JP2014049296A - Nonaqueous electrolyte for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte for a lithium ion secondary battery which can obtain sufficient initial discharge capacity, and furthermore which has excellent cycle characteristics even at a low temperature, and also to provide the lithium ion secondary battery.SOLUTION: A nonaqueous electrolyte for a lithium ion secondary battery contains a nonaqueous solvent, an electrolyte, 0.01 to 2 mass% of cyclic carbonate ester having an unsaturated bond and 0.001 to 1 mass% of chain ether having tertiary carbon.

Description

  The present invention relates to a non-aqueous electrolyte for a lithium ion secondary battery and a lithium ion secondary battery.

  Lithium ion secondary batteries are lighter and have higher capacity than nickel cadmium batteries and nickel metal hydride batteries, and thus are widely applied as power sources for portable electronic devices. It is also a promising candidate as a power source for hybrid vehicles and electric vehicles. However, with the recent miniaturization and higher functionality of portable electronic devices, further increase in capacity is expected for lithium ion secondary batteries serving as these power sources.

  A lithium ion secondary battery is mainly composed of a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. Generally, a lithium metal composite oxide is used for the positive electrode, and a carbon material that can occlude and release lithium ions is used for the negative electrode. A liquid electrolyte in which a lithium salt is dissolved in an organic solvent that is liquid at room temperature is used as a non-aqueous electrolyte. However, a side reaction involving an organic solvent occurs on the surface of the carbon material of the negative electrode, which adversely affects the characteristics such as an expected discharge capacity cannot be obtained. For this reason, it is an important issue to form a film on the surface of the negative electrode and to control the state and properties of the film so that the negative electrode does not directly react with the organic solvent.

  In order to form and control the negative electrode surface coating, a special additive is added to the electrolytic solution. For example, it is known that cycle characteristics are improved by adding vinylene carbonate to a non-aqueous electrolyte (see Patent Document 1). The addition of vinylene carbonate is considered to be caused by the formation of a film on the surface of the negative electrode active material and the suppression of side reactions on the negative electrode.

JP 2003-308875 A

  However, in the conventional lithium ion secondary battery described in Patent Document 1 described above, an unnecessary reaction or decomposition of the electrolyte is suppressed by forming a film on the negative electrode surface, and improvement in cycle characteristics at room temperature has been confirmed. However, at low temperatures, there was a problem that the capacity was drastically reduced when charging and discharging were repeated several times.

  The present invention has been made in view of the problems described above, and provides a non-aqueous electrolyte for a lithium ion secondary battery and a lithium ion secondary battery that can provide a sufficient initial discharge capacity and also have good cycle characteristics even at low temperatures. The purpose is to provide.

  In order to solve the above problems, the non-aqueous electrolyte for a lithium ion secondary battery of the present invention comprises 0.01 to 2% by mass of a non-aqueous solvent, an electrolyte, and a cyclic carbonate having an unsaturated bond, 3 0.001 to 1% by mass of a chain ether containing secondary carbon.

  Both the cyclic carbonate having an unsaturated bond and the chain ether containing tertiary carbon contribute to the formation of a film at the interface between the negative electrode active material of the lithium ion secondary battery and the non-aqueous electrolyte. Lithium ions with a sufficient initial discharge capacity can be obtained by making the amount of the cyclic carbonate having an unsaturated bond and the amount of the chain ether containing tertiary carbon within the specified ranges, respectively, to form a film having high lithium ion conductivity. A battery can be obtained. In addition, this film has high lithium ion conductivity even at low temperatures, and has the effect of suppressing side effects such as decomposition of non-aqueous solvents, so that it can realize excellent discharge capacity even after low-temperature charge / discharge cycles. Conceivable.

  The chain ether containing tertiary carbon contained in the non-aqueous electrolyte for lithium ion secondary batteries of the present invention is preferably t-butyl methyl ether. Thereby, sufficient initial discharge capacity can be obtained at room temperature.

  The cyclic carbonate having an unsaturated bond in the non-aqueous electrolyte for a lithium ion secondary battery of the present invention is preferably vinylene carbonate. Thereby, cycle characteristics are improved.

  The non-aqueous electrolyte for a lithium ion secondary battery of the present invention preferably contains ethylene carbonate and ethyl methyl carbonate as a non-aqueous solvent. Ethylene carbonate and ethyl methyl carbonate are a suitable combination for improving the initial discharge capacity and the low-temperature cycle characteristics at the same time.

In the nonaqueous electrolytic solution for a lithium ion secondary battery of the present invention, the electrolyte preferably contains LiPF ₆ . Thereby, electroconductivity becomes high and sufficient initial stage discharge capacity can be obtained.

〔式（１）中、Ｒ_１及びＲ_２は、それぞれ独立に水素原子及び炭素数１〜５の炭化水素基からなる群より選ばれる少なくとも一種を表す。〕 The non-aqueous electrolyte for a lithium ion secondary battery of the present invention preferably further contains 4% by mass or less of an ethylene glycol sulfate derivative represented by the following formula (1).

[In Formula (1), R < ₁ > and R < ₂ > represents at least 1 type chosen from the group which consists of a hydrogen atom and a C1-C5 hydrocarbon group each independently. ]

  The ethylene glycol sulfate derivative contributes to film formation at the interface between the negative electrode active material of the lithium ion secondary battery and the non-aqueous electrolyte, together with a cyclic carbonate having an unsaturated bond and a chain ether containing tertiary carbon. By adding an appropriate amount of an ethylene glycol sulfate derivative, it is expected that the internal impedance of the lithium ion secondary battery is further reduced and the initial discharge capacity is further increased. Moreover, since the formed film becomes stronger, cycle characteristics at a lower temperature can be further improved.

In the ethylene glycol sulfate derivative represented by the formula (1), both R ₁ and R ₂ are preferably hydrogen atoms from the viewpoint of improving cycle characteristics.

  The lithium ion secondary battery of the present invention includes a positive electrode, a negative electrode, a separator, a nonaqueous electrolytic solution having a nonaqueous solvent and an electrolyte, and 0.01 to 2% by mass of a cyclic carbonate having an unsaturated bond. 0.001 to 1% by mass of a chain ether containing tertiary carbon.

  The negative electrode of the lithium ion secondary battery of the present invention preferably contains a carbon material as a main component.

  ADVANTAGE OF THE INVENTION According to this invention, sufficient initial discharge capacity can be obtained, and the non-aqueous electrolyte for lithium ion secondary batteries and a lithium ion secondary battery which have favorable cycling characteristics at low temperature can be provided.

It is a schematic cross section of a lithium ion secondary battery.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. However, the non-aqueous electrolyte for lithium ion secondary batteries and the lithium ion secondary battery according to the present invention are not limited to the following embodiments. In addition, the dimensional ratio of drawing is not restricted to the ratio of illustration.

(Lithium ion secondary battery)
Next, the electrode and the lithium ion secondary battery according to this embodiment will be briefly described with reference to FIG.

The lithium ion secondary battery 100 mainly includes a power generation element 30, a case 50 that houses the power generation element 30 in a sealed state, and a pair of leads 60 and 62 connected to the power generation element 30.

  The power generation element 30 is configured such that a pair of electrodes 10 and 20 are disposed to face each other with the separator 18 interposed therebetween. The positive electrode 10 is a product in which a positive electrode active material layer 14 is provided on a positive electrode current collector 12. The negative electrode 20 is a product in which a negative electrode active material layer 24 is provided on a negative electrode current collector 22. The positive electrode active material layer 14 and the negative electrode active material layer 24 are in contact with both sides of the separator 18. The positive electrode active material layer 14, the negative electrode active material layer 24, and the separator 18 contain a non-aqueous electrolyte for a lithium ion secondary battery (hereinafter referred to as “non-aqueous electrolyte”). Leads 60 and 62 are connected to the end portions of the positive electrode current collector 12 and the negative electrode current collector 22, respectively, and the end portions of the leads 60 and 62 extend to the outside of the case 50.

(Negative electrode)
The negative electrode 20 includes a negative electrode active material layer 24 on both sides of a negative electrode current collector 22. Furthermore, the negative electrode active material layer 24 is formed by applying a paint containing a negative electrode active material, a conductive additive, and a binder to the negative electrode current collector 22.

The negative electrode active material includes at least one selected from carbon materials such as natural graphite, artificial graphite (non-graphitizable carbon, graphitizable carbon, low-temperature calcined carbon, etc.), MCF (mesocarbon fiber), and the like. Among these, artificial graphite is preferable because it exhibits a favorable negative electrode capacity and cycle characteristics, and from the viewpoint of improving electrode density, it is more preferable to use artificial graphite mixed with natural graphite.
In addition, for example, a metal capable of forming a compound with lithium such as Al, Si and Sn, an amorphous compound mainly composed of an oxide such as SiO ₂ and SnO ₂ , lithium titanate (Li ₄ Ti ₅ O _A known negative electrode active material such as ₁₂ ) may be mixed with a carbon material.

  The conductive auxiliary agent is not particularly limited, and a known conductive auxiliary agent can be used. Examples thereof include carbon materials such as pyrolytic carbon such as carbon black, cokes, glassy carbons, organic polymer compound fired materials, carbon fibers, and activated carbon. Moreover, you may add negative electrode active material materials, such as non-graphitizable carbon, easily graphitizable carbon, and graphite, changing a shape.

  As carbon black, acetylene black, ketjen black and the like are particularly preferable, and ketjen black is particularly preferable. By including an electron conductive porous body, pores can be formed at the interface between the particles of the negative electrode active material and the binder, and the penetration of the non-aqueous electrolyte into the negative electrode active material layer 20 is facilitated by the pores. This is preferable.

  The binder is not particularly limited as long as it can bind the negative electrode active material particles and the conductive auxiliary particles. For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoro Examples thereof include fluorine resins such as ethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), and polyvinyl fluoride (PVF). Further, the binder contributes not only to the binding of the particles of the negative electrode active material material and the particles of the conductive additive but also the binding to the negative electrode current collector 22.

  As the negative electrode current collector 22, various known metal foils used for current collectors for lithium ion secondary batteries can be used. Specifically, it is preferable to use a copper foil.

(Positive electrode)
The positive electrode 10 includes a positive electrode active material layer 14 on both surfaces of a positive electrode current collector 12. Furthermore, the positive electrode active material layer 14 is formed by applying a paint containing a positive electrode active material, a conductive additive, and a binder to the positive electrode current collector 32.

The positive electrode active material is composed of lithium ion occlusion and release, lithium ion desorption and insertion (intercalation), or doping and dedoping of a lithium ion and a counter anion (for example, ClO ₄ ⁻ ) of the lithium ion. Is not particularly limited as long as it can reversibly proceed, and a known electrode active material can be used. For example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), general formula: LiNi _x Co _y Mn _z O ₂ (x + y + z = 1) or Li _a Ni _x Co _y Al _1-xy O ₂ (0.98 <a <1.2, 0 <x, y <1) A composite metal oxide, olivine-type LiMPO ₄ (where M is Co, Ni , Mn, Fe or V), and composite metal oxides such as lithium vanadium compounds (LiVOPO ₄ ).

  Furthermore, each constituent element (conductive auxiliary agent and binder) other than the positive electrode active material can use the same substances as those used in the negative electrode 20. Therefore, the binder contained in the positive electrode 10 contributes not only to the binding between the particles of the positive electrode active material and the particles of the conductive auxiliary agent, but also to the binding to the positive electrode current collector 12. Yes.

  As the positive electrode current collector 12, various known metal foils used in current collectors for lithium ion secondary batteries can be used. Specifically, it is preferable to use an aluminum foil.

(Separator)
As long as the separator 18 is formed of an insulating porous body, the material, the manufacturing method, and the like are not particularly limited, and a known separator can be used. For example, as the insulating porous material, a known polyolefin resin, specifically, a crystalline homopolymer or copolymer obtained by polymerizing polyethylene, polypropylene, 1-butene, 4-methyl-1-pentene, 1-hexene, or the like. A polymer is mentioned. These homopolymers or copolymers can be used alone or in combination of two or more. Further, it may be a single layer or a multilayer.

(Nonaqueous electrolyte)
The nonaqueous electrolytic solution comprises a nonaqueous solvent, an electrolyte, 0.01 to 2% by mass of a cyclic carbonate having an unsaturated bond, and 0.001 to 1% by mass of a chain ether containing tertiary carbon. Including.

  Although the mechanism of effect expression by the simultaneous addition of a cyclic carbonate having an unsaturated bond and a chain ether containing a tertiary carbon is not clear, the present inventors consider as follows.

  When an appropriate amount of a cyclic carbonate having an unsaturated bond and a chain ether containing tertiary carbon is contained in the non-aqueous electrolyte, the solvation energy for lithium ions in the non-aqueous electrolyte is reduced, and lithium ions are added to the positive electrode. It is considered that the energy required for inserting is reduced. Therefore, a large initial charge capacity can be obtained.

  Moreover, the cyclic carbonate having an unsaturated bond forms a film on the surface of the negative electrode active material, and the film mainly works effectively for cycle characteristics. When a chain ether containing tertiary carbon is added thereto, a film having high lithium ion conductivity can be formed, and a large charge capacity can be obtained even at a low temperature.

  Normally, the negative electrode cannot fully occlude lithium ions during charging at low temperature, and precipitates as lithium dendrite on the negative electrode surface, thereby causing deterioration of cycle characteristics at low temperature. However, when an appropriate amount of a cyclic carbonate having an unsaturated bond and a chain ether containing tertiary carbon is contained in the nonaqueous electrolytic solution, the precipitation is eliminated and good cycle characteristics are exhibited even at a low temperature.

  If the content of the cyclic carbonate having an unsaturated bond is 0.01% by mass or more, a coating film is formed at the interface between the negative electrode active material of the lithium ion secondary battery and the non-aqueous electrolyte, and during charge / discharge cycles It is possible to minimize a decrease in capacity of the battery. In addition, when the content of the cyclic carbonate having an unsaturated bond is within 2% by mass, an increase in the internal impedance of the lithium ion secondary battery can be suppressed, and a sufficient charge capacity and discharge capacity can be obtained even at a low temperature. .

  The content of the cyclic carbonate having an unsaturated bond is preferably 0.05 to 1% by mass, and more preferably 0.05 to 0.5% by mass.

  If the content of the chain ether containing tertiary carbon is 0.001% by mass, the film formation by the cyclic carbonate having an unsaturated bond is smoothly performed, and as a result, the capacity deterioration during the charge / discharge cycle at a low temperature is suppressed. be able to. Moreover, if the content of the chain ether containing tertiary carbon is within 1% by mass, it is difficult to inhibit the formation of a film of a cyclic carbonate having an unsaturated bond at the negative electrode interface, and the charge capacity and discharge capacity at low temperatures are sufficient. It will be obtained.

  The content of 1,4-dioxane is preferably 0.001 to 0.2% by mass, and more preferably 0.05 to 0.5% by mass.

  From the viewpoint of improving cycle characteristics at −10 ° C., the content of the cyclic carbonate having an unsaturated bond and the chain ether containing a tertiary carbon is 0.05 to 1 mass of the cyclic carbonate having an unsaturated bond. %, And the chain ether containing tertiary carbon is preferably 0.001 to 0.5% by mass, the cyclic carbonate having an unsaturated bond is 0.05 to 0.5% by mass, and 3% The chain ether containing secondary carbon is more preferably 0.05 to 0.5% by mass.

  Examples of the chain ether containing tertiary carbon include t-butyl methyl ether, ethyl t-butyl ether, di-t-butyl ether, ethylene glycol mono-t-butyl ether, and the like, and a sufficient discharge capacity is obtained at room temperature. Therefore, t-butyl methyl ether is preferable.

  Examples of the cyclic carbonate having an unsaturated bond include vinylene carbonate and vinyl ethylene carbonate. From the viewpoint of improving cycle characteristics at low temperature, vinylene carbonate is preferable.

〔式（１）中、Ｒ_１及びＲ_２は、それぞれ独立に水素原子及び炭素数１〜５の炭化水素基からなる群より選ばれる少なくとも一種を表す。〕 The non-aqueous electrolyte preferably further contains 4% by mass or less of an ethylene glycol sulfate derivative represented by the following formula (1).

[In Formula (1), R < ₁ > and R < ₂ > represents at least 1 type chosen from the group which consists of a hydrogen atom and a C1-C5 hydrocarbon group each independently. ]

  Similarly to the cyclic carbonate having an unsaturated bond, the ethylene glycol sulfate derivative also forms a film on the surface of the negative electrode active material, and the film also has an effect of suppressing a decrease in capacity during the charge / discharge cycle. Adding an appropriate amount of an ethylene glycol sulfate derivative to the cyclic carbonate and cyclic ether having an unsaturated bond not only suppresses the capacity decrease during charge / discharge cycles, but also increases the lithium ion conductivity of the coating on the negative electrode surface. The internal impedance of the lithium ion secondary battery can be reduced, and the charge capacity and discharge capacity at low temperatures can be increased.

  When the content of ethylene glycol sulfate is within 4% by mass, an increase in the internal impedance of the lithium ion secondary battery is suppressed, and when a lithium ion secondary battery is produced, sufficient initial discharge capacity and low temperature cycle characteristics are obtained. be able to.

The ethylene glycol sulfate derivatives, ethylene glycol sulfate R ₁ and R ₂ are hydrogen atoms, R ₁ and one propane 1,2 cyclic sulfate as the other is a methyl group a hydrogen atom of R _2, R ₁ and Examples include butane 1,2-cyclic sulfate in which one of R ₂ is a hydrogen atom and the other is an ethyl group, and ethylene glycol sulfate is preferable from the viewpoint of improving cycle characteristics.

  The non-aqueous solvent preferably contains a cyclic carbonate and a low viscosity solvent. The cyclic carbonate has a dielectric constant of 20 or more so as to promote dissociation of the lithium salt as an electrolyte. A low viscosity solvent refers to an organic solvent having a viscosity of 1.0 cP or less so as to improve the mobility of lithium ions.

  As the cyclic carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, and the like can be used. Among them, ethylene carbonate is preferably included. You may mix and use ethylene carbonate with propylene carbonate and butylene carbonate.

  Further, as a low viscosity solvent, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, ethyl propyl carbonate, diallyl carbonate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1 , 2-diethoxyethane can be used, and it is preferable to contain ethyl methyl carbonate from the viewpoint of achieving both sufficient initial discharge capacity and good cycle characteristics at low temperatures. Ethyl methyl carbonate may be used by mixing with other low viscosity solvents.

  The ratio of the cyclic carbonate and the low viscosity solvent in the non-aqueous solvent is preferably 1: 9 to 1: 1 by volume.

As electrolytes, LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAsF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CF ₂ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ Examples thereof include CF ₂ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiN (CF ₃ CF ₂ CO) _{2, and the} like. In particular, it is preferable to include LiPF ₆ because the conductivity becomes high.

When LiPF ₆ is dissolved in a non-aqueous solvent, the concentration of the electrolyte in the non-aqueous electrolyte is preferably adjusted to 0.5 to 2.0 mol / L. When the concentration of the electrolyte is 0.5 mol / L or more, the conductivity of the nonaqueous electrolytic solution can be sufficiently secured, and a sufficient capacity can be easily obtained during charging and discharging. Moreover, by suppressing the electrolyte concentration to within 2.0 mol / L, it is possible to suppress an increase in the viscosity of the non-aqueous electrolyte, to sufficiently secure the mobility of lithium ions, and to obtain a sufficient capacity during charging and discharging. It becomes easy.

Even when LiPF ₆ is mixed with another electrolyte, the lithium ion concentration in the non-aqueous electrolyte is preferably adjusted to 0.5 to 2.0 mol / L, and the lithium ion concentration from LiPF ₆ is 50 mol%. More preferably, it is contained.

  The case 50 seals the power generation element 30 and the non-aqueous electrolyte therein. The case 50 is not particularly limited as long as it can prevent leakage of the nonaqueous electrolyte solution to the outside and entry of moisture and the like into the lithium ion secondary battery 100 from the outside. For example, as the case 50, as shown in FIG. 1, a metal laminate film in which a metal foil 52 is coated with a polymer film 54 from both sides can be used. For example, an aluminum foil can be used as the metal foil 52, and a film such as polypropylene can be used as the synthetic resin film 54. For example, the material of the outer polymer film 54 is preferably a polymer having a high melting point such as polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film 54 is preferably polyethylene or polypropylene.

  The leads 60 and 62 are made of a conductive material such as aluminum or nickel.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated in more detail, this invention is not limited to these Examples at all.

  Non-aqueous electrolytes for lithium ion secondary batteries and lithium ion secondary batteries of Examples 1 to 56 and Comparative Examples 1 to 9 were prepared according to the following procedure.

Example 1
First, a negative electrode was produced. In the production of the negative electrode, artificial graphite (90% by mass) as a negative electrode active material, carbon black (2% by mass) as a conductive additive, and polyvinylidene fluoride (hereinafter referred to as “PVDF”) (8% by mass) as a binder. %) And dispersed in a solvent N-methyl-2-pyrrolidone (hereinafter referred to as “NMP”) to obtain a slurry. The obtained slurry was applied to an electrolytic copper foil as a current collector by a doctor blade method and dried at 110 ° C. After drying, rolling was performed to obtain a negative electrode.

Next, a positive electrode was produced. Also in the production of the positive electrode, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (90% by mass) as the positive electrode active material, carbon black (6% by mass) as the conductive additive, and PVDF (4 as the binder) (Mass%) was mixed and dispersed in NMP to obtain a slurry. The obtained slurry was applied to an aluminum foil as a current collector, dried, rolled, and a positive electrode was obtained.

Next, a non-aqueous electrolyte was prepared. LiPF ₆ was added at a rate of 1.0 mol / L to a solution in which ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 3: 7. Furthermore, 0.01% by mass of vinylene carbonate and 0.001% by mass of t-butyl methyl ether were added to the whole non-aqueous electrolyte to obtain a non-aqueous electrolyte.

  A laminated body (element body) was obtained by laminating a separator made of polyethylene between the obtained negative electrode and positive electrode. The obtained laminate is placed in an aluminum laminate pack, a non-aqueous electrolyte is injected into the aluminum laminate pack, and then vacuum-sealed to produce a lithium ion secondary battery (length: 60 mm, width: 85 mm, thickness: 3 mm). did. In addition, on the film of the aluminum laminate pack, the innermost layer (layer made of modified polypropylene) made of synthetic resin that comes into contact with the non-aqueous electrolyte, the metal layer made of aluminum foil, and the layer made of polyamide are sequentially laminated in this order. The laminated body used was used. And two sheets of this composite packaging film were piled up, and the edge part was produced by thermocompression bonding.

(Examples 2-56 and Comparative Examples 1-9)
Tables 1 to 3 show the combinations of the nonaqueous solvent, the lithium ion concentration, the cyclic carbonate having an unsaturated bond added to the nonaqueous electrolyte, the chain ether containing tertiary carbon, and the ethylene glycol sulfate derivative. Except having changed into, it carried out similarly to Example 1, and produced the lithium ion secondary battery of Examples 2-58 and Comparative Examples 1-10.

  In Tables 1 to 3, “EC” of the nonaqueous solvent represents ethylene carbonate, “EMC” represents ethyl methyl carbonate, “PC” represents propylene carbonate, and “DEC” represents diethyl carbonate. A cyclic carbonate having an unsaturated bond is referred to as “compound A”, “VC” represents vinylene carbonate, and “VEC” represents vinyl ethylene carbonate. A chain ether containing a tertiary carbon is referred to as “compound B”, “tBME” represents t-butyl methyl ether, “EtBE” represents ethyl t-butyl ether, and “DtBE” represents di-t-butyl ether. The ethylene glycol sulfate derivative is “compound C”, “EGS” represents ethylene glycol sulfate, “PCS” represents propane 1,2-cyclic sulfate, and “BCS” represents butane 1,2-cyclic sulfate.

(Discharge capacity evaluation test)
After the production of the lithium ion secondary battery, the first charge was performed in an environment set at 25 ° C. in a thermostatic bath, and the first discharge was performed immediately after that. The charging was performed at a constant current and constant voltage up to 4.2 V at 30 mA, and the discharging was performed at a constant current of 2.5 mA at 30 mA. The measured value of the discharge capacity (mAh) in the first discharge is shown as the initial discharge capacity (mAh) in Tables 1-3.

(-10 ° C cycle test)
After the discharge capacity evaluation test, the battery was put into a thermostat set to −10 ° C., and a cycle test was started after 2 hours. In this cycle test, charging was performed up to 4.2 V in a constant current constant voltage charging mode of 150 mA, and discharging was performed up to 2.5 V in a constant current discharging mode of 150 mA, and this cycle was performed 200 times continuously. The pause time between charging and discharging and between discharging and charging was 10 minutes. After 200 cycles at −10 ° C., the battery was returned to a thermostat set at 25 ° C., and constant current / constant voltage charging was performed up to 4.2 V at 30 mA, and constant current discharging was performed up to 2.5 V at 30 mA. A value obtained by recalculating the discharge capacity at the ratio to the initial discharge capacity at that time is shown as “capacity after −10 ° C. cycle” in Tables 1 to 3.

(Examples 1 to 6 and Comparative Examples 1 and 2)
First, Table 1 shows the evaluation results when the cyclic carbonate having an unsaturated bond is vinylene carbonate and only the content of vinylene carbonate is changed. In addition, the chain | strand-shaped ether containing tertiary carbon was made into t-butyl methyl ether, and it fixed and compared by the structure as shown in Table 1 except content of vinylene carbonate.

  The initial discharge capacity is 141 mAh or less in the comparative example, whereas the example in which the vinylene carbonate content is 0.01 to 2% by mass shows a value of 144 mAh or more, both having sufficient initial discharge capacity. I was able to confirm.

  The capacity after the −10 ° C. cycle is 75% or less in the comparative example, whereas the lithium ion secondary batteries of the examples in which the vinylene carbonate content is 0.01 to 2% by mass are all −10 ° C. The capacity after cycling exceeded 75%. In particular, when the vinylene carbonate content is 0.01 to 1% by mass, the capacity after -10 ° C. cycle exceeds 80%, and when 0.05 to 1% by mass, the capacity after -10 ° C. cycle is obtained. Exceeded 87%, indicating excellent cycle characteristics.

(Examples 4, 7 to 11 and Comparative Examples 3 and 4)
Next, Table 2 shows the evaluation results when the chain ether containing tertiary carbon is t-butyl methyl ether and only the content of t-butyl methyl ether is changed. In addition, the cyclic carbonate which has an unsaturated bond was vinylene carbonate, and it fixed and compared by the structure as shown in Table 2 except content of t-butyl methyl ether.

  The initial discharge capacity is 142 mAh in the comparative example, whereas the example in which the content of t-butyl methyl ether is 0.001 to 1% by mass shows a value of 146 mAh or more. I was able to confirm.

  The capacity after the -10 ° C cycle is less than 75% in the comparative example, whereas the lithium ion secondary batteries of the examples in which the content of t-butyl methyl ether is 0.001 to 1% by mass are all The capacity exceeded 75% after -10 ° C cycling. In particular, when the content of t-butyl methyl ether is 0.01 to 0.5% by mass, the capacity after cycling at −10 ° C. exceeds 80%, and further 0.05 to 0.5% by mass. The capacity after cycling at -10 ° C exceeded 87%, indicating excellent cycle characteristics.

(Examples 12 to 56 and Comparative Examples 5 to 9)
Table 3 shows the combinations of nonaqueous solvents, LiPF ₆ concentration, cyclic carbonates having unsaturated bonds contained in nonaqueous electrolytes, chain ethers containing tertiary carbon, ethylene glycol sulfate derivatives and their contents as shown in Table 3. Each measurement was performed.

  As for the initial discharge capacity, the lithium ion secondary battery of the comparative example has an initial discharge capacity of less than 145 mAh, whereas the addition amount of the cyclic carbonate having an unsaturated bond is 0.01 to 2% by mass, and the tertiary class. The lithium ion secondary batteries of the examples in which the amount of the chain ether containing carbon was 0.001 to 1% by mass exceeded 145 mAh, and showed excellent initial discharge capacity.

  The capacity after cycling at −10 ° C. is that the comparative lithium ion secondary battery has a capacity after cycling at −10 ° C. of 78%, whereas the amount of cyclic carbonate having an unsaturated bond is 0.01-2 mass. % And the addition amount of the chain ether containing tertiary carbon was 0.001 to 1% by mass, and the capacity after -10 ° C. cycle exceeded 75% and showed good values. When the addition amount of the cyclic carbonate having an unsaturated bond is 0.01 to 1% by mass and the addition amount of the chain ether containing tertiary carbon is 0.01 to 0.5% by mass, The capacity exceeds 80%, in particular, the addition amount of the cyclic carbonate having an unsaturated bond is 0.05 to 1% by mass, and the addition amount of the chain ether containing tertiary carbon is 0.05 to 0.5%. In the case of mass%, the capacity after cycling exceeded 87%, and excellent low-temperature cycle characteristics were exhibited.

  DESCRIPTION OF SYMBOLS 10 ... Positive electrode, 20 ... Negative electrode, 12 ... Positive electrode collector, 14 ... Positive electrode active material layer, 18 ... Separator, 22 ... Negative electrode collector, 24 ... Negative electrode Active material layer, 30 ... power generation element, 50 ... case, 60, 62 ... lead, 100 ... lithium ion secondary battery

Claims (15)

A non-aqueous solvent;
Electrolyte,
0.01 to 2% by mass of a cyclic carbonate having an unsaturated bond;
0.001 to 1% by mass of a chain ether containing tertiary carbon,
A non-aqueous electrolyte for a lithium ion secondary battery, comprising:   The non-aqueous electrolyte for a lithium ion secondary battery according to claim 1, wherein the chain ether containing tertiary carbon is t-butyl methyl ether.   The non-aqueous electrolyte for a lithium ion secondary battery according to claim 1 or 2, wherein the cyclic carbonate having an unsaturated bond is vinylene carbonate.   The non-aqueous electrolyte for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the non-aqueous solvent includes ethylene carbonate and ethyl methyl carbonate. The non-aqueous electrolyte for a lithium ion secondary battery according to any one of claims 1 to 4, wherein LiPF ₆ is included as the electrolyte. The non-aqueous electrolyte for a lithium ion secondary battery according to any one of claims 1 to 5, comprising 4% by mass or less of an ethylene glycol sulfate derivative represented by the following formula (1).

[In Formula (1), R < ₁ > and R < ₂ > represents at least 1 type chosen from the group which consists of a hydrogen atom and a C1-C5 hydrocarbon group each independently. ] 7. The non-aqueous electrolyte for a lithium ion secondary battery according to claim 6, wherein in the ethylene glycol sulfate derivative represented by the formula (1), R ₁ and R ₂ are both hydrogen atoms. A positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte;
The non-aqueous electrolyte is
0.01 to 2% by mass of a non-aqueous solvent, an electrolyte, a cyclic carbonate having an unsaturated bond, 0.001 to 1% by mass of a cyclic ether,
A lithium ion secondary battery comprising:   9. The lithium ion secondary battery according to claim 8, wherein the chain ether containing tertiary carbon is t-butyl methyl ether.   10. The lithium ion secondary battery according to claim 8, wherein the cyclic carbonate having an unsaturated bond is vinylene carbonate.   The lithium ion secondary battery according to any one of claims 8 to 10, comprising ethylene carbonate and ethyl methyl carbonate as the non-aqueous solvent. The lithium ion secondary battery according to any one of claims 8 to 11, wherein LiPF ₆ is included as the electrolyte.   The lithium ion secondary battery according to claim 8, wherein the negative electrode is made of a carbon material. 14. The lithium ion secondary battery according to claim 8, comprising 4% by mass or less of an ethylene glycol sulfate derivative represented by the following formula (1).

[In Formula (1), R < ₁ > and R < ₂ > represents at least 1 type chosen from the group which consists of a hydrogen atom and a C1-C5 hydrocarbon group each independently. ] The lithium ion secondary battery according to claim 14, wherein in the ethylene glycol sulfate derivative of the formula (1), R ₁ and R ₂ are both hydrogen atoms.

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