Naphthalene Diimide and Pyromellitic Diimide Networks as Cathode Materials in Lithium-ion Batteries: on the Instability of Pyromellitic Diimide.

Aromatic diimides such as naphthalene diimide (NDI) and pyromellitic diimide (MDI) are important building blocks for organic electrode materials. They feature a two-electron redox mechanism that allows for energy storage. Due to the smaller size of MDI compared to NDI its theoretical capacity is higher. Studies on MDI-based small molecule and linear polymer electrodes indicate that MDI is unstable, yet the origin of instability remains unclear. Herein, two cross-linked networks of NDI and MDI are designed. The polymers, termed PNDI-EG and PMDI-EG, are synthesized via cationic polymerization of vinyl ethylene glycol-functionalized NDI and MDI monomers. The cross-linked structures preclude extrinsic degradation pathways (e.g., dissolution in the electrolyte), and thereby facilitate the investigation of intrinsic degradation mechanisms. PMDI-EG-based cathodes are less stable, and the performance of PMDI-EG/Li half cells is markedly inferior compared to PNDI-EG/Li cells. Our comprehensive experimental and quantum-chemical investigation reveals that PMDI-EG undergoes irreversible diimide ring opening upon prolonged charge-discharge cycles, while PNDI-EG remains intact. It is hypothesized that the smaller ring size of the five-membered imide renders MDI more susceptible to side reactions with nucleophiles in the electrolyte, causing rapid loss of capacity during the first cycles.