mol / L to 1.5 mol / L, including but not limited to 0.5 mol / L, 1.0 mol / L, 1.5 mol / L or any value within the range of 0.5 mol / L to 1.5 mol / L; the concentration of the phosphorus source solution is 1 mol / L to 3 mol / L, including but not limited to 1.0 mol / L, 2.0 mol / L, 3.0 mol / L or any value within the range of 1 mol / L to 3 mol / L.

[0053] Furthermore, in some embodiments, the ferrous source includes at least one of ferrous sulfate and ferrous phosphate; the phosphorus source includes at least one of monoammonium phosphate and diammonium phosphate; the oxidant includes at least one of hydrogen peroxide and peracetic acid; the inorganic carbon source includes at least one of graphene oxide, carbon nanotubes and aqueous carbon nanotubes; and the sodium source includes at least one of sodium bicarbonate and sodium carbonate.

[0054] In this application, to improve the dispersibility of the inorganic carbon source, the inorganic carbon source can be added in the form of a slurry. The method of addition can be adapted according to actual conditions and is not limited here. The mass concentration of the graphene oxide slurry or aqueous carbon nanotube slurry is 4-6 wt%. It should be noted that the amount of the inorganic carbon source is 1% to 3% by weight of the ferrous source, calculated based on the solid content of the inorganic carbon source.

[0055] Furthermore, in some embodiments, under acidic conditions, a ferrous source solution, a phosphorus source solution and an oxidant are mixed, and then an inorganic carbon source is added, and the precursor material is obtained through aging, washing and drying: the pH value of the acidic conditions is 1.8 to 2.2; the specific steps of mixing the ferrous source solution, the phosphorus source solution and the oxidant include: at 30°C to 50°C, the phosphorus source solution and the oxidant are simultaneously added to the ferrous source solution within 0.5h to 2h, wherein the temperature includes but is not limited to 30°C, 35°C, 40°C, 45°C, 50°C or any value within the range of 30°C to 50°C; the aging time is 3h to 12h, including but not limited to 3h, 5h, 8h, 10h, 12h or any value within the range of 3h to 12h; the washing is filter press cleaning until the conductivity of the filter press filtrate is <150μs and the pH is <5; the drying method includes blower drying or vacuum drying.

[0056] In the present application, the pH value of the acidic conditions will affect the speed of precipitation formation, and too high a pH value will form iron hydroxide precipitation, and too low a pH value will form iron phosphate. Therefore, the pH value of the present application is preferably maintained at 1.8 to 2.2. Specifically, the pH value can be lowered by an acid such as a phosphoric acid solution, and the pH value can be increased by an alkaline solution such as ammonia water. A reaction temperature that is too high or too low, and an aging time that is too long or too short are not conducive to the generation of a ferric hydrogen phosphate precursor; the drying method has an effect on the micromorphology of the generated ferric hydrogen phosphate precursor. Blast drying or vacuum drying are both beneficial to improving the compaction density of the resulting precursor material, especially the vacuum drying method is more favorable.

[0057] Furthermore, in some embodiments, the dispersion treatment includes grinding, and the D50 particle size of the solid phase particles in the mixed slurry after the dispersion treatment is less than 0.6 μm; the drying includes spray drying, the inlet air temperature of the spray drying is 220°C to 260°C, and the outlet air temperature of the spray drying is 85°C to 95°C; the sintering conditions include: sintering at a temperature of 500°C to 550°C for 16h to 24h under a protective atmosphere, wherein the sintering temperature includes but is not limited to 500°C, 510°C, 520°C, 530°C, 540°C, 550°C or any value within the range of 500°C to 550°C, and the sintering time includes but is not limited to 16h, 18h, 20h, 22h, 24h or any value within the range of 16h to 24h.

[0058] In the present application, by controlling the particle size of the solid phase particles in the mixed slurry, spray drying and subsequent sintering are facilitated to form a uniform carbon-coated sodium iron pyrophosphate positive electrode material.

[0059] Furthermore, after the sintering treatment, the process also includes crushing, screening and iron removal to obtain the sodium ion positive electrode material.

[0060] In the present application, the ambient humidity of the crushing and screening is controlled to be lower than 10%, and the particle size range of the sodium ion positive electrode material after crushing is controlled by screening. For example, the D50 particle size of the obtained sodium ion positive electrode material is 3-6μm, and the D100 particle size is less than 25μm, which is conducive to the rapid transmission of sodium ions and subsequent secondary battery preparation.

[0061] In a third aspect, an embodiment of the present application provides a positive electrode plate, which includes the sodium ion positive electrode material provided in the first aspect of the present application, or includes the sodium ion positive electrode material prepared by the preparation method provided in the second aspect of the present application.

[0062] The positive electrode plate of the present application includes the above-mentioned sodium ion positive electrode material, and thus has the advantages of good rate performance and good cycle performance, and can maintain high capacity at high current density.

[0063] In a fourth aspect, an embodiment of the present application provides a secondary battery, comprising the positive electrode sheet provided in the third aspect of the present application.

[0064] The secondary battery of the present application includes the above-mentioned positive electrode sheet, and thus has the advantages of good rate performance and good cycle performance, and can maintain high capacity at high current density.

[0065] Some specific examples are listed below. It should be noted that the examples described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. Where specific techniques or conditions are not specified in the examples, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used without manufacturer's indication are all commercially available conventional products.

[0066] The inorganic carbon source used in the following examples and comparative examples is graphene oxide slurry or aqueous carbon nanotube slurry, with a mass concentration of 5%. The amount of inorganic carbon source added is calculated based on the solid components in the slurry, namely graphene oxide or carbon nanotubes.

[0067] 1. Preparation method Example 1 A method for preparing a sodium ion positive electrode material comprises the following steps: S1, preparation of precursor materials: S101, adding a 1 mol / L ferrous source solution (ferrous sulfate solution) to a reactor and stirring at a speed of 600 rpm, and simultaneously adding a 2 mol / L phosphorus source solution (ammonium dihydrogen phosphate solution) and 28 wt% hydrogen peroxide, wherein the stoichiometric ratio of ferrous source: phosphorus source: H2O2 is 1:1.5:0.67, and adjusting the pH value of the mixed solution with phosphoric acid and ammonia water to maintain the pH value at 2.0. The reaction temperature is 40°C, so that the phosphorus source solution and hydrogen peroxide solution are added simultaneously within 1 hour, and the mother liquor becomes a yellow-white suspension. At this time (i.e., the beginning of aging), an inorganic carbon source (graphene oxide slurry) is added to the yellow-white suspension and stirred to prepare, wherein the graphene oxide accounts for 2% by weight of the iron source. After aging for 12 hours, a black suspension is obtained; S102, placing the black suspension into a filter press, adding deionized water 4 times the volume of the black suspension, and performing filter pressing and cleaning 6-7 times, monitoring the pH and conductivity of the filtrate each time until the conductivity of the filtrate is less than 150 μs and the pH is less than 5, and collecting the filter residue; S103, placing the collected filter residue in a vacuum drying oven and vacuum drying it at 120° C. overnight to obtain a precursor material.

[0068] S2, the precursor material, sodium source (sodium bicarbonate) and organic carbon source (glucose and PEG-2000 in a mass ratio of 1:1) are configured according to a stoichiometric ratio of 1:4:0.03, and MXene-V2C nanosheets accounting for 0.02% of the mass of the precursor material are added. Deionized water is added at a solid content of 25% and mixed. The resulting mixture is added to a coarse grinding equipment and coarsely ground for 1 hour to obtain a first mixed slurry; the first mixed slurry is then transferred to a fine grinding equipment and finely ground for 3 hours, and the particle size D50 of the finely ground slurry is controlled to be less than 0.6μm to obtain a second mixed slurry.

[0069] S3, injecting the second mixed slurry into the spray equipment, controlling the outlet air temperature at 90℃±5℃, obtaining the first powder, placing the first powder in an atmosphere furnace, introducing nitrogen and keeping it at 550℃ for 20 hours to obtain the second powder, crushing and screening the second powder to remove iron to obtain the sodium ion positive electrode material, which is carbon-coated sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7@C / V2C, abbreviated as NFPP@C / V2C).

[0070] Example 2 The only difference from Example 1 is that in step S101 , the inorganic carbon source is CNT slurry, and CNT accounts for 1% of the mass of the iron source. Other steps and conditions are the same as in Example 1.

[0071] Example 3 The only difference from Example 1 is that in S101, the inorganic carbon source uses CNT slurry, and CNT accounts for 3% of the mass of the iron source. Other steps and conditions are the same as in Example 1.

[0072] Example 4 The only difference from Example 1 is that the amount of graphene oxide in step S101 is adjusted to 1% of the mass of the iron source, and the other steps and conditions are the same as those in Example 1.

[0073] Example 5 The only difference from Example 1 is that the amount of graphene oxide in step S101 is adjusted to 3% of the mass of the iron source, and the other steps and conditions are the same as those in Example 1.

[0074] Example 6 The only difference from Example 1 is that the stoichiometric ratio of ferrous source: phosphorus source: H2O2 in step S101 is adjusted to 1:1.2:0.2. Other steps and conditions are the same as those in Example 1.

[0075] Example 7 The only difference from Example 1 is that the stoichiometric ratio of ferrous source: phosphorus source: H2O2 in step S101 is adjusted to 1:1.8:1.2. Other steps and conditions are the same as those in Example 1.

[0076] Example 8 The only difference from Example 1 is that the stoichiometric ratio of the precursor material, the sodium source (sodium bicarbonate), and the organic carbon source (glucose and PEG-2000 at a mass ratio of 1:1) in step S2 is adjusted to 1:3.5:0.02.

[0077] Example 9 The only difference from Example 1 is that the stoichiometric ratio of the precursor material, the sodium source (sodium bicarbonate), and the organic carbon source (glucose and PEG-2000 at a mass ratio of 1:1) in step S2 is adjusted to 1:4.5:0.04.

[0078] Example 10 The only difference from Example 1 is that in the adjustment step S2, MXene-V2C nanosheets accounting for 0.01% of the mass of the precursor material are added at the same time. The other steps and conditions are the same as those in Example 1.

[0079] Example 11 The only difference from Example 1 is that in the adjustment step S2, 0.03% of the mass of the precursor material is added by MXene-V2C nanosheets. The other steps and conditions are the same as those in Example 1.

[0080] Example 12 The only difference from Example 1 is that air drying is adopted in step S103 , and the other steps and conditions are the same as those in Example 1.

[0081] Comparative Example 1 The only difference from Example 1 is that no inorganic carbon source is added in step S101, and the other steps and conditions are the same as those in Example 1.

[0082] Comparative Example 2 The only difference from Example 1 is that no inorganic carbon source is added in step S101 and air drying is adopted in step S103. Other steps and conditions are the same as those in Example 1.

[0083] Comparative Example 3 The only difference from Example 1 is that no inorganic carbon source is added in step S101, and an inorganic carbon source is added together with the organic carbon source in step S2. Other steps and conditions are the same as in Example 1.

[0084] Comparative Example 4 The only difference from Example 1 is that no organic carbon source is added in step S2, and the other steps and conditions are the same as those in Example 1.

[0085] Comparative Example 5 The only difference from Example 1 is that MXene-V2C nanosheets are not added in step S2, and the other steps and conditions are the same as those in Example 1.

[0086] Comparative Example 6 The only difference from Example 1 is that step S1 is removed, and the precursor material is directly replaced by iron phosphate in step S2. The other steps and conditions are the same as those in Example 1.

[0087] 2. Test Method 1. Property testing of precursor materials and sodium ion cathode materials SEM test: The morphology of the precursor material and the sodium ion cathode material was analyzed using a field emission scanning electron microscope (MERLIN Compact, model Quanta 200FEG) produced by Zeiss.

[0088] XRD test: A Bruker D8Advance X-ray diffractometer produced by Bruker, Germany, was used to perform phase analysis on the precursor material and the sodium ion positive electrode material.

[0089] Compaction density: The compaction density was tested using the UTM7305 battery powder compaction density meter provided by Shenzhen Sansi Zongheng Technology Co., Ltd.

[0090] Specific surface area: The specific surface area was measured using a BELSORP MaxII specific surface area analyzer produced by Japan-Michiko Baier.

[0091] 2. Property test of secondary batteries The sodium ion positive electrode material was mixed with conductive carbon black and PVDF binder in a mass ratio of 90:5:5 to obtain a positive electrode slurry. The positive electrode slurry was coated on an aluminum foil with a thickness of 12 μm to form a positive electrode slurry layer with a thickness of 80 μm. The positive electrode slurry was then placed in an oven at 110°C and dried for 10 hours. After drying, the circular electrode pieces with a diameter of 15 mm were punched and compacted to a density of 1.8 g / cm 3 Roll pressing is performed to obtain a positive electrode sheet.

[0092] Sodium hexafluorophosphate (NaPF6) was used as the sodium salt, and ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) with a volume ratio of 1:1:1 were used as organic solvents. The sodium salt was dissolved in the organic solvent to prepare an electrolyte with a concentration of 1M; a pure sodium sheet with a purity of 99% was used as the counter electrode, and the battery was assembled with the positive electrode sheet and the electrolyte in the LG2400 / 1000TS glove box produced by Weige Gas Purification Technology (Suzhou) Co., Ltd. to obtain a button half-cell.

[0093] The CT3002A battery performance test system produced by Wuhan Blue Electric Electronic Technology Co., Ltd. was used to perform rate performance tests on button half-cells. The test temperature was 25°C, the voltage range was 2-4.2V, and the rate range was 0.1C-20C.

[0094] 3. Analysis of test results of various embodiments and comparative examples 1. SEM test: The morphology of the precursor materials obtained by vacuum drying and blast drying in Comparative Example 1 and Comparative Example 2 are shown in SEM. Figure 1 and Figure 2 As shown, Figure 1 It shows that the precursor material obtained by vacuum drying in comparative example 1 is uniform particles and is tightly stacked; the precursor material obtained by blast drying in comparative example 2 has a certain agglomeration phenomenon, which shows that the drying method has a great influence on the precursor morphology, which in turn leads to differences in the comprehensive performance of the sodium ion positive electrode material.

[0095] The SEM images of the precursor materials obtained in Example 1 and Example 2 are as follows: Figure 3 and Figure 4 As shown by Figure 3 It can be seen that after adding graphene oxide in Example 1, the ferric hydrogen phosphate precursor will be directly precipitated on the graphene sheet during the precipitation process; Figure 4 It shows that after adding CNTs in Example 2, the ferric hydrogen phosphate precursor is deposited on the CNT substrate.

[0096] 2. The XRD patterns of the precursor materials obtained in Example 1 and Example 12 are as follows: Figure 5As shown in the figure, it can be seen that the XRD of the precursor material obtained by vacuum drying in Example 1 has obvious peak intensity, while the XRD of the precursor obtained by air drying has weaker peak intensity and relatively low crystallinity, which indicates that the vacuum dried precursor has higher crystallinity.

[0097] 3. The SEM image of the sodium ion cathode material (NFPP@C / V2C) obtained in Example 1 is as follows: Figure 6 As shown, it is a uniform spherical particle; 4. The XRD pattern of the sodium ion cathode material (NFPP@C / V2C) obtained in Example 1 is as follows: Figure 7 As shown in the figure, it can be seen that the precursor prepared by vacuum drying is more conducive to melting into NFPP with higher crystallinity during the subsequent sintering process of the sodium ion positive electrode material sample, reducing the generation of impurity phases.

[0098] Therefore, the sodium ion positive electrode material of the present application is a core-shell carbon-coated sodium ferric pyrophosphate with sodium ferric pyrophosphate as the core and a carbon coating layer as the shell, the secondary particle size of which is 4μm to 8μm, and the thickness of the carbon coating layer is about 0.2μm to 0.4μm.

[0099] 5. The compaction density and specific surface area test results of the sodium ion positive electrode materials (carbon-coated sodium iron pyrophosphate) obtained in the above Examples 1-5 and Comparative Examples 1 and 3-6, as well as the battery capacity test results of the assembled sodium ion secondary batteries at 0.1C are shown in Table 1 below.

[0100] Table 1 Sodium ion positive electrode materials and battery test results of each embodiment and comparative example

[0101] As can be seen from Table 1, after adding graphene oxide in Examples 1 and 4-5, the excellent conductivity and mechanical properties can improve the electrochemical properties of the material, and the excellent ductility can increase the compaction density; in Examples 2-3, adding CNTs to form channels can further improve the material conductivity and the number of ion transport channels, and it is preferred to add 2% graphene oxide and 3% CNTs.

[0102] In Examples 6-7, the amounts of the phosphorus source and hydrogen peroxide were changed, resulting in the iron-phosphorus ratio in the obtained ferric hydrogen phosphate precursor material deviating from 0.73, and the rate performance of the final sodium ion positive electrode material decreased.

[0103] In Examples 8-9, changing the amount of sodium source and organic carbon source, and in Examples 10-11, changing the amount of MXene-V2C nanosheets added, will have a certain impact on the uniformity of the formed carbon coating layer, and the conductivity of the obtained positive electrode material will be slightly reduced, and the battery capacity will be reduced.

[0104] Compared to Example 1, in Comparative Example 1, no inorganic carbon source was added. The compaction density of the sodium ion positive electrode material in Comparative Example 1 was reduced, and the conductivity of the resulting battery was somewhat reduced. In Comparative Example 2, no inorganic carbon source was added and air drying was used. The performance of the sodium ion positive electrode material was further reduced.

[0105] In Comparative Example 3, the inorganic carbon source was added together with the organic carbon source. The results showed that the inorganic carbon source did not provide better coating or allow the precursor to grow using it as a carrier, thereby affecting the structural stability of the material and the quality of the formed carbon layer.

[0106] In Comparative Example 4, no organic carbon source was added, and in Comparative Example 5, no MXene-V2C nanosheets were added, which would lead to a decrease in the performance of the obtained sodium ion positive electrode material and battery. This is mainly because the organic carbon source alone will only form a conductive network carbon layer, and the MXene alone will only act as a carrier to stabilize the volume change. After the two are composited and sintered, the organic carbon can be coated on the MXene surface, which can simultaneously improve the conductivity and mechanical stability of the material.

[0107] In Comparative Example 6, iron phosphate was used as a precursor, and the compaction density and battery capacity of the obtained sodium ion positive electrode material were reduced.

[0108] 6. Under the condition of rate range of 0.1C~20C, the rate performance test results of the sodium ion secondary battery assembled with the sodium ion cathode material (NFPP@C / V2C) obtained in Example 1 are as follows: Figure 8 shown.

[0109] Depend on Figure 8 It can be seen that the battery assembled with the carbon-coated sodium iron pyrophosphate (NFPP@C / V2C) obtained in this application as the positive electrode material has a discharge specific capacity that does not change significantly with the increase in the number of cycles under 0.1C conditions, indicating that it has good cycle stability; the discharge specific capacity at 5C is 95.4mAh / g, and the discharge specific capacity at 20C can still reach 86.3mAh / g, which shows that the positive electrode material of this application can provide ideal capacity at high current density.

[0110] In summary, this application improves the conductivity and structural stability of NFPP materials by combining inorganic and organic carbon and doping with MXene. The synthesized positive electrode materials perform relatively well in terms of capacity, compaction density, BET and other properties.

[0111] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A sodium ion positive electrode material, characterized in that The sodium ion positive electrode material includes a core and a coating layer covering the core, wherein the core is sodium ferric pyrophosphate, the coating layer is a carbon coating layer, the material of the carbon coating layer includes an organic carbon source, an inorganic carbon source and MXene, and the thickness of the carbon coating layer is 0.2μm to 0.4μm.

2. The sodium ion positive electrode material according to claim 1, characterized in that The secondary particle size of the sodium ion positive electrode material is 4 μm to 8 μm, and the compaction density is 2.05 g / cm 3 ~2.13g / cm 3 , with a specific surface area of 10m 2 / g~11m 2 / g.

3. The sodium ion positive electrode material according to claim 1, characterized in that The organic carbon source comprises at least one of glucose and PEG-2000; and / or The inorganic carbon source comprises at least one of graphene oxide, carbon nanotubes and aqueous carbon nanotubes; and / or The MXene includes MXene-V2C nanosheets.

4. The method for preparing the sodium ion positive electrode material according to any one of claims 1 to 3, wherein: The following steps are involved: Under acidic conditions, a ferrous source solution, a phosphorus source solution and an oxidant are mixed, and then an inorganic carbon source is added, followed by aging, washing and drying to obtain a precursor material; The precursor material is mixed with a sodium source, an organic carbon source and MXene, and a mixed slurry is obtained by dispersion treatment; The mixed slurry is dried and sintered to obtain a sodium ion positive electrode material.

5. The method for preparing a sodium ion positive electrode material according to claim 4, wherein: The molar ratio of the ferrous source, the phosphorus source and the oxidant is 1: (1.2-1.8): (0.2-1.2); and / or The amount of the inorganic carbon source is 1% to 3% of the mass of the ferrous source; and / or The molar ratio of the precursor material, the sodium source and the organic carbon source is 1: (3.5-4.5): (0.02-0.04); and / or The added amount of the MXene is 0.01% to 0.03% of the mass of the precursor material.

6. The method for preparing a sodium ion positive electrode material according to claim 4, wherein: The ferrous source comprises at least one of ferrous sulfate and ferrous phosphate; and / or The phosphorus source comprises at least one of monoammonium phosphate and diammonium phosphate; and / or The oxidant comprises at least one of hydrogen peroxide and peracetic acid; and / or The inorganic carbon source comprises at least one of graphene oxide, carbon nanotubes and aqueous carbon nanotubes; and / or The sodium source includes at least one of sodium bicarbonate and sodium carbonate.

7. The method for preparing a sodium ion positive electrode material according to claim 4, wherein: The pH value of the acidic condition is 1.8 to 2.2; and / or The aging time is 3h to 12h; and / or The washing is filter pressing and cleaning until the conductivity of the filter press filtrate is less than 150 μs and the pH is less than 5; and / or The drying method includes air drying or vacuum drying.

8. The method for preparing a sodium ion positive electrode material according to claim 4, wherein: The dispersion treatment includes grinding, and the D50 particle size of the solid phase particles in the mixed slurry after the dispersion treatment is less than 0.6 μm; and / or The drying comprises spray drying, wherein the air inlet temperature of the spray drying is 220-260° C., and the air outlet temperature of the spray drying is 85-95° C.; and / or The sintering conditions include: sintering at a temperature of 500° C. to 550° C. for 16 hours to 24 hours under a protective atmosphere.

9. A positive electrode plate, characterized in that: The invention comprises the sodium ion positive electrode material according to any one of claims 1 to 3, or comprises the sodium ion positive electrode material prepared by the method for preparing the sodium ion positive electrode material according to any one of claims 4 to 8.

10. A secondary battery, characterized in that: Including the positive electrode sheet according to claim 9.

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