Peter Bieker

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.

Ultra-high cycling stability of poly(vinylphenothiazine) as a battery cathode material resulting from π–π interactions

Organic cathode materials are promising candidates for a new generation of “green batteries”, since they have low toxicity and can be produced from renewable resources or from oil. Especially suitable are organic redox polymers that can be reversibly oxidized and reduced. Because of their often-insulating nature, however, many redox polymers have limited rate capabilities. Their cycling stabilities, which are of high importance for the long cycle-life of a battery cell, rarely exceed 1000 cycles. Here, we present a new concept for redox polymers as cathode materials, in which the oxidized states are stabilized through π–π interactions between redox-active groups. We found that due to these interactions poly(3-vinyl-N-methylphenothiazine) showed excellent cycling stability (after 10 000 cycles at a 10C rate, 93% of the initial capacity was retained) in addition to a high rate capability because of supramolecular hole transport. We propose this concept to be used in the future design of redox polymers for batteries.

Decomposition of Imidazolium-Based Ionic Liquids in Contact with Lithium Metal

Ionic liquids (ILs) are considered to be suitable electrolyte components for lithium-metal batteries. Imidazolium cation based ILs were previously found to be applicable for battery systems with a lithium-metal negative electrode. However, herein it is shown that, in contrast to the well-known IL N-butyl-N-methylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide ([Pyr14 ][TFSI]), 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C2MIm][TFSI]) and 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C4MIm][TFSI]) are chemically unstable versus metallic lithium. A lithium-metal sheet was immersed in pure imidazolium-based IL samples and aged at 60 °C for 28 days. Afterwards, the aged IL samples were investigated to deduce possible decomposition products of the imidazolium cation. The chemical instability of the ILs in contact with lithium metal and a possible decomposition starting point are shown for the first time. Furthermore, the investigated imidazolium-based ILs can be utilized for lithium-metal batteries through the addition of the solid-electrolyte interphase (SEI) film-forming additive fluoroethylene carbonate.

Synthesis of High‐Purity Imidazolium Tetrafluoroborates and Bis(oxalato)borates

The synthesis and purification of imidazolium-based ionic liquids (ILs) with boron-containing anions is reported. The scope was the optimization of the meta thesis reaction and on the purification of the synthesized ILs. It was possible to reduce the reaction and purification times, to avoid the use of acetonitrile as processing solvent, and to increase the yield compared to the known procedure for [BOB]- anion-containing ILs. Furthermore, to the best of our knowledge the flashpoints of the ILs could be determined for the first time by the continuously closed-cup flashpoint (CCCFP) method.