Smart polymeric platforms for potassium batteries

Rechargeable batteries are a key technology in the world rush toward the energy transition. They allow us to have a wide range of portable technologies, to have carbon free transportation and to balance the distribution grid while enhancing the renewable energy conversion systems. Li-ion batteries (LIBs) have reached unprecedent targets of performance and safety, nevertheless it is not logical to think that the LIB technology only is able to bear the world electrification, given the lithium scarcity (0.0017 wt% in the Earth crust) and its uneven distribution. 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). K-ion batteries (KIBs) already proved to have all the requirements for large stationary storage systems, however several aspects still have to be addressed. To improve the KIBs safety and life cycling, but keeping always in mind the potential commercialization of a sustainable technology, our group worked on the design, synthesis and characterization of advanced fully biobased polymers for KIB electrolytes. In this contribution two gel polymer electrolytes are proposed. Primary, the first biosorced electrolyte successfully used in a KIB system is a lignin-based membrane resulted by crosslinking of pre-oxidized Kraft lignin matrix with poly(ethylene glycol) diglycidyl ether (PEGDGE). Once the membrane is activated by soaking liquid electrolyte, the as obtained gel polymer electrolyte has been fully characterized showing suitable ionic conductivity exceeding 10-3 S cm-1, excellent chemical compatibility and tremendous ability at suppressing the formation of metal dendrites. The latter has also been proven by the lab-scale KIB prototype which has managed to retain after as many as 200 cycles the 64% of the 10th cycle capacity, and being able to cycle for over 500 cycles. The second electrolyte proposed for potential potassium-based application was crosslinked via ring-opening reaction of a cardanol-derived epoxy resin with three different cyclic anhydrides as curing agents: glutaric, succinic and hexahydro-4-methylphthalic anhydride. All the three samples showed excellent mechanical and thermal stability, a high electrolyte uptake ability, impressive electrochemical stability and outstanding ionic conductivities (up to 10-2 S cm-1). While the first gel polymer electrolyte already demonstrated its excellent applicability, the second one promises extraordinary performances worthy of further studies. The present findings, along with their potentiality to be printed on foils, pave the way to the development of stationary batteries for self-powered platforms.