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

Abstract

Disclosed are an electrolyte for a lithium secondary battery, which is capable of improving the lifespan of a lithium secondary battery by including a certain amount of magnesium chloride (MgCl2) in an electrolyte to form a stable electrode protection layer that prevents consumption of salts and additives in the electrolyte, and a lithium secondary battery including the same. The electrolyte for a lithium secondary battery includes a first solvent including a heterocyclic compound containing one or more double bonds or not containing double bonds and containing at least one of an oxygen atom and a sulfur atom, a second solvent including at least one of an ether compound, an ester compound, an amide compound, and a carbonate compound, a lithium salt, magnesium chloride, and lithium nitrate.

Description

Technical Field

[0001] This application claims priority based on Korean Patent Application No. 10-2021-0001802, filed on January 7, 2021, 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 containing the same, and more specifically, to an electrolyte for lithium secondary batteries and a lithium secondary battery containing the same, wherein the electrolyte improves the lifespan of the lithium secondary battery by containing a certain amount of magnesium chloride (MgCl2) in the electrolyte to form a stable electrode protective layer that prevents the consumption of salts and additives in the electrolyte. Background Technology

[0003] With the increasing interest in energy storage technology, and its applications expanding from power sources for mobile phones, tablets, laptops, and cameras to electric vehicles (EVs) and hybrid electric vehicles (HEVs), research and development of electrochemical devices are steadily increasing. The field of electrochemical devices is one of the most closely watched areas, with the development of rechargeable / 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 these batteries.

[0004] In such electrochemical devices, lithium metal secondary batteries, such as lithium-sulfur batteries (Li-S batteries), not only utilize lightweight lithium metal as the negative electrode active material but also possess high energy density (theoretical capacity: 3862 mAh / g), thus attracting considerable attention as a next-generation high-capacity secondary battery capable of replacing conventional secondary batteries such as lithium-ion batteries. In this type of lithium-sulfur battery, during discharge, a sulfur reduction reaction and a lithium metal oxidation reaction occur, during which sulfur forms linear lithium polysulfides (LiPS) from the cyclic structure of S8. A characteristic of this lithium-sulfur battery is that it exhibits a gradually decreasing discharge voltage before 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 the charge / discharge process and shortens the battery's lifespan. This shortened lifespan is caused by a variety of factors, including electrolyte side reactions (deposition of byproducts due to electrolyte decomposition), the instability of lithium metal (dendritic growth on the lithium anode, leading to short circuits), and the deposition of cathode byproducts (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 polysulfides undergo dissolution and shuttle during charging / discharging. These polysulfides migrate to the negative electrode, reducing the capacity of the lithium-sulfur battery. Therefore, the main problems with lithium-sulfur batteries are shortened lifespan and reduced reactivity. Specifically, because the polysulfides dissolved from the positive electrode have high solubility in organic electrolytes, they can undesirably migrate to the negative electrode through the electrolyte (PS shuttle). As a result, capacity decreases due to irreversible loss of the positive electrode active material, and the battery life is shortened due to sulfur particle deposition on the lithium metal surface caused by side reactions.

[0007] In particular, lithium, as a negative electrode active material, tends to form large-area dendrites and react with salts and additives in the electrolyte to form an SEI (solid electrolyte interface), which continuously consumes salts and additives in the electrolyte, thus promoting battery degradation.

[0008] On the other hand, the behavior of such lithium-sulfur batteries can vary greatly depending on the electrolyte. An electrolyte in which sulfur in the positive electrode dissolves into the electrolyte as lithium polysulfides (LiPS) is called a cathode electrolyte, and an electrolyte in which sulfur hardly dissolves into lithium polysulfides is called a micro-dissolved or micro-solubilized electrolyte (SSE). That is, in the art, various studies are being conducted on lithium-sulfur batteries in which sulfur, the positive electrode active material, does not dissolve into the electrolyte (e.g., studies on adding LiPS adsorbent materials to positive electrode composites or modifying separators made of existing PE), and in particular, studies are being conducted on electrolytes capable of carrying out solid-to-solid reactions in which sulfur is converted into the final discharge product Li₂S, but such results have not yet been achieved. Therefore, a more thorough method is needed to suppress the migration of lithium polysulfides to the negative electrode, thereby shortening the lifespan of lithium-sulfur batteries, and the phenomenon of reduced reactivity due to the large amount of lithium polysulfides.

[0009] As a result, attempts to prevent side reactions in the electrolyte have been ongoing in this technical field, but a fundamental solution remains elusive. Therefore, there is a need to develop an electrolyte for lithium-ion batteries that can improve performance by forming a stable electrode protective layer that prevents the consumption of salts and additives in the electrolyte. Summary of the Invention

[0010] [Technical Issues]

[0011] Therefore, the object of the present invention is to provide an electrolyte for lithium secondary batteries and a lithium secondary battery containing the same, wherein the electrolyte improves the lifespan of the lithium secondary battery by containing a certain amount of magnesium chloride (MgCl2) in the electrolyte to form a stable electrode protective layer that prevents the consumption of salts and additives in the electrolyte.

[0012] [Technical Solution]

[0013] 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 at least one double bond or not containing a double bond and containing at least one of oxygen and 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; magnesium chloride; and lithium nitrate.

[0014] Furthermore, the present invention provides a lithium secondary battery comprising: a positive electrode; a lithium negative electrode; a separator between the positive electrode and the negative electrode; and an electrolyte for the lithium secondary battery.

[0015] [Beneficial Effects]

[0016] The lithium secondary battery electrolyte and the lithium secondary battery containing the present invention have the advantage of improving the lifespan of the lithium secondary battery by including a certain amount of magnesium chloride (MgCl2) in the electrolyte to form a stable electrode protective layer that prevents the consumption of salts and additives in the electrolyte. Attached Figure Description

[0017] Figure 1 This is a graph showing the cycle life performance of lithium-lithium symmetric batteries manufactured according to embodiments and comparative examples of the present invention.

[0018] Figure 2 This is a graph showing the cycle life performance of lithium-sulfur batteries manufactured according to embodiments and comparative examples of the present invention. Detailed Implementation

[0019] The present invention will be described in detail below.

[0020] The electrolyte for lithium secondary batteries according to the present invention comprises: A) a first solvent, the first solvent comprising a heterocyclic compound containing at least one double bond or not containing a double bond and containing at least one of oxygen and 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) magnesium chloride; and E) lithium nitrate.

[0021] Electrolytes used in lithium-ion secondary batteries, such as lithium-sulfur batteries, can cause variations in performance, such as battery life or efficiency, depending on the type of solvent, lithium salt, and additives contained in the electrolyte. Therefore, after repeated research to improve the performance of lithium-ion secondary batteries, the applicant of this invention discovered that by combining the solvent, lithium salt, and additives (magnesium chloride, MgCl2) contained in the electrolyte of lithium-ion secondary batteries in an optimal ratio, it is possible to improve the capacity and lifespan of lithium-ion secondary batteries. In particular, this invention aims to improve the charging / discharging efficiency of lithium and suppress dendrite growth (i.e., increase battery capacity by improving the efficiency of lithium utilization) by reacting magnesium chloride (MgCl2) with the lithium anode to form a LiCl protective layer, while simultaneously forming a lithiophilic Li-Mg alloy on the anode surface.

[0022] In the following text, the components A) first solvent, B) second solvent, C) lithium salt, D) magnesium chloride and E) lithium nitrate contained in the electrolyte for lithium secondary batteries of the present invention will be described in detail.

[0023] A) First solvent

[0024] The first solvent comprises a heterocyclic compound containing one or more double bonds or without double bonds and containing either oxygen or sulfur atoms. The first solvent has the property of being difficult to dissolve salts due to the delocalization of lone pair electrons of heteroatoms (oxygen or sulfur atoms). Thus, the formation of lithium dendrites can be suppressed by forming a polymer protective film (solid electrolyte interface, SEI layer) on the surface of the lithium metal (negative electrode) through the ring-opening reaction of the heterocyclic compound during the initial discharge stage of the battery. Furthermore, the life characteristics of lithium-sulfur batteries can be further improved by reducing the decomposition of the electrolyte on the lithium metal surface and subsequent side reactions.

[0025] That is, the heterocyclic compound of the present invention must contain one or more double bonds to form a polymer protective film on the surface of lithium metals, and must also contain one or more heteroatoms (oxygen or sulfur atoms) to exhibit effects such as increasing affinity for other solvents in the electrolyte by making the electrolyte polar.

[0026] The heterocyclic compound can be a 3- to 15-membered, preferably 3- to 7-membered, and more preferably 5- to 6-membered heterocyclic compound. Furthermore, the heterocyclic compound can be a heterocyclic compound substituted or unsubstituted with at least one of the following: an alkyl group having 1 to 4 carbon atoms, a cyclic alkyl group having 3 to 8 carbon atoms, an aryl group having 6 to 10 carbon atoms, a halogen group, a nitro group (-NO2), an amino group (-NH2), and a sulfonyl group (-SO2). Additionally, the heterocyclic compound can be a polycyclic compound consisting of a heterocyclic compound and at least one of the following: a cyclic alkyl group having 3 to 8 carbon atoms and an aryl group having 6 to 10 carbon atoms.

[0027] When heterocyclic compounds are substituted with alkyl groups having 1 to 4 carbon atoms, this is preferred because the free radicals are stabilized and side reactions between the electrolytes are suppressed. Furthermore, substitution with halogen or nitro groups is preferred because a functional passivation film can be formed on the surface of the lithium metal. This functional passivation film is a dense passivation layer, which has the advantages of stability, allowing for uniform deposition of lithium metal and suppression of side reactions between polysulfides and lithium metal.

[0028] Specific examples of heterocyclic compounds include 1,3-dioxolane, 4,5-diethyl-1,3-dioxolane, 4,5-dimethyl-1,3-dioxolane, 4-methyl-1,3-dioxolane, 4-ethyl-1,3-dioxolane, and 1,3-dioxolane. Alkane, 1,4-di Alkane, 4-methyl-1,3-di Alkane and 2-methyl-1,3-di Alkane, 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, etc. Preferably, 1,3-dioxolane is used as the first solvent, but it is not limited thereto.

[0029] Relative to 100 parts by volume of all organic solvents (i.e., first solvent + second solvent) contained in the electrolyte for lithium secondary batteries of the present invention, the content of the first solvent containing this heterocyclic compound can be 5 to 50 parts by volume (the remaining parts by volume correspond to the second solvent). If the content of the first solvent relative to 100 parts by volume of all organic solvents of the present invention is less than 5 parts by volume, the following problems may exist: the ability to reduce the dissolution of polysulfides decreases, thereby failing to suppress the increase in electrolyte resistance, or failing to completely form a protective film on the lithium metal surface. Furthermore, if the content of the first solvent relative to 100 parts by volume of all organic solvents of the present invention exceeds 50 parts by volume, there is a problem that the battery capacity and lifespan may be reduced due to the increased surface resistance of the electrolyte and lithium metal.

[0030] B) Second solvent

[0031] The second solvent comprises at least one of ether compounds, ester compounds, amide compounds, and carbonate compounds, and not only dissolves the lithium salt to impart lithium-ion conductivity to the electrolyte, but also dissolves sulfur, which serves as the positive electrode active material, to facilitate the electrochemical reaction with lithium. In the case of carbonate compounds, it can be a linear carbonate compound or a cyclic carbonate compound.

[0032] Specific examples of ether compounds may be, but are not limited to, at least one selected from the following: 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.

[0033] In addition, the ester compounds may be, but are not limited to, at least one selected from the following: 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 compounds may be amide compounds commonly used in the art.

[0034] In addition, the linear carbonate compound can be, but is not limited to, at least one selected from the following: dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), and ethyl propyl carbonate (EPC).

[0035] In addition, cyclic carbonate compounds may be, but are not limited to, at least one selected from the following: ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentene carbonate, 2,3-pentene carbonate, ethylene carbonate, ethylene ethylene carbonate and their halides (fluoroethylene carbonate (FEC), etc.).

[0036] On the other hand, if the content of the second solvent is insufficient, there is a problem of reduced lithium-ion conductivity due to incomplete dissolution of the lithium salt, and the concentration of sulfur, as an active material, exceeds its solubility, which may lead to precipitation. If the content of the second solvent is too high, there may be excessive dissolution of sulfur, as an active material, resulting in severe shuttle phenomena and shortened lifespan of lithium polysulfides and lithium anodes.

[0037] 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 containing the first solvent and the second solvent can be 60 to 97% by weight, preferably 65 to 95% by weight, and more preferably 70 to 95% by weight. If the content of the organic solvent relative to the total weight of the electrolyte for lithium secondary batteries is less than 60% by weight, there may be problems such as increased viscosity of the electrolyte and decreased ionic conductivity, or the lithium salt or additives may not be completely dissolved in the electrolyte. If the content of the organic solvent exceeds 97% by weight, there may be problems such as decreased lithium salt concentration in the electrolyte, leading to decreased ionic conductivity.

[0038] C) lithium salts

[0039] Lithium salts are electrolyte salts used to improve ionic conductivity, and examples of lithium salts can be at least one selected from the following: LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 LiPF6, LiCF3SO3, LiCF3CO2, LiC4BO8, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (C2F5SO2)2NLi, (SO2F)2NLi (i.e., LiFSI), (CF3SO2)2NLi (i.e., LiTFSI), (CF3SO2)3CLi, lithium chloroborane, lower aliphatic carboxylic acids with 4 or fewer carbon atoms, lithium tetraphenylborate, and lithium imide.

[0040] 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 less than 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 lithium ion mobility decreases with increasing electrolyte viscosity, or the decomposition reaction of the lithium salt itself increases, which may degrade the battery performance.

[0041] D) Magnesium chloride

[0042] Magnesium chloride (MgCl2) reacts with the lithium anode to form a LiCl protective layer on the anode surface, while simultaneously forming a lithiophilic Li-Mg alloy. This improves the charging / discharging efficiency of lithium and inhibits dendrite growth, ultimately helping to increase battery capacity by improving lithium utilization efficiency.

[0043] It has been confirmed that, as described above, if magnesium chloride is used as an additive in the electrolyte, the low reactivity of lithium and magnesium chloride caused by the natural oxide layer on the surface of the lithium anode can be overcome, and during the initial charging / discharging process, a LiCl protective layer and a Li-Mg alloy can be formed on the surface of the lithium anode where the natural oxide layer on the surface of the lithium anode is destroyed.

[0044] The magnesium chloride content relative to the total weight of the electrolyte for lithium secondary batteries can be 0.1 to 1 wt%, preferably 0.5 to 1 wt%. If the magnesium chloride content based on the total weight of the electrolyte is less than 0.1 wt%, the effect on improving battery life may not be significant. If the magnesium chloride content exceeds 1 wt%, there may be a problem of further accelerating battery degradation due to side reactions between magnesium chloride and the positive electrode active material.

[0045] E) Lithium nitrate

[0046] Furthermore, the electrolyte for lithium secondary batteries according to the present invention mainly contains lithium nitrate (LiNO3). On the other hand, if desired, the electrolyte may also contain one or more of the following: 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).

[0047] The lithium nitrate content relative to the total weight of the electrolyte for lithium secondary batteries can be from 0.1 to 7 wt%, preferably from 0.5 to 5 wt%. If the lithium nitrate content relative to the total weight of the electrolyte for lithium secondary batteries is less than 0.1 wt%, the coulombic efficiency may decrease sharply. If the content exceeds 7 wt%, the lithium secondary battery will be difficult to operate due to the increased viscosity of the electrolyte.

[0048] Next, a lithium secondary battery according to the present invention will be described. The lithium secondary battery includes a positive electrode, a negative electrode, a separator between the positive and negative electrodes, and an electrolyte for the lithium secondary battery. As described above, the electrolyte for the lithium secondary battery includes A) a first solvent, B) a second solvent, C) a lithium salt, D) magnesium chloride, and E) lithium nitrate, the specific details of which are applicable to the description above. Furthermore, the lithium secondary battery can be any lithium secondary battery commonly used in the art, wherein lithium-sulfur batteries and lithium-lithium symmetric batteries are preferred.

[0049] The positive electrode, negative electrode, and separator in the lithium secondary battery according to the present invention will be described in more detail below.

[0050] As described above, the positive electrode included 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, and may, for example, include lithium nickel cobalt manganese-based compounds (lithium NCM-based compounds), and may also include elemental sulfur (S8), sulfur compounds, or mixtures thereof. Specifically, the sulfur compound may be Li2S. n (n≥1), organic sulfur compounds or carbon-sulfur composite materials (C2S) x ) n (x = 2.5–50, n ≥ 2). Furthermore, the positive electrode active material can include a sulfur-carbon composite material, and because sulfur alone is not conductive, it can 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, and any carbon material can be used, as long as it is commonly used in the art. For example, the porous carbon material can be, but is not limited to, at least one selected from: graphite; graphene; carbon black such as Decco black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermally cracked carbon black; carbon nanotubes (CNTs) such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs); carbon fibers such as graphite nanofibers (GNFs), carbon nanofibers (CNFs), and activated carbon fibers (ACFs); and activated carbon, and its shape can be spherical, rod-shaped, needle-shaped, plate-shaped, tubular, or block-shaped, and it can be used without restriction, as long as it is commonly used in lithium secondary batteries.

[0051] 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%, the pores will act as resistive elements because lithium ions cannot transport properly, causing problems. If the porosity exceeds 90%, a decrease 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 ions may not be able to pass through. If the pore size exceeds 5 μm, a short circuit in the battery may occur due to contact between the electrodes, potentially causing safety issues.

[0052] The adhesive is a component that facilitates the bonding between the positive electrode active material and the conductive material, as well as the bonding to the current collector, and may be, for example, but is not limited to, at least one selected from the following: 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(meth)acrylate, polytetrafluoroethylene (PTFE), polyvinyl chloride, polyacrylonitrile, polyvinylpyridine, polyvinylpyrrolidone, styrene-butadiene rubber, nitrile 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.

[0053] Based on the total weight of 100 parts by weight of the positive electrode, the amount of binder added is typically 1 to 50 parts by weight, preferably 3 to 15 parts by weight. If the binder content is less than 1 part by weight, the adhesive strength between the positive electrode active material and the current collector may be insufficient. If the binder content exceeds 50 parts by weight, the adhesive strength is improved, but the content of the positive electrode active material may be reduced accordingly, thereby reducing the battery capacity.

[0054] 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 without causing chemical changes in the battery. The conductive material can typically be graphite or conductive carbon, and can be, but is not limited to, selected from: graphite such as natural or artificial graphite; carbon black materials such as carbon black, acetylene black, Ketjen black, Deco black, thermal cracking carbon black, channel black, furnace black, and lamp 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 thereof.

[0055] Based on the total weight of 100 parts by weight of the positive electrode, the amount of conductive material added is typically 0.5 to 50 parts by weight, preferably 1 to 30 parts by weight. 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, thereby potentially reducing capacity and energy density. There are no particular limitations on the method of incorporating the conductive material into the positive electrode, and conventional methods known in the relevant art, such as coating on the positive electrode active material, can be used. Furthermore, if desired, adding a second conductive coating to the positive electrode active material can replace the addition of the conductive material as described above.

[0056] Furthermore, 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, and examples may include: olefin polymers such as polyethylene and polypropylene; and fibrous materials such as glass fiber and carbon fiber.

[0057] The positive electrode active material, binder, conductive material, etc., are dispersed in a dispersion medium (solvent) and mixed to form a slurry. The slurry is then coated onto a positive electrode current collector, 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.

[0058] 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 their alloys; or aluminum (Al) or stainless steel with a surface treated with carbon (C), nickel (Ni), titanium (Ti) or silver (Ag). The shape of the positive electrode current collector can be foil, film, sheet, stamped form, porous body, foam, etc.

[0059] The negative electrode is a lithium-based metal, and may also include a current collector 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. The stainless steel can be surface-treated with carbon, nickel, titanium, or silver, and the alloy can be an aluminum-cadmium alloy. Furthermore, sintered carbon, non-conductive polymers surface-treated with conductive materials, or conductive polymers can be used. Typically, a thin copper plate is used as the negative electrode current collector.

[0060] 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 of reduced performance when folding and then assembling the battery.

[0061] Lithium-based metals can be lithium or lithium alloys. In this case, the lithium alloy contains elements capable of forming an alloy with lithium. Specifically, the lithium alloy can be an alloy of lithium with at least one of the following: 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.

[0062] Lithium metals can be in the form of sheets or foils. In some cases, they can be in the form of lithium or lithium alloys deposited or coated on a current collector by a dry process, or in the form of metals and alloys in a particulate phase deposited or coated by a wet process.

[0063] A conventional separator can be inserted between the positive and negative electrodes. The separator is a physical membrane that functions to physically separate the electrodes and can be used without particular limitations, as long as it is used as a conventional separator. In particular, separators with low resistance to ion migration in the electrolyte and excellent impregnation ability with the electrolyte are preferred. Furthermore, the separator can transport lithium ions between the positive and negative electrodes while isolating or insulating them from each other. The separator can be made of porous, non-conductive, or insulating materials. The separator can be a separate component such as a membrane or a coating added to the positive and / or negative electrodes.

[0064] Examples of polyolefin porous membranes that can be used as diaphragms can be membranes formed from any polymer selected alone or mixtures thereof: polyethylene such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, and ultra-high molecular weight polyethylene; and polyolefin polymers such as polypropylene, polybutene, and polypentene. Examples of nonwoven fabrics that can be used as diaphragms are nonwoven fabrics formed from polymers alone or mixtures thereof: polyphenylene ether, polyimide, polyamide, polycarbonate, polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, polyphenylene sulfide, polyacetal, polyethersulfone, polyetheretherketone, polyester, etc. Such nonwoven fabrics include nonwoven fabrics formed into porous fabrics in the form of fibers, i.e., spunbond or meltblown nonwoven fabrics composed of long fibers.

[0065] There are no particular limitations on the thickness of the separator, but it is preferably in the range of 1 to 100 μm, more preferably in the range of 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 are no particular limitations 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.

[0066] The lithium secondary battery of the present invention, comprising the positive electrode, negative electrode, separator, and electrolyte as described above, can be manufactured by placing the positive electrode facing the negative electrode and inserting the separator therebetween, and then injecting the electrolyte for lithium secondary batteries according to the present invention.

[0067] On the other hand, the lithium secondary battery according to the present invention can be used not only as a 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. Furthermore, the present invention provides a battery pack in which battery modules are electrically connected according to conventional techniques in the art. The battery module and 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.

[0068] Preferred implementation scheme

[0069] Preferred examples are provided below to aid in understanding the invention, but these examples are merely illustrative and will be apparent to those skilled in the art that many variations and modifications can be made within the scope and spirit of the invention, and such variations and modifications are all within the scope of the claims.

[0070] [Example 1] Preparation of electrolyte for lithium secondary batteries

[0071] First, lithium nitrate (LiNO3) and magnesium chloride, at a total weight of 1 wt% based on the total weight of the electrolyte, are added and dissolved in an organic solvent obtained by mixing 1,3-dioxolane (first solvent) and dimethoxyethane (second solvent) in a 1:1 volume ratio (volume / volume) to prepare an electrolyte for lithium secondary batteries.

[0072] [Examples 2 to 5 and Comparative Examples 1 and 2] Preparation of electrolyte for lithium secondary batteries

[0073] Except for the changes in components shown in Table 1 below, electrolytes for lithium secondary batteries corresponding to Examples 2 to 5 and Comparative Examples 1 and 2 were prepared in the same manner as in Example 1.

[0074] Table 1:

[0075]

[0076] [Examples 6 to 8 and Comparative Example 3] Manufacturing of lithium-ion symmetric batteries

[0077] Two electrodes were prepared by rolling a 35 μm thick lithium sheet onto one side of a copper current collector, which were then used as the positive and negative electrodes, respectively, with the lithium sheets facing each other. Subsequently, a porous polyethylene (PE) membrane was inserted between the positive and negative electrodes, and the electrolytes prepared in Examples 1, 3, 5 and Comparative Example 1 were injected and sealed to manufacture a coin cell type lithium-lithium symmetric battery.

[0078] [Experimental Example 1] Cycle life evaluation of lithium-ion symmetric batteries

[0079] For the lithium-lithium symmetric batteries prepared in Examples 6 to 8 and Comparative Example 3, at a temperature of 25°C and a current of 1.0 mA / cm², 2 Repeated discharge (lower limit -1V) and charge (upper limit +1V) cycles were performed at a current density to measure the change in potential over time (cycles), and the results are shown in Table 2 below. Figure 1 middle.

[0080] Table 2:

[0081]

[0082] Figure 1 This is a graph showing the cycle life performance of lithium-lithium symmetric batteries manufactured according to embodiments and comparative examples of the present invention. (As shown above...) Figure 1 As shown in Table 2, it was confirmed that all lithium-lithium symmetric batteries in Examples 6 to 8, which contained magnesium chloride (MgCl2) in the electrolyte, had improved lifespan compared to the lithium-lithium symmetric battery in Comparative Example 3, which did not contain magnesium chloride in the electrolyte. Furthermore, by evaluating the cycle life of Examples 6 to 8, it was confirmed that the lifespan of the lithium-lithium symmetric batteries was further extended with increasing magnesium chloride content in the electrolyte.

[0083] [Examples 9 to 13 and Comparative Examples 4 and 5] Manufacturing of lithium-sulfur batteries

[0084] First, 87.5 parts by weight of sulfur-carbon (CNT) composite material (S / C weight ratio 75:25) as the positive electrode active material, 5 parts by weight of Deco Black as the conductive material, and 7.5 parts by weight of styrene-butadiene rubber / carboxymethyl cellulose (SBR / CMC 7:3) as the binder were mixed to prepare a slurry composition for the positive electrode. Then, the prepared slurry composition was 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 point, the loading was 2.9 mg / cm³). 2Subsequently, the prepared positive electrode and the negative electrode having a 35 μm thick lithium rolled on one side of the copper current collector were placed opposite each other, and a porous polyethylene (PE) separator was inserted between them. Then, the electrolytes prepared in Examples 1 to 5 and Comparative Examples 1 and 2 were injected and sealed to manufacture a coin-type lithium-sulfur battery.

[0085] [Experimental Example 2] Evaluation of the cycle life of lithium-sulfur batteries

[0086] For the lithium-sulfur batteries prepared in Examples 9 to 13 and Comparative Examples 4 and 5, 0.2C charge / 0.3C discharge cycles were performed in CC mode at 25°C within a voltage range of 1.8V to 2.5V to measure the capacity-potential variation with charge / discharge cycles (after an initial stabilization process of 2.5 cycles at 0.1C / 0.1C and 3 cycles at 0.2C / 0.2C, operation began at 0.3C / 0.5C from the 7th cycle), and the results are shown in Table 3 below. Figure 2 middle.

[0087] Table 3:

[0088]

[0089] Figure 2 This is a graph showing the cycle life performance of lithium-sulfur batteries manufactured according to embodiments and comparative examples of the present invention. As above. Figure 2 As shown in Table 3, it was confirmed that all lithium-sulfur batteries in Examples 9 to 13, which contained magnesium chloride (MgCl2) in the electrolyte, had extended lifespans compared to the lithium-sulfur battery in Comparative Example 4, which did not contain magnesium chloride in the electrolyte.

[0090] Furthermore, it was confirmed that although the electrolyte also contained magnesium chloride, if the content exceeded 1% by weight (Comparative Example 5), the lifespan performance was lower than that of the lithium-sulfur batteries of Examples 9 to 13, which used magnesium chloride within the content range of the present invention (0.1 to 1% by weight). This is because the battery rapidly deteriorates due to a side reaction occurring between magnesium chloride and the positive electrode active material (sulfur). It can be seen that even when magnesium chloride is used as an electrolyte additive, the objectives of the present invention can only be achieved when it is used in appropriate amounts.

Claims

1. An electrolyte for lithium secondary batteries, the electrolyte comprising: The first solvent comprises a heterocyclic compound containing at least one double bond or no double bond and containing at least one of oxygen and sulfur atoms; The second solvent comprises at least one of ether compounds, ester compounds, amide compounds, and carbonate compounds; Lithium salts; Magnesium chloride; and Lithium nitrate, The content of magnesium chloride relative to the total weight of the electrolyte for lithium secondary batteries is 0.5% to 1% by weight. The lithium secondary battery mentioned above is a lithium-sulfur battery.

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

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

4. The electrolyte for lithium secondary batteries according to claim 1, wherein the heterocyclic compound is: Unsubstituted or substituted with at least one of the following 3- to 15-membered heterocyclic compounds: alkyl groups having 1 to 4 carbon atoms, cyclic alkyl groups having 3 to 8 carbon atoms, aryl groups having 6 to 10 carbon atoms, halogen groups, nitro groups, amino groups, and sulfonyl groups, or Polycyclic compounds containing at least one of the following heterocyclic compounds: cyclic alkyl groups having 3 to 8 carbon atoms and aryl groups having 6 to 10 carbon atoms.

5. The electrolyte for lithium secondary batteries according to claim 1, wherein the heterocyclic compound is selected from: 1,3-dioxolane, 4,5-diethyl-1,3-dioxolane, 4,5-dimethyl-1,3-dioxolane, 4-methyl-1,3-dioxolane, 4-ethyl-1,3-dioxolane, 1,3-dioxolane Alkane, 1,4-di Alkane, 4-methyl-1,3-di Alkane and 2-methyl-1,3-di Alkane, 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.

6. 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 the following: 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.

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

8. The electrolyte for lithium secondary batteries according to claim 1, wherein the electrolyte for lithium secondary batteries contains 1,3-dioxolane as a first solvent, dimethoxyethane as a second solvent, LiTFSI, magnesium chloride, and lithium nitrate as lithium salts.

9. A lithium-sulfur battery, the lithium-sulfur battery comprising: positive electrode; Lithium anode; A membrane disposed between the positive electrode and the negative electrode; and The electrolyte for lithium secondary batteries as described in claim 1.

10. The lithium-sulfur battery according to claim 9, wherein during the initial charging / discharging process of the lithium-sulfur battery, a LiCl protective layer and a Li-Mg alloy are formed on the surface of the lithium anode.

Images