The development of stretchable electronics is indispensable for realizing next-generation wearable devices, such as sensors, health care devices, and electronic skin. A key challenge for achieving complete and independent wearable devices is developing stretchable power sources. This issue should be addressed appropriately before the realization of wearable devices. Very recently, stretchable aqueous rechargeable batteries as power supplies have received much attention for wearable devices owing to their intrinsic safety and high power density. In this Perspective, we present the current status and the latest advances in research on stretchable aqueous batteries, especially aqueous Li-ion batteries and zinc-based batteries. Also, we briefly provide the design of stretchable materials and battery systems for stretchable aqueous batteries. Furthermore, an overview of general technical issues confronting their development is presented, and a brief discussion on the future outlook of this field is provided.

With the rapidly approaching implementation of wearable electronic devices such as implantable devices, stretchable sensors, and healthcare devices, stretchable power sources have aroused worldwide attention as a key component in this emerging field. Among stretchable power sources, batteries, which store electrical energy through redox reactions during charge/discharge processes, are an attractive candidate because of their high energy density, high output voltage, and long‐term stability. In recent years, extensive efforts have been devoted to developing new materials and innovative structural designs for stretchable batteries. This review covers the latest advances in stretchable batteries, focusing on advanced stretchable materials and their design strategies. First, we provide a detailed overview of the materials aspects of components in a stretchable battery, including electrode materials, solid‐state electrolytes, and stretchable separator membranes. Second, we provide an overview on various structural engineering strategies to impart stretchability to batteries (i. e., wavy/buckling structures, island‐bridge structures, and origami/kirigami structures). Third, we summarize recently reported developments in stretchable batteries based on various chemistries, including Li‐based batteries, multivalent‐based batteries, and metal‐air batteries. Finally, we discuss the future perspectives and remaining challenges toward the practical application of stretchable batteries with reliable mechanical robustness and stable electrochemical performance under a physical strain.

High-theoretical capacity and low working potential make silicon ideal anode for lithium ion batteries. However, the large volume change of silicon upon lithiation/delithiation poses a critical challenge for stable battery operations. Here, we introduce an unprecedented design, which takes advantage of large deformation and ensures the structural stability of the material by developing a two-dimensional silicon nanosheet coated with a thin carbon layer. During electrochemical cycling, this carbon coated silicon nanosheet exhibits unique deformation patterns, featuring accommodation of deformation in the thickness direction upon lithiation, while forming ripples upon delithiation, as demonstrated by in situ transmission electron microscopy observation and chemomechanical simulation. The ripple formation presents a unique mechanism for releasing the cycling induced stress, rendering the electrode much more stable and durable than the uncoated counterparts. This work demonstrates a general principle as how to take the advantage of the large deformation materials for designing high capacity electrode.

We show that a high energy density can be achieved in a practical manner with freestanding electrodes without using conductive carbon, binders, and current collectors. We made and used a folded graphene composite electrode designed for a high areal capacity anode. The traditional thick graphene composite electrode, such as made by filtering graphene oxide to create a thin film and reducing it such as through chemical or thermal methods, has sluggish reaction kinetics. Instead, we have made and tested a thin composite film electrode that was folded several times using a water-assisted method; it provides a continuous electron transport path in the fold regions and introduces more channels between the folded layers, which significantly enhances the electron/ion transport kinetics. A fold electrode consisting of SnO/graphene with high areal loading of 5 mg cm has a high areal capacity of 4.15 mAh cm, well above commercial graphite anodes (2.50-3.50 mAh cm), while the thickness is maintained as low as ∼20 μm. The fold electrode shows stable cycling over 500 cycles at 1.70 mA cm and improved rate capability compared to thick electrodes with the same mass loading but without folds. A full cell of fold electrode coupled with LiCoO cathode was assembled and delivered an areal capacity of 2.84 mAh cm after 300 cycles. This folding strategy can be extended to other electrode materials and rechargeable batteries.

Nanowires (NWs) synthesized via chemical vapor deposition (CVD) have demonstrated significant improvement in lithium storage performance along with their outstanding accommodation of large volume changes during the charge/discharge process. Nevertheless, NW electrodes have been confined to the research level due to the lack of scalability and severe side reactions by their high surface area. Here, we present nanoporous Ge nanofibers (NPGeNFs) having moderate nanoporosity via a combination of simple electrospinning and a low-energetic zincothermic reduction reaction. In contrast with the CVD-assisted NW growth, our method provides high tunability of macro/microscopic morphologies such as a porosity, length, and diameter of the nanoscale 1D structures. Significantly, the customized NPGeNFs showed a highly suppressed volume expansion of less than 15% (for electrodes) after full lithation and excellent durability with high lithium storage performance over 500 cycles. Our approach offers effective 1D nanostructuring with highly customized geometries and can be extended to other applications including optoelectronics, catalysis, and energy conversion.

LIBs), stretchable supercapacitors, and stretchable silver-zinc batteries. [5][6][7][8] Most of them mainly focused on the development of deformable current collectors (e.g., embedding conductive materials in soft substrates or elastic substrates) [9,10] or structural layouts (e.g., helically coiled spring design, serpentine interconnected configuration, and origami structure). [11][12][13] In comparison, a stretchable separator membrane for deformable energystorage devices attracts little attention. The separator membrane is basically used to prevent physical and electrical contact between electrodes while offering an ion conduction channel. [14] Various types of stretchable batteries are being developed, and thus the stretchable properties of the separator membrane are also required. Generally, because ionic gel-polymer electrolytes (GPEs) are easily controllable and sufficiently deformable, they have been employed as the separator membrane in deformable energy-storage devices. [13,15,16] Although ion-conducting GPEs can be used as both electrolyte and separator, they have intrinsically lower ionic conductivity than liquid electrolytes [17] and poor mechanical properties which are likely to cause an internal short problem due to the contact of both electrodes under physical deformation. [18,19] In order to fabricate a reliable stretchable energy-storage device without these limitations, the presence of a physical separation barrier having an ion-conducting channel and stretchability is essential. Recently, Liu et al. reported a stretchable separator membrane for wavy-structured stretchable LIBs using electrospinning techniques. [20] Li et al. also used electrospinning process to fabricate a stretchable polyurethane separator for stretchable supercapacitor. [21] However, electrospinning has critical drawbacks such as the use of complex equipment, slow production rate, and possible toxicity of chemical residues in electrospun fibers. Moreover, it has the limitation for largescale production for industry level due to its high cost. [22,23] Given these limitations, it is still necessary to develop and improve the fabrication methodologies for stretchable separator membranes. Although various attempts have been made in the membrane component to achieve the complete stretchability of the battery, the development of the standardized separator membrane that can be applied to various types of stretchable battery has not yet been reported. Therefore, the development of stretchable separator membranes with high processibility With the emergence of stretchable electronic devices, there is growing interest in the development of deformable power accessories that can power them. To date, various approaches have been reported for replacing rigid components of typical batteries with elastic materials. Little attention, however, has been paid to stretchable separator membranes that can not only prevent internal short circuit but also provide an ionic conducting pathway between electrodes under extreme physical deformation. Herei...

A crumply and highly flexible lithium-ion battery is realized by using microfiber mat electrodes in which the microfibers are wound or webbed with conductive nanowires. This electrode architecture guarantees extraordinary mechanical durability without any increase in resistance after folding 1000 times. Its areal energy density is easily controllable by the number of folded stacks of a piece of the electrode mat. Deformable lithium-ion batteries of lithium iron phosphate as cathode and lithium titanium oxide as anode at high areal capacity (3.2 mAh cm ) are successfully operated without structural failure and performance loss, even after repeated crumpling and folding during charging and discharging.

Stretchable electronics have been considered a key technology in wearable and implantable medical devices. Although substantial advances have been made in key stretchable components, a stretchable electronic platform that integrates a stretchable power source and a stretchable printed circuit board (SPCB) has been a great challenge. Here, an intrinsically stretchable electronic device platform powered by a stretchable film battery is proposed so that the platform can be used as a stand-alone. The stretchable battery is used as a substrate for manufacturing device platforms where SPCB is printed and directly connected through via holes, thereby enabling an increase in integrated devices density. To achieve an intrinsically stretchable battery and high-performance circuit board, a novel concept of stretchable, self-healable, and pressure-sensitive polymer composite is designed. The platform is waterproof and maintains its stable electrical performance under extreme physical deformations. As a proof of concept, the integration of lightemitting diodes on the platform that can operate at large biaxial strain (125%) underwater is demonstrated.