Magnesium-based energy storage systems and methods having improved electrolytes
Electrolytes for Mg-based energy storage devices can be formed from non-nucleophilic Mg 2+ sources to provide outstanding electrochemical performance and improved electrophilic susceptibility compared to electrolytes employing nucleophilic sources. The instant electrolytes are characterized by high oxidation stability (up to 3.4 V vs Mg), improved electrophile compatibility and electrochemical reversibility (up to 100% coulombic efficiency). Synthesis of the Mg 2+ electrolytes utilizes inexpensive and safe magnesium dihalides as non-nucleophilic Mg 2+ sources in combination with Lewis acids, MR a X 3-a (for 3≧a≧1). Furthermore, addition of free-halide-anion donors can improve the coulombic efficiency of Mg electrolytes from nucleophilic or non-nucleophilic Mg 2+ sources.
1. A Mg-based energy storage system having an anode comprising Mg metal, the system characterized by an electrochemical window greater than 2.8 V vs. Mg and by an electrolyte solution comprising:
(i) an organic solvent;
(ii) an active Mg 2+ dimer monocation concentration greater than or equal to 0.2 M, wherein the active Mg 2+ dimer monocation comprises a Mg 2 X 3 core complexed with six solvent molecules, and wherein at least a portion of the active Mg 2+ dimer monocation concentration is a reaction product of
a magnesium dihalide MgX 2 , wherein X represents a halogen elemental, and
a Lewis acid MR a X 3-a , wherein 3≧a≧1, M is a metal center, R is an organic ligand, and X is the halogen element; and
(iii) an anion having a formula MR a X 4-a − .
2. The Mg-based energy storage system of claim 1 , wherein the Lewis acid is MRX 2 or MR 3 .
3. The Mg-based energy storage system of claim 1 , wherein the metal center of the Lewis acid comprises a metal selected from the group consisting of B, Al, Ga, In, Fe, and combinations thereof.
4. The Mg-based energy storage system of claim 1 , wherein the organic ligand is selected from the group consisting of alkyl, aryl, alkoxide, aryloxide, thiolate, amide, and combinations thereof.
5. The Mg-based energy storage system of claim 1 , wherein the active Mg 2+ dimer monocation concentration is greater than or equal to 0.4 M.
6. The Mg-based energy storage system of claim 1 , wherein the active Mg 2+ dimer monocation concentration is greater than or equal to 0.7 M.
7. The Mg-based energy storage system of claim 1 , wherein the molar ratio of the magnesium dihalide to the Lewis acid is approximately 2:1.
8. The Mg-based energy storage system of claim 1 , wherein the Lewis acid comprises AlEtCl 2 , AlPhCl 2 , or both, and the electrolyte solution comprises AlEtCl 3 − , AlPhCl 3 31 , or both.
9. The Mg-based energy storage system of claim 1 , wherein the electrolyte comprises MgCl 2 .
10. The Mg-based energy storage system of claim 1 , wherein the organic solvent is an ether solvent.
11. The Mg-based energy storage system of claim 1 , further having a free-halide-anion donor comprising X − , but not Mg, wherein the free-halide-anion donor contributes free anions of X − to the electrolyte solution.
12. The Mg-based energy storage system of claim 11 , wherein the free-halide-anion donor comprises an organic salt having X − .
13. The Mg-based energy storage system of claim 11 , wherein the free-halide-anion donor comprises an inorganic salt having X − .
14. The Mg-based energy storage system of claim 1 , having a coulombic efficiency greater than 99%.
15. The Mg-based energy storage system of claim 1 , further comprising a second Lewis acid, MX 3 .
16. The Mg-based energy storage system of claim 1 , wherein the magnesium dihalide is MgCl 2 , the solvent is tetrahydrofuran, and the Mg 2+ dimer monocation is [(μ−Cl) 3 Mg 2 (THF) 6 ] + .
17. A Mg-based energy storage system having an anode comprising Mg metal, the system characterized by an electrochemical window greater than 2.8 V vs. Mg and by an electrolyte solution comprising:
(i) an organic ether solvent;
(ii) an active Mg 2+ dimer monocation concentration greater than or equal to 0.4 M, wherein at least a portion of the active Mg 2+ dimer monocation concentration is a reaction product of MgCl 2 with a Lewis acid comprising AlEtCl 2 , AlPhCl 2 , or both, and wherein the active Mg 2+ dimer monocation comprises a Mg 2 Cl 3 core complexed with six organic ether solvent molecules; and
(iii) AlEtCl 3 − , AlPhCl 3 − , or both.
18. The Mg-based energy storage system of claim 17 , wherein the molar ratio of the MgCl 2 to the Lewis acid is approximately 2:1.
19. The Mg-based energy storage system of claim 17 , wherein the electrolyte solution further comprises a salt contributing free anions of Cl − to the electrolyte solution, wherein the salt does not comprise Mg.
20. A method for improving performance of Mg-based energy storage systems having an anode comprising Mg metal and having an electrochemical window greater than 2.8 V vs. Mg, the method comprising the steps of:
providing an electrolyte solution according to claim 1 ; and
providing a free-halide-anion donor having X, but not Mg, thereby contributing free anions of X − to the electrolyte solution.
21. The method of claim 20 , wherein the metal center M comprises a metal selected from the group consisting of B, Al, Ga, In, Fe, and combinations thereof.
22. The method of claim 20 , wherein the organic ligand R comprises ethyl, phenyl, or both.
23. The method of claim 20 , wherein said providing a free-halide-anion donor comprises providing an amount of the free-halide-anion donor sufficient to yield a Coulombic efficiency greater than 99%.