TECHNICAL FIELD The present invention relates to the field of electrodes for metal/oxygen galvanic cells, in particular for a vehicular battery such as a battery for electric or hybrid vehicles. Specifically, the invention is directed to an electrode arrangement for a negative electrode (anode) of such a galvanic cell and to a metal/oxygen galvanic cell comprising such an electrode arrangement.

BACKGROUND

While traditionally most long-range vehicles, such as cars, trucks, buses, motorcycles, and non-electric railway locomotives, have been powered by gasoline or diesel engines, in recent years the development of electric or hybrid vehicles, in particular automobiles that are at least partially powered by electric motors has been steadily increasing. To that purpose various different battery systems have been developed as suitable storages for electric energy, including in particular lithium-ion batteries, which are used for most of today's electric and hybrid cars. One disadvantage of such lithium-ion batteries is their limited energy density, i.e. stored electrical energy per battery mass or per battery volume. This limitation is - amongst others - caused by the fact that all chemical components needed for the electrochemical reactions taking place in the battery cells are already contained in the charged battery, thus adding to its weight or volume.

To overcome this limitation another battery type has been conceived, which is commonly known as "metal/air battery" or "metal/oxygen battery". Such a battery comprises one or more electrochemical cells each having an electrically negative electrode - usually referred to as "anode" - made of or at least containing a suitable metal, and a suitable metal and an electrically positive electrode - usually referred to as "cathode" - working with ambient air or oxygen, and a separator arranged between the two electrodes to electrically separate them. In particular, the anode can comprise an alloy having such metal as a first component and one or more further metal or non-metal components, such as carbon (C), tin (Sn) or silicon (Si), wherein the metal component in such anode remains available to participate in the electricity generating chemical reactions of electrochemical, i.e. galvanic cell. Instead of such alloy also a transition metal oxide may be used as an anode material. Furthermore, an electrolyte, which may in particular be of the aqueous or solid type, is present in the cathode and optionally in the separator. In particular, it is known to use zinc, aluminum or lithium as the metal for the anode. At the cathode side, oxygen is the relevant electrochemical component and unlike in lithium-ion batteries it does not have to be present in the charged battery from the beginning, but can rather be tak- en from ambient air or be delivered to the battery in the form of an oxygenous gas or pure oxygen from a source such as a tank or other reservoir during discharging of the battery. In this way, batteries having a much higher energy density than traditional lithium-ion batteries become possible. Furthermore, when such a battery is recharged, oxygen is generated at the cathode and can be re-used in a subsequent discharging cycle.

However, if oxygen present at the cathode side of the battery comes into contact with an active metal electrode of the anode side of the battery, the active metal electrode may be oxidized and effects detrimental to at least one of the capacity, life- time, power, and reliability of the battery may occur.

International patent publication WO 2007/021717 A2 discloses a protected anode architecture having ionically conductive protective membrane architectures that in conjunction with compliant seal structures and anode backplanes efficiently enclose an active metal anode inside the interior of the anode compartment. This enclosure prevents the active metal from deleterious reaction with the environment external to the anode compartment, which may include aqueous, ambient moisture, and/or other materials corrosive to the active metal. SUMMARY OF THE INVENTION

Against this background, the present invention is directed to the problem of providing a further improved electrode arrangement for a metal/oxygen galvanic cell that effectively protects the negative electrode from undesired chemical reactions, in particular with oxygen. In particular the invention is directed to reducing the complexity of such an electrode arrangement and of a galvanic cell based thereon. A solution to this problem is provided by the teaching of the appended independent claims, namely by a electrode arrangement according to claim 1 and a metal/oxygen galvanic cell according to claim 15 that comprises such an electrode arrangement.

Various preferred embodiments and further improvements of the invention are pro- vided in the dependent claims.

A first aspect of the present invention is directed to an electrode arrangement for a metal/oxygen galvanic cell, in particular for a battery of an electric or hybrid vehicle. The electrode arrangement comprises an active metal electrode and a separator structure. The separator structure comprises a first ionically conductive separator portion positioned, directly or indirectly, on a first face of the electrode and at least one electrode protection portion being impervious to molecular oxygen and covering a surface portion of the electrode that is not covered by the first separator portion. The electrode protection portion is formed integral with the first separator portion.

The term "metal/oxygen galvanic cell", as used herein, relates to an electrochemical cell as described in detail above, where the electrochemically relevant chemical component of one of the electrodes is oxygen, in particular 0 ₂ . To support the electrochemical reactions taking place in the cell, oxygenous gas is provided to the cell's cathode side. In particular, a "metal/air galvanic cell", i.e. a battery cell that uses air as such oxygenous gas, is also an embodiment of a "metal/oxygen galvanic cell" in the sense of the present invention.

The term "active metal electrode", as used herein, relates to an electrode for a gal- vanic cell, which comprises a metal as its chemically active component involved in the chemical reactions generating a voltage between the electrodes of the galvanic cell. In particular, such chemical reactions at the active metal electrode may involve an oxidation of said metal. A specific type of metal/oxygen galvanic cell is the so- called Li/air cell respectively Li/oxygen cell, where Lithium metal is the current choice of negative electrode (anode) material. At the anode, electrochemical potential forces the lithium metal to give off electrons as per the oxidation. The term "separator", as used herein, relates to a permeable membrane placed between a galvanic cell's anode electrode and cathode electrode. The main function of a separator is to keep the two electrodes apart to prevent electrical short circuits while also allowing the transport of ionic charge carriers that are needed to close the circuit during the passage of current in the electrochemical cell. In particular, a solid- state electrolyte meeting the above requirements is also a "separator", as used herein.

The term "ionically conductive separator portion", as used herein, relates to a portion, i.e. a spatial section, of the separator, which is adapted to separate the anode side from the cathode side of a battery cell and that allows the transport of ionic charge carriers needed to close the circuit during the passage of current in an electrochemical cell. Accordingly, an ionically conductive separator portion is a section of a separator structure that meets the above identified requirements of a "separator". In addition, a separator structure may have one or more other portions, which do not meet these requirements. In particular, this may apply to electrode protection portions.

The electrode structure according to the first aspect of the invention is based in particular on the concept that the protection of the active metal electrode against dele- terious reactions with the environment external to the electrode structure, in particular the prevention of an oxidation of the electrode, is provided by the separator structure rather than by an additional sealing structure. A separator is already a core component of a galvanic cell and does not have to be added as an extra part. In particular, the electrode structure according to the first aspect of the invention effi- ciently prevents oxidation of the active metal electrode on those of its edges that are not configured to be covered by a respective positive electrode (cathode) when a metal/oxygen galvanic cell is assembled from the electrode structure and one or more positive electrodes. Furthermore, as the at least one electrode protection por- tion of the separator structure is formed integral with the first separate portion, both portions of the separator structure can be mounted as one during the assembly of the galvanic cell, such that the number of manufacturing steps may be reduced compared to known processes, where separate parts and manufacturing steps are used to form a protective compartment for the active metal electrode.

In the following, preferred embodiments and variants of the electrode arrangement according to the first aspect of the invention will be described. Unless explicitly excluded or mutually exclusive, those embodiments and variants may be arbitrarily combined with each other and with the second aspect of the invention, as described thereafter.

According to a first preferred embodiment the separator structure further comprises a second ionically conductive separator portion positioned, directly or indirectly, on a second face of the electrode opposite to its first face. In particular, this may be advantageous for double-sided metal/oxygen galvanic cells, were two positive electrodes (cathodes) are arranged on two different, preferably opposing, sides of the electrode arrangement such that the cell is able to receive oxygenous gas from both sides of the cell. Then, the first separator portion can form an ionically conductive interface to a first one of the positive electrodes, and the second separator portion can similarly form an ionically conductive interface to the second one of the positive electrodes.

According to a preferred variant of this embodiment, the second separator portion is formed integral with the first separator portion and the electrode protection portion. Thus, the separator structure can be formed as a single component that can be easily integrated into a galvanic cell during its assembly, in particular by a single process step. According to a further preferred variant, the electrode comprises a first active metal electrode portion, a second active metal electrode portion, and a current collector arranged between the first electrode portion and the second electrode portion and being in electric contact with both electrode portions. The current collector is electri- cally conducting and serves to provide an electrical connection to the electrode from outside the cell. In particular, the current collector may be connected electrically and preferably also mechanically to a terminal that extends to the exterior of the cell. This variant is particularly useful for double-sided metal/oxygen cells, as described above, as it supports both an easy external electric connection to the negative electrode and the forming of a double-sided cell, where the negative electrode is sandwiched between two positive electrodes with the first and second separator portions separating the electrodes of opposite polarity. Thus, the active area of the galvanic cell can be dramatically increased, in particular doubled, as compared to a single- sided implementation.

According to a further preferred variant thereof, the current collector extends through an opening in the electrode protection portion and the opening is sealed around the current collector such that it is substantially gastight, in particular for oxygen such as from ambient air. Thus, the current collector itself can form the electric terminal of the cell, while the protection of the negative electrode against undesired chemical reactions, in particular oxidation, is maintained.

According to another preferred embodiment the electrode protection portion is ar- ranged between the first separator portion and the second separator portion and connects both. Thus, the two separator portions may cover in particular two opposing faces of the negative electrode, while the electrode's edges connecting the two faces can be covered by the at least one electrode protection portion in order to avoid undesired chemical reactions at those edges, in particular with oxygen from ambient air or oxygen diffusing from a cathode side of the cell.

In particular, according to a preferred variant of this embodiment, the first separator portion, the second separator portion and the electrode protection portion form a ring structure that surrounds the electrode. A "ring structure", as used herein, is a structure having a shape with a cross-section that is defined by the area between to closed lines (loops) one of which surrounds the other with crossing it. Thus, a ring structure may be a circular ring, but it is not limited thereto. According to a further preferred embodiment the first separator portion, the second separator portion and the electrode protection portion form a substantially closed compartment encapsulating the electrode. Thus, the negative electrode is protected on all of its faces against coming into contact with undesired chemical substanc- es, esp. with oxygenous gas. In particular, this allows for implementing the structure for protecting the negative electrode as a single piece. Accordingly, the separator structure may be formed in particular as a flexible bag (pouch) or a rigid container, in which the negative electrode is inserted during assembly before the compartment is closed. This allows for a more efficient assembly of the electrode arrangement and a galvanic cell comprising same than known processes, where an electrode compartment has to be built in several different process steps and from multiple parts.

According to a further preferred embodiment the porosity of at least one of said separator portions is greater than the porosity of the electrode protection portion. This allows for optimizing both the ion conductivity of the separator portions and the protection effect of the electrode protection portion, as the porosities can be selected individually for those different portions of the separator structure. In particular, according to a preferred variant the electrode protection portion is welded to the negative electrode, in particular to a current collector thereof.

Thus, during the welding process the material of the separator is at least partially melted at the at least one electrode protection portion, such that a pre-existing porous structure is destroyed. Accordingly, the separator structure can be formed from a single material having a desired porosity for the separator portions, and then by welding the porosity can be removed in the one or more electrode protection portions in order to enable those portions to efficiently prevent gas, in particular oxygenous gas, from reaching the electrode through the electrode protection portions. This allows for an easy and efficient production of the separator structure and its different portions from a single material.

According to a further preferred embodiment at least one of the separator portions is glued to the current collector of the electrode. Thus, the mechanical coupling between the separator and the electrode is improved. In a particular, the relative posi- tioning of the separator portions to the active areas of the electrode can thus be fixed and protected against shifting. In addition, the glue material, which may particularly comprise resin, may enhance the protection of the negative electrode from contact with undesired chemical components, in particular oxygenous gas, from outside the separator structure.

According to a further preferred embodiment at least one of the separator portions and the at least one electrode protection portion are made of a same material. In particular, the material may be selected such that it meets the requirements of both the separator portion and the electrode protection portion. Alternatively, the separator portion, the electrode protection portion, or both may be modified, such as to make it compliant with its respective requirements. A preferred variant of the latter is the welding of the electrode protection portions, as described above, whereby the porosity of at least one electrode protection portion is modified in order to make it impervious to oxygenous gas, in particular to molecular oxygen (esp. 0 ₂ ).

According to a further preferred embodiment at least one of the separator portions comprises a solid state electrolyte. In particular, this embodiment may be advantageous in that the separator and the electrolyte can be formed as one and in that the solid state electrolyte, e.g. a polymeric foil, can be selected such, that it prevents oxygen from the cathode side from reaching the anode side along a path running through a separator portion of the separator structure.

According to a further preferred embodiment the electrode arrangement further comprises an ionically conductive membrane being impervious to molecular oxygen and being positioned on at least on of the separator portions, such that molecular oxygen is substantially prevented from reaching the electrode through said separator portion. In particular, this embodiment may be advantageous, if a non-solid electrolyte is to be used in the cell or if the solid-state electrolyte does not sufficiently prevent oxygen from passing through it. Accordingly, this embodiment may be used alternatively to or cumulatively with a solid-state electrolyte. A second aspect of the invention is directed to a metal/oxygen galvanic cell, in particular for a vehicular battery, comprising an electrode arrangement according to the first aspect of the invention and a positive electrode adapted to support the chemical reduction of oxygen. The positive electrode is positioned in ionic continuity with a separator portion of the electrode arrangement. The term "ionic continuity", as used herein, relates to a connection between the positive electrode and the separator portion that allows for ionic transport between them, in particular of metal ions corresponding to the metal of the negative active metal electrode. Accordingly, the various embodiments and variants and advantages described above in relation to the first aspect of the invention apply similarly to the second aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages, features and applications of the present invention are provided in the following detailed description in connection with the figures, wherein:

Fig. 1 illustrates schematically a cross-section through an electrode stack of a conventional metal/oxygen galvanic cell and paths A and B along which oxygen might diffuse to the negative electrode;

Fig. 2 illustrates schematically potential detrimental effects caused by oxygen at the negative electrode of the electrode stack of Fig. 1 ;

Fig. 3 illustrates schematically the assembly of a negative electrode (anode) ar- rangement having a ring-shaped separator structure, according to a preferred embodiment of the present invention;

Fig. 4 illustrates schematically a cross-section through an electrode stack comprising an electrode arrangement according to a preferred embodiment of the present invention, wherein the separator structure has the shape of a ring or of a closed compartment; and Fig. 5 illustrates schematically a cross-section through a variant of the electrode stack of Fig. 4, wherein the current collector is protruding through an electrode protection portion of the separator structure. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

At first, reference is made to Fig. 1 , which shows a cross-section through an electrode stack of a conventional metal/oxygen galvanic cell. The cell comprises an active metal negative electrode (anode) 2 and a positive electrode (cathode) 3 adapted to support the reduction of oxygen. The two electrodes are separated by a separator 4, which may be in particular a suitable polymer membrane, as known in the art. Further, a current collector 5 is connected to the anode 2 and serves for the external electrical connection of the anode. In particular, the anode comprises lithium as active metal, which is known to be highly reactive with oxygen, in particular also with 0 ₂ . Paths A and B represent exemplarily two main diffusion paths along which oxygen can reach the anode, if no protective measures are present. Path A represents an "edge attack" of oxygen, where surface portions of the anode that are not in contact with either the separator 4 or the current collector 5 are exposed to oxygen. Path B represents a scenario where oxygen that is present at the cathode 3 (which is regularly the case during operation of a metal/oxygen cell) can diffuse through the separator 4 to the anode.

Fig. 2 shows certain detrimental effects that can occur, if the anode is exposed to oxygen. First of all, the exposed areas of the anode can react with oxygen to form lithium oxide in regions 6 on and below the surface of those exposed areas, thus reducing the effective width of the lithium anode. Furthermore, during a charge cycle when lithium ions are migrating through the separator 4 to the anode 2, lithium ions will be deposited preferably in the vicinity 7 of those oxidized regions 6, which may lead to increased growth of dendrites (symbolized by " ^* "). While a reduced effective anode width can reduce the energy storage capacity and power output level of the cell, dendrite growth may ultimately damage the separator and lead to a short circuit between the two electrodes, thus reducing the effective lifetime and reliability of the cell and ultimately destructing it. The term "energy storage capacity" as used herein relates to the electric energy capacity of a cell respectively battery, i.e. the amount of electrical energy (usually expressed in kWh) it can store when fully charged. The term "power output level", as used herein, relates to a power level, i.e. output energy per time (usually measured in Watt [W]), supplied by the cell at a given point in time. Fig. 3 illustrates schematically the assembly of a negative electrode (anode) arrangement 1 according to a preferred embodiment of the present invention. Electrode arrangement 1 comprises a current collector 8 on both sides of which active metal electrode portions 9a and 9b are disposed to form together a double-sided active metal negative electrode. Furthermore, a separator structure 10 is provided, which has the shape of a ring and comprises separator portions 10a and 10b at two opposing sides and two electrode protection portions 10c and 10d at two further opposing sides arranged between the separator portions 10a and 10b. During assembly of the electrode arrangement 1 , the separator structure 10 is positioned around the double-sided active metal negative electrode and the electrode protec- tion portions 10c and 10d are welded to the laterally protruding portions of the current collector 8.

A galvanic cell 14 with the resulting electrode arrangement 1 is shown in more detail in Fig. 4 in the form of a cross-section through its electrode arrangement which has the form of a stack. Starting with the central current collector 8, the active metal electrode portions 9a and 9b are disposed on both sides of it to form together the anode of the cell. The separator portions 10a and 10b of the separator structure 10 follow on both sides as next layers arranged on the exposed principal faces of the active metal electrode portions 9a and 9b. The electrode protection portions 10c and 10d of the separator structure 10 cover the edge portions of the double-sided anode in order to protect same from exposure to the environment, in particular to oxygen. The separator structure 10 is mechanically connected to the anode in two ways. On the one hand, the electrode protection portions 10c and 10d are welded to the laterally protruding portions of the current collector 8. On the other hand, glue is provided in at least some of the hollow regions between the separator structure 10 and the active metal electrode portions 9a and 9a. The glue serves for both strengthening the mechanical connection between the separator structure 10 and the anode and for providing additional protection of the anode against exposure to oxygen. Above each of the separator portions 10a and 10b an additional, optional, ionically conduc- tive membrane 13 is provided as an additional protection against a migration of oxygen through the respective separator portion 10a, 10b towards the anode. Finally, positive electrodes 1 1 a and 1 1 b are provided on each side of the stack to complete the galvanic cell. The positive electrodes use oxygen as chemically active compo- nent such that the cell 14 can operate as a metal/oxygen galvanic cell, in particular as a Li/oxygen cell.

Fig. 5 shows a further preferred embodiment of the present invention which is similar to that of Fig. 4, with the exception that the optional additional membrane 13 is missing and that the current collector 8 extends through the electrode protection portion 10c of the separator structure 10. Thus, the current collector 8 is accessible from outside of the electrode arrangement 1 and can be used to either serve directly as a terminal of the cell 14, or to be connected to an additional terminal portion (not drawn), such as a piece of metal.

While above at least one exemplary embodiment has been described, it has to be noted that a great number of variations thereto exists. It is also noted that the described exemplary embodiments represent only non-limiting examples and that it is not intended that the scope, the applicability or the configuration of the here- described apparatus and methods is thereby limited. Rather, the preceding description will provide the person skilled in the art with directions for the implementation of at least one of the exemplary embodiments, while it has to be appreciated that various different modifications of the functionality and the arrangement of the elements described in connection with the exemplary embodiments may be made without deviating from the scope of the invention as defined in the appended claims and its legal equivalents. LIST OF REFERENCE SIGNS

A, B oxygen diffusion paths

1 electrode arrangement

2 negative electrode (anode)

3 positive electrode (cathode)

4 separator

5 current collector of negative electrode

6 oxidized region of negative electrode

7 region of preferred lithium deposition

8 current collector of double-sided negative electrode

9a, b active metal electrode portions of double-sided negative electrode

10a, b separator portions of separator structure

10c, d electrode protection portions of separator structure

1 1 a, b positive electrodes (cathodes) of double-sided electrode stack

12 glue

13 membrane

14 metal/oxygen galvanic cell