Studies on electrochemical sodium storage into hard carbons with binder-free monolithic electrodes

Hard carbons emerge as one of the most promising candidate for an anode of Na-ion batteries. This research focuses on the carbon monolith derived from resorcinol-formaldehyde (RF) gels as a model hard carbon electrode. A series of binder-free monolithic carbon electrodes heat-treated at varied temperatures allow the comparative investigation of the correlation between carbon nanotexture and electrochemical Na⁺-ion storage. The increase in carbonization temperature exerts a favorable influence on electrode performance, especially in the range between 1600°C and 2500°C. The comparison between Li⁺- and Na⁺-storage behaviors in the carbon electrodes discloses that the Na⁺-trapping in nanovoids is negligible when the carbonization temperature is higher than 1600°C. On the other hand, the high-temperature sintering at 2500-3000°C enlarges the resistance for Na⁺-insertion into interlayer spacing as well as Na⁺-filling into nanovoids. In addition, the study on the effect of pore size clearly demonstrates that not the BET surface area but the surface area related to meso- and macropores is a predominant factor for the initial irreversible capacity. The outcomes of this work are expected to become a benchmark for other hard carbon electrodes prepared from various precursors.