Sodium-philic current collector and negative-electrode-free sodium ion battery comprising same

Abstract

The invention discloses a sodium-philic current collector and a negative-electrode-free sodium ion battery comprising the same. According to the sodium-philic current collector, carbon-coated aluminum foil is subjected to nitrogen plasma treatment, nitrogen doping sites are formed on a carbon-coated layer, the affinity to sodium can be enhanced, and uniform deposition of sodium is promoted; according to the electrolyte, a cosolvent of diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether is adopted, sodium hexafluorophosphate, sodium tetrafluoroborate and sodium trifluoromethanesulfonate are compounded, (3-fluorophenyl)-oxoacetonitrile and allyl methyl carbonate are added to serve as electrolyte additives, a weak solvation structure is formed, a solid electrolyte interfacial film rich in boron and fluorine is constructed, and side reactions are inhibited; sodium oxalate is added to the positive electrode as a sodium supplementing agent to make up the loss of active sodium caused by low first charge-discharge efficiency. The invention solves the problems of low first charge-discharge efficiency and poor dendritic crystal growth and cycle stability of the non-anode sodium ion battery, has high energy density and process simplicity, and is suitable for large-scale production.

Description

Technical Field

[0001] The invention belongs to the field of sodium ion batteries, and in particular relates to a sodium-philic current collector and a negative electrode-free sodium ion battery comprising the current collector. Background Art

[0002] At present, the energy density of sodium-ion batteries is usually lower than that of lithium-ion batteries. In order to improve the energy density of sodium batteries, one of the solutions is to use sodium metal as the negative electrode. However, sodium metal has poor air stability and difficult manufacturing process, which can easily cause safety problems. Negative electrode-free sodium batteries can solve the above problems. The negative electrode does not need to undergo pulping, coating and other processes. Therefore, negative electrode-free sodium batteries not only improve the energy density but also simplify the manufacturing process. Negative electrode-free sodium-ion batteries have no active materials in their negative electrodes. Carbon-based materials such as hard carbon, soft carbon, etc. are used, and only current collectors are used. Their "actual negative electrode" is formed by in-situ electrochemical deposition during the first charge of the battery, so the energy density of the battery can be greatly improved.

[0003] However, there are currently three main technical problems that hinder the practical application of negative electrode-free sodium ion batteries. First, metallic sodium has high chemical activity and is prone to side reactions with the electrolyte, causing the active sodium deposited in situ to fail; second, the initial charge and discharge efficiency of negative electrode-free sodium batteries is low, and the active sodium is only provided by the positive electrode material. The loss of limited positive electrode sodium will cause the battery capacity to decay rapidly and fail in the first few cycles; third, metallic sodium deposition has a high nucleation potential and is prone to uneven deposition on conventional current collectors, causing unstable growth of sodium dendrites and SEI films, resulting in extremely low coulombic efficiency. CN114843524A, CN117747847B, CN117878259A, etc. improve the sodium affinity of sodium-philic current collectors by chemically modifying the current collectors, but the operation and process are cumbersome, the cost is high, and it is difficult to industrialize.

[0004] Therefore, there is an urgent need for an electrolyte, solvent and formulation that are suitable for a negative electrode-free sodium ion battery system, while compensating for the loss of active sodium caused by the low initial charge and discharge efficiency of the negative electrode-free system. Summary of the invention

[0005] The object of the present invention is to provide a sodium-philic current collector with high energy density and simple process and a negative electrode-free sodium ion battery comprising the current collector.

[0006] The technical solution adopted by the present invention to achieve the above-mentioned purpose is: A negative electrode-free sodium ion battery comprises a positive electrode sheet, a sodium-philic current collector, and an electrolyte, wherein: The positive electrode sheet includes a positive electrode active material, a conductive agent, a binder, and a sodium supplement additive, wherein the positive electrode active material is sodium nickel iron manganese oxide, and the sodium supplement additive is sodium oxalate; The sodium-philic current collector is a carbon-coated aluminum foil treated with nitrogen plasma; The electrolyte comprises a solvent and an electrolyte, wherein the solvent comprises diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether, and the electrolyte comprises sodium hexafluorophosphate, sodium tetrafluoroborate and sodium trifluoromethanesulfonate.

[0007] Preferably, the mass ratio of sodium nickel iron manganate to sodium oxalate is 85-94:4-5.

[0008] Preferably, the volume ratio of diethylene glycol dimethyl ether to tetraethylene glycol dimethyl ether is 4000-38000:250-8000.

[0009] Preferably, the molar ratio of sodium hexafluorophosphate to sodium tetrafluoroborate is 2.5-20:1-8.

[0010] Preferably, the molar ratio of sodium hexafluorophosphate to sodium trifluoromethanesulfonate is 2.5-20:1.5-12.

[0011] Preferably, the molar volume ratio of sodium hexafluorophosphate to diethylene glycol dimethyl ether is 2.5-20 mol:4000-38000 mL.

[0012] Preferably, the electrolyte may further include an electrolyte additive, and the electrolyte additive includes (3-fluorophenyl)-oxyacetonitrile and allyl methyl carbonate.

[0013] Preferably, the volume ratio of (3-fluorophenyl)-oxyacetonitrile to allyl methyl carbonate is 100-500:100-500.

[0014] (3-Fluorophenyl)-oxyacetonitrile and allyl methyl carbonate as electrolyte additives can effectively regulate the ion-solvent interaction in the electrolyte and optimize the transmission path of sodium ions in the negative electrode-free sodium ion battery system; a composite interface film with high mechanical strength is formed at the electrode interface, which effectively blocks the direct contact between the electrolyte and metallic sodium, reduces the irreversible loss of active sodium, and improves the kinetic stability of the electrode interface, thereby improving the energy efficiency and initial charge and discharge performance of the battery, thereby improving the battery cycle life and enhancing the battery performance.

[0015] Preferably, the thickness of the carbon coating layer of the sodium-philic current collector is 0.9-1.1 μm, and the die-cutting size of the sodium-philic current collector is 121-125 mm in length and 60-64 mm in width.

[0016] Preferably, the plasma excitation frequency is 13.54-13.58 MHz, the processing power is 90-110 W, and the gas pressure is 45-55 Pa.

[0017] A method for preparing a negative electrode-free sodium ion battery comprises the following steps: The carbon-coated aluminum foil was treated with nitrogen plasma to obtain a sodium-philic current collector; The positive electrode active material, the conductive agent, the binder and the sodium supplement additive are mixed to prepare a positive electrode slurry, which is then coated on an aluminum foil current collector to obtain a positive electrode sheet; dispersing the mixed electrolyte in a solvent and adding an electrolyte additive to obtain an electrolyte; The battery cells are assembled and subjected to step-by-step formation, exhaust, secondary sealing, and capacity division to obtain a negative electrode-free sodium ion battery.

[0018] The present invention also provides a method for preparing an electrolyte, comprising: Preparation of electrolyte: Add electrolyte to the solvent and stir evenly to obtain the electrolyte.

[0019] Preferably, the solvent includes diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether.

[0020] Preferably, the electrolyte comprises sodium hexafluorophosphate, sodium tetrafluoroborate and sodium trifluoromethanesulfonate.

[0021] Preferably, the volume ratio of diethylene glycol dimethyl ether to tetraethylene glycol dimethyl ether is 4000-38000:250-8000.

[0022] Preferably, the molar ratio of sodium hexafluorophosphate to sodium tetrafluoroborate is 2.5-20:1-8.

[0023] Preferably, the molar ratio of sodium hexafluorophosphate to sodium trifluoromethanesulfonate is 2.5-20:1.5-12.

[0024] Preferably, the molar volume ratio of sodium hexafluorophosphate to diethylene glycol dimethyl ether is 2.5-20 mol:4000-38000 mL.

[0025] More preferably, the electrolyte additive further includes perfluoromethyl (2-methyl-3-oxahexanoate), and the volume ratio of (3-fluorophenyl)-oxyacetonitrile to perfluoromethyl (2-methyl-3-oxahexanoate) is 100-500:200-1000. The use of perfluoromethyl (2-methyl-3-oxahexanoate), (3-fluorophenyl)-oxyacetonitrile and allyl methyl carbonate together can further adjust the polarity distribution of the electrolyte system, promote the uniform deposition of sodium ions, reduce the ion migration resistance, and improve the battery charge and discharge efficiency; promote the formation of a fluorine-containing solid electrolyte interface film, enhance the stability of the film, improve the capacity retention rate of the battery under high-rate charge and discharge conditions, and inhibit the volume expansion during the cycle, so that the battery performance is more significantly improved.

[0026] The present invention also provides a method for preparing a positive electrode sheet, comprising: Preparation of positive electrode sheet: Disperse the positive electrode active material, conductive agent and binder with N-methylpyrrolidone, stir at a low speed of 150-250rpm for 3-10min, increase to 600-1000rpm for high-speed dispersion for 20-40min to form a uniform positive electrode slurry, coat the positive electrode slurry on both sides of the aluminum foil current collector, vacuum dry at 115-125℃ for 10-15h, roll and compact, and then die-cut to obtain the positive electrode sheet.

[0027] Preferably, the positive electrode active material is sodium nickel iron manganese oxide.

[0028] Preferably, the conductive agent is conductive carbon black.

[0029] Preferably, the binder is polyvinylidene fluoride.

[0030] Preferably, the mass ratio of sodium nickel iron manganese oxide to conductive carbon black is 90-98:1.2-6.

[0031] Preferably, the mass ratio of sodium nickel iron manganese oxide to polyvinylidene fluoride is 90-98:0.8-4.

[0032] Preferably, the surface density of the coated double sides is 250-270 g / m 2 .

[0033] Preferably, the rolling pressure is 800-1200 kN and the compaction density is 2.3-2.7 g / cm³.

[0034] Preferably, the die-cutting dimensions of the positive electrode sheet are 119-123 mm in length and 58-62 mm in width.

[0035] The present invention also provides a method for preparing a sodium-philic current collector, comprising: Preparation: Double-sided carbon-coated aluminum foil is die-cut to obtain a sodium-philic current collector.

[0036] Preferably, the carbon coating layer has a thickness of 0.9-1.1 μm.

[0037] Preferably, the sodium-philic current collector has a die-cutting size of 121-125 mm in length and 60-64 mm in width.

[0038] The present invention also provides a method for preparing a dry battery cell, comprising: Preparation of dry cells: Use a polypropylene separator coated with a ceramic layer on one side, with the ceramic layer facing the negative electrode to enhance interface stability. Stack the cells in the order of "positive electrode-separator-sodium-philic current collector". Each cell contains 6-8 positive electrode sheets and 7-9 sodium-philic current collectors. Put the stacked bare cells into an aluminum-plastic film with an airbag bag and use a hot press to package them to form dry cells.

[0039] Preferably, the ceramic layer has a thickness of 3.5-4.5 μm.

[0040] Preferably, the packaging temperature is 125-135° C., the pressure is 450-550N, and the packaging time is 8-12s.

[0041] The present invention also provides a method for preparing a negative electrode-free soft-pack battery cell, comprising: Preparation of negative electrode-free soft-pack battery cells: inject electrolyte into the airbag bag of dry battery cells through a liquid injection machine, and let it stand for 10-15 hours after injection to allow the electrolyte to fully infiltrate the electrode and the diaphragm; put the battery cells after injection into a 24-26℃ constant temperature box and use a step-by-step charging method for formation: first charge to 3.9-4.1V at a constant current of 0.09-0.11C, and then charge at a constant voltage until the current drops to 0.04-0.06C to complete the first sodium deposition; after the formation is completed, transfer the battery cells to a glove box, open the airbag bag to exhaust, and remove CO 2 Gas is then released, and the aluminum-plastic film is sealed with an ultrasonic welding machine to complete the second sealing to obtain a negative electrode-free soft-pack battery cell.

[0042] The present invention also provides a method for preparing a positive electrode sheet, comprising: Preparation of positive electrode sheet: Disperse the positive electrode active material, conductive agent, binder and positive electrode sodium supplement additive with N-methylpyrrolidone, stir at a low speed of 150-250rpm for 3-10min, increase to 600-1000rpm for high-speed dispersion for 20-40min to form a uniform positive electrode slurry, coat the positive electrode slurry on both sides of the aluminum foil current collector, vacuum dry at 115-125℃ for 10-15h, roll and compact, and then die-cut to obtain the positive electrode sheet.

[0043] Preferably, the positive electrode active material is sodium nickel iron manganese oxide.

[0044] Preferably, the conductive agent is conductive carbon black.

[0045] Preferably, the binder is polyvinylidene fluoride.

[0046] Preferably, the positive electrode sodium supplement additive is sodium oxalate.

[0047] Preferably, the mass ratio of sodium nickel iron manganese oxide to conductive carbon black is 85-94:1.2-6.

[0048] Preferably, the mass ratio of sodium nickel iron manganese oxide to polyvinylidene fluoride is 85-94:0.8-4.

[0049] Preferably, the mass ratio of sodium nickel iron manganate to sodium oxalate is 85-94:4-5.

[0050] Preferably, the surface density of the coated double sides is 250-270 g / m 2 .

[0051] Preferably, the rolling pressure is 800-1200 kN and the compaction density is 2.3-2.7 g / cm³.

[0052] Preferably, the die-cutting dimensions of the positive electrode sheet are 119-123 mm in length and 58-62 mm in width.

[0053] The present invention also provides a method for preparing a sodium-philic current collector, comprising: Preparation of sodium-philic current collector: The current collector is treated with a nitrogen plasma treatment instrument, high-purity nitrogen is introduced for 3-10 minutes, and the sodium-philic current collector is obtained by die-cutting.

[0054] Preferably, the current collector is a double-sided carbon-coated aluminum foil, and the carbon coating layer is 0.9-1.1 μm.

[0055] Preferably, the sodium-philic current collector has a die-cutting size of 121-125 mm in length and 60-64 mm in width.

[0056] Preferably, the plasma excitation frequency is 13.54-13.58 MHz, the processing power is 90-110 W, and the gas pressure is 45-55 Pa.

[0057] The present invention also provides a method for preparing an electrolyte, comprising: Preparation of electrolyte: Add electrolyte and electrolyte additive into the solvent and stir evenly to obtain electrolyte.

[0058] Preferably, the solvent includes diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether.

[0059] Preferably, the electrolyte comprises sodium hexafluorophosphate, sodium tetrafluoroborate and sodium trifluoromethanesulfonate.

[0060] Preferably, the electrolyte additive includes (3-fluorophenyl)-oxyacetonitrile and allyl methyl carbonate.

[0061] More preferably, the electrolyte additive includes (3-fluorophenyl)-oxyacetonitrile, allyl methyl carbonate and perfluoromethyl (2-methyl-3-oxahexanoate).

[0062] Preferably, the volume ratio of diethylene glycol dimethyl ether to tetraethylene glycol dimethyl ether is 4000-38000:250-8000.

[0063] Preferably, the molar ratio of sodium hexafluorophosphate to sodium tetrafluoroborate is 2.5-20:1-8.

[0064] Preferably, the molar ratio of sodium hexafluorophosphate to sodium trifluoromethanesulfonate is 2.5-20:1.5-12.

[0065] Preferably, the molar volume ratio of sodium hexafluorophosphate to diethylene glycol dimethyl ether is 2.5-20 mol: 8000-9500 mL.

[0066] Preferably, the volume ratio of (3-fluorophenyl)-oxyacetonitrile to allyl methyl carbonate is 100-500:100-500.

[0067] Preferably, the volume ratio of (3-fluorophenyl)-oxoacetonitrile to perfluoromethyl (2-methyl-3-oxahexanoate) is 100-500:200-1000.

[0068] Preferably, the volume ratio of (3-fluorophenyl)-oxyacetonitrile to diethylene glycol dimethyl ether is 100-500:8000-9500.

[0069] The present invention uses a carbon-coated aluminum foil treated with nitrogen plasma as a sodium-philic current collector; uses a co-solvent and a mixed electrolyte, and adds an electrolyte additive as an electrolyte; and adds sodium oxalate as a sodium supplement to the positive electrode, so it has the following beneficial effects: the carbon-coated layer of the sodium-philic current collector forms a nitrogen-doped site, which enhances the affinity for sodium and promotes uniform sodium deposition; the electrolyte forms a weak solvation structure, constructs a solid electrolyte interface film rich in boron and fluorine, and inhibits side reactions; the positive electrode makes up for the loss of active sodium caused by the low initial charge and discharge efficiency. Therefore, the present invention is a sodium-philic current collector with high energy density and simple process, and a negative electrode-free sodium ion battery including the current collector. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] Figure 1 Schematic diagram of infrared absorption spectrum of electrolyte.

[0071] Figure 2 Schematic diagram of the process of nitrogen plasma treatment of carbon-coated aluminum foil.

[0072] Figure 3 Schematic diagram of X-ray photoelectron spectroscopy of sodium-philic current collector.

[0073] Figure 4 Schematic diagram of the charge and discharge curve of a soft-pack battery cell without a negative electrode.

[0074] Figure 5 Schematic diagram of the charge and discharge cycle performance test of the negative electrode-free soft-pack battery cell.

[0075] Figure 6 This is a schematic diagram of the carbon-coated aluminum foil of the negative electrode after disassembly of the soft-pack battery cell without a negative electrode. DETAILED DESCRIPTION

[0076] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0077] The following first describes the concepts involved in the present application in conjunction with the accompanying drawings. It should be noted that the following description of each concept is only to make the content of the present application easier to understand, and does not limit the scope of protection of the present application; at the same time, the embodiments and features in the embodiments of the present application can be combined with each other in the absence of conflict. The present application will be described in detail with reference to the accompanying drawings and in conjunction with the embodiments.

[0078] Embodiment 1: Preparation of electrolyte: adding electrolyte to solvent and stirring to obtain electrolyte. The solvent includes diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether, and the electrolyte is sodium hexafluorophosphate. The volume ratio of diethylene glycol dimethyl ether to tetraethylene glycol dimethyl ether is 8500:1500, and the molar volume ratio of sodium hexafluorophosphate to diethylene glycol dimethyl ether is 10 mol:8500 mL.

[0079] Preparation of positive electrode sheet: Disperse the positive electrode active material, conductive agent and binder with N-methylpyrrolidone, stir at a low speed of 200 rpm for 5 minutes, increase to 800 rpm for high-speed dispersion for 30 minutes to form a uniform positive electrode slurry, coat the positive electrode slurry on both sides of the aluminum foil current collector, vacuum dry at 120°C for 12 hours, roll and compact, and then die-cut to obtain the positive electrode sheet. The positive electrode active material is sodium nickel iron manganese oxide, the conductive agent is conductive carbon black, the binder is polyvinylidene fluoride, and the positive electrode sodium supplement additive is sodium oxalate. The mass ratio of sodium nickel iron manganese oxide to conductive carbon black is 95:3, the mass ratio of sodium nickel iron manganese oxide to polyvinylidene fluoride is 95:2, and the surface density of the coated double sides is 260g / m 2 The rolling pressure is 1000kN, the compaction density is 2.5g / cm³, and the die-cutting size of the positive electrode sheet is 121mm long and 60mm wide.

[0080] Preparation of sodium-philic current collector: Double-sided carbon-coated aluminum foil is die-cut to obtain a sodium-philic current collector. The thickness of the carbon coating layer is 1 μm, and the die-cut size of the sodium-philic current collector is 123 mm in length and 62 mm in width.

[0081] Preparation of dry cells: A polypropylene separator with a ceramic layer coated on one side is used, with the ceramic layer facing the negative electrode side to enhance the interface stability. The sheets are stacked in the order of "positive electrode-separator-sodium-philic current collector". Each cell contains 7 positive electrode sheets and 8 sodium-philic current collectors. The stacked bare cells are placed in an aluminum-plastic film with an airbag bag and packaged using a hot press to form a dry cell. The ceramic layer is 4μm, the packaging temperature is 130℃, the pressure is 500N, and the packaging time is 10s.

[0082] Preparation of negative electrode-free soft-pack battery cells: inject electrolyte into the airbag bag of dry battery cells through a liquid injection machine, and let it stand for 12 hours after injection to allow the electrolyte to fully infiltrate the electrode and the diaphragm; put the battery cells after injection into a 25°C constant temperature box and use a step-by-step charging method for formation: first charge to 4.0V at a constant current of 0.1C, and then charge at a constant voltage until the current drops to 0.05C to complete the first sodium deposition; after the formation is completed, transfer the battery cells to a glove box, open the airbag bag to exhaust, and remove CO 2 Gas is then released, and the aluminum-plastic film is sealed with an ultrasonic welding machine to complete the second sealing to obtain a negative electrode-free soft-pack battery cell.

[0083] Example 2: The difference between this example and Example 1 lies only in the preparation of the electrolyte.

[0084] Preparation of electrolyte: Add electrolyte to solvent and stir to obtain electrolyte. The solvent includes diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether, and the electrolyte includes sodium hexafluorophosphate, sodium tetrafluoroborate and sodium trifluoromethanesulfonate. The volume ratio of diethylene glycol dimethyl ether to tetraethylene glycol dimethyl ether is 8500:1500, the molar ratio of sodium hexafluorophosphate to sodium tetrafluoroborate is 5:2, the molar ratio of sodium hexafluorophosphate to sodium trifluoromethanesulfonate is 5:3, and the molar volume ratio of sodium hexafluorophosphate to diethylene glycol dimethyl ether is 5mol:8500mL.

[0085] Example 3: Compared with Example 2, the difference between this example and this example is only the preparation of the positive electrode sheet.

[0086] Preparation of positive electrode sheet: The positive electrode active material, conductive agent, binder, and positive electrode sodium supplement additive are dispersed with N-methylpyrrolidone, stirred at a low speed of 200 rpm for 5 minutes, and then dispersed at a high speed of 800 rpm for 30 minutes to form a uniform positive electrode slurry. The positive electrode slurry is coated on both sides of the aluminum foil current collector, vacuum dried at 120°C for 12 hours, rolled and compacted, and then die-cut to obtain a positive electrode sheet. The positive electrode active material is sodium nickel iron manganese oxide, the conductive agent is conductive carbon black, the binder is polyvinylidene fluoride, and the positive electrode sodium supplement additive is sodium oxalate. The mass ratio of sodium nickel iron manganese oxide to conductive carbon black is 90.25:3, the mass ratio of sodium nickel iron manganese oxide to polyvinylidene fluoride is 90.25:2, and the mass ratio of sodium nickel iron manganese oxide to sodium oxalate is 90.25:4.75. The surface density of the coated double sides is 260g / m 2The rolling pressure is 1000kN, the compaction density is 2.5g / cm³, and the die-cutting size of the positive electrode sheet is 121mm long and 60mm wide.

[0087] Example 4: Compared with Example 3, the only difference between this example and Example 3 is the preparation of the sodium-philic current collector.

[0088] Preparation of sodium-philic current collector: The double-sided carbon-coated aluminum foil was treated with a nitrogen plasma treatment instrument, high-purity nitrogen was introduced for 5 minutes, and the sodium-philic current collector was obtained by die-cutting. The thickness of the carbon coating layer was 1 μm, and the die-cut size of the sodium-philic current collector was 123 mm long and 62 mm wide. The plasma treatment instrument used in the present invention comes from Guangzhou Shanzhun Technology Co., Ltd., with a plasma excitation frequency of 13.56 MHz, a processing power of 100 W, and an air pressure of 50 Pa.

[0089] Example 5: The difference between this example and Example 4 lies only in the preparation of the electrolyte.

[0090] Preparation of electrolyte: adding electrolyte and electrolyte additive to the solvent, stirring evenly to obtain electrolyte. The solvent includes diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether, the electrolyte includes sodium hexafluorophosphate, sodium tetrafluoroborate and sodium trifluoromethanesulfonate, and the electrolyte additive includes (3-fluorophenyl)-oxyacetonitrile and allyl methyl carbonate. The volume ratio of diethylene glycol dimethyl ether to tetraethylene glycol dimethyl ether is 8500:1500, the molar ratio of sodium hexafluorophosphate to sodium tetrafluoroborate is 5:2, the molar ratio of sodium hexafluorophosphate to sodium trifluoromethanesulfonate is 5:3, the molar volume ratio of sodium hexafluorophosphate to diethylene glycol dimethyl ether is 5mol:8500mL, the volume ratio of (3-fluorophenyl)-oxyacetonitrile to allyl methyl carbonate is 150:150, and the volume ratio of (3-fluorophenyl)-oxyacetonitrile to diethylene glycol dimethyl ether is 150:8500.

[0091] Example 6: The difference between this example and Example 4 lies only in the preparation of the electrolyte.

[0092] Preparation of electrolyte: adding electrolyte and electrolyte additive to the solvent, stirring evenly to obtain electrolyte. The solvent includes diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether, the electrolyte includes sodium hexafluorophosphate, sodium tetrafluoroborate and sodium trifluoromethanesulfonate, and the electrolyte additive includes (3-fluorophenyl)-oxyacetonitrile and allyl methyl carbonate. The volume ratio of diethylene glycol dimethyl ether to tetraethylene glycol dimethyl ether is 8500:1500, the molar ratio of sodium hexafluorophosphate to sodium tetrafluoroborate is 5:2, the molar ratio of sodium hexafluorophosphate to sodium trifluoromethanesulfonate is 5:3, the molar volume ratio of sodium hexafluorophosphate to diethylene glycol dimethyl ether is 5mol:8500mL, the volume ratio of (3-fluorophenyl)-oxyacetonitrile to allyl methyl carbonate is 300:150, and the volume ratio of (3-fluorophenyl)-oxyacetonitrile to diethylene glycol dimethyl ether is 300:8500.

[0093] Example 7: The difference between this example and Example 4 lies only in the preparation of the electrolyte.

[0094] Preparation of electrolyte: adding electrolyte and electrolyte additive to solvent and stirring to obtain electrolyte. The solvent includes diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether, the electrolyte includes sodium hexafluorophosphate, sodium tetrafluoroborate, sodium trifluoromethanesulfonate and perfluoromethyl (2-methyl-3-oxahexanoate), and the electrolyte additive includes (3-fluorophenyl)-oxyacetonitrile and allyl methyl carbonate. The volume ratio of diethylene glycol dimethyl ether to tetraethylene glycol dimethyl ether is 8500:1500, the molar ratio of sodium hexafluorophosphate to sodium tetrafluoroborate is 5:2, the molar ratio of sodium hexafluorophosphate to sodium trifluoromethanesulfonate is 5:3, the molar volume ratio of sodium hexafluorophosphate to diethylene glycol dimethyl ether is 5 mol:8500 mL, the volume ratio of (3-fluorophenyl)-oxyacetonitrile to allyl methyl carbonate is 150:150, the volume ratio of (3-fluorophenyl)-oxyacetonitrile to perfluoromethyl (2-methyl-3-oxahexanoate) is 150:150, and the volume ratio of (3-fluorophenyl)-oxyacetonitrile to diethylene glycol dimethyl ether is 150:8500.

[0095] Example 8: The difference between this example and Example 4 lies only in the preparation of the electrolyte.

[0096] Preparation of electrolyte: adding electrolyte and electrolyte additive to solvent and stirring to obtain electrolyte. The solvent includes diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether, the electrolyte includes sodium hexafluorophosphate, sodium tetrafluoroborate, sodium trifluoromethanesulfonate and perfluoromethyl (2-methyl-3-oxahexanoate), and the electrolyte additive includes (3-fluorophenyl)-oxyacetonitrile and allyl methyl carbonate. The volume ratio of diethylene glycol dimethyl ether to tetraethylene glycol dimethyl ether is 8500:1500, the molar ratio of sodium hexafluorophosphate to sodium tetrafluoroborate is 5:2, the molar ratio of sodium hexafluorophosphate to sodium trifluoromethanesulfonate is 5:3, the molar volume ratio of sodium hexafluorophosphate to diethylene glycol dimethyl ether is 5 mol:8500 mL, the volume ratio of (3-fluorophenyl)-oxyacetonitrile to allyl methyl carbonate is 150:150, the volume ratio of (3-fluorophenyl)-oxyacetonitrile to perfluoromethyl (2-methyl-3-oxahexanoate) is 150:300, and the volume ratio of (3-fluorophenyl)-oxyacetonitrile to diethylene glycol dimethyl ether is 150:8500.

[0097] Comparative Example 1: This comparative example is different from Example 4 only in the preparation of the electrolyte.

[0098] Preparation of electrolyte: Add electrolyte and electrolyte additive to the solvent, stir evenly to obtain electrolyte. The solvent includes diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether, the electrolyte includes sodium hexafluorophosphate, sodium tetrafluoroborate and sodium trifluoromethanesulfonate, and the electrolyte additive is (3-fluorophenyl)-oxyacetonitrile. The volume ratio of diethylene glycol dimethyl ether to tetraethylene glycol dimethyl ether is 8500:1500, the molar ratio of sodium hexafluorophosphate to sodium tetrafluoroborate is 5:2, the molar ratio of sodium hexafluorophosphate to sodium trifluoromethanesulfonate is 5:3, the molar volume ratio of sodium hexafluorophosphate to diethylene glycol dimethyl ether is 5mol:8500mL, and the volume ratio of (3-fluorophenyl)-oxyacetonitrile to diethylene glycol dimethyl ether is 150:8500.

[0099] Comparative Example 2: This comparative example is different from Example 4 only in the preparation of the electrolyte.

[0100] Preparation of electrolyte: Add electrolyte and electrolyte additive to the solvent, stir evenly to obtain electrolyte. The solvent includes diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether, the electrolyte includes sodium hexafluorophosphate, sodium tetrafluoroborate and sodium trifluoromethanesulfonate, and the electrolyte additive is allyl methyl carbonate. The volume ratio of diethylene glycol dimethyl ether to tetraethylene glycol dimethyl ether is 8500:1500, the molar ratio of sodium hexafluorophosphate to sodium tetrafluoroborate is 5:2, the molar ratio of sodium hexafluorophosphate to sodium trifluoromethanesulfonate is 5:3, the molar volume ratio of sodium hexafluorophosphate to diethylene glycol dimethyl ether is 5mol:8500mL, and the volume ratio of allyl methyl carbonate to diethylene glycol dimethyl ether is 150:8500.

[0101] Test Example 1: Solvation test of electrolyte.

[0102] Test sample: the electrolyte prepared in Example 1 and Example 2.

[0103] Test method: 8500 mL of diethylene glycol dimethyl ether and 1500 mL of tetraethylene glycol dimethyl ether were uniformly mixed to obtain a pure solvent. The pure solvent, the electrolyte prepared in Example 1 and Example 2 were heated at 400-4000 cm -1 Detect infrared absorption in the range of

[0104] The infrared absorption spectrum of the electrolyte prepared by the present invention is as follows: Figure 1 As shown in the figure, it can be seen that the pure solvent has a -1 The characteristic peak at corresponds to the in-plane swing vibration of the methylene group in the solvent. In Example 1, when the electrolyte is only sodium hexafluorophosphate, the characteristic peak undergoes an obvious red shift, and the corresponding wave number is 838 cm -1 , indicating that when sodium hexafluorophosphate is used as electrolyte, Na +The sodium hexafluorophosphate electrolyte has a strong solvation reaction with the solvent. When sodium tetrafluoroborate and sodium trifluoromethanesulfonate are added to Example 2, the characteristic peak is blue-shifted relative to Example 1, and the corresponding wave number is 846 cm -1 , indicating that sodium tetrafluoroborate and sodium trifluoromethanesulfonate weaken the Na + The solvation trend of sodium salt electrolytes, that is, sodium hexafluorophosphate, sodium tetrafluoroborate and sodium trifluoromethanesulfonate, can form a weakly solvated electrolyte-solvent structure in ether-based solvents. At the same time, sodium salts containing boron and fluorine components are conducive to the formation of a boron / fluorine-rich solid electrolyte interface film, thereby achieving stable sodium deposition.

[0105] Experimental Example 2: X-ray photoelectron spectroscopy test of the sodium-philic current collector carbon coating layer.

[0106] Test sample: sodium-philic current collector prepared in Example 1 and Example 3.

[0107] Test method: X-ray photoelectron spectroscopy was used to perform N 1s spectrum test on the carbon-coated layer of the original carbon-coated aluminum foil in Example 1 and the carbon-coated layer after nitrogen plasma treatment in Example 3.

[0108] The schematic diagram of the process of treating carbon-coated aluminum foil by nitrogen plasma in the present invention is as follows Figure 2 As shown, the X-ray photoelectron spectrum of the sodium-philic current collector is Figure 3 As shown, Figure 3 (a) is the X-ray photoelectron spectrum of the sodium-philic current collector prepared in Example 1, Figure 3 (b) is the X-ray photoelectron spectrum of the sodium-philic current collector obtained in Example 3. It can be seen from the figure that after the nitrogen plasma treatment in Example 3, pyridinic N, pyrrolic N, graphitic N, and oxidized N appeared on the surface of the carbon-coated layer. These are all electron-deficient states, and therefore have a strong affinity for Na. The nitrogen plasma treatment process involves ionizing nitrogen to form plasma, including ions, excited molecules, free radicals, etc. The active particles in the plasma interact with the carbon-coated layer on the surface of the carbon-coated aluminum foil, causing the original chemical bonds of the carbon-coated layer, such as CO, CC, and C=C, to break, and the plasma forms a network of cross-linked structures with these bonds. Through this treatment, nitrogen-doped sites with electron-withdrawing ability are formed in the carbon-coated layer structure, and this structure has a strong affinity for Na.

[0109] Test Example 3: The first charge and discharge performance test of a soft-pack battery cell without a negative electrode.

[0110] Test sample: the negative electrode-free soft-pack battery cell prepared in Examples 1-4.

[0111] Test method: At a constant temperature of 25℃, charge and discharge were performed at a current of 0.5C, the charge cut-off voltage was 4.0V, the discharge cut-off voltage was 2.0V, and the first charge and discharge curve was recorded.

[0112] The charge and discharge curves of the negative electrode-free soft-pack battery cell prepared by the present invention are as follows: Figure 4 The first charge and discharge performance test results are shown in Table 1.

[0113] Table 1 The first charge and discharge performance test results of the negative electrode-free soft pack battery

[0114] In Example 1, only sodium hexafluorophosphate is used as the electrolyte. Due to the strong solvation effect of a single electrolyte, it is difficult to desolvate sodium ions, resulting in uneven sodium deposition, many side reactions, and serious loss of active sodium. Compared with Example 1, in Example 2, a mixed electrolyte of sodium hexafluorophosphate, sodium tetrafluoroborate and sodium trifluoromethanesulfonate is used, and the first discharge capacity is greatly improved, and the first charge and discharge efficiency is also improved. The three sodium salts form a weak solvation structure in the co-solvents of diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether, which promotes the desolvation of sodium ions, and the boron / fluorine-rich solid electrolyte interface film inhibits side reactions and reduces "dead sodium". In Example 3, sodium oxalate sodium supplement is added, and the first charge capacity and discharge capacity are improved. The sodium supplement additive provides an additional sodium source during the first charge decomposition to make up for the loss of active sodium caused by the low first charge and discharge efficiency, but the first charge and discharge efficiency is slightly reduced. Due to the decomposition of the additive, CO 2 Not completely removed; Example 4 combines plasma treatment of the negative electrode, the first discharge capacity is improved, and the first charge and discharge efficiency is also improved, indicating that the nitrogen-doped site enhances the sodium affinity, promotes uniform sodium deposition, reduces the nucleation overpotential, and reduces the irreversible capacity loss, verifying the synergistic effect of the negative electrode modification with the electrolyte and the sodium supplement; Example 5-6 adds (3-fluorophenyl)-oxyacetonitrile and allyl methyl carbonate as electrolyte additives. Compared with Example 4, the first charge and discharge efficiency is further improved, indicating that (3-fluorophenyl)-oxyacetonitrile and allyl methyl carbonate are used as electrolyte additives in the negative electrode-free sodium ion battery body. In the system, the structure of the solvated ions in the electrolyte can be changed, the occurrence of side reactions can be suppressed, and the irreversible capacity loss can be reduced; Example 7 further adds perfluoromethyl (2-methyl-3-oxahexanoate) as an electrolyte additive, and Example 8 increases the amount of perfluoromethyl (2-methyl-3-oxahexanoate), and the first charge and discharge efficiency reaches the highest, and the first charge capacity and discharge capacity are improved, verifying that the synergistic effect between the additives significantly enhances the stability of the solid electrolyte interface film, synergistically optimizes the solvation structure, reduces the ion migration resistance, accelerates the sodium deposition kinetics, and improves the battery charge and discharge efficiency. Comparative Examples 1 and 2 use only a single additive, and cannot effectively regulate the solvation structure, and the first charge and discharge efficiency is less than 60%, verifying the necessity of multi-component additives.

[0115] Test Example 4: Charge and discharge cycle performance test of soft-pack battery cells without negative electrode.

[0116] Test samples: negative electrode-free soft-pack batteries prepared in various embodiments and comparative examples.

[0117] Test method: Maintain a charge and discharge rate of 0.5C, record the capacity retention rate after each cycle, and monitor the volume expansion rate of the battery cell by the water drainage method.

[0118] The charge and discharge cycle performance test results of the negative electrode-free soft-pack battery cell prepared by the present invention are as follows: Figure 5 The charge and discharge cycle performance test results of the negative electrode-free soft-pack battery are shown in Table 2.

[0119] Table 2 Test results of charge and discharge cycle performance of negative electrode-free soft-pack battery cells

[0120] In Example 1, due to the side reaction of the electrolyte and the growth of dendrites, the capacity retention rate after 150 weeks was the lowest and the volume expansion rate was the highest, reflecting that the solid electrolyte interface membrane was unstable and the active material was lost; in Example 2, the capacity retention rate was improved and the expansion rate was reduced by optimizing the solid electrolyte interface membrane through mixed electrolyte, proving that the weak solvation structure reduced the side reaction; in Example 3, after adding the sodium supplement, the capacity retention rate was further improved, but the expansion rate was slightly higher than that of Example 2, due to CO 2 The residue may affect the interface stability; in Example 4, the capacity retention rate is further improved and the expansion rate is further reduced in combination with the negative electrode modification, indicating that the nitrogen-doped sites promote uniform deposition of sodium, inhibit dendrite growth, and improve cycle stability, verifying that the coordinated optimization of "electrolyte-sodium supplement-current collector" significantly improves the battery life; in Example 5, (3-fluorophenyl)-oxyacetonitrile and allyl methyl carbonate are added as electrolyte additives to construct a stable solid electrolyte interface film on the electrode surface to prevent direct contact between metallic sodium and the electrolyte, thereby improving the energy efficiency of the battery and the capacity retention rate; in Example 6, the amount of (3-fluorophenyl)-oxyacetonitrile is increased, and the capacity retention rate is improved; in Example 7, perfluoromethyl (2-methyl-3-oxahexanoate) is introduced into the electrolyte additive to make the solid electrolyte interface film denser and more stable, inhibit the occurrence of side reactions, and significantly improve the battery performance; in Example 8, the amount of perfluoromethyl (2-methyl-3-oxahexanoate) is increased, and the capacity retention rate reaches the highest and the expansion rate reaches the lowest. Comparative Examples 1 and 2 have low capacity retention, high volume expansion, and low cycle stability due to incomplete solid electrolyte interface and uneven sodium deposition.

[0121] Test Example 5: Sodium ion migration number test of electrolyte.

[0122] Test samples: electrolytes prepared in Examples 1-2, Examples 5-8, and Comparative Examples 1-2.

[0123] Test method: A sodium metal symmetrical battery is used. In an argon glove box, a sodium sheet, a diaphragm soaked in electrolyte, and another sodium sheet are placed in sequence and loaded into a CR2032 button battery shell. A 10N preload is applied to ensure interface adhesion. The battery is left to stand for 30 minutes to allow the electrolyte to fully penetrate the diaphragm and activate the electrode interface. The purity of the sodium sheet is ≥99.9%, the thickness is 0.5mm, and the diameter is 10mm. A polypropylene ceramic diaphragm is used, with the ceramic layer facing any sodium electrode and a thickness of 20μm. The assembled sodium metal symmetrical battery is connected to the test instrument, and the time-current curve is tested using CHI660D, and the electrode resistance is tested using 2273. A certain external polarization voltage is applied, the initial current is measured and recorded, and the initial resistance of the passivation layer on the sodium electrode is tested at the same time. After the current reaches a steady state, the steady-state current is recorded, the steady-state resistance of the passivation layer on the sodium electrode is tested, and the sodium ion migration number is calculated.

[0124] The sodium ion transference number is calculated as follows:

[0125] Where: t Na+ is the sodium ion migration number, I o is the initial current, I ss is the steady-state current, R o is the initial resistance, R ss is the steady-state resistance, ΔV is the external polarization voltage.

[0126] The sodium ion migration number test results of the electrolyte prepared by the present invention are shown in Table 3.

[0127] Table 3 Test results of sodium ion migration number of electrolyte

[0128] In the single sodium hexafluorophosphate electrolyte of Example 1, sodium ions are strongly coordinated with diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether, the solvated ions are large in size, and the migration resistance is high; compared with Example 1, in the mixed electrolyte of Example 2, sodium tetrafluoroborate and sodium trifluoromethanesulfonate weaken the solvation effect, the desolvation energy of sodium ions is reduced, the ion migration ability is enhanced, and the sodium ion migration number is improved; after adding (3-fluorophenyl)-oxyacetonitrile and allyl methyl carbonate as electrolyte additives in Examples 5-6, the polar groups further optimize the solvation structure, and the sodium ion migration number is improved, proving that the additives and the mixed electrolyte synergistically improve the ion conduction efficiency, reduce concentration polarization, and inhibit dendrite formation; in Examples 7-8, (3-fluorophenyl)-oxyacetonitrile, allyl methyl carbonate and perfluoromethyl (2-methyl-3-oxahexanoate) are used as electrolyte additives together, and the sodium ion migration number is further improved; the single additive effect of Comparative Examples 1-2 is limited, and there is no significant improvement compared with Example 2, which verifies the necessity of multi-component electrolytes and additives.

[0129] Test Example 6: Negative electrode interface disassembly test of a soft-pack battery cell without a negative electrode.

[0130] Test sample: negative electrode-free soft-pack battery cell prepared in Example 3 and Example 4.

[0131] Test method: After the first charge, disassemble the battery cell in the glove box, take out the negative electrode carbon-coated aluminum foil, clean the residual electrolyte on the surface with dimethyl carbonate, and observe it under a scanning electron microscope after drying.

[0132] The negative electrode carbon-coated aluminum foil after disassembly of the negative electrode-free soft-pack battery cell prepared by the present invention is shown in FIG. Figure 6 As shown, the carbon-coated aluminum foil negative electrode of the battery cell that has not been treated with plasma has point-like sodium aggregation, while after plasma treatment, the sodium deposition is more uniform, further indicating that the carbon-coated aluminum foil treated with plasma improves the sodium affinity of the sodium-philic current collector and can promote the flat epitaxial growth of metallic sodium on the sodium-philic current collector, thereby reducing the irreversible capacity loss.

[0133] The embodiments and / or implementation methods described above are only used to illustrate the preferred embodiments and / or implementation methods for realizing the technology of the present invention, and are not intended to limit the implementation methods of the technology of the present invention in any form. Any technical personnel in this field may make slight changes or modifications to other equivalent embodiments without departing from the scope of the technical means disclosed in the content of the present invention, but they should still be regarded as technologies or embodiments that are essentially the same as the present invention.

[0134] This article uses specific examples to illustrate the principles and implementation methods of this application. The description of the above embodiments is only used to help understand the method and its core ideas of this application. The above is only the preferred implementation method of this application. It should be pointed out that due to the limitations of textual expression and the objective existence of infinite specific structures, ordinary technicians in this technical field can make several improvements, modifications or changes without departing from the principles of this application, and can also combine the above technical features in an appropriate manner; these improvements, modifications, changes or combinations, or the direct application of the concept and technical solution of the invention to other occasions without improvement, should be regarded as the scope of protection of this application.

Claims

1. A sodium ion battery without negative electrode, comprising a positive electrode sheet, a sodium-philic current collector, and an electrolyte, wherein: The positive electrode sheet comprises a positive electrode active material, a conductive agent, a binder, and a sodium supplement additive, wherein the positive electrode active material is sodium nickel iron manganese oxide, and the sodium supplement additive is sodium oxalate; The sodium-philic current collector is a carbon-coated aluminum foil treated with nitrogen plasma; The electrolyte includes a solvent and an electrolyte, wherein the solvent includes diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether, and the electrolyte includes sodium hexafluorophosphate, sodium tetrafluoroborate and sodium trifluoromethanesulfonate.

2. A negative electrode-free sodium ion battery according to claim 1, characterized in that: The mass ratio of the sodium nickel iron manganate to sodium oxalate is 85-94:4-5.

3. A negative electrode-free sodium ion battery according to claim 1, characterized in that: The volume ratio of diethylene glycol dimethyl ether to tetraethylene glycol dimethyl ether is 4000-38000:250-8000.

4. A negative electrode-free sodium ion battery according to claim 1, characterized in that: The molar ratio of the sodium hexafluorophosphate to the sodium tetrafluoroborate is 2.5-20:1-8, and the molar ratio of the sodium hexafluorophosphate to the sodium trifluoromethanesulfonate is 2.5-20:1.5-12.

5. A negative electrode-free sodium ion battery according to claim 1, characterized in that: The molar volume ratio of the sodium hexafluorophosphate to diethylene glycol dimethyl ether is 2.5-20 mol:4000-38000 mL.

6. A negative electrode-free sodium ion battery according to claim 1, characterized in that: The electrolyte may further include an electrolyte additive, wherein the electrolyte additive includes (3-fluorophenyl)-oxyacetonitrile and allyl methyl carbonate.

7. A negative electrode-free sodium ion battery according to claim 6, characterized in that: The volume ratio of the (3-fluorophenyl)-oxyacetonitrile to allyl methyl carbonate is 100-500:100-500.

8. The negative electrode-free sodium ion battery according to claim 1, characterized in that: The carbon coating layer thickness of the sodium-philic current collector is 0.9-1.1 μm, and the die-cutting size of the sodium-philic current collector is 121-125 mm in length and 60-64 mm in width.

9. A negative electrode-free sodium ion battery according to claim 1, characterized in that: The plasma excitation frequency is 13.54-13.58 MHz, the processing power is 90-110 W, and the gas pressure is 45-55 Pa.

10. The method for preparing any one of claims 1 to 9 of a negative electrode-free sodium ion battery, comprising the following steps: The carbon-coated aluminum foil was treated with nitrogen plasma to obtain a sodium-philic current collector; The positive electrode active material, the conductive agent, the binder and the sodium supplement additive are mixed to prepare a positive electrode slurry, which is then coated on an aluminum foil current collector to obtain a positive electrode sheet; dispersing the mixed electrolyte in a solvent and adding an electrolyte additive to obtain an electrolyte; The battery cells are assembled and subjected to step-by-step formation, exhaust, secondary sealing, and capacity division to obtain a negative electrode-free sodium ion battery.

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