Lignin as polymer electrolyte precursors for KIB

Introduction Rechargeable batteries are a key technology in the world rush toward the energy transition. In this view, significant amount of stationary energy storage is needed, and potassium-ion batteries (KIBs) can fulfil this task, given potassium abundance (2.09% in the Earth crust with respect to Li, i.e. 0.0017%) and even distribution. Considering also the low K+/K standard equilibrium potential (-2.93 V vs.SHE) and Lewis acidity, this alkali metal already proved to have all the requirements for large stationary storage systems. Still, KIBs safety and life cycling need to be improved, and in this view our group has designed a biobased polymer electrolyte. Material and Methods Pre-oxidized Kraft lignin powder was solubilized in aqueous NaOH solution and mixed with PEGDGE for the crosslinking. The as obtained membrane was activated by swelling in 0.8 M KPF6 in 1:1 EC:DEC and electrochemically characterized. The half-cell was assembled with potassium foil and carbonaceous electrode (8:2SuperP:PvDF). Results FESEM images showed a very porous structure and the swelling test indicated an increase of 80% of the initial mass after only 10 min. The ionic conductivity at ambient temperature was in the order of 10-3 S/cm, while the transport number was 0.24. The lignin-based electrolyte provided a low overpotential even after 150 h of plating and stripping and consequently was able to work in the half-cell for over 500 cycles. Discussion The perfectly homogenously packed 3D porous structure allowed the fast soaking of the liquid electrolyte. Rheological tests revealed an increased linear viscoelastic region for the swollen sample, due to the plasticization and expansion of the porous structure. This is beneficial for the extended life cycling of the cell, as proven by the 150 h of reversible and stable plating and stripping, with no occurrence of dendrite growth or short circuit. As expected, the half-cell delivered 168 mAh/g at the 200th cycle, being also able to stand for over 500 cycles without any failure.