Current collector with integrated leak-proofing means, bipolar battery comprising such a collector

Abstract

The present patent application relates to a device for a lithium electrochemical generator, said device comprising a band (100) of electrical insulating material including at least one polymer, and at least one metallic layer (102) which forms a current collector and is deposited on at least one of the two main faces in the central part of the band. The central part (100C) of the band comprises a plurality of holes (101) emerging on its two opposite main faces, said holes being filled at least partially with a metal that is continuous with each deposited metallic layer. The periphery of the band (100P) is devoid of metallic layer and at least one metallic layer is covered with an electrode of lithium insertion material.

Claims

1. A bipolar battery comprising at least two electrochemical cells stacked on one another and at least one device comprising: a band made of an electrically insulating material comprising at least one polymer; and at least one metal layer forming a current collector, said metal layer being deposited on each of two main faces in a central portion of the band; wherein: the central portion of the band comprises a plurality of holes opening onto two opposite main faces; the holes are filled at least partially with a metal that is continuous with each deposited metal layer; the whole periphery of the main faces of the band is devoid of the at least one metal layer and forms a frame of width sufficient to serve as a part of an electrolyte-tight wall in the electrochemical generator; the at least one metal layer is covered with an electrode made of a lithium insertion material; and one of the two layers forming the current collector is covered with the anode made of a lithium insertion material of one of the two cells, and the other of the two layers forming the current collector is covered with the cathode made of a lithium insertion material of the other of the two cells, the periphery of the band made of at least one polymer forming a peripheral zone of a wall that is leak-tight to the electrolytes of the two cells that encircles the latter.

2. The bipolar battery as claimed in claim 1, further comprising at least one device wherein the dimensions of the holes are determined to minimize internal resistance induced by the metal that fills it, and with the other face not covered by an electrode making contact via the holes filled with metal with a terminal current collector.

3. The bipolar battery as claimed in claim 1, wherein the at least one polymer of the band that is a majority constituent of the band is a polyolefin.

4. The bipolar battery as claimed in claim 1, wherein the at least one polymer that is a majority constituent of the band is a two-component resin.

5. The bipolar battery as claimed in claim 4, wherein the resin is an acrylic resin.

6. The bipolar battery as claimed in claim 1, wherein the holes are of cylindrical section.

7. The bipolar battery as claimed in claim 1, wherein the holes are of frustoconical section.

8. The bipolar battery as claimed in claim 1, wherein the diameter of the holes is between 50 and 500 m.

9. The bipolar battery as claimed in claim 8, wherein the diameter of the holes is between 50 and 200 m.

10. The bipolar battery as claimed in claim 1, wherein the density of the holes is equal to a number between 0.1 and 1/cm.sup.2 of band.

11. The bipolar battery as claimed in claim 10, wherein the density of the holes is equal to a number between 0.1 and 0.3/cm.sup.2 of band.

Description

DETAILED DESCRIPTION

(1) Other advantages and features will become more clearly apparent on reading the following detailed description which is given by way of illustration and with reference to the figures, in which:

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

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

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

(5) FIG. 4 is a schematic cross-sectional view of a hole in a polymer band, said hole being partially filled with a metal according to the invention;

(6) FIGS. 5A to 5C are schematic views each of which illustrates a step of a process for producing holes in a polymer band according to the invention;

(7) FIG. 6 is a schematic front view of a polymer band obtained using the process in FIGS. 5A to 5C;

(8) FIG. 7 is a cross-sectional view of a device according to the invention devoid of electrodes; and

(9) FIGS. 8A to 8D are schematic views each of which illustrates a step of a process for producing a polar battery from three devices according to the invention, step 8D being an alternative step to step 8D.

(10) 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 in all FIGS. 1 to 6.

(11) It will be noted that various elements, in particular the layers of material and holes according to the invention, are shown merely for the sake of clarity and that they are not to scale.

(12) It will also be noted that the metal layers 102 forming the current collector are not shown in FIGS. 8B to 8D for the sake of clarity.

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

(14) Within this battery, a bipolar electrode 1, also called a bipolar current collector, comprises an anode layer 16 and a cathode layer 18 on either side of a conductive aluminum substrate 10 taking the form of a plate. The bottom electrode 20 and top electrode 14 are separated from the bipolar electrode 1 by two separators 15, 19 in which an electrolyte is present in liquid or gel form. The battery is made tight to leakage of the electrolytes between the two completed adjacent electrochemical cells 14, 15, 16 and 18, 19, 20 by a seal 22 that is produced by depositing a resin or adhesives on the periphery of all the electrodes and the plate 10.

(15) A bipolar current collector 10 according to the prior art, depending on the lithium-ion insertion materials employed to produce the electrodes: either consists of two superposed plates, one 10A1 of which, typically made of aluminum, is covered by a cathode 11, and the other 10C of which, typically made of copper, is covered by an anode 12 (FIGS. 2A and 2B); or consists of a single plate 10A1, typically made of aluminum, covered on one of its faces by a cathode 11 and on the other of its faces by an anode 12 (FIGS. 3A and 3B). The main difficulty encountered designing a bipolar battery according to the prior art is how to produce compartments that are perfectly tight against leakage of the electrolyte, which in general is a liquid, from one compartment to another, such as between the two cells C1 and C2, i.e. between the compartments referenced 14, 15, 16 and 18, 19, 20 in FIG. 1.

(16) The prior art solutions of producing seals 22 or increasing the size of the plates 10 of the bipolar electrode are not entirely satisfactory.

(17) Thus, the inventors propose a completely different current-collector design solution that allows a higher electrical conductivity and a lower internal resistance to be obtained relative to a current collector according to patent application JP 2010153224 or according to patent application WO2011/157751.

(18) Thus, the inventors in substance propose to produce holes that open onto each of the opposite main faces of a polymer band, then to deposit on at least one of the faces a metal layer that will fill the holes. The metal filling the holes is thus continuous with the one or more layers forming the current collector for the subsequently deposited electrodes, thereby allowing a high electrical conductivity to be obtained for said collector while its internal electrical resistance remains low.

(19) The electrical resistance of a hole 101 of frustoconical cross section filled with a metal of given electrical conductivity , as schematically shown in FIG. 4, has been calculated.

(20) It will be noted that the hole 101 must be plugged but is not necessarily filled in its entirety.

(21) The equation is written in the following way:

(22) $R_{\mathrm{via}}=\frac{1}{S\left(x\right)}x=\left\{\frac{1}{S_1\left(x\right)}x+\frac{1}{S_2\left(x\right)}\right\}$

(23) in which Rvia designates the resistance of the hole, designates the resistivity of the metal filling the hole, l designates the height of the hole and l1 designates the height of the metal filling the hole.

(24) The first term of the equation is calculated as follows:

(25) $\frac{1}{S_1\left(x\right)}x=\frac{1}{}\frac{1}{{r\left(x\right)}^2}x=\frac{1}{}x$

(26) By setting:

(27) $r={}_{\mathrm{PAR}}-{}_{\mathrm{FAV}},\frac{r}{l}=a\mathrm{et}{}_{\mathrm{PAV}}=b$${}_0^{l_1}\frac{1}{S_1\left(x\right)}x=\frac{1}{}\frac{1}{{\left(\mathrm{ax}+b\right)}^2}x=\frac{1}{}\left[-\frac{1}{a.Math.\left(\mathrm{ax}+b\right)}\right]\begin{array}{c}x=l_1\\ x=0\end{array}$$\frac{1}{S_1\left(x\right)}x=\frac{1}{}\left[\left(-\frac{1}{a.Math.\left({\mathrm{al}}_1+b\right)}\right)+\left(\frac{1}{\mathrm{ab}}\right)\right]$$\frac{1}{S_1\left(x\right)}x=\frac{1}{}\left[\frac{-b+{\mathrm{al}}_1+b}{\mathrm{ab}\left({\mathrm{al}}_1+b\right)}\right]=\frac{1}{}\left[\frac{l_1}{b\left({\mathrm{al}}_1+b\right)}\right]$

(28) The second term of the equation is calculated as follows:

(29) $\frac{1}{S_2\left(x\right)}x=\frac{1}{}\frac{1}{{r\left(x\right)}^2-r_1^2\left(x\right)}x=\frac{1}{}\frac{1}{r^2\left(x\right)-{\left(r\left(x\right)-e\right)}^2}x=\frac{1}{}{}_{l_1}^l\frac{1}{r^2\left(x\right)-r^2\left(x\right)+2r\left(x\right)\mathrm{Xe}-e^2}x$${}_{l_1}^l\frac{1}{S_2\left(x\right)}x=\frac{1}{}{}_{l_1}^l\frac{1}{2a\left(\mathrm{ax}+b\right)-e^2}=\frac{1}{}{}_{l_1}^l\frac{1}{2\mathrm{eax}+2\mathrm{eb}-e^2}=\frac{1}{}{}_{l_1}^l\frac{1}{\mathrm{Ax}+B}$$\mathrm{Where}\text{:}A=2\mathrm{ea}\mathrm{and}B=2\mathrm{eb}-e^2$${}_{l_1}^l\frac{1}{S_2\left(x\right)}x=-\frac{1}{}\left[\mathrm{Ln}\left(\mathrm{Ax}+B\right)\right]\begin{array}{c}l\\ l_1\end{array}$${}_{l_1}^l\frac{1}{S_2\left(x\right)}x=\frac{1}{}\mathrm{Ln}\frac{\mathrm{Al}+B}{{\mathrm{Al}}_1+B}$

(30) Thus, finally, the equation is written in the following way:

(31) $R_{\mathrm{via}}=\left\{\frac{1}{S_1\left(x\right)}x+\frac{1}{S_2\left(x\right)}x\right\}=\frac{}{}\left\{\left[\frac{l_1}{b\left({\mathrm{al}}_1+b\right)}\right]+\frac{1}{A}\mathrm{Ln}\frac{\mathrm{Al}+B}{{\mathrm{Al}}_1+B}\right\}$

(32) Thus, it is easily possible to calculate the best possible dimensions for a hole in order to minimize the internal resistance induced by the metal that fills it.

(33) FIGS. 5A to 5C illustrate a process for producing one portion 10 of a device according to invention.

(34) First a polymer band 100 is produced.

(35) Next, a tool 2 with heated irons, preferably taking the form of a matrix the number of irons of which corresponds to the number of through-holes that it is desired to produce, is brought near. The tool 2 is moved in a direction X orthogonal to the plane of the polymer band (FIG. 5A).

(36) The irons are heated here to a temperature above the melting point of the polymer that is the majority constituent of the band 100. Thus, a band made of PE, the melting point of which is between 85 and 140 C. must be pierced with irons heated to a higher temperature.

(37) The movement occurs along a given path in order to obtain holes 101 of calibrated dimensions (FIG. 5B). In the example illustrated, the irons 200 are all identical and conical in shape and therefore allow identical holes 101 of frustoconical shape to be obtained.

(38) Once the movement along the given path has been carried out, and therefore the holes 101 of calibrated dimensions obtained, the tool 2 is removed, i.e. moved along the same direction X but away from the tool (FIG. 5C).

(39) An example of a polymer band 100 according to the invention is shown in FIG. 6: the band 100 is rectangular in shape and the holes 101, which are all identical to one another, are uniformly distributed over the central portion 100C of the band, the peripheral portion 100P of the latter being devoid of holes.

(40) A metal paste is then deposited on one face 10.1 of the band 100 using a screen-printing technique. This paste is advantageously made of aluminum and the deposition allows the holes 101 to be at least partially filled with this paste. The same thing is done on the other face 10.2 with a paste of the same composition using the same screen-printing technique.

(41) Thus the carrier portion forming the current collector of a device according to the invention is obtained with the two metal layers 102 deposited on each of the two faces 10.1, 10.2 of the band 100, a perfect metal and therefore electrical continuity being obtained by virtue of the holes 101 at least partially filled with the metal, a metal such as aluminum (FIG. 7).

(42) By virtue of the peripheral portion 100P of the polymer band 100 that is devoid of metal, there is no risk of short-circuiting subsequently during operation, i.e. when a current is made to flow through a bipolar battery incorporating devices according to the invention.

(43) As indicated above, the size and density of the holes and the amount of metal in the layers and holes is determined beforehand in order to minimize both the internal electrical resistance and the Joule heating that is liable to occur during operation.

(44) To produce a complete bipolar battery according to invention, the procedure is as follows.

(45) First, the carrier portion 10 of the device according to the invention, obtained such as above (FIG. 8A), is produced, i.e. a hybrid semi-product comprising a functional leak-tight sealing zone 100/current collector 102 the metal of which is filled into the holes 101 of the polymer band.

(46) At least one electrode made of a lithium insertion material is then deposited in order to obtain, finally, the device according to the invention.

(47) Such as illustrated in FIG. 8B, to produce a complete bipolar battery, three separate devices according to the invention are produced: to obtain a bipolar device 1, a layer 12 of negative lithium insertion material (anode), such as a layer of Li.sub.4Ti.sub.5O.sub.12, is deposited on a metal layer 102 forming the collector, and a layer 11 of a positive lithium insertion material (cathode), such as a layer of LiFePO.sub.4, is deposited on the other metal layer forming the collector; to obtain a positive monopolar device 1 only one layer 11 of positive lithium insertion material (cathode), such as a layer of LiFePO.sub.4, is deposited on the only metal layer 102 deposited on the band 10; and to obtain a negative monopolar device 1 only one layer 12 of negative lithium insertion material (anode), such as a layer of Li.sub.4Ti.sub.5O.sub.12, is deposited on the only metal layer 102 deposited on the band 10.

(48) For these three devices 1, 1, 1, the electrode layers are advantageously produced by a printing technique (rotogravure, flexographic or screen printing) on the current-collecting portion 102 itself taking the form of one or more layers filling the holes 101, the periphery 100P of the polymer band 100 methodically being left devoid of any electrode. All the electrode layers 11, 12, 11, 12 and current collectors 102 are substantially the same size, as are all the peripheral zones 100P of the polymer bands.

(49) Optionally, provision may be made for a calendering step in order to ensure a better electronic percolation in each electrode.

(50) The devices 1, 1, 1 are then aligned and stacked with the bipolar device 1 between the two monopolar devices 1, 1, and separated by an electrically insulating and ionically conductive membrane 3 forming a separator: such as illustrated in FIG. 8C, a bipolar battery is then obtained that comprises two electrochemical cells C1, C2 that are stacked one on the other, and a peripheral zone 100P that is continuous over the entire height.

(51) Once the stack has been produced, each separator is impregnated with an electrolyte. Alternatively, each separator may already have been impregnated with electrolyte during its placement in the stack.

(52) To produce the definitive leak-tight seal between the compartments of such a bipolar battery, the peripheries 100P of the three bands 100 of the devices 1, 1, 1 are pressed against one another and the three bands are maintained pressed against one another. Thus, a wall that is leak-tight to the electrolytes 3 of the two cells C1, C2 and that surrounds the latter is produced over the entire height of the stack. Two alternative ways of carrying out this definitive sealing step may be envisioned: either pressure is applied to the periphery 100P of the polymer band 100 of the cells, advantageously using the rigid packaging 4 of the battery (FIG. 8D); or the polymer peripheries 100P are heat sealed, uniformly over the height, advantageously using a U-shaped clamp 5 (FIG. 8D).

(53) Other variants and improvements of the invention described above may be envisioned without however departing from the scope of the invention.

(54) Thus, for example, it may be envisioned to deposit each metal layer using a paste deposited by screen printing through a mask in order to allow the distribution of the paste to be localized.

(55) Furthermore, instead of screen printing it may be envisioned to use rotogravure or flexographic printing to carry out the deposition.

(56) Moreover, the polymer of the peripheral part 100P of the band 100 may be thicker than the polymer of the rest of the band in order to allow, during the heat sealing, a calibrated thickness to be preserved.