Electrolyte for sodium secondary battery and sodium secondary battery comprising same

Abstract

The present invention relates to an electrolyte for a sodium secondary battery and a sodium secondary battery comprising same. More specifically, when an electrolyte comprising a solvent combining a cyclic carbonate solvent, a linear carbonate-based solvent, and / or an ester-based solvent is applied to a sodium secondary battery, the output performance of the sodium secondary battery can be improved.

Description

Electrolyte for sodium secondary batteries and sodium secondary batteries containing the same

[0001] Cross-citation with related applications

[0002] This application claims the benefit of priority based on Korean Patent Application No. 10-2024-0175417 filed November 29, 2024 and Korean Patent Application No. 10-2025-0186978 filed December 1, 2025, and all contents disclosed in the documents of said Korean patent applications are incorporated herein as part of this specification.

[0003] Technology field

[0004] The present invention relates to an electrolyte for a sodium secondary battery and a sodium secondary battery comprising the same.

[0005] As portable electronic devices such as video cameras, mobile phones, and laptops become lighter and more functional, research is being conducted on increasing the capacity and energy density of batteries used as their power sources.

[0006] Lithium batteries are widely used commercially because they have an energy density about three times higher per unit weight compared to conventional lead-acid batteries, nickel-cadmium batteries, nickel-hydrogen batteries, and nickel-zinc batteries, and can be rapidly charged.

[0007] However, due to issues such as the limited availability of lithium mineral resources and the resulting high production costs, there is a demand for new battery systems that can replace lithium-ion batteries.

[0008] Sodium batteries are being actively researched for high-output applications because they are environmentally friendly, highly cost-competitive, and offer superior low-temperature performance compared to lithium batteries. Sodium ions have lower de-solvation energy at the electrode interface than lithium ions. Consequently, sodium secondary batteries are known to be effective in improving output compared to conventional lithium-ion batteries and to have a high energy retention rate at low temperatures.

[0009] Like lithium-ion batteries, sodium secondary batteries also use carbon materials such as graphite as the negative electrode active material, and sodium ions migrated from the positive electrode are inserted or extracted between the carbon materials of the negative electrode.

[0010] However, sodium ions (Na + The radius (1.03 Å) of ) is lithium ions (Li + Because it is larger than the radius (0.71 Å) of ), Na between the carbon materials of the cathode + It is difficult to achieve capacity by inserting [it]. Therefore, hard carbon that achieves capacity by adsorbing sodium ions is mainly used as the negative electrode material for sodium secondary batteries.

[0011] As such, even if hard carbon is used as the negative electrode material for sodium secondary batteries, the problem remains that the rate capability is reduced due to the large ionic radius of sodium ions, and consequently, the output characteristics are also reduced.

[0012] Therefore, there is growing interest in electrolytes for sodium secondary batteries that can improve the degradation of output characteristics caused by the large ionic radius of sodium ions and enable the maximum utilization of sodium secondary battery performance.

[0013] [Prior Art Literature]

[0014] [Patent Literature]

[0015] (Patent Document 1) Korean Published Patent No. 10-2023-0088729

[0016] As a result of conducting multifaceted research to solve the above problem, the inventors confirmed that when an electrolyte prepared by mixing a cyclic carbonate-based solvent, a linear carbonate-based solvent, and / or an ester-based solvent in an appropriate compositional ratio is applied to a sodium secondary battery, the output performance of the battery is improved.

[0017] Accordingly, the objective of the present invention is to provide an electrolyte for a sodium secondary battery that can improve the output performance of the sodium secondary battery.

[0018] Another objective of the present invention is to provide a sodium secondary battery comprising the above electrolyte.

[0019] To achieve the above objective, a first embodiment of the present invention is an electrolyte for a sodium secondary battery comprising a sodium salt, an additive, and a solvent, wherein

[0020] The above solvent comprises a first solvent; and a second solvent, and

[0021] The first solvent comprises a cyclic carbonate-based solvent including one or more selected from the group consisting of ethylene carbonate (EC) and propylene carbonate (PC), and

[0022] The second solvent comprises one or more selected from the group consisting of linear carbonate-based solvents and ester-based solvents, and

[0023] The above linear carbonate-based solvent includes ethyl methyl carbonate (EMC), and

[0024] The present invention provides an electrolyte for a sodium secondary battery, wherein the above-mentioned ester-based solvent comprises one or more selected from the group consisting of methyl acetate (MA), ethyl acetate (EA), n-propyl acetate (PA), dimethyl acetate (DMA), methyl propionate (MP), ethyl propionate (EP), and propyl propionate (PP).

[0025] In one embodiment of the present invention, an electrolyte for a sodium secondary battery is provided, wherein the cyclic carbonate-based solvent is included in an amount of 10 to 50 volume% based on the total volume of the solvent.

[0026] In one embodiment of the present invention, an electrolyte for a sodium secondary battery is provided, wherein the linear carbonate-based solvent is included in an amount of 50 to 70 volume% based on the total volume of the solvent.

[0027] In one embodiment of the present invention, an electrolyte for a sodium secondary battery is provided, wherein the ester-based solvent is included in an amount of 20 to 50 volume% based on the total volume of the solvent.

[0028] In one embodiment of the present invention, the solvent comprises the first solvent and the second solvent, wherein

[0029] The first solvent above includes the cyclic carbonate-based solvent, and

[0030] The above second solvent provides an electrolyte for a sodium secondary battery, wherein the second solvent comprises the linear carbonate-based solvent and the ester-based solvent.

[0031] In one embodiment of the present invention, the cyclic carbonate-based solvent comprises propylene carbonate (PC), and the linear carbonate-based solvent comprises ethyl methyl carbonate (EMC), thereby providing an electrolyte for a sodium secondary battery.

[0032] In one embodiment of the present invention, the electrolyte for a sodium secondary battery is provided, wherein the electrolyte does not contain ethylene carbonate (EC).

[0033] In one embodiment of the present invention, the sodium salt comprises one or more selected from the group consisting of NaPF6, NaFSI, NaClO4, NaBF4, NaTFSI, NaSO3CF3, NaBOB, and NaODFB, thereby providing an electrolyte for a sodium secondary battery.

[0034] In one embodiment of the present invention, an electrolyte for a sodium secondary battery is provided, wherein the sodium salt comprises the NaPF6 and the NaFSI.

[0035] In one embodiment of the present invention, an electrolyte for a sodium secondary battery is provided, wherein the concentration of NaPF6 among the sodium salts is equal to or greater than the concentration of NaFSI.

[0036] In one embodiment of the present invention, an electrolyte for a sodium secondary battery is provided, wherein the concentration of the sodium salt is 0.7 to 1.4 M in the electrolyte.

[0037] In one embodiment of the present invention, the electrolyte for a sodium secondary battery is provided, wherein the additive comprises one or more selected from the group consisting of vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethylene sulfate (ESA), and sodium difluorophosphate (NaDFP).

[0038] In one embodiment of the present invention, an electrolyte for a sodium secondary battery is provided, wherein the additive is included in an amount of 0.1 to 5 weight percent based on the total weight of the electrolyte.

[0039]

[0040] A second embodiment of the present invention provides a sodium secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the two, and an electrolyte according to a first embodiment of the present invention.

[0041] In one embodiment of the present invention, a sodium secondary battery is provided in which the positive active material included in the positive electrode comprises a layered sodium oxide.

[0042] In one embodiment of the present invention, the layered sodium oxide is NaNi 1 / 3 Mn 1 / 3 Co 1 / 3O2, NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2, NaFe 1 / 2 Co 1 / 2 O2, NaMn 1 / 2 Co 1 / 2 O2 and NaNi 1 / 3 Co 1 / 3 Fe 1 / 3 A sodium secondary battery is provided that includes one or more types selected from the group consisting of O2.

[0043] According to the present invention, by applying an electrolyte prepared by mixing a cyclic carbonate-based solvent, a linear carbonate-based solvent, and / or an ester-based solvent in an appropriate compositional ratio to a sodium secondary battery, the output performance of the sodium secondary battery can be improved.

[0044] Hereinafter, the present invention will be described in more detail to aid in understanding the invention.

[0045] Terms and words used in this specification and claims shall not be interpreted as being limited to their ordinary or dictionary meanings, but shall be interpreted in a meaning and concept consistent with the technical spirit of the invention, based on the principle that the inventor can appropriately define the concept of the terms to best describe his invention.

[0046] In this specification, the singular form of the noun corresponding to an object may include one or more of said objects unless the relevant context clearly indicates otherwise.

[0047] In this specification, when a part is described as 'comprising' or 'having' a certain component, it means that, unless specifically stated otherwise, it does not exclude other components but may include additional components.

[0048] In this specification, when it is stated that any layer is located 'on' or 'between' another arbitrary layer, this includes not only cases where any layer is in contact with another arbitrary layer, but also cases where another layer or material, etc. exists between the two layers.

[0049] Where in this specification, when a quantity, concentration, other value, or parameter is given as an enumeration of a range, a preferred range, a preferred upper limit, and a preferred lower limit, it should be understood that any pair of any upper range limit or preferred value and any lower range limit or preferred value specifically discloses all ranges that may be formed, regardless of whether the range is disclosed separately.

[0050] In this specification, descriptions that specify or add components may be applied to all inventions unless specifically limited, and are not limited to a particular invention.

[0051]

[0052] Electrolyte for sodium secondary batteries

[0053] The present invention relates to an electrolyte for a sodium secondary battery, and aims to improve the output characteristics of a sodium secondary battery by optimizing a combination of a cyclic carbonate-based solvent, a linear carbonate-based solvent, and / or an ester-based solvent as the solvent of the electrolyte.

[0054] The electrolyte for a sodium secondary battery according to the present invention comprises a sodium salt, an additive, and a solvent, wherein the solvent is a first solvent; The solvent comprises a first solvent and a second solvent, wherein the first solvent comprises a cyclic carbonate-based solvent comprising one or more selected from the group consisting of ethylene carbonate (EC) and propylene carbonate (PC), the second solvent comprises one or more selected from the group consisting of linear carbonate-based solvents and ester-based solvents, the linear carbonate-based solvent comprises ethyl methyl carbonate (EMC), and the ester-based solvent comprises one or more selected from the group consisting of methyl acetate (MA), ethyl acetate (EA), n-propyl acetate, dimethyl acetate (DMA), methyl propionate (MP), ethyl propionate (EP), and propyl propionate (PP).

[0055]

[0056] In one embodiment of the present invention, the cyclic carbonate-based solvent may be included in an amount of 10 to 50 volume% based on the total volume of the solvent.

[0057] If the content of the above-mentioned cyclic carbonate-based solvent is less than 10 volume%, the ionic conductivity of the electrolyte may decrease, and if it exceeds 50 volume%, the viscosity of the electrolyte may increase, and the electrode wetting may deteriorate. Specifically, the content of the above-mentioned cyclic carbonate-based solvent may be 10 volume% or more, 15 volume% or more, 20 volume% or more, 25 volume% or more, or 30 volume% or more, and may be 50 volume% or less, 45 volume% or less, or 40 volume% or less.

[0058]

[0059] In one embodiment of the present invention, the linear carbonate-based solvent may be included in an amount of 50 to 70 volume% based on the total volume of the solvent.

[0060] If the content of the linear carbonate-based solvent is less than 50 volume%, the proportion of the cyclic carbonate-based solvent in the electrolyte becomes relatively high, which may worsen electrode wettability, and if it exceeds 70 volume%, the proportion of the cyclic carbonate-based solvent in the electrolyte becomes relatively low, which may lower ion conductivity. Specifically, the content of the linear carbonate-based solvent may be 50 volume% or more, 51 volume% or more, 52 volume% or more, 53 volume% or more, 54 volume% or more, or 55 volume% or more, and may be 70 volume% or less, 69 volume% or less, 68 volume% or less, 67 volume% or less, 66 volume% or less, or 65 volume% or less.

[0061]

[0062] In one embodiment of the present invention, the ester-based solvent may be included in an amount of 20 to 50 volume% based on the total volume of the solvent.

[0063] If the content of the ester-based solvent is less than 20 volume%, there is a limit to low-temperature output performance, and if it exceeds 50 volume%, problems with stable operation may occur due to surface side reactions. Specifically, the content of the ester-based solvent may be 20 volume% or more, 25 volume% or more, 30 volume% or more, or 35 volume% or more, and may be 50 volume% or less, 45 volume% or less, or 40 volume% or less.

[0064]

[0065] In one embodiment of the present invention, the solvent comprises a first solvent and a second solvent, wherein the first solvent comprises a cyclic carbonate-based solvent and the second solvent may comprise a linear carbonate-based solvent and an ester-based solvent.

[0066] If the above solvent includes cyclic carbonate-based solvents, linear carbonate-based solvents, and ester-based solvents, it may be more advantageous for improving the output of the battery.

[0067]

[0068] In one embodiment of the present invention, the cyclic carbonate-based solvent may include propylene carbonate (PC), and the linear carbonate-based solvent may include ethyl methyl carbonate (EMC).

[0069] Among the combination of cyclic carbonate-based solvents and linear carbonate-based solvents included in the above solvent, if the cyclic carbonate-based solvent contains PC and the linear carbonate-based solvent contains EMC, it may be advantageous for improving the output of a sodium secondary battery. In addition, if the EMC is 65 volume% or more of the solvent, it may be advantageous in terms of battery durability.

[0070]

[0071] In one embodiment of the present invention, the electrolyte may not contain ethylene carbonate (EC).

[0072] Even if the EC included as the first solvent is a cyclic carbonate-based solvent and is included in the electrolyte within the content range described above, it is possible to secure battery performance of a certain level or higher; however, if the EC is excluded from the electrolyte, the voltage drop during low-temperature discharge is reduced, which is advantageous for output characteristics, and the amount of gas generated can also be significantly reduced.

[0073]

[0074] In one embodiment of the present invention, the sodium salt may include one or more selected from the group consisting of NaPF6, NaFSI, NaClO4, NaBF4, NaTFSI, NaSO3CF3, NaBOB, and NaODFB.

[0075] The above sodium salt acts as a channel for the movement of sodium ions between the positive and negative electrodes in a sodium secondary battery, thereby compensating for the insufficient ionic conductivity of the solvent. Accordingly, the above sodium salt is not particularly limited as long as it is a sodium salt capable of enhancing the movement characteristics of sodium ions.

[0076]

[0077] In one embodiment of the present invention, the sodium salt may include NaPF6 and NaFSI.

[0078] In the case of the dual sodium salt containing the above NaPF6 and NaFSI, compared to a single sodium salt, it can reduce the initial resistance of the battery and reduce the voltage drop during low-temperature discharge, which may be advantageous for output characteristics.

[0079]

[0080] In one embodiment of the present invention, the concentration of NaPF6 among the sodium salts may be equal to or greater than the concentration of NaFSI.

[0081] In the dual sodium salt containing NaPF6 and NaFSI, if the concentration of NaFSI is higher than the concentration of NaPF6, the amount of gas generated increases, which may lead to corrosion. Accordingly, to prevent the generation of the gas, the concentration ratio of NaPF6 and NaFSI (PF6:FSI = PF6 / FSI) may be 1:1 to 5:1. If the concentration ratio is less than 1:1, it implies that the concentration of NaFSI increases relatively, which leads to a significant increase in the amount of gas generated and a high likelihood of performance degradation during evaluation. If it exceeds 5:1, it implies that the concentration of NaPF6 increases relatively, which may result in insufficient output performance. Specifically, the concentration ratio of the above NaPF6 and NaFSI may be 1:1 or greater, 1.1 / 1 or greater, 1.2 / 1 or greater, 1.3 / 1 or greater, 1.4 / 1 or greater, 1.5 / 1 or greater, 1.6 / 1 or greater, 1.7 / 1 or greater, 1.8 / 1 or greater, 1.9 / 1 or greater, 2 / 1 or greater, 2.1 / 1 or greater, 2.3 / 1 or greater, 2.4 / 1 or greater, 2.5 / 1 or greater, 2.6 / 1 or greater, 2.7 / 1 or greater, 2.8 / 1 or greater, 2.9 / 1 or greater, or 3 / 1 or greater, and may be 5:1 or less, 4.9 / 1 or less, 4.8 / 1 or less, 4.7 / 1 or less, 4.6 / 1 or less, 4.5 / 1 or less, 4.4 / 1 or less, 4.3 / 1 or less, 4.2 / 1 or less, 4.1 / 1 It may be less than or equal to 4 / 1, less than or equal to 3.9 / 1, less than or equal to 3.8 / 1, less than or equal to 3.7 / 1, less than or equal to 3.6 / 1, less than or equal to 3.5 / 1, less than or equal to 3.4 / 1, less than or equal to 3.3 / 1, less than or equal to 3.2 / 1, or less than or equal to 3.1.

[0082] In one embodiment, the concentration of NaPF6 may be 0.5M or higher, and the concentration of NaFSI may be 0.5M or lower. If the concentration of NaPF6 is less than 0.5M, the probability of corrosion occurring may increase further.

[0083]

[0084] In one embodiment of the present invention, the concentration of the sodium salt may be 0.5 to 1.4 M in the electrolyte.

[0085] The concentration range of the sodium salt above may be set considering the degree of improvement in sodium ion characteristics. For example, if the concentration of the sodium salt is less than 0.5 M, the mobility of sodium ions may decrease, and if it exceeds 1.4 M, the concentration of the sodium salt is excessive and may act as a resistance. Specifically, the concentration of the sodium salt may be 0.5 M or more, 0.6 M or more, 0.7 M or more, 0.8 M or more, or 0.9 M or more, and may be 1.4 M or less, 1.3 M or less, 1.2 M or less, or 1.1 M or less.

[0086]

[0087] In one embodiment of the present invention, the additive may include one or more selected from the group consisting of vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethylene sulfate (ESA), propenesultone (PS), and sodium difluorophosphate (NaDFP).

[0088] The above additive is a substance added in small amounts to the electrolyte and can play a role in protecting the surface of the electrode so that charging and discharging can be carried out more stably when sodium ions move.

[0089]

[0090] In one embodiment of the present invention, the additive may be included in an amount of 0.1 to 5 weight percent based on the total weight of the electrolyte.

[0091] If the content of the above additive is less than 0.1 weight%, the functionality of protecting the surface of the electrode during charging and discharging may be reduced, and durability may deteriorate; if it exceeds 5 weight%, the additive may be included in excess and act as a resistor within the battery. Specifically, the content of the above additive may be 0.1 weight% or more, 0.2 weight% or more, 0.3 weight% or more, 0.4 weight% or more, 0.5 weight% or more, 0.6 weight% or more, 0.7 weight% or more, 0.8 weight% or more, 0.9 weight% or more, or 1 weight% or more, and may be 5 weight% or less, 4 weight% or less, or 3 weight% or less.

[0092]

[0093] In one embodiment of the present invention, the additive may not contain fluoroethylene carbonate (FEC).

[0094] The above FEC can also serve as an additive to protect the surface of the electrode, thereby enabling more stable charging and discharging when sodium ions move. However, as the content of FEC in the electrolyte decreases, the initial resistance of the battery decreases and the voltage drop during low-temperature discharge also decreases, resulting in a favorable effect on output characteristics. Furthermore, if FEC is not included at all, this effect can be maximized.

[0095] In one embodiment, the content of the FEC may be 0 to 2 weight percent based on the total weight of the electrolyte. Within the above range, as the range of FEC decreases, it may be advantageous for the output characteristics of the battery.

[0096]

[0097] Method for manufacturing an electrolyte for a sodium secondary battery

[0098] The present invention also relates to a method for manufacturing an electrolyte for a sodium secondary battery.

[0099] The method for manufacturing an electrolyte for a sodium secondary battery according to the present invention can produce an electrolyte by adding a sodium salt and an additive to a combination of solvents as described above. At this time, the solvent may be used after adsorbing moisture using molecular sieves or the like and then filtering it out with a filter.

[0100]

[0101] Sodium secondary battery

[0102] The present invention also relates to a sodium secondary battery.

[0103] A sodium secondary battery according to the present invention comprises a positive electrode, a negative electrode, a separator interposed between them, and an electrolyte. The sodium secondary battery can be manufactured by obtaining a battery assembly by interposing a separator between the positive electrode and the negative electrode, placing the battery assembly in a battery case, and injecting an electrolyte.

[0104]

[0105] Hereinafter, the positive electrode, negative electrode, and separator included in the above-described sodium secondary battery will be explained in more detail. The composition and physical properties of the electrolyte are as described above.

[0106]

[0107] anode

[0108] In one embodiment of the present invention, the positive electrode may include a positive electrode current collector; and a positive electrode active material layer formed on the positive electrode current collector.

[0109]

[0110] The above positive active material layer may include a positive active material, a binder, and a conductive material.

[0111] In addition, the above-mentioned positive electrode active material is a compound capable of reversible sodination and disodination of sodium. Preferably, the above-mentioned positive electrode active material may be a layered sodium oxide to improve the capacity of the battery. The above-mentioned layered sodium oxide is NaNi 1 / 3 Mn1 / 3 Co 1 / 3 O2, NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2, NaFe 1 / 2 Co 1 / 2 O2, NaMn 1 / 2 Co 1 / 2 O2 and NaNi 1 / 3 Co 1 / 3 Fe 1 / 3 It may include one or more types selected from the group consisting of O2.

[0112] In addition, the positive active material may be included in an amount of 80 to 99 weight percent based on the total weight of the positive active material layer. If the content of the positive active material is less than 80 weight percent, the battery capacity of the entire cell may decrease, and if it exceeds 99 weight percent, the conductive material and binder excluding the positive active material may decrease relatively, and the conductivity or physical properties of the positive electrode may decrease. Specifically, the content of the positive active material may be 80 weight percent or more, 85 weight percent or more, or 90 weight percent or more, and 99 weight percent or less, or 95 weight percent or less.

[0113]

[0114] In addition, the above binder retains the positive active material on the positive current collector and organically connects the positive active materials to further enhance the binding force between them, and any binder known in the industry may be used.

[0115] The above binder is a fluoropolymer-based binder comprising polyvinylidene fluoride (PVdF) and / or polytetrafluoroethylene (PTFE); a rubber-based binder comprising one or more of styrene butadiene rubber (SBR), acrylonitrile-butidiene rubber, and styrene-isoprene rubber; a cellulose-based binder comprising one or more of carboxyl methyl cellulose (CMC), starch, hydroxypropyl cellulose, and regenerated cellulose; a polyalcohol-based binder; a polyolefin-based binder comprising one or more of polyethylene and polypropylene; a polyimide-based binder; a polyester-based binder; and an acrylic-based binder comprising an acrylic monomer. One or more mixtures or copolymers selected from the group consisting of silane-based binders may be used. According to one embodiment of the present invention, a rubber-based binder combination may be preferred for the binder. Additionally, considering compatibility with carboxymethylcellulose-based thickeners, the binder may include SBR and / or an acrylic-based binder.

[0116] In addition, the binder may be included in an amount of 0.01 to 10 weight% based on the total weight of the positive electrode active material layer. If the content of the binder is less than 0.01 weight%, physical properties of the positive electrode, such as binding strength, may deteriorate, causing the positive electrode active material and conductive material to detach; if it exceeds 30 weight%, the ratio of the positive electrode active material and conductive material may decrease relatively, which may reduce the battery capacity. Specifically, the content of the binder may be 0.01 weight% or more, 1 weight% or more, or 3 weight% or more, and 10 weight% or less, 8 weight% or less, or 6 weight% or less.

[0117]

[0118] In addition, the above-mentioned conductive material is intended to improve electrical conductivity, and there are no specific restrictions as long as it is an electrically conductive material that does not cause chemical changes in the lithium secondary battery.

[0119] The conductive material may include one or more selected from the group consisting of carbon black, graphite, carbon fiber, carbon nanotube, metal powder, conductive metal oxide, and organic conductive material. The carbon black may include one or more selected from the group consisting of Ketjen black, Super P, Denka black, acetylene black, and furnace black.

[0120] The conductive material may be included in an amount of 0.01 to 10 weight% based on the total weight of the positive electrode active material layer. Specifically, the content of the conductive material may be 0.01 weight% or more, 2 weight% or more, or 4 weight% or more, and 10 weight% or less, 8 weight% or less, or 6 weight% or less. If the content of the conductive material is less than 0.01 weight%, the conductivity of the positive electrode may be reduced, and if it exceeds 10 weight%, the flexibility of the positive electrode may be reduced.

[0121] In addition, the thickness of the positive active material layer is not particularly limited and can be set to an appropriate range considering the mechanical strength of the positive, the loading amount, or the capacity of the battery. For example, the thickness of the positive active material layer can typically be 30㎛ to 300㎛.

[0122]

[0123] In addition, the positive current collector is not particularly limited as long as it is conductive without causing chemical changes in the battery and can be used electrochemically stably at the positive charging voltage. For example, the positive current collector may be one or more selected from the group consisting of aluminum, stainless steel, titanium, silver, palladium, nickel, alloys thereof, and combinations thereof. The stainless steel may be surface-treated with carbon, nickel, titanium, or silver.

[0124] In addition, the form of the anode current collector is not particularly limited, and may be in the form of a film, sheet, foil, net, porous body, foam, or nonwoven fabric. If necessary, fine irregularities may be formed on the surface of the anode current collector, and said irregularities may help improve adhesion with the anode active material layer. The method of forming irregularities on the surface of the anode current collector is not particularly limited, and known methods such as mechanical polishing, electrolytic polishing, or chemical polishing may be applied.

[0125] In addition, the thickness of the positive current collector is not particularly limited and can be set within an appropriate range considering the mechanical strength, productivity, or capacity of the positive electrode. For example, the thickness of the positive current collector can typically be 3㎛ to 500㎛.

[0126]

[0127] cathode

[0128] In one embodiment of the present invention, the cathode may include a cathode current collector and a cathode active material layer formed on the cathode current collector. Alternatively, the cathode may include a cathode active material layer alone without a cathode current collector, in which case the cathode active material layer may be a sodium foil.

[0129] The above-mentioned cathode active material is one or more carbon-based materials selected from the group consisting of hard carbon, crystalline synthetic graphite, crystalline natural graphite, low-crystalline soft carbon, carbon black, acetylene black, Ketjen black, Super-P, graphene, and fibrous carbon, Si-based materials, Na x Fe2O3(0≤x≤1), Na x WO2(0≤x≤1), Sn x Me 1-x Me y O z (Me: Mn, Fe, Pb, Ge; Me': Al, B, P, Si, Group 1, 2, and 3 elements of the periodic table, halogens; 0 <x≤1; 1≤y≤3; 1≤z≤8) 등의 금속 복합 산화물; 소듐 금속; 소듐 합금; 규소계 합금; 주석계 합금; SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, Bi2O5등의 금속 산화물; 폴리아세틸렌 등의 도전성 고분자; Na-Co-Ni 계 재료; 티타늄 산화물; 소듐 티타늄 산화물 등을 포함할 수 있지만, 이들만으로 한정되는 것은 아니다.

[0130] In addition, the above-mentioned negative electrode active material is SnxMe 1-x Me y O z (Me: Mn, Fe, Pb, Ge; Me': Al, B, P, Si, Group 1, 2, and 3 elements of the periodic table, halogens; 0 <x≤1; 1≤y≤3; 1≤z≤8) 등의 금속 복합 산화물; SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO22, Bi2O3, Bi2O4및 Bi2O5등의 산화물 등을 사용할 수 있고, 결정질 탄소, 비정질 탄소 또는 탄소 복합체와 같은 탄소계 음극 활물질이 단독으로 또는 2종 이상이 혼용되어 사용될 수 있다.

[0131] In addition, the description of the binder and conductive material included in the cathode is omitted as it is the same as the description of the binder and conductive material included in the anode as described above.

[0132] In addition, the above-mentioned negative current collector is not particularly limited as long as it is conductive without causing chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface treated with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloy may be used. In addition, the above-mentioned negative current collector, like the positive current collector, may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc., having fine irregularities formed on its surface.

[0133] In addition, the thickness of the negative electrode current collector is not particularly limited and can be set within an appropriate range considering the mechanical strength of the negative electrode, productivity, or capacity of the battery. For example, the thickness of the negative electrode current collector can typically be 3㎛ to 500㎛.

[0134]

[0135] Separator

[0136] In one embodiment of the present invention, the separator can serve as a passage for sodium ions to move, while simultaneously serving as a wall to prevent the anode and cathode from coming into contact.

[0137] The above-mentioned separator is not particularly limited as long as it is used as a separator in the relevant industry. In particular, the above-mentioned separator may have low resistance to sodium ion movement and excellent electrolyte moisture retention capacity.

[0138] For example, the separator may include one or more selected from a porous polymer film and a porous nonwoven fabric. The porous polymer film may be a porous polymer film comprising a polyolefin-based polymer. The polyolefin may include one or more selected from the group consisting of ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer. Additionally, the porous nonwoven fabric may include one or more selected from the group consisting of high-melting-point glass fibers and polyethylene terephthalate fibers.

[0139] In addition, the above-mentioned separator may have a coating layer formed thereon containing ceramic components or polymer materials to ensure heat resistance or mechanical strength.

[0140]

[0141] These sodium secondary batteries can be used not only as battery cells serving as power sources for small devices, but also as unit cells in medium-to-large battery modules containing multiple battery cells. Examples of such medium-to-large devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems; in particular, they can be usefully applied in areas requiring high output, such as hybrid electric vehicles and batteries for renewable energy storage.

[0142]

[0143] Preferred embodiments are presented below to aid in understanding the present invention; however, the following embodiments are merely illustrative of the invention, and it is obvious to those skilled in the art that various changes and modifications are possible within the scope and spirit of the invention, and that such changes and modifications fall within the scope of the appended claims.

[0144]

[0145] In the following examples and comparative examples, an electrolyte for a sodium ion battery was prepared according to the composition as described in Table 1 below.

[0146]

[0147] Sodium salt (M) Additive (wt%) Solvent (vol%) 1st Solvent 2nd Solvent Cyclic Carbonate Solvent Linear Carbonate Solvent Ester Solvent NaPF6 NaFSIFECECPCEMCDECEP Example 11-2 151550-20 Example 20.5 0.5 2151550-20 Example 30.8 0.2 2151550-20 Example 4 10.2 2151550-20 Example 5 10.2 252550-20 Example 6 10.2 2-30 50-20 Example 7 10.22 20 1050-10 Example 8 10.22 10 1030-50 Example 9 10.22 20 40-40 Example 10 10.20 5-20 40-40 Example 11 10.20 20 40-40 Comparative Example 1 10.22 15 15-5020 Comparative Example 2 10.22 15 1570--

[0148]

[0149] Examples 1 to 11 and Comparative Examples 1 to 2

[0150] (1) Electrolyte preparation

[0151] According to the composition shown in Table 1, a molecular sieve was placed in a solvent containing the first and second solvents to absorb moisture, and then a sodium salt and an additive were added to prepare an electrolyte.

[0152]

[0153] (2) Sodium secondary battery manufacturing

[0154] Na[Ni 1 / 3 Fe 1 / 3 Mn 1 / 3 A sodium secondary battery was manufactured by inserting a laminate having a polyethylene (PE) polymer separator between a positive electrode containing O2 as a positive active material and a counter electrode containing hard carbon into a pouch-type case, and then injecting the electrolyte.

[0155]

[0156] Experimental Example 1: Battery Performance Test

[0157] The initial resistance and discharge capacity of the sodium secondary batteries prepared in the examples and comparative examples were measured by the following method.

[0158]

[0159] (1) Initial resistance

[0160] 10C was discharged for 10 seconds at 50% discharge capacity (SOC (State of Charge) 50). At this time, the initial resistance was obtained by calculating the resistance by dividing the voltage drop by the applied current.

[0161] The above initial resistance is the result of a 10C resistance for 10 seconds at 50% discharge capacity (SOC (State of Charge) 50), and corresponds to the representative basic performance of the battery. A lower initial resistance indicates superior output.

[0162]

[0163] (2) Voltage drop during low-temperature discharge

[0164] After charging to SOC 100 at room temperature CC (Constant Current) / CV (Constant Voltage) (0.05C current cut), the temperature was lowered and left for 3 to 4 hours. Subsequently, it was discharged at 15C, and the voltage drop was measured using the voltage difference between immediately before discharge and at a specific point during discharge. A lower voltage drop value during low-temperature discharge indicates that it is more favorable for output characteristics.

[0165]

[0166] (3) Amount of gas generated when stored at 70℃ for 4 weeks

[0167] The battery was left for 4 weeks at SOC 100 and 70℃. Afterwards, the battery was discharged to SOC 0, the outer surface of the pouch was punched, the generated gas was extracted under negative pressure, and the amount of gas generated was measured by gas chromatography.

[0168]

[0169] Table 2 below shows the measured initial resistance, voltage drop during low-temperature discharge, and gas generation amount during storage at 70°C for 4 weeks.

[0170]

[0171] Initial Resistance Voltage Drop During Cold Discharge Gas Generation Amount (Ω) During Storage at 70°C for 4 Weeks (% vs. Example 1) Vdrop (mV / initial-End, 10 sec) Example 1 2.64 Ref. 1.627 Ref. Example 2 2.57 - 2.73 1.60 114.70 (% vs. Example 1) Example 3 2.59 - 1.89 1.618 - Example 4 2.71 2.65 1.636 Ref. Example 5 2.78 5.30 1.624 - 4.80% (% vs. Example 4) Example 6 2.79 5.68 1.615 - 8.60% (% vs. Example 4) Example 7 2.909.921.698 - Example 8 2.60 - 1.331.603 - Example 9 2.58 - 2.231.606 - Example 10 2.24 - 15.231.279 - Example 11 1.95 - 26.291.176 - Comparative Example 13.2523.201.848 - Comparative Example 22.826.781.748 -

[0172]

[0173] Referring to Table 2 above, Example 1 uses a single sodium salt, while Examples 2 and 3 use a dual sodium salt. Since Examples 2 and 3 show reduced initial resistance and reduced voltage drop during low-temperature discharge compared to Example 1, it can be seen that using a dual sodium salt is advantageous for output characteristics. The amount of gas generated in Example 2 is expressed as a percentage relative to Example 1.

[0174] In addition, in Examples 2 and 3 containing the double sodium salt containing NaPF6 and NaFSI, the amount of gas generated is significantly increased in Example 2, where the concentrations of NaPF6 and NaFSI are the same, so it can be seen that when the concentration of NaPF6 is greater than the concentration of NaFSI, it is advantageous for battery performance.

[0175] In addition, from Examples 4, 5, and 6 above, it can be seen that as the EC content decreases, the voltage drop during low-temperature discharge decreases, which is advantageous for output characteristics, and the amount of gas generated also decreases. The amount of gas generated in Examples 5 and 6 above is expressed as a percentage of a relative value based on the amount of gas generated in Example 4 above.

[0176] In addition, from Examples 5, 7, and 8 above, it can be seen that as the EP content increases, the initial resistance decreases and the voltage drop during low-temperature discharge also decreases, which is advantageous for output characteristics.

[0177] In addition, from Examples 9, 10, and 11 above, it can be seen that as the FEC content decreases, the initial resistance decreases and the voltage drop during low-temperature discharge also decreases, which is advantageous for output characteristics.

[0178] In addition, it can be seen that Comparative Example 1 above includes DEC instead of EMC as the linear carbonate-based solvent, and that the initial resistance increases and the voltage drop during low-temperature discharge increases, thereby degrading the output characteristics.

[0179] In addition, it can be seen that Comparative Example 2 above does not contain the ester-based solvent, so the initial resistance increases and the voltage drop during low-temperature discharge increases, resulting in a decrease in output characteristics.

[0180]

[0181] Although the present invention has been described above by limited embodiments and drawings, the present invention is not limited thereto, and it is obvious that various modifications and variations are possible within the scope of the technical spirit of the present invention and the equivalent scope of the claims described below by those skilled in the art to which the present invention belongs.

Claims

1. An electrolyte for a sodium secondary battery comprising a sodium salt, an additive, and a solvent, wherein The above solvent comprises a first solvent; and a second solvent, and The first solvent comprises a cyclic carbonate-based solvent including one or more selected from the group consisting of ethylene carbonate (EC) and propylene carbonate (PC), and The second solvent comprises one or more selected from the group consisting of linear carbonate-based solvents and ester-based solvents, and The above linear carbonate-based solvent includes ethyl methyl carbonate (EMC), and The electrolyte for a sodium secondary battery comprises one or more selected from the group consisting of the ester-based solvents: methyl acetate (MA), ethyl acetate (EA), n-propyl acetate (PA), dimethyl acetate (DMA), methyl propionate (MP), ethyl propionate (EP), and propyl propionate (PP).

2. In Paragraph 1, The electrolyte for a sodium secondary battery, wherein the above-mentioned cyclic carbonate-based solvent is included in an amount of 10 to 50 volume% based on the total volume of the solvent.

3. In Paragraph 1, The electrolyte for a sodium secondary battery, wherein the linear carbonate-based solvent is included in an amount of 50 to 70 volume% based on the total volume of the solvent.

4. In Paragraph 1, An electrolyte for a sodium secondary battery, wherein the above ester-based solvent is included in an amount of 20 to 50 volume% based on the total volume of the solvent.

5. In Paragraph 1, The above solvent comprises the first solvent and the second solvent, wherein The first solvent above includes the cyclic carbonate-based solvent, and The electrolyte for a sodium secondary battery, wherein the second solvent comprises the linear carbonate-based solvent and the ester-based solvent.

6. In Paragraph 1, An electrolyte for a sodium secondary battery, wherein the cyclic carbonate-based solvent comprises propylene carbonate (PC) and the linear carbonate-based solvent comprises ethyl methyl carbonate (EMC).

7. In Paragraph 1, The above electrolyte is an electrolyte for a sodium secondary battery that does not contain the above ethylene carbonate (EC).

8. In Paragraph 1, The above sodium salt comprises one or more selected from the group consisting of NaPF6, NaFSI, NaClO4, NaBF4, NaTFSI, NaSO3CF3, NaBOB, and NaODFB, an electrolyte for a sodium secondary battery.

9. In Paragraph 1, The above sodium salt is an electrolyte for a sodium secondary battery comprising the above NaPF6 and the above NaFSI.

10. In Paragraph 9, An electrolyte for a sodium secondary battery, wherein the concentration of NaPF6 among the above sodium salts is equal to or greater than the concentration of NaFSI.

11. In Paragraph 1, An electrolyte for a sodium secondary battery, wherein the concentration of the sodium salt is 0.5 to 1.4 M in the electrolyte.

12. In Paragraph 1, The above additive comprises one or more selected from the group consisting of vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethylene sulfate (ESA), and sodium difluorophosphate (NaDFP), an electrolyte for a sodium secondary battery.

13. In Paragraph 1, The above additive is an electrolyte for a sodium secondary battery that does not contain fluoroethylene carbonate (FEC).

14. In Paragraph 1, The electrolyte for a sodium secondary battery, wherein the above additive is included in an amount of 0.1 to 5 weight percent based on the total weight of the electrolyte.

15. A sodium secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the same, and the electrolyte of claim 1.

16. In Paragraph 15, A sodium secondary battery in which the positive active material included in the above positive electrode comprises layered sodium oxide.

17. In Paragraph 16, The above layered sodium oxide is NaNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2, NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2, NaFe 1 / 2 Co 1 / 2 O2, NaMn 1 / 2 Co 1 / 2 O2 and NaNi 1 / 3 Co 1 / 3 Fe 1 / 3 A sodium secondary battery comprising one or more types selected from the group consisting of O2.