JP4911888B2 - Nonaqueous electrolyte and nonaqueous electrolyte secondary battery including the same - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte and a non-aqueous electrolyte secondary battery including the same, and particularly to a non-aqueous electrolyte having high non-flammability characteristics and a non-aqueous electrolyte secondary battery having excellent battery characteristics. .

Non-aqueous electrolytes are used as electrolytes for lithium batteries, lithium ion secondary batteries, electric double layer capacitors, etc., and these devices have high voltage and high energy density. Widely used as a power source. As these nonaqueous electrolytic solutions, generally used are solutions in which a supporting salt such as LiPF ₆ is dissolved in an aprotic organic solvent such as an ester compound and an ether compound. However, since the aprotic organic solvent is flammable, it may ignite and burn when it leaks from the device, and has a safety problem.

  To solve this problem, methods for making non-aqueous electrolytes flame-retardant have been studied. For example, phosphoric acid esters such as trimethyl phosphate are used for non-aqueous electrolytes, or phosphoric acid is used for aprotic organic solvents. Methods for adding esters have been proposed (see Patent Documents 1 to 3). However, these phosphate esters are gradually reduced and decomposed at the negative electrode by repeated charge and discharge, and battery characteristics such as charge and discharge efficiency and cycle characteristics are greatly deteriorated. .

  In order to solve this problem, methods such as further adding a compound that suppresses the decomposition of the phosphate ester to the nonaqueous electrolytic solution or devising the molecular structure of the phosphate ester itself have been tried (Patent Documents 4 to 6). reference). However, in this case as well, there is a limit to the amount of addition, and the safety of the electrolyte is sufficiently ensured to the extent that the electrolyte is self-extinguishing due to a decrease in the flame retardancy of the phosphate ester itself. Can not do it.

  Japanese Patent Application Laid-Open No. 6-13108 (Patent Document 7) discloses a method of adding a phosphazene compound to a nonaqueous electrolytic solution in order to impart flame retardancy to the nonaqueous electrolytic solution. The phosphazene compound exhibits high nonflammability depending on the type, and the flame retardancy of the nonaqueous electrolyte tends to improve as the amount added to the nonaqueous electrolyte increases. However, phosphazene compounds exhibiting high incombustibility generally have low solubility and dielectric constant of the supporting salt, so increasing the amount added causes precipitation of the supporting salt and a decrease in conductivity, resulting in a decrease in battery discharge capacity. The charging / discharging characteristics may be hindered. Therefore, when adding the phosphazene compound which shows high nonflammability, there exists a problem that the addition amount is restrict | limited.

  On the other hand, in recent years, in order to use it as a main power source or auxiliary power source for electric vehicles and fuel cell vehicles, the size of the battery has been increased and the energy density has been further increased. It has been. On the other hand, in the conventional non-aqueous electrolyte secondary battery, the metal oxide used for the positive electrode when a large current flows suddenly and the battery generates heat abnormally during an abnormality such as overcharge or external short circuit. A thing decomposes | disassembles and a large amount of oxygen gas is generated. As a result, the inside of the battery is in a state where the oxygen concentration is considerably higher than that in the atmosphere, and the battery is exposed to a state where it is extremely easy to ignite and catch fire.

  If the battery is ruptured or ignited by the gas and heat generated in such a situation, or the spark generated at the time of a short circuit is ignited, the damage can be extremely large. For these reasons, it is desirable that the non-aqueous electrolyte be non-flammable not only in a normal atmosphere but also non-flammable even under higher oxygen concentration conditions. By using such a non-flammable electrolyte with high incombustibility, it is considered that the risk of ignition and ignition of the battery is greatly reduced, and the safety of the battery is drastically improved. However, the conventional method of adding the above-mentioned phosphate ester or phosphazene compound has a limit in improving flame retardancy.

JP-A-4-184870 JP-A-8-22839 JP 2000-182669 A Japanese Patent Laid-Open No. 11-67267 JP-A-10-189040 JP 2003-109659 A JP-A-6-13108

  Accordingly, an object of the present invention is to solve the above-mentioned problems of the prior art and to provide a non-aqueous electrolyte that exhibits non-flammability even under high oxygen concentration conditions, and a non-aqueous electrolyte that has the non-aqueous electrolyte and has excellent battery performance. It is to provide a secondary battery.

  As a result of intensive studies to achieve the above object, the present inventors used the electrolyte solution by using a non-aqueous solvent containing a specific phosphazene compound and a specific phosphate compound in the non-aqueous electrolyte solution. It has been found that the nonflammability of the nonaqueous electrolyte can be significantly improved while maintaining the battery characteristics such as the discharge capacity and cycle characteristics of the nonaqueous electrolyte secondary battery, and the present invention has been completed. It was.

［式中、Ｒ²は、それぞれ独立してハロゲン元素、アルコキシ基及びアリールオキシ基のいずれかであり、２つのＲ²のうち少なくとも１つは、アルコキシ基又はアリールオキシ基である］で表されるフルオロリン酸エステル化合物を含む非水溶媒と、支持塩とからなることを特徴とする。 That is, the non-aqueous electrolyte of the present invention has the following general formula (I):
(NPR ¹ ₂ ) _n ... (I)
[Wherein, each R ¹ independently represents a halogen element or a monovalent substituent , provided that four or more of all R ¹ are fluorine ; n represents 3 to 4] Cyclic phosphazene compounds and the following general formula (II):

[Wherein, R ² is independently a halogen element, an alkoxy group or an aryloxy group, and at least one of the two R ² is an alkoxy group or an aryloxy group] A non-aqueous solvent containing a fluorophosphate ester compound and a supporting salt.

In the nonaqueous electrolytic solution of the present invention, as the fluorophosphate compound, in the general formula (II), one of two R ² is fluorine, and the other is an alkoxy group or an aryloxy group. Compounds are preferred.

In the non-aqueous electrolyte solution of the present invention, examples of the cyclic phosphazene compound, in the general formula (I), R ¹ is fluorine independently, compound is either an alkoxy group or an aryloxy group.

  In a preferred example of the nonaqueous electrolytic solution of the present invention, the volume ratio of the cyclic phosphazene compound represented by the general formula (I) to the fluorophosphate ester compound represented by the general formula (II) is 30/70. It is in the range of ~ 70/30.

  In another preferred embodiment of the nonaqueous electrolytic solution of the present invention, the nonaqueous solvent further contains an aprotic organic solvent.

  The non-aqueous electrolyte of the present invention has a total content of the cyclic phosphazene compound represented by the general formula (I) and the fluorophosphate ester compound represented by the general formula (II) in the non-aqueous solvent of 15 The volume is preferably at least volume%, more preferably at least 70 volume%.

  Moreover, the nonaqueous electrolyte secondary battery of the present invention comprises the nonaqueous electrolyte solution, a positive electrode, and a negative electrode.

  According to the present invention, when a non-aqueous solvent containing a specific phosphazene compound and a specific phosphate ester compound is used, the battery characteristics are improved when used in a non-aqueous electrolyte secondary battery. A non-aqueous electrolyte that can be sufficiently maintained can be provided. Moreover, the nonaqueous electrolyte secondary battery which has this nonaqueous electrolyte and has high nonflammability and the outstanding battery characteristic can be provided.

<Non-aqueous electrolyte>
Hereinafter, the nonaqueous electrolytic solution of the present invention will be described in detail. The nonaqueous electrolytic solution of the present invention comprises a nonaqueous solvent containing a cyclic phosphazene compound represented by the above general formula (I) and a fluorophosphate compound represented by the above general formula (II), and a supporting salt. Further, an aprotic organic solvent may be contained as a non-aqueous solvent. Conventionally, when a phosphazene compound or a phosphoric acid ester compound is used alone, there is a limit in achieving both nonflammability of the non-aqueous electrolyte and battery performance, but the phosphazene compound of formula (I) and the formula (II ) In combination with the non-aqueous electrolyte can be highly balanced between the non-flammability and the battery performance. Although the reason is not necessarily clear, a stable film is formed on the electrode surface due to the synergistic effect of the phosphazene compound of the formula (I) and the fluorophosphate ester compound of the formula (II). It is considered that the discharge characteristics are stabilized, and the highly incombustible gas component generated by the reaction and thermal decomposition of the phosphazene compound and the fluorophosphate ester compound exhibits incombustibility even under a high oxygen concentration.

The cyclic phosphazene compound used in the non-aqueous electrolyte of the present invention is represented by the above general formula (I). R ^{1 in} formula (I) is a halogen element or a monovalent substituent , provided that four or more of all R ¹ are fluorine, and each R ¹ may be the same or different. Here, as the halogen element, fluorine, chlorine, bromine and the like are preferable, and among these, fluorine is most preferable in view of low viscosity, and then chlorine is preferable.

Examples of the monovalent substituent in R ¹ of formula (I) include an alkoxy group, an aryloxy group, an alkyl group, an aryl group, an acyl group, a substituted or unsubstituted amino group, an alkylthio group, and an arylthio group. Among these, an alkoxy group and an aryloxy group are preferable in terms of excellent nonflammability. Examples of the alkoxy group include a methoxy group, an ethoxy group, a propoxy group, a butoxy group, an allyloxy group containing a double bond, an alkoxy-substituted alkoxy group such as a methoxyethoxy group and a methoxyethoxyethoxy group, and the like. Examples of the aryloxy group include a phenoxy group, a methylphenoxy group, and a methoxyphenoxy group. Examples of the alkyl group include a methyl group, an ethyl group, a propyl group, a butyl group, and a pentyl group. Includes a phenyl group, a tolyl group, a naphthyl group, and the substituted or unsubstituted amino group includes an amino group, a methylamino group, a dimethylamino group, an ethylamino group, a diethylamino group, an aziridyl group, and a pyrrolidyl group. Examples of the alkylthio group include methylthio group, ethylthio group It includes groups such as the above-described arylthio group, a phenylthio group, and the like. The hydrogen element in these monovalent substituents may be substituted with a halogen element, and is preferably substituted with fluorine.

R ^{1 in} formula (I) is preferably a halogen element from the viewpoint of improving flame retardancy, and more preferably fluorine from the viewpoint of low viscosity. Note that four or more of R ¹ are fluorine in terms of both flame retardancy and low viscosity .

  Further, n in the formula (I) is 3 to 4, and 3 is preferable as n in terms of cost and easy preparation. In addition, the said phosphazene compound may be used individually by 1 type, and may mix and use 2 or more types.

The fluorophosphate compound used in the non-aqueous electrolyte of the present invention is represented by the above general formula (II). R ^{2 in the} formula (II) is any one of a halogen element, an alkoxy group, and an aryloxy group, and at least one of the two R ² is an alkoxy group or an aryloxy group. Here, as the halogen element, fluorine, chlorine, bromine and the like are preferable, and among these, fluorine is most preferable in terms of low viscosity.

Examples of the alkoxy group in R ² of the formula (II) include a methoxy group, an ethoxy group, a propoxy group, a butoxy group and the like, an allyloxy group containing a double bond, and an alkoxy-substituted alkoxy such as a methoxyethoxy group and a methoxyethoxyethoxy group. Groups and the like. The hydrogen element in these alkoxy groups may be substituted with a halogen element, and is preferably substituted with fluorine. Among these, a methoxy group, an ethoxy group, a trifluoroethoxy group, and a propoxy group are more preferable in terms of excellent flame retardancy and low viscosity.

Examples of the aryloxy group for R ² in the formula (II) include a phenoxy group, a methylphenoxy group, and a methoxyphenoxy group. The hydrogen element in these aryloxy groups may be substituted with a halogen element, and is preferably substituted with fluorine. Among these, a phenoxy group and a fluorophenoxy group are more preferable in terms of excellent flame retardancy and low viscosity.

Two R ^{2 s in} the above formula (II) may be the same or different and may be bonded to each other to form a ring. Further, in terms of both flame retardancy and low viscosity, a difluorophosphate ester in which one of the two R ² is fluorine and the other is an alkoxy group or an aryloxy group is most preferable.

  Specific examples of the fluorophosphate ester of the above formula (II) include dimethyl fluorophosphate, diethyl fluorophosphate, bistrifluoroethyl fluorophosphate, ethylene fluorophosphate, dipropyl fluorophosphate, diallyl fluorophosphate, fluorophosphate Dibutyl acid, diphenyl fluorophosphate, difluorophenyl fluorophosphate, methyl chlorofluorophosphate, ethyl chlorofluorophosphate, trifluoroethyl chlorofluorophosphate, propyl chlorofluorophosphate, allyl chlorofluorophosphate, chlorofluorophosphoric acid Butyl, cyclohexyl chlorofluorophosphate, methoxyethyl chlorofluorophosphate, methoxyethoxyethyl chlorofluorophosphate, phenyl chlorofluorophosphate, fluorophenyl chlorofluorophosphate, difluro Methyl orophosphate, ethyl difluorophosphate, trifluoroethyl difluorophosphate, propyl difluorophosphate, allyl difluorophosphate, butyl difluorophosphate, cyclohexyl difluorophosphate, methoxyethyl difluorophosphate, methoxyethoxyethyl difluorophosphate, difluoro Examples thereof include phenyl phosphate and fluorophenyl difluorophosphate. Among these, methyl difluorophosphate, ethyl difluorophosphate, trifluoroethyl difluorophosphate, propyl difluorophosphate, and phenyl difluorophosphate are preferable. These fluorophosphate esters may be used alone or in combination of two or more.

  In the nonaqueous electrolytic solution of the present invention, the volume ratio of the cyclic phosphazene compound to the fluorophosphate ester compound is preferably in the range of 5/95 to 95/5. From the viewpoint of the balance between battery performance and nonflammability, 30 / A range of 70 to 70/30 is more preferable.

  In addition, an aprotic organic solvent can be added to the nonaqueous electrolytic solution as long as the object of the present invention is not impaired. The amount of the aprotic organic solvent added is 85% by volume or less in the non-aqueous electrolyte, so that the non-aqueous electrolyte can be made non-flammable, but higher non-flammability is imparted to the non-aqueous electrolyte. In order to achieve this, it is preferable to make it 30% by volume or less. Specific examples of the aprotic organic solvent include dimethyl carbonate (DMC), diethyl carbonate (DEC), diphenyl carbonate, ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), and vinylene carbonate (VC). ), Etc., 1,2-dimethoxyethane (DME), tetrahydrofuran (THF), diethyl ether (DEE), ethers such as phenyl methyl ether, γ-butyrolactone (GBL), γ-valerolactone, methylform Examples thereof include carboxylic acid esters such as mate (MF), nitriles such as acetonitrile, amides such as dimethylformamide, and sulfones such as dimethyl sulfoxide. These aprotic organic solvents may contain an unsaturated bond or a halogen element. Moreover, these aprotic organic solvents may be used individually by 1 type, and 2 or more types may be mixed and used for them.

As the supporting salt used in the nonaqueous electrolytic solution of the present invention, a supporting salt that is an ion source of lithium ions is preferable. The supporting salt is not particularly limited, and for example, LiClO ₄ , LiBF ₄ , LiPF ₆ , LiCF ₃ SO ₃ , LiAsF ₆ , LiC ₄ F ₉ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N and Li ( Preferable examples include lithium salts such as C ₂ F ₅ SO ₂ ) ₂ N. Among these, LiPF ₆ is more preferable in terms of excellent nonflammability. These supporting salts may be used alone or in combination of two or more.

  The concentration of the supporting salt in the non-aqueous electrolyte is preferably 0.2 to 1.5 mol / L (M), more preferably 0.5 to 1 mol / L (M). If the concentration of the supporting salt is less than 0.2 mol / L, the conductivity of the electrolyte cannot be sufficiently ensured, and the discharge characteristics and charging characteristics of the battery may be hindered. Since the viscosity of the electrolytic solution increases and the mobility of lithium ions cannot be ensured sufficiently, the conductivity of the electrolytic solution cannot be sufficiently ensured in the same manner as described above, which may hinder battery discharge characteristics and charge characteristics. .

<Nonaqueous electrolyte secondary battery>
Next, the nonaqueous electrolyte secondary battery of the present invention will be described in detail. The non-aqueous electrolyte secondary battery of the present invention includes the above-described non-aqueous electrolyte, a positive electrode, and a negative electrode, and is usually used in the technical field of non-aqueous electrolyte secondary batteries such as a separator as necessary. Other members are provided.

As the positive electrode active material of the non-aqueous electrolyte secondary battery of the present invention, metal oxides such as V ₂ O ₅ , V ₆ O ₁₃ , MnO ₂ , MnO ₃ , LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiFeO Preferred examples include lithium-containing composite oxides such as ₂ and LiFePO ₄ , metal sulfides such as TiS ₂ and MoS ₂ , and conductive polymers such as polyaniline. The lithium-containing composite oxide may be a composite oxide containing two or three transition metals selected from the group consisting of Fe, Mn, Co, and Ni. In this case, the composite oxide includes: LiFe _x Co _y Ni [wherein, 0 ≦ x <1,0 ≦ y <1,0 <x + y ≦ 1] (1-xy) O 2, or represented by LiMn _x Fe _y O _2-xy like. Among these, LiCoO ₂ , LiNiO ₂ , and LiMn ₂ O ₄ are particularly preferable in terms of high capacity, high safety, and excellent electrolyte wettability. These positive electrode active materials may be used individually by 1 type, and may use 2 or more types together.

  As the negative electrode active material of the non-aqueous electrolyte secondary battery of the present invention, lithium metal itself, an alloy of lithium and Al, In, Sn, Si, Pb, Zn or the like, a carbon material such as graphite doped with lithium, etc. Among these, carbon materials such as graphite are preferable, and graphite is particularly preferable in view of higher safety and excellent wettability of the electrolyte. Here, examples of graphite include natural graphite, artificial graphite, mesophase carbon microbeads (MCMB), and the like, and widely include graphitizable carbon and non-graphitizable carbon. These negative electrode active materials may be used individually by 1 type, and may use 2 or more types together.

  The positive electrode and the negative electrode can be mixed with a conductive agent and a binder as necessary. Examples of the conductive agent include acetylene black, and the binder includes polyvinylidene fluoride (PVDF) and polytetrafluoro. Examples include ethylene (PTFE), styrene / butadiene rubber (SBR), carboxymethyl cellulose (CMC), and the like. These additives can be used at a blending ratio similar to the conventional one.

  Moreover, there is no restriction | limiting in particular as a shape of the said positive electrode and a negative electrode, It can select suitably from well-known shapes as an electrode. For example, a sheet shape, a columnar shape, a plate shape, a spiral shape, and the like can be given.

  Other members that can be used in the non-aqueous electrolyte secondary battery of the present invention include a separator interposed in the non-aqueous electrolyte secondary battery between positive and negative electrodes to prevent current short-circuit due to contact between both electrodes. It is done. As the material of the separator, it is possible to reliably prevent contact between the two electrodes and to allow the electrolyte to pass through or to contain, for example, synthesis of polytetrafluoroethylene, polypropylene, polyethylene, cellulose, polybutylene terephthalate, polyethylene terephthalate, etc. Preferred examples include resin non-woven fabrics and thin layer films. These may be a single substance, a mixture or a copolymer. Of these, polypropylene or polyethylene microporous films having a thickness of about 20 to 50 μm, cellulose-based films, polybutylene terephthalate, polyethylene terephthalate, and the like are particularly suitable. In the present invention, in addition to the separators described above, known members that are normally used in batteries can be suitably used.

  The form of the non-aqueous electrolyte secondary battery of the present invention described above is not particularly limited, and various known forms such as a coin type, a button type, a paper type, a square type or a spiral type cylindrical battery are available. Preferably mentioned. In the case of the button type, a non-aqueous electrolyte secondary battery can be manufactured by preparing a sheet-like positive electrode and negative electrode and sandwiching a separator between the positive electrode and the negative electrode. In the case of a spiral structure, for example, a non-aqueous electrolyte secondary battery is manufactured by preparing a sheet-like positive electrode, sandwiching a current collector, and stacking and winding up the sheet-like negative electrode on the current collector. Can do.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples.

Example 1
In the above formula (I), 70% by volume of a cyclic phosphazene compound in which n is 3, 2 of all R ¹ are methoxy groups (MeO) and 4 are fluorine (F), and 30 volumes of ethyl difluorophosphate A nonaqueous electrolyte was prepared by dissolving LiPF ₆ at 1 mol / L in a mixed solvent of 2%. The nonflammability and critical oxygen index of the obtained nonaqueous electrolytic solution were evaluated and measured by the following methods, and the results shown in Table 1 were obtained.

(1) Nonflammability of electrolyte solution The combustion length and combustion time of a flame ignited in an atmospheric environment were measured and evaluated by a method in which the UL94HB method of UL (underwriting laboratory) standard was arranged. Specifically, based on the UL test standard, a test piece was prepared by impregnating a 127 mm × 12.7 mm SiO ₂ sheet with the above electrolytic solution 1.0 mL, and evaluated. The evaluation criteria for nonflammability, flame retardancy, self-extinguishing properties, and flammability are shown below.
<Evaluation of Nonflammability> A case where the test flame did not ignite at all (ignition length: 0 mm) was evaluated as nonflammable.
<Evaluation of Flame Retardancy> The case where the ignited flame did not reach the 25 mm line of the apparatus and the fallen object from the net was not ignited was evaluated as flame retardant.
<Evaluation of self-extinguishing property> When the ignited flame was extinguished in the 25 to 100 mm line and no ignition was observed on the falling object from the net, it was evaluated as having self-extinguishing property.
<Evaluation of combustibility> The case where the ignited flame exceeded the 100 mm line was evaluated as combustible.

(2) Limiting oxygen index of electrolyte The limiting oxygen index of the electrolyte was measured according to JIS K7201. The larger the limiting oxygen index, the more difficult the electrolyte is to burn. Specifically, a SiO ₂ sheet (quartz filter paper, non-combustible) 127 mm × 12.7 mm can be reinforced with a U-shaped aluminum foil so that it can be self-supporting, and the SiO ₂ sheet is impregnated with 1.0 mL of the electrolyte solution, and a test piece Was made. The test piece was perpendicular to the test piece support, and a combustion cylinder (with an inner diameter of 75 mm, a height of 450 mm, and a diameter of 4 mm was uniformly filled with a thickness of 100 ± 5 mm from the bottom, and a metal net was placed thereon. It is attached so that it is located at a distance of 100 mm or more from the upper end of the object), and then oxygen (JIS K 1101 or equivalent) or nitrogen (JIS K 1107 grade 2 or equivalent or more) is attached to the combustion cylinder. The test piece was ignited under a predetermined oxygen concentration (the heat source was JIS K 2240 Type 1 No. 1), and the combustion state was examined. However, the total flow rate in the combustion cylinder is 11.4 L / min. This test was performed three times, and the average value is shown in Table 1. The oxygen index refers to the value of the minimum oxygen concentration expressed by the volume percent necessary for the material to continue burning. In this application, the test piece burns continuously for 3 minutes or longer, From the minimum oxygen flow rate required for burning 50 mm or more and the nitrogen flow rate at that time, the following formula:
Critical oxygen index = (oxygen flow rate) / [(oxygen flow rate) + (nitrogen flow rate)] × 100 (volume%)
The limiting oxygen index was calculated according to

Next, 3 parts by mass of acetylene black (conductive agent) and 3 parts by mass of polyvinylidene fluoride (binder) are added to 94 parts by mass of LiCoO ₂ (positive electrode active material), and an organic solvent (ethyl acetate and (50/50 mass% mixed solvent with ethanol), and the kneaded product is applied to a 25 μm thick aluminum foil (current collector) with a doctor blade and further dried with hot air (100 to 120 ° C.). A positive electrode sheet having a thickness of 80 μm was prepared. Also, 10 parts by weight of polyvinylidene fluoride (binder) was added to 90 parts by weight of artificial graphite (negative electrode active material), and kneaded with an organic solvent (50/50% by weight mixed solvent of ethyl acetate and ethanol). Thereafter, the kneaded product was applied to a copper foil (current collector) having a thickness of 25 μm with a doctor blade, and further dried with hot air (100 to 120 ° C.) to prepare a negative electrode sheet having a thickness of 80 μm.

  On the obtained positive electrode sheet, a negative electrode sheet was overlapped and wound up via a separator (microporous film: made of polypropylene) having a thickness of 25 μm to produce a cylindrical electrode. The positive electrode length of the cylindrical electrode was about 260 mm. The above electrolytic solution was injected into the cylindrical electrode and sealed to prepare an AA lithium battery (non-aqueous electrolyte secondary battery). The initial discharge capacity and cycle characteristics of the obtained battery were measured by the following methods, and the results shown in Table 1 were obtained.

(3) Evaluation of initial discharge capacity and cycle characteristics of battery
Under an environment of 20 ° C, charge and discharge are performed under the conditions of an upper limit voltage of 4.3 V, a lower limit voltage of 3.0 V, a discharge current of 50 mA, and a charge current of 50 mA, and the initial discharge capacity is obtained by dividing the discharge capacity at this time by the known electrode weight. (MAh / g) was determined. Furthermore, charge / discharge was repeated up to 50 cycles under the same charge / discharge conditions, and the discharge capacity after 50 cycles was determined.
Capacity remaining rate S = discharge capacity after 50 cycles / initial discharge capacity × 100 (%)
The capacity remaining rate S was calculated according to the above and used as an index of the cycle characteristics of the battery.

(Example 2)
In place of the mixed solvent used in “Preparation of Nonaqueous Electrolyte” in Example 1, in the above formula (I), n is 3, 2 out of all R ¹ are chlorine (Cl), and 4 are A non-aqueous electrolyte was prepared in the same manner as in Example 1 except that a mixed solvent of 50% by volume of a cyclic phosphazene compound as fluorine (F) and 50% by volume of methyl difluorophosphate was used. The nonflammability and critical oxygen index of the water electrolyte were evaluated and measured, and a non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1, and the initial discharge capacity and cycle characteristics were measured and evaluated. The results are shown in Table 1.

Example 3
In place of the mixed solvent used in “Preparation of Nonaqueous Electrolyte” in Example 1, in the above formula (I), n is 3, ^one of all R ¹ is an ethoxy group (EtO), 5 A non-aqueous electrolyte was prepared and obtained in the same manner as in Example 1 except that a mixed solvent of 30% by volume of a cyclic phosphazene compound of which fluorine (F) is one and 70% by volume of propyl difluorophosphate was used. The nonflammability and critical oxygen index of the nonaqueous electrolyte were evaluated and measured, and a nonaqueous electrolyte secondary battery was prepared in the same manner as in Example 1, and the initial discharge capacity and cycle characteristics were measured and evaluated. The results are shown in Table 1.

Example 4
In place of the mixed solvent used in “Preparation of Nonaqueous Electrolytic Solution” in Example 1, in the above formula (I), n is 3, and ^one of all R ¹ is a trifluoroethoxy group (TFEO). Other than using a mixed solvent of 40% by volume of cyclic phosphazene compound, 5 of which is fluorine (F), 30% by volume of methyl difluorophosphate, 10% by volume of ethylene carbonate, and 20% by volume of ethyl methyl carbonate. A nonaqueous electrolytic solution was prepared in the same manner as in Example 1, and the nonflammability and critical oxygen index of the obtained nonaqueous electrolytic solution were evaluated and measured. Further, in the same manner as in Example 1, a nonaqueous electrolytic solution secondary battery was used. The initial discharge capacity and cycle characteristics were measured and evaluated. The results are shown in Table 1.

(Example 5)
Instead of the mixed solvent used in “Preparation of Nonaqueous Electrolyte” in Example 1, a cyclic phosphazene compound having a volume of 40 in the above formula (I), wherein n is 4 and all R ¹ are fluorine (F) %, 30% by volume of dimethyl fluorophosphate, 10% by volume of ethylene carbonate, 5% by volume of vinylene carbonate, and 15% by volume of diethyl carbonate. The electrolyte solution was prepared, the nonflammability and the critical oxygen index of the obtained nonaqueous electrolyte solution were evaluated and measured, and a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1, and the initial discharge capacity and Cycle characteristics were measured and evaluated. The results are shown in Table 1.

(Example 6)
In place of the mixed solvent used in “Preparation of Nonaqueous Electrolyte” in Example 1, in the above formula (I), n is 4, ^one of all R ¹ is a methoxy group (MeO), 7 Example 1 except that a mixed solvent of 10% by volume of a cyclic phosphazene compound, one of which is fluorine (F), 5% by volume of phenyl difluorophosphate, 28% by volume of ethylene carbonate, and 57% by volume of dimethyl carbonate was used. Similarly, a non-aqueous electrolyte is prepared, and the non-flammability and critical oxygen index of the obtained non-aqueous electrolyte are evaluated and measured. In addition, a non-aqueous electrolyte secondary battery is manufactured in the same manner as in Example 1. The initial discharge capacity and cycle characteristics were measured and evaluated. The results are shown in Table 1.

(Comparative Example 1)
Instead of the mixed solvent used in “Preparation of Nonaqueous Electrolyte” in Example 1, a mixed solvent of 33% by volume of ethylene carbonate and 67% by volume of ethyl methyl carbonate was used in the same manner as in Example 1. The non-aqueous electrolyte solution was prepared, and the non-flammability and the critical oxygen index of the obtained non-aqueous electrolyte solution were evaluated and measured. The discharge capacity and cycle characteristics were measured and evaluated. The results are shown in Table 1.

(Comparative Example 2)
Instead of the mixed solvent used in “Preparation of non-aqueous electrolyte solution” in Example 1, a mixed solvent of 30% by volume of trimethyl phosphate, 23% by volume of ethylene carbonate, and 47% by volume of ethyl methyl carbonate was used. Prepared a non-aqueous electrolyte in the same manner as in Example 1, evaluated and measured the nonflammability and the critical oxygen index of the obtained non-aqueous electrolyte, and in the same manner as in Example 1, A secondary battery was prepared, and the initial discharge capacity and cycle characteristics were measured and evaluated. The results are shown in Table 1.

(Comparative Example 3)
Instead of the mixed solvent used in “Preparation of non-aqueous electrolyte solution” in Example 1, a mixed solvent of 30% by volume of phenyl difluorophosphate, 23% by volume of ethylene carbonate, and 47% by volume of ethyl methyl carbonate was used. Other than that, a non-aqueous electrolyte was prepared in the same manner as in Example 1, and the non-flammability and critical oxygen index of the obtained non-aqueous electrolyte were evaluated and measured. A liquid secondary battery was prepared, and the initial discharge capacity and cycle characteristics were measured and evaluated. The results are shown in Table 1.

(Comparative Example 4)
In place of the mixed solvent used in “Preparation of Nonaqueous Electrolyte” in Example 1, in the above formula (I), n is 3, ^one of all R ¹ is phenoxy group (PhO), 5 A non-aqueous electrolyte was prepared in the same manner as in Example 1 except that a mixed solvent of 18% by volume of a cyclic phosphazene compound, one of which is fluorine (F), 27% by volume of ethylene carbonate, and 55% by volume of diethyl carbonate was used. The non-flammability and critical oxygen index of the obtained non-aqueous electrolyte were evaluated and measured, and a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, and the initial discharge capacity and cycle characteristics were measured. ·evaluated. The results are shown in Table 1.

(Comparative Example 5)
In place of the mixed solvent used in “Preparation of Nonaqueous Electrolyte” in Example 1, in the above formula (I), n is 3, ^one of all R ¹ is phenoxy group (PhO), 5 A nonaqueous electrolytic solution was prepared in the same manner as in Example 1 except that a mixed solvent of 50% by volume of a cyclic phosphazene compound, one of which is fluorine (F), and 50% by volume of triethyl phosphate was used. The nonflammability and critical oxygen index of the water electrolyte were evaluated and measured, and a non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1, and the initial discharge capacity and cycle characteristics were measured and evaluated. The results are shown in Table 1.

  From Examples 1 to 3 in Table 1, the non-aqueous solution of the present invention using a mixed solvent comprising a cyclic phosphazene compound represented by the above general formula (I) and a fluorophosphate ester represented by the above general formula (II) It can be seen that the electrolytic solution exhibits nonflammability even under a high oxygen concentration of 40% by volume or more, and a battery using the electrolytic solution exhibits high discharge capacity and excellent cycle characteristics. In addition, it can be seen from Examples 4 and 5 that the nonaqueous electrolytic solution of the present invention exhibits high incombustibility with a critical oxygen index of 30% by volume or more even when 30% by volume of an aprotic organic solvent is added. . Furthermore, from Example 6, the non-aqueous electrolyte of the present invention is nonflammable even if it contains only 15% by volume in total of the cyclic phosphazene compound of general formula (I) and the fluorophosphate ester compound of general formula (II). Can be seen. Thus, the non-aqueous electrolyte solution of the present invention containing a specific phosphazene compound and a specific phosphate compound has a very high critical oxygen index, and a non-aqueous electrolyte secondary solution using the non-aqueous electrolyte solution. The battery had very good discharge capacity and cycle characteristics.

  On the other hand, from Comparative Examples 2 and 3 in Table 1, when the phosphate ester compound was used alone, the initial discharge capacity was smaller than that of the Examples regardless of the structure, and the cycle characteristics were greatly deteriorated. You can see that In Comparative Example 4 of Table 1, when the cyclic phosphazene compound of the above general formula (I) is mixed alone with an aprotic organic solvent (EC / EMC), the cyclic phosphazene compound is added in an amount of 20% by volume or more. Since the phosphazene compound and the aprotic organic solvent (EC / EMC) are separated into two layers (non-uniform) and cannot be used as a non-aqueous electrolyte for batteries, only about 18% by volume can be added. Although the electrolyte solution is nonflammable, its limit oxygen index is limited to about 26% by volume. In addition, from Comparative Example 5 in Table 1, when a phosphoric acid ester compound having a structure different from that of the fluorophosphoric acid ester compound of the general formula (II) and the cyclic phosphazene compound are used in combination, incombustibility is possible. However, it can be seen that the initial discharge capacity and the cycle characteristics are significantly reduced.

  From the above results, by using a non-aqueous electrolyte containing a cyclic phosphazene compound represented by the general formula (I) and a fluorophosphate ester represented by the general formula (II), high nonflammability and excellent It can be seen that a non-aqueous electrolyte secondary battery having both battery performance can be provided.

Claims (8)

The following general formula (I):
(NPR ¹ ₂ ) _n ... (I)
[Wherein, each R ¹ independently represents a halogen element or a monovalent substituent , provided that four or more of all R ¹ are fluorine ; n represents 3 to 4] Cyclic phosphazene compounds and the following general formula (II):

[Wherein, R ² is independently a halogen element, an alkoxy group or an aryloxy group, and at least one of the two R ² is an alkoxy group or an aryloxy group] A nonaqueous electrolytic solution comprising a nonaqueous solvent containing a fluorophosphate ester compound and a supporting salt. 2. The nonaqueous electrolytic solution according to claim 1, wherein in the general formula (II), one of two R ² is fluorine and the other is an alkoxy group or an aryloxy group. 2. The nonaqueous electrolytic solution according to claim 1, wherein, in the general formula (I), R ¹ is each independently any one of fluorine, an alkoxy group, and an aryloxy group.   The volume ratio of the cyclic phosphazene compound represented by the general formula (I) and the fluorophosphate compound represented by the general formula (II) is in the range of 30/70 to 70/30. The nonaqueous electrolytic solution according to claim 1. The nonaqueous electrolytic solution according to claim 1 , wherein the nonaqueous solvent further contains an aprotic organic solvent. In the non-aqueous solvent, the total content of the cyclic phosphazene compound represented by the general formula (I) and the fluorophosphate compound represented by the general formula (II) is 15% by volume or more. The nonaqueous electrolytic solution according to any one of claims 1 to 5 . The total content of the cyclic phosphazene compound represented by the general formula (I) and the fluorophosphate ester compound represented by the general formula (II) in the non-aqueous solvent is 70% by volume or more. The nonaqueous electrolytic solution according to claim 6 . A nonaqueous electrolyte secondary battery comprising the nonaqueous electrolyte solution according to claim 1 , a positive electrode, and a negative electrode.