TECHNICAL FIELD The present invention relates to the field of vehicular battery systems, in particular for electric or hybrid automobiles. Specifically, the invention is directed to a vehicular battery system having a metal/oxygen battery pack and a method of filling a gas reservoir of such vehicular battery system. 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 that use an anode made of or at least containing a suitable metal and an external cathode of ambient air or oxygen, typically with an aqueous or solid electrolyte. In particular, it is known to use zinc, aluminum or lithium as suitable metals for the anode. Instead, the anode can also 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. 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 taken 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 re-charged, oxygen is generated at the cathode and can be stored and then used again in a subsequent discharging cycle. US patent application publication US 2014/027261 1 A1 to Albertus et al. discloses a metal/oxygen battery for a motor vehicle, the battery having an oxygen supply system comprising a first oxygenated gas supply reservoir, a compressor with an outlet fluidly coupled to the first oxygenated gas supply reservoir, and a valve and pressure regulator fluidly coupled to the first oxygenated gas supply reservoir and to a positive electrode of the battery. The valve and pressure regulator is configured to place the first oxygenated gas supply reservoir in fluid communication with the positive electrode during a discharge cycle, and place the positive electrode in fluid communication with an inlet of the compressor during a charge cycle. SUMMARY OF THE INVENTION

Against this background, the present invention is directed to the problem of providing an improved metal/oxygen battery system for vehicles and a method of providing oxygen to it.

A solution to this problem is provided by the teaching of the appended independent claims, namely by a vehicular battery system according to claim 1 and a method of filling a gas reservoir of a vehicular battery system according to claim 10. Various preferred embodiments and further improvements of the invention are provided in the dependent claims.

The first aspect of the present invention is directed to a vehicular battery system. The battery system comprises a battery pack comprising at least one metal/oxygen galvanic cell and having an electrical energy capacity and a pressure gas reservoir. The gas reservoir is configured to store oxygenous gas under pressure and is operatively coupled to the at least one galvanic cell to provide oxygenous gas to at least one oxygen-type electrode of said at least one galvanic cell. The battery system further comprises a compressor operatively coupled to an oxygenous gas outlet of the battery pack and to an inlet of the gas reservoir. The compressor is configured to compress oxygenous gas effluent from said gas outlet and to fill the gas reservoir with said compressed oxygenous gas up to a maximum pressure substantially greater than atmospheric pressure. The volume of the gas reservoir is dimensioned such that, when it is filled with said oxygenous gas with a volume concentration of oxygen between 30 and 90 percent and at said maximum pressure, the electrochemical amount of the oxygen content of said oxygenous gas in the gas reservoir is equivalent to a fraction of more than 10% and less than 75% of the energy storage capacity of the battery pack. This means that the total electrical energy (e.g. in kWh) that the battery pack can deliver during a discharge cycle when consuming all of the oxygen present in the gas reservoir when fully filled at the maximum pressure, the gas having a volume concentration of oxygen between 30 and 90 percent, is a fraction between 10% and 75%, preferably between 33% and 50%, of that total energy storage capacity of the battery pack. The battery system of the first aspect may provide one or more of a number of advantages over existing batteries, as outlined below: First of all, the system is neither operated with ambient air, which typically has a volume concentration of oxygen of only approximately 21 %, nor with pure oxygen. Instead, the battery system uses oxygenous gas having a volume concentration of oxygen well above ambient air and well below pure oxygen. Thus, on the one hand, due to the increased oxygen concentration compared to air the battery system is significantly more power efficient than metal/air battery systems. On the other hand, the efforts needed for securely storing such oxygenous gas in a vehicle are significantly lower than they would have to be for pure oxygen gas. The latter is chemically highly reactive and even poison- ous, such that relatively heavy tanks are needed to store pure oxygen in a vehicle, where unforeseen impacts like in a car accident cannot be fully avoided.

Furthermore, the selection of the volume of the gas reservoir and the maximum pressure are optimized such that on the one hand enough oxygen is supplied so that the capacity provided by the battery pack allows for relatively great driving ranges before another charge cycle is necessary or an alternative motor being not powered by the battery system has to be used. On the other hand, because the reservoir, e.g. a high-pressure tank, only has to hold oxygenous gas corresponding to a fraction of the full capacity of the battery pack, the reservoir can be formed with re- duced size and weight relative to a full capacity reservoir, which in turn reduces the weight of the vehicle and thus extends its electric driving range.

In addition, the battery pack is typically operated under low Depth-of-Discharge (DOD) conditions, which typically contributes to a lowering or even prevention of battery degradation, because the more a metal/oxygen battery pack, in particular a Li/Oxygen battery pack, is operated under higher DOD conditions, the more does the thickness of discharge deposit (i.e. metal oxide such as Li0 ₂ ) within the battery cells increase, which reduces the electrical conductivity in affected areas of the battery pack.

Besides, advantages can also be achieved in relation to gas stations, which would represent a preferred kind of external source of oxygenous gas for refilling the battery system's reservoir, in particular during middle range or long range drives. Also the efforts to be taken by the gas station to securely store oxygenous gas having an oxygen concentration between 30 and 90 volume percent could be significantly lowered as compared to the case of pure oxygen and the gas stations' tanks could have a significantly lower volume/size than would be needed for (cleaned and dried) air containing a same amount of oxygen. The term "oxygenous gas", as used herein, relates to a gas that contains oxygen as one of its components. In particular, the oxygen component may comprise molecular oxygen such as 0 ₂ .

The term "metal/oxygen galvanic cell", as used herein relates to a battery cell of a metal/oxygen battery as described in detail above. To support the electrochemical reactions taking place in the battery, oxygen may be provided to the metal/oxygen galvanic cell, specifically to its cathode, either as pure oxygen or in the form of a gas mix containing oxygen as one of its components. The term "energy storage capacity", as used herein, relates to the electric energy capacity of a battery respectively battery pack, i.e. the amount of electrical energy (usually expressed in kWh) it can store. Frequently, a nominal energy storage capacity of a battery is provided by the manufacturer on an outer surface of the battery system and/or in related documentation. At least when the battery pack is new and has not suffered any degradation yet, the nominal energy storage capacity usually coincides, at least substantially, with the factual energy storage capacity of the battery pack. In the following, preferred embodiments, and variants thereof, of the vehicular battery system 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 a metal electrode, usually designated as "anode", of the at least one metal/oxygen galvanic cell contains a form of lithium. Lithium is a highly reactive metal and lithium/oxygen galvanic cells thus allow for a higher battery energy storage capacity than most other forms of metal/oxygen cells.

According to a further preferred embodiment the gas reservoir comprises an inlet for receiving oxygenous gas from a battery-external gas source. Thus, the gas reservoir, in addition to storing oxygen generated at the cathode during charging cycles of the battery pack, can also be filled with oxygenous gas from a battery external gas source, such as a gas station. This can be used to extend the driving range of the electrically powered vehicle beyond the driving range enabled by the initial filling of the battery system's gas reservoir with oxygenous gas. In particular, the inlet may be sealable or a valve may be arranged to stop gas from flowing back from the reservoir through the inlet.

According to a further preferred embodiment the compressor comprises an inlet for receiving oxygenous gas from a battery-external gas source and is further configured to compress said received oxygenous gas and to fill the gas reservoir with said compressed oxygenous gas. Thus, the compressor may not only be used for putting oxygen created at the cathode during a charging cycle under pressure for storing it in the battery system's gas reservoir, but it can also take-in gas from an external gas source, such as a gas station, to fill it under pressure into the battery system's gas reservoir. The filling of the gas reservoir may be performed via a direct connection of the compressor to the reservoir or indirectly via some inter-mediate device, such as a pressure regulator or a valve, for example. Again, a sealing and/or a valve may be provided to stop gas from leaving the compressor through said inlet.

According to a further preferred embodiment the electrical energy storage capacity of the battery pack is at least 50 kWh, preferably at least 70 kWh. Such energy stor- age capacities are particularly suitable for typical electric or hybrid vehicles, such as cars, and represent battery capacities that allow the battery system to serve as the main or even single electric energy storage of a fully-electrical vehicle in view of range expectations of above 100km, preferably of at least several hundred kilometers.

According to a further preferred embodiment the electrochemical amount of the oxygen content of said oxygenous gas in the gas reservoir is equivalent to a fraction of more than 25% and less than 60%, preferably equivalent to approximately 50%, of the energy storage capacity of the battery pack. These values for the fraction repre- sent a particularly advantageous range, where an optimized balancing between the stored oxygen content and thus effective energy storage capacity on the one hand and the size and weight of the gas reservoir on the other hand is achievable.

According to a further preferred embodiment the gas reservoir is dimensioned to store a gas volume of 120 liters or less, preferably 100 liters or less. Accordingly, the size of the gas reservoir is also suitable for integration into smaller electric or hybrid vehicles, such as small city cars, which have only limited space available.

According to further preferred embodiments said maximum gas pressure is in the range of 10 MPa to 100 MPa, preferably in the range of 30 MPa to 80 MPa, more preferably in the range of 60 MPa to 80 MPa. These ranges represent preferred ranges, because on the one hand the oxygen content of the gas reservoir can be made sufficiently high to achieve reasonable driving ranges for the vehicle, and on the other hand such pressures can be rather easily accommodated by standard gas tanks without a need for extreme pressure containment capability. This supports low dimensions and cost of such tanks. In particular, pressures towards the higher end of the provided ranges may be preferable in view of extended driving ranges, and in particularly when small dimensions of the gas reservoir are important, such as for smaller vehicles.

According to a further preferred embodiment the vehicular battery system further comprises a second battery pack which is not of the metal/oxygen type. Thus, a battery system based on different battery technologies can be provided, which combines the advantages of different battery types in one system. In particular, the sec- ond battery pack may be a conventional lithium-ion type battery as is common in today's electrical and hybrid cars. In such a combined system the metal/oxygen battery pack provides its above-mentioned advantages, when oxygen supply is available, while the conventional battery system is still available to supply electricity, if the metal/oxygen type battery pack runs out of oxygen without adequate supply of ox- ygenous gas being available.

A second aspect of the invention is directed to a method of filling a gas reservoir of a vehicular battery system, in particular of one according to the first aspect. The vehicular battery system comprises a battery pack comprising at least one met- al/oxygen galvanic cell and having an energy storage capacity. Furthermore, the battery system comprises a pressure gas reservoir configured to store oxygenous gas and being operatively coupled to the at least one metal/oxygen galvanic cell. The method comprises a step of filling the gas reservoir up to a maximum pressure with an oxygenous gas having a volume concentration of oxygen between 30 and 90 percent, wherein the maximum pressure is selected such that the electrochemical amount of the oxygen content of said oxygenous gas in the gas reservoir at said maximum pressure is equivalent to a fraction of more than 10% and less than 75%, preferably between 33% and 50% of the energy storage capacity of the battery pack.

According to a preferred embodiment the method further comprises a step of operatively coupling a compressor to an oxygenous gas outlet of the battery pack and to an inlet of the gas reservoir, and operating the compressor to compress oxygenous gas effluent from said gas outlet to fill the gas reservoir with said compressed oxyg- enous gas. Thus, not only oxygen initially contained in the gas reservoir can be used to produce electricity, but also oxygen that is recuperated during a charging cycle at the cathode side of the battery can be guided back to the reservoir under pressure such that it is available for a next discharging cycle. As a result the driving range of the vehicle can be extended.

Furthermore, 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 vehicular battery system according to a preferred embodiment of the present invention; and

Fig. 2 illustrates a flow chart of a method of filling a gas reservoir of a vehicular battery system, according to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

At first, reference is made to Fig. 1 , which shows a vehicular battery system 1 according to a preferred embodiment of the present invention. The battery system 1 comprises a metal/oxygen-type battery pack 2, a gas reservoir 3 for holding oxygenous gas under pressure and a compressor 4 for compressing such gas and filling the gas reservoir 3 with it. The battery pack 2 comprises at least one battery cell (as drawn) having an anode A made from or at least containing a suitable metal, preferably lithium, and a cathode C which uses oxygen from the oxygenous gas as its electrochemical component. To that purpose, the cathode C comprises a porous material, preferably mesoporous carbon with metal catalysts, and is electrically connected to a current collector CC for conducting charges to or from the cathode C to a cathode connector of the battery cell. The anode side of the cathode side of the battery cell separated by an electrolyte E, preferably in solid form (as drawn), such as a ceramic, glass, or glass-ceramic electrolyte. Alternatively, also other forms, in particular apriotic, aqueous or mixed aqueous/aprotic forms, are possible. In practice, the battery pack will typically comprise a plurality of similar battery cells (not drawn) of the type described above, which are connected in series or parallel, or some combination of it. The battery pack 2 has a energy storage capacity, which defines the amount of electrical energy the battery pack 2 can store.

The volume of the gas reservoir 3 is dimensioned such that, when it is filled with said oxygenous gas having a volume concentration of oxygen between 30 and 90 percent and at said maximum pressure, the electrochemical amount of the oxygen content of said oxygenous gas in the gas reservoir 3 is equivalent to a fraction of more than 25% and less than 75% of the energy storage capacity of the battery pack 2. In one exemplary embodiment the gas storage volume of the gas reservoir 3 is approximately 100 L, the energy storage capacity is approximately 70 kWh and the maximum pressure is selected such that electrochemical amount of the oxygen stored in the filled gas reservoir 3 is equivalent to approximately half of the energy storage capacity, i.e. to approximately 35 kWh. A typical maximum pressure for this scenario is 70 MPa.

The cathode C of the battery cell battery pack 2 is fluidly connected to an inlet of the compressor 4 such that a flow of gas from the cathode C to the compressor can be controlled by a valve V2. An outlet of the compressor 4 is fluidly connected to the gas reservoir 3 and a further valve V3 is provided to control a flow of gas from the compressor 4 to the gas reservoir 3. The cathode C of the battery pack 2 is also fluidly connected to the gas reservoir 3 and a further valve V1 is provided at this connection to control the flow of gas from the gas reservoir 3 to the cathode C. Furthermore, in order to allow for a filling of the gas reservoir 3 with oxygenous gas from an external source, inlets 5 and 6 with respective valves V4 and V5 are provid- ed at the compressor 4 respectively the gas reservoir 3. In addition, the inlets 5 and 6 can each be sealable by a suitable sealing means that can be opened during the filling process, such as a gas-tight cap. While inlet 6 can be used to supply oxygenous gas that is already compressed to a target pressure (defined maximum pressure), inlet 5 can be used to such gas at a lower pressure which will then be com- pressed to the target pressure by the compressor before it is stored into the gas reservoir 3.

According to a preferred variant, the battery system 1 also comprises a second bat- tery pack 7 which, unlike battery pack 2, is not of the metal/oxygen type and has an anode A2, a cathode C2 and a separator S2 separating the anode side from the cathode side, as is usual for many types of battery cells including lithium-ion cells.

During a discharging cycle of the battery system 1 , valve V1 is open, while the other valves are closed. Oxygenous gas is supplied from the gas reservoir 3 to the cathode C of the battery pack 2, which then produces electrical energy from known electrochemical reactions which consume oxygen present in the oxygenous gas.

During a charging cycle, on the other hand, valve V1 is closed and valves V2 and V3 are opened and the compressor is operated to receive oxygen generated at the cathode C, to compress it to the target pressure, and to resupply it to the gas reservoir 3.

Reference is now also made to Fig. 2, which illustrates a preferred embodiment of the method according to the second aspect of the invention. The method is described in connection with the exemplary vehicular battery system of Fig. 1. In a first step S1 the compressor inlet 5 is connected to an external source of oxygenous gas having a volume concentration of oxygen (0 ₂ ) of approximately 50%. In a second step S2, the compressor 4 compresses the gas received from the external source up to a set maximum pressure of 70 MPa, while filling the 100L gas reservoir 3 of the battery system 1 with said compressed gas. When in a further step S3 the vehicle is operated and consumes electrical energy stored in the battery pack, the latter is discharged while consuming oxygen contained in the gas flowing from the gas reservoir 3 to the cathode C of the battery pack 2. In a subsequent charging cycle the battery pack 2 is recharged by applying a suitable voltage to its electrical connectors (anode and cathode) and oxygen is generated at the cathode from respective electrochemical reactions taking place in the battery pack 2. Thus, when in a further step S5 a flow of this oxygen to the inlet of the compressor 4 is enabled and in a step S6 the compressor 4 is operated to compress the received gas up to the target pressure, the oxygen is pumped back to the gas reservoir 3 and is available again for consumption in a subsequent discharge cycle. Optionally, the gas reservoir 3 may in addition be filled again according to step S1 from an external gas source.

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

1 vehicular battery system

2 metal/oxygen battery pack

3 gas reservoir

4 compressor

5 compressor inlet (for receiving gas from battery external source)

6 gas reservoir inlet (for receiving gas from battery external source)

7 second battery pack

V1 -V5 valves

A anode of metal/oxygen battery pack

C cathode of metal/oxygen battery pack

CC current collector of the cathode C

S separator of metal/oxygen battery pack

A1 anode of second battery pack

C1 cathode of second battery pack

S1 separator of second battery pack