WO2013038672A1

WO2013038672A1 - Nonaqueous electrolyte secondary cell

Info

Publication number: WO2013038672A1
Application number: PCT/JP2012/005835
Authority: WO
Inventors: 佐藤　俊忠; 古結　康隆; 本田　和義
Original assignee: パナソニック株式会社
Priority date: 2011-09-13
Filing date: 2012-09-13
Publication date: 2013-03-21
Also published as: JP2014225324A

Abstract

The present invention provides a nonaqueous electrolyte secondary cell having high capacity and energy density, and moreover improved cycle life. The nonaqueous electrolyte secondary cell is equipped with: an electrode group in which a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode are wound or stacked; and a nonaqueous electrolyte. The positive electrode includes a positive electrode current collector, and, deposited onto the surface of the positive electrode current collector, a positive electrode active material layer of a positive electrode active material capable of occluding and releasing lithium ions. The negative electrode includes a negative electrode current collector, and, deposited onto the surface of the negative electrode current collector, a negative electrode active material layer of a negative electrode active material capable of occluding and releasing lithium ions. The negative electrode active material layer includes a film formed on the surface of the negative electrode active material, at least a portion of the film including an inorganic lithium compound. When the negative electrode active material layer is divided at a thickness ratio of 7:3 into a layer (A) nearer the negative electrode current collector and a layer (B) nearer the surface of the negative electrode active material layer, the inorganic lithium compound content included in the layer (B) is greater than the inorganic lithium compound content included in the layer (A).

Description

Nonaqueous electrolyte secondary battery

The present invention relates to a non-aqueous electrolyte secondary battery, and specifically relates to an improvement of a negative electrode mixture layer.

In recent years, there has been an increasing demand for small and lightweight secondary batteries capable of rapid charging and large current discharge in response to demands for mounting on automobiles and direct current (DC) of large tools due to environmental problems. Non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries have attracted attention as typical batteries that satisfy such requirements.

The nonaqueous electrolyte secondary battery includes an electrode group in which a positive electrode, a negative electrode, and a separator (porous insulator) interposed therebetween are spirally wound, and a nonaqueous electrolyte. In the nonaqueous electrolyte secondary battery, the electrode group is a power generation element. In the wound electrode group, the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the surface thereof, and the negative electrode includes a negative electrode current collector and a negative electrode active material layer formed on the surface thereof.

As the positive electrode active material, a material that reversibly electrochemically reacts with lithium ions, such as lithium cobalt composite oxide, is used, and as the negative electrode active material, lithium metal, a lithium alloy, graphite having lithium ions as a host material is used. A lithium intercalation compound occluded in a carbonaceous material such as is used. The “host substance” refers to a substance that can occlude and release lithium ions. Further, as the non-aqueous electrolyte, an aprotic organic solvent solution in which a lithium salt such as LiClO ₄ or LiPF ₆ is dissolved is used.

Non-aqueous electrolyte secondary batteries often use a wound electrode group as described above in order to increase the capacity of the battery. In addition, a method of filling as many active materials as possible in the battery by increasing the active material density of the active material layer or using a higher capacity active material is employed. For example, an alloy-based active material containing an element such as silicon or tin used as a negative electrode active material can be expected to increase the capacity because it can store a larger amount of lithium than a graphite material.

However, since an alloy-based active material has a large volume change when it expands and contracts as the battery is charged / discharged, a new reaction surface (new surface) is likely to be exposed during the charge / discharge reaction. Since the new surface is in a state where the active material itself is exposed, the reactivity is extremely high, and a new film is formed on the new surface by directly reacting with the nonaqueous electrolyte. Therefore, by repeating charging and discharging, new surfaces are formed one after another, and a film is formed on this new surface.

A film having an appropriate thickness is advantageous for stabilizing the charge / discharge reaction by suppressing side reactions between the active material and the non-aqueous electrolyte. However, when a large amount of film is formed, the capacity of the negative electrode is reduced by taking in lithium contained in the nonaqueous electrolyte and the active material, and the cycle life of the battery is shortened.

Therefore, various methods are being studied in order to form a stable film on the negative electrode surface. For example, Patent Document 1 uses a nonaqueous electrolyte to which a cyclic carbonate having an unsaturated bond, such as vinylene carbonate (VC), is used to form a stable and strong film on the surface of the negative electrode active material layer at the time of initial charge. Is disclosed. Patent Document 2 proposes a method of forming a lithium carbonate film on the surface of the negative electrode active material layer by previously bringing the negative electrode active material layer occluded with lithium into contact with carbon dioxide.

JP 2004-171877 A JP 2005-216601 A

However, the film formed by the method of Patent Document 1 is a film containing a polymer obtained by polymerization of vinylene carbonate or the like, but the film is easily broken or peeled by expansion and contraction due to repeated charge and discharge.

Further, the lithium carbonate film formed in Patent Document 2 tends to cause an increase in resistance and a decrease in charge acceptance by hindering ionic conductivity.
As described above, in Patent Document 1 and Patent Document 2, the capacity and the capacity maintenance rate are easily lost.

An object of the present invention is to provide a non-aqueous electrolyte secondary battery having high capacity and energy density and improved cycle life.

One aspect of the present invention includes a positive electrode, a negative electrode, and an electrode group in which a separator interposed between the positive electrode and the negative electrode is wound or laminated, and a nonaqueous electrolyte. The positive electrode includes a positive electrode current collector, a positive electrode collector, and the like. A positive electrode active material layer including a positive electrode active material capable of occluding and releasing lithium ions attached to the surface of the electric current body, wherein the negative electrode absorbs the negative electrode current collector and the lithium ions attached to the surface of the negative electrode current collector And a negative electrode active material layer containing a releasable negative electrode active material, the negative electrode active material layer comprising a film formed on the surface of the negative electrode active material, wherein at least a part of the film comprises an inorganic lithium compound, When the active material layer is divided into the A layer on the negative electrode current collector side and the B layer on the surface side of the negative electrode active material layer in a thickness ratio of 7: 3, the inorganic contained in the B layer per area of the negative electrode When the content of the lithium compound is the inorganic Greater than the content of um compounds, it relates to a nonaqueous electrolyte secondary battery.

In the present invention, the negative electrode active material layer includes a film containing an inorganic lithium compound, and the content of the inorganic lithium compound is greater on the surface side than the negative electrode current collector side of the negative electrode active material layer. Therefore, it can suppress that a film | membrane is formed excessively with repetition of charging / discharging. Further, the negative electrode active material layer contains a large amount of an inorganic lithium compound on the surface side, that is, a highly rigid film is formed on the surface side. Thereby, since the expansion | swelling of the negative electrode at the time of charge can be suppressed effectively, deterioration of a negative electrode active material is suppressed and the cycle life of a nonaqueous electrolyte secondary battery can be improved.

While the novel features of the invention are set forth in the appended claims, the invention will be further described by reference to the following detailed description, taken in conjunction with the other objects and features of the invention, both in terms of construction and content. It will be well understood.

FIG. 1 is a longitudinal sectional view schematically showing a nonaqueous electrolyte secondary battery according to an embodiment of the present invention. FIG. 2 is a longitudinal sectional view schematically showing a positive electrode, a negative electrode, and a separator used in the nonaqueous electrolyte secondary battery according to one embodiment of the present invention. FIG. 3 is a scanning electron microscope (SEM) photograph of a longitudinal section of the negative electrode active material layer of the negative electrode prepared in Example 1.

The nonaqueous electrolyte secondary battery of the present invention includes a positive electrode, a negative electrode, and an electrode group in which separators interposed between the positive electrode and the negative electrode are wound or laminated, and a nonaqueous electrolyte. The positive electrode includes a positive electrode current collector and a positive electrode active material layer including a positive electrode active material capable of inserting and extracting lithium ions attached to the surface of the positive electrode current collector. The negative electrode includes a negative electrode current collector and a negative electrode active material layer including a negative electrode active material capable of inserting and extracting lithium ions attached to the surface of the negative electrode current collector.

In the present invention, the negative electrode active material layer includes a film formed on the surface of the negative electrode active material, and at least a part of the film includes an inorganic lithium compound. When the negative electrode active material layer is divided into the A layer on the negative electrode current collector side and the B layer on the surface side of the negative electrode active material layer in a thickness ratio of 7: 3, the B layer per area of the negative electrode The content of the inorganic lithium compound contained is greater than the content of the inorganic lithium compound contained in the A layer. Hereinafter, the content per area of the negative electrode may be simply referred to as the content.

Thus, in the present invention, a film containing an inorganic lithium compound is formed on the surface of the negative electrode active material contained in the negative electrode active material layer. Moreover, the content of the inorganic lithium compound in the B layer on the surface side of the negative electrode (the surface side of the negative electrode active material) is larger than that in the A layer on the negative electrode current collector side.

A fresh non-aqueous electrolyte is easily supplied to the B layer on the surface side of the negative electrode active material layer through the separator interface, and side reactions are likely to occur accordingly. In such a B layer, when the film as described above is formed, the surface of the negative electrode active material is appropriately inactivated, and a side reaction between the negative electrode active material and the nonaqueous electrolyte is suppressed. Therefore, it can suppress that a membrane | film | coat is formed excessively with repetition of charging / discharging.

In addition to the formation of a film containing an inorganic lithium compound inside the negative electrode active material layer, many highly rigid films containing an inorganic lithium compound are formed on the surface side of the negative electrode active material layer. Therefore, the expansion of the negative electrode during charging can be effectively suppressed. Thereby, deterioration of a negative electrode active material is suppressed and the cycle life of a nonaqueous electrolyte secondary battery can be improved. Moreover, since formation of an excessive film | membrane is suppressed even if charging / discharging is repeated, the fall of the reaction efficiency of a battery reaction and the increase in resistance are suppressed.

In addition, since the content of the inorganic lithium compound is less in the A layer than in the B layer, a battery reaction is likely to occur smoothly even with a small amount of non-aqueous electrolyte supplied. As a result, the battery reaction occurs between the A layer and the B layer. It can be performed uniformly.

The coating preferably contains, as the inorganic lithium compound, mainly an inorganic acid lithium salt such as lithium carbonate; lithium hydroxide, lithium oxide; lithium halide such as lithium fluoride, and the like.
The coating preferably contains at least lithium carbonate as an inorganic lithium compound. In particular, it is preferable that the inorganic lithium compound contained most in the above film is lithium carbonate.

Lithium carbonate is a chemically and electrochemically stable compound in the non-aqueous electrolyte and in the working potential range of the negative electrode. If such a film containing a large amount of lithium carbonate is formed on the surface side of the negative electrode active material layer, the side reaction between the negative electrode active material and the nonaqueous electrolyte can be more effectively suppressed. Moreover, since the film | membrane formed many on the surface side of a negative electrode active material layer is firm, the excessive expansion | swelling of the negative electrode at the time of discharge can be suppressed more effectively.

In addition, the composition of the film formed on the negative electrode active material layer can be analyzed by X-ray photoelectron spectroscopy. The lithium carbonate formed in the negative electrode active material layer can be quantified by, for example, inductively coupled plasma (ICP) emission spectroscopy, ion chromatography, or the like. Specifically, when a predetermined amount of the negative electrode active material layer is collected at a predetermined position of the negative electrode active material layer, and added to water and stirred, water-soluble components (lithium carbonate, lithium hydroxide) contained in the negative electrode active material layer Water-soluble lithium compounds, etc.) dissolve in water. By analyzing the water in which the water-soluble component is dissolved by ion chromatography, the amount of carbonate ion is quantified, and the content of lithium carbonate can be calculated based on this value. Moreover, the content of lithium can be quantified by analyzing the water in which the water-soluble component is dissolved by ICP emission spectroscopy.

When the surface of the negative electrode active material layer has irregularities, the surface of the negative electrode active material layer is assumed to be intermediate between the bottom surface of the concave portion and the top surface of the convex portion. Then, the distance between the surface of the negative electrode active material layer and the surface of the negative electrode current collector is divided into 7: 3 from the current collector side, thereby forming an A layer and a B layer.

The content C _A of lithium carbonate contained in the A layer of the current collector side, for example, 10 ~ 65μg / cm ^2, preferably 13 ~ 60μg / cm ^2, more preferably 15 ~ 55μg / cm ^2.
The ratio C _B / C _A of the content C _B of lithium carbonate contained in the B layer to the content C _{A of} lithium carbonate in the A layer is, for example, 1.05 to 2, preferably 1.07 to 1.7. Further, this is preferably 1.1 to 1.5. In such a range, the effect by the film containing lithium carbonate can be obtained more effectively.
The content C _A and C _B are each an amount per unit area of the negative electrode.

It is preferable that at least a part of the film formed in the negative electrode active material layer contains an organolithium compound. Examples of the organic lithium compound include lithium alkyl carbonate.
The identification and quantification of the organolithium compound can be performed based on, for example, X-ray photoelectron spectroscopy.

The content of the organolithium compound contained in the A layer per area of the negative electrode is preferably larger than the content of the organolithium compound contained in the B layer. In other words, the A layer on the current collector side preferably contains a large amount of the organic lithium compound per area of the negative electrode, and the B layer on the surface side preferably contains a large amount of the inorganic lithium compound per area of the negative electrode.
The film containing the organolithium compound is mainly generated by the decomposition reaction of the non-aqueous electrolyte as the battery is charged / discharged. In the layer A on the surface side of the negative electrode, a large amount of a film containing an inorganic lithium compound is formed, so that the content of the organic lithium compound is reduced. On the contrary, in the B layer on the negative electrode current collector side, the content of the organic lithium compound is increased because the amount of the film containing the inorganic lithium compound is small. Moreover, since the A layer has a larger volume than the B layer, the content of the organolithium compound is also increased.

When the negative electrode active material contained in the negative electrode active material layer expands due to charging, the contacts between the active materials, between the active material and the current collector, and between the active material and the conductive agent decrease. Further, with expansion, the non-aqueous electrolyte contained in the negative electrode active material layer is easily depleted. Therefore, the reaction efficiency of the battery reaction tends to decrease inside the negative electrode active material layer. However, when a large amount of the organic lithium compound is present in the A layer on the current collector side, the non-aqueous electrolyte is abundantly present inside the negative electrode active material layer. Therefore, even when the expansion of the negative electrode active material is increased, the shortage or depletion of the nonaqueous electrolyte can be more effectively suppressed in the negative electrode active material layer.

The reason why a large amount of the non-aqueous electrolyte can be held inside the negative electrode active material layer is that the organic group portion of the organolithium compound has a high affinity for the non-aqueous electrolyte and can maintain an easily wetted state. Accordingly, a stable battery reaction can be performed even inside the negative electrode active material layer, and the active material inside the active material layer can be used more effectively. As a result, the capacity of the negative electrode can be increased.

The negative electrode including the negative electrode active material layer having the A layer and the B layer having different inorganic lithium compound contents as described above, occludes lithium in the negative electrode active material layer including the negative electrode active material before forming the electrode group. Can be formed. Such occlusion of lithium can be performed by forming a metallic lithium layer by attaching vacuum deposition or a lithium foil to the surface of the negative electrode active material layer containing the negative electrode active material. When a metal lithium layer is formed on the surface of the negative electrode active material layer, a local battery is formed in the negative electrode, and lithium ions can be introduced into the negative electrode active material layer in advance.

Immediately after forming the metal lithium layer on the surface of the negative electrode active material layer, the lithium in the metal lithium layer is occluded by the negative electrode active material of the negative electrode active material layer even if the electrode group is assembled and a battery is manufactured. However, after the metal lithium layer is formed, the lithium occlusion may be promoted by leaving it under dry conditions for a predetermined period. By leaving it under predetermined conditions, a large amount of an inorganic lithium compound such as lithium carbonate is generated, and the content of the inorganic lithium compound on the surface side of the negative electrode active material layer can be increased more effectively.

The standing period is, for example, 0.5 days to 2 weeks, preferably 0.5 to 10 days.
Further, the leaving is preferably performed in air or in an atmosphere containing carbon dioxide. In order to avoid moisture contamination, the negative electrode on which the metal lithium layer is formed is preferably left under dry conditions. As such drying conditions, a low dew point, for example, a dew point of −25 ° C. or lower, preferably a dew point of −30 ° C. or lower is preferable. In particular, the negative electrode on which the metal lithium layer is formed is preferably left in dry air with a low dew point.

The metal lithium layer can be formed on at least a part of the surface of the negative electrode active material layer.
In addition, the metal lithium layer may remain on at least a part of the surface of the negative electrode active material layer before the assembly of the electrode group, or all of the lithium in the metal lithium layer may be occluded in the negative electrode active material layer. Good.

In general non-aqueous electrolyte secondary batteries, lithium ions are supplied into the battery only from the positive electrode. However, it is preferable to make a design to compensate from a lithium source other than the positive electrode when aiming at further higher capacity. In such a design, as a result of supplementing the irreversible capacity of the negative electrode, the positive electrode active material is effectively used, and it is possible to prevent a decrease in capacity due to the irreversible capacity of the negative electrode and increase the capacity. Thus, when using the negative electrode with which the irreversible capacity | capacitance was compensated, the effect of the above films can be acquired more effectively.

The negative electrode active material layer may be attached to both surfaces of the negative electrode current collector, or may be attached to one surface.
The negative electrode current collector may be a non-porous conductive substrate or a porous conductive substrate having a plurality of through holes. As the non-porous conductive substrate, a metal foil, a metal sheet, or the like can be used. Examples of the porous conductive substrate include a metal foil having a communication hole (perforation), a mesh body, a net body, a punching sheet, an expanded metal, and a lath body.

In the present invention, a wound or stacked electrode group is used to increase the capacity of the battery. Therefore, it is preferable that the current collector has a long band shape.
Examples of the metal material used for the negative electrode current collector include stainless steel, nickel, copper, and copper alloy. Of these, copper or a copper alloy is preferable.
The thickness of the negative electrode current collector is not particularly limited, and can be selected, for example, from a range of 1 to 500 μm, preferably 3 to 50 μm, more preferably 5 to 20 μm.

The negative electrode active material layer may be a deposited film of a negative electrode active material by a vapor phase method, but is preferably a mixture layer containing a binder, and optionally a conductive agent and / or a thickener, in addition to the negative electrode active material. .
The deposited film can be formed by depositing the negative electrode active material on the surface of the negative electrode current collector by a vapor phase method such as a vacuum evaporation method, a sputtering method, or an ion plating method. In this case, as the negative electrode active material, for example, silicon, silicon compound, tin, tin compound, metal or alloy described later can be used.

The negative electrode mixture layer is prepared by preparing a negative electrode mixture slurry containing a negative electrode active material, a dispersion medium, and, if necessary, a binder, a conductive agent and / or a thickener, and applying the slurry to the surface of the negative electrode current collector, followed by drying. If necessary, it can be formed by rolling.
The thickness of the negative electrode active material layer is, for example, 10 to 150 μm, preferably 15 to 100 μm.

The negative electrode active material only needs to be able to occlude and release lithium ions. For example, carbon materials; silicon, silicon compounds; tin, tin compounds; metals or alloys (for example, silicon alloys; tin alloys; tin, aluminum, zinc) And a lithium alloy containing at least one selected from magnesium). Examples of the carbon material (carbonaceous active material) include graphite such as natural graphite and artificial graphite; coke, graphitizing carbon, graphitized carbon fiber, and amorphous carbon. Examples of the silicon compound and tin compound include oxides and nitrides. These negative electrode active materials can be used individually by 1 type or in combination of 2 or more types.
The shape of the negative electrode active material is not particularly limited, and may be fibrous or particulate.

Of these, an active material containing at least one selected from the group consisting of silicon and tin is preferable, and an active material containing silicon and / or tin is more preferably included in an amount of 80% by mass or more of the whole negative electrode active material. Since such an active material has a large volume change accompanying charging / discharging, it easily deteriorates and its capacity tends to decrease. In the present invention, since many highly rigid films are formed on the surface side of the negative electrode active material layer, the cycle life can be effectively improved even when such an active material having a large volume change is used. The content of the active material containing silicon and / or tin is preferably 90% by mass or more based on the whole negative electrode active material, and the whole negative electrode active material may be an active material containing silicon and / or tin. .

An active material containing silicon tin is advantageous because it has a larger capacity density than a carbon material conventionally used as a negative electrode active material and can increase the capacity of the negative electrode. In the carbon material, the site where lithium is occluded and released is limited. However, the negative electrode active material containing silicon or tin often has no such limitation. The effect of can be obtained more effectively.

Among active materials containing silicon, examples of the silicon compound include silicon oxide SiO _α (0.05 ≦ α ≦ 1.95). α is preferably 0.1 to 1.8, more preferably 0.15 to 1.6. In the silicon oxide, a part of silicon may be substituted with one or more elements. Examples of such elements include B, Mg, Ni, Ti, Mo, Co, Ca, Cr, Cu, Fe, Mn, Nb, Ta, V, W, Zn, C, N, and Sn. The at least 1 type of element D selected is mentioned.

Examples of the silicon alloy include an alloy in which a part of silicon is substituted with the above element D, and a substance containing the other metal element (such as the above element D) incorporated between silicon atoms (the element D). It may be a silicon solid solution containing).

Among the compounds containing tin, examples of tin compounds include SnO _β (where 0 <β <2), tin oxides such as SnO ₂ ; intermetallic compounds such as Ni ₂ Sn ₄ and Mg ₂ Sn; SnSiO _{3 and the} like it can.

It is also preferable to use a carbonaceous active material such as graphite as the negative electrode active material. The content of the carbonaceous active material with respect to the entire negative electrode active material is preferably 80% by mass or more, and more preferably 90% by mass or more. Moreover, you may use only a carbonaceous active material as a negative electrode active material.

The dispersion medium is not particularly limited, and examples thereof include water, alcohols such as ethanol, ethers such as tetrahydrofuran, amides such as dimethylformamide, N-methyl-2-pyrrolidone (NMP), or a mixed solvent thereof. .

Examples of the binder include resin materials such as polytetrafluoroethylene, polyvinylidene fluoride (PVDF), or modified products thereof (such as PVDF having a functional group introduced), copolymers containing vinylidene fluoride as monomer units, and the like. Polyolefin resin such as polyethylene and polypropylene; Polyamide resin such as aramid resin; Polyimide resin such as polyimide and polyamideimide; Acrylic resin such as polyacrylic acid, polymethyl acrylate, and ethylene-acrylic acid copolymer; Examples thereof include vinyl resins such as acrylonitrile and polyvinyl acetate; polyvinyl pyrrolidone; polyether sulfone; and rubbery materials such as styrene-butadiene copolymer rubber (SBR). These binders can be used individually by 1 type or in combination of 2 or more types.

In addition, the binder may be in a state dissolved in a dispersion medium in the slurry, or in a state of being dispersed in the form of particles.
The ratio of the binder is, for example, 0.3 to 10 parts by mass, preferably 1 to 6 parts by mass, per 100 parts by mass of the active material.

Examples of the conductive agent include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black; carbon fluoride; conductive fiber such as carbon fiber and metal fiber; metal powder such as aluminum; zinc oxide and potassium titanate. Conductive whiskers such as; conductive metal oxides such as titanium oxide; and organic conductive materials such as phenylene derivatives. These electrically conductive agents can be used individually by 1 type or in combination of 2 or more types. The ratio of the conductive agent is, for example, 0.3 to 10 parts by mass per 100 parts by mass of the active material.

Examples of the thickener include carboxymethylcellulose (CMC) and modified products thereof (including salts such as Na salts), cellulose derivatives such as methylcellulose; saponified polymers having vinyl acetate units such as polyvinyl alcohol; polyethers ( And polyalkylene oxides such as polyethylene oxide). These thickeners can be used singly or in combination of two or more.
The amount of the thickener is, for example, 0.01 to 10 parts by mass with respect to 100 parts by mass of the active material.

Hereinafter, the configuration of the nonaqueous electrolyte secondary battery of the present invention will be described in more detail with reference to the drawings as necessary.

FIG. 1 is a longitudinal sectional view schematically showing a lithium ion secondary battery as a nonaqueous electrolyte secondary battery according to an embodiment of the present invention. The nonaqueous electrolyte secondary battery has an electrode group 4 in which a long strip-shaped positive electrode 5, a long strip-shaped negative electrode 6, and a separator 7 interposed between the positive electrode 5 and the negative electrode 6 are wound. In the bottomed cylindrical metal battery case 1, together with the electrode group 4, a non-aqueous electrolyte (not shown) is accommodated.

In the electrode group 4, a positive electrode lead 9 is electrically connected to the positive electrode 5, and a negative electrode lead 10 is electrically connected to the negative electrode 6.
The electrode group 4 is housed in the battery case 1 together with the lower insulating ring 8b with the positive electrode lead 9 led out. The sealing plate 2 is welded to the end of the positive electrode lead 9, and the positive electrode 5 and the sealing plate 2 are electrically connected.

The lower insulating ring 8 b is disposed between the bottom surface of the electrode group 4 and the negative electrode lead 10 led out from the electrode group 4. The negative electrode lead 10 is welded to the inner bottom surface of the battery case 1, and the negative electrode 6 and the battery case 1 are electrically connected. An upper insulating ring 8 a is mounted on the upper surface of the electrode group 4.

The electrode group 4 is held in the battery case 1 by an inwardly protruding step 11 formed on the upper side surface of the battery case 1 above the upper insulating ring 8a. On the step portion 11, a sealing plate 2 having a resin gasket 3 on the periphery is placed, and the opening end of the battery case 1 is caulked and sealed inward.

FIG. 2 is a longitudinal sectional view schematically showing a positive electrode, a negative electrode, and a separator used in the nonaqueous electrolyte secondary battery according to one embodiment of the present invention.
The negative electrode 6 includes a long strip-shaped negative electrode current collector 6a and a negative electrode active material layer 6b formed on both surfaces of the negative electrode current collector 6a. At one end in the longitudinal direction of the negative electrode current collector 6a, current collector exposed

portions

6a and 6d where the negative electrode active material layer 6b is not formed are formed on both surfaces of the negative electrode current collector 6a. . The negative electrode lead is welded to the current collector exposed portion 6c.

The positive electrode 5 includes a long strip-shaped positive electrode current collector 5a and a positive electrode active material layer 5b formed on both surfaces of the positive electrode current collector 5a. In the central part of the positive electrode current collector 5a in the longitudinal direction, the current collector exposed part 5c in which the positive electrode active material layer 5b is not formed so as to cross the short direction on both surfaces of the positive electrode current collector 5a, 5d is formed. A positive electrode lead is welded to the current collector exposed portion 5c.

The separator 7 which is a porous insulator has a long band shape. The positive electrode 5 and the negative electrode 6 are arranged to face each other with a separator 7 interposed therebetween. And in this state, by winding, an electrode group as shown in FIG. 1 is formed.

Hereinafter, components of the battery other than the negative electrode will be described in detail.
(Positive electrode)
As shown in FIG. 2, the positive electrode active material layer may be attached to both surfaces of the negative electrode current collector, or may be attached to one surface.
As the negative electrode current collector, a nonporous or porous conductive substrate can be used as the positive electrode current collector. The thickness of the positive electrode current collector can be selected from the same range as the thickness of the negative electrode current collector. Examples of the metal material used for the positive electrode current collector include stainless steel, titanium, aluminum, and an aluminum alloy.

The positive electrode active material layer may be, for example, a positive electrode mixture layer containing a positive electrode active material and, if necessary, a binder, a conductive additive, a thickener, and the like. A positive electrode can be formed according to the formation method of a negative electrode.
The thickness of the positive electrode active material layer is, for example, 15 to 100 μm, preferably 20 to 70 μm.

As the positive electrode active material, a known positive electrode active material capable of occluding and releasing lithium ions can be used. The positive electrode active material preferably contains a sufficient amount of lithium, and specifically includes an oxide containing lithium and a metal element (element M) (hereinafter also referred to as lithium-containing metal composite oxide). . Such a lithium-containing metal composite oxide preferably has a layered or hexagonal crystal structure or a spinel structure.

The metal element M includes at least one selected from the group consisting of Co, Ni, Mn, and Fe. The element M is particularly preferably at least one selected from the group consisting of Co, Ni and Mn.

The metal element M may further include a different element in addition to the above metal element. Moreover, the surface of the lithium-containing metal composite oxide particles may be coated with a different element. Examples of the different element M ¹ include Na, Mg, Sc, Y, Cu, Zn, Al, Cr, Pb, Sb, and B. A positive electrode active material may be used individually by 1 type, and may be used in combination of 2 or more type.

Specific positive electrode active material, for example, lithium cobalt oxide Li _m CoO _2, lithium nickelate _{_{_{Li m NiO 2, Li m MnO}}} 2, Li m Co n Ni 1-n O 2, Li m Co n M 1 1 _{_{_{_{-n O p, Li m Ni 1}}}} -n M 1 n O p ( for example, _{_{_{_{LiNi 1/3 Co 1/3 Mn 1/3 O 2}}}} ), Li m Mn 2 O 4, Li m Mn 2-n M ¹ _n O ₄ , LiMPO ₄ , Li ₂ MPO ₄ F and the like can be mentioned. The element M ¹ is the above-mentioned different element, and the element M is the above-described metal element. In the above general formula, 0 <m ≦ 1.2, 0 <n ≦ 0.9, and 2.0 ≦ p ≦ 2.3.

A positive electrode active material can be used individually by 1 type or in combination of 2 or more types. Among these, oxides that can be represented by LiMO ₂ (M is the above metal element), specifically, Li _m CoO ₂ , lithium nickelate Li _m NiO ₂ , Li _m MnO ₂ , Li _m Con _n _{_{Ni 1-n O 2, Li}} m Co n M 1 1-n O p, such as _{_{^{Li m Ni 1-n M 1}}} n O p are preferred.

The ratio x / M _c between the total amount x of lithium contained in the positive electrode and the negative electrode and the amount M _c of the metal element M contained in the oxide is preferably greater than 1.03, for example, 1.05 More preferably, it is larger. When the ratio x / _Mc is within such a range, the ratio of lithium ions supplied into the battery becomes very large. That is, it is advantageous in terms of irreversible capacity compensation and formation of inorganic lithium compounds such as lithium carbonate.

When the negative electrode active material includes a carbonaceous active material, the ratio x / M _c is preferably greater than 1.03, more preferably greater than 1.04 (particularly 1.05).
When the negative electrode active material contains silicon and / or tin, the ratio x / _Mc is preferably greater than 1.05, more preferably 1.06 or more, and even more preferably 1.07 or more (particularly 1 .08 or more).

The ratio x / M _c is the amount M _c of the metal element M contained in the lithium content x and the positive electrode active material contained in the positive electrode and the negative electrode, were quantified, respectively, dividing the amount of x in an amount M _c of the metal element M This can be calculated.
The amount M _c of lithium content x and the metal element M can be quantified as follows.
First, the battery is completely discharged and then decomposed to remove the nonaqueous electrolyte, and the inside of the battery is washed with a solvent such as dimethyl carbonate. Next, a predetermined amount of each of the positive electrode and the negative electrode is collected, and the amount of lithium x is determined by quantifying the amounts of lithium contained in the positive electrode and the negative electrode by ICP analysis. Also, as in the case of the amount of lithium in the positive electrode, the amount M _c of the metal element M contained in the positive electrode is quantified by ICP analysis.

The positive electrode active material is usually used in a particulate form. The average particle diameter of the positive electrode active material is, for example, 5 to 30 μm, preferably 5 to 20 μm, and more preferably 7 to 15 μm. When the average particle diameter is in such a range, the surface area of the active material particles can be maintained in an appropriate range. Therefore, sufficient adhesion strength can be easily obtained in the positive electrode without increasing the amount of the binder, and the binding can be achieved. A decrease in capacity due to an increase in the amount of the adhesive can be suppressed. Moreover, when applying the positive electrode mixture slurry to the current collector, it is possible to more effectively suppress the occurrence of coating streaks.

The positive electrode active material particles may be subjected to various surface treatments as necessary. Examples of the surface treatment include a surface treatment with a metal oxide (for example, alumina, titania, etc.) other than the oxide constituting the positive electrode active material, a surface treatment with a conductive agent, and a hydrophobic treatment.

As the dispersion medium, binder, conductive agent and thickener, those exemplified for the negative electrode can be used. The amount of each component relative to the active material can also be selected from the same range as that of the positive electrode. The thickness of the positive electrode active material layer can also be selected from the same range as the negative electrode active material layer.

The content of the binder in the positive electrode active material layer is, for example, 1 to 10 parts by mass, preferably 3 to 6 parts by mass with respect to 100 parts by mass of the positive electrode active material.

Among the binders used for the positive electrode, from the viewpoint of being chemically stable in the battery and easily obtaining a sufficient binding force, a co-polymer containing PVDF, a modified product thereof, or vinylidene fluoride as a monomer unit. It is preferable to use a polymer or a rubber-like material. When the binding force at the positive electrode is high, deterioration of the positive electrode due to charge / discharge can be more effectively suppressed. Therefore, it is more advantageous to use a copolymer containing PVDF and vinylidene fluoride in order to improve discharge characteristics and cycle characteristics. When such a binder is used, it is easy to obtain a higher binding force. Therefore, it is desirable to use the binder in a state where it is dissolved in a dispersion medium.

(Separator)
The separator is not particularly limited as long as it has high ion permeability and has mechanical strength and insulation necessary for preventing a short circuit between the positive electrode and the negative electrode. For example, a microporous membrane made of resin, woven fabric, etc. A cloth or a nonwoven fabric can be illustrated. Examples of the resin constituting the separator include polyolefins such as polyethylene and polypropylene; polyamide resins such as polyamide; polyimide resins such as polyamideimide and polyimide.

Among these, a separator made of polyolefin is preferable from the viewpoint of excellent durability and shutdown function and easy securing of battery safety. The microporous membrane may contain a known additive as necessary.
The microporous membrane separator may be a single layer film or a multilayer film (composite film) having a different composition.

The thickness of the separator can be appropriately selected from the range of, for example, 10 to 300 μm, preferably 10 to 40 μm, more preferably 10 to 30 μm (particularly 10 to 25 μm).
The separator has a porosity of, for example, 30 to 70%, preferably 35 to 60%. The porosity indicates the ratio of the volume of the void portion to the total volume of the separator.

(Non-aqueous electrolyte)
The nonaqueous electrolyte includes a nonaqueous solvent and a lithium salt (electrolyte) dissolved in the nonaqueous solvent.
Examples of the non-aqueous solvent include known non-aqueous solvents used for non-aqueous electrolytes of non-aqueous electrolyte secondary batteries, such as cyclic carbonates, chain carbonates, and cyclic carboxylic acid esters. Examples of the cyclic carbonate include ethylene carbonate (EC) and propylene carbonate. Examples of the chain carbonate include diethyl carbonate, ethyl methyl carbonate, dimethyl carbonate (DMC) and the like. Examples of the cyclic carboxylic acid ester include γ-butyrolactone and γ-valerolactone. A non-aqueous solvent may be used individually by 1 type, and may be used in combination of 2 or more type.

The non-aqueous solvent preferably contains a cyclic carbonate and a chain carbonate. The cyclic carbonate preferably contains EC, and the chain carbonate preferably contains DEC and / or DMC.

Examples of the lithium salt include a lithium salt of a chlorine-containing acid (LiClO ₄ , LiAlCl ₄ , LiB ₁₀ Cl _10, etc.), a lithium salt of a fluorine-containing acid (LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiCF ₃ SO _3). LiCF ₃ CO ₂ ), lithium salt of fluorine-containing acid imide (LiN (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiN (C ₂ F ₅ SO ₂ ) ₂ ), lithium halide (LiCl, LiBr, LiI, etc.) can be used. These lithium salts can be used singly or in combination of two or more.

The concentration of the lithium salt in the nonaqueous electrolyte is, for example, 0.5 to 2 mol / L, preferably 1.2 to 1.6 mol / L.

The non-aqueous electrolyte may contain a known additive. Examples of such additives include an additive (additive A) that decomposes on the negative electrode to form a film having high lithium ion conductivity and increases the charge / discharge efficiency of the battery, and decomposes on overcharge to form a film on the electrode. And an additive (additive B) that inactivates the battery.

Examples of the additive A include cyclic carbonates having a polymerizable unsaturated bond (vinylene group, vinyl group, etc.). As cyclic carbonates having vinylene groups, VC: C _1-4 alkyl groups such as 4-methyl vinylene carbonate, 4,5-dimethyl vinylene carbonate, 4-ethyl vinylene carbonate and / or C _6-10 aryl groups are substituted. VC which has as a group can be illustrated.

Examples of the cyclic carbonate having a vinyl group include EC having a vinyl group as a substituent, such as vinyl ethylene carbonate (VEC) and divinyl ethylene carbonate. In these compounds, those in which a part of the hydrogen atoms constituting the substituent or cyclic carbonate are partially substituted with fluorine atoms can also be used as the additive. Additive A can be used alone or in combination of two or more.
Among the additives A, at least one selected from the group consisting of VC, VEC and divinylethylene carbonate is preferable.

Examples of the additive B include an aromatic compound having an aliphatic ring and an aromatic compound having a plurality of aromatic rings.
Examples of the aliphatic ring include a cycloalkane ring such as a cyclohexane ring, a cyclic ether, a cyclic ester, and the like. The aromatic compound preferably has these aliphatic rings as substituents. Specific examples of such aromatic compounds include benzene compounds such as cyclohexylbenzene.
Examples of the aromatic compound having a plurality of aromatic rings include biphenyl and diphenyl ether.
These additives B can be used singly or in combination of two or more.

The content of the additive in the nonaqueous electrolyte is, for example, 10% by mass or less, preferably 7% by mass or less. In addition, it is preferable that the additive B is 10 mass parts or less with respect to 100 mass parts of nonaqueous solvents, for example.
The non-aqueous electrolyte may be liquid or gel.
The liquid nonaqueous electrolyte includes a nonaqueous solvent and a lithium salt dissolved in the nonaqueous solvent.

The gel-like nonaqueous electrolyte includes a liquid nonaqueous electrolyte and a polymer material that holds the nonaqueous electrolyte. Examples of the polymer material include fluorine resins such as PVDF and vinylidene fluoride-hexafluoropropylene copolymer; vinyl resins such as polyacrylonitrile and polyvinyl chloride; polyalkylene oxides such as polyethylene oxide; acrylic resins such as polyacrylate Etc.

(Other components)
The electrode group is not limited to the wound one as shown in FIG. 1, but may be a laminated one including one folded in a zigzag manner. The shape of the electrode group may be a cylindrical shape or a flat shape whose end surface perpendicular to the winding axis is an oval shape, depending on the shape of the battery or the battery case.

The battery case may be made of metal or laminate film. As the metal material forming the battery case, aluminum, an aluminum alloy (such as an alloy containing a trace amount of metal such as manganese or copper), a steel plate such as iron or stainless steel, or the like can be used. The battery case may be plated by nickel plating or the like, if necessary.
The shape of the battery case may be other than a cylindrical shape, a rectangular shape, etc., depending on the shape of the electrode group.

Hereinafter, the present invention will be specifically described based on examples and comparative examples, but the present invention is not limited to the following examples.

Example 1
Batteries 1 to 5 were produced by the following procedure.
(Battery 1)
(1) Production of positive electrode LiNi _0.82 Co _0.15 Al _0.03 O ₂ particles (average particle diameter 10 μm) as a positive electrode active material, acetylene black as a conductive agent, and an NMP solution containing PVDF as a binder are mixed. Thus, a positive electrode mixture slurry was prepared. The amount of the conductive agent was 4.5 parts by mass and the amount of the binder was 4.7 parts by mass with respect to 100 parts by mass of the positive electrode active material.

The positive electrode mixture slurry was applied to both sides of an aluminum foil (thickness 15 μm) as a positive electrode current collector and dried. The obtained positive electrode current collector having a dried coating film was rolled to produce a positive electrode plate having a thickness of 0.157 mm. By cutting the positive electrode plate, a positive electrode having a thickness of 0.157 mm, a width of 57 mm, and a length of 564 mm was produced.
In addition, the collector exposure part in which the coating film of positive mix slurry was not formed in both surfaces was formed in the center part of the longitudinal direction of a positive electrode. One end of a positive electrode lead made of aluminum was welded to the exposed portion of the current collector.

(2) Production of negative electrode The flaky artificial graphite was pulverized and classified to obtain a negative electrode active material having an average particle diameter of about 20 μm. A negative electrode mixture slurry was prepared by mixing 100 parts by mass of a negative electrode active material, 3 parts by mass of SBR as a binder, and 100 parts by mass of a CMC aqueous solution (CMC concentration: 1% by mass).

The negative electrode mixture slurry was applied to both sides of a copper foil (thickness 8 μm) as a negative electrode current collector and dried. The obtained negative electrode current collector having a dried coating film was rolled to obtain a negative electrode plate having a thickness of 0.156 mm. Heat treatment was performed by exposing the negative electrode plate to hot air at 190 ° C. for 8 hours in a nitrogen atmosphere. By cutting the heat-treated negative electrode plate, a negative electrode having a thickness of 0.156 mm, a width of 58.5 mm, and a length of 750 mm was formed.
In addition, in the negative electrode region that does not face the positive electrode active material when forming the electrode group at one end in the longitudinal direction of the negative electrode, a collector exposed portion in which a coating film of the negative electrode mixture slurry was not formed on both surfaces was formed. .

Next, lithium was vacuum-deposited on the surface of the negative electrode so that the deposition amount was 0.26 g / m ² (equivalent to 0.5 μm when converted to the thickness of the Li metal deposition film). A negative electrode having a deposited film of Li on both sides was placed in a dry air environment having a dew point of −30 ° C. or lower and allowed to stand for 7 days, whereby lithium was occluded in the negative electrode. By such an operation, the irreversible capacity of the negative electrode was reduced.
And the one end part of the negative electrode lead made from nickel was welded to the collector exposed part of the negative electrode.

(3) Preparation of Nonaqueous Electrolyte A nonaqueous electrolyte was prepared by dissolving LiPF ₆ and VC in a mixed solvent containing EC: DMC (volume ratio) = 1: 3. The concentration of LiPF ₆ in the nonaqueous electrolyte was 1.4 mol / dm ³ , and the concentration of VC was adjusted to 5% by mass.

(4) Production of battery The positive electrode and the negative electrode obtained in the above (1) and (2) were wound in a spiral shape with a polyethylene microporous membrane separator (thickness 20 μm) interposed therebetween. Turned to produce an electrode group.

The upper insulating film and the lower insulating plate were arranged on the upper end and the lower end of the obtained electrode group, respectively, and housed in a bottomed cylindrical metal battery case. The other end portion of the positive electrode lead pulled out from the electrode group was welded to a sealing plate having an internal pressure actuated safety valve, and the other end portion of the negative electrode lead was welded to the inner bottom surface of the battery case. Next, the electrode group was held in the battery case by forming a stepped portion protruding inward on the side surface of the battery case above the upper end of the electrode group. A non-aqueous electrolyte is injected into the battery case by a decompression method, and the opening of the battery case is sealed by caulking the peripheral edge of the sealing plate via a gasket, thereby providing a cylindrical lithium ion secondary battery ( A battery 1) was prepared.

(Batteries 2 to 5)
The battery 2 was produced in the same manner as the battery 1 except that lithium was vacuum-deposited on the negative electrode and the battery was produced immediately after deposition, as with the battery 1.
The

batteries

3 and 4 were produced in the same manner as the battery 1 except that the period of time in the dry air environment after vacuum deposition of lithium was changed to the period shown in Table 1.
Battery 5 was produced in the same manner as Battery 1 except that lithium was not vacuum deposited on the negative electrode and then left untreated.

(Discharge capacity and capacity maintenance rate)
For each of the batteries 1 to 5, the charge / discharge cycle characteristics were evaluated.
Specifically, each battery was charged in a 45 ° C. environment at a constant current of 1.4 A until the voltage reached 4.20 V, and charged at a constant voltage of 4.20 V until the current reached 50 mA. Thereafter, discharging was performed at a constant current of 2.8 A until the voltage reached 2.5V. Charging and discharging were repeated with such charging and discharging as one cycle. A 30-minute rest period was provided between charging and discharging and between discharging and charging.

The discharge capacity (initial discharge capacity) at the first cycle of charge / discharge and the discharge capacity at the 500th cycle are measured, and the ratio (percentage) of the discharge capacity at the 500th cycle of charge / discharge to the initial discharge capacity is determined as the capacity maintenance rate. As sought.
The results are shown in Table 1 together with the deposition of lithium and the standing period after the deposition.

Example 2
(Battery 6)
A lithium ion secondary battery (battery 6) was produced in the same manner as the battery 1 of Example 1, except that the negative electrode produced as follows was used.
(Preparation of negative electrode)
100 parts by mass of silicon powder (average particle diameter 10 μm) as a negative electrode active material prepared by a chemical vapor deposition (CVD) method, 10 parts by mass of PVDF as a binder, and 5 mass of graphite (average particle diameter 3 μm) as a conductive agent A negative electrode mixture slurry was prepared by mixing a part and an appropriate amount of NMP.

A copper foil (thickness: 18 μm) whose surfaces were roughened was used as a negative electrode current collector, and a negative electrode mixture slurry was applied to both sides of the negative electrode current collector and dried. The obtained negative electrode current collector having a dried coating film was rolled to prepare a negative electrode plate having a thickness of 98 μm.
In addition, in the region of the negative electrode plate that does not face the positive electrode active material when forming the electrode group at one end in the longitudinal direction of the negative electrode plate, the collector exposed portion where the negative electrode mixture slurry coating film is not formed on both sides Formed.

Next, lithium was vacuum-deposited on the surface of the negative electrode so that the deposition amount was 1.6 g / m ² (equivalent to 3 μm in terms of the thickness of the Li metal deposition film). A negative electrode having a deposited film of Li on both sides was placed in a dry air environment having a dew point of −30 ° C. or lower and allowed to stand for 7 days, whereby lithium was occluded in the negative electrode. By such an operation, the irreversible capacity of the negative electrode was reduced.
The obtained negative electrode plate was cut to obtain a negative electrode having a width of 58.5 mm and a length of 750 mm.

(Batteries 7 to 10)
Batteries 7 to 10 were produced in the same manner as the battery 6 except that the presence or absence of vacuum vapor deposition and the standing period after the vapor deposition were changed in the same manner as the batteries 2 to 5.
For the batteries 6 to 10, the initial discharge capacity and the capacity retention rate were evaluated in the same manner as in Example 1.
The results are shown in Table 2 together with the presence or absence of lithium deposition and the standing period after the deposition.

Example 3
(Battery 11)
A lithium ion secondary battery (battery 11) was produced in the same manner as the battery 1 of Example 1 except that the negative electrode produced as follows was used.
(Preparation of negative electrode)
100 parts by mass of silicon monoxide (SiO) powder (average particle diameter 8 μm) as a negative electrode active material, 15 parts by mass of PVDF as a binder, 7 parts by mass of graphite (average particle diameter 3 μm) as a conductive agent, and an appropriate amount A negative electrode mixture slurry was prepared by mixing with NMP.

A copper foil (thickness: 18 μm) whose surfaces were roughened was used as a negative electrode current collector, and a negative electrode mixture slurry was applied to both sides of the negative electrode current collector and dried. The obtained negative electrode current collector having a dried coating film was rolled to prepare a 125 μm-thick negative electrode having a negative electrode active material layer on both surfaces.
In addition, in the region of the negative electrode plate that does not face the positive electrode active material when forming the electrode group at one end in the longitudinal direction of the negative electrode plate, the collector exposed portion where the negative electrode mixture slurry coating film is not formed on both sides Formed.

Next, a metal lithium foil having a thickness of 30 μm, a width of 50 mm, and a length of 10 mm was attached to the surface of the negative electrode active material layer at intervals of 30 mm. The negative electrode on which the lithium foil was attached was placed in an environment of 120 ° C. in a carbon dioxide atmosphere and left for 7 days to produce a negative electrode in which lithium was occluded. By such an operation, the irreversible capacity of the negative electrode was reduced.

(Batteries 12 to 15)
Batteries 12 to 15 were fabricated in the same manner as the battery 11, except that the presence or absence of the lithium foil and the standing period after the pasting were changed in the same manner as the standing time after the deposition of the batteries 2 to 5.
For the batteries 12 to 15, the initial discharge capacity and the capacity retention rate were evaluated in the same manner as in Example 1.
The results are shown in Table 3 together with the presence / absence of application of lithium foil and the standing period after application.

Example 4
(Battery 16)
100 parts by mass of tin-cobalt-carbon alloy powder (average particle diameter 4 μm) as a negative electrode active material, 15 parts by mass of PVDF as a binder, 5 parts by mass of graphite (average particle diameter 3 μm) as a conductive agent, and an appropriate amount A negative electrode mixture slurry was prepared by mixing with NMP.

A copper foil (thickness: 18 μm) whose surfaces were roughened was used as a negative electrode current collector, and a negative electrode mixture slurry was applied to both sides of the negative electrode current collector and dried. The obtained negative electrode current collector having a dried coating film was rolled to prepare a negative electrode plate having a thickness of 118 μm.
In addition, in the region of the negative electrode plate that does not face the positive electrode active material when forming the electrode group at one end in the longitudinal direction of the negative electrode plate, the collector exposed portion where the negative electrode mixture slurry coating film is not formed on both sides Formed.

Subsequently, lithium was vacuum-deposited on the surface of the negative electrode so that the deposition amount was 3.2 g / m ² (equivalent to 6 μm when converted to the thickness of the Li metal deposition film). A negative electrode having a deposited film of Li on both sides was placed in a dry air environment having a dew point of −30 ° C. or lower and allowed to stand for 7 days, whereby lithium was occluded in the negative electrode. By such an operation, the irreversible capacity of the negative electrode was reduced.
The obtained negative electrode plate was cut to obtain a negative electrode having a width of 58.5 mm and a length of 750 mm.

(Batteries 17-20)
Batteries 17 to 20 were produced in the same manner as the battery 16 except that the presence or absence of vacuum deposition and the standing period after the deposition were changed in the same manner as the batteries 2 to 5.
For the batteries 16 to 20, the initial discharge capacity and the capacity retention rate were evaluated in the same manner as in Example 1.
The results are shown in Table 4 together with the presence or absence of lithium deposition and the standing period after deposition.

Hereinafter, Examples 1 to 4 will be examined in detail based on Tables 1 to 4.
In Examples 1 to 4, irreversible capacity can be obtained by depositing lithium or pasting lithium foil, compared to

batteries

5, 10, 15, and 20, where neither lithium deposition nor lithium foil is applied. High capacity was obtained in other batteries using negative electrodes with reduced resistance. In particular, in Example 3 in which silicon monoxide was used as the negative electrode active material, the irreversible capacity of the negative electrode was very large, so that a high irreversible capacity filling effect was obtained by occlusion of lithium.

In Examples 1 to 4, the capacity retention ratio increased as the standing period after lithium deposition or lithium foil application increased. The following analysis and analysis were conducted for this factor.
The following evaluation was performed using the negative electrode taken out by disassembling the battery after the first charge / discharge.

(About the formation state of the film and the composition of the film in the negative electrode)
X-ray photoelectron spectroscopic analysis of the negative electrode was performed, and the composition of the film formed on the negative electrode was examined. About the change of the composition of the thickness direction, it evaluated by analyzing in the state which scraped off the active material layer from the negative electrode surface by sputtering.

As a result, it was found that the film was formed over the entire negative electrode active material layer. The film formed on the surface side of the negative electrode mainly contains an inorganic lithium compound such as lithium carbonate, lithium hydroxide, lithium oxide, and lithium fluoride, and particularly contains a large amount of lithium carbonate.

Further, in the thickness direction of the negative electrode active material layer, the amount of the film containing the inorganic lithium compound decreases as the surface of the negative electrode active material layer (the surface of the negative electrode) approaches the current collector, and the amount of the film containing the organic lithium compound decreases. The amount increased. In addition, alkyl lithium carbonate etc. were detected as an organolithium compound. When the peak intensities by X-ray photoelectron spectroscopy were compared, the proportion of the film containing an inorganic lithium compound including lithium carbonate increased as the standing period after lithium deposition or after the lithium foil was attached was longer.

(About the amount of lithium carbonate in the negative electrode)
The amount of lithium carbonate contained in the negative electrode was quantified as follows.
Using a surface cutting device (for example, Dyprawintes Co., Ltd., Psycus), as shown in FIG. 3, the active material from the surface of the negative electrode active material layer (the surface of the negative electrode) to a depth of 30% indicated by the broken line 11 is used. The material layer (B layer) 14 was cut out, and the lithium carbonate contained in the B layer 14 was quantitatively analyzed.

Further, the negative electrode active material layer (A layer) 13 left after cutting out the B layer 14 was similarly subjected to quantitative analysis of the amount of lithium carbonate.
In addition, the quantitative analysis of each layer is carried out by putting each layer into pure water, eluting carbonate ions contained in each layer purely, obtaining the content of carbonate ions by ion chromatography, and based on this content, This was done by calculating the amount of lithium carbonate contained in the water.

Moreover, since the thicknesses of the A layer 13 and the B layer 14 are different, a comparison in terms of the same thickness was also performed. Specifically, the amount of lithium carbonate contained in the A layer 13 is directly compared with the amount of lithium carbonate contained in the B layer 14 by multiplying by 30/70 (= 0.429) the thickness ratio of the B layer to the A layer. The possible values were calculated.

(Expansion amount of negative electrode)
At 10 points in the longitudinal direction of the negative electrode, the thickness of the negative electrode was measured with a micrometer, and the average thickness after the first charge / discharge was calculated. Then, a value obtained by subtracting the initial negative electrode thickness (thickness before battery assembly) from this average thickness was evaluated as the amount of expansion of the negative electrode after the first charge / discharge.

(The ratio x / M _c between the amount M _c of the metal element M contained in the lithium content x and the positive electrode active material in the positive electrode and the negative electrode)
And the amount M _c of the metal element M contained in the lithium content x and the positive electrode material contained in the positive electrode and the negative electrode in these cells was quantified as described above, was calculated x / M _c ratio.
The results are shown in Tables 5-8.

As shown in Tables 5 to 8, it is apparent that the amount of lithium carbonate contained in the A layer is obviously large in the current collector side A layer and the surface side B layer of the negative electrode active material layer. In addition, it can be seen that the longer the standing period after lithium deposition or lithium foil attachment, the greater the amount of lithium carbonate contained in the negative electrode active material layer, and the greater the amount of coating film containing lithium carbonate. As a matter of course, the x / _Mc ratio is larger in the battery in which lithium is vapor-deposited or attached to the negative electrode than in the battery in which lithium is not vapor-deposited or attached to the negative electrode. Its value exceeded 1.03.

Moreover, the larger the amount of lithium carbonate contained in the B layer, the smaller the amount of expansion of the negative electrode. This is presumably because excessive expansion of the negative electrode was suppressed by the formation of many strong films containing an inorganic lithium compound such as lithium carbonate in the vicinity of the surface layer of the negative electrode active material layer. And since expansion | swelling was suppressed, since deterioration of the negative electrode active material was suppressed, it is thought that the favorable charging / discharging cycling characteristics were acquired.

Although the present invention has been described in terms of the presently preferred embodiments, such disclosure should not be construed as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains after reading the above disclosure. Accordingly, the appended claims should be construed to include all variations and modifications without departing from the true spirit and scope of this invention.

The non-aqueous electrolyte secondary battery of the present invention has a high capacity and a high energy density, and a high capacity retention rate can be obtained even after repeated charging and discharging. Therefore, non-aqueous electrolyte secondary batteries are used in hybrid electric vehicles (especially for plug-in hybrid vehicles), power sources for automobiles such as electric vehicles, various consumer power sources such as mobile phones, notebook personal computers, and video camcorders. Useful for applications such as power supplies for tools.

DESCRIPTION OF SYMBOLS 1 Battery case 2 Sealing plate 3 Gasket 4 Electrode group 5 Positive electrode 5a Positive electrode collector 5b Positive electrode

active material layer

5c, 5d Positive electrode collector exposed part 6 Negative electrode 6a Negative electrode collector 6b Negative electrode

active material layer

6c, 6d Negative electrode collector Body exposed portion 7 Separator 8a Upper insulating ring 8b Lower insulating ring 9 Positive electrode lead 10 Negative electrode lead 11 Step portion 12 Boundary line between A layer and B layer 13 A layer 14 B layer

Claims

A positive electrode, a negative electrode, and an electrode group in which a separator interposed between the positive electrode and the negative electrode is wound or laminated, and a non-aqueous electrolyte,
The positive electrode includes a positive electrode current collector, and a positive electrode active material layer including a positive electrode active material capable of inserting and extracting lithium ions attached to the surface of the positive electrode current collector,
The negative electrode includes a negative electrode current collector, and a negative electrode active material layer including a negative electrode active material capable of occluding and releasing lithium ions attached to the surface of the negative electrode current collector,
The negative electrode active material layer includes a film formed on the surface of the negative electrode active material,
At least a part of the coating contains an inorganic lithium compound,
When the negative electrode active material layer is divided into the A layer on the negative electrode current collector side and the B layer on the surface side of the negative electrode active material layer at a thickness ratio of 7: 3, the area per area of the negative electrode A nonaqueous electrolyte secondary battery in which the content of the inorganic lithium compound contained in the B layer is greater than the content of the inorganic lithium compound contained in the A layer.
The nonaqueous electrolyte secondary battery according to claim 1, wherein the film contains at least lithium carbonate as the inorganic lithium compound.
The ratio C B / C A of the content C B of lithium carbonate contained in the B layer to the content C A of lithium carbonate contained in the A layer is 1.05 to 2. Water electrolyte secondary battery.
The ratio C B / C A of the content C B of lithium carbonate contained in the B layer to the content C A of lithium carbonate contained in the A layer is 1.1 to 1.5. 3. The nonaqueous electrolyte secondary battery according to 3.
At least a portion of the coating contains an organolithium compound,
The content of the organolithium compound contained in the A layer per area of the negative electrode is larger than the content of the organolithium compound contained in the B layer. Water electrolyte secondary battery.
The non-aqueous electrolyte secondary battery according to claim 5, wherein the film containing the organic lithium compound contains lithium alkyl carbonate as the organic lithium compound.
The nonaqueous electrolyte secondary battery according to any one of claims 1 to 6, wherein the negative electrode active material includes at least one selected from the group consisting of silicon and tin.
The positive electrode active material includes an oxide containing lithium and a metal element M;
The metal element M includes at least one selected from the group consisting of cobalt, nickel, manganese and iron,
The ratio x / M c between the total amount x of lithium contained in the positive electrode and the negative electrode and the amount M c of the metal element M contained in the oxide is greater than 1.03. The nonaqueous electrolyte secondary battery according to any one of the preceding claims.
The nonaqueous electrolyte secondary battery according to claim 8, wherein the negative electrode active material includes a carbonaceous active material, and the ratio x / M c is greater than 1.04.
The nonaqueous electrolyte secondary battery according to claim 8, wherein the negative electrode active material includes at least one selected from the group consisting of silicon and tin, and the ratio x / M c is greater than 1.05.
The nonaqueous electrolyte secondary battery according to any one of claims 8 to 10, wherein the negative electrode is formed by occluding lithium in the negative electrode active material layer before forming the electrode group.
The non-aqueous electrolyte secondary battery according to claim 11, wherein the occlusion of lithium is performed by forming a metallic lithium layer by applying vacuum deposition or a lithium foil to the surface of the negative electrode active material layer.