JP4731125B2 - Non-aqueous electrolyte additive for secondary battery, non-aqueous electrolyte for secondary battery and non-aqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte additive for a secondary battery, a non-aqueous electrolyte solution for a secondary battery including the additive, and a non-aqueous electrolyte secondary battery including the additive, and particularly to safety and low temperature characteristics. The present invention relates to an excellent nonaqueous electrolyte secondary battery.

  In recent years, lightweight, long-life, high-energy-density secondary batteries have been demanded as memory backups for AV and information devices such as personal computers and VTRs, as well as their drive power sources, or as main power sources or auxiliary power sources for electric vehicles and fuel cell vehicles. It has been. On the other hand, a non-aqueous electrolyte secondary battery using lithium as a negative electrode active material has the lowest lithium electrode potential among metals and a large electric capacity per unit volume. Known as one, some are put into practical use and supplied to the market. For example, a non-aqueous electrolyte secondary battery is used as a driving power source for notebook computers and mobile phones, and a non-aqueous electrolyte secondary battery is used as a main power source or auxiliary power source for electric vehicles and fuel cell vehicles. It is being considered.

  In these non-aqueous electrolyte secondary batteries, the negative electrode active material lithium reacts violently with a compound having active protons such as water and alcohol, so that the electrolyte used in the battery is an ester compound or an ether compound. It is limited to aprotic organic solvents.

  However, although the aprotic organic solvent has low reactivity with the lithium of the negative electrode active material, for example, when a battery is short-circuited, a large current flows suddenly, and when the battery abnormally generates heat, it is vaporized and decomposed. Thus, there is a high risk of generating gas, causing the battery to rupture or ignite due to the generated gas and heat, and sparks generated during a short circuit.

  On the other hand, adding a phosphazene compound to the non-aqueous electrolyte to impart non-flammability, flame retardancy or self-extinguishing properties to the non-aqueous electrolyte may cause the battery to ignite or ignite in the event of an emergency such as a short circuit A non-aqueous electrolyte secondary battery that significantly reduces the above has been developed. Among such phosphazene compounds, a phosphazene compound that is cyclic and has two fluorine atoms bonded to each phosphorus element in the molecule has a much lower viscosity than a phosphazene compound in which an organic group is bonded to a phosphorus element. Is added to the non-aqueous electrolyte so that the viscosity of the non-aqueous electrolyte is reduced and the discharge characteristics of the secondary battery at room temperature can be improved as well as the discharge characteristics at low temperatures (see Patent Document 1). ).

International Publication No. 02/21631 Pamphlet

  However, a phosphazene compound that is cyclic and has two fluorine atoms bonded to each phosphorus element in the molecule has a low boiling point, so that the phosphazene compound may vaporize when used at high temperatures. In addition, when the temperature of the battery rises in the event of an emergency such as a short circuit, the phosphazene compound is vaporized before the aprotic organic solvent. There is a risk that the battery may rupture or ignite due to the generated gas and heat, or the aprotic organic solvent in which the spark generated at the time of short-circuiting will remain ignited.

  Therefore, the object of the present invention is to solve the above-mentioned problems of the prior art, the boiling point is sufficiently high, without being vaporized when used at high temperatures, it is possible to sufficiently ensure the safety of the electrolyte even in an emergency such as a short circuit, An object of the present invention is to provide an additive for a non-aqueous electrolyte for a secondary battery that can impart excellent low-temperature characteristics. Another object of the present invention is to provide a non-aqueous electrolyte for a secondary battery containing such an additive and a non-aqueous electrolyte secondary battery having the non-aqueous electrolyte and having excellent safety and low-temperature characteristics. There is.

  As a result of intensive studies to achieve the above object, the present inventors have found that a cyclic phosphazene compound having a specific structure has a sufficiently high boiling point, a sufficiently low freezing point, and a very high oxygen index, By adding the phosphazene compound to the non-aqueous electrolyte, the phosphazene compound does not vaporize when the battery is used at a high temperature, and the safety of the non-aqueous electrolyte can be sufficiently ensured even in an emergency such as a short circuit. The inventors have found that the low temperature characteristics of the battery are improved, and have completed the present invention.

That is, the additive for non-aqueous electrolyte solution of the secondary battery of the present invention is represented by the following formula (I):
(NPX ₂ ) _n ... (I)
(Wherein, X is a halogen element independently, n represents 3 to 15 of an integer) is represented by, and saw including at least two kinds of halogen, phosphazene compound solidifying point of -20 ° C. or less It is characterized by comprising.

  In a preferred example of the non-aqueous electrolyte additive for secondary batteries of the present invention, the phosphazene compound contains fluorine and chlorine. Here, in the phosphazene compound, it is more preferable that each X in the formula (I) is independently fluorine or chlorine.

  In another preferred embodiment of the additive for a non-aqueous electrolyte solution of the secondary battery of the present invention, n in the formula (I) is 3-5. In this case, since the viscosity of the phosphazene compound is sufficiently low, the charge / discharge characteristics of the secondary battery can be sufficiently ensured without increasing the viscosity of the non-aqueous electrolyte.

The additive for a non-aqueous electrolyte of the secondary battery of the present invention is a phosphazene compound in which n in the formula (I) is 3, 1 to 3 out of 6 Xs and the rest is fluorine and / or More preferably, it is composed of a phosphazene compound in which n in the formula (I) is 4, 2 to 4 of 8 Xs are chlorine, and the remainder is fluorine. More preferably, the phosphazene compound contains two or more chlorine atoms in the molecule, and each chlorine atom is bonded to a different phosphorus atom. In this case, since the freezing point of the phosphazene compound is particularly low, the low temperature characteristics of the secondary battery can be greatly improved.

In the non-aqueous electrolyte additive for a secondary battery of the present invention, the freezing point of the phosphazene compound is at -20 ° C. or less, because a sufficiently low freezing point of the phosphazene compound, significantly improve the low temperature properties of the secondary battery can do.

  Moreover, the non-aqueous electrolyte for secondary batteries of this invention is characterized by including the additive for non-aqueous electrolytes of the said secondary battery, an aprotic organic solvent, and a supporting salt.

  In a preferred example of the non-aqueous electrolyte for secondary battery of the present invention, the difference in boiling point between the aprotic organic solvent and the additive for non-aqueous electrolyte of the secondary battery is 25 ° C. or less. In this case, the safety of the nonaqueous electrolytic solution in an emergency can be sufficiently improved.

  Furthermore, the non-aqueous electrolyte secondary battery of the present invention comprises the above-described non-aqueous electrolyte for secondary batteries, a positive electrode, and a negative electrode, and is particularly excellent in safety and low temperature characteristics.

  According to the present invention, it consists of a cyclic phosphazene compound having a specific structure, does not vaporize when used at high temperatures, can sufficiently ensure the safety of a non-aqueous electrolyte even in an emergency such as a short circuit, and further, a secondary battery. It is possible to provide an additive for a non-aqueous electrolyte of a secondary battery that can greatly improve the low temperature characteristics of the secondary battery. In addition, it is possible to provide a non-aqueous electrolyte for a secondary battery that includes such an additive and has sufficiently high safety and can significantly improve the low-temperature characteristics of the secondary battery. Furthermore, it is possible to provide a non-aqueous electrolyte secondary battery that includes the non-aqueous electrolyte for a secondary battery and is excellent in safety and low-temperature characteristics.

The present invention is described in detail below.
<Additive for secondary battery non-aqueous electrolyte>
Additive for the non-aqueous electrolyte secondary battery of the present invention are represented by the formula (I), and saw including at least two kinds of halogen elements, that consist of cyclic phosphazene compound solidifying point of -20 ° C. or less It is characterized by. Since the phosphazene compound has a sufficiently high boiling point, the phosphazene compound does not vaporize at the time of high temperature use, and the secondary battery including the non-aqueous electrolyte containing the additive of the present invention is likely to swell even at the time of high temperature use. There is no. In addition, since the phosphazene compound has a sufficiently low freezing point, it exists as a liquid even at a low temperature, and the low temperature characteristics of the secondary battery can be improved by adding the phosphazene to the non-aqueous electrolyte of the secondary battery. Can do. Furthermore, the phosphazene compound has a very high oxygen index, and generates nitrogen gas and / or phosphate ester in the emergency of the battery, making the non-aqueous electrolyte incombustible, flame retardant or self-extinguishing. It has the effect | action which reduces significantly dangers, such as ignition of a secondary battery.

  The phosphazene compound constituting the additive for non-aqueous electrolyte of the secondary battery of the present invention is represented by the above formula (I) and contains at least two halogen elements. In the formula (I), X is independently a halogen element, and examples of the halogen element include fluorine, chlorine, bromine and the like. Among these, fluorine and chlorine are preferable. The phosphazene compound preferably contains at least fluorine and chlorine, and all X are preferably fluorine or chlorine. Note that when a compound containing a halogen element is used, the generation of a halogen radical may be a problem. However, since the phosphorus element in the molecule captures the halogen radical and forms phosphorus halide, Such a problem does not occur.

  Moreover, in Formula (I), n is an integer of 3-15, and it is preferable that it is 3-5. When n exceeds 5, the viscosity of the phosphazene compound increases, so the viscosity of the non-aqueous electrolyte increases, the internal resistance of the secondary battery increases, the conductivity of the electrolyte decreases, and the battery charge is reduced. Discharge characteristics tend to decrease. Here, the viscosity of the phosphazene compound at 25 ° C. is preferably 10 mPa · s or less, and more preferably 5 mPa · s or less, from the viewpoint of sufficiently securing the charge / discharge characteristics of the battery. In the present invention, the viscosity is 1 rpm, 2 rpm, 3 rpm, 5 rpm, 7 rpm, 10 rpm, 20 rpm, and 50 rpm using a viscometer [R-type viscometer Model RE500-SL, manufactured by Toki Sangyo Co., Ltd.] It is a value measured at that time, measured at the rotational speed for 120 seconds, with the rotational speed when the indicated value is 50 to 60% as the analysis condition.

  The phosphazene compound preferably has a limiting oxygen index of 30% by volume or more, and more preferably 40% by volume or more. By adding a phosphazene compound having a limiting oxygen index of 40% by volume or more to the non-aqueous electrolyte, the risk of ignition and ignition of the electrolyte can be greatly reduced. Here, the limiting oxygen index means a value of the minimum oxygen concentration expressed by volume% necessary for the material to continue to burn under a predetermined test condition specified in JIS K 7201. A high value means a low risk of ignition or ignition.

The phosphazene compound, the freezing point is at -20 ° C. or less, preferably not more -30 ° C. or less. By adding a phosphazene compound having a freezing point of −20 ° C. or lower to the non-aqueous electrolyte, the low-temperature characteristics of the secondary battery can be reliably improved, and the phosphazene compound is added to the non-aqueous electrolyte. Non-aqueous electrolyte secondary batteries are particularly suitable as mobile (HEV) batteries because of their excellent low-temperature characteristics.

Among the above phosphazene compounds, from the viewpoint of a low freezing point, n in the formula (I) is 3, 1 to 3 out of 6 Xs are chlorine and the rest are fluorines, and in the formula (I) It is particularly preferable that n is 4, 2 to 4 of 8 Xs are chlorine and the rest is fluorine. The freezing points of phosphazene compounds in which X in the formula (I) is fluorine or chlorine are shown in Table 1 together with the boiling point and oxygen index.

As is clear from Table 1, in the phosphazene compound in which X in the formula (I) is fluorine or chlorine, the boiling point rises as the chlorine number increases (increase in molecular weight), but the freezing point is in the specific chlorine number range. in becomes minimum, when n is 3, the range of the chlorine number 1-3 is particularly preferred, when n is 4, the range of 2 to 4 chlorine number are particularly preferred.

  The phosphazene compound is prepared by, for example, using commercially available phosphazene compounds in which X in the formula (I) is all chlorine as a starting material, fluorinating all chlorine with a fluorinating agent, and then at the target chlorine substitution site. After introducing an alkoxy group, an amine group, or the like, chlorinating again with a chlorinating agent such as HCl or phosgene, or a commercially available phosphazene compound in which X in the formula (I) is all chlorine After calculating the equivalent of fluorine to be introduced, it can be synthesized by a method of adding a necessary amount of a fluorinating agent. In addition, the said phosphazene compound may be used individually by 1 type, or may be used as a 2 or more types of mixture.

<Nonaqueous electrolyte for secondary battery>
The non-aqueous electrolyte for a secondary battery of the present invention is characterized by containing the above-described additive for a non-aqueous electrolyte for a secondary battery, an aprotic organic solvent, and a supporting salt.

  The aprotic organic solvent used in the non-aqueous electrolyte for secondary batteries of the present invention is not particularly limited, but is preferably an ether compound or an ester compound from the viewpoint of keeping the viscosity of the electrolyte low. Specific examples include 1,2-dimethoxyethane, tetrahydrofuran, dimethyl carbonate, diethyl carbonate, diphenyl carbonate, ethylene carbonate, propylene carbonate, γ-butyrolactone, γ-valerolactone, ethyl methyl carbonate, methyl formate, and the like. It is done. Among these, cyclic ester compounds such as ethylene carbonate, propylene carbonate, and γ-butyrolactone, chain ester compounds such as dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate, chain ether compounds such as 1,2-dimethoxyethane, and the like are further included. preferable. In particular, the cyclic ester compound has a high relative dielectric constant and is excellent in the solubility of lithium salts and the like, and the chain ester compound and the ether compound are suitable in terms of reducing the viscosity of the electrolytic solution because of low viscosity. is there. These may be used individually by 1 type, may use 2 or more types together, but it is suitable to use 2 or more types together. The viscosity of the aprotic organic solvent at 25 ° C. is not particularly limited, but is preferably 10 mPa · s (10 cP) or less, and more preferably 5 mPa · s (5 cP) or less.

  In the non-aqueous electrolyte for secondary battery of the present invention, the difference in boiling point between the aprotic organic solvent and the additive for non-aqueous electrolyte of the secondary battery is preferably 25 ° C. or less. More specifically, the non-aqueous electrolyte for a secondary battery of the present invention contains one or more aprotic organic solvents and a supporting salt, and further, It is preferable to contain a phosphazene compound having a boiling point difference from that of the protic organic solvent of 25 ° C. or less and containing at least two halogen elements represented by the above formula (I).

  As described above, the phosphazene compound has an effect of reducing the risk of battery ignition and the like, but the non-aqueous electrolyte containing the aprotic organic solvent is a phosphazene compound having a boiling point close to that of the aprotic organic solvent. When not included, the aprotic organic solvent and the phosphazene compound in either the gas phase or the liquid phase have a wide temperature range so that when the battery temperature rises abnormally, the vaporized aprotic organic solvent or The risk of ignition and ignition of the aprotic organic solvent remaining in the battery cannot be reduced. In contrast, when the non-aqueous electrolyte contains a phosphazene compound having a boiling point close to that of the aprotic organic solvent together with the aprotic organic solvent, when the battery temperature rises abnormally, the aprotic organic solvent and the phosphazene Since the compound is vaporized at a close temperature, the aprotic organic solvent and the phosphazene compound coexist in both the case where the aprotic organic solvent is present as a liquid and the case where it is present as a gas. The risk of ignition and ignition is greatly reduced.

  In addition, for example, when the non-aqueous electrolyte contains a low-boiling aprotic organic solvent and a high-boiling aprotic organic solvent, it is possible to cope with it near the temperature at which the low-boiling aprotic organic solvent vaporizes. Since the phosphazene compound is vaporized, the risk of ignition and ignition of the vaporized aprotic organic solvent can be reduced. In addition, after the low-boiling aprotic organic solvent and the phosphazene compound having a boiling point close to that of the low-boiling aprotic organic solvent are vaporized, the high-boiling aprotic organic solvent and the high-boiling aprotic organic solvent are used. Since the phosphazene compound having a boiling point close to that of the electrolyte is present in the electrolytic solution, the risk of ignition and ignition of the remaining nonaqueous electrolytic solution can be reduced.

  In the non-aqueous electrolyte for secondary battery of the present invention, it is preferable to appropriately select and use a phosphazene compound (additive) having a boiling point close to that of the aprotic organic solvent according to the aprotic organic solvent to be used. Here, since the phosphazene compound represented by the above formula (I) and containing at least two kinds of halogen elements can take a wide range of boiling points depending on the number of chlorines in the molecule and the value of n, the molecular structure of the phosphazene compound is appropriately determined. By selecting, the danger in the event of an emergency such as a short circuit of the non-aqueous electrolyte can be greatly reduced.

As the supporting salt used in the non-aqueous electrolyte for a secondary battery of the present invention, a supporting salt serving as 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. 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 for secondary batteries of the present invention is preferably 0.2 to 1.5 mol / L (M), and more preferably 0.5 to 1 mol / L (M). When the concentration of the supporting salt is less than 0.2 mol / L (M), the conductivity of the electrolytic solution cannot be sufficiently ensured, and the discharge characteristics and charging characteristics of the battery may be hindered, and 1.5 mol / L ( (M) exceeds the viscosity of the electrolyte and the mobility of lithium ions cannot be sufficiently secured, so that the conductivity of the electrolyte cannot be sufficiently secured as described above. May cause trouble.

  The content of the phosphazene compound (that is, the content of the additive) in the non-aqueous electrolyte for a secondary battery of the present invention is preferably 1% by volume or more, and 5% by volume from the viewpoint of improving the safety of the electrolyte. The above is more preferable, and from the viewpoint of improving the low-temperature characteristics of the battery, 10% by volume or more is preferable, and 15% by volume or more is more preferable.

<Nonaqueous electrolyte secondary battery>
The non-aqueous electrolyte secondary battery of the present invention includes the above-described non-aqueous electrolyte for a secondary battery, a positive electrode, and a negative electrode, and is usually used in the technical field of a non-aqueous electrolyte battery such as a separator as necessary. It includes other members that are used.

As the positive electrode active material used for the positive electrode 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 , Li-containing composite oxides such as LiFeO ₂ and LiFePO ₄ , metal sulfides such as TiS ₂ and MoS ₂ , and conductive polymers such as polyaniline are preferable. 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 used for the negative electrode of the non-aqueous electrolyte secondary battery of the present invention, lithium metal itself, an alloy of lithium and Al, In, Pb, Zn or the like, a carbon material such as graphite doped with lithium, etc. are suitable. Among these, carbon materials such as graphite are preferable, and graphite is particularly preferable in that it is higher in safety and excellent in wettability of the electrolytic solution. 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 thereof 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.

  Another member used in the non-aqueous electrolyte secondary battery of the present invention is a separator interposed in the non-aqueous electrolyte secondary battery between positive and negative electrodes so as to prevent a short circuit of current 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. 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 the 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. be able to.

  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.

<Synthesis of phosphazene compounds>
(Synthesis Example 1)
(NPCl ₂ ) ₃ and sodium fluoride are mixed in nitrobenzene as a solvent and gradually heated from room temperature to 140 ° C. under reduced pressure (15 kPa) over about 1 hour. Obtained as product.

  The obtained product was analyzed by GC-MS. As a result, n in the formula (I) was 3, and all six Xs were fluorine. A cyclic phosphazene compound having a boiling point of 52 ° C. and a freezing point of 28 ° C. A cyclic phosphazene compound having a viscosity of 0.8 mPa · s at 25 ° C. and n of 3 in formula (I), one of the six X being chlorine and five of fluorine (boiling point is 82 ° C., Cyclic phosphazene compound (boiling point) having a freezing point of −30 ° C. and a viscosity of 0.8 mPa · s at 25 ° C., n in the formula (I) being 3, 2 of 6 X being chlorine and 4 being fluorine Is 115 ° C, the freezing point is -46 ° C, the viscosity at 25 ° C is 1.1 mPa · s), n in the formula (I) is 3, 3 of 6 X are chlorine, 3 are fluorine It was confirmed to be a mixture with a cyclic phosphazene compound (boiling point: 150 ° C., freezing point: −35 ° C., viscosity at 25 ° C .: 1.3 mPa · s). In addition, the mixture was separated by distillation to obtain four kinds of pure cyclic phosphazene compounds.

(Synthesis Example 2)
(NPCl ₂ ) ₄ and potassium fluoride sulfite are mixed in nitrobenzene as a solvent, and the temperature is gradually increased from room temperature to 180 ° C. under reduced pressure (1 kPa) over about one hour, and the fraction that volatilizes. Was obtained as the product.

  When the obtained product was analyzed by GC-MS, n in the formula (I) was 4, and all of the eight Xs were fluorine-containing cyclic phosphazene compounds (boiling point was 80 ° C., freezing point was 30 ° C. A cyclic phosphazene compound having a viscosity of 0.8 mPa · s at 25 ° C. and n of 4 in formula (I), one of eight X being chlorine and seven being fluorine (boiling point is 117 ° C., A cyclic phosphazene compound (boiling point) having a freezing point of −6 ° C. and a viscosity of 1.2 mPa · s at 25 ° C., n in the formula (I) being 4, 2 of 8 X being chlorine and 6 being fluorine. Is 147 ° C, freezing point is -22 ° C, viscosity at 25 ° C is 1.5mPa · s), n in formula (I) is 4, 3 out of 8 X are chlorine, 5 are fluorine Cyclic phosphazene compound (boiling point is 178 ° C, freezing point is -29 ° C, viscosity at 25 ° C is 1.9mPa · s), n in formula (I) is 4, 4 out of 8 X are chlorine , Four are cyclic phosphazene compounds of fluorine (boiling point is 205 ° C, freezing point is -23 ° C, viscosity at 25 ° C is 2.3 mPa · s), n in formula (I) is 4, It was confirmed that the mixture was a cyclic phosphazene compound (boiling point is 232 ° C., freezing point is −11 ° C., viscosity at 25 ° C. is 2.8 mPa · s), 5 of which are chlorine and 3 are fluorine. In addition, the mixture was separated by distillation to obtain five pure cyclic phosphazene compounds.

<Preparation of non-aqueous electrolyte for secondary battery>
Next, a mixed solution (comprising an aprotic organic solvent and a phosphazene compound) having the composition shown in Table 2 is prepared, and LiPF ₆ (supporting salt) is dissolved in the mixed solution at a concentration of 1 mol / L (M). Thus, a non-aqueous electrolyte was prepared. The safety and critical oxygen index of the obtained nonaqueous electrolytic solution were measured and evaluated by the following methods. The results are shown in Table 2.

(1) Safety of electrolyte solution The safety of the non-aqueous electrolyte solution was evaluated from the combustion behavior of flames ignited in an atmospheric environment by the method of arranging the UL94HB method of UL (Underwriting Laboratory) standard. At that time, ignitability, combustibility, formation of carbides, and secondary ignition phenomena were also observed. Specifically, based on the UL test standard, a non-combustible quartz fiber was impregnated with 1.0 mL of the electrolytic solution, and a test piece of 127 mm × 12.7 mm was produced. Here, when the test flame does not ignite the test piece (combustion length: 0 mm), it is “non-flammable”, and when the ignited flame does not reach the 25 mm line and the fallen object is not ignited, “flame retardant” The case where the ignited flame was extinguished on the 25 to 100 mm line and the fallen object was not ignited was evaluated as “self-extinguishing”, and 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. Specifically, a test piece was prepared in the same manner as the safety test of the electrolyte solution, and the test piece was perpendicular to the test piece support, and a combustion cylinder (inner diameter 75 mm, height 450 mm, diameter 4 mm glass particles). Is attached so that the thickness of the metal net is 100 ± 5mm evenly from the bottom and the metal mesh is placed on it at a distance of 100mm or more from the top, and then oxygen (JIS K) is attached to the combustion cylinder. 1101 or equivalent or higher) and nitrogen (second grade of JIS K 1107 or equivalent or higher), and the test piece is ignited in air (the heat source is JIS K 2240 type 1 No. 1), The combustion state was investigated. 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 2. 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

<Preparation of non-aqueous electrolyte secondary battery>
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 LiMn ₂ O ₄ (positive electrode active material), and an organic solvent (acetic acid (50/50 mass% mixed solvent of ethyl and ethanol), kneaded product was coated on aluminum foil (current collector) with a thickness of 25μm with a doctor blade, and dried with hot air (100 ~ 120 ℃) Thus, a positive electrode sheet having a thickness of 80 μm was produced. A lithium electrode foil having a thickness of 150 μm was overlapped and wound on the obtained positive electrode sheet via a separator having a thickness of 25 μm (microporous film: made of polypropylene) 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 obtained battery was tested for cycle characteristics and low temperature characteristics by the following methods. The results are shown in Table 2.

(3) Battery cycle characteristics
Under an environment of 60 ° C, charge / discharge was repeated up to 100 cycles under the conditions of an upper limit voltage of 4.5 V, a lower limit voltage of 3.0 V, a discharge current of 100 mA, and a charge current of 50 mA. From the initial discharge capacity and the discharge capacity after 100 cycles, The following formula:
Capacity remaining rate S = discharge capacity after 100 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.

(4) Low temperature characteristics of the battery
Repeated charging and discharging up to 100 cycles under conditions of upper limit voltage 4.5V, lower limit voltage 3.0V, discharge current 100mA, charge current 50mA under each environment of 20 ℃ and -10 ℃, and measured discharge capacity after 100 cycles did. From the discharge capacity after 100 cycles at 20 ° C and the discharge capacity after 100 cycles at -10 ° C, the following formula:
Capacity remaining rate L = discharge capacity (−10 ° C.) / Discharge capacity (20 ° C.) × 100 (%)
The capacity remaining rate L was calculated according to the above and used as an index of the low temperature characteristics of the battery.

  In Table 1, EC is ethylene carbonate (boiling point 238 ° C.), DEC is diethyl carbonate (boiling point 127 ° C.), DMC is dimethyl carbonate (boiling point 90 ° C.), PC is propylene carbonate (boiling point 242 ° C.), EMC represents ethyl methyl carbonate (boiling point 108 ° C.), and MF represents methyl formate (boiling point 32 ° C.). Phosphazene A is a cyclic phosphazene compound (viscosity at 25 ° C .: 0.8 mPa · s, boiling point 82 ° C.) wherein n is 3 and one of the six Xs is chlorine and five are fluorines in formula (I). Phosphazene B is a cyclic phosphazene compound (viscosity at 25 ° C .: 1.1 mPa · s, boiling point) in which n is 3 and 2 out of 6 X are chlorine and 4 are fluorine in formula (I) Phosphazene C is a cyclic phosphazene compound (viscosity at 25 ° C .: 1.3 mPa · s) in which n is 3 and 3 out of 6 Xs are chlorine and 3 are fluorines in formula (I) Phosphazene D is a cyclic phosphazene compound (viscosity at 25 ° C .: 1.2 mPas) in which n is 4 and one of eight Xs is chlorine and seven are fluorines in formula (I) S, boiling point 117 ° C.), and phosphazene E has the formula (I ), A cyclic phosphazene compound (viscosity at 25 ° C .: 1.5 mPa · s, boiling point 147 ° C.) in which n is 4 and 2 out of 8 X are chlorine and 6 are fluorine, and phosphazene F has the formula In (I), a cyclic phosphazene compound (viscosity at 25 ° C .: 1.9 mPa · s, boiling point 178 ° C.) in which n is 4 and 3 out of 8 Xs are chlorine and 5 are fluorines. A cyclic phosphazene compound (viscosity at 25 ° C .: 2.3 mPa · s, boiling point 205 ° C.) in which n is 4 and 4 out of 8 X are chlorine and 4 are fluorine in formula (I), H is a cyclic phosphazene compound (viscosity at 25 ° C .: 2.8 mPa · s, boiling point 232 ° C.) in which n is 4 and 5 out of 8 X are chlorine and 3 are fluorine in the formula (I) .

  As is clear from Table 2, the non-aqueous electrolyte of the example has a high critical oxygen index and excellent safety, and the non-aqueous electrolyte secondary battery of the example is excellent while having sufficient cycle characteristics. It can be seen that it has low temperature characteristics.

Claims (10)

Formula (I) below:
(NPX ₂ ) _n ... (I)
(Wherein, X is a halogen element independently, n represents 3 to 15 of an integer) is represented by, and saw including at least two kinds of halogen, phosphazene compound solidifying point of -20 ° C. or less A non-aqueous electrolyte additive for secondary batteries comprising:   The additive for a non-aqueous electrolyte of a secondary battery according to claim 1, wherein the phosphazene compound contains fluorine and chlorine.   The additive for a non-aqueous electrolyte in a secondary battery according to claim 2, wherein X in the formula (I) is each independently fluorine or chlorine.   The additive for non-aqueous electrolyte of a secondary battery according to claim 1, wherein n in the formula (I) is 3 to 5.   5. The non-aqueous electrolyte for a secondary battery according to claim 3, wherein n in the formula (I) is 3, 1 to 3 out of 6 Xs are chlorine, and the remainder is fluorine. Additive. 5. The non-aqueous electrolyte for a secondary battery according to claim 3, wherein n in the formula (I) is 4, 2 to 4 of 8 Xs are chlorine, and the remainder is fluorine. Additives.   The non-aqueous electrolyte for a secondary battery according to claim 5 or 6, wherein the phosphazene compound contains two or more chlorine atoms in the molecule, and each chlorine atom is bonded to a different phosphorus atom. Additives. A non-aqueous electrolyte for a secondary battery comprising the additive for a non-aqueous electrolyte for a secondary battery according to any one of claims 1 to 7 , an aprotic organic solvent, and a supporting salt. The non-aqueous electrolyte for a secondary battery according to claim 8 , wherein the difference in boiling point between the aprotic organic solvent and the additive for the non-aqueous electrolyte of the secondary battery is 25 ° C or less. A non-aqueous electrolyte secondary battery comprising the non-aqueous electrolyte for a secondary battery according to claim 8 or 9 , a positive electrode, and a negative electrode.