Understanding the Component-Driven Influence on the Electrochemical Properties in Single-Ion Polymer Electrolytes for Sodium-Based Batteries

Research on polymer electrolytes has attracted growing attention in recent years as they offer increased mechanical and thermal stability compared to liquid electrolytes, making them excellent candidates for metal batteries. Their properties can be fine-tuned depending on the need, allowing, for example, the creation of single-ion polymer electrolytes (SIPEs) that could suppress dendrite formation and charge gradient buildups. This work describes the optimization of a Na-SIPE consisting of sodium salt monomer (SSM), pentaerythritol tetraacrylate (PETA4), pentaerythritol tetrakis(3-mercaptopropionate) (PETMP), and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), by varying the composition ratio of the SIPE components. In addition, the influence on ionic conductivity and overall performance in Na-based cells of three different anionic centers of SSM is reported. The best electrochemical performance is achieved with a content of 17 mol% SSM, 18 mol% PVDF-HFP, 40 mol% PETMP, and 25 mol% PET4A due to the highest ionic conductivity. The optimal SSM is found to be (4-styrenesulfonyl)(dicyano)methanide (NaSDCM), enabling the NaSDCM-based SIPE to achieve a conductivity of 2.9 × 10–4 S cm–1 at 90 °C and 4.2 × 10–5 S cm–1 at 20 °C, thermal stability up to 300 °C, and electrochemical stability window up to 5 V vs Na+/Na. This SIPE with 50 wt% molecular transporter enables the realization of Na∥SIPE∥Prussian White (PW) cells, delivering a specific capacity of 102 mAh g–1 after 200 cycles at 0.1C and 40 °C.