Bipolar Li-ion battery having improved sealing and associated method of production

Abstract

A bipolar battery having at least two electrochemical cells stacked one on top of the other, the bipolar collector including, at the periphery of same, on one of the faces of same, at least one first sealing device including one frame made from an electrically insulating and thermosensitive material, and two adhesive frames arranged individually to either side of the thermosensitive frame, the first or the second adjacent collector also including, at the periphery of same, on the covered face of same, at least one second sealing device including a frame made from an electrically insulating and thermosensitive material, and two adhesive frames arranged individually to either side of the thermosensitive frame, the first and second devices each forming a peripheral wall sealed to the electrolyte of the first or second cell, which surrounds same. Each sealed wall is obtained by heat-sealing at least one first and at least one second sealing device on the face of a current collector not provided with a sealing device.

Claims

1. A bipolar Li-ion battery comprising: at least one first and second electrochemical cells stacked on top of one other in a direction of stacking and each comprising an anode, a cathode and an electrolyte; at least one bipolar current collector having two faces, one face of which is covered with the anode of lithium-based insertion material of the first electrochemical cell and the opposite face is covered with the cathode of lithium-based insertion material of the second electrochemical cell, the bipolar current collector comprising at its periphery, on one of its faces, at least one first sealing device comprising a first heat-sensitive frame of an electrically insulating and heat-sensitive material, and two first adhesive frames of adhesive arranged individually on either side of the first heat-sensitive frame in a radial direction, perpendicular to the direction of stacking of the electrochemical cells, the first device constituting a first peripheral wall impervious to the electrolyte of the first or second electrochemical cell, which surrounds the latter; at least one first current collector adjacent to the bipolar current collector, one face of which is covered with the cathode of the first electrochemical cell; at least one second current collector adjacent to the bipolar current collector, one face of which is covered with the anode of the second electrochemical cell; the first or second current collector adjacent to the bipolar current collector also comprising, at its periphery, on its covered face, at least one second sealing device comprising a second heat-sensitive frame of an electrically insulating and heat-sensitive material, and two second adhesive frames of adhesive arranged individually on either side of the second heat-sensitive frame in the radial direction perpendicular to the direction of stacking of the electrochemical cells, the second device constituting a second peripheral wall impervious to the electrolyte of the first or second electrochemical cell, which surrounds the latter; each impervious peripheral wall being obtained by a heat-sealing technique of at least one first and at least one second sealing device on the face of the first or second current collector adjacent to the bipolar current collector that does not have the first or the second sealing device.

2. The bipolar battery as claimed in claim 1, the heat-sensitive frame of the first or second sealing device being made of semicrystalline thermoplastic polyolefins selected among: polyethylene, polypropylene, polymethylpentene, polybutylene.

3. The bipolar battery as claimed in claim 2, the frame of the first or second sealing device being made of polyethylene.

4. The bipolar battery as claimed in claim 1, the first and second adhesive frames being made of a double-faced acrylic with a core of polypropylene, or of polyethylene terephthalate or of polyurethane.

5. The bipolar battery as claimed in claim 1, the thickness of the first and/or the second sealing device being between 50 and 500 m.

6. The bipolar battery as claimed in claim 1, the width of the first and/or the second sealing device being between 0.3 and 3 cm.

7. The bipolar battery as claimed in claim 1, comprising a stack of a number n of electrochemical cells, a number of n2 of said electrochemical cells being bipolar current collectors and two electrochemical cells being a first and a second adjacent current collectors, one of said adjacent current collectors being a terminal current collector, the other one of said adjacent current collectors being the other terminal current collector.

8. The bipolar battery as claimed in claim 1, the anodes being of Li.sub.4Ti.sub.5O.sub.12 and the cathodes of LiFePO.sub.4.

Description

DESCRIPTION OF THE DRAWINGS

(1) Other advantages and features will become clearer on reading the detailed description, given for purposes of illustration, referring to the following figures, where:

(2) FIG. 1 is a schematic view in longitudinal section of a lithium-based bipolar battery according to the prior art;

(3) FIGS. 2A and 2B are respectively front and sectional views of a bipolar current collector used in a lithium-based bipolar battery according to the prior art;

(4) FIGS. 3A and 3B are respectively front and sectional views of another bipolar current collector used in a lithium-based bipolar battery according to the prior art;

(5) FIG. 4 is a longitudinal sectional view showing the creep of a heat-sealing seal employed in a lithium-based bipolar battery according to the prior art;

(6) FIG. 5 is a longitudinal sectional view showing a sealing device used in a lithium-based bipolar battery according to the invention;

(7) FIG. 6 is a top view of a bipolar current collector with a sealing device according to the invention;

(8) FIGS. 7A to 7J are longitudinal sectional views showing the different steps in the production of a lithium-based bipolar battery with three electrochemical cells (C1, C2, C3) according to the invention;

(9) FIG. 8 is a longitudinal sectional view of a lithium-based bipolar battery with three electrochemical cells (C1, C2, C3) according to the invention.

(10) For clarity, the same references denoting the same bipolar battery elements according to the prior art and according to the invention are used for all the figures (FIGS. 1 to 8).

DETAILED DESCRIPTION OF THE INVENTION

(11) FIG. 1 shows a bipolar Li-ion battery according to the prior art, as illustrated in patent application WO 03/047021. This battery comprises, in the upper portion, a conductive aluminum substrate 13 (positive terminal current collector) and an active layer 14 based on lithium-based positive insertion material, such as Li.sub.1.04Mn.sub.1.96O.sub.4 and, in the lower portion, a conductive aluminum substrate 21 (negative terminal current collector) and an active layer 20 based on lithium-based positive insertion material, such as of Li.sub.4Ti.sub.5O.sub.12.

(12) Within this battery, a bipolar electrode 1, also called bipolar current collector, comprises an anode layer 16 and a cathode layer 18 on either side of a conductive aluminum substrate 10 in the form of a plate. The lower electrodes 20 and upper electrodes 14 are separated from the bipolar electrode 1 by two separators 15, 19 in which an electrolyte is present in the form of liquid or gel. Sealing of the battery against the electrolytes between the two resultant adjacent electrochemical cells 14, 15, 16 and 18, 19, 20 is provided by a seal 22 which is made by depositing resin or glue on the periphery of all the electrodes and the plate 10.

(13) Depending on the lithium-ion insertion materials used for making the electrodes, a bipolar current collector 10 according to the prior art: either consists of two superposed plates, one of which, typically of aluminum 10A1, is covered with a cathode 11 and the other, typically of copper 10C, is covered with an anode 12 (FIGS. 2A and 2B), or consists of a single plate typically of aluminum 10A1 covered on one face with a cathode 11 and on the other face with an anode 12 (FIGS. 3A and 3B). The main difficulty encountered in designing a bipolar battery according to the prior art is to make compartments that are perfectly impervious to the electrolyte, generally in liquid form, relative to one another, such as between the two cells C1 and C2, i.e. between compartments referenced 14, 15, 16 and 18, 19, 20 in FIG. 1.

(14) The making of the seals 22 or enlargement of the plates 10 of the bipolar electrode according to the prior art to achieve this are not completely satisfactory.

(15) A solution for sealing using heat-sealing seals 23 in a bipolar battery already employed by the inventors is illustrated in FIG. 4: a seal 23 of heat-sealing material in the form of a frame is provided at the periphery of a bipolar collector 10 and a single frame of adhesive material 24 is provided on the exterior of the frame 23. As shown schematically in FIG. 4, the material of the frame 23 has flowed toward the interior of the compartment so that the effective gluing zone is of a limited area, shaped somewhat like a bird's beak. This reduced, uncontrolled gluing is likely to cause a risk of escape of the electrolyte. Moreover, owing to this creep, it is not possible to control the thickness of an electrochemical compartment.

(16) Thus, the inventors propose a new solution for sealing a bipolar Li-ion battery with respect to the electrolyte, more particularly a liquid electrolyte, which is robust during operation and over time and is easy to implement.

(17) Surprisingly, the inventors decided to employ a simple solution: add an additional frame 24 of adhesive material, inside frame 23 of heat-sensitive material in a radial direction, perpendicular to the direction of stacking of the cells. Thus, instead of completely abandoning the mixed solution of a frame of heat-sensitive material and a frame of adhesive material, they carefully tried to contain the potential creep of the heat-sensitive material, i.e. to control its dimensions during heating for carrying out heat-sealing thereof.

(18) Thus, as illustrated in FIGS. 5 and 6, a sealing device according to the invention between two bipolar current collectors 1 or between a bipolar current collector and a terminal current collector 13, 21 of a Li-ion battery, consists of a frame 23 of electrically insulating heat-sensitive material, and two frames 24 of adhesive material arranged individually on either side of frame 23.

(19) The steps for production of a bipolar battery with the sealing devices according to the invention are described below with reference to FIGS. 7A to 7J. The battery produced comprises three cells C1, C2, C3 stacked on top of one other and each comprising an anode, a cathode and an electrolyte. It is to be noted that all the substrates 10, 13, 21 are of aluminum. All the anodes are of Li.sub.4Ti.sub.5O.sub.12 and all the cathodes are of LiFePO.sub.4. The separators are all of the same material, such as polyvinylidene fluoride (PVDF). The electrolyte used is a mixture of carbonate and a lithium salt LiPF.sub.6.

(20) It is to be noted here that as an advantageous example, the material of each frame 23 is polyethylene (PE) and that each adhesive frame 24 is made from from adhesive tape marketed under the name 3M 467MP.

(21) It should also be noted that, as illustrated schematically, a frame 23 has an initial thickness, before heat-sealing, greater than that of the two frames 24 of adhesive material bordering it. Thus, when the periphery of the electrochemical compartments is heat-sealed, the material of the frame 23 will tend to flow into the space that is left free for it between the two adhesive frames 24 until it reaches the thickness of the latter.

(22) Step 1: a first bipolar current collector 1 is produced with a face covered with the anode 16 of a first cell C1 and the opposite face covered with the cathode 18 of the second cell C2 (FIG. 7A).

(23) A second bipolar current collector 1 is also produced with a face covered with the cathode 18 of a third cell C3 and the opposite face covered with the anode 16 of the second cell C2 (FIG. 7A).

(24) Step 2: a terminal current collector 21 is produced with a face covered with the anode 20 of a third cell C3 (FIG. 7B).

(25) Step 3: a terminal current collector 13 is produced with a face covered with the cathode 14 of the first cell C1 (FIG. 7C).

(26) Step 4: at the periphery of the face covered with the cathode 18 of the first bipolar current collector 1, a first sealing device is produced comprising a frame 23 of an electrically insulating heat-sensitive material, and two frames 24 of adhesive arranged individually on either side of the heat-sensitive frame 23 in a radial direction, perpendicular to the direction of stacking of the cells (FIG. 7 D).

(27) Step 5: a first separator 15 is interposed, placing it on the cathode 18 of the first bipolar current collector (FIG. 7 E).

(28) Step 6: the second bipolar current collector 1 is stacked on the first bipolar current collector 1, bringing into contact the first frame 23 around the anode 16 of the second bipolar collector (FIG. 7F).

(29) Step 7: heating is carried out using U-shaped heating jaws, not shown, surrounding the stack of the two bipolar current collectors 1: thus, a cycle of pressure according to a force F and of temperature is applied to perform heat-sealing between the two collectors 1 with the aid of frame 23 and to precisely define the thickness of the second cell C2 (compartment) (FIG. 7G).

(30) Step 8: at the periphery of the face covered with the cathode 18 of the second bipolar current collector 1, a second sealing device is produced comprising a frame 23 of an electrically insulating heat-sensitive material, and two frames 24 of adhesive arranged individually on either side of the heat-sensitive frame 23 in a radial direction, perpendicular to the direction of stacking of the cells.

(31) Step 9: a second separator 15 is interposed, placing it on the cathode 18 of the second bipolar current collector (FIG. 7H).

(32) Step 10: the terminal current collector 21 with its face covered with the anode 20 is stacked on the second bipolar current collector 1, bringing the second frame 23 in contact at the periphery of the anode 20.

(33) Step 11: heating is carried out using U-shaped heating jaws, not shown, surrounding the stack of the two bipolar current collectors 1 and of the terminal collector 21: thus, a cycle of pressure according to a force F and of temperature is applied to perform heat-sealing using the second frame 23 and to precisely define the thickness of the third cell C3 (compartment) (FIG. 7I).

(34) Step 12: at the periphery of the face covered with the cathode 14 of the second terminal current collector 13, a third sealing device is produced comprising a frame 23 of an electrically insulating heat-sensitive material, and two frames 24 of adhesive arranged individually on either side of the heat-sensitive frame 23 in a radial direction, perpendicular to the direction of stacking of the cells.

(35) Step 13: a third separator 15 is interposed, placing it on the cathode 14 of the second terminal current collector 13.

(36) Step 14: the assembly already constructed from the bipolar collectors 1 and the terminal current collector 21, heat-sealed together, is stacked on the second terminal current collector 13 with its face covered with the cathode 14, bringing the third frame 23 into contact at the periphery of the anode 16 of the first bipolar collector.

(37) Step 15: heating is carried out using U-shaped heating jaws, not shown, surrounding the stack of the two bipolar current collectors 1, the first terminal collector 21 and the second terminal collector 13, thus a cycle of pressure according to a force F and of temperature is applied to perform heat-sealing using third frame 23 and to precisely define the thickness of the first cell C1 (compartment) (FIG. 7J).

(38) The sealing thus obtained by all the frames 23 and the adhesive frames 24 is thus perfect with respect to the electrolyte.

(39) A bipolar battery has thus been produced with three cells C1, C2, C3, with perfect sealing provided at the level of their periphery, and whose thickness is constant for each cell C1, C2, C3.

(40) Regarding the electrolytes, an electrolyte may be used in the form of polymer or in liquid form impregnated in a separator. For activation of the latter, we may proceed via impregnation of each separator prior to their integration during assembly. Alternatively, assembly may be performed with stacking of the whole battery, carrying out the heat-sealing operations according to the invention and then making, via a tube arranged inside the frames, an inlet for the liquid electrolyte for subsequent filling.

(41) The invention is not limited to the examples that have just been described; notably, features of the examples illustrated may be combined within variants that have not been illustrated.

(42) It goes without saying that although sealing according to the invention with the device with a heat-sensitive frame surrounded by two adhesive frames has been described in relation to a bipolar battery with two cells stacked, it may be carried out in the same way for a battery with a number n of stacked cells by repeating the above steps 1 to 7 with a number equal to n2 of bipolar collectors and two terminal current collectors 13, 21.