High-Capacity Anode for Sodium-Ion Batteries Using Hard Carbons Derived from Polyurea-Cross-Linked Silica Xerogel Powders

In this article, we demonstrated carbon aerogel-silica composites derived from polyurea-cross-linked silica xerogel powders (C-PUA@silica) as anodes for Na-ion batteries. These xerogel-derived hard carbons with embedded silica showed a stable high capacity. Of the several hard carbon samples derived at different temperatures, the pyrolyzed sample at 800 °C (C-PUA@silica-800) showed a high capacity of 236 mAh/g at a current density of 10 mA/g and a stable cycle-life at 200 mAh/g. The galvanostatic charge–discharge curves displayed a sloping voltage profile reminiscent of the adsorption mechanism of Na+ storage. On the other hand, higher temperature-pyrolyzed samples at 1300 °C (C-PUA@silica-1300) displayed a lower capacity of 185 mAh/g at a current density of 10 mA/g, and the charge–discharge profile showed a two-step process, a sloping adsorption mechanism at higher voltage and a flat plateau at lower voltage due to the pore-filling mechanism. The two different types of mechanisms in the two xerogel-derived hard carbons can be attributed to the chemistry and size distribution of pores. The stable cycle life for the C-PUA@silica-800 sample has been attributed to the presence of finely distributed embedded amorphous silica particles, as proven by comparing its performance with hard carbon made without embedded SiO2. These xerogel-derived hard carbons have good electronic conductivity, and the anode can be fabricated without the use of added conductive carbon.