High-Capacity\nAnode for Sodium-Ion Batteries Using\nHard Carbons Derived from Polyurea-Cross-Linked Silica Xerogel Powders

In\nthis article, we demonstrated carbon aerogel-silica composites\nderived from polyurea-cross-linked silica xerogel powders (C-PUA@silica)\nas anodes for Na-ion batteries. These xerogel-derived hard carbons\nwith embedded silica showed a stable high capacity. Of the several\nhard carbon samples derived at different temperatures, the pyrolyzed\nsample at 800 °C (C-PUA@silica-800) showed a high capacity of\n236 mAh/g at a current density of 10 mA/g and a stable cycle-life\nat 200 mAh/g. The galvanostatic charge–discharge curves displayed\na sloping voltage profile reminiscent of the adsorption mechanism\nof Na<sup>+</sup> storage. On the other hand, higher temperature-pyrolyzed\nsamples at 1300 °C (C-PUA@silica-1300) displayed a lower capacity\nof 185 mAh/g at a current density of 10 mA/g, and the charge–discharge\nprofile showed a two-step process, a sloping adsorption mechanism\nat higher voltage and a flat plateau at lower voltage due to the pore-filling\nmechanism. The two different types of mechanisms in the two xerogel-derived\nhard carbons can be attributed to the chemistry and size distribution\nof pores. The stable cycle life for the C-PUA@silica-800 sample has\nbeen attributed to the presence of finely distributed embedded amorphous\nsilica particles, as proven by comparing its performance with hard\ncarbon made without embedded SiO<sub>2</sub>. These xerogel-derived\nhard carbons have good electronic conductivity, and the anode can\nbe fabricated without the use of added conductive carbon.