Per and Polyfluoroalkyl Substances‐Free Poly(ionic liquid) Reinforced with Cellulose Nanofibrils as Sustainable Electrolyte for High‐Voltage Solid‐State Lithium Batteries

Gel‐like poly(ionic liquid) electrolytes are promising candidates to develop solid‐state lithium metal batteries (SSLMBs). However, they hardly prevent lithium dendrite growth due to poor mechanical stability, which remains a critical challenge for the safety and long‐term cycling in SSLMBs. Simultaneously, the increasing demand for environmentally friendly electrolytes has driven the search for sustainable, per and polyfluoroalkyl substances (PFAS)‐free, and biodegradable alternatives. To address these challenges, in this work PFAS‐free composite polymer electrolytes are developed using poly(diallyldimethylammonium) bis(fluorosulfonyl)imide, N ‐methyl‐ N ‐propylpyrrolidinium bis(fluorosulfonyl)imide (PYR 13 FSI), and lithium bis(fluorosulfonyl)imide. In addition, cellulose nanofibrils (CNFs) (from 0–8.5 vol.%, corresponding from 0 to 10 wt.%) are used to mechanically reinforce the electrolyte. The most promising properties are observed with a change in the volume ratio to 2.5% of CNFs, which led to enhanced lithium ionic conductivity, a higher storage modulus ( G ´), and reduced Li metal interface resistance. These improvements enable prolonged room temperature cycling of symmetric Li/Li and high‐voltage Li/NMC622 coin cells compared to reference samples equipped with an electrolyte without CNFs. This study demonstrates that incorporating nanoscale cellulose harvested from renewable resources results a promising strategy to enhance the electrochemical performance of gel poly(ionic liquid) electrolytes.