(Invited) Applications of Ionic Liquids in Electrochemical Energy Conversion and Storage

Introduction

Electrochemical energy conversion and storage are of large importance in future energy technology [1]. The increasing market share of renewable energy sources requires corresponding storage options in order to match electricity supply and demand. In this regard, electrochemical storage or (photo)electrochemical conversion of electricity to hydrogen or methane gas may provide solutions [2]. Even more significant is the role of electrochemistry in electromobility. The electrification of cars is required considering the increasing number of electric vehicles worldwide, especially in Asia, and the associated environmental problems and resources consumption [3]. Li-ion battery powered cars as well as fuel cell vehicles both will play an important role. Aside from these aspects, electrochemistry is of importance in the preparation of (nano-) materials for energy conversion devices. In some of these fields, ionic liquids can offer significant enhancements compared to conventional electrolytes. Therefore the application of ionic liquids in energy conversion and storage is reviewed, before focusing on the more specific topic of batteries for electromobility.

Li and Mg Ion Batteries

Rechargeable lithium-ion batteries have been advanced and success-fully commercialized in the past two decades, particularly in the areas of portable and mobile applications. Nowadays, the advancement of hybrid electrical vehicles (HEVs) and electrical vehicles (EVs) technologies depends strongly on the availability of Li-ion battery systems improved with respect to cyclability and energy density [4]. There is an increasing demand of batteries for transportation. This raises big challenges for the next generation rechargeable batteries concerning cost, safety, power and energy density, cycle life, etc [5]. Those batteries in most cases have a highly flammable organic electrolyte, or in some cases a solid electrolyte suffering from limited conductivity. Here ionic liquids offer an alternative combining a significantly improved safety level with the advantages of a liquid electrolyte. In order to optimize the electrode materials and the electrolyte composition an understanding of the physico-chemical processes in the battery at the nanoscale is required. Therefore, various scanning probe microscopy (SPM) studies have been devoted to investigate the nanoscale electrochemical reactions and the physical basis of imaging mechanisms [6]. In our work, we study the processes of ion insertion and SEI formation on model anodes and cathodes using Electrochemical Scanning Tunneling Microscopy (EC-STM). These measurements are performed both in classic organic electrolytes, i.e. mixtures of ethylene carbonate / dimethyl carbonate and LiPF₆, as well as in ionic liquids. Figure 1 gives an example for the initial stages of SEI formation in a classical organic electrolyte. Studies are also performed for Mg ion batteries that still suffer from a number of problems [7]: For this system it has been shown that using ionic liquids in combination with Grignard reagents, reversible Mg electroplating is possible, thus permitting the use of Mg anodes [8]. Therefore Mg insertion in HOPG and cathode materials, mainly V₂O₅, are studied as well in such electrolytes.

Acknowledgments

Funding by BMBF under Project Number 16N11930 and by the Fuel Cell and Hydrogen Initiative Joint Undertaking under contract number 303492 (CathCat) are gratefully acknowledged.

References

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[8] G.T. Cheek, W.E. O’Grady, S.Z.E. Abedin, E.M. Moustafa, F. Endres, J. Electrochem. Soc. 155 (2008) D91-D95.

Figure 1. HOPG with 1M LiPF₆ electrolyte (a) before and (b) after potential induced SEI formation