Abstract:
A PTC device as well as an electrical device such as a battery pack or a dry-cell type secondary cell containing a PTC device and a secondary cell is made more compact. The PTC device includes (1) a PTC component including a laminar polymer PTC element having an electrically conductive filler and a polymer material, and a metal electrode disposed on a surface of each side of the polymer PTC element; and (2) a lead positioned at least in part on the metal electrode of the PTC component, and connected to the metal electrode by an electrically conductive material An exposed part of the electrically conductive material is covered by a protective member including a polypropylene resin, a nylon resin or an epoxy resin.

Description:
RELATED APPLICATIONS 
       [0001]    This Application is a divisional of U.S. patent application Ser. No. 13/807,699, filed Mar. 28, 2013, entitled PTC Device and Secondary Battery Having the Same, and incorporated by reference herein in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a PTC device as well as an electrical device, for example a secondary battery, comprising same. 
       BACKGROUND ART 
       [0003]    A polymer PTC component, comprising a laminar PTC element containing a conductive filler and a polymer material, and a metal electrode disposed on a surface of each side of the polymer PTC element, is widely used in various electrical devices. For example, such a PTC component is used as a circuit protection device in a secondary battery charging circuit in order to prevent a problem of overcharging and the like when a secondary battery of a cell phone is charged. This PTC component is disposed in a protection circuit via leads, and a PTC device comprising the PTC component and the leads, and the protection circuit are disposed outside the positive electrode terminal; these together make up an electrical device, and are used as the so-called a “battery pack”. Since an electrical apparatus has the battery pack embedded therein, the electrical apparatus may be made more compact if the embedded battery pack could be made more compact. 
         [0004]    On the other hand, various types of dry-cell type secondary batteries are used. Among such secondary batteries, in the case of a size AA or a size AAA cell for example, because there is a constraint in enlarging the volume it occupies, no PTC component is not provided therein. 
         [0005]      FIG. 1  shows a nickel hydride battery, as an example of such a dry-cell type secondary battery, schematically as a cross-sectional view so that its internal structure may be seen. In this battery  100 , nickel hydroxide is used as a positive electrode  102  and a hydrogen absorbing alloy is used as a negative electrode  104 , with a separator and an electrolyte present between the electrodes. When discharging for example, hydrogen absorbed in the alloy discharges electrons, and hydrogen ions migrate in the electrolyte from the negative electrode to the positive electrode. The positive electrode is connected to a positive electrode tab  106 , which is connected to a positive electrode terminal  110  via a sealing plate  108 . 
         [0006]    The electrolyte used comprises an alkali as a base and contains for example a potassium salt such as potassium hydroxide. Various salts, such as those of sodium, lithium, etc., are used as other alkalis in some cases. Such alkalis are generally corrosive so that the positive electrode tab is exposed to an environment of such alkali. Therefore, a material having resistance to the alkaline corrosion needs to be selected for the material to be used as the positive electrode tab. For example, nickel is used as the material of the positive electrode tab. Nickel forms a passive material in the alkaline environment, as a result of which it has resistance to the corrosion. 
         [0007]    A battery is shown in Japanese Patent Publication No. 2008-181855. 
         [0008]    The inventors concentrated their investigation on making a PTC device and an electrical device such as a battery pack comprising the PTC device and a secondary battery, and a dry-cell type secondary battery more compact, as a result of which it has been concluded that, in the above dry-cell type secondary battery, if the PTC device could be disposed in a space  112  where the positive electrode tab  106  exists (this space may herein also be called a “positive electrode tab space”), the volume of the secondary battery may be made smaller. Also, it has been concluded that, in the battery pack, if the PTC device disposed outside the secondary battery could be disposed within the secondary battery, the volume of the battery pack may be made smaller. 
         [0009]    However, in order to dispose the PTC device in the positive electrode tab space, or to dispose the PTC device within the secondary battery of the battery pack, the alkaline corrosion as described above needs to be considered. After further studies as to this, it has been found that, among the various elements forming the PTC device, by covering a part(s) that may be adversely affected by the corrosion, etc., by the alkali contained in the electrolyte of the secondary battery with a polypropylene resin, a nylon resin or an epoxy resin, such adverse effects can be suppressed to a minimum, resulting in the PTC device of the present invention. 
         [0010]    Therefore, according to the first aspect, the present invention provides a PTC device comprising:
   (1) a PTC component comprising:
       (A) a laminar polymer PTC element comprising
           (a1) an electrically conductive filler, and   (a2) a polymer material, and   
           (B) a metal electrode disposed on a surface of each side of the polymer PTC element; and   
       (2) a lead positioned at least in part on the metal electrode of the PTC component, and connected to the metal electrode by an electrically conductive material; characterized by an exposed part of said electrically conductive material being covered by a protective member comprising a polypropylene resin, a nylon resin or an epoxy resin.   
 
         [0017]    Among the elements constituting the PTC device of the present invention, the PTC component is well-known, and such a PTC component may be used in the present invention. The lead is an element required to electrically connect the PTC component to a prescribed circuit; this per se also is well-known, and such a lead, e.g. a nickel lead, a nickel-plated stainless steel lead, a nickel-iron alloy (Kover™) lead, and the like, may be used in the present invention. Further, the electrically conductive material connecting the metal electrode of the PTC component to the lead is well-known, and is not restricted in particular as long as they are able to be electrically connected to each other; in the present invention also, such an electrically conductive material may be used. For example, a solder, a solder paste, an electrically conductive adhesive, and the like, may be used. In the PTC device, such electrically conductive material may be a state (for example, a state after curing of an element of the composition such as a setting resin) which is changed from its original state (for example, a state before curing of the element of the composition such as a setting resin) in order to have a function for connecting. 
         [0018]    The present invention is useful in a case where such an electrically conductive material may be adversely affected, e.g. corroded, by the electrolyte constituting the battery, in particular an alkaline component contained therein, for example a salt and/or an ion of an alkaline metal such as sodium, potassium, lithium, etc. In particular, such case corresponds to a case wherein an electrically conductive material contains, for example, tin, zinc, lead, or the like. The present invention is particularly useful when using in particular a solder as the electrically conductive material, in particular a lead-free solder, for example an alloy solder of tin, silver and copper, an alloy solder of tin and bismuth, an alloy of tin and indium, or the like. 
         [0019]    In the present invention, “a polypropylene resin, a nylon resin, or an epoxy resin” may be any commonly-known resin as long as the problem of the invention can be solved. For example, in the case of the polypropylene resin, a syndiotactic type is preferably used. For example, in the case of the nylon resin, 6,6-nylon is preferably used, and a resin with high crystallinity is more preferably used. For example, in the case of the epoxy resin, particularly a resin with an excellent chemical resistance, for example a two-component epoxy resin formed from an epoxy resin prepolymer and a polyamine resin is preferably used. 
         [0020]    In the PTC device of the present invention, the protective member may be in any suitable form. The protective member may be in the form of a protective coating. In this case, the resin as described above is applied, for example by brushing, spraying, etc., so as to cover the exposed part. As needed, it is preferred that a suitable form for applying the resin in the above way, for example, a solution, a dispersed liquid or the like of the resin is obtained by using a solvent or the like, and this is applied, and as necessary, the solvent or the like is dried (for example, removed by heating). Specifically, it is preferred that this form is used when forming a protective member containing an epoxy resin. 
         [0021]    In another embodiment, a film may be obtained from the resin as described above, and the PTC component may be enveloped by this film so that only the end of the lead protrudes. In other words, the protective member may be in a film form enveloping the PTC component or a form wherein two pieces of film sandwich the PTC component therebetween. The protective member may be formed by using, for example, a polypropylene resin film, specifically a biaxial oriented polypropylene (OPP), or a nylon resin film. The polypropylene resin film may also be a layer which is called a K coat film (KOP). This film is a polypropylene film which is coated with a PVDF (polyvinylidene fluoride) resin on one surface or both surfaces thereof, and is effective in that it has an improved gas barrier property. 
         [0022]    In a preferred embodiment of the present invention, such a film is used with at least one, for example one or two additional layers laminated thereon. Examples of the additional layers include, for example, an EVA (ethylene-vinyl acetate copolymer) layer, an EVOH (ethylene-vinyl alcohol copolymer) layer, a PVDF (polyvinylidene fluoride) layer, a PVA (polyvinyl alcohol) layer, a PVDC (polyvinylidene chloride) layer, a PE (polyethylene) layer, an L-LDPE (linear low density polyethylene) layer, and the like, for the purpose of improving an adhesion of the film to the exposed part of the PTC component, the leads, etc., for the purpose of improving a gas barrier property, and/or for the purpose of improving workability. When using such laminated film, the film is wrapped so that the polypropylene resin film or layer (for example, a biaxial oriented polypropylene (OPP) layer) or the nylon resin film or layer is positioned far from the PTC component. 
         [0023]    The PE layer and the L-LDPE layer are effective in improving adhesion; the PVA layer and the PVDC layer are effective in improving a gas barrier property; the EVA layer is effective in improving both the adhesion and the gas barrier property; the EVOH layer is effective in improving the gas barrier property and an easy workability. 
         [0024]    When using the laminated film, as described above, a laminated film may be used wherein, for example, the following combinations of the resin layers are laminated:
   OPP/PE   OPP/L-LDPE   OPP/EVA   KOP/PE   KOP/L-LDPE   KOP/EVA   OPP/EVOH/PE   OPP/EVOH/L-LDPE   OPP/EVOH/EVA   OPP/PVA/PE   OPP/PVA/L-LDPE   OPP/PVA/EVA   
 
         [0037]    In a further embodiment, most of the PTC component, including the exposed part, may be enveloped by the protective member containing the resin, as described above, by insert molding wherein the above described resin in a molten form is injected into a mold in which the PTC component is disposed on (or inserted into) the mold beforehand. In other words, the protective member may be a molded member of the above described resin enveloping the PTC component. 
         [0038]    In the broadest aspect of the present invention, in the PTC device of the present invention, the “protective member comprising a polypropylene resin, a nylon resin or an epoxy resin” is disposed such that at least a portion of a periphery of the PTC component is covered. In other words, said portion is the exposed part of the electrically conductive material as described above. For example, substantially the exposed part only, or the exposed part and its surrounding part are covered with such member. Needless to say, the protective member may cover substantially the entirety of the PTC component and the exposed part of the lead (in other words, parts which are exposed to an environment atmosphere, if the protective member is absent), with the exception of the lead end. 
         [0039]    Various such PTC devices of the present invention can be disposed inside a secondary battery, in particular within its positive electrode tab space, and function as a positive electrode tab. Therefore, the present invention, in the second aspect, provides a secondary battery as an electrical device. Such a secondary battery may be a dry cell type or in the form of a battery pack including a PTC device on its outside. It is noted that as far as the PTC device of the present invention can be used as a protective component, the present invention also provides for other electrical devices containing such a PTC device. 
         [0040]    This secondary battery comprises a positive electrode, a negative electrode and a separator and an electrolyte disposed therebetween, and is characterized by having the PTC device of the present invention, more specifically the leads of the PTC device, electrically connected, directly or indirectly, to and between the positive electrode and the positive electrode tab. Such a secondary battery of the present invention may be a dry cell type or in the form of a battery pack including a protection circuit on its outside of the secondary battery. 
         [0041]    In the PTC device of the present invention, because the exposed part of the electrically conductive material, which is likely to be adversely affected by the corrosion due to an alkali contained in the electrolyte, is covered by the protective member comprising “a polypropylene resin, a nylon resin or an epoxy resin” and these resins have alkali resistance, even if the PTC device is disposed under an alkaline environment, the protective member suppresses that the alkali substantially reaches the electrically conductive material, as a result of which the effect of the corrosion on the electrically conductive material, and therefore the PTC device, is able to be suppressed to a minimum. 
         [0042]    Thus, even if the PTC device is disposed under an environment of the electrolyte contained in the secondary battery, or the electrolyte and the PTC device accidentally come in contact, the adverse effect that the PTC device receives from the electrolyte is minimal, so that the PTC device can function as a circuit protection device. 
         [0043]    In particular, when the PTC device is disposed in an extra space (in the sense that the PTC device is able to be accommodated therein) within the secondary battery, a space needed for the PTC device may be saved. For example, the PTC device of the present invention may be disposed in the above mentioned positive electrode tab space of the secondary battery. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0044]      FIG. 1  shows a cross-sectional view of a secondary battery with its internal structure illustrated schematically; 
           [0045]      FIG. 2  shows a PTC device according to one embodiment of the present invention schematically in its cross-section; 
           [0046]      FIG. 3  shows a PTC device according to another embodiment of the present invention schematically in its cross-section; 
           [0047]      FIG. 4  shows a PTC device according to a further embodiment of the present invention schematically in its cross-section; 
           [0048]      FIG. 5  shows the secondary battery illustrated in  FIG. 1  having a PTC device of the present invention in a cross-sectional view; and 
           [0049]      FIG. 6  shows the measured results of the resistance-temperature property (R-T property) of the PTC device A of the present invention produced in Example 1. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0050]    The present invention is described in more detail below with reference to the drawings.  FIG. 2  shows a PTC device according to one embodiment of the present invention. The illustrated PTC device  10  comprises a PTC component  22  having a laminar polymer PTC element  12  containing an electrically conductive filler and a polymer material, and metal electrodes (e.g. metal foil electrodes)  18  and  20  on the surfaces  14  and  16  on the both sides of the PTC element, and the leads  24  and  26 . 
         [0051]    The lead  24  is connected electrically to a part of the metal electrode  18  by an electrically conductive material  28 , and the lead  26  is connected electrically to the entire metal electrode  20  by an electrically conductive material  30 . As a result, the electrically conductive material has an exposed part on the side surface  32  of the PTC component  22  if there is no protective member present. 
         [0052]    In the PTC device  10  of the present invention, such exposed parts of the electrically conductive material are covered by the protective members as a protective coating  34  comprising a polypropylene resin, a nylon resin or an epoxy resin. 
         [0053]    The protective coating may be in any form; in the embodiment shown in  FIG. 2 , a side part  32  of the electrically conductive material as the exposed part and a relatively narrow region(s) above and/or below the side part are covered by the protective coating  34 . Such a protective coating may be formed by applying a protective coating forming material (e.g. an epoxy resin or a solution or dispersion containing it) by brushing, spraying, etc., and hardening it, and further removing the solvent as needed. Needless to say, the protective coating may cover substantially the entire PTC component and the leads with the exception of the lead end. In this case, the protective coating may be substantially in a similar form as films ( 58 ,  60 ) in  FIG. 4  below. 
         [0054]      FIG. 3  shows schematically a PTC device according to another embodiment of the present invention. In the embodiment shown, the PTC device  40  comprises a PTC component  42  and leads  44  and  46  on both its sides. This PTC device has a molded member  48  around it. In the embodiment shown, the PTC component  42  is illustrated as a whole for the sake of simplicity, and illustration of the electrically conductive materials positioned between the PTC component  42  and the leads  44 ,  46  is omitted. 
         [0055]    Such a PTC device may be produced by inserting a PTC component connected to leads by a conductive material into a mold, then performing injection molding wherein a molding material containing or consisting of a polypropylene resin, a nylon resin or an epoxy resin in its molten state is injected into the mold and solidified, thereby forming the molded member  48  as a protective member so as to cover the exposed surface of the PTC component  42  including the exposed parts of the electrically conductive material. However, as shown in  FIG. 3 , the ends of the leads protrude from the molded member. Therefore, there is a need for forming the mold and inserting the PTC component in the mold so that the molded member will not be formed on the ends of the leads during the injection molding. 
         [0056]      FIG. 4  shows a PTC device  50  according to a further embodiment of the present invention. In the embodiment shown, the PTC device comprises a PTC component  52  and leads  54  and  56  on both its sides. The PTC device is sandwiched between two pieces of film  58  and  60 , with the ends of the leads  54  and  56  protruding outward. These films are formed of the material comprising a polypropylene resin, a nylon resin or an epoxy resin, and these films constitute the protective member of the PTC device of the present invention. As is easily understood, the films  58  and  60  cover the exposed surface of the PTC component and the exposed parts of the leads (excluding, however, their ends), including the exposed parts of the electrically conductive material, and can function as a protective member of the PTC device of the present invention. As in  FIG. 3 , in the embodiment shown, the PTC component  52  is illustrated as a whole for the sake of simplicity, and illustration of the electrically conductive materials positioned between the PTC component  52  and the leads  54  and  56  is omitted. 
         [0057]    Such a PTC device may be produced by sandwiching a PTC component having leads electrically connected by a conductive material between two pieces of the films, with the ends of the leads protruding from the films, and then compression bonding, for example thermal compression bonding (i.e. heat sealing) the films together. In this embodiment, it is preferred that the film has an additional layer(s) on its inside to secure an improved adhesion between the film and the PTC component and the lead. As to the additional layer, the EVA layer, the EVOH layer, the PVDF layer as described above are suitably used. 
         [0058]      FIG. 5  shows, schematically as in  FIG. 1 , a secondary battery of the present invention with a PTC device of the present invention incorporated therein. In the embodiment shown, the secondary battery  70  has the PTC device  74  of the present invention inside a positive electrode tab space  72 , and is characterized by the end of one lead  76  being connected to a positive electrode terminal  80  via a sealing plate  78  and the other lead  82  being connected to a positive electrode  84 . Thus, with the exception of the PTC device of the present invention being disposed instead of the positive electrode tab, the other parts of the secondary battery of the present invention may be the same as the known secondary battery. 
         [0059]    In another embodiment, the secondary battery of the present invention has a protection circuit on its outside and comprises the PTC device of the present invention disposed on the outside of the secondary battery, for example, on the outside of the positive electrode. In this case, in such a secondary battery, the PTC device is disposed within the protection circuit. These together work to protect the secondary battery, and the secondary battery, the PTC device and the protection circuit constitute a battery pack. Thus, even when the PTC device is disposed outside of the secondary battery, if there is a slight leak of the electrolyte in the secondary battery, the adverse effects from the electrolyte to the electrically conductive material may be suppressed. 
       EXAMPLE 1 
       [0060]    A PTC device A of the present invention as shown in  FIG. 4  was produced by thermal compression-bonding a PTC component sandwiched by films (provided that the end of the lead was exposed). During the thermal compression bonding, only the film portion extending outside from around the PTC component was pressed so that force would not be applied to the PTC component (the lead section was pressed). 
         [0061]    The film used was a laminated film of a polypropylene resin layer and a polyethylene resin layer (thickness: 100 μm). This film was used so that the polypropylene resin layer was on the outside. 
         [0062]    Other conditions are shown below: 
         [0063]    PTC component used (trade name: PolySwitch, manufactured by Tyco Electronics Japan G.K.) 
         [0064]    Lead: nickel (trade name: NB201, manufactured by Neomax Co., Ltd., thickness: 150 μm) 
         [0065]    Conductive Material: lead-free solder (alloy solder of tin, silver and copper) 
         [0066]    Thermal compression bonding condition: Pressed for 10-40 seconds at a temperature of 160° C.-210° C. under a pressure of 0.4 MPa. 
       EXAMPLE 2 
       [0067]    A PTC device B of the present invention shown in  FIG. 3  was produced by using the same PTC component as Example 1 and insert-molding a polypropylene resin. The polypropylene resin used was, the trade name: Prime Polypro, manufactured by Prime Polymer Co., Ltd. The conditions for the PTC device B were the same as for the PTC device A except for the protective member. 
         [0068]    The PTC devices A and B thus obtained were subjected to an alkali immersion experiment which was conducted by immersing them in a KOH aqueous solution (1N) at 60° C. for 3 months. The condition of the interface between the protective member and the lead was observed to evaluate the effect of alkali. A holding current test (I-V test) was performed on the PTC devices after the immersion. Also, for comparison, the same test was conducted on other PTC components having the same specification as the PTC components used in the production of the PTC devices. The results are shown below: 
         [0000]                                                              PTC device A   PTC device B                R initial     I max     R initial     I max                 After alkali immersion   5.5   2.979   5.0   3.332       PTC Component 1   5.3   3.366   5.5   2.983       PTC Component 2   5.7   3.047   5.1   3.345                    
In the table, R initial  is the initial resistance (unit: mΩ), and I max  is the maximum current (unit: Amps). The initial resistance was measured at 60° C. using Milliohm HiTester manufactured by HIOKI E.E. CORPORATION.
 
         [0069]    There was no difference that is visually determinable in the conditions of the interface between the protective member and the lead in the PTC devices A and B before and after the alkali immersion test. 
         [0070]    In the above table, the measured results on the PTC components having the same specification as the PTC components used in the production of the PTC devices A and B (provided that, not immersed in alkali) are shown as “PTC Component 1” and “PTC Component 2”, and those immersed in the KOH aqueous solution (1N) at 60° C. for 3 months are shown as “After alkali immersion”. 
         [0071]    “PTC Component 1” and “PTC Component 2” are the measured values as a component that has not been affected by alkali; in the PTC device A, the initial resistance values are 5.3 mΩ and 5.7 mΩ, and the initial resistance value of “After alkali immersion” is 5.5 mΩ. Thus, the resistance can be said to be substantially unchanged. Also, the I max  values are 3.366 A and 3.047 A for PTC Component 1 and PTC Component 2 respectively, and 2.979 A for “After alkali immersion”. Therefore, the I max  is also substantially unchanged. In the PTC device B, the initial resistance values for PTC component 1 and PTC component 2 are 5.5 mΩ and 5.1 mΩ, and 5.0 mΩ for “After alkali immersion”. In this case also, as in the previous case, these values can be said to be within a resistance range that is substantially unchanged. If it had been affected by immersion, it is predicted that the resistance would have increased greatly beyond the 5 mΩ range. With respect to the Imax, the values which were 3.345 A and 2.983 A for PTC Component 1 and PTC Component 2 respectively, was 3.332 A for “After alkali immersion”. Therefore, it can be also said that this is substantially unchanged. 
         [0072]    From these results, it can be seen that the holding current property of the PTC device is substantially unaffected even if the protective member is provided around the PTC device as in the present invention. 
         [0073]    For confirmation, the resistance-temperature (R-T) property of the PTC device A before and after the above alkali immersion experiment was measured. The result is shown in  FIG. 6 . In the chart of  FIG. 6 , the broken line is the measured result of the PTC device after the alkali immersion experiment, and the solid line is the measured result before the alkali immersion experiment. It can be seen that the PTC device of the present invention is substantially unaffected by alkali. 
         [0074]    The PTC device of the present invention can suppress the effect of electrolyte in the battery to a minimum even when disposed in the extra space within the secondary battery. 
       EXPLANATION OF THE LEGENDS 
       [0000]    
       
           10 —PTC device;  12 —polymer PTC element; 
           14 ,  16 —surface;  18 ,  20 —metal electrode; 
           22 —PTC component;  24 ,  26 —lead; 
           28 ,  30 —electrically conductive material;  32 —side; 
           34 —protective coating;  40 —PTC device; 
           42 —PTC component;  44 ,  46 —lead;  48 —molded member; 
           50 —PTC device;  52 —PTC component;  54 ,  56 —lead; 
           58 ,  60 —film;  70 —secondary battery; 
           72 —positive electrode tab space;  74 —PTC device; 
           76 —lead;  78 —sealing plate;  80 —positive electrode; 
           82 —lead;  100 —nickel hydride battery; 
           102 —positive electrode;  104 —negative electrode; 
           106 —positive electrode tab;  108 —sealing plate; 
           110 —positive electrode terminal.