JP4847675B2 - Nonaqueous electrolyte secondary battery and electrolyte used therefor - Google Patents

Description

  The present invention mainly relates to a secondary battery using a non-aqueous electrolyte containing lithium bisfluorosulfonylimide as a lithium salt.

  In recent years, information electronic devices such as personal computers, mobile phones and PDAs, and audiovisual electronic devices such as video camcorders and mini disc players are rapidly becoming smaller and lighter and cordless. Accordingly, there is a growing demand for secondary batteries having high energy density as power sources for driving these electronic devices. Under such circumstances, the practical application of non-aqueous electrolyte secondary batteries having high energy density, which cannot be achieved with conventional secondary batteries such as lead storage batteries, nickel cadmium storage batteries and nickel metal hydride storage batteries, is being promoted.

In a non-aqueous electrolyte secondary battery represented by a lithium ion secondary battery or a lithium ion polymer secondary battery, as a positive electrode active material, an average discharge potential is in a range of 3.5 V to 4.0 V with respect to a lithium metal potential. Some transition metal oxides such as lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate (LiMn ₂ O ₄ ), and solid solution materials (LiCo _x Ni _y Mn _z) incorporating a plurality of transition metals O ₂ , Li (Co _a Ni _b Mn _c ) ₂ O ₄ ) or the like is used alone or in combination. When these active materials are mixed with a conductive agent, a binder, or the like, and then coated on a current collector made of aluminum, titanium, stainless steel, or the like, and rolled, a positive electrode is obtained.

  On the other hand, a carbon material that occludes and releases lithium is generally used for the negative electrode. As the carbon material, artificial graphite, natural graphite, mesophase fired body made from coal / petroleum pitch, non-graphitizable carbon, and the like are used alone or in combination. When these carbon materials are mixed with a binder or the like, and then coated on a current collector made of copper, iron, nickel or the like and rolled, a negative electrode is obtained.

  In general, a negative electrode using a graphite material has a high voltage and a flat voltage because the average potential for releasing lithium is about 0.2 V lower than that of a lithium metal compared to a negative electrode using non-graphitizable carbon. Many graphite materials are used in the field where properties are desired.

The non-aqueous electrolyte withstands the oxidizing atmosphere of the positive electrode that discharges at a high potential of 3.5 V to 4.0 V with respect to the lithium metal potential as described above, and the reducing atmosphere of the negative electrode that charges and discharges at a potential close to lithium. It is desirable to withstand At present, ethylene carbonate (hereinafter referred to as EC) having a high dielectric constant, diethyl carbonate (hereinafter referred to as DEC), dimethyl carbonate (hereinafter referred to as DMC), ethyl methyl carbonate (hereinafter referred to as EMC), and the like. A solution obtained by dissolving lithium hexafluorophosphate (LiPF ₆ ) in a non-aqueous solvent combined with a low-viscosity chain carbonate is used.

However, since this type of non-aqueous electrolyte contains a chain carbonate having a low viscosity and a boiling point of around 100 ° C., the vapor pressure at a high temperature is increased. Therefore, the battery package may swell. In addition, since LiPF ₆ that is thermally unstable and easily hydrolyzed is used as a solute, gas generation or the like occurs inside the battery, and the swelling of the package is easily promoted.

Therefore, alternative lithium salts of LiPF ₆ have been studied. For example, LiBF ₄ , lithium bisperfluoromethylsulfonylimide (LiN (SO ₂ CF ₃ ) ₂ , hereinafter referred to as LiTFSI), which has higher thermal stability than LiPF _6. Lithium bisperfluoroethylsulfonylimide (LiN (SO ₂ C ₂ F ₅ ) ₂ , hereinafter referred to as LiBETI) and the like lower the ionic conductivity of the non-aqueous electrolyte, so that the discharge characteristics of the battery are reduced. To do. In addition, LiTFSI also has a problem of corroding aluminum frequently used as a positive electrode current collector at a high potential of 3.7 V or higher with respect to the potential of lithium metal. When LiBETI is used, the corrosivity is improved, but since the molecular weight is large, there is a strong tendency to increase the viscosity of the nonaqueous electrolyte.

On the other hand, in recent years, lithium bisfluorosulfonylimide has been developed as an imide salt (see, for example, Patent Document 1).
In order to prevent an increase in vapor pressure at a high temperature, a low-viscosity, low-boiling chain carbonate such as propylene carbonate (hereinafter referred to as PC) or γ-butyrolactone (hereinafter referred to as GBL) has a high boiling point. Changing to a solvent is also under consideration. However, GBL and LiPF ₆ react at a high temperature to increase the polarization resistance of the battery, so that the charge / discharge characteristics are degraded.
JP-T 8-511274

  The present invention minimizes swelling of a non-aqueous electrolyte secondary battery that leads to damage to equipment during exposure to high-temperature environments or storage, and a non-aqueous electrolyte or secondary battery that is stable even at high temperatures or storage And a non-aqueous electrolyte secondary battery having the above-described properties but having the same characteristics as those of the prior art.

The present invention comprises a chargeable / dischargeable positive electrode, a negative electrode that occludes and releases lithium, a diaphragm that electronically shields the positive electrode and the negative electrode, and a nonaqueous electrolyte, and the positive electrode has an average discharge potential of lithium metal. A positive electrode active material having a voltage of 3.5 V to 4.0 V with respect to the potential, and a current collector made of aluminum. The nonaqueous electrolyte is made of a nonaqueous solvent and a solute, and the nonaqueous solvent is made of a lactone. The solute is represented by the formula (1): (F—O ₂ S—N—SO ₂ —F) Li and lithium bisfluorosulfonylimide represented by LiPF _m (C _k F _{2k + 1} ) _6-m ( Another containing at least one fluorine selected from 0 ≦ m ≦ 6, 1 ≦ k ≦ 2) and LiBF _n (C _j F _{2j + 1} ) _4-n (0 ≦ n ≦ 4, 1 ≦ j ≦ 2) Do and a lithium salt of Ri, the lithium bis fluoro sulfonyl Louis bromide before Serial Another ratio of lithium salt (lithium bis-fluoro sulfonyl Louis bromide: said further lithium salt), in a molar ratio of 9: 1 to 5: about 5 der Ru nonaqueous electrolyte secondary battery. However, m, n, j, and k are integers.

It is preferable that the non-aqueous electrolyte further includes an additive that forms a film on the positive electrode and / or the negative electrode.
The additive is preferably at least one selected from the group consisting of vinylene carbonate, vinyl ethylene carbonate, phenyl ethylene carbonate, and propane sultone.

It is preferable that the non-aqueous solvent further contains ethylene carbonate and / or propylene carbonate.
The present invention is particularly effective when it is necessary to charge the positive electrode at a potential of 3.7 V or higher with respect to the potential of the lithium metal.
The lactone preferably contains at least γ-butyrolactone.

The present invention also includes a non-aqueous solvent and a solute, the non-aqueous solvent is a lactone, the lactone is γ-butyrolactone, and the solute is represented by the formula (1): (F—O ₂ S—N—SO _2- F) Lithium bisfluorosulfonylimide represented by LiPF _m (C _k F _{2k + 1} ) _6-m (0 ≦ m ≦ 6, 1 ≦ k ≦ 2) and LiBF _n (C _j F _{2j +1)} Ri Do and a _{4-n (0 ≦ n ≦} 4,1 ≦ j ≦ 2) than another lithium salt containing at least one fluorine selected, said another lithium salt with lithium bis fluoro sulfonyl Louis bromide ratio of (the lithium bis fluoro sulfonyl Louis bromide: said further lithium salt), in a molar ratio of 9: 1 to 5: about 5 der Ru non-aqueous electrolyte secondary cell electrolyte.

  According to the present invention, there is provided a non-aqueous electrolyte secondary battery that minimizes battery swelling that leads to damage to equipment when exposed to high temperatures and during storage, and that is safe and has the same characteristics as conventional batteries. It is possible to provide.

In the non-aqueous electrolyte secondary battery of the present invention, a lactone is used as a solvent for the non-aqueous electrolyte,
Formula (1): Lithium bisfluorosulfonylimide (hereinafter referred to as LiFSI) represented by (F—O ₂ S—N—SO ₂ —F) Li and LiPF _m (C _k F _{2k + 1} ) ₆₋ Contains at least one fluorine selected from _m (0 ≦ m ≦ 6, 1 ≦ k ≦ 2) and LiBF _n (C _j F _{2j + 1} ) _4-n (0 ≦ n ≦ 4, 1 ≦ j ≦ 2) And a ratio of lithium bisfluorosulfonylimide to another lithium salt (lithium bisfluorosulfonylimide: another lithium salt) is used. , in a molar ratio of 9: 1 to 5: Ru 5 der. Thus, by using a lactone having a high melting point and a low vapor pressure as a non-aqueous solvent, and using LiFSI and the above-described another lithium salt containing fluorine, gas generation during high-temperature exposure and gas generation during storage can be achieved. Thus, it is possible to obtain a nonaqueous electrolyte secondary battery having characteristics equivalent to those of conventional batteries.

Since the anion molecule generated when LiFSI is ionically dissociated is smaller in size than other lithium imide salts, its viscosity is lower than that of non-aqueous electrolytes containing other imide salts (such as LiBETI) at the same concentration. It can be suppressed. Further, in the case of LiFSI, since the sulfonyl group shields lithium ions, it is easy to generate lithium ions by ion dissociation even when compared with LiPF ₆ or the like. Therefore, it is considered that the ion concentration in the non-aqueous electrolyte increases and the ionic conductivity increases.

In the present invention, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ) having an average discharge potential in the range of 3.5 V to 4.0 V with respect to the potential of the lithium metal is applied to the positive electrode of the nonaqueous electrolyte secondary battery. ), Transition metal oxides such as lithium manganate (LiMn ₂ O ₄ ), and solid solution materials incorporating a plurality of transition metals (LiCo _x Ni _y Mn _z O ₂ , Li (Co _a Ni _b Mn _c ) ₂ O ₄ ) Etc. are particularly effective when used alone or in combination.

  Examples of the lactone that can be used in the present invention include γ-butyrolactone (GBL), γ-valerolactone (GVL), α-methyl-γ-butyrolactone, and the like. These may be used alone or in combination of two or more. Of these, it is particularly preferable to use GBL.

By using a current collector made of aluminum for the positive electrode and using another lithium salt containing fluorine together with LiFSI as the solute of the non-aqueous electrolyte, corrosion of the current collector is remarkably suppressed . Although the mechanism of corrosion inhibition is not clear, it is considered that another lithium salt containing fluorine generates a small amount of fluorine ions and forms a coating of AlF ₃ on the current collector.

Other lithium salts are LiPF _m (C _k F _{2k + 1} ) _6-m (0 ≦ m ≦ 6, 1 ≦ k ≦ 2) and LiBF _n (C _j F _{2j + 1} ) _4-n (0 ≦ n ≦ 4,1 ≦ j ≦ 2) Ru with at least one member selected from the group consisting of. Of these, LiPF ₆ and LiBF ₄ are preferably used.

Ratio of LiFSI with another lithium salt, in a molar ratio, (LiFSI) :( another lithium salt) = 9: 1 to 5: Ru 5 der.
The solute concentration contained in the nonaqueous electrolyte is preferably 0.5 to 1.5 mol / L, but is not particularly limited.

  When a graphite material is used for the negative electrode, the nonaqueous solvent made of lactone has a property of being easily reductively decomposed by the negative electrode. Therefore, it is preferable to add an additive for forming a film on the negative electrode to the non-aqueous electrolyte. Moreover, since the component which decomposes | disassembles on a positive electrode may be contained in a nonaqueous electrolyte, you may add the additive which forms a film on a positive electrode.

  In addition to graphite materials such as artificial graphite and natural graphite, mesophase fired bodies made from coal / petroleum pitch and carbon materials such as non-graphitizable carbon can be used for the negative electrode. In addition, alloy materials such as Si, Si—Ni alloy, and Sn—Ni alloy can be used alone or together with the carbon material for the negative electrode.

  As an additive for forming a film on the positive electrode and / or the negative electrode, it is preferable to use a cyclic compound, phenylethylene carbonate (hereinafter referred to as PhEC), propane sultone (hereinafter referred to as PS), or the like. These may be used alone or in combination of two or more. Examples of the cyclic compound include vinylene carbonate (hereinafter referred to as VC), vinyl ethylene carbonate (hereinafter referred to as VEC), and the like. Of these, VC and VEC are particularly effective.

  Since VEC forms a denser film on the electrode than VC, the effect of suppressing side reactions is high, but the rate characteristics and low-temperature characteristics are lower than when VC is used. PS is thought to give performance in between VC and VEC.

  The amount of the additive is preferably 10 parts by weight or less, more preferably 5 parts by weight or less, per 100 parts by weight of the nonaqueous solvent. When there is too much quantity of an additive, since a film is formed excessively, charging / discharging reaction is inhibited. On the other hand, in order to sufficiently obtain the effect of the additive, it is preferable to use at least 0.3 part by weight or more of the additive per 100 parts by weight of the non-aqueous solvent.

  From the viewpoint of improving the wettability between the nonaqueous electrolyte and the electrode or separator, the nonaqueous solvent can contain a solvent other than the lactone. The solvent other than lactone is not particularly limited, but is preferably an aprotic solvent, and cyclic carbonates, chain carbonates, cyclic ethers, chain ethers, chain carboxylic acid esters, and the like can be preferably used. A compound having a perfluoro group can also be preferably used as a solvent other than lactone.

  The proportion of lactone in the non-aqueous solvent is preferably 50 to 100% by weight, and more preferably 50 to 70% by weight. The non-aqueous solvent gives better characteristics when it contains at least a cyclic carbonate rather than lactone alone.

  The proportion of the cyclic carbonate in the non-aqueous solvent is preferably 50% by weight or less. The proportion of the chain carbonate in the non-aqueous solvent is preferably 20% by weight or less.

  In particular, as a nonaqueous solvent having a preferred composition, for example, a nonaqueous solvent comprising 50 to 70% by weight of a lactone, 20 to 30% by weight of a cyclic carbonate, and 5 to 30% by weight of a chain carbonate can be exemplified.

  EC, PC or the like is preferably used for the cyclic carbonate, and EMC, DMC, DEC or the like is preferably used for the chain carbonate.

The present invention can be applied to any shape non-aqueous electrolyte secondary battery such as a cylindrical shape, a square shape, a laminate shape, and a coin shape. Further, the nonaqueous electrolyte can be combined with a polymer material and used as a gel electrolyte. By using such a gel electrolyte, a lithium ion polymer secondary battery can be obtained.
The present invention will be specifically described below based on examples.
[ Reference Example 1]

(A) Production of positive electrode 100 parts by weight of lithium cobaltate (LiCoO ₂ ) as an active material, 3 parts by weight of acetylene black as a conductive agent, and 4 parts by weight of polyvinylidene fluoride (hereinafter referred to as PVdF) as a binder, An appropriate amount of N-methyl-2-pyrrolidone (hereinafter referred to as NMP) was added and mixed to obtain a paste-like positive electrode mixture. PVdF was previously dissolved in NMP and then mixed with other components. This paste-like positive electrode mixture was applied to both surfaces of a current collector made of a titanium foil, then dried, and the whole was rolled to obtain a positive electrode.

(B) Preparation of negative electrode 3 parts by weight of an aqueous dispersion of styrene butadiene rubber as a binder was added to 100 parts by weight of non-graphitizable carbon (Carbotron P manufactured by Kureha Chemical Industry Co., Ltd.) as an active material. To obtain a paste-like negative electrode mixture. This paste-like negative electrode mixture was applied to both sides of a current collector made of copper foil, then dried and the whole was rolled to obtain a negative electrode.

(C) Preparation of non-aqueous electrolyte GBL was used alone as a non-aqueous solvent. LiFSI was used alone as the solute. Here, a nonaqueous electrolyte was prepared by dissolving LiFSI in GBL at a concentration of 1 mol / L.

(D) Production of battery A prismatic lithium ion secondary battery as shown in FIG. 1 was assembled.
First, the positive electrode and the negative electrode were wound into an oval shape through a separator made of a polyethylene microporous film having a thickness of 25 μm, thereby constituting the electrode plate group 1. And in order to reduce the water | moisture content in an electrode group, the electrode group was dried at 60 degreeC for 12 hours in the vacuum dryer, and the moisture content in an electrode group was 50 ppm or less.

  A positive electrode lead 2 and a negative electrode lead 3 were welded to the positive electrode and the negative electrode, respectively. An insulating ring made of polyethylene resin (not shown) was attached to the upper part of the electrode plate group 1 and inserted into an aluminum rectangular thin battery case 4 as shown in FIG. The other end of the positive electrode lead 2 was spot welded to the aluminum sealing plate 5. The other end of the negative electrode lead 3 was spot welded to the lower part of the nickel negative electrode terminal 6 at the center of the sealing plate 5 (not welded in FIG. 1).

The open end of the battery case 4 and the peripheral edge of the sealing plate 5 were laser welded, and a predetermined amount of non-aqueous electrolyte was injected from the injection port. Finally, the inlet was closed with an aluminum plug 7 and sealed by laser welding.
The dimensions of the battery thus obtained were 30 mm in width, 48 mm in total height, and 5.3 mm in depth. The design capacity of the battery was 800 mAh.

The obtained battery was repeatedly charged and discharged at an atmospheric temperature of 20 ° C. Here, a constant current charge was performed up to a battery voltage of 4.2 V at a charge current of 0.16 A, and after a pause of 20 minutes, a cycle of discharging at a discharge current of 0.16 A and a final voltage of 3.0 V was repeated. Thereafter, the battery was charged to a battery voltage of 4.1 V with a charging current of 0.16 A. This battery was referred to as the battery of Reference Example 1.
[ Reference Example 2]

A nonaqueous electrolyte similar to that of Reference Example 1 was prepared, except that 2 parts by weight of VC was added as an additive to 100 parts by weight of GBL. A battery was prepared in the same manner as in Reference Example 1 except that this non-aqueous electrolyte was used and flaky graphite was used as the negative electrode active material instead of non-graphitizable carbon. This battery was referred to as the battery of Reference Example 2.
[Example 1 ]

As the non-aqueous solvent, GBL was used alone. For the solute, LiFSI and LiPF ₆ were used in a molar ratio of 7: 3. Here, a nonaqueous electrolyte was prepared by dissolving LiFSI in GBL at a concentration of 0.7 mol / L and LiPF ₆ at a concentration of 0.3 mol / L. Using this non-aqueous electrolyte, a battery similar to that of Reference Example 2 was produced except that an aluminum foil was used instead of the titanium foil as the positive electrode current collector. This battery was referred to as the battery of Example 1 .
[Example 2 ]

As the non-aqueous solvent, a mixed solvent composed of 30% by weight of EC and 70% by weight of GBL was used. For the solute, LiFSI and LiPF ₆ were used in a molar ratio of 7: 3. Here, LiFSI is dissolved in the above mixed solvent at a concentration of 0.7 mol / L and LiPF ₆ at a concentration of 0.3 mol / L, and 2 parts by weight of VC is added as an additive to 100 parts by weight of the mixed solvent. A non-aqueous electrolyte was prepared. A battery was prepared in the same manner as in Example 1 except that this nonaqueous electrolyte was used. This battery was referred to as the battery of Example 2 .
[ Reference Example 3 ]

A nonaqueous electrolyte similar to that of Reference Example 1 was prepared, except that 2 parts by weight of VEC was added as an additive to 100 parts by weight of GBL. A battery was prepared in the same manner as in Reference Example 1 except that this non-aqueous electrolyte was used and flaky graphite was used as the negative electrode active material instead of non-graphitizable carbon. This battery was referred to as the battery of Reference Example 3 .
[ Reference Example 4 ]

A nonaqueous electrolyte similar to that of Reference Example 1 was prepared, except that 2 parts by weight of PS was added as an additive to 100 parts by weight of GBL. A battery was prepared in the same manner as in Reference Example 1 except that this non-aqueous electrolyte was used and flaky graphite was used as the negative electrode active material instead of non-graphitizable carbon. This battery was referred to as the battery of Reference Example 4 .
[Example 3 ]

As the non-aqueous solvent, a mixed solvent composed of 30% by weight of EC and 70% by weight of GBL was used. For the solute, LiFSI and LiPF ₆ were used in a molar ratio of 7: 3. Here, LiFSI is dissolved in the above mixed solvent at a concentration of 0.7 mol / L and LiPF ₆ at a concentration of 0.3 mol / L, and 2 parts by weight of VEC is added as an additive to 100 parts by weight of the mixed solvent. Thus, a non-aqueous electrolyte was prepared. A battery similar to that of Example 1 was produced except that this non-aqueous electrolyte was used, and this battery was referred to as the battery of Example 3 .
[Example 4 ]

As the non-aqueous solvent, a mixed solvent composed of 30% by weight of EC and 70% by weight of GBL was used. For the solute, LiFSI and LiPF ₆ were used in a molar ratio of 7: 3. Here, LiFSI is dissolved in the above mixed solvent at a concentration of 0.7 mol / L and LiPF ₆ at a concentration of 0.3 mol / L, and 2 parts by weight of PS is added as an additive to 100 parts by weight of the mixed solvent. A non-aqueous electrolyte was prepared. A battery was prepared in the same manner as in Example 1 except that this nonaqueous electrolyte was used. This battery was referred to as the battery of Example 4 .
[ Reference Example 5 ]

A battery similar to that of Example 1 was produced except that GVL was used instead of GBL as the non-aqueous solvent. This battery was referred to as the battery of Reference Example 5 .
[ Reference Example 6 ]

A battery was prepared in the same manner as in Reference Example 1 except that a mixed solvent composed of 30% by weight of PC and 70% by weight of GVL was used as the non-aqueous solvent instead of GBL. This battery was referred to as battery of Reference Example 6 .
[Example 5 ]

As the non-aqueous solvent, GBL was used alone. LiFSI and LiBF ₄ were used in combination at a molar ratio of 7: 3 as the solute. Here, a nonaqueous electrolyte was prepared by dissolving LiFSI in GBL at a concentration of 0.7 mol / L and LiBF ₄ at a concentration of 0.3 mol / L. A battery was prepared in the same manner as in Example 1 except that this nonaqueous electrolyte was used. This battery was referred to as the battery of Example 5 .
<< Comparative Example 1 >>

As the non-aqueous solvent, a mixed solvent composed of 25% by weight of EC and 75% by weight of EMC was used. LiPF ₆ was used alone as the solute. Here, LiPF ₆ was dissolved in the above mixed solvent at a concentration of 1 mol / L, and 2 parts by weight of VC was added as an additive to 100 parts by weight of the mixed solvent to prepare a nonaqueous electrolyte. A battery was prepared in the same manner as in Example 1 except that this nonaqueous electrolyte was used. This battery was referred to as the battery of Comparative Example 1.
<< Comparative Example 2 >>

As the non-aqueous solvent, GBL was used alone. LiPF ₆ was used alone as the solute. Here, LiPF ₆ was dissolved in GBL at a concentration of 1 mol / L, and 2 parts by weight of VC was added as an additive to 100 parts by weight of GBL to prepare a nonaqueous electrolyte. A battery was prepared in the same manner as in Example 1 except that this nonaqueous electrolyte was used. This battery was referred to as the battery of Comparative Example 2.
<< Comparative Example 3 >>

As the non-aqueous solvent, GBL was used alone. LiBETI was used alone as the solute. Here, LiBETI was dissolved in GBL at a concentration of 1 mol / L, and 2 parts by weight of VC was added as an additive to 100 parts by weight of GBL to prepare a nonaqueous electrolyte. A battery was prepared in the same manner as in Example 1 except that this nonaqueous electrolyte was used. This battery was referred to as the battery of Comparative Example 3.
<< Comparative Example 4 >>

As the non-aqueous solvent, a mixed solvent composed of 30% by weight of EC and 70% by weight of GBL was used. LiPF ₆ was used alone as the solute. Here, LiPF ₆ was dissolved in the above mixed solvent at a concentration of 1 mol / L, and 2 parts by weight of VC was added as an additive to 100 parts by weight of the mixed solvent to prepare a nonaqueous electrolyte. A battery was prepared in the same manner as in Example 1 except that this nonaqueous electrolyte was used. This battery was referred to as the battery of Comparative Example 4.
<< Comparative Example 5 >>

As the non-aqueous solvent, a mixed solvent composed of 25% by weight of EC and 75% by weight of EMC was used. LiFSI was used alone as the solute. Here, LiFSI was dissolved in the mixed solvent at a concentration of 1 mol / L, and 2 parts by weight of VC was added as an additive to 100 parts by weight of the mixed solvent to prepare a nonaqueous electrolyte. A battery was prepared in the same manner as in Reference Example 2, except that this nonaqueous electrolyte was used. This battery was referred to as the battery of Comparative Example 5.

The storage test was performed as follows in combination with the high temperature exposure test.
In a 20 ° C environment, the battery was discharged at a discharge current of 0.8 A and a final voltage of 3.0 V, and then a constant current and constant voltage charge was performed for 2 hours at a maximum current of 0.56 A and a set voltage of 4.2 V. It was. The charge capacity at this time was defined as the specified capacity of the battery.

The recovery rate of the discharge capacity after high temperature storage was measured as follows.
Each battery charged to a specified capacity was discharged at a discharge current of 0.8 A and a final voltage of 3.0 V in an environment of 0 ° C. or 20 ° C., and the discharge capacity was measured. Then, constant current and constant voltage charging was performed for 2 hours at a maximum current of 0.56 A and a set voltage of 4.2 V, and then stored at an ambient temperature of 85 ° C. for 3 days. The battery after storage was discharged at a discharge current of 0.8 A and a final voltage of 3.0 V in an environment of 0 ° C. or 20 ° C.

  Table 1 shows the discharge capacity before storage, the discharge capacity after high temperature storage, and the swelling (increased thickness) of the battery after high temperature storage.

  In the case of Comparative Example 1, the electric characteristics of the battery after high temperature storage showed good values. However, the swelling of the battery of Comparative Example 1 was extremely large, and it was swollen by about 1 mm, and there was a concern that the swelling of the electronic device would be greatly impaired due to the swelling.

  Compared to Comparative Example 1, the swelling of the battery of Comparative Example 2 using GBL alone as the non-aqueous solvent is about 17% of Comparative Example 1. This is considered to be attributable to the fact that the vapor pressure of the nonaqueous electrolyte was lowered and the reactivity with the active material and the like was reduced by using GBL as a solvent.

Furthermore, in Example 2 and Comparative Example 4, since a mixed solvent of EC and GBL was used, the swelling of the batteries was small in both cases. On the other hand, the electrical characteristics of Example 2 using LiFSI as the solute were greatly improved.

In Example 1 , the swelling of the battery after high temperature storage was small, and the capacity after high temperature storage was relatively good. In Reference Example 1 and Reference Example 2, a current collector made of stainless steel or the like can be used as the positive electrode current collector instead of the titanium foil. In addition, favorable characteristics can be expected even in a coin-type battery.

In this example, LiCoO ₂ having a very high full charge potential of 4.3 V with respect to the potential of lithium metal was used as the positive electrode active material. Therefore, in Examples 1 and 2 , LiPF ₆ containing fluorine was used in combination with LiFSI. Is considered to contribute greatly to the improvement of battery characteristics. Similarly, even when aluminum foil is used for the positive electrode current collector, when a positive electrode active material having a charging potential of less than 3.7 V with respect to the lithium metal potential is used, the battery characteristics change depending on whether or not LiPF ₆ is used. It is not considered. The same can be said when a lithium salt containing fluorine other than LiPF ₆ is used.

In Example 2 , the swelling of the battery was small, and the electrical characteristics were the best among the examples. This is considered to be attributable to the fact that, in addition to the combined use of LiFSI and LiPF ₆ , the use of VC as an additive and the mixing of EC with a non-aqueous solvent contributed. That is, in the battery of Example 2 , it is considered that the reductive decomposition on the negative electrode of GBL is sufficiently suppressed. These additives are particularly effective when a graphite material is used for the negative electrode. However, when the graphite material has high crystallinity, these additives should also be used from the viewpoint of improving the initial charge / discharge efficiency. Is effective.

  As described above, the present invention is particularly useful in the field of non-aqueous electrolyte secondary batteries that require a high level of safety when exposed to high temperatures and during storage.

FIG. 1 is a partially cutaway perspective view of an example of the nonaqueous electrolyte secondary battery of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electrode plate group 2 Positive electrode lead 3 Negative electrode lead 4 Battery case 5 Sealing plate 6 Negative electrode terminal 7 Sealing

Claims (10)

A chargeable / dischargeable positive electrode, a negative electrode that occludes and releases lithium, a diaphragm that electronically shields the positive electrode and the negative electrode, and a non-aqueous electrolyte,
The positive electrode includes a positive electrode active material having an average discharge potential of 3.5 V to 4.0 V with respect to a lithium metal potential, and a current collector made of aluminum,
The non-aqueous electrolyte comprises a non-aqueous solvent and a solute;
The non-aqueous solvent comprises a lactone;
The solute has the formula (1): (F—O ₂ S—N—SO ₂ —F) Li
LiPF _m (C _k F _{2k + 1} ) _6-m (0 ≦ m ≦ 6, 1 ≦ k ≦ 2) and LiBF _n (C _j F _{2j + 1} ) _{4− n (0 ≦ n ≦ 4,1 ≦} j ≦ 2) Ri Do from another lithium salt containing at least one fluorine from selected, the ratio of the another lithium salt with lithium bis fluoro sulfonyl Louis bromide (the lithium bis fluoro sulfonyl Louis bromide: said further lithium salt), in a molar ratio of 9: 1 to 5: 5 der Ru nonaqueous electrolyte secondary battery.   The nonaqueous electrolyte secondary battery according to claim 1, wherein the nonaqueous electrolyte further includes an additive that forms a film on the positive electrode and / or the negative electrode.   The non-aqueous electrolyte secondary battery according to claim 2, wherein the additive is at least one selected from the group consisting of vinylene carbonate, vinyl ethylene carbonate, phenyl ethylene carbonate, and propane sultone.   The nonaqueous electrolyte secondary battery according to claim 1, wherein the nonaqueous solvent further contains ethylene carbonate and / or propylene carbonate.   The nonaqueous electrolyte secondary battery according to claim 1, wherein the positive electrode is a positive electrode that needs to be charged at a potential of 3.7 V or more with respect to a potential of lithium metal.   The nonaqueous electrolyte secondary battery according to claim 1, wherein the lactone is γ-butyrolactone. Consisting of non-aqueous solvent and solute,
The non-aqueous solvent comprises a lactone;
The lactone comprises γ-butyrolactone,
The solute has the formula (1): (F—O ₂ S—N—SO ₂ —F) Li
LiPF _m (C _k F _{2k + 1} ) _6-m (0 ≦ m ≦ 6, 1 ≦ k ≦ 2) and LiBF _n (C _j F _{2j + 1} ) _{4− n (0 ≦ n ≦ 4,1 ≦} j ≦ 2) Ri Do from another lithium salt containing at least one fluorine from selected, the ratio of the another lithium salt with lithium bis fluoro sulfonyl Louis bromide (the lithium bis fluoro sulfonyl Louis bromide: said further lithium salt), in a molar ratio of 9: 1 to 5: 5 der Ru non-aqueous electrolyte secondary cell electrolyte.   Furthermore, the electrolyte for nonaqueous electrolyte secondary batteries of Claim 7 containing the additive which forms a film on a positive electrode and / or a negative electrode.   The electrolyte for a non-aqueous electrolyte secondary battery according to claim 8, wherein the additive is at least one selected from the group consisting of vinylene carbonate, vinyl ethylene carbonate, phenyl ethylene carbonate, and propane sultone.   The electrolyte for a nonaqueous electrolyte secondary battery according to claim 7, wherein the nonaqueous solvent further contains ethylene carbonate and / or propylene carbonate.

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