Electrolyte for use in Sodium-Sulfur Batteries

Abstract

The present disclosure relates to an electrolyte comprising: a) a sodium salt; b) an additive comprising at least one additional metallic/metalloid cation having a standard reduction potential which is at least 2.5V more positive than that of sodium cation; wherein said sodium salt and said additive are dispersed in a solvent comprising at least one alkyl carbonate, and wherein the concentration of said metallic/metalloid cation in the electrolyte is 15 mM to 250 mM. The present disclosure also relates to a sodium-sulfur cell comprising a sodium anode, a microporous sulfur cathode, and the electrolyte as described herein. The present disclosure further provides a method of improving cycling life of a sodium-sulfur cell, wherein the sodium-sulfur cell comprising a sodium anode, a sulfur cathode, and an electrolyte containing a sodium salt dispersed in an alkyl carbonate solvent.

Claims

1-26. (canceled)

27. An electrolyte comprising: a) a sodium salt; b) an additive comprising at least one additional metallic/metalloid cation having a standard reduction potential which is at least 2.5V more positive than that of sodium cation; wherein said sodium salt and said additive are dispersed in a solvent comprising at least one alkyl carbonate, and wherein the concentration of said metallic/metalloid cation in the electrolyte is 15 mM to 250 mM.

28. The electrolyte of claim 27, wherein the metallic/metalloid element of said metallic/metalloid cation is selected from Groups 11, 14 or 15 of the Periodic Table of Elements.

29. The electrolyte of claim 28, wherein said metallic/metalloid element is selected from the group consisting of silver, gold, and copper.

30. The electrolyte of claim 29, wherein the concentration of said metallic/metalloid cation in the electrolyte is 50 to 250 mM.

31. The electrolyte of claim 28, wherein said metallic/metalloid cation comprises a metal/metalloid element selected from the group consisting of tin, antimony, and bismuth.

32. The electrolyte of claim 31, wherein the concentration of said metallic/metalloid cation in the electrolyte is 15 to 50 mM.

33. The electrolyte of claim 27, wherein the additive comprises at least one halogen-containing anion.

34. The electrolyte of claim 33, wherein the halogen containing anion is a monoatomic halogen anion selected from group consisting of F.sup.−, Cl.sup.−, and Br.sup.−.

35. The electrolyte of claim 33, wherein the halogen-containing anion is a polyatomic anion selected from group consisting of BF.sub.4.sup.−, B(C.sub.6F.sub.5).sub.4.sup.−, PF.sub.6.sup.−, ClO.sub.4.sup.− and CF.sub.3SO.sub.3.sup.−.

36. The electrolyte of claim 27, wherein the alkyl carbonate is a cyclic alkyl carbonate, a non-cyclic alkyl carbonate, or a combination thereof.

37. The electrolyte of claim 36, wherein the cyclic alkyl carbonate is selected from the group consisting of fluoroethylene carbonate, ethylene carbonate, propylene carbonate, trimethylene carbonate, vinylene carbonate, and combinations thereof.

38. The electrolyte of claim 36, wherein the non-cyclic alkyl carbonate is selected from the group consisting of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, dibenzyl carbonate, diallyl carbonate, diphenyl carbonate, dipropyl carbonate, and combinations thereof.

39. The electrolyte of claim 27, wherein the concentration of the carbonate in the electrolyte is 0.2 mM to 1.5 mM.

40. The electrolyte of claim 27, wherein the sodium salt is selected from the group consisting of sodium perchlorate (NaClO4), sodium trifluoromethanesulfonate (NaOTf), sodium bis(fluorosulfonyl)imide (NaFSI) and sodium trifluoromethanesulfonimide (NaTFSI).

41. The electrolyte of claim 27, wherein the concentration of the sodium salt in the electrolyte is 0.1 M to 5 M.

42. The electrolyte of claim 27, wherein the electrolyte is substantially free of water.

43. A sodium-sulfur cell comprising: a. a sodium anode; b. a microporous sulfur cathode, and c. an electrolyte comprising: i. a sodium salt; ii. an additive comprising at least one additional metallic/metalloid cation having a standard reduction potential which is at least 2.5V more positive than that of sodium cation; iii. wherein said sodium salt and said additive are dispersed in a solvent comprising at least one alkyl carbonate, and iv. wherein the concentration of said metallic/metalloid cation in the electrolyte is 15 mM to 250 mM.

44. The sodium-sulfur cell of claim 43, wherein the sodium anode comprises a sodium alloy interphase, said sodium alloy being formed between a reduced metal/metalloid of the electrolyte and the sodium anode.

45. A method of improving cycling life of a sodium-sulfur cell, wherein the sodium-sulfur cell comprising a sodium anode, a sulfur cathode, and an electrolyte containing a sodium salt dispersed in an alkyl carbonate solvent, the method comprising: introducing one or more additives into said electrolyte, each additive independently capable of forming a sodium alloy interphase on a surface on the sodium anode; and wherein the one or more additives independently comprise a one metal or metalloid cation having a standard reduction potential which is at least 2.5V more positive than that of sodium cation.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0048] The accompanying figures, together with the description below are incorporated in and form part of the specification. These figures serve to illustrate various embodiments and to explain various principles and advantages in accordance with a present embodiment.

[0049] FIG. 1 is a schematic illustration of an exemplary sodium-sulfur cell as described herein.

[0050] FIG. 1a is an illustration of a sodium-sulfur cell without additive metal cations while FIG. 1b depicts an exemplary sodium-sulfur cell comprising additive metal cations which may form an alloy interphase with the sodium anode.

[0051] FIG. 2 is a X-ray photoelectron spectrum (XPS) of a sodium-tin alloy formed at the sodium anode of a sodium-sulfur cell.

[0052] FIG. 2a shows the presence of tin-species in the alloy, while FIG. 2b show the presence of sodium species in the alloy while FIG. 2c shows the presence of chloride ions in the alloy.

[0053] FIG. 3 is a XPS spectrum of a sodium-silver alloy interphase formed on a sodium anode of a sodium-sulfur cell.

[0054] FIG. 3a is a spectrum of the sodium species in the interphase while FIG. 3b is a XPS spectrum of silver species which are present in the interphase.

[0055] FIGS. 3c and 3d are spectra of boron and organic fluorine species in the alloy interphase, which show that the contribution of boron and fluorine species to the metal-alloy interphase is negligible.

[0056] FIG. 4a is a plot which compares the specific capacity of an exemplary sodium-sulfur cell fabricated with a 1M NaTFSI salt in ethylene carbonate, dimethyl carbonate and fluoroethyl carbonate electrolyte (EC-DMC-FEC at a volume ratio of 1:1:0.16) system with and without the additive tin cations. For comparison, the specific capacity of the sodium sulfur cell fabricated with 1M NaTFSI in an EC-DMC electrolyte (volume ratio of 1:1) system is also shown. FIG. 4b compares the Coulombic efficiency of the sodium sulfur electrochemical cells fabricated with electrolytes provided with the additive tin cation and without the additive tin cations. As before, the Coulombic efficiency of a sodium sulfur cell with a 1M Na TFSI in a EC-DMC (1:1 volume ratio) is also shown. FIG. 4c is the voltage profile of a sodium-sulfur cell fabricated with 1M NaTFSI in EC-DMC-FEC without additive metal cations while FIG. 4d is the voltage profile of an exemplary sodium-sulfur cell fabricated with 1M NaTFSI in EC-DMC-FEC with 20 mM of tin chloride.

[0057] FIG. 5a is a comparative plot of the rate behaviour of sodium-sulfur cells fabricated with 1M NaTFSI in EC-DMC-FEC (volume ratio of 1:1:0.16) with or without 20 mM tin chloride. For comparison, the rate behaviour of a sodium-sulfur cell fabricated with 1M NaTFSI in EC-DMC at a volume ratio of 1:1 is also shown. FIGS. 5b, 5c and 5d are plots of the rate behaviour of an exemplary sodium-sulfur cell assembled with the 1M NaTFSI in EC-DMC-FEC (volume ratio of 1:1:0.16) with 20 mM tin chloride at a charging rate of 5 C, 7 C and 9 C, respectively.

[0058] FIG. 6a is a plot of the specific capacity of exemplary sodium-sulfur cells assembled with a 1M NaTFSI in EC-DMC-FEC electrolyte (volume ratio of 1:1:0.16) comprising various concentrations of silver tetrafluoroborate; while FIG. 6b is a plot of their respective Coulombic efficiencies. A comparison of the specific capacity and Coulombic efficiency of sodium-sulfur electrochemical cells with and without the addition of 100 mM AgBF.sub.4 to the electrolyte is shown in FIGS. 6c and 6d, respectively.

[0059] FIG. 7a is a plot of the rate behaviour of sodium-sulfur electrochemical cells fabricated with 100 mM AgBF.sub.4 in a 1M NaTFSI in EC-DMC-FEC electrolyte (volume ratio of 1:1:0.16). FIG. 7b is a plot of the specific capacity of exemplary sodium sulfur cells with a 1M NaTFSI in EC-DMC-FEC electrolyte having 100 mM AgBF.sub.4 at high charge rates of 5 C, 7 C and 10 C.

[0060] FIG. 8a is a plot of the specific capacity of sodium sulfur cells comprising an electrolyte of various sodium salts at concentrations of 1M, dissolved in a EC-DMC-FEC system having 20 mM tin chloride; while FIG. 8b is a plot of their respective Coulombic efficiencies.

[0061] FIG. 9a is a plot of the specific capacity of sodium sulfur cells comprising an electrolyte having BiCl.sub.3 or SbCl.sub.3 dissolved in 1M NaTFSI in a EC-DMC-FEC system; while FIG. 9b is a plot of their respective Coulombic efficiencies.

EXAMPLES

[0062] Non-limiting examples of the invention 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.

Example 1

Assembly of Electrochemical Cell

[0063] The electrolyte described herein was used in sodium sulfur cells. The sodium-sulfur electrochemical cell was assembled with a sodium metal anode and a sulfur-infused microporous carbon cathode having a surface area of approximately 1 cm.sup.2, separated by a glass fibre separator having a thickness of about 25 μm, and 80 μl of an electrolyte. The electrolyte used in the sodium sulfur cells was a mixture of ethylene carbonate (EC), dimethylcarbonate (DMC) and fluoroethylcarbonate (FEC), having a concentration of 1M sodium trifluorosulfonimide, NaTFSI. The electrolyte was prepared by mixing 0.5 mL ethylene carbonate and 0.5 mL dimethyl carbonate and subsequently adding 80 μL fluoroethylene carbonate to the mixture.

[0064] Unless described otherwise, comparative cells were assembled with an electrolyte comprising 1M NaTFSI in EC-DMC-FEC without any additional salts, as shown in FIG. 1a. Electrolytes having additive metal ions were prepared by dissolving a salt of the additive metal ion in a 1 molar solution of a sodium salt in a EC-DMC-FEC system (volume ratio of 1:1:0.16). Electrochemical cells used for the studies below were assembled with the prepared electrolytes, as shown in FIG. 1b, where a Na-metal alloy may be formed with the additive metal ion at the sodium electrode.

Example 2

Evaluation of the Formation of the Alloy Interphase

[0065] The dissociation of SnCl.sub.2 in an electrolyte and its interaction with sodium metal for the formation of a Na—Sn alloy was studied by X-ray photoelectron spectroscopy (XPS).

[0066] X-ray photoelectron spectroscopy of the formation of an alloy between sodium and tin was studied. FIG. 2a demonstrates that besides the formation of metallic Sn, the Sn.sup.2+ ions react with Na to form a Na—Sn alloy where, highly electronegative Sn form Zintl-ions preferentially, while electropositive Na stabilizes in the form of tetrahedral (FIG. 2b). The existence of Cl.sup.− as shown in FIG. 2c can be credited to the solvated chloride ions, which were formed during dissolution of SnCl.sub.2 in the EC-DMC solvent (volume ratio of 1:1).

[0067] The formation of a sodium-metal alloy for electrochemical cells comprising a silver salt was also studied by XPS. The XPS spectra of the formed alloy is shown in FIG. 3.

[0068] Apparent peaks can be assigned to Na or Na—F/Na—O (E=1071 eV), and Na—Ag alloy (E=1072.8 eV), as depicted in FIG. 3a. The existence of strong Ag doublets ensures formation of a stable Na—Ag alloy phase, as shown in FIG. 3b. The interphase involves mainly of Na—Ag alloys, as peaks corresponding to B and F are negligible. FIGS. 3c and 3d are XPS spectra of B and F, respectively, and a comparison between the spectrum of the sodium anode and the XPS spectra of B and F show that the contribution of B and F are negligible. The high-resolution XPS spectrum of F1s suggests trace amount of organic fluorides (E=687.4 eV) is present in the interphase, which could be due to solvated anions that remained on the surface of the anode.

Example 3

Performance of Sodium-Sulfur Cells Fabricated with Tin-Containing Electrolyte

[0069] To evaluate the performance of the metal cation of the electrolyte additives, electrochemical characteristics such as specific energy and Coulombic efficiency of the assembled electrochemical cells were studied in the presence of the metal cations. In particular, the specific capacity and Coulombic efficiency of electrochemical cells with and without the additive metal cations were studied. The specific energy of electrochemical systems can be expressed in terms of specific capacity of the cell (energy=capacity×voltage), and the state of reversibility is expressed in terms of Coulombic efficiency.

[0070] The performance of an electrolyte comprising tin cations was first examined. The specific capacity and Coulombic efficiency of electrochemical cells fabricated with a 1M NaTFSI in EC-DMC-FEC (volume ratio of 1:1:0.16) electrolyte comprising 20 mM tin chloride was studied. This was compared to electrochemical cells comprising 1M NaTFSI in EC-DMC-FEC electrolyte system, without any additional salts.

[0071] The specific capacity of sodium-sulfur electrochemical cells assembled with tin additive additives was found to be higher and more stable over an extended period of time as compared to electrochemical cells comprising electrolytes without the tin additive, as depicted in FIGS. 4a and 4b. FIG. 4a shows that when tin chloride additives are added to the electrolyte, the specific capacity of the electrochemical cell remains stable even after 600 cycles. This may be observed in FIG. 4a, where a high specific capacity of ˜760 mAh/g, which is ˜62% of the initial capacity (˜1230 mAh/g), is retained after 600 cycles. This is a marked improvement over an equivalent electrolyte system without any additives, where Coulombic efficiency decreases to less than 400 mAh/g after 600 cycles.

[0072] The use of tin chloride in the electrolyte of an electrochemical cell also results in an average Coulombic efficiency of about 99.5% even after 600 cycles, as shown in FIG. 4b. This indicates that an electrochemical cell fabricated with an electrolyte system having additives maintains its state of reversibility after multiple charge-discharge cycles.

[0073] The voltage profiles of sodium-sulfur cells fabricated with a 1M NaTFSI in EC-DMC-FEC (volume ratio of 1:1:0.16) electrolyte without alloying type additives exhibit a slight increase in overpotential as the number of cycles increase, as shown in FIG. 4c. However, traits of side reactions are observed during discharging process, and these may contribute to side reactions which may contribute to low Coulombic efficiency of electrochemical cells comprising electrolytes without additives, even in the presence of FEC. In contrast, the voltage profiles of sodium-sulfur cells fabricated using electrolytes comprising alloying additives exhibited minimal increase in overpotential with cycling, without occurrence of any side reactions as shown in FIG. 4d.

[0074] The rate-behaviour of Na//S cells was determined without and with additives, and identified that the inclusion of additives exerts a positive effect on the rate-behaviour, as depicted in FIG. 5a. As a consequence of additives, sodium-sulfur cells could attain a specific capacity of about ˜500 mAh/g, ˜320 mAh/g, and ˜240 mAh/g, after 200 cycles at relatively high charge-rate, i.e., 5 C, 7 C and 9 C, respectively, as shown in FIGS. 5b-5d

Example 4

Performance of Sodium-Sulfur Cells Fabricated with Silver-Containing Electrolyte

[0075] The performance of electrochemical cells comprising an electrolyte system having silver cations was also evaluated. The sodium-sulfur cell was fabricated according to Example 1 with a 1M NaTFSI in EC-DMC-FEC electrolyte comprising silver cations. Comparative sodium-sulfur cells were fabricated similarly, with a 1M NaTFSI in EC-DMC-FEC electrolyte (volume ratio of 1:1:0.16) without additive metal cations.

[0076] To understand the effect of the metal cation on the performance of sodium-sulfur electrochemical cells, different concentrations of the additive metal cation were dissolved in the reference electrolyte system. It is observed that sodium-sulfur cells can function stably for 300 cycles, irrespective of the concentration of AgBF.sub.4 (FIG. 6a). However, after 300 cycles, the specific capacity of sodium-sulfur cells containing 150 mM and 250 mM of the silver salt, declines sharply (FIG. 6a). This may be due to various reasons, for instance, undesirable side-reactions, sluggish diffusion of ion in the electrochemical cell and a thicker interphase region.

[0077] It was found that a silver additive concentration of 100 mM was highly favorable in achieving long-term cycling stability of sodium-sulfur cells, even after 800 cycles (FIG. 6c). The Coulombic efficiency of the sodium-sulfur cell containing an electrolyte comprising 100 mM of AgBF.sub.4 was observed to be highly stable, and the Coulombic efficiency was calculated to be about 98.4% after over 800 cycles. As shown in FIGS. 6c and 6d, when a silver additive is added to the electrolyte, the specific capacity of the cell is markedly higher than electrochemical cells without the additive; and the average Coulombic efficiency is maintained at 98.4% after 800 cycles.

[0078] The silver cation was also found to have a stabilizing effect on the formation of a localized sodium-silver interphase on the sodium metal electrode. This may be observed in the high specific capacity which is maintained at various charge rates, from 0.5 C to 10 C (FIG. 7a). Moreover, the sodium-sulfur cell fabricated with an electrolyte comprising a silver cation achieved a specific capacity of about 450 mAh/g, 260 mAh/g, and 131 mAh/g, after 200 cycles at relatively high charge-rates of 5 C, 7 C and 10 C, respectively, as shown in FIG. 7b.

Example 5

Influence Of Electrolyte Salt On Performance Of Electrochemical Cell

[0079] In order to investigate the role of electrolyte salt on the performance of the electrochemical cell, various sodium salts were used in the electrolyte. The sodium-sulfur electrochemical cell was then fabricated with electrolytes prepared with various sodium salts at a concentration of 1M in a EC-DMC-FEC solvent (volume ratio of 1:1:0.16) comprising tin chloride at a concentration of 20 mM. In particular, electrolytes comprising 1M sodium perchlorate (NaClO.sub.4), 1M sodium trifluoromethanesulfonate (NaOTf), 1M sodium bis(fluorosulfonyl)imide (NaFSI), and 1M sodium trifluoromethanesulfonimide (NaTFSI) were prepared and used for the fabrication of sodium sulfur cells as detailed above.

[0080] It was found that the electrochemical stability of the cell is the highest in the presence of NaTFSI, as depicted in FIG. 8a. From FIG. 8b, it was observed that the Coulombic efficiency of the sodium-sulfur cells in the presence of various salts was found to be unaffected, which indicates that electrochemical reversibility is not associated with the salt system; and is therefore not affected by the different salts used in the electrolyte.

Example 6

Performance of Electrolytes Comprising Other Metal Cations

[0081] The electrochemical performance of electrolytes comprising other metal cations was also studied. Sodium-sulfur cells fabricated with electrolytes having BiCl.sub.3, and SbCl.sub.3, was also examined. It was observed that the stability of sodium-sulfur cells in the presence of different types of additives was also higher than electrochemical cells without the additives (FIG. 9a). The EC-DMC-FEC electrolyte system was prepared at a volume ratio of 1:1:0.16.

[0082] It was also observed that the Coulombic efficiency is affected by the choice of additives, as depicted in FIG. 9b. The highest Coulombic of about 99.5% is obtained with 20 mM of SnCl.sub.2 in 1 M NaTFSI-EC-DMC-FEC electrolyte system , 20 mM of BiCl.sub.3 in 1 M NaTFSI-EC-DMC-FEC electrolyte system and 20 mM of SbCl2 in 1 M NaTFSI-EC-DMC-FEC electrolyte system , respectively.

INDUSTRIAL APPLICABILITY

[0083] The electrolytes as described herein may be used for the fabrication of electrochemical cells, in particular, sodium-sulfur electrochemical cells. Due to their ability in stabilizing the reactive metal electrodes, such electrolytes may be used for the fabrication of stable sodium-based electrochemical cells.

[0084] When used for preparation of sodium-based electrochemical cells, the electrolyte may be used as a homogenous liquid mixture; or provided on an absorbent material which may be contacted with the electrodes in the cell. The ease of preparing the electrolytes also allows for convenient industrial preparation and assembly of electrochemical cells. Such electrochemical cells may be utilized as batteries in portable electronic devices.