Mechanical analyses and structural design requirements for flexible energy storage devices

Flexible energy storage devices with excellent mechanical deformation performance are highly required to improve the integration degree of flexible electronics. Unlike those of traditional power sources, the mechanical reliability of flexible energy storage devices, including electrical performance retention and deformation endurance, has received much attention. To provide the guideline for the construction design of devices, the strain distribution and failure modes in the entire architecture should be comprehensively investigated during mechanical deformation. This review mainly focuses on the mechanical deformation characterization, analysis, and structural design strategies used in recent flexible lithium-ion batteries (LIBs) and supercapacitors (SCs). The primary theoretical calculation of bending strain in the devices is introduced first, and then several parameters to describe the bending status are summarized. Among those parameters, bending radius and its corresponding test methods and equipment are highlighted. Derivative strategies for structural design and an overview of their application progresses, such as selection of substrates, thickness of devices, employment of encapsulation, and novel architectural designs, are reviewed in detail. Finally, the challenges and prospects of flexible energy storage devices with reliable mechanical performance are discussed.