KR102160704B1 - Electrolyte for lithium secondary battery and lithium secondary battery comprising thereof - Google Patents

Abstract

The present invention relates to an electrolyte solution for a lithium secondary battery and a lithium secondary battery including the same, and more particularly, to a lithium salt, a glycol ether-based solvent and a fluorinated ether-based solvent, wherein the glycol ether-based solvent and the fluorinated ether-based solvent 1 It relates to an electrolyte for a lithium secondary battery containing a volume ratio of :2 to 1:5.
The electrolyte for a lithium secondary battery improves battery performance and life by including two solvents in a predetermined volume ratio, thereby enabling high capacity, high stability, and long life of the lithium secondary battery.

Description

Electrolyte for lithium secondary battery and lithium secondary battery containing the same {ELECTROLYTE FOR LITHIUM SECONDARY BATTERY AND LITHIUM SECONDARY BATTERY COMPRISING THEREOF}

The present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery comprising the same.

BACKGROUND ART As portable electronic devices, electric vehicles, and large-capacity power storage systems are developed in recent years, the demand for large-capacity batteries as an energy source is increasing, and many studies on batteries are being conducted to meet such demands. Among various secondary batteries, lithium secondary batteries, which have advantages such as high energy density, discharge voltage, and output stability, are attracting attention.

The lithium secondary battery has a structure in which an electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode is stacked or wound, and the electrode assembly is built into a battery case and a non-aqueous electrolyte is injected therein. . At this time, the capacity of the lithium secondary battery varies depending on the type of electrode active material, and it is not commercially available because a sufficient capacity as the theoretical capacity is not secured during actual operation.

In order to increase the capacity of a lithium secondary battery, metal-based materials such as silicon (4,200 mAh/g) and tin (990 mAh/g), which exhibit high storage capacity characteristics through an alloying reaction with lithium, are used as negative electrode active materials. However, when a metal such as silicon or tin is used as a negative electrode active material, the volume expands to about 4 times during the charging process of alloying with lithium and contracts during discharge. Due to the large volume change of the electrode that occurs repeatedly during such charging and discharging, the active material is gradually pulverized and detached from the electrode, resulting in a rapid decrease in capacity, which makes it difficult to secure stability and reliability, and thus commercialization has not been reached.

Compared to the aforementioned negative electrode active material, lithium metal has an excellent theoretical capacity of 3,860 mAh/g and a standard reduction potential (Standard Hydrogen Electrode; SHE) of -3.045 V, which is very low, enabling a high-capacity, high-energy-density battery. Recently, as interest in lithium-sulfur and lithium-air batteries has increased, research has been actively conducted as a negative active material for lithium secondary batteries.

However, when lithium metal is used as the negative electrode of a lithium secondary battery, the lithium metal reacts with electrolytes, impurities, and lithium salts to form a solid electrolyte interphase (SEI), and such a passivation layer causes a difference in local current density. During charging, it promotes the formation of dendritic dendrite by lithium metal, and gradually grows during charging and discharging, causing an internal short circuit between the anode and the cathode. In addition, dendrite has a mechanically weak part (bottle neck) and forms inert lithium that loses electrical contact with the current collector during discharge, thereby reducing the capacity of the battery and shortening the cycle life, and the stability of the battery. Adversely affects Due to the non-uniformity of the redox reaction of the lithium metal negative electrode and the reactivity with the electrolyte, a lithium secondary battery using lithium metal as a negative electrode has not yet been put into practical use.

In order to solve this problem, various methods such as introducing a protective layer on the surface of the lithium metal, increasing the lithium salt content of the electrolyte, or introducing an additive have been studied.

For example, Korean Patent Laid-Open No. 2016-0034183 describes the loss of electrolyte and generation of dendrites by forming a protective layer with a polymer matrix capable of accumulating electrolyte while protecting the negative electrode on the negative electrode active layer containing lithium metal or lithium alloy. It discloses that it can be prevented.

In addition, Jiangfeng Qian et al. reported in the paper that the lithium metal battery uses an electrolyte containing a high concentration of 4M LiFSI as a lithium salt and 1,2-dimethoxyethane (DME) as a solvent. (Jiangfeng Qian et al. , High rate and stable cycling of lithium metal anode, Nature Communications , 2015, 6, 6362).

The prior documents contributed to the stabilization of lithium metal to some extent, but the effect is not sufficient. In addition, in the case of the former, the protective layer of the battery is hardened or denatured such as swelling occurs upon contact with the electrolyte, so that the application to the lithium secondary battery is limited. In addition, in the latter case, the viscosity of the electrolyte is increased due to the use of a high concentration of lithium salt, and deterioration is caused during battery operation due to the high flammability of the solvent, resulting in a shortening of the battery life. Therefore, the development of an electrolyte solution capable of obtaining excellent performance and stable life characteristics in a battery using lithium metal as a negative electrode is further required.

Republic of Korea Patent Publication No. 2016-0034183 (2016.03.29), a negative electrode for a lithium secondary battery and a lithium secondary battery including the same

Jiangfeng Qian et al., High rate and stable cycling of lithium metal anode, Nature Communications, 2015, 6, 6362

Accordingly, the present inventors conducted various studies to solve the above problem, and as a result of using an electrolyte containing a glycol ether-based solvent and a fluorinated ether-based solvent in a certain volume ratio range, the cycle characteristics of the lithium metal electrode and the stability of the electrolyte were improved. It was confirmed that the battery performance and lifespan were improved to complete the present invention.

Accordingly, an object of the present invention is to provide an electrolyte for a lithium secondary battery having excellent stability and performance.

In addition, another object of the present invention is to provide a lithium secondary battery comprising the electrolyte.

In order to achieve the above object, the present invention is a lithium salt; A glycol ether solvent represented by the following formula (1); And a fluorinated ether-based solvent represented by the following Formula 2,

The glycol ether solvent and the fluorinated ether solvent are included in a volume ratio of 1:2 to 1:5 to provide an electrolyte for a lithium secondary battery:

[Formula 1]

R ¹ -O-(CH ₂ CH ₂ O) _n -R ²

[Formula 2]

R ³ -OR ⁴

(In Formulas 1 and 2, R ¹ to R ⁴ and n follow what is described in the specification.)

The glycol ether solvent is composed of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol ethyl methyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, and triethylene glycol diethyl ether. It may include one or more selected from the group.

The fluorinated ether solvent may be 1,1,2,2-tetrafluoroethyl-2,2,3,3- tetrafluoropropyl ether; 1,1,2,2-tetrafluoroethyl-2,2,3,3,3-pentafluoropropyl ether; 1,1,2,3,3,3-hexafluoropropyl-2,2,2-trifluoroethyl ether; Ethyl-1,1,2,3,3,3- hexafluoropropyl ether; Difluoromethyl-2,2,3,3,3-pentafluoropropyl ether and difluoromethyl-2,2,3,3- tetrafluoropropyl ether containing at least one selected from the group consisting of can do.

The lithium salt is LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiC ₄ BO ₈ , LiAsF ₆ , LiSbF ₆ , LiAlCl ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F ₅ SO ₂ ) ₂ NLi, (SO ₂ F) ₂ NLi, (CF ₃ SO ₂ ) ₃ CLi, chloroborane lithium , Lower aliphatic lithium carboxylic acid having 4 or less carbon atoms, lithium 4-phenyl borate, and lithium imide may include at least one selected from the group consisting of.

In this case, the electrolyte solution may further include an additive represented by the following Formula 3:

[Formula 3]

MNO _x

(In Chemical Formula 3, M and x are as described in the specification.)

The additive may be included in a concentration of 0.01 to 0.2 M.

In addition, the present invention provides a lithium secondary battery comprising the electrolyte.

The electrolyte for a lithium secondary battery according to the present invention may secure capacity characteristics of a lithium secondary battery and improve lifespan characteristics by including a glycol ether solvent and a fluorinated ether solvent in a specific volume ratio range. In addition, the electrolytic solution for a lithium secondary battery of the present invention exhibits excellent lithium metal cycle characteristics when the negative electrode is lithium metal, thereby enabling high capacity of the lithium secondary battery. In addition, the electrolyte solution for a lithium secondary battery of the present invention can further improve lifespan characteristics through additives, thereby enabling a longer lifespan of a lithium secondary battery.

1 is a graph showing the cycle efficiency of half-cells including the electrolytes prepared in Examples 1 to 3 and Comparative Example 1 of the present invention.
2 is a graph showing the cycle overvoltage of a half-cell containing the electrolyte prepared in Example 1 of the present invention.
3 is a graph showing a cycle overvoltage of a half cell containing the electrolyte prepared in Comparative Example 1 of the present invention.
4 is a graph showing the cumulative irreversible efficiency of half-cells including the electrolytes prepared in Examples 1 to 3 and Comparative Example 1 of the present invention.
5 is a graph showing the capacity of a battery including the electrolyte solution prepared in Example 1 and Comparative Example 2 of the present invention.
6 is a graph showing the capacity of a battery including the electrolyte solution prepared in Example 4 and Comparative Example 3 of the present invention.

Hereinafter, the present invention will be described in more detail.

The terms or words used in the specification and claims should not be construed as being limited to their usual or dictionary meanings, and the inventor may appropriately define the concept of terms in order to describe his own invention in the best way. It should be interpreted as a meaning and concept consistent with the technical idea of the present invention based on the principle that there is.

The present invention provides an electrolyte for a lithium secondary battery comprising a glycol ether solvent and a fluorinated ether solvent in a predetermined volume ratio in order to improve the cycle characteristics of a lithium metal electrode to secure the effect of improving the performance and life of a lithium secondary battery including the same. .

Specifically, the electrolyte for a lithium secondary battery according to the present invention includes a lithium salt; A glycol ether solvent represented by the following formula (1); And a fluorinated ether-based solvent represented by the following Formula 2,

The glycol ether solvent and the fluorinated ether solvent are contained in a volume ratio of 1:2 to 1:5 for a lithium secondary battery electrolyte:

[Formula 1]

R ¹ -O-(CH ₂ CH ₂ O) _n -R ²

[Formula 2]

R ³ -OR ⁴

(In Formulas 1 and 2,

R ¹ and R ² are the same as or different from each other, and each independently a C1 to C6 alkyl group, a C6 to C12 aryl group or a C7 to C13 arylalkyl group,

R ³ and R ⁴ are the same as or different from each other, and each independently a C1 to C6 alkyl group or a C1 to C6 fluorinated alkyl group, but at least one of R ³ and R ⁴ is a C1 to C6 fluorinated alkyl group,

n is an integer of 1 to 3.)

The C1 to C6 alkyl group mentioned in the present specification is a linear or branched alkyl group, such as a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a sec-butyl group, a t-butyl group, a pen And a yl group or a hexyl group, but are not limited thereto.

The C6 to C12 aryl group mentioned in the present specification may be, for example, a phenyl group unsubstituted or substituted with a C1 to C6 alkyl group, or a naphthyl group.

The C7 to C13 arylalkyl group referred to in the present specification may be, for example, a benzyl group, a phenylethyl group, a phenylpropyl group, or a phenylbutyl group unsubstituted or substituted with a C1 to C6 alkyl group.

The C1 to C6 fluorinated alkyl group referred to herein refers to a C1 to C6 alkyl group substituted with fluorine.

In Formula 1, R ¹ and R ² may be the same as or different from each other, preferably a methyl group, an ethyl group, a propyl group, an isopropyl group, or a butyl group, and more preferably a methyl group, an ethyl group, or a propyl group. In addition, n in Formula 1 may preferably be 1.

In Formula 2, R ³ and R ⁴ are the same as or different from each other, preferably methyl group, ethyl group, propyl group, difluoromethyl group, trifluoroethyl group, tetrafluoroethyl group, tetrafluoropropyl group, pentafluoropropyl It may be a group or a hexafluoropropyl group.

The electrolyte solution according to the present invention includes a glycol ether solvent represented by Chemical Formula 1 as a first solvent. The glycol ether solvent has excellent solvation ability for lithium ions, thereby increasing the solubility of lithium ions and inhibiting the growth of lithium dendrites. Accordingly, the cycle characteristics of the lithium metal electrode are improved, so that the effect of improving the performance and life characteristics of the battery can be secured.

For example, as a glycol ether solvent represented by Formula 1, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol ethyl methyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl It may include one or more selected from the group consisting of ether and triethylene glycol diethyl ether. Preferably, it may be one or more selected from the group consisting of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, and ethylene glycol ethylmethyl ether, more preferably ethylene glycol dimethyl ether.

The electrolyte solution according to the present invention includes a fluorinated ether-based solvent represented by Formula 2 as a second solvent. The fluorinated ether solvent exhibits a high flash point of 150° C. or more and a low viscosity of 5 cP or less without solvating lithium ions. Specifically, the fluorinated ether-based solvent serves to lower the high flammability and viscosity resulting from the glycol ether-based solvent described above. This prevents deterioration of the electrolyte and improves the mobility of lithium ions, thereby ensuring high stability even for long-term battery operation.

For example, as the fluorinated ether solvent represented by Formula 2, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (1,1,2,2 -tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether; TTE); 1,1,2,2-tetrafluoroethyl-2,2,3,3,3-pentafluoropropyl ether (1,1,2,2-tetrafluoroethyl-2,2,3,3,3-pentafluoropropyl ether); 1,1,2,3,3,3-hexafluoropropyl-2,2,2-trifluoroethyl ether (1,l,2,3,3,3-hexafluoropropyl-2,2,2-trifluoroethyl ether); Ethyl-1,1,2,3,3,3- hexafluoropropyl ether (ethyl-1,1,2,3,3,3-hexafluoropropyl ether); Difluoromethyl-2,2,3,3,3-pentafluoropropyl ether and difluoromethyl-2,2,3,3- It may include at least one selected from the group consisting of difluoromethyl-2,2,3,3-tetrafluoropropyl ether. Preferably 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3 ,3-pentafluoropropyl ether and 1,1,2,3,3,3-hexafluoropropyl-2,2,2-trisfluoroethyl ether may be one or more selected from the group consisting of, more Preferably, it may be 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

In this case, the volume ratio of the glycol ether solvent and the fluorinated ether solvent may be 1:2 to 1:5, preferably 1:2 to 1:4, more preferably 1:2 to 1:3. The volume ratio of the glycol ether-based solvent and the fluorinated ether-based solvent may be appropriately adjusted depending on the type of electrode active material used, battery capacity, etc., but in the case of a battery using lithium metal as a negative electrode, the above two solvents are used in the volume ratio range. The inclusion is preferable in terms of battery performance and life, since it improves the cycle characteristics of lithium metal and the stability of the electrolyte.

The electrolyte for a lithium secondary battery of the present invention contains a lithium salt as an electrolyte salt to increase ionic conductivity. The lithium salt is not particularly limited in the present invention, and may be used without limitation as long as it is commonly used in a lithium secondary battery. For example, the lithium salt is LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiC ₄ BO ₈ , LiAsF ₆ , LiSbF ₆ , LiAlCl ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F ₅ SO ₂ ) ₂ NLi, (SO ₂ F) ₂ NLi, (CF ₃ SO ₂ ) ₃ CLi , Chloroborane lithium, a lower aliphatic lithium carboxylic acid having 4 or less carbon atoms, lithium 4-phenyl borate, and lithium imide may include at least one selected from the group consisting of. Preferably, the lithium salt may be (SO ₂ F) ₂ NLi (lithium bis(fluorosulfonyl) imide, LiFSI).

The concentration of the lithium salt may be determined in consideration of ionic conductivity, and the like, and may be, for example, 0.1 to 4.0 M, preferably 0.5 to 2.0 M. When the concentration of the lithium salt is less than the above range, it is difficult to secure an ionic conductivity suitable for battery operation. On the contrary, when the concentration of the lithium salt exceeds the above range, the viscosity of the electrolyte increases and the mobility of lithium ions decreases, and the decomposition reaction of the lithium salt itself increases. Since the performance of the battery may be deteriorated, it is appropriately adjusted within the above range.

The electrolyte for a lithium secondary battery of the present invention may further include an additive represented by the following Formula 3 in addition to the above-described composition:

[Formula 3]

MNO _x

(In Chemical Formula 3,

M is lithium, magnesium, cesium, potassium or barium,

x is 2 or 3.)

The additive represented by Chemical Formula 3 has an intramolecular N-O bond to form a stable film on the lithium metal electrode and the anode. Accordingly, as side reactions between the lithium metal and the electrolyte or between the positive electrode and the electrolyte are suppressed, stability is further improved, and the life of the battery can be greatly improved.

For example, the additive represented by Formula 3 is lithium nitrate (LiNO ₃ ), potassium nitrate (KNO ₃ ), cesium nitrate (CsNO ₃ ), magnesium nitrate (MgNO ₃ ), barium nitrate (BaNO ₃ ), lithium nitrite ( LiNO ₂ ), potassium nitrite (KNO ₂ ) and cesium nitrite (CsNO ₂ ) may include at least one selected from the group consisting of. Preferably, it may be at least one selected from the group consisting of lithium nitrate, cesium nitrate, magnesium nitrate, lithium nitrite, and cesium nitrite.

The concentration of the additive may be appropriately adjusted according to the composition of the electrolyte solution, and may be, for example, 0.01 to 0.2 M, preferably 0.01 to 0.1 M. When the concentration of the additive is less than the above range, the above-described effect cannot be secured. Conversely, when the concentration of the additive exceeds the above range, resistance may be caused by the coating.

The electrolyte for a lithium secondary battery according to the first embodiment of the present invention uses at least one selected from the group consisting of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, and ethylene glycol ethyl methyl ether as a glycol ether solvent.

The electrolyte for a lithium secondary battery according to the second embodiment of the present invention is a fluorinated ether solvent, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1,1, 2,2-tetrafluoroethyl-2,2,3,3,3-pentafluoropropyl ether and 1,1,2,3,3,3-hexafluoropropyl-2,2,2-trifluoro At least one selected from the group consisting of roethyl ether is used.

The electrolyte for a lithium secondary battery according to the third embodiment of the present invention uses LiFSI as a lithium salt.

The electrolyte for a lithium secondary battery according to the fourth embodiment of the present invention uses cesium nitrate as an additive.

The electrolyte for a lithium secondary battery according to the fifth embodiment of the present invention includes ethylene glycol dimethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and LiFSI. do.

The electrolyte for a lithium secondary battery according to the sixth embodiment of the present invention is ethylene glycol dimethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, LiFSI, and nitric acid. Contains cesium.

As described above, the electrolyte for a lithium secondary battery according to the present invention includes a glycol ether-based solvent and a fluorinated ether-based solvent in a predetermined volume ratio, thereby ensuring excellent cycle characteristics of the lithium metal electrode and the stability of the electrolyte. · Discharge performance and life can be improved.

According to a preferred embodiment of the electrolyte for a lithium secondary battery according to the present invention, (SO ₂ F) ₂ NLi is included as a lithium salt of the electrolyte, and ethylene glycol dimethyl ether as a glycol ether solvent, 1,1 as a fluorinated ether solvent. ,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and their volume ratio may be from 1:2 to 1:4. In this case, it is possible to improve the performance and life of the battery by securing excellent cycle characteristics of the lithium metal electrode.

In addition, another preferred embodiment of the present invention may include cesium nitrate as an additive in addition to the embodiment of the present invention described above. In this case, in addition to the above-described effects, the stability of the electrolyte solution is greatly improved, so that the life characteristics of the battery can be remarkably improved. Thus, the electrolyte may be suitably used for a high voltage battery that requires high electrolyte stability.

The method of preparing the electrolyte for a lithium secondary battery according to the present invention is not particularly limited in the present invention, and may be prepared by a conventional method known in the art.

In addition, the present invention provides a lithium secondary battery comprising the lithium secondary battery electrolyte.

The lithium secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, and the electrolyte for a lithium secondary battery according to the present invention is used as the electrolyte.

The positive electrode may include a positive electrode current collector and a positive electrode active material applied to one or both surfaces of the positive electrode current collector.

The positive electrode current collector supports a positive electrode active material and is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, calcined carbon, copper or stainless steel surface treated with carbon, nickel, silver, etc., aluminum-cadmium alloy, and the like may be used.

The positive electrode current collector may form fine irregularities on its surface to enhance the bonding strength with the positive electrode active material, and various forms such as films, sheets, foils, meshes, nets, porous bodies, foams, and nonwoven fabrics may be used.

The positive electrode active material may include a positive electrode active material and optionally a conductive material and a binder.

The positive electrode active material may be a layered compound such as lithium cobalt oxide (LiCoO ₂ ) or lithium nickel oxide (LiNiO ₂ ), or a compound substituted with one or more transition metals; Lithium manganese oxides such as formula Li _1+x Mn _2-x O ₄ (0≦ _x ≦0.33), LiMnO ₃ , LiMn ₂ O ₃ , and LiMnO ₂ ; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , LiFe ₃ O ₄ , V ₂ O ₅ , and Cu ₂ V ₂ O ₇ ; Ni site-type lithium nickel oxide represented by the formula LiNi _1-x M _x O ₂ (M = Co, Mn, Al, Cu, Fe, Mg, B or Ga; 0.01≦x≦0.3); Formula LiMn _{2 - x} M _x O ₂ (M = Co, Ni, Fe, Cr, Zn or Ta; 0.01≤x≤0.1) or Li ₂ Mn ₃ MO ₈ (M = Fe, Co, Ni, Cu or Zn A lithium manganese composite oxide represented by ); A lithium manganese composite oxide having a spinel structure represented by LiNi _x Mn _{2 - x} O ₄ ; LiCoPO ₄ ; LiFePO ₄ ; Elemental sulfur (S ₈ ); Li ₂ S _n (n≥≥1), an organosulfur compound or a carbon-sulfur polymer ((C ₂ S _x ) _n : x=2.5 ~ 50, n≥≥2), etc. , It is not limited to these only. Preferably, the positive electrode active material may be at least one selected from the group consisting of lithium cobalt oxide, lithium nickel oxide, and lithium manganese oxide. More preferably, it may be lithium cobalt oxide.

The conductive material is for improving electrical conductivity, and there is no particular limitation as long as it is an electronic conductive material that does not cause chemical changes in a lithium secondary battery.

In general, carbon black, graphite, carbon fiber, carbon nanotubes, metal powder, conductive metal oxides, organic conductive materials, etc. can be used, and products currently marketed as conductive materials include acetylene black series (Chevron Chemical Company (Chevron Chemical Company or Gulf Oil Company), Ketjen Black EC series (Armak Company), Vulcan XC-72 (Cabot Company) (Cabot Company) and Super P (MMM). For example, acetylene black, carbon black, graphite, etc. are mentioned.

In addition, the positive electrode active material may further include a binder having a function of maintaining the positive electrode active material in the positive electrode current collector and connecting the active materials. As the binder, for example, polyvinylidene fluoride-hexafluoropropylene (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, poly Various types of binders, such as polymethyl methacrylate, styrene-butadiene rubber (SBR), and carboxyl methyl cellulose (CMC), can be used.

The negative electrode may include a negative electrode current collector and a negative active material disposed on the negative electrode current collector. Alternatively, the negative electrode may be a lithium metal plate.

The negative electrode current collector is for supporting the negative electrode active material, and is not particularly limited as long as it has excellent conductivity and is electrochemically stable in the voltage range of a lithium secondary battery. For example, copper, stainless steel, aluminum, nickel, titanium, Palladium, calcined carbon, copper or stainless steel surface treated with carbon, nickel, silver, etc., aluminum-cadmium alloy, and the like may be used.

The negative electrode current collector may form fine irregularities on its surface to enhance the bonding strength with the negative electrode active material, and various forms such as films, sheets, foils, meshes, nets, porous bodies, foams, and nonwoven fabrics may be used.

The negative active material includes a material capable of reversibly intercalating or deintercalating lithium (Li ⁺ ), a material capable of reversibly forming a lithium-containing compound by reacting with lithium ions, a lithium metal or a lithium alloy can do. The material capable of reversibly occluding or releasing lithium ions (Li ⁺ ) may be, for example, crystalline carbon, amorphous carbon, or a mixture thereof. A material capable of reversibly forming a lithium-containing compound by reacting with the lithium ions (Li ⁺ ) may be, for example, tin oxide, titanium nitrate, or silicon. The lithium alloy is, for example, lithium (Li) and sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium ( It may be an alloy of a metal selected from the group consisting of Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum (Al), and tin (Sn). Preferably, the negative active material may be lithium metal, and specifically, may be in the form of a lithium metal thin film or lithium metal powder.

The method of forming the negative active material is not particularly limited, and a method of forming a layer or film commonly used in the art may be used. For example, a method such as compression, coating, or vapor deposition may be used. In addition, a case in which a metal lithium thin film is formed on a metal plate by initial charging after assembling a battery without a lithium thin film in the current collector is also included in the negative electrode of the present invention.

The separator is for physically separating both electrodes in the lithium secondary battery of the present invention, and can be used without particular limitation as long as it is used as a separator in a lithium secondary battery. In particular, it has a low resistance to ion movement of the electrolyte and impregnates the electrolyte. It is desirable to have excellent ability.

The separator may be made of a porous substrate, and the porous substrate may be used as long as it is a porous substrate commonly used in an electrochemical device, and for example, a polyolefin-based porous membrane or a nonwoven fabric may be used, but is not particularly limited thereto. .

Examples of the polyolefin-based porous membrane include polyolefin-based polymers such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, and ultra-high molecular weight polyethylene, polyolefin-based polymers such as polypropylene, polybutylene, and polypentene, respectively, or a mixture of them. There is one membrane.

As the nonwoven fabric, in addition to the polyolefin nonwoven fabric, for example, polyethylene terephthalate, polybutyleneterephthalate, polyester, polyacetal, polyamide, polycarbonate ), polyimide, polyetheretherketone, polyethersulfone, polyphenyleneoxide, polyphenylenesulfide, and polyethylene naphthalate, respectively, independently Alternatively, a nonwoven fabric formed of a polymer obtained by mixing them may be mentioned. The structure of the nonwoven fabric may be a sponbond nonwoven fabric composed of long fibers or a melt blown nonwoven fabric.

The thickness of the porous substrate is not particularly limited, but may be 1 to 100 μm, preferably 5 to 50 μm.

The size and porosity of the pores present in the porous substrate are also not particularly limited, but may be 0.001 to 50 μm and 10 to 95%, respectively.

The electrolyte solution contains lithium ions and is used to cause an electrochemical oxidation or reduction reaction at the positive electrode and the negative electrode through this, as described above.

The injection of the electrolyte may be performed at an appropriate step in the manufacturing process of the electrochemical device, depending on the manufacturing process and required physical properties of the final product. That is, it can be applied before assembling the electrochemical device or at the final stage of assembling the electrochemical device.

In the lithium secondary battery according to the present invention, in addition to winding, which is a general process, lamination and stacking of a separator and an electrode and folding are possible.

The shape of the lithium secondary battery is not particularly limited and may be in various shapes such as a cylindrical shape, a stacked type, and a coin type.

A preferred embodiment of the lithium secondary battery according to the present invention may be a lithium metal secondary battery including lithium metal as a negative electrode. This is because the electrolyte for a lithium secondary battery according to the present invention can realize excellent cycle characteristics of lithium metal and secure the stability of the electrolyte, thereby sufficiently satisfying the characteristics required for a lithium metal secondary battery, thereby exhibiting excellent battery performance and lifespan characteristics. .

Hereinafter, a preferred embodiment is presented to aid the understanding of the present invention, but the following examples are only illustrative of the present invention, and it is obvious to those skilled in the art that various changes and modifications are possible within the scope and spirit of the present invention, It is natural that such modifications and modifications fall within the appended claims.

Example 1 to 4 and Comparative example 1 to 3: Preparation of electrolyte

An electrolyte solution for a lithium secondary battery was prepared with the composition shown in Table 1 below.

Lithium salt Solvent (volume ratio) additive Example 1 1.0 M (SO ₂ F) ₂ NLi EGDME ^{1 )} :TTE ^{2 )} (1:3) - Example 2 1.0 M (SO ₂ F) ₂ NLi DEGDME ^{3 )} :TTE ^{2 )} (1:3) - Example 3 1.0 M (SO ₂ F) ₂ NLi TEGDME ^{4 )} :TTE ^{2 )} (1:3) - Example 4 1.0 M (SO ₂ F) ₂ NLi EGDME ^{1 )} :TTE ^{2 )} (1:3) 0.05 M CsNO ₃ Comparative Example 1 4.0 M (SO ₂ F) ₂ NLi EGDME ^{1 )} - Comparative Example 2 3.0 M (SO ₂ F) ₂ NLi EGDME ^{1 )} - Comparative Example 3 3.0 M (SO ₂ F) ₂ NLi EGDME ^{1 )} 0.05 M CsNO ₃ 1) EGDME: ethyleneglycol dimethyl ether
2) TTE: 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether)
3) DEGDME: diethyleneglycol dimethyl ether
4) TEGDME: triethyleneglycol dimethyl ether

Experimental example 1: Evaluation of electrolyte properties

The flash points of the electrolytes prepared in Example 1 and Comparative Example 1 were measured using a manual flash point meter (HFP 382 TAG) manufactured by PAC, and viscosity using a viscometer manufactured by Brookfield. In addition, the ionic conductivity at 25 °C was measured with an ion conductivity measuring device manufactured by Thermo Scientific. The results obtained at this time are shown in Table 2 below.

Flash point (℃) Viscosity (cP) Ion conductivity (mS/cm) Example 1 >95 4.2 5.4 Comparative Example 1 76 36.2 5.7

Referring to Table 2, it can be seen that the electrolyte solution for a lithium secondary battery of Example 1 has a higher flash point and a lower viscosity than the electrolyte solution of Comparative Example 1. Through this, the electrolyte of the present invention has reduced flammability compared to the conventional electrolyte, and when the battery is driven, ion conductivity and wettability to the separator may be improved.

Experimental Example 2: Performance evaluation of half-cell

An electrode assembly was manufactured by placing a polyethylene separator between the electrodes using copper as a working electrode and lithium metal as a counter electrode. Thereafter, a coin-shaped half-cell was manufactured by injecting the electrolyte solution prepared in the above Examples and Comparative Examples.

The performance of the half-cell prepared by the above method was measured by the following method.

(1) Cycle efficiency

Electrodeposition of lithium to the copper electrode by 1.04 ㎃/㎠ at 25℃ with a constant current of 0.5 ㎃/㎠ with respect to the half-cells injected with the electrolyte solution prepared in Examples 1 to 3 and Comparative Example 1, and then the potential of the measuring electrode Cycle efficiency (%) was measured while performing 100 cycles under conditions of dissolving lithium from the copper electrode until it reached 1.0 V (vs. Li/Li ⁺ ). Among the results obtained at this time, the cycle efficiency between 10 cycles and 90 cycles is shown in FIG. 1 below.

(2) cycle overvoltage

For the half cells injected with the electrolytes prepared in Example 1 and Comparative Example 1, the cycle was performed in the same manner as the cycle efficiency measurement, and cycle overvoltages at 5 cycles, 50 cycles, and 100 cycles were measured. The results obtained at this time are shown in FIGS. 2 and 3 below.

(3) cumulative irreversible efficiency

For the half cells injected with the electrolytes prepared in Examples 1 to 3 and Comparative Example 1, the cycle was performed in the same manner as the cycle efficiency measurement, and the cumulative irreversible efficiency (%) between 10 cycles and 90 cycles was measured. The results obtained at this time are shown in FIG. 4 below.

As shown in Figs. 1 to 4, it can be seen that the cycle efficiency and accumulated irreversible efficiency characteristics of the half cell using the electrolyte according to the present invention are excellent, and cycle overvoltage does not occur significantly. Specifically, in the case of the half-cell injected with the electrolyte solution of Comparative Example 1 using only a glycol ether-based solvent through FIG. 1, the cycle efficiency is fluctuated, whereas Examples 1 to 1 including a glycol ether-based solvent and a fluorinated ether-based solvent in a certain volume ratio When the electrolyte of 3 is used, the cycle efficiency remains stable throughout the cycle. In addition, it can be seen from FIGS. 2 and 3 that the cycle overvoltage is significantly reduced in Example 1 compared to Comparative Example 1. From the above results, it can be seen that the performance and life characteristics of the battery are improved through the electrolyte composition of the present invention. In addition, referring to FIG. 4, it is possible to see the difference in the irreversible efficiency of the battery according to the type of glycol ether solvent through Examples 1 to 3, and the retention rate of the charge/discharge capacity of Examples 1 to 3 compared to Comparative Example 1. It can be seen that this is excellent, and in the case of Example 1 using ethylene glycol dimethyl ether, it can be seen that the retention rate of charge and discharge capacity is the best.

Experimental Example 2: Evaluation of full-cell performance

After preparing a positive electrode active material slurry comprising 95% by weight, 2.5% by weight, and 2.5% by weight of LiCoO ₂ as a positive electrode active material, super P as a conductive material, and polyvinylidene fluoride (PVDF) as a binder, respectively, the positive electrode active material slurry Was applied on an aluminum current collector and then dried to prepare a positive electrode.

A lithium metal thin film having a thickness of 20 μm was used as the negative electrode.

After the prepared positive electrode and negative electrode were positioned to face each other, a polyethylene separator was placed therebetween, and then the electrolyte solution prepared in Examples and Comparative Examples was injected to prepare a coin-shaped battery.

The battery manufactured by the above method was charged at a constant current to 4.25 V at a current density of 0.62 ㎃/cm 2, and then maintained at a constant voltage at 4.25 V at a constant voltage. Discharge was completed in CC (constant current) mode up to 3 V with a current density of 0.62 ㎃/㎠. The battery was activated while performing 3 cycles under the same conditions, and the performance was measured by the following method.

The electrolyte prepared in Example 1 and Comparative Example 2 was injected, and the activated battery was charged at a constant current to 4.25 V at a current density of 1.56 ㎃/㎠ and then kept at a constant voltage at 4.25 V at a constant voltage of 0.16 When it reached mA/cm2, charging was terminated. Discharge was completed in CC mode up to 3 V with a current density of 1.56 mA/㎠. The lifespan characteristics were confirmed by measuring the capacity while performing 40 cycles under the same conditions. The results obtained at this time are shown in FIG. 5 below.

For the electrolytes prepared in Example 3 and Comparative Example 3, the lifespan characteristics were confirmed by measuring capacity while performing 150 cycles in the same manner as described above. The results obtained at this time are shown in FIG. 6 below.

Through the comparison of FIGS. 5 and 6, it can be seen that the life characteristics of the battery including the electrolyte solution using the MNO _x compound additive are superior to that of the battery using the electrolyte solution not including the additive.

Specifically, 5, compared to the battery using the electrolyte of Comparative Example 2, when the electrolyte of Example 1 was used, the life characteristics were excellent, but both electrolytes were oxidized before 40 cycles, thereby causing a problem in driving the battery. On the other hand, referring to FIG. 6, it can be seen that lifespan characteristics are significantly improved compared to the electrolyte solutions of Example 1 and Comparative Example 2 without using an additive. In particular, when the electrolyte solution of Example 4 containing all of a glycol ether solvent, a fluorinated ether solvent, and an additive is used, the life characteristics are more improved compared to the battery using the electrolyte solution of Comparative Example 3 that does not contain a fluorinated ether solvent. Can be seen.

The electrolyte for a lithium secondary battery according to the present invention improves battery performance and life by including two solvents in a predetermined volume ratio, thereby enabling high capacity, stability and long life of the lithium secondary battery.

Claims (9)

Lithium salt;
A glycol ether solvent represented by the following formula (1);
A fluorinated ether solvent represented by the following formula (2); And
Including additives,
The glycol ether solvent and the fluorinated ether solvent are included in a volume ratio of 1:2 to 1:5,
The additive is an electrolyte for a lithium secondary battery containing cesium nitrate:
[Formula 1]
R ¹ -O-(CH ₂ CH ₂ O) _n -R ²
[Formula 2]
R ³ -OR ⁴
(In Formulas 1 and 2,
R ¹ and R ² are the same as or different from each other, and each independently a C1 to C6 alkyl group, a C6 to C12 aryl group or a C7 to C13 arylalkyl group,
R ³ and R ⁴ are the same as or different from each other, and each independently a C1 to C6 alkyl group or a C1 to C6 fluorinated alkyl group, but at least one of R ³ and R ⁴ is a C1 to C6 fluorinated alkyl group,
n is an integer of 1 to 3.). The method of claim 1,
The glycol ether-based solvent and the fluorinated ether-based solvent are contained in a volume ratio of 1:2 to 1:4, an electrolyte for a lithium secondary battery. The method of claim 1,
The glycol ether solvent is composed of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol ethyl methyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, and triethylene glycol diethyl ether. An electrolyte for a lithium secondary battery containing at least one selected from the group. The method of claim 1,
The fluorinated ether solvent may be 1,1,2,2-tetrafluoroethyl-2,2,3,3- tetrafluoropropyl ether; 1,1,2,2-tetrafluoroethyl-2,2,3,3,3-pentafluoropropyl ether; 1,1,2,3,3,3-hexafluoropropyl-2,2,2-trifluoroethyl ether; Ethyl-1,1,2,3,3,3- hexafluoropropyl ether; Difluoromethyl-2,2,3,3,3-pentafluoropropyl ether and difluoromethyl-2,2,3,3- tetrafluoropropyl ether containing at least one selected from the group consisting of The electrolyte for lithium secondary batteries. The method of claim 1,
The lithium secondary battery electrolyte is a lithium salt; At least one glycol ether solvent selected from the group consisting of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, and ethylene glycol ethyl methyl ether; 1,1,2,2-tetrafluoroethyl-2,2,3,3- tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3,3- Pentafluoropropyl ether and 1,1,2,3,3,3-hexafluoropropyl-2,2,2-trifluoroethyl ether containing at least one fluorinated ether-based solvent selected from the group consisting of , Lithium secondary battery electrolyte. The method of claim 1,
The lithium salt is LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiC ₄ BO ₈ , LiAsF ₆ , LiSbF ₆ , LiAlCl ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F ₅ SO ₂ ) ₂ NLi, (SO ₂ F) ₂ NLi, (CF ₃ SO ₂ ) ₃ CLi, chloroborane lithium , A lithium secondary battery electrolyte containing at least one selected from the group consisting of lithium lower aliphatic carboxylic acid having 4 or less carbon atoms, lithium 4-phenyl borate, and lithium imide. delete The method of claim 1,
The additive is contained in a concentration of 0.01 to 0.2 M, lithium secondary battery electrolyte. A lithium secondary battery comprising the electrolyte of any one of claims 1 to 6 and 8.

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