JP2011034698A - Nonaqueous electrolyte solution, and lithium secondary battery using nonaqueous electrolyte solution - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary battery using a nonaqueous electrolyte solution capable of improving discharge characteristics at a large current. <P>SOLUTION: The lithium secondary battery 20 is provided with an electrode group with a positive electrode plate 1 and a negative electrode plate 2 wound around through a separator 3. The electrode group is housed in a battery container 6. The nonaqueous electrolyte solution is injected into the battery container 6. The nonaqueous electrolyte solution has an organic compound having a phosphorus-oxygen double bond and lithium salt both dissolved in organic solvent. The organic compound has: one phosphorus atom; one oxygen atom double-bonded to the phosphorus atom; and three substituents bonded to the phosphorus atom. Each of the three substituents has the carbon number of 1 to 10, has at least one oxygen atom and that has at least one out of atoms of elements of hydrogen, sulfur, oxygen, nitrogen, fluorine, chlorine, bromine and iodine. As the lithium salt, LiBF<SB>4</SB>is used. Thus, ion conductivity of the nonaqueous electrolyte solution is improved. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a non-aqueous electrolyte and a lithium secondary battery using the non-aqueous electrolyte, and in particular, a non-aqueous electrolyte containing an organic compound having a phosphorus-oxygen double bond and the non-aqueous electrolyte. The present invention relates to a lithium secondary battery.

  With the development and popularization of mobile communication technologies such as cellular phones and portable personal computers, demands for these power sources are increasing, and there is an increasing demand for miniaturization and high energy density. Furthermore, the development of a storage power source for late-night power and a power storage power source combined with solar cells and wind power generation is also in progress. On the other hand, commercialization of an electric vehicle, a hybrid vehicle using electric power as a part of power, and a hybrid train are being promoted.

As a power source for mobile communication and power storage, a lithium secondary battery using a non-aqueous electrolyte is widely used. In this nonaqueous electrolytic solution, a lithium salt is usually dissolved as an electrolyte in an organic solvent. A chain or cyclic ether compound or ester compound is used as the organic solvent of the non-aqueous electrolyte, and lithium hexafluorophosphate (LiPF ₆ ) is widely used as the lithium salt (for example, Patent Document 1). reference). However, when the battery is exposed to high temperatures during use, the stability of LiPF ₆ may be impaired, and hydrolysis may be caused when a minute amount of water is mixed in the electrolytic solution. For the purpose of improving thermal stability and hydrolysis resistance, for example, lithium salts include LiBF ₄ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, LiClO _4, and LiB (C ₂ O _{4) 2, LiBF 2 (C} 2 O 4) , etc. are also used.

JP-A-5-41244

However, LiPF ₆ described above has advantages such as increased ionic conductivity and difficulty in causing side reactions even on the electrode surface, but also has the disadvantage of poor thermal stability. In addition, hydrolysis in the electrolytic solution causes hydrolysis products to accumulate on the electrode surface, which may cause a problem that the internal resistance of the battery increases and the battery capacity decreases. Even when the above-mentioned various electrolytes are used, there is a decrease in solubility in organic solvents, a decrease in ionic conductivity, an increase in viscosity, a decrease in electrochemical stability under a high voltage atmosphere, corrosion of an aluminum current collector, etc. May cause problems. For this reason, the present situation is that the battery characteristics are not sufficient to cope with lithium secondary batteries in applications requiring high capacity and charge / discharge with a large current.

  In order to solve these problems, the present inventors have conducted extensive research on non-aqueous electrolytes. As a result, non-aqueous electrolysis in which an organic compound having a phosphorus (P) -oxygen (O) double bond is dissolved is used. It has been found that by using the liquid, deterioration of the battery over time is small, and the battery has excellent storage characteristics and high rate discharge characteristics.

  An object of the present invention is to provide a non-aqueous electrolyte for a lithium secondary battery capable of improving discharge characteristics at a large current and a lithium secondary battery using the non-aqueous electrolyte.

In order to solve the above-mentioned problem, a first aspect of the present invention is an organic compound having a phosphorus (P) -oxygen (O) double bond and improving ion conductivity, which has the following chemical formula (1) And each of the substituents R ₁ , R ₂ and R ₃ is an organic group having 1 to 10 carbon atoms, and at least one oxygen atom, hydrogen, sulfur, nitrogen, fluorine, chlorine A non-aqueous electrolyte for a lithium secondary battery, characterized by containing an organic compound having at least one atom selected from atoms of bromine and iodine.

  In the first aspect, since the ionic conductivity of the non-aqueous electrolyte is improved by containing an organic compound having a phosphorus-oxygen double bond, the internal resistance is reduced when used in a lithium secondary battery. The discharge characteristics at a large current can be improved.

In the first embodiment, at least one solvent selected from a linear or cyclic carbonate compound, an ester compound, an ether compound, and a compound in which a part of the functional group constituting each compound is substituted with another functional group; In addition, a lithium salt of an electrolyte may be contained. At this time, the organic compound is preferably contained in a proportion of 10% by weight or less. Further, each of the substituents R ₁ , R ₂ and R ₃ may have the same chemical structure having 6 to 10 carbon atoms. The electrolyte is preferably lithium tetrafluoroborate (LiBF ₄ ). Further, at least one compound selected from vinylene carbonate, carboxylic anhydride, sulfur-containing compound, and boron-containing compound can be contained.

  In the second aspect of the present invention, a positive electrode using a lithium-containing oxide capable of inserting and removing lithium ions and a material capable of inserting and removing lithium ions are used in the non-aqueous electrolyte of the first aspect. A lithium secondary battery characterized in that a negative electrode is infiltrated.

  According to the present invention, since the ionic conductivity of the non-aqueous electrolyte is improved by containing an organic compound having a phosphorus-oxygen double bond, the internal resistance is reduced when used in a lithium secondary battery. The effect that the discharge characteristics at a large current can be improved can be obtained.

It is sectional drawing which shows typically the cylindrical lithium secondary battery using the non-aqueous electrolyte to which this invention is applied.

  Embodiments of a cylindrical lithium secondary battery using a non-aqueous electrolyte to which the present invention is applied will be described below with reference to the drawings.

(Constitution)
As shown in FIG. 1, the cylindrical lithium secondary battery 20 of the present embodiment has an electrode group wound in a cross-sectional spiral shape so that the positive electrode plate 1 and the negative electrode plate 2 face each other with a separator 3 therebetween. ing. The electrode group is housed in a bottomed cylindrical battery case 6 made of SUS.

  A disc-shaped sealing lid 10 serving as a positive external terminal is disposed above the electrode group. The sealing lid 10 is formed with a relief valve (not shown) that is cleaved when the pressure inside the battery rises and releases the pressure inside the battery. A positive electrode lead 4 having one end bonded to the positive electrode plate 1 is led out above the electrode group. The other end of the positive electrode lead 4 is joined to the lower surface of the sealing lid 10 by welding. On the other hand, a negative electrode lead 5 having one end bonded to the negative electrode plate 2 is led out below the electrode group. The other end of the negative electrode lead 5 is joined to the inner bottom surface of the battery container 6 that also serves as a negative electrode external terminal by welding. Between the end surface from which the positive electrode lead 4 of the electrode group is led out and the sealing lid 10, and between the end surface from which the negative electrode lead 5 of the electrode group is led out and the inner bottom surface of the battery container 6, respectively, Is arranged.

  In addition, a non-aqueous electrolyte is injected into the battery container 6. The sealing lid 10 is caulked and fixed to the upper part of the battery container 6 via the packing 8. For this reason, the inside of the lithium secondary battery 20 is sealed. The obtained lithium secondary battery 20 is formed to have an outer diameter of 18 mm and a length of 65 mm.

  In the electrode group accommodated in the battery container 6, the positive electrode plate 1 and the negative electrode plate 2 are wound through the separator 3 so that these two electrode plates do not directly contact each other. In this example, the separator 3 is a microporous polypropylene film having a thickness of 40 μm and a width of 5.8 cm. The positive electrode lead 4 and the negative electrode lead 5 are led out in opposite directions from both end surfaces of the electrode group, respectively.

  The positive electrode plate 1 constituting the electrode group has an aluminum foil as a positive electrode current collector. In this example, the thickness of the aluminum foil is set in the range of 15 to 25 μm. A positive electrode mixture containing a positive electrode active material is applied to both surfaces of the aluminum foil substantially evenly. In addition to the positive electrode active material, the positive electrode mixture contains a conductive material such as carbon material powder and a binder (binder) such as polyvinylidene fluoride (hereinafter abbreviated as PVDF).

As the positive electrode active material, a lithium-containing oxide capable of inserting and releasing lithium ions is used. Examples of such a lithium-containing oxide include compounds having a layered crystal structure such as lithium cobaltate (LiCoO ₂ ) and lithium nickelate (LiNiO ₂ ), and a part of cobalt and nickel as one or more other transition metals. And the chemical formula is Li _{1 + x} Mn _2−x O ₄ (where x = 0 to 0.33), Li _{1 + x} Mn _2−xy M _y O ₄ (where M is Ni, Co, Fe) , Cu, Al, Mg, including at least one metal selected from x = 0 to 0.33, y = 0 to 1.0, ₂ -xy> 0), LiMnO ₄ , LiMn ₂ O ₄ , LiMnO ₂ , LiMn _2−x M _x O ₂ (where M includes at least one metal selected from Ni, Co, Fe, Cu, Al, and Mg, x = 0.01 to 0.1) , _Li 2 _Mn 3 MO ₈ However, M is Ni, Co, Fe, represented by lithium manganate with either at least one metal selected from Cu), copper -Li oxide _(Li 2 _CuO 2), disulfide compounds, Fe ₂ (MoO ₄ ) _{3 and the} like, and compounds such as polyaniline, polypyrrole, and polythiophene can be given, and one or more of these may be used in combination.

  When the positive electrode mixture is applied to the aluminum foil, a slurry whose viscosity is adjusted with a dispersion solvent such as N-methylpyrrolidone (hereinafter abbreviated as NMP) is prepared. In this example, the mixing ratio of the conductive material to the positive electrode active material is adjusted to 5 to 20% by weight, and for example, a stirring means such as a rotary blade is used so that the powder particles of the positive electrode active material are uniformly dispersed in the slurry. It is sufficiently kneaded using the equipped mixer. The sufficiently kneaded slurry is applied to both surfaces of the aluminum foil by, for example, a roll transfer type coating machine. The positive electrode plate 1 is produced by pressing and drying. The thickness (mixture application part) of the positive electrode plate 1 after drying is adjusted to a range of 20 to 150 μm.

  A positive electrode lead 4 for drawing current is joined to a side edge on one side in the longitudinal direction of the aluminum foil by spot welding or ultrasonic welding. The positive electrode lead 4 is formed in a rectangular shape with a metal foil of the same material as that of the positive electrode current collector, that is, an aluminum foil, and is installed in order to take out current from the positive electrode plate 1. In this example, since it is required to flow a large current when applied to a power source for a moving body such as an automobile, a plurality of positive electrode leads 4 are provided.

  On the other hand, the negative electrode plate 2 has a copper foil as a negative electrode current collector. In this example, the thickness of the copper foil is set in the range of 7 to 20 μm. A negative electrode mixture containing a negative electrode active material is applied to both sides of the copper foil substantially evenly. In addition to the negative electrode active material, a binder such as PVDF is blended in the negative electrode mixture. Note that a conductive material such as acetylene black can be blended in the negative electrode mixture.

  As the negative electrode active material, a material capable of inserting and removing lithium ions is used. Such materials include natural graphite, graphitizable materials obtained from petroleum coke and coal pitch coke, etc., mesophase carbon, amorphous carbon, graphite whose surface is coated with amorphous carbon, natural Examples include carbon materials whose surface crystallinity has been lowered by mechanically treating graphite or artificial graphite, carbon fibers, lithium metal, metals alloying with lithium, silicon, or materials having a metal supported on the surface of carbon particles. it can. Examples of the carbon material supporting a metal include a metal or an alloy selected from lithium, aluminum, tin, silicon, indium, gallium, and magnesium. Furthermore, these metals or their oxides can also be used as the negative electrode active material. One or more of these materials may be mixed and used.

  When the negative electrode mixture is applied to the copper foil, a slurry whose viscosity is adjusted with a dispersion solvent such as NMP is produced. In this example, the mixing ratio of the negative electrode active material and the binder is adjusted to 90:10 by weight, and the negative electrode mixture slurry is prepared in the same manner as the positive electrode mixture slurry. The produced slurry is applied to both surfaces of the copper foil by, for example, a roll transfer type coating machine. The negative electrode plate 2 is produced by press drying. The thickness (mixture application part) of the negative electrode plate 2 after pressing and drying is adjusted to a range of 20 to 150 μm. The length of the negative electrode plate 2 and the width of the negative electrode mixture application portion are such that when the positive electrode plate 1 and the negative electrode plate 2 are wound, the positive electrode mixture application portion does not protrude from the negative electrode mixture application portion. In this way, the length of the positive electrode plate 1 and the width of the positive electrode mixture application portion are set longer than each other.

  A negative electrode lead 5 for drawing current is joined to one side edge of the copper foil in the longitudinal direction by spot welding or ultrasonic welding. The negative electrode lead 5 is formed in a rectangular shape with a metal foil of the same material as that of the negative electrode current collector, that is, a copper foil, and is installed to take out current from the negative electrode plate 2. In this example, as with the positive electrode plate 1, a plurality of negative electrode leads 5 are provided.

  In the nonaqueous electrolytic solution poured into the battery container 6, an organic compound having a phosphorus-oxygen double bond and an electrolyte lithium salt are dissolved in an organic solvent. Examples of the organic solvent include cyclic carbonates, chain carbonates, linear carboxylic acid esters, lactones, cyclic ethers, and chain ethers, and one or more of these substances are mixed. It can also be used. Specific examples of the organic solvent include ethylene carbonate (EC), propylene carbonate (PC), gamma butyrolactone, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC). In addition, compounds obtained by substituting some of the functional groups of these compounds with other functional groups, for example, those substituted with a halide such as a fluorine-substituted product or sulfur element can be used. Generally, it is preferable to use a mixed solvent system of a high dielectric constant organic solvent such as cyclic carbonate or cyclic lactone and a low dielectric constant solvent such as chain carbonate or chain ester.

In the organic compound having a phosphorus-oxygen double bond, as represented by the following chemical formula (1), one oxygen atom is bonded to the phosphorus atom by a double bond, and two pentavalent phosphorus atoms have two oxygen atoms. In addition to the oxygen atom bonded by a heavy bond, three alkyl groups (substituents) R ₁ , R ₂ and R ₃ are bonded. The three alkyl groups R ₁ , R ₂ and R ₃ each have 1 to 10 carbon atoms, have at least one oxygen atom, and are hydrogen, sulfur, oxygen, nitrogen, fluorine, chlorine , At least one atom of bromine and iodine.

  Specific examples of such organic compounds include 2-ethylhexyl diphenyl phosphate, cresyl diphenyl phosphate, tris (1,3-dichloro-2-propyl) phosphate, tri-o-cresyl phosphate, triamylphos. Fate, tributyl phosphate, tricresyl phosphate, triethyl phosphate, trimethyl phosphate, triphenyl phosphate, tris (2-butoxyethyl) phosphate, tris (2-chloroethyl) phosphate, tris (2-ethylhexyl) A phosphate etc. can be mentioned, These 1 type (s) or 2 or more types can be mixed and used. The amount of the organic compound having a phosphorus-oxygen double bond is preferably 10 wt% or less in terms of the weight ratio with respect to the non-aqueous electrolyte. When the mixing amount of the organic compound becomes excessive, the decomposition reaction of the organic compound occurs in the initial cycle of the obtained lithium secondary battery, and a gas is generated inside the battery, specifically, between the positive and negative electrodes. May increase, and problems such as deterioration of battery characteristics may occur due to an increase in internal resistance.

Examples of the lithium salt of the electrolyte include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ , Li (CF ₃ SO ₂ ), Li (CF ₃ SO ₂ ) ₂ N, and Li (C ₂ F ₅ SO _2. ) N, LiB (C ₂ O ₄ ) ₂ , LiBF ₂ (C ₂ O ₄ ), LiPF ₂ (C ₂ O ₄ ) _{2 and the} like. Moreover, these lithium salts may be used independently and it is also possible to mix and use 2 or more types. Among lithium salts, it is preferable to use LiBF ₄ which is superior in hydrolysis resistance to LiPF _{6 which} has been conventionally used in consideration of improvement in high-temperature storage characteristics of the battery. The lithium salt concentration (electrolyte concentration) is preferably in the range of 0.6 to 1.5 mol / liter (M).

  Hereinafter, although the example is given and demonstrated about the lithium secondary battery 20 manufactured according to this embodiment, this invention is not restrict | limited to these Examples. In addition, it describes together about the lithium secondary battery of the comparative example manufactured for the comparison.

Example 1
In Example 1, LiMn _1.98 Al _0.02 O ₄ having a spinel crystal structure with an average particle size of 26 μm and a specific surface area of 1.5 m ² / g was used as the positive electrode active material. A slurry was prepared by dispersing a positive electrode active material and a conductive agent in which massive graphite and acetylene black were mixed at a ratio of 9: 2 in a PVDF solution previously dissolved in NMP at a concentration of 5 wt% as a binder. The mixing ratio of the positive electrode active material, the conductive agent, and PVDF was set to 85: 10: 5 by weight. This slurry was applied substantially uniformly and evenly to an aluminum foil (positive electrode current collector) having a thickness of 20 μm. After the coating, the coating was dried at a temperature of 80 ° C., and the coating and drying were performed on both sides of the aluminum foil in the same procedure. After compression molding with a roll press, the positive electrode plate 1 was produced by cutting the coating width to 5.4 cm and the coating length to 50 cm and welding the positive electrode lead 4 made of aluminum foil.

On the other hand, the negative electrode plate 2 was produced by the following method. As the negative electrode active material, natural graphite having an average particle diameter of 20 μm and a specific surface area of 4.0 m ² / g was used. The mixing ratio of the negative electrode active material and PVDF was set to 90:10 by weight. The obtained slurry was applied substantially uniformly and evenly to a rolled copper foil (negative electrode current collector) having a thickness of 10 μm. Coating and drying were performed on both sides of the rolled copper foil in the same procedure as the production of the positive electrode plate 1, and the electrode density of the negative electrode plate 2 was adjusted to 1.3 g / cc. Then, it was compression-molded with a roll press machine, cut to a coating width of 5.6 cm and a coating length of 54 cm, and the negative electrode lead 2 made of copper foil was welded to prepare the negative electrode plate 2.

The non-aqueous electrolyte was produced as follows. LiBF ₄ as an electrolyte was dissolved in a mixed organic solvent in which EC and EMC were mixed at a weight ratio of EC: EMC = 1: 2 so as to have a concentration of 1.0 mol / liter. Vinylene carbonate was mixed so that it might become a ratio of 1 wt% with respect to the weight of the obtained solution. Furthermore, triphenyl phosphate was mixed as an organic compound having a phosphorus-oxygen double bond to prepare a non-aqueous electrolyte. The mixing amount of triphenyl phosphate was set to 5 wt% with respect to the weight of the mixed electrolyte. The battery weight of the lithium secondary battery 20 manufactured using the obtained nonaqueous electrolytic solution was approximately 50 g.

(Example 2)
In Example 2, a lithium secondary battery 20 was manufactured in the same manner as in Example 1 except that the amount of triphenyl phosphate mixed with the non-aqueous electrolyte was set to 10 wt%.

(Example 3)
In Example 3, a lithium secondary battery 20 was manufactured in the same manner as in Example 1 except that the amount of triphenyl phosphate mixed with the nonaqueous electrolyte was set to 20 wt%.

(Comparative Example 1)
In Comparative Example 1, LiBF ₄ as an electrolyte was dissolved in a mixed organic solvent in which EC and EMC were mixed at a weight ratio of EC: EMC = 1: 2 so as to have a concentration of 1.0 mol / liter. A water electrolyte was prepared. A lithium secondary battery was produced in the same manner as in Example 1 using the obtained nonaqueous electrolytic solution.

(Comparative Example 2)
In Comparative Example 2, a non-aqueous electrolyte was prepared as follows. That is, LiPF ₆ as an electrolyte was dissolved in a mixed organic solvent in which EC and EMC were mixed at a weight ratio of EC: EMC = 1: 2 so that the concentration was 1.0 mol / liter. Vinylene carbonate was mixed so that it might become a ratio of 1 wt% with respect to the weight of the obtained solution, and the non-aqueous electrolyte was produced. A lithium secondary battery was produced in the same manner as in Example 1 except that the obtained nonaqueous electrolytic solution was used.

(Evaluation)
The following evaluations were performed on the lithium secondary batteries of the examples and comparative examples. That is, in a constant temperature bath at 25 ° C., a charging current of 1300 mA, a voltage of 4.2 V, and a constant current and a constant voltage of 5 hours were performed, and a discharging current of 1300 mA was performed to a battery voltage of 2.7 V. This charging and discharging process was defined as 1 cycle, and 3 cycles were repeated. Next, the battery was charged to 4.2 V under the same charging conditions, and the discharge capacity when discharged to 2.7 V with a discharge current value of 1300 mA was measured to obtain a 1300 mA discharge capacity. Furthermore, after charging again under the same charging conditions, the discharge capacity when discharged at a discharge current value of 3900 mA was measured to obtain a 3900 mA discharge capacity. The percentage of the 3900 mA discharge capacity to the 1300 mA discharge capacity was taken as the high rate discharge characteristic (%). That is, high rate discharge characteristics (%) = 3900 mA discharge capacity (mAh) / 1300 mA discharge capacity (mAh) × 100. The results of the high rate discharge characteristics are shown in Table 1 below. In Table 1, the mixture represents an organic compound mixed in the non-aqueous electrolyte.

As shown in Table 1, in the lithium secondary batteries 20 of Example 1 and Example 2 in which 5 wt% and 10 wt% of triphenyl phosphate were added, the lithium secondary of Comparative Example 1 in which no triphenyl phosphate was added. It was found that the high rate discharge characteristics were improved compared to the battery. On the other hand, in the lithium secondary battery of Example 3 to which 20 wt% of triphenyl phosphate was added, although the high rate discharge characteristics were improved as compared with the lithium secondary battery of Comparative Example 1, the triphenyl phosphate was not added. it was found to decrease from the lithium secondary battery of Comparative example 2 using LiPF ₆ as a lithium salt. This is considered to be because the viscosity of the non-aqueous electrolyte is increased by adding triphenyl phosphate, and the internal resistance is increased by decreasing the ionic conductivity. Therefore, it was found that in a lithium secondary battery using LiBF ₄ as an electrolyte, high rate discharge characteristics can be improved by adding an organic compound having a phosphorus-oxygen double bond in a proportion of 10% by weight or less. .

In addition, the lithium secondary batteries 20 of Examples 1 and 2 using LiBF ₄ that is a hydrolysis-resistant lithium salt and added with 5 wt% and 10 wt% of triphenyl phosphate also have high temperature storage characteristics. It was confirmed that it was excellent. This is because LiPF _{6 which} is easily hydrolyzed by moisture mixed from the inside or outside of the battery generates hydrofluoric acid by hydrolysis, so that the elution of manganese (Mn) from the positive electrode active material is promoted, and the battery performance This is thought to have led to a decline in sales. On the other hand, since LiBF ₄ is excellent in hydrolysis resistance, it is considered that high temperature storage characteristics are improved by using LiBF ₄ as a lithium salt.

(Action etc.)
Next, the operation and the like of the lithium secondary battery 20 of the present embodiment will be described focusing on the operation and the like of the non-aqueous electrolyte.

In non-aqueous electrolytes conventionally used for lithium secondary batteries, an electrolyte lithium salt is dissolved in an organic solvent, and LiPF ₆ is widely used as the lithium salt. This LiPF ₆ has the advantage that the ionic conductivity of the non-aqueous electrolyte can be increased and side reactions are unlikely to occur on the electrode surface. However, LiPF ₆ has a drawback that it has poor thermal stability and also causes hydrolysis in a non-aqueous electrolyte. When hydrolysis occurs, decomposition products accumulate on the electrode surface, so that the internal resistance of the battery increases and the battery capacity decreases. Although various electrolytes are used to solve these problems, the solubility in organic solvents and the decrease in ionic conductivity, the increase in viscosity, the decrease in electrochemical stability under high-voltage atmosphere, the aluminum collector It may lead to problems such as corrosion, and it is difficult to obtain sufficient battery characteristics in applications where high capacity and charge / discharge with a large current are required.

As a result of intensive studies on non-aqueous electrolyte solutions that can solve these problems, the present inventors have used LiBF ₄ as a lithium salt and have an organic compound having a phosphorus (P) -oxygen (O) double bond. It has been found that by using a nonaqueous electrolytic solution in which a compound is dissolved, the deterioration of the lithium secondary battery over time is small and the storage characteristics and the high rate discharge characteristics are excellent. The high rate discharge characteristic here is the ratio of the discharge capacity when the discharge current value is changed to the discharge capacity at a constant current value. That is, the non-aqueous electrolyte used for the lithium secondary battery 20 of the present embodiment is a non-aqueous electrolyte that can solve the above-described problems.

  In the present embodiment, the non-aqueous electrolyte injected into the battery container 6 of the lithium secondary battery 20 contains an organic compound having a phosphorus-oxygen double bond. The action mechanism of the organic compound having a phosphorus-oxygen double bond is not clear, but a function of improving the ionic conductivity of the non-aqueous electrolyte is recognized. For this reason, since the ionic conductivity of the non-aqueous electrolyte is improved, the internal resistance is reduced in the lithium secondary battery 20 and the discharge characteristics at a large current can be improved.

  Moreover, in this embodiment, the organic compound contained in the non-aqueous electrolyte is adjusted to a ratio of 10% by weight or less. When the content of the organic compound is excessive, a decomposition reaction of the organic compound occurs in the initial cycle of the obtained lithium secondary battery, resulting in an increase in internal resistance and a decrease in battery characteristics. By making content of an organic compound 10 weight% or less, a decomposition reaction can be suppressed and a battery characteristic can be improved (refer Example 1 and Example 2).

Further, in the present embodiment, although not particularly limited to the lithium salt to be mixed with the non-aqueous electrolyte, it is desirable to use LiBF _4. Since LiBF ₄ is superior in hydrolysis resistance compared to LiPF ₆ even if moisture is mixed in the non-aqueous electrolyte, the function of the lithium salt can be maintained (Example 1, Example 2, Comparative Example). 2). When hydrolysis of the lithium salt occurs, decomposition products may accumulate on the electrode surface, which may increase the internal resistance of the battery and decrease the battery capacity. By using a hydrolysis-resistant lithium salt, an increase in internal resistance and a decrease in battery capacity can be suppressed. Considering that hydrolysis in a non-aqueous electrolyte is promoted in a high-temperature atmosphere, high-temperature storage characteristics can be improved by using LiBF ₄ that is excellent in hydrolysis resistance.

  Although not particularly mentioned in the present embodiment, vinylene carbonate, carboxylic acid anhydride, sulfur as a component other than an organic solvent, a lithium salt, and an organic compound having a phosphorus-oxygen double bond in the nonaqueous electrolytic solution. You may make it contain the at least 1 sort (s) of compound selected from a containing compound and a boron containing compound. For example, in the case of vinylene carbonate or propane sultone having a double bond in the molecule, the reductive decomposition product forms a protective film on the negative electrode, so that an effect of suppressing deterioration with time of the lithium battery can be expected.

  Further, in this embodiment, as the organic compound having a phosphorus-oxygen double bond, various examples in which three substituents have 1 to 10 carbon atoms are shown, and among them, all three substituents are the same phenyl group. Specific examples using certain triphenyl phosphates are shown (see each example). The present invention is not limited to this, and the three substituents may have different chemical structures. As for the three substituents of the organic compound, considering the thermal stability of the organic compound and the production of the organic compound, each substituent has the same chemical structure having 6 to 10 carbon atoms. It is preferable.

  Furthermore, in this embodiment, although the cylindrical lithium secondary battery 20 which accommodated the electrode group which wound the positive electrode plate 1 and the negative electrode plate 2 in the battery container 6 was illustrated, this invention is not restrict | limited to this. . For example, a rectangular shape may be used instead of the cylindrical shape, and the electrode group may be formed by stacking instead of winding the positive electrode plate 1 and the negative electrode plate 2. Of course, the positive electrode plate 1 and the negative electrode plate 2 may be wound in a flat shape and accommodated in a rectangular battery container. Moreover, in this embodiment, although the battery container 6 made from SUS was illustrated, as a material of a battery container, you may use stainless steel and aluminum, for example.

  Furthermore, in this embodiment, although the example which uses a microporous polypropylene film for the separator 3 was shown, this invention is not limited to this. For example, a polyethylene film may be used, and a polyethylene film or a polypropylene film may be laminated and used. Moreover, when laminating | stacking the positive electrode plate 1 and the negative electrode plate 2 and comprising an electrode group, these films can also be used in bag shape.

  Furthermore, in the present embodiment, the sizes of the positive electrode current collector, the negative electrode current collector, and the separator are exemplified, but the present invention is not limited to these, and each size is adjusted according to the battery size and the form of the electrode group. You may make it to decide. Of course, the thickness of the positive electrode plate 1 and the negative electrode plate 2 is not limited.

  The present invention provides a non-aqueous electrolyte for a lithium secondary battery capable of improving discharge characteristics at a large current and a lithium secondary battery using the non-aqueous electrolyte. Since it contributes to sales, it has industrial applicability.

DESCRIPTION OF SYMBOLS 1 Positive electrode plate 2 Negative electrode plate 3 Separator 6 Battery can 10 Sealing cover part.
20 Lithium secondary battery

Claims (7)

An organic compound having a phosphorus (P) -oxygen (O) double bond for improving ion conductivity, represented by the following chemical formula (1), wherein the substituents R ₁ , R ₂ and R ₃ are Each is an organic group having 1 to 10 carbon atoms, and has at least one oxygen atom and at least one atom selected from hydrogen, sulfur, nitrogen, fluorine, chlorine, bromine and iodine atoms. A non-aqueous electrolyte for a lithium secondary battery comprising an organic compound.

  At least one solvent selected from a linear or cyclic carbonate compound, an ester compound, an ether compound, and a compound in which a part of the functional group constituting each compound is substituted with another functional group, and a lithium salt of an electrolyte The nonaqueous electrolytic solution according to claim 1, wherein   The non-aqueous electrolyte according to claim 2, wherein the organic compound is contained at a ratio of 10% by weight or less. The non-aqueous electrolyte according to claim 3, wherein the substituents R ₁ , R ₂ and R ₃ all have the same chemical structure having 6 to 10 carbon atoms. The non-aqueous electrolyte according to claim 2, wherein the electrolyte is lithium tetrafluoroborate (LiBF ₄ ).   The non-aqueous electrolyte according to claim 2, further comprising at least one compound selected from vinylene carbonate, carboxylic acid anhydride, sulfur-containing compound, and boron-containing compound.   A positive electrode using a lithium-containing oxide capable of inserting and removing lithium ions in the non-aqueous electrolyte according to any one of claims 1 to 6, and a negative electrode using a material capable of inserting and removing lithium ions Lithium secondary battery characterized in that

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