Influence of the backbone chemistry and side-chain spacer flexibility in sodium single-ion conducting polymer electrolyte for sodium-batteries

Abstract

Single-ion conducting polymer electrolytes (SIPEs) have garnered increasing attention in recent years due to their wide electrochemical stability window (ESW), excellent thermal stability, and superior electrochemical performance when impregnated with a molecular transporter. This work investigates the influence of the flexibility of the sulfonyl(trifluoromethanesulfonyl) imide anionic center on 3D crosslinked SIPEs composed of a sodium salt monomer (SSM), either sodium 4-styrenesulfonyl (trifluoromethanesulfonyl)imide, sodium sulfonyl (trifluoromethanesulfonyl)imide methacrylate (NaMTFSI) or sodium ((1,1,2,2-tetrafluoro-2-(1,1,2,2-tetrafluoro-2-iodoethoxy)ethyl)sulfonyl)-(trifluoro-methanesulfonyl)imide, bound to the pentaerythritol tetrakis(3-mercaptopropionate) (PETMP) and pentaerythritol tetraacrylate (PET4A), also including poly(vinylidene fluoride-co-hexafluoropropylene) to enhance the mechanical properties. Sodium metal-based cells employing Prussian White (PW) as cathode deliver the highest specific capacity and capacity retention by using NaMTFSI as SSM due to its increased flexibility and chemical stability. Additionally, the impact of the polymer backbone chemistry on the porosity, mechanical, thermal, and electrochemical properties has been investigated using either PET4A, hexa-1,5-diene (diene), 1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, or dipentaerythritol hexaacrylate, together with PETMP and NaMTFSI. The results reveal that the diene-based-SIPE leads to a higher pore structure, exhibiting a high ionic conductivity of 1.2 × 10⁻⁴ S cm⁻¹ at RT, thermal stability up to 270 °C, wide ESW (4.2 V vs. Na⁺/Na), and the Na|SIPE|PW cells delivering 119 mAh g⁻¹ after 200 cycles with excellent Coulombic efficiency.