Electrochromic energy storage devices (EESDs) including electrochromic supercapacitors (ESC) and electrochromic batteries (ECB) have received significant recent attention in wearables, smart windows, and colour-changing sunglasses due to their multi-functionality, including colour variation under various charge densities. The performance of EESDs is mainly dependent on the properties of three major components (i) the current collector/substrate (cc/substrate) (ii) the electrolyte and (iii) electrochromic materials (ECM). Among various EESDs, advanced flexible or stretchable devices offer better functionality than conventional rigid glass-based devices and are easily integrated with any curved surface. However, in flexible or stretchable EESDs, delamination, dissociation and degradation critically affect the lifecycle and stable performance and are key issues to solve for widespread deployment of the technology. A detailed review of the materials and their performance as flexible EESDs is therefore propitious for the design and engineering of next-generation ECBs and ESCs. In this review, we considered the importance of various materials and their implementation in flexible and stretchable EESD fabrication along with their potential application in sustainable energy systems.