JP5404567B2 - Non-aqueous electrolyte and lithium secondary battery using the same - Google Patents

Description

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

  2. Description of the Related Art In recent years, lithium secondary batteries having a high energy density have attracted attention as electric products become lighter and smaller.

  An electrolyte for a lithium secondary battery is composed of a solute such as a lithium salt and a non-aqueous solvent. The nonaqueous solvent is required to have a high dielectric constant, to have a high oxidation potential, and to be stable in the battery. Since it is difficult to achieve these requirements with a single solvent, examples of the non-aqueous solvent for the electrolyte of a lithium secondary battery include cyclic carbonates such as ethylene carbonate and propylene carbonate, and cyclic carboxyls such as γ-butyrolactone. A high dielectric constant solvent such as an acid ester and a low viscosity solvent such as a chain carbonate such as diethyl carbonate or dimethyl carbonate or an ether such as dimethoxyethane are used in combination.

  However, lithium secondary batteries using these electrolytes may cause ignition or combustion due to short circuit if the electrolyte leaks due to battery damage or abnormal pressure increase due to some reason. Are known. For this reason, the research which mix | blends a flame retardant with organic non-aqueous electrolyte, and provides a flame retardance is energetically advanced.

  For example, Patent Document 1 describes that a phosphazene derivative is blended and used as a flame retardant in a non-aqueous electrolyte. However, although the phosphazene derivative certainly exhibits excellent flame retardancy, the thermal stability of the battery is not necessarily improved depending on the type of organic solvent to be combined. In general, when a battery is abnormally heated for some reason, there is a risk of thermal runaway triggered by a thermal decomposition reaction at the interface between the negative electrode or the positive electrode and the electrolyte, leading to battery rupture and ignition. This phenomenon is no exception even when a phosphazene derivative is blended. This phenomenon is caused by the thermal stability of the negative electrode film when a phosphazene derivative is blended, and as a result, the battery characteristics such as cycle characteristics are also deteriorated.

  On the other hand, Patent Document 2 describes that a film forming agent is added to a nonaqueous electrolytic solution to protect the electrode, and a reaction between the electrode and the electrolytic solution is suppressed to ensure battery safety. . Furthermore, there is a description that a flame retardant may be included, and the combined use with a phosphazene derivative is exemplified. However, in this technique, the volume ratio of the phosphazene derivative to the total solvent of the electrolytic solution was as high as about 40% by volume. Since the phosphazene derivative has a relatively high viscosity and a low dielectric constant, when it is contained in the electrolytic solution with such a high blending amount, there is a concern that the battery performance is deteriorated due to a decrease in conductivity.

JP-A-6-13108 Japanese Patent Laid-Open No. 2002-25615

  From the above background, there is little risk of thermal runaway even in the event of an abnormal temperature rise due to short circuit or overcharge, etc., and it is possible to ensure the safety and reliability of the battery, and the battery accompanying the decrease in conductivity etc. There has been a demand for an excellent nonaqueous electrolytic solution capable of obtaining good battery performance such as cycle characteristics without causing deterioration of performance.

  The present invention was devised in view of the above problems, and its purpose is to ensure safety and reliability during abnormal heating of the battery and to obtain good battery performance such as cycle characteristics. It is to provide an excellent non-aqueous electrolyte that can be used, and to provide a lithium secondary battery using the same.

  The present inventors include a non-aqueous electrolyte containing a cyclic carbonate having a carbon-carbon unsaturated bond in the molecule and a phosphazene derivative having a specific concentration range, thereby providing sufficient flame resistance to the battery. It is possible to ensure safety and reliability during abnormal heating of the battery, etc., and good cycle characteristics etc. without causing deterioration of battery performance due to decrease in conductivity etc. It has been found that battery performance can be obtained, and that the above problems can be effectively solved.

（一般式（Ｉ）中、Ｘ¹¹，Ｘ¹²は、各々独立に、ハロゲン原子又はＲ−Ｏ−で表わされる基（Ｒは、アルキル基又はアリール基を表わす。）を表わし、ｎは３以上、１０以下の整数を表わす。）
また、環状炭酸エステルとして、ビニレンカーボネート及びその誘導体並びにビニルエチレンカーボネート及びその誘導体からなる群より選ばれる少なくとも一種を含有することが好ましい（請求項２）。 That is, the gist of the present invention, lithium salts consisting of LiBF ₄ and, to a non-aqueous electrolyte solution for a lithium secondary battery containing a nonaqueous solvent composed mainly of γ- butyrolactone, further carbon atoms in the molecule -Containing a cyclic carbonate having a carbon unsaturated bond, and 4 wt% or more and 25 wt% or less of a cyclic phosphazene represented by the following general formula (I) with respect to the nonaqueous electrolyte solution, The present invention resides in a non-aqueous electrolyte for a lithium secondary battery, wherein the concentration of the cyclic carbonate relative to the liquid is 0.01% by mass or more and 10% by mass or less.

(In the general formula (I), X ¹¹ and X ¹² each independently represents a halogen atom or a group represented by R—O— (R represents an alkyl group or an aryl group), and n is 3 or more. Represents an integer of 10 or less.)
The cyclic carbonate preferably contains at least one selected from the group consisting of vinylene carbonate and derivatives thereof, and vinyl ethylene carbonate and derivatives thereof (Claim 2) .

  According to the nonaqueous electrolytic solution of the present invention, a lithium secondary battery having a high safety and reliability can be realized with a low risk of thermal runaway even in the case of an abnormal temperature rise due to a short circuit or overcharge. In addition, since battery performance is not deteriorated due to a decrease in electrical conductivity, good battery performance such as cycle characteristics can be obtained.

It is a graph which shows the result of thermal stability evaluation of the lithium secondary battery using the nonaqueous electrolyte solution of Example 1 and Comparative Examples 1-2.

  Hereinafter, the present invention will be described in detail, but the present invention is not limited to the following description, and can be arbitrarily modified and implemented without departing from the gist of the present invention.

[1. Non-aqueous electrolyte]
The non-aqueous electrolyte of the present invention contains a lithium salt and a non-aqueous solvent, and further contains 1 wt.% Of a cyclic carbonate having a carbon-carbon unsaturated bond in the molecule and the non-aqueous electrolyte. % To 25% by weight of a phosphazene derivative.

[Phosphazene derivatives]
In the present invention, the “phosphazene derivative” refers to a compound having a structural unit represented by —PX ^a X ^b ═N— (where X ^a and X ^b each independently represents a monovalent substituent). Depending on the number of structural units described above and their bonding state, they are classified into monophosphazenes consisting of only one structural unit, cyclic phosphazenes in which the structural units are linked in a cyclic manner, polyphosphazenes in which the structural units are linked in a chain form, etc. Is done. The type of the phosphazene derivative is not particularly limited, and a compound corresponding to any of the above classifications can be used. Among them, the cyclic phosphazene represented by the following general formula (I) and / or the following general formula (II It is preferable to use a chain phosphazene represented by

In the general formulas (I) and (II), X ¹¹ , X ¹² , X ²¹ , X ²² , X ²³ , X ²⁴ , X ²⁵ , X ²⁶ , X ²⁷ (In the following description, these groups are In the case of indicating without distinction, “X” represents each independently a monovalent substituent. The monovalent substituent is not particularly limited as long as it does not impair the gist of the present invention, but is a halogen atom, an alkyl group, an aryl group, an acyl group, a carboxy group, or a group represented by R—O— (R is an alkyl group). A group or an aryl group, hereinafter referred to as “RO group” as appropriate). Among these, a halogen atom or an RO group is preferable from the viewpoint of electrochemical stability. As the halogen atom, fluorine, chlorine and bromine are preferable, and fluorine is particularly preferable from the viewpoint of electrochemical stability and flame retardancy. On the other hand, when R is an alkyl group, the RO group is preferably an alkyl group having 1 to 3 carbon atoms because the flame retardancy is reduced when the carbon number exceeds 3. Specific examples of the alkyl group preferable as R include a methyl group, an ethyl group, an n-propyl group, and an i-propyl group, and a methyl group and an ethyl group are particularly preferable in terms of viscosity. On the other hand, when R is an aryl group, a phenyl group, a tolyl group, and a naphthyl group are preferable, and a phenyl group is particularly preferable in terms of flame retardancy. Note that hydrogen contained in the alkyl group or aryl group of R may be substituted with a halogen element. In particular, substitution with fluorine is preferable because the electrochemical stability is increased. The above Xs may be the same type of substituents as long as they satisfy the basic performance as an electrolyte solution such as viscosity, conductivity, electrochemical stability, and safety, but two or more different types of X may be used. You may combine a substituent.

  In general formula (I), n represents an integer of usually 3 or more and usually 10 or less, preferably 5 or less. In the general formula (II), m represents an integer of usually 0 or more, usually 10 or less, preferably 3 or less. When n or m exceeds 10, when these compounds are contained in the electrolytic solution, the viscosity becomes high, and there is a concern that battery performance such as load characteristics and low temperature charge / discharge characteristics due to decrease in conductivity may be caused.

  The molecular weights of the compounds of general formula (I) and general formula (II) are usually 200 or more, and usually 2000 or less, preferably 1000 or less. If the molecular weight is too high, poor dissolution occurs or low temperature characteristics deteriorate due to increased viscosity, such being undesirable.

  The content concentration of the phosphazene derivative in the non-aqueous electrolyte is usually 1% by weight or more, preferably 4% by weight or more, more preferably 8% by weight or more, and usually 25% by weight or less, preferably 22% by weight or less, more preferably. Is in the range of 18% by weight or less. If the lower limit of this range is not reached, the flame retardant effect of the electrolytic solution tends to be reduced. Conversely, if the upper limit is exceeded, the conductivity tends to decrease and load characteristics tend to deteriorate.

  In general, a technique for making flame retardant by blending phosphorus or a compound containing phosphorus into a resin product or the like is widely known. This is due to (i) the mechanism by which decomposition products trap active hydrogen radicals and oxygen radicals to suppress the combustion reaction, and (ii) the mechanism by which carbonaceous materials are formed that block flammables from heat and oxygen. It is believed that. In particular, in the mechanism (ii), it is known that the use of a nitrogen-containing compound in combination promotes the formation of a bulky foamed carbonized layer. The phosphazene derivative used in the present invention satisfies these mechanisms and can be said to be a compound having an excellent flame retardant effect. However, there is no particular limitation as long as it is used for the resin product as described above, but this is not the case when it is blended in the electrolyte.

[Cyclic carbonate]
In the present invention, the cyclic carbonate is not particularly limited as long as it has a carbon-carbon unsaturated bond in the molecule. Examples of the cyclic carbonate having a carbon-carbon unsaturated bond in the molecule include vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, 4,5-dimethyl vinylene carbonate, 4,5-diethyl vinylene carbonate, fluoro vinylene carbonate, Vinylene carbonate compounds such as trifluoromethyl vinylene carbonate; 4-vinylethylene carbonate, 4-methyl-4-vinylethylene carbonate, 4-ethyl-4-vinylethylene carbonate, 4-n-propyl-4-vinylethylene carbonate, 5 -Vinyl ethylene carbonate such as methyl-4-vinylethylene carbonate, 4,4-divinylethylene carbonate, 4,5-divinylethylene carbonate; 4,4-dimethyl-5-methyl Emissions ethylene carbonate, 4,4-such as methylene ethylene carbonate compounds such as diethyl 5-methylene ethylene carbonate. Among these, vinylene carbonate, 4-vinylethylene carbonate, 4-methyl-4-vinylethylene carbonate, and 4,5-divinylethylene carbonate are preferable, and vinylene carbonate or 4-vinylethylene carbonate is particularly preferable. Any one of these cyclic carbonates may be used alone, or two or more thereof may be used in any combination and ratio.

  The concentration of the cyclic carbonate is usually 0.01% by weight or more, preferably 0.3% by weight or more, more preferably 0.5% by weight or more, and usually 10% by weight or less, preferably with respect to the non-aqueous electrolyte. Is 8% by weight or less, more preferably 6% by weight or less. If the lower limit of this range is not reached, it will be difficult to sufficiently improve the cycle characteristics. If the upper limit is exceeded, gas may be generated during high-temperature storage and the internal pressure of the battery may increase.

According to the present invention, the thermal stability of the battery is remarkably improved by adding the above-described phosphazene derivative and the cyclic carbonate to the nonaqueous electrolytic solution.
As described above, the thermal stability of the electrolytic solution itself is improved by adding a phosphazene derivative, but even if this electrolytic solution is used in a battery, it is not necessarily thermally stable. The process leading to thermal runaway of a battery is complicated, and when the temperature of the battery rises for some reason, (i) the reaction between the charging negative electrode and the electrolytic solution, (ii) the reaction between the charging positive electrode and the electrolytic solution, (iii) the electrolytic solution It is presumed that the thermal decomposition reaction of (iv) and the thermal decomposition reaction of the charged negative electrode and positive electrode act in a complicated manner, leading to thermal runaway. In the electrolytic solution containing phosphazene, the formation of a protective film that suppresses the reaction between the electrolytic solution and the negative electrode and the thermal stability are insufficient, and heat generation due to the negative electrode reaction occurs even at a relatively low temperature. Is thermally unstable. Furthermore, a battery with insufficient formation of the protective film and thermal stability of the negative electrode remarkably deteriorates charge / discharge characteristics by repeated use even at room temperature or lower.

  In the present invention, in addition to the above-described phosphazene derivatives, these problems are solved by using a cyclic carbonate having a carbon-carbon unsaturated bond in the molecule. That is, the cyclic carbonate having a carbon-carbon unsaturated bond in the molecule can form a good protective film on the negative electrode even at a relatively low concentration. As a result, the reaction between the negative electrode and the electrolytic solution is effectively suppressed, so the battery heat generation at a low temperature is reduced, and the thermal stabilization effect originally provided by the electrolytic solution containing the phosphazene derivative works effectively. Thus, the thermal stability of the battery is improved. Furthermore, battery performance such as repeated charge / discharge characteristics is greatly improved by stabilizing the negative electrode protective film. That is, according to the present invention, it is possible to provide a battery having high safety and sufficient battery performance.

[Nonaqueous solvent]
As the main component of the non-aqueous solvent used in the present invention, any known solvent can be used as the solvent for the non-aqueous electrolyte secondary battery. For example, alkylene carbonate such as ethylene carbonate, propylene carbonate, butylene carbonate; dialkyl carbonate such as dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, ethyl methyl carbonate (preferably an alkyl group having 1 to 4 carbon atoms); tetrahydrofuran, Cyclic ethers such as 2-methyltetrahydrofuran; chain ethers such as dimethoxyethane and dimethoxymethane; γ-butyrolactone, γ-valerolactone, γ-caprolactone, γ-octanolactone, β-butyrolactone, δ-valerolactone, and ε -Cyclic carboxylic acid esters such as caprolactone; and chain carboxylic acid esters such as methyl acetate, methyl propionate, and ethyl propionate.

  Any one of the non-aqueous solvents may be used alone, or two or more thereof may be used in any combination and ratio. However, as the main component of the non-aqueous solvent, it is preferable to use a plurality of compounds in combination rather than using one kind of compound. For example, it is preferable to use a high dielectric constant solvent such as alkylene carbonate or cyclic carboxylic acid ester in combination with a low viscosity solvent such as dialkyl carbonate or chain carboxylic acid ester.

  One preferable non-aqueous solvent is mainly composed of alkylene carbonate and dialkyl carbonate. Among them, a combination of an alkylene carbonate having an alkylene group having 2 to 4 carbon atoms and a dialkyl carbonate having an alkyl group having 1 to 4 carbon atoms is preferable. In this case, the volume ratio of the alkylene carbonate to the dialkyl carbonate is usually 10:90. As mentioned above, it is preferable that it is the range of 45:55 or less. If the mixing ratio is within this range, the conductivity is relatively large and good low-temperature characteristics can be obtained. In this specification, the volume of the non-aqueous solvent is a measured value at 25 ° C. in principle, but a measured value at the melting point is used for a solid at 25 ° C. such as ethylene carbonate.

  Examples of the alkylene carbonate having an alkylene group having 2 to 4 carbon atoms include ethylene carbonate, propylene carbonate, and butylene carbonate. Among these, ethylene carbonate or propylene carbonate is preferable.

  Examples of the dialkyl carbonate having an alkyl group having 1 to 4 carbon atoms include dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, ethyl methyl carbonate, methyl-n-propyl carbonate, and ethyl-n-propyl carbonate. Among these, dimethyl carbonate, diethyl carbonate, or ethyl methyl carbonate is preferable.

  Specific examples of preferred combinations of alkylene carbonate and dialkyl carbonate include ethylene carbonate and dimethyl carbonate, ethylene carbonate and diethyl carbonate, ethylene carbonate and ethyl methyl carbonate, ethylene carbonate and dimethyl carbonate and diethyl carbonate, ethylene carbonate and dimethyl carbonate and ethyl methyl. Examples thereof include carbonate, ethylene carbonate, diethyl carbonate and ethyl methyl carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate.

  A combination in which propylene carbonate is further added to the combination of these ethylene carbonate and dialkyl carbonate is also mentioned as a preferable combination.

  When propylene carbonate is contained, the volume ratio of ethylene carbonate to propylene carbonate is usually 40:60 or more, preferably 50:50 or more, and usually 99: 1 or less, preferably 95: 5 or less. .

  Among these, those containing ethyl methyl carbonate, which is an asymmetric dialkyl carbonate, are more preferable, and in particular, ethylene carbonate and dimethyl carbonate and ethyl methyl carbonate, ethylene carbonate and diethyl carbonate and ethyl methyl carbonate, ethylene carbonate and dimethyl carbonate and diethyl. Carbonate and ethyl methyl carbonate, ethylene carbonate, symmetric dialkyl carbonate and asymmetric dialkyl carbonate are preferred because they provide a good balance between cycle characteristics and large current discharge characteristics.

  Other examples of preferred non-aqueous solvents are those based on cyclic carboxylic acid esters. Among them, γ-butyrolactone, γ-valerolactone, γ-caprolactone, γ-octanolactone, β-butyrolactone, δ-valerolactone, and ε-caprolactone are preferable, and γ-butyrolactone and γ-valerolactone are particularly preferable. The cyclic carboxylic acid ester is usually in a range of 40% by volume or more, preferably 60% by volume or more, and usually 100% by volume or less of the nonaqueous solvent. When a non-aqueous solvent containing a cyclic carboxylic acid ester as a main component is used, solvent evaporation and liquid leakage during high temperature use are reduced. The cyclic carboxylic acid ester may be used alone, but when mixed with other non-aqueous solvents, it is selected from alkylene carbonates having 2 to 4 carbon atoms and dialkyl carbonates having 1 to 4 carbon atoms. It is preferable to use one or more selected from the above.

  Specific examples of the alkylene carbonate having an alkylene group having 2 to 4 carbon atoms to be mixed with the cyclic carboxylic acid ester include ethylene carbonate, propylene carbonate, butylene carbonate and the like. Among these, ethylene carbonate or propylene carbonate is preferable. The alkylene carbonate is preferably in the range of 0 to 50% by volume of the nonaqueous solvent, and particularly preferably in the range of 0 to 40% by volume from the viewpoint of large current characteristics.

  Examples of the dialkyl carbonate having an alkyl group having 1 to 4 carbon atoms include dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, ethyl methyl carbonate, methyl-n-propyl carbonate, and ethyl-n-propyl carbonate. Among these, dimethyl carbonate, diethyl carbonate, or ethyl methyl carbonate is preferable. The dialkyl carbonate is preferably in the range of 0 to 30% by volume of the nonaqueous solvent, particularly preferably in the range of 0 to 20% by volume. If the upper limit is exceeded, a problem arises in the thermal stability of the battery.

  Non-aqueous solvents include halogen solvents such as halogen-substituted carbonates, carboxylic esters and ethers, room temperature molten salts such as imidazolium salts and pyridinium salts, and non-flammable or flame retardant solvents such as phosphorus-containing organic solvents. Can also be included. In particular, a phosphorus-containing organic solvent is preferable in terms of electrochemical stability and flame retardancy. Specific examples include phosphoric esters such as trimethyl phosphate, triethyl phosphate, dimethyl ethyl phosphate, methyl diethyl phosphate, ethylene methyl phosphate and ethylene ethyl phosphate, trifluoroethyl dimethyl phosphate, bis ( Trifluoroethyl) methyl, tris (trifluoroethyl) phosphate, pentafluoropropyldimethylphosphate, trifluoroethylmethylethyl phosphate, pentafluoropropylmethylethyl phosphate, trifluoroethylmethylpropyl phosphate, trifluorophosphate Fluorine-substituted phosphates such as ethyl diethyl are included. Among them, trimethyl phosphate, dimethylethyl phosphate, methyl diethyl phosphate, trifluoroethyl dimethyl phosphate, bis (trifluoroethyl) methyl phosphate, and tris (trifluoroethyl) phosphate are particularly preferable because of their great flame retarding effect. . The phosphate ester is usually in the range of 10% by volume or more, preferably 15% by volume or more, and usually 40% by volume or less, preferably 25% by volume or less based on the nonaqueous solvent. When the lower limit is exceeded, the flame retardant effect is reduced, and when the upper limit is exceeded, there is a problem that the conductivity is lowered and load characteristics are deteriorated.

  Moreover, it is preferable to add an overcharge inhibitor in the nonaqueous solvent because the safety of the battery during overcharge is improved. As the overcharge inhibitor, aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, terphenyl hydride, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, dibenzofuran; 2-fluorobiphenyl, o -Partially fluorinated products of the above aromatic compounds such as cyclohexylfluorobenzene and p-cyclohexylfluorobenzene; fluorinated anisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole and 2,6-difluoroaniol, etc. Is mentioned. When these are contained in the nonaqueous solvent at a concentration of usually 0.1 wt% or more and 5 wt% or less, the battery can be prevented from bursting or igniting during overcharge.

[Other ingredients]
Further, the non-aqueous solvent may contain other components, for example, conventionally known additives, dehydrating agents, and deoxidizing agents, as necessary.

  As additives, carbonate compounds such as fluoroethylene carbonate, trifluoropropylene carbonate, phenylethylene carbonate, erythritan carbonate and spiro-bis-dimethylene carbonate; succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, Carboxylic anhydrides such as glutaconic anhydride, itaconic anhydride, diglycolic anhydride, cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic dianhydride and phenylsuccinic anhydride; ethylene sulfite, 1,3-propane sultone 1,4-butane sultone, methyl methanesulfonate, busulfan, sulfolane, sulfolene, dimethyl sulfone, diphenyl sulfone, methyl phenyl sulfone, dibutyl disulfide, dicyclohexane Sulfur-containing compounds such as sildisulfide and tetramethylthiuram monosulfide; 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, 3-methyl-2-oxazolidinone, 1,3-dimethyl-2-imidazolidinone and Nitrogen-containing compounds such as N-methylsuccinimide; hydrocarbon compounds such as heptane, octane and cycloheptane. When these are contained in a non-aqueous solvent at a concentration of usually 0.1% by weight or more and 5% by weight or less, capacity maintenance characteristics and cycle characteristics after high-temperature storage are improved.

〔Electrolytes〕
The electrolyte is not particularly limited as long as it is a lithium salt, and various electrolytes can be used. Examples of those usually _{used, LiPF 6, LiBF 4, LiClO} 4, LiAsF 6, LiSbF inorganic lithium salt such as _{6, LiCF 3 SO 3, LiN} (CF 3 SO 2) 2, LiN (CF 3 CF 2 Fluorine-containing organic lithium salts such as SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , Li [PF ₅ (CF ₂ CF ₂ CF ₃ )] Li [PF ₄ (CF ₂ CF ₂ CF ₃ ) ₂ ], Li [PF ₃ (CF ₂ CF ₂ CF ₃ ) ₃ ], Li [PF ₃ (CF ₂ CF ₃ ) ₃ ], Li [PF ₅ (CF ₂ CF ₂ CF ₂ CF ₃ )], Li [PF ₄ (CF ₂ CF ₂ CF ₂ CF ₃ ) ₂ ], Li [PF ₃ (CF ₂ CF ₂ CF ₂ CF ₃ ) ₃ ], etc. Lithium oxide, Li [BF ₃ (CF ₃ )], Li [BF ₃ (CF ₂ C _{F 3)], Li [BF} 2 (CF 2 CF 3) 2] fluoroalkyl lithium borate such _{as, LiB (CF 3 COO) 4} , LiB (OCOCF 2 COO) 2, LiB (OCOC 2 H 4 COO) 2 , LiB (C ₂ O ₄ ) and the like. These lithium salts may be used alone or in combination of any number and ratio. Of these, LiPF ₆ and LiBF ₄ are preferably used.

  The concentration of the electrolyte is not particularly limited, but in the present invention, the amount of the electrolyte lithium salt per liter of the non-aqueous electrolyte is usually 0.5 mol or more, preferably 0.7 mol or more, and usually 2 mol. The range of 1.5 mol or less is preferable below. If the electrolyte concentration falls outside this range, the electrical conductivity of the non-aqueous electrolyte may deteriorate or the viscosity may be affected, which may reduce the performance of the lithium secondary battery obtained using the electrolyte. is there.

[2. Lithium secondary battery]
A lithium secondary battery of the present invention includes a negative electrode and a positive electrode capable of inserting and extracting lithium, and a non-aqueous electrolyte solution in which an electrolyte is dissolved in a non-aqueous solvent, and the non-aqueous electrolyte solution As described above, the non-aqueous electrolyte solution described above (the non-aqueous electrolyte solution of the present invention) is used.

[Negative electrode]
Examples of the negative electrode active material include lithium metal, lithium alloy (including tin, silicon, lead, germanium, aluminum, etc., and sputtering deposited electrodes and vacuum deposited electrodes of these metals), or materials that can occlude and release lithium. If there is, the type is not particularly limited, but specific examples include, for example, pyrolysis products of organic matter under various pyrolysis conditions, carbon materials such as artificial graphite and natural graphite, metal oxide materials, and various A lithium alloy is mentioned. Of these, artificial graphite and purified natural graphite produced by high-temperature heat treatment of graphitizable pitch obtained from various raw materials, or graphite materials obtained by subjecting these graphite to various surface treatments containing pitch are mainly used. The In addition, in the case of a lithium secondary battery that requires higher energy density, the negative electrode using a lithium alloy as a material generally has a larger capacity per unit weight than a carbon material typified by graphite. Is preferred.

  When a graphite material is used as the negative electrode active material, the d value (interlayer distance) of the lattice plane (002) obtained by X-ray diffraction by the Gakushin method is usually 0.335 nm or more, and usually 0.34 nm or less. Of these, those of 0.337 nm or less are desirable.

  The ash content of the graphite material is usually 1% by weight or less, preferably 0.5% by weight or less, and more preferably 0.1% by weight or less.

  The crystallite size (Lc) of the graphite material determined by X-ray diffraction by the Gakushin method is usually in the range of 30 nm or more, preferably 50 nm or more, more preferably 100 nm or more.

  The median diameter of the graphite material is a median diameter value measured by a laser diffraction / scattering method, and is usually 1 μm or more, preferably 3 μm or more, more preferably 5 μm or more, still more preferably 7 μm or more, and usually 100 μm or less. The range is preferably 50 μm or less, more preferably 40 μm or less, and still more preferably 30 μm or less.

Further, the BET specific surface area of the graphite material is usually 0.5 m ² / g or more, preferably 0.7 m ² / g or more, more preferably 1.0 m ² / g or more, and further preferably 1.5 m ² / g. or more, and usually 25.0 m ² / g or less, preferably 20.0 m ² / g or less, more preferably 15.0 m ² / g or less, more preferably suitably the range 10.0 m ² / g .

Moreover, the graphite material, in the Raman spectrum analysis using an argon ion laser beam, a peak P _B (peak intensity in the range of 1580~1620cm range of ^-1 peak P _A (peak intensity I _A) and 1350 ^-1 I _B) of the intensity ratio R = I _B / I _a is 0 or more and 0.5 or less, the half width of the peak in the range of 1580～1620Cm ^-1 is 26cm ^-1 or less, half of the peak in the range of 1580～1620Cm ^-1 The value width is preferably 25 cm ⁻¹ or less.

  Note that these carbonaceous materials can be used by mixing a metal compound capable of inserting and extracting lithium. Examples of metal compounds capable of inserting and extracting lithium include compounds containing metals such as Ag, Zn, Al, Ga, In, Si, Ge, Sn, Pb, P, Sb, Bi, Cu, Ni, Sr, and Ba. These metals are used as simple substances, oxides, alloys with lithium, and the like. In the present invention, those containing an element selected from Si, Sn, Ge and Al are preferred, and oxides or lithium alloys of metals selected from Si, Sn and Al are more preferred.

  The negative electrode active material demonstrated above may be used individually by 1 type, and may use 2 or more types together by arbitrary combinations and a ratio.

  A method for producing a negative electrode using these negative electrode active materials is not particularly limited. For example, a binder, a thickener, a conductive material, a solvent, and the like are added to the negative electrode active material as necessary to form a slurry. An example is a method in which an active material layer is formed by applying to an electric substrate and drying. In this case, it can be manufactured in the same manner as the positive electrode manufacturing method as described later.

  Also, a negative electrode active material with a binder or conductive material added can be rolled into a sheet electrode, or a pellet electrode by compression molding, or deposited on the current collector by techniques such as vapor deposition, sputtering, or plating. A thin film of a negative electrode active material can also be formed.

[Positive electrode]
The positive electrode active material is not particularly limited as long as it is a material that can occlude and release lithium, and a lithium transition metal composite oxide is preferably used. Examples of such substances include lithium cobalt composite oxides such as LiCoO ₂ , lithium nickel composite oxides such as LiNiO ₂ , lithium manganese composite oxides such as LiMnO _{2, and the} like. In these lithium transition metal composite oxides, some of the main transition metal elements are Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Si, etc. It can also be stabilized by replacing with other metal species, and is preferred.

  These positive electrode active materials may be used individually by 1 type, and 2 or more types can also be used together by arbitrary combinations and a ratio.

  A method for producing a positive electrode using these positive electrode active materials is not particularly limited. For example, a binder, a thickener, a conductive material, a solvent, etc. are added to the positive electrode active material as necessary to form a slurry. The method of apply | coating to the board | substrate of a collector and drying is mentioned.

  Further, the positive electrode active material can be roll-formed as it is to obtain a sheet electrode, or a pellet electrode by compression molding.

[Formation of each electrode]
As the material for the current collector for the negative electrode, metals such as copper, nickel, and stainless steel are used. Among these, copper foil is preferable from the viewpoint of easy processing into a thin film and cost. On the other hand, as the material of the positive electrode current collector, metals such as aluminum, titanium, and tantalum are used. Among these, aluminum foil is preferable from the viewpoint of easy processing into a thin film and cost.

  The binder is not particularly limited as long as it is a material that is stable with respect to the solvent and electrolyte used in electrode production. Specific examples include polyvinylidene fluoride, polytetrafluoroethylene, styrene / butadiene rubber, and isoprene rubber. And butadiene rubber.

  Examples of the thickener include carboxymethylcellulose, methylcellulose, hydroxymethylcellulose, ethylcellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, and casein.

Examples of the conductive material include metal materials such as copper and nickel, and carbon materials such as graphite and carbon black. In particular, the positive electrode preferably contains a conductive material.
The solvent used for manufacturing the electrode may be aqueous or organic. Examples of the aqueous solvent include water and alcohol, and examples of the organic solvent include N-methylpyrrolidone (NMP) and toluene.

[Separator]
In lithium secondary batteries, a separator is usually interposed between the positive electrode and the negative electrode in order to prevent short-circuiting between the electrodes. The material and shape of the separator used in the lithium secondary battery of the present invention are not particularly limited, but it is preferable to select from the materials that are stable with respect to the non-aqueous electrolyte solution of the present invention and have excellent liquid retention properties. It is preferable to use a porous sheet or a nonwoven fabric made of a polyolefin such as polyethylene or polypropylene.

[Battery configuration]
The lithium secondary battery of the present invention is manufactured by assembling the above-described non-aqueous electrolyte solution of the present invention, a positive electrode, a negative electrode, and a separator used as necessary into an appropriate shape. Furthermore, other components such as an outer case can be used as necessary. The method for assembling the battery is not particularly limited, and may be appropriately selected from methods usually employed.

  In addition, the shape of the battery is not particularly limited, and a cylinder type in which a sheet electrode and a separator are spiraled, an inside-out structure cylinder type in which a pellet electrode and a separator are combined, a coin type in which a pellet electrode and a separator are stacked, and the like are applied Is possible.

[Use of battery]
The use of the lithium secondary battery of the present invention is not particularly limited, and can be used for various known uses. Specific examples include notebook computers, pen input computers, mobile computers, electronic book players, mobile phones, mobile faxes, mobile copy, mobile printers, headphone stereos, video movies, LCD TVs, handy cleaners, portable CDs, minidiscs, and transceivers. Electronic notebooks, calculators, memory cards, portable tape recorders, radios, backup power supplies, motors, lighting equipment, toys, game machines, watches, strobes, cameras, etc. In particular, since the lithium secondary battery of the present invention is excellent in cycle characteristics, excellent effects can be obtained in any of these applications.

  EXAMPLES Next, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to these examples, and can be implemented with appropriate modifications as long as the gist thereof is not exceeded. It is.

[Non-aqueous electrolyte]
Under a dry argon atmosphere, a phosphazene derivative and an unsaturated cyclic carbonate are added to the non-aqueous solvent or non-aqueous mixed solvent having the composition shown in Table 1 so that the parts by weight are also shown in Table 1, and then the solute is sufficiently dried. Were added so as to have the parts by weight shown in Table 1 and dissolved to obtain non-aqueous electrolytes (non-aqueous electrolytes of Examples 1 to 3 and Comparative Examples 1 to 5).

  The phosphazene derivative used in each example and each comparative example is a cyclic phosphazene in which n is 3 in the above general formula (I), and the substituent X is a fluorine atom and R is a phenyl group RO group. And a compound (phenoxypentafluorocyclotriphosphazene) having a fluorine atom / RO group ratio of 5/1.

In addition, the symbol in a table | surface shows the following.
GBL: γ-butyrolactone EC: ethylene carbonate EMC: ethyl methyl carbonate VC: vinylene carbonate VEC: vinyl ethylene carbonate PhPFPN: phenoxypentafluorocyclotriphosphazene

  Using the non-aqueous electrolytes of Examples 1 to 3 and Comparative Examples 1 to 5 described above, lithium secondary batteries were produced by the following method and evaluated for each.

[Production of battery]
(Preparation of negative electrode)
X-ray diffraction lattice plane (002 plane) d value is 0.336 nm, crystallite size (Lc) is 652 nm, ash content is 0.07 wt%, median diameter by laser diffraction / scattering method is 12 μm, BET specific surface area There 7.5 m ² / g, a peak P _B (peak range of the peak P _a (peak intensity I _a) in the range of 1570～1620Cm ^-1 in the Raman spectrum analysis using an argon ion laser beam and 1300～1400Cm ^-1 the half-value width of the peak intensity ratio R = I _B / I _a is in the range of 0.12,1570～1620Cm ^-1 intensity I _B) is using natural graphite powder is 19.9Cm ^-1 as the negative electrode active material. 94 parts by weight of this graphite powder was mixed with 6 parts by weight of polyvinylidene fluoride and dispersed with N-methyl-2-pyrrolidone to form a slurry. This was uniformly applied to one side of a 18 μm-thick copper foil as a negative electrode current collector, dried, and then pressed by a press machine so that the density of the negative electrode layer was 1.5 g / cm ³ to obtain a negative electrode.

(Preparation of positive electrode)
LiCoO ₂ was used as the positive electrode active material. To 85 parts by weight of this powder, 6 parts by weight of carbon black and 9 parts by weight of polyvinylidene fluoride KF-1000 (manufactured by Kureha Chemical Co., Ltd., trade name) were added and mixed, and dispersed with N-methyl-2-pyrrolidone to form a slurry. Were applied uniformly on both sides of a 20 μm-thick aluminum foil as a positive electrode current collector, dried, and then pressed with a press so that the density of the positive electrode layer was 3.0 g / cm ³ to obtain a positive electrode. .

(Battery assembly)
A battery element is manufactured by laminating the above positive electrode, negative electrode, and polyethylene separator in the order of the negative electrode, separator, positive electrode, separator, and negative electrode, and this battery element is a laminate in which both sides of bag-shaped aluminum are covered with a resin layer. After installation while taking out the terminals of the positive electrode and the negative electrode in the film, the non-aqueous electrolytes of each Example and each Comparative Example were injected and vacuum sealed to produce a sheet-like lithium secondary battery.

[Battery performance evaluation]
(Battery thermal stability evaluation)
The sheet batteries using the non-aqueous electrolytes of Example 1 and Comparative Examples 1 and 2 were charged and discharged at a constant current corresponding to 0.2 C at 25 ° C. with a charge end voltage of 4.2 V and a discharge end voltage of 3 V. Perform 3 cycles to stabilize, charge to a charge end voltage of 4.2 V with a current corresponding to 0.7 C, and charge until the charge current value reaches a current value corresponding to 0.05 C. 4.2 V-CCCV (0 .05C cut) After charging, it is housed in a predetermined high-pressure sealed cell under a dry argon atmosphere, and a temperature range of 25 to 300 ° C. is set at 1 K / min using a differential scanning calorimeter (C80 calorimeter manufactured by Cetaram). The heat generation start temperature in the thermal decomposition process of the battery when the temperature was raised at the rate of temperature rise was measured. The exothermic start temperature here means the temperature at the intersection of the base line and the tangent of the maximum gradient of the exothermic peak. FIG. 1 shows the measurement result of the battery, and Table 2 shows the heat generation start temperature obtained from the measurement result.

(Battery cycle characteristics)
The sheet batteries using the non-aqueous electrolytes of Examples 1 to 3 and Comparative Examples 1 to 5 were charged and discharged at a constant current of 0.5 mA and a charge end voltage of 4.2 V and a discharge end voltage of 3 V at 25 ° C. Perform 3 cycles to stabilize, charge to a charge end voltage of 4.2 V with a current corresponding to 0.7 C, and charge until the charge current value reaches a current value corresponding to 0.05 C. 4.2 V-CCCV (0 .05C cut) After charging, a cycle test was performed in which discharge was performed at a current value corresponding to 1C to a discharge end voltage of 3V. Table 3 shows the discharge capacity at the fourth cycle of the cycle test and the capacity (cycle maintenance ratio) at the 50th cycle when the discharge capacity is taken as 100.

  As is clear from FIG. 1 and Table 2, it can be seen that the lithium secondary battery using the non-aqueous electrolyte of Example 1 according to the present invention has the highest heat generation starting temperature and exhibits excellent thermal stability. Further, as is clear from Table 3, the comparison between the lithium secondary batteries of Examples 1 and 2 and Comparative Example 2 in which the nonaqueous solvent used in combination is GBL, and Example 3 in which the nonaqueous solvent used in combination is EC + EMC and From the comparison of the lithium secondary battery of Comparative Example 5, it can be seen that the battery according to the present invention exhibits excellent cycle stability even when a phosphazene derivative is contained as a flame retardant.

  Further, as can be seen from the comparison of the lithium secondary batteries of Examples 1 and 2 and Comparative Example 3, if the content of the phosphazene derivative is excessive, a stable cycle can be obtained even if an unsaturated cyclic carbonate is contained. It can be seen that the characteristics cannot be obtained.

  According to the non-aqueous electrolyte of the present invention, it is possible to obtain a lithium secondary battery having high safety and reliability with a low risk of thermal runaway even in the case of an abnormal temperature rise due to short circuit or overcharge. It becomes. Therefore, the present invention can be suitably used in various fields such as an electronic device in which a lithium secondary battery is used.

Claims (2)

A non-aqueous electrolyte solution for a lithium secondary battery containing a lithium salt composed of LiBF ₄ and a non-aqueous solvent mainly composed of γ-butyrolactone , and further having a cyclic structure having a carbon-carbon unsaturated bond in the molecule Containing carbonic acid ester and cyclic phosphazene represented by the following general formula (I) of 4% by mass or more and 25% by mass or less with respect to the non-aqueous electrolyte solution,
The non-aqueous electrolyte for a lithium secondary battery, wherein the concentration of the cyclic carbonate with respect to the non-aqueous electrolyte is 0.01% by mass or more and 10% by mass or less.

(In the general formula (I), X ¹¹ and X ¹² each independently represents a halogen atom or a group represented by R—O— (R represents an alkyl group or an aryl group), and n is 3 or more. Represents an integer of 10 or less.) 2. The non-aqueous electrolyte for a lithium secondary battery according to claim 1, wherein the cyclic carbonate includes at least one selected from the group consisting of vinylene carbonate and derivatives thereof and vinylethylene carbonate and derivatives thereof. .

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