A composite negative electrode for sodium batteries and its preparation method

Abstract

This invention discloses a composite negative electrode for sodium batteries and its preparation method. The composite negative electrode includes: a sodium sheet, a sodium fluoride layer formed in situ on the surface of the sodium sheet, and a fluorinated polymer film. The sodium fluoride layer is formed by reacting the sodium sheet with the fluorinated polymer film attached to its surface under the catalysis of an organic solvent. The organic solvent includes esters and / or ethers. When the composite negative electrode is applied to a sodium battery, the sodium fluoride layer in the composite negative electrode can effectively block the corrosion of the sodium metal sheet by water, while inhibiting the growth of sodium dendrites, reducing the capacity loss of the battery, improving sodium deposition, generating a stable SE I film, realizing the rapid transfer and uniform deposition of sodium ions, and giving the composite negative electrode a more stable cycle voltage, thereby enabling the sodium battery to have better coulombic efficiency and cycle stability.

Description

Technical Field

[0001] This invention relates to the field of sodium battery materials technology, and in particular to a composite negative electrode for sodium batteries and its preparation method. Background Technology

[0002] Due to resource scarcity, the search for alternatives to lithium-ion batteries is urgent. Sodium, with its similar chemical properties to lithium, abundant reserves, superior safety, and better performance at high and low temperatures, makes sodium batteries a very attractive prospect for commercialization. Among these, the sodium metal anode is considered the "holy grail" electrode in sodium-ion batteries due to its lower potential (-2.71V) compared to the standard hydrogen electrode and its high specific capacity of 1166 mAh / g.

[0003] However, sodium metal anodes still have many problems. The uneven deposition of sodium ions and the high reactivity of sodium can react with organic electrolytes to form an unstable solid electrolyte interphase (SEI), which leads to the growth of sodium dendrites. During cycling, the continuous growth of sodium dendrites will greatly affect the stability of the battery and cause battery failure. Furthermore, the loss of electrochemical activity of sodium will lead to battery capacity loss. The large-scale application of sodium metal batteries still faces severe challenges.

[0004] Therefore, constructing a more stable sodium metal anode and regulating the composition of the SEI film to suppress dendrite growth are crucial for extending the lifespan of sodium batteries. Summary of the Invention

[0005] The purpose of this invention is to provide a composite negative electrode for sodium batteries and its preparation method. By pressing a fluorinated polymer film attached to the surface of metallic sodium and introducing an organic solvent to accelerate the defluorination reaction of the polymer film, a sodium fluoride / carbon interface layer is generated on the surface of the metallic sodium sheet. This can induce uniform deposition of lithium sodium ions and effectively suppress the growth of sodium dendrites during charge and discharge, enabling the sodium battery to have a more stable cycle voltage and a better battery cycle life.

[0006] Therefore, in a first aspect, embodiments of the present invention provide a composite negative electrode for sodium batteries, the composite negative electrode comprising: a sodium sheet, a sodium fluoride layer formed in situ on the surface of the sodium sheet, and a fluorinated polymer film;

[0007] The sodium fluoride layer is formed by reacting a sodium sheet with a fluorinated polymer film adhered to its surface under the catalysis of an organic solvent.

[0008] The organic solvents include esters and / or ethers.

[0009] Preferably, the thickness of the fluorinated polymer film is 0.01mm-0.2mm; the fluorinated polymer in the fluorinated polymer film includes one or more of the following: polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCTFE), perfluoroalkoxy resin (PFA), perfluoroethylene propylene (FEP), ethylene-tetrafluoroethylene copolymer (ETFE), and polyvinyl fluoride (PVF).

[0010] Preferably, the thickness of the sodium sheet is 0.1 mm to 1 mm;

[0011] The organic solvent includes one or more of the following: diethylene glycol, triethylene glycol di-2-ethylhexanoate, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and dioxane.

[0012] Preferably, the thickness of the sodium fluoride layer is 1 μm-50 μm.

[0013] Secondly, embodiments of the present invention provide a method for preparing the composite negative electrode described in the first aspect above, the method comprising:

[0014] The bulk sodium metal was cut into small pieces in an argon-filled glove box, rolled, and then cut into sodium flakes.

[0015] In a glove box, a fluorinated polymer film is placed on the surface of a sodium sheet, and pressure is applied to make the fluorinated polymer film adhere to the surface of the sodium sheet to obtain a pre-fabricated negative electrode.

[0016] The pre-fabricated negative electrode is immersed in an organic solvent, which allows the sodium in the sodium sheet to react with the fluorine in the fluorine-containing polymer film to form a sodium fluoride layer, thus obtaining a composite negative electrode.

[0017] Preferably, the thickness of the fluorinated polymer film is 0.01mm-0.2mm; the fluorinated polymer in the fluorinated polymer film includes one or more of the following: polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCTFE), perfluoroalkoxy resin (PFA), perfluoroethylene propylene (FEP), ethylene-tetrafluoroethylene copolymer (ETFE), and polyvinyl fluoride (PVF);

[0018] The thickness of the sodium sheet is 0.1mm-1mm;

[0019] The organic solvents include esters and / or ethers.

[0020] Preferably, the organic solvent includes one or more of diethylene glycol, triethylene glycol di-2-ethylhexanoate, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and dioxapentane. Preferably, the soaking time is 1 minute to 10 minutes.

[0021] Thirdly, embodiments of the present invention provide a sodium battery, the sodium battery comprising a positive electrode, a composite negative electrode as described in the first aspect, an electrolyte or solid electrolyte, and a separator.

[0022] Preferably, the sodium battery includes any one of the following: sodium-ion battery, sodium metal battery, and all-solid-state sodium battery.

[0023] This invention provides a method for preparing a composite negative electrode for sodium batteries. First, a fluorine-containing polymer film is laminated onto the surface of a sodium sheet. Then, under the catalysis of a solvent, the sodium element in the sodium sheet reacts more quickly with the fluorine element in the polymer film to form a sodium fluoride layer, ultimately obtaining a composite negative electrode. This preparation method is simple to operate, low in cost, and suitable for large-scale mass production.

[0024] The composite negative electrode prepared by the above-described method in this invention is applied to sodium batteries. During battery cycling, the sodium fluoride layer in the composite negative electrode can effectively block the corrosion of sodium metal sheets by water, and at the same time, it can inhibit the reaction of sodium with organic electrolyte to form an unstable solid electrolyte membrane (SEI) that leads to the growth of sodium dendrites, thereby reducing battery capacity loss and improving the stability and safety of sodium batteries. This invention can control the thickness of the sodium fluoride layer by changing the thickness of the fluorinated polymer film, thereby improving sodium deposition, generating a stable SEI film, and achieving rapid transfer and uniform deposition of sodium ions. The composite negative electrode has a more stable cycle voltage, thus enabling sodium batteries to have better coulombic efficiency and cycle stability. Attached Figure Description

[0025] The technical solutions of the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings and examples.

[0026] Figure 1 This is a flowchart of the preparation method of the composite negative electrode provided in the embodiments of the present invention. Detailed Implementation

[0027] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. However, it should be understood that these embodiments are only for more detailed description and should not be construed as limiting the present invention in any way, that is, not intended to limit the scope of protection of the present invention.

[0028] This invention provides a composite negative electrode for sodium batteries, comprising: a sodium sheet, a sodium fluoride layer formed in situ on the surface of the sodium sheet, and a fluorinated polymer film; wherein the sodium fluoride layer is formed by reacting a sodium sheet with a fluorinated polymer film adhered to its surface under the catalysis of an organic solvent.

[0029] The organic solvents specifically include one or more of the following: diethylene glycol, triethylene glycol di-2-ethylhexanoate, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and dioxane.

[0030] The organic solvent used in this invention has H n COC n The H(n≥1) structure can quickly remove sodium atoms from the metal lattice to form an O-Na-O structure. Fluoride ions in fluorinated polymer films have strong adsorption properties and combine with sodium in the O-Na-O structure to form NaF.

[0031] The thickness of the aforementioned fluorinated polymer film is 0.01mm-0.2mm; the fluorinated polymer in the fluorinated polymer film includes one or more of the following: polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCTFE), perfluoroalkoxy resin (PFA), perfluoroethylene propylene (FEP), ethylene-tetrafluoroethylene copolymer (ETFE), and polyvinyl fluoride (PVF).

[0032] The sodium sheet is a sodium metal sheet with a thickness of 0.1 mm to 1 mm; the thickness of the sodium fluoride layer is 1 μm to 50 μm, preferably 20 μm.

[0033] This invention provides a method for preparing the above-mentioned composite negative electrode, such as... Figure 1 As shown, the specific steps include:

[0034] Step 110: Cut the bulk sodium metal into small pieces in an argon-filled glove box, roll them, and then cut them into sodium sheets.

[0035] The bulk sodium is stored in kerosene to prevent oxidation; the thickness of the cut sodium flakes is between 0.1 mm and 1 mm.

[0036] Step 120: Place the fluorinated polymer film on the surface of the sodium sheet in the glove box, and apply pressure to make the fluorinated polymer film adhere to the surface of the sodium sheet to obtain the pre-fabricated negative electrode.

[0037] The thickness of the fluorinated polymer film is 0.01mm-0.2mm; the fluorinated polymer in the fluorinated polymer film includes one or more of the following: polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCTFE), perfluoroalkoxy resin (PFA), perfluoroethylene propylene (FEP), ethylene-tetrafluoroethylene copolymer (ETFE), and polyvinyl fluoride (PVF).

[0038] The pressure can be applied manually or using equipment such as a tablet press, as long as it is sufficient to make the fluorinated polymer film adhere to the surface of the sodium tablet.

[0039] Step 130: Immerse the pre-made negative electrode in an organic solvent to allow the sodium in the sodium sheet to react with the fluorine in the fluorine-containing polymer film to form a sodium fluoride layer, thus obtaining a composite negative electrode.

[0040] The organic solvents include esters and / or ethers, specifically including one or more of the following: ethylene carbonate, diethyl carbonate, propylene carbonate, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and dioxane.

[0041] Soaking time is 1-10 minutes;

[0042] The thickness of the formed sodium fluoride layer is 1μm-50μm, and can be any value within the above range, such as 1μm, 5μm, 10μm, 15μm, 20μm, 25μm, 30μm, 40μm, 45μm, 50μm, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable. When the thickness of the sodium fluoride layer is less than 1μm, the waterproof effect and the effect of improving sodium deposition are not obvious. When the thickness of the sodium fluoride layer is greater than 50μm, the excessively thick sodium fluoride layer will affect the transfer rate of sodium ions. The preferred thickness of the sodium fluoride layer is 20μm.

[0043] The composite negative electrode prepared by the above preparation method in the embodiments of the present invention can be assembled with a positive electrode, an electrolyte or a solid electrolyte, and a separator to form a sodium battery; wherein, the separator includes, but is not limited to, any one of polyethylene separator, polypropylene separator, and glass fiber separator.

[0044] The aforementioned sodium batteries include any one of the following: sodium-ion batteries, sodium metal batteries, and all-solid-state sodium batteries; among which sodium-ion batteries also include organic sodium-ion batteries or aqueous sodium-ion batteries.

[0045] To better understand the technical solution provided by this invention, the following uses several specific examples to illustrate the preparation process and characteristics of the composite negative electrode of this invention.

[0046] Example 1

[0047] This embodiment provides a preparation process and performance testing of a composite negative electrode, the specific process of which is as follows:

[0048] (1) Take out 99.7% pure bulk sodium metal from kerosene, cut it into small pieces with a blade in a glove box filled with argon, roll it and then cut it into sodium sheets with a diameter of 12 mm and a thickness of 0.3 mm.

[0049] (2) A 0.02 mm thick polytetrafluoroethylene (PTFE) film is cut into 16 mm films using a slicing machine. The cut PTFE film is then pressed onto the surface of a sodium sheet in an argon-filled glove box to obtain a pre-fabricated negative electrode.

[0050] (3) The pre-made negative electrode is immersed in the organic solvent ethylene glycol dimethyl ether for 2 minutes. The sodium in the catalytic sodium sheet reacts with the fluorine in the fluorine-containing polymer film to form a sodium fluoride layer, thus obtaining a composite negative electrode, which is simply called PTFE-Na negative electrode.

[0051] The composite negative electrode prepared in this embodiment was used to assemble and test PTFE-Na / Cu half-cells, PTFE-Na / PTFE-Na symmetric cells, and full cells. The specific process is as follows:

[0052] Assembly of the PTFE-Na / Cu half-cell: First, the cut copper sheet was washed with dilute hydrochloric acid, then washed with a large amount of deionized water until neutral, and finally washed twice with alcohol and air-dried at room temperature. The washed copper sheet was used as the counter electrode, and ethylene glycol dimethyl ether solution with 1 mol / L NaPF6 as the electrolyte salt was used as the electrolyte. A polypropylene membrane was used as the separator. The half-cell was assembled with the PTFE-Na negative electrode prepared in this invention in an argon glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0053] Assembly of PTFE-Na / PTFE-Na symmetric cell: Take two fluorinated sodium sheets PTFE-Na, use ethylene glycol dimethyl ether solution with 1 mol / L NaPF6 as electrolyte salt as electrolyte solution, and use a polypropylene membrane as separator. Assemble the symmetric cell in an argon glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0054] Assembly of the full cell: First, the positive electrode sheet was prepared by mixing sodium vanadium phosphate, polyvinylidene fluoride binder, and acetylene black conductive agent in a mass ratio of 90:3:7 to form a slurry. The slurry was then coated onto washed aluminum foil using a coater and dried in an oven at 80°C for 8 hours. After drying, the positive electrode sheet was pressed into a positive electrode sheet at 10 MPa using a tablet press. The negative electrode sheet used was the PTFE-Na negative electrode prepared in this embodiment. The electrolyte was a 1 mol / L NaPF6 solution in ethylene glycol dimethyl ether. The separator was a polypropylene separator. The full cell was assembled in an argon glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0055] The testing process is as follows:

[0056] To test the coulombic efficiency of PTFE-Na / Cu half-cells: First, the cell was activated at a current density of 0.05C within a voltage range of 0.01V-1V. Then, a charge-discharge test was performed at a current density of 1C. The coulombic efficiency of the half-cell was calculated by the ratio of the discharge capacity to the charge capacity.

[0057] The test was conducted on a PTFE-Na / PTFE-Na symmetric cell at 1 mA / cm². 2 Sodium ions are extracted and inserted at the same current density. The growth of sodium dendrites is judged by the level of overpotential. Overpotential is the result of the electrode potential deviating from equilibrium during the reaction process. The higher the overpotential, the more sodium metal is deposited, which makes it easier for dendrites to form and cause a short circuit in the battery.

[0058] To test the cycle capacity retention of the full battery: First, the battery was activated at 0.2C, and then tested at 1C with an operating voltage of 2.5V-3.7V (where the charging voltage is 3.7V and the discharge cut-off voltage is 2.5V). The battery was charged and discharged to test the specific capacity retention of the full battery after 200 cycles.

[0059] The test data for the three types of batteries assembled in this embodiment are detailed in Table 1.

[0060] Example 2

[0061] This embodiment provides a preparation process and performance testing of a composite negative electrode, the specific process of which is as follows:

[0062] (1) Take out 99.7% pure bulk sodium metal from kerosene, cut it into small pieces with a blade in a glove box filled with argon, roll it and then cut it into sodium sheets with a diameter of 12 mm and a thickness of 0.2 mm.

[0063] (2) A 0.01 mm thick polytetrafluoroethylene (PCTFE) film is cut into 16 mm films using a slicing machine. The cut PCTFE film is then pressed onto the surface of a sodium sheet in an argon-filled glove box to obtain a pre-fabricated negative electrode.

[0064] (3) The pre-made negative electrode is immersed in the organic solvent diethylene glycol dimethyl ether for 3 minutes. The sodium in the catalytic sodium sheet reacts with the fluorine in the PCTFE film to form a sodium fluoride layer, thus obtaining a composite negative electrode, which is simply referred to as PCTFE-Na negative electrode.

[0065] The composite negative electrode prepared in this embodiment was used to assemble and test PCTFE-Na / Cu half-cells, PCTFE-Na / PCTFE-Na symmetric cells, and full cells. The specific process is as follows:

[0066] Assembly of the PCTFE-Na / Cu half-cell: First, the cut copper sheet was washed with dilute hydrochloric acid, then washed with a large amount of deionized water until neutral, and finally washed twice with alcohol and air-dried at room temperature. The washed copper sheet was used as the counter electrode, and a 1 mol / L NaPF6 solution in ethylene glycol dimethyl ether was used as the electrolyte. A polypropylene membrane was used as the separator. The half-cell was assembled with the PCTFE-Na negative electrode prepared in this invention in an argon glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0067] Assembly of PCTFE-Na / PCTFE-Na symmetric cell: Take two fluorinated sodium sheets PCTFE-Na, use ethylene glycol dimethyl ether solution with 1 mol / L NaPF6 as electrolyte salt as electrolyte solution, and use a polypropylene membrane as separator. Assemble the symmetric cell in an argon glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0068] Assembly of the full cell: First, the positive electrode sheet was prepared by mixing sodium vanadium phosphate, polyvinylidene fluoride binder, and acetylene black conductive agent in a mass ratio of 90:3:7 to form a slurry. The slurry was then coated onto washed aluminum foil using a coater and dried in an oven at 80°C for 8 hours. After drying, the positive electrode sheet was pressed into a positive electrode sheet at 10 MPa using a tablet press. The negative electrode sheet used was the PCTFE-Na negative electrode prepared in this embodiment. The electrolyte was a 1 mol / L NaPF6 solution in ethylene glycol dimethyl ether. The separator was a polypropylene separator. The full cell was assembled in an argon glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0069] The testing process for the three types of batteries assembled in this embodiment is the same as that in Embodiment 1, and the test data are detailed in Table 1.

[0070] Example 3

[0071] This embodiment provides a preparation process and performance testing of a composite negative electrode, the specific process of which is as follows:

[0072] (1) Take out 99.7% pure bulk sodium metal from kerosene, cut it into small pieces with a blade in a glove box filled with argon, roll it and then cut it into sodium sheets with a diameter of 12 mm and a thickness of 0.5 mm.

[0073] (2) A 0.03 mm thick perfluoroalkoxy resin (PFA) film is cut into 16 mm films using a slicing machine. The cut polytetrafluoroethylene film is then pressed onto the surface of a sodium sheet in an argon-filled glove box to obtain a pre-fabricated negative electrode.

[0074] (3) The pre-made negative electrode is immersed in the organic solvent tetraethylene glycol dimethyl ether for 2 minutes. The sodium in the catalytic sodium sheet reacts with the fluorine in the PFA film to form a sodium fluoride layer, thus obtaining a composite negative electrode, which is simply referred to as PFA-Na negative electrode.

[0075] The composite negative electrode prepared in this embodiment was used to assemble and test PFA-Na / Cu half-cells, PFA-Na / PFA-Na symmetric cells, and full cells. The specific process is as follows:

[0076] Assembly of the PFA-Na / Cu half-cell: First, the cut copper sheet was washed with dilute hydrochloric acid, then washed with a large amount of deionized water until neutral, and finally washed twice with alcohol and air-dried at room temperature. The washed copper sheet was used as the counter electrode, and ethylene glycol dimethyl ether solution with 1 mol / L NaPF6 as the electrolyte salt was used as the electrolyte solution. A polypropylene membrane was used as the separator. The half-cell was assembled with the PFA-Na negative electrode prepared in this invention in an argon glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0077] Assembly of PFA-Na / PFA-Na symmetric cell: Take two fluorinated sodium sheets PFA-Na, use ethylene glycol dimethyl ether solution with 1 mol / L NaPF6 as electrolyte salt as electrolyte, and use a polypropylene membrane as separator. Assemble the symmetric cell in an argon glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0078] Assembly of the full cell: First, the positive electrode sheet was prepared by mixing sodium vanadium phosphate, polyvinylidene fluoride binder, and acetylene black conductive agent in a mass ratio of 90:3:7 to form a slurry. The slurry was then coated onto washed aluminum foil using a coater and dried in an oven at 80°C for 8 hours. After drying, the positive electrode sheet was pressed into a positive electrode sheet at 10 MPa using a tablet press. The negative electrode sheet used was the PFA-Na negative electrode prepared in this example. The electrolyte was a 1 mol / L NaPF6 solution in ethylene glycol dimethyl ether. The separator was a polypropylene separator. The full cell was assembled in an argon glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0079] The testing process for the three types of batteries assembled in this embodiment is the same as that in Embodiment 1, and the test data are detailed in Table 1.

[0080] Example 4

[0081] This embodiment provides a preparation process and performance testing of a composite negative electrode, the specific process of which is as follows:

[0082] (1) Take out 99.7% pure bulk sodium metal from kerosene, cut it into small pieces with a blade in a glove box filled with argon, roll it and then cut it into sodium sheets with a diameter of 12 mm and a thickness of 0.4 mm.

[0083] (2) A 0.05 mm thick polytetrafluoroethylene (FEP) film is cut into 16 mm films using a slicing machine. The cut FEP film is then pressed onto the surface of a sodium sheet in an argon-filled glove box to obtain a pre-fabricated negative electrode.

[0084] (3) The pre-made negative electrode is immersed in the organic solvent diethylene glycol for 2 minutes. The sodium in the catalytic sodium sheet reacts with the fluorine in the FEP film to form a sodium fluoride layer, thus obtaining a composite negative electrode, which is simply called FEP-Na negative electrode.

[0085] The composite negative electrode prepared in this embodiment was used to assemble and test FEP-Na / Cu half-cells, FEP-Na / FEP-Na symmetric cells, and full cells. The specific process is as follows:

[0086] Assembly of the FEP-Na / Cu half-cell: First, the cut copper sheet was washed with dilute hydrochloric acid, then washed with a large amount of deionized water until neutral, and finally washed twice with alcohol and air-dried at room temperature. The washed copper sheet was used as the counter electrode, and ethylene glycol dimethyl ether solution with 1 mol / L NaPF6 as the electrolyte salt was used as the electrolyte. A polypropylene membrane was used as the separator. The half-cell was assembled with the FEP-Na negative electrode prepared in this invention in an argon-filled glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0087] Assembly of FEP-Na / FEP-Na symmetric cell: Take two fluorinated sodium sheets FEP-Na, use ethylene glycol dimethyl ether solution with 1 mol / L NaPF6 as electrolyte salt as electrolyte, and use a polypropylene membrane as separator. Assemble the symmetric cell in an argon glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0088] Assembly of the full cell: First, the positive electrode sheet was prepared by mixing sodium vanadium phosphate, polyvinylidene fluoride binder, and acetylene black conductive agent in a mass ratio of 90:3:7 to form a slurry. The slurry was then coated onto washed aluminum foil using a coater and dried in an oven at 80°C for 8 hours. After drying, the positive electrode sheet was pressed into a positive electrode sheet at 10 MPa using a tablet press. The negative electrode sheet used was the FEP-Na negative electrode prepared in this embodiment. The electrolyte was a 1 mol / L NaPF6 solution in ethylene glycol dimethyl ether. The separator was a polypropylene separator. The full cell was assembled in an argon glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0089] The testing process for the three types of batteries assembled in this embodiment is the same as that in Embodiment 1, and the test data are detailed in Table 1.

[0090] Example 5

[0091] This embodiment provides a preparation process and performance testing of a composite negative electrode, the specific process of which is as follows:

[0092] (1) Take out 99.7% pure bulk sodium metal from kerosene, cut it into small pieces with a blade in a glove box filled with argon, roll it and then cut it into sodium sheets with a diameter of 12 mm and a thickness of 0.7 mm.

[0093] (2) A 0.08 mm thick polytetrafluoroethylene (PVF) film is cut into 16 mm films using a slicing machine. The cut PVF film is then pressed onto the surface of a sodium sheet in an argon-filled glove box to obtain a pre-fabricated negative electrode.

[0094] (3) The pre-made negative electrode is immersed in the organic solvent triethylene glycol di-2-ethylhexanoate for 5 minutes. The sodium in the catalytic sodium sheet reacts with the fluorine in the PVF film to form a sodium fluoride layer, thus obtaining a composite negative electrode, which is simply referred to as PVF-Na negative electrode.

[0095] The composite negative electrode prepared in this embodiment was used to assemble and test PVF-Na / Cu half-cells, PVF-Na / PVF-Na symmetric cells, and full cells. The specific process is as follows:

[0096] Assembly of the PVF-Na / Cu half-cell: First, the cut copper sheet was washed with dilute hydrochloric acid, then washed with a large amount of deionized water until neutral, and finally washed twice with alcohol and air-dried at room temperature. The washed copper sheet was used as the counter electrode, and a 1 mol / L NaPF6 solution in ethylene glycol dimethyl ether was used as the electrolyte. A polypropylene membrane was used as the separator. The half-cell was assembled with the PVF-Na negative electrode prepared in this invention in an argon-filled glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0097] Assembly of PVF-Na / PVF-Na symmetric cell: Take two fluorinated sodium sheets PVF-Na, use ethylene glycol dimethyl ether solution with 1 mol / L NaPF6 as electrolyte salt as electrolyte, and use a polypropylene membrane as separator. Assemble the symmetric cell in an argon glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0098] Assembly of the full cell: First, the positive electrode sheet was prepared by mixing sodium vanadium phosphate, polyvinylidene fluoride binder, and acetylene black conductive agent in a mass ratio of 90:3:7 to form a slurry. The slurry was then coated onto washed aluminum foil using a coater and dried in an oven at 80°C for 8 hours. After drying, the positive electrode sheet was pressed into a positive electrode sheet at 10 MPa using a tablet press. The negative electrode sheet used was the PVF-Na negative electrode prepared in this embodiment. The electrolyte was a 1 mol / L NaPF6 solution in ethylene glycol dimethyl ether. The separator was a polypropylene separator. The full cell was assembled in an argon glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0099] The testing process for the three types of batteries assembled in this embodiment is the same as that in Embodiment 1, and the test data are detailed in Table 1.

[0100] Example 6

[0101] This embodiment provides a preparation process and performance testing of a composite negative electrode, the specific process of which is as follows:

[0102] (1) Take out 99.7% pure bulk sodium metal from kerosene, cut it into small pieces with a blade in a glove box filled with argon, roll it and then cut it into sodium sheets with a diameter of 12 mm and a thickness of 0.9 mm.

[0103] (2) A 0.1 mm thick polytetrafluoroethylene (PVF) film is cut into 16 mm films using a slicing machine. The cut PVF film is then pressed onto the surface of a sodium sheet in an argon-filled glove box to obtain a pre-fabricated negative electrode.

[0104] (3) The pre-made negative electrode is immersed in the organic solvent ethylene glycol dimethyl ether for 6 minutes. The sodium in the catalytic sodium sheet reacts with the fluorine in the PVF film to form a sodium fluoride layer, thus obtaining a composite negative electrode, which is simply referred to as PVF-Na negative electrode.

[0105] The composite negative electrode prepared in this embodiment was used to assemble and test PVF-Na / Cu half-cells, PVF-Na / PVF-Na symmetric cells, and full cells. The specific process is as follows:

[0106] Assembly of the PVF-Na / Cu half-cell: First, the cut copper sheet was washed with dilute hydrochloric acid, then washed with a large amount of deionized water until neutral, and finally washed twice with alcohol and air-dried at room temperature. The washed copper sheet was used as the counter electrode, and a 1 mol / L NaPF6 solution in ethylene glycol dimethyl ether was used as the electrolyte. A polypropylene membrane was used as the separator. The half-cell was assembled with the PVF-Na negative electrode prepared in this invention in an argon glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0107] Assembly of PVF-Na / PVF-Na symmetric cell: Take two fluorinated sodium sheets PVF-Na, use ethylene glycol dimethyl ether solution with 1 mol / L NaPF6 as electrolyte salt as electrolyte, and use a polypropylene membrane as separator. Assemble the symmetric cell in an argon glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0108] Assembly of the full cell: First, the positive electrode sheet was prepared by mixing sodium vanadium phosphate, polyvinylidene fluoride binder, and acetylene black conductive agent in a mass ratio of 90:3:7 to form a slurry. The slurry was then coated onto washed aluminum foil using a coater and dried in an oven at 80°C for 8 hours. After drying, the positive electrode sheet was pressed into a positive electrode sheet at 10 MPa using a tablet press. The negative electrode sheet used was the PVF-Na negative electrode prepared in this embodiment. The electrolyte was a 1 mol / L NaPF6 solution in ethylene glycol dimethyl ether. The separator was a polypropylene separator. The full cell was assembled in an argon glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0109] The testing process for the three types of batteries assembled in this embodiment is the same as that in Embodiment 1, and the test data are detailed in Table 1.

[0110] Example 7

[0111] This embodiment provides a preparation process and performance testing of a composite negative electrode, the specific process of which is as follows:

[0112] (1) Take out 99.7% pure bulk sodium metal from kerosene, cut it into small pieces with a blade in a glove box filled with argon, roll it and then cut it into sodium sheets with a diameter of 12 mm and a thickness of 1.0 mm.

[0113] (2) The ethylene-tetrafluoroethylene copolymer (ETFE) with a thickness of 0.12 mm is cut into a 16 mm film using a slicing machine. The cut polytetrafluoroethylene film is then pressed onto the surface of the sodium sheet in an argon-filled glove box to obtain a pre-fabricated negative electrode.

[0114] (3) The pre-made negative electrode is immersed in the organic solvent dioxane for 6 minutes. The sodium in the catalytic sodium sheet reacts with the fluorine in the ETFE film to form a sodium fluoride layer, thus obtaining a composite negative electrode, which is simply referred to as ETFE-Na negative electrode.

[0115] The composite anode prepared in this embodiment was used to assemble and test ETFE-Na / Cu half-cells, ETFE-Na / ETFE-Na symmetric cells, and full cells. The specific process is as follows:

[0116] Assembly of the ETFE-Na / Cu half-cell: First, the cut copper sheet was washed with dilute hydrochloric acid, then washed with a large amount of deionized water until neutral, and finally washed twice with alcohol and air-dried at room temperature. The washed copper sheet was used as the counter electrode, and an ethylene glycol dimethyl ether solution with 1 mol / L NaPF6 as the electrolyte salt was used as the electrolyte. A polypropylene membrane was used as the separator. The half-cell was assembled with the ETFE-Na negative electrode prepared in this invention in an argon-filled glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0117] Assembly of ETFE-Na / ETFE-Na symmetric cell: Take two fluorinated sodium sheets ETFE-Na, use ethylene glycol dimethyl ether solution with 1 mol / L NaPF6 as electrolyte salt as electrolyte, and use a polypropylene membrane as separator. Assemble the symmetric cell in an argon glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0118] Assembly of the full cell: First, the positive electrode sheet was prepared by mixing sodium vanadium phosphate, polyvinylidene fluoride binder, and acetylene black conductive agent in a mass ratio of 90:3:7 to form a slurry. The slurry was then coated onto washed aluminum foil using a coater and dried in an oven at 80°C for 8 hours. After drying, the positive electrode sheet was pressed into a positive electrode sheet at 10 MPa using a tablet press. The negative electrode sheet used was the ETFE-Na negative electrode prepared in this embodiment. The electrolyte was a 1 mol / L NaPF6 solution in ethylene glycol dimethyl ether. The separator was a polypropylene separator. The full cell was assembled in an argon glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0119] The testing process for the three types of batteries assembled in this embodiment is the same as that in Embodiment 1, and the test data are detailed in Table 1.

[0120] Example 8

[0121] This embodiment provides a preparation process and performance testing of a composite negative electrode, the specific process of which is as follows:

[0122] (1) Take out 99.7% pure bulk sodium metal from kerosene, cut it into small pieces with a blade in a glove box filled with argon, roll it and then cut it into sodium sheets with a diameter of 12 mm and a thickness of 0.5 mm.

[0123] (2) A 0.15 mm thick polytetrafluoroethylene (PTFE) film is cut into 16 mm films using a slicing machine. The cut PTFE film is then pressed onto the surface of a sodium sheet in an argon-filled glove box to obtain a pre-fabricated negative electrode.

[0124] (3) The pre-made negative electrode is immersed in the organic solvent tetraethylene glycol dimethyl ether for 4 minutes. The sodium in the catalytic sodium sheet reacts with the fluorine in the PTFE film to form a sodium fluoride layer, thus obtaining a composite negative electrode, which is simply referred to as PTFE-Na negative electrode.

[0125] The composite negative electrode prepared in this embodiment was used to assemble and test PTFE-Na / Cu half-cells, PTFE-Na / PTFE-Na symmetric cells, and full cells. The specific process is as follows:

[0126] Assembly of the PTFE-Na / Cu half-cell: First, the cut copper sheet was washed with dilute hydrochloric acid, then washed with a large amount of deionized water until neutral, and finally washed twice with alcohol and air-dried at room temperature. The washed copper sheet was used as the counter electrode, and ethylene glycol dimethyl ether solution with 1 mol / L NaPF6 as the electrolyte salt was used as the electrolyte. A polypropylene membrane was used as the separator. The half-cell was assembled with the PTFE-Na negative electrode prepared in this invention in an argon glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0127] Assembly of PTFE-Na / PTFE-Na symmetric cell: Take two fluorinated sodium sheets PTFE-Na, use ethylene glycol dimethyl ether solution with 1 mol / L NaPF6 as electrolyte salt as electrolyte solution, and use polypropylene membrane as separator. Assemble the symmetric cell in an argon glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0128] Assembly of the full cell: First, the positive electrode sheet was prepared by mixing sodium vanadium phosphate, polyvinylidene fluoride binder, and acetylene black conductive agent in a mass ratio of 90:3:7 to form a slurry. The slurry was then coated onto washed aluminum foil using a coater and dried in an oven at 80°C for 8 hours. After drying, the positive electrode sheet was pressed into a positive electrode sheet at 10 MPa using a tablet press. The negative electrode sheet used was the PTFE-Na negative electrode prepared in this embodiment. The electrolyte was a 1 mol / L NaPF6 solution in ethylene glycol dimethyl ether. The separator was a polypropylene separator. The full cell was assembled in an argon glove box (H2O < 0.01 ppm, O2 < 0.01 ppm).

[0129] The testing process for the three types of batteries assembled in this embodiment is the same as that in Embodiment 1, and the test data are detailed in Table 1.

[0130] To better illustrate the effects of the embodiments of the present invention, Comparative Example 1 is compared with the embodiments.

[0131] Comparative Example 1

[0132] The difference between this comparative example and the example is that the sodium sheet prepared in step (1) of example 1 is used directly to assemble Na / Cu half-cells, Na / Na symmetric cells and full cells and to conduct tests. The assembly and testing processes of the three cells are the same as those in example 1. The test data are detailed in Table 1.

[0133] Table 1 summarizes the test data of the three types of batteries assembled in Examples 1-8 and Comparative Example 1 as follows:

[0134]

[0135]

[0136] Table 1

[0137] The data comparison in Table 1 shows that the coulombic efficiency of the half-cells in Examples 1-8, the stability of the symmetrical cells, and the capacity retention of the full cells are all higher than those in Comparative Example 1. This is because the battery in Comparative Example 1, assembled directly with sodium metal sheets, suffers from uneven sodium ion deposition. Furthermore, the highly reactive sodium reacts with the organic electrolyte to form an unstable solid electrolyte interphase (SEI), leading to sodium dendrite growth. During cycling, the continuous growth of sodium dendrites significantly affects battery stability, causing battery failure. Moreover, the loss of electrochemical activity in sodium results in capacity loss. Therefore, the performance of the three batteries assembled in Comparative Example 1 is inferior to that in Examples 1-8. In contrast, the three batteries in Examples 1-8 of this invention, assembled with composite negative electrodes, effectively prevent water corrosion of the sodium metal sheets. Simultaneously, they inhibit the formation of an unstable SEI due to the reaction of sodium with the organic electrolyte, thus reducing capacity loss and achieving rapid and uniform sodium ion transfer and deposition, thereby improving the coulombic efficiency and cycle performance of the sodium battery.

[0138] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A composite negative electrode for sodium batteries, characterized in that, The composite negative electrode comprises: a sodium sheet, a sodium fluoride layer formed in situ on the surface of the sodium sheet, and a fluorinated polymer film; The sodium fluoride layer is formed by reacting a sodium sheet with a fluorinated polymer film adhered to its surface under the catalysis of an organic solvent; a sodium fluoride / carbon interface layer is generated on the surface of the sodium sheet; The organic solvents include esters and / or ethers; The organic solvent has H n COC n H structure, n≥1, is used to extract sodium atoms from the metal lattice in the sodium sheet to form O-Na-O structure, and fluoride ions in the fluorine-containing polymer film combine with sodium in the O-Na-O structure to form NaF; The thickness of the sodium fluoride layer is 1μm-50μm.

2. The composite negative electrode according to claim 1, characterized in that, The thickness of the fluorinated polymer film is 0.01mm-0.2mm; the fluorinated polymer in the fluorinated polymer film includes one or more of the following: polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCTFE), perfluoroalkoxy resin (PFA), perfluoroethylene propylene (FEP), ethylene-tetrafluoroethylene copolymer (ETFE), and polyvinyl fluoride (PVF).

3. The composite negative electrode according to claim 1, characterized in that, The thickness of the sodium sheet is 0.1mm-1mm; The organic solvent includes one or more of the following: diethylene glycol, triethylene glycol di-2-ethylhexanoate, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and dioxane.

4. A method for preparing the composite negative electrode according to any one of claims 1-3, characterized in that, The preparation method includes: The bulk sodium metal was cut into small pieces in an argon-filled glove box, rolled, and then cut into sodium flakes. In a glove box, a fluorinated polymer film is placed on the surface of a sodium sheet, and pressure is applied to make the fluorinated polymer film adhere to the surface of the sodium sheet to obtain a pre-fabricated negative electrode. The pre-fabricated negative electrode is immersed in an organic solvent, which allows the sodium in the sodium sheet to react with the fluorine in the fluorine-containing polymer film to form a sodium fluoride layer, thus obtaining a composite negative electrode.

5. The preparation method according to claim 4, characterized in that, The thickness of the fluorinated polymer film is 0.01mm-0.2mm; the fluorinated polymer in the fluorinated polymer film includes one or more of the following: polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCTFE), perfluoroalkoxy resin (PFA), perfluoroethylene propylene (FEP), ethylene-tetrafluoroethylene copolymer (ETFE), and polyvinyl fluoride (PVF). The thickness of the sodium sheet is 0.1mm-1mm; The organic solvents include esters and / or ethers.

6. The preparation method according to claim 4, characterized in that, The organic solvent includes one or more of the following: diethylene glycol, triethylene glycol di-2-ethylhexanoate, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and dioxane.

7. The preparation method according to claim 4, characterized in that, The soaking time is 1 to 10 minutes.

8. A sodium battery, characterized in that, The sodium battery includes a positive electrode, a composite negative electrode as described in any one of claims 1-3, an electrolyte or solid electrolyte, and a separator.

9. The sodium battery according to claim 8, characterized in that, The sodium battery includes any one of the following: sodium-ion battery, sodium metal battery, and all-solid-state sodium battery.

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