A high demand for flexible power sources is being driven by the rapid development of wearable smart electronics. Battery-enabled electronic gadgets are constantly outfitted with new functionalities, and they also change their physical appearance, becoming bendable, stretchable, rollable, and ultrathin. Flexible batteries have the potential to develop an ideal energy storage system for future electronics due to their advantages in safety, working temperature, high energy density, and packaging. The entire battery architecture must be transformed to design flexible batteries, including active materials, electrolyte, and separators. This chapter summarizes the key parameters used to evaluate the flexibility of batteries and materials. A computational simulation is also presented to estimate battery flexibility. Moreover, we discussed the configuration and design strategies for flexible batteries. The current status of flexible current collectors, solid electrolytes, and flexible design approaches for state-of-the-art flexible lithium-ion batteries (LIBs) is also addressed. Future research directions are also discussed based on the existing state of LIBs development. This exploration gives birth to flexible batteries, particularly lithium-based batteries, promising materials for ultra-modern, smart wearable devices.