Patent Description:
In accordance with the technology development for a mobile device and an increase in a demand for the mobile device, a demand for a secondary battery as an energy source has rapidly increased. Therefore, many studies on the secondary battery that may satisfy various needs have been conducted.

An interest in the secondary battery as an energy source for a power device such as an electric bicycle, an electric vehicle, a hybrid electric vehicle or the like, as well as a mobile device such as a cellular phone, a digital camera, a laptop computer or the like has increased.

A small-sized device such as a cellular phone, a camera or the like uses a small battery pack in which one battery cell is packed. However, a medium or large-sized device such as a laptop computer, an electric vehicle or the like uses a medium or large-sized battery pack in which two or more battery cells are packed in parallel and/or or in series.

A lithium secondary battery has excellent electrical characteristics, but has low stability. For example, the lithium secondary battery may generate heat and gas by causing decomposition reaction of an active material, an electrolyte or the like, which are battery components, in an abnormal operation state such as overcharging, overdischarging, exposure to a high temperature, an electrical short-circuit or the like. High temperature and high pressure conditions resulting from this may further promote the decomposition reaction, thereby causing ignition or explosion. The preamble of claim <NUM> is based on <CIT>.

The present invention has been made in effort to provide an electrode assembly capable of terminating overcharging in a stable state, upon abnormal heating due to overcharging or the like, and a method of stabilizing a secondary battery.

However, problems to be solved by embodiments of the present invention are not limited to the above-mentioned problems, and can be variously extended within a scope of technical ideas included in the present invention.

An exemplary embodiment of the present invention provides an electrode assembly according to claim <NUM>.

Another embodiment of the present invention provides a method of stabilizing a secondary battery according to claim <NUM>.

According to exemplary embodiments, the uncoated portion of the separator is shrunk in an overcharging condition, such that the cathode and the anode are short-circuited to each other and discharging is generated. As a result, the secondary battery is in a stable state of charge. In this case, the PTC characteristics may be sufficiently expressed in the state of charge in which thermal runaway due to self-heating does not occur, whereby overcharging may be safely terminated.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art may easily practice the present invention. The present invention may be implemented in various different forms within the scope of the claims.

Portions unrelated to the description will be omitted to obviously describe the present invention, and similar components will be denoted by same reference numerals throughout the specification.

In addition, since sizes and thicknesses of the respective components illustrated in the drawings are arbitrarily illustrated for convenience of explanation, the present invention is not necessarily limited to those illustrated in the drawings. In the following drawings, thicknesses are exaggerated in order to clearly represent several layers and areas. In addition, in the accompanying drawings, thicknesses of some of layers and regions are exaggerated for convenience of explanation.

It will be understood that when an element such as a layer, a membrane, a region, a plate or the like, is referred to as being "on" or "over" another element, it may include not only the case where it is "directly on" another element but also the case where there is another element present therebetween. To the contrary, it will be understood that when any element is referred to as being "directly on" another element, an element may be not present therebeween. In addition, "on" or "over" does not necessarily mean that any element toward the opposite direction of gravity, but means that any element positioned on or below the reference portion.

Throughout the specification, unless explicitly described to the contrary, "comprising" any components will be understood to imply the inclusion of other elements rather than the exclusion of any other elements.

Further, throughout the specification, the expression "on the plane" means the case in which a target is viewed from the top, and the expression "on the cross section" means the case in which a cross section of a target taken along a vertical direction is viewed from the side.

<FIG> is a schematic view illustrating an electrode assembly according to an exemplary embodiment of the present invention.

Referring to <FIG>, the electrode assembly according to the present invention includes a cathode plate <NUM>, an anode plate <NUM>, and a separator <NUM> positioned therebetween. The separator <NUM> serves to electrically insulate the cathode plate <NUM> and the anode plate <NUM>. The cathode plate <NUM> includes a cathode current collector <NUM>, and a cathode active material layer <NUM> positioned between the cathode current collector <NUM> and the separator <NUM>. The cathode plate <NUM> further includes a positive temperature coefficient (PTC) layer <NUM> positioned between the cathode current collector <NUM> and the cathode active material layer <NUM>. The PTC layer <NUM> refers to a layer having a positive temperature coefficient.

The cathode current collector <NUM> may be generally formed at thickness of <NUM> to <NUM> micrometers. The cathode current collector <NUM> is not particularly limited, as long as it has high conductivity without causing a chemical change in a battery including an electrode assembly according to the exemplary embodiment. For example, as the cathode current collector <NUM>, stainless steel, aluminum, nickel, titanium, sintered carbon, aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver or the like may be used. The cathode current collector <NUM> may be provided with fine ruggedness formed on the surface thereof to thereby increase a bonding force of the cathode active material, and may have various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a non-woven body.

The cathode active material layer <NUM> includes a cathode active material, the cathode active material may include, but is not limited thereto, a layered compound such as lithium cobalt oxide (LiCoO<NUM>), lithium nikel oxide (LiNiO<NUM>) or the like, or a compound substituted with one or more transition metals, lithium manganese oxide such as formula Li<NUM>+yMn<NUM>-yO<NUM> (where y = <NUM> to <NUM>), LiMnO<NUM>, LiMn<NUM>O<NUM>, LiMnO<NUM> or the like, lithium cupper oxide (Li<NUM>CuO<NUM>), vanadium oxide such as LiV<NUM>O<NUM>, LiFe<NUM>O<NUM>, V<NUM>O<NUM>, Cu<NUM>V<NUM>O<NUM> or the like, Ni site-type lithium nikel oxide represented by formula LiNi<NUM>-yMyO<NUM> (where M = Co, Mn, Al, Cu, Fe, Mg, B or Ga, and y = <NUM> to <NUM>), lithium manganese complex oxide represented by formula LiMn<NUM>-yMyO<NUM> (where M = Co, Ni, Fe, Cr, Zn or Ta, and y = <NUM> to <NUM>) or Li<NUM>Mn<NUM>MO<NUM> (where M = Fe, Co, Ni, Fe, Cr, or Zn), LiMn<NUM>O<NUM> in which a part of Li in formula is substituted with an alkaline earth metal ion, a disulfide compound, Fe<NUM>(MnO<NUM>)<NUM> or the like.

The cathode active material layer <NUM> may be prepared by applying cathode material containing a mixture of the cathode active material, a conductive material, and a binder onto the remaining portions of the cathode current collector <NUM> except the portion where tabs are to be formed, followed by drying and pressing. A filler may be further added to the mixture, if necessary.

The conductive material typically may be added in an amount of <NUM> to <NUM> wt% based on the total weight of the mixture containing the cathode active material. This conductive material is not particularly limited, as long as it has conductivity without causing a chemical change in the battery. As the conductive material, for example, graphite such as natural graphite or artificial graphite or the like; carbon black such as carbon black, acetylene black, ketchen black, channel black, furnace black, lamp black, summer black or the like; conductive fiber such as carbon fiber or metal fiber or the like; metal powders such as carbon fluoride, aluminum, and nickel powders or the like; conductive whiskey such as zinc oxide and potassium titanate or the like; conductive metal oxides such as titanium oxide or the like; conductive material such as polyphenylene derivatives or the like may be used.

The binder is a component that assists bonding of the active material and conductive material and bonding to the current collector, and typically may be added in an amount of <NUM> to <NUM> wt% based on the total weight of the mixture containing the cathode active material. Examples of this binder may include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer, sulfonated ethyl-propylene-diene terpolymer, styrene butadiene rubber, fluorine rubber, various copolymers.

The filler may be selectively used as a component for suppressing the expansion of the cathode, and is not particularly limited, as long as it is a fibrous material without causing a chemical change in a battery using an electrode assembly according to the exemplary embodiment. As the filler, for example, olefin-based polymers such as polyethylene, polypropylene or the like, fibrous materials such as glass fibers, carbon fibers or the like may be used.

The anode plate <NUM> includes an anode current collector <NUM>, and an anode active material layer <NUM> positioned between the anode current collector <NUM> and the separator <NUM>. The anode plate <NUM> may further include the PTC layer <NUM> positioned between the anode current collector <NUM> and the anode active material layer <NUM>. The PTC layer <NUM> has the same properties as the PTC layer <NUM> described in the cathode plate <NUM>, and may be formed on only any one or both of the cathode plate <NUM> and the anode plate <NUM>.

The anode current collector <NUM> may be generally formed at a thickness of <NUM> to <NUM> micrometers. The anode current collector <NUM> is not particularly limited, as long as it has high conductivity without causing a chemical change in a battery including an electrode assembly according to an exemplary embodiment. As the anode current collector <NUM>, for example, cupper, stainless steel, aluminum-cadmium alloy or the like may be used. In addition, similar to the cathode current collector <NUM>, the anode current collector <NUM> may be provided with fine ruggedness formed on the surface thereof to thereby increase a bonding force of the anode active material, and may have various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a non-woven body.

The anode active material layer <NUM> includes an anode active material, and as the anode active material, for example, carbon such as hard carbon, graphite-based carbon, metal complex oxides such as LixFeO<NUM> (<NUM>≤x≤<NUM>), LixWO<NUM> (<NUM>≤x≤<NUM>), SnxMe<NUM>-xMe'yOz (Me: Mn, Fe, Pb, Ge; Me': Al, B, P, Si, Group <NUM>, Group <NUM> and Group <NUM> elements of the Periodic Table, halogen; <NUM>≤x≤<NUM>; <NUM>≤y≤<NUM>; <NUM>≤z≤<NUM>), metal oxides such as lithium metal, lithium alloy, silicon-based alloy, tin-based alloy, SnO, SnO<NUM>, PbO, PbO<NUM>, Pb<NUM>O<NUM>, Sb<NUM>O<NUM>, Sb<NUM>O<NUM>, GeO, GeO<NUM>, Bi<NUM>O<NUM>, Bi<NUM>O<NUM> or Bi<NUM>O<NUM>, a conductive polymer such as polyacetylene, Li-Co-Ni-based material may be used.

The anode active material layer <NUM> may be prepared by applying anode material containing a mixture of the anode active material, a conductive material, and a binder onto the remaining portions of the anode current collector <NUM> except the portion where tabs are to be formed, followed by drying and pressing. The mixture may further include a filler, if necessary.

As the separator <NUM>, an insulating thin film having high ion permeability and mechanical strength may be used. For example, as the separator <NUM>, an olefin-based polymer such as polypropylene having chemical resistance and hydrophobic property, a sheet or non-woven fabric made of glass fiber or polyethylene or the like may be used.

Coating layers <NUM> and <NUM> are respectively positioned above and below the separator <NUM> according to the present exemplary embodiment. The coating layers <NUM> and <NUM> are for improving a thermal stability of the separator <NUM>, and may be formed of an organic/inorganic complex layer. The coating layers <NUM> and <NUM> may include a ceramic material. Specifically, the separator <NUM> according to the exemplary embodiment includes a first portion A covered with the coating layers <NUM> and <NUM>, and a second portion B exposing a surface of the separator <NUM> facing the cathode plate <NUM> and a surface of the separator <NUM> facing the anode plate <NUM>. In other words, the second portion B refers to a portion of the separator <NUM> not covered with the coating layers <NUM> and <NUM>. Here, when the direction horizontal to the upper surface of the separator <NUM> is a first direction as shown in <FIG>, the direction vertical to the upper surface of the separator <NUM> may be a second direction as shown in <FIG>, and the second portion B of the separator <NUM> may overlap the cathode active material layer <NUM> and the anode active material layer <NUM> in the direction vertical to the upper surface of the separator <NUM>. In this case, the portion of the cathode current collector <NUM> and portion of the anode current collector <NUM> which overlap the entire region occupied by the second portion B of the separator <NUM> in the direction vertical to the upper surface of the separator <NUM> may be covered with the cathode active material layer <NUM> and the anode active material layer <NUM>, respectively.

The first portion A of the separator <NUM> may occupy most of the entire area of the separator <NUM>, and the second portion B of the separator <NUM> may occupy an edge of the separator <NUM>. In <FIG>, although it is shown that an uncoated portion corresponding to the second portion B is formed at only one end of the separator <NUM>, an uncoated portion such as the second portion B may be additionally formed at the other end of the separator <NUM>. Positions and shapes of the uncoated portions may be variously modified. However, when the portions occupied by the uncoated portions are too wide relative to the entire area of the separator <NUM>, it may have an adverse effect on durability in a situation where the battery is normally driven. Therefore, an area of the second portion B corresponding to the uncoated portion is preferably <NUM>% or more to <NUM>% or less of a total area of the separator. When the area of the second portion B is less than <NUM>% of the total area of the separator, the possibility of short-circuiting the cathode plate <NUM> and the anode plate <NUM> to each other is very low due to thermal shrinkage of the separator <NUM>, and the effect according to an exemplary embodiment of the present invention may be less likely to be implemented. In addition, when the area of the second portion B exceeds <NUM>% of the total area of the separator, durability may be deteriorated.

The electrode assembly according to an exemplary embodiment as described above may be impregnated with an electrolyte to constitute a secondary battery.

Hereinafter, the PTC layers <NUM> and <NUM> according to the exemplary embodiment will be described in more detail.

The PTC layers <NUM> and <NUM> may include a PTC material of a conductive filler and a polymeric material which serves as an electrical insulator. As the polymer material, any conventional thermoplastic polymer used in the manufacture of the PTC material may be selected and used. Specifically, a thermoplastic polymer is a semi-crystalline material, as it may be easier to obtain PTC characteristics from the semi-crystalline material as compared to an amorphous thermoplastic material. As an example, the semi-crystalline thermoplastic material may have a crystallinity of <NUM>% or more, preferably <NUM>% or more, and more preferably <NUM>% or more. Here, the term "semi-crystalline" means that a behavior of the thermoplastic material has a degree of crystallinity sufficient to exhibit a significant degree of, but not completely, crystalline thermoplastic behavior.

In the exemplary embodiment, the thermoplastic polymer may include a high density polyethylene, a linear low density polyethylene, a low density polyethylene, a medium density polyethylene, a maleic anhydride functionalized polyethylene, a maleic anhydride functionalized elastomer ethylene copolymer, an ethylene-butene copolymer, an ethylene-octene copolymer, an ethylene-acrylate copolymer such as an ethylene-methyl acrylate, an ethylene-ethyl acrylate and an ethylene butyl acrylate copolymer, a polyethylene containing glycidyl methacrylate-modified polyethylene, a polypropylene, a maleic anhydride functionalized polypropylene, a glycidyl methacrylate-modified polypropylene, a polyvinyl chloride, a polyvinyl acetate, a polyvinyl acetyl, an acrylic resin, a syndiotactic polystyrene. The thermoplastic polymer may include, but is not limited thereto, a polyamide, a poly-tetra-fluoroethylene, a polybutylene-terephthalate, a polyphenylene-sulfide, a polyamideimide, a polyimide, a polyethylene vinyl acetate, a glycidyl methacrylate-modified polyethylene vinyl acetate, a polyvinyl alcohol, a poly (methyl methacrylate), a polyacrylonitrile, a polybutadiene, a polyethylene-terephthalate, a poly (<NUM>-aminocaprylic acid), a poly (vinyl alcohol), a polycaprolactone or a combination of one or more polymers. As an example, as the thermoplastic polymer, a polyethylene such as high density polyethylene may be used, wherein "high density" refers to having a density of greater than <NUM>/cm<NUM>.

The amount of the thermoplastic polymer may be <NUM> to <NUM> wt%, preferably <NUM> to <NUM> wt%, and more preferably <NUM> to <NUM> wt%, based on the total weight of the PTC composition. As the conductive filler, a carbon-based material such as carbon black, carbon fiber, graphite may be used, but is not necessarily limited thereto. As the PTC layers <NUM> and <NUM>, a ceramic material, for examples, BaTiO<NUM> may be used. In addition, the PTC layers <NUM> and <NUM> may be prepared by mixing and synthesizing pure BaTiO<NUM> raw material with Y<NUM>O<NUM> and Nb<NUM>O<NUM> having a valence of +<NUM> and +<NUM>. For temperature transition, Pb and Sr elements may be substituted for Ba.

As the polymer material included in the PTC layers <NUM> and <NUM>, the thermoplastic materials described above may be commonly used, but the use of a thermosetting resin is not completely excluded.

The PTC layers <NUM> and <NUM> are material layers exhibiting PTC characteristics that they are changed from a conductor to a non-conductor at a specific temperature. The PTC layers <NUM> and <NUM> serve to allow the active material to exhibit a constant conductivity regardless of charging and discharging when the battery is normally operated, but are changed to a non-conductor to hinder the battery from being properly operated when an internal temperature of the battery rises due to a short-circuit or an accident.

An effective operating temperature of the PTC layer is preferably <NUM> to <NUM>. Here, the "effective operating temperature of the PTC layer" refers to a temperature at which the PTC layer exhibits the PTC characteristics, i.e., a temperature capable of functioning as a fuse that blocks a current due to a rapid increase in resistance caused by the generation of joule heat when an excessive current is generated. Generally, a discharging temperature of the lithium secondary battery is -<NUM> to <NUM>, and a charging temperature thereof is <NUM> to <NUM>. However, an internal temperature of the battery may rapidly rise to <NUM> or more due to overcharging, an internal short-circuit or the like. In this case, it is preferable that the PTC layer is operated. However, when the temperature at which the PTC characteristics are expressed exceeds <NUM>, the PTC characteristics do not appear until the internal temperature of the battery rises excessively, which is not preferable in terms of stability of the battery. Particularly, an event such as a battery explosion due to thermal runaway caused by self-heating before the PTC characteristics are fully expressed may occur.

According to the present invention, as described above, an uncoated portion not covered with the coating layer is implemented at the edge portion of the separator in order to prevent thermal runaway from being occurred before the PTC characteristics are fully expressed. As described above, a process of safely terminating the state occurred due to overcharging or the like by simultaneously implementing the PTC layer and the uncoated portion will be described later.

<FIG> is a view illustrating that in the electrode assembly of <FIG>, a cathode plate and an anode plate are short-circuited to each other under an overcharging condition.

First, when the battery is overcharged, a voltage and a temperature in the battery may rise simultaneously. In this case, thermal shrinkage occurs in the uncoated portion corresponding to the second portion B of the separator <NUM> described in <FIG>. Due to the thermal shrinkage, the cathode plate <NUM> and the anode plate <NUM> oppose each other, and the cathode plate <NUM> and the anode plate <NUM> are in contact with each other, resulting in a short-circuit. Here, the cathode active material layer <NUM> and the anode active material layer <NUM> may be in direct contact with each other. In the exemplary embodiment, the cathode active material layer <NUM> and the anode active material layer <NUM> are in direct contact with each other, thereby making it possible to reduce the possibility of occurrence of an event such as an explosion or a fire due to a short-circuit between the current collectors <NUM> and <NUM> in a manufacturing process or a normal operation of the battery.

When the cathode plate <NUM> and the anode electrode plate <NUM> are in contact with each other to be short-circuited to each other, a voltage of the secondary battery increased due to overcharging drops. As described above, the battery voltage drops and a stable state of charge is maintained for a predetermined time. Then, the temperature again rises due to continuous charging, and the polymer material are expanded and melted in the PTC layers <NUM> and <NUM> included in the cathode plate <NUM> and the anode plate <NUM>, such that a resistance of the battery cell is increased. Specifically, in the polymer material and the conductive filler contained in the PTC layers <NUM> and <NUM>, an interval between the conductive filler particles present in the polymer material is increased due to a rapid thermal expansion change in the vicinity of a melting temperature, such that a flow of electrons is disturbed. Therefore, a battery resistance is rapidly increased, such that a current is blocked.

The PTC layers <NUM> and <NUM> included in the electrode assembly according to the exemplary embodiment are firstly in a stable state of charge by short-circuiting the cathode plate <NUM> and the anode plate <NUM> caused by thermal shrinkage of the uncoated portion of the separator under the overcharging condition, thereby sufficiently exhibiting the PTC characteristics.

<FIG>, which is Comparative Example, is a graph illustrating a change in a voltage over time when an electrode assembly to which a PTC material is applied without an uncoated portion is overcharged.

Comparative Example of <FIG> shows a result of an overcharging test using an electrode assembly of which most of the configurations are the same as those of the embodiments described in <FIG> and <FIG>, but in which an uncoated portion corresponding to the second portion B of the separator <NUM> described in <FIG> and <FIG> is not present and an edge of the separator <NUM> is covered with coating layers <NUM> and <NUM>. The electrode assembly is charged until a battery voltage reaches a termination voltage, which is <NUM> to <NUM> times the maximum value of a driving voltage of the cell, and when an event such as explosion and ignition does not occur during the charging or after completion of the charging, it can be interpreted that the electrode assembly has passed the overcharging test.

Referring to <FIG>, when an electrode assembly is overcharged in a specific state of charge or more, thermal runaway occurs due to self-heating, resulting in events such as an explosion or a fire. Specifically, in <FIG>, a portion indicated by "Vent" means that an internal mixed gas is released to the outside due to a significant increase in an internal pressure of the cell caused by an abnormal environment. When a "Vent" phenomenon occurs, the voltage may temporarily decrease due to a pre-symptom of an event such as explosion. Then, thermal runaway occurs before the voltage reaches the termination voltage.

<FIG> is a graph illustrating a change in a voltage over time when an electrode assembly according to an exemplary embodiment of the present invention is overcharged.

Referring to <FIG>, the cathode and the cathode plates are short-circuited due to the shrinkage of the separator in the uncoated portion and the battery voltage drops, and a voltage overshoot occurs when the PTC characteristic is expressed in such a stable state of charge. The overcharging test may be terminated after the battery voltage reaches a target voltage, which is a maximum value of the battery voltage that may appear in a test process due to the voltage overshoot, and the battery voltage drops to a stable range as the charging is terminated. In the case of the exemplary embodiment, as shown in <FIG>, the state of overcharge may be safely terminated without explosion or a fire.

<FIG> is a view illustrating a modified embodiment of the electrode assembly described in <FIG>.

The modified embodiment of <FIG> and the embodiment described in <FIG> are the same in view of the most configuration. However, in an embodiment of <FIG>, the PTC layers <NUM> and <NUM> described in <FIG> are not formed separately from the active material layers <NUM> and <NUM> in the cathode plate <NUM> and the anode plate <NUM>, and the PTC material is mixed in the active material layers <NUM> and <NUM>. In relation to this, referring to <FIG>, a cathode active material layer <NUM> in which the PTC material is mixed between the separator <NUM> and the cathode current collector <NUM>; and an anode active material layer <NUM> in which the PTC material is mixed between the separator <NUM> and the anode current collector <NUM> are positioned. In other words, the PTC material is mixed in the cathode active material layer <NUM> and the anode active material layer <NUM> in the cathode plate <NUM> and the anode plate <NUM>.

In the exemplary embodiment, as described in <FIG> and <FIG>, the PTC characteristic may be expressed and the overcharging condition may be terminated without an event such as explosion or fire under a stable state of charge.

Claim 1:
An electrode assembly, comprising:
a cathode plate (<NUM>) and an anode plate (<NUM>);
a separator (<NUM>) positioned between the cathode plate (<NUM>) and the anode plate (<NUM>); and
coating layers (<NUM>, <NUM>) positioned between the separator (<NUM>) and the cathode plate (<NUM>) and between the separator (<NUM>) and the anode plate (<NUM>),
wherein the separator (<NUM>) includes a first portion (A) covered with the coating layer (<NUM>, <NUM>), and a second portion (B) exposing a surface facing the cathode plate (<NUM>) and a surface facing the anode plate (<NUM>), and the second portion (B) is positioned at an edge of the separator (<NUM>),
wherein the cathode plate (<NUM>) includes a cathode current collector (<NUM>) and a cathode active material layer (<NUM>), and the anode plate (<NUM>) includes an anode current collector (<NUM>) and an anode active material layer (<NUM>), and the second portion (B) of the separator (<NUM>) overlaps the cathode active material layer (<NUM>) and the anode active material layer (<NUM>) in a direction vertical to an upper surface of the separator (<NUM>), and
wherein a portion of the cathode current collector (<NUM>) and a portion of the anode current collector (<NUM>) which overlap an entire region occupied by the second portion (B) of the separator (<NUM>) in the direction vertical to the upper surface of the separator (<NUM>) are covered with the cathode active material layer (<NUM>) and the anode active material layer(<NUM>), respectively, characterized in that the separator (<NUM>) includes an olefin-based polymer, and the separator (<NUM>) is capable of thermal shrinkage, and at least one of the cathode plate (<NUM>) and the anode plate (<NUM>) includes a positive temperature coefficient (PTC) material (<NUM>, <NUM>).