Sodium-Based vs. Lithium-Based Dual-Ion Cells: Electrochemical Study of Anion Intercalation/De-Intercalation into/from Graphite and Metal Plating/Dissolution Behavior

Abstract Energy storage systems utilizing a simultaneous intercalation of anions into a graphite host structure at the positive electrode and intercalation/insertion/reduction or deposition of cations at the negative electrode during charge, were introduced under the term dual-ion cells, recently. In this work, the electrochemical intercalation of TFSI − anions into graphite has been studied in sodium‐based dual‐ion cells (SDICs) at different upper cut‐off potentials varying between 4.5 V and 4.7 V vs. Na/Na + . Electrochemical characteristics of SDICs, including the reversible capacity, Coulombic efficiency, energy efficiency and onset potentials for anion intercalation, were evaluated in comparison to the lithium-based dual-ions cells (LDICs). A stable charge/discharge cycling performance over 500 cycles has been found for SDICs providing a specific capacity of ≈ 32 mAh g ‐1 and a Coulombic efficiency exceeding 99% at an upper cut‐off potential of 4.7 V vs. Na/Na + at the graphite cathode. By the addition of the electrolyte additive ethylene sulfite (ES), an increase of the reversible capacity to ≈ 46 mAh g ‐1 was achieved. Furthermore, possible reasons for the overall inferior cycling performance in terms of capacity for SDICs as compared to LDICs such as an increased overpotential for plating/stripping of Na + ions as compared to Li + ions are discussed. In this respect, we also found that the addition of ethylene sulfite particularly decreases the overpotentials for the metal plating process, which at least partially explains the enhanced reversible capacity in LDICs and SDICs by using ES as electrolyte additive.