JP7334207B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

The present invention relates to non-aqueous electrolyte secondary batteries.

A non-aqueous electrolyte secondary battery includes a positive electrode active material layer and a negative electrode active material layer on a sheet-shaped core. During charging of the non-aqueous electrolyte secondary battery, charge carriers are occluded in the negative electrode active material layer. During discharge, charge carriers are released from the negative electrode active material layer. The non-aqueous electrolyte secondary battery is configured such that the negative electrode active material layer is wider than the positive electrode active material layer. That is, the negative electrode active material layer has a region facing the positive electrode active material layer (hereinafter also referred to as a facing region) and a region not facing the positive electrode active material layer (hereinafter also referred to as a non-facing region). By charging and discharging of the non-aqueous electrolyte secondary battery, the charge carriers can diffuse and be fixed in the non-opposed region as well. Such immobilization of charge carriers leads to capacity deterioration of the non-aqueous electrolyte secondary battery.

International Publication No. 2014/038092 discloses that a region of a negative electrode active material layer that does not face a positive electrode active material layer contains a negative electrode active material, a heat-fusible binder, and a temperature-sensitive thickener. A lithium secondary battery is disclosed. Both the hot-melt binder and the temperature-sensitive thickener have a gelation temperature within the range of 45°C to 100°C. Such a lithium secondary battery is said to contain a temperature-sensitive thickener to suppress capacity deterioration during high-temperature storage. In addition, it is said that such a lithium secondary battery can improve or maintain battery performance such as cycle characteristics by containing a heat-melting binder with excellent adhesion.

WO2014/038092

Capacity deterioration due to charge carriers diffusing into the non-facing region of the negative electrode active material layer is likely to occur when the non-aqueous electrolyte secondary battery is stored at high temperature and high SOC (State of Charge). However, according to the studies of the present inventors, such capacity deterioration may occur even when the non-aqueous electrolyte secondary battery is stored at a temperature around room temperature.
An object of the present invention is to provide a non-aqueous electrolyte secondary battery in which capacity deterioration is suppressed.

The nonaqueous electrolyte secondary battery disclosed herein includes a positive electrode having a positive electrode active material layer on a positive electrode core and a negative electrode having a negative electrode active material layer on a negative electrode core. The negative electrode active material layer has a facing region facing the positive electrode active material layer and a non-facing region not facing the positive electrode active material layer. When the specific surface area of the opposing region determined by the BET method is S1 and the specific surface area of the non-facing region determined by the BET method is S2, the relationship S2<S1 is satisfied.
According to such a configuration, a non-aqueous electrolyte secondary battery in which capacity deterioration is suppressed is provided.

In a preferred embodiment of the non-aqueous electrolyte secondary battery, the specific surface area S1 and the specific surface area S2 satisfy the relationship S2/S1≦0.7. According to such a configuration, a non-aqueous electrolyte secondary battery in which capacity deterioration is further suppressed is provided.
In a preferred embodiment of the non-aqueous electrolyte secondary battery, (A1+A2)/A1 is 1.025 or more and 1.2 or less, where A1 is the area of the facing region and A2 is the area of the non-facing region. . According to such a configuration, a non-aqueous electrolyte secondary battery in which capacity deterioration is further suppressed is provided.
As a preferred embodiment of the technology disclosed here, the negative electrode active material layer may contain an alloy-based negative electrode active material. Such a non-aqueous electrolyte secondary battery also exhibits the effect of suppressing capacity deterioration.

1 is a perspective view schematically showing a non-aqueous electrolyte secondary battery according to one embodiment; FIG. FIG. 2 is a schematic longitudinal sectional view taken along line II-II in FIG. 1; FIG. 4 is a perspective view schematically showing an electrode assembly attached to a sealing plate; FIG. 3 is a perspective view schematically showing an electrode assembly to which a positive electrode second current collector and a negative electrode second current collector are attached; 1 is a schematic diagram showing the configuration of a wound electrode body of a non-aqueous electrolyte secondary battery according to one embodiment; FIG. 1 is a cross-sectional view schematically showing a cross-section of a wound electrode body of a non-aqueous electrolyte secondary battery according to one embodiment; FIG. 1 is a plan view schematically showing a negative electrode of a non-aqueous electrolyte secondary battery according to one embodiment; FIG. FIG. 4 is a schematic diagram schematically showing a slot die used when applying the negative electrode mixture slurry.

Hereinafter, embodiments of the technology disclosed herein will be described with reference to the drawings. Matters other than those specifically mentioned in this specification, which are necessary for implementing the technology disclosed here (for example, the general configuration and manufacturing process of the battery), are conventional in the relevant field. It can be grasped as a design matter of a person skilled in the art based on technology. The technology disclosed here can be implemented based on the content disclosed in this specification and common general technical knowledge in the field. In this specification, the notation "X to Y" indicating a numerical range means "X or more and Y or less" unless otherwise specified.

In this specification, the term “non-aqueous electrolyte secondary battery” refers to a general electricity storage device in which charge-discharge reactions occur due to the movement of charge carriers between a pair of electrodes (positive electrode and negative electrode) via an electrolyte. Such nonaqueous electrolyte secondary batteries include not only so-called storage batteries such as lithium ion secondary batteries but also capacitors such as electric double layer capacitors. In the following, among the non-aqueous electrolyte secondary batteries described above, an embodiment will be described in the case of a flat prismatic lithium ion secondary battery having a wound electrode assembly.

Further, in each figure referred to in this specification, X indicates the "depth direction", Y indicates the "width direction", and Z indicates the "height direction". Further, F in the depth direction X indicates "front" and Rr indicates "rear". L in the width direction Y indicates "left" and R indicates "right". U in the height direction Z indicates "up" and "D" indicates down. However, these directions are determined for convenience of explanation, and are not intended to limit the installation form when using the secondary battery disclosed herein.

<Structure of Nonaqueous Electrolyte Secondary Battery>
FIG. 1 is a perspective view schematically showing a non-aqueous electrolyte secondary battery 100 according to this embodiment. FIG. 2 is a schematic longitudinal sectional view along line II-II in FIG.

As shown in FIG. 2 , the non-aqueous electrolyte secondary battery 100 according to this embodiment includes a wound electrode body 40 and a battery case 50 that accommodates the wound electrode body 40 . A specific configuration of the non-aqueous electrolyte secondary battery 100 will be described below.
<Battery case>
The battery case 50 is a housing that accommodates the wound electrode body 40 . Although not shown, the battery case 50 also contains a non-aqueous electrolyte. As shown in FIG. 1, the battery case 50 in this embodiment has a flat bottomed cuboid (square) outer shape. Note that conventionally known materials can be used for the battery case 50 without particular limitations. For example, the battery case 50 may be made of metal. Examples of materials for the battery case 50 include aluminum, aluminum alloys, iron, iron alloys, and the like.

As shown in FIGS. 1 and 2 , the battery case 50 includes an exterior body 52 and a sealing plate 54 . The exterior body 52 is a flat bottomed rectangular container having an opening 52h on the upper surface. The exterior body 52 includes a substantially rectangular bottom wall 52a, a pair of long side walls 52b extending upward in the height direction Z from the long sides of the bottom wall 52a, and a pair of long side walls 52b extending upward in the height direction Z from the short sides of the bottom wall 52a. and a pair of short side walls 52c extending therefrom. On the other hand, the sealing plate 54 is a substantially rectangular planar member that closes the opening 52 h of the outer package 52 . The outer peripheral edge of the sealing plate 54 is joined (for example, welded) to the outer peripheral edge of the opening 52h of the exterior body 52 . As a result, the battery case 50 whose inside is hermetically sealed is produced. In addition, the sealing plate 54 is provided with a liquid injection hole 55 and a gas exhaust valve 57 . The injection hole 55 is a through hole provided for injecting a non-aqueous electrolyte into the battery case 50 after sealing. The liquid injection hole 55 is sealed with a sealing member 56 after the non-aqueous electrolyte is injected. The gas exhaust valve 57 is a thin portion designed to break (open) when a large amount of gas is generated in the battery case 50 and exhaust the gas.

<Electrolyte>
As described above, inside the battery case 50, in addition to the wound electrode assembly 40, an electrolytic solution (not shown) is accommodated. As the electrolyte, those used in conventionally known non-aqueous electrolyte secondary batteries can be used without particular limitation. For example, a non-aqueous electrolytic solution in which a supporting salt is dissolved in a non-aqueous solvent can be used as the electrolytic solution. Examples of this non-aqueous solvent include carbonate-based solvents such as ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate. An example of a supporting salt is a fluorine-containing lithium salt such as _LiPF6 .

<Electrode terminal>
A positive electrode terminal 60 is attached to one end (the left side in FIGS. 1 and 2) of the sealing plate 54 in the width direction Y. As shown in FIG. The positive electrode terminal 60 is connected to a plate-like positive electrode external conductive member 62 outside the battery case 50 . On the other hand, a negative terminal 65 is attached to the other end (the right side in FIGS. 1 and 2) of the sealing plate 54 in the width direction Y. As shown in FIG. A plate-shaped negative electrode external conductive member 67 is attached to the negative electrode terminal 65 . These external conductive members (positive electrode external conductive member 62 and negative electrode external conductive member 67) are connected to other non-aqueous electrolyte secondary batteries and external devices via external connection members (such as bus bars). The external conductive member is preferably made of a highly conductive metal (aluminum, aluminum alloy, copper, copper alloy, etc.).

<Electrode current collector>
FIG. 3 is a perspective view schematically showing the electrode assembly 40 attached to the sealing plate 54. As shown in FIG. FIG. 4 is a perspective view schematically showing the electrode body 40 to which the positive electrode second current collector 72 and the negative electrode second current collector 77 are attached. In the non-aqueous electrolyte secondary battery 100 according to this embodiment, a battery case 50 accommodates a plurality of (three) wound electrode bodies 40 . Although the detailed structure will be described later, each wound electrode body 40 is provided with a positive electrode tab group 42 and a negative electrode tab group 44 . These electrode tab groups (the positive electrode tab group 42 and the negative electrode tab group 44) are folded while the electrode current collectors (the positive electrode current collector 70 and the negative electrode current collector 75) are joined together.

Specifically, as shown in FIG. 2 , the positive electrode tab group 42 of each of the plurality of wound electrode bodies 40 is connected to the positive electrode terminal 60 via the positive electrode current collector 70 . This positive electrode current collector 70 is housed inside the battery case 50 . The positive electrode current collector 70 includes a positive electrode first current collector 71, which is a plate-shaped conductive member extending in the width direction Y along the inner surface of the sealing plate 54, and a plate-shaped conductive member extending along the height direction Z. and a plurality of positive electrode second current collectors 72 that are members. A lower end portion 60 c of the positive electrode terminal 60 is inserted into the battery case 50 through the terminal insertion hole 58 of the sealing plate 54 and connected to the positive electrode first current collector 71 . On the other hand, as shown in FIGS. 3 and 4 , in this secondary battery 100 , the number of positive electrode second current collectors 72 corresponding to the plurality of wound electrode bodies 40 is provided. Each positive electrode second current collector 72 is connected to the positive electrode tab group 42 of the wound electrode body 40 (see FIG. 3). Then, the positive electrode tab group 42 of the wound electrode body 40 is bent so that the positive electrode second current collector 72 faces one side surface of the wound electrode body 40 (see FIG. 3). Thereby, the upper end portion of the positive electrode second current collector 72 and the positive electrode first current collector 71 are electrically connected.

On the other hand, as shown in FIG. 2 , the negative electrode tab group 44 of each of the plurality of wound electrode bodies 40 is connected to the negative electrode terminal 65 via the negative electrode current collector 75 . The connection structure on the negative electrode side is substantially the same as the connection structure on the positive electrode side described above. Specifically, the negative electrode current collector 75 includes the negative electrode first current collector 76, which is a plate-shaped conductive member extending in the width direction Y along the inner surface of the sealing plate 54, and the negative electrode first current collector 76 extending along the height direction Z. It includes a plurality of negative electrode second current collectors 77 that are plate-shaped conductive members. A lower end portion 65 c of the negative electrode terminal 65 is inserted into the battery case 50 through the terminal insertion hole 59 and connected to the negative electrode first current collector 76 . On the other hand, each of the plurality of negative electrode second current collectors 77 is connected to the negative electrode tab group 44 of the wound electrode assembly 40 (see FIG. 3). Then, the negative electrode tab group 44 is bent so that the negative electrode second current collector 77 and the other side surface of the wound electrode body 40 face each other (see FIG. 3). Thereby, the upper end portion of the negative electrode second current collector 77 and the negative electrode first current collector 76 are electrically connected. Also, for the electrode current collectors (the positive electrode current collector 70 and the negative electrode current collector 75), metals with excellent conductivity (aluminum, aluminum alloys, copper, copper alloys, etc.) can be suitably used.

<Insulating material>
Moreover, in this non-aqueous electrolyte secondary battery 100 , various insulating members are attached to prevent conduction between the wound electrode assembly 40 and the battery case 50 . Specifically, an external insulating member 92 is interposed between the positive electrode external conductive member 62 (negative electrode external conductive member 67) and the outer surface of the sealing plate 54 (see FIG. 1). This can prevent the positive electrode external conductive member 62 and the negative electrode external conductive member 67 from being electrically connected to the sealing plate 54 . Further, as shown in FIG. 2, a gasket 90 is attached to each of the terminal insertion holes 58 and 59 of the sealing plate 54 . This prevents the positive terminal 60 (or the negative terminal 65 ) inserted through the terminal insertion holes 58 and 59 from being electrically connected to the sealing plate 54 . An internal insulating member 94 is arranged between the positive electrode first current collector 71 (or the negative electrode first current collector 76 ) and the inner surface of the sealing plate 54 . The internal insulating member 94 has a plate-like base portion 94 a interposed between the positive electrode first current collector 71 (or the negative electrode first current collector 76 ) and the inner surface of the sealing plate 54 . This can prevent the positive electrode first current collector 71 and the negative electrode first current collector 76 from being electrically connected to the sealing plate 54 . Further, the internal insulating member 94 has a protrusion 94b that protrudes from the inner surface of the sealing plate 54 toward the wound electrode assembly 40 . As a result, movement of the wound electrode body 40 in the height direction Z can be restricted, and direct contact between the wound electrode body 40 and the sealing plate 54 can be prevented. In addition, the plurality of wound electrode bodies 40 are housed inside the battery case 50 while being covered by an electrode body holder (not shown) made of an insulating resin sheet. As a result, direct contact between the wound electrode body 40 and the exterior body 52 can be prevented. In addition, the material of each insulating member described above is not particularly limited as long as it has a predetermined insulating property. As an example, synthetic resin materials such as polyolefin-based resins (eg, polypropylene (PP), polyethylene (PE)) and fluorine-based resins (eg, perfluoroalkoxyalkane (PFA), polytetrafluoroethylene (PTFE)) can be used. .

<Wound electrode body>
FIG. 5 is a schematic diagram showing the configuration of the wound electrode body 40 of the non-aqueous electrolyte secondary battery 100 according to this embodiment. As shown in FIG. 5, the electrode body 40 used in the non-aqueous electrolyte secondary battery 100 according to the present embodiment is a flat winding in which the positive electrode plate 10 and the negative electrode plate 20 are wound with the separator 30 interposed therebetween. It is the rotating electrode body 40 . In this non-aqueous electrolyte secondary battery 100, the wound electrodes are arranged in the battery case 50 such that the winding axis WL of the wound electrode body 40 and the width direction Y of the non-aqueous electrolyte secondary battery 100 are substantially aligned. A body 40 is accommodated (see FIG. 2). That is, the "winding axis direction" in the following description is substantially the same direction as the width direction Y in the drawings.

<Positive plate>
As shown in FIG. 5, the positive electrode plate 10 is a long belt-like member. The positive electrode plate 10 includes a positive electrode core 12 which is a strip-shaped metal foil, and a positive electrode active material layer 14 applied to the surface of the positive electrode core 12 . From the viewpoint of battery performance, it is preferable that the positive electrode active material layer 14 is applied to both surfaces of the positive electrode core 12 . Further, in this positive electrode plate 10, a positive electrode tab 12t protrudes outward (to the left in FIG. 5) from one edge in the winding axial direction (width direction Y). A plurality of positive electrode tabs 12t are formed at predetermined intervals in the longitudinal direction of the elongated strip-shaped positive electrode plate 10 . The positive electrode tab 12t is a region where the positive electrode active material layer 14 is not applied and the positive electrode core 12 is exposed. A protective layer 16 extending along the longitudinal direction of the positive electrode plate 10 is formed in a region adjacent to the edge of the positive electrode plate 10 on the side of the positive electrode tab 12t.

For each member constituting the positive electrode plate 10, conventionally known materials that can be used in general non-aqueous electrolyte secondary batteries (for example, lithium ion secondary batteries) can be used without particular limitation. For example, a metal material having a predetermined conductivity can be preferably used for the positive electrode core 12 . Such a positive electrode core 12 is preferably made of, for example, aluminum, an aluminum alloy, or the like.

Moreover, the positive electrode active material layer 14 is a layer containing a positive electrode active material. A positive electrode active material is a particulate material that can reversibly store and release charge carriers. From the viewpoint of stably producing a high-performance positive electrode plate 10, the positive electrode active material is preferably a lithium-transition metal composite oxide. Among the above lithium-transition metal composite oxides, lithium-transition metal composite oxides containing at least one selected from the group consisting of nickel (Ni), cobalt (Co) and manganese (Mn) are particularly suitable. Specific examples include lithium nickel cobalt manganese composite oxide (NCM), lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt aluminum system composite oxide (NCA), lithium-iron-nickel-manganese-based composite oxide, and the like. A suitable example of a lithium transition metal composite oxide that does not contain Ni, Co and Mn is a lithium iron phosphate composite oxide (LFP). The term "lithium-nickel-cobalt-manganese-based composite oxide" used herein is a term that includes oxides containing additional elements in addition to the main constituent elements (Li, Ni, Co, Mn, O). be. Examples of such additive elements include transition metal elements such as Mg, Ca, Al, Ti, V, Cr, Si, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn and Sn. and typical metal elements. Also, the additive elements may be metalloid elements such as B, C, Si and P, and nonmetal elements such as S, F, Cl, Br and I. Although detailed description is omitted, this is the same for other lithium-transition metal composite oxides described as "--based composite oxide". Moreover, the positive electrode active material layer 14 may contain additives other than the positive electrode active material. Examples of such additives include conductive materials, binders, and the like. Specific examples of the conductive material include carbon materials such as acetylene black (AB). A specific example of the binder is a resin binder such as polyvinylidene fluoride (PVdF). The content of the positive electrode active material is approximately 80% by mass or more, and typically 90% by mass or more, when the total solid content of the positive electrode active material layer 14 is 100% by mass.

On the other hand, the protective layer 16 is a layer configured to have lower electrical conductivity than the positive electrode active material layer 14 . By providing the protective layer 16 in the region adjacent to the edge of the positive electrode plate 10, it is possible to prevent an internal short circuit due to direct contact between the positive electrode core 12 and the negative electrode active material layer 24 when the separator 30 is damaged. . For example, it is preferable to form a layer containing insulating ceramic particles as the protective layer 16 . Examples of such ceramic particles include inorganic oxides such as alumina (Al ₂ O ₃ ), magnesia (MgO), silica (SiO ₂ ) and titania (TiO 2 ); nitrides such as aluminum nitride and silicon nitride; Examples include metal hydroxides such as magnesium hydroxide and aluminum hydroxide, clay minerals such as mica, talc, boehmite, zeolite, apatite and kaolin, and glass fibers. Considering insulation and heat resistance, alumina, boehmite, aluminum hydroxide, silica and titania are preferable among the above materials. Moreover, the protective layer 16 may contain a binder for fixing the ceramic particles to the surface of the positive electrode core 12 . Examples of such binders include resin binders such as polyvinylidene fluoride (PVdF). Note that the protective layer is not an essential component of the positive electrode plate. That is, the secondary battery disclosed herein can also use a positive electrode plate without a protective layer formed thereon.

<Negative plate>
As shown in FIG. 5, the negative electrode plate 20 is a long belt-like member. The negative electrode plate 20 includes a negative electrode core 22 that is a strip-shaped metal foil and a negative electrode active material layer 24 applied to the surface of the negative electrode core 22 . From the viewpoint of battery performance, it is preferable that the negative electrode active material layer 24 is applied to both surfaces of the negative electrode core 22 . Further, the negative electrode plate 20 is provided with a negative electrode tab 22t that protrudes outward (to the right in FIG. 5) from one edge in the winding axial direction (width direction Y). A plurality of negative electrode tabs 22t are provided at predetermined intervals in the longitudinal direction of the negative electrode plate 20 . The negative electrode tab 22t is a region where the negative electrode active material layer 24 is not applied and the negative electrode core 22 is exposed.

For each member constituting the negative electrode plate 20, conventionally known materials that can be used in general non-aqueous electrolyte secondary batteries (for example, lithium ion secondary batteries) can be used without particular limitation. For example, a metal material having a predetermined conductivity can be preferably used for the negative electrode core 22 . Such a negative electrode core 22 is preferably made of, for example, copper, a copper alloy, or the like.

The negative electrode active material layer 24 is a layer containing a negative electrode active material. The negative electrode active material is not particularly limited as long as it can reversibly occlude and release charge carriers in relation to the positive electrode active material described above, and materials that can be used in conventional general non-aqueous electrolyte secondary batteries are not particularly limited. Available. As such a negative electrode active material, for example, a carbon-based negative electrode active material can be used. Examples of carbon-based negative electrode active materials that can be used include graphite, hard carbon, soft carbon, and amorphous carbon. Amorphous carbon-coated graphite in which the surface of graphite is coated with amorphous carbon can also be used.

Also, the negative electrode active material layer 24 may contain a negative electrode active material other than the carbon-based negative electrode active material. For example, the negative electrode active material layer 24 may contain an alloy-based negative electrode active material. In this specification, the alloy-based negative electrode active material refers to a negative electrode active material that reversibly absorbs and releases charge carriers with the formation of an alloy with charge carriers (lithium in lithium ion secondary batteries). Say. Examples of alloy-based negative electrode active materials include silicon (Si)-based negative electrode active materials and tin (Sn)-based negative electrode active materials.
Examples of Si-based negative electrode active materials include silicon (Si), silicon oxide represented by SiO _x (0.05<x<1.95), lithium silicate (Li _x Si _y O _z ), SiC _x (0 Examples include silicon carbide represented by <x<1), silicon nitride represented by SiN _x (0<x<4/3), and the like. Also, an alloy composed of Si and an element other than Si can be used. Examples of elements other than Si include Fe, Co, Sb, Bi, Pb, Ni, Cu, Zn, Ge, In, Sn, and Ti.
Examples of Sn-based negative electrode active materials include tin (Sn), tin oxides, tin nitrides, tin-containing alloys, and solid solutions thereof. In addition, part of the tin atoms contained in these may be substituted with one or more elements. Tin oxides include tin oxide represented by SnO _x (0<x<2), tin dioxide (SnO ₂ ), and the like. Tin-containing alloys include Ni--Sn alloys, Mg--Sn alloys, Fe--Sn alloys, Cu--Sn alloys, Ti--Sn alloys, and the like. _SnSiO3 , _Ni2Sn4 , _Mg2Sn etc. are illustrated besides this _. The negative electrode active material layer 24 may contain negative electrode active materials other than those described above as long as the effects of the present invention are not impaired.

Alloy-based negative electrode active materials such as Si-based negative electrode active materials and Sn-based negative electrode active materials have the advantage of high energy density. degree is large. Also, the alloy-based negative electrode active material has a feature of having a higher potential than the carbon-based negative electrode active material.
The alloy-based negative electrode active material can be used by being mixed with the carbon-based negative electrode active material. The amount of the alloy-based negative electrode active material mixed is generally 30% by mass or less, and may be 20% by mass or less, for example, 10% by mass or less, when the total solid content of the negative electrode active material is 100% by mass. may be The mixed amount of the alloy-based negative electrode active material is generally 0.1% by mass or more, and may be, for example, 1% by mass or more.

Further, the negative electrode active material layer 24 may contain additives other than the negative electrode active material. Examples of such additives include binders, thickeners, and the like. A specific example of the binder is a rubber-based binder such as styrene-butadiene rubber (SBR). Moreover, carboxymethyl cellulose (CMC) etc. are mentioned as a specific example of a thickener. The content of the negative electrode active material is approximately 70% by mass or more, and typically 80% by mass or more, when the total solid content of the negative electrode active material layer 24 is 100% by mass. The negative electrode active material may occupy 90% by mass or more of the negative electrode active material layer 24, or may occupy 95% by mass or more.

FIG. 6 is a cross-sectional view schematically showing a cross section of the wound electrode body 40 of the non-aqueous electrolyte secondary battery 100 according to this embodiment. FIG. 7 is a plan view schematically showing the negative electrode 20 of the non-aqueous electrolyte secondary battery 100 according to this embodiment. 6, the positive electrode active material layer 14 and the negative electrode active material layer 24 formed on one side of the positive electrode core 12 and the negative electrode core 22 are illustrated. Moreover, in FIG. 6, illustration of the protective layer is omitted.
As shown in FIG. 6 , the wound electrode body 40 is configured such that the negative electrode active material layer 24 has a larger area and width than the positive electrode active material layer 14 . In other words, both ends of the negative electrode active material layer 24 in the width direction Y protrude from the positive electrode active material layer 14 . That is, the negative electrode active material layer 24 has a facing region 24 a that faces the positive electrode active material layer 14 and a non-facing region 24 b that does not face the positive electrode active material layer 14 . The non-facing regions 24b are formed along both ends in the width direction Y of the facing region 24a.

In the non-aqueous electrolyte secondary battery 100 according to the present embodiment, the facing region 24a and the non-facing region 24b of the negative electrode active material layer 24 have approximately the same basis weight (that is, the weight of the negative electrode active material layer per unit area). but have different specific surface areas. S1 is larger than S2, where S1 is the specific surface area of the opposing region 24a determined by the BET method, and S2 is the specific surface area of the non-facing region 24b determined by the BET method. That is, S1 and S2 satisfy the relationship of S2<S1. Also, S1 and S2 preferably satisfy the relationship S2/S1≦0.7, and may satisfy the relationship S2/S1≦0.5, for example. Moreover, S1 and S2 may satisfy the relationship of S2/S1≦0.3. S1 and S2 may satisfy the relationship 0.1≦S2/S1.
As used herein, the term “specific surface area” refers to, for example, a gas adsorption amount measured by a gas adsorption method (constant volume adsorption method) using nitrogen (N ₂ ) gas as an adsorbate, and a BET method (for example, It refers to the surface area calculated by analyzing by BET single point method).

^The specific surface areas S1 and S2 of the negative ^electrode active material layer 24 are not particularly limited as long as the above relationship is satisfied. It may be 1.0 m ² /g or more. Further, the specific surface areas S1 and S2 of the negative electrode active material layer 24 are, for example, 5.0 m ² /g or less, may be 4.0 m ² /g or less, or may be 3.2 m ² /g or less. good too.

The specific surface areas S1 and S2 can be adjusted so as to satisfy the above relationship, for example, by adjusting the particle size of the negative electrode active material used in the facing region 24a and the non-facing region 24b. The particle size can be evaluated, for example, by the cumulative 50% particle size (D ₅₀ ) (hereinafter also referred to as the average particle size (D ₅₀ )) in the volume-based particle size distribution obtained by a laser diffraction scattering method. The average particle diameter (D ₅₀ ) of the negative electrode active material is not particularly limited as long as the specific surface areas S1 and S2 of the negative electrode active material layer 24 satisfy the above relationship. It may be 5 μm or more. Also, the average particle diameter ( _D50 ) of the negative electrode active material is, for example, 30 μm or less, may be 20 μm or less, or may be 15 μm or less.

The area ratio, dimensional relationship, and the like of the opposing region 24a and the non-facing region 24b of the negative electrode active material layer 24 are not particularly limited. As described above, the negative electrode active material layer 24 is configured to be larger in area and width than the positive electrode active material layer 14 . As shown in FIG. 7, (A1+A2)/A1 is greater than 1, where A1 is the area of the opposing region 24a and A2 is the total area of the non-opposing regions 24b. Also, (A1+A2)/A1 is preferably 1.025 or more, and may be 1.05 or more, for example. (A1+A2)/A1 is preferably 1.2 or less, and may be 1.15 or less. In the longitudinal direction of the negative electrode plate 20, when the facing region 24a and the non-facing region 24b have approximately the same length, the area ratio (A1+A2)/A1 is calculated from the widths of the facing region 24a and the non-facing region 24b. can do. That is, (A1+A2) can be considered to be approximately the same as (w1+w2)/w1, where w1 is the width of the opposing region 24a and w2 is the sum of the widths of the non-opposing regions 24b.

<Separator>
As shown in FIG. 5 , the wound electrode body 40 in this embodiment includes two separators 30 . Each separator 30 is an insulating sheet having a plurality of fine through holes through which charge carriers can pass. By interposing the separator 30 between the positive electrode plate 10 and the negative electrode plate 20, contact between the positive electrode plate 10 and the negative electrode plate 20 is prevented, and a charge carrier (for example, a Lithium ions) can be moved.

As the separator 30, those used in conventionally known secondary battery separators can be used without particular limitation. As the separator 30, for example, a resin-made porous sheet made of polyolefin resin such as polyethylene (PE) or polypropylene (PP) can be used. The separator 30 has a substrate portion made of a porous resin sheet, and a heat resistance layer (HRL) provided on at least one surface of the substrate portion and containing an inorganic filler. good too. Examples of inorganic fillers that can be used include alumina, boehmite, aluminum hydroxide, titania, and the like.

By the way, according to the studies of the present inventors, the capacity deterioration of the non-aqueous electrolyte secondary battery can be suppressed by using a negative electrode active material with a small specific surface area. However, in that case, the internal resistance of the non-aqueous electrolyte secondary battery may also increase.
The present inventor considers the reason why capacity deterioration is suppressed by using a negative electrode active material with a small specific surface area as follows. When a negative electrode active material with a small specific surface area is used, the side reaction between the negative electrode active material and the electrolyte that occurs on the surface of the negative electrode active material when the non-aqueous electrolyte secondary battery is stored at a high SOC is reduced. Furthermore, since the area for diffusion of charge carriers (lithium ions in a lithium ion secondary battery) to the non-facing region becomes smaller, the moving speed of the charge carriers decreases, and the charge carriers move from the facing region to the non-facing region. It is thought that the spread of As a result, capacity deterioration can be suppressed. In addition, the present inventor believes that the reason why the internal resistance of the non-aqueous electrolyte secondary battery increases when using a negative electrode active material with a small specific surface area is that the reaction area decreases as the specific surface area decreases. there is

In this embodiment, in the negative electrode active material layer 24, the specific surface area S1 of the facing region 24a and the specific surface area S2 of the non-facing region 24b satisfy the relationship S2<S1. Such a configuration provides the non-aqueous electrolyte secondary battery 100 in which capacity deterioration is suppressed and an increase in internal resistance is suppressed. Moreover, when the specific surface area S1 and the specific surface area S2 satisfy the relationship of S2/S1≦0.7, the above-mentioned effects are exhibited more preferably.

Regarding the reason why the capacity retention rate is improved by making the specific surface area S1 larger than the specific surface area S2, the present inventors have found that the charge carriers from the facing region 24a to the non-facing region 24b (in lithium ion secondary batteries, lithium ion ) is suppressed. That is, when the specific surface area S2 of the non-facing region 24b is smaller than the specific surface area S1 of the facing region 24a, the area for the charge carriers to diffuse into the non-facing region 24b becomes relatively small. It is believed that this reduces the moving speed of the charge carriers and suppresses the diffusion of the charge carriers to the non-facing region 24b. Moreover, since the specific surface area S1 of the facing region 24a that occupies most of the negative electrode active material layer 24 is large, an increase in internal resistance can be suppressed.

In this embodiment, the total area A1+A2 of the opposing region 24a and the non-opposing region 24b with respect to the area A1 of the opposing region 24a, ie, (A1+A2)/A1, is 1.025 or more and 1.2 or less. When the area ratio (A1+A2)/A1 is within such a range, the effect of improving the capacity retention rate is exhibited more favorably.

Also, the diffusion of charge carriers from the facing region 24a to the non-facing region 24b is promoted by the potential difference between the regions in the high SOC state. When an alloy-based active material with a high potential such as a Si-based negative electrode active material or a Sn-based negative electrode active material is used as the negative electrode active material, charge carriers from the facing region 24a to the non-facing region 24b (in a lithium ion secondary battery, (lithium ions) can occur more significantly. Therefore, the technology disclosed herein is also preferably applied to a non-aqueous electrolyte secondary battery having a negative electrode active material layer containing an alloy-based active material.

EXAMPLES Examples relating to the present invention will be described below, but the present invention is not intended to be limited to those shown in the examples.

<Preparation of non-aqueous electrolyte secondary battery for evaluation>
<Example 1>
LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (LNCMO) as a positive electrode active material, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder, LNCMO: AB : PVdF = 100:1:1 by mixing with N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode mixture slurry.
This positive electrode mixture slurry was applied onto an aluminum foil, dried, and then compressed to a predetermined thickness by pressing to produce a positive electrode plate.

Graphite having a specific surface area of 3.5 m ² /g based on the BET method was prepared. This graphite (C) as a negative electrode active material, styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were mixed at a mass ratio of C:SBR:CMC=100:1:1. was mixed with ion-exchanged water to prepare a first negative electrode mixture slurry.
Graphite having a specific surface area of 2.8 m ² /g was prepared. A second negative electrode mixture slurry was prepared by mixing the same materials and proportions as those of the first negative electrode mixture slurry, except that this graphite (C) was used as the negative electrode active material.

FIG. 8 is a schematic diagram schematically showing a slot die 200 used when applying the first and second negative electrode mixture slurries. The slot die 200 is provided with an upper slit 201 and a lower slit 202 . The slot die 200 is configured so that different slurries can be applied simultaneously from the upper slit 201 and the lower slit 202 . The slit width of the upper slit 201 is set to the width w1 corresponding to the facing area 24a. A portion corresponding to the opposing region 24a of the lower slit 202 was masked. The slit width was set so that the slurry having a total width of w2 could be applied to the portions where the non-opposing regions 24b were formed along both width direction ends of the opposing regions 24a. Here, the slit width w1 of the slit 201 and the slit width w2 of the slit 202 are calculated by dividing the area A1 of the opposing region 24a by the area (A1+A2) of the opposing region 24a and the non-opposing region 24b, ie, (A1+A2)/A1, by 1. The slit width was set to be 1. The first negative electrode mixture slurry was applied from the upper slit 201 and the second negative electrode mixture slurry was applied from the lower slit 202 onto the copper foil as the negative electrode core 22 at the same time. The first and second negative electrode mixture slurries coated on the copper foil were dried and compressed to a predetermined thickness by press processing to produce a negative electrode plate having a facing region and a non-facing region. The coating was performed so that the basis weights of the first and second negative electrode mixture slurries were substantially the same.

A predetermined amount of the negative electrode active material layer was cut out from the facing region, and the specific surface area S1 was measured. A predetermined amount of the negative electrode active material layer was cut out from the non-facing region, and the specific surface area S2 was measured. As the specific surface area, a value analyzed by the BET method from the nitrogen gas adsorption isotherm was adopted. The specific surface area S1 was 3.2 m ² /g, and the specific surface area S2 was 2.6 m ² /g.

A positive electrode plate and a negative electrode plate were cut into predetermined dimensions. A separator having a three-layer structure of polypropylene/polyethylene/polypropylene was prepared. With the positive electrode plate and the negative electrode plate insulated from each other via a separator, they were laminated so that the aluminum foil of the positive electrode plate was exposed from one end and the copper foil of the negative electrode plate was exposed from the other end. The positive electrode plate, the negative electrode plate and the separator that were laminated were spirally wound, flattened and molded to produce an electrode assembly. The aluminum foil of the positive electrode plate and the aluminum plate as the positive electrode external conductive member were welded. The copper foil of the negative electrode plate and the copper plate as the negative electrode external conductive member were welded together. The electrode assembly was housed in a laminated film outer package with the positive electrode conductive member and the negative electrode conductive member exposed to the outside of the outer package.

As an electrolytic solution, a non-aluminum solvent was prepared by dissolving LiPF ₆ at a concentration of 1.0 mol/L in a mixed solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of EC:EMC=1:3. A water electrolyte was prepared. A non-aqueous electrolyte secondary battery of Example 1 was produced by injecting this electrolytic solution and sealing the laminate film.

<Comparative Example 1>
A nonaqueous electrolyte secondary battery of Comparative Example 1 was fabricated in the same manner as in Example 1, except that the first negative electrode mixture slurry of Example 1 was used to form the facing region 24a and the non-facing region 24b. Both the specific surface areas S1 and S2 were 3.2 m ² /g.
<Comparative Example 2>
Graphite having a specific surface area of 2.4 m ² /g was prepared. A negative electrode mixture slurry was prepared by mixing materials and ratios similar to those of the first negative electrode mixture slurry of Example 1, except that this graphite (C) was used as the negative electrode active material. A non-aqueous electrolyte secondary battery of Comparative Example 2 was fabricated in the same manner as in Example 1, except that the opposing region 24a and the non-facing region 24b were formed using this negative electrode mixture slurry. The specific surface areas S1 and S2 were both 2.2 m ² /g.
<Comparative Example 3>
Graphite having a specific surface area of 1.2 m ² /g was prepared. A negative electrode mixture slurry was prepared by mixing materials and ratios similar to those of the first negative electrode mixture slurry of Example 1, except that this graphite (C) was used as the negative electrode active material. A non-aqueous electrolyte secondary battery of Comparative Example 3 was fabricated in the same manner as in Example 1, except that the opposing region 24a and the non-facing region 24b were formed using this negative electrode mixture slurry. The specific surface areas S1 and S2 were both 1.1 m ² /g.

<Example 2>
Graphite having a specific surface area of 2.4 m ² /g was prepared. A second negative electrode mixture slurry was prepared by mixing materials and ratios similar to those of the first negative electrode mixture slurry of Example 1, except that this graphite (C) was used as the negative electrode active material. A non-aqueous electrolyte secondary battery of Example 2 was produced in the same manner as in Example 1, except that the non-facing region 24b was formed using this second negative electrode mixture slurry. The specific surface area S1 was 3.2 m ² /g, and the specific surface area S2 was 2.2 m ² /g.
<Example 3>
Graphite having a specific surface area of 1.2 m ² /g was prepared. A second negative electrode mixture slurry was prepared by mixing materials and ratios similar to those of the first negative electrode mixture slurry of Example 1, except that this graphite (C) was used as the negative electrode active material. A non-aqueous electrolyte secondary battery of Example 3 was produced in the same manner as in Example 1, except that the non-facing region 24b was formed using this second negative electrode mixture slurry. The specific surface area S1 was 3.2 m ² /g, and the specific surface area S2 was 1.1 m ² /g.

<Example 4>
The width of the lower slit 202 was adjusted so that (w1+w2)/w1 was 1.025. A non-aqueous electrolyte secondary battery of Example 3 was produced in the same manner as in Example 3 except for the above.
<Example 5>
The width of the lower slit 202 was adjusted so that (w1+w2)/w1 was 1.2. A non-aqueous electrolyte secondary battery of Example 3 was produced in the same manner as in Example 3 except for the above.
<Example 6>
The width of the lower slit 202 was adjusted so that (w1+w2)/w1 was 1.3. A non-aqueous electrolyte secondary battery for evaluation of Example 3 was produced in the same manner as in Example 3 except for the above.
In Examples 4 to 6, the specific surface area S1 was 3.2 m ² /g and the specific surface area S2 was 1.1 m ² /g.

<Comparative Example 4>
A negative electrode active material having a specific surface area of 3.4 m ² /g was prepared by mixing graphite and silicon oxide (SiO) at a mass ratio of C:SiO=95:5. A negative electrode mixture slurry was prepared by mixing materials and ratios similar to those of the first negative electrode mixture slurry of Example 1, except that this negative electrode active material was used. A non-aqueous electrolyte secondary battery of Comparative Example 4 was fabricated in the same manner as in Comparative Example 1, except that the opposing region 24a and the non-facing region 24b were formed using this negative electrode mixture slurry. The specific surface areas S1 and S2 were both 3.1 m ² /g.
<Comparative Example 5>
A negative electrode active material having a specific surface area of 1.1 m ² /g was prepared by mixing graphite and silicon oxide (SiO) at a mass ratio of C:SiO=95:5. A negative electrode mixture slurry was prepared by mixing materials and ratios similar to those of the first negative electrode mixture slurry of Example 1, except that this negative electrode active material was used. A non-aqueous electrolyte secondary battery of Comparative Example 5 was produced in the same manner as in Comparative Example 4, except that the opposing region 24a and the non-facing region 24b were formed using this negative electrode mixture slurry. The specific surface areas S1 and S2 were both 1.0 m ² /g.
<Example 7>
Example 1 was repeated, except that the negative electrode mixture slurry of Comparative Example 4 was used as the first negative electrode mixture slurry, and the negative electrode mixture slurry of Comparative Example 5 was used as the second negative electrode mixture slurry. A non-aqueous electrolyte secondary battery of Example 7 was produced. The specific surface area S1 was 3.1 m ² /g, and the specific surface area S2 was 1.0 m ² /g.

<Evaluation of Nonaqueous Electrolyte Secondary Battery>
<IV resistance>
Each non-aqueous electrolyte lithium secondary battery was adjusted to an SOC of 50%. This was placed in a temperature environment of 25° C. for 1 hour. After that, discharge was performed for 10 seconds at a current value of 5C. The IV resistance was obtained from the current value of (V0−V1)/5C, where V0 was the open circuit voltage (OCV) immediately before discharge and V1 was the voltage at the time of 10-second discharge. Assuming that the initial resistance of the lithium secondary battery of Comparative Example 1 was 100, the IV resistance ratios of the other non-aqueous electrolyte secondary batteries were calculated. Table 1 shows the results. The "area ratio" in Table 1 means the area (A1+A2) of the facing region and the non-facing region with respect to the area A1 of the facing region, that is, (A1+A2)/A1.

<Capacity retention rate>
<Measurement of initial capacity>
Each non-aqueous electrolyte secondary battery was placed in an environment of 25°C. This was subjected to constant-current-constant-voltage charging to 4.2V at a current value of 1/3C, and constant-current discharging to 2.5V at a current value of 1/3C. The discharge capacity at this time was measured and taken as the initial capacity.

<Measurement of capacity retention rate after storage at 25°C>
In a 25° C. environment, each non-aqueous electrolyte secondary battery was constant-current-constant-voltage charged to 4.2 V at a current value of 1/3 C, and then stored in a 25° C. environment for 20 days. Thereafter, each non-aqueous electrolyte secondary battery was charged and discharged in the same manner as above, and the discharge capacity was measured. The discharge capacity at this time was determined as the battery capacity after storage at 25°C. The capacity retention rate (%) after storage at 25°C was obtained from (battery capacity after storage at 25°C/initial capacity) x 100. Table 1 shows the results.

<Measurement of capacity retention rate after storage at 60°C>
In a 25° C. environment, each non-aqueous electrolyte secondary battery was constant-current-constant-voltage charged to 4.2 V at a current value of 1/3 C, and then stored in a 60° C. environment for 20 days. After that, it was left in an environment of 25° C. for 4 hours. Thereafter, each non-aqueous electrolyte secondary battery was charged and discharged in the same manner as above, and the discharge capacity was measured. The discharge capacity at this time was determined as the battery capacity after storage at 60°C. The capacity retention rate (%) after storage at 60°C was calculated from (battery capacity after storage at 60°C/initial capacity) x 100. Table 1 shows the results.

From the results of Comparative Examples 1 to 3, it can be seen that the smaller the specific surface area of the negative electrode active material layer, the higher the capacity retention rate. However, it can be seen that the smaller the specific surface area of the negative electrode active material layer, the higher the IV resistance.
From the results of Comparative Example 1 and Examples 1 to 3, even if the specific surface area of the facing region of the negative electrode active material layer is the same, by reducing the specific surface area of the non-facing region, the capacity retention rate is improved. In addition, it was found that the increase in IV resistance was suppressed. It is presumed that this is because diffusion of lithium ions from the opposing region to the non-facing region during storage was suppressed. In addition, it is presumed that the increase in IV resistance is suppressed because the specific surface area of the facing region that faces the positive electrode active material layer and occupies most of the area of the negative electrode active material layer is the same. Furthermore, when the ratio of specific surface area S2/specific surface area S1 is 0.7 or less, the effect of improving the capacity retention rate is reduced. It is presumed that the diffusion of lithium ions is effectively suppressed by setting the specific surface area S2/specific surface area S1 to 0.7 or less.

From the results of Examples 3 to 6, even if the ratio of the total area of the facing region and the non-facing region to the area of the facing region (area ratio in Table 1) is changed, the capacity retention rate is improved and the IV resistance is increased. It can be seen that a suppressing effect is obtained. However, the larger the area ratio, the smaller the effect of improving the capacity retention rate. When the area ratio is in the range of 1.025 to 1.2, it can be seen that the above effects are exhibited more favorably.

From the results of Comparative Examples 1 and 4, and the results of Comparative Examples 3 and 5, when a mixed material of graphite and silicon oxide is used as the negative electrode active material, the capacity is higher than when graphite is used. It can be seen that the retention rate is low. This is because the negative electrode active material contains silicon oxide, which is an alloy-based negative electrode active material, so that the potential difference between the facing region and the non-facing region increases, and more lithium ions diffuse into the non-facing region. guessed.
Further, from the results of Comparative Examples 4 and 5, it can be seen that the smaller the specific surface area of the negative electrode active material layer, the higher the capacity retention rate and the higher the IV resistance, as in the case of using graphite as the negative electrode active material. .
From the results of Comparative Example 4 and Example 7, even when the negative electrode active material layer contains silicon oxide, which is an alloy-based negative electrode active material, the specific surface area S1 of the facing region is larger than the specific surface area S2 of the non-facing region. By doing so, it was found that the capacity retention rate was improved while suppressing an increase in IV resistance.

Specific examples of the technology disclosed herein have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology disclosed herein includes various modifications and alterations of the above specific examples.

10 positive electrode plate (positive electrode)
12 positive electrode core 12t positive electrode tab 14 positive electrode active material layer 16 protective layer 20 negative electrode plate (negative electrode)
22 Negative electrode core 22t Negative electrode tab 24 Negative electrode active material layer 24a Opposed region 24b Non-opposed region 30 Separator 40 Wound electrode body (electrode body)
42 positive electrode tab group 44 negative electrode tab group 50 battery case 60 positive electrode terminal 65 negative electrode terminal 70 positive electrode current collector 75 negative electrode current collector 100 non-aqueous electrolyte secondary battery 200 slot die 201 upper slit 202 lower slit

Claims (3)

A non-aqueous electrolyte secondary battery comprising a positive electrode having a positive electrode active material layer on a positive electrode core and a negative electrode having a negative electrode active material layer on a negative electrode core,
The negative electrode active material layer has a facing region facing the positive electrode active material layer and a non-facing region not facing the positive electrode active material layer,
A relationship of S2<S1, where S1 is the specific surface area of the negative electrode active material contained in the facing region based on the BET method, and S2 is the specific surface area of the negative electrode active material contained in the non-facing region based on the BET method. satisfy the
A non-aqueous electrolyte secondary battery, wherein (A1+A2)/A1 is 1.025 or more and 1.2 or less, where A1 is the area of the facing region and A2 is the area of the non-facing region. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein said specific surface area S1 and said specific surface area S2 satisfy a relationship of S2/S1≤0.7. 3. The non-aqueous electrolyte secondary battery according to claim 1, wherein said negative electrode active material layer contains an alloy-based negative electrode active material.

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