Flexible few-layered graphene for the ultrafast rechargeable aluminum-ion battery

Fast ion transport is essential for high rate capability in rechargeable battery operation. Recently, an ultrafast rechargeable aluminum-ion battery was experimentally demonstrated through the reversible intercalation/deintercalation of chloroaluminate anions (AlCl4-) in graphitic-foam cathodes. Using first-principles calculations, herein, we report that the unique structural characteristic of graphitic foam, i.e., mechanical flexibility of few-layered graphene nanomaterials, plays a key role for the ultrafast aluminum-ion battery. We found that AlCl4- is stored by forming doubly stacked ionic layers in the interlayer space between graphene sheets, and their diffusivity increases dramatically once graphene film is less than five layers thick; the diffusivity beg-ins to increase when the film thickness reduces below five layers in such a way that the film thickness of four, three, and two graphene layers enables 48, 153, and 225 times enhanced diffusivity than that of the bulk graphite, respectively, and this nanoscale thickness is mainly responsible for the observed ultrafast rate capability of graphitic foam. The faster anion conductivity with the reduced film thickness is attributed to high elasticity of few-layered graphene, providing more space for facile AlCl4- diffusion. This study indicates that even bulky polyanions can be adopted as carrier ions for ultrahigh rate operation if highly elastic few-layered graphene is used as an active material.