JP7040353B2 - Lithium ion secondary battery - Google Patents

Description

The present invention relates to a lithium ion secondary battery.

In recent years, there is an urgent need to reduce the amount of carbon dioxide in order to cope with global warming. In the automobile industry, expectations are high for the reduction of carbon dioxide emissions through the introduction of electric vehicles (EVs) and hybrid electric vehicles (HEVs), and non-water batteries such as secondary batteries for driving motors, which hold the key to their practical application. The development of secondary electrolyte batteries is being actively carried out.

The secondary battery for driving a motor is required to have extremely high output characteristics and high energy as compared with a consumer lithium ion secondary battery used in mobile phones, notebook computers and the like. Therefore, lithium-ion secondary batteries, which have the highest theoretical energy among all realistic batteries, are attracting attention and are currently being rapidly developed.

In a lithium ion secondary battery, a positive electrode containing a positive electrode active material slurry containing a positive electrode active material or the like is generally applied to the surface of a positive electrode current collector, and a negative electrode active material slurry containing a negative electrode active material or the like is applied to the surface of the negative electrode current collector. The coated negative electrode is connected via an electrolyte layer (separator holding an electrolytic solution) and has a configuration of being housed in a battery case.

In a conventional lithium ion secondary battery, in the electrolytic solution, lithium salts such as LiPF ₆ , CF ₃ SO ₃ Li, and (CF ₃ SO ₂ ) ₂ NLi are dissolved in an organic solvent at a concentration of approximately 1 mol / L. Was common. However, in recent years, lithium ion secondary batteries using an electrolytic solution containing a higher concentration of lithium salt have been developed. By using a high-concentration electrolytic solution, the lithium ion transport rate in the electrolytic solution is improved, the reaction rate at the interface between the electrode and the electrolytic solution is improved, the uneven distribution of the salt concentration of the electrolytic solution that occurs during high-rate charging / discharging is alleviated, and the electric double layer is used. It is expected that the performance of batteries will be improved due to the increase in capacity.

On the other hand, since the high-concentration electrolytic solution has a high viscosity, there is a problem that the permeability of the electrolytic solution into the separator is low and sufficient battery performance cannot be exhibited. In order to solve this problem, Patent Document 1 proposes that the water retention ratio of the separator is 0.1 or more.

JP-A-2015-195166

According to Cited Document 1, one of the factors for increasing the water retention ratio of the separator is the hydrophilicity of the separator. Then, by using a hydrophilic material as the material of the separator or by hydrophilizing the surface of the separator made of a hydrophobic material, it is said that a separator having a desired water retention ratio can be obtained.

However, a lithium ion secondary battery using a separator made of a hydrophilic material may not have a shutdown function due to heat generation, or may have low puncture strength and may easily cause an internal short circuit due to dendrite formation from the negative electrode surface. There is concern about a decrease in safety. Further, when the surface of a separator made of a hydrophobic material such as polyolefin is hydrophilized, there is a problem that the cost is high.

Therefore, an object of the present invention is to provide a method for applying a highly safe polyolefin-based separator in a lithium ion secondary battery using a high-viscosity electrolytic solution.

The present inventors have conducted diligent research in order to solve the above problems. As a result, they have found that the above problems can be solved by using polypropylene as the material of the separator and controlling the air permeability of the separator within a specific range, and have completed the present invention.

That is, the present invention relates to a lithium ion secondary battery having a power generation element including a positive electrode, a negative electrode, and an electrolyte layer interposed between the positive electrode and the negative electrode and holding an electrolytic solution in a separator. The viscosity of the electrolytic solution is 26 mPa · s or more, the separator contains polypropylene, and the air permeability measured by the Garley method of the separator is 120 sec / dl or more and 240 sec / dl or less.

According to the present invention, it is possible to apply a highly safe polyolefin-based separator in a lithium ion secondary battery using a high-viscosity electrolytic solution.

It is sectional drawing which shows the basic structure of the laminated type (flat type) non-bipolar type (internal parallel connection type) lithium ion secondary battery which is one Embodiment of the lithium ion secondary battery which concerns on this invention. It is a perspective view which showed the appearance of the flat lithium ion secondary battery which is a typical embodiment of the lithium ion secondary battery which concerns on this invention.

Hereinafter, embodiments of the present invention will be described. In addition, in this specification, "X to Y" indicating a range means "X or more and Y or less". Unless otherwise specified, the operation and physical properties are measured under the conditions of room temperature of 25 ° C., relative humidity of 40 to 50% RH, and normal pressure. Further, the "lithium ion secondary battery" is also simply referred to as a "battery".

The lithium ion secondary battery according to the present invention has a power generation element including a positive electrode, a negative electrode, and an electrolyte layer interposed between the positive electrode and the negative electrode and holding an electrolytic solution in a separator. The viscosity of the electrolytic solution is 26 mPa · s or more, the separator contains polypropylene, and the air permeability measured by the Garley method of the separator is 120 sec / dl or more and 240 sec / dl or less. Having such a configuration makes it possible to apply a highly safe polyolefin-based separator in a lithium ion secondary battery using a high-viscosity electrolytic solution.

First, the basic configuration of the lithium ion secondary battery according to the present invention will be described. The lithium ion secondary battery according to the present invention is expected to have effects such as improvement of the lithium ion transport rate and the charge / discharge reaction rate by using a high viscosity (high concentration) electrolytic solution. Therefore, it is excellent for a vehicle drive power source and an auxiliary power source, which require high energy density and high output density. As a result, it can be suitably used as a lithium ion secondary battery for driving a vehicle. In addition, it is fully applicable to lithium-ion secondary batteries for mobile devices such as mobile phones.

When the lithium ion secondary battery is distinguished by its form and structure, it can be applied to any conventionally known form and structure such as a laminated type (flat type) battery and a wound type (cylindrical type) battery. Is. By adopting a laminated (flat type) battery structure, long-term reliability can be ensured by simple thermocompression bonding and other sealing technologies, which is advantageous in terms of cost and workability.

Further, when viewed in terms of the electrical connection form (electrode structure) in the lithium ion secondary battery, it can be applied to both non-bipolar type (internal parallel connection type) batteries and bipolar type (internal series connection type) batteries. It is a thing.

In the following description, as an example of a lithium ion secondary battery, a laminated (flat type) non-bipolar type (internal parallel connection type) lithium ion secondary battery will be described very briefly with reference to the drawings. However, the technical scope of the lithium ion secondary battery according to the present invention should not be limited to these.

FIG. 1 shows a flat type (laminated type) non-bipolar type (internal parallel connection type) lithium ion secondary battery (hereinafter, simply “laminated type), which is a typical embodiment of the lithium ion secondary battery of the present invention. It is a cross-sectional schematic diagram schematically showing the whole structure (also referred to as a battery).

As shown in FIG. 1, the laminated battery 10 of the present embodiment has a structure in which a substantially rectangular power generation element 21 in which a charge / discharge reaction actually proceeds is sealed inside a laminated sheet which is an exterior body 29. .. Here, the power generation element 21 has a positive electrode having positive electrode active material layers 13 arranged on both sides of the positive electrode current collector 11, an electrolyte layer 17 composed of a separator containing an electrolytic solution, and negative electrodes on both sides of the negative electrode current collector 12. It has a structure in which a negative electrode on which the active material layer 15 is arranged is laminated. Specifically, the negative electrode, the electrolyte layer, and the positive electrode are laminated in this order so that one positive electrode active material layer 13 and the negative electrode active material layer 15 adjacent thereto face each other via the electrolyte layer 17. ..

As a result, the adjacent positive electrode, electrolyte layer, and negative electrode form one cell cell layer 19. Therefore, it can be said that the laminated battery 10 shown in FIG. 1 has a configuration in which a plurality of single battery layers 19 are stacked and electrically connected in parallel. Although the positive electrode active material layer 13 is arranged on only one side of the positive electrode current collectors of the outermost layers located on both outermost layers of the power generation element 21, active material layers may be provided on both sides. .. That is, instead of using a current collector dedicated to the outermost layer having an active material layer on only one side, a current collector having active material layers on both sides may be used as it is as a current collector for the outermost layer. Further, by reversing the arrangement of the positive electrode and the negative electrode as in FIG. 1, the negative electrode current collector of the outermost layer is located on both outermost layers of the power generation element 21, and one side of the negative electrode current collector of the outermost layer or Negative electrode active material layers may be arranged on both sides.

In this embodiment, the viscosity of the electrolytic solution in the electrolyte layer 17 is 26 mPa · s or more. Further, the separator contains polypropylene as a constituent material thereof, and the air permeability of the separator is 120 sec / dl or more and 240 sec / dl or less.

A positive electrode current collector plate 25 and a negative electrode current collector plate 27 that are conductive to each electrode (positive electrode and negative electrode) are attached to the positive electrode current collector 11 and the negative electrode current collector 12, respectively, and are sandwiched between the ends of the exterior body 29. It has a structure derived to the outside of the exterior body 29 in this way. The positive electrode current collector plate 25 and the negative electrode current collector plate 27 are superposed on the positive electrode current collector 11 and the negative electrode current collector 12 of each electrode via the positive electrode terminal lead and the negative electrode terminal lead (not shown), respectively, as necessary. It may be attached by sonic welding, resistance welding, or the like.

The lithium ion secondary battery according to the present embodiment is characterized by the configuration of the electrolytic solution and the separator contained in the electrolyte layer. Hereinafter, the main components of the battery including the electrolytic solution and the separator will be described.

[Positive electrode]
The positive electrode has a function of generating electric energy by exchanging lithium ions together with the negative electrode. The positive electrode essentially includes a current collector and a positive electrode active material layer, and has a structure in which the positive electrode active material layer is arranged on the surface of the current collector.

(Current collector)
The current collectors (11, 12) are made of a conductive material. The size of the current collector is determined according to the intended use of the battery. For example, if it is used for a large battery that requires high energy density, a current collector having a large area is used.

There is no particular limitation on the thickness of the current collector. The thickness of the current collector is usually about 1 to 100 μm.

The shape of the current collector is also not particularly limited. In the laminated battery 10 shown in FIG. 1, in addition to the current collector foil, a mesh shape (expanded grid or the like) or the like can be used.

When a thin film alloy is directly formed on the negative electrode current collector 12 by a sputtering method or the like for the negative electrode active material, it is preferable to use a current collector foil.

There are no particular restrictions on the materials that make up the current collector. For example, a metal or a resin obtained by adding a conductive filler to a conductive polymer material or a non-conductive polymer material can be adopted.

Specific examples of the metal include aluminum, nickel, iron, stainless steel, titanium, and copper. In addition to these, a clad material of nickel and aluminum, a clad material of copper and aluminum, or a plating material of a combination of these metals can be preferably used. Further, the foil may be a metal surface coated with aluminum. Of these, aluminum, stainless steel, copper, and nickel are preferable from the viewpoints of electron conductivity, battery operating potential, adhesion of the negative electrode active material by sputtering to the current collector, and the like.

Examples of the conductive polymer material include polyaniline, polypyrrole, polythiophene, polyacetylene, polyparaphenylene, polyphenylene vinylene, polyacrylonitrile, and polyoxadiazole. Since such a conductive polymer material has sufficient conductivity without adding a conductive filler, it is advantageous in terms of facilitating the manufacturing process or reducing the weight of the current collector.

Examples of the non-conductive polymer material include polyethylene (PE; high density polyethylene (HDPE), low density polyethylene (LDPE), etc.), polypropylene (PP), polyethylene terephthalate (PET), polyether nitrile (PEN), and polyimide. (PI), Polyethyleneimide (PAI), Polyethylene (PA), Polytetrafluoroethylene (PTFE), Styrene-butadiene rubber (SBR), Polyacrylonitrile (PAN), Polymethylacrylate (PMA), Polymethylmethacrylate (PMMA) , Polyvinyl chloride (PVC), polyvinylidene fluoride (PVdF), polystyrene (PS) and the like. Such non-conductive polymer materials may have excellent potential or solvent resistance.

A conductive filler may be added to the above-mentioned conductive polymer material or non-conductive polymer material, if necessary. In particular, when the resin used as the base material of the current collector is composed of only a non-conductive polymer, a conductive filler is inevitably indispensable in order to impart conductivity to the resin.

The conductive filler can be used without particular limitation as long as it is a conductive substance. For example, examples of the material having excellent conductivity, potential resistance, or lithium ion blocking property include metals and conductive carbon. The metal is not particularly limited, but at least one metal selected from the group consisting of Ni, Ti, Al, Cu, Pt, Fe, Cr, Sn, Zn, In, Sb, and K, or a metal thereof. It is preferable to contain an alloy or a metal oxide containing. Further, the conductive carbon is not particularly limited. Preferably, it is selected from the group consisting of acetylene black, vulcan (registered trademark), black pearl (registered trademark), carbon nanofiber, Ketjen black (registered trademark), carbon nanotube, carbon nanohorn, carbon nanoballoon, and fullerene. It contains at least one species.

The amount of the conductive filler added is not particularly limited as long as it can impart sufficient conductivity to the current collector, and is generally about 5 to 35% by mass.

(Positive electrode active material layer)
The positive electrode active material layer 13 essentially contains the positive electrode active material, and may further contain additives such as a conductive auxiliary agent, a binder, a lithium salt (supporting salt), and an ionic conductive polymer, if necessary.

Positive electrode active material The positive electrode active material has a function of occluding ions at the time of discharge and releasing ions at the time of charging. Examples of the positive electrode active material include LiMn ₂ O ₄ , LiCoO ₂ , LiNiO ₂ , Li (Ni—Mn—Co) O ₂ , and lithium such as those in which some of these transition metals are replaced by other elements. Examples thereof include transition metal composite oxides, lithium-transition metal phosphate compounds, and lithium-transition metal sulfate compounds. In some cases, two or more kinds of positive electrode active materials may be used in combination. Preferably, a lithium-transition metal composite oxide is used as the positive electrode active material from the viewpoint of capacity and output characteristics. More preferably, a composite oxide containing lithium and nickel is used, and more preferably Li (Ni-Mn-Co) O ₂ and a part of these transition metals are substituted with other elements (hereinafter,). Simply referred to as "NMC composite oxide") is used. The NMC composite oxide has a layered crystal structure in which a lithium atomic layer and a transition metal (Mn, Ni and Co are arranged in an orderly manner) atomic layer are alternately stacked via an oxygen atomic layer, and each atom of the transition metal M has a layered crystal structure. Contains one Li atom. Therefore, the amount of Li that can be taken out is twice that of the spinel-based lithium manganese oxide, that is, the supply capacity is doubled, and a high capacity can be obtained.

As described above, the NMC composite oxide also includes a composite oxide in which a part of the transition metal element is replaced with another metal element. Other elements in that case include Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr, Fe, B, Ga, In, Si, Mo, Y, Sn, V, Cu. , Ag, Zn and the like, preferably Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr, and more preferably Ti, Zr, P, Al, Mg, It is Cr, and more preferably Ti, Zr, Al, Mg, and Cr from the viewpoint of improving cycle characteristics.

Since the NMC composite oxide has a high theoretical discharge capacity, it is preferable that the general formula: Li _a Ni _b Mn _{c Code} M _x O ₂ (where a, b, c, d, x are 0 in the formula. .9 ≦ a ≦ 1.2, 0 <b <1, 0 <c ≦ 0.5, 0 <d ≦ 0.5, 0 ≦ x ≦ 0.3, b + c + d = 1. M satisfies Ti, Zr. , Nb, W, P, Al, Mg, V, Ca, Sr, Cr, which is at least one kind). Here, a represents the atomic ratio of Li, b represents the atomic ratio of Ni, c represents the atomic ratio of Mn, d represents the atomic ratio of Co, and x represents the atomic ratio of M. Represents. From the viewpoint of cycle characteristics, it is preferable that 0.4 ≦ b ≦ 0.6 in the above general formula. The composition of each element can be measured by, for example, inductively coupled plasma (ICP) emission spectrometry.

Generally, nickel (Ni), cobalt (Co) and manganese (Mn) are known to contribute to capacity and output characteristics from the viewpoint of improving the purity of the material and improving the electron conductivity. Ti and the like partially replace the transition metal in the crystal lattice. From the viewpoint of cycle characteristics, it is preferable that a part of the transition element is substituted with another metal element, and it is particularly preferable that 0 <x ≦ 0.3 in the above general formula. The crystal structure is stabilized by solid solution of at least one selected from the group consisting of Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr and Cr. As a result, it is considered that the capacity of the battery can be prevented from decreasing even if charging and discharging are repeated, and excellent cycle characteristics can be realized.

As a more preferable embodiment, in the above general formula, b, c and d are 0.44 ≦ b ≦ 0.51, 0.27 ≦ c ≦ 0.31 and 0.19 ≦ d ≦ 0.26. This is preferable from the viewpoint of improving the balance between capacity and life characteristics. For example, LiNi _0.5 Mn _0.3 Co _0.2 O ₂ is LiCo O ₂ , LiMn ₂ O ₄ , LiNi _1/3 Mn _1/3 Co _1/3 O ₂ , etc., which have been proven in general consumer batteries. Compared with, the capacity per unit weight is large, and the energy density can be improved. Therefore, it has the advantage of being able to manufacture a compact and high-capacity battery, which is also preferable from the viewpoint of cruising range. In addition, LiNi _0.8 Co _0.1 Al _0.1 O ₂ is more advantageous in that it has a larger capacity, but it has a difficulty in life characteristics. On the other hand, LiNi _0.5 Mn _0.3 Co _0.2 O ₂ has excellent life characteristics comparable to LiNi _1/3 Mn _1/3 Co _1/3 O ₂ .

In some cases, two or more kinds of positive electrode active materials may be used in combination. Preferably, a lithium-transition metal composite oxide is used as the positive electrode active material from the viewpoint of capacity and output characteristics. Of course, a positive electrode active material other than the above may be used.

The average particle size of the positive electrode active material is not particularly limited, but is preferably 1 to 100 μm, more preferably 1 to 20 μm from the viewpoint of increasing the output. In this specification, the median diameter (D50) obtained by using the laser diffraction method is adopted as the average particle diameter of the particles.

The ratio of the positive electrode active material contained in the positive electrode active material layer is preferably 60 to 99% by mass, more preferably 80 to 95% by mass, based on the total mass of the solid content in the positive electrode active material layer. , 80-85% by mass, more preferably. By containing the positive electrode active material in such a ratio, a battery having a high capacity density can be obtained.

Conductive Auxiliary Agent A conductive additive refers to an additive compounded to improve the conductivity of a positive electrode active material layer or a negative electrode active material layer. Examples of the conductive auxiliary agent include carbon black such as Ketjen Black (registered trademark) and acetylene black. When the active material layer contains a conductive auxiliary agent, an electronic network inside the active material layer is effectively formed, which can contribute to the improvement of the output characteristics of the battery.

The content of the conductive auxiliary agent in the positive electrode active material layer is preferably 1 to 10% by mass, more preferably 1 to 8% by mass. The following effects are exhibited by defining the blending ratio (content) of the conductive auxiliary agent within the above range. That is, the electron conductivity can be sufficiently ensured without inhibiting the electrode reaction, the decrease in energy density due to the decrease in electrode density can be suppressed, and the energy density can be improved by improving the electrode density. It is.

Binder The binder is added for the purpose of binding the active materials to each other or the active material and the current collector to maintain the electrode structure. The binder used for the positive electrode active material layer is not particularly limited, and examples thereof include the following materials. Polyethylene, polypropylene, polyethylene terephthalate (PET), polyether nitrile, polyacrylonitrile, polyimide, polyamide, cellulose, carboxymethyl cellulose (CMC) and its salts, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR) ), Isoprene rubber, butadiene rubber, ethylene / propylene rubber, ethylene / propylene / diene copolymer, styrene / butadiene / styrene block copolymer and its hydrogen additive, styrene / isoprene / styrene block copolymer and its hydrogen addition Thermoplastic polymers such as fluoropolymers, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), tetrafluoroethylene / hexafluoropropylene copolymer (FEP), tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer (PFA) ), Fluororesin such as ethylene / tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene / chlorotrifluoroethylene copolymer (ECTFE), polyvinylfluorovinyl (PVF), vinylidene fluoride -Hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based fluororubber), vinylidene fluoride-pentafluoropropylene-based fluoropolymer Rubber (VDF-PFP-based fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-PFP-TFE-based fluororubber), vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene-based fluororubber (VDF-PFMVE-TFE-based fluororubber), vinylidene fluoride-chlorotrifluoroethylene-based fluororubber (VDF-CTFE-based fluororubber) and the like, vinylidene fluoride-based fluororubber, epoxy resin and the like can be mentioned. These binders may be used alone or in combination of two or more.

The binder content in the positive electrode active material layer is preferably 1 to 10% by mass, more preferably 1 to 8% by mass.

Other Ingredients Lithium salts (supporting salts) include (FSO ₂ ) ₂ NLi, (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F ₅ SO ₂ ) ₂ NLi, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ and the like.

Examples of the ion conductive polymer include polyethylene oxide (PEO) -based and polypropylene oxide (PPO) -based polymers.

The positive electrode active material layer shall be formed by any of a kneading method, a sputtering method, a vapor deposition method, a CVD method, a PVD method, an ion plating method and a thermal spraying method, in addition to a method of applying (coating) a normal slurry. Can be done.

[Negative electrode]
The negative electrode has a function of generating electric energy by exchanging lithium ions together with the positive electrode. The negative electrode essentially includes a current collector and a negative electrode active material layer, and has a structure in which the negative electrode active material layer is arranged on the surface of the current collector.

(Current collector)
Since the current collector that can be used for the negative electrode is the same as the current collector that can be used for the positive electrode, detailed description thereof will be omitted here.

(Negative electrode active material layer)
The negative electrode active material layer 15 essentially contains the negative electrode active material, and may further contain additives such as a conductive auxiliary agent, a binder, a lithium salt (supporting salt), and an ionic conductive polymer, if necessary. Since the materials other than the negative electrode active material that can be used for the negative electrode active material layer and the method for producing the negative electrode active material layer are the same as those for the positive electrode active material layer, detailed description thereof will be omitted here.

Negative electrode active material The negative electrode active material has a function of emitting ions at the time of discharging and occluding the ions at the time of charging. Negative negative active materials include carbon-based negative negative active materials such as natural graphite, artificial graphite, carbon black, activated charcoal, carbon fiber, coke, soft carbon, or hard carbon, pure metals such as Si and Sn, and alloy-based active materials thereof. , Or metal oxides such as TiO, Ti ₂ O ₃ , TiO ₂ , or SiO ₂ , SiO, SnO ₂ , or composite oxidation of lithium and transition metal such as Li _4/3 Ti _5/3 O ₄ or Li ₇ MnN. Examples thereof include Li-Pb-based alloys, Li-Al-based alloys, and Li. Above all, from the viewpoint of achieving high energy density, it is preferable to use a silicon-based negative electrode active material containing silicon (Si).

The silicon-based negative electrode active material is not particularly limited, and examples thereof include Si alone, silicon oxides such as SiO ₂ , and SiO, and silicon-containing alloys. Above all, it is preferable to contain a silicon-containing alloy having the composition of the following chemical formula (1) because high cycle durability can be realized.

In the formula, A is an unavoidable impurity, M is at least one transition metal element, and x, y, z, and a represent a value of% by mass, in which case 0 <x <100, 0 <y <100, 0 <z <100, and 0 ≦ a <0.5, and x + y + z + a = 100.

In the present specification, the “unavoidable impurity” means a silicon-containing alloy that is present in the raw material or is inevitably mixed in the manufacturing process. Although the unavoidable impurities are originally unnecessary, they are acceptable impurities because they are in trace amounts and do not affect the characteristics of the silicon-containing alloy.

The silicon-containing alloy represented by the chemical formula (1) is at least a ternary system of Si, Sn and M (transition metal). As described above, excellent cycle durability can be exhibited by having at least a ternary system of Si, Sn and M.

Among them, by selecting Ti as the additive element (transition metal element M), it is possible to further suppress the phase transition of the amorphous-crystal during Li alloying and improve the cycle life. Further, this makes the capacity higher than that of the conventional negative electrode active material (for example, carbon-based negative electrode active material). Therefore, according to a preferred embodiment of the present invention, it is preferable that M is titanium (Ti) in the composition represented by the above chemical formula (1).

Here, it is preferable to suppress the phase transition of amorphous-crystal during Li alloying for the following reasons. When Si and Li are alloyed during charging, the Si material changes from an amorphous state to a crystalline state and causes a large volume change (about 4 times), so that the active material particles themselves are broken and function as an active material. Is lost. Therefore, by suppressing the phase transition of the amorphous-crystal, it is possible to suppress the disintegration of the particles themselves and maintain the function (high capacity) as an active material, and as a result, it is possible to improve the cycle life. This is to become. By selecting such an additive element (transition metal element M), a silicon-containing alloy having a high capacity and excellent cycle characteristics can be obtained.

In the composition of the above chemical formula (1), the composition ratio z of the transition metal element M (particularly Ti) is preferably 7 <z <100, more preferably 10 <z <100, and 15 <z <. It is more preferably 100, and particularly preferably 20 ≦ z <100. By setting the composition ratio z of the transition metal element M (particularly Ti) in such a range, the cycle characteristics can be further improved.

More preferably, the x, y, and z in the chemical formula (1) are the following formulas (1) or (2):

It is preferable to satisfy. When the content of each component is within the above range, an initial discharge capacity exceeding 1000 Ah / g can be obtained, and the cycle life can also exceed 90% (50 cycles).

From the viewpoint of further improving the above-mentioned characteristics of the negative electrode active material, it is desirable that the content of the transition metal element M (particularly Ti) is in the range of more than 7% by mass. That is, the x, y, and z are the following mathematical formulas (3) or (4):

It is preferable to satisfy. This makes it possible to further improve the cycle characteristics.

Then, from the viewpoint of ensuring better cycle durability, the x, y, and z are the following mathematical formulas (5) or (6):

It is preferable to satisfy.

Then, from the viewpoint of the initial discharge capacity and the cycle durability, the x, y, and z are the following mathematical formulas (7):

It is preferable to satisfy.

As described above, A is an impurity (unavoidable impurity) other than the above three components derived from the raw material and the manufacturing method. The a (that is, the balance) is 0 ≦ a <0.5, and preferably 0 ≦ a <0.1.

Whether or not the negative electrode active material (silicon-containing alloy) has the composition of the above chemical formula (1) is determined by qualitative analysis by fluorescent X-ray analysis (XRF) and quantitative analysis by inductively coupled plasma (ICP) emission spectrometry. It is possible to confirm.

The average particle size of the silicon-based negative electrode active material is not particularly limited, but is preferably 0.1 to 100 μm, more preferably 0.1 to 100 μm from the viewpoint of increasing the capacity of the negative electrode active material, reactivity, and cycle durability. It is 20 μm, more preferably 0.2 to 10 μm. Within such a range, an increase in the internal resistance of the battery during charging / discharging under high output conditions is suppressed, and a sufficient current can be taken out. When the silicon-based negative electrode active material is a secondary particle, it is desirable that the average particle diameter of the primary particles constituting the secondary particle is in the range of 10 nm to 100 μm. When the average particle size of the primary particles is 10 nm or more, the conductive path in the negative electrode active material is satisfactorily formed, and an increase in resistance can be suppressed. Further, when the average particle diameter of the primary particles is 100 μm or less, it is possible to prevent a decrease in battery capacity due to overvoltage. However, although it depends on the production method, the silicon-based negative electrode active material may not be agglomerated or agglomerated into secondary particles.

The shape of the silicon-based negative electrode active material differs depending on the type and manufacturing method, and examples thereof include spherical (powder), plate, needle, columnar, and square, but are limited to these. It is not a thing and can be used without any problem in any shape. Preferably, it is desirable to appropriately select the optimum shape that can improve the battery characteristics such as charge / discharge characteristics.

The ratio of the negative electrode active material contained in the negative electrode active material slurry is preferably 60 to 99% by mass, more preferably 80 to 95% by mass, and 80 to 85% by mass with respect to the total mass of the solid content. Is more preferable. By containing the negative electrode active material in such a ratio, a battery having a high capacity density can be obtained.

[Electrolyte layer]
The electrolyte layer 17 is formed by holding the electrolytic solution in the separator.

(Electrolytic solution)
The electrolytic solution has a form in which a lithium salt (supporting salt) is dissolved in an organic solvent which is a plasticizer. Examples of the organic solvent include ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), acetonitrile, 1,2-dimethoxyethane, trimethyl phosphate, and dimethyl. Examples thereof include sulfoxide and 1,3-dioxolane. These may be used alone or in combination of two or more. In particular, a chain carbonate is preferable from the viewpoint that the viscosity does not become too high even if a high concentration lithium salt is dissolved. Specific examples thereof include carbonates such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). The lithium salts include (FSO ₂ ) ₂ NLi, (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F ₅ SO ₂ ) ₂ NLi, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiTaF ₆ , LiCF. ₃ A compound that can be added to the active material layer of the electrode such as SO ₃ can be similarly adopted. Of these, (FSO ₂ ) ₂ NLi (lithium bis (fluorosulfonyl) imide) is preferably used because Li is easily ionized and has excellent ionic conductivity. The electrolytic solution may further contain additives other than the above-mentioned components. Specific examples of such compounds include, for example, vinylene carbonate, methylvinylene carbonate, dimethylvinylene carbonate, phenylvinylene carbonate, diphenylvinylene carbonate, ethylvinylene carbonate, diethylvinylene carbonate, vinylethylene carbonate, 1,2-divinylethylene carbonate. , 1-Methyl-1-vinylethylene carbonate, 1-methyl-2-vinylethylenecarbonate, 1-ethyl-1-vinylethylenecarbonate, 1-ethyl-2-vinylethylenecarbonate, vinylvinylene carbonate, allylethylenecarbonate, vinyl Oxymethylethylene carbonate, allyloxymethylethylene carbonate, acrylicoxymethylethylene carbonate, methacryloxymethylethylene carbonate, ethynylethylene carbonate, propargylethylene carbonate, ethynyloxymethylethylene carbonate, propargyloxyethylene carbonate, methyleneethylene carbonate, 1,1- Examples thereof include dimethyl-2-methyleneethylene carbonate. Of these, vinylene carbonate, methylvinylene carbonate and vinylethylene carbonate are more preferable, and vinylene carbonate and vinylethylene carbonate are even more preferable. Only one of these carbonates may be used alone, or two or more thereof may be used in combination.

The electrolytic solution of the present embodiment is characterized by having a viscosity of 26 mPa · s or more. A high-viscosity electrolytic solution having a viscosity of 26 mPa · s or more generally has low permeability to a separator. Conventionally, the permeability has been improved by making the separator hydrophilic, but according to the present invention, it is possible to apply a highly safe polyolefin-based separator as described later. That is, in the present invention, the viscosity of the electrolytic solution has a position as a precondition for the problem to be solved by the present invention arises.

In the present embodiment, the viscosity of the electrolytic solution is indispensably 26 mPa · s or more, preferably 50 mPa · s or more, and more preferably 100 mPa · s or more. The higher the viscosity of the electrolytic solution, the more remarkable the effect of the present invention is exhibited, which is preferable. On the other hand, the upper limit of the viscosity of the electrolytic solution is not particularly limited, but in order to operate the battery normally, it is generally 500 mPa · s or less, preferably 300 mPa · s or less, and more preferably 150 mPa · s. It is less than or equal to s. In this specification, the viscosity of the electrolytic solution adopts the value measured by the falling ball type viscosity measuring method using a falling ball type viscometer (Lovis2000M, manufactured by Anton Pearl Co., Ltd.) described in Examples described later. And.

In the present embodiment, the concentration of the lithium salt in the electrolytic solution is not particularly limited. However, from the viewpoint of improving the cycle characteristics by eliminating the variation in the lithium ion transport rate in the electrolytic solution and the reaction, the concentration of the lithium salt is preferably more than 4 mol / L and 8 mol / L or less, more preferably 4. It is 5 mol / L or more and 6 mol / L or less, and more preferably 4.7 mol / L or more and 5.5 mol / L or less. Above all, when (FSO ₂ ) ₂ NLi is used as the lithium salt, it is preferable to set (FSO ₂ ) ₂ NLi in the above concentration range.

(Separator)
The separator has a function of holding an electrolytic solution and ensuring lithium ion conductivity between the positive electrode and the negative electrode, and a function as a partition wall between the positive electrode and the negative electrode.

In the present embodiment, it is essential that the separator contains polypropylene (PP) as a constituent material thereof. Since polypropylene has excellent shutdown performance, it is possible to form a highly safe separator.

Examples of the form of the separator include a microporous membrane made of a polymer or fiber that absorbs and retains the electrolytic solution, a non-woven fabric, and the like.

Examples of the microporous membrane separator include a microporous membrane made of PP. Further, a laminated body (for example, PP / PE / PP) in which a microporous film made of a mixture of PP and a polyolefin such as PE or a microporous film made of PP and a microporous film made of a polyolefin such as polyethylene (PE) is combined. It may be a laminated body having a three-layer structure of the above). Further, a microporous film made of PP is combined with a microporous film made of a hydrocarbon resin such as polyimide, aramid, polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP), glass fiber, etc. to form a laminate. May be good.

Examples of the non-woven fabric separator include non-woven fabric made of PP. Further, in addition to PP, a non-woven fabric obtained by mixing cotton, rayon, acetate, nylon, polyester; polyolefin such as PE; polyimide, aramid and other conventionally known materials can be mentioned. The bulk density of the non-woven fabric may be any one as long as sufficient battery characteristics can be obtained, and should not be particularly limited.

In the present embodiment, it is preferable that the content ratio of PP in the resin contained in the separator is high. Specifically, the ratio of PP to the total amount of the resin is preferably 50% by mass or more, more preferably 75% by mass or more, further preferably 90% by mass or more, and particularly preferably 95% by mass or more. It is most preferably 100% by mass. The higher the ratio of PP, the more the battery performance can be improved while exhibiting the shutdown performance.

Further, it may be a separator (separator with a heat-resistant insulating layer) in which a heat-resistant insulating layer is laminated on the above-mentioned microporous film or non-woven fabric. The heat-resistant insulating layer is a ceramic layer containing inorganic particles and a binder. As the separator with a heat-resistant insulating layer, a separator having a melting point or a thermal softening point of 150 ° C. or higher, preferably 200 ° C. or higher, having high heat resistance is used. By having the heat-resistant insulating layer, the internal stress of the separator, which increases when the temperature rises, is relaxed, so that the effect of suppressing heat shrinkage can be obtained. As a result, it is possible to prevent the induction of a short circuit between the electrodes of the battery, so that the battery configuration is less likely to cause performance deterioration due to a temperature rise. Further, by having the heat-resistant insulating layer, the mechanical strength of the separator with the heat-resistant insulating layer is improved, and the film breakage of the separator is less likely to occur. Further, due to the effect of suppressing heat shrinkage and high mechanical strength, the separator is less likely to curl in the battery manufacturing process.

Inorganic particles in the heat-resistant insulating layer contribute to the mechanical strength of the heat-resistant insulating layer and the effect of suppressing heat shrinkage. The material used as the inorganic particles is not particularly limited. Examples include silicon, aluminum, zirconium, titanium oxides (SiO ₂ , Al _{2O 3} , ZrO ₂ , TiO ₂ ), hydroxides, and nitrides, and composites thereof. These inorganic particles may be derived from mineral resources such as boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, and mica, or may be artificially produced. Further, only one kind of these inorganic particles may be used alone, or two or more kinds thereof may be used in combination. Of these, from the viewpoint of cost, it is preferable to use silica (SiO ₂ ) or alumina (Al ₂ O ₃ ), and it is more preferable to use alumina (Al ₂ O ₃ ).

The basis weight of the heat-resistant particles is not particularly limited, but is preferably 5 to 15 g / m ² . Within this range, sufficient ionic conductivity can be obtained and heat resistance is preferably maintained.

The binder in the heat-resistant insulating layer has a role of adhering the inorganic particles or the inorganic particles to the microporous film or the non-woven fabric. The binder stably forms the heat-resistant insulating layer, and can prevent peeling between the microporous film or the non-woven fabric and the heat-resistant insulating layer.

The binder used for the heat-resistant insulating layer is not particularly limited, and is, for example, carboxymethyl cellulose (CMC), polyacrylonitrile, cellulose, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), isoprene rubber. , Butadiene rubber, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl fluoride (PVF), methyl acrylate and the like can be used as the binder. Of these, carboxymethyl cellulose (CMC), methyl acrylate, or polyvinylidene fluoride (PVDF) is preferably used. Only one of these compounds may be used alone, or two or more of these compounds may be used in combination.

The content of the binder in the heat-resistant insulating layer is preferably 2 to 20% by mass with respect to 100% by mass of the heat-resistant insulating layer. When the binder content is 2% by mass or more, the peel strength between the heat-resistant insulating layer and the microporous film or the non-woven fabric can be increased, and the vibration resistance of the separator can be improved. On the other hand, when the binder content is 20% by mass or less, the gaps between the inorganic particles are appropriately maintained, so that sufficient lithium ion conductivity can be ensured.

The heat shrinkage of the separator with a heat-resistant insulating layer is preferably 10% or less in both MD (resin flow direction) and TD (resin flow orthogonal direction) after holding for 1 hour under the conditions of 150 ° C. and 2 gf / cm ² . By using such a highly heat-resistant material, it is possible to effectively prevent the separator from shrinking even when the amount of heat generated by the positive electrode increases and the internal temperature of the battery reaches 150 ° C. As a result, it is possible to prevent the induction of a short circuit between the electrodes of the battery, so that the battery configuration is less likely to cause performance deterioration due to a temperature rise.

The present embodiment is characterized in that the air permeability of the separator measured by the Garley method is 120 sec / dl or more and 240 sec / dl or less. If the upper limit of the air permeability is more than 240 sec / dl, the permeability of the high-viscosity electrolytic solution becomes low, and sufficient battery performance cannot be exhibited. The upper limit of the air permeability is more preferably 210 sec / dl or less from the viewpoint of further improving the permeability of the high-viscosity electrolytic solution. On the other hand, according to the studies by the present inventors, it has been found that even if the lower limit of the air permeability is less than 120 sec / dl, sufficient battery performance may not be exhibited. The present inventors presume that the occurrence of a minute short circuit in the separator due to the generation of dendrite is a factor that deteriorates the battery performance. From the same viewpoint, the lower limit of the air permeability is preferably 150 sec / dl or more. In this specification, the air permeability of the separator shall be the value measured by the measuring method (Garley method) based on JIS P 8117: 2009 described in Examples described later.

In the present embodiment, the porosity of the separator is not particularly limited, but is preferably 33 to 65%, more preferably 35 to 62%, and even more preferably 38 to 56%. When the porosity of the separator is within the above range, the release of the generated gas is improved, the battery has better long-term cycle characteristics, and the short-circuit prevention and mechanical properties, which are the functions of the separator, are sufficient. It becomes a thing. In this specification, the porosity of the separator shall be the value measured by the mercury intrusion method using a mercury porosimeter described in Examples described later.

In the present embodiment, the thickness of the separator is not particularly limited, but is preferably 6 to 20 μm, more preferably 6 to 18 μm, and particularly preferably 6 to 10 μm. When the thickness of the separator is within the above range, short-circuit prevention and mechanical properties, which are functions as a separator, are sufficient.

[Current collector plate (tab)]
In a lithium ion secondary battery, a current collector plate (tab) electrically connected to a current collector is taken out to the outside of a laminated film which is an exterior body for the purpose of taking out a current to the outside of the battery.

The material constituting the current collector plate is not particularly limited, and a known highly conductive material conventionally used as a current collector plate for a lithium ion secondary battery can be used. As the constituent material of the current collector plate, for example, metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferable. From the viewpoint of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferable, and aluminum is particularly preferable. The same material may be used for the positive electrode current collector plate (positive electrode tab) and the negative electrode current collector plate (negative electrode tab), or different materials may be used.

Further, the removal of the tabs 58 and 59 shown in FIG. 2 is not particularly limited. The positive electrode tab 58 and the negative electrode tab 59 may be pulled out from the same side, or the positive electrode tab 58 and the negative electrode tab 59 may be divided into a plurality of each and taken out from each side, as shown in FIG. It is not limited to. Further, in the winding type lithium ion secondary battery, the terminal may be formed by using, for example, a cylindrical can (metal can) instead of the tab.

[Seal part]
The seal portion is a member peculiar to a bipolar type (internal series connection type) battery and has a function of preventing leakage of the electrolyte layer. In addition to this, it is also possible to prevent adjacent current collectors from coming into contact with each other in the battery and short-circuiting due to slight irregularities at the ends of the laminated electrodes.

The constituent material of the sealing portion is not particularly limited, but a polyolefin resin such as polyethylene and polypropylene, an epoxy resin, rubber, polyimide and the like can be used. Of these, it is preferable to use a polyolefin resin from the viewpoints of corrosion resistance, chemical resistance, film forming property, economy and the like.

[Positive terminal lead and negative electrode terminal lead]
As the material of the positive electrode terminal lead and the negative electrode terminal lead, the lead used in a known laminated battery can be used. The part taken out from the exterior body is heat-resistant and insulating heat so that it does not affect the product (for example, automobile parts, especially electronic devices) by contacting peripheral devices and wiring and causing electric leakage. It is preferable to cover with a shrink tube or the like.

[Exterior body; Laminating film]
As the exterior body, a conventionally known metal can case can be used. In addition, the laminating film 22 as shown in FIG. 1 may be used as an exterior body to pack the power generation element 21. The laminating film can be configured as, for example, a three-layer structure in which polypropylene, aluminum, and nylon are laminated in this order. By using such a laminated film, it is possible to easily open the exterior body, add a capacity recovery material, and reseal the exterior body.

<Manufacturing method of lithium ion secondary battery>
The method for producing the lithium ion secondary battery is not particularly limited, and the lithium ion secondary battery can be produced by a known method. Specifically, it includes (1) fabrication of electrodes, (2) fabrication of a cell cell layer, (3) fabrication of a power generation element, and (4) fabrication of a laminated battery. Hereinafter, a method for manufacturing a lithium ion secondary battery will be described with an example, but the present invention is not limited thereto.

(1) Preparation of Electrode (Positive Electrode and Negative Electrode) For the electrode (positive electrode or negative electrode), for example, an active material slurry (positive electrode active material slurry or negative electrode active material slurry) is prepared, and the active material slurry is applied onto the current collector. It can be made by drying, then pressing. The active material slurry contains the above-mentioned active material (solid solution positive electrode active material or Si-containing alloy negative electrode active material), a binder, a conductive auxiliary agent, and a solvent.

The solvent is not particularly limited, and N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methylformamide, cyclohexane, hexane, water and the like can be used.

The method for applying the active material slurry to the current collector is not particularly limited, and examples thereof include a screen printing method, a spray coating method, an electrostatic spray coating method, an inkjet method, and a doctor blade method.

The method for drying the coating film formed on the surface of the current collector is not particularly limited, and at least a part of the solvent in the coating film may be removed. Examples of the drying method include heating. The drying conditions (drying time, drying temperature, etc.) can be appropriately set according to the volatilization rate of the solvent contained in the active material slurry to be applied, the coating amount of the active material slurry, and the like. A part of the solvent may remain. The remaining solvent can be removed by a pressing step or the like described later.

The pressing means is not particularly limited, and for example, a calendar roll, a flat plate press, or the like can be used.

(2) Preparation of Single Battery Layer The single battery layer can be manufactured by laminating the electrodes (positive electrode and negative electrode) prepared in (1) via the electrolyte layer.

(3) Manufacture of Power Generation Element The power generation element can be manufactured by laminating the cell cell layers in consideration of the output and capacity of the cell cell layer, the output and capacity required as a battery, and the like.

(4) Manufacture of laminated battery As the battery configuration, various shapes such as a square type, a paper type, a laminated type, a cylindrical type, and a coin type can be adopted. Further, the current collector, the insulating plate, and the like of the constituent parts are not particularly limited, and may be selected according to the above-mentioned shape. In the laminated battery, leads are bonded to the current collector of the power generation element obtained above, and these positive electrode terminal leads or negative electrode terminal leads are bonded to the positive electrode tab or the negative electrode tab. Then, the power generation element is put in a laminated sheet so that the positive electrode tab and the negative electrode tab are exposed to the outside of the battery, the high-viscosity electrolytic solution is injected by a liquid injection machine, and then the laminated battery is sealed with a vacuum. Can be manufactured.

In the present embodiment, it is preferable to perform the initial charging after providing an impregnation step for sufficiently infiltrating the separator with the high-viscosity electrolytic solution, if necessary. Specifically, the battery is allowed to stand at 25 to 55 ° C., preferably 35 to 50 ° C., more preferably 40 to 47 ° C. under pressure or atmospheric pressure after sealing. To promote the penetration of the high-viscosity electrolytic solution into the separator. The time of the impregnation step is preferably 3 hours, more preferably 6 hours or more, still more preferably 8 hours or more. The upper limit is not particularly limited, but from the viewpoint of simplifying the process, it is preferably 48 hours or less, more preferably 36 hours or less, still more preferably 24 hours or less.

[Battery set]
The assembled battery is configured by connecting a plurality of batteries. More specifically, it is composed of serialization, parallelization, or both by using at least two or more. By serializing and parallelizing, the capacity and voltage can be freely adjusted.

It is also possible to connect a plurality of batteries in series or in parallel to form a small assembled battery that can be attached / detached. Then, by connecting a plurality of small detachable batteries in series or in parallel, a large capacity and a large capacity suitable for a vehicle drive power source or an auxiliary power source that require a high volume energy density and a high volume output density. It is also possible to form an assembled battery with an output. How many batteries are connected to make an assembled battery, and how many stages of small assembled batteries are stacked to make a large-capacity assembled battery depends on the battery capacity of the vehicle (electric vehicle) to be mounted. It may be decided according to the output.

[vehicle]
The electric device of the present invention, such as the lithium ion secondary battery according to the present embodiment, maintains the discharge capacity even after long-term use and has good cycle characteristics. In addition, the volumetric energy density is high. Vehicle applications such as electric vehicles, hybrid electric vehicles, fuel cell vehicles, and hybrid fuel cell vehicles are required to have higher capacity and larger size than electric and portable electronic devices, as well as high safety and long life. Is required. Therefore, the lithium ion secondary battery (electric device) can be suitably used as a power source for a vehicle, for example, a vehicle drive power source or an auxiliary power source.

Specifically, a battery or a combined battery formed by combining a plurality of batteries can be mounted on the vehicle. In the present invention, a long-life battery with excellent safety, long-term reliability, and output characteristics can be configured. Therefore, when such a battery is installed, a plug-in hybrid electric vehicle having a long EV mileage or a long one-charge mileage can be installed. Can configure an electric vehicle. In addition to hybrid vehicles, fuel cell vehicles, electric vehicles (all four-wheeled vehicles (passenger vehicles, trucks, commercial vehicles such as buses, light vehicles, etc.)) This is because it becomes a highly reliable automobile with a long life by using it for two-wheeled vehicles (including motorcycles) and three-wheeled vehicles. However, the application is not limited to automobiles, and can be applied to various power sources of other vehicles, for example, moving objects such as trains, and power supplies for mounting such as uninterruptible power supplies. It is also possible to use it as.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples.

[Manufacturing of lithium-ion secondary batteries]
(Preparation of electrolyte)
Lithium bis (fluorosulfonyl) imide ((FSO ₂ ) ₂ NLi; LiFSI) or LiPF ₆ in the glove box, dimethyl carbonate (DMC) alone, or ethylene carbonate (EC), diethyl carbonate (DEC) and ethylmethyl. An electrolytic solution was prepared by dissolving carbonate (EMC) in a mixed solution mixed at a ratio of 3: 6: 1 (volume ratio) at the concentrations shown in Table 1 or Table 2 below, respectively.

The viscosity of the electrolytic solution shown in Table 1 was measured by a falling ball type viscosity measuring method using a falling ball type viscometer (Lovis2000M, manufactured by Anton Pearl Co., Ltd.).

(Separator)
Separators having thickness, porosity and air permeability (material: PP or a laminate of PP and PE, size: 50 mm × 50 mm) shown in Table 1 below were prepared.

The porosity of the separator was measured by a mercury intrusion method using a mercury porosimeter.

The air permeability of the separator is measured by a measurement method based on JIS P 8117: 2009, and is an air permeability measuring device (Yasuda Seiki) installed in a dry room where the dew point of the outlet is set to -60 ° C and the temperature is set to 25 ° C. It was measured using (manufactured by Mfg. Co., Ltd.).

(Positive electrode / Negative electrode)
As the positive electrode and the negative electrode, commercially available products used in electric vehicle batteries were used. The positive electrode active material contained in the positive electrode is a mixture of Li _1.1 Mn _1.9 O ₄ having a spinel structure and lithium, nickel, cobalt, and lithium manganate (Ni / Li molar ratio 0.7). The binder was polyvinylidene fluoride, and the conductive additive was carbon black powder. The negative electrode active material contained in the negative electrode was graphite. The porosity and the pore diameter of the positive electrode active material layer and the negative electrode active material layer were general values in the present technical field. The active material layer on one side of the current collector of the positive electrode and the negative electrode was peeled off, and the active material layer was cut out to a size of 29 mm × 40 mm and used.

(Making laminated batteries)
An aluminum positive electrode tab (positive electrode current collector plate) was welded to the positive electrode current collector portion of the positive electrode. Further, a negative electrode tab (negative electrode current collector plate) of electrolytic copper was welded to the negative electrode current collector portion of the negative electrode. The positive electrode active material layer of the positive electrode and the negative electrode active material layer of the negative electrode to which these tabs were welded were opposed to each other, and a separator was sandwiched between them to prepare a power generation element composed of one cell cell layer. Next, both sides of the power generation element were sandwiched between aluminum laminated film exterior materials (60 mm × 60 mm), and the three sides were thermocompression-bonded to house the power generation element. 500 μl of an electrolytic solution was injected into this power generation element using a vacuum impregnation device (TOSPACK V-307GII, manufactured by Tohoku Denshi Co., Ltd.), and the remaining one side was vacuum-sealed by thermocompression bonding. Then, the battery was allowed to stand at 45 ° C. for 8 hours to allow the electrolytic solution to permeate the separator to prepare a laminated battery.

(Evaluation of initial charge / discharge characteristics)
Each laminated battery produced above was charged / discharged three times according to the following charge / discharge test conditions, and the initial charge / discharge characteristics were evaluated.

<Charging / discharging test conditions>
1) Charge / discharge tester: TOSCAT-3000 (manufactured by Toyo System Co., Ltd.)
2) Constant temperature bath: set to 25 ° C using LU-113 (manufactured by Espec Co., Ltd.) 3) Charging / discharging conditions [Charging process] 4.2V, 0.1C (1st time) or 0.2C (2nd and 3rd time) ) (Constant current / constant voltage mode), 40 hours, cutoff I <1.5%
[Discharge process] 2.7V, 0.1C (1st time) or 0.2C (2nd and 3rd time) (constant current mode).

In the above three charge / discharge (pause for 10 minutes after discharge and then start charging in the next cycle), the one showing the capacity according to the design capacity is ○, and the overvoltage is too large to show the design capacity ± 10%. Alternatively, those that do not operate as batteries were evaluated as x. The results are shown in Table 1 below.

From the results in Table 1, it was shown that the lithium ion secondary battery according to the present invention exhibits sufficient charge / discharge performance even when an electrolytic solution having a viscosity of 26 mPa · s or more is used. ..

The lithium ion secondary battery using the separator I having an air permeability of 25 sec / dl showed a behavior in which the charge capacity became a value equal to or larger than the design capacity and an obvious side reaction occurred. The present inventors presume that the battery has a minute short circuit due to the generation of dendrites.

(Evaluation of cycle durability)
The laminated battery using the separators C and H was charged and discharged according to the following charge / discharge test conditions, and the cycle durability was evaluated.

<Charging / discharging test conditions>
1) Charge / discharge tester: TOSCAT-3000 (manufactured by Toyo System Co., Ltd.)
2) Constant temperature bath: set to 25 ° C using LU-113 (manufactured by Espec Co., Ltd.) 3) Charging / discharging conditions [Charging process] 4.2V, 2.0C (constant current / constant voltage mode), 40 hours, 25 ℃
[Discharge process] 2.7V, 0.5C (constant current mode).

The above charge / discharge is performed 100 times, the discharge capacity in the first charge / discharge and the discharge capacity in the 100th charge / discharge are obtained, and the discharge capacity retention rate (100th discharge capacity ÷ 1st discharge capacity × 100 (%)). ) Was calculated. When checking the discharge capacity, the charging conditions were set to 4.2V and 0.5C (constant current / constant voltage mode), and the discharge conditions were set to 2.7V and 0.5C (constant current mode). The results are shown in Table 2 below.

From the results in Table 2, it was shown that a high discharge capacity retention rate can be obtained in the lithium ion secondary battery in which the separator H is applied to the high-viscosity electrolytic solution.

10,50 Lithium-ion secondary battery,
11 Positive electrode current collector,
12 Negative electrode current collector,
13 Positive electrode active material layer,
15 Negative electrode active material layer,
17 Separator,
19 Single battery layer,
21,57 Power generation element,
25 Positive electrode current collector plate,
27 Negative current collector plate,
29, 52 exterior body,
58 Positive tab,
59 Negative electrode tab.

Claims (7)

With the positive electrode
With the negative electrode
An electrolyte layer interposed between the positive electrode and the negative electrode and the electrolytic solution being held by the separator.
A lithium-ion secondary battery having a power generation element including
The viscosity of the electrolytic solution is 26 mPa · s or more, and the viscosity is 26 mPa · s or more.
The separator contains polypropylene and
A lithium ion secondary battery having an air permeability measured by the Garley method of the separator of 120 sec / dl or more and 240 sec / dl or less. The lithium ion secondary battery according to claim 1, wherein the electrolytic solution has a viscosity of 50 mPa · s or more. The lithium ion secondary battery according to claim 1 or 2, wherein the electrolytic solution has a viscosity of 150 mPa · s or less. The lithium ion secondary battery according to any one of claims 1 to 3, wherein the air permeability of the separator measured by the Garley method is 150 sec / dl or more and 210 sec / dl or less. The lithium ion secondary battery according to any one of claims 1 to 4, wherein the separator has a thickness of 6 μm or more and 18 μm or less. The lithium ion secondary battery according to any one of claims 1 to 5, wherein the electrolytic solution contains at least one selected from the group consisting of chain carbonate. The lithium ion secondary battery according to any one of claims 1 to 6, wherein the electrolytic solution contains lithium bis (fluorosulfonyl) imide at a concentration of more than 4 mol / L and 8 mol / L or less.

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