JP2003331914A - Nonaqueous electrolyte secondary cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary cell with good cycle life. <P>SOLUTION: The nonaqueous electrolyte of the nonaqueous electrolyte secondary cell contains non-cyclic carbonic ester having substituent containing ether linkage in the nonaqueous electrolyte. The cycle retention rate and initial discharging capacity are sharply improved due to a chemically stable film having high lithium ion conductivity formed on a carbon negative electrode. Especially, a good cycle retention rate and a good initial discharging capacity are obtained when non-cyclic carbonate is a compound chosen from 2-methoxyethylmethyl- carbonate, 2-ethoxyethylmethyl-carbonate, 2-methoxyethylethyl-carbonate, 2- ethyoxyethylethyl-carbonate, 2-bis(ethoxymethyl)carbonate, and 2-bis(ethoxyethyl) carbonate. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art In general, non-aqueous electrolyte secondary batteries include LiPF ₆ or a mixed solvent of a high dielectric constant solvent such as ethylene carbonate (EC) or propylene carbonate and a low viscosity solvent such as diethyl carbonate (DEC). LiBF
An electrolytic solution in which a supporting salt such as No. ₄ is dissolved is used.

However, in a non-aqueous electrolyte secondary battery using these electrolytes, as the charge / discharge cycle proceeds, decomposition of the supporting salt and solvent in the non-aqueous electrolyte progresses on the positive electrode and the negative electrode,
There has been a problem that the amount of the electrolytic solution is reduced, or a film having poor lithium ion conductivity is deposited on the surface of the negative electrode to hinder the movement of lithium ions and reduce the discharge capacity.

Therefore, ethyl methyl carbonate (EM
There is known a technique of suppressing decomposition of the electrolytic solution by using a carbonic acid ester having an alkyl group such as C).

[0005]

However, even if the carbonic acid ester having an alkyl group is added, the cycle characteristics of the non-aqueous electrolyte secondary battery cannot be said to be sufficient, and further improvement of the cycle characteristics has been earnestly desired. .

The present invention was completed in view of the above circumstances, and an object thereof is to provide a non-aqueous electrolyte secondary battery having good cycle characteristics.

[0007]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies to develop a non-aqueous electrolyte secondary battery capable of solving the above problems. As a result, they have found that the addition of an acyclic ester carbonate having an ether bond improves cycle characteristics, and completed the present invention.

That is, the invention of claim 1 is a non-aqueous electrolyte secondary battery comprising a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and a non-aqueous electrolyte. It is a non-aqueous electrolyte secondary battery in which the non-cyclic carbonic acid ester shown is contained in the non-aqueous electrolyte.

[0009]

[Chemical 2] (Wherein _{R 1} represents a substituent having 1 to 12 carbon atoms containing an ether bond, _{R 2} represents a substituent having 1 to 12 carbon atoms containing alkyl group having 1 to 6 carbon atoms, or an ether bond .)

Further, the invention of claim 2 is the non-aqueous electrolyte secondary battery according to claim 1, wherein the non-cyclic carbonic acid ester is 2-methoxyethyl methyl carbonate, 2-ethoxyethyl methyl carbonate, 2-methoxy. It is characterized in that it is at least one selected from the group consisting of ethyl ethyl carbonate, 2-ethoxyethyl ethyl carbonate, bis (2-methoxyethyl) carbonate, and bis (2-ethoxyethyl) carbonate.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The positive electrode used in the non-aqueous electrolyte secondary battery of the present invention contains a positive electrode active material. The positive electrode active material is not particularly limited as long as it is a material capable of inserting and extracting lithium, and known lithium-containing composite metal oxides,
That is, it is not particularly limited as long as it is a cobalt oxide containing lithium, a manganese oxide containing lithium, a nickel oxide containing lithium, or a composite oxide thereof, or a mixture thereof. For example, LiMO ₂ (where M is one or more kinds). (Transition metal) as a compound mainly composed of a lithium-transition metal composite oxide having a basic structure represented by LiCoO ₂ ,
LiNiO ₂ is also included, and LiMnO ₄ and LiM are also included.
_{n 2 O 4, LiM y Mn} 2-y O 4 (M = Cr, Co,
It is also possible to use Ni) or the like, or a complex oxide or mixture thereof. When a compound mainly composed of a lithium-transition metal composite oxide having a basic structure represented by LiMO ₂ (where M is one or more transition metals) is used, the transition metal M is particularly preferable because of its high discharge voltage. Co, N
It is desirable to select and use from i and Mn.

The negative electrode used in the non-aqueous electrolyte secondary battery of the present invention contains a negative electrode active material. The negative electrode active material is not particularly limited as long as it can absorb and release lithium, and examples thereof include known cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, and carbon. Carbonaceous materials such as fibers, metallic lithium, lithium alloys, polyacene and the like can be used alone or in admixture of two or more, but it is particularly preferable to use carbonaceous materials from the viewpoint of high safety. .

The non-aqueous electrolyte of the present invention is not particularly limited as long as it exhibits lithium ion conductivity, and either a non-aqueous electrolytic solution or a solid electrolyte can be used. When the non-aqueous electrolytic solution is used, it is not particularly limited, and for example, a mixed solvent of ethylene carbonate and methyl ethyl carbonate or a mixed solvent of ethylene carbonate and dimethyl carbonate is used. In the mixed solvent, propylene carbonate, butylene carbonate, vinylene carbonate, trifluoropropylene carbonate, γ-butyrolactone, 2-methyl-γ-butyl lactone, acetyl-γ-butyrolactone, γ-valerolactone, sulfolane, 1,2-dimethoxy. Ethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 3-methyl-1,3-dioxolane, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, dimethyl carbonate, diethyl carbonate,
Methyl ethyl carbonate, dipropyl carbonate,
Methyl propyl carbonate, ethyl isopropyl carbonate, dibutyl carbonate, etc. may be used alone or in admixture of two or more.

The electrolyte salt as a solute of the non-aqueous electrolyte is not particularly limited, and for example, LiClO ₄ , LiAsF ₆ , Li
PF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiCF ₃ C
F ₂ SO ₃ , LiCF ₃ CF ₂ CF ₂ SO ₃ , LiN
_{(CF 3 SO 2) 2,} LiN (C 2 F 5 SO 2) 2 and the like may be used alone or as a mixture of two or more. Among them, LiPF ₆ is preferably used as the electrolyte salt.

As the solid electrolyte, an inorganic solid electrolyte or a polymer solid electrolyte can be used.

In the present invention, the non-aqueous electrolyte further contains an acyclic carbonic acid ester represented by the following general formula (1).

[0017]

[Chemical 3] (Wherein _{R 1} represents a substituent having 1 to 12 carbon atoms containing an ether bond, _{R 2} represents a substituent having 1 to 12 carbon atoms containing alkyl group having 1 to 6 carbon atoms, or an ether bond .)

Here, the substituent having 1 to 12 carbon atoms containing an ether bond of R ₁ is not particularly limited, but a substituent in which an alkoxy group having 1 to 6 carbon atoms is bonded to an alkyl group having 1 to 6 carbon atoms. Groups. Where the carbon number is 1 to 6
Examples of the alkyl group include, for example, a methyl group, an ethyl group,
n-propyl group, isopropyl group, n-butyl group, se
c-butyl group, tert-butyl group, n-pentyl group,
An n-hexyl group and the like are preferable, and examples of the alkoxyl group having 1 to 6 carbon atoms include a methoxy group, an ethoxy group,
n-propoxy group, isopropoxy group, n-butoxy group, sec-butoxy group, tert-butoxy group, n-
Pentoxy group, n-hexoxy group and the like are preferable. As the substituent having an alkoxyl group having 1 to 12 carbon atoms,
Specific examples include a 2-methoxyethyl group and a 2-ethoxyethyl group.

R ₂ is an alkyl group having 1 to 6 carbon atoms or a substituent having 1 to 12 carbon atoms and containing an ether bond. Examples of the alkyl group having 1 to 6 carbon atoms of R ₂ include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, a sec-butyl group, a tert-butyl group, an n-pentyl group, The n-hexyl group and the like are preferable, and as the substituent having a carbon number of 1 to 12 and containing an ether bond of R ₂ , the same substituents as those used for R ₁ described above are preferable. Note that R ₁ and R ₂ may be the same or different from each other. That is, it may be a symmetric acyclic carbonate or an asymmetric acyclic carbonate.

Specific examples of the acyclic carbonic acid ester of the present invention include 2-methoxyethyl methyl carbonate and 2
-Ethoxyethyl methyl carbonate, 2-methoxyethyl ethyl carbonate, 2-ethoxyethyl ethyl carbonate, bis (2-methoxyethyl) carbonate, bis (2-ethoxyethyl) carbonate and the like can be mentioned. The non-cyclic carbonic acid ester contained in the non-aqueous electrolyte may be one kind or a combination of two or more kinds.

The amount of these acyclic carbonates added is
Although not particularly limited, it is preferably 0.
1 wt% to 10 wt%, more preferably 0.1 w
t% to 5 wt%, particularly preferably 1 wt% to 3 wt%
Is good. When the addition amount is in the range of 0.1 wt% to 10 wt%,
This is because the initial discharge capacity is large and the cycle characteristics are particularly improved.

As the separator of the non-aqueous electrolyte battery according to the present invention, woven cloth, non-woven cloth, synthetic resin microporous membrane or the like can be used, and particularly synthetic resin microporous membrane can be preferably used. Among them, a polyolefin-based microporous film such as a polyethylene and polypropylene microporous film or a composite microporous film thereof is preferably used in terms of thickness, film strength, film resistance and the like.

Further, by using a solid electrolyte such as a polymer solid electrolyte, it can also serve as a separator.
In this case, the solid polymer electrolyte may further contain an electrolytic solution, for example, by using a porous solid polymer electrolyte membrane as the solid polymer electrolyte.

The non-aqueous electrolyte secondary battery of the present invention includes a cylindrical type, a square type, a sheet type, a laminated type, a coin type, a pin type, etc.
It can be used for any of them, and the shape is not particularly limited.

[0025]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a schematic cross-sectional view of a prismatic nonaqueous electrolyte secondary battery that is an embodiment of the present invention. The prismatic non-aqueous electrolyte secondary battery 1 includes a positive electrode 3 formed by applying a positive electrode mixture to a positive electrode current collector made of aluminum foil, and a negative electrode formed by applying a negative electrode mixture to a negative electrode current collector made of copper foil. 4 in which a flat-wound electrode group 2 wound with a separator 5 interposed therebetween and a non-aqueous electrolyte are housed in a battery case 6, width 30 mm × height 4
It is 8 mm x 4 mm thick.

A battery lid 7 provided with a safety valve 8 is attached to the battery case 6 by laser welding, and a negative electrode terminal 9 is attached.
Is connected to the negative electrode 4 via the negative electrode lead 11, and the positive electrode 3 is connected to the battery lid via the positive electrode lead 10.

The positive electrode mixture was prepared by mixing 87 parts by weight of LiCoO _{2 as} an active material, 5 parts by weight of acetylene black as a conductive material, and 8 parts by weight of polyvinylidene fluoride as a binder to prepare N-methyl-2-pyrrolidone. Was appropriately added and dispersed to prepare a slurry. This slurry was uniformly applied to an aluminum current collector having a thickness of 20 μm, dried, and then compression-molded by a roll press to produce a positive electrode 3.

The negative electrode mixture was prepared by mixing 95 parts by weight of scaly graphite, 2 parts by weight of carboxymethyl cellulose and 3 parts by weight of styrene-butadiene rubber and adding water appropriately to disperse the mixture to prepare a slurry. The slurry was uniformly applied to a copper current collector having a thickness of 15 μm, dried, and compression-molded with a roll press to prepare a negative electrode 4. For the separator 5, a 25-micron-thick microporous polyethylene film was used. Using the above components, the rated capacity is 600 mAh, the width is 30 mm, the height is 48 mm, and the thickness is 4 m.
m rectangular non-aqueous electrolyte secondary battery was produced.

A non-aqueous electrolyte was used as the non-aqueous electrolyte and prepared as in the following Examples and Comparative Examples.

[0030]

[Table 1]

[0031]

[Table 2]

<Examples 1 to 5> Ethylene carbonate (EC) and ethylmethyl carbonate (EMC) were mixed at a volume ratio of 30:70. As an additive, 0.1 to 10 wt% of 2-methoxyethylmethyl carbonate was added and dissolved in each of the solutions as an additive to prepare an electrolytic solution. In addition, 1 mol / mol of LiPF ₆ was added to the electrolytic solution.
Liter dissolved.

<Examples 6 to 10> 2 as shown in Table 1
-Ethoxyethyl methyl carbonate 0.1-10w
An electrolytic solution was prepared in the same manner as in Example 1 except that t% was added and dissolved.

<Examples 11 to 15> 2-methoxyethyl ethyl carbonate was added in an amount of 0.1 to 10 as shown in Table 1.
An electrolytic solution was prepared in the same manner as in Example 1 except that the solution was added by wt% and dissolved.

<Examples 16 to 20> As shown in Table 2, 0.1 to 10 of 2-ethoxyethyl ethyl carbonate was added.
An electrolytic solution was prepared in the same manner as in Example 1 except that the solution was added by wt% and dissolved.

<Examples 21 to 25> 0.1 to 1 of bis (2-methoxyethyl) carbonate was added as shown in Table 2.
An electrolytic solution was prepared in the same manner as in Example 1 except that 0 wt% was added and dissolved.

<Examples 26 to 30> 0.1 to 1 of bis (2-ethoxyethyl) carbonate was added as shown in Table 2.
An electrolytic solution was prepared in the same manner as in Example 1 except that 0 wt% was added and dissolved.

<Comparative Example 1> Ethylene carbonate (E
C) and ethylmethyl carbonate (EMC) were mixed at a volume ratio of 30:70, and LiPF ₆ was added to this solution at 1 mol / mol.
Liter dissolved. No additive was added to this solution.

<Comparative Example 2> Ethylene carbonate (E
C) and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 30:70. As an additive, 3 wt% of methyltrifluoroethyl carbonate was added and dissolved in this solution as an additive to prepare an electrolytic solution. Note that 1 mol / liter of LiPF ₆ was dissolved in the electrolytic solution.

<Measurement of Cycle Retention of Non-Aqueous Electrolyte Secondary Battery> The non-aqueous electrolyte secondary batteries of Examples 1 to 30 and Comparative Examples 1 and 2 were operated at 4.20 V at a current of 600 mA at 25 ° C.
Charged at constant current and constant voltage for 2.5 hours to reach a fully charged state, and then at a current of 600mA, final voltage 2.75V.
It was made to discharge on condition of. This is one cycle and a total of 50
The cycle of discharge capacity was measured with 0 cycles.

The results are also shown in Tables 1 and 2. In Tables 1 and 2, the cycle retention rate indicates the ratio (%) of the discharge capacity at the 500th cycle to the discharge capacity at the first cycle (initial discharge capacity), and was calculated by the following formula. Cycle retention rate (%) = (discharge capacity at the 500th cycle / initial discharge capacity) x 100

It is understood that the initial capacities of Examples 1 to 30 are all Comparative Examples 1 to 2 or more. In addition, Examples 1 to 1
It can be seen that all of No. 30 have a higher cycle retention rate than Comparative Examples 1 and 2. Further, it can be seen that the addition of 1 to 3 wt% of the acyclic carbonate according to the present invention markedly improves the initial discharge capacity and the cycle retention rate.

The mechanism of action by adding the acyclic carbonate according to the present invention is not clear, but it is considered as follows. The film on the surface of the negative electrode obtained by the reductive decomposition of these acyclic carbonates has an ether bond and is chemically stable, and this film suppresses the reaction between the electrolytic solution and the carbon negative electrode. Disassembly is prevented,
It is considered that the initial discharge capacity and the cycle retention rate are significantly improved.

Further, since this coating film has oxygen atoms derived from the substituents of the acyclic carbonic acid ester, lithium ions are coordinated with the oxygen atoms to improve the lithium ion conductivity of the electrolytic solution. In addition, it is considered that the initial discharge capacity and the cycle retention rate are remarkably improved.

On the other hand, as in Comparative Example 1, ethyl methyl carbonate (EM
In the case of only C), since the coating film on the negative electrode does not have oxygen atoms derived from the substituent, it is considered that the lithium ion conductivity is not good and the initial discharge capacity and the cycle retention rate are low.

When the methyltrifluoroethyl carbonate of Comparative Example 2 was added, LiF (lithium fluoride), which is a decomposition product of this carbonate, was formed on the negative electrode.
Are deposited, the electric conductivity is deteriorated, and Examples 1 to 30 are performed.
It is considered that the initial discharge capacity and cycle retention rate are lower.

The methyltrifluoroethyl carbonate of Comparative Example 2, for example, ditrifluoroethyl carbonate, methyl pentafluoropropyl carbonate,
Acyclic carbonic acid esters containing fluorine such as trichloroethyl methyl carbonate and tribromoethyl methyl carbonate, chlorine, or halogen such as bromine are not preferable in consideration of environmental problems.

The present invention is not limited to the embodiments described with reference to the above description and drawings, and can be variously modified and implemented without departing from the scope of the invention.

[0049]

According to the non-aqueous electrolyte secondary battery of the present invention,
Cycle characteristics are improved.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view of a prismatic nonaqueous electrolyte secondary battery according to an embodiment of the present invention.

[Explanation of symbols]

1 ... Non-aqueous electrolyte secondary battery 2 ... Electrode group 3 ... Positive electrode 4 ... Negative electrode 5 ... Separator 6 ... Battery case 7 ... Battery lid 8 ... Safety valve 9 ... Negative electrode terminal 10 ... Positive electrode lead 11 ... Negative electrode lead

Claims (2)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and a non-aqueous electrolyte, wherein the non-cyclic carbonate ester represented by the general formula (1) is used. A non-aqueous electrolyte secondary battery containing the non-aqueous electrolyte. [Chemical 1] (Wherein _{R 1} represents a substituent having 1 to 12 carbon atoms containing an ether bond, _{R 2} represents a substituent having 1 to 12 carbon atoms containing alkyl group having 1 to 6 carbon atoms, or an ether bond .) 2. The non-cyclic carbonic acid ester is 2-methoxyethylmethyl carbonate, 2-ethoxyethylmethyl carbonate, 2-methoxyethylethyl carbonate, 2-ethoxyethylethyl carbonate, bis (2
The non-aqueous electrolyte secondary battery according to claim 1, which is at least one selected from the group consisting of -methoxyethyl) carbonate and bis (2-ethoxyethyl) carbonate.