Categorizing wearable batteries: Unidirectional and omnidirectional deformable batteries

Wearable batteries suffer from the chaotic state where the wearable concept is indiscriminately used regardless of the significant differences in flexibility, energy density, and other aspects, because of the difference between wearable batteries and commercial wearable electronics. Here, we initially present the chaotic state of wearable batteries based on the battery chemistry. Then, the specific requirements of wearable electronics are refined in terms of multifunctional flexible electronics and close-fitting health monitoring electronics. Restrained by the trade-off between energy density and flexibility, we propose to subdivide wearable batteries into unidirectional (bendable) and omnidirectional (soft) deformable batteries, aiming at better positioning flexible batteries and practically powering wearable electronics. These two types of wearable batteries are conceptually defined, and the promising candidates are proposed based on the match with commercial wearable electronics. Finally, we summarize the advances and present the challenges in wearable batteries to promote the development of this prosperous field.