Performance of carbon xerogels as anodes for sodium dual-ion batteries

Carbon xerogel with different mean pore sizes (10, 50, 100, 500 nm) but analogous structure, chemical composition, surface area, and microporosity, which are produced by an energy-effective microwave-based process, and activated carbon xerogels (100 nm-pore size) with higher microporosity were investigated as anodes for sodium dual-ion batteries. The objective of this study was to optimize the material textural properties for this specific application to be further matched with a carbon xerogel-based cathode in a full battery configuration. To this end, the role of these properties was evaluated, specifically the pore size and the microporosity since the storage of the Na+ ions in these carbon xerogels was demostrated to occur mainly by a pseudocapacitive mechanism based on adsorption on surface, defects (i.e. microporosity) and pores. The balance between the pore size and the associated external surface area of the carbon xerogels was a determining factor for their anodic performance. In this context, a material pore size of ∼ 100 nm and an associated external surface area of ∼ 100 m2 g−1 was the optimum balance. For a given material pore size, the increase of the microporosity by physical activation improved the anode capacity as well as the cycling stability. However, the development of the microporosity in relation to the external surface area must be optimized to avoid an undesirable increase in the first cycle irreversible capacity. Overall, the physically activated carbon xerogel with a pore size of 100 nm, a micropore volume of 0.47 cm3 g−1 and an external surface of 123 m2 g−1 was the most suitable active anode material for sodium dual-ion batteries. This material provided a specific discharge capacity of ∼ 154 mAh g−1 after 300 discharge/charge cycles with excellent cycling stability and coulombic efficiency.