KR20200089237A - Lithium secondary battery - Google Patents

Abstract

Provided is a lithium secondary battery. To this end, a positive electrode active material layer is applied, which includes a sulfur-carbon composite in which the relationship between the specific surface area and conductivity of a carbon material satisfies a specific condition (AC factor). By specifically controlling the porosity (%) of the positive electrode active material layer and the mass of sulfur per unit area (mg/cm^2), the initial discharge capacity of a battery is improved while high energy density is realized.

Description

Lithium secondary battery {LITHIUM SECONDARY BATTERY}

The present invention relates to a lithium secondary battery.

Lithium having a relatively low weight-to-weight energy storage density (~250 Wh/kg) as the application area of secondary batteries expands to electric vehicles (EVs) or energy storage systems (ESSs), etc. Ion secondary batteries have limitations in application to these products. On the other hand, lithium-sulfur secondary batteries are in the spotlight as the next generation secondary battery technology because they can theoretically realize a high weight-to-weight energy storage density (~2,600 Wh/kg).

The lithium-sulfur secondary battery is a battery system using a sulfur-based material having a sulfur-sulfur bond as a positive electrode active material and lithium metal as a negative electrode active material. The lithium-sulfur secondary battery has an advantage that sulfur, which is a main material of the positive electrode active material, is rich in resources worldwide, has no toxicity, and has a low weight per atom.

In a lithium-sulfur secondary battery, lithium, a negative electrode active material, is released and ionized when discharged, and a sulfur-based material, which is a positive electrode active material, is reduced by accepting electrons. At this time, the oxidation reaction of lithium is a process in which lithium metal releases electrons and is converted into lithium cation form. In addition, the reduction reaction of sulfur is a process in which the sulfur-sulfur bond is converted into a sulfur anion form by accepting two electrons. The lithium cation generated by the oxidation reaction of lithium is transferred to the positive electrode through the electrolyte, and combines with the sulfur anion generated by the reduction reaction of sulfur to form a salt. Specifically, sulfur before discharge has a cyclic S ₈ structure, which is converted to lithium polysulfide (lithium polysulfide, Li ₂ S _x , x = 8, 6, 4, 2) by a reduction reaction, and this lithium polysulfide When is completely reduced, lithium sulfide (Li ₂ S) is finally produced.

As a positive electrode active material, sulfur has a characteristic of low electrical conductivity, so it is difficult to secure reactivity with electrons and lithium ions in a solid-state form. In order to improve the reactivity of the sulfur, the existing lithium-sulfur secondary battery generates an intermediate polysulfide in the form of Li ₂ S _x to induce a liquid-state reaction and improve reactivity. In this case, an ether-based solvent such as dioxolane or dimethoxyethane, which is highly soluble in lithium polysulfide, is used as a solvent for the electrolyte. In addition, the existing lithium-sulfur secondary battery constructs a catholyte type lithium-sulfur secondary battery system in order to improve reactivity, in which case, due to the characteristics of lithium polysulfide easily soluble in the electrolyte, the sulfur content is Reactivity and life characteristics are affected. In addition, in order to build a high energy density, a low content of electrolyte must be injected, but as the content of the electrolyte decreases, the concentration of lithium polysulfide in the electrolyte increases, leading to a decrease in fluidity of the active material and an increase in side reactions. It is difficult.

Since the elution of lithium polysulfide adversely affects the capacity and life characteristics of the battery, various techniques for suppressing the elution of lithium polysulfide have been proposed.

As an example, Republic of Korea Patent Publication No. 2016-0037084 is to prevent the melting of lithium polysulfide by using a carbon nanotube aggregate of a three-dimensional structure coated with graphene with a carbon material, the conductivity of the sulfur-carbon nanotube composite It is disclosed that it can be improved.

In addition, Korean Patent Registration No. 1379716 treats hydrofluoric acid on graphene to form pores on the graphene surface, and uses a graphene composite containing sulfur prepared through a method of growing sulfur 　 particles in the pores as a positive electrode active material. It is disclosed that it is possible to minimize the capacity decrease of the battery by suppressing the dissolution of lithium polysulfide through.

These patents prevented the elution of lithium polysulfide by varying the structure or material of the sulfur-carbon composite used as the positive electrode active material, and improved the performance degradation problem of the lithium-sulfur secondary battery to some extent, but the effect is not sufficient. Therefore, in order to build a high-energy density lithium-sulfur secondary battery, a battery system capable of driving an electrode with high loading and low porosity is required, and research on such a battery system has been continuously conducted in the related technical field. .

Republic of Korea Patent Publication No. 2016-0037084 (2016.04.05), sulfur-carbon nanotube composite, a manufacturing method thereof, a cathode active material for a lithium-sulfur battery containing the same, and a lithium-sulfur battery including the same Republic of Korea Registered Patent No. 1379716 (2014.03.25), a lithium-sulfur secondary battery comprising a graphene composite anode containing sulfur and a manufacturing method thereof

Abbas Fotouhi et al., Lithium-Sulfur Battery Technology Readiness and Applications―A Review, Energies 2017, 10, 1937.

Accordingly, the present inventors conducted various studies to solve the above problems, and as a result, applied a positive electrode active material layer containing a sulfur-carbon composite in which the relationship between the specific surface area and conductivity of the carbon material satisfies a specific condition ( AC factor ). , When the porosity (%) of the positive electrode active material layer and the mass of sulfur per unit area (mg/cm 2) are specifically controlled, it is confirmed that the lithium secondary battery having excellent initial discharge capacity and high energy density can be implemented. The invention was completed.

In addition, the present invention was completed by confirming that the above effects can be further enhanced by configuring the battery with the above-described configuration to satisfy the specific conditions.

Accordingly, an object of the present invention is to provide a lithium secondary battery having excellent initial discharge capacity and excellent energy density.

In order to achieve the above object, the present invention

A lithium secondary battery comprising an anode, a cathode, a separator and an electrolyte,

The positive electrode includes a sulfur-carbon composite containing a carbon material that satisfies a condition in which the AC factor represented by Equation 1 below is 100 or higher,

It provides a lithium secondary battery having a SC factor value of 0.45 or more represented by Equation 2 below:

[Equation 1]

(In Equation 1 above,

The specific area is the specific surface area of the carbon material,

Conductivity is the electrical conductivity converted from the resistance value of the powder measured in the state of applying a pressure of 2000 kgf to the carbon material . )

[Equation 2]

(In Equation 2 above,

P is the porosity (%) of the positive electrode active material layer in the positive electrode,

L is the mass of sulfur per unit area of the positive electrode active material layer in the positive electrode (mg/cm 2 ),

α is 10 (constant).

In one embodiment of the invention,

The electrolyte solution includes a solvent and a lithium salt,

The solvent may be characterized by including a first solvent having a DV ² factor value of 1.75 or less and a second solvent being a fluorinated ether-based solvent represented by Equation 3 below:

[Equation 3]

(In Equation 3 above,

DV is the dipole moment per unit volume (Dmol/L),

μ is the viscosity of the solvent (cP, 25℃),

γ is 100 (constant).

In one embodiment of the present invention, the lithium secondary battery may have a characteristic that an NS factor value represented by Equation 4 below is 3.5 or less:

[Equation 4]

(In Equation 4 above,

SC factor is the same as the value defined by Equation 2 above,

The DV ² factor is the same as the value defined by Equation 3 above.).

In one embodiment of the present invention, the lithium secondary battery may have a feature that an ED factor value represented by Equation 5 below is 850 or more:

[Equation 5]

(In Equation 5 above,

V is the discharge nominal voltage (V) for Li/Li ⁺ ,

SC factor is the same as the value defined by Equation 2 above,

C is the discharge capacity (mAh/g) when discharging at 0.1C rate,

D is the density of the electrolyte (g/cm 3 ).)

In the lithium secondary battery according to the present invention, a positive electrode active material layer including a sulfur-carbon composite that satisfies a specific condition ( AC factor ) in which the relationship between the specific surface area and conductivity of a carbon material is applied, and the porosity (%) of the positive electrode active material layer ) And the specific mass of sulfur per unit area (mg/cm 2 ), thereby providing excellent initial discharge capacity and providing an effect of realizing high energy density.

In addition, by configuring the battery so that the electrolyte satisfies a specific condition together with the above configuration, it provides an effect of further enhancing the above effect.

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

The terms or words used in the present specification and claims should not be interpreted as being limited to ordinary or dictionary meanings, and the inventor can appropriately define the concept of terms in order to explain his or her invention in the best way. Based on the principle of being present, it should be interpreted as meanings and concepts consistent with the technical spirit of the present invention.

The terms used in the present invention are only used to describe specific embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In the present invention, terms such as'include' or'have' are intended to indicate that there are features, numbers, steps, operations, components, parts, or combinations thereof described in the specification, and one or more other features. It should be understood that the existence or addition possibilities of fields or numbers, steps, operations, components, parts or combinations thereof are not excluded in advance.

The term “composite” as used herein refers to a substance that combines two or more materials to form physically and chemically different phases and expresses more effective functions.

Among lithium secondary batteries, lithium-sulfur secondary batteries have a high discharge capacity and energy density among various secondary batteries, and the sulfur used as a positive electrode active material is rich in reserves and is inexpensive, so the manufacturing cost of the battery can be lowered and it is environmentally friendly As a result, it is receiving attention as a next-generation secondary battery.

However, in the case of the existing lithium-sulfur secondary battery system, the loss of sulfur occurs because it does not suppress the dissolution of lithium polysulfide, and as a result, the amount of sulfur participating in the electrochemical reaction is drastically reduced, so the theoretical discharge capacity and theoretical energy density in actual operation It doesn't implement all.

The present inventors tried hard to solve the above problems, and in a lithium secondary battery including a positive electrode, a negative electrode, a separator, and an electrolyte, a relationship between a specific surface area of a carbon material and conductivity as a positive electrode active material is a specific condition ( AC factor ). When using a sulfur-carbon composite that satisfies, and specifically controlling the porosity (%) of the positive electrode active material layer and the mass of sulfur per unit area (mg/cm 2 ), the initial discharge capacity of the battery can be improved, and high energy density The present invention was completed by confirming that lithium secondary battery can also be implemented.

In addition, it was confirmed that the above-described effect can be further enhanced by adjusting the electrolyte solution to meet a specific condition together with the above-described configuration.

The present invention is a lithium secondary battery comprising an anode, a cathode, a separator and an electrolyte,

The positive electrode includes a sulfur-carbon composite containing a carbon material that satisfies a condition in which the AC factor represented by Equation 1 below is 100 or higher,

Lithium-sulfur secondary battery having an SC factor value of 0.45 or more represented by the following Equation 2:

[Equation 1]

(In Equation 1 above,

The specific area is the specific surface area of the carbon material,

Conductivity is the electrical conductivity converted from the resistance value of the powder measured in the state of applying a pressure of 2000 kgf to the carbon material . )

[Equation 2]

(In Equation 2 above,

P is the porosity (%) of the positive electrode active material layer in the positive electrode,

L is the mass of sulfur per unit area of the positive electrode active material layer in the positive electrode (mg/cm 2 ),

α is 10 (constant).

In the present invention, the lithium secondary battery may be a lithium-sulfur secondary battery.

When the AC factor represented by Equation 1 above satisfies the condition of 100 or more, the initial discharge capacity of the battery may be improved, and thus it is preferable to realize a lithium-sulfur secondary battery having a high energy density.

The upper limit of the AC factor is not particularly limited, but considering the characteristics of the actual carbon material, the AC factor value may be preferably 1,000 or less.

In the present invention, the carbon material preferably has a specific surface area of 100 to 4500 m 2 /g, and more preferably 250 to 4000 m 2 /g. At this time, the 　 specific surface area can be measured by a conventional 　BET method. If the specific surface area of the carbon material is less than the above range, there is a problem of deterioration of reactivity due to a decrease in the contact area with sulfur. There may be a problem that the amount of binder added increases.

The carbon material preferably has a pore volume of 0.8 to 5 cm 3 /g, more preferably 1 to 4.5 cm 3 /g. At this time, the 　 pore volume can be measured through a conventional 　BET method. When the pore volume of the carbon material is less than the above range, impregnation of sulfur into the pore structure is not well performed. On the contrary, when the pore volume exceeds the above range, the porosity of the electrode increases to increase the amount of the electrolyte to fill the pores, thereby increasing the energy density. There may be problems that can be difficult to achieve.

In the present invention, the carbon material preferably has an electrical conductivity of 10 to 100 S/cm, which is converted to a conductivity value of a powder measured under a pressure of 2000 kgf, and more preferably 20 to 100 S/cm. At this time, when the electrical conductivity is included below the above range, it is not preferable because a limitation in electron movement occurs, resulting in a decrease in efficiency during charging and discharging.

In the present invention, the carbon material may be at least one selected from the group consisting of graphite, carbon nanotubes, graphene, graphite, amorphous carbon, carbon black, and activated carbon, but is not limited thereto.

In the sulfur-carbon composite of the present invention, the carbon material may be included in an amount of 10 to 50% by weight, preferably 20 to 40% by weight based on the total weight of the sulfur-carbon composite.

The sulfur may be one or more selected from the group consisting of inorganic sulfur, Li ₂ S _n (n≧1), disulfide compounds, organic sulfur compounds, and carbon-sulfur polymers.

The sulfur may be included in 50 to 90% by weight based on the total weight of the sulfur-carbon complex, more preferably 60 to 80% by weight. When the sulfur is contained in less than 50% by weight, it is difficult to satisfy the SC factor due to insufficient mass (mg/cm2) ( L ) of sulfur per unit area of the positive electrode active material layer, and when it exceeds 90% by weight, sulfur is a carbon material. It is not preferable because impurity is impregnated into the pores and the surface and the sulfur is separately aggregated, which may cause a problem that the conductivity of the sulfur-carbon composite is significantly reduced.

In addition, the weight ratio of the carbon material and sulfur in the sulfur-carbon composite may be 1:1 to 1:9, preferably 1:1.5 to 1:4. If it is less than the above weight ratio range, as the content of the porous carbon material increases, the amount of binder addition required in preparing the positive electrode slurry increases. The increase in the amount of the binder added eventually increases the sheet resistance of the electrode and acts as an insulator to prevent electron pass, thereby degrading cell performance. Conversely, when the weight ratio range is exceeded, sulfur may be aggregated among them, and it may be difficult to directly participate in the electrode reaction due to difficulty receiving electrons.

The sulfur-carbon composite may be compounded by simply mixing the aforementioned sulfur and carbon materials, or may have a core-shell structured coating form or a supported form. The coating form of the core-shell structure is one of sulfur or microporous carbon materials coated with another material, for example, the surface of the carbon material may be wrapped with sulfur or vice versa. In addition, the supported form may be a form in which sulfur is supported inside the carbon. The form of the sulfur-carbon composite may be any form as long as it satisfies the content ratio of the sulfur and carbon materials, and is not limited in the present invention.

In the present invention, the sulfur-carbon composite preferably has a specific surface area of 3 to 20 m 2 /g, and more preferably 5.5 to 15 m 2 /g. When the specific surface area of the sulfur-carbon composite is less than 3 m 2 /g, it is not preferable in that sulfur does not impregnate the surface of the carbon material and deteriorates cell performance, and when it exceeds 20 m 2 /g, an electrode is manufactured. It is not preferable in that the addition amount of the binder increases.

In the present invention, the sulfur-carbon composite preferably has a pore volume of 0.075 to 1.000 cm 3 /g, more preferably 0.080 to 1.000 cm 3 /g. When the pore volume of the sulfur-carbon composite is less than 0.075 cm 3 /g, it is not preferable because sulfur is not impregnated in the sulfur-carbon composite and is present separately on the surface or agglomeration occurs, and when it exceeds 1.000 cm 3 /g Despite the large amount of space to be impregnated with sulfur, it is not preferable in that it is difficult to manufacture a high energy density electrode because the pores of the sulfur-complex are not utilized.

The positive electrode according to the present invention may include a positive electrode current collector and a positive electrode active material layer coated on one or both surfaces of the positive electrode current collector.

The above-mentioned sulfur-carbon composite agent is used as the positive electrode active material.

The positive electrode current collector supports the 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, or the like, aluminum-cadmium alloy, or the like may be used.

The positive electrode current collector may form fine irregularities on its surface to enhance the bonding force with the positive electrode active material, and may use various forms such as a film, sheet, foil, mesh, net, porous body, foam, and nonwoven fabric.

The thickness of the positive electrode current collector is not particularly limited, but may be, for example, 3 to 500 μm.

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

The average diameter of the sulfur-carbon composite is not particularly limited in the present invention and may vary, but may be preferably 0.1 to 100 μm, and preferably 1 to 50 μm. When the above range is satisfied, there is an advantage that a high loading electrode can be manufactured.

The positive electrode active material may further include at least one additive selected from a transition metal element, a group IIIA element, a group IVA element, a sulfur compound of these elements, and an alloy of sulfur with these elements.

The transition metal elements include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Os, Ir, Pt, Au or Hg, and the like, and the group IIIA elements include Al, Ga, In, and Ti, and the group IVA elements may include Ge, Sn, and Pb.

The conductive material is a material that electrically connects the electrolyte and the positive electrode active material to serve as a path for electrons to move from the current collector to the positive electrode active material, and any conductive material can be used without limitation.

For example, the conductive material includes carbon black such as Super P (Super-P), Denka Black, Acetylene Black, Ketjen Black, Channel Black, Furnace Black, Lamp Black, Summer Black, and Carbon Black; Carbon derivatives such as carbon nanotubes and fullerenes; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride powder, aluminum powder, and nickel powder; Alternatively, conductive polymers such as polyaniline, polythiophene, polyacetylene, and polypyrrole may be used alone or in combination.

The content of the conductive material may be added in an amount of 0.01 to 30% by weight based on the total weight of the mixture containing the positive electrode active material.

The binder maintains the positive electrode active material in the positive electrode current collector, and organically connects the positive electrode active materials to increase the binding force therebetween, and any binder known in the art may be used.

For example, the binder may include a fluorine resin-based binder including polyvinylidene fluoride (PVdF) or polytetrafluoroethylene (PTFE); Rubber-based binders including styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber, and styrene-isoprene rubber; Cellulose-based binders including carboxyl methyl cellulose (CMC), starch, hydroxy propyl cellulose, and regenerated cellulose; Poly alcohol-based binders; Polyolefin-based binders including polyethylene and polypropylene; Polyimide-based binders; Polyester-based binders; And silane-based binder; may be used one or two or more mixtures or copolymers selected from the group consisting of.

The content of the binder may be added in 0.5 to 30% by weight based on the total weight of the mixture containing the positive electrode active material. If the content of the binder is less than 0.5% by weight, the physical properties of the positive electrode may deteriorate, and the active material and the conductive material in the positive electrode may drop off. If the content exceeds 30% by weight, the ratio of the active material and the conductive material in the positive electrode is relatively reduced, thereby reducing battery capacity. can do.

The positive electrode can be produced by the method described above, or by a conventional method known in the art. More specifically, for example, a slurry is prepared by mixing and stirring an additive such as a binder, a conductive material, and a filler in a positive electrode active material, if necessary, and then applying (coating) it to a current collector of a metal material, compressing it, and then drying Thus, an anode can be produced.

Specifically, first, the binder is dissolved in a solvent for preparing a slurry, and then the conductive material is dispersed. As a solvent for preparing the slurry, a positive electrode active material, a binder, and a conductive material can be uniformly dispersed, and it is preferable to use one that is easily evaporated. Representatively, acetonitrile, methanol, ethanol, tetrahydrofuran, water, iso Profile alcohol, etc. can be used. Next, the positive electrode active material, or optionally with an additive, is uniformly dispersed again in a solvent in which the conductive material is dispersed to prepare a positive electrode slurry. The amount of the solvent, positive electrode active material, or optionally additive contained in the slurry does not have a particularly important meaning in the present application, and it is sufficient only to have an appropriate viscosity to facilitate application of the slurry. The slurry thus prepared is applied to a current collector and vacuum dried to form an anode. The slurry may be applied to the current collector at an appropriate thickness according to the viscosity of the slurry and the thickness of the anode to be formed.

The coating may be performed by a method commonly known in the art, but for example, the positive electrode active material slurry is distributed on one side of the positive electrode current collector and then uniformly dispersed using a doctor blade or the like. Can. In addition, it may be performed through methods such as die casting, comma coating, and screen printing.

The drying is not particularly limited, but may be performed within 1 day in a vacuum oven at 50 to 200°C.

The positive electrode of the present invention manufactured by the above-described material and method is divided by the SC factor value represented by Equation 2 below.

[Equation 2]

(In Equation 2 above,

P is the porosity (%) of the positive electrode active material layer in the positive electrode,

L is the mass of sulfur per unit area of the positive electrode active material layer in the positive electrode (mg/cm 2 ),

α is 10 (constant).

The lithium-sulfur secondary battery according to the present invention realizes a high energy density by organic bonding of the cathode, separator and electrolyte as well as the above-described positive electrode, and according to the present invention, the lithium-sulfur secondary battery realizes a high energy density. In order to do this, the SC factor value may be 0.45 or more, preferably 0.5 or more. In the present invention, the upper limit of the SC factor value is not particularly limited, but considering the driving of an actual lithium-sulfur secondary battery, the SC factor value may be 4.5 or less. When the SC factor value is 0.45 or more, the performance of the existing lithium-sulfur secondary battery, such as the energy density of the battery, decreases during actual driving, but in the case of the lithium-sulfur secondary battery according to the present invention, the performance of the battery even during actual driving This is maintained without deterioration.

The negative electrode according to the present invention may be composed of a negative electrode current collector and a negative electrode active material layer formed on one or both sides thereof. Alternatively, the negative electrode may be a lithium metal plate.

The negative active material layer may include a negative active material and optionally a conductive material and a binder.

The negative active material is a material capable of reversibly intercalating or deintercalating lithium ions, a material capable of reversibly reacting with lithium ions to form a lithium-containing compound, lithium metal or lithium alloy Can be used.

The material capable of reversibly intercalating or deintercalating the lithium ions 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 ion 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 ( Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum (Al), and tin (Sn).

The current collector, a conductive material, a binder, etc., except for the negative electrode active material, and a method for manufacturing the negative electrode may be the materials and methods used in the positive electrode described above.

The separation membrane according to the present invention is a physical separation membrane having a function of physically separating the positive electrode and the negative electrode, and can be used without particular limitation as long as it is used as a normal separation membrane. It is desirable to be excellent.

In addition, the separator enables the transport of lithium ions between the positive electrode and the negative electrode while separating or insulating the positive electrode and the negative electrode from each other. The separator may be made of a material having a porosity of 30 to 50% and a non-conductive or insulating material.

Specifically, a porous polymer film such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer and an ethylene/methacrylate copolymer may be used. It is possible to use a non-woven fabric made of high melting point glass fiber or the like. Among them, a porous polymer film is preferably used.

If a polymer film is used as both the buffer layer and the separation membrane, the amount of electrolyte impregnation and ion conduction characteristics decreases, and the effect of reducing overvoltage and improving capacity characteristics is negligible. Conversely, when all non-woven materials are used, mechanical stiffness cannot be secured, resulting in a short circuit of the battery. However, when the film-type separation membrane and the polymer nonwoven fabric buffer layer are used together, the mechanical performance can be secured together with the battery performance improvement effect due to the adoption of the buffer layer.

Preferably, in the present invention, an ethylene homopolymer (polyethylene) polymer film is used as a separator, and a polyimide nonwoven fabric is used as a buffer layer. At this time, the polyethylene polymer film is preferably a thickness of 10 to 25μm, porosity of 40 to 50%.

The electrolytic solution according to the present invention is a non-aqueous electrolytic solution containing a lithium salt and is composed of a lithium salt and a solvent. The electrolyte has a density of less than 1.5 g/cm 3. When the electrolyte has a density of 1.5 g/cm 3 or more, it is difficult to realize a high energy density of the lithium-sulfur secondary battery due to an increase in the weight of the electrolyte.

The lithium salt is a material that can be easily dissolved in a non-aqueous organic solvent, for example, LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiB(Ph) ₄ , LiC ₄ BO ₈ , LiPF ₆ , _{LiCF 3 SO 3, LiCF 3 CO} 2, LiAsF 6, LiSbF 6, LiAlCl 4, LiSO 3 CH 3, LiSO 3 CF 3, LiSCN, LiC (CF 3 SO 2) 3, LiN (CF 3 SO 2) 2, LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN(SO ₂ F) ₂ , chloro borane lithium, lower aliphatic lithium carboxylate, lithium tetraphenyl borate, and lithium imide. In one embodiment of the present invention, the lithium salt may be preferably a lithium imide such as LiTFSI.

The concentration of the lithium salt is 0.1 to 8.0 M, depending on several factors such as the exact composition of the electrolyte mixture, the solubility of the salt, the conductivity of the dissolved salt, the conditions for charging and discharging the battery, the working temperature and other factors known in the lithium secondary battery field. , Preferably 0.5 to 5.0 M, more preferably 1.0 to 3.0 M. If the concentration of the lithium salt is less than the above range, the conductivity of the electrolyte solution may be lowered to deteriorate the battery performance, and if it exceeds the above range, the mobility of the lithium ion (Li ⁺ ) may decrease due to an increase in the viscosity of the electrolyte solution. It is desirable to select an appropriate concentration.

The solvent includes a first solvent and a second solvent. The first solvent has the highest dipole moment per unit volume among the components contained in the solvent by 1% by weight or more, and thus is characterized by having a high dipole moment and a low viscosity. When a solvent having a high dipole moment is used, it has an effect of improving the solid phase reactivity of sulfur, and this effect can be excellently expressed when the solvent itself has a low viscosity. In the present invention, the first solvent is classified by the DV ² factor represented by Equation 3 below.

[Equation 3]

(In Equation 3 above,

DV is the dipole moment per unit volume (Dmol/L),

μ is the viscosity of the solvent (cP, 25℃),

γ is 100 (constant).

According to the present invention, the DV ² factor value may be 1.75 or less, preferably 1.5 or less. In the present invention, the lower limit of the DV ² factor value is not particularly limited, but considering the driving of an actual lithium-sulfur secondary battery, the DV ² factor value may be 0.1 or more. When mixing a solvent having a DV ² factor value of 1.75 or less as in the first solvent, even when a positive electrode having a low porosity and a high loading amount of sulfur as a positive electrode active material is applied to a lithium-sulfur battery, the functionality of the electrolyte can be maintained. There is, the performance of the battery is not lowered.

If the first solvent in the present invention is included in the range of the above-mentioned DV ² factor value, the type is not particularly limited, but propionitrile (Propionitrile), dimethylacetamide (Dimethylacetamide), dimethylformamide (Dimethylformamide), gamma- Butyrolactone (Gamma-Butyrolactone), triethylamine (Triethylamine) and may include one or more selected from the group consisting of 1-iodopropane (1-iodopropane).

The first solvent may include 1 to 50% by weight, preferably 5 to 40% by weight, more preferably 10 to 30% by weight based on the total weight of the solvent constituting the electrolyte solution. When the solvent according to the present invention includes the first solvent within the above-mentioned weight percent range, the performance of the battery may be improved even when used with a positive electrode having a low porosity and a high loading amount of sulfur as a positive electrode active material.

The lithium-sulfur secondary battery of the present invention may be further classified by an NS factor combining the SC factor and the DV ² factor. The NS factor is represented by Equation 4 below.

[Equation 4]

(In Equation 4 above,

SC factor is the same as the value defined by Equation 2 above,

The DV ² factor is the same as the value defined by Equation 3 above.).

In the present invention, the NS factor value may be 3.5 or less, preferably 3.0 or less, and more preferably 2.7 or less. In the present invention, the lower limit of the NS factor value is not particularly limited, but considering the driving of the actual lithium-sulfur secondary battery, the NS factor value may be 0.1 or more. When the NS factor value is adjusted within the above range, the performance improvement effect of the lithium-sulfur secondary battery may be more excellent.

The second solvent in the present invention is a fluorinated ether-based solvent. Conventionally, in order to control the viscosity of the electrolyte solution, a solvent such as dimethoxyethane or dimethylcarbonate was used as a diluent, and when such a solvent is used as a diluent, high loading as in the present invention , It is not possible to drive a battery containing a positive electrode having low porosity. Therefore, in the present invention, a second solvent is added together with the first solvent to drive the positive electrode according to the present invention. If the second solvent is a fluorinated ether-based solvent generally used in the art, the type is not particularly limited, but 1H,1H,2'H,3H-decafluorodipropyl ether (1H,1H,2' H,3H-Decafluorodipropyl ether), Difluoromethyl 2,2,2-trifluoroethyl ether, 1,2,2,2-tetrafluoroethyl trifluoro Methyl ether (1,2,2,2-Tetrafluoroethyl trifluoromethyl ether), 1,1,2,3,3,3-hexafluoropropyl difluoromethyl ether (1,1,2,3,3,3- Hexafluoropropyl difluoromethyl ether), pentafluoroethyl 2,2,2-trifluoroethyl ether and 1H,1H,2′H-perfluorodipropyl ether (1H,1H, 2′H-Perfluorodipropyl ether).

The second solvent may include 50 to 99% by weight, preferably 60 to 95% by weight, more preferably 70 to 90% by weight based on the total weight of the solvent constituting the electrolyte solution. When the solvent according to the present invention includes a second solvent within the above-mentioned weight% range, the performance of the battery is improved even when used with a positive electrode having a low porosity and a positive loading amount of sulfur as a positive electrode active material, as in the first solvent. Can have When mixing the first solvent and the second solvent, considering the effect of improving the performance of the battery, the second solvent may be included in the electrolyte in an amount equal to or greater than the first solvent. According to the present invention, the solvent may include the first solvent and the second solvent in a weight ratio of 1:1 to 1:9, preferably 3:7 to 1:9 (first solvent:second solvent).

The non-aqueous electrolyte solution for a lithium-sulfur battery of the present invention may further include a nitric acid or nitrous acid compound as an additive. The nitric acid or nitrite-based compound has an effect of forming a stable film on the lithium electrode and improving charging and discharging efficiency. The nitric acid or nitrite-based compound is not particularly limited in the present invention, but lithium nitrate (LiNO3), potassium nitrate (KNO3), cesium nitrate (CsNO3), barium nitrate (Ba(NO3)2), ammonium nitrate (NH4NO3), Inorganic nitric acid or nitrite compounds such as lithium nitrite (LiNO2), potassium nitrite (KNO2), cesium nitrite (CsNO2), and ammonium nitrite (NH4NO2); Organic nitric acid such as methyl nitrate, dialkyl imidazolium nitrate, guanidine nitrate, imidazolium nitrate, pyridinium nitrate, ethyl nitrite, propyl nitrite, butyl nitrite, pentyl nitrite, octyl nitrite, etc. Or nitrite compounds; Organic nitro compounds such as nitromethane, nitropropane, nitrobutane, nitrobenzene, dinitrobenzene, nitropyridine, dinitropyridine, nitrotoluene, dinitrotoluene, and combinations thereof. Uses lithium nitrate.

In addition, the electrolyte may further include other additives for the purpose of improving charge/discharge characteristics, flame retardancy, and the like. Examples of the additives include pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexatriphosphate amide, nitrobenzene derivative, sulfur, quinone imine dye, and N-substituted oxazoli Dinon, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride, fluoroethylene carbonate (FEC), propene sulfone (PRS), vinylene carbonate ( VC) and the like.

The lithium-sulfur secondary battery according to the present invention is divided by an ED factor value represented by Equation 5 below.

[Equation 5]

(In Equation 5 above,

V is the discharge nominal voltage (V) for Li/Li ⁺ ,

SC factor is the same as the value defined by Equation 2 above,

C is the discharge capacity (mAh/g) when discharging at 0.1C rate,

D is the density of the electrolyte (g/cm 3 ).)

The higher the ED factor, the higher the energy density in a real lithium-sulfur secondary battery. According to the present invention, the ED factor value may be 850 or more, preferably 870 or more, and more preferably 891 or more. In the present invention, the upper limit of the ED factor value is not particularly limited, but considering the driving of an actual lithium-sulfur secondary battery, the ED factor value may be 10,000 or less. The range of the ED factor value means that the lithium-sulfur secondary battery according to the present invention can realize an improved energy density than the existing lithium-sulfur secondary battery.

The lithium-sulfur secondary battery of the present invention may be prepared by disposing a separator between an anode and a cathode to form an electrode assembly, and the electrode assembly is placed in a cylindrical battery case or a square battery case and then injected with electrolyte. Alternatively, after laminating the electrode assembly, it may be prepared by impregnating it with an electrolyte and sealing the obtained result in a battery case.

In addition, the present invention provides a battery module including the lithium-sulfur secondary battery as a unit cell.

The battery module can be used as a power source for medium and large devices that require high temperature stability, long cycle characteristics, and high capacity characteristics.

Examples of the medium-to-large-sized device include a power tool that moves under power by an all-electric motor; Electric vehicles including electric vehicles (EVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and the like; Electric two-wheeled vehicles including electric bicycles (E-bikes) and electric scooters (E-scooters); Electric golf carts; And a power storage system, but is not limited thereto.

Hereinafter, a preferred embodiment is provided to help the understanding of the present invention, but the following examples are merely illustrative of the present invention, and it is apparent to those skilled in the art that various changes and modifications are possible within the scope and technical thought scope of the present invention. It is no wonder that such variations and modifications fall within the scope of the appended claims.

Preparation Example 1: Preparation of sulfur-carbon composite active material

The specific surface area is 319 m2/g, and the electrical conductivity (the value obtained by converting the resistance value of the powder measured by using 2000kgf pressure using a powder resistance measurement equipment (Hantech, powder resistivitiy measurement system) to conductivity) is 62 S /cm carbon nanotubes ( AC factor = 156) and sulfur (S ₈ ) are evenly mixed in a weight ratio of 1:3, crushed by mortar mixing, and placed in an oven at 155°C for 30 minutes for sulfur- A carbon composite was prepared.

Preparation Example 2: Preparation of sulfur-carbon composite active material

The specific surface area is 1400 m 2 /g, and the electrical conductivity (the value obtained by converting the resistance value of the powder measured with conductivity using a powder resistance measurement equipment (Hantech, powder resistivitiy measurement system) to 2000 kgf) into conductivity is 26 S /cm carbon black ( AC factor = 192) and sulfur (S ₈ ) are evenly mixed in a weight ratio of 1:3, crushed by mortar mixing, and placed in an oven at 155° C. for 30 minutes for sulfur-carbon Composites were prepared.

Preparation Example 3: Preparation of sulfur-carbon composite active material

The specific surface area is 184 m2/g, and the electrical conductivity (the value obtained by converting the resistance value of the powder measured with conductivity using a powder resistance measurement equipment (Hantech, powder resistivitiy measurement system) to 2000kgf) into 41 S /cm carbon nanotubes ( AC factor = 100) and sulfur (S ₈ ) are evenly mixed in a weight ratio of 1:3, crushed by mortar mixing, and placed in an oven at 155°C for 30 minutes for sulfur- A carbon composite was prepared.

Preparation Example 4: Preparation of sulfur-carbon composite active material

The specific surface area is 191 m 2 /g, and the electrical conductivity (a value obtained by converting the resistance value of the powder measured by using 2000kgf pressure using a powder resistance measurement equipment (Hantech, powder resistivitiy measurement system) into conductivity) is 47 S /cm carbon nanotubes ( AC factor = 113) and sulfur (S ₈ ) are evenly mixed in a weight ratio of 1:3, crushed by mortar mixing, and placed in an oven at 155°C for 30 minutes for sulfur- A carbon composite was prepared.

Comparative Preparation Example 1: Preparation of sulfur-carbon composite active material

The specific surface area is 58 m 2 /g, and the electrical conductivity (the value obtained by converting the resistance value of the powder measured with the pressure of 2000 kgf using a powder resistance measurement equipment (Hantech, powder resistivitiy measurement system) into conductivity) is 33 S. /cm carbon nanotubes ( AC factor = 72) and sulfur (S ₈ ) are evenly mixed in a weight ratio of 1:3, crushed by mortar mixing, and placed in an oven at 155° C. for 30 minutes for sulfur- A carbon composite was prepared.

Preparation Examples 5 to 8 and Comparative Preparation Example 2: Preparation of positive electrode for lithium-sulfur battery

88% by weight of the sulfur-carbon composite prepared in Preparation Examples 1 to 4 or 88% by weight of the sulfur-carbon composite prepared in Comparative Preparation Example 1, 5% by weight of denka black as a conductive material, and 7% by weight of SBR and CMC as a binder Was mixed with distilled water to prepare a composition for forming an active material layer.

The composition was coated on an aluminum current collector in an amount of 6 mg/cm ² to prepare a positive electrode, and the porosity of the positive electrode active material layer and the mass of sulfur per unit area of the positive electrode active material layer were measured, and SC factor values were calculated based on this. It is shown in Table 1.

At this time, the porosity of the positive electrode active material layer was calculated by measuring the electrode weight and electrode thickness (using TESA-μHITE equipment manufactured by TESA) in the prepared positive electrode.

Sulfur-carbon complex used Porosity of positive electrode active material layer The mass of sulfur per unit area of the positive electrode active material layer SC factor Preparation Example 5 Preparation Example 1 60% 5.34 0.89 Preparation Example 6 Preparation Example 2 60% 4.5 0.75 Preparation Example 7 Preparation Example 3 60% 5.34 0.89 Preparation Example 8 Preparation Example 4 60% 5.34 0.89 Comparative Preparation Example 2 Comparative Production Example 1 60% 4.56 0.76

compare Manufacturing example 3: Preparation of a positive electrode for a lithium-sulfur battery

A composition for forming a positive electrode active material layer was prepared by mixing water as a solvent and mixing sulfur, super-P, SP, a conductive material, and a binder with a ball mill. At this time, Denka Black was used as the conductive material, and a binder in a mixed form of SBR and CMC was used as the binder, and the mixing ratio was sulfur and SP (weight ratio 9:1) by weight ratio: conductive material: binder was 90:10:10 It was made possible. The prepared positive electrode active material layer forming composition was applied to an aluminum current collector and dried to prepare a positive electrode (energy density of the positive electrode: 6.18 mAh/cm 2 ). In the prepared positive electrode, the porosity of the positive electrode active material layer was 75%, the mass of sulfur per unit area of the positive electrode active material layer was 3.1 mg/cm 2, and the SC factor value calculated based on this was 0.41.

Examples 1 to 4: Preparation of lithium-sulfur secondary battery

Each of the positive and negative electrodes prepared in Preparation Examples 5 to 8, 150 μm thick lithium foil was used as the negative electrode, and 20 μm thick and polyethylene having a porosity of 45% was used as the separator, so as to face the positive electrode and the negative electrode. After that, an electrode assembly was manufactured by interposing a separator therebetween.

Subsequently, after placing the electrode assembly inside the case, an electrolyte was injected to prepare a lithium-sulfur secondary battery.

At this time, the electrolyte solution was prepared by dissolving 3M concentration of lithium bis(trifluoromethyl sulfonyl)imide (LiTFSI) in an organic solvent, wherein the organic solvent was propionitrile (first solvent) and 1H,1H,2. A solvent in which'H,3H-decafluorodipropyl ether (second solvent) was mixed in a 3:7 weight ratio was used. In the first solvent, the dipole moment per unit volume was 97.1 D·mol/L, and the viscosity of the solvent was 0.38 cP (25° C.). The DV ² factor value calculated based on this was 0.39. At this time, the viscosity of the solvent was measured using a BROOKFIELD AMETEK LVDV2T-CP viscometer.

Comparative Example 1: Preparation of lithium-sulfur secondary battery

A lithium-sulfur secondary battery was manufactured in the same manner as in Example 2, except that the positive electrode prepared in Comparative Example 2 was used.

Comparative Example 2: Preparation of lithium-sulfur secondary battery

A lithium-sulfur secondary battery was manufactured in the same manner as in Example 2, except that the positive electrode prepared in Comparative Preparation Example 3 was used.

Comparative Example 3: Preparation of lithium-sulfur secondary battery

The same method as in Comparative Example 1, except that dimethoxyethane (DME) was used instead of 1H,1H,2′H,3H-decafluorodipropyl ether as a second solvent by changing the manufacturing conditions of the electrolyte. A lithium-sulfur secondary battery was prepared.

Comparative Example 4: Preparation of lithium-sulfur secondary battery

Propylene carbonate having a dipole moment of 96.13 D·mol/L per unit volume, a viscosity of the solvent of 1.71 cP, and a DV2 factor value of 1.77 calculated based on this, by changing the manufacturing conditions of the electrolyte, instead of propionitrile as the first solvent A lithium-sulfur secondary battery was manufactured in the same manner as in Comparative Example 1, except that Propylene carbonate) was used.

The conditions of Examples 1 to 4 and Comparative Examples 1 to 4 are summarized in Table 2 below.

Electrolyte composition SC factor DV ² factor NS factor Example 1 First electrolyte composition ¹⁾ 0.89 0.39 0.44 Example 2 0.75 0.39 0.52 Example 3 0.89 0.39 0.44 Example 4 0.89 0.39 0.44 Comparative Example 1 0.76 0.39 0.51 Comparative Example 2 0.41 0.39 0.95 Comparative Example 3 Second electrolyte composition ²⁾ 0.6 0.39 0.65 Comparative Example 4 3rd electrolyte composition ³⁾ 0.6 1.77 2.95 ¹⁾ First electrolyte composition = Propionitrile:1H,1H,2'H,3H-Decafluorodipropyl ether(3:7, w/w) solvent, 3.0M LiTFSI
²⁾ Second electrolyte composition = Propionitrile:Dimethoxyethane (3:7, w/w) solvent, 3.0M LiTFSI
³⁾ 3rd electrolyte composition = Propylene carbonate: 1H,1H,2'H,3H-Decafluorodipropyl ether(3:7, w/w) solvent, 3.0M LiTFSI

Experimental Example 1: Battery performance evaluation (initial discharge capacity evaluation)

The lithium-sulfur secondary batteries prepared in Examples 1 to 4 and Comparative Examples 1 to 4 were measured for capacities from 1.0 V to 3.6 V using a charge/discharge measuring device.

As a result of the experiment, the lithium-sulfur secondary batteries of Examples 1 to 4 showed excellent initial discharge capacity compared to the lithium-sulfur secondary batteries prepared in Comparative Examples 1 to 4.

Experimental Example 2: Battery performance evaluation (ED factor value measurement)

Charge-discharge was performed at a rate of 0.1C for 2.5 cycles of initial discharge-charge-discharge-charge-discharge using a charge/discharge measurement device (LAND CT-2001A, Wuhan, China), and then charge- After discharging 0.2C and 3 cycles, the charging proceeds at a rate of 0.5C and the discharge proceeds at a rate of 0.5C, while performing charge-discharge up to 50 cycles, of the lithium-sulfur secondary batteries according to Examples 1 to 4 and Comparative Examples 1 to 4. ED factor values are measured and are shown in Table 3 below.

Electrolyte composition SC factor ED factor Example 1 First electrolyte composition ¹⁾ 0.89 1895 Example 2 0.75 1740 Example 3 0.89 1751 Example 4 0.89 1764 Comparative Example 1 0.76 28 Comparative Example 2 0.41 802 Comparative Example 3 Second electrolyte composition ²⁾ 0.6 1115 Comparative Example 4 3rd electrolyte composition ³⁾ 0.6 1074 ¹⁾ First electrolyte composition = Propionitrile:1H,1H,2'H,3H-Decafluorodipropyl ether(3:7, w/w) solvent, 3.0M LiTFSI
²⁾ Second electrolyte composition = Propionitrile:Dimethoxyethane (3:7, w/w) solvent, 3.0M LiTFSI
³⁾ 3rd electrolyte composition = Propylene carbonate: 1H,1H,2'H,3H-Decafluorodipropyl ether(3:7, w/w) solvent, 3.0M LiTFSI

According to Table 3, the lithium-sulfur secondary batteries according to Examples 1 to 4 have a relatively large ED factor value compared to a second electrolyte composition, a third electrolyte composition, or a lithium-sulfur secondary battery having an SC factor of 0.41 or less. Can have This means that the lithium-sulfur secondary battery according to the present invention can realize a higher energy density that could not be realized in the existing lithium-sulfur secondary battery.

Claims (16)

A lithium secondary battery comprising an anode, a cathode, a separator and an electrolyte,
The positive electrode includes a sulfur-carbon composite containing a carbon material that satisfies a condition in which the AC factor represented by Equation 1 below is 100 or higher,
Lithium secondary battery having an SC factor value of 0.45 or more represented by Equation 2 below:
[Equation 1]

(In Equation 1 above,
The specific area is the specific surface area of the carbon material,
Conductivity is the electrical conductivity converted from the resistance value of the powder measured in the state of applying a pressure of 2000 kgf to the carbon material . )
[Equation 2]

(In Equation 2 above,
P is the porosity (%) of the positive electrode active material layer in the positive electrode,
L is the mass of sulfur per unit area of the positive electrode active material layer in the positive electrode (mg/cm 2 ),
α is 10 (constant). According to claim 1,
The carbon material is a lithium secondary battery, characterized in that the specific surface area is 100 to 4500 m 2 /g. According to claim 1,
The carbon material is a lithium secondary battery, characterized in that the pore volume is 0.8 to 5 cm 3 /g. According to claim 1,
Lithium secondary battery, characterized in that the electrical conductivity of 10 to 100 S / cm converted to the resistance value of the powder measured in the state of applying the pressure of 2000kgf of the carbon material. According to claim 1,
The carbon material is a lithium secondary battery, characterized in that at least one selected from the group consisting of graphite, carbon nanotubes, graphene, graphite, amorphous carbon, carbon black, and activated carbon. According to claim 1,
The sulfur is a lithium secondary battery, characterized in that at least one selected from the group consisting of inorganic sulfur, Li ₂ S _n (n≥1), disulfide compounds, organic sulfur compounds, and carbon-sulfur polymers. According to claim 1,
The sulfur is a lithium secondary battery, characterized in that contained in 50 to 90% by weight based on the total weight of the sulfur-carbon composite. According to claim 1,
The electrolyte solution includes a solvent and a lithium salt,
The solvent is a lithium secondary battery, characterized in that it comprises a first solvent having a DV ² factor value of 1.75 or less represented by the following equation (3) and a second solvent as a fluorinated ether-based solvent:
[Equation 3]

(In Equation 3 above,
DV is the dipole moment per unit volume (Dmol/L),
μ is the viscosity of the solvent (cP, 25℃),
γ is 100 (constant). The method of claim 8,
The first solvent is a lithium secondary battery, characterized in that the DV ² factor value is 1.5 or less. The method of claim 8,
The lithium-sulfur secondary battery is a lithium secondary battery characterized in that the NS factor value represented by Equation 4 below is 3.5 or less:
[Equation 4]

(In Equation 4 above,
SC factor is the same as the value defined by Equation 2 above,
The DV ² factor is the same as the value defined by Equation 3 above.). According to claim 1,
The lithium-sulfur secondary battery is a lithium secondary battery characterized in that the ED factor value represented by Equation 5 below is 850 or more:
[Equation 5]

(In Equation 5 above,
V is the discharge nominal voltage (V) for Li/Li ⁺ ,
SC factor is the same as the value defined by Equation 2 above,
C is the discharge capacity (mAh/g) when discharging at 0.1C rate,
D is the density of the electrolyte (g/cm 3 ).) The method of claim 8,
The first solvent is lithium secondary, characterized in that it comprises at least one selected from the group consisting of propionitrile, dimethylacetamide, dimethylformamide, gamma-butyrolactone, triethylamine and 1-iodopropane. battery. The method of claim 8,
The second solvent is 1H,1H,2'H,3H-decafluorodipropyl ether, difluoromethyl 2,2,2-trifluoroethyl ether, 1,2,2,2-tetrafluoroethyl tri Fluoromethyl ether, 1,1,2,3,3,3-hexafluoropropyl difluoromethyl ether, pentafluoroethyl 2,2,2-trifluoroethyl ether and 1H,1H,2′H -A lithium secondary battery comprising at least one member selected from the group consisting of perfluorodipropyl ether. The method of claim 8,
The solvent is a lithium secondary battery, characterized in that it comprises a first solvent in an amount of 1 to 50% by weight based on the total weight of the solvent. The method of claim 8,
The solvent is a lithium secondary battery, characterized in that it comprises a second solvent in an amount of 50 to 99% by weight based on the total weight of the solvent. The method of claim 8,
The solvent is a lithium secondary battery, characterized in that it comprises a first solvent and a second solvent in a weight ratio of 3:7 to 1:9.