Amorphous metal current collector

The present invention concerns an electrochemical device comprising a cathode and an anode separated from each other by a separator, the battery further comprising two current collectors so that the anode and cathode are each arranged between the separator and a current collector, characterized in that at least one of the two current collectors is made of an at least partially amorphous material comprising at least one metallic element.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a National Phase Application in the United States of International Patent Application PCT/EP 2012/076499 filed Dec. 20, 2012 which claims priority on EP 11194993.9 filed Dec. 21, 2011 . The entire disclosures of the above patent applications are hereby incorporated by reference.

The present invention concerns an electrochemical device comprising a cathode and an anode separated from each other by a separator, the electrochemical device further including two current collectors so that the anode and the cathode are each arranged between the separator and a current collector. This device may be a battery or cell.

BACKGROUND OF THE INVENTION

Among the multitude of existing batteries and cells, batteries called thin film batteries are known. These batteries, shown inFIG. 1, comprise a cathode and anode separated from each other by a separator. The battery further comprises two current collectors which transport the electrons between the cathode and anode and the electric circuit outside the battery.

One of the applications of these batteries, or electrochemical devices, is to obtain flexible batteries. To achieve this, the current collectors and separator must be flexible. One of the elements which greatly limits the flexibility of thin film batteries is the current collector. The current collector is the element of the battery which must have the best electrical conductivity, since the distance travelled by the electric current is by far the greatest (along the entire dimensions of the battery, whereas in the other elements the current only travels the shortest dimension, i.e. the thickness). Too high resistance in the current collector leads to a battery voltage drop and to energy dissipating in the form of heat. For this reason, metals are generally used for the current collector since they have the best electrical conductivity among ambient temperature materials. When the current collectors take the form of metal sheets and are placed outside the battery, as inFIG. 1, they also act as barrier layers preventing the evaporation of the electrolyte and the entry of gases which are noxious for the battery (depending upon the type of battery, e.g. CO2, O2, H2).

However, very flexible materials, such as polymers or composites, can be used for the other battery elements. The use of a metal sheet as the current collector has a negative effect on the flexibility of the battery. Moreover, since the current collector is generally found at the ends of the battery, it is therefore the element that undergoes the highest curvature stress, i.e. a traction stress at the highest radius of curvature on the outside, and compression stress at the smallest radius of curvature on the inside. Consequently, cracks appear in the current collectors after around a hundred bends at radii of curvature of less than 1.5 cm. These cracks become more marked with an increasing number of bends and form folds which damage the active layers inside the battery. This results in a decrease in capacitance which becomes increasingly marked and eventually destroys the battery.

Furthermore, it is known from the prior art the documents U.S. 2007/003812 and JP 2001 250559 disclosing, respectively, a fuel cell comprising the current collectors made in an amorphous metal and a battery comprising a cathode and an anode separated between them by a separator, said battery further comprising two current collectors, the current collector of the cathode being made in a metal or amorphous alloy.

SUMMARY OF THE INVENTION

The invention concerns an electrochemical device such as a battery which overcomes the aforementioned drawbacks of the prior art by proposing a flexible battery which withstands more bending stress and is more reliable.

The invention therefore concerns a cell including a cathode and an anode separated from each other by a separator. The battery further includes an electrolyte and two current collectors so that the anode and cathode are each arranged between the separator and a current collector. The battery is characterized in that the two current collectors are made of at least partially amorphous metallic material.

Advantageous embodiments of this cell form the subject of the dependent claims.

In a first advantageous embodiment, of the two collectors are made of totally amorphous material.

In a second advantageous embodiment, said material includes at least one of the elements found in the list comprising Ti, Zr, Ni, Cu, Fe, Cr, Mn, V, W, Al.

In a third advantageous embodiment, said material has a maximum resistivity of 10−5Ohm*m.

In another advantageous embodiment, said material has a maximum resistivity of 10−6Ohm*m.

In another advantageous embodiment, said material includes 47.5% weight percent of copper, 47.5% weight percent of zirconium and 5% weight percent of aluminium.

In another advantageous embodiment, the thickness of the current collectors is between 1 μm and 50 μm.

In another advantageous embodiment, the thickness of the current collectors is between 5 μm and 25 μm.

In another advantageous embodiment, the thickness of the current collectors is unequal.

In another advantageous embodiment, the edges of the current collectors are thicker than the central area of said collectors.

In another advantageous embodiment, at least one of the two current collectors has structures on the bottom surface thereof.

In another advantageous embodiment, said at least partially amorphous material comprising at least one metallic element further includes a crystalline element.

In another advantageous embodiment, at least one of the two current collectors is made by melt-spinning.

In another advantageous embodiment, at least one of the two current collectors is made by electrolysis.

In another advantageous embodiment, said device is a cell or battery.

In another advantageous embodiment, the cell or battery is rechargeable.

One advantage of the battery of the present invention is that it has more advantageous elastic characteristics. In fact, in the case of an amorphous material, the ratio σe/E is increased by raising the limit of elasticity σe (E being the Young's modulus). Thus, the stress beyond which the material does not return to its initial shape increases. This improvement in the ratio σe/E thus allows greater deformation. This then enables the battery to undergo greater bending stresses and at a higher frequency.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1shows an electrochemical device1according to the invention. This electrochemical device1includes a cathode2and an anode4separated from each other by a separator6. The battery further includes two current collectors8so that the anode4and cathode2are each arranged between the separator6and a current collector8. There is thus an anode current collector9band a cathode current collector9a. Each current collector has a bottom surface91and a top surface90. Separator6is generally made of polymers or composite materials. Electrochemical device1further includes an electrolyte for exchanging ions between cathode2and anode4. This electrolyte may, as inFIG. 1, be directly integrated in separator6, the latter being a porous separator6whose pores are filled with liquid or gel electrolyte. When anode20and cathode40are porous as seen inFIG. 2, the electrolyte also fills these pores. It is also possible for the electrolyte to be formed of an entirely solid layer3, which then replaces the separator as seen inFIG. 3.

This electrochemical device1can be used for various applications such as, for example, a horological application or a smart card or telecommunications application. This electrochemical device1may be a battery or cell, and the battery or cell may or may not be rechargeable. The terms “electrochemical device”, “battery” or “cell” may be used to designate the present invention.

If it is desired to make electrochemical device1in a flexible film while preventing the appearance of cracks in current collector8, even after a large number of bends, it is necessary to remain within the elastic deformation range of the material and also for the material to have good fatigue resistance in the area subject to stress. Generally, for a given alloy, the number of cycles prior to a fatigue break greatly increases when the level of stress moves away from the limit of elasticity. Below a certain stress, fatigue breakage disappears. This behaviour is generally represented by the Wöhler curve.

The present invention consists in using amorphous metal current collectors8. The elastic deformation of amorphous metals (or metallic glass), which are generally alloys rather than pure metals, is around 2 to 4 times higher than crystalline metals.

Advantageously, at least one a preferably the two current collectors8are made of an at least partially amorphous metal. “At least partially amorphous material” means that, for the thicknesses required for the intended application, the material is able to at least partially solidify in the amorphous phase.

For the applications for which the electrochemical film devices of the present invention are used, the total thickness of electrochemical device1is generally 0.4 mm. The thickness of current collectors8of this electrochemical device1may vary from 1 to 50 μm. Preferably, the thickness is comprised between 5 and 25 μm.

Indeed, the advantage of these amorphous metal alloys arises from the fact that, during manufacture, the atoms forming the amorphous materials are not arranged in a particular structure as is the case for crystalline materials. Thus, even if the Young's modulus E of a crystalline metal and that of an amorphous metal are close, the limit of elasticity σeis different. An amorphous metal differs therefore in that it has a higher limit of elasticity σeAthan that σecof the crystalline metal by a factor of between two and four. This means that amorphous metals can undergo higher stress before reaching the limit of elasticity σe.

Moreover, given that the minimum radius of curvature is inversely proportional to the maximum admissible deformation, this means that a radius of curvature which is at least two times smaller is possible by using an amorphous metal, instead of a crystalline metal as is the case in the prior art. Moreover, for an identical radius of curvature to that of a crystalline metal, the risk of fatigue breakage decreases since the limit of elasticity of an amorphous metal is generally 2-4 times higher than that of a crystalline metal of similar chemical composition. Indeed, the relative cyclical stress will be significantly lower for the amorphous metal and thus the number of cycles prior to breakage will increase considerably.

Surprisingly, the flexibility of cells assembled with amorphous metal current collectors according to the invention is more than 10 times higher than for cells using a crystalline metal. This is due to the fact that the inside of the battery is protected by the collectors. Indeed, in the case of crystalline metal, folds are formed in case of bending of the cell and therefore of the collectors. The folds are locally very high curvatures (for cells typically bent with a radius of 1 cm, the folds have a radius less than 1 mm). As the amorphous metal does not form folds during its flexion, local deformations are avoided (delamination and destruction of active layers). Consequently, the interior of the cell is protected from folds having very small radius of curvature. This in turn allows improved flexibility. Typically, at least 1500 bendings with a radius of 1 cm are possible using two current collectors completely in amorphous metal with a thickness of 25 micrometer in a cell with a total thickness of 0.4 mm, without damaging the latter. Among the film batteries available on the market today, none cell survive 150 flections with a radius of 1 cm. The improvement in the flexibility of cell current collectors made from amorphous metal is much greater than the increase by a factor of 2-4 that a man skilled in the art bright possibly predict by analysing the mechanical properties of amorphous metals.

To be able to be used for current collector8, a material must be stable inside the electrochemical potential window of the electrode, which is between the charged and discharged state of the electrode potential, so as to prevent any corrosion of the current collector. Likewise, the material of current collector8must not react chemically with the substances forming the electrodes and electrolyte3. The stability of current collector8may be thermodynamic, kinetic or achieved by passivation. The electrical resistivity of current collector8must not be too high so that it does not affect the power and efficiency of electrochemical device1. Typically, the resistivity of the alloy used as collector8should not exceed 10−5Ohm*m, but more preferably 10−6Ohm*m.

Consequently, the amorphous alloys formed of the following chemical elements are of particular interest for this application: Ti, Zr, Ni, Cu, Fe, Cr, Mn, V, W, Al. An example of an amorphous alloy that can be used for this application is Cu47.5Zr47.5Al5. The good electrical conductivity of this alloy combined with its high mechanical properties (δe˜1600 MPa; E˜87 GPa; εe˜2%) makes it a particularly advantageous candidate. It is also possible to envisage using composites with an amorphous metal matrix and a second very conductive phase (pure copper for example) to further increase electrical conductivity.

The melt-spinning method is used to make current collectors8. This method, seen inFIG. 8, consists in principle in taking a tank of warm liquid18(molten metal for example) heated by a heating system14, which may be a pressure system, from which there flows a thread which, when it falls, enters into contact with a good heat conducting metal cylinder16(made of copper for example). The melt spinning wheel rotates at high speed and is then cooled, generally by contact with another cold liquid, liquid nitrogen or water, which allows it to stay cold. Pressurising the tank allows the liquid to be ejected. The liquid is cooled on contact with the wheel and can form a solid strip which may be thick or thin. The thickness is adjusted by working on the flow rate of the liquid metal or on the rotational speed of the good heat conducting metal cylinder.

Another method for making current collectors8is electrolytis is deposition. This method is based on the principle of the depositing a metal or metal alloy via a current on an electrically conductive support.

In this method, two electrodes are immersed in a bath containing the metal cations to be deposited. Application of a current or potential difference between the two electrodes causes the desired metal cations to be deposited on the cathode acting as a support. After manufacture, the metal or alloy formed can be insulated from the cathode by physical or chemical means.

To adjust the thickness of current collector8, the duration of electrolysis is adjusted so that the longer the duration, the greater the quantity of material deposited.

If the material deposited is an alloy, several metal cations are contained in the bath. The composition of the alloy which will form current collector8can be modulated by adjusting the parameters of the current, temperature and composition of the bath. The ductile properties of the material can be improved or modified by the use of pulsed currents.

When this process is applied in an aqueous medium at low temperatures compared to metallurgic or physical manufacturing methods, the process results in the formation of metals in the amorphous state.

In a first construction variant of electrochemical device1according to the present invention and visible inFIG. 4, electrochemical device1is closed by current collectors8. This means that current collectors8form a structure11. To achieve this, each current collector8takes the form of a smooth plate, for example a rectangular plate, with a peripheral edge8a. This peripheral edge8athus defines a housing8bin which the anode2or cathode4material is placed. These two collectors8are thus separated by separator6. This separator6includes a first surface7aand a second surface7b. The elements are assembled so that the peripheral edge8aof anode collector9bis welded to the first surface7aof separator6and the peripheral edge8aof cathode collector9ais welded to second surface7bof separator as seen inFIG. 4. The welds are preferably at the ends6aof separator6.

In an alternative, seen inFIG. 5, to this first variant, separator6makes it impossible for current collectors8to be welded. Battery1therefore includes a joint10which is secured to the separator and to which the peripheral edge8aof anode collector9band peripheral edge8aof cathode collector9aare welded.

In a second variant seen inFIG. 6, current collectors8are simply a smooth sheet made of amorphous metal. Anode material4is arranged between separator6and anode collector9bwhereas cathode material2is arranged between separator6and cathode collector9a. Shrewdly, this variant encloses the electrochemical device1thus arranged with a resin layer12or in a laminated polyethylene/aluminium/polyethylene sachet which is vacuum packed. Conductive tongues14are then pre-fixed to collectors8to form the battery contacts.

In a third variant seen inFIG. 7, the bottom surface91of collectors8could be structured. In fact, since electrochemical device1is capable of being bent, shearing forces may appear between current collector8and the electrolyte forming cathode2or anode4. If these shearing stresses are too high, battery1is liable to delaminate.

Structuring the bottom surface91of each current collector8increases the friction forces between current collector8and the electrolyte forming cathode2or anode4. Consequently, during twisting, the additional force of adhesion provided by the structures pushes back the limit of delamination.

For this adhesion to be efficient, the structures93must be arranged so that adhesion is improved. Take the example of an electrochemical device1in the form of a rectangular film. In the case of bending along an axis parallel to the width of electrochemical device1, the structures must be arranged in the same direction, i.e. parallel to the width of battery1. Conversely, in the case of bending along an axis parallel to the length of electrochemical device1, structures93must be arranged in the same direction, i.e. parallel to the length of electrochemical device1.

Nonetheless, the bottom surfaces91of each current collector8could be structured lengthways and widthways. This arrangement allows battery1to bend lengthways or widthways.

It is possible to use various methods to make these structures93. A first solution consists in making the structures immediately during manufacture of current collector8, i.e. during the melt spinning step as seen inFIG. 8. To achieve this, the cylinder to which the molten metal is sent to form the strip is structured immediately. It is clear that cylinder16has at the surface thereof the negative die17of structures93which have to be formed on current collector8. Consequently, during this melt spinning step, the liquid metal is straight away solidified in amorphous form with the negative shape of cylinder16.

Another solution consists in using the hot working principle. This method consists in placing current collector8between two dies, heating it within a temperature range between the vitreous transition temperature Tg and the crystallisation temperature Tx while pressing it for a determined time to preserve a totally or partially amorphous structure. This is carried out for the purpose of keeping the characteristic elastic properties of amorphous metals. Once the pressing has finished, current collector8is cooled rapidly to maintain the amorphous state. This shaping method can very precisely reproduce fine geometries since, between Tg and Tx, the viscosity of the alloy greatly decreases, as the alloy therefore matches all the details of the dies without losing its amorphous state.

To form these structures93it is also possible to form current collector8immediately during manufacture by electrolysis, by selecting a cathode support which is structured. One of the aspects of current deposition is that this method can replicate various surface aspects with a high level of precision, including complex aspects like those described for the requirements of this invention.

In a fourth variant, it is possible to envisage pre-bending thin film battery1. This means that thin film electrochemical device1is naturally curved. Indeed, it is possible for electrochemical device1to be placed in a non-linear place such as a watch bracelet or for the battery to be folded for integration into the apparatus or object for which it is intended. It is thus advantageous for electrochemical device1naturally to have a non-linear shape to make it simpler to integrate. This also means that thin film electrochemical device1does not have to be elastically or plastically deformed and consequently made more brittle.

The hot working technique is used to make this type of electrochemical device1. Each current collector8is placed between two dies and then heated to a temperature comprised between the vitreous temperature Tg and crystallisation temperature Tx. The viscosity of the amorphous metal thus increases without any loss of its amorphous characteristic. Current collector8is then pressed by the two dies, which have a curved profile so that one of the dies has a convex profile and one of the dies has a concave profile. The rapid cooling step preserves the amorphous state and solidifies current collector8. Of course, the profile of the dies is calculated to obtain the desired curvature.

For the same purpose, the pre-bent current collector8can be formed immediately during manufacture by electrolysis by using a cathode-support which has the desired element of curvature.

In a fifth variant seen inFIG. 9, current collectors8could exhibit non-uniform deformation. To achieve this, each collector8has a variable or unequal thickness94. Indeed, for a defined stress, the deformation of current collector8will be different according to the thickness thereof. It is thus clear that a current collector8of defined thickness will deform more than a current collector8which two times thicker. Having a current collector8of variable thickness94means that said current collector8can be configured so that the deformation of areas where the thickness is greater is less than the deformation of areas where the thickness is smaller. In particular, it is advantageous for the peripheral areas81of current collector8to be thicker than central area80. Indeed, central area80is generally the area that has to undergo the greatest deformation. This central area80must consequently be able to be deformed easily so as not to deform too quickly plastically. Conversely, the edges81of current collector8are subject to little stress and deformation. It is therefore possible for the thickness of the edges to be different and in particular thicker.

It will be clear that various alterations and/or improvements and/or combinations evident to those skilled in the art may be made to the various embodiments of the invention set out above without departing from the scope of the invention defined by the annexed claims.

In the first construction variant collectors8may therefore be secured by hot forming.