Deliberate introduction of mesopores into microporous activated carbon toward efficient Se cathode of Na−Se batteries

Sodium-selenium (Na−Se) batteries are garnering increasing attention as promising energy storage systems because of the low cost of Na resources and the high volumetric capacity of Se. Nevertheless, their practical application is hindered by the low utilization rate of Se and the shuttle effect of polyselenide, which lead to unstable cycling performance. Therefore, extensive efforts are necessary to develop suitable carbon-based Se hosts. Here, we propose a simple method to introduce a controlled amount of mesopores into microporous carbon, which can remarkably improve the electrochemical performance of Na−Se batteries. The Se encapsulated in the optimized micro-mesoporous carbon exhibited a substantially improved cycling stability, with a capacity of 572 mA h g⁻¹ at 0.5C after 150 cycles, which represents a 93% capacity retention from the second cycle. In addition, the Se could achieve a high reversible capacity of 214 mA h g⁻¹, even at 20C. The results of this study provide guidance for distinguishing the roles of the micropores and mesopores of carbon in the Se host of Na−Se batteries: (a) Micropores are ideal reservoirs to confine Se in an amorphous form for stable electrochemical reactions with Na, and (b) mesopores provide pathways for Na⁺ ion diffusion and a buffer for the volume change of Se. The proposed method would be widely applicable to other types of microporous carbon with much higher specific surface areas, which can afford higher Se loading for more advanced alkali metal−Se batteries.