Entropy-modulated and interlayer-doped transition metal layered oxides enable high-energy-density sodium-ion capacitors

In recent years, sodium-ion capacitors have attracted attention due to their cost-effectiveness, high power density and similar manufacturing process to lithium-ion capacitors. However, the utilization of oxide electrodes in traditional sodium-ion capacitors restricts their further advancement due to the inherent low operating voltage and electrolyte consumption based on their energy storage mechanism. To address these challenges, we incorporated Zn, Cu, Ti, and other elements into Na0.67Ni0.33Mn0.67O2 to synthesize P2-type Na0.7Ni0.28Mn0.6Zn0.05Cu0.02Ti0.05O2 with a modulated entropy and pillaring Zn. Through the synergistic interplay between the interlayer pillar and the entropy modulation within the layers, the material exhibits exceptional toughness, effectively shielding it from detrimental phase transitions at elevated voltage regimes. As a result, the material showcases outstanding kinetic properties and long-term cycling stability across the voltage range. By integrating these materials with hierarchical porous carbon nanospheres to form a "rocking chair" sodium-ion capacitor, the hybrid full device delivers a high energy density (171 Wh·kg−1) and high power density (5245 W·kg−1), as well as outstanding cycling stability (77% capacity retention after 3000 cycles). This work provides an effective material development route to realize simultaneously high energy and power for next-generation sodium-ion capacitors.