KR20200134354A - Surfur-carbon complex and lithium secondary battery comprising the same - Google Patents

Abstract

The present invention relates to a sulfur-carbon complex and a lithium secondary battery comprising same. More specifically, the lithium secondary battery includes an electrolyte solution including: a sulfur-carbon complex having a form in which sulfur is uniformly supported on carbon, a conductive structure; and a solvent having high dipole moment and low viscosity, thereby showing improved service life properties.

Description

Sulfur-carbon composite and lithium secondary battery containing the same {SURFUR-CARBON COMPLEX AND LITHIUM SECONDARY BATTERY COMPRISING THE SAME}

The present invention relates to a sulfur-carbon composite and a lithium secondary battery including the same.

As the application area of secondary batteries expands to electric vehicles (EV) and energy storage devices (ESS), lithium-ion secondary batteries with relatively low weight-to-energy storage density (~250 Wh/kg) are suitable for these products. There is a limitation in application to In contrast, lithium-sulfur secondary batteries are in the spotlight as a next-generation secondary battery technology because they can theoretically realize a high energy storage density (~2,600 Wh/kg) to weight.

The lithium-sulfur secondary battery refers to a battery system in which a sulfur-based material having an S-S bond is used as a positive electrode active material and lithium metal is used as a negative electrode active material. Sulfur, which is the main material of the positive electrode active material, has the advantage of having abundant resources, non-toxicity, and low weight per atom worldwide.

In a lithium-sulfur secondary battery, lithium, which is a negative electrode active material, is oxidized while ionizing and releasing electrons during discharge, and a sulfur-based material, which is a positive electrode active material, accepts electrons and is reduced. Here, the oxidation reaction of lithium is a process in which lithium metal releases electrons and is converted into lithium cation form. In addition, the sulfur reduction reaction is a process in which the SS bond accepts two electrons and is converted into a sulfur anion form. The lithium cation generated by the oxidation reaction of lithium is transferred to the positive electrode through the electrolyte, and forms a salt by combining with the sulfur anion generated by the reduction reaction of sulfur. Specifically, sulfur before discharging has a cyclic S8 structure, which is converted into lithium polysulfide (LiS _x ) by a reduction reaction. When lithium polysulfide is completely reduced, lithium sulfide (Li ₂ S) is generated.

Sulfur, which is a positive electrode active material, has low electrical conductivity, so it is difficult to secure reactivity with electrons and lithium ions in a solid state. Existing lithium-sulfur secondary batteries induce a liquid phase reaction and improve reactivity by generating Li ₂ S _x type intermediate polysulfide to improve the reactivity of sulfur. In this case, an ether solvent such as dioxolane and dimethoxyethane having high solubility in lithium polysulfide is used as a solvent for the electrolyte solution. In addition, the existing lithium-sulfur secondary battery builds a catholyte-type lithium-sulfur secondary battery system to improve reactivity. In this case, the reactivity of sulfur depends on the content of the electrolyte due to the characteristics of lithium polysulfide dissolved in the electrolyte. And life characteristics are affected. In order to build a high energy density, it is necessary to inject a low content of electrolyte, but as the electrolyte content decreases, the concentration of lithium polysulfide in the electrolyte increases, and it is difficult to operate a normal battery due to a decrease in fluidity of the active material and an increase in side reactions.

In addition, a technology to improve battery performance by applying the surface-modified sulfur-carbon composite to a lithium-sulfur secondary battery by improving the specific surface area and pore volume by treating the sulfur-carbon composite with a solvent has been developed (Online ISBN 978-981-10-3406-0 (Zhou Chapter 2. Revealing Localized Electrochemical Transition of Sulfur in Sub-nanometer Confinement)).

In order to build a lithium-sulfur secondary battery with improved overall performance including improved life characteristics, a battery system capable of driving electrodes with high loading and low porosity is required, and research on such a battery system is continuously conducted in the relevant technical field. Is being carried out.

Online ISBN 978-981-10-3406-0 (Zhou Chapter 2. Revealing Localized Electrochemical Transition of Sulfur in Sub-nanometer Confinement)

Accordingly, the present inventors manufactured a lithium secondary battery using an electrolyte solution including a positive electrode including a sulfur-carbon composite in which sulfur is uniformly distributed on the surface and a solvent having a dipole moment less than a certain value. It was confirmed that the battery showed a high energy density.

Accordingly, an object of the present invention is to provide a lithium secondary battery with improved life characteristics.

In order to achieve the above object, the present invention is a sulfur-carbon composite in which sulfur is supported on a carbon material, wherein the specific surface area of the sulfur-carbon composite is 10 to 60 m 2 /g, and the pore volume is 0.3 to 1.0 cm 3 /g. It provides a sulfur-carbon complex.

The present invention also includes the steps of (P1) heat treatment by mixing sulfur and a carbon material; And (P2) surface-modifying the sulfur-carbon composite obtained in the step (P1) by supporting it in a solvent for dissolving sulfur; containing, a method for preparing a sulfur-carbon composite is provided.

The present invention also includes the steps of (P1) heat treatment by mixing sulfur and a carbon material; And (P2) surface-modifying the sulfur-carbon composite obtained in the step (P1) by supporting it in a solvent for dissolving sulfur; containing, a method for preparing a sulfur-carbon composite is provided.

The present invention also, the anode; cathode; A separator interposed therebetween; And as a lithium secondary battery comprising an electrolyte,

The positive electrode includes the sulfur-carbon composite,

The electrolyte solution contains a solvent and a lithium salt,

The solvent is,

A first solvent having a DV ² factor of 1.5 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 (cP) of the solvent, and γ is 100 (constant).

According to the present invention, there is provided a lithium-sulfur secondary battery, comprising: a positive electrode including a surface-modified sulfur-carbon composite; And a first solvent having a DV ² factor of 1.5 or less. And an electrolyte solution including a second solvent that is a fluorinated ether-based solvent; when it contains, there is an effect of improving life characteristics. In this case, the surface-modified sulfur-carbon composite refers to a sulfur-carbon composite in which sulfur is uniformly distributed on the surface by being treated with a solvent.

1 is a scanning transfer microscope (SEM) photograph of the surface-modified sulfur-carbon composite (EtOH-treated group) and sulfur-carbon composite (non-EtOH-treated group) prepared in Preparation Example 1 and Comparative Preparation Example 1, respectively.
2A and 2B show the specific surface area and porosity measurement results of the surface-modified sulfur-carbon composites (EtOH treated group) and sulfur-carbon composites (EtOH untreated group) prepared in Preparation Example 1 and Comparative Preparation Example 1, respectively. It is a graph for.
3 is a thermogravimetric analysis (TGA, thermogravimetric analysis) results for the surface-modified sulfur-carbon composite (EtOH-treated group) and sulfur-carbon composite (non-EtOH-treated group) prepared in Preparation Example 1 and Comparative Preparation Example 1, respectively. This is the graph shown.
4 is a result of a life characteristic experiment for lithium-sulfur secondary batteries prepared in Example 1 and Comparative Examples 1 to 3, respectively.

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 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 uses a sulfur-carbon composite as a positive electrode active material, but uses a sulfur-carbon composite surface-modified in a form in which sulfur is uniformly distributed, and the sulfur-carbon composite can improve battery performance. It is characterized in that an electrolyte solution having an optimal composition to be used is introduced together.

Sulfur-carbon complex

The sulfur-carbon composite included as a positive electrode active material in the positive electrode of the lithium secondary battery according to the present invention has a form in which sulfur is uniformly supported on a carbon material that is a conductive structure, thereby improving battery performance. Specifically, the sulfur-carbon composite may have a form in which sulfur is uniformly distributed on the surface of the carbon material. At this time, the meaning of uniformly supported means that sulfur is not agglomerated or concentrated in a specific part of the surface of the carbon material, and is distributed quantitatively and evenly, and this uniform distribution of sulfur is defined as the sulfur-carbon composite It can be implemented due to the specific surface area and pore volume characteristics.

The sulfur-carbon composite is obtained by mixing carbon and sulfur in order to reduce the leakage of sulfur into the electrolyte and increase the electrical conductivity of the sulfur-containing electrode.

In addition, the specific surface area of the sulfur-carbon composite may be 10 to 60 m 2 /g, preferably 20 to 50 m 2 /g, more preferably 25 to 45 m 2 /g. If the specific surface area of the sulfur-carbon composite is less than 10 ㎡/g, uniform sulfur cannot be supported on the carbon surface, so that the reactivity of the secondary battery may decrease, and if it exceeds 60 ㎡/g, durability of the anode may be reduced.

In addition, the volume of the sulfur-carbon composite may have a pore volume of 0.3 to 1.0 cm 3 /g, preferably 0.4 to 0.7 cm 3 /g, more preferably 0.4 to 0.6 cm 3 /g. When the pore volume of the sulfur-carbon composite is less than 0.3 cm 3 /g, it may be difficult to maintain a conductive structure and secure a lithium ion path, and when it exceeds 1.0 cm 3 /g, durability of the anode may be deteriorated.

The sulfur-carbon composite may include sulfur and a carbon material, and specifically, the sulfur and 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 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 carbon material is natural graphite, artificial graphite, expanded graphite, graphite such as graphene, activated carbon, channel black, furnace black, thermal black (Thermal black), contact black (Contact black), lamp black (Lamp black), such as a carbon black (Carbon black) such as acetylene black (Acetylene black); It may be at least one selected from the group consisting of carbon nanostructures such as carbon fiber, carbon nanotube (CNT), and fullerene, and combinations thereof.

In addition, 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).

Method for producing sulfur-carbon composite

In the present invention, the sulfur-carbon composite may have a specific surface area and a pore area as described above by being surface-modified by a solvent for dissolving sulfur.

The surface-modified sulfur-carbon composite is heat-treated by mixing sulfur and a carbon material (P1); And (P2) surface-modifying the sulfur-carbon composite obtained in the step (P1) by supporting it in a solvent for dissolving sulfur.

In the step (P1), the mixing ratio of the sulfur and the carbon material may be mixed by the weight ratio of the sulfur and the carbon material as described above.

In addition, the heat treatment may be performed in a temperature range of 120°C to 180°C, preferably 130°C to 170°C, and 140°C to 160°C. If the heat treatment temperature is less than 120°C, a sulfur-carbon composite cannot be formed, and if it exceeds 180°C, sulfur and carbon may be denatured.

In addition, the solvent for dissolving sulfur in step (P2) may be one or more selected from the group consisting of ethanol, acetone and CS ₂ .

When the sulfur-carbon composite is supported and mixed in the sulfur-dissolving solvent, the sulfur accumulated on the surface of the sulfur-carbon composite is dissolved or dispersed in the solvent, and it is possible to move to small pores in the carbon. The surface can be modified by uniform distribution of sulfur in the composite.

anode

The positive electrode according to the present invention includes a sulfur-carbon composite as described above; 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 configuration and shape of the sulfur-carbon composite is the same as described above.

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, and properties such as conductivity or durability 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 be in an amount of 0.05% 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 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 the above-described positive electrode as well as a negative electrode, a separator, and an electrolyte, and according to a specific embodiment of the present invention, a lithium-sulfur secondary battery 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 an embodiment 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, in the case of a conventional lithium-sulfur secondary battery, performance such as energy density of the battery is deteriorated in actual implementation, but in the case of lithium secondary batteries including the lithium-sulfur secondary battery according to the present invention Even in actual implementation, the performance of the battery is maintained 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 the ion conduction characteristics decrease, and the effect of reducing the overvoltage and improving the capacity characteristics becomes 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 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 (cP) of the solvent, and γ is 100 (constant). According to an embodiment of 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 value of the DV ² factor is not particularly limited, but considering an embodiment of an actual lithium-sulfur secondary battery, the value of the DV ² factor may be 0.1 or more. When mixing a solvent having a DV ² factor of 1.5 or less, such as the first solvent, when applied to a lithium-sulfur secondary battery including a surface-modified sulfur-carbon composite as described above, it is possible to improve battery performance, such as improving life characteristics. It can be advantageous.

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, a battery such as improvement in life characteristics when applied to a lithium-sulfur secondary battery including the surface-modified sulfur-carbon composite as described above It can be beneficial to improve 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 1, and the DV ² factor is the same as the value defined by Equation 2 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, in order to control the viscosity of the electrolyte, a solvent such as dimethoxyethane and dimethylcarbonate was used as a diluent. When such a solvent is used as a diluent, the above-described solvent is used as a diluent. When applied to a lithium-sulfur secondary battery including the surface-modified sulfur-carbon composite as described above, it may be advantageous in improving battery performance, such as improving life characteristics.

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 lifespan when applied to a lithium-sulfur secondary battery containing the surface-modified sulfur-carbon composite as described above, like the first solvent It may be advantageous for improving battery performance, such as improving properties.

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 for a lithium-sulfur secondary 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 1 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.

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: (S1) preparing a surface-modified sulfur-carbon composite by supporting a sulfur-carbon composite in a sulfur dissolving solvent; (S2) preparing a positive electrode slurry using the sulfur-carbon composite, a binder, and a conductive material; And (S3) coating and drying the positive electrode slurry on a positive electrode current collector.

The preparation of the surface-modified sulfur-carbon composite in step (S1) is the same as steps (P1) and (P2) as described above.

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.

Manufacturing example 1: Surface-modified sulfur-carbon composite preparation ( EtOH Treatment group)

After mixing sulfur and Ketjen black at a weight ratio of 70:30, heat treatment was performed at 155° C. for 30 minutes to prepare a sulfur-carbon composite.

After the sulfur-carbon complex was loaded in ethanol (EtOH) and induced mixing was performed, the EtOH was dried to prepare a sulfur-carbon complex surface-modified by EtOH treatment.

compare Manufacturing example 1: Preparation of sulfur-carbon composite ( EtOH Untreated)

After mixing sulfur and Ketjen black at a weight ratio of 70:30, heat treatment was performed at 155° C. for 30 minutes to prepare a sulfur-carbon composite.

Example One

(1) anode manufacturing

90% by weight of the surface-modified sulfur-carbon composite (EtOH treated group) prepared in Preparation Example 1, 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. The weight percent was mixed and dissolved in water to prepare a positive electrode slurry having a concentration (20% concentration based on the solid content).

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 electrode thickness (using TESA-μHITE equipment) in the prepared positive electrode was 70%, and the mass of sulfur per unit area of the positive electrode active material layer was 4.7 mg/cm 2. The SC factor value calculated based on this was 0.67.

(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 3.0 M in an organic solvent, and the organic solvent was propionitrile (first solvent) and 1H,1H, It is a solvent obtained by mixing 2'H,3H-decafluorodipropyl ether (second solvent) in a 3:7 weight ratio (w/w). DV ² of the first solvent is 0.39 (electrolyte: Propionitrile:1H,1H,2'H,3H-Decafluorodipropyl ether (3:7, w/w), 3.0M LiTFSI).

Comparative example One

A lithium-sulfur secondary battery was manufactured in the same manner as in Example 1, except that the sulfur-carbon composite (non-EtOH group) prepared in Comparative Preparation Example 1 was used, and a comparative electrolyte was used as an electrolyte. The comparative electrolyte is prepared by adding 1% by weight of LiNO ₃ to an organic solvent, and the organic solvent is a 5:5 weight ratio of DOL (dioxolane) (first solvent) and DME (dimethoxyethane) (second solvent) (Comparative electrolyte: DOL:DME (5:5, v/v), 1.0M LiTFSI, 1.0wt% LiNO ₃ ).

Comparative example 2

A lithium-sulfur secondary battery was manufactured in the same manner as in Example 1, except that a comparative electrolyte was used. The comparative electrolyte is prepared by adding 1% by weight of LiNO ₃ to an organic solvent, and the organic solvent is a 5:5 weight ratio of DOL (dioxolane) (first solvent) and DME (dimethoxyethane) (second solvent) (Comparative electrolyte: DOL:DME (5:5, v/v), 1.0M LiTFSI, 1.0wt% LiNO ₃ ).

Comparative example 3

A lithium-sulfur secondary battery was manufactured in the same manner as in Example 1, except that the sulfur-carbon composite (EtOH untreated group) prepared in Comparative Preparation Example 1 was used.

Experimental example One

Experiments on surface shape, specific surface area, porosity, and thermogravimetric analysis of the surface-modified sulfur-carbon composite (EtOH treatment group) and sulfur-carbon composite (EtOH untreated group) prepared in Preparation Example 1 and Comparative Preparation Example 1, respectively Was carried out.

(1) Observation of surface shape

1 is a scanning transfer microscope (SEM) photograph of the surface-modified sulfur-carbon composite (EtOH-treated group) and sulfur-carbon composite (non-EtOH-treated group) prepared in Preparation Example 1 and Comparative Preparation Example 1, respectively.

Referring to FIG. 1, compared to Comparative Preparation Example 1, it can be seen that the sulfur-carbon composite of Preparation Example 1 reduced the amount of sulfur aggregated on the surface of the sulfur-carbon composite by EtOH treatment.

(2) Specific surface area and porosity

2A and 2B show the specific surface area and porosity measurement results of the surface-modified sulfur-carbon composites (EtOH treated group) and sulfur-carbon composites (EtOH untreated group) prepared in Preparation Example 1 and Comparative Preparation Example 1, respectively. It is a graph for. At this time, the specific surface area and the total pore volume were measured using the barrettjoyner-halenda (BJH) method. The BET specific surface area and porosity can be measured by methods generally known in the art. Specifically, from the amount of nitrogen gas adsorption under liquid nitrogen temperature (77K) using the BELSORP-max equipment of Microtrac BET Can be calculated.

2A and 2B, the surface-modified sulfur-carbon composite (EtOH-treated group) of Preparation Example 1 increased the specific surface area and porosity compared to the sulfur-carbon composite (non-EtOH-treated group) of Comparative Preparation Example 1. Able to know. The specific increase rate is as shown in Table 1 below.

Comparative Preparation Example 1 Manufacturing Example 1 Surface area
(㎡/g) Measure 33.186 42.735 Rate of increase - 1.29 Total pore volume
(Cm 3 /g) Measure 0.3703 0.459 Rate of increase - 1.24

(3) Thermogravimetric analysis

3 is a thermogravimetric analysis (TGA, thermogravimetric analysis) results for the surface-modified sulfur-carbon composite (EtOH-treated group) and sulfur-carbon composite (non-EtOH-treated group) prepared in Preparation Example 1 and Comparative Preparation Example 1, respectively. This is the graph shown.

Referring to Figure 3, there is no difference in the sulfur content of the surface-modified sulfur-carbon composite (EtOH treated group) and sulfur-carbon composite (EtOH untreated group) prepared in Preparation Example 1 and Comparative Preparation Example 1, respectively, from this It can be seen that there is no loss of sulfur by the EtOH treatment.

Experimental example 2

The conditions for the positive electrode and secondary battery each prepared in Examples and Comparative Examples are summarized in Table 2 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 three times at 0.1C-rate and then at 0.3C-rate, and the ED factor was calculated as defined in Equation 4 for the first discharge result. Table 2 shows the calculated results.

EtOH
Whether to process Electrolyte SC factor DV ₂ factor NS factor ED factor Comparative Example 1 X Comparison electrolyte 0.67 2.07 3.09 1290 Comparative Example 2 ○ 0.68 2.07 3.04 1335 Comparative Example 3 X Electrolyte 0.67 0.39 0.58 1443 Example 1 ○ 0.67 0.39 0.58 1450 Comparative electrolyte: DOL:DME(5:5, v/v), 1.0M LiTFSI, 1.0wt% LiNO ₃
Electrolyte: Propionitrile:1H,1H,2'H,3H-Decafluorodipropyl ether (3:7, w/w), 3.0M LiTFSI

Referring to Table 2, Example 1 in which an EtOH-treated sulfur-carbon composite and an electrolyte (a first solvent having a DV ² factor value of 1.5 or less; and an electrolyte including a second solvent that is a fluorinated ether-based solvent) are used together. Showed high ED factor.

4 is a result of a life characteristic experiment for lithium-sulfur secondary batteries prepared in Example 1 and Comparative Examples 1 to 3, respectively.

Referring to FIG. 4, in Example 1, in which an EtOH-treated sulfur-carbon composite and an electrolyte (a first solvent having a DV ² factor value of 1.5 or less; and an electrolyte including a second solvent that is a fluorinated ether-based solvent) are used together. It was found that the lifetime characteristics were the best.

Through these results, it was found that the EtOH-treated sulfur-carbon complex exhibits more effective performance in an electrolyte (a first solvent having a DV ² factor value of 1.5 or less; and an electrolyte including a second solvent, which is a fluorinated ether-based solvent). Able to know.

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 (13)

In the sulfur-carbon composite in which sulfur is supported on a carbon material,
The sulfur-carbon composite has a specific surface area of 10 to 60 m 2 /g and a pore volume of 0.3 to 1.0 cm 3 /g. The method of claim 1,
The sulfur and the carbon material is 55 to 90: that is contained in a weight ratio of 45 to 10, sulfur-carbon composite. (P1) heat-treating by mixing sulfur and a carbon material; And
(P2) surface modification by supporting the sulfur-carbon composite obtained in step (P1) in a solvent for dissolving sulfur;
Containing, sulfur-carbon composite manufacturing method. The method of claim 3,
The sulfur dissolving solvent comprises one or more selected from the group consisting of ethanol, acetone, and CS ₂ , sulfur-carbon composite manufacturing method. anode; cathode; A separator interposed therebetween; And as a lithium secondary battery comprising an electrolyte,
The positive electrode comprises the sulfur-carbon composite of claim 1,
The electrolyte solution contains a solvent and a lithium salt,
The solvent is,
A first solvent having a DV ² factor of 1.5 or less represented by Equation 1 below; And
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 (cP) of the solvent, and γ is 100 (constant). The method of claim 5,
The positive electrode has an SC factor value of 0.45 or more 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 method according to claim 5 or 6,
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 of claim 5 or 6,
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 5,
The first solvent is selected from the group consisting of propionitrile, dimethylacetamide, dimethylformamide, gamma-butyrolactone, triethylamine, 1-iodopropane, and combinations thereof. The method of claim 5,
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 selected from the group consisting of a combination thereof. The method of claim 5,
The solvent is a lithium secondary battery containing 1 to 50% by weight of the first solvent. The method of claim 5,
The solvent is a lithium secondary battery containing 50 to 99% by weight of the second solvent. The method of claim 5,
The solvent comprises a first solvent and a second solvent in a weight ratio of 3:7 to 1:9, a lithium secondary battery.

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