Direct Structure–Performance Comparison of All‐Carbon Potassium and Sodium Ion Capacitors

A hybrid ion capacitor (HIC) based on potassium ions (K(+)) is a new high‐power intermediate energy device that may occupy a unique position on the Ragone chart space. Here, a direct performance comparison of a potassium ion capacitor (KIC) versus the better‐known sodium ion capacitor is provided. Tests are performed with an asymmetric architecture based on bulk ion insertion, partially ordered, dense carbon anode (hard carbon, HC) opposing N‐ and O‐rich ion adsorption, high surface area, cathode (activated carbon, AC). A classical symmetric “supercapacitor‐like” configuration AC–AC is analyzed in parallel. For asymmetric K‐based HC–AC devices, there are significant high‐rate limitations associated with ion insertion into the anode, making it much inferior to Na‐based HC–AC devices. A much larger charge–discharge hysteresis (overpotential), more than an order of magnitude higher impedance R (SEI), and much worse cyclability are observed. However, K‐based AC–AC devices obtained on‐par energy, power, and cyclability with their Na counterpart. Therefore, while KICs are extremely scientifically interesting, more work is needed to tailor the structure of “Na‐inherited” dense carbon anodes and electrolytes for satisfactory K ion insertion. Conversely, it should be possible to utilize many existing high surface area adsorption carbons for fast rate K application.