Hard carbons are promising anodes for sodium-ion batteries. However, there is still considerable controversy regarding the sodium storage behaviors in hard carbons, which are mainly attributed to the varied precursors, confused pyrolysis mechanism, and different characterization methods. Herein, benefiting from the flexible molecular structure of polymers, a series of hard carbons with carefully tuned microstructures are fabricated by adjusting the ratio of aryl and alkyl groups in the epoxy resins. The results of dynamic mechanical analysis, in-situ Fourier transform infrared spectra, and synchronous thermal gravimetric-infrared spectrum-gas chromatography/mass spectrometry reveal that replacing the alkyl with aryl groups in the resin can enhance the crosslink density, inhibit the degradation and rearrangement process, and further lead to a more disordered microstructure. In addition, it is suggested that accessible channels provided by sufficiently wide interlayer spacing are necessary for closed pore filling. The optimized anode delivers a high capacity of 375 mAh/g in half cell with an initial Coulombic efficiency of 80.61%, and an energy density of 252 Wh/kg is attained in full cell. Finally, a reliable relationship among precursor–pyrolysis mechanism–structure–performance is established, and the sodium storage mechanism of “adsorption–insertion–pore filling” is well proved.

2D MXenes are attractive for energy storage applications because of their high electronic conductivity. However, it is still highly challenging for improving the sluggish sodium (Na)-ion transport kinetics within the MXenes interlayers. Herein, a novel nitrogen-doped Ti₃C₂T_x MXene was synthesized by introducing the in situ polymeric sodium dicyanamide (Na-dca) to tune the complex terminations and then utilized as intercalation-type pseudocapacitive anode of Na-ion capacitors (NICs). The Na-dca can intercalate into the interlayers of Ti₃C₂T_x nanosheets and simultaneously form sodium tricyanomelaminate (Na₃TCM) by the catalyst-free trimerization. The as-prepared Ti₃C₂T_x/Na₃TCM exhibits a high N-doping of 5.6 at.% in the form of strong Ti–N bonding and stabilized triazine ring structure. Consequently, coupling Ti₃C₂T_x/Na₃TCM anode with different mass of activated carbon cathodes, the asymmetric MXene//carbon NICs are assembled. It is able to deliver high energy density (97.6 Wh kg⁻¹), high power output (16.5 kW kg⁻¹), and excellent cycling stability (≈ 82.6% capacitance retention after 8000 cycles).

Engineering the structure and increasing the near-surface reaction of hard carbon are promising strategies for designing high-performance sodium ion batteries (SIBs). In this study, a series of N, S-co-doped hierarchical porous carbon materials are designed and prepared through an easy one-pot ionothermal approach. The carbon structure is controlled by designing the precursors and changing the reaction temperature. Benefiting from a molecular-engineering strategy, the obtained porous carbon shows a homogeneous distribution of nitrogen and sulfur atoms at the atomic level, and its application as anode materials for SIBs is reported. pTTPN@600 delivers a high reversible capacity (134 mA h g⁻¹ at 1 A g⁻¹, corresponding to a capacity retention of 88.7% after 100 cycles and excellent rate capabilities of 248 mA h g⁻¹ at 0.05 A g⁻¹ and 95 mA h g⁻¹ at 5 A g⁻¹). Even at a current density of 10 A g⁻¹, a specific capacity of 74 mA h g⁻¹ is maintained after 2000 cycles. The outstanding performance is attributed to the large amount of heteroatoms (N 7.52 wt% and S 1.63 wt%) and several mesopores (mesoporous volume 0.48 cm³ g⁻¹) in pTTPN@600. We propose increasing the mesoporous volume and heteroatom amount to enhance the electrochemical performance of porous carbon materials. This study provides an easy route to fabricate hierarchical porous electrode materials for SIBs and provides new insights into the sodium storage behavior in hierarchical porous materials.