(435c) All-Climate and Ultrafast Rechargeable Aluminum Batteries Enabled By Tertiary Eutectic Electrolytes

Lithium-ion technology is one of the most successful energy storage technologies every hit the market and yet the application towards the grid-storage and electric vehicles is limited. Multivalent metal-ion batteries remain as one of the promising and potential contenders to address the inherent challenges posed by Li-ion chemistry. In recent years, Al has attracted great interest due to advantages envisaged such as high volumetric capacity (8046 mAh.cm^-3 vs. 2262 mAh.cm^-3 for Li), high abundance in earth’s crust (82000 ppm vs. 65ppm for Li), multiple electrons transfer per single metal reduction (3e^- vs. 1e^- for Li) and low cost (1.9 USD.kg^-1 vs. 19.2 USD.kg^-1 for Li). However, the major challenge is the development of electrolytes which offer wide thermal and potential stability window.

Here, we designed a tertiary eutectic electrolyte comprised of 1-ethyl-3-methylimidazolium chloride([C₂mim]Cl):1-butyl-3-methylimidazolium chloride([C₄mim]Cl):AlCl₃ for the application of aluminium batteries, suitable for all-climate conditions. The binary eutectic phase between [C₂mim]Cl:[C₄mim]Cl (1:1 mole ratio) at ~40 °C is identified due to the dominating Van der walls forces of attraction between the different-alkyl chain containing organic molecules than the electrostatic forces present between the cations and anions. Further, the addition of AlCl₃ to the binary eutectic, forms tertiary phase eutectic at room temperature based on the acid-base reactions. The differential scanning calorimetry studies on the tertiary eutectic renders significantly low freezing point (<-50 °C) which is a prerequisite condition for all-climate batteries and appreciably high ionic conductivity of ~8 mS.cm^-1 at room temperature. The eutectic offers improved electrochemical stability window of ~2.8V vs. Al/Al⁺³ against the stainless-steel electrode. Galvanostatic charge-discharge analysis of Al||graphene cell retrieves a high discharge capacity of ~150 mAh.g^-1 at ultra-high current density of 1 A.g^-1 and ~120 mAh.g^-1 at 3A.g^-1. The rapid charge-discharge (<1 minute) at ultra-high current density of ~5A.g^-1 delivers ~70 mAh.g^-1 capacity and shows a very minimum capacity fade of <1% per cycle. The study opens-up a broad area of research to investigate the mixed ionic liquid-based eutectics for the design of highly efficient, safe batteries with improved energy density.