JP2018116929A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a non-aqueous electrolyte secondary battery provided with a non-aqueous electrolyte solution having a composition able to achieve high battery performance even in an extremely low temperature region (for example, -30°C or lower).SOLUTION: The non-aqueous electrolyte solution disclosed herein contains, as non-aqueous solvents, ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and ethyl propionate (EP), and when the total volume of the non-aqueous solvents is 100 vol.%, the content of EC is 20 to 30 vol.%, the content of PC is 5 to 10 vol.%, the content of EP is 5 to 10 vol.%, and the content of DMC+EMC is 50 to 70 vol.%.SELECTED DRAWING: None

Description

  The present invention relates to a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery. In detail, it is related with the electrolyte solution composition of a nonaqueous electrolyte secondary battery.

  In recent years, demand for non-aqueous electrolyte secondary batteries has been increasing as so-called portable power sources for personal computers and portable terminals, or power sources for driving vehicles. In particular, a lithium ion secondary battery that is lightweight and obtains a high energy density is preferably used as a high-output power source for driving vehicles such as electric vehicles and hybrid vehicles.

  A non-aqueous electrolyte provided in this type of secondary battery is selected and used as a solvent and a supporting salt (electrolyte) from the viewpoint of improving battery performance and durability. For example, Patent Document 1 is provided for the purpose of improving the ionic conductivity of a non-aqueous electrolyte in a low temperature region by suppressing an increase in viscosity. A non-aqueous electrolyte for a lithium ion secondary battery is described which is a solvent and 10 wt% to 90 wt% is a carbonate solvent.

Special table 2015-528640 gazette

However, the technique disclosed in Patent Document 1 does not assume a case where the battery is used in an extremely low temperature range of −30 ° C. or lower (for example, −40 ° C. to −30 ° C.). There are no data on the behavior of non-aqueous electrolytes. Therefore, with the idea of simply mixing an ester solvent and a carbonate solvent as described in Patent Document 1, a non-aqueous electrolyte for realizing stable battery performance even in such an extremely low temperature range It is difficult to provide.
Therefore, the present invention has been made in view of such circumstances, and stably exhibits good battery performance even in an extremely low temperature range of −30 ° C. or lower (eg, −40 ° C. to −30 ° C.). It aims at providing the non-aqueous electrolyte for secondary batteries of the composition obtained. Furthermore, an object of the present invention is to provide a non-aqueous electrolyte secondary battery having such a non-aqueous electrolyte and excellent in low temperature characteristics.

The present inventors have studied various carbonate solvents and ester solvents that can be used as nonaqueous solvents in nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries. And the non-aqueous electrolyte of the composition which can implement | achieve a high battery performance also in a cryogenic region (for example, -30 degrees C or less) was created, and it came to complete this invention.
That is, the invention disclosed herein relates to a non-aqueous electrolyte secondary battery including a non-aqueous electrolyte containing a non-aqueous solvent and an electrolyte, and the non-aqueous solvent includes a cyclic carbonate solvent and a chain carbonate solvent. It is prepared mainly with a solvent and further an ester solvent.

In a preferred embodiment of the non-aqueous electrolyte secondary battery disclosed herein, the non-aqueous solvent includes at least ethylene carbonate (EC) and propylene carbonate (PC) as a cyclic carbonate solvent, and as a chain carbonate solvent. It contains at least dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC), and further contains at least ethyl propionate (EP) as an ester solvent.
According to the non-aqueous electrolyte secondary battery in which these five solvents are blended, the battery performance in the cryogenic temperature region can be improved.

One particularly preferred embodiment of the non-aqueous electrolyte secondary battery disclosed herein is that each content rate when the entire non-aqueous solvent is 100 vol%,
Ethylene carbonate (EC) 20-30 vol%;
Propylene carbonate (PC) 5-10 vol%;
Ethyl propionate (EP) 5-10 vol%;
Sum of dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC), ie DMC + EMC 50-70 vol%;
It is characterized by being.
By adopting a non-aqueous electrolyte formed by blending each of the above solvents at such a blending ratio, good battery performance in a cryogenic temperature region is achieved while maintaining the flash point of the electrolyte at 21 ° C. or higher. The non-aqueous electrolyte secondary battery can be provided.

In a further preferred embodiment of the non-aqueous electrolyte secondary battery disclosed herein, the dimethyl carbonate (DMC) content and the ethyl methyl carbonate (EMC) content are both 25 to 35 vol%. Features.
By blending DMC and EMC together at such a content (preferably substantially equal), stable battery performance (for example, stable high-rate charge / discharge) is realized.

In a particularly preferred embodiment of the non-aqueous electrolyte secondary battery disclosed herein, the solidification point of the non-aqueous electrolyte is less than −40 ° C.
By preparing the nonaqueous electrolyte so that the freezing point under atmospheric pressure is less than −40 ° C. (more preferably less than −45 ° C., for example, −60 ° C. or more and less than −45 ° C.), −30 ° C. or less (for example, − The battery characteristics in an extremely low temperature region such as 30 ° C. to −40 ° C. can be further improved.

Further, in another particularly preferable embodiment of the non-aqueous electrolyte secondary battery disclosed herein, the ionic conductivity (mS / cm) at −40 ° C. of the non-aqueous electrolyte is 1.0 or more. Features. Here, the measurement of ion conductivity is based on the AC impedance method.
By maintaining high ionic conductivity in the cryogenic temperature region, it is possible to maintain good output characteristics in the cryogenic temperature region.

  The non-aqueous electrolyte secondary battery (for example, lithium ion secondary battery) disclosed here can exhibit good battery performance in a very low temperature range (for example, −40 ° C. to −30 ° C.). Therefore, for example, electric power for driving a motor can be supplied in a cryogenic temperature region, and the power can be suitably used as a vehicle driving power source in a cold region where high performance is required.

It is a perspective view which shows typically the battery external shape in one Embodiment (lithium ion secondary battery) of the non-aqueous-electrolyte secondary battery disclosed here. It is a longitudinal cross-sectional view which follows the II-II line | wire in FIG. It is a schematic diagram which shows the structure of the wound electrode body which concerns on one Embodiment.

Hereinafter, as a preferred embodiment of the non-aqueous electrolyte secondary battery disclosed herein, the configuration of a sealed prismatic lithium ion secondary battery will be described in detail with reference to the drawings. Matters necessary for implementation other than matters specifically mentioned in the present specification can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in the present specification and common general technical knowledge in the field.
In addition, when the numerical range is described as A to B (here, A and B are arbitrary numerical values) in the present specification and claims, it is the same as a general interpretation. That means.

In the present specification, the “non-aqueous electrolyte secondary battery” refers to a secondary battery in which a solvent constituting the electrolyte mainly includes a non-aqueous solvent (that is, an organic solvent). Here, the “secondary battery” refers to a power storage device that can be charged and discharged and can repeatedly extract predetermined electric energy. For example, a lithium ion secondary battery, a sodium ion secondary battery, and the like, in which alkali metal ions in the non-aqueous electrolyte are responsible for charge transfer, are typical examples included in the non-aqueous electrolyte secondary battery herein.
The “electrode body” refers to a structure constituting the main body of a battery including a porous insulating layer that can function as a separator between a positive electrode, a negative electrode, and positive and negative electrodes. “Positive electrode active material” or “negative electrode active material” (these may be collectively referred to as “electrode active material”) are chemical species that serve as charge carriers (for example, lithium ion, sodium in a lithium ion secondary battery). A compound (positive electrode active material or negative electrode active material) capable of reversibly occluding and releasing (sodium ion in an ion secondary battery).

Hereinafter, as a typical example of a nonaqueous electrolyte secondary battery, a case where the present invention is applied to a lithium ion secondary battery having a configuration in which a wound electrode body and a nonaqueous electrolyte are housed in a rectangular battery case is described. Although an embodiment of the present invention will be specifically described mainly, it is not intended to limit the present invention to such an embodiment.
For example, the wound electrode body is an example, and the technical idea of the present invention can be applied to other shapes (for example, stacked electrode bodies). Moreover, there is no restriction | limiting in particular in the shape (outer shape and size) of a nonaqueous electrolyte secondary battery.

As shown in FIG. 1 and FIG. 2, the lithium ion secondary battery 100 according to the present embodiment has an electrode body (winding electrode body) 20 that is wound flatly and has a flat corner together with a non-aqueous electrolyte. It has a configuration housed in a battery case 30 having a shape (box shape).
The battery case 30 includes a flat rectangular parallelepiped battery case main body 32 having an open upper end, and a lid 34 that closes the opening. On the upper surface (that is, the lid 34) of the battery case 30, a positive terminal 42 for external connection that is electrically connected to the positive electrode of the wound electrode body 20, and a negative electrode that is electrically connected to the negative electrode of the wound electrode body 20. A terminal 44 is provided. The lid 34 is also provided with a safety valve 36 for discharging the gas generated inside the battery case 30 to the outside of the battery case 30 as in the case of the battery case of the conventional lithium ion secondary battery.
Examples of the material of the battery case 30 include metal materials such as aluminum and steel; resin materials such as polyphenylene sulfide resin and polyimide resin. The shape of the case (outer shape of the container) may be, for example, a circular shape (cylindrical shape, coin shape, button shape), hexahedron shape (rectangular shape, cubic shape), bag shape, and a shape obtained by processing and deforming them. Good.

As shown in FIG. 3, the wound electrode body 20 according to the present embodiment has a long sheet structure (sheet-like electrode body) in a stage before assembly. The wound electrode body 20 has a positive electrode sheet 50 in which a positive electrode active material layer 54 is formed along the longitudinal direction on one or both surfaces (here, both surfaces) of a metal positive electrode current collector 52 such as long aluminum. A negative electrode sheet 60 in which a negative electrode active material layer 64 is formed along the longitudinal direction on one or both surfaces (here, both surfaces) of a long metal negative electrode current collector 62 such as copper. The sheet is overlapped via the separator sheet 70, wound in the longitudinal direction, and formed into a flat shape.
In the central portion of the wound electrode body 20 in the winding axis direction, there is a wound core portion (that is, the positive electrode active material layer 54 of the positive electrode sheet 50, the negative electrode active material layer 64 of the negative electrode sheet 60, and the separator sheet 70). A densely stacked portion) is formed. Further, at both ends of the wound electrode body 20 in the winding axis direction, the positive electrode active material layer non-formation part 52a in the positive electrode sheet 50 and the negative electrode active material layer non-formation part 62a in the negative electrode sheet 60 are respectively from the wound core part. It protrudes outward. The positive electrode active material layer non-formation part 52a and the negative electrode active material layer non-formation part 62a are respectively provided with a positive electrode current collector plate 42a and a negative electrode current collector plate 44a, and a positive electrode terminal 42 (FIG. 2) and a negative electrode terminal 44 (FIG. 2) are electrically connected to each other.

The positive electrode active material layer 54 includes at least a positive electrode active material. As a positive electrode active material, 1 type, or 2 or more types of various materials which can be used as a positive electrode active material of a lithium ion secondary battery can be employ | adopted without limitation. Preferred examples include lithium composite metal oxides such as layered and spinel (LiNiO ₂ , LiCoO ₂ , LiFeO ₂ , LiMn ₂ O ₄ , LiNi _0.5 Mn _1.5 O ₄ , LiCrMnO ₄ , LiFePO _4, etc.). Can be mentioned. For example, a lithium nickel cobalt manganese composite oxide (for example, LiNi _1/3 Co _1/2 ) having a layered structure (typically a layered rock salt structure belonging to a hexagonal system) containing Li, Ni, Co and Mn as constituent elements. ₃ Mn _1/3 O ₂₎ is excellent in thermal stability, and the theoretical energy density is high the preferred embodiment.

Although the property of a positive electrode active material is not specifically limited, For example, it may be a particle form or a powder form. The average particle diameter of the particulate positive electrode active material may be 20 μm or less (typically 1 μm to 20 μm, for example, 5 μm to 15 μm). The specific surface area is 0.1 m ² / g or more (typically 0.7 m ² / g or more, for example, 0.8 m ² / g or more), and 5 m ² / g or less (typically 1.m ² / g or more). 3 m ² / g or less, for example 1.2 m ² / g or less). In the present specification, the “average particle size” means a particle size distribution corresponding to 50% cumulative from the fine particle side in a volume-based particle size distribution measured by a particle size distribution measurement based on a general laser diffraction / light scattering method. (D ₅₀ particle diameter, also called median diameter).

  The positive electrode active material layer 54 contains, in addition to the positive electrode active material, one or more materials that can be used as a constituent component of the positive electrode active material layer 54 in a general lithium ion secondary battery as necessary. Can do. Examples of such a material include a conductive material and a binder. As the conductive material, for example, carbon materials such as various carbon blacks (typically acetylene black and ketjen black), coke, activated carbon, graphite, carbon fiber, and carbon nanotube can be suitably used. As the binder, for example, a vinyl halide resin such as polyvinylidene fluoride (PVdF); a polyalkylene oxide such as polyethylene oxide (PEO); and the like can be preferably used.

The proportion of the positive electrode active material in the entire positive electrode active material layer 54 is suitably about 60% by mass or more (typically 60% by mass to 99% by mass), and usually about 70% by mass to 95% by mass. % Is preferred. In the case of using a conductive material, the ratio of the conductive material to the entire positive electrode active material layer 54 can be, for example, about 2% by mass to 20% by mass, and usually about 3% by mass to 10% by mass. preferable. When using a binder, the ratio of the binder to the whole positive electrode active material layer 54 can be, for example, about 0.5 mass% to 10 mass%, and usually about 1 mass% to 5 mass%. preferable.
The average thickness per one side of the positive electrode active material layer 54 is, for example, 20 μm or more (typically 40 μm or more, preferably 50 μm or more), and may be 100 μm or less (typically 80 μm or less). The density of the positive electrode active material layer 54 can be, for example, 1 g / cm ^{3 to} 4 g / cm ³ (eg, 1.5 g / cm ^{3 to} 3.5 g / cm ³ ). Moreover, the porosity (porosity) of the positive electrode active material layer 54 can be, for example, 10 to 50 vol% (typically 20 to 40 vol%). By satisfying such properties, an appropriate gap can be maintained in the positive electrode active material layer 54, and the nonaqueous electrolyte can be sufficiently infiltrated. For this reason, a wide reaction field with the charge carrier can be secured, and high input / output characteristics can be exhibited even in an extremely low temperature region. Incidentally, the "porosity" in the present specification, multiplied by 100 and dividing the total pore volume obtained by the measurement of the mercury porosimeter a (cm ³⁾ in the apparent volume of the active material layer (cm ³⁾ value Say.

  The method for producing the positive electrode sheet 50 is not particularly limited. For example, a positive electrode active material and a material used as necessary are dispersed in a suitable solvent (for example, N-methyl-2-pyrrolidone), and a slurry-like or paste-like composition (hereinafter referred to as “positive electrode active material layer forming slurry”). "). The positive electrode active material layer forming slurry is applied to the elongate positive electrode current collector 52, the solvent contained in the slurry is removed, dried, and pressed as necessary. The positive electrode sheet 50 provided with the positive electrode active material layer 54 having desired properties can be produced.

On the other hand, the negative electrode active material layer 64 contains at least a negative electrode active material. As the negative electrode active material, one or more of various materials that can be used as the negative electrode active material of the lithium ion secondary battery can be used without particular limitation. Preferable examples include graphite (graphite), non-graphitizable carbon (hard carbon), graphitizable carbon (soft carbon), carbon nanotubes, at least part of those having a combination of these, etc. The carbon material containing is mentioned. Since high energy density is obtained, graphite-based materials such as natural graphite (graphite) and artificial graphite can be preferably used.
As the negative electrode active material, it is particularly preferable to use an amorphous coated graphite material having amorphous carbon on the surface of graphite. As a method for forming such an amorphous coat, for example, a vapor phase method such as a CVD method in which a vapor phase coating material is deposited on the surface of the core material (graphite particles) in an inert gas atmosphere, a coating material is suitable. After the solution diluted with the solvent is mixed with the core material, the liquid phase method of firing and carbonizing the coating material in an inert gas atmosphere, after kneading the core material and the coating material without using a solvent A conventionally known method such as a solid phase method of firing and carbonizing in an inert gas atmosphere can be appropriately employed.

Although the property of a negative electrode active material is not specifically limited, For example, it may be a particle form or a powder form. The average particle diameter of the particulate negative electrode active material can be, for example, 50 μm or less (typically 20 μm or less, for example, 1 μm to 20 μm, preferably 5 μm to 15 μm). The specific surface area is 1 m ² / g or more (typically 2.5 m ² / g or more, for example, 2.8 m ² / g or more), and 10 m ² / g or less (typically 3.5 m ² / G or less, for example 3.4 m ² / g or less).

  The negative electrode active material layer 64 contains, in addition to the above negative electrode active material, one or more materials as necessary that can be used as a constituent component of the negative electrode active material layer in a general lithium ion secondary battery. obtain. Examples of such materials include binders and various additives. As the binder, for example, a polymer material such as styrene butadiene rubber (SBR), polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), or the like can be suitably used. In addition, various additives such as a thickener, a dispersant, and a conductive material can be used as appropriate. For example, carboxymethylcellulose (CMC) or methylcellulose (MC) can be suitably used as the thickener.

The proportion of the negative electrode active material in the entire negative electrode active material layer 64 is suitably about 50% by mass or more, and is usually 90% by mass to 99% by mass (eg, 95% by mass to 99% by mass). Is preferred. When using a binder, the ratio of the binder to the whole negative electrode active material layer 64 can be, for example, about 1% by mass to 10% by mass, and usually about 1% by mass to 5% by mass is preferable. .
The average thickness per one side of the negative electrode active material layer 64 is, for example, 40 μm or more (typically 50 μm or more), and can be 100 μm or less (typically 80 μm or less). Moreover, the density of the negative electrode active material layer 64 can be set to, for example, about 0.5 g / cm ^{3 to} 2 g / cm ³ (typically 1 g / cm ^{3 to} 1.5 g / cm ³ ). Moreover, the porosity of the negative electrode active material layer 64 can be about 5-50 vol% (preferably 35-50 vol%), for example. Moreover, by satisfy | filling such a property, a moderate space | gap can be maintained in the negative electrode active material layer 64, and a non-aqueous electrolyte can fully be infiltrated. For this reason, a wide reaction field with the charge carrier can be secured, and high input / output characteristics can be exhibited even in an extremely low temperature region.

  The method for producing the negative electrode sheet 60 is not particularly limited. For example, a negative electrode active material and a material used as needed are dispersed in a suitable solvent (for example, distilled water), and a slurry-like or paste-like composition (hereinafter referred to as “a slurry for forming a negative electrode active material layer”). Prepare. The slurry for forming the negative electrode active material layer is applied to the long negative electrode current collector 62, the solvent contained in the slurry is removed, dried, and pressed as necessary. The negative electrode sheet 60 provided with the negative electrode active material layer 64 can be produced.

The separator sheet 70 interposed between the positive and negative electrode sheets 50 and 60 may be any sheet as long as it insulates the positive electrode active material layer 54 and the negative electrode active material layer 64 and has a nonaqueous electrolyte holding function and a shutdown function. . Preferable examples include porous resin sheets (films) made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide.
The property of the separator sheet 70 is not particularly limited. For example, the average thickness is usually 10 μm or more (typically 15 μm or more, for example, 17 μm or more), and preferably 40 μm or less (typically 30 μm or less, for example, 25 μm or less).

Next, the nonaqueous electrolytic solution according to this embodiment will be described in detail.
The non-aqueous electrolyte is composed of a non-aqueous solvent and an electrolyte (that is, a supporting salt), and the non-aqueous solvent is mainly composed of a cyclic carbonate solvent, a chain carbonate solvent, and an ester solvent. Is done.
By allowing the cyclic carbonate solvent and the chain carbonate solvent to coexist, a suitable high dielectric constant and low viscosity can be achieved. Furthermore, by further coexisting an ester solvent, the freezing point (and melting point) can be lowered, and the battery can be suitably used in a low temperature range.

Preferably, at least ethylene carbonate (EC) and propylene carbonate (PC) are included as the cyclic carbonate solvent. The combination of EC having a high dielectric constant and PC having a low freezing point (and melting point) contributes to improvement of battery performance in a cryogenic temperature range. Furthermore, the addition of PC is preferable from the viewpoint of improving durability (for example, storage characteristics and cycle characteristics).
Preferably, the chain carbonate contains at least dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC). A combination of DMC and EMC having a high oxidation potential (wide potential window) is also preferable from the viewpoint of improving battery performance.
Preferably, at least ethyl propionate (EP) is included as an ester solvent. By adding EP, the freezing point (and melting point) of the nonaqueous electrolytic solution and the viscosity in the low temperature region can be suitably reduced. Furthermore, durability (for example, storage characteristics and cycle characteristics) can be improved satisfactorily by adding EP in combination with PC.
According to the non-aqueous electrolyte in which these five types of solvents coexist, excellent performance (for example, good output) can be obtained even in an extremely low temperature range (for example, −40 ° C. to −30 ° C.). Maintenance) can be realized. Furthermore, durability characteristics (for example, cycle characteristics and storage characteristics) can be improved.

More preferably, the content of each of the above five solvents when the total amount of the non-aqueous solvent is 100 vol% is
EC 20-30 vol%,
PC 5-15 vol% (eg 5-10 vol% or 10-15 vol%),
EP 5-15 vol% (e.g. 5-10 vol% or 10-15 vol%) and the sum of DMC and EMC 50-70 vol%,
A non-aqueous electrolyte is prepared so that
According to the non-aqueous electrolyte solution having such a composition, it is possible to achieve good battery performance in a cryogenic temperature region while maintaining the flash point of the electrolyte solution at 21 ° C. or higher. It is particularly preferable that the PC and EP contents are 5 to 10 vol%, respectively. Furthermore, good durability (for example, storage characteristics and cycle characteristics) can be realized.

As long as the non-aqueous electrolyte is mainly prepared from the above five solvents, other organic solvent components may be contained.
Examples of such optional components that can be added include 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, which are cyclic carbonate solvents. -Fluoro-1,3-dioxolan-2-one, trans or cis-4,5-difluoro-1,3-dioxolan-2-one, vinylene carbonate, vinylethylene carbonate, 4-ethynyl-1,3-dioxolane- 2-one etc. are mentioned.
Other optional components that can be added include chain carbonate solvents such as diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, ethyl propyl carbonate, methyl butyl carbonate, and dibutyl carbonate. .
Other optional components that can be added include ester solvents such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, δ-valerolactone, ε-caprolactone and the like.

Examples of the electrolyte (that is, the supporting salt) include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ). ₃ , 1 type or 2 types or more of lithium compounds (lithium salt), such as LiI, can be used. LiPF ₆ may be mentioned as particularly preferred electrolyte. The concentration of the electrolyte is not particularly limited, but can be about 0.1 to 5 mol / L (for example, 0.5 to 3 mol / L, typically 0.8 to 1.5 mol / L). .
When the non-aqueous electrolyte secondary battery is a sodium ion secondary battery, an electrolyte (supporting salt) containing Na such as NaPF ₆ may be used.

  In addition, the nonaqueous electrolytic solution may contain an optional additive component as necessary as long as the object of the present invention can be realized. For example, various additives can be incorporated for the purpose of improving the output performance of the battery, improving the storage stability (suppressing the capacity decrease during storage, etc.), improving the cycle characteristics, and improving the initial charge / discharge efficiency. Examples of such additives include gas generating agents, film forming agents, dispersing agents, thickeners and the like. And the non-aqueous electrolyte of the target composition can be prepared by mixing the various non-aqueous solvent and electrolyte mentioned above, and the various additives added as needed.

  A lithium ion secondary battery as illustrated is manufactured using the various materials and members described above. Such a manufacturing method (that is, a method of constructing a lithium ion secondary battery) itself can be carried out by appropriately adopting a conventionally used method and does not characterize the present invention. Is omitted.

  Although the lithium ion secondary battery disclosed here can be used for various applications, it has excellent battery performance in an extremely low temperature range (particularly −30 ° C. or less, for example, −40 ° C. to −30 ° C.). It can be suitably used as a power source for driving a vehicle in the area. Although the kind of vehicle is not specifically limited, For example, a plug-in hybrid vehicle (PHV), a hybrid vehicle (HV), an electric vehicle (EV), an electrically assisted bicycle, an electric wheelchair, an electric railway etc. are mentioned. A lithium ion secondary battery provided in a vehicle is normally used in the form of a battery pack in which a plurality of battery packs are connected, and the structure of the battery pack is not shown.

  Hereinafter, some test examples relating to the present invention will be described, but the present invention is not intended to be limited to those shown in the specific examples.

<Preparation of non-aqueous electrolyte>
First, LiPF ₆ is used as an electrolyte, and ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and ethyl propionate (EP) are used as non-aqueous solvents. A total of 14 types of non-aqueous electrolytes for lithium ion secondary batteries (Examples 1 to 14) were prepared by mixing at a blending ratio (volume ratio) shown in Table 1. The concentration of LiPF ₆ was 1.1M in all cases. The freezing point (° C.) and the flash point temperature range of the non-aqueous electrolyte of each example under atmospheric pressure (21 ° C. or higher is indicated by ◯, and less than 21 ° C. is indicated by ×) are as shown in the corresponding columns of Table 1.

As shown in Table 1, the freezing point of the entire non-aqueous electrolyte can be lowered by increasing the blending ratio of EP or EMC having a low melting point. However, the flash point temperature range is less than 21 ° C. (Examples 7, 13, and 14), which can be said to be an unfavorable blend from the viewpoint of safety.
On the other hand, in Example 1 where both PC and EP were blended at 10 vol% and Example 11 where both PC and EP were blended at 5 vol%, the freezing point was −49 ° C. (Example 1) or without lowering the flash point temperature range to less than 21 ° C. It can be lowered to −44 ° C. (Example 11), and it can be seen that this is a desirable blend.
Moreover, although detailed data are not shown, it is suitable as a low-viscosity non-aqueous electrolyte exhibiting good viscosity even in a low temperature range of 0 ° C. or lower.

<Ion conductivity evaluation>
The ionic conductivity [mS / cm] of the non-aqueous electrolytes of Examples 1 to 14 prepared above was measured. The measurement was performed by an AC impedance method using a sealed two-electrode cell manufactured by Toyo System. The non-aqueous electrolyte to be tested was controlled to a certain size and thickness by a polytetrafluoroethylene spacer, and was sandwiched between a pair of stainless steel electrodes. For the Cole-Cole plot obtained by applying a voltage of 10 mV to this cell and changing the frequency from 1 MHz to 10 mV, the bulk resistance is obtained by curve fitting using an equivalent circuit, and the ionic conductivity is determined. Calculated.
The measurement temperature was in three stages of −40 ° C., −30 ° C., and 25 ° C., and the ion conductivity was measured at each measurement temperature. The obtained results are shown in Table 2.

As shown in Table 2, the nonaqueous electrolytic solution of Example 1 containing PC and EP at a mixing ratio of 10 vol% each had relatively good ionic conductivity at −30 ° C. and −40 ° C. Similarly, the non-aqueous electrolyte of Example 11 containing PC and EP at a mixing ratio of 5 vol% each also had relatively good ionic conductivity at −30 ° C. and −40 ° C.
In contrast, the non-aqueous electrolytes of Example 3, Example 7 and Example 13 also have good ionic conductivity at −30 ° C. and −40 ° C., but in Example 3, the freezing point is relative to that of Example 1. Since it is not so good in use in a cryogenic temperature range (for example, −40 ° C. to −30 ° C.), and for Example 7 and Example 13, the flash point temperature range is less than 21 ° C. as described above. It is not preferable.

<Construction of lithium ion secondary battery>
LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (LNCM) was prepared as a positive electrode active material powder. Such LNCM, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder are kneaded so as to have a mass ratio of LNCM: AB: PVdF = 94: 3.5: 2.5. The slurry was put into a machine and kneaded while adjusting the viscosity with N-methylpyrrolidone (NMP) to prepare a positive electrode active material layer forming slurry. A positive electrode active material layer having a density of about 2.9 g / cm ³ is formed on the positive electrode current collector by applying the slurry to both surfaces of an aluminum foil (positive electrode current collector) having a thickness of 12 μm and pressing the slurry after drying. The positive electrode sheet which has this was produced.

Spherical natural graphite (C) whose surface was coated with amorphous carbon was prepared as a negative electrode active material powder. The graphite (C), the stadiene butadiene rubber (SBR) as the binder, and the CMC as the thickener are set to a mass ratio of C: SBR: CMC = 99: 0.5: 0.5. The slurry was put into a kneader and kneaded while adjusting the viscosity with ion exchange water to prepare a negative electrode active material layer forming slurry. A negative electrode active material layer having a density of about 1.4 g / cm ³ is formed on the negative electrode current collector by applying the slurry to both surfaces of a copper foil (negative electrode current collector) having a thickness of 8 μm and pressing the slurry after drying. The negative electrode sheet which has this was produced.

The positive electrode sheet and the negative electrode sheet prepared above are wound together with two separator sheets having a thickness of 3 μm (PP / PE / PP) made of polyethylene (PE) and polypropylene (PP), and formed into a flat shape. Thus, an electrode body was produced.
Next, the positive electrode terminal and the negative electrode terminal were attached to the battery case lid made of aluminum, and these terminals were welded to the positive electrode current collector and the negative electrode current collector exposed at the ends of the wound electrode body, respectively. The wound electrode body thus connected to the lid body was accommodated in the inside of the aluminum battery case body through the opening, and the opening and the lid were welded.
And from the electrolyte solution injection hole provided in the said cover body, the nonaqueous electrolyte solution in any one of Example 1- Example 14 shown in Table 1 was inject | poured, and the said electrolyte solution injection hole was sealed airtightly. Thus, the battery assemblies of Examples 1 to 14 corresponding to the non-aqueous electrolytes of Examples 1 to 14 were constructed.

For the battery assemblies according to Examples 1 to 14, the conditioning process was performed at a rate of 1/3 C (1 C is a current value that can be fully charged and discharged in 1 hour) under a temperature environment of 25 ° C. A constant current (CC) charge of time was performed, and then an operation of charging to 4.1 V at a rate of 1/3 C and an operation of discharging to 3.0 V at a rate of 1/3 C were repeated three times.
And as an aging process, each battery after the said charge process was left to stand for 24 hours in a 60 degreeC temperature environment (typically in the thermostat adjusted to 60 degreeC). In this way, lithium ion secondary batteries for evaluation tests according to Examples 1 to 14 were constructed.

<Measurement of initial battery capacity>
For the lithium ion secondary batteries for evaluation tests according to Examples 1 to 14, after the conditioning treatment, in the temperature environment of 25 ° C., in the voltage range of 3.0 V to 4.1 V, the initial steps were performed according to the following procedures 1 to 3. The capacity was measured.
(Procedure 1) After reaching 3.0 V by 1/3 C constant current discharge, discharge by constant voltage discharge for 2 hours and then rest for 10 minutes.
(Procedure 2) After reaching 4.1 V by 1/3 C constant current charging, constant voltage charging is performed until the current reaches 1/100 C, and then resting for 10 minutes.
(Procedure 3) After reaching 3.0 V by 1/3 C constant current discharge, constant voltage discharge is performed until the current becomes 1/100 C, and then stopped for 10 minutes.
And the discharge capacity (CCCV discharge capacity) in the discharge from the constant current discharge in the procedure 3 to the constant voltage discharge was made into the initial stage battery capacity.

<Measurement of IV resistance>
The IV resistance (reaction resistance) of the lithium ion secondary batteries according to Examples 1 to 14 was measured. That is, each battery is charged at a constant current of 3.0V to 1C, adjusted to a SOC (State of Charge) 60% charge state, and then at a temperature environment of −40 ° C., −30 ° C., or 25 ° C. A constant current (CC) discharge was performed for 2 seconds at (high rate discharge), and the IV resistance (mΩ) was determined from the slope of the linear approximation line of the current (I) -voltage (V) plot value at this time. The results are shown in Table 3.

<Evaluation of storage characteristics>
About the lithium ion secondary battery which concerns on Examples 1-14 which measured the said initial stage capacity | capacitance, it adjusted to the charge state of SOC 80% by the CCCV system. Next, after each battery was stored in a constant temperature bath at 60 ° C. for 120 days, the battery capacity after storage (battery capacity after storage) of each battery was measured by the same method as the measurement of the initial battery capacity. Here, the capacity retention rate (%) after 120 days storage was determined from the formula of (battery capacity after storage / initial battery capacity) × 100, and this is shown in Table 3 as an index of storage characteristics of each battery.

<Evaluation of cycle characteristics>
For the lithium ion secondary batteries according to Examples 1 to 14, the cycle capacity retention rate (cycle characteristics) was further measured. Specifically, after measuring the initial battery capacity in a temperature environment of 25 ° C., 2C CC cycle charge / discharge was repeated 1000 cycles, and the discharge capacity after 1000 cycles was measured. Here, the capacity retention rate (%) after the cycle test is obtained from the formula of (discharge battery capacity / initial battery capacity) after 100 cycles × 100, and this is shown in Table 3 as an index of the cycle characteristics of each battery.

As shown in Table 3, the lithium ion secondary battery provided with the non-aqueous electrolyte of Example 1 containing PC and EP at a mixing ratio of 10 vol%, respectively, and Example 11 containing PC and EP at a mixing ratio of 5 vol%, respectively In the lithium ion secondary battery including the nonaqueous electrolyte solution, the IV resistance values at −30 ° C. and −40 ° C. were relatively low and good. Moreover, it can be said that it is a non-aqueous electrolyte secondary battery that has relatively good 60 ° C. storage characteristics and cycle characteristics, and can achieve both battery performance and durability in a cryogenic temperature range.
On the other hand, the IV resistance values at −30 ° C. and −40 ° C. were relatively low and good for the lithium ion secondary batteries including the non-aqueous electrolyte of Example 3, Example 7, Example 9 or Example 12. However, in Examples 3 and 12, as described above, the freezing point is relatively higher than those in Examples 1 and 11, and it cannot be said that it is good in use in the cryogenic temperature region. In addition, as described above, Example 7 is not preferable because the flash point temperature range is less than 21 ° C. As for Example 9, in addition to the fact that the freezing point is relatively high compared to Example 1, durability characteristics are also provided. It is not good compared with the lithium ion secondary battery provided with the nonaqueous electrolyte solution of Example 1. In Example 5 containing only PC as the cyclic carbonate solvent, peeling of the graphite surface layer was caused and charging / discharging was difficult, and it was not usable.

20 Winding electrode body 30 Battery case 32 Battery case main body 34 Cover body 36 Safety valve 42 Positive electrode terminal 42a Positive electrode current collector plate 44 Negative electrode terminal 44a Negative electrode current collector plate 50 Positive electrode (positive electrode sheet)
52 Positive Current Collector 52a Positive Electrode Active Material Layer Non-Forming Portion 54 Positive Electrode Active Material Layer 60 Negative Electrode (Negative Electrode Sheet)
62 Negative electrode current collector 62a Negative electrode active material layer non-forming portion 64 Negative electrode active material layer 70 Separator 100 Lithium ion secondary battery

Claims (4)

A non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte containing a non-aqueous solvent and an electrolyte,
The non-aqueous solvent is
Including at least ethylene carbonate (EC) and propylene carbonate (PC) as the cyclic carbonate solvent,
Including at least dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) as a chain carbonate solvent,
Further, at least ethyl propionate (EP) as an ester solvent,
Here, the content when each of the non-aqueous solvent is 100 vol% is EC 20-30 vol%;
PC 5-10 vol%;
EP 5-10 vol%;
DMC + EMC 50-70 vol%;
A non-aqueous electrolyte secondary battery.   2. The nonaqueous electrolyte secondary battery according to claim 1, wherein a content ratio of the dimethyl carbonate (DMC) and a content ratio of the ethyl methyl carbonate (EMC) are both 25 to 35 vol%.   The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the non-aqueous electrolyte has a freezing point of less than -40 ° C.   The nonaqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein an ionic conductivity (mS / cm) at -40 ° C of the nonaqueous electrolyte is 1.0 or more.

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