Glyme-based multi-salt electrolytes

Abstract

Various strategies can be used to address dendrite formation in zinc batteries, spanning from advanced electrode materials to optimized operating conditions. Addressing dendrite challenges is pivotal for bolstering safety in metal-based batteries. The invention relates to an electrolyte specifically designed to limit5 dendrite formation in Zn / Mg batteries providing a safer and more reliable metal or metal / air battery.

Description

[0001] GLYME-BASED MULTI-SALT ELECTROLYTES

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to an electrolyte solution, a method of producing the electrolyte solution and a metal or metal-air battery comprising the electrolyte solution.

[0004] BACKGROUND OF THE INVENTION

[0005] Dendrites are undesired metallic structures that can grow on the surface of the metal electrodes, eventually leading to short circuits, reduced battery performance, and even safety hazards.

[0006] Dendrites in zinc-based batteries are primarily composed of metallic zinc, and they grow from the anode surface toward the cathode. These dendrites can pierce the separator, leading to short circuits and compromising the safety and performance of the battery, therefore representing a significant challenge in the development of these energy storage systems.

[0007] Various strategies to mitigate dendrite formation in zinc batteries have been explored.

[0008] Some initiatives focus on the development of advanced electrode materials with properties that help avoid dendrite formation.

[0009] Additionally, optimizing the operating conditions of the battery, such as temperature and current density, has been identified as a crucial factor in preventing dendrite growth.

[0010] One further approach involves the incorporation of additives into the electrolyte, aimed at enhancing the uniform deposition of zinc ions and thereby minimizing the conditions leading to dendrite growth.

[0011] Addressing dendrite formation is not merely a technical challenge but a pivotal step in making metal-based batteries safer and more reliable.

[0012] An electrolyte providing solutions to dendrite formation in metal and metal-air batteries would be therefore advantageous.

[0013] Hence, an improved electrolyte providing higher and better performances of a metal or metal-air battery would be advantageous. OBJECT OF THE INVENTION

[0014] An object of the present invention is to provide an electrolyte mitigating dendrite formation in metal and metal-air batteries.

[0015] A further object of the present invention is to provide a battery comprising an electrolyte mitigating dendrite formation.

[0016] An even further object of the invention may also be seen as to provide an alternative to the prior art.

[0017] In particular, it may be seen as an object of the present invention to provide an electrolyte reducing or eliminating the formation of dendrites in metal and metalair batteries by combining Zn salts and ether glycol solvents.

[0018] SUMMARY OF THE INVENTION

[0019] The idea of the invention is to provide a solution to dendrite formation in Zn / Mg metal and metal-air batteries.

[0020] The above-described object and several other objects are intended to be obtained in the first aspect of the invention by an electrolyte solution for metal batteries and metal-air batteries, the electrolyte solution comprising one or more nonaqueous organic solvents and one or more metal salts.

[0021] Metal batteries include metal, such as Zn or Mg, and metal-air batteries, in which the metal is or has the function of anode.

[0022] The electrolyte of the invention may combine more than one metal salt, such as at least two metal salts, for example at least three metal salts.

[0023] The one or more metal salts may be Zn salts.

[0024] In some embodiments, the one or more Zn salts are compounds selected from the group consisting of Zn(OTf)2, Zn(TFSI)2 and ZnC .

[0025] Zn(OTf)2 is the zinc salt of trifluoromethanesulfonic acid also referred to as zinc trifluoromethanesulfonate, zinc bis(trifluoromethanesulfonate) or zinc triflate, i.e. Zn(CF3SO3)2. Zn(TFSI)2 is a zinc imide also referred to as zinc bis(trifluoromethanesulfonyl)imide or zinc bis(trifluoromethane)sulfonimide or zinc bistriflimide, i.e., Zn(N(CF3SO2)2.

[0026] ZnC may refer to anhydrous zinc chloride or to its hydrated form.

[0027] In some embodiments, the one or more Zn salts are at least two Zn salts.

[0028] In some further embodiments, the one or more Zn salts are at least three Zn salts.

[0029] Concentration of Zn salts in the binary or ternary composition may vary.

[0030] In some other embodiments of the electrolyte solution of the invention, the concentration of Zn(TFSI)2 is higher than a concentration of Zn(OTf)2.

[0031] In some embodiments, the electrolyte solution has a tri-salt composition having Zn(TFSI)2 as dominant salt specie together with Zn(OTf)2 and ZnC in triglyme solvent.

[0032] An electrolyte having the tri-salt composition having Zn(TFSI)2 as dominant salt species together with Zn(OTf)2 and ZnC in triglyme showed optimal performance when employed in a metal or metal-air battery in mitigating formation of dendrites.

[0033] Other bi-salts compositions, despite increasing entropy, did not provide a better solution compared to mono-salt composition. In that, the three-salt composition surprisingly showed better performances.

[0034] Furthermore, the same three-salt composition in tetraglyme did not provide better performances, showing the uniqueness of the tri-salt combination in triglyme.

[0035] In a preferred embodiment, the concentration of Zn(OTf)2 is higher than a concentration of Zn(TFSI)2.

[0036] In some further embodiments, the molar ratio of Zn(TFSI)2 and of Zn(OTf)2 in the electrolyte solution is 1: 1.

[0037] The concentration of ZnC may be lower than the concentration of any other Zn salts in the electrolyte solution. In the electrolyte solution, the one or more non-aqueous solvents are organic solvents such as organic glycol ether.

[0038] In some embodiments, the one or more non-aqueous organic solvents are selected from the group consisting of glyme, diglyme, triglyme, tetraglyme, polyethylene glycol or a mixture thereof.

[0039] In some other embodiments, the one or more non-aqueous organic solvents comprise triglyme.

[0040] In some further embodiments, the one or more non-aqueous organic solvents comprise tetraglyme.

[0041] The use of glyme with an increased ether chain does not increase performance. In that, surprisingly, optimal performances were achieved in triglyme compared to the one achieved in diglyme or tetraglyme.

[0042] The one or more non-aqueous organic solvents may further comprise polyethylene glycol.

[0043] The addition of polyethylene glycol even in small amounts has been shown to improve the cycle life of a metal or metal-air, such as Zn or Mg or Zn-air or Mg- air, battery employing the mixture as electrolyte solution.

[0044] In some further embodiments, the one or more Zn salts are ZnC , Zn(OTf)2 and Zn(TFSI)2 and the one or more non-aqueous organic solvent are triglyme, within a molar ratio 1:5:4: 50, and polyethylene glycol wherein the ratio by weight between triglyme and polyethylene glycol (PEG) is 80:20.

[0045] In some other embodiments, the metals batteries and metal-air batteries are Mg or Mg-air batteries and the one or more metal salts are Mg salts.

[0046] In those embodiments, the one or more Mg salts are compounds selected from the group consisting of Mg(OTf)2, Mg(TFSI)2 and MgC . In a second aspect, the invention relates to a metal or metal-air, such as Zn or Mg or Zn-air or Mg-air, battery comprising an electrolyte solution according to the first aspect of the invention.

[0047] In the metal or metal-air battery, the metal, such as Zn or Mg, is or has the function of being the anode of the battery.

[0048] In a third aspect, the invention relates to a battery module comprising metal or metal-air batteries according to the second aspect of the invention.

[0049] In a further aspect, the invention relates to a method of producing an electrolyte according to the first aspect of the invention, the method comprising the step of dissolving Zn(OTf)2, Zn(TFSI)2 and ZnC in triglyme in a determined molar ratio.

[0050] The first, second and other aspects, embodiments and items of the present invention may each be combined with any of the other aspects, embodiments and items. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

[0051] BRIEF DESCRIPTION OF THE FIGURES

[0052] The electrolyte solution, according to some aspects and embodiments of the invention, will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and are not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

[0053] Figure 1 shows the voltage profile during the plating-stripping of the electrolyte comprising Zn(TFSI)2 in tetraglyme (G4) and in triglyme (G3), according to some embodiments of the invention.

[0054] Figure 2 shows a plot of the overpotential of an electrolyte solution having Zn salts in different ratios.

[0055] Figures 3 a)-f) show critical current measurements and SEM micrographs at 5000x magnifications of the Zn electrodes after 20 cycles at 0.1 mA cm-2of binary and ternary mixtures.

[0056] Figure 4 shows critical current measurements using Zn electrodes and electrolytes according to some embodiments of the invention.

[0057] Figure 5 is a plot showing the overpotential of the different electrolytes, represented along with the cycle life.

[0058] DETAILED DESCRIPTION OF AN EMBODIMENT

[0059] The invention provides a solution to dendrites formation in Zn / Mg metal and metal-air batteries.

[0060] In search of solutions to the dendrites formation, a series of a mixture of salts, being Zn(OTf)2, Zn(TFSI)2 and ZnC in glycol di-ether such as glyme, e.g., di / tri or tetraglyme were tested.

[0061] Zn salts employed:

[0062] • ZnCI2

[0063] • Zn(OTf)2

[0064] • Zn(TFSI)2.

[0065] Solvents employed:

[0066] • Triethylene glycol dimethyl ether (Triglyme, G3) • Tetraethylene glycol dimethyl ether (Tetraglyme, G4)

[0067] • Polyethylene Glycol (PEG).

[0068] Nomenclature employed:

[0069] • Gn(xi:x2:x3:x4)

[0070] Where:

[0071] • n = 3 or 4, for triglyme and tetraglyme

[0072] • (xi:x2:x3:x4) = molar ratio of (ZnCl2:Zn(OTf)2:Zn(TFSI)2:Gn)

[0073] Figure 1 shows voltage profile 1 recorded during the plating-stripping of the electrolyte comprising Zn(TFSI)2 in tetraglyme (G4) with a molar ratio 1:4 tested at 0.1 mA cm-2in a stainless-steel electrode.

[0074] Figure 1 also shows voltage profile 2 recorded during the plating-stripping of the electrolyte comprising ZnTFSb in triglyme (G3) with a molar ratio 1: 5 tested at 0.1 mA cm-2in a stainless-steel electrode.

[0075] The systems Zn(TFSI)2:G4 and Zn(OTf)2:G4 present a reversible plating-striping of zinc. Particularly, the electrolytes Zn(TFSI)2:G4 with a 1:4 molar ratio presents a stable plating-striping at 0.1 mA cm-2for 1000 cycles with a coulombic efficiency of 99 % and an overpotential around 0.4V. Similar results were obtained when G3 is used. Specifically, the sample using Zn(TFSI)2:G3 with a 1: 5 molar ratio also shows a stable cycling behavior for more than 1000 cycles.

[0076] Figure 2 shows the dependency of the overpotential on the ZnCl2 / Zn(OTf)2 ratio during plating of Zn in the mixtures ZnCl2:Zn(OTf)2:G4 cycled at 0.1 mA cm-2. In search of improved performance, mixtures of two different salts, so as to increase the entropy of the system, dropping the solvation strength and thus reducing the reaction overpotential, were tested.

[0077] The data plotted in Figure 2 show a drastic decrease in the cycling overpotential dependent on the ZnC content.

[0078] However, the addition of ZnC also produces an increase in dendritic growth, reducing dramatically the cycling life of the cell.

[0079] Mixtures of three salts, ZnC , Zn(OTf)2, and Zn(TFSI)2, showed an improvement in the electrolyte performance in comparison with the binary mixtures. The critical current density test consists of applying increasing current densities to a cell until its failure.

[0080] Figures 3 a), b) and c) show the potential profiles resultant of the critical current density measurements. The samples a) ZnCl2:Zn(OTf)2:G3 and b) ZnCl2:Zn(TFSI)2:G3 both with a molar ratio of 1: 5: 30 are compared with the sample c) ZnCl2:Zn(OTf)2:Zn(TFSI)2:G3 with a molar ratio 1: 5:4: 50, also referred as G3(l: 5:4: 50). In Figures 3a, 3b and 3c the areas at different values of mA / cm2, i.e. 0.1, 1, 2 and 4 mA / cm2are indicated by the correspondent value of mA / cm2.

[0081] It can be noticed that the critical current density is increased drastically for the ternary mixture. This increment in the critical current density implies that the dendrite growth in this sample is reduced and delayed.

[0082] As it can be seen from the SEM images, in figures 3 d), e) and f), of the Zn electrodes of these 3 samples after 20 cycles at 0.1 mA cm-2at 5000x magnifications, the two binary mixtures show very clear vertical metallic growth, while in the case of the tri-salt electrolyte, the surface roughness is reduced and dendrite formation is, at least partially suppressed.

[0083] The optimized mixture ZnCl2:Zn(OTf)2:Zn(TFSI)2:G3 with a molar ratio of 1: 5:4: 50 has also been cycled at 0.1 mA cm-2.

[0084] As can be seen in Figure 4, showing critical current measurements using Zn electrodes and electrolytes G3(l: 5:4: 50) and G3(l: 5:4: 50)PEG, voltage profile 2 recorded a short circuit after around 125 hours (78 cycles), thus a very short cycle life.

[0085] Addition of a co-solvent to the electrolyte mixture surprisingly increased the cycle life. The cosolvent added was Polyethylene glycol (PEG) with a molecular weight of 400 Dalton. Voltage profile 4 in Figure 4 represents the potential profiles of sample G3(l: 5:4: 50) with the addition of PEG, where the solvent is a mixture of 80:20 by weight of G3 and PEG. The cycle life increases more than 6 times with the addition of PEG, from 125 to 825 hours.

[0086] Figure 5 is a plot showing the overpotential of the different electrolytes, along with the cycle life. On one y-axis, it can be noticed that the sample with the longest cycle life is the electrolyte Zn(TFSI)2:G3, which also presents the highest overpotential. On the other y-axis, it can be noticed that the electrolytes with salt mixtures present a huge decrease in the overpotential, but also in the cycle life. The addition of PEG surprisingly produced the best- performing mixture, drastically increasing the cycle life while keeping a low overpotential.

[0087] Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is possible and advantageous.

Claims

CLAIMS1. An electrolyte solution for zinc batteries and zinc-air batteries, said electrolyte solution comprising:- one or more non-aqueous organic solvents, selected from the group consisting of glyme, diglyme, triglyme, tetraglyme, polyethylene glycol or a mixture thereof; one or more zinc salts selected from the group consisting of Zn(OTf)2, Zn(TFSI)2 and ZnC , wherein said one or more zinc salts are at least two zinc salts, such as at least three metal salts.

2. An electrolyte solution according to claim 1, wherein said one or more nonaqueous organic solvents are triglyme and polyethylene glycol.

3. An electrolyte solution according to any of the preceding claims, wherein said one or more non-aqueous organic solvents are tetraglyme and polyethylene glycol.

4. An electrolyte solution according to any of the preceding claims, wherein a concentration in said electrolyte solution of Zn(OTf)2 is higher than a concentration of Zn(TFSI)2.

5. An electrolyte solution according to any of the preceding claims, wherein a molar ratio of Zn(TFSI)2 and of Zn(OTf)2 in said electrolyte solution is 1: 1.

6. An electrolyte solution according to any of the preceding claims, wherein a concentration of said ZnC is lower than a concentration of any other Zn salts in said electrolyte solution.

7. An electrolyte solution according to any of the preceding claims, wherein said one or more Zn salts are ZnC , Zn(OTf)2 and Zn(TFSI)2 and said one or more non-aqueous organic solvent are triglyme, within a molar ration 1: 5:4: 50, and polyethylene glycol, wherein the ratio by weight between triglyme and polyethylene glycol is 80:20.

8. A zinc or zinc-air battery comprising an electrolyte solution according to any of the preceding claims.

9. A battery module comprising zinc or zinc-air batteries according to claim 8.

10. A method of producing an electrolyte according to any of the preceding claims 1-7, said method comprising: dissolving Zn(OTf)2, Zn(TFSI)2 and ZnC in triglyme in a determined molar ratio.

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