JP4767501B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte secondary battery, and particularly to an improvement of the non-aqueous electrolyte.

Currently, in non-aqueous electrolyte secondary batteries, lithium ion secondary batteries having high voltage and high energy density are actively studied. A lithium-containing transition metal oxide such as LiCoO ₂ is generally used as the positive electrode active material constituting the lithium secondary battery, and a carbon material is generally used as the negative electrode active material. Electrolytic solutions 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 electrolytic solution. For example, Patent Documents 1 and 2 propose a method of adding vinylene carbonate or vinyl ethylene carbonate to the electrolytic solution. The purpose of these is to improve the charge / discharge cycle characteristics, and by inhibiting the side reaction between the electrolyte and the negative electrode active material, vinylene carbonate or vinyl ethylene carbonate decomposes on the negative electrode to form a protective film. is there.
JP 2003-151621 A JP 2003-031259 A

  However, when vinylene carbonate or vinyl ethylene carbonate as proposed is included in the electrolyte, the coating formed on the negative electrode is peeled off at high temperatures, causing side reactions between the electrolyte and the negative electrode active material, resulting in high temperature cycle characteristics. There has been a problem that is extremely lowered.

  An object of the present invention is to solve such problems and to provide a non-aqueous electrolyte secondary battery exhibiting good charge / discharge cycle characteristics even at high temperatures.

The present invention is a nonaqueous electrolytic solution, a main solvent, at least one carbon to both the and exocyclic ring - and a cyclic carbonate having a carbon-carbon unsaturated bond, viewed contains a solute content of the cyclic carbonate Is 0.1 to 10 parts by weight per 100 parts by weight of the main solvent .
When a non-aqueous electrolyte contains a cyclic carbonate having a carbon-carbon unsaturated bond both inside and outside the ring, the cyclic carbonate decomposes on the negative electrode to form a very strong protective coating. To do. This protective film is unlikely to peel off from the negative electrode surface even at high temperatures, and can prevent side reactions between the electrolyte and the negative electrode active material. The reason is considered as follows.

  In general, a cyclic carbonate having a carbon-carbon unsaturated bond undergoes a polymerization reaction by reducing its monomer molecules, and becomes a high molecular polymer to form a protective film. The cyclic ester carbonate having at least one carbon-carbon unsaturated bond both inside and outside the ring has at least two sites that undergo reductive polymerization in one molecule. Therefore, a polymerization reaction can occur from these at least two sites. Therefore, the high molecular polymer produced as the protective film has a very large molecular weight and degree of polymerization, and forms a dense and strong protective film on the negative electrode surface. It is considered that this strong coating suppresses the side reaction between the electrolyte and the negative electrode active material and improves the resistance under a high temperature environment.

    According to the present invention, it is possible to avoid a problem when a conventional cyclic carbonate is added, that is, breakage of a coating film under a high temperature environment, and to realize a nonaqueous electrolyte secondary battery having good charge / discharge cycle characteristics.

  As described above, the present invention comprises a positive electrode capable of inserting and extracting lithium, a negative electrode capable of inserting and extracting lithium, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, Provided is a non-aqueous electrolyte secondary battery in which the non-aqueous electrolyte includes a cyclic carbonate having at least one carbon-carbon unsaturated bond inside and outside the ring.

  The cyclic carbonate ester has the formula:

(In the formula, at least one of R ₁ and R ₂ has at least one carbon-carbon unsaturated bond and is an alkenyl group, alkynyl group, or polyenyl group having 2 to 20 carbon atoms, and the other is carbon-carbon. When the group does not have an unsaturated bond, it is a hydrogen atom or an alkyl group having 2 to 20 carbon atoms.)
It is preferable that it is a compound shown by these. The cyclic carbonate used in the present invention is particularly easy to proceed in the case of a 5-membered ring as shown in the above formula, and forms a higher molecular weight protective film.

A preferred example of the cyclic ester carbonate is vinyl vinylene carbonate. Vinyl vinylene carbonate forms a film with particularly high heat resistance.
It is preferable that the non-aqueous electrolyte further includes at least one selected from the group consisting of vinylene carbonate and vinyl ethylene carbonate. Vinylene carbonate and vinyl ethylene carbonate also decompose on the negative electrode to form a film. The film derived from the cyclic carbonate having at least one carbon-carbon unsaturated bond in both the ring and the ring is composed of these vinylene carbonate and / or vinyl ethylene carbonate. The heat resistance is high, and the side reaction between the electrolyte and the negative electrode active material can be further suppressed.

  As the main body of the nonaqueous electrolytic solution of the present invention, a nonaqueous solvent and a solute dissolved in the nonaqueous solvent are used. As the non-aqueous solvent, for example, a cyclic carbonate, a chain carbonate, a cyclic carboxylic acid ester or the like is used. Here, examples of the cyclic carbonate include propylene carbonate and ethylene carbonate, and examples of the chain carbonate include diethyl carbonate, ethyl methyl carbonate, and dimethyl carbonate. Examples of the cyclic carboxylic acid ester include γ-butyrolactone and γ-valerolactone.

  The non-aqueous solvent can further contain a conventionally well-known benzene derivative that decomposes upon 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% or less of the whole non-aqueous solvent.

  The nonaqueous electrolytic solution of the present invention is composed of the main solvent as described above, a cyclic carbonate having at least one carbon-carbon unsaturated bond inside and outside the ring, and a solute described below. The addition ratio of the cyclic carbonate having at least one carbon-carbon unsaturated bond inside and outside the ring is preferably 0.1 to 10 parts by weight per 100 parts by weight of the main solvent. When the amount is less than 0.1 part by weight, the protective film to be formed is not sufficiently resistant in a high temperature environment. On the other hand, when the amount exceeds 10 parts by weight, the degree of polymerization becomes too large, an excessive protective film is formed on the negative electrode, and the charge / discharge reaction is hindered. When vinylene carbonate and / or vinyl ethylene carbonate is added, the addition ratio is a film derived from a cyclic carbonate having at least one carbon-carbon unsaturated bond inside and outside the ring, and a dense and high heat resistance. From the viewpoint of forming a hybrid film and not inhibiting the charge / discharge reaction, 0.1 to 10 parts by weight per 100 parts by weight of the main solvent is preferable.

Examples of the lithium salt dissolved in the non-aqueous solvent include LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , and Li (CF ₃ SO ₂ ) _{2. , LiAsF 6, LiB 10 Cl 10} , lower aliphatic lithium carboxylate, LiCl, LiBr, LiI, chloroborane lithium, bis (1,2-benzene diolate (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) -Borate salts such as oleate-1-benzenesulfonic acid-O, O ') lithium borate, lithium bistetrafluoromethanesulfonate imide ((CF ₃ SO _{2) 2} NLi), tetrafluoromethane acid nonafluorobutanesulfonic acid imide lithium _{(LiN (CF 3 SO 2)} (C 4 F 9 SO 2)), bispentafluoroethanesulfonyl imide lithium ((C ₂ F ₅ Examples thereof include imide salts such as SO ₂ ) ₂ NLi). These may be used alone or in combination of two or more.

Examples of the positive electrode active material of the non-aqueous electrolyte secondary battery of the present invention include Li _x CoO ₂ , Li _x NiO ₂ , Li _x MnO ₂ , Li _x Co _y Ni _1-y O ₂ , and 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 is, Na, Mg, Sc, Y , Mn, Fe, It is at least one selected from the group consisting of Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B, and 0 ≦ x ≦ 1.2, 0 ≦ y ≦ 0.9, 2.0 ≦ An oxide represented by z ≦ 2.3) is used. The x value is a value before the start of charge / discharge, and increases or decreases due to charge / discharge.

The negative electrode material is not particularly limited. For example, as a lithium metal, a lithium ion host material, an amorphous carbon material, artificial carbon fired at a temperature of 2000 ° C. or higher, carbon material such as natural graphite, and alkali metal Alloying aluminum (Al), lead (Pb), tin (Sn), bismuth (Bi), silicon (Si) and other metals and alkali metal interstitial intermetallic cubic intermetallic compounds (AlSb, Mg ₂ Si, NiSi ₂ ), lithium nitrogen compounds (Li _3−x M _x N (M: transition metal)), and the like. In particular, carbon having an interval (d ₀₀₂ ) of a lattice plane (002) plane capable of inserting and extracting lithium of 3.37 mm or less and a crystallite size (Lc) in the c-axis direction of 200 mm or more is preferable. .

  Examples of the positive electrode or negative electrode binder include polyethylene, polypropylene, polytetrafluoroethylene, polyvinylidene fluoride, tetrafluoroethylene-hexafluoropropylene copolymer, and vinylidene fluoride-hexafluoropropylene copolymer. . Examples of the conductive agent included in the electrode include graphites, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and other carbon blacks, carbon fibers, and metal fibers.

  For the positive electrode current collector, for example, a sheet or foil made of stainless steel, aluminum, titanium, or the like is used. For the negative electrode current collector, for example, a sheet or 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 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.
The non-aqueous electrolyte secondary battery of the present invention is configured by combining the above components.

Examples of the present invention will be described below.
Example 1
(I) Preparation of non-aqueous electrolyte LiPF ₆ at a concentration of 1.0 mol / L in a mixed solvent (volume ratio 1: 3) of ethylene carbonate (hereinafter represented by EC) and ethyl methyl carbonate (hereinafter represented by EMC) Was dissolved. In the obtained solution, various carbonic acid esters having at least one carbon-carbon unsaturated bond in both the ring and the ring described in Table 1 were added as additives to 100 parts by weight of the mixed solvent, respectively. A non-aqueous electrolyte was prepared by adding 2 parts by weight.

(Ii) Preparation of positive electrode plate 85 parts by weight of lithium cobaltate powder was mixed with 10 parts by weight of acetylene black as a conductive agent and 5 parts by weight of polyvinylidene fluoride resin as a binder, and these were dehydrated N-methyl-2 -A slurry-like positive electrode mixture was prepared by dispersing in 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 75 parts by weight of artificial graphite powder was mixed with 20 parts by weight of acetylene black as a conductive agent and 5 parts by weight of polyvinylidene fluoride resin as a binder, and these were dehydrated N-methyl-2 -A slurry-like negative electrode mixture was prepared by dispersing in pyrrolidone. This negative electrode mixture was applied onto 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 with the separator 13 interposed therebetween 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 electrolytic solution was poured into the battery case 18, and the opening of the battery case 18 was sealed using the sealing plate 19.

(V) Evaluation of Battery The battery manufactured as described above was repeatedly charged and discharged at 45 ° C. The charging / discharging conditions are constant current / constant voltage charging for 2 hours and 30 minutes with a maximum current of 1050 mA and an upper limit voltage of 4.2 V, and constant current discharging with a discharge current of 1500 mA and a discharge end voltage of 3.0 V. The discharge capacity at the third cycle was assumed to be 100%, and the capacity maintenance rate of the battery that had passed 500 cycles was calculated as the cycle maintenance rate. The results are shown in Table 1.

<< Comparative Example 1 >>
A battery similar to that of Example 1 except that a solution obtained by dissolving LiPF ₆ at a concentration of 1.0 mol / L in a mixed solvent of EC and EMC (volume ratio 1: 3) was used as the nonaqueous electrolytic solution. And a charge / discharge cycle was performed at 45 ° C. The results are shown in Table 1.

  From Table 1, by including in the nonaqueous electrolyte a cyclic carbonate having at least one carbon-carbon unsaturated bond both inside and outside the ring, a battery having excellent high-temperature cycle characteristics can be obtained. You can see that This is presumably because a cyclic protective ester having a carbon-carbon unsaturated bond both inside and outside the ring was decomposed on the negative electrode to form a stable protective film.

  Among the cyclic carbonates having at least one carbon-carbon unsaturated bond both inside and outside the ring, the 5-membered cyclic carbonates were more excellent in high-temperature cycle characteristics. This is considered to be because the polymerization reaction is particularly likely to proceed particularly when the cyclic carbonate is a 5-membered ring, and a higher molecular weight protective film is formed.

  Also, from Table 1, among the cyclic carbonates having at least one carbon-carbon unsaturated bond both inside and outside the ring, vinyl vinylene carbonate (hereinafter referred to as VVC) is particularly excellent in high temperature cycle characteristics. I understand that. This is considered because VVC forms a strong protective film having particularly high heat resistance on the surface of the negative electrode active material.

<< Example 2 , Reference Examples 1 and 2 >>
As a non-aqueous electrolyte, LiPF ₆ was dissolved at a concentration of 1.0 mol / L in a solution obtained by mixing VVC in the amount shown in Table 2 in 100 parts by weight of a mixed solvent of EC and EMC (volume ratio 1: 3). What was done was used. A battery was assembled in the same manner as in Example 1 except for the nonaqueous electrolytic solution, 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 VVC is increased. A preferable mixing range of VVC 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, vinylene carbonate (hereinafter referred to as VC) and vinyl ethylene carbonate (hereinafter referred to as VC) and 100 parts by weight of a mixed solvent (volume ratio 3: 5: 2) of EC, EMC and diethyl carbonate (hereinafter referred to as DEC). (Represented by VEC) was mixed (mixing amount is described in Table 3), and 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 VVC was used. A battery was assembled in the same manner as in Example 1 except for the nonaqueous electrolytic solution, and a charge / discharge cycle was performed at 45 ° C. The results are shown in Table 3.

<< Comparative Example 2 >>
As a non-aqueous electrolyte, a battery similar to Example 3 was prepared except that VC and VEC were mixed without mixing VVC (the mixing amount is described in Table 3), and a charge / discharge cycle was performed at 45 ° C. It was. The results of Comparative Example 2 are also shown in Table 3.

  From Table 3, it can be seen that the batteries of the present invention containing VVC in the electrolytic solution or containing VVC, VC and VEC are all excellent in high-temperature cycle characteristics.

  The nonaqueous electrolyte secondary battery of the present invention is excellent in cycle characteristics even at high temperatures. Therefore, this non-aqueous electrolyte secondary battery is useful as a drive source for electronic devices such as notebook computers, mobile phones, and digital still cameras.

It is a longitudinal cross-sectional view of the cylindrical nonaqueous electrolyte secondary battery concerning the Example of this 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 (4)

A non-aqueous electrolyte secondary battery comprising a positive electrode capable of inserting and extracting lithium, a negative electrode capable of inserting and extracting lithium, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte the non-aqueous electrolyte is a main solvent, at least one carbon respectively in the exocyclic ring - and cyclic carbonate having a carbon-carbon unsaturated bond, and a solute viewed including the content of the cyclic carbonate is the main A non-aqueous electrolyte secondary battery , wherein the content is 0.1 to 10 parts by weight per 100 parts by weight of the solvent . The cyclic carbonate is represented by the formula:

(In the formula, at least one of R ₁ and R ₂ has at least one carbon-carbon unsaturated bond and is an alkenyl group, alkynyl group, or polyenyl group having 2 to 20 carbon atoms, and the other is carbon-carbon. When the group does not have an unsaturated bond, it is a hydrogen atom or an alkyl group having 2 to 20 carbon atoms.)
The nonaqueous electrolyte secondary battery according to claim 1, which is a compound represented by the formula:   The non-aqueous electrolyte secondary battery according to claim 1, wherein the cyclic carbonate is vinyl vinylene carbonate. The non-aqueous electrolyte solution further includes 0.1 to 10 parts by weight per 100 parts by weight of the main solvent, at least one selected from the group consisting of vinylene carbonate and vinyl ethylene carbonate. The nonaqueous electrolyte secondary battery as described.

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