Hard Carbon Anodes for Na‐Ion Batteries: Toward a Practical Use

Hard carbons have immense potential as anode materials for Na‐ion batteries, because the expanded graphene interlayers and nanovoids between randomly stacked aromatic fragments can accommodate a substantial amount of sodium. However, the large irreversible capacity in the first cycle still remains as a significant issue in terms of a practicable battery technology. Here, we show that hard carbon electrodes derived from a common phenol resin deliver a high reversible capacity within the narrow potential range of 0.1–0.005 V (vs. Na⁺/Na) and an excellent initial coulombic efficiency up to 95 %. The former allows the sustainable high voltage, whereas the latter minimizes the amount of unavailable Na⁺ in a closed cell. The findings in this work put forward a guideline for manufacturing hard carbon electrodes, which goes against the current trend of nanostructuring and downsizing.