Electrochemical device

An electrochemical device comprises an electrode matrix including a multilayer structure composed of a positive electrode, a negative electrode, and a first separator, and first and second dummy electrodes electrically connected to the positive and negative electrodes, respectively. The first and second dummy electrodes have respective opposing parts opposing each other through a second separator at an outer peripheral part of the electrode matrix. One or each of the first and second dummy electrodes has a resistance control layer at least on a side where the opposing parts oppose each other. The resistance control layer has such a resistance value that an estimated internal short circuit current between the first and second dummy electrodes is equivalent to 0.09 C to 1.00 C. The first and second dummy electrodes are adapted to short-circuit each other at a lower temperature than the positive and negative electrodes do.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrochemical device.

2. Related Background Art

As portable electronic devices have been becoming widespread, there have been increasing demands for lightweight, small-size electrochemical devices which can continuously be driven for a long time, such as secondary batteries. While conventional secondary batteries used metal outer cans, it has become possible to reduce the battery weight by employing thin, lightweight films for outer bags, as typified by lithium polymer batteries, thereby increasing the degree of freedom in designing.

When some abnormalities happen in a battery using such a film as its outer bag, gases may be generated or, in the worst case, ignition may occur depending on kinds of electrolytes in use. While chargers are set such as to stop charging when reaching a predetermined time or voltage, electrochemical devices will be overcharged if the charging does not stop before exceeding their capacity by some reason. As the overcharged state further progresses, electrolytes may decompose, so as to yield gases, which may inflate outer bags and cause internal short circuits due to internal deformations, thereby exploding the bags or igniting the electrochemical devices.

It has also been known that secondary batteries using lithium ions are likely to cause thermal runaway when a certain temperature is exceeded. Thermal runaway generates gases and further heat, thereby exploding or igniting the batteries.

For evading such circumstances, mechanisms provided with safety valves so as to release gases when the internal pressure rises have been under study, for example, as described in Japanese Patent Application Laid-Open Nos. 2000-100399 and HEI 11-312506.

SUMMARY OF THE INVENTION

However, safety valves such as those mentioned above do not always act stably. Also, if outer packages are inflated by internal pressures so that pressures apply to electrode matrixes before the safety valves act, the electrode matrixes may deform, thereby generating internal short circuits, so that the inner heat may cause thermal runaway in positive electrodes, which may lead to explosion or ignition in the worst case.

In view of the problems of the prior art mentioned above, it is an object of the present invention to provide an electrochemical device which prevents it from exploding and igniting because of temperature rises within its outer package, thereby dramatically improving its safety.

For achieving the above-mentioned object, the present invention provides an electrochemical device comprising an electrode matrix including a multilayer structure laminating positive and negative electrodes with a first separator interposed therebetween, and first and second dummy electrodes respectively arranged on both end faces in the laminating direction of the electrode matrix; wherein one of the first and second dummy electrodes and the other are electrically connected to the positive and negative electrodes in the electrode matrix, respectively; wherein the first and second dummy electrodes have respective opposing parts opposing each other through a second separator at an outer peripheral part of the electrode matrix; wherein one or each of the first and second dummy electrodes has a resistance control layer at least on a side where the opposing parts oppose each other; wherein the resistance control layer has a resistance value as a total resistance value of the first and second dummy electrodes falling in such a range that an estimated internal short circuit current between the first and second dummy electrodes is equivalent to 0.09 C to 1.00 C; and wherein the first and second dummy electrodes opposing each other through the second separator are adapted to short-circuit each other at a lower temperature than the positive and negative electrodes opposing each other through the first separator in the electrode matrix do.

In the present invention, “estimated internal short circuit current” refers to a current which is tolerable in terms of safety when the first and second dummy electrodes in the present invention are short-circuited at an abnormally high temperature. In other words, it means a safe current range when an internal short circuit occurs between the first and second dummy electrodes. The value of the estimated internal short circuit current can be calculated by Ohm's law (V=IR) according to the capacity of the cell and the fully charged battery voltage. Supposing that the cell capacity and the fully charged battery voltage are 2 [Ah] and 4.2 [V], respectively, for example, the resistance value needed for making the estimated internal short circuit current equivalent to 1 C is 4.2 [V]/(2×1) [A]=2.1 [Ω]. The resistance value needed for making the estimated internal short circuit current equivalent to 0.1 C in the same cell is 4.2 [V]/(2×0.1) [A]=21 [Ω]. Here, “equivalent to 1 C” refers to a current corresponding to an amount charged/discharged for 1 hr with a current corresponding to the cell capacity.

As a result of diligent studies, the inventors have found that the safety of an electrochemical device can be secured by providing the first and second dummy electrodes and setting the total resistance value in the thickness direction of the first and second dummy electrodes to such a range that the estimated internal short circuit current is equivalent to 0.09 C to 1.00 C. That is, when placed in a dangerous temperature atmosphere by a temperature rise within the outer package, the electrochemical device in accordance with the present invention can generate moderate self-discharge, so as to shift active materials used in the electrochemical device to more thermostable regions, whereby the safety of the electrochemical device can be improved dramatically. Such effects are obtained because, before the positive and negative electrodes in the electrode matrix short-circuit each other, the first and second dummy electrodes come into electrical contact with each other through the resistance control layer, thereby causing a moderate internal short circuit. Here, the short circuit occurs when the second separator shrinks or melts, for example, so that the first and second dummy electrodes come into electrical contact with each other through the resistance control layer. The first and second dummy electrodes thus moderately short-circuit each other through the resistance control layer, thereby making it possible to safely lower the battery capacity, evade thermal runaway due to the Joule heat at the time of hard short-circuiting, and prevent the outer package from exploding and the electrochemical device from igniting, thereby dramatically improving the safety of the electrochemical device in abnormally high temperature atmospheres.

Preferably, in the electrochemical device of the present invention, each of the first and second dummy electrodes has the resistance control layer, while the total of the resistance value in the thickness direction of the resistance control layer in the first dummy electrode and the resistance value in the thickness direction of the resistance control layer in the second dummy electrode falls within the range mentioned above. It will also be preferred in the electrochemical device of the present invention if one of the first and second dummy electrodes has the resistance control layer, while the resistance value in the thickness direction of the resistance control layer falls within the above-mentioned range.

In other words, while any or each of the first and second dummy electrodes may be provided with the resistance control layer, its resistance value in the thickness direction is preferably adjusted so as to fall within the above-mentioned range between the first and second dummy electrodes. When a short circuit occurs between the first and second dummy electrodes, an extremely safe and moderate internal short circuit can be generated because the resistance control layer having the above-mentioned resistance value is interposed therebetween, whereby the safety of the electrochemical device in abnormally high temperature atmospheres can dramatically be improved.

Preferably, in the electrochemical device of the present invention, the resistance control layer is a layer containing carbon black, a binder, and a ceramic particle. This makes it easier to adjust the resistance value of the resistance control layer in the thickness direction, so that a thin resistance control layer having a favorable resistance value can be formed, whereby the safety of the electrochemical device in abnormally high temperature atmospheres can further be improved.

Preferably, the ratio of the content of the carbon black to that of the binder and ceramic particle in the resistance control layer is 1:99 to 10:90 in terms of mass ratio. When the contents of these components fall within the range mentioned above, a thin resistance control layer having a favorable resistance value can easily be formed, whereby the safety of the electrochemical device in abnormally high temperature atmospheres can further be improved.

It will also be preferred in the electrochemical device of the present invention if the resistance control layer is a layer made of a high-resistance metal material or a ceramic material. This also makes it easier to adjust the resistance value of the resistance control layer in the thickness direction, so that a thin resistance control layer having a favorable resistance value can be formed, whereby the safety of the electrochemical device in abnormally high temperature atmospheres can further be improved.

Preferably, in the electrochemical device of the present invention, the second separator is a separator made of a drawn polyolefin. The drawn polyolefin exhibits a high shrinkage ratio in the drawn direction while being easy to control its shrinking direction, and thus is very effective in generating a short circuit between the first and second dummy electrodes in preference to a short circuit between the positive and negative electrodes in the electrode matrix.

Preferably, in the electrochemical device of the present invention, the first separator is a separator made of a polyacrylonitrile or polyamide imide. The polyacrylonitrile and polyamide imide exhibit a shrinkage ratio lower than that of the second separator based on a drawn film such as the one mentioned above, for example, so that the short circuit between the positive and negative electrodes is hard to occur at a higher temperature, thereby being very effective in preferentially causing the short circuit between the first and second dummy electrodes.

Preferably, in the electrochemical device of the present invention, at least one of the first and second dummy electrodes is accommodated within the second separator shaped like a bag having an opening. In this case, when the second separator shrinks, the electrode accommodated therewithin is exposed out of the opening of the bag-shaped separator. Since the shrinking direction of the second separator can be thus controlled, forming the respective opposing parts of the first and second dummy electrodes in the vicinity of the opening can more reliably generate the short circuit between the first and second dummy electrodes. It can also prevent unexpected short circuits from occurring anywhere other than the vicinity of the opening, whereby the safety can be enhanced more.

As in the foregoing, the present invention can provide an electrochemical device which prevents it from exploding and igniting because of temperature rises within its outer package, thereby dramatically improving its safety.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, preferred embodiments of the present invention will be explained in detail with reference to the drawings. In the drawings, identical or equivalent parts will be referred to with the same signs while omitting their overlapping explanations. Positional relationships such as upper/lower and left/right are based on those illustrated in the drawings unless otherwise specified. Ratios of dimensions in the drawings are not limited to those depicted.

FIG. 1is a partly broken perspective view illustrating a preferred embodiment of a lithium-ion secondary battery as the electrochemical device of the present invention.FIG. 2is a partial sectional view taken along the line I-I ofFIG. 1. However, an outer package50is omitted inFIG. 2.

As illustrated inFIGS. 1 and 2, the lithium-ion secondary battery100in accordance with this embodiment is mainly constituted by a multilayer structure1, a case (outer package)50accommodating the multilayer structure1in a closed state, and leads12,22for connecting the multilayer structure1to the outside of the case50.

The multilayer structure1is composed of an electrode matrix85constructed by laminating negative electrodes10, positive electrodes20, and first separators40, and a first dummy electrode60and a second dummy electrode70which are laminated on respective second separators45on the outside of the electrode matrix85.

As illustrated inFIG. 2, the electrode matrix85is one in which the first separator40, negative electrode10, first separator40, positive electrode20, first separator40, negative electrode10, first separator40, positive electrode20, first separator40, negative electrode10, and first separator40are laminated in order from the upper side.

In this specification, the “negative electrode”, which is based on the polarity of the battery at the time of discharging, refers to an electrode which releases electrons by an oxidation reaction at the time of discharging. The “positive electrode”, which is based on the polarity of the battery at the time of discharging, refers to an electrode which receives electrons by a reduction reaction at the time of discharging.

The case50accommodates not only the multilayer structure1but also an electrolytic solution (not depicted), which infiltrates the electrode matrix85.

Constituents of the lithium-ion secondary battery100in accordance with this embodiment will now be explained.

Each negative electrode10is constituted by a negative electrode current collector16and negative electrode active material containing layers18formed on both faces of the negative electrode current collector16.

The negative electrode current collector16is not limited in particular as long as it is a good conductor which can sufficiently move electric charges to the negative electrode active material containing layers18; current collectors employed in known lithium-ion secondary batteries can be used. Specific examples of the negative electrode current collector16include metal foils made of copper, nickel, and the like.

Each negative electrode active material containing layer18is a layer containing a negative electrode active material, a conductive auxiliary agent, a binder, and the like.

The negative electrode active material is not limited in particular as long as it allows occlusion and release of lithium ions, desorption and insertion of lithium ions, or doping and undoping of lithium ions and their counter anions (e.g., ClO4−) to proceed reversibly; materials similar to those used in known lithium-ion secondary batteries can be used. Examples include carbon materials such as natural graphite, synthetic graphite, mesocarbon microbeads, mesocarbon fiber (MCF), cokes, glasslike carbon, and fired bodies of organic compounds; metals such as Al, Si, and Sn which are combinable with lithium; amorphous compounds mainly composed of oxides such as SiO2and SnO2; and lithium titanate (Li4Ti5O12).

Preferably, the negative electrode active material containing layer18has a thickness of 15 to 80 μm. Preferably, the amount of the negative electrode active material supported by the negative electrode active material containing layer18is 2 to 12 mg/cm2. Here, the supported amount refers to the mass of the negative electrode active material per unit surface area of the negative electrode current collector16.

The conductive auxiliary agent is not limited in particular as long as it can make the conductivity of the negative electrode active material containing layer18favorable; known conductive auxiliary agents can be used. Examples include carbon blacks; carbon materials; fine powders of metals such as copper, nickel, stainless steel, and iron; mixtures of the carbon materials and metal fine powders; and conductive oxides such as ITO.

The binder is not limited in particular as long as it can bind particles of the negative electrode active material and conductive auxiliary agent to the negative electrode current conductor16; known binders can be used. Examples include fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene/hexafluoropropylene copolymers (FEP), tetrafluoroethylene/perfluoroalkylvinyl ether copolymers (PFA), ethylene/tetrafluoroethylene copolymers (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene/chlorotrifluoroethylene copolymers (ECTFE), and polyvinyl fluoride (PVF); and styrene/butadiene rubber (SBR).

Each positive electrode20is constituted by a positive electrode current collector26and positive electrode active material containing layers28formed on both faces of the positive electrode current collector26.

The positive electrode current collector26is not limited in particular as long as it is a good conductor which can sufficiently move electric charges to the positive electrode active material containing layers28; current collectors employed in known lithium-ion secondary batteries can be used. Specific examples of the positive electrode current collector26include metal foils made of aluminum and the like.

Each positive electrode active material containing layer28is a layer containing a positive electrode active material, a conductive auxiliary agent, a binder, and the like.

The positive electrode active material is not limited in particular as long as it allows occlusion and release of lithium ions, desorption and insertion (intercalation) of lithium ions, or doping and undoping of lithium ions and their counter anions (e.g., ClO4−) to proceed reversibly; known electrode active materials can be used. Examples include lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), lithium manganese spinel (LiMn2O4), and mixed metal oxides such as those expressed by the general formula of LiNixCoyMnzO2(x+y+z=1), a lithium vanadium compound (LiV2O5), olivine-type LiMPO4(where M is Co, Ni, Mn, or Fe), and lithium titanate (Li4Ti5O12).

Preferably, the positive electrode active material containing layer28has a thickness of 15 to 90 μm. The amount of the positive electrode active material supported by the positive electrode active material containing layer28can be set arbitrarily as appropriate in response to the amount of the negative electrode active material supported by the negative electrode active material containing layer18, but is preferably 5 to 25 mg/cm2, for example.

As for the constituents other than the positive electrode active material contained in the positive electrode active material containing layer28, materials similar to those constituting the negative electrode active material containing layer18can be used. Preferably, the positive electrode active material containing layer28also contains a conductive auxiliary agent similar to that in the negative electrode active material containing layer18.

Each first separator40is formed from an electrically insulating porous body. As for the material of the first separator40, known separator materials can be used without any limitations in particular. Examples of the electrically insulating porous body include multilayer bodies of films made of polyacrylonitrile, polyamide imide, polyethylene, polypropylene, polyolefin, and the like; drawn films of mixtures of the resins mentioned above; and fibrous nonwovens made of at least one constituent material selected from the group consisting of cellulose, polyester, and polypropylene. The first separator40is also preferably a fibrous nonwoven of glass, polyacrylonitrile, polyamide imide, or the like coated with polyvinylidene chloride from the viewpoint of restraining it from shrinking in high temperature atmospheres.

Here, as illustrated inFIG. 2, the electrode matrix85seen as a plane reduces its area in the order of the first separator40, negative electrode10, and positive electrode20, so that the end faces of the negative electrode10project out of the end faces of the positive electrode20, while the end faces of the first separator40project out of the end faces of the negative electrode10.

This makes it easier for the electrode matrix85to cause the whole surface of each positive electrode20to oppose its corresponding negative electrode10even when the layers are somewhat shifted from each other in directions intersecting the laminating direction because of errors during their manufacture and the like. Therefore, lithium ions released from the positive electrode20are fully taken into the negative electrode10through the first separator40. When the lithium ions released from the positive electrode20are not fully taken into the negative electrode10, the lithium ions not taken into the negative electrode10are deposited, so that carriers for electric energy decrease, whereby the energy capacity of the battery may deteriorate. Further, since the first separator40is greater than each of the positive and negative electrodes20,10and projects out of their end faces, the positive and negative electrodes20,10can be restrained from coming into contact with each other and generating a short circuit.

The first dummy electrode60is constituted by a conductor layer62and a resistance control layer64formed on one face of the conductor layer62. The second dummy electrode70is constituted by a conductor layer72and a resistance control layer74formed on one face of the conductor layer72.

As illustrated inFIG. 2, the conductor layer62of the first dummy electrode60is electrically connected to the positive electrode current collector26of the positive electrode20through a tongue62aprovided with the conductor layer62, while the conductor layer72of the second dummy electrode70is electrically connected to the negative electrode current collector16of the negative electrode10through a tongue72aprovided with the conductor layer72. The tongue62amay be provided either on a side free of the tongues (e.g.,16a,16a,16a) of current collectors as illustrated inFIGS. 1 and 2or on the same side therewith. The first and second dummy electrodes60,70have respective opposing parts80opposing each other through the second separators45at an outer peripheral part of the electrode matrix85. The first and second dummy electrodes60,70opposing each other through the second separators45are adapted to short-circuit each other at a lower temperature than the positive and negative electrodes20,10opposing each other through the first separators40in the electrode matrix85do.

As for the conductor layers62,72, metal foils similar to those of current collectors to connect therewith are preferably used. In other words, as with the positive electrode current collector26, a metal foil made of aluminum or the like is preferably used for the conductor layer62to be connected to the positive electrode current collector26in this embodiment. As with the negative electrode current collector16, a metal foil made of copper, nickel, or the like is preferably used for the conductor layer72to be connected to the negative electrode current collector16.

The resistance control layers64,74are not limited in particular as long as they can generate a moderate short circuit between the first and second dummy electrodes60,70. The resistance values of the resistance control layers64,74, which are adjusted after being calculated from the cell capacity and estimated internal short circuit current of the electrochemical device, are regulated such as to yield an estimated short circuit current equivalent to 0.09 C to 1.00 C. When the resistance values of the resistance control layers64,74are such that the estimated internal short circuit current is equivalent to less than 0.09 C, self-discharge cannot be performed sufficiently, whereby a thermally stable charged state is hard to achieve. When the resistance values are such that the estimated internal short circuit current is equivalent to more than 1.00 C, the Joule heat is generated in excess by self-discharge, whereby the temperature of the battery rises drastically. Because of the same reason, the estimated short circuit current is preferably at most 2 A as a current value. The resistance values can be adjusted according to the material, thickness, and the like of the resistance control layers64,74.

Each of the resistance control layers64,74is preferably a layer containing a conductive material, a high-resistance material, and a binder, a layer made of a high-resistance metal material, or a layer made of a ceramic material, since a stable resistance value can be obtained with a sufficient thinness thereby.

When any of the resistance control layers64,74is a layer containing a conductive material, a high-resistance material and a binder, examples of the conductive material include carbon black, graphite, carbon nanotubes, acetylene black, and ketjen black, among which carbon black is preferred. Examples of the high-resistance material include ceramic particles and resin particles, among which the ceramic particles are preferred. Examples of the ceramic particles include particles of alumina, silicon dioxide, zirconium oxide, and titanium oxide. Examples of the binder include polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE).

When any of the resistance control layers64,74is a layer containing a conductive material, a high-resistance material, and a binder, the ratio of the content of the conductive material to that of the binder and high-resistance material is preferably 1:99 to 10:90, more preferably 3:97 to 5:95, in terms of mass ratio. When the content of the conductive material is less than the above-mentioned ratio, the resistance of the resistance control layer tends to become higher than necessary, so that self-discharge is not performed sufficiently, whereby the discharge may fail to advance to a thermally stable charging state. When the content is greater than the above-mentioned ratio, the resistance of the resistance control layer tends to become so low that a greater amount of Joule heat is generated by self-discharge, whereby the battery temperature may rise abnormally.

When any of the resistance control layers64,74is a layer made of a high-resistance metal material, examples of the high-resistance metal material include tungsten, tantalum, and nickel-chromium alloys. Among them, tungsten is preferred from the viewpoint of its favorable film thickness controllability which makes it easier to adjust the resistance value.

When any of the resistance control layers64,74is a layer made of a ceramic material, examples of the ceramic material include amorphous silicon, silicon dioxide, aluminum oxide, and titanium oxide. Among them, amorphous silicon is preferred from the viewpoint of its flexibility to deformations from the outside.

The resistance control layers64,74may be either identical to or different from each other in terms of material, thickness, and the like.

Each second separator45, which is formed from an electrically insulating material, is preferably made of a porous body or drawn film since a high shrinkage ratio is obtained thereby. As for the material of the second separator45, known separator materials can be used without any limitations in particular. Examples of the second separator45include multilayer bodies of films made of polyethylene terephthalate, polyacrylonitrile, polyethylene, polypropylene, polyolefin, and the like; drawn films of mixtures of the resins mentioned above; and fibrous nonwovens and drawn films made of at least one constituent material selected from the group consisting of cellulose, polyester, and polypropylene. Preferred among them is a drawn film of polyolefin.

From the viewpoint of generating a short circuit between the first and second dummy electrodes60,70in preference to a short circuit between the positive and negative electrodes20,10in the electrode matrix85, the second separator45preferably has a shrinkage ratio of about 0.5 to 10%, more preferably about 3 to 8%, at 120° C.

Preferably, the second separator45has an area seen as a plane greater than each of the first and second dummy electrodes60,70in at least the opposing parts80between the first and second dummy electrodes60,70, so that the end face of the second separator45projects out of the end faces of the first and second dummy electrodes60,70.

This can restrain the first and second dummy electrodes60,70from unintentionally coming into contact with each other and short-circuiting each other because of errors during their manufacture and the like.

The second separator45is also preferably formed like a bag having an opening, within which the dummy electrodes are preferably accommodated. The opening is preferably provided at one side facing the opposing parts80of the first and second dummy electrodes60,70. Using such a second separator45can regulate the shrinking direction of the second separator45, thereby making it possible to generate a short circuit between the first and second dummy electrodes60,70more reliably.

The method of constructing the first and second dummy electrodes60,70such that they generate a short circuit at a lower temperature than the positive and negative electrodes20,10do is not limited in particular; examples include methods adjusting materials and physical properties of the separators, forms and sizes of the separators and electrodes, and their arrangement. More specific examples include a method of making the second separators45shrinkable at a lower temperature than the first separators40, a method of making the second separators45have a higher shrinkage ratio than the first separators40, and a method of making the end faces of the second separators45project out of the end faces of the dummy electrodes by a smaller area than the end faces of the first separators40do, so that the dummy electrodes preferentially come into contact with each other even when the first and second separators40,45shrink to similar extents. A method of accommodating the dummy electrodes in the bag-like second separators45having openings as mentioned above and controlling the shrinking direction of the second separators45may also be employed. A method of accommodating the electrodes in the bag-like first separators40without openings and restraining the electrodes from being exposed by the shrinkage of the first separators40may also be employed. A plurality of these methods may be used in combinations.

The electrolytic solution is contained within pores of the negative and positive electrode active material containing layers18,28and first separators40. As for the electrolytic solution, electrolytic solutions (aqueous electrolytic solutions and electrolytic liquids using organic solvents) containing lithium salts employed in known lithium-ion secondary batteries can be used without any limitations in particular. However, electrolytic solutions using organic solvents (nonaqueous electrolytic solutions) are preferred, since the endurable voltage of aqueous electrolytic solutions is limited to a low level because of their electrochemically low decomposition voltage. As the electrolytic solution for the secondary battery, one in which a lithium salt is dissolved in a nonaqueous solvent (organic solvent) is preferably used. As the lithium salt, salts such as LiPF6, LiClO4, LiBF4, LiAsF6, LiCF3SO3, LiCF3, LiCF2SO3, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(CF3CF2CO)2, and the like can be used, for example. These salts may be used singly or in combinations of two or more.

As the organic solvent, solvents employed in known secondary batteries can be used. Preferred examples include propylene carbonate, ethylene carbonate, and diethyl carbonate. They may be used singly or in mixtures of two or more at any ratios.

In this embodiment, the electrolytic solution may not only be a liquid but also a gelled electrolyte obtained by adding a gelling agent thereto. Instead of the electrolytic solution, a solid electrolyte (a solid polymer electrolyte or an electrolyte made of an ionically conductive inorganic material) may be contained.

As illustrated inFIG. 1, the leads22,12project out of the case50through a seal part50bwhile having ribbon-like outer forms.

The lead22is formed from a conductor material such as a metal. As this conductor material, aluminum or the like can be employed, for example. As illustrated inFIG. 1, the end part of the lead22within the case50is joined to respective tongues26a,26aof the positive electrode current collectors26,26by resistance welding or the like, whereby the lead22is electrically connected to the positive electrode active material containing layers28through the respective positive electrode current collectors26.

The lead12is also formed from a conductor material such as a metal. As this conductor material, a conductive material such as copper or nickel, for example, can be utilized. The end part of the lead12within the case50is welded to respective tongues16a,16a,16aof the negative electrode current collectors16,16,16, whereby the lead12is electrically connected to the negative electrode active material containing layers18through the respective negative electrode current collectors16.

As illustrated inFIG. 1, the parts of the leads22,12held at the seal part50bof the case50are covered with insulators24,14made of a resin or the like in order to enhance sealability. The insulators24,14are not limited in particular in terms of materials, but preferably formed from a synthetic resin, for example. The leads22,12are separated from each other in a direction orthogonal to the laminating direction of the electrode matrix85.

The case50is not limited in particular as long as it can seal the multilayer structure1and prevent air and moisture from entering the inside of the case; cases employed for known secondary battery elements can be used. For example, synthetic resins such as epoxy resins and resin-laminated sheets of metals such as aluminum can be used. The case50, which is formed by folding a flexible rectangular sheet51C into two at substantially the longitudinal center part thereof, holds the multilayer structure1from both sides in the laminating direction (vertical direction) as illustrated inFIG. 1. Among end parts of the two-folded sheet51C, the seal parts50bat three sides excluding the bent part50aare bonded by heat-sealing or with an adhesive, whereby the multilayer structure1is sealed therewithin. The case50is also bonded to the insulators24,14at the seal part50b, so as to seal the leads22,12.

An example of methods of manufacturing the above-mentioned lithium-ion secondary battery100will now be explained.

First, the above-mentioned first separator40, negative electrode current collector16, and positive electrode current collector26are prepared. Subsequently, respective coating liquids (slurries) containing constituent materials for forming electrode layers to become the negative and positive electrode active material containing layers18,28are made. The negative electrode coating liquid is a solvent containing the above-mentioned negative electrode active material, conductive auxiliary agent, binder, and the like, while the positive electrode coating liquid is a solvent containing the above-mentioned positive electrode active material, conductive auxiliary agent, binder, and the like. The solvent used in the coating liquids is not limited in particular as long as it can dissolve the binders and disperse the active materials and conductive auxiliary agents. For example, N-methyl-2-pyrrolidone, N,N-dimethylformamide, and the like can be used.

Next, the negative electrode coating liquid is applied to both faces of the negative electrode current collector16and dried, so as to form the negative electrode active material containing layers18on the both faces, and the resulting product is cut into rectangular forms each having a tongue16a, so as to yield three negative electrodes10.

Similarly, the positive electrode coating liquid is applied to both faces of the positive electrode current collector26and dried, so as to form the positive electrode active material containing layers28on the both faces, and the resulting product is cut into rectangular forms each having a tongue26a, so as to yield two positive electrodes20.

Here, techniques for applying the coating liquids to the current collectors are not limited in particular, and may be determined appropriately according to the material, form, and the like of metal plates for the current collectors. Examples include metal mask printing, electrostatic coating, dip coating, spray coating, roll coating, doctor blading, gravure coating, and screen printing. After being applied, the coatings are extended by a flat press, calender rolls, or the like if necessary.

Subsequently, the negative and positive electrodes10,20are laminated with the first separators40interposed therebetween in the order ofFIG. 2, i.e., first separator40/negative electrode10/first separator40/positive electrode20/first separator40/negative electrode10/first separator40/positive electrode20/first separator40/negative electrode10/first separator40, and heated while being held at the center parts within the planes on both sides in the laminating direction, so as to yield the electrode matrix85illustrated inFIG. 2.

Then, the leads12,22illustrated inFIG. 1are prepared, and their longitudinal center parts are respectively coated with insulators14,24made of a resin or the like. Subsequently, as illustrated inFIG. 1, the tongues16aare welded to an end part of the lead12, while the tongues26aare welded to an end part of the lead22. This completes the electrode matrix85having the leads12,22connected thereto.

The above-mentioned second separators45and conductor layers62,72are also prepared. The resistance control layer64is formed on one face of the conductor layer62, while the resistance control layer74is formed on one face of the conductor layer72.

When the resistance control layers64,74are layers each containing a conductive material, a high-resistance material, and a binder, a coating liquid (slurry) containing the constituent materials mentioned above for forming the resistance control layers64,74is made, applied to the edge part on respective one faces of the conductor layers62,72, and dried, so as to form the resistance control layers64,74. The solvent used in the coating liquid is not limited in particular as long as it can dissolve the binder and disperse the active material and conductive auxiliary agent. For example, N-methyl-2-pyrrolidone, N,N-dimethylformamide, acetone, and the like can be used.

Techniques for applying the coating liquid to the conductor layers62,72are not limited in particular, and may be determined appropriately according to the materials, forms, and the like of metal plates for the conductor layers. Examples include metal mask printing, electrostatic coating, dip coating, spray coating, roll coating, doctor blading, gravure coating, and screen printing. After being applied, the coatings are extended by a flat press, calender rolls, or the like if necessary.

When the resistance control layers64,74are layers made of a high-resistance metal material, they can be formed by using a film-forming method such as vapor deposition, sputtering, or chemical vapor deposition (CVD).

When the resistance control layers64,74are layers made of a ceramic material, they can be formed by using a film-forming method such as vapor deposition, sputtering, or chemical vapor deposition (CVD).

Next, the respective multilayer bodies in which the resistance control layers64,74are laminated on the conductor layers62,72are cut out into rectangular forms having the tongues62a,72a, so as to yield the first and second dummy electrodes60,70.

Subsequently, the first and second dummy electrode60,70are laminated on respective main faces of the electrode matrix85while interposing the second separators45therebetween so as to have the opposing parts80at an outer peripheral part of the electrode matrix85. Then, the parts holding the electrode matrix85is heated while being held at the center parts within the planes on both sides in the laminating direction, the opposing parts80are heated while being held at the center parts within the planes in the laminating direction, and the dummy electrodes are connected to their corresponding electrodes within the electrode matrix85through the tongues62a,72a, so as to yield the multilayer structure1illustrated inFIG. 1.

Next, a bag-shaped case50formed from a sheet in which aluminum is laminated with a thermally adhesive resin layer is prepared, the multilayer structure1is inserted therein from its opening, and an electrolytic solution is injected into the case50within a vacuum container, so that the multilayer structure1is dipped in the electrolytic solution. Thereafter, each of the leads22,12is partly projected out of the case50, and the opening50cof the case50is sealed with a heat sealer. This completes the making of the lithium-ion secondary battery100.

The present invention can be modified in various ways without being restricted to the above-mentioned embodiment.

FIG. 3is a schematic sectional view illustrating another preferred embodiment of the lithium-ion secondary battery as the electrochemical device of the present invention.FIG. 3omits the outer package50. As illustrated inFIG. 3, the first dummy electrode60in the multilayer structure1may directly be laminated on one main face of the electrode matrix85without the second separator45. In this case, the first and second dummy electrodes60,70oppose each other in the opposing parts80only through the second separator45laminated on the second dummy electrode70. ThoughFIG. 3illustrates a case where the second separator45is not laminated on the first dummy electrode60, the second dummy electrode70may be free of the second separator45if the second separator45is laminated on the first dummy electrode60.

FIG. 4is a schematic sectional view illustrating still another preferred embodiment of the lithium-ion secondary battery as the electrochemical device of the present invention.FIG. 4omits the outer package50. As illustrated inFIG. 4, both of the first and second dummy electrodes60,70in the multilayer structure1may directly be laminated on both main faces of the electrode matrix85. In this case, the second separator45is arranged in at least the opposing parts80between the first and second dummy electrodes60,70.

FIG. 5is a schematic sectional view illustrating still another preferred embodiment of the lithium-ion secondary battery as the electrochemical device of the present invention.FIG. 5omits the outer package50. As illustrated inFIG. 5, the first and second dummy electrodes60,70in the multilayer structure1may be accommodated in respective bag-shaped second separators45. Here, each bag-shaped second separator45has an opening on the side facing the opposing parts80between the first and second dummy electrodes60,70, thereby regulating the shrinking direction at the time of thermal shrinkage.

ThoughFIG. 5illustrates a case where the dummy electrodes are accommodated in the bag-shaped separators, it will also be preferred if at least one of the negative and positive electrodes10,20is accommodated in the bag-shaped first separator40. Preferably, the bag-shaped first separator40has no opening in this case.

FIG. 6is a schematic sectional view illustrating still another preferred embodiment of the lithium-ion secondary battery as the electrochemical device of the present invention.FIG. 6omits the outer package50. As illustrated inFIG. 6, the first dummy electrode60in the multilayer structure1may be free of the resistance control layer64. When the first and second dummy electrodes60,70short-circuit each other in the opposing parts80, they do so only through the resistance control layer74of the second dummy electrode70. ThoughFIG. 6illustrates a case where the first dummy electrode60does not have the resistance control layer64, the second dummy electrode70may be free of the resistance control layer74if the first dummy electrode60has the resistance control layer64.

Though the electrode matrix85in the above-mentioned embodiments has four secondary battery elements as single cells, i.e., combinations of negative electrode/separator/positive electrode, the number of secondary battery elements may be more than 4 or less than 3, e.g., 1.

Though the above-mentioned embodiments exemplify a mode in which each of the outermost two electrodes in the electrode matrix85is a three-tier negative electrode10in which the negative electrode active material containing layers18are formed on both faces of the negative electrode current collector16as a preferred mode, one or each of the outermost two electrodes may be realized as a two-tier negative electrode in which the negative electrode active material containing layer18is formed on one face of the negative electrode current collector16.

Though the above-mentioned embodiments exemplify a mode in which each of the outermost two electrodes in the electrode matrix85is the negative electrode10as a preferred mode, the present invention can be carried out with the outermost two electrodes being the positive electrode20and negative electrode10, respectively, or both the positive electrodes20,20.

Though the above-mentioned embodiments exemplify a mode in which each of the outermost two layers in the electrode matrix85is the first separator40as a preferred mode, the outermost electrodes in the electrode matrix85may be employed as the outermost layers instead of the first separators40as long as these electrodes are securely insulated from the dummy electrodes adjacent thereto. The outermost electrodes may also be employed as the outermost layers instead of the first separators40when there is no need to insulate these electrodes from the dummy electrodes adjacent thereto. For example, it is not always necessary for the first separator40to be arranged between the second dummy electrode70and its neighboring positive electrode20inFIGS. 2 to 5, since the conductor layer62and the positive electrode current collector26are electrically connected to each other.

Though the above-mentioned embodiments relate to a case where the electrochemical device is a lithium-ion secondary battery, the electrochemical device of the present invention is not limited to the lithium-ion secondary battery, but may be any of secondary batteries other than the lithium-ion secondary batteries, such as metal lithium secondary batteries, and electrochemical capacitors such as electric double layer capacitors, pseudocapacity capacitors, pseudocapacitors, and redox capacitors. In the case of electrochemical devices other than the lithium-ion secondary batteries, electrode active materials suitable for the respective electrochemical devices may be used. In the case of an electric double layer capacitor for example, acetylene black, graphite, activated carbon, and the like are used as the active materials contained in the positive and negative electrode active material containing layers.

EXAMPLES

In the following, the present invention will be explained more specifically with reference to examples and comparative examples, though the present invention is not limited to the following examples. The following examples and comparative examples relate to lithium-ion secondary batteries having a designed voltage value of 4.2 V.

Making of Negative Electrode

A negative electrode was made by the following procedure. First, 90 parts by mass of mesocarbon microbeads (MCMB) (manufactured by Osaka Gas Co., Ltd.) and 1 part by mass of graphite (product name: KS-6 manufactured by Lonza) as a negative electrode active material, 2 parts by mass of carbon black (product name: DAB manufactured by Denki Kagaku Kogyo, K. K.) as a conductive auxiliary agent, and 7 parts by mass of polyvinylidene fluoride (product name: Kynar 761 manufactured by Atofina) as a binder were mixed and dispersed, and then an appropriate amount of N-methylpyrrolidone (NMP) as a solvent was added thereto, so as to adjust viscosity, thereby making a slurry-like negative electrode coating liquid.

Subsequently, a copper foil (having a thickness of 20 μm) as a negative electrode current collector was prepared, and the negative electrode coating liquid was applied to both faces of the copper foil and dried, so as to form negative electrode active material containing layers (each having a thickness of 75 μm). Thus obtained negative electrode sheet was punched out into such a form that each active material containing layer surface had a size of 144 mm×102 mm while the current collector had a tongue to become an external output terminal, thus yielding a negative electrode.

Making of Positive Electrode

A positive electrode was made by the following procedure. First, 91 parts by mass of lithium cobaltate (LiCoO2) (product name: Selion manufactured by Seimi Chemical Co., Ltd.) and 4 parts by mass of graphite (product name: KS-6 manufactured by Lonza) as a positive electrode active material, 2 parts by mass of carbon black (product name: DAB manufactured by Denki Kagaku Kogyo, K. K.) as a conductive auxiliary agent, and 3 parts by mass of polyvinylidene fluoride (product name: Kynar 761 manufactured by Atofina) as a binder were mixed and dispersed, and then an appropriate amount of N-methylpyrrolidone (NMP) as a solvent was added thereto, so as to adjust viscosity, thereby making a slurry-like positive electrode coating liquid.

Subsequently, an aluminum foil (having a thickness of 15 μm) as a positive electrode current collector was prepared, and the positive electrode coating liquid was applied to both faces of the aluminum foil and dried, so as to form positive electrode active material containing layers (each having a thickness of 95 μm). Thus obtained positive electrode sheet was punched out into such a form that each active material containing layer surface had a size of 142 mm×100 mm while the current collector had a tongue to become an external output terminal, thus yielding a positive electrode.

Making of Electrode Matrix

As separators (first separators), porous films made of a polyacrylonitrile resin (PAN) (each having a size of 148 mm×106 mm with a thickness of 24 μm) were prepared. Then, four negative electrodes and three positive electrodes were alternately laminated with the separators, so as to yield an electrode matrix having the same multilayer structure as that illustrated inFIG. 2except for the numbers of laminated electrodes and separators.

Making of Dummy Electrode

After mixing and dispersing 10 parts by mass of carbon black (product name: DAB manufactured by Denki Kagalu Kogyo, K. K.), 40 parts by mass of polyvinylidene fluoride (product name: Kynar 761 manufactured by Atofina) as a binder, and 50 parts by mass of zirconia oxide particles (product name: Zirconia Oxide, manufactured by Kojundo Chemical Lab. Co., Ltd., having an average particle size of 1 μm), an appropriate amount of N-methylpyrrolidone (NMP) as a solvent was added thereto, so as to adjust viscosity, thereby making a slurry-like resistance control layer coating liquid.

Subsequently, an aluminum foil (having a thickness of 20 μm) as a conductor layer was prepared, and the resistance control layer coating liquid was applied to one face of the conductor layer and dried, so as to form a resistance control layer (having a thickness of 20 μm). Thus obtained dummy electrode sheet was punched out into such a form that the resistance control layer surface had a size of 144 mm×102 mm while the conductor layer had a tongue to connect with a current collector of the electrode matrix, thus yielding a first dummy electrode.

Also, a copper foil (having a thickness of 20 μm) as a conductor layer was prepared, and the resistance control layer coating liquid was applied to one face of the conductor layer and dried, so as to form a resistance control layer (having a thickness of 20 μm). Thus obtained dummy electrode sheet was punched out into such a form that the resistance control layer surface had a size of 144 mm×102 mm while the conductor layer had a tongue to connect with a current collector of the electrode matrix, thus yielding a second dummy electrode.

Making of Lithium-Ion Secondary Battery

As separators (second separators), porous films made of a drawn polyolefin (product name: SV722, manufactured by Asahi Kasei Corporation, having a size of 148 mm×106 mm with a thickness of 22 μm) were prepared. These separators were laminated on the resistance control layers of the dummy electrodes, so as to yield separator-equipped dummy electrodes. Subsequently, the separator-equipped dummy electrodes were laminated on both end faces in the laminating direction of the electrode matrix, and opposing parts where the first and second dummy electrodes opposed each other through the second separator were formed at an outer peripheral part of the electrode matrix. Further, the conductor layers of the first and second dummy electrodes were electrically connected to the positive and negative electrode current collectors through the tongues provided with the conductor layers, respectively. This yielded a multilayer structure having the same multilayer structure as that illustrated inFIG. 2except for the numbers of laminated electrodes and separators.

Subsequently, thus obtained multilayer structure was put into an outer package made of an aluminum-laminated film. The outer package was sealed after injecting therein an electrolytic solution composed of a mixture of propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC) at a volume ratio of 2:1:7 as a solvent and 1.5 mol/L of LiPF6as a solute, so as to yield a lithium-ion secondary battery having the same structure as that illustrated inFIG. 1except for the numbers of laminated electrodes and separators.

In the making of dummy electrodes, after mixing and dispersing 5 parts by mass of carbon black (product name: DAB manufactured by Denki Kagaku Kogyo, K. K.), 40 parts by mass of polyvinylidene fluoride (product name: Kynar 761 manufactured by Atofina) as a binder, and 55 parts by mass of zirconia oxide particles (product name: Zirconia Oxide, manufactured by Kojundo Chemical Lab. Co., Ltd., having an average particle size of 1 μm), an appropriate amount of N-methylpyrrolidone (NMP) as a solvent was added thereto, so as to adjust viscosity, thereby making a slurry-like resistance control layer coating liquid. Except for using this resistance control layer coating liquid, a lithium-ion secondary battery was made as in Example 1.

In the making of dummy electrodes, after mixing and dispersing 3 parts by mass of carbon black (product name: DAB manufactured by Denki Kagaku Kogyo, K. K.), 40 parts by mass of polyvinylidene fluoride (product name: Kynar 761 manufactured by Atofina) as a binder, and 57 parts by mass of zirconia oxide particles (product name: Zirconia Oxide, manufactured by Kojundo Chemical Lab. Co., Ltd., having an average particle size of 1 μm), an appropriate amount of N-methylpyrrolidone (NMP) as a solvent was added thereto, so as to adjust viscosity, thereby making a slurry-like resistance control layer coating liquid. Except for using this resistance control layer coating liquid, a lithium-ion secondary battery was made as in Example 1.

In the making of dummy electrodes, after mixing and dispersing 2 parts by mass of carbon black (product name: DAB manufactured by Denki Kagaku Kogyo, K. K.), 40 parts by mass of polyvinylidene fluoride (product name: Kynar 761 manufactured by Atofina) as a binder, and 58 parts by mass of zirconia oxide particles (product name: Zirconia Oxide, manufactured by Kojundo Chemical Lab. Co., Ltd., having an average particle size of 1 μm), an appropriate amount of N-methylpyrrolidone (NMP) as a solvent was added thereto, so as to adjust viscosity, thereby making a slurry-like resistance control layer coating liquid. Except for using this resistance control layer coating liquid, a lithium-ion secondary battery was made as in Example 1.

A lithium-ion secondary battery was made as in Example 1 except that the number of negative electrodes was 19 and the number of positive electrodes was 18 in the making of the electrode matrix.

A lithium-ion secondary battery was made as in Example 2 except that the number of negative electrodes was 19 and the number of positive electrodes was 18 in the making of the electrode matrix.

Comparative Example 1

A lithium-ion secondary battery was made as in Example 1 except that no dummy electrode was provided.

Comparative Example 2

In the making of dummy electrodes, after mixing and dispersing 20 parts by mass of carbon black (product name: DAB manufactured by Denki Kagaku Kogyo, K. K.), 40 parts by mass of polyvinylidene fluoride (product name: Kynar 761 manufactured by Atofina) as a binder, and 40 parts by mass of zirconia oxide particles (product name: Zirconia Oxide, manufactured by Kojundo Chemical Lab. Co., Ltd., having an average particle size of 1 μm), an appropriate amount of N-methylpyrrolidone (NMP) as a solvent was added thereto, so as to adjust viscosity, thereby making a slurry-like resistance control layer coating liquid. Except for using this resistance control layer coating liquid, a lithium-ion secondary battery was made as in Example 1.

Comparative Example 3

In the making of dummy electrodes, after mixing and dispersing 1 part by mass of carbon black (product name: DAB manufactured by Denki Kagaku Kogyo, K. K.), 40 parts by mass of polyvinylidene fluoride (product name: Kynar 761 manufactured by Atofina) as a binder, and 59 parts by mass of zirconia oxide particles (product name: Zirconia Oxide, manufactured by Kojundo Chemical Lab. Co., Ltd., having an average particle size of 1 μm), an appropriate amount of N-methylpyrrolidone (NMP) as a solvent was added thereto, so as to adjust viscosity, thereby making a slurry-like resistance control layer coating liquid. Except for using this resistance control layer coating liquid, a lithium-ion secondary battery was made as in Example 1.

Comparative Example 4

A lithium-ion secondary battery was made as in Example 5 except that no dummy electrode was provided.

Measurement of Discharged Capacity

The discharged capacity of each of the lithium-ion secondary batteries of Examples 1 to 6 and Comparative Examples 1 to 4 was measured by performing constant current constant voltage charging to 4.2 V with a current equivalent to 0.5 C and then discharging to 3.3 V with a current equivalent to 0.5 C. Table 1 lists the results.

Measurement of Resistance Value of Resistance Control Layer

The resistance value in the thickness direction of the resistance control layers of dummy electrodes used in the lithium-ion secondary batteries of Examples 1 to 6 and Comparative Examples 2 and 3 was measured by the following method. While both sides (resistance control layer side and current collector side) of each dummy electrode were held with probe electrodes having a diameter of 0.3 mm, the voltage upon application of a constant current of 50 mA was measured with a potentio/galvanostat (product name: HA-151 manufactured by Hokuto Denko Corporation), so as to calculate the resistance value of the resistance control layer. The resistance value was measured for each of the first and second dummy electrodes, so as to calculate their total value. Table 1 lists the results.

Measurement of Battery Voltage

The fully charged battery voltage of each of the lithium-ion secondary batteries of Examples 1 to 6 and Comparative Examples 1 to 4 was verified to be their designed value of 4.2 V. Specifically, the voltage of each of the fully charged lithium-ion secondary batteries was measured by a voltmeter (product name: BATTERY HiTESTER 3555 manufactured by Hioki E. E. Corporation). As a result, it was verified that all the lithium-ion secondary batteries had the fully charged battery voltage of 4.2 V as designed.

Calculation of Estimated Internal Short Circuit Current

An estimated internal short circuit current of each of the lithium-ion secondary batteries of Examples 1 to 6 and Comparative Examples 1 to 4 was calculated according to the following equation (1). Here, the discharged capacity, the resistance value of the resistance control layer, and the fully charged battery voltage were measured by the respective methods mentioned above. Table 1 lists the results.

Safety Evaluation

The lithium-ion secondary batteries of Examples 1 to 6 and Comparative Examples 1 to 4 were subjected to a 155° C. heating test in the fully charged state, so as to evaluate their safety. Specifically, each lithium-ion secondary battery was subjected to constant current constant voltage charging to 4.2 V with a current equivalent to 0.5 C. While being kept in a high-temperature vessel, the charged lithium-ion secondary battery was heated to 155° C. at a heating rate of 5° C./min and then held at 155° C. for 1 hr. Here, the temperature change (reached temperature) of the lithium-ion secondary battery during the heating and holding was measured, so as to evaluate the stability of the lithium-ion secondary battery. For example, batteries with lower thermostability tend to exhibit steeper temperature rises than the heating profile of the high-temperature vessel in the process of heating (in the vicinity of 120° C.). Such a phenomenon is presumed to be caused by gas generation within the lithium-ion secondary batteries. By contrast, lithium-ion secondary batteries having higher thermostability generate less heat from therewithin, so as to exhibit a thermal behavior similar to the heating profile of the high-temperature vessel. Table 1 lists the results. The lower the reached temperature is, the higher the safety becomes. In this test, the lithium-ion secondary batteries exhibiting inflation and smoke, thus causing problems in their safety, were indicated with “inflation, smoke”.

Dummy electrodes were made by the following procedure. An aluminum foil (having a thickness of 20 μm) was prepared as a conductor layer, on which a resistance control layer (having a thickness of 0.18 μm) made of tungsten (having a specific resistance of 5.65×10−3mΩ·cm) was formed by sputtering. Thus obtained dummy electrode sheet was punched out into a form similar to that of Example 1, so as to yield a first dummy electrode. On the other hand, a copper foil (having a thickness of 20 μm) was prepared as a conductor layer, on which a resistance control layer (having a thickness of 0.18 μm) made of tungsten (having a specific resistance of 5.65×10−3mΩ·cm) was formed by sputtering. Thus obtained dummy electrode sheet was punched out into a form similar to that of Example 1, so as to yield a second dummy electrode. Except for using these first and second dummy electrodes, a lithium-ion secondary battery was made as in Example 1.

A lithium-ion secondary battery was made as in Example 7 except that the thickness of the resistance control layer made of tungsten was 0.88 μm in each of the first and second dummy electrodes.

A lithium-ion secondary battery was made as in Example 7 except that the thickness of the resistance control layer made of tungsten was 1.60 μm in each of the first and second dummy electrodes.

Dummy electrodes were made by the following procedure. An aluminum foil (having a thickness of 20 μm) was prepared as a conductor layer, on which a resistance control layer (having a thickness of 0.28 μm) made of amorphous silicon (having a specific resistance of 3.4×10−3mΩ·cm) was formed by CVD using an SiH4+H2gas. Thus obtained dummy electrode sheet was punched out into a form similar to that of Example 1, so as to yield a first dummy electrode. On the other hand, a copper foil (having a thickness of 20 μm) was prepared as a conductor layer, on which a resistance control layer (having a thickness of 0.28 μm) made of amorphous silicon (having a specific resistance of 3.4×10−3mΩ·cm) was formed by CVD as with the first dummy electrode. Thus obtained dummy electrode sheet was punched out into a form similar to that of Example 1, so as to yield a second dummy electrode. Except for using these first and second dummy electrodes, a lithium-ion secondary battery was made as in Example 1.

A lithium-ion secondary battery was made as in Example 10 except that the thickness of the resistance control layer made of amorphous silicon was 1.43 μm in each of the first and second dummy electrodes.

A lithium-ion secondary battery was made as in Example 10 except that the thickness of the resistance control layer made of amorphous silicon was 2.57 μm in each of the first and second dummy electrodes.

Comparative Example 5

A lithium-ion secondary battery was made as in Example 7 except that the thickness of the resistance control layer made of tungsten was 0.10 μm in each of the first and second dummy electrodes.

Comparative Example 6

A lithium-ion secondary battery was made as in Example 7 except that the thickness of the resistance control layer made of tungsten was 2.65 μm in each of the first and second dummy electrodes.

Comparative Example 7

A lithium-ion secondary battery was made as in Example 10 except that the thickness of the resistance control layer made of amorphous silicon was 0.21 μm in each of the first and second dummy electrodes.

Comparative Example 8

A lithium-ion secondary battery was made as in Example 10 except that the thickness of the resistance control layer made of amorphous silicon was 4.24 μm in each of the first and second dummy electrodes.

Evaluations of Battery Characteristics

For the lithium-ion secondary batteries of Examples 7 to 12 and Comparative Examples 5 to 8, evaluations of battery characteristics, i.e., measurement of discharged capacity, the resistance value of the resistance control layer, and battery voltage, calculation of the estimated internal short circuit current, and safety evaluation, were carried out by the same methods as those mentioned above. Table 2 lists the results.