JP2003242964A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium and sulfur non-aqueous electrolyte secondary battery in which self-discharge due to reduction on the negative electrode surface of a polysulfide such as Li<SB>x</SB>S<SB>n</SB>generated on the positive electrode by the battery reaction is suppressed and which is superior in charge and discharge cycle characteristics. <P>SOLUTION: In the non-aqueous electrolyte secondary battery that comprises a positive electrode, a negative electrode that contains lithium, lithium alloy or a material capable of storing and releasing lithium, a non-aqueous electrolyte, and a separator, the positive electrode contains sulfur coated with polymer electrolyte. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a non-aqueous electrolyte secondary battery.

[0002]

2. Description of the Related Art The use of sulfur as a positive electrode active material for non-aqueous electrolyte secondary batteries has been studied for quite some time.
For example, research on a lithium / sulfur secondary battery that uses sulfur for the positive electrode and lithium for the negative electrode and operates at room temperature was carried out 20 years ago by R.S. D. Reported by Rauh et al. (21st
IECEC, P283 (1977), J. Elect
rochem. Soc. , 126 , 523 (197)
9)). In these reports, soluble lithium polysulfide (Li ₂ S _12.2 ) was used as the positive electrode active material, the magnitude of the current with respect to charge / discharge characteristics, the type of solvent used in the non-aqueous electrolyte, the temperature, the active material concentration. However, since the problems of short charge / discharge cycle life and low utilization rate of sulfur could not be solved, a battery of a practical level could not be obtained.

Yamin et al. Have also reported the results of a detailed study of lithium / sulfur secondary batteries (J. Electrochem. Soc., 135 , 1).
045 (1988), J. Power Source
s, 9 , 281 (1983)).

On the other hand, recently, lithium ion secondary batteries have been commercialized, and at the same time, research and development of organic solvents and electrolytes have been actively conducted. As a result, organic electrolytes (organic electrolyte using organic solvent, polymer electrolyte,
The possibility of a lithium / sulfur secondary battery operating at room temperature using a gel electrolyte etc. has emerged.

Further, Cho et al. Attach a protective coating composed of a glass solid electrolyte on the surface of a lithium negative electrode,
By optimizing the electrolytic solution, it is possible to improve the utilization rate of sulfur, which is the positive electrode active material, and to prevent the shuttle current from being generated due to repeated chemical redox of sulfide between the positive and negative electrodes. (USP 5,523,179, US
P5,814,420, USP 6,025,094).

When a glass solid electrolyte is used, high rate charge / discharge characteristics are deteriorated, and when a wound-type power generation element is produced by winding a sheet electrode with a separator interposed therebetween, a force is exerted on the sheet electrode plate. Therefore, there is a risk that the glass solid electrolyte may crack and be destroyed, which limits the application. on the other hand,
When a polymer electrolyte obtained by mixing, for example, polyethylene oxide (PEO) and a lithium salt is used in place of the glass solid electrolyte and is used as a binder for a separator or an electrode, the strength as a separator is inferior, and an active material or a collector is used. There was a drawback that the binding force to the electric body was weak.

[0007]

The theoretical capacity of sulfur is 16
75 mAh / g, low toxicity and resource rich
For any reason, it can be used as a positive electrode active material for non-aqueous electrolyte secondary batteries.
It is a very promising substance. However, sulfur is a positive electrode active material.
When it is used for quality, it has several problems such as
There is. 1) For example, S as sulfur₈When using S₈Is an insulator
Therefore, a large amount of conductive agent is required when using it as an electrode.
Become. Therefore, the energy density of the battery should be small.
It 2) If we pay attention only to the positive electrode using sulfur as the active material,
Due to normal battery reaction, polysulfide (Li
_xS_n) Is generated. Also, the positive electrode becomes over-discharged
And low sulfide (Li_TwoS) will be generated
It These polysulfides (Li _xS_n) And low sulfide (Li
_TwoS) is deposited on the positive electrode to form an inactive insulating film
As a result, the high rate charge / discharge characteristics and discharge capacity of the positive electrode decrease.

Therefore, the high rate charge / discharge characteristics and discharge of the positive electrode
In order to prevent the capacity from decreasing, it is necessary to control the depth of discharge.
Therefore, low sulfide (Li_TwoS) is suppressed
It is necessary to In addition, low sulfide (Li
_TwoWhen S) is generated, diffusion and reaction in the electrolytic solution
Because the response speed is slow, the high rate charge / discharge characteristics of the positive electrode deteriorate.
This is due to the formation of low sulfide (Li_TwoS) is not positive
It reacts with the sulfur in the discharge, and it also contains polysulfur radicals in the electrolyte.
(・ S_n ^2-, Where n> 4) and polysulfuric anion
(S_n ^2-) And soluble polysulfide (Li_x
S_n), The low sulfide (Li_TwoS) is on the positive electrode
High rate of positive electrode by suppressing excessive deposition
The charge / discharge characteristics can be maintained. 3) When a battery system is used, polysulfur is used in the positive electrode due to the battery reaction.
Compound (Li_xS_n) Is generated, this Li_xS_nIs non
Since it is soluble in the water electrolyte, Li_xS_nIs in non-aqueous electrolyte
Dissolved in the non-aqueous electrolyte solution, and a part of
Cal (・ S_n ^2-, Where n> 4) and polysulfuric anion (S_n
^2-). These polysulfides (Li_xS_n), Polysulfur
Radical (・ S_n ^2-Where n> 4), polysulfuric anion
(S_n ^2-) Moves to the negative electrode side and is reduced on the surface of the negative electrode.
Inert low sulfide (Li_TwoS) is generated and is recharged at the negative electrode.
It consumes thium and causes self-discharge. Also positive,
Chemical redox of sulfide is repeated between the negative electrodes,
Generates shuttle current. As a result, charge and discharge cycle life
The life was insufficient at 200 to 300 cycles.

For these reasons, in an actual battery system, in order to improve the high rate charge / discharge characteristics of the positive electrode, the amount of low sulfide (Li ₂ S) on the positive electrode is reduced as much as possible, that is, in the electrolytic solution. It will be removed from the surface of the positive electrode by diffusing into. However, polysulfide (Li
_{When x} S _n ) reaches the negative electrode and reacts at the negative electrode, the charge / discharge cycle characteristics and the self-discharge characteristics of the battery deteriorate. Thus, polysulfide (Li _x S _n ) and low sulfide (Li _x S _n ) on the positive electrode are
Regarding ₂ S), the direction of improving the high rate discharge characteristics of the battery and the direction of improving the charge / discharge cycle characteristics and the self-discharge characteristics conflict with each other.

[0010] It is an object of the present invention, even in the somewhat sacrificed rate discharge characteristics of the battery, self-discharge due to the polysulfides such as Li _x S _n generated in the positive electrode by the battery reaction are reduced on the negative electrode surface It is intended to provide a lithium / sulfur-based non-aqueous electrolyte secondary battery that suppresses the above and has excellent charge / discharge cycle characteristics.

[0011]

The invention according to claim 1 is a non-aqueous electrolyte comprising a positive electrode, a negative electrode containing lithium, a lithium alloy or a material capable of inserting and extracting lithium, a non-aqueous electrolytic solution, and a separator. In a secondary battery, the positive electrode contains sulfur coated with a polymer electrolyte.

According to the invention of claim 1, self-discharge due to reduction of the polysulfide such as Li _x S _n generated on the positive electrode on the surface of the negative electrode is suppressed, the energy density is high, and the self-discharge is small. A non-aqueous electrolyte secondary battery having excellent charge / discharge cycle characteristics can be obtained.

According to a second aspect of the present invention, in the above non-aqueous electrolyte secondary battery, a polymer electrolyte layer is provided on at least one of the positive electrode and the separator or the negative electrode and the separator.

According to the second aspect of the present invention, the polymer electrolyte layer prevents polysulfides such as Li _x S _{n from} migrating to the surface of the negative electrode, and thus the non-aqueous electrolyte secondary having excellent charge-discharge cycle characteristics. You can get a battery.

According to a third aspect of the present invention, in the above non-aqueous electrolyte secondary battery, lithium, a lithium alloy or a material capable of inserting and extracting lithium contained in the negative electrode is coated with a film containing fluorine. To do.

According to the third aspect of the present invention, the surface of lithium, a lithium alloy or a material capable of occluding and releasing lithium is coated with a film containing fluorine, so that lithium, a lithium alloy or lithium can be occluded and released. It is possible to obtain a non-aqueous electrolyte secondary battery in which the reaction between the material and polysulfide is suppressed, the self-discharge is small, and the charge / discharge cycle characteristics are excellent.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention provides a non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode containing lithium, a lithium alloy or a material capable of inserting and extracting lithium, a non-aqueous electrolyte solution, and a separator. The positive electrode contains sulfur coated with a polymer electrolyte.

The battery reaction in the non-aqueous electrolyte secondary battery of the present invention can be described by the following equation (1). In the formula (1), the discharge reaction is in the right direction. 2Li + n / 8 · S ₈ = Li ₂ S _n (1) In formula (1), when n = 1, the maximum theoretical capacity is 167.
5 mAh / g, when n = 2, theoretical capacity is 838 m
It becomes Ah / g. In the present invention, in view of the low activity of Li 2 _S, it is preferable that was discharged to a Li ₂ S ₂ in the actual battery.

In the battery reaction of the present invention, SOC (State of Charge, that is, charge level) or DOD (Depth of) of sulfur of the positive electrode active material.
Various polysulfides are produced depending on the discharge, that is, the depth of discharge. When the charge / discharge cycle is repeated, the reaction represented by the following formula (2) becomes main. Li ₂ S _n + (n-2) Li ⁺ + (n-2) e ⁻ = Li ₂ S ₂ (2) In this formula (2), when n = 8, the maximum available value. The capacity is 595 mAh / g.

In the non-aqueous electrolyte secondary battery of the present invention, deterioration of charge / discharge cycle characteristics and self-discharge are caused by the formation of soluble polysulfide (Li _x S _n ) in the positive electrode due to the battery reaction.
This moves to the negative electrode together with the polysulfuric acid radical (.S _n ²⁻ , where n> 4) and polysulfuric acid anion (S _n ²⁻ ) generated by partial dissociation in the electrolytic solution, and reacts with lithium at the negative electrode. Reduced and inert low sulfide (Li ₂ S)
Is generated. In order to solve this problem, it is effective to suppress the diffusion of polysulfide to the surface of the negative electrode.

In order to suppress the diffusion of polysulfide to the surface of the negative electrode, lithium ions can easily pass through the positive electrode active material and the surface of the positive electrode, but the polysulfide, polysulfur radical and polysulfuric anion hardly pass through. It is possible to cover with.

Therefore, according to the present invention, lithium ions are easily
Can pass, but polysulfides, polysulfur radicals and polysulfuric anions
The polymer electrolyte is used as a material that
With a positive electrode containing sulfur coated with a mer electrolyte.
is there. Coating sulfur as the positive electrode active material with polymer electrolyte
Then, polysulfide (Li
_xS _n) Stays in the vicinity of the positive electrode, diffuses into the electrolyte, and becomes negative.
It is possible to prevent the chemical reaction on the surface of the negative electrode by suppressing its reaching to the surface of the electrode.
The response is suppressed.

As a result, the polysulfide (Li
_x S _n ) can be suppressed from being self-discharged by being reduced on the surface of the negative electrode. In addition, since lithium does not react with polysulfide in the negative electrode and lithium is not consumed, the non-aqueous electrolyte secondary battery has a small capacity decrease even after repeated charge / discharge cycles, a high energy density, and excellent charge / discharge cycle characteristics. You can get a battery.

As the polymer electrolyte, a polymer electrolyte obtained by mixing a polymer and a lithium salt can be used. The polymer has the chemical formula (CH ₂ CHR ₁ X) _n
(Wherein R ₁ is a methyl group or an ethyl group, and X is an S, O, or N element) and has a molecular weight of 100,0.
It is preferable to use those of 00 or more and 4,000,000 or less. Specific examples of the polymer include polyethylene oxide (PEO) and polypropylene oxide (PP
O) may be used alone or as a mixed system or a cross-linked product, or a copolymer or derivative. Examples of the lithium salt mixed with the polymer include LiBF ₄ and LiAs.
_{F 6, LiN (CF 3 SO} 2) 2, LiN (SO 2 C 2
F ₅ ) ₂ (LiBETI) and the like can be used alone or in combination of two or more. These supporting electrolytes have a high ionic conductivity and are also advantageous for improving the coulombic efficiency of lithium dissolution and precipitation.

If the polymer electrolyte has pores, the possibility of polysulfides diffusing through the organic electrolyte in the pores increases. Therefore, the polymer electrolyte used in the present invention is
It may be swollen with an organic electrolytic solution, but it is preferable to use one having no pores.

The "sulfur coated with a polymer electrolyte" of the present invention means that the surface of the sulfur particles is completely covered with the polymer electrolyte or that part of the surface of the sulfur particles is covered with the polymer electrolyte. Good. However, when the sulfur particles are electrically isolated in the positive electrode plate, there is no electrical contact, and the sulfur particles do not participate in the electrochemical reaction.Therefore, the sulfur particles must always be in contact with a conductive agent such as acetylene black. is there.

As an example of a method for producing sulfur in which individual particles are contacted with a conductive agent and which is coated with a polymer electrolyte, sulfur and acetylene black are dispersed in a solution prepared by dissolving a polymer and a lithium salt in an organic solvent. There is a method of stirring, stirring and drying.

The present invention is also characterized in that, in the above non-aqueous electrolyte secondary battery, a polymer electrolyte layer is provided on at least one of the positive electrode and the separator or the negative electrode and the separator. This makes it possible to almost completely prevent polysulfide (Li _x S _n ) generated by the battery reaction from diffusing into the electrolytic solution and reaching the surface of the negative electrode. A water electrolyte secondary battery can be obtained.

The thickness of the polymer electrolyte layer provided between at least one of the positive electrode and the separator or between the negative electrode and the separator is preferably 1 μm or more and 20 μm or less. When the thickness of the polymer electrolyte layer is less than 1 μm, the strength is insufficient and the polysulfide (Li
_The filter effect on _x S _n ) is insufficient, while a thickness greater than 20 μm leads to an increase in the internal resistance of the cell and a decrease in the energy density.

Further, the present invention is characterized in that, in the above non-aqueous electrolyte secondary battery, the surface of lithium, a lithium alloy or a material capable of inserting and extracting lithium is coated with a film containing fluorine. As a result, a reaction between lithium, a lithium alloy or a material capable of occluding and releasing lithium and polysulfide is suppressed, a self-discharge is small, and a non-aqueous electrolyte secondary battery having excellent charge-discharge cycle characteristics can be obtained. .

As an example of a method for coating the surface of a negative electrode active material such as lithium, a lithium alloy or a material capable of inserting and extracting lithium with a film containing fluorine, these negative electrode active materials and hydrogen fluoride (HF) are used. By reacting with, a lithium fluoride (LiF) film is formed on the surface of the negative electrode active material, direct reaction between the negative electrode active material and polysulfide is prevented, and self-discharge can be suppressed. As another example, by immersing these negative electrode active materials in an organic electrolytic solution containing hydrogen fluoride, lithium fluoride (Li
It is possible to form a dense coating containing F) as a main component. The organic electrolyte used is LiP in a mixture of organic solvents such as ethylene carbonate, dimethyl carbonate, diethyl carbonate and methyl ethyl carbonate.
A solution in which F ₆ is dissolved can be used.

Since sulfur itself is electrically insulating,
A large amount of conductive agent is required. Then, it is preferable that the sulfur is uniformly dispersed in the conductive agent. In order to maintain the conductive network, it is desirable that the conductive agent be firmly bound on the current collector. The conductive agent for the positive electrode has a particle size of 200 n
m or more and 20 μm or less carbon particles or Ti, Al, A
Fine metal particles such as g, carbon fibers and mixtures thereof can be used. Further, the active material and the conductive agent are P
It is preferable that the fluorocarbon resin such as VdF, PTFE, PVdF-HFP is firmly bonded to the positive electrode current collector such as aluminum foil, nickel foil, and stainless steel foil.

In the non-aqueous electrolyte secondary battery of the present invention, it is desirable to use an ether solvent as the solvent for the non-aqueous electrolyte. The ether-based organic solvent solvates lithium ions and separates them from the polysulfuric acid anion, and on the other hand, it is possible to obtain an electrolyte having high conductivity. In particular, these solvents react with metallic lithium to form an oligomer SEI or polymer SEI layer (SEI = Solid Electrol) on the surface thereof.
yte Interface) is formed, and there is an advantage that it does not excessively react with metallic lithium.

Here, usable solvents include tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), dimethoxyethane (DME),
Examples include dioxolane and tetrahydropyran.
Since these solvents have both a donor property and an acceptor property, they promote the solvation of lithium ions.

Further, in the present invention, when an ether solvent is used as the solvent of the non-aqueous electrolyte, sulfur over-discharge is prevented and inactive Li ₂ S is electrochemically prevented. The discharge end voltage is 1.8 V or higher, 2.0
It is preferably V or less. As a result, the battery life can be extended.

In the present invention, as the separator, those used in conventional non-aqueous electrolyte secondary batteries, such as a microporous membrane made of polyethylene and polypropylene, or a microporous polyolefin membrane such as a microporous membrane in which these are combined, are used. Can be used.

As a method for producing the positive electrode plate used in the non-aqueous electrolyte secondary battery of the present invention, various methods as described below can be used. Here, an example in which sulfur is used as the starting material of the positive electrode active material will be described.

As the first method for producing the positive electrode plate, sulfur and the conductive agent are uniformly mixed at a predetermined ratio (mass ratio 5: 1 to 1: 1), and the binder is bound on the current collecting substrate. And pressed until a predetermined porosity (30 to 50%) was reached. Here, it is desirable that the ratio of the conductive agent to the binder is in the range of 2: 1 to 1: 1. Then, it is heated and vacuum dried to be impregnated with the polymer electrolyte and further dried to obtain a positive electrode plate.

As the second method for producing the positive electrode plate, only the conductive agent is bound on the current collecting substrate with the binder, and the slurry of sulfur, the conductive material and the polymer is cast on the above base material and dried. The positive electrode plate was then pressed.

As the third method for producing the positive electrode plate, the electrode plate obtained by the above-mentioned first and second production methods and soluble Li ₂ S are used.
_{A combination of n} (n> 8) is used as a positive electrode plate.

The positive electrode plate obtained by the first manufacturing method has a large range in which the sulfur loading density can be adjusted, and it is easy to obtain a high capacity density positive electrode. The positive electrode plate obtained by the second manufacturing method can enhance the current collecting property of the active material. The positive electrode plate obtained by the third manufacturing method is advantageous for further improvement in energy density and charge / discharge cycle life.

As the negative electrode active material of the non-aqueous electrolyte secondary battery of the present invention, lithium, a lithium alloy, a material capable of inserting and extracting lithium, or the like is used. Materials capable of inserting and extracting lithium include graphite, petroleum coke, cresol resin-fired carbon, furan resin-fired carbon, polyacrylonitrile fiber-fired carbon, mesophase pitch-fired carbon and other materials, tin oxide and other oxides, or mixtures thereof. May be used.

When metallic lithium is used as the negative electrode active material, a lithium foil having a negative electrode capacity equal to or more than twice the positive electrode capacity may be used. Further, a polymer layer or an inorganic solid electrolyte layer may be formed on the surface of metallic lithium in order to protect lithium from polysulfuric acid anions.

[0044]

The preferred embodiments of the present invention will be described below.

First, a polymer electrolyte casting solution was prepared. 2.856 g of LiN in 250 ml of acetonitrile
(SO ₂ C ₂ F ₅ ) ₂ (LiBETI) was added and dissolved with stirring. With further stirring, 4 g of polyethylene oxide (PEO, Mw = 4,000,000)
0) was added little by little, and the mixture was stirred for about 6 hours until PEO was completely dissolved and the solution became translucent. The solution thus obtained was used as a casting solution. The [EO] / [Li ⁺ ] ratio of the obtained cast liquid was 12: 1. However,
[EO] indicates the number of ethylene oxide units contained in PEO.

Next, three types of positive electrode plates were prepared. The positive electrode plate Pa was manufactured by the following procedure. Sulfur (S) as a positive electrode active material and acetylene black (A as a conductive agent)
B) and 1) were mixed at a ratio of 1: 1 (weight ratio) and uniformly dispersed by a ball meal to obtain a mixture of sulfur (S) and acetylene black (AB) (S-AB mixture). Next, N-methyl-2-pyrrolidone (NMP) 100m
Polyvinylidene fluoride (PVdF) as a binder
A solution (PVdF-NMP solution) in which 10 g was dissolved was prepared. The PVdF-NMP solution was mixed with the S-AB mixture, and S: AB: PVdF = 2: 2: 0.75 (weight ratio).
And added to obtain a paste. This paste was applied to one side of an aluminum foil, dried and pressed. It was dried at 80 ° C. for 6 hours. The amount of sulfur retained on one surface of the positive electrode plate was 1.1 mg / cm ² .

The positive electrode plate Pb was manufactured by the following procedure. Positive
S-AB mixture 10 used when producing the electrode plate Pa
Stir g little by little into 78 ml of double diluted casting solution
While adding it, it was dispersed uniformly. Allow this solution for 2 hours
It was dried and then vacuum dried at 60 ° C. for 4 hours to obtain
By crushing the lumps, the surface of the S-AB mixture particles is
PEO-LiN (SO_TwoC_TwoF₅)_Two(LiBETI)
A polymer electrolyte consisting of (hereinafter "PEO polymer electrolyte"
A powder coated with a coating of "degradation") was obtained. Next
The same PV used when the positive electrode plate Pa was manufactured
The surface of the dF-NMP solution was coated with PEO polymer electrolyte.
The S-AB mixture powder coated with the film was mixed with S: AB: PV.
Add so that dF = 2: 2: 0.75 (weight ratio)
And stirred to obtain a paste. This paste is aluminum
It was coated on one side of an aluminum foil, dried and pressed. At 80 ° C
It was dried for 6 hours. The amount of sulfur retained on one side of the positive electrode plate is 1.1 mg.
/ Cm ^TwoAnd

The positive electrode plate Pc was manufactured by the following procedure. The positive electrode plate Pb was dipped in the casting liquid, the pressure was reduced to impregnate the casting liquid into the electrode plate, and this was naturally dried. The same procedure was repeated, and the obtained electrode plate was vacuum dried at 80 ° C. for 4 hours to obtain a positive electrode plate Pc having a 10 μm thick PEO polymer electrolyte layer attached to the positive electrode plate surface.

Further, three kinds of negative electrode plates were prepared. A negative electrode plate Na was prepared by using the metallic lithium plate itself.
The negative electrode plate Na was immersed in a casting solution and then naturally dried. Further, the same procedure was repeated and vacuum drying was carried out for 4 hours to obtain a negative electrode plate Nb having a 5 μm thick PEO polymer electrolyte layer attached to the surface of lithium.

The negative electrode plate Nc was manufactured by the following procedure. The negative electrode plate Na was immersed for 2 days in a LiPF ₆ / ethylene carbonate (EC) + diethyl carbonate (DEC) solution containing a trace amount of hydrogen fluoride (HF) and having a concentration of 1.0 mol / l, and the surface of lithium was immersed. Lithium fluoride (L
iF) A film was formed, which was washed with DMC and air dried. The obtained electrode plate was immersed in a casting solution and then naturally dried. Further, the same procedure is repeated, followed by vacuum drying for 4 hours to form lithium fluoride (LiF) on the lithium surface.
Attach a film, and then PEO with a thickness of 5 μm.
A polymer electrolyte layer was attached to obtain a negative electrode plate Nc.

Five types of test cells were prepared by combining the three types of positive electrode plates and the three types of negative electrode plates produced by the above procedure. Each test battery consists of two positive electrodes (without single-sided active material) 25 × 20 mm in size and 25 × 25 mm in size
One negative electrode and a polypropylene / polyethylene / polypropylene (PP) with a thickness of 25 μm and a size of 30 × 30 mm.
/ PE / PP) three-layer separator two sheets were laminated in order of positive electrode / separator / negative electrode / separator / positive electrode to obtain a power generating element. This power generating element was housed in an aluminum resin laminate case in which an aluminum layer was sandwiched between polyolefin layers. Then, a test battery was obtained by injecting 0.5 ml of electrolytic solution under reduced pressure and sealing.

The electrolytic solution was prepared by adding LiN (SO ₂
0.5 mol / l LiBETI / T obtained by adding 94.25 g of C ₂ F ₅ ) ₂ (LiBETI)
HF was used. Table 1 shows the type of test battery and the type of electrode plate used for each battery.

[0053]

[Table 1]

The cross-sectional structure of Battery D is shown in FIG. In FIG. 1, 1 is a positive electrode plate, 2 is a negative electrode plate, 3 is a separator, 4 is a positive electrode terminal, 5 is a negative electrode terminal, 6 is a PEO polymer electrolyte layer between the positive electrode plate and the separator, and 7 is PEO between the negative electrode plate and the separator. It is a polymer electrolyte layer. The sectional structures of the other batteries are basically the same as those in FIG. 1, but in the batteries A and B, the PEO layer between the positive electrode plate 6 and the separator 6 and
There is no PEO layer between the negative electrode plate and the separator, and in battery C, there is no PEO layer between the negative electrode plate and the separator. Further, in the battery E, the negative electrode plate 2 of FIG.
A lithium fluoride (LiF) layer is present between the EO layer 7.

Charge / discharge cycle tests of these five types of batteries were conducted at room temperature. Since the manufactured battery was in a charged state, discharging was performed in the first cycle. Discharge is 0.5
With constant current of mA up to 1.8 V, charging is 0.5 mA constant current, based on the time estimated from the amount of discharged electricity,
The battery was charged up to 1.2 times the amount of discharged electricity.

FIG. 2 compares the discharge curves of the first cycle and the second cycle of the battery D. In FIG.
1 shows the discharge curve of the 1st cycle, 2 shows the discharge curve of the 2nd cycle. The discharge curves after the third cycle were almost the same as the discharge curves at the second cycle, although the discharge capacities were different. As can be seen from FIG. 2, the plateau of about 2.4 V existing in the discharge curve of the first cycle due to the reduction reaction of sulfur is not the discharge curve of the second cycle, which means that after the second cycle, This indicates that the reduction reaction of sulfur is almost eliminated, and the reaction of polysulfide to Li ₂ S ₂ reversibly proceeds.

The charge / discharge cycle test results are summarized in Table 2. The capacity retention rate of each cycle was defined as the ratio (%) of the discharge capacity of each cycle to the discharge capacity of the first cycle. The relationship between the number of cycles and the capacity retention rate of each battery is shown in FIG.

[0058]

[Table 2]

From the results shown in Table 2 and FIG. 3, the following facts became clear. The discharge capacity at the first cycle was the largest in the case of the battery A in which no PEO polymer electrolyte was present on the positive electrode plate and the negative electrode plate, and the discharge capacity was close to the theoretical discharge capacity of 838 mAh / g of the positive electrode active material, sulfur. On the other hand, the surface of sulfur as the positive electrode active material of the present invention is
Battery B coated with EO polymer electrolyte, one cycle of battery C, battery D and battery E, in which the surface of sulfur as a positive electrode active material is coated with PEO polymer electrolyte, and further a PEO polymer electrolyte layer is provided between the positive electrode plate and the separator The discharge capacity of each eye was slightly smaller than that of the battery A. In particular,
In the battery E in which the lithium surface of the negative electrode was covered with the lithium fluoride (LiF) layer like the battery E, the discharge capacity in the first cycle was the smallest.

Further, in the battery A in which the PEO polymer electrolyte was not present at all on the positive electrode plate and the negative electrode plate, the discharge capacity drastically decreased with charge and discharge, and became zero at the 6th cycle. On the other hand, the surface of sulfur as the positive electrode active material of the present invention is
The battery B coated with the EO polymer electrolyte, the battery C having the surface of sulfur as the positive electrode active material coated with the PEO polymer electrolyte, and the battery C, the battery D and the battery E having the PEO polymer electrolyte layer between the positive electrode plate and the separator had 5 The discharge capacity after the cycle was almost stable. This is because by coating the surface of sulfur, which is a positive electrode active material, with a PEO polymer electrolyte, the diffusion of polysulfide (Li _x S _n ) into the electrolytic solution is suppressed by the PEO polymer electrolyte layer and reaches the negative electrode. This is because the chemical reaction with lithium was suppressed.

Further, a battery C having a PEO polymer electrolyte layer between the positive electrode plate and the separator, a battery D having a PEO polymer electrolyte layer between the positive electrode plate and the separator, and a negative electrode plate and the separator, and between the positive electrode plate and the separator and the negative electrode. In the battery E including the PEO polymer electrolyte layer between the plate and the separator and further coating the lithium surface of the negative electrode with the lithium fluoride (LiF) layer, diffusion of polysulfide (Li _x S _n ) to the negative electrode was further suppressed. Therefore, the capacity retention ratio was superior to that of the battery B.

[0062]

INDUSTRIAL APPLICABILITY The non-aqueous electrolyte secondary battery provided with the positive electrode, the negative electrode containing lithium, a lithium alloy or a material capable of inserting and extracting lithium, the non-aqueous electrolyte solution, and the separator of the present invention has a positive electrode. by comprising the coated sulfur polymer electrolyte, to suppress self-discharge due to the polysulfides such as Li _x S _n generated in the positive electrode by the battery reaction is reduced at the surface of the negative electrode, energy density High non-aqueous electrolyte secondary battery with excellent charge / discharge cycle characteristics

[Brief description of drawings]

FIG. 1 is a diagram showing a cross-sectional structure of a battery D according to the present invention.

FIG. 2 is a diagram comparing the discharge curves of the first cycle and the second cycle of the battery D according to the present invention.

FIG. 3 is a diagram showing the relationship between the cycle number and the capacity retention rate of batteries A to E.

[Explanation of symbols]

1 Positive plate 2 Negative electrode plate 3 separator 4 Positive terminal 5 Negative electrode terminal 6 PEO layer between positive electrode plate and separator Polymer electrolyte layer 7 PEO layer between negative electrode plate and separator Polymer electrolyte layer

Claims (3)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode containing lithium, a lithium alloy or a material capable of inserting and extracting lithium, a non-aqueous electrolyte solution, and a separator, wherein the positive electrode is a polymer electrolyte. A non-aqueous electrolyte secondary battery characterized by containing sulfur coated with. 2. The non-aqueous electrolyte secondary battery according to claim 1, further comprising a polymer electrolyte layer provided on at least one of the positive electrode and the separator or the negative electrode and the separator. 3. The non-aqueous electrolyte secondary battery according to claim 1, wherein lithium, a lithium alloy or a material capable of inserting and extracting lithium contained in the negative electrode is covered with a film containing fluorine. .