Effect of Conjugation Degree on Solvation Behavior and Performance of Carboxyl-Containing Polyimide Binders for Silicon Anode of Lithium Battery

Polyimide (PI)-based binders are particularly promising for silicon (Si) anodes in lithium-ion batteries due to their robust mechanical properties and functional designability, which effectively accommodate large volume changes during cycling. However, their practical application is often limited by poor solubility in common solvents. In this work, three carboxyl-containing PIs (PMPI-COOH, NTPI-COOH, PTPI-COOH) with tunable benzene ring conjugation degrees are synthesized from different dianhydride monomers. We demonstrate that the electrode performance is governed by the balance between binder dispersibility and the steric hindrance imposed by conjugated structures on carboxyl-Si interactions: increased conjugation enhances solubility and processability, whereas excessive conjugation attenuates carboxyl-Si bonding strength. Among the series, NTPI-COOH with an intermediate conjugation degree achieves an optimal compromise, enabling the Si electrode to deliver superior cycling stability (47.9% capacity retention after 100 cycles at 0.5 A g^-1) and rate performance. This study highlights that rational conjugation engineering of PI binders is an effective strategy to enhance both processability and electrochemical performance of Si-based anodes, and provides a clear molecular-level guideline for designing high-functional binders in next-generation batteries.