JP5777982B2 - Non-aqueous electrolyte battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte battery, and more particularly, to a non-aqueous electrolyte battery in which a positive electrode plate containing a positive electrode active material and a negative electrode plate containing a negative electrode active material are infiltrated with a non-aqueous electrolyte solution containing ethylene carbonate. .

  Conventionally, sealed batteries have been widely used in household electrical appliances, and in recent years, lithium secondary batteries have been used in particular, among sealed batteries. Further, since lithium secondary batteries have a high energy density, they are being developed as in-vehicle power sources for electric vehicles (EV) and hybrid vehicles (HEV), and some of them are put into practical use.

  Usually, a lithium secondary battery includes a strip-like positive electrode plate in which a positive electrode active material and a negative electrode active material are respectively applied to a metal foil, and a winding group wound so that the negative electrode plate is not in direct contact via a separator. ing. The wound group is infiltrated with the electrolytic solution and accommodated in the battery container. High-capacity batteries are required for in-vehicle power supplies of electric vehicles, and non-aqueous electrolyte type sealed lithium secondary batteries using flammable organic solvents as electrolytes to improve battery performance Is used.

  However, in a sealed lithium secondary battery, for example, when the battery is abnormal when exposed to an abnormally high temperature environment or when it reaches an overcharged state due to a failure of the charging device, the nonaqueous electrolyte is decomposed due to a temperature rise. Vaporization may occur and the battery internal pressure may increase, leading to damage to the battery container. In order to avoid this, in general, a lithium secondary battery employs a current interrupt mechanism (a kind of disconnect switch) that operates in response to an increase in battery internal pressure and an internal pressure release mechanism (safety valve) that releases internal pressure.

  In addition, when the battery container is damaged, there is a possibility that the gas ejected from the battery or the leaked non-aqueous electrolyte is easily ignited and burned by an internal short circuit or an external fire point. In order to avoid this, a technique for improving safety by mixing a phosphazene flame retardant with a nonaqueous electrolytic solution is disclosed (for example, see Patent Document 1). In this technique, since the non-aqueous electrolyte is provided with flame retardancy or self-extinguishing properties, it is possible to extinguish even when the gas discharged when the battery is abnormal or the leaked non-aqueous electrolyte is ignited. On the other hand, a technique using an ionic liquid instead of a nonaqueous electrolytic solution using an organic solvent is disclosed (for example, see Patent Document 2). An ionic liquid is a liquid composed of ions that does not contain a molecular solvent, and has a flame resistance because the vapor pressure below the thermal decomposition temperature of the ions themselves is negligible.

JP 2002-083628 A JP 2009-026542 A

  However, in the technique of Patent Document 1, even if the fire is extinguished once, if the high temperature (abnormal) state continues further, the remaining non-aqueous electrolyte may ignite again, and the thermal runaway reaction may occur. It may also cause explosive combustion of the non-aqueous electrolyte. In order to suppress explosive combustion of the non-aqueous electrolyte, it is necessary to increase the mixing ratio of the phosphazene flame retardant. However, since the viscosity of the phosphazene flame retardant is higher than the viscosity of the non-aqueous electrolyte, the viscosity of the entire non-aqueous electrolyte increases when the mixing ratio is increased. As a result, the ionic conductivity in the nonaqueous electrolytic solution is lowered, and the discharge characteristics and the low temperature characteristics are lowered. In the technique of Patent Document 2, although the ionic liquid is excellent in ionic conductivity, the viscosity is higher than the viscosity of the non-aqueous electrolyte using an organic solvent, and it is difficult to cause dissociation of lithium ions. Low temperature characteristics will be reduced. Therefore, in a sealed lithium secondary battery that is required to have a higher capacity and higher performance, such as those used in the on-vehicle power supply described above, it is possible to ensure safety in the event of battery abnormality, as well as discharge characteristics and low temperature. It is important to suppress deterioration of characteristics, particularly discharge characteristics at a high rate.

  An object of the present invention is to provide a non-aqueous electrolyte battery capable of ensuring safety when a battery is abnormal and suppressing a decrease in high-rate discharge characteristics.

In order to solve the above problems, the present invention provides a positive electrode plate having a positive electrode active material as a main component, a negative electrode plate having a negative electrode active material as a main component, and the lithium salt infiltrating the positive electrode plate and the negative electrode plate into ethylene carbonate. A non-aqueous electrolyte dissolved in an organic solvent containing phosphazene-based flame retardant, the anion component is a bis (fluorosulfonyl) imide anion, and the cation component is 1 An ionic liquid which is ethyl-3-methylimidazolium cation, and the non-aqueous electrolyte has a volume ratio of the phosphazene flame retardant to the volume of the non-aqueous electrolyte x, and the ionic liquid Where the volume ratio x and the volume ratio y are 0 <x ≦ 0.2, 0 <y ≦ 0.5, and y ≧ 0.35x ² −0.45x + 0.15. this to satisfy the relationship The features.

In this case, e Sufazen flame retardant is, in liquid form, the non-aqueous electrolyte solution may be contained in a proportion of at least 5% by volume. The nonaqueous electrolytic solution may contain lithium hexafluorophosphate as a lithium salt. Also it is a lithium-manganese composite oxide of the positive electrode active material having a spinel crystal structure. The negative electrode active material can be a graphite-based carbon material.

According to the present invention, the phosphazene-based flame retardant is mixed with the non-aqueous electrolyte, so that the combustion of the battery is suppressed when the temperature rises due to battery abnormality, so the battery behavior is moderated and the safety is ensured. The ethylene carbonate contained in the non-aqueous electrolyte improves the dissociation property of lithium ions, the anion component is a bis (fluorosulfonyl) imide anion, and the cation component is 1-ethyl-3-methylimidazolium. Since the ion conductivity in the non-aqueous electrolyte is ensured by the ionic liquid that is a cation, it is possible to suppress the deterioration of the high rate discharge characteristics due to the mixing of the phosphazene flame retardant , and the non-aqueous electrolyte When the volume ratio of the phosphazene flame retardant to the volume is x and the volume ratio of the ionic liquid is y, the volume ratio x and the volume ratio y are 0 <x ≦ 0.2 and 0 <y ≦ 0. 5, and, by satisfying the relationship of y ≧ 0.35x ² -0.45x + 0.15, since the mixing amount of the phosphazene flame retardant and an ionic liquid is optimized, while ensuring the safety cell The effect that the fall of performance can be suppressed can be acquired.

It is sectional drawing which shows typically the cylindrical lithium ion secondary battery of embodiment to which this invention is applied. It is a graph which shows the evaluation result of a flame retardance when the mixing amount of the ionic liquid and the phosphazene-type flame retardant with respect to the non-aqueous electrolyte of a cylindrical lithium ion secondary battery is changed, respectively. It is a graph which shows the evaluation result of the discharge capacity in 3CA discharge when the mixing amount of the ionic liquid and the phosphazene-type flame retardant with respect to the nonaqueous electrolyte of a cylindrical lithium ion secondary battery is each changed. It is a graph which shows the suitable range of a flame retardance and a high rate discharge capacity when the mixing amount of the ionic liquid and the phosphazene-type flame retardant with respect to the non-aqueous electrolyte of a cylindrical lithium ion secondary battery is each changed. The graph which shows the suitable range of a flame retardance and a high rate discharge capacity by changing the mixing amount of the ionic liquid and the phosphazene-type flame retardant with respect to the nonaqueous electrolyte of a cylindrical lithium ion secondary battery with three approximate expressions, respectively It is.

  Embodiments of a cylindrical lithium ion secondary battery to which the present invention is applied will be described below with reference to the drawings.

(Constitution)
As shown in FIG. 1, the columnar lithium ion secondary battery 20 of the present embodiment has a bottomed cylindrical battery case 7 made of steel plated with nickel. The battery case 7 accommodates an electrode group 6 in which a strip-like positive electrode plate W1 and a negative electrode plate W3 are wound in a spiral shape with a separator W5 interposed therebetween.

  A hollow cylindrical shaft core 1 made of polypropylene resin is used at the winding center of the electrode group 6. On the upper side of the electrode group 6, an annular conductor positive electrode current collection ring 4 for collecting the electric potential from the positive electrode plate W <b> 1 is disposed on a substantially extension line of the shaft core 1. The positive electrode current collecting ring 4 is fixed to the upper end portion of the shaft core 1. The edge part of the positive electrode lead piece 2 led out from the positive electrode plate W1 is joined to the periphery of the flange part integrally protruding from the periphery of the positive electrode current collecting ring 4 by ultrasonic welding. Above the positive electrode current collecting ring 4, a disc-shaped battery lid 11 is provided that incorporates a safety valve and serves as a positive electrode external terminal. One end of the positive electrode lead is fixed to the upper part of the positive electrode current collecting ring 4, and the other end of the positive electrode lead is welded to the lower surface of the battery lid 11. The positive electrode lead is formed by welding ends of two leads formed by overlapping a plurality of ribbon-shaped conductors.

  On the other hand, an annular conductor negative electrode current collecting ring 5 for collecting the electric potential from the negative electrode plate W3 is disposed below the electrode group 6. The outer peripheral surface of the lower end portion of the shaft core 1 is fixed to the inner peripheral surface of the negative electrode current collecting ring 5. The end of the negative electrode lead piece 3 led out from the negative electrode plate W3 is joined to the outer peripheral edge of the negative electrode current collecting ring 5 by ultrasonic welding. A negative electrode lead plate for electrical conduction is welded to the lower part of the negative electrode current collecting ring 5, and the negative electrode lead plate is joined to the inner bottom portion of the battery container 7 by resistance welding. In this example, the battery container 7 has an outer diameter of 40 mm and an inner diameter of 39 mm.

  The battery lid 11 is caulked and fixed to the upper part of the battery container 7 via an insulating and heat resistant EPDM resin gasket 10. For this reason, the positive electrode lead is accommodated in the battery container 7 so as to be folded, and the lithium ion secondary battery 20 is sealed. The lithium ion secondary battery 20 is given a battery function by performing initial charging at a predetermined voltage and current.

(Nonaqueous electrolyte)
Further, a non-aqueous electrolyte (not shown) is injected into the battery container 7. The non-aqueous electrolyte includes lithium tetrafluoroborate (LiBF ₄ ) and lithium hexafluorophosphate (LiPF ₆ ) as a lithium salt (electrolyte) in a mixed solvent containing ethylene carbonate (EC) and dimethyl carbonate (DMC). Can be used. In this example, the ratio of lithium hexafluorophosphate lithium salt in the range of 0.8 to 1.0 mol / liter (M) in a mixed solvent in which EC and DMC are mixed at a volume ratio of 1: 2. It is dissolved in. In this non-aqueous electrolyte, a phosphazene flame retardant as a flame retardant and an ionic liquid containing an anionic component and a cationic component are mixed.

The phosphazene flame retardant is a cyclic compound mainly composed of phosphorus and nitrogen and represented by the general formula (NPR ₂ ) ₃ or (NPR ₂ ) ₄ . R in the general formula represents a halogen element such as fluorine or chlorine or a monovalent substituent. As monovalent substituents, alkoxy groups such as methoxy group and ethoxy group, aryloxy groups such as phenoxy group and methylphenoxy group, alkyl groups such as methyl group and ethyl group, aryl groups such as phenyl group and tolyl group, Examples thereof include an amino group containing a substituted amino group such as a methylamino group, an alkylthio group such as a methylthio group and an ethylthio group, and an arylthio group such as a phenylthio group. Such a phosphazene flame retardant exhibits a fire prevention effect and a fire extinguishing action under a high temperature environment such as when a battery is abnormal. The phosphazene flame retardant becomes solid or liquid depending on the type of the substituent R, but a liquid phosphazene flame retardant can be mixed in the non-aqueous electrolyte.

  On the other hand, an ionic liquid is a liquid that contains only ions and does not contain a molecular solvent, and contains an anionic component and a cationic component. The anion component includes at least one component selected from imide anions, phosphate anions and borate anions. The cation component includes at least one component selected from imidazolium cations, pyridinium cations, quaternary ammonium cations, and quaternary phosphonium cations.

  Examples of the anion component imide anions include bis (fluorosulfonyl) imide anion, bis (trifluoromethanesulfonyl) imide anion, (fluorosulfonyl) (trifluoromethanesulfonyl) imide anion, and (trifluoroacetyl) (trifluoromethanesulfonyl). An imide anion etc. can be mentioned. Examples of phosphate anions include hexafluorophosphate anions. Examples of the borate anions include tetrafluoroborate anions.

  Examples of the imidazolium cation of the cation component include 1,3-dimethylimidazolium cation, 1-ethyl-3-methylimidazolium cation, 1-methyl-3-propylimidazolium cation, and 1-butyl-3-methyl. Dialkylimidazolium cations such as imidazolium cation, 3-ethyl-1,2-dimethyl-imidazolium cation, 1,2-dimethyl-3-propylimidazolium cation, 1-butyl-2,3-dimethylimidazolium cation, etc. And trialkylimidazolium cations. Examples of pyridinium cations include pyridinium cations substituted with an alkyl group having 1 to 16 carbon atoms such as N-methylpyridinium, N-ethylpyridinium, N-butylpyridinium, and N-propylpyridinium. it can. Examples of quaternary ammonium cations include N, N, N, N-tetramethylammonium cation, N, N, N-trimethylethylammonium cation, N, N, N-trimethylpropylammonium cation, and N-ethyl- Examples thereof include N, N-dimethylpropylammonium cation, 2-methoxy-N, N, N-trimethylethylammonium cation, 2-ethoxy-N, N, N-trimethylethylammonium cation, and the like. Further, examples of the quaternary phosphonium cation include a tetramethylphosphonium cation, a tetraethylphosphonium cation, and a tetrabutylphosphonium cation.

  The mixing amount of the phosphazene flame retardant and the ionic liquid with respect to the non-aqueous electrolyte is determined as follows. That is, when the volume ratio of the phosphazene flame retardant to the volume of the non-aqueous electrolyte is x and the volume ratio of the ionic liquid is y, the volume ratio x and the volume ratio y are expressed by the following formulas (1) and ( 2) and the formula (3) are satisfied.

  As shown in FIG. 1, the electrode group 6 has a positive electrode plate W1 and a negative electrode plate W3 with a shaft core through a polyethylene separator W5 having a thickness of 30 μm and allowing lithium ions to pass therethrough so that these two plates do not directly contact each other. It is wound around 1. The positive electrode lead piece 2 and the negative electrode lead piece 3 are respectively disposed on opposite end surfaces of the electrode group 6. The diameter of the electrode group 6 is set to 38 ± 0.5 mm by adjusting the lengths of the positive electrode plate W1, the negative electrode plate W3, and the separator W5. Insulation coating is applied to the entire circumference of the collar peripheral surface of the electrode group 6 and the positive electrode current collecting ring 4 in order to prevent electrical contact between the electrode group 6 and the battery container 7. For the insulation coating, an adhesive tape in which a hexamethacrylate adhesive is applied to one side of a polyimide base material is used. The pressure-sensitive adhesive tape is wound one or more times from the peripheral surface of the collar portion to the outer peripheral surface of the electrode group 6. The number of turns is adjusted so that the maximum diameter portion of the electrode group 6 becomes an insulating coating existing portion, and the maximum diameter is set slightly smaller than the inner diameter of the battery container 7.

(Positive electrode plate)
The positive electrode plate W1 constituting the electrode group 6 has an aluminum foil having a thickness of 20 μm as a positive electrode current collector. A positive electrode mixture containing a lithium manganese complex oxide having a spinel crystal structure as a positive electrode active material is applied to both surfaces of the aluminum foil substantially uniformly and uniformly to form a positive electrode mixture layer W2. . That is, the thickness of the formed positive electrode mixture layer W2 is substantially uniform, and the positive electrode mixture is dispersed substantially uniformly in the positive electrode mixture layer W2. Examples of the positive electrode mixture include 8 parts by mass of flaky graphite and 2 parts by mass of acetylene black as a conductive material and 100 parts by mass of lithium manganese complex oxide and polyvinylidene fluoride (binder) as a binder (binder). Hereinafter, 5 parts by mass of PVDF is abbreviated. When the positive electrode mixture is applied to the aluminum foil, a dispersion solvent N-methyl-2-pyrrolidone (hereinafter abbreviated as NMP) is used. The positive electrode plate W1 is pressed after drying and cut into a width of 80 mm. In this example, the thickness of the positive electrode mixture layer W2 is adjusted to 80 μm (one side). An uncoated portion of a positive electrode mixture having a width of 30 mm is formed on one side edge along the longitudinal direction of the aluminum foil. The non-coated portion is cut out in a comb shape, and the positive electrode lead piece 2 is formed in the remaining portion of the cutout. In this example, the interval between the adjacent positive electrode lead pieces 2 is set to 20 mm, and the width of the positive electrode lead piece 2 is set to 5 mm.

(Negative electrode plate)
On the other hand, the negative electrode plate W3 has a rolled copper foil having a thickness of 10 μm as a negative electrode current collector. A negative electrode mixture containing carbon powder capable of occluding and releasing lithium ions as a negative electrode active material is applied to both surfaces of the rolled copper foil substantially uniformly and uniformly to form a negative electrode mixture layer W4. That is, the thickness of the formed negative electrode mixture layer W4 is substantially uniform, and the negative electrode mixture is substantially uniformly dispersed in the negative electrode mixture layer W4. As the negative electrode active material, amorphous carbon, graphite, or a mixture thereof can be used. In this example, a carbon material mainly composed of graphite, that is, a graphite-based carbon material is used. In the negative electrode mixture, for example, 10 parts by mass of PVDF is blended as a binder with respect to 90 parts by mass of the graphite-based carbon material. When applying the negative electrode mixture to the rolled copper foil, a dispersion solvent NMP is used. An uncoated portion of a negative electrode mixture having a width of 30 mm is formed on one side edge along the longitudinal direction of the rolled copper foil, as in the positive electrode plate W1, and a negative electrode lead piece 3 is formed. In this example, the interval between the adjacent negative electrode lead pieces 3 is set to 20 mm, and the width of the negative electrode lead piece 3 is set to 5 mm. The negative electrode plate W3 is pressed after drying and cut into a width of 86 mm. In this example, the thickness of the negative electrode mixture layer W4 is adjusted to 60 μm (one side). Note that the length of the negative electrode plate W3 is such that when the positive electrode plate W1 and the negative electrode plate W3 are wound, the positive electrode plate W1 does not protrude from the negative electrode plate W3 in the winding innermost circumference and outermost circumference. Further, the length is set to be 120 mm longer than the length of the positive electrode plate W1. The width of the negative electrode mixture application part (negative electrode mixture layer W4) is such that the positive electrode mixture application part does not protrude from the negative electrode mixture application part in the direction crossing the winding direction. It is set 6 mm longer than the width of (positive electrode mixture layer W2).

  Next, examples of the cylindrical lithium ion secondary battery 20 manufactured according to the present embodiment will be described.

In the examples, lithium manganate (LiMn ₂ O ₄ ) having a spinel crystal structure was used as the positive electrode active material. Also, LiPF ₆ as a lithium salt was dissolved at 0.8 mol / liter (0.8M) in a mixed solvent of EC and DMC in a volume ratio of 1: 2, and a phosphazene flame retardant (product made by Bridgestone Corporation, product) A non-aqueous electrolyte mixed with a name Phoslite (registered trademark, liquid) and an ionic liquid was used. The ionic liquid includes 1-ethyl-3-methylimidazolium cation (hereinafter abbreviated as EMI) as an imidazolium cation as a cation component, and a bis (fluorosulfonyl) imide anion (hereinafter referred to as EMI) as an anion component. , And abbreviated as FSI). The mixing ratio of EMI and FSI was adjusted to a volume ratio of 1: 1 in this example so that the amount of electricity was equal. The mixing amount of the phosphazene flame retardant with respect to the non-aqueous electrolyte is in the range of 0 to 20 vol% (in the volume ratio x, the range of 0 to 0.20), and the mixing amount of the ionic liquid is in the range of 0 to 100 vol% (volume) The ratio y was varied in the range of 0 to 1.00), and a plurality of lithium ion secondary batteries having different combinations of the volume ratio x and the volume ratio y were produced.

(Flame resistance evaluation of non-aqueous electrolyte)
The flame retardancy of the non-aqueous electrolyte used for each lithium ion secondary battery was evaluated as follows. That is, a glass mat (volume: 1.016 cm ³ ) 127 mm long × 40 mm wide × 0.2 mm thick was impregnated with 2.5 to 3.0 g of each non-aqueous electrolyte, and indirectly flamed for 10 seconds. The fire was released and the fire was extinguished twice. The time required for fire extinguishing (combustion time after releasing the flame) was measured for the first time and the second time. Fire extinguishing in less than 10 seconds without burning out non-aqueous electrolyte impregnated into glass mat is determined to be flame retardant, and burning after 10 seconds or non-aqueous electrolyte burned out It was determined that there was no sex.

  As shown in FIG. 2, when the phosphazene flame retardant is not mixed (volume ratio x = 0), it exhibits flame retardancy by mixing 50 vol% or more of ionic liquid (volume ratio y ≧ 0.50). can do. Further, when a phosphazene flame retardant is mixed, the volume ratio y of the ionic liquid necessary for exhibiting flame retardancy can be reduced according to the volume ratio x. In FIG. 2, a circle mark indicates that the flame retardancy is determined, and a cross mark indicates that the flame retardance is determined.

(Battery performance evaluation)
About each lithium ion secondary battery, 3CA discharge was performed in 25 degreeC environment, and the discharge capacity, ie, a high rate discharge capacity, was measured. The discharge capacity when the mixing amount of the ionic liquid with respect to the nonaqueous electrolytic solution is 50 vol% (volume ratio y = 0.50) and the mixing amount of the phosphazene flame retardant is 0 vol% (volume ratio x = 0) is 100. % Relative capacity was determined. FIG. 3 shows the relative capacities of lithium ion secondary batteries produced by changing the volume ratio x and the volume ratio y, and the same relative capacities in the range of 70 to 120% are connected by curves. As shown in FIG. 3, it was found that the 3CA discharge capacity is increased by decreasing the volume ratio y of the ionic liquid. Further, it was found that even when the volume ratio y of the ionic liquid is the same, increasing the volume ratio x of the phosphazene flame retardant reduces the 3CA discharge capacity. This is thought to be because both the ionic liquid and the phosphazene flame retardant improve the flame retardancy, but both increase the viscosity of the non-aqueous electrolyte. On the other hand, since the ionic liquid is excellent in ionic conductivity, the volume ratio of the ionic liquid is reduced even when the viscosity of the non-aqueous electrolyte increases and the ionic conductivity decreases as the volume ratio x of the phosphazene flame retardant increases. By adjusting y, it is possible to suppress a decrease in ion conductivity. From this, it was found that when the volume ratio x of the phosphazene flame retardant is increased, the 3CA discharge capacity can be increased by decreasing the volume ratio y of the ionic liquid.

  Next, the case where it was excellent in both a flame retardance and a high rate discharge capacity was examined. The evaluation results of the flame retardancy shown in FIG. 2 and the evaluation results of the high rate discharge capacity shown in FIG. 3 are shown superimposed on FIG. Further, in the range where the mixing amount of the ionic liquid exceeds 50 vol% (volume ratio y> 0.50), the ratio of EC and DMC, which are organic solvents that mediate the movement of lithium ions, becomes relatively small. Therefore, the mixing amount of the ionic liquid was set to 50 vol% or less (volume ratio y ≦ 0.50). In addition to the increase in viscosity of the non-aqueous electrolyte due to the mixing of the ionic liquid, the viscosity also increases with the mixing of the phosphazene flame retardant, so the mixing amount of the phosphazene flame retardant is 20 vol% or less. (Volume ratio x ≦ 0.20). That is, by setting the mixing amount of the phosphazene-based flame retardant and the ionic liquid, that is, the volume ratio x and the volume ratio y within the range surrounded by the curve shown in FIG. A decrease in discharge capacity can be suppressed. Furthermore, although the ionic liquid itself is difficult to burn, in order to ensure flame retardancy as a whole non-aqueous electrolyte, the mixing amount of the phosphazene flame retardant is at least 5 vol% (volume ratio x ≧ 0.05). It is preferable to do.

In order to clarify the preferred range of FIG. 4, approximate equations were obtained for three curves. As a result, as shown in FIG. 5, three approximate expressions, namely, 0 <x ≦ 0.2 that is the above-described expression (1), 0 <y ≦ 0.5 that is the expression (2), and the expression (3 ) That satisfies the relationship y ≧ 0.35x ² −0.45x + 0.15. Therefore, by setting the volume ratio x of the phosphazene flame retardant and the volume ratio y of the ionic liquid so as to satisfy the formulas (1) to (3), the flame retardancy and the high rate discharge capacity are achieved. It has been found that an excellent lithium ion secondary battery 20 can be obtained.

(Action etc.)
Next, the operation and the like of the lithium ion secondary battery 20 of the present embodiment will be described.

  In the present embodiment, a phosphazene flame retardant is mixed in the nonaqueous electrolytic solution. Because phosphazene flame retardants prevent fires and extinguish in high temperature environments such as when batteries are abnormal, mix phosphazene flame retardants with non-aqueous electrolytes to make them flame retardant or self Fire extinguishing properties are added. Thereby, even when the battery is abnormal, such as an overcharged state, or when exposed to an abnormally high temperature environment, the non-aqueous electrolyte is easily extinguished even if it is ignited, so that the safety of the battery can be ensured.

  Moreover, in this embodiment, the ionic liquid is mixed with the non-aqueous electrolyte in addition to the phosphazene flame retardant. In ionic liquids, the vapor pressure below the thermal decomposition temperature of ions constituting the ionic liquid is so low as to be negligible and has a property of being difficult to burn. For this reason, safety can be improved by using the ionic liquid itself. Moreover, since the phosphazene flame retardant has a higher viscosity than the non-aqueous electrolyte, mixing the phosphazene flame retardant increases the viscosity of the non-aqueous electrolyte. As a result, when only the phosphazene-based flame retardant is added to improve the flame retardancy, the ionic conductivity is lowered due to the increase in the viscosity of the non-aqueous electrolyte, leading to a decrease in battery performance. On the other hand, since the ionic liquid is excellent in ionic conductivity, a decrease in ionic conductivity can be suppressed by mixing the ionic liquid. As a result, the flame retardancy by the phosphazene flame retardant is obtained, and the mobility of lithium ions in the non-aqueous electrolyte is ensured, so that the battery performance, in particular, the reduction of the high rate discharge capacity is suppressed. be able to.

Furthermore, in this embodiment, the volume ratio x of the phosphazene flame retardant with respect to the volume of the non-aqueous electrolyte and the volume ratio y of the ionic liquid are the above-described formulas (1), (2), and (3), In other words, 0 <x ≦ 0.2, 0 <y ≦ 0.5, and y ≧ 0.35x ² −0.45x + 0.15 are set. Thereby, since the mixing amount of the phosphazene flame retardant and the ionic liquid is optimized, it is possible to suppress a decrease in battery performance while ensuring safety (see also FIG. 5).

  Furthermore, in the present embodiment, a mixed solvent of EC and DMC is used as the organic solvent of the nonaqueous electrolytic solution. EC can increase the dissociation property of the lithium salt as compared with DMC, but has a high melting point and is difficult to be liquid. By mixing EC and DMC, DMC which is liquid at room temperature dissolves EC, so that it can be made a solvent having excellent lithium salt dissociability.

  In this embodiment, the ionic liquid is a mixture of cation component EMI and anion component FSI in a volume ratio of 1: 1, but the present invention is not limited to this. The ionic liquid only needs to contain at least one component selected from the above-described cation component and at least one component selected from the anion component, and each of the cation component and the anion component may have two or more components. Moreover, what is necessary is just to mix the mixing ratio of a cation component and an anion component so that the electric quantity of a cation and an anion may become equal amount.

In the present embodiment, a mixed solvent in which EC and DMC are mixed at a volume ratio of 1: 2 is exemplified as the organic solvent of the nonaqueous electrolytic solution, but the present invention is not limited to this. As an organic solvent that can be used in other embodiments, EC may be contained, and dimethyl carbonate, diethyl carbonate, propylene carbonate, ethyl methyl carbonate, vinylene carbonate, 1,2-dimethoxyethane, 1,2- Organic solvents such as diethoxyethane, γ-butyrolactone, tetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether, sulfolane, methylsulfolane, acetonitrile, propionitrile may be mixed. . Also, the mixing ratio of these organic solvents is not particularly limited. Considering that EC is solid at normal temperature, it is preferable that DMC is mixed. In the present embodiment, an example has been shown that the ratio of the range LiBF ₄ and LiPF ₆ of 0.8~1.0M as lithium salt to be dissolved in an organic solvent, the present invention is not limited thereto. As a lithium salt, the lithium salt normally used for a lithium ion secondary battery can be used, What is necessary is just to make it the range normally used also as the quantity. When LiBF ₄ and LiPF ₆ are compared, it is confirmed that the effect on the battery characteristics is different. That is, LiBF ₄ is effective in improving the life characteristics because it suppresses elution of manganese ions from the lithium manganese complex oxide used as the positive electrode active material, and LiPF ₆ is effective in improving the high rate discharge characteristics. Therefore, in consideration of securing ionic conductivity in a non-aqueous electrolyte with an ionic liquid and suppressing a decrease in high rate discharge characteristics due to mixing of a phosphazene flame retardant, a mixed solvent of EC and DMC It is preferable to use a nonaqueous electrolytic solution in which LiPF ₆ is dissolved in a ratio in the range of 0.8 to 1.0 M.

  Furthermore, in this embodiment, lithium manganate having a spinel crystal structure is exemplified as the positive electrode active material, but the present invention is not limited to this, and a transition metal double oxide containing lithium can be used. . Examples of such a positive electrode active material include lithium manganese complex oxide and lithium cobalt complex oxide, but it is preferable to use lithium manganese complex oxide from the viewpoint of cost reduction. Further, a part of the manganese site in the crystal may be substituted with a transition metal other than manganese, for example, magnesium, aluminum, cobalt, nickel or the like. The crystal structure is not particularly limited, but it is preferable to use a lithium manganese complex oxide having a spinel crystal structure in consideration of thermal stability. On the other hand, in the present embodiment, a carbon material mainly composed of graphite, that is, a graphite-based carbon material is exemplified as the negative electrode active material, but the present invention is not limited to this. In addition to the graphite-based carbon material, amorphous carbon can also be used. Considering the flattening of voltage characteristics, it is preferable to use a carbon material mainly composed of graphite.

  Furthermore, in this embodiment, as a positive electrode mixture, 100 parts by mass of the positive electrode active material, 8 parts by mass of flaky graphite as a conductive material, 2 parts by mass of acetylene black, and 5 parts by mass of PVDF as a binder are blended. However, the present invention is not limited to this example. Another conductive material usually used for a lithium ion secondary battery may be used, and the conductive material may not be used. A binder other than PVDF may be used. Examples of binders that can be used in other embodiments include polytetrafluoroethylene (PTFE), polyethylene, polystyrene, polybutadiene, butyl rubber, nitrile rubber, styrene / butadiene rubber, polysulfide rubber, nitrocellulose, cyanoethyl cellulose, and various latexes. And polymers such as acrylonitrile, vinyl fluoride, vinylidene fluoride, propylene fluoride, chloroprene fluoride, and mixtures thereof. Furthermore, it goes without saying that the blending ratio of each material may be changed.

  Furthermore, in the present embodiment, the cylindrical lithium ion secondary battery 20 is exemplified, but the present invention is not limited to this, and can be applied to a battery generally using a non-aqueous electrolyte. There is no restriction | limiting in particular also about the shape of a battery, For example, it is good also as a square shape etc. other than a column shape. In the present embodiment, the electrode group 6 in which the positive electrode plate W1 and the negative electrode plate W3 are wound is illustrated. However, the present invention is not limited to this, and for example, a rectangular positive electrode plate and a negative electrode plate are stacked. An electrode group may be used. Furthermore, the battery to which the present invention can be applied may be other than a battery having a structure in which the battery lid 11 is caulked and sealed to the battery container 7 described above. As an example of such a structure, a battery in a state where positive and negative external terminals penetrate through the battery lid and are pressed through the shaft core in the battery container can be mentioned.

  The present invention provides a non-aqueous electrolyte battery that can ensure safety in the event of battery abnormalities and suppress a decrease in high-rate discharge characteristics, and thus contributes to the manufacture and sale of non-aqueous electrolyte batteries. So it has industrial applicability.

W1 Positive electrode plate W2 Positive electrode mixture layer W3 Negative electrode plate W4 Negative electrode mixture layer W5 Separator 6 Electrode group 20 Cylindrical lithium ion secondary battery (non-aqueous electrolyte battery)

Claims (5)

A positive electrode plate mainly composed of a positive electrode active material;
A negative electrode plate mainly composed of a negative electrode active material;
A non-aqueous electrolyte solution infiltrating the positive electrode plate and the negative electrode plate, and a lithium salt dissolved in an organic solvent containing ethylene carbonate;
With
The non-aqueous electrolyte is mixed with a phosphazene flame retardant and an ionic liquid whose anion component is a bis (fluorosulfonyl) imide anion and whose cation component is a 1-ethyl-3-methylimidazolium cation. And
When the volume ratio of the phosphazene flame retardant to the volume of the non-aqueous electrolyte is x and the volume ratio of the ionic liquid is y, the volume ratio x and volume ratio y are A nonaqueous electrolyte battery characterized by satisfying the relations of 0 <x ≦ 0.2, 0 <y ≦ 0.5, and y ≧ 0.35x ² −0.45x + 0.15 . The non-aqueous electrolyte battery according to claim 1 , wherein the phosphazene flame retardant is in a liquid form and is contained in the non-aqueous electrolyte at a ratio of at least 5% by volume.   The non-aqueous electrolyte battery according to claim 1, wherein the non-aqueous electrolyte contains lithium hexafluorophosphate as a lithium salt.   The non-aqueous electrolyte battery according to claim 1, wherein the positive electrode active material is a lithium manganese complex oxide having a spinel crystal structure. The non-aqueous electrolyte battery according to claim 4 , wherein the negative electrode active material is a graphite-based carbon material.

Images