Elucidating the Interplay Between Structure and Electrochemical Behavior in Lignin‐Based Polymer Electrolytes for Potassium Batteries

ABSTRACT 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.