 The proposed structure for sulfurized polyacrylonitrile, SPAN, is composed of short Sx chains attached to cyclized, partially dehydrogenated, and ribbon-like polyacrylonitrile backbones. This structure explains many unique electrochemical phenomena observed from SPAN cells, such as high-capacity and excellent rate capability, which have not been fully explained by previous studies. This article was authored by Xing S. Zhang.