JPH1027625A - Nonaqueous electrolyte and nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a secondary battery using a nonaqueous electrolyte having large ion conductivity even in use at a low temperature in a cold district. SOLUTION: This secondary battery is a nonaqueous electrolyte consisting of electrolyte salt (A) and a mixed solvent (B) of dimethyl carbonate (DMC), methyl ethyl carbonate(MEC) and cyclic ester carbonate, composition ratio of dimethyl carbonate, methyl ethyl carbonate and cyclic ester carbonate in the mixed solvent, relating to a solvent total unit (total amount of dimethyl carbonate, methyl ethyl carbonate and cyclic ester carbonate), contains 10 to 70vol.% DMC, 10 to 70vol.% MEC and 1vol.% or 20vol.% or less cyclic ester carbonate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte and a non-aqueous electrolyte secondary battery, and more particularly, to a non-aqueous electrolyte and a non-aqueous electrolyte having high ionic conductivity even when used at low temperatures in a cold region. And a secondary battery using the same.

[0002]

2. Description of the Related Art Hitherto, Ni-Cd batteries and lead batteries using an aqueous electrolyte have been widely used as general secondary batteries. These batteries are a camera-integrated V
It is included in portable electronic devices such as TRs, mobile phones, and laptop computers, and is used in various environments such as under cold conditions.

However, these batteries are becoming insufficient in terms of high energy density, and cadmium and lead used in such batteries are not preferable from the viewpoint of global environmental protection. Are being regulated.

[0004] Therefore, as an alternative to the Ni-Cd battery and the lead battery, a non-aqueous electrolyte obtained by dissolving an electrolyte in a non-aqueous solvent, and a non-aqueous electrolyte secondary battery using the non-aqueous electrolyte Has been attracting attention, and various proposals have conventionally been made.

For example, Japanese Patent Application Laid-Open No. 7-45304 discloses an organic electrolyte comprising a positive electrode made of a material capable of inserting and extracting lithium ions, a negative electrode made of a carbon material capable of inserting and extracting lithium ions, and an organic electrolyte. A liquid secondary battery, wherein the electrolyte comprises a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and methyl ethyl carbonate (MEC);
An organic electrolyte secondary battery in which the composition ratio of DMC and MEC is 30 to 50 vol%, 10 to 50 vol%, and 10 to 50 vol%, respectively, based on the entire solvent is described.

[0006] Japanese Patent Application Laid-Open No. 7-14607 discloses that
A negative electrode using a carbon material capable of doping and undoping lithium as a negative electrode active material, a positive electrode using a composite oxide of lithium and transition metal as a positive electrode active material, and a nonaqueous solution obtained by dissolving an electrolyte in a nonaqueous solvent. A non-aqueous electrolyte secondary battery comprising an electrolytic solution, and wherein the non-aqueous solvent contains methyl ethyl carbonate and dimethyl carbonate, is described. Methyl ethyl capacity M, dimethyl carbonate capacity D
When 3/10 ≦ (M + D) ≦ 7/10,
An embodiment in which 1/9 ≦ D / M ≦ 8/2 is described.

[0007] These secondary batteries are often used in cold regions, and therefore, it has been required to further improve the ionic conductivity at low temperatures even in the non-aqueous electrolyte used for the secondary batteries.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and to provide a non-aqueous electrolyte and a non-aqueous electrolyte which have a high ionic conductivity even when used at low temperatures in cold regions. It is intended to provide a secondary battery used.

[0009]

SUMMARY OF THE INVENTION The non-aqueous electrolyte according to the present invention comprises an electrolyte salt (A), dimethyl carbonate (hereinafter sometimes referred to as DMC), methylethyl carbonate (hereinafter sometimes referred to as MEC) and cyclic carbonate. A mixed solvent (B) and a non-aqueous electrolyte solution, wherein the composition ratio of dimethyl carbonate, methyl ethyl carbonate and cyclic carbonate in the mixed solvent is equal to the total amount of solvent (total of dimethyl carbonate, methyl ethyl carbonate and cyclic carbonate) Amount) with respect to 10 to 70% by volume of DMC,
MEC is 10 to 70% by volume and cyclic carbonate is 1
It is characterized by being not less than 20% by volume and not less than 20% by volume.

[0010] In a preferred embodiment of the present invention,
The cyclic ester carbonate is desirably ethylene carbonate or propylene carbonate or a mixture of both. The non-aqueous electrolyte secondary battery according to the present invention is a composite of a negative electrode having an active material of any of a carbon material or a metal chalcogen compound capable of doping / dedoping lithium and lithium ions and a transition metal containing lithium. A non-aqueous electrolyte secondary battery including a positive electrode using an oxide as an active material, a separator, and a non-aqueous electrolyte, wherein the non-aqueous electrolyte comprises the non-aqueous electrolyte.

Such a non-aqueous electrolyte has a high ionic conductivity even when used at low temperatures in a cold region, and a secondary battery using such a non-aqueous electrolyte is suitable for use in a cold region.

[0012]

DETAILED DESCRIPTION OF THE INVENTION Hereinafter, the nonaqueous electrolyte according to the present invention and a secondary battery using the nonaqueous electrolyte will be specifically described.

[Non-Aqueous Electrolyte ] The non-aqueous electrolyte according to the present invention is a non-aqueous electrolyte comprising an electrolyte salt (A) and a mixed solvent of solvent DMC, MEC and cyclic carbonate (B). Thus, the content of the cyclic carbonate was 1 to the entire solvent (total amount of DMC, MEC and cyclic carbonate).
It is not less than 20% by volume, preferably from 5% to 19% by volume, more preferably from 10% to 17% by volume.

As the electrolyte salt contained in such a non-aqueous electrolyte, specifically, for example, LiPF ₆ , LiB
F ₄ , LiAsF ₆ , LiClO ₄ , LiN (SO ₂ C
_{F 3) 2, LiN (SO} 2 C 2 F 5) 2 and the like, preferably _{LiPF 6, LiBF 4, LiAsF 6} , LiClO 4 is used. Such an electrolyte salt can be used alone or in combination of two or more.

[0015] Such an electrolyte salt dissociates in a non-aqueous electrolyte to generate Li ⁺ ions. Such an electrolyte salt depends on the type of the electrolyte salt used, the type of the solvent, and the like, but is usually 0.5 to 2.0 mol / liter, preferably 0.7 to 1. Desirably, it is contained in an amount of 5 mol / liter.

The chain carbonate is dimethyl carbonate (DM)
C) and methyl ethyl carbonate (MEC) in a total amount of solvent (the total amount of DMC, MEC and cyclic carbonate) of 10 to 70% by volume of DMC and 10 to 70% by volume of MEC. And preferably the DMC is 20 to
70% by volume, 20 to 70% by volume of MEC, more preferably 30 to 60% by volume of DMC and 30 to 60% by volume of MEC
% By volume.

The cyclic carbonate of the solvent has the following formula [I]:

[0018]

Embedded image

(In the formula [I], R ¹ , R ² , R ³ and R ⁴ each independently represent hydrogen or an alkyl group having 1 to 2 carbon atoms,
Preferably, each represents hydrogen. Specifically, for example, ethylene carbonate (ethylene carbonate (EC)) in which all of R ^{1 to} R ⁴ are hydrogen, propylene carbonate in which R ^{1 to} R ³ are hydrogen and R ⁴ is a methyl group, R ¹ = R ³ = H and R ² = R ⁴ =
2,3-butylene carbonate of CH _{3 and} the like can be mentioned.

These cyclic carbonates can be used alone or in combination of two or more. In the present invention, among such cyclic carbonates, ethylene carbonate (EC), propylene carbonate (PC), or a mixture of both is preferably used.

In the present invention, the volume ratio (cyclic carbonate / chain carbonate) of the above-mentioned cyclic carbonate and chain carbonate (mixture of DMC and MEC) as a solvent is larger than 80/20. It is used in an amount such that it falls within the range of 99/1 or less.
/ 19 to 95/5, more preferably 83/17
It is used in such an amount as to be in the range of 90/10.

As described above, the quantitative ratio of the cyclic carbonate to the chain carbonate is more than 80/20 and 99/1.
Using these esters in amounts such that:
Ion conductivity under low temperature conditions of the obtained non-aqueous electrolyte,
For example, 2.7 to 3.5 mS / cm, preferably 3.0
-3.5 mS / cm, which is preferable because it is high.

Such a non-aqueous electrolyte has a high ionic conductivity at a low temperature, and can be used, for example, in a temperature range of about -30 to 60 ° C. In the present invention, even when diethyl carbonate (DEC) is used instead of MEC, a nonaqueous electrolyte having high ionic conductivity can be obtained at the same time in a specific content volume ratio range with cyclic carbonate. When DEC is used, the preferred composition of the electrolytic solution for obtaining a non-aqueous electrolyte having a high ionic conductivity is 10 to 70% by volume, preferably 30% by volume, of DMC, DEC and cyclic carbonate in a total of 100% by volume. It is an electrolytic solution comprising -60% by volume, DEC of 10-70% by volume, preferably 30-60% by volume, and cyclic carbonate of 11-20% by volume, preferably 15-19% by volume. EC, PC as cyclic carbonate
Or a mixture of both can be mentioned.

[ Secondary Battery Using Non-Aqueous Electrolyte ] The non-aqueous electrolyte secondary battery according to the present invention comprises a negative electrode containing lithium as an active material and a composite oxide of a transition metal containing lithium as an active material. , A separator, and the non-aqueous electrolyte.

Such a nonaqueous electrolyte secondary battery has, for example, a schematic longitudinal sectional structure as shown in FIG. That is, the non-aqueous electrolyte secondary battery includes a negative electrode 1 formed by applying a negative electrode active material to a negative electrode current collector 9 and a positive electrode 2 formed by applying a positive electrode active material to a positive electrode current collector 10. 3, and is housed in a battery can 5 with the insulating plate 4 placed above and below the obtained wound body. A battery lid 7 is attached to the battery can 5 by caulking through a sealing gasket 6, and a negative electrode lead 11 is provided, respectively.
In addition, it is electrically connected to the negative electrode 1 or the positive electrode 2 through the positive electrode lead 12, and is configured to function as the negative electrode or the positive electrode of the battery.

In this battery, the positive electrode lead 12 is electrically connected to the battery lid 7 via the current interrupting thin plate 8. In this battery, when the pressure inside the battery rises, the current interrupting thin plate 8 is pushed up and deformed, and the positive electrode lead 12 is cut leaving a portion welded to the thin plate 8 to cut off the current.

The non-aqueous electrolyte is filled in the voids in such a battery. In the non-aqueous electrolyte secondary battery according to the present invention, except that the above-described non-aqueous electrolyte is used, the above-described negative electrode active material, positive electrode active material, separator, etc., all of which are disclosed in JP-A-7-14607 Can be used. As the negative electrode active material of the nonaqueous electrolyte secondary battery according to the present invention, various materials can be selected as long as doping / dedoping of lithium ions is possible, and among them, it is preferable to use a carbon material. . Examples of such a carbon material include a graphitizable carbon material, a non-graphitizable carbon material, and a graphite material.
Non-graphitizable carbon materials and graphite materials are preferably used.
As the above non-graphitizable carbon material, the (002) plane spacing obtained by the X-ray diffraction method is 0.37 nm or more, and the true density is 1.70.
Those having one or more exothermic peaks at 700 ° C. or less in a differential thermal analysis (DTA) of less than g / cm ³ in air are preferred. The graphite material has a true density of 2.10 g / cm ³
Above, preferably 2.18 g / cm ³ or more.
The graphite material having such a true density has a (002) plane spacing obtained by the X-ray diffraction method of less than 0.340 nm, preferably 0.335 nm or more and 0.337 nm or less.
2) The plane has a C-axis crystallite thickness of 14.0 nm or more.
In addition to such a carbon material, a compound containing phosphorus, oxygen, and carbon as main components can be used because it has the same physical property parameters as those of the non-graphitizable carbon material. Further, in the nonaqueous electrolyte secondary battery according to the present invention, a metal chalcogen compound capable of doping / dedoping lithium ions can be used as a negative electrode material. As the metal chalcogen compound, it is possible to use a crystalline compound or an amorphous compound mainly composed of a transition metal oxide such as iron oxide, ruthenium oxide, molybdenum oxide, tungsten oxide, titanium oxide, tin oxide, and silicon oxide. Particularly, a compound having a charge / discharge potential close to lithium is desirable.

Specifically, as described in Japanese Patent Application Laid-Open No. 7-14607, for example, a functional group containing oxygen is introduced into petroleum pitch in an amount of 10 to 20% by weight as a negative electrode active material. Crosslink and then carbonize in a stream of inert gas to prepare a carbon precursor. Next, the carbon precursor is fired at a temperature of, for example, about 900 to 1500 ° C. to prepare a carbon material having properties similar to glassy carbon. Then
The powder of the carbon material thus obtained and polyvinylidene fluoride (PVDF) are mixed and dispersed in a solvent such as N-methylpyrrolidone to prepare a negative electrode mixture slurry (paste). The negative electrode mixture slurry is applied to a negative electrode assembly made of a strip-shaped copper foil, dried, and then compression-molded to obtain a strip-shaped negative electrode.

[0029] The thickness of such a negative electrode mixture is
For example, it is about 40 to 160 μm [Example: 80 μm on each surface]. As the positive electrode active material, a general formula Li _x MO
₂ (M: at least one of Co, Ni and Mn.)
And the intercalation compound containing lithium. Among them, LiCoO ₂ is preferable because of its high energy density.

Such a positive electrode 2 is mixed, for example, in a molar amount of 2 times of cobalt carbonate with respect to 1 mol of lithium carbonate,
It is fired in air at about 0 to 110 ° C. to obtain LiCoO ₂ and then finely pulverized to a particle size of about 5 to 30 μm. Next, a mixture of the LiCoO ₂ fine particles and lithium carbonate, graphite as a conductive material, and polyvinylidene fluoride as a binder were mixed to prepare a positive electrode mixture, and the mixture was dispersed in N-methylpyrrolidone. A mixture slurry is obtained. This slurry is applied to both sides of a positive electrode current collector made of a strip-shaped aluminum foil, dried, and compression molded to obtain a desired positive electrode. The thickness of the mixture of such a positive electrode is the same as that of the above-mentioned negative electrode.

In such a positive electrode, a charge / discharge capacity equivalent to at least 250 mAh or more per gram of the negative electrode active material is usually obtained in a steady state after repeated charge and discharge, for example, about five times.
i.

As the separator, for example, a thickness of 10
A microporous polypropylene film having a width of about 60 μm and a width of about 30 to 50 mm is used. Such a non-aqueous electrolyte secondary battery is excellent in energy density and cycle life, can be used at a wide temperature range from a low temperature of about -30 ° C to a high temperature of about 60 ° C, and is excellent in safety. .

A secondary battery using such a non-aqueous electrolyte has a high ionic conductivity even when used at low temperatures in a cold region.
It is suitable for use in cold regions.

[0034]

The non-aqueous electrolyte according to the present invention has a high ionic conductivity even when used at low temperatures in cold regions. Secondary batteries using such non-aqueous electrolytes are suitable for use in cold regions. It is suitable.

[0035]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples.

[0036]

Example 1 <Preparation of non-aqueous electrolyte solution and measurement of its electric conductivity and withstand voltage> 15.2 g of lithium hexafluorophosphate (LiPF ₆ )
(100 mmol) in methyl ethyl carbonate (MEC),
Dimethyl carbonate (DMC) and ethylene carbonate (EC) were mixed at a volume ratio (MEC: DMC: EC) of 50:35:15.
Was dissolved in the mixed solvent prepared in (1) to prepare 100 ml of an electrolytic solution (electrolyte concentration: 1.0 mol / l). The electric conductivity and withstand voltage of these electrolytes were measured. The electric conductivity was measured at 10 kHz using an electric conductivity meter with a platinum black electrode. The withstand voltage of the electrolytic solution was measured by placing the above electrolytic solution in a three-electrode withstand voltage measuring cell using glassy carbon for the working electrode, platinum for the counter electrode, and lithium metal for the reference electrode, and using a potentiogalvanostat at 50 mV / sec. And the oxidative decomposition current is 0.1 m based on the potential of lithium metal.
The range in which A or more did not flow was defined as the withstand voltage. The results are shown in Table 1.

[0037]

Comparative Examples 1-2, Reference Examples 1-2 Various tests such as ionic conductivity were conducted in the same manner as in Example 1 except that the solvent and its composition were changed as shown in Table 1. Was.

The results are shown in Table 1.

[0039]

[Table 1]

[0040]

Example 2 <Preparation of Negative Electrode Active Material> First, a negative electrode active material was prepared as follows.

That is, as described in JP-A-7-14607, a functional group containing oxygen is introduced into petroleum pitch in an amount of 10 to 20% by weight, oxygen is crosslinked, and then an inert gas stream is introduced. To prepare a carbon precursor. Next, the carbon precursor was fired at a temperature of, for example, about 1200 ° C. to prepare a carbon material having properties similar to glassy carbon.

Next, 90 parts by weight of the thus obtained carbon material powder and polyvinylidene fluoride (P
VDF) was mixed with 10 parts by weight of NDF and dispersed in N-methylpyrrolidone as a solvent to prepare a negative electrode mixture slurry (paste).

The negative electrode mixture slurry was applied to a 10 μm-thick negative electrode assembly made of a strip-shaped copper foil, dried, and then compression-molded to obtain a strip-shaped negative electrode 1. The thickness of such a negative electrode mixture was 80 μm on each surface. The electrode width was 41.5 mm and the length was 700 mm.

<Preparation of Positive Electrode> The positive electrode 2 was prepared as follows. That is, 2 moles of cobalt carbonate was mixed with 1 mole of lithium carbonate, and the mixture was fired in air at 90 ° C. for 5 hours to obtain LiCoO ₂ and then pulverized to a 50% cumulative particle size of about 15 μm.

Next, 91 parts by weight of a mixture of 95 parts by weight of the LiCoO ₂ fine particles and 5 parts by weight of lithium carbonate, 6 parts by weight of graphite as a conductive material, and 3 parts by weight of polyvinylidene fluoride as a binder were mixed. A cathode mixture was prepared and dispersed in N-methylpyrrolidone to obtain a cathode mixture slurry.

This slurry was applied to both sides of a 20 μm-thick aluminum foil positive electrode current collector, dried, and compression molded to obtain a belt-shaped positive electrode. The mixture thickness of such a positive electrode was 80 μm on both sides, the electrode width was 40.5 mm, and the length was 650 mm.

<Preparation of spiral type electrode> The strip-shaped negative electrode 1, the strip-shaped positive electrode 2 and the separator 3 [microporous polypropylene film having a thickness of 25 µm and a width of 44 mm] obtained as described above were used as a negative electrode, a separator, a positive electrode, The layers were laminated in the order of the separators and then wound many times to produce a spiral electrode having an outer diameter of 20 mm.

<Preparation of Battery> The spiral electrode was housed in a Ni-plated iron battery can 5. Next, the insulating plates 4 are arranged on the upper and lower surfaces of the spiral electrode, the aluminum positive electrode lead 12 is led out from the positive electrode current collector 10, and the Ni negative electrode lead 11 is drawn out from the negative electrode current collector 9. And welded to the battery can 5.

Next, the battery can 5 containing the spiral-type electrode
Among them, methyl ethyl carbonate (MEC) / dimethyl carbonate (DMC) / ethylene carbonate (E
A non-aqueous electrolyte in which LiPF ₆ as an electrolyte salt was dissolved in an amount of 1.0 mol / liter was injected into a mixed solvent containing C) in an amount of 50/35/15 (volume ratio).

Then, the battery can 5 is caulked through an insulating sealing gasket 6 coated on asphalt to fix the safety valve device 8 having a current cut-off mechanism and the battery lid 7 to improve the airtightness in the battery. Hold, diameter 20m
m, a cylindrical nonaqueous electrolyte secondary battery having a height of 50 mm was produced.

[Brief description of the drawings]

FIG. 1 shows an embodiment of a non-aqueous electrolyte secondary battery according to the present invention.

[Explanation of symbols]

 1 ··· negative electrode, 2 ··· positive electrode, 3 ··· separator, 4 ··· insulating plate, 5 ··· battery can, 6 ··· sealing gasket, 7 ··· Battery cover, 8 safety plate, 9 negative electrode current collector, 10 positive electrode current collector, 11 negative electrode lead, 12 positive electrode lead.

Continuation of the front page (51) Int.Cl. ⁶ Identification number Agency reference number FI Technical indication location H01M 4/58 H01M 4/58 (72) Inventor Atsushi Komaru 1 Shimosugita, Takakura, Wada-cho, Koriyama-shi, Fukushima Prefecture No. 1 within Sony Energy Tech Co., Ltd. (72) Inventor Masayuki Nagamine No. 1 Shimosugishita, Takakura, Wada-cho, Koriyama-shi, Fukushima Prefecture No. 1 within Sony Energy Tech Co., Ltd. (72) Inventor Mio Nishi Tokyo 6-7-35 Kita Shinagawa, Shinagawa-ku Sony Corporation

Claims (8)

[Claims] An electrolyte salt (A), dimethyl carbonate (DMC) and methyl ethyl carbonate (MEC)
And a mixed solvent (B) of a cyclic carbonate and a cyclic carbonate, wherein the composition ratio of dimethyl carbonate, methyl ethyl carbonate and the cyclic carbonate in the mixed solvent is the whole solvent (dimethyl carbonate and methyl ethyl carbonate). Non-aqueous electrolysis, wherein DMC is 10 to 70% by volume, MEC is 10 to 70% by volume, and cyclic carbonate is 1% by volume or more and less than 20% by volume based on the total amount of liquid. 2. The method according to claim 1, wherein the cyclic carbonate is ethylene carbonate or propylene carbonate or a mixture of both, and the content of the cyclic carbonate is 5 to 19% by volume based on the whole solvent. Non-aqueous electrolyte. 3. A negative electrode comprising an active material of lithium, a carbon material capable of doping and undoping lithium ions, and a metal chalcogen compound capable of doping and undoping lithium ions; and a transition metal containing lithium. A non-aqueous electrolyte secondary battery comprising a positive electrode using a composite oxide of the above as an active material, a separator, and a non-aqueous electrolyte, wherein the non-aqueous electrolyte is the one according to claim 1. Non-aqueous electrolyte secondary battery. 4. A negative electrode comprising an active material of lithium, a carbon material capable of doping and undoping lithium ions, and a metal chalcogen compound capable of doping and undoping lithium ions, and a transition metal containing lithium. A non-aqueous electrolyte secondary battery having a positive electrode using a composite oxide as an active material, a separator, and a non-aqueous electrolyte, wherein the non-aqueous electrolyte is that according to claim 2. Non-aqueous electrolyte secondary battery. 5. The non-aqueous electrolyte secondary battery according to claim 3, wherein a carbon material capable of doping / dedoping lithium is used as the negative electrode. 6. The non-aqueous electrolyte secondary battery according to claim 5, wherein the carbon material is a graphite material. 7. The non-aqueous electrolyte secondary battery according to claim 5, wherein the carbon material is a non-graphitizable carbon material. 8. The non-aqueous electrolyte secondary battery according to claim 3, wherein the metal chalcogen compound is a crystalline or amorphous metal oxide.