WO2012001814A1

WO2012001814A1 - Lithium secondary battery

Info

Publication number: WO2012001814A1
Application number: PCT/JP2010/061332
Authority: WO
Inventors: 智彦石田; 祐介小野田
Original assignee: トヨタ自動車株式会社
Priority date: 2010-07-02
Filing date: 2010-07-02
Publication date: 2012-01-05

Abstract

Disclosed is a high performance lithium secondary battery in which deterioration of the performance due to deposition of lithium is suppressed. Specifically disclosed is a lithium secondary battery which comprises a negative electrode (20) that is obtained by forming a negative electrode active material layer (24) on a negative electrode collector (22), said negative electrode active material layer (24) containing a negative electrode active material (26) and a binder (28). The negative electrode active material layer (24) has a thickness of 50 μm or less, and the binder content in the negative electrode active material layer (24) is 1% by mass or less. In this connection, if the negative electrode active material layer (24) is divided into halves in the thickness direction and a half that is closer to the negative electrode collector (22) is defined as a lower layer (24a) and the other half, which is farther from the negative electrode collector (22), is defined as an upper layer (24b), the binder concentration (mass%) of the upper layer (24b) is different from that of the lower layer (24a) and the uneven distribution degree (X) of the binder represented by the value obtained by dividing the binder concentration (B) in the upper layer (24b) by the binder concentration (A) in the lower layer (24a), namely X = B/A is within the range of 0.4 ≤ X < 1.0.

Description

Lithium secondary battery

The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery with improved durability against charge / discharge cycles.

In recent years, lithium ion batteries, nickel metal hydride batteries, and other secondary batteries have become increasingly important as on-vehicle power supplies or personal computers and portable terminals. In particular, a lithium secondary battery that is lightweight and has a high energy density is expected to be preferably used as a high-output power source for mounting on a vehicle. In one typical configuration of this type of secondary battery, an electrode having a configuration in which a material (electrode active material) capable of reversibly occluding and releasing lithium ions is held in a conductive member (electrode current collector) is used. Prepare. For example, as a typical example of the electrode active material (negative electrode active material) used for the negative electrode, carbon-based materials such as graphite carbon and amorphous carbon are exemplified. A typical example of the electrode current collector (negative electrode current collector) used for the negative electrode is a sheet-like or foil-like member mainly composed of copper or a copper alloy.

As one of typical methods for holding a negative electrode active material on a negative electrode current collector in manufacturing a negative electrode having such a structure, a negative electrode in which a negative electrode active material powder and a binder (binder) are dispersed in an appropriate medium A method of forming a layer (negative electrode active material layer) containing a negative electrode active material by applying a paste for forming an active material layer to a negative electrode current collector (copper foil or the like), passing it through a hot air dryer or the like, and drying the paste. Is mentioned. The binder contained in the negative electrode active material layer plays a role of binding between the negative electrode active materials and between the negative electrode active material and the current collector. Patent document 1 is mentioned as a technical document regarding the electrode active material layer containing this kind of binder.

Japanese Patent Application Publication No. 10-270013

However, when manufacturing this type of negative electrode, if a negative electrode active material layer forming paste containing a negative electrode active material and a binder is applied to the negative electrode current collector and dried by applying hot air, convection occurs during drying. In some cases, the binder in the vicinity of the negative electrode current collector floats on the surface of the paste coating and is segregated on the surface layer portion of the negative electrode active material layer. When the binder segregates on the surface layer portion of the negative electrode active material layer, the reactivity of the negative electrode active material in the surface layer portion (activity of lithium ion insertion / desorption reaction) decreases, and the reaction resistance increases. Therefore, lithium ions released from the positive electrode active material may not immediately enter the negative electrode active material but may precipitate in the negative electrode active material layer. Such lithium deposition can cause battery performance deterioration (battery capacity reduction, etc.). In particular, at the time of charging / discharging at a low temperature (for example, 0 ° C. or less), the reactivity of the negative electrode active material tends to further decrease, so that the performance deterioration is likely to occur.

The present invention has been made in view of such a point, and a main object thereof is to provide a high-performance lithium secondary battery in which performance deterioration due to the deposition of lithium is suppressed.

According to the present invention, there is provided a lithium secondary battery including a negative electrode in which a negative electrode active material layer having a negative electrode active material and a binder is formed on a negative electrode current collector. The negative electrode active material layer has a thickness of 50 μm or less, and the binder content in the negative electrode active material layer is 1% by mass or less. Here, in the case where the negative electrode active material layer is divided in half in the thickness direction, the side closer to the negative electrode current collector is the lower layer and the side far from the negative electrode current collector is the upper layer, the binder concentration (mass% ) Differs between the upper layer and the lower layer, and the binder uneven distribution degree X = B / A, which is a value obtained by dividing the binder concentration B on the upper layer side by the binder concentration A on the lower layer side, is 0.4 ≦ X <1. 0.0, preferably 0.4 ≦ X ≦ 0.8, and particularly preferably 0.4 ≦ X ≦ 0.6.

According to the lithium secondary battery having such a configuration, the binder uneven distribution degree X (upper layer side binder concentration B / lower layer side binder concentration A) when the negative electrode active material layer is divided in half in the thickness direction is 0.4 ≦ X <. Since 1.0 is satisfy | filled, precipitation of lithium in a negative electrode active material layer can be suppressed.

If the binder uneven distribution degree X exceeds 1.0, a large amount of binder is disposed in the surface layer portion of the negative electrode active material layer, so the reactivity of the negative electrode active material in the surface layer portion (activity of lithium ion insertion / desorption reaction) Decreases. For this reason, the reaction resistance of the negative electrode increases, and lithium ions released from the positive electrode active material may not immediately enter the negative electrode active material but may precipitate in the negative electrode active material layer. On the other hand, when the binder uneven distribution degree X is less than 0.4, a large amount of the binder is disposed in the vicinity of the negative electrode current collector, so that the interface resistance between the negative electrode current collector and the negative electrode active material layer increases. As a result, the direct current resistance of the negative electrode increases, and the internal resistance of the battery increases accordingly. Therefore, an increase in overvoltage accompanying an increase in internal resistance makes it easier to reach the lithium deposition potential. Such lithium deposition can cause performance deterioration (battery capacity reduction, etc.).

In contrast, in the lithium secondary battery in which the binder uneven distribution degree X satisfies 0.4 ≦ X <1.0, the binder concentration on the upper layer side and the lower layer side is appropriately adjusted. Both can be lowered and lithium precipitation can be suppressed. According to such a lithium secondary battery, discharging and charging are performed at a low temperature (for example, 0 ° C.) and at a high rate (for example, 20 C) as expected in a battery for a power source of a vehicle, for example, where lithium is liable to precipitate. Even in a high-rate charge / discharge pattern that repeats continuously, lithium deposition can be reliably suppressed, and a lithium secondary battery with high durability against a high-rate charge / discharge cycle (for example, a high capacity retention rate) can be provided. .

In a preferred embodiment of the lithium secondary battery disclosed herein, the thickness of the negative electrode active material layer is 50 μm or less. If the negative electrode active material layer is too thick, the effect (effect of suppressing lithium deposition) obtained by setting the binder uneven distribution degree X of the negative electrode active material layer to 0.4 ≦ X <1.0 may not be obtained. On the other hand, if the negative electrode active material layer is too thin, the amount of active material contained per volume of the negative electrode is reduced, so that the capacity per volume of the negative electrode (and thus a lithium secondary battery constructed using the negative electrode) tends to decrease. May be. Accordingly, the thickness of the negative electrode active material layer is suitably 50 μm or less, preferably 10 μm to 50 μm, and particularly preferably 20 μm to 50 μm.

In a preferred embodiment of the lithium secondary battery disclosed herein, the binder content in the negative electrode active material layer is 1% by mass or less. If the binder content is too high, the effect (effect of suppressing lithium deposition) obtained by setting the binder uneven distribution degree X of the negative electrode active material layer to 0.4 ≦ X <1.0 may not be obtained. On the other hand, if the binder content is too small, the adhesion between the negative electrode current collector and the negative electrode active material layer may decrease, and peeling of the negative electrode active material layer may occur. Accordingly, the binder content in the negative electrode active material layer is suitably 1% by mass or less, preferably 0.3% by mass to 1% by mass, and particularly preferably 0.5% by mass to 1% by mass. .

In a preferred embodiment of the lithium secondary battery disclosed herein, the porosity of the negative electrode active material layer is 35% or more. If the porosity of the negative electrode active material layer is too low, the electrolytic solution is less likely to penetrate into the negative electrode active material layer, which is not preferable because battery performance decreases. On the other hand, if the porosity of the negative electrode active material layer is too high, the amount of active material contained per volume of the negative electrode decreases, so the capacity per volume of the negative electrode (and thus a lithium secondary battery constructed using the negative electrode) May tend to decline. Therefore, the porosity of the negative electrode active material layer is appropriately 35% or more, preferably 35% to 53%, and particularly preferably 35% to 48%.

In a preferable aspect of the lithium secondary battery disclosed herein, the negative electrode active material layer is formed by applying at least two types of negative electrode active material layer forming pastes having different binder concentrations and drying the layer. . In this case, the binder concentration (ratio of the binder in the solid content in the negative electrode active material layer forming paste) of at least two types of negative electrode active material layer forming paste applied in layers on the negative electrode current collector is appropriately selected. Thus, a negative electrode active material layer in which the binder uneven distribution degree X satisfies 0.4 ≦ X <1.0 can be easily formed.

Any of the lithium secondary batteries disclosed herein has performance suitable for a battery mounted on a vehicle (for example, high output can be obtained), and can be particularly excellent in durability against high-rate charge / discharge. . Therefore, according to this invention, the vehicle provided with one of the lithium secondary batteries disclosed here is provided. In particular, a vehicle (for example, an automobile) including the lithium secondary battery as a power source (typically, a power source of a hybrid vehicle or an electric vehicle) is provided.

As a preferable application object of the technology disclosed herein, lithium that can be used in a charge / discharge cycle including a high-rate charge / discharge of 50 A or more (for example, 50 A to 250 A), or even 100 A or more (for example, 100 A to 200 A). Secondary battery; a large capacity type having a theoretical capacity of 1 Ah or more (more than 3 Ah), and a charge / discharge cycle including high rate charge / discharge of 10 C or more (for example, 10 C to 50 C), or even 20 C or more (for example, 20 C to 40 C). Examples are lithium secondary batteries that are supposed to be used.

FIG. 1 is a perspective view schematically showing a lithium secondary battery according to an embodiment of the present invention. 2 is a cross-sectional view taken along line II-II in FIG. FIG. 3 is a diagram schematically showing an electrode body of a lithium secondary battery according to an embodiment of the present invention. FIG. 4 is a plan view schematically showing an electrode body of a lithium secondary battery according to an embodiment of the present invention. FIG. 5 is an enlarged cross-sectional view showing a main part of the lithium secondary battery according to one embodiment of the present invention. FIG. 6 is a diagram schematically showing a lithium secondary battery (laminate cell) according to Test Example 3. FIG. 7 is a graph showing the relationship between the degree of binder uneven distribution and resistance according to Test Example 3. FIG. 8 is a graph showing a cycle durability test result according to Test Example 4. FIG. 9 is a graph showing the relationship between the binder uneven distribution degree and resistance according to Test Example 5. FIG. 10 is a side view schematically showing a vehicle including a lithium secondary battery according to an embodiment of the present invention.

Embodiments according to the present invention will be described below with reference to the drawings. In the following drawings, members / parts having the same action are described with the same reference numerals. Note that the dimensional relationship (length, width, thickness, etc.) in each drawing does not reflect the actual dimensional relationship. Further, matters other than the matters specifically mentioned in the present specification and matters necessary for carrying out the present invention (for example, the configuration and manufacturing method of an electrode body including a positive electrode and a negative electrode, the configuration and manufacturing method of a separator and an electrolyte, General techniques relating to the construction of lithium secondary batteries and other batteries, etc.) can be understood as design matters for those skilled in the art based on the prior art in this field.

Although it is not intended to be particularly limited, in the following, a flatly wound electrode body (wound electrode body) and a nonaqueous electrolyte solution are accommodated in a flat box-shaped (cuboid shape) container. The present invention will be described in detail by taking a lithium secondary battery (lithium ion battery) as an example.

1 to 4 show a schematic configuration of a lithium ion battery according to an embodiment of the present invention. The lithium ion battery 100 includes an electrode body (winding electrode body) 80 in which a long positive electrode sheet 10 and a long negative electrode sheet 20 are wound flatly via a long separator 40. In addition to the non-aqueous electrolyte solution (not shown), the wound electrode body 80 is accommodated in a container 50 having a shape (flat box shape) that can be accommodated.

The container 50 includes a flat rectangular parallelepiped container main body 52 having an open upper end, and a lid 54 that closes the opening. As a material constituting the container 50, a metal material such as aluminum or steel is preferably used (in this embodiment, aluminum). Or the container 50 formed by shape | molding resin materials, such as PPS and a polyimide resin, may be sufficient. On the upper surface of the container 50 (that is, the lid 54), a positive electrode terminal 70 that is electrically connected to the positive electrode of the wound electrode body 80 and a negative electrode terminal 72 that is electrically connected to the negative electrode 20 of the electrode body 80 are provided. Yes. Inside the container 50, a flat wound electrode body 80 is accommodated together with a non-aqueous electrolyte (not shown).

The wound electrode body 80 according to the present embodiment is the same as the wound electrode body of a normal lithium ion battery except for the configuration of a layer containing a negative electrode active material (negative electrode active material layer) provided in the negative electrode sheet 20 described later. Similarly, as shown in FIG. 3, a long (strip-shaped) sheet structure is provided before the wound electrode body 80 is assembled.

The negative electrode sheet 20 has a structure in which a negative electrode active material layer 24 containing a negative electrode active material is held on both surfaces of a long sheet-like foil-shaped negative electrode current collector (hereinafter referred to as “negative electrode current collector foil”) 22. ing. However, the negative electrode active material layer 24 is not attached to one side edge (the upper side edge portion in the figure) along the edge in the width direction of the negative electrode sheet 20, and the negative electrode current collector 22 is exposed with a certain width. A negative electrode active material layer non-formed portion is formed.

Similarly to the negative electrode sheet 20, the positive electrode sheet 10 holds a positive electrode active material layer 14 containing a positive electrode active material on both surfaces of a long sheet-like foil-shaped positive electrode current collector (hereinafter referred to as “positive electrode current collector foil”) 12. Has a structured. However, the positive electrode active material layer 14 is not attached to one side edge (the lower side edge portion in the figure) along the edge in the width direction of the positive electrode sheet 10, and the positive electrode current collector 12 has a constant width. An exposed positive electrode active material layer non-forming portion is formed.

In producing the wound electrode body 80, the positive electrode sheet 10 and the negative electrode sheet 20 are laminated via the separator sheet 40. At this time, the positive electrode sheet 10 and the negative electrode sheet 20 are formed such that the positive electrode active material layer non-formed portion of the positive electrode sheet 10 and the negative electrode active material layer non-formed portion of the negative electrode sheet 20 protrude from both sides in the width direction of the separator sheet 40. Are overlapped slightly in the width direction. The laminated body thus stacked is wound, and then the obtained wound body is crushed from the side surface direction and ablated, whereby a flat wound electrode body 80 can be produced.

A wound core portion 82 (that is, the positive electrode active material layer 14 of the positive electrode sheet 10, the negative electrode active material layer 24 of the negative electrode sheet 20, and the separator sheet 40) is densely arranged in the central portion of the wound electrode body 80 in the winding axis direction. Laminated portions) are formed. In addition, the electrode active material layer non-formed portions of the positive electrode sheet 10 and the negative electrode sheet 20 protrude outward from the wound core portion 82 at both ends in the winding axis direction of the wound electrode body 80. A positive electrode lead terminal 74 and a negative electrode lead terminal 76 are respectively provided on the protruding portion 84 (that is, a portion where the positive electrode active material layer 14 is not formed) 84 and the protruding portion 86 (that is, a portion where the negative electrode active material layer 24 is not formed) 86. Attached and electrically connected to the positive terminal 70 and the negative terminal 72 described above.

The constituent elements of the wound electrode body 80 may be the same as those of the conventional wound electrode body of the lithium ion battery except for the negative electrode sheet 20, and are not particularly limited. For example, the positive electrode sheet 10 can be formed by applying a positive electrode active material layer 14 mainly composed of a positive electrode active material for a lithium ion battery on a long positive electrode current collector 12. For the positive electrode current collector 12, an aluminum foil or other metal foil suitable for the positive electrode is preferably used.

As the positive electrode active material, one or more of materials conventionally used in lithium ion batteries can be used without any particular limitation. Preferable examples include oxides containing lithium and a transition metal element as constituent metal elements such as lithium nickel oxide (LiMn ₂ O ₄ ), lithium cobalt oxide (LiCoO ₂ ), and lithium manganese oxide (LiNiO ₂ ). And a positive electrode active material mainly composed of a lithium transition metal oxide). Among them, a positive electrode active material (typically, substantially a lithium nickel cobalt manganese composite oxide substantially composed of lithium nickel cobalt manganese composite oxide (for example, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ). Application to a positive electrode active material comprising:

Here, the lithium nickel cobalt manganese composite oxide is an oxide having Li, Ni, Co, and Mn as constituent metal elements, and at least one other metal element in addition to Li, Ni, Co, and Mn (that is, It also includes oxides containing transition metal elements and / or typical metal elements other than Li, Ni, Co, and Mn. Such a metal element is selected from the group consisting of, for example, B, V, Mg, Sr, Zr, Mo, W, Ti, Al, Cr, Fe, Nb, Cu, Zn, Ga, In, Sn, La, and Ce. Or one or more elements. The same applies to lithium nickel oxide, lithium cobalt oxide, and lithium manganese oxide.

The negative electrode sheet 20 can be formed by applying a negative electrode active material layer 24 mainly composed of a negative electrode active material for a lithium ion battery on a long negative electrode current collector 22. For the negative electrode current collector 22, a copper foil or other metal foil suitable for the negative electrode is preferably used.

As the negative electrode active material, one or more of materials conventionally used in lithium ion batteries can be used without any particular limitation. Preferable examples include carbon-based materials such as graphite carbon and amorphous carbon, lithium-containing transition metal oxides and transition metal nitrides. Among these, application to a negative electrode active material (typically, a negative electrode active material substantially composed of a carbon-based material) mainly composed of a carbon-based material such as graphite carbon or amorphous carbon is preferable.

As such a carbon-based material (typically in particulate form), for example, a carbon-based material powder prepared by a conventionally known method can be used as it is. For example, a carbon-based material powder substantially composed of secondary particles having an average particle diameter in the range of about 1 μm to 25 μm (for example, about 10 μm) can be preferably used as the negative electrode active material.

The negative electrode active material layer 24 contains a binder (binder) that binds the negative electrode active material (typically in particulate form). The binder used for the negative electrode active material layer is for bonding the negative electrode active material particles, and the material constituting the binder itself is the same material as that used for a conventionally known negative electrode for a lithium secondary battery. possible.

For example, when the negative electrode active material layer forming paste described later is an aqueous solvent (a solution using water or a mixed solvent containing water as a main component as a binder dispersion medium), the binder is dispersed or dissolved in water. The polymer to be used can be preferably employed. Examples of the polymer that is dispersed or dissolved in water include styrene butadiene rubber (SBR). Styrene butadiene rubber is a copolymer containing styrene and 1,3-butadiene, and the copolymerization mode and styrene / butadiene copolymer ratio are not particularly limited. Further, it may be a modified SBR obtained by copolymerizing an unsaturated carboxylic acid or an unsaturated nitrile compound. The polymer that is dispersed or dissolved in water is not limited to styrene butadiene rubber, and polytetrafluoroethylene (PTFE), polyethylene (PE), and polyacrylic acid (PAA) can also be used.

When the negative electrode active material layer forming paste is a solvent-based solvent (a solution in which the binder dispersion medium is mainly an organic solvent), a polymer that is dispersed or dissolved in the solvent-based solvent can be used. Examples of the polymer dispersed or dissolved in the solvent-based solvent include polyvinylidene fluoride, polytetrafluoroethylene (PTFE), polyacrylonitrile, polymethyl methacrylate, and the like.

Moreover, the negative electrode active material layer 24 can contain one or two or more materials that can be used as components of the negative electrode active material layer in a general lithium ion battery, if necessary. Examples of such materials include various polymer materials (for example, carboxymethyl cellulose (CMC)) that can function as a thickener for the negative electrode active material layer forming paste.

As a method of forming the negative electrode active material layer 24, a negative electrode active material (typically granular), a binder, and other negative electrode active material layer forming components (for example, a thickener) are used in an appropriate solvent (preferably an aqueous solvent). A method in which the dispersed negative electrode active material layer forming paste is applied in a strip shape on one or both sides of the negative electrode current collector 22 and dried can be preferably employed. After drying the negative electrode active material layer forming paste, an appropriate press treatment (for example, various conventionally known press methods such as a roll press method and a flat plate press method can be adopted) is performed, whereby the negative electrode active material layer The thickness and density of 24 can be adjusted.

Examples of the separator sheet 40 suitable for use between the positive and

negative electrode sheets

10 and 20 include those made of a porous polyolefin resin. For example, a porous separator sheet made of synthetic resin (for example, made of polyolefin such as polyethylene) can be suitably used. When a solid electrolyte or a gel electrolyte is used as the electrolyte, there may be a case where a separator is unnecessary (that is, in this case, the electrolyte itself can function as a separator).

Subsequently, the negative electrode sheet 20 according to the present embodiment will be described in detail with reference to FIG. FIG. 5 is a schematic cross-sectional view showing an enlarged part of a cross section along the winding axis of the wound electrode body 80 according to the present embodiment, which is formed on the negative electrode current collector 22 and one side thereof. The negative electrode active material layer 24 and the separator sheet 40 facing the negative electrode active material layer 24 are shown.

As shown in FIG. 5, the negative electrode sheet 20 according to this embodiment has a structure in which a negative electrode active material layer 24 is held on a negative electrode current collector 22. The negative electrode active material layer 24 includes a negative electrode active material (typically granular) 26 and a binder 28, and the binder 28 provides a space between the negative electrode active material 26 and between the negative electrode active material 26 and the negative electrode current collector 22. Are combined. In addition, a hole 25 is formed in a portion that is not bound by the binder 28 between the adjacent negative electrode active materials 26, and a nonaqueous electrolytic solution is held in the hole 25.

The porosity of the negative electrode active material layer is not particularly limited. However, if the porosity of the negative electrode active material layer is too low, the electrolyte solution is less likely to penetrate into the negative electrode active material layer, which is not preferable. . On the other hand, if the porosity of the negative electrode active material layer is too high, the amount of active material contained per volume of the negative electrode decreases, so the capacity per volume of the negative electrode (and thus a lithium secondary battery constructed using the negative electrode) May tend to decline. Therefore, the porosity of the negative electrode active material layer is appropriately 35% or more, preferably 35% to 53%, and particularly preferably 35% to 48%.

Here, in this embodiment, the thickness of the negative electrode active material layer is 50 μm or less, and the binder content in the negative electrode active material layer is 1% by mass or less. When the negative electrode active material layer 24 is divided in half in the thickness direction 90 and the side closer to the negative electrode current collector 22 is the lower layer 24a and the side far from the negative electrode current collector 22 is the upper layer 24b, the binder concentration ( % By mass) is different between the upper layer 24b and the lower layer 24a, and the binder uneven distribution degree X = B / A, which is a value obtained by dividing the binder concentration B on the upper layer side 24b by the binder concentration A on the lower layer side 24a, is 0.00. 4 ≦ X <1.0, preferably 0.4 ≦ X ≦ 0.8, and particularly preferably 0.4 ≦ X ≦ 0.6.

When the binder uneven distribution degree X exceeds 1.0, a large amount of the binder 28 is disposed in the surface layer portion of the negative electrode active material layer 24. Therefore, the reactivity of the negative electrode active material in the surface layer portion (lithium ion insertion / desorption reaction) Activity). For this reason, the reaction resistance of the negative electrode increases, and lithium ions released from the positive electrode active material may not immediately enter the negative electrode active material 26 but may precipitate in the negative electrode active material layer. On the other hand, when the binder uneven distribution degree X is less than 0.4, a large amount of the binder 28 is disposed in the vicinity of the negative electrode current collector 22, so that the interface resistance between the negative electrode current collector 22 and the negative electrode active material layer 24 increases. To do. As a result, the direct current resistance of the negative electrode increases, and the internal resistance of the battery increases accordingly. Therefore, an increase in overvoltage accompanying an increase in internal resistance makes it easier to reach the lithium deposition potential. Such lithium deposition can cause performance deterioration (battery capacity reduction, etc.).

In the preferred technique disclosed herein, the thickness of the negative electrode active material layer is 50 μm or less. If the negative electrode active material layer is too thick, the effect (effect of suppressing lithium deposition) obtained by setting the binder uneven distribution degree X of the negative electrode active material layer to 0.4 ≦ X <1.0 may not be obtained. On the other hand, if the negative electrode active material layer is too thin, the amount of active material contained per volume of the negative electrode is reduced, so that the capacity per volume of the negative electrode (and thus a lithium secondary battery constructed using the negative electrode) tends to decrease. May be. Accordingly, the thickness of the negative electrode active material layer is suitably 50 μm or less, preferably 10 μm to 50 μm, and particularly preferably 20 μm to 50 μm.

In the preferred technique disclosed herein, the binder content in the negative electrode active material layer is 1% by mass or less. If the binder content is too high, the effect (effect of suppressing lithium deposition) obtained by setting the binder uneven distribution degree X of the negative electrode active material layer to 0.4 ≦ X <1.0 may not be obtained. On the other hand, if the binder content is too small, the adhesion between the negative electrode current collector and the negative electrode active material layer may decrease, and peeling of the negative electrode active material layer may occur. Accordingly, the binder content in the negative electrode active material layer is suitably 1% by mass or less, preferably 0.3% by mass to 1% by mass, and particularly preferably 0.5% by mass to 1% by mass. .

Although not particularly limited, the ratio of the negative electrode active material to the whole negative electrode active material layer is preferably about 90% by mass or more (typically 97% by mass to 99% by mass), and about 98% by mass. % To 99% by mass is preferable. Moreover, when it contains negative electrode active material layer formation components (for example, thickener etc.) other than a negative electrode active material and a binder, it is preferable that the total content rate of these arbitrary components shall be about 3 mass% or less, and about 2 masses. % Or less (for example, approximately 0.5% by mass to 1% by mass).

The negative electrode active material layer satisfying the binder uneven distribution degree X of 0.4 ≦ X <1.0 can be realized by appropriately selecting the formation conditions for forming the negative electrode active material layer. For example, the negative electrode active material layer can be formed by applying and drying a negative electrode active material layer forming paste prepared by mixing a negative electrode active material and a binder in a suitable solvent on a negative electrode current collector. . In this case, the binder uneven distribution degree X of the negative electrode active material layer can be controlled by adjusting drying conditions such as a drying temperature and a drying air speed when the applied negative electrode active material layer forming paste is dried. That is, a negative electrode active material layer having a binder uneven distribution degree X satisfying 0.4 ≦ X <1.0 can be formed by appropriately selecting drying conditions such as a drying temperature and a drying air speed.

In addition, as another method for realizing a negative electrode active material layer in which the binder uneven distribution degree X satisfies 0.4 ≦ X <1.0, a method using a plurality of negative electrode active material layer forming pastes having different binder concentrations can be given. It is done. For example, the negative electrode active material layer is formed by applying and drying two types of negative electrode active material layer forming pastes having different binder concentrations on the upper and lower layers (layered). At that time, by adjusting the binder concentration (ratio of the binder in the solid content in the negative electrode active material layer forming paste) of the upper and lower negative electrode active material layer forming paste, the binder uneven distribution degree X of the negative electrode active material layer Can be controlled. That is, a negative electrode active material layer satisfying a binder uneven distribution degree X of 0.4 ≦ X <1.0 can be formed by appropriately selecting the binder concentration of the upper and lower negative electrode active material layer forming pastes. . Preferably, a high binder concentration paste having a relatively high binder concentration is applied on the negative electrode current collector, and a low binder concentration paste having a relatively low binder concentration is applied thereon to form the negative electrode active material layer. . Thereby, the negative electrode active material layer with the binder uneven distribution degree X satisfying 0.4 ≦ X <1.0 can be easily formed.

Examples of the solvent used in the negative electrode active material layer forming paste include water or a mixed solvent mainly composed of water. As a solvent other than water constituting such a mixed solvent, one or more organic solvents (lower alcohol, lower ketone, etc.) that can be uniformly mixed with water can be appropriately selected and used. Alternatively, it may be an organic solvent such as N-methylpyrrolidone (NMP), pyrrolidone, methyl ethyl ketone, methyl isobutyl ketone, ixahexanone, toluene, dimethylformamide, dimethylacetamide, or a combination of two or more thereof.

The negative electrode active material layer forming paste may contain one or more materials that can be used as necessary in addition to the negative electrode active material and the binder. An example of such a material is a polymer that functions as a thickener for the negative electrode active material layer forming paste. As the polymer that functions as a thickener, for example, carboxymethyl cellulose (CMC) is preferably used.

The operation of applying such a negative electrode active material layer forming paste to the surface of the negative electrode current collector 22 can be performed in the same manner as in the production of a conventional negative electrode for a lithium secondary battery. For example, by coating the negative electrode current collector 22 with a predetermined amount of the negative electrode active material layer forming paste to a uniform thickness using an appropriate coating device (slit coater, die coater, comma coater, etc.) Done. Thereafter, the coating material is dried (for example, a drying temperature of 20 to 200 ° C.) by an appropriate drying means (for example, a hot air dryer) to remove the solvent in the negative electrode active material layer forming paste. By removing the solvent from the negative electrode active material layer forming paste, the negative electrode active material layer 24 having the negative electrode active material and the binder is formed.

Thus, the negative electrode sheet 20 in which the negative electrode active material layer 24 is formed on the negative electrode current collector 22 can be obtained. In addition, after drying, the thickness and density of the negative electrode active material layer 24 can be appropriately adjusted by performing an appropriate press process (for example, a roll press process) as necessary.

When the negative electrode sheet 20 is formed in this manner, the negative electrode sheet 20 and the positive electrode sheet 10 are wound through two separator sheets 40 as shown in FIG. Then, as shown in FIGS. 1 and 2, the wound electrode body 80 is accommodated in the container body 52, and an appropriate nonaqueous electrolytic solution is disposed (injected) into the container body 52. As the non-aqueous electrolyte accommodated in the container main body 52 together with the wound electrode body 80, the same non-aqueous electrolyte as used in conventional lithium ion batteries can be used without any particular limitation. Such a nonaqueous electrolytic solution typically has a composition in which a supporting salt is contained in a suitable nonaqueous solvent. As said non-aqueous solvent, ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), propylene carbonate (PC) etc. can be used, for example. Further, as the supporting salt, for _{_{_{example, LiPF 6, LiBF 4, LiAsF}}} 6, LiCF 3 can be preferably used a lithium salt of SO ₃ and the like. For example, a nonaqueous electrolytic solution in which LiPF ₆ as a supporting salt is contained at a concentration of about 1 mol / liter in a mixed solvent containing EC, EMC, and DMC at a volume ratio of 3: 4: 3 can be preferably used.

The non-aqueous electrolyte is accommodated in the container body 52 together with the wound electrode body 80, and the opening of the container body 52 is sealed by welding or the like with the lid body 54, whereby the lithium secondary battery 100 according to the present embodiment. Construction (assembly) of is completed. In addition, the sealing process of the container main body 52 and the arrangement | positioning (injection) process of electrolyte solution can be performed similarly to the method currently performed by manufacture of the conventional lithium ion battery. Thereafter, the battery is conditioned (initial charge / discharge). You may perform processes, such as degassing and a quality inspection, as needed.

In the lithium secondary battery thus constructed, the binder uneven distribution degree X (binder concentration B on the upper layer side 24b / binder concentration A on the lower layer side 24a) when the negative electrode active material layer is divided in half in the thickness direction is 0. Since 4 ≦ X <1.0, the battery performance may be excellent. For example, it may satisfy at least one (preferably all) of high durability against high rate charge / discharge, high output, and excellent low temperature characteristics.

Hereinafter, the present invention will be described in more detail based on test examples.

<Test Example 1: Preparation of negative electrode active material layer forming paste>
Natural graphite (negative electrode active material) having an average particle diameter of 11 μm, SBR (binder), and CMC (thickener) have a mass ratio of these materials of 98: 1: 1 and a solid content concentration of about 46% by mass. The negative electrode active material layer forming paste was prepared by mixing with water so that the ratio of the binder (binder concentration) in the solid content in the negative electrode active material layer forming paste was 1% by mass. Moreover, several types of pastes for forming a negative electrode active material layer having different binder concentrations were prepared using the blending ratio as a standard.

<Test Example 2: Production of negative electrode sheet>
Two types were selected from the various negative electrode active material layer forming pastes prepared in Test Example 1 above, and layered (upper and lower two layers) on one side of a long sheet-like copper foil (negative electrode current collector 22: thickness 10 μm). 10 types of negative electrode active material layers having different binder uneven distribution degrees X were formed. The amount of paste applied to the upper and lower layers was adjusted to be about 7.6 mg / cm ² (solid content basis). The drying temperature of the applied paste was set to 25 ° C. After drying, the negative electrode active material layer was pressed so as to have a thickness of about 50 μm. After pressing, the porosity based on the gas replacement method of the negative electrode active material layer was about 35%, and the density was 1.5 g / cm ³ . In this way, a total of 10 types of negative electrode sheets were prepared in which the negative electrode active material layer was provided on the negative electrode current collector.

The cross sections of the various negative electrode sheets obtained above were analyzed with an electron beam microanalyzer (EPMA), and the binder uneven distribution degree X (binder concentration on the upper layer side 24b / lower layer) when the cross section of the negative electrode active material layer was divided in half in the thickness direction. The binder concentration on the side 24a) was examined. The ratio of the binder concentration between the upper layer side and the lower layer side was calculated from the Br element detection intensity ratio when the binder (SBR) was dyed with Br element. Table 1 shows the binder uneven distribution degree X of each sample.

<Test Example 3: Production and evaluation of lithium secondary battery>
Lithium secondary batteries (laminate cells) were produced using the various negative electrode sheets produced in Test Example 2, and the direct current resistance and reaction resistance of these batteries were measured. The lithium secondary battery was produced as follows.

These materials include nickel cobalt lithium manganate (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ) powder as a positive electrode active material, polyvinylidene fluoride (PVDF) as a binder, and acetylene black as a conductive material. Is mixed with N-methylpyrrolidone (NMP) so that the mass ratio thereof becomes 90: 5: 5 to prepare a positive electrode active material layer forming paste, which is formed into a long sheet-like aluminum foil (positive electrode current collector 12). : Positive electrode sheet 10 provided with positive electrode active material layer 14 on both surfaces of positive electrode current collector 12 by applying a belt-like shape on both surfaces of the positive electrode current collector 12 and drying. The coating amount of the positive electrode active material layer forming paste was adjusted so as to be about 12 mg / cm ² (solid content basis) per side.

The positive electrode sheet obtained above was punched out to 5 cm × 5 cm to produce a positive electrode. Moreover, the various negative electrode sheets produced in Test Example 2 were punched into 5 cm × 5 cm to produce negative electrodes. An aluminum lead is attached to the positive electrode, a nickel lead is attached to the negative electrode, and they are arranged opposite to each other via a separator (a three-layer structure of polypropylene (PP) -polyethylene (PE) -polypropylene (PP)). The lithium secondary battery (laminate cell) shown in FIG. 6 was constructed by inserting it into a laminate bag together with the non-aqueous electrolyte. In FIG. 6, reference numeral 61 indicates a positive electrode, reference numeral 62 indicates a negative electrode, reference numeral 63 indicates a separator impregnated with an electrolytic solution, and reference numeral 64 indicates a laminate bag. In addition, as a non-aqueous electrolyte solution, LiPF ₆ as a supporting salt is approximately mixed in a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 3: 3: 4. The one contained at a concentration of 1 mol / liter was used.

For each of the lithium secondary batteries thus obtained, AC impedance was measured in an environment of 25 ° C., and DC resistance and reaction resistance were read from the Cole-Cole plot of the obtained impedance. The AC impedance measurement conditions were an AC applied voltage of 5 mV and a frequency range of 0.1 Hz to 1000000 Hz. The results are shown in FIG. In FIG. 7, the plot indicated by Δ is the reaction resistance (mΩ).

As is clear from FIG. 7 and Table 1, the lithium secondary battery constructed using the negative electrode sheet in which the binder uneven distribution degree X of the negative electrode active material layer exceeds 1.2 has a low direct current resistance, but has a low reaction resistance. The value increased significantly. On the other hand, a lithium secondary battery constructed using a negative electrode sheet having a binder uneven distribution degree X of less than 0.4 had a low direct-current resistance value although the reaction resistance was low. In contrast, a lithium secondary battery constructed using a negative electrode sheet having a binder uneven distribution degree X satisfying 0.4 ≦ X <1.0 has a low reaction resistance and direct current resistance, and is used for a vehicle power source. The lithium secondary battery used for other purposes showed particularly good performance.

<Test Example 4: Charge / Discharge Cycle Test>
Furthermore, the charge / discharge pattern which repeats a high rate pulse charge / discharge was provided with respect to the lithium secondary battery of the sample 4 and the sample 10, and the charge / discharge cycle test was done. Specifically, in an environment of 0 ° C., high-rate pulse charging is performed at 21.2 mA / cm ² (corresponding to 18C) for 10 seconds, and high-rate pulse discharging is performed at 21.2 mA / cm ² for 10 seconds. The charging / discharging cycle of resting for 10 minutes was repeated 250 times continuously. And, from the initial capacity before the charge / discharge cycle test and the discharge capacity after the charge / discharge cycle test, the capacity retention rate after the charge / discharge cycle test (= [discharge capacity after the charge / discharge cycle test / before the charge / discharge cycle test) Initial capacity] × 100).

In this example, the charge / discharge cycle test was performed in five ways of 21.2 mA / cm ² , 22.4 mA / cm ² , 23.6 mA / cm ² , 24.8 mA / cm ² , and 26.0 mA / cm ² . The capacity retention rate after each charge / discharge cycle test was determined by performing different pulse currents. The results are shown in FIG. FIG. 8 is a graph showing the relationship between the pulse current (mA / cm ² ) and the capacity retention rate (%).

As shown in the drawing, when the pulse current exceeds 23.6 mA / cm ² , the lithium secondary battery constructed using the negative electrode sheet having the binder uneven distribution degree X exceeding 1.0 has a capacity after the charge / discharge cycle test. The maintenance rate dropped sharply. On the other hand, the lithium secondary battery constructed using the negative electrode sheet satisfying the binder uneven distribution degree X satisfying 0.4 ≦ X <1.0 is 90% or more even at a high rate of 26.0 mA / cm ^2. An extremely high capacity retention rate was achieved. This is considered to be because the precipitation of lithium in the negative electrode active material layer could be suppressed by setting the binder uneven distribution degree X to 0.4 ≦ X <1.0. From this result, it was confirmed that by setting the binder uneven distribution degree X to 0.4 ≦ X <1.0, excellent cycle life characteristics can be realized while performing high-rate charge / discharge.

<Test Example 5>
In this example, the thickness of the negative electrode active material layer is 20 μm (density 1.5 g / cm ⁻³ : porosity 35%) and 150 μm (density 1.5 g / cm ⁻³ : porosity 35%), respectively. In other respects, a total of 15 types of negative electrode sheets were produced in the same manner as in Test Example 2. Then, a lithium secondary battery was constructed in the same manner as in Test Example 3 except that the thickness of the negative electrode active material layer was changed, and the reaction resistance and DC resistance of the battery were measured. The results are shown in FIG. 9 and Table 2. In FIG. 9, the plot indicated by Δ is the reaction resistance (mΩ) when the thickness of the negative electrode active material layer is 50 μm, and the plot indicated by ○ is the reaction resistance when the thickness of the negative electrode active material layer is 20 μm. (MΩ), and the plot indicated by x is the value of reaction resistance (mΩ) when the thickness of the negative electrode active material layer is 150 μm.

As is clear from FIG. 9 and Table 2, the lithium secondary battery constructed using the negative electrode sheet having a negative electrode active material layer thickness of 20 μm has a binder uneven distribution degree X of 0.4 ≦ X <1.0. Sometimes the values of reaction resistance and DC resistance were both low, and the same tendency as in the case where the thickness of the negative electrode active material layer was 50 μm was shown. On the other hand, a lithium secondary battery constructed using a negative electrode sheet having a negative electrode active material layer thickness of 150 μm has values of reaction resistance and DC resistance when the binder uneven distribution degree X is 0.8 ≦ X <1.0. As compared with the case where the thickness of the negative electrode active material layer was 50 μm, the preferred range of the binder uneven distribution degree X became narrower. From this result, the effect (the effect of suppressing lithium precipitation) by setting the binder uneven distribution degree X of the negative electrode active material layer to 0.4 ≦ X <1.0 is that the thickness of the negative electrode active material layer is 50 μm or less (for example, It was confirmed that the difference was more remarkable in the case of 20 μm to 50 μm).

Any of the lithium secondary batteries 100 disclosed herein has performance suitable as a battery mounted on a vehicle, and can be particularly excellent in durability against high-rate charge / discharge. Therefore, according to the present invention, as shown in FIG. 10, a vehicle 1 including any of the lithium secondary batteries 100 disclosed herein is provided. In particular, a vehicle (for example, an automobile) including the lithium secondary battery 100 as a power source (typically, a power source of a hybrid vehicle or an electric vehicle) is provided.

Further, as a preferable application target of the technology disclosed herein, it is assumed that the technology can be used in a charge / discharge cycle including a high rate discharge of 50 A or more (for example, 50 A to 250 A), and further 100 A or more (for example, 100 A to 200 A). Lithium secondary battery; large capacity type with a theoretical capacity of 1 Ah or more (more than 3 Ah), and a charge / discharge cycle including high rate charge / discharge of 10 C or more (for example 10 C to 50 C) or 20 C or more (for example 20 C to 40 C) A lithium secondary battery assumed to be used in

According to the present invention, it is possible to provide a lithium secondary battery with improved durability against charge / discharge cycles.

Claims

A lithium secondary battery including a negative electrode in which a negative electrode active material layer having a negative electrode active material and a binder is formed on a negative electrode current collector,
The negative electrode active material layer has a thickness of 50 μm or less,
The binder content in the negative electrode active material layer is 1% by mass or less,
Here, in the case where the negative electrode active material layer is divided in half in the thickness direction, the side closer to the negative electrode current collector is the lower layer and the side far from the negative electrode current collector is the upper layer, the binder concentration (mass% ) Is different between the upper and lower layers,
Lithium secondary characterized in that a binder uneven distribution degree X = B / A, which is a value obtained by dividing the binder concentration B on the upper layer side by the binder concentration A on the lower layer side, is 0.4 ≦ X <1.0. battery.
The lithium secondary battery according to claim 1, wherein the binder uneven distribution degree X is 0.4 ≦ X ≦ 0.8.
The lithium secondary battery according to claim 1 or 2, wherein the binder is styrene butadiene rubber.
The lithium secondary battery according to any one of claims 1 to 3, wherein a porosity of the negative electrode active material layer is 35% or more.
5. The lithium secondary material according to claim 1, wherein the negative electrode active material layer is formed by applying and drying at least two types of negative electrode active material layer forming pastes having different binder concentrations. battery.
A vehicle equipped with the lithium secondary battery according to any one of claims 1 to 5.