WO2001089023A1

WO2001089023A1 - A lithium secondary battery comprising a super fine fibrous polymer electrolyte and its fabrication method

Info

Publication number: WO2001089023A1
Application number: PCT/KR2000/000501
Authority: WO
Inventors: Kyung Suk Yun; Byung Won Cho; Seong Mu Jo; Wha Seop Lee; Won Il Cho; Kun You Park; Hyung Sun Kim; Un Seok Kim; Seok Ku Ko; Suk Won Chun; Sung Won Choi
Original assignee: Korea Institute Of Science And Technology
Priority date: 2000-05-19
Filing date: 2000-05-19
Publication date: 2001-11-22

Abstract

The present invention provides a lithium secondary battery and its fabrication method. More particularly, the present invention provides a lithium secondary battery comprising super fine fibrous porous polymer electrolyte and its preparation method, wherein the polymer electrolyte is fabricated by the following process: a) dissolving at least one polymer with plasticizers and y organic electrolyte solvents to obtain at least one polymeric electrolyte solution; b) adding the obtained polymeric electrolyte solution to a barrel of an electrospinning machine; and, c) electropinning the polymeric electrolyte solution onto a substrate using a nozzle to form a polymer electrolyte film. The lithium secondary battery of the present invention has advantages of better adhesion with electrodes, good mechanical strength, better performance at low and high temperatures, better compatibility with organic electrolytes of a lithium secondary battery.

Description

A LITHIUM SECONDARY BATTERY COMPRISING A SUPER FINE FIBROUS POLYMER ELECTROLYTE AND ITS FABRICATION METHOD

TECHNICAL FIELD

The present invention relates to a lithium secondary battery comprising a superfine fibrous polymer electrolyte, and to a fabrication method thereof.

BACKGROUND ART

Recently, concomitant with miniaturization and lightweight trends in electronic appliances, research into energy sources having high power and high energy has been performed intensively, A lithium secondary battery has been proposed as one energy source in the aspect that the higher integration of energy is possible because the molecular weight of lithium used in a lithium secondary battery is very low, but its energy density is relatively high. In the earlier developed lithium secondary battery, an anode was fabricated with metallic lithium or lithium alloy. However, a cycle characteristic of such secondary battery using metallic lithium or lithium alloy is. reduced significantly due to dendrites generated on an anode in the course of repeated

charging and discharging of the battery. A lithium ion battery was presented in order to solve the problem of dendrite generation. The lithium ion battery developed by SONY Company in Japan and widely used all over the world comprises an anode active material, a cathode active material, an organic electrolyte solution and a separator film. The separator film functions to prevent an internal short-circuiting of the lithium ion battery caused by contacting of a cathode and an anode, and to permeate ions. Separator films generally used at the present time are polyethylene (hereinafter referred to as "PE") or polypropylene (hereinafter referred to as "PP") separator films. However, the lithium ion battery using the PE or PP separator film has problems such as instability of the battery, intricacy of its fabrication process, restriction on battery shape and limitation of high capacity. There have been attempts to solve the above-mentioned problems, but there is no clear result until now. On the contrary, a lithium polymer battery uses a polymer electrolyte having two functions, as a separator film and as an electrolyte at the same time, and it is now being viewed with keen interest as a battery being able to solve all of the problems. The lithium polymer battery has an advantage in view of productivity because the electrodes and a polymer electrolyte can be laminated in a flat-plate shape and its fabrication process is similar to a fabrication process of a polymer film.

A conventional polymer electrolyte is mainly prepared with polyethylene oxide (hereinafter referred to as "PEO"), but its ionic conductivity is merely 10^"8 S/cm at room temperature, and accordingly it can not be used commonly.

Recently, a gel or hybrid type polymer electrolyte having an ionic conductivity above 10^"3 S/cm at room temperature has been developed.

K. M. Abraham et al. and D. L. Chua et al. disclose a polymer electrolyte of a gel type polyacrylonitrile (hereinafter referred to as "PAN") group in U.S. Patent No. 5,219,679 -and in U.S. Patent No.5,240,790 respectively. The gel type PAN group polymer electrolyte is prepared by injecting a solvent compound (hereinafter referred to as an "organic electrolyte solution") prepared with a lithium salt and organic solvents, such as ethylene carbonate and propylene carbonate, etc. into a polymer matrix. It has the advantages that the contact resistance is small in charging/discharging of a battery and desorption of the active materials rarely takes place because the adhesive force of the polymer electrolyte is good, and accordingly adhesion between a composite electrode and a metal substrate is well developed. However, such a polymer electrolyte has a problem in that its mechanical stability, namely its strength, is low because the electrolyte is a little bit soft. Especially, such deficiency in strength may cause many problems in the fabrication of an electrode and battery.

A. S. Gozdz et al. discloses a polymer electrolyte of -hybrid type polyvinylidenedifluoride (hereinafter referred to as "PVdF") group in U.S. Patent No. 5,460,904. The polymer electrolyte of the hybrid type PVdF group is prepared by preparing a polymer matrix having a porosity not greater than submicron, and then injecting an organic electrolyte solution into the small pores in the polymer matrix. It has the advantages that its compatibility with the organic electrolyte solution is good, the organic electrolyte solution injected into the small pores is not leaked so as to be safe in use and the polymer matrix can be prepared in the atmosphere because the organic electrolyte solution is injected afterwards. However, it has the disadvantages that the fabrication process is intricate because when the polymer electrolyte is prepared, an extraction process of. a plasticizer and an impregnation process of the organic electrolyte solution are required. In addition, it has a critical disadvantage in that a process for forming a thin layer by heating and an extraction process are required in- the fabrication of electrodes and batteries because the mechanical strength of the PVdF group electrolyte is good but its adhesive force is poor.

Recently, a polymer electrolyte of a polymethylmethacrylate (hereinafter referred to as "PMMA") group was presented in Solid State Ionics, 66, 97, 105 (1993) by O. Bohnke and G. Frand, et al. The PMMA polymer electrolyte has the advantages that it has an ionic conductivity of 10^"3 S/cm at room temperature and its adhesive force and compatibility with an organic electrolyte solution are good. However, its mechanical strength is very poor, and accordingly it is unsuitable for the lithium polymer battery.

In addition, a polymer electrolyte of a polyvinylchloride (hereinafter referred to as "PVC") group, which has good mechanical strength and has an ionic conductivity of 10^"3 S/cm at room temperature, was presented in J. Electrochem. Soc, 140, L96 (1993) by M. Alamgir and K. M. Abraham. However, it has problems in that a low-temperature characteristic is poor and a contact resistance is high.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a lithium secondary battery having advantages of both a lithium ion battery and a lithium polymer

battery. It is another object of the present invention to provide a lithium secondary battery having good adhesion with electrodes, mechanical strength, low- and high-temperature characteristics, and compatibility with an organic electrolyte solution for a lithium secondary. battery, etc. The above-mentioned objects and other objects can be achieved by providing a polymer electrolyte constructed in a superfine fibrous form.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 is a microphotograph of the polymer electrolyte of the present invention taken with a transmission electronic microscope.

Figures 2a - 2c are process flow diagrams illustrating fabrication processes of lithium secondary batteries according to the present_. invention. Figure 3 is a graph showing charge and discharge characteristics of the lithium secondary batteries of Examples 1-6 and Comparative Examples 1 and 2.

Figure 4 is a graph showing low- and high-temperature characteristics of the lithium secondary batteries of Example 4 and Comparative Example 2.

Figure 5 is a graph showing high-rate discharge characteristics of the lithium secondary batteries of Example 2 and Comparative Example 2.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a lithium secondary battery comprising a superfine fibrous polymer electrolyte, and to a fabrication method thereof. In particular, the present invention relates to a lithium secondary battery comprising a cathode active material, an anode active material, an organic electrolyte solution dissolving a lithium salt and a superfine fibrous polymer electrolyte. As depicted in Figure 1 , a polymer electrolyte constructed with superfine polymer fibers has a structure in which superfine fibers with a diameter of 1 ~ 3000nm are grouped disorderly and three-dimensionally. Due to the small diameter of the fibers, the ratio of surface area to volume and the void ratio are very high compared to those of a conventional separator film and polymer electrolyte. Accordingly, due to the high void ratio, the amount of electrolyte impregnated is large and the ionic conductivity is increased, and due to the large surface area, the contact area with the electrolyte can be increased and the leakage of electrolyte can be minimized in spite of the high void ratio. Furthermore, if a polymer electrolyte is prepared by electrospinning, it has an advantage in that it can be prepared not in the form of mere pieces of fibers but in the form of a film directly.

The polymers used for preparing the polymer electrolyte are not particularly limited, on condition that they can be formed into superfine fibers by electrospinning. Examples include polyethylene, polypropylene, cellulose, cellulose acetate, cellulose acetate butylate, cellulose acetate propionate, polyvinylpyrrolidone-vinylacetate, poly[bis(2-(2-methoxyethoxyethoxy))- phosphagene], polyethyleneimide, polyethyleneoxide, polyethylenesuccinate, polyethylenesulfide, poly(oxymethylene-oligo-oxyethylene), polypropylene- oxide, polyvinylacetate, polyacrylonitrile, poly(acrylonitrile-co-methylacrylate), polymethylmethacrylate, poly(methylmethacrylate-co-ethylacrylate), polyvinyl- chloride, poly(vinylidenechloride-co-acrylonitrile), polyvinylidenedifluoride, poly(vinylidenefluoride-co-hexafluoropropylene) or mixtures thereof.

Although there is no specific limitation on the thickness of the polymer

electrolyte, it is preferable to have a thickness of 1 μm -100 μm. It is more

preferable to have a thickness of 5 μm - 70 μm and most preferable to have

a thickness of 10 μm - 50 μm. Furthermore, the diameter of the fibrous

polymer electrolyte is preferably adjusted to a range of 1 ~ 3000nm, more preferably to a range of 10nm ~ 1000nm, and most preferably to a range of 50nm ~ 500nm.

Lithium salts uaed in the lithium secondary battery of the present invention are conventional lithium salts, such as LiPF₆, LiCIO₄, LiAsF₆, Lι^'BF₄ or UCF₃SO₃, and it is more preferable to use LiPF₆.

Examples of the organic solvent used in the organic electrolyte solution can include ethyiene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate or mixtures thereof. In order to improve the low-temperature characteristic of the battery, methyl acetate,

methyl propionate, ethyl acetate, ethyl propionate, butylenecarbonate, γ-

butyrolactone, 1,2-dimethoxyethane, 1 ,2-dimethoxyethane, dimethyl-

acetamide, tetrahydrofuran or mixtures thereof can be further added to the organic solvent.

The polymer electrolyte of the present invention can further include a filling agent in order to improve porosity and mechanical strength. Examples of a filling agent include substances such as Ti0₂, BaTi0₃, Li₂O, LiF, LiOH, Li₃N, BaO, Na₂0, MgO, Li₂CO₃, LiAIO₂, SiO₂, AI₂O₃, PTFE or mixtures thereof. Generally, the content of the filling agent is not greater than 20wt% of the polymer electrolyte.

The method for preparing the superfine fibrous polymer electrolyte of the present invention comprises the steps of obtaining a polymeric solution by dissolving a polymer for the polymer electrolyte in an organic electrolyte solution and a plasticizer, and of forming a polymer electrolyte with the obtained polymeric solution.

The step of forming a polymeric solution is achieved by adding the polymer to an appropriate amount of an organic electrolyte solution and a plasticizer, and by raising the temperature of the resulting mixture to obtain a clear polymeric solution for the polymer electolyte.

Examples of the plasticizer which may be used include propylene carbonate, butylene carbonate, 1 ,4-butyrolactone, diethyl carbonate/dimethyl carbonate, 1 ,2-dimethoxyethane, 1,3-dimethyl-2-imidazoldinone, dimethyl- sulfoxide, ethyiene carbonate, ethymethyl carbonate, N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, polyethylenesulforane, tetraethylene glycol dimethyl ether, acetone, alcohol or mixtures thereof. However, solvents which might influence on the characteristics of battery can even be used because the plasticizer is removed while preparing the polymer

electrolyte by electrospinning_.._.

The formation of the polymer electrolyte of the present invention is achieved by electrospinning. In more detail, a polymer electrolyte can be prepared by filling the polymeric solution obtained by the above-described method into the barrel of an electrospinning apparatus, applying a high voltage to the nozzle, and discharging the polymeric solution through the nozzle onto a metal substrate or a Mylar film at a constant rate. The thickness of the porous polymer separator film can be optionally adjusted by varying the discharging rate and time. As mentioned before, its preferable thickness

range is within 1 - 100 μm. If the above-described method is used, a polymer

separator film built up three-dimensionally with fibers having a diameter of 1 ~ 3000nm, not just the polymer fibers forming a separator film, can be prepared directly. In order to simplify the preparation process, a polymer electrolyte can be formed onto electrodes directly. Accordingly, although the above-mentioned method is a preparation method in fibrous form, no additional apparatus is required and an economical efficiency can be achieved by simplifying the preparation process because the final product can be prepared not just as fibers but as a film directly. A polymer electrolyte using two or more polymers can be obtained by the following two methods: 1) After two or more polymers are dissolved in a plasticizer and an organic electrolyte solution, the polymeric solution is filled into the barrel of an electrospinning apparatus arid then discharged using a nozzle to prepare a polymer electrolyte in a state that the polymer fibers are entangled with each other; and 2) After two or more polymers are dissolved separately in a plasticizer and an organic electrolyte solution respectively, the polymeric solutions are filled into the different barrels of an electrospinning apparatus respectively and then discharged respectively using different nozzles, to prepare polymer electrolyte in a state that the polymer fibers are entangled with each other.

The present invention also relates to a fabrication method of a lithium secondary battery, and Figure 2 illustrates the fabrication processes of lithium secondary batteries of the present invention in detail. Figure 2a illustrates a fabrication process of a battery, comprising inserting a polymer electrolyte prepared by electrospinning between a cathode and an anode, making the electrolyte and electrodes into one body by a certain heat lamination process, inserting the resulting plate into a battery casing after laminating or rolling it, injecting an organic electrolyte solution into the battery casing, and then finally sealing the casing. Figure 2b illustrates a fabrication process of a battery, comprising coating a polymer electrolyte onto both sides of a cathode or an anode, adhering an electrode having opposite polarity to the coated electrode onto a polymer electrolyte, making the electrolytes and electrodes into one body by a heat lamination process, inserting the resulting plate into a battery casing after laminating or rolling it, injecting an organic electrolyte solution into the battery casing, and then finally sealing the battery casing. Figure 2c illustrates a fabrication process of a battery, comprising coating a polymer electrolyte onto both sides of one of two electrodes and onto one side of the other electrode, adhering the electrodes closely so as to face the polymer electrolytes to each other, making the electrolytes and electrodes into one body by a certain heat lamination process, inserting the resulting plate into a battery casing after laminating or rolling it, injecting an organic electrolyte solution into the battery casing, and sealing the battery casing.

The anode and cathode used in the lithium secondary battery of the present invention may be prepared by a conventional method used in the secondary batteries of the prior art, such as mixing a certain amount of an active material, a conducting material, a bonding agent and an organic solvent, casting the resulting mixture onto both sides of a copper or aluminum foil plate grid, and then dry-compressing the plate. In more detail, the anode active material comprises one or more materials selected from the group consisting of graphite, cokes, hard carbon, tin oxide, lithiated compounds thereof, metallic lithium and lithium alloys. The cathode active material comprises one or more materials selected from the group consisting of LiCIO₂, LiNi0₂, LiNiCoO₂, LiMn₂O₄, V₂O₅, and V₆O₁₃.

Examples

The present invention will be described in more-detail by way of the following examples, but those examples are given for the purpose to illustrate the present invention, not to limit the scope of it. Example 1

1-1) Preparation of a polymer electrolyte

To a mixture of 100g of 1M LiPF₆ solution in EC-DMC and 10g of propylene carbonate (hereinafter referred to as "PC") as a plasticizer, 20g of polyvinylidenefluoride (Kynar 761) was added. The resulting mixture was stirred at 80 °C for 2 hours to give a clear polymeric solution for a polymer electrolyte. The resulting polymeric solution was filled into the barrel of an electrospinning apparatus and discharged onto a metal plate at a constant rate using a nozzle charged with 9kV, to prepare a polymer electrolyte having a thickness of 50μm.

1-2) Fabrication of a lithium secondary battery The polymer electrolyte prepared in Example 1-1 was inserted between a generally used graphite anode and LiCoO₂ cathode, and the resulting plates were cut so as to be 3 cm x 4 cm in size and laminated. Terminals were welded on to the electrodes and the laminated plate was inserted into a vacuum casing. A 1 M LiPF₆ solution in EC-DMC was injected into the vacuum casing, and then finally the vacuum casing was vacuum- sealed to fabricate a lithium secondary battery. Example 2

2-1) To a mixture of 100g of 1 M LiPF₆ solution in EC-DMC and 10g of PC as a plasticizer, 20g of polyvinylidenefluoride (Kynar 761) was added. The resulting mixture was stirred at 80 °C for 2 hours to prepare a clear polymeric solution for a polymer electrolyte. The resulting polymeric solution was filled into the barrel of an electrospinning apparatus and discharged onto both sides of a graphite anode at a constant rate using a nozzle charged with 9kV, to coat a polymer electrolyte onto both sides of a graphite anode at a thickness of 50 μm.

2-2) A LiCoO₂ cathode was adhered onto the polymer electrolyte obtained in Example 2-1. The resulting plate was cut so as to be 3 cm x 4 cm in size and laminated. Terminals were welded on to the electrodes and the laminated plate was inserted into a vacuum casing. A 1M LiPF₆ solution in EC- DMC was injected into the vacuum casing, and the casing was then finally vacuum-sealed to fabricate a lithium secondary battery. Example 3 3-1) To a mixture of 100g of 1M LiPF₆ solution in EC-DMC and 10g of PC as a plasticizer, 20g of polyvinylidenefluoride (Kynar 761) was added. The resulting mixture was stirred at 80 °C for 2 hours to prepare a clear polymeric solution for a polymer electrolyte. The resulting polymeric solution was filled into the Barrel of an electrospinning apparatus and discharged onto one side of a LiCoO₂ cathode at a constant rate using a nozzle charged with 9kV, to fabricate a LiCoO₂ cathode coated with a polymer electrolyte having a thickness of 50 μm on one side of it.

3-2) The LiCoO₂ cathode obtained in Example 3-1 was adhered onto both sides of the graphite anode obtained in Example 2-1 so as to face the polymer electrolytes to each other. The resulting plate was made into one body by heat lamination at 110°C, followed by cutting so as to be 3 cm x 4 cm in size and then laminated. Terminals were welded on to the electrodes and then the laminated plate was inserted into a vacuum casing. A 1M LiPF₆ solution in EC-DMC was injected into the.casing, and the casing was then finally vacuum-sealed to fabricate a lithium secondary battery. Example 4

4-1) To a mixture of 100g of 1M LiPF₆ solution in EC-DMC-PC and 10g of butylene carbonate (hereinafter referred to as "BC") as a plasticizer, 10g of PAN (prepared by Polyscience Company, molecular weight of about 150,000) was added. The resulting mixture was stirred at 100°C for 2 hours to prepare a clear polymeric solution for a polymer electrolyte. The resulting polymeric solution was filled into the barrel of an electrospinning apparatus and discharged onto both sides of a graphite anode using a nozzle charged with 9kV at a constant rate, to fabricate a graphite anode coated with a polymer electrolyte film of 50 μm thickness.

4-2) The process in Example 4-1 was applied to one side of a LiCo0₂ cathode instead of to both sides of a graphite anode, to fabricate a LiCoO₂ cathode coated with a polymer electrolyte on one side of it. 4-3) The LiCoO₂ cathode obtained in Example 4-2 was adhered onto both sides of the graphite anode obtained in Example 4-1 so as to face the polymer electrolytes to each other. The resulting plate was made into one body by heat lamination at 110°C, followed by cutting so as to be 3 cm x 4 cm in size and then laminated. Terminals were welded on to the electrodes and then the laminated plate was inserted into a vacuum casing. A 1 M LiPF₆ solution in EC-DMC was injected into the casing, and the casing was then finally vacuum-sealed to fabricate a lithium secondary battery. Example 5 5-1) To a mixture of 100g of 1M LiPF₆ solution in EC-DMC and 10g of PC as a plasticizer, 20g of polyvinylidenefluoride (Kynar 761) and 10g of PAN (prepared by Polyscience Company, molecular weight of about 150,000) were added respectively in separate bowls. The resulting mixtures were stirred at ^■ 100°C for 2 hours to prepare two clear polymeric solutions for a polymer electrolyte. The resulting polymeric solutions were respectively filled into the separate barrels of an electrospinning apparatus and discharged onto both sides of a graphite anode using nozzles charged with 9kV respectively at a constant rate, to prepare a graphite anode coated with polymer electrolyte films having a thickness of 50 μm.

5-2) A LiCoO₂ cathode was adhered onto the polymer electrolyte obtained in Example 5-1. The resulting plate was cut so as to be 3 cm x 4 cm in size and then laminated. Terminals were welded on to the electrodes and then the laminated plate was inserted into a vacuum casing. A 1 M LiPF₆ solution in EC-DMC was injected into the casing, and the casing was then finally vacuum-sealed to fabricate a lithium secondary battery. Example 6

6-1) 20g of polyvinylidenefluoride (Kynar 761), 10g of PAN (prepared by Polyscience Company, molecular weight of about 150,000) and 20g of PMMA (prepared by Polyscience Company, molecular weight of about 150,000) were respectively added to a mixture of 100g of 1 M LiPF₆ solution in EC-DMC and 10g of PC as a plasticizer in separate bowls. The resulting mixtures were stirred at 100°C for 2 hours to prepare three clear polymeric solutions for a polymer electrolyte. The resulting polymeric solutions were respectively filled into separate barrels of an electrospinning apparatus and then discharged onto both sides of a graphite anode using nozzles charged with 9kV respectively at a constant rate, to fabricate a graphite anode coated with polymer electrolyte films having a thickness of 50 μm.

6-2) The process of Example 6-1 was applied to one side of a LiCoO₂ cathode, instead of to both sides of a graphite anbcie, to fabricate a LiCo0₂ cathode coated with polymer electrolytes on one side of it. 6-3) The LiCoO₂ cathode obtained in Example 6-2 was adhered onto both sides of the graphite anode obtained in Example 6-1 so as to face the polymer electrolytes to each other. The resulting plate was made into one body by heat lamination at 110°C, followed by cutting so as to be 3 cm x 4 cm in size and then laminated. Terminals were welded on to the electrodes and then the laminated plate was inserted into a vacuum casing. A 1 M LiPF₆ solution in EC-DMC was injected into the casing, and the casing was then finally vacuum-sealed to fabricate a lithium secondary battery. Comparative Examples Comparative example 1 A lithium secondary battery was fabricated by laminating electrodes and electrolytes in order of an anode, a PE separator film, a cathode, a PE separator film and an anode, inserting the resulting laminated plate into a vacuum casing, injecting a 1M LiPF₆ solution in EC-DMC into the casing, and then finally vacuum-sealing the casing. Comparative example 2

According to the conventional preparation method for a gel-polymer electrolyte, 9g of 1M LiPF₆ solution in EC-DMC was added to 3g of PAN. The resulting mixture was mixed for 12 hours and then heated at 130°C for 1 hour to give a clear polymeric solution. When a viscosity of 10,000cps suitable for casting was obtained, the polymeric solution was cast by die-casting to give a polymer electrolyte film. A lithium secondary battery was fabricated by laminating, in order, a graphite anode, an electrolyte, a LiCoO₂ cathode, an electrolyte and a graphite anode, welding terminals on to the electrodes, inserting the resulting laminated plate into a vacuum casing, injecting a 1 M LiPFg solution in EC-DMC into the casing, and then finally vacuum-sealing the casing.

Example 7-

Charge/discharge characteristics of the lithium secondary batteries obtained in Examples 1 - 6 and Comparative Examples 1 and 2 were tested, and Figure 3 shows the results. The tests for obtaining the charge/discharge characteristics were performed by a charge/discharge method of, after charging the batteries with a C/2 constant current and 4.2V constant voltage, discharging with a C/2 constant current. And the electrode capacities and cycle life based on the cathode were tested. Figure 3 shows that the electrode 5 capacities and cycle life of the lithium secondary batteries of Examples 1 - 6 were improved compared to the lithium secondary batteries of Comparative Examples 1 and 2. It is considered that such improvement in the battery characteristics came from a decrease in interface resistance and an increase in ionic conductivity, due to good adherence of the electrodes and separator

10 film. ^'

Example 8

Low- and high-temperature characteristics of the lithium secondary batteries of Example 4 and Comparative Example 2 were tested, and Figures 4a and 4b illustrate the results (wherein Figure 4a is for Example 4 and Figure

15. 4b is for Comparative Example 2). The tests for obtaining the low- and high- temperature characteristics of the lithium secondary batteries were performed by a charge/discharge method of charging the lithium batteries with a C/2 constant current and 4.2 V constant voltage, and then discharging with a C/5 constant current. Figures 4a and 4b show that the low- and high-temperature 0 characteristics of the lithium secondary battery of Example 4 are better than those of the battery of Comparative Example 2. In particular, Figure 4a shows that the battery of Example 4 has an outstanding characteristic of 91 % even at -10°C.

Example 9 5 High rate discharge characteristics of the lithium secondary batteries of Example 2 and Comparative Example 2 were tested, and Figures 5a and 5b illustrate the results (wherein Figure 5a is for Example 2 and Figure 5b is for Comparative Example 2). the tests for obtaining the high rate discharge characteristics of the lithium secondary batteries were performed by a charge/discharge method of charging the lithium batteries with a C/2 constant current and 4.2 V constant voltage, and then discharging while varying the constant current to C/5, C/2.1C and 2C. As depicted in Figures 5a and 5b, the lithium secondary battery of Example 2 exhibited capacities such as 99% at C/2 discharge, 96% at 1C discharge and 90% at 2C discharge, respectively, based on the value of C/5 discharge. However, the lithium secondary battery of Comparative Example 2 exhibited low capacities such as 87% at 1C discharge and 56% at 2C discharge, respectively, based on the value of C/5 discharge. Accordingly, it was discovered that the high rate discharge characteristic of the lithium secondary battery of Example 2 was better than that of the lithium secondary battery of Comparative Example 2.

Claims

1. A lithium secondary battery, comprising a cathode active material, an anode active material, a polymer electrolyte and an organic electrolyte solution dissolving a lithium salt, wherein the polymer electrolyte is constructed in the form of superfine fibers having a diameter of 1 ~ 3000nm.

2. The lithium secondary battery according to claim 1 , wherein the polymer electrolyte is prepared by an electrospinning.

3. The lithium secondary battery according to claim 1 , wherein the polymer electrolyte is prepared by the following process: dissolving a polymer or polymer mixture for forming a polymer electrolyte in a mixture of a plasticizer arid an organic electrolyte solution in order to obtain a polymeric solution; filling the resulting polymeric solution into the barrel of an electrospinning apparatus; and discharging the polymeric solution using a nozzle.

4. The lithium secondary battery according to claim 1 , wherein the polymer electrolyte is prepared by the following process: respectively dissolving two or more polymers for forming a polymer electrolyte in a mixture of a plasticizer and an organic electrolyte solution in separate bowls, to obtain two or more respective polymeric solutions; filling the resulting polymeric solutions into the different barrels of an electrospinning apparatus; and

discharging the respective polymeric solutions using different nozzles.

5. The lithium secondary battery according to claim 3 or 4, wherein the plasticizer dissolving the polymer is selected from the group consisting of propylene carbonate, butylene carbonate, 1 ,4-butyrolactone, diethyl carbonate, dimethyl carbonate, 1 ,2-dimethoxyethane, 1 ,3-dimethyl-2- imidazolidinone, dimethylsulfoxide, ethyiene carbonate, ethylmethyl carbonate, N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2- pyrrolidone, polyethylenesulforane, tetraethylene glycol dimethyl ether, acetone, alcohol and mixtures thereof.

6. The lithium secondary battery according to claim 1 , wherein the

polymer electrolyte has a thickness ^"of 1 μm ~ 100 μm.

7. The lithium secondary battery according to claim 1 , wherein the polymer_^.for forming the polymer electrolyte is selected from the group

consisting of polyethylene, polypropylene, cellulose, cellulose acetate, cellulose acetate butylate, cellulose acetate propionate, polyvinylpyrrolidone- vinylacetate, poly[bis(2-(2-methoxyethoxyethoxy))phosphagene], poly- ethyleneimide, polyethyleneoxide, polyethylenesuccinate, polyethylenesulfide, poly(oxymethylene-oligo-oxyethylene), polypropyleneoxide, polyvinyl acetate, polyacrylonitrile, poly(acrylonitrile-co-methylacrylate), polymethylmethacrylate, poly(methylmethacrylate-co-ethylacrylate), polyvinylchloride, poly(vinylidene- -chloride-co-acrylonitrile), polyvinylidenedifluoride, poly(vinylidenefluoride-co- hexafluoropropylene) and mixtures thereof.

8. The lithium secondary battery according to claim 1 , wherein the lithium salt incorporated in the polymer electrolyte is LiPF₆, LiCI0₄, LiAsF₆, LiBF₄ or

LiCF₃SO₃.

9. The lithium secondary battery according to claim 1 , wherein an organic solvent used in the organic electrolyte solution is ethyiene carbonate,. propylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate or mixtures thereof.

10. The lithium secondary battery according to claim 9, wherein the organic solvent further comprises methyl acetate, methyl propionate, ethyl acetate,

ethyl propionate, butylenecarbonate, γ-butyrolactone, 1 ,2-dimethoxyethane,

1 ,2-dimethoxyethane, dimethylacetamide, tetrahydrofuran or mixtures thereof, in order to improve a low-temperature characteristic.

11. The lithium secondary battery according to claim 1 ; wherein the " pofymer electrolyte further comprises a filling agent.

12. The lithium secondary battery according to claim 11 , wherein the filling agent is selected from the group consisting of Ti0₂, BaTiO₃, Li₂O, LiF, LiOH,

Li₃N, BaO, Na₂O, MgO, Li₂CO₃, LiAIO₂, SiO₂, AI₂O₃, PTFE and mixtures thereof, and its content is not greater than 20wt% (excluding 0%) of the total polymer electrolyte.

13. A method for fabricating a lithium secondary battery, comprising: inserting a polymer electrolyte prepared by an electrospinning between a cathode and an anode;. inserting the resulting plates into a battery casing after laminating or rolling them; injecting an organic electrolyte solution into the battery casing; and sealing the battery casing.

14. A method for fabricating a lithium secondary battery, comprising: inserting a polymer electrolyte prepared by an electrospinning between a cathode and an anode; making the polymer electrolyte and electrodes into one body by a heat lamination process; inserting the resulting plate into a battery casing after laminating or rolling it; injecting an organic electrolyte solution into the battery casing; and ^■ sealing the battery casing.

15. A method for fabricating a lithium secondary battery, comprising: coating a polymerelectrolyte prepared by an electrospinning onto both sides of a cathode or an anode; adhering an electrode having opposite polarity to the ooated electrode closely onto the polymer electrolyte; inserting the resulting plate into a battery casing after laminating or rolling it; injecting an organic electrolyte solution into the battery casing; and sealing the battery casing.

16. A method for fabricating a lithium secondary battery, comprising: coating a polymer electrolyte prepared by an electrospinning onto both sides of a cathode or an anode; adhering an electrode having opposite polarity to the coated electrode closely onto the polymer electrolyte; making the electrolyte and electrodes into one body by a heat lamination process; inserting the resulting plate into a battery casing after laminating or rolling it; injecting an organic electrolyte solution into the battery casing; and sealing the battery casing.

17. A method for fabricating a lithium secondary battery, comprising: coating a polymer electrolyte prepared by an electrospinning onto both sides of one of two electrodes and onto one side of the other electrode; adhering the electrodes closely so as to face the polymer electrolytes to each other; inserting the resulting plate into a battery casing after laminating or rolling it; .. injecting an organic electrolyte solution into the battery casing; and sealing the battery casing.

18. A rnethod for fabricating a lithium secondary battery, comprising: coating a polymer electrolyte prepared by an electrospinning onto both sides of one of two electrodes and onto one side of the other electrode ; adhering the resulting plates closely so as to face the polymer electrolytes to each other ; ^" making the electrolyte and electrodes into one body by a heat lamination process; inserting the resulting plate into a battery casing after laminating or rolling it; injecting an organic electrolyte solution into the battery casing; and sealing the battery casing.