Integrated bipolar electrode, preparation method and use thereof

Abstract

An integrated bipolar electrode includes a laminated structure and a bipolar plate. The laminated structure is formed by interconnecting an anode active material layer with a cathode active material layer. The bipolar plate is sandwiched in a hollow cavity of the laminated structure. Side surfaces of the laminated structure are provided with a sealing layer for mating with a bipolar electrode fixing frame to prevent an anolyte and a catholyte from permeating into each other. The anode active material layer and the cathode active material layer in the integrated bipolar electrode are directly connected. A contact resistance between the anode active material layer and the cathode active material layer is quite low, and a battery prepared finally has better performances.

Claims

1. An integrated bipolar electrode, comprising a laminated structure and a bipolar plate, the laminated structure being formed by interconnecting an anode active material layer with a cathode active material layer, the bipolar plate being sandwiched in a hollow cavity of the laminated structure, side surfaces of the laminated structure being provided with a sealing layer for mating with a bipolar electrode fixing frame to prevent an anolyte and a catholyte from permeating into each other, wherein a material of the anode active material layer is same as a material of the cathode active material layer, the laminated structure is formed by folding a single piece of sheet in half, and the bipolar plate is arranged in the hollow cavity of the laminated structure.

2. The integrated bipolar electrode of claim 1, wherein the bipolar plate is a non-conductive sheet which does not react with the anolyte or the catholyte and is impermeable.

3. The integrated bipolar electrode of claim 1, wherein the anode active material layer and the cathode active material layer are graphite felts, carbon felts, carbon cloths or conductive sheets containing a carbon material.

4. The integrated bipolar electrode of claim 1, wherein a thickness h of the anode active material layer and/or the cathode active material layer at the side surface of the laminated structure is less than 1 mm.

5. The integrated bipolar electrode of claim 1, wherein the anode active material layer and/or the cathode active material layer positioned at the side surface of the laminated structure is in a net shape.

6. An integrated bipolar electrode, comprising a laminated structure and a bipolar plate, the laminated structure being formed by interconnecting an anode active material layer with a cathode active material layer, the bipolar plate being sandwiched in a hollow cavity of the laminated structure, side surfaces of the laminated structure being provided with a sealing layer for mating with a bipolar electrode fixing frame to prevent an anolyte and a catholyte from permeating into each other, wherein the anode active material layer and the cathode active material layer are two separate sheets, the hollow cavity of the laminated structure is a sealed cavity formed by folding the two separate sheets, and the bipolar plate is arranged in the sealed cavity.

7. A preparation method of an integrated bipolar electrode wherein the integrated bipolar electrode comprises a laminated structure and a bipolar plate, the laminated structure being formed by interconnecting an anode active material layer with a cathode active material layer, the bipolar plate being sandwiched in a hollow cavity of the laminated structure, side surfaces of the laminated structure being provided with a sealing layer for mating with a bipolar electrode fixing frame to prevent an anolyte and a catholyte from permeating into each other, wherein the method comprises: wherein the method comprises: S1: inserting a bipolar plate between an anode active material layer and a cathode active material layer to form a “sandwich” structure, folding an edge of the anode active material layer and/or the cathode active material layer along a side edge of the bipolar plate to make the anode active material layer interconnect with the cathode active material layer, and pressing a connected part; S2: gluing the connected part in S1, so as to allow the anode active material layer and the cathode active material layer to form an interconnected whole; and S3: gluing and sealing side surfaces of the “sandwich” structure to form a sealing layer, thereby obtaining the integrated bipolar electrode.

8. The integrated bipolar electrode of claim 1, wherein the integrated bipolar electrode is prepared with a preparation method comprising: S1: folding the single piece of sheet in half to form the anode active material layer and the cathode active material layer which have an identical area, then inserting the bipolar plate between the anode active material layer and the cathode active material layer to form a three-layer overlaid structure; and S2: gluing and sealing side surfaces of the three-layer overlaid structure to form the sealing layer, thereby obtaining the integrated bipolar electrode.

9. The integrated bipolar electrode of claim 1, wherein the integrated bipolar electrode is prepared with a preparation method comprising: S1: folding the single piece of sheet to allow two ends of the single piece of sheet to be connected and form a cavity for receiving the bipolar plate, gluing the two ends of the single piece of sheet, and inserting the bipolar plate into the cavity to form a three-layer overlaid structure; and S2: gluing and sealing side surfaces of the three-layer overlaid structure to form the sealing layer, thereby obtaining the integrated bipolar electrode.

10. The integrated bipolar electrode of claim 6, wherein the integrated bipolar electrode is prepared with a preparation method comprising: S1: inserting the bipolar plate between the two separate sheets to form a “sandwich” structure, then folding an edge of at least one of the two separated sheets along a side edge of the bipolar plate to allow the two separate sheets to be in direct contact with each other, then pressing and gluing a contact part, such that the bipolar plate is completely wrapped by the two separate sheets; and S2: gluing and sealing side surfaces of the “sandwich” structure to form the sealing layer, thereby obtaining the integrated bipolar electrode.

11. Use of an integrated bipolar electrode of claim 1 in an all-vanadium redox flow battery, wherein the all-vanadium redox flow battery comprises at least one integrated bipolar electrode, a bipolar electrode fixing frame for fixing the integrated bipolar electrode, and a diaphragm for separating an anolyte from a catholyte.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In order to explain the embodiments of the disclosure or the technical solution in the prior art more clearly, the drawings required for illustrating the embodiments or the prior art will be briefly introduced below. It is apparent that the drawings described below are some embodiments of the disclosure. Other drawings may further be obtained by those of ordinary skill in the art in accordance with these drawings without creative work.

(2) FIG. 1 illustrates a structure diagram of an integrated bipolar electrode in Example 1.

(3) FIG. 2 illustrates a structure diagram (equivalent to a section view along an A-A line in FIG. 1) of the bipolar electrode in Example 1.

(4) FIG. 3 illustrates another structure diagram of a bipolar electrode in Example 1.

(5) FIG. 4 illustrates a structure diagram when an anode active material layer and a cathode active material layer in Example 1 are unfolded.

(6) FIG. 5 illustrates a structure diagram of a bipolar electrode in Example 2.

(7) FIG. 6 illustrates another structure diagram of a bipolar electrode in Example 2.

(8) FIG. 7 illustrates a structure diagram of a bipolar electrode in Example 3.

(9) FIG. 8 illustrates a structure diagram of a bipolar electrode in Example 4.

(10) FIG. 9 illustrates a structure diagram (before folding) when a bipolar plate is sandwiched between an anode active material layer and a cathode active material layer in Example 5.

(11) FIG. 10 illustrates a structure diagram (before folding) when a bipolar plate is sandwiched between an anode active material layer and a cathode active material layer in Example 6.

REFERENCE NUMBERS

(12) 1: Anode active material layer; 2: Bipolar plate; 3: Cathode active material layer; 4: Sealing layer.

DETAILED DESCRIPTION

(13) In order to facilitate understanding of the disclosure, the disclosure will be more comprehensively and meticulously described below with reference to the drawings and the preferred Examples. However, the scope of protection of the disclosure will not be limited to the following detailed Examples.

(14) Unless otherwise defined, all terminologies to be used below may be identical with meanings usually understood by those skilled in the art. The terminologies to be used below are used to describe the detailed embodiments only and are not intended to limit the scope of protection of the disclosure.

(15) Unless otherwise specified, various raw materials, agents, instruments, devices and the like to be used in the disclosure may be commercially available, or prepared through existing methods.

Example 1

(16) As shown in FIG. 1, an integrated bipolar electrode of this Example may include a laminated structure and a bipolar plate 2. The laminated structure is formed by interconnecting an anode active material layer 1 with a cathode active material layer 3. The bipolar plate 2 is sandwiched in a hollow cavity of the abovementioned laminated structure. Side surfaces of the laminated structure are provided with a sealing layer 4 for mating with a bipolar electrode fixing frame to prevent an anolyte and a catholyte from permeating into each other.

(17) In the bipolar electrode, both a material of the anode active material layer 1 and a material of the cathode active material layer 3 are a polyacrylonitrile graphite felt. The bipolar plate 2 is a polypropylene (PP) film. The sealing layer 4 is an acid-resistant and oxidation-resistant epoxy resin.

(18) Specifically, as shown in FIG. 2, the laminated structure in this Example is formed by folding a single piece of polyacrylonitrile graphite felt in half, and the bipolar plate 2 is arranged in the hollow cavity of the laminated structure.

(19) A preparation method of the integrated bipolar electrode in this Example may include the following steps.

(20) (1) A polyacrylonitrile graphite felt with a length of 80 cm and a width of 40 cm was cut and submerged into deionized water for 10-30 min. Then the polyacrylonitrile graphite felt was taken out and placed in a centrifugal dryer to dewater for 1-2 min at a speed of 800-1200 rpm. A PP film with a length of 40 cm and a width of 40 cm was cut.

(21) (2) The 80 cm×40 cm polyacrylonitrile graphite felt was folded in half along its longitudinal center line. The cut PP film was placed between the folded graphite felt such that the four sides of the PP film were aligned to the four sides of one half of the graphite felt.

(22) (3) Side surfaces of the folded polyacrylonitrile graphite felt/PP/the polyacrylonitrile graphite felt were bonded and sealed with an epoxy resin, thereby obtaining the integrated bipolar electrode of this Example.

(23) During actual sealing, in order to ensure a sealing effect of the side surfaces, edges of surfaces of the anode active material layer 1 and the cathode active material layer 3 may also be coated with a sealant during sealing the side surfaces generally, so as to allow the sealing layer of side surfaces to extend to the edges of the surfaces of the anode active material layer 1 and the cathode active material layer 3, obtaining a better sealing effect. For the convenience of description, the sealant on the edges of the surfaces of the anode active material layer 1 and the cathode active material layer 3 is not shown in the figures of this Example, similarly hereinafter.

(24) The integrated bipolar electrode in this Example is used for an all-vanadium redox flow battery. The all-vanadium redox flow battery may include an end plate provided with a flow inlet and a flow outlet, a conductive lug, at least one abovementioned integrated bipolar electrode, a bipolar electrode fixing frame for fixing the abovementioned integrated bipolar electrode, and a diaphragm for isolating the anolyte from the catholyte.

(25) The integrated bipolar electrode in this Example, a positive electrode, a negative electrode, the diaphragm and the like were assembled into a battery pack consisting of two cells through internal series connection. An energy efficiency of constant-current discharge and constant-current charge in presence of a current density of 100 mA cm.sup.−2 is 79.8%. A coulombic efficiency thereof is 98.0%. The anolyte and the catholyte are completely impermeable.

(26) In this Example, the polyacrylonitrile graphite felt at the side surface of the laminated structure (namely, near a folding crease) may be thinned, as shown in FIG. 3.

(27) In addition, in this Example, the polyacrylonitrile graphite felt at the side surface of the laminated structure (namely, adjacent to a folding crease) may be hollowed into a net shape, and the structure after unfolding the polyacrylonitrile graphite felt is shown in FIG. 4.

(28) The integrated bipolar electrode, as shown in FIG. 3 or FIG. 4, is assembled into the battery pack consisting of two cells through the internal series connection. The energy efficiency of the constant-current discharge and the constant-current charge in presence of the current density of 100 mA cm.sup.−2 is 79.3%. The coulombic efficiency thereof is 97.3%. The anolyte and the catholyte do not permeate into each other at all.

Example 2

(29) An integrated bipolar electrode of this Example may include a laminated structure and a bipolar plate 2. The laminated structure is formed by interconnecting an anode active material layer 1 with a cathode active material layer 3. The bipolar plate 2 is sandwiched in a hollow cavity of the laminated structure. Side surfaces of the laminated structure are provided with a sealing layer 4 for mating with a bipolar electrode fixing frame to prevent an anolyte and a catholyte from permeating into each other.

(30) In the bipolar electrode, both a material of the anode active material layer 1 and a material of the cathode active material layer 3 are a polyacrylonitrile graphite felt. The bipolar plate 2 is a polyethylene (PE) film. The sealing layer 4 is an acid-resistant and oxidation-resistant epoxy resin.

(31) Specifically, as shown in FIG. 5, the anode active material layer 1 and the cathode active material layer 3 are two separate sheets. The laminated structure is formed by folding the two separate sheets. Furthermore, an inner cavity of the laminated structure formed by the two separate sheets is a sealed cavity. The bipolar plate 2 is arranged in the sealed cavity (as shown in FIG. 5, for the convenience of description, an adhesive at a contact part of the two separate sheets is not shown, and there is a small overlapping area at the contact part of the two separate sheets which is pressed, similarly hereinafter).

(32) A preparation method of the integrated bipolar electrode in this Example may include the following steps.

(33) (1) A polyacrylonitrile graphite felt with a length of 65 cm and a width of 41 cm and a polyacrylonitrile graphite felt with a length of 68 cm and a width of 45 cm were cut respectively and submerged into deionized water for 10-30 min. Then the two polyacrylonitrile graphite felts were taken out and placed in a centrifugal dryer to dewater for 1-2 min at a speed of 800-1200 rpm. A PE film with a length of 66 cm and a width of 43 cm was cut.

(34) (2) The 68 cm×45 cm polyacrylonitrile graphite felt was horizontally spread on an operating table. The cut PE film was placed in the center of the graphite felt in parallel. Then the 65 cm×41 cm polyacrylonitrile graphite felt was placed on the PE film in parallel, such that centers of the graphite felts coincide with a center of the PE film.

(35) (3) Four sides of the polyacrylonitrile graphite felt at the bottom were folded upward along four side edges of the PE film and placed on the polyacrylonitrile graphite felt on the top, such that the polyacrylonitrile graphite felt on the top is in direct contact with the polyacrylonitrile graphite felt at the bottom. Then a contact part was glued and sealed with an epoxy resin.

(36) (4) Side surfaces of the polyacrylonitrile graphite felt/PE/the polyacrylonitrile graphite felt were bonded and sealed with the epoxy resin, thereby obtaining the integrated bipolar electrode of this Example.

(37) In addition, as shown in FIG. 6, in this Example, the polyacrylonitrile graphite felt at the bottom may further be thinned at side surfaces of the laminated structure (namely, near a folding crease).

(38) The integrated bipolar electrode shown in FIG. 6 is used for an all-vanadium redox flow battery. The integrated bipolar electrode shown in FIG. 6 was assembled into a battery pack consisting of two cells through internal series connection. An energy efficiency of constant-current discharge and constant-current charge in presence of a current density of 100 mA cm.sup.−2 is 79.8%. A coulombic efficiency thereof is 97.5%. The anolyte and the catholyte do not permeate into each other at all.

Example 3

(39) An integrated bipolar electrode of this Example may include a laminated structure and a bipolar plate 2. The laminated structure is formed by interconnecting an anode active material layer 1 with a cathode active material layer 3. The bipolar plate 2 is sandwiched in a hollow cavity of the laminated structure. Side surfaces of the laminated structure are provided with a sealing layer 4 for mating with a bipolar electrode fixing frame to prevent an anolyte and a catholyte from permeating into each other.

(40) In the bipolar electrode, both a material of the anode active material layer 1 and a material of the cathode active material layer 3 are a polyacrylonitrile graphite felt. The bipolar plate 2 is a PP film. The sealing layer 4 is an acid-resistant and oxidation-resistant epoxy resin.

(41) Specifically, as shown in FIG. 7, the laminated structure in this Example is formed by connecting a single piece of polyacrylonitrile graphite felt end to end after folding for four times. The bipolar plate 2 is arranged in the hollow cavity of the laminated structure.

(42) A preparation method of the integrated bipolar electrode in this Example may include the following steps.

(43) (1) A polyacrylonitrile graphite felt with a length of 82 cm and a width of 40 cm was cut and submerged into deionized water for 10-30 min. The polyacrylonitrile graphite felt was taken out and placed in a centrifugal dryer to dewater for 1-2 min at a speed of 800-1200 rpm. APP film with a length of 40 cm and a width of 40 cm was cut.

(44) (2) The 82 cm×40 cm polyacrylonitrile graphite felt was folded for four times to form the hollow cavity for receiving the bipolar plate 2. The polyacrylonitrile graphite felt was glued end to end with an epoxy resin. Then the cut PP film was placed in the cavity.

(45) (3) Side surfaces of the folded polyacrylonitrile graphite felt/PP/fixe polyacrylonitrile graphite felt were bonded and sealed with the epoxy resin, thereby obtaining the integrated bipolar electrode of this Example.

(46) The integrated bipolar electrode in this Example was assembled into a battery pack consisting of two cells through internal series connection. An energy efficiency of constant-current discharge and constant-current charge in presence of a current density of 100 mA cm.sup.−2 is 79.5%. A coulombic efficiency thereof is 97.4%, An anolyte and a catholyte do not permeate into each other at all.

Example 4

(47) An integrated bipolar electrode of this Example may include a laminated structure and a bipolar plate 2. The laminated structure is formed by interconnecting an anode active material layer 1 with a cathode active material layer 3. The bipolar plate 2 is sandwiched in a hollow cavity of the laminated structure. Side surfaces of the laminated structure are provided with a sealing layer 4 for mating with a bipolar electrode fixing frame to prevent an anolyte and a catholyte from permeating into each other.

(48) In the bipolar electrode, both a material of the anode active material layer 1 and a material of the cathode active material layer 3 are a polyacrylonitrile graphite felt. The bipolar plate 2 is a silicon rubber film. The sealing layer 4 is an acid-resistant and oxidation-resistant epoxy resin.

(49) Specifically, as shown in FIG. 8, in this Example, the anode active material layer 1 and the cathode active material layer 3 are two separate sheets with an identical width and different lengths. Three edges of the anode active material layer 1 are aligned to three edges of the cathode active material layer 3. The fourth edge of the bigger sheet is folded and is in direct contact with the fourth edge of the smaller sheet to form the laminated structure. The bipolar plate 2 is arranged in the hollow cavity of the laminated structure.

(50) A preparation method of the integrated bipolar electrode in this Example may include the following steps.

(51) (1) A polyacrylonitrile graphite felt with a length of 47 cm and a width of 31 cm and a polyacrylonitrile graphite felt with a length of 44 cm and a width of 31 cm were cut and submerged in deionized water for 10-30 min. The polyacrylonitrile graphite felts were taken out and placed in a centrifugal dryer to dewater for 1-2 min at a speed of 800-1200 rpm. A silicon rubber film with a length of 45 cm and a width of 31 cm was cut.

(52) (2) The 31 cm×47 cm polyacrylonitrile graphite felt was horizontally spread on an operating platform. The cut silicon rubber film was placed on a surface of the polyacrylonitrile graphite felt. Then the 31 cm×44 cm polyacrylonitrile graphite felt was placed on a surface of the silicon rubber film such that three edges of each of the two polyacrylonitrile graphite felts were aligned to three edges of the silicon rubber film.

(53) (3) The fourth edge of the polyacrylonitrile graphite felt at the bottom was folded upward along the fourth edge of the silicon rubber film and placed on a surface of the polyacrylonitrile graphite felt on the top. The polyacrylonitrile graphite felt on the top was in direct contact with the polyacrylonitrile graphite felt at the bottom. A contact part was pressed and glued with an epoxy resin.

(54) (4) Side surfaces of the folded polyacrylonitrile graphite felt/the silicon rubber film/the polyacrylonitrile graphite felt were bonded and sealed with the epoxy resin, thereby obtaining the integrated bipolar electrode of this Example.

(55) The integrated bipolar electrode in this Example was assembled into a battery pack consisting of two cells through internal series connection. An energy efficiency of constant-current discharge and constant-current charge in presence of a current density of 100 mA cm.sup.−2 is 79.3%. A coulombic efficiency thereof is 97.2%. The anolyte and the catholyte do not permeate into each other at all.

Example 5

(56) An integrated bipolar electrode of this Example may include a laminated structure and a bipolar plate 2. The laminated structure is formed by interconnecting an anode active material layer 1 with a cathode active material layer 3. The bipolar plate 2 is sandwiched in a hollow cavity of the laminated structure. Side surfaces of the laminated structure are provided with a sealing layer 4 for mating with a bipolar electrode fixing frame to prevent an anolyte and a catholyte from permeating into each other.

(57) In the bipolar electrode, both a material of the anode active material layer 1 and a material of the cathode active material layer 3 are a polyacrylonitrile graphite felt. The bipolar plate 2 is a PP film. The sealing layer 4 is an acid-resistant and oxidation-resistant epoxy resin.

(58) Specifically, the anode active material layer 1 and the cathode active material layer 3 are two separate sheets with different widths and different lengths. Two edges of the anode active material layer 1 are aligned to two edges of the cathode active material layer 3, The other two edges of the bigger sheet are folded and are in direct contact with the other two edges of the smaller sheet to form the laminated structure. The bipolar plate 2 is arranged in the hollow cavity of the laminated structure.

(59) A preparation method of the integrated bipolar electrode in this Example may include the following steps.

(60) (1) A polyacrylonitrile graphite felt with a length of 47 cm and a width of 31 cm and a polyacrylonitrile graphite felt with a length of 44 cm and a width of 28 cm were cut and submerged in deionized water for 10-30 min. The polyacrylonitrile graphite felts were taken out and placed in a centrifugal dryer to dewater for 1-2 min at a speed of 800-1200 rpm. A PP film with a length of 45 cm and a width of 29 cm was cut.

(61) (2) The 47 cm×31 cm polyacrylonitrile graphite felt was horizontally spread on an operating platform. The cut PP film was placed on a surface of the polyacrylonitrile graphite felt in parallel. Then the 44 cm×28 cm polyacrylonitrile graphite felt was placed on the PP film in parallel, such that one corner of each of the two polyacrylonitrile graphite felts was overlapped with one corner of the PP film (as shown in FIG. 9).

(62) (3) Two edges of the polyacrylonitrile graphite felt at the bottom were folded upward along two respective edges of the PP film and placed on the polyacrylonitrile graphite felt on the top, such that two edges of the polyacrylonitrile graphite felt on the top were in direct contact with the two respective edges of the polyacrylonitrile graphite felt at the bottom. Then a contact part was pressed and glued with an epoxy resin.

(63) (4) Side surfaces of the folded polyacrylonitrile graphite felt/PP/the polyacrylonitrile graphite felt were bonded and sealed with the epoxy resin, thereby obtaining the integrated bipolar electrode of this Example.

(64) The integrated bipolar electrode in this Example was assembled into a battery pack consisting of two cells through internal series connection. An energy efficiency of constant-current discharge and constant-current charge in presence of a current density of 100 mA cm.sup.−2 is 79.1%. A coulombic efficiency thereof is 97.2%. The anolyte and the catholyte do not permeate into each other at all.

Example 6

(65) An integrated bipolar electrode of this Example may include a laminated structure and a bipolar plate 2. The laminated structure is formed by interconnecting an anode active material layer 1 with a cathode active material layer 3. The bipolar plate 2 is sandwiched in a hollow cavity of the laminated structure. Side surfaces of the laminated structure are provided with a sealing layer 4 for mating with a bipolar electrode fixing frame to prevent an anolyte and a catholyte from permeating into each other.

(66) In the bipolar electrode, both a material of the anode active material layer 1 and a material of the cathode active material layer 3 are a polyacrylonitrile graphite felt. The bipolar plate 2 is a PP film. The sealing layer 4 is an acid-resistant and oxidation-resistant epoxy resin.

(67) Specifically, as shown in FIG. 10, the anode active material layer 1 and the cathode active material layer 3 are two separate sheets having different widths and different lengths. Only one edge of the anode active material layer 1 is aligned to one edge of the cathode active material layer 3. The other three edges of the bigger sheet are folded and are in direct contact with the other three edges of the smaller sheet to form the laminated structure. The bipolar plate 2 is arranged in the hollow cavity of the laminated structure.

(68) A preparation method of the integrated bipolar electrode in this Example may include the following steps.

(69) (1) A polyacrylonitrile graphite felt with a length of 47 cm and a width of 31 cm and a polyacrylonitrile graphite felt with a length of 44 cm and a width of 28 cm were cut and submerged in deionized water for 10-30 min. The polyacrylonitrile graphite felts were taken out and placed in a centrifugal dryer to dewater for 1-2 min at a speed of 800-1200 rpm. A PP film with a length of 45 cm and a width of 29 cm was cut.

(70) (2) The 47 cm×31 cm polyacrylonitrile graphite felt was horizontally spread on an operating platform. The cut PP film was placed on a surface of the polyacrylonitrile graphite felt in parallel. The 44 cm×28 cm polyacrylonitrile graphite felt was placed on the PP film in parallel, such that one long side of each of the two polyacrylonitrile graphite felts was overlapped with one long side of the PP film.

(71) (3) The other three sides (namely, the three edges which were not aligned) of the polyacrylonitrile graphite felt at the bottom were folded upward along the three respective edges of the PP film and placed on the polyacrylonitrile graphite felt on the top. The three edges of the polyacrylonitrile graphite felt on the top were in direct contact with the three respective edges of the polyacrylonitrile graphite felt at the bottom. A contact part was pressed and glued with an epoxy resin.

(72) (4) Side surfaces of the folded polyacrylonitrile graphite felt/PP/the polyacrylonitrile graphite felt were bonded and sealed with the epoxy resin, thereby obtaining the integrated bipolar electrode of this Example.

(73) The integrated bipolar electrode in this Example was assembled into a battery pack consisting of two cells through internal series connection. An energy efficiency of constant-current discharge and constant-current charge in presence of a current density of 100 mA cm.sup.−2 is 79.1%. A coulombic efficiency thereof is 97.0%. The anolyte and the catholyte do not permeate into each other at all.

Comparison Example

(74) A routine current collector and two graphite felts (the two graphite felts were adhered to the two surfaces of the routine current collector through an external pressure) were assembled into a battery pack consisting of two cells through internal series connection. An energy efficiency of constant-current discharge and constant-current charge in presence of a current density of 100 mA cm.sup.−2 is 75.3%, and a coulombic efficiency thereof is 97.7%.

(75) From the Comparison Example, it can be seen that the energy efficiency of the constant-current discharge and the constant-current charge in presence of the current density of 100 mA cm.sup.−2 is significantly reduced as for the battery pack assembled from the routine current collector and the two graphite felts.