Prof Zijian Zheng and Dr Qiyao Huang recently summarised the key performance metrics of flexible batteries (FBs), and proposed a figure of merit (FOM) for FBs in a paper published in Joule. The FOM is a single metric that not only allows for easy reporting and comparison of the performance of FBs but also provides a direct linkage between lab-based testing and industrial development.
This work is published as “A Figure of Merit for Flexible Batteries” in Joule (2020) 4, 1–3, July 15. Joule is one of the leading and most respected journals in energy and materials related research with a high impact factor of 27.059.
FBs will be in great demand for future flexible and wearable electronics in health care, consumable products, soft robotics, and internet-of-things. The key challenges are finding ways to develop these highly flexible batteries while keeping their high energy density.
While energy density has been a well-defined metric of batteries for decades, it is still difficult to benchmark flexibility among the tremendous amount of findings reported by many different research groups. In most reports, the bending radius is used as the metric to evaluate the flexibility; a smaller bending radius indicates higher flexibility of the battery. However, such an evaluation is somehow misleading because cell failure during flexing is determined by the strain, which not only depends on the bending radius but also the cell thickness. Unfortunately, the size and thickness of test cells usually vary significantly among different reports, and a simple comparison between bending radii becomes less meaningful. On the other hand, there is little discussion in the literature on the industrial requirements of FBs and how to meet them for different flexible and wearable applications. Owing to the disconnect between academic research and industrial requirements, and the vast difference in materials preparation and cell structures in the different studies, it is even more challenging to project the suitability of the lab-made cells to real applications.
Flexible battery enables the seamless implementation of power supply to flexible and wearable electronics. It not only enhances the energy capacity by fully utilizing the available space but also revolutionizes the form factors of future device design. Therefore, it is of paramount importance to critically evaluate the performance of flexible batteries and advocate a figure of merit for flexible batteries, with the aim of providing a benchmark among the state-of-the-arts and projecting the suitability of the laboratory study to industrial requirements.
