Low temperature secondary cell with sodium intercalation electrode

Abstract

The present invention provides a molten sodium secondary cell. In some cases, the secondary cell includes a sodium metal negative electrode, a positive electrode compartment that includes a positive electrode disposed in a molten positive electrolyte comprising NaFSA (sodium-bis(fluorosulonyl)amide), and a sodium ion conductive electrolyte membrane that separates the negative electrode from the positive electrolyte. One disclosed example of electrolyte membrane material includes, without limitation, a NaSICON-type membrane. The positive electrode includes a sodium intercalation electrode. Non-limiting examples of the sodium intercalation electrode include Na.sub.xMnO.sub.2, Na.sub.xCrO.sub.2, Na.sub.xNiO, and Na.sub.xFe.sub.y(PO.sub.4).sub.z. The cell is functional at an operating temperature between about 100 C. and about 150 C., and preferably between about 110 C. and about 130 C.

Claims

1. A molten sodium secondary cell, comprising: a negative electrode compartment having a temperature between 100 C. and 150 C. comprising a molten sodium metal negative electrode, which electrochemically oxidizes to release sodium ions during discharge and electrochemically reduces sodium ions to sodium metal during recharging; a positive electrode compartment comprising a positive electrode disposed in a molten positive electrolyte consisting of NaFSA (sodium bis(fluorosulfonyl)amide) having a temperature between 100 C. and 150 C., wherein the positive electrode comprises a sodium intercalation electrode selected from Na.sub.xMnO.sub.2, Na.sub.xCrO.sub.2, Na.sub.xNiO and Na.sub.xFe.sub.y(PO.sub.4).sub.z, wherein x is between 0 and 4, y is between 0 and 4, and z is between 0 and 4; and a sodium ion conductive electrolyte membrane comprising a NaSICON material that separates the sodium metal negative electrode from the molten positive electrolyte such that the molten positive electrolyte does not contact the molten sodium metal negative electrode, wherein the molten sodium metal negative electrode is in contact with the conductive electrolyte membrane.

2. The secondary cell of claim 1, wherein the NaSICON material comprises a composite membrane having a porous layer and a dense functional layer.

3. The secondary cell of claim 1, wherein the negative electrode compartment has a temperature between 110 C. and 130 C.

4. The secondary cell of claim 1, wherein the sodium intercalation electrode comprises Na.sub.xMnO.sub.2, wherein x is between 0 and 1.

5. The secondary cell of claim 1, wherein the sodium intercalation electrode comprises Na.sub.xCrO.sub.2, wherein x is between 0 and 1.

6. The secondary cell of claim 1, wherein the sodium intercalation electrode comprises Na.sub.xNiO or Na.sub.xFe.sub.y(PO.sub.4).sub.z, wherein x is between 0 and 4, y is between 0 and 4, and z is between 0 and 4.

7. The secondary cell of claim 1, further comprising a heat management system to control an operating temperature of the cell.

8. The secondary cell of claim 1, further comprising a heat management system disposed in the negative electrode compartment to control a temperature of the negative electrode compartment.

9. The secondary cell of claim 1, further comprising a heat management system disposed in the positive electrode compartment to control a temperature of the positive electrode compartment.

10. A molten sodium secondary cell, comprising: a negative electrode compartment comprising a molten sodium metal negative electrode having a temperature between 110 C. and 130 C., which electrochemically oxidizes to release sodium ions during discharge and electrochemically reduces sodium ions to sodium metal during recharging; a positive electrode compartment comprising a positive electrode disposed in a molten positive electrolyte consisting of NaFSA (sodium bis(fluorosulfonyl)amide) having a temperature between 110 C. and 130 C., wherein the positive electrode comprises a sodium intercalation electrode selected from Na.sub.xMnO.sub.2, Na.sub.xCrO.sub.2, Na.sub.xNiO and Na.sub.xFe.sub.y(PO.sub.4).sub.z, wherein x is between 0 and 4, y is between 0 and 4, and z is between 0 and 4; a sodium ion conductive electrolyte membrane comprising a NaSICON material that separates the molten sodium metal negative electrode from the molten positive electrolyte such that the molten positive electrolyte does not contact the molten sodium metal negative electrode; and a heat management system to control an operating temperature of the cell, wherein the sodium metal negative electrode is molten and in contact with the conductive electrolyte membrane as the cell operates.

Description

BRIEF DESCRIPTION OF THE SEVERAL DRAWINGS

(1) In order that the manner in which the above-recited and other features and advantages of the invention are obtained and will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings. Understanding that the drawings are not made to scale, depict only some representative embodiments of the invention, and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

(2) FIG. 1 depicts a schematic diagram of a representative embodiment of a molten sodium secondary cell, wherein the cell is in the process of being discharged.

(3) FIG. 2 depicts a schematic diagram of a representative embodiment of the molten sodium secondary cell, wherein the cell is in the process of being recharged.

DETAILED DESCRIPTION OF THE INVENTION

(4) Reference throughout this specification to one embodiment, an embodiment, or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases in one embodiment, in an embodiment, and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. Additionally, while the following description refers to several embodiments and examples of the various components and aspects of the described invention, all of the described embodiments and examples are to be considered, in all respects, as illustrative only and not as being limiting in any manner.

(5) Furthermore, the described features, structures, or characteristics of the invention may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of suitable sodium negative electrodes, positive electrode materials, liquid positive electrode solutions, sodium ion conductive electrolyte membrane, etc., to provide a thorough understanding of embodiments of the invention. One having ordinary skill in the relevant art will recognize, however, that the invention may be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

(6) As stated above, secondary cells can be discharged and recharged and this specification describes cell arrangements and methods for both states. Although the term recharging in its various forms implies a second charging, one of skill in the art will understand that discussions regarding recharging would be valid for, and applicable to, the first or initial charge, and vice versa. Thus, for the purposes of this specification, the terms recharge, recharged and rechargeable shall be interchangeable with the terms charge, charged and chargeable respectively.

(7) The present invention provides a molten sodium secondary cell that functions at an operating temperature between about 100 C. and about 150 C. While the described cell can comprise any suitable component, FIG. 1 shows a representative embodiment in which the molten sodium secondary cell 10 comprises a negative electrode compartment 15 that includes a sodium metal negative electrode 20 and a positive electrode compartment 25 that comprises a positive electrode. The positive electrode includes a current collector 30 and a sodium intercalation electrode material A disposed in a molten sodium-FSA (sodium-bis(fluoro-sulfonyl)amide) electrolyte 35. A sodium ion conductive electrolyte membrane 40 separates the negative electrode from the positive electrolyte and a first terminal 45 from a second terminal 50. To provide a better understanding of the described cell 10, a brief description of how the cell functions is provided below. Following this discussion, each of the cell's components shown in FIG. 1 is discussed in more detail.

(8) Turning now to the manner in which the molten sodium secondary cell 10 functions, the cell can function in virtually any suitable manner. In one example, FIG. 1 illustrates that as the cell 10 is discharged and electrons (e.sup.) flow from the negative electrode 20 (e.g., via the first terminal 45), sodium is oxidized from the negative electrode 20 to form sodium ions (Na.sup.+). FIG. 1 shows that these sodium ions are respectively transported from the sodium negative electrode 20, through the sodium ion conductive membrane 40, and to the positive electrolyte 35.

(9) In a contrasting example, FIG. 2 shows that as the secondary cell 10 is recharged and electrons (e.sup.) flow into the sodium negative electrode 20 from an external power source (not shown), such as a recharger, the chemical reactions that occurred when the cell 10 was discharged (as shown in FIG. 1) are reversed. Specifically, FIG. 2 shows that as the cell 10 is recharged, sodium ions (Na.sup.+) are respectively transported from the positive electrolyte 35, through the electrolyte membrane 40, and to the negative electrode 20, where the sodium ions are reduced to form sodium metal (Na).

(10) Referring now to the various components of the cell 10, the cell, as mentioned above, can comprise a negative electrode compartment 15 and a positive electrode compartment 25. In this regard, the two compartments can be any suitable shape and have any other suitable characteristic that allows the cell 10 to function as intended. By way of example, the negative electrode and the positive electrode compartments can be tubular, rectangular, or be any other suitable shape. Furthermore, the two compartments can have any suitable spatial relationship with respect to each other. For instance, while FIG. 2 shows that the negative electrode compartment 15 and the positive electrode compartment 25 can be adjacent to each other, in other embodiments (not shown), one compartment (e.g., the negative electrode compartment) is disposed, at least partially, in the other compartment (e.g., the positive electrode compartment), while the contents of the two compartments remain separated by the electrolyte membrane 40 and any other compartmental walls.

(11) With respect to the negative electrode 20, the cell 10 can comprise any suitable sodium negative electrode 20 that allows the cell 10 to function (e.g., be discharged and recharged) as intended. Some examples of suitable sodium negative electrode materials include, but are not limited to, a sodium sample that is substantially pure and a sodium alloy comprising any other suitable sodium-containing negative electrode material. In certain embodiments, however, the negative electrode comprises or consists of an amount of sodium that is substantially pure. In such embodiments, because the melting point of pure sodium is around 98 C., the sodium negative electrode will become molten above that temperature.

(12) With respect to the positive current collector 30, the positive electrode compartment 25 can comprise any suitable positive electrode that allows the cell to be charged and discharged as intended. For instance, the positive electrode can comprise virtually any current collector 30 in combination with a sodium intercalation material, shown generically as A in FIGS. 1 and 2, in a molten sodium-FSA electrolyte 35.

(13) In some embodiments, the positive current collector may comprise a wire, felt, plate, tube, mesh, foam, and/or other suitable current collector configuration. In some non-limiting embodiments, the sodium intercalation material (A) is selected from Na.sub.xMnO.sub.2, Na.sub.xCrO.sub.2, Na.sub.xNiO, and Na.sub.xFe.sub.y(PO.sub.4).sub.z where x, y, and z are between 0 and about 4. In one embodiment, the sodium intercalation materials include Na.sub.xMnO.sub.2, Na.sub.xCrO.sub.2, Na.sub.xNiO where x is between 0 and 1.

(14) In some non-limiting embodiments, the reactions that occur at the negative electrode and at the positive electrode and the overall reaction as the cell 10 is discharged may occur as illustrated below:

(15) Negative electrode xNaxNa.sup.++xe.sup.

(16) Positive electrode A+xe.sup.A.sup.x

(17) Overall xNa+ANa.sub.xA

(18) Accordingly, some embodiments of the describe cell 10, at least theoretically, are capable of producing about 3.2V0.5V at standard temperature and pressure.

(19) Moreover, some examples of overall reactions that may occur during the discharging and charging of a cell in which the positive electrode 30 comprises a Na.sub.xMnO.sub.2 intercalation material, the negative electrode 20 comprises sodium, and the positive electrolyte 35 comprises molten sodium-FSA, are shown below:

(20) (Discharge) xNa+MnO.sub.2Na.sub.xMnO.sub.2

(21) (Charge) Na.sub.xMnO.sub.2.fwdarw.MnO.sub.2+xNa

(22) With respect now to the molten sodium-FSA positive electrolyte 35, the positive electrolyte has been found to have good sodium ion conductivity that allows the cell 10 to function as intended. It is intended for the positive electrolyte to have a higher sodium ion conductivity than the electrolyte membrane 40. The molten sodium-FSA conductivity ranges between about 50 mS/cm and 100 mS/cm. The NaSICON conductivity may range between about 20 and about 50 mS/cm. The NaSICON conductivity may range between about 30 and about 45 mS/cm.

(23) With regards now to the sodium ion conductive electrolyte membrane 40, the membrane can comprise any suitable material that selectively transports sodium ions and permits the cell 10 to function with the molten sodium negative electrode and the positive electrolyte. In some embodiments, the electrolyte membrane comprises a NaSICON-type (sodium Super Ion CONductive) material. In such embodiments, the NaSICON-type material may comprise any known or novel NaSICON-type material that is suitable for use with the described cell 10. Some non-limiting examples of NaSICON-type compositions include, but are not limited to, Na.sub.3Zr.sub.2Si.sub.2PO.sub.12, Na.sub.1+xSi.sub.xZr.sub.2P.sub.3xO.sub.12 (where x is selected from 1.6 to 2.4), Y-doped NaSICON (Na.sub.1+x+yZr.sub.2yY.sub.ySi.sub.xP.sub.3xO.sub.12, Na.sub.1+xZr.sub.2yY.sub.y Si.sub.xP.sub.3xO.sub.12y (where x=2, y=0.12), and Fe-doped NaSICON (Na.sub.3Zr.sub.2/.sub.3Fe.sub.4/.sub.3P.sub.3O.sub.12). Indeed, in certain embodiments, the NaSICON-type membrane comprises Na.sub.3Si.sub.2Zr.sub.2PO.sub.12. In still other embodiments, the NaSICON-type membrane comprises known or novel composite, cermet-supported NaSICON membrane. In such embodiments, the composite NaSICON membrane can comprise any suitable component, including, without limitation, a porous NaSICON-cermet layer that comprises NiO/NaSICON or any other suitable cermet layer, and a dense NaSICON layer. In yet other embodiments, the NaSICON membrane comprises a monoclinic ceramic.

(24) Where the cell's electrolyte membrane 40 comprises a NaSICON-type material, the NaSICON-type material may provide the cell 10 with several beneficial characteristics. In one example, because such membranes selectively transport sodium ions but do not allow the negative electrode 20 and the positive electrolyte 35 to mix, such membranes can help the cell to have minimal capacity fade and to have a relatively stable shelf life at ambient temperatures.

(25) With reference now to the terminals 45 and 50, the cell 10 can comprise any suitable terminals that are capable of electrically connecting the cell with an external circuit, including without limitation, to one or more cells. In this regard, the terminals can comprise any suitable material and any suitable shape of any suitable size.

(26) In addition to the aforementioned components, the cell 10 can optionally comprise any other suitable component. By way of non-limiting illustration FIG. 2 shows an embodiment in which the cell 10 comprises a heat management system 55, 60. Independent heat management systems may be associated with the negative electrode and positive electrode compartments. Alternatively, a single heat management system may be disposed in only one compartment. In such embodiments, the cell can comprise any suitable type of heat management system that is capable of maintaining the cell within a suitable operating temperature range. Some examples of such heat management systems include, but are not limited to, a heater, one or more temperature sensors, and appropriate temperature control circuitry.

(27) The described cell 10 may function at any suitable operating temperature. In other words, as the cell is discharged and/or recharged, the sodium negative electrode and the positive electrolyte may have any suitable temperature. The negative and positive electrode compartments may operate at the same or different temperatures. Indeed, in some embodiments, the cell functions at an operating temperature that is as high as a temperature selected from about 120 C., about 130 C., and about 150 C. Moreover, in such embodiments, as the cell functions, the temperature of the negative and/or positive electrode compartments can be as low as a temperature selected from about 120 C., about 115 C., about 110 C., and about 100 C. Indeed, in some embodiments, as the cell functions, the temperature of the negative and/or positive electrode compartments may be between about 100 C. and about 150 C. In other embodiments, the cell functions at a temperature between about 110 C. and about 130 C. In yet other embodiments, however, as the cell functions, the temperature of the negative and/or positive electrode compartments is about 120 C.about 10 C.

(28) In addition to the aforementioned benefits of the cell 10, the described cell may have several other beneficial characteristics. By way of example, by being able to operate in a temperature range between about 100 and about 150 C., the cell 10 may operate in a temperature range that is significantly lower the operating temperature of certain conventional molten sodium rechargeable batteries. Accordingly, the described cell may require less energy to heat and/or dissipate heat from the cell as the cell functions, may be less dangerous use or handle, and may be more environmentally friendly.

(29) The following examples are given to illustrate various embodiments within, and aspects of, the scope of the present invention. These are given by way of example only, and it is understood that the following examples are not comprehensive or exhaustive of the many types of embodiments of the present invention that can be prepared in accordance with the present invention.

(30) While specific embodiments and examples of the present invention have been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention, and the scope of protection is only limited by the scope of the accompanying claims.