Upside-down sodium ion capacitor: A non-presodiated system based on Na3V2O2(PO4)2F and biomass derived activated carbon

Hybrid systems between batteries and supercapacitors are designed to bridge the gap between both technologies, developing high-power, high-energy, and long cycle life energy storage devices. Moreover, in the urgent need to transition to a more sustainable energy model, sodium-based chemistries are an excellent alternative to lithium-based technologies. While various approaches have been tried in Na-ion capacitor (NIC) cell design, the upside-down NIC configuration remains underexplored. This consists of using a Na-containing battery-type cathode instead of an anode, which eliminates problems such as the need for pre-sodiation or Na-plating. This setup also allows the electrolyte concentration to stay stable, with ions exhibiting a rocking-chair motion, ensuring stable ionic conductivity during cycling. However, the potential window of these systems can be limited by the formation of the SEI on the capacitive negative electrode. Therefore, the aim of this paper is to explore the limitations of activated carbon (AC) in upside-down NICs through ex-situ X-ray photoemission spectroscopy (XPS) characterization and comprehensive monitoring of the electrode under various full-cell conditions, thereby assessing how far this configuration can be pushed in terms of energy. The proposed system features a high-rate capability cathode, a Na3V2O2(PO4)2F/C composite prepared via a novel one-step microwave synthesis, which delivers 64 mAh g−1 at 50C. The negative electrode employs a dried olive pits-derived AC with a surface area of 2138 m2 g−1. Together, they enable a full cell achieving up to 69 Wh kg−1 at 19 W kg−1 and 29 Wh kg–1 at 4915 W kg−1, with an impressive capacity retention of 74 % after 10,000 charge-discharge cycles.