Flexible lithium ion batteries (LIBs) have received considerable attention as a key component to enable future flexible electronic devices. A number of designs for flexible LIBs have been reported in recent years; in this article, we review recent progress. We focus on how flexibility can be introduced into each component of the LIB, including the active materials, electrolytes, separators, and current collectors. Approaches to integrating each component into a single device are described and the corresponding changes in the electrochemical and mechanical properties are discussed. Finally, the key challenges in the development of flexible LIBs are summarized.
