Boosting the performance of soft carbon negative electrode for high power Na-ion batteries and Li-ion capacitors through a rational strategy of structural and morphological manipulation

Graphite ineffectiveness in sodium storage has induced extensive research on non-graphitic carbons as high-performance active materials for negative electrodes of Na-ion batteries. Among these, soft carbons are promising for high-power sodium storage, yet their practical success is jeopardized by their low initial coulombic efficiency (i.e., 65–70%). Herein, a facile and rational strategy is proposed based on mechanical treatment at an intermediate stage of the synthesis, after scrutinizing the journey of a soft carbon precursor (e.g., a thermoplastic polymer) during its carbonization process. The obtained results revealed significant changes in carbon matrix's structural, textural and morphological characteristics depending on whether the mechanical treatment was carried out on the intermediate or final residue. The electrochemical properties of the prepared samples were tested in sodium and lithium-ion half-cells. The optimized sample achieved a high initial coulombic efficiency of 82% vs. sodium with a high reversible capacity over 200 mA⋅h⋅g⁻¹ at C/15 (C = 372 mA⋅g⁻¹) and high-rate capability (e.g., ∼60 mA⋅h⋅g⁻¹ at 10C). Furthermore, the prepared samples provided a maximum specific energy of c.a. 140 Wh⋅kg⁻¹ at a specific power of 114 W⋅kg⁻¹ (with respect to total active mass of both electrodes) when used as negative electrodes in all-carbon Li-ion capacitors. Overall, the present work not only achieves a high-performance soft carbon with appealing high-power lithium and sodium storage properties, but also provides a rational, yet facile strategy applicable to any thermoplastic soft carbon polymer precursor; opening a new horizon towards developing high-performance soft carbon-based electrodes for energy storage.