KR20140142038A - Cathode Active Material For Lithium-Sulfur Battery and Lithium-Sulfur Battery Comprising The Same - Google Patents

Abstract

The present invention relates to a cathode active material for a lithium-sulfur battery and a lithium-sulfur battery including the same and, more specifically, to a sulfur-containing compound substituted with metal or transition metal having potential higher than sulfur, and to a lithium-sulfur battery including the same.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cathode active material for a lithium-sulfur battery, and a lithium-sulfur battery including the cathode active material,

The present invention relates to a positive electrode active material for a lithium-sulfur battery and a lithium-sulfur battery including the same. More particularly, the present invention relates to a sulfur-containing compound substituted with a metal or a transition metal having a high reduction potential value Lt; RTI ID = 0.0 > lithium-sulfur < / RTI >

2. Description of the Related Art [0002] A secondary battery exhibiting a high energy density is required due to miniaturization and high integration of portable electronic devices and development of hybrid electric vehicles (HEV) and electric vehicles.

The lithium sulfur battery means a battery system using lithium metal as a positive electrode active material and sulfur as a negative electrode active material. During the discharge, the sulfur in the anode is reduced by accepting electrons, and lithium in the cathode is ionized while being ionized.

Sulfur reduction is a process in which the sulfur-sulfur (S-S) bond accepts two electrons and converts to sulfur anion form. On the other hand, in the oxidation reaction of lithium, lithium metal converts electrons into lithium ions while releasing electrons, and lithium ions are transferred to the anode through an electrolyte to form salts with sulfur anions.

The sulfur before discharge has a cyclic S ₈ structure and is converted to lithium polysulfide by a reduction reaction. When the lithium polysulfide is completely reduced, lithium sulfide (Li ₂ S) is produced.

The electrolyte located between the anode and cathode serves as a migration medium for lithium ions.

On the contrary, during charging, the sulfur of the anode gives off the battery and is oxidized. At the cathode, a reaction occurs in which lithium ions accept electrons and are reduced to lithium metal.

In the charging process, the sulfur gives off electrons, the reaction in which sulfur bonds are formed, and the reaction in which lithium ions receive electrons from the surface of the lithium anode and are reduced to lithium metal.

When the cyclic sulfur is broken by the reduction reaction, polysulfide ions of Sn ^{2 -} are formed, and the value n is the length of the sulfur chain. The value of n may be greater than or equal to 8, and cyclic sulfur and polysulfide may be converted to polysulfide ions having longer chains by the following chemical reaction.

S ₈ + Sn ^{2 -} > S _{n + 8} ^2-

The most significant feature of this lithium sulfur battery system is that the theoretical energy density is much higher than other battery systems. The high energy density is due to the high specific capacity of sulfur and lithium. However, in order to achieve this, a high utilization rate of sulfur and lithium should be secured.

Another feature of the lithium sulfur battery is that the manufacturing cost can be reduced. Specifically, the manufacturing cost of the battery can be reduced by using sulfur as a cathode active material at a low cost and inexpensive. In addition, although the negative electrode manufacturing process of a lithium ion battery includes a slurry production, coating, drying and rolling process, the lithium sulfur battery uses the lithium metal as the negative electrode without pretreatment, thereby simplifying the manufacturing process. can do. The lithium ion battery removes gas generated during the reaction in which lithium is inserted into the carbon material, which is an anode active material through the activation process. However, the lithium sulfur battery is a reaction in which lithium ions are deposited on the surface of the lithium metal, And no activation process is required. The omission of such a process has a great effect on the production cost reduction of the battery.

Since the sulfur used as the cathode active material of the conventional lithium sulfur battery is an insulator, it has been common that 10 to 40% by weight of the conductive material is added to the sulfur anode. Due to the addition of a large amount of conductive material, the conventional lithium sulfur battery has a problem that the energy density per weight is lowered. In addition, despite the addition of a large amount of conductive material, the conventional lithium sulfur battery has a problem that the output density characteristics are not improved.

Moreover, the conventional lithium sulfur battery still contains a problem of safety against lithium metal, and lithium polysulfide produced on the anode at the time of discharging is reduced to the lithium metal surface upon charging to form an unstable film on the lithium metal surface, As a result, the amount of sulfur is continuously reduced, and as a result, the cycle characteristics are degraded.

DISCLOSURE OF THE INVENTION In order to solve the above problems, the present invention aims to provide a sulfur-containing compound having conductivity by minimizing the production of lithium polysulfide by suppressing the reduction reaction of sulfur and a lithium-sulfur battery including the same.

The sulfur-containing compound represented by the following formula (1) is characterized in that it is substituted with a metal or a transition metal having a higher reduction potential value than sulfur.

M _X S (1)

Wherein 0 < X <1; M is a metal or transition metal having a higher reduction potential than sulfur.

The M is a metal or a transition metal having a higher reduction potential value than that of sulfur, and can be reduced prior to the discharge at the time of discharge, so that cracking of the sulfur-sulfur bond can be suppressed. Therefore, the sulfur-containing compound according to the present invention can reduce the amount of sulfur anion produced. This means that, finally, the amount of lithium polysulfide produced can be minimized.

Further, since M is a conductive metal or a transition metal, the function as a metal conductive material can be performed as the content increases. Therefore, the sulfur-containing compound according to the present invention can reduce the content of the conductive material as compared with the conventional sulfur anode, and this has the effect of increasing the energy density per weight of the lithium-sulfur battery including the sulfur- Can be imported.

Specifically, the M may substitute a part of sulfur within the range of the above X value. The electric conductivity and the theoretical capacity of the electrode can be changed according to the X value. However, when the X value is 1 or more, the energy density per weight is reduced, which is not preferable.

More specifically, X may be a metal or transition metal having a reduction potential higher than -0.5 V _SHE , which is the reduction potential of the sulfur, and may be one or more selected from metals or transition metals corresponding to Groups 3 to 14 on the periodic table As a specific example, the metal or transition metal may be at least one selected from the group consisting of Cr, Fe, Cd, Co, Ni, Sn, Pb, (Cu), and the like.

The present invention also provides a positive electrode comprising a sulfur-containing compound as a positive electrode active material; A negative electrode selected from the group consisting of lithium metal and lithium alloy; And a separator interposed between the anode and the cathode; And an electrode assembly.

The lithium alloy may be, for example, a lithium / aluminum alloy or a lithium / tin alloy, but is not limited thereto.

The anode may further include a conductive material and a binder.

The anode may be manufactured by a manufacturing method including the following processes. The method of manufacturing the positive electrode includes:

Dispersing or dissolving the binder in a solvent to prepare a binder solution;

Preparing a positive electrode slurry by mixing the binder solution, the positive electrode active material, and the conductive material;

Coating the positive electrode slurry on a current collector;

Drying the anode; And

And compressing the anode to a constant thickness. In some cases, it may further include a step of drying the rolled positive electrode.

The binder solution preparation process is a process of dispersing or dissolving the binder in a solvent to prepare a binder solution.

The positive electrode active material and the conductive material may be mixed / dispersed in the binder solution to prepare a positive electrode slurry. The cathode slurry thus prepared can be transferred to a storage tank and stored until the coating process.

In the storage tank, the electrode slurry can be agitated continuously to prevent the positive electrode slurry from hardening.

The process of coating the positive electrode slurry on the current collector is a process of coating the positive electrode slurry on the current collector with a predetermined pattern and a constant thickness by passing through a coater head.

The method of coating the positive electrode slurry on the current collector may include a method of uniformly dispersing the positive electrode slurry on a current collector and then using a doctor blade or the like, a die casting method, a comma coating method coating, screen printing, and the like. Alternatively, the positive electrode slurry may be bonded to the current collector by molding on a separate substrate, followed by pressing or lamination.

The current collector is not particularly limited as long as it has high conductivity without causing a chemical change in the battery. For example, the current collector may be made of copper, stainless steel, aluminum, nickel, titanium, sintered carbon, a surface of copper or stainless steel A surface treated with carbon, nickel, titanium, silver or the like, an aluminum-cadmium alloy, or the like can be used. The positive electrode current collector may be formed into various shapes such as a film, a sheet, a foil, a net, a porous body, a foam, a nonwoven fabric, or the like by forming fine irregularities on the surface to enhance the bonding force of the positive electrode active material.

The drying step is a step of removing the solvent and moisture in the slurry to dry the slurry coated on the metal current collector. In a specific embodiment, the drying step is performed within one day in a vacuum oven at 50 to 80 ° C.

The drying process may further include a cooling process, and the cooling process may be a slow cooling process to a room temperature.

In order to increase the capacity density of the coated electrode and increase the adhesion between the current collector and the active materials, an electrode can be passed between two rolls heated at a high temperature and compressed to a desired thickness. This process is called the rolling process.

The anode can be preheated before passing the anode between two hot, heated rolls. The preheating process is a process of preheating the electrode before it is introduced into the roll to increase the compression effect of the electrode.

The rolled electrode can be dried within one day in a vacuum oven at 50-80 占 폚. The rolled electrode may be cut to a certain length and then dried.

The drying process may further include a cooling process, and the cooling process may be a slow cooling process to a room temperature.

The separator is made of an insulating thin film having high ion permeability and mechanical strength. The pore diameter of the separator is generally 0.01 to 10 mu m and the thickness is generally 5 to 300 mu m. Such separation membranes include, for example, olefinic polymers such as polypropylene, which are chemically resistant and hydrophobic; A sheet or nonwoven fabric made of glass fiber, polyethylene or the like; Kraft paper and the like are used. Representative examples currently on the market include the Celgard ^R 2400, 2300 (from Hoechest Celanese Corp.), polypropylene separator (from Ube Industries Ltd. or Pall RAI), and polyethylene series (from Tonen or Entek).

In some cases, a gel polymer electrolyte may be coated on the separation membrane to increase the stability of the cell. Representative examples of such a gel polymer include polyethylene oxide, polyvinylidene fluoride, and polyacrylonitrile.

When a solid electrolyte such as a polymer is used as an electrolyte, the solid electrolyte may also serve as a separation membrane.

The conductive material is not particularly limited as long as it is excellent in conductivity and can provide a large surface area. For example, graphite such as natural graphite or artificial graphite; Carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fiber and metal fiber; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

The binder may be any binder known in the art and specifically includes a fluororesin binder including polyvinylidene fluoride (PVdF) or polytetrafluoroethylene (PTFE), a binder such as styrene-butadiene But are not limited to, cellulose-based binders including carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, poly Or a mixture or copolymer of one or more binders selected from the group consisting of polyolefin binders, polyolefin binders, polyimide binders, polyester-based binder mussel adhesives, and silane-based binders.

The solvent can be selectively used depending on the type of the binder. For example, organic solvents such as isopropyl alcohol, N-methylpyrrolidone (NMP), and acetone and water can be used.

As a specific example of the present invention, PVDF is dispersed and dissolved in NMP (N-methyl pyrrolidone) or SBR (Styrene-Butadiene Rubber) / CMC (Carboxy Methyl Cellulose) .

The present invention also relates to the electrode assembly; And a lithium-sulfur battery having a structure in which the electrode assembly is embedded in a battery case, impregnated with an electrolyte, and then the battery case is sealed.

The electrolyte is a non-aqueous electrolyte containing a lithium salt. As the non-aqueous electrolyte, a non-aqueous electrolyte, a solid electrolyte, an inorganic solid electrolyte, or the like is used.

The lithium salt is a material that is readily soluble in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO 4, LiBF 4, LiB 10 Cl 10, LiPF 6, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF _{6, LiSbF 6, LiAlCl 4,} CH 3 SO 3 Li, CF 3 SO 3 Li, LiSCN, LiC (CF 3 SO 2) 3, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, 4-phenylborate, imide, and the like can be used.

Examples of the nonaqueous electrolyte include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate , Gamma -butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, Diethyl ether, formamide, dimethyl formamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane A non-protonic organic solvent such as an ether, a methyl pyrophosphate, or an ethyl propionate is used as the solvent, a sulfone, a methyl sulfolane, a 1,3-dimethyl-2-imidazolidinone, a propylene carbonate derivative, a tetrahydrofuran derivative, . The organic solvent may be a mixture of one or more organic solvents.

Examples of the organic solid electrolyte include a polymer electrolyte such as a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, an agitation lysine, a polyester sulfide, a polyvinyl alcohol, a polyvinylidene fluoride, Polymers containing ionic dissociation groups, and the like can be used.

Examples of the inorganic solid electrolyte include Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Nitrides, halides and sulfates of Li such as Li ₄ SiO ₄ -LiI-LiOH and Li ₃ PO ₄ -Li ₂ S-SiS ₂ can be used.

For the purpose of improving the charge / discharge characteristics and the flame retardancy, the electrolytic solution is preferably mixed with an organic solvent such as pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, Benzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, . In some cases, halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride may be further added to impart nonflammability. In order to improve the high-temperature storage characteristics, carbon dioxide gas may be further added. FEC (fluoro-ethylene carbonate, PRS (propene sultone), FPC (fluoro-propylene carbonate), and the like.

The battery pack including the lithium-sulfur battery includes an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV) And can be used as a power source of a storage device.

As described above, since the sulfur-containing compound according to the present invention substitutes a part of sulfur with a metal or a transition metal having a higher reduction potential value than sulfur, a large amount of conductive material may not be contained. Therefore, the lithium-sulfur battery including the lithium-sulfur battery can exhibit the effect of improving the energy density per unit weight, the output characteristic, and the cycle characteristic.

Hereinafter, the contents of the present invention will be described in detail with reference to examples and drawings. However, the following examples and drawings are for illustrating the present invention, and the scope of the present invention is not limited thereto.

&Lt; Example 1 >

Nickel powder and sulfur were mixed at 1: 2 atomic%, and a nickel / sulfur composite was obtained through a wet ball milling process. A positive electrode mixture of 80.0 wt% of the composite of nickel and sulfur, 10.0 wt% of Super-P (conductive material) and 10.0 wt% of PVdF (binder) was added to the solvent N-methyl-2-pyrrolidone (NMP) And then coated on an aluminum current collector to prepare a positive electrode having a loading amount of 3.2 mAh / cm ² .

A lithium foil having a thickness of about 150 micrometers was used as a negative electrode and a mixed electrolyte (volume ratio of 5: 4) of dimethoxyethane and dioxolane in which 1 M LiN (CF ₃ SO ₂ ) ₂ was dissolved as an electrolyte was used, Thick polyolefin was used to fabricate a lithium-sulfur battery.

&Lt; Comparative Example 1 &

A lithium-sulfur battery was fabricated in the same manner as in Example 1, except that sulfur was used as a cathode active material instead of the nickel-sulfur composite.

<Experimental Example 1>

The lithium-sulfur battery manufactured in Example 1 was tested for charge-discharge characteristics at 25 degrees Celsius using a charge-discharge measuring apparatus. The obtained battery was subjected to charging and discharging at 0.1 C / 0.1 C charge / discharge and 0.5 C / 0.5 C charge / discharge, and the charge / discharge of 100 cycles was repeated to measure the capacity retention rate (%) at 100 cycles relative to the initial capacity. The results are shown in Table 1 below.

C-rate 0.1 C 0.5 C Initial capacity (mAh / g) Capacity retention after 100 cycles (%) Initial capacity (mAh / g) Capacity retention after 100 cycles (%) Example 1 850 90 800 85 Comparative Example 1 1000 40 900 30

As shown in Table 1, the battery of Example 1 according to the present invention shows a high capacity cyclic characteristic (capacity retention rate) of at least 85% or more as compared with the initial capacity even after 100 cycles.

Claims (17)

A sulfur-containing compound represented by the following general formula (1) and substituted with a metal or a transition metal having a higher reduction potential value than sulfur:


M _X S (1)
In this formula,
0 &lt; X &lt;1;
M is a metal or transition metal having a higher reduction potential value than sulfur. The sulfur-containing compound according to claim 1, wherein the metal or transition metal is at least one selected from metals or transition metals corresponding to Group 3 to Group 14 on the periodic table. The method of claim 2, wherein the metal or transition metal is at least one selected from the group consisting of Cr, Fe, Cd, Co, Ni, Sn, Pb, 0.0 &gt; (Cu). &Lt; / RTI &gt; The sulfur-containing compound according to claim 2, wherein the transition metal is chromium (Cr). The sulfur-containing compound according to claim 1, wherein the redox potential of the sulfur is -0.5 V _SHE . A cathode comprising the sulfur-containing compound according to claim 1 as a cathode active material;
cathode; And
A separator interposed between the anode and the cathode;
Wherein the electrode assembly comprises: The electrode assembly according to claim 6, wherein the negative electrode is made of lithium metal or a lithium alloy. 8. The electrode assembly of claim 7, wherein the lithium alloy is a lithium / aluminum alloy or a lithium / tin alloy. An electrode assembly according to claim 7; Electrolyte; And a battery case. The lithium-sulfur battery according to claim 9, wherein the electrolyte is a non-aqueous electrolyte, an organic solid electrolyte or an inorganic solid electrolyte including a lithium salt and an organic solvent. The lithium secondary battery according to claim 10,
LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF ₆ , LiSbF ₆ , LiAlCl ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li _{, LiSCN, LiC (CF 3 SO} 2) 3, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate, there are lithium, characterized in that at least one selected from the group consisting of draw - Sulfur battery. 11. The lithium-sulfur battery according to claim 10, wherein the organic solvent is an aprotic organic solvent. 13. The method of claim 12, wherein the aprotic organic solvent comprises
N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, gamma-butylolactone, -Dimethoxyethane, 1,2-diethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, 4-methyl- , Diethyl ether, formamide, dimethyl formamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1 , 3-dimethyl-2-imidazolidinone, a propylene carbonate derivative, a tetrahydrofuran derivative, an ether, methyl pyrophosphate, and ethyl propionate. The organic solid electrolyte according to claim 10, wherein the organic solid electrolyte is selected from the group consisting of a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, an agitation lysine, a polyester sulfide, a polyvinyl alcohol , Polyvinylidene fluoride, and an ionic dissociation group. The lithium-sulfur battery according to claim 1, The method of claim 10, wherein the inorganic solid electrolyte is, for _{example, Li 3 N, LiI, Li} 5 NI 2, Li 3 N-LiI-LiOH, LiSiO 4, LiSiO 4 -LiI-LiOH, Li 2 SiS 3 , A nitride, a halide and a sulfate of Li such as Li ₄ SiO ₄ , Li ₄ SiO ₄ -LiI-LiOH, Li ₃ PO ₄ -Li ₂ S-SiS ₂ and the like. - Sulfur battery. The lithium-sulfur battery according to claim 9, wherein the capacity retention ratio (%) at 100 cycles relative to the initial capacity is 85% or more after charge and discharge are repeated 100 times at 25 degrees Celsius with a current of 0.5C. The lithium-sulfur battery according to claim 9, wherein the capacity maintenance ratio (%) at 100 cycles with respect to the initial capacity is 90% or more after charging and discharging are repeated 100 times at a temperature of 25 ° C with a current of 0.1C.