Dual-ion Cells Based on Anion Intercalation into Graphite from Ionic Liquid-Based Electrolytes

Abstract

Electrochemical energy storage systems using graphite as both the negative and the positive electrode have been proposed as “dual-graphite cells”. In this kind of electrochemical system, the electrolyte cations intercalate into the negative electrode and the electrolyte anions intercalate into the positive electrode, both based on graphite, during the charging process. On discharge, cations and anions are released back into the electrolyte. So far, the systems proposed in literature are primarily based on Li⁺ and PF₆^- intercalation/de-intercalation into/from graphite from non-aqueous organic solvent based electrolytes. As the positive electrode potential during charging always exceeds 4.2 V vs. Li/Li⁺, the organic electrolyte starts to decompose at these highly oxidizing conditions resulting in insufficient discharge/charge efficiencies. The replacement of organic solvent by ionic liquids (ILs) leads an increased stability of the electrolyte towards oxidation and thus to remarkably higher efficiencies as well as an increased cycling stability. In fact, ionic liquids provide extended anodic electrochemical stability and in addition, no solvent co-intercalation occurs in parallel to anion intercalation at high potentials.

Here, we present highly promising results for “dual-ion cells” based on a graphite cathode and an ionic liquid based electrolyte, namely N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr₁₄TFSI). As the compatibility of this IL with graphite anodes is poor, alternative anodes such as metallic lithium or lithium titanate (Li₄Ti₅O₁₂, LTO) are used. Consequently, the “dual-graphite” cell is renamed to “dual-ion” cell. In addition, the calculation of the specific energy of these systems will be in the focus of the discussion.