Ultrafast laser micromachining of hard carbon/fumed silica anodes for high-performance sodium-ion capacitors

The slow sodium-ion storage kinetics of battery-type electrodes limits the performance of sodium-ion capacitors (SICs) operating under high-power conditions. In this study, ultrafast laser micromachining was utilized to accelerate the sodium-ion storage kinetics of hard carbon/fumed silica (HC/f-SiO2) anodes. The ablation process involving an ultrafast femtosecond laser source enabled three-dimensional microstructuring of hot-short HC/f-SiO2 anodes with minimal photothermal damage. The microporous structure of the HC/f-SiO2 anodes facilitated the electrolyte wetting of the active materials as well as the diffusion-limited supply of sodium-ions from the bulk electrolytes. The microstructured HC/f-SiO2 anode exhibited a sodium-ion storage capacity of 370 mAh g- 1, which was higher than those of unstructured HC/f-SiO2 anodes of comparable mass (298 mAh g-1) or thickness (248 mAh g-1). In addition, the rate capability of the microstructured HC/f-SiO2 anode was superior to that of the unstructured samples. Comparative full-cell tests with oxidized single-walled carbon nanotube cathodes confirmed that micromachining of the HC/f-SiO2 anode was crucial for improving the performance of the SIC full cells. This study demonstrates that ultrafast laser micromachining of HC/f-SiO2 electrodes is a facile and reliable strategy for the development of high-performance SICs.