Impact of Preoxidation Treatments on Performances of Pitch-Based Hard Carbons for Sodium-Ion Batteries

Recently, sodium-ion batteries have been intensively studied as an alternative to lithium-ion batteries because of the abundance of sodium and its ability, for example, to answer to smart grid energy storage applications. Among all anode materials, carbonaceous materials have shown promising results, particularly hard carbons because of their high capacity and low insertion voltage (vs Na⁺/Na). However, these materials often suffer from their high cost and low initial Coulombic efficiency. In this paper, we investigate an easy route of hard carbon synthesis from low-cost pitch precursor. A pretreatment under a controlled atmosphere can hinder the graphitization of the pitch upon pyrolysis and induce an amorphous-like microstructure with high Na storage capacity. We also investigate the mechanism of preoxidation and show the importance of parameters optimization such as the atmosphere and the duration. This work highlights the influence of the pretreatment conditions on the hard carbon characteristics which are of key importance to explain and improve its electrochemical performances. The control of the preoxidation conditions allowed us to obtain a unique hard carbon with optimized microstructure and texture and delivering impressive electrochemical performances. This hard carbon was obtained from a 12 h pretreatment at 300 °C under oxygen flow followed up by a 2 h carbonization at 1400 °C under nitrogen with a high yield of 49%. This material delivers remarkable 312 mAh g^–1 of reversible capacity at C/20 for only 10% of irreversibility at the first cycle, which correspond to the best electrochemical performances reported so far for pitch-based hard carbons. This work definitively emphasizes the potential of pitch-based hard carbons for further industrialization of sodium-ion batteries.