A positive electrode sodium supplement, a preparation method thereof, a positive electrode plate and a sodium ion battery

Abstract

The application provides a positive electrode sodium supplement, a preparation method thereof, a positive electrode tab and a sodium ion battery, and particularly relates to the field of secondary batteries. The positive electrode sodium supplement comprises sodium hexafluoroferrate and a carbon material, the carbon material is coated on the surface of the sodium hexafluoroferrate, and the mass ratio of the carbon material to the sodium hexafluoroferrate is (3-10):(90-97). The positive electrode sodium supplement of the application can first deintercalate sodium ions from the positive electrode material, consume sodium ions for forming SEI films on the surface of the negative electrode, compensate for the loss of the first coulomb efficiency of the positive electrode and the negative electrode, stably exist in the air, not release gas in the charging and discharging process, and is non-toxic, simple in preparation process and low in cost.

Description

Technical Field

[0001] This invention relates to the field of secondary battery technology, specifically to a positive electrode sodium supplement agent and its preparation method, as well as a positive electrode sheet and a sodium-ion battery. Background Technology

[0002] Lithium resources are limited in quantity and unevenly distributed in the Earth's crust, while the demand for lithium resources from the secondary battery industry is enormous. This has led to a year-on-year increase in the production cost of lithium-ion batteries, creating an urgent need to find a new generation of battery technology to replace traditional lithium-ion batteries. Recently, sodium-ion batteries, which have attracted much attention, are highly similar to lithium-ion batteries in terms of working principle, battery structure, production process, and production equipment. Moreover, sodium resources are abundant and widely distributed in the Earth's crust, making them particularly suitable for large-scale energy storage devices and the two-wheeled vehicle market.

[0003] During the initial charge and discharge phase of a sodium-ion battery, a solid electrolyte interface (SEI) forms at the interface of the negative electrode material. The formation of the SEI film consumes active sodium, and this process is irreversible. The permanent loss of active sodium results in a capacity loss during the first charge and discharge cycle, and a decrease in the initial coulombic efficiency (ICE) of the first cycle. Therefore, pre-sodiumification (also known as "sodium replenishment") techniques, such as incorporating sacrificial additives into the battery cell, have emerged before sodium-ion batteries can operate.

[0004] Currently, commonly used sodium-replenishing agents for the positive electrode include organic sodium-containing compounds such as Na2C3O5, Na3P, and Na2CO3. However, Na2C3O5 and Na2CO3 decompose to generate CO2 gas during charging, affecting battery performance, and Na3P is toxic. Therefore, developing simple and efficient sodium-replenishing materials is of paramount importance. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the present invention provides a positive electrode sodium supplement agent, its preparation method, a positive electrode sheet, and a sodium-ion battery, so as to improve the problem of the gas generated by the decomposition of the current positive electrode sodium supplement agent affecting the battery performance.

[0006] To achieve the above and other related objectives, the present invention provides a positive electrode sodium supplement agent, which includes sodium hexafluoroferrate (Na3FeF6) and carbon material, wherein the carbon material is coated on the surface of the sodium hexafluoroferrate, and the mass ratio of the carbon material to the sodium hexafluoroferrate is (3-10):(90-97).

[0007] In one example of the present invention, the carbon material includes one or more of carbon nanotubes, graphene, graphyne, soft carbon, and hard carbon.

[0008] In one example of the present invention, the particle size D50 of the positive electrode sodium supplement is 2-3 μm.

[0009] Another aspect of the present invention provides a method for preparing a positive electrode sodium supplement, comprising the following steps: preparing sodium hexafluoroferrate; mixing and ball milling the sodium hexafluoroferrate with carbon material to obtain a positive electrode sodium supplement.

[0010] In one example of the present invention, the steps for preparing sodium hexafluoroferrate include: weighing an iron source, dissolving it in deionized water to prepare an iron-containing solution; weighing an alkaline substance, dissolving it in deionized water to prepare an alkaline solution; adding the alkaline solution dropwise to the iron-containing solution, controlling the pH value at the reaction endpoint to be 5-7, and filtering to obtain ferric hydroxide; stirring the ferric hydroxide with a hydrofluoric acid solution at 150-180°C for 18-24 hours to obtain ferric fluoride; mixing the ferric fluoride, sodium source, and hydrofluoric acid, stirring the mixture at 30-80°C for 1.5-8 hours, and filtering, washing, and drying to obtain sodium hexafluoroferrate.

[0011] In one example of the present invention, the iron source includes one or more of Fe2O3, Fe3O4, Fe2(SO4)3, FeSO4, FeCl3, FeCl2, Fe(NO3)3, Fe(NO3)2, FeO, and Fe(CH3COO)3.

[0012] In one example of the present invention, the sodium source includes one or more of the following: Na2O, Na2CO3, NaOH, Na2C2O4, NaNO3, NaCH3COO, Na2SO4, Na2SO3, NaNO2, Na2S2O3, Na2S2O8, NaCl, Na3PO4, NaH2PO4, NaClO3, NaClO4, NaSiO3, NaF, NaCN, and Na3[Fe(CN)6]·H2O.

[0013] The present invention also provides a positive electrode sheet for a sodium-ion battery, comprising a positive current collector and a positive active material layer disposed on the positive current collector, wherein the positive active material layer comprises a positive electrode material, a conductive agent, a binder, and a positive sodium supplement agent of the present invention or a positive sodium supplement agent prepared by the preparation method of the present invention.

[0014] In one example of the present invention, the mass of the positive electrode sodium supplement accounts for 3% to 5% of the total mass of the positive electrode active material layer.

[0015] In one example of the present invention, the cathode material is selected from one or more of layered oxides, tunnel oxides, polyanionic compounds, and Prussian blue compounds.

[0016] The present invention also provides a sodium-ion battery, the sodium-ion battery comprising the positive electrode, negative electrode, separator and electrolyte of the present invention.

[0017] In one example of the present invention, the negative electrode sheet includes a negative current collector and a negative active material layer disposed on the negative current collector, wherein the negative active material layer includes a negative electrode material, a conductive agent and a binder.

[0018] In one example of the present invention, the negative electrode material is selected from one or more of carbon-based materials, alloy materials, titanium-based compound materials, and organic compound materials.

[0019] The sodium hexafluoroferrate in the positive electrode sodium replenisher of this invention is insensitive to humidity, can exist stably in air, does not release gas during charging and discharging, and is non-toxic. Its preparation process is simple and inexpensive. Coating the surface of the sodium hexafluoroferrate with carbon material increases its stability. When the carbon-coated sodium hexafluoroferrate is added to the positive electrode of a sodium-ion battery as a sodium replenisher, during the first charge after battery formation, this positive electrode sodium replenisher first extracts and inserts sodium ions from the positive electrode material, using these ions to compensate for the sodium ions consumed in forming the SEI film on the negative electrode surface, thus compensating for the initial coulombic efficiency loss of both the positive and negative electrodes. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a scanning electron microscope (SEM) image of the sodium supplement agent for the positive electrode of the present invention;

[0022] Figure 2 This is a particle size distribution diagram of the sodium supplement agent in the positive electrode of the present invention;

[0023] Figure 3 This is a flowchart of the preparation method of the positive electrode sodium supplement of the present invention;

[0024] Figure 4 The present invention provides a method for preparing sodium hexafluoroferrate in one embodiment of the preparation of the positive electrode sodium supplement.

[0025] Figure 5 This is a cross-sectional view of the sodium-ion battery assembled with the positive electrode sodium supplement of the present invention after full charge. Detailed Implementation

[0026] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, unless otherwise specified, the following embodiments and features can be combined with each other. It should also be understood that the terminology used in the embodiments of the present invention is for describing specific implementation schemes and not for limiting the scope of protection of the present invention. Test methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions or according to the conditions recommended by the respective manufacturers.

[0027] It should be noted that the terms such as "upper", "lower", "left", "right", "middle" and "one" used in this specification are only for clarity of description and are not intended to limit the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered as part of the scope of the invention.

[0028] Please see Figures 1 to 5 This invention provides a positive electrode sodium replenishing agent, its preparation method, a positive electrode sheet, and a sodium-ion battery. The positive electrode sodium replenishing agent of this invention uses sodium hexafluoroferrate as the main material. Because it is not sensitive to humidity, it can exist stably in the air and does not release gas during charging and discharging. During the first charge, it releases sodium ions to compensate for the loss of the SEI film formed on the negative electrode surface, thereby achieving the sodium replenishment effect.

[0029] Please see Figure 1 The positive electrode sodium replenisher of this invention comprises sodium hexafluoroferrate and carbon material. Sodium hexafluoroferrate has the molecular formula Na3FeF6 and its microstructure consists of micro / nano particles. The carbon material is coated on the surface of the sodium hexafluoroferrate particles. Further, the mass ratio of carbon material to sodium hexafluoroferrate in the positive electrode sodium replenisher is (3-10):(90-97), for example, 3:97, 5:95, 8:92, or 10:90. Sodium hexafluoroferrate itself is non-toxic, insensitive to humidity, stable in air, and does not release gas during charging and discharging. When used as a sodium replenisher in sodium-ion batteries, it allows sodium ions to be extracted and inserted before the positive electrode material, thus replenishing the sodium ions consumed in forming the SEI film on the negative electrode surface. Coating the surface of sodium hexafluoroferrate with carbon material not only improves conductivity but also further enhances the stability of sodium hexafluoroferrate.

[0030] In some embodiments, the carbon material includes one or more of carbon nanotubes, graphene, graphyne, soft carbon, and hard carbon. That is, the carbon material can be any one of the types of carbon materials listed above, such as carbon nanotubes, graphene, or graphyne. The carbon material can also be any two or more of the types of carbon materials listed above, such as a combination of carbon nanotubes and graphene, or a combination of graphene and graphyne; or a combination of soft carbon and hard carbon, or a combination of graphyne, soft carbon, and hard carbon, etc. These will not be listed exhaustively here. It should be noted that when the carbon material is a combination of two or more types, the ratio between the components in the composition is not limited, and they can be mixed in any proportion. In other embodiments, the carbon material can also be selected from types of carbon materials commonly used in the art that are not listed above.

[0031] Please see Figure 2 The particle size of the positive electrode sodium supplement of the present invention meets the following conditions: D50 is 2-3 μm, for example, 2 μm, 2.3 μm, 2.5 μm, or 3 μm. Further, D10 is 1-1.5 μm, for example, 1 μm, 1.2 μm, or 1.5 μm; D90 is 7-8 μm, for example, 7 μm, 7.2 μm, 7.5 μm, or 8 μm; Dmax is 24-25 μm, for example, 24 μm, 24.5 μm, 24.8 μm, or 25 μm. It should be noted that D10 represents the particle size corresponding to a cumulative particle size distribution number of 10% of the sample; D50 represents the particle size corresponding to a cumulative particle size distribution number of 50% of the sample; D90 represents the particle size corresponding to a cumulative particle size distribution number of 90% of the sample; and Dmax represents the particle size corresponding to a cumulative particle size distribution number of the sample reaching its maximum.

[0032] Please see Figure 3 This invention provides a method for preparing a positive electrode sodium supplement, comprising the following steps:

[0033] S1. Preparation of sodium hexafluoroferrate;

[0034] S2. The sodium hexafluoroferrate is mixed with carbon material and ball-milled to obtain a positive electrode sodium supplement.

[0035] Please see Figure 3 and Figure 4 The preparation process of sodium hexafluoroferrate in step S1 includes at least the following steps:

[0036] S11. Weigh the iron source and dissolve it in deionized water to prepare an iron-containing solution.

[0037] S12. Weigh the alkaline substance, dissolve it in deionized water, and prepare an alkaline solution.

[0038] S13. Add the alkaline solution dropwise to the iron-containing solution, control the pH value at the reaction endpoint to be 5.0-7.0, and filter to obtain ferric hydroxide;

[0039] S14. Ferric hydroxide is reacted with hydrofluoric acid solution at 150-180°C for 18-24 hours to obtain ferric fluoride.

[0040] S15. Mix ferric fluoride, sodium source and hydrofluoric acid, stir and react at 30-80℃ for 1.5-8h, filter, wash and dry to obtain sodium hexafluoroferrate.

[0041] Specifically, the iron source in step S11 includes one or more of Fe2O3, Fe3O4, Fe2(SO4)3, FeSO4, FeCl3, FeCl2, Fe(NO3)3, Fe(NO3)2, and FeO. That is, the iron source can be selected from any of the listed types of Fe2O3, Fe3O4, or Fe2(SO4)3, etc.; the iron source can also be selected from any combination of two or more of the listed types, such as a combination of FeCl3 and FeCl2, or a combination of Fe(NO3)2 and FeO, or a combination of FeCl3, Fe(NO3)3, and Fe2(SO4)3, etc. It should be noted that the iron source includes, but is not limited to, the types listed above, and iron sources not listed above can also be selected; when there are several combinations of iron sources, there is no restriction on the proportions between the components in the composition, and they can be mixed in any proportion.

[0042] When preparing an iron-containing solution, place the weighed iron source in a reaction vessel, add an appropriate amount of deionized water and stir until the iron source is completely dissolved. There is no restriction on the ratio of iron source to deionized water, as long as the iron source is completely dissolved in the deionized water.

[0043] Step S12 involves preparing an alkaline solution. The alkaline substance can be, for example, sodium hydroxide, potassium hydroxide, ammonia, or other substances whose aqueous solutions are alkaline. Furthermore, sodium hydroxide is chosen as the alkaline substance to avoid introducing other ions. The concentration of the alkaline solution is not limited here; for example, the concentration can be 1–3 mol / L, such as 1 mol / L, 2 mol / L, or 3 mol / L.

[0044] Step S13 involves adding the alkaline solution prepared in step S12 dropwise to the iron-containing solution prepared in step S11, causing a reaction to generate ferric hydroxide Fe(OH)3. During the dropwise addition, the pH value of the solution is monitored, and the final pH value of the reaction is controlled to be between 5.0 and 7.0, for example, 5, 6, or 7.

[0045] Step S14: The ferric hydroxide obtained in step S13 is placed in a hydrofluoric acid solution and stirred at 150–180°C for 18–24 hours to react and generate ferric fluoride (FeF3). The molar ratio of ferric hydroxide to hydrofluoric acid satisfies the following condition: Fe... 3+ :F - = 1:3. The reaction temperature can be, for example, 150℃, 160℃, 170℃ or 180℃; the stirring time can be, for example, 18h, 20h, 22h or 24h.

[0046] Step S15 involves mixing and reacting the iron fluoride, sodium source, and hydrofluoric acid solution prepared in step S14 to obtain sodium hexafluoroferrate. The sodium source includes any one or more of the following: Na2O, Na2CO3, NaOH, Na2C2O4, NaNO3, NaCH3COO, Na2SO4, Na2SO3, NaNO2, Na2S2O3, Na2S2O8, NaCl, Na3PO4, NaH2PO4, NaClO3, NaClO4, NaSiO3, NaF, NaCN, and Na3[Fe(CN)6]·H2O. Examples include Na2O, Na2CO3, combinations of Na3PO4 and NaH2PO4, or combinations of Na2C2O4, NaNO3, and NaCH3COO, etc., which will not be listed here. The molar ratio of iron fluoride, sodium source, and hydrofluoric acid solution must satisfy the condition that Fe... 3+ Na + :F - The ratio is 1:3:6. The reaction temperature is 30–80°C, for example, 30°C, 50°C, 70°C, or 80°C; the stirring time is 1.5–8 hours, for example, 1.5 hours, 3 hours, 5 hours, or 8 hours. After the reaction, sodium hexafluoroferrate is obtained through filtration, washing, and drying. The filtration, washing, and drying processes can be performed according to conventional methods in the field, for example, washing multiple times with ethanol, and then placing it in a drying oven and drying at 110°C for 8 hours.

[0047] Please continue reading. Figure 3Step S2 involves coating the surface of sodium hexafluoroferrate with carbon material. The specific process is as follows: The carbon material and sodium hexafluoroferrate are mixed and added to a ball mill. The mixture is stirred at 400–600 rpm for 4–8 hours. Then, it is baked in an oven at 80–90°C for 24–36 hours. After crushing and grading, the final positive electrode sodium supplement (carbon-coated sodium hexafluoroferrate) is obtained. The carbon material includes any one or any combination of two or more of carbon nanotubes, graphene, graphyne, soft carbon, and hard carbon, such as graphyne or graphene, a combination of carbon nanotubes and graphene, a combination of soft carbon and hard carbon, etc. The ball milling speed can be, for example, 400 rpm, 500 rpm, 600 rpm, etc., and the ball milling time can be, for example, 4 hours, 6 hours, or 8 hours. The baking temperature after ball milling can be, for example, 80°C, 85°C, or 90°C, etc., and the baking time can be, for example, 24 hours, 28 hours, 32 hours, or 36 hours, etc.

[0048] This invention also provides a positive electrode sheet for a sodium-ion battery, comprising a positive current collector and a positive active material layer disposed on the positive current collector. The positive current collector can be a conventional type of current collector in the art, such as aluminum foil; the positive active material layer comprises a positive electrode material, a conductive agent, a binder, and the aforementioned positive sodium supplement agent, wherein the positive sodium supplement agent accounts for 3-5% of the total mass of the positive active material layer, for example, 3%, 4%, or 5%. Further, the mass ratio of the positive electrode material, conductive agent, binder, and positive sodium supplement agent in the positive active material layer is 92:3:2:3.

[0049] The positive electrode material is the main material of the positive electrode active material layer, and it can be selected from conventional materials in the art, such as one or more of layered oxides, tunnel oxides, polyanionic compounds, and Prussian blue compounds. A conductive agent is used to improve the conductivity of the positive electrode active material layer, and a binder is used to firmly bond the positive electrode material and binder to the positive electrode current collector. The types of conductive agents and binders are not specifically limited here and can be selected according to actual needs. For example, the conductive agent can be one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers; the binder can be one or more of styrene-butadiene rubber (SBR), water-based acrylic resin, carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-vinyl acetate copolymer (EVA), and polyvinyl alcohol (PVA).

[0050] The present invention also provides a sodium-ion battery, which includes a positive electrode, a negative electrode, an electrolyte, and a separator. The positive electrode is the positive electrode described above, and the negative electrode, separator, and electrolyte are all set in a manner conventional in the art, which will not be described in detail here.

[0051] The technical solution of the present invention will be described in detail below through several specific embodiments and comparative examples. Unless otherwise stated, the raw materials and reagents used in the following embodiments are commercially available products or can be prepared by conventional methods in the art.

[0052] Example 1

[0053] Weigh 5g of Fe2(SO4)3 and dissolve it in 100mL of deionized water to prepare a Fe2(SO4)3 solution; weigh 10g of NaOH and dissolve it in 100mL of deionized water to prepare a NaOH solution; slowly add the NaOH solution to the Fe2(SO4)3 solution, monitor the pH value of the solution, control the pH value at the reaction endpoint to 4.5, and then filter to obtain Fe(OH)3.

[0054] Fe(OH)3 was reacted with HF solution, heated to 150°C, and stirred for 24 hours to produce FeF3. The molar ratio of Fe(OH)3 to HF was as follows: Fe... 3+ :F - The mixture was added to the reactor at a ratio of 1:3 for reaction.

[0055] FeF3, Na2CO3, and HF were mixed in a molar ratio of Na... + :Fe 3+ :F - The mixture was stirred and reacted in a ratio of 3:1:6, with a stirring speed of 120 r / min, a reaction temperature of 40℃, and a reaction time of 1.5 h. The mixture was then filtered, washed (using ethanol as the washing solvent), and dried (at 100℃ for 2 h) to obtain Na3FeF6.

[0056] Carbon nanotubes and Na3FeF6 were mixed in a ball mill at a mass ratio of 5:95 and stirred at 500 rpm for 2 hours. Then, the mixture was baked and dried in an oven at 80°C for 24 hours, and then crushed and graded to obtain the Na3FeF6 / C composite material.

[0057] Example 2

[0058] Weigh 5g of FeCl3 and dissolve it in 100mL of deionized water to prepare a FeCl3 solution. Weigh 10g of NaOH and dissolve it in 100mL of deionized water to prepare a NaOH solution. Slowly add the NaOH solution to the FeCl3 solution, monitor the pH value of the solution, control the pH value of the reaction endpoint to 5.0, and then filter to obtain Fe(OH)3.

[0059] Fe(OH)3 was reacted with HF solution, heated to 180℃, and stirred for 18 hours to produce FeF3; wherein, Fe(OH)3 and HF were reacted in a molar ratio of Fe... 3+ :F - The mixture was added to the reactor at a ratio of 1:3 for reaction.

[0060] FeF3, Na2CO3, and HF were mixed in a molar ratio of Na... + :Fe 3+ :F - The mixture was stirred and reacted in a ratio of 3:1:6, with a stirring speed of 120 r / min, a reaction temperature of 30℃, and a reaction time of 3 h. The mixture was then filtered, washed (using ethanol as the washing solvent), and dried (at 90℃ for 2 h) to obtain Na3FeF6 material.

[0061] Acetylene black and Na3FeF6 were mixed in a mass ratio of 7:93 and added to a ball mill. The mixture was stirred at 400 rpm for 4 hours. Then, it was baked and dried in an oven at 90°C for 24 hours. The mixture was then crushed and graded to obtain the Na3FeF6 / C composite material.

[0062] Example 3

[0063] Weigh 5g of Fe(NO3)3 and dissolve it in 100mL of deionized water to prepare a Fe(NO3)3 solution. Weigh 10g of NaOH and dissolve it in 100mL of deionized water to prepare a NaOH solution. Slowly add the NaOH solution to the Fe(NO3)3 solution, monitor the pH value of the solution, control the pH value at the reaction endpoint to 5.5, and then filter to obtain Fe(OH)3.

[0064] Fe(OH)3 was reacted with HF solution, heated to 150℃, and stirred for 24 hours to produce FeF3; wherein, Fe(OH)3 and HF were reacted in a molar ratio of Fe... 3+ :F - The mixture was added to the reactor at a ratio of 1:3 for reaction.

[0065] FeF3, Na2SO4, and HF were mixed in a molar ratio of Na... + :Fe 3+ :F - The mixture was stirred in a ratio of 3:1:6, with a stirring speed of 150 r / min, a reaction temperature of 50℃, and a reaction time of 4 h. The mixture was then filtered, washed (using ethanol as the washing solvent), and dried (at 80℃ for 6 h) to obtain Na3FeF6 material.

[0066] Graphene and the prepared Na3FeF6 material were mixed in a mass ratio of 3:97 and added to a ball mill. The mixture was stirred at 600 rpm for 5 hours and then dried in an oven at 80°C for 36 hours. After crushing and grading, the Na3FeF6 / C composite material was obtained.

[0067] Example 4

[0068] Weigh 5g of ferric acetate Fe(CH3COO)3 and dissolve it in 100mL of deionized water to prepare Fe(CH3COO)3 solution. Weigh 10g of NaOH and dissolve it in 100mL of deionized water to prepare NaOH solution.

[0069] NaOH solution was slowly added to Fe(CH3COO)3 solution, the pH value of the solution was monitored, and the final pH value of the reaction was controlled at 6.5. Then, Fe(OH)3 was obtained by filtration.

[0070] Fe(OH)3 was reacted with HF solution, heated to 150°C, and stirred for 24 hours to produce FeF3. The molar ratio of Fe(OH)3 to HF was as follows: Fe... 3+ :F - The mixture was added to the reactor at a ratio of 1:3 for reaction.

[0071] FeF3, NaHCO3, and HF were mixed in a molar ratio of Na... + :Fe 3+ :F - The mixture was stirred in a ratio of 3:1:6, with a stirring speed of 180 r / min, a reaction temperature of 60 ℃, and a reaction time of 5 h. The mixture was then filtered, washed (using ethanol as the washing solvent), and dried (at 110 ℃ for 8 h) to obtain Na3FeF6 material.

[0072] (5) The graphdiyne and carbon nanotube mixed powder (mass ratio 1:1) and the prepared Na3FeF6 material were mixed in a mass ratio of 10:90 and added to a ball mill. The mixture was stirred at 500 rpm for 8 hours. Then it was baked and dried in an oven at 80°C for 36 hours, crushed and graded to obtain the Na3FeF6 / C composite material.

[0073] Example 5

[0074] Weigh 5g of FeCl3 and dissolve it in 100mL of deionized water to prepare a FeCl3 solution. Weigh 10g of NaOH and dissolve it in 100mL of deionized water to prepare a NaOH solution. Slowly add the NaOH solution to the FeCl3 solution, monitor the pH value of the solution, control the pH value of the reaction endpoint to 7.0, and then filter to obtain Fe(OH)3.

[0075] Fe(OH)3 was reacted with HF solution, heated to 180℃, and stirred for 20 hours to produce FeF3; wherein, Fe(OH)3 and HF were reacted in a molar ratio of Fe... 3+ :F - The mixture was added to the reactor at a ratio of 1:3 for reaction.

[0076] FeF3, NaF, and HF were mixed in a molar ratio of Na... + :Fe 3+ :F- The mixture was stirred in a 3:1:6 ratio at a stirring speed of 160 r / min, at a reaction temperature of 80 ℃, and for 8 h. The mixture was then filtered, washed (using ethanol as the washing solvent), and dried (at 110 ℃ for 8 h) to obtain Na3FeF6 material.

[0077] The graphene, conductive carbon black SP mixed powder (mass ratio 1:2) and the prepared Na3FeF6 material were mixed in a ball mill at a mass ratio of 8:92. The mixture was stirred at high speed at 500 rpm for 5 hours, and then baked and dried in an oven at 90℃ for 24 hours. After crushing and grading, the Na3FeF6 / C composite material was obtained.

[0078] Figure 1 SEM characterization of the positive electrode sodium supplement prepared in Example 1: Figure (a) is a SEM image at 1k magnification; Figure (b) is a SEM image at 5k magnification; and Figure (c) is a SEM image at 10k magnification. Figure 1 As can be seen, sodium hexafluoroferrate has a micro-nano-scale morphology, with carbon material distributed on the surface of the sodium hexafluoroferrate particles. The carbon-coated sodium hexafluoroferrate has a smooth surface, regular particle morphology, uniform and controllable particle size, and good flowability.

[0079] Figure 2 The particle size distribution of the positive electrode sodium supplement prepared in Example 1 was tested according to the method specified in GB / T19077-2016, Particle Size Analysis by Laser Diffraction. Figure 2 The positive electrode sodium supplement prepared in Example 1 showed the following particle sizes: D10 = 1.234 μm; D50 = 2.914 μm; D90 = 7.966 μm; Dmax = 24.42 μm. The particle size of the positive electrode sodium supplement is micro-nano grade, facilitating uniform mixing with the positive electrode active material layer during use.

[0080] The specific surface area of ​​the positive electrode sodium supplement prepared in Examples 1 to 5 was tested. The test conditions were in accordance with GB / T19587-2017 Gas Adsorption BET Method for determining the specific surface area of ​​solid materials. The test results are shown in Table 1.

[0081] Table 1: Specific surface area of ​​the positive electrode sodium supplement prepared in Examples 1 to 5

[0082]

[0083] The sodium-added positive electrode agents prepared in Examples 1 to 5 were assembled into sodium-ion batteries, as follows: In the positive electrode preparation process, the positive electrode main material (Na3V2(PO4)2O2F): conductive agent (Super P): binder (PVDF): sodium-added positive electrode agent were mixed evenly in a mass ratio of 92:3:2:3. NMP solvent was added at a solid content of 70%, and the viscosity of the prepared positive electrode slurry ranged from 4000 to 6000 mPa·s. The positive electrode slurry was coated onto aluminum foil for the positive electrode current collector, rolled, and formed into a positive electrode sheet. In an argon glove box, using a sodium metal sheet as the negative electrode, glass fiber as the separator, and 1 mol / L NaPF6 / PC as the electrolyte, a CR2032 coin cell was assembled. See also Figure 5 , Figure 5 This is a dissected view of the sodium-ion battery assembled using Na3FeF6 / C as the positive electrode sodium replenisher prepared according to the present invention after full charging. As can be seen from the figure, the battery has a smooth appearance after full charging and there are no abnormalities such as gas swelling.

[0084] Comparative Example

[0085] In contrast, when assembling the battery, the positive electrode active material layer of the positive electrode sheet is not filled with the positive electrode sodium supplement agent of this application, but the rest is the same.

[0086] The battery's initial charge-discharge capacity was tested within a voltage range of 2.0 to 4.0V at a rate of 0.3C. The test results are shown in Table 2.

[0087]

[0088] As can be clearly seen from the test results in Table 2, the initial discharge specific capacity of Examples 1 to 5 was significantly improved compared to the comparative examples. This is because a positive electrode sodium supplement was added to the positive electrode material in Examples 1 to 5. This positive electrode sodium supplement extracts sodium ions before the positive electrode material and is used to compensate for the sodium ions consumed in the formation of the SEI film on the negative electrode surface, thereby improving the initial discharge specific capacity.

[0089] The sodium replenishing agent of this invention is stable in air, does not release gas during charging and discharging, is non-toxic, and has a simple and low-cost preparation process. When added to the positive electrode of a sodium-ion battery, during the first charge after battery formation, the sodium replenishing agent extracts and inserts sodium ions before the positive electrode material, using these ions to compensate for the sodium ions consumed in the formation of the SEI film on the negative electrode surface, thus compensating for the initial coulombic efficiency loss of both the positive and negative electrodes. Therefore, this invention effectively overcomes some practical problems in the prior art, thus possessing high utilization value and practical significance.

[0090] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A positive electrode sheet for a sodium-ion battery, comprising a positive current collector and a positive active material layer disposed on the positive current collector, characterized in that, The positive electrode active material layer includes a positive electrode material, a conductive agent, a binder, and a positive electrode sodium supplement agent; the positive electrode sodium supplement agent includes: Sodium hexafluoroferrate; and Carbon material is coated on the surface of the sodium hexafluoroferrate; The mass ratio of the carbon material to the sodium hexafluoroferrate is (3~10):(90~97). The mass of the positive electrode sodium supplement accounts for 3% to 5% of the total mass of the positive electrode active material layer; During the first charge of the battery formation, the sodium replenishing agent in the positive electrode deintercalates sodium ions before the positive electrode material, in order to compensate for the sodium ions consumed by the formation of the SEI film on the negative electrode surface.

2. The positive electrode sheet according to claim 1, characterized in that, The carbon material includes one or more of carbon nanotubes, graphene, graphyne, soft carbon, and hard carbon.

3. The positive electrode sheet according to claim 1, characterized in that, The particle size D50 of the positive electrode sodium supplement is 2~3μm.

4. The positive electrode sheet according to claim 1, characterized in that, The positive electrode sodium supplement is prepared by the following steps: Preparation of sodium hexafluoroferrate; The sodium hexafluoroferrate was mixed with carbon material and ball-milled to obtain a positive electrode sodium supplement.

5. The positive electrode sheet according to claim 4, characterized in that, The steps for preparing sodium hexafluoroferrate include: Weigh out the iron source, dissolve it in deionized water, and prepare an iron-containing solution; Weigh out the alkaline substance, dissolve it in deionized water, and prepare an alkaline solution; The alkaline solution is added dropwise to the iron-containing solution, and the pH value at the end of the reaction is controlled to be 5.0~7.

0. The solution is then filtered to obtain ferric hydroxide. Ferric hydroxide was reacted with hydrofluoric acid solution at 150-180°C for 18-24 hours to obtain ferric fluoride. Sodium hexafluoroferrate is prepared by mixing the ferric fluoride, sodium source and hydrofluoric acid, stirring and reacting at 30~80℃ for 1.5~8h, filtering, washing and drying.

6. The positive electrode sheet according to claim 5, characterized in that, The iron source includes one or more of Fe2O3, Fe3O4, Fe2(SO4)3, FeSO4, FeCl3, FeCl2, Fe(NO3)3, Fe(NO3)2, FeO, and Fe(CH3COO)3.

7. The positive electrode sheet according to claim 5, characterized in that, The sodium source includes one or more of the following: Na2O, Na2CO3, NaOH, Na2C2O4, NaNO3, NaCH3COO, Na2SO4, Na2SO3, NaNO2, Na2S2O3, Na2S2O8, NaCl, Na3PO4, NaH2PO4, NaClO3, NaClO4, NaSiO3, NaF, NaCN, and Na3[Fe(CN)6]·H2O.

8. A sodium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The positive electrode is the positive electrode as described in any one of claims 1 to 7.

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