Flexible lithium-ion batteries (LIBs) can be seamlessly integrated into flexible devices, such as flexible displays, wearable devices, and smart cards, to provide power for steady operation under mechanical deformation. An ideal flexible battery should have high flexibility, high energy density, and high power density simultaneously, which are often in conflict with each other. In this Perspective, we analyze the flexible batteries based on structural designs from both the component level and device level. Recent progress in flexible LIBs, including advances in porous structures for battery components, superslim designs, topological architectures, and battery structures with decoupling concepts, is reviewed. In the end, perspectives on the future of flexible batteries are presented and discussed. © 2019 American Chemical Society.