AN ELECTROLYTE FOR MAGNESIUM-ION BATTERIES

Abstract

The present disclosure relates to an electrolyte comprising at least one magnesium salt having a polyatomic anion, an aluminium halide salt and a solvent comprising at least one ether group. The electrolyte described herein does not comprise magnesium chloride. The electrolyte described herein may be used in magnesium ion electrochemical cells.

Claims

1. An electrolyte comprising: a) at least one magnesium salt; b) an aluminium halide salt; and c) a solvent comprising at least one ether group; wherein the electrolyte does not comprise magnesium chloride; and wherein said at least one magnesium salt comprises a polyatomic anion.

2. An electrolyte consisting essentially of: a) a magnesium salt; b) an aluminium halide salt; and c) a solvent comprising at least one ether group; wherein said magnesium salt comprises a polyatomic anion.

3. The electrolyte of claim 1, wherein the aluminium halide salt is aluminium fluoride, aluminium chloride, aluminium bromide or aluminium iodide.

4. The electrolyte of claim 1, wherein the aluminium halide salt is aluminium chloride.

5. The electrolyte of claim 1, wherein the electrolyte comprises at least two or more magnesium salts.

6. The electrolyte of claim 1, wherein the polyatomic anion of the magnesium salt is chlorate, trifluoromethanesulfonate (OTf), phosphate, sulfate, sulfite, hexafluorophosphate, hexafluoroarsenate, bis(trifluoromethanesulfonyl)imide (TFSI), bis(fluorosulfonyl)imide (FSI), bis(butanesulfonyl) imide, cyanamide, oligomeric fluorosulfonyl imide, nonafluorobutanesulfonyl imide, bis(oxalato)borate (BOB), difluoro(oxalato)borate (DFOB), or tetrafluoroborate.

7. The electrolyte of claim 1, wherein the polyatomic anion of said magnesium salt is bis(trifluoromethanesulfonyl)imide.

8. The electrolyte of claim 1, wherein the solvent comprises less than 200 ppm water.

9. The electrolyte of claim 1, wherein the solvent comprises less than 20 ppm water.

10. The electrolyte of claim 1, wherein the concentration of the magnesium salt is between 0.01 M to 20 M.

11. The electrolyte of claim 1, wherein the concentration of the magnesium salt is between 0.2 M to 0.5 M.

12. The electrolyte of claim 1, wherein the concentration of the aluminium halide salt is between 0.01 M to 20 M.

13. The electrolyte of claim 1, wherein the concentration of the aluminium halide salt is between 0.2 M to 0.6 M.

14. The electrolyte of claim 1, wherein the molar ratio of the magnesium salt to the aluminium halide salt is between 5:1 to 1:10.

15. The electrolyte of claim 1, wherein the molar ratio of the magnesium salt to the aluminium halide salt is about 1:1.32.

16. The electrolyte of claim 1, wherein the solvent is selected from the group consisting of tetrahydrofuran, monoglyme, diglyme, triglyme, tetraglyme, dioxane, tetrahydropyran and combinations thereof.

17. The electrolyte of claim 1, wherein the solvent is monoglyme.

18. An electrochemical cell comprising: a) a positive electrode; b) a magnesium negative electrode; and c) an electrolyte comprising: i) at least one magnesium salt; ii) an aluminium halide salt; and iii) a solvent comprising at least one ether group; wherein the electrolyte does not comprise magnesium chloride; wherein said at least one magnesium salt comprises a polyatomic anion; wherein said positive electrode and said magnesium negative electrode are in fluid communication with said electrolyte.

19. The electrochemical cell of claim 18, wherein the electrolyte is absorbed on a separator located between said positive electrode and said magnesium negative electrode.

20. The electrochemical cell of claim 18, wherein the positive electrode is fabricated from a sulfur composite or a complex comprising a transition metal.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0065] FIG. 1 is a representation of an exemplary 2032 coin cell which may be assembled. The asymmetric cell utilizes a carbon coated aluminium foil as the working electrode and a magnesium disk as a counter electrode.

[0066] FIG. 2a is a cyclic voltammogram obtained using a carbon-coated aluminium foil working electrode immersed in 0.25 M Mg(TFSI).sub.2 and 0.33 M AlCl.sub.3 in monoglyme. FIG. 2b depicts the linear sweep voltammogram of a carbon coated aluminium foil working electrode in an electrolyte comprising 0.25 M Mg(TFSI).sub.2 and 0.33 M AlCl.sub.3 in monoglyme. The voltammograms were obtained using magnesium as the reference electrode and counter electrode at a scan rate of 25 mV/s.

[0067] FIG. 3a is a voltage profile of the plating and stripping of magnesium on a carbon-coated aluminium electrode in an electrolyte comprising 0.25 M Mg(TFSI).sub.2 and 0.33 M AlCl.sub.3 in monoglyme. A solid magnesium electrode was used as the reference and counter electrode. FIG. 3b is a plot of the cell voltage which was measured during the galvanostatic plating and stripping of magnesium in an aluminium-carbon/magnesium cell having an electrolyte comprising 0.25 M Mg(TFSI).sub.2 and 0.33 M AlCl.sub.3 in monoglyme. The galvanostatic plating/stripping was conducted at a current density of 0.5 mA/cm.sup.2, areal capacity of 0.1 mAh/cm.sup.2, and cut-off voltage at 1.2V.

[0068] FIG. 4 is a plot of the Coulombic efficiency of a magnesium ion cell assembled with an aluminium-carbon working electrode and magnesium disc reference and counter electrode in an electrolyte comprising various concentrations of Mg(TFSI).sub.2 and AlCl.sub.3 in monoglyme.

[0069] FIG. 5a is a voltage profile of the plating and stripping of magnesium on a carbon-coated aluminium working electrode in an electrolyte comprising 0.25 M Mg(TFSI).sub.2 and 0.33 M AlCl.sub.3 in diglyme. FIG. 5b is the voltage profile of magnesium plating and stripping on the aluminium-carbon working electrode in a solution of 0.25 M Mg(TFSI).sub.2 and 0.33 M AlCl.sub.3 in triglyme. FIG. 5c is the voltage profile of the plating and stripping of magnesium on an aluminium-carbon working electrode in 0.25 M Mg(TFSI).sub.2 and 0.33 M AlCl.sub.3 in tetraglyme. In these measurements, a magnesium disc was used as the reference and counter electrode. The individual plots in each voltage profile were recorded at cycle 1, 2, 10, 20 and 50, as indicated. FIG. 5d is a plot of the Coulombic efficiency of the plating and stripping of magnesium in 0.25 M Mg(TFSI).sub.2 and 0.33 M AlCl.sub.3 in various solvents using an aluminium-carbon working electrode and a magnesium counter and reference electrode.

[0070] FIG. 6a is the voltage profile of magnesium plating and stripping on a carbon coated aluminium foil electrode in contact with an electrolyte having 0.25 M Mg(TFSI).sub.2 and 0.33 M AlCl.sub.3 dissolved in monoglyme. A magnesium disc was used as the reference and counter electrode. The individual plots in the voltage profile were recorded after the first, second, tenth, twentieth and fiftieth cycle. FIG. 6b is a plot of the Coulombic efficiency of the magnesium plating and stripping in a magnesium ion cell assembled with an aluminium-carbon working electrode, magnesium disc reference and counter electrode and an electrolyte having 0.25 M Mg(TFSI).sub.2 and 0.33 M AlCl.sub.3 dissolved in monoglyme. The voltage profile and Coulombic efficiency was recorded at 0.5 mAh/cm.sup.2. FIGS. 6c and 6d are scanning electron micrographs of a magnesium deposit on a carbon-coated aluminium working electrode at different scales. FIG. 6e is an energy dispersive X-ray spectrum (EDS) of a magnesium layer deposited on a carbon-coated aluminium working electrode of the magnesium-ion batteries.

[0071] FIG. 7 is a plot of the Coulombic efficiency of aluminium-carbon/magnesium cells comprising an electrolyte having various concentrations of AlCl.sub.3 dissolved in monoglyme.

EXAMPLES

[0072] 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.

[0073] In the study of voltage profiles and Coulombic efficiencies below, the asymmetric magnesium ion electrochemical cell was galvanostatically cycled with a current density of 0.5 mA/cm.sup.2. An areal capacity of 0.1 mAh/cm.sup.2 or 0.5 mAh/cm.sup.2 of Mg was plated onto an aluminium-carbon working electrode and the magnesium layer was stripped at current density of 0.5 mA/cm.sup.2 until the voltage reached 1.2V. The Coulombic efficiency was calculated as the ratio of stripping capacity to plating capacity.

Example 1. Fabrication of an Electrochemical Cell

[0074] The electrochemical performance of the electrolyte described herein was evaluated by fabricating a 2032 coin cell comprising the electrolyte, as illustrated in FIG. 1. Unless described otherwise, the coin-cell configuration described below was adopted for the electrochemical studies of the electrolytes described herein.

[0075] An electrolyte was first prepared by dissolving Mg(TFSI).sub.2 and AlCl.sub.3 at concentrations specified described below in monoglyme. A coin-cell type electrochemical cell was fabricated using a polished Mg disk having an area of 1.27 cm.sup.2 as a counter electrode (also used as a reference electrode), 2 layers of Celgard separator, an Al—C disc (carbon-coated aluminum foil) having an area of 1 cm.sup.2 as a working electrode and 25 μl of Mg(TFSI).sub.2—AlCl.sub.3 electrolyte.

Example 2. Reversibility of Magnesium Deposition

[0076] The reversibility of magnesium deposition on the positive electrode was also studied via cyclic voltammetry.

[0077] An electrochemical cell was fabricated with a positive electrode formed from carbon coated on aluminium (working electrode) and a magnesium electrode as the reference electrode and counter electrode. The electrodes were immersed in an electrolyte comprising 0.25 M Mg(TFSI).sub.2 and 0.33 AlCl.sub.3 in monoglyme. Cyclic voltammograms were recorded at a scan rate of 25 mV/s and are illustrated in FIG. 2.

[0078] A cyclic voltammogram of the Al—C electrode in 0.25 M Mg(TFSI).sub.2 and 0.33 M AlCl.sub.3 in monoglyme is shown in FIG. 2a. An enhancement in the reversibility of the magnesium deposition process is clearly observed during the first 15 cycles, expressed by lower overpotentials and higher cycling efficiencies. The current peak of anodic scan reaches 7.1 mA/cm.sup.2 at 15th cycle. The linear sweep voltammogram shown on FIG. 2b shows an anodic stability of 3.06V.

[0079] The voltage profiles of the plating and stripping of magnesium on the Al—C electrode were also studied. In the electrolyte of 0.25 M Mg(TFSI).sub.2 and 0.33 M AlCl.sub.3 in monoglyme, reversible plating and stripping of Mg on Al—C foil were observed near −0.15 V and 0.15 V vs. Mg/Mg.sup.2+ (FIG. 3a). In the first cycle, Coulombic efficiency of the cell was observed to be relatively high (64%). The irreversible capacity (36%) in the first cycle is likely due to reduction of electrolyte components and/or contaminants (e.g. moisture). With increased cycle number, Coulombic efficiency of the cell increases significantly and reaches 95% in subsequent cycles. The stability of the plating and stripping behaviour is also demonstrated in the stable plotting voltage over up to 5500 minutes (FIG. 3b).

Example 3. Effect of Concentration of Aluminium Halide Salt on Electrolyte Performance

[0080] Electrolytes having different concentrations of Mg(TFSI).sub.2 and aluminium chloride in monoglyme were prepared for the fabrication of an electrochemical cell having an aluminium-carbon working electrode, a magnesium disc counter electrode (also used as a reference electrode). The plating and stripping profiles of magnesium-ion electrochemical cells comprising these electrolytes were studied and the results of the studies are shown in Table 1. Plots of the Coulombic efficiency of the electrochemical cells comprising electrolytes having various concentrations of AlCl.sub.3 and Mg(TFSI).sub.2 in monoglyme are also provided in FIG. 4.

TABLE-US-00001 TABLE 1 Columbic efficiency of aluminium-carbon// magnesium cells in electrolytes having various concentrations of Mg(TFSI).sub.2—AlCl.sub.3 monoglyme Initial Highest Coulombic Coulombic Cycle Electrolyte Composition efficiency (%) efficiency (%) life 0.25M Mg(TFSI).sub.2 and 0.05M 37 74 35 AlCl.sub.3 in monoglyme 0.25M Mg(TFSI).sub.2 and 0.10M 65 89 49 AlCl.sub.3 in monoglyme 0.25M Mg(TFSI).sub.2 and 0.33M 64 95 >250 AlCl.sub.3 in monoglyme 0.25M Mg(TFSI).sub.2 and 0.41M 44 94 >250 AlCl.sub.3 in monoglyme 0.25M Mg(TFSI).sub.2 and 0.50M 61 93 207 AlCl.sub.3 in monoglyme 0.25M Mg(TFSI).sub.2 and 0.66M 44 91 98 AlCl.sub.3 in monoglyme

[0081] Among these electrolyte compositions, the magnesium ion cell comprising an electrolyte with a combination of 0.25 M Mg(TFSI).sub.2 and 0.33 M AlCl.sub.3 in monoglyme shows highest Coulombic efficiency. This cell also delivers the highest initial Coulombic efficiency (64%) and the highest Coulombic efficiency was recorded at 95% in subsequent cycles. Long cycle life (above 250 cycles) was also achieved at this composition.

[0082] It should also be noted that the cycle life of the Al—C/Mg cell, which uses 0.25 M Mg(TFSI).sub.2 and 0.5 M MgCl.sub.2 in monoglyme as an electrolyte, is limited to 100 cycles under the same experimental conditions in prior work. Therefore, the Mg(TFSI).sub.2—AlCl.sub.3 electrolyte system demonstrates significant enhancement of cycle life compared to combinations of Mg(TFSI).sub.2 and other halide-containing salts.

[0083] In addition, 0.25 M Mg(TFSI).sub.2 and 0.33 M AlCl.sub.3 in monoglyme demonstrated high ionic conductivity of 4.95 mS/cm at 26.6° C. At higher concentrations of Mg(TFSI).sub.2 (0.5 M), the cells show lower Coulombic efficiency, which is likely due to high viscosity of electrolyte solution and increased concentration of contaminants.

Example 4. Reversible Plating and Stripping of Magnesium in Ether Solvents

[0084] Electrolytes comprising 0.25 M Mg(TFSI).sub.2 and 0.33 M AlCl.sub.3 dissolved in diglyme, triglyme and tetraglyme were prepared.

[0085] Electrochemical cells comprising a carbon coated aluminium working electrode, magnesium disc counter electrode (also used as a reference electrode) and the prepared electrolytes were assembled. The plating and stripping of magnesium on the working electrode in the presence of the electrolytes prepared with diglyme, triglyme and tetraglyme may be observed from the voltage profiles shown in FIGS. 5a to 5c. The Couloumbic efficiency of these electrochemical cells comprising electrolytes prepared with monoglyme, diglyme, triglyme and tetraglyme are also shown on FIG. 5d.

[0086] Reversible plating or stripping of magnesium is observed in all electrochemical cells studied herein. This implies that all solvents used in the preparation of the electrochemical cells facilitate reversible plating and stripping of magnesium. Among these solvents, diglyme was considered to be a promising alternative solvent for the Mg(TFSI).sub.2—AlCl.sub.3 electrolyte system. The diglyme-based electrolyte delivers high initial Coulombic efficiency of 64% and maintains above 90% for 50 cycles.

Example 5. Homogenous Growth of Magnesium Metal

[0087] The morphology of magnesium metal deposits on the working electrode was studied via scanning electron microscopy. An electrochemical cell comprising a carbon-coated aluminium working electrode, magnesium disc counter electrode (also used as a reference electrode) and an electrolyte comprising 0.25 M Mg(TFSI).sub.2 and 0.33 M AlCl.sub.3 in monoglyme was assembled. The cycling performance of the electrochemical cell is shown in FIGS. 5a and 5b; while scanning electron micrographs of magnesium deposits on a working electrode are provided in FIGS. 6c and 6d.

[0088] The voltage profile and Coulombic efficiency (FIGS. 5a and 5b) show that the assembled cell exhibits cycling performance at a high areal capacity of 0.5 mAh/cm.sup.2. Examination of the magnesium deposition film on the working electrode revealed that there was uniform and non-dendritic morphologies even at high areal capacity (0.5 mAh/cm.sup.2) (FIG. 5c-d). This result indicates that the Mg(TFSI).sub.2—AlCl.sub.3 electrolyte system enables homogeneous Mg deposition on Al—C electrode and therefore reduces short-circuit caused by dendrite growth.

[0089] Energy-dispersive X-ray spectroscopy (EDS) analysis of Mg deposition layer on Al—C electrolyte (FIG. 5e) confirmed the elemental composition of the magnesium film. The deposition layer consists of Mg as main component (93.3 wt %). Other elements, including CI, 0, F, and C, are from the surface film, which is believed to originate from the reduction and/or oxidation of electrolyte components at the surface of the electrode during cycling.

Example 6. Comparison of Electrolyte Performance with Other Magnesium-Based Electrolyte Systems

[0090] The electrochemical performance of the electrolyte described herein was compared with that of a comparative electrolyte. To do so, a comparative electrolyte comprising a combination of 0.26 M MgCl.sub.2 and 0.13 M AlCl.sub.3, and an exemplary electrolyte comprising 0.25 M Mg(TFSI).sub.2 and 0.125 M AlCl.sub.3 in monoglyme was prepared. Electrochemical cells comprising the comparative or exemplary electrolyte, an aluminium-carbon working electrode and a magnesium counter electrode (also used as the reference electrode) were fabricated for this study.

[0091] FIG. 7 compares the Coulombic efficiency (CE) of the exemplary electrolyte described herein and the comparative electrolyte solution. The plot shows that the electrolyte comprising 0.26 M MgCl.sub.2+0.13 M AlCl.sub.3 in monoglyme electrolyte (blue curve) shows higher initial CE (92%) than that of Mg(TFSI).sub.2—AlCl.sub.3 based electrolytes. However, the Coulombic efficiency decreases significantly after 4 cycles and large variations of CE is observed in subsequent cycles. It indicates that severe degradation occurred in the Mg/Al—Cl cell comprising the MgCl.sub.2—AlCl.sub.3 system.

[0092] In contrast, the Mg(TFSI).sub.2—AlCl.sub.3 based electrolyte with similar Mg.sup.2+ and Al.sup.3+ ratio (i.e. 0.25 M Mg(TFSI).sub.2 and 0.125 M AlCl.sub.3 in monoglyme) demonstrated more stable performance. Despite the lower initial CE, the electrolyte comprising 0.25 M Mg(TFSI).sub.2 and 0.125 M AlCl.sub.3 in monoglyme demonstrates a stable cycling performance with CE between 80-90%, which is maintained over 50 cycles.

[0093] Herein, it is noted that the performance of Mg(TFSI).sub.2—AlCl.sub.3 based electrolytes may be dependent on the Mg:Cl ratio. It was found that optimal electrochemical performance of electrolyte solution can be achieved at a Mg:Cl ratio of 1:4, corresponding to the electrolyte formula of 0.25 M Mg(TFSI).sub.2 and 0.33 M AlCl.sub.3 in monoglyme.

INDUSTRIAL APPLICABILITY

[0094] The disclosed electrolyte may be used in electrochemical cells, particularly magnesium ion batteries. As such electrolytes allow efficient plating and stripping of magnesium from a working electrode, such electrolytes may be used for the fabrication and assembly of magnesium-ion batteries which may be used as energy sources in various electrical and electronic devices.

[0095] Due to its ease of manufacture, the electrolytes described herein may also be produced on an industrial scale for easy assembly of magnesium ion electrochemical cells, which may be used as an alternative energy storage system to presently available technologies.