Moreover, Magnesium Batteries have the potential to reach very high volumetric energy densities, partly above that of the theoretical energy density of the Lithium-Air couple. In addition, the active materials in such batteries may be abundant, sustainable, and safe in both handling and operation.
The Fluoride Ion Battery
In search of new concepts to build batteries with high energy densities, electrochemical cells based on metal fluorides may be promising.
We have demonstrated the first reversibly working battery cells based on fluoride shuttle. In secondary fluoride batteries, fluoride anion acts as charge transfer ion between a metal/ metal fluoride pair where it will react with metal or evolve from metal fluoride depending on the flow of current. The theoretical capacity can be several times higher than that of conventional Lithium Ion Batteries, depending on the combination of metal and metal Fluoride.
The Chloride Ion Battery
The Chloride Ion Battery (CIB) is a logical consequence of the development of the Fluoride Ion Battery. Also here, singly charged negative ions shuttle between cathode and anode where metal chlorides are either formed or reduced to the metal depending on whether the battery is charged or discharged.The group has demonstrated the proof-of-principle and built first cells of this kind. Ionic liquids have been used as elecrolytes at room temperature.
The Magnesium Battery
Magnesum as anode material has the potential advantage of a high theoretical volumetric capacity of 3832 mAh/cm3 (Lithium: 2062 mAh/cm3), its electrochemical potential is -2.37 V vs. NHE. Interestingly, Manesium does not form dendrites when electrodeposited and can therefore be used in metallic form, thus avoiding inert host materials like in the Lithium/Graphite system. Magnesium is environmentally benign, safe to handle and of low cost compared to lithium. A particular challenge is the development of an electrolyte for reversible Manesium shuttle. We have developed a non-nucleophilic electrolyte which is synthesized from standard chemicals, shows a high stripping/plating efficiency and has an unprecedented electrochemical stability window of 3.9 V and Coulombic efficiency of >99%. The electrolyte is compatible with a sulfur cathode and it opens the door to the development and application of new high voltage cathodes for Magnesium Batteries.
Using a sulfur/CMK-3 composite as cathode, Mg metal as anode and the designed electrolyte in tetraglyme or a binary solvent of glyme and PP14TFSI, the discharge performance and the cyclability of the batteries was considerably improved compared to the first report on Mg/S battery where an HMDS based electrolyte was used in THF solution. The electrochemical conversion of magnesium and sulfur via the formation of a series of intermediate polysulfide MgSx (2<x<8) has been verified by means of various analytical and electrochemical techniques.
Interestingly, the discharge occurs close to the theoretical voltage of 1.7 V with the new electrolyte.
(Zh. Zhao-Karger, X. Zhao, D. Wang, Th. Diemant, R.J. Behm, and M. Fichtner: Performance Improvement of Magnesium Sulfur Batteries with Modified Non-Nucleophilic Electrolytes. Advanced Energy Materials. Article first published online: 6 OCT 2014. DOI: 10.1002/aenm.201401155)
A major issue in Metal-Sulfur Batteries is the formation of polysulfide intermediates during the transition from neutral S8 to Li2S and Li2S2 during discharge. The longer polysulfides are soluble in the electrolyte, which leads to gradual dissolution of the cathode, to self-discharge and a multistep voltage profile, due to different subsequent reactions.
By extensive XPS studies of the subsurface region we have shown now that a simple, coconut based active carbon with ultramicropores (ca. 0.6 nm diameter) does not allow infiltration of the large S8 rings, nor does it allow formation of soluble polysulfides. Rather, one direct transition from smaller sulfur species to Li sulfide and -disulfide is observed. No polysulfides have been found in the electrolyte and there is only one voltage plateau (M. Helen, M. Anji Reddy, T. Diemant, U. Golla-Schindler, R. J. Behm, and M. Fichtner (2015) submitted)
Analytical Tools TERS
Tip Enhanced Raman Spectroscopy is an analytical tool which gives chemical and topographic information in the same time. It is a possibility to analyse surfaces with a resolution on nanometer scale. In battery research it is often not well-‐known, what is going on at surfaces and interfaces like the Solid Electrolyte Interface. The aim of this project is a better understanding of processes in these layers.