Evaluating Flexibility and Wearability of Flexible Energy Storage Devices

摘要

In summary, we investigated the commonly used approaches to evaluate flexibility of flexible ESDs. For bending tests, all three parameters (L, 0, and R) have vast impacts on the post-test electrochemical performance of flexible ESDs. We would like to suggest that L, 9, and R should all be provided for the precise evaluation of bending durability. Second, we also propose softness as a parameter to evaluate comfortability and wearability of a flexible ESD. Different flexible current collectors and different coating techniques adopted will result in a totally different softness of ESDs. More importantly, the softness is vastly affected by the thickness of the active materials and thus the energy density. A high energy density resulting from a thick active layer will unavoidably sacrifice the softness of a flexible ESD. We suggest that softness should be provided when an energy density, especially a high energy density, is claimed for a flexible ESD. Finally, the residual strains of them after being stretched has been long neglected for many stretchable ESDs that were reported. Our study indicates that with current materials and design of stretchable ESDs, the residual strain is not avoidable and significant. Besides the above-mentioned thin-film devices, fiber-shaped ESDs have gained widespread interests owing to their tiny volume, high flexibility, and excellent wearability, which can be woven into textiles.12 With different structures (coaxial and twisted) and configurations, the evaluation standards of flexibility, stretchability, and wearability may be different from planar-structured devices. We hope to set a starting point for discussion on the evaluation standards on flexible and wearable ESDs and facilitate the development of flexible and wearable ESDs in terms of material selection, electrolyte design, and evaluation methodologies.