large areas available on the skin or within the body. [3] Intrinsically stretchable batteries effectively make use of available space while perfectly complying to deformations and maintaining comfort for the wearer. [4][5][6] Recent developments exploit high-energydensity materials, such as Zn and Li-ion based batteries, [7][8][9][10] to enhance battery capacity and close the gap to rigid-island designs. [11] Despite the progress in capacity and rechargeability, the principle design of planar intrinsically stretchable batteries has largely remained unchanged. The interplay of electrodes, efficient current collectors, highly ionically conductive separators/gelelectrolytes, and their conformal orientation must collectively be optimized to boost battery performance. Promising solutions for single components, including multilayer current collectors, [12] tough electrodes, [13] and self-healing gel-electrolytes [14] were demonstrated recently. However, many prototypes still use the coplanar orientation of electrodes [4,12,[15][16][17] as proposed for the first soft batteries. [18] This design sacrifices performance due to increased ionic pathways in the gel electrolytes. Introducing a sandwich design of stacked battery components (Figure 1a), greatly reduces ionic pathways and therefore the internal resistance of the battery. [19] Soft (sandwich-design) batteries require electron-insulating separators, which have to meet high ion conductivity, extensibility, and chemical inertness. Hydrogels are often used in water-based systems, serving as gel Powering soft embodiments of robots, machines and electronics is a key issue that impacts emerging human friendly forms of technologies. Batteries as energy source enable their untethered operation at high power density but must be rendered elastic to fully comply with (soft) robots and human beings. Current intrinsically stretchable batteries typically show decreased performance when deformed due to design limitations, mainly imposed by the separator material. High quality stretchable separators such as gel electrolytes represent a key component of soft batteries that affects power, internal resistance, and capacity independently of battery chemistry. Here, polymerized high internal phase emulsions (polyHIPEs) are introduced as highly ionically conductive separators in stretchable (rechargeable) batteries. Highly porous (>80%) separators result in electrolyte to polyHIPE conductivity ratios of below 2, while maintaining stretchability of ≈50% strain. The high stretchability, tunable porosity, and fast ion transport enable stretchable batteries with internal resistance below 3 Ω and 16.8 mAh cm −2 capacity that power on-skin processing and communication electronics. The battery/separator architecture is universally applicable to boost battery performance and represents a step towards autonomous operation of conformable electronic skins for healthcare, robotics, and consumers.
