JP4535722B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

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

  Currently, non-aqueous electrolyte secondary batteries that use a non-aqueous electrolyte and charge and discharge by moving lithium ions between a positive electrode and a negative electrode are used as secondary batteries with high energy density.

In such a nonaqueous electrolyte secondary battery, a lithium transition metal composite oxide such as LiCoO ₂ is generally used for the positive electrode, and a carbon material capable of occluding and releasing lithium is used for the negative electrode. In addition, as the nonaqueous electrolytic solution, a solution obtained by dissolving an electrolyte made of a lithium salt such as LiBF ₄ or LiPF _{6 in} an organic solvent such as ethylene carbonate or diethyl carbonate is used.

  In recent years, such a non-aqueous electrolyte secondary battery has been used as a power source for various portable devices, and a non-aqueous electrolyte secondary battery having a higher energy density has been 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 is large in weight and has a small number of reaction electrons, so that the capacity per unit weight is sufficiently increased. It was difficult.

  For this reason, it is indispensable to develop a positive electrode material capable of obtaining a high energy density in capacity. In recent years, studies have been conducted using sulfur alone as a positive electrode material. Sulfur alone has a large theoretical capacity of 1675 mAh / g, and is one of the promising positive electrode materials for the next generation secondary battery. In these studies, charge and discharge characteristics have been studied using lithium metal for the negative electrode and sulfur alone and a sulfur compound for the positive electrode. A battery using a cathode material containing a sulfide or polysulfide using a carbon material for the positive electrode and lithium metal for the negative electrode has also been proposed (Patent Document 1).

However, when lithium metal is used for the negative electrode, there is a problem that dendrid crystals are deposited by charging and discharging, causing a short circuit. In addition, when sulfur is used for the positive electrode, when the battery is manufactured, the positive electrode sulfur is in a charged state, and since lithium metal is used for the negative electrode, the battery is manufactured in a charged state. Therefore, when the negative electrode / separator / positive electrode is wound up in the air, the lithium metal in the negative electrode reacts with moisture in the air, so it is necessary to perform it in a place where there is little moisture in the air. There are problems such as requiring equipment such as. Therefore, it is desirable to produce a battery with both the positive electrode and the negative electrode in a discharged state without using lithium metal for the negative electrode.
Special table 2003-522383 gazette International Publication No. 01/029912 Pamphlet

  An object of the present invention is to provide a non-aqueous electrolyte secondary battery that can be easily manufactured and has a high capacity density.

The present invention is mainly composed of a carbon material, and during charging, sulfide ions are oxidized and sulfur is deposited on the surface of the positive electrode. During discharge, the deposited sulfur is reduced to become sulfide ions. And a non-aqueous electrolyte secondary battery comprising a negative electrode containing a negative electrode active material mainly composed of silicon and a non-aqueous electrolyte, the non-aqueous electrolyte is represented by Li ₂ S _x (1 ≦ x ≦ 8). It is characterized by containing lithium sulfide .

In the present invention, the nonaqueous electrolyte contains lithium sulfide represented by Li ₂ S _x (1 ≦ x ≦ 8). This lithium sulfide is a compound in a discharge state when sulfur is used as a positive electrode active material. In the present invention, since the negative electrode active material mainly composed of silicon is used, the battery can be manufactured in the air. Therefore, the nonaqueous electrolyte secondary battery of the present invention can be easily manufactured.

  In the nonaqueous electrolyte secondary battery of the present invention, the following charging reaction and discharging reaction occur in the negative electrode and the positive electrode.

Charge reaction Negative electrode: Si + 4.4Li ⁺ + 4.4e ⁻ → Li _4.4 Si
Positive electrode: 2Li ⁺ + S ²⁻ → S + 2Li ⁺ + 2e ⁻
Discharge reaction Negative electrode: Li _4.4 Si → Si + _4.4 Li ⁺ + 4.4e ⁻
Positive electrode: S + 2Li ⁺ + 2e ⁻ → 2Li ⁺ + S ²⁻
As described above, during charging, sulfide ions are oxidized and sulfur is deposited on the surface of the positive electrode, and during discharging, the precipitated sulfur is reduced to sulfide ions.

  In the present invention, lithium sulfide is preferably dissolved in a non-aqueous electrolyte and contained in a solid state. When lithium sulfide dissolved in the nonaqueous electrolyte reacts and is consumed during the charging reaction, the solid state lithium sulfide dissolves in the nonaqueous electrolyte and is used for the charging reaction. Therefore, the capacity of the battery can be increased by including solid state lithium sulfide. The solid state lithium sulfide is contained in the nonaqueous electrolyte in the form of a precipitate or the like.

The sulfur in the lithium sulfide used in the present invention is preferably in a discharged state as much as possible. Therefore, it is desirable that the value of _x in Li ₂ S _x is as small as possible, and it is considered most desirable that x = 1 in a fully discharged state. In the case of x = 1, the theoretical charge / discharge capacity density is 1675 mAh / g per gram of sulfur, which is very large compared to 150 mAh / g of LiCoO ₂ currently used as the positive electrode active material.

  As the organic solvent for the non-aqueous electrolyte used in the present invention, a solvent capable of dissolving the lithium sulfide is 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.

  Examples of the cyclic ether include 1,3-dioxolane, 2-methyl-1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1, Examples include at least one selected from 4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineol, and crown ether.

  As chain ether, 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, pentyl phenyl ether , Methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-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, tetraethylene At least one can be cited is selected from glycol dimethyl ether.

  As the room temperature molten salt having a melting point of 60 ° C. or lower, a quaternary ammonium salt is preferably used. It is known that the quaternary ammonium salt is excellent in reduction resistance and does not react with lithium metal compared to other room temperature molten salts such as imidazolium salt and pyrazolium salt. Room temperature molten salts such as imidazolium salts and pyrazolium salts have low reduction resistance and react with lithium metal, so that it is considered undesirable to use them as electrolytes for lithium ion batteries.

The quaternary ammonium salts include trimethylpropylammonium bis (trifluoromethylsulfonyl) imide ((CH ₃ ) ₃ N ⁺ (C ₃ H ₇ ) N ⁻ (SO ₂ CF ₃ ) ₂ ), trimethyloctyl ammonium bis (Trifluoromethylsulfonyl) imide ((CH ₃ ) ₃ N ⁺ (C ₈ H ₁₇ ) N ⁻ (SO ₂ CF ₃ ) ₂ ), trimethylallylammonium bis (trifluoromethylsulfonyl) imide ((CH ₃ ) ₃ N ⁺ (Allyl) N ⁻ (SO ₂ CF ₃ ) ₂ ), trimethylhexylammonium bis (trifluoromethylsulfonyl) imide ((CH ₃ ) ₃ N ⁺ (C ₆ H ₁₃ ) N ⁻ (SO ₂ CF ₃ ) _2), methoxymethyl trimethylammonium bis (trifluoromethylsulfonyl) imide ^{_{((CH 3) 3 N +}} (CH 2 OCH 3 ^{_{N - (SO 2 CF 3)}} 2), trimethylethyl ammonium 2,2,2-trifluoro -N- (trifluoromethylsulfonyl) 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- (trifluoromethylsulfonyl) 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 )), tetraethylammonium · 2,2,2-trifluoro-N- (trifluoromethylsulfonyl) acetamide ((C ₂ H ₅ ) ₄ N ⁺ (CF ₃ CO) N ⁻ (SO ₂ CF ₃ )), triethylmethylammonium · 2,2,2-trifluoro-N- (trifluoromethylsulfonyl) acetamide ((C ₂ H ₅ ) ₃ N ⁺ (CH ₃ ) (CF ₃ CO) N ⁻ (SO ₂ CF ₃ )).

  As the organic solvent for the non-aqueous electrolyte used in the present invention, those having a high solubility of lithium sulfide are preferably used. Specifically, the solubility of lithium sulfide is preferably 0.5 mol / liter or more, more preferably 1 mol / liter or more, and particularly preferably 5 mol / liter or more.

Further, as the electrolyte (solute), such as a lithium salt, a non-aqueous electrolyte secondary battery, there can be used those which are generally used as an electrolyte (solute), for _{example, LiBF 4, LiPF 6, LiCF} 3 SO 3 , LiC ₄ F ₉ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (COCF ₃ ), LiAsF ₆ , lithium difluoro (oxalato) borate At least one selected from can be used. Lithium difluoro (oxalato) borate has the structural formula shown in the following (Chemical Formula 1).

本発明における負極は、ケイ素を主体とする負極活物質を含んでいる。ケイ素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の上にＣＶＤ法、スパッタリング法、蒸着法、溶射法などにより堆積させたシリコン薄膜が挙げられる。シリコン薄膜としては、非晶質シリコン薄膜、微結晶シリコン薄膜などが好ましく用いられる。また、本発明における負極活物質として、ケイ素粉末を用いてもよい。このようなケイ素粉末を用いる場合には、ケイ素粉末とバインダーを含むスラリーを集電体上に塗布して電極を作製することができる。

The negative electrode in the present invention contains a negative electrode active material mainly composed of silicon. Examples of the negative electrode active material mainly composed of silicon include a silicon thin film deposited on a current collector made of metal foil by a CVD method, a sputtering method, a vapor deposition method, a thermal spraying method, or the like. As the silicon thin film, an amorphous silicon thin film, a microcrystalline silicon thin film, or the like is preferably used. Moreover, you may use a silicon powder as a negative electrode active material in this invention. When such silicon powder is used, an electrode can be produced by applying a slurry containing silicon powder and a binder onto a current collector.

  When a silicon thin film is used as the negative electrode active material, the electrode disclosed in Patent Document 2 is preferably used. Specifically, an electrode in which the silicon thin film on the current collector is separated into a columnar shape by a cut in the thickness direction, an electrode in which a current collector component (such as a copper component) diffuses inside the silicon thin film, and the surface is roughened And an electrode using a silicon thin film formed on the current collector.

  Moreover, in this invention, having silicon as a main component means that 50% or more of silicon is contained. Examples of the alloy thin film include a silicon alloy thin film containing cobalt, iron, zirconium, zinc and the like.

The positive electrode material in the present invention has electronic conductivity, and examples thereof include a carbon material. Specifically, at least one selected from carbon black such as acetylene black and ketjen black, activated carbon, and graphite can be used. In the present invention, as described above, it is considered that sulfide ions in the electrolyte are oxidized and deposited on the surface of the positive electrode during charging. Therefore, it is considered that the use of a carbon material having a large specific surface area for the positive electrode increases the amount of sulfur to be deposited and increases the capacity density of the battery. For this reason, a carbon material having a specific surface area of 10 m ² / g or more is particularly preferably used.

The positive electrode in the present invention can be produced by applying a slurry containing a positive electrode material and a binder such as a carbon material, on a current collector such as an aluminum foil.

  According to the present invention, since the battery can be manufactured in a discharged state, the battery can be easily manufactured. Moreover, since the non-aqueous electrolyte contains lithium sulfide involved in the charge / discharge reaction, a non-aqueous electrolyte secondary battery having a high capacity density can be obtained.

  In the present invention, since the negative electrode active material mainly composed of silicon is used, excellent cycle characteristics can be obtained as compared with the case where conventional lithium metal is used for the negative electrode material.

  Hereinafter, the present invention will be described by way of specific examples. However, the present invention is not limited to the following examples, and can be appropriately modified and implemented without departing from the scope of the present invention.

Example 1
[Production of non-aqueous electrolyte]
LiN (SO ₂ CF ₃ ) ₂ was added to a solvent in which 1,2-dimethoxyethane and 1,3-dioxolane were mixed at a volume ratio of 90:10 to a concentration of 0.5 mol / liter. Li ₂ S was further added to this so that it might become 5 mol / liter, it was made the supersaturated state, and it used as a nonaqueous electrolyte.

[Production of positive electrode]
The conductive agent acetylene black (specific surface area 70 m ² / g) is 90% by weight of the whole positive electrode, 5% by weight of polytetrafluoroethylene (PTFE) as a binder and carboxymethyl cellulose (CMC) as a thickener. ) Mixed with 5% by weight, added water for 15 minutes and kneaded to prepare a slurry. The prepared slurry was applied onto an electrolytic aluminum foil using a doctor blade method and dried at 50 ° C. using a hot plate. This was cut into a size of 2 cm × 2 cm and further vacuum dried at 100 ° C. to obtain a positive electrode.

[Production of test cell]
As shown in FIG. 1, a test cell was prepared using the above positive electrode 11 as a working electrode under an inert atmosphere, and using lithium metal for the negative electrode 12 and the reference electrode 13 as counter electrodes. A separator 15 was disposed between the positive electrode 11 and the negative electrode 12, and 2 ml of the nonaqueous electrolyte 14 was injected into the test cell 10 to obtain a test cell of Example 1.

(Charge / discharge test)
For the test cell was charged at a charging current 2.5 .mu.A / cm ² until the charge cutoff potential 2.8V (vs.Li/Li ^+), then the discharge current 2.5 .mu.A / cm ² at the discharge cutoff potential of 1.5V ( vs. Li ^/ Li ⁺ ). FIG. 2 shows the relationship between the charge / discharge capacity density and the potential at this time. In FIG. 2, the relationship between the potential at the time of charging and the capacity density per 1 g of the electrode is indicated by a broken line as a charging curve. Further, the relationship between the potential during discharge and the capacity density per 1 g of electrode is shown as a discharge curve by a solid line. In addition, per 1 g of electrode means per 1 g of the total weight of the conductive agent, the binder, and the thickener.

  As is clear from FIG. 2, the initial discharge capacity density was 84.5 mAh / g, and the subsequent charge capacity density was 136 mAh / g, confirming that charge and discharge can be performed.

(Example 2)
[Production of non-aqueous electrolyte]
A non-aqueous electrolyte was produced in the same manner as in Example 1.

(Production of negative electrode)
An amorphous silicon thin film was formed by sputtering on an electrolytic copper foil roughened by electrolytic treatment of the surface. The thickness of the thin film was 0.5 μm. This was cut into a size of 2 cm × 2 cm and vacuum-dried at 110 ° C. to make a negative electrode.

[Production of test cell]
A test cell was produced in the same manner as in Example 1 except that the negative electrode 12 was used as the working electrode and lithium metal was used as the positive electrode 11 and the reference electrode 13 as the counter electrode.

(Charge / discharge test)
For the test cell was charged at a charging current 0.05 mA / cm ² until the charge cutoff potential 0V (vs.Li/Li ^+), then the discharge current 0.05 mA / cm ² at the discharge cutoff potential 1.5V (vs. Li / Li ⁺ ) was discharged and a charge / discharge test was conducted. The result is shown in FIG.

  In FIG. 3, the relationship between the potential during discharge and the capacity density per gram of electrode is shown by a solid line as a discharge curve. The relationship between the potential during charging and the capacity density per gram of electrode is shown as a charging curve by a broken line. Here, per 1 g of electrode means per 1 g of active material (silicon thin film).

  As is clear from FIG. 3, the initial charge capacity density was 1647 mAh / g, the initial discharge capacity density was 1183 mAh / g, and it was confirmed that charging / discharging was performed.

From the results of Example 1 and Example 2, it is clear that charging and discharging can be performed when an electrolyte containing Li ₂ S is used in combination of a positive electrode mainly composed of a carbon material and a negative electrode mainly composed of silicon. is there.

The perspective view which shows the test cell produced in the Example according to this invention. The figure which shows the initial stage charge / discharge characteristic in the test cell of Example 1. FIG. The figure which shows the initial stage charge / discharge characteristic in the test cell of Example 2. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Test cell container 11 ... Positive electrode 12 ... Negative electrode 13 ... Reference electrode 14 ... Nonaqueous electrolyte

Claims (3)

Mainly composed of carbon material, during charging, sulfide ions are oxidized and sulfur is deposited on the surface of the positive electrode, and during discharge, the deposited sulfur is reduced to sulfide ions , In a non-aqueous electrolyte secondary battery comprising a negative electrode containing a negative electrode active material as a main component and a non-aqueous electrolyte,
A non-aqueous electrolyte secondary battery comprising lithium sulfide represented by Li ₂ S _x (1 ≦ x ≦ 8) in the non-aqueous electrolyte.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the lithium sulfide is dissolved in the non-aqueous electrolyte and is also contained in a solid state.   The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the negative electrode active material is a silicon thin film or a silicon alloy thin film formed by being deposited on a current collector.

Images