Zwitterionic versus organic ionic plastic crystal electrolytes with mixed anions: probing the unique physicochemical and electrolyte properties

Abstract

The feasibility of achieving high performance Li metal batteries, with the associated benefits of high energy density, relies on the development of safe and stable electrolytes suitable for use with the highly reactive Li metal. The use of solid electrolytes can improve safety, decrease the evolution of Li dendrites, and increase the cycle life of a battery. Zwitterions (ZIs) are a unique class of material in which the cationic and anionic moieties are covalently bound, and these have previously been used as additives in electrolytes to improve ion transport. We recently reported a novel class of material, zwitterionic plastic crystals, that have intrinsic molecular disorder and offer a promising alternative to traditional organic ionic plastic crystal (OIPC)-based electrolytes. Here we report an investigation into the use of zwitterionic plastic crystals as matrix electrolytes by the incorporation of different types and concentrations of Li salts, namely lithium bis(fluorosulfonyl)imide (LiFSI) or lithium tetrafluoroborate (LiBF₄), and the influence of lithium salt species and concentration on the thermal behaviour and ion dynamics. To understand the effect of ion tethering on the physicochemical properties of the ZI electrolytes we compared their thermal behaviour and ionic conductivity with the analogous OIPC electrolytes, namely [C₂mpyr][BF₄]. The low and high salt concentrations of ZI/LiBF₄ and ZI/LiFSI mixtures form solid-state electrolytes, and the middle concentrations (30–80 mol%) yield liquids at room temperature. The ion dynamics and diffusion were studied by NMR spectroscopy and pulsed-field gradient NMR. The ZI/LiFSI mixtures exhibited lower glass transition temperatures and higher ionic conductivity and higher Li ion mobility than the ZI/LiBF₄ mixtures. Finally, electrochemical analysis of the high LiFSI content ZI electrolyte demonstrated a high Li transference number and stable stripping/plating of Li in Li symmetrical and full cells. The combination of high ionic conductivity, high Li ion transference number and stable lithium cycling make zwitterion-based electrolytes promising candidates for lithium metal battery applications.