JP3294400B2 - Non-aqueous electrolyte and non-aqueous electrolyte battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a novel non-aqueous electrolyte and a non-aqueous electrolyte battery using the same.

[0002]

2. Description of the Related Art A battery using a non-aqueous electrolyte has a high voltage and a high energy density and is excellent in reliability such as storability, so that it is widely used as a power source for consumer electronic devices. . However, the non-aqueous electrolyte-based electrolyte has a fact that the electrical conductivity is lower by one to two orders of magnitude than the aqueous electrolyte, especially,
In the case of a non-aqueous electrolyte having a low withstand voltage, the charge / discharge efficiency of a battery using the non-aqueous electrolyte is reduced, and the life is shortened. In addition, a battery using a non-aqueous electrolyte may repeatedly deposit and discharge a needle-shaped metal called a dendrite. The problem is that there is a high danger that the dendrite breaks through the separator that separates and short-circuits.

In order to improve the electric conductivity of a non-aqueous electrolyte,
Attempts have been made to add low-viscosity solvents such as dimethoxyethane, tetrahydrofuran, and 1,3-dioxolane to electrolytes such as propylene carbonate, γ-butyrolactone, and sulfolane, which are solvents having a high dielectric constant (for example, electrochemical, 53 No. 3, 173 (1985)). Further, as an attempt to improve the durability of the electrolytic solution, use of a carbonate ester such as diethyl carbonate having a high withstand voltage in place of a solvent having a low withstand voltage such as dimethoxyethane to improve the charge / discharge efficiency of the battery has been proposed (for example, JP-A-2-10666), and adding a phosphoric acid ester to an electrolytic solution (JP-A-4-1848).
No. 70) to give the electrolyte a self-extinguishing property.

[0004]

However, since batteries with high energy density are desired, high voltage batteries have been studied from various fields. For example, when a composite oxide of lithium and a transition metal such as LiCoO ₂ , LiNiO ₂ , and LiMn ₂ O ₄ is used for a positive electrode of a battery and metallic lithium or an alloy of lithium or a compound of lithium and carbon is used for a negative electrode, 4 V Batteries capable of generating phenomena have been studied. In this case, since the decomposition of the electrolyte solution due to oxidation is likely to occur, conventionally used esters such as γ-butyrolactone and ethyl acetate and ethers such as 1,3-dioxolan, tetrahydrofuran and dimethoxyethane have a low withstand voltage. It is not a preferable solvent because it reacts with the positive electrode, and there are problems such as a decrease in the capacity of the battery every time charge and discharge are repeated and an increase in the internal pressure of the battery due to generation of gas, and an electrolyte solvent having oxidation resistance is desired. I was

When lithium metal, a lithium alloy or a lithium compound is used for the negative electrode of a battery, the lithium metal deposited by charging or overcharging has a high reactivity, so that it is difficult to use an electrolyte solvent having excellent oxidation resistance. But it may react. When charging and discharging are repeated, needle-shaped lithium called a dendrite may precipitate, dropping from the electrode to generate highly reactive lithium powder, and short-circuiting due to the dendrite breaking through the separator separating the positive and negative electrodes. And other safety issues have been raised.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a non-aqueous electrolyte having excellent withstand voltage and charge / discharge cycle characteristics, a high flash point and excellent safety. . Another object of the present invention is to provide a non-aqueous electrolyte battery that is safe, can generate a high voltage, and has excellent battery performance.

[0007]

DISCLOSURE OF THE INVENTION The present inventors have found an electrolyte having excellent withstand voltage and charge / discharge cycle characteristics in order to produce a non-aqueous electrolyte battery which is safe, can generate a high voltage, and has excellent battery performance. For this reason, I conducted diligent research. As a result, it has been found that, when the β-position hydrogen of at least one alkyl group of the carbonate ester is substituted, the carbonate ester has improved chemical stability, reduced reactivity with metallic lithium, and also improved oxidation resistance. Was. And β of at least one alkyl group
It has been found that a battery using an electrolytic solution containing a carbonate ester substituted with a hydrogen atom has improved safety and, in addition, has improved charge / discharge cycle life.

That is, the non-aqueous electrolyte of the present invention has the general formula [I]
Which contains a carbonate ester of

[0009]

Embedded image

(In the formula, R ¹ represents an alkyl group or a halogen atom-substituted alkyl group, and R ² represents an alkyl group having no hydrogen at the β-position or a halogen atom-substituted alkyl group.) The liquid battery uses an electrolytic solution containing a carbonate of the general formula [I] as the electrolytic solution. In the nonaqueous electrolyte battery of the present invention, as the negative electrode material, metal materials such as metal lithium and lithium alloy, metal sulfides and various carbon materials can be used, and particularly, they can occlude and release lithium ions. Carbon materials are preferred. Such a carbon material may be graphite or amorphous carbon, and any carbon material such as activated carbon, carbon fiber, carbon black, and mesocarbon microbeads can be used.

As the positive electrode material, MoS ₂ , Ti
Transition metal oxides such as S ₂ , MnO ₂ , V ₂ O ₅ , transition metal sulfides, or LiCoO ₂ , LiMnO ₂ , LiMn
_A composite oxide of lithium and a transition metal such as ₂ O ₄ and LiNiO ₂ can be used, and a composite oxide of lithium and a transition metal oxide is preferably used. According to the present invention, the use of the electrolytic solvent containing the carbonate represented by the general formula [I] as the electrolytic solvent reduces the reactivity with metallic lithium and decomposes the electrolytic solvent by oxidation. And the flash point is increased, and the cycle life of charge and discharge of the battery is prolonged.

Hereinafter, the non-aqueous electrolyte of the present invention will be described in more detail. R ¹ of the carbonate represented by the general formula [I] represents an alkyl group, preferably an alkyl group having 1 to 5 carbon atoms or a halogen-substituted alkyl group, R ² represents an alkyl group having no β-position hydrogen, Preferably 5 to 5 carbon atoms
An alkyl group of 8, or a halogen atom-substituted alkyl group,
It preferably represents a halogen atom-substituted alkyl group having 1 to 5 carbon atoms. Examples of the alkyl group include a methyl group, an ethyl group,
Propyl group, isopropyl group and the like. Examples of the alkyl group having no β-position hydrogen include a neopentyl group (—CH _{2
C (CH 3) 3),} 2,2,2- triethyl ethyl group (-C
H ₂ C (CH ₂ CH ₃ ) ₃ ). The halogen atom-substituted alkyl group, a halogen atom fluorine atom, an alkyl group substituted by a chlorine atom or the like, it is desirable in particular fluorine atom-substituted alkyl group, 2-fluoroethyl group (-CH ₂ CFH _{2), 2} , 2-difluoroethyl group _{(-CH 2 CF 2 H),} 2,2,2- trifluoroethyl group (-CH ₂ CF _3), 2,2,3,3,3-pentafluoro-propyl (-CH ₂ CF ₂ CF ₃ ), _{2, 2, 3} ,
3-tetrafluoropropyl group (—CH ₂ CF ₂ CF
₂ H), 2,2,2,3,3,3-hexafluoroiso
And a propyl group (—CH (CF ₃ ) ₂ ).

Examples of such carbonates include carbonates having no β-position hydrogen such as methyl neopentyl carbonate and methyl 2,2,2-triethylethyl carbonate, methyltrichloroethyl carbonate, methyltribromoethyl carbonate, and methyl carbonate. Triiodoethyl, methyl 2,2,2-trifluoroethyl carbonate, ethyl 2,2,2-trifluoroethyl, methyl 2,2,3,3,3-pentafluoroethyl, methyl 2,2,2 3,3-tetrafluoroethyl, methyl carbonate 2,2,2,3,3,3-hexa
Fluoroisopropyl , di2,2,2-trifluoroethyl carbonate, 2,2,2-trifluoroethyl carbonate2
Examples thereof include halogen atom-substituted carbonates such as 2,3,3,3-pentafluoropropyl. These carbonates can be used alone or in combination of two or more as an electrolyte solvent.

As the solvent for the electrolytic solution, the carbonate represented by the general formula [I] may be used alone, but a mixed solvent with a cyclic carbonate such as propylene carbonate, γ-butyrolactone, sulfolane or the like can be used. Can be increased, and the electric conductivity can be further improved. As the cyclic carbonate, a 5- to 6-membered carbonate can be used, but a 5-membered carbonate is preferable, and ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate are particularly preferable.

When used as a mixed solvent of the carbonate represented by the general formula [I] and the cyclic carbonate,
The mixing ratio of the carbonate represented by the general formula [I] to the cyclic carbonate is in the range of 1: 9 to 9: 1, preferably 2: 8 to 8: 2 by volume. Within this range, the viscosity is low and the dielectric constant is high, so that the electrical conductivity increases, which is preferable.

The solvent for electrolysis according to the present invention includes, in addition to the carbonate and cyclic carbonate represented by the general formula [I], non-aqueous solvents such as ethers and chain carbonates which are usually used as electrolyte solvents for batteries. It can be added as appropriate within a range that does not impair the characteristics of the electrolyte solvent of the present invention. When the electrolyte solution containing the carbonate of the general formula [I] is used as the electrolyte, an electrolyte used in a normal battery electrolyte can be used. LiPF ₆ , LiB
F ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , L
iAlCl ₄ , LiN ( SO ₂ CF ₃ ) ₂ , LiC ₄ F ₉ SO
₃ , lithium salts such as LiC ₈ F ₁₇ SO ₃ are preferable, and in particular, LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiClO
₄ is preferred.

The concentration at which the electrolyte is dissolved in the solvent is usually 0.1
To 3 mol / l, preferably 0.1 to 0.1 mol / l.
It can be used at 5 to 1.5 mol / l. The non-aqueous electrolyte battery of the present invention contains the non-aqueous electrolyte described above as the electrolyte, and its shape, form, and the like can be arbitrarily selected within the scope of the present invention.

[0018]

EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. 1. Synthesis of Carbonate Ester A carbonate (R ¹ -O-CO-OR ² ) shown below was converted to a corresponding alcohol (R ² OH) with dimethyl carbonate or carbonate using a 28% sodium methoxide / methanol solution as a catalyst. It was synthesized by transesterification with diethyl. Compound 3 was synthesized by reacting trifluoroethanol and ethyl chlorocarbonate using diethyl ether as a solvent. For compound 7,
It was synthesized by transesterification between 2,2,3,3,3-pentafluoropropanol and compound 6. Compound 1-methyl neopentyl carbonate Compound 2-methyl 2,2,2-trifluoroethyl compound 3-Ethyl carbonate 2,2,2-trifluoroethyl Compound 4-methyl 2,2,3,3,3- Pentafluoropropyl compound 5-methyl 2,2,3,3-tetrafluoropropyl compound 6-Di-2,2,2-trifluoroethyl carbonate Compound 7-2,2,2-trifluoroethyl-2, carbonate
2,3,3,3-Pentafluoropropyl The viscosity (cP, 25 ° C.) and relative dielectric constant (25 ° C.) of these carbonate compounds are shown in Table 1.

[0019]

[Table 1]

As comparative examples, the viscosity and the relative dielectric constant of the conventional compounds diethyl carbonate (DEC) and diisopropyl carbonate (DIPC) are also shown in Table 1. 2. Reactivity with Metallic Lithium Among the seven carbonate ester compounds obtained as described above, methyl neopentyl carbonate (MNPC) and methyl carbonate 2,2
The reactivity of 2,2-trifluoroethyl (MFEC) with metallic lithium was examined.

In an argon box, 0.1 g of dice-cut metallic lithium was added to 5 g of methyl neopentyl carbonate (MNPC), and a spatula was pressed against the lithium in the solution to expose a clean surface of lithium, and the temperature was changed to 25 ° C. At 48
After standing for a period of time, the surface state of the lithium metal and the state of the liquid part were examined to determine the reactivity. The same experiment was carried out for methyl 2,2,2-trifluoroethyl carbonate (MFEC) and the conventional compounds diethyl carbonate (DEC) and diisopropyl carbonate (DIPC) as comparative examples. Table 2 shows the obtained results. Was.

[0022]

[Table 2]

As is clear from Table 2, MNPC and MF
Both ECs did not react with metallic lithium and had extremely high stability as an electrolytic solution, indicating that they were suitable. 3. Measurement of flash point The flash point of compound 2, compound 3, compound 4, compound 6, compound 7, and dimethyl carbonate (DMC) as a comparative example, and the flash point of each of compound 2, compound 4, and dimethyl carbonate and propylene carbonate (PC) The flash point of the 1: 1 (volume ratio) mixed solution was measured by a closed tag method (JIS-K2265). Table 3 shows the measurement results.

[0024]

[Table 3]

As is clear from Table 3, the carbonate of the present invention exhibited a higher flash point than ordinary carbonates such as dimethyl carbonate. 4. Measurement of electric conductivity and withstand voltage Lithium hexafluorophosphate (LiPF ₆ ) as electrolyte
Dissolve 3.8 g (25 mmol) in the electrolyte solvent to obtain 25 m
1 electrolyte solution was prepared. As the solvent, the carbonate ester obtained in the above synthesis alone or a 1: 1 (volume ratio) mixed solvent of the carbonate ester obtained in the above synthesis and propylene carbonate (PC) was used. The electric conductivity of this electrolytic solution was measured at 10 kHz using an impedance meter. The withstand voltage of the electrolytic solution was measured by placing the solution in a three-electrode withstand voltage measuring cell using platinum as a working electrode and a counter electrode and using lithium metal as a reference electrode, and applying a potential of 50 mV / sec with a potentiostat. A range in which the electrophoresis was performed and the decomposition current did not flow by 0.1 mA or more was defined as a withstand voltage. The results are shown in Table 4.

[0026]

[Table 4]

As is clear from Tables 3 and 4, the electrolytic solution of the present invention exhibited a high withstand voltage and excellent electric conductivity at a practical level. 5. Battery cycle life Battery size as shown in Fig. 1 is 20mm in outer diameter and 2.5mm in height
A coin-shaped nonaqueous electrolyte battery having a thickness of 1 mm was prepared. The negative electrode 1 was made of metallic lithium, and the positive electrode 2 was made by pressing and forming a mixture of 85 parts by weight of LiCoO _{2 and} 12 parts by weight of graphite as a conductive agent and 3 parts by weight of a fluororesin as a binder. The materials constituting the negative electrode 1 and the positive electrode 2 are pressure-bonded to a negative electrode can 4 and a positive electrode can 5, respectively, via a porous separator 3 made of polypropylene. As a battery electrolyte for such a battery, lithium hexafluorophosphate (1.0 mol / l) was used in a solvent in which methyl trifluoroethyl carbonate (MFEC) and propylene carbonate (PC) were mixed at a volume ratio of 1: 1. Using a solution dissolved at a rate of 1 l, it was sealed with a sealing gasket 6.

For the battery thus prepared, 1.0
The charge / discharge efficiency was measured when the battery was charged at a current of mA at an upper limit voltage of 4.1 V for 10 hours and then discharged at a current of 1.0 mA until the voltage reached 3.0 V. In addition, such charge / discharge was repeated for a predetermined cycle, and a change in charge / discharge efficiency was observed. FIG. 2 shows the results, in which the charge / discharge efficiency is plotted against the number of cycles (（). Also MN
A battery prepared in the same manner as the MFEC-PC system except that a mixed solvent of PC and PC (volume ratio 1: 1) was used,
FIG. 2 shows the results (△). As a comparative example, the same charge / discharge efficiency was measured for a coin-shaped battery prepared in the same manner as the MFEC-PC system, using a mixed solvent of diethyl carbonate and propylene carbonate (volume ratio 1: 1) as the electrolyte solvent. (●)

As is clear from FIG. 2, the battery using the electrolyte solvent of the present example maintains a high energy density even when exposed to a high voltage of 4 V or more, and exhibits extremely excellent cycle characteristics. Indicated.

[0030]

As is apparent from the above description, according to the present invention, by using a novel organic solvent containing a chain carbonate as an electrolyte solvent, a high flash point, high electric conductivity, and high withstand voltage can be obtained. In both cases, an excellent non-aqueous electrolyte could be provided. Further, according to the present invention, by applying these non-aqueous electrolytes to batteries, batteries having excellent charge / discharge efficiency and cycle characteristics and high energy density can be produced.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing one embodiment of a non-aqueous electrolyte secondary battery of the present invention.

FIG. 2 is a diagram showing cycle characteristics of the nonaqueous electrolyte battery of the present invention.

[Explanation of symbols]

 1 ... Negative electrode 2 ... Positive electrode

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Keiichi Yokoyama 580-32, Takuji, Nagaura, Sodegaura-shi, Chiba (72) Inside of Mitsui Petrochemical Industries Co., Ltd. No. 32 Mitsui Petrochemical Industry Co., Ltd. (56) References JP-A-6-199992 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 10/40

Claims (7)

(57) [Claims] 1. A non-aqueous electrolyte containing a carbonate represented by the general formula [I]. Embedded image (In the formula, R ¹ represents an alkyl group or a halogen atom-substituted alkyl group, and R ² represents an alkyl group having no hydrogen at the β-position or a halogen atom-substituted alkyl group.) 2. In the general formula [I] according to claim 1, R
A non-aqueous electrolyte containing at least one of R ^{1 and} R ² containing a carbonate ester which is an alkyl group having no hydrogen at the β-position or a fluorine atom-substituted alkyl group. 3. The method according to claim 1, wherein R is
¹ is -CH _3, a group selected from the group consisting of -CH ₂ CH ₃ and -CH ^₂ CF _3, R ₂ is _{-CH 2 C (CH 3) 3} , -CH 2
_{CF 3, -CH 2 CF 2 CF} 3, -CH 2 CF 2 CF 2 H, -
A non-aqueous electrolyte containing a carbonate selected from the group of CH (CF ₃ ) ₂ . 4. The non-aqueous electrolyte according to claim 1, further comprising a cyclic carbonate in addition to the carbonate represented by the general formula [I]. 5. The mixing ratio of the carbonate represented by the general formula [I] to the cyclic carbonate is 1: 9 to 9: 1 by volume ratio.
5. The non-aqueous electrolyte according to claim 4, wherein: 6. An electrolyte comprising: LiPF ₆ , LiBF ₄ , LiC
10 ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiAlC
l ₄ , LiN (SO ₃ CF ₃ ) ₂ , LiC ₄ F ₉ SO ₃ , Li
Nonaqueous electrolytic solution according to claim 1, wherein is a lithium compound selected from the group consisting of C ₈ F ₁₇ SO _3. 7. A non-aqueous electrolyte battery comprising the non-aqueous electrolyte according to claim 1 as an electrolyte.