(Invited) Tuning the Formation and Structure of the Silicon Electrode/Electrolyte Interphase in Superconcentrated Ionic Liquids

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. Our recent study using superconcentrated electrolytes revealed the superior electrochemical behaviour of triethyl(methyl)phosphonium bis(fluorosulfonyl)imide (P1222FSI) in a half-cell investigation of Si negative electrodes (Fig. 1). This result was linked to the stability of the electrolyte upon cycling and to improved lithium ion transport which is facilitated at high Li salt content [6]. 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. In the present work, we report the importance of tuning the Si/IL electrolyte interface as a function of IL cation nature and salt concentration. Experimental techniques such as 7Li, 19F MAS-NMR, SEM, and STEM-EDX were used to determine the SEI composition and morphology in order to further investigate its influence on the electrochemical performance as well as the relationship of the electrolyte chemistry (P1222FSI, N-methyl-N-propylpyrrolidinium (C3mpyr) FSI-based electrolytes and carbonate-based electrolyte) to the SEI properties. In addition, computational methods and differential capacitance (DC) experiments using AC impedance were applied to gain a better understanding of the SEI formation by examining the structuring at the electrode/electrolyte interphase.