A lignin-based potassium battery ensuring stable and sustainable stationary energy storage

Rechargeable batteries are a key technology in the world rush toward the energy transition. Li-ion batteries (LIBs) have reached unprecedent targets of performance and safety, nevertheless it is not logical thinking that the LIB technology alone is able to provide for the whole world electrification, given lithium scarcity (0.0017 wt% in the Earth crust) and its uneven distribution. In this scenario, it is not surprising the increasing attention coming from the research community on potassium-based batteries. 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). 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. Once the membrane was activated by soaking a liquid electrolyte, the as obtained gel polymer electrolyte was fully characterized, showing suitable ionic conductivity, excellent chemical compatibility and tremendous ability at suppressing the formation of metal dendrites. In parallel, Kraft lignin was used as a precursor for the synthesis of a highly porous carbon material, which was successfully adopted in the preparation of a carbonaceous anode. 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 a turbostratic disordered structure with very low graphitization degree (i.e., ideal for K-ion insertion). The biobased anode was able to cycle with capacity values comparable to commercial ones and even more promising 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.