Electrolyte for lithium secondary battery and lithium secondary battery containing the same

Abstract

This invention discloses an electrolyte for lithium secondary batteries and a lithium secondary battery comprising the same. The electrolyte, by optimally combining the solvent, lithium salt, and additives contained therein, can improve the capacity and lifespan of the lithium secondary battery. The electrolyte comprises: a first solvent, which includes a heterocyclic compound containing one or more double bonds and simultaneously containing either oxygen or sulfur atoms; a second solvent, which includes at least one of ether compounds, ester compounds, amide compounds, and carbonate compounds; a lithium salt; zirconium oxynitrate; and lithium nitrate.

Description

Technical Field

[0001] This application claims priority to Korean Patent Application No. 10-2020-0140155, filed on October 27, 2020, the entire contents of which are incorporated herein by reference.

[0002] This invention relates to an electrolyte for lithium secondary batteries and a lithium secondary battery comprising the same, and more specifically, to an electrolyte for lithium secondary batteries and a lithium secondary battery comprising the electrolyte for lithium secondary batteries, wherein the electrolyte for lithium secondary batteries can improve the capacity and lifespan of the lithium secondary battery by combining solvents, lithium salts and additives contained in the electrolyte of the lithium secondary battery in an optimal ratio. Background Technology

[0003] As energy storage technologies expand into mobile phones, tablets, laptops, and cameras, and further into electric vehicles (EVs) and hybrid electric vehicles (HEVs), research and development of electrochemical devices are also increasing. Electrochemical devices are the most closely watched area in this field, with the development of rechargeable and dischargeable secondary batteries, such as lithium-sulfur batteries, becoming a focal point. In recent years, research and development have focused on novel electrode and battery designs to improve capacity density and specific energy in the development of such batteries.

[0004] Among such electrochemical devices, lithium-sulfur batteries (Li-S batteries) exhibit high energy density (theoretical capacity), thus attracting considerable attention as a next-generation secondary battery capable of replacing lithium-ion batteries. In these lithium-sulfur batteries, sulfur reduction and lithium metal oxidation occur during discharge, during which sulfur transforms from the cyclic structure of S8 into chain-like lithium polysulfides (LiPS). A characteristic of this type of lithium-sulfur battery is its stepped discharge voltage, which persists until the polysulfides are completely reduced to Li2S.

[0005] However, the biggest obstacle to the commercialization of lithium-sulfur batteries is their lifespan, which decreases during charging / discharging and leads to a decline in battery life. The causes of this lifespan degradation in lithium-sulfur batteries are varied, including side reactions in the electrolyte (byproduct deposition due to electrolyte decomposition), the instability of lithium metal (dendritic growth on the lithium anode, causing short circuits), and the deposition of byproducts at the cathode (lithium polysulfides dissolving from the cathode).

[0006] In batteries using sulfur compounds as the positive electrode active material and alkali metals such as lithium as the negative electrode active material, lithium polysulfide dissolution and shuttle phenomena occur during charge / discharge. Lithium polysulfides are transported to the negative electrode, thus reducing the capacity of the lithium-sulfur battery. Therefore, the main problems of lithium-sulfur batteries are reduced lifespan and decreased reactivity. Specifically, because polysulfides dissolved from the positive electrode have high solubility in organic electrolytes, they can migrate to the negative electrode through the electrolyte (PS shuttle), which is undesirable. As a result, capacity decreases due to irreversible loss of the positive electrode active material, and battery life decreases due to sulfur particle deposition on the lithium metal surface caused by side reactions.

[0007] On the other hand, the characteristics of such lithium-sulfur batteries vary greatly depending on the electrolyte. When sulfur in the positive electrode dissolves into the electrolyte in the form of lithium polysulfides (LiPS), the electrolyte is called a cathode electrolyte, while when almost no sulfur dissolves in the form of lithium polysulfides, the electrolyte is called a slightly soluble electrolyte or slightly solubilized electrolyte (SSE). That is, in this field, various studies are being conducted on lithium-sulfur batteries where sulfur in the positive electrode active material does not dissolve into the electrolyte (such as studies on adding LiPS adsorbent materials to positive electrode composites or modifying separators made of existing PE). Specifically, studies are also being conducted on electrolytes capable of carrying out solid-solid reactions that convert sulfur into the final discharge product Li2S, but such results have not yet been achieved. Therefore, a more fundamental method is needed to suppress the migration of lithium polysulfides to the negative electrode, which reduces the lifespan of lithium-sulfur batteries and the decrease in reactivity due to large amounts of lithium polysulfides.

[0008] Therefore, while current technological efforts continuously attempt to prevent problems such as electrolyte side reactions, a fundamental solution remains elusive. Consequently, there is a need to develop an electrolyte for lithium-ion batteries that can significantly improve the reactivity and lifespan performance of lithium-ion batteries.

[0009] [Existing Technical Documents]

[0010] [Patent Literature]

[0011] Chinese Patent Publication No. 109088101 Summary of the Invention

[0012] Technical issues

[0013] Therefore, the object of the present invention is to provide an electrolyte for lithium secondary batteries, wherein the electrolyte for lithium secondary batteries can improve the capacity and lifespan of the lithium secondary battery by combining the solvent, lithium salt and additives contained in the electrolyte of the lithium secondary battery in an optimal ratio, and to provide a lithium secondary battery comprising the electrolyte for lithium secondary batteries.

[0014] Technical solution

[0015] To achieve the above objectives, the present invention provides an electrolyte for lithium secondary batteries, the electrolyte comprising a first solvent, the first solvent comprising a heterocyclic compound containing one or more double bonds and simultaneously containing either oxygen or sulfur atoms; a second solvent, the second solvent comprising at least one of ether compounds, ester compounds, amide compounds, and carbonate compounds; a lithium salt; zirconium oxynitrate; and lithium nitrate.

[0016] Furthermore, the present invention provides a lithium secondary battery comprising a positive electrode; a negative electrode; a separator disposed between the positive electrode and the negative electrode; and an electrolyte for the lithium secondary battery.

[0017] Beneficial effects

[0018] The lithium secondary battery electrolyte and the lithium secondary battery containing the present invention have the advantage that the capacity and lifespan of the lithium secondary battery can be improved by combining the solvent, lithium salt and additives contained in the electrolyte of the lithium secondary battery in the optimal proportion. More specifically, the battery performance, such as cycle life, can be improved by further incorporating zirconium oxynitrate, which has the advantages of improving the coulombic efficiency of lithium-sulfur batteries. Attached Figure Description

[0019] Figure 1 The graph shows the cycle life performance of lithium secondary batteries manufactured according to embodiments and comparative examples of the present invention.

[0020] Figure 2 The graph shows the cycle life performance of lithium secondary batteries manufactured according to embodiments and comparative examples of the present invention.

[0021] Figure 3 The graph shows the cycle life performance of lithium secondary batteries manufactured according to embodiments and comparative examples of the present invention. Detailed Implementation

[0022] The invention will be described in more detail below.

[0023] The electrolyte for lithium secondary batteries according to the present invention comprises A) a first solvent, the first solvent comprising a heterocyclic compound containing one or more double bonds and simultaneously containing either oxygen or sulfur atoms; B) a second solvent, the second solvent comprising at least one of ether compounds, ester compounds, amide compounds and carbonate compounds; C) a lithium salt; D) zirconium oxynitrate; and E) lithium nitrate.

[0024] In light of the current need to develop novel electrolytes for lithium-ion batteries that can significantly improve battery life performance, the applicant has developed an electrolyte for lithium-ion batteries that can improve the capacity and lifespan of lithium-ion batteries by combining solvents, lithium salts, and additives contained in the electrolyte of the lithium-ion battery in an optimal ratio. More specifically, it improves battery performance, such as cycle life, by combining zirconium oxynitrate, which has been proven to have advantages such as improved coulombic efficiency of lithium-sulfur batteries, with compounds used as components in existing electrolytes.

[0025] That is, depending on the type of solvent, lithium salt, and additives contained in the electrolyte, electrolytes suitable for lithium secondary batteries, such as lithium-sulfur batteries, involve differences in performance aspects such as battery life or efficiency. Therefore, in order to find a method to improve the performance of lithium secondary batteries, after repeated research, the applicant of this invention obtained the present invention with improved reactivity and lifespan, etc., through the following means:

[0026] i) Incorporating zirconium oxynitrate (ZrO(NO3)2), which has advantages such as improving the coulombic efficiency of lithium-sulfur batteries, into the electrolyte (more precisely, replacing a portion of the LiNO3, which prevents the migration of lithium polysulfides from the positive to the negative electrode, thus avoiding self-discharge and overcharging).

[0027] ii) Replace 1,3-dioxolane (DOL) used as a component of the existing electrolyte with "a solvent comprising a heterocyclic compound containing one or more double bonds and simultaneously containing either an oxygen atom or a sulfur atom (the first solvent)", and

[0028] iii) In addition, similarly, LiTFSI, which is used as an existing electrolyte component, will be replaced with "another lithium salt such as LiFSI".

[0029] The electrolyte for lithium secondary batteries of the present invention contains, in detail below, A) a first solvent, B) a second solvent, C) a lithium salt, D) zirconium oxynitrate and E) lithium nitrate.

[0030] A) First solvent

[0031] The first solvent comprises a heterocyclic compound containing one or more double bonds and simultaneously containing either oxygen or sulfur atoms. The heterocyclic compound has the property of being difficult to dissolve salts due to the delocalization of lone pair electrons of heteroatoms (oxygen or sulfur atoms). Therefore, the formation of lithium dendrites can be suppressed by forming a polymer protective film (solid electrolyte interface, SEI layer) on the surface of lithium metal (negative electrode) through the ring-opening reaction of the heterocyclic compound during the initial discharge stage of the battery. Furthermore, the lifespan characteristics of lithium-sulfur batteries can be further improved by reducing electrolyte decomposition and subsequent side reactions on the surface of lithium metal.

[0032] That is, the heterocyclic compound of the present invention must contain one or more double bonds to form a polymer protective film on the lithium metal surface, and must also contain heteroatoms (oxygen or sulfur atoms) to exhibit effects such as increased affinity for other solvents in the electrolyte by making it polar.

[0033] The heterocyclic compound may be a 3- to 15-membered heterocyclic compound, preferably a 3- to 7-membered heterocyclic compound, and more preferably a 5- to 6-membered heterocyclic compound. Furthermore, the heterocyclic compound may be a heterocyclic compound substituted or unsubstituted with at least one group selected from alkyl groups having 1 to 4 carbon atoms, cycloalkyl groups having 3 to 8 carbon atoms, aryl groups having 6 to 10 carbon atoms, halogen groups, nitro (-NO2), amino (-NH2), and sulfonyl (-SO2). Additionally, the heterocyclic compound may be a polycyclic compound containing at least one heterocyclic compound and at least one cycloalkyl group having 3 to 8 carbon atoms and an aryl group having 6 to 10 carbon atoms.

[0034] When the heterocyclic compound is substituted with an alkyl group having 1 to 4 carbon atoms, it is preferred because the group is stabilized and side reactions between electrolytes are suppressed. Furthermore, when substituted with a halogen group or a nitro group, a functional protective film can be formed on the lithium metal surface, and the resulting functional passivation layer is a dense passivation layer. Therefore, it is preferred because it offers advantages such as stability, uniform deposition of lithium metal, and suppression of side reactions between polysulfides and lithium metal.

[0035] Specific examples of the heterocyclic compound may be furan, 2-methylfuran, 3-methylfuran, 2-ethylfuran, 2-propylfuran, 2-butylfuran, 2,3-dimethylfuran, 2,4-dimethylfuran, 2,5-dimethylfuran, pyran, 2-methylpyran, 3-methylpyran, 4-methylpyran, benzofuran, 2-(2-nitrovinyl)furan, thiophene, 2-methylthiophene, 2-ethylthiophene, 2-propylthiophene, 2-butylthiophene, 2,3-dimethylthiophene, 2,4-dimethylthiophene, 2,5-dimethylthiophene, etc. Preferably, 2-methylfuran is used as the first solvent.

[0036] Relative to the total volume ratio of all organic solvents (i.e., first solvent + second solvent) in the electrolyte for lithium secondary batteries of the present invention, the volume ratio of the first solvent containing such heterocyclic compounds can be 5 to 50, preferably 10 to 40, more preferably 15 to 30 (the remaining volume ratios correspond to the second solvent). If the volume ratio of the first solvent is less than 5% relative to the total volume ratio of all organic solvents of the present invention, there may be a decrease in the ability to reduce the dissolution of polysulfides, thereby failing to suppress the increase in electrolyte resistance or the incomplete formation of a protective film on the lithium metal surface. In addition, if the content of the first solvent exceeds 50% relative to the total volume ratio of all organic solvents of the present invention, there is concern that the battery capacity and lifespan may decrease due to the increase in surface resistance of the electrolyte and lithium metal.

[0037] B) Second solvent

[0038] The second solvent comprises at least one of ether compounds, ester compounds, amide compounds, and carbonate compounds, which not only dissolves lithium salts to impart lithium-ion conductivity to the electrolyte but also dissolves sulfur, which serves as the positive electrode active material, allowing the electrochemical reaction with lithium to proceed smoothly. In the case of carbonate compounds, it can be a chain carbonate compound or a cyclic carbonate compound.

[0039] Specific examples of the ether compound may be, but are not limited to, at least one selected from dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, dimethoxyethane, diethoxyethane, methoxyethoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methyl ethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methyl ethyl ether, polyethylene glycol dimethyl ether, polyethylene glycol diethyl ether, and polyethylene glycol methyl ethyl ether. Preferably, dimethoxyethane is used as the second solvent.

[0040] In addition, the ester compound may be, but is not limited to, at least one selected from methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone. Furthermore, the amide compound may be a conventional amide compound used in the art.

[0041] In addition, the chain carbonate compound may be, but is not limited to, at least one selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl ethyl carbonate (EMC), methyl propyl carbonate (MPC), and ethyl propyl carbonate (EPC).

[0042] In addition, the cyclic carbonate compound may be, but is not limited to, at least one selected from ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentene carbonate, 2,3-pentene carbonate, vinylene carbonate, vinyl ethylene carbonate and its halides (such as fluoroethylene carbonate (FEC)).

[0043] On the other hand, if the content of the second solvent is below an appropriate level, there is a concern that insufficient dissolution of the lithium salt may lead to reduced lithium-ion conductivity and that the concentration of sulfur, as an active material, may exceed its soluble concentration, potentially causing precipitation problems. If the content of the second solvent is excessive, there may be excessive dissolution of sulfur, as an active material, leading to severe shuttle phenomena and reduced lifespan of lithium polysulfides and the lithium anode.

[0044] On the other hand, relative to the total weight of the electrolyte for lithium secondary batteries of the present invention, the content of the organic solvent comprising the first solvent and the second solvent can be 70 to 97% by weight, preferably 75 to 95% by weight, and more preferably 80 to 95% by weight. If the content of the organic solvent is less than 70% by weight based on the total weight of the electrolyte for lithium secondary batteries, there may be problems such as increased electrolyte viscosity and decreased ionic conductivity, or incomplete dissolution of lithium salts or additives in the electrolyte. If the content of the organic solvent exceeds 97% by weight, there may be problems such as a decreased concentration of lithium salts in the electrolyte, thus reducing ionic conductivity.

[0045] C) lithium salts

[0046] The lithium salt is an electrolyte salt used to increase ionic conductivity, and examples of the lithium salt may be selected from LiCl, LiBr, LiI, LiClO4, LiBF4, and LiB. 10 Cl 10 The electrolyte comprises at least one of the following: LiPF6, LiCF3SO3, LiCF3CO2, LiC4BO8, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (C2F5SO2)2NLi, (SO2F)2NLi, (CF3SO2)3CLi, lithium chloroborane, lower aliphatic carboxylic acids having four or fewer carbon atoms, lithium tetraphenylborate, and lithium imide. LiFSI ((SO2F)2NLi) is preferably used as the main component. Furthermore, the electrolyte of the present invention is characterized by being free of LiTFSI ((CF3SO2)2NLi).

[0047] The concentration of the lithium salt can be determined by considering factors such as ionic conductivity, and can be, for example, 0.2 to 2 M, preferably 0.5 to 1 M. If the concentration of the lithium salt is below the above range, it may be difficult to ensure suitable ionic conductivity for battery operation. If the concentration of the lithium salt exceeds the above range, the activity of lithium ions decreases or the decomposition reaction of the lithium salt itself increases with the increase of electrolyte viscosity, thus the performance of the battery may deteriorate.

[0048] D) Zirconium oxynitrate

[0049] Zirconium oxynitrate (ZrO(NO3)2) is a component used to improve the coulombic efficiency (CE) of batteries, ultimately improving battery life. There have been past cases of it being used as an electrolyte component (Chinese Patent Publication No. 109088101). However, in that case, LiTFSI was used as the lithium salt and nitrate compounds such as lithium nitrate, described later, were not included; therefore, it did not fundamentally improve the problem of battery life degradation.

[0050] However, the improved reactivity and lifetime of the present invention are achieved by replacing 1,3-dioxolane (DOL), which is used as a component of the existing electrolyte, with “a solvent (first solvent) comprising a heterocyclic compound containing one or more double bonds and simultaneously containing either an oxygen atom or a sulfur atom”, even when using zirconium oxynitrate; and replacing LiTFSI, which is used as a component of the existing electrolyte, with “another lithium salt such as LiFSI”, and also using nitrate compounds such as lithium nitrate.

[0051] The content of zirconium oxynitrate, as described above, can be no less than 0.1% by weight and no more than 2% by weight, preferably 0.5% by weight and 1.5% by weight, relative to the total weight of the electrolyte. If the content of zirconium oxynitrate is less than 0.1% by weight relative to the total weight of the electrolyte, the improvement in the coulombic efficiency of the battery may become negligible, and therefore the improvement in battery life may also become negligible. If the content of zirconium oxynitrate exceeds 2% by weight, it will not dissolve or disperse in the solvent, and precipitation may occur.

[0052] E) lithium nitrate

[0053] Furthermore, the electrolyte for lithium secondary batteries according to the present invention mainly contains lithium nitrate (LiNO3). However, if necessary, it may also contain at least one selected from lanthanum nitrate (La(NO3)3), potassium nitrate (KNO3), cesium nitrate (CsNO3), magnesium nitrate (Mg(NO3)2), barium nitrate (Ba(NO3)2), lithium nitrite (LiNO2), potassium nitrite (KNO2), and cesium nitrite (CsNO2).

[0054] The lithium nitrate content, relative to the total weight of the electrolyte for lithium secondary batteries, can be from 1% to 7% by weight, preferably from 2% to 6% by weight, and more preferably from 3% to 5% by weight. If the lithium nitrate content is less than 1% by weight relative to the total weight of the electrolyte for lithium secondary batteries, the coulombic efficiency will decrease rapidly. If the lithium nitrate content exceeds 7% by weight, the viscosity of the electrolyte may increase, making it difficult to handle. On the other hand, the content of lithium nitrate and zirconium oxynitrate is preferably from 2% to 8% by weight relative to the total weight of the electrolyte for lithium secondary batteries. In this case, the weight ratio of lithium nitrate to zirconium oxynitrate can be from 15:1 to 3:1, preferably from 9.5:1 to 4:1, more preferably from 9:1 to 4:1, but not limited thereto. If the total content of lithium nitrate and zirconium oxynitrate is less than 2% by weight relative to the total weight of the electrolyte for lithium secondary batteries, the coulombic efficiency will decrease rapidly. If the total content of lithium nitrate and zirconium oxynitrate exceeds 8% by weight, the viscosity of the electrolyte may increase, making it difficult to handle.

[0055] The lithium secondary battery according to the present invention will now be described. The lithium secondary battery comprises a positive electrode; a negative electrode; a separator disposed between the positive and negative electrodes; and an electrolyte for the lithium secondary battery. The electrolyte for the lithium secondary battery comprises A) a first solvent, B) a second solvent, C) a lithium salt, D) zirconium oxynitrate, and E) lithium nitrate as described above, the details of which are described above. Furthermore, the lithium secondary battery can be any lithium secondary battery commonly used in the art, wherein a lithium-sulfur battery is most preferably preferred.

[0056] The positive electrode, negative electrode, and separator in the lithium secondary battery according to the present invention will be described in more detail below.

[0057] As described above, the positive electrode in the lithium secondary battery of the present invention comprises a positive electrode active material, a binder, and a conductive material. The positive electrode active material can be a positive electrode active material applicable to conventional lithium secondary batteries, for example, it may contain elemental sulfur (S₈), sulfur compounds, or mixtures thereof. Specifically, the sulfur compound may be Li₂S₂. n (n≥1), organic sulfur compounds or carbon-sulfur composite materials (C2S) x ) n(x = 2.5~50, n ≥ 2). Additionally, the positive electrode active material may include a sulfur-carbon composite material, as sulfur alone is not conductive and can therefore be used in combination with conductive materials. The carbon material (or carbon source) constituting the sulfur-carbon composite material can have a porous structure or a high specific surface area; any carbon material can be used, as long as it is commonly used in the art. For example, the porous carbon material may be, but is not limited to, at least one selected from: graphite; graphene; carbon black such as Denca black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermally cracked black; carbon nanotubes (CNTs) such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs); carbon fibers such as graphite nanofibers (GNF), carbon nanofibers (CNF), and activated carbon fibers (ACF); and activated carbon, which can be spherical, rod-shaped, needle-shaped, plate-shaped, tubular, or block-shaped; it can be used without restriction, as long as it is commonly used in lithium secondary batteries.

[0058] Furthermore, pores are formed in the carbon material, with a porosity of 40% to 90%, preferably 60% to 80%. If the porosity is less than 40%, lithium ions cannot be properly transported, so it will act as a resistive component and cause problems. If the porosity exceeds 90%, a reduction in mechanical strength may occur. Additionally, the pore size of the carbon material is 10 nm to 5 μm, preferably 50 nm to 5 μm. If the pore size is less than 10 nm, lithium ion transport may be impaired. If the pore size exceeds 5 μm, battery short circuits and safety issues may occur due to contact between electrodes.

[0059] The adhesive is a component that facilitates the bonding between the positive electrode active material and the conductive material, as well as the bonding with the current collector. For example, it may be, but is not limited to, at least one selected from: polyvinylidene fluoride (PVdF), polyvinylidene fluoride-polyhexafluoropropylene copolymer (PVdF / HFP), polyvinyl acetate, polyvinyl alcohol, polyvinyl ether, polyethylene, polyethylene oxide, alkylated polyethylene oxide, polypropylene, poly(meth)acrylate, poly(ethyl)acrylate, polytetrafluoroethylene (PTFE), polyvinyl chloride, polyacrylonitrile, polyvinylpyridine, polyvinylpyrrolidone, styrene-butadiene rubber, acrylonitrile-butadiene rubber, ethylene propylene diene monomer (EPDM) rubber, sulfonated EPDM rubber, styrene-butene rubber, fluororubber, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, and mixtures thereof.

[0060] Based on 100 parts by weight of the total positive electrode weight, typically 1 to 50 parts by weight, preferably 3 to 15 parts by weight, of the binder are added. If the binder content is less than 1 part by weight, the adhesion strength between the positive electrode active material and the current collector may be insufficient. If the binder content exceeds 50 parts by weight, the adhesion strength is improved, but the content of the positive electrode active material may be reduced accordingly, thereby reducing the battery capacity.

[0061] There are no particular restrictions on the conductive material contained in the positive electrode, as long as it does not cause side reactions in the internal environment of the battery and has excellent conductivity, and does not cause chemical changes in the battery. The conductive material can typically be graphite or conductive carbon, and can be, for example, but not limited to, a conductive material selected from the following: graphite such as natural or artificial graphite; carbon black such as carbon black, acetylene black, Ketjen black, Tenca black, thermal cracking carbon black, channel black, furnace black, lampblack, and summer black; carbon materials with a crystal structure of graphene or graphite; conductive fibers such as carbon fibers and metal fibers; fluorinated carbon; metal powders such as aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive oxides such as titanium oxide; conductive polymers such as polyphenylene derivatives; or mixtures of two or more of these.

[0062] The amount of conductive material added is typically 0.5 to 50 parts by weight, preferably 1 to 30 parts by weight, based on 100 parts by weight of the total weight of the positive electrode. If the content of the conductive material is too low, i.e., less than 0.5 parts by weight, it is difficult to achieve the effect of improving conductivity, or the electrochemical characteristics of the battery may deteriorate. If the content of the conductive material exceeds 50 parts by weight, i.e., if it is too much, the amount of positive electrode active material is relatively small, and therefore the capacity and energy density may decrease. There are no particular limitations on the method of incorporating the conductive material into the positive electrode; conventional methods known in the relevant art can be used, such as coating the positive electrode active material. Furthermore, if necessary, adding a second conductive coating to the positive electrode active material can replace adding the conductive material as described above.

[0063] Additionally, fillers can be selectively added to the positive electrode of the present invention as components to suppress positive electrode expansion. There are no particular limitations on such fillers, as long as they can suppress electrode expansion without causing chemical changes in the battery; examples may include olefin polymers such as polyethylene and polypropylene; and fibrous materials such as glass fiber and carbon fiber.

[0064] A slurry is formed by dispersing and mixing positive electrode active materials, binders, conductive materials, etc., in a dispersion medium (solvent). The slurry can be coated onto a positive electrode current collector, and then dried and rolled to prepare the positive electrode. The dispersion medium can be, but is not limited to, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethanol, isopropanol, water, or mixtures thereof.

[0065] The positive electrode current collector can be, but is not limited to, platinum (Pt), gold (Au), palladium (Pd), iridium (Ir), silver (Ag), ruthenium (Ru), nickel (Ni), stainless steel (STS), aluminum (Al), molybdenum (Mo), chromium (Cr), carbon (C), titanium (Ti), tungsten (W), ITO (In-doped SnO2), FTO (F-doped SnO2), or alloys thereof, or aluminum (Al) or stainless steel with a surface treated with carbon (C), nickel (Ni), titanium (Ti), or silver (Ag). The positive electrode current collector can be in the form of foil, film, sheet, stamped form, porous body, foam, etc.

[0066] The negative electrode is a lithium-based metal, and may also include a current collector located on one side of the lithium-based metal. The current collector can be a negative electrode current collector. There are no particular limitations on the negative electrode current collector, as long as it has high conductivity without causing chemical changes in the battery, and it can be selected from copper, aluminum, stainless steel, zinc, titanium, silver, palladium, nickel, iron, chromium, and their alloys and combinations. Stainless steel can be surface-treated with carbon, nickel, titanium, or silver, and the alloy can be an aluminum-cadmium alloy. Alternatively, calcined carbon, non-conductive polymers surface-treated with conductive materials, or conductive polymers can be used. Thin copper plates are typically used as negative electrode current collectors.

[0067] Furthermore, the negative electrode current collector can take various shapes, such as films, sheets, foils, meshes, porous bodies, foams, nonwoven fabrics, etc., with or without micro-uneven surfaces. Additionally, the thickness of the negative electrode current collector ranges from 3 to 500 μm. If the thickness of the negative electrode current collector is less than 3 μm, the current collection efficiency decreases. On the other hand, if the thickness exceeds 500 μm, there is a problem with reduced processability during folding and subsequent battery assembly.

[0068] The lithium metal can be lithium or a lithium alloy. In that case, the lithium alloy contains an element capable of forming an alloy with lithium. Specifically, the lithium alloy can be an alloy of lithium with at least one selected from Si, Sn, C, Pt, Ir, Ni, Cu, Ti, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Sb, Pb, In, Zn, Ba, Ra, Ge, and Al.

[0069] The lithium metal can be in the form of sheets or foils, and in some cases, it can be in the form of lithium or lithium alloys deposited or coated onto the current collector by dry deposition, or it can be in the form of particulate metals and alloys deposited or coated by wet deposition, etc.

[0070] A conventional diaphragm can be inserted between the positive and negative electrodes. The diaphragm is a physical diaphragm that functions to physically separate the electrodes and can be used without particular restrictions, as long as it is a conventional diaphragm. Specifically, a diaphragm with low resistance to ion migration in the electrolyte and excellent impregnation ability with respect to the electrolyte is preferred.

[0071] Furthermore, the separator is capable of transporting lithium ions between the positive and negative electrodes while simultaneously isolating or insulating the positive and negative electrodes from each other. The separator can be made of a porous, non-conductive, or insulating material. The separator can be a standalone component, such as a membrane, or a coating added to the positive and / or negative electrodes.

[0072] Examples of polyolefin porous membranes that can be used as diaphragms can be membranes formed alone or from mixtures of any polymer selected from polyethylene such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, and ultra-high molecular weight polyethylene, as well as polyolefin polymers such as polypropylene, polybutene, and polypentene. Examples of nonwoven fabrics that can be used as diaphragms are nonwoven fabrics formed alone or from mixtures of polymers such as polyphenylene ether, polyimide, polyamide, polycarbonate, polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, polyphenylene sulfide, polyacetal, polyethersulfone, polyetheretherketone, polyester, etc. Such nonwoven fabrics include nonwoven fabrics in the form of fibers forming porous fabrics, i.e., spunbond or meltblown nonwoven fabrics composed of long fibers.

[0073] There is no particular limitation on the thickness of the separator, but it is preferably in the range of 1 to 100 μm, more preferably 5 to 50 μm. If the separator thickness is less than 1 μm, mechanical properties cannot be maintained. If the separator thickness exceeds 100 μm, the separator acts as a resistive layer, thereby degrading the performance of the battery. There is no particular limitation on the pore size and porosity of the separator, but a pore size of 0.1 to 50 μm and a porosity of 10% to 95% are preferred. If the separator pore size is less than 0.1 μm or the porosity is less than 10%, the separator acts as a resistive layer. If the separator pore size exceeds 50 μm or the porosity exceeds 95%, mechanical properties cannot be maintained.

[0074] The lithium secondary battery of the present invention, comprising the above-described positive electrode, negative electrode, separator, and electrolyte, can be manufactured by placing the positive electrode opposite to the negative electrode, inserting a separator therebetween, and then injecting the lithium secondary battery electrolyte according to the present invention.

[0075] On the other hand, the lithium secondary battery according to the present invention can be used not only as a single battery cell for use as a power source in small devices, but is also particularly suitable as a unit cell in a battery module for use as a power source in medium to large devices. In this regard, the present invention also provides a battery module in which at least two lithium secondary batteries are electrically connected (in series or in parallel). Needless to say, the number of lithium secondary batteries included in the battery module can be adjusted in various ways, taking into account the intended use and capacity of the battery module. Additionally, the present invention provides a battery pack in which the battery modules are electrically connected according to conventional techniques in the art. The battery modules and the battery pack can be used as a power source for at least one medium to large device selected from: power tools; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); electric trucks; electric commercial vehicles; or energy storage systems, but the present invention is not limited thereto.

[0076] Preferred implementation scheme

[0077] Preferred embodiments are provided below to aid in understanding the invention, but these embodiments are merely illustrative of the invention. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope and spirit of the invention, and all such changes and modifications are within the scope of the claims.

[0078] [Example 1] Manufacturing of lithium secondary batteries

[0079] Preparation of electrolyte

[0080] First, an organic solvent prepared by mixing 2-methylfuran (first solvent) and dimethoxyethane (second solvent) at a volume ratio (v / v) of 2:8 was added to dissolve 4.5 wt% lithium nitrate (LiNO3) and 0.5 wt% zirconium oxynitrate (ZrO(NO3)2) based on the total weight of the electrolyte, so that the concentration of LiFSI (lithium salt) was 0.75 M, to prepare the electrolyte. Here, zirconium oxynitrate was prepared by vacuum drying ZrO(NO3)2·xH2O from ALFA AESAR at 80 °C for 48 hours to remove H2O.

[0081] Manufacturing of the positive electrode

[0082] In addition, a slurry composition for the positive electrode was prepared by mixing 87.5 parts by weight of a sulfur-carbon (CNT) composite material (S / C 75:25 weight ratio) as the positive electrode active material, 5 parts by weight of Denca Black as the conductive material, and 7.5 parts by weight of a styrene-butadiene rubber / carboxymethyl cellulose (SBR / CMC 7:3) as a binder. The prepared slurry composition was then coated onto a current collector (Al foil), dried at 80°C for 12 hours, and pressed using a roller press to manufacture the positive electrode (at this time, the loading was 5.0–5.5 mAh / cm²). 2 (Electrode porosity is 65%).

[0083] Manufacturing of lithium secondary batteries (lithium-sulfur batteries)

[0084] A positive electrode and a negative electrode (manufactured by rolling a 35 μm thick lithium foil on a copper current collector, using Honzo) are placed opposite each other, with a polyethylene (PE) separator inserted between them. The prepared electrolyte is then injected and the battery is sealed to manufacture a coin cell type lithium-sulfur battery. Alternatively, in battery manufacturing, a Φ14 circular electrode is stamped and used as the positive electrode, a Φ19 polyethylene separator is stamped and used, and a Φ16 lithium foil is stamped and used.

[0085] [Example 2, Comparative Examples 1 to 5] Manufacturing of lithium secondary batteries

[0086] The lithium-sulfur batteries corresponding to Example 2 and Comparative Examples 1 to 5 were manufactured in the same manner as in Example 1 above, but with the composition shown in Table 1 below.

[0087] Table 1:

[0088]

[0089] [Experimental Example 1] Evaluation of cycle life of lithium secondary batteries

[0090] For the lithium-sulfur batteries manufactured in Examples 1 and 2 above, and Comparative Examples 1 to 7, they were discharged at 0.1C in CC mode at 25°C until reaching 1.8V with OCV (open circuit voltage), and then charged at 0.1C until reaching 2.5V again for 2.5 cycles. After the batteries were stabilized, they were subjected to 0.3C charge / 0.5C discharge cycles within a voltage range between 1.8V and 2.5V to evaluate the baseline cycle life of maintaining 80% of the initial high-rate capacity. The results are shown in Table 2 below.

[0091] Table 2:

[0092]

[0093] Figures 1 to 3This is a graph illustrating the cycle life performance of lithium secondary batteries manufactured according to embodiments and comparative examples of the present invention. Figures 1 to 3 As shown in Table 2, it was confirmed that the lithium-sulfur battery of Comparative Example 1, which used 1,3-dioxolane (DOL) as the first solvent, LiTFSI as the lithium salt, and did not use lithium nitrate, was the same as that of Comparative Example 2, which did not contain zirconium oxynitrate, Comparative Example 3, which did not contain lithium nitrate, Comparative Example 4, which used LiTFSI as the lithium salt, Comparative Example 5, which used 1,3-dioxolane (DOL) as the first solvent, and Comparative Example 6, which contained lithium nitrate and zirconium oxynitrate, were the same as those of Comparative Example 6. Compared to each of “Lithium-sulfur battery with a weight ratio of about 16:1” and “Lithium-sulfur battery with a weight ratio of about 2.3:1 for lithium nitrate and zirconium oxynitrate in Comparative Example 7”, the lithium-sulfur batteries of Examples 1 and 2, which use 2-methylfuran instead of 1,3-dioxolane (DOL) as the first solvent, use LiFSI instead of LiTFSI as the lithium salt, and use lithium nitrate and zirconium oxynitrate in a weight ratio of 15:1 to 3:1 (preferably 9.5:1 to 4:1), have superior cycle life.

[0094] In other words, it has been confirmed that there are limitations in improving cycle life if 1,3-dioxolane (DOL) is used instead of 2-methylfuran as the first solvent, or if neither zirconium oxynitrate nor lithium nitrate is used, or if both lithium nitrate and zirconium oxynitrate are used but their content ratio is not in the range of 15:1 to 3:1 (preferably 9.5:1 to 4:1 by weight), or if LiTFSI is used as the lithium salt. Therefore, cycle life can only be improved as in this invention if all the above conditions are met. Furthermore, a comparison / contrast of Examples 1 and 2 above confirms that increasing the content of lithium nitrate relatively within the range of lithium nitrate to zirconium oxynitrate by weight ratio further improves the battery's cycle life.

Claims

1. An electrolyte for lithium secondary batteries, the electrolyte comprising... The first solvent comprises a heterocyclic compound containing one or more double bonds and simultaneously containing either an oxygen atom or a sulfur atom; The second solvent comprises at least one of ether compounds, ester compounds, amide compounds, and carbonate compounds; Lithium salts; Zirconium oxynitrate; as well as Lithium nitrate, The ratio of lithium nitrate to zirconium oxynitrate by weight is 15:1 to 3:

1. The lithium salt described therein does not contain LiTFSI. The content of lithium nitrate and zirconium oxynitrate is 2% to 8% by weight relative to the total weight of the electrolyte for the lithium secondary battery.

2. The electrolyte for lithium secondary batteries according to claim 1, wherein the lithium salt comprises a compound selected from LiCl, LiBr, LiI, LiClO4, LiBF4, and LiB. 10 Cl 10 At least one of the following: LiPF6, LiCF3SO3, LiCF3CO2, LiC4BO8, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (C2F5SO2)2NLi, LiFSI((SO2F)2NLi), (CF3SO2)3CLi, lithium chloroborane, lithium lower aliphatic carboxylic acids having 4 or fewer carbon atoms, lithium tetraphenylborate, and lithium imide.

3. The electrolyte for lithium secondary batteries according to claim 2, wherein the lithium salt contains LiFSI.

4. The electrolyte for lithium secondary batteries according to claim 1, wherein the concentration of the lithium salt is 0.2 to 2.0 M.

5. The electrolyte for lithium secondary batteries according to claim 1, wherein the heterocyclic compound is: A heterocyclic compound consisting of 3 to 15 members substituted or unsubstituted with at least one group selected from alkyl groups having 1 to 4 carbon atoms, cycloalkyl groups having 3 to 8 carbon atoms, aryl groups having 6 to 10 carbon atoms, halogen groups, nitro groups, amino groups, and sulfonyl groups; A polycyclic compound consisting of at least one of a cycloalkyl group having 3 to 8 carbon atoms and an aryl group having 6 to 10 carbon atoms, and a heterocyclic compound.

6. The electrolyte for lithium secondary batteries according to claim 1, wherein the heterocyclic compound is selected from: furan, 2-methylfuran, 3-methylfuran, 2-ethylfuran, 2-propylfuran, 2-butylfuran, 2,3-dimethylfuran, 2,4-dimethylfuran, 2,5-dimethylfuran, pyran, 2-methylpyran, 3-methylpyran, 4-methylpyran, benzofuran, 2-(2-nitrovinyl)furan, thiophene, 2-methylthiophene, 2-ethylthiophene, 2-propylthiophene, 2-butylthiophene, 2,3-dimethylthiophene, 2,4-dimethylthiophene, and 2,5-dimethylthiophene.

7. The electrolyte for lithium secondary batteries according to claim 1, wherein the ether compound of the second solvent is selected from at least one of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, dimethoxyethane, diethoxyethane, methoxyethoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methyl ethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methyl ethyl ether, polyethylene glycol dimethyl ether, polyethylene glycol diethyl ether, and polyethylene glycol methyl ethyl ether.

8. The electrolyte for lithium secondary batteries according to claim 1, wherein the electrolyte for lithium secondary batteries further comprises at least one selected from lanthanum nitrate, potassium nitrate, cesium nitrate, magnesium nitrate, barium nitrate, lithium nitrite, potassium nitrite, and cesium nitrite.

9. The electrolyte for lithium secondary batteries according to claim 1, wherein the electrolyte for lithium secondary batteries comprises 2-methylfuran as a first solvent, dimethoxyethane as a second solvent, LiFSI, zirconium oxynitrate, and lithium nitrate as lithium salts.

10. A lithium secondary battery, the lithium secondary battery comprising: positive electrode; negative electrode; A diaphragm inserted between the positive and negative electrodes; and The electrolyte for lithium secondary batteries as described in claim 1.

11. The lithium secondary battery according to claim 10, wherein the lithium secondary battery is a lithium-sulfur battery.

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