Unveiling cathodic and anodic materials for potassium-based batteries

The impact of society on the climate of our planet is becoming even clearer, starting from our energy demand, which is in constant growth. This energy used to be generated through fossil fuels. Due to the rise in awareness of the repercussions they have on the whole world, a decarbonized energy generation is evidently needed. Renewables are the medium to achieve this goal, although the main challenges to face are their intermittency and unpredictability. Energy storage plays a critical role in smoothing out their variations, with electrochemical energy storage being the main actor in terms of volumetric and gravimetric energy density. Nowadays, lithium-ion batteries owe the largest market share. However, the high cost and environmentally concerning production hamper their deployment as stationary electricity storage systems. In this context, potassium represents an encouraging alternative, that witnessed exponential interest growth over the last few years. Its chemical and physical similarity to lithium addressed potassium, which exploits the same rocking-chair working mechanism, as a possible substitute in specific applications. K+/K redox potential vs. SHE is -2.93 V, really close to Li+/Li (-3.04 V), suggesting a considerable energy density. Alongside, its natural abundance and low cost, together with the weak Lewis acidity, that allows a fast ionic transport in the electrolyte, outlined a research-worthy solution. This work is devoted to the study of some electrode materials, through their morphological and electrochemical characterization, and aims to broaden the knowledge around such a new technology. The investigated anode is titanium dioxide, both in the form of nanoparticles and nanotubes, with different lengths, crystalline structures, or thicknesses, in order to understand the impact of the nanostructure on the performances. The cathode material studied is Prussian blue in the form of nanoparticles, with different film thickness, crucial to enhance the operating voltage and provide stability to the cell in the long term. All the systems were studied employing two different electrolytes, KPF6 0.8 M in EC:DEC and KFSI 0.8M in DME, in t-cell and coin-cell configuration. The electrochemical tests carried were galvanostatic cycling, cyclic voltammetry, impedance spectroscopy, and Mott-Schottky’s analysis, together with XRD and FE-SEM for the morphology. The TiO2 amorphous nanotubes showed very good cycling stability in KFSI, delivering 40 µAh cm-2, CE 97.8%, after 200 cycles. Conversely, the anatase nanotubular structure was not able to deliver a decent capacity, which was attributed to the difficult insertion of large-sized K+ ions in the tight lattice. KFSI-based was proven to be the best electrolyte in terms of reversibility and cycling stability (CE close to 100%), ascribed to the homogeneous and robust inorganic SEI layer able to prevent excessive electrolyte/electrode reaction. Despite imide-based electrolytes were reported to cause corrosion issues on aluminium current collectors, Prussian blue thickness of 200 µm was able to provide adequate protection and deliver 20 mAh g-1 after 2350 cycles at an operating voltage of approximately 3.2 V. This result is important because aluminium is cheaper and lighter than copper, which is commonly used in LIBs due to the alloy reaction prevention.