Unveiling commercial carbon electrodes for potassium batteries: an in-depth characterization

Potassium-ion batteries (PIBs) are witnessing during the last years an unprecedent interest from the research community. This trend is given by PIBs potential in replacing Li-ion batteries for stationary energy storage plants. Indeed, potassium is abundant on Earth (2.09 wt%), evenly distributed and characterized by a very low standard equilibrium potential (-2.93 V vs. SHE with respect to -3.09 V vs. SHE of Li+/Li) and Lewis acidity (smaller solvated ions and thus faster conduction). Nevertheless, potassium provides ions the radius of which almost doubles the lithium one, i.e. 1.38 Å vs. 0.76 Å, respectively. This divergence requires a different kind of anode with respect to Li-ion batteries and, thus, a new approach in understanding their electrochemical storage performance is essential. Indeed, the confined space of a graphitic layered anode is not suitable for the accommodation of bigger K-ions as for the smaller Li-ions. The mechanical stress caused by the K-ions diffusion leads to the pulverization of graphitic materials and this shouts the need for anodes with disordered amorphous phases. Their high densitiy of voids and defects provides a high surface area, where K-ions can deposit and be adsorbed storing energy. For this reason, soft and hard amorphous carbon represent the most performing anode materials for PIBs, showing better life cycle, stability and capacity provided. Anyway, their storage mechanism is very different with respect to their Li-graphite system counterpart, where most of the electrochemical capacity is ascribed to the ion intercalation between the graphene layers, and only a limited amount is due to ion adsorption on graphite edges. Electrochemical storage on amorphous carbons occurs mainly as a Faradaic accumulation of ions on the carbon surface. The process is battery-like, nonetheless they behave as a capacitor, and so they are called pseudocapacitive materials. In this work, commercial carbon materials, known as Super P, C65 and C45, and their pseudocapacitive behaviour are in-depth characterized through electrochemical and morphological analysis. Indeed, depending on their graphitization degree, cyclic voltammetry curve and voltage vs. specific capacity profiles can identify the voltage windows for both insertion and adsorption processes. These results can be confirmed by differential capacity curves, above all if extracted from increasing current data. The Cottrell statement relates the current response with the voltage scan rate and, from this relationship, Dunn’s method, Trasatti’s method, b-value measurement and diffusion coefficient spectra allow to identify and quantify the amount of pseudocapacitive behaviour of the material. To conclude, in-depth characterization allows us to better classify the electrochemical behaviour of these electrode materials, that is of vital significance when choosing the proper anode material for potassium-based batteries.