Ionic Liquid Electrolytes with Mixed Organic Cations for Low-Temperature Rechargeable Aluminum–Graphite Batteries

Recently, aluminum–graphite batteries using chloroaluminate ionic liquid electrolytes have been shown to store charge with significant pseudocapacitive contributions, features attractive for energy storage systems that must function with high capacity retention and high specific power at low temperatures. Herein, we present results toward rechargeable aluminum–graphite batteries designed specifically for low-temperature applications, focusing on electrolyte design to reduce the melting point and enhance ion transport properties down to −40 °C. Chloroaluminate ionic liquid electrolytes with mixtures of organic cations, particularly imidazolium cations with different asymmetric functional groups, were used to impart disorder, disrupt ion clustering, and enhance low-temperature ion mobilities. Graphite cathodes with higher pseudocapacitive contributions showed improved specific capacity retention at lower temperatures, while aluminum anodes with higher surface roughness reduced the nucleation energy for aluminum electrodeposition. An aluminum–natural graphite battery with mixed imidazolium cations was shown to exhibit 87% specific capacity retention at −20 °C and 10 mA g–1. Overall, the results demonstrate multiple approaches to improve low-temperature aluminum battery performance and illustrate electrochemical methods that quantify electrolyte properties relevant to ion transport and clustering, which can be universally translated into other battery systems.