WO2021010605A1 - Lithium secondary battery - Google Patents

Abstract

The present invention relates to a lithium secondary battery, and more specifically, to a lithium secondary battery comprising a positive electrode slurry having a particle size (based on D₅₀) of 15-50 µm, wherein a condition for an electrolyte is specified, and thus high energy density and a long lifespan compared to conventional lithium secondary batteries may be implemented.

Description

Lithium secondary battery

This application claims the interests of priority based on Korean patent application No. 10-2019-0085623 filed on July 16, 2019 and Korean patent application No. 10-2020-0073784 filed on June 17, 2020, and All contents disclosed in the literature are included as part of this specification.

The present invention relates to a lithium secondary battery.

As the application area of secondary batteries expands to electric vehicles (EV) or energy storage systems (ESS), lithium-ion has a relatively low energy storage density (~250 Wh/kg) compared to its weight. Ion secondary batteries have limitations in their application to these products. 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 is a battery system using a sulfur-based material having a sulfur-sulfur bond as a positive electrode active material and lithium metal as a negative electrode active material. This lithium-sulfur secondary battery has the advantage that sulfur, which is the main material of the positive electrode active material, has an abundance of resources worldwide, is non-toxic, and has a low weight per atom.

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. At this time, the oxidation reaction of lithium is a process in which lithium metal releases electrons and is converted into lithium cation form. In addition, the sulfur reduction reaction is a process in which a sulfur-sulfur 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 discharge has a cyclic S ₈ structure, which is converted into lithium polysulfide (Li ₂ S _x , x = 8, 6, 4, 2) by a reduction reaction, and this lithium polysulfide When is completely reduced, lithium sulfide (Li ₂ S) is finally produced.

Sulfur, which is a positive active material, has low electrical conductivity, so it is difficult to secure reactivity with electrons and lithium ions in a solid-state form. Existing lithium-sulfur secondary batteries induce a liquid-state reaction and improve reactivity by generating an intermediate polysulfide in the form of Li ₂ S _x 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, due to the characteristic of lithium polysulfide, which is easily soluble in the electrolyte, the amount of sulfur Reactivity and lifetime characteristics will be affected. In addition, in order to build a high energy density, a low content of electrolyte must be injected, but as the electrolyte content decreases, the concentration of lithium polysulfide in the electrolyte increases, so that normal operation of the battery is difficult due to a decrease in fluidity of the active material and an increase in side reactions it's difficult.

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

For example, Korean Patent Laid-Open No. 2016-0037084 uses a carbon nanotube aggregate of a three-dimensional structure coated with graphene with a carbon material to block the melting of lithium polysulfide and improve the conductivity of the sulfur-carbon nanotube composite. It discloses that it can be improved.

In addition, Korean Patent Registration No.1379716 uses a graphene composite containing sulfur prepared by treating graphene with hydrofluoric acid to form pores on the surface of graphene and growing sulfur particles in the pores as a positive electrode active material. It is disclosed that lithium polysulfide elution can be suppressed to minimize a decrease in capacity of the battery.

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

[Prior technical literature]

[Patent Literature]

Republic of Korea Patent Publication No. 10-2016-0037084 (2016.04.05), sulfur-carbon nanotube composite, method for manufacturing the same, cathode active material for lithium-sulfur battery including the same, and lithium-sulfur secondary battery including the same

Republic of Korea Patent Registration No. 10-1379716 (2014.03.25), a lithium-sulfur secondary battery including a graphene composite positive electrode containing sulfur and a manufacturing method thereof

Accordingly, the present inventors manufactured a lithium secondary battery using an electrolyte solution including a positive electrode slurry with a particle size adjusted and a solvent having a dipole moment less than a certain value, and that the lithium secondary battery thus manufactured exhibits improved life characteristics. Confirmed.

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

To achieve the above object,

The present invention is a positive electrode; cathode; A separator interposed therebetween; And as a lithium secondary battery comprising an electrolyte,

The positive electrode includes a positive electrode slurry including a sulfur-carbon composite, a binder, and a conductive material,

The particle size (based on D ₅₀ ) of the positive electrode slurry is 15 to 50 μm,

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]

Where 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 lithium secondary battery of the present invention includes a positive electrode including a positive electrode slurry having a particle size (based on D ₅₀ ) of 15 to 50 μm; And an electrolytic solution including a first solvent having a DV ² factor of 1.75 or less and a second solvent that is a fluorinated ether-based solvent; when it contains, there is an effect of improving high energy density and lifespan characteristics.

1 is a graph showing the ED factor value according to the number of cycles of the lithium-sulfur secondary battery of Experimental Example 1.

Hereinafter, the present invention will be described in detail.

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

The terms used in the present invention are only used to describe specific embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In the present invention, terms such as'include' or'have' are intended to designate the presence of features, numbers, steps, actions, components, parts, or a combination thereof described in the specification, but one or more other features It is to be understood that the presence or addition of elements or numbers, steps, actions, components, parts, or combinations thereof, does not preclude in advance the possibility.

The term "polysulfide" used herein, "polysulfide ions ^{_{(S x 2-, x = 8}} , 6, 4, 2)" and "lithium polysulfide (Li ₂ S _x, or LiS _x ^-, _x = 8 , 6, 4, 2)”.

For the physical properties described in the present specification, if the measurement conditions and methods are not specifically described, the physical properties are measured according to the measurement conditions and methods generally used by those skilled in the art.

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

However, in the case of the existing lithium-sulfur secondary battery system, the loss of sulfur occurs due to the inability to suppress the elution of lithium polysulfide as described above, and the amount of sulfur participating in the electrochemical reaction rapidly decreases. And not all of the theoretical energy density. In particular, the eluted lithium polysulfide, in addition to being suspended or precipitated in the electrolyte, reacts directly with lithium metal, which is the negative electrode, and is fixed in the form of Li ₂ S on the surface of the negative electrode, thereby corroding the lithium metal negative electrode, and initial capacity and cycle after a certain cycle. It causes a problem of rapidly deteriorating properties.

In the prior art, methods such as introducing a material capable of inhibiting the elution of lithium polysulfide into a positive electrode or separator in the form of an additive or protective layer, changing the structure or material of the positive electrode active material, and changing the composition of the electrolyte have been proposed. Not only the dissolution improvement effect was insignificant, but also the amount of sulfur, which is the positive electrode active material, is limited (ie, the loading amount), and it causes serious problems in the stability of the battery or is inefficient in terms of the process.

Accordingly, in the present invention, in a lithium-sulfur secondary battery including a positive electrode, a negative electrode, a separator, and an electrolyte, the porosity (or porosity) of the positive electrode active material layer is low, the loading amount of sulfur as the positive electrode active material is high, and the particle size (based on D ₅₀ ) is 15. And a positive electrode including a positive electrode slurry having a thickness of 50 μm.

In general, when the porosity of the positive electrode is lowered and the content of the positive electrode active material is increased, the energy density of a secondary battery including the same increases. However, in a lithium-sulfur secondary battery, if the porosity of the positive electrode is reduced to a minimum and the content of sulfur is increased to the maximum, the proportion of electrolyte per unit sulfur content decreases, as well as problems due to the dissolution and shuttle phenomenon of lithium polysulfide remain. When the positive electrode is applied to a lithium-sulfur secondary battery, it is difficult to achieve the target performance.

Therefore, in the present invention, while using a positive electrode slurry having a particle size (based on D ₅₀ ) of 15 to 50 μm, the conditions related to the electrolyte are specified to have a high energy density compared to the existing lithium-sulfur secondary battery when actually implemented, A lithium-sulfur secondary battery having excellent properties can be provided.

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

The positive electrode includes a positive electrode slurry including a sulfur-carbon composite, a binder, and a conductive material,

The particle size (based on D ₅₀ ) of the positive electrode slurry is 15 to 50 μm,

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 relates to a lithium secondary battery comprising a second solvent which is a fluorinated ether solvent.

[Equation 1]

Where 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).

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

anode

The positive electrode for a lithium secondary battery of the present invention includes a positive electrode slurry containing a sulfur-carbon composite, a binder, and a conductive material, and the particle size (based on D ₅₀ ) of the positive electrode slurry is 15 to 50 μm.

Specifically, the positive electrode for a lithium secondary battery of the present invention includes a positive electrode current collector; And a positive electrode active material layer formed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer may be formed of a positive electrode slurry having a particle size of 15 to 50 μm (based on D ₅₀ ).

In the present invention, the particle size of the positive electrode slurry refers to the particle size of the positive electrode slurry itself finally obtained by mixing a sulfur-carbon composite, a binder, and a conductive material in a solvent. In general, the particle size of the positive electrode slurry may depend on the particle size of the sulfur-carbon composite contained therein, but the particle size of the sulfur-carbon composite decreases due to the process of mixing for dispersion during the manufacturing process of the positive electrode slurry. The particle size of the slurry and the particle size of the sulfur-carbon composite are distinct.

The particle size (based on D ₅₀ ) of the positive electrode slurry including the sulfur-carbon composite, the binder, and the conductive material may be 15 to 50 µm, preferably more than 15 µm and 30 µm or less, and more preferably 17 to 30 µm.

As the particle size (based on D ₅₀ ) of the positive electrode slurry is 15 to 50 μm, life characteristics of a lithium-sulfur secondary battery including the same may be improved.

If the particle size of the positive electrode slurry (based on D ₅₀ ) is less than 15 µm, adhesion to the positive electrode current collector decreases, and there is a problem of being separated from the positive electrode current collector, and an overvoltage occurs when the lithium-sulfur secondary battery is driven, resulting in a decrease in lifespan characteristics. I can. On the contrary, when the particle size (based on D ₅₀ ) of the positive electrode slurry exceeds 50 μm, the particle size distribution of the particles present in the positive electrode active material layer increases, thereby increasing the non-uniformity of the positive electrode active material layer, which may cause battery performance degradation. In addition, when manufacturing the positive electrode, the particles of the sulfur-carbon composite may be too large to cause scratches, which may cause difficulties in manufacturing the positive electrode.

The sulfur-carbon composite is a cathode active material for a lithium secondary battery, and includes sulfur and carbon nanotubes, and more specifically, is a sulfur-carbon composite in which sulfur is uniformly supported inside and outside the carbon nanotubes.

In addition, the carbon nanotubes may be entangled type carbon nanotubes having a particle size (based on D ₅₀ ) of 15 to 50 μm. The entangled type refers to a form made of one particle by lumping together entangled carbon nanotubes, and is also referred to as a non-bundle type. Here, the particle shape means a particle whose specific shape is not determined.

As the particle size of the entangled carbon nanotubes in the form of particles is controlled through a milling process, the particle size of the sulfur-carbon composite including the same and the particle size of the positive electrode slurry can be controlled.

The sulfur may be one or more selected from the group consisting of inorganic sulfur (S ₈ ), Li ₂ S _n (n≥1) and organic sulfur compounds, preferably inorganic sulfur (S ₈ ).

The sulfur-carbon composite may include the sulfur and carbon nanotubes in a weight ratio of 55:45 to 90:10. 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 sulfur-carbon composite may be prepared by mixing the carbon nanotubes and sulfur, and then impregnating the sulfur into the carbon nanotubes through a melt diffusion method.

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 electrode slurry. If the sulfur-carbon composite is less than 60% by weight, battery performance may deteriorate, and if it exceeds 95% by weight, the content of conductive materials or binders other than the positive electrode active material is relatively reduced, resulting in a decrease in properties such as conductivity or durability. .

The conductive material is not particularly limited, but may include 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 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 slurry. If it is less than the above range, the bonding force between the positive electrode active material or between the positive electrode active material and the current collector may be reduced, thereby deteriorating electrode stability. 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.

The positive electrode slurry is prepared as a slurry having a particle size of 15 to 50 μm (based on D ₅₀ ) by mixing a sulfur-carbon composite, a conductive material, and a binder in a solvent, and after applying the slurry having the particle size on a current collector Drying and selectively rolling may be performed to prepare a positive electrode having a positive electrode active material layer formed thereon.

In this case, a solvent for preparing the sulfur-carbon composite, the conductive material, and the binder in a slurry state may include acetonitrile, methanol, ethanol, tetrahydrofuran, water, isopropyl alcohol, etc., but is not limited thereto.

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/cm ³ ), and α is 10 (constant).

The lithium secondary battery according to the present invention, preferably a lithium-sulfur secondary battery, implements 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, lithium In order for a secondary battery, preferably a lithium-sulfur secondary battery, to achieve a high energy density, the SC factor value may be greater than 0.45, preferably greater than 0.5. 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 more than 0.45, 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 the lithium-sulfur secondary battery according to the present invention, The performance of the battery is maintained without deterioration.

Electrolyte

In the present invention, the electrolyte is a non-aqueous electrolyte containing a lithium salt, and includes 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 secondary battery, preferably 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 8.0 M, 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 0.5 to 5.0 M, more preferably 1.0 to 3.0 M may be. 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 1 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 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 DV ² factor value is not particularly limited, but considering an embodiment of an actual lithium secondary battery, preferably a lithium-sulfur secondary battery, the DV ² factor value 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 one or more selected from the group consisting of butyrolactone (Gamma-Butyrolactone), triethylamine (Triethylamine) and 1-iodopropane (1-iodopropane).

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, as described above, a positive electrode slurry having a low porosity, a high loading amount of sulfur, and a particle size (based on D ₅₀ ) of 15 to 50 μm When applied to a lithium-sulfur secondary battery including a, it may be advantageous in improving battery performance, such as improving life characteristics.

The lithium 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 secondary battery, preferably a lithium-sulfur secondary battery, the NS factor value may be 0.1 or more. When the NS factor value is adjusted within the above range, the 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 adjust the viscosity of the electrolyte. When using such a solvent as a diluent, high loading as in the present invention , It is not possible to drive a battery including a positive electrode having a low porosity and a particle size of the positive electrode slurry within a certain range.

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), pentafluoroethyl 2,2,2-trifluoroethyl ether and 1H,1H,2′H-perfluorodipropyl ether (1H,1H, 2′H-Perfluorodipropyl ether) may be one or more 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 a second solvent within the range of 50 to 99% by weight described above, like the first solvent, the positive electrode slurry having a particle size (based on D ₅₀ ) of 15 to 50 μm as described above is included. When applied to a lithium-sulfur battery, it may be advantageous in improving battery performance, such as improving life characteristics.

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 nitrous acid-based compound has an effect of forming a stable film on the lithium electrode and improving charging/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; Organic nitro compounds such as nitromethane, nitropropane, nitrobutane, nitrobenzene, dinitrobenzene, nitropyridine, dinitropyridine, nitrotoluene, and dinitrotoluene; And one selected from the group consisting of a combination thereof, and lithium nitrate is preferably used.

In addition, the non-aqueous electrolyte may further include other additives for the purpose of improving charging/discharging properties 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 Dione, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride, fluoroethylene carbonate (FEC), propene sultone (PRS), vinylene carbonate ( VC) and the like.

cathode

The negative electrode for a lithium secondary battery of the present invention includes a negative electrode current collector; And a negative active material layer formed on at least one surface of the 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, the conductive material, and the negative electrode current collector may be the same as the material used in the positive electrode.

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 special limitation. In particular, it is preferable that the separator has low resistance against ion migration of the electrolyte and has excellent electrolyte moisture-repelling ability. Do.

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. Such a separator has a porosity of 30 to 50%, and may be made of a non-conductive or insulating material.

Specifically, a porous polymer film, for example, an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, and an ethylene/methacrylate copolymer, etc., are 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%.

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

The lithium secondary battery, preferably 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 (V) for Li/Li ⁺ , 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 SC factor 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 addition, the present invention provides a battery module including the lithium secondary battery as a unit cell.

The battery module can be used as a power supply for medium and large 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, a preferred embodiment is presented to aid the understanding of the present invention, but the following examples are only illustrative of the present invention, and it is obvious to those skilled in the art that various changes and modifications are possible within the scope and spirit of the present invention, It is natural that such modifications and modifications fall within the appended claims.

<Manufacture of lithium-sulfur secondary battery>

Example One.

An entangled type carbon nanotube having a particle size (based on D ₅₀ ) of 100 to 150 µm was manufactured using a ball mill to prepare an entangled type carbon nanotube having a reduced particle size (based on D ₅₀ ).

After mixing the entangled carbon nanotubes with reduced particle size (based on D ₅₀ ) and sulfur, the mixture was melted at a temperature of 155° C. for 30 minutes to prepare a sulfur-carbon composite. .

The weight ratio of sulfur and entangled carbon nanotubes in the sulfur-carbon composite was 70:30.

90% by weight of the sulfur-carbon complex, 5% by weight of styrene butadiene rubber/carboxymethylcellulose as a binder (weight ratio of SBR/CMC 7:3) and 5% by weight of denka black as a conductive material were mixed, dissolved in water, and the concentration ( A positive electrode slurry having a concentration of 20% based on the solid content) was prepared. The particle size of the positive electrode slurry was measured using a particle size analyzer (PSA), and the particle size (based on D ₅₀ ) of the positive electrode slurry was 27 μm.

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 manufactured positive electrode was 60%, and the mass of sulfur per unit area of the positive electrode active material layer was 4.54 mg/cm ³ Was. The SC factor value calculated based on this was 0.757.

After positioning the positive electrode and the negative electrode 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. At this time, a lithium foil having a thickness of 60 μm was used as the negative electrode.

After that, an electrolyte was injected into the case to prepare a lithium-sulfur secondary battery.

At this time, the electrolyte is prepared by dissolving lithium bis (trifluoromethyl sulfonyl) imide (LiTFSI) having a concentration of 3 M in an organic solvent, and the organic solvent is propionitrile (first solvent) and 1H,1H, A solvent obtained by mixing 2'H,3H-decafluorodipropyl ether (second solvent) in a 3:7 weight ratio (w/w) was used. The dipole moment per unit volume in the first solvent was 97.1 D·mol/L, and the viscosity of the solvent measured by using a BROOKFIELD AMETEK LVDV2T-CP viscometer was 0.38 cP (25° C.). The DV ² factor value calculated based on this was 0.39. Charging and discharging of the manufactured battery was performed at 45°C.

Example 2.

An entangled-type carbon nanotube having a particle size (based on D ₅₀ ) of 100 to 150 µm was used with a ball mill to increase the ball mill time compared to Example 1, so that the particle size (based on D ₅₀ ) was reduced compared to Example 1. A type of carbon nanotube was prepared.

Subsequent processes were carried out in the same manner as in Example 1 to prepare a lithium-sulfur secondary battery. The particle size of the positive electrode slurry of Example 2 was measured using a particle size analyzer (PSA), and the particle size (based on D ₅₀ ) of the positive electrode slurry was 24 μm.

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 manufactured positive electrode was 60%, and the mass of sulfur per unit area of the positive electrode active material layer was 4.54 mg/cm ³ Was. The SC factor value calculated based on this was 0.757.

Example 3.

An entangled type carbon nanotube having a particle size (based on D ₅₀ ) of 100 to 150 µm was used in a ball mill to increase the ball mill time compared to Example 2, so that the particle size (based on D ₅₀ ) was reduced compared to Example 2. A type of carbon nanotube was prepared.

Subsequent processes were carried out in the same manner as in Example 1 to prepare a lithium-sulfur secondary battery. The particle size of the positive electrode slurry of Example 3 was measured using a particle size analyzer (PSA), and the particle size (based on D ₅₀ ) of the positive electrode slurry was 18 μm.

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 60%, and the mass of sulfur per unit area of the positive electrode active material layer was 4.35 mg/cm ³ . The SC factor value calculated based on this was 0.725.

Example 4.

An entangled-type carbon nanotube having a particle size (based on D ₅₀ ) of 100 to 150 μm was used in a ball mill to increase the ball mill time compared to Example 3, so that the particle size (based on D ₅₀ ) was reduced compared to Example 3 A type of carbon nanotube was prepared.

Subsequent processes were carried out in the same manner as in Example 1 to prepare a lithium-sulfur secondary battery. The particle size of the positive electrode slurry of Example 4 was measured using a particle size analyzer (PSA), and the particle size (based on D ₅₀ ) of the positive electrode slurry was 15 μm.

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 60%, and the mass of sulfur per unit area of the positive electrode active material layer was 4.6 mg/cm ³ Was. The SC factor value calculated based on this was 0.767.

Comparative Example 1.

An entangled type carbon nanotube having a particle size (based on D ₅₀ ) of 100 to 150 µm was used in a ball mill to increase the ball mill time compared to Example 4, so that the particle size (based on D ₅₀ ) was reduced compared to Example 4. A type of carbon nanotube was prepared.

Subsequent processes were carried out in the same manner as in Example 1 to prepare a lithium-sulfur secondary battery. The particle size of the positive electrode slurry of Comparative Example 1 was measured using a particle size analyzer (PSA), and the particle size (based on D ₅₀ ) of the positive electrode slurry was 11 μm.

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 60%, and the mass of sulfur per unit area of the positive electrode active material layer was 4.73 mg/cm ³ . Based on this, the calculated SC factor value was 0.788.

Comparative Example 2.

An entangled type carbon nanotube having a particle size (based on D ₅₀ ) of 100 to 150 μm was used in a ball mill to increase the ball mill time compared to Example 4, resulting in a reduced particle size (based on D ₅₀ ) than in Example 4. An entangle-type carbon nanotube was prepared.

Subsequent processes were carried out in the same manner as in Example 1 to prepare a lithium-sulfur secondary battery. The particle size of the positive electrode slurry of Comparative Example 2 was measured using a particle size analyzer (PSA), and the particle size (based on D ₅₀ ) of the positive electrode slurry was 8 μm.

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 60%, and the mass of sulfur per unit area of the positive electrode active material layer was 4.37 mg/cm ³ . Based on this, the calculated SC factor value was 0.728.

Comparative Example 3.

An entangled type carbon nanotube having a particle size (based on D ₅₀ ) of 100 to 150 µm was ball milled at the same time as in Example 4, and the entangled type carbon nanotube having the same particle size (based on D ₅₀ ) as in Example 4 The tube was prepared.

Subsequent processes were carried out in the same manner as in Example 1 to prepare a lithium-sulfur secondary battery. The particle size of the positive electrode slurry of Comparative Example 3 was measured using a particle size analyzer (PSA), and the particle size (based on D ₅₀ ) of the positive electrode slurry was 15 μm.

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 60%, and the mass of sulfur per unit area of the positive electrode active material layer was 2.7 mg/cm ³ . The SC factor value calculated based on this was 0.45.

The conditions of Examples 1 to 4 and Comparative Examples 1 to 3 are summarized and shown in Table 1 below.

	Particle size of positive electrode slurry (D 50 , ㎛)	SC factor	DV 2 factor	NS factor	ED factor
Example 1	27	0.757	0.39	0.515	1433.185
Example 2	24	0.757	0.39	0.515	1422.955
Example 3	18	0.725	0.39	0.538	1434.375
Example 4	15	0.767	0.39	0.508	1392.043
Comparative Example 1	11	0.788	0.39	0.495	1359.417
Comparative Example 2	8	0.728	0.39	0.535	992.8758
Comparative Example 3	15	0.45	0.39	0.866	822.1622

Experimental Example 1. Battery performance evaluation

Charge-discharge at a current density of 0.1 C during the initial 5 cycles using a charge/discharge measuring device (LAND CT-2001A, Wuhan, China), and then charge and discharge 0.1 C and discharge 0.3 C. , ED factor values of lithium-sulfur secondary batteries according to Examples 1 to 4 and Comparative Examples 1 to 3 were measured. The results obtained at this time are shown in FIG. 1.

In the results of FIG. 1, the lithium-sulfur secondary batteries of Examples 1 to 4 having a particle size (based on D ₅₀ ) of 15 to 50 μm of the positive electrode slurry exhibited excellent results in life characteristics. In Example 4, the positive electrode slurry had a particle size (based on D ₅₀ ) of 15 µm, and the lifespan characteristics were not superior to those of Examples 1 to 3, but the results were superior to that of Comparative Example 1. Therefore, it can be seen that the particle size (based on D ₅₀ ) of the positive electrode slurry has a critical significance at 15 μm.

In Comparative Examples 1 and 2, the particle size (based on D ₅₀ ) of the positive electrode slurry was less than 15 μm, and it can be seen that the ED factor rapidly decreased as the cycle was repeated. In addition, in Comparative Example 3, the SC factor value is 0.45 or less, and it can be seen that the ED factor shows a low energy density of 800 levels during the charge/discharge cycle.

Therefore, it can be seen that when the particle size (based on D ₅₀ ) of the positive electrode slurry is 15 to 50 μm, the battery life characteristics are excellent.

Claims (14)

1. anode; cathode; A separator interposed therebetween; And as a lithium secondary battery comprising an electrolyte,

   The positive electrode includes a positive electrode slurry including a sulfur-carbon composite, a binder, and a conductive material,

   The particle size (based on D ₅₀ ) of the positive electrode slurry is 15 to 50 μm,

   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]

   

   Where 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).
2. According to claim 1, The positive electrode is a positive electrode current collector; And a positive electrode active material layer formed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer comprises a positive electrode slurry having a particle size (based on D ₅₀ ) of 15 to 50 μm.
3. The lithium secondary battery according to claim 1, wherein the sulfur-carbon composite contains sulfur and carbon nanotubes.
4. The lithium secondary battery according to claim 1, wherein the carbon nanotubes have a particle size (based on D ₅₀ ) of 15 to 50 μm in a particle shape entangled type.
5. The lithium secondary battery according to claim 1, wherein a DV ² factor value of the first solvent is 1.5 or less.
6. The lithium secondary battery according to claim 1, wherein 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/cm ² ), and α is 10 (constant).
7. The lithium secondary battery according to claim 1, wherein 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.
8. The lithium secondary battery according to claim 1, wherein 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 (V) for Li/Li ⁺ , 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 SC factor Is the same as the value defined by Equation 2 above.
9. The method of claim 1, wherein the first solvent is at least one selected from the group consisting of propionitrile, dimethylacetamide, dimethylformamide, gamma-butyrolactone, triethylamine, and 1-iodopropane. Lithium secondary battery.
10. The method of claim 1, wherein the second solvent is 1H,1H,2'H,3H-decafluorodipropyl ether, difluoromethyl 2,2,2-trifluoroethyl ether, 1,2,2,2 -Tetrafluoroethyl trifluoromethyl ether, 1,1,2,3,3,3-hexafluoropropyl difluoromethyl ether, pentafluoroethyl 2,2,2-trifluoroethyl ether and 1H ,1H,2'H-perfluorodipropyl ether, characterized in that at least one selected from the group consisting of a lithium secondary battery.
11. The lithium secondary battery according to claim 1, wherein the solvent comprises 1 to 50% by weight of the first solvent.
12. The lithium secondary battery according to claim 1, wherein the solvent contains 50 to 99% by weight of the second solvent.
13. The lithium secondary battery according to claim 1, wherein the solvent comprises a first solvent and a second solvent in a weight ratio of 1:1 to 1:9.
14. The lithium secondary battery according to claim 1, wherein the lithium secondary battery is a lithium-sulfur secondary battery.

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