JP5766068B2 - Capacitor and manufacturing method thereof - Google Patents

Description

  The present invention relates to a capacitor in which a negative electrode is pre-doped with alkali metal, and a method for manufacturing the same.

  2. Description of the Related Art Lithium ion capacitors and lithium ion secondary batteries are attracting attention as power storage devices for storing regenerative power in hybrid work machines and the like. Generally, a carbon-based material capable of inserting and extracting lithium ions is used for the negative electrode active material of the lithium ion capacitor. A large voltage can be obtained by pre-doping lithium ions into the negative electrode. For the polarizable electrode material, activated carbon similar to the positive electrode of the electric double layer capacitor is used. The negative electrode and the positive electrode are alternately stacked with a separator interposed therebetween.

  A technique for forming a positive electrode and a negative electrode by forming a slurry containing a negative electrode active material and a polarizable electrode material on both sides of a mesh-like current collector is known. The negative electrode can be pre-doped with lithium ions by bringing lithium metal into contact with the laminate of the positive electrode, the negative electrode, and the separator and leaving it in the electrolytic solution. Lithium ions are transported in the thickness direction through the voids of the mesh current collector.

  There has been proposed a method in which a part of a negative electrode active material layer applied to a negative electrode current collector is peeled off and metallic lithium is attached to the part, thereby pre-doping lithium ions into the negative electrode active material. In this method, lithium ions are transported in the in-plane direction within the negative electrode active material layer.

JP 2009-187753 A

  In the method of applying a slurry containing a negative electrode active material or a polarizable electrode material to a mesh current collector, the slurry passes through the voids of the mesh. Since the slurry that has passed through the voids of the mesh needs to be held in the mesh, the coating method is restricted. In the method of transporting lithium ions in the in-plane direction, since the distance to be transported becomes long, it is difficult to shorten the pre-doping time.

  The objective of this invention is providing the capacitor which can shorten pre dope time, such as an alkali metal ion, for example, lithium ion, and its manufacturing method.

According to one aspect of the invention,
A positive electrode comprising a transition metal oxide from which all alkali metals have been released from an alkali metal transition metal composite oxide, and activated carbon;
A negative electrode including a negative electrode active material capable of occlusion and release of alkali metal, wherein the alkali metal is occluded, and the amount of electricity stored by alkali metal occlusion is greater than the amount of electricity stored by cation adsorption;
A capacitor is provided that includes a separator impregnated with an electrolytic solution across the positive electrode and the negative electrode.

According to another aspect of the invention,
(A) a step of disposing a positive electrode containing an alkali metal transition metal complex oxide and activated carbon and a negative electrode containing a negative electrode active material capable of occluding and releasing alkali metal via an electrolytic solution;
(B) All alkali metals are released from the alkali metal transition metal composite oxide forming the positive electrode into the electrolyte as alkali metal ions, and the alkali metal ions in the electrolyte are There is provided a method of manufacturing a capacitor including a step of occluding a negative electrode active material of an electrode.

  The alkali metal in the alkali metal transition metal composite oxide of the positive electrode is pre-doped into the negative electrode active material of the negative electrode. Since the transport distance of alkali metal ions, which are the raw materials for pre-doping, is short, the pre-doping time can be shortened.

1A and 1B are schematic views of a capacitor according to an embodiment before and during pre-doping, respectively. 2A and 2B are schematic views during charging and discharging of the capacitor according to the embodiment, respectively. FIG. 3A is a graph showing the results of measuring the relationship between the amount of pre-doping into the negative electrode active material and the electrode potential, and FIG. 3B is an enlarged graph of the horizontal axis in the vicinity of the origin in FIG. 3A. FIG. 4 is a graph illustrating an example of changes in the potential of the electrodes during pre-doping, charging, and discharging operations of the capacitor according to the example. It is a decomposition | disassembly top view of the electrical storage cell using the capacitor by an Example. It is sectional drawing of the electrical storage cell using the capacitor by an Example.

  Before describing the examples, terms are defined. The physical action of ions gathering on the surface of the polarizable electrode material to form an electric double layer is called “adsorption”. Conversely, the physical action of ions leaving the surface of the polarizable electrode material is called “desorption”. Adsorption and desorption occur at the positive electrode and the negative electrode during charging and discharging of the electric double layer capacitor. The chemical action that causes a redox reaction at the same time that ions are taken into the electrode is called "intercalation". The chemical action that causes a redox reaction at the same time that ions leave the electrode is called "deintercalation". Occlusion and release occur at the positive electrode and the negative electrode during charging and discharging of the lithium ion secondary battery.

  A material that is charged and desorbed mainly by adsorption and desorption is referred to as “polarizable electrode material”. A material that is charged and discharged mainly by occlusion and release is called an “active material”.

  FIG. 1A shows a schematic diagram of the capacitor according to the embodiment before pre-doping. The negative electrode 15 is constituted by the negative electrode current collector 10 and the negative electrode active material layer 11. The positive electrode current collector 20 and the polarizable electrode layer 21 constitute a positive electrode 25.

  A negative electrode active material layer 11 is disposed on the surface of the negative electrode current collector 10. For the negative electrode current collector 10, for example, a copper foil having a thickness of about 14 μm is used. The negative electrode active material layer 11 is formed by binding a negative electrode active material and a conductive additive with a binder. As the negative electrode active material, for example, graphite, graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon nanotube, fullerene and the like are used. As the conductive assistant, for example, conductive fine particles such as carbon black are used. As the binder, carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR) or the like is used.

  A polarizable electrode layer 21 is disposed on the surface of the positive electrode current collector 20. For the positive electrode current collector 20, for example, an aluminum foil having a thickness of 20 μm is used. The polarizable electrode layer 21 is obtained by binding a polarizable electrode material, an alkali metal transition metal composite oxide, and a conductive additive with a binder. For example, activated carbon is used as the polarizable electrode material. The alkali metal transition metal composite oxide is a composite oxide of an alkali metal such as lithium and a transition metal such as V, Fe, Co, Mn, Ni, W, Zn, or Ti. The same material as the conductive assistant and binder of the negative electrode active material layer 11 is used for the conductive assistant and binder.

A separator 30 impregnated with an electrolytic solution is disposed between the negative electrode active material layer 11 and the polarizable electrode layer 21. For the separator 30, for example, a porous film made of polyethylene, polypropylene, aramid, polyethylene terephthalate, cellulose, cellophane, or the like is used. For example, a mixed solution of ethylene carbonate and dimethyl carbonate is used as the solvent of the electrolytic solution, and a lithium salt such as LiPF ₆ is used as the solute. The electrolytic solution contains lithium ions (Li ⁺ ) as cations and hexafluorophosphate ions (PF ₆ ⁻ ) as anions.

FIG. 1B shows a schematic diagram of the capacitor during pre-doping. Hereinafter, an example in which lithium cobalt oxide (LiCoO ₂ ) is used as the alkali metal transition metal composite oxide contained in the polarizable electrode layer 21 will be described.

At the time of pre-doping, the negative electrode current collector 10 and the positive electrode current collector 20 are connected to a DC power source 32, and a positive voltage is applied to the positive electrode current collector 20 with respect to the negative electrode current collector 10. Lithium is released from the polarizable electrode layer 21 into the electrolytic solution. At this time, the same number of free electrons as the number of released lithium atoms is generated. The generated free electrons flow out to the external circuit via the positive electrode current collector 20. Due to the release of lithium, a part of cobalt is oxidized and changed from Co ³⁺ to Co ⁴⁺ .

  Lithium ions in the electrolytic solution are occluded in the negative electrode active material layer 11. The same number of electrons as the number of occluded lithium atoms flows into the negative electrode active material layer 11 from the external circuit via the negative electrode current collector 10.

When all the lithium atoms in the polarizable electrode layer 21 are released into the electrolyte, the pre-doping is finished. At the end of pre-doping, a transition metal oxide in which an alkali metal (for example, Li) is released from an alkali metal transition metal composite oxide (for example, LiCoO ₂ ) remains in the polarizable electrode layer 21. The amount of alkali metal that can be stored in the negative electrode active material layer 11 is larger than the amount of alkali metal contained in the polarizable electrode layer 21. The amount of alkali metal that can be stored in the negative electrode active material layer 11 and the amount of alkali metal contained in the polarizable electrode layer 21 are expressed, for example, in terms of moles. For this reason, alkali metal ions having the same number of moles as the total number of moles of alkali metal ions released from the polarizable electrode layer 21 are occluded in the negative electrode active material layer 11.

  FIG. 2A shows a schematic diagram of the capacitor during charging. At the time of charging, a positive voltage is applied to the positive electrode current collector 20 with respect to the negative electrode current collector 10 by the DC power source 32 as in the case of pre-doping.

  In the positive electrode 25, electrons in the polarizable electrode layer 21 flow out to the external circuit via the positive electrode current collector 20, so that the surface of the polarizable electrode layer 21 is positively charged. Thereby, the anion in electrolyte solution is adsorb | sucked to the surface of the polarizable electrode layer 21, and an electrical double layer is formed. In the negative electrode 15, lithium ions in the electrolytic solution are occluded in the negative electrode active material in the negative electrode active material layer 11. At this time, electrons flow into the negative electrode active material layer 11 from the external circuit via the negative electrode current collector 10.

  An electric double layer is also formed on the surface of a negative electrode active material such as graphite. However, the specific surface area of graphite or the like is significantly smaller than that of activated carbon or the like. Therefore, in the negative electrode active material, the amount of electricity stored by occlusion of alkali metal ions is larger than the amount of electricity stored by cation adsorption. Charging / discharging due to adsorption and desorption of cations in the negative electrode active material layer 11 can be substantially ignored.

  FIG. 2B shows a schematic diagram of the capacitor during discharge. In the negative electrode 15, lithium occluded in the negative electrode active material layer 11 is released into the electrolytic solution. At this time, the same number of electrons as the released lithium ions flow out through the negative electrode current collector 10 to the external circuit in which the load 35 is inserted. In the positive electrode 25, electrons flow into the polarizable electrode layer 21 from the external circuit via the positive electrode current collector 20. Thereby, the anion adsorbed on the surface of the polarizable electrode material is desorbed. As long as the potential of the positive electrode 25 is not lower than the oxidation-reduction potential of the alkali metal transition metal composite oxide contained in the original polarizable electrode layer 21, occlusion of lithium ions does not occur. For this reason, the positive electrode 25 stores and discharges by adsorption and desorption of anions rather than occlusion and release of alkali metals. On the other hand, the negative electrode 15 is charged and discharged by occlusion and release of alkali metal ions.

  FIG. 3A shows the relationship between the pre-doping amount per unit mass of the negative electrode active material in the pre-doping and the potential of the negative electrode 15 (FIG. 1B). The horizontal axis represents the amount of pre-doping in the unit of “mAh / g”. The vertical axis represents the potential of the negative electrode 15 in the unit “V”. A lithium metal electrode was used as the reference electrode. FIG. 3B shows an enlarged range where the pre-doping amount is 50 mAh / g or less. Curves a and b in FIGS. 3A and 3B show measurement results when graphite and non-graphitizable carbon are used as the negative electrode active material, respectively.

  At the beginning of pre-doping, the potential of the negative electrode 15 rapidly decreases as the amount of pre-doping increases. Thereafter, the amount of change in potential with respect to the amount of change in pre-doping amount becomes small.

  FIG. 4 shows an example of potential fluctuations of the negative electrode 15 and the positive electrode 25 during the pre-doping period, the charging period, and the discharging period. The horizontal axis represents the elapsed time, and the vertical axis represents the potential of the electrode. Solid lines Vn and Vp in FIG. 4 indicate the potentials of the negative electrode 15 and the positive electrode 25, respectively. The period from time t0 to t1 corresponds to the pre-doping period. A period from time t1 to t2 and a period from time t3 to t4 correspond to the charging period. A period from time t2 to t3 and a period from time t4 to t5 correspond to the discharge period.

  During the pre-doping period (t0 to t1), the potential Vn of the negative electrode 15 decreases from V0 to V1 with time. From the experimental results shown in FIGS. 3A and 3B, at the beginning of pre-doping, the potential Vn decreases rapidly, and thereafter the potential Vn decreases gradually. At this time, the potential Vp of the positive electrode 25 hardly changes.

  During the charging period (t1 to t2, t3 to t4), the potential Vn of the negative electrode 15 decreases from V1 to V2, and the potential of the positive electrode Vp increases from V0 to V3. However, as can be seen from the experimental results of FIGS. 3A and 3B, the decrease in the potential of the negative electrode 15 is gradual with respect to the elapsed time. Since the electric double layer is formed in the positive electrode 25, the increase in the potential Vp of the positive electrode 25 is linear with respect to the accumulated charge amount. If the charging current is constant, the increase in the potential Vp of the positive electrode 25 is linear with respect to the elapsed time.

  During the discharge period (t2 to t3, t4 to t5), the potential Vn of the negative electrode 15 increases from V2 to V1, and the potential of the positive electrode Vp decreases from V3 to V0. However, the increase in the potential Vn of the negative electrode 15 is gradual with respect to the elapsed time. The decrease in the potential Vp of the positive electrode 25 is linear with respect to the accumulated charge amount. If the discharge current is constant, the decrease in the potential Vp of the positive electrode 25 is linear with respect to the elapsed time. The discharge is preferably terminated in a state where the potential Vp of the positive electrode 25 is higher than the potential V0 at the pre-doping completion time t1.

  In the capacitor according to the embodiment, the fluctuation of the potential Vn of the negative electrode 15 during charging / discharging is smaller than the fluctuation of the potential Vp of the positive electrode 25. On the other hand, when a polarizable electrode such as activated carbon is used as the negative electrode, the variation in the potential Vn of the negative electrode increases. For this reason, the charge / discharge efficiency is reduced due to the irreversible capacity. In the embodiment, since the fluctuation of the potential Vn of the negative electrode 15 is small, it is possible to suppress a decrease in charge / discharge efficiency due to the irreversible capacity.

  FIG. 5 is an exploded plan view of a storage cell including a capacitor according to the embodiment. The electrode laminate 40 is sandwiched and sealed between a pair of laminate films. In FIG. 5, the top view of the state which removed one laminate film is shown. The electrode laminate 40 is disposed on the first laminate film 41A. The electrode laminate 40 includes negative plates 42 and positive plates 43 that are alternately laminated. A separator 44 is inserted between the negative electrode plate 42 and the positive electrode plate 43 adjacent to each other.

  Each of the negative electrode plates 42 includes a square or rectangular negative electrode storage region 42A and a smaller negative electrode connection region 42B. The negative electrode connection region 42B extends outward from a region of one edge of the negative electrode storage region 42A that is biased to one end from the center. Similarly, the positive electrode plate 43 includes a positive electrode storage region 43A and a positive electrode connection region 43B.

  The negative electrode plate 42 and the positive electrode plate 43 are aligned so that the negative electrode storage region 42A and the positive electrode storage region 43A overlap. Further, the negative electrode connection regions 42B of the negative electrode plate 42 overlap each other, and the positive electrode connection regions 43B of the positive electrode plate 43 overlap each other. The negative electrode connection region 42B and the positive electrode connection region 43B do not overlap. The separator 44 is slightly larger than the negative electrode storage region 42A and the positive electrode storage region 43A. The negative electrode connection region 42 </ b> B and the positive electrode connection region 43 </ b> B extend outward from the edge of the separator 44.

  A negative electrode tab 47 is connected to the negative electrode connection region 42B, and a positive electrode tab 48 is connected to the positive electrode connection region 43B. The negative electrode tab 47 and the positive electrode tab 48 are drawn to the outside of the edge of the first laminate film 41A.

  FIG. 6 is a cross-sectional view taken along one-dot chain line 6-6 in FIG. The electrode laminate 40 is accommodated in a container composed of the first laminate film 41A and the second laminate film 41B. The second laminate film 41B is thermally welded to the first laminate film 41A in the vicinity of the outer periphery. The first laminate film 41A and the second laminate film 41B constitute a storage cell container. The container is filled with an electrolytic solution 50. The first laminate film 41A is substantially flat, and the second laminate film 41B is deformed according to the outer shape of the electrode laminate 40. In addition, it is good also as a structure where both laminate films deform | transform according to the external shape of the electrode laminated body 40. FIG.

  Negative electrode plates 42 and positive electrode plates 43 are alternately stacked, and a separator 44 is inserted between them. The negative electrode plate 42 includes the negative electrode current collector 10 and the negative electrode active material layers 11 formed on both surfaces thereof. The positive electrode plate 43 includes the positive electrode current collector 20 and the polarizable electrode layers 21 formed on both surfaces thereof. The negative electrode current collector 10, the negative electrode active material layer 11, the positive electrode current collector 20, and the polarizable electrode layer 21 respectively include the negative electrode current collector 10, the negative electrode active material layer 11, and the positive electrode current collector 20 illustrated in FIG. 1A. The same material as that of the polarizable electrode layer 21 is used.

  The negative electrode connection regions 42 </ b> B of the plurality of negative electrode plates 42 are stacked and connected to the negative electrode tab 47. For example, ultrasonic welding is applied to these connections. The negative electrode tab 47 passes through between the first laminate film 41A and the second laminate film 41B and is led out of the container.

  Similarly, the positive electrode connection regions 43B shown in FIG. 5 are also stacked on each other and ultrasonically welded to the positive electrode tab 48 (FIG. 5).

  Next, the manufacturing method of the electrical storage cell shown in FIG.5 and FIG.6 is demonstrated. A slurry for the negative electrode is prepared by adding a solvent to the fine powder of the negative electrode active material, the fine powder of the conductive additive, and the binder. For example, N-methyl-2-pyrrolidinone (NMP) is used as the solvent. This negative electrode slurry is applied to one surface of the negative electrode current collector 10 and dried. Similarly, the negative electrode slurry is applied to the other surface and dried. The negative electrode collector 42 is completed by punching out the negative electrode current collector 10 with a mold.

  A positive electrode slurry is prepared by adding a solvent to a fine powder of a polarizable electrode material, a fine powder of an alkali metal transition metal composite oxide, a fine powder of a conductive additive, and a binder. A positive electrode slurry is applied to both surfaces of the positive electrode current collector 20 and dried. A positive electrode plate 43 is completed by punching out the positive electrode current collector 20 with a mold.

  The negative electrode plate 42, the separator 44, the positive electrode plate 43, and the separator 44 are stacked in this order on the first laminate film 41A. In this state, the negative electrode tab 47 and the positive electrode tab 48 are ultrasonically welded to the negative electrode connection region 42B and the positive electrode connection region 43B, respectively. The second laminate film 41B is disposed on these laminates and thermally welded to the first laminate film 41A at the outer peripheral portion. At this stage, an opening is left in a part of the outer periphery.

  An electrolytic solution is filled into the container formed of the first laminate film 41A and the second laminate film 41B from the opening. After filling with the electrolytic solution, the opening of the container is vacuum-sealed by heat welding. In this state, a positive voltage is applied to the positive electrode tab 48 with respect to the negative electrode tab 47 to perform pre-doping.

  During pre-doping, alkali metal ions in the electrolytic solution are occluded in the negative electrode active material layer 11 of the negative electrode plate 42. The same amount of alkali metal ions as the alkali metal ions occluded in the negative electrode active material layer 11 is released from the polarizable electrode layer 21 of the positive electrode plate 43 into the electrolytic solution. For this reason, the substantial transport distance of alkali metal ions is substantially equal to the thickness of the separator 50.

  Conventionally, lithium ion pre-doping has been performed by placing a metal lithium plate on the outermost surface of the electrode stack 40. In this method, lithium ions are diffused in the in-plane direction from the end face of the negative electrode active material layer 11 to perform pre-doping. In addition, a technique is known in which a current collector has a mesh structure and lithium ions are diffused in the thickness direction. In this case, the negative electrode active material layer far from the metal lithium plate is pre-doped with lithium yne after the plurality of positive plates, negative plates, and separators are transported in the thickness direction.

  On the other hand, in the embodiment, the distance that the alkali metal ions are transported is substantially equal to the thickness of one separator 50. For this reason, the time required for pre-doping can be shortened.

  Although the present invention has been described with reference to the embodiments, the present invention is not limited thereto. It will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.

DESCRIPTION OF SYMBOLS 10 Negative electrode collector 11 Negative electrode active material layer 20 Positive electrode current collector 21 Polarization electrode layer 30 Separator 32 DC power supply 35 Load 40 Electrode laminated body 41A First laminate film 41B Second laminate film 42 Negative electrode plate 42A Negative electrode storage region 42B Negative electrode connection region 43 Positive electrode plate 43A Positive electrode storage region 43B Positive electrode connection region 44 Separator 47 Negative electrode tab 48 Positive electrode tab 50 Electrolyte

Claims (3)

A positive electrode comprising a transition metal oxide from which all alkali metals have been released from an alkali metal transition metal composite oxide, and activated carbon;
A negative electrode including a negative electrode active material capable of occlusion and release of alkali metal, wherein the alkali metal is occluded, and the amount of electricity stored by alkali metal occlusion is greater than the amount of electricity stored by cation adsorption;
A capacitor having a separator impregnated with an electrolytic solution across the positive electrode and the negative electrode.   The capacitor according to claim 1, wherein the amount of alkali metal released from the positive electrode is smaller than the amount of alkali metal that can be occluded in the negative electrode active material of the negative electrode. (A) a step of disposing a positive electrode containing an alkali metal transition metal complex oxide and activated carbon and a negative electrode containing a negative electrode active material capable of occluding and releasing alkali metal via an electrolytic solution;
(B) All alkali metals are released from the alkali metal transition metal composite oxide forming the positive electrode into the electrolyte as alkali metal ions, and the alkali metal ions in the electrolyte are And a step of occluding the negative electrode active material of the electrode.

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