Facing the bigger K-Ion challenge in potassium organic batteries

Our world is grappling with an energy transition vital for planet survival. To achieve the transition, energy storage is essential. For this reason, Li-ion batteries (LIBs) have reached unprecedent attention from the research community and the global market. Nonetheless, lithium scarcity prevents LIBs from covering the entire demand of storage at global scale. As a result, potassium-ion batteries (PIBs) are strongly emerging as a viable technology for stationary storage and large-scale production for its features: K is 900 time more abundant than Li on the Earth crust, and thus much cheaper; among the alkali metals has the redox potential (-2.93 V vs. SHE) closest to Li one, the smallest Stokes radius, and can work with aluminium current collector at every voltage. Nonetheless, desolvated K-ions show the biggest radius, arising some challenges: once intercalated in the rigid inorganic electrode material structure, K-ions may cause the structure distortion, resulting in drastic capacity decay. As a matter of fact, commercially available graphite, which is the most used anode for alkali-ion batteries, as well as, Prussian blue and its analogs, which are up to now the best option as cathode material in PIBs, have finite interlayer and interstitial volume, respectively, which are too small to reversibly host K-ions. From this starting point, our group has focused on possible solutions to address these issues in both anode and cathode, keeping always into account the imperative of lower impact PIBs production since they are meant for the large-scale. The high electronic conductivity and low redox potential of carbonaceous materials, which make them be the best choice as anodes, can be still exploited in PIBs if the limited interlayer volume is overcame. Here we report a highly porous carbon material, with X-Ray diffraction evidences of no stacking of graphene layers, successfully adopted as anode material. Its disordered structure allows the K-ion to insert without causing any damage to the carbon structure, but instead, upon cycling, they may cause its rearrangement to a structure perfectly suitable for K-ion hosting. This behavior is electrochemically translated in an increase of capacity for the first cycles and null decay of capacity, the values of which are comparable to commercial carbon blacks. Finally, the preparation has been designed to be as green as possible: pre-carbonized Kraft lignin is mixed with a potassium carbonate and urea, and then activated in tubular furnace at 700 °C under nitrogen flow. On the cathode site, in potassium battery field very few satisfying options have been published to now, due to the challenging bigger K-ions. Organic electrodes are not-critical raw materials-based, cheap, environmentally friendly, tunable, and - above all – their intrinsic mechanical flexibility can lead to the reversible and damage-free accommodation of large sized K-ions, resulting in stable PIB performance. Their biggest drawback is the high solubility in organic solvents. In consideration of this, stable aminoxyl radicals like TEMPO have been investigated to exploit its tunability both as monomeric unit and polymeric materials, in pristine for and also functionalized in order to reduce its solubility in organic electrolytes. In our work, we also considered the effect of the amount of free radicals on the electrochemical properties, as well as the possibility to integrate the redox activity of an n-type material in a TEMPO-like-based electrode on an aluminum current collector.