JP2005251473A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery capable of improving charging/discharging cycle property. <P>SOLUTION: The nonaqueous electrolyte secondary battery comprises an anode 1 having an anode active substance containing metal like copper which forms a compound at least with sulfur, a cathode 2 containing a material storing and releasing lithium, and a nonaqueous electrolyte 5 containing lithium sulfide Li<SB>2</SB>S. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery including a positive electrode including a positive electrode active material.

Conventionally, as a secondary battery having a high energy density, a nonaqueous electrolyte secondary battery that performs charge / discharge by moving lithium ions between a positive electrode and a negative electrode using a nonaqueous electrolyte is known. In this conventional nonaqueous electrolyte secondary battery, a lithium transition metal composite oxide such as LiCoO ₂ is used for the positive electrode, and a lithium metal, a lithium alloy, or a carbon material capable of inserting and extracting lithium ions is used for the negative electrode. . As the nonaqueous electrolyte, a nonaqueous electrolyte in which a solute (electrolyte salt) made of a lithium salt such as LiBF ₄ or LiPF ₆ is dissolved in an organic solvent such as ethylene carbonate or diethyl carbonate is used. In recent years, such conventional non-aqueous electrolyte secondary batteries have been used as power sources for various portable devices. Along with the increase in power consumption due to the multifunctional use of portable devices, a non-aqueous electrolyte secondary battery having a higher energy density is demanded.

However, in the conventional non-aqueous electrolyte secondary battery, the lithium transition metal composite oxide such as LiCoO ₂ used for the positive electrode has a large mass and a small number of reaction electrons. Therefore, the capacity per unit mass (capacity density) There is an inconvenience that it is difficult to sufficiently raise the value.

Therefore, conventionally, in order to eliminate the above-described inconvenience, a technique using sulfur having a high theoretical capacity density of about 1675 mAh / g as a positive electrode material has been proposed (for example, see Patent Document 1). A conventional nonaqueous electrolyte secondary battery using sulfur as a positive electrode material disclosed in Patent Document 1 includes a positive electrode containing a simple substance of sulfur and an electron conductive material such as a carbon material, a metal sulfide (for example, And a non-aqueous electrolyte containing sulfur such as lithium sulfide).
Special table 2003-522383 gazette

In the conventional non-aqueous electrolyte secondary battery disclosed in Patent Document 1, sulfide ions S ²⁻ in the electrolyte are first converted into polysulfide ions S _y ²⁻ (2 ≦ ² ) during the charging reaction in the positive electrode. Oxidized to y ≦ 8). Then, most of the generated polysulfide ion S _y ^2-, by being further oxidized, deposited on the surface of the positive electrode as sulfur S. However, as shown in the following formula (1), the sulfur S deposited on the positive electrode reacts with the polysulfide ion S _(y-1) ²⁻ which is not oxidized and remains in the vicinity of the positive electrode, thereby increasing the amount of sulfur again. It elutes in the electrolyte as sulfide ion S _y ^2- . And the polysulfide ion eluted in this electrolyte reacts with the negative electrode and is reduced, whereby discharge (self-discharge phenomenon) occurs on the negative electrode side. As a result, there is a problem that the charge / discharge cycle characteristics of the nonaqueous electrolyte secondary battery deteriorate.

S _(y-1) ² + S → S _y ²⁻ (1)
The present invention has been made to solve the above-described problems, and one object of the present invention is to provide a nonaqueous electrolyte secondary battery capable of improving charge / discharge cycle characteristics. is there.

Means for Solving the Problems and Effects of the Invention

  The inventor of the present application has intensively studied to achieve the above object, and as a result, sulfur does not precipitate on the positive electrode but is fixed as a compound on the positive electrode, so that the sulfur elutes again in the electrolyte. It has been found that the self-discharge phenomenon caused by can be suppressed.

That is, the nonaqueous electrolyte secondary battery according to the first aspect of the present invention includes a positive electrode including a positive electrode active material containing a metal that forms a compound with at least sulfur, a negative electrode including a material that absorbs and releases lithium, and Li _And a non-aqueous electrolyte containing ₂ S _x (1 ≦ x ≦ 8).

In the nonaqueous electrolyte secondary battery according to the first aspect, as described above, in the nonaqueous electrolyte secondary battery using the nonaqueous electrolyte to which Li ₂ S _x (1 ≦ x ≦ 8) is added, the positive electrode active material By using a metal that forms a compound with sulfur as a catalyst, the (multi) sulfide ions in the electrolyte are not deposited as sulfur on the positive electrode during the charging reaction, but instead of the above-described sulfur and the metal that forms the compound. The compound can be fixedly deposited on the positive electrode. As a result, it is possible to prevent the (multi) sulfide ions from eluting into the electrolyte again, and thus it is possible to prevent the self-discharge phenomenon caused by the reaction between the negative electrode and the polysulfide ions in the electrolyte. Thereby, the charge / discharge cycle characteristics of the non-aqueous electrolyte secondary battery can be improved. In addition, since a battery is formed in a discharged state by using a metal that forms a compound with sulfur as a positive electrode active material, lithium metal is used for the negative electrode, unlike a battery in a charged state when sulfur is used as a positive electrode active material. Need not be used. Here, the production of a battery in a charged state means that a battery is produced using a material in an oxidized state for the positive electrode and a material in a reduced state for the negative electrode. On the other hand, the production of a battery in a discharged state means that a battery is produced using a material in a reduced state for the positive electrode and an oxidized material for the negative electrode. Sulfur (S) is a state in which sulfide ions (S ²⁻ ) are oxidized, and the metal that forms a compound with lithium (Li) and sulfur is lithium ion (Li ⁺ ) and the above sulfur and compound. In this state, the metal ions to be formed are reduced. That is, in the case of producing a battery in a charged state using sulfur (S) for the positive electrode, a reduced material must be used for the negative electrode. As the material in such a reduced state, it is most preferable to use a material containing lithium. However, since these all have a property of reacting with moisture in the air, facilities such as a dry room are required. It is necessary to provide it. On the other hand, in the case of producing a battery in a discharged state using a metal that forms a compound with sulfur for the positive electrode, it is not necessary to use a material containing lithium for the negative electrode. As a result, when assembling the non-aqueous electrolyte secondary battery, it is not necessary to separately provide equipment such as a dry room, so that the assembly process of the non-aqueous electrolyte secondary battery can be simplified.

  In the non-aqueous electrolyte secondary battery according to the first aspect, preferably, the metal that forms a compound with sulfur includes at least one of copper, iron, and nickel. If comprised in this way, the (multi) sulfide ion in electrolyte can be fixedly deposited on a positive electrode as a compound with any one of copper, iron, and nickel in the case of charge reaction. As a result, it is possible to prevent the (multi) sulfide ions from eluting into the electrolyte again, and thus it is possible to prevent the self-discharge phenomenon caused by the reaction between the negative electrode and the polysulfide ions in the electrolyte. Thereby, the charge / discharge cycle characteristics of the non-aqueous electrolyte secondary battery can be improved.

  In the nonaqueous electrolyte secondary battery according to the first aspect, preferably, the positive electrode including a metal that forms a compound with sulfur includes a metal foil mainly composed of any one of copper, iron, and nickel. Yes. Note that “mainly metal” means that 50% or more of the main metal is contained. If comprised in this way, a positive electrode can be formed more easily compared with the case where the metal which has any one of powdery copper, iron, and nickel is used.

  In the nonaqueous electrolyte secondary battery according to the first aspect, the positive electrode including a metal that forms a compound with sulfur preferably includes any one of a copper foil, an iron foil, and a nickel foil. If comprised in this way, compared with the positive electrode containing the alloy containing either of copper, iron, and nickel, in the case of charge reaction, the (multi) sulfide ion in electrolyte will be copper foil, iron foil As a compound with any one of nickel foil and nickel foil, more can be fixedly deposited on the positive electrode. As a result, it is possible to effectively prevent the (multi) sulfide ions from eluting into the electrolyte again, so that the self-discharge phenomenon caused by the reaction between the negative electrode and the polysulfide ions in the electrolyte is effectively prevented. be able to. Thereby, the charge / discharge cycle characteristics of the nonaqueous electrolyte secondary battery can be easily improved. Moreover, since (poly) sulfide ions in the electrolyte can be deposited more and more on the positive electrode as a compound with any one of copper foil, iron foil and nickel foil, the battery capacity is improved. There is also an effect that can be done.

In the non-aqueous electrolyte secondary battery according to the first aspect, preferably, the non-aqueous electrolyte contains Li ₂ S _x (1 ≦ x ≦ 8) in a supersaturated state. According to this structure, when the charge reaction, by which is dissolved in the non-aqueous electrolyte (multi) lithium sulfide _{(Li 2 S x (1 ≦} x ≦ 8)) is consumed in the solid state ( Multi) Lithium sulfide dissolves in the non-aqueous electrolyte and is used for the charging reaction. As a result, more (multi) lithium sulfide can be used for the charging reaction than when a nonaqueous electrolyte containing (poly) lithium sulfide in an unsaturated state is used. For this reason, since more metal sulfide is produced | generated by charge reaction, the capacity | capacitance of a nonaqueous electrolyte secondary battery can be enlarged, As a result, charging / discharging cycling characteristics can be improved.

  In the nonaqueous electrolyte secondary battery according to the first aspect, preferably, the negative electrode includes a negative electrode active material containing silicon. If constituted in this way, unlike the case where lithium is used for the negative electrode, dendritic (dendritic) precipitates are not generated on the surface of the negative electrode. Moreover, since the capacity | capacitance of a nonaqueous electrolyte secondary battery can be enlarged by using the silicon | silicone which has a high capacity | capacitance for a negative electrode, charging / discharging cycling characteristics can be improved.

  Examples of the present invention will be specifically described below.

  In the present application, lithium secondary batteries (nonaqueous electrolyte secondary batteries) according to the following Examples 1 to 4 are manufactured as examples corresponding to the present invention, and lithium secondary batteries according to the following Comparative Example 1 are used as comparative examples. Next batteries were fabricated and their characteristics were compared.

[Production of positive electrode]
(Common to Example 1 and Example 2)
According to Example 1 and Example 2, a 99.9% pure copper foil having a thickness of 20 μm formed into a size of 2 cm × 2 cm square was dried at 100 ° C. in a vacuum. A positive electrode of a lithium secondary battery was produced.

(Example 3)
An iron foil having a thickness of 100 μm and having a purity of 99.9% was molded into a size of 2 cm × 2 cm square, and then dried at 100 ° C. in a vacuum, thereby producing a positive electrode of a lithium secondary battery according to Example 3.

Example 4
A nickel foil having a thickness of 100 μm and having a purity of 99.98% was molded into a size of 2 cm × 2 cm square, and then dried at 100 ° C. in a vacuum to produce a positive electrode for a lithium secondary battery according to Example 4.

(Comparative Example 1)
Ketjen black (KB) as a conductive agent, polytetrafluoroethylene (PTFE) as a binder, and carboxymethyl cellulose (CMC) as a thickener have a mass ratio of 90: 5: 5. After being mixed, the mixture was kneaded to obtain a slurry. And after sticking this slurry uniformly on the aluminum foil as a positive electrode collector which surface electrolyzed by the doctor blade method, it was dried at 50 degreeC using the hotplate. This was cut into a 2 cm × 2 cm square and dried at 100 ° C. in a vacuum to produce a positive electrode of a lithium secondary battery according to Comparative Example 1.

[Production of negative electrode]
(Common to Examples 1, 3, 4 and Comparative Example 1)
Negative electrodes of lithium secondary batteries according to Examples 1, 3, 4 and Comparative Example 1 were prepared using lithium metal.

(Example 2)
An amorphous silicon film having a thickness of 0.5 μm as a negative electrode active material was formed on a copper foil as a negative electrode current collector, the surface of which was electrolytically treated, using a sputtering method. This was cut into a 2 cm × 2 cm square and dried at 110 ° C. in a vacuum to produce a negative electrode for a lithium secondary battery according to Example 2.

[Production of non-aqueous electrolyte]
(Common to Examples 1 to 4 and Comparative Example 1)
To an electrolyte in which 1,2-dimethoxyethane and 1,3-dioxolane were mixed at a volume ratio of 9: 1, lithium bis (trifluoromethylsulfonyl) imide LiN (SO ₂ CF ₃ ) ₂ was added at 0.5 mol / l. Dissolved to a concentration. Then, by adding Li 2 _S at a concentration of 2.0 mol / l, by the state of supersaturation, to prepare a non-aqueous electrolyte of a lithium secondary battery according to Examples 1 to 4 and Comparative Example 1 .

[Production of test cell]
(Common to Examples 1 to 4 and Comparative Example 1)
FIG. 1 is a perspective view showing a test cell produced for examining the characteristics of the positive electrode of the nonaqueous electrolyte secondary battery according to Examples 1 to 4 and Comparative Example 1. FIG. Referring to FIG. 1, as a test cell manufacturing process, positive electrode 1 and negative electrode 2 were arranged in test cell container 10 such that positive electrode 1 and negative electrode 2 face each other with separator 3 interposed therebetween. The reference electrode 4 was also disposed in the test cell container 10. And the test cell was produced by inject | pouring the nonaqueous electrolyte 5 in the test cell container 10. FIG. In addition, as the positive electrode 1, the negative electrode 2, and the nonaqueous electrolyte 5, while using the positive electrode, negative electrode, and nonaqueous electrolyte by Examples 1-4 and the comparative example 1 which were produced as mentioned above, as the reference electrode 3, lithium was used. Metal was used.

[Charge / discharge test]
A charge / discharge test was performed on each of the test cells corresponding to Examples 1 to 4 and Comparative Example 1 manufactured as described above. The charge and discharge conditions in Example 1 and Example 2 were 25 μA / cm ² after charging to a charge end potential of 2.8 V (vs. Li / Li ⁺ ) with a charge current of 25 μA / cm ² . Discharge was performed to a discharge end potential of 1.5 V (vs. Li ^/ Li ⁺ ) with a discharge current. And charging / discharging was performed to the 5th cycle and the 11th cycle in Example 1 and Example 2 by making this charging / discharging into 1 cycle. Moreover, in Example 2, only the charge end potential was changed to 3.0 V (vs. Li ^/ Li ⁺ ) from the above-described charging / discharging conditions, and the charging and discharging up to the 11th cycle continued from the 12th cycle to the 19th cycle. Charging / discharging was performed to the eyes. The condition of the charging and discharging in Example 3 and Example 4, at a charging current of 25 .mu.A / cm ^2, respectively 2.9V (vs.Li/Li ⁺⁾ and a 2.5V (vs.Li/Li ⁺⁾ After charging to the end-of-charge potential, discharging was performed to a discharge end potential of 1.5 V (vs. Li / Li ⁺ ) with a discharge current of 25 μA / cm ² . And charging / discharging was performed to the 1st cycle in Example 3 and Example 4 by making this charging / discharging into 1 cycle. The charge / discharge conditions in Comparative Example 1 were 2.5 μA / cm ² after charging to a charge end potential of 2.8 V (vs. Li / Li ⁺ ) with a charge current of 2.5 μA / cm 2. Discharge was performed at a discharge current of ^{2 to} a discharge end potential of 1.5 V (vs. Li ^/ Li ⁺ ). And this charge / discharge was made into 1 cycle, and in the comparative example 1, it charged / discharged to the 7th cycle. The results are shown in FIGS.

  Specifically, in FIGS. 2, 4, 6, 7, and 8, test cells corresponding to Example 1, Example 2, Example 3, Example 4, and Comparative Example 1 were performed, respectively. Initial charge (discharge cycle) charge / discharge characteristics are shown. The battery voltage in FIG. 4 indicates the difference between the positive electrode potential and the negative electrode potential. In Examples 1, 3, 4 and Comparative Example 1, lithium metal is used as the negative electrode and the reference electrode. 2, FIG. 6, FIG. 7 and FIG. 8 show only the potential of the positive electrode with respect to the reference electrode. FIG. 3 shows the charge / discharge cycle characteristics up to the fifth cycle of charge / discharge performed on the test cell of Example 1, and FIG. 5 shows the charge / discharge performed on the test cell of Example 2. The charge / discharge cycle characteristics up to the 19th cycle are shown. FIG. 9 shows the charge / discharge cycle characteristics up to the seventh cycle of charge / discharge performed for the test cell of Comparative Example 1.

Also, the initial (first cycle) charge / discharge reaction equation and two cycles considered to have occurred in the positive electrode and negative electrode of Example 2 using copper foil as the positive electrode, amorphous silicon film as the negative electrode, and Li ₂ S as the nonaqueous electrolyte. The following formulas (2) to (9) show the formulas of the charge and discharge reactions after the first.

Initial (first cycle) charging reaction Negative electrode: Si + 4.4Li ⁺ + 4.4e ⁻ → Li _4.4 Si (2)
The positive _{^{electrode: zCu + S 2- → Cu z}} S + 2e - ... (3)
Initial (first cycle) discharge reaction Negative electrode: Li _4.4 Si → Si + _4.4 Li ⁺ + _4.4 e ⁻ (4)
Positive electrode: Cu _z S + 2Li ⁺ + 2e ⁻ → Li ₂ Cu _z S (5)
Charging reaction after the second cycle Negative electrode: Si + 4.4Li ⁺ + 4.4e ⁻ → Li _4.4 Si (6)
Positive electrode: Li ₂ Cu _z S → Cu _z S + 2Li ⁺ + 2e ⁻ (7)
Discharge reaction after the second cycle Negative electrode: Li _4.4 Si → Si + _4.4 Li ⁺ + 4.4e ⁻ (8)
Positive electrode: Cu _z S + 2Li ⁺ + 2e ⁻ → Li ₂ Cu _z S (9)
However, the value of z in the above formulas (3), (5), (7), and (9) is 0 <z ≦ 2.

  The reaction formula at the negative electrode in the test cells corresponding to Examples 1, 3, and 4 is as follows. It is expressed by the following reaction formula. In addition, the reaction formula at the positive electrode is the same as that in Example 2 in Example 1, and in Examples 3 and 4, the above formulas (3), (5), (7), and In Formula (9), it represents with the reaction formula which replaced Cu with Fe and Ni, respectively.

In Example 1 using a nonaqueous electrolyte containing Li ₂ S and using copper foil and lithium metal as the positive electrode and the negative electrode, respectively, as shown in FIGS. 2 and 3, the initial (first cycle) charge capacity is 0.809 mAh, and the initial (first cycle) discharge capacity was 0.308 mAh, and it was found that charging and discharging can be performed. In this case, in the initial charging reaction, a reaction in which copper sulfide (Cu _z S) is formed at the positive electrode (reaction of the above formula (3)) occurs, and in the subsequent initial discharging reaction, it is formed during the initial charging reaction. It is considered that a reaction in which lithium (Li) was occluded in the copper sulfide thus formed (the reaction of the above formula (5)) occurred. Moreover, the charge capacity in the second cycle of Example 1 was 0.367 mAh, and the discharge capacity in the second cycle was 0.399 mAh. The initial charge / discharge efficiency of Example 1 was 38%, and the charge / discharge efficiency after the second cycle was approximately 100%. From this, in Example 1, it is thought that the charge / discharge reaction can be performed without causing the self-discharge phenomenon, and the copper sulfide formed during the initial charge reaction is not dissolved in the electrolytic solution. In FIG. 3, data indicating charging / discharging efficiency of 100% or more is described, but this is due to an error generated during the experiment, and is actually considered to be a value close to 100%. .

In Example 2 using a nonaqueous electrolyte containing Li ₂ S and using a copper foil and an amorphous silicon film as the positive electrode and the negative electrode, respectively, as shown in FIGS. 4 and 5, the initial (first cycle) The charge capacity was 2.125 mAh, the initial (first cycle) discharge capacity was 0.769 mAh, and it was found that charging / discharging can also be performed in Example 2. In the case of Example 2, as in Example 1 above, in the initial charging reaction, a reaction in which copper sulfide is formed at the positive electrode (reaction of the above formula (3)) occurs, and in the subsequent initial discharging reaction, It is considered that a reaction in which lithium is occluded in the copper sulfide formed during the initial charging reaction (the reaction of the above formula (5)) occurred. Moreover, the charge capacity in the second cycle of Example 2 was 0.775 mAh, and the discharge capacity in the second cycle was 0.879 mAh. The initial charge / discharge efficiency of Example 2 was 36%, and the charge / discharge efficiency after the second cycle was approximately 100%. Thus, also in Example 2, the charge / discharge reaction can be performed without causing the self-discharge phenomenon, and it is considered that the copper sulfide formed in the initial charge reaction is not dissolved in the electrolytic solution. In FIG. 5, data indicating charge / discharge efficiency of 100% or more is described. However, as in FIG. 3, it is due to an error generated during the experiment, and is actually a value close to 100%. it is conceivable that.

Further, the use of non-aqueous electrolyte containing Li 2 _S, as in Example 3 using iron foil and lithium metal respectively as a positive electrode and the negative electrode, together with the use of non-aqueous electrolyte containing Li 2 _S, respectively nickel as the positive electrode and the negative electrode In Example 4 using foil and lithium metal, as shown in FIGS. 6 and 7, the initial (first cycle) charge capacities are 0.34 mAh and 0.514 mAh, and the initial (first cycle) ) The discharge capacities were 0.171 mAh and 0.308 mAh. Thus, it was found that charging and discharging can be performed in the test cells corresponding to Example 3 and Example 4 using iron foil and nickel foil as the positive electrode and using lithium metal as the negative electrode. The initial charge / discharge efficiency was 50% in Example 3 and 59% in Example 4.

In Comparative Example 1 using a non-aqueous electrolyte containing Li ₂ S and a material that does not form a compound with sulfur and lithium metal, respectively, as a positive electrode and a negative electrode, as shown in FIGS. The initial (first cycle) charge capacity was 0.36 mAh, and the initial (first cycle) discharge capacity was 0.15 mAh. The initial charge / discharge efficiency was 41%, and the charge / discharge efficiency after the second cycle was about 70% to about 80%.

  As described above, in Example 1 and Example 2 using copper foil (a metal that forms a compound with sulfur) as the positive electrode, the capacity retention rate of Comparative Example 1 without using a material that forms a compound with sulfur as the positive electrode ( A capacity retention ratio (approximately 100%) higher than about 70% to about 80%) can be obtained, and therefore, Example 1 and Example 2 can improve charge / discharge cycle characteristics more than Comparative Example 1. it can. This is considered to be due to the following reason. That is, in Example 1 and Example 2, by using a copper foil, which is a metal that forms a compound with sulfur, in the positive electrode, sulfide ions in the electrolyte are deposited as sulfur on the positive electrode during the charging reaction. However, it is considered that copper sulfide can be fixedly deposited on the positive electrode. As a result, it is possible to prevent the sulfide ions from eluting again into the electrolyte as polysulfide ions, so that the self-discharge phenomenon caused by the reaction between the negative electrode and the polysulfide ions in the electrolyte can be prevented. It is considered possible. As a result, it is considered that the charge / discharge cycle characteristics of the nonaqueous electrolyte secondary battery could be improved. On the other hand, in Comparative Example 1 in which a material that forms a compound with sulfur is not used as the positive electrode, sulfide ions in the electrolyte are deposited on the positive electrode as sulfur during the charging reaction, and the precipitated sulfur is polysulfide. It is thought that it elutes again into the electrolyte as ions. Thereby, the self-discharge phenomenon resulting from the reaction between the negative electrode and the polysulfide ion in the electrolyte occurs, and as a result, the charge / discharge cycle characteristics of the nonaqueous electrolyte secondary battery are considered to have deteriorated.

  As described above, in Example 3 using iron foil as the positive electrode and Example 4 using nickel foil as the positive electrode, the initial discharge capacity of Comparative Example 1 without using a material that forms a compound with sulfur as the positive electrode ( It has been found that initial discharge capacities (0.171 mAh and 0.308 mAh) higher than 0.15 mAh) can be obtained. Thereby, also in Example 3 and Example 4, similarly to Example 1 and Example 2, it is estimated that a charge / discharge cycle characteristic can be improved rather than the comparative example 1. FIG. As a reason to support this assumption, in Example 3 and Example 4, by using an iron foil and a nickel foil, which are metals that form a compound with sulfur, in the positive electrode, sulfide ions in the electrolyte are reduced during the charging reaction. It is thought that it can be fixedly deposited on the positive electrode as iron sulfide and nickel sulfide rather than being deposited on the positive electrode as sulfur. As a result, it is possible to prevent the sulfide ions from eluting again into the electrolyte as polysulfide ions, so that the self-discharge phenomenon caused by the reaction between the negative electrode and the polysulfide ions in the electrolyte can be prevented. It is considered possible. As a result, it is considered that the charge / discharge cycle characteristics of the nonaqueous electrolyte secondary battery can be improved.

In Example 2, the charge / discharge test was performed by changing the charge end potential to 3.0 V (vs. Li ^/ Li ⁺ ) from the charge / discharge at the 12th cycle. As shown in FIG. It was found that a large discharge capacity can be obtained as compared with the discharge capacity from the first to the 11th cycle. This is considered to be due to the following reason. That is, by changing the charge end potential from 2.8 V (vs. Li ^/ Li ⁺ ) to 3.0 V (vs. Li ^/ Li ⁺ ), more copper sulfide was formed during the charging reaction. Alternatively, it is considered that the number of reaction electrons increased due to the formation of copper sulfide having a larger z value in CuS _z (z ≧ 0.5).

  In addition, it should be thought that the Example disclosed this time is an illustration and restrictive at no points. The scope of the present invention is shown not by the above description of the embodiments but by the scope of claims for patent, and includes all modifications within the meaning and scope equivalent to the scope of claims for patent.

  For example, in the above embodiment, copper foil, iron foil and nickel foil were used as the positive electrode. However, the present invention is not limited to this, and copper capable of forming a compound with sulfur other than copper foil, iron foil and nickel foil. A material mainly composed of a metal such as 50% or more may be used for the positive electrode. For example, when a material mainly composed of copper is used as a material capable of forming a compound with sulfur, nickel, tin, silicon, aluminum, beryllium, tungsten, silver, zinc are used as metals that form an alloy with copper. It is possible to use at least one selected from the group consisting of manganese and lead. Specific examples of the copper alloy include bronze, constantan, white bronze, brass, western white, and red brass. In addition, as the positive electrode active material, a powder in which a powder mainly composed of copper is coated on the positive electrode current collector may be used, but the positive electrode current collector mainly composed of copper has a function as a positive electrode active material. Is more preferable. For example, when copper foil is used for the positive electrode current collector and the copper foil is used as the positive electrode active material, copper sulfide produced by the reaction of copper and sulfur is deposited on the copper foil, and the positive electrode current collector Since the thickness is reduced, an increase in the capacity of the battery can be expected. Further, the current collector is not limited to the copper foil, and a copper alloy foil may be used. The thickness of the current collector is preferably 50 μm or less, more preferably 20 μm or less, and most preferably 10 μm or less from the viewpoint of increasing the battery capacity. .

  Moreover, the material which has as a main the metal which forms sulfur and compounds other than copper can also be used. Examples of such metals include iron, nickel, manganese, cobalt, titanium, molybdenum, tungsten, zinc, silver, tin, zirconium, ruthenium, niobium, tantalum, palladium, and germanium. In addition, when a material mainly composed of iron is used as a material capable of forming a compound with sulfur, nickel, molybdenum, titanium, manganese, chromium, cobalt, tungsten, and aluminum are used as metals that form an alloy with iron. At least one selected from the group consisting of: In addition, when a material mainly composed of nickel is used as a material capable of forming a compound with sulfur, beryllium, copper, silver, cobalt, molybdenum, aluminum, iron, manganese are used as the metal that forms an alloy with nickel. , At least one selected from the group consisting of silicon, titanium and tungsten. Moreover, these metals and alloys may contain carbon. Note that copper and sulfur easily form copper sulfide only by contacting them, and it is considered that the greatest effect can be expected by using copper or a copper alloy among the metals and alloys described above.

In the above embodiment, the non-aqueous there is shown an example of adding lithium sulfide Li 2 _S in the electrolyte, the present invention is not limited thereto, lithium sulfide Li 2 _S other than lithium polysulphides Li 2 _{S x (1} < x ≦ 8) may be added. The (poly) lithium sulfide Li ₂ S _x (1 ≦ x ≦ 8) added to the nonaqueous electrolyte is preferably in a discharged state as much as possible. That is, the smaller the value of x in (poly) lithium sulfide Li ₂ S _x (1 ≦ x ≦ 8), the more preferable, and lithium sulfide Li ₂ S in the case of x = 1 being the complete discharge state is most preferable.

In the above embodiment, a mixed solvent of 1,2-dimethoxyethane and 1,3-dioxolane is used as the organic solvent contained in the non-aqueous electrolyte. However, the present invention is not limited to this, and (poly) sulfurization is used. Other organic solvents other than 1,2-dimethoxyethane and 1,3-dioxolane capable of dissolving lithium Li ₂ S _x (1 ≦ x ≦ 8) may be used. Examples of such an organic solvent include cyclic ethers, chain ethers, and room temperature molten salts having a melting point of 60 ° C. or lower. Specifically, the cyclic ether includes 1,3-dioxolane, 2-methyl-1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2 -Butylene oxide, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, crown ether and the like.

Examples of the chain ether include 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, Pentylphenyl ether, methoxytoluene, benzylethyl ether, diphenyl ether, dibenzyl ether, ₀ -dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1 , 1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether Such fine tetraethylene glycol dimethyl ether. Further, a group consisting of γ-butyrolactone, γ-valerolactone, N, N-dimethylacetamide, N, N-diethylacetamide, N, N-dimethylformamide, N, N-diethylformamide, tetramethylurea, dimethylsulfoxide and acetonitrile At least one selected from the above may be used as the organic solvent contained in the nonaqueous electrolyte.

  Examples of room temperature molten salts having a melting point of 60 ° C. or lower include quaternary ammonium salts, imidazolium salts and pyrazolium salts. Here, the quaternary ammonium salt is a material that is less likely to react with lithium metal than other room temperature molten salts such as imidazolium salt and pyrazolium salt. Therefore, when lithium metal is used as the metal anode, It is preferable to use a quaternary ammonium salt.

Examples of the quaternary ammonium salt include trimethylpropylammonium bis (trifluoromethylsulfonyl) imide ((CH ₃ ) ₃ N ⁺ (C ₃ H ₇ ) N ^— (SO ₂ CF ₃ ) ₂ ), trimethyl octyl ammonium bis (trifluoromethylsulfonyl) imide ^{_{((CH 3) 3 N +}} (C 8 H 17) N - (SO 2 CF 3) 2), trimethyl allyl ammonium bis (trifluoromethylsulfonyl) imide (( ^{_{CH 3) 3 N + (Allyl}} ) N - (SO 2 CF 3) 2), trimethyl hexyl ammonium bis (trifluoromethylsulfonyl) imide ^{_{((CH 3) 3 N +}} (C 6 H 13) N - (SO ₂ CF _{3) 2),} methoxymethyl trimethylammonium bis (tri Le Oro methylsulfonyl) imide ^{_{((CH 3) 3 N +}} (CH 2 OCH 3) N - (SO 2 CF 3) 2), trimethylethyl ammonium 2,2,2-trifluoro -N- (trifluoromethyl sulfonyl) acetamide ^{_{((CH 3) 3 N +}} (C 2 H 5) (CF 3 CO) N - (SO 2 CF 3)), trimethyl allyl ammonium 2,2,2-trifluoro -N- (trifluoro methylsulfonyl) acetamide ^{_{((CH 3) 3 N +}} (Allyl) (CF 3 CO) N - (SO 2 CF 3)), trimethylpropylammonium-2,2,2-trifluoro -N- (trifluoromethylsulfonyl ) acetamide ^{_{((CH 3) 3 N +}} (C 3 H 7) (CF 3 CO) N - (SO 2 CF 3)), Tet Ethyl ammonium 2,2,2-trifluoro -N- (trifluoromethylsulfonyl) acetamide ^{_{((C 2 H 5) 4}} N + (CF 3 CO) N - (SO 2 CF 3)) and triethyl methyl ammonium and the like are _- 2,2,2-trifluoro -N- (trifluoromethylsulfonyl) acetamide _{((SO 2 CF 3) (} C 2 H 5) 3 N + (CH 3) (CF 3 CO) N) .

Further, the above embodiment is used _LiN (SO 2 _{CF 3) 2} was dissolved nonaqueous electrolyte as a solute (lithium salt), the present invention is not limited to this, _LiN (SO 2 _{CF 3) 2} A nonaqueous electrolyte in which a lithium salt other than the above is dissolved may be used. Examples of lithium salts other than LiN (SO ₂ CF ₃ ) ₂ include LiBF ₄ , LiPF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiAsF ₆ and difluoro. (Oxalato) lithium borate (substance represented by the chemical formula of the following chemical formula 1) and the like. Moreover, you may use the mixture which combined 2 or more selected from the group which consists of an above-described lithium salt.

また、上記実施例では、負極活物質としてリチウム金属またはアモルファスシリコン膜を用いたが、本発明はこれに限らず、一般的にリチウムイオン電池に使用されている材料を主体とする負極活物質として用いてもよい。具体的には、リチウムアルミニウム合金、黒鉛、シリコンおよびスズなどが挙げられる。なお、シリコンを主体とする負極活物質としては、金属箔からなる集電体上にＣＶＤ（Ｃｈｅｍｉｃａｌ Ｖａｐｏｒ Ｄｅｐｏｓｉｔｉｏｎ）法、スパッタリング法、蒸着法および溶射法などにより堆積させたシリコン膜およびシリコン合金膜が挙げられる。ここで、シリコンを主体とする負極活物質とは、シリコンが全体の５０％以上の割合で含有されている負極活物質を示している。上記堆積されたシリコン膜としては、非晶質（アモルファス）シリコン膜および微結晶シリコン膜を用いるのが好ましい。また、堆積されたシリコン合金膜としては、たとえば、コバルト、鉄、ジルコニウムおよび亜鉛などを含むシリコン合金膜が挙げられる。また、負極活物質としてシリコン粉末を用いてもよい。なお、このようなシリコン粉末を用いる場合には、シリコン粉末とバインダとを含むスラリーを負極集電体上に塗布することにより負極を作製することができる。また、負極活物質として堆積されたシリコン膜を用いる場合には、集電体上のシリコン膜が厚み方向の切れ目によって柱状に分離された電極、銅成分などの集電体成分がシリコン膜の内部に拡散した電極、または、表面が粗面化された集電体の上に形成されたシリコン膜を用いた電極を用いるのが好ましい。

Further, in the above embodiment, lithium metal or amorphous silicon film was used as the negative electrode active material, but the present invention is not limited to this, and as a negative electrode active material mainly composed of materials generally used in lithium ion batteries. It may be used. Specific examples include lithium aluminum alloy, graphite, silicon, and tin. Note that as the negative electrode active material mainly composed of silicon, a silicon film and a silicon alloy film deposited on a current collector made of metal foil by a CVD (Chemical Vapor Deposition) method, a sputtering method, a vapor deposition method, a thermal spraying method, or the like. Can be mentioned. Here, the negative electrode active material mainly composed of silicon refers to a negative electrode active material containing silicon in a proportion of 50% or more of the whole. As the deposited silicon film, an amorphous silicon film and a microcrystalline silicon film are preferably used. Examples of the deposited silicon alloy film include a silicon alloy film containing cobalt, iron, zirconium, zinc, and the like. Moreover, you may use a silicon powder as a negative electrode active material. In addition, when using such a silicon powder, a negative electrode can be produced by applying a slurry containing silicon powder and a binder on a negative electrode current collector. In addition, when a silicon film deposited as a negative electrode active material is used, an electrode in which the silicon film on the current collector is separated into a columnar shape by a cut in the thickness direction, and a current collector component such as a copper component is inside the silicon film. It is preferable to use an electrode that has diffused into the surface or an electrode that uses a silicon film formed on a current collector having a roughened surface.

FIG. 4 is a perspective view showing a test cell produced for examining the characteristics of the positive electrode of the nonaqueous electrolyte secondary battery according to Examples 1 to 4 and Comparative Example 1. 2 is a graph showing initial charge / discharge characteristics in a charge / discharge test performed on a test cell corresponding to Example 1. FIG. 4 is a graph showing the relationship between the number of cycles, the discharge capacity, and the charge / discharge efficiency in a charge / discharge test performed on a test cell corresponding to Example 1. FIG. 4 is a graph showing initial charge / discharge characteristics in a charge / discharge test performed on a test cell corresponding to Example 2. FIG. 6 is a graph showing the relationship between the number of cycles, the discharge capacity, and the charge / discharge efficiency in a charge / discharge test performed on a test cell corresponding to Example 2. FIG. 4 is a graph showing initial charge / discharge characteristics in a charge / discharge test performed on a test cell corresponding to Example 3. FIG. 6 is a graph showing initial charge / discharge characteristics in a charge / discharge test performed on a test cell corresponding to Example 4. FIG. 5 is a graph showing initial charge / discharge characteristics in a charge / discharge test performed on a test cell corresponding to Comparative Example 1. FIG. It is the graph which showed the relationship between the cycle number in the charging / discharging test done about the test cell corresponding to the comparative example 1, discharge capacity, and charging / discharging efficiency.

Explanation of symbols

1 Positive electrode 2 Negative electrode 5 Nonaqueous electrolyte

Claims (6)

A positive electrode including a positive electrode active material containing a metal that forms a compound with at least sulfur; and
A negative electrode comprising a material that occludes and releases lithium;
A nonaqueous electrolyte secondary battery comprising a nonaqueous electrolyte containing Li ₂ S _x (1 ≦ x ≦ 8).   The nonaqueous electrolyte secondary battery according to claim 1, wherein the metal that forms a compound with sulfur includes at least one of copper, iron, and nickel.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the positive electrode including a metal that forms a compound with sulfur includes a metal foil mainly composed of any one of copper, iron, and nickel.   The nonaqueous electrolyte secondary battery according to claim 1, wherein the positive electrode including a metal that forms a compound with sulfur includes any one of copper foil, iron foil, and nickel foil. 5. The non-aqueous electrolyte secondary battery according to claim 1, wherein the non-aqueous electrolyte includes the Li ₂ S _x (1 ≦ x ≦ 8) in a supersaturated state.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode includes a negative electrode active material containing silicon.

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