Patent Application
20140295237
This invention generally relates to electrochemical devices, such as batteries. In some particular embodiments, the invention relates to sealing systems and current-carrying features within various energy storage devices.
Metal chloride batteries, especially sodium-metal chloride batteries with a molten sodium negative electrode (usually referred to as the anode) and a beta-alumina solid electrolyte, are of considerable interest for energy storage applications. In addition to the anode, the batteries include a positive electrode (usually referred to as the cathode) that supplies/receives electrons during the charge/discharge of the battery. The solid electrolyte, described below, functions as the membrane or “separator” between the anode and the cathode.
The metal chloride batteries and other types of thermal batteries can be employed in a number of applications, such as uninterruptable power supply (UPS) devices; or as part of a battery backup system for a telecommunications (“telecom”) device, sometimes referred to as a telecommunication battery backup system (TBS). The batteries are often capable of providing power surges (high currents) during the discharge cycle. In an ideal situation, the battery power can be achieved without a significant loss in the working capacity and the cycle life of the battery. The advantageous features of these types of batteries provide opportunities for applications in a number of other end use areas as well.
The current collectors are important elements in the operation of these types of electrochemical cells, since they are directly responsible for electrical conductivity characteristics. In particular, the cathode current collector can be an especially important element in a sodium-metal chloride battery. In part, this is because of the belief that the cathode electrochemical reactions are not only concentrated spatially, but include both a spatial and a temporal distribution during the charge/discharge cycles of the cells. Therefore, in order to facilitate the electrochemical reactions that must occur in the cell, it is important for the current collector to provide electronic conductivity to the “reaction front”, with such a distribution characteristic.
Electrochemical cells of this type (such as batteries) operate at high temperatures, usually above about 250 degrees Celsius (° C.); and they include a number of components that need to be sealed (e.g., hermetically sealed), to ensure that each battery cell will function properly. The sodium metal halide (NaMx) batteries, for instance, may contain electrochemical cells that include a sodium metal anode and a metal halide (NiCl2 for example) cathode. A beta”-alumina solid electrolyte (BASE) separator can be used to separate the anode and cathode. The solid electrolyte may allow the transport of sodium ions between anode and cathode. A secondary electrolyte (NaAlCl4) can also be used in the cathode mixture. The cathode mixture typically contains nickel and sodium chloride, along with other additives. The cathode mixture is contained inside the BASE tube, which is closed or sealed on one end after filling. At operating temperatures, the cathode mixture may be in a molten fluid or fluid-like form.
In present, typical designs of NaMx and sodium sulfur cells, the open end of the beta”-alumina ceramic tube is joined to an alpha-alumina collar using a glass seal. Spinel, zirconia, yttria, or other ceramic insulators, or combinations thereof, may also be used as a collar material in NaMx cells. The alpha-alumina collar electrically isolates the anode from the cathode. In order to enable the sealing of this ceramic subassembly to the current collectors (anode and cathode), and thereby at least partially seal the cell, two metallic rings (typically Ni) are typically coupled or bonded to the alpha-alumina collar prior to the sealing glass operation. The inside Ni ring is then typically welded to a cathode current collector assembly, and the current collector assembly includes another weld. The outside Ni ring is typically welded to an anode current collector (e.g., the metallic battery case) via a metal (e.g., Ni) outer bridge member.
Moreover, the various sealing mechanisms within the cell are all critical for its function, reliability, and safety. For example, the integrity (e.g., strength and/or hermeticity) of the glass seal joint between the beta”-alumina ceramic tube and the alpha-alumina collar is very important to the overall integrity of the cell. The same holds true for other joining regions, e.g., the weld between the inside metal ring and the cathode current collector; the weld within the cathode current collector assembly; and the welds between the bridge member and the outer metal ring and the anode current collector, e.g., the battery case. The strength of metal-ceramic joints between the outer and inner metal rings and the ceramic collar can also be critical. As a result, each joint or seal must be formed under specific conditions and process steps particular to the specific type of seal (weld, glass seal, metallization/thermal compression bonding (TCB), etc.) being used to ensure hermeticity.
patent application Ser. No. 13/852,462 (S. Kumar et al, referenced above), provides a description of battery cells with these types of sealing mechanisms used in the prior art, for the sealing of an anodic chamber, as well as other cell structures. Illustrative
Welds and other types of joints and seals in these types of high-temperature electrochemical cells very often represent points of weakness and potential failure. As an illustration, noted in the Kumar patent application, the joints between electrochemical cell bridge members, ring structures, and cell cases are often formed as lap or edge welds. It is known that these types of welds can be relatively difficult to manufacture, and are prone to relatively high failure rates. The welds and joints therefore need to be subjected to numerous inspections and tests to ensure their reliability. This can represent a manufacturing and operational disadvantage—even more so when there are a relatively large number of joints, since they represent a large number of potential failure points.
With these considerations in mind, new types of energy storage devices and other types of electrochemical cells would be welcome in the art. The new devices should exhibit improved electrical conductivity, e.g., by way of unique features within the various cell compartments. Moreover, the devices should be obtainable with lower fabrication costs, and higher reliability, e.g., by reducing the number of sealing mechanisms within the devices.
One embodiment of the invention is directed to an energy storage cell, comprising:
(a) an anodic chamber for containing an anodic material; and a cathodic chamber for containing a cathodic material, separated from each other by an electrolyte separator tube, all contained within a case for the cell;
(b) an electrically insulating ceramic collar positioned at an opening of the cathodic chamber, and defining an aperture in communication with the opening; and
(c) a current collector brazed to the ceramic collar, extending into the cathodic chamber, and in the form of a porous, metallic mesh. In preferred embodiments, the case and the ceramic collar are hermetically sealed to each other by at least one active braze.
Another embodiment of the invention relates to a sodium metal halide thermal battery, comprising a plurality of electrochemical cells that are in electrical communication with each other, wherein each electrochemical cell comprises:
(a) an anodic chamber for containing an anodic material; and a cathodic chamber for containing a cathodic material, separated from each other by an electrolyte separator tube, all contained within a case for the cell;
(b) an electrically insulating ceramic collar positioned at an opening of the cathodic chamber, and defining an aperture in communication with the opening; and
(c) a current collector brazed to the ceramic collar, extending into the cathodic chamber, and in the form of a porous, metallic mesh; and wherein the case and the ceramic collar are hermetically sealed to each other by at least one active braze.
Each embodiment presented below facilitates the explanation of certain aspects of the invention, and should not be interpreted as limiting the scope of the invention. Moreover, approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary, without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about,” is not limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value.
In the following specification and claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. As used herein, the terms “may” and “may be” indicate a possibility of an occurrence within a set of circumstances; a possession of a specified property, characteristic or function; and/or qualify another verb by expressing one or more of an ability, capability, or possibility associated with the qualified verb. Accordingly, usage of “may” and “may be” indicates that a modified term is apparently appropriate, capable, or suitable for an indicated capacity, function, or usage, while taking into account that in some circumstances, the modified term may sometimes not be appropriate, capable, or suitable.
As alluded to previously, one aspect of the present invention relates to energy storage devices that include sealing systems in which device components can be hermetically sealed to each other by at least one active braze. Moreover, preferred embodiments eliminate the need for bridge members, or for a relatively large number of weld-sites.
The general depiction of
In
In many embodiments, the outer-facing sealing surface 122 of the collar 120 may be substantially planar, and may extend about the periphery of the collar 120. In this manner, the sealing surface may be utilized to seal the periphery of the collar 120 to the case 112. More specifically, as shown in
With continued reference to
As also shown in
With continued reference to
It should be understood that an electrochemical cell like that depicted in
Cell 200 in
The cathode current collector 210 includes or defines an internal aperture that may be concentric with the internal aperture 216 of the cathode chamber, when the collar and the cathode current collector are sealed to one another, as shown in
With continued reference to
For most embodiments of the present invention, current collector 210 (not necessarily drawn to scale) is in the form of a porous metallic mesh. The metal, or metal alloy, usually comprises nickel, e.g., alloys containing at least about 25% by weight nickel. However, the current collector can be formed of other metals or metal alloys in some situations, depending on cell design.
In the case of sodium metal halide cells, the current collector material usually must be one that is non-reactive with any of the halide components in the cell, while still retaining the required electrical conductivity characteristics. Non-limiting examples of current collector materials for some embodiments include molybdenum, tungsten, tungsten carbide, noble metals such as gold, platinum, and iridium; as well as various iron-nickel alloys or nickel-cobalt ferrous alloys, e.g., Kovar®-type materials. One illustrative material of this type may include about 29% (by weight) nickel, 17% cobalt, less than 1% (each) of carbon, silicon, and manganese, with the balance being iron.
The present inventors discovered that the presence of a mesh can very advantageously increase the surface area of the current collector, allowing it to exhibit less electrical resistance, and carry more current. These attributes are especially important in the case of cell designs like that of
The size of the mesh can vary to some degree; and will be determined by a number of factors, such as the type and shape of cathode material being used; the particular design and shape of the cathode chamber; and the type of metal forming the mesh. In some embodiments for alkali metal halide cells, the average area of the mesh opening is in the range of about 2 mm to about 4 mm. The mesh can itself have various shapes or “weaves”, e.g., square-like, diamond-shaped, and the like. Preferably, the mesh is formed of an interlaced structure of metallic wire. The size of the wire that forms the mesh can also vary, based on some of the factors set forth above. The wire may have a diameter in the range of about 0.1 mm to about 1 mm, as an example.
As shown in
As is apparent from the drawings, the cathodic chamber is often tubular, and the mesh is in the form of an elongated cylinder. The cylinder is generally concentric with the tubular cathodic chamber.
In the embodiment of
As those skilled in the art understand, the cathode materials are often in the form of granules, e.g., granules of sodium chloride. Sodium chloride in the cathode dissolves to form sodium ions and chloride ions during charging of the electrochemical cell. Sodium ions, under the influence of applied electrical potential, conduct through the separator 252, and combine with electrons from the external electrical circuit, to form the material of the sodium electrode. Chloride ions react with the cathodic material to form metal chloride, donating electrons back to the external circuit. During discharge, sodium ions conduct back through the separator, reversing the reaction, and generating electrons, as described, for example, in U.S. Pat. No. 8,530,090 (Seshadri et al), incorporated herein by reference.
With continued reference to
As mentioned previously, the case and the ceramic collar of an energy storage device according to this invention are preferably sealed to each other by at least one active braze. (Other structures within the cell can also be brazed, as described herein). Typically, “brazing” uses a braze material (usually an alloy) having a lower liquidus temperature than the melting points of the components (i.e. their materials) to be joined, e.g., metal components and an alpha-alumina collar. The braze material is brought to or slightly above its melting (or liquidus) temperature, while protected by a suitable atmosphere. The braze material then flows over the components (known as wetting), and is then cooled to join the components together.
As used herein, “braze alloy composition” or “brazing alloy”, or “braze material”, refers to a composition that has the ability to wet the components to be joined, and to seal them. A braze alloy for a particular application should withstand the service conditions required, and melt at a lower temperature than the base materials, or melt at a very specific temperature. Conventional braze alloys usually do not wet ceramic surfaces sufficiently to form a strong bond at the interface of a joint. In addition, the alloys may be prone to sodium and halide corrosion.
As used herein, the term “brazing temperature” refers to a temperature to which a brazing structure is heated to enable a braze alloy to wet the components to be joined, and to form a brazed joint or seal. The brazing temperature is often higher than or equal to the liquidus temperature of the braze alloy. In addition, the brazing temperature should be lower than the temperature at which the components to be joined may not remain chemically, compositionally, and mechanically stable. There may be several other factors that influence the brazing temperature selection, as those skilled in the art understand.
Embodiments of the present invention utilize a braze alloy composition capable of forming a joint by “active brazing” with one or more “active brazes.” In some specific embodiments, e.g., in the case of sodium-based thermal batteries, the braze composition also has a relatively high resistance to sodium and halide corrosion.
In some embodiments, the braze alloy composition includes nickel and an active metal element; and further comprises a) germanium, b) niobium and chromium, or c) silicon and boron. Alternatively, the braze alloy composition may comprise copper, nickel, and an active metal element. Each of the elements of the alloy contributes to at least one property of the overall braze composition, such as liquidus temperature, coefficient of thermal expansion, flowability or wettability of the braze alloy with a ceramic, and corrosion resistance.
“Active brazing” is a brazing approach often used to join a ceramic to a metal or a metal alloy, or a ceramic to a ceramic. Active brazing uses an active metal element that promotes wetting of a ceramic surface, enhancing the capability of providing a seal (e.g., a hermetic seal). “Sealing”, as used herein, is a function performed by a structure that joins other structures together, to reduce or prevent leakage through the joint between the other structures. The seal structure may also be referred to as a “seal.” An “active metal element”, as used herein, refers to a reactive metal that has higher affinity to the oxygen compared to the affinity of element to the ceramic, and thereby reacts with the ceramic.
A braze alloy composition containing an active metal element can also be referred to as an “active braze alloy.” The active metal element is thought to undergo a decomposition reaction with the ceramic, when the braze alloy is in a molten state, and leads to the formation of a thin reaction layer on the interface of the ceramic and the braze alloy. The thin reaction layer allows the braze alloy to wet the ceramic surface, resulting in the formation of a ceramic-metal joint/bond, which may also be referred to as “active braze seal.”
Thus, an active metal element is an essential constituent of a braze alloy for employing active brazing. A variety of suitable active metal elements may be used to form the active braze alloy. The selection of a suitable active metal element mainly depends on the chemical reaction with the ceramic (e.g., alpha-alumina of the collar) to form a uniform and continuous reaction layer, and the capability of the active metal element of forming an alloy with a base alloy (e.g. Ni—Ge alloy).
An “active” element will react with the ceramic, forming a reaction layer between the ceramic and the molten braze that will reduce the interfacial energy to such a level that wetting of the ceramic takes place. In some preferred embodiments, the active metal element is titanium. Other suitable examples of the active metal element include, but are not limited to, zirconium, hafnium, and vanadium. A combination of two or more active metal elements may also be used.
The presence and the amount of the active metal may influence the thickness and the quality of the thin reactive layer, which contributes to the wettability or flowability of the braze alloy, and therefore, the bond strength of the resulting joint. The active metal element is generally present in small amounts suitable for improving the wetting of the ceramic surface, and forming the thin reaction layer, for example, a layer of less than about 10 microns. A high amount of the active metal layer may cause or accelerate halide corrosion.
The braze alloy composition may further include at least one alloying element. The alloying element may provide further adjustments in several required properties of the braze alloy, for example, the coefficient of thermal expansion, liquidus temperature, and brazing temperature. In one embodiment, the alloying element can include, but is not limited to, cobalt, iron, chromium, niobium or a combination thereof.
Several of the exemplary locations for the active braze are shown in
Those skilled in the art understand that commercial energy storage devices most often include a plurality of the electrochemical cells described herein. The cells are, directly or indirectly, in thermal and/or electrical communication with each other. Those of ordinary skill in the art are familiar with the general principles of such devices.
The present invention has been described in terms of some specific embodiments. They are intended for illustration only, and should not be construed as being limiting in any way. Thus, it should be understood that modifications can be made thereto, which are within the scope of the invention and the appended claims. Furthermore, all of the patents, patent applications, articles, and texts which are mentioned above are incorporated herein by reference.
This Patent Application is a Continuation-in-Part of application Ser. No. 13/852,462, filed on Mar. 28, 2013, the contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13852462 | Mar 2013 | US |
| Child | 14210675 | US |
