Bipolar li-ion battery with improved leaktightness and associated method of production

Abstract

The present invention relates to a bipolar battery with at least two electrochemical cells stacked one above the other, each collector comprising at its periphery at least one bead of an electrical insulating material also constituting a peripheral zone of the electrolyte-leaktight wall. According to the invention, each leaktight wall is constituted of at least one bead consisting of a honeycomb matrix, the matrix being covered, on each of its two main faces, with a layer or leaf made of heat-sealing and electrically insulating material, each layer or leaf being heat-sealed to one of the current collectors, the heat-sealing and electrically insulating material filling at least partly the cells of the honeycomb while interconnecting the two layers or leafs.

Claims

1. A bipolar Li-ion battery, including: at least a first and second electrochemical cells stacked one on top of the other and each comprising an anode, a cathode and an electrolyte, at least one bipolar current collector one face of which is covered by the anode made of lithium insertion material of the first cell and the opposite face is covered by the cathode made of lithium insertion material of the second cell, the bipolar collector comprising at the periphery thereof, on each of the faces thereof, at least one bead of an electrically insulating material forming a peripheral zone of a wall impermeable to the electrolyte of the first or second cells, surrounding same, at least one first current collector adjacent to the bipolar collector one face of which is covered by the cathode of the first cell; the first adjacent collector also comprising at the periphery thereof, at least one bead of an electrically insulating material also forming a peripheral zone of the wall impermeable to the electrolyte of the first cell, at least one second current collector adjacent to the bipolar collector one face of which is covered by the anode of the second cell; the second adjacent collector also comprising at the periphery thereof, at least one bead of an electrically insulating material also forming a peripheral zone of the wall impermeable to the electrolyte of the second cell, wherein each impermeable wall consists of at least one bead formed from a honeycomb matrix covered, on each of its two main faces, with a layer or sheet made of a heat-sealing and electrically insulating material, each layer or sheet being heat sealed to one of the current collectors, the heat-sealing and electrically insulating material at least partially filling the alveoli of the honeycomb in such a way that the two layers or sheets join together.

2. The bipolar battery as claimed in claim 1, each current collector being a substrate made of aluminum.

3. The bipolar battery as claimed in claim 1, the heat-sealing material being an epoxide resin or a methacrylate resin coated, before the installation on a current collector and the heat sealing, on the interior and on each of the two main faces of the honeycomb matrix.

4. The bipolar battery as claimed in claim 1, the heat-sealing material being one of polyethylene hot-molded in the interior of the honeycomb matrix and hot-rolled in the form of a sheet, before the installation on a current collector and the heat sealing, on each of the two main faces of the honeycomb matrix.

5. The bipolar battery as claimed in claim 1, the thickness of each bead being substantially equal to the thickness of an electrochemical cell.

6. The bipolar battery as claimed in claim 1, the thickness of each bead being between 100 and 200 m, preferably equal to 150 m plus or minus 5 m.

7. The bipolar battery as claimed in claim 1, the width of each bead being between 0.1 and 2 cm.

8. The bipolar battery as claimed in claim 1, some of the alveoli of the honeycomb matrix, which are closest to the electrochemical cells, being filled with salts or mucilaginous compounds apt to react with the electrolyte.

9. The bipolar battery as claimed in claim 1, some of the alveoli of the honeycomb matrix, which are furthest from the electrochemical cells, being filled with salts or mucilaginous compounds apt to react with moisture in the ambient air.

10. The bipolar battery as claimed in claim 1, including a stack of n electrochemical cells, with a number of n-2 bipolar current collectors, one of the adjacent collectors being a terminal current collector, the other of the adjacent collectors being the other terminal current collector.

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

12. A bipolar battery production process including least a first and second electrochemical cells stacked one on top of the other and each comprising an anode, a cathode and an electrolyte, comprising the following steps: a/ producing a bipolar current collector with one face covered by the anode made of lithium insertion material of the first cell and the opposite face covered by the cathode made of lithium insertion material of the second cell; b/ producing a first current collector, intended to be adjacent to the bipolar collector one face of which is covered by the cathode of the first cell; c/ producing a second current collector, intended to be adjacent to the bipolar collector one face of which is covered by the anode of the second cell; d/ producing a first bead formed from a honeycomb matrix, at least some of the alveoli of the matrix being filled with a heat-sealing and electrically insulating material that furthermore covers each of its two main faces taking the form of a layer or sheet; e/ installing the bead on the periphery of that face of the second collector which is covered with the anode; f/ installing a first separator on the anode of the second collector in the interior of the first bead; g/ stacking the bipolar current collector such that the anode of the adjacent second collector is facing the cathode of the bipolar collector while being separated from the first separator, and such that the first sealing bead bears against both the second collector and the bipolar collector; and h/ heating the first bead making contact with the periphery of the collectors so as to heat seal the honeycomb matrix, steps d/ to h/ being carried out at least once with a second sealing bead, a second separator and the first current collector.

13. The process as claimed in claim 12, wherein the heating in each step f/ is carried out using U-shaped heating jaws placed about peripheral portions of the previously stacked collectors.

14. The process as claimed in claim 12, wherein the heating in each step f/ is carried out at a temperature below 200 C. and typically at about 80 C.

Description

DETAILED DESCRIPTION

(1) Other advantages and features will better emerge on reading the detailed description, given by way of illustration with reference to the following figures in which:

(2) FIG. 1 is a schematic longitudinal sectional view of a bipolar lithium 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 bipolar lithium 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 bipolar lithium battery according to the prior art,

(5) FIG. 4 is a schematic side view of a bipolar lithium battery according to the invention, the side view allowing impermeable walls produced according to the invention to be seen,

(6) FIG. 4A is a view of a detail of FIG. 4 allowing a sealing bead according to one embodiment of the invention to be seen,

(7) FIG. 5 is a top view of one embodiment of the honeycomb matrix structure of a sealing bead according to the invention,

(8) FIG. 5A is a view of a detail of FIG. 5 allowing the constitution of the walls of the honeycomb matrix alveoli according to the invention to be seen,

(9) FIGS. 6A to 46J are longitudinal sectional views showing the various steps of production of a bipolar lithium battery according to the invention, and

(10) FIG. 7 is a top view of a variant embodiment of a honeycomb matrix structure of a sealing bead according to the invention.

(11) For the sake of clarity, the same references designating the same bipolar battery elements according to the prior art and according to the invention are used for all the FIGS. 1 to 7.

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

(13) Within this battery, a bipolar electrode 1, also called a bipolar current collector, includes an anode layer 16 and a cathode layer 18 on each side of an aluminum conductive substrate 10 in the form of a plate. The lower 20 and upper 14 electrodes are separated from the bipolar electrode 1 by two separators 15, 19 wherein an electrolyte is present in liquid or gel form. The seal for the battery electrolytes between the two adjacent electrochemical cells formed 14, 15, 16 and 18, 19, 20 is provided by a joint 22 which is created by a resin or adhesive deposit on the periphery of all the electrodes and the plate 10.

(14) A bipolar current collector 10 according to the prior art, according to the lithium ion insertion materials used for producing the electrodes: either consists of two superimposed plates, of which one typically made of aluminum 10A1 is covered by a cathode 11 and the other typically made of copper 10C is covered by an anode 12 (FIGS. 2A and 2B), or consists of a single plate typically made of aluminum 10A1 covered on one of the faces by a cathode 11 and on the other of the faces thereof by an anode 12 (FIGS. 3A and 3B). The main difficulty encountered in the design of a bipolar battery according to the prior art is the production of compartments that are perfectly impermeable to the electrolyte, generally in liquid form, with respect to each other, such as between the two cells C1 and C2, i.e. between compartments referenced 14, 15, 16 and 18, 19, 20 in FIG. 1.

(15) The implementation of the joints 22 or the increase in the plates 10 of the bipolar electrode according to the prior art for achieving this are not fully satisfactory.

(16) Consequently, the inventors provide a new solution for sealing a bipolar Li-ion battery with respect to the electrolyte, more particularly a liquid electrolyte, which is robust in operation and in duration and easy to implement, preferably at relatively low temperature.

(17) The inventors thought to produce each impermeable wall with a bead the matrix structure of which is made up of a honeycomb based on polyurethane (PU) or polytetrafluoroethylene (PTFE), the two main faces of the matrix each being covered with a layer or sheet made of a material that not only is electrically insulating but that also heat seals to the material from which the current collectors are made. To ensure complete seal tightness over the height of the honeycomb, the heat-sealing material at least partially fills the alveoli of the honeycomb in such a way that the two opposing layers or sheets join together.

(18) FIGS. 4, 4A, 5 and 5A show the production of sealing beads 23 according to the invention.

(19) A bead 23 comprises a honeycomb matrix structure 24 made of polyurethane (PU) each of the two main faces of which is covered with a polyethylene (PE) sheet 25a, 25b. The walls of the alveoli 240 of the honeycomb are also covered with a coating 25 made of PE joining the sheets 25a, 25b made of PE (FIG. 4A).

(20) To produce such a bead 23, the honeycomb matrix structure 24 made of PU may advantageously be produced by thermoforming or hot molding. Next, the internal coating 25 made of PE is molded on the walls of the alveoli 240 (FIG. 5A) and lastly, a sheet 25a, 25b made of PE is hot rolled onto each of the two main faces of the matrix 24. These sheets 25a, 25b and the internal coating 25 contribute to ensuring the seal tightness of the matrix structure 24 to the various metal current collectors 10, typically made of aluminum, of the stack of a bipolar battery (FIG. 4).

(21) The width of each sealing bead 23 according to the invention corresponds substantially to that of the honeycomb matrix 24 and is advantageously comprised between 0.5 and 1 cm.

(22) The height of each sealing bead 23 according to the invention corresponds substantially to that of the honeycomb matrix 24 plus those of the PE sheets 25a, 25b and is advantageously comprised between 100 and 200 m and preferably equal to 150 m.

(23) With a honeycomb matrix structure 24, a sealing bead 23 according to the invention has many advantages, such as: a good mechanical strength ensuring a constant electrochemical compartment C1, C2 height substantially corresponding to the height of the PU honeycomb matrix structure 24 plus those of the PE sheets 25a, 25b; a propensity for the bipolar battery according to the invention to be more flexible than a battery according to the prior art in which a sealing joint has a solid structure; the sealing beads 23 weigh less compared to sealing joints according to the prior art; after being heated a number of times in succession, all the electrochemical compartments of the bipolar battery are guaranteed to be leak-tight; and the seal tightness is improved compared to those obtained in the prior art, because the many alveoli in themselves form multiple sealing airlocks between each electrochemical compartment and the exterior environment.

(24) Steps for producing a bipolar battery integrating sealing means according to the invention with a honeycomb matrix based on PU or PTFE, covered or coated on each of its faces with a heat-sealing and electrically insulating material, are described below with reference to FIGS. 6A to 6J. The battery produced comprises two cells C1, C2 stacked one on top of the other and each comprising an anode, a cathode and an electrolyte. It is specified that all the substrates 10, 13, 21 are made of aluminum, all the anodes of Li.sub.4Ti.sub.5O.sub.12 and all the cathodes of LiFePO.sub.4. The separators are all made of the same material such as polyvinylidene fluoride (PVDF). The electrolyte used is, for example, a mixture of carbonate and a lithium salt LiPF.sub.6.

(25) It is specified that all the steps 1 to 6 are performed at ambient temperature.

(26) Step 1: a bipolar current collector 1 is produced with one face covered by the cathode 18 of the first cell C1 and the opposite face covered by the anode 16 of the second cell C2 (FIG. 6A).

(27) Step 2: a current collector 21 is produced with one face covered by the anode 20 of the first cell C1 (FIG. 6B).

(28) Step 3: a terminal current collector 13 is produced with one face covered by the cathode 14 of the second cell C2 (FIG. 6C).

(29) Step 4: A first bead 23 is produced comprising a honeycomb matrix 24 based on PU and, in the interior of its alveoli, PE and on each of its main faces a PE sheet 25a, 25b, as described above with reference to FIGS. 4A and 4B. Next, the first bead 23 with the honeycomb matrix 24 is deposited flat on the periphery of and making direct contact with that face of the collector which is covered with the anode 20 (FIG. 6D).

(30) Step 5: A first separator 19 is intercalated by placing it on the anode 20 of the first terminal current collector 21 and in the interior of the first bead 23 (FIG. 6E).

(31) The bipolar current collector 1 is stacked on the first terminal collector 21 in such a way as to bring, on the one hand, the cathode 18 into direct contact with the first separator 19 and, on the other hand, the free face of the first bead 23 into direct contact with the periphery of the actual current collector 10 (FIG. 6F).

(32) Step 6: Next, the first bead 23 is heated using U-shaped heating jaws 26 encircling the electrochemical cell C2 formed (FIG. 6G). This heating heat seals the sheets 25a, 25b of the first bead 23 to one of the faces of the bipolar collector 1 and of the terminal current collector 21.

(33) Step 7: A step that is the same as the aforementioned step 4/ is carried out but with the bipolar collector 1. Thus, a second bead 23 is produced comprising a honeycomb matrix 24 based on PU and, in the interior of its alveoli, PE and on each of its main faces a PE sheet 25a, 25b, as described above with reference to FIGS. 4A and 4B. Next, the second bead 23 with the honeycomb matrix 24 is deposited flat on the periphery of and making direct contact with that face of the collector which is covered with the anode 16 of the bipolar collector.

(34) Step 8: A second separator 15 is intercalated by placing it on the anode 16 of the bipolar collector 1 and in the interior of the second bead 23 (FIG. 6H).

(35) The second terminal current collector 13 is stacked on the bipolar collector 1 in such a way as to bring, on the one hand, the cathode 14 into direct contact with the second separator 15 and, on the other hand, the free face of the second bead 23 into direct contact with the periphery of the terminal current collector 13 (FIG. 6I).

(36) Step 9: Next, the periphery of the two-cell C1, C2 bipolar battery stack is heated using encircling U-shaped heating jaws 26 (FIG. 6J). This heating heat seals the sheets 25a, 25b of the second bead 23 to one of the faces of the bipolar collector and of the terminal current collector 13.

(37) The seal thus obtained by all the beads 23 is thus perfect with respect to the electrolyte and this applies to all the cells C1, C2 of the bipolar battery.

(38) With regard to the electrolytes, an electrolyte in polymer form or that solidifies at low temperature or in liquid form impregnated in a separator may be used. For activating the electrolytes, each separator 15, 19 may be impregnated with an electrolyte in gel form or in a form that solidifies at low temperature, before the integration of same during assembly. Alternatively, the assembly may be carried out with stacking of the whole battery, the seal produced according to the invention, then an entry made for the liquid electrolyte for subsequent filling via a pipe arranged between the two beads.

(39) In order to further reinforce the seal of a bipolar battery produced with beads according to the invention, certain alveoli of the honeycomb structure of the beads may advantageously be filled with salts or mucilaginous compounds. Thus, as shown in FIG. 7, the interior alveoli 240i of a bead 23 matrix 24, which are those closest to the cells C1, C2, i.e. those making contact with the lithiated electrolyte of the cells, may advantageously be filled with salts or mucilaginous compounds apt to react with said electrolyte. The exterior alveoli 240e of a bead 23 matrix 24, which are those closest to the external environment, i.e. those making contact with the ambient air, may also advantageously be filled with salts or mucilaginous compounds apt to react with moisture in the air. By way of mucilaginous compounds, it may be a question of superabsorbent polymers such as sodium polyacrylates.

(40) Instead of coating by hot rolling each main face of the honeycomb matrix 24, alternatively the latter may be coated with an epoxide or methacrylic resin before the deposition on a current collector and before the actual heat sealing step.

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

(42) It goes without saying that while the seal according to the invention, using beads comprising honeycomb matrices based on PU or PE with coating sheets or layers made of a heat-sealing material, has been described in connection with a bipolar battery with two stacked cells, it may be implemented in the same way for a battery with a number n of stacked cells by repeating the preceding steps 1 to 9 with a number equal to n2 of bipolar collectors and two terminal current collectors 13, 21.