FIELD OF THE INVENTION

The present invention relates to an electrochemical cell with an exceptionally high energy density and long operating life. In particular, it refers to a high temperature metal-air battery which includes a metal negative electrode, an air positive electrode and a solid oxide electrolyte being oxygen ion conductor. The invention further relates to particular designs of modular electrochemical cells.

BACKGROUND

Rechargeable, or secondary, batteries have been widely used for electronics, stationary and automotive applications. They have been identified as one of the most important enabling technologies in the 21 ^st century due to their significant roles in a green and sustainable energy future. There are many types of rechargeable batteries such as lead acid, nickel cadmium, nickel metal hydride, vanadium redox flow, sodium sulfur and lithium ion batteries. Among them, Li-based battery is one of the most advanced and has found widespread applications in the past twenty years. However, the current batteries are not keeping up with the demand in terms of energy, power, safety, life and cost. Future batteries will need new chemistries, innovative material concepts, and revolutionary cell design and manufacturing techniques.

One of the potential future systems is metal-air battery such as Li-air or Zn-air, although Zn-air is usually not rechargeable. A lithium-air battery typically includes a lithium- metal negative electrode (or anode), a positive electrode (or cathode) where reaction with oxygen occurs (e.g., from air, sometimes referred to as an "oxygen positive electrode"), and an electrolyte or other ion conducting medium in fluid communication with both the positive electrode and the negative electrode. Typically, the lithium and oxygen react to produce lithium oxides.

During discharge, lithium ions flow from the negative electrode through the electrolyte and/or the ion conducting medium to react with oxygen at the positive electrode to form product such as lithium oxide (Li0 ₂ ) or lithium peroxide (Li ₂ 0 ₂ ) which deposits at the positive electrode. This is coupled with the flow of electrons from the negative to the positive electrode through a load circuit, which can be harnessed to produce power. The system has a discharge voltage around 2.7 V. Theoretically, an energy density of more than 11000 Wh/kg can be given, practically however, it will be much lower due to many issues.

One of the major issues with conventional Li-air battery is the poor recyclability and utilization of lithium anode. After decades of research and development, only a few hundred cycles have been achieved for lithium rechargeable batteries. On repeated stripping from and re-deposition of lithium on the anode, a high surface area dendrite tends to form. This not only reduces the cycling efficiency, because of the reaction with electrolyte to form resistive solid-electrolyte-interface (SEI) layers, but also poses a serious safety problem due to the possibility of thermal runaway. Unless the lithium cycling efficiency is improved, the conventional Li-air batteries will have a hard time competing with Li-ion batteries. It has been estimated that with a three-fold excess of lithium, the volumetric energy density of Li-air battery is even slightly lower than that of current Li-ion battery. Therefore, a tremendous amount of research work has been done by research groups to improve the lithium electrode performance (Kraytsberg, A. et al, Journal of Power Sources, 2011, 196, 886-893; Girishkumar, G. et al, J. Phys. Chem., 2010, 1, 2193-2203). One of the approaches is to restrict the contact between the liquid electrolyte with the lithium anode. Visco et al. (US2007/117007; US2007/172739; WO2007/062220; WO2007/075867 and WO2010/005686) utilized a protective membrane architecture that conducts Li ions but is imperious to electrolyte, moisture and air. Still, it would be a huge challenge to make the conventional Li metal anode to cycle thousands of cycles for auto and utility applications. Another major issue with conventional Li-air battery with organic electrolyte is the air cathode. As the lithium oxide reaction products are formed, they often block the pores of the cathode and effectively stop the electrode reaction. As a result, most of the cell voltage drop occurs at the air cathode (Kraytsberg, A. et al, Journal of Power Sources, 2011, 196, 886-893). To improve air cathode performance, researchers have been developing advanced catalysts in order to reduce the cathode overpotential and increase reaction reversibility. Attention is also given to the cathode architecture to maintain adequate transport of oxygen and Li ions toward the reaction sites and at the same time provide enough room for accommodating solid oxide products (Kraytsberg, A. et al, Journal of Power Sources, 2011, 196, 886-893).

Due to the design, architecture and materials used for the different components configuring the Li-air batteries described in the state of the art, they are not still competitive with other conventional lithium batteries, and therefore their use for auto and utility applications has not been realized. Thus, it is desirable to develop new battery systems that mainly address the low efficiency of the metal anode and the blockage of pores in the air cathode electrode.

On the other hand, solid oxide fuel cells (SOFC) have been developed as a promising technology that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen. The major feature of a solid oxide fuel cell is its solid electrolyte which is an oxygen conductor. It also has a cathode and an anode where half cell reactions take place. At the cathode, oxygen is reduced to oxygen ions that are then transported to the anode through the solid electrolyte under electric load. At the anode, oxygen reacts with hydrogen-containing fuels to form water.

One particular solid oxide fuel cell to be mentioned is the one that contains a liquid tin anode for direct power generation from coal or JP8 fuels (Tao, T. in SOFC-IX, S.C. Singhal and J. Mizusaki Editors, Quebec City, Canada, 2005, pp 353-362; Tao, T. et al, ECS Transactions, 2007, 12, 681-690; McPhee, W.A.G. et al, Energy & Fuels, 2009, 23, 5036-5041; Koslowske, M.T. et al, Advances in Solid Oxide Fuel Cells V, 2009, 30; Tao, T. et al, ECS Transactions, 2009, 25, 1115-1124). In such a system, tin is used as a liquid layer that fully covers the active oxygen exchange area between the electrolyte and the anode. In particular, the liquid anode participates as an intermediary for the oxidation of fuel delivered to the fuel cell. The anode serves as a buffer against fuel contaminants, as it blocks the transport of insoluble or slag-forming constituents to the electrolyte and impedes the transport of soluble fuel contaminants, thereby reducing the rate of reactions between contaminant and the electrolyte. It is also postulated that the usage efficiency of electrolyte surface is improved over existing porous solid anode technology because the liquid layer fully covers the electrolyte. Hence, oxygen reactions can be expected to occur over the full surface of the electrolyte when using a liquid anode, instead of only around triple phase boundaries between the fuel, anode and electrolyte. Jayakumar et al. (J. Electrochem. Soc, 2010, 157(3), B365-B369) reported such a device wherein Sn and Bi were examined at 973 and 1073 K for use as anodes in solid oxide fuel cells with yttria-stabilized zirconia (YSZ) electrolyte. Although open- circuit voltages were close to that expected based on their oxidation thermodynamics, their intention was to use molten metal as a medium to transfer oxygen to solid fuels such as coal. Therefore, these systems were still used as energy conversion devices.

Documents WO03/001617 and WOO 1/80335 describe rechargeable devices having a dual-mode capability, as said devices can operate as a fuel cell and as a battery, and include a liquid metal anode, an electrolyte and a cathode. However, they need to be recharged with a chemical source in order to operate in dual form and only act as a battery providing electric energy for a short period of time when the fuel supply (chemical source) is exhausted or interrupted. Therefore, these systems cannot be considered as an electric energy storage device, and even less with improved properties with respect to metal-air batteries currently available, as intended in this invention.

BRIEF DESCRIPTION OF THE INVENTION

The authors of the present invention have found that a metal-air battery in which oxygen ions diffuse through a solid oxide electrolyte between electrodes and wherein the metal anode works in a melt or semi-melt state, allows the electrochemical reactions to take place and overcomes the problems derived from the use of conventional metal- air batteries of the art.

The metal-air battery of the invention combines the technology of conventional metal- air batteries with that of solid oxide fuel cells to provide a high energy system for many utility applications. This battery operates at high temperature, typically between 300- 1000°C.

Particularly, it uses the cathode and the electrolyte of solid oxide fuel cells (SOFC) and a stored metal-containing fuel as in the metal-air batteries. However, the electrochemical reactions taken place in the new battery system are completely different from those in metal-air batteries or SOFC as shown below. Contrary to other electrochemical devices that combine the technology of metal-air batteries with that of solid oxide fuel cells, the battery of the invention is only electrically rechargeable and used as an electric energy storage device.

More particularly, the invention provides a metal-air battery system with higher energy density than lithium-ion batteries due to the high efficiency of metal utilization. In addition, the use of a solid oxide electrolyte significantly reduces the portion of electrolyte in the system when compared to other metal-air batteries in which up to 70% of the weight is electrolyte. This also results in higher energy densities.

As an additional advantage, it has a longer cycle life than conventional Li-air cells since no lithium dendrite is formed. The absence of dendrite formation avoids also short circuits and thermal runaway, thus making the metal-air battery system of the invention safer than conventional lithium-based batteries.

In particular, the metal-air battery of the invention can be electrically charged and discharged for more than 1000 cycles without any type of fuel or chemical source, thus producing a Coulombic efficiency of approximately 1.

Due to the high energy density, as a result of high loading and utilization of metal anode, and long cycle life, the cost per kWh energy is expected to be comparable to, or lower than, the conventional Li- ion or Li-air batteries.

The metal-air battery of the invention is also rechargeable and thus can be used as an energy storage device.

Compared to solid oxide fuel cells, it uses a stored metal fuel instead of conventional gaseous fuels and therefore it does not need a fuel distribution system. In addition, operating temperature can be potentially lowered and the power density per unit area is much higher than traditional solid-oxide fuel cells. Thus, a first aspect of the present invention refers to a method for electric energy storage, said method comprising: a) providing a metal-air battery comprising:

a.1) a metal-containing negative electrode;

a.2) a porous positive air electrode;

a.3) an oxygen ion conductor electrolyte; and a.4) optionally, a ceramic layer located between the porous positive air electrode and the oxygen ion conductor electrolyte,

b) connecting the metal-air battery to an electric energy source so as said metal air battery is electrically recharged,

wherein said method excludes the connection of the metal-air battery to a chemical energy source,

and wherein the metal-air battery works at temperatures ranging from about 300 to about 1.000°C.

A second aspect of the present invention refers to a metal-air battery comprising: a) a metal-containing negative electrode,

b) a porous positive air electrode;

c) an oxygen ion conductor electrolyte; and

d) optionally, a ceramic layer located between the porous positive air electrode and the oxygen ion conductor electrolyte, wherein the electrolyte is in contact with the metal-containing negative electrode at one side and with the porous positive air electrode on the other side, or when the ceramic layer is present, the solid oxide electrolyte is in contact with the metal-containing negative electrode at one side and with the ceramic layer on the other side, wherein the metal-containing electrode is enclosed in a cover case to isolate the electrode from any gas or chemical source; wherein the metal-air battery is only rechargeable by electricity and operates at temperatures ranging from about 300 to about 1000°C.

In another aspect, the present invention relates to a module system which comprises at least two stacked metal-air batteries as defined above.

Another aspect of the present invention refers to a method for manufacturing the metal- air battery of the present invention, said method comprises: a) providing a solid oxide electrolyte as defined above;

b) placing the porous positive electrode and the metal-containing negative electrode on each side of the solid oxide electrolyte; c) enclosing the metal-containing negative electrode in the cover case.

Another aspect of the present invention refers to a method for electric energy storage, said method comprising: a) providing a metal-air battery as defined above; and

b) connecting the metal-air battery to an electric energy source so as said metal air battery is electrically recharged, wherein said method excludes the connection of the metal-air battery to a chemical energy source, and wherein the metal-air battery works at temperatures ranging from about 300 to about 1.000°C.

Another aspect of the invention refers to the use of the metal-air battery as defined above as an electric power source for utility applications as well as a power source for automotive and power electronic applications.

Finally, another aspect of the invention refers to the use of the metal-air battery as defined above as an electric energy storage device for utility applications as well as for automotive and power electronic applications.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1.Illustration of a high-temperature metal-air cell. Figure 2. Illustration of two options for planar design of a high-temperature metal-air battery.

Figure 3. Illustration of a tubular design of a high-temperature metal-air battery.

Figure 4. Configurations of metal-air battery cells as a function of the material providing mechanical support: (a) electrolyte; (b) anode; (c) inert substrate. Figure 5. Examples on interconnections in planar configuration: a) cells connected in parallel, vertical option; b) cells connected in series, horizontal option. Figure 6. Examples on interconnections in tubular configuration: a) cells connected in parallel within a bundle; b) several bundles connected into a short stack to form a module system.

Figure 7. a) Discharging and charging curves of a tin-air battery at 800°C; b) Efficiency curve of the tin-air battery at 800°C

Figure 8. a) Discharging and charging curves of a tin-air battery using planar design single repeating unit at 800°C (Inset corresponds to the expanded view of the cycling curves); b) Efficiency curve of the tin-air battery conceptual cell at 800°C. Cell fabrication under protective gas Figure 9. a) Discharging and charging curves of a tin-air battery using planar design single repeating unit at 800°C (Inset corresponds to the expanded view of the cycling curves); b) Efficiency curve of the tin-air battery conceptual cell at 800°C. Cell fabrication under air.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to some specific embodiments of the invention including the best modes contemplated by the inventors for carrying out the invention. Examples of these specific embodiments are illustrated in the accompanying drawings. While the invention is described in conjunction with these specific embodiments, it will be understood that it is not intended to limit the invention to the described embodiments. On the contrary, it is intended to cover alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. The present invention may be practiced without some or all of these specific details.

In this specification and the appended claims, the singular forms "a", "an" and "the" include plural reference unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this invention belongs. In a first aspect, the present invention provides a method for electric energy storage. This method is based on the use of a low cost, better safety and higher energy density metal-air battery for utility and other applications, as compared to Li-ion and metal-air batteries of the prior art. In particular, the method of the invention uses a multilayer system comprising a porous air electrode, designed for high catalytic activity towards oxygen reduction/evolution and mixed ionic electronic conductivity (or mostly electronic) over a wide temperature operation range (i.e. 300-1000°C), a metal electrode and an oxygen ion conductor as electrolyte.

Figure 1 illustrates the concept of the method of the invention using a high-temperature metal-air battery. Unlike conventional Li-air batteries where lithium diffuses through the electrolyte between electrodes, the battery used in the method of the invention transports oxygen ions which diffuse through a solid electrolyte between electrodes. The chemical reactions generated during the electrochemical operation of the metal-air battery are shown below: Anode: M + xO ⁼ -» MO _x + 2xe ^~

Cathode: l/2x0 ₂ (g) + 2xe ^~ <-» xCf

Overall: M + l/2x0 ₂ (g) MO _:

More specifically, on discharging, oxygen gas is reduced and it dissociates into oxygen ions at the cathode. The oxygen ions diffuse through a solid oxide electrolyte from the cathode to the anode and there react with the metal and form a metal oxide at the anode. On charging, the metal oxide decomposes and release oxygen ions at the anode which then diffuse to the cathode to form oxygen gas.

Therefore, the electrochemical reaction is reversible and metal oxides formed during discharge can be regenerated into metal when current is reversed, giving rise to a rechargeable system that can be used over many cycles.

The metal-air battery is only rechargeable electrically. Therefore, upon connection to an electric energy source, the metal-air battery is electrically recharged and ready for its use. The method of the invention only contemplates the metal-air battery to be recharged electrically and, therefore, excludes the connection of the metal-air battery to any other source, such as a chemical source.

A detailed explanation of the metal-air battery components are described below.

Metal-containing negative electrode

The metal-containing negative electrode constitutes the anode of the battery system and comprises, as the main component, at least one metal, metal alloy, or metal containing compound either in a molten, solid or semi-solid state. In a particular embodiment of this invention, the metal is selected from alkali elements, alkaline earth metal elements, elements from Periodic Table Group VIB, Group VIIB, Group VIIIB, Group IB, Group IIB, Group IIIA, Group IVA. Preferably, the metal is selected from tin, bismuth, gallium, iron, copper, cobalt, nickel, lead, magnesium, zinc, antimony, indium, sodium, lithium, tungsten, molybdenum, cerium, titanium, manganese, niobium, vanadium and aluminum. More preferably is tin, bismuth, gallium, zinc, sodium, lithium and aluminum, even more preferably the metal is selected form tin, lithium and zinc.

Further, in another embodiment of this invention, mixture of metals can also be used as electrode materials in the metal-air battery system. The mixture of metals can be made from the same elements as mentioned above, for example tin, bismuth, gallium, iron, copper, cobalt, nickel, lead, magnesium, zinc, antimony, indium, sodium, lithium, tungsten, molybdenum, cerium, titanium, manganese, niobium, vanadium and aluminum. The mixture of metals refers to homogeneous mixture of metals and/or inhomogeneous mixture of metals, such as heterogeneous mixture, doped metals, and other forms of materials with more than one metal species such as a mixture of metals either in the form of particles, pressed particles or sintered particles.

Additions in small fractions of the metals described above can also be used to improve wetting properties or to adjust melting points and therefore carefully control the activity of metals. Further, in another embodiment of the invention alloy metal materials can also be used as electrode materials in the metal-air battery system. The alloy metal materials can be made from the same elements as mentioned above, for example tin, bismuth, gallium, iron, copper, cobalt, nickel, lead, magnesium, zinc, antimony, indium, sodium, lithium and aluminum.

Further, in another embodiment of this invention, the electrode-containing metal can be a combination of above mentioned metals, mixture of metals, alloy metal materials, and metal containing compounds.

Preferably, the metal, metal alloy, or metal-containing compound is in a molten or liquid state. This allows a better utilization of the metal, resulting in higher practical energy density. Also, it reduces the mechanical degradation and improves the cycle life of the anode due to the self-healing function of the liquid.

In another preferred embodiment, the metal, metal alloy, or metal-containing compound can be in the form of fine metal powders which can be interdispersed with a conductive material to increase the reaction sites and improve fuel utilization.

In another embodiment, the metal, metal alloy, or metal-containing compound can be in the form of a solid metal and comprises a molten salt carrying oxygen from the electrolyte interface to the metal.

In another embodiment, the metal-containing compound can be in the form of a redox couple such as the Na ₂ S/Na ₂ S04 system.

In another particular embodiment, the metal-containing electrode further comprises a mixed ion-electron or ion-conducting porous matrix which enhances active sites of the electrode for the electrochemical reaction, thus increasing system performance and efficiency. It should be porous in order to allow the metallic fuel to be loaded. Preferably, the mixed ion-electron or ion-conducting porous matrix contains interconnected fine powders or fibers.

Therefore, in a preferred embodiment, the metal-containing negative electrode comprises a mixture of:

1) a metal, metal alloy, or metal-containing compound powder, and

2) a mixed ion-electron or pure ion-conducting powder or fiber. In another preferred embodiment, the metal negative electrode comprises a mixture of:

1) a liquid metal, metal alloy, or metal-containing compound and

2) a mixed ion-electron or pure ion-conducting powder or fiber.

Alternatively, the mixed ion-electron or ion-conducting porous matrix forms a framework wherein which the metal, metal alloy or metal-containing compound is contained.

In a preferred embodiment, the ion-conducting porous matrix is composed of a fluorite- related oxygen ion conductor which comprises a compound of formula (I):

[(A ₁ _ _x _ _y A' _x A" _y )O _s ] ₁ _ _z [(B ₁ _ _v B' _v )0 ₂ ] _z _d (Formula I) wherein:

A, A' and A" are different from each other, and A, A' and A" each independently comprises at least one mono-, di-, or trivalent element selected from yttrium (Y), sodium (Na), scandium (Sc), samarium (Sm), gadolinium (Gd), cerium (Ce), calcium (Ca), magnesium (Mg), aluminum (Al) and bismuth (Bi);

B and B' are different from each other, and B and B' each independently comprises one cation selected from zirconium (Zr) and cerium (Ce). v, x, y and z have values from 0 to 1, with the proviso that x+y is less or equal to 1; s has a value ranging from 0.5 to 1.5; and d corresponds to site deviations from stoichiometry.

In formula I, A' and A" each designates an element which substitutes for A on a portion of the A sites in the metal oxides. In addition, B' substitutes for B on a portion of the B sites in the metal oxide.

The A site in the material of formula I may include at least one metal element selected from yttrium (Y), scandium (Sc), samarium (Sm), gadolinium (Gd), cerium (Ce), calcium (Ca), magnesium (Mg), aluminum (Al) and bismuth (Bi). A' and A" which are substituted for A as doping elements, may include an element different from A, for example, at least one element selected from the group consisting of yttrium (Y), sodium (Na), scandium (Sc), samarium (Sm), gadolinium (Gd), cerium (Ce), calcium (Ca), magnesium (Mg), aluminum (Al) and bismuth (Bi).

The B site in the material of formula I may include one metal element selected from zirconium (Zr) and cerium (Ce). B', which is substituted for B as a doping element, may include an element different from B, for example, one element selected from zirconium (Zr) and cerium (Ce).

The compounds of formula (I) may include minor additions of additives, more particularly metal oxides, such as CaO, Na ₂ 0, Ti0 ₂ , A1 ₂ 0 ₃ , Mn ₂ 0 ₃ , Y ₂ 0 ₃ , Si0 ₂ , Fe ₂ 0 ₃ and Ce0 ₂ . Examples of fluorite-related oxygen ion conductor include Y ₂ 0 ₃ doped-Zr0 ₂ , CaO doped-Zr0 ₂ , Gd ₂ 0 ₃ doped-Ce0 ₂ , Sc ₂ 0 ₃ doped-Zr0 ₂ , Bi ₂ 0 ₃ , Y ₂ 0 ₃ doped-Bi ₂ 0 ₃ .

The fluorite-related oxygen ion conductor can also be combined with metals having melting points higher than 900°C, such as nickel, iron or copper in order to provide a mixed ion-electron conducting porous matrix. Alternatively, the fluorite-related oxygen ion conductor can also be combined with or replaced by a perovskite type transition metal oxide to provide a mixed ion-electron conducting porous matrix. The perovskite type transition metal oxide is an oxide having the same crystalline structure as the mineral CaTi0 ₃ , which is usually expressed as AB0 ₃ in which A and B sites of the metal oxide are each substituted with a different chemical element.

More particularly, a perovskite type transition metal oxide has a formula (II):

(A ₁ _ _x A' _x ) ₁ _ _a (B ₁ _ _y B' _y )i-b0 ₃ _d (Formula II) wherein:

A and A' are different from each other and A and A' each independently comprises at least one element selected from the group consisting of strontium (Sr), yttrium (Y), samarium (Sm), cerium (Ce), bismuth (Bi), lanthanum (La), gadolinium (Gd), neodymium (Nd), praseodymium (Pr), calcium (Ca), barium (Ba), magnesium (Mg) and lead (Pb).; B and B' are different from each other, and B and B' each independently comprises at least one element selected from the group consisting of transition metal ions such as titanium (Ti), vanadium (V), manganese (Mn), cobalt (Co), iron (Fe), chromium (Cr), nickel (Ni) or copper (Cu); and gallium (Ga); x is between 0 and 1 ; and y is between 0 and 1 , a, b and d correspond to site deviations from stoichiometry

In formula II, A' designates an element which substitutes for A on a portion of the A sites in the metal oxides so as to produce an n-type material, improving the electrical conductivity of the metal oxide. In addition, B' substitutes for B on a portion of the B sites in the metal oxide so as to produce a p-type material, and thus atoms of the B site are easily varied to increase oxygen vacancy concentration. The increase in the oxygen vacancy concentration provides ionic conductivity to a perovskite type material, which increases the transport of oxygen ions to or from the triple-phase boundary in which an electrochemical reaction occurs. Specific preparation methods can be used to induce site deficiency and therefore to improve electrochemical activity.

The A site in the material of formula II may include at least one metal element selected from strontium (Sr), yttrium (Y), samarium (Sm), cerium (Ce), bismuth (Bi), lanthanum (La), gadolinium (Gd), neodymium (Nd), praseodymium (Pr), calcium (Ca), barium (Ba), magnesium (Mg) and lead (Pb). A', which is substituted for A as a doping element, may include an electron-donor different from A, for example, at least one transition metal. For example, if the A site includes Sr, A' may include at least one element selected from the group consisting of yttrium (Y), samarium (Sm), lanthanum (La), gadolinium (Gd), neodymium (Nd), praseodymium (Pr), calcium (Ca), magnesium (Mg) barium (Ba).

The B site in the material of formula II may include at least one metal element selected from titanium (Ti), manganese, (Mn), cobalt (Co), iron (Fe), nickel (Ni), chromium (Cr), vanadium (V), gallium (Ga) and copper (Cu). B', which is substituted for B as a doping element, may include an electron-acceptor different from B, for example, at least one transition metal or at least one element selected from the group consisting of titanium (Ti), manganese (Mn), cobalt (Co), iron (Fe), chromium (Cr), gallium (Ga), nickel (Ni) and vanadium (V).

Examples of these perovskite type transition metal oxides are (La,Sr)Ti0 ₃ , (Y,Sr)Ti0 ₃ , (LaSr)Cr0 ₃ , (La,Sr)(Cr,V)0 ₃ , (La,Sr)(Ga,Mn)0 ₃ , (La,Sr,Ca)(Mn,Cr)0 ₃ , (La,Sr)(Ti,Mn)0 ₃ .

An example of a mixed ion-electron conducting material is a composite type perovskite- fluorite such as (SrLa)Ti0 ₃ - Ce(La)0 ₂ _d.

The metal-containing electrode or anode of the metal-air battery system typically does not need any catalyst because the electrochemical reaction is carried out at the surface of the metal itself.

In a particular embodiment, the metal electrode is enclosed in a cover case. By the term "cover case", it should be understood a case designed to isolate the metal-containing electrode (anode) from any gas or chemical source. However, this case should allow the contact of the metal-containing electrode with the oxygen ion conductor electrolyte. Therefore, said cover case is provided with an opened side through which the case is fixed or sealed to the electrolyte, allowing the contact of said electrolyte with the metal- containing electrode.

The cover case also acts as a protective cover case to protect said metal-containing electrode from reactions that induce its degradation or deactivation. For example, this case can prevent it from oxidation in contact with atmosphere. This case can also act as a chamber separator and current collector and/or interconnection system.

This case is typically made of an electronically conducting material so that it may form one electrical lead of the battery. The case may be comprised of any metal that is non- reactive with the other components of the system. In this regard, the case may be comprised of an alloy metal, such as ferritic steels with or without nickel, for example those commonly known as crofer type materials. In another embodiment, the case may be comprised of any ceramic conducting material, non-reactive with the other components. Furthermore, the electronically conducting material may be passivated or coated to prevent chromium poisoning of the air electrode or lateral reactions that induce degradation.

Particularly, the electronically conducting material is preferably treated (passivated) or coated to produce a protective layer that prevents reaction of the case and the metal electrode during processing and operation of the metal-air battery system. The protective layer includes spinel type oxides, for example manganese and cobalt containing spinels or manganese, cobalt and iron containing spinels. Other examples of protective layers include perovskite type oxides of formula II, such as La-Sr-Fe or La- Sr-Fe-Cu containing perovskite type oxides. Further examples include fluorite type materials such as Ce0 ₂ , and other oxides such as Y ₂ 0 ₃ related materials. These protective layers can be generated "in situ" or deposited on top of the electronically conducting material by conventional methods such as reactive sintering, chemical vapor deposition, sputtering, spraying, dip coating, screen printing and others. In another particular embodiment, the case is made of an electrically insulating material, in which instance a separate current collector can be disposed therein.

The set metal electrode-case can be sealed to the electrolyte with a combination of one or more sealing parts such as glass seals, metal-based seals and ceramic parts such as alumina or mica type felts in order to provide a cover case as tight as possible. In a preferred embodiment, said cover case is a gas tight cover case.

Porous positive air electrode

The porous positive air electrode constitutes the cathode of the metal-air battery system used in the method of the invention. It comprises a thin porous layer made of electronically and ionically conductive materials or composites. It should be porous in order to allow oxygen molecules to reach the electrode/electrolyte interface.

In a preferred embodiment of the invention, the porous layer is made of a perovskite type transition metal oxide or composites of perovskite with fluorite-related oxygen ion conductors, such as those defined above for the metal electrode. More preferably, the perovskite type transition metal oxide, or ABO3, has a formula (III):

(Ln ₁ _ _x M _x ) ₁ _ _a (B ₁ _ _y B' _y )i-b0 ₃ -d (Formula III) wherein: Ln is a lanthanide cation selected from lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm) and gadolinium (Gd);

M is at least one alkaline-earth cation selected from calcium (Ca), strontium (Sr) and barium (Ba);

B and B' are different from each other, and B and B' each independently comprises at least one element selected from the group consisting of cobalt (Co), iron (Fe), chromium (Cr), copper (Cu) and manganese (Mn).; x and y are the proportions of A site and B site combination of cations ranging between 0 and 1 ; and a, b and d correspond to atom site deviations with respect to stoichiometry. Examples of these perovskite type transition metal oxides are (LaSr)Co0 ₃ , (La,Sr,Ca)(Mn,Cr)0 ₃ and (La,Sr)(Fe, Co)0 ₃ .

The fluorite-related oxygen ion conductor normally comprises a compound of formula (I) or mixed solutions of two or more oxide systems as shown in formula (I):

[(A ₁ _ _x _ _y A' _x A" _y )O _s ] ₁ _ _z [(B ₁ _ _v B' _v )0 ₂ ] _z _d (Formula I) wherein

A, A' and A" are different from each other, and A, A' and A" each independently comprises at least one mono-, di-or trivalent elements selected from yttrium (Y), sodium (Na), scandium (Sc), samarium (Sm), gadolinium (Gd), cerium (Ce), calcium (Ca), magnesium (Mg), aluminum (Al) and bismuth (Bi);

B and B' are different from each other, and B and B' each independently comprises one cation selected from zirconium (Zr) and cerium (Ce). v, x, y and z have values from 0 to 1, with the proviso that x+y is less or equal to 1; s has a value ranging from 0.5 to 1.5; d corresponds to site deviations from stoichiometry.

The compounds of formula (I) may include minor additions of additives, more particularly metal oxides, such as CaO, Na ₂ 0, Ti0 ₂ , A1 ₂ 0 ₃ , Y ₂ 0 ₃ , Si0 ₂ , Fe ₂ 0 ₃ and Ce0 ₂ .

The ceramic materials used to elaborate the cathode do not become electrically and ionically active until they reach the high temperature and, as a consequence, the metal- air battery of the invention has to run at temperatures ranging from 300-1000°C.

In a particular embodiment, a barrier in the form of a ceramic layer is inserted between the cathode and the electrolyte in order to prevent chemical reaction between cathode and electrolyte materials and to enhance electrochemical performances. The barrier interlay er is normally composed of cerium based fluorite related oxides as described in formula (I).

Although not essentially for carrying out the present invention due to the high temperatures at which the metal-air battery works, the cathode may include a catalytic material that facilitates oxygen reduction. The catalyst can either be physically mixed with the material forming the porous air electrode or can be chemically bound to it. Any metal that can be used as catalyst material for an electrode can be used herein. In one embodiment, the metal includes, but is not limited to a noble metal, a metal oxide, a metal alloy, an intermetallic, or mixtures of the aforementioned metals. Noble metals include silver, platinum, palladium, iridium, osmium, rhodium and ruthenium. However, mixtures or alloys thereof can also be used. Such catalyst may be used either singly or in combination, although other catalytic materials may likewise be incorporated. Additionally, the porous air electrode can also be enclosed in a case which houses the cathode and an associated electrical lead. This case may further include electrode supports, current-collecting and/or interconnect structures and the like. It may also include at least one opening to permit passage of ambient air to the cathode.

In the operation of the metal-air battery, air passes to the cathode. At the cathode oxygen is reduced to form oxygen ions, and in the process consumes electrons. The oxygen ions diffuse through the solid oxide electrolyte and react with the metal of the anode and form a metal oxide, thus generating electrons which flow to the cathode through an external circuit in communication with the anode case and the cathode lead. In the operation of the battery, the metal of the anode is consumed and converted to a metal oxide. When all the metal is consumed, the cell ceases to operate and should be electrically recharged to operate again.

Solid oxide electrolyte

The solid oxide electrolyte is a membrane disposed between the metal electrode and the porous air electrode. Once the molecular oxygen has been converted to oxygen ions in the air cathode, said oxygen ions migrate through the electrolyte to the metal electrode (anode). In order for such migration to occur, the electrolyte must possess a high ionic conductivity. However, its electronic conductivity must be kept as low as possible to prevent losses from leakage currents. The electrolyte must also be gastight to prevent short circuiting of reacting species through it and it is convenient to be as thin as possible to minimize resistive losses in the system. It should also be chemically, thermally and structurally stable across a wide temperature range.

The electrolyte may comprise at least one material selected from the group consisting of zirconium oxide, cerium oxide and a perovskite group consisting of lanthanum-doped gallates (LSGM), or any other material commonly used as SOFC electrolyte materials. The solid oxide electrolyte can also be a fluorite-related oxygen ion conductor, such as yttria-stabilized zirconia ("YSZ"), scandia-stabilized zirconia ("ScSZ"), samaria-doped ceria ("SDC"), gadolinia-doped ceria ("GDC"), or the like. The fluorite-related oxygen ion conductor normally comprises a compound of formula (I):

[(A ₁ _ _x _ _y A' _x A" _y )O _s ] i_ _z [(B ₁ _ _v B' _v )0 ₂ ] _z _d (Formula I) wherein A, A' and A" are different from each other, and A, A' and A" each independently comprises at least one mono-, di-or trivalent element selected from yttrium (Y), sodium (Na), scandium (Sc), samarium (Sm), gadolinium (Gd), cerium (Ce), calcium (Ca), magnesium (Mg), aluminum (Al) and bismuth (Bi); B and B' are different from each other, and B and B' each independently comprises one cation selected from zirconium (Zr) and cerium (Ce). v, x, y and z have values from 0 to 1, with the proviso that x+y is less or equal to 1; s has a value ranging from 0.5 to 1.5; d corresponds to site deviations from stoichiometry. The compounds of formula (I) may include minor additions of additives, more particularly metal oxides, such as CaO, Na ₂ 0, Ti0 ₂ , A1 ₂ 0 ₃ , Y ₂ 0 ₃ , Si0 ₂ , Fe ₂ 0 ₃ and Ce0 ₂ .

Examples of ionic conductive materials which can also be used as electrolyte for the metal-air battery system of the invention are: (BiOi. ₅ )x(YOi. ₅ )i. _x ; (La _x Sri. _x )(GayMgi. _y )0 ₃ ;

(Zr0 ₂ ) _x (ScOi. ₅ )i_ _x ; (Zr0 ₂ ) _x (YOi. ₅ )i_ _x , (CaO)i_ _x (Zr0 ₂ ) _x ; LaCaA10 ₂ , wherein x is a value from 0 to 1, or any other ion conductors of the fluorite type as described above.

The electrolyte is prepared by common methods known by those skilled in the art, particularly following procedures as those used when manufacturing solid oxide fuel cells.

A further aspect of the invention refers to a metal-air battery comprising: a) a metal-containing negative electrode, b) a porous positive air electrode; c) an oxygen ion conductor electrolyte; and d) optionally, a ceramic layer located between the porous positive air electrode and the oxygen ion conductor electrolyte, wherein the electrolyte is in contact with the metal-containing negative electrode at one side and with the porous positive air electrode on the other side, or when the ceramic layer is present the solid oxide electrolyte is in contact with the metal-containing negative electrode at one side and with the ceramic layer on the other side, wherein the metal-containing electrode is enclosed in a cover case to isolate the electrode from any external gas or any chemical source; wherein the metal-air battery is only rechargeable by electricity and operates at temperatures ranging from about 300 to about 1000 °C.

The components of the metal-air battery of the invention, i.e., the metal-containing negative electrode and its tight cover case, the porous positive air electrode, the oxygen ion conductor electrolyte; and the optional ceramic layer located between the porous positive air electrode and the oxygen ion conductor electrolyte are those as mentioned previously in the present document.

The metal-air battery of the invention is characterized in that the metal-containing electrode (anode) is enclosed in a cover case which allows the contact of the metal- containing electrode with the oxygen ion conductor electrolyte. As mentioned before, the cover case is designed to isolate the metal-containing electrode from any gas or chemical source so as the operation of the battery can only occurs through the electrochemical charge and discharge. The cover case enclosing the metal-containing electrode is efficiently enough to guarantee the coulombic efficiency of the battery system. Preferably, the cover case is a gas tight cover case which allows producing a Coulombic efficiency of 1.

Accordingly, the metal-air battery of the invention operates as electric energy storage device and not as energy conversion device and has a high potential storage capacity as no fuel tank or gas supply is used to recharge the battery. The metal-air battery system of the invention can include other additional components such as interlayers, contact layers, buffers and protective layers. For example, when bismuth-based materials are used as solid oxide electrolyte, an interlayer should be placed between the metal-containing electrode and the electrolyte in order to avoid reactions between both components of the metal-air battery system. In a particular embodiment of the invention, the metal-containing electrode enclosed in the cover case, the electrolyte and the cathode are incorporated in a battery case. This battery case has an oxygen supply port in the vicinity of the cathode to supply the oxygen to the cathode. It also includes electrode terminals which are extended from the inside to the outside of the battery case and which are respectively connected to the cathode and the anode to allow the current to flow from one electrode to another.

This battery case can also be provided with an inert/reducing protective gas, such as argon o nitrogen, in order to prevent the penetration of oxygen inside the cover case enclosing the metal-containing electrode. Furthermore, the battery case can be designed in such a way that it also includes air electrode gas distribution, so as to guarantee current collection of the air electrode with sufficient air flow distribution for enhancing the electrochemical reaction.

As will be appreciated by those skilled in the art, the battery system of the present invention may be implemented in a variety of configurations and designs. It can take a planar design (Figure 2) in which a flat cathode (3), electrolyte (1) and anode (2) structure are fabricated to form a metal-air battery or cell. Figure 3 illustrates a tubular design in which metallic fuels are contained inside a tubular assembly to form a cell.

It should be noted that in figures 2 to 6 of the present invention element (1) corresponds to the electrolyte, element (2) to the anode, element (3) to the cathode, element (4) to the cover case, element (5) to a current collector and element (6) to a sealing system.

Multiple cells or metal-air batteries can be stacked to form a stack or module system. Therefore, another aspect of the present invention refers to a module system which comprises at least two stacked metal-air batteries as defined above. This system comprises metal-air batteries repeating units stacked into a module with variable power output depending upon the final application.

The metal-air battery repeating units are connected by means of designs that minimize ohmic losses and guarantee sufficient air flow to the air cathode.

The materials used to interconnect the metal-air battery repeating units can be metallic or ceramic, with the required coating/treatments to assure compatibility with other components. In a particular embodiment, the cover case designed to isolate the metal electrode and to prevent it from the exposure to any gas acts as current collector and/or interconnector. This case can comprise a secondary part connected to the air electrode, acting as current collector and/or gas distributing system and/or cell to cell interconnector. The module system can be sealed by means of ceramic or metal sealing pastes and/or felts that withstand operating conditions.

In a particular embodiment, the module system has a planar or tubular configuration. In the planar configuration (Figure 4), the planar electrochemical device can be mechanically supported by the electrolyte (Figure 4a) or by one of the electrodes (Figure 4b) or by an inert substrate material (8) either metallic or ceramic in nature (Figure 4c). In the tubular configuration, the tubular electrochemical device can be mechanically supported by the electrolyte or by one of the electrodes or an inert substrate material with one or both ends open.

The final design will depend on the pursued application. The metal-air battery repeating units are electrically connected between each other. The electrochemical connections between single repeating units to form the stack or module system can be either in series or in parallel.

In a particular embodiment, each metal-air battery repeating unit, in either planar or tubular configuration, is connected in parallel with the adjacent metal-air battery repeating unit to form a bundle. The set of bundles are further connected in series to build up the specific power specifications.

In another particular embodiment, each metal-air battery repeating unit, in either planar or tubular configuration, is connected in series with the adjacent metal-air battery repeating unit to form a bundle. The set of bundles are further connected in parallel or series to build up the specific power specifications.

Figure 5 illustrates examples of planar single repeating units stacked into bundles with internal connections (7) either in series or in parallel. More into detail, figure 5 a shows for planar cells connected in parallel through the anode conducting case. The parallel connection of the cathodes can be done by a conductive plate that can be perforated, corrugated or designed and machined in such a way that permits sufficient gas flow and electrical conduction. This example can be placed in vertical way to ensure anode contact to the electrolyte and metal case. A bundle can be further connected to the next one either in parallel or in series according to system specifications. Figure 5b shows a modification of the planar configuration in which an additional conductive part is introduced that acts as interconnection of the cathode of one cell with the anode of the next cell in series bundle connection. This additional part can be perforated at certain regions, corrugated or designed and machined in such a way that permits sufficient gas flow and electrical conduction. A bundle can be further connected to the next one either in parallel or in series according to system specifications. Figure 6a shows an example of a tubular system bundle with internal connections between cells, as preferred option, in parallel. Bundles are then connected either in series or combinations of series and parallel in order to build up the required specifications of the system module (Figure 6b).

Another aspect of the present invention refers to a method for manufacturing the metal- air battery of the present invention. Said method comprises: a) providing a solid oxide electrolyte as defined above;

b) placing the porous positive electrode and the metal-containing negative electrode on each side of the solid oxide electrolyte;

c) enclosing the metal negative electrode in the cover case.

It should be noted that the order of steps b) and c) is not particularly limited. However, in a preferred embodiment the metal-containing negative electrode is deposited and processed over the electrolyte prior to the electrode being enclosed in the cover case.

The structure obtained following steps a) to c) can be further installed in a battery case and tightly sealed to produce the metal-air battery.

In a preferred embodiment, this battery case is provided with an inert/reducing protective gas, such as argon or nitrogen, in order to prevent the penetration of oxygen to the inside of the cover case enclosing the metal- containing electrode.

In a particular embodiment, when manufacturing a metal-air battery system with planar design, the solid electrolyte can be prepared from powder formulations by tape casting, slip casting, tape calendering and the like. The electrolyte is subsequently sintered at high temperature to achieve full density prior to depositing the electrodes.

In another particular embodiment, when manufacturing a metal-air battery system with tubular design, the solid electrolyte can be prepared by extrusion, slip casting, isostatic pressure and the like. The electrolyte is subsequently sintered at high temperature to achieve full density prior to depositing the electrodes.

Preferably, the solid electrolyte is manufactured thick and dense in order to act as a substrate or support of the battery system.

In another particular embodiment, step b) of the process of the invention is carried out by depositing and powder processing the porous positive electrode and the metal- containing negative electrode on each side of the solid oxide electrolyte.

The deposition of the electrodes on each side of the electrolyte involves the previous preparation of slurry formulations comprising powdered materials constituting said electrodes. Said slurry formulations can be prepared by milling with zirconia media the powders with water-based or organic-based binders and suitable additives to achieve adequate rheology that provide microstructures with targets in terms of thickness, porosity and permeation. The powdered ceramic material used for manufacturing the electrodes can be milled, calcined and sieved prior to preparing the slurries for deposition of electrodes. Once the slurry formulations are prepared, the deposition of the electrodes on each side of the electrolyte is carried out by tape casting, tape calendaring, dip coating, spraying, screen printing, chemical vapour deposition, physical vapour deposition, sputtering, electrophoretic deposition, reactive sintering and the like.

Once the electrodes are deposited on each side of the solid oxide electrolyte, they are subjected to a powder processing in order to obtain a solid electrode.

In the powder processing step, atmospheres and temperatures are adjusted as a function of the powders, deposition technique used and system geometry or configuration, but include oxidizing, inert or reducing atmospheres.

Once the porous positive electrode and the metal-containing negative electrode have been placed on each side of the solid oxide electrolyte, the metal-containing electrode is enclosed in a cover case. This cover case is fixed or sealed to the electrolyte through its opened side by means of sealing agents such as glass seals, metal-based seals and ceramic parts such as alumina or mica type felts, in order to provide a cover case as tight as possible. This way of placing the electrodes on each side of the electrolyte is particularly useful when manufacturing a metal-air battery system with planar design.

However, as an alternative, step b) can be carried out by first depositing and powder processing the porous positive electrode on one side of the solid oxide electro lye following a procedure as that described above. Subsequently, the cover case is fixed or sealed to the electrolyte/positive cathode block previously obtained, and then the metal negative electrode previously obtained, preferably as a solid electrode, is placed inside the cover case and covered with a sealing cap.

When tubular design is desired, step b) is preferably carried out by first depositing and powder processing the porous positive electrode on one side of the solid oxide electrolyte following a procedure as that described above. Subsequently, the metal negative electrode previously obtained, preferably as a solid electrode, is placed inside a tube which acts as a cover case and then fixed on the free side of the solid oxide electrolyte.

Due to the excellent properties of the metal-air battery of the present invention, in particular its high energy density and long cycle life, it can be used as a power source of small devices as well as a power source for automotive applications, such as electric vehicles and hybrid cars. Moreover, it can also be used as an energy storage device for utility applications as well as for automotive and power electronic applications.

The present invention will now be described in detail by way of examples which serve to illustrate the construction and testing of illustrative embodiments. However, it will be understood that the present invention is in no way limited to the examples set forth below.

Example 1 Laboratory-scale cells using molten tin and other metal electrodes. A thin Yttrium doped zirconia (YSZ) electrolyte was used to deposit a perovskite La-Sr- Fe or La-Sr-Co containing perovskite cathode material and fired in air at temperature between 800 and 1200°C. Pt was co-fired for current collection at laboratory scale tests. An interlayer of Samarium or Gadolinium or Yttrium doped ceria (SDC, GDC, YDC) between cathode and electrolyte layers can be inserted during the processing of the cathode system. The electrolyte-cathode system was then sealed using ceramic based sealant paste to an alumina or quartz tube. The tin metal anode solid material was placed inside the tube over the free side of the electrolyte. The metal current collection system, welded to Pt wires prior to passivation/coating, was then placed inside the tube avoiding contact with the solid anode material. Reducing/inert protective gas was introduced into the anode side and the system was heated up to operation temperature. Once anode material was in molten state, the current collector was introduced into the molten anode and the discharge/charge operation started under inert gas or in a closed embodiment. The discharge and charge profiles were tested at 800°C. As shown in figure 7, the system is electrochemically reversible.

At the laboratory cell level, other materials have been tested as metal anode solid materials under the conditions described above. Initial values of capacities, measured over an active bidimensional area of circa 2 cm ² are given in the following table for Sn, Sn-W, Bi and Sn-Mn.

Example 2

Single repeating unit cells using molten tin as metal electrode were built using planar system design (figure 2a). A thin Yttrium doped zirconia (YSZ) electrolyte was used to deposit a perovskite La-Sr- Fe or La-Sr-Co containing perovskite cathode material and fired in air at temperature between 800 and 1200°C. Pt was co-fired for current collection at laboratory scale tests. An interlayer of Samarium or Gadolinium or Yttrium doped ceria (SDC, GDC, YDC) between cathode and electrolyte layers can be inserted during the processing of the cathode system. The electrolyte-cathode system was then sealed using glass sealing tape to an electronically conducting material, which is going to be used as container for the solid anode metal. First stage sealing was processed in Air. The solid anode was placed in the container and covered with a metallic cap sealed with glass sealing tape. The second stage was fired under Argon. The system is a closed embodiment avoiding gas leakage. Besides, the cell was placed under protective inert gas, assuring no oxygen penetration at the anode side. The current collection was directly done via the anode metal container which was a conducting material, where Pt wires were welded prior to passivation/coating of the metal container. Reducing/inert protective gas was flowed to the closed system and then it was heated up to operation temperature. The discharge and charge profiles were tested at 800°C. As shown in figure 8, the system is electrochemically reversible, obtaining more than 1300 cycles with 100% efficiency.

Example 3 Single repeating unit cells using molten tin as metal electrode were built using planar system design (figure 2a). Alternative cost-efficient processing route.

A thin Yttrium doped zirconia (YSZ) electrolyte was used to deposit a perovskite La-Sr- Fe or La-Sr-Co containing perovskite cathode material and fired in air at temperature between 800 and 1200°C. Pt was co-fired for current collection at laboratory scale tests. An interlayer of Samarium or Gadolinium or Yttrium doped ceria (SDC, GDC, YDC) between cathode and electrolyte layers can be inserted during the processing of the cathode system. The electrolyte-cathode system was then sealed using glass sealing tape to an electronically conducting material, which is going to be used as container for the solid anode metal. The first stage sealing was fired in Air. The solid anode was placed in the container and covered with a metallic cap sealed with glass sealing tape. The second stage was also processed under Air. The system is a gas tight closed embodiment, where no protective inert gas is needed. The current collection was directly done via the anode container which was a conducting material, where Pt wires were welded prior to passivation/coating of the metal cover case. The system was heated up to operation temperature. The discharge and charge profiles were tested at 800°C. As shown in figure 9, the system is electrochemically reversible, obtaining more than 500 cycles with 100% efficiency. This alternative includes full electrical recharging at 800°C as initial conditioning of the cell.