Membrane-free redox flow battery with polymer electrolytes

Li metal-based nonaqueous batteries are valued for high voltage and energy density, but face challenges like Li instability, volatile electrolytes, and costly ion-exchange membranes. To address these, we develop a membrane-free battery employing an ion-immobilized polymer electrolyte as anolyte and organic solvent as catholyte. Two polymer electrolytes are created for Li negative electrode: solid polymer electrolyte of polyvinylidene fluoride-co-hexafluoropropylene and gel polymer electrolyte with polypropylene carbonate. While the solid-state electrolyte offers an initial approach, it is associated with slower Li⁺ ions diffusion and lower ionic conductivity. The gel polymer electrolyte, specifically developed to overcome these limitations, improves Li⁺ diffusion, mass transport, and energy density. These anolytes are coupled with 2,4,6-tri-(1-cyclohexyloxy-4-imino-2,2,6,6-tetramethylpiperidine)-1,3,5-triazine in organic solvents (fluoroethylene carbonate and tetra (ethylene glycol) dimethyl ether) as catholytes, forming membrane-free batteries with solid polymer electrolyte and gel polymer electrolyte. Here we show that, at 0.5 M 2,4,6-tri-(1-cyclohexyloxy-4-imino-2,2,6,6-tetramethylpiperidine)-1,3,5-triazine, the battery with solid polymer electrolyte exhibits capacity retentions of 90.7% and 81.78% and Coulombic efficiencies of 95.4% and 96.7% under static and flow conditions, respectively. The battery with gel polymer electrolyte exhibits capacity retentions of 96.8% and 78.8% and Coulombic efficiencies of 97.8% and 98.4%. These results highlight the polymer electrolyte strategy's potential for enhancing battery performance and safety.