Microwave Exfoliated Few-Layered Graphene Cathode for Aluminum Batteries

Microwave-exfoliated few-layered graphene (meFLG) with oxygen, hydroxyl, and sulfate impurities is tested as a cathode material in aluminum batteries (ABs). The O and S impurities bonded to C atoms are found collectively weighing 2.9-4.7 at. % (4.1-6.9 wt %). meFLG as a cathode in AB pouches delivered stable discharge capacities around 100 mAh/g at 0.1 A/g current rate over 500 cycles, despite high irreversible capacities noticed for the first few cycles. Side reactions of the impurities with the electrolyte species are found to be responsible for the initial high irreversible capacities. The effect of the impurities became negligible when meFLG cycled at higher current rates starting from 1.0 A/g, but initial energy storage capacities and cyclic behavior varied with the voltage window. Postcycling chemical analysis of the electrode matrix revealed that anion species (AlCl4-) of the electrolyte are found covalently bonded to the impurities, while cation species are found intact even after 500 cycles of continuous charge-discharges. We therefore recommend careful characterization of the impurities before using FLG-related materials as aluminum battery cathode materials. Removing the impurities before the electrode fabrication will help meFLG outperform its class of materials because it shows excellent rate capability even with the impurities.