KR20050038897A - An electrolyte for lithium-sulfur batteries and lithium-sulfur batteries comprising the same - Google Patents

Abstract

The present invention relates to an electrolyte for a lithium-sulfur battery, and relates to an electrolyte for a lithium-sulfur battery comprising an ether solvent, a cyclic ether solvent, and a nitrogen-containing cyclic solvent, and to a lithium-sulfur battery including the same. will be.

The electrolyte solution for a lithium-sulfur battery of the present invention has an advantage of increasing the discharging capacity of the lithium-sulfur battery by improving solubility and ion stability of sulfide ions.

Description

An electrolyte for a lithium-sulfur battery and a lithium-sulfur battery including the same {An Electrolyte for Lithium-Sulfur Batteries and Lithium-Sulfur Batteries Comprising The Same}

[Industrial use]

The present invention relates to a lithium-sulfur battery electrolyte and a lithium-sulfur battery including the same, and more particularly, to a lithium-sulfur battery capable of increasing dissolution capacity of sulfide ions and increasing discharge capacity of a lithium-sulfur battery. The present invention relates to an electrolyte and a lithium-sulfur battery including the same.

[Private Technology]

The rapid development of portable electronic devices is increasing the demand for batteries. There is a continuous demand for high energy density batteries to meet the trend of light and small size of portable electronic devices, and to meet these demands, development of batteries that satisfy inexpensive, safe and environmentally friendly aspects is required. .

Among various batteries that meet these requirements, lithium-sulfur batteries are the most promising in terms of energy density among the batteries being developed to date. The energy density of lithium is 3830 mAh / g, sulfur (S ₈ ) of the energy density of 1675 mAh / g is used as the active material itself is an inexpensive, environmentally friendly material has the advantage.

However, there are no examples of commercialization with this battery system yet. The reason is that when the sulfur is used as the active material, the utilization rate indicating the amount of sulfur participating in the electrochemical redox reaction in the battery relative to the amount of sulfur injected is low, resulting in extremely low battery capacity.

In addition, in the redox reaction, if the battery life is deteriorated due to the leakage of sulfur into the electrolyte and the appropriate electrolyte is not selected, lithium sulfide (Li ₂ S), which is a reducing substance of sulfur, is precipitated and no longer participates in the electrochemical reaction. There is a problem that can not be.

At present, research on electrolyte salts and organic solvents showing high oxidation potential as well as high ionic conductivity as an electrolyte component for use in lithium ion batteries has been actively conducted. Lithium salts such as LiClO ₄ , LiBF ₄ , and LiPF ₆ are mainly used in lithium ion batteries.

U. S. Patent No. 5,827, 602 describes a non-aqueous battery using lithium salts comprising triflate, imide, methide anions. However, the above-described lithium ion battery electrolyte shows excellent performance in lithium ion batteries, but there is a problem such as deterioration of battery characteristics in lithium sulfur batteries. Therefore, the electrolyte solution of lithium ion batteries is not used as an electrolyte solution for lithium sulfur batteries.

This is because the electrochemical reaction of polysulfide is very unstable in a carbonate electrolyte mainly used in lithium ion batteries. In order to be used as an electrolyte of a lithium sulfur battery, there is a continuous demand for the development of an electrolyte in which an electrochemical reaction of polysulfide occurs stably and a high concentration of polysulfide is dissolved.

In US Pat. No. 6,030,720, the main solvent is R ₁ (CH ₂ CH ₂ O) _n R ₂ (where n is 2 to 10 and R is an alkyl or alkoxy group), and the cosolvent has a donor number of at least 15 Solvent is used. In addition, a liquid electrolyte including a solvent including at least one of a crown ether, a cryptand, and a donor number is used. The electrolyte is an electrolyte that discharges and subsequently becomes catholyte. However, even when such an electrolyte is used, the capacity, high efficiency, or lifespan characteristics of the lithium-sulfur battery do not reach a satisfactory level.

Many studies have been made to obtain a lithium-sulfur battery having excellent discharge capacity. However, there is a lack of specific studies on the organic solvent and its composition as the most suitable electrolyte solution.

The present invention is to solve the above problems, an object of the present invention is to improve the solubility and ionic stability of sulfide ions to increase the discharge capacity of a lithium-sulfur battery and a lithium-sulfur battery electrolyte comprising the same It is to provide a lithium-sulfur battery.

In order to achieve the above object, the present invention provides an electrolyte for a lithium-sulfur battery comprising an organic solvent and a lithium salt comprising an ether solvent, a cyclic ether solvent, and a nitrogen-containing cyclic solvent.

The present invention also provides a cathode comprising at least one positive electrode active material selected from the group consisting of (a) sulfur elements, sulfur compounds and mixtures thereof, (b) ether solvents, cyclic ether solvents, And an organic solvent including a nitrogen-containing cyclic solvent and an electrolyte including lithium salts, and (c) a material capable of reversibly intercalating or deintercalating lithium ions, and a lithium-containing compound reversibly reacting with lithium ions. Provided is a lithium-sulfur battery comprising a negative electrode including a negative electrode active material selected from the group consisting of a material capable of forming a lithium metal and a lithium alloy.

Hereinafter, the present invention will be described.

When the lithium-sulfur battery is discharged, the sulfur element (S ₈ ) is reduced at the positive electrode to produce sulfide (S ⁻² ) or polysulfide (S _n ⁻¹ , S _n ⁻² , where n ≧ 2). Therefore, the lithium-sulfur battery uses a sulfur element, lithium sulfide (Li ₂ S), or lithium polysulfide (Li ₂ S _n , n = 2, 4, 6, 8) as the positive electrode active material. Dual sulfur has a small polarity, and lithium sulfide or lithium polysulfide is an ionic compound of high polarity, and lithium sulfide is present in an organic solvent as a precipitate, but lithium polysulfide is generally dissolved. In order for a sulfur-based material having such various characteristics to perform an electrochemical reaction smoothly, it is important to select an electrolyte that dissolves these sulfur-based materials well.

MEANS TO SOLVE THE PROBLEM This inventor discovered that the solubility and ion stability of a polysulfide ion improved when the nitrogen-containing cyclic solvent is contained as an electrolyte solution which is excellent in the solubility with respect to a sulfur type positive electrode active material, and has high ion conductivity. To complete.

In the lithium-sulfur battery of the present invention, an electrolyte solution containing a nitrogen-containing cyclic solvent which is excellent in solubility in a sulfur-based positive electrode active material and exhibits high ionic stability is used. The electrolyte solution used in the present invention includes an ether solvent, a cyclic ether solvent, and a nitrogen-containing cyclic solvent as a three component solvent.

Examples of the ether solvent include dimethoxyethane (DME: dimethoxyethane), diethyl ether, diglyme, tetraglyme and the like, the content of which is preferably 10 to 80% by volume with respect to the total organic solvent volume, It is more preferable that it is 20-40 volume%.

Examples of the cyclic ether solvents include 1,3-dioxolane (DOX), 3,5-dimethyl isoxazole, 2,5-dimethylfuran, furan, 2-methyl furan, 1,4-oxane, and 4- Methyldioxolane and the like, and the content thereof is preferably 10 to 30% by volume, more preferably 15 to 25% by volume based on the total organic solvent volume.

In addition, examples of the cyclic solvent including nitrogen include pyridine, methylpyrrole, and N-methylpyrrolidone (NMP: N-methylpyrrolidone), and the content thereof is preferably 10 to 40% by volume based on the total organic solvent volume. More preferably from 15 to 25% by volume.

In the three-component solvent, the effect of increasing the sulfur utilization can be obtained when the content ratio satisfies the optimum range. In particular, when the content of the nitrogen-containing cyclic solvent is less than 10% by volume, the effect of increasing the sulfur utilization is small, and when it exceeds 40% by volume, the solubility of polysulfide is decreased or the viscosity is increased by increasing the viscosity.

The electrolyte solution of the present invention may further include a lithium salt as an electrolytic salt. Examples of the lithium salt are LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , LiSbF ₆ , LiAlO ₄ , LiAlCl ₄ , LiN (C _x F _{2x + 1} SO ₂ ) (C _y F _{2y + 1} SO ₂ ), where x and y are natural water, LiCl, LiI, and the like, and are in the electrolyte at a concentration of 0.5 to 2.0 M. It is preferred to be included.

The lithium sulfur battery 1 according to the present invention includes a battery can 5 including a positive electrode 3, a negative electrode 4, and a separator positioned between the positive electrode 3 and the negative electrode 4 as shown in FIG. 1. ). The electrolyte solution containing pyrrolidone is injected between the positive electrode 3 and the negative electrode 4.

Anode 3 of the lithium sulfur battery of the present invention elemental sulfur, Li ₂ S _n (n≥1), dissolved in the cache solrayiteu (catholyte), Li ₂ S _n (n≥1), organic sulfur compounds, and carbon- Sulfur polymer ((C ₂ S _x ) _n : x = 2.5 to 50, n≥2) at least one positive electrode active material selected from the group consisting of.

The anode 3 may further include one or more additives selected from the group consisting of transition metals, group IIIA, group IVA metals, alloys thereof, or sulfur compounds containing these metals. The transition metal is Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Ta, W, Re, It is preferably selected from the group consisting of Os, Ir, Pt, Au and Hg. The group IIIA metal is preferably selected from the group consisting of Al, Ga, In and Tl. The Group IVA metal is preferably selected from the group consisting of Si, Ge, Sn and Pb.

In addition, it may further include an electrically conductive conductive agent to allow electrons to move smoothly in the positive electrode plate (3). The conductive agent is not particularly limited, but a conductive material such as a graphite material, a carbon material, or a conductive polymer may be preferably used. The graphite material is KS 6 (manufactured by Timcal) and the carbon material is super P (MMM company), ketjen black, denka black, acetylene black, carbon black, and the like. . Examples of the conductive polymer include polyaniline, polythiophene, polyacetylene, polypyrrole, and the like. These conductive conductive agents may be used alone or in combination of two or more thereof.

The positive electrode active material is attached to the current collector by a binder. The binder includes poly (vinyl acetate), polyvinyl alcohol, polyethylene oxide, polyvinyl pyrrolidone, alkylated polyethylene oxide, crosslinked polyethylene oxide, polyvinyl ether, poly (methyl methacrylate), polyvinylidene Copolymers of fluoride, polyhexafluoropropylene and polyvinylidene fluoride (trade name: Kynar), poly (ethyl acrylate), polytetrafluoroethylene, polyvinylchloride, polyacrylonitrile, polyvinylpyridine, polystyrene , Derivatives thereof, blends, copolymers and the like can be used.

In order to manufacture the positive electrode of the present invention, first, a binder is dissolved in a solvent for preparing a slurry, and then a conductive agent is dispersed. As a solvent for preparing the slurry, it is possible to uniformly disperse the sulfur-compound, the binder and the conductive agent, and it is preferable to use one that is easily evaporated. Typically, acetonitrile, methanol, ethanol, tetrahydrofuran, water, Isopropyl alcohol, dimethyl formamide, etc. can be used. Next, the sulfur-based active material and the additive are uniformly dispersed again in the slurry in which the conductive agent is dispersed to prepare a cathode active material slurry. The amount of solvent, sulfur compound or optionally additives included in the slurry does not have a particularly important meaning in the present invention, it is sufficient only to have a suitable viscosity to facilitate the coating of the slurry.

The slurry thus prepared is applied to the current collector, vacuum dried to form the positive electrode plate 3, and then used for assembling the battery 1. It is sufficient that the slurry is coated on the current collector in an appropriate thickness depending on the viscosity and the thickness of the positive electrode plate to be formed. The current collector is not particularly limited, but conductive materials such as stainless steel, aluminum, copper, titanium, and the like are preferably used, and more preferably, a carbon-coated aluminum current collector is used. The use of an aluminum substrate coated with carbon has an advantage in that the adhesion to the active material is excellent, the contact resistance is low, and the corrosion of polysulfide of aluminum is prevented, compared with the non-carbon coated aluminum substrate.

The negative electrode 4 of the lithium-sulfur battery 1 of the present invention is a material capable of reversibly intercalating or deintercalating lithium ions, or a material capable of reacting with lithium ions to form a reversibly lithium-containing compound. And a negative electrode active material selected from the group consisting of lithium metal and lithium alloy.

As a material capable of reversibly intercalating / deintercalating the lithium ions, any carbon-based negative electrode active material generally used in a lithium ion secondary battery may be used, and representative examples thereof include crystalline carbon. , Amorphous carbon or these can be used together. In addition, a representative example of a material capable of reacting with lithium ions to form a lithium-containing compound reversibly may include tin oxide (SnO ₂ ), titanium nitrate, silicon, and the like, but is not limited thereto. As the lithium alloy, an alloy of a metal selected from the group consisting of lithium and Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Al, and Sn may be used.

An inorganic protective layer, an organic protective layer, or a material in which these layers are stacked on a lithium metal surface may also be used as a cathode. As the inorganic protective film, Mg, Al, B, C, Sn, Pb, Cd, Si, In, Ga, lithium silicate, lithium borate, lithium phosphate, lithium proponide, lithium siliconosulfide, lithium borosulfide, lithium It consists of a material selected from the group consisting of aluminosulfide and lithium phosphosulfide. The organic protective film is poly (p-phenylene), polyacetylene, poly (p-phenylene vinylene), polyaniline, polypyrrole, polythiophene, poly (2,5-ethylene vinylene), acetylene, poly (ferry) Naphthalene), polyacene, and poly (naphthalene-2,6-diyl), and a monomer, oligomer or polymer having conductivity selected from the group consisting of.

In addition, in the process of charging and discharging the lithium-sulfur battery, the sulfur used as the positive electrode active material may be changed into an inert material and adhered to the surface of the lithium negative electrode. As such, inactive sulfur refers to a sulfur in which sulfur is no longer able to participate in the electrochemical reaction of the anode through various electrochemical or chemical reactions, and an inert sulfur formed on the surface of the lithium anode is a protective film of the lithium anode. It also has the advantage of acting as a layer. Therefore, lithium metal and inert sulfur formed on the lithium metal, for example lithium sulfide, may be used as the negative electrode.

As the separator present in the positive electrode and the negative electrode, a polymer film such as polyethylene or polypropylene or a multilayer thereof is used.

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are only preferred embodiments of the present invention, and the present invention is not limited thereto.

(Example 1)

67.5 wt% of the sulfur element, 11.4 wt% of Ketjen Black as the conductive agent, and 21.1 wt% of polyethylene oxide as the binder were mixed in an acetonitrile solvent to prepare a cathode active material slurry for a lithium-sulfur battery. The slurry was coated on a carbon-coated aluminum current collector, and the slurry coated current collector was dried in a vacuum oven at 60 ° C. for at least 12 hours to prepare a 2 mAh / cm ² positive electrode plate having a size of 25 × 50 mm ² . It was. Lithium electrodes were sequentially placed on the positive electrode plate, the vacuum dried separator, and the negative electrode, and then inserted into the pouch. Dimethoxyethane / N-methylpyrrolidone (NMP: N-methylpyrrolidone) / dioxolane dissolved in 1.0 M bis (trifluoromethylsulfonylimide) (LiN (CF ₃ SO ₂ ) ₂ ) was dissolved in 70:10: The electrolyte solution mixed at a volume ratio of 20 was injected into the pouch. Pouch-type test cells were assembled by sealing after electrolyte injection.

(Example 2)

In the same manner as in Example 1, except that dimethoxyethane / NMP / dioxolane in which 1.0 M bis (trifluoromethylsulfonylimide) was dissolved in a volume ratio of 60:20:20 was used as an electrolyte solution. The test cell was assembled.

(Example 3)

In the same manner as in Example 1, except that dimethoxyethane / NMP / dioxolane in which 1.0 M bis (trifluoromethylsulfonylimide) was dissolved at a volume ratio of 40:40:20 was used as the electrolyte solution. The test cell was assembled.

(Comparative Example 1)

A test cell was assembled in the same manner as in Example 1, except that a solution obtained by mixing dimethoxyethane / dioxolane in which 1.0 M bis (trifluoromethylsulfonylimide) was dissolved at a volume ratio of 80:20 was used as an electrolyte. It was.

(Comparative Example 2)

In the same manner as in Example 1, except that dimethoxyethane / NMP / dioxolane in which 1.0 M bis (trifluoromethylsulfonylimide) was dissolved in a volume ratio of 10:70:20 was used as the electrolyte solution. The test cell was assembled.

Discharge characteristic evaluation

Cells prepared according to Examples 1 to 3 and Comparative Examples 1 and 2 were discharged at 0.2 C in a cutoff voltage range of 1.5 V to 2.4 V to measure sulfur utilization, and the results are shown in Table 1, and Examples 2 and 2. Sulfur utilization measurement graphs of Comparative Examples 1 and 2 are shown in FIG.

TABLE 1

Sulfur utilization in the primary zone (%) Sulfur utilization in the secondary zone (%) Total Sulfur Utilization (%) Comparative Example 1 5.7 42.7 48.4 Comparative Example 2 6.1 39.6 45.7 Example 1 6.5 44.3 50.8 Example 2 9.0 44.3 53.3 Example 3 9.7 40.9 50.6

As shown in Table 1, the cells of Examples 1 to 3 including NMP increased the sulfur utilization in the primary region and the secondary region, compared to the cells of the comparative example that does not contain NMP. It can be seen that the increase is even greater, from which it can be seen that the discharge capacity of the lithium-sulfur battery is increased when using an electrolyte containing NMP.

Sulfur utilization was measured by discharging the cells prepared according to Examples 1 to 3 and Comparative Examples 1 and 2 at 1.0 C in a cutoff voltage range of 1.5 V to 2.5 V. The sulfur utilization measurement graphs of Example 2 and Comparative Examples 1 and 2 are shown in FIG. 3. As shown in FIG. 3, the sulfur utilization of the cell of Example 2 was also superior to that of Comparative Examples 1 and 2 even at the sulfur utilization at 1.0 C. FIG .

The lithium sulfur battery manufactured using the electrolyte solution containing the nitrogen-containing cyclic solvent of the present invention can be seen that the discharge capacity was increased by increasing the sulfur utilization rate.

1 is a perspective view of a lithium-sulfur battery prepared according to a preferred embodiment of the present invention.

2 is a curve showing sulfur utilization when the discharge current density of the cells prepared according to Example 2, Comparative Examples 1 and 2 is 0.2C.

3 is a curve showing sulfur utilization when the discharge current density of the cells prepared according to Example 2, Comparative Examples 1 and 2 is 1.0 C.

Claims (17)

An electrolyte solution for a lithium-sulfur battery comprising an organic solvent and a lithium salt comprising an ether solvent, a cyclic ether solvent, and a nitrogen-containing cyclic solvent. The method of claim 1, The ether solvent is selected from the group consisting of dimethoxyethane (DME), diethyl ether, diglyme, and tetraglyme, lithium-sulfur battery electrolyte. The method of claim 1, The cyclic ether solvents include 1,3-dioxolane (DOX), 3,5-dimethyl isoxazole, 2,5-dimethylfuran, furan, 2-methyl furan, 1,4-oxane, and 4-methyldioxane The electrolyte solution for lithium-sulfur batteries selected from the group consisting of solans. The method of claim 1, The nitrogen-containing cyclic solvent is pyridine, methylpyrrole, and N-methylpyrrolidone (NMP: N-methylpyrrolidone) electrolyte for lithium-sulfur battery selected from the group consisting of. The method of claim 1, The ether solvent is 10 to 80% by volume relative to the total organic solvent volume, the cyclic ether solvent is 10 to 30% by volume relative to the total organic solvent volume, the nitrogen-containing cyclic solvent to the total organic solvent volume 10 to 40% by volume of the electrolyte for lithium-sulfur battery. The method of claim 5, The ether solvent is 20 to 40% by volume based on the total organic solvent volume of the lithium-sulfur battery electrolyte. The method of claim 5, The cyclic ether solvent is 15 to 25% by volume based on the total organic solvent volume of the lithium-sulfur battery electrolyte. The method of claim 5, The nitrogen-containing cyclic solvent is 15 to 25% by volume with respect to the total organic solvent volume of the lithium-sulfur battery electrolyte. The method of claim 1, The lithium salt is LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , LiSbF ₆ , LiAlO ₄ , LiAlCl ₄ , An electrolyte solution for a lithium-sulfur battery selected from the group consisting of LiN (C _x F _{2x + 1} SO ₂ ) (C _y F _{2y + 1} SO ₂ ), wherein x and y are natural water, LiCl, and LiI. The method of claim 1, The electrolyte solution for lithium-sulfur batteries having a concentration of 0.5 to 2.0 M. A positive electrode comprising at least one positive electrode active material selected from the group consisting of sulfur elements, sulfur compounds and mixtures thereof An electrolyte solution containing an ether solvent, a cyclic ether solvent, and an organic solvent including a nitrogen-containing cyclic solvent and a lithium salt, and A material capable of reversibly intercalating or deintercalating lithium ions, a material capable of reacting with lithium ions to form a lithium-containing compound reversibly, and a negative electrode active material selected from the group consisting of lithium metals and lithium alloys Cathode Lithium-sulfur battery. The method of claim 11, The positive electrode active material is elemental sulfur, Li ₂ S _n (n≥1), the cache solrayiteu dissolved in _{(catholyte) Li 2 S n (} n≥1), organic sulfur compounds, and carbon-sulfur polymers ((C ₂ S _{x n} : x = 2.5 to 50, n≥2) at least one positive electrode active material selected from the group consisting of lithium-sulfur battery. The method of claim 11, And the anode further comprises one or more additives selected from the group consisting of transition metals, group IIIA, group IVA metals, alloys thereof, or sulfur compounds comprising these metals. The method of claim 13, The transition metal is Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Ta, W, Re, At least one element selected from the group consisting of Os, Ir, Pt, Au, and Hg, the Group IIIA metal is at least one element selected from the group consisting of Al, Ga, In, and Tl, and the Group IVA metal is A lithium-sulfur battery which is at least one element selected from the group consisting of Si, Ge, Sn and Pb. The method of claim 11, The lithium-sulfur battery further comprises an electrically conductive conductive agent to help the positive movement of the electrons. The method of claim 11, The anode further comprises a binder for attaching the current collector and the positive electrode active material to the current collector, The binder is poly (vinyl acetate), polyvinyl alcohol, polyethylene oxide, polyvinyl pyrrolidone, alkylated polyethylene oxide, crosslinked polyethylene oxide, polyvinyl ether, poly (methyl methacrylate), polyvinylidene fluorine Lides, copolymers of polyhexafluoropropylene and polyvinylidene fluoride, poly (ethyl acrylate), polytetrafluoroethylene, polyvinylchloride, polyacrylonitrile, polyvinylpyridine, polystyrene, and derivatives thereof, At least one selected from the group consisting of blends or copolymers. The method of claim 11, The electrolyte solution is LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , LiSbF ₆ , LiAlO ₄ , LiAlCl ₄ , LiN Lithium-sulfur battery comprising a lithium salt selected from the group consisting of (C _x F _{2x + 1} SO ₂ ) (C _y F _{2y + 1} SO ₂ ), where x and y are natural water, LiCl, and LiI .