JP2011134690A - Lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a means capable of suppressing corrosion of a positive electrode collector in a lithium ion secondary battery. <P>SOLUTION: A lithium ion secondary battery 10 has a power generation element 21 which includes unit cell layers 19. Each unit cell layer includes: a positive electrode comprising a positive electrode active material layer 15 formed on a surface of a positive electrode collector 12; a negative electrode comprising a negative electrode active material layer 13 formed on a surface of a negative electrode collector 11; and an electrolyte layer 17 interposed between the positive electrode active material layer and the negative electrode active material layer and containing a liquid electrolyte. At least one of the positive electrode collectors contains stainless steel, and the liquid electrolyte contains dodecaborate fluoride expressed by the following chemical formula 1, Li<SB>2</SB>B<SB>12</SB>F<SB>x</SB>H<SB>12-x</SB>. In the formula, x represents 4-12. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a lithium ion secondary battery.

  In recent years, the development of electric vehicles (EV), hybrid electric vehicles (HEV), and fuel cell vehicles (FCV) has been promoted against the background of the increasing environmental protection movement. A secondary battery that can be repeatedly charged and discharged is suitable as a power source for driving these motors, and a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery that can be expected to have a high capacity and a high output is attracting attention.

  The nonaqueous electrolyte secondary battery has, as its constituent elements, a single battery layer including a positive electrode, a negative electrode, and an electrolyte layer interposed between the positive electrode and the negative electrode.

  As a configuration of the electrolyte layer, for example, a separator formed by impregnating a liquid electrolyte in a separator made of a microporous resin sheet is known. This liquid electrolyte contains an organic solvent and a lithium salt as essential components in a lithium ion secondary battery.

Conventionally, as a lithium salt for constituting a liquid electrolyte contained in an electrolyte layer of a lithium ion secondary battery, for example, a fluorine-containing lithium salt such as lithium hexafluorophosphate (LiPF ₆ ) has been used (patent document). 1). Such fluorine-containing lithium salts are widely used in lithium ion secondary batteries in that they have a wide potential window, are easily solventable in organic solvents, and have high electrical conductivity.

US Pat. No. 6,346,351

  Here, a metal material such as aluminum or stainless steel is often used as the positive electrode current collector constituting the positive electrode of the lithium ion secondary battery.

However, when a liquid electrolyte containing a fluorine-containing lithium salt as described in Patent Document 1 is used in a lithium ion secondary battery using these metal materials as a constituent material of a positive electrode current collector, a positive electrode current collector is formed. It has been found that the problem of elution of the metal material occurs. This is presumably because fluoride ions (F ⁻ ) generated by decomposition of the lithium salt during high-temperature operation corrode the metal material that is a constituent material of the positive electrode current collector.

  This invention is made | formed in view of the said situation, and it aims at providing the means which can suppress corrosion of a positive electrode electrical power collector in a lithium ion secondary battery.

  The present inventors have intensively studied to solve the above problems. As a result, it has been found that the above problems can be solved by adopting stainless steel as a constituent material of the positive electrode current collector and configuring a liquid electrolyte using a specific lithium salt, and to complete the present invention. It came.

  The lithium ion secondary battery of the present invention includes a positive electrode in which a positive electrode active material layer is formed on the surface of a positive electrode current collector, a negative electrode in which a negative electrode active material layer is formed on the surface of a negative electrode current collector, and a positive electrode active material. The power generation element includes a single battery layer that is interposed between the material layer and the negative electrode active material layer and includes an electrolyte layer containing a liquid electrolyte. At least one of the positive electrode current collectors includes stainless steel, and the liquid electrolyte has the following chemical formula 1:

Where x is 4-12.
The fluorinated dodecaborate represented by these is included.

The fluoride dodecaborate constituting the liquid electrolyte in the lithium ion secondary battery of the present invention is preferentially adsorbed on the surface of the positive electrode current collector containing stainless steel without producing fluoride ions (F ⁻ ). As a result, exposure of the active surface of the positive electrode current collector is suppressed, and consequently corrosion of the positive electrode current collector is suppressed.

1 is a schematic cross-sectional view showing a lithium ion secondary battery according to an embodiment of the present invention.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. In addition, the dimensional ratios in the drawings are exaggerated for convenience of explanation, and may be different from the actual ratios.

  Lithium ion secondary batteries have various forms and structures such as stacked (flat) batteries and wound (cylindrical) batteries, for example, when distinguished by form and structure. Although these forms can be applied to the following embodiments, a case where a stacked (flat) battery structure is employed will be described here. Of course, a structure other than a stacked (flat) battery structure, such as a wound (cylindrical) battery, can also be implemented. By adopting a laminated (flat) battery structure, long-term reliability can be ensured by a sealing technique such as simple thermocompression bonding, which is advantageous in terms of cost and workability.

  FIG. 1 is a schematic cross-sectional view showing a lithium ion secondary battery according to an embodiment of the present invention.

  As shown in FIG. 1, the lithium ion secondary 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 laminate sheet 29 that is an exterior. Have. Specifically, the power generation element 21 is housed and sealed by using a polymer-metal composite laminate sheet as the battery exterior and joining all of its peripheral parts by thermal fusion.

  The power generation element 21 includes a negative electrode in which the negative electrode active material layer 13 is disposed on both surfaces of the negative electrode current collector 11, an electrolyte layer 17, and a positive electrode in which the positive electrode active material layer 15 is disposed on both surfaces of the positive electrode current collector 12. It has a stacked configuration. Specifically, the negative electrode, the electrolyte layer, and the positive electrode are stacked in this order so that one negative electrode active material layer 13 and the positive electrode active material layer 15 adjacent thereto face each other with the electrolyte layer 17 therebetween. .

  Thereby, the adjacent negative electrode, electrolyte layer, and positive electrode constitute one unit cell layer 19. Therefore, it can be said that the lithium ion battery 10 of the present embodiment has a configuration in which a plurality of single battery layers 19 are stacked and electrically connected in parallel. The outermost negative electrode current collectors located in both outermost layers of the power generation element 21 are each provided with the negative electrode active material layer 12 only on one side. In addition, the arrangement of the negative electrode and the positive electrode is reversed from that in FIG. 1 so that the outermost positive electrode current collector is positioned in both outermost layers of the power generation element 21, and the positive electrode is formed only on one side of the outermost positive electrode current collector An active material layer may be arranged. Of course, when the negative electrodes are located on both outermost layers of the power generation element 21 as shown in FIG. 1, the negative electrode active material layers are arranged on both surfaces of the outermost layer (negative electrode) current collector, so It is good also as a structure which does not make the negative electrode active material layer to function.

  The positive electrode current collector 11 and the negative electrode current collector 12 are respectively attached with a negative electrode current collector plate 25 and a positive electrode current collector plate 27 that are electrically connected to the respective electrodes (positive electrode and negative electrode). These current collector plates (25, 27) are led out of the laminate sheet 29 so as to be sandwiched between the end portions of the laminate sheet 29, respectively. The negative electrode current collector plate 25 and the positive electrode current collector plate 27 are ultrasonically welded to the negative electrode current collector 11 and the positive electrode current collector 12 of each electrode via a negative electrode lead and a positive electrode lead (not shown), respectively, as necessary. Or resistance welding or the like.

  Hereinafter, although the component of the lithium ion secondary battery mentioned above is demonstrated, it is not limited only to the following form.

[Positive electrode (positive electrode current collector, positive electrode active material layer)]
The positive electrode has a structure in which a positive electrode active material layer 15 is formed on the surface of the positive electrode current collector 12.

  One of the features of the lithium ion secondary battery 10 of the present embodiment is that the constituent material of the positive electrode current collector 12 is stainless steel. Corrosion of the positive electrode current collector 12 can be suppressed by forming the positive electrode current collector 12 from stainless steel and including a specific salt described later in the liquid electrolyte.

Stainless steel is an Fe—Cr— (Ni) -based alloy containing about 12% or more of Cr, and is broadly divided into Cr-based stainless steel and Cr—Ni-based stainless steel in terms of composition. Further, the structure is classified into ferritic stainless steel, martensitic stainless steel, austenitic stainless steel, austenitic-ferrite duplex stainless steel, and precipitation hardened stainless steel. Here, examples of the austenite type include SUS201, SUS202, SUS301, SUS302, SUS303, SUS304, SUS305, SUS316, and SUS317. Moreover, SUS329J1 is mentioned as an austenite ferrite system. Furthermore, examples of martensite include SUS403 and SUS420. Examples of the ferrite type include SUS405, SUS430, and SUS430LX. An example of the precipitation hardening system is SUS630. Especially, in this embodiment, Preferably, austenitic stainless steel or austenitic-ferritic duplex stainless steel is used. Such a configuration is preferable in that stress corrosion cracking due to fluoride ions (F ⁻ ) can be suppressed, and corrosion of the positive electrode current collector 12 can be further suppressed. Moreover, although these stainless steels contain nickel (Ni), the fluorinated dodecaborate used for constituting the liquid electrolyte in this embodiment preferentially adsorbs on nickel and covers the surface of the stainless steel. be able to. Of course, stainless steels other than those listed above may be used as the constituent material of the positive electrode current collector 12 by such a mechanism. In the lithium ion secondary battery 10 of the present embodiment, all the positive electrode current collectors 12 constituting the positive electrode are made of stainless steel. However, the technical scope of the present invention is not limited to such a form, and it is sufficient that at least one positive electrode current collector 12 is made of stainless steel.

  In another preferred embodiment, the stainless steel constituting the positive electrode current collector 12 preferably contains nitrogen (N) or molybdenum (Mo). Examples of such stainless steel include SUS316, SUS316L, SUS316N, and SUS354N. Such a form is preferable in that corrosion (pitting corrosion) gap cracking of stainless steel is suppressed, and corrosion of the positive electrode current collector 12 can be further suppressed.

In still another preferred embodiment, the carbon content in the stainless steel constituting the positive electrode current collector 12 is 0.3% by mass or less, or the stainless steel constituting the positive electrode current collector 12 is titanium (Ti). Niobium (Nb) or tantalum (Ta) is preferable. By reducing the carbon content in stainless steel, sensitization (Fe ₂₃ C ₆ ) of stainless steel is suppressed. Similarly, the sensitization of stainless steel can be suppressed by including in the stainless steel an element (Ti, Nb, Ta) that can react with carbon. By suppressing the sensitization of stainless steel in this way, corrosion of stainless steel can be further suppressed, which is preferable. Here, SUS321 is mentioned as a stainless steel containing titanium (Ti), SUS347 is mentioned as a stainless steel containing niobium (Nb), and SUS348 is mentioned as a stainless steel containing tantalum (Ta). .

  Next, the positive electrode active material layer 15 will be described. The positive electrode active material layer 15 includes a positive electrode active material, and may further include a conductive material, a binder, and the like for increasing electrical conductivity as necessary.

The positive electrode active material is not particularly limited as long as it is a material capable of occluding and releasing lithium, and a positive electrode active material usually used for a lithium ion secondary battery can be used. Specifically, a lithium-transition metal composite oxide is preferable. For example, a Li—Mn composite oxide such as LiMn ₂ O _4, a Li—Ni composite oxide such as LiNiO ₂ , LiNi _0.5 Mn _0. A Li—Ni—Mn based composite oxide such as ₅ O ₂ can be given. In some cases, two or more positive electrode active materials may be used in combination.

  The conductive material is blended for the purpose of improving the conductivity of the active material layer. The electrically conductive material that can be used in the present embodiment is not particularly limited, and conventionally known forms can be appropriately referred to. Examples thereof include carbon blacks such as acetylene black, furnace black, channel black, and thermal black; carbon fibers such as vapor grown carbon fiber (VGCF); and carbon materials such as graphite. When the active material layer includes a conductive material, an electronic network inside the active material layer is effectively formed, which can contribute to improvement of output characteristics of the battery.

  The binder is not limited to the following, but heat such as polyvinylidene fluoride (PVDF), carboxymethylcellulose (CMC), polytetrafluoroethylene (PTFE), polyvinyl acetate, and acrylic resin (for example, LSR). Examples thereof include thermosetting resins such as plastic resins, polyimides, epoxy resins, polyurethane resins, and urea resins, and rubber-based materials such as styrene-butadiene rubber (SBR).

[Negative electrode (negative electrode current collector, negative electrode active material layer)]
The negative electrode has a structure in which the negative electrode active material layer 13 is formed on the surface of the negative electrode current collector 11.

  There is no restriction | limiting in particular about the constituent material of the negative electrode electrical power collector 11, A conventionally well-known knowledge can be referred suitably. As a constituent material of the negative electrode current collector 11, for example, a metal or a conductive polymer can be employed. Specific examples include iron, chromium, nickel, manganese, titanium, molybdenum, vanadium, niobium, copper, silver, platinum, stainless steel, and carbon, and these may form a simple substance, an alloy, or a composite. Note that a structure having a structure in which a conductive filler is dispersed in a base material made of a nonconductive polymer can also be adopted as one form of the negative electrode current collector.

  The negative electrode active material layer 13 includes a negative electrode active material, and may further include a conductive material similar to the above, a binder, and the like as necessary.

The negative electrode active material is not particularly limited as long as it is made of a material capable of inserting and extracting lithium, but preferably contains an element that can be alloyed with lithium. Examples of the form containing an element that can be alloyed with lithium include simple elements that can be alloyed with lithium, oxides and carbides containing these elements, and the like. By using an element that can be alloyed with lithium, a high-capacity battery having a higher energy density than that of a conventional carbon material can be obtained. Elements that can be alloyed with lithium are not limited to the following, but specifically, Si, Ge, Sn, Pb, Al, In, Zn, H, Ca, Sr, Ba, Ru, Rh, Ir, Pd, Pt, Ag, Au, Cd, Hg, Ga, Tl, C, N, Sb, Bi, O, S, Se, Te, Cl and the like can be mentioned. Among these, from the viewpoint that a battery excellent in capacity and energy density can be configured, the negative electrode active material contains at least one element selected from the group consisting of Si, Ge, Sn, Pb, Al, In, and Zn. It is preferable to include Si, Sn elements are more preferable, and Si is particularly preferable. As the oxide, silicon monoxide (SiO), tin dioxide (SnO ₂ ), tin monoxide (SnO), or the like can be used. These may be used individually by 1 type and may use 2 or more types together.

In addition, carbon materials such as graphite, soft carbon, and hard carbon, metal materials such as lithium metal, lithium-transition metal composite oxides such as lithium-titanium composite oxide (lithium titanate: Li ₄ Ti ₅ O ₁₂ ), In addition, other conventionally known negative electrode active materials can be used. In some cases, two or more of these negative electrode active materials may be used in combination.

[Electrolyte layer]
The electrolyte layer 17 functions as a spatial partition (spacer) between the positive electrode active material layer and the negative electrode active material layer. In addition, it also has a function of holding an electrolyte that is a lithium ion transfer medium between the positive and negative electrodes during charging and discharging.

  In the present embodiment, the electrolyte layer 17 includes a liquid electrolyte (electrolytic solution). And since a liquid electrolyte (electrolytic solution) is used, it is necessary to use a separator for the electrolyte layer 17. Here, as a specific form of the separator, for example, a polyolefin such as polyethylene or polypropylene, a hydrocarbon such as polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP), a microporous film made of glass fiber, or the like can be given.

  The liquid electrolyte has a form in which a lithium salt as a supporting salt is dissolved in an organic solvent. Examples of the organic solvent include carbonates such as ethylene carbonate (EC) and propylene carbonate (PC). The lithium ion secondary battery 10 of this embodiment is characterized in that the liquid electrolyte contained in the electrolyte layer 17 contains a specific salt. Specifically, the liquid electrolyte of the present embodiment has the following chemical formula 1:

The fluorinated dodecaborate represented by these is included.

According to the prior art, the positive electrode current collector made of aluminum is eluted under the influence of fluoride ions (F ⁻ ) derived from lithium salts. Then, the eluted aluminum is reduced on the negative electrode side and deposited as metallic aluminum. As a result, there is a problem that the internal resistance of the battery increases and the capacity retention rate also decreases. On the other hand, according to the lithium ion secondary battery 10 of the present embodiment, the liquid electrolyte containing the predetermined fluoride dodecaborate is employed in combination with the positive electrode current collector 12 made of stainless steel. It becomes possible to suppress corrosion of the current collector 12. As a result, the above-described problems in the prior art are solved, and a battery having excellent durability can be provided.

In addition, the use of the fluorinated dodecaborate of the above chemical formula 1 is essential in combination with the use of stainless steel made of a positive electrode current collector. For example, even if a liquid electrolyte is formed using the fluorinated dodecaborate in a lithium ion secondary battery using a positive electrode current collector made of aluminum, corrosion (elution) of aluminum cannot be prevented. This is because fluoride ions (F ⁻ ) are not generated from the above-described fluoride dodecaborate, and therefore, a film of aluminum fluoride (AlF ₃ ) is not formed on the aluminum surface as the positive electrode current collector. In the lithium ion secondary battery of the present embodiment, fluoride dodecaborate is preferentially adsorbed on the surface of the positive electrode current collector including stainless steel without generating fluoride ions (F ⁻ ). As a result, exposure of the active surface of the positive electrode current collector is suppressed, and consequently corrosion of the positive electrode current collector is suppressed.

In chemical formula 1, x is 4-12. In a more preferred form, x in Chemical Formula 1 is preferably 12. That is, among the fluorinated dodecaborates represented by Chemical Formula 1, it is preferable that Li ₂ B ₁₂ F ₁₂ where x = 12 is included as a supporting salt for the liquid electrolyte. According to such a form, there is an advantage that it is difficult to generate a decomposition product that may cause a decrease in battery performance, and the durability of the battery can be improved. This is presumably because the fluorinated dodecaborate does not contain a hydrogen atom, so that the durability against the potential on the oxidation side is high even at the battery operating temperature (about 80 ° C.). At this time, it is more preferable that the initial fluoride ion (F ⁻ ) concentration and chloride ion (Cl ⁻ ) concentration in the liquid electrolyte are 1 mass ppm or less, respectively. Such a form is preferable because corrosion of stainless steel due to the presence of these halide ions is sufficiently suppressed, and the durability of the battery can be improved.

Also in this embodiment, the liquid electrolyte may contain a lithium salt other than the fluorinated dodecaborate of the above chemical formula 1 as a supporting salt. The Such lithium salts, _{e.g., LiPF 6, LiBF 4, LiAlF} 4, LiAsF 6, include conventionally known lithium salt such as LiSbF _6. However, for the reasons described above, it is preferable that the content of these halogen atom-containing lithium salts is not too high. On the other hand, these halogen atom-containing lithium salts can improve the dissociation of the lithium salt in the liquid electrolyte. Thereby, the raise of the internal resistance of the battery by use of a battery can be suppressed. Accordingly, in another preferred form, a liquid _{electrolyte, LiPF 6, LiBF 4, LiAlF} 4, one or more lithium salts are selected from the group consisting of LiAsF _6, and LiSbF _6, the following concentrations 0.1M Contains.

  In the present embodiment, the liquid electrolyte may further include additives other than the components described above. One type of additive that can be used in the present embodiment is a cyclic carbonate having an unsaturated bond. Specific examples of such compounds include, for example, vinylene carbonate, methyl vinylene carbonate, dimethyl vinylene carbonate, phenyl vinylene carbonate, diphenyl vinylene carbonate, ethyl vinylene carbonate, diethyl vinylene carbonate, vinyl ethylene carbonate, 1,2-divinyl ethylene carbonate. 1-methyl-1-vinylethylene carbonate, 1-methyl-2-vinylethylene carbonate, 1-ethyl-1-vinylethylene carbonate, 1-ethyl-2-vinylethylene carbonate, vinylvinylene carbonate, allylethylene carbonate, vinyl Oxymethyl ethylene carbonate, allyloxymethyl ethylene carbonate, acryloxymethyl ethylene carbonate, methacrylate Oxy methylethylene carbonate, ethynyl ethylene carbonate, propargyl carbonate, ethynyloxy methylethylene carbonate, propargyloxy ethylene carbonate, methylene carbonate, etc. 1,1-dimethyl-2-methylene-ethylene carbonate. Among these, vinylene carbonate, methyl vinylene carbonate, and vinyl ethylene carbonate are preferable, and vinylene carbonate and vinyl ethylene carbonate are more preferable. These cyclic carbonates may be used alone or in combination of two or more.

  Further, in the present embodiment, the liquid electrolyte may include a sultone compound such as 1,3-propane sultone and 1,4-butane sultone. These sultone compounds may also be used alone or in combination of two or more.

  In the present embodiment, when the liquid electrolyte contains a cyclic carbonate or sultone compound having an unsaturated bond as described above, a stable film can be formed on the surface of the active material constituting the battery. As a result, an increase in the internal resistance of the battery due to the use of the battery can be suppressed, and the durability of the battery can be improved, which is preferable.

  The addition amount (total amount) of these additives (cyclic carbonate, sultone compound) in the present embodiment is not particularly limited, but is preferably 100% by mass relative to the total amount of the organic solvent and the supporting salt described above. It is 0.1-10 mass%, More preferably, it is 0.5-5 mass%, More preferably, it is 0.8-3 mass%, Most preferably, it is 1-2 mass%. If the addition amount of the above-described additive is a value within such a range, the above-described effect due to the addition is necessary and sufficient.

[Positive electrode current collector and negative electrode current collector]
The material which comprises a current collector plate (25, 27) is not restrict | limited in particular, The well-known highly electroconductive material conventionally used as a current collector plate for lithium ion secondary batteries can be used. As a 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. The negative electrode current collector plate 25 and the positive electrode current collector plate 27 may be made of the same material or different from each other as long as they are not alloyed with lithium and the progress of corrosion is acceptable for the battery life. Materials may be used.

[Positive lead and negative lead]
Moreover, although illustration is abbreviate | omitted, you may electrically connect between the collector 11 and the current collector plates (25, 27) via a negative electrode lead or a positive electrode lead. As a constituent material of the negative electrode and the positive electrode lead, materials used in known lithium ion secondary batteries can be similarly employed. In addition, heat-shrinkable heat-shrinkable parts are removed from the exterior so that they do not affect products (for example, automobile parts, especially electronic devices) by touching peripheral devices or wiring and causing leakage. It is preferable to coat with a tube or the like.

[Exterior]
As the exterior, a laminate sheet 29 as shown in FIG. 1 can be used. For example, the laminate sheet may be configured as a three-layer structure in which polypropylene, aluminum, and nylon are laminated in this order. In some cases, a conventionally known metal can case can also be used as an exterior.

In the lithium ion secondary battery 10 of the present embodiment, the liquid electrolyte contains fluorinated dodecaborate. The positive electrode current collector is made of stainless steel. Fluorine dodecaborate is preferentially adsorbed on the surface of the positive electrode current collector containing stainless steel without generating fluoride ions (F ⁻ ). As a result, the exposure of the active surface of the positive electrode current collector is suppressed, and as a result, the effect of suppressing the corrosion of the positive electrode current collector is exhibited.

  Hereinafter, the present invention will be specifically described based on examples. The technical scope of the present invention is not limited to only these examples.

<Example 1>
1. Production of Positive Electrode A solid content comprising 85% by mass of LiFePO ₄ (average particle size: 10 μm) as a positive electrode active material, 5% by mass of acetylene black (average particle size: 0.1 μm) as a conductive additive, and 10% by mass of PVdF as a binder. Prepared. An appropriate amount of N-methyl-2-pyrrolidone (NMP), which is a slurry viscosity adjusting solvent, was added to the solid content to prepare a positive electrode slurry. Next, the positive electrode slurry was applied to one side of a stainless steel (SUS304) foil (thickness: 10 μm) as a positive electrode current collector, dried, and subjected to a press treatment to produce a positive electrode having a positive electrode active material layer.

2. Production of Negative Electrode A solid content comprising 90% by mass of hard carbon (average particle size: 10 μm) as a negative electrode active material and 10% by mass of PVdF as a binder was prepared. An appropriate amount of NMP, which is a slurry viscosity adjusting solvent, was added to the solid content to prepare a negative electrode slurry. Next, the negative electrode slurry was applied to one side of a copper foil (thickness: 10 μm) as a negative electrode current collector, dried, and subjected to a press treatment to produce a negative electrode having a negative electrode active material layer.

3. Preparation of Electrolytic Solution A mixed solvent (1: 1 (volume ratio)) of ethylene carbonate (EC) and propylene carbonate (PC) was used as a solvent. Li ₂ B ₁₂ F ₁₂ that is a lithium salt was added to this solvent at a concentration of 0.4 M to prepare an electrolytic solution.

4). Battery Completion Step The positive electrode and the negative electrode produced above were laminated via a separator (polyethylene microporous film, thickness: 25 μm) so that the active material layers face each other.

  A tab was welded to each of the positive electrode and the negative electrode, and sealed in an exterior made of an aluminum laminate film to complete a laminated lithium ion secondary battery.

<Example 2>
A laminated lithium ion secondary battery was completed by the same method as in Example 1 except that SUS316 was used instead of SUS304 as the stainless steel constituting the positive electrode current collector.

<Example 3>
A laminated lithium ion secondary battery was completed by the same method as in Example 1 except that SUS316N was used instead of SUS304 as the stainless steel constituting the positive electrode current collector.

<Example 4>
A laminated lithium ion secondary battery was completed by the same method as in Example 1 described above, except that SUS354N was used instead of SUS304 as the stainless steel constituting the positive electrode current collector.

<Example 5>
A laminated lithium ion secondary battery was completed by the same method as in Example 1 except that SUS304L was used instead of SUS304 as the stainless steel constituting the positive electrode current collector.

<Example 6>
A laminated lithium ion secondary battery was completed by the same method as in Example 1 except that SUS316L was used instead of SUS304 as the stainless steel constituting the positive electrode current collector.

<Example 7>
A laminated lithium ion secondary battery was completed by the same method as in Example 1 except that SUS329 was used instead of SUS304 as the stainless steel constituting the positive electrode current collector.

<Example 8>
A laminated lithium ion secondary battery was completed by the same method as in Example 1 except that SUS321 was used instead of SUS304 as the stainless steel constituting the positive electrode current collector.

<Example 9>
A laminated lithium ion secondary battery was completed by the same method as in Example 1 except that SUS347 was used instead of SUS304 as the stainless steel constituting the positive electrode current collector.

<Example 10>
A laminated lithium ion secondary battery was completed in the same manner as in Example 1 except that SUS348 was used instead of SUS304 as the stainless steel constituting the positive electrode current collector.

<Example 11>
A laminated lithium ion secondary battery was completed by the same method as in Example 5 described above except that LiPF ₆ was further dissolved at a concentration of 0.1 M as the electrolytic solution.

<Example 12>
A laminated lithium ion secondary battery was completed by the same method as in Example 11 except that SUS316L was used instead of SUS304L as the stainless steel constituting the positive electrode current collector.

<Example 13>
By the same method as in Example 5 described above except that vinylene carbonate (VC) was added at a concentration of 1% by mass with respect to 100% by mass of the electrolytic solution prepared in Example 1, as the electrolytic solution. A laminated lithium ion secondary battery was completed.

<Example 14>
A laminated lithium ion secondary battery was completed by the same method as in Example 13 described above, except that SUS316L was used instead of SUS304L as the stainless steel constituting the positive electrode current collector.

<Example 15>
Except that vinylene carbonate (VC) and 1,3-propane sultone (PS) were added at a concentration of 1% by mass with respect to 100% by mass of the electrolyte prepared in Example 1, respectively. A laminated lithium ion secondary battery was completed by the same method as in Example 5 described above.

<Example 16>
A laminated lithium ion secondary battery was completed by the same method as in Example 15 described above, except that SUS316L was used instead of SUS304L as the stainless steel constituting the positive electrode current collector.

<Example 17>
By the same method as in Example 11 described above except that vinylene carbonate (VC) was added at a concentration of 1% by mass with respect to 100% by mass of the electrolyte prepared in Example 11, as the electrolyte. A laminated lithium ion secondary battery was completed.

<Example 18>
A laminated lithium ion secondary battery was completed by the same method as in Example 17 described above, except that SUS316L was used instead of SUS304L as the stainless steel constituting the positive electrode current collector.

<Example 19>
Except that vinylene carbonate (VC) and 1,3-propane sultone (PS) were added at a concentration of 1% by mass with respect to 100% by mass of the electrolyte prepared in Example 11, respectively. A laminated lithium ion secondary battery was completed by the same method as in Example 11 described above.

<Example 20>
A laminated lithium ion secondary battery was completed by the same method as in Example 19 except that SUS316L was used instead of SUS304L as the stainless steel constituting the positive electrode current collector.

<Comparative Example 1>
As the positive electrode current collector, an aluminum foil (thickness: 20 μm) was used instead of the stainless steel foil. Further, LiPF ₆ was used as a lithium salt at a concentration of 1.0 M instead of Li ₂ B ₁₂ F ₁₂ . Except for these points, a stacked lithium ion secondary battery was completed by the same method as in Example 1 described above.

<Comparative Example 2>
A laminated lithium ion secondary battery was completed by the same method as in Example 1 except that an aluminum foil (thickness: 20 μm) was used instead of the stainless steel foil as the positive electrode current collector.

<Comparative Example 3>
A laminated lithium ion secondary battery was completed by the same method as in Example 5 except that LiPF ₆ was used at a concentration of 1.0 M instead of Li ₂ B ₁₂ F ₁₂ as the lithium salt. .

<Comparative example 4>
A laminated lithium ion secondary battery was completed by the same method as Comparative Example 3 described above, except that SUS316L was used instead of SUS304L as the stainless steel constituting the positive electrode current collector.

<Battery evaluation>
About the battery produced in said each Example and each comparative example, after charging for 8 hours (CCCV, upper limit voltage 3.6V) at 0.2C, it discharges at 0.2C (CC, lower limit voltage 2.0V) ) And the discharge capacity at this time was measured, and this was used as the initial discharge capacity.

  Further, each battery was charged to SOC 50%, and the value obtained by dividing the potential difference ΔE when discharged at 1 C for 10 seconds by the value of the flowing current was used as the initial battery resistance.

  Further, each battery was charged at 0.2 C for 8 hours (CCCV, upper limit 3.6 V), and then stored in a thermostat set at 80 ° C. for 30 days to conduct a storage test.

  After a 30-day storage test, the discharge capacity and battery resistance were measured by the same method as described above. The values of the discharge capacity and the battery resistance after the storage test when the initial discharge capacity and the initial battery resistance are 100% are shown in Table 1 below.

<Discussion>
In Comparative Example 1, a fluorine-containing lithium salt is combined with a positive electrode current collector made of aluminum. As a result, not only a sufficient capacity retention rate is not ensured, but the internal resistance of the battery is significantly increased. This is considered to be a result of corrosion (elution) of the aluminum current collector by fluoride ions (F ⁻ ) derived from the fluorine-containing lithium salt.

In Comparative Example 2, fluorinated dodecaborate is combined with a positive electrode current collector made of aluminum. As a result, there are problems in both capacity retention and internal resistance. This is probably because, unlike the case of using a fluorine-containing lithium salt, a film of aluminum fluoride (AlF ₃ ) is not formed on the surface of the aluminum current collector, and corrosion (elution) of aluminum is not prevented.

In Comparative Examples 3 and 4, a fluorine-containing lithium salt is combined with a positive electrode current collector made of stainless steel. As a result, the capacity retention rate is not sufficiently ensured, and the increase in the internal resistance of the battery is still not sufficiently suppressed. This is considered to be the result of corrosion (elution) of the current collector made of stainless steel by fluoride ions (F ⁻ ) derived from the fluorine-containing lithium salt, as in Comparative Example 1 described above.

  On the other hand, in Examples 1-20 which have the structure of this invention, compared with Comparative Examples 1-4, it turns out that both a capacity | capacitance maintenance factor and a resistance increase rate are improved.

  From a comparison between Example 1 and Examples 2 to 4, in the present invention, when the stainless steel constituting the positive electrode current collector contains nitrogen (N) or molybdenum (Mo), the capacity retention rate is further improved. I understand that.

Further, from comparison between Example 1 and Examples 5 and 6, it can be seen that, in the present invention, when the carbon content in the stainless steel constituting the positive electrode current collector is small, the capacity retention rate is further improved. Similarly, from the comparison between Example 1 and Examples 8 to 10, in the present invention, when the stainless steel constituting the positive electrode current collector contains Ti, Nb, or Ta, the capacity retention rate is further improved. I understand that. These results are thought to be due to the suppression of sensitization (Fe ₂₃ C ₆ ) of stainless steel.

  Furthermore, from comparison between Example 1 and Examples 11 and 12, it can be seen that when the liquid electrolyte further contains a fluorine-containing lithium salt as a supporting salt, the capacity retention rate is further improved. Similarly, the comparison effect of Example 1 and Examples 13-16 confirmed the effect of addition of various additives, and the comparison of Example 1 and Examples 17-20 confirmed the combined effect of the fluorine-containing lithium salt / additive. It was.

10 Lithium ion secondary battery,
11 negative electrode current collector,
12 positive electrode current collector,
13 negative electrode active material layer,
15 positive electrode active material layer,
17 electrolyte layer,
19 cell layer,
21 power generation elements,
25 negative current collector,
27 positive current collector,
29 Laminate sheet.

Claims (7)

A positive electrode in which a positive electrode active material layer is formed on the surface of the positive electrode current collector;
A negative electrode in which a negative electrode active material layer is formed on the surface of the negative electrode current collector;
An electrolyte layer interposed between the positive electrode active material layer and the negative electrode active material layer and containing a liquid electrolyte;
A lithium ion secondary battery having a power generation element comprising a single cell layer comprising:
At least one of the positive electrode current collectors comprises stainless steel;
The liquid electrolyte has the following chemical formula 1:
Where x is 4-12.
The lithium ion secondary battery containing the fluoride dodecaborate represented by these.   The lithium ion secondary battery according to claim 1, wherein the stainless steel is austenitic stainless steel or ferrite-austenite duplex stainless steel.   The lithium ion secondary battery according to claim 1 or 2, wherein the stainless steel contains nitrogen or molybdenum.   The lithium ion secondary battery according to any one of claims 1 to 3, wherein the stainless steel has a carbon content of 0.3% by mass or less, or contains Ti, Nb, or Ta.   5. The lithium ion secondary battery according to claim 1, wherein x is 12, and a fluoride ion concentration and a chloride ion concentration in the initial liquid electrolyte are each 1 ppm by mass or less. The liquid electrolyte contains one or more lithium salts selected from the group consisting of LiPF ₆ , LiBF ₄ , LiAlF ₄ , LiAsF ₆ and LiSbF _{6 at} a concentration of 0.1 M or less. The lithium ion secondary battery according to any one of 5.   The lithium ion secondary battery according to any one of claims 1 to 6, wherein the liquid electrolyte contains vinyl carbonate, vinyl ethylene carbonate, or 1,3-propane sultone.