In order to meet the demands for future energy storage, a broad variety of batteries is desired. Even though lithium ion batteries (LIBs) are currently used for most portable electronic devices and electric vehicles, the abundancy of its raw materials might lead to shortages in the future. One alternative system are dual-ion batteries (DIBs), which might be favored especially for large-scale energy storage applications, since they do not contain transition metals. Different from LIBs, where the lithium-ion is transported from one electrode to the other during charge and discharge, in DIBs both ions are involved in the electrochemical storage. While the cation is stored in the negative electrode (e.g., lithium-ion intercalation into graphite), the anion is for example intercalated in a positive graphite electrode.[1, 2] A further advantage of DIBs is the broad versatility of its components: DIBs have been enabled with numerous different positive and negative electrode materials, as well as anions and several cations, also beyond lithium. Based on their bivalency, especially multivalent cations such as magnesium-ions are beneficial for a high energy density. So far, however, only few reports about magnesium-ion-based DIBs were published and often do not use the high capacity magnesium metal negative electrode, based on the inability to reversibly electrodeposit and -dissolve magnesium from electrolytes commonly used in DIBs, such as TFSI salts in ionic liquids.[3-6]
Herein, a study about different electrolytes for the use in magnesium-ion-based DIBs is presented. Beside the ionic liquid-based electrolyte Mg(TFSI)2 in Pyr14TFSI, also the additive ethylene sulfite (ES) is applied. In addition, a study of highly concentrated organic carbonate solvent-based electrolytes is presented. Since none of these electrolytes enables reversible magnesium electrodeposition and -dissolution, the study was performed in graphit‖activated carbon cells. Beside the electrochemical performance and a direct comparison to a lithium-ion-based system, the TFSI-ion intercalation into graphite is studied via ex situ XRD and in situ Raman. Furthermore, the optimal cycling conditions, but also the limitations regarding the applied current and upper cut-off potential were determined. Combining the most promising electrolyte with optimized cycling conditions, a specific discharge capacity of 93 ± 2 mAh g−1 and Coulombic efficiencies above 99% could be achieved, which showed a stable cycling performance and a capacity retention of 88% after 400 cycles.
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