Tailoring a Phenolic Resin Precursor by Facile Pre-oxidation Tactics to Realize a High-Initial-Coulombic-Efficiency Hard Carbon Anode for Sodium-Ion Batteries.

As the leading anode material for sodium-ion batteries (SIBs), hard carbon (HC) still faces the puzzle of low initial Coulombic efficiency (ICE) in achieving commercialization. From the perspective of precursors, the low ICE has been attributed to the large specific surface area and porosity produced by the rapid decomposition of polymers during the carbonization. Therefore, increasing the cross-linking degree of precursors will be an effective shortcut to improve the ICE. Herein, a facile pre-oxidation tactic was successfully employed to tailor the cross-linking degree of phenolic resin precursors to precisely control the specific surface area of the obtained HC. As the pre-oxidation time is increased, the optimal HC with the lowest specific surface area shows an ICE elevated by 22.2% (from 62.5 to 84.7%) compared to the original pre-oxidation HC and delivers a high reversible capacity of 334.3 mAh g-1 at 20 mA g-1. Besides, the pre-oxidation also introduces abundant carbonyl groups, which increase the disorder degree of HC and supply abundant adsorption sites of Na+, thus enhancing the rate performance. When matched with a layered O3-NaNi1/3Fe1/3Mn1/3O2 cathode, the full cell achieves an energy density of ca. 256.2 Wh kg-1 with superior rate performance. This work sheds light on the positive effect of pre-oxidation in elevating the ICE of HC and provides effective guidance to achieve a high ICE for other HC materials.