JP2007250415A - Nonaqueous electrolyte solution and lithium secondary battery using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrochemical element having a cathode potential of 4.5 V or higher in a full charged condition and nonaqueous electrolyte solution which has limited capacity fall due to charge/discharge cycles at a high temperature especially in a high voltage lithium secondary battery and provide an electrochemical element and a lithium secondary battery using the above electrolyte solution. <P>SOLUTION: The nonaqueous solvent contains a ring-shaped carbonate and a chain carbonate as main compositions and the ring carbonate of 60 wt.% or more is a fluorine ethylene carbonate in which fluorine atoms are combined direct with a carbonate ring, and moreover, the ring-shaped carbonate and the ring-shaped carbonate are contained in a predetermined ratio. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention is a non-aqueous electrolyte for use in an electrochemical device having a fully charged positive electrode potential of 4.35 V or more with respect to the potential of metallic lithium, and has a small capacity drop due to a charge / discharge cycle at a high temperature. Further, the present invention relates to an electrolytic solution that generates little decomposition gas during storage at a high temperature.

The electrochemical element is an element utilizing an electrochemical reaction exemplified by a battery, a capacitor, an electrochromic element and the like.
Among these, lithium batteries are widely used as power sources for consumer electronic devices because of their high voltage and high energy density and high reliability such as storage stability. A typical example of the lithium battery is a lithium ion secondary battery. This includes a negative electrode using a carbon material capable of inserting and extracting lithium as an active material, a positive electrode using a composite oxide of lithium and a transition metal as an active material, and a non-aqueous electrolyte. It is a battery.
A typical example of the capacitor is a non-aqueous electric double layer capacitor. This is comprised including the positive electrode and negative electrode which use high surface area electron conductive materials, such as activated carbon, as an active material, and a non-aqueous electrolyte. Although it has a low energy density, it is characterized by a relatively high voltage and a long lifetime. In the electric double layer capacitor, since electricity is stored by electrostatic adsorption of ions to the electric double layer on the electrode surface, a pure electrochemical reaction does not occur, but it is included in the electrochemical element discussed in the present invention in a broad sense. In recent years, electrochemical capacitors have been proposed as electrochemical elements having intermediate properties between lithium secondary batteries and capacitors. This is a capacitor configured to contain a material capable of inserting and extracting lithium and having a high surface area in at least one of a positive electrode and a negative electrode.

Here, the non-aqueous electrolyte plays a role of transferring ions between the positive electrode and the negative electrode. In order to improve the charge / discharge characteristics of the battery, it is necessary to increase the ion transfer rate between the positive electrode and the negative electrode as much as possible. For this purpose, the ion conductivity of the non-aqueous electrolyte is increased and the viscosity of the non-aqueous electrolyte is decreased. It is necessary to do. Further, in order to improve the high temperature storage characteristics, cycle stability, etc. of the battery, it is necessary that the non-aqueous electrolyte is stable with respect to the positive and negative electrodes having high chemical and electrochemical reactivity.
As a non-aqueous electrolyte satisfying such requirements, in lithium ion batteries, cyclic esters such as propylene carbonate, ethylene carbonate, and γ-butyrolactone, and chain forms such as diethyl carbonate, methyl ethyl carbonate, dimethyl carbonate, and methyl propionate are used. What melt | dissolved lithium salts, such as LiPF ₆ , in the mixed solvent with ester is mentioned. It has also been reported that the charge / discharge cycle characteristics of a battery are improved by containing fluorinated ethylene carbonate (see, for example, Patent Documents 1 and 2) in the nonaqueous electrolytic solution. Patent Document 2 exemplifies only a non-aqueous electrolyte composed of a solvent having a volume ratio of 1: 1 between 4,5-difluoroethylene carbonate and dimethyl carbonate as the fluorinated ethylene carbonate. However, the blending ratio of the electrolyte is described. There is no. The reason why the charge / discharge cycle characteristics are improved by these conventional techniques is considered to be because the electrochemical stability of the non-aqueous electrolyte with respect to the negative electrode is improved.

By the way, with the recent remarkable improvement in functions of portable devices, there is a strong demand for electrochemical elements having a higher energy density than before.
As such an electrochemical element, a lithium battery (hereinafter referred to as “high voltage lithium secondary battery”) in which the potential of the positive electrode in a fully charged state is set to 4.35 V or more with respect to the potential of metallic lithium is exemplified. (For example, refer to Patent Document 3). This lithium battery can increase the occlusion / release amount of lithium per unit volume of the positive electrode and can increase the battery voltage as compared with the conventional lithium ion battery, so that the energy density of the battery can be improved. However, because the potential of the positive electrode during full charge increases, the oxidative decomposition reaction of the electrolytic solution is likely to occur particularly at high temperatures, the capacity decreases with charge / discharge cycles at high temperatures, and the generation of electrolytic decomposition gas during storage at high temperatures. There is a possibility that the battery outer can swells due to, or the safety element that operates by detecting the internal pressure of the battery operates abnormally.

  In order to suppress the capacity reduction associated with the charge / discharge cycle of the high voltage lithium secondary battery, for example, as an electrolyte of the high voltage lithium secondary battery, a fluoroalkylsulfonic acid lithium salt is used as an electrolyte solute, Various methods such as incorporating a sulfone-based additive are exemplified, but it is not sufficient. In order to suppress the storage gas of the high-voltage lithium secondary battery, there is a description regarding a combination of difluoroethylene carbonate and fluorinated chain carbonate as an electrolyte (for example, see Patent Document 4). However, in this document, there is no specific example regarding the combination of fluorinated ethylene carbonate and fluorinated chain carbonate in the means and examples for solving the problems of the invention, and charge / discharge cycle characteristics of the battery. There is no description about improvement.

Patent Publication 2001-501355 Japanese Patent Application Laid-Open No. 07-240232 JP 2004-281158 A JP 2003-168480 A

  An object of the present invention is to provide a battery accompanying a charge / discharge cycle at a high temperature in an electrochemical element (particularly a high-voltage lithium secondary battery) in which the positive electrode potential in a fully charged state is 4.35 V or more with respect to the potential of metallic lithium. It is to obtain a non-aqueous electrolytic solution with a small capacity decrease and a gas generation during high-temperature storage, and an electrochemical device and a lithium secondary battery using the same.

In view of the above problems, the present inventors have intensively studied, and as a result, have completed the present invention. That is, the non-aqueous electrolyte of the present invention is
[1] A non-aqueous solvent containing a cyclic carbonate and a chain carbonate as main components and an electrolyte solute, wherein 60% by weight or more of the cyclic carbonate is a fluorinated ethylene carbonate in which a fluorine atom is directly bonded to the carbonate ring, and the cyclic carbonate A non-aqueous electrolyte for an electrochemical device in which the positive electrode potential in a fully charged state in which the weight ratio of the chain carbonate is from 3:97 to 35:65 is 4.35 V or more based on the potential of metallic lithium,
[2] The nonaqueous electrolytic solution according to [1], wherein 25% by weight or more of the chain carbonate is a fluorinated chain carbonate,
[3] The nonaqueous electrolytic solution according to [1] or [2], wherein the fluorinated ethylene carbonate is 4-fluoroethylene carbonate,
[4] The nonaqueous electrolytic solution according to any one of [1] to [3], a positive electrode having a positive electrode active material capable of reversible electrochemical reaction with lithium ions or anions, and a negative electrode capable of charging and discharging lithium ions A lithium secondary battery including a negative electrode having an active material, wherein the fully charged positive electrode has a potential of 4.35 V or more based on the potential of metallic lithium;
It is about.

  When the non-aqueous electrolyte of the present invention is used for an electrochemical device in which the potential of the fully charged positive electrode is 4.35 V or more with respect to the potential of metallic lithium, the charge / discharge cycle characteristics at high temperature are improved, and storage at high temperature is possible. The swelling of the battery due to the electrolyte decomposition gas at the time can be suppressed. Therefore, when the non-aqueous electrolyte of the present invention is used, a high energy density lithium secondary battery or electric double layer capacitor having excellent charge / discharge cycle characteristics at high temperatures and small swelling due to high temperature storage gas can be obtained.

Hereinafter, the nonaqueous electrolytic solution of the present invention and, in particular, a lithium secondary battery using the nonaqueous electrolytic solution will be described.
The nonaqueous electrolytic solution of the present invention comprises a nonaqueous solvent containing a cyclic carbonate and a chain carbonate as main components and an electrolyte solute, and 60% by weight or more of the cyclic carbonate has a fluorine atom directly bonded to the carbonate ring. And the weight ratio of the cyclic carbonate to the chain carbonate is 3:97 to 35:65 and is used for an electrochemical device in which the positive electrode potential in a fully charged state is 4.35 V or more with respect to the potential of metallic lithium. is there.
Below, the non-aqueous electrolyte for high voltage system lithium secondary batteries is demonstrated to an example as an electrochemical element.

[Cyclic carbonate]
As the cyclic carbonate according to the present invention, fluorinated ethylene carbonate such as 4-fluoroethylene carbonate is an essential component. Other examples include non-fluorinated cyclic carbonates. Non-fluorinated cyclic carbonates include ethylene carbonate, propylene carbonate, butylene carbonate, 1,2-pentene carbonate, 1,2-hexene carbonate, 1,2-heptene carbonate, 1,2-octene carbonate, 1,2 -Nonene carbonate, 1,2-decene carbonate, 1,2-dodecene carbonate, 5,6-dodecene carbonate, vinylene carbonate, vinyl ethylene carbonate, 1,2-divinyl ethylene carbonate and the like are exemplified.

  In the present invention, it is essential to contain 60% by weight or more of fluorinated ethylene carbonate in which fluorine atoms are directly bonded to the carbonate ring with respect to the total amount of cyclic carbonate. The higher the ratio of fluorinated ethylene carbonate in which fluorine atoms are directly linked to the carbonate ring in the cyclic carbonate, the less the oxidative electrolysis of the electrolyte in the positive electrode at high temperature and high voltage. The charge / discharge cycle characteristics at high temperatures are improved. The ratio of fluorinated ethylene carbonate in the cyclic carbonate is 60% by weight or more, preferably 75% by weight or more, more preferably 80% by weight or more, and most desirably 90% by weight or more. If it exists in this range, it is preferable as a non-aqueous electrolyte used for the electrochemical element whose positive electrode potential of a full charge state is 4.35V or more on the basis of the electric potential of metallic lithium.

  On the other hand, among the cyclic carbonates, non-fluorinated cyclic carbonates are preferably not contained as much as possible because they are easily oxidatively decomposed at a high temperature at a positive electrode of 4.35 V or higher with reference to the standard oxidation-reduction potential of lithium. In the case where the non-fluorinated cyclic carbonate is unavoidably contained from the viewpoint of improving the ionic conductivity of the electrolytic solution, ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, vinyl ethylene carbonate, and divinyl ethylene carbonate are desirable. Propylene carbonate is most desirable from the viewpoint of reducing reactivity at the positive electrode and reducing adverse effects on charge / discharge cycle characteristics. Vinylene carbonate, vinyl ethylene carbonate, and divinyl ethylene carbonate are easily oxidatively decomposed at the positive electrode at high temperature and high voltage. However, they have a high effect of suppressing the reductive decomposition reaction of the negative electrode, so they may be contained in trace amounts. When it is contained, it is appropriately adjusted depending on the reductive decomposition reactivity of the negative electrode, but 0.05 to 2% by weight is appropriate.

[Fluorinated ethylene carbonate with a fluorine atom directly attached to the carbonate ring]
The fluorinated ethylene carbonate in which a fluorine atom is directly bonded to the carbonate ring according to the present invention is a compound of ethylene carbonate in which “hydrogen directly bonded to the carbonate ring is substituted with a fluorine atom”. Various known fluorinated ethylene carbonates can be used. For example, 4-fluoroethylene carbonate, 4,4-difluoroethylene carbonate, 4,5-difluoroethylene carbonate, 4,4,5-trifluoroethylene carbonate, 4,4,5,5-tetrafluoroethylene carbonate.

  Among these, 4-fluoroethylene carbonate is less liable to increase in viscosity than the above-mentioned other fluorinated ethylene carbonates and is less likely to cause a decrease in lithium coordinating power. 4-fluoroethylene carbonate is most preferable because 4-fluoroethylene carbonate has a high ability to form a protective film against the decomposition of the electrolytic solution at the negative electrode and has a property of suppressing the reduction reaction of the electrolytic solution at the negative electrode. These fluorinated ethylene carbonates can be used alone or in combination of two or more.

  Examples of cyclic carbonates similar to fluorinated ethylene carbonate include ethylene carbonates such as trifluoromethyl ethylene carbonate, difluoromethyl ethylene carbonate, and fluoromethyl ethylene carbonate, in which hydrogen that does not directly bond to the carbonate ring is substituted with fluorine atoms. However, these compounds are inappropriate because the effects of the present invention are not sufficiently obtained. These compounds are presumed to have high stability at the positive electrode at high temperature and high voltage, but the protective film formed on the negative electrode is destabilized at high temperature and the reductive decomposition reactivity of the electrolyte is increased. ing.

[Chain carbonate]
Examples of the chain carbonate according to the present invention include fluorinated chain carbonate and non-fluorinated chain carbonate. In the present invention, it is desirable that the fluorinated chain carbonate is contained because the charge / discharge cycle characteristics at a high temperature of the high voltage lithium secondary battery are improved and the generation of an electrolyte decomposition gas during storage at a high temperature is reduced.

  Non-fluorinated chain carbonates include dimethyl carbonate, diethyl carbonate, dipropyl carbonate, dibutyl carbonate, dipentyl carbonate, dihexyl carbonate, dioctyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, methyl butyl carbonate, methyl pentyl carbonate, methyl hexyl carbonate, Examples include methyl octyl carbonate, ethyl propyl carbonate, ethyl butyl carbonate, ethyl pentyl carbonate, ethyl hexyl carbonate, and ethyl octyl carbonate. From the viewpoint of excellent ionic conductivity and viscosity of the electrolytic solution and improving the output characteristics of the battery, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, and methyl propyl carbonate are preferable.

  From the viewpoint of improving the output characteristics of the battery and satisfying both the charge / discharge cycle characteristics at high temperature and high voltage and the high temperature storage gas, fluorinated chain carbonate having 4 or less carbon atoms, dimethyl carbonate, diethyl carbonate, It is preferable to mix one or more chain carbonates selected from methyl ethyl carbonate and methyl propyl carbonate. In this case, it is desirable that the fluorinated chain carbonate is contained in the chain carbonate in an amount of 25 wt% or more, more preferably 45 wt% or more.

[Fluorinated chain carbonate]
The fluorinated chain carbonate according to the present invention has a structure in which a part or all of hydrogen atoms of a chain carbonate having a carbonate group (—OCOO—) is substituted with a fluorine atom. Although various things are mentioned as a fluorinated chain carbonate, For example, what has the following structural formula is mentioned.

(Wherein R ₁ and R ₂ represent an alkyl group, and at least one of them is an alkyl group in which a part or all of hydrogen atoms are substituted with fluorine atoms.)

  Examples of such chain fluorinated carbonates include methyl-2,2,2-trifluoroethyl carbonate, ethyl-2,2,2-trifluoroethyl carbonate, methyl 2,2,3,3,3- Pentafluoropropyl carbonate, methyl 2,2,3,3-tetrafluoropropyl carbonate, methyl-3,3,3-trifluoropropyl carbonate, methyl-2,2,3,3,4,4,4-heptafluoro Butyl carbonate, 2,2,2-trifluoroethyl-2,2,3,3,3-pentafluoropropyl carbonate, di (2,2,2-trifluoroethyl) carbonate, fluoromethyl methyl carbonate, (difluoromethyl ) Methyl carbonate, bis (fluoromethyl) carbonate, (1-fluoroethyl) methyl carbonate, (2-fluoro Ethyl) methyl carbonate, ethyl fluoromethyl carbonate, (1-fluoroethyl) fluoromethyl carbonate, (2-fluoroethyl) fluoromethyl carbonate, (1,2-difluoroethyl) methyl carbonate, (1,1-difluoroethyl) methyl Carbonate, (1-fluoroethyl) ethyl carbonate, (2-fluoroethyl) ethyl carbonate, ethyl (1,1-difluoroethyl) carbonate, ethyl (1,2-difluoroethyl) carbonate, bis (1-fluoroethyl) carbonate , Bis (2-fluoroethyl) carbonate, (1-fluoroethyl) (2-fluoroethyl) carbonate, and the like. From the viewpoint of ionic conductivity and viscosity, a fluorinated chain carbonate having 5 or less carbon atoms is desirable. Further, considering the stability at the positive electrode at high temperature and high voltage, methyl-2,2,2-trifluoroethyl carbonate, ethyl-2,2,2-trifluoroethyl carbonate, di-2,2,2- A carbonate having a 2,2,2-trifluoroethyl group such as trifluoroethyl carbonate is particularly desirable. The chain fluorinated carbonates can be used alone or in combination of two or more.

  The higher the content ratio of the fluorinated chain carbonate in the chain carbonate, the better the charge / discharge cycle characteristics at high temperature in the high voltage lithium secondary battery, and the amount of electrolyte decomposition gas generated at high temperature and high voltage. Decreases. From this viewpoint, the ratio of the fluorinated chain carbonate in the chain carbonate is preferably 25% by weight or more, more preferably 45% by weight or more, and most preferably 75% by weight or more.

  On the other hand, the higher the ratio of fluorinated chain carbonate in the chain carbonate, the lower the ionic conductivity of the electrolyte solution related to the output characteristics of the battery. In addition to suppressing charge / discharge cycle characteristics at high temperature and high voltage and gas generation during high temperature storage, the ratio of the fluorinated chain carbonate in the chain carbonate is 25 to 85% by weight in terms of achieving both battery output characteristics. It is preferable to be 45 to 65% by weight. However, when the output characteristics of the battery are particularly demanded, the ratio of the fluorinated chain carbonate in the chain carbonate may be 25% by weight or less. Even in this case, relatively good charge / discharge cycle characteristics and gas suppression action during high-temperature storage are shown, but this may be insufficient.

[Composition of non-aqueous solvent]
The non-aqueous solvent according to the present invention contains the cyclic carbonate and the chain carbonate as main components, and 60% by weight or more of the cyclic carbonate is fluorinated ethylene carbonate in which a fluorine atom is directly bonded to the carbonate ring, The weight ratio of the chain carbonate is 3:97 to 35:65. Here, the meaning of the main component means that 90% by weight or more, preferably 95% by weight or more in the non-aqueous solvent is composed of a cyclic carbonate and a chain carbonate.

  As described above, the higher the ratio of the content of fluorinated ethylene carbonate in the cyclic carbonate and the ratio of the content of fluorinated chain carbonate in the chain carbonate, the higher the temperature in the high voltage lithium secondary battery. The charge / discharge cycle characteristics of the are improved.

  However, although the fluorinated cyclic carbonate is highly stable at a high temperature and high voltage positive electrode, simply increasing the content of the fluorinated cyclic carbonate in the non-aqueous solvent may increase the amount of gas generated. In a lithium secondary battery in which the positive electrode potential in a fully charged state is smaller than 4.3 V based on the potential of metallic lithium, it is known that the amount of gas generated during high-temperature storage decreases as the content of cyclic carbonate increases. However, the lithium secondary battery having a positive electrode potential of 4.3 V or higher has not been clarified. For example, the composition of the electrolytic solution containing a fluorinated cyclic carbonate disclosed in Patent Document 2 (fluorinated ethylene carbonate / chain carbonate = 5/5 (volume ratio)) greatly increases the amount of gas generated during high-temperature storage. Increased.

  As a result of intensive studies to solve the above problems, the composition of the nonaqueous solvent of the nonaqueous electrolytic solution of the present invention is that the ratio of fluorinated ethylene carbonate in which fluorine atoms are directly bonded to the carbonate ring in the cyclic carbonate is 60 wt. % And the weight ratio of cyclic carbonate to chain carbonate is 3:97 to 35:65, preferably 5:95 to 25:75. With such a configuration, the amount of gas generated during high-temperature storage is greatly reduced when the positive electrode potential in a fully charged state is used for an electrochemical element of 4.3 V or higher with respect to the potential of metallic lithium. The degree of decrease is suppressed, and the charge / discharge cycle characteristics are also improved. On the other hand, when the weight ratio of the cyclic carbonate in the total non-aqueous solvent exceeds 35% by weight, even if the ratio of the fluorinated ethylene carbonate in which the fluorine atom is directly bonded to the carbonate ring in the cyclic carbonate is 60% by weight or more, In addition, the production amount of the high-temperature storage gas increases, which is undesirable.

[Other compounds]
The non-aqueous solvent according to the present invention contains the aforementioned cyclic carbonate and the aforementioned chain carbonate as main components, and may contain other compounds in the balance as long as the object of the present invention is not impaired.
In the high voltage lithium secondary battery, other compounds include 1,3-propane sultone, 1,4-butane sultone, 1,3-prop-1-ene sultone, γ-butyrolactone, trimethyl phosphate, tris phosphate ( 2,2,2-trifluoroethyl), tris (trimethylsilyl) phosphate, ethylene sulfate, propylene sulfate, butene sulfate, pentene sulfate, fluorobenzene, di (2,2,2-trifluoroethoxy) ethane and the like. The Of these, 1,3-prop-1-ene sultone and tris (trimethylsilyl) phosphate are particularly preferable.

[Non-aqueous electrolyte]
The nonaqueous electrolytic solution of the present invention contains the nonaqueous solvent and an electrolyte solute. Examples of electrolyte solutes for lithium batteries include lithium salts, and those commonly used in this field can be used. For example, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , Li ₂ SiF ₆ , LiOSO ₂ C _k F _{(2k + 1)} (k = 1 to 8), LiN (SO ₂ C _k F _{(2k + 1)} ) ₂ ( k = 1 to 8 _{integer), LiPF n (C k F} (2k + 1)) (6-n) (n = 1~5, k = 1~8 _{integer), LiBF n (C k F} (2k + 1)) _(4-n) (n = 1 to 3, k = 1 to 8) and the like. Moreover, the lithium salt shown by the following general formula can also be used. LiC (SO ₂ R ¹¹ ) (SO ₂ R ¹² ) (SO ₂ R ¹³ ), LiN (SO ₂ OR ¹⁴ ) (SO ₂ OR ¹⁵ ), LiN (SO ₂ R ¹⁶ ) (SO ₂ OR ¹⁷ ), LiN ( SO ₂ R ¹⁶ ) (SO ₂ F), LiN (SO ₂ F) ₂ (wherein R ^{11 to} R ¹⁷ may be the same as or different from each other, and are perfluoroalkyl having 1 to 8 carbon atoms) Group). Also, as borate ester lithium salt or phosphate ester lithium salt, bis (oxalato) lithium borate, difluoro (oxalato) lithium borate, bis (oxalato) fluorophosphate, trifluoro (oxalato) lithium phosphate Is mentioned. A lithium salt can be used individually by 1 type, or can use 2 or more types together.

Among these lithium salts, LiPF ₆ , LiPF _n (C _k F _{(2k + 1)} ) _(6-n) (n = 1 to 5, k = 1 to 8 ₎ from the viewpoint of ionic conductivity of the nonaqueous electrolytic solution. Integer), LiN (SO ₂ CF ₃ ) ₂ , and LiN (SO ₂ C ₂ F ₅ ) ₂ are preferable. Since the solvent used in the nonaqueous electrolytic solution of the present invention is substituted with fluorine, the coordination power to lithium ions related to ionic conductivity is weak, and the dissociation property of the lithium salt is deteriorated. For this reason, the lithium salt needs to be a salt having good dissociation as much as possible. _{Thus, LiPF 6, LiPF n (C} k F (2k + 1)) (6-n) (n = 1~5, k = 1~8 integer) is most desirable.
The amount of the lithium salt contained in the non-aqueous electrolyte may be within the range normally used in this field, and the non-aqueous electrolyte is non-aqueous at a concentration of 1 to 50% by weight, preferably 5 to 25% by weight. Dissolved in the electrolyte.

[Positive electrode]
The positive electrode of the lithium secondary battery according to the present invention includes a positive electrode active material layer and a positive electrode current collector.
The positive electrode active material is a material that can electrochemically insert and desorb lithium or anions, and is not particularly limited as long as the positive electrode potential in a fully charged state is 4.35 V or more with respect to the potential of metallic lithium. Can be used. Examples of the substance capable of electrochemically inserting and removing lithium include lithium-containing transition metal oxides such as LiCoO ₂ and metal oxides not containing lithium such as MnO ₂ . Examples of substances that can electrochemically insert and desorb anions include carbon materials that can electrochemically dope and dedoped anions, and conductive polymers. A positive electrode active material can be used individually by 1 type, or can use 2 or more types together.
As the positive electrode current collector, known ones can be used. For example, a passive film is formed on the surface by anodic oxidation in a non-aqueous electrolyte such as Al, Ti, Zr, Hf, Nb, Ta, and alloys containing these. Examples thereof include metals to be formed.

In the lithium secondary battery using the non-aqueous electrolyte of the present invention, in order to increase the storage density of lithium in the positive electrode unit volume, the potential of the fully charged positive electrode is 4.35 V or more with respect to the potential of metallic lithium. Preferred for use in. As such a positive electrode material, in addition to the conventional LiCoO ₂ , an oxide obtained by dissolving another transition metal in a spinel type Mn oxide as represented by the general formula of LiMn _x M _y O ₂ , LiNi Examples thereof include oxides in which Mn is partially dissolved in layered Ni or Co oxide to stabilize the structure, as represented by the general formula of _x Co _y Mn _(1-xy) O _2. .

In addition, the non-aqueous electric double layer capacitor has a configuration similar to that of a lithium secondary battery, and the positive electrode uses a high surface area conductive material such as activated carbon as an active material. From the viewpoint of the oxidative decomposition stability of the electrolyte solution at the positive electrode, the positive electrode potential is usually less than 4.3 V on the basis of metallic lithium, but in recent years, it is charged to 4.35 V or more in order to improve the energy density. However, when the potential of the positive electrode exceeds 4.35 V, the decomposition reactivity of the electrolytic solution at the positive electrode increases particularly at high temperatures. Therefore, when a conventional nonaqueous electrolytic solution is used, charging at a high temperature is possible. Discharge cycle characteristics deteriorate, and the amount of gas generated during high-temperature storage increases.
Since the non-aqueous electrolyte of the present invention has low reactivity on the positive electrode of high temperature and high voltage, the charge / discharge cycle characteristics at high temperature and high voltage are improved, and further, the amount of gas generated during high temperature storage can be reduced. In the nonaqueous electrolyte of the present invention, the higher the positive electrode potential, the greater the advantage over the conventional nonaqueous electrolyte, and the positive electrode potential is 4.35 V or more, further 4.40 V or more, further 4.45 V or more. Increases effectiveness.

[Negative electrode]
The negative electrode of the lithium secondary battery according to the present invention includes a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer contains metallic lithium, a carbon material capable of occluding and releasing lithium ions, and a simple substance, compound, or any one of Al, Si, Sn, Sb, or Ge, and alloyed with lithium. Alloys that can be used as a negative electrode active material.

The negative electrode active material layer is formed, for example, by forming negative electrode active material particles and a conductive agent with a binder such as polyvinylidene fluoride into a sheet or film, and embedding the negative electrode active material particles in a metal sheet or on the surface. Examples include a sheet form, a film form, and a negative electrode active material itself made into a thin film form.
The non-aqueous electrolyte of the present invention has not only high stability at a high temperature and high voltage positive electrode but also high reduction stability with respect to the negative electrode. This is presumed to be because a stable protective film is formed on the negative electrode surface by ethylene carbonate substituted with fluorine. In addition, it is thought that such a protective film is formed only with ethylene carbonate “a fluorine atom is directly bonded to a carbonate ring”. “Ethylene carbonate in which hydrogen that is not directly bonded to the carbonate ring is substituted with a fluorine atom” is not suitable because ethylene carbonate is insufficiently formed and may destabilize the protective film.

  The negative electrode active material may be any of the above, but Al, Si, Sn, Sb or Ge alone, compounds such as oxides, sulfides, carbides, etc., which can be expected to further increase the energy density of the battery, or any Or a negative electrode active material containing at least one selected from alloys that can be alloyed with lithium, and if included, Al, Sn, Si alone or an alloy thereof is particularly preferable. These can significantly increase the amount of lithium occlusion per unit volume compared to conventional carbon materials, which are the main negative electrode active materials, so the volume of the negative electrode in the battery can be greatly reduced, and the energy density of the battery Can be increased.

  The electric double layer capacitor has a configuration similar to that of a lithium secondary battery, and the negative electrode uses a high surface area conductive material such as activated carbon as an active material.

[Separator]
The separator of the lithium secondary battery according to the present invention is a film that electrically insulates the positive electrode and the negative electrode and transmits lithium ions, and a porous film, a nonwoven fabric film, a polymer electrolyte, or the like can be used. The porous film is preferably a microporous polymer film, and the material thereof is polyolefin, polyimide, polyvinylidene fluoride, polyester or the like. A porous polyolefin film is particularly preferable, and specific examples thereof include a porous polyethylene film, a porous polypropylene film, and a multilayer film of a porous polyethylene film and polypropylene. The porous polyolefin film may be coated with another resin having excellent thermal stability and chemical stability such as PVDF. Examples of the polymer electrolyte include a polymer in which a lithium salt is dissolved, a polymer swollen with a nonaqueous electrolytic solution, and the like.

[Lithium secondary battery]
The lithium secondary battery of the present invention can take various known configurations, and is usually composed of the non-aqueous electrolyte, the negative electrode, the positive electrode, and the separator, which can be a metal can or a resin bag laminated with aluminum. It is a sealed structure. In addition, a gel-like polymer electrolyte using a non-aqueous electrolyte as a plasticizer can be used instead of the non-aqueous electrolyte, and the non-aqueous electrolyte of the present invention can also be used as a plasticizer for a polymer electrolyte. Can do.
With such a configuration, the lithium secondary battery of the present invention has improved charge / discharge cycle characteristics and can suppress the voltage drop during high-temperature storage and swelling due to gas generation. Capable of increasing capacity. Even when a positive electrode with a fully charged potential of 4.35 V or higher in terms of the standard oxidation-reduction potential is used, the reaction between the non-aqueous electrolyte and the positive electrode is small, so that the generation of high-temperature storage gas can be suppressed, and A lithium battery having excellent discharge cycle characteristics and a high capacity can be obtained.

  The lithium secondary battery of the present invention can have any shape, for example, a cylindrical shape, a coin shape, a square shape, a film shape, or the like. However, the basic structure of the battery is the same regardless of the shape of the battery, and the design can be changed according to the purpose. For example, when the lithium secondary battery of the present invention is cylindrical, a wound body in which a sheet-like negative electrode and a sheet-like positive electrode are wound through a separator is impregnated with the non-aqueous electrolyte described above, The wound body is housed in a battery can so that the insulating plates are placed above and below the wound body. In the case of a coin type, a laminate of a disc-shaped negative electrode, a separator and a disc-shaped positive electrode is impregnated with a non-aqueous electrolyte and stored in a coin-type battery can with a spacer plate inserted if necessary. The configuration is

  The lithium secondary battery of this invention can be used for the same use as the conventional lithium secondary battery. For example, as a power source for various consumer electronic devices, in particular, cellular phones, mobiles, laptop personal computers, cameras, portable video recorders, portable CD players, portable MD players, electric tools, and electric vehicles It can be used suitably.

EXAMPLES Hereinafter, although an Example and a comparative example are shown and this invention is demonstrated concretely, this invention is not limited by this Example.
(Preparation of non-aqueous electrolyte)
Carbonate ester was mixed to prepare a non-aqueous solvent, and this was mixed with LiPF ₆ (electrolyte solute) to prepare a lithium salt concentration in the electrolytic solution of 1 mol / l. The carbonic acid esters used are as follows, and the mixing ratio (weight ratio with respect to the total amount of the nonaqueous solvent) is shown in Table 1.

・ 4-Fluoroethylene carbonate (abbreviation: FEC / fluorinated ethylene carbonate with a fluorine atom directly attached to the carbonate ring)
・ 4,5-difluoroethylene carbonate (abbreviation DFEC / fluorinated ethylene carbonate with a fluorine atom directly attached to the carbonate ring)
・ Trifluoromethylethylene carbonate (abbreviation: TFPC / ethylene carbonate in which hydrogen not directly bonded to carbonate ring is substituted with fluorine atom)
・ Ethylene carbonate (abbreviated EC / non-fluorinated cyclic carbonate)
・ Propylene carbonate (abbreviation PC / non-fluorinated cyclic carbonate)
・ Diethyl carbonate (abbreviated DEC / non-fluorinated chain carbonate)
・ Ethylmethyl carbonate (abbreviated as EMC / non-fluorinated chain carbonate)
・ Dimethyl carbonate (abbreviation DMC / fluorinated chain carbonate)
・ Methyl-2,2,2-trifluoroethyl carbonate (abbreviation: MFEC / fluorinated chain carbonate)

[Production of test battery]
<Negative electrode>
A graphite negative electrode sheet, a lithium negative electrode sheet, and an aluminum negative electrode sheet were prepared.
(1) Graphite negative electrode sheet 20 parts by weight of artificial graphite (manufactured by TIMCAL, SFG6), 80 parts by weight of natural graphite-based graphite (GDR, manufactured by Mitsui Mining Co., Ltd.), 1 part by weight of carboxymethyl cellulose, and 2 parts by weight of SBR latex in an aqueous solvent A paste mixture was kneaded to prepare a negative electrode mixture slurry. Next, this negative electrode mixture slurry was applied to a negative electrode current collector made of a strip-shaped copper foil having a thickness of 18 μm, dried, and then compressed by a roll press to obtain a graphite negative electrode sheet.
(2) Lithium negative electrode sheet A metal lithium foil having a thickness of 20 μm was used as the negative electrode.
(3) Aluminum negative electrode sheet An aluminum foil having a thickness of 15 μm was used as the negative electrode.

<Positive electrode>
LiCoO ₂ (trade name: HLC-22, manufactured by Honjo FMC Energy Systems Co., Ltd.) 82 parts, graphite (conductive agent) 7 parts, acetylene black (conductive agent) 3 parts, and polyvinylidene fluoride (binder) 8 parts The mixture was mixed and dispersed in 80 parts of N-methylpyrrolidone to prepare a LiCoO ₂ mixture slurry. This LiCoO ₂ mixture slurry was applied to an aluminum foil having a thickness of 20 μm, dried and roll-pressed to obtain a positive electrode sheet.

<Production of coin-type battery>
The positive electrode sheet was punched into a diameter of 13 mm to obtain a coin-type positive electrode. The graphite negative electrode sheet and the lithium negative electrode sheet were wound to a diameter of 14 mm to form a coin-type negative electrode. In the negative electrode can of a stainless steel 2032 size battery can, the above-mentioned coin-type negative electrode, a separator made of a microporous polypropylene film (made by Mitsui Chemicals) having a thickness of 25 μm and a diameter of 16 mm, and the coin-type positive electrode are laminated in this order. did. After injecting 20 μl of the non-aqueous electrolyte into the separator and absorbing the separator, an aluminum plate and a spring were stacked on the laminate. Finally, the positive electrode can of the battery was covered with a gasket made of polypropylene, and the can lid was caulked to maintain airtightness in the battery, thereby obtaining a coin-type battery having a diameter of 20 mm and a height of 3.2 mm. The coin-type battery was charged and discharged from 4.2 V to 2.85 V at a current of 0.5 mA, and then charged to 3.8 V to prepare for various evaluations. The discharge capacity discharged from 4.2 V to 2.85 V was about 4 mAh.

<Production of laminated battery>
The said aluminum negative electrode sheet and the said positive electrode sheet were cut out suitably, and it was set as the negative electrode and the positive electrode, respectively. The negative electrode and the positive electrode were laminated via a separator made of a microporous polypropylene film (manufactured by Mitsui Chemicals, Inc.) having a width of 40 mm and a length of 60 mm to produce an electrode group. This electrode group is accommodated in a cylindrical bag made of a multilayer film of aluminum and resin so that the negative and positive leads are drawn from one open part of the cylindrical bag, and the side where the leads are drawn is heated. Fused and closed. After vacuum-drying in this state, 0.4 ml of nonaqueous electrolyte was subsequently injected into the electrode group, and the other open portion was heat-sealed and sealed to produce a laminate type battery. This laminated battery was charged to 3.8 V with a current of 1.4 mA to prepare for various evaluations. The discharge capacity discharged from 4.1 V to 2.85 V was about 40 mAh.

[Charge / discharge cycle test at high temperature and high voltage (positive electrode charge voltage 4.48V)]
Using a coin-type battery (using a graphite negative electrode sheet) obtained as described above by using the electrolytic solution shown in Table 1, under the condition of 50 ° C., 3.5 mA constant current and 4.4 V constant voltage. After charging, charging / discharging was performed by discharging to 2.8 V at a constant current of 3.5 mA as one cycle. In addition, every 100 cycles, charging / discharging at 1 mA was performed after charging at a constant current of 3.5 mA and a constant voltage of 4.4 V. At this time, the potential of the positive electrode in the fully charged state was 4.48 V on the basis of the potential of metallic lithium. The ratio (%) of the discharge capacity at the 200th cycle to the discharge capacity at the 3rd cycle was defined as the high temperature / high voltage cycle capacity retention rate, and the results are shown in Table 2.

[Charge / discharge cycle test when the charge voltage is changed (positive charge voltage 4.38V and 4.28V)]
The charge / discharge cycle test was performed under exactly the same conditions as the “charge / discharge cycle test at high temperature and high voltage” except that the charge voltage was 4.3V. At this time, the potential of the positive electrode in the fully charged state was 4.38 V on the basis of the potential of metallic lithium.
Furthermore, a charge / discharge cycle test was performed under exactly the same conditions as the “charge / discharge cycle test at high temperature and high voltage” except that the charge voltage was set to 4.3V. At this time, the potential of the positive electrode in the fully charged state was 4.38 V on the basis of the potential of metallic lithium.
The above high-temperature cycle test was performed for the batteries of Example 2 and Comparative Example 10 and is shown in Table 2.

[Battery voltage sustainability test at high temperature and high voltage]
Using a coin-type battery (using a lithium negative electrode sheet) obtained as described above with the electrolyte shown in Table 1, the battery was charged under the conditions of a constant current of 0.5 mA and a constant voltage of 4.48 V. The voltage was defined as “voltage before storage”. Note that the potential of the positive electrode in a fully charged state is 4.48 V with respect to the potential of metallic lithium.
The voltage of the battery after the battery was put in a thermostat at 60 ° C. and stored for 170 hours was measured, and the voltage at this time was defined as “voltage after storage”. Since the battery voltage after storage decreases so that the oxidative decomposition reaction of the electrolytic solution at the positive electrode is more likely to occur, the voltage drop level of the battery (= voltage before storage−voltage after storage, mV) is oxidative decomposition of the electrolytic solution at the positive electrode. It is an indicator. Table 2 shows the results.

[Battery swelling test of batteries at high temperature and high voltage]
Using the electrolytic solution shown in Table 1, a laminate type battery (using an aluminum negative electrode sheet) obtained as described above was produced. In a laminated battery, when gas is generated due to decomposition of the electrolytic solution, since the material of the outer package is an aluminum laminated film, the entire laminated battery expands almost uniformly. The “battery volume after initial charge” and “battery volume after high-temperature storage” of the laminate battery were measured, and the difference between them was defined as “battery swelling” due to electrolyte decomposition gas during high-temperature storage.
The volume of the battery when the laminate battery was initially charged at a constant current of 1.4 mA and a constant voltage of 4.1 V was defined as “battery volume after initial charge”. The potential of the positive electrode during initial charging was 4.35 V with respect to the potential of metallic lithium.
The volume of the laminated battery charged as described above after high temperature storage at 85 ° C. for 3 days was defined as “battery volume after high temperature storage”. The volume of the battery was again determined by the Archimedes method. The results are shown in Table 2. In Table 2, “-” indicates that no measurement was performed.

[Explanation of test results]
From the results of Examples shown in Table 2, the weight ratio of the cyclic carbonate to the chain carbonate is 5:95 to 35:65, and 60% by weight or more of the cyclic carbonate is ethylene carbonate in which a fluorine atom is directly connected to the carbonate ring. When the electrolytic solution is used, it can be seen that a lithium battery can be obtained that has excellent voltage drop and gas generation during storage at high temperature and high voltage, and excellent charge / discharge cycle characteristics at high temperature and high voltage.
In particular, from the comparison of the cycle capacity retention rate when the positive electrode charge voltage of Example 2 and Comparative Example 10 was changed, in the case of 4.28 V charge where the positive electrode charge voltage is 4.35 V or less, both have an excellent cycle capacity retention rate. Indicated. On the other hand, in the case of 4.38V charging where the positive electrode charging voltage exceeds 4.35V, Example 2 shows a better cycle capacity retention rate than Comparative Example 10, and when the positive electrode charging voltage is 4.48V, The cycle capacity retention rate was much better than that of Comparative Example 10.
In particular, from the comparison of Examples 7 to 12, it can be seen that as the ratio of the fluorine-substituted chain carbonate in the chain carbonate is increased, both the voltage drop degree, the gas generation amount, and the charge / discharge cycle characteristics are excellent.
Further, from comparison between Examples 6 and 7, it can be seen that fluoroethylene carbonate is superior to difluoroethylene carbonate in producing less gas than ethylene carbonate having a fluorine atom directly bonded to the carbonate ring. Although DFEC with a larger number of fluorine substitutions should have better oxidation resistance than FEC originally, it seems that the thermal stability of FEC is superior to DFEC.

On the other hand, from the results of Comparative Examples 1 to 3, even if 60% by weight or more of the cyclic carbonate is ethylene carbonate in which a fluorine atom is directly bonded to the carbonate ring, the weight ratio of the cyclic carbonate to the chain carbonate is 2:98. If the amount is less, the voltage drop degree and the gas generation amount are excellent, but the charge / discharge cycle characteristics are greatly inferior.
In addition, from the results of Comparative Examples 7 to 9, when the weight ratio of the cyclic carbonate to the chain carbonate exceeds 40:60 even if 60% by weight or more of the cyclic carbonate is ethylene carbonate in which a fluorine atom is directly bonded to the carbonate ring. It is understood that the voltage drop degree and charge / discharge cycle characteristics are excellent, but the gas generation amount is greatly inferior.
Further, from the results of Comparative Examples 4, 5, 6, 10, 11, and 12, even if the weight ratio of the cyclic carbonate to the chain carbonate is 5:95 to 35:65, a fluorine atom is present in the carbonate ring in the cyclic carbonate. When the ratio of ethylene carbonate directly connected to is lower than 60% by weight, it is understood that the degree of voltage drop, gas generation amount, and charge / discharge cycle characteristics are inferior.
Further, from the comparison between Example 2 and Comparative Example 13, when the cyclic carbonate is ethylene carbonate “with hydrogen atoms not directly bonded to the carbonate ring substituted by fluorine atoms”, the electrolytic solution composition is the same as the electrolytic solution of the present invention. Even if it exists, it turns out that the effect of this invention is not acquired.

[Summary]
From the experimental results described above, 60% by weight or more of the cyclic carbonate is fluorinated ethylene carbonate in which fluorine atoms are directly connected to the carbonate ring, and the weight ratio of the cyclic carbonate to the chain carbonate in the non-aqueous solvent is 3 : From 98 to 35:65, the oxidative decomposition reaction of the electrolyte on the positive electrode is suppressed, and a battery with excellent balance in charge / discharge cycle characteristics at high temperature and high voltage, voltage sustainability, and battery swelling is obtained. I understood that I could get it.

Claims (4)

A non-aqueous solvent containing a cyclic carbonate and a chain carbonate as main components and an electrolyte solute, and 60% by weight or more of the cyclic carbonate is a fluorinated ethylene carbonate in which a fluorine atom is directly connected to the carbonate ring, and the cyclic carbonate and the chain carbonate A non-aqueous electrolyte for an electrochemical device in which the positive electrode potential in a fully charged state with a weight ratio of 3:97 to 35:65 is 4.35 V or more with respect to the potential of metallic lithium. The nonaqueous electrolytic solution according to claim 1, wherein 25% by weight or more of the chain carbonate is a fluorinated chain carbonate. The nonaqueous electrolytic solution according to claim 1 or 2, wherein the fluorinated ethylene carbonate is 4-fluoroethylene carbonate. 4. A non-aqueous electrolyte according to claim 1, a positive electrode having a positive electrode active material capable of reversible electrochemical reaction with lithium ions or anions, and a negative electrode having a negative electrode active material capable of charging and discharging lithium ions. A lithium secondary battery in which the potential of the fully charged positive electrode is 4.35 V or more with respect to the potential of the metallic lithium.