A boron-doped carbon-coated modified sodium vanadium fluorophosphate material, a preparation method and application thereof

Abstract

The application discloses a kind of boron-doped carbon-coated modified sodium vanadium fluorophosphate materials and its preparation method and application, including the following steps: (a) vanadium source is dissolved in deionized water to obtain a first solution;(b) adding reducing agent to the first solution to obtain a second solution;(c) phosphorus source, sodium source and fluorine source are added to the second solution according to stoichiometric ratio, heated and stirred continuously until the solution becomes gel;The gel is dried, ground into powder, to obtain sodium vanadium fluorophosphate precursor;(d) the sodium vanadium fluorophosphate precursor is calcined under inert gas condition, to obtain sodium vanadium fluorophosphate material;(e) the sodium vanadium fluorophosphate material is mixed with boric acid in proportion to obtain a first mixture;(f) the first mixture is calcined under inert gas. Boron-doped carbon-coated sodium vanadium fluorophosphate positive electrode material can be obtained.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of nanomaterials, and relates to a fluorinated sodium vanadium phosphate material, in particular to a boron-doped carbon-coated modified fluorinated sodium vanadium phosphate material and a preparation method and application thereof. BACKGROUND

[0002] Since 2020, China has proposed a series of targets and visions for addressing climate change in major international events, and has issued a series of views and initiatives, which have pointed out the direction for green and low-carbon transformation and injected new energy into the global climate governance. In order to implement the resolution, provinces and cities are also planning their own green and low-carbon development paths, and new material energy storage has received extensive attention.

[0003] At present, among all battery types, the total amount of lithium ion batteries still accounts for more than 85%; however, the problems of poor low-temperature performance, slow charging rate and inability to high-rate charge and discharge have always been unable to be completely solved; in addition, in recent years, the price of lithium ore has soared, making the price of lithium ion batteries high, which has hindered the further development of lithium ion batteries. Therefore, finding other secondary batteries that can gradually replace lithium ion batteries has been put on the agenda. In recent years, sodium element, which belongs to the same first main group as lithium, has returned to the sight of people. Because the sodium resources on the earth are more abundant than the lithium resources (400 times more than lithium element), and are widely distributed, the price of sodium resources is lower than that of lithium resources. However, due to the fact that sodium ions are heavier and have a larger ionic radius than lithium ions, the energy density of lithium ion batteries is higher than that of sodium ion batteries under the same volume condition; at the same time, the diffusion rate of sodium ions is lower than that of lithium ions, which also brings certain challenges to the development of sodium ion batteries.

[0004] Fluorinated sodium vanadium phosphate (NVPF) belongs to a polyanion type electrode material, which is evolved from sodium vanadium phosphate (NVP), that is, one phosphate ion is replaced by three fluoride ions, and the theoretical capacity is also increased from 117 mAh / g to 128 mAh / g, and the average working voltage platform is increased from 3.4 V to 3.9 V; therefore, compared with sodium vanadium phosphate, the fluorinated sodium vanadium phosphate material has a broader application prospect. However, the problem of low electronic conductivity of the fluorinated sodium vanadium phosphate positive electrode material limits its further development. If the fluorinated sodium vanadium phosphate material is not modified, its capacity and energy density will not be able to adapt to the existing energy storage system. SUMMARY

[0005] The present application provides a boron-doped carbon-coated modified fluorinated sodium vanadium phosphate material to solve the above-mentioned defects of the prior art.

[0006] To achieve the above-mentioned purpose, the present application provides a preparation method of a boron-doped carbon-coated modified fluorinated sodium vanadium phosphate material, comprising the following steps:

[0007] (a) dissolving a vanadium source in deionized water to obtain a first solution;

[0008] (b) adding a reducing agent to the first solution to obtain a second solution;

[0009] (c) adding a phosphorus source, a sodium source and a fluorine source in stoichiometric ratio to the second solution, heating and continuously stirring until the solution becomes a gel; drying the gel, grinding into powder to obtain a sodium vanadium fluorophosphate precursor;

[0010] (d) calcining the sodium vanadium fluorophosphate precursor under inert gas to obtain a sodium vanadium fluorophosphate material;

[0011] (e) mixing the sodium vanadium fluorophosphate material with boric acid in a certain proportion to obtain a first mixture;

[0012] (f) calcining the first mixture under inert gas to obtain a boron-doped carbon-coated sodium vanadium fluorophosphate positive electrode material.

[0013] Optimally, in step (a), the vanadium source is a mixture selected from one or more of ammonium metavanadate, vanadium pentoxide and vanadium trichloride.

[0014] Optimally, in step (b), the molar ratio of the vanadium source to the reducing agent is 1:1-1.5, and the reducing agent is citric acid.

[0015] Optimally, in step (c), the heating temperature is 50-80℃.

[0016] Optimally, in step (d), the inert gas is nitrogen or argon, the calcination temperature is 500-700℃, and the time is 8-10h.

[0017] Optimally, in step (e), the mass ratio of the sodium vanadium fluorophosphate material to the boric acid is 1:0.02-0.2.

[0018] Optimally, in step (f), the inert gas is nitrogen or argon, the calcination temperature is 300-500℃, and the time is 1-3h.

[0019] Still another object of the present application is to provide a boron-doped carbon-coated modified sodium vanadium fluorophosphate material prepared by the above preparation method.

[0020] Still another object of the present application is to provide an application of the above boron-doped carbon-coated modified sodium vanadium fluorophosphate material as a positive electrode active material for sodium ion batteries.

[0021] The boron-doped carbon-coated modified sodium vanadium fluorophosphate material, a conductive agent and a binder are mixed uniformly in a mass ratio of 8:1:1, a solvent is added to grind into a slurry, the slurry is coated on a current collector, dried and cut into electrode sheets.

[0022] With the above technical solution, the present application has the following advantages compared with the prior art: the preparation method of the boron-doped carbon-coated modified sodium vanadium fluorophosphate material uses a simple sol-gel method to synthesize a carbon-coated sodium vanadium fluorophosphate material, and the carbon-coated sodium vanadium fluorophosphate material is mixed uniformly with boric acid in a certain proportion and calcined to obtain a boron-doped carbon-coated sodium vanadium fluorophosphate positive electrode material; citric acid acts as a reducing agent in the sol-gel process and also serves as a carbon source, so that no additional carbon source needs to be added in the synthesis process, which is simple, efficient, economical and environmentally friendly; boron doping is used to achieve the following two purposes: 1. High-temperature decomposition of boric acid forms boron oxide, which forms a carbon-boron bond with carbon in the carbon layer on the surface of the material, and the electron defects of boron in the carbon-boron bond produce holes, which carry electrons through the conductive carbon layer, so that the bulk material improves the electronic conductivity without accelerating the diffusion of sodium ions in the carbon coating; 2. The boron-doped carbon layer provides more active sites to improve the conductivity of the material, thereby obtaining higher capacity. As a positive electrode material for sodium ion batteries, it has higher cycle stability and high-rate performance, and the process of the present application is simple, green and environmentally friendly, which is conducive to the promotion of all lithium ion battery and sodium ion battery positive electrode materials. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 XRD diffraction pattern of the boron-doped carbon-coated modified sodium vanadium fluorophosphate material prepared in Examples 1-4;

[0024] Figure 2 SEM image of the boron-doped carbon-coated modified sodium vanadium fluorophosphate material prepared in Example 1;

[0025] Figure 3 TEM image of the boron-doped carbon-coated modified sodium vanadium fluorophosphate material prepared in Example 1;

[0026] Figure 4 Elemental distribution map of the boron-doped carbon-coated modified sodium vanadium fluorophosphate material prepared in Example 1;

[0027] Figure 5 Battery cycle diagram of the boron-doped carbon-coated modified sodium vanadium fluorophosphate material prepared in Examples 1-4;

[0028] Figure 6 Battery rate diagram of the boron-doped carbon-coated modified sodium vanadium fluorophosphate material prepared in Examples 1-4. DETAILED DESCRIPTION

[0029] The preparation method of the boron-doped carbon-coated modified sodium vanadium fluorophosphate material includes the following steps: (a) dissolving a vanadium source in deionized water to obtain a first solution; (b) adding a reducing agent to the first solution to obtain a second solution; (c) adding phosphorus source, sodium source and fluorine source in a stoichiometric ratio (usually a molar ratio of 2:3:3) to the second solution, and continuously stirring under heating until the solution becomes a gel; drying the gel, grinding it into powder to obtain a sodium vanadium fluorophosphate precursor; (d) calcining the sodium vanadium fluorophosphate precursor under inert gas conditions to obtain a sodium vanadium fluorophosphate material; (e) mixing the sodium vanadium fluorophosphate material with boric acid in a certain proportion to obtain a first mixture; (f) calcining the first mixture under inert gas to obtain a boron-doped carbon-coated sodium vanadium fluorophosphate positive electrode material. The carbon-coated sodium vanadium fluorophosphate material is synthesized by using a simple sol-gel method, and the boron-doped carbon-coated sodium vanadium fluorophosphate positive electrode material can be obtained by uniformly mixing and calcining the carbon-coated sodium vanadium fluorophosphate material with boric acid. Citric acid acts as a reducing agent in the sol-gel process and also serves as a carbon source, so that no additional carbon source is needed in the synthesis process, which is simple, efficient, economical and environmentally friendly. Moreover, boron doping is used to achieve the following two purposes: 1. Boric acid decomposes at high temperature to form boron oxide, which forms a carbon-boron bond with carbon in the carbon layer on the surface of the material. The electron defect of boron in the carbon-boron bond produces holes, and the holes carry electrons through the conductive carbon layer, thereby improving the electronic conductivity of the bulk material without accelerating the diffusion of sodium ions in the carbon coating; 2. The boron-doped carbon layer provides more active sites, improving the conductivity of the material and thus achieving higher capacity. As a positive electrode material for sodium ion batteries, it has higher cycle stability and high-rate performance, and the process of the present application is simple, green and environmentally friendly, which is conducive to the promotion of all lithium ion battery and sodium ion battery positive electrode materials.

[0030] In step (a), the vanadium source is a mixture of one or more selected from ammonium metavanadate, vanadium pentoxide and vanadium trichloride. In step (b), the molar ratio of the vanadium source to the reducing agent is 1:1-1.5. In step (c), the heating temperature is 50-80℃. In step (d), the inert gas is nitrogen or argon, the calcination temperature is 500-700℃, and the time is 8-10h. In step (e), the mass ratio of the sodium vanadium fluorophosphate material to the boric acid is 1:0.02-0.2. In step (f), the inert gas is nitrogen or argon, the calcination temperature is 300-600℃, and the time is 1-3h.

[0031] The application of the boron-doped carbon-coated modified sodium vanadium fluorophosphate material is as a positive active material for sodium ion batteries. Preferably, the boron-doped carbon-coated modified sodium vanadium fluorophosphate material, a conductive agent and a binder are mixed in a mass ratio of 8:1:1, and a solvent is added to grind a slurry; the slurry is coated on a current collector, dried and cut into an electrode sheet. Specifically, the active material (boron-doped carbon-coated sodium vanadium fluorophosphate material), a conductive agent (acetylene black) and a binder (PVDF) are mixed in a mass ratio of 8:1:1, and a solvent N-methyl pyrrolidone (NMP) is added to grind for a period of time. The above slurry is coated on a current collector (aluminum foil), dried and cut into a circular electrode sheet with a diameter of 12 mm; a CR2032 type button cell is assembled in an argon glove box, with 1M NaClO4 as a sodium salt, a mixed solvent (volume ratio 1:1) of EC and DMC containing 5% FEC by volume as an electrolyte, a sodium sheet as a negative electrode of the half cell and a glass fiber as a separator.

[0032] The preferred embodiments of the application are described in detail below.

[0033] Example 1

[0034] The present embodiment provides a boron-doped carbon-coated modified sodium vanadium fluorophosphate material and a preparation method thereof, comprising the following steps:

[0035] (a) 20 mmol of ammonium metavanadate (2.3421 g) is dissolved in deionized water, heated and stirred at 80°C to obtain a light yellow solution (i.e. a first solution);

[0036] (b) 30 mmol of citric acid (5.7926 g) is added to the first solution, and heating and stirring are continued to obtain a dark blue solution (i.e. a second solution);

[0037] (c) 20 mmol of ammonium dihydrogen phosphate (2.3 g), 30 mmol of sodium fluoride (1.2598 g) and 3 mmol of ammonium fluoride (0.1111 g) are sequentially added to the dark blue solution of step (b), and heating and stirring are continued until the solution is homogeneous; heating and stirring are continued until the liquid becomes a gel, the gel is placed in a forced air drying oven at 90°C for drying, and is ground into a powder in a mortar to obtain a sodium vanadium fluorophosphate precursor;

[0038] (d) the above powder (sodium vanadium fluorophosphate precursor) is calcined at 600°C for 8h under argon protection to obtain a sodium vanadium fluorophosphate positive electrode material;

[0039] (e) the sodium vanadium fluorophosphate powder in step (d) is mixed with boric acid in a mass ratio of 1:0.05 to obtain a first mixture;

[0040] (f) calcining the first mixture at 500℃ for 4h under nitrogen to obtain the boron-doped carbon-coated sodium fluorovanadophosphate positive electrode material.

[0041] Example 2

[0042] The present example provides a preparation method of a boron-doped carbon-coated sodium fluorovanadophosphate material, which is basically the same as the preparation method in Example 1, except that in step (e), the mass ratio of sodium fluorovanadophosphate to boric acid is 1:0.02.

[0043] Example 3

[0044] The present example provides a preparation method of a boron-doped carbon-coated sodium fluorovanadophosphate material, which is basically the same as the preparation method in Example 1, except that in step (f), the mass ratio of sodium fluorovanadophosphate to boric acid is 1:0.1.

[0045] Example 4

[0046] The present example provides a preparation method of a boron-doped carbon-coated sodium fluorovanadophosphate material, which is basically the same as the preparation method in Example 1, except that in step (f), the mass ratio of sodium fluorovanadophosphate to boric acid is 1:0.2.

[0047] Example 5

[0048] The present example provides a preparation method of a boron-doped carbon-coated sodium fluorovanadophosphate material, which is basically the same as the preparation method in Example 1, except that the molar ratio of ammonium metavanadate to citric acid is 1:1.

[0049] Comparative Example 1

[0050] The present example provides a preparation method of a boron-doped carbon-coated sodium fluorovanadophosphate material, which is basically the same as the preparation method in Example 1, except that step (e) is not performed.

[0051] Comparative Example 2

[0052] The present example provides a preparation method of a boron-doped carbon-coated sodium fluorovanadophosphate material, which is basically the same as the preparation method in Example 1, except that in step (e), the mass ratio of sodium fluorovanadophosphate to boric acid is 1:0.01.

[0053] Comparative Example 3

[0054] The present example provides a preparation method of a boron-doped carbon-coated sodium fluorovanadophosphate material, which is basically the same as the preparation method in Example 1, except that in step (e), the mass ratio of sodium fluorovanadophosphate to boric acid is 1:0.3.

[0055] Comparative Example 4

[0056] The example provides a preparation method of boron-doped carbon-coated sodium vanadium fluorophosphate material, which is basically the same as the preparation method in Example 1, except that the reducing agent in step (b) is replaced by oxalic acid instead of citric acid.

[0057] Comparative Example 5

[0058] The example provides a preparation method of boron-doped carbon-coated sodium vanadium fluorophosphate material, which is basically the same as the preparation method in Example 1, except that the calcination temperature in step (f) is 300°C.

[0059] Comparative Example 6

[0060] The example provides a preparation method of boron-doped carbon-coated sodium vanadium fluorophosphate material, which is basically the same as the preparation method in Example 1, except that the calcination temperature in step (f) is 600°C.

[0061] Comparative Example 7

[0062] The example provides a preparation method of boron-doped carbon-coated sodium vanadium fluorophosphate material, which is basically the same as the preparation method in Example 1, except that the molar ratio of ammonium metavanadate to citric acid is 1:2.

[0063] The boron-doped carbon-coated sodium vanadium fluorophosphate material in Examples 1-5 and Comparative Examples 1-7 (as a positive active material for sodium ion batteries) was prepared into button half-cells according to the application method described above and subjected to electrochemical testing, and the results are shown in Table 1 (see Figures 1 to 6 ) for part of the performance.

[0064] Table 1 Battery performance of Examples 1-5 and Comparative Examples 1-7

[0065]

[0066]

[0067] The above examples are only for illustrating the technical concept and characteristics of the present application, and the purpose is to enable those skilled in the art to understand the content of the present application and implement it, and cannot limit the protection scope of the present application. Any equivalent changes or modifications made in accordance with the spirit and essence of the present application should be covered within the protection scope of the present application.

Claims

1. A method for preparing a boron-doped carbon-coated modified sodium vanadium fluorophosphate material, characterized in that, The method comprises the following steps: (a) dissolving a vanadium source in deionized water to obtain a first solution; (b) adding a reducing agent to the first solution to obtain a second solution; the molar ratio of the vanadium source to the reducing agent is 1:1-1.5, and the reducing agent is citric acid; (c) adding a phosphorus source, a sodium source, and a fluorine source in stoichiometric proportions to the second solution, heating, and continuously stirring until the solution becomes a gel; drying the gel, grinding it into a powder, and obtaining a sodium vanadium fluorophosphate precursor; (d) calcining the sodium vanadium fluorophosphate precursor under nitrogen or argon to obtain a sodium vanadium fluorophosphate material; (e) mixing the sodium vanadium fluorophosphate material with boric acid in a certain proportion to obtain a first mixture; the mass ratio of the sodium vanadium fluorophosphate material to the boric acid is 1:0.02-0.2; (f) calcining the first mixture under nitrogen or argon to obtain a boron-doped carbon-coated sodium vanadium fluorophosphate positive electrode material; the calcination temperature is 300-500°C.

2. The method for preparing boron-doped carbon-coated modified sodium vanadium fluorophosphate material according to claim 1, characterized in that: In step (a), the vanadium source is a mixture composed of one or more selected from ammonium metavanadate and vanadium pentoxide.

3. The method for preparing boron-doped carbon-coated modified sodium vanadium fluorophosphate material according to claim 1, characterized in that: In step (c), the heating temperature is 50-80°C.

4. The method for preparing boron-doped carbon-coated modified sodium vanadium fluorophosphate material according to claim 1, characterized in that: In step (d), the calcination temperature is 500-700°C, and the time is 8-10h.

5. The method for preparing boron-doped carbon-coated modified sodium vanadium fluorophosphate material according to claim 1, characterized in that: In step (f), the calcination time is 1-3h.

6. A boron-doped carbon-coated modified sodium vanadium fluorophosphate material, characterized in that, It is prepared by the preparation method of any one of claims 1-5.

7. The use of the boron-doped carbon-coated modified sodium vanadium fluorophosphate material according to claim 6, characterized in that it is used as an electrode material for sodium-ion batteries. It is used as a positive active material for sodium ion batteries.

8. The use of the boron-doped carbon-coated modified sodium vanadium fluorophosphate material according to claim 7, characterized in that: The boron-doped carbon-coated modified sodium vanadium fluorophosphate material, a conductive agent, and a binder are mixed in a mass ratio of 8:1:1, a solvent is added, and grinding is performed to obtain a slurry; the slurry is coated on a current collector, dried, and cut into electrode sheets.

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