JP7027649B2 - Lithium ion secondary battery - Google Patents

Description

The present invention relates to a lithium ion secondary battery.

Lithium-ion secondary batteries have the characteristic of high energy density, and are widely used as power sources for mobile phones, notebook computers, electric vehicles, and the like. As an example of the structure of a lithium ion secondary battery, a lithium ion secondary battery having a zigzag structure is known.

Examples of the technique relating to the zigzag-folded lithium ion secondary battery include those described in Patent Document 1 (Japanese Patent Laid-Open No. 2009-218105) and Patent Document 2 (Japanese Patent Laid-Open No. 2016-143550).

Patent Document 1 has a square battery element in which a flat plate-shaped positive electrode, a separator, and a flat plate-shaped negative electrode are laminated, and one surface of the battery element is a plate-shaped battery element provided on each of the positive electrode and the negative electrode. Both the positive electrode terminal connection portion and the plate-shaped negative electrode terminal connection portion are the extraction surface of the terminal connection portion, and the positive electrode terminal connection portion and the negative electrode terminal connection portion are projected vertically onto the extending surfaces of the positive electrode and the negative electrode. The surfaces do not intersect each other, and the positive electrode and the negative electrode have different surface areas facing each other. Each electrode is arranged so as to be located on the electrode surface of the above, and the separator is provided with an abutting portion where the positive electrode end face and the negative electrode end face abut each other to regulate the movement of the positive electrode and the negative electrode. , Of the separators arranged on each electrode surface, all the separators laminated on the electrode surface are joined by the creases or joints, while being formed by the creases or joints between adjacent separators. A laminated battery characterized by this is described.

Patent Document 2 describes a secondary battery in which the positive electrode plate and the negative electrode plate are sandwiched between a separator in which a plurality of positive electrode plates containing a positive electrode active material and a plurality of negative electrode plates containing a negative electrode active material are folded in a continuous shape. The separator has a plurality of protrusions protruding from the positive electrode plate and the negative electrode plate, and at least a part of the protrusions has a stress having a tensile breaking strength smaller than that of the rest. A spell-folded laminate structure of a secondary battery having a broken portion is described.

Japanese Unexamined Patent Publication No. 2009-218105 Japanese Unexamined Patent Publication No. 2016-143550

According to the study of the present inventor, it has been clarified that the separator of the outermost layer of the conventional lithium ion secondary battery having a zigzag structure may be altered (discolored, embrittled, etc.).

The present invention has been made in view of the above circumstances, and provides a lithium ion secondary battery having a zigzag structure in which deterioration of the separator in the outermost layer is suppressed.

As a result of diligent studies to solve the above problems, the present inventor has found that the separator located on the surface where the SEI film is not formed on the outermost negative electrode is easily decomposed by the reaction with the electrolytic solution. As a result of further diligent studies based on the above findings, the present inventor determined the proportion of separators located on the surface on which the SEI film was not formed by controlling the length of the separator on the outermost negative electrode to a specific range. We have found that the number can be reduced, and as a result, deterioration (discoloration, embrittlement, etc.) of the separator of the outermost layer can be suppressed, and the present invention has been completed.

The present invention has been invented based on such findings.

According to the present invention
A battery body including at least one power generation element composed of a positive electrode, an electrolytic solution, a separator, and a negative electrode.
An exterior body for enclosing the battery body inside, and
A pair of electrode terminals electrically connected to the battery body and at least partially exposed to the outside of the exterior body.
It is a lithium ion secondary battery equipped with
The battery body has a structure in which the positive electrode and the negative electrode are arranged between the separators that are bent in a zigzag manner.
Provided is a lithium ion secondary battery in which the ratio (L ₂ / L ₁ ) of the length L ₂ of the separator on the outermost negative electrode to the length L ₁ of the outermost negative electrode is 0.20 or less.
(Here, the length L ₁ of the outermost negative electrode is the outermost negative electrode from the end of the outermost negative electrode in the center of one side of the lithium ion secondary battery on the side where the electrode terminal is not exposed. It is the length to the other end of the above and the length in the direction perpendicular to the one side. The length L ₂ of the separator is such that the electrode terminal in the lithium ion secondary battery is exposed. It is the length from the end of the separator on the outermost negative electrode at the center of one side of the non-side to the other end of the separator, and is the length in the direction perpendicular to the one side. )

According to the present invention, it is possible to provide a lithium ion secondary battery having a zigzag structure in which deterioration of the separator in the outermost layer is suppressed.

The above-mentioned objectives and other objectives, features and advantages are further clarified by the preferred embodiments described below and the accompanying drawings below.

It is an exploded perspective view schematically showing an example of the structure of the battery body of the embodiment which concerns on this invention. It is a perspective view schematically showing an example of the structure of the battery body of the embodiment which concerns on this invention. It is sectional drawing which shows typically the example of the structure of the battery body of embodiment which concerns on this invention, and is sectional drawing in the direction of AA' shown in FIG. It is a top view schematically showing an example of the structure of the battery body of the embodiment which concerns on this invention. It is a perspective view which showed typically an example of the structure of the lithium ion secondary battery of embodiment which concerns on this invention.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In all drawings, similar components are designated by the same reference numerals, and description thereof will be omitted as appropriate. Further, in the figure, each component schematically shows the shape, size, and arrangement relationship to the extent that the present invention can be understood, and is different from the actual size. In addition, "-" in the numerical range indicates the following from the above unless otherwise specified.

FIG. 1 is an exploded perspective view schematically showing an example of the structure of the battery body 10 according to the embodiment of the present invention. FIG. 2 is a perspective view schematically showing an example of the structure of the battery body 10 according to the embodiment of the present invention. FIG. 3 is a cross-sectional view schematically showing an example of the structure of the battery body 10 according to the embodiment of the present invention, and is a cross-sectional view in the direction of AA'shown in FIG. FIG. 4 is a plan view schematically showing an example of the structure of the battery body 10 according to the embodiment of the present invention. FIG. 5 is a perspective view schematically showing an example of the structure of the lithium ion secondary battery 100 according to the embodiment of the present invention.

As shown in FIGS. 1 to 5, the lithium ion secondary battery 100 according to the present embodiment includes a battery body 10 including at least one power generation element composed of a positive electrode 15, an electrolytic solution, a separator 18, and a negative electrode 20. It includes an exterior body 40 for enclosing the battery body 10 inside, and a pair of electrode terminals 30 electrically connected to the battery body 10 and at least partially exposed to the outside of the exterior body 40. The battery body 10 has a structure in which the positive electrode 15 and the negative electrode 20 are arranged between the separators 18 bent in _a zigzag manner, and the length of the separator 18 on the outermost negative electrode 20A with respect to the length L1 of the outermost negative electrode 20A. The ratio of L ₂ (L ₂ / L ₁ ) is 0.20 or less.
Here, as shown in FIG. 4, the length L ₁ of the outermost negative electrode is the end portion of the outermost negative electrode 20A at the center of one side 28 on the side where the electrode terminal 30 of the lithium ion secondary battery 100 is not exposed. It is the length from the outermost negative electrode 20A to the other end, and is the length in the direction perpendicular to one side 28. The length L ₂ of the separator 18 is the other of the separator 18 from the end of the separator 18 on the outermost negative electrode 20A at the center of one side 28 of the lithium ion secondary battery 100 on the side where the electrode terminal 30 is not exposed. It is the length to the end and the length in the direction perpendicular to one side 28.

According to the study of the present inventor, it has been clarified that the separator of the outermost layer of the conventional lithium ion secondary battery having a zigzag structure may be altered (discolored, embrittled, etc.).
The present inventor has made diligent studies to realize a zigzag-folded lithium-ion secondary battery in which deterioration of the separator in the outermost layer is suppressed. As a result, it was found that the separator located on the surface where the SEI film was not formed on the outermost negative electrode was easily decomposed by the reaction with the electrolytic solution. As a result of further diligent studies based on the above findings, the present inventor found that the ratio (L ₂ / L ₁ ) of the length L ₂ of the separator 18 on the outermost negative electrode 20A to the length L ₁ of the outermost negative electrode 20A was within the above range. By controlling to, the ratio of the separator 18 located on the surface on which the SEI film is not formed can be reduced, and as a result, the deterioration (discoloration, embrittlement, etc.) of the separator 18 of the outermost layer can be suppressed for the first time. I found it.
That is, according to the present embodiment, the ratio (L ₂ / L ₁ ) of the length L ₂ of the separator 18 on the outermost negative electrode 20A to the length L ₁ of the outermost negative electrode 20A is configured to be within the above range. As a result, it is possible to obtain a lithium ion secondary battery 100 having a zigzag-folded structure in which deterioration of the separator in the outermost layer is suppressed.
Further, in the present embodiment, at least the peripheral edge portion 20B of the surface of the outermost negative electrode 20A located on the outermost side of the electrodes composed of the positive electrode 15 and the negative electrode 20 in the battery body 10 is usually a solid electrolyte interface (SEI) film. 25 is formed.

The reason why the deterioration of the separator 18 in the outermost layer of the lithium ion secondary battery 100 according to the present embodiment is suppressed is not always clear, but the following reasons can be considered.
First, the separator located on the outermost negative electrode is decomposed by the reaction with the electrolytic solution and easily deteriorates. In particular, since the ester bond is easily decomposed by the reaction with the electrolytic solution, the deterioration of the separator in the outermost layer tends to be remarkable when the separator contains a polyester resin.
Here, since the separator located on the portion where the SEI film is formed does not come into direct contact with the outermost negative electrode, the decomposition reaction is unlikely to occur. Therefore, by controlling the ratio (L ₂ / L ₁ ) of the length L ₂ of the separator 18 on the outermost negative electrode 20A to the length L ₁ of the outermost negative electrode 20A within the above range, it comes into direct contact with the outermost negative electrode. Since the ratio of the separator can be reduced, the decomposition of the separator is suppressed. As a result, it is considered that a lithium ion secondary battery 100 having a zigzag structure in which deterioration of the separator in the outermost layer is suppressed can be obtained.

The upper limit of L ₂ / L ₁ is 0.20 or less, but from the viewpoint of further suppressing the deterioration of the separator of the outermost layer, it is preferably 0.15 or less, more preferably 0.10 or less, still more preferably 0.10 or less. It is 0.08 or less, particularly preferably 0.06 or less.
The lower limit of L ₂ / L ₁ is not particularly limited, but is, for example, 0.00 or more, preferably more than 0.00, and more preferably 0., from the viewpoint of stably maintaining the position of the outermost negative electrode 20A. It is 001 or more, more preferably 0.01 or more.

Here, the SEI film is usually formed on the surface on the side facing the positive electrode, but the lithium ions for forming the SEI film also wrap around on the surface on the side not facing the positive electrode, so that the outermost negative electrode 20A The SEI film 25 is also formed on at least the peripheral portion 20B of the surface on the side not facing the positive electrode 15.
Further, the presence or absence of the formation of the SEI film 25 on the outermost negative electrode 20A can be measured by, for example, XPS analysis. Specifically, since the ratio of Li is large in the place where the SEI film is formed, the presence or absence of the formation of the SEI film can be investigated by the ratio of Li.

In the lithium ion secondary battery 100 according to the present embodiment, the length L ₂ of the separator 18 is preferably less than 15.0 mm, more preferably 10. It is 0 mm or less, more preferably 8.0 mm or less, even more preferably 5.0 mm or less, and particularly preferably 4.0 mm or less.
The lower limit of the length L ₂ of the separator 18 is not particularly limited, but is, for example, 0.0 mm or more, preferably 0.1 mm or more from the viewpoint of stably holding the position of the outermost negative electrode 20A, and further. It is preferably 0.5 mm or more.

In the lithium ion secondary battery 100 according to the present embodiment, from the viewpoint of further suppressing the deterioration of the separator in the outermost layer, the average length of the separator 18 on the outermost negative electrode 20A with respect to the _average length L3 of the outermost negative electrode 20A. The ratio of L ₄ (L ₄ / L ₃ ) is preferably 0.20 or less.
The upper limit of L ₄ / L ₃ is more preferably 0.15 or less, still more preferably 0.10 or less, still more preferably 0.08 or less, from the viewpoint of further suppressing the deterioration of the separator of the outermost layer. Particularly preferably, it is 0.06 or less.
The lower limit of L ₄ / L ₃ is not particularly limited, but is, for example, 0.00 or more, and is preferably more than 0.00, more preferably 0., from the viewpoint of stably maintaining the position of the outermost negative electrode 20A. It is 001 or more, more preferably 0.01 or more.
Here, on one side 28 of the lithium ion secondary battery 100 on the side where the electrode terminal 30 is not exposed, the length of the outermost negative electrode 20A in the direction perpendicular to the side 28 is measured at 10 points, and the obtained 10 points are obtained. Let the average value of the lengths of the outermost negative electrodes 20A be the average length L3 _, measure the length of the separator 18 in the direction perpendicular to the side 28 at 10 points, and obtain the average value of the lengths of the 10 points obtained. The _average length of the separator 18 is L4. For the above 10 points, for example, one side 28 can be divided into 10 pieces at equal intervals, and the central portion (10 points in total) of each can be selected.

In the lithium ion secondary battery 100 according to the present embodiment, the _average length L4 of the separator 18 is preferably less than 15.0 mm, more preferably 10 from the viewpoint of further suppressing the deterioration of the separator in the outermost layer. It is 9.0 mm or less, more preferably 8.0 mm or less, even more preferably 5.0 mm or less, and particularly preferably 4.0 mm or less.
The lower limit of the average length L ₄ of the separator 18 is not particularly limited, but is, for example, 0.0 mm or more, preferably 0.1 mm or more from the viewpoint of stably holding the position of the outermost negative electrode 20A. More preferably, it is 0.5 mm or more.

Further, the lithium ion secondary battery according to the present embodiment has a cell rated capacity of preferably 7 Ah or more.
Further, in the lithium ion secondary battery according to the present embodiment, the number of laminated positive electrodes or the number of turns in the central portion is preferably 10 or more, more preferably 15 or more, still more preferably 20 or more. ..
This makes it possible to increase the capacity of the lithium ion secondary battery according to the present embodiment. Further, even with such a high capacity, the lithium ion secondary battery according to the present embodiment has excellent short-circuit resistance and can suppress thermal runaway of the battery.

Next, each configuration used in the lithium ion secondary battery of the present embodiment will be described.

<Battery body>
The battery body according to the present embodiment includes, for example, one or more power generation elements in which a positive electrode and a negative electrode are alternately laminated via a separator bent in a zigzag manner. These power generation elements are housed in a container made of an exterior body together with an electrolytic solution (not shown). Electrode terminals (positive electrode terminal and negative electrode terminal) are electrically connected to the power generation element, and a part or all of the electrode terminals are drawn out to the outside of the exterior body.

The positive electrode is provided with a positive electrode active material coated portion (positive electrode active material layer) and an uncoated portion on the front and back of the positive electrode current collector layer, and the negative electrode is provided with a negative electrode active material on the front and back surfaces of the negative electrode current collector layer. A coated portion (negative electrode active material layer) and an uncoated portion are provided.

The uncoated portion of the positive electrode active material in the positive electrode current collector layer is used as a positive electrode tab for connecting to the positive electrode terminal, and the uncoated portion of the negative electrode active material in the negative electrode current collector layer is used as a negative electrode tab for connecting to the negative electrode terminal. ..
The positive electrode tabs are grouped on the positive electrode terminal and connected to each other by ultrasonic welding or the like together with the positive electrode terminal, and the negative electrode tabs are grouped on the negative electrode terminal and connected to each other by ultrasonic welding or the like together with the negative electrode terminal. Then, one end of the positive electrode terminal is pulled out to the outside of the exterior body, and one end of the negative electrode terminal is also pulled out to the outside of the exterior body.
The battery body according to this embodiment can be manufactured according to a known method.

(Positive electrode)
The positive electrode can be appropriately selected from the positive electrodes that can be used in known lithium ion secondary batteries, depending on the intended use and the like. As the positive electrode active material used for the positive electrode, a material having high electron conductivity is preferable so that lithium ions can be reversibly released and occluded and electron transport can be easily performed.

The positive electrode active material used for the positive electrode is not particularly limited, but for example, a lithium composite oxide having a layered rock salt type structure or a spinel type structure, lithium iron phosphate having an olivine type structure, or the like may be used. Can be done. Lithium composite oxides include lithium manganate (LiMn ₂ O ₄ ); lithium cobaltate (LiCoO ₂ ); lithium nickelate (LiNiO ₂ ); at least a portion of the manganese, cobalt, and nickel moieties of these lithium compounds. Substituted with other metal elements such as aluminum, magnesium, titanium, zinc; nickel-substituted lithium manganate in which part of the manganese of lithium manganate is replaced with at least nickel; part of nickel in lithium nickelate is at least cobalt. Substituted Cobalt Substituted Lithium Nickelate; Part of Manganese in Nickel Substituted Lithium Manganate Substituted by Other Metals (eg, at least one of aluminum, magnesium, titanium, zinc); Part of Nickel in Cobalt Substituted Lithium Nylate Is replaced with another metal element (for example, at least one of aluminum, magnesium, titanium, zinc, and manganese).
These positive electrode active materials may be used alone or in combination of two or more.

Examples of the lithium-containing composite oxide having a layered crystal structure include a lithium nickel-containing composite oxide. As this lithium nickel-containing composite oxide, a nickel site in which a part of nickel is replaced with another metal can be used. Examples of the metal other than Ni occupying the nickel site include at least one metal selected from Mn, Co, Al, Mg, Fe, Cr, Ti, and In.

This lithium nickel-containing composite oxide preferably contains Co as a metal other than Ni that occupies nickel sites. Further, the lithium nickel-containing composite oxide preferably contains Mn or Al in addition to Co, that is, lithium nickel cobalt manganese composite oxide (NCM) having a layered crystal structure, and lithium nickel having a layered crystal structure. Cobalt-aluminum composite oxide (NCA), or a mixture thereof, can be preferably used.

As the lithium nickel-containing composite oxide having a layered crystal structure, for example, one represented by the following formula (1) can be used.
Li _{1 + a} (Ni _b Co _c Me1 _d Me2 _1-b-c-d ) O ₂ (1)
(In the formula, Me1 is Mn or Al, and Me2 is at least one selected from the group consisting of Mn, Al, Mg, Fe, Cr, Ti, and In (excluding metals of the same type as Me1). -0.5≤a <0.1, 0.1≤b <1, 0 <c <0.5, 0 <d <0.5)

The average particle size of the positive electrode active material is preferably, for example, 0.1 to 50 μm, more preferably 1 to 30 μm, still more preferably 2 to 25 μm, from the viewpoint of reactivity with the electrolytic solution, rate characteristics, and the like. Here, the average particle size means the particle size (median diameter: D ₅₀ ) at an integrated value of 50% in the particle size distribution (volume basis) by the laser diffraction / scattering method.

The positive electrode is composed of, for example, a positive electrode current collector layer and a positive electrode active material layer on the positive electrode current collector layer. The positive electrode is arranged so that the positive electrode active material layer faces the negative electrode active material layer on the negative electrode current collector layer via the separator.

Further, the positive electrode according to the present embodiment can be manufactured by a known method. For example, the positive electrode active material, the binder resin, and the conductive auxiliary agent are dispersed in an organic solvent to obtain a positive electrode slurry, and then this positive electrode slurry is applied to and dried on the positive electrode current collector layer and pressed as necessary. A method of forming a positive electrode active material layer on the positive electrode current collector layer or the like can be adopted.
As the slurry solvent used for producing the positive electrode, for example, N-methyl-2-pyrrolidone (NMP) can be used.

As the binder resin, for example, those generally used as a binder resin for a positive electrode such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) can be used.

The content of the binder resin in the positive electrode active material layer is preferably 0.1 part by mass or more and 10.0 parts by mass or less, preferably 0.5 part by mass, when the entire positive electrode active material layer is 100 parts by mass. It is more preferably 5.0 parts by mass or less, and further preferably 1.0 part by mass or more and 5.0 parts by mass or less. When the content of the binder resin is within the above range, the balance between the coatability of the positive electrode slurry, the binder binding property and the battery characteristics is further excellent.
Further, when the content of the binder resin is not more than the above upper limit value, the ratio of the positive electrode active material becomes large and the capacity per positive electrode mass becomes large, which is preferable. When the content of the binder resin is at least the above lower limit value, electrode peeling is suppressed, which is preferable.

The positive electrode active material layer can contain a conductive auxiliary agent in addition to the positive electrode active material and the binder resin. The conductive auxiliary agent is not particularly limited as long as it improves the conductivity of the positive electrode, and examples thereof include carbon black, ketjen black, acetylene black, natural graphite, artificial graphite, and carbon fiber. These conductive auxiliaries may be used alone or in combination of two or more.

The content of the conductive auxiliary agent in the positive electrode active material layer is preferably 1.0 part by mass or more and 4.0 parts by mass or less, preferably 1.2 parts by mass, when the entire positive electrode active material layer is 100 parts by mass. It is more preferably 5 parts by mass or more and 3.5 parts by mass or less, further preferably 1.5 parts by mass or more and 3.5 parts by mass or less, and 2.0 parts by mass or more and 3.5 parts by mass or less. Especially preferable. When the content of the conductive auxiliary agent is within the above range, the balance between the coatability of the positive electrode slurry, the binding property of the binder resin and the battery characteristics is further excellent.
Further, when the content of the conductive auxiliary agent is not more than the above upper limit value, the ratio of the positive electrode active material becomes large and the capacity per positive electrode mass becomes large, which is preferable. When the content of the conductive auxiliary agent is at least the above lower limit value, the conductivity of the positive electrode becomes better and the battery characteristics of the lithium ion secondary battery are improved, which is preferable.

As the positive electrode current collector layer, aluminum, stainless steel, nickel, titanium, alloys thereof and the like can be used. Examples of the shape include a foil, a flat plate, a mesh, and the like. In particular, aluminum foil can be preferably used.
The thickness of the positive electrode current collector layer is not particularly limited, but is, for example, 1 μm or more and 30 μm or less.

The density of the positive electrode active material layer is not particularly limited, but is preferably 2.0 g / cm ³ or more and 4.0 g / cm ³ or less, and is preferably 2.4 g / cm ³ or more and 3.8 g / cm ³ or less. It is more preferable that it is 2.8 g / cm ³ or more and 3.6 g / cm ³ or less.

The thickness of the positive electrode active material layer (total thickness of both sides) is not particularly limited, and can be appropriately set according to desired characteristics. For example, it can be set thick from the viewpoint of energy density, and can be set thin from the viewpoint of output characteristics. The thickness of the positive electrode active material layer (total thickness of both sides) can be appropriately set in the range of, for example, 20 μm or more and 500 μm or less, preferably 40 μm or more and 400 μm or less, and more preferably 60 μm or more and 300 μm or less.
Further, the thickness of the positive electrode active material layer (thickness on one side) is not particularly limited, and can be appropriately set according to desired characteristics. For example, it can be set thick from the viewpoint of energy density, and can be set thin from the viewpoint of output characteristics. The thickness of the positive electrode active material layer (thickness on one side) can be appropriately set in the range of 10 μm or more and 250 μm or less, preferably 20 μm or more and 200 μm or less, and more preferably 30 μm or more and 150 μm or less.

(Negative electrode)
The negative electrode can be appropriately selected from the negative electrodes that can be used in known lithium ion secondary batteries, depending on the intended use and the like. The negative electrode active material used for the negative electrode can also be appropriately set according to the intended use as long as it can be used for the negative electrode.
The negative electrode is composed of, for example, a negative electrode current collector layer and a negative electrode active material layer formed on the negative electrode current collector layer. The negative electrode active material layer preferably contains, for example, a negative electrode active material and a binder resin, and further contains a conductive auxiliary agent from the viewpoint of enhancing conductivity.

The negative electrode active material is not particularly limited as long as it is an active material for a negative electrode capable of storing and releasing lithium ions, but a carbonaceous material can be used. Examples of the carbonaceous material include graphite, amorphous carbon (for example, easily graphitizable carbon and non-graphitizable carbon), diamond-like carbon, fullerenes, carbon nanotubes, carbon nanohorns and the like. As the graphite, natural graphite and artificial graphite can be used, and inexpensive natural graphite is preferable from the viewpoint of material cost. Examples of the amorphous carbon include those obtained by heat-treating coal pitch coke, petroleum pitch coke, acetylene pitch coke and the like. As the other negative electrode active material, a lithium metal material, an alloy-based material such as silicon or tin, an oxide-based material such as Nb ₂ O ₅ or TiO ₂ , or a composite thereof can be used.
As the negative electrode active material, only one kind may be used alone, or two or more kinds may be used in combination.

The average particle size of the negative electrode active material is preferably 2 μm or more, more preferably 5 μm or more, from the viewpoint of suppressing side reactions during charging and discharging and suppressing a decrease in charging / discharging efficiency, and from the viewpoint of input / output characteristics and negative electrode fabrication. From the viewpoint of (smoothness of the negative electrode surface, etc.), 40 μm or less is preferable, and 30 μm or less is more preferable. Here, the average particle size means the particle size (median diameter: D ₅₀ ) at an integrated value of 50% in the particle size distribution (volume basis) by the laser diffraction scattering method.

Further, the negative electrode in the present embodiment can be manufactured by a known method. For example, a method in which a negative electrode active material and a binder resin are dispersed in a solvent to obtain a slurry, the slurry is applied to and dried on a negative electrode current collector layer, and pressed as necessary to form a negative electrode active material layer. Can be adopted.
Examples of the method for applying the negative electrode slurry include a doctor blade method, a die coater method, and a dip coating method. Additives such as defoaming agents and surfactants may be added to the slurry, if necessary.

The content of the binder resin in the negative electrode active material layer is preferably 0.1 part by mass or more and 10.0 parts by mass or less, preferably 0.5 part by mass, when the entire negative electrode active material layer is 100 parts by mass. It is more preferably 8.0 parts by mass or less, further preferably 1.0 part by mass or more and 5.0 parts by mass or less, and particularly preferably 1.0 part by mass or more and 3.0 parts by mass or less. preferable. When the content of the binder resin is within the above range, the balance between the coatability of the negative electrode slurry, the binding property of the binder resin and the battery characteristics is further excellent.
Further, when the content of the binder resin is not more than the above upper limit value, the ratio of the negative electrode active material becomes large and the capacity per negative electrode mass becomes large, which is preferable. When the content of the binder resin is at least the above lower limit value, electrode peeling is suppressed, which is preferable.

As the solvent, an organic solvent such as N-methyl-2-pyrrolidone (NMP) or water can be used. When an organic solvent is used as the solvent, a binder resin for an organic solvent such as polyvinylidene fluoride (PVDF) can be used. When water is used as the solvent, a rubber-based binder (for example, SBR (styrene-butadiene rubber)) or an acrylic-based binder resin can be used. As such an aqueous binder resin, one in the form of an emulsion can be used. When water is used as the solvent, it is preferable to use a water-based binder and a thickener such as CMC (carboxymethyl cellulose) in combination.

The negative electrode active material layer may contain a conductive auxiliary agent, if necessary. As the conductive auxiliary agent, a conductive material generally used as a conductive auxiliary agent for the negative electrode, such as a carbonaceous material such as carbon black, Ketjen black, and acetylene black, can be used.
The content of the conductive auxiliary agent in the negative electrode active material layer is preferably 0.1 part by mass or more and 3.0 parts by mass or less, preferably 0.1 part by mass, when the entire negative electrode active material layer is 100 parts by mass. It is more preferably 20 parts by mass or more and 2.0 parts by mass or less, and particularly preferably 0.2 parts by mass or more and 1.0 part by mass or less. When the content of the conductive auxiliary agent is within the above range, the balance between the coatability of the negative electrode slurry, the binding property of the binder resin and the battery characteristics is further excellent.
Further, when the content of the conductive auxiliary agent is not more than the above upper limit value, the ratio of the negative electrode active material becomes large and the capacity per negative electrode mass becomes large, which is preferable. When the content of the conductive auxiliary agent is at least the above lower limit value, the conductivity of the negative electrode becomes better, which is preferable.

The average particle size (primary particle size) of the conductive auxiliary agent used in the positive electrode active material layer and the negative electrode active material layer is preferably in the range of 10 to 100 nm. The average particle size (primary particle size) of the conductive auxiliary agent is preferably 10 nm or more, more preferably 30 nm or more, and a sufficient number of contact points from the viewpoint of suppressing excessive aggregation of the conductive auxiliary agent and uniformly dispersing the conductive auxiliary agent in the negative electrode. 100 nm or less is preferable, and 80 nm or less is more preferable, from the viewpoint of forming a good conductive path. When the conductive auxiliary agent is in the form of fibers, those having an average diameter of 2 to 200 nm and an average fiber length of 0.1 to 20 μm can be mentioned.
Here, the average particle size of the conductive auxiliary agent is the median size (D ₅₀ ), which means the particle size at an integrated value of 50% in the particle size distribution (volume basis) by the laser diffraction scattering method.

The thickness of the negative electrode active material layer (total thickness of both sides) is not particularly limited, and can be appropriately set according to desired characteristics. For example, it can be set thick from the viewpoint of energy density, and can be set thin from the viewpoint of output characteristics. The thickness of the negative electrode active material layer (total thickness of both sides) can be appropriately set in the range of, for example, 40 μm or more and 1000 μm or less, preferably 80 μm or more and 800 μm or less, and more preferably 120 μm or more and 600 μm or less.
Further, the thickness of the negative electrode active material layer (thickness of one side) is not particularly limited, and can be appropriately set according to desired characteristics. For example, it can be set thick from the viewpoint of energy density, and can be set thin from the viewpoint of output characteristics. The thickness of the negative electrode active material layer (thickness on one side) can be appropriately set in the range of, for example, 20 μm or more and 500 μm or less, preferably 40 μm or more and 400 μm or less, and more preferably 60 μm or more and 300 μm or less.

The density of the negative electrode active material layer is not particularly limited, but is preferably 1.2 g / cm ³ or more and 2.0 g / cm ³ or less, and 1.3 g / cm ³ or more and 1.9 g / cm ³ or less. It is more preferable, and it is more preferable that it is 1.4 g / cm ³ or more and 1.8 g / cm ³ or less.

As the negative electrode current collector layer, copper, stainless steel, nickel, titanium or an alloy thereof can be used. Examples of the shape include a foil, a flat plate, and a mesh.
The thickness of the negative electrode current collector layer is not particularly limited, but is, for example, 1 μm or more and 20 μm or less.

(Electrolytic solution)
The electrolytic solution according to this embodiment is obtained by dissolving an electrolyte in a solvent.
The electrolytic solution used in this embodiment is, for example, a non-aqueous electrolytic solution containing a lithium salt, and may be appropriately selected from known ones according to the type of electrode active material, the use of the lithium ion secondary battery, and the like. can.

Specific examples of lithium salts include, for example, LiClO ₄ , LiBF ₆ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF ₆ , LiSbF ₆ , LiB ₁₀ Cl ₁₀ , LiAlCl ₄ , LiCl, LiBr, LiB. (C ₂ H ₅ ) ₄ , CF ₃ SO ₃ Li, CH ₃ SO ₃ Li, LiC ₄ F ₉ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, lithium lower fatty acid carboxylate and the like can be mentioned.

The solvent for dissolving the lithium salt is not particularly limited as long as it is usually used as a liquid for dissolving an electrolyte, and is not particularly limited, and is ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and dimethyl carbonate. Carbonates such as (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), vinylene carbonate (VC); lactones such as γ-butyrolactone and γ-valerolactone; trimethoxymethane , 1,2-Dimethoxyethane, diethyl ether, tetrahydrofuran, 2-methyltetraester and other ethers; dimethyl sulfoxide and other sulfoxides; 1,3-dioxolane, 4-methyl-1,3-dioxolan and other oxolanes; acetonitrile , Nitromethane-containing solvents such as nitromethane, formamide, dimethylformamide; organic acid esters such as methyl formate, methyl acetate, ethyl acetate, butyl acetate, methyl propionate, ethyl propionate; phosphate triesters and diglimes; triglimes; Solvents such as sulfolane and methyl sulfoxide; oxazolidinones such as 3-methyl-2-oxazolidinone; sulton such as 1,3-propanesulton, 1,4-butanesulton and naphthalusulton can be mentioned. These may be used alone or in combination of two or more.

(Separator)
The separator according to the present embodiment is not particularly limited as long as it has a function of electrically insulating the positive electrode and the negative electrode and transmitting lithium ions, and for example, a porous separator can be used.

The separator according to the present embodiment preferably includes a resin layer containing a heat-resistant resin as a main component.
Here, the resin layer is formed of a heat-resistant resin as a main component. Here, the "main component" means that the ratio in the resin layer is 50% by mass or more, preferably 70% by mass or more, more preferably 90% by mass or more, and 100% by mass. It means that it may be.
The resin layer constituting the separator according to the present embodiment may be a single layer or two or more kinds of layers.

Examples of the heat-resistant resin forming the resin layer include polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, poly-m-phenylene terephthalate, poly-p-phenylene isophthalate, polycarbonate, polyester carbonate, and aliphatic polyamides. Aromatic polyamide, semi-aromatic polyamide, total aromatic polyester, polyphenylene sulfide, polyparaphenylene benzobisoxazole, polyimide, polyarylate, polyetherimide, polyamideimide, polyacetal, polyether ether ketone, polysulfone, polyether sulfone, One or more selected from fluororesins, polyether nitriles, modified polyphenylene ethers and the like can be mentioned.

Among these, from the viewpoint of excellent balance of heat resistance, mechanical strength, elasticity, price, etc., polyester resins such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and total aromatic polyester, aliphatic polyamides, and total fragrance. One or more selected from polyamide resins such as group polyamide and semi-aromatic polyamide is preferred, and one or more polyesters selected from polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate and all aromatic polyesters. The based resin is more preferable, and polyethylene terephthalate is further preferable.

The melting point of the separator according to the present embodiment is preferably 220 ° C. or higher, more preferably 230 ° C. or higher, and more preferably 240 ° C. or higher, from the viewpoint of improving the safety of the lithium ion secondary battery. More preferred. Alternatively, the separator according to the present embodiment preferably has no melting point, preferably has a decomposition temperature of 220 ° C. or higher, and preferably has a decomposition temperature of 230 ° C. or higher, from the viewpoint of improving the safety of the lithium ion secondary battery. It is more preferably 240 ° C. or higher, and particularly preferably 250 ° C. or higher.
By setting the melting point or decomposition temperature of the separator according to the present embodiment to the above lower limit value or more, it is possible to suppress the thermal shrinkage of the separator even if the battery generates heat and becomes high temperature, and as a result, the positive electrode and the negative electrode The contact area can be suppressed. As a result, thermal runaway of the lithium ion secondary battery can be suppressed, and safety can be further improved.
The upper limit of the melting point of the separator according to the present embodiment is not particularly limited, but is, for example, 500 ° C. or lower, preferably 400 ° C. or lower from the viewpoint of elasticity. Alternatively, the upper limit of the decomposition temperature of the separator according to the present embodiment is not particularly limited, but is, for example, 500 ° C. or lower, preferably 400 ° C. or lower from the viewpoint of elasticity.

The resin layer constituting the separator according to the present embodiment is preferably a porous resin layer. As a result, an abnormal current is generated in the lithium ion secondary battery, and when the temperature of the battery rises, the micropores in the porous resin layer are blocked and the current flow can be cut off, causing thermal runaway of the battery. It can be avoided.

The porosity of the porous resin layer is preferably 20% or more and 80% or less, more preferably 30% or more and 70% or less, and 40% or more and 60% or less from the viewpoint of the balance between mechanical strength and lithium ion conductivity. Is particularly preferable.
The porosity can be calculated from the following formula.
ε = {1-Ws / (ds · t)} × 100
Here, ε: porosity (%), Ws: basis weight (g / m ² ), ds: true density (g / cm ³ ), t: film thickness (μm).

The planar shape of the separator according to the present embodiment is not particularly limited, and can be appropriately selected according to the shape of the electrode or the current collector, and can be, for example, a rectangle.

The thickness of the separator according to the present embodiment is preferably 5 μm or more and 50 μm or less, more preferably 10 μm or more and 40 μm or less, and further preferably 10 μm or more and 30 μm or less, from the viewpoint of the balance between mechanical strength and lithium ion conductivity. Is.

From the viewpoint of further improving heat resistance, the separator according to the present embodiment preferably further includes a ceramic layer on at least one surface of the resin layer. Here, the ceramic layer is preferably provided on only one surface of the resin layer from the viewpoint of handleability of the separator and productivity according to the present embodiment, but the heat resistance of the separator is further improved. From the viewpoint of making the resin layer, it may be provided on both sides of the resin layer.
By further providing the above-mentioned ceramic layer, the separator according to the present embodiment can further reduce the thermal shrinkage of the separator and further prevent a short circuit between the electrodes.

The ceramic layer can be formed, for example, by applying a ceramic layer forming material on the resin layer and drying it. As the ceramic layer forming material, for example, a material obtained by dissolving or dispersing an inorganic filler and a binder resin in an appropriate solvent can be used.
The inorganic filler used for this ceramic layer can be appropriately selected from known materials used for the separator of the lithium ion secondary battery. For example, highly insulating oxides, nitrides, sulfides, carbides and the like are preferable, and aluminum oxide, boehmite, titanium oxide, silicon oxide, magnesium oxide, barium oxide, zirconium oxide, zinc oxide and iron oxide are selected. It is more preferable to prepare one or more kinds of ceramics in the form of particles. Among these, aluminum oxide, boehmite and titanium oxide are preferable.

The binder resin is not particularly limited, and examples thereof include a cellulosic resin such as carboxymethyl cellulose (CMC); an acrylic resin; a fluororesin such as polyvinylidene fluoride (PVDF); and the like. As the binder resin, only one kind may be used alone, or two or more kinds may be used in combination.

The solvent for dissolving or dispersing these components is not particularly limited, and is appropriately selected from, for example, alcohols such as water and ethanol, N-methylpyrrolidone (NMP), toluene, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and the like. Can be used.

The thickness of the ceramic layer is preferably 0.1 μm or more and 50 μm or less, more preferably 0.5 μm or more and 30 μm or less, and further, from the viewpoint of the balance between heat resistance, mechanical strength, handleability and lithium ion conductivity. It is preferably 1 μm or more and 15 μm or less.

(Electrolyte layer)
The electrolyte layer is a layer arranged so as to be interposed between the positive electrode and the negative electrode. The electrolyte layer contains a separator and an electrolytic solution, and examples thereof include a porous separator impregnated with a non-aqueous electrolytic solution.

<Exterior body>
The exterior body according to the present embodiment has, for example, a substantially quadrangular planar shape. The exterior body according to the present embodiment includes, for example, an accommodating portion accommodating a battery body and a joint portion in which heat-sealing resin layers located at the peripheral edge of the accommodating portion are directly or via electrode terminals. , Have.

The exterior body according to the present embodiment preferably has at least a heat-sealing resin layer and a barrier layer, and can enclose the battery body inside.
From the viewpoint of reducing the weight of the battery, it is preferable to use a laminated film having at least a heat-sealing resin layer and a barrier layer. The barrier layer can be selected to have barrier properties such as preventing leakage of electrolytic solution and invasion of moisture from the outside. For example, stainless steel (SUS) foil, aluminum foil, aluminum alloy foil, copper foil, titanium can be selected. A barrier layer made of metal such as foil can be used. The thickness of the barrier layer is, for example, 10 μm or more and 100 μm or less, preferably 20 μm or more and 80 μm or less, and more preferably 30 μm or more and 50 μm or less.
The resin material constituting the heat-sealing resin layer is not particularly limited, and for example, polyethylene, polypropylene, nylon, polyethylene terephthalate (PET) and the like can be used. The thickness of the heat-bondable resin layer is, for example, 20 μm or more and 200 μm or less, preferably 30 μm or more and 150 μm or less, and more preferably 50 μm or more and 100 μm or less.
Further, the heat-sealing resin layer and the barrier layer of the laminated film according to the present embodiment are not limited to one layer, and may be two or more layers.

In the present embodiment, the exterior body can be formed by facing the heat-sealing resin layers with each other via the battery body and heat-sealing the periphery of the portion for accommodating the battery body. A resin layer such as a nylon film or a polyester film can be provided on the outer surface of the exterior body, which is the surface opposite to the surface on which the heat-sealing resin layer is formed.

The heating temperature at which the heat-sealing resin layers are heat-sealed differs depending on the melting point of the resin material constituting the heat-sealing resin layer. For example, the resin material constituting the heat-sealing resin layer is polypropylene. In the case of, it is preferably 140 ° C. to 185 ° C., and more preferably 150 ° C. to 180 ° C.
The heat sealing time for heat-sealing the heat-sealing resin layers is, for example, 10 seconds to 50 seconds, preferably 12 seconds to 30 seconds.

(Electrode terminal)
In this embodiment, known members can be used for the pair of electrode terminals 30 (positive electrode terminal and negative electrode terminal). For the positive electrode terminal, for example, one made of aluminum or an aluminum alloy, and for the negative electrode terminal, for example, copper or a copper alloy or one obtained by subjecting them to nickel plating can be used. Each terminal is pulled out to the outside of the container, and a heat-weldable resin layer is provided in advance at a portion of each terminal located at a portion where heat welding is performed around the exterior body.

In FIG. 1, the positive electrode terminal and the negative electrode terminal are drawn from the same side of the exterior body, but the positive electrode terminal and the negative electrode terminal may be drawn out from different sides of the outer body.

Although the embodiments of the present invention have been described above, these are examples of the present invention, and various configurations other than the above can be adopted.
The present invention is not limited to the above-described embodiment, and modifications, improvements, and the like to the extent that the object of the present invention can be achieved are included in the present invention.

(Example 1)
<Manufacturing of positive electrode>
3. 93.9 parts by mass of lithium nickel-containing composite oxide (chemical formula: LiNi _0.8 Co _0.15 Al _0.05 O ₂ , average particle size: 6 μm) as the positive electrode active material, and carbon black as the conductive auxiliary agent. 0 part by mass of polyvinylidene fluoride (PVDF) was used as a binder resin, and 0.1 part by mass of anhydrous oxalic acid was used as an additive. These were dispersed in an organic solvent to prepare a positive electrode slurry. This positive electrode slurry is continuously applied and dried on an aluminum foil (tensile elongation: 6%) having a thickness of 15 μm, which is a positive electrode current collector, and then pressed to obtain a coated portion (positive electrode) of the positive electrode current collector. An active material layer: a positive electrode roll having a thickness of 60 μm on one side, a density of 3.35 g / cm ³ ) and an uncoated portion not coated was prepared.
This positive electrode roll was punched out so as to leave an uncoated portion as a tab for connecting to the positive electrode terminal to obtain a positive electrode.

<Manufacturing of negative electrode>
96.7 parts by mass of natural graphite (average particle size: 16 μm) as the negative electrode active material, 0.3 parts by mass of carbon black as a conductive auxiliary agent, 2.0 parts by mass of styrene-butadiene rubber as a binder resin, and a thickener. As a result, 1.0 part by mass of carboxymethyl cellulose was used. These were dispersed in water to prepare a negative electrode slurry. This negative electrode slurry is continuously applied and dried on a copper foil (tensile elongation: 4%) having a thickness of 8 μm, which is a negative electrode current collector, and then pressed to obtain a coated portion (negative electrode) of the negative electrode current collector. An active material layer: a negative electrode roll having a thickness of 90 μm on one side, a density of 1.55 g / cm ³ ) and an uncoated portion not coated was prepared.
This negative electrode roll was punched out so as to leave an uncoated portion as a tab for connecting to the negative electrode terminal to obtain a negative electrode.

<Manufacturing of lithium-ion secondary battery>
A positive electrode and a negative electrode were laminated in a zigzag structure via a separator, and a negative electrode terminal and a positive electrode terminal were provided therein to obtain a laminated body. Next, a laminated laminated battery was formed by accommodating an electrolytic solution in which 1 M of LiPF ₆ was dissolved in a solvent composed of ethylene carbonate, diethyl carbonate and ethyl methyl carbonate, and the obtained laminated body in a flexible film. Obtained. The rated capacity of this laminated laminated battery was 9.2 Ah, the positive electrode was 28 layers, and the negative electrode was 29 layers.
As the separator, a separator 1 (thickness: 25 μm, porosity 56%, resin layer melting point: 250 ° C.) including a porous resin layer made of polyethylene terephthalate (PET) and a ceramic layer made of boehmite particles was used. ..
Further, the length L ₂ and the average length L ₄ of the separator on the outermost negative electrode were adjusted to the values shown in Table 1.

<Evaluation>
(1) Porosity of the porous resin layer It was calculated from the following formula.
ε = {1-Ws / (ds · t)} × 100
Here, ε: porosity (%), Ws: basis weight (g / m ² ), ds: true density (g / cm ³ ), t: film thickness (μm).

(2) Deterioration evaluation of the separator in the outermost layer For the obtained lithium ion secondary battery, a constant current constant voltage (CC-CV) method was used, and a voltage of 0.2 C constant current was applied at 25 ° C. A constant current charge was performed up to 2 V, then a constant voltage charge was performed at a constant voltage of 4.2 V to a charge end current of 0.015 C, and then CC discharge was performed at a discharge rate of 0.2 C and a discharge end voltage of 2.5 V.
Next, the lithium ion secondary battery that had been charged and discharged for the first time was allowed to stand at 45 ° C. for 168 hours for aging treatment.
The obtained lithium ion secondary battery was disassembled, and the deterioration of the separator in the outermost layer was visually observed and evaluated according to the following criteria.
◎ ◎: No discolored part is observed on the surface of the separator in the outermost layer ◎: One or two discolored parts with a diameter of about 1 to 5 mm are observed, but deterioration is suppressed as a whole of the separator ○: Length One or two light brown discolored parts having a width of 5 mm to 20 mm and a width of 1 to 5 mm are observed, but deterioration is suppressed as a whole of the separator. Δ: Light brown discolored parts over the entire surface of the separator in the outermost layer. However, deterioration of the separator as a whole is suppressed. ×: A dark brown discolored part is observed over the entire surface of the separator in the outermost layer, and the entire separator is altered. The obtained evaluation results are shown in Table 1. Shown in.

(Examples 2 to 9 and Comparative Example 1)
Lithium-ion secondary batteries were prepared in the same manner as in Example 1 except that L ₂ and L ₄ were changed to the values shown in Table 1, and the same evaluations as in Example 1 were performed.
The obtained evaluation results are shown in Table 1, respectively.

From Table 1, in the lithium ion secondary battery of the example in which L ₂ / L ₁ was 0.20 or less, the deterioration of the separator in the outermost layer was suppressed. On the other hand, in the lithium ion secondary battery of the comparative example in which L ₂ / L ₁ exceeds 0.20, the surface of the separator in the outermost layer is denatured.

This application claims priority on the basis of Japanese application Japanese Patent Application No. 2018-12507 filed on June 29, 2018, the entire disclosure of which is incorporated herein by reference.

Claims (11)

A battery body including at least one power generation element composed of a positive electrode, an electrolytic solution, a separator, and a negative electrode.
An exterior body for enclosing the battery body inside, and
A pair of electrode terminals electrically connected to the battery body and at least partially exposed to the outside of the exterior body.
It is a lithium ion secondary battery equipped with
The battery body has a structure in which the positive electrode and the negative electrode are arranged between the separators that are bent in a zigzag manner.
A lithium ion secondary battery in which the ratio (L ₂ / L ₁ ) of the length L ₂ of the separator on the outermost negative electrode to the length L ₁ of the outermost negative electrode is 0.20 or less.
(Here, the length L ₁ of the outermost negative electrode is the outermost negative electrode from the end of the outermost negative electrode in the center of one side of the lithium ion secondary battery on the side where the electrode terminal is not exposed. The length to the other end of the separator and the length in the direction perpendicular to the one side. The length L2 of the separator is such that the electrode terminal in the lithium ion _secondary battery is exposed. It is the length from the end of the separator on the outermost negative electrode at the center of one side of the non-side to the other end of the separator, and is the length in the direction perpendicular to the one side. ) In the lithium ion secondary battery according to claim 1,
The separator is a lithium ion secondary battery containing a polyester resin. In the lithium ion secondary battery according to claim 1 or 2.
A lithium ion secondary battery in which the length L ₂ of the separator is less than 15.0 mm. The lithium ion secondary battery according to any one of claims 1 to 3.
A lithium ion secondary battery in which the ratio (L ₄ / L ₃ ) of the average length L ₄ of the separator on the outermost negative electrode to the average length L ₃ of the outermost negative electrode is 0.20 or less.
(Here, on one side of the lithium ion secondary battery on the side where the electrode terminal is not exposed, the length of the outermost negative electrode in the direction perpendicular to the one side is measured at 10 points, and the obtained 10 points are obtained. Let the average value of the lengths of the outermost negative electrodes be the _average length L3 of the outermost negative electrode, measure the length of the separator in the direction perpendicular to one side at 10 points, and obtain the average value of the lengths of the 10 points obtained. The _average length of the separator is L4.) In the lithium ion secondary battery according to claim 4,
A lithium ion secondary battery having an average length L ₄ of the separator of less than 15.0 mm. The lithium ion secondary battery according to any one of claims 1 to 5.
A lithium ion secondary battery in which the exterior body is a laminated film having at least a heat-sealing resin layer and a barrier layer. The lithium ion secondary battery according to any one of claims 1 to 6.
A lithium ion secondary battery in which the positive electrode contains a lithium nickel composite oxide as a positive electrode active material. In the lithium ion secondary battery according to claim 7.
A lithium ion secondary battery in which the lithium nickel-containing composite oxide is represented by the following formula (1).
Li _{1 + a} (Ni _b Co _c Me1 _d Me2 _1-b-c-d ) O ₂ (1)
(In the formula, Me1 is Mn or Al, and Me2 is at least one selected from the group consisting of Mn, Al, Mg, Fe, Cr, Ti, and In (excluding metals of the same type as Me1). -0.5≤a <0.1, 0.1≤b <1, 0 <c <0.5, 0 <d <0.5) The lithium ion secondary battery according to any one of claims 1 to 8.
A lithium ion secondary battery having a cell rated capacity of 7 Ah or more. The lithium ion secondary battery according to any one of claims 1 to 9.
A lithium ion secondary battery in which the number of layers or the number of turns of the positive electrodes in the central portion of the lithium ion secondary battery is 10 or more. The lithium ion secondary battery according to any one of claims 1 to 10.
The separator is a lithium ion secondary battery having a resin layer and a ceramic layer.

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