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

Abstract

The application provides a long-circulation polyanion positive electrode material and a preparation method thereof and a sodium ion battery, wherein the positive electrode material comprises: an inner core, a particle with a chemical formula of Na3V2(PO4)2F3; a middle layer, a multi-level pore carbon layer coated on the surface of the inner core; and an outer layer, a nitrogen-doped graphene three-dimensional conductive network. In the positive electrode material, the multi-level pore carbon layer is coated on the surface of the inner core, which not only improves the conductivity of the material, but also effectively alleviates the volume expansion problem of sodium ions in the charging and discharging process, thereby prolonging the cycle life of the battery; the outer layer adopts the nitrogen-doped graphene three-dimensional conductive network, which further improves the conductivity and structural stability of the material. The three-dimensional conductive network not only provides more electron transmission channels, but also effectively prevents the aggregation between particles, thereby improving the cycle stability and rate performance of the material.

Description

TECHNICAL FIELD

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

[0002] The working principle of a sodium ion battery is similar to that of a lithium ion battery, and sodium salt reserves are more abundant and easier to exploit, so sodium ion batteries have more advantages in large-scale applications. Therefore, based on the abundance of materials and the borrowability of production equipment and process, the sodium ion battery is considered as a promising choice for energy storage. However, the sodium ion radius is much larger than that of lithium ion, which makes the sodium diffusion dynamics slow and it is difficult to find a suitable material with an open framework.

[0003] Common sodium ion positive electrode materials include oxides, prussian blue and polyanions, but in terms of resource abundance, overall cost of materials, electrochemical performance of materials and environmental sustainability, polyanion-type positive electrode materials are the best choice for sodium ion batteries. The current polyanion sodium battery positive electrode material has the performance advantages of low material cost and good cycle performance. Sodium iron sulfate and sodium iron pyrophosphate phosphate have excellent performance in the fields of low cost and long cycle, respectively, and have similar voltage platforms. If they are combined into a composite polyanion sodium battery positive electrode material, they will have the common advantages of low cost and long cycle, which can promote the sodium battery positive electrode material to enter the market faster and expand the application field.

[0004] In the prior art, the performance of sodium ion positive electrode materials is usually improved by doping or coating. For example, patent No. CN202211040949.5 discloses a polyanion-type positive electrode material and a preparation method and application thereof. 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, and obtaining a sintered material after sintering the obtained mixture; mixing a mixed solution of an aluminum source and a fluorine source with the sintered material, drying and primary calcining to obtain the polyanion-type positive electrode material. By generating an aluminum fluoride coating layer in situ on the surface of the positive electrode material, the conductivity of the polyanion-type positive electrode material is improved. However, the traditional aluminum coating layer has limited performance improvement for sodium ion positive electrode materials, and cannot significantly improve the cycle performance of sodium ion batteries.

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

[0006] The present application is made in view of the above problems, and aims 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 application provides a long-circulating polyanion positive electrode material, comprising:

[0008] a core: particles with a chemical formula of Na3V2(PO4)2F3;

[0009] an intermediate layer: a hierarchical porous carbon layer coated on the surface of the core;

[0010] an outer layer: a nitrogen-doped graphene three-dimensional conductive network.

[0011] Further, the hierarchical porous carbon layer contains micropores with a pore size of <2 nm and mesopores with a pore size of 2-50 nm.

[0012] Further, the mass ratio of the nitrogen-doped graphene to the core particles is 1:8-1:12.

[0013] The second aspect of the application provides a preparation method of the long-circulating polyanion positive electrode material, comprising the following steps:

[0014] (a) mixing ZIF-8, F127, and a sucrose ethanol solution, carbonizing after volatilizing the solvent to obtain a hierarchical porous carbon (HPC);

[0015] (b) ball-milling NaF, NH4VO3, NH4H2PO4, and HPC, and adding boric acid for mixing again;

[0016] (c) performing microwave sintering under a nitrogen atmosphere to obtain Na3V2(PO4)2F3@HPC;

[0017] (d) dispersing graphene oxide and melamine with Na3V2(PO4)2F3@HPC, spray drying, and reducing to obtain a composite positive electrode material.

[0018] Further, in step (b), the molar ratio of Na, V, and P in NaF, NH4VO3, and NH4H2PO4 is 2.8-3.1:1.9-2.2:2.

[0019] Further, in step (c), the temperature of microwave sintering is 600-700°C, and / or

[0020] the heating rate is 50-100°C / s, and / or

[0021] the power of microwave sintering is 700-900 W.

[0022] Further, in step (d), the inlet temperature of spray drying is 170-190°C, and the outlet temperature is 70-90°C.

[0023] The third aspect of the application provides a sodium ion battery, comprising a positive electrode, a negative electrode, a separator and an electrolyte, wherein the positive electrode adopts the positive electrode material, the negative electrode adopts hard carbon, and the separator is an Al2O3 / PVDF-HFP composite ceramic coating separator.

[0024] Further, the electrolyte comprises:

[0025] Sodium salt: NaPF6;

[0026] Solvent: a mixed solvent with a volume ratio of EC to PC of 3:6-8;

[0027] Additive: NaPO2F2, butanedinitrile, ethylene sulfate.

[0028] Further, the preparation method of the hard carbon in the negative electrode comprises:

[0029] After the biomass material is crushed, it is immersed in a ZnCl2 solution;

[0030] Carbonization under Ar atmosphere;

[0031] Washing with a solution until neutral.

[0032] The application has the following beneficial effects:

[0033] (1) The long-cycle polyanion positive electrode material of the application adopts particles with a chemical formula of Na3V2(PO4)2F3 as the core, and the material itself has good electrochemical performance; by coating a multi-level pore carbon layer on the surface of the core, the conductivity of the material is improved, and the volume expansion problem of sodium ions in the charging and discharging process is effectively alleviated, thereby prolonging the cycle life of the battery; the multi-level pore carbon layer comprises micropores with a pore size of less than 2 nm and mesopores with a pore size of 2-50 nm, and such a structure is beneficial 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 three-dimensional conductive network of nitrogen-doped graphene, further improving the conductivity and structural stability of the material, and such a 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.

[0034] (2) The application obtains hierarchical porous carbon by mixing ZIF-8, F127 and sucrose ethanol solution and carbonizing, thereby providing a basis for a subsequent coating process, then NaF, NH4VO3, NH4H2PO4 and the hierarchical porous carbon are ball-milled and mixed, and boric acid is added for re-mixing to ensure uniform distribution of raw materials; microwave sintering is performed under a nitrogen atmosphere to obtain a Na3V2(PO4)2F3@HPC composite material; finally, graphene oxide, melamine and Na3V2(PO4)2F3@HPC are dispersed, spray dried and subjected to reduction treatment to obtain the final long-cycle polyanion positive electrode material. This preparation method is simple and easy to industrialize.

[0035] (3) The application also provides a sodium ion battery using the positive electrode material, which comprises a positive electrode, a negative electrode, a separator and an electrolyte, wherein the positive electrode uses the long-cycle polyanion positive electrode material prepared above, the negative electrode uses hard carbon, the separator is an Al2O3 / PVDF-HFP composite ceramic coating separator, and the electrolyte comprises NaPF6, EC, PC mixed solvents and NaPO2F2, butanedinitrile, vinyl sulfate and other additives. This battery has excellent cycle performance and rate performance. DETAILED DESCRIPTION

[0036] In order to make the purpose, technical scheme and advantages of the application clearer, the following describes and explains the application with examples. It should be understood that the specific examples described herein are only used to explain the application and not to limit the application. Based on the examples provided by the application, all other examples obtained by those of ordinary skill in the art without creative labor fall within the scope of the application.

[0037] Obviously, the following description is only some examples or embodiments of the application, and for those of ordinary skill in the art, the application can also be applied to other similar scenarios without creative labor. In addition, it can be understood that although the efforts made in this development process can be complex and lengthy, for those of ordinary skill in the art related to the content disclosed in the application, some design, manufacture or production changes based on the technology disclosed in the application are only routine technical means and should not be understood as insufficient disclosure of the application.

[0038] The embodiments of the first aspect of the application provide a long-cycle polyanion positive electrode material, comprising:

[0039] Core: particles with a chemical formula of Na3V2(PO4)2F3;

[0040] Intermediate layer: a multi-level pore carbon layer coated on the surface of the core, the multi-level pore carbon layer comprising micropores with a pore size of <2 nm and mesopores with a pore size of 2-50 nm;

[0041] Outer layer: a three-dimensional conductive network of nitrogen-doped graphene, the mass ratio of the nitrogen-doped graphene to the core particle being 1:8-1:12.

[0042] In the long-circulation polyanion positive electrode material of the present application, the core is a particle with a chemical formula of Na3V2(PO4)2F3, which has good electrochemical performance itself. By coating a multi-level pore carbon layer on the surface of the core, the conductivity of the material is improved, and the volume expansion problem of sodium ions during charging and discharging is effectively alleviated, thereby prolonging the cycle life of the battery. The multi-level pore carbon layer comprises micropores with a pore size of <2 nm and mesopores with a pore size of 2-50 nm, which is beneficial 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 three-dimensional conductive network of nitrogen-doped graphene, and the nitrogen content in the nitrogen-doped graphene is 5-7 at%, which further improves the conductivity and structural stability of the material. This three-dimensional conductive network not only provides more electron transport channels, but also effectively prevents the agglomeration between particles, thereby improving the cycle stability and rate performance of the material.

[0043] The embodiment of the second aspect of the present application provides a preparation method of the long-circulation polyanion positive electrode material, comprising the following steps:

[0044] (a) mixing ZIF-8, F127 and a sucrose ethanol solution, carbonizing after volatilizing the solvent to obtain a multi-level pore carbon (HPC);

[0045] (b) ball-milling NaF, NH4VO3, NH4H2PO4 and HPC, and adding boric acid for mixing again;

[0046] (c) microwave sintering under a nitrogen atmosphere to obtain Na3V2(PO4)2F3@HPC;

[0047] (d) dispersing graphene oxide and melamine in Na3V2(PO4)2F3@HPC, spray drying, and reducing to obtain a composite positive electrode material.

[0048] In the present embodiment, in step (a), 1 g of ZIF-8 (zeolite imidazole framework) is dispersed in 200 ml of ethanol, 0.5 g of block copolymer F127 (template agent) is added, and 2 g of sucrose ethanol solution (50 wt%) is added dropwise. After ultrasonic treatment for 30 min, the solvent is volatilized at 80°C, and the temperature is raised to 700°C at a rate of 5°C / min under an Ar atmosphere, and the temperature is kept for 2 h to obtain a multi-level pore carbon (HPC).

[0049] In the present embodiment, NaF, NH4VO3, NH4H2PO4 in step (b) are ball-milled in a molar ratio of Na:V:P = 2.8-3.1:1.9-2.2:2. Preferably, NaF, NH4VO3, NH4H2PO4 are ball-milled in a molar ratio of Na:V:P = 3:2:2 with HPC (mass ratio 8-12:1) of step (a) for 2-3 h, a fluxing agent 1wt% boric acid is added, microwave sintering in a tube furnace, the temperature of microwave sintering is 600-700℃, the heating rate is 50-100℃ / s, the power of microwave sintering is 700-900W, to obtain Na3V2(PO4)2F3@HPC, the particle size is 100-300nm, and the carbon layer thickness is 5-8nm.

[0050] In the present embodiment, the bulk density of graphene oxide in step (d) is 1.8-1.9 g / cm 3 , and the oxygen content is 25-30%. The graphene oxide (GO) is purchased from Shanghai Yuan Ye Biological Technology Co., Ltd. 0.5g of the graphene oxide (GO) is dispersed in 100ml of water, 0.1g of melamine (a nitrogen source) is added, and Na3V2(PO4)2F3@HPC of step (b) is added. The mass ratio of the graphene oxide to the Na3V2(PO4)2F3@HPC is 1:8-12. After ultrasonic dispersion, spray drying is performed at an inlet temperature of 170-190℃ and an outlet temperature of 70-90℃. Finally, reduction treatment is performed at 300-320℃ under a H2 / Ar (5:95) atmosphere for 2-3h to obtain a composite cathode material.

[0051] Embodiments of the third aspect of the present application provide a sodium ion battery, which comprises a cathode, an anode, a separator and an electrolyte, wherein the cathode adopts the cathode material, the anode adopts hard carbon, and the separator is an Al2O3 / PVDF-HFP composite ceramic coating separator.

[0052] In the present embodiment, the electrolyte comprises:

[0053] Sodium salt: NaPF6;

[0054] Solvent: a mixed solvent of EC and PC in a volume ratio of 3:6-8;

[0055] Additive: NaPO2F2, butanedinitrile and ethylene sulfate.

[0056] In the present embodiment, the preparation method of the hard carbon in the anode comprises: crushing a biomass material and then immersing it in a ZnCl2 solution; carbonizing under an Ar atmosphere; and washing with a solution until neutral.

[0057] Specifically, the pomelo peel is crushed and immersed in a 0.5M ZnCl2 solution for 12 hours, carbonized at 600°C under Ar for 4 hours, and then washed with acid to remove residual Zn. The pomelo peel is coated with a CMC / SBR binder (8:1:1) on an aluminum foil.

[0058] EMBODIMENT

[0059] The present disclosure is more particularly described in the following examples that are intended to be illustrative only, as numerous modifications and variations within the scope of the present disclosure will be apparent to those skilled in the art. Unless otherwise noted, all parts, percentages, and ratios reported herein are on a weight basis. Unless otherwise noted, all reagents used in the examples are commercially available or synthesized according to conventional methods and used without further purification. Unless otherwise noted, all equipment used in the examples is commercially available.

[0060] EMBODIMENT

[0061] A long-circulation polyanionic positive electrode material, comprising:

[0062] a core: particles with a chemical formula of Na3V2(PO4)2F3;

[0063] an intermediate layer: a multi-level pore carbon layer coated on the surface of the core, the multi-level pore carbon layer comprising micropores with a pore size < 2 nm and mesopores with a pore size of 2-50 nm;

[0064] an outer layer: a nitrogen-doped graphene three-dimensional conductive network, the mass ratio of the nitrogen-doped graphene to the core particles being 1:8-1:12.

[0065] A preparation method of the long-circulation polyanionic positive electrode material, comprising the following steps:

[0066] (a) mixing ZIF-8, F127, and a sucrose ethanol solution, carbonizing after volatilizing the solvent to obtain a multi-level pore carbon (HPC);

[0067] (b) ball-milling NaF, NH4VO3, and NH4H2PO4 in a molar ratio of Na:V:P=3:2:2 and HPC in a mass ratio of 10:1 for 2 hours, and adding 1wt% boric acid flux;

[0068] (c) microwave sintering under a nitrogen atmosphere, the microwave sintering temperature being 650°C, the heating rate being 80°C / s, the microwave sintering power being 800W, and the holding time being 15 minutes, to obtain Na3V2(PO4)2F3@HPC;

[0069] (d) dispersing graphene oxide, melamine and Na3V2(PO4)2F3@HPC and then spray drying, wherein the mass ratio of graphene oxide to Na3V2(PO4)2F3@HPC is 1:10, the inlet temperature of spray drying is 180℃, the outlet temperature is 80℃, and finally, the composite positive electrode material is obtained by reducing treatment at 300℃ in a H2 / Ar (5:95) atmosphere for 2h.

[0070] Example 2

[0071] This example is basically the same as Example 1, except that the temperature of microwave sintering is 700℃, the heating rate is 60℃ / s, and the power of microwave sintering is 700W.

[0072] Example 3

[0073] This example is basically the same as Example 1, except that the mass ratio of graphene oxide to Na3V2(PO4)2F3@HPC is 1:8.

[0074] Example 4

[0075] This example is basically the same as Example 1, except that the inlet temperature of spray drying is 185℃, and the outlet temperature is 85℃.

[0076] Comparative Example 1

[0077] This comparative example is basically the same as Example 1, except that the multi-level porous carbon coating is replaced by acetylene black coating.

[0078] Comparative Example 2

[0079] This comparative example is basically the same as Example 1, except that the microwave sintering (heating rate of 80℃ / s to 650℃ for 15min) is replaced by conventional sintering (heating rate of 5℃ / s to 650℃ for 6h).

[0080] Comparative Example 3

[0081] This comparative example is basically the same as Example 1, except that the nitrogen-doped graphene is replaced by ordinary graphene.

[0082] The positive electrode materials in Examples 1-4 and Comparative Examples 1-3 are used in sodium-ion batteries, wherein

[0083] Positive electrode: positive electrode material in Examples 1-4 and Comparative Examples 1-3 : PVDF : Super-P = 92:4:4, coated on a three-dimensional porous foam copper current collector;

[0084] Negative electrode: hard carbon : CMC : SBR = 94:3:3, coated on an aluminum foil;

[0085] Separator: composite ceramic-coated separator Al2O3 / PVDF-HFP

[0086] After liquid injection, a stepwise pre-circulation was carried out: 0.1C charge-discharge 1 time (rest for 12h) → 0.2C charge-discharge 2 times (rest for 6h) → 0.5C charge-discharge 1 time → form stable SEI / CEI film.

[0087] 2032 type button cells were assembled in an argon glove box (H2O < 0.1 ppm).

[0088] The batteries assembled with the positive electrode materials in Examples 1-4 and Comparative Examples 1-3 above were subjected to performance detection, and the results are shown in Table 1.

[0089] Table 1 Performance detection results of batteries containing the positive electrode materials in Examples 1-4 and Comparative Examples 1-3

[0090]

[0091] From the above table, it can be seen that the sodium ion battery prepared by using the long cycle polyanion positive electrode material provided in the present application exhibits excellent cycle stability and high rate performance, especially after 2000 cycles, the capacity retention rate of the battery of Example 1 is as high as 98%, and the capacity at 10C rate reaches 112mAh / g. It is possible that the core of the positive electrode material adopts Na3V2(PO4)2F3 particles, which have good electrochemical performance. By coating a multi-level pore carbon layer on the surface of the core, the micropores less than 2nm improve the specific capacity through Na + adsorption sites, and the mesopores of 2-50nm accelerate ion transmission, not only significantly improving the conductivity of the material, but also effectively relieving the volume expansion of sodium ions during charging and discharging, thereby greatly prolonging the cycle life of the battery. The micropore and mesopore structure of the multi-level pore carbon layer is beneficial to the rapid diffusion of sodium ions and the penetration of electrolyte, further improving the overall performance of the battery. By using a heating rate of 80℃ / s for microwave sintering at 650℃, the abnormal growth of crystal grains can be inhibited, and the holding time of 15min can reduce the loss of fluorine volatilization. In addition, the three-dimensional conductive network constructed by the nitrogen-doped graphene in the outer layer not only provides rich electron transmission channels, but also effectively prevents the agglomeration between particles, further improving the cycle stability and rate performance of the material.

[0092] In contrast, the battery performance in Comparative Example 1-3 is obviously poor, the capacity retention rate of 2000 cycles in Comparative Example 1 is 72%, because the disordered accumulation of acetylene black leads to ion blockage and particle cracking; in Comparative Example 2, the ordinary sintering technology is also used, which leads to insufficient crystallinity of the material, affecting the performance of the battery; in Comparative Example 3, ordinary graphene is used, which has poorer conductivity and structural stability than nitrogen-doped graphene, so the battery performance also decreases. These comparison results show that the long-cycle polyanion positive electrode material and the preparation method thereof provided by the present application, and the sodium ion battery using the positive electrode material, have significant technical advantages and practical application value.

[0093] It should be noted that the present application is not limited to the above embodiments. The above embodiments are only examples, and embodiments having substantially the same configuration and playing the same role and effect as the technical idea 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 that can be thought of by those skilled in the art are applied to the embodiments, and other ways constructed by combining part of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A long-cycling polyanionic cathode material, characterized in that, Comprise: a core: particles of chemical formula Na3V2(PO4)2F3; an intermediate layer: a multi-level pore carbon layer coated on the surface of the core, the multi-level pore carbon layer comprising micropores with a pore size <2nm and mesopores with a pore size of 2-50nm; an outer layer: a nitrogen-doped graphene three-dimensional conductive network; The preparation method of the long-circulating polyanion positive electrode material comprises the following steps: (a) mixing ZIF-8, F127 and sucrose ethanol solution, carbonizing after volatilizing the solvent to obtain multi-level pore carbon HPC; (b) ball-milling NaF, NH4VO3, NH4H2PO4 and HPC, adding boric acid and mixing again; (c) microwave sintering under a nitrogen atmosphere, the temperature of the microwave sintering being 600-700℃, the heating rate being 50-100℃ / s, the power of the microwave sintering being 700-900W, and the holding time being 15min, to obtain Na3V2(PO4)2F3@HPC; (d) dispersing graphene oxide and melamine in Na3V2(PO4)2F3@HPC, spray drying, and reducing to obtain the composite positive electrode material.

2. The long cycle polyanionic cathode material of claim 1, wherein, The mass ratio of the nitrogen-doped graphene to the core particles is 1:8-1:

12.

3. The long cycle polyanionic cathode material of claim 1, wherein, In step (b), the molar ratio of Na, V and P in NaF, NH4VO3 and NH4H2PO4 is 2.8-3.1:1.9-2.2:

2.

4. The long cycle polyanionic cathode material of claim 1, wherein, In step (d), the inlet temperature of the spray drying is 170-190℃, and the outlet temperature is 70-90℃.

5. A sodium-ion battery, characterized in that, The battery comprises a positive electrode, a negative electrode, a separator and an electrolyte, wherein the positive electrode uses the positive electrode material according to any one of claims 1-4, the negative electrode uses hard carbon, and the separator is an Al2O3 / PVDF-HFP composite ceramic coating separator.

6. The sodium-ion battery of claim 5, wherein, The electrolyte comprises: a sodium salt: NaPF6; a solvent: a mixed solvent of EC and PC in a volume ratio of 3:6-8; an additive: NaPO2F2, butanedinitrile and ethylene sulfate.

7. The sodium-ion battery of claim 5, wherein, The preparation method of the hard carbon in the negative electrode comprises: crushing a biomass material and then immersing it in a ZnCl2 solution; carbonizing under an Ar atmosphere; washing with a solution until neutral.

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