JP2017017186A - Power storage device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power storage device which has a lithium electrode as a lithium ion supply source and in which electrolytic solution is impregnated into the entire area of a lithium foil and a lithium ion doping amount of a negative electrode is made uniform when pre-doping is performed in the power storage device.SOLUTION: A power storage device 1 includes an electrode unit 8 having a plurality of laminated positive electrodes 5 and negative electrodes 4, a lithium electrode 7 having a lithium foil 7b arranged so as to face the outermost layer of the laminate, and external connection tabs (3a, 3b) protruding outwards from the end portions of the positive electrodes 5 and the negative electrodes 4, and an outer packaging bag 2 for enclosing the electrode unit 8. The power storage device 1 seals two sides located at both end portions of the outer packaging bag 2 in the tab protruding direction, and one side located at the end portion in an electrolytic solution injection direction, and electrolytic solution is injected from the other one side located at the end portion in the electrolytic solution injection direction to impregnate the electrode unit 8 with the electrolytic solution, thereby manufacturing the power storage device 1. A plurality of grooves extending along the entire length in the electrolytic solution injection direction are juxtaposed with one another on the surface of the lithium foil 7b.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a power storage device such as a lithium ion capacitor or a lithium ion secondary battery having an electrode unit in which positive electrodes and negative electrodes are alternately stacked via separators, and a lithium electrode disposed opposite to the outermost layer. Regarding devices.

  In recent years, when generating power using natural energy (solar power generation, wind power generation, etc.), it can be stored when surplus power is temporarily stored and the amount of power generation decreases, and can be used as a power source for electric vehicles. The development of large-capacity electricity storage devices is underway.

  As such an electricity storage device, a lithium ion capacitor is particularly attracting attention. The lithium ion capacitor is a power storage component having a hybrid structure in which a power storage component called an electric double layer capacitor and a lithium ion secondary battery are combined. Specifically, the positive electrode of the electric double layer capacitor and the negative electrode of the lithium ion secondary battery are combined.

  In a lithium ion capacitor, using a lithium electrode having a lithium foil as a lithium ion supply source, the potential difference between the positive electrode and the positive electrode is increased by preliminarily occluding (pre-doping) lithium ions in the negative electrode to lower the potential of the negative electrode. High voltage can be achieved.

  As a pre-doping method, for example, an electrode cell is assembled by laminating a plurality of positive electrodes and negative electrodes with a separator interposed therebetween, and an electrolytic solution is injected in a state where the negative electrode and the lithium foil are short-circuited by being sandwiched between pressure plates. Lithium ions can be occluded.

  Patent Document 1 discloses a battery including a positive electrode, a negative electrode, and an aprotic organic solvent solution of a lithium salt as an electrolyte solution. The positive electrode current collector and the negative electrode current collector include through holes, and the negative electrode active material includes lithium. Disclosed is a battery that can be reversibly supported and is supported by electrochemical contact of lithium derived from the negative electrode with lithium disposed opposite the positive electrode, and the opposed area of lithium is 40% or less of the negative electrode area Has been.

Japanese Patent No. 3485935

  However, when a lithium ion capacitor as described in Patent Document 1 is prepared and lithium ions are pre-doped, there are individual differences in device characteristics, and the expected capacitance value is not reached. It was seen.

  When such a device was analyzed, it was found that there was in-plane variation in the lithium ion doping amount of the negative electrode. In particular, the amount of occlusion of lithium ions was smaller in the region away from the injection port in which the electrode cell was sealed in the outer bag and the electrolyte solution in the outer bag was injected, and a part of the lithium foil remained undissolved. As a result of intensive studies, the present inventor has found that this cause may be due to the electrolyte not being impregnated throughout the lithium foil.

  In view of such a problem, the present invention, when performing pre-doping in an electricity storage device including a lithium foil as a lithium ion supply source, the electrolyte solution is impregnated throughout the lithium foil, and the amount of lithium ion dope of the negative electrode is reduced. An electrical storage device that can be made uniform is provided.

  In order to solve the above-described problems, an electricity storage device according to the present invention includes a lithium foil having a positive electrode and a negative electrode that are stacked in plural via separators, and a lithium foil that is disposed so as to face the outermost layer of the stacked positive and negative electrodes. An electrode unit having an electrode unit having an external connection tab connected to each of the positive electrode and the negative electrode so as to protrude outward from the ends of the electrode and the positive electrode and the negative electrode, and a pair of sheet-shaped exterior members are bonded together. An outer bag to be sealed, and two sides positioned at both ends of the outer bag in the direction in which the external connection tab protrudes and one side positioned at the end in the direction orthogonal to the protruding direction are sealed and orthogonal In an electricity storage device manufactured by injecting an electrolyte from another side located at the end of the direction and impregnating the electrode unit, the surface of the lithium foil is orthogonal Wherein the groove extends over the entire length of the is several juxtaposed.

  According to the above configuration of the present invention, since a plurality of grooves are provided on the surface of the lithium foil, the electrolyte impregnated in the vicinity of the end of the lithium foil is filled with the entire area of the wall surface of the lithium foil through the grooves by capillary action. Impregnate into. As a result, the pre-doping progresses uniformly, and the lithium ion pre-doping amount of the negative electrode can be made uniform.

  Furthermore, it is preferable that a plurality of grooves extending over the entire length in the protruding direction are arranged in parallel on the surface of the lithium foil.

  Moreover, it is preferable that the depth of the groove | channel formed in the surface of lithium foil is 60 to 90% of the thickness of lithium foil.

  Moreover, it is preferable that the width | variety of the groove | channel formed in the surface of lithium foil is 0.1 mm or more and 1 mm or less.

  Moreover, it is preferable that the space | interval of the some groove | channel formed in the surface of lithium foil is 3 times or more and 10 mm or less of the depth of a groove | channel.

  According to the electricity storage device of the present invention, when pre-doping is performed, the electrolyte solution is impregnated throughout the lithium foil, and the amount of lithium ion doping of the negative electrode can be made uniform.

1 is a schematic perspective view showing an overall configuration of an electricity storage device according to one embodiment of the present invention. It is a schematic side view which shows an example of a structure of the electrode unit contained in the electrical storage device. It is a schematic plan view which shows an example of a structure of the lithium foil contained in the same electrical storage device. FIG. 4 is a cross-sectional view taken along line A-A ′ of FIG. 3. It is a schematic plan view which shows the structure of the lithium foil which concerns on another one embodiment. It is a graph which shows the result of an Example and a comparative example.

  Hereinafter, an electric storage device according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

  As shown in FIG. 1, the power storage device 1 according to one embodiment of the present invention includes an electrode unit 8 and an outer bag 2 that encloses the electrode unit 8.

  As shown in FIGS. 1 and 2, the electrode unit 8 includes a plurality of positive electrodes 5 and negative electrodes 4 stacked via a separator 6, a lithium electrode 7 disposed in the outermost layer of the stacked positive electrodes 5 and negative electrodes 4, and External connection tabs (3a, 3b) connected to the positive electrode 5 and the negative electrode 4, respectively.

  The positive electrode 5 is formed in a rectangular sheet shape, and includes a positive electrode current collector 5a and a positive electrode mixture layer 5b formed on both surfaces thereof.

  The positive electrode current collector 5a is made of, for example, a metal foil such as aluminum or stainless steel, and a plurality of through-holes through which lithium ions can pass are formed. The positive electrode mixture layer 5b is a mixture of a positive electrode active material, a conductive additive, a binder (binder), and a dispersant. As the positive electrode active material, it is preferable to use a transition metal oxide capable of inserting and extracting lithium ions and anions contained in the electrolyte, or activated carbon. As the conductive aid, it is preferable to use carbon powder such as graphite or carbon black. As the binder, it is preferable to use a fluorine resin such as polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE), or a non-fluorine polymer such as styrene butadiene rubber (SBR).

  The negative electrode 4 is formed in a rectangular sheet shape, and includes a negative electrode current collector 4a and a negative electrode mixture layer 4b formed on both sides thereof.

  The negative electrode current collector 4a is made of, for example, a metal foil such as copper or stainless steel, and a plurality of through-holes through which lithium ions can pass are formed. The negative electrode mixture layer 4b is a mixture of a negative electrode active material, a conductive additive, a binder (binder), and a dispersant. As the negative electrode active material, it is preferable to use a carbon-based material such as polyacene, soft carbon, hard carbon, and graphite that can occlude and release lithium ions. It is preferable to select and use the conductive aid and the binder from those mentioned in the description of the positive electrode. In the present embodiment, the positive electrode mixture layer 5b and the negative electrode mixture layer 4b are formed on both surfaces of the positive electrode current collector 5a and the negative electrode current collector 5a, respectively, but may be formed only on one surface. .

  One of the ends of the positive electrode current collector 5a and the negative electrode current collector 4a is exposed without being covered with the positive electrode mixture layer 5b and the negative electrode mixture layer 4b, and an external connection tab (positive electrode) Tab 3a and negative electrode tab 3b) are connected. Here, as shown in FIGS. 1 to 3, the direction in which the external connection tabs (3a, 3b) protrude is defined as a tab protruding direction, and the direction orthogonal to this is defined as an electrolyte injection direction. As will be described later, in order to manufacture the electricity storage device 1 of the present embodiment, an electrolyte solution is injected along the electrolyte solution injection direction and impregnated in the electrode unit 8.

  The separator 6 disposed between the electrodes (the positive electrode 5, the negative electrode 4, and the lithium electrode 7) in the electrode unit is formed in a rectangular sheet shape. A material that can maintain the insulation between the positive electrode, the negative electrode, and the lithium electrode while allowing the permeation of ions and electrolyte is used. For example, in addition to olefinic porous resins such as polyethylene and polypropylene, it is preferable to use cellulose. .

  The lithium electrode 7 is formed in a rectangular sheet shape, and is composed of a lithium electrode current collector 7a made of copper foil and a metal lithium foil 7b attached to one surface of the lithium electrode current collector 7a. In order to facilitate the supply of lithium ions from the lithium foil 7b during the pre-doping, the lithium electrode 7 is arranged so that the lithium foil 7b faces the outermost layer of the structure in which the positive electrode 5 and the negative electrode 4 are stacked. It is arranged on the outermost layer of the electrode unit 8. As shown in FIGS. 2 to 4, the lithium foil 7 b according to the present embodiment has a plurality of grooves 9 extending continuously over the entire length in the electrolyte injection direction on the surface thereof. .

  The thickness of the lithium foil 7b as a lithium ion supply source is determined by the required lithium ion doping amount calculated from the thickness of the mixture layer and the number of laminated layers of the positive electrode 5 and the negative electrode 4, but usually 0.05 mm or more It is preferable that

  The external connection tabs (3 a, 3 b) are formed in a band shape so as to protrude outward from the end of the positive electrode 5 or the negative electrode 4. As the external connection tabs (3a, 3b), it is preferable to use aluminum, copper, stainless steel, or a metal plate on which these are nickel-plated or tin-plated and on which a sealant film is formed.

  The exterior bag 2 has a size sufficient to enclose the electrode unit 8 therein, and is composed of a pair of sheet-shaped exterior materials. In this embodiment, a laminate of aluminum foil is used as the exterior material.

As the electrolytic solution impregnating the electrode unit 8 in the electricity storage device 1, it is preferable to use a non-aqueous electrolyte solution in which an electrolyte salt containing lithium is dissolved in an organic solvent. As the electrolyte salt, LiBF ₄ , LiPF ₆ , LiClO ₄ , LiB (C ₆ H ₅ ) ₄ , CH ₃ SO ₃ Li and the like are suitable. Examples of organic solvents include cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC), and chain carbonates such as ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and diethyl carbonate (DEC). It is preferable to use alone or in combination. Moreover, you may add a small amount of vinylene carbonate (VC), a fluoroethylene carbonate (FEC), etc. as an additive.

  And in order to produce the electrical storage device 1 in this Embodiment, the positive electrode 5 and the negative electrode 4 are laminated | stacked via the separator 6, and the lithium electrode 7 is arrange | positioned in the outermost layer. The external connection tabs 3 a and 3 b are connected to the ends of the positive electrode 5 and the negative electrode 4 in the tab projecting direction to produce the electrode unit 8. Subsequently, the electrode unit 8 is covered with a pair of sheet-like aluminum laminate films. Two sides (the side to which the positive electrode tab 3a and the negative electrode tab 3b are connected in FIG. 1 and the opposite side) located at both ends in the tab protruding direction of the pair of aluminum laminate films and the electrolyte injection direction One side located at the end is sealed. And electrolyte solution is inject | poured from the opening part of the other one side located in the edge part of electrolyte solution injection direction, and the electrode unit 8 is impregnated. After the electrolyte solution is injected, one side into which the electrolyte solution is injected is sealed, and the electrode unit 8 is sealed in the outer bag 2. In this way, the electricity storage device 1 shown in FIG. 1 is obtained.

  According to the electricity storage device 1 according to the present embodiment, since the groove 9 is formed on the surface of the lithium foil 7b, the injected electrolyte solution is capillary from the end side in the electrolyte solution injection direction of the electrode unit 8. As a result, the lithium foil 7b permeates through the groove 9. The grooves 9 are formed over the entire length in the electrolyte injection direction, and a plurality of the grooves 9 are arranged side by side. Therefore, the region away from the position where the electrolyte solution is injected in the lithium foil 7b, or the lithium foil in which the electrolyte solution is difficult to penetrate. It is possible to make the electrolyte reach the center of 7b. As a result, the entire area of the lithium foil 7b can be impregnated with the electrolytic solution.

  In the case where the groove 9 is not continuously provided over the entire length of the lithium foil 7b in the electrolyte solution injection direction, penetration by capillary action at a location where the groove 9 is discontinuous, that is, at a location where it is interrupted. As a result, the electrolyte solution does not reach the entire area of the lithium foil 7b. However, even if a slight change in depth or width due to a manufacturing error or a slight groove is not formed at the end of the lithium foil 7b, etc., this implementation is possible as long as the groove 9 can exhibit a capillary phenomenon as a whole. It is allowed to be included in the form.

  The depth d of the groove 9 formed in the lithium foil 7b is preferably 60% or more and 90% or less of the thickness of the lithium foil. When the depth of the groove 9 is less than 60% of the thickness of the lithium foil 7b, the lithium foil 7b near the boundary between the lithium electrode current collector 7a and the lithium foil 7b, which is far from the interface between the lithium foil 7b and the electrolyte solution. Therefore, there is a risk that the electrolyte will not reach and the lithium will remain undissolved. Moreover, when the depth of the groove 9 is deeper than 90% of the thickness of the lithium foil, the lithium foil 7b may be broken at the portion of the groove 9 due to processing variations and handling, and may be divided into a plurality of individual pieces. In that case, there is a possibility that the separated lithium will fall off the lithium electrode current collector 7a and remain undissolved.

  The width w of the groove 9 is preferably 0.1 mm or more and 1 mm or less. When the width of the groove 9 is less than 0.1 mm, the amount of the electrolytic solution penetrating from the groove 9 into the lithium foil 7b becomes insufficient, which causes the remaining undissolved lithium. In addition, when the width of the groove 9 exceeds 1 mm, the capillary phenomenon is not sufficiently exhibited, the penetration of the electrolytic solution through the groove 9 is difficult to occur, and this also causes the remaining undissolved lithium.

  The distance p between adjacent grooves 9 is preferably 10 mm or less in order to penetrate the electrolytic solution into the lithium foil 7b between the plurality of grooves 9 arranged in parallel. When it exceeds 10 mm, the electrolyte solution hardly penetrates into the lithium foil 7b between the grooves 9 and the lithium may not be melted. On the other hand, the distance p between the grooves 9 is preferably at least three times the depth of the grooves 9. Since the lithium foil 7b is soft, if it is less than 3 times, the lithium foil 7b is liable to be crushed when pressurized when forming the electrode unit, and the depth of the groove 9 necessary for the expression of the capillary phenomenon cannot be secured. There is.

  Moreover, the groove | channel 9 is formed by pressing the lithium foil 7b with the flat plate with a convex part. Either the groove 9 is formed in the lithium foil 7b and then bonded to the lithium electrode current collector 7a, or the groove 9 is formed after the lithium foil 7b is bonded to the lithium electrode current collector 7a. Good.

(Example)
Hereinafter, the effectiveness of the electricity storage device described in the above embodiment will be described with reference to examples and comparative examples. These examples are illustrative, and the present invention is not limited to the following examples.

(Example 1)
After mixing, stirring, and dispersing coconut shell steam activated charcoal in the positive electrode active material, carbon black as the conductive additive, and SBR as the binder with pure water in which the dispersing agent carboxymethylcellulose (CMC) is dissolved, It applied to the aluminum foil which is an electric body. This was dried under reduced pressure to remove water, and then punched out to a predetermined size to obtain a positive electrode.

  Next, graphite as a negative electrode active material, carbon black as a conductive additive, and SBR as a binder were mixed with pure water in which CMC was dissolved, stirred, and dispersed, and then applied to a copper foil as a current collector. This was dried under reduced pressure to remove water, and then punched out to a predetermined size to obtain a negative electrode. Further, as a pre-doped lithium supply source, a lithium electrode having a thickness of 0.1 mm was cut and adhered to a copper foil as a current collector to produce a lithium electrode. The depth of the groove formed in the lithium foil was 0.08 mm, the width of the groove was 0.5 mm, and the interval was 5 mm.

  The positive electrode, the negative electrode, and the lithium electrode were laminated via a cellulose separator so that the lithium electrode was the outermost layer and the lithium foil was the inner electrode side. A positive electrode tab made of aluminum was welded to the exposed aluminum portion provided in the positive electrode current collector, and a negative electrode tab made of nickel-plated copper was welded to the exposed copper portion provided in the negative electrode current collector. The positive electrode tab and the negative electrode tab protruded from the outer bag and were enclosed in an outer bag made of a pair of sheet-like aluminum laminate films.

A cell was prepared by injecting 1M lithium hexafluorophosphate (LiPF ₆ ) dissolved in an equal volume of EC and DEC, and sealing the outer bag. When injecting the electrolyte, the side of the outer bag on the side where the tab protrudes and the two sides opposite to the side (two sides at the end of the tab protruding direction) and the electrolyte orthogonal to the tab protruding direction One side on the end side in the injection direction was sealed, and the electrolyte solution was injected from the end side in the electrolyte injection direction. Thereafter, this side was also sealed, the negative electrode and the lithium electrode of the cell were short-circuited, and lithium ion pre-doping was performed.

(Example 2)
A cell was produced by the same material and manufacturing method as in Example 1 except that the depth of the groove was 0.09 mm.

Example 3
A cell was produced by the same material and manufacturing method as in Example 1 except that the depth of the groove was 0.06 mm.

  Example 4

  A cell was produced using the same material and manufacturing method as in Example 1 except that the width of the groove was 0.1 mm.

  (Example 5)

  A cell was produced by the same material and manufacturing method as in Example 1 except that the width of the groove was 1 mm.

  Example 6

  A cell was manufactured using the same material and manufacturing method as in Example 1 except that the interval between the grooves was 3 mm.

(Example 7)
A cell was produced by the same material and manufacturing method as in Example 1 except that the interval between the groove portions was 10 mm.

(Comparative Example 1)
A cell was produced by the same material and manufacturing method as in Example 1 except that the groove was not provided in the lithium foil.

  (Comparative Example 2)

  A cell was produced by the same material and manufacturing method as in Example 1 except that the depth of the groove of the lithium foil was 0.05 mm.

  (Comparative Example 3)

  A cell was produced by the same material and manufacturing method as in Example 1 except that the depth of the groove of the lithium foil was 0.095 mm.

(Comparative Example 4)
A cell was produced by the same material and manufacturing method as in Example 1 except that the groove width was 0.05 mm.

(Comparative Example 5)
A cell was produced by the same material and manufacturing method as in Example 1 except that the groove width was 2 mm.

(Comparative Example 6)
A cell was produced by the same material and manufacturing method as in Example 1 except that the groove interval was 15 mm.

(Comparative Example 7)
A cell was produced by the same material and manufacturing method as in Example 1 except that the groove interval was 2 mm.

  (Evaluation test)

  As a result of disassembling each of the cells prepared in Examples 1 to 7 and Comparative Examples 1 to 7 in order to confirm that the pre-doping has progressed, in all the comparative examples, undissolved lithium foil was observed. It was.

  Further, as an evaluation test, a cycle test in the range of 2 A constant current and 2.2 V to 3.8 V was performed at 25 ° C., and the capacitance and internal resistance at the 100,000th cycle with respect to the first cycle were evaluated. The capacity maintenance ratio, which is the capacity maintenance ratio before and after the cycle, was less than 95% in Comparative Examples 1 to 5, and was outside the allowable range. The resistance increase rate, which is the increase rate of the internal resistance before and after the cycle, exceeded 100% in Comparative Example 1 and was outside the allowable range. Although Comparative Examples 2 to 5 satisfied 100% or less, it was confirmed that the rate of increase in resistance was clearly higher when compared with Examples.

  The result of the evaluation test is shown in the chart of FIG. From this result, it is confirmed that the electricity storage device according to each example shows a uniform pre-dope, and shows a high capacitance maintenance rate and a low internal resistance increase rate.

(Modification)
In the above embodiment, an example in which only the grooves 9 extending in the electrolyte injection direction are formed on the surface of the lithium foil 7b used for the electricity storage device. However, as shown in FIG. It may be formed.

  By forming the grooves 9 in a lattice shape, not only the electrolyte injection direction of the lithium foil 7b but also the penetration of the electrolyte due to the capillary phenomenon occurs in the tab protruding direction, so that the entire area of the lithium foil 7b is more reliably impregnated with the electrolyte. It becomes possible to make it.

  The depth, thickness, and distance between the grooves 9 when forming the grooves 9 in a lattice shape are the same as when the grooves 9 shown in FIG. 3 are provided only in the electrolyte injection direction.

  The present invention has been specifically described above based on the embodiments. However, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the scope of the invention. Nor. For example, in the above embodiment, the direction orthogonal to the tab protruding direction is the electrolyte injection direction. However, when the electrolytic solution is injected from the side of the outer bag in the protruding direction of the tab, that is, when injected from the side to which the tab is connected or the opposite side, The liquid injection direction coincides with the tab protruding direction. In this case, it is preferable that a plurality of grooves formed on the surface of the lithium foil extend continuously over the entire length in the tab protruding direction of the lithium foil, and a plurality of grooves are arranged in a direction perpendicular to the tab protruding direction. In the above-described embodiment, the configuration in which the respective electrodes are stacked has been described. However, the present invention is not limited to this, and the present invention can also be applied to a configuration in which the respective electrodes are wound.

DESCRIPTION OF SYMBOLS 1 ... Power storage device 2 ... Outer packaging bag 3a ... Positive electrode tab 3b ... Negative electrode tab 4 ... Negative electrode 4a ... Negative electrode collector 4b ... Negative electrode mixture layer 5 ... Positive electrode 5a ... Positive electrode collector 5b ... Positive electrode mixture layer 6 ... Separator 7 ... Lithium electrode 7a ... Lithium current collector 7b ... Lithium foil 8 ... Electrode unit 9 ... Groove

Claims (5)

A plurality of positive and negative electrodes stacked via a separator, a lithium electrode having a lithium foil disposed so as to face the outermost layer of the stacked positive and negative electrodes, and projecting outward from the ends of the positive and negative electrodes An electrode unit having an external connection tab connected to each of the positive electrode and the negative electrode, and an exterior bag for enclosing the electrode unit by bonding a pair of sheet-shaped exterior materials, 2 sides positioned at both ends of the outer bag in the direction in which the tab protrudes and 1 side positioned at the end in the direction orthogonal to the protruding direction are sealed, and the other is positioned at the end in the orthogonal direction In an electricity storage device produced by injecting an electrolyte from one side of the electrode and impregnating the electrode unit,
A power storage device, wherein a plurality of grooves extending over the entire length in the orthogonal direction are arranged in parallel on the surface of the lithium foil.   2. The electricity storage device according to claim 1, wherein a plurality of grooves extending over the entire length in the protruding direction are provided in parallel on the surface of the lithium foil.   The electrical storage device according to claim 1 or 2, wherein the depth of the groove is 60% or more and 90% or less of the thickness of the lithium foil.   The power storage device according to any one of claims 1 to 3, wherein a width of the groove is 0.1 mm or more and 1 mm or less. The electrical storage device according to any one of claims 1 to 4, wherein an interval between the plurality of grooves is not less than three times the depth of the grooves and not more than 10 mm.

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