From Neutral Zwitterion to Electroactive Disalt: Electrochemical Insights into Double Zwitterionic Viologens for Anion‐Insertion Batteries

In the quest for sustainable and metal-free energy storage systems, viologen-based materials offer exciting prospects as they are synthetically well accessible and show two reversible electrochemical processes with anion insertion. This study introduces and compares two π-extended viologen–carboxylate materials bearing double zwitterionic backbones as model compounds based on 1,1′-bis(4-carboxyphenyl)-4,4′-bipyridinium ([bcbp]): the neutral [bcbp] as a double zwitterionic molecule (1) and its corresponding (Li)$_2$[bcbp](ClO$_4$)$_2$ disalt (2). The choice of these two materials for our electrochemical studies is motivated by the literature available on their synthesis routes and solid-state properties. Electrochemical tests in lithium half-cells revealed that compound (1) is initially inactive but gradually converts into the electroactive disalt (2) via spontaneous chemical insertion of LiClO$_4$ from the electrolyte. Compound (2) directly displays the expected reversible two-electron p-type mechanism involving perchlorate anion (de)insertion, while lithium ions act as spectator species. The system delivers stable cycling performance and high coulombic efficiency, supporting the interest of viologen-based zwitterionic salts as host material for negative electrode application in anionic rocking-chair organic batteries. ... mehrThe bipyridinium-bis(carboxylate) radical ((Li)[bcbp]• (3)) formed during our synthesis optimizations is likewise electrochemically assessed. This fundamental work highlights the tunability of double zwitterionic viologens as molecular platforms to promote optimized p-type negative electrode materials.