KR20180083801A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

The present invention provides a non-aqueous electrolyte secondary battery having a non-aqueous electrolyte having a composition capable of realizing high cell performance even in a cryogenic temperature region (for example, -30 ° C or less). The nonaqueous electrolytic solution disclosed herein is a nonaqueous electrolyte solution which is a mixture of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), and ethyl propionate EP)
When the total amount of the non-aqueous solvent is 100 vol%
EC 20 to 30 vol%;
5 to 10 vol% PC;
EP 5 to 10 vol%;
DMC + EMC 50 to 70 vol%;
to be.

Description

TECHNICAL FIELD [0001] The present invention relates to a non-aqueous electrolyte secondary battery,

The present invention relates to a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery. More particularly, it relates to an electrolyte composition of a non-aqueous electrolyte secondary battery.

2. Description of the Related Art In recent years, demand for non-aqueous electrolyte secondary batteries has become higher as a so-called portable power source for a personal computer or a portable terminal or a power source for driving a vehicle. In particular, a lithium ion secondary battery which is light in weight and has a high energy density is preferably used as a high output power source for driving vehicles such as electric vehicles and hybrid vehicles.

The non-aqueous electrolyte provided in this kind of secondary battery has been selected and used from the viewpoints of improving the performance and durability of the battery and the like and the type of the supporting electrolyte (electrolyte). For example, in Japanese Laid-Open Patent Publication No. 2015-528640 (WO2014 / 081254), there is proposed a method for improving the ionic conductivity of a non-aqueous electrolyte in a low temperature region by suppressing an increase in viscosity, To 90% by weight of the nonaqueous electrolyte solution is an ester solvent and 10% to 90% by weight of the solvent is a carbonate-based solvent.

However, the technique disclosed in the above publication (WO2014 / 081254) does not assume the case where the battery is used in a cryogenic region such as -30 DEG C or lower (for example, -40 DEG C to -30 DEG C) There is no data that examines the behavior of the non-aqueous electrolyte in the non-aqueous electrolyte. Therefore, as a conceivable method in which a simple ester solvent and a carbonate-based solvent described in the above publication (WO2014 / 081254) are mixed, a nonaqueous electrolyte solution for realizing a stable cell performance even in such a cryogenic temperature region is provided It is difficult.

SUMMARY OF THE INVENTION Accordingly, the present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a nonaqueous electrolyte solution for a secondary battery having a composition capable of stably exhibiting good battery performance even in a cryogenic temperature range of -30 ° C or lower (for example, -40 ° C to -30 ° C) . Another object of the present invention is to provide a nonaqueous electrolyte secondary battery having such a nonaqueous electrolyte and excellent low temperature characteristics.

The present inventors have studied a variety of carbonate-based solvents and ester-based solvents that can be used as non-aqueous solvents in non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries. A nonaqueous electrolytic solution having a composition capable of realizing a high battery performance even in a cryogenic temperature region (for example, -30 占 폚 or lower) was produced, and the present invention was completed.

That is, the invention disclosed herein relates to a nonaqueous electrolyte secondary battery having a nonaqueous electrolyte solution containing a nonaqueous solvent and an electrolyte, wherein the nonaqueous solvent comprises a cyclic carbonate solvent, a chain carbonate solvent, An ester-based solvent as a main component.

In a preferred embodiment of the non-aqueous electrolyte secondary cell disclosed herein, the non-aqueous solvent includes at least ethylene carbonate (EC) and propylene carbonate (PC) as a cyclic carbonate-based solvent, and a chain carbonate- (DMC) and ethyl methyl carbonate (EMC) as a solvent, and at least ethyl propionate (EP) as an ester solvent.

The nonaqueous electrolyte secondary battery comprising these five kinds of solvents can improve the battery performance in the cryogenic temperature region.

One particularly preferred form of the nonaqueous electrolyte secondary cell disclosed herein is a nonaqueous electrolyte secondary battery comprising a nonaqueous electrolyte secondary battery,

20 to 30 vol% of ethylene carbonate (EC);

5 to 10% by volume of propylene carbonate (PC);

5 to 10% by volume of ethyl propionate (EP);

The sum of dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC), that is, 50 to 70 vol% of DMC + EMC;

.

A nonaqueous electrolyte secondary battery realizing good battery performance in a cryogenic temperature range while maintaining the flash point of the electrolytic solution at 21 DEG C or higher can be provided by adopting the nonaqueous electrolyte solution formed by blending each solvent at such a blending ratio.

In a further preferred embodiment of the non-aqueous electrolyte secondary cell disclosed herein, the content of the dimethyl carbonate (DMC) and the content of the ethylmethyl carbonate (EMC) are both in the range of 25 to 35 vol%.

By mixing DMC and EMC together at such a content (preferably substantially equal), stable battery performance (for example, stable high rate charging and discharging) is realized.

In a particularly preferred embodiment of the non-aqueous electrolyte secondary cell disclosed herein, the non-aqueous electrolyte has a solidification point of less than -40 占 폚.

(For example, from -30 ° C to less than -30 ° C) by preparing a nonaqueous electrolyte solution such that the solidification point under atmospheric pressure is less than -40 ° C (more preferably less than -45 ° C, for example, -40 < 0 > C) can be further improved.

In another particularly preferred embodiment of the non-aqueous electrolyte secondary cell disclosed herein, the non-aqueous electrolyte has an ion conductivity (mS / cm) at -40 캜 of 1.0 or more. Here, the measurement of the ion conductivity is based on the AC impedance method.

By keeping the ionic conductivity at a cryogenic temperature region high, it is possible to maintain good output characteristics in a cryogenic temperature region.

The nonaqueous electrolyte secondary battery (for example, a lithium ion secondary battery) disclosed herein can exhibit good battery performance in a cryogenic temperature range (for example, -40 ° C to -30 ° C). Therefore, for example, it is possible to supply the electric power for driving the motor in the cryogenic temperature range, and thus it can be suitably used as a power source for driving the vehicle in a cold environment requiring high performance.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view schematically showing an outer shape of a battery in an embodiment (a lithium ion secondary battery) of the non-aqueous electrolyte secondary cell disclosed herein. FIG.
Fig. 2 is a longitudinal sectional view taken along line II-II in Fig. 1; Fig.
3 is a schematic diagram showing the configuration of the wound electrode body according to one embodiment.

Hereinafter, as a preferred embodiment of the nonaqueous electrolyte secondary battery disclosed herein, the configuration of a prismatic lithium ion secondary battery having a closed structure will be described in detail with reference to the drawings. The matters other than the matters specifically mentioned in this specification and the matters necessary for the implementation can be grasped as a design matter of a person skilled in the art based on the prior art in the field. The present invention can be carried out based on the contents disclosed in this specification and the technical knowledge in the field.

In the present specification and claims, when numerical ranges A to B (where A and B are arbitrary numerical values) are described, it is the same as a general interpretation and means A or more and B or less.

As used herein, the term " non-aqueous electrolyte secondary cell " refers to a secondary battery in which the solvent constituting the electrolyte is composed mainly of a non-aqueous solvent (i.e., organic solvent). Here, " secondary battery " refers to a power storage device that can be charged and discharged, and can take out predetermined electric energy repeatedly. For example, a lithium ion secondary battery, a sodium ion secondary battery, and the like in which alkali metal ions in a non-aqueous electrolyte take charge of charge transfer are typical examples of the non-aqueous electrolyte secondary battery.

"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 a negative electrode. The positive electrode active material or the negative electrode active material (collectively referred to as " electrode active material ") may be a chemical species to be a charge carrier (for example, lithium ion secondary battery, lithium ion, Refers to a compound (positive electrode active material or negative electrode active material) capable of reversibly intercalating and deintercalating sodium ions.

Hereinafter, as a typical example of a non-aqueous 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 solution are housed in a rectangular-shaped battery case is described as an embodiment The present invention is not intended to be limited to these embodiments.

For example, the wound electrode body is an example, and the technical idea of the present invention is also applied to other shapes (for example, a laminated electrode body). The shape (external shape and size) of the nonaqueous electrolyte secondary battery is not particularly limited.

As shown in Figs. 1 and 2, the lithium ion secondary battery 100 according to the present embodiment has a flat-rolled electrode body (wound electrode body) 20, Shaped battery case 30 as shown in FIG.

The battery case 30 has a flat rectangular parallelepiped-shaped battery case body 32 with an opened top and a lid 34 for closing the opening. The positive electrode terminal 42 for external connection which is electrically connected to the positive electrode of the wound electrode body 20 and the positive electrode terminal 42 for electrically connecting to the negative electrode of the wound electrode body 20 are formed on the upper surface And a negative electrode terminal 44 connected to the negative electrode terminal. 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 in the same manner as the battery case of the conventional lithium ion secondary battery .

As the material of the battery case 30, metal materials such as aluminum and steel; A polyphenylene sulfide resin, and a 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), a hexahedral shape (rectangular parallelepiped shape, cubic shape), a bag shape,

As shown in Fig. 3, the wound electrode body 20 according to the present embodiment has an elongated sheet structure (sheet-shaped electrode body) in a stage prior to assembly. The wound electrode body 20 includes a positive electrode sheet 50 on which a positive electrode active material layer 54 is formed on one side or both sides (here both sides) of a metal positive electrode collector 52 made of a long metal such as aluminum, The negative electrode sheet 60 having the negative electrode active material layer 64 formed on one side or both sides (here both sides) of the negative electrode current collector 62 made of metal such as long copper or the like along the long side is formed into a long separator sheet 70 And is formed into a flat shape by being wound in a long direction.

The positive electrode active material layer 54 of the positive electrode sheet 50 and the negative electrode active material layer 64 of the negative electrode sheet 60 are formed in the center portion in the winding axis direction of the wound electrode body 20, And the separator sheet 70 are stacked in this order). The positive electrode active material layer non-formed portion 52a in the positive electrode sheet 50 and the negative electrode active material layer non-formed portion 62a in the negative electrode sheet 60 are formed at both ends in the winding axis direction of the wound electrode body 20 And are each pushed outward from the winding core portion. The positive electrode current collector plate 42a and the negative electrode current collector plate 44a are laid on the positive electrode active material layer non-formed portion 52a and the negative electrode active material layer non-formed portion 62a. The positive electrode terminal 42 (FIG. 2) (Fig. 2).

The positive electrode active material layer 54 includes at least a positive electrode active material. As the positive electrode active material, one or more kinds of various materials usable as the positive electrode active material of the lithium ion secondary battery can be employed without particular limitation. As a preferable example, a lithium composite metal oxide (LiNiO ₂ , LiCoO ₂ , LiFeO ₂ , LiMn ₂ O ₄ , LiNi _0.5 Mn _1.5 O ₄ , LiCrMnO ₄ , LiFePO _4, etc.) such as a layered system or a spinel system can be given. For example, a lithium nickel cobalt manganese composite oxide (for example, LiNi _1/3 Co (typically LiNi _1/3 Co) having a layered structure (typically, a layered salt salt structure belonging to a hexagonal system) containing Li, Ni, _1/3 Mn _1/3 O ₂ ) is a suitable example having excellent thermal stability and high theoretical energy density.

The properties of the positive electrode active material are not particularly limited, but may be, for example, particulate or powdery. The average particle size of such particulate positive electrode active material may be 20 占 퐉 or less (typically 1 占 퐉 to 20 占 퐉, for example, 5 占 퐉 to 15 占 퐉). The specific surface area is not less than 0.1 m 2 / g (typically not less than 0.7 m 2 / g, such as not less than 0.8 m 2 / g) and not more than 5 m 2 / g (typically not more than 1.3 m 2 / g, M < 2 > / g or less). In the present specification, the "average particle diameter" means a particle size distribution based on a particle size distribution measurement based on a general laser diffraction / light scattering method, in which the particle diameter (D ₅₀ particle size , Also referred to as median diameter).

The positive electrode active material layer 54 may contain, in addition to the above positive electrode active material, one or more kinds of materials that can be used as constituent components of the positive electrode active material layer 54 in general lithium ion secondary batteries, if necessary . Examples of such a material include a conductive material and a binder. As the conductive material, carbon materials such as various kinds of carbon black (typically, acetylene black, Ketjen black), coke, activated carbon, graphite, carbon fiber, and carbon nanotube can be preferably used. Examples of the binder include vinyl halide resins such as polyvinylidene fluoride (PVdF); Polyalkylene oxides such as polyethylene oxide (PEO); May be preferably used.

The ratio of the positive electrode active material to the entire positive electrode active material layer 54 is suitably about 60 mass% or more (typically 60 mass% to 99 mass%), and usually about 70 mass% to 95 mass% . When a conductive material is used, the proportion of the conductive material in the entire positive electrode active material layer 54 may be, for example, about 2% by mass to 20% by mass, and usually about 3% by mass to 10% . When a binder is used, the proportion of the binder in the whole positive electrode active material layer 54 can be, for example, about 0.5% by mass to 10% by mass, and usually about 1% by mass to 5% desirable.

The average thickness per side of the positive electrode active material layer 54 may be 20 μm or more (typically 40 μm or more, preferably 50 μm or more) and 100 μm or less (typically 80 μm or less) . The density of the positive electrode active material layer 54 may be, for example, 1 g / cm 3 to 4 g / cm 3 (for example, 1.5 g / cm 3 to 3.5 g / cm 3). The porosity (porosity) of the positive electrode active material layer 54 may be, for example, 10 to 50 vol% (typically 20 to 40 vol%). By satisfying such a property, a proper void can be maintained in the positive electrode active material layer 54, and the non-aqueous electrolyte can be sufficiently infiltrated. Therefore, it is possible to secure a wide reaction field with the charge carrier, and high input / output characteristics can be exhibited even in the cryogenic temperature region. In the present specification, the term "porosity" refers to a value obtained by dividing the total pore volume (cm 3) obtained by measurement of mercury porosimeter by the volume (cm 3) of the outer surface of the active material layer and multiplying by 100.

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 needed are dispersed in a suitable solvent (for example, N-methyl-2-pyrrolidone) to prepare a slurry or paste composition (hereinafter referred to as " slurry for forming a positive electrode active material layer & ). The slurry for forming the positive electrode active material layer is applied to the elongated positive electrode collector 52, the solvent contained in the slurry is removed, dried and, if necessary, pressed to form the positive electrode collector 52, The positive electrode sheet 50 having the positive electrode active material layer 54 can be manufactured.

On the other hand, the negative electrode active material layer 64 includes at least a negative electrode active material. As the negative electrode active material, one kind or two or more kinds of materials which can be used as a negative electrode active material of a lithium ion secondary battery can be used without particular limitation. As a preferable example, a graphite structure (layer structure) is included in at least a part of graphite (graphite), non-graphitized carbon (hard carbon), graphitized carbon (soft carbon), carbon nanotubes, Carbon material. Graphite-based materials such as natural graphite (graphite) and artificial graphite can be preferably used because a high energy density can be obtained.

As the negative electrode active material, it is particularly preferable to use an amorphous coated graphite material having amorphous carbon (amorphous carbon) on the surface of graphite. As a method for forming such an amorphous coating, for example, a vapor phase method such as a CVD method in which a vapor phase coating material is vapor-deposited on the surface of a nuclear material (graphite particle) under an inert gas atmosphere, a method in which a solution obtained by diluting a coating material with an appropriate solvent is mixed A solid phase method in which a liquid phase method for calcining and carbonizing the coating material in an inert gas atmosphere, a nuclear material and a coating material are kneaded without using a solvent, and the mixture is sintered and carbonized in an inert gas atmosphere, Can be appropriately adopted.

The properties of the negative electrode active material are not particularly limited, but may be, for example, particulate or powdery. The average particle size of the particulate negative electrode active material may be, for example, 50 占 퐉 or less (typically 20 占 퐉 or less, for example, 1 占 퐉 to 20 占 퐉, preferably 5 占 퐉 to 15 占 퐉). Further, the specific surface area is not less than 1 m 2 / g (typically not less than 2.5 m 2 / g, such as not less than 2.8 m 2 / g) and not more than 10 m 2 / g (typically not more than 3.5 m 2 / g, M < 2 > / g or less).

The negative electrode active material layer 64 may contain, in addition to the negative electrode active material, one or more kinds of materials that can be used as constituent components of the negative electrode active material layer in general lithium ion secondary batteries, if necessary. Examples of such a material include a binder and various additives. As the binder, for example, polymer materials such as styrene butadiene rubber (SBR), polyvinylidene fluoride (PVdF), and polytetrafluoroethylene (PTFE) can be preferably used. (CMC) or methyl cellulose (MC) can be preferably used as the thickening agent. The amount of the thickener to be used in the present invention is not particularly limited.

The ratio of the negative electrode active material to the total of the negative electrode active material layer 64 is suitably about 50 mass% or more, and is usually 90 mass% to 99 mass% (for example, 95 mass% to 99 mass%) . In the case of using a binder, the ratio of the binder to the entire negative electrode active material layer 64 may be, for example, approximately 1% by mass to 10% by mass, and usually approximately 1% by mass to 5% Do.

The average thickness per side of the negative electrode active material layer 64 is, for example, 40 占 퐉 or more (typically 50 占 퐉 or more) and 100 占 퐉 or less (typically 80 占 퐉 or less). The density of the negative electrode active material layer 64 may be, for example, about 0.5 g / cm 3 to 2 g / cm 3 (typically 1 g / cm 3 to 1.5 g / cm 3). The porosity of the negative electrode active material layer 64 may be, for example, about 5 to 50 vol% (preferably 35 to 50 vol%). By satisfying such a property, a proper void can be maintained in the negative electrode active material layer 64, and the non-aqueous electrolyte can be sufficiently infiltrated. Therefore, it is possible to secure a wide reaction field with the charge carrier, and high input / output characteristics can be exhibited even in the cryogenic 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 to be used as needed are dispersed in an appropriate solvent (for example, distilled water) to prepare a slurry or paste-like composition (hereinafter referred to as " slurry for forming negative electrode active material layer "). The slurry for forming the negative electrode active material layer is applied to the long negative electrode collector 62, and the solvent contained in the slurry is removed, dried and, if necessary, pressed to form a negative electrode active material layer The negative electrode sheet 60 having the negative electrode sheet 64 can be manufactured.

The separator sheet 70 sandwiched between the pair of electrode sheets 50 and 60 may be any one having a function of insulating the positive electrode active material layer 54 and the negative electrode active material layer 64 as well as a function of holding the non- . As a preferable example, a porous resin sheet (film) made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, polyamide or the like can be given.

The shape 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 40 μm or less (typically 30 μm or less, for example, 25 μm or less) desirable.

Next, the non-aqueous electrolyte according to the present embodiment will be described in detail.

The nonaqueous electrolyte is composed of a nonaqueous solvent and an electrolyte (that is, a support salt), and such nonaqueous solvent is prepared mainly using a cyclic carbonate-based solvent, a chain carbonate-based solvent, and an ester-based solvent.

The coexistence of a cyclic carbonate-based solvent and a chain carbonate-based solvent makes it possible to achieve both a high dielectric constant and a low viscosity. In addition, coexistence of an ester-based solvent also lowers the solidification point (and melting point), thereby enabling the use of a preferable battery in a low-temperature region.

Preferably, the cyclic carbonate-based solvent includes at least ethylene carbonate (EC) and propylene carbonate (PC). The combination of EC having a high relative dielectric constant and PC having a low freezing point (and melting point) contributes to improvement of cell performance in a cryogenic temperature region. The addition of PC is also preferable from the viewpoint of improving durability (for example, storage characteristics and cycle characteristics).

In addition, preferably, at least a dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) are used as the chain carbonate. The combination of DMC and EMC having a high oxidation potential (wide potential window) is preferable from the viewpoint of improving battery performance.

In addition, preferably, at least ethyl propionate (EP) is contained as an ester-based solvent. By the addition of EP, the freezing point (and the melting point) of the non-aqueous electrolyte and the lowering of the viscosity in the low temperature region can be desirably achieved. Further, by adding EP in combination with a PC, durability (for example, storage characteristics and cycle characteristics) can be improved.

According to the nonaqueous electrolyte solution in which these five kinds of solvents are coexisted, stable performance (for example, good power retention) can be realized even in a cryogenic temperature region such as a cryogenic temperature region (for example, -40 ° C to -30 ° C). Further, the durability characteristics (for example, the cycle characteristics and the storage characteristics) can be improved.

More preferably, the content of each of the above-mentioned five solvents when the total amount of the non-aqueous solvent is 100 vol%

EC 20 to 30 vol%

5 to 15 vol% (for example, 5 to 10 vol% or (10 to 15 vol%),

5 to 15 vol% of EP (for example, 5 to 10 vol% or (10 to 15 vol%) and

The total of DMC and EMC is 50 to 70 vol%

The non-aqueous electrolyte is prepared.

According to the nonaqueous electrolyte solution having such a composition, good cell performance in a cryogenic temperature range can be realized while maintaining the flash point of the electrolytic solution at 21 DEG C or higher. It is particularly preferable that the contents of PC and EP are respectively 5 to 10 vol%. In addition, good durability (for example, storage characteristics and cycle characteristics) can be realized.

Other organic solvent components may be added as long as the nonaqueous electrolyte solution is prepared mainly of the above five kinds of solvents.

Examples of such optional additives include cyclic carbonate-based solvents such as 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2, 4-fluoro-1,3-dioxolan-2-one, trans or cis-4,5-difluoro-1,3-dioxolan- Vinylene carbonate, 4-ethynyl-1,3-dioxolan-2-one, and the like.

Examples of other optional components that can be added include straight chain carbonate-based solvents such as diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, ethyl propyl carbonate, methyl Butyl carbonate, dibutyl carbonate, and the like.

Examples of other optional components that can be added include esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate,? -Butyrolactone,? -Valerolactone,? -Caprolactone, Lactone, epsilon -caprolactone, and the like.

As the electrolyte (i.e. not salts), for example, _{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 , a lithium compound such as LiI (lithium salt), or the like. A particularly preferable electrolyte is LiPF ₆ . The concentration of the electrolyte is not particularly limited, but may 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 nonaqueous electrolyte secondary battery is a sodium ion secondary battery, an electrolyte (support salt) containing Na such as NaPF ₆ may be employed.

The nonaqueous electrolytic solution may contain an optional additive component as necessary so long as the object of the present invention can be achieved. For example, various additives may be added for the purpose of improving the output performance of the battery, improving the preservability (suppressing the decrease in capacity during storage, etc.), improving the cycle characteristics, and improving the initial charge and discharge efficiency. Examples of such an additive include a gas generating agent, a film forming agent, a dispersant, and a thickener. A nonaqueous electrolytic solution having a desired composition can be prepared by mixing the various nonaqueous solvents and electrolytes described above and various additives added as needed.

A lithium ion secondary battery is produced by 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 performed by appropriately employing a conventionally used method, and does not characterize the present invention, so that detailed description thereof will be omitted here.

The lithium ion secondary battery disclosed herein can be used for various applications. Since the battery performance is excellent in a cryogenic temperature region (particularly -30 ° C or less, for example, -40 ° C to -30 ° C), in the cryogenic temperature region And a power source for driving the vehicle. The type of vehicle is not particularly limited, and examples thereof include a plug-in hybrid vehicle (PHV), a hybrid vehicle (HV), an electric vehicle (EV), an electric electric bicycle, an electric wheelchair, and an electric railroad. Generally, a lithium ion secondary battery provided in a vehicle is used in the form of a plurality of connected battery cells, and the structure of the battery cell is not shown.

Hereinafter, some test examples according to the present invention will be described, but the present invention is not intended to be limited to those shown in these 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) (Nos. 1 to 14) for 14 kinds of lithium ion secondary batteries were prepared by mixing Nitrogen (EP) as shown in Table 1 at the compounding ratio (volume ratio) shown in Table 1. The concentration of LiPF ₆ was all 1.1M. The solidification point (° C) and the flash point temperature region (indicated by o in the case of 21 ° C or more and X in 21 ° C or less) of the non-aqueous electrolyte of each example under atmospheric pressure are shown in the corresponding column of Table 1.

As shown in Table 1, by increasing the blending ratio of EP or EMC having a low melting point, the freezing point of the entire non-aqueous electrolyte can be lowered. However, the flash point temperature range becomes less than 21 占 폚 (Examples 7, 13 and 14), which is an undesirable combination from the viewpoint of safety.

On the other hand, in Example 11 in which both PC and EP were blended at 10 vol% and Example 11 in which PC and EP were blended at 5 vol%, the freezing point temperature was lowered to less than 21 캜 and the freezing point was changed to -49 캜 (Example 1) Lt; 0 &gt; C (Example 11), which is a preferable blend.

Further, although detailed data is not shown, it is preferable as a low-viscosity non-aqueous electrolyte showing good viscosity even at a low temperature region of 0 占 폚 or lower.

&Lt; Evaluation of ion conductivity &

The non-aqueous electrolyte ionic conductivities [mS / cm] of the prepared Examples 1 to 14 were measured. The measurement was carried out by using an alternating-current impedance method using a sealed two-electrode cell made of Toyo System. The nonaqueous electrolytic solution to be disclosed was controlled to have a constant size and thickness by a spacer made of polytetrafluoroethylene, and sandwiched between a pair of stainless steel electrodes. A Cole-Cole plot obtained when a voltage of 10 mV was applied to this cell and the frequency was changed from 1 MHz to 10 mHz was subjected to curve fitting using an equivalent circuit to determine the bulk resistance and the ion conductivity was calculated.

The measurement temperatures were -40 ° C, -30 ° C, and 25 ° C, and ion conductivity was measured at each measurement temperature. The obtained results are shown in Table 2.

As shown in Table 2, the nonaqueous electrolyte solution of Example 1 containing PC and EP at a blending ratio of 10 vol%, respectively, had relatively good ionic conductivity at -30 캜 and -40 캜. Likewise, the nonaqueous electrolyte of Example 11 containing PC and EP at a blending ratio of 5 vol% each had a relatively good ionic conductivity at -30 캜 and -40 캜.

On the other hand, the ion conductivity of the non-aqueous electrolyte of Example 3, Example 7 and Example 13 was good at -30 ° C and -40 ° C. In Example 3, the solidifying point was relatively higher than that of Example 1, (For example, -40 ° C to -30 ° C). In Examples 7 and 13, as described above, the flash point temperature region is less than 21 ° C, which is not preferable.

<Construction of Lithium Ion Secondary Battery>

As the positive electrode active material powder, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (LNCM) was prepared. This LNCM, acetylene black AB as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were fed into a kneader so as to be in a mass ratio of LNCM: AB: PVdF = 94: 3.5: 2.5, (NMP) while adjusting the viscosity, thereby preparing a slurry for forming a positive electrode active material layer. This slurry was applied to both sides of an aluminum foil (positive electrode collector) having a thickness of 12 占 퐉 and pressed after drying to prepare a positive electrode sheet having a positive electrode active material layer having a density of about 2.9 g / cm3 on the positive electrode collector .

Spherical natural graphite (C) having a surface coated with amorphous carbon was prepared as a negative electrode active material powder. This graphite (C), styrene butadiene rubber (SBR) as a binder, and CMC as a thickener were fed into a kneader in a mass ratio of C: SBR: CMC = 99: 0.5: 0.5, And kneaded to prepare a slurry for forming the negative electrode active material layer. This slurry was coated on both sides of a copper foil (negative electrode collector) having a thickness of 8 占 퐉 and pressed after drying to produce a negative electrode sheet having a negative electrode active material layer having a density of about 1.4 g / cm3 on the negative electrode collector .

The above prepared positive electrode sheet and negative electrode sheet were wound together with two separator sheets each having a thickness of 20 mu m in a three-layer structure (PP / PE / PP) made of polyethylene (PE) and polypropylene (PP) Thereby forming an electrode body.

Subsequently, a positive electrode terminal and a negative electrode terminal were provided on a battery case cover made of aluminum, and these terminals were respectively welded to the positive electrode collector and the negative electrode collector exposed at the end of the wound electrode body. In this manner, the wound electrode body connected to the lid was housed inside the battery case body made of aluminum through the opening, and the opening and lid were welded.

Then, the nonaqueous electrolyte solution of any of Examples 1 to 14 shown in Table 1 was injected from the electrolyte injection hole provided in the cover, and the electrolyte injection hole was hermetically sealed. Thus, the battery assemblies of Examples 1 to 14 corresponding to the non-aqueous electrolytes of Examples 1 to 14 were constructed.

As a conditioning treatment for the battery assemblies according to Examples 1 to 14, a constant current (CC) of 3 hours at a rate of 1 / 3C (1C is a current value capable of fully discharging in 1 hour) Charging was performed, and then the operation of charging to 4.1 V at a rate of 1 / 3C and the operation of discharging to 3.0 V at a rate of 1 / 3C were repeated three times.

Then, as the aging treatment, each of the cells after the filling treatment was allowed to stand for 24 hours in a temperature environment of 60 占 폚 (typically in a thermostatic chamber at 60 占 폚). Thus, a lithium ion secondary battery for evaluation tests according to Examples 1 to 14 was constructed.

<Measurement of Initial Battery Capacity>

The lithium ion secondary battery for evaluation test according to Examples 1 to 14 was measured for initial capacity in the voltage range of 3.0 V to 4.1 V under the temperature environment of 25 캜 according to the following procedures 1 to 3 after the above- Respectively.

(Procedure 1) After reaching 3.0 V by the constant current discharge of 1 / 3C, discharge by the constant voltage discharge for 2 hours, and then stopping for 10 minutes.

(Procedure 2) After reaching 4.1 V by constant current charging at 1 / 3C, the battery is charged at a constant voltage until the current becomes 1 / 100C, and then the battery is stopped for 10 minutes.

(Procedure 3) After reaching 3.0 V by the constant current discharge of 1 / 3C, the constant-voltage discharge is performed until the current becomes 1 / 100C, and then the operation is stopped for 10 minutes.

The discharge capacity (CCCV discharge capacity) in the discharge from the constant current discharge to the constant voltage discharge in the procedure 3 was taken as the initial cell capacity.

&Lt; Measurement of IV resistance >

The IV resistance (reaction resistance) of the lithium ion secondary batteries of Examples 1 to 14 was measured. That is, each battery was charged at a constant current of from 1 V to 3.0 V, adjusted to a state of charge of SOC (State of Charge) of 60%, and then charged at 10 C (CC) discharge for 2 seconds was carried out by using the voltage (V) and the voltage (V). The IV resistance (m?) Was obtained from the slope of the first approximation straight line of the current (I) -voltage (V) plot value at this time. The results are shown in Table 3.

<Evaluation of Storage Characteristics>

The lithium ion secondary batteries of Examples 1 to 14 in which the initial capacity was measured were adjusted to a charged state of SOC of 80% by the CCCV system. Subsequently, each cell was stored in a thermostatic chamber at 60 캜 for 120 days, and then the cell capacity (after storage) of each cell was measured by the same method as the measurement of the initial cell capacity. Here, the capacity retention rate (%) after 120 days of storage was obtained from the expression ((storage battery capacity after storage) / (initial cell capacity)) × 100, and this is shown in Table 3 as an index of the storage characteristics of each battery.

<Evaluation of cycle characteristics>

The cycle capacity retention (cycle characteristics) of the lithium ion secondary batteries of Examples 1 to 14 was also measured. Specifically, the initial cell capacity was measured in a temperature environment of 25 占 폚, and then the CC cycle charge and discharge of 2C was repeated 1000 times, and the discharge capacity after 1000 cycles was measured. Here, the capacity retention rate (%) after the cycle test is obtained from the expression (discharging cell capacity / initial cell capacity) after 1000 cycles × 100, and this is shown in Table 3 as an index of the cycle characteristics of each cell.

As shown in Table 3, a lithium ion secondary battery having a nonaqueous electrolyte solution of Example 1 containing PC and EP at a blending ratio of 10 vol%, respectively, and a nonaqueous electrolyte of Example 11 containing PC and EP at a blending ratio of 5 vol% In the lithium ion secondary battery having the electrolytic solution, the IV resistance value at -30 캜 and -40 캜 was relatively low, which was good. In addition, the nonaqueous electrolyte secondary battery can be said to be a nonaqueous electrolyte secondary battery capable of satisfying battery performance and durability in a cryogenic temperature region at 60 deg. C with relatively good storage characteristics and cycle characteristics.

On the other hand, with respect to each of the lithium ion secondary batteries having the nonaqueous electrolyte solution of Example 3, Example 7, Example 9, or Example 12, the IV resistance value at -30 占 폚 and -40 占 폚 was relatively low. However, with respect to Examples 3 and 12, as described above, the solidifying point is relatively high as compared with Example 1 or Example 11, and it can not be said that the cryogenic temperature region is satisfactory in use. In Example 7, as described above, since the flash point temperature region is less than 21 ° C, it is not preferable. In Example 9, the solidifying point is relatively higher than that in Example 1, 1 &lt; / RTI &gt; non-aqueous electrolyte. In Example 5 containing only PC as the cyclic carbonate-based solvent, peeling of the graphite surface layer occurred and charging and discharging were difficult, and thus it was impossible to use.

20: wound electrode body
30: Battery case
32: Battery case body
34: Cover
36: Safety valve
42: Positive electrode terminal
42a: positive pole house front plate
44: Negative terminal
44a: anode collector plate
50: positive electrode (positive electrode sheet)
52: positive electrode current collector
52a: Positive electrode active material layer non-
54: Positive electrode active material layer
60: negative electrode (negative electrode sheet)
62: anode collector
62a: Negative electrode active material layer non-
64: Negative electrode active material layer
70: Separator
100: Lithium ion secondary battery

Claims (4)

A non-aqueous electrolyte secondary battery comprising:
And a nonaqueous electrolyte solution containing a nonaqueous solvent and an electrolyte,
The non-aqueous solvent may include,
And at least ethylene carbonate (EC) and propylene carbonate (PC) as the cyclic carbonate-based solvent,
(DMC) and ethyl methyl carbonate (EMC) as a chain carbonate-based solvent,
And at least ethyl propionate (EP) as an ester solvent,
When the total amount of the non-aqueous solvent is 100 vol%
EC 20 to 30 vol%;
5 to 10 vol% PC;
EP 5 to 10 vol%;
DMC + EMC 50 to 70 vol%;
By weight of a nonaqueous electrolyte secondary battery. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of the dimethyl carbonate (DMC) and the content of the ethyl methyl carbonate (EMC) are in the range of 25 to 35 vol%. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the solidification point of the nonaqueous electrolyte solution is lower than -40 ° C. The nonaqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the ionic conductivity (mS / cm) of the nonaqueous electrolyte at -40 캜 is 1.0 or more.

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