JP2008123732A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery which has a high energy concentration and an excellent cycle characteristics even under a high temperature environment. <P>SOLUTION: The nonaqueous electrolyte secondary battery is provided with a group of electrodes, in which a cathode and an anode which can occlude and discharge lithium are wound around or laminated with a separator in-between, and a nonaqueous electrolyte which are both sealed in a case. The nonaqueous electrolyte contains a ring-shaped compound having one unsaturated bond and moreover two nitrogen atoms in a ring. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a non-aqueous electrolyte secondary battery, and particularly to a battery using the preferred electrolyte.

Currently, research on lithium ion secondary batteries having high voltage and high energy density is actively conducted in non-aqueous electrolyte secondary batteries. As the positive electrode active material constituting the lithium ion secondary battery, lithium-containing transition metal oxides such as LiCoO ₂ are generally used, the negative electrode active material is a carbon material, and the separator is polyethylene or polypropylene. In addition, electrolytes used in non-aqueous electrolyte secondary batteries are generally those in which a solute is dissolved in a non-aqueous solvent. Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, and cyclic carboxylates. As the solute, lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), or the like is used.

  Furthermore, for the purpose of improving battery characteristics, attempts have been made to mix various additives into the positive electrode active material, the negative electrode active material, and the electrolyte. For example, Patent Document 1 proposes a method of adding a nitrogen-containing aromatic heterocyclic compound such as pyridine, pyrrole, 1-methylimidazole, and pyrimidine to an electrolyte. The purpose is to improve the cycle characteristics, and when the nitrogen-containing aromatic heterocyclic compound forms a film, the desired effect can be obtained.

Patent Documents 2 and 3 propose a method of adding pyrroline to the electrolyte, which can improve the low-temperature characteristics after storage and increase the capacity of the conductive polymer, respectively.
JP 2002-359002 A JP-A-3-46771 JP-A-4-104477

  However, when a nitrogen-containing cyclic compound as proposed in Patent Documents 1, 2, and 3 is included in the electrolyte, the side reaction between the electrolyte and the negative electrode active material occurs vigorously at a high temperature, resulting in extremely poor cycle characteristics. There was a problem to do.

  The present invention solves such problems and provides a nonaqueous electrolyte secondary battery exhibiting good charge / discharge cycle characteristics even in a high temperature environment.

  The present invention is characterized in that a non-aqueous electrolyte contains a cyclic compound having one unsaturated bond in the ring and containing two nitrogen atoms.

  By incorporating the compound of the present invention into a non-aqueous electrolyte, the compound of the present invention decomposes on the negative electrode to form a very strong protective film, so that it is difficult to peel off from the negative electrode surface even at high temperatures. Side reactions with substances can be suppressed. The reason is considered as follows.

Since the compound of the present invention is a cyclic compound having one unsaturated bond in the ring, it can be reduced on the negative electrode to cause addition or ring-opening polymerization, and can form a protective film as a polymer. In addition, since it has two nitrogen atoms in the ring, the nitrogen-containing polymer produced by polymerization has high heat resistance, and it is difficult to peel off from the negative electrode surface even at high temperatures.
It is considered that side reaction between the electrolyte and the negative electrode active material is suppressed, and resistance under a high temperature environment is improved. Patent Document 1 also proposes nitrogen-containing aromatic heterocyclic compounds such as pyridine, but these compounds are structurally different from the compounds of the present invention in that they contain two or more unsaturated bonds in the ring. In addition, since it contains two or more unsaturated bonds in the ring, the oxidation resistance is lowered, and it oxidatively decomposes on the positive electrode active material at a high temperature. Is not formed, and the cycle characteristics deteriorate.

  Patent Documents 2 and 3 also propose pyrroline, which is a nitrogen-containing cyclic compound, but this compound is structurally different from the compound of the present invention in that it contains only one nitrogen atom. Since it contains only one atom, the heat resistance of the nitrogen-containing polymer produced by polymerization does not improve much, and it tends to peel off from the negative electrode surface at high temperatures, causing a side reaction between the electrolyte and the negative electrode active material. Cycle characteristics will deteriorate.

  On the other hand, since the compound of the present invention has one unsaturated bond in the ring and contains two nitrogen atoms, it does not cause a decrease in oxidation resistance and a decrease in film heat resistance. In particular, a sufficient amount of a heat-resistant film capable of protecting the negative electrode active material is formed, and the cycle characteristics at high temperatures are improved.

  By incorporating the compound of the present invention into a non-aqueous electrolyte, the problems when conventional compounds are added, that is, deterioration of cycle characteristics due to side reaction between the electrolyte and the negative electrode active material in a high temperature environment can be avoided, and good charge and discharge A non-aqueous electrolyte secondary battery having cycle characteristics can be realized.

  The best mode for carrying out the present invention will be described in detail below.

  In the present invention, it is necessary for the nonaqueous electrolyte to contain a cyclic compound having one unsaturated bond in the ring and containing two nitrogen atoms.

  Among the compounds of the present invention, compounds having an imidazoline skeleton are preferable. This is because, since the imidazoline skeleton is a 5-membered ring compound, the strain in the molecule is moderately large, and therefore the ring-opening polymerization reaction with the release of strain as a driving force is easy to proceed, and a strong protective film with a higher degree of polymerization can be obtained. This is because it is formed.

  The nonaqueous electrolyte secondary battery of the present invention is configured by combining the components described in detail below.

The positive electrode active material, for _{example, Li x CoO 2, Li x} NiO 2, Li x MnO 2, Li x Co y Ni 1-y O 2, Li x Co y M 1-y O z, Li x Ni 1- _{y M y O z, Li x} Mn 2 O 4, Li x Mn 2-y M y O 4 (M = Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, (At least one of Pb, Sb, and B), where x is 0 to 1.2, y is 0 to 0.9, and z is 2.0 to 2.3. The x value is a value before the start of charging / discharging and increases / decreases due to charging / discharging.

  Examples of the anode material include carbon such as natural graphite (eg, scaly graphite), graphite such as artificial graphite, acetylene black, ketjen black (registered trademark), channel black, furnace black, lamp black, thermal black, and the like. Blacks, carbon fibers, metal fibers, alloys, lithium metals, tin compounds, silicides, nitrides, and the like can be used.

Examples of the positive electrode or negative electrode binder include polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and vinylidene fluoride-hexa. A fluoropropylene copolymer or the like is used. Examples of the conductive agent included in the electrode include graphite, acetylene black, ketjen black (registered trademark), channel black, furnace black, lamp black, thermal black, and other carbon blacks, carbon fibers, metal fibers, and the like. Is used.

  For the positive electrode current collector, for example, a sheet foil made of stainless steel, aluminum, titanium, or the like is used. For the negative electrode current collector, for example, a sheet foil made of stainless steel, nickel, copper, or the like is used. These thicknesses are not particularly limited, but are 1 to 500 μm.

  As the electrolyte, a solution in which a solute is dissolved in a non-aqueous solvent is used. As the non-aqueous solvent, for example, a cyclic carbonate, a chain carbonate, a cyclic carboxylate, a cyclic sulfone, or the like is used. Here, examples of the cyclic carbonate include propylene carbonate (PC) and ethylene carbonate (EC). Examples of the chain carbonate include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Etc. Examples of the cyclic carboxylic acid ester include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of the cyclic sulfone include sulfolane (SL) and 3-methylsulfolane (3MeSL).

Examples of the lithium salt dissolved in these solvents include LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF ₆ , LiB ₁₀ Cl ₁₀ , lower fat. Lithium group carboxylate, LiCl, LiBr, LiI, chloroborane lithium, bis (1,2-benzenediolate (2-)-O, O ') lithium borate, bis (2,3-naphthalenedioleate (2-) -O, O ') lithium borate, bis (2,2'-biphenyldiolate (2-)-O, O') lithium borate, bis (5-fluoro-2-olate-1-benzenesulfonic acid-O, O ′) borates such as lithium borate, lithium bistetrafluoromethanesulfonate imide ((CF ₃ SO ₂ ) ₂ NLi), tetrafur Olomethanesulfonic acid nonafluorobutane sulfonic acid imidolithium (LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ )), bispentafluoroethane sulfonic acid imidolithium ((C ₂ F ₅ SO ₂ ) ₂ NLi), etc. And imide salts thereof. These may be used alone or in combination of two or more.

  The nonaqueous electrolyte can contain a cyclic carbonate having at least one carbon-carbon unsaturated bond. It is because it decomposes | disassembles on a negative electrode and forms a membrane | film | coat with high lithium ion conductivity, and charge / discharge efficiency becomes high.

  Examples of the cyclic ester carbonate having at least one carbon-carbon unsaturated bond include vinylene carbonate (VC), 4-methyl vinylene carbonate, 4,5-dimethyl vinylene carbonate, 4-ethyl vinylene carbonate, 4,5-diethyl. Examples include vinylene carbonate, 4-propyl vinylene carbonate, 4,5-dipropyl vinylene carbonate, 4-phenyl vinylene carbonate, 4,5-diphenyl vinylene carbonate, vinyl ethylene carbonate (VEC), and divinyl ethylene carbonate. These may be used alone or in combination of two or more. Among these, at least one selected from the group consisting of vinylene carbonate, vinyl ethylene carbonate, and divinyl ethylene carbonate is preferable. In the above compound, part of the hydrogen atoms may be substituted with fluorine atoms.

Further, the non-aqueous electrolyte can contain a conventionally well-known benzene derivative that decomposes during overcharge to form a film on the electrode and inactivate the battery. The benzene derivative is preferably composed of a phenyl group and a cyclic compound group adjacent to the phenyl group. As the cyclic compound group, a phenyl group, a cyclic ether group, a cyclic ester group, a cycloalkyl group, a phenoxy group, and the like are preferable. Specific examples of the benzene derivative include cyclohexylbenzene, biphenyl, diphenyl ether and the like. These may be used alone or in combination of two or more. However, it is preferable that the content rate of a benzene derivative is 10 volume parts or less of the whole non-aqueous solvent.

  As the separator, a microporous thin film having a large ion permeability, a predetermined mechanical strength, and an insulating property is used. For example, a sheet made of an olefin polymer such as polypropylene or polyethylene, a glass fiber, a nonwoven fabric, a woven fabric, or the like is used. The thickness of the separator is generally 10 to 300 μm.

(Example 1)
(I) Preparation of Nonaqueous Electrolyte LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (volume ratio 1: 4) at a concentration of 1.0 mol / L. In the obtained solution, 2 parts by weight of a cyclic compound having one unsaturated bond in various rings described in Table 1 and containing two nitrogen atoms as an additive with respect to 100 parts by weight of the solvent. A non-aqueous electrolyte was prepared by addition.

(Ii) Production of Positive Electrode Plate With respect to 85 parts by weight of lithium cobaltate powder, 10 parts by weight of acetylene black as a conductive agent and 5 parts by weight of polyvinylidene fluoride resin as a binder were mixed, and these were mixed with dehydrated N-methyl- A slurry-like positive electrode mixture was prepared by dispersing in 2-pyrrolidone. This positive electrode mixture was applied onto a positive electrode current collector made of an aluminum foil, dried and rolled to obtain a positive electrode plate.

(Iii) Production of Negative Electrode Plate To 75 parts by weight of artificial graphite powder, 20 parts by weight of acetylene black as a conductive agent and 5 parts by weight of polyvinylidene fluoride resin as a binder were mixed, and these were dehydrated N-methyl- A slurry-like negative electrode mixture was prepared by dispersing in 2-pyrrolidone. This negative electrode mixture was applied on a negative electrode current collector made of copper foil, dried and rolled to obtain a negative electrode plate.

(Iv) Production of Cylindrical Battery A cylindrical battery was produced. A longitudinal sectional view thereof is shown in FIG.

  The positive electrode plate 11 and the negative electrode plate 12 were wound in a spiral shape via the separator 13 to produce an electrode plate group. The electrode plate group was housed in a nickel-plated iron battery case 18. The positive electrode lead 14 made of aluminum was pulled out from the positive electrode plate 11 and connected to the back surface of the sealing plate 19 that was conducted to the positive electrode terminal 20. Further, a nickel negative electrode lead 15 was pulled out from the negative electrode plate 12 and connected to the bottom of the battery case 18. An insulating plate 16 is provided above the electrode plate group, and an insulating plate 17 is provided below the electrode plate group. A predetermined nonaqueous electrolyte was poured into the battery case 18, and the opening of the battery case 18 was sealed using the sealing plate 19.

  In this way, batteries 1 to 8 were prepared.

(V) Battery evaluation [Capacity maintenance rate after cycle]
For the batteries 1 to 8 manufactured as described above, the battery charge / discharge cycle was repeated at 45 ° C., the discharge capacity at the third cycle was regarded as 100%, and the capacity retention rate of the battery after 500 cycles was The cycle maintenance rate was calculated. The results are shown in Table 1.

  In the charging, constant current / constant voltage charging was performed for 2 hours 30 minutes with a maximum current of 1050 mA and an upper limit voltage of 4.2 V. In discharging, constant current discharge was performed at a discharge current of 1500 mA and a discharge end voltage of 3.0 V.

(Comparative Example 1)
As a non-aqueous electrolyte, except that a solution in which LiPF6 was dissolved at a concentration of 1.0 mol / L in a mixed solvent (volume ratio 1: 4) of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) was used, A battery 9 similar to that of Example 1 was produced, and a charge / discharge cycle was performed at 45 ° C.

(Comparative Example 2)
The nonaqueous electrolyte has one unsaturated bond in the ring and does not include a cyclic compound containing two nitrogen atoms, except that various nitrogen-containing cyclic compounds described in Table 1 are mixed. Batteries 10 to 14 similar to those in Example 1 were produced, and a charge / discharge cycle was performed at 45 ° C.

  The results of Comparative Examples 1 and 2 are also shown in Table 1. From Table 1, it can be seen that a battery excellent in high-temperature cycle characteristics can be obtained by including a cyclic compound containing one unsaturated bond in the ring and two nitrogen atoms in the nonaqueous electrolyte. . This is because a cyclic compound containing one unsaturated bond in the ring and containing two nitrogen atoms is reduced and polymerized on the negative electrode, whereby a highly heat-resistant nitrogen-containing polymer is formed as a protective film. Therefore, it is difficult to peel off from the negative electrode surface even under high temperature, and it can be inferred that the side reaction between the electrolyte and the negative electrode active material was suppressed.

  Among the cyclic compounds having one unsaturated bond in the ring and two nitrogen atoms, the compounds having an imidazoline skeleton were more excellent in high-temperature cycle characteristics. This is because, since the imidazoline skeleton is a 5-membered ring compound, the strain in the molecule is moderately large, and therefore the ring-opening polymerization reaction with the release of strain as a driving force is easy to proceed, and a strong protective film with a higher degree of polymerization can be obtained. It is thought that it is because it is formed.

(Example 2)
As a nonaqueous electrolyte, 1.0 mol of LiPF ₆ was added to a solution obtained by mixing 2-methyl-2-imidazoline in an amount shown in Table 2 with respect to 100 parts by weight of a mixed solvent of EC and EMC (volume ratio 1: 4). A solution dissolved at a concentration of / L was used. Except for the nonaqueous electrolyte, batteries 15 to 23 were assembled in the same manner as in Example 1, and a charge / discharge cycle was performed at 45 ° C. The results are shown in Table 2.

  From Table 2, it can be seen that the high-temperature cycle characteristics are improved as the mixing amount of 2-methyl-2-imidazoline is increased. When the amount is less than 0.1 parts by weight, the effect of addition is small, and when more than 10 parts by weight is added, the film becomes too thick and the resistance increases, and the insertion / extraction reaction of lithium ions to the negative electrode is inhibited. It can be seen that the preferable mixing range of 2-methyl-2-imidazoline, which is effective in properties, is 0.1 to 10 parts by weight with respect to 100 parts by weight of the solvent.

(Example 3)
As a non-aqueous electrolyte, a solution obtained by dissolving LiPF ₆ at a concentration of 1.0 mol / L in a solution obtained by mixing 2 parts by weight of 2-methyl-2-imidazoline with 100 parts by weight of various solvents described in Table 3 Using. Except for the nonaqueous electrolyte, batteries 24 to 33 were assembled in the same manner as in Example 1, and a charge / discharge cycle was performed at 45 ° C. The results are shown in Table 3.

  When a mixed solvent was used as the electrolyte solvent, it was prepared by mixing at a volume ratio.

(Comparative Example 3)
Batteries 34 to 43 as in Example 1 were used except that LiPF ₆ was dissolved at a concentration of 1.0 mol / L with respect to 100 parts by weight of various solvents described in Table 3 as the nonaqueous electrolyte. It produced and the charge / discharge cycle was performed at 45 degreeC. When a mixed solvent was used as the electrolyte solvent, it was prepared by mixing at a volume ratio.

  The results of Comparative Example 3 are also shown in Table 3. From Table 3, it can be seen that when any electrolyte solvent is used, a battery having excellent high-temperature cycle characteristics can be obtained by including 2-methyl-2-imidazoline in the nonaqueous electrolyte. Among these, when SL or 3MeSL was used as a solvent, the high temperature cycle characteristics were particularly excellent. This is because the polymer film formed on the negative electrode surface by reduction of 2-methyl-2-imidazoline is changed to a film with higher heat resistance containing sulfur atoms in solvent molecules by incorporating SL or 3MeSL. It can be inferred that

  Since the present invention is a non-aqueous electrolyte secondary battery excellent in high-temperature cycle characteristics, it is useful as a power source for driving portable electronic devices such as mobile phones, notebook computers, and video camcorders.

1 is a longitudinal sectional view of a cylindrical nonaqueous electrolyte secondary battery according to an embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Positive electrode plate 12 Negative electrode plate 13 Separator 14 Positive electrode lead 15 Negative electrode lead 16 Upper insulating plate 17 Lower insulating plate 18 Battery case 19 Sealing plate 20 Positive electrode terminal

Claims (2)

  In a non-aqueous electrolyte secondary battery in which an electrode group in which a positive electrode and a negative electrode capable of occluding and releasing lithium are wound or laminated with a non-aqueous electrolyte is enclosed in a case with a separator, an unsaturated bond is formed in the ring. And a non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte containing a cyclic compound containing one nitrogen atom and two nitrogen atoms.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the cyclic compound is a compound having an imidazoline skeleton.

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