(Invited) 3D-Catenated EMImCl/(TiCl4)1.4/(δ-MgCl2)x Ionic Liquid Electrolyte for Mg Secondary Batteries
Authors: Pagot Gioele , Bertasi Federico , Negro Enrico , Sepehr Fatemeh , Paddison Stephen J. , Di Noto Vito 
Published: February 2020

The rapid advance in the fields of portable electronics, load leveling and peak shaving for the power grid and zero-emission automotive applications require the development of new and improved electrical energy storage systems [1,2]. Since the 90’s major improvements have been achieved in magnesium battery technology [3-6]. In comparison to Li, Mg offers the following advantages: (i) a higher volumetric capacity (3832 vs. 2062 mAh·cm-3); (ii) far greater abundance in the Earth’s crust, lowering the costs; (iii) a safer operation and a better compatibility with the environment; and (iv) an acceptable standard reduction potential (-2.36 vs. -3.04 V) [7-9]. The main roadblock for these devices is the development of an efficient and stable electrolyte that is able to reversibly deposit and strip magnesium. Although Grignard and other organo-magnesium compounds exhibit good electrochemical performances [9], they do not exhibit an optimal stability due to their high vapor pressure and flammability. Ionic liquids dissolving a Mg salt with a high crystalline disorder were proposed as promising alternative electrolytes to organo-Mg systems owing to their good electrochemical performance and lack of flammability and thermal stability issues [10].
In the present work a new family of electrolytes is proposed, based on 1-ethyl-3-methylimidazolium chloride (EMImCl), titanium(IV) chloride (TiCl4) and increasing amounts of δ-MgCl2. Specifically, four EMImCl/(TiCl4)1.4/(δ-MgCl2)xelectrolytes, with 0.00 ≤ x ≤ 0.23 are prepared and extensively characterized. The chemical composition was determined by Inductively-Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). The thermal stability was gauged using High-Resolution Thermo Gravimetric Analysis (HR-TGA) and the phase transitions are highlighted with Modulated Differential Scanning Calorimetry (MDSC). Chemical interactions were studied through Fourier-Transform spectroscopy in the medium and far infrared (FT-MIR and FT-FIR) regions and confocal micro-Raman spectroscopy. The electrochemical performance was studied with: (i) Cyclic Voltammetry (CV), to probe Mg deposition and stripping (Fig. 1); (ii) Linear Sweep Voltammetry (LSV), to evaluate the electrochemical stability window; (iii) Chronopotentiometry (CP) experiments coupled with ICP-AES, to confirm and quantify the Mg deposition; and (iv) Broadband Electrical Spectroscopy (BES), to elucidate the long-range charge migration mechanisms of the electrolytes. High level density functional theory (DFT) based electronic structure calculations were undertaken to elucidate structures and vibrational frequency assignments.

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Figure 1