Enhancing specific energy in sodium-ion hybrid capacitors via quasi-anodeless configuration

Sodium-ion hybrid capacitors (Na-HCs) often experience limited rate capabilities due to the inherent challenges of hard carbon anodes, such as sloped discharge profiles and poor kinetics. Despite this, the effectiveness of hard carbon for sodium plating opens opportunities beyond its conventional use. Leveraging this property, we explore hard carbon as a 3D current collector for Na deposition, aiming to employ it as an efficient anode material in Na-HCs. A key advancement in our approach is the employment of a lean hard carbon anode with ultralow loading (<0.3 mg cm^-2), which significantly elevates the cell’s energy density. This design boosts the energy density of the cell by operating the anode at a stable potential near 0 V, thereby increasing the operating voltage of the hybrid capacitor. Moreover, it enables the voltage profile of hybrid capacitors to resemble more closely that of porous carbon, contrasting with the typical dome-shaped profiles seen in standard hybrid capacitors. The implementation of this method has yielded a remarkable 40% increase in the specific energy. Our research provides a promising direction for the Na-HC community, showcasing a Na-HC that can operate for 25,000 cycles at a current density of 5 A g^-1 while maintaining 71% of its capacity. This quasi-anodeless configuration, coupled with a judicious selection of electrode materials and electrolyte composition, marks a significant advancement in the quest for efficient and sustainable energy storage solutions.