Post-treatments on carbon xerogels to improve their performance as negative electrodes of Na-ion batteries

A carbon xerogel (CX) with ∼2 μm nodules was synthesized via polycondensation of resorcinol with formaldehyde in water, followed by pyrolysis at 800 °C. The resulting sample underwent surface treatments using Chemical Vapor Deposition (CVD) and/or CO2 activation in order to mask the micropores with a secondary carbon layer or develop additional micropores, respectively. This strategy aimed at understanding the impact of surface modification and closed micropores on the performance of hard carbons as negative electrode materials for Na-ion battery. On the one hand, the coating deposited by CVD was found to display more graphitic-like domains and to close the CX microporosity, leading to enhanced Initial Coulombic Efficiency (ICE) and reversible capacity. On the other hand, due to its very high accessible surface area, the activated sample showed very low ICE (18 %) and reversible capacity (62 mAh g−1). However, once the activated sample was covered with a secondary carbon layer by CVD, the capacity reached 294 mAh g−1 with a high ICE of around 88 %, and an enhanced insertion plateau at low voltage was observed. Additionally, this activated-coated sample showed a high-rate capability and much greater stability than the other samples upon cycling. Such surface treatments provide an effective strategy for both understanding the impact of hard carbon surface properties on Na storage and optimizing their performance for negative electrodes in Na-ion batteries.