KR20200110160A - Lithium secondary battery - Google Patents

Abstract

The present invention relates to a lithium secondary battery. More particularly, the lithium secondary battery includes a positive electrode including a sulfur-carbon composite made of a point-shaped carbon material in the form of nanoparticles, and an electrolyte containing a solvent having a high dipole moment and a low viscosity. High energy density can be exhibited.

Description

Lithium secondary battery {LITHIUM SECONDARY BATTERY}

The present invention relates to a lithium secondary battery having a high energy density.

Recently, the demand for secondary batteries is increasing with the rapid development of the electronic device field and the electric vehicle field. In particular, according to the trend of miniaturization and weight reduction of portable electronic devices, there is a growing demand for a secondary battery having a high energy density that can meet the trend.

Among secondary batteries, a lithium-sulfur secondary battery uses a sulfur-based compound having a sulfur-sulfur bond as a positive electrode active material, and an alkali metal such as lithium or a carbon-based material in which metal ions such as lithium ions are inserted and deintercalated, or an alloy with lithium It is a secondary battery that uses silicon or tin to form a negative electrode active material. Specifically, electrical energy is stored by using an oxidation-reduction reaction in which sulfur-sulfur bonds are cut off during discharge, which is a reduction reaction, and the oxidation number of sulfur decreases. And create it.

In particular, sulfur, which is used as a positive electrode active material in lithium-sulfur secondary batteries, has a theoretical energy density of 1675 mAh/g, and has a theoretical energy density that is 5 times higher than that of the positive electrode active material used in conventional lithium secondary batteries. It is a battery capable of expressing the density. In addition, sulfur is attracting attention as an energy source for mid- to large-sized devices such as electric vehicles as well as portable electronic devices because of its low cost, rich reserves, easy supply, and environmental friendliness.

In relation to lithium-sulfur secondary batteries, various studies are being conducted to maintain the conductive structure of the positive electrode and increase the reactivity of sulfur.

For example, a study has been attempted to apply a nanoparticle-shaped 0D (dimension) particulate carbon material to a positive electrode active material of a lithium-sulfur secondary battery. In the case of manufacturing a positive electrode by applying the 0D particulate carbon material to a positive electrode active material, there is an advantage of minimizing a spring-back phenomenon without causing electrode detachment during a rolling process due to its morphological characteristics.

In addition, although the OD particulate carbon material has an advantage of having a large specific surface area, it is difficult to drive a lithium-sulfur secondary battery having a high energy density.

Korean Patent Publication No. 2018-0017654

In order to solve the problems of the prior art, the inventors of the present invention use an electrolyte containing a sulfur-carbon composite made of a nanoparticle-shaped point-like carbon material and a solvent having a high dipole moment and a low viscosity. A battery was prepared, and it was confirmed that the lithium secondary battery thus prepared exhibits a high energy density.

Accordingly, an object of the present invention is to provide a lithium secondary battery having a high energy density.

In order to achieve the above object, the present invention, the anode; cathode; A separator interposed therebetween; And as a lithium secondary battery comprising an electrolyte,

The positive electrode includes a sulfur-carbon composite containing a point-shaped carbon material,

The electrolyte solution contains a solvent and a lithium salt,

The solvent is,

A first solvent having a DV ² factor value of 1.75 or less represented by Equation 1 below; And

It provides a lithium secondary battery comprising a second solvent that is a fluorinated ether-based solvent:

[Equation 1]

Here, DV is the dipole moment per unit volume (D·mol/L), μ is the viscosity of the solvent (cP, 25°C), and γ is 100 (constant).

According to the present invention, a lithium secondary battery having a high energy density can be manufactured.

In addition, since the lithium secondary battery uses a sulfur-carbon composite made of a point-like carbon material in the form of nanoparticles as a positive electrode material, desorption may not occur due to a rolling process when manufacturing the positive electrode.

1 shows photographs of a positive electrode before and after rolling, when manufacturing a positive electrode in Example 1.

Hereinafter, the present invention will be described in more detail to aid understanding of the present invention.

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 term "point-shaped carbon material" as used herein refers to a carbon material in the form of nanoparticles. It may also be referred to as OD (O dimension) particulate carbon material.

The present invention provides a lithium secondary battery including a positive electrode, a negative electrode, a separator interposed therebetween, and an electrolyte. Specifically, the lithium secondary battery may be a lithium-sulfur secondary battery containing sulfur as a positive electrode active material.

The lithium-sulfur secondary battery according to the present invention includes a positive electrode having a low porosity and a high loading amount of sulfur as a positive electrode active material. When the porosity in the positive electrode is lowered and the content of the positive electrode active material is increased, the energy density of a battery including the positive electrode increases. However, in a lithium-sulfur secondary battery, if the porosity of the positive electrode is reduced to a minimum and the sulfur content is increased to the maximum, the electrolyte solution per unit sulfur content decreases. Therefore, when the positive electrode is applied to a lithium-sulfur secondary battery, the target It is difficult to implement performance.

In the present invention, the conditions related to sulfur in the positive electrode, specifically, the carbon material used to prepare the sulfur-carbon composite is limited to a point-shaped carbon material, and by specifying the appropriate electrolyte conditions, the conventional lithium-sulfur secondary battery It is intended to provide a lithium-sulfur secondary battery having a high energy density compared to.

anode

The positive electrode according to the present invention includes a sulfur-carbon composite comprising a point-shaped carbon material; Conductive material; And a binder; may be formed with a positive electrode active material layer.

Specifically, the positive electrode for a lithium secondary battery of the present invention includes a positive electrode current collector; And a positive active material layer formed on at least one surface of the current collector.

In the positive electrode for a lithium secondary battery according to the present invention, the sulfur-carbon composite may be used as a positive electrode active material. The sulfur-carbon composite is a mixture of carbon and sulfur in order to reduce the leakage of sulfur into the electrolyte and increase the electrical conductivity of the sulfur-containing electrode.

The sulfur-carbon composite may include sulfur and a point-type carbon material, and specifically, the sulfur and point-type carbon material may be included in a weight ratio of 55 to 90: 45 to 10. When the weight ratio of the sulfur and the point-like carbon material contained in the sulfur-carbon composite is satisfied, the capacity of the battery can be improved and the conductivity can be maintained.

In addition, the sulfur-carbon composite may be included in an amount of 60 to 95% by weight, preferably 65 to 95% by weight, more preferably 70 to 90% by weight based on the total weight of the positive active material layer. If it is less than the above range, battery performance may be deteriorated, and if it is greater than the above range, the content of a conductive material or a binder other than the positive electrode active material is relatively reduced, so that properties such as conductivity or durability may be deteriorated.

In the present invention, the sulfur may be selected from the group consisting of elemental sulfur (S8), a sulfur-based compound, and a sulfur-carbon complex. Specifically, the sulfur-based compound may be Li ₂ Sn (n≥1), an organosulfur compound or a carbon-sulfur polymer ((C ₂ S _x ) _n :x=2.5-50, n≥2).

In the present invention, the point-shaped carbon material may mean a carbon material in the form of nanoparticles.

The pointed carbon material is carbon black, ketjen black, denka black, acetylene black, channel black, furnace black, and lamp black. Lamp black) and thermal black may include at least one carbon black-based carbon material selected from the group consisting of, and preferably, Ketjen black.

In addition, the point-type carbon material may have a particle diameter of 5 to 100 nm, preferably 10 to 90 nm, more preferably 15 to 80 nm, of the primary particles. If it is less than the above range, the durability of the positive electrode may be reduced, and if it is more than the above range, the reactivity of the positive electrode may be reduced. In this case, the primary particles mean particles in a state in which the point-shaped carbon material particles are not aggregated with each other.

In addition, the specific surface area of the point-shaped carbon material may be 500 to 2000 m2/g, preferably 600 to 1900 m2/g, more preferably 700 to 1800 m2/g. If it is less than the above range, it is difficult to support uniform sulfur, so that the reactivity of the secondary battery may decrease, and if it exceeds the above range, durability of the positive electrode may decrease.

In addition, the pore volume of the point-shaped carbon material may be 1 to 1.3 cm 3 /g, preferably 1.1 to 1.3 cm 3 /g, more preferably 1.2 to 1.3 cm 3 /g. If it is less than the above range, it may be difficult to maintain the conductive structure and secure the lithium ion path, and if it exceeds the above range, durability of the anode may be deteriorated.

In the present invention, the conductive material is not particularly limited, for example, graphite such as natural graphite or artificial graphite; Carbon blacks such as carbon black (super-p), acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and denka black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; It may be a conductive material such as a polyphenylene derivative. Although not particularly limited, for example, graphite such as natural graphite or artificial graphite; Carbon blacks such as carbon black (super-p), acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and denka black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; It may be a conductive material such as a polyphenylene derivative. The conductive material may generally be in an amount of 0.05% by weight to 5% by weight based on the total weight of the positive electrode active material slurry.

In the positive electrode for a lithium secondary battery according to the present invention, the binder is SBR (Styrene-Butadiene Rubber)/CMC (Carboxymethyl Cellulose), poly(vinyl acetate), polyvinyl alcohol, polyethylene oxide, polyvinylpyrrolidone, alkylated Polyethylene oxide, cross-linked polyethylene oxide, polyvinyl ether, poly(methyl methacrylate), polyvinylidene fluoride, a copolymer of polyhexafluoropropylene and polyvinylidene fluoride (trade name: Kynar), poly(ethyl) Acrylate), polytetrafluoroethylene, polyvinyl chloride, polyacrylonitrile, polyvinylpyridine, polystyrene, polyacrylic acid, derivatives, blends, copolymers and the like thereof may be used.

In addition, the content of the binder may be 1 to 20% by weight, preferably 3 to 18% by weight, more preferably 5 to 15% by weight based on the total weight of the positive electrode active material layer. If it is less than the above range, the bonding strength between the positive electrode active material or between the positive electrode active material and the current collector may be greatly improved, and a problem of deteriorating capacity characteristics may be prevented. In addition, suppression of polysulfide elution due to the interaction between polysulfide and specific functional groups of the polymer chain used as a binder can be expected. If it exceeds the above range, the battery capacity may decrease.

In the positive electrode for a lithium secondary battery according to the present invention, the positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, for example, stainless steel, aluminum, nickel, titanium, and calcined carbon. , Or a surface-treated aluminum or stainless steel surface with carbon, nickel, titanium, silver, or the like may be used. In this case, the positive electrode current collector may be in various forms such as a film, sheet, foil, net, porous material, foam, non-woven fabric having fine irregularities formed on the surface so as to increase adhesion to the positive electrode active material.

In the present invention, the positive electrode may exhibit high loading of more than 5 mAh/cm 2, and may also exhibit a low porosity due to the point-like carbon material. In this case, the low porosity may mean a porosity of 60% or less, preferably 50% or less.

In addition, the anode is classified by an SC factor value represented by Equation 2 below.

[Equation 2]

Here, 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/cm2), and α is 10 (constant). The lithium-sulfur secondary battery according to the present invention realizes a high energy density by organic bonding of not only the above-described positive electrode, but also a negative electrode, a separator, and an electrolyte, and according to a specific embodiment of the present invention, a lithium-sulfur secondary battery has high energy. In order to implement the density, 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 actual driving of the lithium-sulfur secondary battery, the SC factor value may be 4.5 or more. When the SC factor value is 0.45 or more, the performance of the existing lithium-sulfur secondary battery, such as energy density, 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. It is kept without deterioration.

cathode

In the present invention, the negative electrode includes a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector.

The negative active material layer includes a negative active material, a binder, and a conductive material. As the negative active material, a material capable of reversibly intercalating or deintercalating lithium ions (Li ⁺ ), a material capable of reversibly forming a lithium-containing compound by reacting with lithium ions, lithium metal or lithium alloy You can use 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).

The binder is not limited to the above-described binder, and any binder that can be used in the relevant technical field may be used.

Materials and methods used in the above-described positive electrode may be used as a configuration of a current collector excluding the negative active material and the conductive material.

Separator

In the present invention, the separator is a physical separator having a function of physically separating an electrode, and if it is used as a conventional separator, it can be used without particular limitation. In particular, it has a low resistance against ion migration of the electrolyte and has excellent electrolyte moisture-absorbing ability. desirable.

In addition, the separator separates or insulates the positive electrode and the negative electrode from each other and enables transport of lithium ions between the positive electrode and the negative electrode. This separator has a porosity of 30-50% and may be made of a non-conductive or insulating material.

Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer such as ethylene homopolymer, propylene homopolymer, ethylene/butene copolymer, ethylene/hexene copolymer, and ethylene/methacrylate copolymer may be used. In addition, nonwoven fabrics made of high melting point glass fibers or the like can be used. Among these, a porous polymer film is preferably used.

If a polymer film is used as both the buffer layer and the separator, the impregnation amount of the electrolyte solution and ion conduction characteristics decrease, and the effect of reducing overvoltage and improving capacity characteristics is insignificant. Conversely, when all non-woven materials are used, mechanical stiffness cannot be secured, resulting in a battery short circuit. However, when a film-type separator and a polymer nonwoven buffer layer are used together, it is possible to secure mechanical strength as well as an effect of improving battery performance due to the adoption of the buffer layer.

According to a preferred embodiment of 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. In this case, the polyethylene polymer film preferably has a thickness of 10 to 25 μm and a porosity of 40 to 50%.

Electrolyte

In the present invention, the electrolytic solution is a non-aqueous electrolytic solution containing a lithium salt and is composed of a lithium salt and a solvent. The electrolyte solution 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 achieve a high energy density of a lithium-sulfur secondary battery due to an increase in 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 , lithium chloroborane, lithium lower aliphatic carboxylic acid, lithium tetraphenylborate, and lithium imide. In one embodiment of the present invention, the lithium salt may be a lithium imide such as LiTFSI.

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

The solvent includes a first solvent and a second solvent. The first solvent is characterized by having the highest dipole moment per unit volume among the constituents contained in the solvent at least 1% by weight, and thus has 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 state 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 a DV ² factor represented by Equation 2 below.

[Equation 1]

Here, DV is the dipole moment per unit volume (debye(D)·mol/L), μ is the viscosity of the solvent (cP, 25°C), and 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 when considering the actual driving of the lithium-sulfur secondary battery, the DV ² factor value may be 0.1 or more. When a solvent having a DV ² factor of 1.75 or less, such as the first solvent, is mixed, the functionality of the electrolyte can be maintained 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. So, the performance of the battery does not deteriorate.

In the present invention, if the first solvent is included in the range of the DV ² factor, the type is not particularly limited, but propionitrile, dimethylacetamide, dimethylformamide, gamma- It may be selected from the group consisting of butyrolactone (Gamma-Butyrolactone), triethylamine (Triethylamine), 1-iodopropane (1-iodopropane), and combinations thereof. According to an embodiment of the present invention, the first solvent may contain 1 to 50% by weight, preferably 5 to 40% by weight, and more preferably 10 to 30% by weight, based on the solvent constituting the electrolyte. When the solvent according to the present invention contains the first solvent within the above-described weight% range, even when used with a positive electrode having a low porosity and a high loading amount of sulfur, a positive electrode active material, may have an effect of improving battery performance.

The lithium-sulfur secondary battery of the present invention may be further classified by an NS factor obtained by combining the SC factor and the DV ² factor. The NS factor is represented by Equation 3 below.

[Equation 3]

Here, the SC factor is the same as the value defined by Equation 2, and the DV ² factor is the same as the value defined by Equation 1 above. According to an embodiment of the present invention, the value of the NS factor 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 value of the NS factor is not particularly limited, but considering an embodiment of an actual lithium-sulfur secondary battery, the value of the NS factor may be 0.1 or more. When the NS factor value is adjusted within the above range, the effect of improving the performance of the lithium-sulfur secondary battery may be more excellent.

In the present invention, the second solvent is a fluorinated ether solvent. Conventionally, a solvent such as dimethoxyethane and dimethylcarbonate has been used as a diluent to control the viscosity of the electrolyte solution.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 a low porosity. Therefore, in the present invention, the second solvent is added together with the first solvent to drive the anode according to the present invention. If the second solvent is a fluorinated ether solvent generally used in the relevant technical field, 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), 1H,1H,2'H,3H-decafluorodipropyl ether (1H,1H,2'H,3H-Decafluorodipropyl ether), pentafluoroethyl 2,2,2-trifluoroethyl ether (Pentafluoroethyl 2,2,2-trifluoroethyl ether), 1H,1H,2'H-perfluorodipropyl ether (1H,1H,2'H-Perfluorodipropyl ether) and a combination thereof may be selected from the group consisting of. According to an embodiment of the present invention, the second solvent may contain 50 to 99% by weight, preferably 60 to 95% by weight, and more preferably 70 to 90% by weight, based on the solvent constituting the electrolyte. When the solvent according to the present invention contains the second solvent within the above-described weight% range, the effect of improving the performance of the battery is 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. Can have. When mixing the first solvent and the second solvent, the second solvent may be included in the electrolyte in an amount equal to or greater than the first solvent in consideration of an effect of improving the performance of the battery. According to an embodiment of the present invention, the solvent may contain a first solvent and a second solvent in a weight ratio of 1:1 to 1:9, preferably 3:7 to 1:9 (first solvent: second solvent). I can.

The non-aqueous electrolyte solution for a lithium-sulfur battery of the present invention may further include nitric acid or nitrous acid-based compounds 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 nitrous acid-based compound is not particularly limited in the present invention, but lithium nitrate (LiNO ₃ ), potassium nitrate (KNO ₃ ), cesium nitrate (CsNO ₃ ), barium nitrate (Ba(NO ₃ ) ₂ ), ammonium nitrate Inorganic nitric acid or nitrite compounds such as (NH ₄ NO ₃ ), lithium nitrite (LiNO ₂ ), potassium nitrite (KNO ₂ ), cesium nitrite (CsNO ₂ ), and ammonium nitrite (NH ₄ NO ₂ ); Organic nitric acids such as methyl nitrate, dialkyl imidazolium nitrate, guanidine nitrate, imidazolium nitrate, pyridinium nitrate, ethyl nitrite, propyl nitrite, butyl nitrite, pentyl nitrite, and octyl nitrite Or nitrous acid compounds; One selected from the group consisting of organic nitro compounds such as nitromethane, nitropropane, nitrobutane, nitrobenzene, dinitrobenzene, nitro pyridine, dinitropyridine, nitrotoluene, dinitrotoluene, and combinations thereof is possible, and preferably Uses lithium nitrate.

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

The lithium-sulfur secondary battery of the present invention may be prepared by disposing a separator between a positive electrode and a negative electrode to form an electrode assembly, and the electrode assembly may be placed in a cylindrical battery case or a prismatic battery case, and then an electrolyte is injected. Alternatively, after laminating the electrode assembly, the electrode assembly may be impregnated with an electrolyte, and the resulting product may be sealed by putting it in a battery case.

The lithium-sulfur secondary battery according to the present invention is classified by the ED factor value represented by Equation 4 below.

[Equation 4]

Where V is the nominal discharge voltage for Li/Li ⁺ (V), D is the density of the electrolyte (g/cm 3 ), C is the discharge capacity (mAh/g) when discharging at a 0.1C rate, and the SC factor is It is the same as the value defined by Equation 2 above. The higher the value of the ED factor, the higher the energy density can be realized in the actual lithium-sulfur secondary battery. According to an embodiment of 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 an embodiment 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 conventional lithium-sulfur secondary battery.

In the present invention, when the positive electrode of the lithium secondary battery includes a point-type carbon material, the ED factor value is compared to the case of including a carbon material other than a point-type carbon material, for example, a linear carbon material or a planar carbon material. It can have a larger value. When the positive electrode of the lithium secondary battery includes a point-shaped carbon material, the ED factor may preferably be 1350 or more, or 1400 or more, or 1450 or more.

The present invention also relates to a method of manufacturing a positive electrode for a lithium secondary battery.

The method of manufacturing a positive electrode for a lithium secondary battery includes the steps of: (S1) preparing a sulfur-carbon composite comprising a point-shaped carbon material; (S2) preparing a positive electrode slurry using the sulfur-carbon composite, a binder, and a conductive material; (S3) coating and drying the positive electrode slurry on a positive electrode current collector; may include.

In the step (S1), specific types and weight ratios of the point-shaped carbon material and sulfur used to prepare the sulfur-carbon composite are the same as described above.

After mixing the point-like carbon material and sulfur at a prescribed weight ratio, heat treatment is performed at 120 to 180° C. for 10 minutes to 1 hour to prepare a sulfur-carbon composite.

In the step (S2), the type and amount of the binder and the conductive material used to prepare the positive electrode slurry are the same as described above.

The solvent used for preparing the positive electrode slurry is water (distilled water), methanol, ethanol, isopropyl alcohol, acetone, dimethyl sulfoxide, formamide, dimethylformamide, dioxolone, acetonitrile, nitromethane, acetic acid, methyl formate, Methyl acetate, phosphoric acid tryester, trimethoxy methane, dioxolone derivative, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ether, It may be one or more selected from the group consisting of methyl pyropionate and ethyl propionate. In particular, when using water (distilled water) or an anhydrous alcohol-based solvent, it is preferable to prevent damage to the positive electrode active material.

The concentration of the positive electrode slurry is not particularly limited as long as it is such that the coating process can be smoothly performed.

In the step (S3), the positive electrode slurry may be coated and dried on a positive electrode current collector to form a positive electrode active material layer to manufacture a lithium secondary battery.

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

Further, the shape of the battery case is not particularly limited, and may be in various shapes such as a cylindrical shape, a stacked type, a square shape, a pouch type, or a coin type. The structure and manufacturing method of these batteries are well known in this field, and thus detailed descriptions are omitted.

In addition, the lithium secondary battery can be classified into various batteries, such as lithium-sulfur secondary batteries, lithium-air batteries, lithium-oxide batteries, and lithium all-solid batteries, depending on the material of the positive electrode/cathode used.

The present invention also provides a battery module including the lithium secondary battery as a unit cell.

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

Examples of the medium and large-sized devices include a power tool that is powered by an omniscient motor and moves; Electric vehicles including electric vehicles (EV), 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 cart; Power storage systems, etc., but are not limited thereto.

Hereinafter, preferred embodiments are 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 changes and modifications fall within the scope of the appended claims.

Example One

(1) anode manufacturing

Sulfur and Ketjen Black, a point-like carbon material, were mixed in a weight ratio of 70:30, and then heat-treated at 155°C for 30 minutes to prepare a sulfur-carbon composite.

90% by weight of the sulfur-carbon composite, 5% by weight of styrene butadiene rubber/carboxymethyl cellulose (SBR/CMC 7:3) as a binder and 5% by weight of denka black as a conductive material were mixed, dissolved in water, and the concentration (solid content A positive electrode slurry having a concentration of 20%) was prepared.

The positive electrode slurry was coated on an aluminum current collector to form a positive electrode active material layer, followed by drying and rolling to prepare a positive electrode. The porosity of the positive electrode active material layer calculated by measuring the electrode weight and the electrode thickness (using TESA-μHITE equipment) in the prepared positive electrode was 63%, and the mass of sulfur per unit area of the positive electrode active material layer was 4.6 mg/cm 2. The SC factor value calculated based on this was 0.73.

(2) Manufacture of lithium-sulfur secondary battery

After positioning the positive electrode and negative electrode prepared by the above-described method to face each other, a polyethylene separator having a thickness of 20 μm and a porosity of 45% was interposed between the positive electrode and the negative electrode.

After that, an electrolyte was injected into the case to prepare a lithium-sulfur secondary battery. At this time, the electrolyte was prepared by dissolving lithium bis(trifluoromethyl sulfonyl)imide (LiTFSI) at a concentration of 3M in an organic solvent. As the organic solvent, a solvent obtained by mixing propionitrile (first solvent) and 1H,1H,2'H,3H-decafluorodipropyl ether (second solvent) in a weight ratio of 3:7 was used. In the first solvent, the dipole moment (DV) per unit volume was 97.1 D·mol/L, and the viscosity (μ) of the solvent was 0.38 cP. 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.

Example 2

The same method as in Example 1, except that a positive electrode having a porosity of 55% of the positive electrode active material layer, a mass of sulfur per unit area of the positive electrode active material layer of 5.0 mg/cm2, and an SC factor value (0.91) calculated based on this was prepared. As a result, a lithium-sulfur secondary battery was prepared.

Comparative example One

By changing the type of organic solvent used in the electrolyte, a solvent in which DOL (dioxolane) (first solvent) and DME (dimethoxyethane) (second solvent) are mixed in a weight ratio of 5:5 was used, and the electrolyte was A lithium-sulfur secondary battery was manufactured in the same manner as in Example 1, except that 1% by weight of LiNO ₃ was added and used as an additive. In the first solvent, the dipole moment (DV) per unit volume was 33.66 D·mol/L, and the viscosity (μ) of the solvent was 0.7 cP. The DV ² factor value calculated based on this was 2.07.

In addition, by changing the manufacturing conditions of the positive electrode, the porosity of the positive electrode active material layer was 70%, the mass of sulfur per unit area of the positive electrode active material layer was 4.7 mg/cm 2, and the calculated SC factor value was 0.67.

Comparative example 2

A lithium-sulfur secondary battery was manufactured in the same manner as in Comparative Example 1, except that a sulfur-carbon composite was prepared using CNT, not a point-shaped carbon material, as the carbon material used in the production of the sulfur-carbon composite.

Comparative example 3

A lithium-sulfur secondary battery was manufactured in the same manner as in Example 1, except that the carbon material used in the production of the sulfur-carbon composite was prepared using CNTs instead of the point-shaped carbon material.

In addition, by changing the manufacturing conditions of the positive electrode, the porosity of the positive electrode active material layer was 65%, the mass of sulfur per unit area of the positive electrode active material layer was 4.6 mg/cm 2, and the SC factor value calculated based on this was set to 0.71.

Experimental example One

In Example 1, when manufacturing the positive electrode, it was observed whether the occurrence of the positive electrode detachment phenomenon before and after rolling.

1 shows photographs of a positive electrode before and after rolling, when manufacturing a positive electrode in Example 1.

Referring to FIG. 1, it was confirmed that when a positive electrode was manufactured using a sulfur-carbon composite made of a point-shaped carbon material, no positive electrode detachment occurred before and after rolling.

Experimental example 2

The conditions for the positive electrode and secondary battery prepared in Examples and Comparative Examples, respectively, are summarized in Table 1 below.

The performance of the batteries prepared in Examples and Comparative Examples was evaluated using a charge/discharge measuring device (LAND CT-2100A, Wuhan, China). Battery performance was evaluated by charging and discharging at a rate of 0.1C, and the ED factor was calculated as defined in Equation 4 for the first discharge result. Table 1 shows the calculated results.

Electrolyte SC factor DV ² factor NS factor ED factor Comparative Example 1 1st electrolyte ^{Note 1 )} 0.67 2.07 3.09 935 Comparative Example 2 0.67 2.07 3.09 388 Comparative Example 3 2nd electrolyte ^{Note 2 )} 0.71 0.39 0.55 1342 Example 1 0.73 0.39 0.53 1498 Example 2 0.91 0.39 0.43 1678 ^{Note 1)} Electrolyte 1: DOL:DME (5:5, v/v), 1.0M LiTFSI, 1.0wt% LiNO ₃ 1wt%
^{Note 2)} Electrolyte 2: Propionitrile: 1H,1H,2'H,3H-Decafluorodipropyl ether (3:7, w/w), 3.0M LiTFSI

Referring to Table 1, when the first electrolyte was used, it was found that the ED factor of Comparative Example 1 using a point-shaped carbon material was lower than that of Comparative Example 2 using CNT.

In addition, when the second electrolyte was used, it was found that the ED factor of Example 1 using a point-shaped carbon material was higher than that of Comparative Example 3 using a linear carbon material.

From these results, it can be seen that the sulfur-carbon composite using a point-shaped carbon material exhibits more effective performance in the first electrolyte solution compared to the sulfur-CNT composite using CNT, which is a linear carbon material.

In addition, it can be seen that a secondary battery including a sulfur-carbon composite using a point-shaped carbon material exhibits a high ED factor value even under a high SC factor condition (~0.91, Example 2).

In the above, although the present invention has been described by limited embodiments and drawings, the present invention is not limited thereto, and the technical idea of the present invention and the following description by those of ordinary skill in the art to which the present invention pertains. It goes without saying that various modifications and variations are possible within the equivalent range of the claims to be made.

Claims (11)

anode; cathode; A separator interposed therebetween; And as a lithium secondary battery comprising an electrolyte,
The positive electrode includes a sulfur-carbon composite containing a point-shaped carbon material,
The electrolyte solution contains a solvent and a lithium salt,
The solvent is,
A first solvent having a DV ² factor value of 1.75 or less represented by Equation 1 below; And
A lithium secondary battery comprising a second solvent which is a fluorinated ether solvent:
[Equation 1]

Here, DV is the dipole moment per unit volume (D·mol/L), μ is the viscosity of the solvent (cP, 25°C), and γ is 100 (constant). The method of claim 1,
The positive electrode is a lithium secondary battery having an SC factor value of 0.45 or more represented by Equation 2:
[Equation 2]

Here, 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/cm2), and α is 10 (constant). The method of claim 1,
The first solvent will have a DV ² factor value of 1.5 or less, the lithium secondary battery. The method according to claim 1 or 2,
The lithium secondary battery has an NS factor value of 3.5 or less represented by Equation 3 below:
[Equation 3]

Here, the SC factor is the same as the value defined by Equation 2, and the DV ² factor is the same as the value defined by Equation 1 above. The method according to claim 1 or 2,
The lithium secondary battery has an ED factor value of 850 or more represented by Equation 4 below:
[Equation 4]

Where V is the nominal discharge voltage for Li/Li ⁺ (V), D is the density of the electrolyte (g/cm 3 ), C is the discharge capacity (mAh/g) when discharging at a 0.1C rate, and the SC factor is It is the same as the value defined by Equation 2 above. The method of claim 1,
The first solvent is a lithium secondary battery, characterized in that selected from the group consisting of propionitrile, dimethylacetamide, dimethylformamide, gamma-butyrolactone, triethylamine, 1-iodopropane, and combinations thereof. The method of claim 1,
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, 1H,1H,2'H,3H-decafluorodipropyl ether, pentafluoroethyl 2,2 ,2-trifluoroethyl ether, 1H,1H,2'H-perfluorodipropyl ether, and a lithium secondary battery, characterized in that selected from the group consisting of a combination thereof. The method of claim 1,
The solvent is a lithium secondary battery, characterized in that containing 1 to 50% by weight of the first solvent. The method of claim 1,
The solvent is a lithium secondary battery, characterized in that containing 50 to 99% by weight of the second solvent. The method of claim 1,
The solvent is a lithium secondary battery comprising a first solvent and a second solvent in a weight ratio of 3:7 to 1:9. The method of claim 1,
The point-shaped carbon material includes at least one carbon black-based carbon material selected from the group consisting of carbon black, Ketjen black, Denka black, acetylene black, channel black, furnace black, lamp black, and thermal black, lithium secondary battery.

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