KR102260832B1 - Electrolyte of rechargeable lithium battery and rechargeable lithium battery including same - Google Patents

Abstract

(상기 화학식 1에서, 각 치환기의 정의는 상세한 설명과 동일하다)An electrolyte for a lithium secondary battery and the same relates to a lithium secondary battery, wherein the electrolyte includes a non-aqueous organic solvent, a lithium salt, and an additive represented by the following formula (1).
[Formula 1]

(In Formula 1, the definition of each substituent is the same as in the detailed description)

Description

Electrolyte for a lithium secondary battery and a lithium secondary battery comprising the same {ELECTROLYTE OF RECHARGEABLE LITHIUM BATTERY AND RECHARGEABLE LITHIUM BATTERY INCLUDING SAME}

It relates to an electrolyte for a lithium secondary battery and a lithium secondary battery including the same.

Lithium secondary batteries have a high discharge voltage and high energy density, attracting attention as a power source for various electronic devices.

As a cathode active material for a lithium secondary battery, LiCoO ₂ , LiMn ₂ O ₄ , LiNi _1-x Co _x O ₂ (0 < x < 1) Lithium having a structure that allows intercalation of lithium ions and a transition metal, such as Oxides are mainly used.

As an anode active material, various types of carbon-based materials including artificial, natural graphite, and hard carbon capable of insertion/desorption of lithium are mainly used.

An organic solvent in which a lithium salt is dissolved is used as an electrolyte for a lithium secondary battery.

When the lithium secondary battery is stored at a high temperature, there may be a problem in that the thickness of the battery is significantly expanded due to gas generation.

One embodiment is to provide an electrolyte for a lithium secondary battery capable of suppressing the generation of gas during high-temperature storage, and consequently suppressing the thickness expansion of the battery.

Another embodiment is to provide a lithium secondary battery including the electrolyte.

According to one embodiment, there is provided an electrolyte for a lithium secondary battery comprising a non-aqueous organic solvent lithium salt and an additive represented by the following Chemical Formula 1.

[Formula 1]

In Formula 1, X1 is carbon or N, and R ^a is a substituted or unsubstituted alkyl group. The substituted alkyl group is a fluorine-substituted alkyl group.

The content of the additive may be 0.1 wt% to 3 wt% based on the total weight of the electrolyte. According to one embodiment, the content of the additive may be 1 wt% to 2 wt% based on the total weight of the electrolyte.

The electrolyte may further include a second additive that is fluoroethylene carbonate, vinylene carbonate, succinonitrile, hexane tricyanide, LiBF _{4 , or a combination thereof.} The content of the second additive may be 5 wt% to 20 wt% based on the total weight of the electrolyte.

Another embodiment is a negative electrode comprising a negative active material; a positive electrode including a positive active material; And it provides a lithium secondary battery comprising the electrolyte.

The details of other implementations are included in the detailed description below.

The electrolyte for a lithium secondary battery according to an embodiment of the present invention may improve high-temperature storage characteristics, particularly high-temperature swelling characteristics.

1 is a view schematically showing a lithium secondary battery according to an embodiment of the present invention.
Figure 2 is a graph showing the CV results of the electrolyte of Example 2 and Comparative Example 1.

Hereinafter, embodiments of the present invention will be described in detail. However, this is presented as an example, and the present invention is not limited thereto, and the present invention is only defined by the scope of the claims to be described later.

One embodiment provides an electrolyte for a lithium secondary battery comprising a non-aqueous organic solvent lithium salt and an additive represented by Formula 1 below.

[Formula 1]

In Formula 1, X1 may be carbon or N, and according to one embodiment, X1 may be N. In Formula 1, R ^a is a substituted or unsubstituted alkyl group. This alkyl group is a C1 to C20 alkyl group, and is a linear or branched alkyl group. In addition, the substituted alkyl group is a fluorine-substituted alkyl group. That is, the substituted alkyl group is an alkyl group in which at least one hydrogen is substituted with fluorine, and may be, for example, -(CF ₂ ) _n -CF ₃ (n is an integer of 0 to 20).

In one embodiment, a specific example of Formula 1 may be Formula 1a or Formula 1b below.

[Formula 1a]

[Formula 1b]

When charging and discharging a battery using an electrolyte including an additive according to one embodiment, the additive is decomposed to have excellent lithium ion transfer properties on the surface of the negative electrode, and forms a stable film when stored at a high temperature. It is possible to suppress the generation of gas, thereby exhibiting improved swelling properties.

As shown in Formula 1, the additive according to an embodiment contains S in the 5-membered ring, and -SO ₂ R ^a is bonded to the 5-membered ring, so it is reduced and decomposed on the surface of the anode to have excellent thermal stability. A film can be formed, thereby improving high-temperature storage characteristics. This property further includes N along with S in the five-membered ring (ie, when X1 is N), and when -SO ₂ R ^a is combined with N included in the five-membered ring, it can be further maximized, so it is appropriate.

If S is not included in the 5-membered ring, or even includes S, when an alkyl group is bonded to the 5-membered ring, there is a problem in that an excessive film is formed on the negative electrode to increase the initial thickness and resistance.

In particular, in the case of propane sultone, which is mainly used to improve the swelling characteristics when stored at a high temperature, the swelling characteristics can be improved when stored at a high temperature, but there is a problem in that the capacity retention rate and capacity recovery rate are deteriorated by increasing the resistance. Since the additive of Chemical Formula 1 according to Formula 1 does not increase resistance while forming a stable film at high temperature, it is possible to effectively improve the swelling characteristics, capacity retention rate, and recovery rate during high temperature storage.

The content of the additive may be 0.1 wt% to 3 wt% based on the total weight of the electrolyte, and the content of the additive may be 1 wt% to 2 wt% based on the total weight of the electrolyte. When the content of the additive is included in the above range, gas generation due to decomposition of the electrolyte hardly occurs when the battery is stored, and in particular, gas generation hardly occurs during high-temperature storage, thereby exhibiting improved high-temperature storage characteristics. If the additive content is less than 0.1% by weight, the film cannot be sufficiently formed on the surface of the anode, so the effect of improving the high-temperature characteristics is somewhat insignificant. When the additive content exceeds 3% by weight, the electrolyte is somewhat severely decomposed, , since the amount of gas generated increases, this may slightly increase the initial thickness and resistance.

The electrolyte may further include a second additive that is fluoroethylene carbonate, vinylene carbonate, succinonitrile, hexane tricyanide, LiBF ₄ , or a combination thereof while including the additive as the first additive. When the second additive is further included, a more robust film is formed on the surface of the anode, and the anode interface is stabilized, so that collateral decomposition of the electrolyte can be better prevented. When the second additive is further included, the content of the second additive may be 5 wt% to 20 wt% based on the total weight of the electrolyte. When the second additive is included in an amount within the above range, the effect of using the second additive may be more appropriately obtained. If the second additive is used in excess of the amount, resistance may be excessively increased to cause deterioration of the battery during the charge/discharge cycle. As the second additive, when fluoroethylene carbonate, vinylene carbonate, succinonitrile, hexane tricyanide or LiBF ₄ is mixed and used, the mixing ratio thereof may be appropriately adjusted, and there is no need to limit it.

The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move.

As the non-aqueous organic solvent, a carbonate-based, ester-based, ether-based, ketone-based, alcohol-based, or aprotic solvent may be used.

Examples of the carbonate-based solvent include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate (MEC), ethylene carbonate ( EC), propylene carbonate (PC), butylene carbonate (BC), and the like may be used. Examples of the ester solvent include methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methylpropionate, ethylpropionate, propylpropionate, γ-butyrolactone, decanolide, valero. Lactone, mevalonolactone (mevalonolactone), caprolactone (caprolactone) and the like may be used.

Dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, etc. may be used as the ether-based solvent, and cyclohexanone, etc. may be used as the ketone-based solvent. have.

Ethyl alcohol, isopropyl alcohol, etc. may be used as the alcohol-based solvent, and the aprotic solvent is T-CN (T is a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms, nitriles such as nitriles (which may contain double bond aromatic rings or ether bonds), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, sulfolanes, and the like can be used.

The non-aqueous organic solvent may be used alone or in combination of one or more. When one or more are mixed and used, the mixing ratio can be appropriately adjusted according to the desired battery performance, which can be widely understood by those skilled in the art.

In addition, in the case of the carbonate-based solvent, it is preferable to use a mixture of a cyclic carbonate and a chain carbonate. In this case, when the cyclic carbonate and the chain carbonate are mixed in a volume ratio of 1:1 to 1:9, the electrolyte may exhibit excellent performance.

The non-aqueous organic solvent may further include an aromatic hydrocarbon-based organic solvent in the carbonate-based solvent. In this case, the carbonate-based solvent and the aromatic hydrocarbon-based organic solvent may be mixed in a volume ratio of 1:1 to 30:1.

As the aromatic hydrocarbon-based organic solvent, an aromatic hydrocarbon-based compound represented by the following Chemical Formula 2 may be used.

[Formula 2]

(In Formula 2, R ₁ to R ₆ are the same as or different from each other and are selected from the group consisting of hydrogen, halogen, an alkyl group having 1 to 10 carbon atoms, a haloalkyl group, and combinations thereof.)

Specific examples of the aromatic hydrocarbon-based organic solvent include benzene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-tri Fluorobenzene, 1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1 ,2,4-trichlorobenzene, iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene, 1,2,3-triiodobenzene, 1, 2,4-triiodobenzene, toluene, fluorotoluene, 2,3-difluorotoluene, 2,4-difluorotoluene, 2,5-difluorotoluene, 2,3,4-trifluoro Rottoluene, 2,3,5-trifluorotoluene, chlorotoluene, 2,3-dichlorotoluene, 2,4-dichlorotoluene, 2,5-dichlorotoluene, 2,3,4-trichlorotoluene, 2, 3,5-trichlorotoluene, iodotoluene, 2,3-diiodotoluene, 2,4-diiodotoluene, 2,5-diiodotoluene, 2,3,4-triiodotoluene, 2,3 ,5-triiodotoluene, xylene, and combinations thereof are selected from the group consisting of.

The electrolyte for a lithium secondary battery may further include an ethylene carbonate-based compound represented by the following Chemical Formula 3 as a lifespan improving additive in order to improve battery life.

[Formula 3]

(In Formula 3, R ₇ and R ₈ are each independently selected from the group consisting of hydrogen, a halogen group, a cyano group (CN), a nitro group (NO ₂ ), and a fluorinated alkyl group having 1 to 5 carbon atoms, the R _{At least one of 7} and R ₈ is selected from the group consisting of a halogen group, a cyano group (CN), a nitro group (NO ₂ ) and a fluorinated alkyl group having 1 to 5 carbon atoms, provided that R ₇ and R ₈ are not both hydrogen .)

Representative examples of the ethylene carbonate-based compound include difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate or fluoroethylene carbonate. can When such a life-enhancing additive is further used, its amount can be appropriately adjusted.

The lithium salt is dissolved in an organic solvent, serves as a source of lithium ions in the battery, enables basic lithium secondary battery operation, and promotes movement of lithium ions between the positive electrode and the negative electrode. Representative examples of such lithium salts include LiPF ₆ , LiSbF ₆ , LiAsF ₆ , LiN(SO ₂ C ₂ F ₅ ) ₂ , Li(CF ₃ SO ₂ ) ₂ N, LiN(SO ₃ C ₂ F ₅ ) ₂ , LiC ₄ F ₉ SO ₃ , LiClO ₄ , LiAlO ₂ , LiAlCl ₄ , LiN(C _x F _2x+1 SO ₂ )(C _y F _2y+1 SO ₂ ), where x and y are natural numbers, for example, 1 to 20 is an integer of), LiCl, LiI, and LiB(C ₂ O ₄ ) ₂ (lithium bis(oxalato) borate: LiBOB) as an electrolyte salt supporting one or two or more selected from the group consisting of The concentration of lithium salt is preferably used within the range of 0.1 M to 2.0 M. When the concentration of lithium salt is included in the above range, the electrolyte has appropriate conductivity and viscosity, so that excellent electrolyte performance can be exhibited, and lithium ions can move effectively.

Another embodiment provides a lithium secondary battery including the electrolyte, the positive electrode and the negative electrode.

The positive electrode includes a current collector and a positive electrode active material layer formed on the current collector and including a positive electrode active material.

In the cathode active material layer, a compound capable of reversible intercalation and deintercalation of lithium (lithiated intercalation compound) may be used as the cathode active material, and specifically, cobalt, manganese, nickel, and these At least one of a complex oxide of a metal selected from a combination of lithium and lithium may be used. As a more specific example, a compound represented by any one of the following formulas may be used. Li _a A _1-b X _b D ₂ (0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5); Li _a A _1-b X _b O _2-c D _c (0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05); Li _a E _1-b X _b O _2-c D _c (0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05); Li _a E _2-b X _b O _4-c D _c (0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05); Li _a Ni _1-bc Co _b X _c D _α (0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.5, 0 < α ≤2); Li _a Ni _1-bc Co _b X _c O _2-α T _α (0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, 0 < α <2); Li _a Ni _1-bc Co _b X _c O _2-α T ₂ (0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, 0 < α <2); Li _a Ni _1-bc Mn _b X _c D _α (0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, 0 < α ≤2); Li _a Ni _1-bc Mn _b X _c O _2-α T _α (0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, 0 < α <2); Li _a Ni _1-bc Mn _b X _c O _2-α T ₂ (0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, 0 < α <2); Li _a Ni _b E _c G _d O ₂ (0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, 0.001 ≤ d ≤ 0.1); Li _a Ni _b Co _c Mn _d G _e O ₂ (0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, 0 ≤ d ≤ 0.5, 0.001 ≤ e ≤ 0.1); Li _a NiG _b O ₂ (0.90 ≤ a ≤ 1.8, 0.001 ≤ b ≤ 0.1) Li _a CoG _b O ₂ (0.90 ≤ a ≤ 1.8, 0.001 ≤ b ≤ 0.1); Li _a Mn _1-b G _b O ₂ (0.90 ≤ a ≤ 1.8, 0.001 ≤ b ≤ 0.1); Li _a Mn ₂ G _b O ₄ (0.90 ≤ a ≤ 1.8, 0.001 ≤ b ≤ 0.1); Li _a Mn _1-g G _g PO ₄ (0.90 ≤ a ≤ 1.8, 0 ≤ g ≤ 0.5); QO ₂ ; QS ₂ ; LiQS ₂ ; V ₂ O ₅ ; LiV ₂ O ₅ ; LiZO ₂ ; LiNiVO ₄ ; Li _(3-f) J ₂ (PO ₄ ) ₃ (0 ≤ f ≤ 2); Li _(3-f) Fe ₂ (PO ₄ ) ₃ (0 ≤ f ≤ 2); Li _a FePO ₄ (0.90 ≤ a ≤ 1.8)

In the above formula, A is selected from the group consisting of Ni, Co, Mn, and combinations thereof; X is selected from the group consisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, rare earth elements and combinations thereof; D is selected from the group consisting of O, F, S, P, and combinations thereof; E is selected from the group consisting of Co, Mn, and combinations thereof; T is selected from the group consisting of F, S, P, and combinations thereof; G is selected from the group consisting of Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, and combinations thereof; Q is selected from the group consisting of Ti, Mo, Mn, and combinations thereof; Z is selected from the group consisting of Cr, V, Fe, Sc, Y, and combinations thereof; J is selected from the group consisting of V, Cr, Mn, Co, Ni, Cu, and combinations thereof.

Of course, a compound having a coating layer on the surface of the compound may be used, or a mixture of the compound and a compound having a coating layer may be used. The coating layer may include at least one coating element compound selected from the group consisting of an oxide of a coating element, a hydroxide of a coating element, an oxyhydroxide of a coating element, an oxycarbonate of a coating element, and a hydroxycarbonate of a coating element. can The compound constituting these coating layers may be amorphous or crystalline. As the coating element included in the coating layer, Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, or a mixture thereof may be used. In the coating layer forming process, any coating method may be used as long as it can be coated by a method that does not adversely affect the physical properties of the positive electrode active material by using these elements in the compound (eg, spray coating, dipping method, etc.). Since the content can be well understood by those engaged in the field, a detailed description thereof will be omitted.

In the positive electrode, the content of the positive active material may be 90 wt% to 98 wt% based on the total weight of the positive active material layer.

In one embodiment, the positive active material layer may further include a binder and a conductive material. In this case, the content of the binder and the conductive material may be 1 wt% to 5 wt%, respectively, based on the total weight of the positive electrode active material layer.

The binder serves to adhere the positive active material particles well to each other and also to the positive electrode active material to the current collector, and representative examples thereof include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl. Chloride, carboxylated polyvinylchloride, polyvinylfluoride, polymers including ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene- Butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon, etc. may be used, but the present invention is not limited thereto.

The conductive material is used to impart conductivity to the electrode, and in the configured battery, any electronically conductive material may be used without causing chemical change. Examples of the conductive material include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, and carbon fiber; metal-based substances such as metal powders such as copper, nickel, aluminum, and silver, or metal fibers; conductive polymers such as polyphenylene derivatives; or a conductive material containing a mixture thereof.

The current collector may be an aluminum foil, a nickel foil, or a combination thereof, but is not limited thereto.

The negative electrode includes a current collector and a negative electrode active material layer formed on the current collector and including a negative electrode active material.

As the anode active material, a material capable of reversibly intercalating/deintercalating lithium ions, lithium metal, an alloy of lithium metal, a material capable of doping and dedoping lithium, or a transition metal oxide may be used.

As a material capable of reversibly intercalating/deintercalating lithium ions, for example, a carbon material, that is, a carbon-based negative active material generally used in a lithium secondary battery may be used. Representative examples of the carbon-based negative active material include crystalline carbon, amorphous carbon, or a combination thereof. Examples of the crystalline carbon include graphite such as amorphous, plate-like, flake, spherical or fibrous natural graphite or artificial graphite, and examples of the amorphous carbon include soft carbon or hard carbon ( hard carbon), mesophase pitch carbide, and calcined coke.

The lithium metal alloy includes lithium and a group consisting of Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al and Sn. An alloy of a metal selected from may be used.

Examples of the material capable of doping and dedoping lithium include Si, SiO _x (0 < x < 2), Si-Q alloy (wherein Q is an alkali metal, alkaline earth metal, group 13 element, group 14 element, group 15 element, 16 an element selected from the group consisting of group elements, transition metals, rare earth elements, and combinations thereof, and not Si), Si-carbon composite, Sn, SnO ₂ , Sn-R (wherein R is an alkali metal, alkaline earth metal, 13 an element selected from the group consisting of a group element, a group 14 element, a group 15 element, a group 16 element, a transition metal, a rare earth element, and a combination thereof (not Sn), Sn-carbon composite, and the like, and these At least one of SiO ₂ may be mixed and used. The elements Q and R include Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb, Bi, S, One selected from the group consisting of Se, Te, Po, and combinations thereof may be used.

Lithium titanium oxide may be used as the transition metal oxide.

The anode active material layer includes an anode active material and a binder, and may optionally further include a conductive material.

The content of the anode active material in the anode active material layer may be 95 wt% to 99 wt% based on the total weight of the anode active material layer. The content of the binder in the anode active material layer may be 1 wt% to 5 wt% based on the total weight of the anode active material layer. In addition, when the conductive material is further included, 90 wt% to 98 wt% of the negative active material, 1 to 5 wt% of the binder, and 1 wt% to 5 wt% of the conductive material may be used.

The binder serves to well adhere the negative active material particles to each other and also to adhere the negative active material to the current collector. As the binder, a water-insoluble binder, a water-soluble binder, or a combination thereof may be used.

Examples of the water-insoluble binder include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, and polyvinylidene fluoride. , polyethylene, polypropylene, polyamideimide, polyimide, or a combination thereof.

Examples of the water-soluble binder include styrene-butadiene rubber, acrylated styrene-butadiene rubber, polyvinyl alcohol, sodium polyacrylate, an olefin copolymer having 2 to 8 propylene and carbon atoms, (meth)acrylic acid and (meth)acrylic acid alkyl ester. copolymers or combinations thereof.

When a water-soluble binder is used as the negative electrode binder, a cellulose-based compound capable of imparting viscosity may be further included as a thickener. As the cellulose-based compound, carboxymethyl cellulose, hydroxypropylmethyl cellulose, methyl cellulose, or alkali metal salts thereof may be mixed and used. As the alkali metal, Na, K or Li may be used. The amount of the thickener used may be 0.1 parts by weight to 3 parts by weight based on 100 parts by weight of the negative active material.

The conductive material is used to impart conductivity to the electrode, and in the configured battery, any electronically conductive material may be used without causing chemical change. Examples of the conductive material include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, denka black, and carbon fiber; metal-based substances such as metal powders such as copper, nickel, aluminum, and silver, or metal fibers; conductive polymers such as polyphenylene derivatives; or a conductive material containing a mixture thereof.

The current collector may be selected from the group consisting of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with conductive metal, and combinations thereof.

The positive active material layer and the negative active material layer are formed by preparing an active material composition by mixing an anode active material, a binder, and optionally a conductive material in a solvent, and applying the active material composition to a current collector. Since such a method for forming an active material layer is widely known in the art, a detailed description thereof will be omitted herein. The solvent may include, but is not limited to, N-methylpyrrolidone. In addition, when a water-soluble binder is used for the anode active material layer, water may be used as a solvent used in preparing the anode active material composition.

In addition, depending on the type of the lithium secondary battery, a separator may exist between the positive electrode and the negative electrode. As such a separator, polyethylene, polypropylene, polyvinylidene fluoride, or a multilayer film of two or more layers thereof may be used. A polyethylene/polypropylene two-layer separator, a polyethylene/polypropylene/polyethylene three-layer separator, and polypropylene/polyethylene/poly It goes without saying that a mixed multilayer film such as a propylene three-layer separator or the like can be used.

1 is an exploded perspective view of a lithium secondary battery according to an embodiment of the present invention. The lithium secondary battery according to an embodiment is described as a pouch-type battery as an example, but the present invention is not limited thereto, and may be applied to various types of batteries such as cylindrical and prismatic.

Referring to FIG. 1 , a lithium secondary pouch battery 100 according to an embodiment includes an electrode assembly 110 wound with a separator 30 interposed between a positive electrode 10 and a negative electrode 20 , and the electrode assembly 110 . ) may include a case 120 therein, and an electrode tab 130 serving as an electrical path for inducing a current formed in the electrode assembly 110 to the outside. The two surfaces of the case 120 overlap and seal the surfaces facing each other. In addition, the electrolyte is injected into the case 120 containing the electrode assembly 110, and the positive electrode 10, the negative electrode 20, and the separator 30 may be impregnated with the electrolyte (not shown). .

Hereinafter, Examples and Comparative Examples of the present invention will be described. The following examples are only examples of the present invention, and the present invention is not limited to the following examples.

(Example 1)

1.15M LiPF ₆ was added to a mixed solvent of ethylene carbonate and ethylmethyl carbonate (3:7 volume ratio), and 0.5 wt% of the additive of Formula 1a below, 5 wt% of fluoroethylene carbonate and vinylene carbonate to 100 wt% of the mixture An electrolyte for a lithium secondary battery was prepared by adding 1 wt%.

[Formula 1a]

(Example 2)

An electrolyte for a lithium secondary battery was prepared in the same manner as in Example 1, except that the additive content of Formula 1a was changed to 1 wt%.

(Example 3)

An electrolyte for a lithium secondary battery was prepared in the same manner as in Example 1, except that the additive content of Formula 1a was changed to 2 wt%.

(Comparative Example 1)

1.15M LiPF ₆ was added to a mixed solvent of ethylene carbonate and ethylene carbonate (3:7 volume ratio), and to 100% by weight of the mixture, 5% by weight of fluoroethylene carbonate, 1% by weight of vinylene carbonate, and 2% by weight of propanesultone was added to prepare an electrolyte for a lithium secondary battery.

(Comparative Example 2)

1.15M LiPF ₆ was added to a mixed solvent of ethylene carbonate and ethylmethyl carbonate (3:7 volume ratio), and to 100% by weight of the mixture, 1% by weight of the additive of Formula 4 below, 5% by weight of fluoroethylene carbonate and vinylene carbonate An electrolyte for a lithium secondary battery was prepared by adding 1 wt%.

[Formula 4]

* Cyclic Voltammetry (CV) characteristics evaluation

The circulating current voltage (scan rate: 0.1 mV/sec) of the three electrodes using the electrolyte of Comparative Example 1, the graphite working electrode, and the lithium counter electrode was measured, and the results are shown in FIG. indicated. In addition, the circulating current voltage (scan speed: 1 mV/sec) of the three electrodes using the electrolyte of Example 2, the graphite working electrode, and the lithium counter electrode was measured, and the results are shown together in FIG. 2 .

As shown in FIG. 2 , since the electrolyte of Example 2 reacts at a higher potential than Comparative Example 1, it can be seen that the reactivity to the negative electrode is increased. In addition, as shown in FIG. 2 , it can be seen that the electrolyte of Example 2 has a decomposition peak, that is, a reduction peak around 0.8V, whereas the electrolyte of Comparative Example 1 does not have a decomposition peak. It can be seen that the electrolyte of Example 2 is reduced before ethylene carbonate because the reaction occurs at a higher potential than ethylene carbonate having a reduction peak at 0.5V. As such, when reduced before ethylene carbonate, a film is formed on the surface of the anode to suppress further decomposition of ethylene carbonate.

As described above, it is possible to prevent the problem of film formation due to ethylene carbonate, which is difficult to move due to solvent depletion of the electrolyte due to decomposition of ethylene carbonate, and may reduce cycle life characteristics and output characteristics by acting as a resistor.

From these results, it can be seen that the lithium secondary battery using the electrolyte of Example 1 can form a film on the negative electrode during the initial one-time charging and discharging, and it can be seen that the film has a small resistance during lithium insertion/desorption.

* Battery manufacturing

96 wt% of LiCoO ₂ cathode active material, 2 wt% of Ketjen black conductive material, and 2 wt% of polyvinylidene fluoride were mixed in an N-methylpyrrolidone solvent to prepare a cathode active material slurry. The positive electrode active material slurry was coated on an aluminum foil, dried and rolled to prepare a positive electrode.

An anode active material slurry was prepared by mixing 96 wt% of an artificial graphite anode active material, 2 wt% of Ketjen Black conductive material, and 2 wt% of polyvinylidene fluoride in an N-methylpyrrolidone solvent. The negative electrode active material slurry was coated on a copper foil, dried and rolled to prepare a negative electrode.

A lithium secondary battery was manufactured by a conventional method using the prepared positive electrode, the negative electrode, and the electrolytes prepared according to Examples 1 to 3 and Comparative Examples 1 and 2. At this time, the amount of electrolyte injection was 5.8 g.

* Battery thickness increase rate at 85℃

Lithium secondary batteries using the prepared electrolytes of Examples 1 to 3 and Comparative Examples 1 to 2 were charged at 4.4V, 0.7C, SOC (State of Charge) 100% (full charge, the battery was charged and discharged at 2.75V to 4.4V) At the time, when the total charge capacity of the battery was 100%, the battery was charged to 100% charge capacity), and then stored at 85° C. for 8 hours. The battery thickness before storage was measured, and the battery thickness after storage was measured, and the results are shown in Table 1 below. In addition, the battery thickness increase rate was calculated from this result, and the results are shown in Table 1 below.

* Capacity retention and recovery rate at 85℃

Lithium secondary batteries using the electrolytes of Examples 1 to 3 and Comparative Examples 1 to 2 were charged to 4.4V and 0.7C to SOC (State of Charge) 100% (full charge, when charging and discharging the battery at 2.75V to 4.4V) , when the total charge capacity of the battery is 100%, the battery is charged to 100% charge capacity), stored at 85°C for 8 hours, discharged at 0.2C, 2.75V cut-off voltage, and stored for one-time discharge capacity The post-discharge capacity ratio (capacity retention rate) was calculated, and the results are shown in Table 1 below.

Then, the discharge capacity was measured by charging and discharging once under the same conditions as above, and the ratio of the discharge capacity to the initial capacity before storage is shown as the capacity recovery rate in Table 1 below.

Thickness before storage (mm) Thickness after storage (mm) Thickness increase rate (%) Capacity retention rate (%) Capacity recovery rate (%) Comparative Example 1 5.145 5.458 6.09 93.5 97.0 Comparative Example 2 5.145 5.568 8.23 92.0 97.0 Example 1 5.140 5.350 4.10 93.6 98.2 Example 2 5.143 5.205 1.21 94.0 98.8 Example 3 5.140 5.213 1.42 93.8 98.9

As shown in Table 1, the batteries using the electrolytes of Examples 1 to 3 using the additive of Formula 1a were compared to the batteries using the electrolytes of Comparative Examples 1 and 2 using propane sultone or the additive of Formula 4 at 85°C high temperature. It can be seen that the thickness increase rate is very small, and the capacity retention rate and capacity recovery rate are excellent results.

In particular, it can be seen that the batteries using the electrolytes of Examples 2 and 3 using 1 to 2 wt% of the additive of Formula 1a exhibit very good swelling and capacity characteristics.

* Battery thickness increase rate at 60℃

Lithium secondary batteries using the prepared electrolytes of Examples 1 to 3 and Comparative Examples 1 to 2 were charged at 4.4V, 0.7C, SOC (State of Charge) 100% (full charge, the battery was charged and discharged at 2.75V to 4.4V) At the time, when the total charge capacity of the battery was 100%, the battery was charged to 100% charge capacity), and then stored at 60° C. for 21 days. The battery thickness before storage was measured, and the battery thickness after storage was measured, and the results are shown in Table 2 below. In addition, the battery thickness increase rate was calculated from this result, and the results are shown in Table 2 below.

* Capacity retention and recovery rate at 60℃

Lithium secondary batteries using the electrolytes of Examples 1 to 3 and Comparative Examples 1 to 2 were charged to 4.4V and 0.7C to SOC (State of Charge) 100% (full charge, when charging and discharging the battery at 2.75V to 4.4V) , When the total charge capacity of the battery is 100%, the battery is charged up to 100% charge capacity) and stored at 60°C for 21 days, then discharged at 0.2C, 2.75V cut-off voltage, and stored for one-time discharge capacity The post-discharge capacity ratio (capacity retention rate) was calculated, and the results are shown in Table 2 below.

Then, the discharge capacity was measured by charging and discharging once under the same conditions as described above, and the ratio of the discharge capacity to the initial capacity before storage is shown as the capacity recovery rate in Table 2 below.

Thickness before storage (mm) Thickness after storage (mm) Thickness increase rate (%) Capacity retention rate (%) Capacity recovery rate (%) Comparative Example 1 5.140 5.521 7.41 86.3 97.0 Comparative Example 2 5.140 6.270 22.01 87.0 96.9 Example 1 5.141 5.360 4.26 87.0 97.2 Example 2 5.139 5.230 1.77 88.0 97.6 Example 3 5.140 5.240 1.95 88.0 97.5

As shown in Table 2, the batteries using the electrolytes of Examples 1 to 3 using the additive of Chemical Formula 1a have a higher temperature of 60° C. than the batteries using the electrolytes of Comparative Examples 1 and 2 using the additive of Propane Sultone or Chemical Formula 4 It can be seen that even during long-term storage, the thickness increase rate was very small, and excellent results were obtained in the capacity retention rate and capacity recovery rate.

In particular, it can be seen that the batteries using the electrolytes of Examples 2 and 3 using 1 to 2 wt% of the additive of Formula 1a exhibit very good swelling and capacity characteristics.

The present invention is not limited to the above embodiments, but can be manufactured in a variety of different forms, and those of ordinary skill in the art to which the present invention pertains can take other specific forms without changing the technical spirit or essential features of the present invention. It will be understood that it can be implemented as Therefore, it should be understood that the embodiments described above are illustrative in all respects and not restrictive.

Claims (7)

non-aqueous organic solvents;
lithium salt; and
An additive represented by the following formula (1)
Electrolyte for lithium secondary batteries.
[Formula 1]

(In Formula 1,
X1 is carbon or N, and R ^a is a substituted or unsubstituted alkyl group) According to claim 1,
The content of the additive is 0.1% to 3% by weight based on the total weight of the electrolyte for a lithium secondary battery. According to claim 1,
The content of the additive is 1% to 2% by weight based on the total weight of the electrolyte for a lithium secondary battery. According to claim 1,
The substituted alkyl group is an electrolyte for a lithium secondary battery that is a fluorine-substituted alkyl group. According to claim 1,
The electrolyte is fluoroethylene carbonate, vinylene carbonate, succinonitrile, hexane tricyanide, LiBF ₄ , or a combination thereof, the electrolyte for a lithium secondary battery further comprising a second additive. 6. The method of claim 5,
The content of the second additive is 5% to 20% by weight based on the total weight of the electrolyte for a lithium secondary battery. a negative electrode including an anode active material;
a positive electrode including a positive active material; and
The electrolyte of any one of claims 1 to 6
A lithium secondary battery comprising a.

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