Patent Application
20130004828
The invention relates generally to an electrochemical cell, and in some specific embodiments, to a high-temperature, rechargeable electrochemical cell, and related devices.
A battery is an essential component used to store a portion of the energy in mobile systems such as electric vehicles, hybrid electric vehicles and non-vehicles (for example locomotives, off-highway mining vehicles, marine applications, buses and automobiles), and for stationary applications such as uninterruptible power supply (UPS) systems and “Telecom” (telecommunication systems). The high-temperature sodium-metal halide electrochemical cells are generally targeted for use in locomotives; telecommunication, and uninterruptible power supply (UPS) batteries. These cells could potentially offer high energy density, high power density, longer cell life, and lower cost-requirements for many of these applications.
Normally, a battery is made up of many cells. Current development of these batteries is focused on performance and cell life. A number of factors can affect performance and limit the cell life. The primary factor may be unreliability of a separator. The most common separator used in these cells is beta″-alumina solid electrolyte (BASE), which is a sodium conducting ceramic. It should be noted that thermal cycling, pressure differential, sodium-wicking, and vibrations in the cell, during use, may damage the separator in some circumstances. The damaged separator may lead to a large thermal excursion which, in turn, causes corrosion of the cell casing (e.g. a steel casing). Usually, when the separator breaks, a metallic short is formed, allowing the battery to operate with the failed cell. In some cells, the cell casing corrodes. The corroded cell casing can allow the cell's chemical components to breach the casing, which can short out the entire battery.
Development work has been undertaken on different battery designs to avoid and/or solve the corrosion problem. One way to avoid the corrosion of the casing is by providing effective thermal and electrical conductance between the core of the cell and the casing. However, the currently available designs are not suitable to address these functions effectively.
It would therefore be desirable to develop a new cell design, with improved thermal and electrical management, for various battery applications.
Some embodiments of the present invention provide an electrochemical cell. The cell comprises a housing having an interior surface defining a volume. An elongated, ion-conducting separator is disposed in the volume, and has a first circumferential surface. A shim structure is disposed between the interior surface of the housing and the first circumferential surface of the separator. The shim structure comprises at least a first shim and a second shim, each having opposing first and second surfaces. At least a portion of the first surface of the first shim is generally parallel to the first circumferential surface of the separator. At least a portion of the second surface of the first shim is generally parallel to a portion of the first surface of the second shim.
In another embodiment, an electrochemical cell comprises a housing having an interior surface defining a volume, and having a base. The housing is polygonal in cross-section, and has a plurality of corner regions. An ion-conducting separator is disposed in the volume, and extends in a vertical direction relative to the base of the housing, so as to define a height dimension of the cell. The separator has a first circumferential surface defining a portion of a first compartment. The separator further has a plurality of lobe portions and valley portions. The plurality of lobe portions corresponds in number to the plurality of corner regions of the housing. The separator is concentrically located in the housing, with each lobe portion of the separator being generally aligned with and projecting towards one of the corner regions of the housing. The cell further includes at least a double-shim structure disposed adjacent to the first circumferential surface of the separator.
An energy storage device is provided in some embodiments of the present invention. The device includes a plurality of electrochemical cells that are in thermal and electrical communication with each other.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings, in which like characters represent like parts throughout the drawings, wherein:
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. For example, in some circumstances, an event or capacity can be expected, while in other circumstances, the event or capacity cannot occur. This distinction is captured by the terms “may” and “may be”.
As used herein, the term “cathodic material” refers to a material that supplies electrons during charging, and is present as part of a redox reaction. The term “anodic material”, as used herein, refers to a material that accepts electrons during charging, and is also present as part of the redox reaction.
As used herein, the term “high temperature” generally refers to a temperature above about 250 degrees Celsius (° C.), unless otherwise indicated.
The electrochemical cell 10 includes an ion-conducting separator 8 disposed in the volume of the housing 2. The separator 8 is usually a sodium ion-conducting solid electrolyte. Suitable materials for the separator 8 may include beta'-alumina, beta″-alumina, beta′-gallate, beta″-gallate, or zeolite. In specific embodiments, the separator 6 includes a beta″-alumina solid electrolyte (BASE).
The separator 8 usually extends in a generally vertical direction relative to the base 6 of the housing 2, so as to define a height dimension of the cell 10. (The particular orientation of the separator, as well as the housing, can vary somewhat). The separator 8 may be cylindrical, elongate, tubular, or cup-shaped, with a closed end 9 and an open end 11. The open-end 11 of the separator may be sealable, and may be part of the separator assembly that defines an aperture 12 for filling the separator 8 with a material during the manufacture process. In one instance, the aperture 12 may be useful for adding the cathodic material. The closed-end 9 of the separator 8 may be pre-sealed, to increase the cell integrity and robustness.
With continued reference to
The separator 8 may have a cross-sectional profile of a circle, an oval or ellipse, a polygon, a cross-shape, or a star-shape, for example.
According to most of the embodiments of the invention, the electrochemical cell 10 (
The shim structure may be mounted in close proximity to the first circumferential surface 14 of the separator 8. The shim structure usually has a shape that allows it to be wrapped around the BASE separator, so as to conform to all or a part of the first circumferential surface of the separator. In one embodiment, the shim structure is shaped so that one or more of its portions (as described below) are in physical contact with the first circumferential surface of the separator. Additionally, the shim structure can be positioned to hold and support the separator in a desired position. For example, the shim structure can be “biased” tightly against the maximum surface area of the BASE separator. Furthermore, the shim structure is designed in a manner that renders the shim portions flexible enough to allow the deposition and wicking of the anodic material, e.g., the sodium, between the shim structure and the first circumferential surface 14 of the separator.
In one embodiment, multiple shim structures (i.e., each with at least a first shim and a second shim, as discussed below) may be combined to cover most of the surface area of the BASE separator from which an ionic current may be collected. If there is more than one shim structure, the structures may be interlocking or partially overlapping. Moreover, each shim structure may be physically connected to at least one other shim structure, or may constitute a portion of the housing. Moreover, other connections or contacts may be made between each shim structure and the housing, using other structures or pieces which may or may not be of the same material or shape. While a multitude of shim structures could be used, e.g., upwards of 10-12 structures, the use of as few shim structures as possible is sometimes desirable (depending on their shape and/or size), because of increased simplicity and lower cost. (The double shim design for embodiments of this invention is sometimes referred to herein as simply the “shim structure”, although strictly speaking, that term also refers to the number of multiple shim structures that circumferentially surround the separator).
An edge of the shim structure can be worked, manufactured and/or formed to facilitate loading of the structure into the cell. The working may include flanging or beveling the edge of the shim structure, for example. A surface of the shim structure can further be worked or modified (e.g., crenellated) to define flowpaths, and to facilitate flow of the anodic material (usually sodium) over the first circumferential surface of the BASE separator, during the cell usage.
The shim structure of the present invention includes at least a first shim and a second shim (the individual shims are not shown in
Each shim may have opposing first and second surfaces. A portion of the first surface of the first shim may be generally parallel to the first circumferential surface of the separator. A portion of the second surface of the first shim may be generally parallel to a portion of the first surface of the second shim. In other words, one shim usually overlaps a substantial portion of the other shim As used herein, the term “substantial portion” refers to greater than about 50 percent of the surface area of a shim, and in some specific embodiments, greater than about 75 percent of the surface area of the shim. In some embodiments, the shims may be substantially identical in shape and size. In some other embodiments, shims of different sizes may be used. In certain embodiments, the shims may be symmetrical, i.e., identical in shape and size; and may fully overlap one another.
The shim structure may be constructed by placing a circumferential surface of one shim parallel to a circumferential surface of the other. For example, the shims may be disposed one over the other, pressed together, or simply placed in contact, e.g., touching one another, face-to-face. In some instances, the structure may be constructed by sliding one shim over another shim. In one embodiment, there is substantially no gap between two shims. In another embodiment, there may be a gap between the shims, though preferably, the gap is relatively small. The anodic material may wick through the gap, although the amount of sodium moving through the gap would usually be insignificant.
The thickness of each shim can vary, but is usually in a range from about 0.01 millimeter to about 1 millimeter. In some embodiments, the thickness may range from about 0.05 millimeter to about 0.5 millimeter, and in some specific embodiments, from about 0.1 millimeter to about 0.2 millimeter.
The material used for the shims is usually thermally, and/or electrically conductive. Some of the preferred materials are chemically and electrochemically inert in the anode environment. In some embodiments, the material may include a single metal, or may be a metal-containing composite or alloy. In one embodiment, the shims may include a metal substrate covered with a different metal layer on one or both sides. In one embodiment, the shims may comprise iron, nickel, copper, aluminum, or any alloy containing the foregoing. Furthermore, each shim of the shim structure may be formed of a same material or of a different material.
The shim design for this invention usually (though not always) performs four primary functions. One function is to provide an intimate and uniform electrical contact between the separator and the anode and/or the housing, to facilitate charge transfer in the initial stages of the cell charging, when no sodium is present in the anode compartment. This may be achieved by shaping the shims forming the shim structure, so as to closely conform to the first circumferential surface of the separator. A second function is to provide thermal management for the cell, by transferring heat from the core i.e., the cathode compartment of the cell, to a location outside of the cell. Moreover, the shims can help to dissipate excess heat after cell failure, to minimize damage to the rest of the device.
The third function of the shim structure is to serve as a structural support for the separator. The shim structure provides dimensional stability by restricting the potential movement of the separator within the housing, and/or by absorbing vibrations and shocks. A fourth function is to provide an optimized distribution of metallic anodic material (e.g., sodium) over the regions of the separator through which ionic current would be passed. This may be achieved by the wicking of the anodic material by capillary forces between the shim structure and the separator.
As described above, the shim structure usually provides electrical and thermal conduction for the operation of the cell. Conventionally, a single, thin shim (e.g. a metal sheet or foil) can be used for that purpose. However, the single, thin shim may not provide sufficient and effective conduction. On the other hand, a thick metal sheet or foil may be used to enhance electrical and thermal conduction. However, the increased thickness of such a shim may lead to less flexibility, making it difficult to bend into a particular shape, and to assemble inside the cell. In contrast, multiple, thin shims according to embodiments of this invention (especially a double-shim structure) advantageously provide improved electrical and thermal conduction, without losing their flexibility.
As discussed above, the separator can be in a variety of shapes, e.g., circular, oval or ellipsoidal, or polygonal cross-sectional profile. In some particular embodiments, the separator may have a cross-sectional profile in the shape of a rugate. Such a shape usually includes a plurality of lobe portions and valley (depression) portions (also referred to as “lobes” and “valleys”), in an alternate fashion, as shown in the multi-lobed depiction of
As alluded to previously, the separator 26 can be designed to have four lobe portions (lobes) 36, and four corresponding valley portions 38 (
In one embodiment, there is substantially no gap between the two shims. In another embodiment, the two shims 42 and 44 may have a gap 60. The gap 60 may be kept as small as possible, although for clarity it is shown, exaggerated, in
With reference to
The shim structure usually includes a separator-facing surface i.e. the inner circumferential surface 46 of the first shim 42; and a housing-facing surface i.e., the outer circumferential surface 49 of the second shim 44. (See
The separator-facing surface usually includes a material e.g., in the form of a layer or a coating, to induce the surface to wick. The layer may be metallic. The surface may also be subjected to a surface-treatment to be made sodium-philic. In certain embodiments, suitable surface-treatments include subjecting the surface to chemical etching, physical etching, or reactive gas exposure. Moreover, the separator-facing surface may be patterned to provide increased surface area. Details of such a surface treatment and patterning are described in Patent Application Publication No. U.S. 2010/0178546 A1, filed on Jan. 9, 2009, which is incorporated herein by reference.
Referring to
Furthermore, the second section 52 usually includes a coil portion 53 that is situated in the valley portion 38, and configured to span a gap 51 between the interior surface 24 (
The selected amount of force generated by the coil-induced pressure is usually configured to achieve a selected distance (gap) 55 between the shim structure 40 and the first circumferential surface 28 of the separator, as shown in
The coil portion 53 may be configured with a selected number of windings. In one embodiment, the coil portion 53 may include at least a one-half winding as depicted in
Once placed in the housing 22 in this configuration, the inner circumferential surface 46 (
It is also possible to place a greater number or a smaller number of the shim structures around the separator, to engage the first surface of the separator. For example,
In the drawings, gaps are sometimes shown between the various parts of the shim structure, and between the overlapping parts of adjacent shim structures, for the purposes of simplifying the illustration. In practice, the shim structures may not have any gaps therebetween, so that sufficient electrical and/or ionic contact can be maintained, e.g., for wicking purposes, and for electrical and thermal communication.
In accordance with an embodiment of the invention, an energy storage device includes a plurality of the electrochemical cells that 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 example that follows is merely illustrative, and should not be construed to be any sort of limitation on the scope of the claimed invention.
4 single-shim structures were manufactured from 4.8 cm×19.8 cm×0.1 mm steel sheets. The steel sheets were coated with nickel on both sides. The thickness of the coating was about 2 microns. The sheets were designed in a particular shape (as described in some of the above exemplary embodiments, and shown in
8 shims of the same shape and size were formed, as described in example 1. Four double-shim structures were constructed by sliding two shims together, so that the two shims were in parallel contact with each other, fully overlapping each other.
2 sodium-nickel-chloride cells; cell 1 and cell 2, were used for measuring the temperature profile within the cells, using single-shim structures (example 1) and double-shim structures (example 2), respectively. The cells used a beta″-alumina separator, and had same design as described for
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
