Non-aqueous electrolyte for lithium secondary battery and lithium secondary battery containing the same

Abstract

The present invention provides a non-aqueous electrolyte for a lithium secondary battery, which comprises a lithium salt, an organic solvent and a pyridine additive containing two nitrile groups.

Description

Technical Field

[0001] The present invention relates to a non-aqueous electrolyte for a lithium secondary battery capable of reducing or alleviating resistance and expansion of the battery at high voltage, and a lithium secondary battery comprising the non-aqueous electrolyte.

[0002] This application claims priority from Korean Patent Application No. 10-2021-0001813, filed on January 7, 2021, which is hereby incorporated by reference in its entirety. Background Art

[0003] Recently, portable electronic devices have become widely distributed. Therefore, for the batteries that serve as their power source, as these portable electronic devices are rapidly miniaturized, reduced in weight, and thinned, there is a strong demand for the development of a secondary battery that is small and light, can be charged and discharged for a long time, and has excellent high-rate characteristics.

[0004] Among the secondary batteries currently used, lithium secondary batteries developed in the early 1990s have attracted much attention because of their advantages in that they have a much higher operating voltage and energy density than conventional batteries (such as Ni-MH, Ni-Cd and lead sulfate batteries) using electrolytes in the form of aqueous solutions. However, due to the use of non-aqueous electrolytes, these lithium secondary batteries have safety issues such as fire and explosion, and these problems become more serious as the capacity density of the battery increases.

[0005] In secondary batteries using non-aqueous electrolytes, the reduction in battery safety that occurs during continuous charging is a major concern. One factor that can influence this is the heat generated by the collapse of the positive electrode structure. The operating principle is as follows. Specifically, the positive electrode active material of batteries using non-aqueous electrolytes is composed of a lithium-containing metal oxide that can intercalate and deintercalate lithium and / or lithium ions. This positive electrode active material transforms into a thermally unstable structure due to the release of large amounts of lithium during overcharging. In this overcharged state, due to external physical impact, such as exposure to high temperatures, when the battery temperature reaches a critical temperature, oxygen is released from the unstable positive electrode active material. The released oxygen causes an exothermic decomposition reaction with the electrolyte solvent. In particular, since the oxygen released from the positive electrode further accelerates the combustion of the electrolyte, this exothermic chain reaction can lead to battery fire and explosion due to thermal runaway.

[0006] To control the fire or explosion caused by the increase in internal battery temperature, a method of adding aromatic compounds as redox shuttle additives to the electrolyte is used. For example, Japanese Patent Laid-Open No. 2002-260725 discloses a non-aqueous lithium-ion battery that uses aromatic compounds (such as biphenyl) to prevent the overcharge current and thermal runaway caused by such compounds. In addition, U.S. Patent No. 5,879,834 also describes a method for improving battery safety by adding a small amount of aromatic compounds (such as biphenyl and 3-chlorothiophene) to increase the internal resistance through electrochemical polymerization under abnormal overvoltage conditions.

[0007] However, when using additives such as biphenyl, when a relatively high voltage occurs locally at a general operating voltage, the additive gradually decomposes during charging and discharging, or when the battery is discharged for a long time at high temperature, the amount of biphenyl gradually decreases, so after 300 charge and discharge cycles, there are problems such as the inability to ensure safety and storage characteristics.

[0008] At the same time, high-voltage batteries (systems above 4.2V) are being continuously researched and developed as a way to increase the charge capacity in order to increase the capacity of the battery while miniaturizing it. Even in the same battery system, when the charging voltage increases, the charge capacity generally increases. However, there are safety issues such as decomposition of the electrolyte, insufficient space for lithium insertion, and dangers caused by increased electrode potential. Therefore, in order to make the battery operate at high voltage, the overall conditions are managed by the system so that the standard reduction potential difference between the negative electrode active material and the positive electrode active material is easily maintained large and the electrolyte does not decompose at this voltage.

[0009] Considering this aspect of high-voltage batteries, when conventional overcharge inhibitors used in conventional lithium-ion batteries (e.g., biphenyl (BP) or cyclohexylbenzene (CHB)) are used, it can be easily seen that even during normal charge and discharge operations, a lot of such decomposition occurs, and even at slightly higher temperatures, the battery characteristics rapidly deteriorate, leading to a problem of shortening the battery life. In addition, if a commonly used non-aqueous carbonate solvent is used as the electrolyte, when charging at a voltage higher than the normal charge potential of 4.0 V, there is a problem that due to the increased oxidizing power, the decomposition reaction of the electrolyte proceeds as the charge / discharge cycle progresses, and the life characteristics rapidly deteriorate.

[0010] Therefore, in high-voltage batteries (systems above 4.2 V), there is a continuous need to develop a method to improve battery expansion characteristics by inhibiting the leaching of transition metals from the positive electrode and reducing gas generation at high temperatures.

[0011] [Prior art literature]

[0012] [Patent Document]

[0013] (Patent Document 1) Japanese Patent Laid-Open No. 2002-260725

[0014] (Patent Document 2) U.S. Patent No. 5,879,834 Summary of the Invention

[0015] [Technical Issues]

[0016] In order to solve the existing problems, the purpose of the present invention is to provide a non-aqueous electrolyte for lithium secondary batteries and a lithium secondary battery containing the same. By adding a pyridine additive containing two nitrile groups as an additive to the non-aqueous electrolyte for lithium secondary batteries, the non-aqueous electrolyte can inhibit the leaching of transition metals from the positive electrode under high voltage conditions and reduce the amount of gas generated at high temperatures, while maintaining basic properties such as good high-rate charge / discharge characteristics and life characteristics, thereby improving the expansion characteristics of the battery.

[0017] Furthermore, another object of the present invention is to provide a lithium secondary battery having improved capacity characteristics and safety by including the above-mentioned nonaqueous electrolyte for lithium secondary batteries while suppressing side reactions between the positive electrode and the electrolyte under high voltage and high temperature conditions.

[0018] [Technical solution]

[0019] In order to achieve the above object, the present invention provides a non-aqueous electrolyte for a lithium secondary battery, which comprises a lithium salt, an organic solvent and a pyridine additive containing two nitrile groups.

[0020] In addition, the present invention provides a non-aqueous electrolyte for a lithium secondary battery, wherein the pyridine additive is represented by the following Chemical Formula 1:

[0021] [Chemical Formula 1]

[0022]

[0023] wherein R is -L-CN and L is an alkylene group.

[0024] In addition, the present invention provides a non-aqueous electrolyte for a lithium secondary battery, wherein Chemical Formula 1 is represented by the following Chemical Formula 1-1:

[0025] [Chemical Formula 1-1]

[0026]

[0027] In addition, the present invention provides a non-aqueous electrolyte for a lithium secondary battery, wherein Chemical Formula 1 is represented by any one of the following compounds:

[0028]

[0029]

[0030] In addition, the present invention provides a non-aqueous electrolyte for a lithium secondary battery, wherein the content of the pyridine additive is 0.01 wt % to 10 wt % relative to the total weight of the electrolyte.

[0031] In addition, the present invention provides a non-aqueous electrolyte for lithium secondary batteries, wherein the lithium salt is selected from LiCl, LiBr, LiI, LiBF4, LiClO4, LiB 10 Cl 10 , a group consisting of LiAlCl4, LiAlO4, LiPF6, LiCF3SO3, LiCH3CO2, LiCF3CO2, LiAsF6, LiSbF6, LiCH3SO3, LiFSI (lithium bis(fluorosulfonyl)imide, LiN(SO2F)2), LiBETI (lithium bis(perfluoroethanesulfonyl)imide, LiN(SO2CF2CF3)2) and LiTFSI (lithium bis(trifluoromethanesulfonyl)imide, LiN(SO2CF3)2).

[0032] Furthermore, the present invention provides a non-aqueous electrolyte for a lithium secondary battery, wherein the concentration of the lithium salt is 0.1M to 3M.

[0033] Furthermore, the present invention provides a non-aqueous electrolyte for a lithium secondary battery, wherein the organic solvent comprises at least one selected from the group consisting of ethers, esters, amides, linear carbonates, and cyclic carbonates.

[0034] In addition, the present invention provides a non-aqueous electrolyte for a lithium secondary battery, wherein the operating voltage of the lithium secondary battery is above 4.0V.

[0035] 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 the non-aqueous electrolyte for a lithium secondary battery as described above.

[0036] [Beneficial Effects]

[0037] The non-aqueous electrolyte for lithium secondary batteries of the present invention contains a pyridine additive containing two nitrile groups, thereby significantly improving the battery expansion phenomenon caused by oxidation / decomposition of the electrolyte at high voltage and high temperature, thereby showing excellent safety and excellent discharge characteristics.

[0038] Furthermore, the lithium secondary battery comprising the nonaqueous electrolyte for a lithium secondary battery of the present invention exhibits an effect of improving battery expansion characteristics by suppressing the leaching of transition metals from the positive electrode at high voltage and reducing the amount of gas generated at high temperature, while maintaining basic performance such as good high-rate charge / discharge characteristics and life characteristics. DETAILED DESCRIPTION

[0039] The embodiments provided by the present invention can all be realized through the following description. It should be understood that the following description describes preferred embodiments of the present invention, and it should be understood that the present invention is not necessarily limited thereto.

[0040] The present invention provides a non-aqueous electrolyte for lithium secondary batteries, which comprises a lithium salt, an organic solvent and a pyridine additive containing two nitrile groups.

[0041] The pyridine additive containing two nitrile groups may be a compound represented by the following Chemical Formula 1:

[0042] [Chemical Formula 1]

[0043]

[0044] wherein R is -L-CN and L is an alkylene group.

[0045] In addition, the compound represented by Chemical Formula 1 may preferably be a compound represented by the following Chemical Formula 1-1:

[0046] [Chemical Formula 1-1]

[0047]

[0048] In addition, the compound represented by Chemical Formula 1 may more preferably be any one of the following compounds:

[0049]

[0050] The non-aqueous electrolyte for lithium secondary batteries of the present invention contains a pyridine additive containing two nitrile groups and thus suppresses the reaction inside the battery, thereby significantly improving the battery expansion caused by oxidation / decomposition of the electrolyte under high voltage conditions, and exhibits excellent storage characteristics under high temperature conditions and excellent discharge characteristics.

[0051] In this regard, when the additive containing two nitrile groups is applied to the electrolyte, the nitrile groups form a bond with the transition metal ions on the positive electrode surface to form a positive electrode film, thereby suppressing side reactions that occur due to direct contact between the positive electrode surface and the electrolyte. Therefore, by suppressing the generation of gases that can be generated at high temperatures and the leaching of transition metals, the performance of the battery is improved, and this performance improvement is due to the binding of the transition metal ions on the positive electrode surface to the nitrile groups themselves.

[0052] Furthermore, when a pyridine-based additive containing two nitrile groups is used, it exhibits a more excellent effect in improving battery performance compared to the case of containing one or three or more nitrile groups.

[0053] Specifically, when there is only one nitrile group, when the additive is applied in small amounts, it is not enough to fully form a film on the positive electrode surface. If the content of the additive is greatly increased to solve this problem, it may cause the problem of increased battery resistance, so an additive containing one nitrile group is not suitable as an additive for protecting the positive electrode of the battery. In addition, when there are three or more nitrile groups, the physical properties of the electrolyte may deteriorate due to reasons such as increased viscosity, and the structure of the additive becomes too bulky and becomes difficult to effectively bind to the transition metal ions on the positive electrode surface due to steric hindrance. Therefore, it is preferred that the additive contain two nitrile groups.

[0054] At the same time, the nitrile group substituted into the pyridine additive is preferably an acetonitrile group. That is, as described above, when an acetonitrile group is included as the nitrile group, it exhibits a more excellent effect in improving battery performance compared to the case where no linking group other than the acetonitrile group is present or the case where the linking group is an allylene group.

[0055] Specifically, when the itrile group substituted into the pyridine additive is connected with arylidene, due to the presence of double bonds in the linking group and the risk of easily reducing reaction, before the pyridine additive acts on the cathode film, the effect of itrile group can not be fully exerted at the positive electrode, and the reactivity at the negative electrode increases simultaneously. In addition, when itrile group is directly substituted into the pyridine structure without a linking group, due to the lower binding energy of itrile group and transition metal ion, even if a key is formed, compared with methylene, it is also easily broken. Therefore, due to this being disadvantageous in terms of the mechanism of long-term formation of cathode film, it is advantageous for film maintenance to be substituted into the pyridine structure by the methylene as a linking group, rather than being directly substituted into the pyridine structure. That is, itrile group is preferably acetonitrile.

[0056] In addition, the non-aqueous electrolyte for lithium secondary batteries of the present invention may further include an additive selected from the group consisting of lithium difluorooxalatoborate (LiFOB), lithium bisoxalatoborate (LiB(C2O4)2, LiBOB), fluoroethylene carbonate (FEC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), divinyl sulfone, ethylene sulfite, propylene sulfite, diallyl sulfonate, ethane sultone, propane sultone (PS), butane sultone, ethylene sultone, butene sultone and propylene sultone (PRS).

[0057] In addition, relative to the gross weight of the electrolyte, the content of the pyridine additive containing two nitrile groups can be 0.01 wt % to 10 wt %, preferably 0.1 wt % to 5 wt %, more preferably 0.5 wt % to 2 wt %. If the content of the pyridine additive is less than the above range, the effect of suppressing battery expansion during high voltage operation is not significant, the improvement of capacity retention is not significant, etc., that is, the additive effect is not shown, and the effects such as improving the discharge capacity or output of the lithium secondary battery are not significant. If the content of the pyridine additive exceeds the above range, there is a problem that the characteristics of the lithium secondary battery are deteriorated instead, such as rapid life degradation. Therefore, it is preferred that the content of the pyridine additive meets the above range.

[0058] The non-aqueous electrolyte for lithium secondary batteries of the present invention may contain a lithium salt, and the lithium salt may contain a selected from the group consisting of LiCl, LiBr, LiI, LiBF4, LiClO4, LiB 10 Cl 10 , at least one of the group consisting of LiAlCl4, LiAlO4, LiPF6, LiCF3SO3, LiCH3CO2, LiCF3CO2, LiAsF6, LiSbF6, LiCH3SO3, LiFSI (lithium bis(fluorosulfonyl)imide, LiN(SO2F)2), LiBETI (lithium bis(perfluoroethanesulfonyl)imide, LiN(SO2CF2CF3)2 and LiTFSI (lithium bis(trifluoromethanesulfonyl)imide, LiN(SO2CF3)2).

[0059] The concentration of the lithium salt may be 0.1M to 3.0M, preferably 0.5M to 2.5M, and more preferably 0.8M to 2.0M. If the concentration of the lithium salt is less than 0.1M, the conductivity of the electrolyte decreases and the performance of the electrolyte deteriorates. If the concentration of the lithium salt exceeds 3.0M, there is a problem of increased viscosity of the electrolyte, thereby reducing the mobility of lithium ions. Therefore, it is preferred that the concentration of the lithium salt satisfy the above range. The lithium salt acts as a lithium ion source in the battery, thereby achieving the basic operation of the lithium secondary battery.

[0060] Furthermore, the non-aqueous electrolyte for lithium secondary batteries of the present invention may contain a mixture of a lithium imide salt and a lithium salt other than the lithium imide salt.

[0061] The lithium imide salt may be at least one selected from LiFSI (lithium bis(fluorosulfonyl)imide, LiN(SO2F)2), LiBETI (lithium bis(perfluoroethanesulfonyl)imide, LiN(SO2CF2CF3)2) and LiTFSI (lithium bis(trifluoromethanesulfonyl)imide, LiN(SO2CF3)2), and the lithium salt other than the lithium imide salt may be selected from LiCl, LiBr, LiI, LiBF4, LiClO4, LiB 10 Cl 10, at least one of LiAlCl4, LiAlO4, LiPF6, LiCF3SO3, LiCH3CO2, LiCF3CO2, LiAsF6, LiSbF6 and LiCH3SO3.

[0062] In addition, the molar ratio of the lithium imide salt to the lithium salt other than the lithium imide salt is 1: 1 to 7: 1, preferably 1: 1 to 6: 1, and more preferably 1: 1 to 4: 1. The lithium imide salt and the lithium salt other than the lithium imide salt satisfy the above molar ratio, thereby stably forming a film capable of suppressing corrosion of the current collector and even suppressing side reactions of the electrolyte.

[0063] The non-aqueous electrolyte for lithium secondary batteries of the present invention may contain an organic solvent, and the organic solvent is a solvent commonly used in lithium secondary batteries, for example, ether compounds, ester (acetate, propionate) compounds, amide compounds, linear carbonate compounds or cyclic carbonate compounds can be used alone or in combination of two or more.

[0064] In the compounds listed above, a mixture of a straight-chain carbonate and a cyclic carbonate can be preferably used as an organic solvent. If a mixture of a straight-chain carbonate and a cyclic carbonate is used as an organic solvent, the dissociation and movement of the lithium salt can be promoted. In this case, the cyclic carbonate compound and the straight-chain carbonate compound are mixed in a volume ratio of 1:9 to 6:4, preferably 1:9 to 4:6, more preferably 2:8 to 4:6.

[0065] Meanwhile, as a specific example, the linear carbonate compound may be a compound selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methylpropyl carbonate (MPC) and ethylpropyl carbonate (EPC), or a mixture of at least two or more thereof, but is not limited thereto.

[0066] In addition, as a specific example, the cyclic carbonate compound may include a compound selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate and their halides, or a mixture of at least two or more thereof.

[0067] The operating voltage of the lithium secondary battery of the present invention can be 4.0V or higher, preferably 4.1V or higher, and more preferably 4.2V or higher. If the operating voltage of the lithium secondary battery is less than 4.0V, the addition of the pyridine additive of the present invention will not significantly improve the performance. However, in lithium secondary batteries with an operating voltage of 4.0V or higher, the addition of the additive will rapidly increase the high-temperature storage stability and capacity characteristics.

[0068] lithium secondary batteries

[0069] Hereinafter, the lithium secondary battery of the present invention will be described.

[0070] The lithium secondary battery of the present invention includes a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte for a lithium secondary battery. More specifically, the lithium secondary battery includes at least one positive electrode, at least one negative electrode, and a separator selectively disposed between the positive electrode and the negative electrode, and the non-aqueous electrolyte for the lithium secondary battery described above. At this time, since the non-aqueous electrolyte for the lithium secondary battery is the same as that described above, its detailed description will be omitted.

[0071] (1) Positive electrode

[0072] The positive electrode can be prepared by coating a positive electrode active material paste containing a positive electrode active material, an electrode binder, an electrode conductive material, and a solvent on a positive electrode current collector.

[0073] The positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the relevant battery. For example, stainless steel, aluminum, nickel, titanium, sintered carbon; or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. can be used as the positive electrode current collector. At this time, the positive electrode current collector can form fine irregularities on the surface to enhance the adhesion to the positive electrode active material, and can be used in various forms, such as films, sheets, foils, meshes, porous bodies, foams, non-woven fabrics, etc.

[0074] The positive electrode active material is a compound capable of reversibly intercalating and deintercalating lithium. Specifically, it may include a lithium composite metal oxide containing lithium and at least one metal (such as cobalt, manganese, nickel, or aluminum). More specifically, the lithium composite metal oxide may be: lithium manganese-based oxides (such as LiMnO2, LiMn2O4, etc.), lithium cobalt-based oxides (such as LiCoO2, etc.), lithium nickel-based oxides (such as LiNiO2, etc.), lithium nickel manganese-based oxides (such as LiNi 1-Y1 Mn Y1 O2 (where 0 < Y1 < 1), LiMn 2-z1 Ni z1 O4 (where 0 < Z1 < 2), etc.), lithium nickel cobalt-based oxides (such as LiNi 1-Y2 Co[[ID=2{]] Y2 O2 (where 0 < Y2 < 1), etc.), lithium manganese cobalt-based oxides (such as LiCo 1-Y3 Mn Y3 O2 (where 0 < Y3 < 1), LiMn 2-z2 Co z2 O4 (where 0 < Z2 < 2), etc.), lithium nickel manganese cobalt-based oxides (such as Li(Ni p1 Co q1 Mn r1)O2 (where 0 < p1 < 1, 0 < q1 < 1, 0 < r1 < 1, p1 + q1 + r1 = 1) or Li(Ni p2 Co q2 Mn r2 )O4 (where 0 < p2 < 2, 0 < q2 < 2, 0 < r2 < 2, p2 + q2 + r2 = 2), etc.), or lithium nickel cobalt transition metal (M) oxide (such as Li(Ni p3 Co q3 Mn r3 M S1 )O2 (where M is selected from Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, and p3, q3, r3, and s1 are the atomic fractions of each independent element, where 0 < p3 < 1, 0 < q3 < 1, 0 < r3 < 1, 0 < s1 < 1, p3 + q3 + r3 + s1 = 1), etc.), and may include any one or more than two of these compounds.

[0075] Among them, considering that the capacity characteristics and stability of the battery can be improved, the lithium composite metal oxide can be LiCoO2, LiMnO2, LiNiO2, lithium nickel manganese cobalt oxide (such as Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2 or Li(Ni 0.8 Mn 0.1 Co 0.1 )O2, etc.) or lithium nickel cobalt aluminum oxide (such as LiNi 0.8 Co 0.15 Al 0.05 O2, etc.). When considering the significant improvement effects brought by controlling the types and content ratios of the elements forming the lithium composite metal oxide, the lithium composite metal oxide can be Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2, Li(Ni 0.7 Mn 0.15 Co 0.15 )O2 or Li(Ni 0.8 Mn 0.1 Co 0.1 )O2, and any one or a mixture of two or more of these can be used.

[0076] The electrode binder is a component that helps the positive electrode active material and the electrode conductive material to adhere to each other and to the current collector. Specifically, the binder can be polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber and various copolymers thereof.

[0077] The conductive material for the electrode is a component for further improving the conductivity of the positive electrode active material. The conductive material for the electrode is not particularly limited, as long as it has conductivity and does not cause chemical changes in the battery. For example, graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black and thermal black; conductive fibers such as carbon fibers and metal fibers; metal powders such as carbon fluoride, aluminum and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives, etc. Specific examples of commercially available conductive materials can include acetylene black series products of Chevron Chemical Company or Denka black (DenkaSingapore Private Limited), products of Gulf Oil Company, Ketjen black, EC series (products of Armak Company), Vulcan XC-72 (products of Cabot Company) and Super P (products of Timcal Company).

[0078] The solvent may include an organic solvent such as N-methyl-2-pyrrolidone (NMP), and the amount of the solvent used can achieve a desired viscosity when a positive electrode active material and optionally a positive electrode binder, a positive electrode conductive material, etc. are included.

[0079] (2) Negative electrode

[0080] In addition, the negative electrode can be prepared by coating a negative electrode active material slurry containing a negative electrode active material, an electrode binder, an electrode conductive material and a solvent on a negative electrode current collector. At the same time, as the negative electrode, the metal negative electrode current collector itself can be used as an electrode.

[0081] The negative electrode current collector is not particularly limited as long as it has high conductivity and does not cause chemical changes in the battery. Examples of the negative electrode current collector include copper, stainless steel, aluminum, nickel, titanium, sintered carbon; copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like; and aluminum-cadmium alloys. Similar to the positive electrode current collector, the negative electrode current collector may have fine surface irregularities to enhance adhesion to the negative electrode active material and may be used in various forms, such as films, sheets, foils, meshes, porous bodies, foams, and non-woven fabrics.

[0082] The negative electrode active material may be at least one negative electrode active material selected from the following group: natural graphite, artificial graphite, carbonaceous material; lithium-containing titanium composite oxide (LTO), Si, Sn, Li, Zn, Mg, Cd, Ce, Ni or Fe metal (Me); alloys composed of the above metals (Me); oxides of the above metals (MeO x ); and a complex of the above metal (Me) and carbon.

[0083] Since the binder for an electrode, the conductive material for an electrode, and the solvent are the same as described above, a detailed description thereof will be omitted.

[0084] (3) Diaphragm

[0085] As a separator, a conventional porous polymer film conventionally used as a separator, such as a porous polymer film made of a polyolefin polymer (such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer and ethylene / methacrylate copolymer) can be used alone, or they can be stacked and used; or a conventional porous non-woven fabric can be used, such as a non-woven fabric made of high-melting point glass fiber, polyethylene terephthalate fiber, etc., but is not limited thereto.

[0086] Hereinafter, preferred examples are provided to help understanding the present invention, but the following examples are provided only to make the present invention easier to understand, and the present invention is not limited thereto.

[0087] Example

[0088] 1. Example 1

[0089] (1) Preparation of non-aqueous electrolyte for lithium secondary batteries

[0090] Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 30:70, and then LiPF6 (lithium hexafluorophosphate) was dissolved to a concentration of 1.0 M to prepare a non-aqueous organic solvent. 0.5 g of 2,6-pyridinediacetonitrile as an additive was added to 99.5 g of the non-aqueous organic solvent to prepare a non-aqueous electrolyte for a lithium secondary battery.

[0091] (2) Manufacturing of lithium secondary batteries

[0092] The positive electrode active material (LiNi 0.8 Co 0.1 Mn 0.1A positive electrode active material slurry was prepared by mixing carbon black (a conductive material) and polyvinylidene fluoride (PVDF) (a binder) in a weight ratio of 94:3:3. The mixture was then added to N-methyl-2-pyrrolidone (NMP) as a solvent. The positive electrode active material slurry was applied to an aluminum (Al) film with a thickness of approximately 20 μm, which served as the positive electrode current collector, and dried to prepare a positive electrode. The positive electrode was then roll-pressed to prepare a positive electrode.

[0093] Graphite as the negative electrode active material, polyvinylidene fluoride (PVDF) as a binder, and carbon black as a conductive material were mixed in a weight ratio of 95:2:3, and then the mixture was added to N-methyl-2-pyrrolidone (NMP) as a solvent to prepare a negative electrode active material slurry. The negative electrode active material slurry was coated on a copper (Cu) film with a thickness of 10 μm as the negative electrode current collector, dried to prepare a negative electrode, and then roll-pressed to prepare a negative electrode.

[0094] A positive electrode, a negative electrode, and a separator made of polypropylene / polyethylene / polypropylene (PP / PE / PP) were stacked in the order of positive electrode / separator / negative electrode, and the stacked structure was placed in a pouch-type battery case, and then the non-aqueous electrolyte for a lithium secondary battery was injected to prepare a lithium secondary battery.

[0095] 2. Example 2

[0096] A non-aqueous electrolyte for a lithium secondary battery and a lithium secondary battery were prepared in the same manner as in Example 1, except that 2 g of 2,6-pyridinediacetonitrile as an additive was added to 98 g of the non-aqueous organic solvent.

[0097] 3. Example 3

[0098] A non-aqueous electrolyte for a lithium secondary battery and a lithium secondary battery were prepared in the same manner as in Example 1, except that 0.5 g of 2,5-pyridinediacetonitrile was added as an additive instead of 0.5 g of 2,6-pyridinediacetonitrile.

[0099] Comparative Example

[0100] 1. Comparative Example 1

[0101] A non-aqueous electrolyte for a lithium secondary battery and a lithium secondary battery were prepared in the same manner as in Example 1, except that 2,6-pyridinediacetonitrile was not added as an additive when preparing the electrolyte for a lithium secondary battery.

[0102] 2. Comparative Example 2

[0103] A non-aqueous electrolyte for a lithium secondary battery and a lithium secondary battery were prepared in the same manner as in Example 1, except that 0.5 g of 1,4-phenylenediacetonitrile was added as an additive instead of 0.5 g of 2,6-pyridinediacetonitrile.

[0104] 3. Comparative Example 3

[0105] A non-aqueous electrolyte for a lithium secondary battery and a lithium secondary battery were prepared in the same manner as in Example 1, except that 10 g of 2,6-pyridinediacetonitrile was added as an additive.

[0106] Table 1 below shows the components and contents of the additives used in Examples 1 to 3 and Comparative Examples 1 to 3.

[0107] Table 1

[0108] Additive type Additive content (weight %) Example 1 2,6-Pyridinediacetonitrile 0.5 Example 2 2,6-Pyridinediacetonitrile 2 Example 3 2,5-Pyridinediacetonitrile 0.5 Comparative Example 1 - 0 Comparative Example 2 Phthalocyanine 0.5 Comparative Example 3 2,6-Pyridinediacetonitrile 10

[0109] Experimental example

[0110] 1. Experimental Example 1: Evaluation of Thickness Increase Rate after High-Temperature Storage

[0111] The lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 3 were charged to 4.2 V / 0.05 C mA at room temperature under 0.33 C / 4.2 V constant current / constant voltage (CC / CV) conditions and discharged to 3 V under 0.33 C constant current (CC) conditions.

[0112] Then, after the state of charge (SOC) of each lithium secondary battery was set to 100%, the thickness of the lithium secondary battery was measured, which was defined as the initial thickness.

[0113] Next, the lithium secondary battery was placed in an oven (OF-02GW, manufactured by Jeotech Company) at 60°C for 4 weeks at high temperature, and then cooled at room temperature for 24 hours, and then the thickness of the lithium secondary battery was measured. The respective measured values ​​of the initial thickness and the thickness after high temperature storage were substituted into the following equation 1 to calculate the thickness increase rate (%), and are shown in Table 2.

[0114] [Equation 1]

[0115] Thickness increase rate (%) = {(thickness after high temperature storage - initial thickness) / initial thickness} × 100 (%)

[0116] Table 2

[0117] Thickness increase rate (%) Example 1 7.4 Example 2 6.8 Example 3 8.4 Comparative Example 1 18.6 Comparative Example 2 14.9 Comparative Example 3 12.2

[0118] Referring to Table 2, it was confirmed that in the case of the lithium secondary batteries of Examples 1 to 3, the thickness increase rate was lower than that of the lithium secondary batteries of Comparative Examples 1 to 3.

[0119] The reason for this is believed to be that 2,6-pyridinediacetonitrile has the effect of suppressing gas generation at high temperatures.

[0120] First, the suppression of gas generation at high temperatures is due to the Lewis base properties of the pyridine structure.

[0121] Specifically, the Lewis basic properties of the pyridine structure combine with the Lewis acid properties of PF5 generated in the electrolyte using LiPF6 salt to stabilize the PF5, thereby suppressing the generation of HF from the PF5. As a result, the generation of HF at high temperatures is reduced, thereby suppressing degradation of the positive electrode surface, the occurrence of side reactions, and decomposition of the electrolyte. This, in turn, reduces gas generation at high temperatures and reduces thickness after high-temperature storage.

[0122] Second, the effect of suppressing gas generation at high temperatures is due to the protective effect of the diacetonitrile functional group on the cathode.

[0123] Specifically, this functional group bonds with transition metal ions on the cathode surface to form a cathode film, and serves to suppress side reactions that occur due to direct contact between the electrolyte and the cathode surface. This suppresses the generation of gases that can be generated at high temperatures and the leaching of transition metals, resulting in a reduced thickness after high-temperature storage.

[0124] Therefore, it was confirmed that the addition of the pyridine-based additive containing two nitrile groups of the present invention can exhibit excellent high-temperature storage stability.

[0125] 2. Experimental Example 2: Evaluation of Capacity Retention During High-Temperature Storage

[0126] The lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 3 were charged to 4.2 V / 0.05 C mA at room temperature under 0.33 C / 4.2 V constant current / constant voltage (CC / CV) conditions and discharged to 3 V under 0.33 C constant current (CC) conditions. At this time, the discharge capacity shown was defined as the initial capacity (mAh).

[0127] Then, after the state of charge (SOC) of each lithium secondary battery was set to 100%, the lithium secondary battery was placed in an oven (OF-02GW, manufactured by Jeotech Company) at 60°C for storage at high temperature for 4 weeks, and then cooled at room temperature for 24 hours. Thereafter, the lithium secondary battery was discharged to 3V under 0.33C constant current (CC) conditions, and then, as when measuring the initial capacity, charged to 4.2V / 0.05C mA under 0.33C / 4.2V constant current / constant voltage (CC / CV) conditions and discharged to 3V under 0.33C constant current (CC), repeated three times. In this case, the final third discharge capacity was defined as the capacity (mAh) after high temperature storage.

[0128] The capacity retention rate (%) was calculated by substituting the respective measured values ​​of the initial capacity and the capacity after high-temperature storage into the following Equation 2, and is shown in Table 3 below.

[0129] [Equation 2]

[0130] Capacity retention (%) = (capacity after high-temperature storage (mAh) / initial capacity (mAh)) × 100 (%)

[0131] Table 3

[0132] Capacity retention rate (%) Example 1 94.7 Example 2 93.3 Example 3 94.4 Comparative Example 1 92.6 Comparative Example 2 93.1 Comparative Example 3 91.4

[0133] Referring to Table 3, it was confirmed that in the case of the lithium secondary batteries of Examples 1 to 3, the capacity retention ratio after high-temperature storage was excellent compared with the lithium secondary batteries of Comparative Examples 1 to 3.

[0134] As described in Experimental Example 2, the excellent effect of capacity retention after high-temperature storage is the result of stabilization of PF5, suppression of HF generation at high temperatures by the 2,6-pyridinediacetonitrile additive, and suppression of cathode degradation by the formation of the cathode film.

[0135] Meanwhile, as in Comparative Example 3, it was confirmed that if an excessive amount of the additive was applied, the resistance at the electrode surface excessively increased, so that the capacity retention ratio was greatly reduced despite the suppression of gas generation.

[0136] Therefore, it was confirmed that the capacity retention rate after high-temperature storage was excellent by adding the pyridine-based additive containing two nitrile groups of the present invention.

[0137] All simple modifications and variations of the present invention are within the scope of the present invention, and the specific scope of protection of the present invention will become apparent from the appended claims.

Claims

1. A non-aqueous electrolyte for a lithium secondary battery, comprising a lithium salt, an organic solvent, and a pyridine additive containing two nitrile groups; in, The pyridine additive is represented by the following chemical formula 1: [Chemical Formula 1] wherein R is -L-CN and L is an alkylene group.

2. The non-aqueous electrolyte for lithium secondary batteries according to claim 1, wherein The above Chemical Formula 1 is represented by the following Chemical Formula 1-1: [Chemical Formula 1-1] 3. The non-aqueous electrolyte for lithium secondary batteries according to claim 1, wherein The above Chemical Formula 1 is represented by any one of the following compounds:

4. The non-aqueous electrolyte for lithium secondary batteries according to claim 1, wherein The content of the pyridine additive is 0.01 wt % to 10 wt % relative to the total weight of the electrolyte.

5. The non-aqueous electrolyte for lithium secondary batteries according to claim 1, wherein The lithium salt is selected from LiCl, LiBr, LiI, LiBF4, LiClO4, LiB 10 Cl 10 , a group consisting of LiAlCl4, LiAlO4, LiPF6, LiCF3SO3, LiCH3CO2, LiCF3CO2, LiAsF6, LiSbF6, LiCH3SO3, LiN(SO2F)2, LiN(SO2CF2CF3)2 and LiN(SO2CF3)2.

6. The non-aqueous electrolyte for lithium secondary batteries according to claim 1, wherein The concentration of the lithium salt is 0.1M to 3M.

7. The non-aqueous electrolyte for lithium secondary batteries according to claim 1, wherein The organic solvent includes at least one selected from the group consisting of ethers, esters, amides, linear carbonates, and cyclic carbonates.

8. The non-aqueous electrolyte for lithium secondary batteries according to claim 1, wherein The operating voltage of the lithium secondary battery is above 4.0V. 9 . A lithium secondary battery comprising a positive electrode, a negative electrode, a separator provided between the positive electrode and the negative electrode, and the non-aqueous electrolyte for a lithium secondary battery according to claim 1 .

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