Strategies for developing flexible lithium batteries with high energy and high safety

Nowadays, flexible technology and related electronics are widely used in personal health monitoring, drug delivery, motion detection, power supply, sensors, and electronic skin, and thus greatly enrich our lives. The rapid increasing demand of flexible power sources for implantable medical and wearable electronic devices has simultaneously prompted extensive research in the scientific community on flexible energy storage devices. However, there are still some tough issues getting in the way of further industrialization of the deep application of flexible and wearable electronics, especially, the lack of suitable flexible power supply devices. The power supply device is the key component that guarantees a continuous, uninterrupted, and long-term operation, which is required to have high flexibility, high energy density, long time durability, and high safety for wearable purpose. The structure characteristics and potential application fields of flexible electronic devices require flexible energy storage devices with properties of not only excellent mechanical deformation ability but also high specific energy and high safety. Lithium batteries are considered as the ideal energy sources and are leading the development direction of flexible energy storage devices because of their low self-discharge rate, high energy density and long cycle life. But lithium-ion batteries that are widely used in current consumer electronics cannot meet these demands due to their rigid package, suboptimal cycle stability, and possible safety issues. Traditional lithium-ion batteries have poor flexibility due to the stacked structures of battery packs for fulfilling high energy density and the low yield strain of materials used as the cathode/anode, electrolyte, separator, and current collector in batteries, such as metal collectors, aluminum-plastic films on encapsulating lithium-ion batteries, etc. When subjected to external forces, the current collectors are prone to crack, inducing mechanical failure such as the separation of electrode active materials and collectors, which eventually result in short circuit in the battery, leading to serious safety issues. Therefore, the main problem still focuses on how to simultaneously endow lithium batteries with high flexibility, high safety, and high energy density. The most effective way to develop high-flexible lithium-ion batteries can be classified into two categories: One is fabrication of flexible materials as the mobile components to assemble power supply devices, e.g., polymers, carbon-based materials, and gel materials, etc. The other is construction of special architectures through rational structural design to attain high flexibility, e.g., origami and kirigami. To this end, the development of flexible lithium batteries with high specific energy and high safety in the future is thoroughly discussed in this review. Firstly, representative scenarios of electronic devices/flexible lithium batteries for common applications are presented to highlight the demand for high specific energy and high safety of flexible batteries. Then, we describe various strategies for effectively improving flexibility, safety, and energy density of lithium batteries from two aspects: Material selection, including collector, electrolyte and electrode active materials, and structural design, e.g., origami/kirigami structures, bionic structures and "triple-phase percolation" structures. Finally, the future opportunities and challenges of flexible lithium batteries are fully discussed in this review.