阅读说明：本技术 锂二次电池 (Lithium secondary battery ) 是由 蚊野聪 宫前亮平 石井圣启 名仓健祐 于 2019-05-23 设计创作，主要内容包括：本公开在使用卷绕式电极群的锂二次电池中抑制与充电相伴的负极膨胀。该锂二次电池具备电极群和非水电解质,电极群是正极、负极(134)以及隔板卷绕而成的,正极包含含锂的正极活性物质,负极(134)包含负极集电体(132)和负极集电体(132)上的多个凸部(133)。负极(134)在充电时析出锂金属,锂金属在放电时溶解于非水电解质中。负极集电体(132)包含朝向电极群的卷绕外侧方向的第1表面和朝向电极群的卷绕内侧方向的第2表面。第1表面和/或第2表面包含第1区域(OW)和相比于第1区域(OW)更接近电极群的卷绕最内周的第2区域(IW)。多个凸部(133)包含第1区域(OW)上的多个外周侧凸部和第2区域(IW)上的多个内周侧凸部。多个外周侧凸部的第1平均高度小于多个内周侧凸部的第2平均高度。(Disclosed is a lithium secondary battery using a coiled electrode group, wherein negative electrode swelling associated with charging is suppressed. The lithium secondary battery comprises an electrode group and a non-aqueous electrolyte, wherein the electrode group is formed by winding a positive electrode, a negative electrode (134) and a separator, the positive electrode comprises a positive electrode active material containing lithium, and the negative electrode (134) comprises a negative electrode current collector (132) and a plurality of convex parts (133) on the negative electrode current collector (132). The negative electrode (134) deposits lithium metal during charging, and the lithium metal dissolves in the nonaqueous electrolyte during discharging. The negative electrode collector (132) includes a 1 st surface facing the outside direction of the electrode group in the winding direction and a 2 nd surface facing the inside direction of the electrode group in the winding direction. The 1 st surface and/or the 2 nd surface include a 1 st region (OW) and a 2 nd region (IW) closer to the innermost periphery of the winding of the electrode group than the 1 st region (OW). The plurality of projections (133) include a plurality of outer circumferential projections in the 1 st region (OW) and a plurality of inner circumferential projections in the 2 nd region (IW). The 1 st average height of the plurality of outer peripheral side protrusions is smaller than the 2 nd average height of the plurality of inner peripheral side protrusions.)

1. A lithium secondary battery comprising an electrode group and a lithium ion conductive nonaqueous electrolyte, wherein the electrode group is formed by winding a positive electrode, a negative electrode and a separator interposed between the positive electrode and the negative electrode,

The positive electrode includes a positive electrode active material containing lithium, the negative electrode includes a negative electrode current collector and a plurality of projections arranged on the negative electrode current collector,

the negative electrode precipitates lithium metal during charging, the lithium metal is dissolved in the nonaqueous electrolyte during discharging,

the negative electrode current collector includes a 1 st surface facing a winding outer direction of the electrode group and a 2 nd surface facing a winding inner direction of the electrode group,

At least one of the 1 st surface and the 2 nd surface includes a 1 st region and a 2 nd region, the 2 nd region being closer to a winding innermost circumference of the electrode group than the 1 st region,

The plurality of convex parts comprise a plurality of outer periphery convex parts arranged on the 1 st area and a plurality of inner periphery convex parts arranged on the 2 nd area,

The 1 st average height of the plurality of outer peripheral side protrusions is smaller than the 2 nd average height of the plurality of inner peripheral side protrusions.

2. the lithium secondary battery according to claim 1,

At least one of the 1 st surface and the 2 nd surface, the 1 st region being farther from the innermost circumference than a center line in a longitudinal direction,

The 2 nd region is proximate to the innermost periphery compared to the centerline at least one of the 1 st surface and the 2 nd surface.

3. The lithium secondary battery according to claim 1 or 2,

The difference between the 1 st average height and the 2 nd average height is 3% or more and 150% or less of the 1 st average height.

4. The lithium secondary battery according to any one of claims 1 to 3,

The negative electrode current collector is provided with a copper foil or a copper alloy foil.

5. The lithium secondary battery according to any one of claims 1 to 4,

The plurality of protrusions are respectively in contact with the separator,

During the charging, the lithium metal is precipitated in a space between the negative electrode current collector and the separator.

6. The lithium secondary battery according to any one of claims 1 to 5,

The material of the plurality of projections is different from the material of the negative electrode current collector.

7. The lithium secondary battery according to any one of claims 1 to 6,

The plurality of projections are made of a resin material.

8. The lithium secondary battery according to any one of claims 1 to 5,

The plurality of projections and the negative electrode current collector are integrally formed of the same material.

9. the lithium secondary battery according to any one of claims 1 to 8,

The 1 st and 2 nd surfaces comprise the 1 st and 2 nd regions respectively,

The plurality of outer circumference-side protrusions include a plurality of 1 st protrusions disposed on the 1 st surface and a plurality of 2 nd protrusions disposed on the 2 nd surface,

The plurality of inner peripheral-side protrusions include a plurality of 1 st protrusions disposed on the 1 st surface and a plurality of 2 nd protrusions disposed on the 2 nd surface.

10. the lithium secondary battery according to claim 9,

The projection shapes of the plurality of 1 st convex parts contained in the plurality of outer periphery convex parts and the projection shapes of the plurality of 1 st convex parts contained in the plurality of inner periphery convex parts to the 1 st surface are respectively linear,

The projection shapes of the plurality of 2 nd convex parts included in the plurality of outer periphery convex parts and the projection shapes of the plurality of 2 nd convex parts included in the plurality of inner periphery convex parts to the 2 nd surface are respectively linear,

a minimum value of a distance between adjacent 21 st convex portions among the plurality of 1 st convex portions included in the plurality of outer peripheral side convex portions and the plurality of 1 st convex portions included in the plurality of inner peripheral side convex portions is larger than a maximum width of the adjacent 21 st convex portions,

Among the plurality of 2 nd convex portions included in the plurality of outer-peripheral-side convex portions and the plurality of 2 nd convex portions included in the plurality of inner-peripheral-side convex portions, a minimum value of a distance between adjacent 2 nd convex portions is larger than a maximum width of the adjacent 2 nd convex portions.

11. The lithium secondary battery according to claim 9 or 10,

The ratio of the total A _1X of the projected area of the 1 st convex parts contained in the plurality of outer peripheral convex parts and the 1 st convex parts contained in the plurality of inner peripheral convex parts to the 1 st surface, namely (A _1X/A ₁) × 100%, in the area A ₁ of the 1 st surface is 0.2% to 70%,

The ratio of the sum of the areas a _2X of the 2 nd convex parts included in the plurality of outer peripheral convex parts and the 2 nd convex parts included in the plurality of inner peripheral convex parts projected onto the 2 nd surface to the area a ₂ of the 2 nd surface, that is, (a _2X/a ₂) × 100% is 0.2% to 70%.

12. the lithium secondary battery according to any one of claims 1 to 11,

The 2 nd average height is 15 μm or more and 120 μm or less.

13. the lithium secondary battery according to any one of claims 1 to 12,

The non-aqueous electrolyte contains lithium ions and anions,

The anion comprises at least one selected from the group consisting of PF ₆ ^- , imide-based anions, and oxalate-based anions.

Technical Field

The present disclosure relates to a lithium secondary battery including a lithium ion conductive nonaqueous electrolyte.

background

Nonaqueous electrolyte secondary batteries are used for ICT applications such as personal computers and smart phones, for vehicles, for power storage, and the like. In such applications, nonaqueous electrolyte secondary batteries are required to have further high capacity. As a high-capacity nonaqueous electrolyte secondary battery, a lithium ion battery is known. The capacity of the lithium ion battery can be increased by using an alloy active material such as graphite and a silicon compound as a negative electrode active material. However, the capacity of lithium ion batteries is increasing to a limit.

Lithium secondary batteries are expected to be a promising candidate for high-capacity nonaqueous electrolyte secondary batteries exceeding lithium ion batteries. In a lithium secondary battery, lithium metal is precipitated on a negative electrode during charging, and the lithium metal is dissolved in a nonaqueous electrolyte during discharging.

In order to suppress the deterioration of battery characteristics due to the dendritic precipitation of lithium metal, studies have been made on the lithium secondary battery in which the shape of the negative electrode current collector is improved. For example, patent document 1 proposes that the ten-point average roughness Rz of the lithium metal deposition surface of the negative electrode current collector be 10 μm or less. Patent document 2 proposes that a negative electrode including a porous metal current collector and lithium metal inserted into pores of the current collector be used in a lithium secondary battery. Patent document 3 proposes a lithium metal polymer secondary battery using a negative electrode current collector having a surface formed with a plurality of concave grooves recessed in a predetermined shape.

Prior art documents

patent document 1: japanese patent laid-open No. 2001 and 243957

Patent document 2: japanese laid-open patent publication No. 2016-527680

Patent document 3: japanese patent laid-open publication No. 2006 and 156351

Disclosure of Invention

Embodiments of the present disclosure provide a lithium secondary battery including a wound electrode group in which negative electrode swelling during charging is suppressed.

A lithium secondary battery according to an aspect of the present disclosure includes an electrode group formed by winding a positive electrode including a positive electrode active material containing lithium, a negative electrode including a negative electrode current collector and a plurality of projections arranged on the negative electrode current collector, and a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte having lithium ion conductivity. The negative electrode precipitates lithium metal during charging, and the lithium metal dissolves in the nonaqueous electrolyte during discharging. The negative electrode current collector includes a 1 st surface and a 2 nd surface, the 1 st surface faces a winding outer side direction of the electrode group, and the 2 nd surface faces a winding inner side direction of the electrode group. At least one of the 1 st surface and the 2 nd surface includes a 1 st region and a 2 nd region, and the 2 nd region is closer to a winding innermost circumference of the electrolytic group than the 1 st region. The plurality of convex portions include a plurality of outer circumferential convex portions disposed on the 1 st region and a plurality of inner circumferential convex portions disposed on the 2 nd region. The 1 st average height of the plurality of outer peripheral side protrusions is smaller than the 2 nd average height of the plurality of inner peripheral side protrusions.

According to the embodiment of the present disclosure, in the lithium secondary battery using the coiled electrode group, the negative electrode swelling associated with the charging can be suppressed.

Drawings

Fig. 1 is a plan view schematically showing a negative electrode for a lithium secondary battery according to an embodiment of the present disclosure.

Fig. 2 is a sectional view as viewed in the direction of the arrow along the line II-II of fig. 1.

Fig. 3 is a longitudinal sectional view schematically showing a lithium secondary battery according to another embodiment of the present disclosure.

Fig. 4 is an enlarged sectional view schematically showing the region IV of fig. 3.

Fig. 5 is an enlarged sectional view schematically showing a V region of fig. 3.

Description of the reference numerals

10 lithium secondary battery

11 positive electrode

12 negative electrode

13 baffle

14 electrode group

15 casing body

16 sealing body

17. 18 insulating board

19 positive electrode lead

20 cathode lead

21 step part

22 filter

23 lower valve body

24 insulating member

25 upper valve body

26 cap

27 sealing gasket

30 positive electrode current collector

31 positive electrode mixture layer

32. 132 negative electrode collector

132a, 132b strip-shaped region

33a 1 st projection

33b 2 nd projection

133 convex part

134 negative electrode

35 space

S1 surface No. 1

S2 surface 2

1 st length direction of LD1

LD2 length 2

LD31, LD32 length direction 3

WD1 Width 1 st direction

CL 4 th center line

IW inner peripheral side winding part

OW outer circumference side winding part

Detailed Description

(insight underlying the present disclosure)

Embodiments of the present disclosure relate to a lithium secondary battery including a wound electrode group and a lithium metal as a negative electrode active material. More specifically, embodiments of the present disclosure relate to improvements in negative electrode collectors in wound electrode groups. Further, the lithium secondary battery is sometimes referred to as a lithium metal secondary battery. In a lithium secondary battery, lithium metal may be precipitated in a dendritic form in a negative electrode during charging. Further, the specific surface area of the negative electrode increases with the formation of dendrites, and side reactions increase. Therefore, the discharge capacity and the cycle characteristics are easily degraded. On the other hand, patent document 1 teaches that the ten-point average roughness Rz of the lithium metal deposition surface of the negative electrode is 10 μm or less to suppress the generation of dendrites and to obtain high charge/discharge efficiency.

In addition, since lithium metal is deposited on the negative electrode during charging, the lithium secondary battery is particularly likely to have a large amount of swelling of the negative electrode. Here, "expansion of the negative electrode" means that the total volume of the negative electrode volume and the deposited lithium metal volume increases. In particular, when lithium metal is precipitated in a dendritic form, the amount of expansion further increases. In the case of a cylindrical lithium battery including a wound electrode group, stress is generated due to excessive expansion of the negative electrode. In order to absorb the volume change of the negative electrode during charge and discharge, patent document 2 proposes to use a porous negative electrode current collector of copper or nickel having a porosity of 50 to 99% and a pore size of 5 to 500 μm, for example. In the negative electrode current collector of patent document 3, a groove is provided to secure a space for forming the dendritic lithium metal.

The stress associated with the deposition of lithium metal is released from the main surface, side surface, and the like of the negative electrode in the coin-type electrode group, and is released from the end portion, and the like of the negative electrode in the laminated electrode group. On the other hand, in the coiled electrode group, as lithium metal is deposited, stress due to tensile strain is generated in the circumferential direction of the cross section perpendicular to the coiling axis of the electrode group. In the wound electrode group, stress caused by deposition of lithium metal is hard to be released from the inner peripheral side and the negative electrode end portion of the electrode group, and therefore, the stress is directed toward the outer peripheral side of the electrode group. Further, when the wound end of the wound electrode group is fixed by a tape and surrounded by the battery case, stress is applied from the outside. In this way, in the wound electrode group, stress is less likely to be dispersed than in other electrode groups such as coin-type or laminated-type, and therefore excessive expansion or uneven expansion of the negative electrode is likely to occur.

The negative electrode collector of the wound electrode group has a 1 st surface facing the outside of the electrode group in the wound state and a 2 nd surface facing the inside of the electrode group in the wound state. That is, the 1 st surface faces a direction away from the winding axis of the electrode group with respect to the negative electrode collector, and the 2 nd surface faces a direction close to the winding axis of the electrode group with respect to the negative electrode collector. Hereinafter, in the negative electrode collector, the side facing the outside of the electrode group in the winding direction may be referred to as the outside, and the side facing the inside of the electrode group in the winding direction may be referred to as the inside. In addition, if at least one of the outer and inner surfaces of the negative electrode current collector includes a 1 st region and a 2 nd region closer to the innermost wound periphery of the electrode group than the 1 st region, in the electrode group, a portion including the 1 st region is referred to as an outer peripheral wound portion, and a portion including the 2 nd region is referred to as an inner peripheral wound portion.

Due to the stress toward the outer peripheral side and the stress from the outside of the electrode group as described above, the pressure applied to the surface of the negative electrode collector is greater in the outer peripheral wound part than in the inner peripheral wound part of the electrode group. Hereinafter, the pressure applied to the surface of the negative electrode current collector may be referred to as a surface pressure. In the wound electrode group, as described above, a large stress and a large surface pressure are applied to the wound portion on the outer peripheral side of the electrode group. Therefore, the lithium metal deposited on the negative electrode surface of the outer peripheral winding portion is pressed more strongly than the lithium metal deposited on the negative electrode surface of the inner peripheral winding portion. Due to this large pressure, the lithium metal deposited on the surface of the negative electrode is compressed in the wound portion on the outer peripheral side of the electrode group. On the other hand, in the inner circumferential wound portion of the electrode group, the lithium metal deposited on the surface of the negative electrode is less likely to be compressed, and the thickness of the lithium metal becomes larger than that of the outer circumferential wound portion.

Due to such a difference in stress and surface pressure between the inner-peripheral wound portion and the outer-peripheral wound portion of the electrode group, lithium metal deposition on the surface of the negative electrode tends to become uneven, and therefore the negative electrode may partially expand excessively. In addition, charge and discharge efficiency may be reduced.

In the negative electrode current collector of patent document 2 or patent document 3, lithium metal is precipitated in the pores or the spaces in the grooves by charging. Patent documents 2 and 3 basically assume a laminated or coin-type electrode group. Therefore, the lithium metal in the pores or in the grooves is less likely to be subjected to the pressure generated in the electrode group. Even if the negative electrode current collector of patent document 2 or patent document 3 is assumed to be used for a wound electrode group, uneven deformation is likely to occur due to winding. As a result, stress applied to the deposited lithium metal becomes uneven, and therefore expansion of the negative electrode during charging tends to become uneven. Therefore, it is difficult to sufficiently suppress the expansion of the negative electrode during charging. In addition, lithium metal in the pores or in the grooves is less likely to be pressed, and therefore is likely to be peeled off from the wall surface of the current collector. The peeled lithium metal is not dissolved at the time of discharge, and thus the charge and discharge efficiency is lowered.

the inventors have intensively studied to solve the above problems, and as a result, have conceived a lithium secondary battery according to the present disclosure. A lithium secondary battery according to an aspect of the present disclosure includes an electrode group in which a positive electrode including a positive electrode active material containing lithium, a negative electrode including a negative electrode current collector and a plurality of protrusions arranged on the negative electrode current collector, and a separator interposed between the positive electrode and the negative electrode are wound, and a nonaqueous electrolyte having lithium ion conductivity. The negative electrode precipitates lithium metal during charging, and the lithium metal dissolves in the nonaqueous electrolyte during discharging. The 1 st average height of the plurality of projections (an example of the plurality of outer-peripheral projections) of the outer-peripheral wound portion of the electrode group is smaller than the 2 nd average height of the plurality of projections (an example of the plurality of inner-peripheral projections) of the inner-peripheral wound portion of the electrode group. That is, the 2 nd average height of the plurality of convex portions of the inner circumferential wound part of the electrode group is larger than the 1 st average height of the plurality of convex portions of the outer circumferential wound part of the electrode group

According to the aspect of the present disclosure, a negative electrode including a negative electrode current collector and a plurality of protrusions arranged on the negative electrode current collector is used in the wound electrode group. Since the plurality of projections can secure a space for lithium metal deposition in the negative electrode, the change in the apparent volume of the negative electrode due to lithium metal deposition can be reduced. Here, the apparent volume of the negative electrode means the total volume of the negative electrode volume, the volume of the deposited lithium metal, and the volume of the space secured by the plurality of projections. By making the 2 nd average height of the plurality of convex portions of the inner peripheral wound portion of the electrode group larger than the 1 st average height of the plurality of convex portions of the outer peripheral wound portion of the electrode group, even if the thickness of the lithium metal deposited in the inner peripheral wound portion due to charging becomes large, the increase in volume can be effectively absorbed. Thus, the increase in the apparent volume of the anode can be further suppressed.

In this way, in each of the outer-peripheral wound portion and the inner-peripheral wound portion of the electrode group, a volume space suitable for the thickness of lithium metal deposited by charging can be secured in advance by the plurality of convex portions. Therefore, it is not necessary to estimate the expansion of the negative electrode, and thus it is not necessary to reduce the volume of the negative electrode and/or the positive electrode at an early stage. As a result, a high discharge capacity is easily ensured. In addition, even if lithium metal is produced in a dendrite form, it can be accommodated in a space formed in the negative electrode by the plurality of projections. Since the electrode group is wound, a certain degree of pressure is applied to the lithium metal deposited in the space of the negative electrode. Therefore, unlike the cases of patent documents 2 and 3, the lithium metal deposited in the space is difficult to be peeled off. Therefore, the charge-discharge efficiency can be suppressed from decreasing. Further, since a suitable pressure is applied to the deposited lithium metal, the lithium metal can be prevented from being deposited in a dendritic form without smoothing the negative electrode as in patent document 1.

for example, on each of the outer surface and the inner surface (i.e., the 1 st surface and the 2 nd surface) of the negative electrode current collector, a region facing the positive electrode active material is divided into 2 parts by a center line in the longitudinal direction of the region. Further, for example, a portion farther from the innermost wound periphery of the electrode group than the center line is defined as a 1 st region located in the outer peripheral wound part of the electrode group, and a portion closer to the innermost wound periphery of the electrode group than the center line is defined as a 2 nd region located in the inner peripheral wound part of the electrode group. The average heights of the plurality of projections arranged in the 1 st region and the 2 nd region are set to the 1 st average height and the 2 nd average height, respectively. The longitudinal direction of the region of the negative electrode current collector facing the positive electrode active material is parallel to the longitudinal direction of the negative electrode current collector. In each of the 1 st surface and the 2 nd surface of the negative electrode current collector, a direction perpendicular to a winding axis of the electrode group is defined as a longitudinal direction of the negative electrode current collector, and a direction parallel to the winding axis is defined as a width direction of the negative electrode current collector. Hereinafter, the longitudinal direction of the negative electrode current collector is referred to as the 1 st longitudinal direction, and the width direction thereof is referred to as the 1 st width direction. In addition, at 2 ends in the longitudinal direction of the negative electrode current collector, a line connecting the respective widthwise centers is defined as a 1 st center line.

the 1 st average height can be obtained by, for example, arbitrarily selecting 10 convex portions on the 1 st region in a cross-sectional photograph of the negative electrode in the thickness direction, calculating the distance from the end portion on the negative electrode current collector side of each of the selected convex portions to the end portion on the side opposite to the negative electrode current collector as the height of the convex portion, and averaging the heights of the convex portions, and the 1 st average height can be obtained by cutting a certain area (for example, 5cm ² or the like) or a plurality of arbitrary regions in the 1 st region of the negative electrode current collector, and averaging the heights of a plurality of arbitrary convex portions existing within the certain area or the plurality of regions.

the 2 nd average height may be similarly obtained by, for example, arbitrarily selecting 10 convex portions on the 2 nd region in a cross-sectional photograph of the negative electrode in the thickness direction, calculating the distance from the end portion on the negative electrode current collector side of each of the selected convex portions to the end portion on the side opposite to the negative electrode current collector as the height of the convex portions, and averaging the heights of the convex portions, and the 2 nd average height may be obtained by cutting a certain area (for example, 5cm ² or the like) or an arbitrary plurality of regions in the 2 nd region of the negative electrode current collector, and averaging the heights of an arbitrary plurality of convex portions existing in the certain area or the plurality of regions.

In the measurement of the 1 st average height and the 2 nd average height, when the end portion of each convex portion on the negative electrode current collector side and/or the end portion on the opposite side are/is not flat, the maximum value of the length between the two end portions of each convex portion in the direction parallel to the thickness direction of the negative electrode is defined as the height of each convex portion. In the measurement of the 1 st average height and the 2 nd average height, when a plurality of protrusions are formed on both the 1 st surface and the 2 nd surface of the negative electrode current collector, the 10 protrusions are arbitrarily selected from the plurality of protrusions formed on both the 1 st surface and the 2 nd surface. The 1 st average height and the 2 nd average height can be obtained based on a cross-sectional photograph of the electrode group in which a cross section in the thickness direction of the negative electrode can be observed.

In the case where the 1 st surface and/or the 2 nd surface are rough, the surface roughness Rz of the 1 st surface and/or the 2 nd surface may be 1 μm or less. In addition, the height of each of the plurality of protrusions on the 1 st protrusion and/or the 2 nd surface may exceed 1 μm. When the 1 st surface and/or the 2 nd surface are rough and the plurality of projections and the negative electrode current collector are integrally composed of the same material, the height of each of the plurality of projections on the 1 st surface and/or the 2 nd surface is measured with reference to the bottom of the roughness. In this case, the winding of the electrode group was unwound, and the 1 st surface and the 2 nd surface were measured in a state of being stretched in a planar state.

The negative electrode current collector generally has a 1 st surface and a 2 nd surface opposite to the 1 st surface. The 1 st surface and the 2 nd surface mean 2 main surfaces of the sheet-like negative electrode collector. The plurality of projections are disposed on the 1 st surface side and/or the 2 nd surface side. The 1 st surface is an outer surface of the negative electrode current collector in the electrode group. The 2 nd surface is an inner side surface of the negative electrode collector in the electrode group. The plurality of convex portions may include a plurality of 1 st convex portions disposed on the 1 st surface side, which is the outer side, or a plurality of 2 nd convex portions disposed on the 2 nd surface side, which is the inner side. That is, the 1 st surface or the 2 nd surface may contain the 1 st region and the 2 nd region.

In both the 1 st surface and the 2 nd surface of the negative electrode current collector and the vicinity thereof, the plurality of projections may include both the plurality of 1 st projections and the plurality of 2 nd projections from the viewpoint of ensuring a space for deposition of lithium metal during charging. That is, the 1 st surface and the 2 nd surface may include the 1 st region and the 2 nd region, respectively. Further, each 1 st projection projects from the 1 st surface side of the negative electrode current collector toward the separator surface opposite to the 1 st surface. Each 2 nd projection projects from the 2 nd surface side of the negative electrode current collector toward the separator surface opposite to the 2 nd surface. From the viewpoint of easily securing a higher discharge capacity while maintaining an excellent expansion suppressing effect of the negative electrode, the difference between the 1 st average height and the 2 nd average height may be 3% or more and 150% or less of the 1 st average height. Further, the difference between the 1 st average height and the 2 nd average height is a value obtained by subtracting the 1 st average height from the 2 nd average height.

The plurality of protrusions may be in contact with the separator, respectively. For example, the plurality of 1 st protrusions may contact a surface of the separator opposite to the 1 st surface. In addition, the plurality of 2 nd protrusions may contact a surface of the separator opposite to the 2 nd surface. By the presence of the plurality of projections, a space is ensured between the negative electrode and the separator. Therefore, lithium metal is precipitated in the space by charging. The effect of the relationship between the 1 st average height and the 2 nd average height is remarkably exhibited by the deposition of lithium metal in such a space. Therefore, even if the thickness of the lithium metal deposited on the inner peripheral side of the electrode group is increased, the apparent volume change of the negative electrode can be suppressed. Further, the protruding portion is in contact with the separator, so that deposition of lithium metal at the separator-side tip of the protruding portion can be suppressed. Therefore, local expansion of the negative electrode can also be suppressed.

Hereinafter, the structure of the lithium secondary battery according to the above-described embodiment will be described in more detail with reference to the drawings as appropriate. First, the structure of the negative electrode will be described.

(cathode)

The negative electrode includes a negative electrode current collector and a plurality of projections arranged on the negative electrode current collector. The negative electrode current collector generally has a 1 st surface and a 2 nd surface opposite to the 1 st surface. The 1 st surface and the 2 nd surface are the outer surface and the inner surface of the negative electrode collector in the wound electrode group, respectively. In the negative electrode of the lithium secondary battery, lithium metal is precipitated by charging. More specifically, lithium ions contained in the nonaqueous electrolyte receive electrons in the negative electrode due to charging, become lithium metal, and are deposited on the surface of the negative electrode. The precipitated lithium metal is dissolved as lithium ions in the nonaqueous electrolyte by the discharge. The lithium ions contained in the nonaqueous electrolyte may be derived from a lithium salt added to the nonaqueous electrolyte, may be supplied from the positive electrode active material by charging, or may be both of them.

Since the negative electrode has a plurality of projections, a space for accommodating lithium metal deposited on the surface of the negative electrode can be secured. Therefore, the expansion of the negative electrode due to the deposition of lithium metal can be reduced by this space. In addition, the 2 nd average height of the plurality of convex portions of the inner circumferential wound portion of the electrode group is made larger than the 1 st average height of the plurality of convex portions of the outer circumferential wound portion of the electrode group. As a result, even if the thickness of the lithium metal deposited on the inner peripheral side winding portion of the electrode group is increased as described above, the volume change caused by the increase in the thickness can be absorbed. Thus, an increase in the apparent volume of the anode can be suppressed. Further, since a certain degree of pressure is applied to the deposited lithium metal in the coiled electrode group, peeling of the lithium metal can be suppressed, and as a result, a decrease in charge-discharge efficiency can be suppressed.

The 2 nd average height may be larger than the 1 st average height, and the difference between the 1 st average height and the 2 nd average height may be adjusted according to the energy density, size, and the like of the battery. The difference between the 1 st average height and the 2 nd average height may be 3% or more of the 1 st average height, or may be 10% or more or 20% or more. Further, the difference between the 1 st average height and the 2 nd average height may be 30% or more of the 1 st average height, or may be 40% or more. When the difference is within such a range, even if the thickness of lithium metal deposited on the inner peripheral side of the electrode group is increased, the volume change of the negative electrode due to the deposition is easily absorbed. The difference between the 1 st average height and the 2 nd average height is, for example, 150% or less, 120% or less, or 60% or less or 50% or less of the 1 st average height. When the difference is within such a range, it is easy to secure a volume space suitable for the amount of precipitated lithium, and therefore it is easy to secure a higher discharge capacity while maintaining the expansion suppressing effect of the negative electrode. These lower and upper limits may be arbitrarily combined.

The height of each projection may be determined by the position where the projection is formed and the amount of lithium metal deposited. The 2 nd average height may be 15 μm or more, 20 μm or more, or 30 μm or more. The 2 nd average height may be 40 μm or more, or 50 μm or more. When the average height of 2 nd is within these ranges, the effect of absorbing the volume change of the negative electrode accompanying the deposition of lithium metal can be further improved. The effect of suppressing damage to the electrode can be improved. The 2 nd average height may be 120 μm or less, or may be 110 μm or less. The 2 nd average height may be 100 μm or less, or 90 μm or less. When the 2 nd average height is within these ranges, the lithium metal deposited on the surface of the negative electrode in the inner circumferential winding portion of the electrode group is moderately pressed by the separator, and the conductivity between the lithium metal and the negative electrode current collector is improved, so that the charge/discharge efficiency can be improved. In addition, excessive pressing of the separator against the convex portion is suppressed, and the electrode can be protected. These lower limit values and upper limit values may be arbitrarily combined.

There is no particular limitation as long as the 1 st average height is less than the 2 nd average height. For example, the 1 st average height may be determined in such a manner that the 2 nd average height and/or the difference between the 1 st average height and the 2 nd average height becomes the above range. At least a portion of the plurality of protrusions may contact the separator. In this case, lithium metal is precipitated by charging in a space formed between the negative electrode current collector and the separator. The convex portion contacts the separator, whereby the influence of the relationship between the 1 st average height and the 2 nd average height is remarkably exhibited, and the effect of suppressing the expansion of the negative electrode can be improved. In addition, since precipitation of lithium metal is suppressed at a portion where each convex portion contacts the separator, such as the tip of the convex portion, local expansion of the negative electrode can be suppressed.

From the viewpoint of further improving the effect of suppressing swelling of the negative electrode, 80% or more of the total a _1X of the areas of the plurality of 1 st projections projected onto the 1 st surface may be in contact with the separator, from the same viewpoint, 80% or more of the total a _2X of the areas of the plurality of 2 nd projections projected onto the 2 nd surface may be in contact with the separator, from the same viewpoint, all of the plurality of 1 st projections and/or all of the plurality of 2 nd projections may be in contact with the separator, further, the total a _1X of the areas of the plurality of 1 st projections projected onto the 1 st surface is the total of the areas of the projected shapes formed when the plurality of 1 st projections are projected onto the 1 st surface in the thickness direction of the negative electrode current collector, and similarly, the total a _2X of the areas of the plurality of 2 nd projections projected onto the 2 nd surface is the total of the areas of the projected shapes formed when the plurality of 2 nd projections are projected onto the 2 nd surface in the thickness direction of the negative electrode current collector.

The ratio (A _1X/A ₁) × 100% of the total area A _1X projected onto the 1 st surface of the plurality of 1 st protrusions to the area A ₁ of the 1 st surface may be 0.2% or more, 1% or more, or 3% or more, the ratio (A _2X/A ₂) × 100% of the total area A _2X projected onto the 2 nd surface of the plurality of 2 nd protrusions to the area A ₂ of the 2 nd surface may be 0.2% or more, 1% or more, or 3% or more.

The ratio of the total area a _1X projected onto the 1 st surface of the plurality of 1 st protrusions to the 1 st surface area a ₁ (a _1X/a ₁) × 100% may be 70% or less, or may be 50% or less, the ratio of the total area a _2X projected onto the 2 nd surface area a ₂ (a _2X/a ₂) × 100% may be 70% or less, or 50% or less, when the above ratio is in this range, a space is easily secured between the negative electrode current collector surface and the separator, and therefore, a higher discharge capacity can be secured while suppressing negative electrode expansion accompanying lithium metal precipitation, these lower limit values and upper limit values may be arbitrarily combined, and in the calculation of a ₁, a _1X, a ₂, and a _2X, a region of the negative electrode surface not facing the positive electrode current collector may not be taken into account, that the 1 st surface and the 2 nd surface do not include the negative electrode active material, and therefore, the regions of the negative electrode collector surface facing the negative electrode ₁, the negative electrode collector surface 8536 a, the positive electrode collector area _2X a, the collector area of the negative electrode 6336, the negative electrode area of.

For example, in a wound electrode group, the area of the negative electrode collector in a band shape extending in the longitudinal direction perpendicular to the winding axis does not face the positive electrode active material in the case where the area of the negative electrode collector in the outer periphery of the winding is not considered in the calculation of the area a ₁ of the 1 st surface and the total area a _1X of the 1 st projection because lithium metal is difficult to deposit in the outer periphery of the winding, and the area of the negative electrode collector in the inner periphery of the winding is not considered in the calculation of the area a ₂ of the 2 nd surface and the total area a _2X of the 2 nd projection in the case where lithium metal is difficult to deposit in the inner periphery of the negative electrode collector in the direction parallel to the winding axis, and the area of the negative electrode collector in a band shape extending in the longitudinal direction perpendicular to the winding axis does not face the positive electrode active material in the case where the width of the negative electrode collector in the direction parallel to the winding axis is larger than the width of the positive electrode collector.

The areas of a ₁, a _1X, a ₂, and a _2X may be determined for the negative electrode in which the 1 st surface and the 2 nd surface are spread in a flat state, or may be determined for the negative electrode before the wound electrode group is produced, or may be determined for the negative electrode taken out from the wound electrode group, each area may be partially calculated for a predetermined region, and the ratio of the areas determined based on the calculated values may be set as the ratio.

The shape of projection of each projection onto the surface of the negative electrode current collector is not particularly limited. The shape of projection of each convex portion on the surface of the negative electrode current collector may be linear or the like, from the viewpoint of easily supporting the separator and easily supplying the nonaqueous electrolyte to the vicinity of the electrode. Further, the linear shape also includes a bar shape. The stripe shape is a shape in which, among linear shapes, each convex portion projects on the surface of the negative electrode current collector, and the ratio of the length of the convex portion in the longitudinal direction to the length of the convex portion in the width direction (length in the longitudinal direction/length in the width direction) is relatively small. The shape of projection of each convex portion on the surface of the negative electrode current collector is a shape formed when each convex portion is projected in the thickness direction of the negative electrode current collector on the surface of the negative electrode current collector on which the convex portion is formed. Some layers may be formed between the negative electrode current collector and the projections, but the projection shape or the projection area may be determined by projecting the projections on the surface of the negative electrode current collector.

From the viewpoint of ensuring a suitable volume space for accommodating the precipitated lithium metal, 2 adjacent projections among the plurality of projections may be spaced apart from each other to some extent in a direction parallel to the surface of the negative electrode. For example, the minimum value of the separation distance between the adjacent 2 projections may be larger than the maximum width of the adjacent 2 projections. The minimum value of the distance between the adjacent 2 projections means the minimum value of the distances between the outer edges of the projected shapes of the adjacent 2 projections when the adjacent 2 projections are arbitrarily selected from the plurality of projections and projected onto the surface of the negative electrode current collector on the side where the projections are formed in the thickness direction of the negative electrode current collector. The maximum width of the adjacent 2 convex portions is the maximum value of the width of the shape of the projection of the adjacent 2 convex portions onto the surface of the negative electrode current collector on the side where these convex portions are formed. When the projected shape is a circle, the maximum width is the largest of the diameters of the projected shapes of the 2 projections.

The shape of the projection of the plurality of projections on the surface of the negative electrode current collector may be linear, and the longitudinal directions of the plurality of projections may be arranged substantially in parallel, for each of the 1 st surface and the 2 nd surface of the negative electrode current collector. In this case, the minimum value of the distance between the adjacent 2 projections may be larger than the maximum width of the adjacent 2 projections. In this case, the partition plate is easily supported by the plurality of projections, and an appropriate volume space is easily secured between the adjacent 2 projections. Hereinafter, the longitudinal direction of each projection having a linear projection shape is referred to as the 2 nd longitudinal direction. More specifically, at 2 ends in the longitudinal direction of the projection having the linear shape, a line connecting the centers in the width direction is defined as a 2 nd center line, and the direction of the 2 nd center line is defined as a 2 nd longitudinal direction. The state where the 2 nd longitudinal directions of the plurality of projections are arranged substantially in parallel means a case where the 2 nd longitudinal directions of the projections are parallel to each other, or an angle on an acute angle side formed by the 2 nd longitudinal directions of the projections is 30 ° or less. The projections may be projected in the thickness direction of the negative electrode current collector with respect to the surface of the negative electrode current collector on which the projections are formed, and the longitudinal direction of the projected shape formed at this time may be the 2 nd longitudinal direction of each projection.

When a plurality of linear projections are arranged substantially in parallel, the distance between the projections can be determined from the distance between the centers of 2 adjacent projections and the width of each projection. The center of the convex portion in this case is referred to as the 2 nd center line of the convex portion. The distance between the 2 nd center lines of the adjacent 2 convex portions may be defined as an inter-center distance. The direction perpendicular to the 2 nd longitudinal direction is referred to as the 2 nd width direction. In this case, the maximum width of the adjacent 21 st convex portions is the maximum value of the 2 nd width direction width in the shape of the projection of the adjacent 21 st convex portions onto the 1 st surface. In addition, the maximum width of the adjacent 2 nd convex portions means the maximum value of the 2 nd width direction width in the shape of the projection of the adjacent 2 nd convex portions onto the 2 nd surface. That is, the maximum width of the adjacent 2 convex portions is set to be larger between the maximum width in the 2 nd width direction of one of the 2 convex portions in the projected shape and the maximum width in the 2 nd width direction of the other projected shape.

The frame-like continuous projection surrounding the entire surface or a partial region of the surface may not be formed on the 1 st surface and/or the 2 nd surface so that the nonaqueous electrolyte can easily penetrate into the electrode group. The edge of the 1 st surface and/or the 2 nd surface may not be formed with a continuous frame-like projection surrounding most of the surfaces. In the case where the frame-like continuous projection is not formed, the nonaqueous electrolyte easily penetrates into the inside of the separator at the portion where the projection is not formed, and the separator easily contacts the precipitated lithium metal. Therefore, the effect of suppressing the uneven precipitation of lithium metal is improved, and therefore, the generation of dendrites can be suppressed, and the decrease in charge-discharge efficiency can be suppressed.

On the 1 st surface and/or the 2 nd surface, a band-shaped region where no convex portion is formed may be provided along at least one of the 1 st length direction and the 1 st width direction. Each surface may have at least 1 band-shaped region, or may have 2 or more. In this case, the nonaqueous electrolyte easily penetrates into the electrode group through the band-shaped region. Since the nonaqueous electrolyte can be easily held between the positive electrode and the negative electrode, precipitation and dissolution of lithium metal proceed smoothly, and a decrease in capacity and a decrease in charge-discharge efficiency can be suppressed. In addition, the separator easily contacts the precipitated lithium metal in the band-shaped region. This improves the effect of suppressing the uneven precipitation of lithium metal, and therefore, the generation of dendrites can be suppressed.

The stripe region may be formed along the 1 st length direction or the 1 st width direction. In addition, the anode current collector may have both a belt-shaped region along one of the 1 st lengthwise direction and the 1 st widthwise direction and a belt-shaped region along the other direction at the 1 st surface and/or the 2 nd surface. The strip-shaped region may be provided along the 1 st longitudinal direction from the viewpoint of easily permeating the nonaqueous electrolyte to the more inner peripheral side of the wound electrode group and easily securing high capacity and high charge/discharge efficiency. If a plurality of linear projections are provided on each surface of the negative electrode current collector, the projections project toward each surface of the negative electrode current collector, a band-shaped region is easily formed. In particular, if the convex portions are provided such that the 2 nd longitudinal direction of the plurality of convex portions is substantially parallel to the 1 st longitudinal direction, a band-shaped region is easily formed between the 2 convex portions adjacent in the 1 st width direction.

The negative electrode current collector may have a region on each of the 1 st surface and the 2 nd surface, in which a plurality of projections are not formed, for example, on the innermost peripheral side wound portion and/or the outermost peripheral side wound portion, as necessary. That is, in the negative electrode collector, the region where the 1 st projection and/or the 2 nd projection are not formed may be provided in the portion closest to the winding axis of the electrode group and/or the portion farthest from the winding axis of the electrode group. The negative electrode lead for electrical connection to the negative electrode may be connected to the 1 st surface or the 2 nd surface of the negative electrode current collector by, for example, welding or the like at a portion where the convex portion is not formed. At least one belt-shaped region in which no convex portion is formed may be provided in at least one of the 1 st longitudinal direction and the 1 st width direction on each of the 1 st surface and the 2 nd surface of the negative electrode collector. By providing the band-shaped region, the nonaqueous electrolyte easily penetrates into the electrode group through the region. This makes it possible to perform charge and discharge reactions in the entire electrode group, and thus a high capacity can be easily ensured.

The arrangement of the band-shaped region along the 1 st longitudinal direction means that the negative electrode current collector has a band-shaped region in which no convex portion is formed along a direction substantially parallel to the 1 st longitudinal direction. The arrangement of the band-shaped region along the 1 st width direction means that the negative electrode current collector has a band-shaped region in which no convex portion is formed along a direction substantially parallel to the 1 st width direction.

hereinafter, the longitudinal direction of the band-shaped region is referred to as the 3 rd longitudinal direction. More specifically, at 2 ends in the longitudinal direction of the belt-shaped region, a line connecting the respective widthwise centers is defined as a 3 rd center line, and the direction of the 3 rd center line is defined as a 3 rd longitudinal direction. The center of each end in the width direction may be determined, for example, with respect to a maximum rectangular band-shaped region that can be formed between the ends of adjacent convex portions. The above-mentioned direction substantially parallel to the 1 st longitudinal direction means a case where the 3 rd longitudinal direction is parallel to the 1 st longitudinal direction, and a case where an angle on an acute angle side formed by the 3 rd longitudinal direction and the 1 st longitudinal direction is 30 ° or less. The above-mentioned direction substantially parallel to the 1 st width direction means a case where the 3 rd longitudinal direction is parallel to the 1 st width direction and a case where the angle on the acute angle side formed by the 3 rd longitudinal direction and the 1 st width direction is 30 ° or less.

Fig. 1 is a plan view schematically showing a negative electrode for a lithium secondary battery according to an embodiment. Fig. 2 is a sectional view taken in the direction of the arrow along the line II-II of fig. 1. In fig. 1, one surface of the negative electrode is shown. The negative electrode 134 includes a negative electrode current collector 132 made of metal foil or the like, and a plurality of protrusions 133 protruding from the surface of the negative electrode current collector 132. The projection shape of the projection 133 on the surface of the negative electrode current collector 132 in the thickness direction of the negative electrode current collector 132 is linear, as is the planar shape of the projection 133 shown in fig. 1. In the case where the surface of the negative electrode current collector 132 shown in fig. 1 is the 1 st surface, the convex portion 133 is the 1 st convex portion, and in the case where the surface is the 2 nd surface, the convex portion 133 is the 2 nd convex portion.

When the surface is viewed from the normal direction, the negative electrode current collector 132 has a rectangular shape in which the length in the direction perpendicular to the winding axis when the electrode group is formed by winding is larger than the length in the direction parallel to the winding axis. In fig. 1, the surface of the negative electrode current collector 132 is indicated by the 1 st longitudinal direction LD1 in the direction perpendicular to the winding axis and by the 1 st width direction WD1 in the direction parallel to the winding axis.

In fig. 1, the plurality of projections 133 are provided on the surface of the negative electrode current collector 132 such that the 2 nd longitudinal direction LD2 of each projection 133 is parallel to the 1 st longitudinal direction LD 1. The negative electrode current collector 132 is divided into a region located at the inner circumferential wound part IW of the electrode group and a region located at the outer circumferential wound part OW, with the 4 th center line CL dividing the length of the region where the negative electrode current collector 132 faces the positive electrode active material in the 1 st longitudinal direction LD1 into two. As shown in fig. 2, a plurality of projections 133 having a larger height than the outer peripheral side winding portion OW are provided in the inner peripheral side winding portion IW. Therefore, the 2 nd average height of the plurality of convex portions 133 of the inner peripheral wound portion IW becomes larger than the 1 st average height of the plurality of convex portions 133 of the outer peripheral wound portion OW. Thus, even if the thickness of the lithium metal deposited on the inner circumferential side of the electrode group at the wound portion IW becomes large, the volume change caused by the increase in the thickness can be absorbed. Thus, the increase in the apparent volume of the negative electrode can be suppressed.

A belt-shaped region 132a in which the protruding portion 133 is not formed along the 1 st longitudinal direction LD1 is provided on the surface of the negative electrode 134. The 3 rd longitudinal direction LD31 of the band-shaped region 132a is parallel to the 1 st longitudinal direction LD 1. In addition, on each of the 1 st surface and the 2 nd surface of the negative electrode current collector 132, a band-shaped region 132b in which the convex portion 133 is not formed is provided at the 4 th center line CL and the vicinity thereof. The 3 rd longitudinal direction LD32 of the band-shaped region 132b is parallel to the 1 st widthwise direction WD 1. The minimum value of the distance between the adjacent 2 projections 133 is larger than the maximum width of the adjacent 2 projections 133.

When such a negative electrode 134 is wound together with the positive electrode and the separator from the end on the inner peripheral winding part IW side to form a wound electrode group and used in a lithium secondary battery, a space is formed between the negative electrode current collector 132 and the separator between the adjacent 2 projections 133. Lithium metal deposited during charging is contained in this space, and therefore expansion of negative electrode 134 can be suppressed.

For example, the projection shape, number, direction, width of the convex portions 133, and the distance between adjacent 2 convex portions 133, or some or all of the features are not limited to those shown in fig. 1, and can be changed as described above. These features may be the same in the inner peripheral side wound portion IW and the outer peripheral side wound portion OW, or may be different. For example, the distance between the adjacent 2 convex portions 133 of the inner peripheral wound portion IW may be made larger than the distance between the adjacent 2 convex portions 133 of the outer peripheral wound portion OW. In this case, it becomes easy to further suppress the expansion of the negative electrode accompanying the precipitation of lithium metal. In addition, when the plurality of projections 133 are formed on both the 1 st surface and the 2 nd surface of the negative electrode collector 132, the above-described characteristics may be the same or different between the 1 st surface side and the 2 nd surface side of the negative electrode collector 132. For example, the distance between the adjacent 2 convex portions 133 on the inner 2 nd surface side may be made larger than the distance between the adjacent 2 convex portions 133 on the outer 1 st surface side. In this case, since the volume change accompanying the deposition of lithium metal is easily absorbed, the effect of suppressing the expansion of the negative electrode is further improved.

When the plurality of projections 133 are formed on both the 1 st surface and the 2 nd surface of the negative electrode collector 132, the height of the plurality of 1 st projections 133 on the 1 st surface side of the negative electrode collector 132 may be the same as or different from the height of the plurality of 2 nd projections 133. For example, in the region of the electrode group that becomes the inner peripheral side wound portion, the height of the plurality of projections 133 on the 2 nd surface side, which is the inner side, may be made larger than the height of the plurality of projections 133 on the 1 st surface side, which is the outer side. In the region of the electrode group that becomes the inner peripheral side wound portion, the height of the plurality of projections 133 on the 2 nd surface side, which is the inner side, may be made larger than the height of the plurality of projections 133 on the 1 st surface side, which is the outer side. In these cases, the volume change associated with the deposition of lithium metal is easily absorbed, and therefore, the effect of suppressing the expansion of the negative electrode can be further improved.

The negative electrode 134 includes a negative electrode current collector 132 that is a conductive sheet such as a metal foil, and a plurality of protrusions 133 formed on the surface of each conductive sheet. In the wound electrode group, the outer surface and the inner surface of the negative electrode collector 132 are the 1 st surface and the 2 nd surface of the negative electrode collector 132, respectively.

The conductive sheet is a conductive material other than lithium metal and lithium alloy, for example. The conductive material may be a metal material such as a metal or an alloy, or may be a carbon material. The conductive material may be a material that does not react with lithium. Such a material includes a material that does not react with lithium metal and/or lithium ions, and more specifically, may be a material that does not form any of an alloy and an intermetallic compound with lithium. As the carbon material, graphite or the like in which the base surface is preferentially exposed can be used. The metal material is, for example, copper (Cu), nickel (Ni), iron (Fe), an alloy containing these metal elements, or the like. As the alloy, a copper alloy, stainless steel, or the like can be used. From the viewpoint of easily obtaining high strength, at least one selected from copper, copper alloys, and stainless steel can be used as the conductive material. The conductive material may be copper and/or a copper alloy from the viewpoint of easily ensuring high capacity and high charge-discharge efficiency by having high conductivity. The conductive sheet may contain one of these conductive materials, or may contain two or more of these conductive materials.

As the conductive sheet, a foil, a film, or the like can be used, and a sheet made of the above-described carbon material can also be used. The conductive sheet may be porous within a range not to impair the winding property. The conductive sheet may be a metal foil or a metal foil containing copper, from the viewpoint of easily ensuring high conductivity. Such metal foil may be a copper foil or a copper alloy foil. The copper content in the metal foil may be 50 mass% or more, or may be 80 mass% or more. The metal foil may be particularly a copper foil containing substantially only copper as a metal element.

In order to provide a plurality of projections on the negative electrode 134, each of the 1 st and 2 nd surfaces of the negative electrode collector 132 may be smooth. Thus, lithium metal is likely to be uniformly deposited on the 1 st surface and the 2 nd surface of the negative electrode current collector 132 during charging. The term "smooth" means that the maximum height roughness Rz of each of the 1 st surface and the 2 nd surface of the negative electrode current collector 132 is 20 μm or less. The maximum height roughness Rz of each of the 1 st surface and the 2 nd surface of the negative electrode current collector 132 may be 10 μm or less. The maximum height roughness Rz is based on JIS B0601: 2013.

In addition, from the viewpoint of easily ensuring a high volumetric energy density, negative electrode 134 may include only negative electrode current collector 132 and a plurality of projections 133 in a fully discharged state of the lithium secondary battery. In addition, from the viewpoint of easily ensuring high charge and discharge efficiency, the negative electrode may include a negative electrode active material layer disposed on the surface of the negative electrode current collector in addition to the negative electrode current collector and the plurality of projections in the fully discharged state. In the present disclosure, the fully discharged State of the lithium secondary battery refers to a State of Charge (SOC: State of Charge) in which the battery is discharged to 0.05 × C or less, assuming that the rated capacity of the battery is C. For example, a state of being discharged to a lower limit voltage at a constant current of 0.05C. The lower limit voltage is, for example, 2.5V.

examples of the negative electrode active material contained in the negative electrode active material layer include metallic lithium, a lithium alloy, and a material that reversibly stores and releases lithium ions. As the negative electrode active material, a negative electrode active material used in a lithium ion battery can be used. Examples of the lithium alloy include a lithium-aluminum alloy and the like. Examples of the material that reversibly stores and releases lithium ions include carbon materials and alloy materials. Examples of the carbon material include at least one selected from a graphite material, soft carbon, hard carbon, and amorphous carbon. Examples of the alloy-based material include materials containing silicon and/or tin. Examples of the alloy-based material include at least one selected from the group consisting of a simple substance of silicon, a silicon alloy, a silicon compound, a simple substance of tin, a tin alloy, and a tin compound. Examples of the silicon compound and the tin compound include an oxide and/or a nitride, respectively.

the negative electrode active material layer can be formed by depositing a negative electrode active material on the surface of a negative electrode current collector by a vapor phase method such as electrodeposition or vapor deposition. The negative electrode mixture may be formed by applying a negative electrode mixture containing a negative electrode active material, a binder, and other components added as needed to the surface of a negative electrode current collector. The other component includes at least one selected from the group consisting of a conductive agent, a tackifier, and an additive. The thickness of the negative electrode active material layer is not particularly limited, and each surface of the negative electrode current collector is, for example, 1 μm or more and 150 μm or less in a fully discharged state of the lithium secondary battery.

The order of forming the anode active material layer and the plurality of projections 133 is not particularly limited, and the plurality of projections 133 may be formed after forming the anode active material layer. In addition, the anode active material layer may be formed after the plurality of projections 133 are formed. More specifically, the plurality of protrusions 133 may protrude from the 1 st surface and/or the 2 nd surface of the anode current collector 132 in a state of being in direct contact with each surface. In addition, the negative electrode active material layer may protrude from the 1 st surface and/or the 2 nd surface of the negative electrode current collector 132 in a state where the negative electrode active material layer is present between each surface and the plurality of projections 133. The thickness of the negative electrode current collector or the conductive sheet is not particularly limited, and is, for example, 5 μm or more and 20 μm or less.

The material constituting the plurality of projections 133 is not particularly limited. The material of the plurality of projections 133 may be different from that of the negative electrode current collector 132. Alternatively, the plurality of projections 133 and the negative electrode collector 132 may be integrally formed of the same material. Each of the plurality of projections 133 may be made of a conductive material and/or an insulating material. The conductive material can be appropriately selected from the materials exemplified for the conductive sheet. The negative electrode current collector 132 having such projections 133 can be obtained by forming the projections 133 on the surface of the conductive sheet by, for example, press working. The negative electrode current collector 132 may be formed by applying a coating material of a conductive material to the surface of the conductive sheet or by attaching a tape of a conductive material.

each of the plurality of projections 133 may be made of a resin material. The resin material may be insulating. If the convex portions 133 are made of an insulating material such as a resin material, deposition of lithium metal at the tips of the convex portions 133 due to charging can be suppressed. The deposited lithium metal is contained in the negative electrode current collector 134, more specifically, in a space formed in the vicinity of the surface of the negative electrode current collector 132 which is a conductive sheet such as a metal foil. Therefore, the effect of suppressing the expansion of the negative electrode can be improved.

Examples of the resin material include at least one selected from olefin resins, acrylic resins, polyamide resins, polyimide resins, and silicone resins. As the resin material, a cured product of a curable resin such as an epoxy resin can be used. The convex portion 133 can be formed by, for example, attaching a resin adhesive tape to the surface of the negative electrode current collector 132 or the surface of the negative electrode current collector 132 with the negative electrode active material layer interposed therebetween. The convex portion 133 can be formed by applying a solution or dispersion containing a resin material to the surface of the negative electrode current collector 132 or the negative electrode active material layer and drying the solution or dispersion. The convex portion 133 may be formed by applying a curable resin to the surface of the negative electrode current collector 132 in a desired shape and curing the curable resin.

Further, the negative electrode 134 may further include a protective layer. The protective layer may be formed on the surface of the anode current collector 132, or may be formed on the surface of the anode active material layer in the case where the anode 134 has the anode active material layer. The protective layer has an effect of making the surface reaction of the electrode more uniform, and lithium metal is more likely to be more uniformly deposited in the negative electrode. The protective layer may be made of, for example, an organic substance and/or an inorganic substance. As these materials, materials that do not inhibit lithium ion conductivity are used. Examples of the organic material include polymers having lithium ion conductivity. Examples of the inorganic substance include ceramics and solid electrolytes. (lithium secondary battery)

Hereinafter, the structure of the lithium secondary battery will be described in more detail. A lithium secondary battery includes a group of wound electrodes and a nonaqueous electrolyte. The wound electrode group is formed by winding a positive electrode, a negative electrode, and a separator interposed between these electrodes. Fig. 3 is a longitudinal sectional view schematically showing a lithium secondary battery according to an embodiment of the present disclosure. Fig. 4 is an enlarged sectional view schematically showing the region IV of fig. 3. Fig. 5 is an enlarged sectional view schematically showing a V region of fig. 3. Fig. 5 is a cross section in a fully discharged state.

The lithium secondary battery 10 is a cylindrical battery including a cylindrical battery case, a wound electrode group 14 housed in the battery case, and a non-aqueous electrolyte not shown. The battery case is composed of a case main body 15 and a sealing member 16, the case main body 15 is a bottomed cylindrical metal container, and the sealing member 16 seals an opening of the case main body 15. A gasket 27 is disposed between the case main body 15 and the sealing body 16, thereby ensuring the sealing property of the battery case. In the case body 15, insulating plates 17 and 18 are disposed at both ends of the electrode group 14 in the winding axis direction, respectively.

The case body 15 has, for example, a step portion 21, and the step portion 21 is formed by partially pressing a side wall of the case body 15 from an outer side. The step portion 21 may be formed in a ring shape along the circumferential direction of the case main body 15 at the side wall of the case main body 15. In this case, the sealing body 16 is supported by the surface of the stepped portion 21 on the opening side.

Sealing body 16 includes filter 22, lower valve element 23, insulating member 24, upper valve element 25, and cap 26. In sealing body 16, these members are laminated in the above-described order. Sealing body 16 is attached to the opening of case body 15 such that cap 26 is positioned outside case body 15 and filter 22 is positioned inside case body 15. The members constituting the sealing body 16 are, for example, circular plate-shaped or ring-shaped. The members other than the insulating member 24 are electrically connected to each other.

The electrode group 14 includes a positive electrode 11, a negative electrode 12, and a separator 13. The positive electrode 11, the negative electrode 12, and the separator 13 are all in the form of a belt. The positive electrode 11 and the negative electrode 12 are wound in a spiral shape with the separator 13 interposed between the electrodes so that the width direction of the strip-shaped positive electrode 11 and the negative electrode 12 is parallel to the winding axis. In a cross section perpendicular to the winding axis of the electrode group 14, the positive electrodes 11 and the negative electrodes 12 are alternately stacked in the radial direction of the electrode group 14 with the separators 13 interposed therebetween.

The positive electrode 11 is electrically connected to a cap 26 also serving as a positive electrode terminal via a positive electrode lead 19. One end of the positive electrode lead 19 is connected to, for example, the vicinity of the center in the longitudinal direction of the positive electrode 11. A positive electrode lead 19 extending from the positive electrode 11 extends to the filter 22 through a through-hole, not shown, formed in the insulating plate 17. The other end of the positive electrode lead 19 is welded to the surface of the filter 22 on the electrode group 14 side. The negative electrode 12 is electrically connected to the case main body 15 also serving as a negative electrode terminal via a negative electrode lead 20. One end of the negative electrode lead 20 is connected to, for example, the end of the negative electrode 12 in the longitudinal direction, and the other end is welded to the inner bottom surface of the case main body 15.

Fig. 4 shows the positive electrode 11 opposite the separator 13. Fig. 5 shows the negative electrode 12 opposite to the separator 13. The positive electrode 11 includes a positive electrode current collector 30 and a positive electrode mixture layer 31 disposed on both the 1 st surface and the 2 nd surface of the positive electrode current collector 30. The negative electrode 12 includes a negative electrode current collector 32, a plurality of 1 st protrusions 33a arranged on the 1 st surface S1 on the outer side of the negative electrode current collector 32, and a plurality of 2 nd protrusions 33b arranged on the 2 nd surface S2 on the inner side of the negative electrode current collector 32. The 1 st surface S1 and the 2 nd surface S2 of the negative electrode current collector 32 are the 1 st surface and the 2 nd surface, respectively, of a conductive sheet such as a metal foil constituting the negative electrode current collector 32. The plurality of 1 st convex portions 33a protrude from the 1 st surface S1 toward a surface of the partition plate 13 opposite to the 1 st surface S1. The plurality of 2 nd convex portions 33b protrude from the 2 nd surface S2 toward the surface of the partition plate 13 opposite to the 2 nd surface S2.

A plurality of 1 st protrusions 33a and a plurality of 2 nd protrusions 33b are formed on the 1 st surface S1 and the 2 nd surface S2, respectively. Between the adjacent 21 st projections 33a, a space 35 is formed between the 1 st surface S1 and the partition plate 13. Further, between the 2 nd convex portions 33b adjacent to each other, a space 35 is formed between the 2 nd surface S2 and the partition plate 13. In the lithium secondary battery 10, lithium metal is deposited in the space 35 by charging, and the deposited lithium metal is dissolved in the nonaqueous electrolyte by discharging. Since lithium metal deposited in the space 35 can be stored, the change in apparent volume of the negative electrode 12 due to deposition of lithium metal can be reduced. Further, by making the 2 nd average height of the inner peripheral side wound portion IW larger than the 1 st average height of the outer peripheral side wound portion OW in advance, which increases the thickness of the deposited lithium metal, it is possible to absorb the volume change accompanying the deposition of lithium metal. Therefore, the expansion of the negative electrode can be suppressed. In addition, since the pressure is also applied to the lithium metal accommodated in the space 35 in the electrode group 14, the peeling of the lithium metal can be suppressed. Therefore, the charge/discharge efficiency of the lithium secondary battery 10 can be suppressed from decreasing.

As the negative electrode 12 including the plurality of projections and the negative electrode current collector 32, the negative electrode 134 including the plurality of projections and the negative electrode current collector 132 described above can be used. Therefore, for the negative electrode 12, the plurality of projections, and the negative electrode current collector 32, the description of the negative electrode 134, the plurality of projections, and the negative electrode current collector 132 described above can be referred to. The following describes the structure of the lithium secondary battery other than the negative electrode 12 in more detail. (Positive electrode 11)

The positive electrode 11 includes, for example, a positive electrode current collector 30 and a positive electrode mixture layer 31 formed on the positive electrode current collector 30. The positive electrode mixture layer 31 may be formed on both the 1 st surface and the 2 nd surface of the positive electrode collector 30. The positive electrode mixture layer 31 may be formed on one surface of the positive electrode current collector 30. For example, the positive electrode mixture layer 31 may be formed only on one surface of the positive electrode current collector 30 in a region where the positive electrode lead 19 is connected and/or a region not facing the negative electrode 12. For example, since there may be a region not facing the negative electrode 12, such as a region located at and near the innermost winding periphery and/or a region located at and near the outermost winding periphery, the positive electrode mixture layer 31 may be formed only on one surface of the positive electrode current collector 30 in such a region, and the positive electrode mixture layer 31 may not be formed on both the 1 st surface and the 2 nd surface.

The positive electrode mixture layer 31 may contain a positive electrode active material, and contain a conductive material and/or a binder as optional components. The positive electrode mixture layer 31 may contain an additive as needed. Between the positive electrode current collector 30 and the positive electrode mixture layer 31, a conductive carbon material may be disposed as necessary.

The positive electrode 11 is obtained by, for example, applying a slurry containing the constituent components of the positive electrode mixture layer and the dispersion medium to the surface of the positive electrode current collector 30, drying the coating film, and then rolling. If necessary, a conductive carbon material may be applied to the surface of the positive electrode current collector 30. Examples of the dispersion medium include water and/or an organic medium.

Examples of the positive electrode active material include materials that occlude and emit lithium ions. As the positive electrode active material, at least one selected from the group consisting of a lithium-containing transition metal oxide, a transition metal fluoride, a polyanion, a fluorinated polyanion, and a transition metal sulfide can be exemplified. The positive electrode active material may be a lithium-containing transition metal oxide from the viewpoint of high average discharge voltage and advantageous cost.

examples of the transition metal element contained in the lithium-containing transition metal oxide include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, W, and the like. The lithium-containing transition metal oxide may contain one transition metal element, or may contain two or more kinds. The transition metal element may be at least one selected from Co, Ni, and Mn. The lithium-containing transition metal oxide may contain one or two or more typical metal elements, as needed. Typical examples of the metal elements include Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb, and Bi. Typical metal elements may be Al and the like.

The conductive material is, for example, a carbon material. Examples of the carbon material include carbon black, carbon nanotubes, and graphite. Examples of the carbon black include acetylene black and ketjen black. The positive electrode mixture layer 31 may contain one or two or more kinds of conductive materials. At least one selected from these carbon materials can be used as the conductive carbon material present between the positive electrode current collector 30 and the positive electrode mixture layer 31.

Examples of the binder include fluororesins, polyacrylonitrile, polyimide resins, acrylic resins, polyolefin resins, and rubbery polymers. Examples of the fluororesin include polytetrafluoroethylene and polyvinylidene fluoride. The positive electrode mixture layer 31 may contain one kind of binder, or may contain two or more kinds.

Examples of the material of the positive electrode current collector 30 include metal materials including Al, Ti, Fe, and the like. The metal material may be Al, Al alloy, Ti alloy, Fe alloy, and the like. The Fe alloy may be stainless steel. Examples of the positive electrode current collector 30 include a foil and a film. The positive electrode collector 30 may be porous. For example, a metal mesh or the like can be used as the positive electrode collector 30.

(partition plate 13)

The separator 13 is a porous sheet having ion permeability and insulation properties. Examples of the porous sheet include a microporous film, woven fabric, and nonwoven fabric. The material of the separator is not particularly limited, and may be a polymer material. Examples of the polymer material include olefin resins, polyamide resins, and cellulose. Examples of the olefin resin include polyethylene, polypropylene, and a copolymer of ethylene and propylene. The separator 13 may contain an additive as needed. Examples of the additive include inorganic fillers.

Separator 13 may comprise multiple layers of differing morphology and/or composition. Such a separator 13 may be, for example, a laminate of a polyethylene microporous membrane and a polypropylene microporous membrane, or a laminate of a nonwoven fabric including cellulose fibers and a nonwoven fabric including thermoplastic resin fibers. A material having a polyamide resin coating film formed on the surface of a microporous membrane, woven fabric, nonwoven fabric, or the like can be used as the separator 13. Such a separator 13 has high durability even when pressure is applied in a state where it is in contact with the plurality of projections. In addition, the separator 13 may be provided with a layer containing an inorganic filler on the side facing the positive electrode 11 and/or the side facing the negative electrode 12, from the viewpoint of ensuring heat resistance and/or strength.

(non-aqueous electrolyte)

As the nonaqueous electrolyte, a material having lithium ion conductivity is used. Such a nonaqueous electrolyte contains a nonaqueous solvent and lithium ions and anions dissolved in the nonaqueous solvent. The nonaqueous electrolyte may be in a liquid state or a gel state. In addition, the nonaqueous electrolyte may be a solid electrolyte.

the liquid nonaqueous electrolyte is prepared by dissolving a lithium salt in a nonaqueous solvent. The non-aqueous electrolyte may contain a non-dissociative lithium salt by dissolving the lithium salt in the non-aqueous solvent to generate lithium ions and anions. As the lithium salt, a salt of a lithium ion and an anion is used.

The gel-like nonaqueous electrolyte includes a liquid nonaqueous electrolyte and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a nonaqueous solvent to be gelled is used. Examples of such a polymer material include at least one selected from a fluororesin, an acrylic resin, and a polyether resin.

₄ ^- ₄ ^- ₆ ^- _{3 3} ^- _{3 2} ^- _{2 m 2m+1 2 n 2n+1} ^- _{2 3 2} ^- _{2 2 5 2} ^- _{2 2} ^- _{2 2 4} ^- _{4 2 4} ^- _{2 2 4 2} ^-The lithium salt or anion may be a known substance used for a nonaqueous electrolyte of a lithium secondary battery, and examples of the anion include BF, ClO, PF, CF SO, CF CO, imide anions, and oxalic acid anions, and examples of the imide anions include N (SO C F) (SO C F) (m and N are each independently an integer of 0 or more), m and N may be 0 to 3, and may be 0, 1, or 2, respectively.

₆ ^- ₆ ^-In particular, if a nonaqueous electrolyte containing an oxalate anion is used, lithium metal is likely to be uniformly precipitated in a fine particle form due to the interaction of the oxalate anion with lithium.

Examples of the nonaqueous solvent include esters, ethers, nitriles, amides, and halogen substitutes thereof. The nonaqueous electrolyte may contain one of these nonaqueous solvents, or may contain two or more of these nonaqueous solvents. Examples of the halogen substituent include fluoride.

Examples of the ester include a carbonate ester and a carboxylic acid ester. Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, fluoroethylene carbonate, and the like. Examples of the chain carbonate include dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate. Examples of the cyclic carboxylic acid ester include γ -butyrolactone and γ -valerolactone. Examples of the chain carboxylic acid ester include ethyl acetate, methyl propionate, and methyl fluoropropionate.

Examples of the ether include cyclic ethers and chain ethers. Examples of the cyclic ether include 1, 3-dioxolane, 4-methyl-1, 3-dioxolane, tetrahydrofuran, and 2-methyltetrahydrofuran. Examples of the chain ether include 1, 2-dimethoxyethane, diethyl ether, ethyl vinyl ether, methylphenyl ether, benzylethyl ether, diphenyl ether, dibenzyl ether, 1, 2-diethoxyethane, and diethylene glycol dimethyl ether.

Examples of the nitrile include acetonitrile, propionitrile, and benzonitrile. Examples of the amide include dimethylformamide and dimethylacetamide.

The concentration of the lithium salt in the nonaqueous electrolyte is, for example, 0.5mol/L to 3.5 mol/L. Here, the lithium salt concentration is the total of the dissociated lithium salt concentration and the undissociated lithium salt concentration. The concentration of anions in the nonaqueous electrolyte can be set to 0.5mol/L to 3.5 mol/L.

The nonaqueous electrolyte may contain an additive. The additive may form a coating on the negative electrode. By forming a coating film derived from the additive on the negative electrode, the formation of dendrites is easily suppressed. Examples of such additives include vinylene carbonate, fluoroethylene carbonate, and vinylethylene carbonate. One additive may be used alone, or two or more additives may be used in combination.

(others)

In the illustrated example, the cylindrical lithium secondary battery is described, but the present embodiment is not limited to this case, and the present embodiment can also be applied to a lithium secondary battery including a rolled electrode group in which the end surface shape in the rolling axis direction of the rolled electrode group is elliptical or oval. In addition, as for the structure other than the electrode group and the nonaqueous electrolyte of the lithium secondary battery, a known structure may be used without particular limitation.