Ballistic ion transport through hierarchically-ordered-structure polymer binder

All-solid-state batteries (ASSBs) have emerged as a promising solution to enhance battery safety by employing solid electrolytes in place of flammable organic electrolytes. However, ASSBs encounter several challenges that limit their practical applications. Silicon (Si) is among the potential negative electrode materials for high-energy-density ASSBs, owing to its specific capacity of approximately 3700 mAh/g. Employing polymer binders is an effective strategy to accommodate the significant volume changes of Si during electrochemical lithiation and delithiation.

Unlike liquid electrolytes, solid electrolytes exhibit a less effective contact interface with active materials. Mixed electronic-ionic conductive (MEIC) polymer binders appear to be a promising solution to mitigate interface issues and the challenge of poor ionic conductivity within electrodes. Cutting-edge approaches to engineering mixed electronic-ionic conductive polymer binders include mixing conjugated polymers with ion-conductive polymers or modifying conjugated polymer backbones by grafting ion-conductive side chains. However, nearly all of these ion-conductive polymers, such as poly(ethylene oxide) (PEO), rely on polymer segmental motions to drive ion diffusion. This ion transport mechanism faces a trade-off between ionic conductivity and mechanical strength. Specifically, it must compromise mechanical strength to enhance ionic conductivity, or vice versa. In addition, the ion transport based on polymer dynamics features low ionic conductivity at room temperature and is highly dependent on temperature. Here, we report a ballistic ion transport mechanism in a MEIC polymer binder, where its hierarchically ordered structure facilitates ion diffusion and achieves solid-state lithium-ion conductivity in the range of 10^-4 to 10^-3 S cm^-1 from -20 to 70 °C. This mechanically robust MEIC polymer is a versatile ionic conductor, allowing Li⁺, Na⁺, or K⁺ to diffuse through polymer matrix, with their cationic charges counterbalanced by electrons on conjugated polymer backbones. Additionally, this ballistic ion transport mechanism results in enhanced cycling performance in ASSBs.