JP3369310B2 - Non-aqueous electrolyte and non-aqueous electrolyte battery - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention 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 Batteries using non-aqueous electrolytes are widely used as power sources for consumer electronic devices because they have high voltage and high energy density and are highly reliable in terms of storability. . As the non-aqueous electrolyte, propylene carbonate which is a high dielectric constant solvent, γ-butyrolactone, sulfolane and the like are mixed with a low viscosity solvent such as dimethoxyethane, tetrahydrofuran or 1,3-dioxolane, and Li.
BF ₄ , LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiC
A mixture of electrolytes such as F ₃ SO ₃ , LiAlCl ₄ , and LiSiF ₆ is used.

However, many of the solvents of such non-aqueous electrolytes have a low withstand voltage, and when an electrolyte using a solvent with a low withstand voltage is used for a secondary battery, the solvent becomes an electric solvent when charging and discharging are repeated. As a result of the decomposition, the generated gas raises the internal pressure of the battery, and the product causes a polymerization reaction to adhere to the electrode. Therefore, there is a problem that the battery charge / discharge efficiency is reduced, the battery energy density is reduced, and the battery life is shortened. As an attempt to improve the durability of the electrolytic solution, conventionally used esters such as γ-butyrolactone and ethyl acetate, and 1,3
-Use of carbonic acid ester such as diethyl carbonate, which has high withstand voltage, instead of solvent with low withstand voltage of ethers such as dioxolane, tetrahydrofuran, dimethoxyethane, etc., to suppress the decrease in battery energy density after repeated charging and discharging. (For example, JP-A-2-10666).

On the other hand, metallic lithium or a lithium alloy is used for the negative electrode of the lithium secondary battery, and lithium ions in the electrolytic solution are deposited on the negative electrode when charging and discharging are repeated, resulting in needle-shaped dendrites. Highly reactive metals were sometimes produced. If the dendrite falls off the electrode, it will self-deplete and the cycle life of the battery will be shortened, and there is a high possibility that the dendrite will penetrate the separator that separates the positive electrode and the negative electrode, resulting in a short circuit. There were some points to be improved, such as not being able to secure it.

[0005]

By the way, since a battery having a high energy density is desired, research on a high-voltage battery is being promoted from various fields. For example, a secondary battery called a rocking chair type, in which a composite oxide of lithium and a transition metal such as LiCoO ₂ , LiNiO ₂ , and LiMn ₂ O ₄ is used for the positive electrode of the battery and a carbon material is used for the negative electrode, has been studied. It was In this case, the battery voltage can generate 4 V or more, and since there is no deposition of metallic lithium, it is confirmed that safety is ensured by experiments such as overcharging, external short circuit, needle stick, and crushing. It has been made available for commercial use. However, it is necessary to further improve the safety such as flame retardancy in the case where the energy density is increased drastically and the size is increased in the future.

The electrolyte solvents currently used do not necessarily have a satisfactory high flash point and are not self-extinguishing. Therefore, it has been proposed to add phosphoric acid esters, which are known as self-extinguishing compounds, to the electrolytic solution (JP-A-4-184870). However, the electrolyte containing 15 wt% or more of this type of compound is flame-retardant and has improved safety, but has problems in terms of battery charge / discharge efficiency, battery energy density, and battery life. Further, there has been proposed one using trimethyl phosphate as a solvent for an electrolytic solution (JP-A-1-10286).
No. 2), trimethyl phosphate is not excellent in self-extinguishing performance, and is suitable for high-voltage secondary batteries that are excellent in withstand voltage and electrical conductivity, and are excellent in safety. There was no electrolyte.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and does not cause a decrease in battery energy density even after repeated charging / discharging, and overcharge, external short circuit, needle stick, crushing, etc. The objective is to provide a non-aqueous electrolyte solution that can ensure sufficient safety even under severe conditions, has excellent withstand voltage and electrical conductivity, has a high flash point, excellent safety, and excellent load characteristics and low temperature characteristics. And Another object of the present invention is to provide a non-aqueous electrolyte battery that is safe, can generate a high voltage, has excellent battery performance, and has a long life.

[0008]

Means for Solving the Problems In order to produce a non-aqueous electrolyte battery which is safe, can generate high voltage, and is excellent in safety, the inventors have excellent withstand voltage and charge / discharge cycle characteristics, are hard to ignite, and An intensive research was conducted to find out an electrolytic solution having self-extinguishing property. As a result, the one containing trimethyl phosphate as an electrolyte solvent, one or more kinds of chain carbonate represented by the general formula [I], and a cyclic carbonate, and containing LiPF ₆ as an electrolyte is flammable. It has been found that it has high points, is hard to ignite, is self-extinguishing, and is excellent in withstand voltage and charge / discharge cycle characteristics.

[0009]

[Chemical 2]

Wherein R ¹ is a methyl group or an ethyl group,
R ² is a chain or branched alkyl group having 1 to 3 carbon atoms. The inventors have found that batteries using such an electrolytic solution have improved safety and, in addition, improved charge / discharge cycle life. That is, the non-aqueous electrolytic solution of the present invention contains trimethyl phosphate as an electrolyte solvent, one or more chain carbonates represented by the general formula [I], and a cyclic carbonate.

[0011]

[Chemical 3]

In the formula, R ¹ represents a methyl group or an ethyl group, and R ² represents a chain or branched alkyl group having 1 to 3 carbon atoms. Trimethyl phosphate has a high flash point and has an action of increasing the flash point of the mixed solvent. Furthermore, trimethyl phosphate is flame-retardant and imparts self-extinguishing properties to the solvent. However, it is important to use an appropriate amount of trimethyl phosphate because it has the properties of simultaneously lowering the charge / discharge efficiency and lowering the battery energy density. In order to impart flame retardancy to the solvent, trimethyl phosphate must be contained in at least 1% by volume of the electrolytic solution solvent. More preferably, the content is 3% by volume or more. When it is contained in an amount of 3% by volume or more, it is possible to impart sufficient flame retardancy to the non-aqueous electrolyte. If trimethyl phosphate is contained in the electrolyte solvent in an amount of 10% by volume or more, the charging efficiency of the battery is reduced. Therefore, the content of trimethyl phosphate is 10% by volume or less, preferably 7% by volume or less. Is desirable.
If it is 7% by volume or less, the charging efficiency of the battery is not lowered.

R ¹ of the chain ester carbonate represented by the general formula [I] represents a methyl group or an ethyl group, and R ² represents a chain or branched alkyl group having 1 to 3 carbon atoms. Examples of such carbonic acid ester include dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate, and methyl isopropyl carbonate. Of these, dimethyl carbonate, methyl ethyl carbonate and diethyl carbonate are particularly preferable. These carbonates are 1
One kind or a mixture of two or more kinds can be used.

When such a chain ester carbonate is used in combination with trimethyl phosphate, the withstand voltage is improved and decomposition of the electrolyte solvent due to oxidation is suppressed. Furthermore, the solubility of the electrolyte can be increased, and an electrolytic solution having excellent electric conductivity at room temperature and low temperature can be obtained. In order to obtain the electrolytic solution having such characteristics, the chain ester carbonate is usually contained in the electrolytic solution solvent in an amount of 20 to 90% by volume, preferably 40 to 75% by volume. When the content of the chain carbonic acid ester is 20% by volume or less, it is easily oxidized,
If the solvent is decomposed, and 90% by volume or more is contained, the self-extinguishing property is impaired. If it is 40 to 75% by volume, a solvent having desired withstand voltage and self-extinguishing property can be obtained.

Further, as the cyclic carbonic acid ester contained in the solvent of the non-aqueous electrolytic solution, butylene carbonate, vinylene carbonate and the like can be used, but propylene carbonate, ethylene carbonate or a mixture of two kinds is preferable. is there. By adding this carbonic acid ester, the solubility of the electrolyte can be increased even at low temperature, the viscosity can be lowered, the electric conductivity can be further improved, and the transport of the electrolyte can be facilitated.
The content of cyclic carbonic acid ester is 10 to the electrolytic solution solvent.
70% by volume, preferably 20 to 60% by volume. Within this range, the viscosity is low and the dielectric constant is high, and the electrical conductivity is high, which is preferable.

The electrolyte solvent of the present invention is not limited to the above-mentioned trimethyl phosphate, chain carbonic acid ester, cyclic carbonic acid ester, and other non-solvents such as ether type, ester type, γ-butyrolactone and sulfolane which are usually used as electrolyte solution solvents for batteries. A water solvent can be appropriately added within a range that does not impair the characteristics of the electrolytic solution solvent of the present invention. LiPF ₆ is preferably used as the electrolyte dissolved in the above-mentioned electrolyte solvent. When LiPF ₆ is used as the electrolyte, the self-extinguishing property can be maintained to a high degree even if the content of trimethyl phosphate is lowered.
What is used as an electrolyte in a normal battery electrolytic solution, for example, LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ S
O ₃ , LiAlCl ₃ , LiSiF ₆ , LiN (SO ₃ CF
₃ ) ₂ , lithium salts such as LiC ₄ F ₉ SO ₃ and LiC ₈ F ₁₇ SO ₃ can also be used, but when these lithium salts are used, the content of trimethyl phosphate is 10
If it is not more than the volume%, sufficient self-extinguishing property cannot be exhibited, and therefore, the charge / discharge efficiency and energy density of the battery will be reduced.

The concentration of the electrolyte dissolved in the solvent is usually 0.1.
~ 3 mol / l, preferably 0.
It can be used at 5 to 2.0 mol / l. Further, the non-aqueous electrolyte battery of the present invention uses the above-mentioned non-aqueous electrolyte and a carbon material capable of doping / dedoping lithium ions as a negative electrode material, and as a positive electrode material, a composite of lithium and a transition metal. Oxides are used.

According to the present invention, an electrolyte containing trimethyl phosphate, a chain carbonic acid ester represented by the general formula [I], and a cyclic carbonic acid ester as an electrolytic solution solvent, which is dissolved in the electrolytic solution solvent. By using LiPF ₆ as the compound, the reactivity with metallic lithium becomes low, the flash point becomes high, and the decomposition of the electrolytic solution solvent due to oxidation hardly occurs, so that the cycle life of charge / discharge of the battery becomes long.

The non-aqueous electrolyte battery of the present invention can be applied to a cylindrical non-aqueous electrolyte secondary battery as an example. The cylindrical non-aqueous electrolyte secondary battery has a negative electrode current collector 9 as shown in FIG.
A negative electrode 1 formed by applying a negative electrode active material to a positive electrode current collector 10
A positive electrode 2 formed by applying a positive electrode active material to a separator 3
It is wound through the battery, and is housed in the battery can 5 with the insulator 4 placed on the upper and lower sides of the wound body. A battery lid 7 is attached to the battery can 5 by caulking via a sealing gasket 6, and is electrically connected to the negative electrode 1 or the positive electrode 2 via a negative electrode lead 11 and a positive electrode lead 12, respectively, and is a negative electrode or a positive electrode of the battery. Is configured to function as.

As the negative electrode active material constituting such a negative electrode 1, metallic lithium, lithium alloys, and carbon materials capable of occluding / releasing lithium ions can be used, and particularly preferably lithium ions are doped / dedoped. A carbon material capable of being applied is applied. Graphite or amorphous carbon may be used as such a carbon material, and any carbon material such as activated carbon, carbon fiber, carbon black, and mesocarbon microbeads can be applied.

As the positive electrode active material constituting the positive electrode 2, transition metal oxides such as MoS ₂ , TiS ₂ , MnO ₂ and V ₂ O ₅ , transition metal sulfides or LiCoO ₂ , LiMn.
A composite oxide composed of lithium and a transition metal such as O ₂ , LiMn ₂ O ₄ , and LiNiO ₂ can be used, but a composite oxide of lithium and a transition metal is particularly preferably used.

The non-aqueous electrolyte battery of the present invention contains the above-mentioned non-aqueous electrolyte solution as an electrolyte solution, and its shape, form, etc. are not limited to those of the above-mentioned embodiment, but may be of a cylindrical type, A square type, a coin type, a large size, etc. can be arbitrarily selected within the scope of the present invention.

[0023]

EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. 1. Self-extinguishing property of electrolyte Manila paper for separator with thickness of 0.04mm is 15m wide
It was cut into a strip having a length of m and a length of 320 mm, and immersed in a beaker containing a sample of an electrolyte solution prepared as shown in the following examples for 1 minute or more. Excess sample dripping from the Manila paper was wiped with a beaker wall, and the Manila paper was pierced at 25 mm intervals on a supporting needle of a sample table having a supporting needle and fixed horizontally. 250mm x 25 sample stand with Manila paper fixed
It was put in a metal box of 0 mm x 500 mm, one end was ignited by a lighter, and the burned length of the Manila paper was measured three times. Three
The average value of the length of the times is shown in the table.

1) Tests were carried out on electrolyte solvent samples in which the solvent composition and the self-extinguishing property of the electrolyte solution were changed such that propylene carbonate (PC), methyl ethyl carbonate (MEC) and trimethyl phosphate (TMPA) were mixed in different proportions. The electrolyte LiPF ₆ was dissolved at a concentration of 1.0 mol / l. The results shown in Table 1 were obtained.

As a comparative example, triethyl phosphate (TEP
A test using A) and a test using no trimethyl phosphate were similarly conducted. The results are shown in Table 1.

[0026]

[Table 1]

As is clear from Table 1, those containing trimethyl phosphate were found to have self-extinguishing properties. 2) Type of electrolyte and self-extinguishing property of electrolyte propylene carbonate / methyl ethyl carbonate /
A test was conducted on an electrolytic solution in which an electrolyte was dissolved in a solvent having a composition of trimethyl phosphate = 40/55/5 (volume ratio) at a concentration of 1.0 mol / l. The results shown in Table 2 were obtained.

[0028]

[Table 2]

As is clear from Table 2, Li was used as the electrolyte.
It was shown that the electrolytic solution containing PF ₆ was extremely excellent in self-extinguishing property. 3) LiPF ₆ concentration and self-extinguishing property of electrolyte propylene carbonate / methyl ethyl carbonate /
The test was carried out by dissolving in a solvent having a composition of trimethyl phosphate = 40/55/5 (volume ratio). The results shown in Table 3 were obtained.

[0030]

[Table 3]

As is clear from Table 3, the self-extinguishing property was exhibited with a small content of LiPF ₆ . 4) Types of chain carbonate and dimethyl carbonate (DM) as self-extinguishing chain carbonate of electrolyte
C), methyl ethyl carbonate (MEC), and diethyl carbonate (DEC), propylene carbonate / chain carbonate / trimethyl phosphate =
A solvent having a composition of 40/55/5 (volume ratio) and a concentration of 1.
A test was conducted by dissolving 0 mol / l of LiPF ₆ to prepare an electrolytic solution. The results shown in Table 4 were obtained.

[0032]

[Table 4]

As is clear from Table 4, the electrolyte containing these chain carbonates was shown to have excellent self-extinguishing properties. 2. Measurement of Withstand Voltage and Electric Conductivity of Electrolyte Solution An electrolyte solution of LiPF ₆ at 1.0 mol / l was prepared as an electrolyte. The electric conductivity of the electrolytic solution is 10 kHz using an impedance meter, and is room temperature (25 ° C.) and low temperature (−20).
(° C). The withstand voltage of the electrolytic solution is as follows: Put the electrolytic solution in a three-electrode type withstand voltage measuring cell using glassy carbon for the working electrode and the counter electrode and metallic lithium for the reference electrode, and conduct a potential sweep at 50 mV / sec with a potentiostat. The breakdown voltage was defined as the range in which the decomposition current did not flow by 0.1 mA or more.

1) Solvent composition and withstand voltage and electric conductivity of electrolyte solution Tests were carried out on electrolyte solvent samples in which the mixing ratio of propylene carbonate (PC), methyl ethyl carbonate (MEC) and trimethyl phosphate (TMPA) was changed. I did. The results shown in Table 5 were obtained. As a comparative example, the same test was carried out for those containing no trimethyl phosphate.
The results obtained are shown in Table 5.

[0035]

[Table 5]

As is clear from Table 5, it was found that the electrolytic solution of the present invention has a high withstand voltage and excellent electric conductivity at a practical level even at room temperature and low temperature. 2) LiPF ₆ concentration of electrolytic solution, withstand voltage and electric conductivity Volume composition ratio of solvent PC / MEC / TMPA = 40/55
For / 5, LiPF6 was dissolved and tested. The results are shown in Table 6.

[0037]

[Table 6]

As is apparent from Table 6, it was found that the electrolytic solution has a high withstand voltage and a practical level of excellent electrical conductivity even at room temperature and low temperature with a slight content of LiPF ₆ . 3. Discharge Capacity and Battery Cycle Characteristics of Battery First, the negative electrode 1 was manufactured as follows. For the negative electrode active material,
Petroleum pitch as the starting material, 1000 ° C in an inert gas stream
A non-graphite carbon material having properties close to those of glassy carbon obtained by firing at was used. As a result of X-ray analysis measurement of this material, the spacing between (002) planes was 3.76 Å and the true specific gravity was 1.58 g / cm ² . The carbon material thus obtained was pulverized into a carbon material powder having an average particle size of 10 μm, and 90 parts by weight of this carbon material powder and 10 parts by weight of polyvinylidene fluoride (PVDF) as a binder were mixed. A negative electrode mixture was prepared and dispersed in N-methyl-2-pyrrolidone to form a slurry. Next, this slurry was uniformly applied to both sides of a strip-shaped copper foil having a thickness of 10 μm, which is the negative electrode current collector 9, dried, and compression-molded with a roll press machine to prepare a negative electrode 1.

Next, the positive electrode 2 was produced as follows. As the positive electrode active material (LiCoO ₂ ), lithium carbonate and cobalt carbonate were mixed at a ratio of 0.5 mol to 1 mol, and the mixture was heated to 900
It was obtained by firing at ℃ for 5 hours. LiCo thus obtained
91 parts by weight of O ₂ , 6 parts by weight of graphite as a conductive agent, polyvinylidene fluoride (PVDF) 3 as a binder
The mixture was mixed at a ratio of parts by weight to prepare a positive electrode mixture, and this was mixed with N-
It was dispersed in methyl-2-pyrrolidone to form a slurry. Next, this slurry is applied to a positive electrode current collector 10 having a thickness of 20.
A positive electrode 2 was produced by uniformly coating both surfaces of a strip-shaped aluminum foil having a thickness of μm, drying the mixture, and then compression-molding it with a roll plater.

The strip-shaped positive electrode 2, negative electrode 1, and separator 3 made of a microporous polypropylene film having a thickness of 25 μm were laminated in this order and then spirally wound many times to accommodate the wound body. Next, the insulator 4 was inserted into the bottom of the nickel-plated iron battery can 5 to house the wound body. Then, in order to collect the current of the negative electrode, one end of a negative electrode lead 11 made of nickel was pressure-bonded to the negative electrode 1, and the other end was welded to the battery can 5. In addition, one end of a positive electrode lead 12 made of aluminum is attached to the positive electrode 2 in order to collect current from the positive electrode.
The other end was electrically connected to the battery lid 7 via a current interrupting thin plate 8 that interrupts the current according to the battery internal pressure.

Then, in the battery can 5, 50% by volume of propylene carbonate (PC) and methyl ethyl carbonate (MEC) were used.
Trimethyl phosphate was added in a volume ratio shown in Table 7 to a mixed solvent of 50% by volume, and an electrolytic solution in which 1 mol / l of LiPF ₆ was dissolved was injected. The battery lid 5 is fixed by caulking the battery can 5 through the insulating sealing gasket 6 coated with asphalt, and the diameter is 18 mm and the height is 65 mm.
mm cylindrical non-aqueous electrolyte batteries (Examples 1 and 2) were produced.

As comparative examples, a cylindrical non-aqueous electrolyte battery (Comparative Example 1) was prepared in the same manner as in Examples except that triethyl phosphate was added in a volume% shown in Table 7 in place of trimethyl phosphate and trimethyl phosphate was not used. ~ 5) were produced. The following examinations were conducted to evaluate the capacity and cycle characteristics of the obtained cylindrical non-aqueous electrolyte battery.

The charging conditions were 4.2 V, 1 A, 2.5 h of constant current and constant voltage charging, and the discharging was a constant current of 1000 mA, and the final voltage was 2.75 V. This charging / discharging was performed up to 100 cycles, and the discharge capacity of the battery of each Example of the 2nd cycle was measured. The results are shown in Table 7. Further, the capacity retention rate (%) at the 100th cycle was measured when the discharge capacity at the first cycle was 100. The results are shown in Table 7.

[0044]

[Table 7]

As is clear from Table 7, the discharge capacity of trimethyl phosphate added in an amount of 10% by volume or less is slightly smaller than that of no addition, and the cycle characteristics are slightly lowered, but the addition amount is 15%. It was found that the deterioration of volume% was remarkable, while the size of the deterioration was slight. Further, it was found that the discharge capacity was larger and the capacity retention rate after the cycle was higher than that in the case where the same amount of triethyl phosphate was added.

[0046]

As is apparent from the above description, according to the present invention, a novel chain ester carbonate as an electrolyte solvent,
By including trimethyl phosphate and cyclic carbonic acid ester and containing LiPF ₆ as an electrolyte, a non-aqueous electrolyte having a high flash point, excellent self-extinguishing property, and excellent electric conductivity and withstand voltage could be obtained. . Further, according to the present invention, by applying these non-aqueous electrolytes to a secondary battery, a battery having excellent charge / discharge efficiency and cycle characteristics and high energy density can be produced.

[Brief description of drawings]

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

[Explanation of symbols]

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

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akio Hinohara 580-2 Takuji Nagaura, Sodegaura-shi, Chiba 32 Mitsui Petrochemical Industries, Ltd. (72) Yoshiaki Naruse 6-7 Kitashinagawa, Shinagawa-ku, Tokyo No. 35 Sonny Co., Ltd. (72) Inventor Masahiro Torida No. 580-32, Takuji Nagaura, Sodegaura City, Chiba Prefecture Mitsui Petrochemical Industry Co., Ltd. (56) Reference JP-A-5-13088 JP-A-4-184870 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01M 10/40 H01M 6/16

Claims (3)

(57) [Claims] 1. Trimethyl phosphate as an electrolyte solvent, one or more kinds of chain carbonic acid ester represented by the general formula [I], and a cyclic carbonic acid ester, and LiPF ₆ as an electrolyte, When the content of the trimethyl phosphate is 1 to 10% by volume of the electrolyte solvent, the
It has a high flash point, excellent self-extinguishing properties, electrical conductivity and withstand voltage.
A non-aqueous electrolyte characterized by having excellent properties . (In the formula, R ¹ represents a methyl group or an ethyl group, and R ² represents a chain or branched alkyl group having 1 to 3 carbon atoms.) 2. The non-aqueous electrolyte solution according to claim 1, wherein the cyclic ester carbonate is one or two of propylene carbonate and ethylene carbonate. 3. A negative electrode containing a carbon material capable of doping and dedoping lithium ions as a negative electrode active material, and a positive electrode containing a composite oxide of lithium and a transition metal as a positive electrode active material,
Having a non-aqueous electrolyte solution of claim 1, wherein as the electrolyte solution
Has excellent charge / discharge efficiency and cycle characteristics,
A non-aqueous electrolyte battery characterized by having high density characteristics .