Enabling Mg‐Based Ionic Liquid Electrolytes for Hybrid Dual‐Ion Capacitors

Abstract

We report on the reversible (de)intercalation of TFSI− anions from a Mg‐based ionic liquid electrolyte, Mg(TFSI)2 in Pyr14TFSI, in graphite ∥ activated carbon hybrid dual‐ion capacitor (DIC) cells. The role of different pseudo reference electrodes (PREs) including Ag‐wire and Li metal is discussed regarding the comparability of different battery cells. We show that an Ag‐wire PRE is not suitable for the designated purpose, while the use of a Li metal PRE results in high reproducibility. Mg‐based DIC cells are compared with cells based on pure Pyr14TFSI and LiTFSI‐Pyr14TFSI electrolytes, and with graphite ∥ Li metal dual‐ion cells employing LiTFSI‐Pyr14TFSI. At 5.2 V vs. Li|Li+, Mg‐containing DIC cells reveal an improved performance compared to Li‐based cells, i. e., higher capacity (87 vs. 85 mAh g−1) and higher Coulombic efficiency (98 vs. 96 %). These results may pave the way to further studies and performance improvements of Mg‐based batteries and to hybrid DIC chemistries with other cations, especially multi‐valent ones.

Recent Progress and Challenges in Multivalent Metal‐Ion Hybrid Capacitors

“…In the device models of MHCs, the mainly used capacitor‐type material is carbon. Some carbon‐based capacitor‐type materials, such as AC, [57] carbon aerogels [6c] and ACF cloth, [6c] have been reported. However, the adsorption/desorption mechanism of a typical carbon significantly limits its capacitance.…”

“…The electrolytes of MHCs can be simply classified into inorganic and organic electrolytes. Examples of inorganic electrolytes are MgSO 4 and MgCl 2 [57] . As an example, a modified ethereal‐Mg organo‐haloaluminate complex was used as the electrolyte of an MHC [6c] .…”

Opportunities and Limitations of Ionic Liquid‐ and Organic Carbonate Solvent‐Based Electrolytes for Mg‐Ion‐Based Dual‐Ion Batteries

“…As pointed out by other studies, [10b,12k] the limitation of the cut‐off potential is crucial to achieve high SDCs in combination with a high C Eff , resulting in severe changes of the specific capacities if the cut‐off potential is changed. This is based on a staging behavior of the TFSI − ‐intercalation into graphite at different potentials [10b,12k] . In order to evaluate if intrinsic differences of the electrolytes or rather potential differences at the Li|Li + RE/QRE lead to the different cycling performances, the Li RE/QRE was calibrated in different electrolytes using ferrocene/ferrocenium (Fc/Fc + ) as standard redox reference system [31] .…”

“…As shown recently, the reversible intercalation of TFSI − into/from graphite is possible from a Mg‐ion‐based electrolyte [12i–k] . However, herein, deeper knowledge of graphite as anion‐intercalation cathode for Mg‐ion‐based DIBs is focused and the differences between Li‐ and Mg‐ion‐based DIBs are systematically evaluated.…”

“…Even though AC does not show a capacity based on intercalation or conversion reactions, the formation of a double layer on its surface enables physical storage of charges in form of capacitance [30] . This double layer formation shows fast kinetics and is usually within the electrochemical stability window of the electrolyte, thus, allowing cathodes to be evaluated with electrolytes, which are not able to electrodeposit and ‐dissolve Mg at adequate overpotentials if the cathode potential is controlled with a reference electrode [12k,30,31] . In addition, Li metal was used as reference electrode (RE, for Li‐ion‐based cells) or quasi‐reference electrode (QRE, for Mg‐ion‐based cells), since the usage of a Mg|Mg 2+ electrode can lead to high variations of the working electrode (WE) potential [12k,30] .…”