Lignin as main precursor of anode and electrolyte for sustainable potassium batteries

Potassium-ion batteries (PIBs) are one of the top candidates to replace Li-ion batteries (LIBs) for stationary applications. Li-ion batteries have reached unprecedent targets of performance and safety, which ensure the Li-ion system as the best choice for portable devices and electric vehicles. Nevertheless, as lithium is now in the list of critical raw materials due to its low abundancy (0.0017 wt% in the Earth crust), alternatives are coming on the scene. Potassium is abundant on Earth (2.09 wt%), evenly distributed and characterized by a very low standard equilibrium potential and Lewis acidity (smaller solvated ions and, thus, faster conduction). Nonetheless, K-ion batteries have lower volumetric and gravimetric energy density with respect to LIBs, but this makes them perfectly suitable for stationary application. In light of this consideration, it is logical to already design potassium-batteries for large scale commercialization to be sustainable and low cost. Looking at this goal, our groups have carried out the preparation of two main components of this battery (i.e., anode and electrolyte) starting from a widely abundant and sustainable biosourced precursor: lignin. On one side, we have worked on the design, synthesis and characterization of a lignin-based membrane by crosslinking a pre-oxidized Kraft lignin matrix with poly(ethylene glycol) diglycidyl ether. The polymer electrolyte showed tremendous ability at suppressing the formation of metal dendrites and glorious cycling stability. In parallel, Kraft lignin was also used as a precursor for the synthesis of a highly porous carbon material to be used as anode material. In particular, after a pre-carbonization process at 250 °C, lignin was mixed with a KOH aqueous solution and urea, and finally activated in tubular furnace at 700 °C under nitrogen flow. Not only the chemical activation requires lower activation temperature, but the resultant material showed a high specific surface area (2900 m2/g) and very low graphitization degree (i.e., ideal for K-ion insertion). The biobased anode showed an evolving structure that cycle by cycle rearranges itself to accommodate more potassium ions, overall increasing its capacity and cycling stability. The resulting lignin-based potassium prototype cell is the answer to the quest of an exponentially increasing electrification and large stationary storage demand, but it also fulfills the responsibility for sustainable and low-cost industrial production.