Electrochemical capacitor

An electrochemical capacitor has a pair of positive and negative electrode elements each having a solid electrode disposed on a surface of a current collector. The positive and negative electrode elements are disposed in confronting relation to each other with a separator interposed therebetween. The positive and negative electrode elements, together with an electrolytic solution, are housed in the casing. The solid electrode of the negative electrode element is made of a lithium vanadium oxide and an electrically conductive filler, and the solid electrode of the positive electrode element is made of activated carbon.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to an electrochemical capacitor.
 2. Description of the Related Art
 There has heretofore been known an electric double-layer capacitor having
 positive and negative electrode elements disposed in confronting relation
 to each other with a separator interposed therebetween. Each of the
 positive and negative electrode elements having a solid electrode made of
 activated carbon on the surface of a current collector such as of metal
 foil or the like. The positive and negative electrode elements are sealed
 together with an electrolytic solution in a casing which has terminals
 connected to the respective current collectors.
 In the above conventional electric double-layer capacitor, the solid
 electrode is made of activated carbon as a substance having a large
 specific surface area. However, the electric double-layer capacitor having
 solid electrodes of activated carbon has an energy density lower than
 secondary cells that operate based on a chemical reaction. In view of such
 a drawback, there has been proposed an electrochemical capacitor having
 solid electrodes made of a material which is capable of producing
 pseudocapacitance owing to an electrochemical reaction. Various materials
 including ruthenium oxide capable of producing pseudocapacitance have been
 reviewed for use as solid electrode materials. However, these materials
 are disadvantageous in that they are expensive.
 SUMMARY OF THE INVENTION
 It is therefore an object of the present invention to provide an
 electrochemical capacitor which is relatively inexpensive to manufacture
 and has excellent discharging characteristics.
 The inventors have employed a lithium vanadium oxide that is available
 inexpensively as a cell material capable of producing pseudocapacitance,
 and produced a solid electrode containing a lithium vanadium oxide. In the
 lithium vanadium oxide, a vanadium oxide can be either tetravalent or
 pentavalent. The lithium vanadium oxide exists stably as either a
 tetravalent or pentavalent oxide because of the presence of lithium. As a
 result, the lithium vanadium oxide can cause an electrochemical reaction
 and operates as a cell in the presence of an electrolytic solution, so
 that it can produce pseudocapacitance in a capacitor.
 The inventors have studied various discharging characteristics of solid
 electrodes containing a lithium vanadium oxide. As a consequence, the
 inventors have found that excellent discharging characteristics are
 obtained by an electrochemical capacitor having a negative solid electrode
 containing a lithium vanadium oxide and a positive solid electrode made of
 activated carbon, and have completed by the invention based on the
 finding.
 To achieve the above object, there is provided in accordance with the
 present invention an electrochemical capacitor comprising a casing and a
 pair of positive and negative electrode elements each having a solid
 electrode disposed on a surface of a current collector, the positive and
 negative electrode elements being disposed in confronting relation to each
 other with a separator interposed therebetween, the positive and negative
 electrode elements, together with an electrolytic solution, being housed
 in the casing, the solid electrode of the negative electrode element being
 made of a lithium vanadium oxide and an electrically conductive filler,
 the solid electrode of the positive electrode element being made of
 activated carbon.
 In the electrochemical capacitor according to the present invention, the
 negative electrode element has a solid electrode made of a lithium
 vanadium oxide and an electrically conductive filler, and positive
 electrode element has a solid electrode made of activated carbon. These
 solid electrodes are effective to prevent a sharp voltage drop from
 occurring at the positive electrode element when the electrochemical
 capacitor starts being discharged, to increase the period of time for
 which the electrochemical capacitor can produce a high voltage, and to
 enable the electrochemical capacitor to discharge a large amount of
 electric energy.
 The electrochemical capacitor according to the present invention can be
 manufactured inexpensively because the lithium vanadium oxide is used as a
 substance for producing pseudocapacitance. The electrically conductive
 filler, which together with the lithium vanadium oxide makes up the solid
 electrode of the negative electrode element, is required to increase the
 electric conductivity of the solid electrode.
 Since the lithium vanadium oxide can cause an electrochemical reaction and
 operates as a cell in the presence of an electrolytic solution, the
 vanadium oxide should preferably be tetravalent or pentavalent. The
 lithium vanadium oxide may comprise at least one oxide selected from the
 group consisting of LiV.sub.3 O.sub.8, LiV.sub.2 O.sub.5, and Li.sub.2
 V.sub.2 O.sub.5. The vanadium in LiV.sub.3 O.sub.8 is pentavalent, the
 vanadium in LiV.sub.2 O.sub.5 is a mixture of tetravalent and pentavalent
 vanadium, and the vanadium in Li.sub.2 V.sub.2 O.sub.5 is tetravalent.
 The electrically conductive filler may comprise carbon black or the like.
 The electrically conductive filler is used to adjust the electric
 conductivity of the solid electrode, and its amount differs depending on
 the application of the electrochemical capacitor. For example, the
 electrically conductive filler is added in an increased amount in order to
 reduce the resistance of the solid electrode for a high output level, and
 added in a reduced amount in order to reduce the output density for a high
 energy density.
 The electrically conductive filler is in the range from 3 through 80 weight
 % of the total weight of the solid electrode. If the amount of the
 electrically conductive filler were less than 3 weight % of the total
 weight of the solid electrode, then the resistance of the electrode would
 be too high to discharge the electrochemical capacitor well. If the amount
 of the electrically conductive filler were in excess of 80 weight % of the
 total weight of the solid electrode, then the energy density of the
 electrode would be lowered.
 The activated carbon of the solid electrode of the positive electrode
 element has a specific surface area in the range from 100 to 3000 m.sup.2
 /g. If the specific surface area of the activated carbon were less than
 100 m.sup.2 /g, then the electrostatic capacitance per volume would be
 excessively small. If the specific surface area of the activated carbon
 were greater than 3000 m.sup.2 /g, then the bulk density would be reduced.
 The electrolytic solution comprises a propylene carbonate solution of
 LiBF.sub.4 or LiPF.sub.6. Positive ions of the electrolytic salt should
 preferably be Li.sup.+ because the vanadium oxide capable of producing
 pseudocapacitance at the negative electrode element charges and discharges
 electric energy by reversibly doping and undoping Li.sup.+. Negative ions
 of the electrolytic salt should preferably be BF.sub.4.sup.- or
 PF.sub.6.sup.- because it has a high withstand voltage and a high electric
 conductivity, can easily be produced industrially, and has a low toxic
 level. The solvent of the electrolytic salt should preferably propylene
 carbonate since it has a high withstand voltage and a high electric
 conductivity and can be used in a wide temperature range.
 The electrolytic solution has a concentration in the range from 0.5 to 1.5
 mol/liter. If the concentration of the electrolytic solution were lower
 than 0.5 mol/liter, then the resistance would be increased. If the
 concentration of the electrolytic solution were greater than 1.5
 mol/liter, then the electrolytic salt might separate out at low
 temperatures.
 The above and other objects, features, and advantages of the present
 invention will become apparent from the following description when taken
 in conjunction with the accompanying drawings which illustrate preferred
 embodiments of the present invention by way of example.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 1st Embodiment
 As shown in FIG. 1, an electrochemical capacitor according to a first
 embodiment of the present invention has a positive solid electrode 1a and
 a negative solid electrode 1b disposed in confronting relation to each
 other with a separator 2 interposed as an insulator therebetween, and a
 disk-shaped casing 3 of aluminum which houses the positive and negative
 solid electrodes 1a, 1b and the separator 2 therein. The separator 2 is
 made of glass fibers, for example. The positive and negative solid
 electrodes 1a, 1b and the separator 2 are housed together with an
 electrolytic solution (not shown) in the casing 3. The casing 3 is sealed
 by a lid 5 of aluminum whose circumferential edge is joined to the casing
 3 by a gasket 4 of synthetic resin.
 The solid electrodes 1a, 1b are bonded to inner surfaces of the casing 3
 and the lid 5, respectively, by an electrically conductive adhesive or the
 like. The inner surfaces of the casing 3 and the lid 5 serve as current
 collectors for the solid electrodes 1a, 1b. The casing 3 has an outer
 surface that can be used as a negative connection terminal, and the lid 5
 has an outer surface that can be used as a positive connection terminal.
 An electrochemical capacitor according to a comparative example will be
 described below. The electrochemical capacitor according to the
 comparative example is of a physical structure identical to the inventive
 electrochemical capacitor shown in FIG. 1. According to the comparative
 example, the solid electrodes 1a, 1b contain a lithium vanadium oxide
 (LiV.sub.3 O.sub.8). Specifically, the solid electrodes 1a, 1b are
 produced by mixing 45 parts by weight of lithium vanadium oxide (LiV.sub.3
 O.sub.8), 45 parts by weight of an electrically conductive filler of
 carbon black (tradename DENKA BLACK, manufactured by Denki Kagaku Kogyo
 K.K.), and 10 parts by weight of polytetrafluoroethylene as a binder, and
 pressing the mixed to a disk shape having a diameter of 20 mm. Each of the
 solid electrodes 1a, 1b has a weight of 150 mg.
 The lithium vanadium oxide can be prepared according to a sol-gel process
 by mixing a predetermined amount of a solution of lithium propoxide
 dissolved by an organic solvent and a predetermined amount of a solution
 of vanadium propoxide dissolved by an organic solvent, and hydrolyzing and
 condensing the mixture.
 The electrolytic solution comprises a propylene carbonate solution of
 lithium tetrafluoroborate (LiBF.sub.4) and has a concentration of 1
 mol/liter.
 FIG. 2 shows a charging and discharging curve plotted when the
 electrochemical capacitor according to the comparative example was charged
 with a constant current at a constant voltage. Specifically, the
 electrochemical capacitor according to the comparative example was charged
 with a constant current of 5 mA, and after the charged voltage reached 2
 V, the electrochemical capacitor according to the comparative example was
 charged at a constant voltage of 2 V for 2 hours. It can be seen from FIG.
 2 that the voltage across the electrochemical capacitor according to the
 comparative example dropped sharply when it started being discharged.
 FIG. 3 shows a charging and discharging curve of the potentials of the
 electrodes measured by a three-electrode process using a lithium electrode
 as a reference electrode, the curve being plotted when the electrochemical
 capacitor according to the comparative example was charged under the same
 conditions as those for the charging and discharging curve shown in FIG.
 2. It can be understood from FIG. 3 that the sharp voltage drop upon start
 of the discharging of the electrochemical capacitor according to the
 comparative example occurred at the positive electrode.
 In the electrochemical capacitor according to the first embodiment of the
 present invention, the negative solid electrode 1a contains a lithium
 vanadium oxide (LiV.sub.3 O.sub.8), and the positive solid electrode 1b is
 made of activated carbon. The negative solid electrode 1a according to the
 first embodiment is identical to the negative solid electrode 1a according
 to the comparative example. That is, the negative solid electrode 1a
 according to the first embodiment is produced by mixing 45 parts by weight
 of lithium vanadium oxide (LiV.sub.3 O.sub.8), 45 parts by weight of an
 electrically conductive filler of carbon black (tradename DENKA BLACK,
 manufactured by Denki Kagaku Kogyo K.K.), and 10 parts by weight of
 polytetrafluoroethylene as a binder, and pressing the mixed to a disk
 shape having a diameter of 20 mm. The solid electrode 1a has a weight of
 150 mg. The electrically conductive filler may be KETJEN BLACK (tradename)
 manufactured by Mitsubishi Chemical Corp. or PRINTEX (tradename)
 manufactured by Dekusa, or the like, rather than DENKA BLACK manufactured
 by Denki Kagaku Kogyo K.K.
 The positive solid electrode 1b is made of steam-activated carbon or
 alkali-activated carbon whose specific surface area is in the range from
 100 to 3000 m.sup.2 /g. The carbon may be BAC-PW (tradename) manufactured
 by Kureha Chemical Industry Co., Ltd., for example. The positive solid
 electrode 1b is prepared by shaping the carbon into a disk form having a
 diameter of 20 mm, and has a weight of 100 mg.
 The electrolytic solution comprises a propylene carbonate solution of
 lithium tetrafluoroborate (LiBF.sub.4) and has a concentration of 1
 mol/liter. However, the electrolytic solution is not limited to the above
 solution, but may comprise another known electrolytic solution, e.g., a
 propylene carbonate solution of lithium hexafluorophosphate (LiPF.sub.6).
 FIG. 4 shows a charging and discharging curve of the potentials of the
 electrodes measured by a three-electrode process using a lithium electrode
 as a reference electrode, the curve being plotted when the electrochemical
 capacitor according to the first embodiment of the present invention was
 charged under the same conditions as those for the charging and
 discharging curve shown in FIG. 2.
 It can be understood from FIG. 4 that no sharp voltage drop occurred at the
 positive electrode upon start of the discharging of the electrochemical
 capacitor according to the first embodiment of the present invention, and
 the electrochemical capacitor is capable of discharging a large amount of
 electric energy.
 The electrochemical capacitors according to the first embodiment and the
 comparative example were charged with the constant current at the constant
 voltage under the same conditions as those for the charging and
 discharging curve shown in FIG. 2, and measured for various properties.
 The measured properties of the electrochemical capacitors according to the
 first embodiment and the comparative example are given in Table 1 shown
 below, and their charging and discharging curves are shown in FIG. 5.
 TABLE 1
 Electric Electric Discharged
 Electrode capacitance capacitance energy
 density (F/cc) (F/g) (Wh/kg)
 Embodiment 1 1.14 53.5 29.7 46.5
 Com. Ex. 2.03 77.3 157.0 41.7
 A review of Table 1 indicates that the electrochemical capacitor according
 to the first embodiment can discharge a greater amount of electric energy
 than the electrochemical capacitor according to the comparative example. A
 study of FIG. 5 shows that since the electrochemical capacitor according
 to the first embodiment does not suffer a sharp voltage drop immediately
 after it starts being discharged, the period of time for which the
 electrochemical capacitor according to the first embodiment can produce a
 high voltage is longer than the period of time for which the
 electrochemical capacitor according to the comparative example can produce
 a high voltage.
 The electrochemical capacitor according to the first embodiment and a
 conventional electrochemical capacitor were charged with a constant
 current at a constant voltage as follows: Each electrochemical capacitor
 was charged with a constant current of 5 mA, and after the charged voltage
 reached 2.5 V, the electrochemical capacitor according to the comparative
 example was charged at a constant voltage of 2.5 V for 2 hours. The
 conventional electrochemical capacitor is an electric double-layer
 capacitor which has the same physical structure as that shown in FIG. 1.
 In the conventional electrochemical capacitor, each of the solid
 electrodes 1a, 1b is prepared by shaping activated carbon (BAC-PW
 (tradename) manufactured by Kureha Chemical Industry Co., Ltd.) into a
 disk form having a diameter of 20 mm. Each of the solid electrodes 1a, 1b
 has a weight of 100 mg.
 A property of the electrochemical capacitors is given in Table 2 below, and
 their charging and discharging curves are shown in FIG. 6. FIG. 6 also
 shows the charging and discharging curve (see FIG. 5) of the
 electrochemical capacitor according to the comparative example.
 TABLE 2
 Discharged energy
 (Wh/kg)
 Embodiment 1 57.1
 Conventional 22.8
 Table 1 reveals that the electrochemical capacitor according to the first
 embodiment can discharge a greater amount of electric energy than the
 conventional electrochemical capacitor (electric double-layer capacitor).
 While it can be seen from FIG. 5 shows that the period of time for which
 the electrochemical capacitor according to the first embodiment can
 produce a high voltage is longer than the period of time for which the
 electrochemical capacitor according to the comparative example can produce
 a high voltage, the period of time for which the electrochemical capacitor
 according to the first embodiment can produce a high voltage is also
 longer than the period of time for which the conventional electrochemical
 capacitor (electric double-layer capacitor) can produce a high voltage.
 2nd Embodiment
 An electrochemical capacitor according to a second embodiment has the
 structure shown in FIG. 1, and is manufactured in the same manner as the
 electrochemical capacitor according to the first embodiment except that
 the lithium vanadium oxide has changed from LiV.sub.3 O.sub.8 used in the
 first embodiment to LiV.sub.2 O.sub.5 containing a mixture of tetravalent
 vanadium and pentavalent vanadium.
 The electrochemical capacitor according to the second embodiment was
 charged with the constant current at the constant voltage under the same
 conditions as those for the charging and discharging curve shown in FIG.
 2, and measured for various properties. The measured properties of the
 electrochemical capacitors according to the second embodiment, together
 with the properties of the electrochemical capacitor according to the
 first embodiment (which are the same as the numerical values in Table 1),
 are given in Table 3 shown below.
 3rd Embodiment
 An electrochemical capacitor according to a third embodiment has the
 structure shown in FIG. 1, and is manufactured in the same manner as the
 electrochemical capacitor according to the first embodiment except that
 the lithium vanadium oxide has changed from LiV.sub.3 O.sub.8 used in the
 first embodiment to Li.sub.2 V.sub.2 O.sub.5 containing tetravalent
 vanadium.
 The electrochemical capacitor according to the third embodiment was charged
 with the constant current at the constant voltage under the same
 conditions as those for the charging and discharging curve shown in FIG.
 2, and measured for various properties. The measured properties of the
 electrochemical capacitors according to the third embodiment, together
 with the properties of the electrochemical capacitor according to the
 first embodiment (which are the same as the numerical values in Table 1),
 are given in Table 3 shown below.
 TABLE 3
 Electric Electric Discharged
 Electrode capacitance capacitance energy
 density (F/cc) (F/g) (Wh/kg)
 Embodiment 1 1.14 53.5 29.7 46.5
 Embodiment 2 1.16 50.2 27.8 43.5
 Embodiment 3 1.10 48.7 27.0 42.2
 It can be seen from Table 3 that the electrochemical capacitor according to
 the second embodiment which employs LiV.sub.2 O.sub.5 containing a mixture
 of tetravalent vanadium and pentavalent vanadium as a lithium vanadium
 oxide, and the electrochemical capacitor according to the third embodiment
 which employs Li.sub.2 V.sub.2 O.sub.5 containing tetravalent vanadium as
 a lithium vanadium oxide offer the same performance as the electrochemical
 capacitor according to the first embodiment which employs LiV.sub.3
 O.sub.8 containing pentavalent vanadium.
 In each of the above embodiments, the electrochemical capacitor has the
 physical structure shown in FIG. 1. However, the electrochemical capacitor
 according to the present invention is not limited to the physical
 structure shown in FIG. 1, but may be of other structures including a
 structure where a plurality of positive and negative electrode elements
 each having a polarized electrode disposed on a current collector are
 alternately laminated with separators interposed therebetween, and leads
 extending from the current collectors are connected to positive and
 negative connection terminals.
 Although certain preferred embodiments of the present invention have been
 shown and described in detail, it should be understood that various
 changes and modifications may be made therein without departing from the
 scope of the appended claims.