Glyoxylic‐Acetal‐Based Electrolytes for Sodium‐Ion Batteries and Sodium‐Ion Capacitors

A comprehensive study on the properties and implementation of glyoxylic‐acetals in sodium‐ion energy storage systems is presented. Electrolytes containing 1,1,2,2‐tetramethoxyethane (tetramethoxyglyoxal, TMG), 1,1,2,2‐tetraethoxyethane (tetraethoxyglyoxal, TEG) and a mixture of the latter with propylene carbonate (PC) exhibit increased thermal stabilities and higher flash points compared to classical electrolytes based on carbonates as solvents. Due to its favorable properties, 1 m NaTFSI in TEG/PC (3 : 7), has been selected and used for sodium‐ion energy storage systems based on a Prussian Blue (PB) positive electrode and a hard carbon (HC) negative electrode. Compared to conventional electrolyte (based on a 1 : 1 mixture of ethylene carbonate, EC, and dimethyl carbonate, DMC), this glyoxylic‐acetal electrolyte provides competitive capacity and prolonged cycle life. Postmortem XPS analysis indicates that the electrode‐electrolyte interphases formed in presence of TEG are thicker and presumably more protective, inhibiting typical degradation processes of the electrodes. Furthermore, it is demonstrated that the suitable properties of TEG on the cycling stability can also be exploited for the construction of highly stable sodium‐ion capacitors. Glyoxylic‐acetals offer high flash points, high boiling points and simultaniously low melting points. Owing to these properties as well as their cost competiviness and large scale availability, Tetramethoxyglyoxal and Tetraethoxyglyoxal represent promising candidates for the realization of safe and durable sodium‐ion energy storage devices.