JPH10116630A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the safety by forming a solvent for non-aqueous electrolyte from fluoro-carbonate and chain carbonate as major components, and arranging so that their proportion by volume lies within the specific range. SOLUTION: A solvent for a non-aqueous electrolyte is prepared by mixing fluoro-carbonate with chain carbonate in a specific mix proportion by volume. The fluoro-carbonate should have such a structure that part or all of the hydrogen atoms of a chain or cyclic carbonate having carbonate group (-OCOO-) are substituted with fluorine atoms. For example, the chain fluoro-carbonate should have such a structure as expressed by the given equation, where R1 and R2 are alkyl group, provided that at least either of them is alkyl group in which part or all of the hydrogen atoms are substituted with fluorine atoms. The mix proportion by volume (fluoro-carbonate vs. chain carbonate) should range from 5:95 to 90:10. This allows enhancement of the safety against a destructive test for the battery.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a secondary battery using a positive electrode, a negative electrode, and a non-aqueous electrolyte.

[0002]

2. Description of the Related Art In recent years, with the spread of portable devices such as video cameras and notebook personal computers, demand for small and high capacity secondary batteries has been increasing. Most of the secondary batteries currently used are nickel-cadmium batteries using an alkaline electrolyte, but the battery voltage is as low as about 1.2 V, and it is difficult to improve the energy density. Therefore, a lithium secondary battery in which the voltage is increased to 3 V or more and lithium having a large discharge capacity per weight is used for the negative electrode has been studied. These are called lithium metal secondary batteries that use lithium metal or lithium alloy for the negative electrode, lithium ion secondary batteries that use lithium ion-doped carbon materials or transition metals as the negative electrode active material, and have a high energy density. Recently, lithium secondary batteries have attracted particular attention.

[0003] In these lithium secondary batteries, water cannot be used as a solvent from the viewpoint of withstand voltage. Therefore, a non-aqueous electrolyte using a non-aqueous solvent is used. Such non-aqueous electrolytes are described in detail in numerous books (eg, Kosuke Izuzu, “Electrochemistry in Non-Aqueous Solutions”, Baifukan (1995)).

Further, regarding the negative electrode carbon material of the lithium ion secondary battery, Japanese Patent Application Laid-Open Nos. 57-208079, 58-93176 and 58-19226 have been disclosed.
No. 6, JP-A-62-90863, JP-A-62-90863
JP-A-122066 is known.

[0005]

However, the lithium secondary batteries described in the above-mentioned known examples have many problems regarding safety such as overcharging, heating, and short circuit. In particular, since a lithium secondary battery uses a non-aqueous electrolyte mainly containing an organic solvent, the battery is short-circuited or thrown into a fire, or a non-aqueous electrolyte is formed by a reaction between a positive electrode or a negative electrode and the non-aqueous electrolyte. If the liquid is ignited, an exothermic runaway reaction occurs, and the battery explodes and ignites. In particular, it cannot be said that sufficient safety is secured in the event of internal short-circuit breakdown such as nail penetration or crushing of a battery.

[0006] Flame retardants and antioxidants are added to the non-aqueous electrolyte of lithium secondary batteries having such a danger to improve safety, or hydrogen atoms in the solvent are replaced with fluorine atoms. Attempts have been made to burn it, but to a lesser extent. One such attempt is disclosed in
-37025, JP-A-8-37025, JP-A-7-31
2227, JP-A-8-88023, JP-A-7-678
6, JP-A-6-219992 and the like are known.

An object of the present invention is to provide a non-aqueous electrolyte secondary battery which is excellent in safety and aims to solve the disadvantages of the prior art.

[0008]

The present invention has the following arrangement to achieve the above object.

[0009] In a secondary battery composed of a positive electrode, a negative electrode and a non-aqueous electrolyte, the solvent of the non-aqueous electrolyte is mainly composed of fluorinated carbonate and chain carbonate, and its volume ratio (fluorinated carbonate: chain Carbonate) is 5:
A secondary battery having a range of 95 to 90:10. "

[0010]

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a lithium ion secondary battery having improved safety by using a solvent in which fluorinated carbonate and chain carbonate are mixed in a specific volume ratio as a non-aqueous electrolyte for a lithium secondary battery. A secondary battery is provided. Therefore, there is no particular limitation on the electrolyte components other than the solvent of the non-aqueous electrolyte, the structure of the positive electrode and the negative electrode, the structure of the secondary battery, and the method of manufacturing the same.

The fluorinated carbonate used in the non-aqueous electrolyte of the present invention has a structure in which part or all of the hydrogen atoms of a chain or cyclic carbonate having a carbonate group (—OCOO—) are substituted with fluorine atoms. Have For example, the chain fluorinated carbonate includes those having the following structural formula.

[0012]

Embedded image R ₁ and R ₂ each represent an alkyl group, at least one of which is an alkyl group in which part or all of the hydrogen atoms have been substituted with fluorine atoms.

Examples of such a chain fluorinated carbonate include, for example, methyl 2,2,2-trifluoroethyl carbonate, methyl 2,2,3,3,3-pentafluoropropyl carbonate, methyl 3,3,3 -Trifluoropropyl carbonate, methyl 2,2,3,3,4,4,4-heptafluorobutyl carbonate, 2,2,2-trifluoroethyl 2,2,3,3,3-pentafluoropropyl carbonate, etc. Is mentioned.

The cyclic fluorinated carbonate has, for example, the following structure.

[0015]

Embedded image R ₃ represents an alkyl group having 2 or more carbon atoms, and a part or all of the hydrogen atoms are substituted with fluorine atoms.

The chain carbonate used in the non-aqueous electrolyte of the present invention is not limited as long as it is a chain carbonate, and examples thereof include compounds having the following structures.

[0017]

Embedded image R ₄ and R ₅ represent an alkyl group having 1 to 5 carbon atoms.

Examples of such a chain carbonate include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl-n-propyl carbonate, methyl-iso-propyl carbonate, ethyl
-n-propyl carbonate, di-n-propyl carbonate, di-iso-propyl carbonate and the like.

According to the present invention, the nonaqueous electrolyte obtained by mixing the above-mentioned fluorinated carbonate and chain carbonate in a specific volume ratio can greatly improve the safety without impairing various performances of the lithium secondary battery. It is characterized by the following. The volume ratio (fluorinated carbonate: chain carbonate) is in the range of 5:95 to 90:10, preferably
10:90 to 80:20 range, more preferably 15:85 to 7
The range is 0:30. When the ratio of the fluorinated carbonate in the non-aqueous electrolyte is small, the effect of improving the safety of the lithium secondary battery cannot be obtained. When the ratio of the fluorinated carbonate is too large, the viscosity of the nonaqueous electrolyte becomes high, and the battery performance other than safety, such as a decrease in the high output capacity of the battery due to a decrease in ionic conductivity, is impaired.

The volume ratio between the fluorinated carbonate and the chain carbonate can be measured by various analytical techniques, and is not particularly limited. For example, gas chromatography (GC), liquid chromatography (L
C), mass spectrometry (MS), nuclear magnetic resonance spectroscopy (NM)
R) and the like. Among them, the GC-MS method combining gas chromatography with mass spectrometry and the NM
The R method is preferably used. When analyzing the volume ratio, the electrolyte solution may be analyzed as it is, or may be analyzed after being diluted or extracted with another solvent. In particular, when the electrolytic solution of the present invention is used for a lithium secondary battery and a trace amount of the electrolytic solution is analyzed, it is necessary to extract G with ethers or the like before extracting G
You may analyze by C-MS method or NMR.

Although the mechanism by which the nonaqueous electrolyte of the present invention is effective in improving the safety of a battery has not been completely elucidated,
Think as follows. When the battery is destroyed with an internal short circuit such as nailing or crushing, a reaction between the positive electrode or the negative electrode and the electrolyte occurs due to the heat generated by Joule heat at the short-circuited portion. Due to these heat generations, oxygen is released from the transition metal oxide of the positive electrode active material, and the oxygen oxidizes the electrolyte and the negative electrode to further raise the temperature. It is thought that it reacts further and leads to fever runaway. Non-aqueous electrolyte of the present invention, by suppressing the reaction with the positive electrode active material and the negative electrode active material,
It seems that the safety of the battery is improved.

The electrolyte used in the non-aqueous electrolyte of the present invention is not particularly limited, but may be LiPF ₆ , LiBF ₄ ,
LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃
SO ₂ ) ₃ , LiSbF ₆ , LiSiF ₅ , LiAlF ₄ , LiSCN or LiClO ₄ are preferably used. Further, when used for a lithium secondary battery, LiPF ₆ and LiBF ₄ are preferable from the viewpoint that the high output capacity of the battery is large. In addition, LiBF ₄ is particularly preferably used from the viewpoint of stability against water and temperature and cost.

The concentration of the electrolyte used in the present invention is 0.1.
4M to 2.5M is preferred. In particular, 0.7M to 1.5M
Is preferable in that a high conductivity can be obtained.

The solvent of the non-aqueous electrolyte used in the present invention is
It is a preferred embodiment to add a minor component up to 10% by volume in addition to the above-mentioned fluorinated carbonate and chain carbonate. The additives used in this case include:
Various organic or inorganic compounds can be mentioned. Such additives include antioxidants, flame retardants, radical scavengers, surfactants, heterocyclic compounds,
It is not particularly limited, such as a halogen compound.

Examples of the negative electrode active material used in the lithium secondary battery of the present invention include lithium compounds such as lithium metal and lithium alloy, carbon materials into which lithium ions can be doped, and transition metal compounds.

Examples of the lithium alloy include a wood alloy made of lithium and aluminum.

The carbon material to which lithium ions can be doped is not particularly limited, and a carbon material obtained by calcining an organic substance or a naturally occurring carbon material is used. Specifically, polyacrylonitrile (P
AN) and a PAN-based carbon body obtained from the copolymer thereof;
Pitch-based carbon bodies obtained from pitch such as coal or petroleum, cellulosic carbon bodies obtained from cellulose, vapor-grown carbon bodies obtained from low-molecular-weight organic gas, and the like, in addition to polyvinyl alcohol, lignin, A carbon body obtained by firing polyvinyl chloride, polyamide, polyimide, phenol resin, furfuryl alcohol, or the like may be used. In addition, naturally occurring natural graphite and other carbon bodies are also examples. Among these, a carbon body satisfying the characteristics is appropriately selected according to the characteristics of the negative electrode and the lithium secondary battery in which the carbon body is used, and they may be one kind or a mixture of two or more kinds.
Among the carbon bodies, when used for the negative electrode of the lithium secondary battery of the present invention, a PAN-based carbon body, a pitch-based carbon body,
A vapor-grown carbon body, natural graphite or a mixture of two or more of these is preferred.

As the PAN-based carbon material, Japanese Patent Publication No. 37-4
No. 405, JP-B-44-21175, JP-B-47-24185, JP-B-51-6244, and many other known methods. In general, in these methods, the PAN-based polymer is calcined in air at 150 to 300 ° C., and then calcined in an inert gas atmosphere at 900 to 2000 ° C. for about 5 minutes as a holding time at the ultimate temperature. A PAN-based carbon body is obtained. Here, the inert gas is a gas that does not react with the carbon material at the exemplified firing temperature, and examples thereof include nitrogen, argon, and a mixed gas thereof. Pitch-based carbon, cellulose, and the like can also be produced by using a known method and the like, and are described in, for example, "Carbon Fiber" (Sugio Otani, Modern Editors). By controlling these manufacturing conditions,
A carbon material having an appropriate structure can be obtained. When used for the negative electrode of the lithium secondary battery of the present invention, the amorphous PAN-based carbon material obtained at a sintering temperature of 1000 to 1500 ° C. has a large lithium ion doping amount and has a high discharge capacity and high output capacity of the battery. It is preferably used because it has good cycle performance and other characteristics.

As the carbon material, in addition to the above-mentioned PAN amorphous carbon, graphite can also be used as a crystalline carbon material. Examples of such graphite include artificial graphite obtained by subjecting pitch or coke of petroleum or coal, a high-molecular compound, a low-molecular-weight organic compound, and the like to high-temperature treatment, and natural graphite produced naturally, and are particularly limited. It can be used without. Examples of natural graphite include flaky graphite mainly produced in Madagascar and China, flaky graphite mainly produced in Sri Lanka, and earthy graphite mainly produced in Mexico and Russia. Among these graphites, natural graphite is preferably used in terms of price. It is also a preferred embodiment to mix an amorphous carbon body with such graphite. The following can be considered as a reason for this. Graphite may have insufficient charge / discharge cycle characteristics of a lithium ion secondary battery, such as expansion and contraction of a graphite negative electrode or collapse of a graphite layer itself due to doping or undoping of lithium ions. However, it is considered that by mixing an amorphous carbon material with graphite, such expansion and contraction is alleviated, and the cycle characteristics become preferable. As such an amorphous carbon body mixed with graphite, a PAN-based amorphous carbon fiber is preferably used.

When carbon fibers are used for the negative electrode active material of the present invention, carbon fibers having an average length of 5 mm or less are preferably used, preferably 100 μm or less, particularly preferably 30 μm or less. Further, the diameter is preferably 1
It is preferably at most 00 μm, particularly preferably at most 20 μm. Furthermore, the ratio of fiber length to fiber diameter (aspect ratio) is 1
The above is preferred. It is also preferable to use several types of carbon fibers having different diameters and fiber lengths. further,
It is also preferable to use a mixture of the above-mentioned carbon powder, short fiber, long fiber, or the like as an electrode material. The average particle size, fiber diameter and fiber length of these carbon materials are, for example, S
It can be determined by measuring 20 or more carbon bodies by microscopic observation such as EM.

The negative electrode of the present invention includes, in addition to the above-mentioned metallic lithium, lithium alloy, and carbon material, groups 13 to 13 of the periodic table.
It is also possible to use a composite oxide or a composite chalcogenide composed of a combination of three or more elements belonging to Group XV. Examples of such a negative electrode material include JP-A-7-122274, JP-A-7-235293, JP-A-7-288123, JP-A-7-220721, JP-A-7-263028, JP-A-7-201318, and JP-A-8-130036. The negative electrode material described in (1) is mentioned as an example.

For example, as such a negative electrode, the following general formula (5) M ¹ M ^2p M ^3q general formula (5) (where M ¹ is at least one selected from Ge, Sn, Pb, As, and Sb) Species, M ² is at least two kinds selected from B, Al, Si, P, M ³ is at least one kind selected from O, S, Se, Te, p is more than 0 and 10 or less, q is 1 or more and 50 or less The composite oxide, the composite chalcogenide, and the like represented by the following formulas are exemplified. Among these, as the M ¹ Sn, as the M ² Si, M ³
Is preferably used as O. Among these composite oxides and composite chalcogens, amorphous particles having a particle size of 0.1 to 60 μm are preferably used.

When using lithium metal, lithium alloy, carbon material, composite oxide or composite chalcogenide as the negative electrode active material used in the present invention as an electrode, the form is not particularly limited. For example, in the case of powder of carbon or oxide, or short carbon fiber, it is preferable to knead them with a small amount of a binder and apply a paste to a conductive current collector. When applying a paste-like material, a conductive agent such as carbon black or graphite may be mixed with the paste-like material in some cases. Among them, acetylene black is preferably used as the conductive agent. In the case of long carbon fibers, for example, it is preferable to arrange them in a uniaxial direction, or to form a fabric-like or felt-like structure. As a structure such as a fabric or a felt, fabrics, knits, braids, laces, nets,
Examples thereof include felt, paper, nonwoven fabric, and mat, and woven fabric and felt are preferable from the viewpoint of the properties of carbon fibers and electrode characteristics.

It is also preferable to add a binder to the active material in order to make the negative electrode used in the present invention into a paste as described above or to enhance the moldability. Examples of such a binder include, but are not particularly limited to, high molecular compounds such as polytetrafluoroethylene, polyvinylidene fluoride, polyacrylonitrile, polyimide, polyethylene, polypropylene, epoxy resin, and phenol resin. These binders are not limited to a specific form, such as being mixed with a powder, or dissolved in a solvent or dispersed as an emulsion and used as a slurry with an active material.

It is also preferable to add a conductive agent to the negative electrode used in the present invention, in addition to the above-mentioned carbon material and composite oxide, in order to improve electron conductivity. By adding the conductive agent, the resistance in the electrode is reduced, which is effective in improving the battery capacity, not only improving the low output capacity,
This is particularly effective in improving the high output capacity. As such a conductive agent, a material having a low electric resistance, such as a metal,
Semiconductors and semimetals are used, and carbonaceous materials or carbon materials such as graphite and carbon black are particularly preferably used. Among them, acetylene black, Ketjen black, aniline black, artificial and natural graphite and the like are preferably used. The shape of the conductive agent is not particularly limited, such as a powder or a fiber. In the case of a powder, the particle size is preferably 0.1 to 100 μm, and more preferably 1 to 50 μm. The amount of the conductive agent added,
0.1 to 20% by weight is preferable from the viewpoint of improving conductivity.

In the negative electrode used in the present invention, a current collector is used to conduct the current from the negative electrode to the terminal. As such a current collector, metals such as copper, stainless steel, nickel, titanium, and platinum can be used in the form of white, net, lath, and the like, but these are not particularly limited. . The method for bringing the negative electrode and the current collector into contact with each other is not particularly limited, such as a method in which a fibrous or powdery mixture containing the negative electrode active material is directly pressed onto the current collector. Further, the distance from the current collector corresponding to the thickness of the negative electrode to the surface of the negative electrode is not particularly limited.

The active material of the positive electrode used in the present invention is not particularly limited. For example, lithium cobaltate, lithium nickelate, lithium manganate,
A transition metal oxide containing lithium such as lithium niobate or lithium vanadate, a transition metal chalcogen such as molybdenum sulfide or titanium sulfide, or a mixture thereof is used. In addition, disulfide compounds such as dimercaptothiadiazole, and polymer compounds containing a hetero atom such as polyalkylene oxide, polyalkylene sulfide, polyaniline, polythiophene, and polypyrrole are also included. Further, conjugated polymer compounds such as polyacetylene, polydiacetylene, polyparaphenylene, and polyphenylenevinylene are also used. The above-described substances capable of inserting and extracting lithium ions or anions can be used as the positive electrode active material without any limitation, and the oxidation potential of these substances is preferably 2.5 V or more with respect to lithium. When this positive electrode active material is used as a powder, the particle size of the powder is 0.1 to 100 μm, preferably 1 to 50 μm.

In the positive electrode used in the present invention, it is also preferable to add a conductive agent or a binder to the active material in order to enhance moldability and improve electron conductivity. As the binder and the conductive agent, the same compounds as those of the negative electrode are used, and the usage form is not particularly limited.

For the positive electrode used in the present invention, a current collector is used to make conduction to the terminal. As such a current collector,
Metals such as aluminum, titanium, platinum, nickel,
It can be used in the form of foil, net, lath, etc., but these are not particularly limited. Further, as a method of bringing the positive electrode into contact with the current collector, the powder mixture containing the positive electrode active material is directly pressed onto the current collector, the slurry containing the positive electrode active material is applied to the current collector, and the solvent is dried and then pressed. The production method is not particularly limited. Further, the distance from the current collector corresponding to the thickness of the positive electrode to the surface of the positive electrode is not particularly limited.

In the lithium secondary battery of the present invention, a separator is used between the positive electrode and the negative electrode. However, a polymer microporous film used in ordinary nonaqueous electrolyte lithium batteries can be used without any particular limitation. . For example, polyethylene, polypropylene, polyvinylidene fluoride, polyvinylidene chloride, polyacrylonitrile, polyacrylamide, polytetrafluoroethylene, polysulfone, polyethersulfone, polycarbonate, polyamide, polyimide, polyethers such as polyethylene oxide and polypropylene oxide, carboxymethyl cellulose And cellulose derivatives such as hydroxypropylcellulose, poly (meth) acrylic acid and various esters thereof, and high molecular compounds and derivatives thereof, and films of copolymers and mixtures thereof. . Further, such a film may be used alone, or a multilayer film in which these films are laminated. Further, various additives may be used for these films, and the types and contents thereof are not particularly limited. Among these microporous films, polyethylene, polypropylene, polyvinylidene fluoride, and polysulfone are preferably used for the lithium secondary battery of the present invention.

[0041] These separator films are microporous so that the electrolyte solution permeates and ions easily permeate. The microporous method includes a phase separation method in which a film of a polymer compound and a solvent is formed while microphase-separating the solution, and the solvent is extracted and removed to form a porous layer. Extrusion film formation and heat treatment,
A “stretching method” in which the crystals are arranged in one direction and a gap is formed between the crystals by stretching to achieve porosity, and the like, may be mentioned, and the method is appropriately selected depending on the polymer film to be used. In particular, a phase separation method is preferably used for polyethylene and polyvinylidene fluoride preferably used in the present invention. In particular, polyvinylidene fluoride is formed into a film by dissolving a solution dissolved in a good solvent such as nitrogen-containing or sulfur-containing polar solvent using a poor solvent such as alcohols, and at the same time, extracting the polar solvent. Good microporosity is also obtained by the method, and is suitably used in the present invention. They are,
By applying a polymer compound solution dissolved in a good solvent directly onto the positive and negative electrodes and phase-separating this using a poor solvent to produce a microporous membrane, a separator integrated with the electrode is obtained. This is a preferred embodiment for the electrolytic solution of the present invention. N-methylpyrrolidone, N, N-dimethylformamide, N, N-dimethylacetamide, dimethylsulfoxide and a mixed solvent thereof are used as good solvents for polyvinylidene fluoride and polysulfone which are good for the secondary battery of the present invention. As a poor solvent, water, methanol, ethanol, n-propanol, iso-propanol and a mixed solvent thereof are preferably used.

The separator composed of these microporous films may be disposed between the positive electrode and the negative electrode as an independent film,
It may be a form integrated with the positive electrode or the negative electrode. The microporous pore diameter of these microporous films is not particularly limited as long as ions of the electrolyte can pass therethrough.
The range of m is chosen. It is also possible to use a solid polymer electrolyte, which will be described later, which is integrated with the positive and negative electrodes and whose pore size is not required exactly, as the separator.

As described above, the lithium secondary battery of the present invention uses an electrolyte containing fluorinated carbonate and chain carbonate as a main solvent. The lithium secondary battery having a form in which this electrolyte is impregnated into a solid electrolyte is used. A secondary battery may be used. Solid electrolytes include inorganic solid electrolytes composed of oxides such as alkali (earth) metals, transition metals, semi-metals, semiconductors, nonmetals, chalcogenides, halides, and the like, and polymer solid electrolytes. Absent. In particular, for a lithium secondary battery, it is a preferred embodiment to use the non-aqueous electrolyte solution of the present invention in combination with a polymer compound as a polymer solid electrolyte.

Examples of the polymer compound used for the polymer solid electrolyte include polyethers such as polyethylene oxide and polypropylene oxide; halogen-containing polymer compounds such as polytetrafluoroethylene, polyvinylidene fluoride and polyvinylidene chloride; Poly (meth) acrylic acid and various esters thereof, polyacrylonitrile, polyacrylamide, polycarbonate, polysulfone, polyethersulfone, polyamide, polyimide, celluloses such as carboxymethylcellulose and hydroxypropylcellulose, polyphosphazene,
Examples thereof include a polymer compound mainly composed of, for example, a derivative thereof, a copolymer, and a mixture, and are used without any particular limitation. Various additives may be added to these polymer compounds, and the types and amounts of the additives are not particularly limited. Among these, a solid polymer electrolyte composed of polyether, polyvinylidene fluoride, polysulfone, and polyacrylonitrile is preferably used for a lithium secondary battery using the electrolyte solution of the present invention.

When a solid polymer electrolyte is used in the lithium secondary battery of the present invention, a non-aqueous electrolytic solution is combined with the above-mentioned polymer compound to form a solid polymer electrolyte, but the production method and shape are not particularly limited. is not. The production method is not particularly limited. For example, a polymer compound may be disposed between the positive electrode and the negative electrode and impregnated or impregnated with the electrolytic solution later, or the electrolytic solution and the polymer compound may be used in advance. May be disposed between the positive electrode and the negative electrode. Also, when disposed between the positive electrode and the negative electrode, the method is not particularly limited, using a polymer compound or a solid polymer electrolyte impregnated with an electrolytic solution as an independent membrane, it may be disposed between the positive and negative electrodes Alternatively, the polymer solid electrolyte impregnated with the polymer compound or the electrolyte and the positive and negative electrodes may be integrated on the positive electrode or the negative electrode.

In such a solid polymer electrolyte, the ion permeability (conductivity) is good and the reaction of the electrolytic solution on the surfaces of the positive electrode and the negative electrode is suppressed. The combination of the microporous polymer compound and the electrolyte of the present invention is a preferred embodiment in that various performances such as high output, cycle, storage, high temperature, and safety are improved.

The method for making the microporous material is not particularly limited, but the “phase separation method” described in the above-mentioned method for making the separator microporous is preferable. Polymer compounds such as polyvinylidene fluoride and polysulfone suitably used in the present invention, a solution dissolved in a polar solvent of nitrogen-containing or sulfur-containing which is a good solvent is subjected to phase separation using a poor solvent such as alcohols. Good microporosity is obtained by a phase separation method in which a film is formed and at the same time, a polar solvent is also extracted, and is suitably used in the present invention. At this time, after directly applying a solution of a polymer compound dissolved in a good solvent to the electrode surface of the positive electrode or the negative electrode, the electrode is immersed in a poor solvent to make microporous and remove the solvent by extraction. Thus, a microporous polymer solid electrolyte integrated with the above is obtained, and the productivity is improved.
Among these, as the good solvent and the poor solvent for polyvinylidene fluoride and polysulfone, the solvents described in the phase separation method of the separator are similarly used.

When a solid electrolyte is used in the lithium secondary battery of the present invention, the ratio between the solid electrolyte and the electrolyte is not particularly limited, and is appropriately selected according to the required characteristics of the battery. .

The secondary battery of the present invention is widely used for small portable electronic devices such as video cameras, personal computers, word processors, radio-cassettes, mobile phones, etc., utilizing the features of light weight, high capacity, and high energy density. It is possible.

[0050]

EXAMPLES Specific embodiments of the present invention will be described below with reference to examples, but the present invention is not limited thereto.

Example 1 (1) Preparation of Negative Electrode A commercially available finely shortened carbon fiber (MLD-
30), acetylene black as a conductive agent and polyvinylidene fluoride as a binder were mixed at a weight ratio of 80: 5: 15 to prepare a slurry using N-methylpyrrolidone as a solvent. This was applied to a copper foil of a current collector, dried and pressed to obtain a molded negative electrode.

(2) Preparation of positive electrode Commercially available lithium carbonate (Li ₂ CO ₃ ) and basic cobalt carbonate
(2CoCO ₃ .3Co (OH) ₂ ) was weighed so that the molar ratio Li / Co = 1/1, mixed by a ball mill, and heat-treated at 900 ° C. for 20 hours to obtain LiCoO ₂ . This was ground with a ball mill to obtain a positive electrode active material. This positive electrode active material, acetylene black as a conductive agent, and polyvinylidene fluoride as a binder were mixed at a weight ratio of 91: 6: 3, and a slurry was prepared using N-methylpyrrolidone as a solvent. This was applied to an aluminum foil of a current collector, dried and pressed to obtain a molded positive electrode.

(3) Preparation of Secondary Battery The negative electrode obtained in the above (1) is overlapped with the positive electrode prepared in the above (2) via a porous polyethylene film as a separator to form a cylindrical shape. It was rolled up, fitted with terminals, and housed in a battery can to make an 18650 cylindrical battery with a diameter of 18 mm and a height of 65 mm. The non-aqueous electrolyte is mixed with a mixed solvent of methyl 2,2,2-trifluoroethyl carbonate (MFEC) and dimethyl carbonate (DMC) at a volume ratio of 25:75.
MLiPF ₆ was dissolved to prepare 1MLiPF ₆ / MFEC-DMC (25:75).

(4) Evaluation The secondary battery obtained in the above (3) was charged at a constant current and a constant voltage of up to 4.3 V at a current of 1000 mA for 4 hours.
Constant current discharge was performed to 2.75 V at mA. The discharge capacity at this time was 1250 mAh. This charge / discharge cycle was performed a total of 5 times, only charging was performed, and a nail penetration test was performed by piercing a nail of 3 mm in diameter into the side of the battery can. There was no burst ignition.

Comparative Example 1 A secondary battery was prepared and evaluated in the same manner as in Example 1 except that the solvent of the nonaqueous electrolyte was a mixed solvent of PC-DMC (50:50). The discharge capacity was 1200 mAh, but in the nail penetration test,
It burst and ignited.

Example 2 A secondary battery was prepared in the same manner as in Example 1 except that the solvent of the non-aqueous electrolyte was a mixed solvent of MFEC-DMC (50:50). The battery had a discharge capacity of 1150 mAh. No nail explosion was found in the nail penetration test.

Example 3 A secondary battery was fabricated in the same manner as in Example 1, except that the solvent of the nonaqueous electrolyte was a mixed solvent of MFEC-DMC (10:90). The discharge capacity of this battery was 1200 mAh, and there was no explosion or ignition in the nail penetration test.

Comparative Example 2 A secondary battery was fabricated in the same manner as in Example 1, except that the solvent of the non-aqueous electrolyte was a sole solvent of DMC alone. This battery had a discharge capacity of 1250 mAh and burst and ignited in a nail penetration test.

Example 4 (1) Preparation of Negative Electrode A commercially available finely shortened carbon fiber (MLD-
30), natural graphite powder, acetylene black as a conductive agent,
The binder polyvinylidene fluoride was added in a weight ratio of 20:60:
The mixture was mixed at a ratio of 5:15 to prepare a slurry using N-methylpyrrolidone as a solvent. This was applied to a copper foil of a current collector, dried and pressed to obtain a molded negative electrode.

(2) Preparation of positive electrode Commercially available lithium hydroxide (LiOH), nickel hydroxide (Ni (OH)
_2), strontium hydroxide octahydrate (Sr (OH) ₂ · 8H ₂
O), cobalt hydroxide (Co (OH) ₂ ) converted to oxide _0.98
Sr _0.02 Ni _0.90 Co _{0.10 O2} , weighed enough and mixed thoroughly in an automatic mortar, filled in an alumina crucible, and _{placed in} a pure oxygen gas stream (flow rate 1 liter /
Min) and held at 650 ° C. for 16 hours for preliminary firing. After cooling to room temperature, the mixture was pulverized again in an automatic mortar for 30 minutes to break up aggregation of secondary particles. And, under the same atmosphere as the pre-firing,
Main firing was carried out at 750 ° C. for 8 hours, cooled to room temperature, and then crushed again in an automatic mortar to obtain a positive electrode active material. This positive electrode active material, acetylene black as a conductive agent, and polyvinylidene fluoride as a binder were mixed at a weight ratio of 89: 4: 7, and a slurry was prepared using N-methylpyrrolidone as a solvent. Apply this to the aluminum foil of the current collector,
After drying, pressing was performed to obtain a molded positive electrode.

(3) Preparation of Secondary Battery On each of the negative and positive electrodes obtained in the above (1) and (2), commercially available PVDF (KF polymer # manufactured by Kureha Chemical Co., Ltd.)
2300) of N-methylpyrrolidone (15% by weight), and immersed in methanol for 20 minutes.
After drying for 20 minutes, positive and negative electrodes of microporous PVDF were obtained. The positive and negative electrodes were overlapped, wound into a cylindrical shape, fitted with terminals, and housed in a battery can to produce an 18650 cylindrical battery having a diameter of 18 mm and a height of 65 mm. Non-aqueous electrolyte is methyl
1M in a mixed solvent of 2,2,2-trifluoroethyl carbonate (MFEC) and dimethyl carbonate (DMC) in a volume ratio of 25:75
It was prepared _{1MLiPF 6 / MFEC-DMC (25:75} ) by dissolving LiPF ₆ in.

(4) Evaluation The secondary battery obtained in the above (3) was charged at a constant current and a constant voltage of up to 4.2 V at a current of 1000 mA for 4 hours, and then charged for 200 hours.
Constant current discharge was performed to 2.25 V at mA. The discharge capacity at this time was 1500 mAh. This charge / discharge cycle was performed a total of 5 times, only charging was performed, and a nail penetration test was performed by piercing a nail of 3 mm in diameter into the side of the battery can. There was no burst ignition.

[0063]

According to the electrolytic solution of the present invention, a lithium ion secondary battery having improved safety against a battery breakdown test can be obtained.

Claims (10)

[Claims] 1. A secondary battery comprising a positive electrode, a negative electrode and a non-aqueous electrolyte, wherein the solvent of the non-aqueous electrolyte is mainly composed of fluorinated carbonate and chain carbonate, and the volume ratio (fluorinated carbonate: chain) Carbonate) is 5: 9
A secondary battery having a range of 5 to 90:10. 2. The secondary battery according to claim 1, wherein said fluorinated carbonate is represented by the following structural formula. Embedded image R ₁ and R ₂ each represent an alkyl group, at least one of which is an alkyl group in which part or all of the hydrogen atoms have been substituted with fluorine atoms. 3. The method according to claim 2, wherein R ₁ and R _{2 each} have 1 carbon atom.
A non-aqueous electrolyte, wherein at least one of the alkyl groups is an alkyl group in which part or all of the hydrogen atoms have been substituted with fluorine atoms. 4. The secondary battery according to claim 1, wherein said positive electrode comprises a lithium-containing transition metal compound as an active material. 5. The secondary battery according to claim 1, wherein the transition metal compound of the positive electrode is at least one transition metal oxide selected from Mn, Ni and Co. 6. The secondary battery according to claim 1, wherein said negative electrode is a carbon material. 7. The secondary battery according to claim 1, wherein said negative electrode contains carbon fiber or a powdery carbon material obtained by finely pulverizing carbon fiber. 8. The secondary battery according to claim 1, wherein the carbon fiber of the negative electrode is a fired body of a polymer compound containing polyacrylonitrile as a main component. 9. The secondary battery according to claim 1, wherein the carbon material of the negative electrode is a mixture of a powdery carbon material obtained by finely pulverizing carbon fibers and graphite powder. 10. The secondary battery according to claim 1, wherein said negative electrode is lithium metal or a lithium alloy.