Elucidating the interplay between structure and electrochemical behavior in lignin-based polymer electrolytes for potassium batteries

Potassium batteries are very appealing for stationary applications and domestic use, offering a promising alternative to lithium-ion systems. To improve their safety and environmental impact, gel polymer electrolytes (GPEs) based on bioderived materials can be employed. In this work, a series of biobased membranes are developed by crosslinking pre-oxidized Kraft lignin as bio-based component and poly(ethylene glycol) diglycidyl ether (PEGDGE) as functional linker with 200, 500, and 1000 g mol-1 molecular weight. The influence of PEGDGE chain length on the physicochemical properties and electrochemical performance of GPEs for potassium batteries is investigated. These membranes exhibit thermal stability above 240°C and tunable glass transition temperatures depending on the PEGDGE molecular weight. Their mechanical properties are determined by rheology measurements in dry and swollen states, evidencing a slight decrease of elastic modulus (G′) by increasing PEGDGE chain length. An approximately one-order-of-magnitude lower G′ value is observed in swollen membranes versus their dry counterpart. Upon successful activation of the lignin-based membranes by swelling in the liquid electrolyte embedding potassium salts, these GPEs are tested in potassium metal cell prototypes. These systems exhibit ionic conductivity of ~10-3 S cm-1 at ambient temperature. Interestingly, battery devices equipped with the GPE based on PEGDGE 1000 g mol−1 withstand current densities as high as 1.5 mA cm-2 during operation. Moreover, the same devices reach specific capacities of 130 mAh g-1 at 0.05 A g-1 in the first 100 cycles and long-term operation for over 2500 cycles, representing outstanding achievements as bio-sourced systems for potassium batteries.