Electrolyte and sodium-ion battery

Abstract

The application provides an electrolyte and a sodium ion battery. The electrolyte comprises a solvent, an electrolyte and an additive. The additive comprises a first additive selected from at least one of the compounds shown in formula 1. In the formula, R1 and R2 are independently selected from an oxygen atom, a C1-C3 alkylene group or a C1-C3 alkylenoxy group, at least one of R1 and R2 is a C1-C3 alkylene group or a C1-C3 alkylenoxy group, n and m are each independently 0 or 1, and n and m are not 0 at the same time. The electrolyte comprises a main salt, and the main salt comprises sodium bisfluorosulfonylimide. Through the synergistic effect of the first additive and sodium bisfluorosulfonylimide, a good interface film can be formed on the surface of the electrode material, and the impedance increase under high-temperature conditions can be inhibited.

Description

Technical Field

[0001] This application relates to the field of electrochemical technology, and in particular to an electrolyte and a sodium-ion battery. Background Technology

[0002] Lithium-ion batteries are widely used in consumer electronics, electric vehicles, and energy storage due to their high energy density and cycle performance. However, rising lithium salt prices have limited their development. Compared to lithium resources, sodium resources are abundant, with an abundance of 2.64% in the Earth's crust, 440 times that of lithium. Sodium resources are also widely distributed, easy to refine, and cost-effective for large-scale commercial use. Sodium-ion batteries operate on a similar principle to lithium-ion batteries, giving them a cost advantage in terms of raw materials. Beyond cost, sodium-ion batteries offer the following advantages over lithium-ion batteries: 1) Superior energy density compared to lead-acid batteries, comparable to lithium iron phosphate batteries; 2) Excellent low-temperature performance, suitable for use in cold regions; 3) Higher safety performance compared to ternary lithium-ion and lithium iron phosphate batteries; 4) Environmentally friendly, aligning with sustainable development strategies.

[0003] However, sodium-ion batteries still face some problems and challenges in practical applications, such as severe gas generation and poor performance at high and low temperatures. Therefore, improving the performance of sodium-ion batteries is particularly important. The electrolyte, as a regulator of electrochemical behavior and a stabilizer of the ion conductors between the interface and electrodes, significantly impacts the performance of sodium-ion batteries. Existing conventional electrolytes exhibit poor thermal and low-temperature stability, and their surfaces are prone to severe oxidative decomposition, leading to deterioration in capacity and cycle performance. The key to solving this problem lies in developing electrolyte systems with excellent high and low temperature performance. Summary of the Invention

[0004] The purpose of this application is to provide an electrolyte and a sodium-ion battery to improve the low-temperature performance of the sodium-ion battery, while simultaneously enhancing its high-temperature performance by suppressing high-temperature gas generation. The specific technical solution is as follows:

[0005] A first aspect of this application provides an electrolyte comprising a solvent, an electrolyte, and an additive, wherein the additive comprises a first additive selected from at least one compound shown in Formula 1.

[0006]

[0007] R1 and R2 are independently selected from oxygen atoms, C1-C3 alkylene atoms, or C1-C3 alkoxide atoms, and at least one of R1 and R2 is a C1-C3 alkylene atom or a C1-C3 alkoxide atom. n and m are each independently 0 or 1, and n and m are not simultaneously 0. The electrolyte includes a main salt, which includes sodium difluorosulfonamide.

[0008] In some embodiments of this application, the compound represented by Formula 1 is selected from at least one of the following compounds:

[0009]

[0010] In some embodiments of this application, the mass percentage of the first additive is W1, 0.03% ≤ W1 ≤ 3%, based on the mass of the electrolyte; and / or, the mass percentage of sodium difluorosulfonamide is W2, 6% ≤ W2 ≤ 18%, based on the mass of the electrolyte.

[0011] In some embodiments of this application, 0.5% ≤ W1 ≤ 2%.

[0012] In some embodiments of this application, the mass ratio of sodium difluorosulfonamide to the first additive is (2-120):1.

[0013] In some embodiments of this application, the electrolyte includes other sodium salts selected from sodium hexafluorophosphate; and / or, based on the mass of the electrolyte, the mass percentage of the other sodium salt is W3, 0% ≤ W3 ≤ 6%.

[0014] In some embodiments of this application, the electrolyte includes other sodium salts and sodium difluorosulfonamide, wherein the mass ratio of the other sodium salts to sodium difluorosulfonamide is (0-1):1.

[0015] In some embodiments of this application, the solvent is selected from at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, propylene carbonate, ethylene carbonate, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, dioxolane, ethyl acetate, ethyl propionate, and propyl propionate.

[0016] A second aspect of this application provides a sodium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte provided in the first aspect of this application. The separator is located between the positive and negative electrode. The positive electrode comprises a positive active material selected from at least one of layered oxides, Prussian compounds, and polyanionic compounds; preferably Na3V2(PO4)3 or Na 0.9 Mn 0.6 Fe 0.4 PO4, Na4Fe3(PO4)2(P2O7), NaFePO4, sodium ferrocyanide and NaNi X Me Y At least one of O2, Me is selected from two or three of Fe, Co, Mn and Al, and X+Y=1.

[0017] In some embodiments of this application, the negative electrode sheet includes a negative electrode active material, which is selected from one or more of carbon-based materials, silicon-based materials, alloy materials and organic materials; preferably hard carbon.

[0018] The beneficial effects of this application are:

[0019] This application provides an electrolyte and a sodium-ion battery. The electrolyte includes a solvent, an electrolyte, and an additive. The additive includes a first additive selected from at least one compound shown in Formula 1. The electrolyte includes a main salt, which includes sodium bis(fluorosulfonyl)imide. Through the synergistic effect of the first additive and sodium bis(fluorosulfonyl)imide, a good interfacial film can be formed on the surface of the electrode material while suppressing the increase in impedance under high-temperature conditions.

[0020] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Detailed Implementation

[0021] The technical solutions of this application will be clearly and completely described below with reference to the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0022] A first aspect of this application provides an electrolyte comprising a solvent, an electrolyte, and an additive, wherein the additive comprises a first additive selected from at least one compound shown in Formula 1.

[0023]

[0024] R1 and R2 are independently selected from oxygen atoms, C1-C3 alkylene atoms, or C1-C3 alkoxide atoms, and at least one of R1 and R2 is a C1-C3 alkylene atom or a C1-C3 alkoxide atom. n and m are each independently 0 or 1, and n and m are not simultaneously 0. The electrolyte includes a main salt, which includes sodium difluorosulfonamide.

[0025] Sodium ions, due to their larger size and greater insertion / extraction losses, exhibit poorer cycle reversibility and interfacial film stability compared to lithium-ion batteries. Therefore, sodium-ion batteries require higher ion transport capabilities. Furthermore, sodium-ion batteries generate a significant amount of gas during cycling, and their high-temperature storage performance needs improvement. The inventors discovered that the large π-bond on the benzene ring in the compound shown in Formula 1 affects the activity of the compound, lowering its energy barrier. This facilitates the ring-opening reaction of the sulfonate structure in Formula 1 during the sodium-ion battery formation stage, forming an interfacial film rich in inorganic sulfates, inorganic sulfites, sodium alkyl sulfonates, and sodium alkyl sulfates on the surface of the negative electrode. This enhances the stability at the interface between the negative electrode and the electrolyte, improving the cycle performance and high-temperature storage performance of the sodium-ion battery.

[0026] Sodium bis(fluorosulfonyl)imide can partially decompose, introducing components such as sodium nitride, sodium sulfate, and sodium fluoride into the SEI or CEI film. This reduces electrode interface polarization, improves the stability of the SEI or CEI film, and enables sodium-ion batteries to exhibit excellent high-temperature performance. However, the hydrolyzed bis(fluorosulfonyl)imide group is weakly alkaline, which can cause degradation during high-temperature storage of sodium-ion batteries. This leads to oxidative decomposition of the electrolyte on the positive electrode surface, causing the dissolution of transition metal ions in the positive electrode material, reducing the stability of the positive electrode material structure and interface. The bis(fluorosulfonyl)imide group can also hydrolyze during battery reactions to generate corrosive products, which can damage the aluminum current collector, thus degrading the battery's high-temperature cycle performance and high-temperature storage performance.

[0027] This application combines a first additive with sodium bis(fluorosulfonyl)imide. The intermediate product obtained from the ring-opening reaction of the compound of Formula 1 has a benzene ring. Under the conjugation effect of the large π bond on the benzene ring, the intermediate free radical formed by the ring-opening reaction of the compound shown in Formula 1 can combine with the bis(fluorosulfonyl)imide group to form a stable conjugated compound. This reduces the damage to the positive electrode material caused by the hydrolysis of sodium bis(fluorosulfonyl)imide and suppresses the side reactions between the positive electrode material interface and the electrolyte. Through the synergistic effect of sodium bis(fluorosulfonyl)imide and the first additive, a good interfacial film can be formed on the surface of the electrode material while suppressing the increase in impedance under high temperature conditions.

[0028] In some embodiments of this application, the compound represented by Formula 1 is selected from at least one of the following compounds:

[0029]

[0030] By selecting compounds of Formula 1 within the scope of this application, it is beneficial to enhance the stability at the interface between the negative electrode and the electrolyte, and to further improve the cycle performance and high-temperature storage performance of the secondary battery. Preferably, the first additive is selected from compounds of Formula 1-1. Compounds of Formula 1-1 form more intermediate product sites after bond breaking, which is more conducive to forming a stable interfacial film on the electrode and electrolyte surface, thereby further improving the cycle performance and high-temperature storage performance of the secondary battery.

[0031] In some embodiments of this application, based on the mass of the electrolyte, the mass percentage content of the first additive is W1, 0.03% ≤ W1 ≤ 3%, preferably 0.5% ≤ W1 ≤ 2%; and / or, based on the mass of the electrolyte, the mass percentage content of sodium difluorosulfonyl imide is W2, 6% ≤ W2 ≤ 18%. For example, based on the mass of the electrolyte, the mass percentage content of the first additive can be 0.03%, 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, or a range consisting of two such values; based on the mass of the electrolyte, the mass percentage content of sodium difluorosulfonyl imide can be 6%, 8%, 10%, 12%, 14%, 16%, 18%, or a range consisting of any two such values. Controlling the mass percentage content of the first additive and sodium difluorosulfonyl imide within the range of this application is beneficial for further enhancing the stability at the interface between the negative electrode and the electrolyte, and improving the cycle performance and high-temperature storage performance of the sodium-ion battery.

[0032] In some embodiments of this application, the mass ratio of sodium bis(fluorosulfonyl)imide to the first additive is (2-120):1. For example, the mass ratio of sodium bis(fluorosulfonyl)imide to the first additive can be 2:1, 10:1, 30:1, 50:1, 70:1, 100:1, 120:1, or a range consisting of any two of these values. Controlling the mass ratio of sodium bis(fluorosulfonyl)imide to the first additive within the range of this application is beneficial for their synergistic effect, further improving the cycle performance and high-temperature storage performance of sodium-ion batteries.

[0033] In some embodiments of this application, the electrolyte includes other sodium salts selected from sodium hexafluorophosphate; and / or, based on the mass of the electrolyte, the mass percentage of the other sodium salt is W3, where 0% ≤ W3 ≤ 6%. For example, based on the mass of the electrolyte, the content of the other sodium salt can be 0%, 1%, 2%, 3%, 4%, 5%, 6%, or a range consisting of any two of these values. By including electrolytes within the above ranges and adjusting the mass percentage of other sodium salts within the scope of this application, the electrolyte can possess higher ionic conductivity and better electrochemical stability, further improving the cycle performance and high-temperature storage performance of sodium-ion batteries.

[0034] In some embodiments of this application, the electrolyte includes other sodium salts and sodium difluorosulfonylimide, with a mass ratio of other sodium salts to sodium difluorosulfonylimide of (0-1):1. For example, the mass ratio of other sodium salts to sodium difluorosulfonylimide can be 0:1, 0.1:1, 0.2:1, 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, or a range consisting of any two of these values. The electrolyte includes electrolytes within the above range, and by adjusting the mass ratio of other sodium salts to sodium difluorosulfonylimide within the scope of this application, sodium difluorosulfonylimide can improve the kinetic performance of the electrolyte and further reduce electrode interface polarization, thereby improving the high-temperature cycle performance of the sodium-ion battery. Other sodium salts, when fully dissolved in organic solvents, can form a more chemically stable electrolyte with higher conductivity, thus enabling the sodium-ion battery to exhibit better electrochemical performance.

[0035] In some embodiments of this application, the electrolyte also includes other additives selected from at least one of sodium bis(oxalateborate), sodium perchlorate, and sodium difluorooxalateborate.

[0036] In some embodiments of this application, the solvent is selected from at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, propylene carbonate, ethylene carbonate, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, dioxolane, ethyl acetate, ethyl propionate, and propyl propionate. This application does not impose any particular limitation on the mass of the solvent, as long as it meets the objectives of this application. For example, based on the mass of the electrolyte, the mass percentage of the solvent is W4, where 73% ≤ W4 ≤ 93.97%. Using solvents within the above range in the electrolyte can give the electrolyte suitable viscosity, high ionic conductivity, and good electrochemical stability, further improving the cycle performance and high-temperature storage performance of sodium-ion batteries.

[0037] A second aspect of this application provides a sodium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte provided in the first aspect of this application. The separator is located between the positive and negative electrode. The positive electrode comprises a positive active material selected from at least one of layered oxides, Prussian compounds, and polyanionic compounds; preferably Na3V2(PO4)3 or Na 0.9 Mn 0.6 Fe 0.4 PO4, Na4Fe3(PO4)2(P2O7), NaFePO4, sodium ferrocyanide and NaNi X Me YAt least one of O2, Me is selected from two or three of Fe, Co, Mn and Al, and X+Y=1. Typically, during the charging and discharging process of a sodium-ion battery, active metal ions (sodium ions) repeatedly insert and extract between the positive and negative electrode plates. The electrolyte acts as a conductor between the positive and negative electrode plates. The separator is disposed between the positive and negative electrode plates, primarily to prevent short circuits between the positive and negative electrodes, while simultaneously allowing ions to pass through. The sodium-ion battery of this application includes the electrolyte provided in the first aspect of this application, which is beneficial for forming a stable interface film between the positive and negative electrodes, further improving the diffusion ability of sodium ions at the electrode interface, further reducing impedance, and improving the low-temperature performance, high-temperature cycle performance, and high-temperature storage performance of the sodium-ion battery. The positive electrode plate includes the positive electrode active material within the scope of this application, which is beneficial for improving the cycle performance and high-temperature storage performance of the sodium-ion battery.

[0038] The positive electrode includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The phrase "positive electrode material layer disposed on at least one surface of the positive current collector" means that the positive electrode material layer can be disposed on one surface of the positive current collector along its thickness direction, or on two surfaces of the positive current collector along its thickness direction. It should be noted that "surface" here can refer to the entire surface of the positive current collector or only a portion thereof; this application does not impose any particular limitation, as long as the purpose of this application is achieved. This application does not impose any particular limitation on the positive current collector, as long as the purpose of this application is achieved. For example, the positive current collector can be aluminum foil, aluminum alloy foil, or a composite positive current collector. The aforementioned composite positive current collector can be a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The material of the aforementioned polymer material base layer can be, but is not limited to, at least one of polypropylene (PP), polyethylene terephthalate (PET), or polybutylene terephthalate (PBT), and the material of the aforementioned metal layer can be, but is not limited to, at least one of aluminum, aluminum alloy, nickel, or nickel alloy. This application does not impose any particular limitation on the thickness of the positive electrode material layer and the positive electrode current collector, as long as the purpose of this application can be achieved. For example, the thickness of the single-sided positive electrode material layer is 50 μm to 250 μm, and the thickness of the positive electrode current collector is 7 μm to 20 μm.

[0039] The positive electrode material layer may further include a positive electrode conductive agent and a positive electrode binder. This application does not impose any particular limitation on the types of positive electrode conductive agents and positive electrode binders, as long as they achieve the purpose of this application. For example, the positive electrode conductive agent may include, but is not limited to, at least one of conductive carbon black (Super P), acetylene black, Ketjen black, carbon nanotubes, graphene, carbon dots, or carbon fibers. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. For example, the positive electrode binder may include, but is not limited to, at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene-propylene terpolymer, ethylene-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, or fluorinated acrylate resin. This application does not impose any particular limitation on the mass ratio of the positive electrode active material, conductive agent, and binder in the positive electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application is achieved.

[0040] This application does not impose any particular restrictions on the preparation method of the positive electrode sheet, as long as it can achieve the purpose of this application. For example, the preparation method of the positive electrode sheet may include, but is not limited to, the following steps: dispersing the above-mentioned components used to prepare the positive electrode sheet, such as positive electrode active material, positive electrode conductive agent and positive electrode binder, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0041] In this application, the negative electrode sheet includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The phrase "the negative electrode material layer is disposed on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be disposed on one surface of the negative electrode current collector along its thickness direction, or on two surfaces of the negative electrode current collector along its thickness direction. It should be noted that the "surface" here can be the entire surface area of ​​the negative electrode current collector, or only a portion of the surface area; this application has no particular limitation, as long as the purpose of this application is achieved. This application has no particular limitation on the negative electrode current collector, as long as the purpose of this application is achieved; for example, the negative electrode current collector can be copper foil, copper alloy foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, foamed nickel or foamed copper, or a composite negative electrode current collector. The aforementioned composite negative electrode current collector can be a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The material of the polymer material base layer can be, but is not limited to, at least one of polypropylene (PP), polyethylene terephthalate (PET), or polybutylene terephthalate (PBT). The material of the metal layer can be, but is not limited to, at least one of copper, copper alloy, nickel, or nickel alloy. This application does not impose any particular limitation on the thickness of the negative electrode material layer and the negative electrode current collector, as long as the purpose of this application can be achieved. For example, the thickness of the single-sided negative electrode material layer can be from 50 μm to 180 μm, and the thickness of the negative electrode current collector can be from 3 μm to 15 μm.

[0042] In some embodiments of this application, the negative electrode sheet includes a negative electrode active material, which is selected from one or more of carbon-based materials, silicon-based materials, alloy materials and organic materials; preferably hard carbon.

[0043] In some embodiments of this application, the negative electrode material layer may further include a negative electrode conductive agent and a negative electrode binder. This application does not particularly limit the types of negative electrode conductive agents and negative electrode binders, as long as they achieve the purpose of this application. For example, the negative electrode conductive agent may include, but is not limited to, at least one of conductive carbon black (Super P), acetylene black, Ketjen black, carbon nanotubes, graphene, carbon dots, or carbon fibers. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. For example, the negative electrode binder may include, but is not limited to, at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethyl methacrylate (PMAA), or carboxymethyl chitosan (CMCS). In some embodiments of this application, the negative electrode material layer may also optionally include other additives, such as thickeners, which may include, but are not limited to, sodium carboxymethyl cellulose (CMC-Na).

[0044] This application does not impose any particular restrictions on the preparation method of the negative electrode sheet, as long as it can achieve the purpose of this application. For example, the preparation method of the negative electrode sheet may include, but is not limited to, the following steps: dispersing the above-mentioned components used to prepare the negative electrode sheet, such as negative electrode active material, negative electrode conductive agent, negative electrode binder and other additives, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto a negative electrode current collector, and obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0045] This application does not impose any particular limitation on the diaphragm, as long as it achieves the purpose of this application. For example, the diaphragm material may include, but is not limited to, at least one of polyethylene (PE), polypropylene (PP), glass fiber, polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), and polyamide (PA). The type of diaphragm may include at least one of woven membrane, nonwoven fabric, microporous membrane, composite membrane, rolled membrane, or spun membrane. In this application, the thickness of the diaphragm is not particularly limited, as long as it achieves the purpose of this application; for example, the thickness of the diaphragm may be from 4 μm to 20 μm.

[0046] In this application, the sodium-ion battery also includes a casing for housing the positive electrode, separator, negative electrode, and electrolyte, as well as other components known in the field of sodium-ion batteries. This application does not limit the scope of these other components. This application does not impose any particular limitation on the casing; it can be a casing known in the art, as long as it achieves the purpose of this application. For example, the casing can be a rigid casing or a flexible casing. The material of the rigid casing can be metal; this application does not limit the type of metal and can use known metal rigid casings, as long as they achieve the purpose of this application. The flexible casing can be a metal plastic film, such as aluminum-plastic film, steel-plastic film, etc.

[0047] The fabrication process of the sodium-ion battery described in this application is well known to those skilled in the art, and this application does not impose any particular limitations. For example, the fabrication process of the sodium-ion battery may include, but is not limited to, the following steps: stacking the positive electrode, separator, and negative electrode in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain a sodium-ion battery. Alternatively, stacking the positive electrode, separator, and negative electrode in sequence, and then fixing the four corners of the entire stacked structure with tape to obtain a stacked electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain a sodium-ion battery. In addition, overcurrent protection elements, conductive plates, etc., may be placed in the housing as needed to prevent pressure rise and overcharging / discharging inside the sodium-ion battery.

[0048] Example

[0049] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0050] Test methods and equipment:

[0051] 25℃ Cyclic Performance Test

[0052] The sodium-ion battery was placed at 25°C for 30 minutes to reach a constant temperature. It was then charged at a constant current of 1C until the voltage reached 4.0V, followed by constant voltage charging at 4.0V until the cutoff current reached 0.05C. Finally, it was discharged at a constant current of 1C until the voltage reached 2.0V. The initial discharge capacity was recorded as C1, constituting one charge-discharge cycle. This charge-discharge cycle was repeated 500 times, and the discharge capacity of the 500th cycle was recorded as C. 500 .

[0053] 25℃ Cyclic Capacity Retention Rate (%) = C 500 / C1×100%; The room temperature cycle performance of sodium-ion batteries is evaluated by the capacity retention rate at 25℃. The higher the capacity retention rate at 25℃, the better the room temperature cycle performance of sodium-ion batteries.

[0054] 45℃ Cyclic Performance Test

[0055] The sodium-ion battery was left to stand at 45°C for 30 minutes to reach a constant temperature. It was then charged at a constant current of 1C until the voltage reached 4.0V, followed by constant voltage charging at 4.0V until the cutoff current reached 0.05C. Finally, it was discharged at a constant current of 1C until the voltage reached 2.0V. The initial discharge capacity was recorded as C. m This constitutes one charge-discharge cycle. Repeat the above charge-discharge cycle up to 400 times, and record the discharge capacity C of the 400th cycle. 400 .

[0056] 45℃ Cyclic Capacity Retention Rate (%) = C 400 / C m ×100%; The high-temperature cycling performance of sodium-ion batteries is evaluated by the capacity retention rate at 45℃. The higher the capacity retention rate at 45℃, the better the high-temperature cycling performance of the sodium-ion battery.

[0057] 55℃ Storage Performance Test

[0058] The sodium-ion battery was left to stand at 25°C for 30 minutes to reach a constant temperature. It was then charged at a constant current of 1C to a voltage of 4.0V, followed by constant voltage charging at 4.0V to a cutoff current of 0.05C. Finally, it was discharged at a constant current of 0.5C to a voltage of 2.0V. The discharge capacity was recorded as C0. Next, it was charged at a constant current of 1C to 4.0V and then charged at a constant voltage of 4.0V to a cutoff current of 0.05C. The thickness of the sodium-ion battery was measured using a plate thickness gauge and recorded as d0. The sodium-ion battery was then transferred to a 55°C explosion-proof oven and left for 14 days. The thickness of the sodium-ion battery was then measured and recorded as d1. Finally, the sodium-ion battery was removed and cooled to room temperature. At 25°C, it was discharged at a constant current of 0.5C to a voltage of 2.0V. The discharge capacity was recorded as C. n Thickness expansion rate at 55℃ = (d1-d0) / d0 × 100%, Storage capacity retention rate at 55℃ = C n / C0×100%.

[0059] The high-temperature storage performance of sodium-ion batteries is evaluated by the storage thickness expansion rate and storage capacity retention rate at 55℃. The smaller the storage thickness expansion rate and the larger the storage capacity retention rate, the better the high-temperature storage performance of sodium-ion batteries.

[0060] -20℃ discharge performance test

[0061] The sodium-ion battery was left to stand at 25°C for 30 minutes to reach a constant temperature. It was then charged at a constant current of 1C to a voltage of 4.0V, and then charged at a constant voltage of 4.0V to a cutoff current of 0.05C. Finally, it was discharged at a constant current of 0.5C to a voltage of 2.0V. The initial discharge capacity was recorded as C2. The sodium-ion battery was then charged at a constant current of 1C to a voltage of 4.0V, and then charged at a constant voltage of 4.0V to a cutoff current of 0.05C. The sodium-ion battery was then transferred to a -20°C constant temperature chamber and left to stand for 4 hours. Finally, it was discharged at a constant current of 0.5C at -20°C to a voltage of 2.0V. The discharge capacity was recorded as C3.

[0062] -20℃ storage capacity retention rate (%) = C3 / C2 × 100%; The low-temperature discharge performance of sodium-ion batteries is evaluated by the -20℃ capacity retention rate. The higher the -20℃ capacity retention rate, the better the low-temperature discharge performance of sodium-ion batteries.

[0063] Example 1

[0064] <Preparation of Electrolyte>

[0065] Under an inert atmosphere with a water content of less than 0.1 ppm and an oxygen content of less than 1 ppm, propylene carbonate, methyl ethyl carbonate, and diethyl carbonate are mixed in a mass ratio of 3:6:1 to obtain a base solvent. Then, sodium bis(fluorosulfonyl)imide (NaFSI) and a first additive (Formula 1-1) are added to the base solvent and mixed thoroughly to obtain an electrolyte. The mass percentage of NaFSI is 12%, and the mass percentage of the first additive (Formula 1-1) is 0.02%, based on the mass of the electrolyte.

[0066] <Preparation of the positive electrode>

[0067] NaNi, the positive electrode active material 1 / 3 Mn 1 / 3 Fe 1 / 3 O2 powder, conductive agent acetylene black, carbon nanotubes, and binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 94:2.5:0.5:3. N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solid content of 50 wt%. After vacuum stirring, a positive electrode slurry was obtained. This positive electrode slurry was uniformly coated onto one surface of a 16 μm thick aluminum foil current collector to obtain a single-sided coated positive electrode sheet. The above steps were repeated on the other surface of the aluminum foil to obtain a double-sided coated positive electrode sheet. After drying at 85°C, the sheet was cold-pressed, trimmed, cut, slit, and vacuum-dried at 75°C for 10 hours. After welding tabs, the positive electrode sheet was ready for use. The total thickness of the positive electrode sheet was 174 μm, and the single-sided coating density of the positive electrode slurry was 15 mg / cm³. 2 .

[0068] <Preparation of Negative Electrode Sheets>

[0069] Hard carbon (negative electrode active material), conductive carbon black (Super P), styrene-butadiene rubber (SBR) (binder), and sodium carboxymethyl cellulose (CMC) (dispersant) were mixed in a mass ratio of 95:1.5:2:1.5. Deionized water was added as a solvent to prepare a slurry with a solid content of 49 wt%. The slurry was stirred evenly in a vacuum mixer to obtain the negative electrode slurry. The negative electrode slurry was uniformly coated onto one surface of a 9 μm thick aluminum foil current collector to obtain a negative electrode sheet with a single-sided coating of negative electrode material. The above steps were repeated on the other surface of the aluminum foil to obtain a negative electrode sheet with a double-sided coating of negative electrode material. After drying at 85℃, the negative electrode sheet was cold-pressed, trimmed, cut, slit, and vacuum-dried at 85℃ for 10 hours. After welding tabs, the negative electrode sheet was prepared for use. The total thickness of the negative electrode sheet was 144 μm, and the single-sided coating density of the negative electrode slurry was 6.4 mg / cm³. 2 .

[0070] <Preparation of the diaphragm>

[0071] The diaphragm is a PE+ceramic+PVDF (7+3+2) diaphragm with a thickness of 12μm.

[0072] <Preparation of Sodium-ion Batteries>

[0073] The positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to isolate them. They are then wound, with the positive tab connected to the positive electrode and the negative tab connected to the negative electrode, thus obtaining the electrode assembly. The electrode assembly is placed in an aluminum foil packaging bag, with the positive and negative tabs extended from the inside to the outside of the bag. After drying at 75°C for 48 hours to remove moisture, it is heat-sealed to obtain the cell ready for electrolyte injection. The prepared electrolyte is injected into the dried cell in a glove box. Following encapsulation, settling, formation, aging, and capacity testing, a sodium-ion battery is obtained.

[0074] The formation conditions are: temperature 45℃, pressure 3 kg / cm². 2 Charge to 4.0V with constant current 0.1C.

[0075] Examples 2 to 24

[0076] Except for adjusting the type and mass percentage of the first additive W1, the mass percentage of sodium difluorosulfonamide W2, and the mass percentage of other sodium salts W3 according to Table 1, the rest is the same as in Example 1.

[0077] Specifically, when the mass percentage content of the first additive W1, the mass percentage content of sodium difluorosulfonamide W2, and the mass percentage content of other sodium salts W3 change, the content of the base solvent changes accordingly.

[0078] Example 25

[0079] Except for changing the type of negative electrode active material to artificial graphite, everything else is the same as in Example 20.

[0080] Example 26

[0081] Except for changing the type of negative electrode active material to a graphene composite material, everything else is the same as in Example 20.

[0082] Example 27

[0083] Besides adjusting the type of positive electrode active material to NaNi 1 / 3 Mn 1 / 3 Co 1 / 3 Except for O2, everything else is the same as in Example 20.

[0084] Example 28

[0085] Besides adjusting the type of positive electrode active material to Na 0.9 Mn0.6 Fe 0.4 Except for PO4, the rest is the same as in Example 20.

[0086] Comparative Examples 1 to 2

[0087] Except for adjusting the mass percentage content of the first additive W1, the mass percentage content of sodium difluorosulfonamide W2, and the mass percentage content of other sodium salt electrolytes W3 according to Table 1, the rest is the same as in Example 1.

[0088] Specifically, when the mass percentage content of the first additive W1, the mass percentage content of sodium difluorosulfonamide W2, and the mass percentage content of other sodium salts W3 change, the content of the base solvent changes accordingly.

[0089]

[0090]

[0091] As can be seen from Examples 1 to 11 and Comparative Example 1, by adding a first additive to the electrolyte and controlling the type and content of the first additive within the scope of this application, the sodium-ion battery exhibits high cycle capacity retention, storage capacity retention, and low storage thickness expansion rate. As can be seen from Examples 12 to 19 and Comparative Example 2, by adding sodium bis(fluorosulfonyl)imide to the electrolyte and controlling the content of sodium bis(fluorosulfonyl)imide within the scope of this application, the synergistic effect of the first additive and sodium bis(fluorosulfonyl)imide improves the cycle performance and storage performance of the sodium-ion battery. As can be seen from Examples 20 to 24, by adding sodium bis(fluorosulfonyl)imide as the main salt and sodium hexafluorophosphate as another sodium salt to the electrolyte, and adjusting the mass ratio of sodium hexafluorophosphate to sodium bis(fluorosulfonyl)imide within the scope of this application, the electrolyte can have high ionic conductivity and good electrochemical stability, thereby further improving the cycle performance and storage performance of the sodium-ion battery. As can be seen from Examples 1 to 28, sodium-ion batteries using the negative electrode active materials and positive electrode active materials within the scope of this application all exhibit high cycle performance and storage performance.

[0092] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. An electrolyte comprising a solvent, an electrolyte, and an additive, said additive comprising a first additive selected from at least one compound of Formula 1; in, R1 and R2 are independently selected from oxygen atoms, C1-C3 alkylene atoms, or C1-C3 alkoxide atoms, and at least one of R1 and R2 is a C1-C3 alkylene atom or a C1-C3 alkoxide atom. n and m are independently 0 or 1, and n and m are not both 0. The electrolyte includes a main salt, which includes sodium difluorosulfonylimide. Based on the mass of the electrolyte, the mass percentage of the first additive is W1, 0.03% ≤ W1 ≤ 3%; Based on the mass of the electrolyte, the mass percentage of sodium difluorosulfonamide is W2, where 6% ≤ W2 ≤ 18%. The mass ratio of sodium difluorosulfonamide to the first additive is (2-120):

1.

2. The electrolyte according to claim 1, wherein, The compound represented by Formula 1 is selected from at least one of the following compounds:

3. The electrolyte according to claim 1, wherein, 0.5％≤W1≤2％。 4. The electrolyte according to claim 1, wherein, The electrolyte includes other sodium salts selected from sodium hexafluorophosphate.

5. The electrolyte according to claim 4, wherein, Based on the mass of the electrolyte, the mass percentage of the other sodium salts is W3, where 0% ≤ W3 ≤ 6%.

6. The electrolyte according to any one of claims 1 to 5, wherein the electrolyte comprises other sodium salts and sodium difluorosulfonamide, wherein the mass ratio of the other sodium salts to the sodium difluorosulfonamide is (0-1):

1.

7. The electrolyte according to claim 6, wherein the solvent is selected from at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, propylene carbonate, ethylene carbonate, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, dioxolane, ethyl acetate, ethyl propionate, and propyl propionate.

8. A sodium-ion battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte according to any one of claims 1 to 7, wherein the separator is located between the positive electrode and the negative electrode, and the positive electrode comprises a positive active material selected from at least one of layered oxides, Prussian compounds, and polyanionic compounds.

9. The sodium-ion battery according to claim 8, wherein, The positive electrode active material is Na3V2(PO4)3, Na 0.9 Mn 0.6 Fe 0.4 PO4, Na4Fe3(PO4)2(P2O7), NaFePO4, sodium ferrocyanide and NaNi X Me Y At least one of O2, Me is selected from two or three of Fe, Co, Mn and Al, and X+Y=1.

10. The sodium-ion battery according to claim 8, wherein the negative electrode sheet comprises a negative electrode active material, and the negative electrode active material is selected from one or more of carbon-based materials, silicon-based materials, alloy materials and organic materials.

11. The sodium-ion battery according to claim 10, wherein, The negative electrode active material is hard carbon.

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