CVD-coated carbon xerogels for negative electrodes of Na-ion batteries

Carbon xerogels (CX) with varying nodule sizes, from 50 nm to 2 μm, are synthesized via polycondensation of resorcinol with formaldehyde in water, followed by pyrolysis at 800 °C to investigate their electrochemical properties as negative electrode material in Na-ion batteries. All samples exhibit high specific surface areas (∼600 m2 g−1 by N2 physisorption) due to the presence of a large volume of micropores. Chemical Vapor Deposition (CVD) is used to fill or mask the micropores to mitigate the typical detrimental effects of high surface areas on the Initial Coulombic Efficiency (ICE). Larger nodules correlate with increased Na+ storage capacity and ICE (up to 80 %), independently of the measured specific surface area. Notably, the sample displaying 2 μm nodule size reach a reversible capacity of 248 mAh g−1 and 80 % ICE at C/20 cycling rate. CVD-deposited carbon layers show a graphitic-like structure and completely block the micropores, reducing the specific surface area and improving both reversible capacity and ICE up to 298 mAh g−1 and 84 %, respectively. Such materials composed of two different carbons show great promise in the advancement of carbon-based materials for Na-ion batteries.