Electrical energy storage device Field of the Invention

The present invention relates to an electrical energy storage device comprising at least one module with a negative

electrode, a positive electrode, and an anion conducting electrolyte located between the negative electrode and the positive electrode.

Description of Related Art High temperature solid oxide electrolyte fuel cells are well known in the art and convert chemical energy into direct current electrical energy, typically at temperatures above about 500°C. This temperature is required to render the solid electrolyte sufficiently conductive. Stabilized zirconia is a prime electrolyte. Such fuel cells are taught, for example, by U.S. Patent No. 4,395,468 (Isenberg) . The general working principles and general reactions of a solid oxide fuel cell ("SOFC") are shown in prior art Fig. 1, which is self- explanatory. Air and a required gaseous fuel, such as natural gas, are both utilized solely to generate electricity at about 800°C to about 1,000°C. This type SOFC utilizes

metal/ceramic fuel electrodes 10, gaseous reformed natural gas fuel and ceramic, dense solid electrolyte 11 and porous ceramic air electrode 12. Fuel 13 is shown by F and oxidant or air A is shown by 14.

An encyclopaedic publication by N. Q. Minh, in Ceramic Fuel Cells, J. Am. Ceramic Soc, 76[3] 563-588, 1993 describes in detail a variety of fuel cell designs, including tubular, triangular and other configurations, as well as materials used and accompanying electrochemical reactions. For example, that article describes segmented cell-in-series (banded and bell-and-spigot ) , monolithic (co-flow and cross-flow) , and flat-plate designs in substantial detail. Cermet fuel

electrode (anode) materials, such as nickel or cobalt/yttria stabilized zirconia are also discussed as well as their coefficient of thermal expansion problems.

In addition to generating energy, batteries also store it. Electrical energy storage is crucial for the effective proliferation of an electrical economy and for the

implementation of many renewable energy technologies. During the past two decades, the demand for the storage of

electrical energy has increased significantly in the areas of portable, transportation, and load-levelling and central backup applications. The present electrochemical energy storage systems are simply too costly to penetrate major new markets, still higher performance is required, and

environmentally acceptable materials are preferred.

Transformational changes in electrical energy storage science and technology are in great demand to allow higher and faster energy storage at the lower cost and longer lifetime

necessary for major market enlargement. Most of these changes require new materials and/or innovative concepts with

demonstration of larger redox capacities that react more rapidly and reversibly with cations and/or anions. Batteries are by far the most common form of storing

electrical energy, ranging from: standard every day lead - acid cells, exotic iron-silver batteries for nuclear

submarines taught by Brown in U.S. Patent No. 4,078,125 and nickel-metal hydride (NiMH) batteries taught by Venkatesan et al. in U.S. Patent No. 5,856,047, Kitayama in U.S. Patent No. 6,399,247 Bl and Young et al . in U.S. Patent No. 7,261,970. Also known are metal-air cells taught in U.S. Patent No.

3,977,901 (Buzzelli), Isenberg in U.S. Patent No. 4,054,729, U.S. Patent Publications 2006/0063051; 2007/0077491;

2007/0259234 (Jang, Burchardt and Chua et al, respectively) and air batteries also taught in U.S. Patent Publications 2003/0143457 and 2004/0241537 (Kashino et al . and Okuyama et al . , respectively) . Lithium-ion batteries are taught by Ohata in U.S. Patent No. 7,396,612 B2.

Batteries range in size from button cells used in watches, to megawatt load levelling applications. They are, in general, efficient storage devices, with output energy typically exceeding 90% of input energy, except at the highest power densities. Rechargeable batteries have evolved over the years from lead-acid through nickelcadmium and nickel-metal hydride (NiMH) to lithium-ion. NiMH batteries were the initial workhorse for electronic devices such as computers and cell phones, but they have almost been completely displaced from that market by lithium-ion batteries because of the latter' s higher energy storage capacity. Today, NiMH technology is the principal battery used in hybrid electric vehicles, but it is likely to be displaced by the higher power energy and now lower cost lithium batteries, if the latter' s safety and lifetime can be improved. Of the advanced batteries, lithium- ion is the dominant power source for most rechargeable electronic devices.

Summary of the invention With respect to the mentioned prior art it is an objective of the present invention to provide an advantageous electrical energy storage device that can easily discharge and charge a high capacity of energy quickly and reversibly, as needed. This objective is solved by an electrical energy storing device as claimed in claim 1. The depending claims define further developments of the invention.

An inventive electrical energy storage device comprises at least one module with a negative electrode, a positive electrode made from an anion generating material, or material combination, that conduct anions and electrons, and an anion conducting solid electrolyte located between the negative electrode and the positive electrode. The negative electrode of each module comprises a porous structure, typically a porous ceramic structure that conducts anions. The pores of the porous structure are at least partially filled by a first redox mass of a redox pair, which comprises a metal and its oxide or two different oxides of the metal. The negative electrode of each module comprises a porous structure, typically a porous ceramic structure that conducts anions. The pores of the porous structure are at least partially filled by a second redox mass of a redox pair, which

comprises a metal and its oxide or two different oxides of the metal in a state with an increased oxidation potential of the species the electrolyte is conducting via anions compared to the first redox mass of a redox pair. Preferably the porous ceramic structure also conducts electrons.

The first redox mass and the second redox mass may comprise the same metal in different pairs of oxidation states.

Alternatively or additionally, the first redox mass may comprise a first metal/metal oxide pair and the second redox mass may comprise a second metal/metal oxide pair with a different oxidation potential compared with the first metal/ metal oxide pair. The porous structure of the negative electrode can be

infiltrated by the first redox mass. The porous structure of the second electrode can be infiltrated by the second redox mass. In case of infiltration the porous structure may comprise an outwards open pore system. The mass can be solid or liquid under operating conditions.

Such an electrical energy storage device can easily be discharged by oxidising the metal or its oxide of the redox mass and charged by applying a voltage between the negative electrode and the positive electrode that leads to reduction of the oxidised metal or its higher oxide in the redox mass. In particular, the metal in said oxidised state can be an oxide of the metal, i.e. an oxygen compound of said metal. Note, that in a more general sense, the metal in the oxidised state may be represented by other compounds in which the metal donates one or more electrons to another element of the compound. For example, an element of the halogen group could be used for oxidising the metal so that a metal halogenide compound would be formed. However, oxygen is a preferred element for oxidising the metal in the first and/or second mass since oxygen is easily available from air.

Typically, in a storage battery with oxygen conducting oxide ceramic electrolyte where the positive electrode is supplied with air the redox system MeOx/MeOx+y acting as storage medium at the negative electrode determines the storage volume at this electrode for a predefined capacity. Thereby it has to be considered that at an insufficient conductivity of the material, i.e. ionic and electronic conductivity, an additional conductive structure is needed. This additional conductive structure must be incorporated into the storage material and may affect the total volume of the negative electrode. If the positive electrode must be supplied with oxygen for the oxidation by means of an air fan, an extensive and costly flow and heat management of the energy storage system is necessary at a working temperature between 600°C and 800°C.

The present invention avoids the need of such an extensive and costly flow and heat management by introducing a storage medium Me' Ox' /Me' Ox' +y' in form of a redox system at the positive electrode as well. The storage medium

Me' Ox' /Me' Ox' +y' at the positive electrode is characterised by an increased or higher oxygen potential compared with the redox system at the negative electrode MeOx/MeOx+y. The storage medium Me' Ox' /Me' Ox' +y' at the positive electrode emits oxygen in the form of oxygen ions (0 ^2~ ) which flow from the positive electrode via the electrolyte to the negative electrode during the discharging process and can be absorbed in the positive electrode again during the charging process.

In the present invention the positive electrode has a similar porous structure, preferably ceramic structure, as the negative electrode has. The positive electrode can comprise a redox system based on the same metal as the negative

electrode but at a different pair of oxidation states.

Alternatively, the positive electrode can comprise a redox system based on a different metal than the anode

Me' Ox' ' /Me' Ox' ' ' with a different oxidation potential. Thus, the oxygen which is emitted from the negative electrode can directly be absorbed from the positive electrode during charging and vice versa can flow from the positive to the negative electrode during discharging.

The first and/or second redox mass is preferably liquid at temperatures above 500°C, in particular at temperatures between 600°C and 800°C. At these temperatures, high ion transport capacities of the solid electrolyte and the porous ceramic structures of the positive electrode and the negative electrode can be achieved.

In a special implementation of the first and/or second redox mass the metal as such is liquid at temperatures below 800°C. Metals fulfilling this condition are, e.g. tin, lead and bismuth .

In a different implementation, in which the metal is not itself liquid at temperature below 800°C the first and/or second redox mass can comprise a molten salt of said metal comprised in the first and/or second redox mass. By this measure, a mixture of the metal and the metal salt can be achieved that is liquid or at least malleable even at

temperatures at which the metal itself would not be liquid. The molten salt can, for example, comprise halogens as an anion, for example chlorine. In this case, the first and/or second redox mass may be formed by a mixture given by the formula : aMe-bMeO _x -cMeCl _x /2

Suitable metals which are liquid in a liquid mass comprising a salt of the respective metal, in particular of the form MeCl _x , are iron, manganese or nickel. Those amounts of the metal which are not bound in the salt compound are then available for the electrochemical redoxreaction .

A suitable geometrical configuration of the at least one module is a tubular configuration in which the module extends along a longitudinal axis and has a circular cross section with the positive electrode at the inside of the circle and the negative electrode at the outside of the circle. The module may further comprise container for the first and for the second redox mass in case these are liquids.

Alternatively, in the tubular configuration, the positive electrode can be at the outside of the tubular module. The negative electrode is then located inside of the tubular module .

Other configurations of the at least one module of the electrical energy storage device are possible as well. For example, flat configurations in which a number of flat modules can be stacked and in which each module comprises a layer stack with a positive electrode, a negative electrode and an electrolyte located between the positive electrode and the negative electrode.

To increase the voltage produced by the electrical energy storage device at least two modules can be connected in series. Similarly, to increase the current of the electrical energy storage device at lest two modules can be connected in parallel. Connecting modules in parallel and in series is also possible. Generally, by providing the positive electrode and the negative electrode with a porous structure, typically a porous ceramic structure, which comprises a metal in a non- oxidised and/or in an oxidised state in different oxidation states at the positive electrode and the negative electrode a costly and extensive air supply at the electrodes, especially at the positive electrode, can be avoided. This reduces the own energy consumption and simplifies the heat management means of the energy storage device.

Further features, properties and advantages of the present invention will become clear from the following description of embodiments in conjunction with the accompanying drawings. The mentioned features are advantageous separately or in any combination with each other. The dimensions of the objects in the figures have been chosen for the sake of clarity and do not necessarily reflect the actual relative dimensions.

Brief Description of the Drawings

FIG. 1 illustrates the working principals of prior art

SOFC' s;

FIG. 2 illustrates the broadest example of the energy storage device of this invention based on an anion A conductor which utilizes A-containing gas and eliminates the need for gaseous fuel ;

FIG. 3 illustrates the working principals of one embodiment of the electrical energy storage device of this invention, which utilizes air and eliminates the need for gaseous fuel;

FIG. 4 illustrates an example of both electrode reactions of the electrical energy storage device of this invention;

FIG. 5 schematically shows a typical tubular configuration of the electrical energy storage device with an inner positive electrode in a sectional view along the longitudinal

direction of the cell;

FIG. 6 schematically shows a typical tubular configuration of the electrical energy storage device with an outer positive electrode in a sectional view along the longitudinal

direction of the cell;

FIG. 7 perceptively shows a typical tubular configuration of the electrical energy storage device with an inner positive electrode in a sectional view along the radial direction of the cell;

FIG. 8 perceptively shows a further typical tubular

configuration of the electrical energy storage device with an outer cathode in a sectional view along the radial direction of the cell;

FIG. 9 perceptively shows a typical tubular configuration of the electrical energy storage device with an outer positive electrode in a sectional view along the radial direction of the cell;

FIG. 10 shows an electrical energy storage device formed of three banks each comprising a number of tubular cell modules.

FIG. 11 schematically shows a typical flat configuration of the electrical energy storage device in a sectional view. FIG. 12 schematically shows an example for the electrical contacting of a tubular cell of the electrical energy storage device as shown in Figure 8 in a sectional view.

Description of the Preferred Embodiments

The broadest working principle of the electrical storage device of this invention is shown in FIG. 2. A non-fuel containing gas 16 which contains an anion forming element or compound A contacts an A-gas electrode 17 made from a

material or material composition that generates and conducts anions from the element or compound A and electrons. An anion A ^z~ conducting conductor/electrolyte 18 is disposed next to the A-gas electrode and an electron or anions conducting electrode 19. Furthermore, there is an electrical circuit, a load 20 and a DC supply 21. Here, there is an anion A ^z~ conducting electrolyte where there is ion transfer between electrodes on either side of the electrolyte, such ions are selected from at least one of 0 ^2~ , CC>3 ^2~ and PC>4 ^3~ .

The working principle of one embodiment of the electrical storage device of this invention is schematically shown in Fig. 3. In this embodiment, the A-gas is air with oxygen (0) as the anion forming element. Hence, Fig. 3 shows an oxide- ion battery configuration. In discharge mode, oxide-ion anions migrate from high partial pressure of oxygen side (air side in this case) to low partial pressure of oxygen side (metal-metal oxide electrode) under the driving force of a gradient of the oxygen chemical potential. In charge mode, the oxide-ions are forced to migrate from low partial

pressure of oxygen side to high partial pressure of oxygen side under the driving force of an electrical field. Here, air 16' is in contact with air electrode (positive electrode) 17'. Oxygen ion conductor electrolyte is between the air electrode and metallic (metal-metal oxide) electrode

(negative electrode) 19'. Load is shown as 20', and D.C.

power supply 21'. The corresponding electrode reactions occurring during charge and discharge course are illustrated in Fig. 4. Under the discharge mode the metal of the anode is oxidized into metal oxide with exothermic heat whereas under the charge mode metal oxide of the anode is reduced into metal with endothermic heat. The discharging process is, where Me stand for metal: yMe + x/2 0 ₂ = Me _y O _: and the charging process is:

Me _y O _x = x/2 0 ₂ + yMe. Here, air electrode is shown as 17'', electrolyte as 18'' and metal electrode as 19''.

Tubular cell configurations are preferred and will be

illustrated throughout this specification for simplicity reasons. However, this should not be construed in any way as restrictive, as other "hollow, elongated tubular cell" structures are herein included, as are described by Isenberg, in U.S. Patent No. 4,728,584 - a corrugated design, and by U.S. Patent Application Publication No. U.S. 2008/0003478 Al (Greiner et al . ) - a triangular, quadrilateral, oval, stepped triangle and meander, are all herein defined as "hollow elongated tubular" cells.

Typical tubular cell configurations are schematically

displayed in FIGS. 5 to 9 in sectional views along the longitudinal direction (FIGS. 5 and 6) and perspective views in a section along the radial direction (FIGS. 7 to 9) .

The cell configurations comprise at least three functional layers: a positive electrode 26, a solid electrolyte 27 and a negative electrode 28. While FIGS. 5 and 7 show cell

configurations with an inner positive electrode 26, FIGS. 6, 8 and 9 show cell configurations with an outer positive electrode 26. In FIGS. 5, 6, 9, 11 and 12 an outer casing 25 forming a container is shown. In the examples shown in FIGS. 5, 6 and 11 the outer casing 25 comprises two casing portions 25a and 25b.

In the cell configurations with the outer positive electrode 26, as shown in FIGS. 6, 8 and 9, the negative electrode 28 may be in the form of a structure forming the centre of the cell. The negative electrode 28 can have circular, square, irregular or any other cross-section, thus the term structure as used herein shall not be restricted to circular cross- sections as they are shown in FIGS. 6, 8 and 9. The negative electrode 28 then forms a substrate for the electrolyte 27 and the positive electrode 26 that are sequentially deposited on the negative electrode 28.

In the cell configurations with the inner positive electrode 26, as shown in FIGS. 5 and 7, the positive electrode 26 may be in the form of a structure forming the centre of the cell. The positive electrode 26 can have circular, square,

irregular or any other cross-section, thus the term structure as used herein shall not be restricted to circular cross- sections as they are shown in FIGS. 5 and 7. The positive electrode 26 then forms a substrate for the electrolyte 27 and the negative electrode 28 that are sequentially deposited on the positive electrode 26.

The electrolyte layer 27 is a dense ceramic membrane that transfers oxygen ions (0 ^2~ ) and can comprise, or consist of, a single ceramic phase that conducts anions but not

electrons, e.g. a single phase of scandia stabilised

zirconia. As an alternative, yttria stabilised zirconia or mixtures of scandia stabilised zirconia and yttria stabilised zirconia could be used instead of scandia stabilised

zirconia. The electrolyte layer 27 is typically about 20 micrometers to 100 micrometers thick.

Important components of the cell of this invention are the negative electrode 28 and the positive electrode 26.

Important criteria for the negative electrode 28 and the positive electrode are: thermodynamic EMF (electromotive force); theoretical energy density (MJoule/kg metal);

thermodynamic electrical efficiency; cost ($/kWatt electrical hours eh) [e = electricity; h = hour] ; maximum current density (determines performance) ; and maximum charge storage (ampere hour/cm ² ) . According to the present invention, the negative electrode 28 and the positive electrode 26 are formed from a porous structure that conducts anions and, in the present

embodiment, also electrons. The porous structure of the negative electrode 28 is partially filled by a first redox mass 31 which comprises a metal in a non-oxidised and/or in an oxidised state. The porous structure of the positive electrode 26 is partially filled by a second redox mass 41 which comprises a metal in a non-oxidised and/or in an oxidised state. The second redox mass comprises a metal in a non-oxidised and/or in an oxidised state with an increased oxidation potential compared with the metal of the first redox mass. The first redox mass and the second redox mass may comprise the same metal in different pairs of oxidation states.

Alternatively, the first redox mass may comprise a first metal and the second redox mass may comprise a second metal with a different oxidation potential compared with the first metal.

In the present invention a first storage medium MeOx/MeOx+y in form of a redox system is introduced at the negative electrode 28 and a second storage medium Me' Ox' /Me' Ox' +y' in form of a redox system is introduced at the positive

electrode 26 as well. Me and Me' indicate two metals but which may also be the same. The storage medium MeOx' /MeOx' +y' at the positive electrode 26 is characterised by an increased or higher oxygen potential compared with the redox system at the negative electrode 28 MeOx/MeOx+y. The storage medium MeOx' /MeOx' +y' at the positive electrode 26 emits oxygen which flows from the positive electrode 26 to the negative electrode 28 where it is absorbed during the discharging process and flows back again during the charging process.

In the present invention the positive electrode 26 has a similar porous structure, preferably ceramic structure, as the negative electrode 28 has. The positive electrode 26 can comprise a redox system based on the same metal as the negative electrode 28 but at a different oxidation state. Alternatively, the positive electrode 26 can comprise a redox system based on a different metal than the negative electrode Me' Ox' ' /Me' Ox' ' ' with a different oxidation potential. Thus, the oxygen which is emitted from the negative electrode 28 can directly be absorbed from the positive electrode 26 during charging and vice versa can flow from the positive electrode 26 to the negative electrode 28 during discharging.

The porous structure (skeleton) of the negative electrode 28 and/or the positive electrode 26 may be formed from a

perovskite (ABO3) . In the present embodiment, the porous structure can be formed from a mixture of LaCr03 doped with Strontium (Sr) and/or Cobalt (Co) and Gadolinium-doped ceria (GDC) . The fraction of GDC may vary from 30% to 70%.

Preferably, the porous structure is formed from a mixture that consist of 50% by volume doped LaCr03 and 50% by volume GDC. A technique available to form a fine porous structure (also called skeleton) is, for example, plasma spraying to form a well-adhered fine structured mixed electrical

conducting skeleton.

The porous ceramic structure of the positive electrode 26 and/or the negative electrode 28 can be infiltrated with a redox mass 41, 31 that is liquid at temperatures above 500°C, in particular at temperatures between 600°C and 800°C.

In a first embodiment of the first or second redox mass, the mass is formed by liquid tin/tin oxide (Sn/SnO _x ) . However, other metals that are liquid at temperatures below 800°C like, for example, lead (Pb) , bismuth (Bi) and thallium (Tl) can also be used in their oxidised and/or non-oxidised form. The liquid metal in the oxidised and/or non-oxidised state infiltrates the porous structure which has a large surface.

Hence, the metal/metal oxide has a large interface to the ion and electron leading porous ceramic structure, which allows a quick loading and a high current of the electrical energy storage device.

In an alternative embodiment of the first or second liquid infiltration mass, the mass is formed by a molten salt that comprises a metal in an oxidised and/or in a non-oxidised state together with a salt of this metal. The salt forming element can be a halogen, in particular chlorine (CI) . Such a redox mass can be represented by the formula aMe-bMeO _x -MeCl _y .

The metal used in the molten salt is chosen such that the molten salt with the metal and/or the metal oxide is liquid at temperatures below 800°C, in particular in the temperature range between 600°C and 800°C. In the present embodiment iron (Fe) is used as metal Me in the molten salt which fulfils the above conditions. An alternative element fulfilling these conditions would be nickel (Ni) .

One or both of the two redox masses may be also solid under operating conditions of the cell.

A cell as described above can itself form an electrical energy storage device. However, a cell can also be used as module in an electrical storage device. Two or more such modules can be connected in series to increase the available voltage or in parallel to increase the available current. In addition, it is possible to form banks, where each bank comprises at least two modules that are connected in parallel to increase the available current. An electrical energy storage device can then comprise two or more such banks that are connected in series to increase the available voltage. An electrical energy storage device formed of three banks 210 each comprising a number of tubular cells 200 as modules is shown in FIG. 10. Generally the electrical contacting of a tubular cell is more difficult compared with a flat cell. Therefore, the flat configuration is advantageous. Figure 12 schematically shows an example for the electrical contacting of a tubular cell of the electrical energy storage device as shown in Figure 8 in a sectional view. In Figure 12 the electrolyte 27 and the positive electrode 26 only

partially surround the circumference of the tubular negative electrode 28 to provide an electrical contacting of the negative electrode 28 via a gas tight, electronically

conducting but anions blocking contacting means 32. A

mechanically compliant and electronically conducting means 33 is placed between the contacting means 32 of the negative electrode 28 and the adjacent positive electrode 26.

There are many advantages presented by a bank of cells to provide a consolidated oxide-ion battery. The cell bank and module system can be much simplified. Since no gaseous fuels are used, the relevant subsystems of SOFC's, such as

reformer, desulfurizer and depleted fuel recirculation loop can be eliminated, resulting in considerable cost reduction. In addition, common combustion of depleted fuel and vitiated air encountered in a SOFC is no longer present. Therefore, the system reliability is also greatly improved.

Up to now only a tubular configuration of the modules of the electrical energy storage device has been described. However, as already mentioned before other configurations are possible is well. For example, Figure 11 shows a flat configuration in a schematic sectional view. The flat electrical energy storage device module of the present embodiment is formed from three layers stacked onto each other, namely a positive electrode 26, a negative electrode 28 and a solid electrolyte 27 located between the negative electrode 28 and the positive electrode 26 and separating them from each other. What has been said with respect to the materials of the positive electrode 26, the electrolyte 27 and the negative electrode 28 in the tubular configuration is also valid in the embodiment shown in Figure 11. In particular, the porous negative electrode 28 and the porous positive electrode 26 are partially filled by a redox mass as it has been described with respect to the tubular configuration.

It should be mentioned that Figure 11 depicts only one example of a flat configuration. Modifications to the flat configuration shown in figure 11 are possible. For example, while the negative electrode 28 is the lowermost layer and the positive electrode 26 the uppermost layer of the layer stack shown in Figure 11 it is also possible to have the negative electrode 28 as the uppermost layer and the positive electrode 26 as the lowermost layer.

Like with the tubular configuration the modules in the flat configuration can be connected in series and/or in parallel. Moreover, the modules of the flat configurations can be piled .

The invention described above provides for an electrical energy storage device that can easily discharge and charge a high capacity of energy quickly and reversibly, as needed. In addition, the electrical energy storage device is simple, can operate for years without major maintenance and does not need to operate on carbonaceous fuel gases such as natural gas fuel, hydrocarbon fuel or its reformed by-products such as ¾ fuel. Moreover, the invention allows for a simple cell and module structure with high theoretical energy density, low system cost, and low power-loss current collection in case of a flat configuration. While specific embodiments of the invention have been

described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and al equivalents thereof.