Carbon vacancies boost plateau sodium storage in sieving carbon anodes

Abstract

Defects in hard carbon are primarily associated with the slope capacity arising from sodium ion adsorption on defect sites for sodium storage. However, the impact of defects on plateau sodium storage remains poorly understood owing to the difficulties for hard carbon in precisely customizing carbon defects in the walls of closed pores without changing other structures such as pore geometry. Here, sieving carbon is selected as a model material to study the role of defects within closed pores in plateau sodium storage, considering that its pore forming and pore sealing by chemical vapor deposition are decoupled during material preparation. Specifically, vacancy-dominated defects are formed within closed pores by hydrogen reduction of porous carbon precursor and an identical external surface with deposited carbon covered is obtained by chemical vapor deposition. Such a vacancy-rich sieving carbon exhibits a high specific capacity of 470 mAh g⁻¹, along with an increased plateau capacity and improved cycling stability, remaining 72.3 % of its initial capacity after 400 cycles (342 mAh g⁻¹). A series of spectroscopic studies demonstrate that increased carbon vacancies promote the formation of sodium clusters with higher reversibility in closed pores. This work provides new insights into the mechanism of plateau sodium storage and offers an effective strategy to improve the capacity and cycling stability of carbon anodes for sodium-ion batteries.