KR100553774B1 - Organic electrolytic solutin and lithium sulfur battery comprising the same - Google Patents

Abstract

The present invention relates to a lithium sulfur battery electrolyte having a high discharge capacity and excellent cycle life characteristics, and a lithium sulfur battery including the same. In a lithium sulfur battery electrolyte including a lithium salt and an organic solvent, a borate compound of the following Chemical Formula 1 is further added: Features include:

[Formula 1]

In Chemical Formula 1,

R ₁ and R ₂ are each independently hydrogen, a straight or branched alkyl group having 1 to 5 carbon atoms, or an alkoxy group, or R ₁ and R ₂ are connected to each other to form a ring,

R ₃ and R ₄ are independently of each other hydrogen, a straight or branched alkyl group having 1 to 5 carbon atoms or an alkoxy group.

Description

Organic electrolytic solution and lithium sulfur battery including the same {Organic electrolytic solutin and lithium sulfur battery comprising the same}

Figure 1 shows the discharge capacity according to the cycle of the lithium sulfur battery of the Examples and Comparative Examples.

The present invention relates to an organic electrolyte and a lithium-sulfur battery including the same, and more particularly, to an electrolyte including a borate-based compound capable of improving the discharge capacity and cycle life of a lithium-sulfur battery and a lithium-sulfur battery including the same. will be.

With the rapid development of portable electronic devices, the demand for secondary batteries is increasing, and the emergence of batteries of high energy density that can meet the trend of light and short of portable electronic devices is continuously required. In order to meet these demands, it is necessary to develop batteries that satisfy inexpensive, safe and environmentally friendly aspects.

Among the various batteries satisfying these needs, lithium sulfur batteries are the most promising in terms of energy density among the batteries developed to date, and the energy density of lithium is 3830 mAh / g and the sulfur (S ₈ ) is 1675 mAh /. g, the active material itself is an inexpensive and environmentally friendly material, but there are no examples of successful commercialization with this battery system.

The reason why the lithium sulfur battery cannot be commercialized is because the utilization rate indicating the amount of sulfur participating in the electrochemical redox reaction in the battery relative to the amount of sulfur introduced is extremely low, resulting in extremely low battery capacity.

For lithium sulfur batteries, monomeric sulfur is used as the initial positive electrode active material. As the discharge of the battery proceeds, the eight sulfur atoms in the ring-shaped molecular state are reduced to linear molecules, and finally S ^2- is obtained by continuous reduction reaction. S ² formed is chemically tightly bonded to the surrounding lithium cation to form lithium sulfide (Li ₂ S). The produced lithium sulfide precipitates on the surface of the positive electrode to reduce the activated area of the battery and also to reduce the capacity of the battery because it cannot be oxidized during charging. Therefore, it is necessary to dissociate such lithium sulfide to maintain the activated area of the battery.

Researches to solve these problems include the following.

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 ₁ and R ₂ are the same or different and are substituted or unsubstituted alkyl or alkoxy groups. ), A mixed solvent in which the cosolvent is crown ether, cryptand, or the like is used. Preferred solvents include donor or acceptor cosolvents, the donor solvent having a donor number of at least 15. In addition, the separation distance of the battery should be 400 μm or less.

In general, during the discharge of a lithium sulfur secondary battery, the formation of Li ₂ S and the precipitation on the electrode surface are known as the biggest causes of the decrease in battery capacity. Much research has been conducted to increase the capacity of the batteries, most of which use ether-based solvents with high sulfide stability, and the current level is 840 mAh / g-sulfur with an initial discharge capacity of about 50% of theoretical capacity. . Attempts have been made to use polar solvents such as DMF and DMAc as attempts to dissociate Li ₂ S, but these solvents have the property of reacting violently with lithium, making it virtually impossible to apply to lithium / sulfur cell systems.

In addition, U.S. Patent No. 5,961,672 discloses 1M LiSO ₃ CF ₃ , 1,3-dioxolane / diglyme / sulforan / dimethoxyethane (50/20) to improve the life and safety of a polymer film on a lithium metal anode. / 10/20) using a mixed solution as the electrolyte is shown.

U.S. Patent No. 5,814,420 allows an electrode with an active material to be in contact with both ionic and electron conductive materials such that active materials such as active sulfur and / or polysulfide polymers are used almost completely.

U. S. Patent No. 5,523, 179 discloses a lithium sulfur battery having excellent performance as the anode includes an active sulfur-based material, an ion conductive material and an electron conductive material.

 The present invention provides a lithium sulfur battery electrolyte and lithium sulfur containing the same to improve the discharge capacity and life characteristics of the battery by dissociating lithium poly sulfide by applying an additive to the electrolyte of the lithium-sulfur battery so that sulfur can continuously participate in the electrochemical reaction It is an object to provide a battery.

In order to achieve the above object, the present invention provides an electrolytic solution, further comprising a borate-based compound represented by the following Chemical Formula 1 in a lithium sulfur battery electrolyte solution containing a lithium salt and an organic solvent:

[Formula 1]

In Chemical Formula 1,

R ₁ and R ₂ are each independently hydrogen, a straight or branched alkyl group having 1 to 5 carbon atoms, or an alkoxy group, or R ₁ and R ₂ are connected to each other to form a ring,

R ₃ and R ₄ are independently of each other hydrogen, a straight or branched alkyl group having 1 to 5 carbon atoms or an alkoxy group

In addition, the present invention is a cathode comprising at least one cathode active material selected from the group consisting of elemental sulfur, sulfur-based compounds and mixtures thereof;

An anode comprising an anode active material of an alloy of lithium or lithium metal;

A separator interposed between the cathode and the anode; And

Provided is a lithium sulfur secondary battery including an organic electrolyte solution including a lithium salt, an organic solvent, and a borate-based compound of Formula 1 below:

[Formula 1]

In Chemical Formula 1,

R ₁ and R ₂ are each independently hydrogen, a straight or branched alkyl group having 1 to 5 carbon atoms, or an alkoxy group, or R ₁ and R ₂ are connected to each other to form a ring,

R ₃ and R ₄ are independently of each other hydrogen, a straight or branched alkyl group having 1 to 5 carbon atoms or an alkoxy group

Hereinafter, the present invention will be described in detail.

The present invention is characterized in that a borate-based compound represented by the following formula (1) is added to a lithium sulfur battery electrolyte solution containing a conventional lithium salt and an organic solvent.

[Formula 1]

In Chemical Formula 1

R ₁ , R ₂ are each independently hydrogen, linear or branched alkyl of 1 to 5 carbon atoms, alkoxy, or R ₁ and R ₂ are connected to each other to form a ring,

R ₃ and R ₄ are independently of each other hydrogen, straight or branched alkyl or alkoxy having 1 to 5 carbon atoms.

The borate compound of Formula 1 is preferably present in an amount of 0.1 to 20% by weight in the organic electrolyte. If the amount is less than 0.1% by weight, there is a quantitative limit to dissociation of a high concentration of polysulfide. If the amount is more than 20% by weight, the decomposition reaction of the borate system itself becomes serious.

Among the borate-based compounds of Formula 1, triethanol amine borate is particularly preferred.

The electrolyte of the present invention can suppress the production of lithium sulfide, which can be oxidized during charging to suppress the reduction of the capacity of the battery in the next discharge cycle. That is, in the present invention, the formation of insoluble lithium sulfide is suppressed by adding an anion acceptor capable of stabilizing S ^2- to the electrolyte.

The borate compound of the present invention is characterized by enhancing the stability of sulfide anion by inhibiting the binding of lithium ions with lithium ions by binding and coordinating with S ^2- or LiS ⁻ in lithium sulfide.

Representative triethanol amine borate of the borate compound of the present invention is described by way of example, as shown in the following scheme, nitrogen atoms with relatively high electrons capture lithium ions and boron atoms with relatively few electrons capture sulfide anions and It inhibits the binding of lithium ions.

The lithium sulfur secondary battery of the present invention comprises a cathode comprising at least one positive electrode active material selected from the group consisting of elemental sulfur, sulfur-based compounds and mixtures thereof;

An anode comprising a negative electrode active material of lithium metal or an alloy of lithium metal;

A separator interposed between the cathode and the anode; And

It comprises an electrolyte comprising a lithium salt, an organic solvent and the borate compound of the formula (1).

It is preferable to use a lithium metal electrode, an alloy of lithium metal, or a composite electrode of lithium / inactive sulfur as the anode active material, and as the cathode active material, single sulfur, solid Li ₂ S _n (n ≧ 1), Li ₂ S _n ( at least one active material selected from the group consisting of catholyte, organic sulfur, and carbon-sulfur composite polymer ((C ₂ S _x ) _n : x = 2.5 to 50, n ≥ 2) in which n ≥ 1) is dissolved Preference is given to using.

Lithium salts used in the electrolyte include LiPF ₆ , LiSO ₃ CF ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiC (CF ₃ SO ₂ ) ₃ , LiN (C ₂ F ₅ SO ₂ ) It is preferable to use a salt selected from the group consisting of ₂ , LiN (CR ₃ SO ₂ ) ₂ and combinations thereof.

As the organic solvent used in the electrolyte solution of the present invention, any organic solvent used in a conventional lithium sulfur battery can be used. For example, oligoether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, dimethyl carbonate, diethyl carbonate, dipropyl carbonate Ester carbonate compounds such as methyl ethyl carbonate and methyl propyl carbonate, alkyl ester compounds such as methyl permate, methyl acetate and methyl propionate, lactone compounds such as aromatic nitrile compounds, amide compounds, and butyl lactones; And at least one selected from the group consisting of sulfur compounds. The organic solvent may be used as a single solvent or as a mixed solvent of two or more.

The electronic conductive material may further include an electron conductive material to allow electrons to smoothly move in the anode plate. The conductive material is not particularly limited, but an electron conductive compound having carbon black, graphite, carbon fiber, conjugate carbon-carbon and / or carbon-nitrogen double bonds such as polyaniline, polythiophene, polyacetylene, Electron conducting polymers such as polypyrrole and mixtures of these electron conducting materials.

The positive electrode active material is attached to a current collector by a binder, and the binder includes polyvinyl acetate, polyvinyl alcohol, polyethylene oxide, polyvinylpyrrolidone, alkylated polyethylene oxide, crosslinked polyethylene oxide, and polyvinyl ether. , Polymethyl methacrylate, polyvinylidene fluoride, copolymers of polyhexafluoropropylene and polyvinylidene fluoride, polyethyl acrylate, polytetrafluoroethylene, polyvinylchloride, polyacrylonitrile, polyvinyl Pyridine, polystyrene and derivatives thereof, mixtures, copolymers and the like can be used.

As the separator, any one commonly used in lithium batteries can be used. That is, a composition using a wound separator such as polyethylene (PE), polypropylene (PP) film, or the like as a separator, coating a gelling polymer on the PE or PP film, or containing a polymerizable monomer for forming a gelling polymer, is contained in a battery. After the injection, polymerization is performed to form a gel polymer layer on a PE or PP film.

A lithium sulfur battery using the organic electrolyte according to the present invention is prepared as follows.

First, a cathode active material composition is prepared by mixing a cathode active material, a conductive agent, a binder, and a solvent. The cathode active material composition is directly coated and dried on an aluminum current collector to prepare a cathode electrode plate. Alternatively, the cathode active material composition may be cast on a separate support, and then a film obtained by peeling from the support may be laminated on an aluminum current collector to manufacture a cathode electrode plate. Here, a mylar film or the like is used as the support.

As a negative electrode, a lithium metal plate, a sodium metal plate, a lithium alloy plate, a sodium alloy plate, etc. are cut | disconnected to predetermined dimension, and are used. A current collector plate made of a conductive metal plate such as a copper plate may also be stacked on the cathode.

The separator is disposed between the cathode electrode plate and the anode electrode plate as described above to form an electrode assembly. The electrode assembly is wound or folded and placed in a cylindrical battery case or a square battery case, and then an organic electrolyte solution according to the present invention is injected to complete a lithium sulfur battery.

The present invention will be described in more detail with reference to the following examples. The following examples are intended to illustrate the invention and are not intended to limit the scope and content of the invention.

Example

Example 1

The positive electrode was coated with a mixture of elemental sulfur (80% by weight), polymer binder (styrenebutadiene rubber, 15% by weight) and carbon black conductor (5% by weight) on an aluminum thin film, and the negative electrode was 150 microns thick. The metallic lithium of was used, and a separator of 25 microns thick PE / PP / PE obtained from Asahi Corporation was used. As an electrolyte solution, an organic electrolyte solution in which 5% by weight of triethanol amine borate was added to 2M LiN (SO ₂ CF ₃ ) ₂ / dimethoxyethane (DME) / dioxolane (DOX) (1: 1 volume ratio) was used. After the battery was assembled using the positive electrode, the negative electrode, and the organic electrolyte, a charge and discharge test was performed, and the results are shown in FIG. 1. In the charge and discharge test, the discharge current density was discharged at 1.2 mA / cm 2, and the charge current density was fixed at 2.4 mA / cm 2, and then the discharge current was changed to 1.2, 2.4, 6, 12, and then performed one cycle. 100 cycles of charge and discharge were performed by fixing the discharge current. The cut-off voltage during charging and discharging was 1.5V to 2.8V.

Comparative Example 1

A battery was manufactured in the same composition and method as in Example 1, except that triethanol amine borate was not added to the electrolyte. Charge and discharge experiments were similarly performed on the obtained battery, and the results are shown in FIG. 1.

1 is a graph showing the discharge capacity according to the cycle when the triethanol amine borate is added to the electrolyte and the non-added electrolyte in the lithium sulfur secondary battery.

As can be seen in FIG. 1, it can be seen that the lithium / sulfur secondary battery employing the organic electrolyte solution to which triethanol amine borate is added has a higher discharge capacity and better life characteristics.

When the borate-based compound of Formula 1 of the present invention is used as an additive in an existing electrolyte for lithium sulfur secondary batteries, not only improves the cycle life of the battery, but also reacts with lithium metal, unlike Li ₂ S dissociation materials reported previously. Since it is low, it is more effective.

Claims (7)

An electrolyte solution for a lithium sulfur battery containing a lithium salt and an organic solvent, the electrolyte solution further comprising a borate compound of the following general formula (1): [Formula 1]

In Chemical Formula 1 R ₁ and R ₂ are each independently hydrogen, a straight or branched alkyl group having 1 to 5 carbon atoms, or an alkoxy group, or R ₁ and R ₂ are connected to each other to form a ring, R ₃ and R ₄ are independently of each other hydrogen, a straight or branched alkyl group having 1 to 5 carbon atoms or an alkoxy group. The electrolyte of claim 1, wherein the compound of Formula 1 is present in the electrolyte in an amount of 0.1 wt% to 20 wt%. The electrolyte of claim 1, wherein the compound of Formula 1 is triethanol amine borate. A cathode comprising at least one positive electrode active material selected from the group consisting of elemental sulfur, sulfur compounds and mixtures thereof; An anode comprising an anode active material of an alloy of lithium or lithium metal; A separator interposed between the cathode and the anode; And A lithium sulfur battery comprising an organic electrolyte solution including a lithium salt, an organic solvent, and a borate compound of Formula 1 below: [Formula 1]

In Chemical Formula 1, R ₁ and R ₂ are each independently hydrogen, a straight or branched alkyl group having 1 to 5 carbon atoms, or an alkoxy group, or R ₁ and R ₂ are connected to each other to form a ring, R ₃ and R ₄ are independently of each other hydrogen, a straight or branched alkyl group having 1 to 5 carbon atoms or an alkoxy group. The lithium-sulfur battery according to claim 4, wherein the compound of Formula 1 is present in an electrolyte in an amount of 0.1% to 20% by weight. The lithium-sulfur battery according to claim 4, wherein the compound of Formula 1 is triethanol amine borate. The method of claim 4, wherein the positive electrode active material, a sulfur group, Li ₂ S _n (n≥1), the cache dissolved in solrayiteu Li ₂ S _n (n≥1), organo-sulfur and carbon-sulfur composite polymer (C ₂ S _x ) _n (x is 2.5 to 50, n≥2) lithium-sulfur battery, characterized in that at least one member selected from the group consisting of.

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