The emerging direction toward the ever-growing market of wearable electronics has contributed to the progress made in energy storage systems that are flexible while maintaining their electrochemical performance. Endowing lithium-ion batteries with high flexibility is currently considered to be one of the most essential choices in future. Here, we first propose the basic deformation mode according to the manifestation of flexibility and constructively reevaluate the concept of flexible lithium-ion batteries. Furthermore, the failure mechanism of flexible lithium-ion batteries is investigated with regard to their mechanical failure and electrochemical failure, and the related strategies of battery design and manufacturing are analyzed. More importantly, an in-depth analysis is conducted on the approaches to overcome mechanical failure through stress dispersion, stress absorption, prestress concentration, stress transfer, and other flexible reinforcement methods. Additionally, the advantages of suppressing electrochemical failure are discussed by enhancing the surface roughness, pore formation, surface coating, chemical bonding, in situ encapsulation, etc. Regarding self-healing technology, the general approaches taken for self-healing batteries to achieve flexibility are explained through the classification of macroscopic self-healing (to avert mechanical failure) and microscopic self-healing (to respond to electrochemical failure). Finally, after considering the current state of flexible lithium-ion batteries, future challenges are presented.