WO2010032362A1

WO2010032362A1 - Bipolar electrode and production method thereof

Info

Publication number: WO2010032362A1
Application number: PCT/JP2009/003672
Authority: WO
Inventors: Yasuo Ohta; Hideaki Horie; Kana Sato; Chiduru Matsuyama
Original assignee: Nissan Motor Co., Ltd.
Priority date: 2008-09-17
Filing date: 2009-08-03
Publication date: 2010-03-25
Also published as: JP2010073421A

Abstract

According to the present invention, there is provided a bipolar electrode that includes a collector, a positive electrode active material layer formed on one side of the collector and a negative electrode active material layer formed on the other side of the collector. The bipolar electrode is characterized in that the collector has at least two polymer-containing layers laminated in a thickness direction thereof.

Description

BIPOLAR ELECTRODE AND PRODUCTION METHOD THEREOF

The present invention relates to a bipolar electrode, particularly suitable for use in a vehicle-mounted bipolar lithium-ion secondary battery, and a production method thereof.

In recent years, the reduction of carbon dioxide (CO₂) emissions has been sincerely desired in view of air pollution and global warming. The automotive industry has a growing expectation on the introduction of electric vehicles (EV) and hybrid electric vehicles (HEV) for CO₂ emission reduction and has increasingly developed chargeable/dischargeable power sources, which are essential as motor-driving power sources in the electric vehicles and become key to the practical application of the electric vehicles.

As the motor-driving power sources, there are widely used secondary batteries such as lithium-ion batteries and nickel-metal-hydride batteries and electric double layer capacitors. In particular, attentions are given to the lithium-ion secondary batteries because of the highest theoretical energy and high resistance to repeated charge/discharge cycles. Developments are thus being rapidly made in the lithium-ion secondary batteries. One type of the lithium-ion secondary batteries is known as a bipolar lithium-ion secondary battery in which a plurality of bipolar electrodes are connected to each other via electrolyte layers and disposed in a battery package or casing. Each of the bipolar electrodes consists of a collector, a positive electrode active material layer formed on one side of the collector and a negative electrode active material layer formed on the other side of the collector.

In the bipolar lithium-ion secondary battery, the collectors are generally made of metal foil. As an alternative to the metal foil collectors, conductive collectors of resin materials have recently been proposed as disclosed in Patent Literature 1. These so-called resin collectors are lighter in weight than the metal foil collectors and are expected to bring improvements in battery power output.

JP 61-285664 A

However, there is a problem that minute cracks occurs in the resin collectors and become a cause of liquid short circuits between the bipolar electrodes due to electrolyte leakage through the cracks. This results in battery performance deterioration.

It is accordingly an object of the present invention to provide a bipolar electrode having a collector formed of polymeric material and capable of preventing battery performance deterioration even in the event of a crack in the collector. It is also an object of the present invention to provide a method for production of the bipolar electrode.

According to one aspect of the present invention, there is provided a bipolar electrode, comprising: a collector; a positive electrode active material layer laminated on one side of the collector; and a negative electrode active material layer laminated on the other side of the collector, the collector having at least two polymeric material layers laminated in a thickness direction thereof.

According to another aspect of the present invention, there is provided a production method of a bipolar electrode, comprising: providing at least two, first and second polymeric material layers; applying a positive electrode active material slurry to one side of the first polymeric material layer and drying the positive electrode active material slurry to form a positive electrode active material layer on the one side of the first polymeric material layer; applying a negative electrode active material slurry to one side of the second polymeric material layer and drying the negative electrode active material slurry to form a negative electrode active material layer on the one side of the second polymeric material layer; pressing the polymeric material layers separately, with the positive and negative electrode active material layers formed on the respective one sides of the first and second polymeric material layers; and, after the pressing, joining together the other side of the first polymeric material layer and the other side of the second polymeric material layer in such a manner that the polymeric material layers form a collector between the positive and negative electrode active material layers.

Fig. 1 is a schematic section view showing the overall structure of a bipolar lithium-ion secondary battery with bipolar electrodes according to one embodiment of the present invention. Fig. 2A is a schematic view showing a state of a conventional collector during pressing. Fig. 2B is a schematic view showing a state of a collector of the bipolar secondary battery during pressing according to one embodiment of the present invention. Fig. 3 is a schematic view showing how the collector changes in form during pressing. Fig. 4A is a plan view of a battery assembly using a plurality of the bipolar secondary batteries according to one embodiment of the present invention. Fig. 4B is an elevation view of the battery assembly of Fig. 4A. Fig. 4C is a side view of the battery assembly of Fig. 4A. Fig. 5 is a schematic view of a vehicle on which the battery assembly of Figs. 4A, 4B and 4C is mounted. Fig. 6 is a schematic view showing a process of production of the bipolar secondary battery according to one embodiment of the present invention. Fig. 7 is a schematic view showing a process of production of the secondary battery according to another embodiment of the present invention.

The present invention will be described below with reference to the drawings.

Referring to Fig. 1, the following embodiment refers to a bipolar lithium-ion secondary battery 10 (occasionally simply called a bipolar secondary battery) in which a substantially rectangular battery element 17 is sealed in a battery package 22. The battery element 17 includes a plurality of bipolar electrodes 14 and electrolyte layers 15 and serves as a power generation unit that actually undergoes a charge/discharge reaction. Each of the bipolar electrodes 14 has a collector 11, a positive electrode active material layer 12 formed on one side of the collector 11 and a negative electrode active material layer 13 formed on the other side of the collector 11. The bipolar electrodes 14 and the electrolyte layers 15 are alternately laminated together in such a manner that the positive electrode active material layer 12 of one of two adjacent bipolar electrodes 14 faces the negative electrode active material layer 13 of the other of two adjacent bipolar electrodes 14 via the insulating layer 15. These adjacent positive electrode active material layer 12, insulating layer 15 and negative electrode active material layer 13 function together as one electric cell layer (unit cell) 16. It can thus also be said that the bipolar secondary battery 10 has a plurality of unit cells 16 connected in series.

The bipolar secondary battery 10 also includes insulating layers 23 at outer peripheries of the unit cells 16 so as to not only prevent liquid short circuits due to electrolyte leakage from the electrolyte layers 15, but also establish insulation between the adjacent collectors 11 and prevent short circuits due to contact of the adjacent bipolar electrodes 14 or slight irregularities in outer peripheral ends of the bipolar electrodes 14.

The

outermost electrodes

14a and 14b, located outermost of the battery element 17, are not necessarily of bipolar electrode structure. In the

outermost electrode

14a, 14b, only either the positive electrode active material layer 12 or the negative electrode active material layer 13 may be formed on one side of the

collector

11a, 11b. For example, it is feasible to form only the positive active material layer 12 on the collector 11a (or terminal plate) of the positive electrode side outermost electrode 14a and form only the negative electrode active material layer 13 on the collector 11b (or terminal plate) of the negative electrode side outermost electrode 14b as shown in Fig. 1. The bipolar secondary electrode 10 further includes a positive electrode tab 18 connected to the collector 11a of the positive electrode side outermost electrode 14a via a terminal lead 20 and a negative electrode tab 19 connected to the collector 11b of the negative electrode side outermost electrode 14b via a terminal lead 21. Alternatively, the

collectors

11a and 11b of the

outermost electrodes

14a and 14b may have portions extended to function as the

electrode tabs

18 and 19.

In order to protect the battery element 17 from external impact and environmental deterioration during use, the battery element 17 is preferably vacuum-sealed in the battery package 22 with parts of the

electrode tabs

18 and 19 led out of the battery package 22.

Conventionally, a bipolar electrode for a bipolar secondary battery is produced by preparing a collector material, applying a positive electrode active material slurry to one side of the collector material, applying a negative electrode active material slurry to the other side of the collector and then pressing the slurry-coated collector material. Namely, the conventional bipolar electrode has a single-layer collector 31 sandwiched between positive and negative electrode

active material layers

32 and 33 as shown in Fig. 2A.

It has been found by the present inventors that, when the collector is formed of a conductive resin material by the above conventional production method, a minute crack(s) occurs in the collector during the pressing.

In the event of a crack A in the single-layer resin collector 31 of the conventional bipolar electrode, the crack A passes through the collector 31 from the positive electrode active material layer 32 to the negative electrode active material layer 33 and becomes a cause of liquid short circuit due to electrolyte leakage through the crack A. This results in battery performance deterioration.

In view of the foregoing, the bipolar electrode 14 of the present embodiment is characterized in that the collector 11 has a multilayer structure in which at least two polymeric material layers 34 are laminated together in a thickness direction of the collector 11 (i.e. a lamination direction of the collector 11 and the positive and negative electrode active material layers 12 and 13) as shown in Fig. 2B.

Even if minute cracks A occurs in the respective polymeric material layers 34, it is very unlikely that the positions of the cracks A in the polymeric material layers 34 agree with each other. As long as the positions of the cracks A in the polymeric material layers 34 are different, the cracks A do not pass through the collector 11 in the thickness direction from the positive electrode active material layer 12 to the negative electrode material layer 13. It is therefore possible that the collector 11 avoids electrolyte leakage from the crack A in one polymeric material layer 34 to the crack A in the other polymeric material layer 34 and thus effectively prevents the occurrence of liquid short circuits between the cells 16. This liquid short prevention effect is more enhanced when the collector 11 has three or more polymeric material layers 34. However, the bipolar electrode 14 (and, by extension, the bipolar secondary battery 10) increases in size with increase in the number of the polymeric material layers 34 of the collector 11. It is thus advisable to select the number of the polymeric material layers 34 as appropriate depending on the intended use or purpose. The number of the polymeric material layers 34 of the collector 11 is preferably 2 to 5, more preferably 2 to 3, most preferably 2, in terms of compatibility between sufficient liquid short prevention effect and small collector thickness.

For convenience of explanation, the outermost polymeric material layer 34 of the collector 11 on which the positive electrode active material layer 12 is formed (e.g. the upper polymeric material layer in Fig. 2B) and the outermost polymeric material layer 34 of the collector 11 on which the negative electrode active material layer 13 is formed (e.g. the lower polymeric material layer in Fig. 2B) are hereinafter occasionally called first and second polymeric material layers. It is further noted that the

collectors

11a and 11b of the

outermost electrodes

14a and 14b are not necessarily of the above multilayer polymeric material structure but may be formed of metal foil such as stainless steel foil or aluminum foil.

There is no particular restriction on the composition of the polymeric material layers 34 of the collector 11 as long as polymeric material layers 34 exhibit electrical conductivity such that the collector 11 can serve its purpose properly.

In particular, the multilayer structure of the collector 11 is effective in liquid short prevention when the polymeric material layers 34 are of a polymer containing a conductive filler (conductive particles). In the production of the collector 11, the collector material spreads horizontally during pressing as shown in Fig. 3. As the conductive filler is less likely to spread than the polymer due to a difference in elastic modulus between the polymer and the conductive filler, the collector 11 (polymeric material layers 34) tends to have a nonuniform concentration of the conductive filler by aggregation of the conductive filler etc. and thus readily sustains cracks. Even in the event the cracks occur during the pressing, however, it is possible for the multilayer collector 11 to avoid electrolyte leakage through these cracks and prevent battery performance deterioration effectively.

The conductive filler is selected from those having electrical conductivity, but not allowing conduction of ions (as charge-transfer medium), and having the ability to withstand positive and negative electrode potentials to be applied. Examples of the conductive filler are conductive particles such as aluminum particles, SUS particles, carbon particles, silver particles, gold particles, copper particles, titanium particles and any conductive alloy particles. The conductive filler is not limited to the above. Any other commercially practical conductive fillers such as carbon nanotubes and conductive resin fillers can suitably be used.

The distribution of the conductive filler in the collector 11 is not necessarily uniform and can be changed depending on the position within the collector 11. Two or more kinds of conductive fillers may be used in combination. For example, the conductive filler of the first polymeric material layer 34 can be different in kind from that of the second polymeric material layer 34. The conductive filler of the first polymeric material layer 34 is preferably selected from aluminum particles, SUS particles and carbon particles. The conductive filler of the second polymeric material layer 34 is preferably selected from silver particles, gold particles, copper particles, titanium particles, SUS particles and carbon particles. In each polymeric material layer 34, it is especially preferable to use carbon particles such as carbon black or graphite. The carbon particles have a large potential window and are highly conductive and lightweight. By the use of such carbon particles as the conductive filler, the collector 11 has the advantages of stability to a wide range of positive and negative electrode potentials, high electrical conductivity and minimum increase in weight. Further, the carbon particles are often contained as conductive aids in the active material layers 12 and 13 as will be explained later. It is feasible to reduce the contact resistance between the collector 11 and the

active material layer

12, 13 significantly by using the same carbon material as the conductive filler of the collector 11 and as the conductive aid of the

active material layer

12, 13. Before using the carbon particles as the conductive filler, the carbon particles may be subjected to hydrophobic treatment so as to decrease the electrolyte compatibility of the collector 11 and thereby make it unlikely that an electrolyte of the electrolyte layer 15 will penetrate into pores of the collector 11.

The particle size of the conductive filler is not particularly restricted. Preferably, the conductive filler has an average particle size of 0.1 to 50 micrometers, more preferably 1 to 20 micrometers, still more preferably 1 to 3 micrometers.

The particle form of the conductive filler is not also particularly restricted. The conductive filler can be in fiber form, in plate form or in massive form.

The polymer, when used in combination with the conductive filler, does not necessarily show electrical conductivity and can be a nonconductive polymer. The nonconductive polymer is selected from those having the ability to withstand positive and negative electrode potentials to be applied to enhance binding of the condutive filler for battery reliability improvement. Preferred examples of the nonconductive polymer are: thermoplastic polymers such as polyethylene, polypropylene, polyethylene terephthalate, polyacrylonitrile, polyethernitrile, polymethylacrylate, polymethylmethacrylate, polyamide, polyimide, cellulose, carboxymethyl cellulose, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber, isoprene rubber, butadiene rubber, ethylene-propylene rubber, ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer, hydrogenated derivative thereof, styrene-isoprene-styrene block copolymer and hydrogenated derivative thereof; hydroxyl-containing polymers such as ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, 1,3-butylene glycol, polyvinyl alcohol, polyvinyl propynal, polyvinyl butyral, polyacrylamide, polyethylene glycol, polypropylene glycol and polybutylene glycol; fluoropolymers such as polyvinylidene difluoride (PVdF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE) and polyvinyl fluoride (PVF); and vinylidene fluoride rubbers such as vinylidene fluoride-hexafluoropropylene fluoro-rubber (VDF-HFP fluoro-rubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluoro-rubber (VDF-HFP-TFE fluoro-rubber), vinylidene fluoride-pentafluoropropylene fluoro-rubber (VDF-PFP fluoro-rubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene fluoro-rubber (VDF-PFP-TFE fluoro-rubber), vinylidene fluoride-perfluoromethylvinylether-tetrafluoroethylene fluoro-rubber (VDF-PFMVE-TFE fluoro-rubber) and vinylidene fluoride-chlorotrifluoroethylene fluoro-rubber (VDF-CTFE fluoro-rubber). These polymer compounds can be used alone or in combination of two or more thereof. The above nonconductive polymers have not only a large potential window to show stability to a wide range of positive and negative potentials but also the ability to increase the adhesion of the collector 11 to the

active material layer

12, 13. It is especially preferable to use polypropylene because the polypropylene is also stable to the electrolyte and has a low thermal expansion coefficient.

The content ratio of the conductive filler and the polymer is not particularly restricted. The content of the conductive filler is preferably in the range of 5 to 35 vol%, more preferably 5 to 25 vol%, still more preferably 5 to 15 vol%, based on the total volume of the collector 11 (i.e. the total of the polymer and the conductive filler). It is possible to provide the collector 11 with adequate conductivity by the addition of such a sufficient amount of conductive filler.

The polymeric material layers 34 of the collector 11 do not necessarily contain a conductive filler and can alternatively be formed of a conductive polymer.

The conductive polymer is selected from those having electrical conductivity, but not allowing conduction of ions (as charge-transfer medium), and having the ability to withstand positive and negative electrode potentials to be applied. There can be used as the conductive polymer conjugated polyene, which is currently proceeding toward practical use in electrolytic capacitors etc. It is assumed that the conjugated polyene forms an energy band to show electrical conductivity. Examples of the conjugated polyene are polyaniline, polypyrrole, polythiophene, polyacetylene, polyparaphenylene, polyphenylenevinylene, polyacrylonitrile, polyoxadiazole and any mixture thereof. The conductive polymer is not limited to the above. Any other conductive polymer may alternatively be adopted as appropriate.

The distribution of the polymer (conductive polymer, nonconductive polymer) in the collector 11 is not necessarily uniform and may be changed depending on the position in the collector 11. Two or more kinds of polymers may be used in combination. For example, the polymer of the first polymeric material layer 34 can be different in kind from that of the second polymeric material layer 34. The polymeric material of the collector 11 may contain any other additive or additives as needed.

The thickness of the collector 11 is not particularly restricted. It is desirable that the thickness of the collector 11 is small in terms of high battery output density. In view of the fact that the performance of the bipolar secondary battery 10 will not be affected even when the electrical resistance of the collector 11 is high in a direction horizontal (perpendicular) to the lamination direction, it is feasible to use anisotropic conductive films as the polymeric material layers 34 of the collector 11 so as to decrease the thickness of the collector 11. In particular, the thickness of the polymeric material layers 34 of the collector 11 is preferably in the range of 1 to 100 micrometers, more preferably 1 to 50 micrometers, still more preferably 5 to 30 micrometers.

The resistance of the collector 11 is not also particularly restricted. It is preferable to select the conductive polymer or adjust the content of the conductive filler in the polymer in such a manner that the resistance of the collector 11 is one-hundredth of the total resistance of the battery 10 or lower.

The positive and negative electrode material layers 12 and 13 contain positive and negative electrode active materials to play leading parts in the charge/discharge reaction, respectively.

Preferred examples of the positive electrode active material are lithium-transition metal composite oxides such as Li-Mn composite oxides e.g. LiMn₂O₄ and Li-Ni composite oxides e.g. LiNiO₂. Two or more kinds of positive electrode active materials may be used in combination.

Preferred examples of the negative electrode active material are lithium-transition metal composite oxide such as LiTi₅O₁₂ and carbon materials such as graphitic carbon materials (graphite materials) e.g. natural graphite, artificial graphite and expanded graphite, carbon black, acetylene black, activated carbon, carbon fiber, coke, soft carbon and hard carbon. Among the carbon materials, graphite materials such as natural graphite, artificial graphite and expanded graphite are preferred. As the natural graphite, there can be used flake graphite and massive graphite. As the artificial graphite, there can be used massive graphite, vapor grown graphite, flake graphite and fibrous graphite. The flake graphite and massive graphite are especially preferred for high packing density. Two or more kinds of negative electrode active materials may be used in combination.

The average particle size of the active material is not particularly restricted and is preferably in the range of 1 to 100 micrometers, more preferably 1 to 50 micrometers, still more preferably 1 to 20 micrometers. The active material, even if having an average particle size out of the above specified range, can be adopted within the scope of the present invention. The particle size of the active material is herein measured by laser diffraction particle size analysis (laser diffraction scattering) in the present embodiment.

For high energy density, the content of the active material in the

active material layer

12, 13 is preferably in the range of 70 to 98 mass%, more preferably 80 to 98 mass%, based on the total mass of the

active material layer

12, 13.

The thickness of each of the positive and negative electrode active material layers 12 and 13 is preferably in the range of 20 to 500 micrometers, more preferably 20 to 300 micrometers, still more preferably 20 to 150 micrometers.

Each of the active material layers 12 and 13 may optionally contain a binder, a conductive aid and a support salt (lithium salt). The mixing ratio of the respective additive components in the

active material layer

12, 13 is not particularly restricted and can be adjusted as appropriate in the light of any knowledge about lithium-ion secondary batteries.

Examples of the binder are polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyethernitrile (PEN), polyimide (PI), polyamide (PA), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polymethylacrylate (PMA), polymethylmethacrylate (PMMA), polyvinyl chloride (PVC), polyvinylidene difluoride (PVdF) and any mixture thereof.

Examples of the conductive aid are those added for improvement in conductivity, such as carbon powder e.g. graphite and carbon fiber e.g. vapor grown carbon fiber (VGCF).

Examples of the support salt (lithium salt) are those contained in the electrolyte layers 15 as will be explained later.

Preferably, the collector 11 has a layer 53 of conductive paste between the adjacent polymeric material layers 34 as shown in Fig. 7. The lamination of the polymeric material layers 34 results in contact resistance at the interface between the polymeric material layers 34. There is a possibility that the performance of the bipolar secondary battery 10 may be affected due to such contact resistance between the polymeric material layers 34. It is however possible to reduce the contact resistance between the polymeric material layers 34 of the collector 11 by joining the polymeric material layers 34 via the conductive paste layer 53. It is also possible to increase the mechanical strength of the collector 11 because the conductive paste generally contains a binder and thus, at the time the polymeric material layers 34 are joined before the conductive paste gets dried, serves as an adhesive between the polymeric material layers 34.

Preferred examples of the conductive paste are metal oxide conductive pastes such as those containing zinc oxide, indium oxide and titanium oxide and carbon conductive pastes such as those containing carbon black, carbon nanotube and graphite.

The thickness of the conductive paste layer 53 is preferably in the range of 5 to 100 micrometers, more preferably 5 to 40 micrometers, still more preferably 5 to 20 micrometers.

It is preferable (but not necessary) that, when the collector 11 has three or more polymeric material layers 34, the conductive paste layer 53 is formed between each adjacent two polymeric material layers 34 so as to obtain a larger contact resistance reduction effect. Even if the conductive paste layer 53 is not formed between each adjacent two polymeric material layers 34, the contact resistance reduction effect can be obtained as long as at least one conductive paste layer 53 is formed between any adjacent two polymeric material layers 34.

There is no particular restriction on the form of the electrolyte layers 15. The electrolyte layers 15 can be in the form of a solid polymer electrolyte, a gel polymer electrolyte or a separator supporting therein an electrolyte solution (liquid electrolyte), a gel polymer electrolyte or a solid polymer electrolyte. The electrolyte solution refers to a liquid formed by dissolving a support salt (lithium salt) in an organic solvent. The gel polymer electrolyte refers to a gel formed by impregnating a pre-gel electrolyte solution in a matrix polymer. The solid polymer electrolyte refers to a solid formed by curing a solution of a support salt (lithium salt) and a matrix polymer. The use of the solid polymer electrolyte or gel polymer electrolyte provides the insulating layer 15 with improved mechanical strength due to the cross-link structure of the matrix polymer.

As the electrolyte solvent, there can be used any organic solvent capable of dissolving therein the support salt. Examples of the organic solvent are aprotic organic solvents (plasticizers) containing one or two or more kinds of: cyclic carbonates such as propylene carbonate (PC) and ethylene carbonate (EC); linear carbonates such as dimethyl carbonate (DMC), methyl ethyl carbonate and diethyl carbonate (DEC); ethers such as tetrahydrofuran, 2-methyl tetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane and 1,2-dibutoxyethane; lactones such as gamma-butyrolactone; nitriles such as acetonitrile; esters such as methyl propionate; amides such as dimethylformamide; methyl acetate; and methyl formate. These solvents can be used alone or in combination of two or more thereof. Among others, preferred are carbonates such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC) and diethyl carbonate (DEC). The electrolyte solvent is not limited to the above. Any other electrolyte solvent can suitably be used. Further, the organic solvent can also be used as a viscosity adjusting agent.

Any known lithium salts can be used as the support salt. Examples of the support salt (lithium salt) are Li(C₂F₅SO₂)₂N (LiBETI), LiPF₆, LiBF₄, LiClO₄, LiAsF₆, LiCF₃SO₃ and Li(CF₃SO₂)₂N.

Any ion conductive polymers can be used as the matrix polymer. Examples of the ion conductive polymers are polyalkylene oxide polymers, which are known for their high ability to dissolve/disperse therein electrolyte salts such as lithium salts, including polyethylene oxide (PEO), polypropylene oxide (PPO), polyethylene glycol (PEG), polyacrylonitrile (PAN), polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP) copolymer, poly(methylmethacrylate) (PMMA) and copolymer thereof.

As the separator, there can be used a porous film separator or a non-woven fabric separator. The separator may be subjected to heating at about 80 Celsius degrees during battery manufacturing as will be explained later. It is thus preferable that the separator has sufficient heat resistance to withstand such a heating temperature as well as no reactivity to the electrolyte. Examples of such separator material include, but are not limited to, polyolefins such as polyethylene and polypropylene and polyethylene terephthalate (PET).

There is no particular restriction on the material of the insulating layers 23. The insulating materials 23 can be made of any insulating material as long as it is capable of providing insulation between the adjacent collectors 11, seal against leakage of the electrolyte from the electrolyte layers 15 and seal against moisture from the surrounding air. Examples of such insulating material are urethane resin, epoxy resin, polyethylene resin, polypropylene resin, polyimide resin and rubber material. Among others, preferred are urethane resin and epoxy resin in terms of corrosion resistance, resistance to chemical attack, ease of layer formation (film forming properties) and cost effectiveness.

There is no particular restriction on the material of the

electrode tabs

18 and 19. Any known (commonly used) tab material can be used. Examples of the tab material include, but are not limited to, aluminum, copper, nickel, stainless steel and alloy thereof.

As the material of the battery package 22, there can be used a metal can or a bag of aluminum laminate film such as that in which a polypropylene layer, an aluminum layer and a nylon layer are laminated in this order. The battery package material is not limited to the above. Any known battery package/casing material can be used.

There is also no particular restriction on the form of the bipolar secondary battery 10. The bipolar secondary battery 10 can have any known structure such as laminated (flat) battery structure or winding (cylindrical) battery structure.

Referring to Figs. 4A, 4B and 4C, the present embodiment also refers to a battery assembly 300 in which a plurality of (two or more) bipolar secondary batteries 10 are electrically connected in series, in parallel or in combination thereof. The output capacity and voltage of the battery assembly 300 can be adjusted freely depending on the series and/or parallel connection of the bipolar secondary batteries 10. For example, the battery assembly 300 can be produced by forming a plurality of attachable/detachable battery modules 250, in each of which a plurality of plurality of bipolar secondary batteries 10 are connected in series or in parallel, laminating the battery modules 250 in layers using a connection jig 310, and then, connecting the battery modules 250 in series or in parallel via any electrical connection means such as busbars.

The bipolar secondary battery(s) 10 and the battery assembly 300 fit for a wide range of uses as power sources in automotive vehicles such as hybrid electric vehicles, electric vehicles and fuel-cell vehicles. These automotive vehicles includes only four-wheel vehicles (passenger cars, commercial cars e.g. trucks and buses, light cars etc.) but also two-wheel vehicles (motorbikes etc.) and three-wheel vehicles.

One example of use of the battery assembly 300 is as a power source in an electric vehicle 400 as shown in Fig. 5. In this case, the battery assembly 300 is placed at a position under a seat in the center of a vehicle body of the electric vehicle 400 in order to secure a wide vehicle interior space and trunk rooms. The mounting position of the battery assembly 300 is not limited to the position under the seat. The battery assembly 300 may alternatively be placed in a lower section of the rear trunk room or an engine room in the vehicle front side. The electric vehicle 400 with the battery assembly 300 attains high durability and ensures sufficient output during long-term use. Further, the electric vehicle 400 with the battery assembly 300 offers high fuel efficiency and running performance.

The uses of the bipolar secondary battery(s) 10 and the battery assembly 300 are not limited to the automotive vehicles. The bipolar secondary battery(s) 10 and the battery assembly 300 can also be used as power sources in any other transportation means such as trains and mountable/installable power supplies such as uninterruptible power supplies.

The above-structured bipolar secondary battery 10 can be manufactured by the following procedure.

The bipolar electrodes 14 are first produced. In the present embodiment, each of the bipolar electrodes 14 is produced by providing at least two separate polymeric material layers 34, forming the positive electrode active material layer 12 on one side of the first polymeric material layer 34, forming the negative electrode active material layer 13 on one side of the second polymeric material layer 34,
pressing the polymeric material layers 34 separately with the active material layers 12 and 13 formed thereon, and then, joining the other sides of the polymeric material layers 34 to form the collector 11 between the active material layers 12 and 13.

More specifically, the polymeric material layers 34 are provided as shown in Step A of Fig. 6 and Step A of Fig. 7 by e.g. preparing a slurry of the polymeric material (polymer with conductive filler, or conductive polymer) and applying and drying the slurry onto resin films such as PET films, or forming the polymeric material into a desired film shape.

There is no particular restriction on the slurry application technique or film forming technique. There can be used any known application technique using e.g. an applicator or a coater or any thin-film forming technique such as doctor blade technique, spray coating technique, screen printing technique or ink jet printing technique. It is alternatively feasible to use thermo-compressed sheets of blends of the polymer with the conductive filler as the polymeric material layers 34. These polymer sheets are commercially available from e.g. Hitachi Chemical Co., Ltd. and Fujikura Ltd.

The positive electrode active material layer 12 is next formed by preparing a slurry of the positive electrode active material, applying the slurry to the one side of the first polymeric material layer 34, and then, drying the slurry. Similarly, the negative electrode active material layer 13 is formed by preparing a slurry of the negative electrode active material, applying the slurry to the one side of the second polymeric material layer 34, and then, drying the slurry. Also see Step A of Fig. 6 and Step A of Fig. 7.

There is no particular restriction on the slurry preparation technique. The active material slurry is prepared by mixing the active material, optionally together with the binder, the conductive aid, the support salt and the other additive(s) such as electrolytic material (solid electrolyte, matrix polymer, electrolyte solution etc.) and polymerization initiator, into a solvent (viscosity adjusting agent) at a given mixing ratio. As the solvent, there can be used any slurry solvents such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), dimethylacetamide and methylformamide. At this time, it is feasible to blend the binder in powder form with the other solid components (the active material etc.) and then add the solvent to the resulting blend, or to disperse the binder in a small amount of the solvent and then add the active material etc. and the remaining solvent into the resulting dispersion system. Any known mixing technique can be used without particular limitation.

There is no particular restriction on the slurry application technique. The active material slurry can be applied by any known application technique using e.g. an applicator, a conventional bar coater or a self-running coater. The active material slurry may alternatively be applied by any thin-film forming technique such as doctor blade technique, spray coating technique, screen printing technique, ink jet printing technique or combination thereof.

There is also no particular restriction on the drying technique as long as the slurry is dried by the drying technique to remove the solvent and thereby form the

active material layer

12, 13 on the polymeric material layer 34. Any known drying technique such as heating or vacuum drying can be used without particular limitation. The drying conditions (drying time and temperature) are set as appropriate according to the application amount of the slurry and the volatilization speed of the slurry solvent etc. Depending on the kind of the slurry, the binder may be cross-linked to increase the mechanical strength of the

active material layer

12, 13. As the drying conditions vary with the kind of the slurry and thus cannot be uniquely determined, the drying operation is generally performed at 40 to 150 Celsius degrees for 5 minutes to 20 hours.

The polymeric material layers 34, on which the active material layers 12 and 13 have been formed, are separately pressed to a desired thickness for improvements in surface smoothness and thickness uniformity as shown in Step B of Fig. 6 and Step B of Fig. 7. As the polymeric material layers 34 are separately subjected to pressing, it is very unlikely that, even in the event of minute cracks in the polymeric material layers 34, the positions of these cracks agree with each other. It is therefore possible to, when the polymeric material layers 34 are joined together in the subsequent joining step, prevent the cracks from passing through the collector 11 from the positive electrode active material layer 12 to the negative electrode active material layer 13.

The pressing step can be performed by either cold roll pressing or hot roll pressing. In the case of hot roll pressing, it is preferable to adjust the pressing temperature to a degree that does not cause decomposition of the support salt and polymerizable polymer if contained in the

active material layer

12, 13. Further, the pressing pressure is preferably 20 to 100 t/m, more preferably 30 to 80 t/m, still more preferably 40 to 70 t/m, in terms of the line pressure. The pressing conditions (pressing pressure and time) are not limited to the above and can be varied as appropriate depending on the materials used and the desired layer thickness. There is no particular restriction on the roll press machine. Any known roll press machine such as a calender roll can suitably be used. The pressing step may alternatively be performed by any other known technique such as platen pressing.

After that, the other sides of the polymeric material layers 34 on which the active material layers 12 and 13 have not been formed are mated and joined together, thereby completing the bipolar electrode 14 as shown in Step C of Fig. 6 and Step D of Fig. 7. When the collector 11 has three or more polymeric material layers 34, the active material layers 12 and 13 are formed only on the outermost first and second polymeric material layers 34. Any polymeric material layer other than the outermost first and second polymeric material layers 34 is pressed to a desired thickness and inserted between the mating sides of the first and second polymeric material layers 34.

Before the joining step, the conductive paste may preferably be applied as

thin coatings

53a and 53b to either one or both of the mating sides of the polymeric material layers 34 as shown in Step C of Fig. 7.

There is no particular restriction on the paste application technique. The conductive paste can be applied by any known application technique using e.g. an applicator, a conventional bar coater, a self-running coater or a doctor blade. The amount of the conductive paste applied is adjusted as appropriate depending on the size and thickness of the resulting conductive paste layer 53.

The conductive paste can be dried before and during/after the joining step. It is however preferable to join the mating sides of the polymeric material layers 34 before the conductive paste gets dried, such that the conductive paste can function as an adhesive between the polymeric material layers 34 so as to increase the mechanical strength of the collector 11.

On the other hand, the positive electrode side outermost electrode 14a is produced in the same manner as above except that the only the positive electrode active material layer 12 is formed on the outermost collector 11a. The negative electrode side outermost electrode 14b is also produced in the same manner as above except that only the negative electrode active material layer 13 is formed on the outermost collector 11b.

The electrolyte layers 15 are next produced as shown in Step D of Fig. 6 and Step E of Fig. 7.

In the case of using the solid polymer electrolyte, the electrolyte layer 15 is produced by dissolving the lithium salt, together with a material of the matrix polymer and optionally a polymerization initiator etc., into the appropriate electrolyte solvent and curing the resulting solid polymer electrolyte material solution. The solid polymer electrolyte material solution may be applied to the separator and then cured to form the electrolyte layer 15 in which the separator supports the solid polymer electrolyte.

In the case of using the gel polymer electrolyte, the electrolyte layer 15 is produced by mixing the lithium salt, together with a material of the matrix polymer and optionally a polymerization initiator etc., into the appropriate electrolyte solvent and heating and drying the resulting pre-gel solution in an inert atmosphere and simultaneously carrying out polymerization (cross-linking) of the polymer. The pre-gel solution may be applied to the separator and then polymerized by heating/drying to form the electrolyte layer 15 in which the separator supports the gel polymer electrolyte.

For example, it is feasible to produce the insulating layer 15 by applying the solid polymer electrolyte material solution or pre-gel solution at a given thickness to the bipolar electrode 14 (positive electrode active material layer 12 and/or negative electrode active material layer 13), or placing the separator on the bipolar electrode 14 and applying the solid polymer electrolyte material solution or pre-gel solution to the separator, and then, subjecting the applied electrolyte solution to curing or polymerization by heating/drying in an inert atmosphere. It is alternatively feasible to produce the insulating layer 15 separately from the electrodes 14 by e.g. applying the solid polymer electrolyte material solution or pre-gel solution to the separator and then subjecting the solid polymer electrolyte material solution or pre-gel solution to curing or polymerization by heating/drying in an inert atmosphere. The separator material may be removed. In this case, it is preferable to select the separator material with good releasability.

The compositions and mixing ratios of the solid polymer electrolyte material solution and the pre-gel solution are selected as appropriate depending on the intended use or purpose. Further, the curing operation and the heating/drying operation can be performed using e.g. a vacuum drier (vacuum oven). Although the curing conditions and the heating/drying conditions depend on the solid polymer electrolyte material solution and the pre-gel solution and cannot be uniquely determined, the curing operation and the heating/drying operation are generally performed at 30 to 110 Celsius degrees for 0.5 to 12 hours. The thickness of the electrolyte layers 15 can be adjusted using e.g. a spacer. In the case of using a photo polymerization initiator, for example, the electrolyte solution is poured into a light-transmitting gap and dried and subjected to photo polymerization (cross-linking) by irradiation with ultraviolet using an ultraviolet radiation unit. Any other curing/polymerization reaction system such as radiation induced polymerization, electron-beam induced polymerization or thermal polymerization can be adopted as appropriate depending on the kind of the polymerization initiator.

In the case using the liquid electrolyte (electrolyte solution), the electrolyte layer 15 is produced by preparing the electrolyte solution and impregnating the prepared electrolyte solution in the separator. The electrolyte solution may be impregnated in the separator after placing the separator on the bipolar electrode 14. Any impregnation technique such as vacuum impregnation can be used.

In general, the width of the electrolyte layers 15 is made slightly smaller than that of the collectors 11 of the bipolar electrodes 14.

The electrodes 14 and the electrolyte layers 15 are dried sufficiently by heating in a high vacuum, cut into proper sizes and alternately laminated to each other so as to yield the battery element 17. See also Step D of Fig. 6 and Step E of Fig. 7. The number of the electrodes 14 and the electrolyte layers 15 laminated in the battery element 17 depends on the performance required of the bipolar secondary battery 10. This lamination operation is preferably performed in an inert atmosphere such as an argon atmosphere or nitrogen atmosphere in order to prevent moisture from entering into the battery element 17.

The insulating layers 23 are subsequently formed as shown in Step E of Fig. 6 and Step F of Fig. 7 by applying the insulating material e.g. epoxy resin (precursor solution) by a given width to the outer peripheries of the unit cells 16 around the electrodes 14 and curing the insulating material. Before the application of the insulating material, masking treatment may be performed to apply a releasable masking material to any portions of the collectors 11 for connection to the

electrode tabs

18 and 19, electrode terminal plates and leads and voltage detection tabs. The masking material is removed after curing the insulating material.

The

electrode tabs

18 and 19 are electrically joined to the

collectors

11a and 11b of the

outermost electrodes

14a and 14b, respectively. Also see Step E of Fig. 6 and Step F of Fig. 7. There is no particular restriction on the electrical joining technique. The electrical joining can be performed by any suitable technique such as ultrasonic welding technique in which the joining temperature is relatively low.

Finally, the battery element 17 is sealed in the battery package 22, with parts of the

electrode tabs

18 and 19 led out of the battery package 22, as shown in Step F of Fig. 6 and Step G of Fig. 7. With this, the bipolar secondary battery 10 is completed.

The present invention will be described below in more detail with reference to the following examples. It should be however noted that the following examples are only illustrative and not intended to limit the invention thereto.

In Example 1, each of 20 battery samples was produced by the process as shown in Fig. 6.

Formation of Electrodes
A bipolar electrode was first produced as follows.

Two commercially available sheets of polymeric material containing 80 vol% polypropylene (as a polymer) and 20 vol% carbon fine particles with an average particle size of 0.8 micrometers (as a conductive filler) and having a thickness of 20 micrometers were prepared as polymeric material layers for each collector.

A positive electrode active material slurry was prepared by charging an appropriate amount of high-purity anhydrous NMP (as a solvent) into a dispersion mixer, dispersing 10 mass% PVdF (as a binder) into the NMP, dispersing 85 mass% LiMn₂O₄ (as a positive electrode active material) and 5 mass% acetylene black (as a conductive aid) gradually into the dispersion system, and then, adjusting the viscosity of the dispersion system with the addition of an appropriate amount of NMP (as a viscosity adjusting solvent). Then, the positive electrode active material slurry was applied by a coating machine to one side of one of the polymeric material layers. The thickness of the slurry coating was adjusted by a uniform-thickness doctor blade. The slurry coating was then dried at 100 Celsius degrees on a hot stirrer, thereby forming a positive electrode active material layer with a thickness of 15 micrometers and a density of 2.5 g/cm³ on the one side of the one of the polymeric material layers.

A negative electrode active material slurry was prepared by charging an appropriate amount of high-purity anhydrous NMP (as a solvent) into a dispersion mixer, dispersing 10 mass% PVdF (as a binder) into the NMP, dispersing 85 mass% Li₄Ti₅O₁₂ (as a negative electrode active material) and 5 mass% acetylene black (as a conductive aid) gradually into the dispersion system, and then, adjusting the viscosity of the dispersion system with the addition of an appropriate amount of NMP (as a viscosity adjusting solvent). The resulting negative electrode active material slurry was applied by a coating machine to one side of the other polymeric material layer. The thickness of the slurry coating was adjusted by a uniform-thickness doctor blade. The slurry coating was then dried at 100 Celsius degrees on a hot stirrer, thereby forming a negative electrode active material layer with a thickness of 15 micrometers and a density of 1.5 g/cm³ on the one side of the other polymeric material layer.

The polymeric material layers were pressed separately by a roll press machine with a roll pressure with a press pressure of 50 t/m, with the active material layers formed on the one sides of the polymeric material layers.

After that, a conductive carbon paste containing graphite, carbon black, phenol resin and butylcarbitol (available under the trade name of Varniphite from Nippon Graphite Industries, Ltd.) was applied as coatings with a thickness of 10 micrometers onto the other sides of the polymeric material layers on which the active material layers have not been formed. The paste-coated sides of the polymeric material layers were mated together to form the collector before drying the applied paste coatings.

The bipolar electrode was completed when the paste coatings were dried to form a conductive paste layer between the polymeric material layers. The resulting bipolar electrode had an overall thickness of 40 micrometers, an overall size of 140 mm by 90 mm and a sealing margin of 100 mm around the active material layers on the collector. Further, the positive and negative electrode active material layers were the same in size.

On the other hand, two sheets of stainless steel foil were provided as outermost collectors. One of the outermost collectors was coated with only the above-prepared positive electrode active material slurry, whereas the other outermost collector was coated with only the above-prepared negative electrode active material slurry.

Formation of Electrolyte Layers
An electrolyte solvent was first prepared by mixing propylene carbonate and ethylene carbonate at a mass ratio of 1:1. Then, a gel polymer electrolyte solution was prepared by mixing 90 mass% 1M LiPF6 (as an electrolyte solute) and VdF-HFP copolyer containing 10 mass% HFP (as a matrix polymer) into the electrolyte solvent and adding DMC (as a viscosity adjusting solvent) to the electrolyte solution. The resulting gel polymer electrolyte solution was applied to and impregnated into the positive and negative electrode material layers of the bipolar electrode, the positive electrode active material layer of the positive electrode side outermost collector and the negative electrode active material layer of the negative electrode side outermost collector by a coating machine. The DMC was removed by drying. The gel polymer electrolyte solution was also applied to and impregnated into porous film separators of polypropylene having a thickness of 20 micrometers and a size of 130 mm by 80 mm. The DMC was removed by drying. In this example, the porous film separator formed an electrolyte layer when laminated on the electrolyte-impregnated active material layer of the electrode.

Battery Assembling
The positive electrode side outermost collector, the separator, the bipolar electrode, the separator and the negative electrode side outermost collector were laminated sequentially so that the positive and negative electrode active material layers faced each other via the separator, thereby providing a battery element. Polyethylene film sheets were then arranged as seal material on the sealing margins of the respective collectors.

The thus-obtained battery element was subjected to thermal pressing with 0.2 MPa at 160 Celsius degrees for 5 seconds in such a manner as to eliminate the interfaces between the electrolyte layers and the electrodes. The battery element was further heated for about 1 hour to establish a seal by the seal material on an outer periphery of the battery element. Current lead plates of aluminum having a thickness of 100 micrometers and a size of 130 mm by 80 mm were placed on the respective outermost collectors. Further, current lead terminals having a size of 30 mm by 50 mm were provided on the respective current lead plates. Finally, the battery element was vacuum-sealed in a laminate film package with the current lead terminals led out of the laminate film package. With this, the battery sample was completed.

In Example 2, 20 battery samples were produced in the same manner as in Example 1 except that no conductive paste layer was formed between polymeric material layers of each bipolar electrode collector.

Comparative Example

In Comparative Example, 20 battery samples were produced in the same manner as in Example 1 except that each bipolar electrode was prepared by providing a single polymeric material sheet as a collector, forming positive and negative electrode material layers on respective opposite sides of the collector, and then, pressing the collector. Needless to say, the bipolar electrode collector had a single polymeric material layer with no conductive paste layer in Comparative Example.

Performance Evaluation
Each of the above-prepared battery samples was subjected to evaluation test as follows.

The battery sample was charged with a constant current of 0.5 mA up to 8.2 V, and then, charged at a constant voltage. The total charge time was 10 hours. After that, the battery sample was discharged with 1 mA for 5 seconds. The internal resistance of the battery sample was calculated based on the amount of voltage change in the battery sample during the discharge. The average value of the internal resistance calculation results of the 20 samples was determined. Further, the amount of voltage drop in the battery sample was measured after leaving the battery sample at 60 Celsius degrees for 1 week. The battery sample was judged as defective when the voltage of the battery sample was decreased to 7.5 V or lower. The test results are shown in Table 1. In Table 1, the internal resistance of Examples 1 and 2 are indicated as relative values when the internal resistance of Comparative Example was set to 100.

In Comparative Example, 7 out of 20 samples had a relatively large pressure drop and thus were judged as defective after left standing for 1 week. In each of Examples 1 and 2, by contrast, all of 20 samples had no pressure drop even after left standing for 1 week. In view of the fact what a conventional bipolar secondary battery with metal foil collectors shows almost no such a voltage drop, it is assumed that the voltage drop occurred in Comparative Example due to a liquid short circuit caused by a minute crack or cracks in the polymeric material layer of the collector. In other words, it has been shown that it is possible for the bipolar electrode of the present invention to prevent battery performance deterioration from being affected by a minute crack or cracks in the collector.

Furthermore, the internal resistance of the battery sample was slightly higher in Examples 1 and 2 than in Comparative Example. It is assumed that this resistance increase occurred due to contact resistance of the polymeric material layers of the collector, which was at a level of no problem in practical use. As apparent from the comparison of Examples 1 and 2, it is possible for the multilayer collector to largely reduce the contact resistance of the polymeric material layers by the formation of the conductive paste layer between the polymeric material layers so as to achieve battery performance almost comparable to the case of the single layer collector as in Comparative example.

The present invention is not limited to the above specific embodiments. Various modification and variation of the embodiments described above will occur to those skilled in the art in light of the above teaching. The scope of the invention is defined with reference to the following claims.

Claims

A bipolar electrode, comprising:
a collector;
a positive electrode active material layer formed on one side of the collector; and
a negative electrode active material layer formed on the other side of the collector,
the collector having at least two polymeric material layers laminated in a thickness direction thereof.
A bipolar electrode according to claim 1, wherein the number of the polymeric material layers of the collector is two.
A bipolar electrode according to claim 1 or 2, wherein said at least two polymeric material layers are made of a polymer containing a conductive filler.
A bipolar electrode according to any one of claims 1 to 3, wherein the collector has a conductive paste layer between the polymeric material layers.
A bipolar secondary battery using a bipolar electrode according to any one of claims 1 to 4.
A battery assembly comprising a plurality of electrically connected bipolar secondary batteries according to claim 5.
A vehicle comprising as a power source either a bipolar secondary battery according to claim 5 or a battery assembly according to claim 6.
A production method of a bipolar electrode, comprising:
providing at least two, first and second polymeric material layers;
applying a positive electrode active material slurry to one side of the first polymeric material layer and drying the positive electrode active material slurry to form a positive electrode active material layer on the one side of the first polymeric material layer;
applying a negative electrode active material slurry to one side of the second polymeric material layer and drying the negative electrode active material slurry to form a negative electrode active material layer on the one side of the second polymeric material layer;
pressing the polymeric material layers separately, with the positive and negative electrode active material layers formed on the respective one sides of the first and second polymeric material layers; and
, after the pressing, joining together the other side of the first polymeric material layer and the other side of the second polymeric material layer in such a manner that the polymeric material layers form a collector between the positive and negative electrode active material layers.
A production method according to claim 8, further comprising: before said joining, forming a conductive paste layer on either one of the other side of the first polymeric material layer and the other of the second polymeric material layer.