KR100373835B1 - A Lithium-Sulfur Secondary Battery - Google Patents

Abstract

The present invention relates to a lithium sulfur secondary battery, i) a positive electrode using an active material selected from the group consisting of simple sulfur, lithium sulfide, and lithium polysulfide, ii) a negative electrode using lithium metal or an alloy of lithium metal; Iii) The lithium sulfur secondary battery can improve the lifespan of the lithium sulfur secondary battery by providing a lithium sulfur secondary battery having both ion conductivity and electrical conductivity at the same time and including a polymer film coating the positive electrode active material.

Description

Lithium sulfur secondary battery {A Lithium-Sulfur Secondary Battery}

The present invention relates to a lithium sulfur secondary battery, and more particularly, an improved lithium sulfur 2 which can increase the capacity of a lithium sulfur secondary battery by improving conductivity of a positive electrode plate used as a positive electrode for a lithium sulfur secondary battery. Relates to a car battery.

With the rapid development of portable electronic devices, the demand for secondary batteries is increasing. 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. .

Lithium sulfur battery is the most promising in terms of energy density among the batteries developed so far, and the energy density of lithium is 3830 mAh / g, and the sulfur (S ₈ ) energy density is 1675 mAh. The active material used in / g itself is an inexpensive and environmentally friendly material, but there are no examples of commercialization with this battery system.

The reason why the lithium sulfur battery cannot be commercialized is that when 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 added is very low, resulting in extremely low battery capacity.

In addition, the degradation of the battery life caused by sulfur leakage into the electrolyte during the redox reaction and when the appropriate electrolyte solution is not selected, lithium sulfide (Li ₂ S), a reducing substance of sulfur, is precipitated so that it can no longer participate in the electrochemical reaction. There is a problem.

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 as the electrolyte solution to solve the above problems by using a liquid electrolyte containing a solvent containing at least one of crown ether, cryptand, donor number (donor number).

In addition, US Pat. No. 5,961,672 discloses 1M LiSO ₃ CF ₃ , 1,3-dioxolane / diglyme / sulforan / dimethoxyethane (50 / The use of the liquid mixture of the ratio of 20/10/20 as electrolyte solution is shown.

U.S. Patent No. 5,523,179 and U.S. Patent No. 5,814,420 also present technical improvement directions for solving the above problems.

On the other hand, a lithium sulfur secondary battery has a problem in that deterioration of battery life is solved by using lithium metal as a negative electrode.

The reason for the above is that due to the charge and discharge process, the dendrites, which are deposited on the lithium metal surface and grow on the anode surface, reach the anode surface and short-circuit the battery so that the battery can no longer function as a battery. There is a reduction in battery capacity due to corrosion of lithium.

To solve this problem, US Pat. No. 6,017,651, US Pat. No. 6,025,094, US Pat. No. 5,961,672 and the like disclose a technique for forming a protective film on the surface of a lithium electrode.

The prerequisite for the lithium protective film to function properly is that lithium ions should be free to enter and prevent contact between lithium and the electrolyte. However, the methods known to date present some problems.

Most of the lithium protective films allow the lithium protective film to be formed by the reaction between the additive and the lithium in the electrolyte after the battery is assembled, but this method is not dense so that a large amount of electrolyte penetrates into the lithium metal and contacts the lithium metal. There is a problem that can be.

Another method is to form a lithium nitride (Li ₃ N) layer on the surface of lithium by reacting a nitrogen plasma on the surface of lithium, but this method also allows the electrolyte to penetrate through grain boundaries. Nitride is weak in moisture and the layer is easy to decompose, and above all, the potential window (potential window) is low (0.45 V) is difficult to actually use.

Another method is to deposit a thin film by sputtering LiPON (Litium phosphorus oxynitride) on the lithium surface. However, LiPON produced by this method is also porous, which is not suitable for use, and the method itself is expensive and thus disadvantageous in battery processing.

U.S. Patent No. 5,919,587 discloses a lithium sulfur secondary battery in which the anode consists of an electrically active sulfur containing material comprising a -SSS- moiety and an electrically active transition metal chalcogenide surrounding the same. It is starting.

The method used in this patent is a method in which an electrically active sulfur-containing material is added to a solution of an electrically active transition metal chalcogenide compound, followed by a conductive agent to make a positive electrode mixture. Since it effectively wraps and catches it, it catches with a chemical electrostatic attraction, and thus, there is a problem in that the anode cannot be effectively wrapped because the bond is weaker than the physical force.

The present invention has been made to solve the problems described above, an object of the present invention is to provide a lithium sulfur secondary battery with improved life characteristics by improving the positive electrode plate used in the lithium sulfur secondary battery.

The present invention to achieve the above object, the present invention

Iii) a positive electrode using an active material selected from the group consisting of simple sulfur, lithium sulfide, and lithium polysulfide;

Ii) a negative electrode using lithium metal or an alloy of lithium metal; And

Iii) a polymer film having an ion conductivity and an electrical conductivity at the same time, and coating the cathode active material

It provides a lithium sulfur secondary battery comprising a.

Hereinafter, the present invention will be described in detail.

In the present invention, during the electrode reaction of S ₈ , lithium polysulfide, and lithium sulfide used as a positive electrode active material of a lithium sulfur secondary battery, it melts or precipitates in an electrolyte and escapes from the positive electrode plate, thereby preventing it from participating in the electrochemical reaction any more. In order to prevent the dropping of the positive electrode active material by coating an active sulfur used as a positive electrode active material such as S ₈ , lithium sulfide, lithium polysulfide in the production of a positive electrode plate with a suitable polymer.

The polymer is preferably a polymer having ion conductivity and electric conductivity.

Polymers coated on the positive electrode active material include polyvinylidene fluoride (PVDF), copolymers of polyvinylidene fluoride (PVDF) and hexafluorophosphate (HFP), poly (methyl methacrylate), and poly (vinyl). Acetate), poly (vinyl butyral-co-vinylalcohol-co-vinylacetate), poly (methylmethacrylate-co-ethylacrylate), polyacrylonitrile, polyethylene oxide, polyvinylchloride-co-vinylacetate, It is preferable to use a polymer selected from the group consisting of polyvinyl alcohol, poly (1-vinylpyrrolidone-co-vinylacetate), cellulose acetate, and polyvinylpyrrolidone, and ion conductivity in the electrolyte solution used Preference is given to using polymers capable of having.

The polymer coating the cathode active material does not play a role as a coating film if the solubility in the electrolyte is too high or swelled, so the solubility in the electrolyte should be appropriate.

The amount of the polymer is preferably 20% by weight or less, more preferably 10% by weight or less with respect to the total amount of the mixture constituting the positive electrode plate. Increasing the amount of the binder not only decreases the ion conductivity and the electrical conductivity, but also decreases the energy density of the electrode plate. When the amount of the binder is small, the desired effect does not occur.

In addition, a conductive material may be further included to improve conductivity of the positive electrode plate. Carbon or metal powder may be used as the conductive material.

The amount of the conductive material is preferably used 20% by weight or less. If the amount of the conductive material exceeds 20% by weight, it is not preferable because the energy density of the electrode plate is lowered.

The conductive material is prepared by dispersing the conductive material together in the polymer film during the production of the positive electrode plate.

On the other hand, in order to prevent loss of the active material due to the dissolution of the positive electrode active material into the electrolyte during the electrochemical reaction or due to undissolved lithium sulfide, lithium polysulfide, etc., the coating thickness of the polymer film is 20 μm or less on average. Preferably, more preferably 5 μm or less.

Electrolytes that can be used in the present invention include benzene, fluorobenzene, toluene, trifluorotoluene (FT), xylene, cyclohexane, tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF ), Ethanol, isopropyl alcohol (IPA), dimethyl carbonate (DMC), ethylene methylene carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate (MPC), methyl propionate (MP), ethyl propionate (EP), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), dimethyl ester (DME), 1,3-dioxolane (1,3-Dioxolane), diglyme (DGM), tetra Glyme (TGM), ethylene carbonate (EC), propylene carbonate (PC), gamma butyrolactone (GBL), sulfolane, dimethyl sulfone, dialkyl carbonate, dimethyl carbonate (DMC), butyrolactone, N-methylpy Ralidone, Tetramethylurea, Glyme, Ether, Crownether, Dimethoxyethane, Diglyme, Methoxy ethane, hexamethylphosphoamide, pyridine, N, N-diethylacetamide, N, N-diethylformamide, dimethylsulfoxide, tetramethylurea, N, N-dimethylacetamide, N, N- One or more electrolyte solutions selected from the group consisting of dimethylformamide, tributyl phosphate, trimethyl phosphate, tetramethylenediamine, tetramethylpropylenediamine, pentamethyldiethylenetriamine trimethylphosphate, and tetramethylenediamine can be used.

On the other hand, it is preferable to use at least one lithium salt selected from the group consisting of LiPF ₆ , LiSO ₃ CF ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiClO ₄ , LiBF ₄ , and LiAsF ₆ .

Hereinafter, a preferred embodiment of the present invention. However, the following examples are only presented to aid the understanding of the present invention, and the present invention is not limited to the following examples.

Manufacture of batteries

Example

(Anode manufacturing)

Melt poly (methyl methacrylate) using acetonitrile as a solvent to prepare a gel-like binder solution, and disperse it by adding carbon powder (super-P) as a conductive agent to ensure electrical conductivity. Sulfur (S ₈ ) powder pulverized to the degree was added and stirred with a ball mill for at least one day. By spraying the air using an air spray, the solvent was volatilized to obtain instantaneously dried fine particles (<100 μm).

The polymer coated around the sulfur was prepared to obtain ionic conductivity when it is subsequently deposited in the electrolyte due to the fine pores generated by evaporation of acetonitrile, and the carbon powder added together imparts electrical conductivity.

The powder (sulfur + conductor + polymer) thus prepared is placed in isopropylalcohol having no solubility in a polymer film, and super-P is used as a polyvinylpylidone and a conductive agent. After additionally, the mixture was stirred for at least one day using a ball mill and then coated on a carbon-coated aluminum substrate to form a positive electrode plate, followed by drying for about 1 hour in a 60 ° C. hot air drying furnace, to complete the positive electrode plate.

The composition of the finished positive electrode mixture was such that the weight% ratio of sulfur: PMMA; PVP: super-P was 60: 10: 10: 20.

(Cathode production)

As the cathode, a lithium metal foil having a thickness of 50 microns without oxidation was used.

(Battery assembly)

The prepared anode was left in a vacuum oven (60 ℃) for more than one day and then moved to a glove box in which moisture and oxygen are controlled. Cut the positive and negative plates to a certain size, and ask for the positive and negative tabs. Winding is applied with a certain tension between the polyethylene separators and inserted into a pouch. The remaining part was sealed except for the part to be injected.

(Electrolyte injection)

The electrolyte solution was used in a ratio of 50/20/10/20 of 1 M LiSO ₃ CF ₃ , 1,3-dioxolane / diglyme / sulfolane / dimethoxy ethane in a volume ratio, which was injected through the pouch inlet After sealing, the cells were left for one day or more to age and then battery characteristics were evaluated.

Comparative example

Only the positive electrode was different from the embodiment, and the rest of the procedure was carried out in the same manner as in the embodiment to prepare a lithium sulfur battery.

The positive electrode was prepared by dissolving polyvinylpyrrolidone, which is used as a battery binder, using isopropyl alcohol as a solvent to prepare a binder solution, and adding and dispersing carbon powder (Super-P) as a conductive agent to ensure electrical conductivity. Sulfur (S ₈ ) powder, which had a particle size of about 20 microns, was added and stirred with a ball mill for at least one day (S ₈ : binder: conductive agent = 60: 20: 20). It was coated on a carbon coated aluminum foil substrate using a doctor blade with a constant thickness and then dried in a 60 ° C. drying furnace for 1 hour.

The battery prepared by Example 1 and Comparative Example 1 was repeated charging and discharging at a constant current density of 1 mA / ㎠ to compare the life characteristics and the charge and discharge cut-off (cut-off) is 2.75 V and At 1.8 V. The results are shown in Table 1 below.

Table 1

Example Comparative example 1 cycle capacity 2.2 mAh / ㎠ (100%) 2.1 mAh / ㎠ (100%) 10 cycle capacity 93% 92% 100 cycle capacity 85% 72% 200 cycle capacity 75% 55%

Looking at the results of the Examples and Comparative Examples described in Table 1, it is found that as the number of cycles is repeated, the battery using the positive electrode plate of the present invention produced by the Example has a smaller decrease in capacity than the battery of the Comparative Example Can be.

It can be seen that the lithium sulfur secondary battery manufactured using the positive electrode plate of the present invention has improved life characteristics.

Claims (7)

(Correction) i) a positive electrode using an active material selected from the group consisting of simple sulfur, lithium sulfide, and lithium polysulfide; Ii) a negative electrode using lithium metal or an alloy of lithium metal; And Iii) A film having both ion conductivity and electrical conductivity and coating the positive electrode active material is a copolymer of polyvinylidene fluoride (PVdF), polyvinylidene fluoride and hexafluorophosphate (HFP), and poly (methyl methacrylate). ), Poly (vinylacetate), poly (vinyl butyral-co-vinylalcohol-co-vinyl acetate), poly (methylmethacrylate-co-ethylacrylate), polyacrylonitrile, polyethylene oxide, polyvinylchloride And one polymer film selected from the group consisting of copolymers of vinyl acetate, polyvinyl alcohol, poly (1-vinylpyrrolidone-co-vinylacetate), cellulose acetate and polyvinylpyrrolidone Lithium sulfur secondary battery comprising a. (delete) (Correction) The method of claim 1, A lithium sulfur secondary battery further comprising carbon or metal powder added to the polymer film. (Correction) The method of claim 1, A lithium sulfur secondary battery having an amount of polymer constituting the polymer film is 20% by weight or less based on the total amount of the positive electrode mixture. The method of claim 1, A lithium sulfur secondary battery having an amount of the polymer constituting the polymer film is 10% by weight or less based on the total amount of the positive electrode mixture. The method of claim 1, The thickness of the coated polymer film is an average of 20 ㎛ or less lithium sulfur secondary battery. The method of claim 1, The thickness of the coated polymer film is an average of 5 μm or less lithium sulfur secondary battery.