Long-circulation polyanion positive electrode material, preparation method thereof and sodium ion battery

Abstract

The invention provides a long-circulation polyanion positive electrode material, a preparation method thereof and a sodium ion battery. The positive electrode material comprises an inner core and an outer core, wherein the inner core is particles with the chemical formula of Na3V2 (PO4) 2F3; the middle layer is a hierarchical porous carbon layer coated on the surface of the inner core; and the outer layer is a nitrogen-doped graphene three-dimensional conductive network. The surface of the inner core in the positive electrode material is coated with the hierarchical porous carbon layer, so that the conductivity of the material is improved, the problem of volume expansion of sodium ions in the charging and discharging process is effectively relieved, and the cycle life of a battery is prolonged; the outer layer adopts the nitrogen-doped graphene three-dimensional conductive network, so that the conductivity and the structural stability of the material are further improved, and the three-dimensional conductive network not only provides more electron transmission channels, but also effectively prevents agglomeration among particles, so that the cycling stability and the rate capability of the material are improved.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a long-cycle polyanion cathode material, a preparation method thereof, and a sodium ion battery. Background Art

[0002] The working principle of sodium-ion batteries is similar to that of lithium-ion batteries, and sodium salt reserves are more abundant and simple to mine, making them more advantageous in large-scale applications. Therefore, based on the abundance of materials, production equipment, and the referenceability of processes, sodium-ion batteries are regarded as a very promising energy storage option; however, the radius of sodium ions is much larger than that of lithium ions, which makes the sodium diffusion kinetics slow, making it difficult to find suitable materials with an open framework.

[0003] Common sodium-ion cathode materials include oxides, Prussian blue, and polyanions. However, in terms of resource abundance, overall material cost, electrochemical performance, and environmental sustainability, polyanion cathode materials are the best choice for sodium-ion batteries. Polyanion sodium cathode materials currently offer comprehensive advantages such as low cost and excellent cycling performance. Sodium ferric sulfate and sodium ferric pyrophosphate excel in low cost and long cycling, respectively. Both materials also share similar voltage platforms. If synthesized together into a composite polyanion sodium cathode material, they will possess the combined advantages of low cost and long cycling, potentially accelerating the market entry of sodium cathode materials and expanding their application areas.

[0004] In the prior art, doping or coating methods are commonly used to improve the performance of sodium-ion cathode materials. For example, patent application number CN202211040949.5 discloses a polyanionic cathode material, its preparation method, and application. The preparation method comprises the following steps: mixing a sodium source, a phosphorus source, a transition metal source, a first carbon source, and a reducing agent, sintering the resulting mixture to obtain a sintered material; and mixing a mixed solution of an aluminum source and a fluorine source with the sintered material, drying, and calcining it once to obtain the polyanionic cathode material. The conductive properties of the polyanionic cathode material are enhanced by in-situ formation of an aluminum fluoride coating on the surface of the cathode material. However, conventional aluminum coatings have limited performance improvements for sodium-ion cathode materials and cannot significantly improve the cycling performance of sodium-ion batteries.

[0005] Therefore, it is necessary to provide a long-cycle polyanion positive electrode material and a preparation method thereof to significantly improve the cycle performance of sodium ion batteries. Summary of the Invention

[0006] The present application is made in view of the above problems, and its purpose is to provide a long-cycle polyanion positive electrode material and a preparation method thereof and a sodium ion battery.

[0007] Specifically, the first aspect of the present application provides a long-cycle polyanion cathode material, comprising: Core: particles with the chemical formula Na3V2(PO4)2F3; Middle layer: a multi-level porous carbon layer covering the surface of the inner core; Outer layer: nitrogen-doped graphene three-dimensional conductive network.

[0008] Furthermore, the multi-level porous carbon layer comprises micropores with a pore diameter of less than 2 nm and mesopores with a pore diameter of 2-50 nm.

[0009] Furthermore, the mass ratio of the nitrogen-doped graphene to the core particles is 1:8-1:12.

[0010] The second aspect of the present application provides a method for preparing the long-cycle polyanion cathode material, comprising the following steps: (a) ZIF-8 and F127 were mixed with sucrose ethanol solution, and the solvent was evaporated before carbonization to produce hierarchical porous carbon (HPC). (b) NaF, NH4VO3, NH4H2PO4 and HPC were ball-milled and mixed again by adding boric acid; (c) Microwave sintering under nitrogen atmosphere to obtain Na3V2(PO4)2F3@HPC; (d) Graphene oxide, melamine and Na3V2(PO4)2F3@HPC were dispersed, spray-dried, and reduced to obtain a composite cathode material.

[0011] Furthermore, in step (b), the molar ratio of Na, V, and P in the NaF, NH4VO3, and NH4H2PO4 is 2.8-3.1:1.9-2.2:2.

[0012] Furthermore, the microwave sintering temperature in step (c) is 600-700°C, and / or Heating rate is 50-100℃ / s, and / or The power of microwave sintering is 700-900W.

[0013] Furthermore, in step (d), the inlet temperature of the spray drying is 170-190°C, and the outlet temperature is 70-90°C.

[0014] The third aspect of the present application provides a sodium ion battery, comprising a positive electrode, a negative electrode, a diaphragm and an electrolyte, wherein the positive electrode adopts the positive electrode material, the negative electrode adopts hard carbon, and the diaphragm is an Al2O3 / PVDF-HFP composite ceramic coating diaphragm.

[0015] Furthermore, the electrolyte comprises: Sodium salt: NaPF6; Solvent: a mixed solvent of EC and PC with a volume ratio of 3:6-8; Additives: NaPO2F2, succinonitrile, vinyl sulfate.

[0016] Furthermore, the method for preparing the hard carbon in the negative electrode includes: The biomass material is crushed and then immersed in a ZnCl2 solution; Carbonization under Ar atmosphere; Wash with solution until neutral.

[0017] The present invention has the following beneficial effects: (1) The core of the long-cycle polyanion positive electrode material of the present application is a particle with the chemical formula Na3V2(PO4)2F3, which has good electrochemical properties. By coating the surface of the core with a multi-level porous carbon layer, not only the conductivity of the material is improved, but also the volume expansion problem of sodium ions during the charge and discharge process is effectively alleviated, thereby extending the cycle life of the battery. The multi-level porous carbon layer contains micropores with a pore size of less than 2nm and mesopores with a pore size between 2-50nm. This structure is conducive to the rapid diffusion of sodium ions and the penetration of electrolyte, further improving the performance of the battery. Secondly, the outer layer adopts a nitrogen-doped graphene three-dimensional conductive network, which further improves the conductivity and structural stability of the material. This three-dimensional conductive network not only provides more electron transmission channels, but also effectively prevents the agglomeration between particles, thereby improving the cycle stability and rate performance of the material.

[0018] (2) In this application, ZIF-8, F127 and sucrose ethanol solution are mixed and carbonized to obtain hierarchical porous carbon, which provides a basis for the subsequent coating process. NaF, NH4VO3, NH4H2PO4 and hierarchical porous carbon are then ball-milled and mixed again with boric acid to ensure uniform distribution of the raw materials. Microwave sintering is performed under a nitrogen atmosphere to obtain Na3V2(PO4)2F3@HPC composite material. Finally, graphene oxide, melamine and Na3V2(PO4)2F3@HPC are dispersed and spray-dried, and then reduced to obtain the final long-cycle polyanion positive electrode material. This preparation method is simple and easy to implement and can be easily industrialized.

[0019] (3) The present application also provides a sodium ion battery using the positive electrode material, which includes a positive electrode, a negative electrode, a diaphragm and an electrolyte, wherein the positive electrode uses the long-cycle polyanion positive electrode material prepared above, the negative electrode uses hard carbon, the diaphragm is an Al2O3 / PVDF-HFP composite ceramic coating diaphragm, and the electrolyte includes a mixed solvent of NaPF6, EC, and PC and additives such as NaPO2F2, succinonitrile, and vinyl sulfate. This battery has excellent cycle performance and rate performance. DETAILED DESCRIPTION

[0020] In order to make the purpose, technical solutions and advantages of this application more clearly understood, the present application is described and illustrated below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are merely used to explain this application and are not intended to limit this application. Based on the embodiments provided in this application, all other embodiments obtained by those of ordinary skill in the art without making any creative work are within the scope of protection of this application.

[0021] Obviously, the following descriptions are merely some examples or embodiments of the present application. Those skilled in the art can apply the present application to other similar scenarios without inventive effort. Furthermore, it is also understood that, although the effort involved in such a development process may be complex and lengthy, for those skilled in the art related to the content disclosed in the present application, changes in design, manufacturing, or production based on the technical content disclosed in the present application are merely conventional technical means and should not be construed as an insufficiency of the content disclosed in the present application.

[0022] An embodiment of the first aspect of the present application provides a long-cycle polyanion cathode material, comprising: Core: particles with the chemical formula Na3V2(PO4)2F3; Intermediate layer: a hierarchical porous carbon layer coated on the surface of the inner core, wherein the hierarchical porous carbon layer comprises micropores with a pore size of <2 nm and mesopores with a pore size of 2-50 nm; Outer layer: nitrogen-doped graphene three-dimensional conductive network, the mass ratio of the nitrogen-doped graphene to the core particles is 1:8-1:12.

[0023] The core of the long-cycle polyanion positive electrode material of the present application adopts particles with the chemical formula Na3V2(PO4)2F3, and the material itself has good electrochemical properties; by coating the surface of the core with a multi-level porous carbon layer, not only the conductivity of the material is improved, but also the volume expansion problem of sodium ions during the charge and discharge process is effectively alleviated, thereby extending the cycle life of the battery; the multi-level porous carbon layer contains micropores with a pore size less than 2nm and mesopores with a pore size between 2-50nm. This structure is conducive to the rapid diffusion of sodium ions and the penetration of electrolyte, further improving the performance of the battery; secondly, the outer layer adopts a nitrogen-doped graphene three-dimensional conductive network, and the nitrogen content in the nitrogen-doped graphene is 5-7at%, which further improves the conductivity and structural stability of the material. This three-dimensional conductive network not only provides more electron transmission channels, but also effectively prevents agglomeration between particles, thereby improving the cycle stability and rate performance of the material.

[0024] The embodiment of the second aspect of the present application provides a method for preparing the long-cycle polyanion cathode material, comprising the following steps: (a) ZIF-8 and F127 were mixed with sucrose ethanol solution, and the solvent was evaporated before carbonization to produce hierarchical porous carbon (HPC). (b) NaF, NH4VO3, NH4H2PO4 and HPC were ball-milled and mixed again by adding boric acid; (c) Microwave sintering under nitrogen atmosphere to obtain Na3V2(PO4)2F3@HPC; (d) Graphene oxide, melamine and Na3V2(PO4)2F3@HPC were dispersed, spray-dried, and reduced to obtain a composite cathode material.

[0025] In this embodiment, step (a) disperses 1 g of ZIF-8 (zeolite imidazolate framework) in 200 ml of ethanol, adds 0.5 g of block copolymer F127 (template), and drops 2 g of sucrose ethanol solution (50 wt %). After ultrasonication for 30 min, the solvent is evaporated at 80°C, and the temperature is increased to 700°C at a rate of 5°C / min under an Ar atmosphere for carbonization, and the temperature is maintained for 2 h to obtain hierarchical porous carbon (HPC).

[0026] In this embodiment, the NaF, NH4VO3, and NH4H2PO4 described in step (b) are ball-milled at a molar ratio of Na:V:P of 2.8-3.1:1.9-2.2:2. Preferably, NaF, NH4VO3, and NH4H2PO4 are ball-milled with the HPC (mass ratio of 8-12:1) from step (a) at a molar ratio of Na:V:P of 3:2:2 for 2-3 hours, followed by the addition of 1 wt% boric acid as a flux, and microwave sintering in a tube furnace at a temperature of 600-700°C, a heating rate of 50-100°C / s, and a power of 700-900 W. The resulting Na3V2(PO4)2F3@HPC has a particle size of 100-300 nm and a carbon layer thickness of 5-8 nm.

[0027] In this embodiment, the bulk density of graphene oxide in step (d) is 1.8-1.9 g / cm 3 , with an oxygen content of 25-30%, purchased from Shanghai Yuanye Biotechnology Co., Ltd. 0.5g of graphene oxide (GO) was dispersed in 100ml of water, 0.1g of melamine (nitrogen source) was added, and the Na3V2(PO4)2F3@HPC from step (b) was added. The mass ratio of graphene oxide to Na3V2(PO4)2F3@HPC was 1:8-12. After ultrasonic dispersion, the material was spray-dried at an inlet temperature of 170-190°C and an outlet temperature of 70-90°C. Finally, the material was reduced in an H2 / Ar (5:95) atmosphere at 300-320°C for 2-3h to obtain a composite cathode material.

[0028] An embodiment of the third aspect of the present application provides a sodium ion battery, comprising a positive electrode, a negative electrode, a diaphragm and an electrolyte, wherein the positive electrode adopts the positive electrode material, the negative electrode adopts hard carbon, and the diaphragm is an Al2O3 / PVDF-HFP composite ceramic coating diaphragm.

[0029] In this embodiment, the electrolyte includes: Sodium salt: NaPF6; Solvent: a mixed solvent of EC and PC with a volume ratio of 3:6-8; Additives: NaPO2F2, succinonitrile, vinyl sulfate.

[0030] In this embodiment, the preparation method of the hard carbon in the negative electrode includes: pulverizing the biomass material and then immersing it in a ZnCl2 solution; carbonizing it under an Ar atmosphere; and washing it with a solution until it becomes neutral.

[0031] Specifically, the grapefruit peel was crushed and immersed in 0.5 M ZnCl2 solution for 12 h, carbonized at 600 °C under Ar for 4 h, acid-washed to remove residual Zn, and coated on aluminum foil with CMC / SBR binder (8:1:1).

[0032] Example The following examples describe the present disclosure in more detail and are intended to be illustrative only, as various modifications and variations within the scope of the present disclosure will be apparent to those skilled in the art. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are by weight. Unless otherwise stated, all reagents used in the examples are available through conventional commercial sources or synthesized according to conventional methods and can be used directly without further processing. Unless otherwise stated, all instruments used in the examples are available through conventional commercial sources.

[0033] Example 1 A long-cycle polyanion cathode material comprising: Core: particles with the chemical formula Na3V2(PO4)2F3; Intermediate layer: a hierarchical porous carbon layer coated on the surface of the inner core, wherein the hierarchical porous carbon layer comprises micropores with a pore size of <2 nm and mesopores with a pore size of 2-50 nm; Outer layer: nitrogen-doped graphene three-dimensional conductive network, the mass ratio of the nitrogen-doped graphene to the core particles is 1:8-1:12.

[0034] The method for preparing the long-cycle polyanion cathode material comprises the following steps: (a) ZIF-8 and F127 were mixed with sucrose ethanol solution, and the solvent was evaporated before carbonization to produce hierarchical porous carbon (HPC). (b) NaF, NH4VO3, and NH4H2PO4 were ball-milled at a molar ratio of Na:V:P = 3:2:2 with HPC at a mass ratio of 10:1 for 2 h, and 1 wt% boric acid flux was added; (c) Microwave sintering was performed under nitrogen atmosphere at a temperature of 650°C, a heating rate of 80°C / s, a power of 800 W, and a holding time of 15 min to obtain Na3V2(PO4)2F3@HPC; (d) Graphene oxide, melamine and Na3V2(PO4)2F3@HPC were dispersed and then spray-dried. The mass ratio of graphene oxide to Na3V2(PO4)2F3@HPC was 1:10. The inlet temperature of the spray drying was 180°C and the outlet temperature was 80°C. Finally, the composite cathode material was reduced at 300°C in a H2 / Ar (5:95) atmosphere for 2 h to obtain the composite cathode material.

[0035] Example 2 This embodiment is basically the same as embodiment 1, except that the microwave sintering temperature is 700° C., the heating rate is 60° C. / s, and the microwave sintering power is 700W.

[0036] Example 3 This embodiment is substantially the same as embodiment 1, except that the mass ratio of graphene oxide to Na3V2(PO4)2F3@HPC is 1:8.

[0037] Example 4 This embodiment is basically the same as embodiment 1, except that the inlet temperature of the spray drying is 185°C and the outlet temperature is 85°C.

[0038] Comparative Example 1 This comparative example is basically the same as Example 1, except that the multi-level pore carbon coating is replaced by acetylene black coating.

[0039] Comparative Example 2 This comparative example is basically the same as Example 1, except that microwave sintering (heating at 80°C / s to 650°C and holding for 15 minutes) is replaced by conventional sintering (heating at 5°C / s to 650°C and holding for 6 hours).

[0040] Comparative Example 3 This comparative example is substantially the same as Example 1, except that the nitrogen-doped graphene is replaced by ordinary graphene.

[0041] The positive electrode materials in Examples 1-4 and Comparative Examples 1-3 are used in sodium ion batteries, wherein Positive electrode: The positive electrode material of Examples 1-4 and Comparative Examples 1-3: PVDF: Super-P = 92:4:4, coated on a three-dimensional porous foam copper current collector; Anode: hard carbon: CMC: SBR = 94:3:3, coated on aluminum foil; Diaphragm: composite ceramic coating diaphragm Al2O3 / PVDF-HFP; After injection, a step-by-step pre-circulation was performed: 0.1C charge and discharge once (standing for 12 hours) → 0.2C charge and discharge twice (standing for 6 hours) → 0.5C charge and discharge once → forming a stable SEI / CEI film.

[0042] 2032-type coin cells were assembled in an argon glove box (H2O < 0.1 ppm).

[0043] The performance of batteries assembled using the positive electrode materials in Examples 1-4 and Comparative Examples 1-3 was tested, and the results are shown in Table 1.

[0044] Table 1 Performance test results of batteries containing the positive electrode materials in Examples 1-4 and Comparative Examples 1-3

[0045] As can be seen from the above table, the sodium ion battery prepared by using the long-cycle polyanion cathode material provided by the present application exhibits excellent cycle stability and high rate performance. In particular, after 2000 cycles, the battery capacity retention rate of Example 1 is as high as 98%, and the capacity at 10C rate also reaches 112 mAh / g. This may be because the core of the positive electrode material uses Na3V2(PO4)2F3 particles, which have good electrochemical properties. By coating the surface of the core with a multi-level porous carbon layer, micropores less than 2 nm are formed through Na + The adsorption sites increase the specific capacity, and the 2-50nm mesopores accelerate ion transport through low-tortuosity channels, which not only significantly improves the conductivity of the material, but also effectively alleviates the volume expansion of sodium ions during the charge and discharge process, thereby greatly extending the cycle life of the battery. The micropore and mesoporous structure of the multi-level pore carbon layer is conducive to the rapid diffusion of sodium ions and the penetration of electrolyte, further improving the overall performance of the battery. By adopting a heating rate of 80℃ / s and microwave sintering at 650℃, the abnormal growth of grains can be suppressed, and the holding time of 15min can reduce the loss of fluorine volatilization. In addition, the three-dimensional conductive network constructed with nitrogen-doped graphene in the outer layer not only provides abundant electron transmission channels, but also effectively prevents the agglomeration between particles, further improving the cycle stability and rate performance of the material.

[0046] In contrast, the battery performance in Comparative Examples 1-3 is significantly poorer. The 2000 cycle capacity retention rate in Comparative Example 1 is 72%, which is because the disordered accumulation of acetylene black leads to ion blockage and particle cracks; in Comparative Example 2, ordinary sintering technology is also used, resulting in insufficient material crystallinity, which affects the performance of the battery; in Comparative Example 3, due to the use of ordinary graphene, its conductivity and structural stability are not as good as nitrogen-doped graphene, so the battery performance is also reduced. These comparative results show that the long-cycle polyanion cathode material and its preparation method provided by this application, as well as the sodium ion battery using the cathode material, have significant technical advantages and practical application value.

[0047] 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 long-cycle polyanion cathode material, characterized in that: include: Core: particles with the chemical formula Na3V2(PO4)2F3; Middle layer: a multi-level porous carbon layer covering the surface of the inner core; Outer layer: nitrogen-doped graphene three-dimensional conductive network.

2. The long-cycle polyanion cathode material according to claim 1, characterized in that The multi-level porous carbon layer comprises micropores with a pore diameter of less than 2 nm and mesopores with a pore diameter of 2-50 nm.

3. The long-cycle polyanion cathode material according to claim 1, characterized in that The mass ratio of the nitrogen-doped graphene to the core particles is 1:8-1:

12.

4. A method for preparing a long-cycle polyanion cathode material according to any one of claims 1 to 3, characterized in that: The following steps are involved: (a) ZIF-8 and F127 were mixed with sucrose ethanol solution, and the solvent was evaporated before carbonization to produce hierarchical porous carbon (HPC). (b) NaF, NH4VO3, NH4H2PO4 and HPC were ball-milled and mixed again by adding boric acid; (c) Microwave sintering under nitrogen atmosphere to obtain Na3V2(PO4)2F3@HPC; (d) Graphene oxide, melamine and Na3V2(PO4)2F3@HPC were dispersed, spray-dried, and reduced to obtain a composite cathode material.

5. The method for preparing a long-cycle polyanion cathode material according to claim 4, characterized in that: Step (b) The molar ratio of Na, V and P in the NaF, NH4VO3 and NH4H2PO4 is 2.8-3.1:1.9-2.2:

2.

6. The method for preparing a long-cycle polyanion cathode material according to claim 4, characterized in that: The microwave sintering temperature in step (c) is 600-700°C, and / or Heating rate is 50-100℃ / s, and / or The power of microwave sintering is 700-900W.

7. The method for preparing a long-cycle polyanion cathode material according to claim 4, characterized in that: The inlet temperature of the spray drying in step (d) is 170-190°C, and the outlet temperature is 70-90°C.

8. A sodium ion battery, characterized in that: The invention comprises a positive electrode, a negative electrode, a diaphragm and an electrolyte, wherein the positive electrode adopts the positive electrode material according to any one of claims 1 to 3, the negative electrode adopts hard carbon, and the diaphragm is an Al2O3 / PVDF-HFP composite ceramic coating diaphragm.

9. The sodium ion battery according to claim 8, characterized in that The electrolyte comprises: Sodium salt: NaPF6; Solvent: a mixed solvent of EC and PC with a volume ratio of 3:6-8; Additives: NaPO2F2, succinonitrile, vinyl sulfate.

10. The sodium ion battery according to claim 8, characterized in that The method for preparing the hard carbon in the negative electrode comprises: The biomass material is crushed and then immersed in a ZnCl2 solution; Carbonization under Ar atmosphere; Wash with solution until neutral.

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