Lithium secondary battery with improved safety

Abstract

The present invention relates to an electrolyte composition for a lithium secondary battery, which contains a cyclic ester-based solvent as a main component and a predetermined amount of an electrolyte additive, and thus can uniformly form a solid electrolyte interface layer (SEI layer) having high lithium ion conductivity and excellent heat resistance on a negative electrode surface when a lithium secondary battery is activated. As a result, the reactivity between the negative electrode active material and the electrolyte composition is significantly reduced, and the temperature at which heat generation between the negative electrode active material and the electrolyte composition begins (i.e., the heat release initiation temperature) is raised, thereby suppressing an exothermic reaction caused by degradation of the negative electrode or the like. Further, the lithium secondary battery comprising the electrolyte composition can reduce side reactions of the electrolyte composition on the surface of the negative electrode as much as possible when exposed to high temperatures, thereby exhibiting excellent high-temperature safety.

Description

Technical Field

[0001] This invention relates to lithium secondary batteries with enhanced high-temperature safety.

[0002] This application is based on and claims priority to Korean Patent Application No. 10-2024-0037345, filed on March 18, 2024, with the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. Background Technology

[0003] Recently, lithium secondary batteries have been used not only in small devices such as portable electronic devices, but also in large devices such as hybrid electric vehicles or battery packs or power storage devices for electric vehicles.

[0004] In the manufacture of lithium-ion secondary batteries, the positive and negative electrodes are produced by coating and drying a composition containing electrode active materials onto a current collector at a suitable thickness and length, or by molding the electrode active materials themselves into a film. These electrodes are then wound or laminated together with a separator, which serves as an insulator disposed therebetween, to produce an electrode assembly. The electrode assembly is then placed in a can or similar container and an electrolyte is injected, thus producing a lithium-ion secondary battery.

[0005] As the demand for such lithium-ion secondary batteries increases, research to improve their safety is also actively underway. See, for example, Korean Patent Publication No. 10-2022-0105936. Summary of the Invention

[0006] Technical issues

[0007] Therefore, the present invention provides an electrolyte for lithium secondary batteries and a lithium secondary battery containing the electrolyte, which can more effectively suppress the side reactions between the negative electrode and the electrolyte occurring on the negative electrode surface when the lithium secondary battery is exposed to high temperatures.

[0008] Technical solution

[0009] To solve the above problems, In one embodiment, the present invention provides an electrolyte composition for lithium secondary batteries, the electrolyte composition comprising a lithium salt, an electrolyte additive, and a non-aqueous organic solvent. The electrolyte additive comprises an inorganic compound, and the non-aqueous organic solvent comprises about 60% by volume and less than 100% by volume of a cyclic ester solvent represented by Formula 1, and when a heat flow measurement is performed on a mixture comprising the negative electrode active material of a lithium secondary battery in a 100% state of charge at a weight ratio of about 1:0.5, the heat flow of the electrolyte composition in the range of about 250°C to 350°C is about 30.0 W / g or less. [Formula 1]

[0010] In Equation 1, Is it a single or double bond? X is hydrogen, fluorine, or vinyl, and p is an integer from 1 to 5.

[0011] According to one embodiment, the heat flux of the electrolyte composition can be from about 5 W / g to 25 W / g.

[0012] The negative electrode active material used for heat flow measurement may include at least one of natural graphite, artificial graphite, expanded graphite, non-graphitizable carbon, acetylene black, and Ketjen black.

[0013] In addition, the inorganic compound may include at least one of lithium nitrate (LiNO3), lithium chloride (LiCl), lithium fluoride (LiF), lithium bromide (LiBr), lithium borate (Li3BO3), lithium carbonate (Li2CO3), lithium sulfate (Li2SO4), and lithium phosphate (Li3PO4).

[0014] The content of the electrolyte additive may be greater than about 0% by weight and equal to or less than 5% by weight relative to the total weight of the electrolyte composition.

[0015] In addition to the inorganic compound, the electrolyte composition may also contain other electrolyte additives. The electrolyte additives may contain at least one cyclic carbon compound selected from vinylene carbonate (VC), 1,3-propanesulfonyl lactone (PS), ethylene sulfate (ESa), propylene sulfate (PSa), butylene sulfate (BSa), and fluoroethylene carbonate (FEC).

[0016] In this case, the content of the cyclic carbon compound may be from about 100 parts by weight to 1000 parts by weight relative to 100 parts by weight of the inorganic compound.

[0017] The cyclic ester solvent represented by Formula 1 may contain at least one of dihydrofuranone, vinyl dihydrofuranone, fluorodihydrofuranone, furanone, tetrahydropyranone, methyl dihydrofuranone, propyl tetrahydropyranone, and oxetanetanone.

[0018] In addition, the non-aqueous organic solvent may also include at least one carbonate solvent selected from ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylene carbonate (EC), propylene carbonate (PC), and butyl carbonate (BC).

[0019] The content of the carbonate solvent can be greater than about 0% by volume and equal to or less than 40% by volume relative to the total weight of the non-aqueous organic solvent.

[0020] The lithium salt may contain Li + as a cation, and PF6 - , PF4C2O4 - , PF2C4O8 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , (C2O4)2PF2 - , BF2C2O4 - , B(C2O4)2 - , (CF3SO2)2N - , (FSO2)2N - , (CF3CF2SO2)2N - and at least one of ((C(CN))2NC(CF3))N - as an anion.

[0021] In addition, in one embodiment, the present invention provides a lithium secondary battery comprising: an electrode assembly including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode; and the above electrolyte composition of the present invention, the electrolyte composition impregnating the electrode assembly.

[0022] Herein, the positive electrode may include a positive electrode active layer disposed on at least one surface of a positive electrode current collector and containing a positive electrode active material represented by Formula 2: [Formula 2] Li x [Ni y Co z Mn w M 1 v O2 In Formula 2, M 1 is at least one element selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and x, y, z, w, and v satisfy 0.9 ≤ x ≤ 1.30, 0.6 ≤ y < 1, 0 < z ≤ 0.2, 0 < w ≤ 0.2, and 0 ≤ v ≤ 0.1, and y + z + w + v = 1.

[0023] Beneficial effects

[0024] The electrolyte composition for lithium secondary batteries of the present invention can uniformly form a solid electrolyte interface layer (SEI layer) with high lithium-ion conductivity and excellent heat resistance on the surface of the negative electrode during the activation of the lithium secondary battery. Therefore, the reactivity between the negative electrode active material and the electrolyte composition is significantly reduced. Consequently, the temperature at which heat generation begins between the negative electrode active material and the electrolyte composition (e.g., the heat generation onset temperature) increases, and heat generation caused, for example, by the degradation of the negative electrode, can be suppressed. Furthermore, when the lithium secondary battery containing the electrolyte composition is exposed to high temperatures, side reactions occurring on the negative electrode surface of the electrolyte composition can be minimized, thus improving the high-temperature safety of the lithium secondary battery. Attached Figure Description

[0025] The accompanying drawings illustrate embodiments of the invention and serve to further illustrate the technical concept of the invention and the detailed description of the invention that follows. Therefore, the invention should not be construed as limited to the contents shown in the drawings.

[0026] Figure 1 This is a graph showing the heat flow in an electrolyte composition that varies with the type of electrolyte additive, according to one embodiment of the present invention.

[0027] In some of the accompanying drawings, corresponding components are given the same reference numerals. Those skilled in the art will understand that the drawings depict elements simply and clearly, and are not necessarily drawn to scale. For example, to facilitate understanding of the various embodiments, the dimensions of some elements shown in the drawings may be exaggerated compared to other elements. Furthermore, to avoid interfering with the spirit of the various embodiments of the invention, elements of well-known technology useful or necessary in commercially viable embodiments may generally not be depicted. Detailed Implementation

[0028] Since the present invention is susceptible to various modifications and can have various implementations, specific implementations will be described in detail in the detailed description.

[0029] However, this is not intended to limit the invention to a particular implementation, and should be understood to include all modifications, equivalents or substitutions contained within the spirit and scope of the invention.

[0030] In this invention, it should be understood that terms such as “comprising” or “having” are intended to specify the presence of features, numbers, steps, operations, components, parts or combinations thereof described in the specification, but do not preclude the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts or combinations thereof.

[0031] Furthermore, in this invention, when a portion such as a layer, film, region, or plate is described as being "on" other portions, this includes not only the case where the portion is "directly above" other portions, but also the case where another portion exists between them. Conversely, when a portion such as a layer, film, region, or plate is described as being "below" other portions, this includes not only the case where the portion is "directly below" other portions, but also the case where another portion exists between them. Additionally, in this application, "placed on" can include both placement on the lower and upper portions.

[0032] Furthermore, in this invention, "comprising...as a major component" can mean that, relative to the total weight (or total volume), the content of the defined component is 50% by weight or more (or 50% by volume or more), 60% by weight or more (or 60% by volume or more), 70% by weight or more (or 70% by volume or more), 80% by weight or more (or 80% by volume or more), 90% by weight or more (or 90% by volume or more), or 95% by weight or more (or 95% by volume or more). For example, "comprising a cyclic ester solvent as a major component of a non-aqueous organic solvent" can mean that, relative to the total volume of the non-aqueous organic solvent, the content of the cyclic ester solvent is 50% by volume or more, 60% by volume or more, 70% by volume or more, 80% by volume or more, 90% by volume or more, or 95% by volume or more. In some cases, it can also mean that all non-aqueous organic solvents consist of cyclic ester solvents (content: 100% by volume).

[0033] Because lithium-ion batteries operate at high voltages, aqueous electrolytes, which are highly reactive with lithium, cannot be used. Instead, organic electrolytes are typically used. Organic electrolytes are prepared by dissolving lithium salts in organic solvents. The organic solvents used are materials that are stable at high voltages and possess high ionic conductivity, high dielectric constant, and low viscosity.

[0034] Typically, carbonate-based polar non-aqueous solvents are used for such organic solvents. Carbonate-based non-aqueous solvents cause an irreversible reaction during the initial charging of a lithium-ion secondary battery, resulting in the use of excess charge due to side reactions between the negative / positive electrode and the electrolyte. This irreversible reaction forms a passivation layer on the negative electrode surface, such as a solid electrolyte interphase (SEI) layer. The SEI layer prevents electrolyte decomposition on the negative electrode surface during charging and discharging and acts as an ion channel. Therefore, due to the high stability and low resistance of the SEI layer, the lifespan of lithium-ion secondary batteries can be improved.

[0035] Meanwhile, various additives are used in the electrolyte to stabilize the SEI layer. Since SEI layers formed using conventional general-purpose additives are prone to degradation at high temperatures, the stability of SEI layers formed by applying conventional general-purpose additives at high temperatures is reduced.

[0036] In view of these points, the present invention provides a technology that suppresses the side reactions with the electrolyte that occur on the surface of the negative electrode at high temperatures, thereby further improving the high-temperature safety of lithium secondary batteries.

[0037] The invention will be described in more detail below with reference to the accompanying drawings.

[0038] Electrolyte composition for lithium secondary battery

[0039] In one embodiment, the present invention provides an electrolyte composition for lithium secondary batteries, wherein the electrolyte composition comprises a lithium salt, an electrolyte additive, and a non-aqueous organic solvent. The electrolyte additive comprises an inorganic compound, and the non-aqueous organic solvent comprises about 60% by volume and less than 100% by volume of a cyclic ester solvent represented by Formula 1, and when a heat flow measurement is performed on a mixture comprising the negative electrode active material of a lithium secondary battery in a 100% state of charge at a weight ratio of about 1:0.5, the heat flow of the electrolyte composition in the range of 250°C to 350°C is about 30.0 W / g or less. [Formula 1]

[0040] In Equation 1, Is it a single or double bond? X is hydrogen, fluorine, or vinyl, and p is an integer from 1 to 5.

[0041] The electrolyte composition for lithium secondary batteries of the present invention is a liquid electrolyte. This electrolyte allows for the uniform formation of a solid electrolyte interphase (SEI) layer with high lithium-ion conductivity and excellent heat resistance on the negative electrode surface during lithium secondary battery activation. Therefore, since the reactivity between the negative electrode active material and the electrolyte composition is significantly reduced, the temperature at which heat generation begins between the negative electrode active material and the electrolyte composition (e.g., the heat generation onset temperature) increases. This can suppress heat generation, for example, caused by the degradation of the negative electrode. Furthermore, the lithium secondary battery containing the electrolyte composition exhibits excellent high-temperature safety because side reactions occurring on the negative electrode surface of the electrolyte composition when exposed to high temperatures are minimized.

[0042] Therefore, the electrolyte composition comprises a lithium salt, an electrolyte additive, and a non-aqueous organic solvent, and the lithium salt, electrolyte additive, and non-aqueous organic solvent may have a predetermined composition.

[0043] According to one embodiment, the electrolyte additive may include an inorganic compound, and the inorganic compound may include at least one of lithium nitrate (LiNO3), lithium chloride (LiCl), lithium fluoride (LiF), lithium bromide (LiBr), lithium borate (Li3BO3), lithium carbonate (Li2CO3), lithium sulfate (Li2SO4), and lithium phosphate (Li3PO4).

[0044] As an example, the inorganic compound may include lithium nitrate (LiNO3) and / or lithium sulfate (Li2SO4).

[0045] When an inorganic compound is included as the electrolyte additive, the reactivity between the electrolyte composition and the lithium ions intercalated into and / or deintercalated from the negative electrode active material can be directly reduced, thereby reducing the heat generation amount. In addition, the inorganic compound indirectly participates in the formation of the SEI layer during the activation process of the lithium secondary battery. Therefore, an SEI layer having lithium nitride (Li3N), lithium oxide (Li2O), partially reduced lithium nitrate (LiNO x , 0 < x < 3), etc. can be provided, and this SEI layer has high lithium ion transport ability and excellent heat resistance. Then, an SEI layer with excellent heat resistance can be uniformly formed on the surface of the negative electrode.

[0046] As an example, when performing differential scanning calorimetry (DSC) analysis on the electrolyte composition containing an inorganic compound, the temperature at which the negative electrode active material starts to generate heat during high-temperature exposure (for example, the onset temperature) may be 250 °C or higher, or may be 260 °C or higher, 270 °C or higher, 250 °C to 310 °C, 260 °C to 310 °C, 270 °C to 310 °C, 265 °C to 300 °C, greater than 270 °C and equal to or less than 310 °C, greater than 270 °C and equal to or less than 300 °C, or 275 °C to 300 °C.

[0047] In addition to the inorganic compound, the electrolyte additive may further include a cyclic carbon compound. For example, the cyclic carbon compound may include at least one of vinylene carbonate (VC), 1,3-propane sultone (PS), ethylene sulfite (ESa), propylene sulfite (PSa), butylene sulfite (BSa), and fluoroethylene carbonate (FEC).

[0048] In one embodiment, the cyclic carbon compound may include vinylene carbonate (VC), 1,3-propane sultone (PS), ethylene sulfite (ESa), and / or propylene sulfite (PSa).

[0049] When the electrolyte composition contains cyclic carbon compounds in addition to inorganic compounds, the initial charge-discharge capacity of the lithium secondary battery can be increased. Furthermore, the decomposition of non-aqueous organic solvents, which constitute a large portion of the electrolyte composition, can be suppressed under high-temperature conditions. Therefore, the cycle characteristics of the lithium secondary battery can be improved while suppressing gas production.

[0050] The electrolyte composition may contain a predetermined amount of the aforementioned electrolyte additive. According to one embodiment, the content of the electrolyte additive relative to the total weight of the electrolyte composition may be greater than about 0% by weight and equal to or less than 5% by weight; or, relative to the total weight of the electrolyte composition, the content of the electrolyte additive may, for example, be greater than about 0% by weight and equal to or less than 4% by weight; greater than 0% by weight and equal to or less than 3% by weight; greater than 0% by weight and equal to or less than 2% by weight; greater than 0% by weight and equal to or less than 1% by weight; greater than 0% by weight and equal to or less than 0.9% by weight; 0.1% by weight to 5% by weight; 0.1% by weight to 4% by weight; 0.1% by weight to 3% by weight; 0.1% by weight to 2% by weight; 0.1% by weight to 1% by weight; 0.5% by weight to 4% by weight; 1% by weight to 5% by weight; 2% by weight to 4.5% by weight; 2.5% by weight to 5% by weight; 3% by weight to 5% by weight; 2% by weight to 4% by weight; or 3% by weight to 4.5% by weight.

[0051] In this invention, the viscosity of the electrolyte composition is appropriately maintained by controlling the total content of the electrolyte additives within the aforementioned range. Then, while preventing a decrease in the ionic conductivity of the electrolyte composition and a deterioration in battery performance, the deterioration of the wettability of the separator can be prevented or suppressed.

[0052] Here, when the electrolyte additive contains only inorganic compounds, the content of inorganic compounds in the electrolyte composition can be within the same range as the total content of the electrolyte additives mentioned above.

[0053] Furthermore, when the electrolyte additive contains both inorganic compounds and cyclic carbon compounds, the total content of the inorganic compounds and cyclic carbon compounds can meet the aforementioned total content range for electrolyte additives. In this case, the content of inorganic compounds and the content of cyclic carbon compounds can have a predetermined ratio.

[0054] For example, when the electrolyte additive contains inorganic compounds and cyclic carbon compounds, the content of the cyclic carbon compounds relative to 100 parts by weight of the inorganic compounds can be approximately 100 parts by weight to 1000 parts by weight. For example, the content of the cyclic carbon compounds relative to 100 parts by weight can be approximately 100 parts by weight to 900 parts by weight; 100 parts by weight to 800 parts by weight; 100 parts by weight to 700 parts by weight; 300 parts by weight to 900 parts by weight; 450 parts by weight to 900 parts by weight; 400 parts by weight to 800 parts by weight; or 500 parts by weight to 700 parts by weight.

[0055] Increased acidity in the electrolyte composition can lead to concentration polarization and potentially reduce lithium-ion transport rates. However, in this invention, by controlling the ratio of inorganic compounds to cyclic carbon compounds as described above, the increase in acidity of the electrolyte composition due to a high ratio of inorganic compounds can be prevented or suppressed. Furthermore, by controlling the ratio of inorganic compounds to cyclic carbon compounds as described above, the heat resistance and thickness uniformity of the SEI layer formed on the negative electrode surface can be sufficiently improved in this invention.

[0056] In addition, the electrolyte composition contains a non-aqueous organic solvent, which comprises a cyclic ester solvent represented by Formula 1 as a major component: [Formula 1]

[0057] In Equation 1, Is it a single or double bond? X is hydrogen, fluorine, or vinyl, and p is an integer from 1 to 5.

[0058] According to one embodiment, the cyclic ester solvent represented by Formula 1 may comprise at least one of the following cyclic ester compounds:

[0059] For electrolytes used in lithium-ion secondary batteries, in order to suppress the decomposition of the electrolyte composition on the surfaces of the positive and / or negative electrodes and improve high-temperature safety, existing technologies use fluorine-substituted non-aqueous organic solvents such as fluorinated ethylene carbonate (FEC). However, due to the low reduction potential of these fluorinated organic solvents, using them in electrolytes can lead to reduced battery capacity or deteriorated cycle characteristics.

[0060] In contrast, the cyclic ester solvents represented by Formula 1 have high dielectric constants, thus exhibiting excellent electrical properties (e.g., cycling characteristics). Furthermore, to function as an additive, inorganic compounds such as lithium nitrate (LiNO3) need to be completely dissociated in the electrolyte composition. However, due to the low solubility of inorganic compounds in carbonate solvents, they may not fully perform their function as electrolyte additives when carbonate solvents are used as the main component of non-aqueous solvents. However, because of the high solubility of inorganic compounds in the cyclic ester solvents represented by Formula 1, complete dissociation of the inorganic compounds contained in the electrolyte composition can be induced. Moreover, due to the cyclic ester solvents represented by Formula 1, external heat transfer to the negative electrode active material can be minimized when the lithium secondary battery is exposed to high temperatures. Therefore, the solvent can possess excellent characteristics that improve the high-temperature safety of lithium secondary batteries.

[0061] The content of cyclic ester solvent relative to the total weight of the non-aqueous solvent in the electrolyte composition may be about 60% by volume or more, or the content of cyclic ester solvent may be, for example, about 60% by volume or more and less than 100% by volume; 70% by volume or more and less than 100% by volume; 75% by volume or more and less than 100% by volume; 80% by volume or more and less than 100% by volume; 85% by volume or more and less than 100% by volume; 90% by volume or more and less than 100% by volume; 60% by volume to 99% by volume; 70% by volume to 99% by volume; 80% by volume to 99% by volume; 90% by volume to 99% by volume; 95% by volume to 99% by volume; 60% by volume to 80% by volume; or 70% by volume to 90% by volume.

[0062] In this invention, the dissociation of the inorganic compound can be sufficiently achieved by controlling the content of the cyclic ester solvent contained in the non-aqueous solvent as described above, and the heat transferred to the negative electrode active material can be sufficiently reduced when the lithium secondary battery is exposed to high temperatures. Furthermore, since cyclic ester solvents have excellent heat resistance, the electrolyte composition is less prone to decomposition at high temperatures when its content is controlled within the aforementioned range. Then, when combined with the predetermined lithium salt of this invention, it is advantageous to minimize the heat transferred to the negative electrode active material.

[0063] In addition to cyclic ester solvents, non-aqueous organic solvents may also include carbonate solvents. Carbonate solvents assist cyclic ester solvents, which are the main components of non-aqueous organic solvents, in controlling the dielectric constant and viscosity of the electrolyte composition. Thus, carbonate solvents can ensure high ionic conductivity of the electrolyte composition.

[0064] Furthermore, carbonate solvents can improve the low-temperature performance of electrolyte compositions and prevent side reactions and gas generation induced in the electrolyte compositions during exposure to high temperatures. Therefore, in terms of electrochemical stability for oxidation / reduction and chemical stability for reactions with heat or solutes, a carbonate solvent can be mixed alone with a cyclic ester solvent represented by Formula 1, or two or more carbonate solvents can be arbitrarily combined according to the intended use and then mixed with the cyclic ester solvent before use.

[0065] This type of carbonate solvent may include ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylene carbonate (EC), propylene carbonate (PC), and butyl carbonate (BC), which can be used alone or in combination.

[0066] When the auxiliary solvent is mixed with a cyclic ester solvent, a certain volume ratio can be achieved, allowing control over the dielectric constant and viscosity of the electrolyte composition without reducing the solubility of the inorganic compound used as an electrolyte additive. According to one embodiment, the content of the auxiliary solvent relative to the total volume of the non-aqueous organic solvent can be 40% by volume or less, or, relative to the total volume of the non-aqueous organic solvent, the content of the auxiliary solvent can, for example, be greater than about 0% by volume and equal to or less than 40% by volume; greater than 0% by volume and equal to or less than 30% by volume; greater than 0% by volume and equal to or less than 25% by volume; greater than 0% by volume and equal to or less than 20% by volume; greater than 0% by volume and equal to or less than 15% by volume; greater than 0% by volume and equal to or less than 10% by volume; 1% to 40% by volume; 1% to 30% by volume; 1% to 20% by volume; 1% to 10% by volume; 1% to 5% by volume; 20% to 40% by volume; or 10% to 30% by volume.

[0067] In this invention, the high compatibility between ester solvents and auxiliary solvents can be maintained by adjusting the content of auxiliary solvents in all non-aqueous organic solvents to the above ratio. At the same time, the charge mobility and / or ion mobility of the battery can be improved, thereby improving the battery performance.

[0068] To reduce the heat transferred to the positive electrode active material, the electrolyte composition may contain a specific lithium salt. According to one embodiment, the lithium salt may be at least one of the following: a phosphate-based lithium salt containing Li. + As a cation, and PF6 - PF4C2O4 - PF2C4O8 - (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2- (CF3)5PF - (CF3)6P - and (C2O4)2PF2 - As an anion; borate lithium salts, which contain BF₂C₂O₄ - and B(C2O4)2 - ; and sulfonylimide lithium salts, which contain (CF3SO2)2N - (FSO2)2N - (CF3CF2SO2)2N - and ((C(CN))2NC(CF3))N - Alternatively, if necessary, two or more can be used in combination.

[0069] Lithium salts essentially impart conductivity to the organic solvents constituting the electrolyte and also play a role in inducing negative electrode passivation by forming an SEI. In this invention, the lithium salt can be dissolved in cyclic ester solvents, thereby delaying the side reactions between the electrolyte composition and lithium ions deintercalated from the negative electrode during exposure to high temperatures in lithium secondary batteries. Therefore, the lithium salts used in this invention may selectively comprise two or more of the aforementioned lithium salts.

[0070] As an example, lithium salts can contain lithium hexafluorophosphate (hereinafter referred to as LiPF6) and lithium bis(trifluoromethanesulfonyl)imide (anion: (CF3SO2)2N). - (hereinafter referred to as LiFSI).

[0071] LiPF6 is a lithium phosphate salt commonly used in electrolyte compositions for lithium secondary batteries, and exhibits PF6 as an anion upon dissociation. - Furthermore, LiPF6 possesses the characteristic of imparting high conductivity to carbonate electrolytes. However, when LiPF6 is used alone as a lithium salt, it decomposes into PF5 at temperatures above approximately 200°C upon exposure to high temperatures. This can lead to an unstable state and may also reduce the high-temperature durability of the electrode due to the removal of the film located on the surface of the positive electrode active layer. This film is derived from organic matter (e.g., solvents) and the electrode active material. Therefore, in this embodiment, not only LiPF6 is used as the lithium salt, but two or more lithium salts are used as described above.

[0072] Furthermore, LiFSI, as a sulfonylimide lithium salt, has a high decomposition temperature in carbonate solvents, thus exhibiting high safety at high temperatures. Moreover, due to its high water resistance, the amount of hydrogen fluoride (HF) generated is minimal even when LiFSI comes into contact with moisture. However, the sulfonylimide groups of LiFSI themselves have a significantly high reactivity with aluminum, which can lead to corrosion of the positive electrode current collector when used in electrolyte compositions. For example, when the inorganic additives of this invention are excluded, the heat flux transferred to the negative electrode active material is high. Therefore, there are limitations in its use in electrolyte compositions.

[0073] When LiPF6 is used in conjunction with LiFSI, a significant improvement in the relatively low high-temperature safety of LiPF6 is achieved. Furthermore, the electrolyte composition can significantly reduce the heat flux transferred to the negative electrode active material.

[0074] Here, in the lithium salts used together, when the phosphate-based lithium salt is referred to as the first lithium salt and the sulfonylimide-based lithium salt as the second lithium salt, the first and second lithium salts can have a predetermined mixing ratio. For example, based on the molar concentration (M), the mixing ratio of the first and second lithium salts can be approximately 1:0.1 to 1.0, or for example, approximately 1:0.4 to 0.9 or 1:0.6 to 0.8. In this invention, by controlling the ratio of the first and second lithium salts as described above, the corresponding disadvantages of the lithium salts can be minimized while maximizing the effect of reducing heat flux in the electrolyte composition.

[0075] To maintain the inherent properties of lithium salts and improve high-temperature safety, the concentration of these lithium salts can meet predetermined requirements. For example, the lower limit of the lithium salt concentration can be above approximately 0.5 mol / L, such as above approximately 0.7 mol / L or above approximately 0.9 mol / L; and the upper limit can be below approximately 2.5 mol / L, below approximately 2.0 mol / L or below approximately 1.5 mol / L. By maintaining the lithium salt concentration within the above range, the ionic conductivity can be appropriately maintained, and the cycle characteristics and output characteristics of the non-aqueous electrolyte battery can be improved. Furthermore, by maintaining the lithium salt concentration within the above range, the viscosity of the electrolyte used in the non-aqueous electrolyte battery is maintained at an appropriate value. Then, the ionic conductivity can be appropriately maintained without decreasing, and the cycle characteristics and output characteristics of the non-aqueous electrolyte battery can be improved.

[0076] Furthermore, when a large amount of lithium salt is dissolved in a non-aqueous organic solvent at once, the liquid temperature may rise in some cases due to the heat of solution of the lithium salt. Consequently, when the temperature of the non-aqueous organic solvent rises significantly due to the amount of lithium salt dissolved, decomposition is promoted in the case of fluorine-containing lithium salts, resulting in the production of hydrogen fluoride (HF), which degrades battery performance. Therefore, there is no particular limitation on the temperature at which lithium salts dissolve in non-aqueous organic solvents, but it can be adjusted to approximately -20°C to 80°C, or, for example, approximately 0°C to 60°C.

[0077] Because the electrolyte composition of the present invention has the above-described composition, an SEI layer can be uniformly formed on the negative electrode surface when the lithium secondary battery is activated. This minimizes side reactions with the electrolyte composition occurring on the negative electrode surface, thereby improving the high-temperature safety of the lithium secondary battery.

[0078] As an example, when measuring the heat flow of a mixture containing a negative electrode active material and an electrolyte composition of a lithium secondary battery in a weight ratio of approximately 1:0.5 at 1:0.5, the heat flow of the electrolyte composition in the range of 250°C to 350°C can be less than approximately 30.0 W / g. For example, in the heat flow measurement, the heat flow of the electrolyte composition in the range of 250°C to 350°C can be 5 W / g to 25 W / g; 5 W / g to 20 W / g; 10 W / g to 25 W / g; 15 W / g to 25 W / g; 17 W / g to 23 W / g; or 19 W / g to 22 W / g.

[0079] The negative electrode active material can be a carbon-based negative electrode active material commonly used as a negative electrode active material in lithium secondary batteries. For example, the negative electrode active material can include at least one of natural graphite, artificial graphite, expanded graphite, non-graphitizable carbon, acetylene black, and Ketjen black.

[0080] "Heat flow" represents the amount of heat flow per unit weight and can indicate, for example, the degree of heat generation in a secondary battery. As a result of analyzing the electrolyte composition of the present invention using thermal analysis, the electrolyte composition of the present invention exhibits a relatively low heat flow compared to conventional electrolyte compositions. Thermal analysis simulates the degradation of a secondary battery when it is exposed to high temperatures due to external mechanical factors or spontaneous heat generation. Here, the heat flow measured according to the thermal analysis results indicates the degree of heat (e.g., heat) generated in the simulation of high-temperature degradation of a secondary battery due to the high-temperature reaction between the electrolyte composition and the negative electrode active material of a charged lithium secondary battery. The heat flow can increase or decrease depending on the type or content ratio of the components constituting the electrolyte composition, and can increase or decrease depending on whether an inorganic negative electrode film containing lithium ions is formed on the surface of the negative electrode active layer. A decrease in heat flow indicates a decrease in heat generation between the negative electrode active material and the electrolyte composition. For example, a decrease in heat indicates a reduced degree of reaction between lithium ions embedded in the negative electrode active material and the electrolyte composition, thus reducing heat generation. The results show that the reactivity of the electrolyte composition with lithium ions in the negative electrode active material decreases under high temperature conditions, thus indicating that the thermal safety of lithium secondary batteries is improved.

[0081] Lithium secondary battery

[0082] According to one embodiment of the present invention, a lithium secondary battery is provided, comprising: An electrode assembly comprising a positive electrode, a negative electrode, and a diaphragm disposed between the positive and negative electrodes; and The present invention also includes the electrolyte composition described above, which impregnates the electrode assembly. A lithium secondary battery according to one embodiment of the present invention may further include a casing, and may be manufactured in, for example, a prismatic, pouch, coin, or cylindrical shape, depending on the manufacturing process.

[0083] The lithium secondary battery of the present invention comprises an electrode assembly and an electrolyte composition impregnated with the electrode assembly. The electrode assembly has a structure in which a separator is disposed between alternately stacked positive and negative electrodes.

[0084] The electrolyte composition can uniformly form an SEI layer with high lithium-ion conductivity and excellent heat resistance on the negative electrode surface during the activation of a lithium secondary battery.

[0085] Therefore, a lithium secondary battery containing the electrolyte composition has a significantly reduced reactivity between the negative electrode active material (e.g., lithium ions intercalated into the negative electrode active material) and the electrolyte composition. As a result, the temperature at which heat generation starts between the negative electrode active material and the electrolyte composition (e.g., heat generation start temperature) increases, and then heat generation caused by, for example, deterioration of the negative electrode can be suppressed. In addition, when a lithium secondary battery containing the electrolyte composition is exposed to high temperatures, side reactions occurring on the surface of the negative electrode by the electrolyte composition can be minimized, and thus, it has the advantage of excellent high-temperature safety.

[0086] The lithium secondary battery includes an electrode assembly and an electrolyte composition impregnating the electrode assembly. Here, since the electrolyte composition has the same composition as described above, a detailed description thereof will be omitted.

[0087] Hereinafter, each component of the electrode assembly will be described in detail.

[0088] The electrode assembly includes a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode.

[0089] Here, the positive electrode includes a positive electrode active layer provided on at least one surface of a positive electrode current collector. The positive electrode active layer contains a lithium metal oxide represented by Formula 2 and allowing reversible intercalation / deintercalation of lithium ions as a main component, which is a material capable of achieving electroactivity by causing an electrochemical reaction on the positive electrode current collector: [Formula 2] Li x [Ni y Co z Mn w M 1 v O2 In Formula 2, M 1 is at least one element selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and x, y, z, w, and v respectively satisfy about 0.9 ≤ x ≤ 1.30, about 0.6 ≤ y < 1, about 0 < z ≤ 0.2, about 0 < w ≤ 0.2, and about 0 ≤ v ≤ 0.1, and y + z + w + v = 1.

[0090] The lithium metal oxide represented by Formula 2 is an oxide in which lithium (Li) is mixed with nickel (Ni), cobalt (Co), and manganese (Mn) as transition metals, and the content of nickel (Ni) is about 60% or more (e.g., 60 mol% or more) of the total molar fraction of the transition metals.

[0091] The three-component NCM cathode active material with nickel (Ni), cobalt (Co) and manganese (Mn) as the main components has the advantages of high capacity of LiNiO2 (LNO), excellent electrochemical performance of LiCoO2 (LCO) and stability of LiMn2O4 (LMO).

[0092] Regarding positive electrode active materials, LiNi can be mentioned. 0.95 Co 0.03 Mn 0.02 O2, LiNi 0.9 Co 0.05 Mn 0.05 O2, LiNi 0.9 Co 0.06 Mn 0.04 O2, LiNi 0.85 Co 0.1 Mn 0.05 O2, LiNi 0.85 Co 0.05 Mn 0.1 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.75 Co 0.1 Mn 0.15 O2, LiNi 0.7 Co 0.2 Mn 0.1 O2, LiNi 0.7 Co 0.15 Mn 0.15 O2, LiNi 0.7 Co 0.1 Mn 0.2 O2, LiNi 0.7 Co 0.05 Mn 0.25 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.6 Co 0.2 Mn 0.1 Al 0.1 O2, LiNi 0.6 Co 0.2 Mn 0.15 Al 0.05 O2 and LiNi 0.7 Co 0.1 Mn 0.1 Al 0.1 At least one of O2.

[0093] Furthermore, relative to the total weight of the positive electrode active layer, the content of the positive electrode active material can be approximately 85 parts by weight or more, and the content of the positive electrode active material can be, for example, approximately 90 parts by weight or more, 93 parts by weight or more, or 95 parts by weight or more.

[0094] If necessary, in addition to the positive electrode active material, the positive electrode active layer may also contain conductive materials, binders and other additives.

[0095] Conductive materials are used to improve the electrical properties of the positive electrode, and those commonly used in the art can be applied. Specifically, the conductive material may comprise at least one selected from graphite, such as natural graphite and artificial graphite; carbon black, such as acetylene black, channel black, furnace black, lamp black, and thermally cracked carbon black; graphene; and carbon nanotubes.

[0096] Furthermore, relative to the total weight of the positive electrode active layer, the content of conductive material can be approximately 0.1 parts by weight to 5 parts by weight, or the content of conductive material can be, for example, approximately 0.1 parts by weight to 4 parts by weight; 2 parts by weight to 4 parts by weight; 1.5 parts by weight to 5 parts by weight; 0.5 parts by weight to 3.5 parts by weight; 1 part by weight to 3 parts by weight; 0.1 parts by weight to 2.5 parts by weight; 0.1 parts by weight to 2 parts by weight; or 0.1 parts by weight to 1 part by weight.

[0097] Furthermore, the adhesive serves to bond the positive electrode active material, positive electrode additive, and conductive material together, and any adhesive can be used without particular limitation as long as it has this function. For example, the adhesive may contain at least one resin selected from polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVdF), polyacrylonitrile, polymethyl methacrylate, and copolymers thereof. As an example, the adhesive may contain polyvinylidene fluoride (PVdF).

[0098] In addition, relative to the total weight of the positive electrode active layer, the content of the binder can be from about 0.1 parts by weight to 5 parts by weight, or the content of the binder can be, for example, from about 0.1 parts by weight to 4 parts by weight; 2 parts by weight to 4 parts by weight; 1.5 parts by weight to 5 parts by weight; 0.5 parts by weight to 3.5 parts by weight; 1 part by weight to 3 parts by weight; 0.1 parts by weight to 2.5 parts by weight; 0.1 parts by weight to 2 parts by weight; or from about 0.1 parts by weight to 1 part by weight.

[0099] There is no particular limitation on the total thickness of the positive electrode active layer, but it can be, for example, about 50 μm to 300 μm, or about 100 μm to 200 μm; 80 μm to 150 μm; 120 μm to 170 μm; 150 μm to 300 μm; 200 μm to 300 μm; or 150 μm to 190 μm.

[0100] For the positive electrode current collector, a material with high conductivity can be used without causing chemical changes in the corresponding battery. For example, stainless steel, aluminum, nickel, titanium, or calcined carbon can be used, and in the case of aluminum or stainless steel, the aforementioned materials surface-treated with carbon, nickel, titanium, or silver can also be used. Furthermore, regarding the average thickness of the current collector, considering the conductivity and total thickness of the positive electrode to be manufactured, a thickness of approximately 3 μm to 500 μm can be appropriately applied.

[0101] Similar to the positive electrode, the negative electrode includes a negative electrode active layer containing negative electrode active material on at least one side of the negative electrode current collector.

[0102] In order to achieve electroactivity through reversible redox reactions during the charging and discharging process of the battery, the negative electrode active layer contains carbon-based negative electrode active materials as negative electrode active materials.

[0103] Carbon-based anode active materials refer to materials whose main component is carbon atoms, and these carbon-based anode active materials can include graphite. Graphite can include at least one of natural graphite and artificial graphite.

[0104] As an example, carbon-based negative electrode active materials may comprise mixed graphite, which is a mixture of natural graphite and artificial graphite. In this case, the mixed graphite may be a mixture obtained by mixing natural graphite and artificial graphite in a weight ratio of about 10 to 50:50 to 90 or about 10 to 30:70 to 90. By adjusting the content ratio of natural graphite to artificial graphite in the mixed graphite as described above, it is possible to prevent or suppress the decrease in adhesion between the negative electrode current collector and the negative electrode active layer due to the natural graphite content being less than about 10 parts by weight relative to the total weight, and it is possible to prevent or suppress the decrease in the charge / discharge capacity of the negative electrode due to the natural graphite content being greater than 50 parts by weight.

[0105] The shape of carbon-based anode active materials is not particularly limited, but can be the shape of spherical graphite assemblies formed by the aggregation of flake graphite. Examples of flake graphite can include not only natural and artificial graphite, but also materials obtained by graphitizing mesophase calcined carbon (bulk mesophase) using coal tar pitch as a raw material, as well as coke (e.g., coke, green coke, pitch coke, needle coke, and petroleum coke). For example, carbon-based anode active materials can be assembled using multiple highly crystalline natural graphites. Furthermore, a graphite assembly can be formed by aggregating approximately 2 to 100 flake graphites (e.g., approximately 3 to 20 graphites).

[0106] Furthermore, relative to the total weight of the negative electrode active layer, the content of the negative electrode active material can be approximately 85 parts by weight or more, or for example, approximately 90 parts by weight or more, 93 parts by weight or more, or 95 parts by weight or more.

[0107] If necessary, in addition to the carbon-based anode active material as the main component, the anode active layer of the present invention may optionally further include conductive materials, binders and other additives.

[0108] The conductive material may include, but is not limited to, carbon black, such as acetylene black and Ketjen black; carbon nanotubes; and at least one of carbon fibers.

[0109] As an example, the negative electrode active layer may contain only carbon black, carbon nanotubes or carbon fibers as conductive materials, or a combination thereof.

[0110] The content of the conductive material can be from about 0.1 parts by weight to 10 parts by weight of the entire negative electrode active layer, or for example, from about 0.1 parts by weight to 8 parts by weight, 0.1 parts by weight to 5 parts by weight, 0.1 parts by weight to 3 parts by weight, 2 parts by weight to 6 parts by weight, or from about 0.5 parts by weight to 2 parts by weight. In this invention, by controlling the content of the conductive material within the above range, an increase in the resistance of the negative electrode can be prevented or suppressed, and a decrease in charging capacity due to a low content of conductive material can be prevented or suppressed. Then, the problem of reduced charging capacity due to a decrease in the content of the negative electrode active material caused by excessive conductive material, or the problem of reduced fast charging characteristics due to an increase in the loading of the negative electrode active layer, can be prevented or suppressed.

[0111] Furthermore, the adhesive is a component that facilitates the adhesion of the negative electrode active material to conductive materials and to the current collector, and can be appropriately applied within a range that does not degrade the electrical properties of the electrode. For example, the adhesive may contain at least one selected from polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVdF), polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber.

[0112] Relative to 100 parts by weight of the total negative electrode active layer, the binder content can be approximately 0.1 parts by weight to 10 parts by weight, or for example, approximately 0.1 parts by weight to 8 parts by weight, 0.1 parts by weight to 5 parts by weight, 0.1 parts by weight to 3 parts by weight, or 2 parts by weight to 6 parts by weight. In this invention, by controlling the binder content in the negative electrode active layer within the above ranges, it is possible to prevent or suppress a decrease in the adhesion of the active layer due to a low binder content, or to prevent or suppress a decrease in the electrical performance of the electrode due to an excessive amount of binder.

[0113] The average thickness of the negative electrode active layer can be approximately 100 μm to 300 μm, or for example, approximately 100 μm to 250 μm; 100 μm to 250 μm; or approximately 130 μm to 190 μm. In this invention, by controlling the average thickness of the negative electrode active layer within the above range, the crystal planes of the carbon-based negative electrode active material contained in each region can be uniformly arranged. This can improve the high-rate charge-discharge performance and energy density of the battery containing this negative electrode.

[0114] Furthermore, there are no particular limitations on the negative electrode current collector, as long as it has high conductivity without causing chemical changes in the corresponding battery. For example, copper, stainless steel, nickel, titanium, or calcined carbon can be used, and in the case of copper or stainless steel, the aforementioned materials surface-treated with carbon, nickel, titanium, or silver can also be used. Moreover, considering the conductivity and total thickness of the negative electrode to be manufactured, the average thickness of the negative electrode current collector can appropriately be approximately 1 μm to 500 μm.

[0115] Meanwhile, the separator disposed between the positive and negative electrodes of each cell is an insulating film with high ion permeability and mechanical strength, and there are no particular limitations, as long as it is a separator commonly used in the art. However, for example, a separator comprising at least one polymer selected from chemically resistant and hydrophobic polypropylene, polyethylene, and polyethylene-propylene copolymer can be used. The separator can be in the form of a porous polymer substrate (e.g., sheet or nonwoven fabric) comprising the above-mentioned polymers, and in some cases, it can be in the form of a composite separator in which organic or inorganic particles are coated onto a porous polymer substrate by an organic adhesive. Furthermore, the average pore size of the separator can be from about 0.01 μm to 10 μm, and the average thickness can be from about 5 μm to 300 μm.

[0116] Furthermore, the lithium secondary battery of the present invention is not particularly limited, but can be used in various forms such as cylindrical, prismatic, pouch, or coin-shaped, depending on the performance purpose. One embodiment of the lithium secondary battery of the present invention can be a pouch-shaped secondary battery.

[0117] Method for manufacturing electrolyte composition for lithium secondary battery

[0118] In addition, the present invention provides a method for manufacturing the above-described electrolyte composition.

[0119] Specifically, the method for manufacturing the electrolyte composition includes the steps of providing a lithium salt, an electrolyte additive, and a non-aqueous organic solvent.

[0120] The electrolyte additive contains an inorganic compound, and the non-aqueous organic solvent contains about 60% by volume and less than 100% by volume a cyclic ester solvent represented by Formula 1 below. Furthermore, when the heat flow of a mixture comprising the negative electrode active material of a lithium secondary battery in a 100% state of charge and the electrolyte composition is measured at a weight ratio of about 1:0.5, the heat flow of the electrolyte composition in the range of about 250°C to 350°C is about 30.0 W / g or less. [Formula 1]

[0121] In Equation 1, Is it a single or double bond? X is hydrogen, fluorine, or vinyl, and p is an integer from 1 to 5.

[0122] The electrolyte composition manufactured by the above-described method can uniformly form a solid electrolyte interphase (SEI) layer with high lithium-ion conductivity and excellent heat resistance on the negative electrode surface during the activation process of a lithium secondary battery. Therefore, the reactivity between the negative electrode active material and the electrolyte composition is significantly reduced, increasing the temperature at which heat generation begins between the negative electrode active material and the electrolyte composition (i.e., the exothermic initiation temperature), thereby suppressing exothermic reactions caused by, for example, negative electrode degradation. Furthermore, when a lithium secondary battery containing this electrolyte composition is exposed to high temperatures, side reactions occurring on the negative electrode surface of the electrolyte composition can be minimized, thus exhibiting excellent high-temperature safety.

[0123] In the method for manufacturing the electrolyte composition, since the components and contents constituting the electrolyte additive, the type of cyclic ester solvent represented by Formula 1 contained in the non-aqueous organic solvent, etc. are the same as those previously described for the electrolyte composition, their detailed description will be omitted.

[0124] The invention will be described in more detail below with reference to examples and experimental cases.

[0125] However, the following embodiments and experimental examples are merely illustrative of the present invention, and the content of the present invention is not limited to the following embodiments and experimental examples.

[0126] Examples 1 to 11 and Comparative Examples 1 to 9. Preparation of electrolyte compositions for lithium secondary batteries

[0127] For non-aqueous solvents, prepare dihydrofuranone (DHF), fluorodihydrofuranone (FDHF), ethylene carbonate (EC), ethyl methyl carbonate (EMC), and fluoroethylene carbonate (FEC), and prepare lithium salts LiPF6, LiBF4, and LiFSI.

[0128] In addition, for electrolyte additives, inorganic compounds such as lithium nitrate (LiNO3) and lithium sulfate (Li2SO4) are prepared, and for additional electrolyte additives, LIBOB, LiODFB, LiDFOP, vinylene carbonate (VC), 1,3-propane sulpholactone (PS), ethylene sulfate (ESa) and fluoroethylene carbonate (FEC) are prepared.

[0129] The prepared lithium salts were added to a non-aqueous organic solvent at a temperature of 30°C to 40°C to meet the molar concentrations (M) shown in Table 1. Electrolyte additives were then added to prepare the electrolyte composition.

[0130] The types of non-aqueous organic solvents, lithium salts, and electrolyte additives used in the electrolyte composition are shown in Table 1. Furthermore, the content of each component is adjusted as shown in Table 1 below. The content ratio of each component of the non-aqueous organic solvent is adjusted based on the total volume of the non-aqueous organic solvent, and the electrolyte additives are adjusted based on the total weight of the electrolyte composition.

[0131] [Table 1]

[0132] Examples 12-22 and Comparative Examples 10-18. Manufacturing of lithium secondary batteries

[0133] Prepare LiNi with a particle size of 5 μm 0.86 Co .05 Mn .07 Al 0.02 O2, used as the positive electrode active material, is mixed with carbon-based conductive materials and polyvinylidene fluoride (PVDF) as a binder in N-methylpyrrolidone (NMP) at a weight ratio of 94:3:3 to prepare a positive electrode slurry with a solid content of 45%. The prepared positive electrode slurry is cast onto an aluminum sheet, dried in a vacuum oven at 120°C, and then calendered to prepare a positive electrode with a positive electrode active layer of 160 μm thickness.

[0134] Furthermore, a carbon-based negative electrode active material was prepared by mixing natural graphite and artificial graphite in a weight ratio of 3:7. 97 parts by weight of the prepared negative electrode active material and 3 parts by weight of styrene-butadiene rubber (SBR) were mixed with water to prepare a negative electrode slurry with a solids content of 40%. The negative electrode slurry was cast onto a copper sheet, dried in a vacuum oven at 130°C, and then calendered to prepare a negative electrode with a negative electrode active layer of 180 μm thickness.

[0135] A separator made of 18 μm polypropylene was placed between the obtained positive and negative electrodes, and then placed in a casing. Then, as shown in Table 2 below, the electrolyte compositions previously prepared in Examples 1 to 11 and Comparative Examples 1 to 9 were injected to assemble the lithium secondary battery.

[0136] Each assembled lithium secondary battery was initially charged. Specifically, under the conditions shown in Table 2 below, the lithium secondary batteries were initially charged at 55±2°C to a charging termination voltage of 4.2 V. Then, activated lithium secondary batteries were manufactured.

[0137] [Table 2]

[0138] Experimental Example

[0139] The following experiments were conducted to evaluate the high-temperature safety of the electrolyte composition for lithium secondary batteries and the lithium secondary batteries containing the electrolyte composition of the present invention.

[0140] 1) Evaluation of electrolyte additive solubility

[0141] The solubility of inorganic additives contained in the electrolyte compositions prepared in Examples 1 to 11 and Comparative Examples 1 to 9 was evaluated. Specifically, each electrolyte composition was filtered under reduced pressure using filter paper. Subsequently, it was determined whether any inorganic additives remained on the filter paper.

[0142] As a result, it was found that in electrolyte compositions containing dihydrofuranone (DHF) as a non-aqueous organic solvent, inorganic additives did not remain on the filter paper. However, in electrolyte compositions without dihydrofuranone (DHF), most of the inorganic additives remained, insoluble in the non-aqueous organic solvent.

[0143] This means that inorganic additives such as lithium nitrate and lithium sulfate have high solubility in cyclic ester solvents represented by Formula 1, but low solubility in carbonate organic solvents commonly used in electrolyte compositions.

[0144] 2) Measurement of heat generation onset temperature and heat flow between negative electrode active material and electrolyte composition

[0145] The lithium secondary batteries manufactured in Examples 12 to 22 and Comparative Examples 10 to 18 were fully charged to 4.25V at 25°C under CC-CV conditions at a rate of 0.5 C, and the fully charged lithium secondary batteries were disassembled. In the disassembled lithium secondary batteries, a mixed graphite and electrolyte composition, which served as the negative electrode active material, were weighed and mixed at a weight ratio of 1:0.5.

[0146] The prepared mixture was injected into the sample vessel of a differential scanning calorimeter (DSC) to prepare a sample. The heat generation onset temperature and heat flux of the prepared sample were then measured. Here, the temperature change and heat flux of the sample were measured within the range of 100°C to 400°C while the temperature was increased at a heating rate of 10 ± 0.1°C / min.

[0147] Based on the measurement results, the heat flow between the negative electrode active material and the electrolyte composition in each lithium secondary battery was evaluated. In the measurement results, if more than two effective heat flow peaks exist within the temperature range of 100℃ to 400℃, the peak with the largest heat flow value is identified as the corresponding heat flow. The results are shown in Table 3 below. Figure 1 As shown in the image.

[0148] 3) Thermal runaway test evaluation

[0149] The lithium secondary batteries manufactured in Examples 12 to 22 and Comparative Examples 10 to 18 were each charged at 25°C under a constant current (CC) of 1.25 A until 4.2V, and then maintained at 4.2V. After charging was completed, the batteries were discharged and activated under a constant current (CC) of 1.25 A until 2.85V. Thereafter, each activated lithium secondary battery was left to stand at 25°C for 6 hours.

[0150] Next, the batteries were fully charged to 4.2V at 25°C under a constant current (CC) of 1.25 A, and thermal runaway testing was performed using an accelerated calorimeter (ARC). For the ARC, a product from THT (ThermalHazard Technology) was used, and the thermal runaway test was performed using the Heated-Wait-Search (HWS) method. In the HWS method, an adiabatic furnace chamber equipped with each lithium-ion secondary battery was heated by 5°C at a rate of 10°C / min and held for 10 minutes until its temperature increased from 50°C to 190°C. During the 10-minute standby period of the lithium-ion secondary batteries, when each heated lithium-ion secondary battery exhibited a temperature change greater than 0.02°C / min, it was identified as undergoing self-heating corresponding to thermal runaway (TR). Thereafter, no additional heat energy was applied, allowing the temperature to change due to the self-heating of each secondary battery. After the secondary battery underwent self-heating, the self-heating curve of the secondary battery was measured, and the maximum temperature of the secondary battery was calculated from the measured self-heating curve. The results are shown in Table 3 below.

[0151] [Table 3]

[0152] Therefore, it can be seen that the electrolyte composition for lithium secondary batteries and the lithium secondary batteries containing the electrolyte composition of the present invention have excellent high-temperature safety.

[0153] Referring to Table 3, it was found that in the case of the embodiments, the heat generation onset temperature, which begins to generate heat due to the reaction between the negative electrode active material and the electrolyte composition during high-temperature exposure, was approximately 265°C to 300°C. Meanwhile, in the case of the comparative examples, the heat generation onset temperature was approximately 170°C to 275°C, significantly lower than the aforementioned values. Furthermore, it was found that in the case of the embodiments, the heat flux was approximately 22 W / g to 30 W / g, while in the case of the comparative examples, the heat flux was approximately 30 W / g to 60 W / g, greater than the aforementioned values. Moreover, in the thermal runaway test, it was found that in the case of the embodiments, the maximum temperature was approximately 245°C to 282.5°C, while in the case of the comparative examples, the maximum temperature was 310.7°C to 521°C, much higher than the aforementioned values.

[0154] Reference Figure 1 It was found that the electrolyte composition of Example 2, shown by the dashed line, had a low heat flux of approximately 20 W / g, and the electrolyte composition of Comparative Example 9, shown by the solid line, had a heat flux of approximately 60 W / g.

[0155] This means that the electrolyte composition prepared in the examples effectively inhibits the reaction between the negative electrode active material and the electrolyte composition during high-temperature exposure, thereby improving the heat generation caused by the reaction between the negative electrode active material and the electrolyte composition.

[0156] Based on these results, it can be found that the electrolyte composition for lithium secondary batteries of the present invention has excellent electrical performance. Furthermore, controlling the heat flow between the negative electrode active material and the electrolyte composition to fall within a predetermined low range has an excellent effect on improving safety issues caused by the negative electrode active material at high temperatures.

[0157] Although the foregoing has been described with reference to embodiments of the invention, those skilled in the art will understand that various modifications and alterations can be made to the invention without departing from the technical scope of the various embodiments of the invention as described in the claims which are described below.

[0158] Therefore, the scope of the present invention is not limited to the contents described in the detailed description of the present invention, but should be defined by the claims.

Claims

1. An electrolyte composition for a lithium secondary battery, the electrolyte composition comprising: a lithium salt, an electrolyte additive, and a non-aqueous organic solvent, the electrolyte additive comprising an inorganic compound, the non-aqueous organic solvent comprising 60% by volume or more and less than 100% by volume of a cyclic ester-based solvent represented by Formula 1 below, and the electrolyte composition having a heat flow of 30.0 W / g or less in a range of 250°C to 350°C when a mixture comprising a negative electrode active material of a lithium secondary battery in a 100% charged state and the electrolyte composition at a weight ratio of 1:0.5 is subjected to a heat flow measurement: [Formula 1] wherein, X is hydrogen, a fluoro group, or a vinyl group, and p is an integer of 1 to 5. the heat flow is 5 W / g to 25 W / g. wherein the inorganic compound comprises at least one of lithium nitrate (LiNO3), lithium chloride (LiCl), lithium fluoride (LiF), lithium bromide (LiBr), lithium borate (Li3BO3), lithium carbonate (Li2CO3), lithium sulfate (Li2SO4), and lithium phosphate (Li3PO4). the content of the electrolyte additive is greater than 0% by weight and equal to or less than 5% by weight with respect to the total weight of the electrolyte composition. 5.The electrolyte composition for a lithium secondary battery according to claim 1, further comprising at least one cyclic carbon compound of vinylene carbonate (VC), 1,3-propane sultone (PS), ethylene sulfate (ESa), propylene sulfate (PSa), butylene sulfate (BSa), and fluoroethylene carbonate (FEC). the content of the cyclic carbon compound is 100 parts by weight to 1000 parts by weight with respect to 100 parts by weight of the inorganic compound. the cyclic ester-based solvent represented by Formula 1 comprises at least one of dihydrofuranone, vinyl dihydrofuranone, fluoro dihydrofuranone, furanone, tetrahydropyranylone, methyl dihydrofuranone, propyl tetrahydropyranylone, and oxepanone. is a single or double bond, the non-aqueous organic solvent further comprises at least one carbonate-based solvent of ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC). the content of the carbonate-based solvent is greater than 0% by volume and equal to or less than 40% by volume with respect to the total weight of the non-aqueous organic solvent.

2. The electrolyte composition for a lithium secondary battery according to claim 1, wherein, the lithium salt comprises:

3. The electrolyte composition for a lithium secondary battery according to claim 1, wherein, the negative electrode active material for the heat flow measurement comprises at least one of natural graphite, artificial graphite, expanded graphite, hard graphitizable carbon, acetylene black, and ketjen black.

4. The electrolyte composition for a lithium secondary battery according to claim 1, wherein, 12.A lithium secondary battery comprising: an electrode assembly comprising a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode; and the electrolyte composition of claim 1 impregnating the electrode assembly. the positive electrode comprises a positive electrode active layer disposed on at least one side of a positive electrode current collector and comprising a positive electrode active material represented by Formula 2: [Formula 2] wherein, 6. The electrolyte composition for a lithium secondary battery according to claim 5, wherein, ​ 7. The electrolyte composition for a lithium secondary battery according to claim 1, wherein, ​ 8. The electrolyte composition for a lithium secondary battery according to claim 1, wherein, ​ 9. The electrolyte composition for a lithium secondary battery according to claim 8, wherein, ​ 10. The electrolyte composition for a lithium secondary battery according to claim 1, wherein, ​ Li + as cations, and PF6 - , PF4C2O4 - , PF2C4O8 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , (C2O4)2PF2 - , BF2C2O4 - , B(C2O4)2 - , (CF3SO2)2N - , (FSO2)2N - , (CF3CF2SO2)2N - , and ((C(CN))2NC(CF3))N - as an anion.

11. The electrolyte composition for a lithium secondary battery according to claim 1, wherein, ​ ​ ​ ​ 13. The lithium secondary battery as claimed in claim 12, wherein, ​ ​ Li x [Ni y Co z Mn w M 1 v ]O2 ​ M 1 is at least one element selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and x, y, z, w, and v satisfy 0.9 ≤ x ≤ 1.30, 0.6 ≤ y < 1, 0 < z ≤ 0.2, 0 < w ≤ 0.2, and 0 ≤ v ≤ 0.1, and y + z + w + v = 1.

14. A method for manufacturing an electrolyte composition for a lithium secondary battery, the method comprising: providing a lithium salt, an electrolyte additive, and a non-aqueous organic solvent, wherein the electrolyte additive includes an inorganic compound, the non-aqueous organic solvent includes 60% by volume or more and less than 100% by volume of a cyclic ester-based solvent represented by Formula 1 below, and when a mixture including a negative active material of a lithium secondary battery in a 100% charged state and the electrolyte composition at a weight ratio of 1:0.5 is subjected to heat flow measurement, the electrolyte composition has a heat flow of 30.0 W / g or less in a range of 250°C to 350°C: [Formula 1] wherein, is a single or double bond, X is hydrogen, a fluoro group, or a vinyl group, and p is an integer of 1 to 5.

15. The method for manufacturing an electrolyte composition for a lithium secondary battery according to claim 14, wherein the heat flow is 5 W / g to 25 W / g.

16. The method for manufacturing an electrolyte composition for a lithium secondary battery according to claim 14, wherein the inorganic compound includes at least one of lithium nitrate (LiNO3), lithium chloride (LiCl), lithium fluoride (LiF), lithium bromide (LiBr), lithium borate (Li3BO3), lithium carbonate (Li2CO3), lithium sulfate (Li2SO4), and lithium phosphate (Li3PO4).

17. The method for manufacturing an electrolyte composition for a lithium secondary battery according to claim 14, wherein The content of the electrolyte additive is greater than 0% by weight and equal to or less than 5% by weight with respect to the total weight of the electrolyte composition.

18. The method for manufacturing an electrolyte composition for a lithium secondary battery according to claim 14, wherein, The electrolyte additive further includes at least one cyclic carbon compound of vinylene carbonate (VC), 1,3-propane sultone (PS), ethylene sulfate (ESa), propylene sulfate (PSa), butylene sulfate (BSa), and fluoroethylene carbonate (FEC).

19. The method for manufacturing an electrolyte composition for a lithium secondary battery according to claim 18, wherein The content of the cyclic carbon compound is 100 parts by weight to 1000 parts by weight with respect to 100 parts by weight of the inorganic compound.

20. The method for manufacturing an electrolyte composition for a lithium secondary battery according to claim 14, wherein The cyclic ester-based solvent represented by Formula 1 includes at least one of dihydrofuranone, vinyl dihydrofuranone, fluoro dihydrofuranone, furanone, tetrahydropyranyl ketone, methyl dihydrofuranone, propyl tetrahydropyranyl ketone, and oxepanone.

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