JP2009272585A - Electrochemical capacitor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrochemical capacitor which ensures a sufficient withstand voltage by resolving the problem that a part of lithium in contact with a carbon material peels off a lithium layer, and performing stable predoping. <P>SOLUTION: When a capacitor element 1 is formed, a force with which a separator 4 contracts to return to a tension-free state after winding of the capacitor element 1 acts by applying a tension in a lengthwise direction to the wound separator 4. This force becomes a pressure acting upon a positive electrode 2 and a negative electrode 3, and this pressure becomes a force for firmly fixing the formed lithium layer to the negative electrode 3. Thus stable potential drop due to predoping of lithium is possible. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an electrochemical capacitor used as a backup power source for electronic devices, a regenerative brake for a hybrid vehicle, a fuel cell vehicle, or a power storage device.

  Conventionally, an electric double layer capacitor has an excellent reliability with respect to rapid discharge and rapid charge as a power storage device, and thus has been used mainly as a backup power source in a wide range of fields.

  In general, an electric double layer capacitor uses mainly activated carbon for a positive electrode and a negative electrode, and performs charging / discharging by adsorbing and desorbing ions contained in the electrolytic solution on the surface of the activated carbon by impregnating the electrolytic solution.

  The type of electrolyte can be classified into aqueous and non-aqueous. Aqueous electrolytes have the characteristics of high electrostatic capacity but low withstand voltage, and non-aqueous electrolytes have certain trade-off characteristics of low electrostatic capacity but high withstand voltage. It was. However, generally, since the amount of energy stored in a capacitor is proportional to the square of the withstand voltage of the capacitor, a non-aqueous organic electrolyte has been used.

  In recent years, in order to develop new application fields, development of capacitors with higher withstand voltage has been active.

  The capacitor developed as a part of this was an electrochemical capacitor having a higher withstand voltage than a conventional electric double layer capacitor.

  Electrochemical capacitors form a polarizable electrode layer using activated carbon on the front and back surfaces of an aluminum foil current collector on the positive electrode as before, but the structure of the negative electrode is carbon on the front and back surfaces of a metal current collector such as copper. It is characterized in that a carbon electrode layer made of a material and a lithium layer formed thereon are further formed. By adopting this configuration, when the capacitor element is once impregnated into a large amount of electrolyte during assembly of the electrochemical capacitor device, lithium is melted from the lithium layer formed on the negative electrode to become positively charged lithium ions, which are ionized. Sometimes energy is released and negative charges accumulate in the negative electrode. Almost at the same time, the eluted lithium ions and the lithium ions originally present in the electrolyte are attracted to the negative electrode and are occluded by the carbon material of the negative electrode, thereby lowering the potential of the negative electrode (pre-doping). As a result, the potential difference from the positive electrode is widened, and the withstand voltage can be improved.

  In the development of electric double layer capacitors, as described below, in parallel with the research on withstand voltage improvement, the development of new value-added members has been promoted for each component of the electrochemical capacitor. Attempts have been made to improve the performance of chemical capacitors from another aspect.

  In this attempt, regarding the invention of a separator that is interposed between a positive electrode and a negative electrode for the purpose of insulating both electrodes, it has been proposed to use a sponge-like porous insulator as a separator component. By using this sponge-like porous insulator, a large amount of electrolytic solution can be contained in the sponge-like interior, and shortage of the electrolytic solution during operation of the electrochemical capacitor can be prevented. Furthermore, since it is sponge-like, even if the polarizable electrode layer expands during charging / discharging, it is possible to prevent a short circuit by easily changing the shape of the separator (Patent Document 1).

In addition to the above, one of the positive electrode and the negative electrode and the separator provided on both surfaces of the electrode are formed in a ninety-nine fold shape so as to fill a gap between the other stacked electrodes. The invention of being installed is proposed. By adopting this configuration, it was possible to suppress a short circuit caused by turning over of the separator that occurred during the manufacture of a conventional multilayer capacitor element (Patent Document 2).
JP 2001-185454 A JP-A-2005-243455

  However, in the conventional electrochemical capacitor, when pre-doping the negative electrode, some lithium having a surface in contact with the carbon electrode layer may be peeled off from the lithium layer by some impact. This lithium peeling reduces the contact area between the lithium layer and the carbon material, hinders the reaction due to the potential drop performed between the carbon material surface and the lithium layer during pre-doping, and the potential of the pre-doped negative electrode is sufficient. It has been a problem that the withstand voltage of the electrochemical capacitor may be lowered without dropping.

  An object of the present invention is to provide an electrochemical capacitor having a stable withstand voltage by preventing a part of lithium in contact with the carbon electrode layer from peeling from the lithium layer and performing pre-doping under a stable condition. .

  In order to solve this problem, in the present invention, a tension is applied to the elastic separator disposed so as to be interposed between the electrodes inside the capacitor element when the capacitor element is wound, and the winding is performed while maintaining the tension. A spiral capacitor element was formed, and a stress in a direction substantially perpendicular to the winding center axis was generated inside the spiral capacitor element by the tension applied to the separator.

  As a result, the electrochemical capacitor according to the present invention is subjected to a stress in a direction substantially perpendicular to the winding center axis of the capacitor element, which is applied to the entire capacitor element as a pressure. This pressure also acts on the lithium to be pre-doped as a force for sandwiching and pressing the lithium using a negative electrode and a separator. Even if a part of lithium having a surface in contact with the carbon electrode layer is peeled off from the lithium layer by this acting force, the part of the peeled lithium stays at the original position and is sufficient. It becomes possible to continue the pre-doping in a state where it contacts the surface.

  As a result, the electrode potential can be greatly lowered from the lithium layer formed on the carbon electrode layer.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(Embodiment of the present invention)
FIG. 1 is a cutaway perspective view of an electrochemical capacitor according to an embodiment of the present invention.

  FIG. 2 is a top view of a state in which a capacitor element used in the electrochemical capacitor is fixed with a fixture.

  FIGS. 3A and 3B are views showing the state of the separator from before and after the fixing member provided in the capacitor element is peeled off.

  In FIG. 1, reference numeral 1 denotes a capacitor element. The capacitor element 1 includes a positive electrode 2, a negative electrode 3, and a separator 4.

  The positive electrode 2 is obtained by applying a polarizable electrode layer 2b mainly composed of activated carbon on both front and back surfaces of a current collector 2a of a high-purity aluminum foil having a thickness of about 30 μm. Incidentally, the polarizable electrode layer 2b is composed of activated carbon, a binder, and a conductive additive. Specifically, a mixed solution in which, for example, carboxymethyl cellulose (CMC) is dissolved in water as a binder in phenol resin-based activated carbon having an average particle diameter of 5 μm, and acetylene black, for example, is used in a quantity of 10: 2: 1 as a conductive assistant. What was mixed in was applied and water was removed.

  The negative electrode 3 is obtained by forming a carbon electrode layer 3b mainly composed of carbon on both front and back surfaces of a copper foil current collector 3a having a thickness of 15 μm. Incidentally, the carbon electrode layer 3b is composed of a carbon material, a binder, and a conductive additive. Specifically, for example, graphite is used as the carbon material, polytetrafluoroethylene (PTFE) and CMC are used as the binder in a weight ratio of 8: 2, and acetylene black is used as the conductive auxiliary agent in the same manner as the positive electrode. It was. When these materials are mixed, the mixing ratio of the carbon material, the conductive additive and the binder is used at a ratio of about 80:10:10. Then, this mixture is applied to the front and back surfaces of the current collector so that the thickness of the layer after application is about 50 μm with a comma coater or the like, and water is removed.

  Furthermore, when assembling the electrochemical capacitor, the negative electrode 3 has a configuration in which lithium foil is disposed on both the front and back surfaces of the carbon material. Since this lithium foil becomes lithium ions and is pre-doped into the carbon electrode layer, it is not shown in FIG.

  For the separator 4, for example, polyolefin resin or aramid fiber such as polypropylene and polyethylene having excellent elasticity is used. In addition, a fluorine-based material such as polyimide resin or polyvinylidene fluoride is used as the separator material in the embodiment of the present invention.

  When the separator 4 is wound together with the positive electrode and the negative electrode to form the capacitor element 1, the separator 4 is disposed so as to be interposed between the positive electrode and the negative electrode. At that time, the winding is performed by a known equipment such as a winder. For example, the reel 4 is rotated by rotating the rotation speed of the reel for feeding the separator 4 to the winding core in the reverse direction of the winding process of the conventional capacitor element. The tension of the separator 4 delivered to the core part can be increased. The separator 4 using the elastic material is stretched by the increased tension, and is wound around the winding core portion while maintaining the stretched state.

  After the end of winding, the end of the separator 4 in the longitudinal direction was fixed together with the electrodes so that the tension applied to the separator 4 in the longitudinal direction was maintained. As a result, the separator 4 as a whole is maintained in a fixed state by the positive electrode and the negative electrode interposed between the separators 4, and the separator 4 wound around the winding core portion has a contracting force to return to the form before the tension is applied. work. The contracting force acts in a direction substantially perpendicular to the winding center axis, thereby compressing the entire capacitor element 1 and becomes a pressure between the electrode and the separator 4.

  By this pressure, the lithium foil formed on the negative electrode 3 is firmly fixed by the separator 4 and the negative electrode 3, and even if a part of lithium having a surface in contact with the surface of the carbon electrode layer 3 b is peeled off from the negative electrode 3, it is fixed by the pressure. Stays in the original position. In this way, it is possible to prevent peeling of lithium and secure a contact area. Thereby, it is possible to perform a stable potential drop of the negative electrode 3 without hindering the pre-doping, and a sufficient withstand voltage can be obtained.

  In addition, regarding the thickness of the separator 4, the thickness of about 10-50 micrometers conventionally used is used, for example. Further, in the present embodiment, the capacitor element 1 has a configuration in which the center part obtained by extracting the winding core part after the winding has a hollow shape. However, if the winding core part has insulating properties, the capacitor element 1 is used as it is. It is good also as a structure which left the winding core part in the center axis | shaft.

  Then, the member which comprises the electrochemical capacitor by embodiment of this invention other than the capacitor element 1 using FIG. 1 is demonstrated.

  Reference numeral 5 denotes a lead wire. Since the lead wire 5 is connected to the positive electrode 2 and the negative electrode 3, it can be classified into a positive electrode lead wire 5a and a negative electrode lead wire 5b. The lead wire 5 is connected to the polarizable electrode layer 2b and the carbon electrode layer 3b not formed in the positive electrode 2 and the negative electrode 3, that is, the exposed surfaces of the positive electrode current collector 2a and the negative electrode current collector 3a, and is connected to an external circuit. . Therefore, in order to reduce the connection resistance of the lead wire 5 with the positive electrode current collector 2a and the negative electrode current collector 3a as much as possible, for example, the positive electrode lead wire 5a is made of aluminum, and the negative electrode lead wire 5b is made of copper or nickel. .

Reference numeral 6 denotes an exterior case. The outer case 6 has a bottomed cylindrical shape, and accommodates the capacitor element 1 connected to the lead wire 5 and the driving electrolyte impregnated in the capacitor element 1. The driving electrolyte is not shown in FIG. For example, aluminum is used for the base material of the exterior case from the viewpoint of processability. Incidentally The driving electrolyte, for example, Li ⁺ as electrolyte cations, BF ₄ as an electrolyte anion ^- or PF ₆ ^-, and the weight ratio high dielectric constant of ethylene carbonate (EC) and low viscosity diethyl carbonate (DEC) as a solvent A mixed solvent mixed 1: 1 was used. Other than this, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), etc. may be used instead of DEC, and when mixing with EC, multiple selections from DEC, DMC, EMC, etc. may be used. It is done. Moreover, you may mix a propylene carbonate (PC) with the said mixed solvent for the improvement of a low-temperature characteristic.

  Reference numeral 7 denotes a sealing member. The sealing member 7 was disposed so as to be in close contact with the inner peripheral surface of the outer case 6 inside the opening end of the outer case 6. The sealing member 7 was subjected to drawing processing from the outer peripheral surface of the outer case 6 toward the inside of the outer case 6 with respect to a part of the inner peripheral surface of the outer end of the outer case 6 in contact with the sealing member 7. The sealing member 7 was fixed to the arrangement location by this drawing process.

  Further, a part of the opening end portion of the exterior case 6 protruding outward from the sealing member 7 was bent toward the inside of the exterior case 6 to enhance the fixing strength of the sealing member 7. In the embodiment of the present invention, a through hole is provided in a part of the sealing member 7 so that the lead wire 5 connected to the capacitor element 1 penetrates the sealing member 7 and is connected to an external circuit. The sealing member 7 is made of, for example, fluoro rubber.

  Further, the external connection method is not limited to the lead wire 5, for example, the width direction of the positive electrode current collector 2 a and the negative electrode current collector 3 a on the front and back surfaces of the positive electrode current collector 2 a and the negative electrode current collector 3 a of the capacitor element 1. Are provided with unformed portions of the polarizable electrode layer 2b and the carbon electrode layer 3b, respectively. When the separator 4 and the positive electrode 2 and the negative electrode 3 are wound as the capacitor element 1, the non-formed portions of the polarizable electrode layer 2b and the carbon electrode layer 3b are opposed to each other through the separator. The positive electrode 2 and the negative electrode 3 are wound in a state where they are partially shifted in the opposite direction, and are composed of the non-formed portions of the polarizable electrode layer 2b and the carbon electrode layer 3b of the positive electrode current collector 2a and the negative electrode current collector 3a. The end faces where the both end faces of the wound capacitor element 1 are in contact with and electrically connected to the inner surface of the terminal board and the inner bottom of the outer case 6 used as members for sealing the opening of the outer case 6. A configuration called current collection may be taken. In this case, for example, the outer surface of the terminal board and the outer surface of the outer case 6 are electrically connected to the external circuit.

  As a method for further improving the connection between the both end surfaces of the wound capacitor element 1 and the terminal plate and the inner bottom portion of the outer case 6 in order to reduce the resistance when the end face current collection is employed, A configuration may be employed in which current collecting plates, which are metal flat plates, are disposed on both end faces of the element 1 and then end face current collection is performed. With this configuration, the member connected to the terminal plate having a flat connection surface is not the capacitor element 1 but also a current collecting plate having a flat connection surface, so that stable connection can be achieved.

  Here, the confirmation method at the time of using the separator 4 by this Embodiment is demonstrated.

  As shown in FIG. 2, when the capacitor element 1 has a winding shape, the winding shape is not deformed by sandwiching the outer surface of the capacitor element 1 with the fixture 8. Crimped and fixed with moderate force. Then, the fixing member 9 that has maintained the winding shape of the capacitor element 1 as shown in FIG. 3A is removed as shown in FIG.

  At this time, since the tension is applied to the separator 4, a portion from the one end in the longitudinal direction fixed by the fixing member 9 to the fixing portion of the separator 4 is released from the tension and contracts. Therefore, by measuring the length of the separator 4 from the fixing member 9 fixing position to the fixing tool 8 fixing position before removing the fixing member 9, whether the separator 4 has contracted as described above or after the fixing member 9 is removed. It can be confirmed by measuring the length from one end of the separator 4 in the longitudinal direction to the fixing part 8 fixing portion.

  As described above, the electrochemical capacitor according to the present invention applies a longitudinal tension to the separator 4 when the separator 4 interposed between the positive electrode 2 and the negative electrode 3 is overlapped with the positive electrode 2 and the negative electrode 3 to be wound. Thus, a force toward the winding core acts on the wound capacitor element 1, and becomes a pressure acting on each member constituting the wound capacitor element 1. Thereby, the lithium foil formed on the carbon electrode surface of the negative electrode 3 is firmly fixed by the separator 4 and the negative electrode 3, and a part of lithium of the lithium foil having a surface in contact with the carbon material surface is peeled off due to various factors. It is possible to prevent contact and secure a contact area. And by preventing peeling, the potential drop of the negative electrode 3 can be stabilized without hindering the pre-doping, and a sufficient withstand voltage can be obtained.

  According to the electrochemical capacitor according to the present invention, the pre-doping of the lithium foil on the carbon material of the negative electrode is more stable than before, and a stable voltage drop of the negative electrode due to lithium is possible and has an excellent withstand voltage. As a result, it is expected to be used as a backup power source or regenerative power for a hybrid vehicle or a fuel cell vehicle that instantaneously consumes high energy.

Cutaway perspective view showing an electrochemical capacitor used in the present embodiment Schematic showing a state in which the capacitor element is fixed at two points opposite to each other for verification work of the electrochemical capacitor used in the present embodiment (A) The top view which extracted and showed a part before fixation member removal in the verification confirmation operation | work of the electrochemical capacitor used for this Embodiment, (b) The plane which extracted and showed a part after the fixation member removal Figure

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Separator element 2 Positive electrode 2a Positive electrode collector 2b Polarization electrode layer 3 Negative electrode 3a Negative electrode collector 3b Carbon electrode layer 4 Separator 5 Lead wire 5a Positive electrode lead wire 5b Negative electrode lead wire 6 Outer case 7 Sealing member 8 Fixing tool 9 Fixing Element

Claims (2)

A positive electrode provided with a polarizable electrode layer mainly composed of activated carbon on the front and back surfaces of a positive electrode current collector made of metal, and a carbon electrode layer mainly composed of a carbon material on the front and back surfaces of a negative electrode current collector made of metal. Forming and stacking a negative electrode having a lithium foil disposed on the outer surface of the carbon electrode layer and a porous separator having elasticity and elasticity;
A capacitor element having a configuration in which the separator is wound so that the separator is interposed between the positive electrode and the negative electrode;
A driving electrolyte containing lithium ions;
A bottomed cylindrical outer case containing the capacitor element and the driving electrolyte;
A sealing member for sealing the opening end of the exterior case;
A lead wire connected to the capacitor element so that a part thereof is exposed from the outer case and the sealing member,
In the equipped electrochemical capacitor,
An electrochemical capacitor in which the wound capacitor element has a stress acting therein in a direction substantially perpendicular to a winding central axis of the capacitor element due to the elastic force of the extended separator. The electrochemical capacitor according to claim 1, wherein the separator is made of any one of polyolefin, aramid, polyimide, and fluorine material.

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