Electrochemical cells for electrical energy storage

Abstract

The present invention relates to an electrochemical cell comprising: an anode comprising potassium (K), sodium (Na), and carbon (C); a cathode comprising polysulfide; and a catholyte comprising amide. The present invention also relates to an electrochemical cell comprising: an anode comprising K-Na liquid alloy and carbon powder; a cathode comprising K2S8; and a catholyte comprising acetamide and ε-caprolactam. The present invention also relates to a use of an electrochemical cell as disclosed herein for energy storage.

Description

[0001] DESCRIPTION

[0002] Title of Invention: Electrochemical Cells For Electrical Energy Storage

[0003] Technical Field

[0004] The present disclosure generally relates to electrochemical cells, and more particularly relates to electrochemical cells for use in energy storage.

[0005] Background Art

[0006] Grid-level energy storage is important for addressing climate change and realizing a sustainable future, since it can stabilize intermittent power generation from renewable solar and wind energy. In particular, lithium-ion batteries, flow batteries, and high-temperature Na-S batteries are three of the most promising grid-scale battery technologies.

[0007] However, lithium-ion batteries arc hindered by its high cost and flow batteries show limited energy density and efficiency. Sodium-sulfur (Na-S) batteries have recently gained increasing interest as an electrical energy storage device for renewable integration and grid applications, along with commercial or fleet transportation. They show high theoretical energy density, high roundtrip efficiency and long cycle life. The materials of Na-S batteries are non-toxic, inexpensive and earth-abundant. All these features make it extremely competitive in comparison with other technologies for grid energy storage such as lithium-ion and flow batteries. Conventional Na-S batteries operate at 300-350 °C and use P-alumina solid electrolytes (BASE) to separate the molten sulfur cathode from the molten sodium anode. The full cell reaction is

[0008] 2 Na + x S Na2Sx (x = 3-5)

[0009] However, the high operating temperature of the cell comes with severe drawbacks, which includes:

[0010] 1) the intrinsic corrosive behavior of polysulfide, which increases the manufacturing cost of the cell; and

[0011] 2) thermal runaway and open circuit cell failure mode. If the BASE is broken during battery operation, molten sulfides may directly contact and react vigorously with molten sodium, which could result in fires or explosions. Adjacent cells may also experience significant power loss due to open circuits. In addition, it is also difficult to improve on the energy density of the cell due to the incomplete conversion of the polysulfides.

[0012] Thus, there is a need to provide an electrochemical cell that overcomes, or at least ameliorates one or more of the disadvantages described above.

[0013] Summary

[0014] In an aspect of the present disclosure, there is provided an electrochemical cell comprising: a. an anode comprising potassium (K), sodium (Na), and carbon (C); b. a cathode comprising polysulfide; and c. a catholyte comprising amide.

[0015] In another aspect of the present disclosure, there is provided an electrochemical cell comprising: a. an anode comprising K-Na liquid alloy and carbon powder; b. a cathode comprising K2S8; and c. a catholyte comprising acetamide and E-caprolactam.

[0016] In a further aspect of the present disclosure, there is provided a use of an electrochemical cell as disclosed herein for energy storage.

[0017] Advantageously, the electrochemical cell of the present disclosure may exhibit increased cell density and lower operating temperatures as compared to conventional electrochemical cells. This may allow the electrochemical cell to be safer, cheaper to operate and maintain than conventional electrochemical cells. Further advantageously, the electrochemical cell of the present disclosure may exhibit significantly lower capacity decay and increased capacity.

[0018] Definitions

[0019] Unless otherwise defined herein, scientific and technical terms used in this application shall have the meanings that are commonly understood by those of ordinary skill in the art. Generally, nomenclature used in connection with, and techniques of, chemistry described herein, are those well- known and commonly used in the art.

[0020] Unless the context requires otherwise or specifically stated to the contrary, integers, steps, or elements of the invention recited herein as singular integers, steps or elements clearly encompass both singular and plural forms of the recited integers, steps or elements.

[0021] As used herein, the term "eutectic" refers to a mixture of substances that freezes at a temperature that is lower than the freezing points of the separate constituents.

[0022] As used herein, the phrase “flow battery” refers to an electrochemical energy storage device in which chemical energy is stored in one or more liquid electrolytes that flow through electrochemical cells during operation. The system typically comprises two separate electrolyte tanks (anolyte and catholyte), a pump mechanism to circulate the electrolytes, and an ion-exchange membrane or separator that facilitates ionic exchange while preventing cross -mixing of the electrolytes.

[0023] The word “substantially” does not exclude “completely” e.g. a composition which is “substantially free” from Y may be completely free from Y. Where necessary, the word “substantially” may be omitted from the definition of the invention.

[0024] As used herein in the specification and in the claims, the phrase "at least," in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase "at least one" refers, whether related or unrelated to those elements specifically identified. Thus, as a nonlimiting example, "at least one of A and B" (or, equivalently, "at least one of A or B," or, equivalently "at least one of A and / or B") can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc.

[0025] Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements.

[0026] As used herein, the term "about", in the context of concentrations of components of the formulations, typically means + / - 5% of the stated value, more typically + / - 4% of the stated value, more typically + / - 3% of the stated value, more typically, + / - 2% of the stated value, even more typically + / - 1% of the stated value, and even more typically + / - 0.5% of the stated value.

[0027] Throughout this disclosure, certain embodiments may be disclosed in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub -ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of tire range.

[0028] Certain embodiments may also be described broadly and genetically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the disclosure. This includes the generic description of the embodiments with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

[0029] Brief Description of Drawings

[0030] The accompanying drawings illustrate disclosed embodiments and serve to explain the principles of the disclosed embodiments. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

[0031] Fig. 1

[0032] Fig. 1 is a diagram illustrating the reaction mechanism of an embodiment of the present invention, a K / Na- S battery system.

[0033] Fig. 2

[0034] Fig. 2 is a series of graphs showing the impedance results of a symmetric cell comprising (a) an embodiment of the present invention (K-Na-C electrode), (b) a comparative embodiment (K-Na electrode), and the cycling of a symmetric cell comprising (c) an embodiment of the present invention (K-Na-C electrode), and (d) a comparative embodiment (K-Na electrode at 2.6 mA cm-2).

[0035] Fig. 3

[0036] Fig. 3 is a series of graphs, with (a) showing the charge / discharge profiles of an embodiment of the present invention comprising 1 M [S] in the catholyte, (b) showing the cycling performance of said embodiment at 0.5 mA cm2, (c) showing the charge / discharge profiles of an embodiment of the present invention comprising 4 M [S] in the catholyte, and (d) showing the cycling performance of said embodiment at 2.0 mA cm2. Fig. 4 is a diagram illustrating an embodiment of the present invention, a K / Na-S flow battery system for future grid energy storage.

[0037] Detailed Disclosure of Embodiments

[0038] Conventional grid scale energy storage technologies include lithium-ion batteries, flow batteries, and high-temperature Na-S batteries. However, lithium-ion batteries are limited by cost, while flow batteries show limited energy density and efficiency. While sodium-sulfur (Na-S) batteries are able to overcome these problems, they require molten sodium and sulfur in order to operate, and hence have a typical operating temperature of 300 to 350 °C. This relatively high operating temperature leads to increased costs in thermal management, as well as safety issues.

[0039] Hence, to avoid the high operating temperatures, the present invention provides a potassium / sodium-sulfur (K / Na-S) flow battery that is useful for grid scale electrical energy storage. The electrochemical cell comprises an anode comprising liquid K / Na at room temperatures, as well as sulfur dissolved in the catholyte, which advantageously allow the electrochemical cell to operate at much lower operating temperatures, which not only decreases significant cost in thermal management, but also solves safety and reliability issues arising from the high operating temperature, such as corrosion and thermal runaway. In addition, the present invention also exhibits high energy density at the cell level as well as a lower operating cost as compared to conventional energy storage technologies. In addition, the present invention also exhibits a larger capacity and significantly less capacity decay when compared to conventional energy storage technologies.

[0040] The present invention provides an electrochemical cell comprising: a. an anode comprising potassium (K), sodium (Na), and carbon (C); b. a cathode comprising polysulfide; and c. a catholyte comprising amide.

[0041] In some embodiments, the electrochemical cell may have an operating temperature in a range of about 25 °C to about 120 °C, from about 25 °C to about 115 °C, from about 25 °C to about 110 °C, from about 25 °C to about 105 °C, from about 25 °C to about 100 °C, from about 25 °C to about 95 °C, from about 25 °C to about 90 °C, from about 25 °C to about 85 °C, from about 25 °C to about 80

[0042] °C, from about 25 °C to about 75 °C, from about 25 °C to about 70 °C, from about 25 °C to about 65

[0043] °C, from about 25 °C to about 60 °C, from about 25 °C to about 55 °C, from about 25 °C to about 50

[0044] °C, from about 25 °C to about 45 °C, from about 25 °C to about 40 °C, from about 25 °C to about 35

[0045] °C, from about 25 °C to about 30 °C, or from about 30 °C to about 120 °C, from about 35 °C to about 120 °C, from about 40 °C to about 120 °C, from about 45 °C to about 120 °C, from about 50 °C to about 120 °C, from about 55 °C to about 120 °C, from about 60 °C to about 120 °C, from about 65 °C to about 120 °C, from about 70 °C to about 120 °C, from about 75 °C to about 120 °C, from about 80 °C to about 120 °C, from about 85 °C to about 120 °C, from about 90 °C to about 120 °C, from about 95 °C to about 120 °C, from about 100 °C to about 120 °C, from about 105 °C to about 120 °C, from about 1 10 °C to about 120 °C, from about 1 15 °C to about 120 °C, or about 25 °C, about 30 °C, about 35 °C, about 40 °C, about 45 °C, about 50 °C, about 55 °C, about 60 °C, about 65 °C, about 70 °C, about 75 °C, about 80 °C, about 85 °C, about 90 °C, about 95 °C, about 100 °C, about 105 °C, about 110 °C, about 115 °C, about 120 °C, or any value or range therein. It is to be appreciated that the above ranges should be interpreted as including and supporting any sub-ranges or discrete values (which may or may not be a whole number) that are within the stated range(s) .

[0046] Advantageously, an electrochemical cell that operates at a temperature in this range may allow the cell to avoid potential safety issues, such as corrosion and thermal runaway from handling molten sodium or sulfur. Additionally, an electrochemical cell that operates at this temperature may also require lower manufacturing and maintenance costs in thermal management.

[0047] In some embodiments, the anode of the electrochemical cell may comprise K and Na as a liquid alloy (K-Na alloy).

[0048] Advantageously, the presence of the K-Na alloy may allow the solid electrolyte to be “wet”, thus leading to low anode / solid electrolyte interfacial impedance. This may advantageously lead to increased energy density in the electrochemical cell. The inventors have surprisingly found that only a small amount of K and Na is required to support the ion mobility required for the electrochemical reaction. The K-Na alloy is also liquid at room temperature, which enables its use at temperatures in this range, in contrast with sodium metal, which has a melting temperature of about 98 °C.

[0049] In some embodiments, the anode of the electrochemical cell may comprise K and Na in a molar ratio of in a range of about 1:1.5 to about 4:1 , from about 1:1.5 to about 3.75: 1 , from about 1:1.5 to about 3.5:1 , from about 1:1.5 to about 3.25:1 , from about 1:1.5 to about 3:1 , from about 1:1.5 to about 2.75:1 , from about 1:1.5 to about 2.5:1 , from about 1:1.5 to about 2.25:1 , from about 1:1.5 to about 2:1 , from about 1:1.5 to about 1.75:1 , from about 1:1.5 to about 1.5:1 , from about 1 :1.5 to about 1.25:1 , from about 1 :1.5 to about 1 :1 , from about 1 :1.5 to about 1 :1.25, or from about 1: 1.25 to about 4: 1 , from about 1:1 to about 4: 1 , from about 1.25: 1 to about 4:1 , from about 1.5:1 to about 4:1 , from about 1.75:1 to about 4:1 , from about 2:1 to about 4:1 , from about 2.25:1 to about 4:1 , from about 2.5:1 to about 4:1 , from about 2.75:1 to about 4:1 , from about 3:1 to about 4:1 , from about 3.25:1 to about 4:1 , from about 3.5:1 to about 4:1 , from about 3.75:1 to about 4:1 , or about 1 :1.5 , about 1 :1.25 , about 1 :1 , about 1.25:1 , about 1.5:1 , about 1.75:1 , about 2:1 , about 2.25:1 , about 2.5:1 , about 2.75:1 , about 3:1 , about 3.25:1 , about 3.5:1 , about 3.75:1 , about 4:1, or any value or range therein. It is to be appreciated that the above ranges should be interpreted as including and supporting any sub-ranges or discrete values (which may or may not be a whole number) that are within the stated range(s) . .

[0050] In some embodiments, the range of K in K-Na may vary according to the working temperature of the electrochemical cell. For example, when the working temperature of the electrochemical cell is about 60 °C, the weight ratio of K in the K-Na alloy may be in a range of about 20 wt% to about 98 wt%, from about 20 wt% to about 95 wt%, from about 20 wt% to about 90 wt%, from about 20 wt% to about 85 wt%, from about 20 wt% to about 80 wt%, from about 20 wt% to about 75 wt%, from about 20 wt% to about 70 wt%, from about 20 wt% to about 65 wt%, from about 20 wt% to about 60 wt%, from about 20 wt% to about 55 wt%, from about 20 wt% to about 50 wt%, from about 20 wt% to about 45 wt%, from about 20 wt% to about 40 wt%, from about 20 wt% to about 35 wt%, from about 20 wt% to about 30 wt%, from about 20 wt% to about 25 wt%, or from about 25 wt% to about 98 wt%, from about 30 wt% to about 98 wt%, from about 35 wt% to about 98 wt%, from about 40 wt% to about 98 wt%, from about 45 wt% to about 98 wt%, from about 50 wt% to about 98 wt%, from about 55 wt% to about 98 wt%, from about 60 wt% to about 98 wt%, from about 65 wt% to about 98 wt%, from about 70 wt% to about 98 wt%, from about 75 wt% to about 98 wt%, from about 80 wt% to about 98 wt%, from about 85 wt% to about 98 wt%, from about 90 wt% to about 98 wt%, from about 95 wt% to about 98 wt%, or about 20 wt%, about 25 wt%, about 30 wt%, about 35 wt%, about 40 wt%, about 45 wt%, about 50 wt%, about 55 wt%, about 60 wt%, about 65 wt%, about 70 wt%, about 75 wt%, about 80 wt%, about 85 wt%, about 90 wt%, about 95 wt%, about 98 wt% or any value or range therein. Tt is to be appreciated that the above ranges should be interpreted as including and supporting any sub-ranges or discrete values (which may or may not be a whole number) that are within the stated range(s).

[0051] In another example, when the working temperature of the electrochemical cell is about 75 °C, the weight ratio of K in the K-Na alloy may be in a range of about 15 wt% to about 100 wt%, from about 15 wt% to about 95 wt%, from about 15 wt% to about 90 wt%, from about 15 wt% to about 85 wt%, from about 15 wt% to about 80 wt%, from about 15 wt% to about 75 wt%, from about 15 wt% to about 70 wt%, from about 15 wt% to about 65 wt%, from about 15 wt% to about 60 wt%, from about 15 wt% to about 55 wt%, from about 15 wt% to about 50 wt%, from about 15 wt% to about 45 wt%, from about 15 wt% to about 40 wt%, from about 15 wt% to about 35 wt%, from about 15 wt% to about 30 wt%, from about 15 wt% to about 25 wt%, from about 15 wt% to about 20 wt, or from about 20 wt% to about 100 wt%, from about 25 wt% to about 100 wt%, from about 30 wt% to about 100 wt%, from about 35 wt% to about 100 wt%, from about 40 wt% to about 100 wt%, from about 45 wt% to about 100 wt%, from about 50 wt% to about 100 wt%, from about 55 wt% to about 100 wt%, from about 60 wt% to about 100 wt%, from about 65 wt% to about 100 wt%, from about 70 wt% to about 100 wt%, from about 75 wt% to about 100 wt%, from about 80 wt% to about 100 wt%, from about 85 wt% to about 100 wt%, from about 90 wt% to about 100 wt%, from about 95 wt% to about 100 wt%, or about 15 wt%, about 20 wt%, about 25 wt%, about 30 wt%, about 35 wt%, about 40 wt%, about 45 wt%, about 50 wt%, about 55 wt%, about 60 wt%, about 65 wt%, about 70 wt%, about 75 wt%, about 80 wt%, about 85 wt%, about 90 wt%, about 95 wt%, about 100 wt% or any value or range therein. Tt is to be appreciated that the above ranges should be interpreted as including and supporting any sub-ranges or discrete values (which may or may not be a whole number) that are within the stated range(s).

[0052] In some embodiments, the alloy may comprise carbon as a carbon powder.

[0053] Advantageously, the presence of carbon in the anode of the electrochemical cell may further lower the interfacial impedance of the anode / solid electrolyte interface, thus making it even more suitable for use together with a 0” -alumina solid electrolyte. The inventors have surprisingly found that the interfacial impedance can be lowered even at temperatures as low as room temperature.

[0054] In some embodiments, the carbon powder may be selected from the group consisting of Super C, Super P, acetylene black, ketjen black, and conductive carbon nanotube.

[0055] In some embodiments, the amount of carbon to K-Na alloy may be in the range of about 2 mg to about 30 mg for every 100 pl of K-Na alloy, or from about 2 mg to about 29 mg, from about 2 mg to about 28 mg, from about 2 mg to about 27 mg, from about 2 mg to about 26 mg, from about 2 mg to about 25 mg, from about 2 mg to about 24 mg, from about 2 mg to about 23 mg, from about 2 mg to about 22 mg, from about 2 mg to about 21 mg, from about 2 mg to about 20 mg, from about 2 mg to about 19 mg, from about 2 mg to about 18 mg, from about 2 mg to about 17 mg, from about 2 mg to about 16 mg, from about 2 mg to about 15 mg, from about 2 mg to about 14 mg, from about 2 mg to about 13 mg, from about 2 mg to about 12 mg, from about 2 mg to about 11 mg, from about 2 mg to about 10 mg, from about 2 mg to about 9 mg, from about 2 mg to about 8 mg, from about 2 mg to about 7 mg, from about 2 mg to about 6 mg, from about 2 mg to about 5 mg, from about 2 mg to about 4 mg, from about 2 mg to about 3 mg, or from about 3 mg to about 30 mg, from about 4 mg to about 30 mg, from about 5 mg to about 30 mg, from about 6 mg to about 30 mg, from about 7 mg to about 30 mg, from about 8 mg to about 30 mg, from about 9 mg to about 30 mg, from about 10 mg to about 30 mg, from about 11 mg to about 30 mg, from about 12 mg to about 30 mg, from about 13 mg to about 30 mg, from about 14 mg to about 30 mg, from about 15 mg to about 30 mg, from about 16 mg to about 30 mg, from about 17 mg to about 30 mg, from about 18 mg to about 30 mg, from about 19 mg to about 30 mg, from about 20 mg to about 30 mg, from about 21 mg to about 30 mg, from about 22 mg to about 30 mg, from about 23 mg to about 30 mg, from about 24 mg to about 30 mg, from about 25 mg to about 30 mg, from about 26 mg to about 30 mg, from about 27 mg to about 30 mg, from about 28 mg to about 30 mg, from about 29 mg to about 30 mg, or about 2 mg, about 3 mg, about 4 mg, about 5 mg, about 6 mg, about 7 mg, about 8 mg, about 9 mg, about 10 mg, about 11 mg, about 12 mg, about 13 mg, about 14 mg, about 15 mg, about 16 mg, about 17 mg, about 18 mg, about 19 mg, about 20 mg, about 21 mg, about 22 mg, about 23 mg, about 24 mg, about 25 mg, about 26 mg, about 27 mg, about 28 mg, about 29 mg, about 30 mg, per 100 pel of K-Na alloy or any value or range therein. It is to be appreciated that the above ranges should be interpreted as including and supporting any sub-ranges or discrete values (which may or may not be a whole number) that are within the stated range(s).

[0056] In some embodiments, a discharge product of the electrochemical cell may comprise K2S. Advantageously, when the discharge product of the electrochemical cell is K2S, which has a theoretical specific capacity of 1675 mAh / g, it suggests that the electrochemical cell is utilizing the high theoretical capacity of sulfur. Conversely, in other electrochemical cells where the discharge product is K2S3, for example, the theoretical specific capacity of the discharge product is 560 mAh / g, thus potentially undcrutilizing the capacity of sulfur in the cell.

[0057] In some embodiments, the cathode of the electrochemical cell may be dissolved in the catholyte.

[0058] In some embodiments, the concentration of sulfur in the catholyte may be in a range of about

[0059] I.0 M to about 12.0 M, from about 1.0 M to about 11.5 M, from about 1.0 M to about 11.0 M, from about 1.0 M to about 10.5 M, from about 1 .0 M to about 10.0 M, from about 1.0 M to about 9.5 M, from about 1.0 M to about 9.0 M, from about 1.0 M to about 8.5 M, from about 1.0 M to about 8.0 M, from about 1.0 M to about 7.5 M, from about 1.0 M to about 7.0 M, from about 1.0 M to about

[0060] 6.5 M, from about 1.0 M to about 6.0 M, from about 1.0 M to about 5.5 M, from about 1.0 M to about 5.0 M, from about 1.0 M to about 4.5 M, from about 1.0 M to about 4.0 M, from about 1.0 M to about

[0061] 3.5 M, from about 1.0 M to about 3.0 M, from about 1.0 M to about 2.5 M, from about 1.0 M to about 2.0 M, from about 1.0 M to about 1.5 M, or from about 1.0 M to about 12.0 M, from about 1.5 M to about 12.0 M, from about 2.0 M to about 12.0 M, from about 2.5 M to about 12.0 M, from about 3.0 M to about 12.0 M, from about 3.5 M to about 12.0 M, from about 4.0 M to about 12.0 M, from about

[0062] 4.5 M to about 12.0 M, from about 5.0 M to about 12.0 M, from about 5.5 M to about 12.0 M, from about 6.0 M to about 12.0 M, from about 6.5 M to about 12.0 M, from about 7.0 M to about 12.0 M, from about 7.5 M to about 12.0 M, from about 8.0 M to about 12.0 M, from about 8.5 M to about 12.0 M, from about 9.0 M to about 12.0 M, from about 9.5 M to about 12.0 M, from about 10.0 M to about 12.0 M, from about 10.5 M to about 12.0 M, from about 11.0 M to about 12.0 M, from about

[0063] I I.5 M to about 12.0 M, or about 1.0 M, about 1.5 M, about 2.0 M, about 2.5 M, about 3.0 M, about

[0064] 3.5 M, about 4.0 M, about 4.5 M, about 5.0 M, about 5.5 M, about 6.0 M, about 6.5 M, about 7.0 M, about 7.5 M, about 8.0 M, about 8.5 M, about 9.0 M, about 9.5 M, about 10.0 M, about 10.5 M, about 11.0 M, about 11.5 M, about 12.0 M, or any value or range therein. It is to be appreciated that the above ranges should be interpreted as including and supporting any sub -ranges or discrete values (which may or may not be a whole number) that arc within the stated rangc(s) . In some embodiments, the polysulfidc may be selected from the group consisting of K 'S;;, K2S6, K2S5, K2S4, K2S3, K2S2, K2S, and combinations thereof.

[0065] In some embodiments, the amide may be selected from the group consisting of acetamide, e- caprolactam, formamide, and combinations thereof.

[0066] Advantageously, the amides arc able to dissolve a high amount of polysulfidcs, including but not limited to short-chain poly sulfides. The inventors have surprisingly found that this may occur at the relatively low operating temperatures of the electrochemical cell as disclosed herein, which may enhance energy density, and thus the applicability and flexibility of the electrochemical cell.

[0067] In some embodiments, the catholyte may further comprise ether.

[0068] In some embodiments, the catholyte may comprise amide and ether combined as a liquid deep eutectic solvent.

[0069] Advantageously, the addition of an ether to the catholyte may further enhance the solubility of polysulfides, thus allowing a larger concentration of sulfur in the catholyte, which may further increase the energy density of the electrochemical cell.

[0070] In some embodiments, the catholyte may comprise ether in a range of about 15 wt% to about 90 wt%, from about 15 wt% to about 85 wt%, from about 15 wt% to about 80 wt%, from about 15 wt% to about 75 wt%, from about 15 wt% to about 70 wt%, from about 15 wt% to about 65 wt%, from about 15 wt% to about 60 wt%, from about 15 wt% to about 55 wt%, from about 15 wt% to about 50 wt%, from about 15 wt% to about 45 wt%, from about 15 wt% to about 40 wt%, from about 15 wt% to about 35 wt%, from about 15 wt% to about 30 wt%, from about 15 wt% to about 25 wt%, from about 15 wt% to about 20 wt%, or from about 20 wt% to about 90 wt%, from about 25 wt% to about 90 wt%, from about 30 wt% to about 90 wt%, from about 35 wt% to about 90 wt%, from about 40 wt% to about 90 wt%, from about 45 wt% to about 90 wt%, from about 50 wt% to about 90 wt%, from about 55 wt% to about 90 wt%, from about 60 wt% to about 90 wt%, from about 65 wt% to about 90 wt%, from about 70 wt% to about 90 wt%, from about 75 wt% to about 90 wt%, from about 80 wt% to about 90 wt%, from about 85 wt% to about 90 wt%, or about 15 wt%, about 20 wt%, about 25 wt%, about 30 wt%, about 35 wt%, about 40 wt%, about 45 wt%, about 50 wt%, about 55 wt%, about 60 wt%, about 65 wt%, about 70 wt%, about 75 wt%, about 80 wt%, about 85 wt%, about 90 wt%, or any value or range therein. It is to be appreciated that the above ranges should be interpreted as including and supporting any sub-ranges or discrete values (which may or may not be a whole number) that arc within the stated rangc(s).

[0071] In some embodiments, the ether may be selected from the group consisting of dioxolane (DOL), dimethoxyethane (DME), diethylene glycol dimethyl ether (DEGDME), triethylene glycol dimethyl ether (TEGDME-1), tetraethylene glycol dimethyl ether (TEGDME-2), and combinations thereof.

[0072] In some embodiments, the electrochemical cell further comprises a p”-alumina solid electrolyte.

[0073] The present invention further provides an electrochemical cell comprising: a. an anode comprising K-Na liquid alloy and carbon powder; b. a cathode comprising K2S8; and c. a catholyte comprising acetamide and E-caprolactam.

[0074] In some embodiments, the catholyte may further comprise DEGDME.

[0075] In some embodiments, the catholyte may comprise acetamide and E-caprolactam in a ratio of about 1 :0.5 to about 1 :1.5 , from about 1 :0.5 to about 1 :1.4 , from about 1 :0.5 to about 1 :1.3 , from about 1 :0.5 to about 1:1.2 , from about 1:0.5 to about 1:1.1 , from about 1:0.5 to about 1:1 , from about 1:0.5 to about 1:0.9 , from about 1:0.5 to about 1:0.8 , from about 1:0.5 to about 1:0.7 , from about 1:0.5 to about 1:0.6, or from about 1:0.6 to about 1:1.5 , from about 1:0.7 to about 1:1.5 , from about 1:0.8 to about 1:1.5 , from about 1:0.9 to about 1:1.5 , from about 1:1 to about 1:1.5 , from about 1:1.1 to about 1:1.5 , from about 1:1.2 to about 1:1.5 , from about 1:1.3 to about 1:1.5 , from about 1:1.4 to about 1:1.5, or about 1:0.5 , about 1:0.6 , about 1:0.7 , about 1:0.8 , about 1:0.9 , about 1:1 , about 1:1.1 , about 1:1.2 , about 1 :1.3 , about 1 :1.4 , about 1 :1 .5, or any value or range therein, and about 15 wt% to about 90 wt%, from about 15 wt% to about 85 wt%, from about 15 wt% to about 80 wt%, from about 15 wt% to about 75 wt%, from about 15 wt% to about 70 wt%, from about 15 wt% to about 65 wt%, from about 15 wt% to about 60 wt%, from about 15 wt% to about 55 wt%, from about 15 wt% to about 50 wt%, from about 15 wt% to about 45 wt%, from about 15 wt% to about 40 wt%, from about 15 wt% to about 35 wt%, from about 15 wt% to about 30 wt%, from about 15 wt% to about 25 wt%, from about 15 wt% to about 20 wt%, or from about 20 wt% to about 90 wt%, from about 25 wt% to about 90 wt%, from about 30 wt% to about 90 wt%, from about 35 wt% to about 90 wt%, from about 40 wt% to about 90 wt%, from about 45 wt% to about 90 wt%, from about 50 wt% to about 90 wt%, from about 55 wt% to about 90 wt%, from about 60 wt% to about 90 wt%, from about 65 wt% to about 90 wt%, from about 70 wt% to about 90 wt%, from about 75 wt% to about 90 wt%, from about 80 wt% to about 90 wt%, from about 85 wt% to about 90 wt%, or about 15 wt%, about 20 wt%, about 25 wt%, about 30 wt%, about 35 wt%, about 40 wt%, about 45 wt%, about 50 wt%, about 55 wt%, about 60 wt%, about 65 wt%, about 70 wt%, about 75 wt%, about 80 wt%, about 85 wt%, about 90 wt%, or any value or range therein of DEGDME when present. It is to be appreciated that the above ranges should be interpreted as including and supporting any sub-ranges or discrete values (which may or may not be a whole number) that are within the stated range(s).

[0076] In some embodiments, the electrochemical cell may be a flow battery.

[0077] The present invention also provides a use of an electrochemical cell as disclosed herein for energy storage.

[0078] Examples

[0079] Non-limiting examples of the invention and comparative examples will be further described in greater detail by reference to specific examples, which should not be construed as in any way limiting the scope of the invention.

[0080] Materials and Methods

[0081] Potassium, sodium, sulfur, potassium sulfide, KTFS1, carbon powder, acetamide, CPL, DEGDME are purchased from Sigma- Aldrich. Characterization

[0082] Fourier transform infrared (FTIR) spectroscopy

[0083] A PerkinElmer Frontier Spectrometer was used to conduct FTIR experiments. The scanning range was from 400 to 4000 cm . and the spectrum resolution was 4 cm

[0084] Viscosity Measurement and Contact Angle Measurement

[0085] Viscosity measurements were conducted using a Brookfield AMETEK LV DV3T viscometer . The contact angle between the electrolyte solutions and K-|3” -alumina solid electrolyte (K-BASE) was measured using a DataPhysics OCA 15Pro contact angle meter.

[0086] Electrochemical measurements

[0087] Galvanostatic cycling tests were conducted using a Neware battery tester (CT-4008Tn- 5V50mA-HWX). Electrochemical impedance spectroscopy (EIS) was performed on a Bio -logic SP- 150, with a frequency range of 0.01 Hz to 1 MHz and an AC voltage amplitude of 10 mV. Cyclic voltammetry (CV) was conducted over the potential window of 1.5-2.9 V with a scan rate of 0.1 mV / s. All electrochemical measurements were carried out in an oven at 60 °C.

[0088] Example 1: Preparation and characterization of a symmetric K / Na cell

[0089] A deep eutectic solvent (DES) was prepared by mixing s-caprolactam (CPL, 99 %, Sigma Aldrich) and acetamide (99 %, Sigma Aldrich) in a 1:1 molar ratioThen, 0.5M (159 mg) potassium bis(trifluoromethanesulfonyl)imide (KTFSI, 99%, Dodochem) was added to 1 g of the DES create catholyte solutions. For the catholyte solutions containing DEGDME, the DES and DEGDME (99 %, Sigma Aldrich) were mixed in weight ratios of 3:7 (333 mg DES and 667 mg DEGDME).

[0090] To form catholyte solutions containing 4M sulfur concentrations, 0.5M (39 mg) anhydrous NasS (99 %, Dodochem) and 3.5 M (112 mg) sulfur were mixed with the above catholyte solutions, and the mixture was stirred at 60 °C.

[0091] To form catholyte solutions containing 1 M sulfur concentrations, 0.125M NajS and0.875M sulfur were mixed with the above catholyte solutions, and the mixture was stirred at 60 °C.

[0092] The K-Na alloy anode was prepared by mixing 400 mg of potassium (98 %, Sigma Aldrich), 100 mg of sodium (99 %, Sigma Aldrich), and 40 mg of conductive carbon black uniformly in an aluminum container within an argon-filled glove box.

[0093] CR2032 coin cells were used for assembly. The K-BASE disks were cut into 15 mm diameter discs. Stainless steel foils were cut into round disks to fit the size of O rings. Then a thin layer of K- Na alloy with carbon black was coated on each foil to form the negative and positive casings. The components were stacked in the following order: negative casing, 0.2 mm SAE-316L stainless steel spacer, wave spring, stainless steel foil, PTFE O-ring, K-BASE disc, PTFE O-ring, carbon cloth, aluminum current collector, 0.2 nun stainless steel spacer, and positive casing. Finally, the assembled cell was compressed at 800 psi using a hydraulic press. For the symmetric cells, the addition of carbon black (CB) to the K-Na alloy reduced the overpotential from ~40 mV to ~20 mV at a current density of 2.6 mA cm2(Figs 2c and 2d). This reduction in overpotential indicates that CB lowers the interfacial resistance by improving the electronic conductivity and promoting uniform reaction interfaces. These factors enhance charge transfer and minimize polarization, contributing to improved electrochemical performance.

[0094] Example 2: Characterization and evaluation of the K / Na-S cell

[0095] For K-Na / BASE / S full cells, carbon paper was cut into round disks to fit the size of the O-rings. During the assembly process, 26.7 pL cm2of catholyte was dispensed onto two layers of carbon paper. The components were then stacked in the same order as the K-Na symmetric cell, except that one K-Na electrode was replaced by catholyte-soaked carbon paper.

[0096] The K-Na / BASE / S full cell was tested using galvanostatic charge -discharge (GCD) at 0.954 mA to evaluate its capacity, cycling stability, and reaction kinetics. In the full cells, a high specific capacity of 1590 mAh g1was achieved, nearly matching the theoretical value of 1675 mAh g (Figs. 3a and 3b). This exceptional capacity is attributed to the electrolyte's ability to dissolve intermediate discharge products (K2S2 and K2S), which enhances reaction kinetics and maximizes sulfur utilization (Fig. 2). Additionally, increasing the sulfur concentration to 4 M resulted in an initial discharge capacity of 840 mAh g with 773 mAh g1retained after 200 cycles (Figs. 3c and 3d). This high retention reflects the electrolyte's excellent electrochemical stability and resistance to evaporation, ensuring consistent cycling performance.

[0097] The DES enhanced polysulfide solubility and reaction kinetics, facilitating the controlled conversion of K2S2 and K2S, reducing intermediate accumulation, and improving sulfur utilization. The liquid K-Na alloy anode improved wettability with K-BASE, reduced interfacial resistance, and prevented dendrite formation, ensuring long-term anode stability. The K-BASE solid electrolyte suppressed polysulfide shuttling, stabilized Na+transport, and minimized side reactions, contributing to extended cycle life.

[0098] When 70 wt% of DEGDME was added to the catholyte, the K / Na-S cell achieved 1000 cycles without capacity decay. This stability is primarily due to the liquid K-Na alloy anode and the K-BASE solid electrolyte. The liquid alloy prevents dendrite formation, while the K-BASE electrolyte suppresses the polysulfide shuttle effect, maintaining a stable electrochemical environment and enabling long-term operation. The DEGDME addition improved ionic conductivity and reduced viscosity, enhancing ion transport and ensuring uniform electrolyte infiltration into the carbon paper. This enabled 1000 cycles without capacity decay.

[0099] Through the electrolyte composition, anode interface engineering, and solid-state electrolyte design of the present invention, the system is able to achieve high capacity, long lifespan, and stable cycling performance, demonstrating its potential for advanced potassium-sulfur batteries.

[0100] Example 3: Grid Scale Energy Storage

[0101] It is possible to use a K / Na-S full cell for grid level energy storage, which is shown in Fig. 4. The battery has four main components, a flowable polysulfide as catholyte, an external storage tank for the catholyte, a solution pump, and a current extracting stack housing a BASE tube filled with the K / Na / C anode. Catholyte may be pumped into the space between the BASE tube and the casing wall, and kept at 60 °C for more favourable reaction kinetics. The K / Na-S battery could achieve a specific energy of -250 Wh / kg at the cell level, and an estimated system cost of ~$30 / kWh for 24 h-operation. In comparison, grid-scale commercialized lithium- ion battery pack exhibits an energy density of around 120 Wh / kg with a cost of $350 / kWh and power-type commercialized lithium-ion battery pack has an energy density of 130 Wh / kg and a cost of $151 / kWh. Therefore, the K / Na-S liquid flow battery system proves highly competitive in long-duration energy storage technology for renewable grids.

[0102] Comparative Examples

[0103] Comparative Example 1

[0104] An electrochemical cell was synthesized following the above methods, except that the catholyte used was TEGDME instead of CPL / acetamide. The electrochemical cell was then compared with

[0105] From the results shown above, it can he seen that when the catholyte was changed from CPL / acetamide or CPL / acetamide / DEGDME to TEGDME, the capacity of the electrochemical cell sharply decreases. This is due to the formation of solid K2S2 / K2S discharge products with low diffusivity and poor kinetics, as the discharge products are significantly less soluble in TEGDME as compared to CPL / acetamide. Hence, it can be shown that the selection of a suitable catholyte is important for tire electrochemical properties of the electrochemical cell of the present invention.

[0106] Industrial Applicability

[0107] The present invention relates electrochemical cells useful for energy storage. The electrochemical cell of the present invention may exhibit higher energy density, lower operating temperatures as compared to conventional electrochemical cells. This allows the electrochemical cell of the present invention to have increased safety, lower operating and maintenance costs. Thus, this invention is capable of industrial applicability.

[0108] It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.

Claims

Claims1. An electrochemical cell comprising: a. an anode comprising potassium (K), sodium (Na), and carbon (C); b. a cathode comprising polysulfide; and c. a catholyte comprising amide.

2. The electrochemical cell of claim 1, wherein electrochemical cell has an operating temperature of about 25 °C to about 120 °C.

3. The electrochemical cell of claim 1 or 2, wherein the anode comprises K and Na as a liquid alloy (K-Na alloy).

4. The electrochemical cell of any one of claims 1 to 3, wherein the anode comprises K and Na in a ratio of about 1 : 1.5 to about 4:1.

5. The electrochemical cell of any one of claims 1 to 4, wherein the anode comprises carbon as a carbon powder.

6. The electrochemical cell of claim 5, wherein the carbon powder is selected from the group consisting of Super C, Super P, acetylene black, ketjen black, and conductive carbon nanotube.

7. The electrochemical cell of any one of claims 3 to 6, wherein the amount of carbon to K-Na alloy is about 2 mg to about 30 mg for every 100 pl of K-Na alloy.

8. The electrochemical cell of any one of claims 1 to 7, wherein a discharge product of the electrochemical cell comprises K2S.

9. The electrochemical cell of any one of claims 1 to 8, wherein the cathode is dissolved in the catholyte.

10. The electrochemical cell of any one of claims 1 to 9, wherein the cathode is dissolved in the catholyte and the concentration of sulfur in the catholyte is about 1 M to about 12 M.

11. The electrochemical cell of any one of claims 1 to 10, wherein the polysulfide is selected from the group consisting of K2S8, 2S6, K2S5, K2S4, K2S3, K2S2, K2S, and combinations thereof.

12. The electrochemical cell of any one of claims 1 to 11 , wherein the amide is selected from the group consisting of acetamide, E-caprolactam, formamide, and combinations thereof.

13. The electrochemical cell of any one of claims 1 to 12, wherein the catholyte further comprises ether.

14. The electrochemical cell of claim 13, wherein the catholyte comprises amide and ether combined as a liquid deep eutectic solvent.

15. The electrochemical cell of claim 13 or 14, wherein the catholyte comprises about 15 wt% to about 90 wt% ether.

16. The electrochemical cell of any one of claims 13 to 15, wherein the ether is selected from the group consisting of dioxolane (DOL), dimethoxy ethane (DME), diethylene glycol dimethyl ether (DEGDME), triethylene glycol dimethyl ether (TEGDME-1), tetraethylene glycol dimethyl ether (TEGDME-2), and combinations thereof.

17. The electrochemical cell of any one of claims 1-16, further comprising a P’’-alumina solid electrolyte.

18. An electrochemical cell comprising: a. an anode comprising K-Na liquid alloy and carbon powder; b. a cathode comprising K2S8; and c. a catholyte comprising acetamide and E-caprolactam.

19. The electrochemical cell of claim 18, wherein the catholyte further comprises DEGDME.

20. The electrochemical cell of claim 18 or 19, wherein the catholyte comprises acetamide and e- caprolactam in a ratio of about 1 :0.5 to about 1 :1.5, and about 65 wt% to about 75 wt% of DEGDME when present.

21. The electrochemical cell of any one of claims 1 to 20, wherein the electrochemical cell is a flow battery.

22. Use of an electrochemical cell of any one of claims 1 to 21 for energy storage.

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