KR20220063447A - Secondary battery and manufacturing method of the same - Google Patents

Abstract

A secondary battery according to an embodiment of the present invention includes: a jelly roll-shaped electrode assembly; a battery case in which the electrode assembly is accommodated and a slit is formed; and a reference electrode having one end passed through the slit to be positioned below the electrode assembly, and the other end exposed to the outside of the battery case.

Description

Secondary battery and manufacturing method thereof

The present invention relates to a secondary battery and a method for manufacturing the same, and more particularly, to a secondary battery including a reference electrode for measuring an electrode potential of a three-electrode system and a method for manufacturing such a secondary battery.

Recently, an increase in the price of energy sources due to the depletion of fossil fuels, interest in environmental pollution is amplified, and the demand for eco-friendly alternative energy sources is becoming an indispensable factor for future life. Accordingly, research on various power production technologies such as nuclear power, solar power, wind power, and tidal power is continuing, and power storage devices for using the generated energy more efficiently are also of great interest.

In particular, as technology development and demand for mobile devices increase, the demand for batteries as an energy source is rapidly increasing, and accordingly, many studies on batteries capable of meeting various needs are being conducted.

Typically, there is a high demand for lithium secondary batteries such as lithium ion batteries and lithium ion polymer batteries having advantages such as high energy density, discharge voltage, and output stability.

In addition, secondary batteries are classified according to the structure of an electrode assembly in which a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode are stacked. Typically, a jelly roll electrode assembly having a structure in which a long sheet-shaped anode and cathode are wound with a separator interposed therebetween, a plurality of cathodes and anodes cut into units of a predetermined size are sequentially stacked with a separator interposed therebetween. and stacked electrode assemblies. Recently, in order to solve the problems of the jelly roll type electrode assembly and the stack type electrode assembly, as a mixed type of the jelly roll type and the stack type, unit cells in which positive and negative electrodes of a predetermined unit are stacked with a separator interposed therebetween are separated A stack/folding type electrode assembly having a structure in which it is sequentially wound while placed on a film has been developed.

In addition, depending on the shape of the case, the secondary battery includes a cylindrical battery in which the electrode assembly is built into a cylindrical case, a prismatic battery in which the electrode assembly is built in a prismatic case, and a pouch in which the electrode assembly is built in a pouch-type case of an aluminum laminate sheet. classified as a type battery.

In order to check the performance of the newly developed and manufactured battery cell, the electrode potential of the battery cell is measured.

A three-electrode system electrode potential measurement method comprising a reference electrode, a working electrode, and a counter electrode is mainly used to measure the electrode potential.

The reference electrode is an electrode used to make a battery circuit for electrode potential measurement to measure the potential of the electrode constituting the battery or the electrode undergoing electrolysis, and serves as a potential reference when measuring the relative value of the electrode potential.

1 is a cross-sectional view for explaining a method for measuring an electrode potential of a three-electrode system for a conventional cylindrical battery.

Referring to FIG. 1 , in the cylindrical secondary battery 10 , the jelly roll type electrode assembly 20 is accommodated in a cylindrical battery case 30 , and electrolyte is injected into the battery case 30 , It can be manufactured by combining the cap assembly 70 in which the electrode terminal is formed on the open upper end. At this time, a gasket 80 for sealing maintenance is disposed between the battery case 30 and the cap assembly 70 , and the battery case 30 and the cap assembly 70 are crimped and coupled to the crimping part 31 . ), coupling of the cap assembly 70 may be made. In addition, the beading part 32 recessed in the center direction may be formed in the battery case 30 for stable coupling of the cap assembly 70 and preventing the electrode assembly 20 from flowing.

Meanwhile, the reference electrode 40 for measuring the three-electrode system electrode potential may be located inside the battery case 30 . The reference electrode 40 may be exposed to the outside by passing between the crimping part 31 and the gasket 80 while being immersed in the electrolyte inside the battery case 30 . However, since the reference electrode 40 is disposed to pass between the crimping part 31 and the gasket 80 as described above, a problem in which the reference electrode 40 is damaged and disconnected during the crimping bonding process may occur frequently. Accordingly, there is a disadvantage in that the production yield is lowered.

There is a need to develop a technology that can solve the problems of the conventional method for measuring the three-electrode electrode potential.

An object of the present invention is to provide a secondary battery capable of measuring an electrode potential of a three-electrode system without the risk of damage or disconnection of a reference electrode, and a method for manufacturing the same.

However, the problems to be solved by the embodiments of the present invention are not limited to the above problems and may be variously expanded within the scope of the technical idea included in the present invention.

A secondary battery according to an embodiment of the present invention includes an electrode assembly in the form of a jelly roll; a battery case in which the electrode assembly is accommodated and a slit is formed; and a reference electrode having one end positioned under the electrode assembly through the slit and the other end exposed to the outside of the battery case.

The battery case may include a bottom part and a side part, and the slit may be formed in the bottom part, a lower end of the side part, or a corner between the bottom part and the side part.

The secondary battery may further include an insulating tape attached to a lower surface of the electrode assembly, and the reference electrode may be positioned between the electrode assembly and the insulating tape.

The secondary battery may further include a sealing member surrounding the reference electrode and sealing the slit.

The reference electrode may include a metal plate positioned under the electrode assembly and a connection part connected to the metal plate and exposed to the outside of the battery case.

The metal plate may include at least one of lithium (Li), lithium titanate (LTO), lithium iron phosphate (LFP), platinum (Pt), and palladium (Pd), and the connection part It may include copper (Cu).

The secondary battery may further include a separation member positioned between the electrode assembly and the reference electrode.

A method of manufacturing a secondary battery according to an embodiment of the present invention includes forming a slit in a battery case; accommodating the electrode assembly in the battery case; and inserting a reference electrode into the slit. In the step of inserting the reference electrode, one end of the reference electrode is located under the electrode assembly, and the other end of the reference electrode is exposed to the outside of the battery case.

The method of manufacturing the secondary battery may further include a sealing step of sealing the slit through which the reference electrode passes.

The battery case may include a bottom part and a side part, and the slit may be formed in the bottom part, a lower end of the side part, or a corner between the bottom part and the side part.

According to embodiments of the present invention, it is possible to provide a secondary battery capable of accurately measuring an actual electrode potential without damage or disconnection of the reference electrode by providing a reference electrode passing through a slit formed in the battery case.

Effects of the present invention are not limited to the effects mentioned above, and other effects not mentioned will be clearly understood by those skilled in the art from the description of the claims.

1 is a cross-sectional view for explaining a method for measuring an electrode potential of a three-electrode system for a conventional cylindrical battery.
2 is a perspective view illustrating a secondary battery according to an embodiment of the present invention.
3 is an exploded perspective view of the secondary battery of FIG. 2 .
4 is an exploded perspective view showing a state before the electrode assembly included in the secondary battery of FIG. 3 is wound.
FIG. 5 is a partial cross-sectional view showing a part of the cross-section taken along the cutting line A-A' of FIG. 2 .
6 is a perspective view illustrating a reference electrode and an insulating tape according to an embodiment of the present invention.
7 is a partial perspective view illustrating a state in which a reference electrode is positioned on a lower surface of an electrode assembly according to an embodiment of the present invention.
8A and 8B are views for explaining various positions of slits formed in a battery case according to embodiments of the present invention.
9 is a perspective view for explaining a method of manufacturing a secondary battery according to an embodiment of the present invention.

Hereinafter, with reference to the accompanying drawings, various embodiments of the present invention will be described in detail so that those of ordinary skill in the art to which the present invention pertains can easily implement them. The present invention may be embodied in several different forms and is not limited to the embodiments described herein.

In order to clearly explain the present invention, parts irrelevant to the description are omitted, and the same reference numerals are assigned to the same or similar components throughout the specification.

In addition, since the size and thickness of each component shown in the drawings are arbitrarily indicated for convenience of description, the present invention is not necessarily limited to the illustrated bar. In order to clearly express various layers and regions in the drawings, the thicknesses are enlarged. And in the drawings, for convenience of description, the thickness of some layers and regions are exaggerated.

Also, when a part of a layer, film, region, plate, etc. is said to be “on” or “on” another part, it includes not only cases where it is “directly on” another part, but also cases where there is another part in between. . Conversely, when we say that a part is "just above" another part, we mean that there is no other part in the middle. In addition, to be "on" or "on" the reference part means to be located above or below the reference part, and to necessarily mean to be located "on" or "on" in the direction opposite to gravity not.

In addition, throughout the specification, when a part "includes" a certain component, this means that other components may be further included, rather than excluding other components, unless otherwise stated.

In addition, throughout the specification, when referring to "planar", it means when the target part is viewed from above, and "cross-sectional" means when viewed from the side when a cross-section of the target part is vertically cut.

2 is a perspective view illustrating a secondary battery according to an embodiment of the present invention. 3 is an exploded perspective view of the secondary battery of FIG. 2 . 4 is an exploded perspective view showing a state before the electrode assembly included in the secondary battery of FIG. 3 is wound.

2 to 4 , a secondary battery 100 according to an embodiment of the present invention includes an electrode assembly 200 in the form of a jelly roll, a battery case 300 in which the electrode assembly 200 is accommodated, and a reference electrode. (400). A slit 300S is formed in the battery case 300 . One end of the reference electrode 400 passes through the slit 300S formed in the battery case 300 and is located under the electrode assembly 200 , and the other end of the reference electrode 400 is exposed to the outside of the battery case 300 . . Here, the slit 300S refers to a penetrating portion formed in the battery case 300 .

First, in the secondary battery 100 according to this embodiment, the electrode assembly 200 is accommodated in the battery case 300 , and electrolyte is injected into the battery case 300 , and then a cap assembly ( 700) can be combined.

The battery case 300 is a structure for accommodating the electrode assembly 200 impregnated with the electrolyte, and may include a metal material and may be a cylindrical case.

The electrode assembly 200 according to the present embodiment may include a first electrode 210 , a second electrode 220 , and a separator 230 . The first electrode 210 , the second electrode 220 , and the separator 230 may be wound together to form the jelly roll-type electrode assembly 200 . The separator 230 may be interposed between the first electrode 210 and the second electrode 220 . In addition, it is preferable that the separator 240 is additionally disposed under the second electrode 220 to prevent the first electrode 210 and the second electrode 220 from contacting each other when wound in the form of a jelly roll.

The first electrode 210 includes a first electrode current collector 211 and a first active material layer 212 formed by coating an electrode active material on the first electrode current collector 211 . Specifically, the electrode active material is applied on the first electrode current collector 211 to form the first active material layer 212 , and the electrode active material is not applied among the first electrode current collectors 211 , so the first electrode current collector ( The first electrode tab 213 may be attached to the portion where the 211 is exposed by welding or the like.

The second electrode 220 includes a second electrode current collector 221 and a second active material layer 222 formed by coating an electrode active material on the second electrode current collector 221 . Specifically, the electrode active material is applied on the second electrode current collector 221 to form the second active material layer 222 , and the electrode active material is not applied among the second electrode current collectors 221 , so the second electrode current collector ( The second electrode tab 223 may be attached to the portion where the 221 is exposed by welding or the like.

The first electrode 210 may be a cathode, and the second electrode 220 may be an anode. That is, the first electrode current collector 211 and the first electrode tab 213 may be a negative electrode current collector and a negative electrode tab, respectively, and the second electrode current collector 221 and the second electrode tab 223 are each a positive electrode current collector. and a positive electrode tab. The first electrode current collector 211, as a negative electrode current collector, may include at least one of copper, stainless steel, aluminum, and nickel, and a negative electrode active material is applied thereon to form the first active material layer 212 . there is. The second electrode current collector 221 is a positive electrode current collector, and may include at least one of stainless steel, aluminum, nickel, and titanium, and a positive electrode active material is applied thereon to form the second active material layer 222 . there is.

On the other hand, in the electrode assembly 200 according to the present embodiment, the first electrode 210 , the second electrode 220 , and the separators 230 and 240 are wound together with a tape to prevent them from being unwound. can be

Hereinafter, the reference electrode according to the present embodiment will be described in detail with reference to FIGS. 2 and 5 to 7 .

5 is a partial cross-sectional view showing a part of the cross-section taken along the cutting line A-A' of FIG. 2 . 6 is a perspective view illustrating a reference electrode and an insulating tape according to an embodiment of the present invention. 7 is a partial perspective view illustrating a state in which a reference electrode is positioned on a lower surface of an electrode assembly according to an embodiment of the present invention. In particular, FIG. 7 shows a state in which the electrode assembly 200 is turned over so that the lower surface 200B of the electrode assembly 200 faces upward.

Referring to FIGS. 2 and 5 to 7 , as described above, one end of the reference electrode 400 passes through the slit 300S formed in the battery case 300 and is located under the electrode assembly 200, the reference The other end of the electrode 400 is exposed to the outside of the battery case 300 .

The battery case 300 may be a cylindrical case with an open top, and may include a bottom part 310 and a side part 320 . The bottom portion 310 corresponds to a circular flat surface, and the side portion 320 corresponds to a curved portion connected to the bottom portion 310 . In this case, the slit 300S may be formed on the side portion 320 , and more specifically, may be formed on the lower end of the side portion 320 , that is, in a region close to the bottom portion 310 . The position of the slit 300S will be described again with reference to FIGS. 8A and 8B below.

The reference electrode 400 may include a metal plate 410 positioned under the electrode assembly 200 and a connection part 420 connected to the metal plate 410 and exposed to the outside of the battery case 300 . The metal plate 410 immersed in the electrolyte functions as a reference electrode terminal, and the connection part 420 is connected to an external electric wire, etc., so that the three-electrode system electrode potential can be measured. The metal plate 410 may have a plate shape, and among lithium (Li), lithium titanate (LTO), lithium iron phosphate (LFP), platinum (Pt) and palladium (Pd). It may include at least one. The connection part 420 may take the form of a metal wire or a metal tab, and there is no limitation on a material thereof as long as it includes an electrically conductive metal, but may include copper (Cu) as an example.

As described above, the reference electrode 400 passes through the slit 300S and is inserted into the battery case 300 . In this case, the secondary battery 100 according to the present embodiment may further include an insulating tape 500 attached to the lower surface 200B of the electrode assembly 200 , and the reference electrode 400 includes the electrode assembly 200 and It may be positioned between the insulating tapes 500 . Here, the lower surface 200B of the electrode assembly 200 means a surface facing the bottom 310 of the battery case 300 , and the first electrode tab is formed from the lower surface 200B of the electrode assembly 200 . 213 may protrude.

The secondary battery 100 according to this embodiment has a separate slit ( 300S), it is possible to avoid the risk that the reference electrode 400 is damaged or disconnected.

On the other hand, one end of the reference electrode 400 according to the present embodiment is located under the electrode assembly 200 as shown in FIG. 5 , and more specifically, as shown in FIG. 7 , an insulating tape 500 . may be attached to the lower surface 200B of the electrode assembly 200 . That is, the reference electrode 400 according to the present embodiment is located under the electrode assembly 200 and can be stably immersed in the electrolyte, and more accurate and consistent three-electrode system electrode potential measurement is possible.

In addition, since the conventional reference electrode 40 is located on the side of the electrode assembly 20 and accommodated in the electrode case 30 together with the electrode assembly 20 , the reference electrode 40 itself accommodates the electrode assembly 20 . may interfere with On the other hand, since the reference electrode 400 according to the present embodiment is inserted into the battery case 300 through the slit 300S separately from the housing of the electrode assembly 200, this problem does not occur.

In addition, since one end of the reference electrode 400 is located under the electrode assembly 200 , the metal plate 410 and the first and second electrodes 210 of the electrode assembly 200 by the weight of the electrode assembly 200 , 220), there is an advantage in that the deviation of the interval between them is reduced.

With respect to the position of the reference electrode 400, the position of the slit will be described with reference to FIGS. 8A and 8B. 8A and 8B are views for explaining various positions of slits formed in a battery case according to embodiments of the present invention.

First, referring to FIG. 2 , as described above, the slit 300S may be formed at the lower end of the side part 320 . As a next modified embodiment, referring to FIG. 8A , a slit 300S1 may be formed in the bottom part 310 , and referring to FIG. 8B , the slit 300S2 is formed by the bottom part 310 and the side part 320 . It can be formed at the edge between. In other words, the slits 300S, 300S1, and 300S2 according to the present embodiments are the bottom part 310 of the battery case 300 , the lower end of the side part 320 , or a corner between the bottom part 310 and the side part 320 . can be formed in

In order to guide the reference electrode 400 to be positioned under the electrode assembly 200, the slits 300S, 300S1, and 300S2 according to the present embodiment are preferably formed at one of the aforementioned positions.

The insulating tape 500 is an electrically insulating tape, and may include at least one of polyimide (PI), polyethylene terephthalate (PET), and polypropylene (PP).

Specifically, the insulating tape 500 extends together in the direction in which the reference electrode 400 is connected, except for the other end of the reference electrode 400 exposed to the outside, in particular, the connection part 420 portion. can cover The insulating tape 500 prevents the reference electrode 400 from contacting the battery case 300 including the bottom part 310 and the side part 320 , thereby causing a risk of a short circuit resulting from the reference electrode 400 . can be blocked in advance.

Meanwhile, referring back to FIGS. 2 and 5 , the secondary battery 100 according to the present embodiment may further include a sealing member 400S that surrounds the reference electrode 400 and seals the slit 300S. .

When the sealing member 400S fills the gap between the slit 300S and the reference electrode 400 , the electrolyte inside the battery case 300 leaks or external moisture or foreign matter flows into the battery case 300 . it can be prevented As long as the gap between the slit 300S and the reference electrode 400 can be effectively sealed, the material of the sealing member 400S is not particularly limited. For example, it may include at least one of a rubber material such as natural rubber or synthetic rubber, an epoxy (epoxide)-based resin, and a plastic material.

There is no particular limitation on the method of forming the sealing member 400S. For example, after the sealing member 400S is formed to surround the reference electrode 400, the sealing member 400S and the reference electrode 400 are inserted together into the slit 300S, or the sealing member 400S is inserted into the slit 300S. After forming , the reference electrode 400 may be inserted before the sealing member 400S is cured.

Since the sealing is maintained by the sealing member 400S, even when the reference electrode 400 is inserted, it is possible to measure the electrode potential of the three-electrode system while maintaining the actual operating state. Since the electrode potential is measured with respect to the secondary battery 100 in an actual operating state, consistent data can be obtained.

Meanwhile, referring back to FIGS. 2, 5 and 6 , the secondary battery 100 according to the present embodiment further includes a separation member 600 positioned between the electrode assembly 200 and the reference electrode 400 . may be included, and the separation member 600 may be in the form of a thin film.

The reference electrode 400 located under the electrode assembly 200 may come into contact with the electrode assembly 200 to cause a short circuit. Such a short circuit may induce an overcurrent of the secondary battery 100 to cause fire or explosion.

Accordingly, since the separation member 600 is positioned between the electrode assembly 200 and the reference electrode 400 , a short circuit between the electrode assembly 200 and the reference electrode 400 may be prevented. In order to effectively prevent a short circuit, the separation member 600 preferably covers both the metal plate 410 and the portion of the connection part 420 adjacent to the metal plate 410 . The separation member 600 may include a porous nonwoven material or a material applied to the separation membrane 230 . There is no particular limitation on the material applied to the separator 230 , and a material such as polypropylene, glass fiber, or polyethylene may be used.

Meanwhile, a bottom insulating member 900 (bottom insulator) may be positioned between the lower portion of the electrode assembly 200 and the bottom portion 310 of the battery case 300 . Although not specifically illustrated, a top insulator may be positioned above the electrode assembly 200 . The upper and lower insulating members may include insulating fibers, and these insulating fibers may be entangled without direction to form a non-woven fabric-like shape. The insulating fiber may include at least one of polyethylene, polybutylene, polystyrene, polyethylene terephthalate, polypropylene, glass fiber, natural rubber, and synthetic rubber.

Hereinafter, a method of manufacturing a secondary battery according to an embodiment of the present invention will be described in detail with reference to FIG. 9 and the like. 9 is a perspective view illustrating a method of manufacturing a secondary battery according to an embodiment of the present invention.

3 and 9 , the method of manufacturing the secondary battery 100 according to an embodiment of the present invention includes the steps of forming a slit 300S in the battery case 300 , and an electrode assembly in the battery case 300 . and inserting the reference electrode 400 into the slit 300S. In the step of inserting the reference electrode 400 , one end of the reference electrode 400 is positioned under the electrode assembly 200 , and the other end of the reference electrode 400 is exposed to the outside of the battery case 300 .

As described above, the slit 300S may be formed in the bottom part 310 of the battery case 300 , the lower end of the side part 320 , or a corner between the bottom part 310 and the side part 320 . There is no particular limitation on the method of forming the slit 300S, and for example, a laser or the like may be used.

Since the reference electrode 400 according to this embodiment is not inserted into the open upper end of the battery case 300 together with the electrode assembly 200, but is inserted into a separate slit 300S, the electrode assembly 200 It does not interfere with storage.

Meanwhile, in the step of inserting the reference electrode 400 , not only the reference electrode 400 , but also the insulating tape 500 and the separation member 600 described above may be inserted into the slit 300S together.

The method of manufacturing the secondary battery according to the present embodiment may further include a sealing step of sealing the slit 300S through which the reference electrode 400 passes. For example, at least one of a rubber material such as natural rubber or synthetic rubber, an epoxy (epoxide)-based resin, and a plastic material surrounds the reference electrode 400, so that the sealing member 400S surrounding the reference electrode 400 is formed. can be formed Thereafter, the sealing member 400S and the reference electrode 400 may be inserted into the slit 300S together. Alternatively, after the sealing member 400S is first formed in the slit 300S, the reference electrode 400 may be inserted before the sealing member 400S is cured.

Thereafter, the step of injecting the electrolyte into the battery case 300 and the step of coupling the cap assembly 700 to the open top of the battery case 300 may be followed. Specifically, the crimping part may be formed by positioning the gasket 800 between the battery case 300 and the cap assembly 700 and bending the end of the battery case 300 . Through this, the cap assembly 700 may be coupled.

Meanwhile, referring back to FIG. 3 , the cap assembly 700 may include an upper cap 710 and a safety vent 720 . The upper cap 710 may be positioned on the safety vent 720 and may be electrically connected to each other by forming a structure in close contact with the safety vent 720 . The upper cap 710 has a center that protrudes upward, and is indirectly connected to the second electrode 220 of the electrode assembly 200 through a second electrode tab 223 (see FIG. 4 ), etc., so as to be connected to an external circuit. It can perform a function as an electrode terminal by

In this embodiment, terms indicating directions such as front, rear, left, right, up, and down are used, but these terms are for convenience of explanation only, and may vary depending on the location of the object or the position of the observer. .

The secondary battery according to the present embodiment described above can be applied to various devices. Specifically, it may be applied to transportation means such as electric bicycles, electric vehicles, hybrids, etc., but is not limited thereto, and may be applied to various devices capable of using a secondary battery.

Although preferred embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements by those skilled in the art using the basic concept of the present invention as defined in the following claims are also provided. is within the scope of the

200: electrode assembly
300: battery case
300S, 300S1, 300S2: Slit
400: reference electrode
500: insulation tape

Claims (10)

electrode assembly in the form of a jelly roll;
a battery case in which the electrode assembly is accommodated and a slit is formed; and
and a reference electrode having one end passed through the slit and positioned under the electrode assembly and the other end exposed to the outside of the battery case. In claim 1,
The battery case includes a bottom portion and a side portion,
The slit is formed in the bottom part, the lower end of the side part, or a corner between the bottom part and the side part. In claim 1,
Further comprising an insulating tape attached to the lower surface of the electrode assembly,
The reference electrode is a secondary battery positioned between the electrode assembly and the insulating tape. In claim 1,
The secondary battery further comprising a sealing member surrounding the reference electrode and sealing the slit. In claim 1,
The reference electrode includes a metal plate positioned under the electrode assembly and a connection part connected to the metal plate and exposed to the outside of the battery case. In claim 5,
The metal plate includes at least one of lithium (Li), lithium titanate (LTO), lithium iron phosphate (LFP), platinum (Pt), and palladium (Pd),
The connection part is a secondary battery including copper (Cu). In claim 1,
The secondary battery further comprising a separation member positioned between the electrode assembly and the reference electrode. forming a slit in the battery case;
accommodating the electrode assembly in the battery case; and
Inserting a reference electrode into the slit,
In the step of inserting the reference electrode, one end of the reference electrode is positioned under the electrode assembly, and the other end of the reference electrode is exposed to the outside of the battery case. In claim 8,
The method of manufacturing a secondary battery further comprising a sealing step of sealing the slit through which the reference electrode passes. In claim 8,
The battery case includes a bottom portion and a side portion,
The slit is formed in the bottom part, the lower end of the side part, or a corner between the bottom part and the side part.

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