Interchain bottlenecks and helical-channel pathways as the origin of low Li+ conductivity in crystalline PEO.

Polyethylene oxide (PEO) is among the most promising next-generation electrolyte materials for Li-ion batteries. However, its tendency to crystallize at room temperature hinders Li-ion movement, leading to degradation of battery performance, especially at high charge/discharge rates. To overcome this bottleneck, various strategies based on the core idea of suppressing the degree of crystallization in the PEO system have been tested. Despite some achievements, the design of PEO-based electrolytes is still largely trial-and-error in nature, as there is not yet a clear understanding of what the actual limiting factors are for Li+ diffusion inside crystalline PEO. Motivated by this, herein, we combined molecular dynamics (MD) and density functional theory (DFT) calculations to gain such an understanding. We find that, due to the unique nature of crystalline PEO, Li+ diffusion is highly anisotropic. Specifically, Li+ moves preferentially along the helical direction of the molecular chain but moves slowly between chains, which we attribute mainly to the local Li+ environment. This behaviour arises from the high energetic penalty associated with the Li+ coordination rearrangement required to hop between oxygen-rich helical channels. These findings suggest that rather than solely suppressing crystallinity, aligning PEO domains or even controlling their size could unlock higher ionic conductivity.