Si Anode in High-Salt Concentration Ionic Liquid Electrolytes Based on Pyrrolidinium and Phosphonium Systems for High-Energy Li-Ion Batteries

Abstract

In order to push further the development of next generation Li-ion batteries (LiBs), high energy density, good capacity retention and increased safety are key features that need to be taken into account when designing energy storage devices. To this date, commercial batteries for portable devices mainly use graphite as anode. However, with the advent of electric vehicles and intelligent power grids, there is an increasing demand for high capacity materials to meet the energy and power requirement of these applications. More recently, Si anode has received considerable attention because of its natural abundance and its ability to deliver high energy density, with a gravimetric capacity of 3579 mAhg-1 which is almost 10 times that of graphite (372 mAhg-1) [1-2]. Bearing in mind that commercial LiBs use carbonate electrolytes which are recognized to pose safety issues and have been identified to suffer from degradation during charge-discharge cycles, we explored in this study the performance of Si anode in ionic liquid electrolytes based on phosphonium and pyrrolidinium cations, systems that are currently the most studied in the field of Li batteries [3-4]. Ionic liquid electrolytes are an appealing alternative to carbonates mainly because of their low volatility and better stability. In the recent years, good electrochemical behavior has been observed in ionic liquids with high salt concentration [3-5]. In this work, we were able to demonstrate better electrochemical performance of Si anode in ionic liquid electrolytes with 3.2 m LiFSI compared to conventional carbonate electrolytes in terms of capacity and capacity retention at 50°C. Electrochemical performance obtained in the phosphonium and pyrrolidinium systems are compared for different concentrations and temperatures. Moreover, initial investigation of the SEI using NMR and XPS spectroscopy revealed that less degradation products are formed upon cycling in ionic liquid electrolytes compared to usual carbonates electrolytes. Overall, these results highlight the stability of ionic liquids which translate to good cycling performance and show promising potential of Si anode as a high-capacity material for high-energy and high-temperature applications. Cyclability and capacity retention for the phosphonium and pyrrolidinium system will be discussed in terms of ionic liquids components degradation and SEI formation and evolution, as probed by combined 19F, 7Li MAS-NMR, XPS and impedance spectroscopy.